On top of plain vanilla RFC3261, there’s a series of “Extension” methods added to SIP to expand it’s functionality, common extension methods are INFO, MESSAGE, NOTIFY, PRACK and UPDATE. Although now commonplace, of these is not defined in RFC3261 so is considered an “extension” to SIP.
It’s worth just pausing here to reiterate we’re not talking extensions like in a PBX context, like extra phones, we’re talking extensions like you’d add to a house, like extra functionality.
A SIP client can request functionality from a server (UAC to a UAS), if the server does not have support for that functionality, it can reject the session on those grounds and send back a response indicating it doesn’t know how to handle that extension, like a 420 Bad Extension – Bad SIP Protocol Extension used, not understood by the server. Response.
So clients can determine what functionality a server doesn’t support if it rejects the request, but there was no way to see what functionality the server does support, and what functionality the client requires.
Enter the Supported header, initially drafted by Rosenberg & Schulzrinne in 2000, it made it into the SIP we know today (SIP v2 / RFC3261).
If a UAC or UAS requires support for an extension – For example a Media Gateway has to understand PRACK, it can use the Require header to specify the request should be rejected if support for the listed extensions is not provided.
These headers are most commonly seen in SIP OPTIONS requests.