[Serusers] How to change INVITE From field

[SIP]

Olaf Bergmann wrote:
> x-ser at sidell.org wrote:
>   
>> For outbound pstn calls, I want to change the INVITE message's From:
>> header field to specify my caller id.  For example, I want to change
>> it from this:
>>
>> From: 201 <sip:201 at 67.32.47.145:5060>;tag=4f3c1a5892c015f
>>
>> to this:
>>
>> From: Forte Inc <sip:9195551234 at 67.32.47.145:5060>;tag=4f3c1a5892c015f
>>
>> What's the best way to change the "comment" and LHS of the address?
>>     
>
> The general recommendation is not to do this. Besides
> RFC-2543-related reasons, a compelling argument is that the branch
> tag created by SER is (IIRC) built from a hash over specific parts
> of the From field.
>
> Regards,
> Olaf
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>   
Out of curiosity, is this just a limitation because of the way SER
handles it? Reading 2543, I simply see:


	"Call-ID, To and From are needed to identify a call leg.
        The distinction between call and call leg matters in calls
        with multiple responses to a forked request. The format is
        similar to the equivalent RFC 822 [24] header, but with a
        URI instead of just an email address."


There's a reference that the tag header MAY (or may not) be used in a
URI, but MUST be used if one is to identify unique calls from the same
location, which to me implies that the important part of the From/To
headers for identification is the tag.

	"The "tag" MAY appear in the From field of a request. It MUST be
   	present when it is possible that two instances of a user sharing a
   	SIP address can make call invitations with the same Call-ID."


The implication of this is that the ENTIRE From/To headers are not
necessarily immutable, as the tag is used to identify them as unique.

The question is, is there something elsewhere which contradicts this
(and therefore enforced the rule of immutable From/To headers), or is
this just a SER idiosyncrasy? If it's just a SER thing, might it not be
different in other systems (something we might have to watch out for),
or might it not be coded around within SER to allow modification of the
 From header?

While I'm not in a rush to go out and change headers, I was just curious.

N.