WebSockets ping/pong, why not TCP keepalive?

Solution 1:

The problems with TCP keepalive are:

1. It is off by default.
2. It operates at two-hour intervals by default, instead of on-demand as the Ping/Pong protocol provides.
3. It operates between proxies rather than end to end.
4. As pointed out by @DavidSchwartz, it operates between TCP stacks, not between the applications so therefore it doesn't tell us whether the application is alive or not.

The comparison with WebSockets ping/pong isn't meaningful. TCP keepalive is automatic, and timed, when enabled, whereas WebSocket ping/pong is executed as required by the application.

Solution 2:

Besides the answer of EJP I think it might be also related to HTTP proxy mechanisms. Websocket connections can also run through a (HTTP) proxy server. In such cases the TCP keepalive would only check the connection up to the proxy and not the end-to-end connection.

Solution 3:

http://www.whatwg.org/specs/web-apps/current-work/multipage/network.html#ping-and-pong-frames

.3.4 Ping and Pong frames

The WebSocket protocol specification defines Ping and Pong frames that can be used for keep-alive, heart-beats, network status probing, latency instrumentation, and so forth. These are not currently exposed in the API.

User agents may send ping and unsolicited pong frames as desired, for example in an attempt to maintain local network NAT mappings, to detect failed connections, or to display latency metrics to the user. User agents must not use pings or unsolicited pongs to aid the server; it is assumed that servers will solicit pongs whenever appropriate for the server's needs.

WebSockets have been developed with RTC in mind, so when I look at the ping/pong functionality, I see a way of measuring latency as well. The fact that the pong must return the same payload as the ping, make it very convenient to send a timestamp, and then calculate latency from client to server or vice verse.

Solution 4:

TCP keepalive doesn't get passed through a web proxy. The websocket ping/pong will be forwarded by through web proxies. TCP keepalive is designed to supervise a connection between TCP endpoints. Web socket endpoints are not equal to TCP endpoints. A websocket connection can use several TCP connections between two websocket endpoints.