[SIPForum-discussion] [Sip] What's the difference between session and dialog in SIP?

gaurav katiyar gaurav_katiyar at yahoo.com
Wed Apr 30 06:23:49 UTC 2008 

Hi,
     I am getting confused now. what session exactly is ? Why ACK is an end to end request for 200 OK of INVITE request? does it relate to session any where ??? :-). Is it right to generate ACK locally by any proxy including B2BUA for 200 OK of initial INVITE request? 
   
  Gaurav

Mahesh Anjanappa <mahesha at samsung.com> wrote:
  I'm surprised to see that a fundamental concept "SIP Session" does not have
a clear definition documented anywhere. I was eagerly waiting for a IETF 
Guru
to clarify this. I wish somebody writes a draft clarifying all the 
undocumented
fundamental concepts in SIP. If i knew it all, i would have done it :-)

My 2 cents on it.
A Session is a Rendezvous Contract between 2 or more UA's for communnicating
with some modality. SIP dialog is a context over which such a Contract is 
negotiated
decided, and upheld.
Normally the SDP Session Id should identify the Contract/Session. But not 
necessarily
always. If its a multiparty the Conference ID would serve such a purpose.
I can get into a session with a non-INVITE dialog as well. Example: 
SUBSCRIBE to
a specialized Event package. In this case some Identifier in the Event 
package probably will
serve as the Session Id.
As you can see there can be more than 1 dialog related a Session.
But there is no documentation saying what exactly threads all the dialogs to 
a Session except
the mentioning of a ID in the SDP RFC to identify the session being 
described in a SDP.

My understanding is that a Session Identifier can take the form of 
Conference ID or any
other depending on the method used to setup the rendezvoud/session.

Well, the above is my understanding of it, not based on any existing draft 
or RFC, so i welcome
whatever corrections need be.

regards
Mahesh

----- Original Message ----- 
From: "Paul Kyzivat" 

To: "gaurav katiyar" 
Cc: ; ; 
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 /* wrote:
>>
>> old good questions in sip, also add another "transaction".
>>
>>
>> On Fri, Apr 25, 2008 at 8:18 AM, å&shy;™å®—君 >> > 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 >> >:
>> > 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: vkg@{alcatel-lucent.com
>> ,bell-labs.com
>> ,acm.org }
>> > WWW: http://www.alcatel-lucent.com/bell-labs
>> >
>> _______________________________________________
>> Sip mailing list https://www.ietf.org/mailman/listinfo/sip
>> This list is for NEW development of the core SIP Protocol
>> Use sip-implementors at cs.columbia.edu
>> for questions on
>> current sip
>> Use sipping at ietf.org for new
>> developments on the application of sip
>>
>>
>>
>>
>> -- 
>> BR
>> Donald _______________________________________________
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>>
>>
>>
>> Gaurav Katiyar
>> Induslogic india pvt. ltd
>> B-34/1 sector 59 NOIDA
>> Phone:9818381368
>>
>>
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>>
>>
>> ------------------------------------------------------------------------
>>
>> _______________________________________________
>> Sip mailing list https://www.ietf.org/mailman/listinfo/sip
>> This list is for NEW development of the core SIP Protocol
>> Use sip-implementors at cs.columbia.edu for questions on current sip
>> Use sipping at ietf.org for new developments on the application of sip
>


--------------------------------------------------------------------------------


> _______________________________________________
> Sip mailing list https://www.ietf.org/mailman/listinfo/sip
> This list is for NEW development of the core SIP Protocol
> Use sip-implementors at cs.columbia.edu for questions on current sip
> Use sipping at ietf.org for new developments on the application of sip
> 




Gaurav Katiyar
  Induslogic india pvt. ltd
  B-34/1 sector 59 NOIDA
  Phone:9818381368
   

       
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