Network Working Group C. Bormann
Request for Comments: 5049 Universitaet Bremen TZI
Category: Standards Track Z. Liu
Nokia Research Center
R. Price
EADS Defence and Security Systems Limited
G. Camarillo, Ed.
Ericsson
December 2007
Applying Signaling Compression (SigComp)
to the Session Initiation Protocol (SIP)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This document describes some specifics that apply when Signaling
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 and SigComp, and in addition, support the SIP and
Session Description Protocol (SDP) static dictionary.
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RFC 5049 Applying SigComp to SIP December 2007
Table of Contents
1. Introduction ....................................................3
2. Terminology .....................................................3
3. Compliance with This Specification ..............................3
4. Minimum Values of SigComp Parameters for SIP/SigComp ............3
4.1. decompression_memory_size (DMS) for SIP/SigComp ............4
4.2. state_memory_size (SMS) for SIP/SigComp ....................4
4.3. cycles_per_bit (CPB) for SIP/SigComp .......................5
4.4. SigComp_version (SV) for SIP/SigComp .......................5
4.5. locally available state (LAS) for SIP/SigComp ..............5
5. Delimiting SIP Messages and SigComp Messages on the Same Port ...5
6. Continuous Mode over TCP ........................................6
7. Too-Large SIP Messages ..........................................7
8. SIP Retransmissions .............................................7
9. Compartment and State Management for SIP/SigComp ................7
9.1. Remote Application Identification ..........................8
9.2. Identifier Comparison Rules ...............................10
9.3. Compartment Opening and Closure ...........................11
9.4. Lack of a Compartment .....................................13
10. Recommendations for Network Administrators ....................13
11. Private Agreements ............................................14
12. Backwards Compatibility .......................................14
13. Interactions with Transport Layer Security (TLS) ..............14
14. Example .......................................................15
15. Security Considerations .......................................17
16. IANA Considerations ...........................................17
17. Acknowledgements ..............................................17
18. References ....................................................18
18.1. Normative References .....................................18
18.2. Informative References ...................................19
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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 be 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.
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 RFC 2119 [RFC2119].
3. Compliance with This Specification
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].
4. 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.
The new minimum values are specific to SIP/SigComp and, thus, do not
apply to any other application protocols. A SIP/SigComp endpoint MAY
offer additional resources over and above the minimum values
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specified in this document if available; these resources can be
advertised to remote endpoints as described in Section 9.4.9 of
[RFC3320].
4.1. 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). Therefore, if DMS is
too small, at least one of C, B, R, or S will be severely restricted.
On the other hand, DMS is memory that is only temporarily needed
during decompression of a SigComp message (the memory can be
reclaimed when the message has been decompressed). Therefore, a
requirement of 8 KB should not cause any problems for an endpoint
that already implements SIP, SigComp, and applications that use SIP.
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 circular buffer in UDVM memory
S any additional state uploaded other than that created from the
content of the circular 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
4.2. 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 the remote endpoint will have enough memory to store such
a state. 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
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SigComp function. Stateful implementations should have little
problem to keep 2K additional state for each compartment (see Section
9).
Note: SMS is a parameter that applies to each individual compartment.
An endpoint MAY offer different SMS values for different compartments
as long as the SMS value is not less than 2048 bytes.
4.3. 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).
4.4. 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).
Note that this implies that the provisions of [RFC4077] apply. That
is, decompression failures result in SigComp NACK messages sent back
to the originating compressor. It also implies that the compressor
need not make use of the methods detailed in Section 2.4 of [RFC4077]
(Detecting Support for NACK); for example, it can use optimistic
compression methods right from the outset.
4.5. 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].
Note that, since support for the static SIP/SDP dictionary is
mandatory, it does not need to be advertised.
5. 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 Stream Control
Transmission Protocol (SCTP), distinguishing between SigComp and
non-SigComp messages can be done per message. The receiving endpoint
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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 it is passed directly
to the application with no further effect on the SigComp layer.
For a stream-based transport such as TCP, distinguishing between
SigComp and non-SigComp messages has to be done 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 it is passed 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].
Section 11 of [RFC4896] details a simple set of bytecodes, intended
to be "well-known", that implement a null decompression algorithm.
These bytecodes effectively allow SigComp peers to send selected
SigComp messages with uncompressed data. If a SIP implementation has
reason to send both compressed and uncompressed SIP messages on a
single TCP connection, the compressor can be instructed to use these
bytecodes to send uncompressed SIP messages that are also valid
SigComp messages.
6. 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.
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7. 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 MUST NOT send the message over the same TCP connection.
The SIP application SHOULD send the message over a different
transport connection (to do this, the SIP application may need to
establish a new transport connection).
8. SIP Retransmissions
When SIP messages are retransmitted, they need to be re-compressed,
taking into account any SigComp states that may have been created or
invalidated since the previous transmission. Implementations MUST
NOT cache the result of compressing the message and retransmit such a
cached result.
The reason for this behavior is that it is impossible to know whether
the failure causing the retransmission occurred on the message being
retransmitted or on the response to that message. If the response
was lost, 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.
Note that a SIP retransmission may be caused by the original message
or its response being lost by a decompression failure. In this case,
a NACK will have been sent by the decompressor to the compressor,
which may use the information in this NACK message to adjust its
compression parameters. Note that, on an unreliable transport, such
a NACK message may still be lost, so if a compressor used some form
of optimistic compression, it MAY want to switch to a method less
likely to cause any form of decompression failure when compressing a
SIP retransmission.
9. 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.
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9.1. 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, due to the
difficulties in equality comparisons for other kinds of URNs. The
UUID URN [RFC4122] allows for non-centralized computation of a URN
based on time, unique names (such as a Media Access Control (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 [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.
Note: 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
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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 'sigcomp-id' SIP URI parameter:
sigcomp-id=urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128
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"
The following is an example of a REGISTER request that carries
'sigcomp-id' parameters in a Via entry and in the Contact header
field. Additionally, it also carries a '+sip.instance' Contact
header field parameter.
REGISTER sip:example.net SIP/2.0
Via: SIP/2.0/UDP 192.0.2.247:2078;branch=z9hG4bK-et736vsjirav;
rport;sigcomp-id="urn:uuid:2e5fdc76-00be-4314-8202-1116fa82a473"
From: "Joe User" <sip:2145550500@example.net>;tag=6to4gh7t5j
To: "Joe User" <sip:2145550500@example.net>
Call-ID: 3c26700c1adb-lu1lz5ri5orr
CSeq: 215196 REGISTER
Max-Forwards: 70
Contact: <sip:2145550500@192.0.2.247:2078;
sigcomp-id=urn:uuid:2e5fdc76-00be-4314-8202-1116fa82a473>;
q=1.0; expires=3600;
+sip.instance="<urn:uuid:2e5fdc76-00be-4314-8202-1116fa82a473>"
Content-Length: 0
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
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application identifier is the IP address plus port of the datagram
carrying the request for connectionless 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
that of the previously sent request that initiated the transaction
to which the response belongs.
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 connectionless 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
that of the previously received request that initiated the
transaction to which the response belongs. 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 9.3.
9.2. 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
that provides the URN is the SIP/SigComp application identified by
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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 lexicographically different from
previous identifiers.
9.3. 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.
Note: Linking the lifetime of SIP/SigComp compartments to
registration state limits the applicability of this specification.
In particular, SIP user agents that do not register but, for
example, only handle PUBLISH or SUBSCRIBE/NOTIFY transactions are
not able create SIP/SigComp compartments following this
specification. Previous revisions of this specification also
defined compartments valid during a SIP transaction or a SIP
dialog. Those compartments covered all possible SIP entities,
including those that do not handle REGISTER transactions.
However, it was decided to eliminate those types of compartments
because the complexity they introduced (e.g., edge proxy servers
were required to keep dialog state) 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 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
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Acknowledgement (NACK) Mechanism [RFC4077] in SIP/SigComp ensures
that it is possible to recover from synchronization errors regarding
compartment 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.
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.
Note: 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 will be configured so
that subsequent requests follow the same path as the initial
REGISTER transaction in order to achieve the best possible
compression. Section 10 provides network administrators with
recommendations so that they can configure the networks properly.
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If, following the rules above, a SIP application is supposed to open
a compartment for a remote application identifier for which it
already has a compartment (e.g., the SIP application registers
towards a second registrar using the same edge proxy server as for
its registration towards its first registrar), the SIP application
MUST use the already existing compartment. That is, the SIP
application MUST NOT open a new compartment.
9.4. 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 MAY choose not to
compress it even in the presence of the 'comp=sigcomp' parameter.
Section 5 describes how a SIP application can send compressed and
uncompressed messages over the same TCP connection. 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, in most cases, not worthwhile. That is why
it is not recommended to use it any longer.
10. 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.
11. 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 4).
o Use of continuous mode (Section 6).
o Compartment definition (Section 9).
12. 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 were already 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 compartment mapping.
13. Interactions with Transport Layer Security (TLS)
Endpoints exchanging SIP traffic over a TLS [RFC4346] connection can
use the compression provided by TLS. Two endpoints exchanging SIP/
SigComp traffic over a TLS connection that provides compression need
to first compress the SIP messages using SigComp and then pass them
to the TLS layer, which will compress them again. When receiving
data, the processing order is reversed.
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However, compressing messages this way twice does not typically bring
significant gains. Once a message is compressed using SigComp, TLS
is not usually able to compress it further. Therefore, TLS will
normally only be able to compress SigComp code sent between
compressor and decompressor. Since the gain of having SigComp code
compressed should be minimal in most cases, it is NOT RECOMMENDED to
use TLS compression when SigComp compression is being used.
14. 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).
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
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The user agent in Figure 1 is initially configured (e.g., using the
SIP configuration framework [CONFIG]) with the URI of its outbound
proxy. That URI contains the outbound proxy's 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 for the duration of
the registration, at least.
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
'UA-id'. The compartment opened by the outbound proxy will be valid
for the duration of the registration, at least. The outbound proxy
adds a 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) will follow 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.
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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.
15. Security Considerations
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 because
the state has been allowed to be created in the first place.
16. IANA Considerations
The IANA has registered the 'sigcomp-id' Via header field parameter,
which is defined in Section 9.1, 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 [RFC5049]
The IANA has registered the 'sigcomp-id' SIP URI parameter, which is
defined in Section 9.1, under the SIP/SIPS URI Parameters subregistry
within the SIP Parameters registry:
Parameter Name Predefined Values Reference
-------------- ----------------- ---------
sigcomp-id No [RFC5049]
17. 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, Robert Sparks, Juergen
Schoenwaelder, and Tuukka Karvonen. Abigail Surtees and Adam Roach
performed thorough reviews of this document.
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18. References
18.1. 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.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
Bormann, et al. Standards Track [Page 18]
RFC 5049 Applying SigComp to SIP December 2007
[RFC4896] Surtees, A., West, M., and A. Roach, "Signaling
Compression (SigComp) Corrections and Clarifications", RFC
4896, June 2007.
18.2. Informative References
[CONFIG] Petrie, D. and S. Channabasappa, "A Framework for Session
Initiation Protocol User Agent Profile Delivery", Work in
Progress, June 2007.
[OUTBOUND] Jennings, C. and R. Mahy, "Managing Client Initiated
Connections in the Session Initiation Protocol (SIP)",
Work in Progress, March 2007.
Bormann, et al. Standards Track [Page 19]
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Authors' Addresses
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28334
Germany
Phone: +49 421 218 63921
Fax: +49 421 218 7000
EMail: cabo@tzi.org
Zhigang Liu
Nokia Research Center
955 Page Mill Road
Palo Alto, CA 94304
USA
Phone: +1 650 796 4578
EMail: zhigang.c.liu@nokia.com
Richard Price
EADS Defence and Security Systems Limited
Meadows Road
Queensway Meadows
Newport, Gwent NP19 4SS
Phone: +44 (0)1633 637874
EMail: richard.price@eads.com
Gonzalo Camarillo (editor)
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
EMail: Gonzalo.Camarillo@ericsson.com
Bormann, et al. Standards Track [Page 20]
RFC 5049 Applying SigComp to SIP December 2007
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Bormann, et al. Standards Track [Page 21]