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Versions: 00                                                            
Internet Engineering Task Force                                   SIP WG
Internet Draft                                 J.Rosenberg,H.Schulzrinne
draft-rosenberg-sip-guidelines-00.txt            dynamicsoft,Columbia U.
March 10, 2000
Expires: September, 2000


                Guidelines for Authors of SIP Extensions

STATUS OF THIS MEMO

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as work in progress.

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.


Abstract

   The Session Initiation Protocol (SIP) is a flexible, yet simple tool
   for establishing interactive connections across the Internet. Part of
   this flexibility is the ease with which it can be extended. In order
   to facilitate effective and interoperable extensions to SIP, some
   guidelines need to be followed when developing SIP extensions. This
   document outlines a set of such guidelines for authors of SIP
   extensions.


1 Introduction

   The Session Initiation Protocol (SIP) [1] is a flexible, yet simple
   tool for establishing interactive connections across the Internet.
   Part of this flexibility is the ease with which it can be extended.
   SIP can be extended in numerous ways. New methods can be defined, new



J.Rosenberg,H.Schulzrinne                                     [Page 1]


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   headers can be added, new body types can be used, and new parameters
   for existing headers can be added. This flexibility also means that
   caution should be exercised when defining extensions, in order to
   ensure interoperability.

   In order to facilitate interoperability, this document serves as a
   set of guidelines for authors of SIP extensions. It points out issues
   to consider when deciding whether a SIP extension is the right answer
   for a specific problem. It then points out issues which extensions
   should deal with from within the specification. Finally, it discusses
   common interactions with existing SIP features which often cause
   difficulties in extensions.

2 Should I define a SIP Extension?

   The first question to be addressed when defining a SIP extension is:
   is a SIP extension the best solution to my problem? SIP has been
   proposed as a solution for numerous problems, including mobility,
   configuration and management, QoS control, call control, caller
   preferences, device control, third party call control, and MPLS path
   setup, to name a few. Clearly, not every problem can be solved by a
   SIP extension. More importantly, some problems that could be solved
   by a SIP extension, probably shouldn't.

   To assist engineers in determining whether a SIP extension is an
   appropriate solution to their problem, we present two broad criteria.
   First, the problem should fit into the general purvey of SIPs
   solution space. Secondly, the solution must conform to the general
   SIP architectural model.

   While the first criteria might seem obvious, we have observed that
   numerous extensions to SIP have been proposed because some function
   is needed in a device which also speaks SIP. The argument is
   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
   obtain their IP address. This is generally done through DHCP [2].
   SIPs multicast registration mechanisms might supply an alternate way
   to obtain an IP address. This would eliminate the need for DHCP in
   clients. However, we do not believe such extensions are appropriate.
   We believe that protocols should be defined to provide specific,
   narrow functions, rather than being defined based on all
   communications requirements between a pair of devices. The latter
   approach to protocol design yields modular protocols with broad
   application. It also facilitates extensibility and growth; single
   protocols can be removed and changed without affecting the entire
   system. We observe that this approach to protocol engineering mirrors
   object oriented software engineering.




J.Rosenberg,H.Schulzrinne                                     [Page 2]


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   Our second criteria, that the extension must conform to the general
   SIP architectural model, ensures that the protocol remains manageable
   and broadly applicable.

2.1 SIP's Solution Space

   In order to evaluate the first criteria, it is necessary to define
   exactly what SIPs solution space is, and what it is not.

   SIP is a protocol for initiating, modifying, and terminating
   interactive sessions. This process involves the discovery of a user
   wherever they may be located, so that a description of the session
   can be delivered to the user. SIP itself is independent of the
   session, and the session description is delivered as an opaque body.
   Much of SIP focuses on this discovery component. Its ability to fork,
   its registration capabilities, and its routing capabilities are all
   present for the singular purpose of finding the called user wherever
   they may be. As such, features and capabilities such as personal
   mobility, automatic call distribution, and follow-me are well within
   the SIP solution space.

   Session initiation also depends on the ability of the called party to
   have enough information about the session itself in order to make a
   decision on whether to join or not. That information includes data
   about the caller, the purpose for the invitation, and parameters of
   the session itself. For this reason, SIP includes this kind of
   information.

   Part of the process of session initiation is the communication of
   progress and the final results of establishment of the session. SIP
   provides this information as well.

   There are many functions that SIP explicitly does not provide. It is
   not a session management protocol or a conference control protocol.
   The particulars of the communications within the session are outside
   of SIP. This includes features such as media transport, voting and
   polling, virtual microphone passing, chairman election, floor
   control, and feedback on session quality.

   SIP is not a resource reservation protocol for sessions. This is
   fundamentally because (1) SIP is independent of the underlying
   session it establishes, and (2) the path of SIP messages is
   completely independent from the path that packets for a session may
   take. The path independence refers to paths within a providers
   network, and the set of providers itself. For example, it is
   perfectly reasonable for a SIP message to traverse a completely
   different set of autonomus systems than the audio in a session SIP
   establishes.



J.Rosenberg,H.Schulzrinne                                     [Page 3]


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   SIP is not a transfer protocol. It is not meant to send large amounts
   of data unrelated to SIPs operation. It is not meant as a replacement
   for HTTP. This is for numerous reasons, one of which is that SIP's
   recommended mode of operation is over UDP. Sending large messages
   over SIP can lead to fragmentation at the IP layer and thus poor
   performance in even mildly lossy networks. This is not to say that
   carrying payloads in SIP messages is never a good thing; in many
   cases, the data is very much related to SIPs operation. However, SIP
   is not meant to carry large amounts of data unrelated to SIPs general
   function.

2.2 SIP Architectural Model

   We describe here some of the primary architectual assumptions which
   underly SIP. Extensions which violate these assumptions should be
   examined more carefully to determine their appropriateness for SIP.

        Session independence: SIP is independent of the session it
             establishes. This includes the type of session, be it
             audio, video, game, chat session, or virtual reality. SIP
             operation should never be dependent on some characteristic
             of the session.

        SIP and Session Path Independence: We have already touched on
             this once, but it is worth noting again. The set of routers
             and/or networks and/or autonomous systems traversed by SIP
             messages and the packets in the session are unrelated. They
             may be the same in some cases, bit it is fundamental to
             SIPs architecture that they need not be the same.
             Extensions which only work under some assumption of overlap
             are not generally applicable to SIPs operation and should
             be scrutinized carefully.

        Multi-provider and Multi-hop: SIP assumes that its messages will
             traverse the "Big I". That is, SIP works through multiple
             networks administered by different providers. It is also
             assumed that SIP messages traverse many hops (where each
             hop is a proxy). Extensions which only work in single hop
             or single provider networks may not be appropriate for SIP.

        Transactional: SIP is a request/response protocol. Many of the
             rules of operation in SIP are based on general processing
             of requests and responses. This includes the reliability
             mechanisms, routing mechanisms, and state maintenance
             rules. Extensions which add new messages that are not
             within the request-response model will likely break many
             aspects of SIP.




J.Rosenberg,H.Schulzrinne                                     [Page 4]


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        Proxies can ignore bodies: In order for proxies to scale well,
             they must be able to operate with minimal message
             processing. SIP has been engineered so that proxies can
             always ignore bodies. Extensions which require proxies to
             examine bodies in order to work will likely lead to serious
             scaling problems.

        Proxies don't need to understand the method: Processing of
             requests in proxies does not depend on the method, except
             for the well known methods INVITE, ACK, and CANCEL. This
             allows for extensibility. Extensions that define new
             methods which must be understood by proxies are NOT
             RECOMMENDED.

        INVITE messages carry full state: An initial INVITE message for
             a session is nearly identical (the exception is the tag) to
             a re-INVITE message to modify some characteristic of the
             session. This is strongly coupled to the idempotency of SIP
             requests, but is a different characteristic. Extensions
             which modify INVITE processing such that data spanning
             multiple INVITEs must be collected in order to perform some
             feature, are frowned upon.

        Generality over efficiency: Wherever possible, SIP has favored
             general purpose components rather than narrow ones. If some
             capability is added to support one service, but a slightly
             broader capability can support a larger variety of services
             (at the cost of complexity or message sizes), the broader
             capability is generally preferred.

3 Issues to be Addressed

   Given an extension has met the litmus tests in the previous section,
   there are several issues that all extension should take into
   consideration.

3.1 Backwards Compatibility

   One of the most important issues to consider is whether the new
   extension is backwards compatible with baseline SIP. This is tightly
   coupled with how the Require, Proxy-Require, and Supported [3]
   headers are used.

   If an extension consists of new headers inserted by a user agent in a
   request, and the request cannot be processed reasonably by a proxy
   and/or user agent without understanding the headers, the extension
   MUST mandate the usage of the Require and/or Proxy-Require headers in
   the request. These extensions are not backwards compatible with SIP.



J.Rosenberg,H.Schulzrinne                                     [Page 5]


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   The result of mandating usage of these headers means that requests
   cannot be serviced unless the entities being communicated with also
   understand the extension. If 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 first, the request simply fails, and the
   service cannot be provided. This is basically an interoperability
   failure. In the second case, the UAC retries the request without the
   extension. This will preserve interoperability, at the cost of a
   "dual stack" implementation in a UAC (processing rules for operation
   with and without the extension). As the number of extensions
   increases, this leads to an exponential explosion in the sets of
   processing rules a UAC may need to implement. The result is excessive
   complexity.

   Because of the possibility of interoperability and complexity
   problems that result from the usage of Require and Proxy-Require, we
   believe the following guidelines are appropriate:

        o The usage of these headers in requests for basic SIP services
          (in particular, session initiation and termination) is NOT
          RECOMMENDED. The less frequently a particular extension is
          needed in a request, the more reasonable it is to use these
          headers.

        o The Proxy-Require header SHOULD be avoided at all costs. As
          the number of hops for a request increases, the likelihood a
          particular proxy doesn't support some extension increases
          exponentially. On the other hand, the Require header only
          mandates that a single entity, the UAS, support the extension.
          Usage of Proxy-Require is thus considered exponentially worse
          than usage of the Require header.

   Extensions which define new methods do not need to use the Require
   header. SIP defines mechanisms which allow a UAC to know whether a
   new method is understood by a UAS. This includes both the OPTIONS
   request, and the 405 (Method Not Allowed) response with the Accept
   header. It is fundamental to SIP that proxies do not need to
   understand the semantics of a new method in order to process it. If
   an extension defines a new method which must be understood by proxies
   in order to be processed, a Proxy-Require header is needed. As
   discussed above, these kinds of extensions are frowned upon.

   In order to achieve backwards compatibility for extensions that
   define new methods, a "probing" mechanism SHOULD generally be defined
   as an integral component of the extension. In this mechanism, some
   header is included by the UAC in a standard SIP request. The UAS
   places some information in the response if it understands this header
   (and thus, the extension). If the UAC sees this information in the



J.Rosenberg,H.Schulzrinne                                     [Page 6]


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   response, it knows it is safe to send a request with the new method.

   Another type of extension are those which require a proxy to insert
   headers into a request as it traverses the network, or for the UAS to
   insert headers into a response. Some extensions can simply insert
   these headers. If the UAC or UAS does not understand them, the
   message can still be processed correctly. These extensions are
   completely backwards compatible.

   Most other extensions of this type will need to make use of the
   Supported request header mechanism. This mechanism allows a server to
   determine if the client can understand some extension applied to the
   response. If an extension is such that it requires a server to insert
   information into a response which must be understood in order for the
   response to be correctly processed, that extension SHOULD make use of
   [3]. By their nature, these extensions may not always be able to be
   applied to every response.

   If an extension requires a proxy to insert a header into a request,
   and this header needs to be understood by both UAC and UAS to be
   executed correctly, a combination of the probing mechanism above, and
   the Supported mechanism will need to be used. An example of such an
   extension is the SIP Session Timer [4].

   Yet another type of extension are those which define new body types
   to be carried in SIP messages. If the body type is to be conveyed in
   a request without usage of multipart, the compatibility issues mirror
   those of new methods. A probing mechanism is RECOMMENDED to determine
   if the body type is understood. If a body type is to be conveyed in a
   response, that type MUST only be sent if support for it was indicated
   in an Accept header in the request. If the body type is to be
   conveyed in a request with multipart, that body can either be
   mandatory or optional. Mandatory implies that the request cannot be
   processed unless the body is understood. Optional implies that the
   request can be processed if the body is understood. It is RECOMMENDED
   that extensions specify optional bodies if at all possible.


        We note that there is no defined way right now through MIME
        headers to indicate whether a body is mandatory or
        optional. This can be accomplished through a Require
        header, but a MIME parameter somehow seems more appropriate

3.2 Security

   Security is an important component of any protocol. SIP extensions
   SHOULD consider how (or if) they affect usage of the general SIP
   security mechanisms. Most extensions should not require any new



J.Rosenberg,H.Schulzrinne                                     [Page 7]


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   security capabilities beyond general purpose SIP. If they do, it is
   likely that the security mechanism has more general purpose
   application, and should be considered an extension in its own right.

3.3 Usage Guidelines

   All SIP extensions MUST contain guidelines defining when the
   extension is to be used.

   For new headers, the extension MUST define the request methods the
   header can appear in, and what responses it can be used in. It is
   RECOMMENDED that this information be represented as a new row of
   Table 4 of RFC 2543 [1]. The extension SHOULD specify which entities
   (UAC, UAS, proxy, redirect, registrar) are allowed to insert the
   header.

3.4 Syntactic Issues

   Extensions that define new methods SHOULD use all capitals for the
   method name. Method names SHOULD be less than 10 characters, and
   SHOULD attempt to convey the general meaning of the request.

   Extensions that define new headers SHOULD define a compact form
   representation if the non-compact header is more than four
   characters.

   Case sensitivity of parameters and values is a constant source of
   confusion. SIP extensions MUST clearly indicate the case sensitivity
   or insensitivity of every parameter, value or field they define. In
   general, case sensitivity is preferred because of the reduced
   processing requirements.

   Extensions which contain freeform text MUST allow that text to be
   UTF-8. This ensures that SIP remains an internationalized standard.
   As a general guideline, freeform text is never needed by programs in
   order to perform protocol processing. It is usually entered by and
   displayed to the user. If an extension uses a parameter which can
   contain UTF-8 encoded characters, and that extension requires a
   comparison to be made of this parameter to other parameters, the
   comparison MUST be case sensitive. Case insensitive comparison rules
   for UTF-8 text are extremely complicated and are to be avoided.

   Extensions which make use of dates and times MUST use the SIP-Date
   BNF defined in RFC 2543. No other date formats are allowed.

   Extensions which include network layer addresses SHOULD permit dotted
   quad IPv4 addresses, IPv6 addresses in the format described in , and
   domain names.



J.Rosenberg,H.Schulzrinne                                     [Page 8]


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   Extensions which have headers containing URLs SHOULD allow any URI,
   not just SIP URLs.

3.5 Semantics, Semantics, Semantics

   Developers of protocols often get caught up in syntax issues, without
   spending enough time on semantics. The semantics of a protocol are
   far more important. SIP extensions MUST clearly define the semantics
   of the extensions. Specifically, the extension MUST specify the
   behaviors expected of a UAC, UAS and proxy in processing the
   extension. This is often best described by having separate sections
   for each of these three elements. Each section SHOULD step through
   the processing rules in temporal order of the most common messaging
   scenario.

   Processing rules generally specify actions to take (in terms of
   messages to send, variables to store, rules to follow) on receipt of
   messages and expiration of timers. If an action requires transmission
   of a message, the rule SHOULD outline requirements for insertion of
   headers or other information in the message.

   The extension SHOULD specify procedures to take in exceptional
   conditions. This usually includes receipt of messages that are not
   expected, expiration of timers that handle timeouts, and presence of
   headers in messages when they are not expected.

3.6 Examples Section

   Presence of sections in the extension giving examples of call flows
   and message formatting is RECOMMENDED. Extensions which define
   substantial new syntax SHOULD include examples of messages containing
   that syntax. Examples of message flows SHOULD be given to cover
   common cases and at least one failure or unusual case.

3.7 Overview Section

   Too often, extension documents dive into detailed syntax and
   semantics without giving a general overview of operation. This makes
   understanding of the extension harder. It is RECOMMENDED that
   extensions have a protocol overview section which discusses the basic
   operation of the extension. Basic operation usually consists of the
   message flow, in temporal order, for the most common case covered by
   the extension. The most important processing rules for the elements
   in the call flow SHOULD be mentioned. Usage of the RFC 2119 [5]
   terminology in the overview section is RECOMMENDED.

3.8 Additional Considerations for New Methods




J.Rosenberg,H.Schulzrinne                                     [Page 9]


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   Extensions which define new methods SHOULD take into consideration,
   and discuss, the following issues:

        o Can it contain bodies? If so, what is the meaning of the
          presence of those bodies? What body types are allowed?

        o Can a transaction with this request method occur while another
          transaction, in the same and/or reverse direction, is in
          progress?

        o What headers are allowed in requests of this method? It is
          recommended that this information be presented through a
          column of Table 4 in RFC 2543 [1].

        o All SIP requests can generally be cancelled. However, an
          extension MAY mandate that a new method not be cancelled. In
          either case, handling of CANCEL SHOULD be described. In
          particular, the rules a UAS should follow upon cancellation of
          an unanswered request SHOULD be described.

        o Can the request be sent within a call or not? In this context,
          within means that the request is sent with the same Call-ID,
          To and From field as an INVITE that was sent or received
          previously. For, example, the REGISTER method is not
          associated with a call, whereas the BYE method is.

3.9 Additional Considerations for New Headers or Header Parameters

   The most important issue for extensions that define new headers is
   backwards compatibility. See Section 3.1 for a discussion of the
   issues. The extension MUST detail how backwards compatibility is
   addressed.

   It is often tempting to avoid creation of a new method by overloading
   an existing method through a header. Headers are not meant to
   fundamentally alter the meaning of the method of the request. A new
   header SHOULD NOT change the basic semantic and processing rules of a
   method.

3.10 Additional Considerations for New Body Types

   Because SIP can run over UDP, extensions that specify the inclusion
   of large bodies are frowned upon. If at all possible, the content
   should be included indirectly through an http URL.

4 Interactions with SIP Features

   We have observed that certain capabilities of SIP continually



J.Rosenberg,H.Schulzrinne                                    [Page 10]


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   interact with extensions in unusual ways. Writers of extensions
   SHOULD consider the interactions of their extensions with these SIP
   capabilities, document any unusual interactions if they exist. The
   most common causes of problems are:

        Forking: Forking by far presents the most troublesome
             interactions with extensions. This is generally because it
             can cause (1) a single transmitted request to be received
             by an unknown number of UASs, and (2) a single request to
             have multiple responses.

        Tags: Tags are used to uniquely identify call legs. Their
             presence is neccesitated as a result of forking. They are
             an unfortunate exception to many SIP processing rules.
             Extensions should carefully consider their effect.

        CANCEL and ACK: CANCEL and ACK are "special" SIP requests, in
             that they are exceptions to many of the general request
             processing rules. That is because CANCEL and ACK are always
             associated with another request. New methods SHOULD
             consider the meaning of cancellation. New headers in INVITE
             requests SHOULD consider whether they also need to be
             included in ACK.

        Routing: The Route, Record-Route and Via headers are used to
             support message routing. New request methods SHOULD
             carefully consider how these headers are used.

        Stateless Proxies: SIP allows a proxy to be stateless. Stateless
             proxies are unable to retransmit messages and cannot
             execute certain services. Extensions which depend on some
             kind of proxy processing SHOULD consider how stateless
             proxies affect that processing.

5 Security Considerations

   The nature of this document is such that it does not introduce any
   new security considerations.

6 Authors Addresses



   Jonathan Rosenberg
   dynamicsoft
   200 Executive Drive
   Suite 120
   West Orange, NJ 07052



J.Rosenberg,H.Schulzrinne                                    [Page 11]


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   email: jdrosen@dynamicsoft.com

   Henning Schulzrinne
   Columbia University
   M/S 0401
   1214 Amsterdam Ave.
   New York, NY 10027-7003
   email: schulzrinne@cs.columbia.edu




7 Bibliography

   [1] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP:
   session initiation protocol," Request for Comments (Proposed
   Standard) 2543, Internet Engineering Task Force, Mar. 1999.

   [2] R. Droms, "Dynamic host configuration protocol," Request for
   Comments (Draft Standard) 2131, Internet Engineering Task Force, Mar.
   1997.

   [3] J. Rosenberg and H. Schulzrinne, "Mandating SIP extension support
   by servers," Internet Draft, Internet Engineering Task Force, Jan.
   2000.  Work in progress.

   [4] S. Donovan, "SIP session timer," Internet Draft, Internet
   Engineering Task Force, Oct. 1999.  Work in progress.

   [5] S. Bradner, "Key words for use in RFCs to indicate requirement
   levels," Request for Comments (Best Current Practice) 2119, Internet
   Engineering Task Force, Mar. 1997.



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J.Rosenberg,H.Schulzrinne                                    [Page 12]


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J.Rosenberg,H.Schulzrinne                                    [Page 13]