Early%20Media%20in%20SIP:%20Problem%20Statement,%20Requirements,%20and%20Analysis%20of%20Solutions%20draft-barnes-sip-em-ps-req-sol

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Early Media in SIP Problem Statement,
Requirements, and Analysis of Solutionsdraft-ba
rnes-sip-em-ps-req-sol

• Richard Barnes
• BBN Technologies
• IETF 68, Prague, CZ

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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

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What is Early Media?

• Media that flows before the 200/OK response in an
  INVITE transaction.

UAC / offeror
UAS / answerer
INVITE SDP offer
Early Media
200/OK SDP answer
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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 Cant 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

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Whats 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

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Sketch of a Solution
UAC / offeror
UAS / answerer
INVITE SDP offer
pre-200 SDP answer
pre-200 Media
200/OK (SDP answer)
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Existing Solutions

• Lots of solutions have been come and gone
• RFC 3960 - too vague
• Reliable 183 - requires PRACK
• ICE - Uses non-SIP messages
• None were mandatory to implement
• All met with objections
• So lets consolidate those objections into
  requirements

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Requirements

• Ensure an answer before media
• Minimize additional messaging
• Backward-compatibility with SIP as defined today
• Well-defined PSTN interactions
• No new DoS opportunities
• Free from IPR constraints
• Others?

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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