Expires Value

A refresher on the expires value: If the Contacts: header does not contain an expires parameter, the SBC adds one with the value in the Expires: header in the 200 OK returned to the UA who sent the INVITE. If there is no Expires: header, the SBC adds expires parameter with a value of 3600 and an Expires header with a value of 3600 to the 200 OK.

As the SBC forwards the final 200 OK to the initiating endpoint, the Expires (888) AVP contains the largest expires value of all expires parameters in Contact: headers.

When registration caching is enabled and the SBC receives a reREGISTER from an endpoint before the halftime of the local registration has expired, the SBC inserts the maximum of all associates contacts’ remaing time until expiry in the Expires AVP.