RFC 2145 (rfc2145) - Page 2 of 7

Use and Interpretation of HTTP Version Numbers

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 "." 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  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  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 RFC 1945 [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