Subject: [SIP] Destination addresses in Via/Contact

From: "Piotr S. Kossowski" <[email-masked]>
Date: Wed, 18 Oct 2000 00:15:18 +0200

Hello,

After studying of Via and Contact description in RFC-2543, My thinking
about these headers is not fully clear (My apologies, if it is basic
question). So, please someone to explain if something below is wrong:

Let's imagine an INVITE sent from UAC directly to UAS that contain 
(amongst others):

Via: SIP/2.0/UDP wonderland.com:6000 
Contact: [email-masked]:5080

After getting that INVITE by UAS, UAS should send a response
to UDP socket 6000 on wonderland.com host. 
This "wonderland.com:6000" destination is valid only for this
single request from UAC. In next requests (e.g. re-INVITE or BYE)
UAC can put in Via not the same data, for example "new" 6001 port.

As I figured out, Contact has no influence on destination of response.
Port 5080 on wonderland.com host will be used only if UAS (callee) would
like to send some request (e.g. BYE) to UAC (caller) during this call.

Is that true ?

Regards, Piotr

PS

BTW (For Henning, I suppose): There is incorrect year of expiration 
on the first side of rfc2354bis-02...

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---

From: "Vijay K. Gurbani" <[email-masked]>
Date: Tue, 17 Oct 2000 17:52:01 -0500

> to UDP socket 6000 on wonderland.com host.

Right.  A FQDN (fully qualified domain name) is recommended for Via
lists, though.  Responses are sent to the topmost Via.

> UAC can put in Via not the same data, for example "new" 6001 port.

Right.  The next request is a new transaction which has no bearing on
the previous one.  Thus the topmost Via for the new request could be,
as you indicated,

   Via: SIP/2.0/UDP wonderland.com:6001 

> As I figured out, Contact has no influence on destination of response.

Nope.  Responses follow the Via list.

> Port 5080 on wonderland.com host will be used only if UAS (callee) 
> would like to send some request (e.g. BYE) to UAC (caller) during this 
> call.
> 
> Is that true ?

Right.  It gets somewhat more complicated when a proxy is involved and
it wants to do Record-Route'ing.  In that case, a Route list has to be
constructed by the UACs, with Contact being at the bottom of the list.  
This Route list basically acts the same for requests as the Via list 
does for responses; i.e. requests are proxied to the topmost Route entry 
(which is removed before proxying).  See Section 6.34 of the bis for 
more information.

Regards,

- vijay
-- 
Vijay K. Gurbani  [email-masked] [email-masked] [email-masked]
Internet Software Group/Intelligent Network and Messaging Systems
Lucent Technologies/Bell Labs Innovations, 263 Shuman Blvd, Rm 1A-413
Naperville, Illinois 60566  Voice: +1 630 224 0216 Fax: +1 630 713 0184