Next Steps in Signaling                                   H. Schulzrinne
Internet-Draft                                               Columbia U.
Expires: April 19, 2004                                       R. Hancock
                                                        October 20, 2003

       GIMPS:  General Internet Messaging Protocol for Signaling

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.


   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 solution uses existing transport and
   security protocols under a common messaging layer, the Generic
   Internet Messaging Protocol for Signaling (GIMPS), which provides a
   universal service for diverse signaling applications. GIMPS 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 GIMPS and the
   lower layer protocols provides a solution for the base protocol
   component of the "Next Steps in Signaling" framework.

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Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   Requirements Notation and Terminology  . . . . . . . . . . .   4
   3.   Design Methodology . . . . . . . . . . . . . . . . . . . . .   6
   3.1  Overall Approach . . . . . . . . . . . . . . . . . . . . . .   6
   3.2  Design Attributes  . . . . . . . . . . . . . . . . . . . . .   8
   4.   Overview of Operation  . . . . . . . . . . . . . . . . . . .  10
   4.1  GIMPS State  . . . . . . . . . . . . . . . . . . . . . . . .  10
   4.2  Basic Message Processing . . . . . . . . . . . . . . . . . .  12
   4.3  Routing State and Messaging Association Maintainance . . . .  15
   5.   Message Formats and Encapsulations . . . . . . . . . . . . .  19
   5.1  Message Formats  . . . . . . . . . . . . . . . . . . . . . .  19
   5.2  Encapsulation in Datagram Mode . . . . . . . . . . . . . . .  20
   5.3  Encapsulation Options in Connection Mode . . . . . . . . . .  20
   6.   Advanced Protocol Features . . . . . . . . . . . . . . . . .  28
   6.1  Route Changes  . . . . . . . . . . . . . . . . . . . . . . .  28
   6.2  Policy-Based Forwarding  . . . . . . . . . . . . . . . . . .  31
   6.3  Route Recording  . . . . . . . . . . . . . . . . . . . . . .  32
   6.4  NAT Traversal  . . . . . . . . . . . . . . . . . . . . . . .  32
   6.5  Interaction with IP Tunnelling . . . . . . . . . . . . . . .  33
   6.6  IPv4-IPv6 Transition and Interworking  . . . . . . . . . . .  34
   7.   Security Considerations  . . . . . . . . . . . . . . . . . .  36
   7.1  Message Confidentiality and Integrity  . . . . . . . . . . .  36
   7.2  Peer Node Authentication . . . . . . . . . . . . . . . . . .  37
   7.3  Routing State Integrity  . . . . . . . . . . . . . . . . . .  37
   7.4  Denial of Service Prevention . . . . . . . . . . . . . . . .  38
   8.   Open Issues  . . . . . . . . . . . . . . . . . . . . . . . .  40
   8.1  Protocol Naming  . . . . . . . . . . . . . . . . . . . . . .  40
   8.2  General IP Layer Issues  . . . . . . . . . . . . . . . . . .  40
   8.3  Encapsulation and Addressing for Datagram Mode . . . . . . .  40
   8.4  Intermediate Node Bypass and Router Alert Values . . . . . .  42
   8.5  Messaging Association Flexibility  . . . . . . . . . . . . .  43
   8.6  Messaging Association Setup Message Sequences  . . . . . . .  43
   8.7  Connection Mode Encapsulation  . . . . . . . . . . . . . . .  45
   8.8  GIMPS State Teardown . . . . . . . . . . . . . . . . . . . .  45
   8.9  Datagram Mode Retries and Single Shot Message Support  . . .  45
   8.10 GIMPS Support for Message Scoping  . . . . . . . . . . . . .  46
   8.11 Mandatory or Optional Reverse Routing State  . . . . . . . .  46
   8.12 Additional Discovery Mechanisms  . . . . . . . . . . . . . .  47
   8.13 Protocol Design Details  . . . . . . . . . . . . . . . . . .  47
        Normative References . . . . . . . . . . . . . . . . . . . .  49
        Informative References . . . . . . . . . . . . . . . . . . .  50
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  51
   A.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  52
   B.   Example Routing State Table  . . . . . . . . . . . . . . . .  53
   C.   Connection Mode Messaging Association Configurations . . . .  54
        Intellectual Property and Copyright Statements . . . . . . .  55

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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.

   The case of path-coupled signaling 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 router on the path to
   participate. Path-coupled signaling thus excludes end-to-end
   higher-layer application signaling (except as a degenerate case); an
   example of such application signaling would be ISUP (telephony
   signaling for Signaling System #7) messages being transported by SCTP
   between two nodes.

   In the context of path-coupled signaling, examples of state
   management include network resource allocation (for "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
   path-coupled signaling is referred to as a signaling application.

   Every signaling application involves 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 applications, and hence should be developed as a common
   standard. The framework given in [18] 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).

   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 Generic Internet Messaging Protocol for
   Signaling (GIMPS). Different signaling applications may make use of
   different services provided by GIMPS, but GIMPS does not handle
   signaling application state itself; in that crucial respect, it
   differs from application signaling protocols such as the control
   component of FTP, SIP and RTSP. Instead, GIMPS 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

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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 [2].

   The terminology used in this specification is fully defined in this
   section. The basic entities relevant at the GIMPS level are shown in
   Figure 1.

                          GIMPS (adjacent) peer nodes
                           IP addresses = Signaling
   IP address            Source/Destination Addresses         IP address
   = Flow             (depending on signaling direction)      = Flow
   Source                   |                   |             Destination
   Address                  |                   |             Address
                            V                   V
   +--------+  Data     +------+             +------+         +--------+
   |  Flow  |-----------|------|-------------|------|-------->|  Flow  |
   | Sender |  Flow     |      |             |      |         |Receiver|
   +--------+           |GIMPS |============>|GIMPS |         +--------+
                        | Node |<============| Node |
                        +------+  Signaling  +------+

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

                      Figure 1: Basic Terminology

   [Data] Flow: A set of packets following a unique path through the
      network, identified by some fixed combination of header fields.
      Only unicast, unidirectional flows are considered.

   Session: A single application layer flow of information for which
      some network control state information is to be manipulated or
      monitored. IP mobility may cause the mapping between sessions and
      flows to change, and IP multihoming may mean there is more than
      one flow for a given session.

   [Flow] Sender: The node in the network which is the source of the
      packets in a flow. Could be a host or a router (if the flow is
      actually an aggregate).

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

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   Downstream: In the same direction as the data flow.

   Upstream: In the opposite direction to the data flow.

   GIMPS Node: Any node along the data path supporting GIMPS (regardless
      of what signaling applications it supports).

   Adjacent peer: The next GIMPS node along the data path, in the
      upstream or downstream direction. Whether two nodes are adjacent
      or not is determined implicitly by the GIMPS peer discovery
      mechanisms; it is possible for adjacencies to 'skip over'
      intermediate nodes if they have no interest in the signaling
      messages being exchanged.

   Datagram mode: A mode of sending GIMPS messages between nodes without
      using any transport layer state or security protection. Upstream
      messages are sent UDP encapsulated directly to the signaling
      destination; downstream messages are sent towards the flow
      receiver with a router alert option.

   Connection mode: A mode of sending GIMPS messages directly between
      nodes using point to point transport protocols and security
      associations, i.e. messaging associations (see below).

   Messaging association: A single connection between two explicitly
      identified GIMPS adjacent peers, i.e. between a given signaling
      source and destination address. A messaging association uses a
      specific transport protocol and known ports, and may be run over
      specific network layer security associations, or use a transport
      layer security association internally. A messaging association is

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3. Design Methodology

3.1 Overall Approach

   The generic requirements identified in [18] for transport of
   path-coupled signaling messages are essentially two-fold:

   "Routing": Determine how to reach the adjacent signaling node along
      the data path (the GIMPS peer);

   "Transport": Deliver the signaling information to that peer.

   To meet the routing requirement, for downstream signaling the node
   can either use local state information (e.g. gathered during previous
   signaling exchanges) to determine the identity of the GIMPS peer
   explicitly, or it can just send the signaling towards the flow
   destination address and rely on the peer to intercept it. For
   upstream signaling, only the first technique is possible.

   Once the routing decision has been made, the node has to select a
   mechanism for transport of the message to the peer. GIMPS divides the
   transport problems into two categories, the easy and the difficult
   ones. It handles the easy cases within GIMPS itself, avoiding
   complexity and latency, while drawing on the services of
   well-understood reliable transport protocols for the harder cases.
   Here, with details discussed later, "easy" messages are those that
   are sized well below the lowest MTU along a path, are infrequent
   enough not to cause concerns about congestion and flow control, and
   do not need transport or network-layer security support.

   In addition, in many cases, signaling information needs to be
   delivered reliably between GIMPS peers.  Some applications may
   implement their own reliability mechanism, but experience with RSVP
   has shown [12] that relying on soft-state refreshes itself may yield
   unsatisfactory performance if signaling messages are lost even
   occasionally. The provision of this type of reliability is also the
   responsibility of the underlying transport protocols.

   In [18] 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, as a specialised GIMPS 'messaging'
   layer running over standard transport protocols, as shown in Figure

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          ^^                       +-------------+
          ||                       |  Signaling  |
          ||          +------------|Application 2|
          ||          |  Signaling +-------------+
         NSIS         |Application 1|         |
       Signaling      +-------------+         |
      Application         |   +-------------+ |
         Level            |   |  Signaling  | |
          ||              |   |Application 3| |
          ||              |   +-------------+ |
          VV              |          |        |
          ^^          +----------------------------------------------+
          ||          |+-----------------------+      +------------+ |
          ||          ||         GIMPS         |      |   GIMPS    | |
          ||          ||     Encapsulation     |<<<>>>|Maintainance| |
         NSIS         |+-----------------------+      +------------+ |
       Transport      |GIMPS: messaging layer                        |
         Level        +----------------------------------------------+
       ("NTLP")               |       |       |       |
          ||                +----+  +----+  +----+  +----+
          ||                |UDP |  |TCP |  |SCTP|  |DCCP|....
          VV                +----+  +----+  +----+  +----+
                              |       |       |       |
                      |                      IP                      |

           Figure 2: Protocol Stacks for Signaling Transport

   For efficiency, GIMPS offers two modes of transport operation:

   Datagram mode: for small, infrequent messages with modest delay

   Connection mode: for larger data objects or where fast setup in the
      face of packet loss is desirable, or where channel security is

   The datagram mode uses a lower-layer unreliable unsecured datagram
   transport mechanism, with UDP as the initial choice. The connection
   mode can use any stream or message-oriented transport protocol,
   including TCP and SCTP. It may employ specific network layer security
   associations (using IPsec), or an internal transport layer security
   association (using TLS).

   It is possible to combine these two modes along a chain of nodes,

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   without coordination or manual configuration.  This allows, for
   example, the use of datagram mode at the edges of the network and
   connection 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 GIMPS are not totally independent. If the message transfer has
   performance requirements that enforce the use of connection mode
   (e.g. because fragmentation is required), this can only be used
   between explicitly identified nodes. In such cases, the GIMPS node
   must carry out signaling in datagram mode to identify the peer node
   and set up the necessary transport connection - even for downstream
   signaling, the datagram mode option of sending the message in the
   direction of the flow receiver and relying on interception is not

   In general, the state associated with connection 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". There may be any number of
   messaging associations between two GIMPS peers (although the usual
   case is 0 or 1), and they are set up and torn down by managment
   actions within the GIMPS layer itself.

3.2 Design Attributes

   Soft state: All parts of GIMPS state are subject to time-out
      ("soft-state"). Explicit removal is also logically possible (see
      Section 8.8. 'State' here includes the transport associations
      managed by GIMPS.

   Application-neutral: GIMPS is designed to support the largest range
      of signaling applications.  While a number of such applications
      have been identified, it appears likely that new applications will
      emerge.  (This was the case after the development of RSVP, for

   Mobility support: End systems can change their network attachment
      point and network address during a session. GIMPS minimises the
      use of IP addresses as identifiers for non-topological information
      (e.g. authentication state).

   Efficient: Signaling often occurs before an application such as an IP
      telephone conversation can commence, so that any signaling delay
      becomes noticeable to the application.  Signaling delays are
      incurred by the delay in finding signaling nodes along the path

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      (peer discovery), in retransmitting lost signaling messages and in
      setting up security associations between nodes, among other
      factors. GIMPS attempts to minimise these delays by several
      mechanisms, such as the use of high performance transport
      protocols to circumvent message loss, and the re-use of messaging
      associations to avoid setup latency. If discovery is needed it is
      a lightweight process which only probes local topology, and GIMPS
      also allows it to be bypassed completely for downstream datagram
      mode messages.

   IP version neutral: GIMPS supports both IPv4 and IPv6: it can use
      either for transport, largely as a result of their support in the
      underlying transport protocols, and can signal for either type of
      flow. In addition, GIMPS is able to operate on dual-stack nodes
      (to bridge between v4 and v6 regions) and also to operate across
      v4/v6 boundaries and other addressing boundaries. Specific
      transition issues are considered in Section 6.6.

   Transport neutral: GIMPS can operate over any message or
      stream-oriented transport layer, including UDP, DCCP, TCP and
      SCTP. Messages sent over protocols that do not offer a native
      fragmentation service, such as UDP, are strictly limited in size
      and rate to avoid network congestion and loss-amplification

   Proxy support: The end systems in a session may not be capable of
      handling either the signaling transport or the application and may
      instead rely on proxies to initiate and terminate signaling
      sessions. GIMPS decouples the operation of the messaging functions
      from the flow source and destination addresses, treating these
      primarily as data.

   Scaleable: As will be discussed in Section 4.3, up to one messaging
      association is generally kept for each next-hop node and thus
      transport state scales better than the number of sessions. (Many
      next-hops may not have transport state at all, if there are no
      messages on sessions visiting those nodes that warrant such
      treatment.) Transport associations are removed based on policy at
      each node, depending on trade-offs between fast peer-to-peer
      communication and state overhead. In short, transport state can be
      removed immediately after the last signaling session to a
      particular next-hop is removed, after some delay to wait for new
      sessions or only if resource demands warrant it.

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4. Overview of Operation

   This section describes the basic structure and operation of GIMPS. It
   is divided into three parts. Section 4.1 gives an overview of the
   per-flow and per-peer state that GIMPS maintains for the purpose of
   routing messages. Section 4.2 describes how messages are processed in
   the case where any necessary messaging associations and associated
   routing state already exist; this includes the simple scenario of
   pure datagram mode operation, where no messaging associations are
   necessary in the first place (equivalent to the transport
   functionality of base RSVP as defined in [8].) Section 4.3 describes
   how routing state is maintained and how messaging associations are
   initiated and terminated.

4.1 GIMPS State

   For each flow, the GIMPS 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 {flow
   routing info, session id, signaling application id}:

   Flow: the header N-tuple that determines the route taken by the flow
      through the network; in particular, including the flow destination

   Session: a cryptographically random and (probabilistically) globally
      unique identifier of the application layer session that is using
      the flow. For a given flow, different signaling applications may
      or may not use the same session identifier. Often there will only
      be one flow for a given session, but in mobility/multihoming
      scenarios there may be more than one and they may be differently

   Signaling application: an IANA assigned identifier of the signaling
      application which is generating messages for this flow. The
      inclusion of this identifier allows the routing state to be
      different for different signaling applications (e.g. because of
      different adjacencies or different messaging requirements).

   For a given flow and signaling application there can only be a single
   row in the table.

   The state information for a given key is as follows:

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   Upstream peer: the adjacent GIMPS peer closer to the flow source for
      this flow and signaling application. This could be an IP address
      (learned from previous signaling) or a pointer to a messaging
      association. It could also be null, if this node is the flow
      sender or this node is not storing reverse routing state, or a
      special value to indicate that this node is the last upstream node
      (but not the sender).

   Downstream peer: the adjacent GIMPS peer closer to the flow
      destination for this flow and signaling application. This could be
      a pointer to a messaging association, or it could be null, if this
      node is the flow receiver or this node is only sending downstream
      datagram mode messages for this flow and signaling application, or
      a special value to indicate that this node is the last downstream
      node (but not the receiver).

   Note that both the upstream and downstream peer state may be null,
   and the session identifier information is not required for message
   processing; in that case, no state information at all needs to be
   stored in the table. Both items of peer identification state have
   associated timers for how long the identification can be considered
   accurate; when these timers expire, the peer identification (IP
   address or messaging association pointer) is purged if it has not
   been refreshed. An example of a routing state table for a simple
   scenario is given in Appendix B.

   Note also that the information is described as a table of flows, but
   that there is no implied constraint on how the information is stored.
   For example, in a network using pure destination address routing
   (without load sharing or any form of policy-based forwarding), the
   downstream peer information might be possible to store in an
   aggregated form in the same manner as the IP forwarding table. In
   addition, many of the per-flow entries may point to the same per-peer
   state (e.g. the same messaging association) if the flows go through
   the same adjacent peer.

   The per-flow message routing state is not the only state stored by
   GIMPS. There is also the state implied in the messaging associations.
   Since we assume that these associations are typically per-peer rather
   than per-flow, they are stored separately. As well as the messaging
   association state itself, this table also stores per-association
   information including:

   o  messages pending transmission while an association is being

   o  an inactivity timer for how long the association has been idle.

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4.2 Basic Message Processing

   This section describes how signaling application messages are
   processed in the simple case where any necessary messaging
   associations and routing state are already in place. The description
   is divided into several parts: message reception, local processing
   and delivery to signaling applications, and message transmission. An
   overview is given in Figure 3.

       |            Signaling Application Processing             |
       |                                                         |
                  ^                                   V
                  ^           NSLP Payloads           V
                  ^                                   V
       |                    GIMPS Processing                     |
       |                                                         |
          x           u  d                     d  u           x
          x           u  d>>>>>>>>>>>>>>>>>>>>>d  u           x
          x           u  d      Bypass at      d  u           x
       +--x-----+  +--u--d--+  GIMPS level  +--d--u--+  +-----x--+
       | C-mode |  | D-mode |               | D-mode |  | C-mode |
       |Handling|  |Handling|               |Handling|  |Handling|
       +--x-----+  +--u--d--+               +--d--u--+  +-----x--+
          x          u   d                     d   u          x
          x    uuuuuu    d>>>>>>>>>>>>>>>>>>>>>d    uuuuuu    x
          x   u          d      Bypass at      d          u   x
       +--x--u--+  +-----d--+    router     +--d-----+  +--u--x--+
       |IP Host |  |  RAO   |  alert level  |  RAO   |  |IP Host |
       |Handling|  |Handling|               |Handling|  |Handling|
       +--x--u--+  +-----d--+               +--d-----+  +--u--x--+
          x  u           d                     d           u  x
       +--x--u-----------d--+               +--d-----------u--x--+
       |      IP Layer      |               |      IP Layer      |
       |   (Receive Side)   |               |  (Transmit Side)   |
       +--x--u-----------d--+               +--d-----------u--x--+
          x  u           d                     d           u  x
          x  u           d                     d           u  x
          x  u           d                     d           u  x

            uuuuuuuuuuuuuu = upstream datagram mode messages
            dddddddddddddd = downstream datagram mode messages
            xxxxxxxxxxxxxx = connection mode messages
                       RAO = Router Alert Option

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              Figure 3: Message Paths through a GIMPS Node

   Note that the same messages are also used for internal GIMPS state
   maintenance operations. The state maintenance takes place as a result
   of processing specific GIMPS payloads in these messages. The
   processing of these payloads is the subject of Section 4.3.

   Message Reception: Messages can be received in connection or datagram
      mode, and from upstream or downstream peers.

      Reception in connection mode is simple: incoming packets undergo
      the security and transport treatment associated with the messaging
      association, and the messaging association provides complete
      messages to the GIMPS layer for further processing.

      Reception in datagram mode depends on the message direction.
      Upstream messages (from a downstream peer) will arrive UDP
      encapsulated and addressed directly to the receiving signaling
      node. Each datagram contains a single complete message which is
      passed to the GIMPS layer for further processing, just as in the
      connection mode case.

      Downstream datagram mode messages are UDP encapsulated and
      addressed to the flow receiver with an IP router alert option to
      cause interception. The signaling node will therefore 'see' all
      such messages, including those in which it is not interested.
      There are then two cases. If the node categorises the message as
      'not interesting', it is passed on for message transmission
      without further processing (criteria and mechanisms for
      categorisation are discussed in Section 8.4). If the message is
      determined to be interesting, it is passed up to the GIMPS layer
      for further processing as in the other cases.

   Local Processing: Once a message has been received, by any method, it
      is processed locally within the GIMPS layer. The GIMPS processing
      to be done depends on the payloads carried; most of the
      GIMPS-internal payloads are associated with state maintenance and
      are covered in the next subsection.

      One GIMPS-internal payload which can be carried in any message and
      requires processing is the GIMPS hop count. This is decremented on
      input processing, and checked to be greater than zero on output
      processing. The primary purpose of the GIMPS hop count is to
      prevent message looping.

      The remainder of the GIMPS message consists of an NSLP payload.
      This is delivered locally to the signaling application identified
      at the GIMPS level; the format of the NSLP payload is not

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      constrained by GIMPS, and the content is not interpreted.

      Signaling applications can generate their messages for
      transmission, either asynchronously, or in response to an input
      message. Messages may also be forwarded in the GIMPS layer if
      there is no appropriate local signaling application to process
      them. Regardless of the source, outgoing messages are passed
      downwards for message transmission processing.

   Message Transmission: When a message is available for transmission,
      GIMPS 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 GIMPS or signaling application levels.

      The main decision is whether the message must be sent in
      connection mode or datagram mode. Reasons for using the former
      could be:

      *  NSLP requirements: for example, the signaling application has
         requested channel secured delivery, or reliable delivery;

      *  protocol specification: for example, this document could
         specify that a message that requires fragmentation MUST be sent
         over a messaging association;

      *  local GIMPS policy: for example, a node may prefer to send
         messages over a messaging association to benefit from
         congestion control.

      In principle, as well as determining that some messaging
      association must be used, GIMPS could select between a set of
      alternatives, e.g. for load sharing or because different messaging
      associations provide different transport or security attributes
      (see Section 8.5 for further discussion).

      If a messaging association is not required, the message is sent in
      datagram mode. The processing in this case depends on whether the
      message is directed upstream or downstream.

      *  If the upstream peer IP address is available from the per-flow
         routing table, the message is UDP encapsulated and sent
         directly to that address. Otherwise, the message cannot be
         forwarded (i.e. this is an error condition).

      *  In the downstream direction, messages can always be sent. They
         are simply UDP encapsulated and IP addressed to the flow
         receiver, with the appropriate router alert option.

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      If the use of a messaging association is selected, the message is
      queued on the association (found from the upstream or downstream
      peer state table), and further output processing is carried out
      according to the details of the protocol stack used for the
      association. If no appropriate association exists, the message is
      queued while one is created (see next subsection). If no
      association can be created, this is again an error condition.

4.3 Routing State and Messaging Association Maintainance

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

   The complete sequence of possible messages for state setup between
   adjacent peers is shown in Figure 4 and described in detail in the
   following text.

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          +----------+              +----------+
          | Querying |              |Responding|
          |   Node   |              |   Node   |
          +----------+              +----------+
                   Router Alert Option
                 Flow/Session/Sig App ID
                Querying Node Addressing
            [Query cookie, response request]
                     [NSLP Payload]

                 Flow/Session/Sig App ID
                      Query cookie
                Responder node addressing
           [Responder cookie, response request]
                     [NSLP Payload]

               Messaging Association Setup

                     Final handshake
                 Flow/Session/Sig App ID
                    Responder cookie
                 Querying Node Addressing
                     [NSLP Payload]

               Figure 4: Message Sequence at State Setup

   The initial message in any routing state maintenance operation is a
   downstream datagram mode message, sent from the querying node and
   intercepted at the responding node. This is encapsulated and
   addressed just as in the normal case; in particular, it has
   addressing and other identifiers appropriate for the flow and
   signaling application that state maintenance is being done for, and
   it is allowed to contain an NSLP payload. Processing at the querying
   and responding nodes is also essentially the same. However, the
   querying node includes additional payloads: its own address
   information, and optionally a 'discover-query' payload, which
   contains a response request flag and a query cookie. This message is
   informally referred to as a 'GIMPS-query', although it is just a
   normal message with a particular payload set.

   In the responding node, the GIMPS level processing of the
   discover-query payload triggers the generation of a 'GIMPS-response'

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   message. This is also a normally encapsulated and addressed message
   with particular payloads, this time in the upstream direction. Again,
   it can contain an NSLP payload (possibly a response to the NSLP
   payload in the initial message). It includes its own addressing
   information and the query cookie, and optionally a
   'discover-response' payload, which contains another response request
   flag and a responder cookie. Note that if a messaging association
   already exists towards the querying node, this can be used to deliver
   the GIMPS-response message; otherwise, datagram mode is used.

   The querying node installs the responder address as downstream peer
   state information after verifying the query cookie in the
   GIMPS-response. The responding node can install the querying address
   as upstream peer state information after the receipt of the initial
   GIMPS-query, or after a third message in the downstream direction
   containing the responder cookie. The detailed constraints on
   precisely when state information is installed are driven by security
   considerations on prevention of denial-of-service attacks and state
   poisoning attacks, which are discussed further in Section 7.

   Setup of messaging associations begins when both downstream peer
   addressing information is available and a new messaging association
   is actually needed. (In many cases, the GIMPS-response message above
   will identify a downstream peer for whom an appropriate messaging
   association already exists, in which case no further GIMPS management
   is needed.) Setup of the messaging association always starts from the
   upstream node, but once the querying node has sent any queued
   signaling application messages it can be used equally in both

   Refresh and expiration of all types of state is controlled by timers.
   State in the routing table has a per-flow, per-direction timer, which
   expires after a routing state lifetime (RSL). It is the
   responsibility of the querying node to generate a GIMPS-query
   message, optionally with a discover-query payload, before this timer
   expires, if it believes that the flow is still active. Receipt of the
   message at the responding node will refresh upstream peer addressing
   state, and receipt of a GIMPS-response at the querying node will
   refresh any downstream peer addressing state if it exists. Note that
   nodes do not control the refresh of upstream peer state themselves,
   they are dependent on the upstream peer to manage this.

   Messaging associations can be managed by either end. Management
   consists of tearing down unneeded associations. Whether an
   association is needed is a local policy decision, 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 are flows still in place which might

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   generate messages that would use it). Because messaging associations
   can always be set up on demand, and messaging association status is
   not made directly visible outside the GIMPS layer, even if GIMPS
   tears down and later re-establishes a messaging association,
   signaling applications cannot distinguish this from the case where
   the association is kept permanently open.

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5. Message Formats and Encapsulations

5.1 Message Formats

   All GIMPS messages begin with a common header which includes a
   version number field. For messages sent over a messaging association,
   it may also include transport protocol specific format information as
   discussed in Section 5.3.1. The remainder of the message is is
   encoded in an RSVP-style format, i.e., consisting of
   type-length-value (TLV) objects. A later version of this
   specification will contain more details on rules for object encodings
   which enable protocol extensibility.

   These items are contained in each GIMPS message:

   Flow routing information: Information sufficient to define the route
      that the flow will take through the network. 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 used (see Section 6.2 and Section 6.4 for
      further discussion). This object also contains a flag to indicate
      whether the signaling message is being sent upstream or downstream
      with respect to the flow.

   Session identifier: The GIMPS session identifier is a long,
      cryptographically random identifier chosen by the initiator.  The
      length is open, but 128 bits should be more than sufficient to
      make the probability of collisions orders of magnitude lower than
      other failure reasons.

   Signaling application identifier: This describes the signaling
      application, such as resource reservation or firewall control.

   GIMPS Hop counter: A hop counter prevents a message from looping
      indefinitely. (Since messages may get translated between different
      lower-layer transport protocols, the IP hop count cannot be relied

   The following items are optional:

   Lifetime: The lifetime of a routing state in the absence of
      refreshes, measured in seconds. Defaults to 30 seconds.

   Node addressing: Minimally, this is the IP address at which the GIMPS
      node originating the message can be reached; this will be used to
      fill in peer routing state. Additional information can be provided
      on node capabilities and policy on messaging association
      management, as well as currently available associations. The level

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      of flexibility required in this field is discussed in Section 8.5.

   Cookies: A query-cookie is optional in a GIMPS-query message and if
      present must be echoed in a GIMPS-response; A response-cookie is
      optional in a GIMPS-response message, and if present must be
      echoed in the following downstream message. The optional cookies
      and response request flags make up the discover-query and
      discover-response payloads. Cookies are X-octets long and need to
      be designed so that a node can determine the validity of a cookie
      without keeping state.

   Message identifier: A four-octet message counter, used to associate
      GIMPS messages with their confirmations. The initial use for this
      is in state maintenance exchanges (GIMPS-query/response); possible
      use to provide a simple one-off reliable exchange for signaling
      application messages is considered in Section 8.9.

   NSLP Payload: The payload carries one or more NSLP objects. GIMPS
      does not interpret the payload content.

5.2 Encapsulation in Datagram Mode

   Encapsulation in datagram mode is simple. The complete set of GIMPS
   payloads for a single message is concatenated together with the
   common header, and placed in the data field of a UDP datagram. UDP
   checksums should be enabled. Upstream messages are addressed to the
   adjacent peer, downstream messages are addressed to the flow receiver
   and encapsulated with a Router Alert Option. Open issues about
   alternative encapsulations, addressing possibilities, and router
   alert option value field setting are discussed in Section 8.2,
   Section 8.3 and Section 8.4 respectively.

   The source UDP port is selected by the message sender. A destination
   UDP port should be allocated by IANA. Note that GIMPS may send
   messages addressed as {flow sender, flow receiver} which could make
   their way to the flow receiver even if that receiver were
   GIMPS-unaware. This should be rejected (with an ICMP message) rather
   than delivered to another application. Therefore, a well-known port
   would seem to be required.

5.3 Encapsulation Options in Connection Mode

   Encapsulation in connection mode is more complex, firstly because of
   the increased demands on transport functionality. This issue is
   treated in Section 5.3.1. In addition, a consequence of the
   restriction to the use of existing transport and security protocols
   is that messaging associations must run between fixed nodes. However,

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   there are still several options for how the IP packets for the
   messaging association should be carried on the wire between these
   nodes. These alternatives have different functionality, flexibility
   and performance tradeoffs, and are considered in Section 5.3.2 -
   Section 5.3.4.

5.3.1 General Considerations

   It is a general requirement of the NTLP defined in [18] 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.

   SCTP [5] satisfies all requirements. (The bundling requirement is met
      implicitly by the use of Nagle-like algorithms inside the SCTP

   DCCP [6] is message based but does not provide bundling or
      fragmentation. These could both be provided by an additional
      (simple) encapsulation above the DCCP level.

   TCP provides both bundling and fragmentation, but not message
      boundaries. Again, this could be done with a simple length field
      embedded in the TCP data stream to mark each message.

   UDP could be augmented as in the DCCP case. (However, note that it
      should probably not be used for messages requiring fragmentation,
      because of UDP's lack of congestion control functionality.)

   If it is desired to support all these protocol options, it is
   probably easiest to handle this as the addition of transport protocol
   specific shim layers at the top of the messaging association, so that
   the GIMPS layer sees a uniform functionality transport service
   interface. The control information for these shim layers would be
   carried inside the GIMPS fixed header.

5.3.2 Raw Encapsulation

   The simplest encapsulation is to carry the GIMPS and signaling
   application payloads directly in the transport protocol, and to carry
   that protocol on the wire in the normal way. This situation is shown
   in Figure 5; it applies to both upstream and downstream messages.

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      |                  L2 Header                  |
      |                  IP Header                  |   ^
      |      Source address = signaling source      |   ^
      | Destination address = signaling destination |   .
      +---------------------------------------------+   .
      |                  L4 Header                  |   . ^
      |     (Standard TCP/SCTP/DCCP/UDP header)     |   . ^
      +---------------------------------------------+   . .
      |             GIMPS Fixed Header              |   . . ^
      |   (Optional shim fields, depending on L4)   |   . . ^  Scope of
      +---------------------------------------------+   . . .  security
      |               GIMPS Payloads                |   . . . protection
      |(Sequence of TLV Elements, as in Section 6.1)|   . . . (depending
      +---------------------------------------------+   . . . on channel
      |                NSLP Payloads                |   . . .  security
      |              (Opaque to GIMPS)              |   V V V  mechanism
      +---------------------------------------------+   V V V   in use)

           Figure 5: Raw Messaging Association Encapsulation

   The advantages and disadvantages of this approach are as follows:

   +) Practically all transport and security protocols can be used.

   +) Use of existing protocol stack implementations is simple.

   +) There is very little protocol overhead.

   -) In order to do any processing on any of the GIMPS or signaling
      application payloads, the complete messaging association must be
      terminated at the processing node. This applies even if the
      processing to be done is very lightweight (e.g. an address
      translation) and/or the node does not support the signaling
      application in question. This property makes it hard to offer
      useful transport and security properties between signaling
      application peers which need them, particularly flow control/
      reliable delivery and signaling application payload
      confidentiality and integrity.

   -) More generally, for a given signaling application on two nodes,
      there is very little room for flexibility in the NSIS processing
      at nodes in between. Nodes have to process everything or nothing,
      igoring the application completely.

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   -) Signaling messages are only ever delivered between peers
      established in GIMPS-query/response exchanges. Any route change is
      not detected until another GIMPS-query/response procedure takes
      place; in the meantime, signaling messages are misdelivered.

   The raw encapsulation is most appropriate (has very few
   disadvantages) in an environment where routes are stable and the
   signaling nodes are highly homogeneous (all nodes have identical
   processing requirements). A scenario which illustrates the problems
   that arise in more heterogeneous environments is given in Appendix C.

5.3.3 Peer-Peer Tunnelled Encapsulation

   In order to allow intermediate nodes to do limited processing of some
   GIMPS payloads without interfering with the overall transport and
   security service being provided by the messaging association, it is
   necessary to carry some of the payloads outside the messaging
   association protocols. One method for doing this is shown in Figure
   6; it also applies to both upstream and downstream messages.

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      |                  L2 Header                  |
      |                  IP Header                  |
      |      Source address = signaling source      |<---+
      | Destination address = signaling destination |    |
      +---------------------------------------------+    |  Repeated
      |             Router Alert Option             |    |  IP header
      +---------------------------------------------+    |
      |                 UDP Header                  |    |
      |    (Possibly using datagram mode ports)     |    |
      +---------------------------------------------+    |
      |      Mutable GIMPS Header and Payloads      |    |
      |     (Subset of full GIMPS payload set)      |    |
      +---------------------------------------------+    |
      |                  IP Header                  |    |  ^
      |      Source address = signaling source      |<---+  ^
      | Destination address = signaling destination |       .
      +---------------------------------------------+       .
      |                  L4 Header                  |       .
      |     (Standard TCP/SCTP/DCCP/UDP header)     |       .Similar
      +---------------------------------------------+       .to 'raw'
      |             GIMPS Fixed Header              |       .  case
      |   (Optional shim fields, depending on L4)   |       .
      +---------------------------------------------+       .
      |       'End to end' GIMPS Payloads           |       .
      |(Sequence of TLV Elements, as in Section 6.1)|       .
      +---------------------------------------------+       .
      |                NSLP Payloads                |       .
      |              (Opaque to GIMPS)              |       V
      +---------------------------------------------+       V

   Figure 6: Peer-Peer Tunnelled Messaging Association Encapsulation

   The 'raw' encapsulation is carried within a tunnel; the outer tunnel
   encapsulation is similar to datagram mode (except with fixed
   addresses for both the upstream and downstream directions).
   Advantages and disadvantages are as follows:

   +) Any message-based transport and security protocol can be used.

   +) Intermediate nodes (between the signaling source and destination,
      which are assumed to be peers processing the full signaling
      application) can intercept the packet because of the router alert
      option and process the 'mutable' GIMPS payloads. This allows for
      lighter-weight intermediate nodes. To avoid disrupting the
      operation of the transport protocols, this processing should be
      restricted to operations that can be carried out on a 'fast

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      signaling path', for example avoiding AAA operations.

   +) However, reliability and other transport functions continue to run
      between the 'outer' signaling application peers, and channel
      security also applies (except of course for the mutable fields)
      between them.

   +) In principle, signaling application payloads could also be carried
      in the 'mutable' area.

   -) Stream-based transport and security protocols would be hard to use
      in this way (i.e. ruling out TCP/TLS).

   -) Integration of existing protocol stacks implementations is more
      complex. When a message is sent or received at the messaging
      association endpoints, the mutable field contents have to be
      provided as ancilliary data somehow.

   -) The mutable area is vulnerable to spoofing and interception.
      (However, such attacks can only be launched by on-path nodes.)

   -) There is some additional protocol overhead, mainly the repeated IP
      header. (It might be possible to omit this.)

   -) This encapsulation is 'blind' to route changes as in the raw case.

   -) If policy-based forwarding is in use between the signaling source
      and destination, the intermediate nodes traversed may not be the
      correct ones for the flow path, because the outer IP header does
      not match the flow packet IP header. (This could be handled with
      the functionality of Section 6.3.)

   The application of this method to a heterogeneous signaling scenario
   is shown in Appendix C. This tunnelled encapsulation is conceptually
   similar to the use of a new IP option to carry the mutable fields.
   However, the use of a new IP option format would be problematic for
   IPv4; for IPv6, we would either have to allow the intermediate nodes
   to process destination options on seeing the router alert, or use a
   hop-by-hop option, with similar effects to the IPv4 case. Although
   the cost is a duplicate IP header, the UDP tunnelling method seems
   more attractive.

5.3.4 End-to-End Tunnelled Encapsulation

   In order to get round the need for parallel route change discovery,
   an alternative tunnel encapsulation can be considered for downstream
   signaling messages. One method for doing this is shown in Figure 7.
   (The corresponding upstream encapsulation would be the point-to-point

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   tunnel mode.)

      |                  L2 Header                  |
      |                  IP Header                  |
      |        Source address = flow sender         |<---+
      |   Destination address = flow receiver       |    |
      +---------------------------------------------+    |  Different
      |             Router Alert Option             |    |  IP header
      +---------------------------------------------+    |
      |                 UDP Header                  |    |
      |    (Possibly using datagram mode ports)     |    |
      +---------------------------------------------+    |
      |      Mutable GIMPS Header and Payloads      |    |
      |     (Subset of full GIMPS payload set)      |    |
      +---------------------------------------------+    |
      |                  IP Header                  |    |  ^
      |      Source address = signaling source      |<---+  ^
      | Destination address = signaling destination |       .
      +---------------------------------------------+       .
      |                  L4 Header                  |       .
      |     (Standard TCP/SCTP/DCCP/UDP header)     |       .Similar
      +---------------------------------------------+       .to 'raw'
      |             GIMPS Fixed Header              |       .  case
      |   (Optional shim fields, depending on L4)   |       .
      +---------------------------------------------+       .
      |       'End to end' GIMPS Payloads           |       .
      |(Sequence of TLV Elements, as in Section 6.1)|       .
      +---------------------------------------------+       .
      |                NSLP Payloads                |       .
      |              (Opaque to GIMPS)              |       V
      +---------------------------------------------+       V

   Figure 7: End-to-End Tunnelled Messaging Association Encapsulation

   The sole change is the use of flow sender and receiver addresses in
   the outer IP header. This has a number of significant implications
   compared to the previous case:

   +) Route changes are automatically detected by messages sent in the
      downstream direction. The receiving node will determine that the
      messaging association headers (in fact, the inner IP header) is
      incorrect and can notify the signaling source that a discovery
      exchange is needed.

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   +) Intermediate nodes can still process the 'mutable' GIMPS payloads
      without breaking end-to-end security and transport properties of
      the messaging association.

   +) Again, signaling application payloads could also be carried in the
      'mutable' area.

   +) Policy-based forwarding will be automatically handled in the
      downstream direction.

   -) Stream-based transport and security protocols would be hard to use
      in this way (i.e. ruling out TCP/TLS). In addition, because some
      messages will be 'lost' from the messaging association on a route
      change, the protocol must allow for partial reliability (e.g.
      PR-SCTP [7] rather than full SCTP, also allowing only IPsec and
      not TLS for channel security.)

   -) For the same reason, the congestion processing of the transport
      protocols becomes more complex, because there is an additional
      source of non-congestive message loss which the transport protocol
      itself is not aware of.

   -) Integration of existing protocol stacks implementations is as
      complex as in the peer-to-peer case, and there is the same
      additional protocol overhead.

   The attractiveness of this encapsulation depends on the perceived
   value of the automatic route change detection. If route change
   detection is primarily done by tracking events outside GIMPS itself,
   background rediscovery at a lower rate is no longer a significant
   overhead, and the advantages over peer-to-peer tunnelling are

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6. Advanced Protocol Features

6.1 Route Changes

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

6.1.1 Route Change Detection

   There are three primary mechanisms for a GIMPS node to detect that
   the downstream flow path has changed:

   1.  In trigger mode, a node finds out that the next hop has changed.
       This is the RSVP trigger mechanism where some form of
       notification mechanism from the routing table to the protocol
       handler is assumed. Clearly, in RSVP and GIMPS, this only works
       if the routing change is local, not if the routing change happens
       somewhere a few routing hops away.

   2.  An extended trigger, where the node checks the link-state routing
       table to discover that the path has changed.  This makes certain
       assumptions on consistency of route computation (but you probably
       need to make those to avoid routing loops) and only works
       intra-domain and for OSPF and similar link-state protocols.
       Where available, this offers the most accurate and expeditious
       indication of route changes, but requires more access to the
       routing internals than a typical OS may provide.

   3.  In probing mode, each GIMPS node periodically repeats the
       discovery operation.  This is similar to RSVP behavior, except
       that there is an extra degree of freedom since not every message
       needs to repeat the discovery, depending on the likely stability
       of routes.  All indications are that leaving mobility aside that
       routes are stable for hours and days, so this may not be
       necessary on a 30-second interval.  (Sender mobility is handled
       differently issue since the signaling message will visit "virgin"
       territory, rather than nodes with existing sessions.)

   For the first two mechanisms, once the change has been detected, the
   discovery-query/response sequence is then triggered at the detecting
   node (for the third case this sequence is part of the detection

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   mechanism anyway). This propagates downstream until it rejoins the
   old path; the node where this happens may also have to carry out
   local repair actions.

   In addition, there are several semi-heuristic techniques to detect
   that the flow path might have changed, based on monitoring changes in
   flow or signaling packet behaviour at downstream nodes (these are
   discussed in [18]). These are mainly implementation options at the
   detecting node; exploiting them might require an additional GIMPS
   payload to be sent in the upstream direction with the semantics 'the
   route for this flow may have changed', hence triggering discovery

6.1.2 Local Repair Storms

   Even where a node has detected a route change in the downstream
   direction, there are still two possible cases:

   o  the detecting node may be the true crossover router, i.e. the
      point in the network where old and new paths diverge, or

   o  the path change may actually have taken place upstream of the
      detecting node (so that this node is no longer on the path at all)

   In some circumstances, it is hard to distinguish these cases; an
   example is shown in Figure 8. Here, after the link failure, node A is
   the true crossover, but nodes B1/C1/D1 may all also initiate local
   repair operations. A later version of this document will consider how
   to control this.

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               x  +--+        +--+        +--+  x      Initial
              x  .|B1|_.......|C1|_.......|D1|   x     Configuration
             x  . +--+.      .+--+.      .+--+\.  x
            x  .       .    .      .    .       .  x
    >>xxxxxx  .         .  .        .  .         .  xxxxxx>>
         +-+ .           ..          ..           . +-+
    .....|A|/            ..          ..            .|E|_....
         +-+ .          .  .        .  .          . +-+
              .        .    .      .    .        .
               .      .      .    .      .      .
                . +--+        +--+        +--+ .
                  +--+        +--+        +--+

                  +--+        +--+        +--+         Configuration
                 .|B1|........|C1|........|D1|         after failure
                . +--+       .+--+        +--+         of D1-E link
               .      \.    .     \.    ./
              .         .  .        .  .
         +-+ .           ..          ..             +-+
    .....|A|.            ..          ..            .|E|_....
         +-+\.          .  .        .  .          . +-+
    >>xxxxxx  .        .    .      .    .        .  xxxxxx>>
            x  .      .      .    .      .      .  x
             x  . +--+        +--+        +--+ .  x
              x  .|B2|_.......|C2|_.......|D2|/  x
               x  +--+        +--+        +--+  x

               ........... = physical link topology

               >>xxxxxxx>> = flow direction

               _.......... = indicates outgoing link
                             for flow xxxxxx given
                             by local forwarding table

                      Figure 8: A Re-Routing Event

6.1.3 Propagating Route Change Notifications

   A second problem that may occur with route change detection is that
   the detecting GIMPS node may not implement all the signaling
   applications that need to be informed. Therefore, the GIMPS node
   needs to be able to send a notification back along the unchanged path

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   to trigger the nearest signaling application aware node to take
   action. If multiple signaling applications are in use, it will be
   hard to define when to stop propagating this notification.

   If the intermediate node bypass capabilities described in Section
   4.3, Section 5.3 and Section 8.4 are fully used, one consequence is
   that this separation between the nodes which do route change
   detection and signaling application processing no longer occurs, so
   this problem in its raw form no longer arises. Conversely, the
   ability of GIMPS to detect route changes by purely local monitoring
   of forwarding tables is more limited. (This is probably an
   appropriate limitation of GIMPS functionality. If we need a protocol
   for distributing notifications about local changes in forwarding
   table state, a flow signaling protocol is probably not the right
   starting point.)

6.2 Policy-Based Forwarding

   Signaling messages almost by definition need to contain address and
   port information to identify the flow they are signaling for. We can
   divide this information into two categories:

   Flow Routing Information: This is the information needed to determine
      how a flow is routed within the network. Minimally it is the flow
      destination address, but to handle load sharing, QoS routing, and
      other forms of policy based forwarding it can be extended to
      include the full IPv4 5-tuple or IPv6 3-tuple, and possibly
      traffic class information as well. It is carried as an object in
      each GIMPS message (see Section 5.1).

   Additional Packet Classification Information: This is any further
      higher layer information needed to select a subset of packets for
      special treatment by the signaling application. The need for this
      is highly signaling application specific, and so this information
      is invisible to GIMPS (if indeed it exists).

   The correct pinning of signaling messages to the data path depends on
   how well the downstream messages in datagram mode can be made to be
   routed correctly. Two strategies are used:

      The messages themselves have the flow destination address, and
      possibly source address and traffic class (see Section 8.3 for
      further discussion). In many cases, this will cause the messages
      to be routed correctly even by GIMPS unaware nodes.

      A GIMPS aware node carrying out policy based forwarding on higher
      layer identifiers (in particular, the protocol and port numbers
      for IPv4) should take into account the entire flow routing

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      information object in selecting the outgoing interface rather than
      relying on the IP layer.

6.3 Route Recording

   The basic procedures of Section 4.3 describe how messages between
   adjacent GIMPS peers are used to fill in address information for use
   in subsequent message exchanges. When GIMPS messages only go between
   adjacent peers, this is all that is needed.

   However, the more flexible encapsulations of Section 5.3.3 and
   Section 5.3.4 allow signaling messages to be sent via intermediate
   GIMPS nodes which do some limited processing of GIMPS or signaling
   application payloads. It is necessary to ensure that the correct
   sequence of intermediate nodes is traversed, in the downstream
   direction and possibly also in the upstream direction. (For example,
   if the intermediate node is a NAT, payload translations must be made
   for both directions of signaling message.) However, this may not
   happen automatically, in the downstream direction because of policy
   based forwarding, and in the upstream direction because of asymmetric

   One solution for this problem is to generalise the notion of the
   upstream/downstream peer address into a sequence of addresses, i.e. a
   recorded route. This route can be built up in GIMPS-query messages:
   nodes which need to be kept on the signaling path but which do not
   wish to maintain per-flow forwards or reverse routing state can
   append their outgoing address to a GIMPS 'route record' payload, a
   generalisation of the 'peer addressing information' object of Section
   5.1. This is then stored at the receiver in place of the upstream
   peer address. The GIMPS-response message sent back can be source
   routed using this, and can gather another route record of the
   upstream path which replaces the downstream peer address at the
   querying node. The address manipulations at intermediate nodes are
   very similar to source routing in IPv4 or IPv6, and it might be
   possible to use these underlying protocol features instead of a
   specific GIMPS function.

6.4 NAT Traversal

   A already noted, GIMPS messages must carry packet addressing and
   higher layer information as payload data in order to define the flow
   signalled for. (This applies to all GIMPS 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 likewise translated,
   the signaling messages will refer to incorrect (and probably

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   meaningless) flows after passing through the boundary.

   The simplest solution to this problem is to require that a NAT is
   GIMPS aware, and to allow it to update/rewrite the flow routing
   information payload described in Section 6.2. (This is making the
   implicit assumption that NATs only rewrite the header fields included
   in this payload, and not 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. For this reason, the flow
   routing information is a candidate to be carried in the 'mutable'
   area if either tunnelled encapsulation for connection mode messages
   is used (Section 5.3.3 and Section 5.3.4), since then the NAT does
   not even have to terminate the transport association protocols. It
   does raise security issues about unauthorised modifications to this

   The case of traversing a GIMPS unaware NAT is for further study.
   There is a dual problem of whether the GIMPS 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 flow routing information
   from what is done to the signaling packet headers. The fundamental
   problem is that GIMPS mssages contain 3 or 4 interdependent addresses
   which all have to be consistently translated, and existing generic
   NAT traversal techniques such as STUN [17] can process only two.

6.5 Interaction with IP Tunnelling

   The interaction between GIMPS and IP tunnelling is very simple. An IP
   packet carrying a GIMPS 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 have the router alert option or the
   standard GIMPS protocol encapsulation (e.g. port numbers). They will
   not be identifiable as GIMPS messages until they leave the tunnel
   again. 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, we are assuming that 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, one tunnel endpoint must be signaling application aware.

   In some cases, it is the tunnel exit point (i.e. the node where

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   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. This information could be
   carried as additional data (an additional GIMPS payload) in the
   tunnelled signaling packets if the tunnel entry point was at least
   GIMPS aware. This payload would be the GIMPS equivalent of the RSVP
   SESSION_ASSOC object of [9]. Whether this functionality should really
   be part of GIMPS and if so how the payload should be handled will be
   considered in a later version.

6.6 IPv4-IPv6 Transition and Interworking

   GIMPS itself is essentially IP version neutral (version dependencies
   are isolated in the formats of certain GIMPS payloads, and GIMPS also
   depends on the version independence of the protocols that underlie
   messaging associations). In mixed environments, GIMPS operation will
   be influenced by the IP transition mechanisms in use. This section
   provides a high level overview of how GIMPS is affected, considering
   only the currently predominant mechanisms.

   Dual Stack: (This applies both to the basic approach described in
      [11] as well as the dual-stack aspects of more complete
      architectures such as [21].) In mixed environments, GIMPS should
      use the same IP version as the flow it is signaling for; hosts
      which are dual stack for applications and routers which are dual
      stack for forwarding should have GIMPS implementations which can
      support both IP versions.

      In theory, for some connection mode encapsulation options, a
      single messaging association could carry signaling messages for
      flows of both IP versions, but the saving seems of limited value.
      The IP version used in datagram mode is closely tied to the IP
      version used by the data flow, so it is intrinsically impossible
      for a IPv4-only or IPv6-only GIMPS node to support signaling for
      flows using the other IP version.

      Applications with a choice of IP versions might select a version
      for which GIMPS support was available in the network, which could
      be established by running parallel discovery procedures. In
      theory, a GIMPS message related to a flow of one IP version could
      flag support for the other; however, given that IPv4 and IPv6
      could easily be separately routed, the correct GIMPS peer for a
      given flow might well depend on IP version anyway, making this
      flagged information irrelevant.

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   Packet Translation: (Applicable to SIIT [4] and NAT-PT [10].) Some
      transition mechanisms allow IPv4 and IPv6 nodes to communicate by
      placing packet translators between them. From the GIMPS
      perspective, this should be treated essentially the same way as
      any other NAT operation (e.g. between 'public' and 'private'
      addresses) as described in Section 6.4. In other words, the
      translating node needs to be GIMPS aware; it will run GIMPS with
      IPv4 on some interfaces and with IPv6 on others, and will have to
      translate the flow routing information payload between IPv4 and
      IPv6 formats for flows which cross between the two. The
      translation rules for the fields in the payload (including e.g.
      traffic class and flow label) are as defined in [4].

   Tunnelling: (Applicable to 6to4 [13] and a whole host of other
      tunnelling schemes.) 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 overhead.

      From the GIMPS perspective, the treatment should be as similar as
      possible to any other IP tunnelling mechanism, as described in
      Section 6.5. 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. For example, [14] is
      based on using an anycast address as the destination tunnel
      endpoint. It might be unwise to carry out signaling for the tunnel
      to such an address, and the GIMPS implementation there would not
      be able to use it as a source address for its own signaling
      messages (e.g. GIMPS-responses). Further analysis will be
      contained in a future version of this specification.

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7. Security Considerations

   The security requirement for the GIMPS layer is to protect the
   signaling plane against identified security threats. For the
   signaling problem as a whole, these threats have been outlined in
   [19]; the NSIS framework [18] assigns a subset of the responsibility
   to the NTLP. The main issues to be handled can be summarised as:

   Message Protection: Signaling message content should be protected
      against interception, modification etc. while in transit. This
      applies both to GIMPS payloads, and GIMPS should also provide such
      protection as a service to signaling applications between adjacent

   State Integrity Protection: It is important that signaling messages
      are delivered to the right nodes (and not delivered to the wrong
      ones). GIMPS 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: GIMPS nodes and the network
      have finite resources (state storage, processing power,
      bandwidth). The protocol should try to minimise exhaustion attacks
      against these resources and not allow GIMPS nodes to be used to
      launch attacks on other network elements.

   The main missing 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, GIMPS 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 GIMPS protocol
   to be understood and verified.

7.1 Message Confidentiality and Integrity

   GIMPS can use messaging association functionality, such as TLS or
   IPsec, to ensure message confidentiality and integrity. In many
   cases, confidentiality of GIMPS 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.  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, particularly if this is provided efficiently
   and if it runs unbroken between signaling application peers.

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7.2 Peer Node Authentication

   Cryptographic protection (of confidentiality or integrity) typically
   requires session keys, which can established during authentication
   exchanges based on shared secrets or public key techniques.
   Authentication and key agreement is possible using the protocols
   associated with the messaging association being secured (TLS
   incorporates this functionality directly, IKE provides it for IPsec).
   GIMPS nodes rely on these protocols to authenticate the identity of
   the next hop, and GIMPS has no authentication capability of its own.

   However, with discovery, there are few effective ways to know what is
   the legitimate next or previous hop as opposed to an impostor. In
   other words, 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. the correct peer to carry out
   signaling with 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 7.3 below.

7.3 Routing State Integrity

   The internal state in a node (see Section 4.1), specifically the
   upstream and downstream peer identification, is used to route
   messages along the data flow path. If this state is corrupted,
   signaling messages may be misdirected.

   The routing state table is the local GIMPS view of what routes are
   being taken by flows through the network. Since routes are only
   weakly secured (e.g. there is usually no cryptographic binding of a
   flow to a route), and there is no other authoritative information
   about flow routes than the current state of the network itself.
   Therefore, consistency between GIMPS and network routing state has to
   be ensured by directly interacting with the routing mechanisms to
   ensure that the upstream and downstream signaling peers are the
   appropriate ones for any given flow. A good overview of security
   issues and techniques in this sort of context is provided in [20].

   Downstream peer identification is installed and refreshed only on
   receiving a GIMPS-reponse message. This must echo the cookie from a
   previous GIMPS-query message, which will have been sent downstream
   along the flow path (in datagram mode, 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.

   Upstream peer identification can be installed directly on receiving a

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   GIMPS-query message containing addressing information for the
   upstream peer. However, any node in the network could generate such a
   message (indeed, almost any node in the network could be the genuine
   upstream peer for a given flow). To protect against this, two
   strategies are possible:

   Filtering: the receiving node may be able to reject signaling
      messages which claim to be for flows with flow source addresses
      which would 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 datagram mode signaling packets. (If they are
      not, it is likely that at last one of the signaling or flow
      packets is being spoofed.) Signaling applications should only
      install state on the route taken by the signaling itself.

   Authentication: the receiving node may refuse to install upstream
      state until it has handshaked by some means with the upstream
      peer. This handshaking could be as simple as requesting the
      upstream peer to echo the response cookie in the discover-response
      payload of a GIMPS-response message (to discourage nodes
      impersonating upstream peers from using forged source addresses);
      or, it could be full peer authentication within the messaging
      association, the reasoning being that an authenticated peer can be
      trusted not to pretend to be on path when it is not.

   The second technique also plays a role in denial of service
   prevention, see below. In practice, a combination of both techniques
   may be appropriate.

7.4 Denial of Service Prevention

   GIMPS is designed so that each connectionless discovery message only
   generates at most one response, so that a GIMPS node cannot become
   the source of a denial of service amplification attack.

   However, GIMPS 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.  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 GIMPS-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
   GIMPS state to be established by bogus messages.  A related

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   computational/network-resource attack uses unverified messages to
   cause a node to make AAA queries or attempt to cryptographically
   verify a digital signature.  (RSVP is vulnerable to this type of

   There are at least three defenses against these attacks:

   1.  The responding node does not establish a session or discover its
       next hop on receiving the GIMPS-query message, but can wait for a
       setup message on a reliable channel.  If the reliable channel
       exists, the additional delay is a 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.

   2.  The response to the initial discovery message contains a cookie.
       The previous hop repeats the discovery with the cookie included.
       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 [5].)

   3.  If there is a chance that the next-hop node shares a secret with
       the previous hop, the sender could include a hash of the session
       ID and the sender's secret. The receiver can then verify that the
       message was likely sent by the purported source. This does not
       scale well, but may work if most nodes tend to communicate with a
       small peer clique of nodes. (In that case, however, they might as
       well establish more-or-less permanent transport sessions with
       each other.)

   These techniques are complementary; we chose a combination of the
   first and second method.

   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 additional denial of service
   attacks. GIMPS relies on the lower layer protocols that make up
   messaging associations to mitigate such attacks. The current
   description assumes that the upstream node is always the one wishing
   to establish a messaging association, so it is typically the
   downstream node that is protected. Extensions are considered in
   Section 8.6; these would require further analysis.

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8. Open Issues

8.1 Protocol Naming

   Alternate names:
   GIST: Generic Internet Signaling Transport
   GIMPS: Generic Internet Messaging Protocol for Signaling
   LUMPS: Lightweight Universal Messaging for Path associated Signaling

   There is a danger of some ambiguity as to whether the protocol name
   refers to the complete transport stack below the signaling
   applications, or only to the additional protocol functionality above
   the standard transport protocols (UDP, TCP etc.) The NSIS framework
   uses the term NTLP for the first, but this specification uses the
   GIST/variants names for the second (see Figure 2 in Section 3.1). In
   other words, this specification proposes to meet the requirements for
   NTLP functionality by layering GIMPS/... over existing standard
   transport protocols. It isn't clear if additional terminological
   surgery is needed to make this clearer.

8.2 General IP Layer Issues

   Some NSIS messages have to be addressed end-to-end but intercepted at
   intermediate routers, and this imposes some special constraints on
   how they can be encapsulated. RSVPv1 [8] primarily uses raw IP with
   specific protocol number (46); a UDP encapsulation is also possible
   for hosts unable to perform raw network i/o. RSVP aggregation [15]
   uses an additional protocol number (134) to bypass certain interior

   The critical requirements for the encapsulation at this level are
   that routers should be able to identify signaling packets for
   processing, and that they should not mis-identify packets for
   'normal' end-to-end user data flows as signaling packets. The current
   assumption is that UDP encapsulation can be used for such messages,
   by allocating appropriate (new) value codes for the router alert
   option (RAO) [1][3] to identify NSIS messages. Specific open issues
   about how to allocate such values are discussed in Section 8.4.

   An alternative approach would be to use raw IP with the RSVP protocol
   numbers and a new RSVP version number.

8.3 Encapsulation and Addressing for Datagram Mode

   The discussion in Section 4 essentially assumes that datagram mode
   messages are UDP encapsulated. This leaves open the question of
   whether other encapsulations are possible, and exactly how these
   messages should be addressed.

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   As well as UDP/IP (and raw IP as discussed and temporarily ruled out
   in Section 8.2), DCCP/IP and UDP/IPsec could also be considered as
   'datagram' encapsulations. However, they still require explicit
   addressing between GIMPS peer nodes and some per-peer state to be set
   up and maintained. Therefore, it seems more appropriate to consider
   these encapsulation options as possible messaging association types,
   for use where there is a need for congestion control or security
   protection but without reliability. This would leave UDP/IP as the
   single encapsulation allowed for all datagram mode messages.

   Addressing for upstream datagram mode messages is simple: the IP
   source address is the signaling source address, and the IP
   destination address is the signaling destination address (compare
   Figure 1). For downstream datagram mode messages, the IP destination
   address will be the flow destination address, but the IP source
   address could be either of the flow source address or signaling
   source address. Some of the relative merits of these options are as

   o  Using the flow source address makes it more likely that the
      message will be correctly routed through any intermediate
      NSIS-unaware region which is doing load sharing or policy routing
      on the {source, destination} address pair. If the signaling source
      address is used, the message will be intercepted at some node
      closer to the flow destination, but it may not be the same as the
      next node for the data flow packets.

   o  Conversely, using the signaling source address means that ICMP
      error messages (specifically, unreachable port or address) will be
      correctly delivered to the message originator, rather than being
      sent back to the flow source. Without seeing these messages, it is
      very difficult for the querying node to recognise that it is the
      last NSIS node on the path. In addition, using the signaling
      source address may make it possible to exchange messages through
      GIMPS unaware NATs (although it isn't clear how useful the
      resulting messages will be, see Section 6.4).

   It is not clear which of these situations it is more important to
   handle correctly and hence which source addressing option to use.
   (RSVP uses the flow source address, although this is primarily for
   multicast routing reasons.) A conservative approach would be to allow
   both, possibly even in parallel (although this might open up the
   protocol to amplification attacks).

   As well as the addressing, downstream datagram mode messages could be
   given the same traffic class (and, in the case of IPv6, flow label)
   as data flow messages. Doing this would increase the faithfulness of
   the routing of such messages. The traffic class may not be strictly

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   appropriate for signaling; however, in many cases the bulk of the
   signaling messages will be sent in connection mode (for which a
   different traffic class can be freely chosen).

8.4 Intermediate Node Bypass and Router Alert Values

   We assume that the primary mechanism for intercepting messages is the
   use of the RAO. The RAO contains a 16 bit value field, within which
   35 values have currently been assigned by IANA. It is open how to
   assign values for use by GIMPS messages to optimise protocol
   processing, i.e. to minimise the amount of slow-path processing that
   nodes have to carry out for messages they are not actually interested
   in the content of.

   There are two basic reasons why a GIMPS node might wish to ignore a

   o  because it is for a signaling application that the node does not

   o  because even though the signaling application is present, the node
      is only processing signaling messages at the aggregate level and
      not for individual flows (compare [15]).

   Once a message is ignored and forwarded onwards towards the next
   node, the current node has essentially dropped out of the signaling
   path for this flow and signaling application. Its address will not be
   discovered and used for upstream datagram mode messages or any
   connection mode messages.

   Some or all of this information could be encoded in the RAO value
   field, which would then allow these messages to be filtered on the
   fast path. (Even if the information is encoded deeper into the
   message, such as in the GIMPS fixed header, it may still be possible
   to process it on the fast path, and the semantics of the node
   dropping out of the signaling path are the same however the filtering
   is done. However, using the RAO seems the most efficient method if
   this is possible.) It is not clear whether the signaling application
   and aggregation level should be directly identified in the RAO value,
   or whether IANA should allocate special values for 'popular'
   applications or groups of applications or network configurations. The
   former is most flexible but is liable to be expensive in terms of
   value allocations.

   There is a special consideration in the case of the aggregation
   level. In this case, whether a message should be processed depends on
   the network region it is in. There are then two basic possibilities:

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   1.  All routers have essentially the same algorithm for which
       messages they process, i.e. all messages at aggregation level 0.
       However, messages have their aggregation level incremented on
       entry to an aggregation region and decremented on exit.

   2.  Routers process messages only above a certain aggregation level
       and ignore all others. The aggregation level of a message is
       never changed; signaling messages for end to end flows have level
       0, but signaling messages for aggregates are generated with a
       higher level.

   The first technique requires aggregating/deaggregating routers to be
   configured as being at a particular aggregation level, and also
   requires consistent message rewriting at these boundaries. The second
   technique eliminates the rewriting, but requires interior routers to
   be configured also. It is not clear what the right trade-off between
   these options is.

8.5 Messaging Association Flexibility

   The language of Section 4 is mainly based on the use of 0 or 1
   messaging associations between any pair of GIMPS nodes. However,
   logically it would be quite possible to have more than one
   association, for example:

   o  to allow different reliability characteristics;

   o  to provide different levels of security protection or to have
      security separation between different signaling streams;

   o  even simply to have load split between different connections
      according to priority (so there could be two associations with
      identical protocol stacks).

   It is possible to imagine essentially infinite flexibility in these
   options, both in terms of how many possibilities are allowed and how
   nodes signal their capabilities and preferences, without much
   changing the overall GIMPS structure. (The GIMPS-query and
   GIMPS-response messages described in section Section 4.3 can be used
   to exchange this information.) What is not clear is how much
   flexibility is actually needed.

8.6 Messaging Association Setup Message Sequences

   The discussion of Section 4.3 assumes a simple fixed message
   sequence, which we can picture as follows:

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   |            Direction            | Message                         |
   |               --->              | GIMPS-query message             |
   |                                 |                                 |
   |               <---              | GIMPS-response message          |
   |                                 |                                 |
   |               ===>              | Querying node initiates         |
   |                                 | messaging association setup     |
   |                                 | messages                        |
   |                                 |                                 |
   |               <-->              | Signaling messages exchanged    |

   There are several variants which could be considered at this level,
   for example whether the messaging association could be set up by the
   responding node:

   |            Direction            | Message                         |
   |               --->              | GIMPS-query message             |
   |                                 |                                 |
   |               <===              | Responding node initiates       |
   |                                 | messaging association setup     |
   |                                 | messages                        |
   |                                 |                                 |
   |               <-->              | Signaling messages exchanged    |

   This saves a message but may be vulnerable to additional denial of
   service attacks.

   Another open area is how the responding node propagates the signaling
   message downstream. It could initiate the downstream discovery
   process as soon as it received the initial GIMPS-query message, or it
   could wait until the first signaling application message has been
   received (which might not be until a messaging association has been
   established). A similar timing question applies to when it should
   initiate its own downstream messaging associations. It is possible
   that all these options are simply a matter for implementation
   freedom, although leaving them open will make mobility and re-routing
   behaviour rather harder to analyse, and again there are denial of
   service implications for some approaches (see Section 7.4).

   A final open area is how to manage the protocol exchanges that take
   place in setting up the messaging association itself. It is probably
   an implementation matter to consider whether to carry out, for

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   example, the SCTP 4-way handshake only after IKE exchanges (for IPsec
   SA initialisation) have completed, or whether these can be done
   partly in parallel. A more radical step is to carry the initial
   request and response messages of both exchanges as payloads in the
   GIMPS-query/response exchange, with the request message initially
   formatted by the querying node with an unspecified destination IP
   address. This would require modifications to the protocol
   implementations (especially at the querying node) similar to what is
   needed for NAT traversal; it would have to be evaluated whether this
   was worth the one or two round trip times that are saved.

8.7 Connection Mode Encapsulation

   Section 5.3.2 - Section 5.3.4 present 3 options for encapsulation in
   connection mode. Allowing all three would be a significant complexity
   cost, especially given the interaction between encapsulation mode and
   feasible protocols. If one option has to be chosen, point-to-point
   tunnelling currently seems most attractive.

   There might be efficiency saving in being able to use raw mode in the
   core of the networks, for example on single-hop interdomain links
   where these intermediate node problems do not arise. The absence of
   intermediate nodes could be determined during the GIMPS-query/
   response exchange, and then raw mode encapsulation chosen only when

8.8 GIMPS State Teardown

   The description of Section 4.3 provides for GIMPS state (per-flow
   routing state and per-peer messaging association state) to be removed
   on timer expiry; routing state can also be replaced (updated). This
   is the fundamental technique. An additional possibility would be to
   have explicit removal, i.e. a protocol mechanism to tear down GIMPS
   state immediately for a particular flow. On recieving such a message,
   a GIMPS node would clear routing entries and possibly take down
   messaging associations that were no longer used.

   Even if one peer indicates that routing state is no longer required,
   the receiving GIMPS node would have to ensure that no other peers
   (e.g. supporting different signaling applications) might generate
   messages of their own still needing the state. In addition, it is not
   clear how useful it is to remove GIMPS state promptly, since
   maintaining it only requires table storage without retention of any
   actual network resources.

8.9 Datagram Mode Retries and Single Shot Message Support

   The GIMPS-query and GIMPS-response messages may suffer from message

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   loss (e.g. due to congestion or corruption). Because a successful
   handshake is necessary before a messaging association can even be
   initiated, GIMPS must provide its own recovery method for these
   cases. A working assumption is that the querying node can repeat the
   GIMPS-query with an exponential backoff until a response is received
   or some retry threshold is reached.

   The same technique could be used by the GIMPS layer to provide a
   'low-cost' reliable message transfer service, restricted to short
   messages, without incurring the cost of setting up a messaging
   association. (If a messaging association exists, it will often be
   cheaper to discover and use that.) Providing such a service would
   require some minor extensions to the basic GIMPS protocol. It isn't
   clear if this additional option fills an important gap in the
   spectrum between datagram and connection mode message transfer.

8.10 GIMPS Support for Message Scoping

   Many signaling applications are interested in sending messages over a
   specific region of the network. Message scoping of this nature seems
   to be hard to achieve in a topologically robust way, because such
   region boundaries are not well defined in the network layer.

   It may be that the GIMPS layer can assist such scoping, by detecting
   and counting different types of nodes in the signaling plane. The
   simplest solution would be to count GIMPS nodes supporting the
   relevant signaling application - this is already effectively done by
   the GIMPS hop count. A more sophisticated approach would be to track
   the crossing of aggregation region boundaries, as introduced in
   Section 8.4. Whether this is plausible depends on the willingness of
   operators to configure such boundary information in their routers.

8.11 Mandatory or Optional Reverse Routing State

   Reverse routing state (i.e. the upstream peer addressing information
   in the routing state table) is installed per-flow when receiving a
   downstream datagram mode message containing an addressing information
   payload for the signaling source.

   Technically, the presence of this payload (and hence the installation
   of the state) is optional. This allows for very lightweight sending
   of multi-hop downstream signaling messages (even all the way from
   flow sender to flow receiver) because no state needs to be installed
   or managed by GIMPS at the intermediate nodes. However, this rules
   out the possibility of any downstream node sending signaling
   responses (including error messages) directly upstream; they have to
   be sent via the flow endpoints, leading to additional processing
   there, as well as more complex security considerations.

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   It is possible that the requirement for lightweight intermediate
   nodes can be better matched by using one of the tunnelled
   encapsulations described in Section 5.3. This would allow for a
   restricted subset of processing at the intermediate nodes, while
   still allowing the use of bidirectional communication between the
   'outer' GIMPS peers, including reverse routing state at the
   downstream one. This would eliminate the complexity of considering
   reverse routing state maintenance as optional.

8.12 Additional Discovery Mechanisms

   The routing state maintenance procedures described in Section 4.3 are
   strongly focussed on the problem of discovering, implicitly or
   explicitly, the neighbouring peers on the flow path - which is the
   necessary functionality for path-coupled signaling.

   As well as the GIMPS-query/response discovery mechanism, other
   techniques may sometimes also be possible. For example, in many
   environments, a host has a single access router, i.e. the downstream
   peer (for outgoing flows) and the upstream peer (for incoming ones)
   are known a priori. More generally, a link state routing protocol
   database can be analysed to determine downstream peers in more
   complex topologies, and maybe upstream ones if strict ingress
   filtering is in effect. More radically, much of the GIMPS protocol is
   unchanged if we consider off-path signaling nodes, although there are
   significant differences in some of the security analysis (Section
   7.3). However, none of these possibilities are considered further in
   this specification.

8.13 Protocol Design Details

   Clearly, not all details of GIMPS operation have been defined so far.
   This section provides a list of slightly non-trivial areas where more
   detail is need, where these have not been mentioned elsewhere in the

   o  Datagram mode still requires (primitive) transport functions for
      backoff on retransmission and rate limiting in general.

   o  Processing of the GIMPS hop count and IP TTL needs to be
      clarified, especially for messages which are being bypassed
      without going through full GIMPS processing.

   o  Receiver initiated signaling applications need to have reverse
      path state set up in the network. Should this be done by GIMPS
      carrying out the discovery for the specific signaling application
      (which requires the flow sender to know what signaling
      applications are going to be used), or should the discovery

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      attempt to find every GIMPS node and the signaling applications
      they support?

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

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

   [5]  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.

   [6]  Kohler, E., "Datagram Congestion Control Protocol (DCCP)",
        draft-ietf-dccp-spec-04 (work in progress), July 2003.

   [7]  Stewart, R., "SCTP Partial Reliability Extension",
        draft-ietf-tsvwg-prsctp-01 (work in progress), August 2003.

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Informative References

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

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

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

   [11]  Gilligan, R. and E. Nordmark, "Transition Mechanisms for IPv6
         Hosts and Routers", RFC 2893, August 2000.

   [12]  Berger, L., Gan, D., Swallow, G., Pan, P., Tommasi, F. and S.
         Molendini, "RSVP Refresh Overhead Reduction Extensions", RFC
         2961, April 2001.

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

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

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

   [16]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
         Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
         Session Initiation Protocol", RFC 3261, June 2002.

   [17]  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.

   [18]  Hancock, R., "Next Steps in Signaling: Framework",
         draft-ietf-nsis-fw-04 (work in progress), September 2003.

   [19]  Tschofenig, H. and D. Kroeselberg, "Security Threats for NSIS",
         draft-ietf-nsis-threats-02 (work in progress), July 2003.

   [20]  Nikander, P., "Mobile IP version 6 Route Optimization Security
         Design Background", draft-nikander-mobileip-v6-ro-sec-01 (work
         in progress), July 2003.

   [21]  Bound, J., "Dual Stack Transition Mechanism",

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         draft-bound-dstm-exp-00 (work in progress), August 2003.

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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Appendix A. 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: Bob Braden, Xiaoming Fu, Ruediger Geib, Eleanor
   Hepworth, Georgios Karagiannis, John Loughney, Jukka Manner, Andrew
   McDonald, Charles Shen, Melinda Shore, Mike Thomas, Hannes
   Tschofenig, Sven van den Bosch, and Lars Westberg. We look forward to
   inputs and comments from many more in the future.

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Appendix B. Example Routing State Table

   Figure 9 shows a signaling scenario for a single flow being managed
   by two signaling applications. The flow sender and receiver and one
   router support both, two other routers support one each.

       A                        B          C          D           E
   +------+                  +-----+    +-----+    +-----+    +--------+
   | Flow |    +-+    +-+    |NSLP1|    |NSLP1|    |     |    |  Flow  |
   |Sender|====|R|====|R|====|NSLP2|====|     |====|NSLP2|====|Receiver|
   |      |    +-+    +-+    |GIMPS|    |GIMPS|    |GIMPS|    |        |
   +------+                  +-----+    +-----+    +-----+    +--------+

                              Flow Direction

                     Figure 9: A Signaling Scenario

   |    Flow    | Session ID |  Signaling  |   Upstream  |  Downstream |
   |   Routing  |            | Application |     Peer    |     Peer    |
   | Informatio |            |             |             |             |
   |      n     |            |             |             |             |
   |   {IP-#A,  |  0xABCDEF  |    NSLP1    |    IP-#A    |    (null)   |
   |   IP-#E,   |            |             |             |             |
   | protocol & |            |             |             |             |
   |   ports}   |            |             |             |             |
   |            |            |             |             |             |
   |   {IP-#A,  |  0x123456  |    NSLP2    |    IP-#A    |  Pointer to |
   |   IP-#E,   |            |             |             |     B-D     |
   | protocol & |            |             |             |  messaging  |
   |   ports}   |            |             |             | association |

   The table shows the routing state at Node B for the single flow from
   A to E. The upstream state is just the same address for each
   application. For the downstream case, NSLP1 only requires datagram
   mode messages and so no explicit routing state towards C is needed.
   NSLP2 requires a messaging association for its messages, and node C
   has elected to bypass the GIMPS-query/response exchange, so the
   downstream 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
   flow routing information); routing state is stored only for adjacent

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Appendix C. Connection Mode Messaging Association Configurations

   Figure 10 shows a segment of a path between flow sender and receiver
   (not shown). Nodes A and F are GIMPS nodes which also fully process
   NSLP1. They need to carry out relatively complex negotiations, maybe
   exchanging large messages or requiring a secured channel. There are
   several other nodes on the path between them:

   o  Node B is GIMPS aware but only processes NSLP2;

   o  Node C is a GIMPS aware NAT (see Section 6.4) but processes no
      signaling applications;

   o  Node D is a router;

   o  Node E processes a subset of NSLP1 (e.g. insertion of local
      resource availability status for use by Node F).

            A          B          C        D        E          F
         +-----+    +-----+    +-----+           +-----+    +-----+
         |NSLP1|    |     |    |     |    +-+    |NSLP1|    |NSLP1|
    >>>>>|     |>>>>|NSLP2|>>>>| NAT |>>>>|R|>>>>|-lite|>>>>|     |>>>>>
         |GIMPS|    |GIMPS|    |GIMPS|    +-+    |GIMPS|    |GIMPS|
         +-----+    +-----+    +-----+           +-----+    +-----+

                   Figure 10: Heterogeneous Signaling

   The possible messaging association arrangements depend on the
   connection mode encapsulation:

   Raw (as in Section 5.3.2): Messaging associations are needed from
      A-C, C-E and E-F (node D is invisible to GIMPS and node B can
      ignore discovery for NSLP1). Achieving properties such as
      reliability or flow control or channel security between A and F
      depends on the quality of implementation and trust in the
      administration of nodes C and E.

   Tunnelled (as in Section 5.3.3 and Section 5.3.4): There is a single
      messaging association from A-F directly. C can do NAT processing
      if the flow routing information is placed in the mutable area, and
      E can manipulate NSLP1 objects in the mutable area also.

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