Re: [sipcore] UPDATE, reINVITE rejections, and session-state rollback

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Hi Byron Campen,

Before getting into the details, I'd like to check does the "INVITE/500" 
just means the 5xx of INVITE?

Thanks,

Gao

>    Sending to the authors of the offeranswer and reinvite drafts, 
> since I am not sure which bucket this would fall in:
> 
>    Consider the following. In all cases, this is within a reINVITE 
> transaction, after an offer/answer exchange has completed using 
> 100rel (which kind is not important), that is being rejected by the 
> server side due to some unspecified error:
> 
>                  A          B
>                  |<---------| INVITE/500
>                  |          |
>                  |          |
>   UPDATE (offer) |--------->|
>                  |          |
>                  |<---------| UPDATE/200 (answer)
> 
>                  A          B
>   UPDATE (offer) |---\  /---| INVITE/500
>                  |    \/    |
>                  |    /\    |
>                  |<--/  \-->|
>                  |          |
>                  |<---------| UPDATE/200 (answer)
> 
>                  A          B
>                  |        /-| INVITE/500
>                  |       /  |
>   UPDATE (offer) |------/-->|
>                  |     /    |
>                  |<---/-----| UPDATE/200 (answer)
>                  |   /      |
>                  |<-/       |
> 
>    All of these look identical to B; UPDATE offer/answer completes 
> after the end of the INVITE transaction. However, A sees them very 
> differently. B cannot know that something unusual has happened. Also
> consider the following cases:
> 
>                  A          B
>   UPDATE (offer) |--------->|
>                  |          |
>                  |<---------| UPDATE/200 (answer)
>                  |          |
>                  |<---------| INVITE/500
> 
>                  A          B
>   UPDATE (offer) |--------->|
>                  |          |
>                  |       /--| UPDATE/200 (answer)
>                  |      /   |
>                  |<----/----| INVITE/500
>                  |    /     |
>                  |<--/      |
> 
>    Again, these look the same to B, but different to A. What is the 
> best approach to ensuring that session-state is consistent on both 
> ends in all of these cases? It seems to me that one way would be to 
> specify that rollback of a reINVITE does not invalidate session-
> state established with an UPDATE, regardless of whether that UPDATE 
> transaction occurred (or appeared to have occurred) during the 
> reINVITE transaction. If we insist that session-state established 
> with an in-reINVITE UPDATE be rolled back (the current language says
> that both sides must roll back to the session-state in use prior to 
> the reINVITE), then there is no way to ensure consistent rollback on
> both ends, even if everything has the same rollback logic, because 
> what appears to be an in-reINVITE UPDATE on one end may appear to be
> after the reINVITE transaction on the other end. If any portion of 
> the UPDATE transaction appeared to have happened within the reINVITE
> transaction, then the observing UA must send a sub
>  sequent UPDATE to re-sync (since there is no guarantee that the 
> other end saw the UPDATE within the reINVITE transaction). This is 
> likely to cause glare, unless we put asymmetric timers on the re-
> sync, and even then there will be a delay similar to that caused by 
glare.
> 
>    My gut says that it would be better to not rollback session-state
> established by an UPDATE, under any circumstances. But, there may be
> situations where the session-state established in an UPDATE is 
> contingent on the reINVITE succeeding it. Would it be appropriate to
> say that if a UA "feels" this way, it is responsible for re-syncing?
> Or is there something I am missing here?
> 
> Best regards,
> Byron Campen
> 

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