MMUSIC Working Group F. Andreasen Internet Draft Cisco Systems Expires: July 2007 January
2,28, 2007 SDP Capability Negotiation draft-ietf-mmusic-sdp-capability-negotiation-00.txtdraft-ietf-mmusic-sdp-capability-negotiation-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 July 2,28, 2007. Abstract The Session Description Protocol (SDP) was intended for describing multimedia sessions for the purposes of session announcement, session invitation, and other forms of multimedia session initiation. SDP was not intended to provide capability indication or capability negotiation, however over the years, SDP has seen widespread adoption and as a result it has been gradually extended to provide limited support for these. SDP and its current extensions however do not have the ability to negotiate one or more alternative transport protocols (e.g. RTP profiles) which makes it particularly difficult to deploy new RTP profiles such as secure RTP or RTP with RTCP-based feedback. The purpose of this document is to address that and other real-life limitations by extending SDP with capability negotiation parameters and associated offer/answer procedures to use those parameters in a backwards compatible manner. The solution provided in this document provides a general SDP capability negotiation framework. It also defines specifically how to provide attributes and transport protocols as capabilities and negotiate them using the framework. Extensions for other types of capabilities (e.g. media types and formats) may be provided in other documents. Conventions used in this document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Table of Contents 1. Introduction...................................................3 2. SDP Capability Negotiation Solution............................5 2.1. Solution Overview.........................................5 2.2. Version and Extension Indication Attributes...............8 2.2.1. SDP Capability Negotiation Version Attribute.........8 2.2.2.Supported Capability Negotiation Extensions Attribute9 2.2.3.Attribute8 2.2.2. Required Capability Negotiation Extension Attribute.10 2.3. Capability Attributes....................................11 2.3.1. Media Type and Format Capability Attribute..........11 2.3.2.Attribute ParameterCapability Attribute............14 2.3.3.Attribute......................11 2.3.2. Transport Protocol Capability Attribute.............15Attribute.............13 2.4. Configuration Attributes.................................16Attributes.................................13 2.4.1. Potential Configuration Attribute...................16Attribute...................13 2.4.2. Actual Configuration Attribute......................19Attribute......................17 2.5. Offer/Answer Model Extensions............................20Extensions............................18 2.5.1. Generating the Initial Offer........................21Offer........................18 2.5.2. Generating the Answer...............................22Answer...............................19 2.5.3. Offerer Processing of the Answer....................22Answer....................20 2.5.4. Modifying the Session...............................23Session...............................20 3. Examples......................................................23Examples......................................................21 3.1. Best-Effort Secure RTP...................................23RTP...................................21 4. Security Considerations.......................................25Considerations.......................................23 5. IANA Considerations...........................................25Considerations...........................................23 6. To Do and Open Issues.........................................26Issues.........................................23 7. Acknowledgments...............................................26Acknowledgments...............................................23 8. Change Log....................................................26Log....................................................24 8.1. draft-ietf-mmusic-sdp-capability-negotiation-00..........26draft-ietf-mmusic-sdp-capability-negotiation-01..........24 8.2. draft-ietf-mmusic-sdp-capability-negotiation-00..........24 9. References....................................................27References....................................................26 9.1. Normative References.....................................27References.....................................26 9.2. Informative References...................................27References...................................26 Author's Addresses...............................................29Addresses...............................................28 Intellectual Property Statement..................................29Statement..................................28 Disclaimer of Validity...........................................30Validity...........................................29 Copyright Statement..............................................30 Acknowledgment...................................................30Statement..............................................29 Acknowledgment...................................................29 1. Introduction The Session Description Protocol (SDP) was intended for describing multimedia sessions for the purposes of session announcement, session invitation, and other forms of multimedia session initiation. The SDP contains one or more media stream descriptions with information such as IP-address and port, type of media stream (e.g. audio or video), transport protocol (possibly including profile information, e.g. RTP/AVP or RTP/SAVP), media formats (e.g. codecs), and various other session and media stream parameters that define the session. Simply providing media stream descriptions is sufficient for session announcements for a broadcast application, where the media stream parameters are fixed for all participants. When a participant wants to join the session, he obtains the session announcement and uses the media descriptions provided, e.g., joins a multicast group and receives media packets in the encoding format specified. If the media stream description is not supported by the participant, he is unable to receive the media. Such restrictions are not generally acceptable to multimedia session invitations, where two or more entities attempt to establish a media session that uses a set of media stream parameters acceptable to all participants. First of all, each entity must inform the other of its receive address, and secondly, the entities need to agree on the media stream parameters to use for the session, e.g. transport protocols and codecs. We here make a distinction between the capabilities supported by each participantparticipant, the way in which those capabilities can be supported and the parameters that can actually be used for the session. More generally, we can say that we have the following: o A set of capabilities and potential configurations offor the session and its associated media stream components, supported by each side. o A set of actualpotential configurations indicating which of those capabilities can be used for the session and its associated media stream components. o A set of actual configurations for the session and its associated media stream components, which specifies which session parameters to use and which media stream components to use and with what parameters. o A negotiation process that takes the set of potential configurations (capabilities)(lists of capabilities) as input and provides the actual configurations as output. SDP by itself was designed to provide only the secondone of these, i.e.,namely the actual configurations, however over the years, use of SDP has been extended beyond its original scope. Session negotiation semantics were defined by the offer/answer model in RFC 3264. It defines how two entities, an offerer and an answerer, exchange session descriptions to negotiate a session. The offerer can include one or more media formats (codecs) per media stream, and the answerer then selects one or more of those offered and returns them in an answer. Both the offer and the answer contain actual configurations - capabilities and potential configurations are not supported. The answer however may reduce the set of actual configurations from the offer. The answer may also extend the set of actual configurations that can be used to receive media by the answerer. Other relevant extensions have been defined. Simple capability declarations, which define how to provide a simple and limited set of capability descriptions in SDP was defined in RFC 3407. Grouping of media lines, which defines how media lines in SDP can have other semantics than the traditional "simultaneous media streams" semantics, was defined in RFC 3388, etc. Each of these extensions was designed to solve a specific limitation of SDP. Since SDP had already been stretched beyond its original intent, a more comprehensive capability declaration and negotiation process was intentionally not defined. Instead, work on a "next generation" of a protocol to provide session description and capability negotiation was initiated [SDPng]. SDPng however has not gained traction and has remained as work in progress for an extended period of time. Existing real-time multimedia communication protocols such as SIP, RTSP, Megaco, and MGCP continue to use SDP. SDP and its current extensions however do not address an increasingly important problem: the ability to negotiate one or more alternative transport protocols (e.g., RTP profiles). This makes it difficult to deploy new RTP profiles such as secure RTP (SRTP) [SRTP], RTP with RTCP-Based Feedback [AVPF], etc. This particular problem is exacerbated by the fact that RTP profiles are defined independently. When a new profile is defined and N other profiles already exist, there is a potential need for defining N additional profiles, since profiles cannot be combined automatically. For example, in order to support the plain and secure RTP version of RTP with and without RTCP-based feedback, four separate profiles (and hence profile definitions) are needed: RTP/AVP [RFC3551], RTP/SAVP [SRTP], RTP/AVPF [AVPF], and RTP/SAVPF [SAVPF]. In addition to the pressing profile negotiation problem, other important real-life constraints have been found as well. The purpose of this document is to define a mechanism that enables SDP to provide limited support for indicating capabilities and their associated potential configurations and negotiate the use of those potential configurations as actual configurations. It is not the intent to provide a full-fledged capability indication and negotiation mechanism along the lines of SDPng or ITU-T H.245. Instead, the focus is on addressing a set of well-known real-life limitations. More specifically, the solution provided in this document provides a general SDP capability negotiation framework. It also defines specifically how to provide attributes and transport protocols as capabilities and negotiate them using the framework. Extensions for other types of capabilities (e.g. media types and formats) may be provided in other documents. As mentioned above, SDP is used by several protocols, and hence the mechanism should be usable by all of these. One particularly important protocol for this problem however is the Session Initiation Protocol (SIP) [RFC3261]. SIP uses the offer/answer model (which is not specific to SIP) to negotiate sessions and hence any mechanism must at least consider how it either interacts with offer/answer, or how it should extend it. The rest of the document is structured as follows. In Section 0we2. we present our SDP capability negotiation solution followed by examples in Section 3. and security considerations in Section 4. 2. SDP Capability Negotiation Solution In this section we first provide an overview of the SDP Capability negotiation solution. This is followed by definitions of new SDP attributes for the solution and its associated updated offer/answer procedures. 2.1. Solution Overview The solution consists of the following: o ThreeTwo new attributes to support versioning and extensions to the framework itself as follows: o A new attribute ("a=cver") that lists the version of the SDP capability negotiation framework being used. o A new attribute("a=csup") that lists the supported extensionsbase and extension options to the framework. o A new attribute ("a=creq") that lists the base and or extensions to the framework that are required to be supported by the entity receiving the SDP. o ThreeTwo new attributes used to express capabilities as follows (additional attributes can be defined as extensions): o A new attribute ("a=cmed") that defines how to list the media types and media formats supported as capabilities. o A new attribute("a=capar") that defines how to list theattribute parameter values ("a=" values) as capabilities. o A new attribute ("a=ctrpr") that defines how to list transport protocols (e.g. "RTP/AVP") as capabilities. o Two new attributes to negotiate configurations as follows: o A new attribute ("a=pcfg") that lists the potential configurations supported. This is done by reference to the abovecapabilities from the SDP in question. TheMultiple potential configurations are listed in order of preference. Extensionshave an explicitly indicated ordering associated with them. Extension capabilities can be defined as welland included in the potential configurations. o A new attribute ("a=acfg") to be used in an answer SDP. The attribute identifies which of the potential configurations from an offer SDP were used as actual configurations to form the answer SDP. Extension capabilities can included. o Extensions to the offer/answer model that allow for capabilities and potential configurations to be included in an offer, whereoffer. When included at the session level, they constitute latent capabilities that may be used to guide a subsequent offer. When included at the media level, they constitute offers that may be accepted by the answerer instead of the actual configuration(s) included in the "m=" line(s). The answerer indicates which (if any) of the potential configurations it used to form the answer by including the actual configuration attribute ("a=cfg")("a=acfg") in the answer. Capabilities and potential configurations may be included in answers as well, where they can aid in guiding a subsequent new offer. 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 184.108.40.206 s= c=IN IP4 220.127.116.11 t=0 0 m=audio 3456 RTP/AVP 0 18 a=cver:0 a=cmed:1 audio RTP/AVP 0 18 96a=creq: v0 a=ctrpr:1 RTP/SAVP a=capar:1 a=crypto:1 AES_CM_128_HMAC_SHA1_32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 a=capar:2 a=rtpmap:96 iLBC/8000 a=pcfg: m=1,2|3 p=1 a=1,2 a=pcfg: m=1,2|3 a=2a=pcfg:1 t=1 a=1 The "m=" line indicates that Alice is offering to use plain RTP with PCMU or G.729. The required base and extensions are provided by the "a=creq" attribute, which indicates that the option tag "v0", which indicates the base framework defined here, must be supported. The capabilities are provided by the "a=cver", "a=cmed","a=ctrpr" and "a=capar" attributes. The capabilities indicate that PCMU, G.729 and iLBC aresecure RTP under the AVP profile ("RTP/SAVP") is supported with either RTP or secure RTP.an associated transport capability handle of 1. The first"capar" attribute provides aan attribute capability parameterwith a handle of 1. The attribute capability parameteris a "crypto" attribute in the capability set,attribute, which provides the keying material for SRTP using SDP security descriptions [SDES]. The second "capar" attribute provides the "rtpmap" for the dynamic payload type 96, which is mapped to the iLBC codec. The"a=pcfg" attribute provides the potential configurationsconfiguration included in the offer by reference to the capability declarations. Twoparameters. One alternatives are provided; the first one, and hence the preferred oneis using media capabilitiesprovided; it has a preference of 1 and 2, i.e. PCMU and G.729, or media capability 3, i.e. iLBC. Furthermore,it consists of transport protocol capability 1 (i.e. the RTP/SAVP profile - secure RTP), and the attribute capability parameter1, i.e. the crypto attribute provided, and the attribute capability parameter 2, i.e. the rtpmap for iLBC is included. The second one is simply using media capabilities 1 and 2, i.e. PCMUprovided. Potential configurations are always preferred over actual configurations, and G.729, or media capability 3, i.e. iLBC under the RTP/AVP profile as listed in the "m=" line. The "capar" parameterhence Alice is still needed to provide the rtpmapexpressing a preference for iLBC.using secure RTP. Bob receives the SDP offer from Alice. Bob supports RTP, but not SRTP,SRTP and the SDP Capability Negotiation framework, and hence he accepts the (preferred) potential configuration for Secure RTP provided by Alice. Furthermore, Bob wants to use the iLBC codec and hence generates the following answer:Alice: v=0 o=- 24351 621814 IN IP4 18.104.22.168 s= c=IN IP4 22.214.171.124 t=0 0 m=audio 4567 RTP/AVP 96 a=rtpmap:96 iLBC/8000 a=cver:RTP/SAVP 0 18 a=crypto:1 AES_CM_128_HMAC_SHA1_80 inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4 a=acfg: m=3 a=2t=1 a=1 Bob includes the "a=cver" and"a=acfg" attribute in the answer to inform Alice that he based his answer on an offer containing the potential configuration with mediatransport protocol capability 31 and attribute capability 1 from the offer SDP (i.e. iLBC under the RTP/AVP profile) andthe attribute capability parameter 2, i.e.RTP/SAVP profile using the associated rtpmap. Note that in this particular example,keying material provided). Bob also includes his keying material in a crypto attribute. If Bob supported one or more extensions to the capability negotiation framework, he would have included those in the answer as well. Note that in this particular example, the answerer supported the capability negotiation extensions defined here, however had he not, hethe answerer would simply have processed the offer based onignored the offered PCMUnew attributes and G.729 codecs underaccepted the RTP/AVP profile only. Consequently,offer to use normal RTP. In that case, the following answer would have omitted the "a=cver" and "a=acfg" attribute line and chosen one or both of the PCMU and G.729 codecs instead.been generated instead: v=0 o=- 24351 621814 IN IP4 126.96.36.199 s= c=IN IP4 188.8.131.52 t=0 0 m=audio 4567 RTP/AVP 0 18 2.2. Version and Extension Indication Attributes In this section, we present the new attributes associated with indicating the SDP capability negotiation version and extensions supported and required. 2.2.1. SDP Capability Negotiation Version Attribute The SDP Capability Negotiation Version attribute ("a=cver") lists the version of the SDP Capability Negotiation supported by the entity that generated the SDP. The attribute is defined as follows: a=cver: <version> where <version> is a non-zero positive integer. White space is permitted, but not required, before <version>. The value of <version> defined by this document is 0 as illustrated by the following example: a=cver: 0 The SDP Capability Negotiation version attribute MUST be present in each SDP that uses the SDP Capability negotiation solution defined in this document. The attribute can be provided at either the session- or media-level, however there MUST NOT be more than one occurrence of it. Furthermore, the attribute SHOULD be the first of the SDP capability negotiation attributes provided. 2.2.2.Supported Capability Negotiation Extensions Attribute The SDP Capability negotiation solution allows for capability negotiation extensions to be defined. Associated with each such extension is an option tag that identifies the extension in question. Option-tags MUST be registered with IANA per the procedures defined in Section 5. The Supported Capability Negotiation Extensions attribute ("a=csup") contains a comma-separated list of option tags identifying the SDP Capability negotiation extensions supported by the entity that thatgenerated the SDP. The attribute is defined as follows: a=csup: <option-tag-list> where <option-tag-list> is defined by the following ABNF: option-tag-list = option-tag *(COMMA option-tag) option-tag = token ; defined in [SDP] COMMA = *WSP "," *WSP ; defined in [RFC4234] White-space is permitted before the <option-tag-list>. Implementers familiar with SIP should note that the above definition of COMMA differs from the one in [RFC3261]. [EDITOR'S NOTE: There's nothing specific toA special base option tag with a value of "v0" is defined for the basic SDP Capability Negotiation Solution for this parameter. Should consider generalizing and/or providingcapability negotiation framework specified in a separate document.] The following examples illustrates thethis document. Entities can use this option tag with the "a=csup" attribute to indicate support for the SDP capability negotiation framework specified in this document. The following examples illustrates the use of the "a=csup" attribute with two hypothetical option tags, "foo" and "bar": a=csup: foo a=csup: bar a=csup: foo, bar The "a=csup" attribute can be provided at the session and the media- level. When provided at the session-level, it applies to the entire SDP. When provided at the media-level, it applies to the media-stream in question only.only (option-tags provided at the session level apply as well). There can be one or more "a=csup" attributes at both the session and media-level (one or more per media stream in the latter case). Whenever an entity that supports one or more extensions to the SDP Capability Negotiation framework generates an SDP, it SHOULD include the "a=csup" attribute with the option tags for the extensions it supports. 2.2.3.supports at the session and/or media-level, unless those option tags are already provided in one or more "a=creq" attribute (see Section 2.2.2. ) at the relevant levels. The base option tag MAY be included. 2.2.2. Required Capability Negotiation Extension Attribute The SDP Capability negotiation solution allows for capability negotiation extensions to be defined. Associated with each such extension is an option tag that identifies the extension in question. Option-tags MUST be registered with IANA per the procedures defined in Section 5. The Required Capability Negotiation Extensions attribute ("a=csup") contains a comma-separated list of option tags identifying the SDP Capability negotiation extensions that MUST be supported by the entity receiving the SDP in order for that entity to properly process the SDP Capability negotiation. The attribute is defined as follows: a=creq: <option-tag-list> where <option-tag-list> is defined in Section 184.108.40.206.2.1. White-space is permitted before the <option-tag-list>. [EDITOR'S NOTE: There's nothing specific to the SDP Capability Negotiation Solution for this parameter. Should consider generalizing and/or providing in a separate document.]The following examples illustratesillustrate the use of the "a=creq" attribute with two hypothetical option tags, "foo" and "bar": a=creq: foo a=creq: bar a=creq: foo, bar The "a=creq" attribute can be provided at the session and the media- level. When provided at the session-level, it applies to the entire SDP. When provided at the media-level, it applies to the media-stream in question only.only (required option tags provided at the session level apply as well). There can be one or more "a=creq" attributes at both the session and media-level (one or more per media stream in the latter case). WheneverWhen an entity generates an SDP and it requires the recipient of that SDP to support one or more SDP capability negotiation extensions in order to properly process the SDP Capability negotiation, the "a=creq" attribute MUST be included with option-tags that identify the required extensions.extensions at the session and/or media level, unless it is already known that the receiving entity supports those option-tags at the relevant levels (in which case their inclusion is OPTIONAL). An example of this is when generating an answer to an offer. If the answerer supports the required option-tags from the offer, and the answerer does not require any additional option-tags beyond what was listed in either the required ("creq") or supported ("csup") attributes from the offer, then the answerer is not required to include a required ("creq") attribute with any option-tags that may need to be supported (such as the base option tag - "v0"). A recipient that receives suchan SDP and does not support one or more of the required extensions,extensions listed in a "creq" attribute, MUST NOT perform the SDP capability negotiation defined in this document. For non-supported extensions provided at the session-level, this implies that SDP capability negotiation MUST NOT be performed at all. For non-supported extensions at the media-level, this implies that SDP capability negotiation MSUTMUST NOT be performed for the media stream in question. When an entity does not support one or more required SDP capability negotiation extensions, the entity SHOULD proceed as if the SDP capability negotiation attributes were not included in the first place.place, i.e. all the capability negotiation attributes should be ignored. This ensures that introduction of the SDP capability negotiation mechanism does not introduce any new failure scenarios. The above rules apply to the base option tag as well. Thus, entities compliant to this specification MUST include a "creq" attribute (at least in an offer) that includes the option tag "v0" as illustrated below: a=creq: v0 2.3. Capability Attributes In this section, we present the new attributes associated with indicating the capabilities for use by the SDP Capability negotiation. 2.3.1. Media Type and FormatAttribute Capability Attribute Media types and media formatsAttributes can be expressed as capabilities by use of the "a=cmed" attribute, which is defined as follows: a=cmed: <med-cap-num> <media> [<proto> <fmt list>] where <med-cap-num> is an integer between 1 and 2^32-1 (both included) used to number the media capabilities, and <media>, <proto>, and <fmt list> are defined as in the SDP "m=" line. The <fmt list> may contain multiple media formats. In that case, the media format capability number associated with the first one provided is the value of <med-cap-num>, the number associated with the second one is one higher, etc. Each occurrence of the attribute MUST use a different value of <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 session description (explicitly or implicitly). When <proto> and <fmt list> are omitted, the media capability merely indicates support for the <media> type in question, without any details as to what kind of transport protocol and media formats are supported. This can for example be used to indicate support for additional types of media than those included as actual configurations in an offer or answer. 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 2.4.1. 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 - see Section 2.4.1. [EDITOR'S NOTE: This is clear as mud. If a media capability applies to a media-stream only, then why can it still be referenced and hence used as capabilities in other media streams (by the "a=pcfg") attribute. The motivation is message size efficiency, but the means are not clean. Session versus media-level syntax and semantics need further consideration] The <proto> parameter indicates the default transport protocol associated with the media capability. As described in [RFC4566], the value of the <proto> parameter guides the interpretation of the <fmt list>, which is why it is included here. Note that <proto> is also a capability that can be negotiated separately (see Section 2.3.3. The <fmt list> contains one or more media formats supported, the interpretation of which depends on the value of <proto>. The rules that apply to "m=" lines (as defined in [SDP]) for interpretation of these apply here as well. For RTP-based transports, this implies that the <fmt list> contains one or more RTP payload type numbers. When those payload type numbers are dynamic, SDP requires an "a=rtpmap" attribute to determine the actual codec. In the case of SDP capability negotiation, such additional attribute parameters MUST be provided in conjunction with the media capability. There are two different cases to consider for this: o The media capability is provided at the session level: In this case, the required parameters MUST be provided in one or more attribute parameter capabilities (see Section 2.3.2. listed before the first "m=" line as well as before any other media capability attributes ("a=cmed"). o The media capability is provided at the media stream level: In this case, when the payload type numbers are part of the "m=" line itself, this is done by use of the "a=rtpmap" attribute as usual. In all other cases, the required parameters MUST be provided in one or more attribute parameter capabilities (see Section 2.3.2. within the media stream description (i.e. before the next "m=" line). [EDITOR'S NOTE: The above assumes that intermediaries will not reorder session-level attributes. It would be safer to explicitly link the two, but that will require yet another attribute. Also, it sends us down the path of building more complicated capabilities (made up of multiple parameters). Another issue here is that payload type numbers, which really have only media-level scope, are ill-suited to be used at the session-level or across multiple media streams (as is being done here). However, an alternative (and more proper) solution seems to involve significantly more work and deviations from the current SDP framework. This in turn makes it more difficult to automatically use new media types, formats, protocols, etc. defined elsewhere within this framework, and that is a major disadvantage. Another option is to forgo the session- level media capabilities as well as the ability to reference across media streams - it will make the solution less efficient though and difficult to express latent capabilities for media streams not included in the offer or answer.] The following example illustrates the first case above: v=0 o=- 25678 753849 IN IP4 220.127.116.11 s= c=IN IP4 18.104.22.168 t=0 0 a=cmed: 1 audio RTP/AVP 96 a=capar: 1 a=rtpmap:96 G729/8000 m=audio... The following example illustrates the second case above: v=0 o=- 25678 753849 IN IP4 22.214.171.124 s= c=IN IP4 126.96.36.199 t=0 0 m=audio 2345 RTP/AVP 96 a=rtpmap:96 G729/8000 a=cver:0 a=cmed: 1 audio RTP/AVP 96 97 a=capar: 1 a=rtpmap:97 iLBC/8000 Note: Readers familiar with RFC 3407 may notice the similarity between the "cmed" attribute defined above and the "cdsc" attribute defined in RFC 3407. There are however a couple of important differences, namely an increase in the capability numbering space as well as a relaxation of certain requirements found in RFC 3407. To simplify overall operation, the "cmed" parameter is limited to media-level operation only as well. 2.3.2. Attribute Parameter Capability Attribute Attributes can be expressed as negotiable parametersnegotiable parameters by use of a new attribute parametercapability attribute ("a=capar"), which is defined as follows: a=capar: <att-cap-num> <att-par> where <att-cap-num> is an integer between 1 and 2^32-1 (both included) used to number the attribute parametercapability and <att- par><att-par> is an attribute ("a=") in its full '<type>=<value>' form (see [SDP]). The "capar" attribute can be provided at the session level for session-level attributes and the media level.level for media-level attributes. The "capar" attribute MUST NOT be used to provide a media-level attribute at the session-level or vice versa. Each occurrence of the "capar" attribute in the entire session description MUST use a different value of <app-cap-num>. There is a need to be able to reference both session-level and media-level attributes in potential configurations at the media level, and this provides for a simple solution to avoiding overlap between the handle references. The <att-cap-num> values provided are independent of similar <cap-num><cap- num> values provided for other attributes, i.e., they form a separate name-space for attribute parameter capabilities. Attribute parameter capabilities are generally used for two things. First of all, they may be necessary to interpret a media format capability (e.g. by including an rtpmap), or they may provide attribute value parameters that are referenced in potential configurations (see Section 2.4.1. )The following examples illustrate use of the "capar" attribute: a=capar: 1 a=ptime:20 a=capar: 2 a=ptime:30 a=capar: 3 a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyONQ6gAA AAAGEEoo2pee4hp2UaDX8ZE22YwKAAAPZG9uYWxkQGR1Y2suY29tAQAAAAAAAQAk0 JKpgaVkDaawi9whVBtBt0KZ14ymNuu62+Nv3ozPLygwK/GbAV9iemnGUIZ19fWQUO SrzKTAv9zV a=capar: 4 a=crypto:1 AES_CM_128_HMAC_SHA1_32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 The first two provide attribute values for the ptime attribute. The third one provides SRTP parameters by using MIKEY with the key-mgmt attribute [KMGMT]. The fourth one provides SRTP parameters by use of security descriptions with the crypto attribute [SDES]. Readers familiar with RFC 3407 may notice the similarity between the RFC 3407 "cpar" attribute and the above. There are however a couple of important differences, most notably that the "capar" attribute contains a handle that enables referencing it and it furthermore supports attributes only (the "cpar" attribute defined in RFC 3407 supports bandwidth information as well). The "capar" attribute also is not automatically associated with any particular capabilities. 188.8.131.52.3.2. Transport Protocol Capability Attribute Transport Protocols can be expressed as capabilities by use of a new Transport Protocol Capability attribute ("a=ctrpr") defined as follows: a=ctrpr: <trpr-cap-num> <proto-list> where <trpr-cap-num> is an integer between 1 and 2552^32-1 (both included) used to number the transport address capability for later reference, and <proto-list> is one or more <proto>, separated by white space, as defined in the SDP "m=" line. The "ctrpr" attribute can be provided at the session- and media- level. Each occurrence of the "ctrpr" attribute in the entire session description MUST use a different value of <trpr-cap-num>. When multiple <proto> values are provided, the first one is associated with the value <trpr-cap-num>, the second one with the value one higher, etc. The <trpr-cap-num> values provided are independent of similar <cap-num> values provided for other attributes, i.e., they form a separate name-space for transport protocol capabilities. Below, we provide examples of the "a=ctrpr" attribute: a=ctrpr: 1 RTP/AVP a=ctrpr: 2 RTP/AVPF a=ctrpr: 3 RTP/SAVP RTP/SAVPF The first one provides a capability for the "RTP/AVP" profile defined in [RFC3551] and the second one provides a capability for the RTP with RTCP-Based Feedback profile defined in [AVPF]. The third one provides capabilities for the "RTP/SAVP" and "RTP/SAVPF" profiles. Note that the "cmed" attribute provides a similar functionality by including <proto>, however having this as a separate capability indication can provide significant message size reduction when negotiating alternative profiles (of which there can be many). In particular, there is no need to repeat supported payload types. Also, use of this attribute combined with the potential configuration attribute (see Section 2.4. )a capability for the RTP with RTCP-Based Feedback profile defined in [AVPF]. The third one provides capabilities for more expressive power.the "RTP/SAVP" and "RTP/SAVPF" profiles. 2.4. Configuration Attributes 2.4.1. Potential Configuration Attribute Potential Configurations can be expressed by use of a new Potential Configuration Attribute ("a=pcfg") defined as follows: a=pcfg: <preference> <pot-cfg-list> where <preference> is an integer between 1 and 2^32-1 (both included) and <pot-cfg-list> is defined as pot-cfg-list = pot-config *(1*WSP pot-config) pot-config = pot-media-config |pot-attribute-parameter-config | pot-transport-protocol-config | pot-extension-config The potential configuration attribute includes a preference indication (lowest number is most preferred) followed by one or more setsof potential media configurations,attribute parameter configurationsconfiguration and transport protocol configurations.configuration. Each of these MUST NOT be present more than once in a particular potential configuration attribute. Potential extension configurations can be included as well. There can be more than one potential extension configuration, however each particular potential extension configuration MUST NOT be present more than once in a given potential configuration attribute. Together, these values define a set ofpotential configurations.configuration. There can be one or moremultiple potential configuration attributesconfigurations provided at the session-level as well as the media-level. The semantics for each media stream. The attributes are provided in orderof preference. [EDITOR'S NOTE: We run into another issue with session-level media capabilities here. In the offer/answer model,these levels differ. A potential configurationsconfiguration at the media-level constitute alternative offers,session level provides a set of latent capabilities. A latent capability is merely an indication that the potential configuration could be supported, however it does not represent a willingness to do so at the session-level, that would/shouldcurrent time. A potential configuration at the media level on the other hand indicates not beonly a willingness, but in fact a desire to use the potential configuration. In the case for media capabilities. Other parameters however may be used as alternative offersof offer/answer, this implies that a potential configuration at the session level (e.g. key-mgmt attributesdoes not constitute an alternative offer whereas it does at the session level)] pot-media-config is defined by the following ABNF: pot-media-config = "m=" med-cap-list *(BAR med-cap-list) med-cap-list = med-cap-num *(COMMA med-cap-num) med-cap-num = 1*DIGIT ; defined in [RFC4234] BAR = *WSP "|" *WSP ; defined in [RFC4234] Each potentialmedia level. Associated with each potential configuration is a comma-separated list of media capability numbers where med-cap-num refers to media capability numbers and hence MUST bepreference indication, which is an integer between 1 and 2^32-1 (both included). Alternativeincluded) to indicate the relative preference of potential media configurations are separated by a vertical bar ("|").configurations. The alternatives are ordered by preference. Whenscope of the preference (and in fact each occurrence of a potential configuration attribute) is the session-level, when provided there, or the particular media stream it is provided at. Attribute capabilities are notincluded in a potential configuration at the media level, the media type and media format fromby use of the associated "m=" line will be used.pot-attribute-parameter-config parameter, which is defined by the following ABNF: pot-attribute-parameter-config = "a=" capar-cap-list *(BAR capar-cap-list) capar-cap-list = att-cap-num *(COMMA att-cap-num) att-cap-num = 1*DIGIT ;defined in [RFC4234] Each potential attribute parameter configuration list is a comma- separated list of attribute capability parameternumbers where att- cap-numatt-cap-num refers to attribute parametercapability numbers defined above and hence MUST be between 1 and 2^32-1 (both included). Alternative potential attribute parameter configurations are separated by a vertical bar ("|").("|"), the scope of which extends to the next alternative (i.e. "," has higher precedence than "|"). The alternatives are ordered by preference. Transport protocol capabilities are included in a potential configuration by use of the pot-transport-protocol-config parameter, which is defined by the following ABNF: pot-transport-protocol-config = "p=""t=" trpr-cap-num *(BAR trpr-cap-num) trpr-cap-num = 1*DIGIT ; defined in [RFC4234] The trpr-cap-num refers to transport protocol capability numbers defined above and hence MUST be between 1 and 2^32-1 (both included). Alternative potential transport protocol configurations are separated by a vertical bar ("|"). The alternatives are ordered by preference. When transport protocol capabilities are not included in a potential configuration,configuration at the media level, the transport protocol information from an included potential media configurationthe associated "m=" line will be used. If a potential media configuration is not included,At the session-level, lack of a transport protocol from the media description ("m=" line) will be used instead. pot-extension-configcapability indication simply implies that no such information is defined byprovided. Extension capabilities can be included in a potential configuration as well. Such extensions MUST adhere to the following ABNF: pot-extension-config=pot-extension-config = ext-cap-name "=" ext-cap-list *(BAR ext-cap-list) ext-cap-name = token ; defined in [SDP] ext-cap-list = ext-cap-num *(COMMA ext-cap-num) ext-cap-num = 1*DIGIT ; defined in [RFC4234] The ext-cap-name refers to the type of extension capability and the ext-cap-num refers to a capability number associated with that particular type of extension capability. The number MUST be between 1 and 2^32-1 (both included). Alternative potential extension configurations for a particular extension are separated by a vertical bar ("|").("|"),the scope of which extends to the next alternative (i.e. "," has higher precedence than "|"). Unsupported or unknown potential extension configs MUST be ignored, unless an option tag showing the extension as beingignored. The "creq" attribute and its associated rules can be used to ensure that required was included (see Section 2.2.3.extensions are supported in the first place. Potential configurations can be provided at the session level and the media level and in either case, it is syntactically possible to reference attribute capabilities provided at either the session or the media level. There are however semantic rules and limitations associated with this: At the session-level, a potential configuration MUST NOT reference any attribute capabilities provided at the media- level. The converse however is permitted, i.e. a media-level potential configuration can reference a session-level attribute capability. The semantics of doing so (should that potential configuration ("a=pcfg")be chosen), is that the corresponding attribute can(provided within that attribute capability) will be considered part of the active configuration at the *session* level. In other words, it will be as-if that attribute was simply provided at the sessionsession- level andin the media-level. Each occurrence offirst place. Note that individual media streams perform capability negotiation individually, and hence it is possible that another media stream (where the attribute within a given media description ("m=" line) defines a setwas part of a potential configurationsconfiguration) chose a configuration without that can be used forsession level attribute. The session-level attribute however remains "active" and hence applies to the entire session. It is up to the entity that media description. TO DO: Needgenerates the SDP to decide on relationshipensure that the resulting active configuration SDP is still meaningful. [EDITOR'S NOTE: There are too many subtle differences between session-levelpotential configurations at the session and media-level (how should conflicts, overlap, etc. be handled - simplicitymedia level. I'm inclined to have two similarly looking (but different) attributes instead as that will make it more straightforward and intuitive. That also leaves the door open to have more than latent capabilities at the possible expense of expressive power is preferablesession level in the editor's opinion).case that is needed later] Below, we provide an example of the "a=pcfg" attribute in a complete media description in order to properly indicate the supporting attributes: v=0 o=- 25678 753849 IN IP4 184.108.40.206 s= c=IN IP4 220.127.116.11 t=0 0 m=audio 3456 RTP/SAVPFRTP/AVPF 0 18 a=crypto:1a=creq: v0 a=capar:1 crypto:1 AES_CM_128_HMAC_SHA1_32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 a=cver: 0 a=cmed: 1 audio RTP/SAVP 0 4 18a=ctrpr: 1 RTP/AVPRTP/AVPF RTP/AVP a=ctrpr: 3 RTP/SAVPRTP/SAVPF a=pcfg: m=1|3 p=1|2|3|4 a=pcfg: m=2 p=1RTP/SAVP a=pcfg:1 t=3|4 a=1 a=pcfg:8 t=1|2 We have two potential configurations listed here. The first one (and most preferred, since it's preference is "1") indicates that PCMU (payload type number 0 referenced by media capability number 1) or G.729 (payload type number 18 referenced by media capability number 3) can be supported witheither of the profiles RTP/AVP, RTP/AVPF, RTP/SAVP, orRTP/SAVPF or RTP/SAVP (specified by the transport protocol capability numbers 1, 2,3 and 4).4) can be supported with attribute capability 1 (the "crypto" attribute); RTP/SAVPF is preferred since it is listed first. The second potential configuration indicates that G.723 (payload type number 4 referenced by media capability number 2)the RTP/AVPF of RTP/AVP profile can be supportedused, with RTP/AVPF being the RTP/AVP profile only (transport protocol capability number 1).preferred one. This non secure RTP alternative is the less preferred one since it's preference is "8". 2.4.2. Actual Configuration Attribute The actual configuration attribute identifies which of the potential configurations from an offer SDP were used as actual configurations in an answer SDP. This is done by reference to the media capabilities,attribute parametercapabilities and transport protocol capabilities from the offer that were actually used by the answerer in his offer/answer procedure. If extension capabilities were used, those will be included by reference as well. The Actual Configuration Attribute ("a=acfg") is defined as follows: a=acfg: <act-cfg-list> where <act-cfg-list> is defined as act-cfg-list = capability *(1*WSP capability) capability = act-media-config |act-attribute-parameter-config | act-transport-protocol-config | act-extension-config act-media-config is defined by the following ABNF: act-media-config = "m=" med-cap-list where med-cap-list is as defined in Section 2.4.1.act-attribute-parameter-config is defined by the following ABNF: act-attribute-parameter-config = "a=" capar-cap-list where capar-cap-list is as defined in Section 2.4.1. act-transport-protocol-config is defined by the following ABNF: act-transport-protocol-config = "p=""t=" trpr-cap-num where trpr-cap-num is as defined in Section 2.4.1. trpr-cap-num = 1*3DIGIT ; defined in [RFC4234] act-extension-config is defined by the following ABNF: act-extension-config = ext-cap-name "=" ext-cap-list where ext-cap-name and ext-cap-list are as defined in Section 2.4.1. The actual configuration ("a=acfg") attribute can be provided at the session-level and the media-level.media-level only. There MUST NOT be more than one occurrence of an actual configuration attribute at the session level, and there MUST NOT be more than one occurrence of an actual configuration attributewithin a given media description. Below, we provide an example of the "a=acfg" attribute (building on the previous example with the potential configuration attribute): v=0 o=- 24351 621814 IN IP4 18.104.22.168 s= c=IN IP4 22.214.171.124 t=0 0 m=audio 4567 RTP/AVPFRTP/SAVPF 0 a=cver:a=creq: 0 a=acfg: m=1 p=2t=3 a=1 It indicates that the answerer used an offer consisting of media capability 1 from the offer (PCMU) andtransport protocol capability 2 from the offer (RTP/AVPF).(RTP/SAVPF) and attribute capability 1 (the "crypto" attribute. 2.5. Offer/Answer Model Extensions In this section, we define extensions to the offer/answer model defined in [RFC3264] to allow for potential configurations to be included in an offer, where they constitute offers that may be accepted by the answerer instead of the actual configuration(s) included in the "m=" line(s). [EDITOR'S NOTE: Multicast considerations have been omitted for now.] TO DO: Elaborate and firm up offer/answer procedures. 2.5.1. Generating the Initial Offer An offerer that wants to use the SDP capability negotiation extensions defined in this document MUST include the following in the offer: o an SDP capability negotiation versionrequired extensions attribute with("a- creq") that contains the version set to 0 o oneoption tag "v0". It must either be provided at the session-level or more media capabilities (as defined in Section 2.3.1. ), if alternative media types andfor each individual media formats are tostream. Option tags for any other required extensions MUST be indicatedincluded as offerer capabilities or be negotiated.well (in accordance with Section 2.2.2. ) o one or more attribute parametercapability attributes (as defined in Section 126.96.36.199.3.1. ) if alternative attribute parameter values are to be indicated as offerer capabilities or be negotiated. o one or more transport protocol capability attributes (as defined in Section 188.8.131.52.3.2. ) if alternative transport protocols are to be to be indicated as offerer capabilities or be negotiated. o one or more potential configuration attributes (as defined in Section 2.4. ) if alternative potential configurations are to be negotiated. o one or more required capability negotiation extension attributes (as defined in Section 184.108.40.206.2.2. ), if the answerer is required to support one or more SDP capability negotiation extensions. The offerer SHOULD furthermore include the following: o one or more supported capability negotiation extension attributes (as("a=csup" as defined in Section 220.127.116.11.2.1. ), if the offerer supports one or more SDP capability negotiation extensions.extensions that have not been included in one or more "a=creq" attributes at the relevant session and media level(s). The capabilities provided merely indicate what the offerer is capable of doing. They do not constitute a commitment or even an indication to actually use them. This applies to potential configurations listed at the session level as well. Conversely, each of the potential configurations listed at the media level constitutes an alternative offer which may be used to negotiate and establish the session. [EDITOR'S NOTE: This is only partially true for potential configurations listed at the session level. The only thing we want to offer up as alternative offers at the session level is attributes - not media types or media formats, which should be capabilities only at the session level]The current actual configuration is included in the "m=" line (as defined by [RFC3264]). 2.5.2. Generating the Answer When the answerer receives an offer with valid SDP capability negotiation information in it and in particular with one or more valid potential configuration information attributes present, it may use any of the potential configurations as an alternative offer. A potential configuration information attribute is valid if all of the capabilities (media, attribute(attribute capabilities, transport protocol capabilities and any extension capabilities) it references are present and valid themselves. The actual configuration is contained in the media description's "m=" line. The answerer can send media to the offerer in accordance with the actual configuration, however if it chooses to use one of the alternative potential configurations, media sent to the offerer may be discarded by the offerer until the answer is received. If the answerer chooses to accept one of the alternative potential configurations instead of the actual configuration, the answerer MUST generate an answer as if the offer contained that potential configuration instead of the actual configuration included. The answerer MUST also include an actual configuration attribute in the answer that identifies the potential configuration from the offer used by the answerer. The actual configuration attribute in the answer MUST include information about the media capabilities,attribute capability parameters,capabilities, transport protocol parameters, and extension capabilities from the potential configuration that were used to generate the answer. 2.5.3. Offerer Processing of the Answer When the offerer included potential configurations for a media stream, it MUST examine the answer for the presence of an actual configuration attribute for each such media stream. If the attribute is missing, offerer processing of the answer MUST proceed as defined by [RFC3264]. If the attribute is present, processing continues as follows: The actual configuration attribute specifies which of the potential configurations were used by the answerer to generate the answer. This includes all the types of capabilities from the potential configuration offered, i.e. the media formats ("cmed" capabilities),attribute capability parameters ("capar"),capabilities ("a=capar"), transport protocol capabilities ("ctrpr"),("a=ctrpr"), and any extension capability parameters included. The offerer MUST now process the answer as if the offer had contained the potential configuration as the actual configuration in the media description ("m=" line) and relevant attributes in the offer. 2.5.4. Modifying the Session Potential configurations may be included in subsequent offers as defined in [RFC3264, Section 8]. The procedure for doing so is similar to that described above with the answer including an indication of the actual configuration used by the answerer. 3. Examples In this section, we provide examples showing how to use the SDP Capability Negotiation. 3.1. Best-Effort Secure RTP The following example illustrates how to use the SDP Capability negotiation extensions to support so-called Best-Effort Secure RTP. In that scenario, the offerer supports both RTP and Secure RTP. If the answerer does not support secure RTP (or the SDP capability negotiation extensions), an RTP session will be established. However, if the answerer supports Secure RTP and the SDP Capability Negotiation extensions, a Secure RTP session will be established. The best-effort Secure RTP negotiation 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 18.104.22.168 s= c=IN IP4 22.214.171.124 t=0 0 m=audio 3456 RTP/AVP 0 18 a=cver:0a=creq: v0 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=pcfg: p=1a=pcfg:5 t=1 a=1 a=pcfg: p=2a=pcfg:10 t=2 The "m=" line indicates that Alice is offering to use plain RTP with PCMU or G.729. The capability declarationAlice indicates that support for the base protocol defined here is required by including the "a=creq" attribute containing the value "v0". The capabilities are provided by the "a=cver","a=ctrpr" and "a=capar" attributes. The capabilities indicate that both Secure RTP and normal RTP are supported. The "capar" attribute provides a capability parameter with a handle of 1. The capability parameter is a "crypto" attribute in the capability set, which provides the keying material for SRTP using SDP security descriptions [SDES]. The "a=pcfg" attribute provides the potential configurations included in the offer by reference to the capabilities. Two alternatives are provided; the first one, andone with preference "5" (and hence the preferredpreferred one since the preference on the second one is "10") is transport protocol capability 1 (RTP/SAVP, i.e. secure RTP) together with the attribute capability parameter1, i.e. the crypto attribute provided. The second one is using transport protocol capability 2. Since there are no media format capabilities included,Note that we could have omitted the media format parameters fromsecond potential configuration since it equals the media description itselfactual configuration (which is used.always the least preferred configuration). Bob receives the SDP offer from Alice. Bob supports SRTP and the SCPSDP Capability Negotiation extensions, and hence he accepts the potential configuration for Secure RTP provided by Alice: v=0 o=- 24351 621814 IN IP4 126.96.36.199 s= c=IN IP4 188.8.131.52 t=0 0 m=audio 4567 RTP/SAVP 0 18 a=crypto:1 AES_CM_128_HMAC_SHA1_80 inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:4 a=cver: 0a=csup: foo a=acfg: p=1t=1 a=1 Bob includes the "a=cver" and"a=acfg" attribute in the answer to inform Alice that he based his answer on an offer containing the potential configuration with transport protocol capability 1 and attribute parametercapability 1 from the offer SDP (i.e. the RTP/SAVP profile using the keying material provided). Bob also includes his keying material in a crypto attribute. Finally, Bob supports an SDP capability negotiation extension with the option tag "foo" and hence he includes the "a=csup" parameter containing value "foo" in the answer. Note that in this particular example, the answerer supported the capability extensions defined here, however had he not, the answerer would simply have ignored the new attributes and accepted the offer to use normal RTP. In that case, the following answer would have been generated instead: v=0 o=- 24351 621814 IN IP4 184.108.40.206 s= c=IN IP4 220.127.116.11 t=0 0 m=audio 4567 RTP/AVP 0 18 4. Security Considerations TBD. 5. IANA Considerations TBD. [EDITOR'S NOTE: Need to define registry and procedures for option tags] [EIDTOR'S NOTE: Need to define registry and procedures for extension capabilities] 6. To Do and Open Issues o Capability descriptions, potential configurations and actual configurations can be provided at both the session level and media level. It needs to be decided what the relationship between the session level and media level parameters are. o Look for "EDITOR'S NOTE" throughout the document. 7. Acknowledgments ThanksThis 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 in here: Roni Even, Robert Gilman, Cullen Jennings, Matt Lepinski, Joerg Ott, Colin Perkins, and Thomas Stach. Francois Audet and Dan Wing forprovided useful comments on earlier versions of this document. 8. Change Log 8.1. draft-ietf-mmusic-sdp-capability-negotiation-01 The following are the major changes compared to version -00: o Media capabilities are no longer considered a core capability and hence have been removed. This leaves transport protocols and attributes as the only capabilities defined by the core. o Version attribute has been removed and an option tag to indicate the actual version has been defined instead. o Clarified rules for session-level and media level attributes provided at either level as well how they can be used in potential configurations. o Potential configuration parameters no longer have implicit ordering; an explicit preference indicator is now included. o The parameter name for transport protocols in the potential and actual configuration attributes have been changed "p" to "t". o Clarified operator precedence within potential and actual configuration attributes. o Potential configurations at the session level now limited to indicate latent capability configurations. Consequently, an actual configuration attribute can no longer be provided at the session level. o Cleaned up capability and potential configuration terminology - they are now two clearly different things. 8.2. draft-ietf-mmusic-sdp-capability-negotiation-00 Version 00 is the initial version. The solution provided in this initial version is based on an earlier (individual submission) version of [SDPCapNeg]. The following are the major changes compared to that document: o Solution no longer based on RFC 3407, but defines a set of similar attributes (with some differences). o Various minor changes to the previously defined attributes. o Multiple transport capabilities can be included in a single "ctrpr" attribute o A version attribute is now included. o Extensions to the framework are formally supported. o Option tags and the ability to list supported and required extensions are supported. o A best-effort SRTP example use case has been added. o Some terminology change throughout to more clearly indicate what constitutes capabilities and what constitutes configurations. 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. [RFC2234] Crocker, D. and Overell, P.(Editors), "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, Internet Mail Consortium and Demon Internet Ltd., November 1997. [RFC3264] Rosenberg, J., and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002. [RFC3407] F. Andreasen, "Session Description Protocol (SDP) Simple Capability Declaration", RFC 3407, October 2002. [RFC3605] C. Huitema, "Real Time Control Protocol (RTCP) attribute in Session Description Protocol (SDP)", RFC 3605, October 2003. [RFC4234] Crocker, D., and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 4234, October 2005. [SDP] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006. 9.2. Informative References [RFC2046] Freed, N., and N. Borensteain, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046, November 1996. [RFC2327] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 2327, April 1998. [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. [RFC3388] Camarillo, G., Eriksson, G., Holler, J., and H. Schulzrinne, "Grouping of Media Lines in the Session Description Protocol (SDP)", RFC 3388, December 2002. [RFC3551] Schulzrinne, H., and S. Casner, "RTP Profile for Audio and Video Conferences with Minimal Control", RFC 3551, July 2003. [SRTP] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004. [RFC3851] B. Ramsdell, "Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.1 Message Specification", RFC 3851, July 2004. [RFC4091] Camarillo, G., and J. Rosenberg, The Alternative Network Address Types (ANAT) Semantics for the Session Description Protocol (SDP) Grouping Framework, RFC 4091, June 2005. [AVPF] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, "Extended RTP Profile for RTCP-Based Feedback (RTP/AVPF)", Work in Progress, August 2004. [I-D.jennings-sipping-multipart] Wing, D., and C. Jennings, "Session Initiation Protocol (SIP) Offer/Answer with Multipart Alternative", Work in Progress, March 2006. [SAVPF] Ott, J., and E Carrara, "Extended Secure RTP Profile for RTCP-based Feedback (RTP/SAVPF)", Work in Progress, December 2005. [SDES] Andreasen, F., Baugher, M., and D. Wing, "Session Description Protocol Security Descriptions for Media Streams", RFC 4568, July 2006. [SDPng] Kutscher, D., Ott, J., and C. Bormann, "Session Description and Capability Negotiation", Work in Progress, February 2005. [BESRTP] Kaplan, H., and F. Audet, "Session Description Protocol (SDP) Offer/Answer Negotiation for Best-Effort Secure Real- Time Transport Protocol, Work in progress, August 2006. [KMGMT] Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E. Carrara, "Key Management Extensions for Session Description Protocol (SDP) and Real Time Streaming Protocol (RTSP)", RFC 4567, July 2006. [SDPCapNegRqts] Andreasen, F. "SDP Capability Negotiation: Requirementes and Review of Existing Work", work in progress, December 2006. [SDPCapNeg] Andreasen, F. "SDP Capability Negotiation", work in progress, December 2006. [MIKEY] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, and K. Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830, August 2004. Author's Addresses Flemming Andreasen Cisco Systems Edison, NJ Email: email@example.com Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. 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