draft-ietf-sip-guidelines-07.txt   draft-ietf-sip-guidelines-08.txt 
SIP J. Rosenberg SIP J. Rosenberg
Internet-Draft dynamicsoft Internet-Draft dynamicsoft
Expires: April 26, 2004 H. Schulzrinne Expires: January 16, 2005 H. Schulzrinne
Columbia University Columbia University
October 27, 2003 July 18, 2004
Guidelines for Authors of Extensions to the Session Initiation Guidelines for Authors of Extensions to the Session Initiation
Protocol (SIP) Protocol (SIP)
draft-ietf-sip-guidelines-07 draft-ietf-sip-guidelines-08
Status of this Memo Status of this Memo
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved. Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract Abstract
The Session Initiation Protocol (SIP) is a flexible, yet simple tool The Session Initiation Protocol (SIP) is a flexible, yet simple tool
for establishing interactive connections across the Internet. Part of for establishing interactive connections across the Internet. Part
this flexibility is the ease with which it can be extended. In order of this flexibility is the ease with which it can be extended. In
to facilitate effective and interoperable extensions to SIP, some order to facilitate effective and interoperable extensions to SIP,
guidelines need to be followed when developing SIP extensions. This some guidelines need to be followed when developing SIP extensions.
document outlines a set of such guidelines for authors of SIP This document outlines a set of such guidelines for authors of SIP
extensions. extensions.
Table of Contents Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Should I define a SIP Extension? . . . . . . . . . . . . . . 5 3. Should I define a SIP Extension? . . . . . . . . . . . . . . 5
3.1 SIP's Solution Space . . . . . . . . . . . . . . . . . . . . 5 3.1 SIP's Solution Space . . . . . . . . . . . . . . . . . . . 5
3.2 SIP Architectural Model . . . . . . . . . . . . . . . . . . 7 3.2 SIP Architectural Model . . . . . . . . . . . . . . . . . 7
4. Issues to be Addressed . . . . . . . . . . . . . . . . . . . 10 4. Issues to be Addressed . . . . . . . . . . . . . . . . . . . 10
4.1 Backwards Compatibility . . . . . . . . . . . . . . . . . . 10 4.1 Backwards Compatibility . . . . . . . . . . . . . . . . . 10
4.2 Security . . . . . . . . . . . . . . . . . . . . . . . . . . 12 4.2 Security . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.3 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 12 4.3 Terminology . . . . . . . . . . . . . . . . . . . . . . . 12
4.4 Syntactic Issues . . . . . . . . . . . . . . . . . . . . . . 13 4.4 Syntactic Issues . . . . . . . . . . . . . . . . . . . . . 13
4.5 Semantics, Semantics, Semantics . . . . . . . . . . . . . . 15 4.5 Semantics, Semantics, Semantics . . . . . . . . . . . . . 15
4.6 Examples Section . . . . . . . . . . . . . . . . . . . . . . 16 4.6 Examples Section . . . . . . . . . . . . . . . . . . . . . 16
4.7 Overview Section . . . . . . . . . . . . . . . . . . . . . . 16 4.7 Overview Section . . . . . . . . . . . . . . . . . . . . . 16
4.8 IANA Considerations Section . . . . . . . . . . . . . . . . 16 4.8 IANA Considerations Section . . . . . . . . . . . . . . . 16
4.9 Document Naming Conventions . . . . . . . . . . . . . . . . 17 4.9 Document Naming Conventions . . . . . . . . . . . . . . . 17
4.10 Additional Considerations for New Methods . . . . . . . . . 18 4.10 Additional Considerations for New Methods . . . . . . . 18
4.11 Additional Considerations for New Header Fields or Header 4.11 Additional Considerations for New Header Fields or
Field Parameters . . . . . . . . . . . . . . . . . . . . . . 19 Header Field Parameters . . . . . . . . . . . . . . . . 19
4.12 Additional Considerations for New Body Types . . . . . . . . 19 4.12 Additional Considerations for New Body Types . . . . . . 19
5. Interactions with SIP Features . . . . . . . . . . . . . . . 20 5. Interactions with SIP Features . . . . . . . . . . . . . . . 20
6. Security Considerations . . . . . . . . . . . . . . . . . . 21 6. Security Considerations . . . . . . . . . . . . . . . . . . 21
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 22 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 22
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 23 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 23
Normative References . . . . . . . . . . . . . . . . . . . . 24 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
Informative References . . . . . . . . . . . . . . . . . . . 25 9.1 Normative References . . . . . . . . . . . . . . . . . . . . 24
9.2 Informative References . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 26 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 26
Intellectual Property and Copyright Statements . . . . . . . 27 Intellectual Property and Copyright Statements . . . . . . . 27
1. Terminology 1. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED", In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [1] and and "OPTIONAL" are to be interpreted as described in RFC 2119 [1] and
indicate requirement levels for compliant implementations. indicate requirement levels for compliant implementations.
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While the first criteria might seem obvious, we have observed that While the first criteria might seem obvious, we have observed that
numerous extensions to SIP have been proposed because some function numerous extensions to SIP have been proposed because some function
is needed in a device which also speaks SIP. The argument is is needed in a device which also speaks SIP. The argument is
generally given that "I'd rather implement one protocol than many". generally given that "I'd rather implement one protocol than many".
As an example, user agents, like all other IP hosts, need some way to As an example, user agents, like all other IP hosts, need some way to
obtain their IP address. This is generally done through DHCP [9]. obtain their IP address. This is generally done through DHCP [9].
SIP's multicast registration mechanisms might supply an alternate way SIP's multicast registration mechanisms might supply an alternate way
to obtain an IP address. This would eliminate the need for DHCP in to obtain an IP address. This would eliminate the need for DHCP in
clients. However, we do not believe such extensions are appropriate. clients. However, we do not believe such extensions are appropriate.
We believe that protocols should be defined to provide specific, We believe that protocols should be defined to provide specific,
narrow functions, rather than being defined based on all narrow functions, rather than being defined based on all protocols
communications requirements between a pair of devices. The latter needed between a pair of devices. The latter approach to protocol
approach to protocol design yields modular protocols with broad design yields modular protocols with broad application. It also
application. It also facilitates extensibility and growth; single facilitates extensibility and growth; single protocols can be removed
protocols can be removed and changed without affecting the entire and changed without affecting the entire system. We observe that
system. We observe that this approach to protocol engineering mirrors this approach to protocol engineering mirrors object oriented
object oriented software engineering. software engineering.
Our second criteria, that the extension must conform to the general Our second criteria, that the extension must conform to the general
SIP architectural model, ensures that the protocol remains manageable SIP architectural model, ensures that the protocol remains manageable
and broadly applicable. and broadly applicable.
3.1 SIP's Solution Space 3.1 SIP's Solution Space
In order to evaluate the first criteria, it is necessary to define In order to evaluate the first criteria, it is necessary to define
exactly what SIP's solution space is, and what it is not. exactly what SIP's solution space is, and what it is not.
SIP is a protocol for initiating, modifying, and terminating SIP is a protocol for initiating, modifying, and terminating
interactive sessions. This process involves the discovery of users, interactive sessions. This process involves the discovery of users,
(or more generally, entities that can be communicated with, including (or more generally, entities that can be communicated with, including
services, such as voicemail or translation devices) wherever they may services, such as voicemail or translation devices) wherever they may
be located, so that a description of the session can be delivered to be located, so that a description of the session can be delivered to
the user. It is assumed that these users or communications entities the user. It is assumed that these users or communications entities
are mobile, and their point of attachment to the network changes over are mobile, and their point of attachment to the network changes over
time. The primary purpose of SIP is a rendezvous function, to allow a time. The primary purpose of SIP is a rendezvous function, to allow
request initiator to deliver a message to a recipient wherever they a request initiator to deliver a message to a recipient wherever they
may be. Such rendezvous is needed to establish a session, but can be may be. Such rendezvous is needed to establish a session, but can be
used for other purposes related to communications, such as querying used for other purposes related to communications, such as querying
for capabilities or delivery of an instant message. for capabilities or delivery of an instant message.
Much of SIP focuses on this discovery and rendezvous component. Its Much of SIP focuses on this discovery and rendezvous component. Its
ability to fork, its registration capabilities, and its routing ability to fork, its registration capabilities, and its routing
capabilities are all present for the singular purpose of finding the capabilities are all present for the singular purpose of finding the
desired user wherever they may be. As such, features and capabilities desired user wherever they may be. As such, features and
such as personal mobility, automatic call distribution, and follow-me capabilities such as personal mobility, automatic call distribution,
are well within the SIP solution space. and follow-me are well within the SIP solution space.
Session initiation also depends on the ability of the called party to Session initiation also depends on the ability of the called party to
have enough information about the session itself in order to make a have enough information about the session itself in order to make a
decision on whether to join or not. That information includes data decision on whether to join or not. That information includes data
about the caller, the purpose for the invitation, and parameters of about the caller, the purpose for the invitation, and parameters of
the session itself. For this reason, SIP includes this kind of the session itself. For this reason, SIP includes this kind of
information. information.
Part of the process of session initiation is the communication of Part of the process of session initiation is the communication of
progress and the final results of establishment of the session. SIP progress and the final results of establishment of the session. SIP
provides this information as well. provides this information as well.
SIP itself is independent of the session, and the session description SIP itself is independent of the session, and the session description
is delivered as an opaque body within SIP messages. Keeping SIP is delivered as an opaque body within SIP messages. Keeping SIP
independent of the sessions it initiates and terminates is independent of the sessions it initiates and terminates is
fundamental. As such, there are many functions that SIP explicitly fundamental. As such, there are many functions that SIP explicitly
does not provide. It is not a session management protocol or a does not provide. It is not a session management protocol or a
conference control protocol. The particulars of the communications conference control protocol. The particulars of the communications
within the session are outside of SIP. This includes features such as within the session are outside of SIP. This includes features such
media transport, voting and polling, virtual microphone passing, as media transport, voting and polling, virtual microphone passing,
chairman election, floor control, and feedback on session quality. chairman election, floor control, and feedback on session quality.
SIP is not a resource reservation protocol for sessions. This is SIP is not a resource reservation protocol for sessions. This is
fundamentally because (1) SIP is independent of the underlying fundamentally because (1) SIP is independent of the underlying
session it establishes, and (2) the path of SIP messages is session it establishes, and (2) the path of SIP messages is
completely independent from the path that session packets may take. completely independent from the path that session packets may take.
The path independence refers to paths within a providers network, and The path independence refers to paths within a provider's network and
the set of providers itself. For example, it is perfectly reasonable the set of providers itself. For example, it is perfectly reasonable
for a SIP message to traverse a completely different set of for a SIP message to traverse a completely different set of
autonomous systems than the audio in a session SIP establishes. autonomous systems than the audio in a session SIP establishes.
SIP is not a general purpose transfer protocol. It is not meant to SIP is not a general purpose transfer protocol. It is not meant to
send large amounts of data unrelated to SIP's operation. It is not send large amounts of data unrelated to SIP's operation. It is not
meant as a replacement for HTTP. This is not to say that carrying meant as a replacement for HTTP. This is not to say that carrying
payloads in SIP messages is never a good thing; in many cases, the payloads in SIP messages is never a good thing; in many cases, the
data is very much related to SIP's operation. In those cases, data is very much related to SIP's operation. In those cases,
congestion controlled transports end-to-end are critical. congestion controlled transports end-to-end are critical.
SIP is not meant to be a general Remote Procedure Call (RPC) SIP is not meant to be a general Remote Procedure Call (RPC)
mechanism. None of its user discovery and registration capabilities mechanism. None of its user discovery and registration capabilities
are needed for RPC, neither are most of its proxy functions. are needed for RPC, neither are most of its proxy functions.
SIP is not meant to be used as a strict PSTN signaling replacement. SIP is not meant to be used as a strict Public Switched Telephone
It is not a superset of ISUP. While it can support gatewaying of PSTN Network (PSTN) signaling replacement. It is not a superset of the
signaling, and can provide many features present in the PSTN, the Integrated Services Digital Network (ISDN) User Part (ISUP). While
mere existence of a feature or capability in the PSTN is not a it can support gatewaying of PSTN signaling, and can provide many
justification for its inclusion in SIP. Extensions needed to support features present in the PSTN, the mere existence of a feature or
telephony MUST meet the other criteria described here. capability in the PSTN is not a justification for its inclusion in
SIP. Extensions needed to support telephony MUST meet the other
criteria described here.
SIP is a poor control protocol. It is not meant to be used for one SIP is a poor control protocol. It is not meant to be used for one
entity to tell another to pick up or answer a phone, send audio using entity to tell another to pick up or answer a phone, send audio using
a particular codec, or to provide a new value for a configuration a particular codec, or to provide a new value for a configuration
parameter. Control protocols have different trust relationships than parameter. Control protocols have different trust relationships than
is assumed in SIP, and are more centralized in architecture than SIP, is assumed in SIP, and are more centralized in architecture than SIP,
which is a very distributed protocol. which is a very distributed protocol.
There are many network layer services needed to make SIP function. There are many network layer services needed to make SIP function.
These include quality of service, mobility, and security, among These include quality of service, mobility, and security, among
others. Rather than building these capabilities into SIP itself, they others. Rather than building these capabilities into SIP itself,
SHOULD be developed outside of SIP, and then used by it. they SHOULD be developed outside of SIP, and then used by it.
Specifically, any protocol mechanisms that are needed by SIP, but are Specifically, any protocol mechanisms that are needed by SIP, but are
also needed by many other application layer protocols, SHOULD NOT be also needed by many other application layer protocols, SHOULD NOT be
addressed within SIP. addressed within SIP.
3.2 SIP Architectural Model 3.2 SIP Architectural Model
We describe here some of the primary architectual assumptions which We describe here some of the primary architectual assumptions which
underly SIP. Extensions which violate these assumptions should be underly SIP. Extensions which violate these assumptions should be
examined more carefully to determine their appropriateness for SIP. examined more carefully to determine their appropriateness for SIP.
Session independence: SIP is independent of the session it Session independence: SIP is independent of the session it
establishes. This includes the type of session, be it audio, establishes. This includes the type of session, be it audio,
video, game, chat session, or virtual reality. SIP operation video, game, chat session, or virtual reality. SIP operation
SHOULD NOT be dependent on some characteristic of the session. SIP SHOULD NOT be dependent on some characteristic of the session.
is not specific to VoIP only. Any extensions to SIP MUST consider SIP is not specific to voice only. Any extensions to SIP MUST
the application of SIP to a variety of different session types. consider the application of SIP to a variety of different session
types.
SIP and Session Path Independence: We have already touched on this SIP and Session Path Independence: We have already touched on this
once, but it is worth noting again. The set of routers and/or once, but it is worth noting again. The set of routers and/or
networks and/or autonomous systems traversed by SIP messages are networks and/or autonomous systems traversed by SIP messages are
unrelated to the set of routers and/or networks and/or autonomous unrelated to the set of routers and/or networks and/or autonomous
systems traversed by session packets. They may be the same in some systems traversed by session packets. They may be the same in
cases, but it is fundamental to SIP's architecture that they need some cases, but it is fundamental to SIP's architecture that they
not be the same. Extensions which only work under some assumption need not be the same. Extensions which only work under some
of overlap are not generally applicable to SIP's operation and assumption of overlap are not generally applicable to SIP's
should be scrutinized carefully. operation and should be scrutinized carefully.
Multi-provider and Multi-hop: SIP assumes that its messages will Multi-provider and Multi-hop: SIP assumes that its messages will
traverse the Internet. That is, SIP works through multiple traverse the Internet. That is, SIP works through multiple
networks administered by different providers. It is also assumed networks administered by different providers. It is also assumed
that SIP messages traverse many hops (where each hop is a proxy). that SIP messages traverse many hops (where each hop is a proxy).
Extensions SHOULD NOT work only under the assumption of a single Extensions SHOULD NOT work only under the assumption of a single
hop or single provider. hop or single provider.
Transactional: SIP is a request/response protocol, possibly enhanced Transactional: SIP is a request/response protocol, possibly enhanced
with intermediate responses. Many of the rules of operation in SIP with intermediate responses. Many of the rules of operation in
are based on general processing of requests and responses. This SIP are based on general processing of requests and responses.
includes the reliability mechanisms, routing mechanisms, and state This includes the reliability mechanisms, routing mechanisms, and
maintenance rules. Extensions SHOULD NOT add messages that are not state maintenance rules. Extensions SHOULD NOT add messages that
within the request-response model. are not within the request-response model.
Proxies can ignore bodies: In order for proxies to scale well, they Proxies can ignore bodies: In order for proxies to scale well, they
must be able to operate with minimal message processing. SIP has must be able to operate with minimal message processing. SIP has
been engineered so that proxies can always ignore bodies. been engineered so that proxies can always ignore bodies.
Extensions SHOULD NOT require proxies to examine bodies. Extensions SHOULD NOT require proxies to examine bodies.
Proxies don't need to understand the method: Processing of requests Proxies don't need to understand the method: Processing of requests
in proxies does not depend on the method, except for the well in proxies does not depend on the method, except for the well
known methods INVITE, ACK, and CANCEL. This allows for known methods INVITE, ACK, and CANCEL. This allows for
extensibility. Extensions MUST NOT define new methods which must extensibility. Extensions MUST NOT define new methods which must
be understood by proxies. be understood by proxies.
INVITE messages carry full state: An initial INVITE message for a INVITE messages carry full state: An initial INVITE message for a
session is nearly identical (the exception is the tag) to a session is nearly identical (the exception is the tag) to a
re-INVITE message to modify some characteristic of the session. re-INVITE message to modify some characteristic of the session.
This full state property is fundamental to SIP, and is critical This full state property is fundamental to SIP, and is critical
for robustness of SIP systems. Extensions SHOULD NOT modify for robustness of SIP systems. Extensions SHOULD NOT modify
INVITE processing such that data spanning multiple INVITEs must be INVITE processing such that data spanning multiple INVITEs must be
collected in order to perform some feature. collected in order to perform some feature.
Generality over efficiency: Wherever possible, SIP has favored Generality over efficiency: Wherever possible, SIP has favored
general purpose components rather than narrow ones. If some general purpose components rather than narrow ones. If some
capability is added to support one service, but a slightly broader capability is added to support one service, but a slightly broader
capability can support a larger variety of services (at the cost capability can support a larger variety of services (at the cost
of complexity or message sizes), the broader capability SHOULD be of complexity or message sizes), the broader capability SHOULD be
preferred. preferred.
The Request URI is the primary key for forwarding: Forwarding logic The Request URI is the primary key for forwarding: Forwarding logic
at SIP servers depends primarily on the request URI (this is at SIP servers depends primarily on the request URI (this is
different from request routing in SIP, which uses the Route header different from request routing in SIP, which uses the Route header
fields to pass a request through intermediate proxies). It is fields to pass a request through intermediate proxies). It is
fundamental to the operation of SIP that the request URI indicate fundamental to the operation of SIP that the request URI indicate
a resource that, under normal operations, resolves to the desired a resource that, under normal operations, resolves to the desired
recipient. Extensions SHOULD NOT use other components of the SIP recipient. Extensions SHOULD NOT modify the semantics of the
message as the primary forwarding key, and SHOULD NOT modify the request URI.
semantics of the request URI.
Heterogeneity is the norm: SIP supports hetereogeneous devices. It Heterogeneity is the norm: SIP supports hetereogeneous devices. It
has built in mechanisms for determining the set of overlapping has built in mechanisms for determining the set of overlapping
protocol functionalities. Extensions SHOULD NOT be defined which protocol functionalities. Extensions SHOULD NOT be defined which
only function if all devices support the extension. only function if all devices support the extension.
4. Issues to be Addressed 4. Issues to be Addressed
Given an extension has met the litmus tests in the previous section, Given an extension has met the litmus tests in the previous section,
there are several issues that all extensions should take into there are several issues that all extensions should take into
consideration. consideration.
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and/or user agent without understanding the header fields or and/or user agent without understanding the header fields or
parameters, the extension MUST mandate the usage of the Require and/ parameters, the extension MUST mandate the usage of the Require and/
or Proxy-Require header fields in the request. These extensions are or Proxy-Require header fields in the request. These extensions are
not backwards compatible with SIP. The result of mandating usage of not backwards compatible with SIP. The result of mandating usage of
these header fields means that requests cannot be serviced unless the these header fields means that requests cannot be serviced unless the
entities being communicated with also understand the extension. If entities being communicated with also understand the extension. If
some entity does not understand the extension, the request will be some entity does not understand the extension, the request will be
rejected. The UAC can then handle this in one of two ways. In the rejected. The UAC can then handle this in one of two ways. In the
first, the request simply fails, and the service cannot be provided. first, the request simply fails, and the service cannot be provided.
This is basically an interoperability failure. In the second case, This is basically an interoperability failure. In the second case,
the UAC retries the request without the extension. This will preserve the UAC retries the request without the extension. This will
interoperability, at the cost of a "dual stack" implementation in a preserve interoperability, at the cost of a "dual stack"
UAC (processing rules for operation with and without the extension). implementation in a UAC (processing rules for operation with and
As the number of extensions increases, this leads to an exponential without the extension). As the number of extensions increases, this
explosion in the sets of processing rules a UAC may need to leads to an exponential explosion in the sets of processing rules a
implement. The result is excessive complexity. UAC may need to implement. The result is excessive complexity.
Because of the possibility of interoperability and complexity Because of the possibility of interoperability and complexity
problems that result from the usage of Require and Proxy-Require, we problems that result from the usage of Require and Proxy-Require, we
believe the following guidelines are appropriate: believe the following guidelines are appropriate:
o The usage of these header fields in requests for basic SIP o The usage of these header fields in requests for basic SIP
services (in particular, session initiation and termination) is services (in particular, session initiation and termination) is
NOT RECOMMENDED. The less frequently a particular extension is NOT RECOMMENDED. The less frequently a particular extension is
needed in a request, the more reasonable it is to use these header needed in a request, the more reasonable it is to use these header
fields. fields.
o The Proxy-Require header field SHOULD be avoided at all costs.
o The Proxy-Require header field SHOULD be avoided at all costs. The The failure likelihood in an individual proxy stays constant, but
failure likelihood in an individual proxy stays constant, but the the path failure grows exponentially with the number of hops. On
path failure grows exponentially with the number of hops. On the the other hand, the Require header field only mandates that a
other hand, the Require header field only mandates that a single single entity, the UAS, support the extension. Usage of
entity, the UAS, support the extension. Usage of Proxy-Require is Proxy-Require is thus considered exponentially worse than usage of
thus considered exponentially worse than usage of the Require the Require header field.
header field.
o If either Require or Proxy-Require are used by an extension, the o If either Require or Proxy-Require are used by an extension, the
extension SHOULD discuss how to fall back to baseline SIP extension SHOULD discuss how to fall back to baseline SIP
operation if the request is rejected with a 420 response. operation if the request is rejected with a 420 response.
Extensions which define new methods do not need to use the Require Extensions which define new methods do not need to use the Require
header field. SIP defines mechanisms which allow a UAC to know header field. SIP defines mechanisms which allow a UAC to know
whether a new method is understood by a UAS. This includes both the whether a new method is understood by a UAS. This includes both the
OPTIONS request, and the 405 (Method Not Allowed) response with the OPTIONS request, and the 405 (Method Not Allowed) response with the
Allow header field. It is fundamental to SIP that proxies do not need Allow header field. It is fundamental to SIP that proxies do not
to understand the semantics of a new method in order to process it. need to understand the semantics of a new method in order to process
If an extension defines a new method which must be understood by it. If an extension defines a new method which must be understood by
proxies in order to be processed, a Proxy-Require header field is proxies in order to be processed, a Proxy-Require header field is
needed. As discussed above, these kinds of extensions are frowned needed. As discussed above, these kinds of extensions are frowned
upon. upon.
In order to achieve backwards compatibility for extensions that In order to achieve backwards compatibility for extensions that
define new methods, the Allow header field is used. There are two define new methods, the Allow header field is used. There are two
types of new methods - those that are used for established dialogs types of new methods - those that are used for established dialogs
(initiated by INVITE, for example), and those that are sent as the (initiated by INVITE, for example), and those that are sent as the
initial request to a UA. Since INVITE and its response both SHOULD initial request to a UA. Since INVITE and its response both SHOULD
contain an Allow header field, a UA can readily determine whether the contain an Allow header field, a UA can readily determine whether the
new method can be supported within the dialog. For example, once an new method can be supported within the dialog. For example, once an
INVITE dialog is established, a user agent could determine if the INVITE dialog is established, a user agent could determine if the
REFER method [10] is supported if it is present in an Allow header. REFER method [10] is supported if it is present in an Allow header.
If it was, the "transfer" button on the UI could be ``greyed out'' If it was, the "transfer" button on the UI could be "greyed out" once
once the call is established. the call is established.
Another type of extension are those which require a proxy to insert Another type of extension are those which require a proxy to insert
header fields or header field parameters into a request as it header fields or header field parameters into a request as it
traverses the network, or for the UAS to insert header fields or traverses the network, or for the UAS to insert header fields or
header field parameters into a response. For some extensions, if the header field parameters into a response. For some extensions, if the
UAC or UAS does not understand these header fields, the message can UAC or UAS does not understand these header fields, the message can
still be processed correctly. These extensions are completely still be processed correctly. These extensions are completely
backwards compatible. backwards compatible.
Most other extensions of this type require that the server only Most other extensions of this type require that the server only
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If an extension requires a proxy to insert a header field or If an extension requires a proxy to insert a header field or
parameter into a request, and this header field or parameter needs to parameter into a request, and this header field or parameter needs to
be understood by both UAC and UAS to be executed correctly, a be understood by both UAC and UAS to be executed correctly, a
combination of the Require and the Supported mechanism will need to combination of the Require and the Supported mechanism will need to
be used. The proxy can insert a Require header field into the be used. The proxy can insert a Require header field into the
request, given the Supported header field is present. An example of request, given the Supported header field is present. An example of
such an extension is the SIP Session Timer [11]. such an extension is the SIP Session Timer [11].
Yet another type of extension is that which defines new body types to Yet another type of extension is that which defines new body types to
be carried in SIP messages. According to the SIP specification, be carried in SIP messages. According to the SIP specification,
bodies must be understood in order to process a request. As such, the bodies must be understood in order to process a request. As such,
interoperability issues are similar to new methods. However, the the interoperability issues are similar to new methods. However, the
Content-Disposition header field has been defined to allow a client Content-Disposition header field has been defined to allow a client
or server to indicate that the message body is optional [2]. Usage of or server to indicate that the message body is optional [2]. Usage
optional bodies, as opposed to mandatory ones, is RECOMMENDED of optional bodies, as opposed to mandatory ones, is RECOMMENDED
wherever possible. wherever possible.
When a body must be understood to properly process a request or When a body must be understood to properly process a request or
response, it is preferred that the sending entity know ahead of time response, it is preferred that the sending entity know ahead of time
whether the new body is understood by the recipient. For requests whether the new body is understood by the recipient. For requests
that establish a dialog, inclusion of Accept in the request and its that establish a dialog, inclusion of Accept in the request and its
success responses is RECOMMENDED. This will allow both parties to success responses is RECOMMENDED. This will allow both parties to
determine what body types are supported by their peers. Subsequent determine what body types are supported by their peers. Subsequent
messaging between the peers would then only include body types that messaging between the peers would then only include body types that
were indicated as being understood. were indicated as being understood.
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4.2 Security 4.2 Security
Security is an important component of any protocol. Designers of SIP Security is an important component of any protocol. Designers of SIP
extensions need to carefully consider if additional security extensions need to carefully consider if additional security
requirements are required over those described in RFC 3261. requirements are required over those described in RFC 3261.
Frequently authorization requirements, and requirements for Frequently authorization requirements, and requirements for
end-to-end integrity are the most overlooked. end-to-end integrity are the most overlooked.
SIP extensions MUST consider how (or if) they affect usage of the SIP extensions MUST consider how (or if) they affect usage of the
general SIP security mechanisms. Most extensions should not require general SIP security mechanisms. Most extensions should not require
any new security capabilities beyond general purpose SIP. If they do, any new security capabilities beyond general purpose SIP. If they
it is likely that the security mechanism has more general purpose do, it is likely that the security mechanism has more general purpose
application, and should be considered an extension in its own right. application, and should be considered an extension in its own right.
4.3 Terminology 4.3 Terminology
RFC 3261 has an extensive terminology section that defines terms like RFC 3261 has an extensive terminology section that defines terms like
caller, callee, user agent, header field, and so on. All SIP caller, callee, user agent, header field, and so on. All SIP
extensions MUST conform to this terminology. They MUST NOT define new extensions MUST conform to this terminology. They MUST NOT define
terms that describe concepts already defined by a term in another SIP new terms that describe concepts already defined by a term in another
specification. If new terminology is needed, it SHOULD appear in a SIP specification. If new terminology is needed, it SHOULD appear in
separate section towards the beginning of the document. a separate section towards the beginning of the document.
Careful attention must be paid to the actual usage of terminology. Careful attention must be paid to the actual usage of terminology.
Many documents misuse the terms header, header field, and header Many documents misuse the terms header, header field, and header
field values, for example. Document authors SHOULD do a careful field values, for example. Document authors SHOULD do a careful
review of their documents for proper usage of these terms. review of their documents for proper usage of these terms.
4.4 Syntactic Issues 4.4 Syntactic Issues
Extensions that define new methods SHOULD use all capitals for the Extensions that define new methods SHOULD use all capitals for the
method name. Method names SHOULD be less than 10 characters, and method name. Method names SHOULD be less than 10 characters, and
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Careful attention must be paid to the actual usage of terminology. Careful attention must be paid to the actual usage of terminology.
Many documents misuse the terms header, header field, and header Many documents misuse the terms header, header field, and header
field values, for example. Document authors SHOULD do a careful field values, for example. Document authors SHOULD do a careful
review of their documents for proper usage of these terms. review of their documents for proper usage of these terms.
4.4 Syntactic Issues 4.4 Syntactic Issues
Extensions that define new methods SHOULD use all capitals for the Extensions that define new methods SHOULD use all capitals for the
method name. Method names SHOULD be less than 10 characters, and method name. Method names SHOULD be less than 10 characters, and
SHOULD attempt to convey the general meaning of the request. SHOULD attempt to convey the general meaning of the request.
Method names are case sensitive, and therefore there is no Method names are case sensitive, and therefore there is no
requirement that they be capitalized. However, using capitalized requirement that they be capitalized. However, using capitalized
method names keeps with a long-standing convention in SIP and many method names keeps with a long-standing convention in SIP and many
similar protocols, such as HTTP [12] and RTSP [13] similar protocols, such as HTTP [13] and RTSP [14]
Extensions that define new header fields that are anticipated to be Extensions that define new header fields that are anticipated to be
heavily used SHOULD define a compact form if those header fields are heavily used SHOULD define a compact form if those header fields are
more than four characters. Compact header fields MUST be a single more than four characters. Compact header fields MUST be a single
character. When all 26 characters are exhausted, new compact forms character. When all 26 characters are exhausted, new compact forms
will no longer be defined. Header field names SHOULD be composed will no longer be defined. Header field names SHOULD be composed
primarily of ASCII characters and marks. They SHOULD be descriptive primarily of ASCII characters and marks. They SHOULD be descriptive
but reasonably brief. Although header field names are case but reasonably brief. Although header field names are case
insensitive, a single common capitalization SHOULD be used throughout insensitive, a single common capitalization SHOULD be used throughout
the document. It is RECOMMENDED that each English word present in the the document. It is RECOMMENDED that each English word present in
header field name have its first letter capitalized. For example, the header field name have its first letter capitalized. For
"ThisIsANewHeader". example, "ThisIsANewHeader".
As an example, the following are poor choices for header field names: As an example, the following are poor choices for header field names:
ThisIsMyNewHeaderThatDoesntDoVeryMuchButItHasANiceName ThisIsMyNewHeaderThatDoesntDoVeryMuchButItHasANiceName
--.!A --.!A
Function Function
Case sensitivity of parameters and values is a constant source of Case sensitivity of parameters and values is a constant source of
confusion, a difficulty that plagued RFC 2543 [14]. This has been confusion, a difficulty that plagued RFC 2543 [15]. This has been
made simple through the usage of the BNF constructs of RFC 2234 [5], made simple through the usage of the BNF constructs of RFC 2234 [5],
which have clear rules of case sensivitity and insensitivity. which have clear rules of case sensivitity and insensitivity.
Therefore, the BNF for an extension completely defines the matching Therefore, the BNF for an extension completely defines the matching
rules. rules.
Extensions MUST be consistent with the SIP conventions for Extensions MUST be consistent with the SIP conventions for case
sensitivity. Methods MUST be case sensitive. Header field names MUST sensitivity. Methods MUST be case sensitive. Header field names
be case insensitive. Header field parameter names MUST be case MUST be case insensitive. Header field parameter names MUST be case
insensitive. Header field values and parameter values are sometimes insensitive. Header field values and parameter values are sometimes
case sensitive, and sometimes case insensitive. However, generally case sensitive, and sometimes case insensitive. However, generally
they SHOULD be case insensitive. Definiting a case sensitive they SHOULD be case insensitive. Definiting a case sensitive
component requires explicitly listing each character through its component requires explicitly listing each character through its
ASCII code. ASCII code.
Extensions which contain freeform text MUST allow that text to be Extensions which contain freeform text MUST allow that text to be
UTF-8, as per the IETF policies on character set usage [3]. This UTF-8, as per the IETF policies on character set usage [3]. This
ensures that SIP remains an internationalized standard. As a general ensures that SIP remains an internationalized standard. As a general
guideline, freeform text is never needed by programs in order to guideline, freeform text is never needed by programs in order to
perform protocol processing. It is usually entered by and displayed perform protocol processing. It is usually entered by and displayed
to the user. If an extension uses a parameter which can contain UTF-8 to the user. If an extension uses a parameter which can contain
encoded characters, and that extension requires a comparison to be UTF-8 encoded characters, and that extension requires a comparison to
made of this parameter to other parameters, the comparison MUST be be made of this parameter to other parameters, the comparison MUST be
case sensitive. Case insensitive comparison rules for UTF-8 text are, case sensitive. Case insensitive comparison rules for UTF-8 text
at this time, impossible and MUST be avoided. are, at this time, impossible and MUST be avoided.
Extensions which make use of dates MUST use the SIP-Date BNF defined Extensions which make use of dates MUST use the SIP-Date BNF defined
in RFC 3261 \cite{RFC3261}. No other date formats are allowed. in RFC 3261. No other date formats are allowed. However, the usage
However, the usage of absolute dates in order to determine intervals of absolute dates in order to determine intervals (for example, the
(for example, the time at which some timer fires) is NOT RECOMMENDED. time at which some timer fires) is NOT RECOMMENDED. This is because
This is because it requires synchronized time between peers, and this it requires synchronized time between peers, and this is frequently
is frequently not the case. Therefore, relative times, expressed in not the case. Therefore, relative times, expressed in numbers of
numbers of seconds, SHOULD be used. seconds, SHOULD be used.
Extensions which include network layer addresses SHOULD permit dotted Extensions which include network layer addresses SHOULD permit dotted
quad IPv4 addresses, IPv6 addresses in the format described in [4], quad IPv4 addresses, IPv6 addresses in the format described in [4],
and domain names. and domain names.
Extensions which have header fields containing URIs SHOULD allow any Extensions which have header fields containing URIs SHOULD allow any
URI, not just SIP URIs. URI, not just SIP URIs.
Header fields MUST follow the standard formatting for SIP, defined Header fields MUST follow the standard formatting for SIP, defined
as: as:
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In some cases, this form is not sufficient. That is the case for In some cases, this form is not sufficient. That is the case for
header fields that express descriptive text meant for human header fields that express descriptive text meant for human
consumption. An example is the Subject header field in SIP [2]. In consumption. An example is the Subject header field in SIP [2]. In
this case, an alternate form is: this case, an alternate form is:
header = header-name HCOLON [TEXT-UTF8-TRIM] header = header-name HCOLON [TEXT-UTF8-TRIM]
Developers of extensions SHOULD allow for extension parameters in Developers of extensions SHOULD allow for extension parameters in
their header fields. their header fields.
Header Fields that contain a list of URIs SHOULD follow the same Header fields that contain a list of URIs SHOULD follow the same
syntax as the Contact header field in SIP. Implementors are also syntax as the Contact header field in SIP. Implementors are also
encouraged to always wrap these URI in angle brackets "<" and ">". We encouraged to always wrap these URI in angle brackets "<" and ">".
have found this to be a frequently misimplemented feature. We have found this to be a frequently misimplemented feature.
Beyond compact form, there is no need to define compressed versions Beyond compact form, there is no need to define compressed versions
of header field values. Compression of SIP messages SHOULD be handled of header field values. Compression of SIP messages SHOULD be
at lower layers, for example, using IP payload compression [15] or handled at lower layers, for example, using IP payload compression
signalling compression [16]. [16] or signalling compression [18].
Syntax for header fields is expressed in Augmented Backus-Naur Form Syntax for header fields is expressed in Augmented Backus-Naur Form
and MUST follow the format of RFC 2234 [5]. Extensions MUST make use and MUST follow the format of RFC 2234 [5]. Extensions MUST make use
of the primitive components defined in RFC 3261 [2]. If the of the primitive components defined in RFC 3261 [2]. If the
construction for a BNF element is defined in another specification, construction for a BNF element is defined in another specification,
it is RECOMMENDED that the construction be referenced rather than it is RECOMMENDED that the construction be referenced rather than
copied. The reference SHOULD include both the document and section copied. The reference SHOULD include both the document and section
number. All BNF elements must be either defined or referenced. number. All BNF elements must be either defined or referenced.
It is RECOMMENDED that BNF be collected into a single section near It is RECOMMENDED that BNF be collected into a single section near
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far more important. SIP extensions MUST clearly define the semantics far more important. SIP extensions MUST clearly define the semantics
of the extensions. Specifically, the extension MUST specify the of the extensions. Specifically, the extension MUST specify the
behaviors expected of a UAC, UAS and proxy in processing the behaviors expected of a UAC, UAS and proxy in processing the
extension. This is often best described by having separate sections extension. This is often best described by having separate sections
for each of these three elements. Each section SHOULD step through for each of these three elements. Each section SHOULD step through
the processing rules in temporal order of the most common messaging the processing rules in temporal order of the most common messaging
scenario. scenario.
Processing rules generally specify actions to take (in terms of Processing rules generally specify actions to take (in terms of
messages to send, variables to store, rules to follow) on receipt of messages to send, variables to store, rules to follow) on receipt of
messages and expiration of timers. If an action requires transmission messages and expiration of timers. If an action requires
of a message, the rule SHOULD outline requirements for insertion of transmission of a message, the rule SHOULD outline requirements for
header fields or other information in the message. insertion of header fields or other information in the message.
The extension SHOULD specify procedures to take in exceptional The extension SHOULD specify procedures to take in exceptional
conditions which are recoverable, or which require some kind of user conditions which are recoverable, or which require some kind of user
intervention. Recovering from unrecoverable problems generally does intervention. Recovering from unrecoverable problems generally does
not require specification. not require specification.
4.6 Examples Section 4.6 Examples Section
The specification SHOULD contain a section that gives examples of The specification SHOULD contain a section that gives examples of
call flows and message formatting. Extensions which define call flows and message formatting. Extensions which define
substantial new syntax SHOULD include examples of messages containing substantial new syntax SHOULD include examples of messages containing
that syntax. Examples of message flows should be given to cover that syntax. Examples of message flows should be given to cover
common cases and at least one failure or unusual case. common cases and at least one failure or unusual case.
For an example of how to construct a good examples section, see the For an example of how to construct a good examples section, see the
message flows and message formatting defined in the Basic Call Flows message flows and message formatting defined in the Basic Call Flows
specification [17]. Note that complete messages SHOULD be used. Be specification [19]. Note that complete messages SHOULD be used. Be
careful to include tags, Via header fields (with the branch ID careful to include tags, Via header fields (with the branch ID
cookie), Max-Forwards, Content-Lengths, Record-Route and Route header cookie), Max-Forwards, Content-Lengths, Record-Route and Route header
fields. Example INVITE messages MAY omit session descriptions, and fields. Example INVITE messages MAY omit session descriptions, and
Content-Length values MAY be set to "..." to indicate that the value Content-Length values MAY be set to "..." to indicate that the value
is not provided. However, the specification MUST explicitly call out is not provided. However, the specification MUST explicitly call out
the meaning of the "..." and explicitly indicate that session the meaning of the "..." and explicitly indicate that session
descriptions were not included. descriptions were not included.
4.7 Overview Section 4.7 Overview Section
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specification should explicitly state that the overview is tutorial specification should explicitly state that the overview is tutorial
in nature only. in nature only.
4.8 IANA Considerations Section 4.8 IANA Considerations Section
Documents which define new SIP extensions will invariably have IANA Documents which define new SIP extensions will invariably have IANA
Considerations sections. Considerations sections.
If your extension is defining a new event package, you MUST register If your extension is defining a new event package, you MUST register
that package. RFC 3265 [6] provides the registration template. See that package. RFC 3265 [6] provides the registration template. See
[18] for an example of the registration of a new event package. [20] for an example of the registration of a new event package.
If your extension is defining a new header field, you MUST register If your extension is defining a new header field, you MUST register
that header field. RFC 3261 [2] provides a registration template. See that header field. RFC 3261 [2] provides a registration template.
Section 8.2 of RFC 3262 [19] for an example of how to register new See Section 8.2 of RFC 3262 [21] for an example of how to register
SIP header fields. new SIP header fields.
If your extension is defining a new response code, you MUST register If your extension is defining a new response code, you MUST register
that response code. RFC 3261 [2] provides a registration template. that response code. RFC 3261 [2] provides a registration template.
See [11] for an example of how to register a new response code. See Section 6.4 of RFC 3329 [17] for an example of how to register a
new response code.
If your extension is defining a new SIP method, you MUST register If your extension is defining a new SIP method, you MUST register
that method. RFC 3261 [2] provides a registration template. See that method. RFC 3261 [2] provides a registration template. See
Section 10 of RFC 3311 [20] for an example of how to register a new Section 10 of RFC 3311 [22] for an example of how to register a new
SIP method. SIP method.
Many SIP extensions make use of option tags, carried in the Require, Many SIP extensions make use of option tags, carried in the Require,
Proxy-Require and Supported header fields. Section Section 4.1 Proxy-Require and Supported header fields. Section 4.1 discusses
discusses some of the issues involved in the usage of these header some of the issues involved in the usage of these header fields. If
fields. If your extension does require them, you MUST register an your extension does require them, you MUST register an option tag for
option tag for your extension. RFC 3261 [2] provides a registration your extension. RFC 3261 [2] provides a registration template. See
template. See Section 8.1 of RFC 3262 [19] for an example of how to Section 8.1 of RFC 3262 [21] for an example of how to register an
register an option tag. option tag.
Some SIP extensions will require establishment of their own IANA Some SIP extensions will require establishment of their own IANA
registries. RFC 2434 [21] provides guidance on how and when IANA registries. RFC 2434 [23] provides guidance on how and when IANA
registries are established. For an example of how to set one up, see registries are established. For an example of how to set one up, see
Section 6 of RFC 3265 [6] for an example. Section 6 of RFC 3265 [6] for an example.
4.9 Document Naming Conventions 4.9 Document Naming Conventions
An important decision to be made about the extension is its title. An important decision to be made about the extension is its title.
The title MUST indicate that the document is an extension to SIP. It The title MUST indicate that the document is an extension to SIP. It
is RECOMMENDED that the title follow the basic form of "A [summary of is RECOMMENDED that the title follow the basic form of "A [summary of
function] for the Session Initiation Protocol (SIP)", where the function] for the Session Initiation Protocol (SIP)", where the
summary of function is a one to three word description of the summary of function is a one to three word description of the
extension. For example, if an extension defines a new header field, extension. For example, if an extension defines a new header field,
called Make-Coffee, for making coffee, the title would read, "Making called Make-Coffee, for making coffee, the title would read, "Making
Coffee with the Session Initiation Protocol (SIP)". It is RECOMMENED Coffee with the Session Initiation Protocol (SIP)". It is RECOMMENED
that these additional words be descriptive rather than naming the that these additional words be descriptive rather than naming the
header field. For example, the extension for making coffee should not header field. For example, the extension for making coffee should
be named "The Make-Coffee Header for the Session Initiation not be named "The Make-Coffee Header for the Session Initiation
Protocol". Protocol".
For extensions that define new methods, an acceptable template for For extensions that define new methods, an acceptable template for
titles is "The Session Initiation Protocol (SIP) X Method" where X is titles is "The Session Initiation Protocol (SIP) X Method" where X is
the name of the method. the name of the method.
Note that the acronymn SIP MUST be expanded in the titles of RFCs, as Note that the acronymn SIP MUST be expanded in the titles of RFCs, as
per [22]. per [24].
4.10 Additional Considerations for New Methods 4.10 Additional Considerations for New Methods
Extensions which define new methods SHOULD take into consideration, Extensions which define new methods SHOULD take into consideration,
and discuss, the following issues: and discuss, the following issues:
o Can it contain bodies? If so, what is the meaning of the presence o Can it contain bodies? If so, what is the meaning of the presence
of those bodies? What body types are allowed? of those bodies? What body types are allowed?
o Can a transaction with this request method occur while another o Can a transaction with this request method occur while another
transaction, in the same and/or reverse direction, is in progress? transaction, in the same and/or reverse direction, is in progress?
o The extension MUST define which header fields can be present in o The extension MUST define which header fields can be present in
requests of that method. It is RECOMMENDED that this information requests of that method. It is RECOMMENDED that this information
be represented as a new column of Table 2/3 of RFC 3261 [2]. The be represented as a new column of Table 2/3 of RFC 3261 [2]. The
table MUST contain rows for all header fields defined in standards table MUST contain rows for all header fields defined in standards
track RFCs at the time of writing of the extension. track RFCs at the time of writing of the extension.
o Can the request be sent within a dialog, or does it establish a o Can the request be sent within a dialog, or does it establish a
dialog? dialog?
o Is it a target refresh request? o Is it a target refresh request?
o Extensions to SIP that define new methods MAY specify whether o Extensions to SIP that define new methods MAY specify whether
offers and answers can appear in requests of that method or its offers and answers can appear in requests of that method or its
responses. However, those extensions MUST adhere to the protocol responses. However, those extensions MUST adhere to the protocol
rules specified in [23], and MUST adhere to the additional rules specified in [25], and MUST adhere to the additional
constraints for offers and answers as specified in SIP [2]. constraints for offers and answers as specified in SIP [2].
o Because of the nature of reliability treatment of requests with o Because of the nature of reliability treatment of requests with
new methods, those requests need to be answered immediately by the new methods, those requests need to be answered immediately by the
UAS. Protocol extensions that require longer durations for the UAS. Protocol extensions that require longer durations for the
generation of a response (such as a new method that requires human generation of a response (such as a new method that requires human
interaction) SHOULD instead use two transactions - one to send the interaction) SHOULD instead use two transactions - one to send the
request, and another in the reverse direction to convey the result request, and another in the reverse direction to convey the result
of the request. An example of that is SUBSCRIBE and NOTIFY [6]. of the request. An example of that is SUBSCRIBE and NOTIFY [6].
o The SIP specification [2] allows new methods to specify whether
transactions using that new method can be canceled using a CANCEL
request. Further study of the non-INVITE transaction [12] has
determined that non-INVITE transactions must complete as soon as
possible. New methods must not plan for the transaction to pend
long enough for CANCEL to be meaningful. Thus, new methods MUST
declare that transactions initiated by requests with that method
cannot be canceled. Future work may relax this restriction, at
which point these guidelines will be revised.
o New methods that establish a new dialog must discuss the impacts
of forking. The design of such new methods should follow the
pattern of requiring an immediate request in the reverse direction
from the request establishing a dialog, similar to the immediate
NOTIFY sent when a subscription is created per RFC 3265 [6].
o The CANCEL request can be used for a particular extension method The reliability mechanisms for all new methods must be the same as
on a method-by-method basis. SIP [2] only allows cancellation of for BYE. The delayed response feature of INVITE is only available in
INVITE. Extensions that define new methods MUST state whether or INVITE, never for new methods. The design of new methods must
not transactions initiated by requests with that method can be encourage an immediate response. If the application being enabled
cancelled. Furthermore, the rules a UAS should follow upon requires a delay, the design SHOULD follow a pattern using multiple
cancellation of an unanswered request MUST be described. Note transactions similar to RFC 3265's use of NOTIFYs with different
that, since non-INVITE requests are generally answered Subscription-State header field values (pending and active in
immediately, cancellation ususally serves no purpose. particular) in response to SUBSCRIBE [6].
Note that the reliability mechanisms for all new methods must be the
same as for BYE. The delayed response feature of INVITE is only
available in INVITE, never for new methods. This means requests with
new SIP methods need to be responded to within short time periods (on
the order of seconds).
4.11 Additional Considerations for New Header Fields or Header Field 4.11 Additional Considerations for New Header Fields or Header Field
Parameters Parameters
The most important issue for extensions that define new header fields The most important issue for extensions that define new header fields
or header field parameters is backwards compatibility. See Section or header field parameters is backwards compatibility. See Section
4.1 for a discussion of the issues. The extension MUST detail how 4.1 for a discussion of the issues. The extension MUST detail how
backwards compatibility is addressed. backwards compatibility is addressed.
It is often tempting to avoid creation of a new method by overloading It is often tempting to avoid creation of a new method by overloading
an existing method through a header field or parameter. Header fields an existing method through a header field or parameter. Header
and parameters are not meant to fundamentally alter the meaning of fields and parameters are not meant to fundamentally alter the
the method of the request. A new header field cannot change the basic meaning of the method of the request. A new header field cannot
semantic and processing rules of a method. There is no shortage of change the basic semantic and processing rules of a method. There is
method names, so when an extension changes the basic meaning of a no shortage of method names, so when an extension changes the basic
request, a new method SHOULD be defined. meaning of a request, a new method SHOULD be defined.
For extensions that define new header fields, the extension MUST For extensions that define new header fields, the extension MUST
define the request methods the header field can appear in, and what define the request methods the header field can appear in, and what
responses it can be used in. It is RECOMMENDED that this information responses it can be used in. It is RECOMMENDED that this information
be represented as a new row of Table 2/3 of RFC 3261 [2]. The table be represented as a new row of Table 2/3 of RFC 3261 [2]. The table
MUST contain columns for all methods defined in standards track RFCs MUST contain columns for all methods defined in standards track RFCs
at time of writing of the extension. at time of writing of the extension.
4.12 Additional Considerations for New Body Types 4.12 Additional Considerations for New Body Types
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with processing a request of a particular method or a response. with processing a request of a particular method or a response.
Bodies enhance this processing by providing additional data. Bodies enhance this processing by providing additional data.
5. Interactions with SIP Features 5. Interactions with SIP Features
We have observed that certain capabilities of SIP continually We have observed that certain capabilities of SIP continually
interact with extensions in unusual ways. Writers of extensions interact with extensions in unusual ways. Writers of extensions
SHOULD consider the interactions of their extensions with these SIP SHOULD consider the interactions of their extensions with these SIP
capabilities, document any unusual interactions if they exist. The capabilities, document any unusual interactions if they exist. The
most common causes of problems are: most common causes of problems are:
Forking: Forking by far presents the most troublesome interactions Forking: Forking by far presents the most troublesome interactions
with extensions. This is generally because it can cause (1) a with extensions. This is generally because it can cause (1) a
single transmitted request to be received by an unknown number of single transmitted request to be received by an unknown number of
UASs, and (2) a single INVITE request to have multiple responses. UASs, and (2) a single INVITE request to have multiple responses.
CANCEL and ACK: CANCEL and ACK are "special" SIP requests, in that CANCEL and ACK: CANCEL and ACK are "special" SIP requests, in that
they are exceptions to many of the general request processing they are exceptions to many of the general request processing
rules. The main reason for this special status is that CANCEL and rules. The main reason for this special status is that CANCEL and
ACK are always associated with another request. New methods SHOULD ACK are always associated with another request. New methods
consider the meaning of cancellation, as described above. SHOULD consider the meaning of cancellation, as described above.
Extensions which defined new header fields in INVITE requests Extensions which defined new header fields in INVITE requests
SHOULD consider whether they also need to be included in ACK and SHOULD consider whether they also need to be included in ACK and
CANCEL. Frequently they do, in order to allow a stateless proxy to CANCEL. Frequently they do, in order to allow a stateless proxy
route the CANCEL or ACK identically to the INVITE. to route the CANCEL or ACK identically to the INVITE.
Routing: The presence of Route header fields in a request can cause Routing: The presence of Route header fields in a request can cause
it to be sent through intermediate proxies. Requests that it to be sent through intermediate proxies. Requests that
establish dialogs can be record-routed, so that the initial establish dialogs can be record-routed, so that the initial
request goes through one set of proxies, and subsequent requests request goes through one set of proxies, and subsequent requests
through a different set. These SIP features can interact in through a different set. These SIP features can interact in
unusual ways with extensions. unusual ways with extensions.
Stateless Proxies: SIP allows a proxy to be stateless. Stateless Stateless Proxies: SIP allows a proxy to be stateless. Stateless
proxies are unable to retransmit messages and cannot execute proxies are unable to retransmit messages and cannot execute
certain services. Extensions which depend on some kind of proxy certain services. Extensions which depend on some kind of proxy
processing SHOULD consider how stateless proxies affect that processing SHOULD consider how stateless proxies affect that
processing. processing.
6. Security Considerations 6. Security Considerations
The nature of this document is such that it does not introduce any The nature of this document is such that it does not introduce any
new security considerations. new security considerations.
skipping to change at page 23, line 7 skipping to change at page 23, line 7
The nature of this document is such that it does not introduce any The nature of this document is such that it does not introduce any
new security considerations. new security considerations.
7. IANA Considerations 7. IANA Considerations
There are no IANA considerations associated with this specification. There are no IANA considerations associated with this specification.
8. Acknowledgements 8. Acknowledgements
The authors would like to thank Rohan Mahy for his comments. The authors would like to thank Rohan Mahy for his comments. Robert
Sparks contributed important text on CANCEL issues.
Normative References 9. References
9.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997. Levels", BCP 14, RFC 2119, March 1997.
[2] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., [2] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002. Session Initiation Protocol", RFC 3261, June 2002.
[3] Alvestrand, H., "IETF Policy on Character Sets and Languages", [3] Alvestrand, H., "IETF Policy on Character Sets and Languages",
BCP 18, RFC 2277, January 1998. BCP 18, RFC 2277, January 1998.
skipping to change at page 25, line 5 skipping to change at page 24, line 33
Specifications: ABNF", RFC 2234, November 1997. Specifications: ABNF", RFC 2234, November 1997.
[6] Roach, A., "Session Initiation Protocol (SIP)-Specific Event [6] Roach, A., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002. Notification", RFC 3265, June 2002.
[7] Olson, S., "A Mechanism for Content Indirection in Session [7] Olson, S., "A Mechanism for Content Indirection in Session
Initiation Protocol (SIP) Messages", Initiation Protocol (SIP) Messages",
draft-ietf-sip-content-indirect-mech-03 (work in progress), June draft-ietf-sip-content-indirect-mech-03 (work in progress), June
2003. 2003.
Informative References 9.2 Informative References
[8] Mankin, A., Bradner, S., Mahy, R., Willis, D., Ott, J. and B. [8] Mankin, A., Bradner, S., Mahy, R., Willis, D., Ott, J. and B.
Rosen, "Change Process for the Session Initiation Protocol Rosen, "Change Process for the Session Initiation Protocol
(SIP)", BCP 67, RFC 3427, December 2002. (SIP)", BCP 67, RFC 3427, December 2002.
[9] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, [9] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997. March 1997.
[10] Sparks, R., "The Session Initiation Protocol (SIP) Refer [10] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003. Method", RFC 3515, April 2003.
[11] Donovan, S. and J. Rosenberg, "Session Timers in the Session [11] Donovan, S. and J. Rosenberg, "Session Timers in the Session
Initiation Protocol (SIP)", draft-ietf-sip-session-timer-12 Initiation Protocol (SIP)", draft-ietf-sip-session-timer-14
(work in progress), October 2003. (work in progress), May 2004.
[12] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., Masinter, L., [12] Sparks, R., "Problems identified associated with the Session
Initiation Protocol's non-INVITE Transaction",
draft-sparks-sip-nit-problems-00 (work in progress), February
2004.
[13] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol -- Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999. HTTP/1.1", RFC 2616, June 1999.
[13] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming [14] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC 2326, April 1998. Protocol (RTSP)", RFC 2326, April 1998.
[14] Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg, [15] Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg,
"SIP: Session Initiation Protocol", RFC 2543, March 1999. "SIP: Session Initiation Protocol", RFC 2543, March 1999.
[15] Shacham, A., Monsour, R., Pereira, R. and M. Thomas, "IP [16] Shacham, A., Monsour, R., Pereira, R. and M. Thomas, "IP
Payload Compression Protocol (IPComp)", RFC 2393, December Payload Compression Protocol (IPComp)", RFC 2393, December
1998. 1998.
[16] Price, R., Bormann, C., Christoffersson, J., Hannu, H., Liu, Z. [17] Arkko, J., Torvinen, V., Camarillo, G., Niemi, A. and T.
Haukka, "Security Mechanism Agreement for the Session
Initiation Protocol (SIP)", RFC 3329, January 2003.
[18] Price, R., Bormann, C., Christoffersson, J., Hannu, H., Liu, Z.
and J. Rosenberg, "Signaling Compression (SigComp)", RFC 3320, and J. Rosenberg, "Signaling Compression (SigComp)", RFC 3320,
January 2003. January 2003.
[17] Johnston, A., "Session Initiation Protocol Basic Call Flow [19] Johnston, A., Donovan, S., Sparks, R., Cunningham, C. and K.
Examples", draft-ietf-sipping-basic-call-flows-02 (work in Summers, "Session Initiation Protocol (SIP) Basic Call Flow
progress), April 2003. Examples", BCP 75, RFC 3665, December 2003.
[18] Rosenberg, J., "A Session Initiation Protocol (SIP) Event [20] Rosenberg, J., "A Session Initiation Protocol (SIP) Event
Package for Registrations", draft-ietf-sipping-reg-event-00 Package for Registrations", RFC 3680, March 2004.
(work in progress), October 2002.
[19] jdrosen@dynamicsoft.com and schulzrinne@cs.columbia.edu, [21] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional
"Reliability of Provisional Responses in Session Initiation Responses in Session Initiation Protocol (SIP)", RFC 3262, June
Protocol (SIP)", RFC 3262, June 2002. 2002.
[20] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE [22] Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
Method", RFC 3311, October 2002. Method", RFC 3311, October 2002.
[21] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA [23] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October Considerations Section in RFCs", BCP 26, RFC 2434, October
1998. 1998.
[22] Reynolds, J. and R. Braden, "Instructions to Request for [24] Reynolds, J. and R. Braden, "Instructions to Request for
Comments (RFC) Authors", draft-rfc-editor-rfc2223bis-07 (work Comments (RFC) Authors", draft-rfc-editor-rfc2223bis-07 (work
in progress), August 2003. in progress), August 2003.
[23] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with [25] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002. Session Description Protocol (SDP)", RFC 3264, June 2002.
Authors' Addresses Authors' Addresses
Jonathan Rosenberg Jonathan Rosenberg
dynamicsoft dynamicsoft
600 Lanidex Plaza 600 Lanidex Plaza
Parsippany, NJ 07054 Parsippany, NJ 07054
US US
skipping to change at page 27, line 8 skipping to change at page 27, line 8
1214 Amsterdam Ave. 1214 Amsterdam Ave.
New York, NY 10027 New York, NY 10027
US US
EMail: schulzrinne@cs.columbia.edu EMail: schulzrinne@cs.columbia.edu
URI: http://www.cs.columbia.edu/~hgs URI: http://www.cs.columbia.edu/~hgs
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