rfc1738.txt

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Network Working Group                                     T. Berners-Lee
Request for Comments: 1738                                          CERN
Category: Standards Track                                    L. Masinter
                                                       Xerox Corporation
                                                             M. McCahill
                                                 University of Minnesota
                                                                 Editors
                                                           December 1994


                    Uniform Resource Locators (URL)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   This document specifies a Uniform Resource Locator (URL), the syntax
   and semantics of formalized information for location and access of
   resources via the Internet.

1. Introduction

   This document describes the syntax and semantics for a compact string
   representation for a resource available via the Internet.  These
   strings are called "Uniform Resource Locators" (URLs).

   The specification is derived from concepts introduced by the World-
   Wide Web global information initiative, whose use of such objects
   dates from 1990 and is described in "Universal Resource Identifiers
   in WWW", RFC 1630. The specification of URLs is designed to meet the
   requirements laid out in "Functional Requirements for Internet
   Resource Locators" [12].

   This document was written by the URI working group of the Internet
   Engineering Task Force.  Comments may be addressed to the editors, or
   to the URI-WG <[e-mail]>. Discussions of the group are archived
   at <URL:http://www.acl.lanl.gov/URI/archive/uri-archive.index.html>








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2. General URL Syntax

   Just as there are many different methods of access to resources,
   there are several schemes for describing the location of such
   resources.

   The generic syntax for URLs provides a framework for new schemes to
   be established using protocols other than those defined in this
   document.

   URLs are used to `locate' resources, by providing an abstract
   identification of the resource location.  Having located a resource,
   a system may perform a variety of operations on the resource, as
   might be characterized by such words as `access', `update',
   `replace', `find attributes'. In general, only the `access' method
   needs to be specified for any URL scheme.

2.1. The main parts of URLs

   A full BNF description of the URL syntax is given in Section 5.

   In general, URLs are written as follows:

       <scheme>:<scheme-specific-part>

   A URL contains the name of the scheme being used (<scheme>) followed
   by a colon and then a string (the <scheme-specific-part>) whose
   interpretation depends on the scheme.

   Scheme names consist of a sequence of characters. The lower case
   letters "a"--"z", digits, and the characters plus ("+"), period
   ("."), and hyphen ("-") are allowed. For resiliency, programs
   interpreting URLs should treat upper case letters as equivalent to
   lower case in scheme names (e.g., allow "HTTP" as well as "http").

2.2. URL Character Encoding Issues

   URLs are sequences of characters, i.e., letters, digits, and special
   characters. A URLs may be represented in a variety of ways: e.g., ink
   on paper, or a sequence of octets in a coded character set. The
   interpretation of a URL depends only on the identity of the
   characters used.

   In most URL schemes, the sequences of characters in different parts
   of a URL are used to represent sequences of octets used in Internet
   protocols. For example, in the ftp scheme, the host name, directory
   name and file names are such sequences of octets, represented by
   parts of the URL.  Within those parts, an octet may be represented by



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RFC 1738            Uniform Resource Locators (URL)        December 1994


   the chararacter which has that octet as its code within the US-ASCII
   [20] coded character set.

   In addition, octets may be encoded by a character triplet consisting
   of the character "%" followed by the two hexadecimal digits (from
   "0123456789ABCDEF") which forming the hexadecimal value of the octet.
   (The characters "abcdef" may also be used in hexadecimal encodings.)

   Octets must be encoded if they have no corresponding graphic
   character within the US-ASCII coded character set, if the use of the
   corresponding character is unsafe, or if the corresponding character
   is reserved for some other interpretation within the particular URL
   scheme.

   No corresponding graphic US-ASCII:

   URLs are written only with the graphic printable characters of the
   US-ASCII coded character set. The octets 80-FF hexadecimal are not
   used in US-ASCII, and the octets 00-1F and 7F hexadecimal represent
   control characters; these must be encoded.

   Unsafe:

   Characters can be unsafe for a number of reasons.  The space
   character is unsafe because significant spaces may disappear and
   insignificant spaces may be introduced when URLs are transcribed or
   typeset or subjected to the treatment of word-processing programs.
   The characters "<" and ">" are unsafe because they are used as the
   delimiters around URLs in free text; the quote mark (""") is used to
   delimit URLs in some systems.  The character "#" is unsafe and should
   always be encoded because it is used in World Wide Web and in other
   systems to delimit a URL from a fragment/anchor identifier that might
   follow it.  The character "%" is unsafe because it is used for
   encodings of other characters.  Other characters are unsafe because
   gateways and other transport agents are known to sometimes modify
   such characters. These characters are "{", "}", "|", "\", "^", "~",
   "[", "]", and "`".

   All unsafe characters must always be encoded within a URL. For
   example, the character "#" must be encoded within URLs even in
   systems that do not normally deal with fragment or anchor
   identifiers, so that if the URL is copied into another system that
   does use them, it will not be necessary to change the URL encoding.








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   Reserved:

   Many URL schemes reserve certain characters for a special meaning:
   their appearance in the scheme-specific part of the URL has a
   designated semantics. If the character corresponding to an octet is
   reserved in a scheme, the octet must be encoded.  The characters ";",
   "/", "?", ":", "@", "=" and "&" are the characters which may be
   reserved for special meaning within a scheme. No other characters may
   be reserved within a scheme.

   Usually a URL has the same interpretation when an octet is
   represented by a character and when it encoded. However, this is not
   true for reserved characters: encoding a character reserved for a
   particular scheme may change the semantics of a URL.

   Thus, only alphanumerics, the special characters "$-_.+!*'(),", and
   reserved characters used for their reserved purposes may be used
   unencoded within a URL.

   On the other hand, characters that are not required to be encoded
   (including alphanumerics) may be encoded within the scheme-specific
   part of a URL, as long as they are not being used for a reserved
   purpose.

2.3 Hierarchical schemes and relative links

   In some cases, URLs are used to locate resources that contain
   pointers to other resources. In some cases, those pointers are
   represented as relative links where the expression of the location of
   the second resource is in terms of "in the same place as this one
   except with the following relative path". Relative links are not
   described in this document. However, the use of relative links
   depends on the original URL containing a hierarchical structure
   against which the relative link is based.

   Some URL schemes (such as the ftp, http, and file schemes) contain
   names that can be considered hierarchical; the components of the
   hierarchy are separated by "/".













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RFC 1738            Uniform Resource Locators (URL)        December 1994


3. Specific Schemes

   The mapping for some existing standard and experimental protocols is
   outlined in the BNF syntax definition.  Notes on particular protocols
   follow. The schemes covered are:

   ftp                     File Transfer protocol
   http                    Hypertext Transfer Protocol
   gopher                  The Gopher protocol
   mailto                  Electronic mail address
   news                    USENET news
   nntp                    USENET news using NNTP access
   telnet                  Reference to interactive sessions