• The HTTP2 Upgrade Protocol

    Table of Contents

    Introduction

    The HTTP Upgrade Protocol element represents an Upgrade Protocol component that supports the HTTP/2 protocol. An instance of this component must be associated with an existing HTTP/1.1 Connector.

    HTTP/2 connectors use non-blocking I/O, only utilising a container thread from the thread pool when there is data to read and write. However, because the Servlet API is fundamentally blocking, each HTTP/2 stream requires a dedicated container thread for the duration of that stream.

    Attributes

    Common Attributes

    All implementations of Upgrade Protocol support the following attributes:

    Attribute Description
    className

    This must be org.apache.coyote.http2.Http2Protocol.

    Standard Implementation

    The HTTP/2 Upgrade Protocol implementation supports the following attributes in addition to the common attributes listed above.

    Attribute Description
    allowedTrailerHeaders

    By default Tomcat will ignore all trailer headers when processing HTTP/2 connections. For a header to be processed, it must be added to this comma-separated list of header names.

    compressibleMimeType

    The value is a comma separated list of MIME types for which HTTP compression may be used. The default value is text/html,text/xml,text/plain,text/css,text/javascript,application/javascript,application/json,application/xml .

    compression

    The HTTP/2 protocol may use compression in an attempt to save server bandwidth. The acceptable values for the parameter is "off" (disable compression), "on" (allow compression, which causes text data to be compressed), "force" (forces compression in all cases), or a numerical integer value (which is equivalent to "on", but specifies the minimum amount of data before the output is compressed). If the content-length is not known and compression is set to "on" or more aggressive, the output will also be compressed. If not specified, this attribute is set to "off".

    Note: There is a tradeoff between using compression (saving your bandwidth) and using the sendfile feature (saving your CPU cycles). If the connector supports the sendfile feature, e.g. the NIO2 connector, using sendfile will take precedence over compression. The symptoms will be that static files greater that 48 Kb will be sent uncompressed. You can turn off sendfile by setting useSendfile attribute of the protocol, as documented below, or change the sendfile usage threshold in the configuration of the DefaultServlet in the default conf/web.xml or in the web.xml of your web application.

    compressionMinSize

    If compression is set to "on" then this attribute may be used to specify the minimum amount of data before the output is compressed. If not specified, this attribute is defaults to "2048".

    initialWindowSize

    Controls the initial size of the flow control window for streams that Tomcat advertises to clients. If not specified, the default value of 65535 is used.

    keepAliveTimeout

    The time, in milliseconds, that Tomcat will wait between HTTP/2 frames before closing the connection. Negative values will be treated as an infinite timeout. If not specified, a default value of -1 will be used.

    maxConcurrentStreamExecution

    The controls the maximum number of streams for any one connection that can be allocated threads from the container thread pool. If more streams are active than threads are available, those streams will have to wait for a stream to become available. If not specified, the default value of 200 will be used.

    maxConcurrentStreams

    The controls the maximum number of active streams permitted for any one connection. If a client attempts to open more active streams than this limit, the stream will be reset with a STREAM_REFUSED error. If not specified, the default value of 200 will be used.

    maxHeaderCount

    The maximum number of headers in a request that is allowed by the container. A request that contains more headers than the specified limit will be rejected. A value of less than 0 means no limit. If not specified, a default of 100 is used.

    maxHeaderSize

    The maximum total size for all headers in a request that is allowed by the container. Total size for a header is calculated as the uncompressed size of the header name in bytes, plus the uncompressed size of the header value in bytes plus an HTTP/2 overhead of 3 bytes per header. A request that contains a set of headers that requires more than the specified limit will be rejected. A value of less than 0 means no limit. If not specified, a default of 8192 is used.

    maxTrailerCount

    The maximum number of trailer headers in a request that is allowed by the container. A request that contains more trailer headers than the specified limit will be rejected. A value of less than 0 means no limit. If not specified, a default of 100 is used.

    maxTrailerSize

    The maximum total size for all trailer headers in a request that is allowed by the container. Total size for a header is calculated as the uncompressed size of the header name in bytes, plus the uncompressed size of the header value in bytes plus an HTTP/2 overhead of 3 bytes per header. A request that contains a set of trailer headers that requires more than the specified limit will be rejected. A value of less than 0 means no limit. If not specified, a default of 8192 is used.

    noCompressionUserAgents

    The value is a regular expression (using java.util.regex) matching the user-agent header of HTTP clients for which compression should not be used, because these clients, although they do advertise support for the feature, have a broken implementation. The default value is an empty String (regexp matching disabled).

    readTimeout

    The time, in milliseconds, that Tomcat will wait for additional data when a partial HTTP/2 frame has been received. Negative values will be treated as an infinite timeout. If not specified, a default value of 10000 will be used.

    writeTimeout

    The time, in milliseconds, that Tomcat will wait to write additional data when an HTTP/2 frame has been partially written. Negative values will be treated as an infinite timeout. If not specified, a default value of 10000 will be used.

    The HTTP/2 upgrade protocol will also inherit the following limits from the HTTP Connector it is nested with:

    • maxCookieCount
    • maxParameterCount
    • maxPostSize
    • maxSavePostSize

    Nested Components

    This component does not support any nested components.

    Special Features

    This component does not support any special features.

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