cnp3-ebook/protocols/http2 — English @ Weblate

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31 ../../protocols/http2.rst:228

	English By using multiple streams, the server can multiplex different web objects over the same underlying transport connection. However, these objects are only sent in response to requests from clients. There are some situations where the server might know in advance that the client will request a given object. It could speedup the transfer by sending it before having received a client request. This is the `push` feature of HTTP/2.0. A server can independently push web objects to a client without having received any request. This feature can only be used by the server if the client has enabled it by sending `SETTINGS_ENABLE_PUSH` in its `Settings` frame. A classical use case for this `push` feature is to enable a server to automatically send an object which cannot be cached by the client, such as a dynamic javascript code, when another web object that references it is requested. However, measurement studies indicate that very few web servers seem to have adopted this feature [ZWH2018]_. 994/10000 · 994
32 ../../protocols/http2.rst:230

	English Another feature of HTTP/2.0 is that it is possible to assign different priorities to different streams. A high priority stream should carry more `Data` frames than a lower priority ones. The HTTP/2.0 specification defines `Priority` frames which can be used for this purpose. 275/10000 · 275
33 ../../protocols/http2.rst:232

	English As the server can send multiple objects at the same time, there is a risk of overloading the client buffers. To cope with this potential problem, HTTP/2.0 includes its own flow control mechanism. When an HTTP/2.0 session starts, a receiver agrees to receive up to 65,535 bytes over this connection (unless it has indicated a different initial window in its `Settings` frame). This limits the amount of data that a sender can transmit over the HTTP/2.0 session. The receiver can advertise a large receive window by sending a `Window_Update` frame at any time. This flow control mechanism can be applied to the entire connection or to a specific stream. In practice, using a small `HTTP/2.0 window` could severely limit the throughput over an HTTP/2.0 session. 758/10000 · 758
34 ../../protocols/http2.rst:234

	English HTTP/2.0 includes much more than what we have covered in this short introduction. There is for example a `Ping` frame that allows measuring the round-trip-time between a client and a server or the `GoAway` frame that indicates the termination of an HTTP/2.0 session. This frame contains an error code that indicates why the session has been terminated. Several error codes are defined in :rfc:`7540`, including `ENHANCE_YOUR_CALM` that is used to indicate that the other endpoint exhibits an behavior that could cause excessive load. 533/10000 · 533
35 ../../protocols/http2.rst:237

	English Detecting whether a server supports HTTP/2.0 44/10000 · 44
36 ../../protocols/http2.rst:239

	English HTTP/2.0 is a new version of the HTTP protocol that still uses port 80. When a client contacts an HTTP server, it must be able to determine whether it supports HTTP/1.x or HTTP/2.0. If the client sends a binary encoded HTTP/2.0 request to a server that only supports the ASCII encoded HTTP/1.x, it could cause problems on the server and even crash it. To minimize the risk of crashing HTTP/1.x servers, an HTTP/2.0 session starts like an HTTP/1.1 session and the first request contains the `Connection`, `Upgrade` and `HTTP2-Settings` headers. An example of such a request to upgrade the version of HTTP is shown below. 619/10000 · 619
37 ../../protocols/http2.rst:251

	English The `HTTP2-Settings` line contains the HTTP/2.0 settings frame that the client would server over an HTTP/2.0 session encoded in Base64. The server replies with a response that indicates that it has accepted to upgrade the connection to HTTP/2.0. A sample response is shown below. 279/10000 · 279
38 ../../protocols/http2.rst:260

	English Finally, the client and the server need to confirm the utilization of HTTP/2.0. A client confirms this by sending the following Magic string `PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n` or `0x505249202a20485454502f322e300d0a0d0a534d0d0a0d0a` in hex. This string is followed by a SETTINGS frame. The server must send a possibly empty SETTINGS frame. 338/10000 · 338
39 ../../protocols/http2.rst:266

	English Footnotes 9/10000 · 9
40 ../../protocols/http2.rst:267

	English See https://en.wikipedia.org/wiki/HTTP_pipelining for additional information. 77/10000 · 77
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