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I was discussing Websocket overhead with a fellow engineer and we were both uncertain of how a Websocket actually detects the status of a client connection.

  1. Is there a "status" packet sent to the client/server periodically?
  2. Does it have anything to do with ping or pong in the low level API?
  3. What about frames?
  4. How does the Websocket detect that it's disconnected on the client? the server?

I'm surprised I couldn't find this answered on SO, but that may be my mistake. I found this answer addresses scalability, but that's not what I'm asking here. This answer touches on implementation, but not at the depth I'm pursuing here.

4Z4T4R
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1 Answers1

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A webSocket connection is a TCP connection that uses the webSocket protocol. By default, a server or client knows when the connection has disappeared only when the underlying TCP realizes that the connection has been closed and the webSocket layer is listening for a close event on the connection so it is informed that way.

The webSocket protocol does not, by itself, require heartbeat packets that can regularly test if the connection is still working. The TCP socket may still appear to be alive, but the connection may not actually still work. The other end could have disappeared or been interrupted in between and one or both endpoints might not know that at any given time.

Socket.io which is built on top of webSocket, uses the ping and pong packets to implement a heartbeat which regularly tests the connection and will, in fact, detect a non-functioning connection at the client, close the socket and then reconnect automatically.

Is there a "status" packet sent to the client/server periodically?

Not by default for a regular webSocket connection.

Does it have anything to do with ping or pong in the low level API?

It is up to a client or server if they want to send ping or pong packets themselves to implement some sort of connection validation detection.

What about frames?

webSocket frames are the data format for sending data over a webSocket. They don't have anything to do with this issue.

How does the Websocket detect that it's disconnected on the client? the server?

Described above. Unless the client/server implement their own ping/pong system to detect when a connection has gone awry, they just rely on TCP signaling to know when a connection has been closed by the other end. A webSocket connection can be non-functional without the client or server knowing it until they try to send.

When a browser window/tab opens a webSocket connection and then the window/tab is retargeted to a new URL, the browser will close all resources associated with that window/tab including any webSocket connections. If the link between client and server is functional at that point, then the server will be told the underlying TCP connection (and thus the webSocket) has been closed. If the network link goes down and then the user moves that window/tab to a new URL, the server will not necessarily know that the connection is non-functional without relying on ping/pong type signalling to regularly test the connection.

If the browser crashes, the OS should close any sockets opened by that process.

If the computer/OS crashes, the sockets will likely not get closed normally (though this may be somewhat OS dependent and may be crash-dependent too).

jfriend00
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  • This is good info. I'm looking for specifics about the TCP connection protocols for disconnections or dropped connections (half-open connections?) as they relate to the low-level implementation within Websockets sources. An RFC or source link would help my deeper investigation. Thank you for what you've offered so far, @jfriend00! – 4Z4T4R Aug 16 '17 at 04:14
  • @toszter - There's lots to find with Google. Here are some: [What happens to TCP connections when I remove the ethernet cable](https://superuser.com/questions/911808/what-happens-to-tcp-connections-when-i-remove-the-ethernet-cable/911829) and [Detection of Half Open (dropped) Connections](https://blog.stephencleary.com/2009/05/detection-of-half-open-dropped.html) and [How does TCP terminate if one machine dies](https://stackoverflow.com/questions/9449209/how-does-tcp-connection-terminate-if-one-of-the-machine-dies). Also, if this is useful info, do you care to at least upvote my answer? – jfriend00 Aug 16 '17 at 04:22
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    @toszter - TCP itself won't know a connection has died until either it receives a close packet from the other end or until it tries to send data and gets an error and all retry attempts fail. Some sort of keep-alive packet and response is required to reliably detect connections that have died. That's why socket.io implements such a feature. – jfriend00 Aug 16 '17 at 04:24