[Serusers] Curious interop bug.

[Klaus Darilion]

David Gilbert wrote:
> In short, the problem is that level3 abuses the RFC by sending from a
> port and expecting you to reply on another port (specified in the
> Contact: header).  This goes almost right except for level3's equally
> literal read of the Contact header that SER sends.

Using different ports is valid. You have to differ between replys and 
new transactions.

1. The replies (180, 200) must be sent to the port indicated in the Via 
header. (except there is a rport parameter in the Via header from the 
gateway). Thus, if the level3 gateway is "un-symmetrical" make sure to 
no call force_rport for messages received from the level3 gateway.

2. Requests by the ATA sent to the gateway, must be sent to the IP:port 
indicated in the Contact: header of the INVITE. Thus, make sure to not 
call fix_nated_contact for requests received from the level3 gateway.

3. use ngrep -d any ... or tcpdump -i any ... to see also the packets on 
the loopback device (between ser and asterisk)

regards
klaus


> 
> Packet			    Sip Command	    Contact
> ======			    ===========	    =======
> level3:38999 ser:5070	    INVITE	    sip:+1416...@<level3 ip>:5060
> ser:5070 level3:38999	    100 Trying	    sip:+1866...@<ser ip>
> (ser invites asterisk, not in dump)
> asterisk:5060 ata:5060	    INVITE	    sip:+1416...@<asterisk ip>
> ata:5060 asterisk:5060	    100 Trying	    None
> ata:5060 asterisk:5060	    180 Ringing	    None
> (asterisk relays 180 to ser, not in dump)
> ser:5070 level3:38999	    180 Ringing	    sip:+1866...@<ser ip>
> ata:5060 asterisk:5060	    200 OK	    sip:604...@<ata ip>:5060
> asterisk:5060 ata:5060	    ACK		    sip:+1416...@<asterisk ip>
> (asterisk relays 200 OK to ser, not in dump)
> ser:5070 level3:5060	    200 OK	    sip:+1866...@<ser ip>
> level3:5060 ser:5060 !?!    ACK		    sip:+1866...@<level3 ip>:5060
> 
> Now... the problem is that each time ser repsonds, the contact line
> should be appended with ":5070" and it is not.  Therefore, being the
> literal people they are, level3 repsonds to port 5060 with their ACK.
> This means that the ACK goes directly to asterisk rather than to SER
> and it means that when the call ends, the BYE notification from
> asterisk goes directly to the wrong port at level3.
> 
> 
> 
> ------------------------------------------------------------------------
> 
> 
> 
> Dave.
> 
> 
> 
> ------------------------------------------------------------------------
> 
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