MMUSIC Working Group F. Andreasen Internet-Draft Cisco Systems Intended Status: Proposed Standard February 13, 2007 Expires: August 2007 SDP Capability Negotiation draft-ietf-mmusic-sdp-capability-negotiation-02.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This Internet-Draft will expire on August 13, 2007. Copyright Notice Copyright (C) The IETF Trust (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 Andreasen Expires August 13, 2007 [Page 1] Internet-Draft SDP Capability Negotiation February 2007 (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. Table of Contents 1. Introduction...................................................3 2. Conventions used in this document..............................5 3. SDP Capability Negotiation Solution............................6 3.1. Solution Overview.........................................6 3.2. Version and Extension Indication Attributes...............9 3.2.1. Supported Capability Negotiation Extensions Attribute9 3.2.2. Required Capability Negotiation Extension Attribute.10 3.3. Capability Attributes....................................12 3.3.1. Attribute Capability Attribute......................12 3.3.2. Transport Protocol Capability Attribute.............13 3.4. Configuration Attributes.................................15 3.4.1. Potential Configuration Attribute...................15 3.4.2. Actual Configuration Attribute......................18 3.5. Offer/Answer Model Extensions............................20 3.5.1. Generating the Initial Offer........................20 3.5.2. Generating the Answer...............................21 3.5.3. Offerer Processing of the Answer....................22 3.5.4. Modifying the Session...............................22 3.6. Interactions with ICE....................................23 3.7. Processing Media before Answer...........................24 4. Examples......................................................24 4.1. Best-Effort Secure RTP...................................24 4.2. Multiple Transport Protocols.............................27 4.3. Session-Level MIKEY and Media Level Security Descriptions30 4.4. Capability Negotiation with Interactive Connectivity Establishment.................................................30 5. Security Considerations.......................................30 6. IANA Considerations...........................................30 7. To Do and Open Issues.........................................30 8. Acknowledgments...............................................30 Andreasen Expires August 13, 2007 [Page 2] Internet-Draft SDP Capability Negotiation February 2007 9. Change Log....................................................31 9.1. draft-ietf-mmusic-sdp-capability-negotiation-02..........31 9.2. draft-ietf-mmusic-sdp-capability-negotiation-01..........31 9.3. draft-ietf-mmusic-sdp-capability-negotiation-00..........32 10. References...................................................34 10.1. Normative References....................................34 10.2. Informative References..................................34 Author's Addresses...............................................36 Intellectual Property Statement..................................36 Full.............................................................37 Copyright Statement..............................................37 Acknowledgment...................................................37 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 participant, 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 for the session and its associated media stream components, supported by each side. Andreasen Expires August 13, 2007 [Page 3] Internet-Draft SDP Capability Negotiation February 2007 o A set of potential 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 combinations of session parameters and media stream components to use and with what parameters. o A negotiation process that takes the set of potential configurations (combinations of capabilities) as input and provides the actual configurations as output. SDP by itself was designed to provide only one of these, 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 as well as 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 Andreasen Expires August 13, 2007 [Page 4] Internet-Draft SDP Capability Negotiation February 2007 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 limitations 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 is the Session Initiation Protocol (SIP) [RFC3261]. SIP uses the offer/answer model (which is not specific to SIP) to negotiate sessions and hence the mechanism defined here defines the offer/answer procedures to use for the capability negotiation framework. The rest of the document is structured as follows. In Section 3. we present our SDP capability negotiation solution, which consists of new SDP attributes and associated offer/answer procedures. In Section 4. we provide examples illustrating its use and in Section 5. we provide the security considerations. 2. 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]. Andreasen Expires August 13, 2007 [Page 5] Internet-Draft SDP Capability Negotiation February 2007 3. 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. 3.1. Solution Overview The solution consists of the following: o Two new attributes to support versioning and extensions to the framework itself as follows: o A new attribute ("a=csup") that lists the supported base 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 in order to do capability negotiation. o Two new attributes used to express capabilities as follows (additional attributes can be defined as extensions): o A new attribute ("a=acap") that defines how to list attribute parameter values ("a=" values) as capabilities. o A new attribute ("a=tcap") 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 capabilities from the SDP in question. Multiple potential configurations have an explicitly indicated ordering associated with them. Extension capabilities can be defined and referenced 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 be included as well. Andreasen Expires August 13, 2007 [Page 6] Internet-Draft SDP Capability Negotiation February 2007 o Extensions to the offer/answer model that allow for capabilities and potential configurations to be included in an offer. Capabilities can be provided at the session level or the media level. Potential configurations can be included at the media level only, where they constitute alternative 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=acfg") in the answer. Capabilities 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 (SRTP) | |<---------------------------------| | | Alice's offer includes RTP and SRTP as alternatives. RTP is the default (actual configuration), but SRTP is the preferred one (potential configuration): v=0 o=- 25678 753849 IN IP4 128.96.41.1 s= c=IN IP4 128.96.41.1 t=0 0 m=audio 3456 RTP/AVP 0 18 a=creq: v0 a=tcap:1 RTP/SAVP a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 a=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 includes the option tag "v0" to indicate that the base framework defined here must be supported. The capabilities are provided by the "a=tcap" and "a=acap" attributes. The transport capabilities ("a=tcap") indicate that secure RTP under the AVP profile ("RTP/SAVP") is supported with an associated Andreasen Expires August 13, 2007 [Page 7] Internet-Draft SDP Capability Negotiation February 2007 transport capability handle of 1. The "acap" attribute provides an attribute capability with a handle of 1. The attribute capability is a "crypto" attribute, which provides the keying material for SRTP using SDP security descriptions [SDES]. The "a=pcfg" attribute provides the potential configuration included in the offer by reference to the capability parameters. One alternative is provided; it has a configuration number of 1 and it consists of transport protocol capability 1 (i.e. the RTP/SAVP profile - secure RTP), and the attribute capability 1, i.e. the crypto attribute provided. Potential configurations are always preferred over actual configurations, and hence Alice is expressing a preference for using secure RTP. Bob receives the SDP offer from Alice. Bob supports SRTP and the SDP Capability Negotiation framework, and hence he accepts the (preferred) potential configuration for Secure RTP provided by Alice: v=0 o=- 24351 621814 IN IP4 128.96.41.2 s= c=IN IP4 128.96.41.2 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=acfg:1 t=1 a=1 Bob includes the "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 capability 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. If Bob supported one or more extensions to the capability negotiation framework, he would have included those in the answer as well (in an "a=csup" attribute). Note that in this particular example, the answerer supported the capability negotiation extensions defined here, however had he not, the answerer would simply have ignored the new attributes and accepted the (actual configuration) offer to use normal RTP. In that case, the following answer would have been generated instead: v=0 o=- 24351 621814 IN IP4 128.96.41.2 s= c=IN IP4 128.96.41.2 Andreasen Expires August 13, 2007 [Page 8] Internet-Draft SDP Capability Negotiation February 2007 t=0 0 m=audio 4567 RTP/AVP 0 18 3.2. Version and Extension Indication Attributes In this section, we present the new attributes associated with indicating the SDP capability negotiation extensions supported and required. 3.2.1. 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 6. 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 generated the SDP. The attribute is defined as follows: a=csup: RFC 4566, Section 9, provides the ABNF for SDP attributes. The "csup" attribute adheres to the RFC 4566 "attribute" production, with an att-value defined as follows: att-value = *WSP option-tag-list option-tag-list = option-tag *(COMMA option-tag) option-tag = token ; defined in [SDP] COMMA = *WSP "," *WSP ; defined in [RFC4234] Note that white-space is permitted before the option-tag-list. Also, implementers familiar with SIP should note that the above definition of COMMA differs from the one in [RFC3261]. A special base option tag with a value of "v0" is defined for the basic SDP capability negotiation framework. Entities 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 the "v0" option tags and two hypothetical option tags, "foo" and "bar": Andreasen Expires August 13, 2007 [Page 9] Internet-Draft SDP Capability Negotiation February 2007 a=csup: v0 a=csup: foo a=csup: bar a=csup: v0, 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 (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 at the session and/or media-level, unless those option tags are already provided in one or more "a=creq" attribute (see Section 3.2.2. ) at the relevant levels. The base option tag MAY be included. 3.2.2. 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 6. The Required Capability Negotiation Extensions attribute ("a=creq") 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: The "creq" attribute adheres to the RFC 4566 "attribute" production, with an att-value defined as follows: att-value = *WSP option-tag-list where "option-tag-list" is defined in Section 3.2.1. Note that white-space is permitted before the option-tag-list. The following examples illustrate the use of the "a=creq" attribute with the "v0" base option tag and two hypothetical option tags, "foo" and "bar": Andreasen Expires August 13, 2007 [Page 10] Internet-Draft SDP Capability Negotiation February 2007 a=creq: v0 a=creq: foo a=creq: bar a=creq: v0, 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 (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). When 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 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 ("a=creq") or supported ("a=csup") attributes from the offer, then the answerer is not required to include a required ("a=creq") attribute with any option-tags that may need to be supported (such as the base option tag - "v0"). A recipient that receives an SDP and does not support one or more of the required 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 MUST 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, i.e. all the capability negotiation attributes should be ignored. In that case, the entity SHOULD include a "csup" attribute listing the SDP capability negotiation extensions it actually supports. Andreasen Expires August 13, 2007 [Page 11] Internet-Draft SDP Capability Negotiation February 2007 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 3.3. Capability Attributes In this section, we present the new attributes associated with indicating the capabilities for use by the SDP Capability negotiation. 3.3.1. Attribute Capability Attribute Attributes can be expressed as negotiable parameters by use of a new attribute capability attribute ("a=acap"), which is defined as follows: a=acap: where is an integer between 1 and 2^31-1 (both included) used to number the attribute capability and is an attribute ("a=") in its full '=' form (see [SDP]). The "acap" attribute adheres to the RFC 4566 "attribute" production, with an att-value defined as follows: att-value = *WSP att-cap-num 1*WSP att-par att-cap-num = 1*DIGIT ;defined in [RFC4234] att-par = attribute ;defined in RFC 4266 Note that white-space is permitted before the att-cap-num. The "acap" attribute can be provided at the session level for session-level attributes and the media level for media-level attributes. The "acap" attribute MUST NOT be used to provide a media-level attribute at the session-level or vice versa. Each occurrence of the "acap" attribute in the entire session description MUST use a different value of . There is a need to be able to reference both session-level and media-level attributes in potential configurations at the media Andreasen Expires August 13, 2007 [Page 12] Internet-Draft SDP Capability Negotiation February 2007 level, and this provides for a simple solution to avoiding overlap between the references (handles) to each attribute capability. The values provided are independent of similar values provided for other capability attributes, i.e., they form a separate name-space for attribute capabilities. The following examples illustrate use of the "acap" attribute: a=acap: 1 a=ptime:20 a=acap: 2 a=ptime:30 a=acap: 3 a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyONQ6gAA AAAGEEoo2pee4hp2UaDX8ZE22YwKAAAPZG9uYWxkQGR1Y2suY29tAQAAAAAAAQAk0 JKpgaVkDaawi9whVBtBt0KZ14ymNuu62+Nv3ozPLygwK/GbAV9iemnGUIZ19fWQUO SrzKTAv9zV a=acap: 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 provides SRTP parameters by using MIKEY with the key-mgmt attribute [KMGMT]. The fourth provides SRTP parameters by use of security descriptions with the crypto attribute [SDES]. Note that the line-wrapping and new-lines in example three and four are provided for formatting reasons only - they are not permitted in actual SDP. 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 "acap" 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 "acap" attribute also is not automatically associated with any particular capabilities. 3.3.2. Transport Protocol Capability Attribute Transport Protocols can be expressed as capabilities by use of a new Transport Protocol Capability attribute ("a=tcap") defined as follows: a=tcap: where is an integer between 1 and 2^31-1 (both included) used to number the transport address capability for later Andreasen Expires August 13, 2007 [Page 13] Internet-Draft SDP Capability Negotiation February 2007 reference, and is one or more , separated by white space, as defined in the SDP "m=" line. The "tcap" attribute adheres to the RFC 4566 "attribute" production, with an att-value defined as follows: att-value = *WSP trpr-cap-num 1*WSP proto-list trpr-cap-num = 1*DIGIT ;defined in [RFC4234] proto-list = proto *(1*WSP proto) ; defined in RFC 4566 Note that white-space is permitted before the trpr-cap-num. The "tcap" attribute can be provided at the session- and media-level. Each occurrence of the "tcap" attribute in the entire session description MUST use a different value of . When multiple values are provided, the first one is associated with the value , the second one with the value one higher, etc. The values provided are independent of similar values provided for other capability attributes, i.e., they form a separate name-space for transport protocol capabilities. Below, we provide examples of the "a=tcap" attribute: a=tcap: 1 RTP/AVP a=tcap: 2 RTP/AVPF a=tcap: 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. Transport capabilities are inherently included in the "m=" line, however they still need to be specified explicitly in a "tcap" attribute, if they are to be used as a capability. This may seem redundant (and indeed it is from the offerer's point of view), however it is done to protect against middle-boxes that may modify "m=" lines while passing unknown attributes through. If an implicit capability were used instead (e.g. a reserved transport capability number could be used to refer to the transport protocol in the "m=" line), and a middle-box were to modify the transport protocol in the "m=" line (e.g. to translate between plain RTP and secure RTP), then the potential configuration referencing that implicit transport capability may no longer be correct. With explicit capabilities, we avoid this pitfall, although the potential configuration preference (see Section 3.4.1. ) may not reflect that of the middle-box (which some may view as a feature). Andreasen Expires August 13, 2007 [Page 14] Internet-Draft SDP Capability Negotiation February 2007 3.4. Configuration Attributes 3.4.1. Potential Configuration Attribute Potential Configurations can be expressed by use of a new Potential Configuration Attribute ("a=pcfg") defined as follows: a=pcfg: where is an integer between 1 and 2^31-1 (both included). The "pcfg" attribute adheres to the RFC 4566 "attribute" production, with an att-value defined as follows: att-value = *WSP config-number 1*WSP pot-cfg-list config-number = 1*DIGIT ;defined in [RFC4234] pot-cfg-list = pot-config *(1*WSP pot-config) pot-config = pot-attribute-parameter-config / pot-transport-protocol-config / pot-extension-config The missing productions are defined below. Note that white-space is permitted before the config-number. The potential configuration attribute can be provided at the media- level only. The attribute includes a configuration number, which is an integer between 1 and 2^31-1 (both included). The configuration number MUST be unique within the media stream. The configuration number also indicates the relative preference of potential configurations; lower numbers are preferred over higher numbers. After the configuration number, one or more potential configuration parameters MUST be provided. This document defines potential attribute parameter configurations and potential transport protocol configurations. Each of these MUST NOT be present more than once in a particular potential configuration attribute. Potential extension configurations can be included as well; unknown potential extension configurations MUST be ignored (if support is required, then the "a=creq" with a suitable option tag should be used). 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 potential configuration. Andreasen Expires August 13, 2007 [Page 15] Internet-Draft SDP Capability Negotiation February 2007 There can be multiple potential configurations provided within a media description. Each of these indicates not only a willingness, but in fact a desire to use the potential configuration. Attribute capabilities are included in a potential configuration by use of the pot-attribute-parameter-config parameter, which is defined by the following ABNF: pot-attribute-parameter-config = "a=" acap-cap-list *(BAR acap-cap-list) acap-cap-list = att-cap-num *(COMMA att-cap-num) att-cap-num = 1*DIGIT ;defined in [RFC4234] BAR = *WSP "|" *WSP ; defined in [RFC4234] Each potential attribute parameter configuration list is a comma- separated list of attribute capability numbers where att-cap-num refers to attribute capability numbers defined above and hence MUST be between 1 and 2^31-1 (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 with the most preferred listed first. 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 = "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 2^31-1 (both included). Alternative potential transport protocol configurations are separated by a vertical bar ("|"). The alternatives are ordered by preference with the most preferred listed first. When transport protocol capabilities are not included in a potential configuration at the media level, the transport protocol information from the associated "m=" line will be used. In the presence of middle-boxes (the existence of which may not be known), care should be taken with assuming that the transport protocol in the "m=" line will not be modified by a middle-box. Use of an explicit capability will guard against the capability indications of that. Andreasen Expires August 13, 2007 [Page 16] Internet-Draft SDP Capability Negotiation February 2007 Extension capabilities can be included in a potential configuration as well. Such extensions MUST adhere to the following ABNF: 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 2^31-1 (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. The "creq" attribute and its associated rules can be used to ensure that required extensions are supported in the first place. Potential configurations can be provided at the media level only, however it is possible to reference capabilities provided at either the session or media level. There are certain semantic rules and restrictions associated with this: A (media level) potential configuration in a given media description MUST NOT reference a media-level capability provided in a different media description; doing so invalidates that potential configuration. A potential configuration can however reference a session-level capability. The semantics of doing so (should that potential configuration be chosen), depends on the type of capability. In the case of transport capabilities, this has no particular implication. In the case of attribute capabilities however, it does. More specifically, the corresponding attribute value (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 with that value at the session- level in the first place. Note that individual media streams perform capability negotiation individually, and hence it is possible that another media stream (where the attribute was part of a potential configuration) chose a configuration without that session level attribute. The session-level attribute however remains "active" and hence applies to the entire session. It is up to the entity that generates the SDP to ensure that in such cases, the resulting active configuration SDP is still meaningful. Andreasen Expires August 13, 2007 [Page 17] Internet-Draft SDP Capability Negotiation February 2007 The session-level operation of extension capabilities is undefined: Consequently, if session-level extension capabilities are defined, they MUST specify the implication of making them part of an active configuration at the media level. 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 128.96.41.1 s= c=IN IP4 128.96.41.1 t=0 0 m=audio 3456 RTP/AVPF 0 18 a=creq: v0 a=acap:1 crypto:1 AES_CM_128_HMAC_SHA1_32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 a=tcap: 1 RTP/AVPF RTP/AVP a=tcap: 3 RTP/SAVP RTP/SAVPF a=pcfg:1 t=4|3 a=1 a=pcfg:8 t=1|2 We have two potential configurations listed here. The first one (and most preferred, since its configuration number is "1") indicates that either of the profiles RTP/SAVPF or RTP/SAVP (specified by the transport protocol capability numbers 4 and 3) can be supported with attribute capability 1 (the "crypto" attribute); RTP/SAVPF is preferred over RTP/SAVP since its capability number (4) is listed first in the preferred potential configuration. The second potential configuration indicates that the RTP/AVPF of RTP/AVP profile can be used, with RTP/AVPF being the preferred one. This non secure RTP alternative is the less preferred one since its configuration number is "8". 3.4.2. 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 configuration number and the attribute capabilities 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. Note that the configuration number and all capability numbers used are those from the offer; not the answer. Andreasen Expires August 13, 2007 [Page 18] Internet-Draft SDP Capability Negotiation February 2007 The Actual Configuration Attribute ("a=acfg") is defined as follows: a=acfg: The "acfg" attribute adheres to the RFC 4566 "attribute" production, with an att-value defined as follows: att-value = *WSP config-number 1*WSP act-cfg-list ;config-number defined in Section 3.4.1. act-cfg-list = capability *(1*WSP capability) capability = act-attribute-parameter-config / act-transport-protocol-config / act-extension-config act-attribute-parameter-config = "a=" acap-cap-list ; defined in Section 3.4.1. act-transport-protocol-config = "t=" trpr-cap-num ; defined in Section 3.4.1. act-extension-config = ext-cap-name "=" ext-cap-list ; defined in Section 3.4.1. Note that white-space is permitted before the config-number. The actual configuration ("a=acfg") attribute can be provided at the media-level only. There MUST NOT be more than one occurrence of an actual configuration attribute within 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 128.96.41.2 s= c=IN IP4 128.96.41.2 t=0 0 m=audio 4567 RTP/SAVPF 0 a=creq: 0 a=acfg:1 t=4 a=1 It indicates that the answerer used an offer consisting of potential configuration number 1 with transport protocol capability 4 from the offer (RTP/SAVPF) and attribute capability 1 (the "crypto" attribute). Andreasen Expires August 13, 2007 [Page 19] Internet-Draft SDP Capability Negotiation February 2007 3.5. 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. 3.5.1. 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 required extensions attribute ("a- creq") that contains the option tag "v0". It must either be provided at the session-level or for each individual media stream. Option tags for any other required extensions MUST be included as well (in accordance with Section 3.2.2. ) o one or more attribute capability attributes (as defined in Section 3.3.1. ) 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 3.3.2. ) 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 3.4. ) if alternative potential configurations are to be negotiated. o one or more required capability negotiation extension attributes (as defined in Section 3.2.2. ), if the answerer is required to support one or more SDP capability negotiation extensions. The offerer SHOULD furthermore include the following: Andreasen Expires August 13, 2007 [Page 20] Internet-Draft SDP Capability Negotiation February 2007 o one or more supported capability negotiation extension attributes ("a=csup" as defined in Section 3.2.1. ), if the offerer supports one or more SDP capability negotiation 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. The current actual configuration is included in the "m=" line (as defined by [RFC3264]). Per [RFC3264], once the offerer generates the offer, he must be prepared to receive incoming media in accordance with that offer. That rule applies here as well, but for the actual configurations only; media received by the offerer according to one of the potential configurations MAY be discarded, until the offerer receives an answer indicating what the actual configuration is. Once that answer is received, incoming media MUST be processed in accordance with the actual configuration indicated and the answer received. 3.5.2. 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 (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 Andreasen Expires August 13, 2007 [Page 21] Internet-Draft SDP Capability Negotiation February 2007 used by the answerer. The actual configuration attribute in the answer MUST include information about the attribute capabilities, transport protocol parameters, and extension capabilities from the potential configuration that were used to generate the answer. 3.5.3. 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 attribute capabilities ("a=acap"), transport protocol capabilities ("a=tcap"), 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. If the answerer selected one of the potential configurations from the offer as the actual configuration, then the offerer SHOULD perform another offer/answer exchange, where the offer contains the selected potential configuration as the actual configuration, i.e. with the actual configuration used in the "m=" line and any other relevant attributes. This second offer/answer exchange will not modify the session anyway, however it will help intermediaries that look at the SDP, but do not understand the capability negotiation extensions, to understand the details of the negotiated media streams. 3.5.4. 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. If the answer indicates use of a potential configuration from the offer, then a second offer/answer exchange using that potential configuration as the actual configuration SHOULD be performed. Andreasen Expires August 13, 2007 [Page 22] Internet-Draft SDP Capability Negotiation February 2007 3.6. Interactions with ICE Interactive Connectivity Establishment (ICE) [ICE] provides a mechanism for verifying connectivity between two endpoints by sending STUN messages directly between the media endpoints. The basic ICE specification [ICE] is defined to support UDP-based connectivity only, however it allows for extensions to support other transport protocols, such as TCP, which is being specified in [ICETCP]. ICE defines a new "a=candidate" attribute, which, among other things, indicates the possible transport protocol(s) to use and then associates a priority with each of them. The most preferred transport protocol that *successfully* verifies connectivity will end up being used. When using ICE, it is thus possible that the transport protocol that will be used differs from what is specified in the "m=" line. Furthermore, since both ICE and SDP Capability Negotiation may now specify alternative transport protocols, there is a potentially unintended interaction when using these together. We provide the following guidelines for addressing that. [EDITOR'S NOTE: This requires more work] There are two basic scenarios to consider here: 1) A particular media stream can run over different transport protocols (e.g. UDP, TCP, or TCP/TLS), and the intent is simply to use the one that works (in the preference order specified). 2) A particular media stream can run over different transport protocols (e.g. UDP, TCP, or TCP/TLS) and the intent is to have the negotiation process decide which one to use (e.g. T.38 over TCP or UDP). In scenario 1, there should be ICE "a=candidate" attributes for UDP, TCP, etc. but otherwise nothing special in the potential configuration attributes to indicate the desire to use different transport protocols (e.g. UDP, or TCP). The ICE procedures essentially cover the capability negotiation required (by having the answerer select something it supports and then use of trial and error). Scenario 2 does not require a need to support or use ICE. Instead, we simply use transport protocol capabilities and potential configuration attributes to indicate the desired outcome. Andreasen Expires August 13, 2007 [Page 23] Internet-Draft SDP Capability Negotiation February 2007 The scenarios may be combined, e.g. by offering potential configuration alternatives where some of them can support one transport protocol only (e.g. UDP), whereas others can support multiple transport protocols (e.g. UDP or TCP). In that case, the ICE candidate attributes should be defined as attribute capabilities and the relevant ones should then be included in the proper potential configurations (for example candidate attributes for UDP only for potential configurations that are restricted to UDP, whereas there could be candidate attributes for UDP, TCP, and TCP/TLS for potential configurations that can use all three). 3.7. Processing Media before Answer The offer/answer model requires an offerer to be able to receive media in accordance with the offer prior to receiving the answer. This property is retained with the SDP capability negotiation extensions defined here, but only when the actual configuration is selected by the answerer. If a potential configuration is chosen, it is permissible for the offerer to not process any media received before the answer is received. This however may lead to clipping. In the case of SIP, this issue could be solved easily by defining a precondition [RFC3312] for capability negotiation, however preconditions are viewed as complicated to implement and they add to overall session establishment delay by requiring an extra offer/answer exchange. An alternative is therefore desirable. The SDP capability negotiation framework does not define such an alternative, however extensions may do so. For example, one technique proposed for best-effort SRTP in [BESRTP] is to provide different RTP payload type mappings for different transport protocols used. The basic SDP capability negotiation framework defined here does not include the ability to do so, however extensions that enable that may be defined. 4. Examples In this section, we provide examples showing how to use the SDP Capability Negotiation. 4.1. 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, Andreasen Expires August 13, 2007 [Page 24] Internet-Draft SDP Capability Negotiation February 2007 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 (SRTP) | |<---------------------------------| | | | (3) Offer (SRTP) | |--------------------------------->| | | | (4) Answer (SRTP) | |<---------------------------------| | | 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 128.96.41.1 s= c=IN IP4 128.96.41.1 t=0 0 m=audio 3456 RTP/AVP 0 18 a=creq: v0 a=tcap:1 RTP/SAVP RTP/AVP a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_80 inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4 FEC_ORDER=FEC_SRTP a=pcfg:1 t=1 a=1 The "m=" line indicates that Alice is offering to use plain RTP with PCMU or G.729. Alice 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=tcap" and "a=acap" attributes. The "tcap" capability indicates that both Secure RTP and normal RTP are supported. The "acap" attribute provides a capability parameter with a handle of 1. The capability parameter is a "crypto" attribute, which provides the keying material for SRTP using SDP security descriptions [SDES]. The Andreasen Expires August 13, 2007 [Page 25] Internet-Draft SDP Capability Negotiation February 2007 "a=pcfg" attribute provides the potential configurations included in the offer by reference to the capabilities. A single potential configuration with a configuration number of "1" is provided. It includes is transport protocol capability 1 (RTP/SAVP, i.e. secure RTP) together with the attribute capability 1, i.e. the crypto attribute provided. Bob receives the SDP offer from Alice. Bob supports SRTP and the SDP Capability Negotiation extensions, and hence he accepts the potential configuration for Secure RTP provided by Alice: v=0 o=- 24351 621814 IN IP4 128.96.41.2 s= c=IN IP4 128.96.41.2 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=acfg:1 t=1 a=1 Bob includes the "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 capability 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. When Alice receives Bob's answer, session negotiation has completed, however Alice nevertheless generates a new offer using the actual configuration. This is done purely to assist any middle-boxes that may reside between Alice and Bob but do not support the capability negotiation extensions (and hence may not understand the negotiation that just took place): Alice's updated offer includes only SRTP, and it is not using the SDP capability negotiation extensions (Alice could have included the capabilities as well is she wanted to): v=0 o=- 25678 753850 IN IP4 128.96.41.1 s= c=IN IP4 128.96.41.1 t=0 0 m=audio 3456 RTP/SAVP 0 18 a=crypto:1 AES_CM_128_HMAC_SHA1_80 Andreasen Expires August 13, 2007 [Page 26] Internet-Draft SDP Capability Negotiation February 2007 inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4 FEC_ORDER=FEC_SRTP The "m=" line now indicates that Alice is offering to use secure RTP with PCMU or G.729. The "crypto" attribute, which provides the SRTP keying material, is included with the same value again. Bob receives the SDP offer from Alice, which he accepts, and then generates an answer to Alice: v=0 o=- 24351 621815 IN IP4 128.96.41.2 s= c=IN IP4 128.96.41.2 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 Bob includes the same crypto attribute as before, and the session proceeds without change. Although Bob did not include any capabilities in his answer, he could of course have done so if he wanted to. 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 received in step 1 and accepted the offer to use normal RTP. In that case, the following answer would have been generated in step 2 instead: v=0 o=- 24351 621814 IN IP4 128.96.41.2 s= c=IN IP4 128.96.41.2 t=0 0 m=audio 4567 RTP/AVP 0 18 4.2. Multiple Transport Protocols [EDITOR'S NOTE: Example to be updated - old copy below] 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, Andreasen Expires August 13, 2007 [Page 27] Internet-Draft SDP Capability Negotiation February 2007 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 (SRTP) |@@ |<---------------------------------| | | | (3) Offer (SRTP) | |--------------------------------->| | | | (4) Answer (SRTP) | |<---------------------------------| 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 128.96.41.1 s= c=IN IP4 128.96.41.1 t=0 0 m=audio 3456 RTP/AVP 0 18 a=creq: v0 a=tcap:1 RTP/SAVP RTP/AVP a=acap:1 a=crypto:1 AES_CM_128_HMAC_SHA1_80 inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4 FEC_ORDER=FEC_SRTP a=pcfg:5 t=1 a=1 a=pcfg:10 t=2 The "m=" line indicates that Alice is offering to use plain RTP with PCMU or G.729. Alice 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=tcap" and "a=acap" attributes. The capabilities indicate that both Secure RTP and normal RTP are supported. The "acap" 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 Andreasen Expires August 13, 2007 [Page 28] Internet-Draft SDP Capability Negotiation February 2007 alternatives are provided; the first one with preference "5" (and hence the preferred 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 1, i.e. the crypto attribute provided. The second one is using transport protocol capability 2. Note that we could have omitted the second potential configuration since it equals the actual configuration (which is always the least preferred configuration). Bob receives the SDP offer from Alice. Bob supports SRTP and the SDP Capability Negotiation extensions, and hence he accepts the potential configuration for Secure RTP provided by Alice: v=0 o=- 24351 621814 IN IP4 128.96.41.2 s= c=IN IP4 128.96.41.2 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=csup: foo a=acfg:1 t=1 a=1 Bob includes the "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 capability 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 128.96.41.2 s= c=IN IP4 128.96.41.2 t=0 0 Andreasen Expires August 13, 2007 [Page 29] Internet-Draft SDP Capability Negotiation February 2007 m=audio 4567 RTP/AVP 0 18 4.3. Session-Level MIKEY and Media Level Security Descriptions [EDITOR'S NOTE: Example to be added] 4.4. Capability Negotiation with Interactive Connectivity Establishment [EDITOR'S NOTE: Example to be added] 5. Security Considerations TBD. 6. 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] 7. To Do and Open Issues o Look for "EDITOR'S NOTE" throughout the document. 8. Acknowledgments 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 in here: Roni Even, Robert Gilman, Cullen Jennings, Matt Lepinski, Joerg Ott, Colin Perkins, and Thomas Stach. Andreasen Expires August 13, 2007 [Page 30] Internet-Draft SDP Capability Negotiation February 2007 Francois Audet and Dan Wing provided useful comments on earlier versions of this document. 9. Change Log 9.1. draft-ietf-mmusic-sdp-capability-negotiation-02 The following are the major changes compared to version -01: o Potential configurations are no longer allowed at the session level o Renamed capability attributes ("capar" to "acap" and "ctrpr" to "tcap") o Changed name and semantics of the initial number (now called configuration number) in potential configuration attributes; must now be unique and can be used as a handle o Actual configuration attribute now includes configuration number from the selected potential configuration attribute o Added ABNF throughout o Specified that answerer should include "a=csup" in case of unsupported required extensions in offer. o Specified use of second offer/answer exchange when answerer selected a potential configuration o Updated rules (and added restrictions) for referencing media- and session-level capabilities in potential configurations (at the media level) o Added initial section on ICE interactions o Added initial section on receiving media before answer 9.2. 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. Andreasen Expires August 13, 2007 [Page 31] Internet-Draft SDP Capability Negotiation February 2007 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. 9.3. 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 "tcap" 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. Andreasen Expires August 13, 2007 [Page 32] Internet-Draft SDP Capability Negotiation February 2007 o Some terminology change throughout to more clearly indicate what constitutes capabilities and what constitutes configurations. Andreasen Expires August 13, 2007 [Page 33] Internet-Draft SDP Capability Negotiation February 2007 10. References 10.1. 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. 10.2. 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. Andreasen Expires August 13, 2007 [Page 34] Internet-Draft SDP Capability Negotiation February 2007 [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. Andreasen Expires August 13, 2007 [Page 35] Internet-Draft SDP Capability Negotiation February 2007 [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. [ICE] J. Rosenberg, "Interactive Connectivity Establishment (ICE): A Methodology for Network Address Translator (NAT) Traversal for Offer/Answer Protocols", work in progress, January 2007. [ICETCP] J. Rosenberg, "TCP Candidates with Interactive Connectivity Establishment (ICE)", work in progress, October 2006. [RFC3312] G. Camarillo, W. Marshall, and J. Rosenberg, "Integration of Resource Management and Session Initiatio Protocol (SIP)", RFC 3312, October 2002. Author's Addresses Flemming Andreasen Cisco Systems Edison, NJ Email: fandreas@cisco.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. Andreasen Expires August 13, 2007 [Page 36] Internet-Draft SDP Capability Negotiation February 2007 Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Andreasen Expires August 13, 2007 [Page 37]