[SR-Users] Interpretation of Contact header

[Joachim Büchse]

Good day,

I’m experiencing some problems with our VoiP providers handling of REGISTER requests. We are using a Gigaset PRO N720 as UAC behind a Juniper SSG 140 with SIP-Alg enabled. This setup kind of works with UDP but our provider wants us to use TCP. With TCP enforced incoming calls don’t work. I’ve done some wire tracing and to me it seems that the providers configuration is to blame, but then - there are many RFCs out there and many NAT and UAC bug workarounds. Anyway, I wanted to get the opinion of “the" experts about how the requests send to the UAS  SHOULD  be correctly interpreted.


The REGISTER requests/responses look like this (outside of the firewall):

Protocol TCP!
client port 19091 <-> server port 5060

REGISTER sip:pbx.peoplefone.ch SIP/2.0
Via: SIP/2.0/TCP 212.126.160.92:6717;rport;branch=z9hG4bKc1375589832468de63a719eac31156ec
From: "Michel" <sip:90780408050 at pbx.peoplefone.ch>;tag=2153084485
To: "Michel" <sip:90780408050 at pbx.peoplefone.ch>
Call-ID: 2825358480 at 10_10_128_10
CSeq: 1 REGISTER
Contact: <sip:90780408050 at 212.126.160.92:6717;transport=tcp>
Max-Forwards: 70
User-Agent: N720-DM-PRO/70.089.00.000.000
Expires: 180
Allow: INVITE, ACK, CANCEL, BYE, OPTIONS, INFO, REFER, SUBSCRIBE, NOTIFY
Content-Length: 0

SIP/2.0 401 Unauthorized
Via: SIP/2.0/TCP 212.126.160.92:6717;rport=19091;branch=z9hG4bKc1375589832468de63a719eac31156ec
From: "Michel" <sip:90780408050 at pbx.peoplefone.ch>;tag=2153084485
To: "Michel" <sip:90780408050 at pbx.peoplefone.ch>;tag=a0440f545f39b2694d387b475a5f6bc9.b8fc
Call-ID: 2825358480 at 10_10_128_10
CSeq: 1 REGISTER
WWW-Authenticate: Digest realm="pbx.peoplefone.ch", nonce="VNqJBVTah9m57ZGGs8b5XCTM3GyaExDy"
Server: kamailio (3.2.1 (x86_64/linux))
Content-Length: 0

REGISTER sip:pbx.peoplefone.ch SIP/2.0
Via: SIP/2.0/TCP 212.126.160.92:6717;rport;branch=z9hG4bK9c27afea96e2af4baab2f8d144a588e0
From: "Michel" <sip:90780408050 at pbx.peoplefone.ch>;tag=2153084485
To: "Michel" <sip:90780408050 at pbx.peoplefone.ch>
Call-ID: 2825358480 at 10_10_128_10
CSeq: 2 REGISTER
Contact: <sip:90780408050 at 212.126.160.92:6717;transport=tcp>
Authorization: Digest username="90780408050", realm="pbx.peoplefone.ch", uri="sip:pbx.peoplefone.ch", nonce="VNqJBVTah9m57ZGGs8b5XCTM3GyaExDy", response="764f371a08d258157a249f8d1b852514"
Max-Forwards: 70
User-Agent: N720-DM-PRO/70.089.00.000.000
Expires: 180
Allow: INVITE, ACK, CANCEL, BYE, OPTIONS, INFO, REFER, SUBSCRIBE, NOTIFY
Content-Length: 0

SIP/2.0 200 OK
Via: SIP/2.0/TCP 212.126.160.92:6717;rport=19091;branch=z9hG4bK9c27afea96e2af4baab2f8d144a588e0
From: "Michel" <sip:90780408050 at pbx.peoplefone.ch>;tag=2153084485
To: "Michel" <sip:90780408050 at pbx.peoplefone.ch>;tag=a0440f545f39b2694d387b475a5f6bc9.6bda
Call-ID: 2825358480 at 10_10_128_10
CSeq: 2 REGISTER
Contact: <sip:90780408050 at 212.126.160.92:6717;transport=tcp>;q=0;expires=180;received="sip:212.126.160.92:19091;transport=TCP"
Server: kamailio (3.2.1 (x86_64/linux))
Content-Length: 0


The ip:port the firewall is sending those requests from is  ip 212.126.160.92 port 19091. So this does NOT match the port from the Contact header. For TCP this seems rather logical to me, as one cant be listening on a TCP port and use it for sending at the same time. The UAC closes this “register connection” with TCP FIN after the register, and so does the firewall.

However unfortunately subsequent requests from the provider (ie UAS) come in on port 19091 (not port 6717 from the Contact header) and the firewall simply drops them.

Observations:
	- the server does NOT include received=212.126.160.92 in the Via of the reponse. According to RFC3581 this is mandatory when rport is present in the request, so this is probably an error in the server.
	- the server does include received="sip:212.126.160.92:19091;transport=TCP” in the Contact of the response. I didnt see this in any RFC (which means nothing;-) but it could be an error.
	- after the client received the 200 OK it closes the TCP connection.
	- the server tries several times to re-contact the client (incoming TCP SYN). However not on port 6717 (defined in the Contact header) but on port 19091 (where the REGISTER came from).

RFC3581 defines special behaviour when “rport” is defined in the request (i.e. response should go to the same port the request came from) - however it’s not so clear if this should apply to subsequent (INVITE/OPTIONS) requests from the server to the client. Those are strictly spoken not replies (or are they?).

RFC5626 defines that a “proxy” should keep track of the flows over which it received a registration and send requests over the same flow. It is not clear if RFC5626 should be applied. The RFC5626 defines that a UAC includes an “ob” parameter in the Contact field if it whishes further requests over the same flow. Also the RFC mandates a client to add a "reg-id=x" in the Contact field. Both are not the case here, so in short I think RFC5626 should NOT be applied. In which case conecting to the originating port (instead of the Contact port) would be a server error.

So in short and if I interpret the RFCs correctly, the client is reachable and should be contacted on
	Transport:	TCP
	IP:		212.126.160.92
	Port:		6717


If anyone who lives and breathes SIP could enlighten me if the UAS is right to call back on 19091 instead of 6717 I would really appreciate it;-))

Best regards,
Joachim