Simple definition
A SIP dialog is the persistent signaling relationship identified by Call-ID and dialog tags.
Purpose and use
A SIP dialog relates two participants across multiple signaling exchanges, for example during an established call.
How does it work?
Its identity combines Call-ID with local and remote tags. Stored state includes routing and sequencing information for subsequent requests.
Example and verification
To investigate a dropped call, correlate INVITE, responses, re-INVITE and BYE using dialog identity rather than only the dialed number.
Points to watch
One call can produce multiple dialogs, particularly when an invitation forks to several destinations. The dialog is not the RTP audio stream.
In short
A dialog describes an ongoing signaling relationship; individual operations within it have their own transactions.