Early Media in SIP: Problem Statement, Requirements, and Analysis of Solutions draft-barnes-sip-em-ps-req-sol Richard Barnes BBN Technologies IETF 68, Prague, CZ

Overview Early media is a longstanding problem Lots of proposed solutions, no consensus This document tries to provide structure for the discussion –Clear problem statement –Consolidated list of requirements –Initial analysis of solution space

What is Early Media? Media that flows before the 200/OK response in an INVITE transaction. UAC / offerorUAS / answerer INVITE SDP offer 200/OK SDP answer Early Media

Why Early Media a problem? UAC gets media with no SDP answer The asymmetry of information is what causes problems –Forking: Multiple un-signaled streams –Security: Can’t use signaling to complete a Diffie- Hellman exchange Draft uses the definition “early” = “before the offeror has received an answer” This definition still allows pre-200 media, as long as an answer is sent

What’s the solution A normative update to RFC 3261 and RFC 3264 (et al.) that requires an SDP answer to be received by the offeror before anyone sends media. –Provide reliable transport for an SDP answer –Forbid sending media until receipt of answer is acknowledged

Sketch of a Solution UAC / offerorUAS / answerer INVITE SDP offer 200/OK (SDP answer) pre-200 Media pre-200 SDP answer

Existing Solutions Lots of solutions have been come and gone –RFC too vague –Reliable requires PRACK –ICE - Uses non-SIP messages None were mandatory to implement All met with objections So lets consolidate those objections into requirements …

Requirements 1.Ensure an answer before media 2.Minimize additional messaging 3.Backward-compatibility with SIP as defined today 4.Well-defined PSTN interactions 5.No new DoS opportunities 6.Free from IPR constraints Others?

Solution space Messages with SIP INVITE transaction –Early 200/OK –Offer-less INVITE –Reliable provisional response –Separate INVITE 3960 App. Server Model / EMIND Non-INVITE SIP mechanisms –INFO / UPDATE (3960 Gateway Model) –Other response codes (e.g. 401 w/ null auth) Non-SIP mechanisms –Lower-layer protocol