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Offer/Answer Usage

Offer/Answer Usage. draft-sawada-sipping-sip-offeranswer-01 IETF-67 7-Nov-2006 P. Kyzivat, for Takuya Sawada. Overview / Scope. Intended as a clarification on offer/answer usage (BCP? Informational?) Consolidates material from many sources: RFCs 3261, 3264, 3262, 3311, 3312

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Offer/Answer Usage

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  1. Offer/Answer Usage draft-sawada-sipping-sip-offeranswer-01 IETF-67 7-Nov-2006 P. Kyzivat, for Takuya Sawada P.Kyzivat, IETF-67 SIPPING WG

  2. Overview / Scope • Intended as a clarification on offer/answer usage • (BCP? Informational?) • Consolidates material from many sources: • RFCs 3261, 3264, 3262, 3311, 3312 • Captures folk wisdom and informal agreements reached through list discussions • Includes no normative changes • Recommend future work that does make normative changes P.Kyzivat, IETF-67 SIPPING WG

  3. Status • Has already been valuable as resource in answering questions that come up frequently. • Some planned clarifications and additions yet to be incorporated. • A few significant open issues need to be addressed. • Has received limited review & comment • Needs a broader and more thorough review P.Kyzivat, IETF-67 SIPPING WG

  4. Planned Clarifications & Additions • New section of guidelines for constructing offer after receiving offerless INVITE • Clarify that a reliable provisional to an invite w/ offer need not carry an answer • Clarify that a PRACK may carry an answer only if it confirms a response containing an answer. • Clarify handling of unacceptable offers in responses. • Recommend future work to define how to reject offer in PRACK without rejecting the PRACK itself. • For details, see • http://www1.ietf.org/mail-archive/web/sipping/current/msg11309.html P.Kyzivat, IETF-67 SIPPING WG

  5. Issues • Terminology • SDP that isn’t an offer/answer • When to be prepared to receive media • Commit/Rollback of offer/answer P.Kyzivat, IETF-67 SIPPING WG

  6. Issue: Terminology – SDP that isn’t an offer/answer • What do you call the SDP in an unreliable provisional response? • The lack of consistent terminology makes discussion about behavior difficult. • In the draft we have chosen to call this a “preview” of the offer or answer. P.Kyzivat, IETF-67 SIPPING WG

  7. When to be prepared to receive media • Some alternatives: • When the offer or answer is sent • When a “preview” of the offer/answer is sent • Proposal: pass the buck • Consider early media a separate subject • Let it be handled independently P.Kyzivat, IETF-67 SIPPING WG

  8. Issue: Commit / Rollback of offer/answer • If an offer/answer initiated by a reINVITE • is completed prior to the completion of the reINVITE transaction, • and the reINVITE subsequently fails, • Is the offer/answer rolled back to the pre-reINVITE state? • Corollary issue: multiple offers/answers followed by failure (precondition resolution). • No concensus on this issue P.Kyzivat, IETF-67 SIPPING WG

  9. Issue: Commit / Rollback of offer/answer • Potential resolutions: • Each answer is committed when it is received reliably. • No going back if the reINVITE fails. • Nothing is committed until the reINVITE succeeds. • Candidate new state is built from all offers and answers. • Discarded on failure, • Committed on success of reINVITE. P.Kyzivat, IETF-67 SIPPING WG

  10. stable audio call reINVITE (offer high BW codec) PRACK 183 (unresolved preconditions) 200 (PRACK) 580 (INVITE) can’t get BW Example 1: Commit / Rollback of offer/answer P.Kyzivat, IETF-67 SIPPING WG

  11. stable audio call reINVITE (offer high BW codec) PRACK 183 (unresolved preconditions) 200 (PRACK) can’t get BW 580 (INVITE) UPDATE (offer) Example 2: Commit / Rollback of offer/answer P.Kyzivat, IETF-67 SIPPING WG

  12. stable audio call reINVITE (offer, add video) PRACK CANCEL 180 (answer, reliable) 200 (PRACK) 200 (CANCEL) 487 (INVITE) Example 3: Commit / Rollback of offer/answer (alert) P.Kyzivat, IETF-67 SIPPING WG

  13. stable audio call reINVITE (offer, add video) PRACK (alert) 200 (PRACK) 180 (answer, reliable) (reject) 403 (INVITE) Example 4: Commit / Rollback of offer/answer P.Kyzivat, IETF-67 SIPPING WG

  14. Issue: Commit / Rollback of offer/answer • Observation: • The use cases for this are obscure • Cases examined can be handled so as to avoid the problem. • Conjecture: this is always so. • Can someone provide counter example? • Possible Resolution: • Best practices that prevent the case from arising. P.Kyzivat, IETF-67 SIPPING WG

  15. INVITE (offer 1) PRACK 183 (answer 1, reliable) 200 (PRACK) 180 (no offer, reliable) 200 (no offer) 200 (INVITE) PRACK (offer 2) Race between PRACK & 200 OK P.Kyzivat, IETF-67 SIPPING WG

  16. Next Steps • Make planned changes • Reach consensus on issues on sipping list • Issue new version as WG draft • Hopefully, WGLC – before IETF-68 P.Kyzivat, IETF-67 SIPPING WG

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