[SIPForum-discussion] clarification on contact header

Abhishek Mishra abhishek.mishra at globallogic.com
Mon Jul 30 08:02:27 UTC 2007

Hi Andrew, Halit
I think, the Contact header works in a different was from your perseption.
>From RFC 3261: Contact contains a SIP or SIPS URI that represents a direct route to
   contact Alice, usually composed of a username at a fully qualified
   domain name (FQDN)
Contact header contains a routable address of the party sending contact header. It is used for sending future request. The basic difference between Contact and From header is that URI in the From header is the 'address-of-record' which is registered to the proxy. For example, you have a e-mail account on gmail then you would be allocated an ID of the form yourname at gmail.com. Similarly, the AOR in the From header contains the domain of the Proxy where user is registering.
Contact header denotes the actual address where other endpoint can directly send the request. So, in case of INVITE, Contact header would contact the IP/FQDN of the originating endpont (not AOR) and its in 200OK, Contact header would contain IP/FQDN of the terminating endpoint. Since 200 OK contains the Contact header therefore ACK is usually sent directly to the terminating endpoing (unless Record-Route is used).
Contact header is mandatory in INVITE request so that ACK and future request could be sent directly and does not required re-routing.
Contact header is optional in REGISTER method. When used, registrar uses its value to maintain mapping between user's AOR and actual IP/FQDN where user could be used. If contact header is not sent in the REIGSTER request, registrar would simply return the present bindings.
Please tell if this does not answers your query.
Kind Regards,


From: discussion-bounces at sipforum.org on behalf of Halit Sakca
Sent: Sun 7/29/2007 4:44 PM
To: andrew at asiatel.com.sg; discussion at sipforum.org
Subject: Re: [SIPForum-discussion] clarification on contact header

Hi Andrew,
As far as I know about the Contact header is, when SBC sends a request to Redirect server it founds the related proxy and turns back to SBC with contact header (f.e in 302) that contains the IP address of this Element.
It is clear that with contact header info, related element directly send respond to this element (could be element or UA)
hope its helpful,


> Date: Sat, 28 Jul 2007 01:43:20 +0800
> From: andrew at asiatel.com.sg
> To: discussion at sipforum.org
> Subject: [SIPForum-discussion] clarification on contact header
> Dear members,
> I'm some difficulties on understanding the purpose of the contact 
> header. Please advice what role does it play and how it is different 
> when compared with the FROM header? and why is that contact header is 
> not mandatory in all SIP requests as described in 8.1.1 RFC3261. Thank you.
> -- 
> Cheers,
> Asiatel Singapore Pte Ltd
> Andrew Yu
> 19 Jalan Kilang Barat
> #06-01, Acetech Centre
> Singapore 159361
> Tel: +65 6271 8233
> Fax: +65 6274 4266
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