Next Steps in Signaling                                   H. Schulzrinne
Internet-Draft                                               Columbia U.
Intended status: Standards Track                              R. Hancock
Expires: March 4, 2007                                       Siemens/RMR
                                                         August 31, 2006

              GIST:  General Internet Signaling Transport

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Copyright Notice

   Copyright (C) The Internet Society (2006).

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   This document specifies protocol stacks for the routing and transport
   of per-flow signaling messages along the path taken by that flow
   through the network.  The design uses existing transport and security
   protocols under a common messaging layer, the General Internet
   Signaling Transport (GIST), which provides a universal service for
   diverse signaling applications.  GIST does not handle signaling
   application state itself, but manages its own internal state and the
   configuration of the underlying transport and security protocols to
   enable the transfer of messages in both directions along the flow
   path.  The combination of GIST and the lower layer transport and
   security protocols provides a solution for the base protocol
   component of the "Next Steps in Signaling" framework.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Restrictions on Scope . . . . . . . . . . . . . . . . . .   5
   2.  Requirements Notation and Terminology . . . . . . . . . . . .   6
   3.  Design Overview . . . . . . . . . . . . . . . . . . . . . . .   9
     3.1.  Overall Design Approach . . . . . . . . . . . . . . . . .   9
     3.2.  Modes and Messaging Associations  . . . . . . . . . . . .  10
     3.3.  Message Routing Methods . . . . . . . . . . . . . . . . .  12
     3.4.  GIST Messages . . . . . . . . . . . . . . . . . . . . . .  13
     3.5.  Signaling Sessions  . . . . . . . . . . . . . . . . . . .  14
     3.6.  Signaling Applications and NSLPIDs  . . . . . . . . . . .  15
     3.7.  Example of Operation  . . . . . . . . . . . . . . . . . .  15
   4.  GIST Processing Overview  . . . . . . . . . . . . . . . . . .  18
     4.1.  GIST Service Interface  . . . . . . . . . . . . . . . . .  18
     4.2.  GIST State  . . . . . . . . . . . . . . . . . . . . . . .  20
     4.3.  Basic GIST Message Processing . . . . . . . . . . . . . .  21
     4.4.  Routing State and Messaging Association Maintenance . . .  27
   5.  Message Formats and Transport . . . . . . . . . . . . . . . .  35
     5.1.  GIST Messages . . . . . . . . . . . . . . . . . . . . . .  35
     5.2.  Information Elements  . . . . . . . . . . . . . . . . . .  37
     5.3.  D-mode Transport  . . . . . . . . . . . . . . . . . . . .  41
     5.4.  C-mode Transport  . . . . . . . . . . . . . . . . . . . .  44
     5.5.  Message Type/Encapsulation Relationships  . . . . . . . .  46
     5.6.  Error Message Processing  . . . . . . . . . . . . . . . .  47
     5.7.  Messaging Association Setup . . . . . . . . . . . . . . .  48
     5.8.  Specific Message Routing Methods  . . . . . . . . . . . .  52
   6.  Formal Protocol Specification . . . . . . . . . . . . . . . .  57
     6.1.  Node Processing . . . . . . . . . . . . . . . . . . . . .  59
     6.2.  Query Node Processing . . . . . . . . . . . . . . . . . .  60
     6.3.  Responder Node Processing . . . . . . . . . . . . . . . .  63
     6.4.  Messaging Association Processing  . . . . . . . . . . . .  66

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   7.  Advanced Protocol Features  . . . . . . . . . . . . . . . . .  70
     7.1.  Route Changes and Local Repair  . . . . . . . . . . . . .  70
     7.2.  NAT Traversal . . . . . . . . . . . . . . . . . . . . . .  77
     7.3.  Interaction with IP Tunnelling  . . . . . . . . . . . . .  80
     7.4.  IPv4-IPv6 Transition and Interworking . . . . . . . . . .  80
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  83
     8.1.  Message Confidentiality and Integrity . . . . . . . . . .  83
     8.2.  Peer Node Authentication  . . . . . . . . . . . . . . . .  84
     8.3.  Routing State Integrity . . . . . . . . . . . . . . . . .  85
     8.4.  Denial of Service Prevention  . . . . . . . . . . . . . .  86
     8.5.  Requirements on Cookie Mechanisms . . . . . . . . . . . .  87
     8.6.  Security Protocol Selection Policy  . . . . . . . . . . .  89
     8.7.  Residual Threats  . . . . . . . . . . . . . . . . . . . .  90
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  91
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  96
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  97
     11.1. Normative References  . . . . . . . . . . . . . . . . . .  97
     11.2. Informative References  . . . . . . . . . . . . . . . . .  97
   Appendix A.  Bit-Level Formats and Error Messages . . . . . . . . 100
     A.1.  The GIST Common Header  . . . . . . . . . . . . . . . . . 100
     A.2.  General Object Format . . . . . . . . . . . . . . . . . . 101
     A.3.  GIST TLV Objects  . . . . . . . . . . . . . . . . . . . . 102
     A.4.  Errors  . . . . . . . . . . . . . . . . . . . . . . . . . 111
   Appendix B.  API between GIST and Signaling Applications  . . . . 119
     B.1.  SendMessage . . . . . . . . . . . . . . . . . . . . . . . 119
     B.2.  RecvMessage . . . . . . . . . . . . . . . . . . . . . . . 121
     B.3.  MessageStatus . . . . . . . . . . . . . . . . . . . . . . 122
     B.4.  NetworkNotification . . . . . . . . . . . . . . . . . . . 123
     B.5.  SetStateLifetime  . . . . . . . . . . . . . . . . . . . . 123
     B.6.  InvalidateRoutingState  . . . . . . . . . . . . . . . . . 124
   Appendix C.  Example Routing State Table and Handshake Message
                Sequence . . . . . . . . . . . . . . . . . . . . . . 125
   Appendix D.  Change History . . . . . . . . . . . . . . . . . . . 127
     D.1.  Changes In Version -11  . . . . . . . . . . . . . . . . . 127
     D.2.  Changes In Version -10  . . . . . . . . . . . . . . . . . 128
     D.3.  Changes In Version -09  . . . . . . . . . . . . . . . . . 131
     D.4.  Changes In Version -08  . . . . . . . . . . . . . . . . . 132
     D.5.  Changes In Version -07  . . . . . . . . . . . . . . . . . 134
     D.6.  Changes In Version -06  . . . . . . . . . . . . . . . . . 135
     D.7.  Changes In Version -05  . . . . . . . . . . . . . . . . . 136
     D.8.  Changes In Version -04  . . . . . . . . . . . . . . . . . 137
     D.9.  Changes In Version -03  . . . . . . . . . . . . . . . . . 138
     D.10. Changes In Version -02  . . . . . . . . . . . . . . . . . 139
     D.11. Changes In Version -01  . . . . . . . . . . . . . . . . . 140
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 143
   Intellectual Property and Copyright Statements  . . . . . . . . . 144

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1.  Introduction

   Signaling involves the manipulation of state held in network
   elements.  'Manipulation' could mean setting up, modifying and
   tearing down state; or it could simply mean the monitoring of state
   which is managed by other mechanisms.

   This specification concentrates mainly on path-coupled signaling,
   which involves network elements which are located on the path taken
   by a particular data flow, possibly including but not limited to the
   flow endpoints.  Indeed, there are almost always more than two
   participants in a path-coupled signaling session, although there is
   no need for every node on the path to participate.  Path-coupled
   signaling thus excludes end-to-end higher-layer application
   signaling.  In the context of path-coupled signaling, examples of
   state management include network resource reservation, firewall
   configuration, and state used in active networking; examples of state
   monitoring are the discovery of instantaneous path properties, such
   as available bandwidth or cumulative queuing delay.  Each of these
   different uses of signaling is referred to as a signaling
   application.  Note that GIST can be extended to cover other types of
   signaling pattern, where the message routing is not related to any
   end-to-end flow in the network, in which case the distinction between
   GIST and end-to-end higher-layer signaling will be drawn differently
   or not at all.

   Every signaling application requires a set of state management rules,
   as well as protocol support to exchange messages along the data path.
   Several aspects of this protocol support are common to all or a large
   number of signaling applications, and hence can be developed as a
   common protocol.  The NSIS framework given in [26] provides a
   rationale for a function split between the common and application
   specific protocols, and gives outline requirements for the former,
   the 'NSIS Transport Layer Protocol' (NTLP).  The application specific
   protocols are referred to as 'NSIS Signaling Layer Protocols'
   (NSLPs), and are defined in separate documents.

   This specification provides a concrete solution for the NTLP.  It is
   based on the use of existing transport and security protocols under a
   common messaging layer, the General Internet Signaling Transport
   (GIST).  GIST does not handle signaling application state itself; in
   that crucial respect, it differs from application signaling protocols
   such as SIP, RTSP, and the control component of FTP.  Instead, GIST
   manages its own internal state and the configuration of the
   underlying transport and security protocols to ensure the transfer of
   signaling messages on behalf of signaling applications in both
   directions along the flow path.

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   The structure of this specification is as follows.  Section 2 defines
   terminology, and Section 3 gives an informal overview of the protocol
   design principles and operation.  The normative specification is
   contained mainly in Section 4 to Section 8.  Section 4 describes the
   message sequences and Section 5 their format and contents.  Note that
   the detailed bit formats are given in Appendix A.  The protocol
   operation is captured in the form of state machine language in
   Section 6.  Section 7 describes some more advanced protocol features
   and security considerations are contained in Section 8.  In addition,
   Section 9 gives the IANA considerations, Appendix B describes an
   abstract API for the service which GIST provides to signaling
   applications, and Appendix C provides an example message flow.

1.1.  Restrictions on Scope

   This section briefly lists some important restrictions on GIST
   applicability and functionality.  In some cases, these are implicit
   consequences of the functionality split developed in the NSIS
   framework; in others, they are restrictions on the types of scenario
   in which GIST can operate correctly.

   Flow splitting:  In some cases, e.g. where packet-level load sharing
      has been implemented, the path taken by a single flow in the
      network may not be well defined.  If this is the case, GIST cannot
      route signaling meaningfully.  In some circumstances, GIST
      implementations could detect this condition, but even this cannot
      be guaranteed.

   Multicast:  GIST does not handle multicast flows.  This includes
      classical IP multicast and any of the small group multicast
      schemes recently proposed.

   Legacy NATs:  GIST messages will generally pass through NATs, but
      unless the NAT is GIST-aware, any addressing data carried in the
      payload will not be handled correctly.  There is a dual problem of
      whether the GIST peers either side of the boundary can work out
      how to address each other, and whether they can work out what
      translation to apply to the signaling packet payloads.  The
      fundamental problem is that GIST messages contain three or four
      interdependent addresses which all have to be consistently
      translated, and existing generic NAT traversal techniques such as
      STUN [23] or TURN [24] can process only two.  Appropriate
      behaviour for a GIST-aware NAT is discussed in Section 7.2.

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2.  Requirements Notation and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [2].

   The terminology used in this specification is defined in this
   section.  The basic entities relevant at the GIST level are shown in
   Figure 1.  In particular, this diagram distinguishes the different
   address types as being associated with a flow (end-to-end addresses)
   or signaling (addresses of adjacent signaling peers).

   Source                 GIST (adjacent) peer nodes         Destination

   IP address              IP addresses = Signaling          IP address
   = Flow                Source/Destination Addresses        = Flow
   Source             (depending on signaling direction)     Destination
   Address                  |                   |            Address
                            V                   V
   +--------+           +------+  Data Flow  +------+         +--------+
   |  Flow  |-----------|------|-------------|------|-------->|  Flow  |
   | Sender |           |      |             |      |         |Receiver|
   +--------+           | GIST |============>| GIST |         +--------+
                        | Node |<============| Node |
                        +------+  Signaling  +------+
                          GN1       Flow       GN2

                  >>>>>>>>>>>>>>>>>  =  Downstream direction
                  <<<<<<<<<<<<<<<<<  =  Upstream direction

                        Figure 1: Basic Terminology

   [Data] Flow:  A set of packets identified by some fixed combination
      of header fields.  Flows are unidirectional; a bidirectional
      communication is considered a pair of unidirectional flows.

   Session:  A single application layer flow of information for which
      some state information is to be manipulated or monitored.  See
      Section 3.5 for further detailed discussion.

   Session Identifier (SID):  A long, opaque identifier for a session.

   [Flow] Sender:  The node in the network which is the source of the
      packets in a flow.  A sender could be a host, or a router if for
      example the flow is actually an aggregate.

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   [Flow] Receiver:  The node in the network which is the sink for the
      packets in a flow.

   Downstream:  In the same direction as the data flow.

   Upstream:  In the opposite direction to the data flow.

   GIST Node:  Any node along the data path supporting GIST, regardless
      of what signaling applications it supports.

   [Adjacent] Peer:  The next node along the data path, in the upstream
      or downstream direction, with which a GIST node explicitly
      interacts.  The GIST peer discovery mechanisms implicitly
      determine whether two nodes will be adjacent.  It is possible for
      adjacencies to skip over intermediate nodes which decide not to
      take part in the signaling exchange at the NTLP layer; even if
      such nodes process parts of the signaling messages, they store no
      state about the session and are never visible at the GIST level to
      the nodes on either side.

   Datagram Mode (D-mode):  A mode of sending GIST messages between
      nodes without using any transport layer state or security
      protection.  Datagram mode uses UDP encapsulation, with source and
      destination IP addresses derived either from the flow definition
      or previously discovered adjacency information.

   Connection Mode (C-mode):  A mode of sending GIST messages directly
      between nodes using point-to-point messaging associations (see
      below).  Connection mode allows the re-use of existing transport
      and security protocols where such functionality is required.

   Messaging Association (MA):  A single connection between two
      explicitly identified GIST adjacent peers, i.e. between a given
      signaling source and destination address.  A messaging association
      may use a specific transport protocol and known ports.  If
      security protection is required, it may use a specific network
      layer security association, or use a transport layer security
      association internally.  A messaging association is bidirectional;
      signaling messages can be sent over it in either direction, and
      can refer to flows of either direction.

   Message Routing Method (MRM):  There can be different algorithms for
      discovering the route that signaling messages should take.  These
      are referred to as message routing methods, and GIST supports
      alternatives within a common protocol framework.  See Section 3.3.

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   Message Routing Information (MRI):  The set of data item values which
      is used to route a signaling message according to a particular
      MRM; for example, for routing along a flow path, the MRI includes
      flow source and destination addresses, protocol and port numbers.
      See Section 3.3.

   Transfer Attributes:  A description of the requirements which a
      signaling application has for the delivery of a particular
      message; for example, whether the message should be delivered
      reliably.  See Section 4.1.2.

   Router Alert Option (RAO):  An option that can be included in IP v4
      and v6 headers to assist in the packet interception process; see
      [1] and [5].

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3.  Design Overview

3.1.  Overall Design Approach

   The generic requirements identified in the NSIS framework [26] for
   transport of signaling messages are essentially two-fold:

   Routing:  Determine how to reach the adjacent signaling node along
      each direction of the data path (the GIST peer), and if necessary
      explicitly establish addressing and identity information about
      that peer;

   Transport:  Deliver the signaling information to that peer.

   To meet the routing requirement, one possibility is for the node to
   use local routing state information to determine the identity of the
   GIST peer explicitly.  GIST defines a three-way handshake which
   probes the network to set up the necessary routing state between
   adjacent peers, during which signaling applications can also exchange
   data.  Once the routing decision has been made, the node has to
   select a mechanism for transport of the message to the peer.  GIST
   divides the transport problems into two categories, the easy and the
   difficult.  It handles the easy cases internally, and uses well-
   understood transport protocols for the harder cases.  Here, with
   details discussed later, "easy" messages are those that are sized
   well below the lowest maximum transmission unit (MTU) along a path,
   are infrequent enough not to cause concerns about congestion and flow
   control, and do not need security protection or guaranteed delivery.

   In [26] all of these routing and transport requirements are assigned
   to a single notional protocol, the NSIS Transport Layer Protocol
   (NTLP).  The strategy of splitting the transport problem leads to a
   layered structure for the NTLP, of a specialised GIST messaging layer
   running over standard transport and security protocols, as shown in
   Figure 2.  This also shows GIST offering its services to upper layers
   at an abstract interface, the GIST API, further discussed in
   Section 4.1.

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          ^^                      +-------------+
          ||                      |  Signaling  |
         NSIS        +------------|Application 2|
       Signaling     |  Signaling +-------------+
      Application    |Application 1|         |
         Level       +-------------+         |
          ||             |                   |
          VV             |                   |
                 ========|===================|=====  <-- GIST API
                         |                   |
          ^^       +------------------------------------------------+
          ||       |+-----------------------+      +--------------+ |
          ||       ||         GIST          |      | GIST State   | |
          ||       ||     Encapsulation     |<<<>>>| Maintenance  | |
          ||       |+-----------------------+      +--------------+ |
          ||       | GIST: Messaging Layer                          |
          ||       +------------------------------------------------+
         NSIS              |       |       |       |
       Transport      ..................................
         Level        . Transport Layer Security (TLS) .
        (NTLP)        ..................................
          ||               |       |       |       |
          ||             +----+  +----+  +----+  +----+
          ||             |UDP |  |TCP |  |SCTP|  |DCCP| ... other
          ||             +----+  +----+  +----+  +----+     protocols
          ||               |       |       |       |
          ||             .............................
          ||             .     IP Layer Security     .
          ||             .............................
          VV               |       |       |       |
                           |       |       |       |
                   |                      IP                      |

             Figure 2: Protocol Stacks for Signaling Transport

3.2.  Modes and Messaging Associations

   Internally, GIST has two modes of operation:

   Datagram mode (D-mode):  used for small, infrequent messages with
      modest delay constraints and no security requirements; it must
      also be used when no routing state exists.

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   Connection mode (C-mode):  used for larger data objects or where fast
      state setup in the face of packet loss is desirable, or where
      channel security is required.

   D-mode uses UDP, as this is the only encapsulation which does not
   require per-message shared state to be maintained between the peers.
   C-mode can in principle use any stream or message-oriented transport
   protocol; this specification defines TCP as the initial choice.  It
   can in principle employ specific network layer security associations,
   or an internal transport layer security association; this
   specification defines TLS as the initial choice.  When GIST messages
   are carried in C-mode, they are treated just like any other traffic
   by intermediate routers between the GIST peers.  Indeed, it would be
   impossible for intermediate routers to carry out any processing on
   the messages without terminating the transport and security protocols

   It is possible to mix these two modes along a path.  This allows, for
   example, the use of D-mode at the edges of the network and C-mode in
   the core of the network.  Such combinations may make operation more
   efficient for mobile endpoints, while allowing multiplexing of
   signaling messages across shared security associations and transport
   connections between core routers.

   It must be understood that the routing and transport decisions made
   by GIST are not independent.  If the message transfer has
   requirements that require C-mode, for example if the message is so
   large that fragmentation is required, this can only be used between
   explicitly identified nodes.  In such cases, GIST carries out the
   three-way handshake initially in D-mode to identify the peer and then
   sets up the necessary connections if they do not already exist.  It
   must also be understood that the signaling application does not make
   the D-mode/C-mode selection directly; rather, this decision is made
   by GIST on the basis of the message characteristics and the transfer
   attributes stated by the application.  The distinction is not visible
   at the GIST service interface.

   In general, the state associated with C-mode messaging to a
   particular peer (signaling destination address, protocol and port
   numbers, internal protocol configuration and state information) is
   referred to as a messaging association (MA).  There may be any number
   of MAs between two GIST peers although the usual case is zero or one.
   They are set up and torn down by management actions within GIST

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3.3.  Message Routing Methods

   The baseline message routing functionality in GIST is that signaling
   messages follow a route defined by an existing flow in the network,
   visiting a subset of the nodes through which it passes.  This is the
   appropriate behaviour for application scenarios where the purpose of
   the signaling is to manipulate resources for that flow.  However,
   there are scenarios for which other behaviours are applicable.  Two
   examples are:

   Predictive Routing:  Here, the intent is to signal along a path that
      the data flow may follow in the future.  Possible cases are pre-
      installation of state on the backup path that would be used in the
      event of a link failure, and predictive installation of state on
      the path that will be used after a mobile node handover.

   NAT Address Reservations:  This applies to the case where a node
      behind a NAT wishes to reserve an address at which it can be
      reached by a sender on the other side.  This requires a message to
      be sent outbound from what will be the flow receiver although no
      reverse routing state for the flow yet exists.

   Most of the details of GIST operation are independent of which
   alternative is being used.  Therefore, the GIST design encapsulates
   the routing-dependent details as a message routing method (MRM), and
   allows multiple MRMs to be defined.  The default is the path-coupled
   MRM, which corresponds to the baseline functionality described above;
   a second MRM for the NAT Address Reservation case is also defined.

   The content of a MRM definition is as follows, using the path-coupled
   MRM as an example:

   o  The format of the information that describes the path that the
      signaling should take, the Message Routing Information (MRI).  For
      the path-coupled MRM, this is just the Flow Identifier (see
      Section and some additional control information.
      Specifically, the MRI always includes a flag to distinguish
      between the two directions that signaling messages can take,
      denoted 'upstream' and 'downstream'.

   o  A specification of the IP-level encapsulation of the messages
      which probe the network to discover the adjacent peers.  A
      downstream encapsulation must be defined; an upstream
      encapsulation is optional.  For the path-coupled MRM, this
      information is given in Section and Section

   o  A specification of what validation checks GIST should apply to the
      probe messages, for example to protect against IP address spoofing

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      attacks.  The checks may be dependent on the direction (upstream
      or downstream) of the message.  For the path-coupled MRM, the
      downstream validity check is basically a form of ingress
      filtering, also discussed in Section

   o  The mechanism(s) available for route change detection, i.e. any
      change in the neighbour relationships that the MRM discovers.  The
      default case for any MRM is soft-state refresh, but additional
      supporting techniques may be possible; see Section 7.1.2.

   In addition, it should be noted that NAT traversal may require
   translation of fields in the MRI object carried in GIST messages (see
   Section 7.2).  The generic MRI format includes a flag that must be
   given as part of the MRM definition, to indicate if some kind of
   translation is necessary.  Development of a new MRM therefore
   includes updates to the GIST specification, and may include updates
   to specifications of NAT behaviour.  These updates may be done in
   separate documents as is the case for the base GIST specification, as
   described in Section 7.2.2.

   The MRI is passed explicitly between signaling applications and GIST;
   therefore, signaling application specifications must define which
   MRMs they require.  Signaling applications may use fields in the MRI
   in their packet classifiers; if they use additional information for
   packet classification, this would be carried at the NSLP level and so
   would be invisible to GIST.  Any node hosting a particular signaling
   application MUST use a GIST implementation that supports the
   corresponding MRMs.  The GIST processing rules enforce that nodes
   which do not host the signaling application are not forced to handle
   messages for it at the GIST level, so it does not matter if they
   support the MRM or not.

3.4.  GIST Messages

   GIST has six message types: Query, Response, Confirm, Data, Error,
   and MA-Hello.  Apart from the invocation of the messaging association
   protocols, all GIST communication consists of these messages.  In
   addition, all signaling application data is carried as additional
   payloads in these messages, alongside the GIST information.

   The first three messages implement the handshake that GIST uses to
   set up routing state and messaging associations.  The handshake is
   initiated from the Querying node towards the Responding node.  The
   first message is the Query, which is encapsulated in a special way
   depending on the message routing method, in order to probe the
   network infrastructure so that the correct peer will intercept it and
   become the Responding node.  A Query always triggers a Response in
   the reverse direction as the second message of the handshake.  As

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   part of the defence against denial of service attacks, the Responding
   node can delay state installation until a return routability check,
   and require the Querying node to complete the handshake with the
   Confirm.  All of these three messages can optionally carry signaling
   application data.

   The Data message is used purely to encapsulate signaling application
   data.  Usually it is sent using pre-established routing state.
   However, if there are no security or transport requirements and no
   need for persistent reverse routing state, it can also be sent in the
   same way as the Query.  Finally, Error messages are used to indicate
   error conditions at the GIST level, and the MA-Hello message can be
   used as a keepalive for the messaging association protocols.

3.5.  Signaling Sessions

   GIST allows signaling applications to associate each of their
   messages with a signaling session.  Informally, given an application
   layer exchange of information for which some network control state
   information is to be manipulated or monitored, the corresponding
   signaling messages should be associated with the same session.
   Signaling applications provide the session identifier (SID) whenever
   they wish to send a message, and GIST reports the SID when a message
   is received.

   Most GIST processing and state information is related to the flow
   (defined by the MRI, see above) and signaling application (given by
   the NSLP identifier, see below).  There are several possible
   relationships between flows and sessions, for example:

   o  The simplest case is that all messages for the same flow have the
      same SID.

   o  Messages for more than one flow may use the same SID, for example
      because one flow is replacing another in a mobility or multihoming

   o  A single flow may have messages for different SIDs, for example
      from independently operating signaling applications.

   Because of this range of options, GIST does not perform any
   validation on how signaling applications map between flows and
   sessions, nor does it perform any validation on the properties of the
   SID itself.  In particular, when a new SID is needed, logically it
   should be generated by the signaling application.  NSIS
   implementations could provide common functionality to generate SIDs
   for use by any signaling application, but this is not part of GIST.
   GIST only defines the syntax of the SID as an opaque 128-bit

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   The SID assignment has the following impact on GIST processing:

   o  Messages with the same SID that are to be delivered reliably
      between the same GIST peers are delivered in order.

   o  All other messages are handled independently.

   o  GIST identifies routing state (upstream and downstream peer) by
      the triplet (MRI, NSLP, SID).

   Strictly, the routing state should not depend on the SID.  However,
   if the routing state is keyed only by (MRI, NSLP), there is a trivial
   denial of service attack (see Section 8.3) where a malicious off-path
   node asserts that it is the peer for a particular flow.  Instead, the
   routing state is also segregated between different SIDs, which means
   that the attacking node can only disrupt a signaling session if it
   can guess the corresponding SID.  A consequence of this design is
   that signaling applications SHOULD choose SIDs so that they are
   cryptographically random, and SHOULD NOT use several SIDs for the
   same flow, to avoid additional load from routing state maintenance.
   Guidance on secure randomness generation can be found in [28].

3.6.  Signaling Applications and NSLPIDs

   The functionality for signaling applications is supported by NSIS
   signaling layer protocols (NSLPs).  Each NSLP is identified by a 16
   bit NSLP identifier (NSLPID), assigned by IANA (Section 9).  A single
   signaling application, such as resource reservation, may define a
   family of NSLPs to implement its functionality, for example to carry
   out signaling operations at different levels in a hierarchy (cf.
   [19]).  However, the interactions between the different NSLPs (for
   example, to relate aggregation levels or aggregation region
   boundaries in the resource management case) are handled at the
   signaling application level; the NSLPID is the only information
   visible to GIST about the signaling application being used.

3.7.  Example of Operation

   This section presents an example of GIST usage in a relatively simple
   (in particular, NAT-free) signaling scenario, to illustrate its main

   Consider the case of an RSVP-like signaling application which
   allocates resources for a single unicast flow.  We will consider how
   GIST transfers messages between two adjacent peers along the path,
   GN1 and GN2 (see Figure 1 in Section 2).  In this example, the end-

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   to-end exchange is initiated by the signaling application instance in
   the sender; we take up the story at the point where the first message
   is being processed above the GIST layer by the signaling application
   in GN1.

   1.  The signaling application in GN1 determines that this message is
       a simple description of resources that would be appropriate for
       the flow.  It determines that it has no special security or
       transport requirements for the message, but simply that it should
       be transferred to the next downstream signaling application peer
       on the path that the flow will take.

   2.  The message payload is passed to the GIST layer in GN1, along
       with a definition of the flow and description of the message
       transfer attributes {unsecured, unreliable}.  GIST determines
       that this particular message does not require fragmentation and
       that it has no knowledge of the next peer for this flow and
       signaling application; however, it also determines that this
       application is likely to require secured upstream and downstream
       transport of large messages in the future.  This determination is
       a function of node-local policy interactions between GIST and the
       signaling application.

   3.  GN1 therefore constructs a GIST Query carrying the NSLP payload,
       and additional payloads at the GIST level to be used to initiate
       a messaging association.  The Query is encapsulated in a UDP
       datagram and injected into the network, addressed towards the
       flow destination and with an IP Router Alert Option (RAO)

   4.  The Query passes through the network towards the flow receiver,
       and is seen by each router in turn.  GIST-unaware routers will
       not recognise the RAO value and will forward the message
       unchanged; GIST-aware routers which do not support the NSLP in
       question will also forward the message basically unchanged,
       although they may need to process more of the message to decide

   5.  The message is intercepted at GN2.  The GIST layer identifies the
       message as relevant to a local signaling application, and passes
       the NSLP payload and flow description upwards to it.  The
       signaling application in GN2 indicates to GIST that it will peer
       with GN1 and so GIST should proceed to set up any routing state.
       In addition, the signaling application continues to process the
       message as in GN1 (compare step 1), forwarding the message
       downstream, and this will eventually result in the message
       reaching the flow receiver.

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   6.  In parallel, the GIST instance in GN2 now knows that it should
       maintain routing state and a messaging association for future
       signaling with GN1.  This is recognised because the message is a
       Query, and because the local signaling application has indicated
       that it will peer with GN1.  There are two possible cases for
       sending back the necessary GIST Response:

       Association Exists:  GN1 and GN2 already have an appropriate MA.
          GN2 simply records the identity of GN1 as its upstream peer
          for that flow and NSLP, and sends a Response back to GN1 over
          the MA identifying itself as the peer for this flow.

       No Association:  No MA exists.  GN2 sends the Response in D-mode
          directly to GN1, identifying itself and agreeing to the
          association setup.  The protocol exchanges needed to complete
          this will proceed in parallel with the following stages.

   7.  Eventually, another NSLP message works its way upstream from the
       receiver to GN2.  This message contains a description of the
       actual resources requested, along with authorisation and other
       security information.  The signaling application in GN2 passes
       this payload to the GIST level, along with the flow definition
       and transfer attributes {secured, reliable}.

   8.  The GIST layer in GN2 identifies the upstream peer for this flow
       and NSLP as GN1, and determines that it has an MA with the
       appropriate properties.  The message is queued on the MA for
       transmission; this may incur some delay if the procedures begun
       in step 6.B have not yet completed.

   Further messages can be passed in each direction in the same way.
   The GIST layer in each node can in parallel carry out maintenance
   operations such as route change detection (see Section 7.1).

   It should be understood that several of these details of GIST
   operations can be varied, either by local policy or according to
   signaling application requirements.  The authoritative details are
   contained in the remainder of this document.

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4.  GIST Processing Overview

   This section defines the basic structure and operation of GIST.
   Section 4.1 describes the way in which GIST interacts with local
   signaling applications in the form of an abstract service interface.
   Section 4.2 describes the per-flow and per-peer state that GIST
   maintains for the purpose of transferring messages.  Section 4.3
   describes how messages are processed in the case where any necessary
   messaging associations and routing state already exist; this includes
   the simple scenario of pure D-mode operation, where no messaging
   associations are necessary in the first place.  Finally, Section 4.4
   describes how routing state and messaging associations are created
   and managed.

4.1.  GIST Service Interface

   This section defines the service interface that GIST presents to
   signaling applications in terms of abstract properties of the message
   transfer.  Note that the same service interface is presented at every
   GIST node; however, applications may invoke it differently at
   different nodes, depending for example on local policy.  In addition,
   the service interface is defined independently of any specific
   transport protocol, or even the distinction between D-mode and
   C-mode.  The initial version of this specification defines how to
   support the service interface using a C-mode based on TCP; if
   additional protocol support is added, this will support the same
   interface and so the change will be invisible to applications, except
   as a possible performance improvement.  A more detailed description
   of this service interface is given in Appendix B.

4.1.1.  Message Handling

   Fundamentally, GIST provides a simple message-by-message transfer
   service for use by signaling applications: individual messages are
   sent, and individual messages are received.  At the service
   interface, the NSLP payload, which is opaque to GIST, is accompanied
   by control information expressing the application's requirements
   about how the message should be routed, and the application also
   provides the session identifier (see Section 3.5).  Additional
   message transfer attributes control the specific transport and
   security properties that the signaling application desires.

   The distinction between GIST D- and C-mode is not visible at the
   service interface.  In addition, the functionality to handle
   fragmentation and reassembly, bundling together of small messages for
   efficiency, and congestion control are not directly visible at the
   service interface; GIST will take whatever action is necessary based
   on the properties of the messages and local node state.

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4.1.2.  Message Transfer Attributes

   Message transfer attributes are used to define certain performance
   and security related aspects of message processing.  The attributes
   available are as follows:

   Reliability:  This attribute may be 'true' or 'false'.  When 'true',
      messages MUST be delivered to the signaling application in the
      peer exactly once or not at all; if there is a chance that the
      message was not delivered, an error MUST be indicated to the local
      signaling application identifying the routing information for the
      message in question.  GIST implements reliability by using an
      appropriate transport protocol within a messaging association, so
      mechanisms for the detection of message loss depend on the
      protocol in question; for the current specification, the case of
      TCP is considered in Section 5.7.2.  Messages with the same SID to
      the same peer MUST be delivered in order.  When 'false', a message
      may be delivered, once, several times or not at all, with no error
      indications in any case.

   Security:  This attribute defines the security properties that the
      signaling application requires for the message, including the type
      of protection required, and what authenticated identities should
      be used for the signaling source and destination.  This
      information maps onto the corresponding properties of the security
      associations established between the peers in C-mode.  It can be
      specified explicitly by the signaling application, or reported by
      GIST to the signaling application.  The latter can take place
      either on receiving a message, or just before sending a message
      but after configuring or selecting the messaging association to be
      used for it.  This attribute can also be used to convey
      information about any address validation carried out by GIST, such
      as whether a return routability check has been carried out.
      Further details are discussed in Appendix B.

   Local Processing:  An NSLP may provide hints to GIST to enable more
      efficient or appropriate processing.  For example, the NSLP may
      select a priority from a range of locally defined values to
      influence the sequence in which messages leave a node.  Any
      priority mechanism MUST respect the ordering requirements for
      reliable messages within a session, and priority values are not
      carried in the protocol or available at the signaling peer or
      intermediate nodes.  An NSLP may also indicate that reverse path
      routing state will not be needed for this flow, to inhibit the
      node requesting its downstream peer to create it.

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4.2.  GIST State

4.2.1.  Message Routing State

   For each flow, the GIST layer can maintain message routing state to
   manage the processing of outgoing messages.  This state is
   conceptually organised into a table with the following structure.
   The primary key (index) for the table is the combination of the
   information about how the message is to be routed, the session being
   signaled for, and the signaling application itself:

   Message Routing Information (MRI):  This defines the method to be
      used to route the message, the direction in which to send the
      message, and any associated addressing information; see
      Section 3.3.

   Session Identification (SID):  The signaling session with which this
      message should be associated; see Section 3.5.

   NSLP Identification (NSLPID):  This is an IANA-assigned identifier
      associated with the NSLP which is generating messages for this
      flow.  The inclusion of this identifier allows the routing state
      to be different for different NSLPs, for example because of
      different adjacencies.

   The information associated with a given key consists of the routing
   state to reach the peer in the direction given by the MRI.  For any
   flow, there will usually be two entries, one each for the upstream
   and downstream MRI.  The routing state includes information about the
   peer identity (see Section 4.4.2), and a UDP port number for D-mode,
   or a reference to one or more MAs for C-mode.  All of this
   information is learned from prior GIST exchanges.

   It is also possible for the state information for either direction to
   be empty.  There are several possible cases:

   o  The signaling application has indicated that no messages will
      actually be sent in that direction.

   o  The node is a flow endpoint, so there can be no signaling peer in
      one or other direction.

   o  The node is the endpoint of the signaling path, for example
      because it is acting as a proxy, or because it has determined that
      there are no further signaling nodes in that direction.

   o  The node is using other techniques to route the message.  For
      example, it can encapsulate it the same way as a Query and rely on

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      the peer to intercept it.

   Each entry in the routing state table has an associated validity
   timer for how long it can be considered accurate; when this timer
   expires, the entry MUST be purged if it has not been refreshed.
   Installation and maintenance of routing state is described in more
   detail in Section 4.4.

   Note also that the routing state is described as a table of per-flow
   entries, but that there is no implied constraint on how the
   information is stored.  However, in general, and especially if GIST
   peers are several IP hops away, there is no way to identify the
   correct downstream peer for a flow and signaling application from the
   local forwarding table using prefix matching, and the same applies
   always to upstream peer state because of the possibility of
   asymmetric routing: prefix aggregation is not possible, and per-flow
   state has to be stored, just as for RSVP [12].

4.2.2.  Peer-Peer Messaging Association State

   The per-flow message routing state is not the only state stored by
   GIST.  There is also the state required to manage the MAs.  Since
   these are not per-flow, they are stored separately from the routing
   state, including the following per-MA information:

   o  a queue of messages pending transmission while an MA is being

   o  a timer for how long since the peer re-stated its desire to keep
      the MA open (see Section 4.4.3).

   In addition, per-MA state is held in the messaging association
   protocols themselves.  However, the details of this state are not
   directly visible to GIST, and they do not affect the rest of the
   protocol description.

4.3.  Basic GIST Message Processing

   This section describes how signaling application messages are
   processed in the case where any necessary messaging associations and
   routing state are already in place.  The description is divided into
   several parts.  Firstly, message reception, local processing and
   message transmission are described for the case where the node hosts
   the NSLPID identified in the message.  Secondly, the case where the
   message is handled directly in the IP or GIST layer (because there is
   no matching signaling application on the node) is given.  An overview
   is given in Figure 3.

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       |        >>  Signaling Application Processing   >>        |
       |                                                         |
                ^                                       V
                ^             NSLP Payloads             V
                ^                                       V
       |                    >>    GIST    >>                     |
       |  ^           ^  ^     Processing      V  V           V  |
          x           N  Q                     Q  N           x
          x           N  Q>>>>>>>>>>>>>>>>>>>>>Q  N           x
          x           N  Q      Bypass at      Q  N           x
       +--x-----+  +--N--Q--+  GIST level   +--Q--N--+  +-----x--+
       | C-mode |  | D-mode |               | D-mode |  | C-mode |
       |Handling|  |Handling|               |Handling|  |Handling|
       +--x-----+  +--N--Q--+               +--Q--N--+  +-----x--+
          x          N   Q                     Q   N          x
          x    NNNNNN    Q>>>>>>>>>>>>>>>>>>>>>Q    NNNNNN    x
          x   N          Q      Bypass at      Q          N   x
       +--x--N--+  +-----Q--+  IP (router   +--Q-----+  +--N--x--+
       |IP Host |  |  RAO   |  alert) level |  RAO   |  |IP Host |
       |Handling|  |Handling|               |Handling|  |Handling|
       +--x--N--+  +-----Q--+               +--Q-----+  +--N--x--+
          x  N           Q                     Q           N  x
       +--x--N-----------Q--+               +--Q-----------N--x--+
       |      IP Layer      |               |      IP Layer      |
       |   (Receive Side)   |               |  (Transmit Side)   |
       +--x--N-----------Q--+               +--Q-----------N--x--+
          x  N           Q                     Q           N  x
          x  N           Q                     Q           N  x

        NNNNNNNNNNNNNN = Normal D-mode messages
        QQQQQQQQQQQQQQ = D-mode messages which are Q-mode encapsulated
        xxxxxxxxxxxxxx = C-mode messages
                   RAO = Router Alert Option

                Figure 3: Message Paths through a GIST Node

4.3.1.  Message Reception

   Messages can be received in C-mode or D-mode.  In the D-mode case,
   there are two possible message encapsulations, described below.

   Reception in C-mode is simple: incoming packets undergo the security
   and transport treatment associated with the MA, and the MA provides
   complete messages to the GIST layer for further processing.

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   Reception in D-mode depends on the message type.

   Normal encapsulation:  Normal messages arrive UDP-encapsulated and
      addressed directly to the receiving signaling node, at an address
      and port learned previously.  Each datagram contains a single
      message which is passed to the GIST layer for further processing,
      just as in the C-mode case.

   Q-mode encapsulation:  Where GIST is sending messages to be
      intercepted by the appropriate peer rather than directly addressed
      to it (in particular, Query messages), these are UDP encapsulated,
      usually with an IP router alert option.  Each signaling node will
      therefore see all such messages.  Several RAO values may be used
      by NSIS; the assignment rationale is discussed in [11].  The case
      where the NSLPID does not match a local signaling application at
      all is considered below in Section 4.3.4; otherwise, the message
      is again passed up to the GIST layer for further processing.

4.3.2.  Local Processing and Validation

   Once a message has been received, it is processed locally within the
   GIST layer.  The GIST hop count, which every message contains to
   prevent looping, MUST be checked and decremented immediately the
   message has been received.  Further processing depends on the message
   type and payloads carried; most of the GIST payloads are associated
   with state maintenance and details are covered in Section 4.4.

   In the case of a Query, there is an interaction with signaling
   application policy to determine which of two courses to follow:

   1.  The receiving signaling application wishes to become a signaling
       peer with the Querying node.  GIST MUST continue with the
       handshake process to set up message routing state, as described
       in Section 4.4.1.  The application MAY provide an NSLP payload
       for the same NSLPID, which GIST will transfer in the Response.

   2.  The signaling application does not wish to set up state with the
       Querying node and become its peer.  GIST MUST propagate the
       Query, similar to the case described in Section 4.3.4.  No
       message is sent back to the Querying node.  The application MAY
       provide an updated NSLP payload for the same NSLPID, which will
       be used in the Query forwarded by GIST.

   This interaction with the signaling application, including the
   generation or update of an NSLP payload, SHOULD take place
   synchronously as part of the Query processing.  In terms of the GIST
   service interface, this can be implemented by providing appropriate
   return values for the primitive that is triggered when such a message

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   is received; see Appendix B.2 for further discussion.

   For all other message types, the GIST payloads are processed as
   described in Section 4.4.  The remainder of the GIST message consists
   of an NSLP payload, which is delivered locally to the signaling
   application identified by the NSLPID.  The format of the payload is
   not constrained by GIST, and the content is not interpreted.
   Delivery is subject to the following validation checks:

   o  if the message was explicitly routed (see Section 7.1.4) or is a
      Data message delivered without routing state (see Section 5.3.2),
      the payload is delivered but flagged to the receiving NSLP to
      indicate that routing state was not validated;

   o  else, if there is no routing state for the MRI/SID/NSLPID the
      message MUST be rejected with a "No Routing State" error message
      (Appendix A.4.4.5);

   o  else, if the message arrived on an association which is not
      associated with the MRI/NSLPID/SID combination given in the
      message, the message MUST be rejected with an "Incorrectly
      Delivered Message" error message (Appendix A.4.4.4);

   o  else, the payload is delivered as normal.

4.3.3.  Message Transmission

   Signaling applications can generate their messages for transmission,
   either asynchronously, or in response to a normal input message, and
   GIST can also generate messages autonomously.  When a message is
   available for transmission, GIST uses internal policy and the stored
   routing state to determine how to handle it.  The following
   processing applies equally to locally generated messages and messages
   forwarded from within the GIST or signaling application levels.
   However, see Section 5.6 for special rules applying to the
   transmission of error messages by GIST.

   The main decision is whether the message must be sent in C-mode or
   D-mode.  Reasons for using the former are:

   o  signaling application requirements: for example, it has requested
      channel secured delivery, or reliable delivery;

   o  protocol specification: a message that requires fragmentation MUST
      be sent over a messaging association;

   o  local policy: for example, a node MAY send messages over a
      messaging association to benefit from adaptive congestion control.

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   In principle, as well as determining that some messaging association
   must be used, GIST MAY select between a set of alternatives, e.g. for
   load sharing or because different messaging associations provide
   different transport or security attributes.

   If the use of a messaging association is selected, the message is
   queued on the association found from the routing state table, and
   further output processing is carried out according to the details of
   the protocol stacks used.  If no appropriate association exists, the
   message is queued while one is created (see Section 4.4.1).  If no
   association can be created, this is an error condition, and should be
   indicated back to the local signaling application.

   If a messaging association is not required, the message is sent in
   D-mode.  The processing in this case depends on the message type and
   whether routing state exists or not.

   o  If the message is not a Query, and routing state exists, it is UDP
      encapsulated and sent directly to the address from the routing
      state table.

   o  If the message is a Query, then it is UDP encapsulated with IP
      address and if necessary router alert option determined from the
      MRI and NSLPID; the details depend on the message routing method.

   o  If no routing state exists, GIST can attempt to use the same
      Q-mode encapsulation as in the Query case.  If this is not
      possible, e.g. because the encapsulation for the MRM is only
      defined for one message direction, then this is an error condition
      which is reported back to the local signaling application.

4.3.4.  Nodes not Hosting the NSLP

   A node may receive messages where it has no signaling application
   corresponding to the message NSLPID.  There are several possible
   cases depending mainly on the encapsulation:

   1.  A Q-mode encapsulated message contains an RAO value which is
       relevant to NSIS but not to the specific node, but the IP layer
       is unable to recognise whether it needs to be passed to GIST for
       further processing or whether the packet should be forwarded just
       like a normal IP datagram.

   2.  A Q-mode encapsulated message contains an RAO value which is
       relevant to the node, but the specific signaling application for
       the NSLPID in the message is not processed there.

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   3.  A directly addressed message (in D-mode or C-mode) is delivered
       to a node for which there is no corresponding signaling
       application.  With the current specification, this should never
       happen.  While future versions might find a use for such a
       feature, currently this MUST cause an "Unknown NSLPID" error
       message, Appendix A.4.4.6.

   4.  A Q-mode encapsulated message arrives at the end-system which
       does not handle the signaling application.  This is possible in
       normal operation, and MUST be indicated to the sender with an
       "Endpoint Found" informational message (Appendix A.4.4.7).  The
       end-system includes the MRI and SID from the original message in
       the error message without interpreting them.

   5.  The node is GIST-aware NAT.  See Section 7.2.

   In cases (1) and (2), the role of GIST is to forward the message
   essentially unchanged, and it will not become a peer to the node
   sending the message.  Forwarding with modified NSLP payloads is
   covered above in Section 4.3.2.  However, a GIST implementation must
   ensure that the IP-layer TTL field and GIST hop count are managed
   correctly to prevent message looping, and this should be done
   consistently independently of whether the processing takes place on
   the fast path or in GIST-specific code.  The rules are that in cases
   (1) and (2), the IP-layer TTL MUST be decremented just as if the
   message was a normal IP forwarded packet; in case (2) the GIST hop
   count MUST be decremented as in the case of normal input processing,
   which indeed applies to cases (3) and (4).

   A GIST node processing Q-mode encapsulated messages in this way
   SHOULD make the routing decision based on the full contents of the
   MRI and not only the IP destination address.  It MAY also apply a
   restricted set of sanity checks and under certain conditions return
   an error message rather than forward the message.  These conditions

   1.  The message is so large that it would be fragmented on downstream
       links, for example because the downstream MTU is very small.  The
       error "Message Too Large" (Appendix A.4.4.8) SHOULD be returned
       to the sender, which SHOULD begin messaging association setup.

   2.  The GIST hop count has reached zero.  The error "Hop Limit
       Exceeded" (Appendix A.4.4.2) SHOULD be returned to the sender,
       which MAY retry with a larger initial hop count if it is clear
       that a loop has not been formed.

   3.  The MRI represents a flow definition which is too general to be
       forwarded along a unique path (e.g. the destination address

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       prefix is too short).  The error "MRI Validation Failure"
       (Appendix A.4.4.12) with subcode 0 ("MRI Too Wild") SHOULD be
       returned to the sender, which MAY retry with restricted MRIs,
       possibly starting additional signaling sessions to do so.  If the
       GIST node does not understand the MRM in question it MUST NOT
       apply this check, instead forwarding the message transparently.

   In the first two cases, only the common header of the GIST message is
   examined; in the third case, the MRI is also examined.  The rest of
   the message MUST never be inspected or modified.

   Note that the GIST hop count is only intended to prevent message
   looping at the GIST level, and by default NSLPs must take their own
   measures to prevent looping at the application level.  However, the
   GIST API (Appendix B) provides the incoming hop count to the NSLPs,
   which can preserve it on outgoing messages as they are forwarded
   further along the path.  This provides a lightweight loop-prevention
   mechanism for NSLPs which do not define anything more sophisticated.

4.4.  Routing State and Messaging Association Maintenance

   The main responsibility of GIST is to manage the routing state and
   messaging associations which are used in the message processing
   described above.  Routing state is installed and maintained by
   specific GIST messages.  Messaging associations depend on the
   existence of routing state, but are actually set up by the normal
   procedures of the transport and security protocols that comprise
   them.  Timers control routing state and messaging association refresh
   and expiration.

   There are two different cases for state installation and refresh:

   1.  Where routing state is being discovered or a new association is
       to be established; and

   2.  Where an existing association can be re-used, including the case
       where routing state for the flow is being refreshed.

   These cases are now considered in turn, followed by the case of
   background general management procedures.

4.4.1.  State Setup

   The complete sequence of possible messages for GIST state setup
   between adjacent peers is shown in Figure 4 and described in detail
   in the following text.  The figure informally summarises the contents
   of each message, including optional elements in [].  An example is
   given in Appendix C.

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            +----------+                     +----------+
            | Querying |                     |Responding|
            |   Node   |                     |   Node   |
            +----------+                     +----------+
                       ---------------------->        .............
                       Router Alert Option            .  Routing  .
                       MRI/SID/NSLPID                 .   state   .
                       Q-Node Network Layer Info      . installed .
                       Query Cookie                   .    at     .
                       [Q-Node Stack-Proposal         . R-node(1) .
                        Q-Node Stack-Config-Data]     .............
                       [NSLP Payload]

               .  The responder can use an existing .
               . messaging association if available .
               . from here onwards to short-circuit .
               .     messaging association setup    .

   .............       <----------------------
   .  Routing  .       MRI/SID/NSLPID
   .   state   .       R-Node Network Layer Info (D-mode only)
   . installed .       Query cookie
   .    at     .       [Responder Cookie
   .  Q-Node   .        [R-Node Stack-Proposal
   .............         R-Node Stack-Config-Data]]
                       [NSLP Payload]

                . If a messaging association needs .
                . to be created, it is set up here .
                .     and the Confirm uses it      .

                     MRI/SID/NSLPID                   .............
                     Q-Node Network Layer Info        .  Routing  .
                     [Responder Cookie                .   state   .
                      [R-Node Stack-Proposal          . installed .
                       [Q-Node Stack-Config-Data]]]   .    at     .
                     [NSLP Payload]                   . R-node(2) .

                 Figure 4: Message Sequence at State Setup

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   The initial message in any routing state maintenance operation is a
   Query, sent from the querying node and intercepted at the responding
   node.  This message has addressing and other identifiers appropriate
   for the flow and signaling application that state maintenance is
   being done for, addressing information about the node itself, and it
   MAY contain an NSLP payload.  It also includes a Query Cookie, and
   optionally capability information about messaging association
   protocol stacks.  The role of the cookies in this and subsequent
   messages is to protect against certain denial of service attacks and
   to correlate the various events in the message sequence (see
   Section 8.5 for further details).

   Provided that the signaling application has indicated that message
   routing state should be set up (see Section 4.3.2), reception of a
   Query MUST elicit a Response.  This is a normally encapsulated D-mode
   message with additional payloads.  It contains network layer
   information about the responding node, echoes the Query Cookie, and
   MAY contain an NSLP payload, possibly a response to the NSLP payload
   in the initial message.  In case a messaging association was
   requested, it MUST also contain a Responder Cookie and its own
   capability information about messaging association protocol stacks.
   Even if a messaging association is not requested, the Response MAY
   still include a Responder Cookie if the node's routing state setup
   policy requires it (see below).

   Setup of a new messaging association begins when peer addressing
   information is available and a new messaging association is actually
   needed.  Any setup MUST take place immediately after the specific
   Query/Response exchange, because the addressing information used may
   have a limited lifetime, either because it depends on limited
   lifetime NAT bindings or because it refers to agile destination ports
   for the transport protocols.  The Stack-Proposal and Stack-
   Configuration-Data objects carried in the exchange carry capability
   information about what messaging association protocols can be used,
   and the processing of these objects is described in more detail in
   Section 5.7.  With the protocol options currently defined, setup of
   the messaging association always starts from the Querying node,
   although more flexible configurations are possible within the overall
   GIST design.  In any case, once set up, the association itself can be
   used equally in both directions.

   Finally, after any necessary messaging association setup has
   completed, a Confirm MUST be sent if the Response requested it.  If a
   messaging association is being used, the Confirm MUST be sent over it
   before any other messages for the same flow, and it echoes the
   Responder Cookie and Stack-Proposal from the Response.  The former is
   used to allow the receiver to validate the contents of the message
   (see Section 8.5), and the latter is to prevent certain bidding-down

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   attacks on messaging association security (see Section 8.6).  The
   Confirm MAY also contain an abbreviated form of the original Stack-
   Configuration-Data to finalise details of the messaging association
   configuration.  The association can be used in the upstream direction
   for the MRI and NSLPID carried in the Confirm, after the Confirm has
   been received.

   The querying node MUST install the responder address, derived from
   the R-Node Network Layer info, as routing state information after
   verifying the Query Cookie in the Response.  The responding node MAY
   install the querying address as peer state information at two points
   in time:

   1.  after the receipt of the initial Query, or

   2.  after a Confirm containing the Responder Cookie.

   The responding node SHOULD derive the peer address from the Q-Node
   Network Layer Info if this was decoded successfully.  Otherwise, it
   MAY be derived from the IP source address of the message if the
   common header flags this as being the signalling source address.  The
   precise constraints on when state information is installed are a
   matter of security policy considerations on prevention of denial-of-
   service attacks and state poisoning attacks, which are discussed
   further in Section 8.  Because the responding node MAY choose to
   delay state installation as in case (2), the Confirm must contain
   sufficient information to allow it to be processed in the same way as
   the original Query.  This places some special requirements on NAT
   traversal and cookie functionality, which are discussed in
   Section 7.2 and Section 8 respectively.

4.4.2.  Messaging Association Re-use

   It is a design goal of GIST that, as far as possible, messaging
   associations should be re-used for multiple flows and sessions,
   rather than setting up a new MA for each.  This is to ensure that the
   MA cost scales only with the number of peers, and to avoid the
   latency of new MA setup where possible.

   However, re-use requires the identification of an existing MA which
   matches the same routing state and desired properties that would be
   the result of a full handshake in D-mode, and this identification
   must be done as reliably and securely as continuing with the full
   procedure.  Note that this requirement is complicated by the fact
   that NATs may remap the node addresses in D-mode messages, and also
   interacts with the fact that some nodes may peer over multiple
   interfaces (and thus with different addresses).

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   MA re-use is controlled by the Network-Layer-Information (NLI)
   object, which is carried in Query/Confirm and optionally Response
   messages.  The NLI object includes:

   Peer-Identity:  For a given node, this is an interface independent
      value with opaque syntax.  It MUST be chosen so as to have a high
      probability of uniqueness between peers, and SHOULD be stable at
      least until the next node restart.  Note that there is no
      cryptographic protection of this identity; attempting to provide
      this would essentially duplicate the functionality in the
      messaging association security protocols.

   Interface-Address:  This is an IP address through which the signaling
      node can be reached.  There may be several choices available for
      the Interface-Address, and further discussion of this is contained
      in Section 5.2.2.

   By default, a messaging association is associated with the NLI object
   that was provided by the peer in the Query/Response/Confirm at the
   time the association was set up.  There may be more than one
   association for a given NLI object, for example with different
   security or transport properties.

   MA re-use is controlled by matching the NLI provided in a GIST
   message with those associated with existing MAs.  This can be done on
   receiving either a Query or Response, although the former is more

   o  If there is a perfect match to the NLI of an existing association,
      that association SHOULD be re-used, provided it has the
      appropriate properties in other respects.  This is indicated by
      sending the remaining messages in the handshake over that
      association.  This will only fail, that is, lead to re-use of an
      association to the wrong node, if signaling nodes have colliding
      Peer-Identities and one is reachable at the same Interface-Address
      as another.  This could be done by an on-path attacker.

   o  In all other cases, the full handshake MUST be executed in D-mode
      as usual.  There are in fact four possibilities:

      1.  Nothing matches: this is clearly a new peer.

      2.  Only the Peer-Identity matches: this may be either a new
          interface on an existing peer, or a changed address mapping
          behind a NAT, or an attacker attempting to hijack the Peer-
          Identity.  These should be rare events, so the expense of a
          new association setup is acceptable.

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      3.  Only the Interface-Address matches: this is probably a new
          peer behind the same NAT as an existing one.  A new
          association setup is required.

      4.  The full NLI object matches: this is a degenerate case, where
          one node recognises an existing peer, but wishes to allow the
          option to set up a new association in any case, for example to
          create an association with different properties.

4.4.3.  State Maintenance Procedures

   Refresh and expiration of all types of GIST state is controlled by

   Each item of routing state expires after a lifetime which is
   negotiated during the Query/Response/Confirm handshake.  The Network
   Layer Info (NLI) object in the Query contains a proposal for the
   lifetime value, and the NLI in the Response contains the value the
   Responding node requires.  A default timer value of 30 seconds is
   RECOMMENDED.  Nodes which can exploit alternative, more powerful,
   route change detection methods such as those described in
   Section 7.1.2 MAY choose to use much longer times.  Nodes MAY use
   shorter times to provide more rapid change detection, but MUST take
   into account the fact that the Query messages generated may stress
   the rate limits applied to D-mode traffic (Section 5.3.3).

   The Querying node MUST generate a Query before this timer expires, if
   it believes that the signaling session is still active; otherwise,
   the Responding node MAY delete the state.  Receipt of the message at
   the Responding node will refresh peer addressing state for one
   direction, and receipt of a Response at the querying node will
   refresh it for the other.  There is no mechanism at the GIST level
   for explicit teardown of routing state.  However, GIST MUST NOT
   refresh routing state if a signaling session is known to be inactive,
   either because upstream state has expired, or because the signaling
   application has indicated via the GIST API (Appendix B.5) that the
   state is no longer required, because this would prevent correct state
   repair in the case of network rerouting.

   Unneeded MAs are torn down by GIST, using the teardown mechanisms of
   the underlying transport or security protocols if available, for
   example by simply closing a TCP connection.  The teardown can be
   initiated by either end.  Whether an MA is needed is a combination of
   two factors:

   o  local policy, which could take into account the cost of keeping
      the messaging association open, the level of past activity on the
      association, and the likelihood of future activity, e.g. if there

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      is routing state still in place which might generate messages to
      use it.

   o  whether the peer still wants the MA in place.  During MA setup,
      each node indicates its own MA-Hold-Time as part of the Stack-
      Configuration-Data.  A node MUST NOT tear down the MA if it has
      received traffic from its peer over that period.  A peer which has
      generated no traffic but still wants the MA retained may use a
      special null message (MA-Hello) to indicate the fact.  A default
      value for MA-Hold-Time of 30 seconds is RECOMMENDED.  Nodes MAY
      use shorter times to achieve more rapid peer failure detection,
      but need to take into account the load on the network created by
      the MA-Hello messages.  Nodes MAY use longer times, but need to
      take into account the cost of retaining idle MAs for extended

      Because the Responding node can choose not to retain state until a
      Confirm, an abbreviated Stack-Configuration-Data object containing
      just this information MUST be repeated by the Querying node in the
      first Confirm sent on a new MA.

   Messaging associations can always be set up on demand, and messaging
   association status is not made directly visible outside the GIST
   layer.  Therefore, even if GIST tears down and later re-establishes a
   messaging association, signaling applications cannot distinguish this
   from the case where the MA is kept permanently open.  To maintain the
   transport semantics described in Section 4.1, GIST MUST close
   transport connections carrying reliable messages gracefully or report
   an error condition, and MUST NOT open a new association for a given
   session and peer while messages on a previous association may still
   be outstanding.

   This specification defines precisely only the time at which routing
   state or messaging associations expire; it does not define when
   refresh handshakes or keepalives should be initiated.
   Implementations MUST select timer settings which take at least the
   following into account:

   o  The transmission latency between source and destination;

   o  The need for retransmissions, either explicitly or within the
      messaging association protocols;

   o  The need to avoid network synchronisation of control traffic (cf.

   In most cases, a reasonable policy is to initiate the refresh process
   when between 1/2 and 3/4 of the appropriate validity time has elapsed

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   since the last successful refresh.  The actual moment is chosen
   randomly within this interval to avoid synchronisation effects.

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5.  Message Formats and Transport

5.1.  GIST Messages

   All GIST messages begin with a common header, followed by a sequence
   of type-length-value (TLV) objects.  This subsection describes the
   various GIST messages and their contents at a high level in ABNF [9];
   a more detailed description of the header and each object is given in
   Section 5.2.  Note that the NAT traversal mechanism for GIST involves
   the insertion of an additional NAT-Traversal-Object in Query,
   Response, and some Data and Error messages; the rules for this are
   given in Section 7.2.

   GIST-Message: The primary messages are either one of the stages in
   the three-way handshake, or a simple message carrying NSLP data.
   Additional types are defined for errors and keeping messaging
   associations alive.

       GIST-Message = Query / Response / Confirm /
                      Data / Error / MA-Hello

   The common header includes a version number, message type and size,
   and NSLPID.  It also carries a hop count to prevent message looping
   and various control flags, including one (the R flag) to indicate if
   a reply of some sort is requested.  The objects following the common
   header MUST be carried in a fixed order, depending on message type.
   Messages with missing, duplicate or invalid objects for the message
   type MUST be rejected with an "Object Type Error" error message with
   the appropriate subcode (Appendix A.4.4.9).

   Query: A Query MUST be sent in D-mode, in fact with the special
   Q-mode encapsulation.  In addition to the common header, it contains
   certain mandatory control objects, and MAY contain a signaling
   application payload.  A stack proposal and configuration data MUST be
   included if the message exchange relates to setup of a messaging
   association.  The R flag MUST always be set (R=1) in a Query, since
   this message always elicits a Response.

       Query = Common-Header
               [ NAT-Traversal-Object ]
               [ Stack-Proposal Stack-Configuration-Data ]
               [ NSLP-Data ]

   Response: A Response may be sent in D-mode, or C-mode if a messaging

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   association is being re-used.  It MUST echo the MRI SID and Query-
   Cookie of the Query, and in D-mode carries its own Network-Layer-
   Information; if the message exchange relates to setup of a messaging
   association, which can only take place in D-mode, a Responder cookie
   MUST be included, as well as its own stack proposal and configuration
   data.  The R flag MUST be set (R=1) if a Responder cookie is present
   but otherwise is optional; if the R flag is set, a Confirm MUST be
   sent as a reply.  Note that the direction of this MRI will be
   inverted compared to that in the Query, that is, an upstream MRI
   becomes downstream and vice versa (see Section 3.3).

       Response = Common-Header
                  [ NAT-Traversal-Object ]
                  [ Network-Layer-Information ]
                  [ Responder-Cookie
                    [ Stack-Proposal Stack-Configuration-Data ] ]
                  [ NSLP-Data ]

   Confirm: A Confirm may be sent in D-mode, or C-mode if a messaging
   association has been re-used.  It MUST echo the MRI (with inverted
   direction), SID, and Responder-Cookie if the Response carried one; if
   the message exchange relates to setup of a new messaging association
   or re-use of an existing one (which can only take place in C-mode),
   the message MUST also echo the Stack-Proposal from the Response so it
   can be verified that this has not been tampered with.  The first
   message on an association MUST also repeat the Stack-Configuration-
   Data from the original Query in an abbreviated form, just containing
   the MA-Hold-Time.

       Confirm = Common-Header
                 [ Responder-Cookie
                   [ Stack-Proposal
                     [ Stack-Configuration-Data ] ] ]
                 [ NSLP-Data ]

   Data: The Data message is used to transport NSLP data without
   modifying GIST state.  It contains no control objects, but only the
   MRI and SID associated with the NSLP data being transferred.
   Network-Layer-Information (NLI) MUST be carried in the D-mode case,
   but MUST NOT be included otherwise.

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       Data = Common-Header
              [ NAT-Traversal-Object ]
              [ Network-Layer-Information ]

   Error: An Error message reports a problem determined at the GIST
   level.  (Errors generated by signaling applications are reported in
   NSLP-Data payloads and are not treated specially by GIST.)  The
   message includes a Network-Layer-Information object for the
   originator of the error message if it is being sent in D-mode; all
   other information related to the error is carried in a GIST-Error-
   Data object.

       Error = Common-Header
               [ NAT-Traversal-Object ]
               [ Network-Layer-Information ]

   MA-Hello: This message MUST be sent only in C-mode to indicate that a
   node wishes to keep a messaging association open.  It contains only
   the common header, with a NSLPID of zero.  The R flag MAY be set
   (R=1); if so, the peer MUST send another message back along the
   messaging association.  This allows a node to test the liveness of
   the peer.

       MA-Hello = Common-Header

5.2.  Information Elements

   This section describes the content of the various objects that can be
   present in each GIST message, both the common header, and the
   individual TLVs.  The bit formats are provided in Appendix A.

5.2.1.  The Common Header

   Each message begins with a fixed format common header, which contains
   the following information:

   Version:  The version number of the GIST protocol.  This
      specification defines GIST version 1.

   Length:  The number of 32 bit words in the message following the
      common header.

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   Upper layer identifier (NSLPID):  This gives the specific NSLP that
      this message is used for.

   GIST hop count:  A hop count to prevent a message from looping.

   Message type:  The message type (Query, Response, etc.)

   Source addressing mode:  If set (S=1), this indicates that the IP
      source address of the message is the same as the IP address of the
      signaling peer, so replies to this message can be sent safely to
      this address.  S is always set in C-mode.  It is cleared (S=0) if
      the IP source address was derived from the message routing
      information in the payload and this is different from the
      signaling source address.

   Response requested:  A flag which if set (R=1) indicates that a GIST
      message should be sent in response to this message.  The
      appropriate message type for the response depends on the type of
      the initial message.

   Explicit routing:  A flag which if set (E=1) indicates that the
      message was explicitly routed (see Section 7.1.4).

5.2.2.  TLV Objects

   All data following the common header is encoded as a sequence of
   type-length-value objects.  Currently, each object can occur at most
   once; the set of required and permitted objects is determined by the
   message type and encapsulation (D-mode or C-mode).

   Message-Routing-Information (MRI):  Information sufficient to define
      how the signaling message should be routed through the network.

       Message-Routing-Information = message-routing-method

      The format of the method-specific-information depends on the
      message-routing-method requested by the signaling application.
      Note that it always includes a flag defining the direction as
      either 'upstream' or 'downstream' (see Section 3.3).  It is
      provided by the NSLP in the message sender and used by GIST to
      select the message routing.

   Session-Identification (SID):  The GIST session identifier is a 128
      bit, cryptographically random identifier chosen by the node which
      originates the signaling exchange.  See Section 3.5.

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   Network-Layer-Information (NLI):  This object carries information
      about the network layer attributes of the node sending the
      message, including data related to the management of routing
      state.  This includes a peer identity and IP address for the
      sending node.  It also includes IP-TTL information to allow the IP
      hop count between GIST peers to be measured and reported, and a
      validity time (RS-validity-time) for the routing state.

       Network-Layer-Information = peer-identity

      The use of the RS-validity-time field is described in
      Section 4.4.3.  The peer-identity and interface-address are used
      for matching existing associations, as discussed in Section 4.4.2.

      The interface-address must be routable, i.e. it MUST be usable as
      a destination IP address for packets to be sent back to the node
      generating the signaling message, whether in D-mode or C-mode.  If
      this object is carried in a Query or Confirm, the interface-
      address MUST specifically be set to an address bound to the
      interface associated with the MRI, to allow its use in route
      change handling as discussed in Section 7.1.  A suitable choice is
      the interface that is carrying the outbound flow.  A node may have
      several choices for which of its addresses to use as the
      interface-address.  For example, there may be a choice of IP
      versions, or addresses of limited scope (e.g. link-local), or
      addresses bound to different interfaces in the case of a router or
      multi-homed host.  However, some of these interface addresses may
      not be usable by the peer.  A node MUST follow a policy of using a
      global address of the same IP version as in the MRI, unless it can
      establish that an alternative address would also be usable.

      The setting and interpretation of the IP-TTL field depends on the
      message direction (upstream/downstream as determined from the MRI
      as described above) and encapsulation.

      *  If the message is sent downstream, if the TTL that will be set
         in the IP header for the message can be determined, the IP-TTL
         value MUST be set to this value, or else set to 0.

      *  On receiving a downstream message in D-mode, a non-zero IP-TTL
         is compared to the TTL in the IP header, and the difference is
         stored as the IP-hop-count-to-peer for the upstream peer in the
         routing state table for that flow.  Otherwise, the field is

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      *  If the message is sent upstream, the IP-TTL MUST be set to the
         value of the IP-hop-count-to-peer stored in the routing state
         table, or 0 if there is no value yet stored.

      *  On receiving an upstream message, the IP-TTL is stored as the
         IP-hop-count-to-peer for the downstream peer.

      In all cases, the IP-TTL value reported to signaling applications
      is the one stored with the routing state for that flow, after it
      has been updated if necessary from processing the message in

   Stack-Proposal:  This field contains information about which
      combinations of transport and security protocols are available for
      use in messaging associations, and is also discussed further in
      Section 5.7.

       Stack-Proposal = 1*stack-profile

       stack-profile = 1*protocol-layer

      Each protocol-layer field identifies a protocol with a unique tag;
      any additional data, such as higher-layer addressing or other
      options data associated with the protocol, will be carried in a
      MA-protocol-options field in the Stack-Configuration-Data TLV (see

   Stack-Configuration-Data (SCD):  This object carries information
      about the overall configuration of a messaging association.

       Stack-Configuration-Data = MA-Hold-Time

      The MA-Hold-Time field indicates how long a node will hold open an
      inactive association; see Section 4.4.3 for more discussion.  The
      MA-protocol-options fields give the configuration of the protocols
      (e.g.  TCP, TLS) to be used for new messaging associations, and
      they are described in more detail in Section 5.7.

   Query-Cookie/Responder-Cookie:  A Query-Cookie is contained in a
      Query and MUST be echoed in a Response; a Responder-Cookie MAY be
      sent in a Response, and if present MUST be echoed in the following
      Confirm.  Cookies are variable length bit strings, chosen by the
      cookie generator.  See Section 8.5 for further details on
      requirements and mechanisms for cookie generation.

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   NSLP-Data:  The NSLP payload to be delivered to the signaling
      application.  GIST does not interpret the payload content.

   GIST-Error-Data:  This contains all the information to determine the
      cause and context of an error.

       GIST-Error-Data = error-class error-code error-subcode
                         [ Message-Routing-Information-content ]
                         [ Session-Identification-content ]
                         [ comment ]

      The error-class indicates the severity level, and the error-code
      and error-subcode identify the specific error itself.  A full list
      of GIST errors and their severity levels is given in Appendix A.4.
      The common-error-header carries the Common-Header from the
      original message, and contents of the Message-Routing-Information
      (MRI) and Session-Identification (SID) objects are also included
      if they were successfully decoded.  For some errors, additional
      information fields must be included according to a fixed format;
      finally, an optional free-text comment may be added.

5.3.  D-mode Transport

   This section describes the various encapsulation options for D-mode
   messages.  Although there are several possibilities, depending on
   message type, MRM, and local policy, the general design principle is
   that the sole purpose of the encapsulation is to ensure that the
   message is delivered to or intercepted at the correct peer.  Beyond
   that, minimal significance is attached to the type of encapsulation
   or the values of addresses or ports used for it.  This allows new
   options to be developed in the future to handle particular deployment
   requirements without modifying the overall protocol specification.

5.3.1.  Normal Encapsulation

   Normal encapsulation MUST be used for all D-mode messages where the
   signaling peer is already known from previous signaling.  This
   includes Response and Confirm messages, and Data messages except if
   these are being sent without using local routing state.  Normal
   encapsulation is simple: the complete set of GIST payloads is
   concatenated together with the common header, and placed in the data
   field of a UDP datagram.  UDP checksums MUST be enabled.  The message
   is IP addressed directly to the adjacent peer as given by the routing
   state table.  Where the message is a direct reply to a Query and no
   routing state exists, the destination address is derived from the
   input message using the same rules as in Section 4.4.1.  The UDP port

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   numbering MUST be compatible with that used on Query messages (see
   below), that is, the same for messages in the same direction and with
   port numbers swapped for messages in the opposite direction.

5.3.2.  Q-mode Encapsulation

   Q-mode encapsulation MUST be used for messages where no routing state
   is available or where the routing state is being refreshed, in
   particular for Query messages.  Q-mode encapsulation is similar to
   normal encapsulation, with changes in IP address selection, IP
   options, and a defined method for selecting UDP ports.

   In general, the IP addresses are derived from information in the MRI;
   the exact rules depend on the MRM.  For all current MRMs, the IP
   header is given a Router Alert Option ([1] and [5]) to assist the
   peer in intercepting the message depending on the NSLPID.  Each
   NSLPID maps to a unique RAO value, but one RAO value may refer to
   multiple NSLPIDs; further details are discussed in [11].

   The source UDP port is selected by the message sender as the port at
   which it is prepared to receive UDP messages in reply, and a
   destination UDP port is allocated for GIST by IANA (see Section 9).
   Note that for some MRMs, GIST nodes anywhere along the path can
   generate GIST packets with source addresses that spoof the source
   address of the data flow.  Therefore, destinations cannot distinguish
   these packets from genuine end-to-end data purely on address
   analysis.  Instead, it must be possible to distinguish such GIST
   packets by port analysis; furthermore, the mechanism to do so must
   remain valid even if the destination is GIST-unaware.  GIST solves
   this problem by using a fixed destination UDP port from the "well
   known" space for the Q-mode encapsulation.  This port should never be
   allocated on a GIST-unaware host, and therefore Q-mode encapsulated
   messages will always be rejected with an ICMP error.

5.3.3.  Retransmission and Rate Control

   D-mode uses UDP, and hence has no automatic reliability or congestion
   control capabilities.  Signaling applications requiring reliability
   should be serviced using C-mode, which should also carry the bulk of
   signaling traffic.  However, some form of messaging reliability is
   required for the GIST control messages themselves, as is rate control
   to handle retransmissions and also bursts of unreliable signaling or
   state setup requests from the signaling applications.

   Query messages which do not receive Responses MAY be retransmitted;
   retransmissions MUST use a binary exponential backoff.  The initial
   timer value is T1, which the backoff process can increase up to a
   maximum value of T2 seconds.  The default value for T1 is 500 ms.  T1

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   is an estimate of the round-trip time between the querying and
   responding nodes.  Elements MAY use smaller values of T1 if it is
   known that the Query should be answered within the local network.  T1
   MAY be chosen larger, and this is RECOMMENDED if it is known in
   advance (such as on high latency access links) that the round-trip
   time is larger.  The default value of T2 is 64*T1.

   Note that Queries may go unanswered either because of message loss
   (in either direction), or because there is no reachable GIST peer.
   Therefore, implementations MAY trade off reliability (large T2)
   against promptness of error feedback to applications (small T2).  If
   the NSLP has indicated a timeout on the validity of this payload (see
   Appendix B.1), T2 MUST be chosen so that the process terminates
   within this timeout.

   Retransmitted Queries MUST use different Query-Cookie values.  If the
   Query carries NSLP data, it may be delivered multiple times to the
   signaling application.  These rules apply equally to the message that
   first creates routing state, and those that refresh it.

   This algorithm is sufficient to handle lost Queries and Responses.
   The case of a lost Confirm is more subtle.  Notionally, we can
   distinguish between two cases:

   1.  Where the Responding node is already prepared to store per-flow
       state after receiving a single (Query) message.  This would
       include any cases where the node has NSLP data queued to send.
       Here, the Responding node MAY run a retransmission timer to
       resend the Response until a Confirm is received, since the node
       is already managing state for that flow.  The problem of an
       amplification attack stimulated by a malicious Query is handled
       by requiring the cookie mechanism to enable the node receiving
       the Response to discard it efficiently if it does not match a
       previously sent Query.

   2.  Where the responding node is not prepared to store per-flow state
       until receiving a properly formed Confirm.

   Case (2) should be handled without requiring a retransmission timer,
   since this would require per-flow state at the Responding node.
   However, we can assume that the next signaling message will be in the
   direction Querying Node -> Responding Node (if there is no next
   signaling message, the fact that the Confirm has been lost is moot).
   In this case, the responding node will start to receive messages at
   the GIST level for a MRI/NSLP combination for which there is no
   stored routing state, since this state is only created on receipt of
   a Confirm.

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   Therefore, the error condition is detected at the Responding node
   when such a message arrives, without the need for a specific timer.
   Recovery requires a Confirm to be transmitted and successfully
   received.  The mechanism to cause this is that the Responding node
   MUST reject the incoming message with a "No Routing State" error
   message (Appendix A.4.4.5) back to the Querying node, which MUST
   interpret this as caused by a lost Confirm; the Querying node MUST
   regenerate the Confirm purely from local state.  In particular, it
   needs to remember a valid Responder Cookie.

   In all cases, Responses MUST be sent promptly to avoid spurious
   retransmissions.  Nodes generating any type of retransmission MUST be
   prepared to receive and match a reply to any of them, not just the
   one most recently sent.

   The basic rate-control requirements for D-mode traffic are
   deliberately minimal.  A single rate limiter applies to all traffic,
   for all interfaces and message types.  It applies to retransmissions
   as well as new messages, although an implementation MAY choose to
   prioritise one over the other.  Rate-control applies only to locally
   generated D-mode messages, not to messages which are being forwarded.
   When the rate limiter is in effect, D-mode messages MUST be queued
   until transmission is re-enabled, or an error condition MAY be
   indicated back to local signaling applications.  The rate limiting
   mechanism is implementation-defined, but it is RECOMMENDED that a
   token bucket limiter as described in [31] be used.  The token bucket
   MUST be sized to ensure that a node cannot saturate the network with
   D-mode traffic, for example when re-probing the network for multiple
   flows after a route change.  A suitable approach is to restrict the
   token bucket parameters so that the mean output rate is a small
   fraction, such as 5%, of the node's lowest-speed interface.

5.4.  C-mode Transport

   Encapsulation in C-mode is more complex, because of the variation in
   available transport functionality.  This issue is treated in
   Section 5.4.1.  The actual encapsulation is given in Section 5.4.2.

5.4.1.  Choice of Transport Protocol

   It is a general requirement of the NTLP defined in [26] that it
   should be able to support bundling of small messages, fragmentation
   of large messages, and message boundary delineation.  Not all
   transport protocols natively support all these features.

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   TCP  provides both bundling and fragmentation, but not message
      boundaries.  However, the length information in the GIST common
      header allows the message boundary to be discovered during

   SCTP  [16] satisfies all requirements.

   DCCP  [30] is message-based but does not provide bundling or
      fragmentation, nor does it provide reliability.  Bundling can be
      carried out by the GIST layer sending multiple messages in a
      single datagram; because the common header includes length
      information, the message boundaries within the datagram can be
      discovered during parsing.  Fragmentation of GIST messages over
      multiple datagrams should be avoided, because of amplification of
      message loss rates that this would cause.

   The bundling together of small messages is either built into the
   transport protocol or can be carried out by the GIST layer during
   message construction.  Either way, two approaches can be

   1.  As messages arrive for transmission they are gathered into a
       bundle until a size limit is reached or a timeout expires (cf.
       the Nagle algorithm of TCP or similar optional functionality in
       SCTP).  This provides maximal efficiency at the cost of some

   2.  Messages awaiting transmission are gathered together while the
       node is not allowed to send them, for example because it is
       congestion controlled.

   The second type of bundling is always appropriate.  For GIST, the
   first type MUST NOT be used for trigger messages (i.e. messages that
   update GIST or signaling application state), but may be appropriate
   for refresh messages (i.e. messages that just extend timers).  These
   distinctions are known only to the signaling applications, but MAY be
   indicated (as an implementation issue) by setting the priority
   transfer attribute (Section 4.1.2).

   It can be seen that all of these transport protocol options can be
   supported by the basic GIST message format already presented.  GIST
   messages requiring fragmentation must be carried using a reliable
   transport protocol, TCP or SCTP.  This specification defines only the
   use of TCP, but other possibilities could be included without
   additional work on message formatting.

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5.4.2.  Encapsulation Format

   The GIST message, consisting of common header and TLVs, is carried
   directly in the transport protocol, possibly incorporating transport
   layer security protection.  Further messages can be carried in a
   continuous stream (for TCP), or up to the next transport layer
   message boundary (for SCTP, DCCP, or UDP).  This is shown in
   Figure 5.

      |                  L2 Header                  |
      |                  IP Header                  |   ^
      |      Source address = signaling source      |   ^
      | Destination address = signaling destination |   .
      +---------------------------------------------+   .
      |                  L4 Header                  |   . ^
      |     (Standard TCP/SCTP/DCCP/UDP header)     |   . ^
      +---------------------------------------------+   . .
      |                GIST Message                 |   . . ^
      | (Common header and TLVs as in section 5.1)  |   . . ^  Scope of
      +---------------------------------------------+   . . .  security
      |  Additional GIST messages, each with its    |   . . . protection
      | own common header, either as a continuous   |   . . . (depending
      |   stream, or continuing to the next L4      |   . . . on channel
      .             message boundary                .   . . .  security
      .                                             .   V V V  mechanism
      .                                             .   V V V   in use)

                      Figure 5: C-mode Encapsulation

5.5.  Message Type/Encapsulation Relationships

   GIST has four primary message types (Query, Response, Confirm, and
   Data) and three possible encapsulation methods (normal D-mode,
   Q-mode, and C-mode).  The possible combinations of message type and
   encapsulation are given in the table below.  In some cases there are
   several possible choices, depending on the existence of routing state
   or messaging associations.  The rules governing GIST policy,
   including whether or not to create such state to handle a message,
   are described normatively in the other sections of this
   specification.  If a message arrives with an invalid encapsulation
   (e.g. a Query arrives over a messaging association), this MUST be
   rejected with an "Incorrect Encapsulation" error message
   (Appendix A.4.4.3).  However, it should be noted that the processing
   of the message at the receiver is not otherwise affected by the
   encapsulation method used, except that that the decapsulation process
   may provide additional information, such as translated addresses or

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   IP hop count to be used in the subsequent message processing.

   |  Message |  Normal D-mode  |     Query D-mode    |     C-mode     |
   |          |                 |       (Q-mode)      |                |
   |   Query  |      Never      |        Always       |      Never     |
   |          |                 |                     |                |
   | Response |     Unless a    |        Never        | If a messaging |
   |          |    messaging    |                     | association is |
   |          |  association is |                     |  being re-used |
   |          |  being re-used  |                     |                |
   |          |                 |                     |                |
   |  Confirm |     Unless a    |        Never        | If a messaging |
   |          |    messaging    |                     |   association  |
   |          | association has |                     |  has been set  |
   |          |  been set up or |                     | up or is being |
   |          |     is being    |                     |     re-used    |
   |          |     re-used     |                     |                |
   |          |                 |                     |                |
   |   Data   |    If routing   | If no routing state | If a messaging |
   |          |   state exists  |  exists and the MRI |   association  |
   |          |   for the flow  |    can be used to   |     exists     |
   |          |      but no     |  derive the Q-mode  |                |
   |          |    messaging    |    encapsulation    |                |
   |          |   association   |                     |                |

5.6.  Error Message Processing

   Special rules apply to the encapsulation and transmission of error

   GIST only generates error messages in response to incoming messages.
   Error messages MUST NOT be generated in response to incoming error
   messages.  The routing and encapsulation of the error message is
   derived from that of the message that caused the error; in
   particular, local routing state is not consulted.  Routing state and
   messaging association state MUST NOT be created to handle the error,
   and error messages MUST NOT be retransmitted explicitly by GIST,
   although they are subject to the same rate control as other messages.

   o  If the incoming message was received in D-mode, the error MUST be
      sent in D-mode using the normal encapsulation, using the
      addressing information from the NLI object in the incoming
      message.  If the NLI could not be determined, the error MUST be
      sent to the IP source of the incoming message if the S flag was
      set in it.  The NLI object in the Error message reports

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      information about the originator of the error.

   o  If the incoming message was received over a messaging association,
      the error MUST be sent back over the same messaging association.

   The NSLPID in the common header of the Error message has the value
   zero.  If for any reason the message cannot be sent, for example,
   because it is too large to send in D-mode, an error SHOULD be logged

5.7.  Messaging Association Setup

5.7.1.  Overview

   A key attribute of GIST is that it is flexible in its ability to use
   existing transport and security protocols.  Different transport
   protocols may have performance attributes appropriate to different
   environments; different security protocols may fit appropriately with
   different authentication infrastructures.  Even given an initial
   default mandatory protocol set for GIST, the need to support new
   protocols in the future cannot be ruled out, and secure feature
   negotiation cannot be added to an existing protocol in a backwards-
   compatible way.  Therefore, some sort of capability discovery is

   Capability discovery is carried out in Query and Response messages,
   using Stack-Proposal and Stack-Configuration-Data objects.  If a new
   messaging association is required it is then set up, followed by a
   Confirm.  Messaging association re-use is achieved by short-
   circuiting this exchange by sending the Response or Confirm messages
   on an existing association (Section 4.4.2); whether to do this is a
   matter of local policy.  The end result of this process is a
   messaging association which is a stack of protocols.  If multiple
   associations exist, it is a matter of local policy how to distribute
   messages over them, subject to respecting the transfer attributes
   requested for each message.

   Every possible protocol for a messaging association has the following

   o  MA-Protocol-ID, a 1-byte IANA assigned value (see Section 9).

   o  A specification of the (non-negotiable) policies about how the
      protocol should be used; for example, in which direction a
      connection should be opened.

   o  [Depending on the specific protocol:] Formats for an MA-protocol-
      options field to carry the protocol addressing and other

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      configuration information in the Stack-Configuration-Data object.
      The format may differ depending on whether the field is present in
      the Query or Response.  Some protocols do not require the
      definition of such additional data, in which case no corresponding
      MA-protocol-options field will occur in the SCD object.

   A Stack-Proposal object is simply a list of profiles; each profile is
   a sequence of MA-Protocol-IDs.  A profile lists the protocols in 'top
   to bottom' order (e.g.  TLS over TCP, or TCP over IPsec).  A Stack-
   Proposal is generally accompanied by a Stack-Configuration-Data
   object which carries an MA-protocol-options field for any protocol
   listed in the Stack-Proposal which needs it.  An MA-protocol-options
   field may apply globally, to all instances of the protocol in the
   Stack-Proposal; or it can be tagged as applying to a specific
   instance.  The latter approach can be used to carry different port
   numbers for TCP depending on whether it is to be used with or without
   TLS.  An MA-protocol-options field may also be flagged as not usable;
   for example, a NAT which could not handle SCTP would set this in an
   MA-protocol-options field about SCTP.  A protocol flagged this way
   MUST NOT be used for a messaging association.  If the Stack-Proposal
   and Stack-Configuration-Data are both present but not consistent, for
   example, if they refer to different protocols, or an MA-protocol-
   options field refers to a non-existent profile, an "Object Value
   Error" error message (Appendix A.4.4.10) with subcode 5 ("Stack-
   Proposal - Stack-Configuration-Data Mismatch") MUST be returned and
   the message dropped.

   A node generating a Stack-Configuration-Data object MUST honour the
   implied protocol configurations for the period during which a
   messaging association might be set up; in particular, it MUST be
   immediately prepared to accept incoming datagrams or connections at
   the protocol/port combinations advertised.  This MAY require the
   creation of listening endpoints for the transport and security
   protocols in question, or a node MAY keep a pool of such endpoints
   open for extended periods.  However, the received object contents
   MUST be retained only for the duration of the Query/Response exchange
   and to allow any necessary association setup to complete.  They may
   become invalid because of expired bindings at intermediate NATs, or
   because the advertising node is using agile ports.  Once the setup is
   complete, or if it is not necessary, or fails for some reason, the
   object contents MUST be discarded.  A default time of 30 seconds to
   keep the contents is RECOMMENDED.

   A Query requesting association setup always contains a Stack-Proposal
   and Stack-Configuration-Data object.  The Stack-Proposal MUST only
   include protocol configurations that are suitable for the transfer
   attributes of the messages that the Querying node wishes use the
   messaging association for.  For example, it should not simply include

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   all configurations that the Querying node is capable of supporting.

   The Response always contains a Stack-Proposal and Stack-
   Configuration-Data object, unless re-use (where the Responder decides
   to use an existing association) occurs.  For such a Response, the
   Stack-Proposal MUST NOT depend on the Query.  A node MAY make
   different proposals depending on the combination of interface and
   NSLPID.  If re-use does occur, which is indicated by sending the
   Response over an existing messaging association, the following rules

   o  The re-used messaging association MUST NOT have weaker security
      properties than would have been offered in the full Response that
      would have been sent without re-use.

   o  The re-used messaging association MUST have equivalent or better
      transport and security characteristics as at least one of the
      protocol configurations that was offered in the Query.

   Once the messaging association is set up, the querying node repeats
   the responder's Stack-Proposal over it in the Confirm.  The
   responding node MUST verify that this has not been changed as part of
   bidding-down attack prevention.  If a difference is detected, the
   responding node MUST terminate the messaging association and SHOULD
   log an error condition locally.  See Section 8.6 for further

5.7.2.  Protocol Definition: Forwards-TCP

   This MA-Protocol-ID denotes a basic use of TCP between peers.
   Support for this protocol is REQUIRED.  If this protocol is offered,
   MA-protocol-options data MUST also be carried in the SCD object.  The
   MA-protocol-options field formats are:

   o  in a Query: no information apart from the field header.

   o  in a Response: 2 byte port number at which the connection will be
      accepted, followed by 2 pad bytes.

   The connection is opened in the forwards direction, from the querying
   node towards the responder.  The querying node MAY use any source
   address and source port.  The destination information MUST be derived
   from information in the Response: the address from the interface-
   address from the Network-Layer-Information object and the port from
   the SCD object as described above.

   Associations using Forwards-TCP can carry messages with the transfer
   attribute Reliable=True.  If an error occurs on the TCP connection

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   such as a reset, as can be detected for example by a socket exception
   condition, GIST MUST report this to NSLPs as discussed in
   Section 4.1.2.

5.7.3.  Protocol Definition: Transport Layer Security

   This MA-Protocol-ID denotes a basic use of transport layer channel
   security.  Support for this protocol is mandatory; associations using
   it can carry messages with the transfer attribute Secure=True.  For
   use with TCP, implementation of TLS1.0 [7] is REQUIRED and
   implementation of TLS1.1 [10] is RECOMMENDED.  (If an unreliable
   transport such as DCCP or UDP is defined for GIST in the future, this
   MA-Protocol-ID would be implemented for it using DTLS [41].)  GIST
   nodes supporting TLS1.0 or TLS1.1 MUST be able to negotiate the TLS
   ciphersuite TLS_RSA_WITH_3DES_EDE_CBC_SHA and SHOULD be able to
   negotiate the TLS ciphersuite TLS_RSA_WITH_AES_128_CBC_SHA.

   The default mode of TLS authentication, which applies in particular
   to the above ciphersuites, uses a client/server certificate exchange.
   The Querying node acts as a TLS client, and the Responding node acts
   as a TLS server.  Where one of the above ciphersuites is negotiated,
   the GIST node acting as a server MUST provide a certificate, and MUST
   request one from the GIST node acting as a TLS client.  This allows
   either server-only or mutual authentication, depending on the
   certificates available to the client and the policy applied at the

   GIST nodes MAY negotiate other TLS ciphersuites.  In some cases, the
   negotiation of alternative ciphersuites is used to trigger
   alternative authentication procedures, such as the use of pre-shared
   keys [29].  The use of other authentication procedures may require
   additional specification work to define how they can be used as part
   of TLS within the GIST framework, and may or may not require the
   definition of additional MA-Protocol-IDs.

   No MA-protocol-options field is required for this use of TLS.

5.7.4.  Additional Protocol Options

   Further protocols or configurations could be defined in the future
   for additional performance or flexibility.  Examples are:

   o  SCTP or DCCP as alternatives to TCP, with essentially the same

   o  SigComp [21] for message compression.

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   o  IPsec [35], ssh [36], or HIP/IPsec [37] for channel security.

   o  Alternative modes of TCP operation, for example where it is set up
      from the responder to the querying node.

5.8.  Specific Message Routing Methods

   Each message routing method (see Section 3.3) requires the definition
   of the format of the message routing information (MRI) and Q-mode
   encapsulation rules.  These are given in the following subsections
   for the various possible MRMs.

5.8.1.  The Path-Coupled MRM  Message Routing Information

   For the path-coupled MRM, this is conceptually the Flow Identifier as
   in the NSIS Framework [26].  Minimally, this could just be the flow
   destination address; however, to account for policy based forwarding
   and other issues a more complete set of header fields SHOULD be
   specified if possible (see Section 4.3.4 and Section 7.2 for further

       MRI = network-layer-version
             source-address prefix-length
             destination-address prefix-length
             [ flow-label ]
             [ ipsec-SPI / L4-ports]

   Additional control information defines whether the flow-label, IPsec
   Security Parameters Index (SPI), and port information are present,
   and whether the IP-protocol and diffserv-codepoint fields should be
   interpreted as significant.  The source and destination addresses
   MUST be real node addresses, but prefix lengths other than 32/128
   (for IPv4/6) MAY be used to implement address wildcarding, allowing
   the MRI to refer to traffic to or from a wider address range.

   The MRI format allows a potentially very large number of different
   flag and field combinations.  A GIST implementation that cannot
   interpret the MRI in a message MUST return an "Object Value Error"
   message (Appendix A.4.4.10) with subcodes 1 ("Value Not Supported")
   or 2 ("Invalid Flag-Field Combination") and drop the message.

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   Where the signaling message is travelling in the same ('downstream')
   direction as the flow defined by the MRI, the IP addressing for
   Q-mode encapsulated messages is as follows.  Support for this
   encapsulation is REQUIRED.

   o  The destination IP address MUST be the flow destination address as
      given in the MRI of the message payload.

   o  By default, the source address is the flow source address, again
      from the MRI.  This provides the best likelihood that the message
      will be correctly routed through any region performing per-packet
      policy-based forwarding or load balancing which takes the source
      address into account.  However, there may be circumstances where
      the use of the signaling source address is preferable, such as:

      *  In order to receive ICMP error messages about the signaling
         message (such as unreachable port or address).  If these are
         delivered to the flow source rather than the signaling source,
         it will be very difficult for the querying node to detect that
         it is the last GIST node on the path.

      *  In order to receive GIST Error messages where the error message
         sender could not interpret the NLI in the original message.

      *  In order to attempt to run GIST through an unmodified NAT,
         which will only process and translate IP addresses in the IP

      Because of these considerations, use of the signaling source
      address is allowed as an option, with use based on local policy.
      A node SHOULD use the flow source address for initial Query
      messages, but SHOULD transition to the signaling source address
      for some retransmissions or as a matter of static configuration,
      for example if a NAT is known to be in the path out of a certain
      interface.  The S-flag in the common header tells the message
      receiver which option was used.

   A router alert option is also included in the IP header.  The option
   value depends on the NSLP being signaled for.  In addition, it is
   vital that the Query mimics the actual data flow as closely as
   possible, since this is the basis of how the signaling message is
   attached to the data path.  To this end, GIST SHOULD set the DiffServ
   codepoint and (for IPv6) flow label to match the values in the MRI.

   Any message sent in D-mode MUST have a size below a conservative
   estimate of the path MTU, for which this specification takes the

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   value 512 bytes as a default.  It is possible that fragmented
   datagrams including an RAO will not be correctly handled in the
   network, so the sender MAY set the Don't Fragment (DF) bit in the
   IPv4 header in order to detect that a message has encountered a link
   with an unusually low MTU.  If the sender sets DF for any reason, it
   SHOULD use the signaling source address for the IP source address in
   order to receive the ICMP error.

   A GIST implementation SHOULD apply validation checks to the MRI, to
   reject Query messages that are being injected by nodes with no
   legitimate interest in the flow being signalled for.  In general, if
   the GIST node can detect that no flow could arrive over the same
   interface as the Query, it MUST be rejected with an appropriate error
   message.  Such checks apply only to messages with the Q-mode
   encapsulation, since only those messages are required to track the
   flow path.  The main checks are that the IP version should match the
   version(s) used on that interface, and that the full range of source
   addresses (the source-address masked with its prefix-length) would
   pass ingress filtering checks.  For these cases, the error message is
   "MRI Validation Failure" (Appendix A.4.4.12) with subcodes 1 or 2
   ("IP Version Mismatch" or "Ingress Filter Failure") respectively.  Upstream Q-mode Encapsulation

   In some deployment scenarios it is desirable to set up routing state
   in the upstream direction, (i.e. from flow receiver towards the
   sender).  This could be used to support firewall signaling to control
   traffic from an un-cooperative sender, or signaling in general where
   the flow sender was not NSIS-capable.  This capability is
   incorporated into GIST by defining an encapsulation and processing
   rules for sending Query messages upstream.

   In general, it is not possible to determine the hop-by-hop route
   upstream because of asymmetric routing.  However, in particular
   cases, the upstream peer can be discovered with a high degree of
   confidence, for example:

   o  The upstream GIST peer is 1 IP hop away, and can be reached by
      tracing back through the interface on which the flow arrives.

   o  The upstream peer is a border router of a single-homed (stub)

   This section defines an upstream Q-mode encapsulation and validation
   checks for when it can be used.  The functionality to generate
   upstream Queries is OPTIONAL, but if received they MUST be processed
   in the normal way.  No special functionality is needed for this.

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   It is possible for routing state at a given node, for a specific MRI
   and NSLPID, to be created by both an upstream Query exchange
   (initiated by the node itself), and a downstream Query exchange
   (where the node is the responder).  If the SIDs are different, these
   items of routing state MUST be considered as independent; if the SIDs
   match, the routing state installed by the downstream exchange MUST
   take precedence, provided that the downstream Query passed ingress
   filtering checks.  The rationale for this is that the downstream
   Query is in general a more reliable way to install state, since it
   directly probes the routing infrastructure along the flow path,
   whereas use of the upstream Query depends on the correctness of the
   Querying node's understanding of the topology.

   The details of the encapsulation are as follows:

   o  The destination address SHOULD be the flow source address as given
      in the MRI of the message payload.  An implementation with more
      detailed knowledge of local routing MAY use an alternative
      destination address (e.g. the address of its default router).

   o  The source address SHOULD be the signaling node address.

   o  A router alert option is included as in the downstream case.

   o  The DiffServ codepoint and (for IPv6) flow label MAY be set to
      match the values from the MRI, as in the downstream case.  The
      same considerations about message size and fragmentation also
      apply as in the downstream case, and RAO setting and UDP port
      selection are also the same.

   o  The IP layer TTL of the message MUST be set to 255.

   The sending GIST implementation SHOULD attempt to send the Query via
   the same interface and to the same link layer neighbour from which
   the data packets of the flow are arriving.

   The receiving GIST node MAY apply validation checks to the message
   and MRI, to reject Query messages which have reached a node at which
   they can no longer be trusted.  In particular, a node SHOULD reject a
   message which has been propagated more than one IP hop, with an
   "Invalid IP layer TTL" error message (Appendix A.4.4.11).  This can
   be determined by examining the received IP layer TTL, similar to the
   generalised IP TTL security mechanism described in [25].
   Alternatively, receipt of an upstream Query at the flow source MAY be
   used to trigger setup of GIST state in the downstream direction.
   These restrictions may be relaxed in a future version.

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5.8.2.  The Loose-End MRM

   The Loose-End MRM is used to discover GIST nodes with particular
   properties in the direction of a given address, for example to
   discover a NAT along the upstream data path as in [32].  Message Routing Information

   For the loose-end MRM, only a simplified version of the Flow
   Identifier is needed.

       MRI = network-layer-version

   The source address is the address of the node initiating the
   discovery process, for example the node that will be the data
   receiver in the NAT discovery case.  The destination address is the
   address of a node which is expected to be the other side of the node
   to be discovered.  Additional control information defines the
   direction of the message relative to this flow as in the path-coupled
   case.  Downstream Q-mode Encapsulation

   Only one encapsulation is defined for the loose-end MRM; by
   convention, this is referred to as the downstream encapsulation, and
   is defined as follows:

   o  The IP destination address MUST be the destination address as
      given in the MRI of the message payload.

   o  By default, the IP source address is the source address, again
      from the MRI.  However, the use of the signaling source address is
      allowed as in the case of the path-coupled MRM.

   A router alert option is included in the IP header.  The option value
   depends on the NSLP being signaled for.  There are no special
   requirements on the setting of the DiffServ codepoint, IP layer TTL,
   or (for IPv6) the flow label.  Nor are any special validation checks
   applied.  Restrictions on message size and setting of the Don't
   Fragment (DF) bit apply as in the case of the path-coupled MRM.

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6.  Formal Protocol Specification

   This section provides a more formal specification of the operation of
   GIST processing, in terms of rules for transitions between states of
   a set of communicating state machines within a node.  The following
   description captures only the basic protocol specification;
   additional mechanisms can be used by an implementation to accelerate
   route change processing, and these are captured in Section 7.1.

   Conceptually, GIST processing at a node may be seen in terms of four
   types of cooperating state machine:

   1.  There is a top-level state machine which represents the node
       itself (Node-SM).  It is responsible for the processing of events
       which cannot be directed towards a more specific state machine,
       for example, inbound messages for which no routing state
       currently exists.  This machine exists permanently, and is
       responsible for creating per-MRI state machines to manage the
       GIST handshake and routing state maintenance procedures.

   2.  For each flow and signaling direction where the node is
       responsible for the creation of routing state, there is an
       instance of a Query-Node state machine (Querying-SM).  This
       machine sends Query and Confirm messages and waits for Responses,
       according to the requirements from local API commands or timer
       processing, such as message repetition or routing state refresh.

   3.  For each flow and signaling direction where the node has accepted
       the creation of routing state by a peer, there is an instance of
       a Responding-Node state machine (Responding-SM).  This machine is
       responsible for managing the status of the routing state for that
       flow.  Depending on policy, it MAY be responsible for
       [re]transmission of Response messages, or this MAY be handled by
       the Node-SM, and a Responding-SM is not even created for a flow
       until a properly formatted Confirm has been accepted.

   4.  Messaging associations have their own lifecycle, represented by
       MA-SM, from when they are first created (in an incomplete state,
       listening for an inbound connection or waiting for outbound
       connections to complete), to when they are active and available
       for use.

   Apart from the fact that the various machines can be created and
   destroyed by each other, there is almost no interaction between them.
   The machines for different flows do not interact; the Querying-SM and
   Responding-SM for a single flow and signaling direction do not
   interact.  That is, the Responding-SM which accepts the creation of
   routing state for a flow on one interface has no direct interaction

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   with the Querying-SM which sets up routing state on the next
   interface along the path.  This interaction is mediated instead
   through the NSLP.

   The state machine descriptions use the terminology rx_MMMM, tg_TTTT
   and er_EEEE for incoming messages, API/lower layer triggers and error
   conditions respectively.  The possible events of these types are
   given in the table below.  In addition, timeout events denoted
   to_TTTT may also occur; the various timers are listed independently
   for each type of state machine in the following subsections.

   | Name                | Meaning                                     |
   | rx_Query            | A Query has been received.                  |
   |                     |                                             |
   | rx_Response         | A Response has been received.               |
   |                     |                                             |
   | rx_Confirm          | A Confirm has been received.                |
   |                     |                                             |
   | rx_Data             | A Data message has been received.           |
   |                     |                                             |
   | rx_Message          | rx_Query||rx_Response||rx_Confirm||rx_Data. |
   |                     |                                             |
   | rx_MA-Hello         | A MA-Hello message has been received.       |
   |                     |                                             |
   | tg_NSLPData         | A signaling application has requested data  |
   |                     | transfer (via API SendMessage).             |
   |                     |                                             |
   | tg_Connected        | The protocol stack for a messaging          |
   |                     | association has completed connecting.       |
   |                     |                                             |
   | tg_RawData          | GIST wishes to transfer data over a         |
   |                     | particular messaging association.           |
   |                     |                                             |
   | er_NoRSM            | A "No Routing State" error was received.    |
   |                     |                                             |
   | er_MAConnect        | A messaging association protocol failed to  |
   |                     | complete a connection.                      |
   |                     |                                             |
   | er_MAFailure        | A messaging association failed.             |

                              Incoming Events

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6.1.  Node Processing

   The Node level state machine is responsible for processing events for
   which no more appropriate messaging association state or routing
   state exists.  Its structure is trivial: there is a single state
   ('Idle'); all events cause a transition back to Idle.  Some events
   cause the creation of other state machines.  The only events that are
   processed by this state machine are incoming GIST messages (Query/
   Response/Confirm/Data) and API requests to send data; no other events
   are possible.  In addition to this event processing, the Node level
   machine is responsible for managing listening endpoints for messaging
   associations.  Although these relate to Responding node operation,
   they cannot be handled by the Responder state machine since they are
   not created per flow.  The processing rules for each event are as

   Rule 1 (rx_Query):

   use the GIST service interface to determine the signaling application
       policy relating to this peer
   if (the signaling application indicates that routing state should
       be created) then
     if (routing state can be created without a 3-way handshake) then
       create Responding-SM and transfer control to it
       send Response
     propagate the Query with any updated NSLP payload provided

   Rule 2 (rx_Response):

   // should already have a Querying-SM to handle this
   discard message
   send "No Routing State" error message

   Rule 3 (rx_Confirm):

   if (routing state can be created before receiving a Confirm) then
     // we should already have Responding-SM for it,
     // which would handle this message
     discard message
     send "No Routing State" error message
     create Responding-SM and pass message to it

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   Rule 4 (rx_Data):

   if (node policy will only process Data messages with matching
       routing state) then
     send "No Routing State" error message
     pass directly to NSLP

   Rule 5 (tg_NSLPData):

   if Q-mode encapsulation is not possible for this MRI
     reject message with an error
     if (local policy & transfer attributes say routing
         state is not needed) then
       send message statelessly
       create Querying-SM and pass message to it

6.2.  Query Node Processing

   The Querying-Node state machine (Querying-SM) has three states:

   o  Awaiting Response

   o  Established

   o  Awaiting Refresh

   The Querying-SM is created by the Node-SM machine as a result of a
   request to send a message for a flow in a signaling direction where
   the appropriate state does not exist.  The Query is generated
   immediately and the No_Response timer is started.  The NSLP data MAY
   be carried in the Query if local policy and the transfer attributes
   allow it, otherwise it MUST be queued locally pending MA
   establishment.  Then the machine transitions to the Awaiting Response
   state, in which timeout-based retransmissions are handled.  Data
   messages (rx_Data events) should not occur in this state; if they do,
   this may indicate a lost Response and a node MAY also retransmit a
   Query for this reason.

   Once a Response has been successfully received and routing state
   created, the machine transitions to Established, during which NSLP
   data can be sent and received normally.  Further Responses received
   in this state (which may be the result of a lost Confirm) MUST be
   treated the same way.  The Awaiting Refresh state can be considered
   as a substate of Established, where a new Query has been generated to
   refresh the routing state (as in Awaiting Response) but NSLP data can

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   be handled normally.

   The timers relevant to this state machine are as follows:

   Refresh_QNode:  Indicates when the routing state stored by this state
      machine must be refreshed.  It is reset whenever a Response is
      received indicating that the routing state is still valid.
      Implementations MUST set the period of this timer based on the
      value in the RS-validity-time field of a Response to ensure that a
      Query is generated before the peer's routing state expires.

   No_Response:  Indicates that a Response has not been received in
      answer to a Query.  This is started whenever a Query is sent and
      stopped when a Response is received.

   Inactive_QNode:  Indicates that no traffic is currently being handled
      by this state machine.  This is reset whenever the state machine
      handles NSLP data, in either direction.  When it expires, the
      state machine MAY be deleted.  The period of the timer can be set
      at any time via the API (SetStateLifetime), and if the period is
      reset in this way the timer itself MUST be restarted.

   The main events (including all those that cause state transitions)
   are shown in the figure below, tagged with the number of the
   processing rule that is used to handle the event.  These rules are
   listed after the diagram.  All events not shown or described in the
   text above are assumed to be impossible in a correct implementation.

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              [Initialisation]   +-----+
       |                         +-----+
       |                                                 rx_Response[4]
       |                                               || tg_NSLPData[5]
       |      tg_NSLPData[1]             er_NoRSM[3]      || rx_Data[7]
       |        --------                ------------------   -------
       |       |        V              |                  V |       V
       |       |        V              |                  V |       V
       |      +----------+             |                 +-----------+
        ---->>| Awaiting |              ---------------- |Established|
        ------| Response |---------------------------->> |           |
       |      +----------+       rx_Response[4]          +-----------+
       |       ^        |                                     ^   |
       |       ^        |                                     ^   |
       |        --------                                      |   |
       |    to_No_Response[2]                                 |   |
       |    [!nResp_reached]     tg_NSLPData[5]               |   |
       |                         || rx_Data[7]                |   |
       |                          --------                    |   |
       |                         |        V                   |   |
       |    to_No_Response[2]    |        V                   |   |
       |     [nResp_reached]    +-----------+  rx_Response[4] |   |
        ----------   -----------|  Awaiting |-----------------    |
                  | |           |  Refresh  |<<-------------------
                  | |           +-----------+    to_Refresh_QNode[8]
                  | |            ^        |
                  | |            ^        |
                  | |             --------
                  | |         to_No_Response[2]
                  | |         [!nResp_reached]
                  V V
                  V V
                +-----+   to_Inactive_QNode[6]
                          (from all states)

                    Figure 6: Query Node State Machine

   The processing rules are as follows:

   Rule 1:  Store the message for later transmission

   Rule 2:

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   if number of Queries sent has reached the threshold
     // nQuery_isMax is true
     indicate No Response error to NSLP
     destroy self
     send Query
     start No_Response timer with new value

   Rule 3:

   // Assume the Confirm was lost in transit so resend it
   // for the last Response we received
   send Confirm
   restart Refresh_QNode and Inactive_QNode timers

   Rule 4:

   if a new MA-SM is needed create one
   if the R flag was set send a Confirm
   pass any NSLP data to the NSLP
   send any stored Data messages
   stop No_Response timer
   start Refresh_QNode and Inactive_QNode timers

   Rule 5:

   send Data message
   restart Inactive_QNode timer

   Rule 6:  Terminate

   Rule 7:

   pass any data to the NSLP
   (re)start Inactive_QNode timer

   Rule 8:

   send Query
   start No_Response timer
   stop Refresh_QNode timer

6.3.  Responder Node Processing

   The Responding-Node state machine (Responding-SM) has three states:

   o  Awaiting Confirm

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   o  Established

   o  Awaiting Refresh

   The policy governing the creation of the Responding-SM has three

   1.  It is created on receiving a Query, no Confirm is requested.

   2.  It is created on receiving a Query, but a Confirm is requested.
       A timer is used to retransmit Response messages and the
       Responding-SM is destroyed if no valid Confirm is received.

   3.  It cannot be created until a valid Confirm is received; the
       initial Query will have been handled by the Node level machine.

   In case 2 the Responding-SM is created in the Awaiting Confirm state,
   and remains there until a Confirm is received, at which point it
   transitions to Established.  In cases 1 and 3 the Responding-SM is
   created directly in the Established state.  Note that if the machine
   is created on receiving a Query, some of the message processing will
   already have been performed in the Node state machine.  In the
   Established state the NSLP can send and receive data normally, and
   any additional rx_Confirm events MUST be silently ignored.  The
   Awaiting Refresh state can be considered a substate of Established,
   where a Query has been received to begin the routing state refresh.

   In the Awaiting Refresh state the Responding-SM behaves as in the
   Awaiting Confirm state, except that the NSLP can still send and
   receive data.  In particular, in both states there is timer-based
   retransmission of Response messages until a Confirm is received;
   additional rx_Query events in these states MUST also generate a
   response and restart the no_Confirm timer.

   The timers relevant to the operation of this state machine are as

   Expire_RNode:  Indicates when the routing state stored by this state
      machine needs to be expired.  It is reset whenever a Query or
      Confirm (depending on local policy) is received indicating that
      the routing state is still valid.  Note that state cannot be
      refreshed from the R-Node.

   No_Confirm:  Indicates that a Confirm has not been received in answer
      to a Response.  This is started/reset whenever a Response is sent
      and stopped when a Confirm is received.

   The detailed state transitions and processing rules are described

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   below as in the Query node case.

               rx_Query[1]                      rx_Query[5]
            [confirmRequired]    +-----+    [!confirmRequired]
       |                         +-----+                            |
       |                            |         rx_Confirm[2]         |
       |                             ----------------------------   |
       |                                                         |  |
       |                                       rx_Query[5]       |  |
       |     tg_NSLPData[7]                 [!confirmRequired]   |  |
       |      || rx_Query[1]                  || rx_Data[4]      |  |
       |      || rx_Data[6]                   || tg_NSLPData[3]  |  |
       |        --------                        --------------   |  |
       |       |        V                      |              V  V  V
       |       |        V                      |              V  V  V
       |      +----------+                     |           +-----------+
        ---->>| Awaiting |     rx_Confirm[8]    -----------|Established|
        ------| Confirm  |------------------------------>> |           |
       |      +----------+                                 +-----------+
       |       ^        |                                      ^   |
       |       ^        |         tg_NSLPData[3]               ^   |
       |        --------          || rx_Query[1]               |   |
       |    to_No_Confirm[9]      || rx_Data[4]                |   |
       |    [!nConf_reached]       --------                    |   |
       |                          |        V                   |   |
       |    to_No_Confirm[9]      |        V                   |   |
       |    [nConf_reached]      +-----------+  rx_Confirm[8]  |   |
        ----------   ------------|  Awaiting |-----------------    |
                  | |            |  Refresh  |<<-------------------
                  | |            +-----------+      rx_Query[1]
                  | |             ^        |     [confirmRequired]
                  | |             ^        |
                  | |              --------
                  V V          to_No_Confirm[9]
                  V V          [!nConf_reached]
                +-----+  to_Expire_RNode[10]
                          (from all states)

                  Figure 7: Responder Node State Machine

   The processing rules are as follows:

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   Rule 1:

   // a Confirm is required
   send Response
   (re)start No_Confirm timer

   Rule 2:

   pass any piggybacked data to the NSLP
   start Expire_RNode timer

   Rule 3:  send the Data message

   Rule 4:  pass data to NSLP

   Rule 5:

   // no Confirm is required
   send Response
   start Expire_RNode timer

   Rule 6:  send "No Routing State" error message

   Rule 7:  store Data message

   Rule 8:

   pass any piggybacked data to the NSLP
   send any stored Data messages
   stop No_Confirm timer
   start Expire_RNode timer

   Rule 9:

   if number of Responses sent has reached threshold
     // nResp_isMax is true
     destroy self
     send Response
     start No_Response timer

   Rule 10:  destroy self

6.4.  Messaging Association Processing

   Messaging associations (MAs) are modelled for use within GIST with a
   simple three-state process.  The Awaiting Connection state indicates
   that the MA is waiting for the connection process(es) for every

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   protocol in the messaging association to complete; this might involve
   creating listening endpoints or attempting active connects.  Timers
   may also be necessary to detect connection failure (e.g. no incoming
   connection within a certain period), but these are not modelled
   explicitly.  The Connected state indicates that the MA is open and
   ready to use.  In addition there is an Idle state in which the local
   node no longer requires the messaging association but the remote node
   still wants it to be kept open.

   Clearly, many internal details of the messaging association protocols
   are hidden in this model, especially where the messaging association
   uses multiple protocol layers.  Note also that although the existence
   of messaging associations is not directly visible to signaling
   applications, there is some interaction between the two because
   security-related information becomes available during the open
   process, and this may be indicated to signaling applications if they
   have requested it.

   The timers relevant to the operation of this state machine are as

   SendHello:  Indicates that an MA-Hello message should be sent to the
      remote node.  The period of this timer is determined by the MA-
      Hold-Time sent by the remote node during the Query/Response/
      Confirm exchange.

   NoHello:  Indicates that no MA-Hello has been received from the
      remote node for a period of time.  The period of this timer is
      sent to the remote node as the MA-Hold-Time during the Query/
      Response exchange.

   NoActivity:  Indicates that the link has been inactive for a period
      of time.  The period of this timer is implementation-specific but
      is likely to be related to the period of the NoHello timer.

   The detailed state transitions and processing rules are described
   below as in the Query node case.

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         [Initialisation]       +-----+
   |                            +-----+
   |                                          tg_RawData[1]
   |                                          || rx_Message[2]
   |                                          || rx_MA-Hello[3]
   |       tg_RawData[5]                      || to_SendHello[4]
   |        --------                             --------
   |       |        V                           |        V
   |       |        V                           |        V
   |      +----------+                         +-----------+
    ---->>| Awaiting |    tg_Connected[6]      | Connected |
    ------|Connection|----------------------->>|           |
   |      +----------+                         +-----------+
   |                                              ^    |
   |                             tg_RawData[1]    ^    |
   |                           || rx_Message[2]   |    |to_NoActivity[7]
   |                                              |    V
   |                                              |    V
   | er_MAConnect[8]  +-----+   to_NoHello[8]  +-----------+
    ---------------->>|Death|<<----------------|   Idle    |
                      +-----+                  |           |
                        ^                      +-----------+
                        ^                       ^        |
                        |                       ^        |
                         ---------------         --------
                         er_MAFailure[8]        rx_MA-Hello[9]
                        (from all states)

               Figure 8: Messaging Association State Machine

   The processing rules are as follows:

   Rule 1:

   pass message to transport layer
   (re)start NoActivity timer
   (re)start SendHello

   Rule 2:

   pass message to Node-SM
   (re)start NoActivity timer

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   Rule 3:

   if reply requested
     send MA-Hello
     restart SendHello timer

   Rule 4:

   send MA-Hello message
   restart SendHello timer

   Rule 5:  queue message for later transmission

   Rule 6:

   pass outstanding queued messages to transport layer
   stop any timers controlling connection establishment
   start NoActivity timer
   start SendHello timer

   Rule 7:

   stop NoActivity timer
   stop SendHello timer
   start NoHello timer

   Rule 8:  destroy self

   Rule 9:

   if reply requested
     send MA-Hello
   restart NoHello timer

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7.  Advanced Protocol Features

7.1.  Route Changes and Local Repair

7.1.1.  Introduction

   When re-routing takes place in the network, GIST and signaling
   application state need to be updated for all flows whose paths have
   changed.  The updates to signaling application state depend mainly on
   the signaling application: for example, if the path characteristics
   have actually changed, simply moving state from the old to the new
   path is not sufficient.  Therefore, GIST cannot carry out the
   complete path update processing.  Its responsibilities are to detect
   the route change, update its local routing state consistently, and
   inform interested signaling applications at affected nodes.

                       x  +--+      +--+      +--+  x      Initial
                      x  .|C1|_.....|D1|_.....|E1|   x     Configuration
                     x  . +--+.    .+--+.    .+--+\.  x
      >>xxxxxxxxxxxxx  .       .  .      .  .       .  xxxxxx>>
        +-+       +-+ .         ..        ..         . +-+
     ...|A|_......|B|/          ..        ..          .|F|_....
        +-+       +-+ .        .  .      .  .        . +-+
                       .      .    .    .    .      .
                        . +--+      +--+      +--+ .
                          +--+      +--+      +--+

                          +--+      +--+      +--+         Configuration
                         .|C1|......|D1|......|E1|         after failure
                        . +--+     .+--+      +--+         of E1-F link
                       .      \.  .     \.  ./
        +-+       +-+ .         ..        ..           +-+
     ...|A|_......|B|.          ..        ..          .|F|_....
        +-+       +-+\         .  .      .  .        . +-+
      >>xxxxxxxxxxxxx .       .    .    .    .      .  xxxxxx>>
                     x  . +--+      +--+      +--+ .  x
                      x  .|C2|_.....|D2|_.....|E2|/  x
                       x  +--+      +--+      +--+  x

               ........... = physical link topology
               >>xxxxxxx>> = flow direction
               _.......... = outgoing link for flow xxxxxx given
                             by local forwarding table

                       Figure 9: A Re-Routing Event

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   Route change management is complicated by the distributed nature of
   the problem.  Consider the re-routing event shown in Figure 9.  An
   external observer can tell that the main responsibility for
   controlling the updates will probably lie with nodes B and F;
   however, E1 is best placed to detect the event quickly at the GIST
   level, and C1 and D1 could also attempt to initiate the repair.

   On the assumption that signaling applications are soft-state based
   and operate end to end, and because GIST also periodically updates
   its picture of routing state, route changes will eventually be
   repaired automatically.  The specification as already given includes
   this functionality.  However, especially if upper layer refresh times
   are extended to reduce signaling load, the duration of inconsistent
   state may be very long indeed.  Therefore, GIST includes logic to
   exchange prompt notifications with signaling applications, to allow
   local repair if possible.  The additional mechanisms to achieve this
   are described in the following subsections.  To a large extent, these
   additions can be seen as implementation issues; the protocol messages
   and their significance are not changed, but there are extra
   interactions through the API between GIST and signaling applications,
   and additional triggers for transitions between the various GIST

7.1.2.  Route Change Detection Mechanisms

   There are two aspects to detecting a route change at a single node:

   o  Detecting that the outgoing path, in the direction of the Query,
      has or may have changed.

   o  Detecting that the incoming path, in the direction of the
      Response, has (or may have) changed, in which case the node may no
      longer be on the path at all.

   At a single node, these processes are largely independent, although
   clearly a change in one direction at a node corresponds to a change
   the opposite direction at its peer.  Note that there are two possible
   forms for a route change: the interface through which a flow leaves
   or enters a node may change, and the adjacent peer may change.  In
   general, a route change can include one or the other or both (or
   indeed neither, although such changes are very hard to detect).

   The route change detection mechanisms available to a node depend on
   the MRM in use and the role the node played in setting up the routing
   state in the first place (i.e. as Querying or Responding node).  The
   following discussion is specific to the case of the path-coupled MRM
   using downstream Queries only; other scenarios may require other
   methods.  However, the repair logic described in the subsequent

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   subsections is intended to be universal.

   There are five mechanisms for a node to detect that a route change
   has occurred, which are listed below.  They apply differently
   depending on whether the change is in the Query or Response
   direction, and these differences are summarised in the following

   Local Trigger:  In local trigger mode, GIST finds out from the local
      forwarding table that the next hop has changed.  This only works
      if the routing change is local, not if it happens a few routing
      hops away, including the case that it happens at a GIST-unaware

   Extended Trigger:  Here, GIST checks a link-state topology database
      to discover that the path has changed.  This makes certain
      assumptions on consistency of route computation and only works
      within a single area for OSPF [13] and similar link-state
      protocols.  Where available, this offers the most accurate and
      rapid indication of route changes, but requires more access to the
      routing internals than a typical operating system may provide.

   GIST C-mode Monitoring:  GIST may find that C-mode packets are
      arriving (from either peer) with a different IP layer TTL or on a
      different interface.  This provides no direct information about
      the new flow path, but indicates that routing has changed and that
      rediscovery may be required.

   Data Plane Monitoring:  The signaling application on a node may
      detect a change in behaviour of the flow, such as IP layer TTL
      change, arrival on a different interface, or loss of the flow
      altogether.  The signaling application on the node is allowed to
      notify this information locally to GIST (Appendix B.6).

   GIST Probing:  According to the specification, each GIST node MUST
      periodically repeat the discovery (Query/Response) operation.
      Values for the probe frequency are discussed in Section 4.4.3.
      The querying node will discover the route change by a modification
      in the Network-Layer-Information in the Response.  The period can
      be negotiated independently for each GIST hop, so nodes that have
      access to the other techniques listed above MAY use long periods
      for the probing operation.

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   | Method      | Query direction          | Response direction       |
   | Local       | Discovers new interface  | Not applicable           |
   | Trigger     | (and peer if local)      |                          |
   |             |                          |                          |
   | Extended    | Discovers new interface  | May determine that route |
   | Trigger     | and may determine new    | from peer will have      |
   |             | peer                     | changed                  |
   |             |                          |                          |
   | C-mode      | Provides hint that       | Provides hint that       |
   | Monitoring  | change has occurred      | change has occurred      |
   |             |                          |                          |
   | Data Plane  | Not applicable           | NSLP informs GIST that a |
   | Monitoring  |                          | change may have occurred |
   |             |                          |                          |
   | Probing     | Discovers changed NLI in | Discovers changed NLI in |
   |             | Response                 | Query                    |

7.1.3.  GIST Behaviour Supporting Re-Routing

   The GIST behaviour necessary to support re-routing can be modelled
   using a 3-level classification of the validity of each item of
   routing state.  (This classification applies separately to the
   Querying and Responding node for each pair of GIST peers.)  The
   levels are:

   Bad:  The routing state is either missing altogether, or not safe to
      use to send data.

   Tentative:  The routing state may have changed, but it is still
      usable for sending NSLP data pending verification.

   Good:  The routing state has been established and no events affecting
      it have since been detected.

   These classifications are not identical to the states described in
   Section 6, but there are dependencies between them.  Specifically,
   routing state is considered Bad until the machine first enters the
   Established state, at which point it becomes Good.  Thereafter, the
   status may be invalidated for any of the reasons discussed above; it
   is an implementation issue to decide which techniques to implement in
   any given node, and how to reclassify routing state (as Bad or
   Tentative) for each.  The status returns to Good, either when the
   state machine re-enters the Established state, or if GIST can
   determine from direct examination of the routing or forwarding tables
   that the peer has not changed.  When the status returns to Good, GIST

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   MUST if necessary update its routing state table so that the
   relationships between MRI/SID/NSLPID tuples and messaging
   associations are up to date.

   When classification of the routing state for the downstream direction
   changes to Bad/Tentative because of local routing indications, GIST
   MAY automatically change the classification in the upstream direction
   to Tentative unless local routing indicates that this is not
   necessary.  This SHOULD NOT be done in the case where the initial
   change was indicated by the signaling application.  This mechanism
   accounts for the fact that a routing change may affect several nodes,
   and so can be an indication that upstream routing may also have
   changed.  In any case, whenever GIST updates the routing status, it
   informs the signaling application with the NetworkNotification API
   (Appendix B.4), unless the change was caused via the API in the first

   The GIST behaviour for state repair is different for the Querying and
   Responding node.  At the Responding node, there is no additional
   behaviour, since the Responding node cannot initiate protocol
   transitions autonomously, it can only react to the Querying node.
   The Querying node has three options, depending on how the transition
   from 'Good' was initially caused:

   1.  To inspect the routing/forwarding table and verifying that the
       next peer has not changed.  This technique MUST NOT be used if
       the transition was caused by a signaling application, but SHOULD
       be used otherwise if available.

   2.  To move to the 'Awaiting Refresh' state.  This technique MUST NOT
       be used if the current status is 'Bad', since data is being
       incorrectly delivered.

   3.  To move to the 'Awaiting Response' state.  This technique may be
       used at any time, but has the effect of freezing NSLP
       communication while GIST state is being repaired.

   The second and third techniques trigger the execution of a GIST
   handshake to carry out the repair.  It may be desirable to delay the
   start of the handshake process, either to wait for the network to
   stabilise, to avoid flooding the network with Query traffic for a
   large number of affected flows, or to wait for confirmation that the
   node is still on the path from the upstream peer.  One approach is to
   delay the handshake until there is NSLP data to be transmitted.
   Implementation of such delays is a matter of local policy; however,
   GIST MUST begin the handshake immediately if the status change was
   caused by an InvalidateRoutingState API call marked as 'Urgent', and
   SHOULD begin it if the upstream routing state is still known to be

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7.1.4.  Signaling Application Operation

   Signaling applications can use these functions as provided by GIST to
   carry out rapid local repair following re-routing events.  The
   signaling application instances carry out the multi-hop aspects of
   the procedure, including crossover node detection, and tear-down/
   reinstallation of signaling application state; they also trigger GIST
   to carry out the local routing state maintenance operations over each
   individual hop.  The local repair procedures depend heavily on the
   fact that stateful NSLP nodes are a single GIST hop apart; this is
   enforced by the details of the GIST peer discovery process.

   The following outline description of a possible set of NSLP actions
   takes the scenario of Figure 9 as an example.

   1.  The signaling application at node E1 is notified by GIST of route
       changes affecting the downstream and upstream directions.  The
       downstream status was updated to Bad because of a trigger from
       the local forwarding tables, and the upstream status changed
       automatically to Tentative as a consequence.  The signaling
       application at E1 MAY begin local repair immediately, or MAY
       propagate a notification upstream to D1 that re-routing has

   2.  The signaling application at node D1 is notified of the route
       change, either by signaling application notifications or from the
       GIST level (e.g. by a trigger from a link-state topology
       database).  If the information propagates faster within the
       routing protocol, GIST will change the upstream/downstream
       routing state to Tentative/Bad automatically, and this will cause
       the signaling application to propagate the notification further

   3.  This process continues until the notification reaches node A.
       Here, there is no downstream routing change, so GIST only learns
       of the update via the signaling application trigger.  Since the
       upstream status is still Good, it therefore begins the repair
       handshake immediately.

   4.  The handshake initiated by node A causes its downstream routing
       state to be confirmed as Good and unchanged there; it also
       confirms the (Tentative) upstream routing state at B as Good.
       This is enough to identify B as the crossover router, and the
       signaling application and GIST can begin the local repair

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   An alternative way to reach step (4) is that node B is able to
   determine autonomously that there is no likelihood of an upstream
   route change.  For example, it could be an area border router and the
   route change is only intra-area.  In this case, the signaling
   application and GIST will see that the upstream state is Good and can
   begin the local repair directly.

   After a route change, a signaling application may wish to remove
   state at another node which is no longer on the path.  However, since
   it is no longer on the path, in principle GIST can no longer send
   messages to it.  In general, provided this state is soft, it will
   time out anyway; however, the timeouts involved may have been set to
   be very long to reduce signaling load.  The requirement to remove
   state in a specific peer node is identified in [33].

   This requirement can be met provided that GIST is able to use the old
   path to the signaling application peer for some period while the
   signaling application still needs it.  Since NSLP peers are a single
   GIST hop apart, the necessary information is just the old entry in
   the node's routing state table for that flow.  Rather than requiring
   GIST to maintain multiple generations of this information, it can
   just be provided to the signaling application in the same node in an
   opaque form for each message that is received.  The signaling
   application can store it if necessary and provide it back to the GIST
   layer in case it needs to be used.  Because this is a reference to
   information about the source of a prior signaling message, it is
   denoted 'SII-Handle' (for Source Identification Information) in the
   abstract API of Appendix B.  Note that GIST if possible SHOULD use
   the same SII-Handle for multiple sessions to the same peer, since
   this then allows signaling applications to aggregate some signaling,
   such as summary refreshes or bulk teardowns.

   Messages sent this way MUST bypass the GIST routing state tables at
   the sender, and this MUST be indicated by setting the E flag in the
   common header (Appendix A.1).  Messages other than Data messages MUST
   NOT be sent in this way.  At the receiver, GIST MUST NOT validate the
   MRI/SID/NSLPID against local routing state and instead indicates the
   mode of reception to signaling applications through the API
   (Appendix B.2).  Signaling applications should validate the source
   and effect of the message themselves, and if appropriate should in
   particular indicate to GIST (see Appendix B.5) that routing state is
   no longer required for this flow.  This is necessary to prevent GIST
   in nodes on the old path initiating routing state refresh and thus
   causing state conflicts at the crossover router.

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7.2.  NAT Traversal

7.2.1.  Overview

   GIST messages must carry packet addressing and higher layer
   information as payload data in order to define the flow signalled
   for.  (This applies to all GIST messages, regardless of how they are
   encapsulated or which direction they are travelling in.)  At an
   addressing boundary the data flow packets will have their headers
   translated; if the signaling payloads are not translated
   consistently, the signaling messages will refer to incorrect (and
   probably meaningless) flows after passing through the boundary.  In
   addition, GIST handshake messages carry additional addressing
   information about the GIST nodes themselves, and this must also be
   processed appropriately when traversing a NAT.

   The simplest solution to this problem is to require that a NAT is
   GIST-aware, and to allow it to modify messages based on the contents
   of the MRI.  This makes the assumption that NATs only rewrite the
   header fields included in this payload, and not other higher layer
   identifiers.  Provided this is done consistently with the data flow
   header translation, signaling messages will be valid each side of the
   boundary, without requiring the NAT to be signaling application
   aware.  Note, however, that if the NAT does not understand the MRI,
   and the N-flag in the MRI is clear (see Appendix A.3.1), it should
   reject the message with an "Object Type Error" message
   (Appendix A.4.4.9) with subcode 4 ("Untranslated Object").

   This specification defines an additional object that a NAT inserts
   into all Q-mode encapsulated messages and which is echoed back in any
   replies, i.e.  Response or Error messages.  NATs apply GIST-specific
   processing only to Q-mode encapsulated messages or replies carrying
   the NAT traversal object.  All other GIST messages, either in C-mode,
   or D-mode messages with no NAT-Traversal object, should be treated as
   normal data traffic by the NAT, i.e. with IP and transport layer
   header translation but no GIST-specific processing.

   The new object, the NAT-Traversal object (Appendix A.3.8), carries
   the translation between the MRIs which are appropriate for the
   internal and external sides of the NAT.  It also carries a list of
   which other objects in the message have been translated.  This should
   always include the NLI, and the Stack-Configuration-Data if present;
   if GIST is extended with further objects that carry addressing data,
   this list allows a message receiver to know if the new objects were
   supported by the NAT.  Finally, the NAT-Traversal object MAY be used
   to carry data to be used in back-translating D-mode responses; this
   could be the original NLI or SCD, or opaque equivalents in the case
   of topology hiding.

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   A consequence of this approach is that the routing state tables at
   the signaling application peers each side of the NAT are no longer
   directly compatible.  In particular, the values for Message-Routing-
   Information are different, which is why the unmodified MRI is
   propagated in the NAT-Traversal object to allow subsequent C-mode
   messages to be interpreted correctly.

7.2.2.  Message Processing Rules

   This specification normatively defines the behaviour of a GIST node
   receiving a message containing a NAT-Traversal object.  However, it
   does not define normative behaviour for a NAT translating GIST
   messages, since much of this will depend on NAT implementation and
   policy about allocating bindings.  In addition, it is not necessary
   for a GIST implementation itself.  Therefore, those aspects of the
   following description are informative; full details of NAT behaviour
   for handling GIST messages can be found in [40].

   A possible set of operations for a NAT to process a Q-mode
   encapsulated message is as follows.  Note that for a Data message,
   only a subset of the operations is applicable.

   1.  Verify that bindings for any data flow are actually in place.

   2.  Create a new Message-Routing-Information object with fields
       modified according to the data flow bindings.

   3.  Create bindings for subsequent C-mode signaling based on the
       information in the Network-Layer-Information and Stack-
       Configuration-Data objects.

   4.  Create new Network-Layer-Information and if necessary Stack-
       Configuration-Data objects with fields to force D-mode response
       messages through the NAT, and to allow C-mode exchanges using the
       C-mode signaling bindings.

   5.  Add a NAT-Traversal object, listing the objects which have been
       modified and including the unmodified MRI and any other data
       needed to interpret the response.  If a NAT-Traversal object is
       already present, in the case of a sequence of NATs, the list of
       modified objects may be updated and further opaque data added,
       but the MRI contained in it is left unchanged.

   6.  Encapsulate the message according to the normal rules of this
       specification for the Q-mode encapsulation.  If the S-flag was
       set in the original message, the same IP source address selection
       policy should be applied to the forwarded message.

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   7.  Forward the message with these new payloads.

   A GIST node receiving such a message MUST verify that all mandatory
   objects containing addressing have been translated correctly, or else
   reject the message with an "Object Type Error" message
   (Appendix A.4.4.9) with subcode 4 ("Untranslated Object").  The error
   message MUST include the NAT-Traversal object as the first TLV after
   the common header, and this is also true for any other error message
   generated as a response.  Otherwise, the message is processed
   essentially as normal.  If no state needs to be updated for the
   message, the NAT-Traversal object can be effectively ignored.  The
   other possibility is that a Response must be returned, either because
   the message is the beginning of a handshake for a new flow, or it is
   a refresh for existing state.  In both cases, the GIST node MUST
   create the Response in the normal way using the local form of the
   MRI, and its own NLI and (if necessary) SCD.  It MUST also include
   the NAT-Traversal object as the first object in the Response after
   the common header.

   A NAT will intercept D-mode messages with the normal encapsulation
   containing such echoed NAT-Traversal objects.  The NAT processing is
   a subset of the processing for the Q-mode encapsulated case:

   1.  Verify the existence of bindings for the data flow.

   2.  Leave the Message-Routing-Information object unchanged.

   3.  Modify the NLI and SCD objects for the Responding node if
       necessary, and create or update any bindings for C-mode signaling

   4.  Forward the message.

   A GIST node receiving such a message MUST use the MRI from the NAT-
   Traversal object as the key to index its internal routing state; it
   MAY also store the translated MRI for additional (e.g. diagnostic)
   information, but this is not used in the GIST processing.  The
   remainder of GIST processing is unchanged.

   Note that Confirm messages are not given GIST-specific processing by
   the NAT.  Thus, a Responding node which has delayed state
   installation until receiving the Confirm, only has available the
   untranslated MRI describing the flow, and the untranslated NLI as
   peer routing state.  This would prevent the correct interpretation of
   the signaling messages; also, subsequent Query (refresh) messages
   would always be seen as route changes because of the NLI change.
   Therefore, a Responding node that wishes to delay state installation
   until receiving a Confirm must somehow reconstruct the translations

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   when the Confirm arrives.  How to do this is an implementation issue;
   one approach is to carry the translated objects as part of the
   Responder cookie which is echoed in the Confirm.  Indeed, for one of
   the cookie constructions in Section 8.5 this is automatic.

7.3.  Interaction with IP Tunnelling

   The interaction between GIST and IP tunnelling is very simple.  An IP
   packet carrying a GIST message is treated exactly the same as any
   other packet with the same source and destination addresses: in other
   words, it is given the tunnel encapsulation and forwarded with the
   other data packets.

   Tunnelled packets will not be identifiable as GIST messages until
   they leave the tunnel, since any router alert option and the standard
   GIST protocol encapsulation (e.g. port numbers) will be hidden within
   the standard tunnel encapsulation.  If signaling is needed for the
   tunnel itself, this has to be initiated as a separate signaling
   session by one of the tunnel endpoints - that is, the tunnel counts
   as a new flow.  Because the relationship between signaling for the
   microflow and signaling for the tunnel as a whole will depend on the
   signaling application in question, it is a signaling application
   responsibility to be aware of the fact that tunnelling is taking
   place and to carry out additional signaling if necessary; in other
   words, at least one tunnel endpoint must be signaling application

   In some cases, it is the tunnel exit point (i.e. the node where
   tunnelled data and downstream signaling packets leave the tunnel)
   that will wish to carry out the tunnel signaling, but this node will
   not have knowledge or control of how the tunnel entry point is
   carrying out the data flow encapsulation.  The information about how
   the inner MRI/SID relate to the tunnel MRI/SID needs to be carried in
   the signaling data from the tunnel entry point; this functionality is
   the equivalent to the RSVP SESSION_ASSOC object of [14].  In the NSIS
   protocol suite, these bindings are managed by the signaling
   applications, either implicitly (e.g. by SID re-use) or explicitly by
   carrying objects that bind the inner and outer SIDs as part of the
   NSLP payload.

7.4.  IPv4-IPv6 Transition and Interworking

   GIST itself is essentially IP version neutral: version dependencies
   are isolated in the formats of the Message-Routing-Information,
   Network-Layer-Information and Stack-Configuration-Data objects, and
   GIST also depends on the version independence of the protocols that
   support messaging associations.  In mixed environments, GIST
   operation will be influenced by the IP transition mechanisms in use.

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   This section provides a high level overview of how GIST is affected,
   considering only the currently predominant mechanisms.

   Dual Stack:  (As described in [34].)  In mixed environments, GIST
      MUST use the same IP version for Q-mode encapsulated messages as
      the flow it is signaling for, and SHOULD do so for other signaling
      also (see Section 5.2.2).  The IP version used in D-mode is
      closely tied to the IP version used by the data flow, so it is
      intrinsically impossible for an IPv4-only or IPv6-only GIST node
      to support signaling for flows using the other IP version.  Hosts
      which are dual stack for applications and routers which are dual
      stack for forwarding need GIST implementations which can support
      both IP versions.  Applications with a choice of IP versions might
      select a version based on which could be supported in the network
      by GIST, which could be established by invoking parallel discovery

   Packet Translation:  (Applicable to SIIT [6] and NAT-PT [15].)  Some
      transition mechanisms allow IPv4 and IPv6 nodes to communicate by
      placing packet translators between them.  From the GIST
      perspective, this should be treated essentially the same way as
      any other NAT operation (e.g. between internal and external
      addresses) as described in Section 7.2.  The translating node
      needs to be GIST-aware; it will have to translate the addressing
      payloads between IPv4 and IPv6 formats for flows which cross
      between the two.  The translation rules for the fields in the MRI
      payload (including e.g.  DiffServ-codepoint and flow-label) are as
      defined in [6].

   Tunnelling:  (Applicable to 6to4 [17].)  Many transition mechanisms
      handle the problem of how an end to end IPv6 (or IPv4) flow can be
      carried over intermediate IPv4 (or IPv6) regions by tunnelling;
      the methods tend to focus on minimising the tunnel administration

      From the GIST perspective, the treatment should be as similar as
      possible to any other IP tunnelling mechanism, as described in
      Section 7.3.  In particular, the end to end flow signaling will
      pass transparently through the tunnel, and signaling for the
      tunnel itself will have to be managed by the tunnel endpoints.
      However, additional considerations may arise because of special
      features of the tunnel management procedures.  In particular, [18]
      is based on using an anycast address as the destination tunnel
      endpoint.  GIST MAY use anycast destination addresses in the
      Q-mode encapsulation of D-mode messages if necessary, but MUST NOT
      use them in the Network-Layer-Information addressing field; normal
      unicast addresses MUST be used instead.  Note that the addresses
      from the IP header are not used by GIST in matching requests and

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      responses, so there is no requirement to use anycast source

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8.  Security Considerations

   The security requirement for GIST is to protect the signaling plane
   against identified security threats.  For the signaling problem as a
   whole, these threats have been outlined in [27]; the NSIS framework
   [26] assigns a subset of the responsibilities to the NTLP.  The main
   issues to be handled can be summarised as:

   Message Protection:  Signaling message content can be protected
      against eavesdropping, modification, injection and replay while in
      transit.  This applies both to GIST payloads, and GIST should also
      provide such protection as a service to signaling applications
      between adjacent peers.

   Routing State Integrity Protection:  It is important that signaling
      messages are delivered to the correct nodes, and nowhere else.
      Here, 'correct' is defined as 'the appropriate nodes for the
      signaling given the Message-Routing-Information'.  In the case
      where the MRI is based on the Flow Identification for path-coupled
      signaling, 'appropriate' means 'the same nodes that the
      infrastructure will route data flow packets through'.  GIST has no
      role in deciding whether the data flow itself is being routed
      correctly; all it can do is ensure the signaling is routed
      consistently with it.  GIST uses internal state to decide how to
      route signaling messages, and this state needs to be protected
      against corruption.

   Prevention of Denial of Service Attacks:  GIST nodes and the network
      have finite resources (state storage, processing power,
      bandwidth).  The protocol tries to minimise exhaustion attacks
      against these resources and not allow GIST nodes to be used to
      launch attacks on other network elements.

   The main additional issue is handling authorisation for executing
   signaling operations (e.g. allocating resources).  This is assumed to
   be done in each signaling application.

   In many cases, GIST relies on the security mechanisms available in
   messaging associations to handle these issues, rather than
   introducing new security measures.  Obviously, this requires the
   interaction of these mechanisms with the rest of the GIST protocol to
   be understood and verified, and some aspects of this are discussed in
   Section 5.7.

8.1.  Message Confidentiality and Integrity

   GIST can use messaging association functionality, specifically in
   this version TLS (Section 5.7.3), to ensure message confidentiality

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   and integrity.  Implementation of this functionality is REQUIRED but
   its use for any given flow or signaling application is OPTIONAL.  In
   some cases, confidentiality of GIST information itself is not likely
   to be a prime concern, in particular since messages are often sent to
   parties which are unknown ahead of time, although the content visible
   even at the GIST level gives significant opportunities for traffic
   analysis.  Signaling applications may have their own mechanism for
   securing content as necessary; however, they may find it convenient
   to rely on protection provided by messaging associations, since it
   runs unbroken between signaling application peers.

8.2.  Peer Node Authentication

   Cryptographic protection (of confidentiality or integrity) requires a
   security association with session keys.  These can be established by
   an authentication and key exchange protocol based on shared secrets,
   public key techniques or a combination of both.  Authentication and
   key agreement is possible using the protocols associated with the
   messaging association being secured.  TLS incorporates this
   functionality directly; IKE, IKEv2 or KINK could provide it for
   IPsec.  GIST nodes rely on these protocols to authenticate the
   identity of the next hop, and GIST has no authentication capability
   of its own.

   With routing state discovery, there are few effective ways to know
   what is the legitimate next or previous hop as opposed to an
   impostor.  In other words, cryptographic authentication here only
   provides assurance that a node is 'who' it is (i.e. the legitimate
   owner of identity in some namespace), not 'what' it is (i.e. a node
   which is genuinely on the flow path and therefore can carry out
   signaling for a particular flow).  Authentication provides only
   limited protection, in that a known peer is unlikely to lie about its
   role.  Additional methods of protection against this type of attack
   are considered in Section 8.3 below.

   It is an implementation issue whether peer node authentication should
   be made signaling application dependent; for example, whether
   successful authentication could be made dependent on presenting
   credentials related to a particular signaling role (e.g. signaling
   for QoS).  The abstract API of Appendix B leaves open such policy and
   authentication interactions between GIST and the NSLP it is serving.
   However, it does allow applications to inspect the authenticated
   identity of the peer to which a message will be sent before

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8.3.  Routing State Integrity

   Internal state in a node (see Section 4.2) is used to route messages.
   If this state is corrupted, signaling messages may be misdirected.

   In the case where the MRM is path-coupled, the messages need to be
   routed identically to the data flow described by the MRI, and the
   routing state table is the GIST view of how these flows are being
   routed through the network in the immediate neighbourhood of the
   node.  Routes are only weakly secured (e.g. there is no cryptographic
   binding of a flow to a route), and there is no authoritative
   information about flow routes other than the current state of the
   network itself.  Therefore, consistency between GIST and network
   routing state has to be ensured by directly interacting with the
   routing mechanisms to ensure that the signaling peers are the
   appropriate ones for any given flow.  An overview of security issues
   and techniques in this context is provided in [38].

   In one direction, peer identification is installed and refreshed only
   on receiving a Response (compare Figure 4).  This MUST echo the
   cookie from a previous Query, which will have been sent along the
   flow path with the Q-mode encapsulation, i.e. end-to-end addressed.
   Hence, only the true next peer or an on-path attacker will be able to
   generate such a message, provided freshness of the cookie can be
   checked at the querying node.

   In the other direction, peer identification MAY be installed directly
   on receiving a Query containing addressing information for the
   signaling source.  However, any node in the network could generate
   such a message; indeed, many nodes in the network could be the
   genuine upstream peer for a given flow.  To protect against this,
   three strategies are used:

   Filtering:  the receiving node MAY reject signaling messages which
      claim to be for flows with flow source addresses which could be
      ruled out by ingress filtering.  An extension of this technique
      would be for the receiving node to monitor the data plane and to
      check explicitly that the flow packets are arriving over the same
      interface and if possible from the same link layer neighbour as
      the D-mode signaling packets.  If they are not, it is likely that
      at least one of the signaling or flow packets is being spoofed.

   Authentication (weak or strong):  the receiving node MAY refuse to
      install upstream state until it has completed a Confirm handshake
      with the peer.  This echoes the Response cookie of the Response,
      and discourages nodes from using forged source addresses.  This
      also plays a role in denial of service prevention, see below.  A
      stronger approach is to require full peer authentication within

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      the messaging association, the reasoning being that an
      authenticated peer can be trusted not to pretend that it is on
      path when it is not.

   SID segregation:  The routing state lookup for a given MRI and NSLPID
      MUST also take the SID into account.  A malicious node can only
      overwrite existing routing state if it can guess the corresponding
      SID; it can insert state with random SID values, but generally
      this will not be used to route messages for which state has
      already been legitimately established.

8.4.  Denial of Service Prevention

   GIST is designed so that in general each Query only generates at most
   one Response which is at most only slightly larger than the Query, so
   that a GIST node cannot become the source of a denial of service
   amplification attack.  (There is a special case of retransmitted
   Response messages, see Section 5.3.3.)

   However, GIST can still be subjected to denial-of-service attacks
   where an attacker using forged source addresses forces a node to
   establish state without return routability, causing a problem similar
   to TCP SYN flood attacks.  Furthermore, an adversary might use
   modified or replayed unprotected signaling messages as part of such
   an attack.  There are two types of state attacks and one
   computational resource attack.  In the first state attack, an
   attacker floods a node with messages that the node has to store until
   it can determine the next hop.  If the destination address is chosen
   so that there is no GIST-capable next hop, the node would accumulate
   messages for several seconds until the discovery retransmission
   attempt times out.  The second type of state-based attack causes GIST
   state to be established by bogus messages.  A related computational/
   network-resource attack uses unverified messages to cause a node
   query an authentication or authorisation infrastructure, or attempt
   to cryptographically verify a digital signature.

   We use a combination of two defences against these attacks:

   1.  The responding node need not establish a session or discover its
       next hop on receiving the Query, but MAY wait for a Confirm,
       possibly on a secure channel.  If the channel exists, the
       additional delay is one one-way delay and the total is no more
       than the minimal theoretically possible delay of a three-way
       handshake, i.e., 1.5 node-to-node round-trip times.  The delay
       gets significantly larger if a new connection needs to be
       established first.

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   2.  The Response to the Query contains a cookie, which is repeated in
       the Confirm.  State is only established for messages that contain
       a valid cookie.  The setup delay is also 1.5 round-trip times.
       This mechanism is similar to that in SCTP [16] and other modern

   Once a node has decided to establish routing state, there may still
   be transport and security state to be established between peers.
   This state setup is also vulnerable to denial of service attacks.
   GIST relies on the lower layer protocols that make up messaging
   associations to mitigate such attacks.  In the current specification,
   the querying node is always the one wishing to establish a messaging
   association, so it is the responding node that needs to be protected.

   Signaling applications can use the services provided by GIST to
   defend against certain (e.g. flooding) denial of service attacks.  In
   particular, they can elect to process only messages from peers that
   have passed a return routability check or been authenticated at the
   messaging association level (see Appendix B.2).  Signaling
   applications that accept messages under other circumstances (in
   particular, before routing state has been fully established at the
   GIST level) need to take this into account when designing their
   denial of service prevention mechanisms, for example by not creating
   local state as a result of processing such messages.

8.5.  Requirements on Cookie Mechanisms

   The requirements on the Query cookie can be summarised as follows:

   Liveness:  The cookie must be live, that is, it must change from one
      handshake to the next.  To prevent replay attacks.

   Unpredictability:  The cookie must not be guessable e.g. from a
      sequence or timestamp.  To prevent direct forgery based on seeing
      a history of captured messages.

   Easily validated:  It must be efficient for the Q-Node to validate
      that a particular cookie matches an in-progress handshake, for a
      routing state machine which already exists.  To discard responses
      which have been randomly generated by an adversary, or to discard
      responses to queries which were generated with forged source
      addresses or an incorrect address in the included NLI object.

   Uniqueness:  The cookie must be unique to a given handshake since it
      is actually used to match the Response to a handshake anyway, e.g.
      during messaging association re-use.

   Likewise, the requirements on the Responder cookie can be summarised

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   as follows:

   Liveness:  The cookie must be live as above.  To prevent replay

   Creation simplicity:  The cookie must be lightweight to generate.  To
      avoid resource exhaustion at the responding node.

   Validation simplicity:  It must be simple for the R-node to validate
      that an R-cookie was generated by itself and no-one else, without
      storing state about the handshake it was generated for.

   Binding:  The cookie must be bound to the routing state that will be
      installed.  To prevent use with different routing state e.g. in a
      modified Confirm.  The routing state here includes the NLI of the
      Query, the MRI/NSLPID for the messaging, and the interface on
      which the Query was received.

   A suitable implementation for the Q-Cookie is a cryptographically
   strong random number which is unique for this routing state machine
   handshake.  A node MUST implement this or an equivalently strong
   mechanism.  Guidance on random number generation can be found in

   A suitable implementation for the R-Cookie is as follows:

       R-Cookie = liveness data + hash (locally known secret,
                                        Q-Node NLI, MRI, NSLPID,
                                        reception interface,
                                        liveness data)

   A node MUST implement this or an equivalently strong mechanism.
   There are several alternatives for the liveness data.  One is to use
   a timestamp like SCTP.  Another is to give the local secret a (rapid)
   rollover, with the liveness data as the generation number of the
   secret, like IKEv2.  In both cases, the liveness data has to be
   carried outside the hash, to allow the hash to be verified at the
   Responder.  Another approach is to replace the hash with encryption
   under a locally known secret, in which case the liveness data does
   not need to be carried in the clear.  Any symmetric cipher immune to
   known plaintext attacks can be used.

   To support the validation simplicity requirement, the Responder can
   check the liveness data to filter out some blind (flooding) attacks
   before beginning any cryptographic cookie verification.  To support
   this usage, the liveness data must be carried in the clear and not be
   easily guessable; this rules out the timestamp approach, and suggests
   the use of sequence of secrets with the liveness data identifying the

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   position in the sequence.  The secret strength and rollover frequency
   must be high enough that the secret cannot be brute-forced during its
   lifetime.  Note that any node can use a Query to discover the current
   liveness data, so it remains hard to defend against sophisticated
   attacks which disguise such probes within a flood of Queries from
   forged source addresses.  Therefore, it remains important to use an
   efficient hashing mechanism or equivalent.

   If a node receives a message for which cookie validation fails, it
   MAY return an "Object Value Error" error message (Appendix A.4.4.10)
   with subcode 4 ("Invalid Cookie") to the sender, as well as dropping
   the message.  However, doing so in general makes a node a source of
   backscatter.  Therefore, this MUST only be enabled selectively, e.g.
   during initial deployment or debugging.

8.6.  Security Protocol Selection Policy

   This specification defines a single mandatory-to-implement security
   protocol (TLS, Section 5.7.3).  However, it is possible to define
   additional security protocols in the future, for example to allow re-
   use with other types of credentials, or migrate towards protocols
   with stronger security properties.  In addition, use of any security
   protocol for a messaging association is optional.  Security protocol
   selection is carried out as part of the GIST handshake mechanism
   (Section 4.4.1).

   The selection process may be vulnerable to downgrade attacks, where a
   man in the middle modifies the capabilities offered in the Query or
   Response to mislead the peers into accepting a lower level of
   protection than is achievable.  There is a two part defence against
   such attacks (the following is based the same concepts as [22]):

   1.  The Response does not depend on the Stack-Proposal in the Query
       (see Section 5.7.1).  Therefore, tampering with the Query has no
       effect on the resulting messaging association configuration.

   2.  The Responding node's Stack-Proposal is echoed in the Confirm.
       The Responding node checks this to validate that the proposal it
       made in the Response is the same as the one received by the
       Querying node.  Note that as a consequence of the previous point,
       the Responding node does not have to remember the proposal
       explicitly, since it is a static function of local policy.

   The validity of the second part depends on the strength of the
   security protection provided for the Confirm.  If the Querying node
   is prepared to create messaging associations with null security
   properties (e.g.  TCP only), the defence is ineffective, since the
   man in the middle can re-insert the original Responder's Stack-

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   Proposal, and the Responding node will assume that the minimal
   protection is a consequence of Querying node limitations.  However,
   if the messaging association provides at least integrity protection
   that cannot be broken in real-time, the Confirm cannot be modified in
   this way.  Therefore, if the Querying node does not apply a security
   policy to the messaging association protocols to be created that
   ensures at least this minimal level of protection is met, it remains
   open to the threat that a downgrade has occurred.  Applying such a
   policy ensures capability discovery process will result in the setup
   of a messaging association with the correct security properties as
   appropriate for the two peers involved.

8.7.  Residual Threats

   Taking the above security mechanisms into account, the main residual
   threats against NSIS are three types of on-path attack.

   An on-path attacker who can intercept the initial Query can do most
   things it wants to the subsequent signaling.  It is very hard to
   protect against this at the GIST level; the only defence is to use
   strong messaging association security to see whether the Responding
   node is authorised to take part in NSLP signaling exchanges.  To some
   extent, this behaviour is logically indistinguishable from correct
   operation, so it is easy to see why defence is difficult.  Note that
   an on-path attacker of this sort can do anything to the traffic as
   well as the signaling.  Therefore, the additional threat induced by
   the signaling weakness seems tolerable.

   At the NSLP level, there is a concern about transitivity of trust of
   correctness of routing along the signaling chain.  The NSLP at the
   querying node can have good assurance that it is communicating with
   an on-path peer or a node delegated by the on-path node.  However, it
   has no assurance that the node beyond the responder is also on-path,
   or that the MRI (in particular) is not being modified by the
   responder to refer to a different flow.  Therefore, if it sends
   signaling messages with payloads (e.g. authorisation tokens) which
   are valuable to nodes beyond the adjacent hop, it is up to the NSLP
   to ensure that the appropriate chain of trust exists, which must in
   general use strong messaging association security.

   There is a further residual attack by a node which is not on the path
   of the Query, but is on the path of the Response, or is able to use a
   Response from one handshake to interfere with another.  The attacker
   modifies the Response to cause the Querying node to form an adjacency
   with it rather than the true peer.  In principle, this attack could
   be prevented by including an additional cryptographic object in the
   Response which ties the Response to the initial Query and the routing
   state and can be verified by the Querying node.

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9.  IANA Considerations

   This section defines the registries and initial codepoint assignments
   for GIST.  It also defines the procedural requirements to be followed
   by IANA in allocating new codepoints.  Note that the guidelines on
   the technical criteria to be followed in evaluating requests for new
   codepoint assignments are covered normatively in a separate document
   which considers the NSIS protocol suite in a unified way.  That
   document discusses the general issue of NSIS extensibility, as well
   as the technical criteria for particular registries; see [11] for
   further details.

   The registry definitions that follow leave large blocks of codes
   reserved as unused.  This is to allow a future revision of this
   specification to modify the allocation policies without having to
   retrospectively change the initial rules if they turn out to have
   been suboptimal, e.g. if the space for one particular policy is
   exhausted too quickly.

   The allocation policies used in this section follow the guidance
   given in [3].  In addition, for a number of the GIST registries, this
   specification also defines private/experimental ranges as discussed
   in [8].  Note that the only environment in which these codepoints can
   validly be used is a closed one in which the experimenter knows all
   the experiments in progress.

   This specification allocates the following codepoints in existing

      Well-known UDP port XXX as the destination port for Q-mode
      encapsulated GIST messages (Section 5.3).

   This specification creates the following registries with the
   structures as defined below:

   NSLP Identifiers:  Each signaling application requires the assignment
      of one of more NSLPIDs.  The following NSLPID is allocated by this

   | NSLPID  | Application                                             |
   | 0       | Used for GIST messages not related to any signaling     |
   |         | application.                                            |

      Every other NSLPID MUST be associated with a specific RAO value;
      multiple NSLPIDs MAY be associated with the same value.  The

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      NSLPID is a 16 bit integer, and allocation policies for further
      values are as follows:

      1-32703:  IESG Approval

      32704-32767:  Private/Experimental Use

      32768-65536:  Reserved

   GIST Message Type:  The GIST common header (Appendix A.1) contains a
      1 byte message type field.  The following values are allocated by
      this specification:

                          | MType   | Message  |
                          | 0       | Query    |
                          |         |          |
                          | 1       | Response |
                          |         |          |
                          | 2       | Confirm  |
                          |         |          |
                          | 3       | Data     |
                          |         |          |
                          | 4       | Error    |
                          |         |          |
                          | 5       | MA-Hello |

      Allocation policies for further values are as follows:

      6-63:  Standards Action

      64-119:  Expert Review

      120-127:  Private/Experimental Use

      128-255:  Reserved

   Object Types:  There is a 12-bit field in the object header
      (Appendix A.2).  The following values for object type are defined
      by this specification:

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                 | OType   | Object Type                 |
                 | 0       | Message Routing Information |
                 |         |                             |
                 | 1       | Session ID                  |
                 |         |                             |
                 | 2       | Network Layer Information   |
                 |         |                             |
                 | 3       | Stack Proposal              |
                 |         |                             |
                 | 4       | Stack Configuration Data    |
                 |         |                             |
                 | 5       | Query Cookie                |
                 |         |                             |
                 | 6       | Responder Cookie            |
                 |         |                             |
                 | 7       | NAT Traversal               |
                 |         |                             |
                 | 8       | NSLP Data                   |
                 |         |                             |
                 | 9       | Error                       |

      Allocation policies for further values are as follows:

      10-1023:  Standards Action

      1024-1999:  Specification Required

      2000-2047:  Private/Experimental Use

      2048-4095:  Reserved

      When a new object type is defined, the object format MUST be
      provided, and the setting of the extensibility bits (A/B, see
      Appendix A.2.1) MUST also be defined.

   Message Routing Methods:  GIST allows multiple message routing
      methods (see Section 3.3).  The MRM is indicated in the leading
      byte of the MRI object (Appendix A.3.1).  This specification
      defines the following values:

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                  | MRM-ID     | Message Routing Method |
                  | 0          | Path Coupled MRM       |
                  |            |                        |
                  | 1          | Loose End MRM          |

      Allocation policies for further values are as follows:

      2-63:  Standards Action

      64-119:  Expert Review

      120-127:  Private/Experimental Use

      128-255:  Reserved

      When a new MRM is defined, the specification MUST provide the
      information described in Section 3.3.

   MA-Protocol-IDs:  Each upper layer protocol that can be used in a
      messaging association is identified by a 1-byte MA-Protocol-ID
      (Section 5.7).  This is used as a tag in the Stack-Proposal and
      Stack-Configuration-Data objects (Appendix A.3.4 and
      Appendix A.3.5).  The following values are defined by this

     | MA-Protocol-ID      | Higher Layer Protocol                   |
     | 0                   | Reserved - not to be allocated          |
     |                     |                                         |
     | 1                   | TCP opened in the forwards direction    |
     |                     |                                         |
     | 2                   | TLS initiated in the forwards direction |

      Allocation policies for further values are as follows:

      3-63:  Standards Action

      64-119:  Expert Review

      120-127:  Private/Experimental Use

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      128-255:  Reserved

      Allocation of a new MA-Protocol-ID MUST define the format for the
      MA-protocol-options field (if any) in the Stack-Configuration-Data
      object that is needed to define its configuration.  If a protocol
      is to be used for reliable message transfer, it MUST be described
      how delivery errors are to be detected by GIST.  Note that the MA-
      Protocol-ID is not an IP Protocol number; indeed, some of the
      messaging association protocols - such as TLS - do not have an IP
      Protocol number.

   Error Codes/Subcodes:  There is a 2 byte error code and 1 byte
      subcode in the Value field of the Error object (Appendix A.4.1).
      Error codes 1-12 are defined in Appendix A.4.4 together with
      subcodes 0-4 for code 1, 0-5 for code 9, 0-5 for code 10, and 0-2
      for code 12.  Additional codes and subcodes are allocated on a
      first-come, first served basis.  When a new error code/subcode
      combination is allocated, the Error Class and the format of any
      associated error-specific information MUST also be defined.

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10.  Acknowledgements

   This document is based on the discussions within the IETF NSIS
   working group.  It has been informed by prior work and formal and
   informal inputs from: Cedric Aoun, Attila Bader, Roland Bless, Bob
   Braden, Marcus Brunner, Benoit Campedel, Yoshiko Chong, Luis
   Cordeiro, Elwyn Davies, Christian Dickmann, Pasi Eronen, Alan Ford,
   Xiaoming Fu, Bo Gao, Ruediger Geib, Eleanor Hepworth, Thomas Herzog,
   Cheng Hong, Jia Jia, Cornelia Kappler, Georgios Karagiannis, Ruud
   Klaver, Chris Lang, John Loughney, Allison Mankin, Jukka Manner, Pete
   McCann, Andrew McDonald, Glenn Morrow, Dave Oran, Andreas Pashalidis,
   Henning Peters, Tom Phelan, Akbar Rahman, Takako Sanda, Charles Shen,
   Melinda Shore, Martin Stiemerling, Martijn Swanink, Mike Thomas,
   Hannes Tschofenig, Sven van den Bosch, Michael Welzl, Lars Westberg,
   and Mayi Zoumaro-djayoon.  Parts of the TLS usage description
   (Section 5.7.3) were derived from the Diameter base protocol
   specification, RFC3588.  In addition, Hannes Tschofenig provided a
   detailed set of review comments on the security section, and Andrew
   McDonald provided the formal description for the initial packet
   formats.  Chris Lang's implementation work provided objective
   feedback on the clarity and feasibility of the specification, and he
   also provided the state machine description and the initial error
   catalogue and formats.  Finally, Magnus Westerlund carried out a
   detailed AD review which identified a number of issues and led to
   significant clarifications.

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11.  References

11.1.  Normative References

   [1]   Katz, D., "IP Router Alert Option", RFC 2113, February 1997.

   [2]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

   [3]   Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", BCP 26, RFC 2434,
         October 1998.

   [4]   Nichols, K., Blake, S., Baker, F., and D. Black, "Definition of
         the Differentiated Services Field (DS Field) in the IPv4 and
         IPv6 Headers", RFC 2474, December 1998.

   [5]   Partridge, C. and A. Jackson, "IPv6 Router Alert Option",
         RFC 2711, October 1999.

   [6]   Nordmark, E., "Stateless IP/ICMP Translation Algorithm (SIIT)",
         RFC 2765, February 2000.

   [7]   Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
         RFC 2246, January 1999.

   [8]   Narten, T., "Assigning Experimental and Testing Numbers
         Considered Useful", BCP 82, RFC 3692, January 2004.

   [9]   Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
         Specifications: ABNF", RFC 4234, October 2005.

   [10]  Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)
         Protocol Version 1.1", RFC 4346, April 2006.

   [11]  Loughney, J., "NSIS Extensibility Model",
         draft-loughney-nsis-ext-02 (work in progress), March 2006.

11.2.  Informative References

   [12]  Braden, B., Zhang, L., Berson, S., Herzog, S., and S. Jamin,
         "Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
         Specification", RFC 2205, September 1997.

   [13]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

   [14]  Terzis, A., Krawczyk, J., Wroclawski, J., and L. Zhang, "RSVP
         Operation Over IP Tunnels", RFC 2746, January 2000.

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   [15]  Tsirtsis, G. and P. Srisuresh, "Network Address Translation -
         Protocol Translation (NAT-PT)", RFC 2766, February 2000.

   [16]  Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer,
         H., Taylor, T., Rytina, I., Kalla, M., Zhang, L., and V.
         Paxson, "Stream Control Transmission Protocol", RFC 2960,
         October 2000.

   [17]  Carpenter, B. and K. Moore, "Connection of IPv6 Domains via
         IPv4 Clouds", RFC 3056, February 2001.

   [18]  Huitema, C., "An Anycast Prefix for 6to4 Relay Routers",
         RFC 3068, June 2001.

   [19]  Baker, F., Iturralde, C., Le Faucheur, F., and B. Davie,
         "Aggregation of RSVP for IPv4 and IPv6 Reservations", RFC 3175,
         September 2001.

   [20]  Grossman, D., "New Terminology and Clarifications for
         Diffserv", RFC 3260, April 2002.

   [21]  Price, R., Bormann, C., Christoffersson, J., Hannu, H., Liu,
         Z., and J. Rosenberg, "Signaling Compression (SigComp)",
         RFC 3320, January 2003.

   [22]  Arkko, J., Torvinen, V., Camarillo, G., Niemi, A., and T.
         Haukka, "Security Mechanism Agreement for the Session
         Initiation Protocol (SIP)", RFC 3329, January 2003.

   [23]  Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, "STUN
         - Simple Traversal of User Datagram Protocol (UDP) Through
         Network Address Translators (NATs)", RFC 3489, March 2003.

   [24]  Rosenberg, J., "Obtaining Relay Addresses from Simple Traversal
         of UDP Through NAT (STUN)", draft-ietf-behave-turn-01 (work in
         progress), June 2006.

   [25]  Gill, V., Heasley, J., and D. Meyer, "The Generalized TTL
         Security Mechanism (GTSM)", RFC 3682, February 2004.

   [26]  Hancock, R., Karagiannis, G., Loughney, J., and S. Van den
         Bosch, "Next Steps in Signaling (NSIS): Framework", RFC 4080,
         June 2005.

   [27]  Tschofenig, H. and D. Kroeselberg, "Security Threats for Next
         Steps in Signaling (NSIS)", RFC 4081, June 2005.

   [28]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness

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         Requirements for Security", BCP 106, RFC 4086, June 2005.

   [29]  Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites for
         Transport Layer Security (TLS)", RFC 4279, December 2005.

   [30]  Kohler, E., Handley, M., and S. Floyd, "Datagram Congestion
         Control Protocol (DCCP)", RFC 4340, March 2006.

   [31]  Conta, A., Deering, S., and M. Gupta, "Internet Control Message
         Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6)
         Specification", RFC 4443, March 2006.

   [32]  Stiemerling, M., "NAT/Firewall NSIS Signaling Layer Protocol
         (NSLP)", draft-ietf-nsis-nslp-natfw-12 (work in progress),
         June 2006.

   [33]  Manner, J., "NSLP for Quality-of-Service Signaling",
         draft-ietf-nsis-qos-nslp-11 (work in progress), June 2006.

   [34]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms for
         IPv6 Hosts and Routers", RFC 4213, October 2005.

   [35]  Kent, S. and K. Seo, "Security Architecture for the Internet
         Protocol", RFC 4301, December 2005.

   [36]  Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Protocol
         Architecture", RFC 4251, January 2006.

   [37]  Moskowitz, R., "Host Identity Protocol", draft-ietf-hip-base-06
         (work in progress), June 2006.

   [38]  Nikander, P., Arkko, J., Aura, T., Montenegro, G., and E.
         Nordmark, "Mobile IP Version 6 Route Optimization Security
         Design Background", RFC 4225, December 2005.

   [39]  Floyd, S. and V. Jacobson, "The Synchronisation of Periodic
         Routing Messages", SIGCOMM Symposium on Communications
         Architectures and Protocols pp. 33--44, September 1993.

   [40]  Pashalidis, A. and H. Tschofenig, "GIST NAT Traversal",
         draft-pashalidis-nsis-gimps-nattraversal-03 (work in progress),
         June 2006.

   [41]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
         Security", RFC 4347, April 2006.

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Appendix A.  Bit-Level Formats and Error Messages

   This appendix provides formats for the various component parts of the
   GIST messages defined abstractly in Section 5.2.

   Each GIST message consists of a header and a sequence of objects.
   The GIST header has a specific format, described in more detail in
   Appendix A.1 below.  An NSLP message is one object within a GIST
   message.  Note that GIST itself provides the NSLP message length
   information and signaling application identification.  General object
   formatting guidelines are provided in Appendix A.2 below, followed in
   Appendix A.3 by the format for each object.  Finally, Appendix A.4
   provides the formats used for error reporting.

   In the following object diagrams, '//' is used to indicate a variable
   sized field and ':' is used to indicate a field that is optionally

A.1.  The GIST Common Header

   This header begins all GIST messages.  It has a fixed format, as
   shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |    Version    |   GIST hops   |        Message Length         |
   |           NSLPID              |   Type        |S|R|E| Reserved|

   Version (8 bits):  The GIST protocol version number.

   GIST hops (8 bits):  A hop count for the number of GIST-aware nodes
      this message can still pass through.

   Message Length (16 bits):  The total number of 32-bit words in the
      message after the common header itself.

   NSLPID (16 bits):  IANA assigned identifier of the signaling
      application the message refers to.

   Type (8 bits):  The GIST message type (Query, Response, etc.).

   S flag:  S=1 if the IP source address is the same as the signaling
      source address, S=0 if it is different.

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   R flag:  R=1 if a reply to this message is explicitly requested.

   E flag:  E=1 if the message was explicitly routed (Section 7.1.4).

   The rules governing the use of the R-flag depend on the GIST message
   type.  It MUST always be set (R=1) in Query messages, since these
   always elicit a Response, and never in Confirm, Data or Error
   messages.  It is optional in an MA-Hello; if set, another MA-Hello is
   sent in reply.  It is optional in a Response, but MUST be set if the
   Response contains a Responder cookie; if set, a Confirm is sent in
   reply.  The E flag MUST NOT be set unless the message type is a Data

   Parsing failures may be caused by unknown Version or Type values,
   inconsistent R or E flag setting, or a Message Length inconsistent
   with the set of objects carried.  In all cases the receiver MUST if
   possible return a "Common Header Parse Error" message
   (Appendix A.4.4.1) with the appropriate subcode, and not process the
   message further.

A.2.  General Object Format

   Each object begins with a fixed header giving the object Type and
   object Length.  This is followed by the object Value, which is a
   whole number of 32-bit words long.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |A|B|r|r|         Type          |r|r|r|r|        Length         |
   //                             Value                           //

   A/B flags:  The bits marked 'A' and 'B' are extensibility flags which
      are defined in Appendix A.2.1 below; the remaining bits marked 'r'
      are reserved.

   Type (12 bits):  An IANA-assigned identifier for the type of object.

   Length (12 bits):  Length has the units of 32-bit words, and measures
      the length of Value.  If there is no Value, Length=0.  If the
      Length is not consistent with the contents of the object, an
      "Object Value Error" message (Appendix A.4.4.10) with subcode 0
      "Incorrect Length" MUST be returned and the message dropped.

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   Value (variable):  Value is (therefore) a whole number of 32 bit
      words.  If there is any padding required, the length and location
      must be defined by the object-specific format information; objects
      which contain variable length (e.g. string) types may need to
      include additional length subfields to do so.

   Any part of the object used for padding or defined as reserved
   (marked 'Reserved' or 'Rsv' or, in the case of individual bits, 'r'
   in the diagrams below) MUST be set to 0 on transmission and MUST be
   ignored on reception.

A.2.1.  Object Extensibility

   The leading two bits of the TLV header are used to signal the desired
   treatment for objects whose Type field is unknown at the receiver.
   The following three categories of object have been identified, and
   are described here.

   AB=00 ("Mandatory"):  If the object is not understood, the entire
      message containing it MUST be rejected with an "Object Type Error"
      message (Appendix A.4.4.9) with subcode 1 ("Unrecognised Object").

   AB=01 ("Ignore"):  If the object is not understood, it MUST be
      deleted and the rest of the message processed as usual.

   AB=10 ("Forward"):  If the object is not understood, it MUST be
      retained unchanged in any message forwarded as a result of message
      processing, but not stored locally.

   The combination AB=11 is reserved.  If a message is received
   containing and object with AB=11, it MUST be rejected with an "Object
   Type Error" message (Appendix A.4.4.9) with subcode 5 ("Invalid
   Extensibility Flags").

A.3.  GIST TLV Objects

A.3.1.  Message-Routing-Information

   Type:  Message-Routing-Information

   Length:  Variable (depends on MRM)

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |     MRM-ID    |N|  Reserved   |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   //     Method-specific addressing information (variable)       //

   MRM-ID (8 bits):  An IANA-assigned identifier for the message routing

   N flag:  If set (N=1), this means that NATs do not need to translate
      this MRM; if clear (N=0) it means that the method-specific
      information contains network or transport layer information that a
      NAT must process.

   The remainder of the object contains method-specific addressing
   information, which is described below.

A.3.1.1.  Path-Coupled MRM

   In the case of basic path-coupled routing, the addressing information
   takes the following format.  The N-flag N=0 for this MRM.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                   |IP-Ver |P|T|F|S|A|B|D|Reserved |
   //                       Source Address                        //
   //                      Destination Address                    //
   | Source Prefix |  Dest Prefix  |   Protocol    | DS-field  |Rsv|
   :       Reserved        |              Flow Label               :
   :                              SPI                              :
   :          Source Port          :       Destination Port        :

   IP-Ver (4 bits):  The IP version number, 4 or 6.

   Source/Destination address (variable):  The source and destination
      addresses are always present and of the same type; their length
      depends on the value in the IP-Ver field.

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   Source/Dest Prefix (each 8 bits):  The length of the mask to be
      applied to the source and destination addresses for address
      wildcarding.  In the normal case where the MRI refers only to
      traffic between specific host addresses, the Source/Dest Prefix
      values would both be 32/128 for IPv4/6 respectively.

   P flag:  P=1 means that IP Protocol should be interpreted.

   Protocol (8 bits):  The IP protocol number.  Ignored if P=0.  In the
      case of IPv6, the Protocol field refers to the true upper layer
      protocol carried by the packets, i.e. excluding any IP option
      headers.  This is therefore not necessarily the same as the Next
      Header value from the base IPv6 header.

   T flag:  T=1 means that DiffServ field (DS-field) should be

   DS-field (6 bits):  The DiffServ field.  See [4] and [20].

   F flag:  F=1 means that flow label is present and should be

   Flow Label (20 bits):  The flow label; only present if F=1.  If F=0,
      the entire 32 bit word containing the Flow Label is absent.  F may
      only be set if IP-Ver is 6.

   S flag:  S=1 means that the SPI field is present.  Can only be set if

   SPI field (32 bits):  The SPI field; see [35].  Only present if S=1.

   A/B flags:  These can only be set if P=1.  If either is set, the port
      fields are also present.

   Source/Destination Port (each 16 bits):  If either of A, B is set the
      word containing the port numbers is included in the object.
      However, the contents of each field is only significant if the
      corresponding flag is set; otherwise, the contents of the field is
      regarded as padding, and the MRI refers to all ports (i.e. acts as
      a wildcard).  If the flag is set and Port=0x0000, the MRI will
      apply to a specific port, whose value is not yet known.  If
      neither of A or B is set, the word is absent.

   D flag:  The Direction flag has the following meaning: the value 0
      means 'in the same direction as the flow' (i.e. downstream), and
      the value 1 means 'in the opposite direction to the flow' (i.e.

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A.3.1.2.  Loose-End MRM

   In the case of the loose-end MRM, the addressing information takes
   the following format.  The N-flag N=0 for this MRM.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                   |IP-Ver |D|      Reserved       |
   //                       Source Address                        //
   //                      Destination Address                    //

   IP-Ver (4 bits):  The IP version number, 4 or 6.

   Source/Destination address (variable):  The source and destination
      addresses are always present and of the same type; their length
      depends on the value in the IP-Ver field.

   D flag:  The direction flag.  Note that for Q-mode messages, the only
      valid value is D=0 (see Section 5.8.2).

A.3.2.  Session Identification

   Type:  Session-Identification

   Length:  Fixed (4 32-bit words)

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |                                                               |
   +                                                               +
   |                                                               |
   +                          Session ID                           +
   |                                                               |
   +                                                               +
   |                                                               |

A.3.3.  Network-Layer-Information

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   Type:  Network-Layer-Information

   Length:  Variable (depends on length of Peer-Identity and IP version)

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |   PI-Length   |    IP-TTL     |IP-Ver |        Reserved       |
   |                  Routing State Validity Time                  |
   //                       Peer Identity                         //
   //                     Interface Address                       //

   PI-Length (8 bits):  The byte length of the Peer Identity field.

   Peer Identity (variable):  The Peer Identity field.  Note that the
      Peer-Identity field itself is padded to a whole number of words.

   IP-TTL (8 bits):  Initial or reported IP layer TTL.

   IP-Ver (4 bits):  The IP version for the Interface Address field.

   Interface Address (variable):  The IP address allocated to the
      interface, matching the IP-Ver field.

   Routing State Validity Time (32 bits):  The time for which the
      routing state for this flow can be considered correct without a
      refresh.  Given in milliseconds.

A.3.4.  Stack Proposal

   Type:  Stack-Proposal

   Length:  Variable (depends on number of profiles and size of each

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |  Prof-Count   |     Reserved                                  |
   //                    Profile 1                                //
   :                                                               :
   //                    Profile 2                                //

   Prof-Count (8 bits): The number of profiles listed. MUST be > 0.

   Each profile is itself a sequence of protocol layers, and the profile
   is formatted as a list as follows:

   o  The first byte is a count of the number of layers in the profile.

   o  This is followed by a sequence of 1-byte MA-Protocol-IDs as
      described in Section 5.7.

   o  The profile is padded to a word boundary with 0, 1, 2 or 3 zero

   If there are no profiles (i.e. all bytes are null), then an "Object
   Value Error" message (Appendix A.4.4.10) with subcode 3 ("Empty
   List") MUST be returned and the message dropped.

A.3.5.  Stack-Configuration-Data

   Type:  Stack-Configuration-Data

   Length:  Variable (depends on number of protocols and size of each
      MA-protocol-options field)

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |   MPO-Count   |     Reserved                                  |
   |                           MA-Hold-Time                        |
   //                     MA-protocol-options 1                   //
   :                                                               :
   //                     MA-protocol-options N                   //

   MPO-Count (8 bits):  The number of MA-protocol-options fields present
      (these contain their own length information).

   MA-Hold-Time (32 bits):  The time for which the messaging association
      will be held open without traffic or a hello message.  Given in

   The MA-protocol-options fields are formatted as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |MA-Protocol-ID |     Profile   |    Length     |D|  Reserved   |
   //                         Options Data                        //

   MA-Protocol-ID (8 bits):  Protocol identifier as described in
      Section 5.7.

   Profile (8 bits):  Tag indicating which profile from the accompanying
      Stack-Proposal object this applies to.  Profiles are numbered from
      1 upwards; the special value 0 indicates 'applies to all

   Length (8 bits):  The byte length of MA-protocol-options field that
      follows.  This will be zero-padded up to the next word boundary.

   D flag:  If set (D=1), this protocol MUST NOT be used for a messaging

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   Options Data (variable):  Any options data for this protocol.  Note
      that the format of the options data may differ depending on
      whether the field is in a Query or Response.

A.3.6.  Query Cookie

   Type:  Query-Cookie

   Length:  Variable (selected by querying node)

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   //                        Query Cookie                         //

   The contents are implementation defined.  See Section 8.5 for further

A.3.7.  Responder Cookie

   Type:  Responder-Cookie

   Length:  Variable (selected by responding node)

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   //                      Responder Cookie                       //

   The contents are implementation defined.  See Section 8.5 for further

A.3.8.  NAT Traversal

   Type:  NAT-Traversal

   Length:  Variable (depends on length of contained fields)

   This object is used to support the NAT traversal mechanisms described
   in Section 7.2.

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   | MRI-Length    | Type-Count    | NAT-Count    |  Reserved      |
   //            Original Message-Routing-Information             //
   //                 List of translated objects                  //
   | Length of opaque information  |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                              //
   //                Information replaced by NAT #1                |
   :                                                               :
   | Length of opaque information  |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                              //
   //                Information replaced by NAT #N                |

   MRI-Length (8 bits):  The length of the included MRI payload in 32-
      bit words.

   Original Message-Routing-Information (variable):  The MRI data from
      when the message was first sent, not including the object header.

   Type-Count (8 bits):  The number of objects in the 'List of
      translated objects' field.

   List of translated objects (variable):  This field lists the object
      types of the objects that were translated by every NAT through
      which the message has passed.  It is initialised by the first NAT
      on the path; subsequent NATs may delete elements in the list.
      Padded with 2 null bytes if necessary.

   NAT-Count (8 bits):  The number of NATs traversed by the message, and
      the number of opaque payloads at the end of the object.  The
      length fields for each opaque payload are byte counts, not
      including the 2 bytes of the length field itself.  Note that each
      opaque information field is zero-padded to the next 32-bit word
      boundary if necessary.

A.3.9.  NSLP Data

   Type:  NSLP-Data

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   Length:  Variable (depends on NSLP)

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   //                          NSLP Data                          //

A.4.  Errors

A.4.1.  Error Object

   Type:  Error

   Length:  Variable (depends on error)

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |  Error Class  |           Error Code          | Error Subcode |
   |S|M|C|D|Q|       Reserved      |  MRI Length   |  Info Count   |
   |                                                               |
   +                         Common Header                         +
   |                    (of original message)                      |
   :                          Session Id                           :
   :                    Message Routing Information                :
   :                    Additional Information                     :
   :                       Debugging Comment                       :

   The flags are:
   S - S=1 means the Session ID object is present
   M - M=1 means MRI object is present
   C - C=1 means a debug Comment is present after header.
   D - D=1 means the original message was received in D-mode
   Q - Q=1 means the original message was received Q-mode encapsulated
       (can't be set if D=0).

   A GIST Error object contains an 8 bit error-class (see

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   Appendix A.4.3), a 16 bit error-code, an 8 bit error-subcode, and as
   much information about the message which triggered the error as is
   available.  This information MUST include the Common header of the
   original message and MUST also include the Session Id and MRI objects
   if these could be decoded correctly.  These objects are included in
   their entirety, except for their TLV Headers.  The MRI Length field
   gives the length of the MRI object in 32-bit words.

   The Info Count field contains the number of Additional Information
   fields in the object, and the possible formats for these fields are
   given in Appendix A.4.2.  The precise set of fields to include
   depends on the error code/subcode.  For every error description in
   the error catalogue Appendix A.4.4, the line "Additional Info:"
   states what fields MUST be included, and in what order if there can
   be more than one; if this line is given as 'None' then additional
   information MUST NOT be included.  The Debugging Comment is a null-
   terminated UTF-8 string, padded if necessary to a whole number of 32-
   bit words with more null characters.

A.4.2.  Additional Information Fields

   The Common Error Header may be followed by some Additional
   Information objects.  The possible formats of these objects are shown

   Message Length Info:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |     Calculated Length         |           Reserved            |
   Calculated Length (16 bits): the length of the original message
   calculated by adding up all the objects in the message. Measured in
   32-bit words.

   MTU Info:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |           Link MTU            |           Reserved            |
   Link MTU (16 bits): the MTU for a link along which a message could
   not be sent.

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   Object Type Info:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |         Object Type           |           Reserved            |
   Object type (16 bits): This provides information about the type
   of object which caused the error.

   Object Value Info:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |  Rsv  |  Real Object Length   |            Offset             |
   //                           Object                            //
   This object carries information about a TLV object which was found
   to be invalid in the original message. An error message may contain
   more than one Object Value Info object.

      Real Object Length (12 bits)  Since the length in the original TLV
         header may be inaccurate, this field provides the actual length
         of the object (including the TLV Header) included in the error
         message.  Measured in 32-bit words.

      Offset (16 bits):  The byte in the object at which the GIST node
         found the error.  The first byte in the object has offset=0.

      Object (variable):  The invalid TLV object (including the TLV

A.4.3.  Error Classes

   The first byte of the error object, "Error Class", indicates the
   severity level.  The currently defined severity levels are:

   0 (Informational):  response data which should not be thought of as
      changing the condition of the protocol state machine.

   1 (Success):  response data which indicates that the message being
      responded to has been processed successfully in some sense.

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   2 (Protocol-Error):  the message has been rejected because of a
      protocol error (e.g. an error in message format).

   3 (Transient-Failure):  the message has been rejected because of a
      particular local node status which may be transient (i.e. it may
      be worthwhile to retry after some delay).

   4 (Permanent-Failure):  the message has been rejected because of
      local node status which will not change without additional out of
      band (e.g. management) operations.

   Additional error class values are reserved.

   The allocation of error classes to particular errors is not precise;
   the above descriptions are deliberately informal.  Actual error
   processing should take into account the specific error in question;
   the error class may be useful supporting information (e.g. in network

A.4.4.  Error Catalogue

   This section lists all the possible GIST errors, including when they
   are raised and what additional information fields should be carried
   in the error object.

A.4.4.1.  Common Header Parse Error

   Class:              Protocol-Error
   Code:               1
   Additional Info:    For subcode 3 only, Message Length Info carries
                       the calculated message length.

   This message is sent if a GIST node receives a message where the
   common header cannot be parsed correctly, or where an error in the
   overall message format is detected.  Note that in this case the
   original MRI and Session ID are not included in the Error Object.
   This error code is split into subcodes as follows:

   0: Unknown Version:  The GIST version is unknown.  The (highest)
      supported version supported by the node can be inferred from the
      Common Header of the Error message itself.

   1: Unknown Type:  The GIST message type is unknown.

   2: Invalid R-flag:  The R flag in the header is inconsistent with the
      message type.

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   3: Incorrect Message Length:  The overall message length is not
      consistent with the set of objects carried.

   4: Invalid E-flag:  The E flag is set in the header but this is not a
      Data message.

A.4.4.2.  Hop Limit Exceeded

   Class:              Permanent-Failure
   Code:               2
   Additional Info:    None

   This message is sent if a GIST node receives a message with a GIST
   hop count of zero, or a GIST node decrements a packet's GIST hop
   count to zero on reception.  This message indicates either a routing
   loop or too small an initial hop count value.

A.4.4.3.  Incorrect Encapsulation

   Class:              Protocol-Error
   Code:               3
   Additional Info:    None

   This message is sent if a GIST node receives a message which uses an
   incorrect encapsulation method (e.g. a Query arrives over an MA).

A.4.4.4.  Incorrectly Delivered Message

   Class:              Protocol-Error
   Code:               4
   Additional Info:    None

   This message is sent if a GIST node receives a message over an MA
   which is not associated with the MRI/NSLPID/SID combination in the

A.4.4.5.  No Routing State

   Class:              Protocol-Error
   Code:               5
   Additional Info:    None

   This message is sent if a node receives a message for which routing
   state should exist, but has not yet been created and thus there is no
   appropriate Querying-SM or Responding-SM.  This can occur either on
   receiving a Response to an unknown Query, or on receiving a Data
   message at a node whose policy requires routing state to exist before
   such messages can be accepted.  See also Section 6.1 and Section 6.3.

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A.4.4.6.  Unknown NSLPID

   Class:              Permanent-Failure
   Code:               6
   Additional Info:    None

   This message is sent if a router receives a directly addressed
   message for an NSLP which it does not support.

A.4.4.7.  Endpoint Found

   Class:              Informational
   Code:               7
   Additional Info:    None

   This message is sent if a GIST node at a flow endpoint receives a
   Query message for an NSLP which it does not support.

A.4.4.8.  Message Too Large

   Class:              Permanent-Failure
   Code:               8
   Additional Info:    MTU Info

   A router receives a message which it can't forward because it exceeds
   the MTU on the next or subsequent hops.

A.4.4.9.  Object Type Error

   Class:              Protocol-Error
   Code:               9
   Additional Info:    Object Type Info

   This message is sent if a GIST node receives a message containing a
   TLV object with an invalid type.  The message includes the object at
   fault.  This error code is split into subcodes as follows:

   0: Duplicate Object:  This subcode is used if a GIST node receives a
      message containing multiple instances of an object which may only
      appear once in a message.  In the current specification, this
      applies to all objects.

   1: Unrecognised Object:  This subcode is used if a GIST node receives
      a message containing an object which it does not support, and the
      extensibility flags AB=00.

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   2: Missing Object:  This subcode is used if a GIST node receives a
      message which is missing one or more mandatory objects.  This
      message is also sent if a Stack-Proposal is sent without a
      matching Stack-Configuration-Data object when one was necessary,
      or vice versa.

   3: Invalid Object Type:  This subcode is used if the object type is
      known, but it is not valid for this particular GIST message type.

   4: Untranslated Object:  This subcode is used if the object type is
      known and is mandatory to interpret, but it contains addressing
      data which has not been translated by an intervening NAT.

   5: Invalid Extensibility Flags:  This subcode is used if an object is
      received with the extensibility flags AB=11.

A.4.4.10.  Object Value Error

   Class:              Protocol-Error
   Code:               10
   Additional Info:    1 or 2 Object Value Info fields as given below

   This message is sent if a router receives a packet containing an
   object which cannot be properly parsed.  The message contains a
   single Object Value Info object, except for subcode 5 as stated
   below.  This error code is split into subcodes as follows:

   0: Incorrect Length:  The overall length does not match the object
      length calculated from the object contents.

   1: Value Not Supported:  The value of a field is not supported by the
      GIST node.

   2: Invalid Flag-Field Combination:  An object contains an invalid
      combination of flags and/or fields.  At the moment this only
      relates to the Path-Coupled MRM object, but in future there may be

   3: Empty List:  At the moment this only relates to Stack-Proposals.
      The error message is sent if a stack proposal with a length > 0 (a
      length of 0 is handled as "Value Not Supported") contains only
      null bytes.

   4: Invalid Cookie:  The message contains a cookie which could not be
      verified by the node.

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   5: Stack-Proposal - Stack-Configuration-Data Mismatch:  This subcode
      is used if a GIST node receives a message in which the data in the
      Stack-Proposal object is inconsistent with the information in the
      Stack Configuration Data object.  In this case, both the Stack-
      Proposal object and Stack-Configuration-Data object MUST be
      included in separate Object Value Info fields in that order.

A.4.4.11.  Invalid IP layer TTL

   Class:              Permanent-Failure
   Code:               11
   Additional Info:    None

   This error indicates that a message was received with an IP layer TTL
   outside an acceptable range; for example, that an upstream Query was
   received with an IP layer TTL of less than 254 (i.e. more than one IP
   hop from the sender).  The actual IP distance can be derived from the
   IP-TTL information in the NLI object carried in the same message.

A.4.4.12.  MRI Validation Failure

   Class:              Permanent-Failure
   Code:               12
   Additional Info:    Object Value Info

   This error indicates that a message was received with an MRI that
   could not be accepted, e.g. because of too much wildcarding or
   failing some validation check (cf. Section  The Object
   Value Info includes the MRI so the error originator can indicate the
   part of the MRI which caused the problem.  The error code is divided
   into subcodes as follows:

   0: MRI Too Wild:  The MRI contained too much wildcarding (e.g. too
      short a destination address prefix) to be forwarded correctly down
      a single path.

   1: IP Version Mismatch:  The MRI in a path-coupled Query message uses
      an IP version which is not implemented on the interface used.

   2: Ingress Filter Failure:  The MRI in a path-coupled Query message
      describes a flow which would not pass ingress filtering on the
      interface used.

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Appendix B.  API between GIST and Signaling Applications

   This appendix provides an abstract API between GIST and signaling
   applications.  It should not constrain implementors, but rather help
   clarify the interface between the different layers of the NSIS
   protocol suite.  In addition, although some of the data types carry
   the information from GIST information elements, this does not imply
   that the format of that data as sent over the API has to be the same.

   Conceptually the API has similarities to the sockets API,
   particularly that for unconnected UDP sockets.  An extension for an
   API like that for UDP connected sockets could be considered.  In this
   case, for example, the only information needed in a SendMessage
   primitive would be NSLP-Data, NSLP-Data-Size, and NSLP-Message-Handle
   (which can be null).  Other information which was persistent for a
   group of messages could be configured once for the socket.  Such
   extensions may make a concrete implementation more efficient but do
   not change the API semantics, and so are not considered further here.

B.1.  SendMessage

   This primitive is passed from a signaling application to GIST.  It is
   used whenever the signaling application wants to initiate sending a

   SendMessage ( NSLP-Data, NSLP-Data-Size, NSLP-Message-Handle,
                 NSLPID, Session-ID, MRI, SII-Handle,
                 Transfer-Attributes, Timeout, IP-TTL, GIST-Hop-Count )

   The following arguments are mandatory.

   NSLP-Data:  The NSLP message itself.

   NSLP-Data-Size:  The length of NSLP-Data.

   NSLP-Message-Handle:  A handle for this message, that can be used by
      GIST as a reference in subsequent MessageStatus notifications
      (Appendix B.3).  Notifications could be about error conditions or
      about the security attributes that will be used for the message.
      A NULL handle may be supplied if the NSLP is not interested in
      such notifications.

   NSLPID:  An identifier indicating which NSLP this is.

   Session-ID:  The NSIS session identifier.  Note that it is assumed
      that the signaling application provides this to GIST rather than
      GIST providing a value itself.

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   MRI:  Message routing information for use by GIST in determining the
      correct next GIST hop for this message.  The MRI implies the
      message routing method to be used and the message direction.

   The following arguments are optional:

   SII-Handle:  A handle, previously supplied by GIST, to a data
      structure that should be used to route the message explicitly to a
      particular GIST next hop.

   Transfer-Attributes:  Attributes defining how the message should be
      handled (see Section 4.1.2).  The following attributes can be

      Reliability:  Values 'unreliable' or 'reliable'.

      Security:  This attribute allows the NSLP to specify what level of
         security protection is requested for the message (selected from
         'integrity' and 'confidentiality'), and can also be used to
         specify what authenticated signaling source and destination
         identities should be used to send the message.  The
         possibilities can be learned by the signaling application from
         prior MessageStatus or RecvMessage notifications.  If an NSLP-
         Message-Handle is provided, GIST will inform the signaling
         application of what values it has actually chosen for this
         attribute via a MessageStatus callback.  This might take place
         either synchronously (where GIST is selecting from available
         messaging associations), or asynchronously (when a new
         messaging association needs to be created).

      Local Processing:  This attribute contains hints from the
         signaling application about what local policy should be applied
         to the message; in particular, its transmission priority
         relative to other messages, or whether GIST should attempt to
         set up or maintain forward routing state.

   Timeout:  Length of time GIST should attempt to send this message
      before indicating an error.

   IP-TTL:  The value of the IP layer TTL that should be used when
      sending this message (may be overridden by GIST for particular

   GIST-Hop-Count:  The value for the hop count when sending the

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B.2.  RecvMessage

   This primitive is passed from GIST to a signaling application.  It is
   used whenever GIST receives a message from the network, including the
   case of null messages (zero length NSLP payload), typically initial
   Query messages.  For Queries, the results of invoking this primitive
   are used by GIST to check whether message routing state should be
   created (see the discussion of the 'Routing-State-Check' argument

   RecvMessage ( NSLP-Data, NSLP-Data-Size, NSLPID, Session-ID, MRI,
                 Routing-State-Check, SII-Handle, Transfer-Attributes,
                 IP-TTL, IP-Distance, GIST-Hop-Count,
                 Inbound-Interface )

   NSLP-Data:  The NSLP message itself (may be empty).

   NSLP-Data-Size:  The length of NSLP-Data (may be zero).

   NSLPID:  An identifier indicating which NSLP this is message is for.

   Session-ID:  The NSIS session identifier.

   MRI:  Message routing information that was used by GIST in forwarding
      this message.  Implicitly defines the message routing method that
      was used and the direction of the message relative to the MRI.

   Routing-State-Check:  This boolean is True if GIST is checking with
      the signaling application to see if routing state should be
      created with the peer or the message should be forwarded further
      (see Section 4.3.2).  If True, the signaling application should
      return the following values via the RecvMessage call:

         A boolean indicating whether to set up the state.

         Optionally, an NSLP-Payload to carry in the generated Response
         or forwarded Query respectively.

      This mechanism could be extended to enable the signaling
      application to indicate to GIST whether state installation should
      be immediate or deferred (see Section 5.3.3 and Section 6.3 for
      further discussion).

   SII-Handle:  A handle to a data structure, identifying a peer address
      and interface.  Can be used to identify route changes and for
      explicit routing to a particular GIST next hop.

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   Transfer-Attributes:  The reliability and security attributes that
      were associated with the reception of this particular message.  As
      well as the attributes associated with SendMessage, GIST may
      indicate the level of verification of the addresses in the MRI.
      Three attributes can be indicated:

      *  Whether the signaling source address is one of the flow
         endpoints (i.e. whether this is the first or last GIST hop);

      *  Whether the signaling source address has been validated by a
         return routability check.

      *  Whether the message was explicitly routed (and so has not been
         validated by GIST as delivered consistently with local routing

   IP-TTL:  The value of the IP layer TTL this message was received with
      (if available).

   IP-Distance:  The number of IP hops from the peer signaling node
      which sent this message along the path, or 0 if this information
      is not available.

   GIST-Hop-Count:  The value of the hop count the message was received
      with, after being decremented in the GIST receive-side processing.

   Inbound-Interface:  Attributes of the interface on which the message
      was received, such as whether it lies on the internal or external
      side of a NAT.  These attributes have only local significance and
      are implementation defined.

B.3.  MessageStatus

   This primitive is passed from GIST to a signaling application.  It is
   used to notify the signaling application that a message that it
   requested to be sent could not be dispatched, or to inform the
   signaling application about the transfer attributes that have been
   selected for the message (specifically, security attributes).  The
   signaling application can respond to this message with a return code
   to abort the sending of the message if the attributes are not

   MessageStatus (NSLP-Message-Handle, Transfer-Attributes, Error-Type)

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   NSLP-Message-Handle:  A handle for the message provided by the
      signaling application in SendMessage.

   Transfer-Attributes:  The reliability and security attributes that
      will be used to transmit this particular message.

   Error-Type:  Indicates the type of error that occurred.  For example,
      'no next node found'.

B.4.  NetworkNotification

   This primitive is passed from GIST to a signaling application.  It
   indicates that a network event of possible interest to the signaling
   application occurred.

   NetworkNotification ( NSLPID, MRI, Network-Notification-Type )

   NSLPID:  An identifier indicating which NSLP this is message is for.

   MRI:  Provides the message routing information to which the network
      notification applies.

   Network-Notification-Type:  Indicates the type of event that caused
      the notification and associated additional data.  Two events have
      been identified:

      Last Node:  GIST has detected that this is the last NSLP-aware
         node in the path.  See Section 4.3.4.

      Routing Status Change:  GIST has installed new routing state, or
         has detected that the routing state may no longer be valid, or
         has re-established the routing state.  See Section 7.1.3.  The
         new status is reported; if the status is Good, the SII-Handle
         of the peer is also reported, as for RecvMessage.

B.5.  SetStateLifetime

   This primitive is passed from a signaling application to GIST.  It
   indicates the duration for which the signaling application would like
   GIST to retain its routing state.  It can also give a hint that the
   signaling application is no longer interested in the state.

   SetStateLifetime ( NSLPID, MRI, State-Lifetime )

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   NSLPID:  Provides the NSLPID to which the routing state lifetime

   MRI:  Provides the message routing information to which the routing
      state lifetime applies; includes the direction (in the D flag).

   State-Lifetime:  Indicates the lifetime for which the signaling
      application wishes GIST to retain its routing state (may be zero,
      indicating that the signaling application has no further interest
      in the GIST state).

B.6.  InvalidateRoutingState

   This primitive is passed from a signaling application to GIST.  It
   indicates that the signaling application has knowledge that the next
   signaling hop known to GIST may no longer be valid, either because of
   changes in the network routing or the processing capabilities of
   signaling application nodes.  See Section 7.1.

   InvalidateRoutingState ( NSLPID, MRI, Status, Urgent )

   NSLPID:  The NSLP originating the message.  May be null (in which
      case the invalidation applies to all signaling applications).

   MRI:  The flow for which routing state should be invalidated;
      includes the direction of the change (in the D flag).

   Status:  The new status that should be assumed for the routing state,
      one of Bad or Tentative (see Section 7.1.3).

   Urgent:  A hint as to whether rediscovery should take place
      immediately, or only with the next signaling message.

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Appendix C.  Example Routing State Table and Handshake Message Sequence

   Figure 10 shows a signaling scenario for a single flow being managed
   by two signaling applications using the path-coupled message routing
   method.  The flow sender and receiver and one router support both,
   two other routers support one each.  The figure also shows the
   routing state table at node B.

       A                        B          C          D           E
   +------+                  +-----+    +-----+    +-----+    +--------+
   | Flow |    +-+    +-+    |NSLP1|    |NSLP1|    |     |    |  Flow  |
   |Sender|====|R|====|R|====|NSLP2|====|     |====|NSLP2|====|Receiver|
   |      |    +-+    +-+    |GIST |    |GIST |    |GIST |    |        |
   +------+                  +-----+    +-----+    +-----+    +--------+
             Flow Direction ------------------------------>>

   |     Message Routing Information    | Session | NSLPID |  Routing  |
   |                                    |    ID   |        |   State   |
   |    MRM = Path Coupled; Flow ID =   |  0xABCD |  NSLP1 |    IP-A   |
   |   {IP-A, IP-E, proto/ports}; D=up  |         |        |           |
   |                                    |         |        |           |
   |    MRM = Path Coupled; Flow ID =   |  0xABCD |  NSLP1 |   (null)  |
   |  {IP-A, IP-E, proto/ports}; D=down |         |        |           |
   |                                    |         |        |           |
   |    MRM = Path Coupled; Flow ID =   |  0x1234 |  NSLP2 |    IP-A   |
   |   {IP-A, IP-E, proto/ports}; D=up  |         |        |           |
   |                                    |         |        |           |
   |    MRM = Path Coupled; Flow ID =   |  0x1234 |  NSLP2 | Points to |
   |  {IP-A, IP-E, proto/ports}; D=down |         |        |   B-D MA  |

                      Figure 10: A Signaling Scenario

   The upstream state is just the same address for each application.
   For the downstream direction, NSLP1 only requires D-mode messages and
   so no explicit routing state towards C is needed.  NSLP2 requires a
   messaging association for its messages towards node D, and node C
   does not process NSLP2 at all, so the peer state for NSLP2 is a
   pointer to a messaging association that runs directly from B to D.
   Note that E is not visible in the state table (except implicitly in
   the address in the message routing information); routing state is
   stored only for adjacent peers.  (In addition to the peer
   identification, IP hop counts are stored for each peer where the
   state itself if not null; this is not shown in the table.)

   Figure 11 shows a GIST handshake setting up a messaging association

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   for B-D signaling, with the exchange of Stack Proposals and MA-
   protocol-options in each direction.  The Querying node selects TLS/
   TCP as the stack configuration and sets up the messaging association
   over which it sends the Confirm.

    -------------------------- Query ---------------------------->
    IP(Src=IP#A; Dst=IP#E; RAO for NSLP2); UDP(Src=6789; Dst=GIST)
    GIST(Header(Type=Query; NSLPID=NSLP2; R=1; S=0)
         MRI(MRM=Path-Coupled; Flow=F; Direction=down)
         NLI(Peer='string1'; IA=IP#B)
         StackProposal(#Proposals=3;1=TLS/TCP; 2=TLS/SCTP; 3=TCP)
         StackConfigurationData(HoldTime=300; #MPO=2;
           TCP(Applicable: all; Data: null)
           SCTP(Applicable: all; Data: null)))

    <---------------------- Response ----------------------------
    IP(Src=IP#D; Dst=IP#B); UDP(Src=GIST; Dst=6789)
    GIST(Header(Type=Response; NSLPID=NSLP2; R=1; S=1)
         MRI(MRM=Path-Coupled; Flow=F; Direction=up)
         NLI(Peer='stringr2', IA=IP#D)
         StackProposal(#Proposals=3; 1=TCP; 2=SCTP; 3=TLS/TCP)
         StackConfigurationData(HoldTime=200; #MPO=3;
           TCP(Applicable: 3; Data: port=6123)
           TCP(Applicable: 1; Data: port=5438)
           SCTP(Applicable: all; Data: port=3333)))

    -------------------------TCP SYN----------------------->
    <----------------------TCP SYN/ACK----------------------
    -------------------------TCP ACK----------------------->
    TCP connect(IP Src=IP#B; IP Dst=IP#D; Src Port=9166; Dst Port=6123)
    <-----------------------TLS INIT----------------------->

    ------------------------ Confirm ---------------------------->
    [Sent within messaging association]
    GIST(Header(Type=Confirm; NSLPID=NSLP2; R=0; S=1)
         MRI(MRM=Path-Coupled; Flow=F; Direction=down)
         NLI(Peer='string1'; IA=IP#B)
         StackProposal(#Proposals=3; 1=TCP; 2=SCTP; 3=TLS/TCP)

                Figure 11: GIST Handshake Message Sequence

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Appendix D.  Change History

D.1.  Changes In Version -11

   1.   Added some text in Section 1 to clarify the scope of GIST
        applicability with non-path-coupled message routing methods.

   2.   Loosened the text about the Query encapsulation to indicate that
        a Router Alert Option is needed for all the current message
        routing methods but not necessarily for future ones.

   3.   Clarified the rules for deriving protocol encapsulation
        addresses for the Response and other messages in Section 4.4.1
        and Section 5.3.1.

   4.   Updated the ABNF and message descriptions in Section 3.4 to
        cover the case of NAT traversal for stateless data messages;
        also minor changes in Section 7.2.

   5.   Re-corrected the timeout processing rules in Section 6.4 (update
        in version 10 changed rule 3 but should have changed rule 4).
        In addition, the rule 3 processing is made conditional on the
        state (i.e. split) since different timers are running in the two

   6.   Clarified that the E flag can only be set on Data messages, and
        added notes to the flag description in Section 7.1.4 and the
        format description in Appendix A.1.  Also, included a new error
        condition to cover incorrect setting in Appendix A.4.4.1.

   7.   Clarified the text in Section 8.4 to note the issues about
        Response size contributing to reflection attacks, and also the
        defence against various forms of message spoofing in
        Section 8.5.

   8.   Stated that MA-Protocol-ID value 0 is reserved (not allocated)
        in Section 9.

   9.   Clarified the units (bytes, 32-bit words) for all length fields
        in Appendix A.

   10.  Clarified that the restriction on the D flag value for the
        loose-end MRM applies only to Q-mode messages in
        Appendix A.3.1.2.

   11.  Added the Hold Time to the example in Appendix C.

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D.2.  Changes In Version -10

   1.   Added further guidance on parameter setting for initial backoff
        and rate control for D-mode to Section 5.3.3 [AD review comment

   2.   Rephrased the end of Section 8.6 to highlight the threat left
        open when the Querying node does not apply a strong security
        policy to offered Stack-Proposal [AD review comment M2].

   3.   Clarified in Section 7.2 that although NAT behaviour is only
        informatively described in this specification, it is being
        defined in a separate document [AD review M3].

   4.   Strengthened and clarified the reference to the extensibility
        document for technical guidance on codepoint allocation, and
        made the reference normative.  Added rationale for the
        'Reserved' blocks in the various registries, and added further
        notes on what information must be provided to support an
        allocation request [AD review comment M4].

   5.   Fixed an identifier collision in the ABNF for the GIST messages
        in Section 5.2.2 (Common-Header in the message header and
        common-header as a payload in error messages) and re-verified
        the ABNF [AD review comment L1].

   6.   Clarified the text in Section 3.3 about the impact on NATs of
        defining a new MRM, referring to the specification split
        described in Section 7.2.2.  Also added a flag to the MRM format
        (Appendix A.3.1) to denote MRIs which do not contain network or
        transport addresses, and made more specific the error message to
        be returned if a NAT does not understand an MRM in Section 7.2.1
        [AD review comment L2].

   7.   Added discussion in Section 4.1.2 on delivery failure detected
        for reliable messaging in general, and for the case of Forwards-
        TCP in particular in Section 5.7.2.  Also noted that this needs
        to be considered for future MA-Protocol-IDs used for reliable
        messaging (Section 9) [AD review comment L3].

   8.   Added clarifying text to Section 5.1 on what it means to invert
        the direction of an MRI [AD review comment L5].

   9.   Enhanced the format descriptions in Appendix A to include
        descriptions of all message and object fields and also field
        lengths [AD review comment L6].

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   10.  Added more explanation in Section 5.2.2 of how a message
        direction is defined, in particular in the context of TTL
        measurement [AD review comment L7].

   11.  Added a new explanation of why a well-known port is needed for
        the query encapsulation in Section 5.3.2 [AD review comment L8].

   12.  Added a note that DCCP does not provide reliability in
        Section 5.4.1 [AD review comment L9].

   13.  Clarified the rules on how long to retain stack configuration
        data in Section 5.7.1 and included a default timer value [AD
        review comment L10].

   14.  Modified the text about stack-proposal verification as part of
        downgrade protection in Section 5.7.1, to clarify that the MUST
        applies directly to the object verification itself; also noted
        the action to be taken in case of a failed verification [AD
        review comment L11].

   15.  Added further information on the addressing used in opening a
        forwards-TCP connection in Section 5.7.2 [AD review comment

   16.  Modified the text in Section to say that using the
        signaling source address is a consequence of setting DF itself
        rather than why DF was set in the first place; also weakened the
        instruction from MUST to SHOULD [AD review comment L14].

   17.  Added further clarification of why routing state installed by a
        downstream Query should supersede that from an upstream Query in
        Section [AD review comment L15].

   18.  Corrected a timer in the Messaging Association state machine
        (Section 6.4) from NoHello to SendHello.  Also, added default
        values for MA-Hold-Time and route change probe frequency, and
        explanatory text for each, to Section 4.4.3 [AD review comment

   19.  Re-arranged the text in Section 7.2 to highlight the rules about
        precisely which messages are and are not translated in a GIST-
        specific way by NATs [AD review comment L19].

   20.  Explicitly noted that 'r' bits are also reserved in Appendix A.2
        [AD review comment L20].

   21.  Added an error condition for processing messages which have the
        extensibility flags AB set to 11 in Appendix A.2.1 [AD review

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        comment L21].

   22.  Fixed the table of MRM identifiers in Section 9 so the field
        name matches that in Appendix A.3.1 [AD review comment L22].

   23.  Clarified why only D=0 is valid for the loose-end MRM in
        Appendix A.3.1.2 [AD review comment L23].

   24.  Clarified the rules about processing the NAT traversal object in
        Appendix A.3.8 to cover the case where there are several NATs
        along the path with different capabilities [AD review comment

   25.  Strengthened the text in Appendix A.4.1 to be clearer about what
        additional information fields must be included in error messages
        [AD review comment L25].

   26.  Tidied up the use of acronyms throughout the document, including
        adding some to the terminology list in Section 2 [AD review
        comment N1].

   27.  Added references to RFC4086 and updated 2119 language for
        cryptographic randomness of SIDs and cookies in Section 3.5 and
        Section 8.5 respectively [AD review comment N2].

   28.  Modified the transition labelling in Figure 7 to make it clearer
        that in the Established-Established transition, the
        [!confirmRequired] qualification applies only to the rx_Query
        case [AD review comment N4].

   29.  Added a reference for OSPF in Section 7.1.2 [AD review comment

   30.  Changed NAT terminology from public/private to external/internal
        to match BEHAVE usage in Section 7.2 and Section 7.4 [AD review
        comment N6].

   31.  Updated a number of i-d references to published RFCs or working
        group documents [AD review comment N7 partial].

   32.  Fixed rfc2119 capitalisation of MUST not in Appendix A.3.5 [AD
        review comment N8].

   33.  Fixed an error subcode name from 'Invalid Object' to 'Invalid
        Object Type' in Appendix A.4.4.9 [AD review comment N9].

   34.  Added the NTO to the GIST message ABNF in Section 5.1 and
        updated the forward reference to the NAT traversal section

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        [tracker issue 104].

   35.  Removed a spurious rule about creating listening MAs in
        Section 6.3 and strengthened the rules about needing to have
        these available but with an open policy on when to create and
        destroy them in Section 5.7.1 [tracker issue 105].

   36.  Added text that limits the applicability of the private/
        experimental space to closed network environments [tracker issue

   37.  Added text in Section 7.1.4 encouraging GIST to use a single SII
        across multiple sessions if possible to allow signaling
        application aggregation [tracker issue 107].

   38.  Specified that this document would define GIST version 1 in
        Section 5.2.1 [tracker issue 108].

   39.  Added the ability for RecvMessage to pass up interface
        attributes in Appendix B.2 [tracker issue 110].

   40.  Added additional text on rules for selecting stack proposals and
        MA re-use in Section 5.7.1 to ensure that re-used associations
        have properties that the Querying node actually needs [tracker
        issue 111].

   41.  Added a brief introduction to the GIST message types in a new
        Section 3.4.

   In addition, the following AD review comments did not lead to text
   changes.  See the mailing list discussion at

   L4:  Direct use of PMTUD by GIST.

   L13:  Use of TLS 1.0 rather than 1.1.

   L17:  Guidance on NSLP behaviour during rerouting,

   L18:  Behaviour of GIST-unaware NATs.

   N3:  Node state machine logic.

D.3.  Changes In Version -09

   1.   Added a new Section 3.6 clarifying the relationship between
        signaling applications and NSLPIDs; modified terminology in the
        remainder of the document likewise.

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   2.   Added a new Section 8.6 explaining the rationale behind the
        downgrade attack prevention mechanism.

   3.   Re-wrote parts of Section 4.3.2, Section 6.1 and Appendix B.2 to
        clarify the way that GIST is assumed to interact with signaling
        applications to exercise policy control over whether or not two
        nodes become signaling peers during a GIST handshake.

   4.   Generalised an error message Appendix A.4.4.12 to cover
        additional MRI validation checks in Section 4.3.4 and

   5.   Allowed an optional Stack-Configuration-Data object in Confirm
        messages to allow messaging association lifetime to be
        negotiated even in the case of late state installation at the
        Responding node (see Section 4.4.1 and Section 4.4.3).

   6.   Removed the option in Section 4.4.2 of allowing a node to treat
        messaging associations with the same authenticated end points as

   7.   Include additional guidance in Section 4.4.3 to prevent routing
        state being erroneously refreshed in the case of rerouting
        events; also included general guidance notes on timer setting.

   8.   Clarified that the Stack-Proposal lists protocols in top-to-
        bottom order (see Section 5.7.1).

   9.   Enhanced the definition of TLS usage in Section 5.7.3 with
        details on ciphersuite requirements and authentication methods.

   10.  Tidied up terminology and discussion of how protocol options
        data is carried in the SCD; renamed higher-layer-addressing to

D.4.  Changes In Version -08

   1.   Changed the protocol name from GIMPS to GIST (everywhere).

   2.   Inserted RFC2119 language (MUST etc.) in the appropriate places.

   3.   Added references to the actions to be taken in various error
        conditions, including the error messages to be send

   4.   Added legacy NAT traversal to the list of excluded functions in
        Section 1.1.

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   5.   Included some text at the end of Section 3.3 analysing the case
        of a GIST node which does not support a particular MRM.

   6.   Added a flag to mark when messages have been explicitly routed,
        so they can bypass validation against current routing state (see
        Section 4.3.1, TBD).

   7.   Re-wrote the discussion in Section 4.3.4 to cover all cases of
        nodes not hosting an NSLP (including end systems), in particular
        the validations that can be performed at intermediate GIST nodes
        (this replaces the old section 7.2).

   8.   Clarified the rules about R and S flag setting in the common
        header and D flag in the MRI (Section 5).

   9.   Included discussion of how a node with a choice of interfaces or
        IP versions should select one to use in the NLI (Section 5.2.2).

   10.  Modified the description of messaging association protocol
        selections (Section 5.7 and elsewhere) to clarify that this is
        essentially capability discovery rather than an open ended
        protocol negotiation.

   11.  Modified the description of how higher layer addressing
        information is carried (Section 5.7.1 and Appendix A.3.5) to
        allow the data to be tagged against a specific profile if
        necessary, or omitted if the protocol does not need it.

   12.  Added a higher layer protocol definition for TLS in
        Section 5.7.3.

   13.  Simplified and restructured the state machine presentation in
        Section 6, in particular using a single list for the events and
        eliminating the transition tables.  Also modified the operation
        of the Responder machine to handle retransmitted Query messages

   14.  Re-wrote the route change handling text in Section 7.1 to
        clarify the relative responsibilities of GIST and NSLPs and
        their interaction through the API.  Notifications are now
        assumed to be a signaling application responsibility, and GIST
        behaviour is defined in terms of handling changes in a 3-state
        model of the correctness of the routing state for each

   15.  Updated the NAT traversal description in Section 7.2, including
        normative text about how GIST nodes should handle messages
        containing NAT-Traversal objects.

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   16.  Likewise, clarified that the responsibility for session/flow
        binding in the case of tunnelling is handled by NSLPs
        (Section 7.3).

   17.  Formalised the IANA considerations (Section 9).

   18.  Extended the routing state example (Appendix C) to include a
        message sequence for association setup.

   19.  Re-arranged the sequence of sections, including placing this
        change history at the end.

D.5.  Changes In Version -07

   1.  The open issues section has finally been removed in favour of the
       authoritative list of open issues in an online issue tracker at h

   2.  Clarified terminology on peering and adjacencies that there may
       be NSIS nodes between GIMPS peers that do some message
       processing, but that are not explicitly visible in the peer state

   3.  Added a description of the loose-end MRM (Section 5.8.2 and
       Appendix A.3.1.2).

   4.  Added a description of an upstream Query encapsulation for the
       path-coupled MRM, Section, including rationale for and
       restrictions on its use.

   5.  The formal description of the protocol in Section 6 has been
       significantly updated and extended in terms of detail.

   6.  Modified the description of the interaction between NSLPs and
       GIMPS for handling inbound messages for which no routing state
       exists, to allow the NSLP to indicate whether state setup should
       proceed and to provide NSLP payloads for the Response or
       forwarded message (Section 3.7, Section 4.3.2 and Appendix B).

   7.  Included new text, Section 5.6, on the processing and
       encapsulation of error messages.  Also added formats and an error
       message catalogue in Appendix A.4, including a modified format
       for the overall GIMPS-Error message and the GIMPS-Error-Data

   8.  Removed the old section 5.3.3 on NSLPID/RAO setting on the
       assumption that this will be covered in the extensibility

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   9.  Included a number of other minor corrections and clarifications.

D.6.  Changes In Version -06

   Version -06 does not introduce any major structural changes to the
   protocol definition, although it does clarify a number of details and
   resolve some outstanding open issues.  The primary changes are as

   1.   Added a new high level Section 3.3 which gathers together the
        various aspects of the message routing method concept.

   2.   Added a new high level Section 3.5 which explains the concept
        and significance of the session identifier.  Also clarified that
        the routing state always depends on the session identifier.

   3.   Added notes about the level of address validation performed by
        GIMPS in Section 4.1.2 and extensions to the API in Appendix B.

   4.   Split the old Node-Addressing object into a Network-Layer-
        Information object and Stack-Configuration-Data object.  The
        former refers to basic information about a node, and the latter
        carries information about messaging association configuration.
        Redefined the content of the various handshake messages
        accordingly in Section 4.4.1 and Section 5.1.

   5.   Re-wrote Section 4.4.3 to clarify the rules on refresh and purge
        of routing state and messaging associations.  Also, moved the
        routing state lifetime into the Network-Layer-Information object
        and added a messaging association lifetime to the Stack-
        Configuration-Data object (Section 5.2).

   6.   Added specific message types for errors and MA-Refresh in
        Section 5.1.  The error object is now GIMPS-specific
        (Appendix A.4.1).

   7.   Moved the Flow-Identifier information about the message routing
        method from the general description of the object to the path-
        coupled MRM section (Section, and made a number of
        clarifications to the bit format (Appendix A.3.1.1).

   8.   Removed text about assumptions on the version numbering of
        NSLPs, and restricted the scope of the description of TLV object
        formats and extensibility flags to GIMPS rather than the whole
        of NSIS (Appendix A).

   9.   Added a new Section 5.5 explaining the possible relationships
        between message types and encapsulation formats.

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   10.  Added a new Section 6 in outline form, to capture the formal
        specification of the protocol operation.

   11.  Added new security sections on cookie requirements (Section 8.5)
        and residual threats (Section 8.7).

D.7.  Changes In Version -05

   Version -05 reformulates the specification, to describe routing state
   maintenance in terms of exchanging explicitly identified Query/
   Response/Confirm messages, leaving the upstream/downstream
   distinction as a specific detail of how Query messages are
   encapsulated.  This necessitated widespread changes in the
   specification text, especially Section 4.2.1, Section 4.4,
   Section 5.1 and Section 5.3 (although the actual message sequences
   are unchanged).  A number of other issues, especially in the area of
   message encapsulation, have also been closed.  The main changes are
   the following:

   1.   Added a reference to an individual draft on the Loose End MRM as
        a concrete example of an alternative message routing method.

   2.   Added further text (particularly in Section 2) on what GIMPS
        means by the concept of 'session'.

   3.   Firmed up the selection of UDP as the encapsulation choice for
        D-mode, removing the open issue on this topic.

   4.   Defined the interaction between GIMPS and signaling applications
        for communicating about the cryptographic security properties of
        how a message will be sent or has been received (see
        Section 4.1.2 and Appendix B).

   5.   Closed the issue on whether Query messages should use the
        signaling or flow source address in the IP header; both options
        are allowed by local policy and a flag in the common header
        indicates which was used.  (See Section

   6.   Added the necessary information elements to allow the IP hop
        count between adjacent GIMPS peers to be measures and reported.
        (See Section 5.2.2 and Appendix A.3.3.)

   7.   The old open-issue text on selection of IP router alert option
        values has been moved into the main specification to capture the
        technical considerations that should be used in assigning such
        values (in old section 5.3.3).

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   8.   Resolved the open issue on lost Confirm messages by allowing a
        choice of timer-based retransmission of the Response, or an
        error message from the responding node which causes the
        retransmission of the Confirm (see Section 5.3.3).

   9.   Closed the open issue on support for message scoping (this is
        now assumed to be a NSLP function).

   10.  Moved the authoritative text for most of the remaining open
        issues to an online issue tracker.

D.8.  Changes In Version -04

   Version -04 includes mainly clarifications of detail and extensions
   in particular technical areas, in part to support ongoing
   implementation work.  The main details are as follows:

   1.   Substantially updated Section 4, in particular clarifying the
        rules on what messages are sent when and with what payloads
        during routing and messaging association setup, and also adding
        some further text on message transfer attributes.

   2.   The description of messaging association protocol setup
        including the related object formats has been centralised in a
        new Section 5.7, removing the old Section 6.6 and also closing
        old open issues 8.5 and 8.6.

   3.   Made a number of detailed changes in the message format
        definitions (Appendix A), as well as incorporating initial rules
        for encoding message extensibility information.  Also included
        explicit formats for a general purpose Error object, and the
        objects used to discover supported messaging association
        protocols.  Updated the corresponding open issues section (old
        section 9.3) with a new item on NSLP versioning.

   4.   Updated the GIMPS API (Appendix B), including more precision on
        message transfer attributes, making the NSLP hint about storing
        reverse path state a return value rather than a separate
        primitive, and adding a new primitive to allow signaling
        applications to invalidate GIMPS routing state.  Also, added a
        new parameter to SendMessage to allow signaling applications to
        'bypass' a message statelessly, preserving the source of an
        input message.

   5.   Added an outline for the future content of an IANA
        considerations section (Section 9).  Currently, this is
        restricted to identifying the registries and allocations
        required, without defining the allocation policies and other

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        considerations involved.

   6.   Shortened the background design discussion in Section 3.

   7.   Made some clarifications in the terminology section relating to
        how the use of C-mode does and does not mandate the use of
        transport or security protection.

   8.   The ABNF for message formats in Section 5.1 has been re-written
        with a grammar structured around message purpose rather than
        message direction, and additional explanation added to the
        information element descriptions in Section 5.2.

   9.   The description of the D-mode transport in Section 5.3 has been
        updated.  The encapsulation rules (covering IP addressing and
        UDP port allocation) have been corrected, and a new subsection
        on message retransmission and rate limiting has been added,
        superseding the old open issue on the same subject (section

   10.  A new open issue on IP TTL measurement to detect non-GIMPS
        capable hops has been added (old section 9.5).

D.9.  Changes In Version -03

   Version -03 includes a number of minor clarifications and extensions
   compared to version -02, including more details of the GIMPS API and
   messaging association setup and the node addressing object.  The full
   list of changes is as follows:

   1.  Added a new section pinning down more formally the interaction
       between GIMPS and signaling applications (Section 4.1), in
       particular the message transfer attributes that signaling
       applications can use to control GIMPS (Section 4.1.2).

   2.  Added a new open issue identifying where the interaction between
       the security properties of GIMPS and the security requirements of
       signaling applications should be identified (old section 9.10).

   3.  Added some more text in Section 4.2.1 to clarify that GIMPS has
       the (sole) responsibility for generating the messages that
       refresh message routing state.

   4.  Added more clarifying text and table to GHC and IP TTL handling
       discussion of Section 4.3.4.

   5.  Split Section 4.4 into subsections for different scenarios, and
       added more detail on Node-Addressing object content and use to

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       handle the case where association re-use is possible in
       Section 4.4.2.

   6.  Added strawman object formats for Node-Addressing and Stack-
       Proposal objects in Section 5.1 and Appendix A.

   7.  Added more detail on the bundling possibilities and appropriate
       configurations for various transport protocols in Section 5.4.1.

   8.  Included some more details on NAT traversal in Section 7.2,
       including a new object to carry the untranslated address-bearing
       payloads, the NAT-Traversal object.

   9.  Expanded the open issue discussion in old section 9.3 to include
       an outline set of extensibility flags.

D.10.  Changes In Version -02

   Version -02 does not represent any radical change in design or
   structure from version -01; the emphasis has been on adding details
   in some specific areas and incorporation of comments, including early
   review comments.  The full list of changes is as follows:

   1.   Added a new Section 1.1 which summarises restrictions on scope
        and applicability; some corresponding changes in terminology in
        Section 2.

   2.   Closed the open issue on including explicit GIMPS state teardown
        functionality.  On balance, it seems that the difficulty of
        specifying this correctly (especially taking account of the
        security issues in all scenarios) is not matched by the saving
        of state enabled.

   3.   Removed the option of a special class of message transfer for
        reliable delivery of a single message.  This can be implemented
        (inefficiently) as a degenerate case of C-mode if required.

   4.   Extended Appendix A with a general discussion of rules for
        message and object formats across GIMPS and other NSLPs.  Some
        remaining open issues are noted in old section 9.3 (since

   5.   Updated the discussion of RAO/NSLPID relationships to take into
        account the proposed message formats and rules for allocation of
        NSLP id, and propose considerations for allocation of RAO

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   6.   Modified the description of the information used to route
        messages (first given in Section 4.2.1 but also throughout the
        document).  Previously this was related directly to the flow
        identification and described as the Flow-Routing-Information.
        Now, this has been renamed Message-Routing-Information, and
        identifies a message routing method and any associated

   7.   Modified the text in Section 4.3 and elsewhere to impose sanity
        checks on the Message-Routing-Information carried in C-mode
        messages, including the case where these messages are part of a
        GIMPS-Query/Response exchange.

   8.   Added rules for message forwarding to prevent message looping in
        a new Section 4.3.4, including rules on IP TTL and GIMPS hop
        count processing.  These take into account the new RAO
        considerations described above.

   9.   Added an outline mechanism for messaging association protocol
        stack setup, with the details in a new Section 6.6 and other
        changes in Section 4.4 and the various sections on message

   10.  Removed the open issue on whether storing reverse routing state
        is mandatory or optional.  This is now explicit in the API
        (under the control of the local NSLP).

   11.  Added an informative annex describing an abstract API between
        GIMPS and NSLPs in Appendix B.

D.11.  Changes In Version -01

   The major change in version -01 is the elimination of
   'intermediaries', i.e. imposing the constraint that signaling
   application peers are also GIMPS peers.  This has the consequence
   that if a signaling application wishes to use two classes of
   signaling transport for a given flow, maybe reaching different
   subsets of nodes, it must do so by running different signaling
   sessions; and it also means that signaling adaptations for passing
   through NATs which are not signaling application aware must be
   carried out in D-mode.  On the other hand, it allows the elimination
   of significant complexity in the C-mode handling and also various
   other protocol features (such as general route recording).

   The full set of changes is as follows:

   1.   Added a worked example in Section 3.7.

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   2.   Stated that nodes which do not implement the signaling
        application should bypass the message (Section 4.3).

   3.   Decoupled the state handling logic for routing state and
        messaging association state in Section 4.4.  Also, allow
        messaging associations to be used immediately in both directions
        once they are opened.

   4.   Added simple ABNF for the various GIMPS message types in a new
        Section 5.1, and more details of the common header and each
        object in Section 5.2, including bit formats in Appendix A.  The
        common header format means that the encapsulation is now the
        same for all transport types (Section 5.4.1).

   5.   Added some further details on D-mode encapsulation in
        Section 5.3, including more explanation of why a well known port
        is needed.

   6.   Removed the possibility for fragmentation over DCCP
        (Section 5.4.1), mainly in the interests of simplicity and loss

   7.   Removed all the tunnel mode encapsulations (old sections 5.3.3
        and 5.3.4).

   8.   Fully re-wrote the route change handling description
        (Section 7.1), including some additional detection mechanisms
        and more clearly distinguishing between upstream and downstream
        route changes.  Included further details on GIMPS/NSLP
        interactions, including where notifications are delivered and
        how local repair storms could be avoided.  Removed old
        discussion of propagating notifications through signaling
        application unaware nodes (since these are now bypassed
        automatically).  Added discussion on how to route messages for
        local state removal on the old path.

   9.   Revised discussion of policy-based forwarding (old Section 7.2)
        to account for actual Flow-Routing-Information definition, and
        also how wildcarding should be allowed and handled.

   10.  Removed old route recording section (old Section 6.3).

   11.  Extended the discussion of NAT handling (Section 7.2) with an
        extended outline on processing rules at a GIMPS-aware NAT and a
        pointer to implications for C-mode processing and state

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   12.  Clarified the definition of 'correct routing' of signaling
        messages in Section 8 and GIMPS role in enforcing this.  Also,
        opened the possibility that peer node authentication could be
        signaling application dependent.

   13.  Removed old open issues on C-mode Encapsulation (section 8.7);
        added new open issues on Message Routing (old Section 9.3 of
        version -05, later moved to Section 3.3) and D-mode congestion

   14.  Added this change history.

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Authors' Addresses

   Henning Schulzrinne
   Columbia University
   Department of Computer Science
   450 Computer Science Building
   New York, NY  10027

   Phone: +1 212 939 7042

   Robert Hancock
   Siemens/Roke Manor Research
   Old Salisbury Lane
   Romsey, Hampshire  SO51 0ZN


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