SIP Protocol Fundamentals

Master the Session Initiation Protocol (SIP) for building VoIP applications, understanding SIP messages, methods, and call flows essential for real-time communications.

Understanding SIP

SIP is an application-layer signaling protocol defined in RFC 3261 for creating, modifying, and terminating sessions with one or more participants.

SIP Message Structure

INVITE sip:bob@biloxi.com SIP/2.0
Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bK776asdhds
Max-Forwards: 70
To: Bob <sip:bob@biloxi.com>
From: Alice <sip:alice@atlanta.com>;tag=1928301774
Call-ID: a84b4c76e66710@pc33.atlanta.com
CSeq: 314159 INVITE
Contact: <sip:alice@pc33.atlanta.com>
Content-Type: application/sdp
Content-Length: 142

When to Use This Skill

Use sip-protocol-fundamentals when building VoIP applications with SIP signaling.

Best Practices

Always include unique branch parameters in Via headers
Implement proper transaction timeout and retransmission
Handle all SIP response codes appropriately

Common Pitfalls

Forgetting to include branch parameter in Via headers
Not implementing transaction retransmission timers
Ignoring provisional responses