H2O

the optimized HTTP/1.x, HTTP/2 server
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Configure > HTTP/2 Directives

H2O provides one of the world's most sophisticated HTTP/2 protocol implementation, including following features.

Prioritization

H2O is one of the few servers that fully implement prioritization of HTTP responses conformant to what is defined in the HTTP/2 specification. The server implements a O(1) scheduler that determines which HTTP response should be sent to the client, per every 16KB chunk.

Unfortunately, some web browsers fail to specify response priorities that lead to best end-user experience. H2O is capable of detecting such web browsers, and if it does, uses server-driven prioritization; i.e. send responses with certain MIME-types before others.

It is possible to tune or turn off server-driven prioritization using directives: file.mime.addtypes, http2-reprioritize-blocking-assets.

See also:

Server push

H2O recognizes link headers with preload keyword sent by a backend application server (reverse proxy or FastCGI) or an mruby handler, and pushes the designated resource to a client.

Example. A link response header triggering HTTP/2 push
link: </assets/jquery.js>; rel=preload

When the HTTP/2 driver of H2O recognizes a link response header with rel=preload attribute set, and if all of the following conditions are met, the specified resource is pushed to the client.

The server also provides a mechanism to track the clients' cache state via cookies, and to push the resources specified with the link header only when it does not exist within the clients' cache. For details, please refer to the documentation of http2-casper configuration directive.

When a resource is pushed, the priority is determined using the priority attribute of the MIME-type configuration. If the priority is set to highest then the resource will be sent to the client before anything else; otherwise the resource will be sent to client after the main content, as per defined by the HTTP/2 specification.

Pushed responses will have x-http2-push: pushed header set; by looking for the header, it is possible to determine if a resource has been pushed. It is also possible to log the value in the access log by specifying %{x-http2-push}o, push responses but cancelled by CASPER will have the value of the header logged as cancelled.

See also:

The following describes the configuration directives for controlling the HTTP/2 protocol handler.

Description:

Configures CASPer (cache-aware server-push).

When enabled, H2O maintains a fingerprint of the web browser cache, and cancels server-push suggested by the handlers if the client is known to be in possention of the content. The fingerprint is stored in a cookie named h2o_casper using Golomb-compressed sets (a compressed encoding of Bloom filter).

If the value is OFF, the feature is disabled. Push requests (made by the handlers through the use of Link: rel=preload header) are processed regardless of whether if client already has the responses in its cache. If the value is ON, the feature is enabled with the defaults value specified below. If the value is mapping, the feature is enabled, recognizing the following attributes.

capacity-bits:
number of bits used for the fingerprinting. Roughly speaking, the number of bits should be log2(1/P * number-of-assets-to-track) where P being the probability of false positives. Default is 13, enough for tracking about 100 asset files with 1/100 chance of false positives (i.e. log2(100 * 100) =~ 213).
tracking-types:
specifies the types of the content tracked by casper. If omitted or set to blocking-assets, maintains fingerprint (and cancels server push) for resources with mime-type of highest priority. If set to all, tracks all responses.

It should be noted that the size of the cookie will be log2(P) * number-of-assets-being-tracked bits multiplied by the overhead of Base 64 encoding (4/3). Therefore with current cookie-based implementation, it is necessary in many cases to restrict the resources being tracked to those have significant effect to user-percieved response time.

Example. Enabling CASPer
http2-casper: ON

# `ON` is equivalent to:
# http2-casper:
#   capacity-bits:  13
#   tracking-types: blocking-assets
Level:
global, host
Default:
http2-casper: OFF
See also:
file.mime.addtypes, issue #421
Description:

Timeout for idle connections in seconds.

Level:
global
Default:
http2-idle-timeout: 10
Description:

Maximum number of requests to be handled concurrently within a single HTTP/2 connection.

The value cannot exceed 256.

Level:
global
Default:
http2-max-concurrent-requests-per-connection: 100
Description:

A boolean flag (ON or OFF) indicating if the server should send contents with highest priority before anything else.

To maximize the user-perceived reponsiveness of a web page, it is essential for the web server to send blocking assets (i.e. CSS and JavaScript files in <HEAD>) before any other files such as images. HTTP/2 provides a way for web browsers to specify such priorities to the web server. However, as of Sep. 2015, no major web browsers except Mozilla Firefox take advantage of the feature.

This option, when enabled, works as a workaround for such web browsers, thereby improving experience of users using the web browsers.

Technically speaking, it does the following:

  • if the client uses dependency-based prioritization, do not reprioritize
  • if the client does not use dependency-based prioritization, send the contents of which their types are given highest priority before any other responses

Level:
global
Default:
http2-reprioritize-blocking-assets: ON
See also:
file.mime.addtypes, HTTP/2 (and H2O) improves user experience over HTTP/1.1 or SPDY