server to direct clients to discard these cached credentials. This is
a significant defect that requires further extensions to HTTP.
Circumstances under which credential caching can interfere with the
application's security model include but are not limited to:
- Clients which have been idle for an extended period following
which the server might wish to cause the client to reprompt the
user for credentials.
- Applications which include a session termination indication
(such as a `logout' or `commit' button on a page) after which
the server side of the application `knows' that there is no
further reason for the client to retain the credentials.
This is currently under separate study. There are a number of work-
arounds to parts of this problem, and we encourage the use of
password protection in screen savers, idle time-outs, and other
methods which mitigate the security problems inherent in this
problem. In particular, user agents which cache credentials are
encouraged to provide a readily accessible mechanism for discarding
cached credentials under user control.
15.7 Proxies and Caching
By their very nature, HTTP proxies are men-in-the-middle, and
represent an opportunity for man-in-the-middle attacks. Compromise of
the systems on which the proxies run can result in serious security
and privacy problems. Proxies have access to security-related
information, personal information about individual users and
organizations, and proprietary information belonging to users and
content providers. A compromised proxy, or a proxy implemented or
configured without regard to security and privacy considerations,
might be used in the commission of a wide range of potential attacks.
Proxy operators should protect the systems on which proxies run as
they would protect any system that contains or transports sensitive
information. In particular, log information gathered at proxies often
contains highly sensitive personal information, and/or information
about organizations. Log information should be carefully guarded, and
appropriate guidelines for use developed and followed. (Section
15.1.1).
Caching proxies provide additional potential vulnerabilities, since
the contents of the cache represent an attractive target for
malicious exploitation. Because cache contents persist after an HTTP
request is complete, an attack on the cache can reveal information
long after a user believes that the information has been removed from
the network. Therefore, cache contents should be protected as
sensitive information.
Proxy implementors should consider the privacy and security
implications of their design and coding decisions, and of the
configuration options they provide to proxy operators (especially the
default configuration).
Users of a proxy need to be aware that they are no trustworthier than
the people who run the proxy; HTTP itself cannot solve this problem.
The judicious use of cryptography, when appropriate, may suffice to
protect against a broad range of security and privacy attacks. Such
cryptography is beyond the scope of the HTTP/1.1 specification.
15.7.1 Denial of Service Attacks on Proxies
They exist. They are hard to defend against. Research continues.
Beware.
16 Acknowledgments
This specification makes heavy use of the augmented BNF and generic
constructs defined by David H. Crocker for RFC822 [9]. Similarly, it
reuses many of the definitions provided by Nathaniel Borenstein and
Ned Freed for MIME [7]. We hope that their inclusion in this
specification will help reduce past confusion over the relationship
between HTTP and Internet mail message formats.
The HTTP protocol has evolved considerably over the years. It has
benefited from a large and active developer community--the many
people who have participated on the www-talk mailing list--and it is
that community which has been most responsible for the success of
HTTP and of the World-Wide Web in general. Marc Andreessen, Robert
Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois
Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob
McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc
VanHeyningen deserve special recognition for their efforts in
defining early aspects of the protocol.
This document has benefited greatly from the comments of all those
participating in the HTTP-WG. In addition to those already mentioned,
the following individuals have contributed to this specification:
Gary Adams Ross Patterson
Harald Tveit Alvestrand Albert Lunde
Keith Ball John C. Mallery
Brian Behlendorf Jean-Philippe Martin-Flatin
Paul Burchard Mitra
Maurizio Codogno David Morris
Mike Cowlishaw Gavin Nicol
Roman Czyborra Bill Perry
Michael A. Dolan Jeffrey Perry
David J. Fiander Scott Powers
Alan Freier Owen Rees
Marc Hedlund Luigi Rizzo
Greg Herlihy David Robinson
Koen Holtman Marc Salomon
Alex Hopmann Rich Salz
Bob Jernigan Allan M. Schiffman
Shel Kaphan Jim Seidman
Rohit Khare Chuck Shotton
John Klensin Eric W. Sink
Martijn Koster Simon E. Spero
Alexei Kosut Richard N. Taylor
David M. Kristol Robert S. Thau
Daniel LaLiberte Bill (BearHeart) Weinman
Ben Laurie Francois Yergeau
Paul J. Leach Mary Ellen Zurko
Daniel DuBois Josh Cohen
Much of the content and presentation of the caching design is due to
suggestions and comments from individuals including: Shel Kaphan,
Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
Most of the specification of ranges is based on work originally done
by Ari Luotonen and John Franks, with additional input from Steve
Zilles.
Thanks to the "cave men" of Palo Alto. You know who you are.
Jim Gettys (the current editor of this document) wishes particularly
to thank Roy Fielding, the previous editor of this document, along
with John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen
Holtman, John Franks, Josh Cohen, Alex Hopmann, Scott Lawrence, and
Larry Masinter for their help. And thanks go particularly to Jeff
Mogul and Scott Lawrence for performing the "MUST/MAY/SHOULD" audit.
The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik
Frystyk implemented RFC2068 early, and we wish to thank them for the
discovery of many of the problems that this document attempts to
rectify.
17 References
[1] Alvestrand, H., "Tags for the Identification of Languages", RFC
1766, March 1995.
[2] Anklesaria, F., McCahill, M., Lindner, P., Johnson, D., Torrey,
D. and B. Alberti, "The Internet Gopher Protocol (a distributed
document search and retrieval protocol)", RFC1436, March 1993.
[3] Berners-Lee, T., "Universal Resource Identifiers in WWW", RFC
1630, June 1994.
[4] Berners-Lee, T., Masinter, L. and M. McCahill, "Uniform Resource
Locators (URL)", RFC1738, December 1994.
[5] Berners-Lee, T. and D. Connolly, "Hypertext Markup Language -
2.0", RFC1866, November 1995.
[6] Berners-Lee, T., Fielding, R. and H. Frystyk, "Hypertext Transfer
Protocol -- HTTP/1.0", RFC1945, May 1996.
[7] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC2045, November 1996.
[8] Braden, R., "Requirements for Internet Hosts -- Communication
Layers", STD 3, RFC1123, October 1989.
[9] Crocker, D., "Standard for The Format of ARPA Internet Text
Messages", STD 11, RFC822, August 1982.
[10] Davis, F., Kahle, B., Morris, H., Salem, J., Shen, T., Wang, R.,
Sui, J., and M. Grinbaum, "WAIS Interface Protocol Prototype
Functional Specification," (v1.5), Thinking Machines
Corporation, April 1990.
[11] Fielding, R., "Relative Uniform Resource Locators", RFC1808,
June 1995.
[12] Horton, M. and R. Adams, "Standard for Interchange of USENET
Messages", RFC1036, December 1987.
[13] Kantor, B. and P. Lapsley, "Network News Transfer Protocol", RFC
977, February 1986.
[14] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
Three: Message Header Extensions for Non-ASCII Text", RFC2047,
November 1996.
[15] Nebel, E. and L. Masinter, "Form-based File Upload in HTML", RFC
1867, November 1995.
[16] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC821,
August 1982.
[17] Postel, J., "Media Type Registration Procedure", RFC1590,
November 1996.
[18] Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9, RFC
959, October 1985.
[19] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC1700,
October 1994.
[20] Sollins, K. and L. Masinter, "Functional Requirements for
Uniform Resource Names", RFC1737, December 1994.
[21] US-ASCII. Coded Character Set - 7-Bit American Standard Code for
Information Interchange. Standard ANSI X3.4-1986, ANSI, 1986.
[22] ISO-8859. International Standard -- Information Processing --
8-bit Single-Byte Coded Graphic Character Sets --
Part 1: Latin alphabet No. 1, ISO-8859-1:1987.
Part 2: Latin alphabet No. 2, ISO-8859-2, 1987.
Part 3: Latin alphabet No. 3, ISO-8859-3, 1988.
Part 4: Latin alphabet No. 4, ISO-8859-4, 1988.
Part 5: Latin/Cyrillic alphabet, ISO-8859-5, 1988.
Part 6: Latin/Arabic alphabet, ISO-8859-6, 1987.
Part 7: Latin/Greek alphabet, ISO-8859-7, 1987.
Part 8: Latin/Hebrew alphabet, ISO-8859-8, 1988.
Part 9: Latin alphabet No. 5, ISO-8859-9, 1990.
[23] Meyers, J. and M. Rose, "The Content-MD5 Header Field", RFC
1864, October 1995.
[24] Carpenter, B. and Y. Rekhter, "Renumbering Needs Work", RFC
1900, February 1996.
[25] Deutsch, P., "GZIP file format specification version 4.3", RFC
1952, May 1996.
[26] Venkata N. Padmanabhan, and Jeffrey C. Mogul. "Improving HTTP
Latency", Computer Networks and ISDN Systems, v. 28, pp. 25-35,
Dec. 1995. Slightly revised version of paper in Proc. 2nd
International WWW Conference '94: Mosaic and the Web, Oct. 1994,
which is available at
http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/mogul/HTTPLat
ency.html.
[27] Joe Touch, John Heidemann, and Katia Obraczka. "Analysis of HTTP
Performance", <URL: http://www.isi.edu/touch/pubs/http-perf96/>,
ISI Research Report ISI/RR-98-463, (original report dated Aug.
1996), USC/Information Sciences Institute, August 1998.
[28] Mills, D., "Network Time Protocol (Version 3) Specification,
Implementation and Analysis", RFC1305, March 1992.
[29] Deutsch, P., "DEFLATE Compressed Data Format Specification
version 1.3", RFC1951, May 1996.
[30] S. Spero, "Analysis of HTTP Performance Problems,"
http://sunsite.unc.edu/mdma-release/http-prob.html.
[31] Deutsch, P. and J. Gailly, "ZLIB Compressed Data Format
Specification version 3.3", RFC1950, May 1996.
[32] Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P.,
Luotonen, A., Sink, E. and L. Stewart, "An Extension to HTTP:
Digest Access Authentication", RFC2069, January 1997.
[33] Fielding, R., Gettys, J., Mogul, J., Frystyk, H. and T.
Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC
2068, January 1997.
[34] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.
[35] Troost, R. and Dorner, S., "Communicating Presentation
Information in Internet Messages: The Content-Disposition
Header", RFC1806, June 1995.
[36] Mogul, J., Fielding, R., Gettys, J. and H. Frystyk, "Use and
Interpretation of HTTP Version Numbers", RFC2145, May 1997.
[jg639]
[37] Palme, J., "Common Internet Message Headers", RFC2076, February
1997. [jg640]
[38] Yergeau, F., "UTF-8, a transformation format of Unicode and
ISO-10646", RFC2279, January 1998. [jg641]
[39] Nielsen, H.F., Gettys, J., Baird-Smith, A., Prud'hommeaux, E.,
Lie, H., and C. Lilley. "Network Performance Effects of
HTTP/1.1, CSS1, and PNG," Proceedings of ACM SIGCOMM '97, Cannes
France, September 1997.[jg642]
[40] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC2046, November
1996. [jg643]
[41] Alvestrand, H., "IETF Policy on Character Sets and Languages",
BCP 18, RFC2277, January 1998. [jg644]
[42] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
Identifiers (URI): Generic Syntax and Semantics", RFC2396,
August 1998. [jg645]
[43] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., Sink, E. and L. Stewart, "HTTP
Authentication: Basic and Digest Access Authentication", RFC
2617, June 1999. [jg646]
[44] Luotonen, A., "Tunneling TCP based protocols through Web proxy
servers," Work in Progress. [jg647]
[45] Palme, J. and A. Hopmann, "MIME E-mail Encapsulation of
Aggregate Documents, such as HTML (MHTML)", RFC2110, March
1997.
[46] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC2026, October 1996.
[47] Masinter, L., "Hyper Text Coffee Pot Control Protocol
(HTCPCP/1.0)", RFC2324, 1 April 1998.
[48] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Five: Conformance Criteria and Examples",
RFC2049, November 1996.
[49] Troost, R., Dorner, S. and K. Moore, "Communicating Presentation
Information in Internet Messages: The Content-Disposition Header
Field", RFC2183, August 1997.
18 Authors' Addresses
Roy T. Fielding
Information and Computer Science
University of California, Irvine
Irvine, CA 92697-3425, USA
Fax: +1 (949) 824-1715
EMail: fielding@ics.uci.edu
James Gettys
World Wide Web Consortium
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, USA
Fax: +1 (617) 258 8682
EMail: jg@w3.org
Jeffrey C. Mogul
Western Research Laboratory
Compaq Computer Corporation
250 University Avenue
Palo Alto, California, 94305, USA
EMail: mogul@wrl.dec.com
Henrik Frystyk Nielsen
World Wide Web Consortium
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, USA
Fax: +1 (617) 258 8682
EMail: frystyk@w3.org
Larry Masinter
Xerox Corporation
3333 Coyote Hill Road
Palo Alto, CA 94034, USA
EMail: masinter@parc.xerox.com
Paul J. Leach
Microsoft Corporation
1 Microsoft Way
Redmond, WA 98052, USA
EMail: paulle@microsoft.com
Tim Berners-Lee
Director, World Wide Web Consortium
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, USA
Fax: +1 (617) 258 8682
EMail: timbl@w3.org
19 Appendices
19.1 Internet Media Type message/http and application/http
In addition to defining the HTTP/1.1 protocol, this document serves
as the specification for the Internet media type "message/http" and
"application/http". The message/http type can be used to enclose a
single HTTP request or response message, provided that it obeys the
MIME restrictions for all "message" types regarding line length and
encodings. The application/http type can be used to enclose a
pipeline of one or more HTTP request or response messages (not
intermixed). The following is to be registered with IANA [17].
Media Type name: message
Media subtype name: http
Required parameters: none
Optional parameters: version, msgtype
version: The HTTP-Version number of the enclosed message
(e.g., "1.1"). If not present, the version can be
determined from the first line of the body.
msgtype: The message type -- "request" or "response". If not
present, the type can be determined from the first
line of the body.
Encoding considerations: only "7bit", "8bit", or "binary" are
permitted
Security considerations: none
Media Type name: application
Media subtype name: http
Required parameters: none
Optional parameters: version, msgtype
version: The HTTP-Version number of the enclosed messages
(e.g., "1.1"). If not present, the version can be
determined from the first line of the body.
msgtype: The message type -- "request" or "response". If not
present, the type can be determined from the first
line of the body.
Encoding considerations: HTTP messages enclosed by this type
are in "binary" format; use of an appropriate
Content-Transfer-Encoding is required when
transmitted via E-mail.
Security considerations: none
19.2 Internet Media Type multipart/byteranges
When an HTTP 206 (Partial Content) response message includes the
content of multiple ranges (a response to a request for multiple
non-overlapping ranges), these are transmitted as a multipart
message-body. The media type for this purpose is called
"multipart/byteranges".
The multipart/byteranges media type includes two or more parts, each
with its own Content-Type and Content-Range fields. The required
boundary parameter specifies the boundary string used to separate
each body-part.
Media Type name: multipart
Media subtype name: byteranges
Required parameters: boundary
Optional parameters: none
Encoding considerations: only "7bit", "8bit", or "binary" are
permitted
Security considerations: none
For example:
HTTP/1.1 206 Partial Content
Date: Wed, 15 Nov 1995 06:25:24 GMT
Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT
Content-type: multipart/byteranges; boundary=THIS_STRING_SEPARATES
--THIS_STRING_SEPARATES
Content-type: application/pdf
Content-range: bytes 500-999/8000
...the first range...
--THIS_STRING_SEPARATES
Content-type: application/pdf
Content-range: bytes 7000-7999/8000
...the second range
--THIS_STRING_SEPARATES--
Notes:
1) Additional CRLFs may precede the first boundary string in the
entity.
2) Although RFC2046 [40] permits the boundary string to be
quoted, some existing implementations handle a quoted boundary
string incorrectly.
3) A number of browsers and servers were coded to an early draft
of the byteranges specification to use a media type of
multipart/x-byteranges, which is almost, but not quite
compatible with the version documented in HTTP/1.1.
19.3 Tolerant Applications
Although this document specifies the requirements for the generation
of HTTP/1.1 messages, not all applications will be correct in their
implementation. We therefore recommend that operational applications
be tolerant of deviations whenever those deviations can be
interpreted unambiguously.
Clients SHOULD be tolerant in parsing the Status-Line and servers
tolerant when parsing the Request-Line. In particular, they SHOULD
accept any amount of SP or HT characters between fields, even though
only a single SP is required.
The line terminator for message-header fields is the sequence CRLF.
However, we recommend that applications, when parsing such headers,
recognize a single LF as a line terminator and ignore the leading CR.
The character set of an entity-body SHOULD be labeled as the lowest
common denominator of the character codes used within that body, with
the exception that not labeling the entity is preferred over labeling
the entity with the labels US-ASCII or ISO-8859-1. See section 3.7.1
and 3.4.1.
Additional rules for requirements on parsing and encoding of dates
and other potential problems with date encodings include:
- HTTP/1.1 clients and caches SHOULD assume that an RFC-850 date
which appears to be more than 50 years in the future is in fact
in the past (this helps solve the "year 2000" problem).
- An HTTP/1.1 implementation MAY internally represent a parsed
Expires date as earlier than the proper value, but MUST NOT
internally represent a parsed Expires date as later than the
proper value.
- All expiration-related calculations MUST be done in GMT. The
local time zone MUST NOT influence the calculation or comparison
of an age or expiration time.
- If an HTTP header incorrectly carries a date value with a time
zone other than GMT, it MUST be converted into GMT using the
most conservative possible conversion.
19.4 Differences Between HTTP Entities and RFC2045 Entities
HTTP/1.1 uses many of the constructs defined for Internet Mail (RFC
822 [9]) and the Multipurpose Internet Mail Extensions (MIME [7]) to
allow entities to be transmitted in an open variety of
representations and with extensible mechanisms. However, RFC2045
discusses mail, and HTTP has a few features that are different from
those described in RFC2045. These differences were carefully chosen
to optimize performance over binary connections, to allow greater
freedom in the use of new media types, to make date comparisons
easier, and to acknowledge the practice of some early HTTP servers
and clients.
This appendix describes specific areas where HTTP differs from RFC
2045. Proxies and gateways to strict MIME environments SHOULD be
aware of these differences and provide the appropriate conversions
where necessary. Proxies and gateways from MIME environments to HTTP
also need to be aware of the differences because some conversions
might be required.
19.4.1 MIME-Version
HTTP is not a MIME-compliant protocol. However, HTTP/1.1 messages MAY
include a single MIME-Version general-header field to indicate what
version of the MIME protocol was used to construct the message. Use
of the MIME-Version header field indicates that the message is in
full compliance with the MIME protocol (as defined in RFC2045[7]).
Proxies/gateways are responsible for ensuring full compliance (where
possible) when exporting HTTP messages to strict MIME environments.
MIME-Version = "MIME-Version" ":" 1*DIGIT "." 1*DIGIT
MIME version "1.0" is the default for use in HTTP/1.1. However,
HTTP/1.1 message parsing and semantics are defined by this document
and not the MIME specification.
19.4.2 Conversion to Canonical Form
RFC2045 [7] requires that an Internet mail entity be converted to
canonical form prior to being transferred, as described in section 4
of RFC2049 [48]. Section 3.7.1 of this document describes the forms
allowed for subtypes of the "text" media type when transmitted over
HTTP. RFC2046 requires that content with a type of "text" represent
line breaks as CRLF and forbids the use of CR or LF outside of line
break sequences. HTTP allows CRLF, bare CR, and bare LF to indicate a
line break within text content when a message is transmitted over
HTTP.
Where it is possible, a proxy or gateway from HTTP to a strict MIME
environment SHOULD translate all line breaks within the text media
types described in section 3.7.1 of this document to the RFC2049
canonical form of CRLF. Note, however, that this might be complicated
by the presence of a Content-Encoding and by the fact that HTTP
allows the use of some character sets which do not use octets 13 and
10 to represent CR and LF, as is the case for some multi-byte
character sets.
Implementors should note that conversion will break any cryptographic
checksums applied to the original content unless the original content
is already in canonical form. Therefore, the canonical form is
recommended for any content that uses such checksums in HTTP.
19.4.3 Conversion of Date Formats
HTTP/1.1 uses a restricted set of date formats (section 3.3.1) to
simplify the process of date comparison. Proxies and gateways from
other protocols SHOULD ensure that any Date header field present in a
message conforms to one of the HTTP/1.1 formats and rewrite the date
if necessary.
19.4.4 Introduction of Content-Encoding
RFC2045 does not include any concept equivalent to HTTP/1.1's
Content-Encoding header field. Since this acts as a modifier on the
media type, proxies and gateways from HTTP to MIME-compliant
protocols MUST either change the value of the Content-Type header
field or decode the entity-body before forwarding the message. (Some
experimental applications of Content-Type for Internet mail have used
a media-type parameter of ";conversions=<content-coding>" to perform
a function equivalent to Content-Encoding. However, this parameter is
not part of RFC2045.)
19.4.5 No Content-Transfer-Encoding
HTTP does not use the Content-Transfer-Encoding (CTE) field of RFC
2045. Proxies and gateways from MIME-compliant protocols to HTTP MUST
remove any non-identity CTE ("quoted-printable" or "base64") encoding
prior to delivering the response message to an HTTP client.
Proxies and gateways from HTTP to MIME-compliant protocols are
responsible for ensuring that the message is in the correct format
and encoding for safe transport on that protocol, where "safe
transport" is defined by the limitations of the protocol being used.
Such a proxy or gateway SHOULD label the data with an appropriate
Content-Transfer-Encoding if doing so will improve the likelihood of
safe transport over the destination protocol.
19.4.6 Introduction of Transfer-Encoding
HTTP/1.1 introduces the Transfer-Encoding header field (section
14.41). Proxies/gateways MUST remove any transfer-coding prior to
forwarding a message via a MIME-compliant protocol.
A process for decoding the "chunked" transfer-coding (section 3.6)
can be represented in pseudo-code as:
length := 0
read chunk-size, chunk-extension (if any) and CRLF
while (chunk-size > 0) {
read chunk-data and CRLF
append chunk-data to entity-body
length := length + chunk-size
read chunk-size and CRLF
}
read entity-header
while (entity-header not empty) {
append entity-header to existing header fields
read entity-header
}
Content-Length := length
Remove "chunked" from Transfer-Encoding
19.4.7 MHTML and Line Length Limitations
HTTP implementations which share code with MHTML [45] implementations
need to be aware of MIME line length limitations. Since HTTP does not
have this limitation, HTTP does not fold long lines. MHTML messages
being transported by HTTP follow all conventions of MHTML, including
line length limitations and folding, canonicalization, etc., since
HTTP transports all message-bodies as payload (see section 3.7.2) and
does not interpret the content or any MIME header lines that might be
contained therein.
19.5 Additional Features
RFC1945 and RFC2068 document protocol elements used by some
existing HTTP implementations, but not consistently and correctly
across most HTTP/1.1 applications. Implementors are advised to be
aware of these features, but cannot rely upon their presence in, or
interoperability with, other HTTP/1.1 applications. Some of these
describe proposed experimental features, and some describe features
that experimental deployment found lacking that are now addressed in
the base HTTP/1.1 specification.
A number of other headers, such as Content-Disposition and Title,
from SMTP and MIME are also often implemented (see RFC2076 [37]).
19.5.1 Content-Disposition
The Content-Disposition response-header field has been proposed as a
means for the origin server to suggest a default filename if the user
requests that the content is saved to a file. This usage is derived
from the definition of Content-Disposition in RFC1806 [35].
content-disposition = "Content-Disposition" ":"
disposition-type *( ";" disposition-parm )
disposition-type = "attachment" | disp-extension-token
disposition-parm = filename-parm | disp-extension-parm
filename-parm = "filename" "=" quoted-string
disp-extension-token = token
disp-extension-parm = token "=" ( token | quoted-string )
An example is
Content-Disposition: attachment; filename="fname.ext"
The receiving user agent SHOULD NOT respect any directory path
information present in the filename-parm parameter, which is the only
parameter believed to apply to HTTP implementations at this time. The
filename SHOULD be treated as a terminal component only.
If this header is used in a response with the application/octet-
stream content-type, the implied suggestion is that the user agent
should not display the response, but directly enter a `save response
as...' dialog.
See section 15.5 for Content-Disposition security issues.
19.6 Compatibility with Previous Versions
It is beyond the scope of a protocol specification to mandate
compliance with previous versions. HTTP/1.1 was deliberately
designed, however, to make supporting previous versions easy. It is
worth noting that, at the time of composing this specification
(1996), we would expect commercial HTTP/1.1 servers to:
- recognize the format of the Request-Line for HTTP/0.9, 1.0, and
1.1 requests;
- understand any valid request in the format of HTTP/0.9, 1.0, or
1.1;
- respond appropriately with a message in the same major version
used by the client.
And we would expect HTTP/1.1 clients to:
- recognize the format of the Status-Line for HTTP/1.0 and 1.1
responses;
- understand any valid response in the format of HTTP/0.9, 1.0, or
1.1.
For most implementations of HTTP/1.0, each connection is established
by the client prior to the request and closed by the server after
sending the response. Some implementations implement the Keep-Alive
version of persistent connections described in section 19.7.1 of RFC
2068 [33].
19.6.1 Changes from HTTP/1.0
This section summarizes major differences between versions HTTP/1.0
and HTTP/1.1.
19.6.1.1 Changes to Simplify Multi-homed Web Servers and Conserve IP
Addresses
The requirements that clients and servers support the Host request-
header, report an error if the Host request-header (section 14.23) is
missing from an HTTP/1.1 request, and accept absolute URIs (section
5.1.2) are among the most important changes defined by this
specification.
Older HTTP/1.0 clients assumed a one-to-one relationship of IP
addresses and servers; there was no other established mechanism for
distinguishing the intended server of a request than the IP address
to which that request was directed. The changes outlined above will
allow the Internet, once older HTTP clients are no longer common, to
support multiple Web sites from a single IP address, greatly
simplifying large operational Web servers, where allocation of many
IP addresses to a single host has created serious problems. The
Internet will also be able to recover the IP addresses that have been
allocated for the sole purpose of allowing special-purpose domain
names to be used in root-level HTTP URLs. Given the rate of growth of
the Web, and the number of servers already deployed, it is extremely
important that all implementations of HTTP (including updates to
existing HTTP/1.0 applications) correctly implement these
requirements:
- Both clients and servers MUST support the Host request-header.
- A client that sends an HTTP/1.1 request MUST send a Host header.
- Servers MUST report a 400 (Bad Request) error if an HTTP/1.1
request does not include a Host request-header.
- Servers MUST accept absolute URIs.
19.6.2 Compatibility with HTTP/1.0 Persistent Connections
Some clients and servers might wish to be compatible with some
previous implementations of persistent connections in HTTP/1.0
clients and servers. Persistent connections in HTTP/1.0 are
explicitly negotiated as they are not the default behavior. HTTP/1.0
experimental implementations of persistent connections are faulty,
and the new facilities in HTTP/1.1 are designed to rectify these
problems. The problem was that some existing 1.0 clients may be
sending Keep-Alive to a proxy server that doesn't understand
Connection, which would then erroneously forward it to the next
inbound server, which would establish the Keep-Alive connection and
result in a hung HTTP/1.0 proxy waiting for the close on the
response. The result is that HTTP/1.0 clients must be prevented from
using Keep-Alive when talking to proxies.
However, talking to proxies is the most important use of persistent
connections, so that prohibition is clearly unacceptable. Therefore,
we need some other mechanism for indicating a persistent connection
is desired, which is safe to use even when talking to an old proxy
that ignores Connection. Persistent connections are the default for
HTTP/1.1 messages; we introduce a new keyword (Connection: close) for
declaring non-persistence. See section 14.10.
The original HTTP/1.0 form of persistent connections (the Connection:
Keep-Alive and Keep-Alive header) is documented in RFC2068. [33]
19.6.3 Changes from RFC2068
This specification has been carefully audited to correct and
disambiguate key word usage; RFC2068 had many problems in respect to
the conventions laid out in RFC2119 [34].
Clarified which error code should be used for inbound server failures
(e.g. DNS failures). (Section 10.5.5).
CREATE had a race that required an Etag be sent when a resource is
first created. (Section 10.2.2).
Content-Base was deleted from the specification: it was not
implemented widely, and there is no simple, safe way to introduce it
without a robust extension mechanism. In addition, it is used in a
similar, but not identical fashion in MHTML [45].
Transfer-coding and message lengths all interact in ways that
required fixing exactly when chunked encoding is used (to allow for
transfer encoding that may not be self delimiting); it was important
to straighten out exactly how message lengths are computed. (Sections
3.6, 4.4, 7.2.2, 13.5.2, 14.13, 14.16)
A content-coding of "identity" was introduced, to solve problems
discovered in caching. (section 3.5)
Quality Values of zero should indicate that "I don't want something"
to allow clients to refuse a representation. (Section 3.9)
The use and interpretation of HTTP version numbers has been clarified
by RFC2145. Require proxies to upgrade requests to highest protocol
version they support to deal with problems discovered in HTTP/1.0
implementations (Section 3.1)
Charset wildcarding is introduced to avoid explosion of character set
names in accept headers. (Section 14.2)
A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
was introduced to add this missing case. (Sections 13.4, 14.8, 14.9,
14.9.3)
The Cache-Control: max-age directive was not properly defined for
responses. (Section 14.9.3)
There are situations where a server (especially a proxy) does not
know the full length of a response but is capable of serving a
byterange request. We therefore need a mechanism to allow byteranges
with a content-range not indicating the full length of the message.
(Section 14.16)
Range request responses would become very verbose if all meta-data
were always returned; by allowing the server to only send needed
headers in a 206 response, this problem can be avoided. (Section
10.2.7, 13.5.3, and 14.27)
Fix problem with unsatisfiable range requests; there are two cases:
syntactic problems, and range doesn't exist in the document. The 416
status code was needed to resolve this ambiguity needed to indicate
an error for a byte range request that falls outside of the actual
contents of a document. (Section 10.4.17, 14.16)
Rewrite of message transmission requirements to make it much harder
for implementors to get it wrong, as the consequences of errors here
can have significant impact on the Internet, and to deal with the
following problems:
1. Changing "HTTP/1.1 or later" to "HTTP/1.1", in contexts where
this was incorrectly placing a requirement on the behavior of
an implementation of a future version of HTTP/1.x
2. Made it clear that user-agents should retry requests, not
"clients" in general.
3. Converted requirements for clients to ignore unexpected 100
(Continue) responses, and for proxies to forward 100 responses,
into a general requirement for 1xx responses.
4. Modified some TCP-specific language, to make it clearer that
non-TCP transports are possible for HTTP.
5. Require that the origin server MUST NOT wait for the request
body before it sends a required 100 (Continue) response.
6. Allow, rather than require, a server to omit 100 (Continue) if
it has already seen some of the request body.
7. Allow servers to defend against denial-of-service attacks and
broken clients.
This change adds the Expect header and 417 status code. The message
transmission requirements fixes are in sections 8.2, 10.4.18,
8.1.2.2, 13.11, and 14.20.
Proxies should be able to add Content-Length when appropriate.
(Section 13.5.2)
Clean up confusion between 403 and 404 responses. (Section 10.4.4,
10.4.5, and 10.4.11)
Warnings could be cached incorrectly, or not updated appropriately.
(Section 13.1.2, 13.2.4, 13.5.2, 13.5.3, 14.9.3, and 14.46) Warning
also needed to be a general header, as PUT or other methods may have
need for it in requests.
Transfer-coding had significant problems, particularly with
interactions with chunked encoding. The solution is that transfer-
codings become as full fledged as content-codings. This involves
adding an IANA registry for transfer-codings (separate from content
codings), a new header field (TE) and enabling trailer headers in the
future. Transfer encoding is a major performance benefit, so it was
worth fixing [39]. TE also solves another, obscure, downward
interoperability problem that could have occurred due to interactions
between authentication trailers, chunked encoding and HTTP/1.0
clients.(Section 3.6, 3.6.1, and 14.39)
The PATCH, LINK, UNLINK methods were defined but not commonly
implemented in previous versions of this specification. See RFC2068
[33].
The Alternates, Content-Version, Derived-From, Link, URI, Public and
Content-Base header fields were defined in previous versions of this
specification, but not commonly implemented. See RFC2068 [33].
20 Index
Please see the PostScript version of this RFCfor the INDEX.
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