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



Mahesh Anjanappa 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 :-)
IMO the reason there is no definition of such a "fundamental concept" is that it isn't fundamental. While it might be interesting to discuss, it doesn't affect anything. And in fact, when you start to dig into what this "fundamental concept" means it starts to fall apart. 

Yes, it starts to fall apart,:-),  which annoys me & is the reason why i
dig further because i feel i'm missing something here. We use
the word Session so often in discussions and IMHO it is dangerous
if i cant put down a fullfilling definition for 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 
From sip-bounces at ietf.org  Wed Apr 30 07:22:03 2008

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Date: Wed, 30 Apr 2008 19:56:10 +0530
From: Mahesh Anjanappa <mahesha at samsung.com>
To: Paul Kyzivat <pkyzivat at cisco.com>
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	gaurav katiyar <gaurav_katiyar at yahoo.com>
Subject: Re: [Sip] [SIPForum-discussion] What's the difference
between	session and dialog in SIP?
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Mahesh Anjanappa 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 :-)
IMO the reason there is no definition of such a "fundamental concept" is that it isn't fundamental. While it might be interesting to discuss, it doesn't affect anything. And in fact, when you start to dig into what this "fundamental concept" means it starts to fall apart. 

Yes, it starts to fall apart,:-),  which annoys me & is the reason why i
dig further because i feel i'm missing something here. We use
the word Session so often in discussions and IMHO it is dangerous
if i cant put down a fullfilling definition for 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 negoti> 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.
Is each modality a separate contract? Or is is a contract for a *collection* of modalities? (E.g. voice + video) 

It is a contract for a *collection* of modalities.
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.
Do you mean to imply that the event notifications are then the communication modality? 

Yes, thats right (example:PIDF doc if Presence )


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.
Its a decent try. As I mentioned in a prior reply, IMO there is no real need to define this.
Frankly, i didnt understand why you say that there is no real need to define this.
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.
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.



Paul


regards
Mahesh

----- Original Message ----- From: "Paul Kyzivat" <pkyzivat at cisco.com>
To: "gaurav katiyar" <gaurav_katiyar at yahoo.com>
Cc: <sip at ietf.org>; <discussion at sipforum.org>; <bmwg at ietf.org>
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
 ated

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.
Is each modality a separate contract? Or is is a contract for a *collection* of modalities? (E.g. voice + video) 

It is a contract for a *collection* of modalities.
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.
Do you mean to imply that the event notifications are then the communication modality? 

Yes, thats right (example:PIDF doc if Presence )


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.
Its a decent try. As I mentioned in a prior reply, IMO there is no real need to define this.
Frankly, i didnt understand why you say that there is no real need to define this.
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.
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.



Paul


regards
Mahesh

----- Original Message ----- From: "Paul Kyzivat" <pkyzivat at cisco.com>
To: "gaurav katiyar" <gaurav_katiyar at yahoo.com>
Cc: <sip at ietf.org>; <discussion at sipforum.org>; <bmwg at ietf.org>
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 <baolovebao at gmail.com>/* wrote:

    old good questions in sip, also add another "transaction".


    On Fri, Apr 25, 2008 at 8:18 AM, 孙宗� <szj087 at gmail.com
    <mailto:szj087 at gmail.com>> 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 <vkg at alcatel-lucent.com
        <mailto:vkg at alcatel-lucent.com>>:
         > 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 at {alcatel-lucent.com
        <http://alcatel-lucent.com/>,bell-labs.com
        <http://bell-labs.com/>,acm.org <http://acm.org/>}
         >  WWW:   http://www.alcatel-lucent.com/bell-labs
         >
        _______________________________________________
        Sip mailing list  https://www.ietf.org/mailman/listinfo/sip
        This list is f        /-------- 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 <baolovebao at gmail.com>/* wrote:

    old good questions in sip, also add another "transaction".


    On Fri, Apr 25, 2008 at 8:18 AM, 孙宗� <szj087 at gmail.com
    <mailto:szj087 at gmail.com>> 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 <vkg at alcatel-lucent.com
        <mailto:vkg at alcatel-lucent.com>>:
         > 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 at {alcatel-lucent.com
        <http://alcatel-lucent.com/>,bell-labs.com
        <http://bell-labs.com/>,acm.org <http://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 dor NEW development of the core SIP Protocol
        Use sip-implementors at cs.columbia.edu
        <mailto:sip-implementors at cs.columbia.edu> for questions on
        current sip
        Use sipping at ietf.org <mailto:sipping at ietf.org> for new
        developments on the application of sip




    --     BR
    Donald _______________________________________________
    This is the SIP Forum discussion mailing list
    TO UNSUBSCRIBE, or edit your delivery options, please visit
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    Post to the list at discussion at sipforum.org




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