SplitHTTP

Uses HTTP chunked-transfer encoding for download, and multiple HTTP requests for upload.

Can be deployed on CDNs that do not support WebSocket. However, the CDN must support HTTP chunked transfer encoding in a streaming fashion, no response buffering.

This transport serves the same purpose as Meek (support non-WS CDN). It has the above streaming requirement to the CDN so that download can be much faster than (v2fly) Meek, close to WebSocket performance. The upload is also optimized, but still much more limited than WebSocket.

Like WebSocket transport, SplitHTTP parses the X-Forwarded-For header for logging.

SplitHttpObject

The SplitHttpObject corresponds to the splithttpSettings section under transport configurations.

{
  "path": "/",
  "host": "xray.com",
  "headers": {
    "key": "value"
  },
  "maxUploadSize": 1000000,
  "maxConcurrentUploads": 10
}

path: string

HTTP path used by the connection. Defaults to "/".

host: string

HTTP Host sent by the connection. Empty by default. If this value is empty on the server, the host header sent by clients will not be validated.

If the Host header has been defined on the server in any way, the server will validate if the Host header matches.

The current priority of the Host header sent by clients: host > headers > address

headers: map {string: string}

Customized HTTP headers defined in key-value pairs. Defaults to empty.

maxUploadSize

The largest possible chunk to upload. Defaults to 1 MB. This should be less than the max request body size your CDN allows. Decrease this if the client prints HTTP 413 errors. Increase this to improve upload bandwidth.

maxConcurrentUploads

The number of concurrent uploads to run. Defaults to 10. Connections are reused wherever possible, but you may want to lower this value if the connection is unstable, or if the server is using too much memory.

The value on the client must not be higher than on the server. Otherwise, connectivity issues will occur.

HTTP versions

Added in 1.8.21: HTTP/3 support

SplitHTTP supports http/1.1, h2 and h3 ALPN values. If the value is not set, h2 (prior-knowledge) is assumed when TLS is enabled, and http/1.1 without TLS. If the value is set to h3, the client will attempt to connect as HTTP/3, so UDP instead of TCP.

The server listens to HTTP/1.1 and h2 by default, but if h3 ALPN is set on the server, it will listen as HTTP/3.

Please note that nginx, Caddy and all CDN will almost certainly translate client requests to a different HTTP version for forwarding, and so the server may have to be configured with a different ALPN value than the client. If you use a CDN, it is very unlikely that h3 is a correct value for the server, even if the client speaks h3.

Troubleshooting

If a connection hangs, the CDN may not support streaming downloads. You can use curl -Nv https://example.com/abcdef to initiate a download and see for yourself (see protocol details).

If you do not see 200 OK and a response body of ok, then the CDN is buffering the response body. Please ensure that all HTTP middleboxes along the path between client and server observe X-Accel-Buffering: no from their origin server. If your chain is xray -> nginx -> CDN -> xray, nginx may strip this response header and you have to re-add it.

Browser Dialer

If uTLS is not enough, SplitHTTP's TLS can be handled by a browser using Browser Dialer

Protocol details

See #3412 and #3462 for extensive discussion and revision of the protocol. Here is a summary, and the minimum needed to be compatible:

GET /<UUID> opens the download. The server immediately responds with 200 OK, and immediately sends the string ok (arbitrary length, such as ooook) to force HTTP middleboxes into flushing headers.

The server will send the X-Accel-Buffering: no and Content-Type: text/event-stream headers to force CDN into not buffering the response body. In HTTP/1.1 it may also send Transfer-Encoding: chunked.
Client uploads using POST /<UUID>/<seq>. seq starts at 0 and can be used like TCP seq number, and multiple "packets" may be sent concurrently. The server has to reassemble the "packets" live. The sequence number never resets for simplicity reasons.

The client may open upload and download in any order, either one starts a session. However, eventually GET needs to be opened (current deadline is hardcoded to 30 seconds) If not, the session will be terminated.
The GET request is kept open until the tunneled connection has to be terminated. Either server or client can close.

How this actually works depends on the HTTP version. For example, in HTTP/1.1 it is only possible to disrupt chunked-transfer by closing the TCP connection, in other versions the stream is closed or aborted.

Recommendations:

Do not assume any custom headers are transferred correctly by the CDN. This transport is built for CDN who do not support WebSocket, these CDN tend to not be very modern (or good).
It should be assumed there is no streaming upload within a HTTP request, so the size of a packet should be chosen to optimize between latency, throughput, and any size limits imposed by the CDN (just like TCP, nagle's algorithm and MTU...)
HTTP/1.1 and h2 should be supported by server and client, and it should be expected that the CDN will translate arbitrarily between versions. A HTTP/1.1 server may indirectly end up talking to a h2 client, and vice versa.

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