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

[Yuantao Zhang]

Dear all 

Thanks for your reply. Please let me ask my question in another way.

===============================================================
UDP:
Src port: 5060
Dst port: 5555        *<--------------
===============================================================
SIP:
INVITE sip:123101 at 192.168.100.100:5555 SIP/2.0  *<--------------
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
-----Original Message-----
From: Paul Kyzivat [mailto:pkyzivat at cisco.com] 
Sent: 2008年6月4日 20:45
To: Attila Sipos
Cc: Yuantao Zhang; sip at ietf.org; discussion at sipforum.org
Subject: Re: [Sip] What is the port number in "Invite" request-line? Thanks



Attila Sipos wrote:
>  
> It is possible to have two instances of a UA (for example) on one unit.
> 
> So using the port is theoretically one way to differentiate between 
> instances.
>  
> It's not useless.  If you get a Contact header with a port then you have
> to send to that port (discounting NAT traversal scenarios) otherwise 
> everything will break.
>  
>> >Or is the port number is which the UAS is listening to? How the UAC 
> knows it is 5555 if so?
>  
> A UAC might not know but a proxy/registrar might - for example if a user 
> was registered with
> the Contact as port 5555, then requests to that user will get forwarded 
> to port 5555.
> Another user using, for example, port 5556 will register using a Contact 
> with 5556
> and so will receive requests to that port.

Right. This illustrates a good point:

In general a UAC should not be expected to know "what port a UAS listens 
on". All A UAC should be doing is sending requests to URIs that it has 
been given. If the URI it is given contains a port number, then it 
should honor that.

This gets bent out of shape a bit by numeric dialing, where the UAC is 
only given a dial string and must derive a URI from it. The "standard" 
way to do this is via ENUM, where effectively the numeric address is 
looked up in a DB to obtain a URI. In that case again the UAC has been 
"given" the URI, which may have a port or not.

In the non-standard but common case where the UAC *constructs* a URI 
from a dial string, the UAC must still be *told* what to use for the 
remainder of the URI.

	Paul


From: sip-bounces at ietf.org [mailto:sip-bounces at ietf.org] On Behalf Of Yuantao Zhang
Sent: 04 June 2008 11:10
To: sip at ietf.org; discussion at sipforum.org
Subject: [Sip] What is the port number in "Invite" request-line? Thanks
Dear all
INVITE sip:bob at biloxi.com SIP/2.0:5555
INVITE sip:bob at biloxi.com SIP/2.0
Above are 2 “Invite” request-line examples. In example one, it indicates a port number, 5555. What does the port number mean? 
Is it the port number of SIP proxy? Why we include port number here? The SIP proxy(biloxi.com) application layer who receives this "Invite" should receive this via port 5555 from lowe layer(UDP). Therefore, The SIP proxy(biloxi.com) application layer already know the port is 5555. Actualy, the SIP proxy application layer keeps listening port 5555. 
The second “Invite” request-line example is from RFC3261 and does not include port number. Can I say the port number is useless? So why example one includes a useless part in “Invite” request-line URL? 
Or is the port number is which the UAS is listening to? How the UAC knows it is 5555 if so? 
Any standard on this port number issue? Thanks. 
Best regards
Steven

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