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 to the Session! (What Even is SIP?)
SIP GAMES
SIP GAMES

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📡 You’re Invited
 to the Session! (What Even is SIP?)

#voip #sip #telephony #beginners

“SIP sets up the game. RTP plays it.”

đŸ§© So... What is SIP?

SIP (Session Initiation Protocol) is the VoIP equivalent of the game master — it sets the rules, gets the players into the arena, and then steps back once the game (the voice conversation) begins.

In plain terms:

SIP handles call setup, ringing, answering, and tear-down. It does not carry your voice — that’s RTP’s job.

Think of SIP like sending someone a party invite. It tells them:

  • Who’s calling
  • Where the call should happen
  • How they can reach you
  • And whether they want to accept it or ghost you

☎ The Call Flow (Simplified)

Here’s how a basic SIP call between two softphones works:

User A            SIP Server            User B
  |                   |                   |
  |--- INVITE ------->|                   |
  |                   |--- INVITE ------->|
  |                   |<-- 180 RINGING ---|
  |<-- 180 RINGING ---|                   |
  |                   |<-- 200 OK --------|
  |<-- 200 OK --------|                   |
  |--- ACK ---------->|                   |
  |                   |--- ACK ---------->|
  |                   |                   |
  |<==== RTP Media Stream (Voice) ======>|
  |                   |                   |
  |--- BYE ---------->|                   |
  |                   |--- BYE ---------->|
  |                   |<-- 200 OK --------|
  |<-- 200 OK --------|                   |
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🧠 Key SIP Messages

Message What it Means
INVITE “Hey, want to talk?”
180 RINGING “They’re being notified”
200 OK “They picked up!”
ACK “Cool, let’s connect”
BYE “Gotta go, bye!”

🎯 SIP in Action — What You See in Logs

If you’ve ever looked at SIP logs (Wireshark, anyone?), you’ll see something like:

INVITE sip:bob@example.com SIP/2.0
From: "Alice" sip:alice@example.net
To: sip:bob@example.com
Call-ID: 789abc123@example.net
CSeq: 1 INVITE
Contact: sip:alice@192.168.1.10:5060
Content-Type: application/sdp
Content-Length: 151

Notice that last part? Content-Type: application/sdp 👀

Yup — SIP includes something called SDP inside it. It’s like the fine print in your call invitation — it lists what codecs you'll use, what port to send media to, and more.

đŸ§© SIP ≠ Voice

SIP doesn’t carry your voice.

Instead, once the SIP call is accepted, RTP (Real-time Transport Protocol) takes over to actually move the media (voice, video, etc.) between endpoints.

So:

  • SIP = The call setup (the RSVP)
  • RTP = The actual voice (the conversation)

🔁 What Happens When You Hang Up?

A typical SIP BYE exchange:

User A --> BYE --> SIP Server --> BYE --> User B
User B --> 200 OK --> SIP Server --> 200 OK --> User A

Simple. Fast. And very polite.

🎯 TL;DR

  • SIP is a signaling protocol, not a media protocol
  • It helps set up, modify, and end VoIP calls
  • It uses messages like INVITE, RINGING, ACK, and BYE
  • It wraps media setup details using SDP (coming next)
  • After the call setup, RTP moves the actual voice packets

🧠 Up Next in SIP GAMES:

“What’s in the Envelope? Unpacking SDP”

SDP (Session Description Protocol) tells your devices what media codecs to use, how to send the audio, and more — it’s like the secret note inside your SIP envelope.

Stay tuned as we dive into how a voice call actually finds its voice.

đŸ•č Follow @sip_games for more VoIP magic — minus the RFC migraines.

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