SIP Cheat Sheet 2026

The 30 highest-yield SIP facts, distilled from real exam questions. Print it, save it as a PDF, or study it here — free, no sign-up.

1. Which SIP header field is used to carry authentication credentials from a client to a server? → Authorization
2. What is the most important foundational concept in SIP Architecture? → Understanding core principles and their practical application in SIP Architecture
3. When a SIP proxy performs Record-Routing, what is the primary reason? → To remain in the signaling path for all subsequent requests within the dialog
4. For SIP interworking with MGCP-controlled media gateways, which protocol carries the call control signaling to the gateway? → MGCP from a Media Gateway Controller
5. Which SIP method is used to modify an existing session's parameters, such as changing the codec mid-call? → RE-INVITE
6. What SIP response is sent by a UAS when it receives a BYE request that references a dialog it cannot find? → 481 Call/Transaction Does Not Exist
7. In SIP trunking interoperability, Early Media is used to: → Carry media before a 200 OK is received, typically for ringback or announcements
8. Which transport protocol is recommended for SIP interoperability when traversing firewalls and NATs due to its reliable delivery? → TCP
9. Which SIP method is used to establish a call between two endpoints? → INVITE
10. What is the primary purpose of standardized procedures in Call Setup and Teardown? → To ensure consistency, safety, and quality across all practitioners
11. In SIP session timers, what does the 'refresher' parameter in the Session-Expires header indicate? → Which UA is responsible for sending the refresh request
12. Which SIP method is used to query the capabilities of a SIP server or UA? → OPTIONS
13. What role does continuing education play in Call Setup and Teardown? → Keeping practitioners current with evolving best practices and new developments
14. In a SIP environment that uses SIPREC for call recording, the Session Recording Client (SRC) sends a copy of the media to: → The Session Recording Server (SRS)
15. Which SIP method is used to end an established call? → BYE
16. What is Timer F used for in non-INVITE SIP client transactions? → The non-INVITE client transaction timeout before giving up
17. Which SIP header prevents a response from being replayed by an attacker to a different transaction? → CSeq
18. In SIP, what does 'SPIT' stand for and what threat does it represent? → Spam over Internet Telephony — unsolicited bulk voice calls
19. What is the purpose of the 'qop' (quality of protection) parameter in SIP Digest Authentication? → It specifies whether authentication, integrity, or both protections are applied
20. In a SIP-to-WebRTC interworking scenario, which protocol adaptation is typically required for media? → RTP to SRTP with DTLS-SRTP keying
21. A call center deploys SIP and needs to fork an incoming call to multiple agents simultaneously. Which SIP feature supports this? → Parallel forking by a proxy
22. Which protocol is commonly used in conjunction with SIP to handle the actual transmission of voice and video media? → RTP (Real-time Transport Protocol)
23. What does the SIP 'Require' header with the value 'sec-agree' enforce according to RFC 3329? → Both endpoints must negotiate and agree on a security mechanism before proceeding
24. Which header enables a SIP UA behind NAT to advertise an externally reachable contact address learned from a STUN server? → Contact
25. What SIP method is typically used to modify an existing session, such as changing media parameters? → RE-INVITE
26. What is the significance of the 'tag' parameter in the From and To headers for session management? → Together they uniquely identify a dialog alongside the Call-ID
27. Which SIP method is used to send mid-session information that does not change the session state? → INFO
28. What role does continuing education play in Security and NAT Traversal? → Keeping practitioners current with evolving best practices and new developments
29. What is the significance of the branch parameter in the SIP Via header? → It uniquely identifies a SIP transaction across the network
30. What is a SIP Dialog? → A call leg maintained between two endpoints, which provides state and context for the call