WebRTC Architecture

We will be looking at:

• Networking
• Protocol stack
• Codec Negotiation
• Case: Video Conferencing

TCP versus UDP

Networking

A client connects

Second clients connects

In the home network

Client connects to server

The gateway performs NAT

And does housekeeping

Peer-to-peer TCP won’t work

STUN to the rescue

STUN - Session Traversal Utilities for NAT

Have an external server answer the questions:

• “What is my external IP?”
• “What is my external port?”

var ice = {"iceServers": [
    {"url": "stun:23.21.150.121"},
    {"url": "stun:stun.l.google.com:19302"},
]};

Bad Luck

Sometimes a connection fails:

• (company) firewall
• WiFi without client interconnect
• network topologies

Then use TURN - Traversal Using Relays around NAT

⇒ Traffic is relayed, no longer P2P!

var ice = {"iceServers": [
    {"url": "stun:23.21.150.121"},
    {"url": "stun:stun.l.google.com:19302"},
    {"url": "turn:user@turnserver.com", "account": "********"}
]};

WebRTC simplifies with ICE

ICE - Interactive Connectivity Establishment

Finds a PeerConnection for you

• on the local network
• using STUN
• using TURN

Security

W3C WebRTC working group requires all transmissions to be encrypted

• Media
• Data
• Control

For UDP media traffic solutions are available, e.g. SRTP. For connection-oriented traffic, DTLS was invented.

WebRTC protocol stack

SDP

Session description
    v=  (protocol version number, currently only 0)
    o=  (originator and session identifier : username, id, version number, network address)
    s=  (session name : mandatory with at least one UTF-8-encoded character)
    i=* (session title or short information)
    u=* (URI of description)
    e=* (zero or more email address with optional name of contacts)
    p=* (zero or more phone number with optional name of contacts)
    c=* (connection information—not required if included in all media)
    b=* (zero or more bandwidth information lines)
    One or more Time descriptions ("t=" and "r=" lines; see below)
    z=* (time zone adjustments)
    k=* (encryption key)
    a=* (zero or more session attribute lines)
    Zero or more Media descriptions (each one starting by an "m=" line; see below)

SDP (cont)

Time description (mandatory)
    t=  (time the session is active)
    r=* (zero or more repeat times)

Media description (if present)
    m=  (media name and transport address)
    i=* (media title or information field)
    c=* (connection information — optional if included at session level)
    b=* (zero or more bandwidth information lines)
    k=* (encryption key)
    a=* (zero or more media attribute lines — overriding the Session attribute lines)

Case: Video Conferencing

Using a media gateway