“SDP: The tiny contract that decides how we talk.”
Remember that SIP message we saw last time? It was polite. It said “Hi, want to talk?” But it also had a little payload, tucked inside like a secret note in a game invitation. That note was SDP — Session Description Protocol.
Let’s unpack it, casually.
🧠 What Is SDP?
SDP is not a protocol that sends media — it’s just a description.
It tells the other side things like:
- What IP and port to send media to
- What kind of media we support (audio, video, fax, etc.)
- Which codecs we can use
- How many media streams we want
Think of SDP as the cheat sheet you give your opponent before a Squid Game round. It says:
“We’ll play audio only. You send it to this IP, this port. Here are the rules (codecs) I speak.”
📃 A Sample SDP Offer
Here’s a simplified version of what might be inside the body of an INVITE:
v=0
o=alice 2890844526 2890844526 IN IP4 192.168.1.10
s=-
c=IN IP4 192.168.1.10
t=0 0
m=audio 49170 RTP/AVP 0 96
a=rtpmap:0 PCMU/8000
a=rtpmap:96 opus/48000/2
Let’s decode this bit by bit.
✏️ Mandatory Fields in SDP
| Field | What It Means |
|---|---|
v= |
Version — always 0 |
o= |
Origin — username, session ID, IP of sender |
s= |
Session Name — can be - if not used |
c= |
Connection Info — IP address to send media |
t= |
Timing — start and stop time (0 0 = forever) |
m= |
Media Description — media type, port, transport, codecs |
a= |
Attributes — like codec details, encryption, etc. |
🎙 What Are Codecs?
A codec (short for coder-decoder) compresses and decompresses media like voice or video. In VoIP, we use codecs to:
- Save bandwidth (fewer bits = faster calls)
- Improve audio quality
- Handle different devices and network conditions
🔊 Common Audio Codecs
| Codec | Description | Bandwidth | Notes |
|---|---|---|---|
| PCMU (G.711 µ-law) | Classic PSTN quality | ~64 kbps | Good quality, high bandwidth |
| PCMA (G.711 A-law) | EU version of µ-law | ~64 kbps | Similar to PCMU |
| G.729 | Compressed voice | ~8 kbps | Low bandwidth, licensed |
| Opus | Modern, flexible | 6–510 kbps | Great quality, dynamic bitrate |
| G.722 | HD voice | ~64 kbps | Wideband audio |
📹 Common Video Codecs
| Codec | Description | Use Case |
|---|---|---|
| H.264 | Standard for video calls | Widely used |
| VP8 / VP9 | Open-source codecs | Used in WebRTC |
| H.265 (HEVC) | Next-gen, better compression | Not always supported |
🎮 SIP + SDP: Setting the Game Rules
Let’s update our SIP message to imagine it has the voice rules included:
INVITE sip:bob@example.com SIP/2.0
...
Content-Type: application/sdp
Content-Length: 151v=0
o=alice 2890844526 2890844526 IN IP4 192.168.1.10
s=-
c=IN IP4 192.168.1.10
t=0 0
m=audio 49170 RTP/AVP 0 96
a=rtpmap:0 PCMU/8000
a=rtpmap:96 opus/48000/2
It’s saying:
“Hey Bob, send me voice on 192.168.1.10:49170. I speak PCMU and Opus. You pick.”
Bob can then reply with:
- “Cool, I speak Opus too” (and use
a=rtpmap:96 opus/48000/2) - Or “Let’s just stick with PCMU.”
📦 TL;DR
- SDP is metadata — it tells the other side how to stream media
- SIP uses SDP to offer codecs, IPs, and ports
- Audio codecs like PCMU, Opus, G.729 define voice quality + size
- Video codecs like H.264 and VP8 make video calls possible
- SDP doesn’t send media — it just describes it
🧠 Up Next in SIP GAMES:
“The Real MVP — RTP: The Voice Inside the Wires”
Now that SIP + SDP agreed on how we’ll talk, it’s time to learn how your voice actually hops across the internet in real time.
Stay tuned for packet magic!
🎮 Follow @sip_games for more VoIP fun with less RFC-induced stress.
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