Network Working Group A. Mankin
Request for Comments: 3427 Bell Labs, Lucent Corporation
BCP: 67 S. Bradner
Category: Best Current Practice Harvard University
ipDialog / Uni Bremen TZI
December 2002 Change Process for the Session Initiation Protocol (SIP)
Status of this Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2002). All Rights Reserved.
This memo documents a process intended to apply architectural
discipline to the future development of the Session Initiation
Protocol (SIP). There have been concerns with regards to new SIP
proposals. Specifically, that the addition of new SIP features can
be damaging towards security and/or greatly increase the complexity
of the protocol. The Transport Area directors, along with the SIP
and Session Initiation Proposal Investigation (SIPPING) working group
chairs, have provided suggestions for SIP modifications and
In this document, the key words "MAY", "MUST, "MUST NOT", "SHOULD",
and "SHOULD NOT", are to be interpreted as described in Keywords .
2. History and Development
The IETF's Session Initiation Protocol (SIP)  was originally
developed for initiation of multimedia sessions. Internet
multimedia, voice over IP, IP telephony, and SIP have become quite
popular, both inside IETF and with other standards groups, and the
applications of SIP have grown. One result of this popularity has
been a continual flood of suggestions for SIP modifications and
extensions. The task for IETF management of SIP has been to keep the
protocol development focused on SIP's core strengths and the
applications it does best.
2.1 The IETF SIP Working Group
The IETF SIP Working Group has been chartered to be the "owner" of
the SIP protocol , as long as the working group exists. All
changes or extensions to SIP must first exist as SIP Working Group
documents. The SIP Working group is charged with being the guardian
of the SIP protocol for the Internet, and therefore should only
extend or change the SIP protocol when there are compelling reasons
to do so.
Documents that must be handled by the SIP working group include new
SIP methods, new SIP option tags, new response codes, and new
standards track SIP headers. With the exception of "P-" headers
described in Section 4.1, all SIP extensions must be standards track
and must be developed in the IETF based upon requirements provided by
the SIPPING Working Group.
IETF working groups do not live forever; typically, mailing lists
continue after the working group is concluded. If the SIP Working
Group has closed and no suitable replacement or follow-on working
group is active, the Transport Area directors will the use the non-
working group standards track document process (described in section
6.1.2 of RFC 2026--IETF Standards Process ) using the SIP and
SIPPING mailing lists and designated experts from the SIP community
for advice. The IETF will remain the home of extensions of SIP and
the requirement of standards track action will remain as defined in
the rest of this document. The rate of growth of extensions of any
protocol in the IETF is hoped to be low.
It is appropriate for any working group to develop SIP event packages
, but the working group must have charter approval to do so. The
IETF will also require (Individual) RFC publication for the
registration of event packages developed outside the scope of an IETF
working group. Requirements for publishing event packages are
described in detail in Section 4.3.
2.2 The IETF SIPPING Working Group
The IETF Session Initiation Protocol Proposal Investigation (sipping)
Working Group is chartered to be a filter in front of the SIP Working
Group. This working group will investigate requirements for
applications of SIP, some of which may lead to requests for
extensions to SIP. These requirements may come from the community at
large, or from individuals who are reporting the requirements as
determined by another standards body. The SIPPING Working Group will
also not live forever, with similar consideration to the sections
The SIPPING Working Group may determine: that these requirements can
be satisfied by SIP without modifications, that the requirements are
not sufficiently general to warrant a change to SIP, that the
requirements justify a change to SIP, or that the requirements should
be combined with other requirements to solve a more general problem
or solve the same problem in a more flexible way.
Because the SIP protocol gets so much attention, some application
designers may want to use it just because it is there, such as for
controlling household appliances. SIPPING should act as a filter,
accepting only requirements which play to the best strengths of SIP,
such as realtime presence.
When the SIPPING working group decides on a set of requirements, it
forwards them to the SIP working group. The SIPPING Working Group
may also document usage or applications of SIP which do not require
any protocol extensions.
The SIPPING working group also acts as a filter for proposed event
packages as described in Section 4.3.
3. SIP Change Process
Anyone who thinks that the existing SIP protocol is applicable to
their application, yet not sufficient for their task must write an
individual Internet-Draft explaining the problem they are trying to
solve, why SIP is the applicable protocol, and why the existing SIP
protocol will not work. The Internet-Draft must include a detailed
set of requirements (distinct from solutions) that SIP would need to
meet to solve the particular problem. The Internet-Draft must also
describe in detail any security issues that arise from meeting those
requirements. After the Internet-Draft is published, the authors
should send a note to the SIPPING Working Group mailing list to start
discussion on the Internet-Draft.
The SIPPING working group chairs, in conjunction with the Transport
Area Directors, will determine if the particular problems raised in
the requirements Internet-Draft warrants being added to the SIPPING
charter based on the mailing list discussion. The SIPPING working
group should consider whether the requirements can be merged with
other requirements from other applications, and refine the ID
If the chairs and the ADs both feel that the particular new problems
should be added to the SIPPING Working Group charter, then the ADs
will present the proposed SIPPING charter modifications to the IESG
and IAB, in accordance with the usual process for charter expansion.
If the IESG (with IAB advice) approves of the charter changes, the
SIPPING working group can then work on the problems described in the
In a separate Internet-Draft, the authors may describe a set of
changes to SIP that would meet the requirements. The Internet-Draft
would then be passed to the SIP working group for consideration (if
warranted). The SIP working group is not required to adopt the
proposed solution from this additional Internet-Draft.
The SIPPING working group may also evaluate such proposals for
extensions if the requirements are judged to be appropriate to SIP,
but are not sufficiently general for standards track activity. The
SIPPING working group will attempt to determine if the new proposal
meets the requirements for publication as a "P-" header, as described
in Section 4.1, within a specific scope of applicability.
The Transport ADs may, on a case by case basis, support a process in
which the requirements analysis is implicit and the SIP working group
requests the addition of a charter item for an extension without a
full SIPPING process as described. This will be the exception.
With respect to standardization, this process means that SIP
extensions come only from the IETF, the body that created SIP. The
IETF will not publish a SIP extension RFC outside of the processes
The SIP Working Group is required to protect the architectural
integrity of SIP and must not add features that do not have general
use beyond the specific case. Also, they must not add features just
to make a particular function more efficient at the expense of
simplicity or robustness.
Some working groups besides SIPPING generate requirements for SIP
solutions and/or extensions as well. At the time this document was
written, these include SIP for Instant Messaging and Presence
Leveraging Extensions (simple), Service in the PSTN/IN Requesting
InTernet Service (spirits), and Telephone Number Mapping (enum).
4. Extensibility and Architecture
In an idealized protocol model, extensible design would be self-
contained, and it would be inherent that new extensions and new
headers would naturally have an architectural coherence with the
However, this idealized vision has not been attained in the world of
standards track protocols. While, interoperability implications can
be addressed by capabilities negotiation rules, the effects of adding
features that overlap, or that deal with a point solution and are not
general, are much harder to control with rules. Therefore, the
Transport Area calls for architectural guardianship and application
of Occam's Razor by the SIP Working Group.
In keeping with the IETF tradition of "running code and rough
consensus", it is valid to allow for the development of SIP
extensions that are either not ready for standards track, but might
be understood for that role after some running code, or are private
or proprietary in nature, because a characteristic motivating them is
usage that is known not to fit the Internet architecture for SIP. We
call these "P-" headers, for "preliminary", "private", or
There are two key issues to consider with respect to keeping the "P-"
header extension space "safe":
1. Clearly indicating the unarchitected or not-yet understood nature
of the extension.
2. Preventing identity conflicts between extensions.
4.1 Indicating a "P-" Header:
Use of an "X-" prefix on textual identifiers has been widely used to
indicate experimental extensions in other protocols. This approach
is applied in modified form here by use of a "P-" header extension.
However, there are a number of stronger constraints for "P-" headers,
including documentation that get Expert and IESG review, and other
SIP protocol criteria described below.
Informational SIP Headers can be registered as "P-" headers if all of
the following conditions are met:
1. A designated expert (as defined in RFC 2434 ) MUST review the
proposal for applicability to SIP and conformance to these
guidelines. The Expert Reviewer will send email to the Transport
Area Directors on this determination. The expert reviewer can
cite one or more of the guidelines that haven't been followed in
2. The proposed extension MUST NOT define SIP option tags, response
codes, or methods.
3. The function of the proposed header MUST NOT overlap with current
or planned chartered extensions.
4. The proposed header MUST be of a purely informational nature, and
MUST NOT significantly change the behavior of SIP entities which
support it. Headers which merely provide additional information
pertinent to a request or a response are acceptable. If the
headers redefine or contradict normative behavior defined in
standards track SIP specifications, that is what is meant by
significantly different behavior.
5. The proposed header MUST NOT undermine SIP security in any sense.
The Internet Draft proposing the new header MUST address security
issues in detail as if it were a Standards Track document. Note
that, if the intended application scenario makes certain
assumptions regarding security, the security considerations only
need to meet the intended application scenario rather than the
general Internet case. In any case, security issues need to be
discussed for arbitrary usage scenarios (including the general
6. The proposed header MUST be clearly documented in an (Individual
or Working Group) Informational RFC, and registered with IANA.
7. An applicability statement in the Informational RFC MUST clearly
document the useful scope of the proposal, and explain its
limitations and why it is not suitable for the general use of SIP
in the Internet.
Any implementation of a "P-" header (meaning "not specified by a
standards-track RFC issued through the SIP Working Group") MUST
include a "P-" prefix on the header, as in "P-Headername". Note that
"P-" extensions are not IETF standards of any kind, and MUST NOT be
required by any production deployment considered compliant to IETF
specifications. Specifically, implementations are only SIP compliant
if a) they fall back to baseline behavior when they ignore all P-
headers, and b) when using P- headers they do not contradict any
4.2 Preventing Identity Conflicts Between P-Extensions:
In order to prevent identity conflicts between P-headers, this
document provides an IANA process (See: "IANA Considerations" below)
to register the P-headers. The handling of unknown P-headers is to
ignore them, however, section 4.1 is to be taken seriously, and users
of P-headers will have best results with adherence. All implemented
P-headers SHOULD meet the P-Header requirements in 4.1. Any P-header
used outside of a very restricted research or teaching environment
(such as a student lab on implementing extensions) MUST meet those
requirements and MUST be documented in an RFC and be IANA registered.
IANA registration is permitted when the IESG approves the internet-
4.3 SIP Event Packages
events  defines two different types of event packages: normal
event packages, and event template-packages. Event template-packages
can only be created and registered by the publication of a Standards
Track RFC (from an IETF Working Group). Normal event packages can be
created and registered by the publication of any Working Group RFC
(Informational, Standards Track, Experimental), provided that the RFC
is a chartered working group item.
Individuals may also wish to publish SIP Event packages. Individual
proposals for registration of a SIP event package MUST first be
published as Internet-drafts for review by the SIPPING Working Group,
or the working group, mailing list, or expert designated by the
Transport Area Directors if the SIPPING Working Group has closed.
Proposals should include a strong motivational section, a thorough
description of the proposed syntax and semantics, event package
considerations, security considerations, and examples of usage. The
author should submit his or her proposal as an individual Internet-
Draft, and post an announcement to the working group mailing list to
begin discussion. The SIPPING Working Group will determine if the
proposed package is a) an inappropriate usage of SIP, b) applicable
to SIP but not sufficiently interesting, general, or in-scope to
adopt as a working group effort, c) contrary to similar work planned
in the Working Group, or d) should be adopted as or merged with
The IETF requires (Individual) RFC publication for registration of
event packages developed outside the scope of an IETF working group,
according to the following guidelines:
1. A designated expert (as defined in RFC 2434 ) MUST review the
proposal for applicability to SIP and conformance with these
guidelines. The Expert Reviewer will send email to the IESG on
this determination. The expert reviewer can cite one or more of
the guidelines that have not been followed in his/her opinion.
2. The proposed extension MUST NOT define an event template-package.
3. The function of the proposed package MUST NOT overlap with
current or planned chartered packages.
4. The event package MUST NOT redefine or contradict the normative
behavior of SIP events , SIP , or related standards track
5. The proposed package MUST NOT undermine SIP security in any
sense. The Internet Draft proposing the new package MUST address
security issues in detail as if it were a Standards Track
document. Security issues need to be discussed for arbitrary
usage scenarios (including the general Internet case).
6. The proposed package MUST be clearly documented in an
(Individual) Informational RFC, and registered with IANA. The
package MUST document all the package considerations required in
Section 5 of SIP events .
7. If determined by the expert reviewer or the chairs or ADs of the
SIPPING WG, an applicability statement in the Informational RFC
MUST clearly document the useful scope of the proposal, and
explain its limitations and why it is not suitable for the
general use of SIP in the Internet.
5. Security Considerations
Complexity and indeterminate or hard to define protocol behavior,
depending on which of many extensions operate, is a fine breeding
ground for security flaws.
All Internet-Drafts that present new requirements for SIP must
include a discussion of the security requirements and implications
inherent in the proposal. All RFCs that modify or extend SIP must
show that they have adequate security and do not worsen SIP's
existing security considerations.
6. IANA Considerations
RFC 3261  directs the Internet Assigned Numbers Authority (IANA)
to establish a registry for SIP method names, a registry for SIP
option tags, and a registry for SIP response codes, and to amend the
practices used for the existing registry for SIP headers.
With the exception of P-headers, entries go into these registries
only by approval of an Internet-Draft as a standards track RFC.
Each RFC shall include an IANA Considerations section which directs
IANA to create appropriate registrations. Registration shall be done
at the time the IESG announces its approval of the draft containing
the registration requests.
Standard headers and messages MUST NOT begin with the leading
"P-" header names MUST begin with the leading characters "P-". No
"P-" header which conflicts with (would, without the "P-" prefix have
the same name as) an existing standards track header is allowed.
Each registration of a "P-" header will also reserve the name of the
header as it would appear without the "P-" prefix. However, the
reserved name without the "P-" will not explicitly appear in the
registry. It will only appear if there is a later standards track
document (which is unlikely in most cases!). Please do not accept
the registration of IANA-Greeting when you see: P-IANA-Greeting.
P-header's "reserved standard names" MUST NOT be used in a SIP
implementation prior to standardization of the header.
Short forms of headers MUST only be assigned to standards track
headers. In other words, P-headers MUST NOT have short forms.
Similarly, RFC 3265  directs the IANA to establish a registry for
SIP event packages and SIP event template packages. For event
template packages, entries go into this registry only by approval of
a draft for standards track RFC. For ordinary event packages,
entries go into this registry only by approval of a draft for RFC (of
any type). In either case, the IESG announcement of approval
authorizes IANA to make the registration.
The Transport ADs thank our IESG and IAB colleagues (especially Randy
Bush, Harald Alvestrand, John Klensin, Leslie Daigle, Patrik
Faltstrom, and Ned Freed) for valuable discussions of extensibility
issues in a wide range of protocols, including those that our area
brings forward and others. Thanks to the many members of the SIP
community engaged in interesting dialogue about this document as
well; Jonathan Rosenberg and Jon Peterson gave us useful reviews.
Thanks also to Henning Schulzrinne and William Marshall.
8. Normative References
 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
 Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
 Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
 Roach, A., "Session Initiation Protocol (SIP) - Specific Event
Notification", RFC 3265, June 2002.
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