F. Andreasen Internet-Draft Cisco Systems Expires: August 16, 2007R. Gilman Internet-Draft Avaya, Inc. Intended status: Standards Track R. Even, Ed. Expires: August 24, 2007 Polycom F. Andreasen Cisco Systems February 12,20, 2007 SDP media capabilities Negotiation draft-ietf-mmusic-sdp-media-capabilities-00.txtdraft-ietf-mmusic-sdp-media-capabilities-01.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 16,24, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract Session Description Protocol (SDP) capability negotiation provides a general framework for negotiationnegotiating capabilities in SDP. The base framework defines only capabilities for negotiationnegotiating 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 attributes.parameters. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. SDP Media capabilities . . . . . . . . . . . . . . . . . . . . 5 3.1. Solution Overview . . . . . . . . . . . . . . . . . . . . 5 3.2. Capability Attributes . . . . . . . . . . . . . . . . . . 8 3.2.1. Media Type and Subtype Capability Attribute . . . . . 8 3.2.2. The Capability Encoding Parameters Attribute . . . . . 9 3.2.3. The CapabilityMedia Format ParametersParameter Capability Attribute . . . 10 3.3. Extensions to the Potential Configuration Attribute . . . 10 184.108.40.206 3.3.1. The Media Line CapabilitiesCapability Extension to the Potential Configuration Attribute . . . . . . . . . . . . . . . 11 3.3. Extensions3.3.2. The Payload Type Mapping Extension to the Potential Configuration Attribute . . . 11 3.3.1. The Media Capability Extension. . . . . . . 12 3.4. Extensions to the PotentialActual Configuration Attribute . . . . . 12 3.5. The Latent Configuration Attribute . . . . . . . . . . . 11. 13 3.5.1. The crypto: Attribute in Latent Configurations . . . . 14 3.6. Offer/Answer Model Extensions . . . . . . . . . . . . . . 14 3.6.1. Generating the Initial Offer . . . . . . . . . . . . . 14 3.6.2. Generating the Answer . . . . . . . . . . . . . . . . 14 3.6.3. Offerer Processing of the Answer . . . . . . . . . . . 14 3.6.4. Modifying the Session . . . . . . . . . . . . . . . . 14 4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 1615 4.1. Alternative Codecs . . . . . . . . . . . . . . . . . . . . 1615 4.2. Latent Media Streams . . . . . . . . . . . . . . . . . . . 1615 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 1716 6. Security Considerations . . . . . . . . . . . . . . . . . . . 1817 7. Changes from version 00 . . . . . . . . . . . . . . . . . . . 18 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19 8.9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20 9.1. Normative References . . . . . . . . . . . . . . . . . . . 20 9.2. Informative References . . . . . . . . . . . . . . . . . . 20 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21 Intellectual Property and Copyright Statements . . . . . . . . . . 22 1. Introduction Session Description Protocol (SDP) capability negotiation [SDPCapNeg] provides a general framework for negotiationnegotiating capabilities in SDP[RFC4566]. The base framework defines only capabilities for negotiationnegotiating 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 attributes.parameters. SDP Simple Capability Declaration (simcap) is defined in RFC 3407.3407 [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 - 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. 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 the SDP media Capability negotiation solution. 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 consist of the following new attributes extending the base attributes from (reference)[SDPCapNeg]. Three attributes are used to make up media capabilities o A new media attribute ("a=cmed") that lists potentialmedia typesformats 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") that lists potentialencoding parametersparameter capabilities associated with a particular media format capability. o A new attribute ("a=cfmt") that lists potentialmedia format parameter capabilities A new media-level attribute ("a=mcap:") that assigns identifying numbers to media capabilities specified in the correspondingassociated with a particular media ("m=") line.format capability. A new attribute ("a=lcfg") that specifies latent configurations when no corresponding media line is offered. An example is a potentiallatent configuration for video even though no video is currently offered. A new parameter type ("m=") to the proposed-configurationpotential configuration ("a=pcfg:") attribute and the acceptedactual configuration ("a=acfg:") attribute,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 PCUM and telephonytelephone events or G.729B and telephonytelephone events as acceptable configurations. The "a=acfg:" line in the answer would specify the accepted choice. A new parameter type ("pt=") to the proposed-configurationpotential configuration ("a=pcfg:") attribute which permits specification ofassociates RTP payload types forwith the referenced media capabilities. 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. When included at the session level, theyMedia capabilities constitute latentcapabilities that maycan be used to guide a subsequent offer. When included at the media level, theyin 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). The answerer replies by including the accepted configuration inline(s), latent configurations merely inform the media lineother side of possible configurations supported by the answer. Capabilities,entity. Those latent configurations and potential configurationsmay be included in answers as well, where they can aid in guiding aused to guide subsequent new offer.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 192.0.2.1/24192.0.2.1 s= c=IN IP4 192.0.2.1/24192.0.2.1 t=0 0 a=creq:v1 a=cmed:1 audio g729 iLBC PCMU g729 a=cenc:2 8000 a=cfmt:1 annexb:no a=ctrpr:1 RTP/SAVP m=audio 3456 RTP/AVP 0 18 a=mcap:3a=capar:1 a=crypto:1 AES_CM_128_HMAC_SHA1_32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 a=cptmap:1 1:100 2:101 3:102a=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, see base protocol, which indicates thatattribute defined in [SDPCapNeg], with the option tag "v1", 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" line defines twofour audio media subtype capabilities (mime subtypes),, 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" line specifies the clock rate and encoding parmeters (see [RFC4566]) for capability 2, iLBC. The "a=cfmt:1" line specifies media format parametersparameter capabilities for codec 1 (no comfort noise( no Annex B for G.729). The "a=ctrpr:1" line, specified in the base protocol, defines a transport profile,protocol capability, in this case using SRTP.Secure RTP. The "m=" line indicates that Alice is offering to use plain RTP with PCMU or G.729B.G.729. The media line implicitly defines the default transport profileprotocol (RTP/AVP in this case) and the default proposedactual configuration. The "a=mcap:3""a=capar:1" line assigns numbers 3 and 4 to the media capabilities,specified in the m-line (PCMU and G.729B, respectively) [Editors' note: we need some way to refer tobase protocol provides the "crypto" attribute which provides the keying material for SRTP using SDP security descriptions. The "a=pcfg:" attributes provide the capspotential configurations included in the m= line so that we don't haveoffer by reference to repeat them in an a=cmed: line. I think we can always refer to the current transport protocol from the m-line by omission - it's always the default. Since we may want to refer to the media caps outside the media session when/if we specify constraints between media encodings, we might define a new media- level attribute that lets us associate a unique set of numbers with the media types in the m-line. For the sake of discussion, let's call it "a=mcap:<n>", where <n> is the ordinal assigned to the first payload or subtype in the m-line, and the subsequent payloads/ subtypes, if any, are assigned consecutive ordinals. This will permit us to use those media caps in pcfg: attributes as well, and it's constant under a reordering ofthe media linescapabilities, transport capabilities, and associated attributes. Unique numbering will also let us refer to those caps outside of the media session if we need to specify multimedia constraints.] The "a=capar: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 capability declarations.payload type mappings. Two explicit alternatives are provided; the first one, numbered 1 is the preferred one,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 parameter1, 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 . 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 potentialactual 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 192.0.2.2/24192.0.2.2 s= c=IN IP4 192.0.2.2/24192.0.2.2 t=0 0 a=csup:v1 m=audio 4567 RTP/AVP 103 a=rtpmap:103 8000 a=acfg:2 Bob includes the "a=csup" and "a=acfg" attribute in the answer to inform Alice that he can support the v1 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.729BG.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 should 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 rtpmap:media descriptions ("m=" lines) and fmtp:the associated "rtpmap" and "fmtp" attributes, but using "media capability numbers" instead of payload types to identify eachlink the relevant media capability.capability parameters. 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=cmed" attribute, which is defined as follows: a=cmed:<med-cap-num> <type> <subtype>*[ <subtype>] where <med-cap-num> is an integer between 1 and 2^31-1 (both included) used to number the media format capabilities, <type> is a media type (e.g., audio or video), and the <subtype> is the media subtype e.g. H263-1998, PMCU.PCMU (Editors' note: can specify in cmed anything that can be specified in an m-line?). 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>.<med-cap-num>. Furthermore, when a "cmed" attribute indicates more than one media format, the capability numbers implied MUST NOT be used by any other "cmed" attribute in the entire session or media description (explicitly or implicitly). In ABNF, we have: media-capability-line = "a=cmed:" media-cap-num WSP media-type WSP media-cap *(WSP media-cap) media-cap-num = 1*DIGIT media-type = token ; MIME typeType name (audio, video, etc.) media-cap = token ; MIME subtype (PCMU,Subtype name(PCMU, G729, etc.) Media subtypes identified in "a=cmed" lines may be qualified via otherthe attributes, "a=cenc" and "a=cfmt", in much the same way as media formats in "m=" lines payload types arecan have their clock rate and encoding parameters qualified by an "a=rtpmap" line and media format specific parameters can be provided by "a=fmtp" lines. 3.2.2. The Capability Encoding Parameters Attribute Capabilities defined in "a=cmed" linesMedia format capabilities may require additional encoding parameters, such as sample rate, to be precisely defined. The "a=cenc" encoding attribute is defined as a=cenc:<med-cap-num> <clock rate>[/<encoding parameters>] The clock rate and other encoding parameters are as defined for the "a=rtpmap:" lineattribute defined in RFC 4566.4566 [RFC4566]. For example, a capability for low-bit-rate encoding at 8000 samples per second could be specified by a=cmed:1 iLBC a=cenc: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:1 L16 L16 a=cenc:1 8000 a=cenc: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 better to put the encoding information in the cmed line or cptmap,pcfg "pt", along with the MIMEmedia subtype, and eliminate the cenc attribute. This would make 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 can be done so long as the encoding attributes never contain whitespace. Is this the case? or instead use semi colon as separator] 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. ). Media capabilities can be provided at the session-level and the media-level. Media capabilities provided at the session level apply to the session description in general, whereas media capabilities provided at the media level apply to 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 referenced across the entire session description (e.g. in a potential configuration.) 3.2.3. The CapabilityMedia Format ParametersParameter Capability Attribute This attribute is used to associate media format parameters with a media capability. The form of the attribute is: a=cfmt:<med-cap-num> <list of format parameters> where the format parameters are specific to the type of codec, as described for the fmtp: attribute defined in RFC 4566. As an 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:1 audio G729 G729 a=cfmt:2 annexb:no Example for H.263 video: a=cmed:1 video H263-1998 H263-2000 a=cfmt:1 CIF=4;QCIF=2;F=1;K=1 a=cftp:2a=cfmt:2 profile=2;level=2.2 3.2.4. The Media Line Capabilities Attribute This attribute is used to assign unique ordinals to the media capabilities listed in the m= line. It is a media-level attribute, but the numbers assigned must be unique across the session. The form of the attribute is: a=mcap:<med-cap-num> where <media-cap-num> is the ordinal assigned to the first media capability listed in the m-line; subsequent capabilities, if present, are assigned consecutive numbers following the first. For example, the following sequence defines three capabilies, 1 = GSM, 6 = PCMU, and 7 = G.729. The m-line configuration is the preferred one, but GSM support is offered as an alternative: a=cmed:1 audio GSM m=audio 4567 RTP/AVP 0 18 a=mcap:6 a=pcfg:1 m=6|7 a=pcfg:2 m=13.3. Extensions to the Potential Configuration Attribute The extension protocol of capabilities negotiation requires two new extensions for the pcfg: attribute defined in the base protocol. One extension permits the specification of selectedmedia capabilities, or combinations thereof; the other permits the assignment of payload types to those capabilities when used in the specified configuration. 3.3.1. The Media Capability Extension to the 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 extension (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>]*["|"<med-cap-list>] where <med-cap-list> is a comma-separated list of media capability numbers (media-cap-num) as defined by a=cmed: lines and media lines. In ABNF form,form (adhering to the ABNF for pot-extension-config in [SDPCapNeg]: pot-media-config = "m=" med-cap-list *("/"*(BAR med-cap-list) med-cap-list = med-cap-num *("," med-cap-num) med-cap-num = 1*DIGIT ; defined in SDP BAR = *WSP "|" *WSP Each 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=cmed attributes or implicitly by the media line, and hence MUST be between 1 and 2^31-1 (both included). Alternative potential media configurations are separated by a forward slash ("/").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. For example: v=0 o=- 25678 753849 IN IP4 192.0.2.1/24192.0.2.1 s= c=IN IP4 192.0.2.1/24192.0.2.1 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 eventstelephone-events a=fmtp:100 0-15 a=pcfg:1 m=2,3/1,3m=2,3|1,3 pt=1:0, 2:18, 3:100 In this example, G729 is media capability 2, PCMU is media capability 1, and events is media capability 3. The a=pcfg: line specifies that the preferred configuration is G.729BG.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. 220.127.116.11.preferred (the actual 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 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 different payload types to media capabilities defined in the 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 a payload-type-map attribute which define a (set of) mappings from media capability number to payload type a = cptmap: 1*[<med-cap-num>:<payload-type>] payload-type-map = 1*(WSP <med-cap-num)":"<payload-type>) med-cap-num = 1*DIGIT ; from cmed: attribute payload-type = 1*DIGIT ; RTP payload type This is illustrated by extending an example from the version 0 document showing an offer of an audio session with or without SRTP. The following specifies RTP/SAVP transport using payload type 98 for PCMU and payload type 99 for G.729; RTP/AVP transport uses the standard payload types from the media line. v=0 o=- 25678 753849 IN IP4 192.0.2.1/24 s= c=IN IP4 192.0.2.1/24 t=0 0 a=creq:v1 m=audio 3456 RTP/AVP 0 18 a=ctrpr:1 RTP/SAVP RTP/AVP a=capar:1 a=crypto:1 AES_CM_128_HMAC_SHA1_80 inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4 FEC_ORDER=FEC_SRTP a=cptmap:1 1:98 2:99 a=pcfg:5 t=1 a=1 pt=1 a=pcfg:10 t=2 [Editor's note: it seems to me thatOne example of the cptmap: attributelatter is only really useful within a media block, so perhaps we should drop itwhen configurations for AVP and just defineSAVP are offered: the pt= parameter as a comma-separated list of PT mappingsofferer would like "pt=1:98,2:99" inthe above example. Alternative isanswerer to use the cenc=1:98/8000. What do you think?] 18.104.22.168. Extensions to the Actual Configuration Attribute [Editor's note: I'm thinking moredifferent payload types for encrypted and moreunencrypted media so that we don't needit (the offerer) can decide whether or not to render early media which arrives before the acfg: attribute at all, givenanswer is received. We define the presence ofmedia type mapping configuration parameter, pot-media- map, in accordance with the csup: attribute. Allfollowing format: pot-media-map = "pt=" med-map-num *("," *WSP med-map-num) med-map-num = 1*DIGIT ":" 1*DIGIT ; The example in the significant reply information isprevious section shows how the parameters from the cmed line are mapped to payload type in the m=line and its associated attributes. We should be ablepcfg "pt" parameter. 3.4. Extensions to infer that they came from a pcfg: attribute orthe default offer. What do you think?] (will it be OK with offer answer)Actual Configuration Attribute We define an actual configuration extension parameter act-media- config in accordance with the following ABNF: act-media-config =a=acfg:<med-cap-num> *WSP "m=" med-cap-list where med-cap-list is as defined in Section 2.3.1. Thus, a<med-cap-list> A response to the exampleprevious offer example in thatthe above section might be: v=0 o=- 24351 621814 IN IP4 192.0.2.2/24192.0.2.2 s= c=IN IP4 192.0.2.2/24192.0.2.2 t=0 0 a=csup:v1 m=audio 5432 RTP/AVP 18 100 a=rtpmap:100 events a=fmtp:100 0-15 a=acfg:1 m=2,3 Note that the capability numbers expressed in the acfg: attribute are based on the offered capability numbering, not on those listed in the answer. In this particular example, it's somewhat redundantThe acfg identify to includethe acfg: line at all , but it is important in cases inoffrer which potetial configuration was selected by the payload type mappings are not the same in both directions. 22.214.171.124.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. 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>] ["t="<transport>] ["a="<attributes>] The m=, t= and a= parameters are identical in format and meaning to those defined for the pcfg: attribute. Note that the pt= parameter is not permitted in the lcfg: attribute because no actual media session is being offered or accepted. Latent Configurations may be specified at the session level in offers and in answers. [Editor's note: Do you have a good example of an offer of an audio stream with a latent video stream?][Editor's note (rrg): What if, as Roni suggested, we exclude the a= parameters from the lcfg: lines? We can include the t= parameter as the 'cheapest' way to indicate SRTP. That conveys most of the necessary info; the details can be firmed up in the actual offer when/if it's actually made.] 126.96.36.199.188.8.131.52. The crypto: Attribute in Latent Configurations If the sdescriptions crypto: attribute is necessary as part of any latent configuration which announces sdescriptions capabilities. then 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] 184.108.40.206. 220.127.116.11. Offer/Answer Model Extensions In this section, we define extensions to the offer/answer model defined in RFC3264 to allow for media capabilities to be used with the SDP Capability Negotation framework. 18.104.22.168.22.214.171.124. Generating the Initial Offer 126.96.36.199.188.8.131.52. Generating the Answer 184.108.40.206.220.127.116.11. Offerer Processing of the Answer 18.104.22.168.22.214.171.124. Modifying the Session 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 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. 8.9. 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-02 (work in progress), February 2007. 9.2. Informative References [RFC3407] Andreasen, F., "Session Description Protocol (SDP) Simple Capability Declaration", RFC 3407, October 2002. Authors' Addresses Flemming Andreasen Cisco Systems Edison, NJ USA Email: firstname.lastname@example.orgRobert R Gilman Avaya, Inc. 1300 West 120th Avenue Westminster, CO 80234 USA Email: email@example.com Roni Even (editor) Polycom 94 Derech Em Hamoshavot Petach Tikva 49130 Israel Email: firstname.lastname@example.org Flemming Andreasen Cisco Systems Edison, NJ USA Email: email@example.com 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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