Internet-Draft RTCP LNTF June 2024
Majali Expires 5 December 2024 [Page]
Workgroup:
AVTCORE Working Group
INTERNET-DRAFT:
draft-majali-avtcore-lntf-feedback-message-00
Published:
Intended Status:
Experimental
Expires:
Author:
S. Majali
Nvidia

RTCP feedback Message for Loss Notification

Abstract

Real-time media streams that use RTP, use multiple mechanisms to recover from packet and frame loss. Sender to recover frame loss effectively, it can use information on the last frame receiver successfully decoded and latest packet it received on network. This document describes an RTP Control Protocol (RTCP) feedback message intended to notify the sender with loss details.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on 5 December 2024.

Table of Contents

1. Introduction

This document proposes RTCP feedback message Loss Notification (LNTF) during packet loss, containing information about the latest frame decoded and latest packet received by the receiver. This feedback is beneficial in video streaming; video encoder can use this information to encode future frames by referencing successfully decoded frames.

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

2. LNTF Message

LNTF message is identified by payload type defined as Payload-Specific Feedback message (PSFB).

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |V=2|P| FMT=15  |   PT=206      |             length            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                   SSRC of packet sender                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                   SSRC of media source                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |              Unique identifier 'L' 'N' 'T' 'F'                |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | Last Decoded Sequence Number  | Last Received SeqNum Delta  |D|
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1

The fields V, P, FMT, PT, length and SSRC are defined in the RTP specification Section 6.1 of [RFC4585], the respective meaning being summarized below:

version (V): 2 bits
This field identifies the RTP version. The current version is 2
padding (P): 1 bit
If set, the padding bit indicates that the packet contains additional padding octets at the end that are not part of the control information but are included in the length field.
feedback message type (FMT): 5 bits
This field identifies the type of the FB message. It must have the value 15, Application layer FB message
payload type (PT): 8 bits
This is the RTCP packet type that identifies the packet as being an RTCP FB message. The value must be PT=206, Payload-specific message (PSFB)
length: 16 bits
The length of this packet in 32-bit words minus one, including the header and any padding. This is in line with the definition of the length field used in RTCP sender and receiver reports Section 6.4.1 of [RFC3550]
SSRC of packet sender: 32 bits
The synchronization source identifier for the originator of this packet
SSRC of media source: 32 bits
The synchronization source identifier of the media source that this piece of feedback information is related to.
Unique identifier: 32 bits
LNTF message is identified by ASCII characters ‘L’, ‘N’, ‘T’, ‘F’ (0x4C4E5446).
Last Decoded Sequence Number: 16 bits
RTP sequence number of last frame decoded, first packet sequence number of the frame is chosen.
Last Received Sequence Number Delta: 15 bits
Delta between last received sequence number and last decoded sequence number.
Decodability flag (D): 1 bit
Decodability flag indicates if last packet received is part of the frame which potentially can be decoded. D=true for key frame without RTP sequence gaps in that keyframe or frame with all dependencies decodable. Frame with sequence number gap has D=false.

3. SDP Definitions

This section defines Session Description Protocol (SDP) signaling for LNTF feedback that can be employed by applications.

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

rtcp-fb-pt
= "*" ; wildcard: applies to all formats
= /fmt ; as defined in SDP
rtcp-fb-val
=/ "lntf"

The literals of the above grammar have the following semantics: Feedback type "lntf": This feedback type indicates loss notification.

4. IANA Considerations

Upon publication of this document as an RFC (if it is decided to publish it), IANA is requested to register the string "lntf" in its registry of "rtcp-fb" values in the SDP attribute registry group.

5. Security Considerations

This is not different from security considerations for any other RTCP message.

6. Acknowledgements

The authors would like to thank Alfred Heggestad and Harald Alvestrand for their reviews and valuable comments.

7. References

7.1. Normative References

[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.
[RFC4585]
Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, "Extended RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, DOI 10.17487/RFC4585, , <https://www.rfc-editor.org/info/rfc4585>.
[RFC3550]
Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, , <https://www.rfc-editor.org/info/rfc3550>.

Author's Address

Shridhar Majali
Nvidia
2788 San Tomas Expressway
Santa Clara, CA 95051
United States of America