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

HTTP General Headers 1 2 3 4 HTTP Response Headers

If-Match

Makes a method conditional by specifying the entity tag (or tags) corresponding to the specific entity that the client wishes to access. This is usually used in a GET method, and the server responds with the entity only if it matches the one specified in this header. Otherwise, a 412 (“Precondition Failed”) reply is sent.

Makes a method conditional by telling the server to return the requested entity only if it has been modified since the time specified in this header. Otherwise, the server sends a 304 (“Not Modified”) response. This is used to check if a resource has changed since it was last accessed, to avoid unnecessary transfers.

This is the opposite of If-Match; it creates a conditional request that is only filled if the specified tag(s) do not match the requested entity.

This header is used in combination with the Range header to effectively allow a client to both check for whether an entity has changed and request that a portion of it be sent in a single request. (The alternative is to first issue a conditional request, which if it fails would then require a second request.) When present, If-Range tells the server to send to the client the part of the entity indicated in the Range header if the entity has not changed. If the entity has changed, the entire entity is sent in response.

The logical opposite of the If-Modified-Since header; the request is filled only if the resource has not been modified since the specified time; otherwise a 412 reply is sent.

Specifies a limit on the number of times a request can be forwarded to the next device in the request chain. This header is used with the TRACE or OPTIONS methods only, to permit diagnosis of forwarding failures or looping. When present in one of these methods, each time a device forwards the request, the number in this header is decremented. If a device receives a request with a Max-Forwards value of 0, it must not forward it but rather respond back to the client. (In a way, this is somewhat analogous to how the Time To Live field is used in the Internet Protocol datagram format.)

This is like the Authorization header, but is used to present credentials to a proxy server for authentication, rather than the end server. It is created using information sent by a proxy in a response containing a Proxy-Authenticate header. This is a hop-by-hop header, sent only to the first proxy that receives the request. If authentication is required with more than one proxy, multiple Proxy-Authorization headers may be put in a message, with each proxy in turn “consuming” one of the headers.

HTTP General Headers 1 2 3 4 HTTP Response Headers