Subject: [Sip] RE: [Sip-implementors] Via: processing

From: "Paul D.Smith" <[email-masked]>
Date: Fri, 30 Jan 2004 16:25:38 -0000

Brett, Arlie,

RFC3261 clearly allows a response to be "rerouted" if it can be determined
that the response did not reach the first-attempt target.  For example, if
the request is received over TCP, and the connection has gone and cannot be
reestablished by the time the response is ready to be sent back, RFC3261
states the following

--- from RFC3261, section 18.2.2, 1st bullet.

If that connection attempt fails, the server SHOULD use the procedures in
[4] for servers in order to determine the IP address and port to open the
connection and send the response to.

--- end of cut

RFC3263 then gives an algorithm for attempting to determine alternate IP
addresses to which the response might be sent.  Clearly having an FQDN in
the sent-by will give a chance of success which having an IP address will
not, especially if the sent-by and received IP addresses are the same!

Paul DS.

Paul D.Smith
Network Protocols Group 
Data Connection Ltd (DCL)
Tel: +44 20 8366 1177  Email: [email-masked] 
Fax: +44 20 8363 1039  Web:   http://www.dataconnection.com

-----Original Message-----
From: Brett Tate [mailto:[email-masked]]
Sent: 29 January 2004 00:27
To: SIP (E-mail)
Subject: RE: [Sip-implementors] Via: processing

> I'm looking at the spec for Via: processing.  
> RFC 3261 clearly shows that a Via: line can 
> contain DNS names, as well as textual
> representations of network addresses.
> 
> How many products actually implement support 
> for resolving DNS names provided in Via: lines?  

Not very many support DNS resolution of a
Via entry.  However many support the concept
of adding the Via's received parameter when
it does not match the Via's sent-by address.

> This seems...  well, it seems unnecessary,
> potentially insecure (allowing for easy DDoS 
> and potentially amplification attacks), and 
> just a general nuisance.  Is this kind of
> thing ever actually used for a good purpose?  

Some people have toyed with the idea of
advancing response addresses based upon
rfc3263 during failure situations.

> Does any product do anything *but* just put 
> real network addresses in Via: lines in
> requests?

Yes.

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