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rfc:rfc2396

Network Working Group T. Berners-Lee Request for Comments: 2396 MIT/LCS Updates: 1808, 1738 R. Fielding Category: Standards Track U.C. Irvine

                                                           L. Masinter
                                                     Xerox Corporation
                                                           August 1998
         Uniform Resource Identifiers (URI): Generic Syntax

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.

Copyright Notice

 Copyright (C) The Internet Society (1998).  All Rights Reserved.

IESG Note

 This paper describes a "superset" of operations that can be applied
 to URI.  It consists of both a grammar and a description of basic
 functionality for URI.  To understand what is a valid URI, both the
 grammar and the associated description have to be studied.  Some of
 the functionality described is not applicable to all URI schemes, and
 some operations are only possible when certain media types are
 retrieved using the URI, regardless of the scheme used.

Abstract

 A Uniform Resource Identifier (URI) is a compact string of characters
 for identifying an abstract or physical resource.  This document
 defines the generic syntax of URI, including both absolute and
 relative forms, and guidelines for their use; it revises and replaces
 the generic definitions in RFC 1738 and RFC 1808.
 This document defines a grammar that is a superset of all valid URI,
 such that an implementation can parse the common components of a URI
 reference without knowing the scheme-specific requirements of every
 possible identifier type.  This document does not define a generative
 grammar for URI; that task will be performed by the individual
 specifications of each URI scheme.

Berners-Lee, et. al. Standards Track [Page 1] RFC 2396 URI Generic Syntax August 1998

1. Introduction

 Uniform Resource Identifiers (URI) provide a simple and extensible
 means for identifying a resource.  This specification of URI syntax
 and semantics 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"
 [RFC1630].  The specification of URI is designed to meet the
 recommendations laid out in "Functional Recommendations for Internet
 Resource Locators" [RFC1736] and "Functional Requirements for Uniform
 Resource Names" [RFC1737].
 This document updates and merges "Uniform Resource Locators"
 [RFC1738] and "Relative Uniform Resource Locators" [RFC1808] in order
 to define a single, generic syntax for all URI.  It excludes those
 portions of RFC 1738 that defined the specific syntax of individual
 URL schemes; those portions will be updated as separate documents, as
 will the process for registration of new URI schemes.  This document
 does not discuss the issues and recommendation for dealing with
 characters outside of the US-ASCII character set [ASCII]; those
 recommendations are discussed in a separate document.
 All significant changes from the prior RFCs are noted in Appendix G.

1.1 Overview of URI

 URI are characterized by the following definitions:
    Uniform
       Uniformity provides several benefits: it allows different types
       of resource identifiers to be used in the same context, even
       when the mechanisms used to access those resources may differ;
       it allows uniform semantic interpretation of common syntactic
       conventions across different types of resource identifiers; it
       allows introduction of new types of resource identifiers
       without interfering with the way that existing identifiers are
       used; and, it allows the identifiers to be reused in many
       different contexts, thus permitting new applications or
       protocols to leverage a pre-existing, large, and widely-used
       set of resource identifiers.
    Resource
       A resource can be anything that has identity.  Familiar
       examples include an electronic document, an image, a service
       (e.g., "today's weather report for Los Angeles"), and a
       collection of other resources.  Not all resources are network
       "retrievable"; e.g., human beings, corporations, and bound
       books in a library can also be considered resources.

Berners-Lee, et. al. Standards Track [Page 2] RFC 2396 URI Generic Syntax August 1998

       The resource is the conceptual mapping to an entity or set of
       entities, not necessarily the entity which corresponds to that
       mapping at any particular instance in time.  Thus, a resource
       can remain constant even when its content---the entities to
       which it currently corresponds---changes over time, provided
       that the conceptual mapping is not changed in the process.
    Identifier
       An identifier is an object that can act as a reference to
       something that has identity.  In the case of URI, the object is
       a sequence of characters with a restricted syntax.
 Having identified a resource, a system may perform a variety of
 operations on the resource, as might be characterized by such words
 as `access', `update', `replace', or `find attributes'.

1.2. URI, URL, and URN

 A URI can be further classified as a locator, a name, or both.  The
 term "Uniform Resource Locator" (URL) refers to the subset of URI
 that identify resources via a representation of their primary access
 mechanism (e.g., their network "location"), rather than identifying
 the resource by name or by some other attribute(s) of that resource.
 The term "Uniform Resource Name" (URN) refers to the subset of URI
 that are required to remain globally unique and persistent even when
 the resource ceases to exist or becomes unavailable.
 The URI scheme (Section 3.1) defines the namespace of the URI, and
 thus may further restrict the syntax and semantics of identifiers
 using that scheme.  This specification defines those elements of the
 URI syntax that are either required of all URI schemes or are common
 to many URI schemes.  It thus defines the syntax and semantics that
 are needed to implement a scheme-independent parsing mechanism for
 URI references, such that the scheme-dependent handling of a URI can
 be postponed until the scheme-dependent semantics are needed.  We use
 the term URL below when describing syntax or semantics that only
 apply to locators.
 Although many URL schemes are named after protocols, this does not
 imply that the only way to access the URL's resource is via the named
 protocol.  Gateways, proxies, caches, and name resolution services
 might be used to access some resources, independent of the protocol
 of their origin, and the resolution of some URL may require the use
 of more than one protocol (e.g., both DNS and HTTP are typically used
 to access an "http" URL's resource when it can't be found in a local
 cache).

Berners-Lee, et. al. Standards Track [Page 3] RFC 2396 URI Generic Syntax August 1998

 A URN differs from a URL in that it's primary purpose is persistent
 labeling of a resource with an identifier.  That identifier is drawn
 from one of a set of defined namespaces, each of which has its own
 set name structure and assignment procedures.  The "urn" scheme has
 been reserved to establish the requirements for a standardized URN
 namespace, as defined in "URN Syntax" [RFC2141] and its related
 specifications.
 Most of the examples in this specification demonstrate URL, since
 they allow the most varied use of the syntax and often have a
 hierarchical namespace.  A parser of the URI syntax is capable of
 parsing both URL and URN references as a generic URI; once the scheme
 is determined, the scheme-specific parsing can be performed on the
 generic URI components.  In other words, the URI syntax is a superset
 of the syntax of all URI schemes.

1.3. Example URI

 The following examples illustrate URI that are in common use.
 ftp://ftp.is.co.za/rfc/rfc1808.txt
    -- ftp scheme for File Transfer Protocol services
 gopher://spinaltap.micro.umn.edu/00/Weather/California/Los%20Angeles
    -- gopher scheme for Gopher and Gopher+ Protocol services
 http://www.math.uio.no/faq/compression-faq/part1.html
    -- http scheme for Hypertext Transfer Protocol services
 mailto:mduerst@ifi.unizh.ch
    -- mailto scheme for electronic mail addresses
 news:comp.infosystems.www.servers.unix
    -- news scheme for USENET news groups and articles
 telnet://melvyl.ucop.edu/
    -- telnet scheme for interactive services via the TELNET Protocol

1.4. Hierarchical URI and Relative Forms

 An absolute identifier refers to a resource independent of the
 context in which the identifier is used.  In contrast, a relative
 identifier refers to a resource by describing the difference within a
 hierarchical namespace between the current context and an absolute
 identifier of the resource.

Berners-Lee, et. al. Standards Track [Page 4] RFC 2396 URI Generic Syntax August 1998

 Some URI schemes support a hierarchical naming system, where the
 hierarchy of the name is denoted by a "/" delimiter separating the
 components in the scheme. This document defines a scheme-independent
 `relative' form of URI reference that can be used in conjunction with
 a `base' URI (of a hierarchical scheme) to produce another URI. The
 syntax of hierarchical URI is described in Section 3; the relative
 URI calculation is described in Section 5.

1.5. URI Transcribability

 The URI syntax was designed with global transcribability as one of
 its main concerns. A URI is a sequence of characters from a very
 limited set, i.e. the letters of the basic Latin alphabet, digits,
 and a few special characters.  A URI may be represented in a variety
 of ways: e.g., ink on paper, pixels on a screen, or a sequence of
 octets in a coded character set.  The interpretation of a URI depends
 only on the characters used and not how those characters are
 represented in a network protocol.
 The goal of transcribability can be described by a simple scenario.
 Imagine two colleagues, Sam and Kim, sitting in a pub at an
 international conference and exchanging research ideas.  Sam asks Kim
 for a location to get more information, so Kim writes the URI for the
 research site on a napkin.  Upon returning home, Sam takes out the
 napkin and types the URI into a computer, which then retrieves the
 information to which Kim referred.
 There are several design concerns revealed by the scenario:
    o  A URI is a sequence of characters, which is not always
       represented as a sequence of octets.
    o  A URI may be transcribed from a non-network source, and thus
       should consist of characters that are most likely to be able to
       be typed into a computer, within the constraints imposed by
       keyboards (and related input devices) across languages and
       locales.
    o  A URI often needs to be remembered by people, and it is easier
       for people to remember a URI when it consists of meaningful
       components.
 These design concerns are not always in alignment.  For example, it
 is often the case that the most meaningful name for a URI component
 would require characters that cannot be typed into some systems.  The
 ability to transcribe the resource identifier from one medium to
 another was considered more important than having its URI consist of
 the most meaningful of components.  In local and regional contexts

Berners-Lee, et. al. Standards Track [Page 5] RFC 2396 URI Generic Syntax August 1998

 and with improving technology, users might benefit from being able to
 use a wider range of characters; such use is not defined in this
 document.

1.6. Syntax Notation and Common Elements

 This document uses two conventions to describe and define the syntax
 for URI.  The first, called the layout form, is a general description
 of the order of components and component separators, as in
    <first>/<second>;<third>?<fourth>
 The component names are enclosed in angle-brackets and any characters
 outside angle-brackets are literal separators.  Whitespace should be
 ignored.  These descriptions are used informally and do not define
 the syntax requirements.
 The second convention is a BNF-like grammar, used to define the
 formal URI syntax.  The grammar is that of [RFC822], except that "|"
 is used to designate alternatives.  Briefly, rules are separated from
 definitions by an equal "=", indentation is used to continue a rule
 definition over more than one line, literals are quoted with "",
 parentheses "(" and ")" are used to group elements, optional elements
 are enclosed in "[" and "]" brackets, and elements may be preceded
 with <n>* to designate n or more repetitions of the following
 element; n defaults to 0.
 Unlike many specifications that use a BNF-like grammar to define the
 bytes (octets) allowed by a protocol, the URI grammar is defined in
 terms of characters.  Each literal in the grammar corresponds to the
 character it represents, rather than to the octet encoding of that
 character in any particular coded character set.  How a URI is
 represented in terms of bits and bytes on the wire is dependent upon
 the character encoding of the protocol used to transport it, or the
 charset of the document which contains it.
 The following definitions are common to many elements:
    alpha    = lowalpha | upalpha
    lowalpha = "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" | "i" |
               "j" | "k" | "l" | "m" | "n" | "o" | "p" | "q" | "r" |
               "s" | "t" | "u" | "v" | "w" | "x" | "y" | "z"
    upalpha  = "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" | "I" |
               "J" | "K" | "L" | "M" | "N" | "O" | "P" | "Q" | "R" |
               "S" | "T" | "U" | "V" | "W" | "X" | "Y" | "Z"

Berners-Lee, et. al. Standards Track [Page 6] RFC 2396 URI Generic Syntax August 1998

    digit    = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" |
               "8" | "9"
    alphanum = alpha | digit
 The complete URI syntax is collected in Appendix A.

2. URI Characters and Escape Sequences

 URI consist of a restricted set of characters, primarily chosen to
 aid transcribability and usability both in computer systems and in
 non-computer communications. Characters used conventionally as
 delimiters around URI were excluded.  The restricted set of
 characters consists of digits, letters, and a few graphic symbols
 were chosen from those common to most of the character encodings and
 input facilities available to Internet users.
    uric          = reserved | unreserved | escaped
 Within a URI, characters are either used as delimiters, or to
 represent strings of data (octets) within the delimited portions.
 Octets are either represented directly by a character (using the US-
 ASCII character for that octet [ASCII]) or by an escape encoding.
 This representation is elaborated below.

2.1 URI and non-ASCII characters

 The relationship between URI and characters has been a source of
 confusion for characters that are not part of US-ASCII. To describe
 the relationship, it is useful to distinguish between a "character"
 (as a distinguishable semantic entity) and an "octet" (an 8-bit
 byte). There are two mappings, one from URI characters to octets, and
 a second from octets to original characters:
 URI character sequence->octet sequence->original character sequence
 A URI is represented as a sequence of characters, not as a sequence
 of octets. That is because URI might be "transported" by means that
 are not through a computer network, e.g., printed on paper, read over
 the radio, etc.
 A URI scheme may define a mapping from URI characters to octets;
 whether this is done depends on the scheme. Commonly, within a
 delimited component of a URI, a sequence of characters may be used to
 represent a sequence of octets. For example, the character "a"
 represents the octet 97 (decimal), while the character sequence "%",
 "0", "a" represents the octet 10 (decimal).

Berners-Lee, et. al. Standards Track [Page 7] RFC 2396 URI Generic Syntax August 1998

 There is a second translation for some resources: the sequence of
 octets defined by a component of the URI is subsequently used to
 represent a sequence of characters. A 'charset' defines this mapping.
 There are many charsets in use in Internet protocols. For example,
 UTF-8 [UTF-8] defines a mapping from sequences of octets to sequences
 of characters in the repertoire of ISO 10646.
 In the simplest case, the original character sequence contains only
 characters that are defined in US-ASCII, and the two levels of
 mapping are simple and easily invertible: each 'original character'
 is represented as the octet for the US-ASCII code for it, which is,
 in turn, represented as either the US-ASCII character, or else the
 "%" escape sequence for that octet.
 For original character sequences that contain non-ASCII characters,
 however, the situation is more difficult. Internet protocols that
 transmit octet sequences intended to represent character sequences
 are expected to provide some way of identifying the charset used, if
 there might be more than one [RFC2277].  However, there is currently
 no provision within the generic URI syntax to accomplish this
 identification. An individual URI scheme may require a single
 charset, define a default charset, or provide a way to indicate the
 charset used.
 It is expected that a systematic treatment of character encoding
 within URI will be developed as a future modification of this
 specification.

2.2. Reserved Characters

 Many URI include components consisting of or delimited by, certain
 special characters.  These characters are called "reserved", since
 their usage within the URI component is limited to their reserved
 purpose.  If the data for a URI component would conflict with the
 reserved purpose, then the conflicting data must be escaped before
 forming the URI.
    reserved    = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+" |
                  "$" | ","
 The "reserved" syntax class above refers to those characters that are
 allowed within a URI, but which may not be allowed within a
 particular component of the generic URI syntax; they are used as
 delimiters of the components described in Section 3.

Berners-Lee, et. al. Standards Track [Page 8] RFC 2396 URI Generic Syntax August 1998

 Characters in the "reserved" set are not reserved in all contexts.
 The set of characters actually reserved within any given URI
 component is defined by that component. In general, a character is
 reserved if the semantics of the URI changes if the character is
 replaced with its escaped US-ASCII encoding.

2.3. Unreserved Characters

 Data characters that are allowed in a URI but do not have a reserved
 purpose are called unreserved.  These include upper and lower case
 letters, decimal digits, and a limited set of punctuation marks and
 symbols.
    unreserved  = alphanum | mark
    mark        = "-" | "_" | "." | "!" | "~" | "*" | "'" | "(" | ")"
 Unreserved characters can be escaped without changing the semantics
 of the URI, but this should not be done unless the URI is being used
 in a context that does not allow the unescaped character to appear.

2.4. Escape Sequences

 Data must be escaped if it does not have a representation using an
 unreserved character; this includes data that does not correspond to
 a printable character of the US-ASCII coded character set, or that
 corresponds to any US-ASCII character that is disallowed, as
 explained below.

2.4.1. Escaped Encoding

 An escaped octet is encoded as a character triplet, consisting of the
 percent character "%" followed by the two hexadecimal digits
 representing the octet code. For example, "%20" is the escaped
 encoding for the US-ASCII space character.
    escaped     = "%" hex hex
    hex         = digit | "A" | "B" | "C" | "D" | "E" | "F" |
                          "a" | "b" | "c" | "d" | "e" | "f"

2.4.2. When to Escape and Unescape

 A URI is always in an "escaped" form, since escaping or unescaping a
 completed URI might change its semantics.  Normally, the only time
 escape encodings can safely be made is when the URI is being created
 from its component parts; each component may have its own set of
 characters that are reserved, so only the mechanism responsible for
 generating or interpreting that component can determine whether or

Berners-Lee, et. al. Standards Track [Page 9] RFC 2396 URI Generic Syntax August 1998

 not escaping a character will change its semantics. Likewise, a URI
 must be separated into its components before the escaped characters
 within those components can be safely decoded.
 In some cases, data that could be represented by an unreserved
 character may appear escaped; for example, some of the unreserved
 "mark" characters are automatically escaped by some systems.  If the
 given URI scheme defines a canonicalization algorithm, then
 unreserved characters may be unescaped according to that algorithm.
 For example, "%7e" is sometimes used instead of "~" in an http URL
 path, but the two are equivalent for an http URL.
 Because the percent "%" character always has the reserved purpose of
 being the escape indicator, it must be escaped as "%25" in order to
 be used as data within a URI.  Implementers should be careful not to
 escape or unescape the same string more than once, since unescaping
 an already unescaped string might lead to misinterpreting a percent
 data character as another escaped character, or vice versa in the
 case of escaping an already escaped string.

2.4.3. Excluded US-ASCII Characters

 Although they are disallowed within the URI syntax, we include here a
 description of those US-ASCII characters that have been excluded and
 the reasons for their exclusion.
 The control characters in the US-ASCII coded character set are not
 used within a URI, both because they are non-printable and because
 they are likely to be misinterpreted by some control mechanisms.
 control     = <US-ASCII coded characters 00-1F and 7F hexadecimal>
 The space character is excluded because significant spaces may
 disappear and insignificant spaces may be introduced when URI are
 transcribed or typeset or subjected to the treatment of word-
 processing programs.  Whitespace is also used to delimit URI in many
 contexts.
 space       = <US-ASCII coded character 20 hexadecimal>
 The angle-bracket "<" and ">" and double-quote (") characters are
 excluded because they are often used as the delimiters around URI in
 text documents and protocol fields.  The character "#" is excluded
 because it is used to delimit a URI from a fragment identifier in URI
 references (Section 4). The percent character "%" is excluded because
 it is used for the encoding of escaped characters.
 delims      = "<" | ">" | "#" | "%" | <">

Berners-Lee, et. al. Standards Track [Page 10] RFC 2396 URI Generic Syntax August 1998

 Other characters are excluded because gateways and other transport
 agents are known to sometimes modify such characters, or they are
 used as delimiters.
 unwise      = "{" | "}" | "|" | "\" | "^" | "[" | "]" | "`"
 Data corresponding to excluded characters must be escaped in order to
 be properly represented within a URI.

3. URI Syntactic Components

 The URI syntax is dependent upon the scheme.  In general, absolute
 URI are written as follows:
    <scheme>:<scheme-specific-part>
 An absolute URI 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.
 The URI syntax does not require that the scheme-specific-part have
 any general structure or set of semantics which is common among all
 URI.  However, a subset of URI do share a common syntax for
 representing hierarchical relationships within the namespace.  This
 "generic URI" syntax consists of a sequence of four main components:
    <scheme>://<authority><path>?<query>
 each of which, except <scheme>, may be absent from a particular URI.
 For example, some URI schemes do not allow an <authority> component,
 and others do not use a <query> component.
    absoluteURI   = scheme ":" ( hier_part | opaque_part )
 URI that are hierarchical in nature use the slash "/" character for
 separating hierarchical components.  For some file systems, a "/"
 character (used to denote the hierarchical structure of a URI) is the
 delimiter used to construct a file name hierarchy, and thus the URI
 path will look similar to a file pathname.  This does NOT imply that
 the resource is a file or that the URI maps to an actual filesystem
 pathname.
    hier_part     = ( net_path | abs_path ) [ "?" query ]
    net_path      = "//" authority [ abs_path ]
    abs_path      = "/"  path_segments

Berners-Lee, et. al. Standards Track [Page 11] RFC 2396 URI Generic Syntax August 1998

 URI that do not make use of the slash "/" character for separating
 hierarchical components are considered opaque by the generic URI
 parser.
    opaque_part   = uric_no_slash *uric
    uric_no_slash = unreserved | escaped | ";" | "?" | ":" | "@" |
                    "&" | "=" | "+" | "$" | ","
 We use the term <path> to refer to both the <abs_path> and
 <opaque_part> constructs, since they are mutually exclusive for any
 given URI and can be parsed as a single component.

3.1. Scheme Component

 Just as there are many different methods of access to resources,
 there are a variety of schemes for identifying such resources.  The
 URI syntax consists of a sequence of components separated by reserved
 characters, with the first component defining the semantics for the
 remainder of the URI string.
 Scheme names consist of a sequence of characters beginning with a
 lower case letter and followed by any combination of lower case
 letters, digits, plus ("+"), period ("."), or hyphen ("-").  For
 resiliency, programs interpreting URI should treat upper case letters
 as equivalent to lower case in scheme names (e.g., allow "HTTP" as
 well as "http").
    scheme        = alpha *( alpha | digit | "+" | "-" | "." )
 Relative URI references are distinguished from absolute URI in that
 they do not begin with a scheme name.  Instead, the scheme is
 inherited from the base URI, as described in Section 5.2.

3.2. Authority Component

 Many URI schemes include a top hierarchical element for a naming
 authority, such that the namespace defined by the remainder of the
 URI is governed by that authority.  This authority component is
 typically defined by an Internet-based server or a scheme-specific
 registry of naming authorities.
    authority     = server | reg_name
 The authority component is preceded by a double slash "//" and is
 terminated by the next slash "/", question-mark "?", or by the end of
 the URI.  Within the authority component, the characters ";", ":",
 "@", "?", and "/" are reserved.

Berners-Lee, et. al. Standards Track [Page 12] RFC 2396 URI Generic Syntax August 1998

 An authority component is not required for a URI scheme to make use
 of relative references.  A base URI without an authority component
 implies that any relative reference will also be without an authority
 component.

3.2.1. Registry-based Naming Authority

 The structure of a registry-based naming authority is specific to the
 URI scheme, but constrained to the allowed characters for an
 authority component.
    reg_name      = 1*( unreserved | escaped | "$" | "," |
                        ";" | ":" | "@" | "&" | "=" | "+" )

3.2.2. Server-based Naming Authority

 URL schemes that involve the direct use of an IP-based protocol to a
 specified server on the Internet use a common syntax for the server
 component of the URI's scheme-specific data:
    <userinfo>@<host>:<port>
 where <userinfo> may consist of a user name and, optionally, scheme-
 specific information about how to gain authorization to access the
 server.  The parts "<userinfo>@" and ":<port>" may be omitted.
    server        = [ [ userinfo "@" ] hostport ]
 The user information, if present, is followed by a commercial at-sign
 "@".
    userinfo      = *( unreserved | escaped |
                       ";" | ":" | "&" | "=" | "+" | "$" | "," )
 Some URL schemes use the format "user:password" in the userinfo
 field. This practice is NOT RECOMMENDED, because the passing of
 authentication information in clear text (such as URI) has proven to
 be a security risk in almost every case where it has been used.
 The host is a domain name of a network host, or its IPv4 address as a
 set of four decimal digit groups separated by ".".  Literal IPv6
 addresses are not supported.
    hostport      = host [ ":" port ]
    host          = hostname | IPv4address
    hostname      = *( domainlabel "." ) toplabel [ "." ]
    domainlabel   = alphanum | alphanum *( alphanum | "-" ) alphanum
    toplabel      = alpha | alpha *( alphanum | "-" ) alphanum

Berners-Lee, et. al. Standards Track [Page 13] RFC 2396 URI Generic Syntax August 1998

    IPv4address   = 1*digit "." 1*digit "." 1*digit "." 1*digit
    port          = *digit
 Hostnames take the form described in Section 3 of [RFC1034] and
 Section 2.1 of [RFC1123]: a sequence of domain labels separated by
 ".", each domain label starting and ending with an alphanumeric
 character and possibly also containing "-" characters.  The rightmost
 domain label of a fully qualified domain name will never start with a
 digit, thus syntactically distinguishing domain names from IPv4
 addresses, and may be followed by a single "." if it is necessary to
 distinguish between the complete domain name and any local domain.
 To actually be "Uniform" as a resource locator, a URL hostname should
 be a fully qualified domain name.  In practice, however, the host
 component may be a local domain literal.
    Note: A suitable representation for including a literal IPv6
    address as the host part of a URL is desired, but has not yet been
    determined or implemented in practice.
 The port is the network port number for the server.  Most schemes
 designate protocols that have a default port number.  Another port
 number may optionally be supplied, in decimal, separated from the
 host by a colon.  If the port is omitted, the default port number is
 assumed.

3.3. Path Component

 The path component contains data, specific to the authority (or the
 scheme if there is no authority component), identifying the resource
 within the scope of that scheme and authority.
    path          = [ abs_path | opaque_part ]
    path_segments = segment *( "/" segment )
    segment       = *pchar *( ";" param )
    param         = *pchar
    pchar         = unreserved | escaped |
                    ":" | "@" | "&" | "=" | "+" | "$" | ","
 The path may consist of a sequence of path segments separated by a
 single slash "/" character.  Within a path segment, the characters
 "/", ";", "=", and "?" are reserved.  Each path segment may include a
 sequence of parameters, indicated by the semicolon ";" character.
 The parameters are not significant to the parsing of relative
 references.

Berners-Lee, et. al. Standards Track [Page 14] RFC 2396 URI Generic Syntax August 1998

3.4. Query Component

 The query component is a string of information to be interpreted by
 the resource.
    query         = *uric
 Within a query component, the characters ";", "/", "?", ":", "@",
 "&", "=", "+", ",", and "$" are reserved.

4. URI References

 The term "URI-reference" is used here to denote the common usage of a
 resource identifier.  A URI reference may be absolute or relative,
 and may have additional information attached in the form of a
 fragment identifier.  However, "the URI" that results from such a
 reference includes only the absolute URI after the fragment
 identifier (if any) is removed and after any relative URI is resolved
 to its absolute form.  Although it is possible to limit the
 discussion of URI syntax and semantics to that of the absolute
 result, most usage of URI is within general URI references, and it is
 impossible to obtain the URI from such a reference without also
 parsing the fragment and resolving the relative form.
    URI-reference = [ absoluteURI | relativeURI ] [ "#" fragment ]
 The syntax for relative URI is a shortened form of that for absolute
 URI, where some prefix of the URI is missing and certain path
 components ("." and "..") have a special meaning when, and only when,
 interpreting a relative path.  The relative URI syntax is defined in
 Section 5.

4.1. Fragment Identifier

 When a URI reference is used to perform a retrieval action on the
 identified resource, the optional fragment identifier, separated from
 the URI by a crosshatch ("#") character, consists of additional
 reference information to be interpreted by the user agent after the
 retrieval action has been successfully completed.  As such, it is not
 part of a URI, but is often used in conjunction with a URI.
    fragment      = *uric
 The semantics of a fragment identifier is a property of the data
 resulting from a retrieval action, regardless of the type of URI used
 in the reference.  Therefore, the format and interpretation of
 fragment identifiers is dependent on the media type [RFC2046] of the
 retrieval result.  The character restrictions described in Section 2

Berners-Lee, et. al. Standards Track [Page 15] RFC 2396 URI Generic Syntax August 1998

 for URI also apply to the fragment in a URI-reference.  Individual
 media types may define additional restrictions or structure within
 the fragment for specifying different types of "partial views" that
 can be identified within that media type.
 A fragment identifier is only meaningful when a URI reference is
 intended for retrieval and the result of that retrieval is a document
 for which the identified fragment is consistently defined.

4.2. Same-document References

 A URI reference that does not contain a URI is a reference to the
 current document.  In other words, an empty URI reference within a
 document is interpreted as a reference to the start of that document,
 and a reference containing only a fragment identifier is a reference
 to the identified fragment of that document.  Traversal of such a
 reference should not result in an additional retrieval action.
 However, if the URI reference occurs in a context that is always
 intended to result in a new request, as in the case of HTML's FORM
 element, then an empty URI reference represents the base URI of the
 current document and should be replaced by that URI when transformed
 into a request.

4.3. Parsing a URI Reference

 A URI reference is typically parsed according to the four main
 components and fragment identifier in order to determine what
 components are present and whether the reference is relative or
 absolute.  The individual components are then parsed for their
 subparts and, if not opaque, to verify their validity.
 Although the BNF defines what is allowed in each component, it is
 ambiguous in terms of differentiating between an authority component
 and a path component that begins with two slash characters.  The
 greedy algorithm is used for disambiguation: the left-most matching
 rule soaks up as much of the URI reference string as it is capable of
 matching.  In other words, the authority component wins.
 Readers familiar with regular expressions should see Appendix B for a
 concrete parsing example and test oracle.

5. Relative URI References

 It is often the case that a group or "tree" of documents has been
 constructed to serve a common purpose; the vast majority of URI in
 these documents point to resources within the tree rather than

Berners-Lee, et. al. Standards Track [Page 16] RFC 2396 URI Generic Syntax August 1998

 outside of it.  Similarly, documents located at a particular site are
 much more likely to refer to other resources at that site than to
 resources at remote sites.
 Relative addressing of URI allows document trees to be partially
 independent of their location and access scheme.  For instance, it is
 possible for a single set of hypertext documents to be simultaneously
 accessible and traversable via each of the "file", "http", and "ftp"
 schemes if the documents refer to each other using relative URI.
 Furthermore, such document trees can be moved, as a whole, without
 changing any of the relative references.  Experience within the WWW
 has demonstrated that the ability to perform relative referencing is
 necessary for the long-term usability of embedded URI.
 The syntax for relative URI takes advantage of the <hier_part> syntax
 of <absoluteURI> (Section 3) in order to express a reference that is
 relative to the namespace of another hierarchical URI.
    relativeURI   = ( net_path | abs_path | rel_path ) [ "?" query ]
 A relative reference beginning with two slash characters is termed a
 network-path reference, as defined by <net_path> in Section 3.  Such
 references are rarely used.
 A relative reference beginning with a single slash character is
 termed an absolute-path reference, as defined by <abs_path> in
 Section 3.
 A relative reference that does not begin with a scheme name or a
 slash character is termed a relative-path reference.
    rel_path      = rel_segment [ abs_path ]
    rel_segment   = 1*( unreserved | escaped |
                        ";" | "@" | "&" | "=" | "+" | "$" | "," )
 Within a relative-path reference, the complete path segments "." and
 ".." have special meanings: "the current hierarchy level" and "the
 level above this hierarchy level", respectively.  Although this is
 very similar to their use within Unix-based filesystems to indicate
 directory levels, these path components are only considered special
 when resolving a relative-path reference to its absolute form
 (Section 5.2).
 Authors should be aware that a path segment which contains a colon
 character cannot be used as the first segment of a relative URI path
 (e.g., "this:that"), because it would be mistaken for a scheme name.

Berners-Lee, et. al. Standards Track [Page 17] RFC 2396 URI Generic Syntax August 1998

 It is therefore necessary to precede such segments with other
 segments (e.g., "./this:that") in order for them to be referenced as
 a relative path.
 It is not necessary for all URI within a given scheme to be
 restricted to the <hier_part> syntax, since the hierarchical
 properties of that syntax are only necessary when relative URI are
 used within a particular document.  Documents can only make use of
 relative URI when their base URI fits within the <hier_part> syntax.
 It is assumed that any document which contains a relative reference
 will also have a base URI that obeys the syntax.  In other words,
 relative URI cannot be used within a document that has an unsuitable
 base URI.
 Some URI schemes do not allow a hierarchical syntax matching the
 <hier_part> syntax, and thus cannot use relative references.

5.1. Establishing a Base URI

 The term "relative URI" implies that there exists some absolute "base
 URI" against which the relative reference is applied.  Indeed, the
 base URI is necessary to define the semantics of any relative URI
 reference; without it, a relative reference is meaningless.  In order
 for relative URI to be usable within a document, the base URI of that
 document must be known to the parser.
 The base URI of a document can be established in one of four ways,
 listed below in order of precedence.  The order of precedence can be
 thought of in terms of layers, where the innermost defined base URI
 has the highest precedence.  This can be visualized graphically as:
    .----------------------------------------------------------.
    |  .----------------------------------------------------.  |
    |  |  .----------------------------------------------.  |  |
    |  |  |  .----------------------------------------.  |  |  |
    |  |  |  |  .----------------------------------.  |  |  |  |
    |  |  |  |  |       <relative_reference>       |  |  |  |  |
    |  |  |  |  `----------------------------------'  |  |  |  |
    |  |  |  | (5.1.1) Base URI embedded in the       |  |  |  |
    |  |  |  |         document's content             |  |  |  |
    |  |  |  `----------------------------------------'  |  |  |
    |  |  | (5.1.2) Base URI of the encapsulating entity |  |  |
    |  |  |         (message, document, or none).        |  |  |
    |  |  `----------------------------------------------'  |  |
    |  | (5.1.3) URI used to retrieve the entity            |  |
    |  `----------------------------------------------------'  |
    | (5.1.4) Default Base URI is application-dependent        |
    `----------------------------------------------------------'

Berners-Lee, et. al. Standards Track [Page 18] RFC 2396 URI Generic Syntax August 1998

5.1.1. Base URI within Document Content

 Within certain document media types, the base URI of the document can
 be embedded within the content itself such that it can be readily
 obtained by a parser.  This can be useful for descriptive documents,
 such as tables of content, which may be transmitted to others through
 protocols other than their usual retrieval context (e.g., E-Mail or
 USENET news).
 It is beyond the scope of this document to specify how, for each
 media type, the base URI can be embedded.  It is assumed that user
 agents manipulating such media types will be able to obtain the
 appropriate syntax from that media type's specification.  An example
 of how the base URI can be embedded in the Hypertext Markup Language
 (HTML) [RFC1866] is provided in Appendix D.
 A mechanism for embedding the base URI within MIME container types
 (e.g., the message and multipart types) is defined by MHTML
 [RFC2110].  Protocols that do not use the MIME message header syntax,
 but which do allow some form of tagged metainformation to be included
 within messages, may define their own syntax for defining the base
 URI as part of a message.

5.1.2. Base URI from the Encapsulating Entity

 If no base URI is embedded, the base URI of a document is defined by
 the document's retrieval context.  For a document that is enclosed
 within another entity (such as a message or another document), the
 retrieval context is that entity; thus, the default base URI of the
 document is the base URI of the entity in which the document is
 encapsulated.

5.1.3. Base URI from the Retrieval URI

 If no base URI is embedded and the document is not encapsulated
 within some other entity (e.g., the top level of a composite entity),
 then, if a URI was used to retrieve the base document, that URI shall
 be considered the base URI.  Note that if the retrieval was the
 result of a redirected request, the last URI used (i.e., that which
 resulted in the actual retrieval of the document) is the base URI.

5.1.4. Default Base URI

 If none of the conditions described in Sections 5.1.1--5.1.3 apply,
 then the base URI is defined by the context of the application.
 Since this definition is necessarily application-dependent, failing

Berners-Lee, et. al. Standards Track [Page 19] RFC 2396 URI Generic Syntax August 1998

 to define the base URI using one of the other methods may result in
 the same content being interpreted differently by different types of
 application.
 It is the responsibility of the distributor(s) of a document
 containing relative URI to ensure that the base URI for that document
 can be established.  It must be emphasized that relative URI cannot
 be used reliably in situations where the document's base URI is not
 well-defined.

5.2. Resolving Relative References to Absolute Form

 This section describes an example algorithm for resolving URI
 references that might be relative to a given base URI.
 The base URI is established according to the rules of Section 5.1 and
 parsed into the four main components as described in Section 3.  Note
 that only the scheme component is required to be present in the base
 URI; the other components may be empty or undefined.  A component is
 undefined if its preceding separator does not appear in the URI
 reference; the path component is never undefined, though it may be
 empty.  The base URI's query component is not used by the resolution
 algorithm and may be discarded.
 For each URI reference, the following steps are performed in order:
 1) The URI reference is parsed into the potential four components and
    fragment identifier, as described in Section 4.3.
 2) If the path component is empty and the scheme, authority, and
    query components are undefined, then it is a reference to the
    current document and we are done.  Otherwise, the reference URI's
    query and fragment components are defined as found (or not found)
    within the URI reference and not inherited from the base URI.
 3) If the scheme component is defined, indicating that the reference
    starts with a scheme name, then the reference is interpreted as an
    absolute URI and we are done.  Otherwise, the reference URI's
    scheme is inherited from the base URI's scheme component.
    Due to a loophole in prior specifications [RFC1630], some parsers
    allow the scheme name to be present in a relative URI if it is the
    same as the base URI scheme.  Unfortunately, this can conflict
    with the correct parsing of non-hierarchical URI.  For backwards
    compatibility, an implementation may work around such references
    by removing the scheme if it matches that of the base URI and the
    scheme is known to always use the <hier_part> syntax.  The parser

Berners-Lee, et. al. Standards Track [Page 20] RFC 2396 URI Generic Syntax August 1998

    can then continue with the steps below for the remainder of the
    reference components.  Validating parsers should mark such a
    misformed relative reference as an error.
 4) If the authority component is defined, then the reference is a
    network-path and we skip to step 7.  Otherwise, the reference
    URI's authority is inherited from the base URI's authority
    component, which will also be undefined if the URI scheme does not
    use an authority component.
 5) If the path component begins with a slash character ("/"), then
    the reference is an absolute-path and we skip to step 7.
 6) If this step is reached, then we are resolving a relative-path
    reference.  The relative path needs to be merged with the base
    URI's path.  Although there are many ways to do this, we will
    describe a simple method using a separate string buffer.
    a) All but the last segment of the base URI's path component is
       copied to the buffer.  In other words, any characters after the
       last (right-most) slash character, if any, are excluded.
    b) The reference's path component is appended to the buffer
       string.
    c) All occurrences of "./", where "." is a complete path segment,
       are removed from the buffer string.
    d) If the buffer string ends with "." as a complete path segment,
       that "." is removed.
    e) All occurrences of "<segment>/../", where <segment> is a
       complete path segment not equal to "..", are removed from the
       buffer string.  Removal of these path segments is performed
       iteratively, removing the leftmost matching pattern on each
       iteration, until no matching pattern remains.
    f) If the buffer string ends with "<segment>/..", where <segment>
       is a complete path segment not equal to "..", that
       "<segment>/.." is removed.
    g) If the resulting buffer string still begins with one or more
       complete path segments of "..", then the reference is
       considered to be in error.  Implementations may handle this
       error by retaining these components in the resolved path (i.e.,
       treating them as part of the final URI), by removing them from
       the resolved path (i.e., discarding relative levels above the
       root), or by avoiding traversal of the reference.

Berners-Lee, et. al. Standards Track [Page 21] RFC 2396 URI Generic Syntax August 1998

    h) The remaining buffer string is the reference URI's new path
       component.
 7) The resulting URI components, including any inherited from the
    base URI, are recombined to give the absolute form of the URI
    reference.  Using pseudocode, this would be
       result = ""
       if scheme is defined then
           append scheme to result
           append ":" to result
       if authority is defined then
           append "//" to result
           append authority to result
       append path to result
       if query is defined then
           append "?" to result
           append query to result
       if fragment is defined then
           append "#" to result
           append fragment to result
       return result
    Note that we must be careful to preserve the distinction between a
    component that is undefined, meaning that its separator was not
    present in the reference, and a component that is empty, meaning
    that the separator was present and was immediately followed by the
    next component separator or the end of the reference.
 The above algorithm is intended to provide an example by which the
 output of implementations can be tested -- implementation of the
 algorithm itself is not required.  For example, some systems may find
 it more efficient to implement step 6 as a pair of segment stacks
 being merged, rather than as a series of string pattern replacements.
    Note: Some WWW client applications will fail to separate the
    reference's query component from its path component before merging
    the base and reference paths in step 6 above.  This may result in
    a loss of information if the query component contains the strings
    "/../" or "/./".
 Resolution examples are provided in Appendix C.

Berners-Lee, et. al. Standards Track [Page 22] RFC 2396 URI Generic Syntax August 1998

6. URI Normalization and Equivalence

 In many cases, different URI strings may actually identify the
 identical resource. For example, the host names used in URL are
 actually case insensitive, and the URL <http://www.XEROX.com> is
 equivalent to <http://www.xerox.com>. In general, the rules for
 equivalence and definition of a normal form, if any, are scheme
 dependent. When a scheme uses elements of the common syntax, it will
 also use the common syntax equivalence rules, namely that the scheme
 and hostname are case insensitive and a URL with an explicit ":port",
 where the port is the default for the scheme, is equivalent to one
 where the port is elided.

7. Security Considerations

 A URI does not in itself pose a security threat.  Users should beware
 that there is no general guarantee that a URL, which at one time
 located a given resource, will continue to do so.  Nor is there any
 guarantee that a URL will not locate a different resource at some
 later point in time, due to the lack of any constraint on how a given
 authority apportions its namespace.  Such a guarantee can only be
 obtained from the person(s) controlling that namespace and the
 resource in question.  A specific URI scheme may include additional
 semantics, such as name persistence, if those semantics are required
 of all naming authorities for that scheme.
 It is sometimes possible to construct a URL such that an attempt to
 perform a seemingly harmless, idempotent operation, such as the
 retrieval of an entity associated with the resource, will in fact
 cause a possibly damaging remote operation to occur.  The unsafe URL
 is typically constructed by specifying a port number other than that
 reserved for the network protocol in question.  The client
 unwittingly contacts a site that is in fact running a different
 protocol.  The content of the URL contains instructions that, when
 interpreted according to this other protocol, cause an unexpected
 operation.  An example has been the use of a gopher URL to cause an
 unintended or impersonating message to be sent via a SMTP server.
 Caution should be used when using any URL that specifies a port
 number other than the default for the protocol, especially when it is
 a number within the reserved space.
 Care should be taken when a URL contains escaped delimiters for a
 given protocol (for example, CR and LF characters for telnet
 protocols) that these are not unescaped before transmission.  This
 might violate the protocol, but avoids the potential for such

Berners-Lee, et. al. Standards Track [Page 23] RFC 2396 URI Generic Syntax August 1998

 characters to be used to simulate an extra operation or parameter in
 that protocol, which might lead to an unexpected and possibly harmful
 remote operation to be performed.
 It is clearly unwise to use a URL that contains a password which is
 intended to be secret. In particular, the use of a password within
 the 'userinfo' component of a URL is strongly disrecommended except
 in those rare cases where the 'password' parameter is intended to be
 public.

8. Acknowledgements

 This document was derived from RFC 1738 [RFC1738] and RFC 1808
 [RFC1808]; the acknowledgements in those specifications still apply.
 In addition, contributions by Gisle Aas, Martin Beet, Martin Duerst,
 Jim Gettys, Martijn Koster, Dave Kristol, Daniel LaLiberte, Foteos
 Macrides, James Marshall, Ryan Moats, Keith Moore, and Lauren Wood
 are gratefully acknowledged.

9. References

 [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
           Languages", BCP 18, RFC 2277, January 1998.
 [RFC1630] Berners-Lee, T., "Universal Resource Identifiers in WWW: A
           Unifying Syntax for the Expression of Names and Addresses
           of Objects on the Network as used in the World-Wide Web",
           RFC 1630, June 1994.
 [RFC1738] Berners-Lee, T., Masinter, L., and M. McCahill, Editors,
           "Uniform Resource Locators (URL)", RFC 1738, December 1994.
 [RFC1866] Berners-Lee T., and D. Connolly, "HyperText Markup Language
           Specification -- 2.0", RFC 1866, November 1995.
 [RFC1123] Braden, R., Editor, "Requirements for Internet Hosts --
           Application and Support", STD 3, RFC 1123, October 1989.
 [RFC822]  Crocker, D., "Standard for the Format of ARPA Internet Text
           Messages", STD 11, RFC 822, August 1982.
 [RFC1808] Fielding, R., "Relative Uniform Resource Locators", RFC
           1808, June 1995.
 [RFC2046] Freed, N., and N. Borenstein, "Multipurpose Internet Mail
           Extensions (MIME) Part Two: Media Types", RFC 2046,
           November 1996.

Berners-Lee, et. al. Standards Track [Page 24] RFC 2396 URI Generic Syntax August 1998

 [RFC1736] Kunze, J., "Functional Recommendations for Internet
           Resource Locators", RFC 1736, February 1995.
 [RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997.
 [RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities",
           STD 13, RFC 1034, November 1987.
 [RFC2110] Palme, J., and A. Hopmann, "MIME E-mail Encapsulation of
           Aggregate Documents, such as HTML (MHTML)", RFC 2110, March
           1997.
 [RFC1737] Sollins, K., and L. Masinter, "Functional Requirements for
           Uniform Resource Names", RFC 1737, December 1994.
 [ASCII]   US-ASCII. "Coded Character Set -- 7-bit American Standard
           Code for Information Interchange", ANSI X3.4-1986.
 [UTF-8]   Yergeau, F., "UTF-8, a transformation format of ISO 10646",
           RFC 2279, January 1998.

Berners-Lee, et. al. Standards Track [Page 25] RFC 2396 URI Generic Syntax August 1998

10. Authors' Addresses

 Tim Berners-Lee
 World Wide Web Consortium
 MIT Laboratory for Computer Science, NE43-356
 545 Technology Square
 Cambridge, MA 02139
 Fax: +1(617)258-8682
 EMail: timbl@w3.org
 Roy T. Fielding
 Department of Information and Computer Science
 University of California, Irvine
 Irvine, CA  92697-3425
 Fax: +1(949)824-1715
 EMail: fielding@ics.uci.edu
 Larry Masinter
 Xerox PARC
 3333 Coyote Hill Road
 Palo Alto, CA 94034
 Fax: +1(415)812-4333
 EMail: masinter@parc.xerox.com

Berners-Lee, et. al. Standards Track [Page 26] RFC 2396 URI Generic Syntax August 1998

A. Collected BNF for URI

    URI-reference = [ absoluteURI | relativeURI ] [ "#" fragment ]
    absoluteURI   = scheme ":" ( hier_part | opaque_part )
    relativeURI   = ( net_path | abs_path | rel_path ) [ "?" query ]
    hier_part     = ( net_path | abs_path ) [ "?" query ]
    opaque_part   = uric_no_slash *uric
    uric_no_slash = unreserved | escaped | ";" | "?" | ":" | "@" |
                    "&" | "=" | "+" | "$" | ","
    net_path      = "//" authority [ abs_path ]
    abs_path      = "/"  path_segments
    rel_path      = rel_segment [ abs_path ]
    rel_segment   = 1*( unreserved | escaped |
                        ";" | "@" | "&" | "=" | "+" | "$" | "," )
    scheme        = alpha *( alpha | digit | "+" | "-" | "." )
    authority     = server | reg_name
    reg_name      = 1*( unreserved | escaped | "$" | "," |
                        ";" | ":" | "@" | "&" | "=" | "+" )
    server        = [ [ userinfo "@" ] hostport ]
    userinfo      = *( unreserved | escaped |
                       ";" | ":" | "&" | "=" | "+" | "$" | "," )
    hostport      = host [ ":" port ]
    host          = hostname | IPv4address
    hostname      = *( domainlabel "." ) toplabel [ "." ]
    domainlabel   = alphanum | alphanum *( alphanum | "-" ) alphanum
    toplabel      = alpha | alpha *( alphanum | "-" ) alphanum
    IPv4address   = 1*digit "." 1*digit "." 1*digit "." 1*digit
    port          = *digit
    path          = [ abs_path | opaque_part ]
    path_segments = segment *( "/" segment )
    segment       = *pchar *( ";" param )
    param         = *pchar
    pchar         = unreserved | escaped |
                    ":" | "@" | "&" | "=" | "+" | "$" | ","
    query         = *uric
    fragment      = *uric

Berners-Lee, et. al. Standards Track [Page 27] RFC 2396 URI Generic Syntax August 1998

    uric          = reserved | unreserved | escaped
    reserved      = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+" |
                    "$" | ","
    unreserved    = alphanum | mark
    mark          = "-" | "_" | "." | "!" | "~" | "*" | "'" |
                    "(" | ")"
    escaped       = "%" hex hex
    hex           = digit | "A" | "B" | "C" | "D" | "E" | "F" |
                            "a" | "b" | "c" | "d" | "e" | "f"
    alphanum      = alpha | digit
    alpha         = lowalpha | upalpha
    lowalpha = "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" | "i" |
               "j" | "k" | "l" | "m" | "n" | "o" | "p" | "q" | "r" |
               "s" | "t" | "u" | "v" | "w" | "x" | "y" | "z"
    upalpha  = "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" | "I" |
               "J" | "K" | "L" | "M" | "N" | "O" | "P" | "Q" | "R" |
               "S" | "T" | "U" | "V" | "W" | "X" | "Y" | "Z"
    digit    = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" |
               "8" | "9"

Berners-Lee, et. al. Standards Track [Page 28] RFC 2396 URI Generic Syntax August 1998

B. Parsing a URI Reference with a Regular Expression

 As described in Section 4.3, the generic URI syntax is not sufficient
 to disambiguate the components of some forms of URI.  Since the
 "greedy algorithm" described in that section is identical to the
 disambiguation method used by POSIX regular expressions, it is
 natural and commonplace to use a regular expression for parsing the
 potential four components and fragment identifier of a URI reference.
 The following line is the regular expression for breaking-down a URI
 reference into its components.
    ^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))?
     12            3  4          5       6  7        8 9
 The numbers in the second line above are only to assist readability;
 they indicate the reference points for each subexpression (i.e., each
 paired parenthesis).  We refer to the value matched for subexpression
 <n> as $<n>.  For example, matching the above expression to
    http://www.ics.uci.edu/pub/ietf/uri/#Related
 results in the following subexpression matches:
    $1 = http:
    $2 = http
    $3 = //www.ics.uci.edu
    $4 = www.ics.uci.edu
    $5 = /pub/ietf/uri/
    $6 = <undefined>
    $7 = <undefined>
    $8 = #Related
    $9 = Related
 where <undefined> indicates that the component is not present, as is
 the case for the query component in the above example.  Therefore, we
 can determine the value of the four components and fragment as
    scheme    = $2
    authority = $4
    path      = $5
    query     = $7
    fragment  = $9
 and, going in the opposite direction, we can recreate a URI reference
 from its components using the algorithm in step 7 of Section 5.2.

Berners-Lee, et. al. Standards Track [Page 29] RFC 2396 URI Generic Syntax August 1998

C. Examples of Resolving Relative URI References

 Within an object with a well-defined base URI of
    http://a/b/c/d;p?q
 the relative URI would be resolved as follows:

C.1. Normal Examples

    g:h           =  g:h
    g             =  http://a/b/c/g
    ./g           =  http://a/b/c/g
    g/            =  http://a/b/c/g/
    /g            =  http://a/g
    //g           =  http://g
    ?y            =  http://a/b/c/?y
    g?y           =  http://a/b/c/g?y
    #s            =  (current document)#s
    g#s           =  http://a/b/c/g#s
    g?y#s         =  http://a/b/c/g?y#s
    ;x            =  http://a/b/c/;x
    g;x           =  http://a/b/c/g;x
    g;x?y#s       =  http://a/b/c/g;x?y#s
    .             =  http://a/b/c/
    ./            =  http://a/b/c/
    ..            =  http://a/b/
    ../           =  http://a/b/
    ../g          =  http://a/b/g
    ../..         =  http://a/
    ../../        =  http://a/
    ../../g       =  http://a/g

C.2. Abnormal Examples

 Although the following abnormal examples are unlikely to occur in
 normal practice, all URI parsers should be capable of resolving them
 consistently.  Each example uses the same base as above.
 An empty reference refers to the start of the current document.
    <>            =  (current document)
 Parsers must be careful in handling the case where there are more
 relative path ".." segments than there are hierarchical levels in the
 base URI's path.  Note that the ".." syntax cannot be used to change
 the authority component of a URI.

Berners-Lee, et. al. Standards Track [Page 30] RFC 2396 URI Generic Syntax August 1998

    ../../../g    =  http://a/../g
    ../../../../g =  http://a/../../g
 In practice, some implementations strip leading relative symbolic
 elements (".", "..") after applying a relative URI calculation, based
 on the theory that compensating for obvious author errors is better
 than allowing the request to fail.  Thus, the above two references
 will be interpreted as "http://a/g" by some implementations.
 Similarly, parsers must avoid treating "." and ".." as special when
 they are not complete components of a relative path.
    /./g          =  http://a/./g
    /../g         =  http://a/../g
    g.            =  http://a/b/c/g.
    .g            =  http://a/b/c/.g
    g..           =  http://a/b/c/g..
    ..g           =  http://a/b/c/..g
 Less likely are cases where the relative URI uses unnecessary or
 nonsensical forms of the "." and ".." complete path segments.
    ./../g        =  http://a/b/g
    ./g/.         =  http://a/b/c/g/
    g/./h         =  http://a/b/c/g/h
    g/../h        =  http://a/b/c/h
    g;x=1/./y     =  http://a/b/c/g;x=1/y
    g;x=1/../y    =  http://a/b/c/y
 All client applications remove the query component from the base URI
 before resolving relative URI.  However, some applications fail to
 separate the reference's query and/or fragment components from a
 relative path before merging it with the base path.  This error is
 rarely noticed, since typical usage of a fragment never includes the
 hierarchy ("/") character, and the query component is not normally
 used within relative references.
    g?y/./x       =  http://a/b/c/g?y/./x
    g?y/../x      =  http://a/b/c/g?y/../x
    g#s/./x       =  http://a/b/c/g#s/./x
    g#s/../x      =  http://a/b/c/g#s/../x

Berners-Lee, et. al. Standards Track [Page 31] RFC 2396 URI Generic Syntax August 1998

 Some parsers allow the scheme name to be present in a relative URI if
 it is the same as the base URI scheme.  This is considered to be a
 loophole in prior specifications of partial URI [RFC1630]. Its use
 should be avoided.
    http:g        =  http:g           ; for validating parsers
                  |  http://a/b/c/g   ; for backwards compatibility

Berners-Lee, et. al. Standards Track [Page 32] RFC 2396 URI Generic Syntax August 1998

D. Embedding the Base URI in HTML documents

 It is useful to consider an example of how the base URI of a document
 can be embedded within the document's content.  In this appendix, we
 describe how documents written in the Hypertext Markup Language
 (HTML) [RFC1866] can include an embedded base URI.  This appendix
 does not form a part of the URI specification and should not be
 considered as anything more than a descriptive example.
 HTML defines a special element "BASE" which, when present in the
 "HEAD" portion of a document, signals that the parser should use the
 BASE element's "HREF" attribute as the base URI for resolving any
 relative URI.  The "HREF" attribute must be an absolute URI.  Note
 that, in HTML, element and attribute names are case-insensitive.  For
 example:
    <!doctype html public "-//IETF//DTD HTML//EN">
    <HTML><HEAD>
    <TITLE>An example HTML document</TITLE>
    <BASE href="http://www.ics.uci.edu/Test/a/b/c">
    </HEAD><BODY>
    ... <A href="../x">a hypertext anchor</A> ...
    </BODY></HTML>
 A parser reading the example document should interpret the given
 relative URI "../x" as representing the absolute URI
    <http://www.ics.uci.edu/Test/a/x>
 regardless of the context in which the example document was obtained.

Berners-Lee, et. al. Standards Track [Page 33] RFC 2396 URI Generic Syntax August 1998

E. Recommendations for Delimiting URI in Context

 URI are often transmitted through formats that do not provide a clear
 context for their interpretation.  For example, there are many
 occasions when URI are included in plain text; examples include text
 sent in electronic mail, USENET news messages, and, most importantly,
 printed on paper.  In such cases, it is important to be able to
 delimit the URI from the rest of the text, and in particular from
 punctuation marks that might be mistaken for part of the URI.
 In practice, URI are delimited in a variety of ways, but usually
 within double-quotes "http://test.com/", angle brackets
 <http://test.com/>, or just using whitespace
                           http://test.com/
 These wrappers do not form part of the URI.
 In the case where a fragment identifier is associated with a URI
 reference, the fragment would be placed within the brackets as well
 (separated from the URI with a "#" character).
 In some cases, extra whitespace (spaces, linebreaks, tabs, etc.) may
 need to be added to break long URI across lines. The whitespace
 should be ignored when extracting the URI.
 No whitespace should be introduced after a hyphen ("-") character.
 Because some typesetters and printers may (erroneously) introduce a
 hyphen at the end of line when breaking a line, the interpreter of a
 URI containing a line break immediately after a hyphen should ignore
 all unescaped whitespace around the line break, and should be aware
 that the hyphen may or may not actually be part of the URI.
 Using <> angle brackets around each URI is especially recommended as
 a delimiting style for URI that contain whitespace.
 The prefix "URL:" (with or without a trailing space) was recommended
 as a way to used to help distinguish a URL from other bracketed
 designators, although this is not common in practice.
 For robustness, software that accepts user-typed URI should attempt
 to recognize and strip both delimiters and embedded whitespace.
 For example, the text:

Berners-Lee, et. al. Standards Track [Page 34] RFC 2396 URI Generic Syntax August 1998

    Yes, Jim, I found it under "http://www.w3.org/Addressing/",
    but you can probably pick it up from <ftp://ds.internic.
    net/rfc/>.  Note the warning in <http://www.ics.uci.edu/pub/
    ietf/uri/historical.html#WARNING>.
 contains the URI references
    http://www.w3.org/Addressing/
    ftp://ds.internic.net/rfc/
    http://www.ics.uci.edu/pub/ietf/uri/historical.html#WARNING

Berners-Lee, et. al. Standards Track [Page 35] RFC 2396 URI Generic Syntax August 1998

F. Abbreviated URLs

 The URL syntax was designed for unambiguous reference to network
 resources and extensibility via the URL scheme.  However, as URL
 identification and usage have become commonplace, traditional media
 (television, radio, newspapers, billboards, etc.) have increasingly
 used abbreviated URL references.  That is, a reference consisting of
 only the authority and path portions of the identified resource, such
 as
    www.w3.org/Addressing/
 or simply the DNS hostname on its own.  Such references are primarily
 intended for human interpretation rather than machine, with the
 assumption that context-based heuristics are sufficient to complete
 the URL (e.g., most hostnames beginning with "www" are likely to have
 a URL prefix of "http://").  Although there is no standard set of
 heuristics for disambiguating abbreviated URL references, many client
 implementations allow them to be entered by the user and
 heuristically resolved.  It should be noted that such heuristics may
 change over time, particularly when new URL schemes are introduced.
 Since an abbreviated URL has the same syntax as a relative URL path,
 abbreviated URL references cannot be used in contexts where relative
 URLs are expected.  This limits the use of abbreviated URLs to places
 where there is no defined base URL, such as dialog boxes and off-line
 advertisements.

Berners-Lee, et. al. Standards Track [Page 36] RFC 2396 URI Generic Syntax August 1998

G. Summary of Non-editorial Changes

G.1. Additions

 Section 4 (URI References) was added to stem the confusion regarding
 "what is a URI" and how to describe fragment identifiers given that
 they are not part of the URI, but are part of the URI syntax and
 parsing concerns.  In addition, it provides a reference definition
 for use by other IETF specifications (HTML, HTTP, etc.) that have
 previously attempted to redefine the URI syntax in order to account
 for the presence of fragment identifiers in URI references.
 Section 2.4 was rewritten to clarify a number of misinterpretations
 and to leave room for fully internationalized URI.
 Appendix F on abbreviated URLs was added to describe the shortened
 references often seen on television and magazine advertisements and
 explain why they are not used in other contexts.

G.2. Modifications from both RFC 1738 and RFC 1808

 Changed to URI syntax instead of just URL.
 Confusion regarding the terms "character encoding", the URI
 "character set", and the escaping of characters with %<hex><hex>
 equivalents has (hopefully) been reduced.  Many of the BNF rule names
 regarding the character sets have been changed to more accurately
 describe their purpose and to encompass all "characters" rather than
 just US-ASCII octets.  Unless otherwise noted here, these
 modifications do not affect the URI syntax.
 Both RFC 1738 and RFC 1808 refer to the "reserved" set of characters
 as if URI-interpreting software were limited to a single set of
 characters with a reserved purpose (i.e., as meaning something other
 than the data to which the characters correspond), and that this set
 was fixed by the URI scheme.  However, this has not been true in
 practice; any character that is interpreted differently when it is
 escaped is, in effect, reserved.  Furthermore, the interpreting
 engine on a HTTP server is often dependent on the resource, not just
 the URI scheme.  The description of reserved characters has been
 changed accordingly.
 The plus "+", dollar "$", and comma "," characters have been added to
 those in the "reserved" set, since they are treated as reserved
 within the query component.

Berners-Lee, et. al. Standards Track [Page 37] RFC 2396 URI Generic Syntax August 1998

 The tilde "~" character was added to those in the "unreserved" set,
 since it is extensively used on the Internet in spite of the
 difficulty to transcribe it with some keyboards.
 The syntax for URI scheme has been changed to require that all
 schemes begin with an alpha character.
 The "user:password" form in the previous BNF was changed to a
 "userinfo" token, and the possibility that it might be
 "user:password" made scheme specific. In particular, the use of
 passwords in the clear is not even suggested by the syntax.
 The question-mark "?" character was removed from the set of allowed
 characters for the userinfo in the authority component, since testing
 showed that many applications treat it as reserved for separating the
 query component from the rest of the URI.
 The semicolon ";" character was added to those stated as being
 reserved within the authority component, since several new schemes
 are using it as a separator within userinfo to indicate the type of
 user authentication.
 RFC 1738 specified that the path was separated from the authority
 portion of a URI by a slash.  RFC 1808 followed suit, but with a
 fudge of carrying around the separator as a "prefix" in order to
 describe the parsing algorithm.  RFC 1630 never had this problem,
 since it considered the slash to be part of the path.  In writing
 this specification, it was found to be impossible to accurately
 describe and retain the difference between the two URI
    <foo:/bar>   and   <foo:bar>
 without either considering the slash to be part of the path (as
 corresponds to actual practice) or creating a separate component just
 to hold that slash.  We chose the former.

G.3. Modifications from RFC 1738

 The definition of specific URL schemes and their scheme-specific
 syntax and semantics has been moved to separate documents.
 The URL host was defined as a fully-qualified domain name.  However,
 many URLs are used without fully-qualified domain names (in contexts
 for which the full qualification is not necessary), without any host
 (as in some file URLs), or with a host of "localhost".
 The URL port is now *digit instead of 1*digit, since systems are
 expected to handle the case where the ":" separator between host and
 port is supplied without a port.

Berners-Lee, et. al. Standards Track [Page 38] RFC 2396 URI Generic Syntax August 1998

 The recommendations for delimiting URI in context (Appendix E) have
 been adjusted to reflect current practice.

G.4. Modifications from RFC 1808

 RFC 1808 (Section 4) defined an empty URL reference (a reference
 containing nothing aside from the fragment identifier) as being a
 reference to the base URL.  Unfortunately, that definition could be
 interpreted, upon selection of such a reference, as a new retrieval
 action on that resource.  Since the normal intent of such references
 is for the user agent to change its view of the current document to
 the beginning of the specified fragment within that document, not to
 make an additional request of the resource, a description of how to
 correctly interpret an empty reference has been added in Section 4.
 The description of the mythical Base header field has been replaced
 with a reference to the Content-Location header field defined by
 MHTML [RFC2110].
 RFC 1808 described various schemes as either having or not having the
 properties of the generic URI syntax.  However, the only requirement
 is that the particular document containing the relative references
 have a base URI that abides by the generic URI syntax, regardless of
 the URI scheme, so the associated description has been updated to
 reflect that.
 The BNF term <net_loc> has been replaced with <authority>, since the
 latter more accurately describes its use and purpose.  Likewise, the
 authority is no longer restricted to the IP server syntax.
 Extensive testing of current client applications demonstrated that
 the majority of deployed systems do not use the ";" character to
 indicate trailing parameter information, and that the presence of a
 semicolon in a path segment does not affect the relative parsing of
 that segment.  Therefore, parameters have been removed as a separate
 component and may now appear in any path segment.  Their influence
 has been removed from the algorithm for resolving a relative URI
 reference.  The resolution examples in Appendix C have been modified
 to reflect this change.
 Implementations are now allowed to work around misformed relative
 references that are prefixed by the same scheme as the base URI, but
 only for schemes known to use the <hier_part> syntax.

Berners-Lee, et. al. Standards Track [Page 39] RFC 2396 URI Generic Syntax August 1998

H. Full Copyright Statement

 Copyright (C) The Internet Society (1998).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Berners-Lee, et. al. Standards Track [Page 40]

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