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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9  TCP/IP Key Applications and Application Protocols
           9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)
                9  TCP/IP World Wide Web (WWW, "The Web") and the Hypertext Transfer Protocol (HTTP)
                     9  TCP/IP Hypertext Transfer Protocol (HTTP)
                          9  HTTP Message Headers

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HTTP General Headers
(Page 3 of 3)

Upgrade

Allows a client device to specify what additional protocols it supports. If the server also supports one of the protocols the client listed, the server may agree to “upgrade” the connection to the alternative protocol. It indicates the protocol to which it is upgrading by including an Upgrade header in a 101 (“Switching Protocols”) response to the client. This is a hop-by-hop header.

Via

Included by intermediary devices to indicate to the recipient what gateways, proxies and/or tunnels were used in conveying a request or response. This header allows easy tracing of the path a message took over a potentially complex chain of devices between a client and server.

Warning

Used when needed to provide additional information about the status of a message. Many of the defined warning header types are related to caching. More than one Warning header may appear in a message, and each typically includes a three-digit numeric code as well as a plain text message—the same basic format used in HTTP response status codes.

Table 277 briefly lists the warnings defined in RFC 2616.


Table 277: HTTP Warning Header Codes

Warning Code

Warning Text

Description

110

Response is stale

Must be included when a response provided by a cache is stale (that is, has passed the expiration time set for it.)

111

Revalidation failed

A cache attempted to revalidate a cached entry but was unsuccessful, so it returned its (stale) cached entry.

112

Disconnected operation

The cache is disconnected from the rest of the network.

113

Heuristic expiration

Included if the cache chose a freshness lifetime of more than 24 hours, and the age of the response is also greater than 24 hours.

199

Miscellaneous warning

Catch-all code for other, non-specific warnings.

214

Transformation applied

Warns the recipient that an intermediate cache or proxy applied a transformation of some type to change the content coding or media type of the message or message body.

299

Miscellaneous persistent warning

Similar to code 199 but indicates a persistent warning.


Key Concept: HTTP general headers can appear in either an HTTP Request or HTTP Response message. They are used to communicate information about the message itself, as opposed to its contents. General headers are used for functions such as specifying the date and time of a message, controlling how the message is cached, and indicating its transfer encoding method.



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