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:

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

Related sections for more details:

Flow Control API

Question: How can I automate the flow control? ↕

Short Answer:

Yes, you can. WebRTC provides a set of methods and an event for it.

Related sections for more details:

Flow Control API

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

Short Answer:

Yes. Optionally, you could tell the data channel to deliver message as it they arrive even those messages reordered over the network.

Related sections for more details:

Data Channel API
SCTP

Question: When do you use unordered delivery option? ↕

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 can 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, while it is automatically done when you use media channels.

Related sections for more details:

Audio and Video Communication

Functional Overview #

What does data channels solve? #

Application examples
- Real-time network games
- Game player action events
- Asset exchange
- Text chat
- Collaborative file transfer (distributed storage p2p file sharing)
- Monitoring IoT devices

What is Data Channel? #

Questions From The Curious #

Functional Overview #

What does data channels solve? #

Protocol Stack #

Basic Features #

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 #