What is TLS handshake in SIP?

• A. The negotiation that establishes an encrypted TLS session and validates certificates
• B. The process of starting a SIP session
• C. The exchange of codecs
• D. The exchange of user IDs

Answer: (A)

The TLS handshake in SIP is the negotiation that establishes a secure TLS session and validates certificates. Before SIP signaling messages travel over TLS, the client and server negotiate the TLS protocol version and a cipher suite, exchange and verify certificates (and optionally perform mutual authentication with a client certificate), and perform the key exchange to derive the symmetric keys that will encrypt and authenticate the SIP signaling. This process ensures confidentiality and integrity of all SIP messages on the signaling path.

This isn't about starting a SIP session, which is about initiating calls; it doesn't cover media codec negotiation, which happens later for the RTP stream; and it doesn't involve exchanging user IDs, which occur within SIP messages themselves rather than during the TLS handshake.