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"?
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)