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
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