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

[孙宗君]

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?

Thanks for your further instructions

Zongjun

2008/4/24, Lincoln Y. Lavoie <lylavoie at iol.unh.edu>:
> Hello BR,
>
>    In the strictest context, the dialog exists only in SIP and can exist
> without the establishment of a media session (setup through SDP).  To define
> a dialog, the SIP specification RFC-3261 defines several qualifiers, which
> all must be present in the SIP messages, off the top of my head, they are
> tag parameters in the TO/FROM headers, and a CALLID header.  You should
> check our RFC-3261 to look for the others.
>    For a simple example of a dialog without SDP, think of the presence
> subscription case.  In this case, the SUBSCRIBE request message and 200OK
> response establish the SIP dialog.  After that, all the NOTIFY messages use
> the same dialog to transmit the requested information.  The actual
> information is usually carried directly in the same packet, encoded as DSL
> or something.
>
>  Cheers,
>  Lincoln
>
>
>  孙宗君 wrote:
>
> > Thanks for  all your kindly explanation.
> >
> > Do you mean that dialog and session is one-2-one relationship and only
> > for different scenario and different context?
> >
> > Thanks
> > BR
> > Zongjun
> >
> > 2008/4/24, James M. Polk <jmpolk at cisco.com>:
> >
> >
> > > At 01:28 PM 4/23/2008, Dale.Worley at comcast.net wrote:
> > >  >    From: "=?GB2312?B?y+/X2r79?=" <szj087 at gmail.com>
> > >  >
> > >  >    I am confused about the conception of session and dialog in RFC
> 3261.
> > >  >    Although someone pointed out the dialog is a relationship between
> > >  >    peer-2-peer and session is a multimedia relative relation, I am
> still
> > >  >    not clear about that.
> > >  >
> > >  >As I understand the terms, a "session" is the set of media
> > >  >transmisstions described/controlled by a series of SDP offer/answer
> > >  >exchanges.  These exchanges are usually carried by the messages in one
> > >  >SIP dialog.
> > >
> > >
> > > To this too, these terms are often meant to be interchangeable - even
> > >  though technically they are separate terms with subtlety different
> > >  actual meanings (SDP "sessions", and SIP "dialogs"). Sessions and
> > >  Dialogs can, sometimes mean the same thing. Thus it requires a bit of
> > >  understanding the context in which either is stated in to understand
> > >  if they are meaning one or the other, or were misused for the same
> meaning.
> > >
> > >
> > >
> > >  >Dale
> > >  >_______________________________________________
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> > >
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> > >
> > >
> > _______________________________________________
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> >
> >
>
>  --
> **********************************************************************
>  Lincoln Lavoie
>  Senior Engineer
>  UNH InterOperability Laboratory
>  121 Technology Drive, Suite 2, Durham, NH 03824
>  +1-603-862-4809
>  Email: lylavoie at iol.unh.edu
>  AIM: LincolnAtIOL
>
>  Ars sine scientia nihil est! -- Art without science is nothing.
>  Scientia sine ars est vacua! -- Science without art is empty.
> **********************************************************************
>
>