[SIPForum-discussion] [Sip] What is the port number in "Invite" request-line? Thanks

[Yuantao Zhang]

------------------     -------------------         -------------------
|UA1		  |	|SIP proxy	    |	      |UA2		  |
|IP:192.168.100.1|-----|IP: 192.168.100.2 |--------|IP: 192.168.100.3 |
|Port: 1111	  |	|Port: 2222        |	      |Port: 3333        |
------------------	--------------------         -------------------

If UA1 makes call to UA2, UA1 should send the following message to the SIP proxy:

===============================================================
IP: 
Src: 192.168.100.1
Dst: 192.168.100.2
===============================================================
UDP:
Src port: 1111
Dst port: 2222        						   !
===============================================================
SIP:
INVITE sip:UA2 at 192.168.100.2:2222 SIP/2.0  			   !
Via: SIP/2.0/UDP 192.168.100.1:1111;
===============================================================


Then SIP proxy sends the following message to UA2:

===============================================================
IP: 
Src: 192.168.100.2
Dst: 192.168.100.3
===============================================================
UDP:
Src port: 2222
Dst port: 3333        						   !
===============================================================
SIP:
INVITE sip:UA2 at 192.168.100.2:3333 SIP/2.0  			   !
Via: SIP/2.0/UDP 192.168.100.2:2222;
Via: SIP/2.0/UDP 192.168.100.1:1111;
===============================================================

UAC(UA1) does not know the port of UAS(UA2). All the UAC needs to know is the proxy's port. And the proxy knows all the ports of UAs under it (UAs tell proxy when register). Therefore, the proxy does not care the port number in the received "Invite" request-linen (because the port number is proxy itself)

Therefore, the port number in the "Invite" request-line is always same with the port number in the lower layer UDP. Why we insert redundant information? How about we drop this redundant and confusing port number in the URL? Please correct me if I am wrong. Thanks.

Best regards
Yuantao Zhang

-----Original Message-----
From: Paul Kyzivat [mailto:pkyzivat at cisco.com] 
Sent: 2008年6月6日 19:55
To: Yuantao Zhang
Cc: Attila Sipos; sip at ietf.org; discussion at sipforum.org
Subject: Re: [Sip] What is the port number in "Invite" request-line? Thanks

The URI typically exists before the message is constructed. Then a
message is constructed and the URI included in it. The URI specifies
where the message is *intended* to go. And it might not go there
immediately. Instead it may first go to a proxy. When it gets there the
proxy needs to consult the R-URI to know where the message is intended
to go.

When the last hop to the final destination is made it is true that the
port number, and the ip address, may be redundant. But we don't expect
the intermediate nodes to remove the information. The recipient may do
as it wishes if it receives a request with addressing that is
inconsistent with where it is received.

	Paul

Yuantao Zhang wrote:
> Dear all
> 
> Thanks for your reply. Please let me ask my question in another way.
> 
> ===============================================================
> 
> UDP:
> 
> Src port: 5060
> 
> Dst port: 5555        *ß------------
> 
> ===============================================================
> 
> SIP:
> 
> INVITE sip:123101 at 192.168.100.100:5555 SIP/2.0  *ß------------
> 
> Via: SIP/2.0/UDP 192.168.100.3:5060;rport;branch=z9hG4bK27195
> 
> From: <sip:123102 at 192.168.100.100>;tag=27266
> 
> To: <sip:123101 at 192.168.100.100:5555>
> 
> Call-ID: 24658 at 192.168.100.3
> 
> CSeq: 20 INVITE
> 
> Contact: <sip:123102 at 192.168.100.3:5060>
> 
> ===============================================================
> 
> The above "Invite" message is over UDP. In the UDP part (Transport 
> layer), the Destination port is 5555. In the SIP part (application 
> layer), the port in the "Invite" request-line URL is 5555 too. Both of 
> them are same. In which case the port in "Invite" request-line URL is 
> different from the Destination port in lower layer UDP? Furthermore, if 
> they are always same why we include redundant information (the port 
> number) in "Invite" request-line URL? What happens if the port in 
> "Invite" request-line URL is different from the Destination port in 
> lower layer UDP?
> 
> Thank you very much
> 
> Best Regards
> 
> Yuantao
> 
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