Re: INVITE 200 OK - TCP retransmission

On Sep 15, 2008, at 10:00 PM, Rockson Li (zhengyli) wrote:

> check RFC3261 sec 13.3.1.4
>
>       Since 2xx is retransmitted end-to-end, there may be hops between
>       UAS and UAC that are UDP.  To ensure reliable delivery across
>       these hops, the response is retransmitted periodically even if  
> the
>       transport at the UAS is reliable.
>
>

Even more critically, there may be insane or overloaded proxies in the  
path that drop or defer the message. So even if TCP is used end-to- 
end, SIP requires retransmission-until-acknowledgement at the protocol  
level.

I have long maintained that if we'd thought this through that instead  
of the three-message INVITE transaction that we'd have been better off  
using opposing pairs of two-message transactions. Carrying payloads in  
what are essentially acknowledgement messages (200) was a bad idea.  
But it's what we do, hence the retransmission.

--
Dean
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