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Network Working Group                                       F. AndreasenRequest for Comments: 3407                                 Cisco SystemsCategory: Standards Track                                   October 2002Session Description Protocol (SDP) Simple Capability DeclarationStatus of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2002).  All Rights Reserved.Abstract   This document defines a set of Session Description Protocol (SDP)   attributes that enables SDP to provide a minimal and backwards   compatible capability declaration mechanism.  Such capability   declarations can be used as input to a subsequent session   negotiation, which is done by means outside the scope of this   document.  This provides a simple and limited solution to the general   capability negotiation problem being addressed by the next generation   of SDP, also known as SDPng.1. Conventions Used in this Document   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this   document are to be interpreted as described inRFC-2119 [2].2. Introduction   The Session Description Protocol (SDP) [3] describes multimedia   sessions for the purposes of session announcement, session   invitation, and other forms of multimedia session initiation.  SDP   was not intended to provide capability negotiation.  However, as the   need for this has become increasingly important, work has begun on a   "next generation SDP" (SDPng) [4,5] that supports both session   description and capability negotiation.  SDPng is not anticipated to   be backwards compatible with SDP and work on SDPng is currently in   the early stages.  However, several other protocols, e.g. SIP [6] and   Media Gateway Control Protocol (MGCP) [7], use SDP and are likely toAndreasen                   Standards Track                     [Page 1]

RFC 3407           SDP Simple Capability Declaration        October 2002   continue doing so for the foreseeable future.  Nevertheless, in many   cases these signaling protocols have an urgent need for some limited   form of capability negotiation.   For example, an endpoint may support G.711 audio (over RTP) as well   as T.38 fax relay (over UDP or TCP).  Unless the endpoint is willing   to support two media streams at the same time, this cannot currently   be expressed in SDP.  Another example involves support for multiple   codecs.  An endpoint indicates this by including all the codecs in   the "m=" line in the session description.  However, the endpoint   thereby also commits to simultaneous support for each of these   codecs.  In practice, Digital Signal Processor (DSP) memory and   processing power limitations may not make this feasible.   As noted in [4], the problem with SDP is that media descriptions are   used to describe session parameters as well as capabilities without a   clear distinction between the two.   In this document, we define a minimal and backwards compatible   capability declaration feature in SDP by defining a set of new SDP   attributes.  Together, these attributes define a capability set,   which consists of a capability set sequence number followed by one or   more capability descriptions.  Each capability description in the set   contains information about supported media formats, but the endpoint   is not committing to use any of these.  In order to actually use a   declared capability, session negotiation will have to be done by   means outside the scope of this document, e.g., using the   offer/answer model [8].   It should be noted that the mechanism is not intended to solve the   general capability negotiation problem targeted by SDPng.  It is   merely intended as a simple and limited solution to the most urgent   problems facing current users of SDP.3. Simple Capability Declaration Attributes   The SDP Simple Capability Declaration (simcap) is defined by a set of   SDP attributes.  Together, these attributes form a capability set   which describes the complete media capabilities of the endpoint.  Any   previous capability sets issued by the endpoint for the session in   question no longer apply.  The capability set consists of a sequence   number and one or more capability descriptions.  Each such capability   description describes the media type and media formats supported and   may include one or more capability parameters to further define the   capability.  A session description MUST NOT contain more than one   capability set, however the capability set can describe capabilities   at both the session and media level.  Capability descriptions   provided at the session level apply to all media streams of the mediaAndreasen                   Standards Track                     [Page 2]

RFC 3407           SDP Simple Capability Declaration        October 2002   type indicated, whereas capability descriptions provided at the media   level apply to that particular media stream only.  We refer to these   respectively as session capabilities and media stream capabilities.   A media stream capability may or may not be of the same media type as   the media stream to which it applies.   The capability set MUST begin with a single sequence number followed   by one or more capability descriptions listing all media formats the   endpoint is currently able and willing to support.  More   specifically, if a media format is included in a media ("m=") line,   then by definition the media format MUST be included in either a   session capability or a media stream capability for that media line.   The endpoint MAY include additional media formats in a capability if   it is capable of supporting those media formats in a session with its   peer.  An endpoint MUST NOT include capabilities it knows it cannot   use in a particular session.  An endpoint receiving a capability set   from another endpoint MAY use any of the media formats included in   that capability set in a later attempt to negotiate media streams   with the other endpoint, e.g., using the offer/answer model [8].  If   a new capability set is received from the other endpoint, the old   capability set MUST NOT be used any longer.  Session capabilities can   be used for any media streams of the indicated media type, whereas   media stream capabilities can only be used for their associated media   line.  However, an endpoint receiving a capability set with a given   media format MUST NOT assume that a subsequent attempt to negotiate a   media stream using just this media format will succeed.   The individual capability descriptions in a capability set can be   provided contiguously or they can be scattered throughout the session   description.  The first capability description MUST, however, follow   immediately after the sequence number.   The sequence number is on the form:     a=sqn: <sqn-num>   where <sqn-num> is an integer between 0 and 255 (both included).  The   initial sequence number MUST be 0 (zero) and it MUST be incremented   by 1 modulo 256 with each new capability set issued by the endpoint.   Receivers may not necessarily see all capability sets issued and   hence MUST NOT reject a capability set due to gaps in sequence   numbers.  The sequence number MUST either be provided as a session-   level or media-level attribute, however there MUST NOT be more than   one occurrence of the sequence number attribute in the session   description (since there cannot be more than one capability set).Andreasen                   Standards Track                     [Page 3]

RFC 3407           SDP Simple Capability Declaration        October 2002   Each capability description in the capability set is on the form:     a=cdsc: <cap-num> <media> <transport> <fmt list>   where <cap-num> is an integer between 1 and 255 (both included) used   to number the capabilities, and <media>, <transport>, and <fmt list>   are defined as in the SDP "m=" line.  The capability description   refers to a send and receive capability by default.  When generating   a capability set, the capability number MUST start with 1 in the   first capability description, and be incremented by the number of   media formats in the <fmt list> for each subsequent capability   description.  The media formats in the <fmt list> are numbered from   left to right.  Receivers of a capability set MUST NOT, however,   reject capability descriptions due to gaps in the capability numbers.   The capability number provides a convenient handle within the context   of the capability set (as referenced by the sequence number) which   may be used to reference a particular capability by means outside of   this specification.   A capability description can include one or more capability parameter   lines on the form:     a=cpar: <cap-par>     a=cparmin: <cap-par>     a=cparmax: <cap-par>   where <cap-par> is either bandwidth information ("b=") or an   attribute ("a=") in its full  '<type>=<value>' form (see [3]).  A   capability parameter line provides additional parameters for the   preceding "cdsc" attribute line.  Capability parameter lines for a   capability description SHOULD immediately follow the "cdsc" line they   refer to.  Nevertheless, the capability description includes all   capability parameter lines until the next capability description   ("cdsc") or media ("m=") line in the session description.   The "cpar" attribute should normally be used when capability   parameter values are to be specified. When provided, it means that   the endpoint is declaring that it supports the media formats in the   preceding "cdsc" line in accordance with the <cap-par> value   specified.  This can, for example, be used to specify "fmtp"   parameters.  If a session negotiation is attempted without   considering the <cap-par> value, it may fail due to lack of endpoint   support.  A capability description may contain zero, one, or more   "cpar" attribute lines describing either the same or different   parameters.  Describing the same parameter more than once can be used   to specify alternatives.Andreasen                   Standards Track                     [Page 4]

RFC 3407           SDP Simple Capability Declaration        October 2002   Where a minimum numerical value is to be specified, the "cparmin"   attribute should be used.  There may be zero, one, or more "cparmin"   attribute lines in a capability description, however a given   parameter MUST NOT be described by a "cparmin" attribute more than   once.   Where a maximum numerical value is to be specified, the "cparmax"   attribute should be used.  There may be zero, one, or more "cparmax"   attribute lines in a capability description, however a given   parameter MUST NOT be described by a "cparmax" attribute more than   once.   Ranges of numerical values can be expressed by using a "cparmin" and   a "cparmax" attribute for a given parameter.  It follows from the   previous rules, that only a single range can be specified for a given   parameter.   Capability descriptions may be provided at both the session-level and   media-level.  A capability description provided at the session-level   applies to all the media streams of the indicated media type in the   session description.  A capability description provided at the   media-level only applies to that particular media stream (regardless   of media type).  If a capability description with media type X is   provided at the session-level, and there are no media streams of type   X in the session description, then it is undefined which of the media   streams the capability description applies to (except if there is   only one media stream).  It is therefore RECOMMENDED, that such   capabilities are provided at the media-level instead.   Below we show an example session description using the above simple   capability declaration mechanism:     v=0     o=- 25678 753849 IN IP4 128.96.41.1     s=     c=IN IP4 128.96.41.1     t=0 0     m=audio 3456 RTP/AVP 18 96     a=rtpmap:96 telephone-event     a=fmtp:96 0-15,32-35     a=sqn: 0     a=cdsc: 1 audio RTP/AVP 0 18 96     a=cpar: a=fmtp:96 0-16,32-35     a=cdsc: 4 image udptl t38     a=cdsc: 5 image tcp t38Andreasen                   Standards Track                     [Page 5]

RFC 3407           SDP Simple Capability Declaration        October 2002   The sender of this session description is currently prepared to send   and receive G.729 audio as well as telephone-events 0-15 and 32-35.   The sender is furthermore capable of supporting:   *  PCMU encoding for the audio media stream,   *  telephone events 0-16 and 32-35,   *  T.38 fax relay using udp or tcp (see [9]).   Note, that the first capability number specified is 1, whereas the   next is 4 since three media formats were included in the first   capability description.  Also note that the rtpmap for payload type   96 was not included in the capability description, as it was already   specified for the media ("m=") line.  Conversely, it would of course   not have been valid to provide the rtpmap in the capability   description and then omit the "a=rtpmap" line.   Below, we show another example of the simple capability declaration   mechanism, this time with multiple media streams:     v=0     o=- 25678 753849 IN IP4 128.96.41.1     s=     c=IN IP4 128.96.41.1     t=0 0     m=audio 3456 RTP/AVP 18     a=sqn: 0     a=cdsc: 1 audio RTP/AVP 0 18     m=video 3458 RTP/AVP 31     a=cdsc: 3 video RTP/AVP 31 34   The sender of this session description is currently prepared to send   and receive G.729 audio and H.261 video.  The sender is furthermore   capable of supporting:   *  PCMU encoding for the audio media stream,   *  H.263 for the video media stream.   Note that the first capability number specified is 1, whereas the   next is 3, since two media formats were included in the first   capability description.  Also note that the sequence number applies   to the entire capability set, i.e. both audio and video, and hence is   only supplied once.  Finally, note that the media formats 18 and 31   are listed in both the media lines and the capability set as   required.  The above session description could have equally been   supplied as follows:Andreasen                   Standards Track                     [Page 6]

RFC 3407           SDP Simple Capability Declaration        October 2002     v=0     o=- 25678 753849 IN IP4 128.96.41.1     s=     c=IN IP4 128.96.41.1     t=0 0     a=sqn: 0     a=cdsc: 1 audio RTP/AVP 0 18     a=cdsc: 3 video RTP/AVP 31 34     m=audio 3456 RTP/AVP 18     m=video 3458 RTP/AVP 31   i.e., with the capability set provided at the session-level.4. Security Considerations   Capability negotiation of security-sensitive parameters is a delicate   process, and should not be done without careful evaluation of the   design, including the possible susceptibility to downgrade attacks.   Use of capability re-negotiation may make the session susceptible to   denial of service, without design care as to authentication.5. IANA Considerations   This document defines the following new SDP parameters of type "att-   field" (attribute names):   Attribute name:      sqn   Long form name:      Sequence number.   Type of attribute:   Session-level and media-level.   Subject to charset:  No.   Purpose:             Capability set numbering.   Appropriate values:  SeeSection 3.   Attribute name:      cdsc   Long form name:      Capability description.   Type of attribute:   Session-level and media-level.   Subject to charset:  No.   Purpose:             Describe capabilities in a capability set.   Appropriate values:  SeeSection 3.   Attribute name:      cpar   Long form name:      Capability parameter line.   Type of attribute:   Session-level and media-level.   Subject to charset:  No.   Purpose:             Provide capability description parameters.   Appropriate values:  SeeSection 3.Andreasen                   Standards Track                     [Page 7]

RFC 3407           SDP Simple Capability Declaration        October 2002   Attribute name:      cparmin   Long form name:      Minimum capability parameter line.   Type of attribute:   Session-level and media-level.   Subject to charset:  No.   Purpose:             Provide minimum capability description                        parameters.   Appropriate values:  SeeSection 3.   Attribute name:      cparmax   Long form name:      Maximum capability parameter line.   Type of attribute:   Session-level and media-level.   Subject to charset:  No.   Purpose:             Provide maximum capability description                        parameters.   Appropriate values:  SeeSection 3.6. Normative References   [1] Bradner, S., "The Internet Standards Process -- Revision 3",BCP9,RFC 2026, October 1996.   [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement       Levels",BCP 14,RFC 2119, March 1997.   [3] Handley, M. and V. Jacobson, "SDP: session description protocol",       Request for Comments 2327, April 1998.7. Informative References   [4] Kutscher, Ott, Bormann and Curcio, "Requirements for Session       Description and Capability Negotiation", Work in Progress.   [5] Kutscher, Ott and Borman, "Session Description and Capability       Negotiation", Work in Progress.   [6] Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg,       "SIP: session initiation protocol",RFC 2543, March 1999.   [7] Arango, M., Dugan, A., Elliott, I., Huitema, C. and S. Pickett,       "Media Gateway Control Protocol (MGCP) Version 1.0",RFC 2705,       October 1999.   [8] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with       SDP", Work in Progress.   [9] ITU-T Recommendation T.38 Annex D, "SIP/SDP Call Establishment       Procedures".Andreasen                   Standards Track                     [Page 8]

RFC 3407           SDP Simple Capability Declaration        October 20028. Acknowledgments   This work draws upon the ongoing work on SDPng in the IETF MMUSIC   Working Group; in particular [4].  Furthermore this work was inspired   by the CableLabs PacketCable project.  The author would like to   recognize and thank Joerg Ott and Jonathan Rosenberg who provided   many detailed comments and suggestions to improve this specification.   Colin Perkins, Orit Levin and Tom Taylor provided valuable feedback   as well.9. Author's Address   Flemming Andreasen   Cisco Systems   499 Thornall Street, 8th floor   Edison, NJ   EMail: fandreas@cisco.comAndreasen                   Standards Track                     [Page 9]

RFC 3407           SDP Simple Capability Declaration        October 200210. Full Copyright Statement   Copyright (C) The Internet Society (2002).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Andreasen                   Standards Track                    [Page 10]