Transcript Document 7485139

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 / offeror
UAS / answerer
INVITE
SDP offer
Early Media
200/OK
SDP answer
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 DiffieHellman 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 / offeror
UAS / answerer
INVITE
SDP offer
pre-200
SDP answer
pre-200 Media
200/OK
(SDP answer)
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 …
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