Network Working Group T. Berners-Lee
Internet-Draft MIT/LCS
Updates: 1738 (if approved) R. Fielding
Obsoletes: 2732, 2396, 1808 (if approved) Day Software
Expires: September 1, 2003 L. Masinter
Adobe
March 3, 2003
Uniform Resource Identifier (URI): Generic Syntax
draft-fielding-uri-rfc2396bis-01
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
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 a URI, including both absolute and
relative forms, and guidelines for their use.
This document defines a grammar that is a superset of all valid URIs,
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
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grammar for all URIs; that task will be performed by the individual
specifications of each URI scheme.
Editorial Note
Discussion of this draft and comments to the editors should be sent
to the uri@w3.org mailing list. An issues list and version history
is available at <http://www.apache.org/~fielding/uri/rev-2002/>.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Overview of URIs . . . . . . . . . . . . . . . . . . . . . . 4
1.2 URI, URL, and URN . . . . . . . . . . . . . . . . . . . . . 5
1.3 Example URIs . . . . . . . . . . . . . . . . . . . . . . . . 6
1.4 Hierarchical URIs and Relative Forms . . . . . . . . . . . . 6
1.5 URI Transcribability . . . . . . . . . . . . . . . . . . . . 7
1.6 Syntax Notation and Common Elements . . . . . . . . . . . . 8
2. URI Characters and Escape Sequences . . . . . . . . . . . . 9
2.1 URIs and non-ASCII characters . . . . . . . . . . . . . . . 9
2.2 Reserved Characters . . . . . . . . . . . . . . . . . . . . 10
2.3 Unreserved Characters . . . . . . . . . . . . . . . . . . . 11
2.4 Escape Sequences . . . . . . . . . . . . . . . . . . . . . . 11
2.4.1 Escaped Encoding . . . . . . . . . . . . . . . . . . . . . . 11
2.4.2 When to Escape and Unescape . . . . . . . . . . . . . . . . 11
2.4.3 Excluded US-ASCII Characters . . . . . . . . . . . . . . . . 12
3. URI Syntactic Components . . . . . . . . . . . . . . . . . . 14
3.1 Scheme Component . . . . . . . . . . . . . . . . . . . . . . 15
3.2 Authority Component . . . . . . . . . . . . . . . . . . . . 15
3.2.1 Registry-based Naming Authority . . . . . . . . . . . . . . 16
3.2.2 Server-based Naming Authority . . . . . . . . . . . . . . . 16
3.3 Path Component . . . . . . . . . . . . . . . . . . . . . . . 18
3.4 Query Component . . . . . . . . . . . . . . . . . . . . . . 19
4. URI References . . . . . . . . . . . . . . . . . . . . . . . 20
4.1 Fragment Identifier . . . . . . . . . . . . . . . . . . . . 20
4.2 Same-document References . . . . . . . . . . . . . . . . . . 21
4.3 Parsing a URI Reference . . . . . . . . . . . . . . . . . . 21
5. Relative URI References . . . . . . . . . . . . . . . . . . 22
5.1 Establishing a Base URI . . . . . . . . . . . . . . . . . . 23
5.1.1 Base URI within Document Content . . . . . . . . . . . . . . 24
5.1.2 Base URI from the Encapsulating Entity . . . . . . . . . . . 24
5.1.3 Base URI from the Retrieval URI . . . . . . . . . . . . . . 25
5.1.4 Default Base URI . . . . . . . . . . . . . . . . . . . . . . 25
5.2 Resolving Relative References to Absolute Form . . . . . . . 25
6. URI Normalization and Comparison . . . . . . . . . . . . . . 29
6.1 URI Equivalence . . . . . . . . . . . . . . . . . . . . . . 29
6.2 Comparison Ladder . . . . . . . . . . . . . . . . . . . . . 29
6.2.1 Simple String Comparison . . . . . . . . . . . . . . . . . . 30
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6.2.2 Syntax-based Normalization . . . . . . . . . . . . . . . . . 31
6.2.3 Scheme-based Normalization . . . . . . . . . . . . . . . . . 32
6.2.4 Protocol-based Normalization . . . . . . . . . . . . . . . . 32
6.3 Good Practice When Using URIs . . . . . . . . . . . . . . . 32
7. Security Considerations . . . . . . . . . . . . . . . . . . 34
7.1 Reliability and Consistency . . . . . . . . . . . . . . . . 34
7.2 Malicious Construction . . . . . . . . . . . . . . . . . . . 34
7.3 Rare IP Address Formats . . . . . . . . . . . . . . . . . . 35
7.4 Sensitive Information . . . . . . . . . . . . . . . . . . . 35
7.5 Semantic Attacks . . . . . . . . . . . . . . . . . . . . . . 36
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 37
Normative References . . . . . . . . . . . . . . . . . . . . 38
Non-normative References . . . . . . . . . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 40
A. Collected BNF for URI . . . . . . . . . . . . . . . . . . . 42
B. Parsing a URI Reference with a Regular Expression . . . . . 43
C. Examples of Resolving Relative URI References . . . . . . . 44
C.1 Normal Examples . . . . . . . . . . . . . . . . . . . . . . 44
C.2 Abnormal Examples . . . . . . . . . . . . . . . . . . . . . 44
D. Embedding the Base URI in HTML documents . . . . . . . . . . 46
E. Recommendations for Delimiting URI in Context . . . . . . . 47
F. Abbreviated URIs . . . . . . . . . . . . . . . . . . . . . . 49
G. Summary of Non-editorial Changes . . . . . . . . . . . . . . 50
G.1 Additions . . . . . . . . . . . . . . . . . . . . . . . . . 50
G.2 Modifications from RFC 2396 . . . . . . . . . . . . . . . . 50
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
Intellectual Property and Copyright Statements . . . . . . . 55
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1. Introduction
A Uniform Resource Identifier (URI) provides 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], and 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 obsoletes [RFC2396], which merged "Uniform Resource
Locators" [RFC1738] and "Relative Uniform Resource Locators"
[RFC1808] in order to define a single, generic syntax for all URIs.
It excludes those portions of RFC 1738 that defined the specific
syntax of individual URI schemes; those portions will be updated as
separate documents. The process for registration of new URI schemes
is defined separately by [RFC2717].
All significant changes from RFC 2396 are noted in Appendix G.
1.1 Overview of URIs
URIs 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.
The resource is the conceptual mapping to an entity or set of
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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 a 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 URIs
that, in addition to identifying the resource, provide a means of
locating the resource by describing its primary access mechanism
(e.g., its network "location"). The term "Uniform Resource Name"
(URN) refers to the subset of URIs that are required to remain
globally unique and persistent even when the resource ceases to exist
or becomes unavailable.
An individual scheme does not need to be cast into one of a discrete
set of URI types such as "URL", "URN", "URC", etc. Any given URI
scheme may define subspaces that have the characteristics of a name,
a locator, or both, often depending on the persistence and care in
the assignment of identifiers by the naming authority, rather than on
any quality of the URI scheme. For that reason, this specification
deprecates use of the terms URL or URN to distinguish between
schemes, instead using the term URI throughout.
Each 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.
Although many URI schemes are named after protocols, this does not
imply that use of such a URI will result in access to the resource
via the named protocol. URIs are often used in contexts that are
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purely for identification, just like any other identifier. Even when
a URI is used to obtain a representation of a resource, that access
might be through gateways, proxies, caches, and name resolution
services that are independent of the protocol of the resource origin,
and the resolution of some URIs may require the use of more than one
protocol (e.g., both DNS and HTTP are typically used to access an
"http" URI's resource when it can't be found in a local cache).
A parser of the generic URI syntax is capable of parsing any URI
reference into its major components; once the scheme is determined,
further scheme-specific parsing can be performed on the components.
In other words, the URI generic syntax is a superset of the syntax of
all URI schemes.
1.3 Example URIs
The following examples illustrate URIs that are in common use.
ftp://ftp.is.co.za/rfc/rfc1808.txt
-- ftp scheme for File Transfer Protocol services
gopher://gopher.tc.umn.edu:70/11/Mailing%20Lists/
-- gopher scheme for Gopher and Gopher+ Protocol services
http://www.ietf.org/rfc/rfc2396.txt
-- http scheme for Hypertext Transfer Protocol services
mailto:John.Doe@example.com
-- 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 TELNET services
1.4 Hierarchical URIs 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.
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
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`relative' form of URI reference that can be used in conjunction with
a `base' URI of a hierarchical scheme to produce the `absolute' URI
form of the reference. The syntax of a 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
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.
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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 formal grammar defined using the Augmented
Backus-Naur Form (ABNF) notation of [RFC2234]. Although the ABNF
defines syntax in terms of the ASCII character encoding [ASCII], the
URI syntax should be interpreted in terms of the character that the
ASCII-encoded octet represents, rather than the octet encoding
itself. 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 that contains it.
The complete URI syntax is collected in Appendix A.
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2. URI Characters and Escape Sequences
A URI consists 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 a URI are excluded. The restricted set of
characters consists of digits, letters, and a few graphic symbols
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 URIs and non-ASCII characters
The relationship between URIs 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 a URI might be "transported" by means that
are not through a computer network, e.g., printed on paper, read over
the radio, etc.
Within a delimited component of a URI, a sequence of characters is
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).
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
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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. For example, a new scheme "foo" might be defined such
that any escaped octet is keyed to the UTF-8 encoding in order to
determine the corresponding Unicode character.
It is expected that a systematic treatment of character encoding
within URIs 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.
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.
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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 = ALPHA / DIGIT / 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.
URI normalization processes may unescape sequences in the ranges of
ALPHA (%41-%5A and %61-%7A), DIGIT (%30-%39), underscore (%5F), or
tilde (%7E) without fear of creating a conflict, but unescaping the
other mark characters is usually counterproductive.
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 in . For example, "%20" is the escaped
encoding for the US-ASCII space character.
escaped = "%" HEXDIG HEXDIG
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
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.
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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 URI
path, but the two are equivalent for an http URI.
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 (CTL) 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.
The space character (SP) is excluded because significant spaces may
disappear and insignificant spaces may be introduced when a URI is
transcribed or typeset or subjected to the treatment of
word-processing programs. Whitespace is also used to delimit a URI
in many contexts.
The angle-bracket "<" and ">" and double-quote (") characters are
excluded because they are often used as the delimiters around a URI
in text documents and protocol fields. The character "#" is excluded
because it is used to delimit a URI from a fragment identifier in a
URI reference (Section 4). The percent character "%" is excluded
because it is used for the encoding of escaped characters.
delims = "<" / ">" / "#" / "%" / DQUOTE
Other characters are excluded because gateways and other transport
agents are known to sometimes modify such characters, or they are
used as delimiters.
unwise = "{" / "}" / "|" / "\" / "^" / "`"
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Data corresponding to excluded characters must be escaped in order to
be properly represented within a URI.
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3. URI Syntactic Components
The URI syntax is dependent upon the scheme. In general, absolute
URIs 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
URIs. 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.
absolute-URI = scheme ":" ( hier-part / opaque-part )
URIs 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
URIs 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 / "[" / "]" / ";" / "?" /
":" / "@" / "&" / "=" / "+" / "$" / ","
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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 a 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.
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.
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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 / ";" /
":" / "@" / "&" / "=" / "+" / "$" / "," )
3.2.2 Server-based Naming Authority
URI schemes that involve the direct use of an IP-based protocol to a
specified server on the Internet use a common syntax for the server
component of the URI's scheme-specific data:
<userinfo>@<host>:<port>
where <userinfo> may consist of a user name and, optionally,
scheme-specific information about how to gain authorization to access
the server. The parts "<userinfo>@" and ":<port>" may be omitted. If
<host> is omitted, the default host is defined by the scheme-specific
semantics of the URI (e.g., the "file" URI scheme defaults to
"localhost", whereas the "http" URI scheme does not allow host to be
omitted).
server = [ [ userinfo "@" ] hostport ]
The user information, if present, is followed by a commercial
at-sign "@".
userinfo = *( unreserved / escaped / ";" /
":" / "&" / "=" / "+" / "$" / "," )
Some URI schemes use the format "user:password" in the userinfo
field. This practice is NOT RECOMMENDED, because the passing of
authentication information in clear text has proven to be a security
risk in almost every case where it has been used. Note also that
userinfo which is crafted to look like a trusted domain name might be
used to mislead users, as described in Section 7.5.
The server is identified by a network host --- as described by an
IPv6 literal encapsulated within square brackets, an IPv4 address in
dotted-decimal form, or a domain name --- and an optional port
number. The server's port, if any is required by the URI scheme, can
be specified by a port number in decimal following the host and
delimited from it by a colon (":") character. If no explicit port
number is given, the default port number, as defined by the URI
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scheme, is assumed. The type of network port identified by the URI
(e.g., TCP, UDP, SCTP, etc.) is defined by the scheme-specific
semantics of the URI scheme.
hostport = host [ ":" port ]
host = IPv6reference / IPv4address / hostname
port = *DIGIT
A hostname takes 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.
hostname = domainlabel qualified
qualified = *( "." domainlabel ) [ "." toplabel "." ]
domainlabel = alphanum [ 0*61( alphanum | "-" ) alphanum ]
toplabel = alpha [ 0*61( alphanum | "-" ) alphanum ]
alphanum = ALPHA / DIGIT
A host identified by an IPv4 literal address is represented in
dotted-decimal notation (a sequence of four decimal numbers in the
range 0 to 255, separated by "."), as described in [RFC1123] by
reference to [RFC0952]. Note that other forms of dotted notation may
be interpreted on some platforms, as described in Section 7.3, but
only the dotted-decimal form of four octets is allowed by this
grammar.
IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet
dec-octet = DIGIT / ; 0-9
( %x31-39 DIGIT ) / ; 10-99
( "1" 2DIGIT ) / ; 100-199
( "2" %x30-34 DIGIT ) / ; 200-249
( "25" %x30-35 ) ; 250-255
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A host identified by an IPv6 literal address [RFC2373] is
distinguished by enclosing the IPv6 literal within square-brakets
("[" and "]"). This is the only place where square-bracket
characters are allowed in the hierarchical URI syntax.
IPv6reference = "[" IPv6address "]"
IPv6address = ( 6( h4 ":" ) ls32 )
/ ( "::" 5( h4 ":" ) ls32 )
/ ( [ h4 ] "::" 4( h4 ":" ) ls32 )
/ ( [ *1( h4 ":" ) h4 ] "::" 3( h4 ":" ) ls32 )
/ ( [ *2( h4 ":" ) h4 ] "::" 2( h4 ":" ) ls32 )
/ ( [ *3( h4 ":" ) h4 ] "::" h4 ":" ls32 )
/ ( [ *4( h4 ":" ) h4 ] "::" ls32 )
/ ( [ *5( h4 ":" ) h4 ] "::" h4 )
/ ( [ *6( h4 ":" ) h4 ] "::" )
ls32 = ( h4 ":" h4 ) / IPv4address
; least-significant 32 bits of address
h4 = 1*4HEXDIG
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
pchar = unreserved / escaped / ";" /
":" / "@" / "&" / "=" / "+" / "$" / ","
The path may consist of a sequence of path segments separated by a
single slash "/" character. Within a path segment, the characters "/
", ";", "=", and "?" are reserved. The semicolon (";") and equals
("=") characters have the reserved purpose of delimiting parameters
and parameter values within a path segment. However, parameters are
not significant to the parsing of relative references.
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3.4 Query Component
The query component is a string of information to be interpreted by
the resource.
query = *( pchar / "/" / "?" )
Within a query component, the characters ";", "/", "?", ":", "@",
"&", "=", "+", ",", and "$" are reserved.
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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 = [ absolute-URI / relative-URI ] [ "#" fragment ]
Many protocol elements allow only the absolute form of a URI with an
optional fragment identifier.
absolute-URI-reference = absolute-URI [ "#" fragment ]
The syntax for a relative URI is a shortened form of that for an
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 = *( pchar / "/" / "?" )
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
for a 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.
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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 [HTML], 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.
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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 URIs in
these documents point to resources within the tree rather than
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 URIs 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 URIs.
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 URIs.
The relative URI syntax takes advantage of the <hier-part> syntax of
<absolute-URI> (Section 3) in order to express a reference that is
relative to the namespace of another hierarchical URI.
relative-URI = [ 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).
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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.
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 a relative URI is
used within a particular document. Documents can only make use of a
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, a
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:
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.----------------------------------------------------------.
| .----------------------------------------------------. |
| | .----------------------------------------------. | |
| | | .----------------------------------------. | | |
| | | | .----------------------------------. | | | |
| | | | | <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 |
`----------------------------------------------------------'
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) [HTML] 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
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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 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 a relative URI to ensure that the base URI for that
document can be established. It must be emphasized that a 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 algorithm
is intended to provide a definitive result that can be used to test
the output of other implementations. Implementation of the algorithm
itself is not required, but the result given by an implementation
must match the result that would be given by this algorithm.
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 (R), the following pseudocode describes an
algorithm for transforming R into its target (T), which is either an
absolute URI or the current document, and R's optional fragment:
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(R.scheme, R.authority, R.path, R.query, fragment) = parse(R);
-- The URI reference is parsed into the four components and
-- fragment identifier, as described in Section 4.3.
if ((not validating) and (R.scheme == Base.scheme)) then
-- A non-validating parser may ignore a scheme in the
-- reference if it is identical to the base URI's scheme.
undefine(R.scheme);
endif;
if defined(R.scheme) then
T.scheme = R.scheme;
T.authority = R.authority;
T.path = R.path;
T.query = R.query;
else
if defined(R.authority) then
T.authority = R.authority;
T.path = R.path;
T.query = R.query;
else
if (R.path == "") then
if defined(R.query) then
T.path = Base.path;
T.query = R.query;
else
-- An empty reference refers to the current document
return (current-document, fragment);
endif;
else
if (R.path starts-with "/") then
T.path = R.path;
else
T.path = merge(Base.path, R.path);
endif;
T.query = R.query;
endif;
T.authority = Base.authority;
endif;
T.scheme = Base.scheme;
endif;
return (T, fragment);
The pseudocode above refers to a merge routine for merging a
relative-path reference with the path of the base URI to obtain the
target path. Although there are many ways to do this, we will
describe a simple method using a separate string buffer:
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1. 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. If the
base URI's path component is the empty string, then a single
slash character ("/") is copied to the buffer.
2. The reference's path component is appended to the buffer string.
3. All occurrences of "./", where "." is a complete path segment,
are removed from the buffer string.
4. If the buffer string ends with "." as a complete path segment,
that "." is removed.
5. 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.
6. If the buffer string ends with "<segment>/..", where <segment> is
a complete path segment not equal to "..", that "<segment>/.." is
removed.
7. 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.
8. The remaining buffer string is the target URI's path component.
Some systems may find it more efficient to implement the merge
algorithm as a pair of path 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. This may result in a loss of
information if the query component contains the strings "/../" or
"/./".
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The resulting target URI components and fragment can be recombined to
provide the absolute form of the URI reference. Using pseudocode,
this would be:
result = ""
if defined(T.scheme) then
append T.scheme to result;
append ":" to result;
endif;
if defined(T.authority) then
append "//" to result;
append T.authority to result;
endif;
append T.path to result;
if defined(T.query) then
append "?" to result;
append T.query to result;
endif;
if defined(fragment) then
append "#" to result;
append fragment to result;
endif;
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.
Resolution examples are provided in Appendix C.
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6. URI Normalization and Comparison
One of the most common operations on URIs is simple comparison:
determining if two URIs are equivalent without using the URIs to
access their respective resource(s). A comparison is performed every
time a response cache is accessed, a browser checks its history to
color a link, or an XML parser processes tags within a namespace.
Extensive normalization prior to comparison of URIs is often used by
spiders and indexing engines to prune a search space or reduce
duplication of request actions and response storage.
URI comparison is performed in respect to some particular purpose,
and software with differing purposes will often be subject to
differing design trade-offs in regards to how much effort should be
spent in reducing duplicate identifiers. This section describes a
variety of methods that may be used to compare URIs, the trade-offs
between them, and the types of applications that might use them.
6.1 URI Equivalence
Since URIs exist to identify resources, presumably they should be
considered equivalent when they identify the same resource. However,
such a definition of equivalence is not of much practical use, since
there is no way for software to compare two resources without
knowledge of their origin. For this reason, determination of
equivalence or difference of URIs is based on string comparison,
perhaps augmented by reference to additional rules provided by URI
scheme definitions. We use the terms "different" and "equivalent" to
describe the possible outcomes of such comparisons, but there are
many application-dependent versions of equivalence.
Even though it is possible to determine that two URIs are equivalent,
it is never possible to be sure that two URIs identify different
resources. Therefore, comparison methods are designed to minimize
false negatives while strictly avoiding false positives.
In testing for equivalence, it is generally unwise to directly
compare relative URI references; they should be converted to their
absolute forms before comparison. Furthermore, when URI references
are being compared for the purpose of selecting (or avoiding) a
network action, such as retrieval of a representation, it is often
necessary to separate fragment identifiers from the URIs prior to
comparison.
6.2 Comparison Ladder
A variety of methods are used in practice to test URI equivalence.
These methods fall into a range, distinguished by the amount of
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processing required and the degree to which the probability of false
negatives is reduced. As noted above, false negatives cannot in
principle be eliminated. In practice, their probability can be
reduced, but this reduction requires more processing and is not
cost-effective for all applications.
If this range of comparison practices is considered as a ladder, the
following discussion will climb the ladder, starting with those that
are cheap but have a relatively higher chance of producing false
negatives, and proceeding to those that have higher computational
cost and lower risk of false negatives.
6.2.1 Simple String Comparison
If two URIs, considered as character strings, are identical, then it
is safe to conclude that they are equivalent. This type of
equivalence test has very low computational cost and is in wide use
in a variety of applications, particularly in the domain of parsing.
Testing strings for equivalence requires some basic precautions. This
procedure is often referred to as "bit-for-bit" or "byte-for-byte"
comparison, which is potentially misleading. Testing of strings for
equality is normally based on pairwise comparison of the characters
that make up the strings, starting from the first and proceeding
until both strings are exhausted and all characters found to be
equal, or a pair of characters compares unequal or one of the strings
is exhausted before the other.
Such character comparisons require that each pair of characters be
put in comparable form. For example, should one URI be stored in a
byte array in EBCDIC encoding, and the second be in a Java String
object, bit-for-bit comparisons applied naively will produce both
false-positive and false-negative errors. Thus, in principle, it is
better to speak of equality on a character-for-character rather than
byte-for-byte or bit-for-bit basis.
Unicode defines a character as being identified by number
("codepoint") with an associated bundle of visual and other
semantics. At the software level, it is not practical to compare
semantic bundles, so in practical terms, character-by-character
comparisons are done codepoint-by-codepoint.
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6.2.2 Syntax-based Normalization
Software may use logic based on the definitions provided by this
specification to reduce the probability of false negatives. Such
processing is (moderately) higher in cost than
character-for-character string comparison. For example, an
application using this approach could reasonably consider the
following two URIs equivalent:
example://a/b/c/%7A
eXAMPLE://a/./b/../b/c/%7a
Web user agents, such as browsers, typically apply this type of URI
normalization when determining whether a cached response is
available. Syntax-based normalization includes such techniques as
case normalization, escape normalization, and removal of leftover
relative path segments.
6.2.2.1 Case Normalization
When a URI 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 therefore can be normailized to
lowercase. For example, the URI <HTTP://www.EXAMPLE.com/> is
equivalent to <http://www.example.com/>.
6.2.2.2 Escape Normalization
The %-escape mechanism described in Section 2.4 is a frequent source
of variance among otherwise identical URIs. One cause is the choice
of upper-case or lower-case letters for the hexadecimal digits within
the escape sequence (e.g., "%3a" versus "%3A"). Such sequences are
always equivalent; for the sake of uniformity, URI generators and
normalizers are strongly encouraged to use upper-case letters for the
hex digits A-F.
Only characters that are excluded from or reserved within the URI
syntax must be escaped when used as data. However, some URI
generators go beyond that and escape characters that do not require
escaping, resulting in URIs that are equivalent to their unescaped
counterparts. Such URIs can be normalized by unescaping sequences
that represent the unreserved characters, as described in Section
2.3.
6.2.2.3 Path Segment Normalization
The complete path segments "." and ".." have a special meaning within
hierarchical URI schemes. As such, they should not appear in
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absolute URI paths; if they are found, they can be removed by
splitting the URI just after the "/" that starts the path, using the
left half as the base URI and the right as a relative reference, and
normalizing the URI by merging the two in in accordance with the
relative URI processing algorithm (Section 5).
6.2.3 Scheme-based Normalization
The syntax and semantics of URIs vary from scheme to scheme, as
described by the defining specification for each scheme. Software
may use scheme-specific rules, at further processing cost, to reduce
the probability of false negatives. For example, Web spiders that
populate most large search engines would consider the following two
URIs to be equivalent:
http://example.com/
http://example.com:80/
This behavior is based on the rules provided by the syntax and
semantics of the "http" URI scheme, which defines an empty port
component as being equivalent to the default TCP port for HTTP (port
80). In general, a URI scheme that uses the generic syntax of
hostport is defined such that a URI with an explicit ":port", where
the port is the default for the scheme, is equivalent to one where
the port is elided.
6.2.4 Protocol-based Normalization
Web spiders, for which substantial effort to reduce the incidence of
false negatives is often cost-effective, are observed to implement
even more aggressive techniques in URI comparison. For example, if
they observe that a URI such as
http://example.com/data
redirects to
http://example.com/data/
they will likely regard the two as equivalent in the future.
Obviously, this kind of technique is only appropriate in special
situations.
6.3 Good Practice When Using URIs
It is in the best interests of everyone to avoid false-negatives in
comparing URIs, and to only require the minimum amount of software
processing for such comparisons. Those who generate and make
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reference to URIs can reduce the cost of processing and the risk of
false negatives by consistently providing them in a form that is
reasonably canonical with respect to their scheme. Specifically:
Always provide the URI scheme in lower-case characters.
Always provide the hostname, if any, in lower-case characters.
Only perform %-escaping where it is essential.
Always use upper-case A-through-F characters when %-escaping.
Use the UTF-8 character-to-octet mapping, whenever possible.
Prevent /./ and /../ from appearing in absolute URI paths.
The choices listed above are motivated by observations that a high
proportion of deployed software already use these techniques in
practice for the purposes of normalization.
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7. Security Considerations
A URI does not in itself pose a security threat. However, since URIs
are often used to provide a compact set of instructions for access to
network resources, care must be taken to properly interpret the data
within a URI, to prevent that data from causing unintended access,
and to avoid including data that should not be revealed in plain
text.
7.1 Reliability and Consistency
There is no guarantee that, having once used a given URI to retrieve
some information, that the same information will be retievable by
that URI in the future. Nor is there any guarantee that the
information retrievable via that URI in the future will be observably
similar to that retrieved in the past. The URI syntax does not
constrain how a given scheme or authority apportions its namespace or
maintains it over time. Such a guarantee can only be obtained from
the person(s) controlling that namespace and the resource in
question. A specific URI scheme may define additional semantics,
such as name persistence, if those semantics are required of all
naming authorities for that scheme.
7.2 Malicious Construction
It is sometimes possible to construct a URI such that an attempt to
perform a seemingly harmless, idempotent operation, such as the
retrieval of a representation associated with a resource, will in
fact cause a possibly damaging remote operation to occur. The unsafe
URI 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 URI contains instructions that, when
interpreted according to this other protocol, cause an unexpected
operation. An example has been the use of a gopher URI to cause an
unintended or impersonating message to be sent via a SMTP server.
Caution should be used when using any URI that specifies a TCP 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 URI 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
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 being performed.
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7.3 Rare IP Address Formats
Although the URI syntax for IPv4address only allows the common,
dotted-decimal form of IPv4 address literal, many implementations
that process URIs make use of platform-dependent system routines,
such as gethostbyname() and inet_aton(), to translate the string
literal to an actual IP address. Unfortunately, such system routines
often allow and process a much larger set of formats than those
described in Section 3.2.2.
For example, many implementations allow dotted forms of three
numbers, wherein the last part is interpreted as a 16-bit quantity
and placed in the right-most two bytes of the network address (e.g.,
a Class B network). Likewise, a dotted form of two numbers means the
last part is interpreted as a 24-bit quantity and placed in the right
most three bytes of the network address (Class A), and a single
number (without dots) is interpreted as a 32-bit quantity and stored
directly in the network address. Adding further to the confusion,
some implementations allow each dotted part to be interpreted as
decimal, octal, or hexadecimal, as specified in the C language (i.e.,
a leading 0x or 0X implies hexadecimal; otherwise, a leading 0
implies octal; otherwise, the number is interpreted as decimal).
These additional IP address formats are not allowed in the URI syntax
due to differences between platform implementations. However, they
can become a security concern if an application attempts to filter
access to resources based on the IP address in string literal format.
If such filtering is performed, it is recommended that literals be
converted to numeric form and filtered based on the numeric value,
rather than a prefix or suffix of the string form.
7.4 Sensitive Information
It is clearly unwise to use a URI that contains a password which is
intended to be secret. In particular, the use of a password within
the userinfo component of a URI is strongly discouraged except in
those rare cases where the 'password' parameter is intended to be
public.
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7.5 Semantic Attacks
Because the userinfo component is rarely used and appears before the
hostname in the authority component, it can be used to construct a
URI that is intended to mislead a human user by appearing to identify
one (trusted) naming authority while actually identifying a different
authority hidden behind the noise. For example
http://www.example.com&story=breaking_news@10.0.0.1/top_story.htm
might lead a human user to assume that the authority is
'www.example.com', whereas it is actually '10.0.0.1'. Note that the
misleading userinfo could be much longer than the example above.
A misleading URI, such as the one above, is an attack on the user's
preconceived notions about the meaning of a URI, rather than an
attack on the software itself. User agents may be able to reduce the
impact of such attacks by visually distinguishing the various
components of the URI when rendered, such as by using a different
color or tone to render userinfo if any is present, though there is
no general panacea. More information on URI-based semantic attacks
can be found in [Siedzik].
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8. Acknowledgements
This document is derived from RFC 2396 [RFC2396], RFC 1808 [RFC1808],
and RFC 1738 [RFC1738]; the acknowledgements in those specifications
still apply. It also incorporates the update (with corrections) for
IPv6 literals in the host syntax, as defined by Robert M. Hinden,
Brian E. Carpenter, and Larry Masinter in [RFC2732]. In addition,
contributions by Reese Anschultz, Tim Bray, Dan Connolly, Adam M.
Costello, Jason Diamond, Martin Duerst, Henry Holtzman, Graham Klyne,
Dan Kohn, Bruce Lilly, Michael Mealling, Julian Reschke, Tomas
Rokicki, Miles Sabin, Ronald Tschalaer, Marc Warne, Henry Zongaro,
and Zefram are gratefully acknowledged.
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Normative References
[ASCII] American National Standards Institute, "Coded Character
Set -- 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
[RFC2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
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Non-normative 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, "Uniform
Resource Locators (URL)", RFC 1738, December 1994.
[RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[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.
[RFC2518] Goland, Y., Whitehead, E., Faizi, A., Carter, S. and D.
Jensen, "HTTP Extensions for Distributed Authoring --
WEBDAV", RFC 2518, February 1999.
[RFC0952] Harrenstien, K., Stahl, M. and E. Feinler, "DoD Internet
host table specification", RFC 952, October 1985.
[RFC2373] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[RFC2732] Hinden, R., Carpenter, B. and L. Masinter, "Format for
Literal IPv6 Addresses in URL's", RFC 2732, December 1999.
[RFC1736] Kunze, J., "Functional Recommendations for Internet
Resource Locators", RFC 1736, February 1995.
[RFC1737] Masinter, L. and K. Sollins, "Functional Requirements for
Uniform Resource Names", RFC 1737, December 1994.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
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[RFC2110] Palme, J. and A. Hopmann, "MIME E-mail Encapsulation of
Aggregate Documents, such as HTML (MHTML)", RFC 2110,
March 1997.
[RFC2717] Petke, R. and I. King, "Registration Procedures for URL
Scheme Names", BCP 35, RFC 2717, November 1999.
[HTML] Raggett, D., Le Hors, A. and I. Jacobs, "Hypertext Markup
Language (HTML 4.01) Specification", December 1999.
[Siedzik] Siedzik, R., "Semantic Attacks: What's in a URL?", April
2001.
[UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.
Authors' Addresses
Tim Berners-Lee
World Wide Web Consortium
MIT/LCS, Room NE43-356
200 Technology Square
Cambridge, MA 02139
USA
Phone: +1-617-253-5702
Fax: +1-617-258-5999
EMail: timbl@w3.org
URI: http://www.w3.org/People/Berners-Lee/
Roy T. Fielding
Day Software
2 Corporate Plaza, Suite 150
Newport Beach, CA 92660
USA
Phone: +1-949-999-2523
Fax: +1-949-644-5064
EMail: roy.fielding@day.com
URI: http://www.apache.org/~fielding/
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Larry Masinter
Adobe Systems Incorporated
345 Park Ave
San Jose, CA 95110
USA
Phone: +1-408-536-3024
EMail: LMM@acm.org
URI: http://larry.masinter.net/
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Appendix A. Collected BNF for URI
To be filled-in later.
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Appendix 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 of Section 5.2.
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Appendix 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/d;p?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.
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../../../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
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
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, but is allowed for backwards compatibility.
http:g = http:g ; for validating parsers
/ http://a/b/c/g ; for backwards compatibility
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Appendix 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) [HTML] 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 "-//W3C//DTD HTML 4.01 Transitional//EN">
<HTML><HEAD>
<TITLE>An example HTML document</TITLE>
<BASE href="http://www.example.com/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.example.com/Test/a/x>
regardless of the context in which the example document was obtained.
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Appendix E. Recommendations for Delimiting URI in Context
URIs are often transmitted through formats that do not provide a
clear context for their interpretation. For example, there are many
occasions when a URI is 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://example.com/", angle brackets <http://
example.com/>, or just using whitespace
http://example.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 a 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 a URI that contains whitespace.
The prefix "URL:" (with or without a trailing space) was formerly
recommended as a way to help distinguish a URI from other bracketed
designators, though it is not commonly used in practice and is no
longer recommended.
For robustness, software that accepts user-typed URI should attempt
to recognize and strip both delimiters and embedded whitespace.
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For example, the text:
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
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Appendix F. Abbreviated URIs
The URI syntax was designed for unambiguous reference to network
resources and extensibility via the URI scheme. However, as URI
identification and usage have become commonplace, traditional media
(television, radio, newspapers, billboards, etc.) have increasingly
used abbreviated URI 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 URI (e.g., most hostnames beginning with "www" are likely to have
a URI prefix of "http://"). Although there is no standard set of
heuristics for disambiguating abbreviated URI 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 URI schemes are introduced.
Since an abbreviated URI has the same syntax as a relative URI path,
abbreviated URI references cannot be used in contexts where relative
URIs are expected. This limits the use of abbreviated URIs to places
where there is no defined base URI, such as dialog boxes and off-line
advertisements.
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Appendix G. Summary of Non-editorial Changes
G.1 Additions
IPv6 literals have been added to the list of possible identifiers for
the host portion of a server component, as described by [RFC2732],
with the addition of "[" and "]" to the reserved, uric, and
uric-no-slash sets. Square brackets are now specified as reserved
for the authority component, allowed within the opaque part of an
opaque URI, and not allowed in the hierarchical syntax except for
their use as delimiters for an IPv6reference within host. In order
to make this change without changing the technical definition of the
path, query, and fragment components, those rules were redefined to
directly specify the characters allowed rather than continuing to be
defined in terms of uric.
Since [RFC2732] defers to [RFC2373] for definition of an IPv6 literal
address, which unfortunately has an incorrect ABNF description of
IPv6address, we created a new ABNF rule for IPv6address that matches
the text representations defined by Section 2.2 of [RFC2373].
Likewise, the definition of IPv4address has been improved in order to
limit each decimal octet to the range 0-255, and the definition of
hostname has been improved to better specify length limitations and
partially-qualified domain names.
Section 6 on URI normalization and comparison has been completely
rewritten and extended using input from Tim Bray and discussion
within the W3C Technical Architecture Group.
G.2 Modifications from RFC 2396
The ad-hoc BNF syntax has been replaced with the ABNF of [RFC2234].
This change required all rule names that formerly included underscore
characters to be renamed with a dash instead. Likewise, absoluteURI
and relativeURI have been changed to absolute-URI and relative-URI,
respectively, for consistency.
The ABNF of hier-part and relative-URI (Section 3) has been corrected
to allow a relative URI path to be empty. This also allows an
absolute-URI to consist of nothing after the "scheme:", as is present
in practice with the "DAV:" namespace [RFC2518] and the "about:" URI
used by many browser implementations.
The ABNF of qualified has been simplified to remove a parsing
ambiguity without changing the allowed syntax.
The resolving relative references algorithm of [RFC2396] has been
rewritten using pseudocode for this revision to improve clarity and
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fix the following issues:
o [RFC2396] section 5.2, step 6a, failed to account for a base URI
with no path.
o Restored the behavior of [RFC1808] where, if the the reference
contains an empty path and a defined query component, then the
target URI inherits the base URI's path component.
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Index
A
abs-path 14
absolute-URI 14
absolute-URI-reference 20
alphanum 17
authority 15
D
dec-octet 17
delims 12
domainlabel 17
E
escaped 11
F
fragment 20
H
h4 18
hier-part 14
host 16
hostname 17
hostport 16
I
IPv4 17
IPv4address 17
IPv6 18
IPv6address 18
IPv6reference 18
L
ls32 18
M
mark 11
N
net-path 14
O
opaque-part 14
P
path 18
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path-segments 18
pchar 18
port 16
Q
qualified 17
query 19
R
reg-name 16
rel-path 22
rel-segment 22
relative-URI 22
reserved 10
S
scheme 15
segment 18
server 16
T
toplabel 17
U
unreserved 11
unwise 12
URI grammar
abs-path 14
absolute-URI 14
absolute-URI-reference 20
alphanum 17
authority 15
dec-octet 17
delims 12
domainlabel 17
escaped 11
fragment 20
h4 18
hier-part 14
host 17
hostname 17
hostport 17
IPv4address 17
IPv6address 18
IPv6reference 18
ls32 18
mark 11
net-path 14
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opaque-part 14
path 18
path-segments 18
pchar 18
port 17
qualified 17
query 19
reg-name 16
rel-path 22
rel-segment 22
relative-URI 22
reserved 10
scheme 15
segment 18
server 16
toplabel 17
unreserved 11
unwise 12
URI-reference 20
uric 9
uric-no-slash 14
userinfo 16
URI-reference 20
uric 9
uric-no-slash 14
userinfo 16
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Intellectual Property Statement
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement
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