Mahesh Anjanappa wrote:
Mahesh Anjanappa wrote:
I'm not fully understanding what you are proposing (see questions
above). I expect that it may start to fall apart in the details of
particular use cases. Will you explain to me how your definition
relates to the use case with A/B/C/D that I gave?
OK,let me see.
A, C and D get into a Session/Contract to communicate using Video and
Audio.
I guess you misunderstand what I was describing.
The SDP negotiated between A and B contains both audio and video.
The SDP negotiated between B and C contains only audio. (B didn't offer
the video to C.)
The SDP negotiated between B and D contains only video. (B didn't offer
the video to D.)
The purpose of B is to consolidate an audio-only device and a video-only
device into what seems, to A, like one audio/video device.
OK. In this case, there is one session between A and B. A doesnt know
anything about C & D. As far A is concerned it is communicating with B
with
modality of Audio+Video. B sets up individual sessions with C and D.
(B is purpose-built just to send the Audio and Video to C & D resp.)
It seems to A as though B is one audio/video device.
C does not know about A & D nor does D know about A and C. So basically
there are 3 individual sessions and not a conference since A,C and D
are not aware of each other. All they do, is have their individual
contract with
B. In contrast to a Conference wherein A,C and D would have been aware
about each other and a common contract hence a single session .
OK, we are closer to agreement.
Note that in a regular conference, a participant may or may not know it is
in a conference. If not, then it thinks it is in a session with the focus.
The Participant can be ignorant of the Conference, but that is only by
choice
and not because the framework doesnt allow the Pariticpant to know it.
What i mean is that the Participant still is part of the same
Contract(single
session or confernce) but just chooses not to see it. So i feel my defintion
of
the *Session* stands good (survives:-) ) this scenario.
They don't all have the same contract. So by your definition I don't
think they can all have one session.
In some conceptual sense you may think of this as one session. But its
hard to find any tangible item that they all have that refers to a
single session. In the end, from the point of view of B it may indeed be
one session. But A, C, and D can't perceive that.
The contract allows for either of the Media to be used hence C uses
audio
and D uses video. B facilitates the Contract by holding the three
dialogs.
The 3 dialogs are tied together by a Conference Id (i suppose)
thus COnference Id representing the Session Identifier.B too is
considered a party in the
session though its role is different from that of A,C and D.
If this were done with a conference mixer, and hence there was a
conference id, then I guess you could say that the conference id was an
identifier for the "session".
But if B was purpose-built for this particular functionality, and there
is no conference id, then what?
There are separate Session Id's for each of the three sessions, probably
the
Session id carried in the SDP(but not neccessarily). A, C & D, each will
be
bothered about ONLY their respective Session Id's (one each) with B.
B is aware of all the session id's and would want to collate them for an
app
specific reason but not for the notion of *SIP Session* that we discuss
here.
In simple words,even B sees it as 3 SIP Sessions rather than 1 though the
three collated by some logic by B.
So, is there one session, three, or four? Does it matter?
And in a simple call between A and C there would be no conference id, so
something else must serve as the "session" identifier. But in that case,
it would seem that these aren't the *same* notion of session.
You asked why I didn't think it was important to define session. I will
admit that it seems odd to have a Session Initiation Protocol without a
clear idea of what this "session" is that it is initiating. So it indeed
would be *nice* to have a definition that covers all the cases.
But not having one doesn't seem to be preventing anyone from doing
anything, does it?
Thats right, it doesnt prevent anyone from doing "anything", everybody
does their own thing since no single guiding principle. Look at what,
has happened with INFO :-). Anyway, its just my opinion, i shall not
cause further degression from the actual technical topic at hand :-)
Do you mean that you think there are things people are doing that they
shouldn't be doing, and wouldn't be doing if there was a better definition
of session?
Well, i dont have enough data to state that "there are things people are
doing that they shouldnt be doing", but i can definitely say that by not
providing
a definition for Session it is gives huge room for different perceptions and
hence for different incorrent usages which later on will come to dog IETF
discussion rooms when implementors get into trouble.
My main requirement for a definition is that it shall serve as a base
principle
whenever a designer gets into a situation and wants to make a decision
as to which is the right way while designing some SIP based mechanism.
Let me give an example:
Consider design of a Rich Chat Application which provides for Text Chat
as well as Video share.
The Application CAN have two SIP dialogs, one for realising Text Mesaging
session/contract and another for Video sharing session/contract.
Lets assume for a moment that the concept of session is non-existent
because it hasnt been concretely defined.
Now if a "is typing" kind of signal information has to be sent to the
remote, a
Designer can argue that it can be sent on either of the two dialogs because
both are between the same Application/UA and there is no other basis to
discrimnate the two to favour one over the other.
But instead if we DID have a defintion for Session, the designer would refer
to the
base principle of Session and decide to send it over the dialog being used
for the
Text Messaging Session.
From a practical perspective I think the thing that is being initiated
by SIP is the contract established by a matching offer/answer. That is
fairly straightforward when the UAC and UAS are the offerer and
answerer, or visa versa. It gets messy when other UAs are involved in
the construction of the offer and the answer. And in those cases it gets
really hard to figure out what the session is, because not all the
parties will agree.
Right, so dont you feel there a loose end somewhere and need to be tied.
I guess possibly it might be helpful to more carefully define the term
"session" as used in sip. But I think only to *narrow* its scope so that
people don't read too much into it. But I see no urgency to that.
:-) :-), Paul when would you see the urgency for it ? :-) probably until
then
we can defer the discussion :-):-)
Paul
Paul
Paul
regards
Mahesh
----- Original Message ----- From: "Paul Kyzivat"
<[EMAIL PROTECTED]>
To: "gaurav katiyar" <[EMAIL PROTECTED]>
Cc: <[email protected]>; <[EMAIL PROTECTED]>; <[EMAIL PROTECTED]>
Sent: Tuesday, April 29, 2008 10:32 PM
Subject: Re: [Sip] [SIPForum-discussion] What's the difference
between session and dialog in SIP?
gaurav katiyar wrote:
Sip "Session Initiation Protocol". It talks about media session, i
mean
how to initiate, modify and terminate a session and Dialog provides
a
sort of context (like caller, callee, location, routing etc.) to
modify
this session. So dialog is more related to route the sip message to
right node and body (SDP) handles media session if present. Dialogs
may
have media sessions Or may not.
Take an example of B2BUA. It is a type of statefull proxy having
two
dialog and one media session if doing media routing.
Maybe. IMO these concepts aren't sufficiently defined.
One view is that a session is whatever SDP describes. Thus its one
session regardless of how many media streams it contains. And in the
case of an SDP containing multicast media addresses, and possibly
advertised via SAP, there could be many participants in the same
session.
But then in sip it takes a pair of SDPs to establish a call. Is that
one
session, or two? And in your case above with the B2BUA, is it one
session, or two, or four?
Another view is that a session in the context of sip is whatever is
established as a result of an INVITE. Then I guess the pair of SDPs
together describe one session, regardless of how many media streams.
But
in that case the B2BUA example would be two sessions.
Then consider a more complex case:
audio
/-------- C
/
A ========== B
audio \
video \-------- D
video
Here B is a B2BUA and media relay as in your example, but one stream
is
relayed to C and the other to D.
In this case, how many sessions are there? A, C, and D each have
one,
but are any of those the same? Or are there three altogether?
When I see "session" used, I just assume it means something vague,
and
read on to see what is actually meant.
Paul
*/Donald Lee <[EMAIL PROTECTED]>/* wrote:
old good questions in sip, also add another "transaction".
On Fri, Apr 25, 2008 at 8:18 AM, å™å®—å�› <[EMAIL PROTECTED]
<mailto:[EMAIL PROTECTED]>> wrote:
Hi, Vijay
Thanks very much for your timely answers.
From this specification, session is a combination of
signaling
plane
and media plane messages and processes that enable two or
more
participants to communicate. I think session is a larger
scope than
dialog. In another word, one session can contain more than
one
dialog.
Is what I understand right?
Zongjun
2008/4/24, Vijay K. Gurbani <[EMAIL PROTECTED]
<mailto:[EMAIL PROTECTED]>>:
> There is some work progressing in the BMWG WG to create
performance
> metrics around SIP. One of the first tasks in such an
endeavor
> is to define the notion of dialogs and sessions.
Please see
> Section 3.1.1 of
>
https://svn.resiprocate.org/rep/ietf-drafts/gurbani/bmwg-sip-bench-term/draft-ietf-bmwg-sip-term-00.txt
> of the above draft for some insight.
>
> Ëï×Ú¾ý wrote:
>
> > Hi, Lincoln
> >
> > thanks for your instructions.
> >
> > I know that dialog can exist without session since
SUBSCRIBE/REFER can
> > create dialog and without any media between
communication
peers.
> >
> > What I want to know is the relationship of dialog and
session when
> > they both exist in one communication activity. Take a
example, when
> > there are 2 person participating talks with voice, we
say
there is a
> > dialog and a session.
> >
> > But when one caller invites another callee and gets
five
200 final
> > responses from 5 UA every of which has its own session
description. We
> > can say that there are 5 dialogs between caller and
the other 5
> > callees, right? And then what is the exact number of
session in this
> > scenario? One session or five session? That is what I
want
to know.
> >
> > Dialog is determined by dialogID (call-id, from/to
tag) and
session id
> > is determined by session id given in the SDP message.
My
answer is
> > there are 5 dialog and one session now, right?
> >
>
> - vijay
> --
> Vijay K. Gurbani, Bell Laboratories, Alcatel-Lucent
> 2701 Lucent Lane, Rm. 9F-546, Lisle, Illinois 60532
(USA)
> Email: [EMAIL PROTECTED]
<http://alcatel-lucent.com/>,bell-labs.com
<http://bell-labs.com/>,acm.org <http://acm.org/>}
> WWW: http://www.alcatel-lucent.com/bell-labs
>
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