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Say I have an websocket that may receive an event at any time, but is mostly idle, how much bandwidth will be consumed after the initial connection in order to keep it alive?

For what it's worth, the server is NodeJS using ws, and the client is using QtWebSockets.

Thanks!

vtortola
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Jacob
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  • What level of bandwdith are you concerned about? This is likely to be very, very small - less for a day than a single good-sized image. – jfriend00 Oct 14 '14 at 07:50
  • It's basically a push notification server. Say it's on 8 hours a day. – Jacob Oct 14 '14 at 08:45
  • You didn't answer my question. Are you worried about 100kb of bandwidth, 200MB of bandwidth, 200GB of bandwidth? Where is your worry level? Keep alive messages are very small so they don't add up to a lot. Typically, a bigger problem is battery life on mobile devices. – jfriend00 Oct 14 '14 at 08:57
  • Say >5mb is unwanted, and >20mb is unacceptable. – Jacob Oct 14 '14 at 09:35

1 Answers1

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Once established (meaning the three-way handshake completes), a raw TCP connection uses zero bandwidth unless:

A server or client can enable TCP keepalives. A keepalive is a zero-length packet sent with the ACK flag set, and is only 54 bytes sent on the wire plus another 54 for the response. By default, TCP keepalives are sent every two hours. In other words, completely negligible.

WebSockets also have their own keepalive mechanism (to deal with proxies). The server or the client may send a PING frame, and the other end MUST respond with a PONG frame. While there is no browser-side JS API to send PINGs, a node server may send them and a compliant browser will automatically respond. (QtWebSockets does have an API to send PINGs.) This does not happen by default; you must do it manually. WebSocket PING and PONG frames are at least 7 bytes and at most 131 bytes each (plus 54 bytes of TCP/IP overhead). Thus a single PING/PONG costs between 122 and 370 bytes.

ws does not do any keepalives automatically, nor does QtWebSockets. So to answer your question, a default configuration does use zero bandwidth to maintain the TCP connection.

However...

Keepalives are important because intermediate devices (ie NAT routers) will drop inactive TCP connections. Depending on the networks between your server and your clients, this means that if you don't have any keepalives, your clients will lose their connections (possibly without knowing it, which is bad), and will have to re-eastablish the WebSocket session. This will likely be far more expensive in terms of bandwidth than enabling sane keepalives.

A PING/PONG every 5 minutes costs 1.5 - 4.4 kB per hour (per client).

Note: socket.io has its own keepalive mechanism (separate from WebSockets') that is enabled by default. The sio keepalive is managed by the sio library and happens over the WebSocket data channel (as opposed to WebSocket PING/PONG, which are control frames). The socket.io server sends an 8 byte (+overhead) keepalive message every 30 seconds, which amounts to about 15 kB per hour.

Community
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josh3736
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  • Thanks for the informative answer! Is 5 minutes sufficient to not have a dropped connection? – Jacob Oct 14 '14 at 19:31
  • Totally depends on the devices between endpoints, but 5 minutes should be more than enough. To really know, you'd have to do some experimentation with your clients. – josh3736 Oct 14 '14 at 19:37