## The HTTP2 Upgrade Protocol

### Table of Contents

- Introduction
- Attributes
  1. Common Attributes
  2. Standard Implementation
- Nested Components
- Special Features

### 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.

Requests processed using HTTP/2 will have the following additional request attributes available:

- `org.apache.coyote.connectionID` will return the HTTP/2 connection ID
- `org.apache.coyote.streamID` will return the HTTP/2 stream ID

### 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 when there is no active Stream before closing the connection. Negative values will be treated as an infinite timeout. If not specified, a default value of `20000` 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 `20` 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 `100` 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.

`noCompressionStrongETag` | This flag configures whether resources with a stong ETag will be considered for compression. If `true`, resources with a strong ETag will not be compressed. The default value is `true`. This attribute is deprecated. It will be removed in Tomcat 10 onwards where it will be hard-coded to `true`.
`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).

`overheadContinuationThreshold`

The threshold below which the payload size of a non-final `CONTINUATION` frame will trigger an increase in the overhead count (see overheadCountFactor). The overhead count will be increased by `overheadContinuationThreshold/payloadSize` so that the smaller the `CONTINUATION` frame, the greater the increase in the overhead count. A value of zero or less disables the checking of non-final `CONTINUATION` frames. If not specified, a default value of `1024` will be used.

`overheadCountFactor`

The factor to apply when counting overhead frames to determine if a connection has too high an overhead and should be closed. The overhead count starts at `-10`. The count is decreased for each data frame sent or received and each headers frame received. The count is increased by the `overheadCountFactor`for each setting received, priority frame received and ping received. If the overhead count exceeds zero, the connection is closed. A value of less than `1` disables this protection. In normal usage a value of `3` or more will close the connection before any streams can complete. If not specified, a default value of `1` will be used.

`overheadDataThreshold`

The threshold below which the average payload size of the current and previous non-final `DATA` frames will trigger an increase in the overhead count (see overheadCountFactor). The overhead count will be increased by `overheadDataThreshold/average` so that the smaller the average, the greater the increase in the overhead count. A value of zero or less disables the checking of non-final `DATA` frames. If not specified, a default value of `1024` will be used.

`overheadWindowUpdateThreshold`

The threshold below which the average size of current and previous `WINDOW_UPDATE` frame will trigger an increase in the overhead count (see overheadCountFactor). The overhead count will be increased by `overheadWindowUpdateThreshold/average` so that the smaller the average, the greater the increase in the overhead count. A value of zero or less disables the checking of `WINDOW_UPDATE` frames. If not specified, a default value of `1024` will be used.

`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 `5000` will be used.
`streamReadTimeout` | The time, in milliseconds, that Tomcat will wait for additional data frames to arrive for the stream when an application is performing a blocking I/O read and additional data is required. Negative values will be treated as an infinite timeout. If not specified, a default value of `20000` will be used.

`streamWriteTimeout`

The time, in milliseconds, that Tomcat will wait for additional window update frames to arrive for the stream and/or connection when an application is performing a blocking I/O write and the stream and/or connection flow control window is too small for the write to complete. Negative values will be treated as an infinite timeout. If not specified, a default value of `20000` will be used.

`useSendfile` | Use this boolean attribute to enable or disable sendfile capability. The default value is `true`.
`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 `5000` 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.