[Users] CANCEL/ACK processing

[Mark Kent]

Observant readers will recognize from the following that this question
is related to earlier posts from me...  that is: there is something
fundamental that I just don't get.

If someone is eager to answer all my questions, then
please search ahead for the question marks (?).

Also, what is the bigger picture that I am missing?
Please use as many words as necessary to explain.

Thanks,
-mark

 The players:

  CUSTOMER-IP: the UserAgent
   OPENSER-IP: my openser box
 OPENSER.FQDN: the fully qualified domain name of my openser box
    PSTNGW-IP: the PSTN gateway

 My openser box gets this CANCEL:

  U 2006/11/06 18:40:10.689785 CUSTOMER-IP:5060 -> OPENSER-IP:5060
  CANCEL sip:011445551212 at OPENSER.FQDN:5060 SIP/2.0
  From: <sip:CUST-PHONE-NUMBER at CUSTOMER-IP:5060;transport=UDP>;tag=TAG
  To: <sip:011445551212 at OPENSER.FQDN:5060;transport=UDP>
  Call-ID: CALLID at CUSTOMER-IP
  CSeq: 2 CANCEL
  Via: SIP/2.0/UDP CUSTOMER-IP:5060;branch=BRANCH
  Max-Forwards: 70
  Supported: 100rel,replaces
  Content-Length: 0

 It ends up in my standard stateful relay handler where I 
 t_relay() it (should I?) and this gets generated:

  #
  U 2006/11/06 18:40:10.691301 OPENSER-IP:5060 -> PSTNGW-IP:5060
  CANCEL sip:445551212 at PSTNGW-IP;user=phone SIP/2.0
  Record-Route: <sip:011445551212 at OPENSER-IP;lr=on;ftag=TAG>
  From: <sip:CUST-PHONE-NUMBER at CUSTOMER-IP:5060;transport=UDP>;tag=TAG
  To: <sip:011445551212 at OPENSER.FQDN:5060;transport=UDP>
  Call-ID: CALLID at CUSTOMER-IP
  CSeq: 2 CANCEL
  Via: SIP/2.0/UDP OPENSER-IP;branch=BRANCH2.0
  Via: SIP/2.0/UDP CUSTOMER-IP:5060;branch=BRANCH
  Max-Forwards: 69
  Supported: 100rel,replaces
  Content-Length: 0

 and this also happens:

  #
  U 2006/11/06 18:40:10.691342 OPENSER-IP:5060 -> CUSTOMER-IP:5060
  SIP/2.0 200 canceling
  From: <sip:CUST-PHONE-NUMBER at CUSTOMER-IP:5060;transport=UDP>;tag=TAG
  To: <sip:011445551212 at OPENSER.FQDN:5060;transport=UDP>;tag=TAG2
  Call-ID: CALLID at CUSTOMER-IP
  CSeq: 2 CANCEL
  Via: SIP/2.0/UDP CUSTOMER-IP:5060;branch=BRANCH
  Content-Length: 0

 How was that last packet generated, where was it done?

 It's not in my openser.cfg, should I have sent back just such
 a message?  If so, would this automagically sent message still
 have been sent?

 Next I get these two packets:

  #
  U 2006/11/06 18:40:10.761582 PSTNGW-IP:5060 -> OPENSER-IP:5060
  SIP/2.0 200 OK
  Call-ID: CALLID at CUSTOMER-IP
  From:  <sip:CUST-PHONE-NUMBER at CUSTOMER-IP:5060;transport=UDP>;tag=TAG
  To: <sip:011445551212 at OPENSER.FQDN:5060;transport=UDP>
  Content-Length: 0
  CSeq: 2 CANCEL
  Via: SIP/2.0/UDP OPENSER-IP;branch=BRANCH2.0,SIP/2.0/UDP CUSTOMER-IP:5060;branch=BRANCH
  Contact: sip:445551212 at PSTNGW-IP:5060;user=phone
  Record-Route: <sip:011445551212 at OPENSER-IP;lr=on;ftag=TAG>

  #
  U 2006/11/06 18:40:10.761616 PSTNGW-IP:5060 -> OPENSER-IP:5060
  SIP/2.0 487 Request Terminated
  Call-ID: CALLID at CUSTOMER-IP
  From:  <sip:CUST-PHONE-NUMBER at CUSTOMER-IP:5060;transport=UDP>;tag=TAG
  To: <sip:011445551212 at OPENSER.FQDN:5060;transport=UDP>;tag=20505
  Content-Length: 0
  CSeq: 2 INVITE
  Via: SIP/2.0/UDP OPENSER-IP;branch=BRANCH2.0,SIP/2.0/UDP CUSTOMER-IP:5060;branch=BRANCH

 and my openser box replies with :

  #
  U 2006/11/06 18:40:10.762238 OPENSER-IP:5060 -> PSTNGW-IP:5060
  ACK sip:445551212 at PSTNGW-IP;user=phone SIP/2.0
  Via: SIP/2.0/UDP OPENSER-IP;branch=BRANCH2.0
  From: <sip:CUST-PHONE-NUMBER at CUSTOMER-IP:5060;transport=UDP>;tag=TAG
  Call-ID: CALLID at CUSTOMER-IP
  To: <sip:011445551212 at OPENSER.FQDN:5060;transport=UDP>;tag=20505
  CSeq: 2 ACK
  Content-Length: 0

 and sends on this to the UAC:

  #
  U 2006/11/06 18:40:10.762377 OPENSER-IP:5060 -> CUSTOMER-IP:5060
  SIP/2.0 487 Request Terminated
  Call-ID: CALLID at CUSTOMER-IP
  From:  <sip:CUST-PHONE-NUMBER at CUSTOMER-IP:5060;transport=UDP>;tag=TAG
  To: <sip:011445551212 at OPENSER.FQDN:5060;transport=UDP>;tag=20505
  Content-Length: 0
  CSeq: 2 INVITE
  Via: SIP/2.0/UDP CUSTOMER-IP:5060;branch=BRANCH

 Now, how was that generated?   Is this part of the "tm" module?

 And then when I get this ACK:

  #
  U 2006/11/06 18:40:10.772875 CUSTOMER-IP:5060 -> OPENSER-IP:5060
  ACK sip:011445551212 at OPENSER.FQDN:5060 SIP/2.0
  From: <sip:CUST-PHONE-NUMBER at CUSTOMER-IP:5060;transport=UDP>;tag=TAG
  To: <sip:011445551212 at OPENSER.FQDN:5060;transport=UDP>;tag=20505
  Call-ID: CALLID at CUSTOMER-IP
  CSeq: 2 ACK
  Via: SIP/2.0/UDP CUSTOMER-IP:5060;branch=BRANCH
  Max-Forwards: 70
  Contact: <sip:CUST-PHONE-NUMBER at CUSTOMER-IP:5060;transport=UDP>
  Content-Length: 0

 What should I do with it?  I do nothing... if I should do nothing
 is there any special way I should do it?  Or is this just handled
 by the tm module?

 What I don't get is the CANCEL is directed AT my openser box:

  CANCEL sip:011445551212 at OPENSER.FQDN:5060 SIP/2.0
  To: <sip:011445551212 at OPENSER.FQDN:5060;transport=UDP>

 just like the ACK:

  ACK sip:011445551212 at OPENSER.FQDN:5060 SIP/2.0
  To: <sip:011445551212 at OPENSER.FQDN:5060;transport=UDP>;tag=20505

 and I t_relay() the CANCEL  but sit on the ACK...