RFC3264] to allow for potential configurations to be included in an offer, 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 procedures defined in the following subsections apply to both unicast and multicast streams. Section 3.4.1 for each attribute name and associated value (if any) that needs to be indicated as a capability in the offer. Attribute capabilities may be included irrespective of whether or not they are referenced by a potential configuration. Session-level attributes and associated values MUST be provided in attribute capabilities only at the session level, whereas media- level attributes and associated values can be provided in attribute capabilities at either the media level or session level. Attributes that are allowed at either the session or media level can be provided in attribute capabilities at either level. o Zero or more transport protocol capability attributes. There MUST be transport protocol capabilities as defined in Section 3.4.2 with values for each transport protocol that needs to be indicated as a capability in the offer. Transport protocol capabilities may be included irrespective of whether or not they are referenced by a potential configuration. Transport protocols that apply to multiple media descriptions SHOULD be provided as transport protocol capabilities at the session level whereas transport protocols that apply only to a specific media description ("m=" line), SHOULD be provided as transport protocol capabilities within that particular media description. In either case, there MUST NOT be more than a single "a=tcap" attribute at the session level and a single "a=tcap" attribute in each media description.
o Zero or more extension capability attributes. There MUST be one or more extension capability attributes (as outlined in Section 3.4.3) for each extension capability that is referenced by a potential configuration. Extension capability attributes that are not referenced by a potential configuration can be provided as well. o Zero or more potential configuration attributes. There MUST be one or more potential configuration attributes ("a=pcfg"), as defined in Section 3.5.1, in each media description where alternative potential configurations are to be negotiated. Each potential configuration attribute MUST adhere to the rules provided in Section 3.5.1 and the additional rules provided below. If the offerer requires support for one or more extensions (besides the base protocol defined here), then the offerer MUST include one or more "a=creq" attributes as follows: o If support for one or more capability negotiation extensions is required for the entire session description, then option tags for those extensions MUST be included in a single session-level "creq" attribute. o For each media description that requires support for one or more capability negotiation extensions not listed at the session level, a single "creq" attribute containing all the required extensions for that media description MUST be included within the media description (in accordance with Section 3.3.2). Note that extensions that only need to be supported by a particular potential configuration can use the "mandatory" extension prefix ("+") within the potential configuration (see Section 3.5.1). The offerer SHOULD furthermore include the following: o A supported capability negotiation extension attribute ("a=csup") at the session level and/or media level as defined in Section 3.3.2 for each capability negotiation extension supported by the offerer and not included in a corresponding "a=creq" attribute (i.e., at the session level or in the same media description). Option tags provided in a "a=csup" attribute at the session level indicate extensions supported for the entire session description, whereas option tags provided in a "a=csup" attribute in a media description indicate extensions supported for only that particular media description.
Capabilities provided in an offer merely indicate what the offerer is capable of doing. They do not constitute a commitment or even an indication to use them. In contrast, each potential configuration constitutes an alternative offer that the offerer would like to use. The potential configurations MUST be used by the answerer to negotiate and establish the session. The offerer MUST include one or more potential configuration attributes ("a=pcfg") in each media description where the offerer wants to provide alternative offers (in the form of potential configurations). Each potential configuration attribute in a given media description MUST contain a unique configuration number and zero, one or more potential configuration lists, as described in Section 3.5.1. Each potential configuration list MUST refer to capabilities that are provided at the session level or within that particular media description; otherwise, the potential configuration is considered invalid. The base SDP Capability Negotiation framework REQUIRES that potential configurations not reference any session- level attribute capabilities that contain media-level-only attributes; however, extensions may modify this behavior, as long as it is fully backwards compatible with the base specification. Furthermore, it is RECOMMENDED that potential configurations avoid use of session-level capabilities whenever possible; refer to Section 3.5.1. The current actual configuration is included in the "m=" line (as defined by [RFC3264]) and any associated parameters for the media description (e.g., attribute ("a=") and bandwidth ("b=") lines). Note that the actual configuration is by default the least-preferred configuration, and hence the answerer will seek to negotiate use of one of the potential configurations instead. If the offerer wishes a different preference for the actual configuration, the offerer MUST include a corresponding potential configuration with the relevant configuration number (which indicates the relative preference between potential configurations); this corresponding potential configuration should simply duplicate the actual configuration. This can either be done implicitly (by not referencing any capabilities), or explicitly (by providing and using capabilities for the transport protocol and all the attributes that are part of the actual configuration). The latter may help detect intermediaries that modify the actual configuration but are not SDP Capability Negotiation aware. 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 only for the actual configurations provided in the offer: 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 selected configuration is. Once that answer is received, incoming media MUST be processed in accordance with the actual selected configuration indicated and the answer received (provided the offer/answer exchange completed successfully). The above rule assumes that the offerer can determine whether incoming media adheres to the actual configuration offered or one of the potential configurations instead; this may not always be the case. If the offerer wants to ensure he does not play out any garbage, the offerer SHOULD discard all media received before the answer SDP session description is received. Conversely, if the offerer wants to avoid clipping, he SHOULD attempt to play any incoming media as soon as it is received (at the risk of playing out garbage). In either case, please note that this document does not place any requirements on the offerer to process and play media before answer. For further details, please refer to Section 3.9. RFC3264]. Furthermore, the answerer MUST include a session-level supported capability negotiation extensions attribute ("a=csup") with option tags for the capability negotiation extensions supported by the answerer. o If a media-level "creq" attribute is provided, and it contains an option tag that the answerer does not support, then the answerer MUST NOT use any of the potential configuration attributes
provided for that particular media description. Instead, the offer/answer procedures for that media description MUST continue as per [RFC3264] (SDP Capability Negotiation is still performed for other media descriptions in the SDP session description). Furthermore, the answerer MUST include a supported capability negotiation extensions attribute ("a=csup") in that media description with option tags for the capability negotiation extensions supported by the answerer for that media description. Assuming all required capability negotiation extensions are supported, the answerer now proceeds as follows. For each media description where capability negotiation is to be performed (i.e., all required capability negotiation extensions are supported and at least one valid potential configuration attribute is present), the answerer MUST perform capability negotiation by using the most preferred potential configuration that is valid to the answerer, subject to any local policies. A potential configuration is valid to the answerer if: 1. It is in accordance with the syntax and semantics provided in Section 3.5.1. 2. It contains a configuration number that is unique within that media description. 3. All attribute capabilities referenced by the potential configuration are valid themselves (as defined in Section 3.4.1) and each of them is provided either at the session level or within this particular media description. For session-level attribute capabilities referenced, the attributes contained inside them MUST NOT be media-level-only attributes. Note that the answerer can only determine this for attributes supported by the answerer. If an attribute is not supported, it will simply be ignored by the answerer and hence will not trigger an "invalid" potential configuration. 4. All transport protocol capabilities referenced by the potential configuration are valid themselves (as defined in Section 3.4.2) and each of them is furthermore provided either at the session level or within this particular media description. 5. All extension capabilities referenced by the potential configuration and supported by the answerer are valid themselves (as defined by that particular extension) and each of them are furthermore provided either at the session level or within this particular media description. Unknown or unsupported extension
capabilities MUST be ignored, unless they are prefixed with the plus ("+") sign, which indicates that the extension MUST be supported in order to use that potential configuration. If the extension is not supported, that potential configuration is not valid to the answerer. The most preferred valid potential configuration in a media description is the valid potential configuration with the lowest configuration number. The answerer MUST now process the offer for that media stream based on the most preferred valid potential configuration. Conceptually, this entails the answerer constructing an (internal) offer as follows. First, all capability negotiation parameters from the offer SDP session description are removed, thereby yielding an offer SDP session description with the actual configuration as if SDP Capability Negotiation was not done in the first place. Secondly, this actual configuration SDP session description is modified as follows for each media stream offered, based on the capability negotiation parameters included originally: o If a transport protocol capability is included in the potential configuration, then it replaces the transport protocol provided in the "m=" line for that media description. o If attribute capabilities are present with a delete-attributes session indication ("-s") or media and session indication ("-ms"), then all session-level attributes from the actual configuration SDP session description MUST be deleted in the resulting potential configuration SDP session description in accordance with the procedures in Section 3.5.1. If attribute capabilities are present with a delete-attributes media indication ("-m") or media and session indication ("-ms"), then all attributes from the actual configuration SDP session description inside this media description MUST be deleted. o If a session-level attribute capability is included, the attribute (and its associated value, if any) contained in it MUST be added to the resulting SDP session description. All such added session- level attributes MUST be listed before the session-level attributes that were initially present in the SDP session description. Furthermore, the added session-level attributes MUST be added in the order they were provided in the potential configuration (see also Section 3.5.1). This allows for attributes with implicit preference ordering to be added in the desired order; the "crypto" attribute [RFC4568] is one such example.
o If a media-level attribute capability is included, then the attribute (and its associated value, if any) MUST be added to the resulting SDP session description within the media description in question. All such added media-level attributes MUST be listed before the media-level attributes that were initially present in the media description in question. Furthermore, the added media- level attributes MUST be added in the order they were provided in the potential configuration (see also Section 3.5.1). o If a supported extension capability is included, then it MUST be processed in accordance with the rules provided for that particular extension capability. The above steps MUST be performed exactly once per potential configuration, i.e., there MUST NOT be any recursive processing of any additional capability negotiation parameters that may (illegally) have been nested inside capabilities themselves. As an example of this, consider the (illegal) attribute capability a=acap:1 acap:2 foo:a The resulting potential configuration SDP session description will, after the above processing has been done, contain the attribute capability a=acap:2 foo:a However, since we do not perform any recursive processing of capability negotiation parameters, this second attribute capability parameter will not be processed by the offer/answer procedure. Instead, it will simply appear as a (useless) attribute in the SDP session description that will be ignored by further processing. Note that a transport protocol from the potential configuration replaces the transport protocol in the actual configuration, but an attribute capability from the potential configuration is simply added to the actual configuration. In some cases, this can result in having one or more meaningless attributes in the resulting potential configuration SDP session description, or worse, ambiguous or potentially even illegal attributes. Use of delete-attributes for the session- and/or media-level attributes MUST be done to avoid such scenarios. Nevertheless, it is RECOMMENDED that implementations ignore meaningless attributes that may result from potential configurations.
For example, if the actual configuration was using Secure RTP and included an "a=crypto" attribute for the SRTP keying material, then use of a potential configuration that uses plain RTP would make the "crypto" attribute meaningless. The answerer may or may not ignore such a meaningless attribute. The offerer can here ensure correct operation by using delete-attributes to remove the "crypto" attribute (but will then need to provide attribute capabilities to reconstruct the SDP session description with the necessary attributes deleted, e.g., rtpmaps). Also note, that while it is permissible to include media-level attribute capabilities at the session level, the base SDP Capability Negotiation framework defined here does not define any procedures for use of them, i.e., the answerer effectively ignores them. Please refer to Section 22.214.171.124 for examples of how the answerer may conceptually "see" the resulting offered alternative potential configurations. The answerer MUST check that he supports all mandatory attribute capabilities from the potential configuration (if any), the transport protocol capability (if any) from the potential configuration, and all mandatory extension capabilities from the potential configuration (if any). If he does not, the answerer MUST proceed to the second most preferred valid potential configuration for the media description, etc. o In the case of attribute capabilities, support implies that the attribute name contained in the capability is supported and it can (and will) be negotiated successfully in the offer/answer exchange with the value provided. This does not necessarily imply that the value provided is supported in its entirety. For example, the "a=fmtp" parameter is often provided with one or more values in a list, where the offerer and answerer negotiate use of some subset of the values provided. Other attributes may include mandatory and optional parts to their values; support for the mandatory part is all that is required here. A side effect of the above rule is that whenever an "fmtp" or "rtpmap" parameter is provided as a mandatory attribute capability, the corresponding media format (codec) must be supported and use of it negotiated successfully. If this is not the offerer's intent, the corresponding attribute capabilities must be listed as optional instead.
o In the case of transport protocol capabilities, support implies that the transport protocol contained in the capability is supported and the transport protocol can (and will) be negotiated successfully in the offer/answer exchange. o In the case of extension capabilities, the extension MUST define the rules for when the extension capability is considered supported and those rules MUST be satisfied. If the answerer has exhausted all potential configurations for the media description, without finding a valid one that is also supported, then the answerer MUST process the offered media stream based on the actual configuration plus any session-level attributes added by a valid and supported potential configuration from another media description in the offered SDP session description. The above process describes potential configuration selection as a per-media-stream process. Inter-media stream coordination of selected potential configurations however is required in some cases. First of all, session-level attributes added by a potential configuration for one media description MUST NOT cause any problems for potential configurations selected by other media descriptions in the offer SDP session description. If the session-level attributes are mandatory, then those session-level attributes MUST furthermore be supported by the session as a whole (i.e., all the media descriptions if relevant). As mentioned earlier, this adds additional complexity to the overall processing and hence it is RECOMMENDED not to use session-level attribute capabilities in potential configurations, unless absolutely necessary. Once the answerer has selected a valid and supported offered potential configuration for all of the media streams (or has fallen back to the actual configuration plus any added session attributes), the answerer MUST generate a valid virtual answer SDP session description based on the selected potential configuration SDP session description, as "seen" by the answerer using normal offer/answer rules (see Section 126.96.36.199 for examples). The actual answer SDP session description is formed from the virtual answer SDP session description as follows: if the answerer selected one of the potential configurations in a media description, the answerer MUST include an actual configuration attribute ("a=acfg") within that media description. The "a=acfg" attribute MUST identify the configuration number for the selected potential configuration as well as the actual parameters that were used from that potential configuration; if the potential configuration included alternatives, the selected alternatives only MUST be included. Only the known and supported parameters will be included. Unknown or unsupported parameters MUST NOT be included in the actual configuration attribute. In the case
of attribute capabilities, only the known and supported capabilities are included; unknown or unsupported attribute capabilities MUST NOT be included. If the answerer supports one or more capability negotiation extensions that were not included in a required capability negotiation extensions attribute in the offer, then the answerer SHOULD furthermore include a supported capability negotiation attribute ("a=csup") at the session level with option tags for the extensions supported across media streams. Also, if the answerer supports one or more capability negotiation extensions for only particular media descriptions, then a supported capability negotiation attribute with those option tags SHOULD be included within each relevant media description. The required capability negotiation attribute ("a=creq") MUST NOT be used in an answer. The offerer's originally provided actual configuration is contained in the offer media description's "m=" line (and associated parameters). The answerer MAY send media to the offerer in accordance with that actual configuration as soon as it receives the offer; however, it MUST NOT send media based on that actual configuration if it selects an alternative potential configuration. If the answerer selects one of the potential configurations, then the answerer MAY immediately start to send media to the offerer in accordance with the selected potential configuration; however, the offerer MAY discard such media or play out garbage until the offerer receives the answer. Please refer to Section 3.9. for additional considerations and possible alternative solutions outside the base SDP Capability Negotiation framework. If the answerer selected a potential configuration instead of the actual configuration, then it is RECOMMENDED that the answerer send back an answer SDP session description as soon as possible. This minimizes the risk of having media discarded or played out as garbage by the offerer. In the case of SIP [RFC3261] without any extensions, this implies that if the offer was received in an INVITE message, then the answer SDP session description should be provided in the first non-100 provisional response sent back (per RFC 3261, the answer would need to be repeated in the 200 response as well, unless a relevant extension such as [RFC3262] is being used).
Note that the transport protocol in the media descriptions indicate use of Secure RTP. Below, we show the offer the answerer "sees" when using potential configuration 1 for both audio and video and furthermore using attribute capability 2 and 3, respectively, (SDP security descriptions) for the audio and video stream -- note the order in which the resulting attributes are provided: v=0 o=alice 2891092738 2891092738 IN IP4 lost.example.com s= t=0 0 c=IN IP4 lost.example.com a=tool:foo m=audio 59000 RTP/SAVP 98 a=crypto:1 AES_CM_128_HMAC_SHA1_32 inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32 a=rtpmap:98 AMR/8000 m=video 52000 RTP/SAVP 31 a=crypto:1 AES_CM_128_HMAC_SHA1_80 inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32 a=rtpmap:31 H261/90000 Again, note that the transport protocol in the media descriptions indicate use of Secure RTP. And finally, we show the offer the answerer "sees" when using potential configuration 1 with attribute capability 1 (MIKEY) for the audio stream, and potential configuration 1 with attribute capability 3 (SDP security descriptions) for the video stream: v=0 o=alice 2891092738 2891092738 IN IP4 lost.example.com s= t=0 0 c=IN IP4 lost.example.com a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO... a=tool:foo m=audio 59000 RTP/SAVP 98 a=rtpmap:98 AMR/8000 m=video 52000 RTP/SAVP 31 a=crypto:1 AES_CM_128_HMAC_SHA1_80 inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32 a=rtpmap:31 H261/90000
Section 3.6.2. If a valid actual configuration attribute is not present in a media description, then the offerer MUST process the answer SDP session description for that media stream per the normal offer/answer rules defined in [RFC3264]. However, if a valid one is found, the offerer MUST instead process the answer as follows: o The actual configuration attribute specifies which of the potential configurations was used by the answerer to generate the answer for this media stream. This includes all the supported attribute capabilities and the transport capabilities referenced by the potential configuration selected, where the attribute capabilities have any associated delete-attributes included. Extension capabilities supported by the answerer are included as well. o The offerer MUST now process the answer in accordance with the rules in [RFC3264], except that it must be done as if the offer consisted of the selected potential configuration instead of the original actual configuration, including any transport protocol changes in the media ("m=") line(s), attributes added and deleted by the potential configuration at the media and session level, and any extensions used. If this derived answer is not a valid answer to the potential configuration offer selected by the answerer, the offerer MUST instead continue further processing as it would have for a regular offer/answer exchange, where the answer received does not adhere to the rules of [RFC3264].
If the offer/answer exchange was successful, and if the answerer selected one of the potential configurations from the offer as the actual configuration, and the selected potential configuration differs from the actual configuration in the offer (the "m=", "a=", etc., lines), then the offerer SHOULD initiate another offer/answer exchange. This second offer/answer exchange will not modify the session in any way; however, it will help intermediaries (e.g., middleboxes), which look at the SDP session description but do not support the capability negotiation extensions, understand the details of the media stream(s) that were actually negotiated. This new offer MUST contain the selected potential configuration as the actual configuration, i.e., with the actual configuration used in the "m=" line and any other relevant attributes, bandwidth parameters, etc. Note that, per normal offer/answer rules, the second offer/answer exchange still needs to update the version number in the "o=" line (<sess-version> in [RFC4566]). Attribute lines carrying keying material SHOULD repeat the keys from the previous offer, unless re-keying is necessary, e.g., due to a previously forked SIP INVITE request. Please refer to Section 3.12 for additional considerations related to intermediaries. RFC3264], Section 8. The procedure for doing so is similar to that described above with the answer including an indication of the actual selected configuration used by the answerer. If the answer indicates use of a potential configuration from the offer, then the guidelines provided in Section 3.6.3 for doing a second offer/answer exchange using that potential configuration as the actual configuration apply. RFC5245] provides a mechanism for verifying connectivity between two endpoints by sending Session Traversal Utilities for NAT (STUN) messages directly between the media endpoints. The basic ICE specification [RFC5245] is only defined to support UDP-based connectivity; 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. Since both ICE and SDP Capability Negotiation may specify alternative transport protocols, there is a potentially unintended interaction when using these together. We provide the following guidelines for addressing that. There are two basic scenarios to consider: 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 connectivity checks). 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. The scenarios may be combined, e.g., by offering potential configuration alternatives where some of them can support only one transport protocol (e.g., UDP), whereas others can support multiple transport protocols (e.g., UDP or TCP). In that case, there is a need for tight control over the ICE candidates that will be used for a particular configuration, yet the actual configuration may want to use all of the ICE candidates. In that case, the ICE candidate attributes can 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). Furthermore, use of the delete-attributes in a potential configuration can be used to ensure that ICE will not end up using a transport protocol that is not desired for a particular configuration.
SDP Capability Negotiation recommends use of a second offer/answer exchange when the negotiated actual configuration was one of the potential configurations from the offer (see Section 3.6.3). Similarly, ICE requires use of a second offer/answer exchange if the chosen candidate is not the same as the one in the m/c-line from the offer. When ICE and capability negotiation are used at the same time, the two secondary offer/answer exchanges SHOULD be combined to a single one. RFC3261] allows for SIP extensions to define a SIP option tag that identifies the SIP extension. Support for one or more such extensions can be indicated by use of the SIP Supported header, and required support for one or more such extensions can be indicated by use of the SIP Require header. The "a=csup" and "a=creq" attributes defined by the SDP Capability Negotiation framework are similar, except that support for these two attributes by themselves cannot be guaranteed (since they are specified as extensions to the SDP specification [RFC4566] itself). SIP extensions with associated option tags can introduce enhancements to not only SIP, but also SDP. This is for example the case for SIP preconditions defined in [RFC3312]. When using SDP Capability Negotiation, some potential configurations may include certain SDP extensions, whereas others may not. Since the purpose of the SDP Capability Negotiation is to negotiate a session based on the features supported by both sides, use of the SIP Require header for such extensions may not produce the desired result. For example, if one potential configuration requires SIP preconditions support, another does not, and the answerer does not support preconditions, then use of the SIP Require header for preconditions would result in a session failure, in spite of the fact that a valid and supported potential configuration was included in the offer. In general, this can be alleviated by use of mandatory and optional attribute capabilities in a potential configuration. There are however cases where permissible SDP values are tied to the use of the SIP Require header. SIP preconditions [RFC3312] is one such example, where preconditions with a "mandatory" strength-tag can only be used when a SIP Require header with the SIP option tag "precondition" is included. Future SIP extensions that may want to use the SDP Capability Negotiation framework should avoid such coupling.
RFC3264] 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, the offerer may decide not to process any media received before the answer is received. This may lead to clipping. Consequently, the SDP Capability Negotiation framework recommends sending back an answer SDP session description as soon as possible. The issue can be resolved by introducing a three-way handshake. In the case of SIP, this can, for example, be done by defining a precondition [RFC3312] for capability negotiation (or by using an existing precondition that is known to generate a second offer/answer exchange before proceeding with the session). However, preconditions are often viewed as complicated to implement and they may add to overall session establishment delay by requiring an extra offer/answer exchange. An alternative three-way handshake can be performed by use of ICE [RFC5245]. When ICE is being used, and the answerer receives a STUN Binding Request for any one of the accepted media streams from the offerer, the answerer knows the offer has received his answer. At that point, the answerer knows that the offerer will be able to process incoming media according to the negotiated configuration and hence he can start sending media without the risk of the offerer either discarding it or playing garbage. Please note that, the above considerations notwithstanding, this document does not place any requirements on the offerer to process and play media before answer; it merely provides recommendations for how to ensure that media sent by the answerer and received by the offerer prior to receiving the answer can in fact be rendered by the offerer. In some use cases, a three-way handshake is not needed. An example is when the offerer does not need information from the answer, such as keying material in the SDP session description, in order to process incoming media. The SDP Capability Negotiation framework does not define any such solutions; 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, outside of the actual configuration, while still allowing them to be used by the answerer (exchange of keying
material is still needed, e.g., inband). The basic SDP Capability Negotiation framework defined here does not include the ability to do so; however, extensions that enable that may be defined. RFC3711] with integrity protection requires more bandwidth than plain RTP [RFC3551]. SDP defines the bandwidth ("b=") parameter to indicate the proposed bandwidth for the session or media stream. In SDP, as defined by [RFC4566], each media description contains one transport protocol and one or more codecs. When specifying the proposed bandwidth, the worst case scenario must be taken into account, i.e., use of the highest bandwidth codec provided, the transport protocol indicated, and the worst case (bandwidth-wise) parameters that can be negotiated (e.g., a 32-bit Hashed Message Authentication Code (HMAC) or an 80-bit HMAC). The base SDP Capability Negotiation framework does not provide a way to negotiate bandwidth parameters. The issue thus remains; however, it is potentially worse than with SDP per [RFC4566], since it is easier to negotiate additional codecs, and furthermore possible to negotiate different transport protocols. The recommended approach for addressing this is the same as for plain SDP; the worst case (now including potential configurations) needs to be taken into account when specifying the bandwidth parameters in the actual configuration. This can make the bandwidth value less accurate than in SDP per [RFC4566] (due to potential greater variability in the potential configuration bandwidth use). Extensions can be defined to address this shortcoming. Note, that when using RTP retransmission [RFC4588] with the RTCP- based feedback profile [RFC4585] (RTP/AVPF), the retransmitted packets are part of the media stream bandwidth when using synchronization source (SSRC) multiplexing. If a feedback-based protocol is offered as the actual configuration transport protocol, a non-feedback-based protocol is offered as a potential configuration transport protocol and ends up being used, the actual bandwidth usage may be lower than the indicated bandwidth value in the offer (and vice versa).
Section 3.6.3). However, there are several limitations with this approach. First of all, although the second offer/answer exchange is RECOMMENDED, it is not required and hence may not be performed. Secondly, the intermediary may refuse the initial answer, e.g., due to perceived transport protocol mismatch. Thirdly, the strategy is not foolproof since the offer/answer procedures [RFC3264] leave the original offer/answer exchange in effect when a subsequent one fails. Consider the following example: 1. Offerer generates an SDP session description offer with the actual configuration specifying a low-bandwidth configuration (e.g., plain RTP) and a potential configuration specifying a high(er) bandwidth configuration (e.g., Secure RTP with integrity).
2. An intermediary (e.g., an SBC or P-CSCF), that does not support SDP Capability Negotiation, authorizes the session based on the actual configuration it sees in the SDP session description. 3. The answerer chooses the high(er) bandwidth potential configuration and generates an answer SDP session description based on that. 4. The intermediary passes through the answer SDP session description. 5. The offerer sees the accepted answer, and generates an updated offer that contains the selected potential configuration as the actual configuration. In other words, the high(er) bandwidth configuration (which has already been negotiated successfully) is now the actual configuration in the offer SDP session description. 6. The intermediary sees the new offer; however, it does not authorize the use of the high(er) bandwidth configuration, and consequently generates a rejection message to the offerer. 7. The offerer receives the rejected offer. After step 7, per RFC 3264, the offer/answer exchange that completed in step 5 remains in effect; however, the intermediary may not have authorized the necessary network resources and hence the media stream may experience quality issues. The solution to this problem is to upgrade the intermediary to support the SDP Capability Negotiation framework.
Only the media formats (e.g., RTP payload types) provided in the "m=" line are actually offered; inclusion of "rtpmap" attributes with other RTP payload types in a potential configuration does not change this fact and hence they do not provide any useful information there. They may still be useful as pure capabilities though (outside a potential configuration) in order to inform a peer of additional codecs supported. It is possible to provide an "rtpmap" attribute capability with a payload type mapping to a different codec than a corresponding actual configuration "rtpmap" attribute for the media description has. Such practice is permissible as a way of indicating a capability. If that capability is included in a potential configuration, then delete- attributes (see Section 3.5.1) MUST be used to ensure that there is not multiple "rtpmap" attributes for the same payload type in a given media description (which would not be allowed by SDP [RFC4566]). Similar considerations and rules apply to the "fmtp" attribute. An "fmtp" attribute capability for a media format not included in the "m=" line is useless in a potential configuration (but may be useful as a capability by itself). An "fmtp" attribute capability in a potential configuration for a media format that already has an "fmtp" attribute in the actual configuration may lead to multiple fmtp format parameters for that media format and that is not allowed by SDP [RFC4566]. The delete-attributes MUST be used to ensure that there are not multiple "fmtp" attributes for a given media format in a media description. Extensions to the base SDP Capability Negotiation framework may change the above behavior.
RFC 3407 defines capability descriptions with limited abilities to describe attributes, bandwidth parameters, transport protocols and media formats. RFC 3407 does not define any negotiation procedures for actually using those capability descriptions. This document defines new attributes for describing attribute capabilities and transport capabilities. It also defines procedures for using those capabilities as part of an offer/answer exchange. In contrast to RFC 3407, this document does not define bandwidth parameters, and it also does not define how to express ranges of values. Extensions to this document may be defined in order to fully cover all the capabilities provided by RFC 3407 (for example, more general media capabilities). It is RECOMMENDED that implementations use the attributes and procedures defined in this document instead of those defined in [RFC3407]. If capability description interoperability with legacy RFC 3407 implementations is desired, implementations MAY include both RFC 3407 capability descriptions and capabilities defined by this document. The offer/answer negotiation procedures defined in this document will not use the RFC 3407 capability descriptions.