Next Steps in Signaling H. Schulzrinne
Internet-Draft Columbia U.
Expires: August 16, 2004 R. Hancock
Siemens/RMR
February 16, 2004
GIMPS: General Internet Messaging Protocol for Signaling
draft-ietf-nsis-ntlp-01
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Copyright Notice
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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
Messaging Protocol for Signaling (GIMPS), which provides a universal
service for diverse signaling applications. GIMPS does not handle
signaling application state itself, but manages its own internal
state and the configuration of the underlying transport and security
protocols to enable the transfer of messages in both directions along
the flow path. The combination of GIMPS and the lower layer protocols
provides a solution for the base protocol component of the "Next
Steps in Signaling" framework.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Requirements Notation and Terminology . . . . . . . . . . . 5
3. Design Methodology . . . . . . . . . . . . . . . . . . . . . 7
3.1 Overall Approach . . . . . . . . . . . . . . . . . . . . . . 7
3.2 Design Attributes . . . . . . . . . . . . . . . . . . . . . 9
3.3 Example of Operation . . . . . . . . . . . . . . . . . . . . 11
4. GIMPS Processing Overview . . . . . . . . . . . . . . . . . 13
4.1 GIMPS State . . . . . . . . . . . . . . . . . . . . . . . . 13
4.2 Basic Message Processing . . . . . . . . . . . . . . . . . . 14
4.3 Routing State and Messaging Association Maintainance . . . . 18
5. Message Formats and Encapsulations . . . . . . . . . . . . . 21
5.1 GIMPS Messages . . . . . . . . . . . . . . . . . . . . . . . 21
5.2 Information Elements . . . . . . . . . . . . . . . . . . . . 22
5.3 Encapsulation in Datagram Mode . . . . . . . . . . . . . . . 24
5.4 Encapsulation in Connection Mode . . . . . . . . . . . . . . 25
6. Advanced Protocol Features . . . . . . . . . . . . . . . . . 27
6.1 Route Changes and Local Repair . . . . . . . . . . . . . . . 27
6.2 Policy-Based Forwarding and Flow Wildcarding . . . . . . . . 33
6.3 NAT Traversal . . . . . . . . . . . . . . . . . . . . . . . 33
6.4 Interaction with IP Tunnelling . . . . . . . . . . . . . . . 35
6.5 IPv4-IPv6 Transition and Interworking . . . . . . . . . . . 35
7. Security Considerations . . . . . . . . . . . . . . . . . . 38
7.1 Message Confidentiality and Integrity . . . . . . . . . . . 38
7.2 Peer Node Authentication . . . . . . . . . . . . . . . . . . 39
7.3 Routing State Integrity . . . . . . . . . . . . . . . . . . 39
7.4 Denial of Service Prevention . . . . . . . . . . . . . . . . 41
8. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . 43
8.1 Protocol Naming . . . . . . . . . . . . . . . . . . . . . . 43
8.2 General IP Layer Issues . . . . . . . . . . . . . . . . . . 43
8.3 Encapsulation and Addressing for Datagram Mode . . . . . . . 43
8.4 Intermediate Node Bypass and Router Alert Values . . . . . . 45
8.5 Messaging Association Flexibility . . . . . . . . . . . . . 46
8.6 Messaging Association Setup Message Sequences . . . . . . . 47
8.7 GIMPS State Teardown . . . . . . . . . . . . . . . . . . . . 48
8.8 Datagram Mode Retries and Single Shot Message Support . . . 48
8.9 GIMPS Support for Message Scoping . . . . . . . . . . . . . 49
8.10 Mandatory or Optional Reverse Routing State . . . . . . . . 49
8.11 Additional Discovery Mechanisms . . . . . . . . . . . . . . 49
8.12 Alternative Message Routing Requirements . . . . . . . . . . 50
8.13 Congestion Control in Datagram Mode . . . . . . . . . . . . 50
8.14 Protocol Design Details . . . . . . . . . . . . . . . . . . 51
9. Change History . . . . . . . . . . . . . . . . . . . . . . . 52
9.1 Changes In Version -01 . . . . . . . . . . . . . . . . . . . 52
Normative References . . . . . . . . . . . . . . . . . . . . 54
Informative References . . . . . . . . . . . . . . . . . . . 55
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 56
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A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 57
B. Example Routing State Table . . . . . . . . . . . . . . . . 58
C. Information Element Formats . . . . . . . . . . . . . . . . 59
C.1 The Common Header . . . . . . . . . . . . . . . . . . . . . 59
C.2 TLV Objects . . . . . . . . . . . . . . . . . . . . . . . . 59
Intellectual Property and Copyright Statements . . . . . . . 63
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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 specifically on the case of
"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 router 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 support are common to all or a large number
of applications, and hence should be developed as a common protocol.
The framework given in [20] provides a rationale for a function split
between the common and application specific protocols, and gives
outline requirements for the former, the 'NSIS Transport Layer
Protocol' (NTLP).
This specification provides a concrete solution for the NTLP. It is
based on the use of existing transport and security protocols under a
common messaging layer, the General Internet Messaging Protocol for
Signaling (GIMPS). Different signaling applications may make use of
different services provided by GIMPS, but GIMPS does not handle
signaling application state itself; in that crucial respect, it
differs from application signaling protocols such as the control
component of FTP, SIP and RTSP. Instead, GIMPS manages its own
internal state and the configuration of the underlying transport and
security protocols to ensure the transfer of signaling messages on
behalf of signaling applications in both directions along the flow
path.
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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 GIMPS level are shown in
Figure 1.
GIMPS (adjacent) peer nodes
IP addresses = Signaling
IP address Source/Destination Addresses IP address
= Flow (depending on signaling direction) = Flow
Source | | Destination
Address | | Address
V V
+--------+ Data +------+ +------+ +--------+
| Flow |-----------|------|-------------|------|-------->| Flow |
| Sender | Flow | | | | |Receiver|
+--------+ |GIMPS |============>|GIMPS | +--------+
| Node |<============| Node |
+------+ Signaling +------+
GN1 Flow GN2
>>>>>>>>>>>>>>>>> = Downstream direction
<<<<<<<<<<<<<<<<< = Upstream direction
Figure 1: Basic Terminology
[Data] Flow: A set of packets following a unique path through the
network, identified by some fixed combination of header fields.
Only unicast, unidirectional flows are considered.
Session: A single application layer flow of information for which
some network control state information is to be manipulated or
monitored. IP mobility may cause the mapping between sessions and
flows to change, and IP multihoming may mean there is more than
one flow for a given session.
[Flow] Sender: The node in the network which is the source of the
packets in a flow. Could be a host or a router (if the flow is
actually an aggregate).
[Flow] Receiver: The node in the network which is the sink for the
packets in a flow.
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Downstream: In the same direction as the data flow.
Upstream: In the opposite direction to the data flow.
GIMPS Node: Any node along the data path supporting GIMPS (regardless
of what signaling applications it supports).
Adjacent peer: The next GIMPS node along the data path, in the
upstream or downstream direction. Whether two nodes are adjacent
is determined implicitly by the GIMPS peer discovery mechanisms;
it is possible for adjacencies to 'skip over' intermediate nodes
if they have no interest in the signaling messages being
exchanged.
Datagram mode: A mode of sending GIMPS messages between nodes without
using any transport layer state or security protection. Upstream
messages are sent UDP encapsulated directly to the signaling
destination; downstream messages are sent towards the flow
receiver with a router alert option.
Connection mode: A mode of sending GIMPS messages directly between
nodes using point to point "messaging associations" (see below),
i.e. transport protocols and security associations.
Messaging association: A single connection between two explicitly
identified GIMPS adjacent peers, i.e. between a given signaling
source and destination address. A messaging association uses a
specific transport protocol and known ports, and may be run over
specific network layer security associations, or use a transport
layer security association internally. A messaging association is
bidirectional.
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3. Design Methodology
3.1 Overall Approach
The generic requirements identified in [20] for transport of
path-coupled signaling messages are essentially two-fold:
"Routing": Determine how to reach the adjacent signaling node along
the data path (the GIMPS peer);
"Transport": Deliver the signaling information to that peer.
To meet the routing requirement, for downstream signaling the node
can either use local state information (e.g. gathered during previous
signaling exchanges) to determine the identity of the GIMPS peer
explicitly, or it can just send the signaling towards the flow
destination address and rely on the peer to intercept it. For
upstream signaling, only the first technique is possible.
Once the routing decision has been made, the node has to select a
mechanism for transport of the message to the peer. GIMPS divides the
transport problems into two categories, the easy and the difficult
ones. It handles the easy cases within GIMPS itself, avoiding
complexity and latency, while drawing on the services of
well-understood reliable transport protocols for the harder cases.
Here, with details discussed later, "easy" messages are those that
are sized well below the lowest MTU along a path, are infrequent
enough not to cause concerns about congestion and flow control, and
do not need transport or network-layer security protection.
However, in many cases, signaling information needs to be delivered
between GIMPS peers with additional transport or security properties.
For example, some applications could implement their own reliability
mechanism, but experience with RSVP has shown [13] that relying
solely on soft-state refreshes itself may yield unsatisfactory
performance if signaling messages are lost even occasionally. The
provision of this type of reliability would therefore also be the
responsibility of the underlying transport protocols.
In [20] all of these routing and transport requirements are assigned
to a single notional protocol, the 'NSIS Transport Layer Protocol'
(NTLP). The strategy of splitting the transport problem leads to a
layered structure for the NTLP, as a specialised GIMPS 'messaging'
layer running over standard transport and security protocols, as
shown in Figure 2.
For efficiency, GIMPS offers two modes of transport operation:
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Datagram mode: for small, infrequent messages with modest delay
constraints;
Connection mode: for larger data objects or where fast setup in the
face of packet loss is desirable, or where channel security is
required.
^^ +-------------+
|| | Signaling |
|| +------------|Application 2|
|| | Signaling +-------------+
NSIS |Application 1| |
Signaling +-------------+ |
Application | +-------------+ |
Level | | Signaling | |
|| | |Application 3| |
|| | +-------------+ |
VV | | |
=======================|==========|========|=======================
^^ +------------------------------------------------+
|| |+-----------------------+ +--------------+ |
|| || GIMPS | | GIMPS | |
|| || Encapsulation | |Internal State| |
|| || |<<<>>>| Maintenance | |
|| |+-----------------------+ +--------------+ |
|| |GIMPS: Messaging Layer |
|| +------------------------------------------------+
NSIS | | | |
Transport .............................
Level . Transport Layer Security .
("NTLP") .............................
|| | | | |
|| +----+ +----+ +----+ +----+
|| |UDP | |TCP | |SCTP| |DCCP|....
|| +----+ +----+ +----+ +----+
|| | | | |
|| .............................
|| . IP Layer Security .
|| .............................
VV | | | |
=========================|=======|=======|=======|===============
| | | |
+----------------------------------------------+
| IP |
+----------------------------------------------+
Figure 2: Protocol Stacks for Signaling Transport
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The datagram mode uses a lower-layer unreliable unsecured datagram
transport mechanism, with UDP as the initial choice. The connection
mode can use any stream or message-oriented transport protocol. It
may employ specific network layer security associations (e.g. IPsec),
or an internal transport layer security association (e.g. TLS).
It is possible to mix these two modes along a chain of nodes, without
coordination or manual configuration. This allows, for example, the
use of datagram mode at the edges of the network and connection mode
in the core of the network. Such combinations may make operation
more efficient for mobile endpoints, while allowing multiplexing of
signaling messages across shared security associations and transport
connections between core routers.
It must be understood that the routing and transport decisions made
by GIMPS are not totally independent. If the message transfer has
requirements that enforce the use of connection mode (e.g. because
fragmentation is required), this can only be used between explicitly
identified nodes. In such cases, the GIMPS node must carry out
signaling in datagram mode to identify the peer and then set up the
necessary transport connection. The datagram mode option of sending
the message in the direction of the flow receiver and relying on
interception is not available, even for downstream signaling.
In general, the state associated with connection mode messaging to a
particular peer (signaling destination address, protocol and port
numbers, internal protocol configuration and state information) is
referred to as a "messaging association". There may be any number of
messaging associations between two GIMPS peers (although the usual
case is 0 or 1), and they are set up and torn down by managment
actions within GIMPS itself.
3.2 Design Attributes
Soft state: All parts of GIMPS state are subject to time-out
("soft-state"). Explicit removal is also logically possible (see
Section 8.7). 'State' here includes the messaging associations
managed by GIMPS.
Application-neutral: GIMPS is designed to support the largest range
of signaling applications. While a number of such applications
have been identified, it appears likely that new ones will emerge.
Mobility support: End systems can change their network attachment
point and network address during a session. GIMPS minimises the
use of IP addresses as identifiers for non-topological information
(e.g. authentication state).
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Efficient: Signaling often occurs before an application such as an IP
telephone conversation can commence, so that any signaling delay
becomes noticeable to the application. Signaling delays are
incurred by the delay in finding signaling nodes along the path
(peer discovery), in retransmitting lost signaling messages and in
setting up security associations between nodes, among other
factors. GIMPS attempts to minimise these delays by several
mechanisms, such as the use of high performance transport
protocols to circumvent message loss, and the re-use of messaging
associations to avoid setup latency. If discovery is needed it is
a lightweight process which only probes local topology, and GIMPS
also allows it to be bypassed completely for downstream datagram
mode messages.
IP version neutral: GIMPS supports both IPv4 and IPv6: it can use
either for transport, largely as a result of their support in the
underlying transport protocols, and can signal for either type of
flow. In addition, GIMPS is able to operate on dual-stack nodes
(to bridge between v4 and v6 regions) and also to operate across
v4/v6 boundaries and other addressing boundaries. Specific
transition issues are considered in Section 6.5.
Transport neutral: GIMPS can operate over any message or
stream-oriented transport layer, including UDP, DCCP, TCP and
SCTP. Messages sent over protocols that do not offer a native
fragmentation service, such as UDP or DCCP, are strictly limited
in size to avoid loss-amplification; in the case of UDP, they must
also be limited in rate to avoid network congestion.
Proxy support: The end systems in a session may not be capable of
handling either the signaling transport or the application and may
instead rely on proxies to initiate and terminate signaling
sessions. GIMPS decouples the operation of the messaging functions
from the flow source and destination addresses, treating these
primarily as data.
Scaleable: As will be discussed in Section 4.3, up to one messaging
association is generally kept for each adjacent GIMPS peer and
thus association state scales better than the number of sessions.
(Many peers may not have association state at all, if there are no
messages on sessions visiting those nodes that warrant such
treatment.) Messaging associations are managed based on policy at
each node, depending on trade-offs between fast peer-to-peer
communication and state overhead. Messaging association state can
be removed immediately after the last signaling session to a
particular next-hop is removed, after some delay to wait for new
sessions or only if resource demands warrant it.
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3.3 Example of Operation
This section presents an example of GIMPS usage in a relatively
simple signaling scenario, to illustrate its main features.
Consider the case of an RSVP-like signaling application which
allocates resources for a flow from sender to receiver; we will
consider how GIMPS 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 has been received 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 GIMPS layer in GN1, along
with a definition of the flow and description of the transfer
requirements {downstream, unsecured, unreliable}. GIMPS
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
(e.g. from local policy for that signaling application
identifier) that this application is likely to require secured
upstream and downstream transport of large messages in the
future.
3. GN1 therefore constructs a UDP datagram with the signaling
application payload, and additional payloads at the GIMPS level
to be used to initiate the possible setup of a messaging
association. This datagram is injected into the network,
addressed towards the flow destination and with a Router Alert
Option included.
4. This D-mode message passes through the network towards the flow
receiver, and is seen by each router in turn. GIMPS-unaware
routers will not recognise the RAO value and will forward the
message unchanged; GIMPS-aware routers which do not support the
signaling application in question will also forward the message
unchanged, although they may need to process more of the message
to decide this.
5. The message is finally intercepted at GN2. The GIMPS layer
identifies that the message is relevant to a local signaling
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application, and passes the signaling application payload and
flow description to upwards to it. From there, the signaling
application in GN2 can continue to process this message as in GN1
(compare step 1), and this will eventually result in the message
reaching the flow receiver.
6. In parallel, GIMPS internally recognises that GN1 is attempting
to discover GN2 in order to set up a messaging association for
future signaling for the flow. There are two possible cases:
A. GN1 and GN2 already have an appropriate association. GN2
simply records the identity of GN1 as its upstream peer for
that flow and signaling application, and sends a GIMPS
message back to GN1 over the association identifying itself.
B. No messaging association exists. Again, GN2 records the
identity of GN1 as before, but sends an upstream D-mode
message to GN1, identifying itself and agreeing to the
association setup. The protocol exchanges needed to complete
this will proceed in the background, controlled by GN1.
7. Eventually, another signaling application 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 GIMPS level, along
with the flow definition and transfer requirements {upstream,
secured, reliable}.
8. The GIMPS layer in GN2 identifies the upstream peer for this flow
and signaling application 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 negotiations begun in step 6.B have not
yet completed).
Further messages can be passed in each direction in the same way. The
GIMPS layer in each node can in parallel carry out maintenance
operations such as route change detection (this can be done by
sending additional GIMPS-only datagram mode messages, see Section 6.1
for more details).
It should be understood that many of these details of GIMPS
operations can be varied, either by local policy or according to
signaling application requirements, and they are also subject to
development and refinement as the protocol design proceeds. The
authoritative details are contained in the remainder of this
document.
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4. GIMPS Processing Overview
This section defines the basic structure and operation of GIMPS. It
is divided into three parts. Section 4.1 gives an overview of the
per-flow and per-peer state that GIMPS maintains for the purpose of
transferring messages. Section 4.2 describes how messages are
processed in the case where any necessary messaging associations and
associated routing state already exist; this includes the simple
scenario of pure datagram mode operation, where no messaging
associations are necessary in the first place (equivalent to the
transport functionality of base RSVP as defined in [9]). Section 4.3
describes how routing state is maintained and how messaging
associations are initiated and terminated.
4.1 GIMPS State
For each flow, the GIMPS layer can maintain message routing state to
manage the processing of outgoing messages. This state is
conceptually organised into a table with the following structure.
The primary key (index) for the table is the combination of {flow
routing info, session id, signaling application id}:
Flow: the header N-tuple that determines the route taken by the flow
through the network; in particular, including the flow destination
address.
Session: a cryptographically random and (probabilistically) globally
unique identifier of the application layer session that is using
the flow. For a given flow, different signaling applications may
or may not use the same session identifier. Often there will only
be one flow for a given session, but in mobility/multihoming
scenarios there may be more than one and they may be differently
routed.
Signaling application: an IANA assigned identifier of the signaling
application which is generating messages for this flow. The
inclusion of this identifier allows the routing state to be
different for different signaling applications (e.g. because of
different adjacencies).
The state information for a given key is as follows:
Upstream peer: the adjacent GIMPS peer closer to the flow source.
This could be an IP address (learned from previous signaling) or a
pointer to a messaging association. It could also be null, if this
node is the flow sender or this node is not storing reverse
routing state, or a special value to indicate that this node is
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the last upstream node (but not the sender).
Downstream peer: the adjacent GIMPS peer closer to the flow
destination. This could be a pointer to a messaging association,
or it could be null, if this node is the flow receiver or this
node is only sending downstream datagram mode messages for this
flow and signaling application, or a special value to indicate
that this node is the last downstream node (but not the receiver).
Note that both the upstream and downstream peer state may be null,
and that the session identifier information is not actually required
for message processing; in that case, no state information at all
needs to be stored in the table. Both items of peer identification
state have associated timers for how long the identification can be
considered accurate; when these timers expire, the peer
identification (IP address or messaging association pointer) is
purged if it has not been refreshed. An example of a routing state
table for a simple scenario is given in Appendix B.
Note also that the information is described as a table of flows, but
that there is no implied constraint on how the information is stored.
For example, in a network using pure destination address routing
(without load sharing or any form of policy-based forwarding), the
downstream peer information might be possible to store in an
aggregated form in the same manner as the IP forwarding table. In
addition, many of the per-flow entries may point to the same per-peer
state (e.g. the same messaging association) if the flows go through
the same adjacent peer.
The per-flow message routing state is not the only state stored by
GIMPS. There is also the state implied in the messaging associations.
Since we assume that these associations are typically per-peer rather
than per-flow, they are stored in a separate table. As well as the
messaging association state itself, this table also stores
per-association information including:
o messages pending transmission while an association is being
established;
o an inactivity timer for how long the association has been idle.
4.2 Basic Message Processing
This section describes how signaling application messages are
processed in the simple case where any necessary messaging
associations and routing state are already in place. The description
is divided into several parts: message reception, local processing
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and delivery to signaling applications, and message transmission. An
overview is given in Figure 3.
+---------------------------------------------------------+
| >> Signaling Application Processing >> |
| |
+--------^---------------------------------------V--------+
^ V
^ NSLP Payloads V
^ V
+--------^---------------------------------------V--------+
| >> GIMPS >> |
| ^ ^ ^ Processing V V V |
+--x-----------u--d---------------------d--u-----------x--+
x u d d u x
x u d>>>>>>>>>>>>>>>>>>>>>d u x
x u d Bypass at d u x
+--x-----+ +--u--d--+ GIMPS level +--d--u--+ +-----x--+
| C-mode | | D-mode | | D-mode | | C-mode |
|Handling| |Handling| |Handling| |Handling|
+--x-----+ +--u--d--+ +--d--u--+ +-----x--+
x u d d u x
x uuuuuu d>>>>>>>>>>>>>>>>>>>>>d uuuuuu x
x u d Bypass at d u x
+--x--u--+ +-----d--+ router +--d-----+ +--u--x--+
|IP Host | | RAO | alert level | RAO | |IP Host |
|Handling| |Handling| |Handling| |Handling|
+--x--u--+ +-----d--+ +--d-----+ +--u--x--+
x u d d u x
+--x--u-----------d--+ +--d-----------u--x--+
| IP Layer | | IP Layer |
| (Receive Side) | | (Transmit Side) |
+--x--u-----------d--+ +--d-----------u--x--+
x u d d u x
x u d d u x
x u d d u x
uuuuuuuuuuuuuu = upstream datagram mode messages
dddddddddddddd = downstream datagram mode messages
xxxxxxxxxxxxxx = connection mode messages
RAO = Router Alert Option
Figure 3: Message Paths through a GIMPS Node
Note that the same messages are used for maintaining internal GIMPS
state and carrying signaling application payloads. The state
maintenance takes place as a result of processing specific GIMPS
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payloads in these messages. The processing of these payloads is the
subject of Section 4.3.
Message Reception: Messages can be received in connection or datagram
mode, and from upstream or downstream peers.
Reception in connection mode is simple: incoming packets undergo
the security and transport treatment associated with the messaging
association, and the messaging association provides complete
messages to the GIMPS layer for further processing.
Reception in datagram mode depends on the message direction.
Upstream messages (from a downstream peer) will arrive UDP
encapsulated and addressed directly to the receiving signaling
node. Each datagram contains a single complete message which is
passed to the GIMPS layer for further processing, just as in the
connection mode case.
Downstream datagram mode messages are UDP encapsulated and
addressed to the flow receiver with an IP router alert option to
cause interception. The signaling node will therefore 'see' all
such messages, including those in which it is not interested.
There are then two cases. If the node categorises the message as
'not interesting' (which includes the case that the node does not
implement the signaling application which the message is about),
it is passed on to the IP layer for message transmission without
further processing (further criteria and mechanisms for
categorisation are discussed in Section 8.4). If the message is
determined to be interesting, it is passed up to the GIMPS layer
for further processing as in the other cases.
Local Processing: Once a message has been received, by any method, it
is processed locally within the GIMPS layer. The GIMPS processing
to be done depends on the payloads carried; most of the
GIMPS-internal payloads are associated with state maintenance and
are covered in the next subsection.
One GIMPS-internal payload which can be carried in any message and
requires processing is the GIMPS hop count. This is decremented on
input processing, and checked to be greater than zero on output
processing. The primary purpose of the GIMPS hop count is to
prevent message looping.
The remainder of the GIMPS message consists of an NSLP payload.
This is delivered locally to the signaling application identified
at the GIMPS level; the format of the NSLP payload is not
constrained by GIMPS, and the content is not interpreted.
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Signaling applications can generate their messages for
transmission, either asynchronously, or in response to an input
message. Messages may also be forwarded in the GIMPS layer if
there is no appropriate local signaling application to process
them. Regardless of the source, outgoing messages are passed
downwards for message transmission.
Message Transmission: When a message is available for transmission,
GIMPS uses internal policy and the stored routing state to
determine how to handle it. The following processing applies
equally to locally generated messages and messages forwarded from
within the GIMPS or signaling application levels.
The main decision is whether the message must be sent in
connection mode or datagram mode. Reasons for using the former
could be:
* NSLP requirements: for example, the signaling application has
requested channel secured delivery, or reliable delivery;
* protocol specification: for example, this document could
specify that a message that requires fragmentation MUST be sent
over a messaging association;
* local GIMPS policy: for example, a node may prefer to send
messages over a messaging association to benefit from
congestion control.
In principle, as well as determining that some messaging
association must be used, GIMPS could select between a set of
alternatives, e.g. for load sharing or because different messaging
associations provide different transport or security attributes
(see Section 8.5 for further discussion).
If the use of a messaging association is selected, the message is
queued on the association (found from the upstream or downstream
peer state table), and further output processing is carried out
according to the details of the protocol stack used for the
association. If no appropriate association exists, the message is
queued while one is created (see next subsection). If no
association can be created, this is again an error condition.
If a messaging association is not required, the message is sent in
datagram mode. The processing in this case depends on whether the
message is directed upstream or downstream.
* If the upstream peer IP address is available from the per-flow
routing table, the message is UDP encapsulated and sent
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directly to that address. Otherwise, the message cannot be
forwarded (i.e. this is an error condition).
* In the downstream direction, messages can always be sent. They
are simply UDP encapsulated and IP addressed to the flow
receiver, with the appropriate router alert option.
4.3 Routing State and Messaging Association Maintainance
The main responsibility of the GIMPS layer is to manage the routing
state and messaging associations which are used in the basic message
processing described above. Routing state is installed and maintained
by datagram mode messages containing specific GIMPS payloads.
Messaging associations are dependent on the existence of routing
state, but are actually set up by the normal procedures of the
transport and security protocols that comprise the messaging
association. Timers control routing state and messaging association
refresh and expiration.
The complete sequence of possible messages for state setup between
adjacent peers is shown in Figure 4 and described in detail in the
following text.
The initial message in any routing state maintenance operation is a
downstream datagram mode message, sent from the querying node and
intercepted at the responding node. This is encapsulated and
addressed just as in the normal case; in particular, it has
addressing and other identifiers appropriate for the flow and
signaling application that state maintenance is being done for, and
it is allowed to contain an NSLP payload. Processing at the querying
and responding nodes is also essentially the same. However, the
querying node includes additional payloads: its own address
information, and optionally 'Discover-Query' information, including a
Response Request flag and a Query Cookie. This message is informally
referred to as a 'GIMPS-query'. The role of the cookies in this and
subsequent messages is to protect against certain denial of service
attacks.
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+----------+ +----------+
| Querying | |Responding|
| Node | | Node |
+----------+ +----------+
GIMPS-query
---------------------->
Router Alert Option ...........
Flow/Session/Sig App ID . Routing .
Q-Node Addressing . state . ^
[Query Cookie] .installed. | Existing
[Response Request] . at . | messaging
[NSLP Payload] . R-node . | associations
........... | can be used
GIMPS-response | from here
........... <---------------------- | onwards
. Routing . Flow/Session/Sig App ID |
. state . Query cookie |
.installed. R-Node Addressing | ^
. at . (D Mode only) | |
. Q-node . [Responder Cookie] | |
........... [Response Request] | | New
[NSLP Payload] | | messaging
| | associations
Final handshake | | can be set
----------------------> | | up from here
Flow/Session/Sig App ID ........... | | onwards
Responder Cookie . Routing . | |
Q-Node Addressing . state . | |
(D Mode only) .installed. | |
[NSLP Payload] . at . | |
. R-node . | |
........... | |
Figure 4: Message Sequence at State Setup
In the responding node, the GIMPS level processing of the
Discover-Query information triggers the generation of a
'GIMPS-response' message. This is also a normally encapsulated and
addressed message with particular payloads, this time in the upstream
direction. Again, it can contain an NSLP payload (possibly a response
to the NSLP payload in the initial message). It includes its own
addressing information and the Query Cookie, and optionally
'Discover-Response' information, including another Response Request
flag and a Responder Cookie. Note that if a messaging association
already exists towards the querying node, this can be used to deliver
the GIMPS-response message; otherwise, datagram mode is used.
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The querying node installs the responder address as downstream peer
state information after verifying the Query Cookie in the
GIMPS-response. The responding node can install the querying address
as upstream peer state information after the receipt of the initial
GIMPS-query, or after a third message in the downstream direction
containing the Responder Cookie. The detailed constraints on
precisely when state information is installed are driven by security
considerations on prevention of denial-of-service attacks and state
poisoning attacks, which are discussed further in Section 7.
Setup of messaging associations begins when both downstream peer
addressing information is available and a new messaging association
is actually needed. (In many cases, the GIMPS-response message above
will identify a downstream peer for whom an appropriate messaging
association already exists, in which case no further action is
needed.) Setup of the messaging association always starts from the
upstream node, but it can be used equally in both directions.
Refresh and expiration of all types of state is controlled by timers.
State in the routing table has a per-flow, per-direction timer, which
expires after a routing state lifetime. It is the responsibility of
the querying node to generate a GIMPS-query message, optionally with
a discover-query payload, before this timer expires, if it believes
that the flow is still active. Receipt of the message at the
responding node will refresh upstream peer addressing state, and
receipt of a GIMPS-response at the querying node will refresh any
downstream peer addressing state if it exists. Note that nodes do not
control the refresh of upstream peer state themselves, they are
dependent on the upstream peer for this.
Messaging associations can be managed by either end. Management
consists of tearing down unneeded associations. Whether an
association is needed is a local policy decision, which could take
into account the cost of keeping the messaging association open, the
level of past activity on the association, and the likelihood of
future activity (e.g. if there are flows still in place which might
generate messages that would use it). Messaging associations can
always be set up on demand, and messaging association status is not
made directly visible outside the GIMPS layer. Therefore, even if
GIMPS tears down and later re-establishes a messaging association,
signaling applications cannot distinguish this from the case where
the association is kept permanently open.
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5. Message Formats and Encapsulations
5.1 GIMPS Messages
All GIMPS messages begin with a common header, which includes a
version number, information about message type, signaling
application, and additional control information. The remainder of the
message is encoded in an RSVP-style format, i.e., as a sequence of
type-length-value (TLV) objects. This subsection describes the
possible GIMPS messages and their contents at a high level; a more
detailed description of each information element is given in Section
5.2.
The following gives the syntax of GIMPS messages in ABNF [3].
GIMPS-message: A message is either a datagram mode message or a
connection mode message. GIMPS can detect which by the encapsulation
the message arrives over.
GIMPS-message = D-message / C-message
D-message: A datagram mode message is either upstream or downstream
(slightly different contents are allowed); the common header contains
a flag to say which.
D-message = D-upstream-message / D-downstream-message
C-message: A connection mode message is either upstream or downstream
(again, slightly different contents are allowed); the common header
contains a flag to say which. Note that upstream and downstream
messages can be mixed on a single messaging association.
C-message = C-upstream-message / C-downstream-message
D-downstream-message: A downstream datagram mode message is used for
the GIMPS-query and final handshake in the discovery procedure, and
can also be used simply for carrying NSLP data.
D-downstream-message = Common-Header
Flow-Routing-Information
Node-Addressing
Session-Identification
[ Query-Cookie / Responder-Cookie ]
[ Routing-State-Lifetime ]
[ NSLP-Data ]
D-upstream-message: An upstream datagram mode message is used for the
GIMPS-response in the discovery procedure, and can also be used
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simply for carrying NSLP data.
D-upstream-message = Common-Header
Flow-Routing-Information
Node-Addressing
Session-Identification
[ Query-Cookie [ Responder-Cookie ] ]
[ Routing-State-Lifetime ]
[ NSLP-Data ]
C-downstream-message: A downstream connection mode message is used
primarily for carrying NSLP data, but can also be used for the final
handshake during discovery. Connection mode messages do not carry
node addressing, since this can be inferred from the messaging
association.
C-downstream-message = Common-Header
Flow-Routing-Information
Session-Identification
[ Responder-Cookie ]
[ Routing-State-Lifetime ]
[ NSLP-Data ]
C-upstream-message: An upstream connection mode message is used
primarily for carrying NSLP data, but can also be used for the
GIMPS-response during discovery.
C-upstream-message = Common-Header
Flow-Routing-Information
Session-Identification
[ Query-Cookie [ Responder-Cookie ] ]
[ Routing-State-Lifetime ]
[ NSLP-Data ]
5.2 Information Elements
This section describes the content of the various information
elements that can be present in each GIMPS message, both the common
header, and the individual TLVs. The format description in terms of
bit patterns is provided (in an extremely preliminary form) in
Appendix C.
5.2.1 The Common Header
Each message begins with a fixed format common header, which contains
the following information:
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Version: The version number of the GIMPS protocol.
Length: The number of TLVs following in this message.
Signaling application identifier: This describes the signaling
application, such as resource reservation or firewall control.
GIMPS hop counter: A hop counter prevents a message from looping
indefinitely. (Since messages may get translated between different
lower-layer transport protocols, the IP hop count may be reset and
cannot be relied upon.)
U/D flag: A bit to indicate if this message is to propagate upstream
or downstream relative to the flow.
Response requested flag: A bit to indicate that this message contains
a cookie which must be echoed in the response.
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 further information in the common header.
These items are contained in each GIMPS message:
Flow-Routing-Information: Information sufficient to define the route
that the flow will take through the network. Minimally, this could
just be the flow destination address; however, to account for
policy based forwarding and other issues a more complete set of
header fields should be used (see Section 6.2 and Section 6.3 for
further discussion).
Flow-Routing-Information = network-layer-version
source-address prefix-length
destination-address prefix-length
IP-protocol
traffic-class
[ 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
traffic-class fields should be interpreted as significant.
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Session-Identification: The GIMPS session identifier is a long,
cryptographically random identifier chosen by the node which
begins the signaling exchange (the signaling application at the
node may specify it explicitly, or leave it up to GIMPS to select
a value). The length is open, but 128 bits should be more than
sufficient to make the probability of collisions orders of
magnitude lower than other failure reasons. The session identifier
should be considered immutable end-to-end along the flow path
(GIMPS never changes it, and signaling applications should
propagate it unchanged on messages for the same flow).
The following items are optional:
Node addressing: Minimally, this is the IP address at which the GIMPS
node originating the message can be reached; this will be used to
fill in peer routing state. A logical interface identifier is also
included to assist in route change handling, see Section 6.1.
Additional information can be provided on node capabilities and
policy on messaging association management, as well as currently
available associations. The level of flexibility required in this
field is discussed in Section 8.5.
Query-Cookie/Responder-Cookie: A query-cookie is optional in a
GIMPS-query message and if present must be echoed in a
GIMPS-response; a response-cookie is optional in a GIMPS-response
message, and if present must be echoed in the following downstream
message. Cookies are variable length (chosen by the cookie
generator) and need to be designed so that a node can determine
the validity of a cookie without keeping state.
Routing-State-Lifetime: The lifetime of GIMPS routing state in the
absence of refreshes, measured in seconds. Defaults to 30 seconds.
NSLP-Data: The NSLP payload to be delivered to the signaling
application. GIMPS does not interpret the payload content.
5.3 Encapsulation in Datagram Mode
Encapsulation in datagram mode is simple. The complete set of GIMPS
payloads for a single message is concatenated together with the
common header, and placed in the data field of a UDP datagram. UDP
checksums should be enabled. Upstream messages are directly addressed
to the adjacent peer. Downstream messages are addressed to the flow
receiver and encapsulated with a Router Alert Option; GIMPS may set
the traffic class and (for IPv6) flow label to match the values in
the Flow-Routing-Information if this would be needed to ensure
correct routing. Open issues about alternative encapsulations,
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addressing possibilities, and router alert option value-field setting
are discussed in Section 8.2, Section 8.3 and Section 8.4
respectively.
The source UDP port is selected by the message sender. A destination
UDP port should be allocated by IANA. Note that GIMPS may send
messages addressed as {flow sender, flow receiver} which could make
their way to the flow receiver even if that receiver were
GIMPS-unaware. This should be rejected (with an ICMP message) rather
than delivered to 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 would seem to be required.
5.4 Encapsulation in Connection Mode
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 [20] that it
should be able to support bundling (of small messages), fragmentation
(of large messages), and message boundary delineation. Not all
transport protocols natively support all these features.
SCTP [6] satisfies all requirements. (The bundling requirement is met
implicitly by the use of Nagle-like algorithms inside the SCTP
stack.)
DCCP [7] is message based but does not provide bundling or
fragmentation. Bundling can be carried out by the GIMPS layer
sending multiple messages in a single datagram; because the common
header includes length information (number of TLVs), the message
boundaries within the datagram can be discovered during parsing.
Fragmentation of GIMPS messages over multiple datagrams should be
avoided, because of amplification of message loss rates that this
would cause.
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.
UDP can be augmented as in the DCCP case. (An additional reason for
avoiding fragmentation is the lack of congestion control
functionality in UDP.)
It can be seen that all of these protocol options can be supported by
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the basic GIMPS message format already presented. GIMPS messages
requiring fragmentation must be carried using a reliable transport
protocol, TCP or SCTP.
5.4.2 Encapsulation Format
The GIMPS message, consisting of common header and TLVs, is carried
directly in the transport protocol (possibly incorporating transport
layer security protection). Further GIMPS 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; it applies to both upstream and downstream messages.
+---------------------------------------------+
| L2 Header |
+---------------------------------------------+
| IP Header | ^
| Source address = signaling source | ^
| Destination address = signaling destination | .
+---------------------------------------------+ .
| L4 Header | . ^
| (Standard TCP/SCTP/DCCP/UDP header) | . ^
+---------------------------------------------+ . .
| GIMPS Message | . . ^
| (Common header and TLVs as in section 5.1) | . . ^ Scope of
+---------------------------------------------+ . . . security
| Additional GIMPS 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
Note that when GIMPS messages are carried in connection mode in this
way, between the GIMPS peers they are treated just like any other
traffic by intermediate routers. Indeed, it would be impossible for
intermediate routers to carry out any processing on the messages
without terminating the transport and security protocols used.
Signaling messages are only ever delivered between peers established
in GIMPS-query/response exchanges. Any route change is not detected
until another GIMPS-query/response procedure takes place; in the
meantime, signaling messages are misdelivered. GIMPS is responsible
for prompt detection of route changes to minimise the period during
which this can take place.
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6. Advanced Protocol Features
6.1 Route Changes and Local Repair
6.1.1 Introduction
When re-routing takes place in the network, GIMPS and signaling
application state needs to be updated for all flows whose paths have
changed. The updates to signaling application state 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, GIMPS cannot carry
out the complete path update processing. Its responsibilities are to
detect the route change, update its own routing state consistently,
and inform interested signaling applications at affected nodes.
Route change management is complicated by the distributed nature of
the problem. Consider the re-routing event shown in Figure 6. An
external observer can tell that the main responsibility for
controlling the updates will probably lie with nodes A and E;
however, D1 is best placed to detect the event quickly at the GIMPS
level, and B1 and C1 could also attempt to initiate the repair.
On the assumption that NSLPs are soft-state based and operate end to
end, and because GIMPS also periodically updates its picture of
routing state, route changes will eventually be repaired
automatically. However, especially if NSLP refresh times are extended
to reduce signaling load, the duration of inconsistent state may be
very long indeed. Therefore, GIMPS includes logic to deliver prompt
notifications to NSLPs, to allow NSLPs to carry out local repair if
possible.
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xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
x +--+ +--+ +--+ x Initial
x .|B1|_.......|C1|_.......|D1| x Configuration
x . +--+. .+--+. .+--+\. x
x . . . . . . x
>>xxxxxx . . . . . . xxxxxx>>
+-+ . .. .. . +-+
.....|A|/ .. .. .|E|_....
+-+ . . . . . . +-+
. . . . . .
. . . . . .
. +--+ +--+ +--+ .
.|B2|_.......|C2|_.......|D2|/
+--+ +--+ +--+
+--+ +--+ +--+ Configuration
.|B1|........|C1|........|D1| after failure
. +--+ .+--+ +--+ of D1-E link
. \. . \. ./
. . . . .
+-+ . .. .. +-+
.....|A|. .. .. .|E|_....
+-+\. . . . . . +-+
>>xxxxxx . . . . . . xxxxxx>>
x . . . . . . x
x . +--+ +--+ +--+ . x
x .|B2|_.......|C2|_.......|D2|/ x
x +--+ +--+ +--+ x
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
........... = physical link topology
>>xxxxxxx>> = flow direction
_.......... = indicates outgoing link
for flow xxxxxx given
by local forwarding table
Figure 6: A Re-Routing Event
6.1.2 Route Change Detection
There are two aspects to detecting a route change at a single node:
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Detecting that the downstream path has (or may have) changed.
Detecting that the upstream path 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 downstream path at a node corresponds to a change
in upstream path at the downstream peer. Note that there are two
primary elements to route change:
Interface: The interface through which a flow leaves or enters a node
may change.
Peer: The adjacent upstream peer or downstream peer may change.
In general, a route change could include one or the other or both.
(In theory it could include neither, although such changes are hard
to detect and even harder to do anything useful about.)
There are five mechanisms for a GIMPS node to detect that a route
change has occurred, which are listed below. They apply differently
depending on whether the change is in the upstream or downstream
path, and these differences are given in the following table.
Local Trigger: In trigger mode, a node finds out that the next hop
has changed. This is the RSVP trigger mechanism where some form of
notification mechanism from the routing table to the protocol
handler is assumed. Clearly this only works if the routing change
is local, not if the routing change happens somewhere a few
routing hops away (including the case that the change happens at a
GIMPS-unaware node).
Extended Trigger: An extended trigger, where the node checks a
link-state routing table to discover that the path has changed.
This makes certain assumptions on consistency of route computation
(but you probably need to make those to avoid routing loops) and
only works intra-domain for OSPF and similar link-state protocols.
Where available, this offers the most accurate and expeditious
indication of route changes, but requires more access to the
routing internals than a typical OS may provide.
GIMPS C-mode Monitoring: A node may find that C-mode packets are
arriving (from upstream or downstream 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.
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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 GIMPS.
GIMPS D-mode Probing: In probing mode, each GIMPS node periodically
repeats the discovery (GIMPS-query/GIMPS-response) operation. The
querying node will discover the route change by a modification in
the Node-Addressing information in the GIMPS-response. This is
similar to RSVP behavior, except that there is an extra degree of
freedom since not every message needs to repeat the discovery,
depending on the likely stability of routes. All indications are
that, leaving mobility aside, routes are stable for hours and
days, so this may not be necessary on a 30-second interval,
especially if the other techniques listed above are available.
When these methods discover a route change in the upstream direction,
this cannot be handled directly by GIMPS at the detecting node, since
route discovery proceeds only in the downstream direction. Therefore,
to exploit these mechanisms, it must be possible for GIMPS to send a
notification message in the upstream direction to initiate this.
(This would be possible for example by setting an additional flag in
the Common-Header of an upstream message.)
+----------------------+----------------------+---------------------+
| Method | Downstream | Upstream |
+----------------------+----------------------+---------------------+
| Local Trigger | Discovers new | Not applicable |
| | downstream interface | |
| | (and peer if local) | |
| | | |
| Extended Trigger | Discovers new | May determine that |
| | downstream interface | route from upstream |
| | and may determine | peer will have |
| | new downstream peer | changed |
| | | |
| C-Mode Monitoring | Provides hint that | Provides hint that |
| | change has occurred | change has occurred |
| | | |
| Data Plane | Not applicable | NSLP informs GIMPS |
| Monitoring | | that a change may |
| | | have occurred |
| | | |
| D-Mode Probing | Discovers changed | Discovers changed |
| | Node-Addressing in | Node-Addressing in |
| | GIMPS-response | GIMPS-query |
+----------------------+----------------------+---------------------+
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6.1.3 Local Repair
Once a node has detected that a change may have occurred, there are
three possible cases:
1. Only an upstream change is indicated. There is nothing that can
be done locally; GIMPS must propagate a notification to its
upstream peer.
2. A downstream change has been detected and an upstream change is
indicated. Although some local repair may be appropriate, it is
difficult to decide what, since the path change may actually have
taken place upstream of the detecting node (so that this node is
no longer on the path at all).
3. A downstream change has been detected, but there is no upstream
change. In this case, the detecting node is the true crossover
router, i.e. the point in the network where old and new paths
diverge. It is the correct node to initiate the local repair
process.
In case (3), i.e. at the upstream crossover node, the local repair
process is initiated by the GIMPS level as follows:
o GIMPS marks its downstream routing state information for this flow
as 'invalid', unless the route change was actually detected by
D-mode probing (in which case the new state has already been
installed).
o GIMPS notifies the local NSLP that local repair is necessary.
It is assumed that the second step will typically trigger the NSLP to
generate a downstream message, and the attempt to send it will
stimulate a GIMPS-query/response. This signaling application message
will propagate downstream, also discovering the new route, until it
rejoins the old path; the node where this happens may also have to
carry out local repair actions.
A problem is that there is usually no robust technique to distinguish
case (2) from case (3), because of the relative weakness of the
techniques in determining that upstream change has not occurred.
(They can be effective in determining that a change has occurred;
however, even where they can tell that the route from the upstream
peer has not changed, they cannot rule out a change beyond that
peer.) There is therefore a danger that multiple nodes within the
network would attempt to carry out local repair in parallel.
One possible technique to address this problem is that a GIMPS node
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that detects case (3) locally, rather than initiating local repair
immediately, still sends a route change notification upstream, just
in case (2) actually applies. If the upstream peer locally detects no
downstream route change, it can signal this to the downstream node
(e.g. by setting another flag in the Common-Header of a GIMPS
message). This acts to damp the possibility of a 'local repair
storm', at the cost of an additional peer-peer round trip time.
6.1.4 Local Signaling Application State Removal
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 GIMPS 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 [23].
This requirement can be met provided that GIMPS is able to 'remember'
the old path to the signaling application peer for the period while
the NSLP wishes to be able to use it. Since NSLP peers are a single
GIMPS hop apart, the necessary information is just the old entry in
the node's routing state table for that flow. Rather than requiring
the GIMPS 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 GIMPS layer in case it needs to be used.
6.1.5 Operation with Heterogeneous NSLPs
A potential problem with route change detection is that the detecting
GIMPS node may not implement all the signaling applications that need
to be informed. Therefore, it would need to be able to send a
notification back along the unchanged path to trigger the nearest
signaling application aware node to take action. If multiple
signaling applications are in use, it would be hard to define when to
stop propagating this notification. However, given the rules on
message interception and routing state maintenance in Section 4.2,
Section 4.3 and Section 8.4, this situation cannot arise: all NSLP
peers are exactly one GIMPS hop apart.
The converse problem is that the ability of GIMPS to detect route
changes by purely local monitoring of forwarding tables is more
limited. (This is probably an appropriate limitation of GIMPS
functionality. If we need a protocol for distributing notifications
about local changes in forwarding table state, a flow signaling
protocol is probably not the right starting point.)
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6.2 Policy-Based Forwarding and Flow Wildcarding
Signaling messages almost by definition need to contain address and
port information to identify the flow they are signaling for. We can
divide this information into two categories:
Flow-Routing-Information: This is the information needed to determine
how a flow is routed within the network. Minimally it is the flow
destination address, but to handle load sharing, QoS routing, and
other forms of policy based forwarding, additional information may
be required. It is carried as an object in each GIMPS message (see
Section 5.1).
Additional Packet Classification Information: This is any further
higher layer information needed to select a subset of packets for
special treatment by the signaling application. The need for this
is highly signaling application specific, and so this information
is invisible to GIMPS (if indeed it exists); it will be carried
only in the corresponding NSLP.
The correct pinning of signaling messages to the data path depends on
how well the downstream messages in datagram mode can be made to be
routed correctly. Two strategies are used:
The messages themselves have the flow destination address, and
possibly source address and traffic class (see Section 8.3 for
further discussion). In many cases, this will cause the messages
to be routed correctly even by GIMPS-unaware nodes.
A GIMPS-aware node carrying out policy based forwarding on higher
layer identifiers (in particular, the protocol and port numbers
for IPv4) should take into account the entire
Flow-Routing-Information object in selecting the outgoing
interface rather than relying on the IP layer.
The current Flow-Routing-Information format allows a limited degree
of 'wildcarding', for example by applying a prefix length to the
source or destination address, or by leaving certain fields
unspecified. A GIMPS-aware node must verify that all flows matching
the Flow-Routing-Information would be routed identically in the
downstream direction, or else reject the message with an error.
6.3 NAT Traversal
A already noted, GIMPS messages must carry packet addressing and
higher layer information as payload data in order to define the flow
signalled for. (This applies to all GIMPS messages, regardless of how
they are encapsulated or which direction they are travelling in.) At
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an addressing boundary the data flow packets will have their headers
translated; if the signaling payloads are not likewise translated,
the signaling messages will refer to incorrect (and probably
meaningless) flows after passing through the boundary.
The simplest solution to this problem is to require that a NAT is
GIMPS-aware, and to allow it to modify datagram mode messages based
on the contents of the Flow-Routing-Information payload. (This is
making the implicit assumption that NATs only rewrite the header
fields included in this payload, and not higher layer identifiers.)
Provided this is done consistently with the data flow header
translation, signaling messages will be valid each side of the
boundary, without requiring the NAT to be signaling application
aware. An outline of the set of operations necessary on a downstream
datagram mode message is as follows:
1. Verify that bindings for the data flow are actually in place.
2. Create bindings for subsequent C-mode signaling (based on the
information in the Node-Addressing field).
3. Create a new Flow-Routing-Information payload with fields
modified according to the data flow bindings.
4. Create a new Node-Addressing payload with fields to force
upstream D-mode messages through the NAT, and to allow C-mode
exchanges using the C-mode signaling bindings.
5. Forward the message with these new payloads.
The original Flow-Routing-Information and Node-Addressing payloads
should be retained in the message, but encapsulated in a new TLV
type. (In the case of a sequence of NATs, this TLV would become a
list.) This TLV essentially becomes a recorded route for the D-mode
message; GIMPS node that wished to do topology hiding could replace
this original payloads with opaque tokens, or omit them altogether.
Note that a consequence of this approach is that the routing state
tables at the actual signaling application peers (either side of the
NAT) are no longer directly compatible (the values of
Flow-Routing-Information are different, and the Node-Addressing may
be different between C- and D-mode). Details are for further study.
The case of traversing a GIMPS unaware NAT is also for further study.
There is a dual problem of whether the GIMPS peers either side of the
boundary can work out how to address each other, and whether they can
work out what translation to apply to the flow routing information
from what is done to the signaling packet headers. The fundamental
problem is that GIMPS messages contain 3 or 4 interdependent
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addresses which all have to be consistently translated, and existing
generic NAT traversal techniques such as STUN [19] can process only
two.
6.4 Interaction with IP Tunnelling
The interaction between GIMPS and IP tunnelling is very simple. An IP
packet carrying a GIMPS message is treated exactly the same as any
other packet with the same source and destination addresses: in other
words, it is given the tunnel encapsulation and forwarded with the
other data packets.
Tunnelled packets will not have the router alert option or the
standard GIMPS protocol encapsulation (e.g. port numbers). They will
not be identifiable as GIMPS messages until they leave the tunnel
again. If signaling is needed for the tunnel itself, this has to be
initiated as a separate signaling session by one of the tunnel
endpoints - that is, the tunnel counts as a new flow. Because the
relationship between signaling for the 'microflow' and signaling for
the tunnel as a whole will depend on the signaling application in
question, we are assuming that it is a signaling application
responsibility to be aware of the fact that tunnelling is taking
place and to carry out additional signaling if necessary; in other
words, one tunnel endpoint must be signaling application aware.
In some cases, it is the tunnel exit point (i.e. the node where
tunnelled data and downstream signaling packets leave the tunnel)
that will wish to carry out the tunnel signaling, but this node will
not have knowledge or control of how the tunnel entry point is
carrying out the data flow encapsulation. This information could be
carried as additional data (an additional GIMPS payload) in the
tunnelled signaling packets if the tunnel entry point was at least
GIMPS aware. This payload would be the GIMPS equivalent of the RSVP
SESSION_ASSOC object of [11]. Whether this functionality should
really be part of GIMPS and if so how the payload should be handled
will be considered in a later version.
6.5 IPv4-IPv6 Transition and Interworking
GIMPS itself is essentially IP version neutral (version dependencies
are isolated in the formats of the Flow-Routing-Information and
Node-Addressing TLVs, and GIMPS also depends on the version
independence of the protocols that underly messaging associations).
In mixed environments, GIMPS operation will be influenced by the IP
transition mechanisms in use. This section provides a high level
overview of how GIMPS is affected, considering only the currently
predominant mechanisms.
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Dual Stack: (This applies both to the basic approach described in
[24] as well as the dual-stack aspects of more complete
architectures such as [26].) In mixed environments, GIMPS should
use the same IP version as the flow it is signaling for; hosts
which are dual stack for applications and routers which are dual
stack for forwarding should have GIMPS implementations which can
support both IP versions.
In theory, for some connection mode encapsulation options, a
single messaging association could carry signaling messages for
flows of both IP versions, but the saving seems of limited value.
The IP version used in datagram mode is closely tied to the IP
version used by the data flow, so it is intrinsically impossible
for a IPv4-only or IPv6-only GIMPS node to support signaling for
flows using the other IP version.
Applications with a choice of IP versions might select a version
for which GIMPS support was available in the network, which could
be established by running parallel discovery procedures. In
theory, a GIMPS message related to a flow of one IP version could
flag support for the other; however, given that IPv4 and IPv6
could easily be separately routed, the correct GIMPS peer for a
given flow might well depend on IP version anyway, making this
flagged information irrelevant.
Packet Translation: (Applicable to SIIT [5] and NAT-PT [12].) Some
transition mechanisms allow IPv4 and IPv6 nodes to communicate by
placing packet translators between them. From the GIMPS
perspective, this should be treated essentially the same way as
any other NAT operation (e.g. between 'public' and 'private'
addresses) as described in Section 6.3. In other words, the
translating node needs to be GIMPS aware; it will run GIMPS with
IPv4 on some interfaces and with IPv6 on others, and will have to
translate the Flow-Routing-Information payload between IPv4 and
IPv6 formats for flows which cross between the two. The
translation rules for the fields in the payload (including e.g.
traffic class and flow label) are as defined in [5].
Tunnelling: (Applicable to 6to4 [14] and a whole host of other
tunnelling schemes.) Many transition mechanisms handle the problem
of how an end to end IPv6 (or IPv4) flow can be carried over
intermediate IPv4 (or IPv6) regions by tunnelling; the methods
tend to focus on minimising the tunnel administration overhead.
From the GIMPS perspective, the treatment should be as similar as
possible to any other IP tunnelling mechanism, as described in
Section 6.4. In particular, the end to end flow signaling will
pass transparently through the tunnel, and signaling for the
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tunnel itself will have to be managed by the tunnel endpoints.
However, additional considerations may arise because of special
features of the tunnel management procedures. For example, [15] is
based on using an anycast address as the destination tunnel
endpoint. It might be unwise to carry out signaling for the tunnel
to such an address, and the GIMPS implementation there would not
be able to use it as a source address for its own signaling
messages (e.g. GIMPS-responses). Further analysis will be
contained in a future version of this specification.
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7. Security Considerations
The security requirement for the GIMPS layer is to protect the
signaling plane against identified security threats. For the
signaling problem as a whole, these threats have been outlined in
[21]; the NSIS framework [20] assigns a subset of the responsibility
to the NTLP. The main issues to be handled can be summarised as:
Message Protection: Signaling message content should be protected
against eavesdropping, modification, injection and replay while in
transit. This applies both to GIMPS payloads, and GIMPS should
also provide such protection as a service to signaling
applications between adjacent peers.
State Integrity Protection: It is important that signaling messages
are delivered to the correct nodes, and nowhere else. Here,
'correct' is defined as 'the same nodes that the infrastructure
will route data flow packets through'. (GIMPS 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.) GIMPS uses internal state to decide how to route signaling
messages, and this state needs to be protected against corruption.
Prevention of Denial of Service Attacks: GIMPS nodes and the network
have finite resources (state storage, processing power,
bandwidth). The protocol should try to minimise exhaustion attacks
against these resources and not allow GIMPS nodes to be used to
launch attacks on other network elements.
The main missing issue is handling authorisation for executing
signaling operations (e.g. allocating resources). This is assumed to
be done in each signaling application.
In many cases, GIMPS relies on the security mechanisms available in
messaging associations to handle these issues, rather than
introducing new security measures. Obviously, this requires the
interaction of these mechanisms with the rest of the GIMPS protocol
to be understood and verified.
7.1 Message Confidentiality and Integrity
GIMPS can use messaging association functionality, such as TLS or
IPsec, to ensure message confidentiality and integrity. In many
cases, confidentiality of GIMPS information itself is not likely to
be a prime concern, in particular since messages are often sent to
parties which are unknown ahead of time, although the content visible
even at the GIMPS level gives significant opportunities for traffic
analysis. Signaling applications may have their own mechanism for
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securing content as necessary; however, they may find it convenient
to rely on protection provided by messaging associations,
particularly if this is provided efficiently and if it runs unbroken
between signaling application peers.
7.2 Peer Node Authentication
Cryptographic protection (of confidentiality or integrity) requires a
security association with session keys, which can be established
during an authentication and key exchange protocol run 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 can
provide it for IPsec). GIMPS nodes rely on these protocols to
authenticate the identity of the next hop, and GIMPS has no
authentication capability of its own.
However, with discovery, there are few effective ways to know what is
the legitimate next or previous hop as opposed to an impostor. In
other words, 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 7.3 below.
It is open whether peer node authentication should be made signaling
application dependent; for example, whether successful authentication
could be made dependent on presenting authorisation to act in a
particular signaling role (e.g. signaling for QoS).
7.3 Routing State Integrity
The internal state in a node (see Section 4.1), specifically the
upstream and downstream peer identification, is used to route
messages along the data flow path. If this state is corrupted,
signaling messages may be misdirected.
The routing state table is the GIMPS view of how flows are being
routed through the network in the immediate neighbourhood of the
node. Routes are only weakly secured (e.g. there is usually no
cryptographic binding of a flow to a route), and there is no other
authoritative information about flow routes than the current state of
the network itself. Therefore, consistency between GIMPS and network
routing state has to be ensured by directly interacting with the
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routing mechanisms to ensure that the upstream and downstream
signaling peers are the appropriate ones for any given flow. A good
overview of security issues and techniques in this sort of context is
provided in [25].
Downstream peer identification is installed and refreshed only on
receiving a GIMPS-reponse message (compare Figure 4). This must echo
the cookie from a previous GIMPS-query message, which will have been
sent downstream along the flow path (in datagram mode, i.e.
end-to-end addressed). Hence, only the true next peer or an on-path
attacker will be able to generate such a message, provided freshness
of the cookie can be checked at the querying node.
Upstream peer identification can be installed directly on receiving a
GIMPS-query message containing addressing information for the
upstream peer. However, any node in the network could generate such a
message (indeed, almost any node in the network could be the genuine
upstream peer for a given flow). To protect against this, two
strategies are possible:
Filtering: the receiving node may be able to reject signaling
messages which claim to be for flows with flow source addresses
which would be ruled out by ingress filtering. An extension of
this technique would be for the receiving node to monitor the data
plane and to check explicitly that the flow packets are arriving
over the same interface and if possible from the same link layer
neighbour as the datagram mode signaling packets. (If they are
not, it is likely that at last one of the signaling or flow
packets is being spoofed.) Signaling applications should only
install state on the route taken by the signaling itself.
Authentication (weak or strong): the receiving node may refuse to
install upstream state until it has handshaked by some means with
the upstream peer. This handshaking could be as simple as
requesting the upstream peer to echo the response cookie in the
discover-response payload of a GIMPS-response message (to
discourage nodes impersonating upstream peers from using forged
source addresses); or, it could be full peer authentication within
the messaging association, the reasoning being that an
authenticated peer can be trusted not to pretend to be on path
when it is not.
The second technique also plays a role in denial of service
prevention, see below. In practice, a combination of both techniques
may be appropriate.
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7.4 Denial of Service Prevention
GIMPS is designed so that each connectionless discovery message only
generates at most one response, so that a GIMPS node cannot become
the source of a denial of service amplification attack.
However, GIMPS can still be subjected to denial-of-service attacks
where an attacker using forged source addresses forces a node to
establish state without return routability, causing a problem similar
to TCP SYN flood attacks. In addition to vulnerabilities of a next
peer discovery an unprotected path discovery procedure might
introduce more denial of service attacks since a number of nodes
could possibly be forced to allocate state. Furthermore, an
adversary might modify or replay unprotected signaling messages.
There are two types of state attacks and one computational resource
attack. In the first state attack, an attacker floods a node with
messages that the node has to store until it can determine the next
hop. If the destination address is chosen so that there is no
GIMPS-capable next hop, the node would accumulate messages for
several seconds until the discovery retransmission attempt times out.
The second type of state-based attack causes GIMPS state to be
established by bogus messages. A related computational/
network-resource attack uses unverified messages to cause a node to
make AAA queries or attempt to cryptographically verify a digital
signature. (RSVP is vulnerable to this type of attack.) Relying only
on upper layer security, for example based on CMS, might open a
larger door for denial of service attacks since the messages are
often only one-shot-messages without utilizing multiple roundtrips
and DoS protection mechanisms.
There are at least three defenses against these attacks:
1. The responding node does not establish a session or discover its
next hop on receiving the GIMPS-query message, but can wait for a
setup message on a reliable channel. If the reliable channel
exists, the additional delay is a one one-way delay and the total
is no more than the minimal theoretically possible delay of a
three-way handshake, i.e., 1.5 node-to-node round-trip times.
The delay gets significantly larger if a new connection needs to
be established first.
2. The response to the initial discovery message contains a cookie.
The previous hop repeats the discovery with the cookie included.
State is only established for messages that contain a valid
cookie. The setup delay is also 1.5 round-trip times. (This
mechanism is similar to that in SCTP [6] and other modern
protocols.)
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3. If there is a chance that the next-hop node shares a secret with
the previous hop, the sender could include a hash of the session
ID and the sender's secret. The receiver can then verify that the
message was likely sent by the purported source. This does not
scale well, but may work if most nodes tend to communicate with a
small peer clique of nodes. (In that case, however, they might as
well establish more-or-less permanent transport sessions with
each other.)
These techniques are complementary; we chose a combination of the
first and second method.
Once a node has decided to establish routing state, there may still
be transport and security state to be established between peers. This
state setup is also vulnerable to additional denial of service
attacks. GIMPS relies on the lower layer protocols that make up
messaging associations to mitigate such attacks. The current
description assumes that the upstream node is always the one wishing
to establish a messaging association, so it is typically the
downstream node that is protected. Extensions are considered in
Section 8.6; these would require further analysis.
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8. Open Issues
8.1 Protocol Naming
Alternate names:
GIST: General Internet Signaling Transport
GIMPS: General Internet Messaging Protocol for Signaling
LUMPS: Lightweight Universal Messaging for Path associated Signaling
There is a danger of some ambiguity as to whether the protocol name
refers to the complete transport stack below the signaling
applications, or only to the additional protocol functionality above
the standard transport protocols (UDP, TCP etc.) The NSIS framework
uses the term NTLP for the first, but this specification uses the
GIST/variants names for the second (see Figure 2 in Section 3.1). In
other words, this specification proposes to meet the requirements for
NTLP functionality by layering GIMPS/... over existing standard
transport protocols. It isn't clear if additional terminological
surgery is needed to make this clearer.
8.2 General IP Layer Issues
Some NSIS messages have to be addressed end-to-end but intercepted at
intermediate routers, and this imposes some special constraints on
how they can be encapsulated. RSVPv1 [9] primarily uses raw IP with a
specific protocol number (46); a UDP encapsulation is also possible
for hosts unable to perform raw network i/o. RSVP aggregation [17]
uses an additional protocol number (134) to bypass certain interior
nodes.
The critical requirements for the encapsulation at this level are
that routers should be able to identify signaling packets for
processing, and that they should not mis-identify packets for
'normal' end-to-end user data flows as signaling packets. The current
assumption is that UDP encapsulation can be used for such messages,
by allocating appropriate (new) value codes for the router alert
option (RAO) [1][4] to identify NSIS messages. Specific open issues
about how to allocate such values are discussed in Section 8.4.
An alternative approach would be to use raw IP with the RSVP protocol
numbers and a new RSVP version number. Although this would provide
some more commonality with existing RSVP implementations, the NAT
traversal problems for such an encapsulation seem much harder to
solve.
8.3 Encapsulation and Addressing for Datagram Mode
The discussion in Section 4 essentially assumes that datagram mode
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messages are UDP encapsulated. This leaves open the question of
whether other encapsulations are possible, and exactly how these
messages should be addressed.
As well as UDP/IP (and raw IP as discussed and temporarily ruled out
in Section 8.2), DCCP/IP and UDP/IPsec could also be considered as
'datagram' encapsulations. However, they still require explicit
addressing between GIMPS peer nodes and some per-peer state to be set
up and maintained. Therefore, it seems more appropriate to consider
these encapsulation options as possible messaging association types,
for use where there is a need for congestion control or security
protection but without reliability. This would leave UDP/IP as the
single encapsulation allowed for all datagram mode messages.
Addressing for upstream datagram mode messages is simple: the IP
source address is the signaling source address, and the IP
destination address is the signaling destination address (compare
Figure 1). For downstream datagram mode messages, the IP destination
address will be the flow destination address, but the IP source
address could be either of the flow source address or signaling
source address. Some of the relative merits of these options are as
follows:
o Using the flow source address makes it more likely that the
message will be correctly routed through any intermediate
NSIS-unaware region which is doing load sharing or policy routing
on the {source, destination} address pair. If the signaling source
address is used, the message will be intercepted at some node
closer to the flow destination, but it may not be the same as the
next node for the data flow packets.
o Conversely, using the signaling source address means that ICMP
error messages (specifically, unreachable port or address) will be
correctly delivered to the message originator, rather than being
sent back to the flow source. Without seeing these messages, it is
very difficult for the querying node to recognise that it is the
last NSIS node on the path. In addition, using the signaling
source address may make it possible to exchange messages through
GIMPS unaware NATs (although it isn't clear how useful the
resulting messages will be, see Section 6.3).
It is not clear which of these situations it is more important to
handle correctly and hence which source addressing option to use.
(RSVP uses the flow source address, although this is primarily for
multicast routing reasons.) A conservative approach would be to allow
both, possibly even in parallel (although this might open up the
protocol to amplification attacks).
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As well as the addressing, downstream datagram mode messages could be
given the same traffic class (and, in the case of IPv6, flow label)
as data flow messages. Doing this would increase the faithfulness of
the routing of such messages. The traffic class may not be strictly
appropriate for signaling; however, in many cases the bulk of the
signaling messages will be sent in connection mode (for which a
different traffic class can be freely chosen).
8.4 Intermediate Node Bypass and Router Alert Values
We assume that the primary mechanism for intercepting messages is the
use of the RAO. The RAO contains a 16 bit value field, within which
35 values have currently been assigned by IANA. It is open how to
assign values for use by GIMPS messages to optimise protocol
processing, i.e. to minimise the amount of slow-path processing that
nodes have to carry out for messages they are not actually interested
in the content of.
There are two basic reasons why a GIMPS node might wish to ignore a
message:
o because it is for a signaling application that the node does not
process;
o because even though the signaling application is present, the node
is only processing signaling messages at the aggregate level and
not for individual flows (compare [17]).
Once a message is ignored and forwarded onwards towards the next
node, the current node has essentially dropped out of the signaling
path for this flow and signaling application. Its address will not be
discovered and used for upstream datagram mode messages or any
connection mode messages.
Some or all of this information could be encoded in the RAO value
field, which would then allow these messages to be filtered on the
fast path. (Even if the information is encoded deeper into the
message, such as in the GIMPS fixed header, it may still be possible
to process it on the fast path, and the semantics of the node
dropping out of the signaling path are the same however the filtering
is done. However, using the RAO seems the most efficient method if
this is possible.) It is not clear whether the signaling application
and aggregation level should be directly identified in the RAO value,
or whether IANA should allocate special values for 'popular'
applications or groups of applications or network configurations. The
former is most flexible but is liable to be expensive in terms of
value allocations.
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There is a special consideration in the case of the aggregation
level. In this case, whether a message should be processed depends on
the network region it is in. There are then two basic possibilities:
1. All routers have essentially the same algorithm for which
messages they process, i.e. all messages at aggregation level 0.
However, messages have their aggregation level incremented on
entry to an aggregation region and decremented on exit.
2. Routers process messages only above a certain aggregation level
and ignore all others. The aggregation level of a message is
never changed; signaling messages for end to end flows have level
0, but signaling messages for aggregates are generated with a
higher level.
The first technique requires aggregating/deaggregating routers to be
configured as being at a particular aggregation level, and also
requires consistent message rewriting at these boundaries. The second
technique eliminates the rewriting, but requires interior routers to
be configured also. It is not clear what the right trade-off between
these options is.
8.5 Messaging Association Flexibility
The language of Section 4 is mainly based on the use of 0 or 1
messaging associations between any pair of GIMPS nodes. However,
logically it would be quite possible to have more than one
association, for example:
o to allow different reliability characteristics;
o to provide different levels of security protection or to have
security separation between different signaling streams;
o even simply to have load split between different connections
according to priority (so there could be two associations with
identical protocol stacks).
It is possible to imagine essentially infinite flexibility in these
options, both in terms of how many possibilities are allowed and how
nodes signal their capabilities and preferences, without much
changing the overall GIMPS structure. (The GIMPS-query and
GIMPS-response messages described in section Section 4.3 can be used
to exchange this information.) What is not clear is how much
flexibility is actually needed.
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8.6 Messaging Association Setup Message Sequences
The discussion of Section 4.3 assumes a simple fixed message
sequence, which we can picture as follows:
+---------------------------------+---------------------------------+
| Direction | Message |
+---------------------------------+---------------------------------+
| ---> | GIMPS-query message |
| | |
| <--- | GIMPS-response message |
| | |
| ===> | Querying node initiates |
| | messaging association setup |
| | messages |
| | |
| <--> | Signaling messages exchanged |
+---------------------------------+---------------------------------+
There are several variants which could be considered at this level,
for example whether the messaging association could be set up by the
responding node:
+---------------------------------+---------------------------------+
| Direction | Message |
+---------------------------------+---------------------------------+
| ---> | GIMPS-query message |
| | |
| <=== | Responding node initiates |
| | messaging association setup |
| | messages |
| | |
| <--> | Signaling messages exchanged |
+---------------------------------+---------------------------------+
This saves a message but may be vulnerable to additional denial of
service attacks.
Another open area is how the responding node propagates the signaling
message downstream. It could initiate the downstream discovery
process as soon as it received the initial GIMPS-query message, or it
could wait until the first signaling application message has been
received (which might not be until a messaging association has been
established). A similar timing question applies to when it should
initiate its own downstream messaging associations. It is possible
that all these options are simply a matter for implementation
freedom, although leaving them open will make mobility and re-routing
behaviour rather harder to analyse, and again there are denial of
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service implications for some approaches (see Section 7.4).
A final open area is how to manage the protocol exchanges that take
place in setting up the messaging association itself. It is probably
an implementation matter to consider whether to carry out, for
example, the SCTP 4-way handshake only after IKE exchanges (for IPsec
SA initialisation) have completed, or whether these can be done
partly in parallel. A more radical step is to carry the initial
request and response messages of both exchanges as payloads in the
GIMPS-query/response exchange, with the request message initially
formatted by the querying node with an unspecified destination IP
address. This would require modifications to the protocol
implementations (especially at the querying node) similar to what is
needed for NAT traversal; it would have to be evaluated whether this
was worth the one or two round trip times that are saved.
8.7 GIMPS State Teardown
The description of Section 4.3 provides for GIMPS state (per-flow
routing state and per-peer messaging association state) to be removed
on timer expiry; routing state can also be replaced (updated). This
is the fundamental technique. An additional possibility would be to
have explicit removal, i.e. a protocol mechanism to tear down GIMPS
state immediately for a particular flow. On recieving such a message,
a GIMPS node would clear routing entries and possibly take down
messaging associations that were no longer used.
Even if one peer indicates that routing state is no longer required,
the receiving GIMPS node would have to ensure that no other peers
(e.g. supporting different signaling applications) might generate
messages of their own still needing the state. In addition, it is not
clear how useful it is to remove GIMPS state promptly, since
maintaining it only requires table storage without retention of any
actual network resources.
8.8 Datagram Mode Retries and Single Shot Message Support
The GIMPS-query and GIMPS-response messages may suffer from message
loss (e.g. due to congestion or corruption). Because a successful
handshake is necessary before a messaging association can even be
initiated, GIMPS must provide its own recovery method for these
cases. A working assumption is that the querying node can repeat the
GIMPS-query with an exponential backoff until a response is received
or some retry threshold is reached.
The same technique could be used by the GIMPS layer to provide a
'low-cost' reliable message transfer service, restricted to short
messages, without incurring the cost of setting up a messaging
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association. (If a messaging association exists, it will often be
cheaper to discover and use that.) Providing such a service would
require some minor extensions to the basic GIMPS protocol. It isn't
clear if this additional option fills an important gap in the
spectrum between datagram and connection mode message transfer.
8.9 GIMPS Support for Message Scoping
Many signaling applications are interested in sending messages over a
specific region of the network. Message scoping of this nature seems
to be hard to achieve in a topologically robust way, because such
region boundaries are not well defined in the network layer.
It may be that the GIMPS layer can assist such scoping, by detecting
and counting different types of nodes in the signaling plane. The
simplest solution would be to count GIMPS nodes supporting the
relevant signaling application - this is already effectively done by
the GIMPS hop count. A more sophisticated approach would be to track
the crossing of aggregation region boundaries, as introduced in
Section 8.4. Whether this is plausible depends on the willingness of
operators to configure such boundary information in their routers.
8.10 Mandatory or Optional Reverse Routing State
Reverse routing state (i.e. the upstream peer addressing information
in the routing state table) is installed per-flow when receiving a
downstream datagram mode message containing an addressing information
payload for the signaling source.
Technically, the presence of this payload (and hence the installation
of the state) is optional. This allows for very lightweight sending
of multi-hop downstream signaling messages (even all the way from
flow sender to flow receiver) because no state needs to be installed
or managed by GIMPS at the intermediate nodes. However, this rules
out the possibility of any downstream node sending signaling
responses (including error messages) directly upstream; they have to
be sent via the flow endpoints, leading to additional processing
there, as well as more complex security considerations.
8.11 Additional Discovery Mechanisms
The routing state maintenance procedures described in Section 4.3 are
strongly focussed on the problem of discovering, implicitly or
explicitly, the neighbouring peers on the flow path - which is the
necessary functionality for path-coupled signaling.
As well as the GIMPS-query/response discovery mechanism, other
techniques may sometimes also be possible. For example, in many
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environments, a host has a single access router, i.e. the downstream
peer (for outgoing flows) and the upstream peer (for incoming ones)
are known a priori. More generally, a link state routing protocol
database can be analysed to determine downstream peers in more
complex topologies, and maybe upstream ones if strict ingress
filtering is in effect. More radically, much of the GIMPS protocol is
unchanged if we consider off-path signaling nodes, although there are
significant differences in some of the security analysis (Section
7.3). However, none of these possibilities are considered further in
this specification.
8.12 Alternative Message Routing Requirements
The basic assumption of GIMPS is that signaling messages are to be
routed identically to data flow messages. GIMPS messages include
information (the Flow-Routing-Information TLV and upstream/downstream
flag in the Common-Header) which defines the flow and the
relationship of the signaling to it sufficiently for GIMPS to route
its messages correctly. However, some additional modes of routing
signaling messages have been identified:
Predictive Routing: Here, the intent is to send signaling along a
path that the data flow may or will 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. It is currently unclear whether these cases can be
met using the existing GIMPS routing capabilities (and if they
cannot, whether they are in the initial scope of the work).
NAT Address Reservations: This applies to the case where a node
behind a NAT wishes to use NSIS signaling to reserve an address
from 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 exists. One possible
solution (assumed in [22]) is to construct a message with the
Flow-Routing-Information matching the possible senders and send it
as though it was downstream signaling. It is not clear whether
signaling for the 'wrong direction' in this way will always be
treated consistently by GIMPS, especially if routing policies and
encapsulations for inbound and outbound traffic are treated very
differently within the rest of the network.
8.13 Congestion Control in Datagram Mode
Datagram mode still requires (primitive) transport functions for
backoff on retransmission (for example, when failing to receive a
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response to a GIMPS-query), and rate limiting in general.
Retransmission should probably be handled with a simple exponential
backoff process. This is cautious, but since by definition it is
required when the peer is actually unknown, it is probably the best
that can be achieved anyway.
More subtle is the case where there is a stream of D-mode messages
with no immediate feedback from the neighbour node. This could be the
case where a signaling application was generating messages for
stateless processing in the interior of the network. Here, the
appropriate approach may be to use rate-limiting algorithms such as
in ICMPv6 [10]. Another possibility would be to use ECN [16], if the
datagram mode messages can be correlated with a congestion controlled
messaging association which also supports ECN. Details are clearly
for further study.
8.14 Protocol Design Details
Clearly, not all details of GIMPS operation have been defined so far.
This section provides a list of slightly non-trivial areas where more
detail is need, where these have not been mentioned elsewhere in the
text.
o Processing of the GIMPS hop count and IP TTL needs to be
clarified, especially for messages which are being bypassed
without going through full GIMPS processing.
o Receiver initiated signaling applications need to have reverse
path state set up in the network. Should this be done by GIMPS
carrying out the discovery for the specific signaling application
(which requires the flow sender to know what signaling
applications are going to be used), or should the discovery
attempt to find every GIMPS node and the signaling applications
they support?
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9. Change History
9.1 Changes In Version -01
The major change in version -01 is the elimination of
'intermediaries', i.e. imposing the constraint that signaling
application peers are also GIMPS peers. This has the consequence that
if a signaling application wishes to use two classes of signaling
transport for a given flow, maybe reaching different subsets of
nodes, it must do so by running different signaling sessions; and it
also means that signaling adaptations for passing through NATs which
are not signaling application aware must be carried out in 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 3.3.
2. Stated that nodes which do not implement the signaling
application should bypass the message (Section 4.2).
3. Decoupled the state handling logic for routing state and
messaging association state in Section 4.3. 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 C. 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
it needed.
6. Removed the possibility for fragmentation over DCCP (xref
target='shims'/>), 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 (xref
target='route-changes'/>), including some additional detection
mechanisms and more clearly distinguishing between upstream and
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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 (Section 6.2) to
account for actual FLow-Routing-Information definition, and also
how wildcarding should be allowed and handled.
10. Removed old route recording section (old Section 6.3).
11. Extended the discussion of NAT handling (Section 6.3) 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 7 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 (Section
8.12) and Datagram Mode congestion control (Section 8.13).
14. Added this change history.
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Normative References
[1] Katz, D., "IP Router Alert Option", RFC 2113, February 1997.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[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] 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.
[7] Kohler, E., "Datagram Congestion Control Protocol (DCCP)",
draft-ietf-dccp-spec-05 (work in progress), October 2003.
[8] Stewart, R., "SCTP Partial Reliability Extension",
draft-ietf-tsvwg-prsctp-03 (work in progress), January 2004.
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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] Conta, A. and S. Deering, "Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6)
Specification", RFC 2463, December 1998.
[11] Terzis, A., Krawczyk, J., Wroclawski, J. and L. Zhang, "RSVP
Operation Over IP Tunnels", RFC 2746, January 2000.
[12] Tsirtsis, G. and P. Srisuresh, "Network Address Translation -
Protocol Translation (NAT-PT)", RFC 2766, February 2000.
[13] Berger, L., Gan, D., Swallow, G., Pan, P., Tommasi, F. and S.
Molendini, "RSVP Refresh Overhead Reduction Extensions", RFC
2961, April 2001.
[14] Carpenter, B. and K. Moore, "Connection of IPv6 Domains via
IPv4 Clouds", RFC 3056, February 2001.
[15] Huitema, C., "An Anycast Prefix for 6to4 Relay Routers", RFC
3068, June 2001.
[16] Ramakrishnan, K., Floyd, S. and D. Black, "The Addition of
Explicit Congestion Notification (ECN) to IP", RFC 3168,
September 2001.
[17] Baker, F., Iturralde, C., Le Faucheur, F. and B. Davie,
"Aggregation of RSVP for IPv4 and IPv6 Reservations", RFC 3175,
September 2001.
[18] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[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] Hancock, R., "Next Steps in Signaling: Framework",
draft-ietf-nsis-fw-05 (work in progress), October 2003.
[21] Tschofenig, H. and D. Kroeselberg, "Security Threats for NSIS",
draft-ietf-nsis-threats-03 (work in progress), October 2003.
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[22] Stiemerling, M., "A NAT/Firewall NSIS Signaling Layer Protocol
(NSLP)", draft-ietf-nsis-nslp-natfw-00 (work in progress),
October 2003.
[23] Bosch, S., "NSLP for Quality-of-Service signaling",
draft-ietf-nsis-qos-nslp-01 (work in progress), October 2003.
[24] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms for
IPv6 Hosts and Routers", draft-ietf-v6ops-mech-v2-02 (work in
progress), February 2004.
[25] Nikander, P., "Mobile IP version 6 Route Optimization Security
Design Background", draft-nikander-mobileip-v6-ro-sec-02 (work
in progress), December 2003.
[26] Bound, J., "Dual Stack Transition Mechanism",
draft-bound-dstm-exp-00 (work in progress), August 2003.
Authors' Addresses
Henning Schulzrinne
Columbia University
Department of Computer Science
450 Computer Science Building
New York, NY 10027
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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Appendix A. Acknowledgements
This document is based on the discussions within the IETF NSIS
working group. It has been informed by prior work and formal and
informal inputs from: Cedric Aoun, Attila Bader, Bob Braden, Marcus
Brunner, Xiaoming Fu, Ruediger Geib, Eleanor Hepworth, Georgios
Karagiannis, John Loughney, Jukka Manner, Andrew McDonald, Dave Oran,
Charles Shen, Melinda Shore, Martin Stiemerling, Mike Thomas, Hannes
Tschofenig, Sven van den Bosch, Michael Welzl, and Lars Westberg. In
particular, 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. We look forward to
inputs and comments from many more in the future.
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Appendix B. Example Routing State Table
Figure 7 shows a signaling scenario for a single flow being managed
by two signaling applications. The flow sender and receiver and one
router support both, two other routers support one each.
A B C D E
+------+ +-----+ +-----+ +-----+ +--------+
| Flow | +-+ +-+ |NSLP1| |NSLP1| | | | Flow |
|Sender|====|R|====|R|====|NSLP2|====| |====|NSLP2|====|Receiver|
| | +-+ +-+ |GIMPS| |GIMPS| |GIMPS| | |
+------+ +-----+ +-----+ +-----+ +--------+
------------------------------>>
Flow Direction
Figure 7: A Signaling Scenario
Routing state table at node B:
+-------------+-----------+--------------+-----------+--------------+
| Flow | Session | Signaling | Upstream | Downstream |
| Routing | ID | Application | Peer | Peer |
| Information | | | | |
+-------------+-----------+--------------+-----------+--------------+
| {IP-#A, | 0xABCDEF | NSLP1 | IP-#A | (null) |
| IP-#E, | | | | |
| protocol, | | | | |
| ports} | | | | |
| | | | | |
| {IP-#A, | 0x123456 | NSLP2 | IP-#A | Pointer to |
| IP-#E, | | | | B-D |
| protocol, | | | | messaging |
| ports} | | | | association |
+-------------+-----------+--------------+-----------+--------------+
The table shows the routing state at Node B for the single flow from
A to E. The upstream state is just the same address for each
application. For the downstream case, NSLP1 only requires datagram
mode messages and so no explicit routing state towards C is needed.
NSLP2 requires a messaging association for its messages towards node
D, and node C does not process NSLP2 at all, so the downstream peer
state for NSLP2 is a pointer to a messaging association that runs
directly from B to D. Note that E is not visible in the state table
(except implicitly in the address in the flow routing information);
routing state is stored only for adjacent peers.
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Appendix C. Information Element Formats
This appendix provides initial formats for the various component
parts of the GIMPS messages defined abstractly in Section 5.2. It
should be noted that these formats are extremely preliminary and
should be expected to change completely several times during the
further development of this specification.
C.1 The Common Header
This header precedes all GIMPS 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 | GIMPS hops | Number of TLVs |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Signalling Application ID |D|R| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The flags are:
D - Direction (Set for "Upstream", Unset for "Downstream")
R - Response requested
C.2 TLV Objects
C.2.1 Standard Object Header
Each object begins with a fixed header giving the object type and
object length. A later version of this specification will contain
more details on rules for object encodings which enable protocol
extensibility.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In the following object diagrams, '//' is used to indicate a variable
sized field and ':' is used to indicate a field that is optionally
present.
C.2.2 Flow-Routing-Information
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Type: Flow-Routing-Information
Length: Variable (depends on address version and how much upper layer
information is included)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version |P|T|F|S|O| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Source Address //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Destination Address //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Prefix | Dest Prefix | Protocol | Traffic Class |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Reserved : Flow Label :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: SPI :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Source Port : Destination Port :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The flags are:
P - Protocol
T - Traffic Class
F - Flow Label
S - SPI
O - Source/Destination Ports
C.2.3 Session Identification
Type: Session-Identification
Length: Fixed (TBD 128 bits)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Session ID +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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C.2.4 Node Addressing
Type: Node-Addressing
Length: Variable (depends on detailed format and what optional fields
are present)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// Node Addressing //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
C.2.5 Query Cookie
Type: Query-Cookie
Length: Variable (selected by querying node)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// Query Cookie //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
C.2.6 Responder Cookie
Type: Responder-Cookie
Length: Variable (selected by querying node)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// Responder Cookie //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
C.2.7 Lifetime
Type: Lifetime
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Length: Fixed (TBD 4 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
C.2.8 NSLP Data
Type: NSLP-Data
Length: Variable (depends on NSLP)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// NSLP Data //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Internet-Draft GIMPS February 2004
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Schulzrinne & Hancock Expires August 16, 2004 [Page 63]
Internet-Draft GIMPS February 2004
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Schulzrinne & Hancock Expires August 16, 2004 [Page 64]
