How SIP hold using RFC6336 is implemented and how it looks in production.
Function of Supported & Require headers in SIP.
The rules for which headers to use when creating a SIP request and knowing where to route it.
Explanation of what a SIP Registrar does, Contact header and Address on Record, and how it all fits together to route messages correctly.
Explanation of Record-Routing, why it exists and how it’s used.
Why does every SIP branch start with z9hG4bK?
How the SIP Authentication mechanism works, and how we can reverse it to check the password we’re expecting is the encrypted value we’re seeing in captures.