Re: Query related to Expires header with value 0 in INVITE

Anita Tailor wrote:
> Hi All,
> 
>  
> 
> I have some queries related to expires header in SIP message.
> 
>  
> 
> /RFC 3261 Section 20.19///
> 
> /" The value of this field is an integral number of seconds (in decimal)
>    between 0 and (2**32)-1, measured from the receipt of the request"/
> 
>  
> 
> Does this mean 0 is a valid value of Expires header ? I am confused 
> because of "between" word here as meaning might different if "from 0 to  
> (2**32)-1" have used.

Its certainly valid in *some* contexts. For instance in REGISTER and 
SUBSCRIBE this is the way to terminate a registration or subscription.

> Does Expires value in INVITE is modified by intermittent SIP entities 
> before it reaches to its destination?

According to table 2 of 3261, proxies are not permitted to add, modify, 
or delete this header. But of course a B2BUA can do as it wishes.

> If yes, then what should be UAS behavior when it receives INVITE with 
> value 0 in Expires header ? Should UAS respond with 487 ?
> 
> /Section 13.1.1 [1]/
> 
> /  "If the request is an INVITE that contains an Expires header
>  field, the UAS core sets a timer for the number of seconds
>  indicated in the header field value.  When the timer fires, the
>  invitation is considered to be expired.  If the invitation
>  expires before the UAS has generated a final response, a 487
>  (Request Terminated) response SHOULD be generated."/

The above is SHOULD strength. I expect you will find many UAs that don't 
bother with this.

But if you are implementing the UAS, this would be a good thing to do. 
There is no point in establishing a session if the caller no longer 
wants it.

	Paul
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