NonINVITE Transaction Issues Robert Sparks dynamicsoft The Race

Non-INVITE Transaction Issues Robert Sparks dynamicsoft

The Race Condition UAS UAC Request 64*T 1 Timeout Response

The Race Condition • UAS believes it responded in time • UAC and UAS have different ideas of the result of the transaction • UAS has no way to know how Timeout much its transaction has been offset from the UAC • Each proxy makes the race easier to lose UAC Request UAS 64*T 1 Response 64*T 1 RESULT: UAS Non-INVITE transactions must complete immediately!

408 is Not Useful UAS UAC Request 64*T 1 Timeout t Timeout s e u q e R 408

408 is Not Useful Proxy UAC Request Proxy Request UAS Request 64*T 1 408 408 408

408 is Not Useful Proxy UAC Request Proxy Request UAS Request 64*T 1 408 200

Non-INVITE Timeouts Doom Forking Proxies Proxy UAC Proxy UAS 1 UAS 2 Request 415 64*T 1 408 415 UAS 2’s failure to respond forces both proxies to lose the race

Failure to Respond Gets You Blacklisted • RFC 3263 discusses moving to next SRV on a no-response timeout (or explicit transport failure) • Non-INVITE can’t use next SRV on this request – transaction is already over • If failure isn’t cached (address temporarily blacklisted), next request is doomed to the same failure.

Non-INVITE Provisional Responses Should they be banned? • Arguments for Banning – Wasteful since transaction must complete immediately – (Early) provisionals harm recovery from lost final responses • Arguments Against – No response causes blacklisting – Losing the race is the same as not responding at all

Proposals • Proposal A – Make minor changes to use of current Non. INVITE transaction • Proposal B – Reform the Non-INVITE transaction to allow it to pend indefinitely

Proposal A 1. Disallow non-100 provisionals 2. Disallow 100 Trying before Timer E reaches T 2 3. Encourage 100 Trying if no final response after Timer E reaches T 2 4. Disallow 408 to non-INVITE requests 5. Absorb late non-INVITE responses

Proposal A 1. 2. 3. Disallow non-100 provisionals Disallow 100 Trying before Timer E reaches T 2 Encourage 100 Trying if no final response after Timer E reaches T 2 • Prevents blacklisting • Doesn’t harm lost final response recovery • Doesn’t allow reliable provisional responses

Proposal A 4. Disallow 408 to non-INVITE requests • Currently they waste network 5. Absorb late non-INVITE responses • Don’t statelessly forward after the transaction times out

Proposal B • • Allow Non-INVITEs to pend (no timer F) Allow CANCEL ACK is not needed Backwards safe

Contrast • Proposal A – Removes waste from the network – Improves recovery from a failed node – UAS NI transactions still have to complete immediately or risk losing the race – Does not address proxy doom • Proposal B – 408 s become useful again – Relieves proxies from timeout doom – Still need to address harmful provisionals for UDP • Either deprecate UDP or take Proposal A 1 -3