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答复: Re: PRACK: Does non-200 response cease re-transmission ofreliable 18x?




Hi,

Brett Tate has listed some.
Also:
 o. Services can be supplied with fewer messages.It helps lighten the
servers stress.
 o. You can recognize the session for the tone easily.
 o. It's useful for services such as CF(call forward),in which case there
may be several early diologs.

Regards,
Eric




> > Do you find some advantage to it over simply using early media
> > with a different to-tag?
>
> It allows the early media session and answered media session to be
> negotiated prior to INVITE 2xx.  The main benefit is to reduce
> clipping upon INVITE 2xx if forking has occurred and all UAS
> actually support early-session.
>
> More specifically, UAS may assume only answered media will be sent
> to the port for answer; thus it could quickly start rendering
> answered media prior to receiving INVITE 2xx.  UAC can allocate
> other ports for early media; or it can refuse or hold early media
> without it subsequently delaying answered media (by requiring
> another offer/answer exchange).
>
> Works good in theory if all user-agents support it.  I have no idea
> how well it works in the real world. :)
>
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