draft-ietf-rohc-sigcomp-sip-03.txt   draft-ietf-rohc-sigcomp-sip-04.txt 
Robust Header Compression C. Bormann Robust Header Compression C. Bormann
Internet-Draft Universitaet Bremen TZI Internet-Draft Universitaet Bremen TZI
Expires: April 15, 2007 Z. Liu Expires: May 30, 2007 Z. Liu
Nokia Research Center Nokia Research Center
R. Price R. Price
Cogent Defence and Security Cogent Defence and Security
Networks Networks
G. Camarillo G. Camarillo
Ericsson Ericsson
October 12, 2006 November 26, 2006
Applying Signaling Compression (SigComp) to the Session Initiation Applying Signaling Compression (SigComp) to the Session Initiation
Protocol (SIP) Protocol (SIP)
draft-ietf-rohc-sigcomp-sip-03.txt draft-ietf-rohc-sigcomp-sip-04.txt
Status of this Memo Status of this Memo
By submitting this Internet-Draft, each author represents that any By submitting this Internet-Draft, each author represents that any
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have been or will be disclosed, and any of which he or she becomes have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79. aware will be disclosed, in accordance with Section 6 of BCP 79.
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This Internet-Draft will expire on April 15, 2007. This Internet-Draft will expire on May 30, 2007.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2006). Copyright (C) The Internet Society (2006).
Abstract Abstract
This document describes some specifics that apply when Signaling This document describes some specifics that apply when Signaling
Compression (SigComp) is applied to the Session Initiation Protocol Compression (SigComp) is applied to the Session Initiation Protocol
(SIP), such as default minimum values of SigComp parameters, (SIP), such as default minimum values of SigComp parameters,
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2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Minimum Values of SigComp Parameters for SIP/SigComp . . . . . 3 3. Minimum Values of SigComp Parameters for SIP/SigComp . . . . . 3
3.1. decompression_memory_size (DMS) for SIP/SigComp . . . . . 4 3.1. decompression_memory_size (DMS) for SIP/SigComp . . . . . 4
3.2. state_memory_size (SMS) for SIP/SigComp . . . . . . . . . 4 3.2. state_memory_size (SMS) for SIP/SigComp . . . . . . . . . 4
3.3. cycles_per_bit (CPB) for SIP/SigComp . . . . . . . . . . . 5 3.3. cycles_per_bit (CPB) for SIP/SigComp . . . . . . . . . . . 5
3.4. SigComp_version (SV) for SIP/SigComp . . . . . . . . . . . 5 3.4. SigComp_version (SV) for SIP/SigComp . . . . . . . . . . . 5
3.5. locally available state (LAS) for SIP/SigComp . . . . . . 5 3.5. locally available state (LAS) for SIP/SigComp . . . . . . 5
4. Delimiting SIP Messages and SigComp Messages on the Same 4. Delimiting SIP Messages and SigComp Messages on the Same
Port . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Port . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Continuous Mode over TCP . . . . . . . . . . . . . . . . . . . 6 5. Continuous Mode over TCP . . . . . . . . . . . . . . . . . . . 6
6. Compartment and State Management for SIP/SigComp . . . . . . . 6 6. Too Large SIP Messages . . . . . . . . . . . . . . . . . . . . 6
6.1. Remote Application Identification . . . . . . . . . . . . 7 7. SIP Retransmissions . . . . . . . . . . . . . . . . . . . . . 6
6.2. Identifier Comparison Rules . . . . . . . . . . . . . . . 9 8. Compartment and State Management for SIP/SigComp . . . . . . . 7
6.3. Compartment Opening and Closure . . . . . . . . . . . . . 10 8.1. Remote Application Identification . . . . . . . . . . . . 7
6.4. Compartment Valid During a Transaction . . . . . . . . . . 11 8.2. Identifier Comparison Rules . . . . . . . . . . . . . . . 10
6.5. Compartment Valid During a Registration . . . . . . . . . 11 8.3. Compartment Opening and Closure . . . . . . . . . . . . . 10
6.6. Compartment Valid During a Dialog . . . . . . . . . . . . 12 8.4. Compartment Valid During a Registration . . . . . . . . . 11
7. Recommendations for Network Administrators . . . . . . . . . . 12 8.5. Lack of a Compartment . . . . . . . . . . . . . . . . . . 12
8. Private Agreements . . . . . . . . . . . . . . . . . . . . . . 13 9. Recommendations for Network Administrators . . . . . . . . . . 12
9. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 13 10. Private Agreements . . . . . . . . . . . . . . . . . . . . . . 13
10. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 11. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 13
11. Security Considerations . . . . . . . . . . . . . . . . . . . 16 12. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 13. Security Considerations . . . . . . . . . . . . . . . . . . . 15
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17 14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17 15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
14.1. Normative References . . . . . . . . . . . . . . . . . . . 17 16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
14.2. Informative References . . . . . . . . . . . . . . . . . . 18 16.1. Normative References . . . . . . . . . . . . . . . . . . . 17
Appendix A. Shim header for sending uncompressed messages . . . . 18 16.2. Informative References . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . . . . 21 Intellectual Property and Copyright Statements . . . . . . . . . . 20
1. Introduction 1. Introduction
SigComp [RFC3320] is a solution for compressing messages generated by SigComp [RFC3320] is a solution for compressing messages generated by
application protocols. Although its primary driver is to compress application protocols. Although its primary driver is to compress
SIP [RFC3261] messages, the solution itself has been intentionally SIP [RFC3261] messages, the solution itself has been intentionally
designed to be application agnostic so that it can be applied to any designed to be application agnostic so that it can be applied to any
application protocol. (This is denoted as ANY/SigComp.) application protocol; this is denoted as ANY/SigComp. Consequently,
Consequently, many application dependent specifics are left out of many application dependent specifics are left out of the base
the base standard. It is intended that a separate specification is standard. It is intended that a separate specification is used to
used to describe those specifics when SigComp is applied to a describe those specifics when SigComp is applied to a particular
particular application protocol. application protocol.
This document binds SigComp and SIP (denoted as SIP/SigComp). Any This document binds SigComp and SIP; this is denoted as SIP/SigComp.
SigComp implementation that is used for the compression of SIP Any SigComp implementation that is used for the compression of SIP
messages must conform to this document, as well as to [RFC3320]. messages must conform to this document, as well as to [RFC3320].
Additionally, it must support the SIP/SDP static dictionary as Additionally, it must support the SIP/SDP static dictionary as
specified in [RFC3485] and the mechanism for discovering SigComp specified in [RFC3485] and the mechanism for discovering SigComp
support at the SIP layer as specified in [RFC3486]. support at the SIP layer as specified in [RFC3486].
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14 [RFC2119]. document are to be interpreted as described in BCP 14 [RFC2119].
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these resources can be advertised to remote endpoints as described in these resources can be advertised to remote endpoints as described in
section 9.4.9 of [RFC3320]. section 9.4.9 of [RFC3320].
3.1. decompression_memory_size (DMS) for SIP/SigComp 3.1. decompression_memory_size (DMS) for SIP/SigComp
Minimum value for ANY/SigComp: 2048 bytes, as specified in section Minimum value for ANY/SigComp: 2048 bytes, as specified in section
3.3.1 of [RFC3320]. 3.3.1 of [RFC3320].
Minimum value for SIP/SigComp: 8192 bytes. Minimum value for SIP/SigComp: 8192 bytes.
Reason: a DMS of 2048 bytes is too small for SIP message compression, Reason: a DMS of 2048 bytes is too small for SIP message compression
as it seriously limits the compression ratio and even makes as it seriously limits the compression ratio and even makes
compression impossible for certain messages. For example, the compression impossible for certain messages. For example, the
condition set by [RFC3320] for SigComp over UDP means: C + 2*B + R + 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, 2*S + 128 < DMS (each term is described below). On the other hand,
8KB additional memory should not cause any problem for an endpoint 8KB additional memory should not cause any problem for an endpoint
that already implements SIP, SigComp, and applications that use SIP, that already implements SIP, SigComp, and applications that use SIP
as DMS is memory only temporarily needed during decompression of a as DMS is memory only temporarily needed during decompression of a
SigComp message (the memory can be reclaimed when the message has SigComp message (the memory can be reclaimed when the message has
been decompressed). been decompressed).
C size of compressed application message, depending on R C size of compressed application message, depending on R
B size of bytecode (note: two copies -- one as part of the SigComp B size of bytecode. Note: two copies -- one as part of the SigComp
message and one in UDVM memory) message and one in UDVM (Universal Decompressor Virtual Machine)
memory.
R size of ring buffer in UDVM memory R size of ring buffer in UDVM memory
S any additional state uploaded other than that created from the S any additional state uploaded other than that created from the
content of the ring buffer at the end of decompression (similar to content of the ring buffer at the end of decompression (similar to
B, two copies of S are needed) B, two copies of S are needed)
128 the smallest address in UDVM memory to copy bytecode to 128 the smallest address in UDVM memory to copy bytecode to
3.2. state_memory_size (SMS) for SIP/SigComp 3.2. state_memory_size (SMS) for SIP/SigComp
Minimum value for ANY/SigComp: 0 (zero) bytes, as specified in Minimum value for ANY/SigComp: 0 (zero) bytes, as specified in
section 3.3.1 of [RFC3320]. section 3.3.1 of [RFC3320].
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Minimum value for SIP/SigComp: 2048 bytes. Minimum value for SIP/SigComp: 2048 bytes.
Reason: a non-zero SMS allows an endpoint to upload a state in the 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 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- of whether or not it can be created in the remote endpoint. A non-
zero SMS obviously requires the SIP/SigComp implementation to keep zero SMS obviously requires the SIP/SigComp implementation to keep
state. Based on the observation that there is little gain from state. Based on the observation that there is little gain from
stateless SigComp compression, the assumption is that purely stateless SigComp compression, the assumption is that purely
stateless SIP implementations are unlikely to provide a SigComp stateless SIP implementations are unlikely to provide a SigComp
function. Stateful implementations should have little problem to function. Stateful implementations should have little problem to
keep 2K additional state for each compartment (see Section 6). keep 2K additional state for each compartment (see Section 8).
Note: SMS is a parameter that applies to each individual compartment. Note: SMS is a parameter that applies to each individual compartment.
An endpoint MAY offer different SMS values for different compartments An endpoint MAY offer different SMS values for different compartments
as long as the SMS value is not less than 2048 bytes. as long as the SMS value is not less than 2048 bytes.
Compressors that make use of initial state memory MUST implement the
SigComp Negative Acknowledgement (NACK) Mechanism [RFC4077]. (Note
that there is no such requirement on decompressors, but see also
Section 6.) For this requirement, initial state memory is defined as
the assumption of a non-zero SMS value before having received an
advertisement of non-zero SMS (e.g., via returned parameters as
specified in section 9.4.9 of [RFC3320]); ANY/SigComp as defined in
[RFC3320] does not have initial state memory.
3.3. cycles_per_bit (CPB) for SIP/SigComp 3.3. cycles_per_bit (CPB) for SIP/SigComp
Minimum value for ANY/SigComp: 16, as specified in section 3.3.1 of Minimum value for ANY/SigComp: 16, as specified in section 3.3.1 of
[RFC3320]. [RFC3320].
Minimum value for SIP/SigComp: 16 (same as above) Minimum value for SIP/SigComp: 16 (same as above)
3.4. SigComp_version (SV) for SIP/SigComp 3.4. SigComp_version (SV) for SIP/SigComp
For ANY/SigComp: 0x01, as specified in section 3.3.2 of [RFC3320]. For ANY/SigComp: 0x01, as specified in section 3.3.2 of [RFC3320].
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4. Delimiting SIP Messages and SigComp Messages on the Same Port 4. Delimiting SIP Messages and SigComp Messages on the Same Port
In order to limit the number of ports required by a SigComp-aware In order to limit the number of ports required by a SigComp-aware
endpoint, it is possible to allow both SigComp messages and 'vanilla' endpoint, it is possible to allow both SigComp messages and 'vanilla'
SIP messages (i.e. uncompressed SIP messages with no SigComp header) SIP messages (i.e. uncompressed SIP messages with no SigComp header)
to arrive on the same port. to arrive on the same port.
For a message-based transport such as UDP or SCTP, this can be done 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 per message. The receiving endpoint checks the first octet of the
UDP/SCTP payload to determine whether the message has been compressed UDP/SCTP payload to determine whether the message has been compressed
using SigComp. If the MSBs of the octet are "11111" then the message using SigComp. If the MSBs (Most Significant Bits) of the octet are
is considered to be a SigComp message and is parsed as per [RFC3320]. "11111" then the message is considered to be a SigComp message and is
If the MSBs of the octet take any other value, then the message is parsed as per [RFC3320]. If the MSBs of the octet take any other
assumed to be an uncompressed SIP message, and is passed directly to value, then the message is assumed to be an uncompressed SIP message,
the application with no further effect on the SigComp layer. 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 For a stream-based transport such as TCP, the distinction is per
connection. The receiving endpoint checks the first octet of the TCP connection. The receiving endpoint checks the first octet of the TCP
data stream to determine whether the stream has been compressed using data stream to determine whether the stream has been compressed using
SigComp. If the MSBs of the octet are "11111" then the stream is SigComp. If the MSBs of the octet are "11111" then the stream is
considered to contain SigComp messages and is parsed as per considered to contain SigComp messages and is parsed as per
[RFC3320]. If the MSBs of the octet take any other value, then the [RFC3320]. If the MSBs of the octet take any other value, then the
stream is assumed to contain uncompressed SIP messages, and is passed stream is assumed to contain uncompressed SIP messages, and is passed
directly to the application with no further effect on the SigComp directly to the application with no further effect on the SigComp
layer. Note that SigComp message delimiters MUST NOT be used if the layer. Note that SigComp message delimiters MUST NOT be used if the
stream contains uncompressed SIP messages. stream contains uncompressed SIP messages.
Applications MUST NOT mix SIP messages and SigComp messages on a Applications MUST NOT mix SIP messages and SigComp messages on a
single TCP connection. If the TCP connection is used to carry single TCP connection. If the TCP connection is used to carry
SigComp messages then all messages sent over the connection MUST have SigComp messages then all messages sent over the connection MUST have
a SigComp header and be delimited by the use of 0xFFFF as described a SigComp header and be delimited by the use of 0xFFFF as described
in [RFC3320]. in [RFC3320].
Note: Appendix A shows how to send uncompressed messages in a SigComp [I-D.ietf-rohc-sigcomp-impl-guide] shows how to send uncompressed
structured TCP connection using a "well-known shim header". Should messages in a SigComp structured TCP connection using a "well-known
it for any reason not be desirable to set up more than one TCP shim header". Should it for any reason not be desirable to set up
connection to a SIP implementation, but the flexibility to send both more than one TCP connection to a SIP implementation, but the
compressed and uncompressed SIP messages be required, the compressor flexibility to send both compressed and uncompressed SIP messages be
can set up a SigComp structured connection and send any uncompressed required, the compressor can set up a SigComp structured connection
SIP messages using the well-known shim header. and send any uncompressed SIP messages using the well-known shim
header.
5. Continuous Mode over TCP 5. Continuous Mode over TCP
Continuous Mode is a special feature of SigComp, which is designed to Continuous Mode is a special feature of SigComp, which is designed to
improve the overall compression ratio for long-lived connections. improve the overall compression ratio for long-lived connections.
Its use requires pre-agreement between the SigComp compressor and Its use requires pre-agreement between the SigComp compressor and
decompressor. Continuous mode is not used with SIP/SigComp. decompressor. Continuous mode is not used with SIP/SigComp.
Reason: continuous mode requires the transport itself to provide a Reason: continuous mode requires the transport itself to provide a
certain level of protection against denial of service attacks. TCP certain level of protection against denial of service attacks. TCP
alone is not considered to provide enough protection. alone is not considered to provide enough protection.
6. Compartment and State Management for SIP/SigComp 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.
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 An application exchanging compressed traffic with a remote
application has a compartment that contains state information needed application has a compartment that contains state information needed
to compress outgoing messages and to decompress incoming messages. to compress outgoing messages and to decompress incoming messages.
To increase the compression efficiency, the application must assign To increase the compression efficiency, the application must assign
distinct compartments to distinct remote applications. distinct compartments to distinct remote applications.
6.1. Remote Application Identification 8.1. Remote Application Identification
SIP/SigComp applications identify remote applications by their SIP/ SIP/SigComp applications identify remote applications by their SIP/
SigComp identifiers. Each SIP/SigComp application MUST have SIP/ SigComp identifiers. Each SIP/SigComp application MUST have a SIP/
SigComp identifier URN (Uniform Resource Name) that uniquely SigComp identifier URN (Uniform Resource Name) that uniquely
identifies the application. Usage of a URN provides a persistent and identifies the application. Usage of a URN provides a persistent and
unique name for the SIP/SigComp identifier. It also provides an easy unique name for the SIP/SigComp identifier. It also provides an easy
way to guarantee uniqueness. This URN MUST be persistent across way to guarantee uniqueness. This URN MUST be persistent as long as
power cycles of the device or devices hosting the SIP/SigComp the application stores compartment state related to other SIP/SigComp
application. The SIP/ SigComp identifier MUST NOT change as the applications.
application moves from one network to another.
A SIP/Sigcomp application SHOULD use a UUID (Universally Unique A SIP/Sigcomp application SHOULD use a UUID (Universally Unique
IDentifier) URN as its SIP/SigComp identifier. The UUID URN IDentifier) URN as its SIP/SigComp identifier. The UUID URN
[RFC4122] allows for non-centralized computation of a URN based on [RFC4122] allows for non-centralized computation of a URN based on
time, unique names (such as a MAC address), or a random number 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 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/ selected such that the application can be certain that no other SIP/
SigComp application would choose the same URN value. SigComp application would choose the same URN value.
Note that the definition of SIP/SigComp identifier is similar to the Note that the definition of SIP/SigComp identifier is similar to the
definition of instance identifier in [I-D.ietf-sip-outbound]. One definition of instance identifier in [I-D.ietf-sip-outbound]. One
difference is that instance identifiers are only required to be difference is that instance identifiers are only required to be
unique within their AoR (Address of Record) while SIP/SigComp unique within their AoR (Address of Record) while SIP/SigComp
identifiers are required to be globally unique. identifiers are required to be globally unique.
Nevertheless, devices may choose to generate globally unique instance 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 identifiers. A device with a globally unique instance identifier
SHOULD use its instance identifier as its SIP/SigComp identifier. SHOULD use its instance identifier as its SIP/SigComp identifier.
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 Server farms that share SIP/SigComp state across servers MUST use the
same SIP/SigComp identifier for all their servers. same SIP/SigComp identifier for all their servers.
SIP/SigComp identifiers are carried in the 'sigcomp-id' SIP URI SIP/SigComp identifiers are carried in the 'sigcomp-id' SIP URI
(Uniform Resource Identifier) or Via header field parameter. The (Uniform Resource Identifier) or Via header field parameter. The
'sigcomp-id' SIP URI parameter is a 'uri-parameter', as defined by '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 SIP ABNF (Augmented Backus-Naur Form, Section 25.1 of [RFC3261]).
The following is its ABNF [RFC4234]: The following is its ABNF [RFC4234]:
sip-sigcomp-id = "sigcomp-id=" instance-val uri-sip-sigcomp-id = "sigcomp-id=" 1*paramchar
instance-val = *uric ; defined in RFC 2396 The SIP URI 'sigcomp-id' parameter MUST contain a URN [RFC2141].
The Via 'sigcomp-id' parameter is a 'via-extension', as defined by 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 the SIP ABNF (Section 25.1 of [RFC3261]). The following is its ABNF
[RFC4234]: [RFC4234]:
via-sip-sigcomp-id = "sigcomp-id" EQUAL via-sip-sigcomp-id = "sigcomp-id" EQUAL
LDQUOT "<" instance-val ">" RDQUOT LDQUOT *( qdtext / quoted-pair ) RDQUOT
instance-val = *uric ; defined in RFC 2396 The Via 'sigcomp-id' parameter MUST contain a URN [RFC2141].
The following is an example of a Via header field with a 'sigcomp-id' The following is an example of a Via header field with a 'sigcomp-id'
parameter: parameter:
Via: SIP/2.0/UDP server1.example.com:5060 Via: SIP/2.0/UDP server1.example.com:5060
;branch=z9hG4bK87a7 ;branch=z9hG4bK87a7
;comp=sigcomp ;comp=sigcomp
;sigcomp-id="<urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128>" ;sigcomp-id="urn:uuid:0C67446E-F1A1-11D9-94D3-000A95A0E128"
Note that some characters that are allowed to appear in a Via header 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 field parameter, such as ':' (colon), are not allowed to appear in a
SIP URI parameter. Those characters need to be escaped when they SIP URI parameter. Those characters need to be escaped when they
appear in a SIP URI parameter. appear in a SIP URI parameter.
The need to escape characters in parameters could be avoided by The need to escape characters in parameters could be avoided by
defining Contact, Route, Record-Route, Path, and Service-Route defining Contact, Route, Record-Route, Path, and Service-Route
header field 'sigcomp-id' parameters instead of the 'sigcomp-id' header field 'sigcomp-id' parameters instead of the 'sigcomp-id'
SIP URI parameter. For example, instance identifiers typically SIP URI parameter. For example, instance identifiers typically
appear in '+sip.instance' Contact header field parameters, and not appear in '+sip.instance' Contact header field parameters, and not
in SIP URI parameters. We have chosen to define 'sigcomp-id' as a 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- SIP URI parameter to be consistent with the use of the already-in-
use 'comp=sigcomp' parameter, which is a SIP URI parameter as use 'comp=sigcomp' parameter, which is a SIP URI parameter as
well. well.
The following is an example of a 'sigcomp-id' SIP URI parameter: The following is an example of a 'sigcomp-id' SIP URI parameter:
sigcomp-id:urn%3auuid%3a0C67446E-F1A1-11D9-94D3-000A95A0E128 sigcomp-id=urn%3auuid%3a0C67446E-F1A1-11D9-94D3-000A95A0E128
SIP messages are matched with remote application identifiers as SIP messages are matched with remote application identifiers as
follows. follows.
Outgoing requests: the remote application identifier is the SIP/ Outgoing requests: the remote application identifier is the SIP/
SigComp identifier of the URI to which the request is sent. If SigComp identifier of the URI to which the request is sent. If
the URI does not contain a SIP/SigComp identifier but is the URI does not contain a SIP/SigComp identifier, the remote
associated with an instance identifier (e.g., the URI appears in a application identifier is the IP address plus port of the datagram
Contact header field with a '+sip.instance' parameter), the carrying the request for connection-less transport protocols, and
instance identifier is used as the remote application identifier. the transport connection (e.g., a TCP connection) carrying the
In other cases, the remote application identifier is the host part request for connection-oriented transport protocols (this is to
of the URI to which the request is sent (this is to support legacy support legacy SIP/SigComp applications).
SIP/SigComp applications).
Incoming responses: the remote application identifier is the same as Incoming responses: the remote application identifier is the same as
the one of the previously-sent request that initiated the the one of the previously-sent request that initiated the
transaction the response belongs to. transaction the response belongs to.
Incoming requests: the remote application identifier is the SIP/ Incoming requests: the remote application identifier is the SIP/
SigComp identifier of the top-most Via entry. If the Via header SigComp identifier of the top-most Via entry. If the Via header
field does not contain a SIP/SigComp identifier, the remote field does not contain a SIP/SigComp identifier, the remote
application identifier is the sent-by parameter of the top-most application identifier is the source IP address plus port of the
Via entry (this is to support legacy SIP/SigComp applications). 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 Outgoing responses: the remote application identifier is the same as
the previously-received request that initiated the transaction the the previously-received request that initiated the transaction the
response belongs to. 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' A SIP/SigComp application placing its URI with the 'comp=sigcomp'
parameter in a header field MUST add a 'sigcomp-id' parameter with parameter in a header field MUST add a 'sigcomp-id' parameter with
its SIP/SigComp identifier to that URI unless the URI is associated its SIP/SigComp identifier to that URI.
with an instance identifier (e.g., the URI appears in a Contact
header field with a '+sip.instance' parameter). If the URI is
associated with an instance identifier, the SIP/SigComp application
SHOULD NOT add a 'sigcomp-id' parameter to the URI.
A SIP/SigComp application generating its own Via entry containing the A SIP/SigComp application generating its own Via entry containing the
'comp=sigcomp' parameter MUST add a 'sigcomp-id' parameter with its 'comp=sigcomp' parameter MUST add a 'sigcomp-id' parameter with its
SIP/SigComp identifier to that Via entry. SIP/SigComp identifier to that Via entry.
A given remote application identifier is mapped to a particular A given remote application identifier is mapped to a particular
SigComp compartment ID following the rules given in Section 6.3, SigComp compartment ID following the rules given in Section 8.3 and
Section 6.4, Section 6.5, Section 6.6. Section 8.4.
6.2. Identifier Comparison Rules 8.2. Identifier Comparison Rules
Equality comparisons between SIP/SigComp identifiers are performed Equality comparisons between SIP/SigComp identifiers are performed
using the rules for URN equality that are specific to the scheme in using the rules for URN equality that are specific to the scheme in
the URN. If the element performing the comparisons does not the URN. If the element performing the comparisons does not
understand the URN scheme, it performs the comparisons using the understand the URN scheme, it performs the comparisons using the
lexical equality rules defined in RFC 2141 [RFC2141]. Lexical lexical equality rules defined in RFC 2141 [RFC2141]. Lexical
equality may result in two URNs being considered unequal when they equality may result in two URNs being considered unequal when they
are actually equal. In this specific usage of URNs, the only element are actually equal. In this specific usage of URNs, the only element
which provides the URN is the SIP/SigComp application identified by which provides the URN is the SIP/SigComp application identified by
that URN. As a result, the SIP/SigComp application SHOULD provide that URN. As a result, the SIP/SigComp application SHOULD provide
lexically equivalent URNs in each registration it generates. This is lexically equivalent URNs in each registration it generates. This is
likely to be normal behavior in any case; applications are not likely likely to be normal behavior in any case; applications are not likely
to modify the value of their SIP/SigComp identifiers so that they to modify the value of their SIP/SigComp identifiers so that they
remain functionally equivalent yet lexigraphically different from remain functionally equivalent yet lexigraphically different from
previous identifiers. previous identifiers.
Comparisons between SIP/SigComp identifiers and instance identifiers 8.3. Compartment Opening and Closure
are performed following the same rules.
6.3. Compartment Opening and Closure
SIP applications need to know when to open a new compartment and when SIP applications need to know when to open a new compartment and when
to close it. The lifetime of a compartment depends on how the SIP to close it. The lifetime of SIP/SigComp compartments is linked to
application obtained the remote application identifier (e.g., in a registration state. Compartments are opened at SIP registration time
Record-Route header field of an incoming SIP message). There are and are typically closed when the registration expires or is
compartments that are valid for the duration of a registration, of a canceled.
dialog, and of a single transaction. The following sections specify
how a SIP application decides the lifetime of a particular
compartment.
If following the rules in the following sections, 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. Additionally, the SIP application MUST
adjust the closure time for the compartment so that it is only closed
when the SIP application does not need it any longer.
For example, a SIP application may open a compartment valid for the Previous revisions of this document also defined compartments
duration of a registration for a particular remote application valid during a SIP transaction or a SIP dialog. It was decided to
identifier. At a later point, the application is supposed to open a eliminate those types of compartments because the complexity they
new compartment for the duration of a particular dialog for the same introduced was higher than the benefits they brought in most
remote application identifier. Following the previous rule, the SIP deployment scenarios.
application does not open a new compartment but use the already
existing one for that remote application identifier. However, the
SIP application must not close that compartment until both, the
registration and the dialog are over. So, if the registration
finishes before the dialog, the compartment will not be closed
(because the dialog is still active) even though the compartment was
originally opened for the registration.
Usually, any states created during the lifetime of a compartment will Usually, any states created during the lifetime of a compartment will
be "logically" deleted when the compartment is closed. As described be "logically" deleted when the compartment is closed. As described
in section 6.2 of [RFC3320], a logical deletion can become a physical in section 6.2 of [RFC3320], a logical deletion can become a physical
deletion only when no compartment continues to exist that created the deletion only when no compartment continues to exist that created the
(same) state. (same) state.
A SigComp endpoint may offer to keep a state created upon request A SigComp endpoint may offer to keep a state created upon request
from a SigComp peer endpoint beyond the default lifetime of a from a SigComp peer endpoint beyond the default lifetime of a
compartment. This may be used to improve compression efficiency of 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 subsequent SIP messages generated by the same remote application at
the SigComp peer endpoint. To indicate that such state will continue the SigComp peer endpoint. To indicate that such state will continue
to be available, the SigComp endpoint can inform its peer SigComp to be available, the SigComp endpoint can inform its peer SigComp
endpoint by announcing the (partial) state ID in the returned SigComp endpoint by announcing the (partial) state ID in the returned SigComp
parameters at the end of the registration, dialog, or transaction parameters at the end of the registration, dialog, or transaction
that was supposed to limit the lifetime of the SigComp state. That that was supposed to limit the lifetime of the SigComp state. That
signals the state will be maintained. As there is no way to signal signals the state will be maintained. The mandatory support for the
any limit to the lifetime of this state, both decompressors that SigComp Negative Acknowledgement (NACK) Mechanism [RFC4077] in SIP/
intend to offer state with possibly limited lifetimes as well as SigComp ensures that it is possible to recover from synchronization
compressors that make use of such state use the SigComp Negative errors regarding comparment lifetimes.
Acknowledgement (NACK) Mechanism [RFC4077] to recover from
synchronization errors.
As an operational concern, bugs in the compartment management As an operational concern, bugs in the compartment management
implementation are likely to lead to sporadic, hard to diagnose implementation are likely to lead to sporadic, hard to diagnose
failures. Decompressors may therefore want to cache old state and, failures. Decompressors may therefore want to cache old state and,
if still available, allow access while logging diagnostic if still available, allow access while logging diagnostic
information. Both compressors and decompressors use the SigComp information. Both compressors and decompressors use the SigComp
Negative Acknowledgement (NACK) Mechanism [RFC4077] to recover from Negative Acknowledgement (NACK) Mechanism [RFC4077] to recover from
situations where such old state may no longer be available. situations where such old state may no longer be available.
6.4. Compartment Valid During a Transaction 8.4. Compartment Valid During a Registration
A SIP application that needs to send a compressed SIP request SHOULD
open a compartment for the request's remote application identifier.
This compartment will be used to receive compressed responses for the
request. The application SHOULD NOT close the compartment until the
transaction is over.
A SIP application that receives a compressed SIP request SHOULD open
a compartment for the request's remote application identifier. This
compartment will be used to send compressed responses for the
request. The application SHOULD NOT close the compartment until the
transaction is over.
The previous rules ensure that SIP applications always have a
compartment to send and receive responses.
6.5. Compartment Valid During a Registration
A REGISTER transaction can cause an application to open a new A REGISTER transaction causes an application to open a new
compartment to be valid for the duration of the registration compartment to be valid for the duration of the registration
established by the REGISTER transaction. established by the REGISTER transaction.
A 200 (OK) response for a register may contain a Path [RFC3327] and a A SIP application that needs to send a compressed SIP REGISTER (i.e.,
Service-Route [RFC3308] header field. These header fields indicate a user agent generating a REGISTER or a proxy server relaying one to
the route future incoming and outgoing requests will follow. 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.
A SIP application generating a 200 (OK) response for a REGISTER or These compartments MAY be closed if the REGISTER request is responded
receiving such a response proceeds as follows. If the application with a non-2xx final response, or when the registration expires or is
inserted itself in the Contact (i.e., because it is the user agent) canceled. However, applications MAY also choose to keep these
or in the Path header field of the REGISTER, or it appears in the compartments open for a longer period of time, as discussed
Service-Route header field, the application constructs the route previously. For a given successful registration, applications SHOULD
future incoming requests will follow (using the Contact and the Path NOT close their associated compartments until the registration is
header fields) and the route future outgoing requests will follow over.
(using the Contact and the Service-Route header fields). The
application checks whether the URIs of its adjacent applications in
both routes have the 'comp=sigcomp' parameter. The application
SHOULD open a new compartment for the remote application identifier
of the URIs with that parameter. The application SHOULD NOT close
the compartments until the registration is over.
Note that the route for incoming requests is typically the same A SIP network can be configured so that regular SIP traffic to and
(although traversed in the opposite direction) as the route for from a user agent traverses a different set of proxies than the
outgoing requests. 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
transaction. Section 9 provides network administrators with
recommendations so that they configure they networks properly.
6.6. Compartment Valid During a Dialog 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.
A transaction that establishes a dialog can cause an application to 8.5. Lack of a Compartment
open a new compartment to be valid for the duration of the dialog
established by the transaction.
A SIP message that establishes a dialog (e.g., a 2xx response for an The use of stateless compression (i.e., compression without a
INVITE) may contain a Record-Route header field. This header field compartment) is not typically worthwhile and may even result in
indicates the route future requests within the dialog will follow. 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:
On generating or receiving a SIP message that establishes a dialog, a "If the next-hop URI contains the parameter comp=sigcomp, the
SIP application that inserted itself in the Contact (i.e., because it client SHOULD compress the request using SigComp"
is the user agent) or in the Record-Route header field of the
request, constructs (using the Contact, and the Record-Route header
fields) the route requests within the dialog will follow. The
application checks whether the URIs of its adjacent applications in
that route have the 'comp=sigcomp' parameter. The application SHOULD
open a new compartment for the remote application identifier of the
URIs with that parameter. The application SHOULD NOT close the
compartments until the dialog is over.
7. Recommendations for Network Administrators 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 Network administrators can configure their networks so that the
compression efficiency achieved is increased. The following compression efficiency achieved is increased. The following
recommendations help network administrators perform their task. recommendations help network administrators perform their task.
For a given user agent, the route sets for incoming requests (created For a given user agent, the route sets for incoming requests (created
by a Path header field) and for outgoing requests (created by a by a Path header field) and for outgoing requests (created by a
Service-Route header field) are typically the same. However, Service-Route header field) are typically the same. However,
registrars can, if they wish, insert proxies in the latter route that 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 do not appear in the former route and vice versa. It is RECOMMENDED
skipping to change at page 13, line 14 skipping to change at page 13, line 5
compression appear in both routes. compression appear in both routes.
The routes described previously apply to requests sent outside a The routes described previously apply to requests sent outside a
dialog. Requests inside a dialog follow a route constructed using dialog. Requests inside a dialog follow a route constructed using
Record-Route header fields. It is RECOMMENDED that the proxies Record-Route header fields. It is RECOMMENDED that the proxies
performing SigComp that are in the route for requests outside a performing SigComp that are in the route for requests outside a
dialog are configured to place themselves (by inserting themselves in dialog are configured to place themselves (by inserting themselves in
the Record-Route header fields) in the routes used for requests the Record-Route header fields) in the routes used for requests
inside dialogs. inside dialogs.
8. Private Agreements 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 SIP/SigComp implementations that are subject to private agreements
MAY deviate from this specification, if the private agreements MAY deviate from this specification, if the private agreements
unambiguously specify so. Plausible candidates for such deviations unambiguously specify so. Plausible candidates for such deviations
include: include:
o Minimum values (Section 3). o Minimum values (Section 3).
o Compartment definition (Section 6).
o Use of continuous mode (Section 5). o Use of continuous mode (Section 5).
o Compartment definition (Section 8).
9. Backwards Compatibility 11. Backwards Compatibility
SigComp has a number of parameters that can be configured per SigComp has a number of parameters that can be configured per
endpoint. This document specifies a profile for SigComp when used endpoint. This document specifies a profile for SigComp when used
for SIP compression that further constrains the range that some of for SIP compression that further constrains the range that some of
these parameters may take. Examples of this are Decompressor Memory these parameters may take. Examples of this are Decompressor Memory
Size, State Memory Size, and SigComp Version (support for NACK). Size, State Memory Size, and SigComp Version (support for NACK).
Additionally, this document specifies how SIP/SigComp applications Additionally, this document specifies how SIP/SigComp applications
should perform compartment mapping. should perform compartment mapping.
When this document was written, there already were a few existing When this document was written, there already were a few existing
SIP/SigComp deployments. The rules in this document have been SIP/SigComp deployments. The rules in this document have been
designed to maximize interoperability with those legacy SIP/SigComp designed to maximize interoperability with those legacy SIP/SigComp
implementations. Nevertheless, implementers should be aware that implementations. Nevertheless, implementers should be aware that
legacy SIP/SigComp implementations may not conform to this legacy SIP/SigComp implementations may not conform to this
specification. Examples of problems with legacy applications would specification. Examples of problems with legacy applications would
be smaller DMS than mandated in this document, lack of NACK support, be smaller DMS than mandated in this document, lack of NACK support,
or a different comparment mapping. or a different comparment mapping.
10. Example 12. Example
Figure 1 shows an example message flow where the user agent and the Figure 1 shows an example message flow where the user agent and the
outbound proxy exchange compressed SIP traffic. Compressed messages outbound proxy exchange compressed SIP traffic. Compressed messages
are marked with a (c). are marked with a (c).
User Agent Outbound Proxy Registrar User Agent Outbound Proxy Registrar
|(1) REGISTER (c) | | |(1) REGISTER (c) | |
|---------------->| | |---------------->| |
| |(2) REGISTER | | |(2) REGISTER |
skipping to change at page 14, line 46 skipping to change at page 14, line 46
Figure 1: Example message flow Figure 1: Example message flow
The user agent in Figure 1 is initialy configured (e.g., using the The user agent in Figure 1 is initialy configured (e.g., using the
SIP configuration framework [I-D.ietf-sipping-config-framework]) with SIP configuration framework [I-D.ietf-sipping-config-framework]) with
the URI of its outbound proxy. That URI contains the outbound's the URI of its outbound proxy. That URI contains the outbound's
proxy SIP/SigComp identifier, referred to as 'Outbound-id', in a proxy SIP/SigComp identifier, referred to as 'Outbound-id', in a
'sigcomp-id' parameter. 'sigcomp-id' parameter.
When the user agent sends an initial REGISTER request (1) to the 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 proxy's URI, the user agent opens a new compartment for
'Outbound-id'. This compartment will, in principle, be valid for the 'Outbound-id'. This compartment will be valid, at least, for the
duration of the REGISTER transaction, as discussed in Section 6.4. duration of the registration.
On receiving this REGISTER request (1), the outbound proxy opens a On receiving this REGISTER request (1), the outbound proxy opens a
new compartment for the SIP/SigComp identifier that appears in the new compartment for the SIP/SigComp identifier that appears in the
'sigcomp-id' parameter of the top-most Via entry. This identifier, 'sigcomp-id' parameter of the top-most Via entry. This identifier,
which is the user agent's SIP/SigComp identifier, is referred to as which is the user agent's SIP/SigComp identifier, is referred to as
'UA-id'. The compartment opened by the outbound proxy will, in 'UA-id'. The compartment opened by the outbound proxy will be valid,
principle, be valid for the duration of the REGISTER transaction, as at least, for the duration of the registration. The outbound proxy
discussed in Section 6.4. The outbound proxy adds Path header field adds Path header field with its own URI, which contains the
with its own URI to the REGISTER request and relays it to the 'Outbound-id' SIP/SigComp identifier, to the REGISTER request and
registrar (2). relays it to the registrar (2).
When the outbound proxy receives a 200 (OK) response (3) for the
REGISTER request, it constructs the route future incoming requests
will follow (using the Contact and the Path header fields) and the
route future outgoing requests will follow (using the Contact and the
Service-Route header fields). Both the Path and the Service-Route
header fields contain the outbound proxy's URI. The Contact header
field contains the user agent's URI, which carries the user agent's
SIP/SigComp identifier 'UA-id'.
Consequently, the outbound proxy is supposed to open a new
compartment for 'UA-id' for the duration of the registration, as
discussed in Section 6.4. However, since the outbound proxy has
already a compartment for 'UA-id', it reuses that comparment, as
discussed in Section 6.3.
On receiving the 200 (OK) response (4), the user agent constructs the When the registrar receives the REGISTER request (2), it constructs
route future incoming requests will follow (using the Path header the route future incoming requests (to the user agent) will follow
field) and the route future outgoing requests will follow (using the using the Contact and the Path header fields. Future incoming
Service-Route header field). The user agent is supposed to open a requests will traverse the outbound proxy before reaching the user
new compartment for 'Outbound-id' for the duration of the agent.
registration, as discussed in Section 6.4. However, since the user
agent has already a compartment for 'Outbound-id', it reuses that
comparment, as discussed in Section 6.3.
At a later point, the user agent needs to send an INVITE request (5). The registrar also constructs the route future outgoing requests
The user agent is supposed to open a new compartment for (from the user agent) and places it in a Service-Route header field
'Outbound-id' for the duration of the INVITE transaction, as in a 200 (OK) response (3). Future outgoing requests will always
discussed in Section 6.4. However, since the user agent has already traverse the outbound proxy. The registrar has ensured that the
a compartment for 'Outbound-id', it reuses that comparment, as outbound proxy performing compression handles both incoming and
discussed in Section 6.3. outgoing requests.
On receiving the INVITE request (5), the outbound proxy is supposed When the outbound proxy receives a 200 (OK) response (3), it inspects
to open a new compartment for 'UA-id' for the duration of the INVITE the top-most Via entry. This entry's SIP/SigComp identifier 'UA-id'
transaction, as discussed in Section 6.4. However, since the matches that of the compartment created before. Therefore, the
outbound proxy has already a compartment for 'UA-id', it reuses that outbound proxy uses that compartment to compress it and relay it to
comparment, as discussed in Section 6.3. The outbound proxy Record- the user agent.
Routes and relays the INVITE request (6) forward.
When the outbound proxy receives a dialog-establishing 200 (OK) On receiving the 200 (OK) response (4), the user agent stores the
response (7) for the INVITE request, it constructs the route future Service-Route header field in order to use it to send future outgoing
requests within the dialog will follow (using the Contact and Route requests. The Service-Route header field contains the outbound
header fields). The outbound proxy is supposed to open a new proxy's URI, which contains the 'Outbound-id' SIP/SigComp identifier.
compartment for 'UA-id' for the duration of the dialog, as discussed
in Section 6.6. However, since the outbound proxy has already a
compartment for 'UA-id', it reuses that comparment, as discussed in
Section 6.3.
On receiving the 200 (OK) response (8), the user agent constructs the At a later point, the user agent needs to send an INVITE request (5).
route future requests within the dialog will follow (using the Route According to the Service-Route header field received previously, the
header field). The user agent is supposed to open a new compartment user agent sends the INVITE request (5) to the outbound proxy's URI.
for 'Outbound-id' for the duration of the dialog, as discussed in Since this URI's SIP/SigComp identifier 'Outbound-id' matches that of
Section 6.4. However, since the user agent has already a compartment the compartment created before, this compartment is used to compress
for 'Outbound-id', it reuses that comparment, as discussed in the INVITE request.
Section 6.3.
When the dialog is terminated by a BYE transaction (11), the user On receiving the INVITE request (5), the outbound proxy Record Routes
agent is supposed to close the compartment for 'Outbound-id' and the and relays the INVITE request (6) forward. The outbound proxy Record
outbound proxy is supposed to close the compartment for 'UA-id', as Routes to ensure that all SIP messages related to this new dialog are
discussed in Section 6.6. However, both compartments should be kept routed through the outbound proxy.
open until the current registration expires. Therefore, none of them
close their compartments yet.
11. Security Considerations Finally the dialog is terminated by a BYE transaction (11) that also
traverses the outbound proxy.
13. Security Considerations
The same security considerations as described in [RFC3320] apply to The same security considerations as described in [RFC3320] apply to
this document. Note that keeping SigComp states longer than the this document. Note that keeping SigComp states longer than the
duration of a SIP dialog should not pose new security risks for two 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 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 place; and b) this is on voluntary basis and a SigComp endpoint can
choose not to offer it. choose not to offer it.
12. IANA Considerations 14. IANA Considerations
The IANA is requested to register the 'sigcomp-id' Via header field The IANA is requested to register the 'sigcomp-id' Via header field
parameter, which is defined in Section 6.1, under the Header Field parameter, which is defined in Section 8.1, under the Header Field
Parameters and Parameter Values subregistry within the SIP Parameters Parameters and Parameter Values subregistry within the SIP Parameters
registry: registry:
Predefined Predefined
Header Field Parameter Name Values Reference Header Field Parameter Name Values Reference
---------------------------- --------------- --------- --------- ---------------------------- --------------- --------- ---------
Via sigcomp-id No [RFCxxxx] Via sigcomp-id No [RFCxxxx]
The IANA is requested to register the 'sigcomp-id' SIP URI parameter, The IANA is requested to register the 'sigcomp-id' SIP URI parameter,
which is defined in Section 6.1, under the SIP/SIPS URI Parameters which is defined in Section 8.1, under the SIP/SIPS URI Parameters
subregistry within the SIP Parameters registry: subregistry within the SIP Parameters registry:
Parameter Name Predefined Values Reference Parameter Name Predefined Values Reference
-------------- ----------------- --------- -------------- ----------------- ---------
sigcomp-id No [RFCxxxx] sigcomp-id No [RFCxxxx]
Note to the RFC Editor: please, substitute RFCxxxx with the RFC Note to the RFC Editor: please, substitute RFCxxxx with the RFC
number this document will get. number this document will get.
13. Acknowledgements 15. Acknowledgements
Abigail Surtees provided the code and text for Appendix A.
The authors would like to thank the following people for their The authors would like to thank the following people for their
comments and suggestions: Abigail Surtees, Jan Christoffersson, Joerg comments and suggestions: Jan Christoffersson, Joerg Ott, Mark West,
Ott, Mark West, Pekka Pessi, Robert Sugar, Adam Roach, Jonathan Pekka Pessi, Robert Sugar, Jonathan Rosenberg, and Robert Sparks.
Rosenberg, and Robert Sparks. Abigail Surtees and Adam Roach performed thorough reviews of this
document.
14. References 16. References
14.1. Normative References 16.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997. [RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E. A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261, Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002. June 2002.
skipping to change at page 18, line 23 skipping to change at page 17, line 52
July 2005. July 2005.
[RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax [RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005. Specifications: ABNF", RFC 4234, October 2005.
[I-D.ietf-sip-outbound] [I-D.ietf-sip-outbound]
Jennings, C. and R. Mahy, "Managing Client Initiated Jennings, C. and R. Mahy, "Managing Client Initiated
Connections in the Session Initiation Protocol (SIP)", Connections in the Session Initiation Protocol (SIP)",
draft-ietf-sip-outbound-04 (work in progress), June 2006. draft-ietf-sip-outbound-04 (work in progress), June 2006.
14.2. Informative References [I-D.ietf-rohc-sigcomp-impl-guide]
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] [I-D.ietf-sipping-config-framework]
Petrie, D., "A Framework for Session Initiation Protocol Petrie, D., "A Framework for Session Initiation Protocol
User Agent Profile Delivery", User Agent Profile Delivery",
draft-ietf-sipping-config-framework-08 (work in progress), draft-ietf-sipping-config-framework-08 (work in progress),
March 2006. March 2006.
Appendix A. Shim header for sending uncompressed messages
This appendix presents bytecode that simply instructs the
decompressor to output the entire message (effectively sending it
uncompressed but within a SigComp message).
The mnemonic code is:
at (0)
:udvm_memory_size pad (2)
:cycles_per_bit pad (2)
:sigcomp_version pad (2)
:partial_state_id_length pad (2)
:state_length pad (2)
:reserved pad (2)
at (64)
:byte_copy_left pad (2)
:byte_copy_right pad (2)
:input_bit_order pad (2)
:stack_location pad (2)
; Simple loop
; Read a byte
; Output a byte
; Until there are no more bytes!
at (128)
:start
INPUT-BYTES (1, byte_copy_left, end)
OUTPUT (byte_copy_left, 1)
JUMP (start)
:end
END-MESSAGE (0,0,0,0,0,0,0)
which translates to give the following initial 13 bytes of the
SigComp message (in hexadecimal):
f8 00 a1 1c 01 86 09 22 86 01 16 f9 23
As an implementation optimization, a SigComp implementation MAY
compare the initial 13 bytes of each incoming message with the 13
bytes given (the "well-known shim header"), and, in case of a match,
simply copy the SigComp message data that follow the shim header
without even setting up a UDVM. (Note that, before a SigComp message
is formed from the incoming TCP data, the record marking protocol
defined in section 4.2.2 of [RFC3320] has to be performed.)
To obtain the maximum benefit from this optimization, compressors
SHOULD employ exactly the well-known shim header given (and none of
the other conceivable byte code sequences for just copying input to
output) to send uncompressed data in a SigComp channel.
Authors' Addresses Authors' Addresses
Carsten Bormann Carsten Bormann
Universitaet Bremen TZI Universitaet Bremen TZI
Postfach 330440 Postfach 330440
Bremen D-28334 Bremen D-28334
Germany Germany
Phone: +49 421 218 7024 Phone: +49 421 218 7024
Fax: +49 421 218 7000 Fax: +49 421 218 7000
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