[Sip] RE: [Sip-implementors] RFC3263 - Server Usage query

["Brett Tate" <brett at broadsoft.com>]

> My question is regarding the first step of the above response 
> processing logic. 
> 
>  
> 
> Since SRV lookups are performed on a domain-name only when no 
> port is specified, this obligates the sender of a request to 
> not encode a port in its Via header if that sender wishes to 
> be reachable using SRV resolution. Consequently, this appears 
> to imply that the sender needs to be listening for responses 
> on the default SIP port in addition to any of the ports 
> resolved from the SRV query, since the first transmission of 
> any incoming response will always be directed towards the 
> default port as per section 18.2.2 of RFC3261.  

Yes, when sending over a non reliable transport.
  
 
> As I understand it, based on how this rule is currently 
> worded, if none of the SRV resolutions of Via Sent-By 
> domain-name resolve to port 5060 AND if a UAC is not 
> listening for responses on port 5060, the initial 
> transmission of every response will always encounter a 
> failure before resorting to SRV procedures.

Yes.  If the sender does not include a port and does not listen on 5060,
the response will fail to be delivered over the non reliable transport.
And if the sender of the response is unable to obtain knowledge of the
failure (ICMP errors suppressed), the rfc3263 procedures will not occur.
If UAS is able to detect the failure, the rfc3263 procedures should
occur.

The rfc3261 text (and limitation) appears intentional.  However a SIP
extension similar to rfc3581 could created and used.

The maddr usage has a similar limitation; thus it would not help bypass
the limitation.

http://www1.ietf.org/mail-archive/web/sip/current/msg07554.html


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