Subject: [Sip] Via syntax question

From: <[email-masked]>
Date: Fri, 14 Apr 2006 08:48:11 +0200

Dear SIP experts,
I have the following question:

In RFC 3261 I found the syntax for the Via header being:

---------------------------------------------------------
Via               =  ( "Via" / "v" ) HCOLON via-parm *(COMMA via-parm)
via-parm          =  sent-protocol LWS sent-by *( SEMI via-params )
via-params        =  via-ttl / via-maddr
                     / via-received / via-branch
                     / via-extension
---------------------------------------------------------

In a response message in a call trace I found the following:

---------------------------------------------------------
Via: SIP/2.0/UDP
10.127.128.230;branch=z9hG4bK02aa.428ac753.0,SIP/2.0/UDP
10.127.246.8:5060;rport=5060;branch=z9hG4bK-1145bc-43786746-21c68c42
-----------------------------------

Syntactically this seems to be correct, but how should I interprete this
Via header.
Is this the same as having 2 Via headers (showing the retrun path to be
usedby this response) or does this indicate 2 alternative return paths ?

Many thanks

Jan Van Geel
IT and Network Development Specialist
Belgacom ITN/NIS/NST/NCP
Tel : + 32 2 202 1035
Email : [email-masked]
           [email-masked]
www.cyberwezz.tk

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---

From: "Kasthuri Ram V." <[email-masked]>
Date: Fri, 14 Apr 2006 15:47:52 +0530

On Fri, 2006-04-14 at 08:48 +0200, [email-masked] wrote:
> Syntactically this seems to be correct, but how should I interprete
> this Via header. 
> Is this the same as having 2 Via headers (showing the retrun path to
> be usedby this response) or does this indicate 2 alternative return
> paths ?
> 

Reference: RFC 3261, Section "7.3.1 Header Field Format"

<quote>
   Multiple header field rows with the same field-name MAY be present in
   a message if and only if the entire field-value for that header field
   is defined as a comma-separated list (that is, if follows the grammar
   defined in Section 7.3).  It MUST be possible to combine the multiple
   header field rows into one "field-name: field-value" pair, without
   changing the semantics of the message, by appending each subsequent
   field-value to the first, each separated by a comma.  The exceptions
   to this rule are the WWW-Authenticate, Authorization, Proxy-
   Authenticate, and Proxy-Authorization header fields.
</quote>

Via header is not an exception and hence in my understanding, your example
should be interpreted same as having two Via headers in the respective order.

HTH,
KasthuriRam
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