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What exactly is the difference between a dialog and a session? Does dialog mean a "call has already been setup" in the signaling sense, while a session mean the "media has already been setup" in the media sense?

Also, what does "part of a dialog" mean? Are "INVITE", "ACK" and the responses considered "part of a dialog"?

Chowdhury Md Rajib Sarwar
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  • See also [*Difference between session, dialog and transaction in SIP?*](https://stackoverflow.com/questions/35133331/difference-between-session-dialog-and-transaction-in-sip "Difference between session, dialog and transaction in SIP? - Stack Overflow"), a question that was asked later but got more attention and a more detailed answer. – Wolf Jun 20 '23 at 18:54

1 Answers1

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From the Definition section in RFC 3261:

Dialog: A dialog is a peer-to-peer SIP relationship between two
   UAs that persists for some time.  A dialog is established by
   SIP messages, such as a 2xx response to an INVITE request.  A
   dialog is identified by a call identifier, local tag, and a
   remote tag.  A dialog was formerly known as a call leg in RFC
   2543.

while

Session: From the SDP specification: "A multimedia session is a
   set of multimedia senders and receivers and the data streams
   flowing from senders to receivers.  A multimedia conference is
   an example of a multimedia session." (RFC 2327 [1]) (A session
   as defined for SDP can comprise one or more RTP sessions.)  As
   defined, a callee can be invited several times, by different
   calls, to the same session.  If SDP is used, a session is
   defined by the concatenation of the SDP user name, session id,
   network type, address type, and address elements in the origin
   field.

In my own words: A SIP dialog refers to the signaling relationship between two peers. A SIP dialog can be used to establish calls (INVITE), but there are other SIP methods that could establish a dialog (e.g. SUBSCRIBE)

A (multimedia) session is exchanged through a SIP dialog initiated by an INVITE method, using the SDP offer/answer (see RFC 3264)

Wolf
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jsantander
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  • Your answer should also contains information on what it means to be "part of dialogs"! Thus, I would add that dialog information are re-used among all other request/response sent within the established dialog, such as INFO, BYE, and other possible request/response sent to the remote peer. – AymericM Jan 02 '16 at 13:31