Use and Interpretation of HTTP Version Numbers
RFC 2145

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Network Working Group                                        J. C. Mogul
Request for Comments: 2145                                           DEC
Category: Informational                                      R. Fielding
                                                               UC Irvine
                                                               J. Gettys
                                                                     DEC
                                                              H. Frystyk
                                                                 MIT/LCS
                                                                May 1997

                       Use and Interpretation of
                          HTTP Version Numbers

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

   Distribution of this document is unlimited.  Please send comments to
   the HTTP working group at <http-wg@cuckoo.hpl.hp.com>.  Discussions
   of the working group are archived at
   <URL:http://www.ics.uci.edu/pub/ietf/http/>.  General discussions
   about HTTP and the applications which use HTTP should take place on
   the <www-talk@w3.org> mailing list.

Abstract

   HTTP request and response messages include an HTTP protocol version
   number.  Some confusion exists concerning the proper use and
   interpretation of HTTP version numbers, and concerning
   interoperability of HTTP implementations of different protocol
   versions.  This document is an attempt to clarify the situation.  It
   is not a modification of the intended meaning of the existing
   HTTP/1.0 and HTTP/1.1 documents, but it does describe the intention
   of the authors of those documents, and can be considered definitive
   when there is any ambiguity in those documents concerning HTTP
   version numbers, for all versions of HTTP.

Mogul, et. al.               Informational                      [Page 1]
RFC 2145                  HTTP Version Numbers                  May 1997

TABLE OF CONTENTS

   1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . .  2
        1.1 Robustness Principle . . . . . . . . . . . . . . . . . .  3
   2 HTTP version numbers. . . . . . . . . . . . . . . . . . . . . .  3
   2.1 Proxy behavior. . . . . . . . . . . . . . . . . . . . . . . .  4
        2.2 Compatibility between minor versions of the same major
            version. . . . . . . .  . . . . . . . .  . . . . . . . .  4
        2.3 Which version number to send in a message. . . . . . . .  5
   3 Security Considerations . . . . . . . . . . . . . . . . . . . .  6
   4 References. . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   5 Authors' addresses. . . . . . . . . . . . . . . . . . . . . . .  6

1 Introduction

   HTTP request and response messages include an HTTP protocol version
   number.  According to section 3.1 of the HTTP/1.1 specification [2],

         HTTP uses a "<major>.<minor>" numbering scheme to indicate
      versions of the protocol. The protocol versioning policy is
      intended to allow the sender to indicate the format of a message
      and its capacity for understanding further HTTP communication,
      rather than the features obtained via that communication.  No
      change is made to the version number for the addition of message
      components which do not affect communication behavior or which
      only add to extensible field values.  The <minor> number is
      incremented when the changes made to the protocol add features
      which do not change the general message parsing algorithm, but
      which may add to the message semantics and imply additional
      capabilities of the sender. The <major> number is incremented when
      the format of a message within the protocol is changed.

   The same language appears in the description of HTTP/1.0 [1].

   Many readers of these documents have expressed some confusion about
   the intended meaning of this policy.  Also, some people who wrote
   HTTP implementations before RFC1945 [1] was issued were not aware of
   the intentions behind the introduction of version numbers in
   HTTP/1.0.  This has led to debate and inconsistency regarding the use
   and interpretation of HTTP version numbers, and has led to
   interoperability problems in certain cases.

Mogul, et. al.               Informational                      [Page 2]
RFC 2145                  HTTP Version Numbers                  May 1997

   This document is an attempt to clarify the situation.  It is not a
   modification of the intended meaning of the existing HTTP/1.0 and
   HTTP/1.1 documents, but it does describe the intention of the authors
   of those documents.  In any case where either of those two documents
   is ambiguous regarding the use and interpretation of HTTP version
   numbers, this document should be considered the definitive as to the