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7. SDP Definitions

Section 4 of [RFC4585] defines a new SDP [RFC4566] attribute, rtcp-fb, that may be used to negotiate the capability to handle specific AVPF commands and indications, such as Reference Picture Selection, Picture Loss Indication, etc. The ABNF for rtcp-fb is described in section 4.2 of [RFC4585]. In this section, we extend the rtcp-fb attribute to include the commands and indications that are described for codec control in the present document. We also discuss the Offer/Answer implications for the codec control commands and indications.

7.1. Extension of the rtcp-fb Attribute​

As described in AVPF [RFC4585], the rtcp-fb attribute indicates the capability of using RTCP feedback. AVPF specifies that the rtcp-fb attribute must only be used as a media level attribute and must not be provided at session level. All the rules described in [RFC4585] for rtcp-fb attribute relating to payload type and to multiple rtcp-fb attributes in a session description also apply to the new feedback messages defined in this memo.

The ABNF [RFC4234] for rtcp-fb as defined in [RFC4585] is

"a=rtcp-fb: " rtcp-fb-pt SP rtcp-fb-val CRLF

where rtcp-fb-pt is the payload type and rtcp-fb-val defines the type of the feedback message such as ack, nack, trr-int, and rtcp-fb-id. For example, to indicate the support of feedback of Picture Loss Indication, the sender declares the following in SDP

v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Media with feedback
t=0 0
c=IN IP4 host.example.com
m=audio 49170 RTP/AVPF 98
a=rtpmap:98 H263-1998/90000
a=rtcp-fb:98 nack pli

In this document, we define a new feedback value "ccm", which indicates the support of codec control using RTCP feedback messages. The "ccm" feedback value SHOULD be used with parameters that indicate the specific codec control commands supported. In this document, we define four such parameters, namely:

  • "fir" indicates support of the Full Intra Request (FIR).
  • "tmmbr" indicates support of the Temporary Maximum Media Stream Bit Rate Request/Notification (TMMBR/TMMBN). It has an optional sub-parameter to indicate the session maximum packet rate (measured in packets per second) to be used. If not included, this defaults to infinity.
  • "tstr" indicates support of the Temporal-Spatial Trade-off Request/Notification (TSTR/TSTN).
  • "vbcm" indicates support of H.271 Video Back Channel Messages (VBCMs). It has zero or more subparameters identifying the supported H.271 "payloadType" values.

In the ABNF for rtcp-fb-val defined in [RFC4585], there is a placeholder called rtcp-fb-id to define new feedback types. "ccm" is defined as a new feedback type in this document, and the ABNF for the parameters for ccm is defined here (please refer to section 4.2 of [RFC4585] for complete ABNF syntax).

rtcp-fb-val        =/ "ccm" rtcp-fb-ccm-param

rtcp-fb-ccm-param  = SP "fir"   ; Full Intra Request
                   / SP "tmmbr" [SP "smaxpr=" MaxPacketRateValue]
                                ; Temporary max media bit rate
                   / SP "tstr"  ; Temporal-Spatial Trade-Off
                   / SP "vbcm" *(SP subMessageType) ; H.271 VBCMs
                   / SP token [SP byte-string]
                           ; for future commands/indications
subMessageType = 1*8DIGIT
byte-string = <as defined in section 4.2 of [RFC4585] >
MaxPacketRateValue = 1*15DIGIT

7.2. Offer-Answer​

The Offer/Answer [RFC3264] implications for codec control protocol feedback messages are similar to those described in [RFC4585]. The offerer MAY indicate the capability to support selected codec commands and indications. The answerer MUST remove all CCM parameters corresponding to the CCMs that it does not wish to support in this particular media session (for example, because it does not implement the message in question, or because its application logic suggests that support of the message adds no value). The answerer MUST NOT add new ccm parameters in addition to what has been offered.

The answer is binding for the media session and both offerer and answerer MUST NOT use any feedback messages other than what both sides have explicitly indicated as being supported. In other words, only the joint subset of CCM parameters from the offer and answer may be used.

Note that including a CCM parameter in an offer or answer indicates that the party (offerer or answerer) is at least capable of receiving the corresponding CCM(s) and act upon them. In cases when the reception of a negotiated CCM mandates the party to respond with another CCM, it must also have that capability. Although it is not mandated to initiate CCMs of any negotiated type, it is generally expected that a party will initiate CCMs when appropriate.

The session maximum packet rate parameter part of the TMMBR indication is declarative, and the highest value from offer and answer SHALL be used. If the session maximum packet rate parameter is not present in an offer, it SHALL NOT be included by the answerer.

7.3. Examples​

Example 1: The following SDP describes a point-to-point video call with H.263, with the originator of the call declaring its capability to support the FIR and TSTR/TSTN codec control messages. The SDP is carried in a high-level signaling protocol like SIP.

v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Point-to-Point call
c=IN IP4 192.0.2.124
m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000
m=video 51372 RTP/AVPF 98
a=rtpmap:98 H263-1998/90000
a=rtcp-fb:98 ccm tstr
a=rtcp-fb:98 ccm fir

In the above example, when the sender receives a TSTR message from the remote party it is capable of adjusting the trade-off as indicated in the RTCP TSTN feedback message.

Example 2: The following SDP describes a SIP end point joining a video mixer that is hosting a multiparty video conferencing session. The participant supports only the FIR (Full Intra Request) codec control command and it declares it in its session description.

v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Multiparty Video Call
c=IN IP4 192.0.2.124
m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000
m=video 51372 RTP/AVPF 98
a=rtpmap:98 H263-1998/90000
a=rtcp-fb:98 ccm fir

When the video MCU decides to route the video of this participant, it sends an RTCP FIR feedback message. Upon receiving this feedback message, the end point is required to generate a full intra request.

Example 3: The following example describes the Offer/Answer implications for the codec control messages. The offerer wishes to support "tstr", "fir" and "tmmbr". The offered SDP is

-------------> Offer
      v=0
      o=alice 3203093520 3203093520 IN IP4 host.example.com
      s=Offer/Answer
      c=IN IP4 192.0.2.124
      m=audio 49170 RTP/AVP 0
      a=rtpmap:0 PCMU/8000
      m=video 51372 RTP/AVPF 98
      a=rtpmap:98 H263-1998/90000
      a=rtcp-fb:98 ccm tstr
      a=rtcp-fb:98 ccm fir
      a=rtcp-fb:* ccm tmmbr smaxpr=120

The answerer wishes to support only the FIR and TSTR/TSTN messages and the answerer SDP is

<---------------- Answer

      v=0
      o=alice 3203093520 3203093524 IN IP4 otherhost.example.com
      s=Offer/Answer
      c=IN IP4 192.0.2.37
      m=audio 47190 RTP/AVP 0
      a=rtpmap:0 PCMU/8000
      m=video 53273 RTP/AVPF 98
      a=rtpmap:98 H263-1998/90000
      a=rtcp-fb:98 ccm tstr
      a=rtcp-fb:98 ccm fir

Example 4: The following example describes the Offer/Answer implications for H.271 Video Back Channel Messages (VBCMs). The offerer wishes to support VBCM and the sub-messages of payloadType 1 (one or more pictures that are entirely or partially lost) and 2 (a set of blocks of one picture that are entirely or partially lost).

-------------> Offer
      v=0
      o=alice 3203093520 3203093520 IN IP4 host.example.com
      s=Offer/Answer
      c=IN IP4 192.0.2.124
      m=audio 49170 RTP/AVP 0
      a=rtpmap:0 PCMU/8000
      m=video 51372 RTP/AVPF 98
      a=rtpmap:98 H263-1998/90000
      a=rtcp-fb:98 ccm vbcm 1 2

The answerer only wishes to support sub-messages of type 1 only

<---------------- Answer

      v=0
      o=alice 3203093520 3203093524 IN IP4 otherhost.example.com
      s=Offer/Answer
      c=IN IP4 192.0.2.37
      m=audio 47190 RTP/AVP 0
      a=rtpmap:0 PCMU/8000
      m=video 53273 RTP/AVPF 98
      a=rtpmap:98 H263-1998/90000
      a=rtcp-fb:98 ccm vbcm 1

So, in the above example, only VBCM indications comprised of "payloadType" 1 will be supported.

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