CompTIA Security+ Exam Notes

CompTIA Security+ Exam Notes
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Friday, October 10, 2025

Session Initiation Protocol Explained: Components, Call Flow, and Security

 SIP (Session Initiation Protocol) 

Session Initiation Protocol (SIP) is a signaling protocol used to initiate, maintain, and terminate real-time communication sessions over IP networks. These sessions can include voice, video, messaging, and other multimedia elements. SIP is widely used in VoIP (Voice over IP) systems, video conferencing, and instant messaging.

Core Functions of SIP
SIP is responsible for:
1. Establishing a session:  locating users and negotiating session parameters.
2. Managing the session:  modifying session parameters during the call.
3. Terminating the session: ending the communication.

SIP Components
SIP operates with several key components:
1. User Agents (UA)
User Agent Client (UAC): Initiates requests.
User Agent Server (UAS): Responds to requests.

2. SIP Servers
  • Registrar Server: Manages user registrations.
  • Proxy Server: Routes SIP requests to their intended destinations.
  • Redirect Server: Directs clients to contact another SIP address.

SIP Call Flow Example
Here’s a simplified flow of a SIP call:
1. INVITE: Sent by the caller to initiate a session.
2. 100 TRYING: A provisional response from the server.
3. 180 RINGING: Indicates the callee's device is ringing.
4. 200 OK: The callee accepts the call.
5. ACK: Confirms the session establishment.
6. BYE: Ends the session.
7. 200 OK: Acknowledges the termination.

SIP Message Format
SIP messages are similar to HTTP and consist of:
  • Request Line / Status Line
  • Headers (e.g., From, To, Call-ID, CSeq)
  • Body (often contains SDP – Session Description Protocol – for media negotiation)
Example SIP INVITE:

INVITE sip:bob@domain.com SIP/2.0
Via: SIP/2.0/UDP client.domain.com:5060
From: Alice <sip:alice@domain.com>
To: Bob <sip:bob@domain.com>
Call-ID: 123456789@client.domain.com
CSeq: 1 INVITE
Content-Type: application/sdp
Content-Length: ...

Security in SIP
SIP can be secured using:
  • TLS (Transport Layer Security) for encrypting signaling.
  • S/MIME for message integrity and authentication.
  • SRTP (Secure Real-Time Transport Protocol) for encrypting media streams.
Protocols SIP Works With
SIP is not a standalone protocol. It works alongside:
SDP: for media negotiation.
RTP: for media transport.
DNS: for resolving SIP addresses.
STUN/TURN/ICE: for NAT traversal.

SIP vs. Other Protocols


Acronyms:
RTP: Real-time Transport Protocol
SDP: Session Description Protocol
STUN: Session Traversal Utilities for NAT
TURN: Traversal Using Relay around NAT
ICE: Interactive Connectivity Establishment 

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