MMUSIC R. Gilman Internet-Draft Avaya, Inc. Intended status: Standards Track R. Even, Ed. Expires:
August 24, 2007May 18, 2008 Polycom F. Andreasen Cisco Systems February 20,November 15, 2007 SDP media capabilities Negotiation draft-ietf-mmusic-sdp-media-capabilities-01.txtdraft-ietf-mmusic-sdp-media-capabilities-02.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- 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. This Internet-Draft will expire on August 24, 2007.May 18, 2008. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract Session Description Protocol (SDP) capability negotiation provides a general framework for negotiating capabilities in SDP. The base framework defines only capabilities for negotiating transport protocols and attributes. In this document, we extend the framework by defining media capabilities that can be used to negotiate media types and their associated parameters. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 34 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 45 3. SDP Media capabilities . . . . . . . . . . . . . . . . . . . . 56 3.1. Solution Overview . . . . . . . . . . . . . . . . . . . . 56 3.2. Capability Attributes . . . . . . . . . . . . . . . . . . 89 3.2.1. Media Type and Subtype Capability Attribute . . . . . 810 3.2.2. The CapabilityMedia Encoding ParametersParameter Capability Attribute . . . . . 911 3.2.3. The Media Format Parameter Capability Attribute . . . 1012 3.2.4. The Bandwidth Capability Attribute . . . . . . . . . . 13 3.3. Extensions to the Potential Configuration Attribute . . . 1115 3.3.1. The Media Capability Extension to the Potential Configuration Attribute . . . . . . . . . . . . . . . 1115 3.3.2. The Payload Type Mapping Extension to the Potential Configuration Attribute . . . . . . . . . . 1216 3.3.3. The Bandwidth Extension to the Potential Configuration Attribute . . . . . . . . . . . . . . . 17 3.4. Extensions to the Actual Configuration Attribute . . . . . 1217 3.5. The Latent Configuration Attribute . . . . . . . . . . . . 1318 3.5.1. The crypto: Attribute in Latent Configurations . . . . 1418 3.6. Offer/Answer Model Extensions . . . . . . . . . . . . . . 1419 3.6.1. Generating the Initial Offer . . . . . . . . . . . . . 1419 3.6.2. Generating the Answer . . . . . . . . . . . . . . . . 1419 3.6.3. Offerer Processing of the Answer . . . . . . . . . . . 1419 3.6.4. Modifying the Session . . . . . . . . . . . . . . . . 1420 4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 1521 4.1. Alternative Codecs . . . . . . . . . . . . . . . . . . . . 1521 4.2. Latent Media Streams . . . . . . . . . . . . . . . . . . . 1521 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 1622 6. Security Considerations . . . . . . . . . . . . . . . . . . . 1723 7. Changes from previous versions . . . . . . . . . . . . . . . . 24 7.1. Changes from version 0001 . . . . . . . . . . . . . . . . . 24 7.2. Changes from version 00 . . 18. . . . . . . . . . . . . . . 24 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 1925 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 2026 9.1. Normative References . . . . . . . . . . . . . . . . . . . 2026 9.2. Informative References . . . . . . . . . . . . . . . . . . 2026 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 2127 Intellectual Property and Copyright Statements . . . . . . . . . . 2228 1. Introduction Session Description Protocol (SDP) capability negotiation [SDPCapNeg] provides a general framework for negotiating capabilities in SDP[RFC4566]. The base framework defines only capabilities for negotiating transport protocols and attributes. In this document, we extend the framework by defining media capabilities that can be used to negotiate media types and their associated parameters.SDP Simple Capability Declaration (simcap) is defined in RFC 3407 [RFC3407]. It defines a set of SDP attributes that enables a limited set of capabilities to be described at a session level or on a per media stream basis. RFC 3407 defines capability declaration only - actualonly. Actual negotiation procedures taking advantage of such capabilities have not been defined. The SDP capability negotiation framework adds this required functionality. This document updates RFC3407 and new implementation SHOULD use the functionality defined in the current draft to negotiate media capabilities. The [SDPCapNeg] document lists some of the issues with the current SDP capability negotiation process. An additional real life case is to be able to offer one media stream (e.g. audio) but list the capability to support another media stream (e.g. video) without actually offering it currently. In this document, we extend the framework by defining media capabilities that can be used to negotiate media types and their associated parameters like bandwidth. This document also adds the capability to declare support for media streams that can be offered later rather than as a potential configuration for current use. 2. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC2119 [RFC2119] and indicate requirement levels for compliant RTP implementations. 3. SDP Media capabilities In this section we first provide an overview of theThe SDP media Capabilitycapability negotiation [SDPCapNeg] discusses the use of any SDP [RFC4566] attribute (a=) under the acap attribute. The limitations of using acap for fmtp and rtpmap as potential configuration are listed, for example they can be used only at the media level since they are media level attributes. The [SDPCapNeg] addresses the issue of bandwidth usage but does not offer a way to negotiate different bandwidth for different codecs and profiles. This section provides an overview of extensions providing SDP media Capability negotiation solution.solution offering more robust capabilities negotiation. This is followed by definitions of new SDP attributes for the solution and its associated updated offer/ answeroffer/answer procedures [RFC3264] 3.1. Solution Overview The solution consist of the following new attributes extending the base attributes from [SDPCapNeg]. ThreeFour attributes are used to make up media capabilities in a manner that can be related to the capabilities specified in a media line, its attributes and the bandwidth parameter lines. o A new media attribute ("a=cmed")("a=mcap") that lists media formats as capabilities in the form a media type (e.g. "audio") and one or more subtypes (e.g. "PCMU"), and associates a handle with each subtype o A new attribute ("a=cenc")("a=ecap") that lists encoding parameter capabilities associated with a particularone or more media format capability.capabilities. o A new attribute ("a=cfmt")("a=fcap") that lists media format parameter capabilities associated with a particularone or more media format capability.capabilities. o A new attribute ("a=bcap") that is used to specify the bandwidth parameter(s) for media capabilities. o A new attribute ("a=lcfg") that specifies latent configurations when no corresponding media line is offered. An example is a latent configuration for video even though no video is currently offered. o A new parameter type ("m=") to the potential configuration ("a=pcfg:") attribute and the actual configuration ("a=acfg:") attribute defined in [SDPCapNeg], which permits specification of media capabilities (including their associated parameters) and combinations thereof for the configuration. For example, the "a=pcfg:" line might specify PCUMPCMU and telephone events or G.729B and telephone events as acceptable configurations. The "a=acfg:" line in the answer would specify the accepted choice. o A new parameter type ("pt=") to the potential configuration ("a=pcfg:") attribute which associates RTP payload types with the referenced media capabilities. o A new parameter type ("b=") used to specify bandwidth parameters in a potential configuration. The document extends the base protocol extensions to the offer/answer model that allow for capabilities and potential configurations to be included in an offer. Media capabilities constitute capabilities that can be used in potential and latent configurations. Whereas potential configurations constitute alternative offers that may be accepted by the answerer instead of the actual configuration(s) included in the "m=" line(s), latent configurations merely inform the other side of possible configurations supported by the entity. Those latent configurations may be used to guide subsequent offer/answer exchanges. The mechanism is illustrated by the offer/answer exchange below, where Alice sends an offer to Bob: Alice Bob | (1) Offer (SRTP and RTP) | |--------------------------------->| | | | (2) Answer (RTP) | |<---------------------------------| | | Alice's offer includes RTP and SRTP as alternatives. RTP is the default, but SRTP is the preferred one: v=0 o=- 25678 753849 IN IP4 192.0.2.1 s= c=IN IP4 192.0.2.1 t=0 0 a=creq:v1 a=cmed:1a=creq:med-v0 a=mcap:1 audio g729 iLBC PCMU g729 a=cenc:2a=ecap:2 8000 a=cfmt:1a=fcap:1 annexb:no a=ctrpr:1a=fcap:2 mode=20 a=tcap:1 RTP/SAVP m=audio 3456 RTP/AVP 0 18 a=capar:1 a=crypto:1a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 a=pcfg:1 m=1,3|4,3 t=1 a=1 pt=1:100,4:101,3:102 a=pcfg:2 m=2 pt=2:103 The required base and extensions are provided by the "a=creq" attribute defined in [SDPCapNeg], with the option tag "v1","med-v0", which indicates that the extension framework defined here, must be supported. The Base level support is implied since it is required for the extensions. The "a=cmed:1""a=mcap:1" line defines four audio media subtype capabilities , to be numbered consecutively starting with 1. Note that the media subtypes specified in the m-line (PCMU and G729) are explicitly specified here. The "a=cenc:2""a=ecap:2" line specifies the clock rate and encoding parmeters (see [RFC4566]) for capability 2, iLBC. The "a=cfmt:1""a=fcap:1" line specifies media format parameter capabilities for codec 1 ( no Annex B for G.729). The "a=ctrpr:1""a=fcap:2" line specifies a media format parameter for codec 2 (20ms packets for iLBC. [RFC3952] The "a=tcap:1" line, specified in the base protocol, defines a transport protocol capability, in this case Secure RTP. The "m=" line indicates that Alice is offering to use plain RTP with PCMU or G.729. The media line implicitly defines the default transport protocol (RTP/AVP in this case) and the default actual configuration. The "a=capar:1""a=acap:1" line ,specified in the base protocol provides the "crypto" attribute which provides the keying material for SRTP using SDP security descriptions. The "a=pcfg:" attributes provide the potential configurations included in the offer by reference to the media capabilities, transport capabilities, and associated payload type mappings. Two explicit alternatives are provided; the first one, numbered 1 is the preferred one. It specifies media capabilities 1 and 3, i.e. G.729 and PCMU, or media capability 4 and 3, i.e., G.729B and PCMU. Furthermore, it specifies transport protocol capability 1 (i.e. the RTP/SAVP profile - secure RTP), and the attribute capability 1, i.e. the crypto attribute provided. Lastly, it specifies, a payload type mapping for codecs 1, 3, and 4 thereby permitting the offerer to distinguish between encrypted media and unencrypted media received prior to receipt of the answer. For SRTP the offerer will still need to receive the answer before being able to decrypt the stream. The second alternative specifies media capability 2, i.e. iLBC, under the default RTP/AVP profile .profile. The media line, with any qualifying attributes such as fmtp or rtpmap, is itself considered a valid configuration; it is assumed to be the lowest preference. Bob receives the SDP offer from Alice. Bob supports RTP, but not SRTP, and hence he accepts the actual configuration for RTP provided by Alice. Furthermore, Bob wants to use the iLBC codec and hence generates the following answer: v=0 o=- 24351 621814 IN IP4 192.0.2.2 s= c=IN IP4 192.0.2.2 t=0 0 a=csup:v1a=csup:med-v0 m=audio 4567 RTP/AVP 103 a=rtpmap:103 8000iLBC/8000 a=fmtp:103 mode=20 a=acfg:2 Bob includes the "a=csup" and "a=acfg" attribute in the answer to inform Alice that he can support the v1med-v0 level of capability negotiations. Note that in this particular example, the answerer supported the capability extensions defined here, however had he not, he would simply have processed the offer based on the offered PCMU and G.729 codecs under the RTP/AVP profile only. Consequently, the answer would have omitted the "a=csup" attribute line and chosen one or both of the PCMU and G.729 codecs instead. The answer carries the accepted configuration in the m line along with corresponding rtpmap: and/or fmtp: parameters, as appropriate. Note that per the base protocol, after the above, Alice shouldMAY generate a new offer with an actual configuration ("m=" line, etc.) corresponding to the actual configuration referenced in Bob's answer (not shown here). 3.2. Capability Attributes In this section, we present the new attributes associated with indicating the media capabilities for use by the SDP Capability negotiation. The approach taken is to keep things similar to the existing media capabilities defined by the existing media descriptions ("m=" lines) and the associated "rtpmap" and "fmtp" attributes, but usingattributes. We use media subtypes and "media capability numbers" instead of payload types to link the relevant media capability parameters. 3.2.1. Media Type and Subtype Capability Attribute Media types and subtypes can be expressed as media format capabilities by use ofThis permits the "a=cmed" attribute, which iscapabilities to be defined as follows: a=cmed:<med-cap-num> <type> <subtype>*[ <subtype>] where <med-cap-num> is an integer between 1at the session level and 2^31-1 (both included)used to numberfor multiple streams, if desired. Payload types are then specified at the media format capabilities, <type> is alevel (see Section 3.2.2). A media capability merely indicates possible support for the media type (e.g., audio or video),and the <subtype> is themedia subtype e.g. H263-1998, PCMU (Editors' note: can specify in cmed anything that can be specifiedformat(s) in an m-line?). The <med-cap-num> is thequestion. In order to actually use a media capability in an offer/answer exchange, it must be referenced in a potential configuration (see Section 2.3.1). Media capabilities can be provided at the session-level and/or the media-level. Media capabilities provided at the session level may be referenced in an lcfg attribute at the session level, or by any pcfg attribute at the media level, whereas media capabilities provided at the media level may be referenced by a pcfg attribute within that media stream only. In either case, the scope of the <med-cap-num> is the entire session description. This enables each media capability to be uniquely referenced across the entire session description (e.g. in a potential configuration) 3.2.1. Media Type and Subtype Capability Attribute Media types and subtypes can be expressed as media format capabilities by use of the "a=mcap" attribute, which is defined as follows: a=mcap:<med-cap-num> <subtype>*[ <subtype>] where <med-cap-num> is an integer between 1 and 2^31-1 (both included) used to number the media format capabilities and the <subtype> is the media subtype e.g. H263-1998, PCMU. The <med-cap- num> is the media capability number associated with the first subtype in the list, the number associated with the second subtype is one higher, etc. Each occurrence of the attribute MUST use a different value of <med-cap-num>. Furthermore, when a "cmed"an "mcap" attribute indicates more than one media format, the capability numbers implied MUST NOT be used by any other "cmed""mcap" attribute in the entire session or media description (explicitly or implicitly). In ABNF, we have: media-capability-line = "a=cmed:""a=mcap:" media-cap-num WSP media-type WSP1*WSP media-cap *(WSP media-cap) media-cap-num = 1*DIGIT media-type = token ; Type name (audio, video, etc.)media-cap = token ; Subtype name(PCMU, G729, etc.) Media subtypes identified in "a=cmed" lines mayThe "mcap" attribute can be qualified viaprovided at the attributes, "a=cenc"session-level and "a=cfmt", in muchthe same way as media formats in "m=" lines payload types can have their clock rate and encoding parameters qualified by an "a=rtpmap" line and media format specific parametersmedia-level. There can be provided by "a=fmtp" lines. 3.2.2. The Capability Encoding Parameters Attribute Media format capabilities may require additional encoding parameters, such as sample rate, to be precisely defined. The "a=cenc" encodingmore than one mcap attribute isat the session or media level. The unique media-cap-num is used to identify it in potential configurations. When used in a potential configuration it is a media level attribute regardless if it is specified at the session or media level. For example: v=0 a=mcap:1 iLBC a= mcap:2 H263-1998, H264 3.2.2. The Media Encoding Parameter Capability Attribute Media format capabilities may require additional encoding parameters, such as sample rate, to be precisely defined. The "a=ecap" encoding attribute is defined as a=cenc:<med-cap-num>a=ecap:<med-cap-num-list> <clock rate>[/<encoding parameters>] where med-cap-num-list = <med-cap-num> *[COMMA<med-cap-num>] The clock rate and other encoding parameters are as defined for the "a=rtpmap:" attribute defined in RFC 4566 [RFC4566]. The "ecap" attribute can be provided at the session-level and the media-level. There can be more than one ecap attribute at the session or media level. The unique media-cap-num is used to identify it in potential configurations. When used in a potential configuration it is a media level attribute regardless if it is specified at the session or media level. For example, a capability for low-bit-rate encoding at 8000 samples per second could be specified by a=cmed:1a=mcap:1 iLBC a=cenc:1a=ecap:1 8000 The encoding becomes part of the media capability. Thus, if it is desirable to specify the same subtype with, e.g., two different encoding rates, then the subtype should be listed twice, and each should be modified appropriately. For example: a=cmed:1a=mcap:1 L16 L16 a=cenc:1a=ecap:1 8000 a=cenc:2a=ecap:2 16000/2 defines two low-data-rate codecs, codec 1 uses 8000 samples per second, and codec 2 uses 16000 samples per second and 2 channels. [EDITOR'S NOTE: I'm thinking that it might be better3.2.3. The Media Format Parameter Capability Attribute This attribute is used to put the encoding information in the cmed line pcfg "pt", alongassociate media format parameters with thea media subtype, and eliminatecapability. The form of the cenc attribute. This would makeattribute is: a=fcap:<med-cap-num-list> <list of format parameters> where <med-cap-num-list> permits the above example look like: a=cmed:1 L16/8000 L16/16000/2 or a=pcfg:1 m=1,3|4,3 t=1 a=1 pt=1:100/8000, 4:101/16000/2, 3:102 This canformat parameter(s) to be done so long asassociated with one or more media capabilities, and the encoding attributes never contain whitespace. Is thisformat parameters are specific to the case? or instead use semi colontype of codec(s), as separator] A media capability merely indicates possible supportdescribed for the media type and media format(s) in question. In order to actually use a media capabilityfmtp: attribute defined in an offer/answer exchange, it mustRFC 4566[RFC4566]. Unlike the fmtp attribute, multiple fcap attributes may be referenced inassociated with a potential configuration (see Section 2.3.1. ). Media capabilities canspecified med-cap-num. This provides the ability to mix and match format parameters when using codecs with multiple format options. The fcap attribute adheres to RFC 4566 attribute production rules with media-format-capability-line = "a=fcap:" <att-value> att-value = <med-cap-num-list> WSP <format-specific-parameter- list> med-cap-num-list = <med-cap-num> *["," <med-cap-num>] ; med-cap- num is defined in Section 3.2.1 format-specific-parameter-list = <format-specfic-parameter> *[";"<format-specfic-parameter>] ; format-specific-parameter- list are defined per codec/capability. The "fcap" attribute can be provided at the session-level and the media-level. Media capabilities providedThere can be more than one fcap attribute at the session level apply to the session description in general, whereas media capabilities provided at the media level apply to thator media stream only. In either case, the scope of the <med-cap-num>level. The unique media-cap-num is the entire session description. This enables each media capabilityused to be referenced across the entire session description (e.g.identify it in potential configurations. When used in a potential configuration.) 3.2.3. The Media Format Parameter Capability Attribute This attributeconfiguration it is used to associate media format parameters witha media capability. The form of thelevel attribute is: a=cfmt:<med-cap-num> <list of format parameters> where the format parameters are specific to the type of codec, as described forregardless if it is specified at the fmtp: attribute defined in RFC 4566.session or media level. As ana simple example, a G.729 capability is, by default, considered to support comfort noise as defined by Annex B. Capabilities for G.729 with and without comfort noise support may thus be identified by: a=cmed:1a=mcap:1 audio G729 G729 a=cfmt:2a=fcap:2 annexb:no Example for H.263 video: a=cmed:1a=mcap:1 video H263-1998 H263-2000 a=cfmt:1a=fcap:1 CIF=4;QCIF=2;F=1;K=1 a=cfmt:2a=fcap:2 profile=2;level=2.2 3.3. Extensions to the Potential Configuration Attribute The extension protocol of capabilities negotiation requires two new extensionsFinally, for the pcfg: attribute defined in the base protocol. One extension permits the specification of media capabilities, orsix format combinations thereof; the other permits the assignmentof payload types to those capabilities when used inthe Adaptive MultiRate codec: a=mcap:1 audio AMR AMR AMR AMR AMR AMR a=ecap:1,3,5 16000/1 a=ecap:2,4,6 8000/1 a=fcap:1,2,3,4 mode-change-capability=1 a=fcap:5,6 mode-change-capability=2 a=fcap:1,2,3,5 max-red=220 a=fcap:3,4,5,6 octet-align=1 a=fcap:1,3,5 mode-set=0,2,4,7 a=fcap:2,4,6 mode-set=0,3,5,6 So that AMR codec #1, when specified configuration. 3.3.1. The Media Capability Extension to the Potential Configuration Attribute The potential configurationin a pcfg attribute ("a=pcfg")(and assigned payload type 98) as definedin SDP capabilities negotiation, permits alternate attributesa=pcfg:1 m=1 pt=1:98 is essentially equivalent to be associatedthe following m=audio 49170 RTP/AVP 98 a=rtpmap:98 AMR/16000/1 a=fmtp:98 mode-change-capability=1; max-red=220; mode- set=0,2,4,7 and AMR codec #4 with payload type 99, is essentially equivalent to the media types defined in a media line. In this extension (this document), we define an extension parameterfollowing: m=audio 49170 RTP/AVP 99 a=rtpmap:99 AMR/8000/1 a=fmtp:99 mode-change-capability=1; octet-align=1; mode- set=0,3,5,6 and so on for the specification of media configurations in additionother four combinations. 3.2.4. The Bandwidth Capability Attribute In some cases it is desirable to the one specified on the media line. We define thespecify different bandwidth limits for different media capability configuration parameter, pot-media- config, in accordance withconfigurations. This may be done by use of the following format: m=<med-cap-list> *["|"<med-cap-list>] where <med-cap-list>"a=bcap" attribute, which is a comma-separated list of media capability numbers (media-cap-num) asdefined by a=cmed: linesas follows: a=bcap:<bw-cap-num> <bwtype>:<bandwidth> where <bw-cap-num> is an integer between 1 and media lines. In ABNF form (adhering2^31-1 (both included) used to identify the ABNFbandwidth capability, <bwtype> is the bandwidth type, and <bandwidth> is the bandwidth value, as defined for pot-extension-configthe b= line in [SDPCapNeg]: pot-media-configRFC4566[RFC4566] In ABNF, we have: media-bandwidth-cap-line = "m=" med-cap-list *(BAR med-cap-list) med-cap-list"a=bcap:" bw-cap-num 1*WSP bwtype ":" bandwidth where bw-cap-num = med-cap-num *("," med-cap-num) med-cap-num1*DIGIT; the bandwidth "handle" bwtype = 1*DIGIT ;token; as defined in SDP BARRFC4566 bandwidth = *WSP "|" *WSP Each potential media configuration is a comma-separated list of media capability numbers where med-cap-num refers to media capability numbers1*DIGIT; as defined explicitlyin RFC4566 The "bcap" attribute can appear at the session level, where it can be referenced by a=cmed attributeslcfg or implicitly bypcfg attributes, or at the media line, and hence MUSTlevel, where it can be between 1 and 2^31-1 (both included). Alternative potential media configurations are separatedreferenced by pcfg attributes. When invoked by a vertical bar ("|").pcfg or lcfg attribute, the resulting bandwidth line (b=) is to be interpreted at the media-level. There can be more then one bcap attribute. The alternatives are ordered by preference. When media capabilities are not includedunique bw-cap-num is used to identify it in apotential configuration atconfigurations. Note: The session level bandwidth gives a maximum total values for all the media level,in the media type and media format fromsession. Currently the associated "m=" line willauthors suggest that enforcing a potential limit can be used. For example: v=0 o=- 25678 753849 IN IP4 192.0.2.1 s= c=IN IP4 192.0.2.1 t=0 0 a=creq:v1 a=cmed:1 audio PCMU g729 telephone-event m=audio 3456 RTP/AVP 0 18 100 a=rtpmap:100 telephone-events a=fmtp:100 0-15 a=pcfg:1 m=2,3|1,3 pt=1:0, 2:18, 3:100 In this example, G729 is media capability 2, PCMU isdone at the media capability 1,level and events is mediaby using re-INVITE for the session level bandwidth. Bandwidth capabilities may be included in a potential configuration via the "b=" parameter (see below). Any bandwidth capability 3. The a=pcfg: line specifies thatincluded replaces any media-level bandwidth of the same type declared in a "b=" SDP line. The following example offers a preferred potential configuration is G.729 with dtmf events, second is G.711 mu-law with dtmf events. Intermixing of G.729, G.711,for H.263 QCIF at 360 Kbit/sec and dtmf events is least preferred (the actuala second potential configuration provided by the "m=" line, which is alwaysfor H.263 CIF at the least preferred configuration). 3.3.2. The Payload Type Mapping Extensionoffered 500 Kbit/sec m=video 49170 RTP/AVP 99 b=TIAS:500000 a=rtpmap:99 H263-1998/90000 a=fmtp:99 CIF=4; QCIF=2 a=mcap:1 video H263-1998 H263-1998 a=fcap:1 QCIF=2 a=fcap:2 CIF=4; QCIF=2;F=1;K=1 a=bcap:1 TIAS:360000 a=pcfg:1 m=1 b=1 pt:100 a=pcfg:2 m=2 pt:101 3.3. Extensions to the Potential Configuration Attribute When mediaThe extension protocol of capabilities negotiation requires three new extensions for the pcfg: attribute defined in cmed: attributes are used in potential configuration lines, it is necessary to assign payload types to them. In some cases, it is desirable to assign differentthe base protocol. The first extension permits the specification of media capabilities, or combinations thereof; the second permits the assignment of payload types to mediathose capabilities definedwhen used in the media line. One example of the latter is when configurations for AVP and SAVP are offered:specified configuration; the offerer would likethird permits the answererspecification of bandwidth limits for a media stream. 3.3.1. The Media Capability Extension to use different payloadthe Potential Configuration Attribute The potential configuration attribute ("a=pcfg") as defined in SDP capabilities negotiation, permits alternate attributes to be associated with the media types defined in a media line. In this document, we define an extension parameter for encrypted and unencryptedthe specification of media so that it (the offerer) can decide whether or notconfigurations in addition to render early media which arrives beforethe answer is received.one specified on the media line. We define the media type mappingcapability configuration parameter, pot-media- map,config, in accordance with the following format: pot-media-mapm=<med-cap-list> *["|"<med-cap-list>] where <med-cap-list> is a comma-separated list of media capability numbers (media-cap-num) as defined by a=mcap: lines and media lines. In ABNF form (adhering to the ABNF for pot-extension-config in [SDPCapNeg]: pot-media-config = "pt=" med-map-num"m=" med-cap-list *(BAR med-cap-list) med-cap-list = med-cap-num *("," *WSP med-map-num) med-map-nummed-cap-num) med-cap-num = 1*DIGIT ":" 1*DIGIT; The exampledefined in the previous section shows how the parameters from the cmed line are mappedSDP BAR = *WSP "|" *WSP Each potential media configuration is a comma-separated list of media capability numbers where med-cap-num refers to payload typemedia capability numbers defined explicitly by a=mcap attributes and hence MUST be between 1 and 2^31-1 (both included). Alternative potential media configurations are separated by a vertical bar ("|"). The alternatives are ordered by preference. When media capabilities are not included in a potential configuration at the pcfg "pt" parameter. 3.4. Extensions tomedia level, the Actual Configuration Attribute We define an actual configuration extension parameter act-media- config in accordance withmedia type and media format from the following ABNF: a=acfg:<med-cap-num> *WSPassociated "m=" <med-cap-list> A response to the previous offer example in the above section might be:line will be used. For example: v=0 o=- 24351 62181425678 753849 IN IP4 192.0.2.219220.127.116.11 s= c=IN IP4 192.0.2.21918.104.22.168 t=0 0 a=csup:v1a=creq:med-v0 m=audio 54323456 RTP/AVP 0 18 100 a=rtpmap:100 eventstelephone-events a=fmtp:100 0-15 a=acfg:1 m=2,3 Note that thea=mcap:1 audio PCMU g729 telephone-event a=ecap:1,2,3 8000 a=fcap:3 0-15 a=pcfg:1 m=2,3|1,3 pt=1:0, 2:18, 3:100 In this example, G729 is media capability numbers expressed in the acfg: attribute are based on the offered2, PCMU is media capability numbering, not on those listed in the answer.1, and events is media capability 3. The acfg identify toa=pcfg: line specifies that the offrer which potetialpreferred configuration was selected by the answerer. 3.5. The Latent Configuration Attribute Oneis G.729 with dtmf events, second is G.711 mu-law with dtmf events. Intermixing of G.729, G.711, and dtmf events is least preferred (the actual configuration provided by the goals of this work"m=" line, which is always the least preferred configuration). 3.3.2. The Payload Type Mapping Extension to permitthe exchange ofPotential Configuration Attribute When media configurationscapabilities defined in addition to those offered for immediate use. Such configurationsmcap: attributes are referred to as "latent configurations". For example, a party may offerused in potential configuration lines, it is necessary to establish an audio session, and, at the same time, announce its abilityassign payload types to support a video session. Latent configurations may be announced by use of the latent configuration attribute, whichthem. In some cases, it is defined in a manner very similardesirable to the potential configuration attribute: a=lcfg:<preference> ["m="<media-caps>] ["t="<transport>] ["a="<attributes>] The m=, t= and a= parameters are identical in format and meaningassign different payload types to thosemedia capabilities defined forin the pcfg: attribute.media line. One example of the latter is when configurations for AVP and SAVP are offered: the offerer would like the answerer to use different payload types for encrypted and unencrypted media so that it (the offerer) can decide whether or not to render early media which arrives before the answer is received. We define the media type mapping configuration parameter, pt-media- map, in accordance with the following format: pt-media-map = "pt=" med-map *("," med-map); med-map = <med-cap-num> ":" <'payload-type> ; med-cap-num is defined above payload-type = 1*DIGIT ; RTP payload type The example in the previous section shows how the parameters from the mcap line are mapped to payload type in the pcfg "pt" parameter. 3.3.3. The Bandwidth Extension to the Potential Configuration Attribute A bandwidth parameter is added to the pcfg attribute in order to provide the flexibility to specify different bandwidth limits for different configurations. The bandwidth parameter, bw-param, is defined in accordance with the following format: bw-param = "b=" bw-cap-list; bw-cap-list = bw-cap-num *("," bw-cap-num); bw-cap-num = 1*DIGIT; as defined above Note that it is considered an error condition if the same bandwidth type is referenced more than once in the bw-cap-list. 3.4. Extensions to the Actual Configuration Attribute The Actual configuration attribute is specified in [SDPCapNeg]. The actual configuration MUST list the potential configuration selected by the answerer. This section adds extensions parameters enabling the answerer to specify the potential configuration attributes defined in this document and used in forming the answer. We define actual configuration extensions based on the sel-extension- config specified in [SDPCapNeg]. The new parameters are: parameter act-media-config, act-med-map and act-bw-param in accordance with the following ABNF: act-media-config = "m=" med-cap-list ; defined in section 3.3.1 act-med-map = "pt=" med-map *("," med-map) ; defined in section 3.3.2 act-bw-param = "b=" bw-cap-list ; defined in section 3.3.3 A response to the previous offer example in the above section might be: v=0 o=- 24351 621814 IN IP4 192.0.2.2 s= c=IN IP4 192.0.2.2 t=0 0 a=csup:med-v0 m=audio 5432 RTP/AVP 18 100 a=rtpmap:100 events a=fmtp:100 0-15 a=acfg:1 m=2,3 pt=2:18,3:100 Note that the capability numbers expressed in the acfg: attribute are based on the offered capability numbering, not on those listed in the answer, if any. The acfg attribute identifies to the offrer which potential configuration and media encoding combination was selected by the answerer. 3.5. The Latent Configuration Attribute One of the goals of this work is to permit the exchange of media configurations in addition to those offered for immediate use. Such configurations are referred to as "latent configurations". For example, a party may offer to establish an audio session, and, at the same time, announce its ability to support a video session and supply its video capabilities. Latent configurations may be announced by use of the latent configuration attribute, which is defined in a manner very similar to the potential configuration attribute: a=lcfg:<preference> ["m="<media-caps-list>] ["t="<transport>] ["a="<attributes>] ["b="<bw-cap-list>] The m=, t=, b= and a= parameters are identical in format and meaning to those defined for the pcfg: attribute. Note that the pt= parameter is not permitted inpt= parameter is not permitted in the lcfg: attribute because no actual media session is being offered or accepted. Latent Configurations MUST be specified at the session level when they represent an additional media stream to those in the offer or answer. If an acap: attribute is declared at the session level for use in a session-level lcfg line, it SHOULD NOT be used in a pcfg line at the media level unless it is to become a session-level attribute in the answer. 3.5.1. The crypto: Attribute in Latent Configurations If the sdescriptions crypto: attribute is necessary as part of any latent configuration which announces sdescriptions capabilities, it presents a slight problem in that the rather long key/salt string is useless and should be ignored. This problem is avoided if we exclude the a= parameters from the lcfg: attribute. [Editor's note: Should we define a new crypto: key-method, e.g. "latent", in which the key- salt portion of key-info is empty? I think it may be sufficient to just include the RTP/SAVP transport to indicate SRTP capability since there will be a new offer when the latent configuration will be offered.] 3.6. Offer/Answer Model Extensions In this section, we define extensions to the offer/answer model defined in RFC3264 [RFC3264] to allow for media capabilities to be used with the SDP Capability Negotiation framework. 3.6.1. Generating the Initial Offer When an endpoint generates an initial offer and want to use the functionality described in the current document, it should identify and define the codecs it can support via mcap , ecap , and fcap attributes. The SDP media line(s) should be made up with the default (least preferred) configuration. Typically, the lcfg: attribute because no actualmedia session is being offered or accepted. Latent Configurationsline configuration will contain the minimum acceptable capabilities. The offer MUST include the required extensions needed to support this functionality. Preferred configurations for each media stream are identified following the media line. The present offer may be specifiedalso include latent configuration (lcfg) attributes, at the session level in offers andlevel, describing media streams and/or configurations the offeror is not now offering, but which it is willing to support in answers. [Editor's note: Do you havea goodfuture offer/answer exchange. A simple example might be the inclusion of a latent video configuration in an offer offor an audio stream withstream. 3.6.2. Generating the Answer When the answering party receives the offer and if it supports the required extensions, it should select the most-preferred configuration for each media stream, and build its answer accordingly. Each selected media configuration is placed into the answer as a media line with associated parameters, It must include the supported extension attribute and an actual configuration (acfg) attribute to indicate just which pcfg attribute was used to build the answer. The answer should may also include any latent video stream?][Editor'sconfigurations the answerer can support, especially any configurations compatible with latent configurations received in the offer. The answerer should make note (rrg): What if, as Roni suggested, we excludeof those configurations it might wish to offer in the a= parameters fromfuture. 3.6.3. Offerer Processing of the lcfg: lines? We can includeAnswer When the offerer receives the answer, it should make note of any capabilities and/or latent configurations for future use. The media line(s) must be processed in the normal way to identify the media stream(s) accepted by the answer, if any. The acfg attribute, if present, may be used to verify the t= parameter asproposed configuration used to form the 'cheapest' wayanswer, and to indicate SRTP. That conveys most ofinfer the necessary info;lack of acceptability of higher- preference configurations that were not chosen. Note that the details can be firmed up inbase specification [SDPCapNeg] requires the actual offer when/if it's actually made.] 3.5.1. The crypto: Attribute in Latent Configurations Ifanswerer to choose the sdescriptions crypto: attribute is necessary as part of any latenthighest preference configuration which announces sdescriptions capabilities. thenit presentscan support. 3.6.4. Modifying the Session If, at a slight problem in thatlater time, one of the rather long key/salt string is useless and should be ignored. This problem is avoided if we excludeparties wishes to modify the a=operating parameters from the lcfg: attribute. [Editor's note: should we defineof a session, e.g., by adding a new crypto: key-method, e.g. "latent", in which the key-salt portion of key-info is empty? I thinkmedia stream, or changing a codec used on an existing stream, it may be sufficient to just include the RTP/SAVP transport to indicate SRTP capability] 3.6. Offer/Answer Model Extensions In this section, we define extensions todo so via the offer/answer modelmechanisms defined in RFC3264 to allowfor media capabilities to be used withoffer/answer[RFC3264]. If the SDP Capability Negotation framework. 3.6.1. Generatinginitiating party has remembered the Initial Offer 3.6.2. Generatingcodecs, potential configurations, and latent configurations announced by the Answer 3.6.3. Offerer Processingother party in the earlier negotiation, it may use this knowledge to maximize the likelihood of a successful modification of the Answer 3.6.4. Modifyingsession. Alternatively, it may perform a new capabilities exchange as part of the Sessionreconfiguration. 4. Examples In this section, we provide examples showing how to use the Media Capabilities with the SDP Capability Negotiation. 4.1. Alternative Codecs 4.2. Latent Media Streams 5. IANA Considerations 6. Security Considerations 7. Changes from previous versions 7.1. Changes from version 01 The documents adds a new attribute for specifying bandwidth capability and a parametr to list in the potential configuration. Other changes are to align the document with the terminolgy and attribute names from draft-ietf-mmusic-sdp-capability-negotiation-07. The document also clarifies some previous open issues. 7.2. Changes from version 00 The major changes include taking out the "mcap" and "cptmap" parameter. The mapping of payload type is now in the "pt" parameter of "pcfg". Media subtype need to explictly definesd in the "cmed" attribute if referenced in the "pcfg" 8. Acknowledgements This document is heavily influenced by the discussions and work done by the SDP Capability Negotiation Design team. The following people in particular provided useful comments and suggestions to either the document itself or the overall direction of the solution defined herein: Cullen Jennings, Matt Lepinski, Joerg Ott, Colin Perkins, and Thomas Stach. 9. References 9.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002. [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006. [SDPCapNeg] Andreasen, F., "SDP Capability Negotiation", draft-ietf-mmusic-sdp-capability-negotiation-02draft-ietf-mmusic-sdp-capability-negotiation-07 (work in progress), FebruaryOctober 2007. 9.2. Informative References [RFC3407] Andreasen, F., "Session Description Protocol (SDP) Simple Capability Declaration", RFC 3407, October 2002. [RFC3952] Duric, A. and S. Andersen, "Real-time Transport Protocol (RTP) Payload Format for internet Low Bit Rate Codec (iLBC) Speech", RFC 3952, December 2004. Authors' Addresses Robert R Gilman Avaya, Inc. 1300 West 120th Avenue Westminster, CO 80234 USA Email: firstname.lastname@example.org Roni Even (editor) Polycom 94 Derech Em Hamoshavot Petach Tikva 49130 Israel Email: email@example.com Flemming Andreasen Cisco Systems Edison, NJ USA Email: firstname.lastname@example.org Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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