Subject: [SIP] Basic questions

From: [email-masked]
Date: Mon, 29 May 2000 15:15:04 -0500

Hello, all. Just starting with SIP, but I already have a few basic questions
regarding the RFC. Since this is my first query, let me know if these
questions should be posted elsewhere or I have otherwise violated some other
rule of etiquette.

- "...SIP...is...for...sessions with one or more participants." - How can
there be a session with only one participant? A session is defined elsewhere
as comprising one or more RTP sessions, which implies a sender and a
receiver, or two participants. Should the sentence have been written as
"...with two or more participants?"

- "...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.

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

- "Outbound proxy" - Is there also such a thing as an inbound proxy?

- Regarding UA, UAC, and UAS, can this change for an entity on a per-call
basis? Can this change for an entity during a call?

- "Servers are either proxy, redirect or user agent servers or
registrars." - I have also seen references to "location servers" and "SIP
servers." Is a location server the same thing as a redirect server? Is "SIP
server" just a general reference to servers in "SIPdom?"

- Is "server" only a role played by an entity in a single request-response
dialogue, or are some entities always servers or always clients or always
both?

Paul Long
Smith micro Software, Inc.

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From: Jonathan Rosenberg <[email-masked]>
Date: Mon, 29 May 2000 23:31:23 -0400

> questions should be posted elsewhere or I have otherwise violated some other

Many common questions are answered in the FAQ:
http://www.cs.columbia.edu/~hgs/sip/faq.html

> "...with two or more participants?"

Mulicast sessions can have a single participant. Not that interesting,
but possible.

> is even used, the address is conveyed during call establishment and not

You are of course correct; this should be H.323 call signaling address.

> entity regardless of implementation and not exclusively a "software

Of course. 

> - "Outbound proxy" - Is there also such a thing as an inbound proxy?

Not formally. That would be the proxy which receives a request where the
request URI is its own domain. This is the general proxy described in
the specification. Note that there can be many such proxies of each type
on a call request path.

> basis? Can this change for an entity during a call?

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.

> server" just a general reference to servers in "SIPdom?"

Location server is a non-SIP server, like an LDAP server or SQL
database. "SIP server" is any entity that receives requests (UAS, proxy,
redirect, registrar).

> dialogue, or are some entities always servers or always clients or always

Same as for UAS/UAC - its on a per request basis. 

-Jonathan R.
-- 
Jonathan D. Rosenberg                       72 Eagle Rock Ave.
Chief Scientist                             First Floor
dynamicsoft                                 East Hanover, NJ 07936
[email-masked]                     FAX:   (732) 741-4778
http://www.cs.columbia.edu/~jdrosen         PHONE: (732) 741-7244
http://www.dynamicsoft.com

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From: Dean Willis <[email-masked]>
Date: Tue, 30 May 2000 07:44:37 -0500

> Mulicast sessions can have a single participant. Not that interesting,

Note that a "session" does not have to be RTP. It might have an RTP
component, might have several RTP components, or none at all. I might
also have an associated "Quake" game, SIGTRAN connection, or anything
else that can be described  from a third-person perspective.

> database. "SIP server" is any entity that receives requests (UAS, proxy,

This may be a slightly misleading statements. Many implemented Location
Servers are in fact SIP Servers hat have access to additional routing
data such as LDAP.

--
Dean

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From: charles Agboh <[email-masked]>
Date: Tue, 30 May 2000 15:05:53 +0200

Can somebody please answer the following question? "

     How can you have a "session" of any kind with only one participant?

charles

--

Charles Agboh

Universite Libre de Bruxelles
Service Telematique et Communication
Bd du Triomphe, CP 230
B-1050 Brussels
Belgium

Tel. +32 2 6505717
Fax. +32 2 6505088, +32 2 6293816
RFC-822 : [email-masked]

----------------------------------
"Perilous to us all are the devices of an art deeper than we possess ourselves."
--J.R.R Tolkien, The Two Towers

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From: Lars Berggren <[email-masked]>
Date: Tue, 30 May 2000 15:34:40 +0200 (CEST)

>      How can you have a "session" of any kind with only one participant?

Jonathan gave a good example: multicast sessions. Imagine you are the only
one which sends and/or receives "session data" to a multicast ip-address.
Then you might end up "talking to yourself", but still, it *is* a session
with only one participant.

/Lars

Lars Berggren       <mailto:[email-masked]>
Intertex Data AB    tel: +46-8-6282828
Sundbyberg, Sweden  fax: +46-8-6286414

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From: Dean Willis <[email-masked]>
Date: Tue, 30 May 2000 08:42:11 -0500

I regularly multicast myself to no particular audience. Occasionally I
join the multicast to see if I'm still there and tell people I'm
checking up on myself. Lacking WAN mulicast routing, my traffic doesn't
go all that far -- but it's still a session, even when nobody is joined.

Hence "one or more" participants rather than "zero or more".

--
Dean

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From: [email-masked]
Date: Tue, 30 May 2000 09:16:08 -0500

Dean,

So is this statement incorrect? Section 1.3: "A session as defined for SDP
can comprise one or more RTP sessions."

Paul Long (an admitted hair splitter)
Smith Micro Software, Inc.

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From: "Dave Oran" <[email-masked]>
Date: Tue, 30 May 2000 14:30:31 -0400

> -----Original Message-----
> From: [email-masked]
> [mailto:[email-masked]]On Behalf Of charles Agboh
> Sent: Tuesday, May 30, 2000 9:06 AM
> To: Dean Willis; [email-masked]
> Cc: Jonathan Rosenberg
> Subject: Re: [SIP] Basic questions
> 
> 
> 

What is the sound of one hand clapping?

SIP can set up Zen sessions. It's completely general.

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From: Jonathan Rosenberg <[email-masked]>
Date: Tue, 30 May 2000 18:11:40 -0400

> can comprise one or more RTP sessions."

This is a different question. I think SDP requires a single m line,
although I think this has also been the subject of some discussion...

-Jonathan R.

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From: Dean Willis <[email-masked]>
Date: Tue, 30 May 2000 17:03:05 -0500

The statement is CORRECT -- a session can comprise one or more RTP
sessions. However, it is not COMPLETE -- a session can comprise one or
more network sessions of any sort established by SIP, including RTP and
other protocols.

Historically, there has been great interest in using SIP for voice
communications, hence the emphasis on SDP. But from a design
perspective, it is not limited to RTP. Foe example, I saw last year a
demo of Quake initiation using SIP. 

--
Dean

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From: Dean Willis <[email-masked]>
Date: Tue, 30 May 2000 18:22:53 -0500

As some of you may have noticed, my office mailing address is now:

	[email-masked]

However, you may also see me writing from (or send mail to me at)

	[email-masked]
	[email-masked]
	[email-masked]

--
Dean

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From: "Paul E. Jones" <[email-masked]>
Date: Wed, 31 May 2000 01:49:22 -0400

Jonathan,

SDP does not mandate a single m= line.  Am I missing something?  It's
perfectly valid to do this in SDP I believe:

   m=audio 2500 RTP/AVP 0
   m=video 2502 RTP/AVP 31

The ABNF grammar allows it and there are several examples in RFC 2327.

Paul

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From: Jonathan Rosenberg <[email-masked]>
Date: Wed, 31 May 2000 01:55:57 -0400

Sorry; I meant at least a single m line (i.e., zero m lines not
allowed).

-Jonathan R.

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From: Dubois <[email-masked]>
Date: Wed, 31 May 2000 09:19:25 +0200

Jonathan,

If you need to have at least a single m line, why is it written page 8 of the
RFC 2327 (SDP) that
you can have ZERO or more media descriptions ?

-Alain

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From: James Undery <[email-masked]>
Date: Wed, 31 May 2000 09:15:26 +0100

> Sorry; I meant at least a single m line (i.e., zero m lines not

4th paragraph of section 6 of RFC 2327 starts "An announcement consists of a
session-level section followed by zero or more media-level sections". While this
may be useful in a generic case the lack of media means the it is of no use to
SIP. I would suggest adding a line along the lines of, SDP used in SIP must
contain one or more media-level descriptions, to the bis draft. (Obviously
reworded to avoid my awkward prose ;^) )

James Undery