On Dec 11, 2012, at 6:29 AM, ikuzar RABE <[email protected]> wrote:

> Hi all,
> 
> I'd like to know what are the differences between Request-URI and to URI ?
> from URI and contact URI ?

All are URIs, Universal Resource Indicators, which aren't really universal (of 
they were, we wouldn't have GRUU) but still serve to designate resources within 
a limited scope.

The RequestURI is the thing that is used to select routes across the network to 
find the destination. In early SIP, it got changed at each proxy hop, which was 
a confusing and bad idea (see "strict routing"). This was sort-of fixed in RFC 
3261. In theory, it should be meaningful to the terminating UA by the time the 
request reaches the terminating UA, but might not be.

The To: URI represents the identity to which the originating UA thought it was 
sending the request, which may or may not be of any meaning to the eventual 
recipient, and is only vaguely related to the Request-URI.

The From: URI represents the identity that the originating UA uses to describes 
itself, which may or may not be of any meaning to the terminating UA. This 
might possibly be useful for display of caller-ID, as long as you don't put too 
much faith in it.

We apparently suffered from a plethora of identity-determination issues when 
writing SIP.

--
Dean
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