Network Working Group J. C. Mogul

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 <[email protected]>. 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 <[email protected]> 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.

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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.

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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
intentions of the designers of HTTP.

The specification described in this document is not part of the
specification of any individual version of HTTP, such as HTTP/1.0 or
HTTP/1.1. Rather, this document describes the use of HTTP version
numbers in any version of HTTP (except for HTTP/0.9, which did not
include version numbers).

No vendor or other provider of an HTTP implementation should claim
any compliance with any IETF HTTP specification unless the
implementation conditionally complies with the rules in this
document.

1.1 Robustness Principle

RFC791 [4] defines the "robustness principle" in section 3.2:

an implementation must be conservative in its sending
behavior, and liberal in its receiving behavior.

This principle applies to HTTP, as well. It is the fundamental basis
for interpreting any part of the HTTP specification that might still
be ambiguous. In particular, implementations of HTTP SHOULD NOT
reject messages or generate errors unnecessarily.

2 HTTP version numbers

We start by restating the language quoted above from section 3.1 of
the HTTP/1.1 specification [2]:

It is, and has always been, the explicit intent of the
HTTP specification that the interpretation of an HTTP message
header does not change between minor versions of the same major
version.

It is, and has always been, the explicit intent of the
HTTP specification that an implementation receiving a message
header that it does not understand MUST ignore that header. (The
word "ignore" has a special meaning for proxies; see section 2.1
below.)

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To make this as clear as possible: The major version sent in a
message MAY indicate the interpretation of other header fields. The
minor version sent in a message MUST NOT indicate the interpretation
of other header fields. This reflects the principle that the minor
version labels the capability of the sender, not the interpretation
of the message.

Note: In a future version of HTTP, we may introduce a mechanism
that explicitly requires a receiving implementation to reject a
message if it does not understand certain headers. For example,
this might be implemented by means of a header that lists a set of
other message headers that must be understood by the recipient.
Any implementation claiming at least conditional compliance with
this future version of HTTP would have to implement this
mechanism. However, no implementation claiming compliance with a
lower HTTP version (in particular, HTTP/1.1) will have to
implement this mechanism.

This future change may be required to support the Protocol
Extension Protocol (PEP) [3].

One consequence of these rules is that an HTTP/1.1 message sent to an
HTTP/1.0 recipient (or a recipient whose version is unknown) MUST be
constructed so that it remains a valid HTTP/1.0 message when all
headers not defined in the HTTP/1.0 specification [1] are removed.

2.1 Proxy behavior

A proxy MUST forward an unknown header, unless it is protected by a
Connection header. A proxy implementing an HTTP version >= 1.1 MUST
NOT forward unknown headers that are protected by a Connection
header, as described in section 14.10 of the HTTP/1.1 specification
[2].

We remind the reader that that HTTP version numbers are hop-by-hop
components of HTTP messages, and are not end-to-end. That is, an
HTTP proxy never "forwards" an HTTP version number in either a
request or response.

2.2 Compatibility between minor versions of the same major version

An implementation of HTTP/x.b sending a message to a recipient whose
version is known to be HTTP/x.a, a < b, MAY send a header that is not
defined in the specification for HTTP/x.a. For example, an HTTP/1.1
server may send a "Cache-control" header to an HTTP/1.0 client; this
may be useful if the immediate recipient is an HTTP/1.0 proxy, but
the ultimate recipient is an HTTP/1.1 client.

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An implementation of HTTP/x.b sending a message to a recipient whose
version is known to be HTTP/x.a, a < b, MUST NOT depend on the
recipient understanding a header not defined in the specification for
HTTP/x.a. For example, HTTP/1.0 clients cannot be expected to
understand chunked encodings, and so an HTTP/1.1 server must never
send "Transfer-Encoding: chunked" in response to an HTTP/1.0 request.

2.3 Which version number to send in a message

The most strenuous debate over the use of HTTP version numbers has
centered on the problem of implementations that do not follow the
robustness principle, and which fail to produce useful results when
they receive a message with an HTTP minor version higher than the
minor version they implement. We consider these implementations
buggy, but we recognize that the robustness principle also implies
that message senders should make concessions to buggy implementations
when this is truly necessary for interoperation.

An HTTP client SHOULD send a request version equal to the highest
version for which the client is at least conditionally compliant, and
whose major version is no higher than the highest version supported
by the server, if this is known. An HTTP client MUST NOT send a
version for which it is not at least conditionally compliant.

An HTTP client MAY send a lower request version, if it is known that
the server incorrectly implements the HTTP specification, but only
after the client has determined that the server is actually buggy.

An HTTP server SHOULD send a response version equal to the highest
version for which the server is at least conditionally compliant, and
whose major version is less than or equal to the one received in the
request. An HTTP server MUST NOT send a version for which it is not
at least conditionally compliant. A server MAY send a 505 (HTTP
Version Not Supported) response if cannot send a response using the
major version used in the client's request.

An HTTP server MAY send a lower response version, if it is known or
suspected that the client incorrectly implements the HTTP
specification, but this should not be the default, and this SHOULD
NOT be done if the request version is HTTP/1.1 or greater.

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3 Security Considerations

None, except to the extent that security mechanisms introduced in one
version of HTTP might depend on the proper interpretation of HTTP
version numbers in older implementations.

4 References

1. Berners-Lee, T., R. Fielding, and H. Frystyk. Hypertext
Transfer Protocol -- HTTP/1.0. RFC 1945, HTTP Working Group, May,
1996.

2. Fielding, Roy T., Jim Gettys, Jeffrey C. Mogul, Henrik Frystyk
Nielsen, and Tim Berners-Lee. Hypertext Transfer Protocol --
HTTP/1.1. RFC 2068, HTTP Working Group, January, 1997.

3. Khare, Rohit. HTTP/1.2 Extension Protocol (PEP). HTTP Working
Group, Work in Progress.

4. Postel, Jon. Internet Protocol. RFC 791, NIC, September, 1981.

5 Authors' addresses

Jeffrey C. Mogul
Western Research Laboratory
Digital Equipment Corporation
250 University Avenue
Palo Alto, California, 94305, USA
Email: [email protected]

Roy T. Fielding
Department of Information and Computer Science
University of California
Irvine, CA 92717-3425, USA
Fax: +1 (714) 824-4056
Email: [email protected]

Jim Gettys
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, USA
Fax: +1 (617) 258 8682
Email: [email protected]

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RFC 2145 HTTP Version Numbers May 1997

Henrik Frystyk Nielsen
W3 Consortium
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, USA
Fax: +1 (617) 258 8682
Email: [email protected]

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