1 Coping with Early Media Brian Stucker Nortel Systems/Standards Architect November 6th, 2006

2 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

3 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

4 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).

5 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.

6 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’.

7 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.

8 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.

9 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™.