[Users] TCP-alias and NAT - one more try to get an answer

[Daniel-Constantin Mierla]

Hello Phillipp,

the problem exposed by you here is somehow similar to the topic 
discussed on devel at openser.org in the mail thread "Processing REGISTER 
requests".

Mainly is how to attach a resource to a contact address and identify it 
properly (in this case is the tcp connection). Since we have all over 
NATs, we cannot rely on the contact address. The latest version of 
openser stores in the usrloc database the associated NAT ip and port 
along with contact address. This is not enough when you have multiple 
levels of NATs (e.g., two different networks with same private IPs being 
visible from outside behind same nat - this gets a conflict when same 
user has phones in both networks).

It is clear that in your case you have to keep the tcp connection open 
for the whole call. The problem is how to identify the connection from 
the contact address. We have to address this soon and find the proper 
solution for this case as well as for usrloc.

Cheers,
Daniel


On 10/26/05 12:51, Alexander Ph. Lintenhofer wrote:
> Hi All,
>
> I just wanted to ask you once again about the TCP-alias riddle. I 
> found out, that there is a problem with the combination of 
> fix_nated_contact(), force_tcp_alias() and NAT:
>
> Imagine following situation:
>
> Alice behind NAT: socket 172.16.0.6:2421
> Nat-Box translates this to 192.168.0.13:6007
>
> The Outbound-Proxy of Alice is 192.168.0.1
>
> Bob is registered with 192.168.1.1
>
> 1.)
> Her INVITE:
> ====================================================================
> INVITE sip:bob at 192.168.1.1:2331;transport=tcp;line=wxqurd1s SIP/2.0
> [...]
> Via: SIP/2.0/TCP 172.16.0.6:2421;received=192.168.0.13;
>                branch=z9hG4bK-wm9jcstcboys;rport=6007
> From: "Alice" <sip:alice at atlanta.com>;tag=sufzmxi0us
> To: "Bob" <sip:bob at biloxi.com>;tag=c9550czwtn
> [...]
> Contact: <sip:alice at 172.16.0.6:2421;transport=tcp;line=fyyuh6tl>
> [...]
> ====================================================================
>
> 2.)
> fix_nated_contact() doesn't work with TCP (look at nathelper.c).
> force_tcp_alias() now creates following tuple as TCP-alias: 
> 192.168.0.13:6007 to 192.168.0.13:2421
>
> Reason:
> The TCP-alias is not built solely from the Via-header as suggested in 
> the draft. The portnumber is taken from the Via-header and the 
> IP-address is taken from the source of the incoming datagram. I read 
> it in the sourcecode and assured it by contacting Andrei!
>
> 3.)
> So as a result of the notfixed Contact-header of Alice's INVITE the 
> BYE of Bob is addressed to 172.16.0.6:2421. But no TCP-alias exists 
> for this socket :-(
>
> 4.)
> I made following test by rewriting the Contact-header....
> ====================================================================
> if (method=="INVITE")
> {
>   replace("Contact: <sip:alice at 172.16.0.6:2421;transport=tcp;",
>           "Contact: <sip:alice at 192.168.0.13:2421;transport=tcp;");
> }
> ====================================================================
> ...with success. Now TCP-alias works as you can see on my 
> Ethereal-trace below!
> Compare the destination port of the packet to the destination port of 
> the RURI!
> ====================================================================
> [...]
> Transmission Control Protocol, Src Port: 5060 (5060), Dst Port: 6007 
> (6007), ...
> Session Initiation Protocol
> Request-Line: BYE 
> sip:alice at 192.168.0.13:2421;transport=tcp;line=fyyuh6tl SIP/2.0
> Message Header
> [...]
> From: "Bob" <sip:bob at biloxi.com>;tag=kcsveifugd
> To: "Alice" <sip:alice at atlanta.com>;tag=ricaq5cy15
> Contact: <sip:bob at 192.168.1.1:2331;transport=tcp;line=wxqurd1s>
> [...]
> ====================================================================
>
> Another solution:
> Comment the lines in nathelper.c which force the return in case of TCP 
> or TLS. Now all works well!
>
> But why??????????????
>
> regards,
> Philipp
>
>
>
>
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