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Hypertext Transfer Protocol (HTTP) Tim Berners-Lee, CERN
Internet Draft
Expires 5 May 1994 5 Nov 1993
Hypertext Transfer Protocol (HTTP)
A Stateless Search, Retrieve and Manipulation Protocol
Status of this memo
This document is an Internet Draft. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas,
and its Working Groups. Note that other groups may also distribute
working documents as Internet Drafts.
Internet Drafts are working documents valid for a maximum of six
months. Internet Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use Internet
Drafts as reference material or to cite them other than as a
"working draft" or "work in progress".
This document is a DRAFT specification of a protocol in use on the
internet and to be proposed as an Internet standard. Discussion
of this protocol takes place on the www-talk@info.cern.ch mailing
list -- to subscribe mail to www-talk-request@info.cern.ch.
Distribution of this memo is unlimited.
Abstract
HTTP is a protocol with the lightness and speed necessary for a
distributed collaborative hypermedia information system. It is a
generic stateless object-oriented protocol, which may be used for
many similar tasks such as name servers, and distributed
object-oriented systems, by extending the commands, or "methods",
used. A feature if HTTP is the negotiation of data representation,
allowing systems to be built independently of the development of
new advanced representations.
Note: This specification
This HTTP protocol is an upgrade on the original protocol as
implemented in the earliest WWW releases. It is back-compatible
with that more limited protocol.
This specification includes the following parts:
The Request
T. Berners-Lee 1
Methods
A list of headers in the request message
The response
Status codes
A list of headers on any object transmitted
The content of any object content transmitted
Format negotiation algorithm
The HTTP Registration Authority
Security Considerations
Unresolved points
References
The following notes form recommended practice not part of the
specification:
Servers tolerating clients
Clients tolerating servers
Purpose
When many sources of networked information are available to a
reader, and when a discipline of reference between different
sources exists, it is possible to rapidly follow references
between units of information which are provided at different remote
locations. As response times should ideally be of the order of
100ms in, for example, a hypertext jump, this requires a fast,
stateless, information retrieval protocol.
Practical information systems require more functionality than
simple retrieval, including search, front-end update and
annotation. This protocol allows an open-ended set of methods to be
used. It builds on the discipline of reference provided by the
Universal Resource Identifier (URI) as a name (URN, RFCxxxx) or
address (URL, RFCxxxx) allows the object of the method to be
specified.
Reference is made to the Multipurpose Internet Mail Extensions
(MIME, RFC1341) which are used to allow objects to be transmitted
in an open variety of representations.
Overall operation
T. Berners-Lee 2
On the internet, the communication takes place over a TCP/IP
connection. This does not preclude this protocol being implemented
over any other protocol on the internet or other networks. In
these cases, the mapping of the HTTP request and response
structures onto the transport data units of the protocol in
question is outside the scope of this specification. It should not
however be at all complicated.
The protocol is basically stateless, a transaction consisting of
Connection The establishment of a connection by the
client to the server - when using TCP/IP
port 80 is the well-known port, but other
non-reserverd ports may be specified in the
URL;
Request The sending, by the client, of a request
message to the server;
Response The sending, by the server, of a response to
the client;
Close The closing of the connection by either both
parties.
The format of the request and response parts is defined in this
specification. Whilst header information defined in this
specification is sent in ISO Latin-1 character set in CRLF
terminated lines, object transmission in binary is possible.
Character sets
In all cases in HTTP where RFC822 characters are allowed, these may
be extended to use the full ISO Latin 1 character set. 8-bit
transmission is always used.
tableofcontents
REQUEST
The request is sent with a first line containing the method to be
applied to the object requested, the identifier of the object, and
the protocol version in use, followed by further information
encoded in the RFC822 header style. The format of the request is:
Request = SimpleRequest | FullRequest
SimpleRequest = GET <uri> CrLf
FullRequest = Method UR ProtocolVersion CrLf
[*<HTRQ Header>]
[<CrLf> <data>]
T. Berners-Lee 3
<Method> = <InitialAlpha>
ProtocolVersion = HTTP/V1.0
uri = <as defined in URL spec>
<HTRQ Header> = <Fieldname> : <Value> <CrLf>
<data> = MIME-conforming-message
The URI is the Uniform Resource Locator (URL) as defined in the
specification, or may be (when it is defined) a Uniform Resource
Name (URN) when a specification for this is settled, for servers
which support URN resolution.
Unless the server is being used as a gateway, a partial URL shall
be given with the assuptions of the protocol (HTTP:) and server
(the server) being obvious.
The URI should be encoded using the escaping scheme described in
the URL specification to a level such that (at least) spaces and
control characters (decimal 0-31 and 128-159) do not appear
unesacaped.
Note. The rest of an HTTP url after the host name and optional port
number is completely opaque to the client: The client may make no
deductions about the object from its URL.
Protocol Version
The Protocol/Version field defines the format of the rest of the
request.. At the moment only HTRQ is defined .
If the protocol version is not specified, the server assumes that
the browser uses HTTP version 0.9.
Uniform Resource Identifier
This is a string identifying the object. It contains no blanks. It
may be a Uniform Resource Locator [ URL ] defining the address of
an object as described in RFCxxxx, or it may be a representation of
the name of an object (URN, Universal Resource Name) where that
object has been registered in some name space. At the time of
writing, no suitable naming system exists, but this protocol will
accept such names so long as they are distinguishable from the
existing URL name spaces.
Methods
Method field indicates the method to be performed on the object
identified by the URL. More details are with the list of method
names below .
T. Berners-Lee 4
Request Headers
These are RFC822 format headers with special field names given in
the list below , as well as any other HTTP object headers or MIME
headers.
Object Body
The content of an object is sent (depending on the method) with the
request and/or the reply.
Methods
The Method field in HTTP indicates the method to be performed on
the object identified by the URL. The method GET below is always
supported, The list of other methods acceptable by the object are
returned in response to either of these two requests.
This list may be extended from time to time by a process of
registration with the design authority. Method names are case
sensitive. Currently specified methods are as follows:
GET means retrieve whatever data is identified
by the URI, so where the URI refers to a
data-producing process, or a script which can
be run by such a process, it is this data
which will be returned, and not the source
text of the script or process. Also used for
searches .
HEAD is the same as GET but returns only HTTP
headers and no document body.
CHECKOUT Similar to GET but locks the object against
update by other people. The lock may be
broken by a higher authority or on timeout:
in this case a future CHECKIN will fail. (
Phase out? )
SHOWMETHOD Returns a description (perhaps a form) for a
given method when applied to the given
object. The method name is specified in a
For-Method: field. (TBS)
PUT specifies that the data in the body section
is to be stored under the supplied URL. The
URL must already exist. The new contenst of
the document are the data part of the
request. POST and REPLY should be used for
creating new documents.
DELETE Requests that the server delete the
information corresponding to the given URL.
T. Berners-Lee 5
After a successfull DELETE method, the URL
becomes invalid for any future methods.
POST Creates a new object linked to the specified
object. The message-id field of the new
object may be set by the client or else will
be given by the server. A URL will be
allocated by the server and returned to the
client. The new document is the data part of
the request. It is considered to be
subordinate to the specified object, in the
way that a file is subordinate to a directory
containing it, or a news article is
subordinate to a newsgroup to which it is
posted.
LINK Links an existing object to the specified
object.
UNLINK Removes link (or other meta-) information
from an object.
CHECKIN Similar to PUT, but releases the lock set on
the object. Fails if no lock has been set by
CHECKOUT. Suggestion : phase out this
(rcs-like) model in favor of the PUT
(cvs-like, non-locking) model of code
management.
TEXTSEARCH The object may be queried with a text
string. The search form of the GET method is
used to query the object.
SPACEJUMP The object will accept a query whose terms
are the cooridnates of a point within the
object. The method is implemented using GET
with a derived URL .
SEARCH Proposed only. The index (etc) identified
by the URL is to be searched for something
matching in some sense the enclosed message.
How does the client know what message fromats
are acceptable to the server? (Suggestion of
Fred Williams)
(Some of these methods require more detailed specification)
Note: case sensitivity of method names
Although lack of case sensitivity in methos names would be a
tolerant approach with a limited method set, we require the
extensibility of HTTP to cover an arbitrary underlying object
system. In such a system, method names may be case sensitive, and
T. Berners-Lee 6
so we must preserve case in HTTP.
GET
A representation of the object is transferred to the client.
Some URIs refer to specific variants of an object, and some refer
to objects with many variants. In the latter case, the
representations, encodings, and languages acceptable may be
specified in the header request fields, and may affect the
particular value which is returned.
Other possible replies allow a set of URIs to be returned to the
client, who may use them to retrieve the object. This allows name
servers to be implemented using HTTP, and also forwarding address
to be given when objects have been moved.
Annotation replies
Some servers keep 'parallel webs" -- separate stores of information
about other server's objects. Typically this incldues annotation
links. This information may be retrieved using the GET method,
but a special reply is returned.
Searching using GET
When the TEXTSEARCH or SPACEJUMP methods are supported, their
implementation is in fact by means of the GET method, by
constructing a derived URL.
The URL used with GET is the URL of the object, suffixed by a "?"
character and the text to be searched. If the object being searched
is in fact itself the result of a search (ie the URL contains a
"?"), then those search terms are first stripped off, so the search
is performed on the original object.
In the URL, keywords are seperated with plus signs ("+"). Real
spaces, plus signs, and other illegal characters [who defines
illegal characters?] are represented as hexadecimal ASCII escape
sequences (%##, e.g., space = %20, plus = %2B)
Browsers accepting text input should define keywords as seperated
by spaces (and therefore map them to plus signs). More advanced
browsers may allow separate keyword input and, therefore, a finer
level of control over the content without users needing to
understand the underlying mechanisms (e.g., that they must use %20
to get a real space).
Examples
The following is a request on an object supporting TEXTSEARCH:
T. Berners-Lee 7
GET /indexes/botany?annual+plants HTTP/1.0
This request on an object supporting SPACEJUMP selects point
(0.2,3.8) within a map:
GET /maps/uk/dorset/winspit/0.2+3.8 HTTP/1.0
LINK
The link method of HTTP adds meta information (Object header
information) to an object, without touching the object's content.
For example, it requests the creation of a link from the specified
object to another object.
The request is followed by a set of object headers which are to be
added.
In cases in which a new header is added and the semantics of the
header do not allow it to coexist with a similar header, then the
previous header is deleted. For example, an object may only have
one title, so specifying a title overwrites and preexsiting title.
Link Types
The link type unless void is specified in the WWW-Link object field
for each link. If not specified, void is assumed.
Unresolved points
As this is generalised to allow any metainformation to be added, a
better name might be DESCRIBE or attribute (as a verb). I don't
like "describe" as it does not suggest alteration.
"Bestow"/"Rescind" might be better. For example, one could bestow a
title or author on something previously title-less. We are looking
at a general data model behind here a little like a relational
database. This function adds records. "Set" and "Reset"?
Related Methods
LINK is like POST except that
no storage is requested for the destination object, and
the destination is not assumed to be subordinate to the
specified object.
See also: UNLINK.
Tim BL
POST
T. Berners-Lee 8
This method of HTTP creates a new object linked to and subordinate
to the specified object. The content of the new object is
enclosed as the body of the request.
The POST method is desiged to allow a uniform function to cover
Annotation existing documents;
Posting a message to a bulletin board topic, newsgroup, mailing
list, or similar group of articles;
Adding a file to a directory;
Extending a document during authorship.
The client may not assume any postconditions of the method in terms
of web topology. For example, if a POST is accepted, the effect may
be delayed or overruled by human moderation, batch processing, etc.
The client should not be surprised if a link is not immediately
(or never) created.
However, the semantics are a request for a link to be made from the
object whose URL is quoted to the new, enclosed, object.
If no URL for the NEW object is given by the client, the server is
requested to see to the storage of the new object. That is, the
server does not have to store it but will have to return a URL be
which it can later be retrieved. The semantics of this method
(currently) imply nothing of any undertakings by the server to
maintain the availability of the object.
If the client gives a URL, then the server is not obliged to store
the object, but may take a copy and may in that case issue a new
URL.
Return object headers
The method shall return a set (possibly empty) of object headers
for the newly posted object. If a URL has been assigned by the
server, then that may be included. Similarly, if a URN has been
assigned, then that shall be returned. Other things which may be
returned include for example the expiry-date if any. The server may
return the entire metadata for the object (as in the HEAD command),
or a subset of it.
The object body shall not be returned, so the transaction shall end
with the blank line terminating the headers.
Link type
The link type may be specified by explicitly giving a (reverse)
link in the object header of the linked object. If a link or links
between the two objects are present in those headers, then that
T. Berners-Lee 9
link is used. If no such link is specified, then the server should
generate a link. The link type in this case is determined by the
server. The server may perform other operations as a result of the
new object being added: lists and indexes might be updated, for
example.
Submission
When articles are submitted, the analogy of being addeed to a body
of knowledge by being linked is close. When a form is submitted,
this can be done with POST, though in this case side-effects will
be expected.
(Should submission for action have a different method -- see
showmethod -- or should it be just POST for simplicity? When
interfacing with other systems such as bews and mail, the
distinction is not made as the system does not have the ability to
distinguish different methods. We now have a possibility of making
a separate action, though.)
Unresolved points
The client has no way of knowing what data formats the server is
prepared to accept. This may not be a serious problem, but it may
be if the server has some restrictions. This applies to all
submissions of object content from client to server, for all
methods.
Related methods.
When a new URL has been returned by the server, it may in general
(typically, but not necessarily) be usable as the argument of
DELETE , GET , PUT , etc, methods.
To make a link between two existing objects, see LINK.
Tim BL
SEARCH
Some correspondance about SEARCH:
Q
How did the client know that the server could search for tex? Maybe t
his server can only serach for text and GIF patterns, even though it g
ave out tex. How can the client know that a GIF search is possible?
Answer
I assume the following has happened:
client requests with ACCEPT: text/x-TeX
T. Berners-Lee 10
If the server, which it should if possible, replies with the doc
requested in ``TeX''.
Therefore a server can obviously do an ``internal representation'' to
``TeX'' conversion.
Since the server responded in a particular representation (ie TeX)
The client should be able to search in ``TeX'' provide the ``SEARCH''
method is in the required ALLOWED: section of the response.
The server should only respond with ALLOWED: SEARCH ... if it can do a
Tex to ``internal representation'' conversion for searching.
Fred Williams fwilliam@ccs.carleton.ca
SHOWMETHOD
When an object can support more operations than are defined in this
specification, SHOWMETHOD allows a client to understand the
interface to that operation sufficiently to allow the user to
perform it interactively.
Required parameter field
For-Method: This filed contains only the method name
about which the client is inquiring.
Preconditions
The methodname spacified in the For-Method field must have been
previously issued in a "Allowed:" field returned with the given
object.
The client should specify an Accept: field which includes at least
one form langauge it it wants to be able to interpret the result.
Postcondidtion
SHOWMETHOD returns, if possible, a form in a representation
acceptable to the client. This form will contain instructions for
ordering the operation, and fields for the parameters.
A suitable language for the form might be HTTP2, but any language
may be used which is acceptable to the client.
SPACEJUMP
This method is similar to the TEXTSEARCH method, but instead of the
search criterion being a text string, it is a set of coordinates
defining a point within the image. The semantics of the operation
are not defined here. Typically, the user clicks on a point within
the image with a mouse or other pointing device.
T. Berners-Lee 11
Two or more coordinates are supplied, in the order x, y z, t. All
coordinates are scaled so that 0 represents the bottom left hand
point and 1.0 represents the top right hand point.
The z access direction follows the normal right-hand rule, that is
extends toward the viewer when the x and y axes are flat as in the
normal two-dimensional representation.
In the case of a time-occupying object, 0 represents the starting
instance, and 1.0 represents the finishing instant.
The method is implemented using GET with a derived URL .
TEXTSEARCH
This is a simple form of search. The text is assumed to derive from
the requesting user, and is in no special format.
The exact algorithm to be applied is not defined in this
specification, but techniques such as vocabulary proximity matching
between the request data portion and the contents or titles of
documents, keyword matching, stemming, and the use of a thesaurus
are quite appropriate.
Whilst this method name is given as a flag to specify that the
function is available, the search form of the GET method is in fact
used to query the object.
UNLINK
This method deletes metainformation about an object. The request
contains object geaders which are to be removed. Only headers
exactly matching the headers given are removed.
Obviously the operation may be used for unlinking objects. It may
also be used for removing other metainformation such as object
title, expiry date, etc.
Related methods
See LINK.
Tim BL
HTTP Request fields
These header lines are sent by the client in a HTTP protocol
transaction. All lines are RFC822 format headers. The list of
headers is terminated by an empty line.
FROM:
In Internet mail format, this gives the name of the requesting
T. Berners-Lee 12
user. This field may be used for logging purposes and an insecure
form of access protection. The interpretation of this field is
that the request is being performed on behalf of the person given,
who accepts responsability for the method performed.
The Internet mail address in this field does not have to correspond
to the internet host which issued the request. (For example, when a
request is passed through a gateway, then the original issuer's
address should be used).
The mail address should, if possible, be a valid mail address,
whether or not it is in fact an internet mail address or the
internet mail representation of an address on some other mail
system.
ACCEPT:
This field contains a comma-separated list of representation
schemes (Content-Type metainformation values) which will be
accepted in the response to this request.
The set given may of course vary from request to request from the
same user.
This field may be wrapped onto several lines according to RCFC822,
and also more than one occurence of the field is allowed with the
signifiance being the same as if all the entries has been in one
field. The format of each entry in the list is (/ meaning "or")
<field> = Accept: <entry> *[ ; <entry> ]
<entry> = <content type> *[ , <param> ]
<param> = <attr> = <float>
<attr> = q / mxs / mxb
<float> = <ANSI-C floating point text represntation>
See the appendix on the negotiation algorithm as a function and
penalty model.
If no Accept: field is present, then it is assumed that text/plain
and text/html are accepted.
Example
Accept: text/plain; text/html
Accept: text/x-dvi, q=.8, mxb=100000, mxt=5.0; text/x-
c
Wildcards
In order to save time, and also allow clients to receive content
types of which they may not be aware, an asterisk "*" may be used
T. Berners-Lee 13
in place of either the second half of the content-type value, or
both halves. This only applies to the Accept: filed, and not to
the content-type field of course.
Example
Accept: *.*, q=0.1
Accept: audio/*, q=0.2
Accept: audio/basic q=1
may be interpreted as "if you have basic audio, send it; otherwise
send me some other audio, or failing that, just give me what
you've got."
ACCEPT-ENCODING:
Similar to Accept, but lists the Content-Encoding types which are
acceptable in the response.
<field> = Accept-Encoding: <entry> *[ , <entry> ]
<entry> = <content transfer encoding> *[ , <param> ]
Example
Accept-Encoding: x-compress; x-zip
ACCEPT-LANGUAGE:
Similar to Accept, but lists the Language values which are
preferable in the response. A response in an unspecifies language
is not illegal. See also: Language .
Language coding TBS. (ISO standard xxxx)
USER-AGENT:
This line if present gives the software program used by the
original client. This is for statistical purposes and the tracing
of protocol violations. It should be included. The first white
space delimited word must be the software product name, with an
optional slash and version designator. Other products which form
part of the user agent may be put as separate words.
<field> = User-Agent: <product>+
<product> = <word> [/<version>]
<version> = <word>
Example:
UserAgent: LII-Cello/1.0 libwww/2.5
REFERER:
T. Berners-Lee 14
This optional header field allows the client to specify, for the
server's benefit, the address ( URI ) of the document (or element
within the document) from which the URI in the request was
obtained.
This allows a server to generate lists of back-links to documents,
for interest, logging, etc. It allows bad links to be traced for
maintenance.
If a partial URI is given, then it should be parsed relative to the
URI of the object of the request.
Example:
Referer: http://info.cern.ch/hypertext/DataSources/Over
view.html
AUTHORIZATION:
If this line is present it contains authorization information. The
format is To Be Specified (TBS). The format of this field is in
extensible form. The first word is a specification of the
authorisation system in use.
Proposals have been as follows: (see the specification for current
one implemented by AL Sep 1993)
User/Password scheme
Authorization: user fred:mypassword
The scheme name is "user". The second word is a user name
(typically derived from a USER environment variable or prompted
for), with an optional password separated by a colon (as in the URL
syntax for FTP). Without a password, this povides very low level
security. With the password, it provides a low-level security as
used by unmodified FTP, Telnet, etc.
Kerberos
Authorization: kerberos kerberosauthenticationsparam
eters
The format of the kerberosauthenticationsparameters is to be
specified.
CHARGETO:
This line if present contains account information for the costs of
the application of the method requested. The format is TBS. The
format of this field must be in extensible form. The first word
starts with a specification of the namespace in which the account
is . (This is similar to extensible URL definition.) No namespaces
T. Berners-Lee 15
are currently defined. Namespaces will be registered with the
registration authority .
The format of the rest of the line is a function of the charging
system, but it is recommended that this include a maximum cost
whose payment is authorized by the client for this transaction, and
a cost unit.
Note: Server tolerance of bad clients
Whilst it is seen appropriate for testing parsers to check full
conformance to this specification, it is recommended that
operational parsers be tolerant of deviations.
In particular, lines should be regarded as terminated by the Line
Feed, and the preceeding Carriage Return character ignored.
Any HTTP Header Field Name which is not recognised should be
ignored in operational parsers.
It is recommended that servers use URIs free of "variant"
characters whose representation differs in some of the national
variant character sets, punctuation characters, and spaces. This
will make URIs easier to handle by humans when the need (such as
debugging, or transmission through non hypertext systems) arises.
RESPONSE
The response from the server shall start with the following syntax
(See also: note on client tolerance ):
<status line> ::= <http version> <status code> <reason line>
<CrLf>
<http version> ::= 3*<digit>
<status code> ::= 3*<digit>
<digit> ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
<reason line> ::= * <printable>
<http version> identifies the HyperText Transfer Protocol
version being used by the server. For the
version described by this document version it
is "HTTP/1.0" (without the quotes).
< status code > gives the coded results of the attempt to
understand and satisfy the request. A three
digit ASCII decimal number.
<reason string> gives an explanation for a human reader,
except where noted for particular status
codes.
Fields on the status line are delimited by a single blank (parsers
T. Berners-Lee 16
should accept any amount of white space). The possible values of
the status code are listed below .
Response headers
The headers on returned objects those RFC822 format headers listed
as object headers , as well as any MIME conforming headers, notably
the Content-Type field. Note that this specification doesnot
define any headers particular to the response which are not also
apropriate to any transmission of an object with a request.
Response data
Additional information may follow, in the format of a MIME message
body. The significance of the data depends on the status code.
The Content-Type used for the data may be any Content-Type which
the client has expressed his ability to accept, or text/plain, or
text/html. That is, one can always assume that the client can
handle text/plain and text/html.
Status codes
The values of the numeric status code to HTTP requests are as
follows. The data sections of messages Error, Forward and
redirection responses may be used to contain human-readable
diagnostic information.
SUCCESS 2XX
These codes indicate success. The body section if present is the
object returned by the request. It is a MIME format object. It is
in MIME format, and may only be in text/plain, text/html or one fo
the formats specified as acceptable in the request.
OK 200
The request was fulfilled.
CREATED 201
Following a POST command, this indicates success, but the textual
part of the response line indicates the URI by which the newly
created document should be known.
Accepted 202
The request has been accepted for processing, but the processing
has not been completed. The request may or may not eventually be
acted upon, as it may be disallowed when processing actually takes
place. there is no facility for status returns from asynchronous
operations such as this.
T. Berners-Lee 17
Partial Information 203
When received in the response to a GET command, this indicates that
the returned metainformation is not a definitive set of the object
from a server with a copy of the object, but is from a private
overlaid web. This may include annotation information about the
object, for example
ERROR 4XX, 5XX
The 4xx codes are intended for cases in which the client seems to
have erred, and the 5xx codes for the cases in which the server is
aware that the server has erred. It is impossible to distinguish
these cases in general, so the difference is only informational.
The body section may contain a document describing the error in
human readable form. The document is in MIME format, and may only
be in text/plain, text/html or one for the formats specified as
acceptable in the request.
Bad request 400
The request had bad syntax or was inherently impossible to be
satisfied.
Unauthorized 401
The parameter to this message gives a specification of
authorization schemes which are acceptable. The client should
retry the request with a suitable Authorization header.
PaymentRequired 402
The parameter to this message gives a specification of charging
schemes acceptable. The client may retry the request with a
suitable ChargeTo header.
Forbidden 403
The request is for something forbidden. Authorization will not
help.
Not found 404
The server has not found anything matching the URI given
Internal Error 500
The server encountered an unexpected condition which prevented it
from fulfilling the request.
Not implemented 501
T. Berners-Lee 18
The server does not support the facility required.
REDIRECTION 3XX
The codes in this section indicate action to be taken (normally
automatically) by the client in order to fulfill the request.
Moved 301
The data requested has been assigned a new URI, the change is
permanent. (N.B. this is an optimisation, which must,
pragmatically, be included in this definition. Browsers with link
editing capabiliy should automatically relink to the new reference,
where possible)
The response contains one or more header lines of the form
URI: <url> String CrLf
Which specify alternative addresses for the object in question.
The String is an optional comment field. If the response is to
indicate a set of variants which each correspond to the requested
URI, then the multipart/alternative wrapping may be used to
distinguish different sets
Found 302
The data requested actually resides under a different URL, however,
the redirection may be altered on occasion (when making links to
these kinds of document, the browser should default to using the
Udi of the redirection document, but have the option of linking to
the final document) as for "Forward".
The response format is the same as for Moved .
Method 303
Method: <method> <url>
body-section
Like the found response, this suggests that the client go try
another network address. In this case, a different method may be
used too, rather than GET.
The body-section contains the parameters to be used for the method.
This allows a document to be a pointer to a complex query
operation.
@@TBS in more detail.
The body may be preceded by the following additional fields as
listed .
T. Berners-Lee 19
Object MetaInformation
The header fields given with or in relation to objects in HTTP are
as follows. All are optional. These headers specify
metainformation: that is, information about the object, not the
information which is contained in the object.
There is no reason for limiting these fields to HTTP use, as any
other system which requires metainformation is encouraged to use
them.
The order of header lines withing the HTTP header has no
significance. However, those fields which are not MIME fields
should occur before the MIME fields, so that the MIME fields and
following form a valid MIME document. This is not mandatory.
Any header fields which are not understood should be ignored.
(TBS in more detail)
ALLOWED: *METHOD
Lists the set of requests which the requesting user is allowed to
issue for this URL. If this header line is omitted, the default
allowed methods are "GET HEAD"
Example of use:
Allow: GET HEAD PUT
PUBLIC: *METHOD
As "Allow" but lists those requests which anyone may use. If
omitted, the default is "GET" only.
Example of use:
Public: GET HEAD TEXTSEARCH
CONTENT-LENGTH: INT
Implies that the body is binary and should be read directly from
the communications link, without parsing lines, etc. When the
data is part of the request, prevents the escaping and de-escaping
of the termination sequence .
@@@ This should be part of the MIME header, as it applies to any
binary encoded part. Note HTML is the first internet protocol to
allow MIME "binary" encoding. In MIME, the use of Content-Length is
currently allowed only for external messages.
CONTENT-TYPE:
T. Berners-Lee 20
As defined in MIME, except where noted here.
Extra non-MIME types
It is reasonable to put strict limits on transfer formats for mail,
where there is no guarantee that the receiver will understand a
weird format. However, in HTTP one knows that the receiver will be
able to receive it because it will have been sent in the Accept:
field. There is therefore a lot to be gained from a very complete
registry of well-defined types for HTTP which may nevertheless not
be recommended for mail. In this case, the content-type list for
HTTP may be a superset of the MIME list.
The x- convention for experimental types is of course still
available as well.
Type parameters
Parameters on the content type are extremely useful for describing
resolutions, colour depths, etc. They will allow a client to
specify in the Accept: field the resolution of its device. This may
allow the server to economise greatly on transmission time by
reducing the resultion of an image, for example.
These parameters are to be specified when types are registered..
@@ TBS.
Multipart types
MIME provides for a number of "multipart" types. These are
encapsulations of several body parts in the one message. In HTTP,
Multipart types may be returned on the condition that the client
has indicated acceptability (using Accept:)of the multipart type
and also of the content types of each consitutent body part.
The body parts (unlike in MIME) MAY contain HTTP metainformation
header fields which ARE significant.
Multipart/alternative
This is normally used in mail to send different content-type
variants when the receiver's capabilities are not known. This is
not the case with HTTP. Multipart/alternate may however be used to
provide meta information of many instances of an object, in the
case of a indirection response. This allows, for example, pointers
to be returned by a name server to a set of instances of an object.
Multipart/related
This is the type to be used when the first body part contains
references to other parts which the server wishes to send at the
same time. For example, the first part could be an HTML document,
and the included bodyparts could be the inline images mentioned
T. Berners-Lee 21
within the text.
The body parts may have URI: fields if the body parts have URIs,
and so they may be referred to by these URIs in the body-parts. If
the body-parts are transient (as in speech insertions in mail
messages) then the [propsed] "cid:" URI type may be used to refer
to them by content-identifier.
Multipart/mixed
This may be used to simply transfer an unrelated unstructured set
of objects.
Multipart/parallel
This may be used as in MIME to indicate simultaneous presentation.
[It is the author's belief that this is a trivial case of a
compound presentation which in general should be described by a
script which would be teh first bodypart of a multipart/related
document].
DATE: DATE
Creation date of object. (or last modified, and separately have a
Created: field?) Format as in RFC850 but GMT MUST BE USED.
EXPIRES: DATE
Gives the date after which the information given ceases to be valid
and should be retrieved again. This allows control of caching
mechanisms, and also allows for the periodic refreshing of displays
of volatile data. Format as for Date:. This does NOT imply that
the original object will cease to exist.
LAST-MODIFIED: DATE
Last time object was modified, i.e. the date of this version if the
document is a "living document". Format as for Date:.
MESSAGE-ID: URI
A unique identifier for the message. As in RFC850 , except that the
unlimited lifetime of HTTP objects requires that the Message-ID be
unique in all time, not just in two years.
A document may only have one Message-ID.
No two documents, even if different versions of the same live
document, may have the same Message-id.
Note: Unlike the URI field, this does not fgive a way of accessing
the document, so the Message-Id cannot be used to refer to the
document. In the case of NNTP articles, the message-id may in fact
T. Berners-Lee 22
be used within the URI for retrieval using NNTP.
URI: 1*URI
This gives a URI with which the object may be found. There is no
guarantee that the object can be retrieved using the URI specified.
However, it is guaranteed that if an object is successfully
retrieved using that URI it will be to a certain given degree the
same object as this one.
If the URI is used to refer to a set of variants, then the
dimensiosn in which the variants may differ must be given with the
"vary" parameter:
Syntax URI: <uri> [ ; vary = dimension [ , dimension
]* ]
dimension content-type | language | version
If no "vary" parameters are given, then the URI may not return
anything other than the same bit stream as this object.
Multiple occurencies of this field give alternative access names or
addresses for the object.
Examples
URI: http://info.cern.ch/pub/www/doc/url6.multi; vary
=content-type
This indicates that retrieval given the URI will return the same
document, never an updated version, but optionally in a different
rendition.
URI: http://info.cern.ch/pub/www/doc/url.multi;
vary=content-type, language, version
This indicates that the URI will return the smae document, possibly
in a different rendition, possibly updated, and without excluding
the provision of translations into different languages.URI:
http://info.cern.ch/pub/www/doc/url6.ps vary=content-typeThis
indicates that accessing the URI in question will return exactly
the same bitstream.
VERSION:
This is a string defining the version of an evolving object. Its
format is currently undefined , and so it should be treated as
opaque to the reader, defined by the informatiuon provider. The
version field is used to indicate evolution along a single path of
a partucular work. It is NOT used to indicate derived works (use a
link), translations , or renditions in different representations .
T. Berners-Lee 23
Note: It would be useful to have sufficient semantics to be able
to deduce whether one version predated or postdated another.
However, it may also be useful to be able to insert a particular
local code management system's own version stamp in this field.
Typically, publishers will have quite complex version information
containing hidden local semantics, giving value to the idea of this
field being opaque to other readers ofthe document.
DERIVED-FROM:
When an editied object is resubmitted using PUT for example, this
field gives the value of the Version . This typically allows a
server to check for example that two concurrent modifications by
different parties will not be lost, and for example to use
established version management techniques to merge both
modifications.
LANGUAGE: CODE
The language code is the ISO code for the language in which the
document is written. If the language is not known, this field
should be omitted of course .
The language code is an ISO 3316 language code with an optional
ISO639 country code to specify a national variant.
Example
Language: en_UK
means that the content of the message is in British English, while
Language: en
means that the language is English in one of its forms. (@@ If a
document is in more than one language, for example requires both
Greek Latin and French to be understood, should this be
representable?)
See also: Accept-Language .
COST: TBS
The cost of retrieving the object is given. This is the cost of
access of a copyright work. Format of units to be specified.
Currently refers to an unspecified charging scheme to be agreed out
of band between parties.
WWW-LINK:
Note. It is proposed that any HTML metainformation element (allowed
withing the HEAD as opposed to BODY element of the document) be a
valid candidate for an HTTP object header. WWW_Link is a required
T. Berners-Lee 24
example. The suggestion was that the isomorphism should be realized
by prepending "WWW-" t the HTML element name to make the HTTP
header name, and the HTML attributes imply identically named
semicolon-separated MIME-style header parameters. Other clear
candidates include WWW-Title.
It is open to discussion whether the "WWW-" should be removed.
This is semantically equivalent to the LINK element in an HTML
document which should be consulted for a full explanation.
Examples
WWW-Link: href="http://info.cern.ch/a/b/c"; rel="includes"
WWW-Link: href="mailto:timbl@info.cern.ch"; rev=made
The first example indicates that this object includes the specified
/a/b/c object. The second indicates that the author of the object
is identified by the given mail address.
Note: Client tolerance of bad servers
Servers not implementing the specification as written are not HTTP
compiant. Servers should always be made completely copmpliant.
However, clients should also tolerate deviant servers where
possible.
BACK COMPATIBILITY
In order that clients using the HTTP protocol should be able to
communicate with servers using the protocol originally implemented
in the W3 data model, clients should tolerate responses which do
not start with a numeric version number and response codes.
In this case, they should assume that the rest of the response is a
document body in type text/html.
WHITE SPACE
Clients should be tolerant in parsing response status lines, in
particular they should accept any sequence of white space (SP and
TAB) characters between fields.
Lines should be regarded as terminated by the Line Feed, and the
preceeding Carriage Return character ignored.
HTTP NEGOTIATION ALGORITM
This note defines the significance of the q, mxb and mxs values
optionally sent in the Accept: field of the HTTP protocol request
message.
It is assumed that there is a certain value of the presentation of
T. Berners-Lee 25
the document, optimally rendered using all the information
available in its original source.
It is further assumed that one can allocate a number between 0 and
1 to represent the loss of value which occurs when a document is
rendered into a representation with loss of information. Whilst
this is a very subjective measurement, and in fact largely a
function of the document in question, the approximation is made
that one can define this "degradation" figure as a function of
merely the representation involved.
The next assumption is that the other cost to the user of viewing
the document is a function of the time taken for presentation. We
first assume that the cost is linear in time, and then assume that
the time is linear in the size of the message.
The final net value to the user can therefore be written
presented_value = initial_value * total-degradation - a - b *
size
for a document in a given incoming representation. Suppose we
normalize the initial value of the document to be 1. The server
may judge that the value in a particular format is less than 1 is a
conversion on the server side has lost information. The total
degradation is then the product of any degradation due to
conversions internal to the server, and the degradation "q" sent in
the Accept field. If q is not sent, it defaults to 1.
The values of a and b have components from processing time on the
server, network delays, and processing time on the client. These
delays are not additive as a good system will pipeline the
processing, and whilst the result may be linear in message size,
calculation of it in advance is not simple. The amount of
pipelining and the loads on machines and network are all difficult
to predict, so a very rough assumption must be made.
We make the client responsible for taking into account network
delays. The client will in fact be in a better position to do this,
as the client will after one transaction be aware of the round-trip
time.
We assume that the delays imposed by the server and by the client
(including network) are additive. We assume that the client's
delay is proportional to message size.
The three parameters given by the client to the server are
q The degradation (quality) factor between 0
and 1. If omitted, 1 is assumed.
mxb The size of message (in bytes) which even if
immediately available from the server will
T. Berners-Lee 26
cause the value to the reader to become zero
mxs The delay (in seconds) which, even for a very
small message with no length-related penalty,
will cause the value to the reader to become
zero.
These parameters are chosen in part because they are easy to
visualize as the largest tolerable delay and size. If not sent,
they default to infinity.
The server may optimize the presented value for the user when
deciding what to return. The hope is that fine decisions will not
have to be made, as in most cases the results for different formats
will be very different, and there will be a clear winner.
A suitable algorithm is that the assumed value v of a document of
initial value u delivered to the network after a delay t whose
transfer length on the net is b bytes is
v = u * q - b/mxb - t/mxs
Note that t is the time from the arrival of the request to the
first byte being available on the net. [[See also: Design issues
discussions around this point.]]
HTTP Negotiation algoritm
This note defines the significance of the d, a and b values
optionally sent in the Accept: field of the HTTP protocol request
message.
It is assumed that there is a certain value of the presentation of
the document, optimally rendered using all the information
available in its original source.
It is further assumed that one can allocate a number between 0 and
1 to represent the loss of value which occurs when a document is
rendered intop a represenmtation with loss of information. Whilst
this is a very subjective measurement, and in fact largely a
function of the document in question, the approximation is made
that one can define this "degradation" figure as a function of
merely the representation involved.
The next assumption is that there the other cost to the user of
viewing the doument is a function of the time taken for
presentation. We first assume that the cost is linear in time, and
then assume that the time is linear in the size of the message.
The final net value to the user can therefore be written
presented_value = initial_value * total-degradation - a - b *
size
T. Berners-Lee 27
for a document in a given incoming represenattion. Suppose we
normalize the initial value of the document to be 1. The total
degradation is the product of any degradation due to conversions
internal to the server, and the degradation "d" sent in the Accept
field. The values of a and b are also sent by the protocol. If not
sent, they default to no cost (d=1, a=b=0).
The server may optimize the presented value for the user when
deciding what to return. The hope is that fine decisions will not
have to be made, as in most cases the results for different formats
will be very different, and there will be a clear winner.
See also: Design issues discussions around this point.
Tim BL
Note: The cost of retrieval time
The assumption that the cost to the user associated with a certain
retrieval time is linear in that time is wildly innaccurate. The
real function could be very dependent on circumstances (like go to
infinity at a deadline).
A better general approximation might be logarithmic for large time
delays, and linear for small ones, like a*log(b*t-1) which has two
parameters.
REGISTRATION AUTHORITY
The HTTP Registration Authority is responsible for maintaining
lists of:
Charge account name spaces (see ChargeTo: field above)
Authorization schemes (see Authorization: field above)
Data format names (as MIME Content-Types)
Data encoding names (as MIME Content-Encoding))
It is proposed that the Internet Assigned Numbers Authority or
their successors take this role.
Unregistered values may be used for experimental purposes if they
are start with "X-".
SECURITY CONSIDERATIONS
The HTTP protocol allows requests to communication to a remote
server machine, and all the expetant security considerations for
client-server systems apply, including
Authentication of requests
T. Berners-Lee 28
Authenticationtion of servers
Privacy of request and response
Abuse of server features
Abuse of servers by exploiting server bugs
Unwitting actions on the net
Abuse of log information
The bulk of these are well known problems, tackled in part by some
featured of this protocol. Some aspects particular to HTTP are
mentioned below.
Unwitting actions on the net
The writers of client software should be aware that the software
represents the user in his interactions over the net, and should be
careful to allow the user to be aware of any actions he may take
which may be taken as having an unexpected significance by others.
TCP PORT NUMBERS
Clients should prompt a user before allowing HTTP access to
reserved ports other than the port resrverd for HTTP (port 80).
Otherwise, the user may unwittingly cause a transaction to occur in
some other (present or future) protocol.
IDEMPOTENT METHODS
The convention should be established that the GET and HEAD methods
never have the significance of taking an action. The link "click
here to subscribe" causing the reading of a special "magic"
document is open to abuse by others making a link "click here to
see a pretty picture". These methods should be considered "safe"
and should not have side effects. This allows the client software
to represent other, methods (such as POST) in a special way, so
that the use is aware of the fact that an action is being
requested.
Abuse of log information
A server is in the position to save large amount of personal data
about information requested by different readers. This information
is clearly confidential in nature, and its handling may be
constrained by law in certain countries. Server providers shall
ensure that such material is not distributed without the
permission of any people or groups of people mentioned in the
results published.
T. Berners-Lee 29
A feature which increases the amount of personal data transferred
is the Referer: field. This allow reading patters to be studied,
reverse links drawn, and so is very useful. Its power can be
abused of course if user details are not separated from the
Referee-Referer pairs. Even when the personal information has been
removed, the Referer field may in fact be a secure document's URI,
whose revelation itself is breach of security. A method of
suppressing the Referer information in such cases may be the
subject of further study.
REFERENCES
RFC 822 "Standard for ARPA Internet Text Messages".
David H. Crocker, describes Internet mail
message fromat.
RFC850 "Standard for Interchange of USENET
Messages" This RFC uses some field names in
common with this specification, and is
relevant reading.
RFC977 "Network News Transfer Protocol", Kantor and
Lampsley.
RFC 1341 Multipurpose Internet Mail Extensions
(MIME), Nathaniel Borenstien and Ned Freed,
Internet RFC 1341, 1992. Now obsoleted by
RFC1521:
RFC 1521 MIME. Not available in hypertext form yet.
RFC 1522 K Moore. "MIME: Part Two: Message Header
Extensions for Non-ASCII Text".
RFC1523 MIME text/enriched.
RFC 1524 MIME...
URL Universal Resource Locators. RFCxxx.
Currently available by anonymous FTP from
info.cern.ch as /pub/ietf/url3.{ps,txt}.
MIME and PEM Internet Draft only
OBJECT CONTENTS
The data (if any) sent with an HTTP request or reply is in a format
and encoding defined by the object header fields, the default being
"plain/text" type with "8bit" encoding. Note that while all the
other information in the request (just as in the reply) is in ISO
Latin1 with lines delimited by Carriage Return/Line Feed pairs, the
data may contain 8-bit binary data.
T. Berners-Lee 30
Termination
The delimiting of the message is determined by the Content-Length:
field. If this is present, then the message contains the specified
number of bytes.
Failing that, the content-type filed may contain a "bounday"
attribute which gives the boundary string with exacly the same
syntax as for a MIME multipart message.
Failing either of the above conditions, the data is terminated by
the closing of the connection by the sending party. Note that this
method can not be used for data sent with the request.
See also: note on server tolerance for back-compatibility, etc.
T. Berners-Lee 31
