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137 lines
4.9 KiB
Markdown
137 lines
4.9 KiB
Markdown
<!--
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Copyright (C) 2000 - 2022 Daniel Stenberg, <daniel@haxx.se>, et al.
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SPDX-License-Identifier: curl
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-->
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# WebSocket in curl
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## URL
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WebSocket communication with libcurl is done by setting up a transfer to a URL
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using the `ws:/` or `wss://` URL schemes. The latter one being the secure
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version done over HTTPS.
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When using `wss://` to do WebSocket over HTTPS, the standard TLS and HTTPS
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options will be acknowledged for the CA, verification of server certificate
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etc.
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WebSocket communication is done by upgrading a connection from either HTTP or
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HTTPS. When given a WebSocket URL to work with, libcurl considers it a
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transfer failure if the upgrade procedure fails. This means that a plain HTTP
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200 response code is considered an error for this work.
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## API
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The WebSocket API is described in the individual man pages for the new API.
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WebSocket with libcurl can be done two ways.
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1. Get the WebSocket frames from the server sent to the write callback. You
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can then respond with `curl_ws_send()` from within the callback (or outside
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of it).
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2. Set `CURLOPT_CONNECT_ONLY` to 2L (new for WebSocket), which makes libcurl
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do an HTTP GET + `Upgrade:` request plus response in the
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`curl_easy_perform()` call before it returns and then you can use
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`curl_ws_recv()` and `curl_ws_send()` to receive and send WebSocket frames
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from and to the server.
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The new options to `curl_easy_setopt()`:
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`CURLOPT_WS_OPTIONS` - to control specific behavior. `CURLWS_RAW_MODE` makes
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libcurl provide all WebSocket traffic raw in the callback.
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The new function calls:
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`curl_ws_recv()` - receive a WebSocket frame
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`curl_ws_send()` - send a WebSocket frame
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`curl_ws_meta()` - return WebSocket metadata within a write callback
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## Max frame size
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The current implementation only supports frame sizes up to a max (64K right
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now). This is because the API delivers full frames and it then cannot manage
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the full 2^63 bytes size.
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If we decide we need to support (much) larger frames than 64K, we need to
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adjust the API accordingly to be able to deliver partial frames in both
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directions.
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## Errors
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If the given WebSocket URL (using `ws://` or `wss://`) fails to get upgraded
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via a 101 response code and instead gets another response code back from the
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HTTP server - the transfer will return `CURLE_HTTP_RETURNED_ERROR` for that
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transfer. Note then that even 2xx response codes are then considered error
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since it failed to provide a WebSocket transfer.
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## Test suite
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I looked for an existing small WebSocket server implementation with maximum
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flexibility to dissect and cram into the test suite but I ended up deciding
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that extending the existing test suite server sws to deal with WebSocket
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might be the better way.
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- This server is already integrated and working in the test suite
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- We want maximum control and ability to generate broken protocol and negative
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tests as well. A dumber and simpler TCP server could then be easier to
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massage into this than a "proper" WebSocket server.
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## Command line tool WebSocket
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The plan is to make curl do WebSocket similar to telnet/nc. That part of the
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work has not been started.
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Ideas:
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- Read stdin and send off as messages. Consider newline as end of fragment.
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(default to text? offer option to set binary)
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- Respond to PINGs automatically
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- Issue PINGs at some default interval (option to switch off/change interval?)
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- Allow `-d` to specify (initial) data to send (should the format allow for
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multiple separate frames?)
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- Exit after N messages received, where N can be zero.
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## Future work
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- Verify the Sec-WebSocket-Accept response. It requires a sha-1 function.
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- Verify Sec-WebSocket-Extensions and Sec-WebSocket-Protocol in the response
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- Make WebSocket work with hyper
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- Consider a `curl_ws_poll()`
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- Make sure WebSocket code paths are fuzzed
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- Add client-side PING interval
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- Provide option to disable PING-PONG automation
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- Support compression (`CURLWS_COMPRESS`)
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## Why not libWebSocket
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[libWebSocket](https://libWebSockets.org/) is said to be a solid, fast and
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efficient WebSocket library with a vast amount of users. My plan was
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originally to build upon it to skip having to implement the low level parts of
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WebSocket myself.
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Here are the reasons why I have decided to move forward with WebSocket in
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curl **without using libWebSocket**:
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- doxygen generated docs only makes them hard to navigate. No tutorial, no
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clearly written explanatory pages for specific functions.
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- seems (too) tightly integrated with a specific TLS library, while we want to
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support WebSocket with whatever TLS library libcurl was already made to
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work with.
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- seems (too) tightly integrated with event libraries
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- the references to threads and thread-pools in code and APIs indicate too
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much logic for our purposes
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- "bloated" - it is a *huge* library that is actually more lines of code than
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libcurl itself
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- WebSocket is a fairly simple protocol on the network/framing layer so
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making a homegrown handling of it should be fine
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