Please Whitelist This Site?

I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :)

If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads.

If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK.

Thanks for your understanding!

Sincerely, Charles Kozierok
Author and Publisher, The TCP/IP Guide


NOTE: Using software to mass-download the site degrades the server and is prohibited.
If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The Book is Here... and Now On Sale!

Read offline with no ads or diagram watermarks!
The TCP/IP Guide

Custom Search







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

Previous Topic/Section
HTTP General Headers
Previous Page
Pages in Current Topic/Section
12
3
4
Next Page
HTTP Response Headers
Next Topic/Section

HTTP Request Headers
(Page 3 of 4)

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.

If-Modified-Since

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.

If-None-Match

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.

If-Range

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.

If-Unmodified-Since

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.

Max-Forwards

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.)

Proxy-Authorization

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.


Previous Topic/Section
HTTP General Headers
Previous Page
Pages in Current Topic/Section
12
3
4
Next Page
HTTP Response Headers
Next Topic/Section

If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support!
Donate $2
Donate $5
Donate $10
Donate $20
Donate $30
Donate: $



Home - Table Of Contents - Contact Us

The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005

© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.