The Expires header field gives the relative time after which the message
(or content) expires. The precise meaning of this is method dependent.
The expiration time in an INVITE does not affect the duration of the
actual session that may result from the invitation. Session description
protocols may offer the ability to express time limits on the session
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. Malformed
values SHOULD be treated as equivalent to 3600.
This interface represents the Expires entity-header. The ExpiresHeader is
optional in both REGISTER and INVITE Requests.
- REGISTER - When a client sends a REGISTER request, it MAY suggest an
expiration interval that indicates how long the client would like the
registration to be valid. There are two ways in which a client can suggest
an expiration interval for a binding: through an Expires header field or an
"expires" Contact header parameter. The latter allows expiration intervals
to be suggested on a per-binding basis when more than one binding is given
in a single REGISTER request, whereas the former suggests an expiration
interval for all Contact header field values that do not contain the
- INVITE - The UAC MAY add an Expires header field to limit the validity
of the invitation. If the time indicated in the Expires header field is
reached and no final answer for the INVITE has been received, the UAC core
SHOULD generate a CANCEL request for the INVITE.
- BEA Systems, NIST