[SIPForum-discussion] what is the diffrence b/w a transaction, session , dialog

[abhishek.chattopadhyay at wipro.com]

Hi,

Transaction, dialog and session they are like one makes the other. No
dialog without transaction and no session without a dialog. But there
can be transactions without dialogs and dialogs without sessions.

Transaction: every request and response pair would make a transaction.
[Please note that when I say that I mean a request and a successful
response for that request. In case there is a failure response for a
particular request then the transaction is not over till it receives
message which can clearly tell that the UAC to kill the transaction]
Basically transaction is a way of keeping a track of UAS and the UAC
interactions done independently.

Register--->
<-------200 OK   is a complete transaction.

Invite --->
<------3xx,4xx,5xx
ACK------> is also a transaction.


[3261] Dialog: Dialog can only be formed by the INVITE request when
there is a non-failure [101-199]/ [200] response for it.
Further RFC3265 also explains how subscribe notify pair can also form a
dialog. I think it is never wrong to consider a dialog as special
transaction which merely matures as a dialog. Dialog can have states.

Invite----->
<--------100 {optional}
<--------200
Ack ----->   is a full dialog [but it has two transactions]

Sessions: we use sip just for signaling. It is not a complete protocol
within itself to serve any purpose. So sip itself works with other
protocols like rtp and sdp to inform the two ends about the type of
conversation that can be possible. So plainly whenever a dialog (couple
of transactions) or a sequence of message exchange are capable of
educating the other end about the required information regarding the
intended conversation the dialog can support a session where the
protocols like RTP or SRTP would be used to facilitate the service for
which all the signaling was done.
So we can easily say (in a crude way) when a sip signaling facilitates
the two ends to exchange information by using some other protocol (whose
information was carried by SIP signals) we have a session.
A session would always have some capabilities which would be conveyed by
the dialog which made it possible.
So far I have always seen Sessions in dialogs initiated by the INVITE
method only and I think it can be only that way. [Anyone else in the
forum if has some other experiences please put in your views]

Further RFC3261 is equipped with definitions of all these but what
confuses me is the place where it says: "A dialog is a peer-to-peer SIP
relationship between two UAs that persists for some time"
As I have never been able to understand how much this some time should
be.


Regards 
ABHISHEK



-----Original Message-----
From: discussion-bounces at sipforum.org
[mailto:discussion-bounces at sipforum.org] On Behalf Of VG
Sent: Friday, June 05, 2009 6:40 PM
To: discussion at sipforum.org
Subject: [SIPForum-discussion] what is the diffrence b/w a
transaction,session , dialog

Hi,

I m confused with these three things :
transaction
session
dialog

Please specify me with an example.

Thanks & Regards,
VG
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