[Sip] Query related to Expires header with value 0 in INVITE

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.

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

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."

Thanks in Advance

Anita

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