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Coping with Early Media. Brian Stucker Nortel Systems/Standards Architect November 6th, 2006. Overview of Draft. Identify common problems with early media in SIP: Identify common ways that proxies and endpoints deal with early media

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coping with early media

Coping with Early Media

Brian StuckerNortel Systems/Standards ArchitectNovember 6th, 2006

overview of draft
Overview of Draft
  • Identify common problems with early media in SIP:
  • Identify common ways that proxies and endpoints deal with early media
  • Identify a set of requirement boundaries that any changes to the protocol should fit within
  • Identify a set of recommendations to start working the problems
problems with early media non exhaustive
Problems with Early Media (non-exhaustive)
  • Signaling path and media are disconnected
    • Clipping
    • No correlation b/w signaling and media stream
  • “SDP in a bottle”
    • SDP offerer must render all media streams sent to it
    • Untrusted data may be sent prior to answer
  • Forking interactions
    • Mob of media streams can result
    • Offerer may have little or no control over where media streams are coming from
types of early media
Types of Early Media
  • Pre-routing
    • Triggered by proxy prior to routing step via SDP.
  • Pre-presentation
    • Triggered by proxy after initial routing step, but before forwarding INVITE to next hop via SDP.
  • Post-presentation
    • Triggered by subsequent hops after proxy has forwarded request (includes forking) via SDP.
  • Non-SDP
    • Triggered (typically by an end-device) through SIP signaling (Ex: Alert-Info header).
coping mechanisms
Coping Mechanisms
  • Proxy SDP manipulation
    • Stripping/Delaying – SDP answer is manipulated in 18x responses.
    • Weighting – May be used in conjunction with stripping/delaying to allow early media it prefers to be presented.
  • Client Forking Detection
    • Client may ignore early media once forking is detected
  • Client Slow-Start
    • Client makes potential early media generators the offerer.
problems w coping mechanisms
Problems w/ Coping Mechanisms
  • Proxy SDP Stripping/Delaying –
    • Clipping
    • Fake forking may confuse client
    • PRACK
    • Breaks other early media
  • Proxy SDP weighting –
    • Breaks other early media
  • Client Forking Detection –
    • Breaks early media either all the time, or in a semi-random manner
    • Bad interaction with fake forking.
  • Client Slow-Start
    • Clipping
    • When multiple SDP offers are received, no idea which one ‘wins’.
requirements identified
Requirements Identified
  • Cannot deprecate forking from SIP.
  • Cannot deprecate early media from SIP.
  • Elimination of blind rendering of media packets by SDP offerer.
  • Creation of a mechanism by which an originating SIP UA can signal early media acceptance outside of codec negotiation/preconditions.
  • Creation of a mechanism by which a terminating SIP UA can signal the rendering requirements of early media.
  • Backward-compatibility is secondary goal, some clients will never get fixed (e.g. interworking).
  • Mechanisms must be able to deal with recursive forking scenarios.
  • Mechanisms must not require exchange of packets on the media path due to common gating mechanisms.
draft recommendations
Draft Recommendations
  • Recommendations included are intended to START conversation.
  • Early Media Classification and Prioritization.
    • Provide a means of signaling what the early media might contain and how important it is that it be rendered.
    • Exact mechanism is left open.
  • Early Media Flow Negotiation
    • Provide a means of signaling that a particular endpoint is ready or not to begin sending/receiving early media and an identifier for the RTP packets.
    • New emflow SDP attribute defined.
  • Need more investigation into how SRTP specification in SDP, key exchanges and operation with early media should work.
desired result of draft
Desired Result of Draft
  • A set of procedures that fixes early media to a large extent, or at least makes it more predictable without major deprecations to the protocol.
  • In cases where fixes can’t be made without major changes:
    • A clear, one-stop-shop explanation of what is fundamentally broken and why it can’t be fixed.
    • Recommendations on the best way to deal with early media in these cases that causes the least interruption to other parts of the protocol.
    • Discouragement of egregiously bad behavior that is likely to break other things.
  • SIP is complex. Having a reference to guide implementers and other SDOs is a good thing™.