RTCP and AVPF related missing features


Most of the missing features are AVPF related, which is defined in RFC4585 and RFC5104.

RFC4585: Extended RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/AVPF)
https://www.rfc-editor.org/rfc/rfc4585.txt
RFC5104:  Codec Control Messages in the RTP Audio-Visual Profile with Feedback (AVPF)
https://www.rfc-editor.org/rfc/rfc5104.txt

AVPF contains a mechanism for conveying such a message, but did not specify for which codec and according to which syntax the message should conform.  Recently, the ITU-T finalized Rec.H.271, which (among other message types) also includes a feedback message.  It is expected that this feedback message will fairly quickly enjoy wide support.  Therefore, a mechanism to convey feedback messages according to H.271 appears to be desirable.

RTCP Receiver Report Extensions
1. CCM – Codec Control Message
2. FIR – Full Intra Request Command
A Full Intra Request (FIR) Command, when received by the designated
media sender, requires that the media sender sends a Decoder Refresh
Point (see section 2.2) at the earliest opportunity.  The evaluation
of such an opportunity includes the current encoder coding strategy
and the current available network resources.

FIR is also known as an “instantaneous decoder refresh request”,
“fast video update request” or “video fast update request”.

3. TMMBR – Temporary Maximum Media Stream Bit Rate Request
4. TMMBN – Temporary Maximum Media Stream Bit Rate Notification

Example from RFC5104:

Receiver A: TMMBR_max total BR = 35 kbps, TMMBR_OH = 40 bytes
Receiver B: TMMBR_max total BR = 40 kbps, TMMBR_OH = 60 bytes

For a given packet rate (PR), the bit rate available for media
payloads in RTP will be:

Max_net media_BR_A =
TMMBR_max total BR_A – PR * TMMBR_OH_A * 8 … (1)

Max_net media_BR_B =
TMMBR_max total BR_B – PR * TMMBR_OH_B * 8 … (2)

For a PR = 20, these calculations will yield a Max_net media_BR_A =
28600 bps and Max_net media_BR_B = 30400 bps, which suggests that
receiver A is the limiting one for this packet rate.  However, at a
certain PR there is a switchover point at which receiver B becomes
the limiting one.  The switchover point can be identified by setting
Max_media_BR_A equal to Max_media_BR_B and breaking out PR:

TMMBR_max total BR_A – TMMBR_max total BR_B
PR =  ——————————————- … (3)
8*(TMMBR_OH_A – TMMBR_OH_B)

5. TSTR – Temporal-Spatial Trade-off Request

5. TSTN – Temporal-Spatial Trade-off Request

6. VBCM – H.271 Video Back Channel Message

7. RTT – Round Trip Time
A receiver that receives a request closely after
sending a decoder refresh point — within 2 times the longest round
trip time (RTT) known, plus any AVPF-induced RTCP packet sending
delays — should await a second request message to ensure that the
media receiver has not been served by the previously delivered
decoder refresh point.  The reason for the specified delay is to
avoid sending unnecessary decoder refresh points.

8. NACK
8a. PLI – Picture Loss Indication
8b. SLI – Slice Loss Indication
8c. RPSI – Reference Picture Selection Indication

Here’s a sample INVITE command relaying from FreeSWITCH:

Sample SDP of WebRTC for Firefox

 

 

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