Subject: Re: [SIP] Basic questions

From: Henning Schulzrinne <[email-masked]>
Date: Sun, 04 Jun 2000 10:27:00 -0400

Jonathan Rosenberg wrote:

> >
> > - "...the H.245 [8] gateway and user address and then use H.225.0 [9] to
> > establish the call." - What is meant by an "H.245 gateway address" and an
> > "H.245 user address?" There is no such thing. I assume that the author is
> > referring to the H.323 call-signaling addresses of a gateway and endpoint.
> > There is an H.245 control-protocol address, but if a separate H.245 channel
> > is even used, the address is conveyed during call establishment and not
> > before.
> 
> You are of course correct; this should be H.323 call signaling address.

Actually, this was intentional. In the days of H.323v1, pre-Fast
Connect, one could imagine a scenario where you do a normal SIP INVITE
and then obtain the address of the peer. H.245 would then be used to
negotiate codecs and open logical channels. Clearly, this would not work
with any existing H.323 implementation, but I don't see why it couldn't
be made to work. However, the example may be too contrived to be useful.
(However, the straight combination with H.323 doesn't make a whole lot
of sense. Why would you have the mailman ring twice?)

> 
> >
> > - The word, "program,: is used throughout the RFC. Is this any functional
> > entity regardless of implementation and not exclusively a "software
> > program?"
> 
> Of course.

It is somewhat unlikely that you would implement SIP as hard-wired logic
gates or via air valves (yes, there were air valve logic gates at some
point...), so I'm confused as to why program is restricting your
choices.

> 
> The role of UAC and UAS is on a request by request basis, NOT a call by
> call basis. This is probably confusing from the spec, as much of it was
> written considering the initial INVITE request, for which caller and UAC
> are the same.

I tried to clarify the per-request nature of the UA* notion.

-- 
Henning Schulzrinne   http://www.cs.columbia.edu/~hgs

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From: Paul Long <[email-masked]>
Date: Sun, 4 Jun 2000 21:19:34 -0500

Henning,

Re: H.245 addresses
Intentional? I don't understand. There is no such thing, per se, as an H.245
gateway address or an H.245 user address. A gateway and endpoint (I assume
this is what is meant by "user") are canonically identified through their
respective call-signaling addresses. The address of the H.245 control
channel may then be conveyed via call signaling. One cannot first get the
H.245 gateway and user address and _then_ use H.225.0 (I assume this refers
to call signaling) to establish the call. It's the other way around.

Re: "program"
Yeah, I was being pedantic. :-) It just gives me the willies to see
implementation-specific terminology in a functional specification, even
though, yes, it would be very silly to use mechanical relays, air valves, or
uh... sticks and stones :-) ... to implement a SIP entity.

Re: UAC/UAS
I was just reading rfc2543bis-00 and don't remember seeing a clarification
of this. It would have been helpful because one usually thinks of clients
and servers as types or roles of entities rather than types of transactions.
Not a big deal, though.

Paul Long
Smith Micro Software, Inc.

-----Original Message-----
From: [email-masked] [mailto:[email-masked]]On Behalf Of
Henning Schulzrinne
Sent: Sunday, June 04, 2000 9:27 AM
To: Jonathan Rosenberg
Cc: [email-masked]; [email-masked]
Subject: Re: [SIP] Basic questions

Jonathan Rosenberg wrote:

---

From: Henning Schulzrinne <[email-masked]>
Date: Sun, 04 Jun 2000 22:27:20 -0400

> to call signaling) to establish the call. It's the other way around.

That's exactly my point. SIP in this fictitious scenario replaces
H.225.0. You then use the IP address and port obtained by SIP for the
H.245 control channel. Yes, it's contrived.

> uh... sticks and stones :-) ... to implement a SIP entity.

I take concreteness over vagueness any day :-)

> and servers as types or roles of entities rather than types of transactions.

It will appear whenever I get a chance to LaTeX the sources again.