Rtcp xr blocks for synchronization delay and offset metrics reporting
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RTCP XR Blocks for Synchronization Delay and Offset Metrics Reporting. draft-ietf-xrblock-rtcp-xr-synchronization-01. Hitoshi Asaeda ([email protected] ) R. Huang ([email protected]) Qin Wu ([email protected]). Updates Since Last Version.

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Rtcp xr blocks for synchronization delay and offset metrics reporting
RTCP XR Blocks for Synchronization Delay and Offset Metrics Reporting

draft-ietf-xrblock-rtcp-xr-synchronization-01

Hitoshi Asaeda ([email protected] )

R. Huang ([email protected])

Qin Wu ([email protected])


Updates Since Reporting Last Version

  • Clarified definition of synchronization offset

    • The relative time difference of two media streams that need to be synchronized.

  • Explain how to use in-band mapping of RTP and NTP-format timestamps to calculate initial synchronization delay

    • The average time taken to receive the first RTP header extension containing in-band mapping of RTP and NTP-format timestamps.

  • Improved SDP section

    • - Added a subsection to include the extended syntax.

    • Added a subsection to clarify SDP Offer/Answer usage

  • Updated references

    • - Adding one reference, TR-126, to normative references

    • - Updating some references to the latest version.


  • In RTP flow synchronization offset metrics block, how to choose the reference stream hasn’t been addressed in previous version of the draft.

  • We propose to let the implementation choose which stream as the reference stream :

    • To support this, the SSRC of the reference stream field has been added to the report block in current draft.


  • RTP flow synchronization offset metrics block Lacks indications to tell whether the reporting stream lags behind or heads before the reference stream

    • synchronization offset value is a 64-bit unsigned fixed-point number.

  • Proposal: Sign flag bit (in current version of the draft):

    • splitting one bit from “Reserved” field of Block header to do the indication.

  • Proposal: Signed value (proposed by Colin):

    • Using a signed 64-bit NTP timestamp format for synchronization offset .

  • Suggest to adopt the “signed value” proposal.

    • Easier for implementations.

    • Few bits are used in the most significant 32 bits.


  • How to calculate the “beginning of session” for initial synchronization delay?

  • According to RFC 6051 Section 2.1.1:

    • The single RTP session is considered to be joined once any in-band signaling for NAT traversal has concluded and/or security keying has concluded, and the media path is open.

  • For a group of RTP sessions, the beginning of session could be:

    • The time when the first RTP session was joined as the beginning of the whole multimedia session.

    • The time when the last RTP session was joined as the beginning of the whole multimedia session.

    • The time when the session with the longest report interval was joined.

      • The report interval may change as session size change and stabilization takes several report interval .

  • We should get consensus in WG.


Next Step Reporting

  • Address all the remaining issues we received.

  • Comments?


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