Subject: [Sip] Using domain name instead of IP address in Contact header

From: "Karapetkov, Stefan H" <[email-masked]>
Date: Fri, 24 Oct 2003 10:00:01 -0700

Hello,

RFC 3236 section 6 discusses "constructing SIP URIs" for inclusion in a
Contact header. The RFC talks about clients and servers, and the problem
that it is trying to solve concerns a proxy client trying to locate a proxy
server. It is not clear to me if this RFC is intended to cover the case of a
proxy trying to locate a UA, e.g. a SIP phone or a SIP soft client. 

The only reason the RFC gives for using a domain name instead of an IP
address is because of the possible need to use a back-up server. By using
the domain name the DNS server can resolve the domain name to either the
main server or the back-up server. However, in the case of a SIP phone or
SIP client, back-up does not apply.

All products we have seen at SIPit use an IP address in the Contact header,
not a domain name. Does anyone see value in using a domain name in a SIP UA
implementation instead of the currently used IP address?

Regards,

Stefan

---

From: Paul Kyzivat <[email-masked]>
Date: Fri, 24 Oct 2003 15:27:45 -0400

Karapetkov, Stefan H wrote:
> that it is trying to solve concerns a proxy client trying to locate a 
> proxy server. It is not clear to me if this RFC is intended to cover the 
> case of a proxy trying to locate a UA, e.g. a SIP phone or a SIP soft 
> client.
> 
> address is because of the possible need to use a back-up server. By 
> using the domain name the DNS server can resolve the domain name to 
> either the main server or the back-up server. However, in the case of a 
> SIP phone or SIP client, back-up does not apply.
 >
> All products we have seen at SIPit use an IP address in the Contact 
> header, not a domain name. Does anyone see value in using a domain name 
> in a SIP UA implementation instead of the currently used IP address?

Certainly there are common cases where a UA does not have a backup, and 
in those cases an IP address may be just as useful as a domain name.

BUT, there are important cases where a UA may be backed up. E.g. 
Gateways, IVRs, music-on-hold servers.

Thus it is important that domain names be supported. In a particular 
case, where it is your device and you are providing the address, you may 
elect to use an IP address. But please don't assume that others do the same.

	Paul

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