7 Network Working Group J. C. Mogul
8 Request for Comments: 2145 DEC
9 Category: Informational R. Fielding
17 Use and Interpretation of
22 This memo provides information for the Internet community. This memo
23 does not specify an Internet standard of any kind. Distribution of
24 this memo is unlimited.
26 Distribution of this document is unlimited. Please send comments to
27 the HTTP working group at <http-wg@cuckoo.hpl.hp.com>. Discussions
28 of the working group are archived at
29 <URL:http://www.ics.uci.edu/pub/ietf/http/>. General discussions
30 about HTTP and the applications which use HTTP should take place on
31 the <www-talk@w3.org> mailing list.
35 HTTP request and response messages include an HTTP protocol version
36 number. Some confusion exists concerning the proper use and
37 interpretation of HTTP version numbers, and concerning
38 interoperability of HTTP implementations of different protocol
39 versions. This document is an attempt to clarify the situation. It
40 is not a modification of the intended meaning of the existing
41 HTTP/1.0 and HTTP/1.1 documents, but it does describe the intention
42 of the authors of those documents, and can be considered definitive
43 when there is any ambiguity in those documents concerning HTTP
44 version numbers, for all versions of HTTP.
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60 RFC 2145 HTTP Version Numbers May 1997
65 1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . 2
66 1.1 Robustness Principle . . . . . . . . . . . . . . . . . . 3
67 2 HTTP version numbers. . . . . . . . . . . . . . . . . . . . . . 3
68 2.1 Proxy behavior. . . . . . . . . . . . . . . . . . . . . . . . 4
69 2.2 Compatibility between minor versions of the same major
70 version. . . . . . . . . . . . . . . . . . . . . . . . 4
71 2.3 Which version number to send in a message. . . . . . . . 5
72 3 Security Considerations . . . . . . . . . . . . . . . . . . . . 6
73 4 References. . . . . . . . . . . . . . . . . . . . . . . . . . . 6
74 5 Authors' addresses. . . . . . . . . . . . . . . . . . . . . . . 6
78 HTTP request and response messages include an HTTP protocol version
79 number. According to section 3.1 of the HTTP/1.1 specification [2],
81 HTTP uses a "<major>.<minor>" numbering scheme to indicate
82 versions of the protocol. The protocol versioning policy is
83 intended to allow the sender to indicate the format of a message
84 and its capacity for understanding further HTTP communication,
85 rather than the features obtained via that communication. No
86 change is made to the version number for the addition of message
87 components which do not affect communication behavior or which
88 only add to extensible field values. The <minor> number is
89 incremented when the changes made to the protocol add features
90 which do not change the general message parsing algorithm, but
91 which may add to the message semantics and imply additional
92 capabilities of the sender. The <major> number is incremented when
93 the format of a message within the protocol is changed.
95 The same language appears in the description of HTTP/1.0 [1].
97 Many readers of these documents have expressed some confusion about
98 the intended meaning of this policy. Also, some people who wrote
99 HTTP implementations before RFC1945 [1] was issued were not aware of
100 the intentions behind the introduction of version numbers in
101 HTTP/1.0. This has led to debate and inconsistency regarding the use
102 and interpretation of HTTP version numbers, and has led to
103 interoperability problems in certain cases.
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116 RFC 2145 HTTP Version Numbers May 1997
119 This document is an attempt to clarify the situation. It is not a
120 modification of the intended meaning of the existing HTTP/1.0 and
121 HTTP/1.1 documents, but it does describe the intention of the authors
122 of those documents. In any case where either of those two documents
123 is ambiguous regarding the use and interpretation of HTTP version
124 numbers, this document should be considered the definitive as to the
125 intentions of the designers of HTTP.
127 The specification described in this document is not part of the
128 specification of any individual version of HTTP, such as HTTP/1.0 or
129 HTTP/1.1. Rather, this document describes the use of HTTP version
130 numbers in any version of HTTP (except for HTTP/0.9, which did not
131 include version numbers).
133 No vendor or other provider of an HTTP implementation should claim
134 any compliance with any IETF HTTP specification unless the
135 implementation conditionally complies with the rules in this
138 1.1 Robustness Principle
140 RFC791 [4] defines the "robustness principle" in section 3.2:
142 an implementation must be conservative in its sending
143 behavior, and liberal in its receiving behavior.
145 This principle applies to HTTP, as well. It is the fundamental basis
146 for interpreting any part of the HTTP specification that might still
147 be ambiguous. In particular, implementations of HTTP SHOULD NOT
148 reject messages or generate errors unnecessarily.
150 2 HTTP version numbers
152 We start by restating the language quoted above from section 3.1 of
153 the HTTP/1.1 specification [2]:
155 It is, and has always been, the explicit intent of the
156 HTTP specification that the interpretation of an HTTP message
157 header does not change between minor versions of the same major
160 It is, and has always been, the explicit intent of the
161 HTTP specification that an implementation receiving a message
162 header that it does not understand MUST ignore that header. (The
163 word "ignore" has a special meaning for proxies; see section 2.1
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172 RFC 2145 HTTP Version Numbers May 1997
175 To make this as clear as possible: The major version sent in a
176 message MAY indicate the interpretation of other header fields. The
177 minor version sent in a message MUST NOT indicate the interpretation
178 of other header fields. This reflects the principle that the minor
179 version labels the capability of the sender, not the interpretation
182 Note: In a future version of HTTP, we may introduce a mechanism
183 that explicitly requires a receiving implementation to reject a
184 message if it does not understand certain headers. For example,
185 this might be implemented by means of a header that lists a set of
186 other message headers that must be understood by the recipient.
187 Any implementation claiming at least conditional compliance with
188 this future version of HTTP would have to implement this
189 mechanism. However, no implementation claiming compliance with a
190 lower HTTP version (in particular, HTTP/1.1) will have to
191 implement this mechanism.
193 This future change may be required to support the Protocol
194 Extension Protocol (PEP) [3].
196 One consequence of these rules is that an HTTP/1.1 message sent to an
197 HTTP/1.0 recipient (or a recipient whose version is unknown) MUST be
198 constructed so that it remains a valid HTTP/1.0 message when all
199 headers not defined in the HTTP/1.0 specification [1] are removed.
203 A proxy MUST forward an unknown header, unless it is protected by a
204 Connection header. A proxy implementing an HTTP version >= 1.1 MUST
205 NOT forward unknown headers that are protected by a Connection
206 header, as described in section 14.10 of the HTTP/1.1 specification
209 We remind the reader that that HTTP version numbers are hop-by-hop
210 components of HTTP messages, and are not end-to-end. That is, an
211 HTTP proxy never "forwards" an HTTP version number in either a
214 2.2 Compatibility between minor versions of the same major version
216 An implementation of HTTP/x.b sending a message to a recipient whose
217 version is known to be HTTP/x.a, a < b, MAY send a header that is not
218 defined in the specification for HTTP/x.a. For example, an HTTP/1.1
219 server may send a "Cache-control" header to an HTTP/1.0 client; this
220 may be useful if the immediate recipient is an HTTP/1.0 proxy, but
221 the ultimate recipient is an HTTP/1.1 client.
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228 RFC 2145 HTTP Version Numbers May 1997
231 An implementation of HTTP/x.b sending a message to a recipient whose
232 version is known to be HTTP/x.a, a < b, MUST NOT depend on the
233 recipient understanding a header not defined in the specification for
234 HTTP/x.a. For example, HTTP/1.0 clients cannot be expected to
235 understand chunked encodings, and so an HTTP/1.1 server must never
236 send "Transfer-Encoding: chunked" in response to an HTTP/1.0 request.
238 2.3 Which version number to send in a message
240 The most strenuous debate over the use of HTTP version numbers has
241 centered on the problem of implementations that do not follow the
242 robustness principle, and which fail to produce useful results when
243 they receive a message with an HTTP minor version higher than the
244 minor version they implement. We consider these implementations
245 buggy, but we recognize that the robustness principle also implies
246 that message senders should make concessions to buggy implementations
247 when this is truly necessary for interoperation.
249 An HTTP client SHOULD send a request version equal to the highest
250 version for which the client is at least conditionally compliant, and
251 whose major version is no higher than the highest version supported
252 by the server, if this is known. An HTTP client MUST NOT send a
253 version for which it is not at least conditionally compliant.
255 An HTTP client MAY send a lower request version, if it is known that
256 the server incorrectly implements the HTTP specification, but only
257 after the client has determined that the server is actually buggy.
259 An HTTP server SHOULD send a response version equal to the highest
260 version for which the server is at least conditionally compliant, and
261 whose major version is less than or equal to the one received in the
262 request. An HTTP server MUST NOT send a version for which it is not
263 at least conditionally compliant. A server MAY send a 505 (HTTP
264 Version Not Supported) response if cannot send a response using the
265 major version used in the client's request.
267 An HTTP server MAY send a lower response version, if it is known or
268 suspected that the client incorrectly implements the HTTP
269 specification, but this should not be the default, and this SHOULD
270 NOT be done if the request version is HTTP/1.1 or greater.
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284 RFC 2145 HTTP Version Numbers May 1997
287 3 Security Considerations
289 None, except to the extent that security mechanisms introduced in one
290 version of HTTP might depend on the proper interpretation of HTTP
291 version numbers in older implementations.
295 1. Berners-Lee, T., R. Fielding, and H. Frystyk. Hypertext
296 Transfer Protocol -- HTTP/1.0. RFC 1945, HTTP Working Group, May,
299 2. Fielding, Roy T., Jim Gettys, Jeffrey C. Mogul, Henrik Frystyk
300 Nielsen, and Tim Berners-Lee. Hypertext Transfer Protocol --
301 HTTP/1.1. RFC 2068, HTTP Working Group, January, 1997.
303 3. Khare, Rohit. HTTP/1.2 Extension Protocol (PEP). HTTP Working
304 Group, Work in Progress.
306 4. Postel, Jon. Internet Protocol. RFC 791, NIC, September, 1981.
311 Western Research Laboratory
312 Digital Equipment Corporation
313 250 University Avenue
314 Palo Alto, California, 94305, USA
315 Email: mogul@wrl.dec.com
318 Department of Information and Computer Science
319 University of California
320 Irvine, CA 92717-3425, USA
321 Fax: +1 (714) 824-4056
322 Email: fielding@ics.uci.edu
325 MIT Laboratory for Computer Science
326 545 Technology Square
327 Cambridge, MA 02139, USA
328 Fax: +1 (617) 258 8682
338 Mogul, et. al. Informational [Page 6]
340 RFC 2145 HTTP Version Numbers May 1997
343 Henrik Frystyk Nielsen
345 MIT Laboratory for Computer Science
346 545 Technology Square
347 Cambridge, MA 02139, USA
348 Fax: +1 (617) 258 8682
349 Email: frystyk@w3.org
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