Next Steps in Signaling H. Schulzrinne
Internet-Draft Columbia U.
Expires: August 13, 2006 R. Hancock
Siemens/RMR
February 9, 2006
GIST: General Internet Signaling Transport
draft-ietf-nsis-ntlp-09
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Abstract
This document specifies protocol stacks for the routing and transport
of per-flow signaling messages along the path taken by that flow
through the network. The design uses existing transport and security
protocols under a common messaging layer, the General Internet
Signaling Transport (GIST), which provides a universal service for
diverse signaling applications. GIST does not handle signaling
application state itself, but manages its own internal state and the
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configuration of the underlying transport and security protocols to
enable the transfer of messages in both directions along the flow
path. The combination of GIST and the lower layer transport and
security protocols provides a solution for the base protocol
component of the "Next Steps in Signaling" framework.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Restrictions on Scope . . . . . . . . . . . . . . . . . . 5
2. Requirements Notation and Terminology . . . . . . . . . . . . 6
3. Design Overview . . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Overall Design Approach . . . . . . . . . . . . . . . . . 8
3.2. Modes and Messaging Associations . . . . . . . . . . . . 9
3.3. Message Routing Methods . . . . . . . . . . . . . . . . . 10
3.4. Signaling Sessions . . . . . . . . . . . . . . . . . . . 12
3.5. Signaling Applications and NSLPIDs . . . . . . . . . . . 13
3.6. Example of Operation . . . . . . . . . . . . . . . . . . 14
4. GIST Processing Overview . . . . . . . . . . . . . . . . . . 16
4.1. GIST Service Interface . . . . . . . . . . . . . . . . . 16
4.2. GIST State . . . . . . . . . . . . . . . . . . . . . . . 18
4.3. Basic Message Processing . . . . . . . . . . . . . . . . 19
4.4. Routing State and Messaging Association Maintenance . . . 25
5. Message Formats and Transport . . . . . . . . . . . . . . . . 32
5.1. GIST Messages . . . . . . . . . . . . . . . . . . . . . . 32
5.2. Information Elements . . . . . . . . . . . . . . . . . . 34
5.3. Datagram Mode Transport . . . . . . . . . . . . . . . . . 38
5.4. Connection Mode Transport . . . . . . . . . . . . . . . . 40
5.5. Message Type/Encapsulation Relationships . . . . . . . . 42
5.6. Error Message Processing . . . . . . . . . . . . . . . . 43
5.7. Messaging Association Setup . . . . . . . . . . . . . . . 44
5.8. Specific Message Routing Methods . . . . . . . . . . . . 47
6. Formal Protocol Specification . . . . . . . . . . . . . . . . 52
6.1. Node Processing . . . . . . . . . . . . . . . . . . . . . 53
6.2. Query Node Processing . . . . . . . . . . . . . . . . . . 55
6.3. Responder Node Processing . . . . . . . . . . . . . . . . 58
6.4. Messaging Association Processing . . . . . . . . . . . . 62
7. Advanced Protocol Features . . . . . . . . . . . . . . . . . 65
7.1. Route Changes and Local Repair . . . . . . . . . . . . . 65
7.2. NAT Traversal . . . . . . . . . . . . . . . . . . . . . . 71
7.3. Interaction with IP Tunnelling . . . . . . . . . . . . . 74
7.4. IPv4-IPv6 Transition and Interworking . . . . . . . . . . 75
8. Security Considerations . . . . . . . . . . . . . . . . . . . 77
8.1. Message Confidentiality and Integrity . . . . . . . . . . 77
8.2. Peer Node Authentication . . . . . . . . . . . . . . . . 78
8.3. Routing State Integrity . . . . . . . . . . . . . . . . . 78
8.4. Denial of Service Prevention . . . . . . . . . . . . . . 80
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8.5. Requirements on Cookie Mechanisms . . . . . . . . . . . . 81
8.6. Security Protocol Selection Policy . . . . . . . . . . . 83
8.7. Residual Threats . . . . . . . . . . . . . . . . . . . . 84
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 86
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 91
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 92
11.1. Normative References . . . . . . . . . . . . . . . . . . 92
11.2. Informative References . . . . . . . . . . . . . . . . . 92
Appendix A. Bit-Level Formats and Error Messages . . . . . . . . 95
A.1. The GIST Common Header . . . . . . . . . . . . . . . . . 95
A.2. General Object Format . . . . . . . . . . . . . . . . . . 96
A.3. GIST TLV Objects . . . . . . . . . . . . . . . . . . . . 97
A.4. Errors . . . . . . . . . . . . . . . . . . . . . . . . . 104
Appendix B. API between GIST and Signaling Applications . . . . 112
B.1. SendMessage . . . . . . . . . . . . . . . . . . . . . . . 112
B.2. RecvMessage . . . . . . . . . . . . . . . . . . . . . . . 113
B.3. MessageStatus . . . . . . . . . . . . . . . . . . . . . . 115
B.4. NetworkNotification . . . . . . . . . . . . . . . . . . . 115
B.5. SetStateLifetime . . . . . . . . . . . . . . . . . . . . 116
B.6. InvalidateRoutingState . . . . . . . . . . . . . . . . . 116
Appendix C. Example Routing State Table and Handshake Message
Sequence . . . . . . . . . . . . . . . . . . . . . . 118
Appendix D. Change History . . . . . . . . . . . . . . . . . . . 120
D.1. Changes In Version -09 . . . . . . . . . . . . . . . . . 120
D.2. Changes In Version -08 . . . . . . . . . . . . . . . . . 120
D.3. Changes In Version -07 . . . . . . . . . . . . . . . . . 122
D.4. Changes In Version -06 . . . . . . . . . . . . . . . . . 123
D.5. Changes In Version -05 . . . . . . . . . . . . . . . . . 124
D.6. Changes In Version -04 . . . . . . . . . . . . . . . . . 125
D.7. Changes In Version -03 . . . . . . . . . . . . . . . . . 126
D.8. Changes In Version -02 . . . . . . . . . . . . . . . . . 127
D.9. Changes In Version -01 . . . . . . . . . . . . . . . . . 128
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 131
Intellectual Property and Copyright Statements . . . . . . . . . 132
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1. Introduction
Signaling involves the manipulation of state held in network
elements. 'Manipulation' could mean setting up, modifying and
tearing down state; or it could simply mean the monitoring of state
which is managed by other mechanisms.
This specification concentrates on "path-coupled" signaling, which
involves network elements which are located on the path taken by a
particular data flow, possibly including but not limited to the flow
endpoints. Indeed, there are almost always more than two
participants in a path-coupled signaling session, although there is
no need for every node on the path to participate. Path-coupled
signaling thus excludes end-to-end higher-layer application signaling
(except as a degenerate case) such as 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 requires a set of state management rules,
as well as protocol support to exchange messages along the data path.
Several aspects of this protocol support are common to all or a large
number of signaling applications, and hence can be developed as a
common protocol. The NSIS framework given in [22] provides a
rationale for a function split between the common and application
specific protocols, and gives outline requirements for the former,
the 'NSIS Transport Layer Protocol' (NTLP). The application specific
protocols are referred to as 'NSIS Signaling Layer Protocols'
(NSLPs), and are defined in separate documents.
This specification provides a concrete solution for the NTLP. It is
based on the use of existing transport and security protocols under a
common messaging layer, the General Internet Signaling Transport
(GIST). GIST does not handle signaling application state itself; in
that crucial respect, it differs from application signaling protocols
such as SIP, RTSP, and the control component of FTP. Instead, GIST
manages its own internal state and the configuration of the
underlying transport and security protocols to ensure the transfer of
signaling messages on behalf of signaling applications in both
directions along the flow path.
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The structure of this specification is as follows. Section 2 defines
terminology, and Section 3 gives an informal overview of the protocol
design principles and operation. The normative specification is
contained mainly in Section 4 to Section 8. Section 4 describes the
message sequences and Section 5 their format and contents (note that
the detailed bit formats are given in Appendix A). The protocol
operation is captured in the form of state machine language in
Section 6. Section 7 describes some more advanced protocol features
and security considerations are contained in Section 8. In addition,
Section 9 gives the IANA considerations, Appendix B describes an
abstract API for the service which GIST provides to signaling
applications, and Appendix C provides an example message flow.
1.1. Restrictions on Scope
This section briefly lists some important restrictions on GIST
applicability and functionality. In some cases, these are implicit
consequences of the functionality split developed in the NSIS
framework; in others, they are restrictions on the types of scenario
in which GIST can operate correctly.
Flow splitting: In some cases, e.g. where packet-level load sharing
has been implemented, the path taken by a single flow in the
network may not be well defined. If this is the case, GIST cannot
route signaling meaningfully. (In some circumstances, GIST
implementations could detect this condition, but even this cannot
be guaranteed.)
Multicast: GIST does not handle multicast flows. This includes
'classical' IP multicast and any of the 'small group multicast'
schemes recently proposed.
Legacy NATs: GIST messages will generally pass through NATs, but
unless the NAT is GIST-aware, any addressing data carried in the
payload will not be handled correctly. There is a dual problem of
whether the GIST peers either side of the boundary can work out
how to address each other, and whether they can work out what
translation to apply to the signaling packet payloads. The
fundamental problem is that GIST messages contain 3 or 4
interdependent addresses which all have to be consistently
translated, and existing generic NAT traversal techniques such as
STUN [19] or TURN [20] can process only two. (Appropriate
behaviour for a GIST-aware NAT is discussed in Section 7.2.)
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2. Requirements Notation and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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 GIST level are shown in
Figure 1.
Source GIST (adjacent) peer nodes Destination
IP address IP addresses = Signaling IP address
= Flow Source/Destination Addresses = Flow
Source (depending on signaling direction) Destination
Address | | Address
V V
+--------+ +------+ Data Flow +------+ +--------+
| Flow |-----------|------|-------------|------|-------->| Flow |
| Sender | | | | | |Receiver|
+--------+ | GIST |============>| GIST | +--------+
| Node |<============| Node |
+------+ Signaling +------+
GN1 Flow GN2
>>>>>>>>>>>>>>>>> = Downstream direction
<<<<<<<<<<<<<<<<< = Upstream direction
Figure 1: Basic Terminology
[Data] Flow: A set of packets identified by some fixed combination of
header fields. Flows are unidirectional (a bidirectional
communication is considered a pair of unidirectional flows).
Session: A single application layer flow of information for which
some state information is to be manipulated or monitored. See
Section 3.4 for further detailed discussion.
[Flow] Sender: The node in the network which is the source of the
packets in a flow. Could be a host, or a router (e.g. 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.
GIST Node: Any node along the data path supporting GIST (regardless
of what signaling applications it supports).
[Adjacent] Peer: The next node along the data path, in the upstream
or downstream direction, with which a GIST node explicitly
interacts. The GIST peer discovery mechanisms implicitly
determine whether two nodes will be adjacent. It is possible for
adjacencies to 'skip over' intermediate nodes which decide not to
take part in the signaling exchange at the NTLP layer; even if
such nodes process parts of the signaling messages, they store no
state about the session and are never explicitly visible at the
GIST level to the nodes either side.
Datagram Mode: A mode of sending GIST messages between nodes without
using any transport layer state or security protection. Datagram
mode uses UDP encapsulation, with IP addresses derived either from
the flow definition or previously discovered adjacency
information.
Connection Mode: A mode of sending GIST messages directly between
nodes using point to point "messaging associations" (see below).
Connection mode allows the re-use of existing transport and
security protocols where such functionality is required.
Messaging Association: A single connection between two explicitly
identified GIST adjacent peers, i.e. between a given signaling
source and destination address. A messaging association may use a
specific transport protocol and known ports. If security
protection is required, it may use a specific network layer
security association, or use a transport layer security
association internally. A messaging association is bidirectional;
signaling messages can be sent over it in either direction, and
can refer to flows of either direction.
Message Routing Method: There can be different algorithms for
discovering the route that signaling messages should take. These
are referred to as message routing methods, and GIST supports
alternatives within a common protocol framework. See Section 3.3.
Transfer Attributes: A description of the requirements which a
signaling application has for the delivery of a particular
message; for example, whether the message should be delivered
reliably. See Section 4.1.2.
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3. Design Overview
3.1. Overall Design Approach
The generic requirements identified in the NSIS framework [22] for
transport of path-coupled signaling messages are essentially two-
fold:
"Routing": Determine how to reach the adjacent signaling node along
each direction of the data path (the GIST peer), and if necessary
explicitly establish addressing and identity information about
that peer;
"Transport": Deliver the signaling information to that peer.
To meet the routing requirement, one possibility is for the node to
use local routing state information to determine the identity of the
GIST peer explicitly. GIST defines a 3-way handshake (Query/
Response/optional Confirm) which sets up the necessary routing state
between adjacent peers, during which signaling applications can also
exchange data; the Query message is encapsulated in a special way,
depending on the message routing method, in order to probe the
network infrastructure so that the correct peer will intercept it.
If the routing state does not exist, GIST may be able to send a
message anyway, with the same encapsulation as for a Query message.
Once the routing decision has been made, the node has to select a
mechanism for transport of the message to the peer. GIST divides the
transport problems into two categories, the easy and the difficult.
It handles the easy cases internally, and uses well-understood
transport protocols for the harder cases. Here, with details
discussed later, "easy" messages are those that are sized well below
the lowest MTU along a path, are infrequent enough not to cause
concerns about congestion and flow control, and do not need security
protection or guaranteed delivery.
In [22] all of these routing and transport requirements are assigned
to a single notional protocol, the 'NSIS Transport Layer Protocol'
(NTLP). The strategy of splitting the transport problem leads to a
layered structure for the NTLP, of a specialised GIST 'messaging'
layer running over standard transport and security protocols, as
shown in Figure 2. This also shows GIST offering its services to
upper layers at an abstract interface, the GIST API, further
discussed in Section 4.1.
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^^ +-------------+
|| | Signaling |
NSIS +------------|Application 2|
Signaling | Signaling +-------------+
Application |Application 1| |
Level +-------------+ |
|| | |
VV | |
========|===================|===== <-- GIST API
| |
^^ +------------------------------------------------+
|| |+-----------------------+ +--------------+ |
|| || GIST | | GIST State | |
|| || Encapsulation |<<<>>>| Maintenance | |
|| |+-----------------------+ +--------------+ |
|| | GIST: Messaging Layer |
|| +------------------------------------------------+
NSIS | | | |
Transport .............................
Level . Transport Layer Security .
("NTLP") .............................
|| | | | |
|| +----+ +----+ +----+ +----+
|| |UDP | |TCP | |SCTP| |DCCP| ... other
|| +----+ +----+ +----+ +----+ protocols
|| | | | |
|| .............................
|| . IP Layer Security .
|| .............................
VV | | | |
========================|=======|=======|=======|===============
| | | |
+----------------------------------------------+
| IP |
+----------------------------------------------+
Figure 2: Protocol Stacks for Signaling Transport
3.2. Modes and Messaging Associations
Internally, GIST has two modes of operation:
Datagram mode ('D mode') is used for small, infrequent messages with
modest delay constraints; it is also used at least for the Query
message of the 3-way handshake.
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Connection mode ('C mode') is used for larger data objects or where
fast state setup in the face of packet loss is desirable, or where
channel security is required.
Datagram mode uses UDP, as this is the only encapsulation which does
not require per-message shared state to be maintained between the
peers. The connection mode can in principal use any stream or
message-oriented transport protocol; this specification defines TCP
as the initial choice. It can in principal employ specific network
layer security associations, or an internal transport layer security
association; this specification defines TLS as the initial choice.
When GIST messages are carried in connection mode, they are treated
just like any other traffic by intermediate routers between the GIST
peers. Indeed, it would be impossible for intermediate routers to
carry out any processing on the messages without terminating the
transport and security protocols used.
It is possible to mix these two modes along a path. 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 GIST are not independent. If the message transfer has
requirements that require connection mode (e.g. the message is so
large that fragmentation is required), this can only be used between
explicitly identified nodes. In such cases, GIST carries out the
3-way handshake initially in datagram mode to identify the peer and
then sets up the necessary connections if they do not already exist.
It must also be understood that the signaling application does not
make the D/C mode selection directly; rather, this decision is made
by GIST on the basis of the message characteristics and the transfer
attributes stated by the application. The distinction is not visible
at the GIST service interface.
In general, the state associated with 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 GIST peers (although the usual
case is 0 or 1), and they are set up and torn down by management
actions within GIST itself.
3.3. Message Routing Methods
The baseline message routing functionality in GIST is that signaling
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messages follow a route defined by an existing flow in the network,
visiting a subset of the nodes through which it passes. This is the
appropriate behaviour for application scenarios where the purpose of
the signaling is to manipulate resources for that flow. However,
there are scenarios for which other behaviours are applicable. Two
examples are:
Predictive Routing: Here, the intent is to signal along a path that
the data flow may follow in the future. Possible cases are pre-
installation of state on the backup path that would be used in the
event of a link failure; and predictive installation of state on
the path that will be used after a mobile node handover.
NAT Address Reservations: This applies to the case where a node
behind a NAT wishes to reserve an address at which it can be
reached by a sender on the other side. This requires a message to
be sent outbound from what will be the flow receiver although no
reverse routing state for the flow yet exists.
Most of the details of GIST operation are independent of which
alternative is being used. Therefore, the GIST design encapsulates
the routing-dependent details as a message routing method (MRM), and
allows multiple MRMs to be defined. The default is the path-coupled
MRM, which corresponds to the baseline functionality described above;
a second MRM for the NAT Address Reservation case is also defined.
The content of a MRM definition is as follows, using the path-coupled
MRM as an example:
o The format of the information that describes the path that the
signaling should take, the Message Routing Information (MRI). For
the path-coupled MRM, this is just the Flow Identifier (see
Section 5.8.1.1) and some additional control information.
Specifically, the MRI always includes a flag to distinguish
between the two directions that signaling messages can take,
denoted 'upstream' and 'downstream'.
o A specification of the IP level encapsulation of the Query
messages which probe the network to discover the adjacent peers.
A downstream encapsulation must be defined; an upstream
encapsulation is optional. For the path-coupled MRM, this
information is given in Section 5.8.1.2 and Section 5.8.1.3.
o A specification of what validation checks GIST should apply to the
probe messages, for example to protect against IP address spoofing
attacks. The checks may be dependent on the direction (upstream/
downstream) of the message. For the path-coupled MRM, the
downstream validity check is basically a form of ingress
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filtering, also discussed in Section 5.8.1.2.
o The mechanism(s) available for route change detection, i.e. any
change in the neighbour relationships that the MRM discovers. The
default case for any MRM is soft-state refresh, but additional
supporting techniques may be possible; see Section 7.1.2.
In addition, it should be noted that NAT traversal almost certainly
requires transformation of the MRI field in GIST messages (see
Section 7.2). Although the transformation does not have to be
defined as part of the standard, the impact on existing GIST-aware
NAT implementations should be considered.
The MRI is passed explicitly between signaling applications and GIST;
therefore, signaling application specifications must define which
MRMs they require (they may use more than one, e.g. depending on the
type of message). Signaling applications may use fields in the MRI
in their packet classifiers; if they use additional information for
packet classification, this would be carried at the NSLP level and so
would be invisible to GIST. Any node hosting a particular signaling
application MUST use a GIST implementation that supports the
corresponding MRMs. The GIST processing rules enforce that nodes
which do not host the signaling application are not forced to handle
messages for it at the GIST level, so it does not matter if they
support the MRM or not.
3.4. Signaling Sessions
GIST allows signaling applications to associate each message with a
"signaling session". Informally, given an application layer exchange
of information for which some network control state information is to
be manipulated or monitored, the corresponding signaling messages
should be associated with the same session. Signaling applications
provide the session identifier (SID) whenever they wish to send a
message, and GIST reports the SID when a message is received.
Most GIST processing and state information is related to the flow
(defined by the MRI, see above) and signaling application (given by
the NSLPID, see below). There are several possible relationships
between flows and sessions, for example:
o The simplest case is that all messages for the same flow have the
same SID.
o Messages for more than one flow may use the same SID, for example
because one flow is replacing another in a mobility or multihoming
scenario.
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o A single flow may have messages for different SIDs, for example
from independently operating NSLPIDs.
Because of this range of options, GIST does not perform any
validation on how signaling applications map between flows and
sessions, nor does it perform any validation on the properties of the
SID itself. In particular, when a new SID is needed, logically it
should be generated by the signaling application. (NSIS
implementations could provide common functionality to generate SIDs
for use by any signaling application, but this is not part of GIST.)
GIST only defines the syntax of the SID as an opaque 128-bit
identifier.
The SID assignment has the following impact on GIST processing:
o Messages with the same SID to be delivered reliably between the
same GIST peers are delivered in order.
o All other messages are handled independently.
o GIST identifies routing state (upstream and downstream peer) by
the triplet (MRI, NSLPID, SID).
Strictly, the routing state should not depend on the SID. However,
if the routing state is keyed only by (MRI, NSLPID) there is a
trivial denial of service attack (see Section 8.3) where a malicious
off-path node asserts that it is the peer for a particular flow.
Instead, the routing state is also segregated between different SIDs,
which means that the attacking node can only disrupt a signaling
session if it can guess the corresponding SID. A consequence of this
design is that signaling applications should choose SIDs so that they
are cryptographically random, and should not use several SIDs for the
same flow unless strictly necessary, to avoid additional load from
routing state maintenance.
3.5. Signaling Applications and NSLPIDs
The functionality for signaling applications is supported by NSIS
signaling layer protocols (NSLPs). Each NSLP is identified by a 16
bit NSLPID, assigned by IANA (Section 9). A single signaling
application (e.g. "resource reservation") may define a family of
NSLPs to implement its functionality, for example to carry out
signaling operations at different levels in a hierarchy (cf. [15]).
However, the interactions between the different NSLPs (for example,
to relate aggregation levels or aggregation region boundaries in the
resource management case) are handled at the signaling application
level; the NSLPID is the only information visible to GIST about the
signaling application being used.
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3.6. Example of Operation
This section presents an example of GIST usage in a relatively simple
(in particular, NAT-free) signaling scenario, to illustrate its main
features.
Consider the case of an RSVP-like signaling application which
allocates resources for a single unicast flow. We will consider how
GIST transfers messages between two adjacent peers along the path,
GN1 and GN2 (see Figure 1). In this example, the end-to-end exchange
is initiated by the signaling application instance in the sender; we
take up the story at the point where the first message is being
processed (above the GIST layer) by the signaling application in GN1.
1. The signaling application in GN1 determines that this message is
a simple description of resources that would be appropriate for
the flow. It determines that it has no special security or
transport requirements for the message, but simply that it should
be transferred to the next downstream signaling application peer
on the path that the flow will take.
2. The message payload is passed to the GIST layer in GN1, along
with a definition of the flow and description of the message
transfer attributes {unsecured, unreliable}. GIST determines
that this particular message does not require fragmentation and
that it has no knowledge of the next peer for this flow and
signaling application; however, it also determines that this
application is likely to require secured upstream and downstream
transport of large messages in the future. This determination is
a function of node-local policy interactions between GIST and the
signaling application.
3. GN1 therefore constructs a GIST-Query message, a UDP datagram
carrying the NSLP payload and additional payloads at the GIST
level to be used to initiate a messaging association. The Query
is injected into the network, addressed towards the flow
destination and with a Router Alert Option included.
4. The Query message passes through the network towards the flow
receiver, and is seen by each router in turn. GIST-unaware
routers will not recognise the RAO value and will forward the
message unchanged; GIST-aware routers which do not support the
NSLP in question will also forward the message basically
unchanged, although they may need to process more of the message
to decide this.
5. The message is intercepted at GN2. The GIST layer identifies the
message as relevant to a local signaling application, and passes
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the NSLP payload and flow description upwards to it. The
signaling application in GN2 indicates to GIST that it will peer
with GN1 and so GIST should proceed to set up any routing state.
In addition, the signaling application continues to process the
message as in GN1 (compare step 1), and this will eventually
result in the message reaching the flow receiver.
6. In parallel, the GIST instance in GN2 now knows that it should
maintain routing state and a messaging association for future
signaling with GN1. This is recognised because the message is a
GIST-Query, and because the local signaling application has
indicated that it will peer with GN1. There are two basic
possible cases for sending back the necessary GIST-Response:
A. GN1 and GN2 already have an appropriate messaging
association. GN2 simply records the identity of GN1 as its
upstream peer for that flow and NSLP, and sends a GIST-
Response back to GN1 over the association identifying itself
as the peer for this flow.
B. No messaging association exists. GN2 sends the GIST-Response
in D mode directly to GN1, identifying itself and agreeing to
the association setup. The protocol exchanges needed to
complete this will proceed in the background.
7. Eventually, another NSLP message works its way upstream from the
receiver to GN2. This message contains a description of the
actual resources requested, along with authorisation and other
security information. The signaling application in GN2 passes
this payload to the GIST level, along with the flow definition
and transfer attributes {secured, reliable}.
8. The GIST layer in GN2 identifies the upstream peer for this flow
and NSLP as GN1, and determines that it has a messaging
association with the appropriate properties. The message is
queued on the association for transmission (this may mean some
delay if the procedures begun in step 6.B have not yet
completed).
Further messages can be passed in each direction in the same way.
The GIST layer in each node can in parallel carry out maintenance
operations such as route change detection (see Section 7.1).
It should be understood that several of these details of GIST
operations can be varied, either by local policy or according to
signaling application requirements. The authoritative details are
contained in the remainder of this document.
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4. GIST Processing Overview
This section defines the basic structure and operation of GIST.
Section 4.1 describes the way in which GIST interacts with (local)
signaling applications in the form of an abstract service interface.
Section 4.2 describes the per-flow and per-peer state that GIST
maintains for the purpose of transferring messages. Section 4.3
describes how messages are processed in the case where any necessary
messaging associations and routing state already exist; this includes
the simple scenario of pure datagram mode operation, where no
messaging associations are necessary in the first place. Finally,
Section 4.4 describes how routing state and messaging associations
are created and managed.
4.1. GIST Service Interface
This section defines the service interface that GIST presents to
signaling applications in terms of abstract properties of the message
transfer. Note that the same service interface is presented at every
GIST node; however, applications may invoke it differently at
different nodes (e.g. depending on local policy). In addition, the
service interface is defined independently of any specific transport
protocol, or even the distinction between datagram and connection
mode. The initial version of this specification defines how to
support the service interface using a connection mode based on TCP;
if additional protocol support is added, this will support the same
interface and so the change will be invisible to applications (except
as a possible performance improvement). A more detailed description
of this service interface is given in Appendix B.
4.1.1. Message Handling
Fundamentally, GIST provides a simple message-by-message transfer
service for use by signaling applications: individual messages are
sent, and individual messages are received. At the service
interface, the NSLP payload (which is opaque to GIST) is accompanied
by control information expressing the application's requirements
about how the message should be routed, and the application also
provides the session identifier (see Section 3.4). Additional
message transfer attributes control the specific transport and
security properties that the signaling application desires for the
message.
The distinction between GIST connection and datagram modes is not
visible at the service interface. In addition, the invocation of
GIST functionality to handle fragmentation and reassembly, bundling
together of small messages (for efficiency), and congestion control
is not directly visible at the service interface; GIST will take
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whatever action is necessary based on the properties of the messages
and local node state.
4.1.2. Message Transfer Attributes
Message transfer attributes are used to define certain performance
and security related aspects of message processing. The attributes
available are as follows:
Reliability: This attribute may be 'true' or 'false'. For the case
'true', messages MUST be delivered to the signaling application in
the peer exactly once or not at all; if there is a chance that the
message was not delivered, an error MUST be indicated to the local
signaling application identifying the routing information for the
message in question. Messages with the same SID to the same peer
MUST be delivered in order. For the case 'false', a message may
be delivered, once, several times or not at all, with no error
indications in any case.
Security: This attribute defines the security properties that the
signaling application requires for the message, including the type
of protection required, and what authenticated identities should
be used for the signaling source and destination. This
information maps onto the corresponding properties of the security
associations established between the peers in connection mode. It
can be specified explicitly by the signaling application, or
reported by GIST to the signaling application (either on receiving
a message, or just before sending a message but after configuring
or selecting the messaging association to be used for it). This
attribute can also be used to convey information about any address
validation carried out by GIST (for example, whether a return
routability check has been carried out). Further details are
discussed in Appendix B.
Local Processing: An NSLP may provide hints to GIST to enable more
efficient or appropriate processing. For example, the NSLP may
select a priority from a range of locally defined values to
influence the sequence in which messages leave a node. Any
priority mechanism MUST respect the ordering requirements for
reliable messages within a session, and priority values are not
carried in the protocol or available at the signaling peer or
intermediate nodes. An NSLP may also indicate that reverse path
routing state will not be needed for this flow, to inhibit the
node requesting its downstream peer to create it.
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4.2. GIST State
4.2.1. Message Routing State
For each flow, the GIST layer can maintain message routing state to
manage the processing of outgoing messages. This state is
conceptually organised into a table with the following structure.
The primary key (index) for the table is the combination of the
information about how the message is to be routed, the session being
signalled for, and the signaling application itself:
Message Routing Information (MRI): This defines the method to be used
to route the message, the direction in which to send the message,
and any associated addressing information; see Section 3.3.
Session Identification (SID): The signaling session with which this
message should be associated; see Section 3.4.
NSLP Identification (NSLPID): This is an IANA assigned identifier
associated with the NSLP which is generating messages for this
flow. The inclusion of this identifier allows the routing state
to be different for different NSLPs (e.g. because of different
adjacencies).
The information for a given key consists of the routing state to
reach the peer in the direction given by the MRI. For any flow,
there will usually be two entries (for the upstream and downstream
MRI). The routing state includes information about the peer identity
(see Section 4.4.2), and a UDP port number (for datagram mode) or a
reference to one or more messaging associations (for connection
mode). All of this information is learned from prior GIST exchanges.
It is also possible for the state information for either direction to
be null. There are several possible cases:
o The signaling application has indicated that no messages will
actually be sent in that direction.
o The node is a flow endpoint, so there can be no signaling peer in
one or other direction.
o The node is the endpoint of the signaling path (for example,
because it is acting as a proxy, or because it has determined
explicitly that there are no further signaling nodes in that
direction).
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o The node is using other techniques to route the message. For
example, it can encapsulate it the same way as a Query message and
rely on the peer to intercept it.
Each item of routing state has an associated validity timer for how
long it can be considered accurate; when this timer expires, it MUST
be purged if it has not been refreshed. Installation and maintenance
of routing state is described in more detail in Section 4.4.
Note also that the routing state is described as a table of flows,
but that there is no implied constraint on how the information is
stored. However, in general, and especially if GIST peers are
several IP hops away, there is no way to identify the correct
downstream peer for a flow and signaling application from the local
forwarding table using prefix matching, and the same applies always
to upstream peer state because of the possibility of asymmetric
routing: per-flow state has to be stored, just as for RSVP [9].
4.2.2. Messaging Association State
The per-flow message routing state is not the only state stored by
GIST. There is also the state required to manage the messaging
associations. Since these are typically per-peer rather than per-
flow, they are stored separately, including the following
information:
o messages pending transmission while an association is being
established;
o a timer for how long since the peer re-stated its desire to keep
the association open (see Section 4.4.3).
In addition, per-association state is held in the messaging
association protocols themselves. However, the details of this state
are not directly visible to GIST, and they do not affect the rest of
the protocol description.
4.3. Basic Message Processing
This section describes how signaling application messages are
processed in the case where any necessary messaging associations and
routing state are already in place. The description is divided into
several parts. Firstly, message reception, local processing and
message transmission are described for the case where the node hosts
the NSLPID identified in the message. Secondly, the case where the
message is handled directly in the IP or GIST layer (because there is
no matching signaling application on the node) is given. An overview
is given in Figure 3.
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+---------------------------------------------------------+
| >> Signaling Application Processing >> |
| |
+--------^---------------------------------------V--------+
^ V
^ NSLP Payloads V
^ V
+--------^---------------------------------------V--------+
| >> GIST >> |
| ^ ^ ^ Processing V V V |
+--x-----------N--Q---------------------Q--N-----------x--+
x N Q Q N x
x N Q>>>>>>>>>>>>>>>>>>>>>Q N x
x N Q Bypass at Q N x
+--x-----+ +--N--Q--+ GIST level +--Q--N--+ +-----x--+
| C-mode | | D-mode | | D-mode | | C-mode |
|Handling| |Handling| |Handling| |Handling|
+--x-----+ +--N--Q--+ +--Q--N--+ +-----x--+
x N Q Q N x
x NNNNNN Q>>>>>>>>>>>>>>>>>>>>>Q NNNNNN x
x N Q Bypass at Q N x
+--x--N--+ +-----Q--+ IP (router +--Q-----+ +--N--x--+
|IP Host | | RAO | alert) level | RAO | |IP Host |
|Handling| |Handling| |Handling| |Handling|
+--x--N--+ +-----Q--+ +--Q-----+ +--N--x--+
x N Q Q N x
+--x--N-----------Q--+ +--Q-----------N--x--+
| IP Layer | | IP Layer |
| (Receive Side) | | (Transmit Side) |
+--x--N-----------Q--+ +--Q-----------N--x--+
x N Q Q N x
x N Q Q N x
NNNNNNNNNNNNNN = 'Normal' datagram mode messages
QQQQQQQQQQQQQQ = Datagram mode messages which
are Queries or likewise encapsulated
xxxxxxxxxxxxxx = connection mode messages
RAO = Router Alert Option
Figure 3: Message Paths through a GIST Node
4.3.1. Message Reception
Messages can be received in connection or datagram mode, and in the
latter case with two types of message encapsulation.
Reception in connection mode is simple: incoming packets undergo the
security and transport treatment associated with the messaging
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association, and the messaging association provides complete messages
to the GIST layer for further processing.
Reception in datagram mode depends on the message type. 'Normal'
messages arrive UDP encapsulated and addressed directly to the
receiving signaling node, at an address and port learned previously.
Each datagram contains a single message which is passed to the GIST
layer for further processing, just as in the connection mode case.
Where GIST is sending messages to be intercepted by the appropriate
peer rather than directly addressed to it (in particular, Query
messages), these are UDP encapsulated with an IP router alert option.
Each signaling node will therefore 'see' all such messages. The case
where the NSLPID does not match a local signaling application at all
is considered below in Section 4.3.4; otherwise, it is again passed
up to the GIST layer for further processing.
4.3.2. Local Processing and Validation
Once a message has been received, it is processed locally within the
GIST layer. The GIST hop count MUST be decremented immediately the
message has been received. Further processing depends on the message
type and payloads carried; most of the GIST payloads are associated
with state maintenance and details are covered in Section 4.4.
In the case of a GIST-Query, there is an interaction with signaling
application policy to determine which of two courses to follow:
1. The signaling application wishes to become a signaling peer with
the Querying node. GIST MUST continue with the handshake process
to set up message routing state, as described in Section 4.4.1.
The application MAY provide an NSLP payload for the same NSLPID,
which GIST will transfer in the GIST-Response.
2. The signaling application does not wish to set up state with the
Querying node and become its peer. GIST MUST propagate the
Query, similar to the case described in Section 4.3.4. No
message is sent back to the Querying node. The application MAY
provide an updated NSLP payload for the same NSLPID (which will
be used in the Query message forwarded by GIST).
This interaction with the signaling application, including the
generation or update of an NSLP payload, SHOULD take place
synchronously as part of the Query message processing . In terms of
the GIST service interface, this can be implemented by providing
appropriate return values for the primitive that is triggered when
such a message is received; see Appendix B.2 for further discussion.
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For all other message types, the GIST payloads are processed as
described in Section 4.4. The remainder of the GIST message consists
of an NSLP payload, which is delivered locally to the signaling
application identified by the NSLPID. The format of the payload is
not constrained by GIST, and the content is not interpreted.
Delivery is subject to the following validation checks:
o if the message was explicitly routed (see Section 7.1.4) or is a
Data message delivered without routing state (see Section 5.3.2),
the payload is delivered but flagged to the receiving NSLP to
indicate that routing state was not validated;
o else, if there is no routing state for the MRI/SID/NSLPID the
message MUST be rejected with a "No Routing State" error message
(Appendix A.4.4.5);
o else, if the message arrived on an association which is not
associated with the MRI/NSLPID/SID combination given in the
message, the message MUST be rejected with an "Incorrectly
Delivered Message" error message (Appendix A.4.4.4);
o else, the payload is delivered as normal.
4.3.3. Message Transmission
Signaling applications can generate their messages for transmission,
either asynchronously, or in response to a normal input message, and
GIST can also generate messages autonomously. Regardless of the
source, outgoing messages are passed downwards for message
transmission. When a message is available for transmission, GIST
uses internal policy and the stored routing state to determine how to
handle it. The following processing applies equally to locally
generated messages and messages forwarded from within the GIST or
signaling application levels. (However, see Section 5.6 for special
rules applying to the transmission of error messages by GIST.)
The main decision is whether the message must be sent in connection
mode or datagram mode. Reasons for using the former are:
o signaling application requirements: for example, it has requested
channel secured delivery, or reliable delivery;
o protocol specification: a message that requires fragmentation MUST
be sent over a messaging association;
o local policy: for example, a node MAY send messages over a
messaging association to benefit from adaptive congestion control.
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In principle, as well as determining that some messaging association
must be used, GIST MAY select between a set of alternatives, e.g. for
load sharing or because different messaging associations provide
different transport or security attributes.
If the use of a messaging association is selected, the message is
queued on the association found from the routing state table, and
further output processing is carried out according to the details of
the protocol stacks used. If no appropriate association exists, the
message is queued while one is created (see Section 4.4.1). If no
association can be created, this is an error condition, and should be
indicated back to the local signaling application.
If a messaging association is not required, the message is sent in
datagram mode. The processing in this case depends on the message
type and whether routing state exists or not.
o If the message is not a Query, and routing state exists, it is UDP
encapsulated and sent directly to the address from the routing
state table.
o If the message is a Query, then it is UDP encapsulated with IP
address and router alert option determined from the MRI and NSLPID
(further details depend on the message routing method).
o If no routing state exists, GIST can attempt to use the same
encapsulation as in the Query case. If this is not possible (e.g.
because the encapsulation for the message routing method is only
defined for one message direction), then this is an error
condition which is reported back to the local signaling
application.
4.3.4. Nodes not Hosting the NSLP
A node may receive messages where it has no signaling application
corresponding to the message NSLPID. There are several possible
cases depending mainly on the encapsulation:
1. A Query-encapsulated message contains an RAO value which is
relevant to NSIS but not to the specific node, but the IP layer
is unable to recognise whether it needs to be passed to GIST for
further processing or whether the packet should be forwarded just
like a normal IP datagram.
2. A Query-encapsulated message contains an RAO value which is
relevant to the node, but the specific signaling application
functionality for the actual NSLPID in the message is not
processed there.
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3. A directly addressed message (in datagram or connection mode) is
delivered to a node for which there is no corresponding signaling
application. (With the current specification, this should never
happen. While future versions might find a use for such a
feature, currently this MUST cause an "Unknown NSLPID" error
message, Appendix A.4.4.6.)
4. A Query-encapsulated message arrives at the end-system which does
not handle the signaling application. This is possible in normal
operation, and MUST be notified to the sender with an "Endpoint
Found" informational message (Appendix A.4.4.7). The end-system
includes the MRI and SID from the original message in the error
message without interpreting them.
5. The node is GIST-aware NAT. See Section 7.2.
In cases (1) and (2), the role of GIST is to forward the message
essentially unchanged, and it will not become a peer to the node
sending the message. (Forwarding with modified NSLP payloads is
covered above in Section 4.3.2.) However, a GIST implementation must
ensure that the IP TTL field and GIST hop count are managed correctly
to prevent message looping, and this should be done consistently
independently of whether the processing (e.g. for case (1)) takes
place on the fast path or in GIST-specific code. The rules are that
in cases (1) and (2), the IP TTL MUST be decremented just as if the
message was a normal IP forwarded packet; in case (2) the GIST hop
count MUST be decremented as in the case of normal input processing,
which indeed applies to cases (3) and (4).
A GIST node processing Query-encapsulated messages in this way SHOULD
make the routing decision based on the full contents of the MRI and
not only the IP destination address. It MAY also apply a restricted
set of sanity checks and under certain conditions return an error
message rather than forwarding the message. These conditions are:
1. The message is so large that it would be fragmented on downstream
links (e.g. because the downstream MTU is very small). The error
"Message Too Large" (Appendix A.4.4.8) SHOULD be returned to the
sender, which SHOULD begin messaging association setup.
2. The GIST hop count has been exceeded. The error "Hop Limit
Exceeded" (Appendix A.4.4.2) SHOULD be returned to the sender,
which MAY retry with a larger initial hop count if it is clear
that a loop has not been formed.
3. The MRI represents a flow definition which is too general to be
forwarded along a unique path (e.g. the destination address
prefix is too short). The error "MRI Validation Failure"
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(Appendix A.4.4.12) with subcode 0 ("MRI Too Wild") SHOULD be
returned to the sender, which MAY retry with restricted MRIs,
possibly starting additional signaling sessions to do so. If the
GIST node does not understand the MRM in question it MUST NOT
apply this check, instead forwarding the message transparently.
In the first two cases, only the common header is examined; in the
third case, the MRI is also examined. The rest of the message MUST
never be inspected or modified.
Note that the hop count is only intended to prevent message looping
at the GIST level, and by default NSLPs must take their own measures
to prevent looping at the application level. However, the GIST API
(Appendix B) provides the incoming hop count to the NSLPs, which can
preserve it on outgoing messages as they are forwarded further along
the path. This provides a lightweight loop-prevention mechanism for
NSLPs which do not define anything more sophisticated.
4.4. Routing State and Messaging Association Maintenance
The main responsibility of GIST is to manage the routing state and
messaging associations which are used in the message processing
described above. Routing state is installed and maintained by
specific GIST messages. Messaging associations depend on the
existence of routing state, but are actually set up by the normal
procedures of the transport and security protocols that comprise
them. Timers control routing state and messaging association refresh
and expiration.
There are two different cases for state installation and refresh:
1. Where routing state is being discovered or a new association is
to be established; and
2. Where an existing association can be re-used, including the case
where routing state for the flow is being refreshed.
These cases are now considered in turn, followed by the case of
background general management procedures.
4.4.1. State Setup
The complete sequence of possible messages for state setup between
adjacent peers is shown in Figure 4 and described in detail in the
following text. A concrete example is given in Appendix C.
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+----------+ +----------+
| Querying | |Responding|
| Node | | Node |
+----------+ +----------+
GIST-Query
----------------------> .............
Router Alert Option . Routing .
MRI/SID/NSLPID . state .
Q-Node Network Layer Info . installed .
Query Cookie . at .
[Q-Node Stack-Proposal . R-node(1) .
Q-Node Stack-Config-Data] .............
[NSLP Payload]
......................................
. The responder can use an existing .
. messaging association if available .
. from here onwards to short-circuit .
. messaging association setup .
......................................
GIST-Response
............. <----------------------
. Routing . MRI/SID/NSLPID
. state . R-Node Network Layer Info (D Mode only)
. installed . Query cookie
. at . [Responder Cookie
. Q-Node . [R-Node Stack-Proposal
............. R-Node Stack-Config-Data]]
[NSLP Payload]
....................................
. If a messaging association needs .
. to be created, it is set up here .
....................................
GIST-Confirm
---------------------->
MRI/SID/NSLPID .............
Q-Node Network Layer Info . Routing .
[Responder Cookie . state .
[R-Node Stack-Proposal . installed .
[Q-Node Stack-Config-Data]]] . at .
[NSLP Payload] . R-node(2) .
.............
Figure 4: Message Sequence at State Setup
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The initial message in any routing state maintenance operation is a
GIST-Query message, sent from the querying node and intercepted at
the responding node. This message has addressing and other
identifiers appropriate for the flow and signaling application that
state maintenance is being done for, addressing information about the
node itself, and it MAY contain an NSLP payload. It also includes a
Query Cookie, and optionally capability information about messaging
association protocol stacks. The role of the cookies in this and
subsequent messages is to protect against certain denial of service
attacks and to correlate the various events in the message sequence.
Provided that the signaling application has indicated that message
routing state should be set up (see Section 4.3.2), reception of a
GIST-Query MUST elicit a GIST-Response message. This is a 'normally'
encapsulated datagram mode message with additional payloads. It
contains network layer information about the responding node, echoes
the Query Cookie, and MAY contain an NSLP payload (possibly a
response to the NSLP payload in the initial message). In case a
messaging association was requested, it MUST also contain a Responder
Cookie and its own capability information about messaging association
protocol stacks. Even if a messaging association is not requested,
the Response MAY still include a Responder Cookie if the node's
routing state setup policy requires it (see below).
Setup of a new messaging association begins when peer addressing
information is available and a new messaging association is actually
needed. The setup MUST be contemporaneous with a specific GIST-
Query/Response exchange, because the addressing information used may
have a limited lifetime (either because it depends on limited
lifetime NAT bindings, or because it refers to agile destination
ports for the transport protocols). The Stack-Proposal and Stack-
Configuration-Data objects carried in the exchange carry capability
information about what messaging association protocols can be used,
and the processing of these objects is described in more detail in
Section 5.7. With the protocol options currently defined, setup of
the messaging association always starts from the Querying node,
although more flexible configurations are possible within the overall
GIST design. In any case, once set up the association itself can be
used equally in both directions.
Finally, a GIST-Confirm MUST be sent if the GIST-Response requested
it. If a messaging association is being used, the GIST-Confirm MUST
be sent over it before any other messages for the same flow, and it
echoes the Responder Cookie and Stack-Proposal from the GIST-
Response. The former is used to allow the receiver to validate the
contents of the message (see Section 8.5), and the latter is to
prevent certain bidding-down attacks on messaging association
security. The Confirm MAY also contain an abbreviated form of the
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original Stack-Configuration-Data to finalise details of the
messaging association configuration. The association can be used in
the upstream direction for that MRI and NSLPID after the Confirm has
been received.
The querying node MUST install the responder address as routing state
information after verifying the Query Cookie in the GIST-Response.
The responding node MAY install the querying address as peer state
information at two points in time:
1. after the receipt of the initial GIST-Query, or
2. after a GIST-Confirm message containing the Responder Cookie.
The precise constraints on when state information is installed are a
matter of security policy considerations on prevention of denial-of-
service attacks and state poisoning attacks, which are discussed
further in Section 8. Because the responding node MAY choose to
delay state installation as in case (2), the GIST-Confirm must
contain sufficient information to allow it to be processed
identically to the original Query. This places some special
requirements on NAT traversal and cookie functionality, which are
discussed in Section 7.2 and Section 8 respectively.
4.4.2. Association Re-use
It is a design goal of GIST that, so far as possible, messaging
associations should be re-used for multiple flows and sessions,
rather than a new association set up for each. This is to ensure
that the association cost scales only like the number of peers, and
to avoid the latency of new association setup where possible.
However, re-use requires the identification of an existing
association which matches the same routing state and desired
properties that would be the result of a full handshake in D-mode,
and this identification must be done as reliably and securely as
continuing with the full procedure. Note that this requirement is
complicated by the fact that NATs may remap the node addresses in
D-mode messages, and also interacts with the fact that some nodes may
peer over multiple interfaces (and so with different addresses).
Association re-use is controlled by the Network-Layer-Information
(NLI) object, which is carried in GIST-Query/Confirm and optionally
GIST-Response messages. The NLI object includes:
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Peer-Identity: For a given node, this is an interface independent
value with opaque syntax. It MUST be chosen so as to have a high
probability of uniqueness between peers, and SHOULD be stable (at
least between restarts). Note that there is no cryptographic
protection of this identity (attempting to provide this would
essentially duplicate the functionality in the messaging
association security protocols).
Interface-Address: This is an IP address through which the signaling
node can be reached. There may be several choices available for
the Interface-Address, and further discussion of this is contained
in Section 5.2.2.
By default, a messaging association is associated with the NLI object
that was provided by the peer in the Query/Response/Confirm at the
time the association was set up. There may be more than one
association for a given NLI object (e.g. with different properties).
Association re-use is controlled by matching the NLI provided in a
GIST message with those associated with existing associations. This
can be done on receiving either a GIST-Query or GIST-Response (the
former is more likely):
o If there is a perfect match to the NLI of an existing association,
that association SHOULD be re-used (provided it has the
appropriate properties in other respects). This is indicated by
sending the remaining messages in the handshake over that
association. This will only fail (i.e. lead to re-use of an
association to the 'wrong' node) if signaling nodes have colliding
Peer-Identities, and one is reachable at the same Interface-
Address as another. (This could be done by an on-path attacker.)
o In all other cases, the full handshake MUST be executed in
datagram mode as usual. There are in fact four possibilities:
1. Nothing matches: this is clearly a new peer.
2. Only the Peer-Identity matches: this may be either a new
interface on an existing peer, or a changed address mapping
behind a NAT, or an attacker attempting to hijack the Peer-
Identity. These should be rare events, so the expense of a
new association setup is acceptable.
3. Only the Interface-Address matches: this is probably a new
peer behind the same NAT as an existing one. A new
association setup is required.
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4. The full NLI object matches: this is a degenerate case, where
one node recognises an existing peer, but wishes to allow the
option to set up a new association in any case (for example to
create an association with different properties).
4.4.3. State Maintenance Procedures
Refresh and expiration of all types of state is controlled by timers.
Each item of routing state expires after a validity lifetime which is
negotiated during the Query/Response/Confirm handshake. The NLI
object in the Query contains a proposal for the lifetime value, and
the NLI in the Response contains the value the Responding node
requires. The Querying node MUST generate a GIST-Query message
before this timer expires, if it believes that the flow is still
active; otherwise, the Responding node MAY delete the state. Receipt
of the message at the Responding node will refresh peer addressing
state for one direction, and receipt of a GIST-Response at the
querying node will refresh it for the other. There is no mechanism
at the GIST level for explicit teardown of routing state. However,
GIST MUST NOT refresh routing state if a flow is known to be
inactive, either because upstream state has expired, or because the
signaling application has indicated via the GIST API (Appendix B.5)
that the state is no longer required, because this would prevent
correct state repair in the case of network rerouting.
Unneeded messaging associations are torn down by GIST, using the
teardown mechanisms of the underlying transport or security protocols
if available (for example, simply by closing a TCP connection). The
teardown can be initiated by either end. Whether an association is
needed is a combination of two factors:
o local policy, which could take into account the cost of keeping
the messaging association open, the level of past activity on the
association, and the likelihood of future activity (e.g. if there
is routing state still in place which might generate messages to
use it).
o whether the peer still wants the association in place. During
messaging association setup, each node indicates its own MA-Hold-
Time as part of the Stack-Configuration-Data. (Because the
Responding node can choose not to retain state until a Confirm
message, an abbreviated Stack-Configuration-Data object containing
just this information MUST be repeated by the Querying node in the
first Confirm sent on a new messaging association.) A node MUST
NOT tear down the association if it has received traffic from its
peer over that period. A peer which has generated no traffic but
still wants the association retained SHOULD use a special 'null'
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message (GIST-MA-Hello) to indicate the fact.
Messaging associations can always be set up on demand, and messaging
association status is not made directly visible outside the GIST
layer. Therefore, even if GIST tears down and later re-establishes a
messaging association, signaling applications cannot distinguish this
from the case where the association is kept permanently open. To
maintain the transport semantics described in Section 4.1, GIST MUST
close transport connections carrying reliable messages gracefully or
report an error condition, and MUST NOT open a new association for a
given session and peer while messages on a previous association may
still be outstanding.
This specification defines precisely only the time at which state or
messaging associations expire; it does not define when refresh
transactions should be initiated. Implementations SHOULD select
timer settings which take at least the following into account:
o The transmission latency between source and destination;
o The need for retransmissions (either explicitly or within the
messaging association protocols);
o The need to avoid network synchronisation of control traffic (cf.
[34]).
In most cases, a reasonable policy is (for example) to initiate the
refresh process when between 1/2 and 3/4 of the appropriate validity
time has elapsed since the last successful refresh. The actual
moment is chosen randomly within this interval to avoid
synchronisation effects.
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5. Message Formats and Transport
5.1. GIST Messages
All GIST messages begin with a common header, followed by a sequence
of type-length-value (TLV) objects. This subsection describes the
various GIST messages and their contents at a high level; a more
detailed description of the header and each object is given in
Section 5.2.
The common header includes a version number, message type and size,
and NSLPID. It also carries a hop count to prevent message looping
and various control flags, including one to indicate if a reply of
some sort is requested. The objects following the common header MUST
be carried in a fixed order, depending on message type. Messages
with missing, duplicate or invalid objects for the message type MUST
be rejected with an "Object Type Error" error message with the
appropriate subcode (Appendix A.4.4.9).
The following gives the basic syntax of GIST messages in ABNF [7].
Note that the NAT traversal mechanism for GIST involves the insertion
of an additional NAT-Traversal object in certain messages; the rules
for this are given in Section 7.2.
GIST-Message: The main messages are either one of the stages in the
3-way handshake, or a simple message carrying NSLP data. Additional
types are allocated for errors and messaging association keepalive.
GIST-Message = GIST-Query / GIST-Response /
GIST-Confirm / GIST-Data /
GIST-Error / GIST-MA-Hello
GIST-Query: A GIST-Query MUST be sent in datagram mode. As well as
the common header, it contains certain mandatory control objects, and
MAY contain a signaling application payload. A stack proposal and
configuration data MUST be included if the message exchange relates
to setup of a messaging association. The R flag MUST always be set
(R=1) in a Query, since this message always elicits a Response.
GIST-Query = Common-Header
Message-Routing-Information
Session-Identification
Network-Layer-Information
Query-Cookie
[ Stack-Proposal Stack-Configuration-Data ]
[ NSLP-Data ]
GIST-Response: A GIST-Response may be sent in datagram or connection
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mode (if a messaging association is being re-used). It MUST echo the
MRI (with inverted direction), SID and Query-Cookie of the Query, and
in D-mode carries its own Network-Layer-Information; if the message
exchange relates to setup of a messaging association (which can only
take place in datagram mode), a Responder cookie MUST be included, as
well as its own stack proposal and configuration data. The R flag
MUST be set (R=1) if a Responder cookie is present but otherwise is
optional; if the R flag is set, a Confirm MUST be sent as a reply.
GIST-Response = Common-Header
Message-Routing-Information
Session-Identification
[ Network-Layer-Information ]
Query-Cookie
[ Responder-Cookie
[ Stack-Proposal Stack-Configuration-Data ] ]
[ NSLP-Data ]
GIST-Confirm: A GIST-Confirm may be sent in datagram or connection
mode (if a messaging association has been re-used). It MUST echo the
MRI (with inverted direction), SID, and Responder-Cookie if the
Response carried one; if the message exchange relates to setup of a
new messaging association or reuse of an existing one (which can only
take place in connection mode), the message MUST also echo the Stack-
Proposal from the GIST-Response so it can be verified that this has
not been tampered with. The first message on an association MUST
also repeat the Stack-Configuration-Data from the original Query in
an abbreviated form (just containing the MA-Hold-Time).
GIST-Confirm = Common-Header
Message-Routing-Information
Session-Identification
Network-Layer-Information
[ Responder-Cookie
[ Stack-Proposal
[ Stack-Configuration-Data ] ] ]
[ NSLP-Data ]
GIST-Data: A plain data message contains no control objects, but only
the MRI and SID associated with the NSLP data being transferred.
Network-Layer-Information MUST be carried in the datagram mode case
and not otherwise.
GIST-Data = Common-Header
Message-Routing-Information
Session-Identification
[ Network-Layer-Information ]
NSLP-Data
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GIST-Error: A GIST-Error message reports a problem determined at the
GIST level. (Errors generated by signaling applications are reported
in NSLP-Data payloads and are not treated specially by GIST.) The
message includes a Network-Layer-Information object for the
originator of the error message it if is being sent in datagram mode;
all other information related to the error is carried in a GIST-
Error-Data object.
GIST-Error = Common-Header
[ Network-Layer-Information ]
GIST-Error-Data
GIST-MA-Hello: This message MUST be sent only in C-Mode to indicate
that a node wishes to keep a messaging association open. It contains
only the common header, with a null NSLPID. The R flag MAY be set
(R=1) to indicate that a reply is requested, thus allowing a node to
test the liveness of the peer.
GIST-MA-Hello = Common-Header
5.2. Information Elements
This section describes the content of the various objects that can be
present in each GIST message, both the common header, and the
individual TLVs. The bit formats are provided in Appendix A.
5.2.1. The Common Header
Each message begins with a fixed format common header, which contains
the following information:
Version: The version number of the GIST protocol.
Length: The number of 32 bit words in the message following the
common header.
Upper layer identifier (NSLPID): This gives the specific NSLP that
this message is used for.
GIST hop counter: A hop counter to prevent a message from looping.
Message type: The message type (Query, Response, etc.)
Source addressing mode: If set (S=1), this indicates that the IP
source address of the message is the same as the signaling source
address, in which case replies to this message can be sent safely
to this address. It is cleared (S=0) if the IP source address was
derived from the message routing information in the payload and
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this is different from the signaling source address.
Response requested: A flag which if set (R=1) indicates that a
message should be sent in response to this message.
Explicit routing: A flag which if set (E=1) indicates that the
message was explicitly routed (see Section 7.1.4).
5.2.2. TLV Objects
All data following the common header is encoded as a sequence of
type-length-value objects. Currently, each object can occur at most
once; the set of required and permitted objects is determined by the
message type and encapsulation.
Message-Routing-Information (MRI): Information sufficient to define
how the signaling message should be routed through the network.
Message-Routing-Information = message-routing-method
method-specific-information
The format of the method-specific-information depends on the
message-routing-method requested by the signaling application. It
is provided by the NSLP in the message sender and used by GIST to
select the message routing.
Session-Identification (SID): The GIST session identifier is a 128
bit, cryptographically random identifier chosen by the node which
originates the signaling exchange. See Section 3.4.
Network-Layer-Information: This object carries information about the
network layer attributes of the node sending the message,
including data related to the management of routing state. This
includes a peer identity and IP address for the sending node. It
also includes IP TTL information to allow the hop count between
GIST peers to be measured and reported, and a validity time for
the routing state.
Network-Layer-Information = peer-identity
interface-address
RS-validity-time
IP-TTL
The use of the RS-validity-time field is described in
Section 4.4.3. The peer-identity and interface-address are used
for matching existing associations, as discussed in Section 4.4.2.
The interface-address must be routable, i.e. it MUST be usable as
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a destination IP address for packets to be sent back to the node
generating the signaling message (whether in datagram or
connection mode). Where this object is carried in a GIST-Query or
GIST-Confirm, the interface-address MUST specifically be set to an
address bound to the interface associated with the MRI (e.g. the
one carrying the outbound flow), to allow its use in route change
handling, see Section 7.1. A node may have several choices for
which of its addresses to use as the interface-address. For
example, there may be a choice of IP versions, or addresses of
limited scope (e.g. link-local), or addresses bound to different
interfaces in the case of a router or multi-homed host. However,
some of these interface addresses may not be usable by the peer.
A node SHOULD follow a default policy of using a global address of
the same IP version as in the MRI, unless it can establish that an
alternative address would also be usable.
The setting and interpretation of the IP-TTL field depends on the
message direction (as determined from the MRI) and encapsulation.
* If the message is downstream, the IP-TTL MUST be set to the TTL
that will be set in the IP header for the message (if this can
be determined), or else set to 0.
* On receiving a downstream message in datagram mode, a non-zero
IP-TTL is compared to the TTL in the IP header, and the
difference is stored as the IP-hop-count-to-peer for the
upstream peer in the routing state table for that flow.
Otherwise, the field is ignored.
* If the message is upstream, the IP-TTL MUST be set to the value
of the IP-hop-count-to-peer stored in the routing state table,
or 0 if there is no value yet stored.
* On receiving an upstream message, the IP-TTL is stored as the
IP-hop-count-to-peer for the downstream peer.
In all cases, the TTL value reported to signaling applications is
the one stored with the routing state for that flow, after it has
been updated (if appropriate) from processing the message in
question.
Stack-Proposal: This field contains information about which
combinations of transport and security protocols are available for
use in messaging associations, and is also discussed further in
Section 5.7.
Stack-Proposal = 1*stack-profile
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stack-profile = 1*protocol-layer
Each protocol-layer field identifies a protocol with a unique tag;
any additional data (e.g. higher-layer addressing or other options
data) associated with the protocol will be carried in a MA-
protocol-options field in the Stack-Configuration-Data TLV (see
below).
Stack-Configuration-Data: This object carries information about the
overall configuration of a messaging association.
Stack-Configuration-Data = MA-Hold-Time
0*MA-protocol-options
The MA-Hold-Time field indicates how long a node will hold open an
inactive association; see Section 4.4.3 for more discussion. The
MA-protocol-options fields give the configuration of the protocols
to be used for new messaging associations, and they are described
in more detail in Section 5.7.
Query-Cookie/Responder-Cookie: A Query-Cookie is contained in a GIST-
Query message and MUST be echoed in a GIST-Response; a Responder-
Cookie MAY be sent in a GIST-Response message, and if present MUST
be echoed in the following GIST-Confirm message. Cookies are
variable length (chosen by the cookie generator). See Section 8.5
for further details on requirements and mechanisms for cookie
generation.
NSLP-Data: The NSLP payload to be delivered to the signaling
application. GIST does not interpret the payload content.
GIST-Error-Data: This contains all the information to determine the
cause and context of an error.
GIST-Error-Data = error-class error-code error-subcode
common-header
[ Message-Routing-Information-content ]
[ Session-Identification-content ]
0*additional-information
[ comment ]
The error-class indicates the severity level, and the error-code
and error-subcode identify the specific error itself. A full list
of GIST errors and their severity levels is given in Appendix A.4.
The common-header from the original message is always included, as
are the contents of the Message-Routing-Information and Session-
Identification objects if they were successfully decoded. For
some errors, additional information fields must be included
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according to a fixed format; finally, an optional free-text
comment may be added.
5.3. Datagram Mode Transport
This section describes the various encapsulation options for datagram
mode messages. Although there are several possibilities, depending
on message type, message routing method, and local policy, the
general design principle is that the sole purpose of the
encapsulation is to ensure that the message is delivered to or
intercepted at the correct peer. Beyond that, minimal significance
is attached to the type of encapsulation or the values of addresses
or ports used for it. This allows new options to be developed in the
future to handle particular deployment requirements without modifying
the overall protocol specification.
5.3.1. Normal Encapsulation
Normal encapsulation MUST be used for all datagram mode messages
where the signaling peer is already known from previous signaling.
This includes Response and Confirm messages, and Data messages except
if these are being sent without using local routing state. Normal
encapsulation is simple: the complete set of GIST payloads is
concatenated together with the common header, and placed in the data
field of a UDP datagram. UDP checksums MUST be enabled. The message
is IP addressed directly to the adjacent peer; the UDP port numbering
MUST be compatible with that used on Query messages (see below), that
is, the same for messages in the same direction and swapped
otherwise.
5.3.2. Query Encapsulation
Query encapsulation MUST be used for messages where no routing state
is available or where the routing state is being refreshed, in
particular for GIST-Query messages. Query encapsulation is similar
to normal encapsulation, with changes in IP address selection, IP
options, and a defined method for selecting UDP ports.
In general, the IP addresses are derived from information in the MRI;
the exact rules depend on the message routing method. In addition,
the IP header is given a Router Alert Option ([1] and [4]) to assist
the peer in intercepting the message depending on the NSLPID. Each
NSLPID corresponds to a unique RAO value, but not necessarily vice
versa; further details are discussed in [36].
The source UDP port is selected by the message sender as the port at
which it is prepared to receive UDP messages in reply, and a
destination UDP port is allocated by IANA (see Section 9). Note that
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GIST may send messages addressed as {flow sender, flow receiver}
which could make their way to the flow receiver even if that receiver
were GIST-unaware. These should be rejected (with an ICMP message)
rather than delivered to the user application (which would be unable
to use the source address to identify it as not being part of the
normal data flow). Therefore, a "well-known" port is required.
5.3.3. Retransmission and Rate-Control
Datagram mode uses UDP, and hence has no automatic reliability or
congestion control capabilities. Signaling applications requiring
reliability should be serviced using C-mode, which should also carry
the bulk of signaling traffic. However, some form of messaging
reliability is required for the GIST control messages themselves, as
is rate control to handle retransmissions and also bursts of
unreliable signaling or state setup requests from the signaling
applications.
Query messages which do not receive Responses MAY be retransmitted;
retransmissions MUST use a binary exponential backoff, with an
initial timeout of T1 up to a maximum of T2 seconds. Retransmitted
Queries MUST use different Query-Cookie values. These rules apply
equally to the message that first creates routing state, and those
that refresh it. The values of T1 and T2 are implementation defined.
Note that Queries may go unanswered either because of message loss
(in either direction), or because there is no reachable GIST peer.
Therefore, implementations should trade off reliability (large T2)
against promptness of error feedback to applications (small T2). If
the Query message carries NSLP data, it may be delivered multiple
times to the signaling application. If the NSLP has indicated a
timeout on the validity of this payload (see Appendix B.1), T2 SHOULD
be chosen to be less than this value.
This algorithm is sufficient to handle lost Queries and Responses.
The case of a lost Confirm is more subtle. Notionally, we can
distinguish between two cases:
1. Where the Responding node is already prepared to store per-flow
state after receiving a single (Query) message. This would
include any cases where the node has NSLP data queued to send.
Here, the Responding node MAY run a retransmission timer to
resend the Response message until a Confirm is received, since
the node is already managing state for that flow. The problem of
an amplification attack stimulated by a malicious Query is
handled by requiring the cookie mechanism to enable the node
receiving the Response to discard it efficiently if it does not
match a previously sent Query.
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2. Where the responding node is not prepared to store per-flow state
until receiving a properly formed Confirm message.
In case (2), a retransmission timer should not be required. However,
we can assume that the next signaling message will be in the
direction Querying Node -> Responding Node (if there is no 'next
signaling message' the fact that the Confirm has been lost is moot).
In this case, the responding node will start to receive messages at
the GIST level for a MRI/NSLP combination for which there is no
stored routing state (since this state is only created on receipt of
a Confirm).
Therefore, the error condition is detected at the Responding node
when such a message arrives, without the need for a specific timer.
Recovery requires a Confirm to be transmitted and successfully
received. The mechanism to cause this is that the Responding node
MUST reject the incoming message with a "No Routing State" error
message (Appendix A.4.4.5) back to the Querying node, which MUST
interpret this as caused by a lost Confirm; the Querying node MUST
regenerate the Confirm purely from local state (e.g. in particular it
needs to remember a valid Responder Cookie).
In all cases, Responses MUST be sent promptly to avoid spurious
retransmissions. Nodes generating any type of retransmission MUST be
prepared to receive (and match) a reply to any of them (not just the
one most recently sent).
The basic rate-control requirements for datagram mode traffic are
deliberately minimal. A single rate limiter applies to all traffic
(for all interfaces and message types). It applies to
retransmissions as well as new messages, although an implementation
MAY choose to prioritise one over the other. When the rate limiter
is in effect, datagram mode messages are queued until transmission is
re-enabled, or an error condition MAY be indicated back to local
signaling applications. The rate limiting mechanism is
implementation defined, but it is RECOMMENDED that a token bucket
limiter as described in [26] be used.
5.4. Connection Mode Transport
Encapsulation in connection mode is more complex, because of the
variation in available transport functionality. This issue is
treated in Section 5.4.1. The actual encapsulation is given in
Section 5.4.2.
5.4.1. Choice of Transport Protocol
It is a general requirement of the NTLP defined in [22] that it
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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.
TCP provides both bundling and fragmentation, but not message
boundaries. However, the length information in the common header
allows the message boundary to be discovered during parsing.
SCTP [12] satisfies all requirements.
DCCP [25] is message based but does not provide bundling or
fragmentation. Bundling can be carried out by the GIST layer
sending multiple messages in a single datagram; because the common
header includes length information, the message boundaries within
the datagram can be discovered during parsing. Fragmentation of
GIST messages over multiple datagrams should be avoided, because
of amplification of message loss rates that this would cause.
The bundling together of small messages is either built into the
transport protocol or can be carried out by the GIST layer during
message construction. Either way, two approaches can be
distinguished:
1. As messages arrive for transmission they are gathered into a
bundle until a size limit is reached or a timeout expires (cf.
the Nagle algorithm of TCP or similar optional functionality in
SCTP). This provides maximal efficiency at the cost of some
latency.
2. Messages awaiting transmission are gathered together while the
node is not allowed to send them (e.g. because it is congestion
controlled).
The second type of bundling is always appropriate. For GIST, the
first type SHOULD NOT be used for 'trigger' (i.e. state-changing)
messages, but may be appropriate for refresh messages. These
distinctions are known only to the signaling applications, but MAY be
indicated (as an implementation issue) by setting the priority
transfer attribute.
It can be seen that all of these transport protocol options can be
supported by the basic GIST message format already presented. GIST
messages requiring fragmentation must be carried using a reliable
transport protocol, TCP or SCTP. This specification defines only the
use of TCP, but other possibilities could be included without
additional work on message formatting.
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5.4.2. Encapsulation Format
The GIST message, consisting of common header and TLVs, is carried
directly in the transport protocol (possibly incorporating transport
layer security protection). Further messages can be carried in a
continuous stream (for TCP), or up to the next transport layer
message boundary (for SCTP/DCCP/UDP). This situation is shown in
Figure 5.
+---------------------------------------------+
| L2 Header |
+---------------------------------------------+
| IP Header | ^
| Source address = signaling source | ^
| Destination address = signaling destination | .
+---------------------------------------------+ .
| L4 Header | . ^
| (Standard TCP/SCTP/DCCP/UDP header) | . ^
+---------------------------------------------+ . .
| GIST Message | . . ^
| (Common header and TLVs as in section 5.1) | . . ^ Scope of
+---------------------------------------------+ . . . security
| Additional GIST messages, each with its | . . . protection
| own common header, either as a continuous | . . . (depending
| stream, or continuing to the next L4 | . . . on channel
. message boundary . . . . security
. . V V V mechanism
. . V V V in use)
Figure 5: Connection Mode Encapsulation
5.5. Message Type/Encapsulation Relationships
GIST has four primary message types (Query/Response/Confirm/Data) and
three possible encapsulation methods (D-Mode Normal/D-Mode Query/
C-Mode). For information, the possible combinations of message type
and encapsulation are given in the table below. In some cases there
are several possible choices, depending on the existence of routing
state or messaging associations. The rules governing GIST policy,
including whether or not to create such state to handle a message,
are described normatively in the other sections of this
specification. If a message arrives with an invalid encapsulation
(e.g. a Query arrives over a messaging association), this MUST be
rejected with an "Incorrect Encapsulation" error message
(Appendix A.4.4.3). However, it should be noted that the processing
of the message at the receiver is not otherwise affected by the
encapsulation method used, with the exception that the decapsulation
process may provide additional information (e.g. translated addresses
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or IP hop count) which is used in the subsequent message processing.
+---------------+---------------+-------------------+---------------+
| Message | D-Mode Normal | D-Mode Query | C-Mode |
+---------------+---------------+-------------------+---------------+
| GIST-Query | Never | Always | Never |
| | | | |
| GIST-Response | Unless a | Never | If a |
| | messaging | | messaging |
| | association | | association |
| | is being | | is being |
| | re-used | | re-used |
| | | | |
| GIST-Confirm | Unless a | Never | If a |
| | messaging | | messaging |
| | association | | association |
| | has been set | | has been set |
| | up or is | | up or is |
| | being re-used | | being re-used |
| | | | |
| GIST-Data | If routing | If no routing | If a |
| | state exists | state exists and | messaging |
| | for the flow | the MRI can be | association |
| | but no | used to derive | exists |
| | messaging | the query | |
| | association | encapsulation | |
+---------------+---------------+-------------------+---------------+
5.6. Error Message Processing
Special rules apply to the encapsulation and transmission of error
messages.
GIST only generates error messages in response to incoming messages.
(Error messages MUST NOT be generated in response to incoming error
messages.) The routing and encapsulation of the error message is
derived from that of the message that caused the error; in
particular, local routing state is not consulted. Routing state and
messaging association state MUST NOT be created to handle the error,
and error messages MUST NOT be retransmitted explicitly by GIST,
although they are subject to the same rate control as other messages.
o If the incoming message was received in datagram mode, the error
MUST be sent in datagram mode using the 'normal' encapsulation,
using the addressing information from the NLI object in the
incoming message. If the NLI could not be determined, the error
MUST be sent to the IP source of the incoming message if the S
flag was set in it. The NLI object in the GIST-Error message
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reports information about the generator of the error.
o If the incoming message was received over a messaging association,
the error MUST be sent back over the same messaging association.
The NSLPID in the common header of the GIST-Error is the null value
(as for GIST-MA-Hello). If for any reason the error message cannot
be sent (for example, because an error message is too large to send
in datagram mode), an error should be logged locally.
5.7. Messaging Association Setup
5.7.1. Overview
A key attribute of GIST is that it is flexible in its ability to use
existing transport and security protocols. Different transport
protocols may have performance attributes appropriate to different
environments; different security protocols may fit appropriately with
different authentication infrastructures. Even given an initial
default mandatory protocol set for GIST, the need to support new
protocols in the future cannot be ruled out, and secure feature
negotiation cannot be added to an existing protocol in a backwards-
compatible way. Therefore, some sort of capability discovery is
required.
Capability discovery is carried out in GIST-Query/Response messages,
using Stack-Proposal and Stack-Configuration-Data objects. If a new
messaging association is required it is then set up, followed by a
GIST-Confirm. Messaging association re-use is achieved by short-
circuiting this exchange by sending the GIST-Response or GIST-Confirm
messages on an existing association (Section 4.4.2); whether to do
this is a matter of local policy. The end result of this process is
a messaging association which is a stack of protocols. If multiple
associations exist, it is a matter of local policy how to distribute
messages over them, subject to respecting the transfer attributes
requested for each message.
Every possible protocol for a messaging association has the following
attributes:
o MA-Protocol-ID, a 1-byte IANA assigned value (see Section 9).
o A specification of the (non-negotiable) policies about how the
protocol should be used (for example, in which direction a
connection should be opened).
o [Depending on the specific protocol:] Formats for an MA-protocol-
options field to carry the protocol addressing and other
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configuration information in the Stack-Configuration-Data object.
The format may differ depending on whether the field is present in
the Query or Response. Some protocols do not require the
definition of such additional data, in which case no corresponding
MA-protocol-options field will occur in the SCD object.
A Stack-Proposal object is simply a list of profiles; each profile is
a sequence of MA-Protocol-IDs. A profile lists the protocols in 'top
to bottom' order (e.g. TLS over TCP, or TCP over IPsec, etc.) A
Stack-Proposal is generally accompanied by a Stack-Configuration-Data
object which carries an MA-protocol-options field for any protocol
listed in the Stack-Proposal which needs it. An MA-protocol-options
field may apply globally (to all instances of the protocol in the
Stack-Proposal) or be tagged as applying to a specific instance; for
example, this can be used to carry different port numbers for TCP
depending on whether it is to be used with or without TLS. An MA-
protocol-options field may also be flagged as 'not usable'; for
example, a NAT which could not handle SCTP would set this in an MA-
protocol-options field about SCTP. A protocol flagged this way MUST
NOT be used for a messaging association. If the Stack-Proposal and
Stack-Configuration-Data are both present but not consistent (e.g.
they refer to different protocols, or an MA-protocol-options field
refers to a non-existent profile), an "Object Value Error" error
message (Appendix A.4.4.10) with subcode 5 ("SP-SCD Mismatch") MUST
be returned and the message dropped.
A node generating a Stack-Configuration-Data object MUST honour the
implied protocol configurations for the period during which a
messaging association might be set up; in particular, it MUST be
immediately prepared to accept incoming datagrams or connections at
the protocol/port combinations advertised. However, the object
contents MUST be retained only for the duration of the Query/Response
exchange and any following association setup, and afterwards
discarded. (They may become invalid because of expired bindings at
intermediate NATs, or because the advertising node is using agile
ports.)
A GIST-Query requesting association setup always contains a Stack-
Proposal and Stack-Configuration-Data object, and unless re-use
occurs, the GIST-Response does so also. For a GIST-Response, the
Stack-Proposal MUST NOT depend on the GIST-Query. A node MAY make
different proposals depending on the combination of interface and
NSLPID. Once the messaging association is set up, the querying node
repeats the responder's Stack-Proposal over it in the GIST-Confirm.
The responding node MUST verify this to ensure that no bidding-down
attack has occurred; see Section 8.6 for further discussion.
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5.7.2. Protocol Definition: Forwards-TCP
This MA-Protocol-ID denotes a basic use of TCP between peers.
Support for this protocol is REQUIRED; associations using it can
carry messages with the transfer attribute Reliable=True. The
connection is opened in the forwards direction, from the querying
node, towards the responder at a previously advertised port. If this
protocol is offered, MA-protocol-options data MUST also be carried in
the SCD object. The MA-protocol-options field formats are:
o in a Query: no information apart from the field header.
o in a Response: 2 byte port number at which the connection will be
accepted, followed by 2 pad bytes.
5.7.3. Protocol Definition: Transport Layer Security
This MA-Protocol-ID denotes a basic use of transport layer channel
security. Support for this protocol is mandatory; associations using
it can carry messages with the transfer attribute Secure=True. For
use with TCP, implementation of TLS1.0 [6] is REQUIRED and
implementation of TLS1.1 [8] is RECOMMENDED. (If an unreliable
transport such as DCCP or UDP is defined for GIST in the future, this
MA-Protocol-ID would be implemented for it using DTLS [35].) GIST
nodes supporting TLS1.0 or TLS1.1 MUST be able to negotiate the TLS
ciphersuite TLS_RSA_WITH_3DES_EDE_CBC_SHA and SHOULD be able to
negotiate the TLS ciphersuite TLS_RSA_WITH_AES_128_CBC_SHA.
The default mode of TLS authentication (which applies in particular
to the above ciphersuites) uses a client/server certificate exchange.
The Querying node acts as a TLS client, and the Responding node acts
as a TLS server. Where one of the above ciphersuites is negotiated,
the GIST node acting as a server MUST provide a certificate, and MUST
request one from the GIST node acting as a TLS client. This allows
either server-only or mutual authentication, depending on the
certificates available to the client and the policy applied at the
server.
GIST nodes MAY negotiate other TLS ciphersuites. In some cases, the
negotiation of alternative ciphersuites is used to trigger
alternative authentication procedures (for example, the use of pre-
shared keys, [24]). The use of other authentication procedures may
require additional specification work to define how they can be used
as part of TLS within the GIST framework, and may or may not require
the definition of additional MA-Protocol-IDs.
No MA-protocol-options field is required for this use of TLS.
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5.7.4. Additional Protocol Options
Further protocols or configurations could be defined in the future
for additional performance or flexibility. Examples are:
o SCTP or DCCP as alternatives to TCP, with essentially the same
configuration.
o SigComp [17] for message compression.
o IPsec [30], ssh [31], or HIP/IPsec [32] for channel security.
o Alternative modes of TCP operation, for example where it is set up
from the responder to the querying node.
5.8. Specific Message Routing Methods
Each message routing method (see Section 3.3) requires the definition
of the format of the message routing information (MRI) and Query-
encapsulation rules. These are given in the following subsections
for the various possible message routing methods.
5.8.1. The Path-Coupled MRM
5.8.1.1. Message Routing Information
For the path-coupled MRM, this is essentially the Flow Identifier as
in [22]. 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 4.3.4
and Section 7.2 for further discussion).
MRI = network-layer-version
source-address prefix-length
destination-address prefix-length
IP-protocol
diffserv-codepoint
[ flow-label ]
[ ipsec-SPI / L4-ports]
Additional control information defines whether the flow-label, SPI
and port information are present, and whether the IP-protocol and
diffserv-codepoint fields should be interpreted as significant. The
source and destination addresses MUST be real node addresses, but
prefix lengths other than 32/128 (for IPv4/6) MAY be used to
implement address 'wildcarding', allowing the MRI to refer to traffic
to or from a wider address range. The MRI format allows a
potentially very large number of different flag and field
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combinations. A GIST implementation that cannot interpret the MRI in
a message MUST return an "Object Value Error" message
(Appendix A.4.4.10) with subcodes 1 ("Value Not Supported") or 2
("Invalid Flag-Field Combination") and drop the message.
5.8.1.2. Downstream Query Encapsulation
Where the signaling message is travelling in the same ('downstream')
direction as the flow defined by the MRI, the IP addressing for Query
messages is as follows. Support for this encapsulation is REQUIRED.
o The destination address MUST be the flow destination address as
given in the MRI of the message payload.
o By default, the source address is the flow source address, again
from the MRI. This provides the best likelihood that the message
will be correctly routed through any region performing per-packet
policy-based forwarding or load balancing which takes the source
address into account. However, there may be circumstances where
the use of the signaling source address is preferable, such as:
* In order to receive ICMP error messages about the Query message
(such as unreachable port or address). If these are delivered
to the flow source rather than the signaling source, it will be
very difficult for the querying node to detect that it is the
last GIST node on the path.
* In order to receive GIST error messages where the error message
sender could not interpret the NLI in the original message.
* In order to attempt to run GIST through an unmodified NAT,
which will only process and translate IP addresses in the IP
header.
Because of these considerations, use of the signaling source
address is allowed as an option, with use based on local policy.
A node SHOULD use the flow source address for initial Query
messages, but SHOULD transition to the signaling source address
for some retransmissions or as a matter of static configuration
(e.g. if a NAT is known to be in the path out of a certain
interface). A flag in the common header tells the message
receiver which option was used.
It is vital that the Query message mimics the actual data flow as
closely as possible, since this is the basis of how the signaling
message is attached to the data path. To this end, GIST SHOULD set
the DiffServ codepoint and (for IPv6) flow label to match the values
in the MRI.
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Any message sent in datagram mode SHOULD be below a conservative
estimate of the path MTU, for which this specification takes the
value 512 bytes as a default. It is possible that fragmented
datagrams including an RAO will not be correctly handled in the
network, so the sender SHOULD set the DF (do not fragment) bit in the
IPv4 header in order to detect that a message has encountered a link
with an unusually low MTU. In this case, it MUST use the signaling
source address for the IP source address in order to receive the ICMP
error.
A GIST implementation SHOULD apply validation checks to the MRI, to
reject Query messages that are being injected by nodes with no
legitimate interest in the flow being signalled for. In general, if
the GIST node can detect that no flow could arrive over the same
interface as the Query message, it MUST be rejected with an
appropriate error message. (Such checks apply only to messages with
the query encapsulation, since only those messages are required to
track the flow path.) The main checks are that the IP version should
match the version(s) used on that interface, and that the full range
of source addresses (the source-address masked with its prefix-
length) would pass ingress filtering checks. For these cases, the
error message is "MRI Validation Failure" (Appendix A.4.4.12) with
subcodes 1 or 2 ("IP Version Mismatch" or "Ingress Filter Failure")
respectively.
5.8.1.3. Upstream Query Encapsulation
In some deployment scenarios it is desirable and logically possible
to set up routing state in the upstream direction (from flow receiver
towards the sender). This could be used to support firewall
signaling to control traffic from an 'un-cooperative' sender, or
signaling in general where the flow sender was not NSIS-capable.
This is incorporated into GIST by defining an encapsulation and
processing rules for sending Query messages upstream.
In general, it is not possible to determine the hop-by-hop route
upstream because of asymmetric routing. However, in particular
cases, the upstream peer can be discovered with a high degree of
confidence, for example:
o The upstream GIST peer is 1 IP hop away, and can be reached by
tracing back through the interface on which the flow arrives.
o The upstream peer is a border router of a single-homed (stub)
network.
This section defines an upstream Query encapsulation and validation
checks for when it can be used. The functionality to generate
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upstream Queries is OPTIONAL, but if received they MUST be processed
in the normal way (no special functionality is needed for this). It
is possible for routing state (for a given MRI and NSLPID) to be
installed by both upstream and downstream Query exchanges. If the
SIDs are different, these items of routing state MUST be considered
as independent; if they match, that installed by the downstream
exchange MUST take precedence.
The details of the encapsulation are as follows:
o The destination address SHOULD be the flow source address as given
in the MRI of the message payload. An implementation with more
detailed knowledge of local routing MAY use an alternative
destination address (e.g. the address of its default router).
o The source address SHOULD be the signaling node address.
o The DiffServ codepoint and (for IPv6) flow label MAY be set to
match the values from the MRI, as in the downstream case. The
same considerations about message size and fragmentation also
apply as in the downstream case, and RAO setting and UDP port
selection are also the same.
o The IP-TTL of the message MUST be set to 255.
The sending GIST implementation SHOULD attempt to send the Query
message out of the same interface and to the same link layer
neighbour from which the data packets of the flow are arriving.
The receiving GIST node MAY apply validation checks to the message
and MRI, to reject Query messages which have reached a node at which
they can no longer be trusted. In particular, a node SHOULD reject a
message which has been propagated more than one IP hop, with an
"Invalid IP TTL" error message (Appendix A.4.4.11). This can be
determined by examining the received IP TTL, similar to the
generalised IP TTL security mechanism described in [21].
Alternatively, receipt of an upstream Query at the flow source MAY be
used to trigger setup of NTLP state in the downstream direction.
These restrictions may be relaxed in a future version.
5.8.2. The Loose-End MRM
This MRM is used to discover GIST nodes with particular properties in
the direction of a given address, for example to discover a NAT along
the upstream data path (e.g. as in [27].
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5.8.2.1. Message Routing Information
For the loose-end MRM, only a simplified version of the Flow
Identifier is needed.
MRI = network-layer-version
source-address
destination-address
The source address is the address of the node initiating the
discovery process, for example the node that will be the data
receiver in the NAT discovery case. The destination address is the
address of a node which is expected to be the 'other side' of the
node to be discovered. Additional control information defines the
direction of the message relative to this flow as in the path-coupled
case.
5.8.2.2. Downstream Query Encapsulation
Only one encapsulation is defined for the loose-end MRM; by
convention, this is referred to as the downstream encapsulation, and
is defined as follows:
o The IP destination address MUST be the destination address as
given in the MRI of the message payload.
o By default, the IP source address is the source address, again
from the MRI. However, the use of the signaling source address is
allowed as in the case of the path-coupled MRM.
There are no special requirements on the setting of the DiffServ
codepoint, IP TTL, or (for IPv6) the flow label. Nor are any special
validation checks applied.
Restrictions on message size and setting of the DF (do not fragment)
bit apply as in the case of the path-coupled MRM.
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6. Formal Protocol Specification
This section provides a more formal specification of the operation of
GIST processing, in terms of rules for transitions between states of
a set of communicating state machines within a node. The following
description captures only the basic protocol specification;
additional mechanisms can be used by an implementation to accelerate
route change processing, and these are captured in Section 7.1.
Conceptually, GIST processing at a node may be seen in terms of 4
types of cooperating state machine:
1. There is a top-level state machine which represents the node
itself (Node-SM). It is responsible for the processing of events
which cannot be directed towards a more specific state machine,
for example, inbound messages for which no routing state
currently exists. This machine exists permanently, and is
responsible for creating 'per-flow' state machines to manage the
GIST handshake and routing state maintenance procedures.
2. For each flow and signaling direction where the node is
responsible for the creation of routing state, there is an
instance of a Query-Node state machine (Query-SM). This machine
sends Query and Confirm messages and waits for Responses,
according to the requirements from local API commands or timer
processing (e.g. message repetition or routing state refresh).
3. For each flow and signaling direction where the node has accepted
the creation of routing state by a peer, there is an instance of
a Responding-Node state machine (Response-SM). This machine is
responsible for managing the status of the routing state for that
flow. Depending on policy, it MAY be responsible for
[re]transmission of Response messages, or this MAY be handled by
the Node-SM, and a Response-SM is not even created for a flow
until a properly formatted Confirm has been accepted.
4. Messaging associations have their own lifecycle, represented by
MA-SM, from when they are first created (in an 'incomplete'
state, listening for an inbound connection or waiting for
outbound connections to complete), to when they are active and
available for use.
Apart from the fact that the various machines can be created and
destroyed by each other, there is almost no interaction between them.
The machines for different flows do not interact; the Query-SM and
Response-SM for a single flow and signaling direction do not
interact. That is, the Response-SM which accepts the creation of
routing state for a flow on one interface has no direct interaction
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with the Query-SM which sets up routing state on the next interface
along the path. This interaction is mediated instead through the
NSLP.
The state machine descriptions use the terminology rx_MMMM, tg_TTTT
and er_EEEE for incoming messages, API/lower layer triggers and error
conditions respectively. The possible events of these types are
given in the table below. In addition, timeout events denoted
to_TTTT may also occur; the various timers are listed independently
for each type of state machine in the following subsections.
+---------------------+---------------------------------------------+
| Name | Meaning |
+---------------------+---------------------------------------------+
| rx_Query | A GIST Query message has been received. |
| | |
| rx_Response | A GIST Response message has been received. |
| | |
| rx_Confirm | A GIST Confirm message has been received. |
| | |
| rx_Data | A GIST Data message has been received. |
| | |
| rx_Message | rx_Query||rx_Response||rx_Confirm||rx_Data. |
| | |
| rx_Hello | A GIST MA-Hello message has been received. |
| | |
| tg_NSLPData | A signaling application has requested data |
| | transfer (via API SendMessage). |
| | |
| tg_Connected | The protocol stack for a messaging |
| | association has completed connecting. |
| | |
| tg_RawData | GIST wishes to transfer data over a |
| | particular messaging association. |
| | |
| er_NoRSM | A "No Routing State" error was received. |
| | |
| er_MAConnect | A messaging association protocol failed to |
| | complete a connection. |
| | |
| er_MAFailure | A messaging association failed. |
+---------------------+---------------------------------------------+
6.1. Node Processing
The Node level state machine is responsible for processing events for
which no more appropriate messaging association state or routing
state exists. Its structure is trivial: there is a single state
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('Idle'); all events cause a transition back to Idle. Some events
cause the creation of other state machines. The only events that are
processed by this state machine are incoming GIST messages (Query/
Response/Confirm/Data) and API requests to send data; all other
events are impossible. In addition to this event processing, the
Node level machine is responsible for managing listening endpoints
for messaging associations (although these relate to Responding node
operation, they cannot be handled by the Responder state machine
since they are not created per flow). The processing rules for each
event are as follows:
Rule 1 (rx_Query):
use the GIST service interface to determine the signaling application
policy relating to this peer
if (the signaling application indicates that routing state should
be created) then
if (routing state can be created without a 3-way handshake) then
create R-SM and transfer control to it
else
send Response
else
propagate the Query with any updated NSLP payload provided
Rule 2 (rx_Response):
// should already have a Q-SM to handle this
discard message
send "No Routing State" error message
Rule 3 (rx_Confirm):
if (routing state can be created before receiving a Confirm) then
// should already have R-SM for it which would handle this message
discard message
send "No Routing State" error message
else
create R-SM and pass message to it
Rule 4 (rx_Data):
if (node policy will only process Data messages with matching
routing state) then
send "No Routing State" error message
else
pass directly to NSLP
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Rule 5 (tg_NSLPData):
if Q-mode encapsulation is not possible for this MRI
reject message with an error
else
if (local policy & transfer attributes say routing
state is not needed) then
send message statelessly
else
create Q-SM and pass message to it
6.2. Query Node Processing
The Querying-Node state machine (Q-SM) has three states:
o Awaiting Response
o Established
o Awaiting Refresh
The Q-SM is created by the N-SM machine as a result of a request to
send a message for a flow in a signaling direction where the
appropriate state does not exist. The Query is generated immediately
and the No_Response timer is started. The NSLP data MAY be carried
in the Query if local policy and the transfer attributes allow it,
otherwise it MUST be queued locally pending MA establishment. Then
the machine transitions to the Awaiting Response state, in which
timeout-based retransmissions are handled. Data messages (rx_Data
events) should not occur in this state; if they do, this may indicate
a lost Response and a node MAY also retransmit a Query for this
reason.
Once a Response has been successfully received and routing state
created, the machine transitions to Established, during which NSLP
data can be sent and received normally. Further Responses received
in this state MUST be treated the same way (this may be the result of
a lost Confirm). The Awaiting Refresh state can be considered as a
substate of Established, where a new Query has been generated to
refresh the routing state (as in Awaiting Response) but NSLP data can
be handled normally.
The timers relevant to this state machine are as follows:
Refresh_QNode: Indicates when the routing state stored by this state
machine must be refreshed. It is reset whenever a Response is
received indicating that the routing state is still valid.
Implementations MUST set the period of this timer based on the
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value in the RS-validity-time field of a Response message to
ensure that a Query is generated before the peer's routing state
expires.
No_Response: Indicates that a Response has not been received in
answer to a Query. This is started whenever a Query is sent and
stopped when a Response is received.
Inactive_QNode: Indicates that no traffic is currently being handled
by this state machine. This is reset whenever the state machine
handles NSLP data (in either direction). When it expires, the
state machine MAY be deleted. The period of the timer can be set
at any time via the API (SetStateLifetime), and if the period is
reset in this way the timer itself SHOULD be restarted.
The main events (including all those that cause state transitions)
are shown in the figure below, tagged with the number of the
processing rule that is used to handle the event. These rules are
listed after the diagram. All events not shown or described in the
text above are assumed to be impossible in a correct implementation.
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[Initialisation] +-----+
-------------------------|Birth|
| +-----+
| rx_Response[4]
| || tg_NSLPData[5]
| tg_NSLPData[1] er_NoRSM[3] || rx_Data[7]
| -------- ------------------ -------
| | V | V | V
| | V | V | V
| +----------+ | +-----------+
---->>| Awaiting | ---------------- |Established|
------| Response |---------------------------->> | |
| +----------+ rx_Response[4] +-----------+
| ^ | ^ |
| ^ | ^ |
| -------- | |
| to_No_Response[2] | |
| [!nResp_reached] tg_NSLPData[5] | |
| || rx_Data[7] | |
| -------- | |
| | V | |
| to_No_Response[2] | V | |
| [nResp_reached] +-----------+ rx_Response[4] | |
---------- -----------| Awaiting |----------------- |
| | | Refresh |<<-------------------
| | +-----------+ to_Refresh_QNode[8]
| | ^ |
| | ^ |
| | --------
| | to_No_Response[2]
| | [!nResp_reached]
V V
V V
+-----+
|Death|<<---------------
+-----+ to_Inactive_QNode[6]
(from all states)
Figure 6: Query Node State Machine
The processing rules are as follows:
Rule 1: Store the message for later transmission
Rule 2:
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if number of Queries sent has reached the threshold
// nQuery_isMax is true
indicate No Response error to NSLP
destroy self
else
send Query message
start No_Response timer with new value
Rule 3:
// Assume the Confirm was lost in transit so resend it
// for the last Response we received
send Confirm message
restart Refresh_QNode and Inactive_QNode timers
Rule 4:
if a new MA-SM is needed create one
if the R flag was set send a Confirm message
pass any NSLP data to the NSLP
send any stored Data messages
stop No_Response timer
start Refresh_QNode and Inactive_QNode timers
Rule 5:
send Data message
restart Inactive_QNode timer
Rule 6: Terminate
Rule 7:
pass any data to the NSLP
(re)start Inactive_QNode timer
Rule 8:
send Query message
start No_Response timer
stop Refresh_QNode timer
6.3. Responder Node Processing
The Responding-Node state machine (R-SM) has three states:
o Awaiting Confirm
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o Established
o Awaiting Refresh
The policy governing the creation of the R-SM has 3 cases (ignoring
the case of pure stateless operation where a Response may be
generated or the message propagated forwards, but no routing state is
created at the GIST level):
1. It is created on receiving a Query, no Confirm is requested.
2. It is created on receiving a Query, but a Confirm is requested.
A timer is used to retransmit Response messages and the R-SM is
destroyed if no valid Confirm is received.
3. It cannot be created until a valid Confirm is received (the
initial Query will have been handled by the Node level machine).
In case 2 the R-SM is created in the Awaiting Confirm state, and
remains there until a Confirm is received, at which point it
transitions to Established. In cases 1 and 3 the R-SM is created
directly in the Established state. Note that if the machine is
created on receiving a Query, some of the message processing will
already have been performed in the Node state machine. In the
Established state the NSLP can send and receive data normally, and
any additional rx_Confirm events MUST be silently ignored. The
Awaiting Refresh state can be considered a substate of Established,
where a Query has been received to begin the routing state refresh.
In the Awaiting Refresh state the R-SM behaves as in the Awaiting
Confirm state, except that the NSLP can still send and receive data.
In particular, in both states there is timer-based retransmission of
Response messages until a Confirm is received; additional rx_Query
events in these states MUST also generate a response and restart the
no_Confirm timer.
The timers relevant to the operation of this state machine are as
follows:
Expire_RNode: Indicates when the routing state stored by this state
machine needs to be expired. It is reset whenever a Query or
Confirm (depending on local policy) is received indicating that
the routing state is still valid. Note that state cannot be
refreshed from the R-Node.
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No_Confirm: Indicates that a Confirm has not been received in answer
to a Response. This is started/reset whenever a Response is sent
and stopped when a Confirm is received.
The detailed state transitions and processing rules are described
below as in the Query node case.
rx_Query[1] rx_Query[5]
[confirmRequired] +-----+ [!confirmRequired]
-------------------------|Birth|----------------------------
| +-----+ |
| | rx_Confirm[2] |
| ---------------------------- |
| | |
| tg_NSLPData[3] | |
| tg_NSLPData[7] || rx_Query[5] | |
| || rx_Query[1] || rx_Data[4] | |
| || rx_Data[6] [!confirmRequired] | |
| -------- -------------- | |
| | V | V V V
| | V | V V V
| +----------+ | +-----------+
---->>| Awaiting | rx_Confirm[8] -----------|Established|
------| Confirm |------------------------------>> | |
| +----------+ +-----------+
| ^ | ^ |
| ^ | tg_NSLPData[3] ^ |
| -------- || rx_Query[1] | |
| to_No_Confirm[9] || rx_Data[4] | |
| [!nConf_reached] -------- | |
| | V | |
| to_No_Confirm[9] | V | |
| [nConf_reached] +-----------+ rx_Confirm[8] | |
---------- ------------| Awaiting |----------------- |
| | | Refresh |<<-------------------
| | +-----------+ rx_Query[1]
| | ^ | [confirmRequired]
| | ^ |
| | --------
V V to_No_Confirm[9]
V V [!nConf_reached]
+-----+
|Death|<<---------------------
+-----+ to_Expire_RNode[10]
(from all states)
Figure 7: Responder Node State Machine
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The processing rules are as follows:
Rule 1:
// a Confirm message is required
send Response message
(re)start No_Confirm timer
Rule 2:
pass any piggybacked data to the NSLP
if a new MA-SM would be needed for this peer
create one in listening state
start Expire_RNode timer
Rule 3: send the Data message
Rule 4: pass data to NSLP
Rule 5:
// no Confirm message is required
send Response message
start Expire_RNode timer
Rule 6: send "No Routing State" error message
Rule 7: store Data message
Rule 8:
pass any piggybacked data to the NSLP
send any stored Data messages
stop No_Confirm timer
start Expire_RNode timer
Rule 9:
if number of Responses sent has reached threshold
// nResp_isMax is true
destroy self
else
send Response message
start No_Response timer
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Rule 10: destroy self
6.4. Messaging Association Processing
Messaging associations are modelled for use within GIST with a simple
3-state process. The Awaiting Connection state indicates that the MA
is waiting for the connection process(es) for every protocol in the
messaging association to complete; this might involve creating
listening endpoints or attempting active connects. Timers may also
be necessary to detect connection failure (e.g. no incoming
connection within a certain period), but these are not modelled
explicitly. The Connected state indicates that the MA is open and
ready to use. In addition there is an Idle state in which the local
node no longer requires the messaging association but the remote node
still wants it to be kept open.
Clearly, many internal details of the messaging association protocols
are hidden in this model, especially where the messaging association
uses multiple protocol layers. Note also that although the existence
of messaging associations is not directly visible to signaling
applications, there is some interaction between the two because
security-related information becomes available during the open
process, and this may be indicated to signaling applications if they
have requested it.
The timers relevant to the operation of this state machine are as
follows:
SendHello: Indicates that an MAHello message should be sent to the
remote node. The period of this timer is determined by the MA-
Hold-Time sent by the remote node during the Query/Response/
Confirm exchange.
NoHello: Indicates that no MAHello has been received from the remote
node for a period of time. The period of this timer is sent to
the remote node as the MA-Hold-Time during the Query/Response
exchange.
NoActivity: Indicates that the link has been inactive for a period of
time. The period of this timer is implementation specific but is
likely to be related to the period of the NoHello timer.
The detailed state transitions and processing rules are described
below as in the Query node case.
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[Initialisation] +-----+
----------------------------|Birth|
| +-----+
| tg_RawData[1]
| || rx_Message[2]
| || rx_Hello[3]
| tg_RawData[5] || to_SendHello[4]
| -------- --------
| | V | V
| | V | V
| +----------+ +-----------+
---->>| Awaiting | tg_Connected[6] | Connected |
------|Connection|----------------------->>| |
| +----------+ +-----------+
| ^ |
| tg_RawData[1] ^ |
| || rx_Message[2] | |to_NoActivity[7]
| | V
| | V
| er_MAConnect[8] +-----+ to_NoHello[8] +-----------+
---------------->>|Death|<<----------------| Idle |
+-----+ | |
^ +-----------+
^ ^ |
| ^ |
--------------- --------
er_MAFailure[8] rx_Hello[3]
(from all states)
Figure 8: Messaging Association State Machine
The processing rules are as follows:
Rule 1:
pass message to transport layer
(re)start NoActivity timer
(re)start SendHello
Rule 2:
pass message to N-SM
(re)start NoActivity timer
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Rule 3:
if reply requested
send MA-Hello
restart NoHello timer
Rule 4:
send MA-Hello message
restart NoHello timer
Rule 5: queue message for later transmission
Rule 6:
pass outstanding queued messages to transport layer
stop any timers controlling connection establishment
start NoActivity timer
start SendHello timer
Rule 7:
stop NoActivity timer
stop sendHello timer
start NoHello timer
Rule 8: destroy self
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7. Advanced Protocol Features
7.1. Route Changes and Local Repair
7.1.1. Introduction
When re-routing takes place in the network, GIST and signaling
application state need to be updated for all flows whose paths have
changed. The updates to signaling application state are usually
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, GIST cannot carry
out the complete path update processing. Its responsibilities are to
detect the route change, update its local routing state consistently,
and inform interested signaling applications at affected nodes.
xxxxxxxxxxxxxxxxxxxxxxxxxxxx
x +--+ +--+ +--+ x Initial
x .|C1|_.....|D1|_.....|E1| x Configuration
x . +--+. .+--+. .+--+\. x
>>xxxxxxxxxxxxx . . . . . . xxxxxx>>
+-+ +-+ . .. .. . +-+
...|A|_......|B|/ .. .. .|F|_....
+-+ +-+ . . . . . . +-+
. . . . . .
. +--+ +--+ +--+ .
.|C2|_.....|D2|_.....|E2|/
+--+ +--+ +--+
+--+ +--+ +--+ Configuration
.|C1|......|D1|......|E1| after failure
. +--+ .+--+ +--+ of E1-F link
. \. . \. ./
+-+ +-+ . .. .. +-+
...|A|_......|B|. .. .. .|F|_....
+-+ +-+\ . . . . . +-+
>>xxxxxxxxxxxxx . . . . . . xxxxxx>>
x . +--+ +--+ +--+ . x
x .|C2|_.....|D2|_.....|E2|/ x
x +--+ +--+ +--+ x
xxxxxxxxxxxxxxxxxxxxxxxxxxxx
........... = physical link topology
>>xxxxxxx>> = flow direction
_.......... = outgoing link for flow xxxxxx given
by local forwarding table
Figure 9: A Re-Routing Event
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Route change management is complicated by the distributed nature of
the problem. Consider the re-routing event shown in Figure 9. An
external observer can tell that the main responsibility for
controlling the updates will probably lie with nodes B and F;
however, E1 is best placed to detect the event quickly at the GIST
level, and C1 and D1 could also attempt to initiate the repair.
On the assumption that signaling applications are soft-state based
and operate end to end, and because GIST also periodically updates
its picture of routing state, route changes will eventually be
repaired automatically. The specification as already given includes
this functionality. However, especially if upper layer refresh times
are extended to reduce signaling load, the duration of inconsistent
state may be very long indeed. Therefore, GIST includes logic to
exchange prompt notifications with signaling applications, to allow
local repair if possible. The additional mechanisms to achieve this
are described in the following subsections. To a large extent, these
additions can be seen as implementation issues; the protocol messages
and their significance are not changed, but there are extra
interactions through the API between GIST and signaling applications,
and additional triggers for transitions between the various GIST
states.
7.1.2. Route Change Detection Mechanisms
There are two aspects to detecting a route change at a single node:
o Detecting that the outgoing path, in the direction of the Query,
has (or may have) changed.
o Detecting that the incoming path, in the direction of the
Response, has (or may have) changed (in which case the node may no
longer be on the path at all).
At a single node, these processes are largely independent, although
clearly a change in one direction at a node corresponds to a change
the opposite direction at its peer. Note that there are two possible
forms for a route change: the interface through which a flow leaves
or enters a node may change, and the adjacent peer may change. In
general, a route change can include one or the other or both (or
indeed neither, although such changes are very hard to detect).
The route change detection mechanisms available to a node depend on
the MRM in use and the role the node played in setting up the routing
state in the first place (i.e. as Querying or Responding node). The
following discussion is specific to the case of the path-coupled MRM
using downstream Queries only; other scenarios may require other
methods. However, the repair logic described in the subsequent
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subsections is intended to be universal.
There are five mechanisms for a node to detect that a route change
has occurred, which are listed below. They apply differently
depending on whether the change is in the Query or Response
direction, and these differences are summarised in the following
table.
Local Trigger: In trigger mode, GIST finds out from the local
forwarding table that the next hop has changed. This only works
if the routing change is local, not if it happens a few routing
hops away (including the case that it happens at a GIST-unaware
node).
Extended Trigger: Here, GIST checks a link-state topology database to
discover that the path has changed. This makes certain
assumptions on consistency of route computation and only works
within a single area for OSPF and similar link-state protocols.
Where available, this offers the most accurate and rapid
indication of route changes, but requires more access to the
routing internals than a typical operating system may provide.
GIST C-mode Monitoring: GIST may find that C-mode packets are
arriving (from either peer) with a different TTL or on a different
interface. This provides no direct information about the new flow
path, but indicates that routing has changed and that rediscovery
may be required.
Data Plane Monitoring: The signaling application on a node may detect
a change in behaviour of the flow, such as TTL change, arrival on
a different interface, or loss of the flow altogether. The
signaling application on the node is allowed to notify this
information locally to GIST (Appendix B.6).
GIST Probing: According to the specification, each GIST node MUST
periodically repeat the discovery (GIST-Query/GIST-Response)
operation. The querying node will discover the route change by a
modification in the Network-Layer-Information in the GIST-
Response. The period can be negotiated independently for each
GIST hop, so nodes that have access to the other techniques listed
above MAY use long periods for the probing operation.
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+------------+--------------------------+---------------------------+
| Method | Query direction | Response direction |
+------------+--------------------------+---------------------------+
| Local | Discovers new interface | Not applicable |
| Trigger | (and peer if local) | |
| | | |
| Extended | Discovers new interface | May determine that route |
| Trigger | and may determine new | from peer will have |
| | peer | changed |
| | | |
| C-Mode | Provides hint that | Provides hint that change |
| Monitoring | change has occurred | has occurred |
| | | |
| Data Plane | Not applicable | NSLP informs GIST that a |
| Monitoring | | change may have occurred |
| | | |
| Probing | Discovers changed NLI in | Discovers changed NLI in |
| | GIST-Response | GIST-Query |
+------------+--------------------------+---------------------------+
7.1.3. GIST Behaviour Supporting Re-Routing
The GIST behaviour necessary to support re-routing can be modelled
using a 3-level classification of the validity of each item of
routing state. (This classification applies separately to the
Querying and Responding node for each pair of GIST peers.) The
levels are:
Bad: The routing state is either missing altogether, or not safe to
use to send data.
Tentative: The routing state may have changed, but it is still usable
for sending NSLP data pending verification.
Good: The routing state has been established and no events affecting
it have since been detected.
These classifications are not identical to the states described in
Section 6, but there are dependencies between them. Specifically,
routing state is considered Bad until the machine first enters the
Established state, at which point it becomes Good. Thereafter, the
status may be invalidated for any of the reasons discussed above; it
is an implementation issue to decide which techniques to implement in
any given node, and how to reclassify routing state (as Bad or
Tentative) for each. The status returns to Good, either when the
state machine re-enters the Established state, or if GIST can
determine from direct examination of the routing or forwarding tables
that the peer has not changed. When the status returns to Good, GIST
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MUST if necessary update its routing state table so that the
relationships between MRI/SID/NSLPID tuples and messaging
associations are up to date.
When classification of the routing state for the downstream direction
changes to Bad/Tentative because of local routing indications, GIST
MAY automatically change the classification in the upstream direction
to Tentative unless local routing indicates that this is not
necessary. This SHOULD NOT be done in the case where the initial
change was indicated by the signaling application. This mechanism
accounts for the fact that a routing change may affect several nodes,
and so can be an indication that upstream routing may also have
changed. In any case, whenever GIST updates the routing status, it
informs the signaling application with the NetworkNotification API
(Appendix B.4), unless the change was caused via the API in the first
place.
The GIST behaviour for state repair is different for the Querying and
Responding node. At the Responding node, there is no additional
behaviour, since the Responding node cannot initiate protocol
transitions autonomously, it can only react to the Querying node.
The Querying node has three options, depending on how the transition
from 'Good' was initially caused:
1. To inspect the routing/forwarding table and verifying that the
next peer has not changed. This technique MUST NOT be used if
the transition was caused by a signaling application, but SHOULD
be used otherwise if available.
2. To move to the 'Awaiting Refresh' state. This technique MUST NOT
be used if the current status is 'Bad', since data is being
incorrectly delivered.
3. To move to the 'Awaiting Response' state. This technique may be
used at any time, but has the effect of freezing NSLP
communication while GIST state is being repaired.
The second and third techniques trigger the execution of a GIST
handshake to carry out the repair. It may be desirable to delay the
start of the handshake process, either to wait for the network to
stabilise, to avoid flooding the network with Query traffic for a
large number of affected flows, or to wait for confirmation that the
node is still on the path from the upstream peer. One approach is to
delay the handshake until there is NSLP data to be transmitted.
Implementation of such delays is a matter of local policy; however,
GIST MUST begin the handshake immediately if the status change was
caused by an InvalidateRoutingState API call marked as 'Urgent', and
SHOULD begin it if the upstream routing state is still known to be
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Good.
7.1.4. Signaling Application Operation
Signaling applications can use these functions as provided by GIST to
carry out rapid local repair following re-routing events. The
signaling application instances carry out the multi-hop aspects of
the procedure, including crossover node detection, and tear-down/
reinstallation of signaling application state; they also trigger GIST
to carry out the local routing state maintenance operations over each
individual hop. The local repair procedures depend heavily on the
fact that stateful NSLP nodes are a single GIST hop apart; this is
enforced by the details of the GIST peer-discovery process.
The following outline description of a possible set of NSLP actions
takes the scenario of Figure 9 as an example.
1. The signaling application at node E1 is notified by GIST of route
changes affecting the downstream and upstream directions. The
downstream status was updated to Bad because of a trigger from
the local forwarding tables, and the upstream status changed
automatically to Tentative as a consequence. The signaling
application at E1 MAY begin local repair immediately, or MAY
propagate a notification upstream to D1 that re-routing has
occurred.
2. The signaling application at node D1 is notified of the route
change, either by signaling application notifications or from the
GIST level (e.g. by a trigger from a link-state topology
database). If the information propagates faster within the
routing protocol, GIST will change the upstream/downstream
routing state to Tentative/Bad automatically, and this will cause
the signaling application to propagate the notification further
upstream.
3. This process continues until the notification reaches node A.
Here, there is no downstream routing change, so GIST only learns
of the update via the signaling application trigger. Since the
upstream status is still Good, it therefore begins the repair
handshake immediately.
4. The handshake initiated by node A causes its downstream routing
state to be confirmed as Good and unchanged there; it also
confirms the (Tentative) upstream routing state at B as Good.
This is enough to identify B as the crossover router, and the
signaling application and GIST can begin the local repair
process.
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An alternative way to reach step (4) is that node B is able to
determine autonomously that there is no likelihood of an upstream
route change (e.g. it is an area border router and the route change
is only intra-area). In this case, the signaling application and
GIST will see that the upstream state is Good and can begin the local
repair directly.
After a route change, a signaling application may wish to remove
state at another node which is no longer on the path. However, since
it is no longer on the path, in principle GIST can no longer send
messages to it. (In general, provided this state is soft, it will
time out anyway; however, the timeouts involved may have been set to
be very long to reduce signaling load.) The requirement to remove
state in a specific peer node is identified in [28].
This requirement can be met provided that GIST is able to use the old
path to the signaling application peer for some period while the
signaling application still needs it. Since NSLP peers are a single
GIST hop apart, the necessary information is just the old entry in
the node's routing state table for that flow. Rather than requiring
the GIST level to maintain multiple generations of this information,
it can just be provided to the signaling application in the same node
(in an opaque form), which can store it if necessary and provide it
back to the GIST layer in case it needs to be used. This information
is denoted as 'SII-Handle' in the abstract API of Appendix B.
Messages sent this way MUST bypass the GIST routing state tables at
the sender, and this is indicated by setting the E flag in the common
header (Appendix A.1); at the receiver, GIST MUST NOT validate the
MRI/SID/NSLPID against local routing state and instead indicates the
mode of reception to signaling applications through the API
(Appendix B.2). Signaling applications should validate the source
and effect of the message themselves, and if appropriate should in
particular indicate to GIST (see Appendix B.5) that routing state is
no longer required for this flow. This is necessary to prevent GIST
in nodes on the old path initiating routing state refresh and thus
causing state conflicts at the crossover router.
7.2. NAT Traversal
GIST messages must carry packet addressing and higher layer
information as payload data in order to define the flow signalled
for. (This applies to all GIST messages, regardless of how they are
encapsulated or which direction they are travelling in.) At an
addressing boundary the data flow packets will have their headers
translated; if the signaling payloads are not translated
consistently, the signaling messages will refer to incorrect (and
probably meaningless) flows after passing through the boundary. In
addition, GIST handshake messages carry additional addressing
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information about the GIST nodes themselves, and this must also be
processed appropriately when traversing a NAT.
The simplest solution to this problem is to require that a NAT is
GIST-aware, and to allow it to modify messages based on the contents
of the MRI. (This makes the assumption that NATs only rewrite the
header fields included in this payload, and not other higher layer
identifiers.) Provided this is done consistently with the data flow
header translation, signaling messages will be valid each side of the
boundary, without requiring the NAT to be signaling application
aware. (Note, however, that if the NAT does not understand the MRI,
it should reject the message with an appropriate error.)
This specification defines an additional object that a NAT can insert
into Query-encapsulated messages and which is echoed back in any
responses to those messages. The new object, the NAT-Traversal
object (Appendix A.3.8), carries the translation between the 'public'
and 'private' MRI. It also carries a list of which other objects in
the message have been translated. This should always include the
NLI, and the Stack-Configuration-Data (if present); if GIST is
extended with further objects that carry addressing data, this list
allows a message receiver to know if the new objects were supported
by the NAT. Finally, the NAT-Traversal object MAY be used to carry
data to be used in back-translating datagram mode responses; this
could be the original NLI or SCD, or opaque equivalents in the case
of topology hiding.
A consequence of this approach is that the routing state tables at
the signaling application peers (each side of the NAT) are no longer
directly compatible. In particular, the values for Message-Routing-
Information are different, which is why the unmodified MRI is
propagated in the NAT-Traversal payload to allow subsequent C-mode
messages to be interpreted correctly.
This specification does not define normative behaviour for a NAT
translating GIST messages, since much of this will depend on NAT
policy about allocating bindings; the description is purely
informative. However, it does define the behaviour of a GIST node
receiving a message containing a NAT-Traversal object.
A possible set of operations for a NAT to process a Query-
encapsulated message is as follows:
1. Verify that bindings for any data flow are actually in place.
2. Create a new Message-Routing-Information object with fields
modified according to the data flow bindings.
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3. Create bindings for subsequent C-mode signaling (based on the
information in the Network-Layer-Information and Stack-
Configuration-Data objects).
4. Create new Network-Layer-Information and if necessary Stack-
Configuration-Data objects with fields to force D-mode response
messages through the NAT, and to allow C-mode exchanges using the
C-mode signaling bindings.
5. Add a NAT-Traversal payload, listing the objects which have been
modified and including the unmodified MRI and any other data
needed to interpret the response. (If a NAT-Traversal object is
already present, in the case of a sequence of NATs, the list of
modified objects may be updated and further opaque data added,
but the MRI contained in it is left unchanged.)
6. Encapsulate the message according to the normal rules of this
specification for the Query-encapsulation. If the S-flag was set
in the original message, the same IP source address selection
policy should be applied to the forwarded message.
7. Forward the message with these new payloads.
A GIST node receiving such a message MUST verify that all mandatory
objects containing addressing have been translated correctly, or else
reject the message with an 'Object Type Error' message
(Appendix A.4.4.9) with subcode 4 ('Untranslated Object'). The error
message MUST include the NAT-Traversal object as the first TLV after
the common header (this is true for any other error message generated
as a response). Otherwise, the message is processed essentially as
normal. If no state needs to be updated for the message, the NAT-
Traversal object can be effectively ignored. The other possibility
is that a Response must be returned, either because the message is
the beginning of a handshake for a new flow, or it is a refresh for
existing state. In both cases, the GIST node MUST create the
Response message in the normal way using the 'local' form of the MRI,
and its own NLI and (if necessary) SCD. It MUST also include the
NAT-Traversal object as the first object in the Response after the
common header.
A NAT will intercept datagram mode messages with the normal
encapsulation containing such echoed NAT-Traversal objects. (All
other GIST messages, either in connection mode, or datagram mode
messages with no NAT-Traversal object, should be treated as 'normal'
data traffic by the NAT, i.e. with IP and transport layer translation
but no GIST-specific processing.) The NAT processing is a subset of
the processing for the Query-encapsulated case:
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1. Verify the existence of bindings for the data flow.
2. Leave the Message-Routing-Information object unchanged.
3. Modify the NLI and SCD objects for the Responding node if
necessary, and create or update any bindings for C-mode signaling
traffic.
4. Forward the message.
A GIST node receiving such a message MUST use the MRI from the NAT-
Traversal object as the key to index its internal routing state; it
MAY also store the 'translated' MRI for additional (e.g. diagnostic)
information, but this is not used in the GIST processing. The
remainder of GIST processing is unchanged.
Note that Confirm messages are not translated. Thus, a Responding
node has available only the untranslated MRI describing the flow, and
the untranslated NLI as peer routing state. This would prevent the
correct interpretation of the signaling messages; also, subsequent
Query (refresh) messages would always be seen as route changes
because of the NLI change. Therefore, a Responding node that wishes
to delay state installation until receiving a Confirm must somehow
reconstruct the translations when the Confirm arrives. How to do
this is an implementation issue; one approach is to carry the
translated objects as part of the Responder cookie which is echoed in
the Confirm. (Indeed, for one of the cookie constructions in
Section 8.5 this is automatic.)
7.3. Interaction with IP Tunnelling
The interaction between GIST and IP tunnelling is very simple. An IP
packet carrying a GIST message is treated exactly the same as any
other packet with the same source and destination addresses: in other
words, it is given the tunnel encapsulation and forwarded with the
other data packets.
Tunnelled packets will not be identifiable as GIST messages until
they leave the tunnel, since any router alert option and the standard
GIST protocol encapsulation (e.g. port numbers) will be hidden within
the standard tunnel encapsulation. If signaling is needed for the
tunnel itself, this has to be initiated as a separate signaling
session by one of the tunnel endpoints - that is, the tunnel counts
as a new flow. Because the relationship between signaling for the
'microflow' and signaling for the tunnel as a whole will depend on
the signaling application in question, it is a signaling application
responsibility to be aware of the fact that tunnelling is taking
place and to carry out additional signaling if necessary; in other
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words, at least one tunnel endpoint must be signaling application
aware.
In some cases, it is the tunnel exit point (i.e. the node where
tunnelled data and downstream signaling packets leave the tunnel)
that will wish to carry out the tunnel signaling, but this node will
not have knowledge or control of how the tunnel entry point is
carrying out the data flow encapsulation. The information about how
the inner MRI/SID relate to the tunnel MRI/SID needs to be carried in
the signaling data from the tunnel entry point (this functionality is
the equivalent to the RSVP SESSION_ASSOC object of [10]). In the
NSIS protocol suite, these bindings are managed by the signaling
applications, either implicitly (e.g. by SID re-use) or explicitly
(by carrying objects that bind the inner and outer SIDs as part of
the NSLP payload).
7.4. IPv4-IPv6 Transition and Interworking
GIST itself is essentially IP version neutral: version dependencies
are isolated in the formats of the Message-Routing-Information,
Network-Layer-Information and Stack-Configuration-Data objects, and
GIST also depends on the version independence of the protocols that
support messaging associations. In mixed environments, GIST
operation will be influenced by the IP transition mechanisms in use.
This section provides a high level overview of how GIST is affected,
considering only the currently predominant mechanisms.
Dual Stack: (As described in [29].) In mixed environments, GIST MUST
use the same IP version for Query-encapsulated messages as the
flow it is signaling for, and SHOULD do so for other signaling
also (see Section 5.2.2). The IP version used in 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 GIST node to
support signaling for flows using the other IP version. Hosts
which are dual stack for applications and routers which are dual
stack for forwarding need GIST implementations which can support
both IP versions. Applications with a choice of IP versions might
select a version based on which could be supported in the network
by GIST, which could be established by invoking parallel discovery
procedures.
Packet Translation: (Applicable to SIIT [5] and NAT-PT [11].) Some
transition mechanisms allow IPv4 and IPv6 nodes to communicate by
placing packet translators between them. From the GIST
perspective, this should be treated essentially the same way as
any other NAT operation (e.g. between 'public' and 'private'
addresses) as described in Section 7.2. The translating node
needs to be GIST-aware; it will have to translate the addressing
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payloads between IPv4 and IPv6 formats for flows which cross
between the two. The translation rules for the fields in the MRI
payload (including e.g. DiffServ-codepoint and flow-label) are as
defined in [5].
Tunnelling: (Applicable to 6to4 [13].) 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 GIST perspective, the treatment should be as similar as
possible to any other IP tunnelling mechanism, as described in
Section 7.3. In particular, the end to end flow signaling will
pass transparently through the tunnel, and signaling for the
tunnel itself will have to be managed by the tunnel endpoints.
However, additional considerations may arise because of special
features of the tunnel management procedures. In particular, [14]
is based on using an anycast address as the destination tunnel
endpoint. GIST MAY use anycast destination addresses in the
Query-encapsulation of D-mode messages if necessary, but MUST NOT
use them in the Network-Layer-Information addressing field; normal
unicast addresses MUST be used instead. Note that the addresses
from the IP header are not used by GIST in matching requests and
responses, so there is no requirement to use anycast source
addresses.
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8. Security Considerations
The security requirement for GIST is to protect the signaling plane
against identified security threats. For the signaling problem as a
whole, these threats have been outlined in [23]; the NSIS framework
[22] assigns a subset of the responsibilities to the NTLP. The main
issues to be handled can be summarised as:
Message Protection: Signaling message content can be protected
against eavesdropping, modification, injection and replay while in
transit. This applies both to GIST payloads, and GIST should also
provide such protection as a service to signaling applications
between adjacent peers.
Routing State Integrity Protection: It is important that signaling
messages are delivered to the correct nodes, and nowhere else.
Here, 'correct' is defined as 'the appropriate nodes for the
signaling given the Message-Routing-Information'. In the case
where the MRI is based on the Flow Identification for path-coupled
signaling, 'appropriate' means 'the same nodes that the
infrastructure will route data flow packets through'. (GIST has
no role in deciding whether the data flow itself is being routed
correctly; all it can do is ensure the signaling is routed
consistently with it.) GIST uses internal state to decide how to
route signaling messages, and this state needs to be protected
against corruption.
Prevention of Denial of Service Attacks: GIST nodes and the network
have finite resources (state storage, processing power,
bandwidth). The protocol tries to minimise exhaustion attacks
against these resources and not allow GIST nodes to be used to
launch attacks on other network elements.
The main additional issue is handling authorisation for executing
signaling operations (e.g. allocating resources). This is assumed to
be done in each signaling application.
In many cases, GIST relies on the security mechanisms available in
messaging associations to handle these issues, rather than
introducing new security measures. Obviously, this requires the
interaction of these mechanisms with the rest of the GIST protocol to
be understood and verified, and some aspects of this are discussed in
Section 5.7.
8.1. Message Confidentiality and Integrity
GIST can use messaging association functionality, specifically in
this version TLS (Section 5.7.3), to ensure message confidentiality
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and integrity. Implementation of this functionality is REQUIRED but
its use for any given flow or signaling application is OPTIONAL. In
some cases, confidentiality of GIST information itself is not likely
to be a prime concern, in particular since messages are often sent to
parties which are unknown ahead of time, although the content visible
even at the GIST level gives significant opportunities for traffic
analysis. Signaling applications may have their own mechanism for
securing content as necessary; however, they may find it convenient
to rely on protection provided by messaging associations, since it
runs unbroken between signaling application peers.
8.2. Peer Node Authentication
Cryptographic protection (of confidentiality or integrity) requires a
security association with session keys. These can be established by
an authentication and key exchange protocol based on shared secrets,
public key techniques or a combination of both. Authentication and
key agreement is possible using the protocols associated with the
messaging association being secured (TLS incorporates this
functionality directly; IKE, IKEv2 or KINK could provide it for
IPsec). GIST nodes rely on these protocols to authenticate the
identity of the next hop, and GIST has no authentication capability
of its own.
With routing state discovery, there are few effective ways to know
what is the legitimate next or previous hop as opposed to an
impostor. In other words, cryptographic authentication here only
provides assurance that a node is 'who' it is (i.e. the legitimate
owner of identity in some namespace), not 'what' it is (i.e. a node
which is genuinely on the flow path and therefore can carry out
signaling for a particular flow). Authentication provides only
limited protection, in that a known peer is unlikely to lie about its
role. Additional methods of protection against this type of attack
are considered in Section 8.3 below.
It is an implementation issue whether peer node authentication should
be made signaling application dependent; for example, whether
successful authentication could be made dependent on presenting
credentials related to a particular signaling role (e.g. "signaling
for QoS"). The abstract API of Appendix B leaves open such policy
and authentication interactions between GIST and the NSLP it is
serving. However, it does allow applications to inspect the
authenticated identity of the peer to which a message will be sent
before transmission.
8.3. Routing State Integrity
Internal state in a node (see Section 4.2) is used to route messages.
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If this state is corrupted, signaling messages may be misdirected.
In the case where the message routing method is path-coupled, the
messages need to be routed identically to the data flow described by
the MRI, and the routing state table is the GIST view of how these
flows are being routed through the network in the immediate
neighbourhood of the node. Routes are only weakly secured (e.g.
there is no cryptographic binding of a flow to a route), and there is
no authoritative information about flow routes other than the current
state of the network itself. Therefore, consistency between GIST and
network routing state has to be ensured by directly interacting with
the routing mechanisms to ensure that the signaling peers are the
appropriate ones for any given flow. An overview of security issues
and techniques in this context is provided in [33].
In one direction, peer identification is installed and refreshed only
on receiving a GIST-Response message (compare Figure 4). This MUST
echo the cookie from a previous GIST-Query message, which will have
been sent 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.
In the other direction, peer identification MAY be installed directly
on receiving a GIST-Query message containing addressing information
for the signaling source. However, any node in the network could
generate such a message (indeed, many nodes in the network could be
the genuine upstream peer for a given flow). To protect against
this, three strategies are used:
Filtering: the receiving node MAY reject signaling messages which
claim to be for flows with flow source addresses which could be
ruled out by ingress filtering. An extension of this technique
would be for the receiving node to monitor the data plane and to
check explicitly that the flow packets are arriving over the same
interface and if possible from the same link layer neighbour as
the datagram mode signaling packets. (If they are not, it is
likely that at least one of the signaling or flow packets is being
spoofed.) Signaling applications SHOULD only install state on the
route taken by the data itself.
Authentication (weak or strong): the receiving node MAY refuse to
install upstream state until it has completed a GIST-Confirm
handshake with the peer. This echoes the Response cookie of the
GIST-Response, and discourages nodes from using forged source
addresses. This also plays a role in denial of service
prevention, see below. A stronger approach is to require full
peer authentication within the messaging association, the
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reasoning being that an authenticated peer can be trusted not to
pretend that it is on path when it is not.
SID segregation: The routing state lookup for a given MRI and NSLPID
MUST also take the SID into account. A malicious node can only
overwrite existing routing state if it can guess the corresponding
SID; it can insert state with random SID values, but generally
this will not be used to route messages for which state has
already been legitimately established.
8.4. Denial of Service Prevention
GIST is designed so that in general each Query message only generates
at most one Response, so that a GIST node cannot become the source of
a denial of service amplification attack. (There is a special case
of retransmitted Response messages, see Section 5.3.3.)
However, GIST can still be subjected to denial-of-service attacks
where an attacker using forged source addresses forces a node to
establish state without return routability, causing a problem similar
to TCP SYN flood attacks. Furthermore, an adversary might use
modified or replayed unprotected signaling messages as part of such
an attack. There are two types of state attacks and one
computational resource attack. In the first state attack, an
attacker floods a node with messages that the node has to store until
it can determine the next hop. If the destination address is chosen
so that there is no GIST-capable next hop, the node would accumulate
messages for several seconds until the discovery retransmission
attempt times out. The second type of state-based attack causes GIST
state to be established by bogus messages. A related computational/
network-resource attack uses unverified messages to cause a node to
make AAA queries or attempt to cryptographically verify a digital
signature.
We use a combination of two defences against these attacks:
1. The responding node need not establish a session or discover its
next hop on receiving the GIST-Query message, but MAY wait for a
GIST-Confirm message, possibly on a secure channel. If the
channel exists, the additional delay is one one-way delay and the
total is no more than the minimal theoretically possible delay of
a three-way handshake, i.e., 1.5 node-to-node round-trip times.
The delay gets significantly larger if a new connection needs to
be established first.
2. The Response to the Query message contains a cookie, which is
repeated in the Confirm. State is only established for messages
that contain a valid cookie. The setup delay is also 1.5 round-
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trip times. (This mechanism is similar to that in SCTP [12] and
other modern protocols.)
Once a node has decided to establish routing state, there may still
be transport and security state to be established between peers.
This state setup is also vulnerable to denial of service attacks.
GIST relies on the lower layer protocols that make up messaging
associations to mitigate such attacks. In the current specification,
the querying node is always the one wishing to establish a messaging
association, so it is the responding node that needs to be protected.
Signaling applications can use the services provided by GIST to
defend against certain (e.g. flooding) denial of service attacks. In
particular, they can elect to process only messages from peers that
have passed a return routability check or been authenticated at the
messaging association level (see Appendix B.2). Signaling
applications that accept messages under other circumstances (in
particular, before routing state has been fully established at the
GIST level) need to take this into account when designing their
denial of service prevention mechanisms, for example by not creating
local state as a result of processing such messages.
8.5. Requirements on Cookie Mechanisms
The requirements on the Query cookie can be summarised as follows:
Liveness: The cookie must be live (must change from one handshake to
the next). To prevent replay attacks.
Unpredictability: The cookie must not be guessable (e.g. not from a
sequence or timestamp). To prevent direct forgery based on seeing
a history of captured messages.
Easily validated: It must be efficient for the Q-Node to validate
that a particular cookie matches an in-progress handshake, for a
routing state machine which already exists. To discard spoofed
responses, or responses to spoofed queries.
Uniqueness: The cookie must be unique to a given handshake (since it
is actually used to match the Response to a handshake anyway, e.g.
during messaging association re-use).
Likewise, the requirements on the Responder cookie can be summarised
as follows:
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Liveness: The cookie must be live (must change from one handshake to
the next). To prevent replay attacks.
Creation simplicity: The cookie must be lightweight to generate. To
avoid resource exhaustion at the responding node.
Validation simplicity: It must be simple for the R-node to validate
that an R-cookie was generated by itself (and no-one else),
without storing state about the handshake it was generated for.
Binding: The cookie must be bound to the routing state that will be
installed. To prevent use with different routing state e.g. in a
modified Confirm. The routing state here includes:
The NLI of the Query
The MRI/NSLPID for the messaging
The interface on which the Query was received
A suitable implementation for the Q-Cookie is a cryptographically
strong random number which is unique for this routing state machine
handshake. A node SHOULD implement this or an equivalently strong
mechanism.
A suitable implementation for the R-Cookie is as follows:
R-Cookie = liveness data + hash (locally known secret,
Q-Node NLI, MRI, NSLPID,
reception interface,
liveness data)
A node SHOULD implement this or an equivalently strong mechanism.
There are several alternatives for the liveness data. One is to use
a timestamp like SCTP. Another is to give the local secret a (rapid)
rollover, with the liveness data as the generation number of the
secret, like IKEv2. In both cases, the liveness data has to be
carried outside the hash, to allow the hash to be verified at the
Responder. Another approach is to replace the hash with encryption
under a locally known secret, in which case the liveness data does
not need to be carried in the clear. Any symmetric cipher immune to
known plaintext attacks can be used.
To support the validation simplicity requirement, the Responder can
check the liveness data to filter out some blind (flooding) attacks
before beginning any cryptographic cookie verification. To support
this usage, the liveness data must be carried in the clear and not be
easily guessable; this rules out the timestamp approach, and suggests
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the use of sequence of secrets with the liveness data identifying the
position in the sequence. The secret strength and rollover frequency
must be high enough that the secret cannot be brute-forced during its
lifetime. Note that any node can use a Query to discover the current
liveness data, so it remains hard to defend against sophisticated
attacks which disguise such 'probes' within a flood of Queries from
forged source addresses. Therefore, it remains important to use an
efficient hashing mechanism or equivalent.
If a node receives a message for which cookie validation fails, it
MAY return an "Object Value Error" error message (Appendix A.4.4.10)
with subcode 4 ("Invalid Cookie") to the sender, as well as dropping
the message. However, doing so in general makes a node a source of
backscatter. Therefore, this SHOULD only be enabled selectively,
e.g. during initial deployment or debugging.
8.6. Security Protocol Selection Policy
This specification defines a single mandatory-to-implement security
protocol (TLS, Section 5.7.3). However, it is possible to define
additional security protocols in the future, for example to allow re-
use with other types of credentials, or migrate towards protocols
with stronger security properties. In addition, use of any security
protocol for a messaging association is optional. Security protocol
selection is carried out as part of the GIST handshake mechanism
(Section 4.4.1).
The selection process may be vulnerable to downgrade attacks, where a
man in the middle modifies the capabilities offered in the Query or
Response to mislead the peers into accepting a lower level of
protection than is achievable. There is a two part defence against
such attacks (the following is based the same concepts as [18]):
1. The Response does not depend on the Stack-Proposal in the Query
(see Section 5.7.1). Therefore, tampering with the Query message
has no effect on the resulting messaging association
configuration.
2. The Responding node's Stack-Proposal is echoed in the Confirm.
The Responding node checks this to validate that the proposal it
made in the Response is the same as the one received by the
Querying node. (Note that as a consequence of the previous
point, the Responding node does not have to remember the proposal
explicitly, since it is a static function of local policy.)
The validity of the second part depends on the strength of the
security protection provided for the Confirm message. If the
Querying node is prepared to create messaging associations with null
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security properties (e.g. TCP only), the defence is ineffective,
since the man in the middle can re-insert the original Responder's
Stack-Proposal, and the Responding node will assume that the minimal
protection is a consequence of Querying node limitations. However,
if the messaging association provides at least integrity protection
that cannot be broken in real-time, the Confirm cannot be modified in
this way. Therefore, provided that the Querying node applies a
security policy on the messaging association protocols it will create
that ensures at least this minimal level of protection is met, it can
be assured that the capability discovery process will result in the
setup of a messaging association with the correct security properties
as appropriate for the two peers involved.
8.7. Residual Threats
Taking the above security mechanisms into account, the main residual
threats against NSIS are three types of on-path attack.
An on-path attacker who can intercept the initial Query can do most
things it wants to the subsequent signaling. It is very hard to
protect against this at the GIST level; the only defence is to use
strong messaging association security to see whether the Responding
node is authorised to take part in NSLP signaling exchanges. To some
extent, this behaviour is logically indistinguishable from correct
operation, so it is easy to see why defence is difficult. Note that
an on-path attacker of this sort can do anything to the traffic as
well as the signaling. Therefore, the additional threat induced by
the signaling weakness seems tolerable.
At the NSLP level, there is a concern about transitivity of trust of
correctness of routing along the signaling chain. The NSLP at the
querying node can have good assurance that it is communicating with
an on-path peer (or a node delegated by the on-path node). However,
it has no assurance that the node beyond the responder is also on-
path, or that the MRI (in particular) is not being modified by the
responder to refer to a different flow. Therefore, if it sends
signaling messages with payloads (e.g. authorisation tokens) which
are "valuable" to nodes beyond the adjacent hop, it is up to the NSLP
to ensure that the appropriate chain of trust exists, which must in
general use messaging association (strong) security.
There is a further residual attack by a node which is not on the path
of the flow, but is on the path of the Response, or is able to use a
Response from one handshake to interfere with another. The attacker
modifies the Response to cause the Querying node to form an adjacency
with it rather than the true downstream node. In principle, this
attack could be prevented by including an additional cryptographic
object in the Response message which ties the Response to the initial
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Query and the routing state and can be verified by the Querying node.
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9. IANA Considerations
This section defines the registries and initial codepoint assignments
for GIST. It also defines the procedural requirements to be followed
by IANA in allocating new codepoints. Guidelines on the technical
criteria to be followed in evaluating requests for new codepoint
assignments are given for the overall NSIS protocol suite in a
separate NSIS extensibility document [36].
This specification allocates the following codepoints in existing
registries:
Well-known UDP port XXX as the destination port for Query-
encapsulated GIST messages (Section 5.3).
This specification creates the following registries with the
structures as defined below:
NSLP Identifiers: Each signaling application requires the assignment
of one of more NSLPIDs. The following NSLPID is allocated by this
specification:
+---------+---------------------------------------------------------+
| NSLPID | Application |
+---------+---------------------------------------------------------+
| 0 | Used for GIST messages not related to any signaling |
| | application. |
+---------+---------------------------------------------------------+
Every other NSLPID MUST be associated with a specific RAO value
(multiple NSLPIDs MAY be associated with the same value). The
NSLPID is a 16 bit integer, and allocation policies for further
values are as follows:
1-32703: IESG Approval
32704-32767: Private/Experimental Use
32768-65536: Reserved
GIST Message Type: The GIST common header (Appendix A.1) contains a 1
byte message type field. The following values are allocated by
this specification:
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+---------+----------+
| MType | Message |
+---------+----------+
| 0 | Query |
| | |
| 1 | Response |
| | |
| 2 | Confirm |
| | |
| 3 | Data |
| | |
| 4 | Error |
| | |
| 5 | MAHello |
+---------+----------+
Allocation policies for further values are as follows:
6-63: Standards Action
64-119: Expert Review
120-127: Private/Experimental Use
128-255: Reserved
Object Types: There is a 12-bit field in the object header
(Appendix A.2). The following values for object type are defined
by this specification:
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+---------+-----------------------------+
| OType | Object Type |
+---------+-----------------------------+
| 0 | Message Routing Information |
| | |
| 1 | Session ID |
| | |
| 2 | Network Layer Information |
| | |
| 3 | Stack Proposal |
| | |
| 4 | Stack Configuration Data |
| | |
| 5 | Query Cookie |
| | |
| 6 | Responder Cookie |
| | |
| 7 | NAT Traversal |
| | |
| 8 | NSLP Data |
| | |
| 9 | Error |
+---------+-----------------------------+
Allocation policies for further values are as follows:
10-1023: Standards Action
1024-1999: Specification Required
2000-2047: Private/Experimental Use
2048-4095: Reserved
When a new object type is defined, the extensibility bits (A/B,
see Appendix A.2.1) must also be defined.
Message Routing Methods: GIST allows multiple message routing methods
(see Section 3.3). The message routing method is indicated in the
leading byte of the MRI object (Appendix A.3.1). This
specification defines the following values:
+---------+------------------------+
| MRM | Message Routing Method |
+---------+------------------------+
| 0 | Path Coupled MRM |
| | |
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| 1 | Loose End MRM |
+---------+------------------------+
Allocation policies for further values are as follows:
2-63: Standards Action
64-119: Expert Review
120-127: Private/Experimental Use
128-255: Reserved
When a new MRM is defined, the information described in
Section 3.3 must be provided.
MA-Protocol-IDs: Each upper layer protocol that can be used in a
messaging association is identified by a 1-byte MA-Protocol-ID
(Section 5.7). This is used as a tag in the Stack-Proposal and
Stack-Configuration-Data objects (Appendix A.3.4 and
Appendix A.3.5). The following values are defined by this
specification:
+---------------------+-----------------------------------------+
| MA-Protocol-ID | Higher Layer Protocol |
+---------------------+-----------------------------------------+
| 1 | TCP opened in the forwards direction |
| | |
| 2 | TLS initiated in the forwards direction |
+---------------------+-----------------------------------------+
Allocation policies for further values are as follows:
3-63: Standards Action
64-119: Expert Review
120-127: Private/Experimental Use
128-255: Reserved
Allocating a new MA-Protocol-ID requires defining the format for
the MA-protocol-options field (if any) in the Stack-Configuration-
Data object that is needed to define its configuration. Note that
the MA-Protocol-ID is not an IP Protocol number (indeed, some of
the messaging association protocols - such as TLS - do not have an
IP Protocol number).
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Error Codes/Subcodes: There is a 2 byte error code and 1 byte subcode
in the Value field of the Error object (Appendix A.4.1). Error
codes 1-12 are defined in Appendix A.4.4 together with subcodes
0-3 for code 1, 0-4 for code 9, 0-5 for code 10, and 0-2 for code
12. Additional codes and subcodes are allocated on a first-come,
first served basis. When a new error code/subcode combination is
allocated, the Error Class and the format of any associated error-
specific information must also be defined.
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10. Acknowledgements
This document is based on the discussions within the IETF NSIS
working group. It has been informed by prior work and formal and
informal inputs from: Cedric Aoun, Attila Bader, Roland Bless, Bob
Braden, Marcus Brunner, Benoit Campedel, Luis Cordeiro, Elwyn Davies,
Christian Dickmann, Pasi Eronen, Alan Ford, Xiaoming Fu, Ruediger
Geib, Eleanor Hepworth, Thomas Herzog, Cheng Hong, Jia Jia, Cornelia
Kappler, Georgios Karagiannis, Chris Lang, John Loughney, Allison
Mankin, Jukka Manner, Pete McCann, Andrew McDonald, Glenn Morrow,
Dave Oran, Andreas Pashalidis, Henning Peters, Tom Phelan, Takako
Sanda, Charles Shen, Melinda Shore, Martin Stiemerling, Martijn
Swanink, Mike Thomas, Hannes Tschofenig, Sven van den Bosch, Michael
Welzl, Lars Westberg, and Mayi Zoumaro-djayoon. Parts of the TLS
usage description (Section 5.7.3) were derived from the Diameter base
protocol specification, RFC3588. In addition, Hannes Tschofenig
provided a detailed set of review comments on the security section,
and Andrew McDonald provided the formal description for the initial
packet formats. Chris Lang's implementation work provided objective
feedback on the clarity and feasibility of the specification, and he
also provided the state machine description and the initial error
catalogue and formats.
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11. References
11.1. Normative References
[1] Katz, D., "IP Router Alert Option", RFC 2113, February 1997.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Nichols, K., Blake, S., Baker, F., and D. Black, "Definition of
the Differentiated Services Field (DS Field) in the IPv4 and
IPv6 Headers", RFC 2474, December 1998.
[4] Partridge, C. and A. Jackson, "IPv6 Router Alert Option",
RFC 2711, October 1999.
[5] Nordmark, E., "Stateless IP/ICMP Translation Algorithm (SIIT)",
RFC 2765, February 2000.
[6] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[7] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[8] Dierks, T. and E. Rescorla, "The TLS Protocol Version 1.1",
draft-ietf-tls-rfc2246-bis-13 (work in progress), June 2005.
11.2. Informative References
[9] Braden, B., Zhang, L., Berson, S., Herzog, S., and S. Jamin,
"Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
Specification", RFC 2205, September 1997.
[10] Terzis, A., Krawczyk, J., Wroclawski, J., and L. Zhang, "RSVP
Operation Over IP Tunnels", RFC 2746, January 2000.
[11] Tsirtsis, G. and P. Srisuresh, "Network Address Translation -
Protocol Translation (NAT-PT)", RFC 2766, February 2000.
[12] 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.
[13] Carpenter, B. and K. Moore, "Connection of IPv6 Domains via
IPv4 Clouds", RFC 3056, February 2001.
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[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] Grossman, D., "New Terminology and Clarifications for
Diffserv", RFC 3260, April 2002.
[17] Price, R., Bormann, C., Christoffersson, J., Hannu, H., Liu,
Z., and J. Rosenberg, "Signaling Compression (SigComp)",
RFC 3320, January 2003.
[18] Arkko, J., Torvinen, V., Camarillo, G., Niemi, A., and T.
Haukka, "Security Mechanism Agreement for the Session
Initiation Protocol (SIP)", RFC 3329, January 2003.
[19] 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.
[20] Rosenberg, J., "Traversal Using Relay NAT (TURN)",
draft-rosenberg-midcom-turn-08 (work in progress),
September 2005.
[21] Gill, V., Heasley, J., and D. Meyer, "The Generalized TTL
Security Mechanism (GTSM)", RFC 3682, February 2004.
[22] Hancock, R., Karagiannis, G., Loughney, J., and S. Van den
Bosch, "Next Steps in Signaling (NSIS): Framework", RFC 4080,
June 2005.
[23] Tschofenig, H. and D. Kroeselberg, "Security Threats for Next
Steps in Signaling (NSIS)", RFC 4081, June 2005.
[24] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites for
Transport Layer Security (TLS)", RFC 4279, December 2005.
[25] Kohler, E., "Datagram Congestion Control Protocol (DCCP)",
draft-ietf-dccp-spec-13 (work in progress), December 2005.
[26] Conta, A., "Internet Control Message Protocol (ICMPv6) for the
Internet Protocol Version 6 (IPv6) Specification",
draft-ietf-ipngwg-icmp-v3-07 (work in progress), July 2005.
[27] Stiemerling, M., "NAT/Firewall NSIS Signaling Layer Protocol
(NSLP)", draft-ietf-nsis-nslp-natfw-09 (work in progress),
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February 2006.
[28] Manner, J., "NSLP for Quality-of-Service signalling",
draft-ietf-nsis-qos-nslp-09 (work in progress), February 2006.
[29] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms for
IPv6 Hosts and Routers", RFC 4213, October 2005.
[30] Kent, S. and K. Seo, "Security Architecture for the Internet
Protocol", RFC 4301, December 2005.
[31] Ylonen, T. and C. Lonvick, "SSH Protocol Architecture",
draft-ietf-secsh-architecture-22 (work in progress),
March 2005.
[32] Moskowitz, R., "Host Identity Protocol", draft-ietf-hip-base-04
(work in progress), October 2005.
[33] Nikander, P., Arkko, J., Aura, T., Montenegro, G., and E.
Nordmark, "Mobile IP Version 6 Route Optimization Security
Design Background", RFC 4225, December 2005.
[34] Floyd, S. and V. Jacobson, "The Synchronisation of Periodic
Routing Messages", SIGCOMM Symposium on Communications
Architectures and Protocols pp. 33--44, September 1993.
[35] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", draft-rescorla-dtls-05 (work in progress),
June 2005.
[36] Loughney, J., "NSIS Extensibility Model",
draft-loughney-nsis-ext-01 (work in progress), July 2005.
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Appendix A. Bit-Level Formats and Error Messages
This appendix provides formats for the various component parts of the
GIST messages defined abstractly in Section 5.2.
Each GIST message consists of a header and a sequence of objects.
The GIST header has a specific format, described in more detail in
Appendix A.1 below. An NSLP message is one object within a GIST
message. Note that GIST itself provides the message length
information and signaling application identification. General object
formatting guidelines are provided in Appendix A.2 below, followed in
Appendix A.3 by the format for each object. Finally, Appendix A.4
provides the formats used for error reporting.
In the following object diagrams, '//' is used to indicate a variable
sized field and ':' is used to indicate a field that is optionally
present.
A.1. The GIST Common Header
This header precedes all GIST messages. It has a fixed format, as
shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version | GIST hops | Message length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NSLPID | Type |S|R|E| Reserved|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message length = the total number of words in the message after
the common header itself
Type = the GIST message type (Query, Response, etc.)
S flag = S=1 if the IP source address is the same as the
signaling source address, S=0 if it is different
R flag = R=1 if a reply to this message is explicitly
requested
E flag = E=1 if the message was explicitly routed
Section 7.1.4
The rules governing the use of the R-flag depend on the GIST message
type. It MUST always be set (R=1) in Query messages (these always
elicit a Response), and never in Confirm, Data or Error messages. It
is optional in a MA-Hello; if set, another MA-Hello is sent in reply.
It is optional in a Response (but MUST be set if the Response
contains a Responder cookie); if set, a Confirm is sent in reply.
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Parsing failures may be caused by unknown Version or Type values,
inconsistent R flag setting, or a Message Length inconsistent with
the set of objects carried. In all cases the receiver MUST if
possible return a "Common Header Parse Error" message
(Appendix A.4.4.1) with the appropriate subcode, and not process the
message further.
A.2. General Object Format
Each object begins with a fixed header giving the object Type and
object Length. This is followed by the object Value, which is a
whole number of 32-bit words long.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|A|B|r|r| Type |r|r|r|r| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Value //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The individual components are as follows:
o The bits marked 'A' and 'B' are extensibility flags which are
defined below; the remaining bits marked 'r' are reserved.
o Length has the units of 32 bit words, and measures the length of
Value. If there is no Value, Length=0. If the Length is not
consistent with the contents of the object, an "Object Value
Error" message (Appendix A.4.4.10) with subcode 0 "Incorrect
Length" MUST be returned and the message dropped.
o Value is (therefore) a whole number of 32 bit words. If there is
any padding required, the length and location must be defined by
the object-specific format information; objects which contain
variable length (e.g. string) types may need to include additional
length subfields to do so.
Any part of the object used for padding or defined as reserved
(marked 'Reserved' or 'Rsv' in the diagrams below) MUST be set to 0
on transmission and MUST be ignored on reception.
A.2.1. Object Extensibility
The leading two bits of the TLV header are used to signal the desired
treatment for objects whose Type field is unknown at the receiver.
The following three categories of object have been identified, and
are described here.
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AB=00 ("Mandatory"): If the object is not understood, the entire
message containing it MUST be rejected with an "Object Type Error"
message (Appendix A.4.4.9) with subcode 1 ("Unrecognised Object").
AB=01 ("Ignore"): If the object is not understood, it MUST be deleted
and the rest of the message processed as usual.
AB=10 ("Forward"): If the object is not understood, it MUST be
retained unchanged in any message forwarded as a result of message
processing, but not stored locally.
The combination AB=11 is reserved.
A.3. GIST TLV Objects
A.3.1. Message-Routing-Information
Type: Message-Routing-Information
Length: Variable (depends on message routing method)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MRM-ID | Reserved | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
// Method-specific addressing information (variable) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A.3.1.1. Path-Coupled MRM
In the case of basic path-coupled routing, the addressing information
takes the following format:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|IP-Ver |P|T|F|S|A|B|D|Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Source Address //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Destination Address //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Prefix | Dest Prefix | Protocol | DS-field |Rsv|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Reserved | Flow Label :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: SPI :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Source Port : Destination Port :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The flags are:
P - P=1 means that IP Protocol should be interpreted
T - T=1 means that DS-Field should be interpreted; see [3] and [16]
F - F=1 means that flow Label is present and should be interpreted
S - S=1 means that SPI is present and should be interpreted; see [30]
A/B - Source/Destination Port (see below)
D - Direction of message relative to flow
The source and destination addresses are always present and of the
same type; their length depends on the value in the IP-Ver field. In
the normal case where the MRI refers only to traffic between specific
host addresses, the Source/Dest Prefix values would both be 32/128
for IPv4/6 respectively.
In the case of IPv6, the Protocol field refers to the true upper
layer protocol carried by the packets, i.e. excluding any IP option
headers. This is therefore not necessarily the same as the Next
Header value from the base IPv6 header.
F may only be set if IP-Ver is 6. If F is not set, the entire 32 bit
word for the Flow Label is absent.
The S/A/B flags can only be set if P is set. The SPI field is only
present if the S flag is set.
If either of A, B is set (value=1), the word containing the port
numbers is included in the object. However, the contents of each
field is only significant if the corresponding flag is set;
otherwise, the contents of the field is regarded as padding, and the
MRI refers to all ports (i.e. acts as a wildcard). If the flag is
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set and Port=0x0000, the MRI will apply to a specific port, whose
value is not yet known. If neither of A or B is set, the word is
absent.
The Direction flag has the following meaning: the value 0 means 'in
the same direction as the flow' (or "downstream"), and the value 1
means 'in the opposite direction to the flow' (or "upstream").
A.3.1.2. Loose-End MRM
In the case of the loose-end message routing method, the addressing
information takes the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|IP-Ver |D| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Source Address //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Destination Address //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The only flag defined is:
D - Direction (always 0 for "downstream")
The source and destination addresses are always present and of the
same type; their length depends on the value in the IP-Ver field.
A.3.2. Session Identification
Type: Session-Identification
Length: Fixed (4 32-bit words)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Session ID +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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A.3.3. Network-Layer-Information
Type: Network-Layer-Information
Length: Variable (depends on length of Peer-Identity and IP version)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PI-Length | IP-TTL |IP-Ver | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Routing State Validity Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Peer Identity //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Interface Address //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Routing State Validity Time = the time for which the routing state
for this flow can be considered correct without a
refresh. Given in milliseconds.
PI-Length = the byte length of the Peer-Identity field
(note that the Peer-Identity field itself is padded
to a whole number of words)
IP-TTL = initial or reported IP-TTL
IP-Ver = the IP version for the Interface-Address field
A.3.4. Stack Proposal
Type: Stack-Proposal
Length: Variable (depends on number of profiles and size of each
profile)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prof-Count | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Profile 1 //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Profile 2 //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Prof-Count = The number of profiles in the proposal. MUST be > 0.
Each profile is itself a sequence of protocol layers, and the profile
is formatted as a list as follows:
o The first byte is a count of the number of layers in the profile.
o This is followed by a sequence of 1-byte MA-Protocol-IDs as
described in Section 5.7.
o The profile is padded to a word boundary with 0, 1, 2 or 3 zero
bytes.
If there are no profiles (i.e. all bytes are null), then a "Object
Value Error" message (Appendix A.4.4.10) with subcode 3 ("Empty
List") MUST be returned and the message dropped.
A.3.5. Stack-Configuration-Data
Type: Stack-Configuration-Data
Length: Variable (depends on number of protocols and size of each MA-
protocol-options field)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MPO-Count | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MA-Hold-Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// MA-protocol-options 1 //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// MA-protocol-options N //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
MA-Hold-Time = the time for which the messaging association will
be held open without traffic or a hello message.
Given in milliseconds.
MPO-Count = the number of MA-protocol-options fields present
(these contain their own length information)
The MA-protocol-options fields are formatted as follows:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|MA-Protocol-ID | Profile | Length |D| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Options Data //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
MA-Protocol-ID = Protocol identifier as described in
Section 5.7
.
Profile = Tag indicating which profile from the accompanying
Stack-Proposal object this applies to. Profiles
are numbered from 1 upwards; the special value 0
indicates 'applies to all profiles'.
Length = the byte length of MA-protocol-options field
that follows. This will be zero-padded
up to the next word boundary.
D flag = If set (D=1), this protocol MUST not be used for a
messaging association.
Note that the format of the options data may differ depending on
whether the field is in a GIST-Query or GIST-Response.
A.3.6. Query Cookie
Type: Query-Cookie
Length: Variable (selected by querying node)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Query Cookie //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The contents are implementation defined. See Section 8.5 for further
discussion.
A.3.7. Responder Cookie
Type: Responder-Cookie
Length: Variable (selected by responding node)
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Responder Cookie //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The contents are implementation defined. See Section 8.5 for further
discussion.
A.3.8. NAT Traversal
Type: NAT-Traversal
Length: Variable (depends on length of contained fields)
This object is used to support the NAT traversal mechanisms described
in Section 7.2.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MRI-Length | Type-Count | NAT-Count | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Original Message-Routing-Information //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// List of translated objects //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of opaque information | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ //
// Information replaced by NAT #1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of opaque information | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ //
// Information replaced by NAT #N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
MRI-Length = the word length of the included MRI payload
Type-Count = the number of GIST payloads translated by the
NAT; the Type numbers are included as a list
(padded with 2 null bytes if necessary)
NAT-Count = the number of NATs traversed by the message, and the
number of opaque payloads at the end of the object
The length fields in the body of the message are byte counts, not
including the 2 bytes of the length field itself. Note that each
opaque information field is zero-padded to the next 32-bit word
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boundary if necessary.
A.3.9. NSLP Data
Type: NSLP-Data
Length: Variable (depends on NSLP)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// NSLP Data //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A.4. Errors
A.4.1. Error Object
Type: Error
Length: Variable (depends on error)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Class | Error Code | Error Subcode |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S|M|C|D|Q| Reserved | MRI Length | Info Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Common Header +
| (of original message) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Session Id :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Message Routing Information :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Additional Information :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Debugging Comment :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The flags are:
S - S=1 means the Session ID object is present
M - M=1 means MRI object is present
C - C=1 means a debug Comment is present after header.
D - D=1 means the original message was received in D-Mode
Q - Q=1 means the original message was received Q-Mode encapsulated
(can't be set if D=0).
A GIST Error object contains an error-class (see Appendix A.4.3), an
error-code, an error-subcode, and as much information about the
message which triggered the error as is available. This information
MUST include the Common header of the original message and SHOULD
also include the Session Id and MRI objects if these could be decoded
correctly. These objects are included in their entirety, except for
their TLV Headers.
The Info Count field contains the number of Additional Information
fields in the object. This count is usually 0 or 1, but may be more
for certain messages; the precise set of fields to include is defined
with the error code/subcode. The field formats are given in
Appendix A.4.2 and their use for the different errors is given in the
error catalogue Appendix A.4.4. The Debugging Comment is a null-
terminated UTF-8 string, padded if necessary to a whole number of 32-
bit words with more null characters.
A.4.2. Additional Information Fields
The Common Error Header may be followed by some Additional
Information objects. The possible formats of these objects are shown
below.
Message Length Info:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Calculated Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Calculated Length = the length of the original message calculated
by adding up all the objects in the message.
MTU Info:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link MTU | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This object provides information about the MTU for a link along
which a message could not be sent.
Object Type Info:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Object Type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This object provides information about the type of object which
caused the error.
Object Value Info:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rsv | Real Object Length | Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Object //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Real Object Length: Since the length in the original TLV header
may be inaccurate, this field provides the actual
length of the object (including the TLV Header)
included in the error message.
Offset: The byte in the object at which the GIST node
found the error.
Object: The invalid TLV object (including the TLV Header)
This object carries information about a TLV object which was found
to be invalid in the original message. An error message may contain
more than one Object Value Info object.
A.4.3. Error Classes
The first byte of the error object, "Error Class", indicates the
severity level. The currently defined severity levels are:
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0 (Informational): response data which should not be thought of as
changing the condition of the protocol state machine.
1 (Success): response data which indicates that the message being
responded to has been processed successfully in some sense.
2 (Protocol-Error): the message has been rejected because of a
protocol error (e.g. an error in message format).
3 (Transient-Failure): the message has been rejected because of a
particular local node status which may be transient (i.e. it may
be worthwhile to retry after some delay).
4 (Permanent-Failure): the message has been rejected because of local
node status which will not change without additional out of band
(e.g. management) operations.
Additional error class values are reserved.
The allocation of error classes to particular errors is not precise;
the above descriptions are deliberately informal. Actual error
processing should take into account the specific error in question;
the error class may be useful supporting information (e.g. in network
debugging).
A.4.4. Error Catalogue
This section lists all the possible GIST errors, including when they
are raised and what additional information fields should be carried
in the error object.
A.4.4.1. Common Header Parse Error
Class: Protocol-Error
Code: 1
Additional Info: Depends on subcode
This message is sent if a GIST node receives a message where the
common header cannot be parsed correctly, or where an error in the
overall message format is detected. Note that in this case the
original MRI and Session ID are not included in the Error Object.
This error code is split into subcodes as follows:
0: Unknown Version: The GIST version is unknown. The (highest)
supported version supported by the node can be inferred from the
Common Header of the GIST-Error message itself.
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1: Unknown Type: The GIST message type is unknown.
2: Invalid R-flag: The R flag in the header is inconsistent with the
message type.
3: Incorrect Message Length: The overall message length is not
consistent with the set of objects carried. An Additional Info
field of Message Length Info carries the calculated message
length.
A.4.4.2. Hop Limit Exceeded
Class: Permanent-Failure
Code: 2
Additional Info: None
This message is sent if a GIST node receives a message with a GIST
Hop Limit of zero, or a GIST node decrements a packet's GIST Hop
Limit to zero. This message indicates either a routing loop or too
small an initial Hop Limit value.
A.4.4.3. Incorrect Encapsulation
Class: Protocol-Error
Code: 3
Additional Info: None
This message is sent if a GIST node receives a message which uses an
incorrect encapsulation method (e.g. a Query arrives over an MA).
A.4.4.4. Incorrectly Delivered Message
Class: Protocol-Error
Code: 4
Additional Info: None
This message is sent if a GIST node receives a message over an MA
which is not associated with the MRI/NSLPID/SID combination in the
message.
A.4.4.5. No Routing State
Class: Protocol-Error
Code: 5
Additional Info: None
This message is sent if a node receives a message for which routing
state should exist, but has not yet been created (and thus there is
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no appropriate Q/R-SM). This can occur either on receiving a
Response to an unknown Query, or on receiving a Data message at a
node whose policy requires routing state to exist before such
messages can be accepted. See also Section 6.1 and Section 6.3.
A.4.4.6. Unknown NSLPID
Class: Permanent-Failure
Code: 6
Additional Info: None
This message is sent if a router receives a directly addressed
message for an NSLP which it does not support.
A.4.4.7. Endpoint Found
Class: Informational
Code: 7
Additional Info: None
This message is sent if a GIST node at a flow endpoint receives a
Query message for an NSLP which it does not support.
A.4.4.8. Message Too Large
Class: Permanent-Failure
Code: 8
Additional Info: MTU Info
A router receives a message which it can't forward because it exceeds
the MTU on the next or subsequent hops.
A.4.4.9. Object Type Error
Class: Protocol-Error
Code: 9
Additional Info: Object Type Info
This message is sent if a GIST node receives a message containing a
TLV object with an invalid type. The message includes the type of
the object at fault. This error code is split into subcodes as
follows:
0: Duplicate Object: This subcode is used if a GIST node receives a
message containing multiple instances of an object which may only
appear once in a message (in the current specification this
applies to all objects).
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1: Unrecognised Object: This subcode is used if a GIST node receive a
message containing an object which it does not support, and the
extensibility flags AB=00.
2: Missing Object: This subcode is used if a GIST node receives a
message which is missing one or more mandatory objects. This
message is also sent if a Stack-Proposal is sent without a
matching Stack-Configuration-Data object when one was necessary,
or vice versa.
3: Invalid Object: This subcode is used if the object type is known,
but it is not valid for this particular GIST message type.
4: Untranslated Object: This subcode is used if the object type is
known and is mandatory to interpret, but it contains addressing
data which has not been translated by an intervening NAT.
A.4.4.10. Object Value Error
Class: Protocol-Error
Code: 10
Additional Info: Object Value Info
This message is sent if a router receives a packet containing an
object which cannot be properly parsed. The message contains a
single Object Value Info object, unless otherwise stated below. This
error code is split into subcodes as follows:
0: Incorrect Length: The overall length does not match the object
length calculated from the object contents.
1: Value Not Supported: The value of a field is not supported by the
GIST node.
2: Invalid Flag-Field Combination: An object contains an invalid
combination of flags and/or fields. At the moment this only
relates to the Path-Coupled MRM object, but in future there may be
more.
3: Empty List: At the moment this only relates to Stack-Proposals.
The error message is sent if a stack proposal with a length > 0 (a
length of 0 is handled as "Value Not Supported") contains only
null bytes.
4: Invalid Cookie: The message contains a cookie which could not be
verified by the node.
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5: SP-SCD Mismatch: This subcode is used if a GIST node receives a
message in which the data in the Stack-Proposal object is
inconsistent with the information in the Stack Configuration Data
object. In this case, both the Stack-Proposal object and Stack-
Configuration-Data object are included in the message, in separate
Object Value Info fields.
A.4.4.11. Invalid IP TTL
Class: Permanent-Failure
Code: 11
Additional Info: None
This error indicates that a message was received with an IP-TTL
outside an acceptable range; for example, that an upstream Query was
received with an IP-TTL of less than 254 (i.e. more than one IP hop
from the sender). The actual IP distance can be derived from the IP-
TTL information in the NLI object carried in the same message.
A.4.4.12. MRI Validation Failure
Class: Permanent-Failure
Code: 12
Additional Info: Object Value Info
This error indicates that a message was received with an MRI that
could not be accepted, e.g. because of too much wildcarding or
failing some validation check (cf. Section 5.8.1.2). The Object
Value Info includes the MRI so the error originator can indicate the
part of the MRI which caused the problem. The error code is divided
into subcodes as follows:
0: MRI Too Wild: The MRI contained too much wildcarding (e.g. too
short a destination address prefix) to be forwarded correctly down
a single path.
1: IP Version Mismatch: The MRI in a path-coupled Query message uses
an IP version which is not implemented on the interface used.
2: Ingress Filter Failure: The MRI in a path-coupled Query message
describes a flow which would not pass ingress filtering on the
interface used.
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Appendix B. API between GIST and Signaling Applications
This appendix provides an abstract API between GIST and signaling
applications. It should not constrain implementors, but rather help
clarify the interface between the different layers of the NSIS
protocol suite. In addition, although some of the data types carry
the information from GIST information elements, this does not imply
that the format of that data as sent over the API has to be the same.
Conceptually the API has similarities to the sockets API,
particularly that for unconnected UDP sockets. An extension for an
API like that for UDP connected sockets could be considered. In this
case, for example, the only information needed in a SendMessage
primitive would be NSLP-Data, NSLP-Data-Size, and NSLP-Message-Handle
(which can be null). Other information which was persistent for a
group of messages could be configured once for the socket. Such
extensions may make a concrete implementation more efficient but do
not change the API semantics, and so are not considered further here.
B.1. SendMessage
This primitive is passed from a signaling application to GIST. It is
used whenever the signaling application wants to initiate sending a
message.
SendMessage ( NSLP-Data, NSLP-Data-Size, NSLP-Message-Handle,
NSLPID, Session-ID, MRI,
SII-Handle, Transfer-Attributes, Timeout, IP-TTL, GHC )
The following arguments are mandatory.
NSLP-Data: The NSLP message itself.
NSLP-Data-Size: The length of NSLP-Data.
NSLP-Message-Handle: A handle for this message, that can be used by
GIST as a reference in subsequent MessageStatus notifications
(Appendix B.3). Notifications could be about error conditions or
about the security attributes that will be used for the message.
A NULL handle may be supplied if the NSLP is not interested in
such notifications.
NSLPID: An identifier indicating which NSLP this is.
Session-ID: The NSIS session identifier. Note that it is assumed
that the signaling application provides this to GIST rather than
GIST providing a value itself.
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MRI: Message routing information for use by GIST in determining the
correct next GIST hop for this message. The MRI implies the
message routing method to be used and the message direction.
The following arguments are optional.
SII-Handle: A handle, previously supplied by GIST, to a data
structure that should be used to route the message explicitly to a
particular GIST next hop.
Transfer-Attributes: Attributes defining how the message should be
handled (see Section 4.1.2). The following attributes can be
considered:
Reliability: Values 'unreliable' or 'reliable'.
Security: This attribute allows the NSLP to specify what level of
security protection is requested for the message (selected from
'integrity' and 'confidentiality'), and can also be used to
specify what authenticated signaling source and destination
identities should be used to send the message. The
possibilities can be learned by the signaling application from
prior MessageStatus or RecvMessage notifications. If an NSLP-
Message-Handle is provided, GIST will inform the signaling
application of what values it has actually chosen for this
attribute via a MessageStatus callback. This might take place
either synchronously (where GIST is selecting from available
messaging associations), or asynchronously (when a new
messaging association needs to be created).
Local Processing: This attribute contains hints from the signaling
application about what local policy should be applied to the
message; in particular, its transmission priority relative to
other messages, or whether GIST should attempt to set up or
maintain forward routing state.
Timeout: Length of time GIST should attempt to send this message
before indicating an error.
IP-TTL: The value of the IP TTL that should be used when sending this
message (may be overridden by GIST for particular messages).
GHC: The value for the GIST hop count when sending the message.
B.2. RecvMessage
This primitive is passed from GIST to a signaling application. It is
used whenever GIST receives a message from the network, including the
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case of 'null' messages (zero length NSLP payload), typically initial
Query messages. For Queries, the results of invoking this primitive
are used by GIST to check whether message routing state should be
created (see the discussion of the 'Routing-State-Check' argument
below).
RecvMessage ( NSLP-Data, NSLP-Data-Size, NSLPID, Session-ID, MRI,
Routing-State-Check, SII-Handle, Transfer-Attributes,
IP-TTL, IP-Distance, GHC )
NSLP-Data: The NSLP message itself (may be empty).
NSLP-Data-Size: The length of NSLP-Data (may be zero).
NSLPID: An identifier indicating which NSLP this is message is for.
Session-ID: The NSIS session identifier.
MRI: Message routing information that was used by GIST in forwarding
this message. Implicitly defines the message routing method that
was used and the direction of the message relative to the MRI.
Routing-State-Check: This boolean is True if GIST is checking with
the signaling application to see if routing state should be
created with the peer or the message should be forwarded further
(see Section 4.3.2). If True, the signaling application should
return the following values via the RecvMessage call:
A boolean indicating whether to set up the state.
Optionally, an NSLP-Payload to carry in the generated GIST-
Response or forwarded Query message respectively.
This mechanism could be extended to enable the signaling
application to indicate to GIST whether state installation should
be immediate or deferred (see Section 5.3.3 and Section 6.3 for
further discussion).
SII-Handle: A handle to a data structure, identifying a peer address
and interface. Can be used to identify route changes and for
explicit routing to a particular GIST next hop.
Transfer-Attributes: The reliability and security attributes that
were associated with the reception of this particular message. As
well as the attributes associated with SendMessage, GIST may
indicate the level of verification of the addresses in the MRI.
Three attributes can be indicated:
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* Whether the signaling source address is one of the flow
endpoints (i.e. whether this is the first or last GIST hop);
* Whether the signaling source address has been validated by a
return routability check.
* Whether the message was explicitly routed (and so has not been
validated by GIST as delivered consistently with local routing
state).
IP-TTL: The value of the IP TTL this message was received with (if
available).
IP-Distance: The number of IP hops from the peer signaling node which
sent this message along the path, or 0 if this information is not
available.
GHC: The value of the GIST hop count the message was received with,
after being decremented in the GIST receive-side processing.
B.3. MessageStatus
This primitive is passed from GIST to a signaling application. It is
used to notify the signaling application that a message that it
requested to be sent could not be dispatched, or to inform the
signaling application about the transfer attributes that have been
selected for the message (specifically, security attributes). The
signaling application can respond to this message with a return code
to abort the sending of the message if the attributes are not
acceptable.
MessageStatus (NSLP-Message-Handle, Transfer-Attributes, Error-Type)
NSLP-Message-Handle: A handle for the message provided by the
signaling application in SendMessage.
Transfer-Attributes: The reliability and security attributes that
will be used to transmit this particular message.
Error-Type: Indicates the type of error that occurred. For example,
'no next node found'.
B.4. NetworkNotification
This primitive is passed from GIST to a signaling application. It
indicates that a network event of possible interest to the signaling
application occurred.
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NetworkNotification ( NSLPID, MRI, Network-Notification-Type )
NSLPID: An identifier indicating which NSLP this is message is for.
MRI: Provides the message routing information to which the network
notification applies.
Network-Notification-Type: Indicates the type of event that caused
the notification and associated additional data. Two events have
been identified:
Last Node: GIST has detected that this is the last NSLP-aware node
in the path. See Section 4.3.4.
Routing Status Change: GIST has installed new routing state, or
has detected that the routing state may no longer be valid, or
has re-established the routing state. See Section 7.1.3. The
new status is reported; if the status is Good, the SII-Handle
of the peer is also reported, as for RecvMessage.
B.5. SetStateLifetime
This primitive is passed from a signaling application to GIST. It
indicates the duration for which the signaling application would like
GIST to retain its routing state. It can also give a hint that the
signaling application is no longer interested in the state.
SetStateLifetime ( NSLPID, MRI, State-Lifetime )
NSLPID: Provides the NSLPID to which the routing state lifetime
applies.
MRI: Provides the message routing information to which the routing
state lifetime applies; includes the direction (in the D flag).
State-Lifetime: Indicates the lifetime for which the signaling
application wishes GIST to retain its routing state (may be zero,
indicating that the signaling application has no further interest
in the GIST state).
B.6. InvalidateRoutingState
This primitive is passed from a signaling application to GIST. It
indicates that the signaling application has knowledge that the next
signaling hop known to GIST may no longer be valid, either because of
changes in the network routing or the processing capabilities of
signaling application nodes. See Section 7.1.
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InvalidateRoutingState ( NSLPID, MRI, Status, Urgent )
NSLPID: The NSLP originating the message. May be null (in which case
the invalidation applies to all signaling applications).
MRI: The flow for which routing state should be invalidated; includes
the direction of the change (in the D flag).
Status: The new status that should be assumed for the routing state,
one of Bad or Tentative (see Section 7.1.3).
Urgent: A hint as to whether rediscovery should take place
immediately, or only with the next signaling message.
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Appendix C. Example Routing State Table and Handshake Message Sequence
Figure 10 shows a signaling scenario for a single flow being managed
by two signaling applications using the path-coupled message routing
method. The flow sender and receiver and one router support both,
two other routers support one each. The figure also shows the
routing state table at node B.
A B C D E
+------+ +-----+ +-----+ +-----+ +--------+
| Flow | +-+ +-+ |NSLP1| |NSLP1| | | | Flow |
|Sender|====|R|====|R|====|NSLP2|====| |====|NSLP2|====|Receiver|
| | +-+ +-+ |GIST | |GIST | |GIST | | |
+------+ +-----+ +-----+ +-----+ +--------+
Flow Direction ------------------------------>>
+------------------------------------+---------+--------+-----------+
| Message Routing Information | Session | NSLP | Routing |
| | ID | ID | State |
+------------------------------------+---------+--------+-----------+
| MRM = Path Coupled; Flow ID = | 0xABCD | NSLP1 | IP-A |
| {IP-A, IP-E, proto/ports}; D=up | | | |
| | | | |
| MRM = Path Coupled; Flow ID = | 0xABCD | NSLP1 | (null) |
| {IP-A, IP-E, proto/ports}; D=down | | | |
| | | | |
| MRM = Path Coupled; Flow ID = | 0x1234 | NSLP2 | IP-A |
| {IP-A, IP-E, proto/ports}; D=up | | | |
| | | | |
| MRM = Path Coupled; Flow ID = | 0x1234 | NSLP2 | Points to |
| {IP-A, IP-E, proto/ports}; D=down | | | B-D MA |
+------------------------------------+---------+--------+-----------+
Figure 10: A Signaling Scenario
The upstream state is just the same address for each application.
For the downstream direction, NSLP1 only requires datagram mode
messages and so no explicit routing state towards C is needed. NSLP2
requires a messaging association for its messages towards node D, and
node C does not process NSLP2 at all, so the peer state for NSLP2 is
a pointer to a messaging association that runs directly from B to D.
Note that E is not visible in the state table (except implicitly in
the address in the message routing information); routing state is
stored only for adjacent peers. (In addition to the peer
identification, IP hop counts are stored for each peer where the
state itself if not null; this is not shown in the table.)
Figure 11 shows the message sequence for a GIST handshake that sets
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up the messaging association for B-D signaling. It shows the
exchange of Stack Proposals and MA-protocol-options data in each
direction. Then the Querying node selects TLS/TCP as the stack
configuration to use and sets up the messaging association over which
it sends the Confirm.
-----------------------GIST-Query --------------------->
IP(Src=IP#A; Dst=IP#E; RAO for NSLP2); UDP(Src=6789; Dst=GIST)
GIST(Header(Type=Query; NSLPID=NSLP2; R=1; S=0)
MRI(MRM=Path-Coupled; Flow=F; Direction=down)
SessionID(0x1234)
NLI(Peer='string1'; IA=IP#B)
QueryCookie(0x139471239471923526)
StackProposal(#Proposals=3;1=TLS/TCP; 2=TLS/SCTP; 3=TCP)
StackConfigurationData(#MPO=2;
TCP(Applicable: all; Data: null)
SCTP(Applicable: all; Data: null)))
<---------------------GIST-Response---------------------
IP(Src=IP#D; Dst=IP#B); UDP(Src=GIST; Dst=6789)
GIST(Header(Type=Response; NSLPID=NSLP2; R=1; S=1)
MRI(MRM=Path-Coupled; Flow=F; Direction=up)
SessionID(0x1234)
NLI(Peer='stringr2', IA=IP#D)
QueryCookie(0x139471239471923526)
ResponderCookie(0xacdefedcdfaeeeded)
StackProposal(#Proposals=3; 1=TCP; 2=SCTP; 3=TLS/TCP)
StackConfigurationData(#MPO=3;
TCP(Applicable: 3; Data: port=6123)
TCP(Applicable: 1; Data: port=5438)
SCTP(Applicable: all; Data: port=3333)))
-------------------------TCP SYN----------------------->
<----------------------TCP SYN/ACK----------------------
-------------------------TCP ACK----------------------->
TCP connect(IP Src=IP#B; IP Dst=IP#D; Src Port=9166; Dst Port=6123)
<-----------------------TLS INIT----------------------->
----------------------GIST-Confirm--------------------->
[Sent within messaging association]
GIST(Header(Type=Confirm; NSLPID=NSLP2; R=0; S=1)
MRI(MRM=Path-Coupled; Flow=F; Direction=down)
SessionID(0x1234)
NLI(Peer='string1'; IA=IP#B)
ResponderCookie(0xacdefedcdfaeeeded)
StackProposal(#Proposals=3; 1=TCP; 2=SCTP; 3=TLS/TCP))
Figure 11: GIST Handshake Message Sequence
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Appendix D. Change History
D.1. Changes In Version -09
1. Added a new Section 3.5 clarifying the relationship between
signaling applications and NSLPIDs; modified terminology in the
remainder of the document likewise.
2. Added a new Section 8.6 explaining the rationale behind the
downgrade attack prevention mechanism.
3. Re-wrote parts of Section 4.3.2, Section 6.1 and Appendix B.2 to
clarify the way that GIST is assumed to interact with signaling
applications to exercise policy control over whether or not two
nodes become signaling peers during a GIST handshake.
4. Generalised an error message Appendix A.4.4.12 to cover
additional MRI validation checks in Section 4.3.4 and
Section 5.8.1.2.
5. Allowed an optional Stack-Configuration-Data object in Confirm
messages to allow messaging association lifetime to be
negotiated even in the case of late state installation at the
Responding node (see Section 4.4.1 and Section 4.4.3).
6. Removed the option in Section 4.4.2 of allowing a node to treat
messaging associations with the same authenticated end points as
equivalent.
7. Include additional guidance in Section 4.4.3 to prevent routing
state being erroneously refreshed in the case of rerouting
events; also included general guidance notes on timer setting.
8. Clarified that the Stack-Proposal lists protocols in top-to-
bottom order (see Section 5.7.1).
9. Enhanced the definition of TLS usage in Section 5.7.3 with
details on ciphersuite requirements and authentication methods.
10. Tidied up terminology and discussion of how protocol options
data is carried in the SCD; renamed higher-layer-addressing to
MA-protocol-options.
D.2. Changes In Version -08
1. Changed the protocol name from GIMPS to GIST (everywhere).
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2. Inserted RFC2119 language (MUST etc.) in the appropriate places.
3. Added references to the actions to be taken in various error
conditions, including the error messages to be send
(throughout).
4. Added legacy NAT traversal to the list of excluded functions in
Section 1.1.
5. Included some text at the end of Section 3.3 analysing the case
of a GIST node which does not support a particular MRM.
6. Added a flag to mark when messages have been explicitly routed,
so they can bypass validation against current routing state (see
Section 4.3.1, TBD).
7. Re-wrote the discussion in Section 4.3.4 to cover all cases of
nodes not hosting an NSLP (including end systems), in particular
the validations that can be performed at intermediate GIST nodes
(this replaces the old section 7.2).
8. Clarified the rules about R and S flag setting in the common
header and D flag in the MRI (Section 5).
9. Included discussion of how a node with a choice of interfaces or
IP versions should select one to use in the NLI (Section 5.2.2).
10. Modified the description of messaging association protocol
selections (Section 5.7 and elsewhere) to clarify that this is
essentially capability discovery rather than an open ended
protocol negotiation.
11. Modified the description of how higher layer addressing
information is carried (Section 5.7.1 and Appendix A.3.5) to
allow the data to be tagged against a specific profile if
necessary, or omitted if the protocol does not need it.
12. Added a higher layer protocol definition for TLS in
Section 5.7.3.
13. Simplified and restructured the state machine presentation in
Section 6, in particular using a single list for the events and
eliminating the transition tables. Also modified the operation
of the Responder machine to handle retransmitted Query messages
correctly.
14. Re-wrote the route change handling text in Section 7.1 to
clarify the relative responsibilities of GIST and NSLPs and
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their interaction through the API. Notifications are now
assumed to be a signaling application responsibility, and GIST
behaviour is defined in terms of handling changes in a 3-state
model of the correctness of the routing state for each
direction.
15. Updated the NAT traversal description in Section 7.2, including
normative text about how GIST nodes should handle messages
containing NAT-Traversal objects.
16. Likewise, clarified that the responsibility for session/flow
binding in the case of tunnelling is handled by NSLPs
(Section 7.3).
17. Formalised the IANA considerations (Section 9).
18. Extended the routing state example (Appendix C) to include a
message sequence for association setup.
19. Re-arranged the sequence of sections, including placing this
change history at the end.
D.3. Changes In Version -07
1. The open issues section has finally been removed in favour of the
authoritative list of open issues in an online issue tracker at h
ttp://nsis.srmr.co.uk/cgi-bin/roundup.cgi/nsis-ntlp-issues/index.
2. Clarified terminology on peering and adjacencies that there may
be NSIS nodes between GIMPS peers that do some message
processing, but that are not explicitly visible in the peer state
tables.
3. Added a description of the loose-end MRM (Section 5.8.2 and
Appendix A.3.1.2).
4. Added a description of an upstream Query encapsulation for the
path-coupled MRM, Section 5.8.1.3, including rationale for and
restrictions on its use.
5. The formal description of the protocol in Section 6 has been
significantly updated and extended in terms of detail.
6. Modified the description of the interaction between NSLPs and
GIMPS for handling inbound messages for which no routing state
exists, to allow the NSLP to indicate whether state setup should
proceed and to provide NSLP payloads for the Response or
forwarded message (Section 3.6, Section 4.3.2 and Appendix B).
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7. Included new text, Section 5.6, on the processing and
encapsulation of error messages. Also added formats and an error
message catalogue in Appendix A.4, including a modified format
for the overall GIMPS-Error message and the GIMPS-Error-Data
object.
8. Removed the old section 5.3.3 on NSLPID/RAO setting on the
assumption that this will be covered in the extensibility
document.
9. Included a number of other minor corrections and clarifications.
D.4. Changes In Version -06
Version -06 does not introduce any major structural changes to the
protocol definition, although it does clarify a number of details and
resolve some outstanding open issues. The primary changes are as
follows:
1. Added a new high level Section 3.3 which gathers together the
various aspects of the message routing method concept.
2. Added a new high level Section 3.4 which explains the concept
and significance of the session identifier. Also clarified that
the routing state always depends on the session identifier.
3. Added notes about the level of address validation performed by
GIMPS in Section 4.1.2 and extensions to the API in Appendix B.
4. Split the old Node-Addressing object into a Network-Layer-
Information object and Stack-Configuration-Data object. The
former refers to basic information about a node, and the latter
carries information about messaging association configuration.
Redefined the content of the various handshake messages
accordingly in Section 4.4.1 and Section 5.1.
5. Re-wrote Section 4.4.3 to clarify the rules on refresh and purge
of routing state and messaging associations. Also, moved the
routing state lifetime into the Network-Layer-Information object
and added a messaging association lifetime to the Stack-
Configuration-Data object (Section 5.2).
6. Added specific message types for errors and MA-Refresh in
Section 5.1. The error object is now GIMPS-specific
(Appendix A.4.1).
7. Moved the Flow-Identifier information about the message routing
method from the general description of the object to the path-
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coupled MRM section (Section 5.8.1.1), and made a number of
clarifications to the bit format (Appendix A.3.1.1).
8. Removed text about assumptions on the version numbering of
NSLPs, and restricted the scope of the description of TLV object
formats and extensibility flags to GIMPS rather than the whole
of NSIS (Appendix A).
9. Added a new Section 5.5 explaining the possible relationships
between message types and encapsulation formats.
10. Added a new Section 6 in outline form, to capture the formal
specification of the protocol operation.
11. Added new security sections on cookie requirements (Section 8.5)
and residual threats (Section 8.7).
D.5. Changes In Version -05
Version -05 reformulates the specification, to describe routing state
maintenance in terms of exchanging explicitly identified Query/
Response/Confirm messages, leaving the upstream/downstream
distinction as a specific detail of how Query messages are
encapsulated. This necessitated widespread changes in the
specification text, especially Section 4.2.1, Section 4.4,
Section 5.1 and Section 5.3 (although the actual message sequences
are unchanged). A number of other issues, especially in the area of
message encapsulation, have also been closed. The main changes are
the following:
1. Added a reference to an individual draft on the Loose End MRM as
a concrete example of an alternative message routing method.
2. Added further text (particularly in Section 2) on what GIMPS
means by the concept of 'session'.
3. Firmed up the selection of UDP as the encapsulation choice for
datagram mode, removing the open issue on this topic.
4. Defined the interaction between GIMPS and signaling applications
for communicating about the cryptographic security properties of
how a message will be sent or has been received (see
Section 4.1.2 and Appendix B).
5. Closed the issue on whether Query messages should use the
signaling or flow source address in the IP header; both options
are allowed by local policy and a flag in the common header
indicates which was used. (See Section 5.8.1.2.)
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6. Added the necessary information elements to allow the IP hop
count between adjacent GIMPS peers to be measures and reported.
(See Section 5.2.2 and Appendix A.3.3.)
7. The old open-issue text on selection of IP router alert option
values has been moved into the main specification to capture the
technical considerations that should be used in assigning such
values (in old section 5.3.3).
8. Resolved the open issue on lost Confirm messages by allowing a
choice of timer-based retransmission of the Response, or an
error message from the responding node which causes the
retransmission of the Confirm (see Section 5.3.3).
9. Closed the open issue on support for message scoping (this is
now assumed to be a NSLP function).
10. Moved the authoritative text for most of the remaining open
issues to an online issue tracker.
D.6. Changes In Version -04
Version -04 includes mainly clarifications of detail and extensions
in particular technical areas, in part to support ongoing
implementation work. The main details are as follows:
1. Substantially updated Section 4, in particular clarifying the
rules on what messages are sent when and with what payloads
during routing and messaging association setup, and also adding
some further text on message transfer attributes.
2. The description of messaging association protocol setup
including the related object formats has been centralised in a
new Section 5.7, removing the old Section 6.6 and also closing
old open issues 8.5 and 8.6.
3. Made a number of detailed changes in the message format
definitions (Appendix A), as well as incorporating initial rules
for encoding message extensibility information. Also included
explicit formats for a general purpose Error object, and the
objects used to discover supported messaging association
protocols. Updated the corresponding open issues section (old
section 9.3) with a new item on NSLP versioning.
4. Updated the GIMPS API (Appendix B), including more precision on
message transfer attributes, making the NSLP hint about storing
reverse path state a return value rather than a separate
primitive, and adding a new primitive to allow signaling
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applications to invalidate GIMPS routing state. Also, added a
new parameter to SendMessage to allow signaling applications to
'bypass' a message statelessly, preserving the source of an
input message.
5. Added an outline for the future content of an IANA
considerations section (Section 9). Currently, this is
restricted to identifying the registries and allocations
required, without defining the allocation policies and other
considerations involved.
6. Shortened the background design discussion in Section 3.
7. Made some clarifications in the terminology section relating to
how the use of C-mode does and does not mandate the use of
transport or security protection.
8. The ABNF for message formats in Section 5.1 has been re-written
with a grammar structured around message purpose rather than
message direction, and additional explanation added to the
information element descriptions in Section 5.2.
9. The description of the datagram mode transport in Section 5.3
has been updated. The encapsulation rules (covering IP
addressing and UDP port allocation) have been corrected, and a
new subsection on message retransmission and rate limiting has
been added, superseding the old open issue on the same subject
(section 8.10).
10. A new open issue on IP TTL measurement to detect non-GIMPS
capable hops has been added (old section 9.5).
D.7. Changes In Version -03
Version -03 includes a number of minor clarifications and extensions
compared to version -02, including more details of the GIMPS API and
messaging association setup and the node addressing object. The full
list of changes is as follows:
1. Added a new section pinning down more formally the interaction
between GIMPS and signaling applications (Section 4.1), in
particular the message transfer attributes that signaling
applications can use to control GIMPS (Section 4.1.2).
2. Added a new open issue identifying where the interaction between
the security properties of GIMPS and the security requirements of
signaling applications should be identified (old section 9.10).
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3. Added some more text in Section 4.2.1 to clarify that GIMPS has
the (sole) responsibility for generating the messages that
refresh message routing state.
4. Added more clarifying text and table to GHC and IP TTL handling
discussion of Section 4.3.4.
5. Split Section 4.4 into subsections for different scenarios, and
added more detail on Node-Addressing object content and use to
handle the case where association re-use is possible in
Section 4.4.2.
6. Added strawman object formats for Node-Addressing and Stack-
Proposal objects in Section 5.1 and Appendix A.
7. Added more detail on the bundling possibilities and appropriate
configurations for various transport protocols in Section 5.4.1.
8. Included some more details on NAT traversal in Section 7.2,
including a new object to carry the untranslated address-bearing
payloads, the NAT-Traversal object.
9. Expanded the open issue discussion in old section 9.3 to include
an outline set of extensibility flags.
D.8. Changes In Version -02
Version -02 does not represent any radical change in design or
structure from version -01; the emphasis has been on adding details
in some specific areas and incorporation of comments, including early
review comments. The full list of changes is as follows:
1. Added a new Section 1.1 which summarises restrictions on scope
and applicability; some corresponding changes in terminology in
Section 2.
2. Closed the open issue on including explicit GIMPS state teardown
functionality. On balance, it seems that the difficulty of
specifying this correctly (especially taking account of the
security issues in all scenarios) is not matched by the saving
of state enabled.
3. Removed the option of a special class of message transfer for
reliable delivery of a single message. This can be implemented
(inefficiently) as a degenerate case of C-mode if required.
4. Extended Appendix A with a general discussion of rules for
message and object formats across GIMPS and other NSLPs. Some
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remaining open issues are noted in old section 9.3 (since
removed).
5. Updated the discussion of RAO/NSLPID relationships to take into
account the proposed message formats and rules for allocation of
NSLP id, and propose considerations for allocation of RAO
values.
6. Modified the description of the information used to route
messages (first given in Section 4.2.1 but also throughout the
document). Previously this was related directly to the flow
identification and described as the Flow-Routing-Information.
Now, this has been renamed Message-Routing-Information, and
identifies a message routing method and any associated
addressing.
7. Modified the text in Section 4.3 and elsewhere to impose sanity
checks on the Message-Routing-Information carried in C-mode
messages, including the case where these messages are part of a
GIMPS-Query/Response exchange.
8. Added rules for message forwarding to prevent message looping in
a new Section 4.3.4, including rules on IP TTL and GIMPS hop
count processing. These take into account the new RAO
considerations described above.
9. Added an outline mechanism for messaging association protocol
stack setup, with the details in a new Section 6.6 and other
changes in Section 4.4 and the various sections on message
formats.
10. Removed the open issue on whether storing reverse routing state
is mandatory or optional. This is now explicit in the API
(under the control of the local NSLP).
11. Added an informative annex describing an abstract API between
GIMPS and NSLPs in Appendix B.
D.9. Changes In Version -01
The major change in version -01 is the elimination of
'intermediaries', i.e. imposing the constraint that signaling
application peers are also GIMPS peers. This has the consequence
that if a signaling application wishes to use two classes of
signaling transport for a given flow, maybe reaching different
subsets of nodes, it must do so by running different signaling
sessions; and it also means that signaling adaptations for passing
through NATs which are not signaling application aware must be
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carried out in datagram mode. On the other hand, it allows the
elimination of significant complexity in the connection mode handling
and also various other protocol features (such as general route
recording).
The full set of changes is as follows:
1. Added a worked example in Section 3.6.
2. Stated that nodes which do not implement the signaling
application should bypass the message (Section 4.3).
3. Decoupled the state handling logic for routing state and
messaging association state in Section 4.4. Also, allow
messaging associations to be used immediately in both directions
once they are opened.
4. Added simple ABNF for the various GIMPS message types in a new
Section 5.1, and more details of the common header and each
object in Section 5.2, including bit formats in Appendix A. The
common header format means that the encapsulation is now the
same for all transport types (Section 5.4.1).
5. Added some further details on datagram mode encapsulation in
Section 5.3, including more explanation of why a well known port
is needed.
6. Removed the possibility for fragmentation over DCCP
(Section 5.4.1), mainly in the interests of simplicity and loss
amplification.
7. Removed all the tunnel mode encapsulations (old sections 5.3.3
and 5.3.4).
8. Fully re-wrote the route change handling description
(Section 7.1), including some additional detection mechanisms
and more clearly distinguishing between upstream and downstream
route changes. Included further details on GIMPS/NSLP
interactions, including where notifications are delivered and
how local repair storms could be avoided. Removed old
discussion of propagating notifications through signaling
application unaware nodes (since these are now bypassed
automatically). Added discussion on how to route messages for
local state removal on the old path.
9. Revised discussion of policy-based forwarding (old Section 7.2)
to account for actual Flow-Routing-Information definition, and
also how wildcarding should be allowed and handled.
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10. Removed old route recording section (old Section 6.3).
11. Extended the discussion of NAT handling (Section 7.2) with an
extended outline on processing rules at a GIMPS-aware NAT and a
pointer to implications for C-mode processing and state
management.
12. Clarified the definition of 'correct routing' of signaling
messages in Section 8 and GIMPS role in enforcing this. Also,
opened the possibility that peer node authentication could be
signaling application dependent.
13. Removed old open issues on Connection Mode Encapsulation
(section 8.7); added new open issues on Message Routing (old
Section 9.3 of version -05, later moved to Section 3.3) and
Datagram Mode congestion control.
14. Added this change history.
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Authors' Addresses
Henning Schulzrinne
Columbia University
Department of Computer Science
450 Computer Science Building
New York, NY 10027
US
Phone: +1 212 939 7042
Email: hgs+nsis@cs.columbia.edu
URI: http://www.cs.columbia.edu
Robert Hancock
Siemens/Roke Manor Research
Old Salisbury Lane
Romsey, Hampshire SO51 0ZN
UK
Email: robert.hancock@roke.co.uk
URI: http://www.roke.co.uk
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