Hi,

Found an interesting call-flow today where a client (pjsip) is behind
NAT and behaves like this during registration (kamailio-3.1.3):

REGISTER sip:somedomain.com
Contact: <sip:me@10.0.0.7:2001;ob>
Expires: 900

SIP/2.0 200 OK
Contact: <sip:me@10.0.0.7:2001;ob>;expires=900;received="sip:1.2.3.4:3001"

REGISTER sip:somedomain.com
Contact: <sip:me@1.2.3.4:3001;transport=UDP;ob>
Contact: <sip:me@10.0.0.7:2001;ob>;expires=0
Expires: 900

SIP/2.0 200 OK
Contact:
<sip:me@10.0.0.7:2001;ob>;expires=900;received="sip:1.2.3.4:3001",
<sip:me@1.2.3.4:3001;transport=UDP;ob>;expires=900

So obviously the client is trying to unregister its private contact (for
whatever reason - probably to play nice with registrars not supporting
far-end NAT traversal), and tries to register the Contact with the
"received" address instead. Kamailio however ignores the ";expires=0"
for the private Contact and just saves both of them with the expiry
value given in the Expires header.

Taking the client behavior aside for now: is this the correct behavior
of the registrar module? In RFC3261 chapter 10.2.1.1 it says:

#+
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 "expires"
parameter.
#-

So I'd say that in my specific case "expires" Contact header param
(which is 0 for the binding having 10.0.0.7) should de-register this
particular binding, whereas the Expires header should be applied to the
second binding (having the 1.2.3.4) because it doesn't carry an
"expires" param.

Opinions on that one?

Andreas