Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests
RFC 7232

Internet Engineering Task Force (IETF)                  R. Fielding, Ed.
Request for Comments: 7232                                         Adobe
Obsoletes: 2616                                          J. Reschke, Ed.
Category: Standards Track                                     greenbytes
ISSN: 2070-1721                                                June 2014

      Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests

Abstract

   The Hypertext Transfer Protocol (HTTP) is a stateless application-
   level protocol for distributed, collaborative, hypertext information
   systems.  This document defines HTTP/1.1 conditional requests,
   including metadata header fields for indicating state changes,
   request header fields for making preconditions on such state, and
   rules for constructing the responses to a conditional request when
   one or more preconditions evaluate to false.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7232.

Fielding & Reschke           Standards Track                    [Page 1]
RFC 7232              HTTP/1.1 Conditional Requests            June 2014

Copyright Notice

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   document authors.  All rights reserved.

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   This document may contain material from IETF Documents or IETF
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   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Fielding & Reschke           Standards Track                    [Page 2]
RFC 7232              HTTP/1.1 Conditional Requests            June 2014

Table of Contents

   1. Introduction ....................................................4
      1.1. Conformance and Error Handling .............................4
      1.2. Syntax Notation ............................................4
   2. Validators ......................................................5
      2.1. Weak versus Strong .........................................5
      2.2. Last-Modified ..............................................7
           2.2.1. Generation ..........................................7
           2.2.2. Comparison ..........................................8
      2.3. ETag .......................................................9
           2.3.1. Generation .........................................10
           2.3.2. Comparison .........................................10
           2.3.3. Example: Entity-Tags Varying on
                  Content-Negotiated Resources .......................11
      2.4. When to Use Entity-Tags and Last-Modified Dates ...........12
   3. Precondition Header Fields .....................................13
      3.1. If-Match ..................................................13
      3.2. If-None-Match .............................................14
      3.3. If-Modified-Since .........................................16
      3.4. If-Unmodified-Since .......................................17
      3.5. If-Range ..................................................18
   4. Status Code Definitions ........................................18
      4.1. 304 Not Modified ..........................................18