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Document how libnghttp2 schedules HTTP/2 frames internally

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Tatsuhiro Tsujikawa 2016-09-23 22:41:49 +09:00
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doc/programmers-guide.rst
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@ -177,6 +177,51 @@ Any deviation results in stream error of type PROTOCOL_ERROR. If
error is found in PUSH_PROMISE frame, stream error is raised against
promised stream.
The order of transmission of the HTTP/2 frames
----------------------------------------------
This section describes the internals of libnghttp2 about the
scheduling of transmission of HTTP/2 frames. This is pretty much
internal stuff, so the details could change in the future versions of
the library.
libnghttp2 categorizes HTTP/2 frames into 4 categories: urgent,
regular, syn_stream, and data in the order of higher priority.
The urgent category includes PING and SETTINGS. They are sent with
highest priority. The order inside the category is FIFO.
The regular category includes frames other than PING, SETTINGS, DATA,
and HEADERS which does not create stream (which counts toward
concurrent stream limit). The order inside the category is FIFO.
The syn_stream category includes HEADERS frame which creates stream,
that counts toward the concurrent stream limit.
The data category includes DATA frame, and the scheduling among DATA
frames are determined by HTTP/2 dependency tree.
If the application wants to send frames in the specific order, and the
default transmission order does not fit, it has to schedule frames by
itself using the callbacks (e.g.,
:type:`nghttp2_on_frame_send_callback`).
For example, suppose that application wants to send RST_STREAM after
sending response HEADERS and DATA. Because of the reason we mentioned
above, the following code does not work:
.. code-block:: c
nghttp2_submit_response(...)
nghttp2_submit_rst_stream(...)
This is because HEADERS submitted by `nghttp2_submit_response()` is
scheduled after RST_STREAM submitted by `nghttp2_submit_rst_stream()`.
The correct way is use :type:`nghttp2_on_frame_send_callback`, and
after HEADERS and DATA frames are sent, issue
`nghttp2_submit_rst_stream()`.
Implement user defined HTTP/2 non-critical extensions
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