The TCP/IP Guide - HTTP General Headers

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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.

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.

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: 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.