One document matched: draft-ietf-rohc-sigcomp-sip-04.txt
Differences from draft-ietf-rohc-sigcomp-sip-03.txt
Robust Header Compression C. Bormann
Internet-Draft Universitaet Bremen TZI
Expires: May 30, 2007 Z. Liu
Nokia Research Center
R. Price
Cogent Defence and Security
Networks
G. Camarillo
Ericsson
November 26, 2006
Applying Signaling Compression (SigComp) to the Session Initiation
Protocol (SIP)
draft-ietf-rohc-sigcomp-sip-04.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document describes some specifics that apply when Signaling
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Compression (SigComp) is applied to the Session Initiation Protocol
(SIP), such as default minimum values of SigComp parameters,
compartment and state management, and a few issues on SigComp over
TCP. Any implementation of SigComp for use with SIP must conform to
this document, in addition to SigComp and support of the SIP and
Session Description Protocol (SDP) static dictionary.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Minimum Values of SigComp Parameters for SIP/SigComp . . . . . 3
3.1. decompression_memory_size (DMS) for SIP/SigComp . . . . . 4
3.2. state_memory_size (SMS) for SIP/SigComp . . . . . . . . . 4
3.3. cycles_per_bit (CPB) for SIP/SigComp . . . . . . . . . . . 5
3.4. SigComp_version (SV) for SIP/SigComp . . . . . . . . . . . 5
3.5. locally available state (LAS) for SIP/SigComp . . . . . . 5
4. Delimiting SIP Messages and SigComp Messages on the Same
Port . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Continuous Mode over TCP . . . . . . . . . . . . . . . . . . . 6
6. Too Large SIP Messages . . . . . . . . . . . . . . . . . . . . 6
7. SIP Retransmissions . . . . . . . . . . . . . . . . . . . . . 6
8. Compartment and State Management for SIP/SigComp . . . . . . . 7
8.1. Remote Application Identification . . . . . . . . . . . . 7
8.2. Identifier Comparison Rules . . . . . . . . . . . . . . . 10
8.3. Compartment Opening and Closure . . . . . . . . . . . . . 10
8.4. Compartment Valid During a Registration . . . . . . . . . 11
8.5. Lack of a Compartment . . . . . . . . . . . . . . . . . . 12
9. Recommendations for Network Administrators . . . . . . . . . . 12
10. Private Agreements . . . . . . . . . . . . . . . . . . . . . . 13
11. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 13
12. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
13. Security Considerations . . . . . . . . . . . . . . . . . . . 15
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
16.1. Normative References . . . . . . . . . . . . . . . . . . . 17
16.2. Informative References . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . . . . 20
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1. Introduction
SigComp [RFC3320] is a solution for compressing messages generated by
application protocols. Although its primary driver is to compress
SIP [RFC3261] messages, the solution itself has been intentionally
designed to be application agnostic so that it can be applied to any
application protocol; this is denoted as ANY/SigComp. Consequently,
many application dependent specifics are left out of the base
standard. It is intended that a separate specification is used to
describe those specifics when SigComp is applied to a particular
application protocol.
This document binds SigComp and SIP; this is denoted as SIP/SigComp.
Any SigComp implementation that is used for the compression of SIP
messages must conform to this document, as well as to [RFC3320].
Additionally, it must support the SIP/SDP static dictionary as
specified in [RFC3485] and the mechanism for discovering SigComp
support at the SIP layer as specified in [RFC3486].
2. 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 BCP 14 [RFC2119].
3. Minimum Values of SigComp Parameters for SIP/SigComp
In order to support a wide range of capabilities among endpoints
implementing SigComp, SigComp defines a few parameters to describe
SigComp behavior (see section 3.3 of [RFC3320]). For each parameter,
[RFC3320] specifies a minimum value that any SigComp endpoint MUST
support for ANY/SigComp. Those minimum values were determined with
the consideration of all imaginable devices in which SigComp may be
implemented. Scalability was also considered as a key factor.
However, some of the minimum values specified in [RFC3320] are too
small to allow good performance for SIP message compression.
Therefore, they are increased for SIP/SigComp as specified in the
following sections. For completeness, those parameters that are the
same for SIP/SigComp as they are for ANY/SigComp are also listed.
Note: the new minimum values are specific to SIP/SigComp. They do
not apply to any other application protocols.
Note: a SigComp endpoint MAY offer additional resources if available;
these resources can be advertised to remote endpoints as described in
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section 9.4.9 of [RFC3320].
3.1. decompression_memory_size (DMS) for SIP/SigComp
Minimum value for ANY/SigComp: 2048 bytes, as specified in section
3.3.1 of [RFC3320].
Minimum value for SIP/SigComp: 8192 bytes.
Reason: a DMS of 2048 bytes is too small for SIP message compression
as it seriously limits the compression ratio and even makes
compression impossible for certain messages. For example, the
condition set by [RFC3320] for SigComp over UDP means: C + 2*B + R +
2*S + 128 < DMS (each term is described below). On the other hand,
8KB additional memory should not cause any problem for an endpoint
that already implements SIP, SigComp, and applications that use SIP
as DMS is memory only temporarily needed during decompression of a
SigComp message (the memory can be reclaimed when the message has
been decompressed).
C size of compressed application message, depending on R
B size of bytecode. Note: two copies -- one as part of the SigComp
message and one in UDVM (Universal Decompressor Virtual Machine)
memory.
R size of ring buffer in UDVM memory
S any additional state uploaded other than that created from the
content of the ring buffer at the end of decompression (similar to
B, two copies of S are needed)
128 the smallest address in UDVM memory to copy bytecode to
3.2. state_memory_size (SMS) for SIP/SigComp
Minimum value for ANY/SigComp: 0 (zero) bytes, as specified in
section 3.3.1 of [RFC3320].
Minimum value for SIP/SigComp: 2048 bytes.
Reason: a non-zero SMS allows an endpoint to upload a state in the
first SIP message sent to a remote endpoint without the uncertainty
of whether or not it can be created in the remote endpoint. A non-
zero SMS obviously requires the SIP/SigComp implementation to keep
state. Based on the observation that there is little gain from
stateless SigComp compression, the assumption is that purely
stateless SIP implementations are unlikely to provide a SigComp
function. Stateful implementations should have little problem to
keep 2K additional state for each compartment (see Section 8).
Note: SMS is a parameter that applies to each individual compartment.
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An endpoint MAY offer different SMS values for different compartments
as long as the SMS value is not less than 2048 bytes.
3.3. cycles_per_bit (CPB) for SIP/SigComp
Minimum value for ANY/SigComp: 16, as specified in section 3.3.1 of
[RFC3320].
Minimum value for SIP/SigComp: 16 (same as above)
3.4. SigComp_version (SV) for SIP/SigComp
For ANY/SigComp: 0x01, as specified in section 3.3.2 of [RFC3320].
For SIP/SigComp: >= 0x02 (at least SigComp + NACK)
3.5. locally available state (LAS) for SIP/SigComp
Minimum LAS for ANY/SigComp: none, see section 3.3.3 of [RFC3320].
Minimum LAS for SIP/SigComp: the SIP/SDP static dictionary as defined
in [RFC3485].
4. Delimiting SIP Messages and SigComp Messages on the Same Port
In order to limit the number of ports required by a SigComp-aware
endpoint, it is possible to allow both SigComp messages and 'vanilla'
SIP messages (i.e. uncompressed SIP messages with no SigComp header)
to arrive on the same port.
For a message-based transport such as UDP or SCTP, this can be done
per message. The receiving endpoint checks the first octet of the
UDP/SCTP payload to determine whether the message has been compressed
using SigComp. If the MSBs (Most Significant Bits) of the octet are
"11111" then the message is considered to be a SigComp message and is
parsed as per [RFC3320]. If the MSBs of the octet take any other
value, then the message is assumed to be an uncompressed SIP message,
and is passed directly to the application with no further effect on
the SigComp layer.
For a stream-based transport such as TCP, the distinction is per
connection. The receiving endpoint checks the first octet of the TCP
data stream to determine whether the stream has been compressed using
SigComp. If the MSBs of the octet are "11111" then the stream is
considered to contain SigComp messages and is parsed as per
[RFC3320]. If the MSBs of the octet take any other value, then the
stream is assumed to contain uncompressed SIP messages, and is passed
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directly to the application with no further effect on the SigComp
layer. Note that SigComp message delimiters MUST NOT be used if the
stream contains uncompressed SIP messages.
Applications MUST NOT mix SIP messages and SigComp messages on a
single TCP connection. If the TCP connection is used to carry
SigComp messages then all messages sent over the connection MUST have
a SigComp header and be delimited by the use of 0xFFFF as described
in [RFC3320].
[I-D.ietf-rohc-sigcomp-impl-guide] shows how to send uncompressed
messages in a SigComp structured TCP connection using a "well-known
shim header". Should it for any reason not be desirable to set up
more than one TCP connection to a SIP implementation, but the
flexibility to send both compressed and uncompressed SIP messages be
required, the compressor can set up a SigComp structured connection
and send any uncompressed SIP messages using the well-known shim
header.
5. Continuous Mode over TCP
Continuous Mode is a special feature of SigComp, which is designed to
improve the overall compression ratio for long-lived connections.
Its use requires pre-agreement between the SigComp compressor and
decompressor. Continuous mode is not used with SIP/SigComp.
Reason: continuous mode requires the transport itself to provide a
certain level of protection against denial of service attacks. TCP
alone is not considered to provide enough protection.
6. Too Large SIP Messages
SigComp does not support the compression of messages larger than 64k.
Therefore, if a SIP application sending compressed SIP messages to
another SIP application over a transport connection (e.g., a TCP
connection) needs to send a SIP message larger than 64k, the SIP
application SHOULD establish a new transport connection and send the
(uncompressed) SIP message over the new connection.
7. SIP Retransmissions
SIP retransmissions need to be compressed again before being sent.
That is, SIP applications MUST NOT retransmit already-compressed
information.
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The reason for this behavior is that it is impossible to know whether
the failure causing the retransmission occurred to the message being
retransmitted or to the response to that message. If the loss
occurred to the response, any state changes effected by the first
instance of the retransmitted message would already have taken place.
If these state changes removed a state that the previously-
transmitted message relied upon, then retransmission of the same
compressed message would lead to a decompression failure.
8. Compartment and State Management for SIP/SigComp
An application exchanging compressed traffic with a remote
application has a compartment that contains state information needed
to compress outgoing messages and to decompress incoming messages.
To increase the compression efficiency, the application must assign
distinct compartments to distinct remote applications.
8.1. Remote Application Identification
SIP/SigComp applications identify remote applications by their SIP/
SigComp identifiers. Each SIP/SigComp application MUST have a SIP/
SigComp identifier URN (Uniform Resource Name) that uniquely
identifies the application. Usage of a URN provides a persistent and
unique name for the SIP/SigComp identifier. It also provides an easy
way to guarantee uniqueness. This URN MUST be persistent as long as
the application stores compartment state related to other SIP/SigComp
applications.
A SIP/Sigcomp application SHOULD use a UUID (Universally Unique
IDentifier) URN as its SIP/SigComp identifier. The UUID URN
[RFC4122] allows for non-centralized computation of a URN based on
time, unique names (such as a MAC address), or a random number
generator. If a URN scheme other than UUID is used, the URN MUST be
selected such that the application can be certain that no other SIP/
SigComp application would choose the same URN value.
Note that the definition of SIP/SigComp identifier is similar to the
definition of instance identifier in [I-D.ietf-sip-outbound]. One
difference is that instance identifiers are only required to be
unique within their AoR (Address of Record) while SIP/SigComp
identifiers are required to be globally unique.
Even if instance identifiers are only required to be unique within
their AoR, devices may choose to generate globally unique instance
identifiers. A device with a globally unique instance identifier
SHOULD use its instance identifier as its SIP/SigComp identifier.
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Using the same value for an entity's instance and SIP/SigComp
identifiers improves the compression ratio of header fields that
carry both identifiers (e.g., a Contact header field in a REGISTER
request).
Server farms that share SIP/SigComp state across servers MUST use the
same SIP/SigComp identifier for all their servers.
SIP/SigComp identifiers are carried in the 'sigcomp-id' SIP URI
(Uniform Resource Identifier) or Via header field parameter. The
'sigcomp-id' SIP URI parameter is a 'uri-parameter', as defined by
the SIP ABNF (Augmented Backus-Naur Form, Section 25.1 of [RFC3261]).
The following is its ABNF [RFC4234]:
uri-sip-sigcomp-id = "sigcomp-id=" 1*paramchar
The SIP URI 'sigcomp-id' parameter MUST contain a URN [RFC2141].
The Via 'sigcomp-id' parameter is a 'via-extension', as defined by
the SIP ABNF (Section 25.1 of [RFC3261]). The following is its ABNF
[RFC4234]:
via-sip-sigcomp-id = "sigcomp-id" EQUAL
LDQUOT *( qdtext / quoted-pair ) RDQUOT
The Via 'sigcomp-id' parameter MUST contain a URN [RFC2141].
The following is an example of a Via header field with a 'sigcomp-id'
parameter:
Via: SIP/2.0/UDP server1.example.com:5060
;branch=z9hG4bK87a7
;comp=sigcomp
;sigcomp-id="urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128"
Note that some characters that are allowed to appear in a Via header
field parameter, such as ':' (colon), are not allowed to appear in a
SIP URI parameter. Those characters need to be escaped when they
appear in a SIP URI parameter.
The need to escape characters in parameters could be avoided by
defining Contact, Route, Record-Route, Path, and Service-Route
header field 'sigcomp-id' parameters instead of the 'sigcomp-id'
SIP URI parameter. For example, instance identifiers typically
appear in '+sip.instance' Contact header field parameters, and not
in SIP URI parameters. We have chosen to define 'sigcomp-id' as a
SIP URI parameter to be consistent with the use of the already-in-
use 'comp=sigcomp' parameter, which is a SIP URI parameter as
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well.
The following is an example of a 'sigcomp-id' SIP URI parameter:
sigcomp-id=urn%3auuid%3a0C67446E-F1A1-11D9-94D3-000A95A0E128
SIP messages are matched with remote application identifiers as
follows.
Outgoing requests: the remote application identifier is the SIP/
SigComp identifier of the URI to which the request is sent. If
the URI does not contain a SIP/SigComp identifier, the remote
application identifier is the IP address plus port of the datagram
carrying the request for connection-less transport protocols, and
the transport connection (e.g., a TCP connection) carrying the
request for connection-oriented transport protocols (this is to
support legacy SIP/SigComp applications).
Incoming responses: the remote application identifier is the same as
the one of the previously-sent request that initiated the
transaction the response belongs to.
Incoming requests: the remote application identifier is the SIP/
SigComp identifier of the top-most Via entry. If the Via header
field does not contain a SIP/SigComp identifier, the remote
application identifier is the source IP address plus port of the
datagram carrying the request for connection-less transport
protocols, and the transport connection (e.g., a TCP connection)
carrying the request for connection-oriented transport protocols
(this is to support legacy SIP/SigComp applications).
Outgoing responses: the remote application identifier is the same as
the previously-received request that initiated the transaction the
response belongs to. Note that, due to standard SIP Via header
field processing, this identifier will be present in the top-most
Via entry in such responses (as long as it was present in the top-
most Via entry of the previously-received request).
A SIP/SigComp application placing its URI with the 'comp=sigcomp'
parameter in a header field MUST add a 'sigcomp-id' parameter with
its SIP/SigComp identifier to that URI.
A SIP/SigComp application generating its own Via entry containing the
'comp=sigcomp' parameter MUST add a 'sigcomp-id' parameter with its
SIP/SigComp identifier to that Via entry.
A given remote application identifier is mapped to a particular
SigComp compartment ID following the rules given in Section 8.3 and
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Section 8.4.
8.2. Identifier Comparison Rules
Equality comparisons between SIP/SigComp identifiers are performed
using the rules for URN equality that are specific to the scheme in
the URN. If the element performing the comparisons does not
understand the URN scheme, it performs the comparisons using the
lexical equality rules defined in RFC 2141 [RFC2141]. Lexical
equality may result in two URNs being considered unequal when they
are actually equal. In this specific usage of URNs, the only element
which provides the URN is the SIP/SigComp application identified by
that URN. As a result, the SIP/SigComp application SHOULD provide
lexically equivalent URNs in each registration it generates. This is
likely to be normal behavior in any case; applications are not likely
to modify the value of their SIP/SigComp identifiers so that they
remain functionally equivalent yet lexigraphically different from
previous identifiers.
8.3. Compartment Opening and Closure
SIP applications need to know when to open a new compartment and when
to close it. The lifetime of SIP/SigComp compartments is linked to
registration state. Compartments are opened at SIP registration time
and are typically closed when the registration expires or is
canceled.
Previous revisions of this document also defined compartments
valid during a SIP transaction or a SIP dialog. It was decided to
eliminate those types of compartments because the complexity they
introduced was higher than the benefits they brought in most
deployment scenarios.
Usually, any states created during the lifetime of a compartment will
be "logically" deleted when the compartment is closed. As described
in section 6.2 of [RFC3320], a logical deletion can become a physical
deletion only when no compartment continues to exist that created the
(same) state.
A SigComp endpoint may offer to keep a state created upon request
from a SigComp peer endpoint beyond the default lifetime of a
compartment (i.e., beyond the duration of its associated
registration). This may be used to improve compression efficiency of
subsequent SIP messages generated by the same remote application at
the SigComp peer endpoint. To indicate that such state will continue
to be available, the SigComp endpoint can inform its peer SigComp
endpoint by announcing the (partial) state ID in the returned SigComp
parameters at the end of the registration, dialog, or transaction
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that was supposed to limit the lifetime of the SigComp state. That
signals the state will be maintained. The mandatory support for the
SigComp Negative Acknowledgement (NACK) Mechanism [RFC4077] in SIP/
SigComp ensures that it is possible to recover from synchronization
errors regarding comparment lifetimes.
As an operational concern, bugs in the compartment management
implementation are likely to lead to sporadic, hard to diagnose
failures. Decompressors may therefore want to cache old state and,
if still available, allow access while logging diagnostic
information. Both compressors and decompressors use the SigComp
Negative Acknowledgement (NACK) Mechanism [RFC4077] to recover from
situations where such old state may no longer be available.
8.4. Compartment Valid During a Registration
A REGISTER transaction causes an application to open a new
compartment to be valid for the duration of the registration
established by the REGISTER transaction.
A SIP application that needs to send a compressed SIP REGISTER (i.e.,
a user agent generating a REGISTER or a proxy server relaying one to
its next hop) SHOULD open a compartment for the request's remote
application identifier. A SIP application that receives a compressed
SIP REGISTER (i.e., the registrar or a proxy relaying the REGISTER to
its next-hop) SHOULD open a compartment for the request's remote
application identifier.
These compartments MAY be closed if the REGISTER request is responded
with a non-2xx final response, or when the registration expires or is
canceled. However, applications MAY also choose to keep these
compartments open for a longer period of time, as discussed
previously. For a given successful registration, applications SHOULD
NOT close their associated compartments until the registration is
over.
A SIP network can be configured so that regular SIP traffic to and
from a user agent traverses a different set of proxies than the
initial REGISTER transaction. The path the REGISTER transaction
follows is typically determined by configuration data. The path
subsequent requests traverse is determined by the Path [RFC3327]
and the Service-Route [RFC3308] header fields in the REGISTER
transaction and by the Record-Route and the Route header fields in
dialog-creating transactions. Previous revisions of this document
supported the use of different paths for different types of
traffic. However, for simplicity reasons, this document now
assumes that networks using compression are configured so that
subsequent requests follow the same path as the initial REGISTER
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transaction. Section 9 provides network administrators with
recommendations so that they configure they networks properly.
If following the previous rules, a SIP application is supposed to
open a compartment for a remote application identifier for which it
already has a compartment, the SIP application MUST use the already
existing compartment. That is, the SIP application MUST NOT open a
new compartment.
8.5. Lack of a Compartment
The use of stateless compression (i.e., compression without a
compartment) is not typically worthwhile and may even result in
message expansion. Therefore, if a SIP application does not have a
compartment for a message it needs to send, it SHOULD NOT compress it
even in the presence of the comp=sigcomp parameter. Note that RFC
3486 [RFC3486] states the following:
"If the next-hop URI contains the parameter comp=sigcomp, the
client SHOULD compress the request using SigComp"
Experience since RFC 3486 [RFC3486] was written has shown that
stateless compression is not worthwhile. That is why now it is not
recommended to use it any longer.
9. Recommendations for Network Administrators
Network administrators can configure their networks so that the
compression efficiency achieved is increased. The following
recommendations help network administrators perform their task.
For a given user agent, the route sets for incoming requests (created
by a Path header field) and for outgoing requests (created by a
Service-Route header field) are typically the same. However,
registrars can, if they wish, insert proxies in the latter route that
do not appear in the former route and vice versa. It is RECOMMENDED
that registrars are configured so that proxies performing SigComp
compression appear in both routes.
The routes described previously apply to requests sent outside a
dialog. Requests inside a dialog follow a route constructed using
Record-Route header fields. It is RECOMMENDED that the proxies
performing SigComp that are in the route for requests outside a
dialog are configured to place themselves (by inserting themselves in
the Record-Route header fields) in the routes used for requests
inside dialogs.
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When a user agent's registration expires, proxy servers performing
compression may close their associated SIP/SigComp compartment. If
the user agent is involved in a dialog that was established before
the registration expired, subsequent requests within the dialog may
not be compressed any longer. In order to avoid this situation, it
is RECOMMENDED that user agents are registered as long as they are
involved in a dialog.
10. Private Agreements
SIP/SigComp implementations that are subject to private agreements
MAY deviate from this specification, if the private agreements
unambiguously specify so. Plausible candidates for such deviations
include:
o Minimum values (Section 3).
o Use of continuous mode (Section 5).
o Compartment definition (Section 8).
11. Backwards Compatibility
SigComp has a number of parameters that can be configured per
endpoint. This document specifies a profile for SigComp when used
for SIP compression that further constrains the range that some of
these parameters may take. Examples of this are Decompressor Memory
Size, State Memory Size, and SigComp Version (support for NACK).
Additionally, this document specifies how SIP/SigComp applications
should perform compartment mapping.
When this document was written, there already were a few existing
SIP/SigComp deployments. The rules in this document have been
designed to maximize interoperability with those legacy SIP/SigComp
implementations. Nevertheless, implementers should be aware that
legacy SIP/SigComp implementations may not conform to this
specification. Examples of problems with legacy applications would
be smaller DMS than mandated in this document, lack of NACK support,
or a different comparment mapping.
12. Example
Figure 1 shows an example message flow where the user agent and the
outbound proxy exchange compressed SIP traffic. Compressed messages
are marked with a (c).
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User Agent Outbound Proxy Registrar
|(1) REGISTER (c) | |
|---------------->| |
| |(2) REGISTER |
| |---------------->|
| |(3) 200 OK |
| |<----------------|
|(4) 200 OK (c) | |
|<----------------| |
|(5) INVITE (c) | |
|---------------->| |
| |(6) INVITE |
| |------------------------------>
| |(7) 200 OK |
| |<------------------------------
|(8) 200 OK (c) | |
|<----------------| |
|(9) ACK (c) | |
|---------------->| |
| |(10) ACK |
| |------------------------------>
|(11) BYE (c) | |
|---------------->| |
| |(12) BYE |
| |------------------------------>
| |(13) 200 OK |
| |<------------------------------
|(14) 200 OK (c) | |
|<----------------| |
Figure 1: Example message flow
The user agent in Figure 1 is initialy configured (e.g., using the
SIP configuration framework [I-D.ietf-sipping-config-framework]) with
the URI of its outbound proxy. That URI contains the outbound's
proxy SIP/SigComp identifier, referred to as 'Outbound-id', in a
'sigcomp-id' parameter.
When the user agent sends an initial REGISTER request (1) to the
outbound proxy's URI, the user agent opens a new compartment for
'Outbound-id'. This compartment will be valid, at least, for the
duration of the registration.
On receiving this REGISTER request (1), the outbound proxy opens a
new compartment for the SIP/SigComp identifier that appears in the
'sigcomp-id' parameter of the top-most Via entry. This identifier,
which is the user agent's SIP/SigComp identifier, is referred to as
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'UA-id'. The compartment opened by the outbound proxy will be valid,
at least, for the duration of the registration. The outbound proxy
adds Path header field with its own URI, which contains the
'Outbound-id' SIP/SigComp identifier, to the REGISTER request and
relays it to the registrar (2).
When the registrar receives the REGISTER request (2), it constructs
the route future incoming requests (to the user agent) will follow
using the Contact and the Path header fields. Future incoming
requests will traverse the outbound proxy before reaching the user
agent.
The registrar also constructs the route future outgoing requests
(from the user agent) and places it in a Service-Route header field
in a 200 (OK) response (3). Future outgoing requests will always
traverse the outbound proxy. The registrar has ensured that the
outbound proxy performing compression handles both incoming and
outgoing requests.
When the outbound proxy receives a 200 (OK) response (3), it inspects
the top-most Via entry. This entry's SIP/SigComp identifier 'UA-id'
matches that of the compartment created before. Therefore, the
outbound proxy uses that compartment to compress it and relay it to
the user agent.
On receiving the 200 (OK) response (4), the user agent stores the
Service-Route header field in order to use it to send future outgoing
requests. The Service-Route header field contains the outbound
proxy's URI, which contains the 'Outbound-id' SIP/SigComp identifier.
At a later point, the user agent needs to send an INVITE request (5).
According to the Service-Route header field received previously, the
user agent sends the INVITE request (5) to the outbound proxy's URI.
Since this URI's SIP/SigComp identifier 'Outbound-id' matches that of
the compartment created before, this compartment is used to compress
the INVITE request.
On receiving the INVITE request (5), the outbound proxy Record Routes
and relays the INVITE request (6) forward. The outbound proxy Record
Routes to ensure that all SIP messages related to this new dialog are
routed through the outbound proxy.
Finally the dialog is terminated by a BYE transaction (11) that also
traverses the outbound proxy.
13. Security Considerations
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The same security considerations as described in [RFC3320] apply to
this document. Note that keeping SigComp states longer than the
duration of a SIP dialog should not pose new security risks for two
reasons: a) the state has been allowed to be created in the first
place; and b) this is on voluntary basis and a SigComp endpoint can
choose not to offer it.
14. IANA Considerations
The IANA is requested to register the 'sigcomp-id' Via header field
parameter, which is defined in Section 8.1, under the Header Field
Parameters and Parameter Values subregistry within the SIP Parameters
registry:
Predefined
Header Field Parameter Name Values Reference
---------------------------- --------------- --------- ---------
Via sigcomp-id No [RFCxxxx]
The IANA is requested to register the 'sigcomp-id' SIP URI parameter,
which is defined in Section 8.1, under the SIP/SIPS URI Parameters
subregistry within the SIP Parameters registry:
Parameter Name Predefined Values Reference
-------------- ----------------- ---------
sigcomp-id No [RFCxxxx]
Note to the RFC Editor: please, substitute RFCxxxx with the RFC
number this document will get.
15. Acknowledgements
The authors would like to thank the following people for their
comments and suggestions: Jan Christoffersson, Joerg Ott, Mark West,
Pekka Pessi, Robert Sugar, Jonathan Rosenberg, and Robert Sparks.
Abigail Surtees and Adam Roach performed thorough reviews of this
document.
16. References
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16.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997.
[RFC3261] 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.
[RFC3308] Calhoun, P., Luo, W., McPherson, D., and K. Peirce, "Layer
Two Tunneling Protocol (L2TP) Differentiated Services
Extension", RFC 3308, November 2002.
[RFC3320] Price, R., Bormann, C., Christoffersson, J., Hannu, H.,
Liu, Z., and J. Rosenberg, "Signaling Compression
(SigComp)", RFC 3320, January 2003.
[RFC3327] Willis, D. and B. Hoeneisen, "Session Initiation Protocol
(SIP) Extension Header Field for Registering Non-Adjacent
Contacts", RFC 3327, December 2002.
[RFC3485] Garcia-Martin, M., Bormann, C., Ott, J., Price, R., and A.
Roach, "The Session Initiation Protocol (SIP) and Session
Description Protocol (SDP) Static Dictionary for Signaling
Compression (SigComp)", RFC 3485, February 2003.
[RFC3486] Camarillo, G., "Compressing the Session Initiation
Protocol (SIP)", RFC 3486, February 2003.
[RFC4077] Roach, A., "A Negative Acknowledgement Mechanism for
Signaling Compression", RFC 4077, May 2005.
[RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122,
July 2005.
[RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[I-D.ietf-sip-outbound]
Jennings, C. and R. Mahy, "Managing Client Initiated
Connections in the Session Initiation Protocol (SIP)",
draft-ietf-sip-outbound-04 (work in progress), June 2006.
[I-D.ietf-rohc-sigcomp-impl-guide]
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Surtees, A., "Implementer's Guide for SigComp",
draft-ietf-rohc-sigcomp-impl-guide-06 (work in progress),
March 2006.
16.2. Informative References
[I-D.ietf-sipping-config-framework]
Petrie, D., "A Framework for Session Initiation Protocol
User Agent Profile Delivery",
draft-ietf-sipping-config-framework-08 (work in progress),
March 2006.
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Authors' Addresses
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28334
Germany
Phone: +49 421 218 7024
Fax: +49 421 218 7000
Email: cabo@tzi.org
Zhigang Liu
Nokia Research Center
6000 Connection Drive
Irving, TX 75039
USA
Phone: +1 972 894-5935
Email: zhigang.c.liu@nokia.com
Richard Price
Cogent Defence and Security Networks
Queensway Meadows Industrial Estate
Meadows Road
Newport, Gwent NP19 4SS
Phone: +44 (0)1794 833681
Email: richard.price@cogent-dsn.com
URI: http://www.cogent-dsn.com
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: Gonzalo.Camarillo@ericsson.com
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