MMUSIC R. Gilman Internet-Draft
Avaya, Inc.NDCI Intended status: Standards Track R. Even, Ed. Expires: May 18,August 28, 2008 Polycom F. Andreasen Cisco Systems November 15, 2007February 25, 2008 SDP media capabilities Negotiation draft-ietf-mmusic-sdp-media-capabilities-02.txtdraft-ietf-mmusic-sdp-media-capabilities-03.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 May 18,August 28, 2008. Copyright Notice Copyright (C) The IETF Trust (2007).(2008). Abstract Session Description Protocol (SDP) capability negotiation provides a general framework for indicating and 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. This extension is designed to map easily to existing and future SDP media attributes. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. SDP Media capabilitiesCapabilities . . . . . . . . . . . . . . . . . . . . 6 3.1. Solution Overview . . . . . . . . . . . . . . . . . . . . 6 3.2. Capability AttributesDesign Goals . . . . . . . . . . . . . . . . . . 9 3.2.1. Media Type and Subtype Capability Attribute. . . . . 10 188.8.131.52.3. New Capability Attributes . . . . . . . . . . . . . . . . 11 3.3.1. The Media Encoding ParameterCapability Attribute . . . . . . . 11 184.108.40.206.3.2. The Media Format Parameter Capability Attribute . . . 12 220.127.116.11.3.3. The Media-Specific Capability Attribute . . . . . . . 15 3.3.4. The Bandwidth Capability Attribute . . . . . . . . . . 13 3.3.16 3.4. Extensions to the Potential Configuration Attribute . . . 15 18.104.22.168 3.4.1. The Media Capability Extension to the Potential Configuration Attribute . . .Parameter . . . . . . . . . . . . 15 22.214.171.124 3.4.2. The Payload Type Mapping Extension to the Potential Configuration AttributeParameter . . . . . . . . . . 16 126.96.36.199 3.4.3. The Bandwidth Extension to the Potential Configuration AttributeParameter . . . . . . . . . . . . . . . 17 3.4.20 3.5. Extensions to the Actual Configuration Attribute . . . . . 17 3.5.20 3.6. The Latent Configuration Attribute . . . . . . . . . . . . 18 3.5.1. The crypto: Attribute21 3.6.1. Cryptographic Attributes in Latent Configurations . . . . 18 3.6.23 3.7. Offer/Answer Model Extensions . . . . . . . . . . . . . . 19 188.8.131.52 3.7.1. Generating the Initial Offer . . . . . . . . . . . . . 19 184.108.40.206 3.7.2. Generating the Answer . . . . . . . . . . . . . . . . 19 220.127.116.11 3.7.3. Offerer Processing of the Answer . . . . . . . . . . . 19 18.104.22.168 3.7.4. Modifying the Session . . . . . . . . . . . . . . . . 2024 4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 2126 4.1. Alternative Codecs . . . . . . . . . . . . . . . . . . . . 2126 4.2. Latent Media Streams . . . . . . . . . . . . . . . . . . . 2129 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 2231 6. Security Considerations . . . . . . . . . . . . . . . . . . . 2332 7. Changes from previous versions . . . . . . . . . . . . . . . . 2433 7.1. Changes from version 0102 . . . . . . . . . . . . . . . . . 2433 7.2. Changes from version 01 . . . . . . . . . . . . . . . . . 33 7.3. Changes from version 00 . . . . . . . . . . . . . . . . . 2433 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 2534 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 2635 9.1. Normative References . . . . . . . . . . . . . . . . . . . 2635 9.2. Informative References . . . . . . . . . . . . . . . . . . 2635 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 2736 Intellectual Property and Copyright Statements . . . . . . . . . . 2837 1. Introduction Session Description Protocol (SDP) capability negotiation [SDPCapNeg] provides a general framework for indicating and negotiating capabilities in SDP[RFC4566]. The base framework defines only capabilities for negotiating transport protocols and attributes. 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. The capabilities include a simple form of media capabilities. RFC 3407 defines capability declaration only. Actual negotiation procedures taking advantage of such capabilities have not been defined. The SDP capability negotiation framework defined in [SDPCapNeg] adds this required functionality.functionality, but does not define media capabilities. This document updates RFC3407 and [SDPCapNeg] and new implementationimplementations SHOULD use the functionality defined in the current draftdocument 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 indicate and negotiate media types and their associated parameters like bandwidth. This document also adds the capabilityability to declare support for media streams thatstreams, the use of which 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 documentnegotiated later. The definitions of new attributes for media capability negotiation are chosen to be interpretedmake the translation from these attributes to "conventional" SDP [RFC4566] media attributes as describedstraightforward as possible in RFC2119order to simplify implementation. This goal is intended to reduce processing in two ways: each proposed configuration in an offer may be easily translated into a conventional SDP media stream record for processing by the receiver; and the construction of an answer based on a selected proposed configuration is straightforward. 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. "Base Configuration": the media configuration represented by a media block exclusive of all the capability negotiation attributes defined in this document and [SDPCapNeg] "Conventional SDP": all SDP lines exclusive of the capability negotiation attributes 3. SDP Media capabilitiesCapabilities The SDP mediacapability negotiation [SDPCapNeg] discusses the use of any SDP [RFC4566] attribute (a=) under the acap attribute.attribute capability "acap". The limitations of using acap for fmtp and rtpmap asin a potential configuration are listed,described in [SDPCapNeg]; for example they can be used only at the media level since they are media level attributes. The [SDPCapNeg] partially addresses the issue of bandwidth usage for different configurations but does not offer a way to negotiate different bandwidth for different codecs and profiles. This section provides an overview of extensions providing SDP mediaMedia Capability and Bandwidth negotiation solution offering more robust capabilities negotiation. This is followed by definitions of new SDP attributes for the solution and its associated updated offer/answer procedures [RFC3264] 3.1. Solution Overview The solution consistconsists of the followingfive new attributes and four new parameters for the pcfg attribute extending the base attributes from [SDPCapNeg]. Four new attributes are used to make up media capabilities in a manner that can be related to the capabilities specified in a media line, its attributesattributes, and the bandwidth parameter lines. o A new media attribute ("a=mcap") that lists media formats as capabilities in the form of a media type (e.g. "audio") and one or more subtypessubtype (e.g. "PCMU"), and associates a handle with eachits encoding parameters (e.g. "/8000/2"). Each resulting media format type / subtype capability has an associated handle. The encoding parameters are as specified for the rtpmap attribute defined in [RFC4566] o A new attribute ("a=ecap")("a=mfcap") that lists encoding parameter capabilitiesspecifies media format parameters associated with aone or more media formatcapabilities. The mfcap attribute is used to associate the formatting capabilities normally carried in the fmtp attribute. o A new attribute ("a=fcap")("a=mscap") that listsspecifies media format parameter capabilitiesparameters associated with aone or more media formatcapabilities. The mscap attribute is used to associate capabilities other than fmtp or rtpmap attributes, for example the rtcp-fb attribute. o A new attribute ("a=bcap") that is used to specify the bandwidth parameter(s) for media capabilities.parameter capability. o A new attribute ("a=lcfg") that specifies latent media stream configurations when no corresponding media linestream is offered. An example is a latent configuration for video even though no video is currently offered. New parameters are defined for the potential configuration (pcfg), latent configuration (lcfg), and accepted configuration (acfg) attributes to associate the new attributes with particular configurations. o A new parameter type ("m=")("m=")is added to the potential configuration ("a=pcfg:") attribute and the actual configuration ("a=acfg:") attribute defined in [SDPCapNeg], and to the new latent configuration ("a=lcfg:") attribute which permits specification of media capabilities (including their associated parameters) and combinations thereof for the configuration. For example, the "a=pcfg:" line might specify PCMU 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=") is added to the potential configuration, actual configuration, and latent configuration ("a=pcfg:") attribute whichattributes. This parameter associates RTP payload types with the referenced media capabilities.capabilities, and is appropriate only when the transport protocol uses RTP. o A new parameter type ("b=") is used to specify bandwidth parameters in a potential configuration. o A new parameter type ("mt=") is used to specify the MIME type for latent configurations that are declared at the session level. 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),line(s) and associated parameters, 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.exchanges, but they are not part of the current offer/answer exchange. 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:med-v0 a=mcap:1 audio g729 iLBC PCMU g729 a=ecap:2 8000 a=fcap:1 annexb:no a=fcap:2 mode=20 a=tcap:1 RTP/SAVPm=audio 3456 RTP/AVP 0 18 a=tcap:1 RTP/SAVP a=rtpmap:0 PCMU/8000/1 a=rtpmap:18 G729/8000/1 a=fmtp:18 annexb=yes a=mcap:1,4 g729/8000/1 a=mcap:2 PCMU/8000/1 a=mcap:5 telephone-event/8000 a=mfcap:1 annexb=no a=mfcap:4 annexb=yes a=mfcap:5 0-11 a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 a=pcfg:1 m=1,3|4,3m=4,5|1,5 t=1 a=1 pt=1:100,4:101,3:102pt=1:100,4:101,5:102 a=pcfg:2 m=2 t=1 a=1 pt=2:103 a=pcfg:3 m=4 t=2 pt=4:18 The required base and extensions are provided by the "a=creq" attribute defined in [SDPCapNeg], with the option tag "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=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=ecap:2" line specifies the clock rate and encoding parmeters (see [RFC4566]) for capability 2, iLBC. The "a=fcap:1" line specifies media format parameter capabilities for codec 1 ( no Annex B for G.729). The "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.G.729B. The media line implicitly defines the default transport protocol (RTP/AVP in this case) and the default actual configuration. The "a=acap:1" line ,specified"a=tcap:1" line, specified in the base protocol, defines a transport protocol provides the "crypto" attribute which provides the keying material for SRTP using SDP security descriptions. The "a=pcfg:" attributes provide the potential configurations includedcapabilities, in this case Secure RTP (SAVP profile) as the offer by reference tofirst option and RTP (AVP profile) as the second option. The "a=mcap:1,4" line defines two G.729 media format capabilities, transport capabilities,numbered 1 and associated4, and their encoding rate. The capabilities are of subtype G729. Note that the media subtype is explicitly specified here, rather than RTP payload type mappings. Two explicit alternativesnumber. In this example, two G.729 subtype capabilities are provided;defined. This permits the first one,declaration of two sets of formatting parameters for G.729. The "a=mcap:2" line defines a G.711 mu-law capability, numbered 1 is the preferred2. The "a=mcap:5" line defines an audio telephone-event capability, numbered 5. The "a=mfcap:1" line specifies the fmtp formatting parameters for capability 1 (no comfort noise packets). The "a=mfcap:4" line specifies the fmtp formatting parameters for capability 4 (G.729B). The "a=mfcap:5" line specifies the fmtp formatting parameters for capability 5 (the DTMF touchtones 0-9,*,#). The "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. Three explicit alternatives are provided; the first one, numbered 1 is the preferred one. It specifies media capabilities 14 and 3, i.e. G.7295, i.e., G.729B and PCMU,DTMF, or media capability 41 and 3,5, i.e., G.729BG.729 and PCMU.DTMF. 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 codecsmedia capabilities 1, 3,4, and 45, thereby permitting the offererofferor to distinguish between encrypted media and unencrypted media received prior to receipt of the answer. Use of unique payload types is not required; codecs such as AMR-WB [RFC4867] have the potential for so many combinations of options that it may be impractical to define unique payload types for all supported combinations. For SRTP using SDES inline keying [RFC4568], the offererofferor will still need to receive the answer before being able to decrypt the stream. The second alternative specifies media capability 2, i.e. iLBC,PCMU, under the default RTP/AVP profile.RTP/SAVP profile, with the same SRTP key material. The third alternative offers G.729B unsecured; it's only purpose in this example is to show a preference for G.729B over PCMU. 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 G.729B, PCMU, and telephone events over RTP, but not SRTP, and hence he accepts the actualpotential configuration 3 for RTP provided by Alice. Furthermore,Hence, Bob wants to use the iLBC codec and hencegenerates 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:med-v0 m=audio 4567 RTP/AVP 103 a=rtpmap:103 iLBC/8000 a=fmtp:103 mode=20 a=acfg:218 a=rtpmap:18 G729/8000 a=fmtp:18 annexb=yes a=acfg:3 m=4 pt=4:18 Bob includes the "a=csup" and "a=acfg" attributeattributes in the answer to inform Alice that he can support the med-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:rtpmap and/or fmtp:fmtp parameters, as appropriate. Note that per the base protocol, after the above, Alice MAY 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 presentDesign Goals The design of the new attributes associated with indicatingelements added to the media capabilities for usecapability negotiation framework by the SDP Capability negotiation.this document has been driven by two basic goals: o The approach taken is to keep things similar toresulting SDP should be as compact as possible while preserving the existingflexibility necessary to declare multiple options for complex media capabilitiesencodings. o The capability attributes defined byshould be easily mapped to and from "conventional" SDP elements, that is, to and from one or more SDP records that do not contain any capability attributes. It is hoped that satisfaction of these goals will facilitate simple implementations of capability negotiation in SDP. 3.3. New 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. We use media subtypes and "media capability numbers" instead of payload types to link the relevant media capability parameters. This permits the capabilities to be defined at the session level and be used for multiple streams, if desired. Payload types are then specified at the media level (see Section 3.2.2).3.4.2). A media capability merely indicates possible support for the media type and media format(s) in question. 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).3.4.1). 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,level (consistent with the MIME type), 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.configuration). 3.3.1. The Media Type and SubtypeEncoding Capability Attribute Media types andsubtypes can be expressed as media formatencoding capabilities by use of the "a=mcap" attribute, which is defined as follows: a=mcap:<med-cap-num> <subtype>*[ <subtype>]a=mcap:<med-cap-num-list> <subtype>[/<encoding-parms>] <med-cap-num-list> = <med-cap-num> *[COMMA <med-cap-num>] where <med-cap-num> is an integer between 1 and 2^31-1 (both included) used to number thea media format capabilitiescapability, or a range of such integers and the <subtype> is the media subtype e.g. H263-1998,H263-1998 or PCMU. The <med-cap- num> isand <encoding-parms> are the media capability number associated withencoding parameters for the first subtype<subtype>. All media format capabilities in the list, the number associated withlist are assigned to the second subtype is one higher, etc.same media type/subtype. Each occurrence of the mcap attribute MUST use a different value of <med-cap-num>. Furthermore, when an "mcap" attribute indicates more than one media format,unique values in its <med-cap-num-list>; the media capability numbers implied MUST NOTmust be used by any other "mcap" attribute inunique across the entire session or media description (explicitly or implicitly).description. In short, the mcap attribute defines media capabilities and associates them with a media capability number in the same manner as the rtpmap attribute defines them and associates them with a payload type. In ABNF, we have: media-capability-line = "a=mcap:" media-cap-num WSP media-typemedia-cap-num-list 1*WSP media-cap *(WSP media-cap)["/" clock-rate ["/" encoding-parms]] media-cap-num-list = media-cap-num *[COMMA media-cap-num] media-cap-num = 1*DIGIT | media-cap-range media-cap-range = 1*DIGIT "-" 1*DIGIT media-cap = token ; Subtype name(PCMU, G729, etc.) clock-rate = 1*DIGIT encoding-parms = token The clock-rate and encoding-params are as defined to appear in an rtpmap attribute for each MIME type/subtype. Thus, it is easy to convert an mcap attribute line into one or more rtpmap attribute lines, once a payload type is assigned to a media-cap-num (see section 3.4.2). The "mcap" attribute can be provided at the session-level andand/or the media-level. There can be more than one mcap attribute at the session or media level. The unique media-cap-num is used to identify iteach media capability in potential and actual configurations. When used in a potential configuration it is a media level attribute capability 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=ecap:<med-cap-num-list> <clock rate>[/<encoding parameters>] where med-cap-num-list = <med-cap-num> *[COMMA<med-cap-num>] The clock rate andIn 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 andwords, 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 ormedia level. For example, acapability for low-bit-rate encoding at 8000 samples per second could be specified by a=mcap:1 iLBC a=ecap:1 8000 The encoding becomes part of the media capability. Thus, if it is desirableapplies to specifythe same subtype with, e.g., two different encoding rates, thenspecific media description associated with the subtype should be listed twice, and each should be modified appropriately.potential configuration in question. For example: v=0 a=mcap:1 L16 L16 a=ecap:1 8000 a=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. 3.2.3.L16/8000/1 a=mcap:2 L16/16000/2 a=mcap:3,4 H263-1998/90000 3.3.2. The Media Format Parameter Capability Attribute This attribute is used to associate media formatmedia-specific parameters with aone or more media capability.capabilities. The form of the attribute is: a=fcap:<med-cap-num-list>a=mfcap:<med-cap-num-list> <list of formatparameters> where <med-cap-num-list> permits the formatparameter(s) to be associated with one or more media capabilities,capabilities and the format parameters are specific to the type of codec(s), as described for the fmtp:codec. The mfcap lines map to a single traditional SDP fmtp attribute line (one per <media-cap-num>) of the form a=fmtp:<fmt> <list of parameters> where <fmt> is the media format description defined in RFC 4566[RFC4566]. Unlike4566 [RFC4566], as appropriate for the fmtp attribute, multiple fcap attributes mayparticular media stream. The mfcap attribute MUST be associatedused to encode attributes for media capabilities, which would conventionally appear in an fmtp attribute. The appearance of media subtypes with a specified med-cap-num. This provideslarge number of formatting options (e.g., AMR-WB [RFC4867]) coupled with the abilityrestriction that only a single fmtp attribute can appear per media format, suggests that it is useful to create a combining rule for mfcap parameters which are associated with the same media capability number. Therefore, different mfcap lines MAY include the same <med-cap-num> in their <med-cap-num-list>. When a particular media capability is selected for processing, the parameters from each mfcap line which references the particular capability number in its med-cap-num-list are concatenated together via ";" to form the equivalent of a single fmtp attribute line. This permits one to mixdefine a separate mfcap line for a single parameter and matchvalue that is to be applied to each media capability designated in the med-cap-num-list. This provides a compact method to specify multiple combinations of format parameters when using codecs with multiple format options. The fcapmfcap attribute adheres to RFC 4566 attribute production rules with media-format-capability-linemedia-format-capability = "a=fcap:" <att-value> att-value = <med-cap-num-list>"a=mfcap:"<media-caps> WSP <format-specific-parameter- list> med-cap-num-list<fmt-specific-param-list> med-caps = <med-cap-num> *["," <med-cap-num>]"*" ; wildcard: all media caps / <med-cap-num-list> ; med-cap- num isdefined in Section 22.214.171.124.1 format-specific-parameter-list = <format-specfic-parameter> *[";"<format-specfic-parameter>] ; format-specific-parameter- listformat-specific-parameter = text Format parameters are not parsed by SDP; their content is specific to the media type/subtype. When format parameters for a specific media capability are combined from multiple a=mfcap lines which reference that media capability, the format-specific parameters are concatenated together and separated by "; " for construction of the corresponding format attribute (a=fmtp): a= fmtp:<fmt> WSP <format-specfic-parameter-list> [1*[";"<format-specfic-parameter-list>] ; where <fmt> depends on the transport protocol in the manner defined per codec/capability.in RFC4566. SDP cannot assess the legality of the resulting parameter list in the "a=fmtp" line; the user must take care to insure that legal parameter lists are generated. The "fcap""mfcap" attribute can be provided at the session-level and the media-level. There can be more than one fcapmfcap 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 atassociate the session orparameters with a media level.capability. As a 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 identifieddefined by: a=mcap:1a=mcap:1,2 audio G729 G729 a=fcap:2G729/8000 a=mfcap:2 annexb:no Media format capability 1 supports G.729 with Annex B, whereas media format capability 2 supports G.729 without Annex B. Example for H.263 video: a=mcap:1 video H263-1998 H263-2000 a=fcap:1H263-1998/90000 a=mcap:2 video H263-2000/90000 a=mfcap:1 CIF=4;QCIF=2;F=1;K=1 a=fcap:2a=mfcap:2 profile=2;level=2.2 Finally, for six format combinations of the 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:1,2,3,4a=mcap:1-3 AMR/8000/1 a=mcap:4-6 AMR/16000/1 a=mfcap:1,2,3,4 mode-change-capability=1 a=fcap:5,6a=mfcap:5,6 mode-change-capability=2 a=fcap:1,2,3,5a=mfcap:1,2,3,5 max-red=220 a=fcap:3,4,5,6a=mfcap:3,4,5,6 octet-align=1 a=fcap:1,3,5a=mfcap:1,3,5 mode-set=0,2,4,7 a=fcap:2,4,6a=mfcap:2,4,6 mode-set=0,3,5,6 So that AMR codec #1, when specified in a pcfg attribute within an audio stream block (and assigned payload type 98) as in a=pcfg:1 m=1 pt=1:98 is essentially equivalent to the following m=audio 49170 RTP/AVP 98 a=rtpmap:98 AMR/16000/1AMR/8000/1 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 following: m=audio 49170 RTP/AVP 99 a=rtpmap:99 AMR/8000/1AMR/16000/1 a=fmtp:99 mode-change-capability=1; octet-align=1; mode- set=0,3,5,6 and so on for the other four combinations. 3.2.4. The BandwidthSDP could thus convert the media capabilities specifications into one or more alternative media stream specifications, one of which can be chosen for the answer. In some cases, particularly when an RFC 2198 redundancy audio subtype (RED) capability is defined, the parameters to the mscap attribute may contain payload type numbers. These numbers are bound to actual payload types by means of the payload type parameter (pt=) in a potential, actual, or latent configuration. See sections 3.4.2, 3.5, and 3.6. 3.3.3. The Media-Specific Capability Attribute Media-specific attributes, beyond the rtpmap and fmtp attributes, may be associated with media capability numbers via a new media-specific attribute, mscap, as follows: media-specific-capability = "a=mscap:" <media-caps> ; defined in 3.3.2 WSP <att-field> ; from [RFC4566] WSP <ms-parameters> ms-parameters = byte-string ; as defined per attribute. Given an association between a media capability and a payload type number as specified by the pt= parameters in an lcfg or pcfg attribute line, a mscap line may be translated easily into a conventional attribute line of the form a=<att-field>":"<fmt> <ms-parameters> ; fmt defined in [RFC4566] A single mscap line may refer to multiple media capabilities; this is equivalent to multiple mscap lines, each with the same attribute values, one line per media capability. Multiple mscap lines may refer to the same media capability, but, unlike the mfcap attribute, no concatenation operation is defined. Hence, multiple mscap lines applied to the same media capability is equivalent to multiple lines of the specified attribute in a conventional media record. Here is example with the rtcp-fb attribute, modified from an example in[I-D.ietf-avt-avpf-ccm] (with the session-level and audio media omitted). If the offer contains a media block like the following, m=video 51372 RTP/AVP 98 a=rtpmap:98 H263-1998/90000 a=tcap:1 RTP/AVPF a=mcap:1 H263-1998/90000 a=mscap:1 rtcp-fb ccm tstr a=mscap:1 rtcp-fb ccm fir a=mscap:* rtcp-fb ccm tmmbr smaxpr=120 a=pcfg:1 t=1 m=1 pt=1:98 and if the proposed configuration is chosen, then the equivalent media block would look like m=video 51372 RTP/AVP 98 a=rtpmap:98 H263-1998/90000 a=rtcp-fb:98 ccm tstr a=rtcp-fb:98 ccm fir a=rtcp-fb:* ccm tmmbr smaxpr=120 3.3.4. The Bandwidth Capability Attribute In some cases it is desirable to specify different bandwidth limits for different media configurations. This may be done by use of the "a=bcap" attribute, which is defined as follows: a=bcap:<bw-cap-num> <bwtype>:<bandwidth> where <bw-cap-num> is an integer between 1 and 2^31-1 (both included) used to identify the bandwidth capability, <bwtype> is the bandwidth type, and <bandwidth> is the bandwidth value, as defined for the b= line in RFC4566[RFC4566] In ABNF, we have: media-bandwidth-cap-line = "a=bcap:" bw-cap-num 1*WSP bwtype ":" bandwidth where bw-cap-num = 1*DIGIT; the bandwidth "handle" bwtype = token; as defined in RFC4566 bandwidth = 1*DIGIT; as defined in RFC4566 The "bcap" attribute can appear at the session level, where it can be referenced by lcfg or pcfg attributes, or at the media level, where it can be referenced by pcfg attributes. When invoked by a pcfg or lcfg attribute, the resulting bandwidth line (b=) is to be interpreted at the media-level.media-level for that configuration. There can be more thenthan one bcap attribute. The unique bw-cap-num is used to identify it in potential configurations. Note: The session level bandwidth gives a maximum total values for all the media in the session. Currently the authors suggest that enforcing a potential limit can be done at the media level and by using re-INVITENo provision has been made for thenegotiation of total session level bandwidth.bandwidth capabilities. Bandwidth capabilities may be included in a potential configuration via the "b=" parameter (see below). Any bandwidth capability included replaces any media-level bandwidth of the same type declared in a "b=" SDP line. The following example offers a preferred potential configuration for H.263 QCIF at 360 Kbit/sec and a second potential configuration for H.263 CIF at the offered 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:1H263-1998/90000 a=mcap:2 video H263-1998/90000 a=mfcap:1 QCIF=2 a=fcap:2a=mfcap:2 CIF=4; QCIF=2;F=1;K=1 a=bcap:1 TIAS:360000 a=pcfg:1 m=1 b=1 pt:100pt=1:100 a=pcfg:2 m=2 pt:101 3.3.pt=2:101 3.4. Extensions to the Potential Configuration Attribute The extension protocol of capabilities negotiation requires three new extensions for the pcfg: attribute defined in the base protocol. The first extension permits the specification of media capabilities, or combinations thereof; the second permits the assignment of payload types to those capabilities when used in the specified configuration; the third permits the specification of bandwidth limits for a media stream. 126.96.36.199.4.1. The Media Capability Extension to the Potential Configuration AttributeParameter 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 the specification of media configurations in addition to the one specified on the media line. We define the media capability configuration parameter, pot-media- config, in accordance with the following format: m=<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 = "m=" med-cap-list *(BAR med-cap-list) ; BAR is defined in [SDPCapNeg] med-cap-list = med-cap-num *("," med-cap-num) med-cap-num = 1*DIGIT ; defined in SDP BAR = *WSP "|" *WSPEach potential media configuration is a comma-separated list of media capability numbers where med-cap-num refers to media 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 media level, the media type and media format from the associated "m=" line will be used. When one or more media capabilities (a=mcap) are invoked in a potential configuration (via m=), and associated with a payload type number by default or by a payload type number parameter (pt=), special processing must be invoked on the attributes associated with that payload type. If the media capability is associated with any mfcap or mscap attributes, then all corresponding conventional attributes (e.g., fmtp or rtcp-fb attribute lines) in the media block are ignored for that configuration. If no mfcap parameters are specified, then an fmtp attribute line within the media block with the correct payload type number, if any, will apply. Similarly, any other media-specific attributes (e.g., rtcp-fb) in the media block with the correct payload type number will apply unless there is an applicable mscap attribute for the same attribute type (e.g., rtcp-fb), in which case all media-block attributes of the same type and payload type number will be ignored. Any media-specific attributes in the media block which refer to payload type numbers not used by the potential configuration are to be ignored. These rules are intended to avoid the need to duplicate attributes and use the a=-m: form of invoking attributes in a potential configuration just to replace an rtpmap or fmtp attribute. 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:med-v0 m=audio 3456 RTP/AVP 0 18 100 a=rtpmap:100 telephone-events a=fmtp:100 0-15 a=mcap:1 audio PCMU g729 telephone-event a=ecap:1,2,3 8000 a=fcap:3PCMU/8000 a=mcap:2 g729/8000 a=mcap:3 telephone-event/8000 a=mfcap:3 0-15 a=pcfg:1 m=2,3|1,3 pt=1:0, 2:18, 3:100 In this example, G729PCMU is media capability 2, PCMU1, G729 is media capability 1,2, and eventstelephone-event is media capability 3. The a=pcfg: line specifies that the preferred configuration is G.729 with extended dtmf events, second is G.711 mu-law with extended dtmf events. Intermixing of G.729, G.711, and "commercial" dtmf events is least preferred (the actualbase configuration provided by the "m=" line, which is always the least preferred configuration). 3.3.2.The Payload Type Mapping Extension to the Potential Configuration Attribute When media capabilities defined in mcap:rtpmap and fmtp attributes of the base configuration are replaced by the mcap and mfcap attributes when invoked by the proposed configuration. 3.4.2. The Payload Type Mapping Parameter When media capabilities defined in mcap attributes are used in potential configuration lines, and the transport protocol uses RTP, it is necessary to assign payload types to them. In some cases, it is desirable to assign different payload types to media capabilities defined inthe same media line.capability when used in different potential configurations. One example of the latteris 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. This association of distinct payload type(s) with different transport protocols requires a separate pcfg line for each protocol. Clearly, this technique cannot be used if the number of potential configurations exceeds the number of possible payload types. We define the media type mapping configuration parameter, pt-media- map, in accordance with the following format: pt-media-map = "pt=" med-map-list med-map-list = med-map *("," med-map);*["," med-map] med-map = <med-cap-num>med-cap-num ":" <'payload-type> ;'payload-type; med-cap-num is defined abovein section 3.4.1 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.As described in section 3.4.1, the choice of payload type numbers is especially important since they are also used to refer to base media -specific attributes (e.g., fmtp or rtcp-fb) if mfcap or mscap attributes are not specified for a media subtype. 3.4.3. The Bandwidth Extension to the Potential Configuration AttributeParameter 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 aboveby the bcap attribute Multiple bandwidth capabilities are permitted in case it is desired to specify multiple bandwidth types. 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.If a bandwidth capability is referenced in a potentical configuration, and that configuration is selected, then the bandwidth capability will replace any bandwidth attribute of the same <bwtype> which appears in the base media configuration. Bandwidth specifications in the base configuration will apply to any potential configuration for which no correspoinding bandwidth capability is specified. 3.5. 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 188.8.131.52.1 act-med-map = "pt=" med-map *("," med-map) ; defined in section 184.108.40.206.2 act-bw-param = "b=" bw-cap-list ; defined in section 220.127.116.11.3 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 offrerofferer which potential configuration and media encoding combination(and parameter combination) was selected by the answerer. 3.5.Note that payload types, if specified, refer to the payloads to be received by the offeror; if the answerer wishes to receive different payload types, those values MUST be specified in the m= line of the SDP answer. 3.6. The Latent Configuration Attribute One of the goals of this work is to permit the exchange of supportable media configurations in addition to those offered or accepted 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 announcedcapabilities by use of theoffering one or more latent configuration attribute, which is defined in a manner very similarconfigurations; the responding party may indicate its ability and willingness to support such a video session by returning one or more latent configurations. 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> ["mt="<media>] ["m="<media-caps-list>] ["t="<transport>] ["a="<attributes>] ["b="<bw-cap-list>] ["pt="<med-map-list>] [future-extensions] <media> is defined in [RFC4566] and future-extensions = extension- config-list as defined in [SDPCapNeg]. The m=, t=, b= and a= parameters are identical in format and meaning to those defined for the pcfg: attribute. Note that the media type (mt=) and media capabilities list MUST be present if the latent configuration is defined at the session level. The mt= parameter provides the MIME type (audio, video, etc.) for the configuration; it is OPTIONAL when the latent configuration is declared at the media level, but if present, MUST match the type of the m= line. The pt= parameter is not permitteddirectly meaningful in the lcfg: attribute because no actual media session is being offered or accepted.accepted, but it is included in order to tie any payload type parameters within attributes to the media. A primary example is the case of format parameters for the RED payload, which are payload type numbers. Specific payload type numbers used in a latent configuration MAY be interpreted as suggestions to be used in any future offer based on the latent configuration, but they are not binding; the offeror and/or answerer may use any payload type numbers each deems appropriate. Future extensions are also permitted. Latent Configurationsconfigurations 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 inLatent Configurations Ifconfigurations placed at the sdescriptions crypto: attribute is necessary as part of any latent configuration which announces sdescriptions capabilities, it presents a slight problem inmedia level represent configurations that are supportable, but are not desired for immediate use. They represent potential future replacements for 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",stream description 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 willthey appear. For example, one party might make an offer with several potential configurations (different codecs, say). The answer can be made with one particular configuration specified in the m= and "a=acfg" lines, and a new offer whennumber of "a=lcfg" lines, one for each "a=pcfg" line which could be supported by the answerer, but which was not chosen for this media stream. This permits the answerer to tell the offeror that, although a specific codec will be used in the current media session, it can also support one or more of the offered alternatives. This could be useful, for example, in the case that PCMU audio encoding is chosen, but subsequent network congestion suggests that the media stream should be renegotiated to G729; the offeror will know in advance that the other party supports G729. The rules for replacement of base-level attributes by mcap and mscap attributes invoked within a latent configuration willare the same as specified for invocation in a potential configuration (see section 3.4) 3.6.1. Cryptographic Attributes in Latent Configurations If a cryptographic attribute, such as the SDES "a=crypto:" attribute [RFC4568], is referenced by a latent configuration, any key material REQUIRED in the attribute, such as the SDES key/salt string, MUST be offered.] 3.6.included. The receiver of the lcfg: attribute MUST ignore any key material associated with the latent configuration. 3.7. Offer/Answer Model Extensions In this section, we define extensions to the offer/answer model defined in RFC3264 [RFC3264] and [SDPCapNeg] to allow for media capabilitiescapabilities, bandwidth capabilities, and latent configurations to be used with the SDP Capability Negotiation framework. 3.6.1.The [SDPCapNeg] provides a relatively compact means to offer the equivalent of an ordered list of alternative media stream configurations (as would be described by separate m= lines and associated attributes). The attributes acap, bcap, mscap, mfcap and mcap are designed to map somewhat straightforwardly into equivalent m= lines and conventional attributes when invoked by a pcfg, lcfg, or acfg attribute with appropriate parameters. The "a=pcfg:" lines, along with the m= line itself, represent offered media configurations. The "a=lcfg:" lines represent alternative capabilities for future use. 3.7.1. Generating the Initial Offer When an endpoint generates an initial offer and wantwants to use the functionality described in the current document, it should identify and define the codecs it can support via mcap , ecap ,mcap, mfcap and fcapmscap attributes. The SDP media line(s) should be made up with the default (least preferred) configuration.configuration to be used if the other party does not understand capability negotiations (by default, this is the least preferred configuration). Typically, the media line configuration will contain the minimum acceptable capabilities. The offer MUST include the requiredlevel of capability negotiation extensions needed to support this functionality.functionality in a "creq" attribute. Preferred configurations for each media stream are identified following the media line. The present offer may also include latent configuration (lcfg) attributes, at the session level, describing media streams and/or configurations the offeror is not now offering, but which it is willing to support in a future offer/answer exchange. A simple example might be the inclusion of a latent video configuration in an offer for an audio stream. 18.104.22.168.7.2. Generating the Answer When the answering party receives the offer and if it supports the required capability negotiation extensions, it should select the most-preferred configuration it can support for each media stream, and build its answer accordingly. EachThe configuration selected for each accepted media configurationstream is placed into the answer as a media line with associated parameters, Itparameters and attributes. If a proposed configuration is chosen, the answer must include the supported extension attribute and each media stream for which a proposed configuration was chosen must contain an actual configuration (acfg) attribute to indicate just which pcfg attribute was used to build the answer. The answer should mayalso include any latent configurations the answerer can support, especially any configurations compatible with other proposed or latent configurations received in the offer. The answerer should make note of those configurations it might wish to offer in the future. 22.214.171.124.7.3. Offerer Processing of the Answer When the offererofferor 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 proposed configuration used to form the answer, and to infer the lack of acceptability of higher- preference configurations that were not chosen. Note that the base specification [SDPCapNeg] requires the answerer[SDPCapNeg] requires the answerer to choose the highest preference configuration it can support. 3.7.4. Modifying the Session If, at a later time, one of the parties wishes to modify the operating parameters of a session, e.g., by adding a new media stream, or changing the properties used on an existing stream, it may do so via the mechanisms defined for offer/answer[RFC3264]. If the initiating party has remembered the codecs, potential configurations, and latent configurations announced by the other party in the earlier negotiation, it may use this knowledge to maximize the likelihood of a successful modification of the session. Alternatively, it may perform a new capabilities exchange as part of the reconfiguration. 4. Examples In this section, we provide examples showing how to use the Media Capabilities with the SDP Capability Negotiation. 4.1. Alternative Codecs This example provide a choice of one of six variations of the adaptive multirate codec. In this example, the default configuration as specified by the media line is the same as the most preferred configuration. Each configuration uses a different payload type so the offeror can interpret early media. 1. v=0 2. o=- 25678 753849 IN IP4 192.0.2.1 3. s= 4. c=IN IP4 192.0.2.1 5. t=0 0 6. a=creq:med-v0 7. m=audio 54321 RTP/AVP 96 8. rtpmap:96 AMR/16000/1 9. a=fmtp:96 mode-change-capability=1; max-red=220; mode- set=0,2,4,7 10. a=cmed:1,3,5 audio AMR/16000/1 11. a=cmed:2,4,6 audio AMR/8000/1 12. a=mfcap:1,2,3,4 mode-change-capability=1 13. a=mfcap:5,6 mode-change-capability=2 14. a=mfcap:1,2,3,5 max-red=220 15. a=mfcap:3,4,5,6 octet-align=1 16. a=mfcap:1,3,5 mode-set=0,2,4,7 17. a=mfcap:2,4,6 mode-set=0,3,5,6 18. pcfg:1 m=1 pt=1:96 19. pcfg:2 m=2 pt=2:97 20. pcfg:3 m=3 pt=3:98 21. pcfg:4 m=4 pt=4:99 22. pcfg:5 m=5 pt=5:100 23. pcfg:6 m=6 pt=6:101 In the above example, media capability 1 could have been excluded from the cmed declaration in line 10 and from the mfcap attributes in lines 12, 14, and 16. The pcfg line 18 could then have been simply "pcfg:1". The next example offers a video stream with three options of H.264 and 4 transports. It also includes an audio stream with different audio qualities: four variations of AMR, or AC3. The offer looks something like: v=0 o=- 25678 753849 IN IP4 192.0.2.1 s=SDP Media NEG example c=IN IP4 192.0.2.1 t=0 0 a=creq:med-v0 a=ice-pwd:speEc3QGZiNWpVLFJhQX m=video 49170 RTP/AVP 100 c=IN IP4 192.0.2.56 a=maxprate:1000 a=rtcp:51540 a=sendonly a=candidate 12345 1 UDP 9 192.0.2.56 49170 host a=candidate 23456 2 UDP 9 192.0.2.56 51540 host a=candidate 34567 1 UDP 7 10.0.0.1 41345 srflx raddr 192.0.2.56 rport 49170 a=candidate 45678 2 UDP 7 10.0.0.1 52567 srflx raddr 192.0.2.56 rport 51540 a=candidate 56789 1 UDP 3 192.0.2.100 49000 relay raddr 192.0.2.56 rport 49170 a=candidate 67890 2 UDP 3 192.0.2.100 49001 relay raddr 192.0.2.56 rport 51540 b=AS:10000 b=TIAS:10000000 b=RR:4000 b=RS:3000 a=rtpmap:100 H264/90000 a=fmtp:100 profile-level-id=42A01E; packetization-mode=2; sprop-parameter-sets=Z0IACpZTBYmI,aMljiA==; sprop-interleaving-depth=45; sprop-deint-buf-req=64000; sprop-init-buf-time=102478; deint-buf-cap=128000 a=tcap:1 RTP/SAVPF RTP/SAVP RTP/AVPF a=mcap:1-3,7-9 H264/90000 a=mcap:4-6 rtx/90000 a=mfcap:1-9 profile-level-id=42A01E a=mfcap:1-9 aMljiA== a=mfcap:1,4,7 packetization-mode=0 a=mfcap:2,5,8 packetization-mode=1 a=mfcap:3,6,9 packetization-mode=2 a=mfcap:1-9 sprop-parameter-sets=Z0IACpZTBYmI a=mfcap:1,7 sprop-interleaving-depth=45; sprop-deint-buf- req=64000; sprop-init-buf-time=102478; deint-buf-cap=128000 a=mfcap:4 apt=100 a=mfcap:5 apt=99 a=mfcap:6 apt=98 a=mfcap:4-6 rtx-time=3000 a=mscap:1-6 rtcp-fb nack a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_80 inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|220|1:32 a=pcfg:1 t=1 m=1,4 a=1 pt=1:100,4:97 a=pcfg:2 t=1 m=2,5 a=1 pt=2:99,4:96 a=pcfg:3 t=1 m=3,6 a=1 pt=3:98,6:95 a=pcfg:4 t=2 m=7 a=1 pt=7:100 a=pcfg:5 t=2 m=8 a=1 pt=8:99 a=pcfg:6 t=2 m=9 a=1 pt=9:98 a=pcfg:7 t=3 m=1,3 pt=1:100,4:97 a=pcfg:8 t=3 m=2,4 pt=2:99,4:96 a=pcfg:9 t=3 m=3,6 pt=3:98,6:95 m=audio 49176 RTP/AVP 101 100 99 98 c=IN IP4 192.0.2.56 a=ptime:60 a=maxptime:200 a=rtcp:51534 a=sendonly a=candidate 12345 1 UDP 9 192.0.2.56 49176 host a=candidate 23456 2 UDP 9 192.0.2.56 51534 host a=candidate 34567 1 UDP 7 10.0.0.1 41348 srflx raddr 192.0.2.56 rport 49176 a=candidate 45678 2 UDP 7 10.0.0.1 52569 srflx raddr 192.0.2.56 rport 51534 a=candidate 56789 1 UDP 3 192.0.2.100 49002 relay raddr 192.0.2.56 rport 49176 a=candidate 67890 2 UDP 3 192.0.2.100 49003 relay raddr 192.0.2.56 rport 51534 b=AS:512 b=TIAS:512000 b=RR:4000 b=RS:3000 a=maxprate:120 a=rtpmap:98 AMR-WB/16000 a=fmtp:98 octet-align=1; mode-change-capability=2 a=rtpmap:99 AMR-WB/16000 a=fmtp:99 octet-align=1; crc=1; mode-change-capability=2 a=rtpmap:100 AMR-WB/16000/2 a=fmtp:100 octet-align=1; interleaving=30 a=rtpmap:101 AMR-WB+/72000/2 a=fmtp:101 interleaving=50; int-delay=160000; a=mcap:14 ac3/48000/6 a=acap:23 crypto:1 AES_CM_128_HMAC_SHA1_80 inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|220|1:32 a=tcap:4 RTP/SAVP a=pcfg:10 t=4 a=23 a=pcfg:11 t=4 m=14 a=23 pt=14:102 4.2. Latent Media Streams Consider a case in which the offeror can support either G.711 mu-law, or G.729B, along with DTMF telephony events for the 12 common touchtone signals, but is willing to support simple G.711 mu-law audio as a last resort. In addition, the offeror wishes to announce its ability to support video in the future, but does not wish to offer a video stream at present. The offer might look like the following: 1. v=0 2. o=- 25678 753849 IN IP4 192.0.2.1 3. s= 4. c=IN IP4 192.0.2.1 5. t=0 0 6. a=creq:med-v0 7. a=mcap:10 H263-1998/90000 8. a=mcap:11 H264/90000 9. a=lcfg:10 mt=video m=10|11 10. m=audio 23456 RTP/AVP 0 11. a=rtpmap:0 PCMU/8000 12. a=mcap:1 PCMU/8000 13. a=mcap:2 g729/8000 14. a=mcap:3 telephone-event/8000 15. a=mfcap:3 0-11 16. a=pcfg:1 m=1,3|2,3 pt=1:0,2:18,3:100 Lines 7-9 announce support for H.263 and H.264 video (H.263 preferred) for future reference. Lines 10 and 11 offer an audio stream and provide the lowest precedence configuration (PCMU without any DTMF encoding). Lines 12-14 define the media capabilities to be offered: PCMU, G729, and telephone-event. Line 15 provides the format parameters for telephone-events, specifying the 12 commercial DTMF 'digits'. Line 16 defines the most-preferred media configuration as PCMU plus DTMF events and the next-most-preferred configuration as G.729B plus DTMF events. If the answerer is able to support all the potential configurations, and also support H.263 video (but not H.264), it would reply with an answer like: 1. v=0 2. o=- 24351 621814 IN IP4 192.0.2.2 3. s= 4. c=IN IP4 192.0.2.2 5. t=0 0 6. a=csup:med-v0 7. a=mcap:10 H263-1998/90000 8. a=lcfg:1 mt=video m=10 9. m=audio 54321 RTP/AVP 0 100 10. a=rtpmap:0 PCMU/8000 11. a=rtpmap:100 telephone-event/8000 12. a=fmtp:100 0-11 13. a=acfg:1 m=1,3 pt=1:0,3:100 14. a=mcap:1 G729/8000 15. a=mcap:2 telephone-event/8000 16. a=mfcap:2 0-11 17. a=lcfg:2 m=1,2 pt=1:18,2:100 Lines 7 and 8 announce the capability to support H.263 video at a later time. Lines 9-12 of the answer present the selected configuration for the media stream. Line 13 identifies the potential configuration from which it was taken, and lines 14-17 announce the latent capability to support G.711 mu-law with DTMF events as well. If, at some later time, congestion becomes a problem in the network, either party may offer a reconfiguration of the media stream to use G.729 in order to reduce packet sizes. Note that line 13 uses media capability numbering as provided in the original offer, whereas lines 14-17 must use their own numbering. 5. IANA Considerations The IANA is hereby requested to register the following new SDP attributes: Attribute name: mcap Long form name: media capability Type of attribute: Session-level and media-level Subject to charset: No Purpose: associate media capability number(s) with media subtype and encoding parameters Appropriate Values: See Section 3.3.1 Attribute name: mfcap Long form name: media format capability Type of attribute: Session-level and media-level Subject to charset: No Purpose: associate media format attributes and parameters with media format capabilities Appropriate Values: See Section 3.3.2 Attribute name: mscap Long form name: media-specific capability Type of attribute: Session-level and media-level Subject to charset: No Purpose: associate media-specific attributes and parameters with media capabilities Appropriate Values: See Section 3.3.3 Attribute name: bcap Long form name: Bandwidth capability attribute Type of attribute: Session-level or media-level Subject to charset: No Purpose: associate bandwidth limitations with potential potential or latent configurations. Appropriate Values: See Section 3.3.4 Attribute name: lcfg Long form name: Latent Configuration Type of attribute: Session-level or media-level Subject to charset: No Purpose: To announce supportable media configurations without offering them for immediate use. Appropriate Values: See Section 3.6 6. Security Considerations The security considertions of [SDPCapNeg] apply for this document. No additional security considerations are introduced here. 7. Changes from previous versions 7.1. Changes from version 02 This version contains several detail changes intended to simplify capability processing and mapping into conventional SDP media blocks. o The "mcap" attribute is enhanced to include the role of the "ecap" attribute; the latter is eliminated. o The "fcap" attribute has been renamed "mfcap". New replacement rules vis-a-vis fmtp attributes in the base media specification have been added. o A new "mscap" attribute is defined to choose the highest preference configuration it can support. 3.6.4. Modifyinghandle the Session If, at a later time, oneproblem of the parties wishesattributes (other than rtpmap and fmtp) that are specific to modify the operating parameters of a session, e.g., by adding a new media stream, or changinga codec used on an existing stream, it may do so via the mechanisms definedparticular payload type. o New rules for offer/answer[RFC3264]. If the initiating party has rememberedprocessing the codecs, potential configurations,mcap, mfcap, and latent configurations announced by themscap attributes, and overriding standard rtpmap, fmtp, or other party inmedia-specific attributes, are put forward to reduce the earlier negotiation, it may use this knowledgeneed to maximizeuse the likelihood of a successful modificationdeletion option in the a= parameter of the session. Alternatively, it may perform apotential configuration (pcfg) attribute. o A new capabilities exchange as part ofparameter, "mt=" is added to the reconfiguration. 4. Examples In this section, we provide examples showing howlatent configuration attribute (lcfg) to usespecify the Media Capabilities withmedia stream type (audio, video, etc.) when 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.lcfg is declared at the session level. o The examples are expanded. o Numerous typos and misspellings have been corrected. 7.2. 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. 126.96.36.199. 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. We thank Ingemar Johansson and Magnus Westerlund for examples that stimulated this work. 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-07draft-ietf-mmusic-sdp-capability-negotiation-08 (work in progress), OctoberDecember 2007. 9.2. Informative References [I-D.ietf-avt-avpf-ccm] Wenger, S., Chandra, U., Westerlund, M., and B. Burman, "Codec Control Messages in the RTP Audio-Visual Profile with Feedback (AVPF)", draft-ietf-avt-avpf-ccm-10 (work in progress), October 2007. [RFC3407] Andreasen, F., "Session Description Protocol (SDP) Simple Capability Declaration", RFC 3407, October 2002. [RFC3952] Duric, A. and S. Andersen, "Real-time Transport[RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session Description Protocol (RTP)(SDP) Security Descriptions for Media Streams", RFC 4568, July 2006. [RFC4867] Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie, "RTP Payload Format and File Storage Format for internet Low Bit Rate Codec (iLBC) Speech",the Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband (AMR-WB) Audio Codecs", RFC 3952, December 2004.4867, April 2007. Authors' Addresses Robert R Gilman Avaya, Inc. 1300 West 120th Avenue Westminster,NDCI Broomfield, CO 8023480020 USA Email: firstname.lastname@example.org_gilman@comcast.net 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).(2008). 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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