What is Data Channel? #

Questions From The Curious #

I know many of you have a bunch of questions similar to the ones shown below. Let’s get them quickly answered first! (Click on the questions to expand for the answer.)

Question: What’s the difference from TCP? ↕

Short Answer:

Peer-to-peer connection (NAT traversal) is possible. While it provides “guaranteed delivery” like TCP, it also provides none-reliable delivery like UDP.

Related sections for more details:

SCTP

Question: How many channels can we have? ↕

Short Answer:

Unlimited (theoretically 65536 channels as stream identifier has 16 bits)

Related sections for more details:

SCTP

Question: What are data channels good for? ↕

Short Answer:

Any data transfer other than audio and video. As it has various delivery options (partial reliability), it has broader applications than TCP.

Related sections for more details:

Functional Overview

Question: Any limit in bandwidth? ↕

Short Answer:

As much as the underlying network allows. The underlying SCTP protocol is designed so that you cannot overwhelm the network.

Question: Can we send binary data? ↕

Short Answer:

Yes, you can send both text and binary data.

Related sections for more details:

Data Channel API

Question: How about congestion control? ↕

Short Answer:

Yes it has congestion control.

Related sections for more details:

SCTP

Question: How is the latency compared to that of TCP? ↕

Short Answer: Even though the WebRTC’s SCTP lives in “user space”, performance is comparable to that of TCP. Data channels support “Partial-reliability” option which can reduce latency caused by data retransmissions over a lossy connection. Related sections for more details:

Partial Reliability

Question: What technologies are used in data channels? ↕

Short Answer: WebRTC (primarily) uses UDP. On top of UDP, there’s SCTP, then DCEP which controls establishment of data channels.

Related sections for more details:

Protocol Stack

Question: How do I know if I am sending too much? ↕

Short Answer:

Calls on the send method on a data channel always return immediately. It does not mean the data was sent on the wire. It will first be written to a buffer… When you call send() faster than your network can process, the buffered size will grow. RTCDataChannel.bufferedAmount is your friend.

Related sections for more details:

Flow Control API

Question: Do messages arrive in the order they were sent? ↕

Short Answer:

By default, Yes. Optionally, you can disable ordered transmission to receive messages as they arrive.

Related sections for more details:

Data Channel API
SCTP

Question: When should you use unordered delivery? ↕

Short Answer:

When newer information obsoletes the old such as positional information of an object, or each message is independent from the others and need to avoid head-of-line blocking delay.

Related sections for more details:

Data Channel Options
SCTP

Question: Can we send audio or video via data channel? ↕

Short Answer:

You could send any data over data channel. In a browser case, it is your responsibility to decode the data and pass it to a media player for rendering, where it is automatically done when you use media channels.

Related sections for more details:

Audio and Video Communication

Functional Overview #

Data channel can deliver any types of data. If you wish, you send send audio or video data over the data channel too, but if you need to playback the media in real-time, using media channels (See Media Communication) that uses RTP/RTCP protocols are the better options.

What are the applications of data channel? #

Applicability of the data channel is unlimited! The data channel solves packet loss, congestion problems and many other stuff for you and provide you with very simple API to deliver you your data to your peer in real-time. You just need to focus on what your application wants to achieve, and be creative.

Here are some example applications using the data channel:

Real-time network games
Serverless home automation from remote
Text chat
Animation (series of still images)
P2P CDN (Content Delivery Network)
Monitoring data from IoT
Continuous ingestion of time-series data from sensor devices
Real-time image recognition with AI from cameras

Protocol Stack Overview #

The data channel is comprised of the following 3 layers:

Data Channel Layer
DCEP (Data channel Establishment Protocol) Layer
SCTP (Stream Control Transmission Protocol) Layer

Data Channel Layer #

This layer provides API.

Data channel Establishment Protocol Layer #

This layer is responsible for the data channel handshake with the peer. It uses a SCTP stream as a control channel to negotiate capabilities such as ordered delivery, maxRetransmits/maxPacketLifeTime (a.k.a. “Partial-reliability”), etc.

SCTP Protocol Layer #

This is the heart of the data channel. What it does includes:

Channel multiplexing (In SCTP, channels are called “streams”)
Reliable delivery with TCP-like retransmission mechanism
Partial-reliability options
Congestion Avoidance
Flow Control

What is Data Channel? #

Questions From The Curious #

Functional Overview #

What are the applications of data channel? #

Protocol Stack Overview #

Data Channel Layer #

Data channel Establishment Protocol Layer #

SCTP Protocol Layer #

Data Channel API #

Connection / Teardown #

Data Channel Options #

Flow Control API #

Data Channel in Depth #

SCTP #

Connection establishment flow #

Connection teardown flow #

Keep-alive mechanism #

How does a user message get sent? #

TSN and retransmission #

Congestion avoidance #

Selective ACK #

Fast retransmission/recovery #

Partial Reliability #

DCEP #

Open/ACK handshake #

PPID #

Parameter exchange #

Reference to RFCs #