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

[Bogdan-Andrei Iancu]

Hi Alexander,

indeed, it seams you have case. Even if the draft says that the alias 
tuple must include the VIA ip:port, currently the IP is considered as 
received IP. Why? not sure (is Andrei's work), but I can speculate on 
the overhead introduce in getting the address from via and convert it to 
IP (via may contain DNS name which should be resolved!).

If we go ahead on this road, right, you need a way to set the NAT IP in 
the contact -> use fix_nated_contact() which does not support TCP :D.  I 
do not like this approach because the alias is a mixture between the NAT 
IP and the inside port :/

If we go as the draft says, and use via ip:port we have to face the need 
of performing DNS lookup only to generate the alias ....this is even 
uglier and more dangerous. and also we will end having private IPs in 
RURIs and in aliases..which is a little bit tricky - a private IP is not 
unique ;).

so, I would say the best way is the first one....and to allow 
fix_nated_contact() also for TCP/TLS.........

regards,
bogdan



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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