Network Working Group Zhigang Liu, Editor, Nokia
INTERNET-DRAFT Richard Price, Siemens/Roke ManorRobust Header Compression C. Bormann, Ed.
Internet-Draft Universitaet Bremen TZI
Expires: August 12, 2004 18, 2006 Z. Liu
Nokia Research Center
Cogent Defence and Security
February 12, 2004 14, 2006
Applying Signaling Compression (SigComp) to the Session Initiation
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is an Internet-Draft aware
have been or will be disclosed, and is any of which he or she becomes
aware will be disclosed, in full conformance accordance with
all provisions of Section 10 6 of RFC2026. BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at http://
The list of Internet-Draft Shadow Directories can be accessed at
This document is a submission of the IETF ROHC WG. Comments should be
directed to its mailing list, firstname.lastname@example.org. Internet-Draft will expire on August 18, 2006.
Copyright (C) The Internet Society (2006).
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, in addition to SigComp and support of the SIP and
Session Description Protocol (SDP) static dictionary.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Minimum Values of SigComp Parameters for SIP/SigComp . . . . . 3
3.1. decompression_memory_size (DMS) for SIP/SigComp . . . . . 4
3.2. state_memory_size (SMS) for SIP/SigComp . . . . . . . . . 4
3.3. cycles_per_bit (CPB) for SIP/SigComp . . . . . . . . . . . 5
3.4. SigComp_version (SV) for SIP/SigComp . . . . . . . . . . . 5
3.5. locally available state (LAS) for SIP/SigComp . . . . . . 5
4. Delimiting SIP Messages and SigComp Messages on the Same
Port . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Continuous Mode over TCP . . . . . . . . . . . . . . . . . . . 6
6. Compartment and State Management for SIP/SigComp . . . . . . . 7
6.1. Remote Application Identifiers . . . . . . . . . . . . . . 7
6.2. Compartment Opening and Closure . . . . . . . . . . . . . 7
6.3. Compartment Valid During a Transaction . . . . . . . . . . 9
6.4. Compartment Valid During a Registration . . . . . . . . . 9
6.5. Compartment Valid During a Dialog . . . . . . . . . . . . 10
7. Recommendations for Network Administrators . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. Private Agreements . . . . . . . . . . . . . . . . . . . . . . 11
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
12. Normative References . . . . . . . . . . . . . . . . . . . . . 11
Appendix A. Shim header for sending uncompressed messages . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . . . 15
SigComp [RFC-3320] [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
protocols. protocol. (This is denoted as ANY/SigComp.)
Consequently, many application dependent specifics are left out. out of
the base standard. It is intended that a separate document will be needed specification is
used to describe those specifics when SigComp is applied to a
particular application protocol.
This document binds SigComp and SIP (denoted as SIP/SigComp). Any
SigComp implementation that is used for the compression of SIP
messages must conform to this document, as well as to SigComp
[RFC-3320] [RFC3320] and
must support the SIP/SDP static dictionary as specified in [RFC-3485]. [RFC3485].
Note: the mechanism of discovering SigComp support at the SIP layer
is specified in [RFC-3486]. [RFC3486].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
2. 14 [RFC2119].
3. Minimum Values of SigComp Parameters for SIP/SigComp
In order to support a wide range of capabilities among endpoints
implementing SigComp, SigComp defines a few parameters to describe
SigComp behaviour behavior (see section 3.3 of [RFC-3320]). [RFC3320]). For each parameter, [RFC-3320]
[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 [RFC-3320] [RFC3320] are too
small to allow good performance for SIP message compression.
Therefore, they are increased for SIP/SigComp as specified in the
following sections. For completeness, those parameters that are the
same for SIP/SigComp as they are for ANY/SigComp are also listed.
Note: the new minimum values are specific to SIP/SigComp. They do
not apply to any other application protocols.
Note: a SigComp endpoint MAY offer additional resources if available;
these resources can be advertised to remote endpoints as described in
section 9.4.9 of [RFC-3320].
3.1. decompression_memory_size (DMS) for SIP/SigComp
Minimum value for ANY/SigComp: 2048 bytes, as specified in section
3.3.1 of [RFC-3320]. [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 [RFC-3320] [RFC3320] for SigComp over UDP means: C + 2*B + R +
2*S + 128 < DMS (each term is described below). On the other hand,
8KB additional memory should not cause any problem for an endpoint
that already implements SIP, SigComp, and applications that use SIP. SIP,
as DMS is memory only temporarily needed during decompression of a
SigComp message (the memory can be reclaimed when the message has
C = size of compressed application message, depending on R. R
B = size of bytecode (note: two copies - -- one as part of the SigComp
message and one in UDVM memory)
R = size of ring buffer in UDVM memory
S = any additional state uploaded other than that created from the
content of the ring buffer at the end of decompression. Similar decompression (similar to
B, two copies of S are needed needed)
128 = the smallest address in UDVM memory to copy bytecode
Note: DMS is per SigComp message and the memory can be reclaimed
after the message has been decompressed.
3.2. state_memory_size (SMS) for SIP/SigComp
Minimum value for ANY/SigComp: 0 (zero) bytes, as specified in
section 3.3.1 of [RFC-3320]. [RFC3320].
Minimum value for SIP/SigComp: 2048 bytes.
Reason: a non-zero SMS allows an endpoint to upload a state in the
first SIP message sent to a remote endpoint without the uncertainty
of whether or not it can be created in the remote endpoint due to
lack of memory.
Note: SMS is per compartment. An endpoint MAY offer different endpoint. A non-
zero SMS for
different compartments as long as obviously requires the SMS SIP/SigComp implementation to keep
state. Based on the observation that there is not less than little gain from
stateless SigComp compression, the assumption is that purely
stateless SIP implementations are unlikely to provide a SigComp
function. Stateful implementations should have little problem to
keep 2K additional state for each compartment (see Section 6).
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.
[Editor's note: this would prevent the
Compressors that make use of initial state memory MUST implement the
SigComp in conjunction
with stateless SIP proxies. Is this acceptable? Discussions Negative Acknowledgement (NACK) Mechanism [I-D.ietf-rohc-
sigcomp-nack]. (Note that there is no such requirement on
benefit/cost are ongoing in
decompressors, but see also Section 6.) For this requirement,
initial state memory is defined as the ROHC mailing list.]
2.3. 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
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 SIP/SigComp: 16 (same as above)
3.4. SigComp_version (SV) for SIP/SigComp
For ANY/SigComp: 0x01, as specified in section 3.3.2 of [RFC-3320]. [RFC3320].
OPEN ISSUE: what SigComp version(s) should be required for SIP/
Version >= 0x01 (i.e. any SigComp)
Version >= 0x02 (i.e. at least SigComp + NACK)
Version == 0x01 | 0x02 (base SigComp or SigComp + NACK)
Version == 0x02 (only SigComp with NACK)
For SIP/SigComp: 0x01 (same as above)
3.5. locally available state (LAS) for SIP/SigComp
Minimum LAS for ANY/SigComp: none, see section 3.3.3 of [RFC-3320] [RFC3320].
Minimum LAS for SIP/SigComp: the SIP/SDP static dictionary as defined
3. Compartment [RFC3485].
4. Delimiting SIP Messages and State Management for SIP/SigComp
When SigComp is applied Messages on the Same Port
In order to SIP, there is a one-to-one relationship
between a SIP dialog (see section 12 limit the number of [RFC-3261]) and ports required by a pair of
peer SigComp compartments. The mapping SigComp-aware
endpoint, it is handled by possible to allow both SigComp messages and 'vanilla'
SigComp) as follows:
- When a messages (i.e. uncompressed SIP user agent client (UAC) compresses and sends a request
that can establish a dialog (such as INVITE), it creates a
compartment associated messages with the dialog.
- When a SIP user agent server (UAS) receives a compressed request
and decides no SigComp header)
to respond with a response that establishes arrive on the same port.
For a dialog
(such message-based transport such as a 2xx to INVITE), it creates a compartment associated with
- Conceptually, the SIP layer of each SigComp UDP or SCTP, this can be done
per message. The receiving endpoint uses checks the
dialog ID as the SigComp compartment ID for first octet of the compartment
UDP/SCTP payload to determine whether the dialog. However, message has been compressed
using SigComp. If the actual representation MSBs of the compartment ID is a local implementation issue. The only
requirement octet are "11111" then the message
is considered to maintain a local one-to-one mapping between a
dialog ID and be a compartment ID.
- A SigComp compartment will be closed by SIP when the corresponding
dialog message and is terminated.
Usually, any states created during parsed as per [RFC3320].
If the lifetime MSBs of a compartment will
be "logically" deleted when the compartment octet take any other value, then the message is closed. A logical
deletion becomes a physical one when all
assumed to be an uncompressed SIP message, and is passed directly to
the compartments that
created application with no further effect on the (same) state are closed. However, a SigComp endpoint may
offer to keep layer.
For a state created upon request from its peer stream-based transport such as TCP, the distinction is per
connection. The receiving endpoint
during a dialog beyond checks the duration of that dialog. This may improve
compression efficiency first octet of SIP messages generated by the same peer
endpoint in subsequent dialogs. In that case, it can inform its peer
SigComp endpoint by announcing the (partial) state ID in TCP
data stream to determine whether the returned
SigComp parameters at stream has been compressed using
SigComp. If the end MSBs of the dialog. That signals the state
will be maintained until octet are "11111" then the associated timer times out. Since there stream is no mechanism in
considered to contain SigComp messages and SIP to convey is parsed as per
[RFC3320]. If the MSBs of the timeout octet take any other value, a
default value needs to be specified [Editor's note: actual value will
be determined later]. The default value can be overwritten by
different means in a particular SIP configuration so long as then the
stream is known assumed to and agreed by all SigComp endpoints involved. If
one SigComp endpoint is a contain uncompressed SIP registrar server messages, and its peer endpoints
register with it, the lifetime of the state can be specified is passed
directly to be the same as that of application with no further effect on the registration.
[Editor's note: this is more complicated than it appears. Above is
only preliminary text serving as a starting point. There are
improvements and alternatives being discussed in SigComp
layer. Note that SigComp message delimiters MUST NOT be used if the ROHC WG.]
stream contains uncompressed SIP Messages messages.
Applications MUST NOT mix SIP messages and SigComp Messages messages on a
single TCP connection. If the Same Port
In order TCP connection is used to limit carry
SigComp messages then all messages sent over the number of ports required connection MUST have
a SigComp header and be delimited by the use of 0xFFFF as described
Note: Appendix A shows how to send uncompressed messages in a SigComp-aware
structured TCP connection using a "well-known shim header". Should
it is possible for any reason not be desirable to multiplex SigComp messages set up more than one TCP
connection to a SIP implementation, but the flexibility to send both
compressed and 'vanilla' uncompressed SIP messages (i.e. be required, the compressor
can set up a SigComp structured connection and send any uncompressed
SIP messages with no SigComp header)
on using the same port.
For well-known shim header.
5. Continuous Mode over TCP
Continuous Mode is a message-based transport such as UDP, the receiving endpoint
checks the first octet special feature of the UDP payload SigComp, which is designed to determine whether the
message has been compressed using SigComp. If the MSBs of
improve the octet
are "11111" then overall compression ratio for long-lived connections.
Its use requires pre-agreement between the message SigComp compressor and
decompressor. Continuous mode is not used with SIP/SigComp.
Reason: continuous mode requires the transport itself to provide a
certain level of protection against denial of service attacks. TCP
alone is not considered to provide enough protection.
6. 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.
6.1. Remote Application Identifiers
SIP/SigComp applications identify remote applications by their FQDN
(Fully Qualified Domain Name) or by their IP address. For outgoing
requests, the remote application identifier is the host part of the
URI to which the request is sent. For incoming responses, the remote
application identifier is the same as the one for the previously-sent
request that initiated the transaction the response belongs to. For
incoming requests and outgoing responses, the remote application
identifier is the sent-by parameter of the top-most Via entry.
A given remote application identifier is mapped to a particular
SigComp compartment ID following the rules given in the following
OPEN ISSUE: this is an implicit way of identifying remote
applications. It assumes that two remote applications are
different if the host parts of their URIs are different. However,
if a proxy farm shares dictionary state among its proxies and
these proxies use different host parts (e.g., proxy1.example.com
and proxy2.example.com), they will be considered like different
remote applications, when they should have been considered a
single remote application. If implementers intend to implement
state sharing this way, we could use explicit application
identifiers instead. These identifiers could be placed in a SIP
URI parameter (e.g., sip:p1.example.net;id="12wsfeQ45") and in a
6.2. Compartment Opening and Closure
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
application obtained the remote application identifier (e.g., in a
Record-Route header field of an incoming SIP message). There are
compartments that are valid for the duration of a registration, of a
dialog, and of a single transaction. The following sections specify
how a SIP application decides the lifetime of a particular
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
duration of a registration for a particular remote application
identifier. At a later point, the application is supposed to open a
new compartment for the duration of a particular dialog for the same
remote application identifier. Following the previous rule, the SIP
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 open for the registration.
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
A SigComp endpoint may offer to keep a state created upon request
from a SigComp peer endpoint beyond the default lifetime of a
compartment. This may be used to improve compression efficiency of
subsequent SIP messages generated by the same remote application at
the SigComp peer endpoint. To indicate that such state will continue
to be available, the SigComp endpoint can inform its peer SigComp
endpoint by announcing the (partial) state ID in the returned SigComp
parameters at the end of the registration, dialog, or transaction
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
any limit to the lifetime of this state, both decompressors that
intend to offer state with possibly limited lifetimes as well as
compressors that make use of such state SHOULD implement the SigComp
Negative Acknowledgement (NACK) Mechanism [I-D.ietf-rohc-sigcomp-
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 are RECOMMENDED to
implement the SigComp Negative Acknowledgement (NACK) Mechanism
[I-D.ietf-rohc-sigcomp-nack], which facilitates recovery in a
situation where such old state may no longer be available.
6.3. Compartment Valid During a Transaction
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 can close the compartment when the
transaction is over.
A SIP application 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 an
already existing compartment to send and receive responses.
6.4. Compartment Valid During a Registration
A REGISTER transaction can cause an application to open a new
compartment to be valid for the duration of the registration
established by the REGISTER transaction.
A 200 (OK) response for a register may contain a Path [RFC3327] and a
Service-Route [RFC3308] header field. These header fields indicate
the route future incoming and outgoing requests will follow.
On receiving a 200 (OK) response for a REGISTER, a SIP application
that inserted itself in the Contact (i.e., because it is the user
agent) or in the Path header field of the REGISTER, 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). 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
(although traversed in the opposite direction) as the route for
Note that some user agents use several registration in parallel to
improve service reliability. Different registration typically have
different associated route vectors. Messages sent to different
remote application identifiers will use different compartments, even
if those messages are generated by the same user agent. It is
assumed that the remote applications do not share SIP/SigComp state
6.5. Compartment Valid During a Dialog
A transaction that establishes a dialog can cause an application to be
open a SigComp message
and is parsed as per [RFC-3320]. If new compartment to be valid for the MSBs duration of the octet take any
other value, then dialog
established by the transaction.
A SIP message is assumed to be that establishes a dialog (e.g., a 2xx response for an uncompressed
INVITE) may contain a Record-Route header field. This header field
indicates the route future requests within the dialog will follow.
On receiving such a SIP message, and is passed directly to the a SIP application with no further
effect on that inserted
itself in the SigComp layer.
For a stream-based transport such as TCP, Contact (i.e., because it is the receiving endpoint
checks user agent) or in the first octet
Record-Route header field of the TCP data stream to determine whether request, constructs (using the stream has been compressed using SigComp. If
Contact, and the MSBs of Record-Route header fields) the
octet are "11111" then route requests
within the stream is considered to contain SigComp
messages and is parsed as per [RFC-3320]. If dialog will follow. The application checks whether the MSBs
URIs of its adjacent applications in that route have the octet
take any other value, then the stream is assumed to contain
uncompressed SIP messages, and is passed directly to
"comp=sigcomp" parameter. The application SHOULD open a new
compartment for the remote application
with no further effect on identifier of the SigComp layer. Note URIs with
message delimiters MUST parameter. The application SHOULD NOT be used if close the stream contains
uncompressed SIP messages.
Applications MUST NOT mix SIP messages and SigComp messages on a
single TCP connection. If compartments
until the TCP connection dialog is used to carry
SigComp messages then all messages sent over over.
7. Recommendations for Network Administrators
Network administrators can configure their networks so that the connection MUST have
compression efficiency achieved is increased. The following
recommendations help network administrators perform their task.
For a SigComp given user agent, the route sets for incoming requests (created
by a Path header field) and be delimited for outgoing requests (created by a
Service-Route header field) are typically the same. However,
registrars can, if they wish, insert proxies in the use of 0xFFFF as described latter route that
do not appear in [RFC 3320].
[Editor's note: there have been discussions on the need of
multiplexing SigComp former route and non-SigComp messages on the same TCP
connection. However, no conclusion was reached.]
5. Continuous Mode over TCP
Continuous Mode 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 special feature of SigComp, which
dialog. Requests inside a dialog follow a route constructed using
Record-Route header fields. It is designed to
improve RECOMMENDED that the overall compression ratio for long-lived connections.
However, it requires proxies
performing SigComp that are in the transport itself to provide route for requests outside a certain level
of protection against denial of service attacks. TCP is not
dialog are configured to provide enough protection, and so Continuous Mode MUST
NOT be used over TCP.
[Editor's note: is this too restrictive place themselves (by inserting themselves in cases where IPsec is
enabled below TCP? There have been no comments on this issue so far.]
the Record-Route header fields) in the routes used for requests
8. Security Considerations
The same security considerations as described in [RFC-3320] [RFC3320] apply to
this document. Note that keeping SigComp states longer than the
duration of a SIP dialog should not pose new security risks for two
reasons: a) the state has been allowed to be created in the first
place; and b) this is on voluntary basis and a SigComp endpoint can
choose not to offer it.
9. 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
o Minimum values (Section 3).
o Compartment definition (Section 6).
o Use of continuous mode (Section 5).
10. IANA Considerations
This specification does not require any actions from the IANA.
Abigail Surtees provided the code and text for Appendix A.
The authors would like to thank the following people for their
comments and suggestions: Carsten Bormann, Abigail Surtees, Jan Christoffersson, Joerg
Ott, Mark West, Pekka Pessi, Robert Sugar, Abigail Surtees, and Mark West.
8. Adam Roach.
12. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M. M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
[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. 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. Adam,
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.
9. Session Initiation
Protocol (SIP)", RFC 3486, February 2003.
Roach, A., "A Negative Acknowledgement Mechanism for
Signaling Compression", draft-ietf-rohc-sigcomp-nack-02
(work in progress), October 2004.
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:
: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)
: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!
INPUT-BYTES (1, byte_copy_left, end)
OUTPUT (byte_copy_left, 1)
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.
Carsten Bormann (editor)
Universitaet Bremen TZI
Phone: +49 421 218 7024
Fax: +49 421 218 7000
Nokia Research Center
6000 Connection Drive
Irving, TX 75039
Phone: +1 972 894-5935
Roke Manor Research Ltd
Romsey, Hants, SO51 0ZN
Cogent Defence and Security Networks
Queensway Meadows Industrial Estate
Newport, Gwent NP19 4SS
Phone: +44 1794 (0)1794 833681
10. Full copyright statement
Copyright (C) email@example.com
Intellectual Property Statement
The Internet Society (2004). All Rights Reserved.
This document and translations IETF takes no position regarding the validity or scope of it may any
Intellectual Property Rights or other rights that might be copied and furnished claimed to
others, and derivative works that comment on or otherwise explain it
or assist in its
pertain to the implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction use of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, technology described in
this document itself may or the extent to which any license under such rights
might or might not be modified in available; nor does it represent that it has
made any independent effort to identify any way, such as by removing rights. Information
on the copyright notice or references procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the Internet Society IETF Secretariat and any
assurances of licenses to be made available, or other
Internet organizations, except as needed for the purpose result of
developing Internet standards in which case the procedures an
attempt made to obtain a general license or permission for
copyrights defined in the Internet Standards process must be
followed, use of
such proprietary rights by implementers or as required to translate it into languages other than
The limited permissions granted above are perpetual and will not users of this
specification can be
revoked by obtained from the Internet Society or IETF on-line IPR repository at
The IETF invites any interested party to bring to its successors attention any
copyrights, patents or patent applications, or assigns. other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
Disclaimer of Validity
This document and the information contained herein is are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIMS DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright (C) The Internet Society (2006). This Internet-Draft will expire on August 12, 2004. document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Funding for the RFC Editor function is currently provided by the