Questions about RTP header extension

[Piasy Xu]

Hi folks,

When browsing WebRTC's source code, I find there is a RTPExtensionType enum definition:

enum RTPExtensionType : int {

  kRtpExtensionNone,

  kRtpExtensionTransmissionTimeOffset,

  kRtpExtensionAudioLevel,

  kRtpExtensionAbsoluteSendTime,

  kRtpExtensionVideoRotation,

  kRtpExtensionTransportSequenceNumber,

  kRtpExtensionPlayoutDelay,

  kRtpExtensionVideoContentType,

  kRtpExtensionVideoTiming,

  kRtpExtensionFrameMarking,

  kRtpExtensionRtpStreamId,

  kRtpExtensionRepairedRtpStreamId,

  kRtpExtensionMid,

  kRtpExtensionGenericFrameDescriptor,

  kRtpExtensionNumberOfExtensions  // Must be the last entity in the enum.

};

https://webrtc.googlesource.com/src/+/ab09039d2ab6d66d3a57c6caa626034d9effb62e/modules/rtp_rtcp/include/rtp_rtcp_defines.h#100

and header extension data format graph:

// Coordination of Video Orientation in RTP streams.

//

// Coordination of Video Orientation consists in signaling of the current

// orientation of the image captured on the sender side to the receiver for

// appropriate rendering and displaying.

//

//    0                   1

//    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5

//   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

//   |  ID   | len=0 |0 0 0 0 C F R R|

//   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

https://webrtc.googlesource.com/src/+/ab09039d2ab6d66d3a57c6caa626034d9effb62e/modules/rtp_rtcp/source/rtp_header_extensions.cc#154

Where could I find reference materials about these header extensions definition? I googled a lot, but didn't find any RFC or RFC draft.

Thanks!

[Lorenzo Miniero]

It's actually a 3GPP specification:

https://tools.ietf.org/html/rfc7742#ref-TS26.114

Lorenzo

[Piasy Xu]

Thanks for your reply! The 3GPP spec defines the format of CVO header extension, but what about other header extensions?

在 2018年10月8日星期一 UTC+8下午11:11:36，Lorenzo Miniero写道：

[Steve Anton]

Hi,

Each RTP header extension has a URI associated with it used for negotiation. The URI target usually describes the format/purpose of the header extension. Here's the URIs for the header extensions supported in WebRTC: https://codesearch.chromium.org/chromium/src/third_party/webrtc/modules/rtp_rtcp/source/rtp_header_extensions.h

Best,

Steve

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[Piasy Xu]

Hi Steve,

Thanks for your reply!

The kUri field is a good clue to find reference materials, but I still have one question: where defines the id of each extension?

For example, the AudioLevel extension, in rfc6464, I can find the extension format, but I can't find where defines the id should be 2.

在 2018年10月9日星期二 UTC+8上午1:13:12，Steve Anton写道：

[Steve Anton]

The RTP header extension IDs are negotiated during the offer/answer exchange. See the a=extmap: lines: https://webrtchacks.com/sdp-anatomy/

To view this discussion on the web visit https://groups.google.com/d/msgid/discuss-webrtc/813e39de-dbf7-4f74-9df5-ef1efc32d1b3%40googlegroups.com.

[Piasy Xu]

I get it, thank you Steve!

在 2018年10月9日星期二 UTC+8上午8:33:13，Steve Anton写道：