WDIFF

draft-fielding-uri-rfc2396bis-01.txt --> draft-fielding-uri-rfc2396bis-02.txt

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
Expires: November 21, 2003 Adobe
March 3,
May 23, 2003

Uniform Resource Identifier (URI): Generic Syntax
draft-fielding-uri-rfc2396bis-01
draft-fielding-uri-rfc2396bis-02

Status of this Memo

This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on September 1, 2003.
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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.

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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/>. <http://www.apache.org/~fielding/uri/rev-2002/
issues.html>.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Overview of URIs . . . . . . . . . . . . . . . . . . . . . . 4
1.2 URI, URL, and URN
1.1.1 Generic Syntax . . . . . . . . . . . . . . . . . . . . . . . 5
1.3 Example URIs
1.1.2 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.4 Hierarchical URIs
1.1.3 URI, URL, and Relative Forms URN . . . . . . . . . . . . 6
1.5 URI Transcribability . . . . . . . . . 6
1.2 Design Considerations . . . . . . . . . . . 7
1.6 Syntax Notation and Common Elements . . . . . . . . 6
1.2.1 Transcription . . . . 8
2. URI Characters and Escape Sequences . . . . . . . . . . . . 9
2.1 URIs and non-ASCII characters . . . . . . . 6
1.2.2 Separating Identification from Interaction . . . . . . . . 9
2.2 Reserved Characters . 7
1.2.3 Hierarchical Identifiers . . . . . . . . . . . . . . . . . . 9
1.3 Syntax Notation . 10
2.3 Unreserved Characters . . . . . . . . . . . . . . . . . . . 11
2.4 Escape Sequences . . 9
2. Characters . . . . . . . . . . . . . . . . . . . . 11
2.4.1 Escaped Encoding . . . . . 10
2.1 Encoding of Characters . . . . . . . . . . . . . . . . . 11
2.4.2 When to Escape and Unescape . . 10
2.2 Reserved Characters . . . . . . . . . . . . . . 11
2.4.3 Excluded US-ASCII Characters . . . . . . 10
2.3 Unreserved Characters . . . . . . . . . . 12
3. URI Syntactic Components . . . . . . . . . 11
2.4 Escaped Characters . . . . . . . . . 14
3.1 Scheme Component . . . . . . . . . . . . 12
2.4.1 Escaped Encoding . . . . . . . . . . 15
3.2 Authority Component . . . . . . . . . . . . 12
2.4.2 When to Escape and Unescape . . . . . . . . 15
3.2.1 Registry-based Naming Authority . . . . . . . . 12
2.5 Excluded Characters . . . . . . 16
3.2.2 Server-based Naming Authority . . . . . . . . . . . . . . 13
3. Syntax Components . 16
3.3 Path Component . . . . . . . . . . . . . . . . . . . . 15
3.1 Scheme . . . 18
3.4 Query Component . . . . . . . . . . . . . . . . . . . . . . 19
4. URI References . . 15
3.2 Authority . . . . . . . . . . . . . . . . . . . . . 20
4.1 Fragment Identifier . . . . 16
3.2.1 User Information . . . . . . . . . . . . . . . . 20
4.2 Same-document References . . . . . . 16
3.2.2 Host . . . . . . . . . . . . 21
4.3 Parsing a URI Reference . . . . . . . . . . . . . . . . 17
3.2.3 Port . . 21
5. Relative URI References . . . . . . . . . . . . . . . . . . 22
5.1 Establishing a Base URI . . . . . . . . 18
3.3 Path . . . . . . . . . . 23
5.1.1 Base URI within Document Content . . . . . . . . . . . . . . 24
5.1.2 Base URI from the Encapsulating Entity . . . . 19
3.4 Query . . . . . . . 24
5.1.3 Base URI from the Retrieval URI . . . . . . . . . . . . . . 25
5.1.4 Default Base URI . . . . . . 20
3.5 Fragment . . . . . . . . . . . . . . . . 25
5.2 Resolving Relative References to Absolute Form . . . . . . . 25
6. URI Normalization and Comparison . . . 20
4. Usage . . . . . . . . . . . 29
6.1 URI Equivalence . . . . . . . . . . . . . . . . 22
4.1 URI Reference . . . . . . 29
6.2 Comparison Ladder . . . . . . . . . . . . . . . . . 22
4.2 Relative URI . . . . 29
6.2.1 Simple String Comparison . . . . . . . . . . . . . . . . . . 30

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6.2.2 Syntax-based Normalization . . 22
4.3 Absolute URI . . . . . . . . . . . . . . . 31
6.2.3 Scheme-based Normalization . . . . . . . . . 23
4.4 Same-document Reference . . . . . . . . 32
6.2.4 Protocol-based Normalization . . . . . . . . . . 23
4.5 Suffix Reference . . . . . . 32
6.3 Good Practice When Using URIs . . . . . . . . . . . . . . . 32
7. Security Considerations . 23
5. Relative Resolution . . . . . . . . . . . . . . . . . 34
7.1 Reliability and Consistency . . . 25
5.1 Establishing a Base URI . . . . . . . . . . . . . 34
7.2 Malicious Construction . . . . . 25
5.1.1 Base URI within Document Content . . . . . . . . . . . . . . 34
7.3 Rare IP Address Formats 26
5.1.2 Base URI from the Encapsulating Entity . . . . . . . . . . . 26
5.1.3 Base URI from the Retrieval URI . . . . . . . 35
7.4 Sensitive Information . . . . . . . 27
5.1.4 Default Base URI . . . . . . . . . . . . 35
7.5 Semantic Attacks . . . . . . . . . . 27

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5.2 Obtaining the Referenced URI . . . . . . . . . . . . 36
8. Acknowledgements . . . . 27
5.3 Recomposition of a Parsed URI . . . . . . . . . . . . . . . 29
5.4 Examples of Relative Resolution . . . 37
Normative References . . . . . . . . . . . 30
5.4.1 Normal Examples . . . . . . . . . 38
Non-normative References . . . . . . . . . . . . . 30
5.4.2 Abnormal Examples . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . 31
6. Normalization and Comparison . . . . . 40
A. Collected BNF for URI . . . . . . . . . . . 33
6.1 Equivalence . . . . . . . . 42
B. Parsing a URI Reference with a Regular Expression . . . . . 43
C. Examples of Resolving Relative URI References . . . . . . . 44
C.1 Normal Examples . . . . 33
6.2 Comparison Ladder . . . . . . . . . . . . . . . . . . 44
C.2 Abnormal Examples . . . 33
6.2.1 Simple String Comparison . . . . . . . . . . . . . . . . . . 44
D. Embedding the Base URI in HTML documents 34
6.2.2 Syntax-based Normalization . . . . . . . . . . 46
E. Recommendations for Delimiting URI in Context . . . . . . . 47
F. Abbreviated URIs 35
6.2.3 Scheme-based Normalization . . . . . . . . . . . . . . . . . 36
6.2.4 Protocol-based Normalization . . . . . 49
G. Summary of Non-editorial Changes . . . . . . . . . . . 36
6.3 Canonical Form . . . 50
G.1 Additions . . . . . . . . . . . . . . . . . . . . 36
7. Security Considerations . . . . . 50
G.2 Modifications from RFC 2396 . . . . . . . . . . . . . 38
7.1 Reliability and Consistency . . . 50
Index . . . . . . . . . . . . . 38
7.2 Malicious Construction . . . . . . . . . . . . . . 52
Intellectual Property and Copyright Statements . . . . . 38
7.3 Rare IP Address Formats . . 55

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1. Introduction

A Uniform Resource Identifier (URI) provides a simple and extensible
means . . . . . . . . . . . . . . . . 39
7.4 Sensitive Information . . . . . . . . . . . . . . . . . . . 39
7.5 Semantic Attacks . . . . . . . . . . . . . . . . . . . . . . 39
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 41
Normative References . . . . . . . . . . . . . . . . . . . . 42
Informative References . . . . . . . . . . . . . . . . . . . 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 45
A. Collected ABNF for identifying URI . . . . . . . . . . . . . . . . . . . 46
B. Parsing a resource. This specification of URI syntax
and semantics is derived from concepts introduced by Reference with a Regular Expression . . . . . 47
C. Embedding the Base URI in HTML documents . . . . . . . . . . 48
D. Delimiting a URI in Context . . . . . . . . . . . . . . . . 49
E. Summary of Non-editorial Changes . . . . . . . . . . . . . . 51
E.1 Additions . . . . . . . . . . . . . . . . . . . . . . . . . 51
E.2 Modifications from RFC 2396 . . . . . . . . . . . . . . . . 51
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
Intellectual Property and Copyright Statements . . . . . . . 57

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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 identifiers
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: as follows:

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

Anything that can be anything that has identity. named or described can be a resource.
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"; A resource is not necessarily
accessible via the Internet; e.g., human beings, corporations, and
bound books in a library can also be considered resources.

The resource is Likewise, abstract
concepts can be resources, such as the conceptual mapping to an entity or set operators and operands of a

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entities, not necessarily

mathematical equation or 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. types of a relationship (e.g.,
"parent" or "employee").

Identifier

An identifier embodies the information required to distinguish
what is being identified from all other things within its scope of
identification.

A URI is an object that can act as a reference to
something identifier that has identity. In the case consists 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
matching the resource, as might be characterized restricted syntax defined by such words
as `access', `update', `replace', or `find attributes'.

1.2 URI, URL, and URN this specification. A URI
can be further classified as used to refer to a locator, resource. This specification does not
place any limits on the nature of a name, resource or both. The
term "Uniform Resource Locator" (URL) refers to the subset of URIs
that, in addition reasons why an
application might wish to refer to identifying the resource, provide a means resource. URIs have a global
scope and should be interpreted consistently regardless of
locating context,
but that interpretation may be defined in relation to the resource by describing its primary access mechanism user's
context (e.g., its network "location"). The term "Uniform Resource Name"
(URN) "http://localhost/" refers to the subset of URIs a resource that are required is
relative to remain
globally unique and persistent even when the resource ceases to exist
or becomes unavailable.

An individual scheme does user's network interface and yet not need specific to be cast into any
one of a discrete
set of URI types such as "URL", "URN", "URC", etc. Any given user).

1.1.1 Generic Syntax

Each URI
scheme may define subspaces that have the characteristics of begins with a scheme name, as defined in Section 3.1, that
refers to a locator, or both, often depending on the persistence and care in
the assignment of specification for assigning identifiers by the naming authority, rather than on
any quality of the URI scheme. For within that reason, this specification
deprecates use of the terms URL or URN to distinguish between
schemes, instead using
scheme. As such, the term URI throughout.

Each URI scheme (Section 3.1) defines the namespace of the URI, syntax is a federated and
thus extensible naming
system wherein each scheme's specification 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 Likewise, protocols and data
formats that make use of such a URI will result in access references can refer to this
specification as defining 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 range of a resource, that access
might be through gateways, proxies, caches, and name resolution
services syntax allowed for all URIs,
including those schemes 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 have yet to access an
"http" URI's resource when it can't be found in a local cache). defined.

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

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1.1.2 Examples

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

1.1.3 URI, URL, and Relative Forms

An absolute identifier refers to URN

A URI can be further classified as a resource independent of the
context in which the identifier is used. In contrast, locator, a relative
identifier name, or both. The
term "Uniform Resource Locator" (URL) refers to a resource by describing the difference within a
hierarchical namespace between the current context and an absolute
identifier subset of URIs
that, in addition to identifying the resource.

Some URI schemes support resource, provide a hierarchical naming system, where the
hierarchy means of
locating the name is denoted resource by a "/" delimiter separating the
components in describing its primary access mechanism
(e.g., its network "location"). The term "Uniform Resource Name"
(URN) refers to the scheme. This document defines a scheme-independent

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`relative' form subset of URI reference URIs that can 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 used in conjunction with
a `base' URI classified as being just one
of a hierarchical "name" or "locator". Instances of URIs from any given scheme to produce may
have the `absolute' URI
form characteristics of names or locators or both, often
depending on the reference. The syntax of a hierarchical URI is described persistence and care in Section 3; the relative URI calculation is described assignment of
identifiers by the naming authority, rather than any quality of the
scheme. This specification deprecates use of the term "URN" for
anything but URIs in Section 5.

1.5 URI Transcribability the "urn" scheme [RFC2141]. This specification
also deprecates the term "URL".

1.2 Design Considerations

1.2.1 Transcription

The URI syntax was has been designed with global transcribability transcription as one of

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its main concerns. considerations. A URI is a sequence of characters from a
very limited set, i.e. set: 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 transcription 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 considerations revealed by the scenario:

o A URI is a sequence of characters, which characters that is not always represented
as a sequence of octets.

o A URI may might be transcribed from a non-network source, and thus
should consist of characters that are most likely to be able to be
typed
entered 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 or
familiar components.

These design concerns considerations 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 a resource identifier from one
medium to another was has been considered more important than having its a
URI consist of the most meaningful of components. In local and or
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.

Berners-Lee,

1.2.2 Separating Identification from Interaction

A common misunderstanding of URIs is that they are only used to refer
to accessible resources. In fact, the URI alone only provides
identification; access to the resource is neither guaranteed nor
implied by the presence of a URI. Instead, an operation (if any)
associated with a URI reference is defined by the protocol element,

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1.6 Syntax Notation and Common Elements

This document uses two conventions

data format attribute, or natural language text in which it appears.

Given a URI, a system may attempt to describe and define the syntax
for URI. The first, called the layout form, is perform a general description variety of operations
on the order of components and component separators, resource, as in

The component names are enclosed in angle-brackets and any characters
outside angle-brackets are literal separators. Whitespace should might be
ignored. These descriptions characterized by such words as "denote",
"access", "update", "replace", or "find attributes". Such operations
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 by the character protocols that the
ASCII-encoded octet represents, rather than the octet encoding
itself. How make use of URIs, not by this
specification. However, we do use a few general terms for describing
common operations on URIs. URI "resolution" is represented in terms the process of bits
determining an access mechanism and bytes the appropriate parameters
necessary to dereference a URI; such resolution may require several
iterations. Using that access mechanism to perform some action on
the
wire URI's resource is dependent upon the character encoding termed a "dereference" of the protocol URI.

When URIs are used within information systems to
transport it, or the charset identify sources of
information, the document that contains it.

The complete most common form of URI syntax dereference is collected in Appendix A.

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2. URI Characters and Escape Sequences

A URI consists "retrieval":
making use of a restricted set of characters, primarily chosen
to aid transcribability and usability both in computer systems and URI in
non-computer communications. Characters used conventionally as
delimiters around order to retrieve a URI are excluded. The restricted set of
characters consists representation of digits, letters, and its
associated resource. A "representation" is a few graphic symbols
chosen from sequence of octets,
along with metadata describing those common to most octets, that constitutes a
record 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 state of data (octets) within the delimited portions.
Octets are either represented directly by a character (using resource at the
US-ASCII character for time that octet [ASCII]) or by an escape encoding.
This the
representation is elaborated below.

2.1 URIs and non-ASCII characters

The relationship between URIs and characters has been generated. Retrieval is achieved by a source of
confusion for characters process that are not part of US-ASCII. To describe
might include using the relationship, it is useful to distinguish between URI as a "character"
(as cache key to check for a distinguishable semantic entity) and an "octet" (an 8-bit
byte). There are two mappings, one from locally
cached representation, resolution of the URI characters to octets, determine an
appropriate access mechanism (if any), and
a second from octets to original characters:

URI character sequence->octet sequence->original character sequence

A URI is represented as a sequence dereference of characters, not as a sequence the URI for
the sake of octets. That is because applying a retrieval operation.

URI might be "transported" by means that references in information systems are not through a computer network, e.g., printed on paper, read over designed to be
late-binding: the radio, etc.

Within a delimited component of a URI, a sequence result of characters an access is
used generally determined at the
time it is accessed and may vary over time or due to represent a sequence other aspects of octets. For example,
the character
"a" represents interaction. When an author creates a reference to such a
resource, they do so with the octet 97 (decimal), while intention that the character sequence
"%", "0", "a" represents reference be used in
the octet 10 (decimal).

There future; what is a second translation for being identified is not some resources: specific result that
was obtained in the sequence of
octets defined past, but rather some characteristic that is
expected to be true for future results. In such cases, the resource
referred to by a component of the URI is subsequently used to
represent actually a sequence sameness of characters. A 'charset' defines this mapping.
There are characteristics as
observed over time, perhaps elucidated by additional comments or
assertions made by the resource provider.

Although many charsets in URI schemes are named after protocols, this does not
imply that use in Internet protocols. For example,
UTF-8 [UTF-8] defines a mapping from sequences of octets to sequences of characters such a URI will result in access to the repertoire of ISO 10646.

In the simplest case, resource
via the original character sequence contains only
characters that named protocol. URIs are defined in US-ASCII, and often used simply for the two levels sake of

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identification. Even when a URI Generic Syntax March 2003

mapping are simple and easily invertible: each 'original character' is represented as used to retrieve a representation
of a resource, that access might be through gateways, proxies,
caches, and name resolution services that are independent of the octet for
protocol associated with 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, scheme name, and the situation is more difficult. Internet protocols that
transmit octet sequences intended to represent character sequences
are expected to provide some way resolution of identifying some
URIs may require the charset used, if
there might be use of more than one [RFC2277]. However, there is currently
no provision within the generic protocol (e.g., both DNS
and HTTP are typically used to access an "http" URI's origin server
when a representation isn't found in a local cache).

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1.2.3 Hierarchical Identifiers

The URI syntax is organized hierarchically, with components listed in
decreasing order from left to accomplish this
identification. An individual right. For some URI scheme may require a single
charset, define a default charset, or provide a way schemes, the
visible hierarchy is limited to indicate the
charset used. For example, a new scheme "foo" might be defined such
that any escaped octet itself: everything after
the scheme component delimiter is keyed considered opaque to URI
processing. Other URI schemes make the UTF-8 encoding in order hierarchy explicit and visible
to
determine generic parsing algorithms.

The URI syntax reserves the corresponding Unicode character.

It is expected that a systematic treatment of character encoding
within URIs will be developed as a future modification slash ("/"), question-mark ("?"), and
crosshatch ("#") characters for the purpose of this
specification.

2.2 Reserved Characters

Many URI include delimiting components consisting of or delimited by, certain
special characters. These characters
that are called "reserved", since
their usage within significant to the URI component is limited generic parser's hierarchical
interpretation of an identifier. In addition to their reserved
purpose. If aiding the data for a URI component would conflict with the
reserved purpose, then
readability of such identifiers through the conflicting data must consistent use of
familiar syntax, this uniform representation of hierarchy across
naming schemes allows scheme-independent references to be escaped before
forming the URI.

reserved = "[" / "]" / ";" / "/" / "?" /
":" / "@" / "&" / "=" / "+" / "$" / ","

The "reserved" syntax class above refers made
relative to those characters that are
allowed within hierarchy.

An "absolute" URI refers to a URI, but resource independent of the naming
hierarchy in which may not be allowed the identifier is used. In contrast, a "relative"
URI refers to a resource by describing the difference within a
particular component of
hierarchical name space between the generic current context and an absolute
URI syntax; they are used as
delimiters of the components described in resource. Section 3.

Characters in the "reserved" set are not reserved in all contexts.
The set 4.2 defines a scheme-independent form
of characters actually reserved within any given relative URI
component is defined by reference that component. In general, can be used in conjunction with a character is
reserved if base
URI of a hierarchical scheme to produce the semantics absolute URI form of that
reference.

1.3 Syntax Notation

This document uses the Augmented Backus-Naur Form (ABNF) notation of
[RFC2234] to define the URI changes if syntax. Although the ABNF defines syntax
in terms of the character is
replaced with its escaped US-ASCII encoding.

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2.3 Unreserved Characters

Data characters that are allowed syntax
should be interpreted 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 terms of punctuation marks and
symbols.

unreserved = ALPHA / DIGIT / mark

mark = "-" / "_" / "." / "!" / "~" / "*" / "'" / "(" / ")"

Unreserved characters can be escaped without changing the semantics
of character that the URI, but this should not be done unless
ASCII-encoded octet represents, rather than the URI is being used
in octet encoding
itself. How a context that does not allow the unescaped character to appear. URI normalization processes may unescape sequences is represented in the ranges terms of
ALPHA (%41-%5A bits and %61-%7A), DIGIT (%30-%39), underscore (%5F), or
tilde (%7E) without fear of creating a conflict, but unescaping bytes on the
other mark characters
wire 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 dependent upon the character encoding of the US-ASCII coded character set, or that
corresponds protocol used 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
transport it, or the charset of the percent character "%" followed document that contains it.

The following core ABNF productions are used 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 this specification as
defined by Section 6.1 of [RFC2234]: ALPHA, CR, CTL, DIGIT, DQUOTE,
HEXDIG, LF, OCTET, and Unescape

A SP. The complete URI syntax is always collected 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.
Appendix A.

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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

2. Characters

A URI scheme defines consists of a canonicalization algorithm, then
unreserved characters may be unescaped according to that algorithm.
For example, "%7e" is sometimes used instead restricted set of "~" characters, primarily chosen
to aid transcription and usability both in an http computer systems and in
non-computer communications. Characters used conventionally as
delimiters around a URI
path, but the two are equivalent for an http URI.

Because excluded. The set of URI characters
consists of digits, letters, and a few graphic symbols chosen from
those common to most of the percent "%" character always has the encodings and input facilities
available to Internet users.

uric = reserved purpose of
being the escape indicator, it must be / unreserved / escaped as "%25" in order

Within a URI, reserved characters are used to
be delimit syntax
components, unreserved characters are used as to describe registered
names, and unreserved, non-delimiting reserved, and escaped
characters are used to represent strings of data (1*OCTET) within a URI. Implementers should be careful the
components.

2.1 Encoding of Characters

As described above (Section 1.3), the URI syntax is defined in terms
of characters by reference to the US-ASCII encoding of characters to
octets. This specification does not mandate the use of any
particular mapping between its character set and the octets used to
escape
store or unescape transmit those characters.

URI characters representing strings of data within a component may,
if allowed by the same string more than once, since unescaping component production, represent an already unescaped string arbitrary
sequence of octets. For example, portions of a given URI might lead
correspond to misinterpreting a percent filename on a non-ASCII file system, a query on
non-ASCII data, numeric coordinates on a map, etc. Some URI schemes
define a specific encoding of raw data character to US-ASCII characters as another escaped character, or vice versa part
of their scheme-specific requirements. Most URI schemes represent
data octets by the US-ASCII character corresponding to that octet,
either directly in the
case form of escaping an already escaped string.

2.4.3 Excluded US-ASCII Characters

Although they are disallowed within the character's glyph or by use of an
escape triplet (Section 2.4).

When a URI syntax, we include here scheme defines a
description component that represents textual data
consisting of those US-ASCII characters from the Unicode (ISO 10646) character set,
we recommend that have been excluded and the reasons for their exclusion.

The control characters (CTL) in data be encoded first as octets according to
the US-ASCII coded UTF-8 [UTF-8] character set encoding, and then escaping any octets
that are not used within a URI, both because they are non-printable in the unreserved character set.

2.2 Reserved Characters

URIs include components and
because they sub-components that are likely to be misinterpreted delimited 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 (")
certain special characters. These characters are
excluded because they are often used as the delimiters around called "reserved",
since their usage within a URI
in text documents and protocol fields. The character "#" is excluded
because it component is used limited to delimit a their reserved

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purpose within that component. If data for a URI reference (Section 4). The percent character "%" is excluded
because it is used for component would
conflict with the encoding of reserved purpose, then the conflicting data must be
escaped characters.

delims (Section 2.4) before forming the URI.

reserved = "<" "/" / ">" "?" / "#" / "%" "[" / 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 "=" / "+" / "$" / ","

Reserved 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 used as follows:

An absolute URI contains the name delimiters 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 generic URI
components described in Section 3, as well as within those components
for delimiting sub-components. A component's ABNF syntax does rule will
not require use the "reserved" production directly; instead, each rule lists
those reserved characters that are allowed within that component.
Allowed reserved characters that are not assigned a sub-component
delimiter role by this specification should be considered reserved
for special use by whatever software generates the scheme-specific-part have
any general structure URI (i.e., they
may be used to delimit or set of semantics which indicate information that is common among all
URIs. However, a subset significant to
interpretation of URI do share a common syntax for
representing hierarchical relationships within the namespace. This
"generic URI" syntax consists identifier, but that significance is outside
the scope of a sequence this specification). Outside of four main components:

each the URI's origin, a
reserved character cannot be escaped without fear of which, except <scheme>, may changing how it
will be absent from a particular URI.
For example, some URI schemes do not allow interpreted; likewise, an <authority> component,
and others do not use escaped octet that corresponds to a <query> component.

absolute-URI = scheme ":" ( hier-part / opaque-part )

URIs
reserved character cannot be unescaped outside the software that is
responsible for interpreting it during URI resolution.

The slash ("/"), question-mark ("?"), and crosshatch ("#") characters
are hierarchical reserved in nature use the slash "/" character all URI for
separating hierarchical components. For some file systems, a "/"
character (used the purpose of delimiting components that
are significant to denote the generic parser's hierarchical structure interpretation
of an identifier. The hierarchical prefix of a URI) is URI, wherein the
delimiter used to construct
slash ("/") character signifies a file name hierarchy, and thus hierarchy delimiter, extends from
the URI
path will look similar scheme (Section 3.1) through to a file pathname. This does NOT imply that the resource is a file first question-mark ("?"),
crosshatch ("#"), 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 end of the URI string. In other words, the
slash "/" ("/") character for separating is not treated as a hierarchical separator
within the query (Section 3.4) and fragment (Section 3.5) components are
of a URI, but is still considered opaque by reserved within those components
for purposes outside the generic scope of this specification.

2.3 Unreserved Characters

Data characters that are allowed in a URI
parser.

opaque-part = uric-no-slash *uric

uric-no-slash = but do not have a reserved
purpose are called unreserved. These include uppercase and lowercase
letters, decimal digits, and a limited set of punctuation marks and
symbols.

unreserved = ALPHA / escaped / "[" / "]" DIGIT / ";" mark

mark = "-" / "?" "_" /
":" "." / "@" "!" / "&" "~" / "=" "*" / "+" "'" / "$" "(" / "," ")"

Unreserved characters can be escaped without changing the semantics
of a URI, but this should not be done unless the URI is being used in

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We use

a context that does not allow the term <path> to refer unescaped character to both the <abs-path> and
<opaque-part> constructs, since they are mutually exclusive for any
given appear. URI
normalization processes may unescape sequences in the ranges of ALPHA
(%41-%5A and can be parsed as %61-%7A), DIGIT (%30-%39), hyphen (%2D), underscore
(%5F), or tilde (%7E) without fear of creating a single component.

3.1 Scheme Component

Just as there are many different methods of access conflict, but
unescaping the other mark characters is usually counterproductive.

2.4 Escaped Characters

Data must be escaped if it does not have a representation using an
unreserved character; this includes data that does not correspond to resources,
there are
a variety of schemes for identifying such resources. The
URI syntax consists printable character of the US-ASCII coded character set or
corresponds to a sequence of components separated by reserved
characters, with US-ASCII character that delimits the first component defining the semantics from
others, is reserved in that component for the
remainder of the delimiting sub-components,
or is excluded from any use within a URI string.

Scheme names consist of (Section 2.5).

2.4.1 Escaped Encoding

An escaped octet is encoded as a sequence character triplet, consisting of characters beginning with a
lower case letter and
the percent character "%" followed by any combination of lower case
letters, digits, plus ("+"), period ("."), or hyphen ("-"). the two hexadecimal digits
representing that octet's numeric value. For
resiliency, programs interpreting a URI should treat upper case
letters example, "%20" is the
escaped encoding for the US-ASCII space character (SP). This is
sometimes referred to as "percent-encoding" the octet.

escaped = "%" HEXDIG HEXDIG

The uppercase hexadecimal digits 'A' through 'F' are equivalent to lower
the lowercase digits 'a' through 'f', respectively. Two URIs that
differ only in the case of hexadecimal digits used in scheme names (e.g., allow
"HTTP" as well as "http").

scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )

Relative URI references escaped octets
are distinguished from absolute equivalent. For consistency, we recommend that uppercase digits
be used by URI in generators and normalizers.

2.4.2 When to Escape and Unescape

Under normal circumstances, the only time that
they do not begin with characters within 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 string are escaped is during the remainder process of generating the URI is governed by
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 authority. This authority component is
typically defined by an Internet-based server can determine whether or
not escaping a scheme-specific
registry of naming authorities.

authority = server / reg-name character will change its semantics. The authority component exception is preceded by
when a double slash "//" and URI is
terminated by the next slash "/", question-mark "?", or by the end of
the URI. Within the authority component, being used within a context where the unreserved "mark"
characters ";", ":",
"@", "?", "/", "[", and "]" are reserved.

An authority component is not required might need to be escaped, such as when used for a
command-line argument or within a single-quoted attribute.

Once generated, a URI scheme to make use
of relative references. A base URI without is always in an authority component
implies escaped form. When a URI is
resolved, the components significant to that any relative reference will also scheme-specific
resolution process (if any) must be without an authority
component. parsed and separated before the
escaped characters within those components can be safely unescaped.

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3.2.1 Registry-based Naming Authority

The structure

In some cases, data that could be represented by an unreserved
character may appear escaped; for example, some 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 /
"mark" characters are automatically escaped / ";" /
":" / "@" / "&" / "=" / "+" / "$" / "," )

3.2.2 Server-based Naming Authority by some systems. A URI schemes
normalizer may unescape escaped octets that involve are represented by
characters in the direct use unreserved set. For example, "%7E" is sometimes
used instead of tilde ("~") in an IP-based protocol "http" URI path and can be
converted to "~" without changing the interpretation of the URI.

Because the percent ("%") character serves as the escape indicator,
it must be escaped as "%25" in order for that octet to be used as
data within a
specified server on URI. Implementers should be careful not to escape or
unescape the Internet use same string more than once, since unescaping an already
unescaped string might lead to misinterpreting a common syntax for percent data
character as another escaped character, or vice versa in the server
component case of
escaping an already escaped string.

2.5 Excluded Characters

Although they are disallowed within the URI's scheme-specific data:

where <userinfo> may consist URI syntax, we include here
a description of those characters that have been excluded and the
reasons for their exclusion.

excluded = invisible / delims / unwise

The control characters (CTL) in the US-ASCII coded character set are
not used within a user name and, optionally,
scheme-specific information about how to gain authorization URI, both because they are non-printable and
because they are likely to access
the server. be misinterpreted by some control
mechanisms. The parts "<userinfo>@" space character (SP) is excluded because significant
spaces may disappear and ":<port>" insignificant spaces may be omitted. If
<host> is omitted, the default host introduced when
a URI is defined by transcribed, typeset, or subjected to the scheme-specific
semantics treatment of the URI (e.g., the "file" URI scheme defaults
word-processing programs. Whitespace is also used to
"localhost", whereas the "http" delimit a URI scheme does not allow host to be
omitted).

server
in many contexts. Characters outside the US-ASCII set are excluded as
well.

invisible = [ [ userinfo "@" ] hostport ] CTL / SP / %x80-FF

The user information, if present, is followed by angle-bracket ("<" and ">") and double-quote (") characters are
excluded because they are often used as the delimiters around a commercial
at-sign "@".

userinfo URI
in text documents and protocol fields. The percent character ("%")
is excluded because it is used for the encoding of escaped (Section
2.4) characters.

delims = *( unreserved "<" / escaped ">" / ";" "%" /
":" DQUOTE

Other characters are excluded because gateways and other transport
agents are known to sometimes modify such characters.

unwise = "{" / "&" "}" / "=" "|" / "+" "\" / "$" "^" / "," )

Some "`"

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field. This practice is NOT RECOMMENDED, because the passing of
authentication information in clear text has proven Generic Syntax May 2003

Data octets corresponding to excluded characters must be a security
risk escaped in almost every case where it has been used. Note also that
userinfo which is crafted
order to look like a trusted domain name might be
used to mislead users, as described in Section 7.5. represented within a URI.

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3. Syntax Components

The server is identified by generic URI syntax consists of a network host --- hierarchical sequence of
components referred to 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 authority, path, query, 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
fragment.

URI scheme.

hostport = host [ scheme ":" port hier-part [ "?" query ]
host [ "#" fragment ]

hier-part = IPv6reference net-path / IPv4address abs-path / hostname
port rel-path

net-path = *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. "//" authority [ abs-path ]
abs-path = "/" path-segments
rel-path = path-segments

The rightmost
domain label of a fully qualified domain name will never start with a
digit, thus syntactically distinguishing domain names from IPv4
addresses, scheme and path components are required, though path may be followed empty
(no characters). An ABNF-driven parser of hier-part will find that
the three productions in the rule are ambiguous: they are
disambiguated by the "first-match-wins" (a.k.a. "greedy") algorithm.
In other words, if the string begins with two slash characters ("//
"), then it is a single "." net-path; if it begins with only one slash
character, then 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 abs-path; otherwise, it 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]. a rel-path. Note
that other forms of dotted notation may rel-path does not necessarily contain any slash ("/")
characters; a non-hierarchical path will be interpreted on some platforms, treated as described in Section 7.3, but opaque data by
a generic URI parser.

The authority component is only present when a string matches the dotted-decimal form
net-path production. Since the presence 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 authority component
restricts the IPv6 literal within square-brakets
("[" and "]"). This remaining syntax for path, we have not included a
specific "path" rule in the syntax. Instead, what we refer to as the
URI path is that part of the only place where square-bracket
characters are allowed parsed URI string matching the abs-path
or rel-path production 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 syntax above, since they are mutually
exclusive for any given URI and can be parsed as a single component.

3.1 Scheme

Each URI begins with a scheme name that refers to a specification for
assigning identifiers within that scheme. As such, the authority (or the
scheme if there URI syntax is no authority component), identifying the resource
within
a federated and extensible naming system wherein each scheme's
specification may further restrict the scope syntax and semantics of
identifiers using that scheme.

Scheme names consist of a sequence of characters beginning with a
letter and followed by any combination of letters, digits, plus
("+"), period ("."), or hyphen ("-"). Although scheme is
case-insensitive, the canonical form is lowercase 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 documents that
specify schemes must do so using lowercase letters. An
implementation should accept uppercase letters as equivalent to the parsing of relative references.
lowercase in scheme names (e.g., allow "HTTP" as well as "http"), for

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3.4 Query Component

The query component is a string of information to be interpreted by

the resource.

query sake of robustness, but should only generate lowercase scheme
names, for consistency.

scheme = ALPHA *( pchar ALPHA / "/" DIGIT / "?" "+" / "-" / "." )

Within a query component, the characters ";", "/", "?", ":", "@",
"&", "=", "+", ",", and "$"

Individual schemes are reserved.

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4. URI References not specified by this document. The term "URI-reference" is used here to denote the common usage process
for registration of
a resource identifier. A new URI reference may be absolute or relative,
and may have additional information attached in schemes is defined separately by
[RFC2717]. The scheme registry maintains the form of mapping between scheme
names and their specifications.

3.2 Authority

Many URI schemes include a
fragment identifier. However, "the URI" that results from such hierarchical element for a
reference includes only the absolute URI after naming
authority, such that governance of the name space defined by the
remainder of the fragment
identifier (if any) is removed and after any relative URI is resolved delegated to its absolute form. Although that authority (which may, in
turn, delegate it is possible to limit the
discussion of URI further). The generic syntax provides a common
means for distinguishing an authority based on a registered domain
name or server address, along with optional port and semantics to that of the absolute
result, most usage of URI user
information.

The authority component is within general URI references, preceded by a double slash ("//") and it is
impossible to obtain
terminated by the URI from such a reference without also
parsing next slash ("/"), question-mark ("?"), or
crosshatch ("#") character, or by the fragment and resolving end of the relative form.

URI-reference URI.

authority = [ absolute-URI / relative-URI userinfo "@" ] host [ "#" fragment ":" port ]

Many protocol elements

The parts "<userinfo>@" and ":<port>" may be omitted.

Some schemes do not allow only the absolute form of a URI userinfo and/or port sub-components.
When presented 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 violates one or more scheme-specific
restrictions, the scheme-specific 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 resolution process should flag
the
identified resource, reference as an error rather than ignore the optional fragment identifier, separated from unused parts; doing
so reduces the URI by a crosshatch ("#") character, consists number of equivalent URIs and helps detect abuses of additional
reference information to be interpreted by
the user agent after generic syntax that might indicate the
retrieval action URI has been successfully completed. As such, it is not
part constructed
to mislead the user (Section 7.5).

3.2.1 User Information

The userinfo sub-component may consist of a URI, but user name and,
optionally, scheme-specific information about how to gain
authorization to access the server. The user information, if
present, is often used in conjunction with followed by a URI.

fragment commercial at-sign ("@") that delimits it
from the host.

userinfo = *( pchar unreserved / "/" escaped / "?" ";" /
":" / "&" / "=" / "+" / "$" / "," )

The semantics of a fragment identifier is a property of the data
resulting from a retrieval action, regardless of the type of

Some URI used
in the reference. Therefore, schemes use 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 "user:password" 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. userinfo

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A fragment identifier is only meaningful when a URI reference

field. This practice is
intended for retrieval and NOT RECOMMENDED, because the result passing of that retrieval is
authentication information in clear text has proven to be a document
for which the identified fragment is consistently defined.

4.2 Same-document References

A URI reference security
risk in almost every case where it has been used. Note also that does not contain a URI is a reference
userinfo might be crafted to the
current document. In other words, an empty URI reference within a
document is interpreted as look like a reference trusted domain name in order
to the start mislead users, as described in Section 7.5.

3.2.2 Host

The host sub-component of that document,
and a reference containing only a fragment identifier authority is a reference
to the identified fragment of that document. Traversal of such a
reference should not result in by an additional retrieval action.
However, if the URI reference occurs IPv6 literal
encapsulated within square brackets, an IPv4 address in
dotted-decimal form, or a context that domain name.

host = [ IPv6reference / IPv4address / hostname ]

If host is always
intended to result in omitted, a new request, as in default may be defined by the case scheme-specific
semantics of HTML's FORM
element [HTML], then an empty URI reference represents the base URI
of URI. For example, the current document and should be replaced by that URI when
transformed into a request.

4.3 Parsing a URI Reference

A "file" URI reference is typically parsed according scheme defaults to
"localhost", whereas the four main
components and fragment identifier in order "http" URI scheme does not allow host to determine what
components are present and whether the reference is relative or
absolute. be
omitted.

The individual components are then parsed production for their
subparts and, if not opaque, to verify their validity.

Although the BNF defines what is allowed in each component, it host is ambiguous in terms of differentiating because it does not completely
distinguish between an authority component IPv4address and a path component that begins with two slash characters. The
greedy algorithm is used for disambiguation: hostname. Again, the left-most matching
rule soaks up as much of
"first-match-wins" algorithm applies: If host matches the URI reference string as production
for IPv4address, then 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 be considered an IPv4 address literal
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 not a common purpose; hostname.

A hostname takes the vast majority of URIs form described 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 Section 3 of their location [RFC1034] and access scheme. For instance, it is
possible for
Section 2.1 of [RFC1123]: a single set sequence of hypertext documents to be simultaneously
accessible and traversable via domain labels separated by
".", each of the "file", "http", domain label starting 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. ending with an alphanumeric
character and possibly also containing "-" characters. The relative URI syntax takes advantage of the <hier-part> syntax rightmost
domain label of
<absolute-URI> (Section 3) in order to express a reference that fully qualified domain name may be followed by a
single "." if it is
relative necessary to distinguish between the namespace of another hierarchical URI.

relative-URI complete
domain name and some local domain.

hostname = domainlabel qualified
qualified = *( "." domainlabel ) [ net-path / abs-path / rel-path "." ]
domainlabel = alphanum [ "?" query 0*61( alphanum / "-" ) alphanum ]
alphanum = ALPHA / DIGIT

A relative reference beginning with two slash characters host identified by an IPv4 literal address is termed a
network-path reference, as defined represented in
dotted-decimal notation (a sequence of four decimal numbers in the
range 0 to 255, separated by <net-path> "."), as described in Section 3. Such
references are rarely used.

A relative [RFC1123] by
reference beginning with a single slash character is
termed an absolute-path reference, to [RFC0952]. Note that other forms of dotted notation may
be interpreted on some platforms, as defined by <abs-path> described in Section 3.

A relative reference that does not begin with a scheme name or a
slash character 7.3, but
only the dotted-decimal form of four octets is termed a relative-path reference.

rel-path = rel-segment [ abs-path ]

rel-segment allowed by this
grammar.

IPv4address = 1*( unreserved / escaped / ";" /
"@" / "&" / "=" / "+" / "$" / "," )

Within a relative-path reference, the complete path segments dec-octet "." 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). dec-octet "." dec-octet "." dec-octet

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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

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

A host identified by an IPv6 literal address [RFC3513] 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
distinguished by enclosing the IPv6 literal within square-brackets
("[" and "]"). This is not necessary for all the only place where square-bracket
characters are allowed in the 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

The presence of host within a given URI does not imply that the scheme to be
restricted
requires access to the <hier-part> syntax, since given host on the hierarchical
properties of that Internet. In many cases,
the host syntax are only necessary when a relative URI is used within a particular document. Documents can only make use for the sake of a
relative URI when their base URI fits within reusing the <hier-part> syntax.
It is assumed that any document which contains a relative reference
will also have existing
registration process created and deployed for DNS, thus obtaining a base URI that obeys
globally unique name without the syntax. In other words, a
relative URI cannot be used within a document that has an unsuitable
base URI.

Some URI schemes do cost of deploying another registry.
However, such use comes with its own costs: domain name ownership may
change over time for reasons not allow a hierarchical syntax matching anticipated by the
<hier-part> syntax, and thus cannot use relative references.

5.1 Establishing a Base URI creator.

3.2.3 Port

The term "relative URI" implies that there exists some absolute "base
URI" against which the relative reference port sub-component of authority is applied. Indeed, designated by an optional
port number in decimal following the
base URI host and delimited from it by a
single colon (":") character.

port = *DIGIT

If port is necessary to define omitted, a default may be defined by the scheme-specific
semantics of any relative URI
reference; without it, a relative reference the URI. Likewise, the type of network port designated
by the port number (e.g., TCP, UDP, SCTP, etc.) is meaningless. In order
for relative URI to be usable within a document, defined by the base URI of that
document must be known to
scheme. For example, the parser.

The base "http" URI of scheme defines 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 default 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

port 80.

3.3 Path

The path component contains hierarchical data that, along with data
in the | | | |
| | | | document's content | | | |
| | | `----------------------------------------' | | |
| | | (5.1.2) Base URI optional query (Section 3.4) component, serves to identify a
resource within the scope of that URI's scheme and naming authority
(if any). There is no specific "path" syntax production in the encapsulating entity | | |
| | | (message, document, or none). | | |
| | `----------------------------------------------' | |
| | (5.1.3)
generic URI used syntax. Instead, what we refer to retrieve as the entity | |
| `----------------------------------------------------' |
| (5.1.4) Default Base URI path is application-dependent |
`----------------------------------------------------------'

5.1.1 Base URI within Document Content

Within certain document media types,
that part of the base parsed URI of string matching either the document can
be embedded within abs-path or
the content itself such that it can be readily
obtained by a parser. This rel-path production, since they are mutually exclusive for any
given URI and can be useful for descriptive documents,
such parsed 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 a single component. The path is beyond
terminated by the scope first question-mark ("?") or crosshatch ("#")
character, or by the end of this document to specify how, for each
media type, the base URI can be embedded. It URI.

path-segments = segment *( "/" segment )
segment = *pchar

pchar = unreserved / escaped / ";" /
":" / "@" / "&" / "=" / "+" / "$" / ","

The path consists of a sequence of path segments separated by a slash
("/") character. A path is assumed that user
agents manipulating such media types will always defined for a URI, though the
defined path may be able to obtain empty (zero length) or opaque (not containing any
"/" delimiters). For example, the
appropriate syntax from that media type's specification. An example URI <mailto:fred@example.com> has
a path of how "fred@example.com".

Within a path segment, the base semicolon (";") and equals ("=") reserved
characters are often used for delimiting parameters and parameter
values applicable to that segment. The comma (",") reserved
character is often used for similar purposes. For example, one URI can
generator might use a segment like "name;v=1.1" to indicate a
reference to version 1.1 of "name", whereas another might use a
segment like "name,1.1" to indicate the same. Parameter types may be embedded
defined by scheme-specific semantics, but in most cases the Hypertext Markup Language
(HTML) [HTML] meaning
of a parameter is provided in Appendix D.

A mechanism for embedding specific to the base URI within MIME container types
(e.g., the message and multipart types) is defined by MHTML
[RFC2110]. Protocols that do originator. Parameters are not use
significant to the MIME message header syntax,
but which do allow some form parsing of tagged metainformation to be included relative references.

The path segments "." and ".." are defined for relative reference
within messages, may define their own syntax the path name hierarchy. They are intended for defining use at the base
URI as part
beginning of a message.

5.1.2 Base URI from the Encapsulating Entity

If no base URI is embedded, relative path reference (Section 4.2) for indicating
relative position within the base URI hierarchical tree of names, with a document is defined by
similar effect to how they are used within some operating systems'
file directory structure to indicate the document's retrieval context. For current directory and parent
directory, respectively. Unlike a document that is enclosed file system, however, these
dot-segments are only interpreted within another entity (such as a message or another document), the
retrieval context is that entity; thus, the default base URI path hierarchy and
must be removed as part of the URI normalization or resolution
process, in accordance with the process described in Section 5.2.

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document is the base URI of the entity

3.4 Query

The query component contains non-hierarchical data that, along with
data 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 path (Section 3.3) component, serves to identify a
resource within some other entity (e.g., the top level scope 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 URI's scheme and naming authority
(if any). The query component is indicated by the retrieval was first question-mark
("?") character and terminated by a crosshatch ("#") character or by
the
result end 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

query = *( pchar / "/" / "?" )

The characters slash ("/") and question-mark ("?") are allowed to
define the base URI using one of the other methods may result in
represent data within the
same content being interpreted differently by different types of
application.

It query component, but such use is the responsibility of the distributor(s)
discouraged; incorrect implementations of a document
containing a relative URI resolution
often fail to ensure that the base URI for that
document can be established. It must be emphasized that a distinguish them from hierarchical separators, thus
resulting in non-interoperable results while parsing relative
URI cannot be
references. However, since query components are often used reliably to carry
identifying information in situations where the document's base
URI form of "key=value" pairs, and one
frequently used value is not well-defined.

5.2 Resolving Relative References a reference to Absolute Form

This section describes an example algorithm another URI, it is sometimes
better for resolving URI
references that might be relative usability to a given base URI. include those characters unescaped.

3.5 Fragment

The algorithm
is intended fragment identifier component allows indirect identification of
a secondary resource by reference to provide a definitive result primary resource and
additional identifying information that can is selective within that
resource. The identified secondary resource may be used to test
the output some portion or
subset of other implementations. Implementation the primary resource, some view on representations of the algorithm
itself
primary resource, or some other resource that is not required, but merely named within
the result given primary resource. A fragment identifier component is indicated
by an implementation
must match the result that would be given presence of a crosshatch ("#") character and terminated by this algorithm.

The base URI is established according to the rules
end of Section 5.1 and
parsed into the four main components as described in Section 3. Note URI string.

fragment = *( pchar / "/" / "?" )

The semantics of a fragment identifier are defined by the set of
representations that only might result from a retrieval action on the scheme
primary resource. Therefore, the format and interpretation of a
fragment identifier component is required to be present in the base
URI; dependent on the other components media type
[RFC2046] of a potential retrieval result. Individual media types
may be empty define their own restrictions on, or undefined. A component is
undefined if its preceding separator does not appear in structure within, the URI
reference;
fragment identifier syntax for specifying different types of subsets,
views, or external references that are identifiable as fragments by
that media type. If the path component is never undefined, though it may be
empty. The base URI's query component primary resource is not used represented by multiple
media types, as is often the resolution
algorithm and may be discarded.

For each URI reference (R), the following pseudocode describes an
algorithm case for transforming R into its target (T), which resources whose representation
is either an
absolute URI or selected based on attributes of the current document, and R's optional fragment: retrieval request, then
interpretation of the given fragment identifier must be consistent
across all of those media types in order for it to be viable as an

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(R.scheme, R.authority, R.path, R.query, fragment) = parse(R);
-- The

identifier.

As with any URI, use of a fragment identifier component does not
imply that a retrieval action will take place. A URI reference is parsed into with a fragment
identifier may be used to refer to the four components secondary resource without any
implication that the primary resource is accessible. However, if
that URI is used in a context that does call for retrieval and
-- is not
a same-document reference (Section 4.4), the fragment identifier, identifier is
only valid as described in Section 4.3. a reference if ((not validating) a retrieval action on the primary
resource succeeds and (R.scheme == Base.scheme)) then
-- A non-validating parser may ignore results in a scheme representation that defines the
fragment.

Fragment identifiers have a special role in information systems as
the
-- reference if it is identical primary form of client-side indirect referencing, allowing an
author to specifically identify those aspects of an existing resource
that are only indirectly provided by 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 resource owner. As such,
interpretation of 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 fragment identifier during a
relative-path reference with retrieval action
is performed solely by the path of user agent; the base URI fragment identifier is not
passed to obtain other systems during the
target path. process of retrieval. Although there are many ways
this is often perceived to do this, we will
describe be a simple method using loss of information, particularly in
regards to accurate redirection of references as content moves over
time, it also serves to prevent information providers from denying
reference authors the right to selectively refer to information
within a separate string buffer: resource.

The characters slash ("/") and question-mark ("?") are allowed to
represent data within the fragment identifier, but such use is
discouraged for the same reasons as described above for query.

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1. All but

4. Usage

When applications make reference to a URI, they do not always use the last segment
full form of reference defined by the base URI's path component is
copied "URI" syntax production. In
order to save space and take advantage of hierarchical locality, many
Internet protocol elements and media type formats allow an
abbreviation of a URI, while others restrict the buffer. In other words, any characters after syntax to a
particular form of URI. We define the
last (right-most) slash character, if any, are excluded. If most common forms of reference
syntax in this specification because they impact and depend upon the
base URI's path component is
design of the empty string, then generic syntax, requiring a single
slash character ("/") is copied uniform parsing algorithm
in order to the buffer.

2. be interpreted consistently.

4.1 URI Reference

The reference's path component ABNF rule URI-reference is appended used to denote the buffer string.

3. All occurrences most common usage
of "./", where "." is a complete path segment,
are removed from the buffer string.

4. If resource identifier.

URI-reference = URI / relative-URI

A URI-reference may be absolute or relative: if the buffer string ends with "." as a complete path segment,
that "." is removed.

5. All occurrences reference
string's prefix matches the syntax of "<segment>/../", where <segment> a scheme followed by its colon
separator, then the reference is a complete
path segment not equal URI rather than a relative-URI.

A URI-reference is typically parsed first into the five URI
components, in order to "..", determine what components are removed from present and
whether the buffer
string. Removal of these path segments reference is performed iteratively,
removing the leftmost matching pattern on relative or absolute, and then each iteration, until
no matching pattern remains.

6. If the buffer string ends
component is parsed for its subparts and their validation. The ABNF
of URI-reference, along with "<segment>/..", where <segment> the "first-match-wins" disambiguation
rule, is
a complete path segment not equal sufficient to "..", that "<segment>/.." is
removed.

7. If define a validating parser for the resulting buffer string still begins generic
syntax. Readers familiar with one or more
complete path segments regular expressions should see
Appendix B for an example of a non-validating URI-reference parser
that will take any given string and extract the URI components.

4.2 Relative URI

A relative URI reference takes advantage of "..", then the hier-part syntax
(Section 3) in order to express a reference that is considered relative to be in error. Implementations may handle this error by
retaining these components in the resolved path (i.e., treating
them as part
name space of the final URI), another hierarchical URI.

relative-URI = hier-part [ "?" query ] [ "#" fragment ]

The URI referred to by removing them from the
resolved path (i.e., discarding a relative levels above the root),
or URI reference is obtained by avoiding traversal of
applying 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 relative resolution algorithm as a pair of path segment stacks being merged, rather than
as Section 5.

A relative reference that begins with two slash characters is termed
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 network-path reference; such references are rarely used. A relative
reference paths. This may result in that begins with a loss of
information if the query component contains the strings "/../" or
"/./". single slash character is termed an
absolute-path reference. A relative reference that does not begin

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The resulting target URI components and fragment can

with a slash character is termed a relative-path reference.

A path segment that contains a colon character (e.g., "this:that")
cannot be recombined used as the first segment of a relative-path reference
because it might be mistaken for a scheme name. Such a segment must
be preceded by a dot-segment (e.g., "./this:that") to
provide make a
relative-path reference.

4.3 Absolute URI

Some protocol elements allow only the absolute form of a URI without
a fragment identifier. For example, defining the base URI reference. Using pseudocode,
this would be:

result for later
use by relative references calls for an absolute-URI production that
does not allow a fragment.

absolute-URI = ""

if defined(T.scheme) then
append T.scheme to result;
append scheme ":" 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 hier-part [ "?" query ]

4.4 Same-document Reference

When a URI reference occurring within a document or message refers to result;
append T.query to result;
endif;

if defined(fragment) then
append "#" to result;
append
a URI that is, aside from its fragment component (if any), identical
to result;
endif;

return result;

Note the base URI (Section 5), that we must be careful to preserve reference is called a
"same-document" reference. The most frequent examples of
same-document references are relative references that are empty or
include only the distinction between crosshatch ("#") separator followed by a
component fragment
identifier.

When a same-document reference is dereferenced for the purpose of a
retrieval action, the target of that reference is undefined, meaning defined to be
within that its separator was current document or message; the dereference should not
present
result in a new retrieval.

4.5 Suffix Reference

The URI syntax is designed for unambiguous reference to resources and
extensibility via the reference, URI scheme. However, as URI identification and
usage have become commonplace, traditional media (television, radio,
newspapers, billboards, etc.) have increasingly used a component that is empty, meaning that suffix of the separator was present
URI as a reference, consisting of only the authority and was immediately followed by path
portions of the next
component separator URI, such as

www.w3.org/Addressing/

or simply the end of DNS hostname on its own. Such references are primarily
intended for human interpretation rather than machine, with the reference.

Resolution examples
assumption that context-based heuristics are provided in Appendix C. sufficient to complete
the URI (e.g., most hostnames beginning with "www" are likely to have

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a URI Normalization and Comparison

One prefix of the most common operations on URIs "http://"). Although there 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 no standard set of URIs is often used by
spiders and indexing engines to prune
heuristics for disambiguating a search space or reduce
duplication of request actions and response storage. URI comparison is performed in respect suffix, many client
implementations allow them to some particular purpose,
and software with differing purposes will often be subject to
differing design trade-offs in regards to how much effort entered by the user and
heuristically resolved. It should be
spent in reducing duplicate identifiers. This section describes a
variety of methods noted that such heuristics may be used to compare URIs, the trade-offs
between them, and the types of applications that might use them.

6.1
change over time, particularly when new URI Equivalence schemes are introduced.

Since URIs exist to identify resources, presumably they should be
considered equivalent when they identify a URI suffix has the same resource. However,
such syntax as a definition of equivalence is not relative path reference,
a suffix reference cannot be used in contexts where relative URIs are
expected. This limits use of much practical use, since suffix references to those places where
there is no way for software defined base URI, such as dialog boxes and off-line
advertisements.

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5. Relative Resolution

It is often the case that a group or "tree" of documents has been
constructed to compare two resources without
knowledge of their origin. For this reason, determination of
equivalence or difference serve a common purpose; the vast majority 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" in
these documents point to
describe resources within the possible outcomes tree rather than
outside of such comparisons, but there it. Similarly, documents located at a particular site are
many application-dependent versions of equivalence.

Even though it is possible
much more likely to determine that two URIs are equivalent,
it is never possible refer to be sure other resources at that two site than to
resources at remote sites.

Relative referencing of URIs identify different
resources. Therefore, comparison methods are designed allows document trees to minimize
false negatives while strictly avoiding false positives.

In testing for equivalence, be partially
independent of their location and access scheme. For instance, it is generally unwise
possible for a single set of hypertext documents to directly
compare relative URI references; they should be converted simultaneously
accessible and traversable via each of the "file", "http", and "ftp"
schemes if the documents refer to their
absolute forms before comparison. each other using relative URIs.
Furthermore, when URI references
are being compared for the purpose of selecting (or avoiding) a
network action, such document trees can be moved, as retrieval of a representation, it whole, without
changing any of the relative references. Experience within the WWW
has demonstrated that the ability to perform relative referencing is often
necessary to separate fragment identifiers from for the URIs prior to
comparison.

6.2 Comparison Ladder

A variety long-term usability of methods are used in practice to test URI equivalence.
These methods fall into embedded URIs.

5.1 Establishing a range, distinguished by the amount of

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processing required and

The term "relative URI" implies that there exists some absolute "base
URI" against which the degree relative reference is applied. Indeed, the
base URI is necessary to which define the probability semantics of false
negatives any relative URI
reference; without it, a relative reference is reduced. As noted above, false negatives cannot in
principle be eliminated. meaningless. In practice, their probability can be
reduced, but this reduction requires more processing and is not
cost-effective order
for all applications.

If this range of comparison practices is considered as relative URI to be usable within a ladder, document, the
following discussion will climb base URI of that
document must be known to the ladder, starting with those parser.

A document that
are cheap but contains relative references must have a relatively higher chance of producing false
negatives, and proceeding to those base URI
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 contains a hierarchical path component. In other words, a
relative-URI cannot be used within a document that they are equivalent. This type of
equivalence test has very low computational cost an unsuitable
base URI. Some URI schemes do not allow a hierarchical path component
and are thus restricted to full URI references.

An authority component is in wide use
in not required for a variety of applications, particularly in the domain of parsing.

Testing strings for equivalence requires some basic precautions. This
procedure is often referred URI scheme 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 use
of the strings
is exhausted before the other.

Such character comparisons require relative references. A base URI without an authority component
implies that each pair of characters any relative reference will also be
put in comparable form. For example, should one without an authority
component.

The base URI be stored in of a
byte array in EBCDIC encoding, and the second document can be established in a Java String
object, bit-for-bit comparisons applied naively will produce both
false-positive and false-negative errors. Thus, one of four ways,
listed below in principle, it is
better to speak order 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 precedence. The order of precedence can be
thought 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. terms of layers, where the innermost defined base URI
has the highest precedence. This can be visualized graphically as:

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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

| | | | .----------------------------------. | | | |
| | | | | <relative-reference> | | | | |
| | | | `----------------------------------' | | | |
| | | | (5.1.1) Base URI embedded 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 | | | |
| | | | 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
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 application-dependent |
`----------------------------------------------------------'

5.1.1 Base URI within Document Content

Within certain document media types, the base URI scheme uses elements of the common syntax, it will also
use document can
be embedded within the common syntax equivalence rules, namely content itself such that the scheme and
hostname are case insensitive and therefore it 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 readily
obtained by a frequent source parser. This can be useful for descriptive documents,
such as tables of variance among otherwise identical URIs. One cause 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 choice scope 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; this document to specify how, for each
media type, the sake of uniformity, base URI generators and
normalizers are strongly encouraged can be embedded. It is assumed that user
agents manipulating such media types will be able to use upper-case letters for obtain the
hex digits A-F.

Only characters that are excluded
appropriate syntax from or reserved within that media type's specification. An example
of how the base URI
syntax must can be escaped when used as data. However, some embedded in the Hypertext Markup Language
(HTML) [HTML] is provided in Appendix D.

A mechanism for embedding the base URI
generators go beyond that within MIME container types
(e.g., the message and escape characters multipart types) is defined by MHTML
[RFC2110]. Protocols that do not require
escaping, resulting in URIs that are equivalent use the MIME message header syntax,
but do allow some form of tagged metadata to their unescaped
counterparts. Such URIs can be normalized by unescaping sequences
that represent included within
messages, may define their own syntax for defining the unreserved characters, base URI as described in Section
2.3.

6.2.2.3 Path Segment Normalization

The complete path segments "." and ".." have
part of a special meaning within
hierarchical message.

5.1.2 Base URI schemes. As such, they should not appear 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.

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absolute

5.1.3 Base URI paths; if they are found, they can be removed by
splitting from the Retrieval URI just after the "/" that starts

If no base URI is embedded and the path, using document is not encapsulated
within some other entity (e.g., the
left half as top level of a composite entity),
then, if a URI was used to retrieve the base document, that URI and shall
be considered the right as base URI. Note that if the retrieval was the
result of a relative reference, and
normalizing redirected request, the last URI by merging the two in used (i.e., that which
resulted in accordance with the
relative actual retrieval of the document) is the base URI.

5.1.4 Default Base URI processing algorithm (Section 5).

6.2.3 Scheme-based Normalization

The syntax and semantics

If none of URIs vary from scheme to scheme, as the conditions described in above apply, then the base URI
is defined by the defining specification for each scheme. Software
may use scheme-specific rules, at further processing cost, context of the application. Since this definition
is necessarily application-dependent, failing to reduce define the probability base URI
using one 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 other methods may result in the rules provided same content being
interpreted differently by different types of application.

It is the syntax and
semantics responsibility of the "http" distributor(s) of a document
containing a relative URI scheme, which defines an empty port
component as being equivalent to ensure that the default TCP port for HTTP (port
80). In general, a base URI scheme for that uses the generic syntax of
hostport is defined such
document can be established. It must be emphasized that a relative
URI with an explicit ":port", cannot be used reliably in situations where the port document's base
URI is not well-defined.

5.2 Obtaining the default Referenced URI

This section describes an example algorithm for the scheme, is equivalent resolving URI
references that might be relative to one where
the port a given base URI. The algorithm
is elided.

6.2.4 Protocol-based Normalization

Web spiders, for which substantial effort intended to reduce provide a definitive result that can be used to test
the incidence output of
false negatives other implementations. Implementation of the algorithm
itself is often cost-effective, are observed to implement
even more aggressive techniques in URI comparison. For example, if
they observe not required, but the result given by an implementation
must match the result that a would be given by this algorithm.

The base URI such as

http://example.com/data

redirects (Base) is established according to

http://example.com/data/

they will likely regard the two as equivalent rules of Section
5.1 and parsed into the five main components described in Section 3.
Note that only the future.
Obviously, this kind of technique scheme component is only appropriate required to be present in special
situations.

6.3 Good Practice When Using URIs

It the
base URI; the other components may be empty or undefined. A
component is undefined if its preceding separator does not appear in
the best interests of everyone to avoid false-negatives in
comparing URIs, and to only require URI reference; the minimum amount of software
processing for such comparisons. Those who generate and make

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may be empty.

For each URI Generic Syntax March 2003 reference to URIs can reduce the cost of processing and (R), the risk of
false negatives by consistently providing them in a form that following pseudocode describes an
algorithm for transforming R into its target URI (T):

(R.scheme, R.authority, R.path, R.query, R.fragment) = parse(R);
-- The URI reference is
reasonably canonical with respect to their scheme. Specifically:

Always provide parsed into the five URI scheme in lower-case characters.

Always provide the hostname, components

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 /./ ((not validating) and /../ from appearing in absolute URI paths.

The choices listed above are motivated by observations that (R.scheme == Base.scheme)) then
-- A non-validating parser may ignore a high
proportion of deployed software already use these techniques scheme 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

-- reference if it is identical to properly interpret the data
within a URI, to prevent that data from causing unintended access,
and 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
T.path = Base.path;
if defined(R.query) then
T.query = R.query;
else
T.query = Base.query;
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;

T.fragment = R.fragment;

The pseudocode above refers 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 merge routine for merging a
relative-path reference with the path of the base URI to retrieve
some information, that obtain the same information
target path. Although there are many ways to do this, we will be retievable by
that URI in
describe a simple method using a separate string buffer:

1. All but the future. Nor last segment of the base URI's path component is there
copied to the buffer. In other words, any guarantee that characters after the
information retrievable via that URI in
last (right-most) slash character, if any, are excluded. If the future will be observably
similar to that retrieved in
base URI's path component is the past. The URI syntax does not
constrain how a given scheme or authority apportions its namespace or
maintains it over time. Such empty string, then a guarantee can only be obtained from
the person(s) controlling that namespace and single
slash character ("/") is copied to the resource in
question. A specific buffer.

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Internet-Draft 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 Generic Syntax May 2003

2. The reference's path component is sometimes possible to construct a URI such that an attempt appended to
perform a seemingly harmless, idempotent operation, such as the
retrieval buffer string.

3. All occurrences of a representation associated with a resource, will in
fact cause a possibly damaging remote operation to occur. The unsafe
URI "./", where "." is typically constructed by specifying a port number other than
that reserved for complete path segment,
are removed from the network protocol in question. The client
unwittingly contacts buffer string.

4. If the buffer string ends with "." as a site complete path segment,
that "." is in fact running a different
protocol. The content removed.

5. All occurrences of the URI contains instructions that, when
interpreted according "<segment>/../", where <segment> is a complete
path segment not equal to this other protocol, cause an unexpected
operation. An example has been "..", are removed from the use 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 gopher URI complete path segment not equal to cause an
unintended "..", that "<segment>/.." is
removed.

7. If the resulting buffer string still begins with one or impersonating message more
complete path segments of "..", then the reference is considered
to be sent via a SMTP server.

Caution should be used when using any URI that specifies a TCP port
number other than in error. Implementations may handle this error by
removing them from the default for resolved path (i.e., discarding relative
levels above the protocol, especially when it root) or by avoiding traversal of the reference.

8. The remaining buffer string is
a number within the reserved space.

Care should be taken when target URI's path component.

Some systems may find it more efficient to implement the merge
algorithm as a URI contains escaped delimiters for pair of path segment stacks being merged, rather than
as a
given protocol (for example, CR and LF characters for telnet
protocols) that these are not unescaped series of string pattern replacements.

Note: Some WWW client applications will fail to separate the
reference's query component from its path component before transmission. merging
the base and reference paths. This
might violate may result in a loss of
information if the protocol, but avoids query component contains the potential for such
characters to strings "/../" or
"/./".

5.3 Recomposition of a Parsed URI

Parsed URI components can be used recombined to simulate an extra operation or parameter in
that protocol, which might lead obtain the referenced URI.
Using pseudocode, this would be:

result = ""

if defined(T.scheme) then
append T.scheme to an unexpected and possibly harmful
remote operation being performed. result;
append ":" to result;
endif;

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7.3 Rare IP Address Formats

if defined(T.authority) then
append "//" to translate the string
literal result;
append T.authority 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 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 are careful to preserve the network address (e.g.,
a Class B network). Likewise, distinction between a dotted form of two numbers means the
last part
component that is interpreted as a 24-bit quantity and placed undefined, meaning that its separator was not
present in the right
most three bytes of the network address (Class A), reference, and a single
number (without dots) component that is interpreted as a 32-bit quantity and stored
directly in empty, meaning that
the network address. Adding further to separator was present and was immediately followed by the confusion,
some implementations allow each dotted part to be interpreted as
decimal, octal, next
component separator or hexadecimal, as specified in the C language (i.e., end of the reference.

5.4 Examples of Relative Resolution

Within an object with a leading 0x or 0X implies hexadecimal; otherwise, well-defined base URI of

http://a/b/c/d;p?q

a leading 0
implies octal; otherwise, the number is interpreted as decimal).

These additional IP address formats are not allowed in the relative 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 reference would be
public. resolved as follows:

5.4.1 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" = "http://a/b/c/d;p?q#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"

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7.5 Semantic Attacks

Because

"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"

5.4.2 Abnormal Examples

Although the userinfo component is rarely used and appears before following abnormal examples are unlikely to occur in
normal practice, all URI parsers should be capable of resolving them
consistently. Each example uses the
hostname same base as above.

An empty reference refers to the current base URI.

"" = "http://a/b/c/d;p?q"

Parsers must be careful in handling the authority component, it can 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 construct a
URI that is intended to mislead change
the authority component of 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 URI.

"../../../g" = "http://a/g"
"../../../../g" = "http://a/g"

Similarly, parsers should remove the meaning dot-segments "." and ".." when
they are complete components 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 path, but not when they are only
part of a segment.

"/./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 when rendered, such as by using a different
color uses unnecessary 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].
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"

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8. Acknowledgements

"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 document error is derived from RFC 2396 [RFC2396], RFC 1808 [RFC1808],
and RFC 1738 [RFC1738]; rarely noticed, since typical usage of a
fragment never includes the acknowledgements in those specifications
still apply. It also incorporates hierarchy ("/") character, and the update (with corrections) for
IPv6 literals in 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 host syntax, scheme name to be present in a relative URI if
it is the same as defined by Robert M. Hinden,
Brian E. Carpenter, and Larry Masinter the base URI scheme. This is considered to be a
loophole 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 [RFC1630]. Its use
should be avoided, but is allowed for Information
Interchange", ANSI X3.4, 1986.

[RFC2234] Crocker, D. and P. Overell, "Augmented BNF backward compatibility.

"http:g" = "http:g" ; for Syntax
Specifications: ABNF", RFC 2234, November 1997. validating parsers
/ "http://a/b/c/g" ; for backward compatibility

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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

6. Normalization and Addresses Comparison

One of Objects on the Network as used in most common operations on URIs is simple comparison:
determining if two URIs are equivalent without using 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. 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 L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998.

[RFC1123] Braden, R., "Requirements for Internet Hosts - Application indexing engines to prune a search space or reduce
duplication of request actions and Support", STD 3, RFC 1123, October 1989.

[RFC1808] Fielding, R., "Relative Uniform Resource Locators", RFC
1808, June 1995.

[RFC2046] Freed, N. response storage.

URI comparison is performed in respect to some particular purpose,
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. 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 D.
Jensen, "HTTP Extensions the types of applications that might use them.

6.1 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 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. 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 L. Masinter, "Format for
Literal IPv6 Addresses in URL's", RFC 2732, December 1999.

[RFC1736] Kunze, J., "Functional Recommendations "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 Internet
Resource Locators", RFC 1736, February 1995.

[RFC1737] Masinter, L. and K. Sollins, "Functional Requirements 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
Uniform Resource Names", RFC 1737, December 1994.

[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, the purpose of selecting (or avoiding) a
network action, such as retrieval of a representation, it is often
necessary to remove 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, 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 components of the generic syntax, it will also
use the common syntax equivalence rules, namely that the scheme and
hostname are case insensitive and therefore can be normalized to
lowercase. For example, the URI <HTTP://www.EXAMPLE.com/> is
equivalent to <http://www.example.com/>.

6.2.2.2 Escape Normalization

The percent-escape mechanism described in Section 2.4 is a frequent
source of variance among otherwise identical URIs. One cause is the
choice of uppercase or lowercase 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 uppercase 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
absolute URI paths; if they are found, they can be removed by

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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 for
authority 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 Canonical Form

It is in the best interests of everyone to avoid false-negatives in
comparing URIs and to minimize the amount of software processing for
such comparisons. Those who generate and make reference to URIs can
reduce the cost of processing and the risk of false negatives by

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consistently providing them in a form that is reasonably canonical
with respect to their scheme. Specifically:

Always provide the URI scheme in lowercase characters.

Always provide the hostname, if any, in lowercase characters.

Only perform percent-escaping where it is essential.

Always use uppercase A-through-F characters when percent-escaping.

Prevent /./ and /../ from appearing in non-relative URI paths.

The good practices listed above are motivated by observations that a
high proportion of deployed software use these techniques 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 retrievable 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 name space
or maintains it over time. Such a guarantee can only be obtained
from the person(s) controlling that name space 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, 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
running a different protocol service. 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 dereferencing a URI that specifies a TCP
port number other than the default for the scheme, 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 octets 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

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

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[RFC2110] Palme, J. and A. Hopmann, "MIME E-mail Encapsulation of
Aggregate Documents,

might lead a human user to assume that the host 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 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, the one above, is an attack on the user's
preconceived notions about the meaning of a transformation format URI, rather than an
attack on the software itself. User agents may be able to reduce the
impact 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/ 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. Acknowledgments

This document is derived from RFC 2396 [RFC2396], RFC 1808 [RFC1808],
and RFC 1738 [RFC1738]; the acknowledgments 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
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/ in [RFC2732]. In addition,
contributions by Reese Anschultz, Tim Bray, Rob Cameron, Dan
Connolly, Adam M. Costello, Jason Diamond, Martin Duerst, Stefan
Eissing, Clive D.W. Feather, Pat Hayes, Henry Holtzman, Graham Klyne,
Dan Kohn, Bruce Lilly, Andrew Main, Michael Mealling, Julian Reschke,
Tomas Rokicki, Miles Sabin, Ronald Tschalaer, Marc Warne, Stuart
Williams, and Henry Zongaro are gratefully acknowledged.

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Appendix A. Collected

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 URI

To be filled-in later. Syntax
Specifications: ABNF", RFC 2234, November 1997.

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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

Informative References

[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and commonplace to use a regular expression for parsing the
potential four components
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 fragment identifier Addresses
of a URI reference.

The following line is Objects on the regular expression for breaking-down a URI
reference into its components.

^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))?
12 3 4 5 6 7 8 9

The numbers Network as used in the second line above are only to assist readability;
they indicate the reference points 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 each subexpression (i.e., each
paired parenthesis). We refer to the value matched 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 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 Distributed Authoring --
WEBDAV", RFC 2518, February 1999.

[RFC0952] Harrenstien, K., Stahl, M. and E. Feinler, "DoD Internet
host table specification", RFC 952, October 1985.

[RFC3513] Hinden, R. and S. Deering, "Internet Protocol Version 6
(IPv6) Addressing Architecture", RFC 3513, April 2003.

[RFC2732] Hinden, R., Carpenter, B. and L. Masinter, "Format for the query component
Literal IPv6 Addresses in the above example. Therefore, we
can determine the value of the four components URL's", RFC 2732, December 1999.

[RFC1736] Kunze, J., "Functional Recommendations for Internet
Resource Locators", RFC 1736, February 1995.

[RFC1737] Masinter, L. and fragment K. Sollins, "Functional Requirements for
Uniform Resource Names", RFC 1737, December 1994.

[RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997.

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[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

scheme = $2
authority = $4
path = $5
query = $7
fragment = $9

and, going 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 the opposite direction, we can recreate a URI reference
from its components using the algorithm URL?", April
2001.

[UTF-8] Yergeau, F., "UTF-8, a transformation format of Section 5.2. ISO
10646", RFC 2279, January 1998.

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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/

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 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 A. Collected ABNF for 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

To be used to change
the authority component of a URI. filled-in later.

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../../../g = http://a/../g
../../../../g = http://a/../../g

In practice, some implementations strip leading relative symbolic
elements (".", "..") after applying

Appendix B. Parsing a relative URI calculation, based
on Reference with a Regular Expression

Since the "first-match-wins" algorithm is identical to the "greedy"
disambiguation method used by POSIX regular expressions, it is
natural and commonplace to use a regular expression for parsing the
potential five components of a URI reference.

The following line is the regular expression for breaking-down a
well-formed URI reference into its components.

^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))?
12 3 4 5 6 7 8 9

The numbers in the theory that compensating for obvious author errors is better
than allowing second line above are only to assist readability;
they indicate the request reference points for each subexpression (i.e., each
paired parenthesis). We refer to fail. Thus, the above two references
will be interpreted as "http://a/g" by some implementations.

Similarly, parsers must avoid treating "." and ".." value matched for subexpression
<n> 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 $<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://a/b/c/.g
g.. http:
$2 = http://a/b/c/g..
..g http
$3 = 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 //www.ics.uci.edu
$4 = http://a/b/g
./g/. www.ics.uci.edu
$5 = http://a/b/c/g/
g/./h /pub/ietf/uri/
$6 = http://a/b/c/g/h
g/../h <undefined>
$7 = http://a/b/c/h
g;x=1/./y <undefined>
$8 = http://a/b/c/g;x=1/y
g;x=1/../y #Related
$9 = 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 Related

where <undefined> indicates that the
base path. This error component is not present, as is rarely noticed, since typical usage of a
fragment never includes
the hierarchy ("/") character, and case for 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 above example. Therefore, we
can determine the base URI scheme. This is considered to be a
loophole in prior specifications value of partial URI [RFC1630]. Its use
should be avoided, but is allowed for backwards compatibility.

http:g the four components and fragment as

scheme = $2
authority = $4
path = $5
query = http:g ; for validating parsers
/ http://a/b/c/g ; for backwards compatibility $7
fragment = $9

and, going in the opposite direction, we can recreate a URI reference
from its components using the algorithm of Section 5.3.

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Appendix D. C. 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 D. Delimiting a 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, line-breaks, 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

net/rfc/>. Note the same syntax as a relative URI path,
abbreviated URI references cannot be used warning in contexts where relative
URIs are expected. This limits <http://www.ics.uci.edu/pub/
ietf/uri/historical.html#WARNING>.

contains the use of abbreviated URIs to places
where there is no defined base URI, such as dialog boxes and off-line
advertisements. 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 G. E. Summary of Non-editorial Changes

G.1

E.1 Additions

IPv6 literals have been added to the list of possible identifiers for
the host portion of a server authority component, as described by [RFC2732],
with the addition of "[" and "]" to the reserved, uric, reserved and
uric-no-slash uric sets.
Square brackets are now specified as reserved
for the authority component, allowed within the opaque part of an
opaque URI, authority
component and not allowed in the hierarchical syntax except for outside 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] [RFC3513] for definition of an IPv6 literal
address, which unfortunately has lacks 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]. [RFC3513].
Likewise, the definition of IPv4address has been improved in order 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 (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. Likewise,
Section 2.1 on the encoding of characters has been replaced.

An ABNF production for URI has been introduced to correspond to the
common usage of the term: an absolute URI with optional fragment.

E.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.

Section 2.2 on reserved characters has been rewritten to clearly
explain what characters are reserved, when they are reserved, and why
they are reserved even when not used as delimiters by the generic
syntax. Likewise, the section on escaped characters has been
rewritten, and URI normalizers are now given license to unescape any
octets corresponding to unreserved characters. The crosshatch ("#")
character has been moved back from the excluded delims to the
reserved set.

The ABNF for URI and URI-reference has been redesigned to
limit each decimal octet make them
more friendly to the range 0-255, LALR parsers and significantly reduce complexity. As

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a result, the definition layout form of
hostname syntax description has been improved removed,
along with the uric-no-slash, opaque-part, and rel-segment
productions. All references to "opaque" URIs have been replaced with
a better specify length limitations and
partially-qualified domain names.

Section 6 on URI normalization and comparison description of how the path component may be opaque to
hierarchy. The fragment identifier has been completely
rewritten and extended using input from Tim Bray moved back into the
section on generic syntax components and discussion within the W3C Technical Architecture Group.

G.2 Modifications URI and
relative-URI productions, though it remains excluded from RFC 2396
absolute-URI. The ad-hoc BNF syntax has been replaced with ambiguity regarding the ABNF parsing of [RFC2234].
This change required all rule names that formerly included underscore
characters to be renamed URI-reference as
a URI or a relative-URI with a dash instead. Likewise, absoluteURI
and relativeURI have been changed to absolute-URI colon in the first segment is now
explained and relative-URI,
respectively, for consistency. disambiguated in the section defining relative-URI.

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 ambiguity regarding the parsing of
net-path, abs-path, and rel-path is now explained and disambiguated
in the same section.

Registry-based naming authorities that use the hierarchical authority
syntax component are now limited to DNS hostnames, since those have
been the only such URIs in deployment. This change was necessary to
enable internationalized domain names to be processed in their native
character encodings at the application layers above URI processing.
The reg_name, server, and hostport productions have been removed to
simplify parsing of the URI syntax.

The ABNF of qualified has been simplified to remove a parsing
ambiguity without changing the allowed syntax. allowed syntax. The toplabel
production has been removed because it served no useful purpose. The
ambiguity regarding the parsing of host as IPv4address or hostname is
now explained and disambiguated in the same section.

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.

o Removed the special-case treatment of same-document references in
favor of a section that explains that a new retrieval action
should not be made if the target URI and base URI, excluding
fragments, match.

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Index

A
ABNF 9
abs-path 14 15
absolute 9
absolute-path 22
absolute-URI 14
absolute-URI-reference 20 23
access 7
alphanum 17
authority 15 15, 16

D
dec-octet 17
delims 12 13
dereference 8
domainlabel 17
dot-segments 19

E
escaped 11 12
excluded 13

F
fragment 20

G
generic syntax 5

H
h4 18
hier-part 14 15
hierarchical 9
host 16 17
hostname 17
hostport 16

I
identifier 5
invisible 13
IPv4 17
IPv4address 17
IPv6 18
IPv6address 18
IPv6reference 18

L
locator 6
ls32 18

M
mark 11

N
net-path 14

O
opaque-part 14

P
path 18

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M
mark 11

N
name 6
net-path 15
network-path 22

P
path 15, 19
path-segments 18 19
pchar 18 19
port 16 18

Q
qualified 17
query 19 20

R
reg-name 16
rel-path 22
rel-segment 15
relative 9
relative-path 22
relative-URI 22
representation 8
reserved 10
resolution 8
resource 4
retrieval 8

S
same-document 23
sameness 8
scheme 15
segment 18
server 16 19
suffix 23

T
toplabel 17
transcription 6

U
uniform 4
unreserved 11
unwise 12 13
URI grammar
abs-path 14 15
absolute-URI 14
absolute-URI-reference 20 23
ALPHA 9
alphanum 17

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authority 15 15, 16
CR 9
CTL 9
dec-octet 17
delims 12
DIGIT 9
domainlabel 17
DQUOTE 9
escaped 11 12
fragment 20 15, 20, 22
h4 18
HEXDIG 9
hier-part 14 15, 22, 23
host 16, 17
hostname 17
hostport 17
IPv4address 17
IPv6address 18
IPv6reference 18
LF 9
ls32 18
mark 11
net-path 14

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opaque-part 14
path 18 15
OCTET 9
path-segments 18 15, 19
pchar 18 19, 20, 20
port 17 16, 18
qualified 17
query 19
reg-name 16 15, 20, 22, 23
rel-path 22
rel-segment 22 15
relative-URI 22, 22
reserved 10 11
scheme 15 15, 16, 23
segment 18
server 16
toplabel 17 19
SP 9
unreserved 11
unwise 12
URI 15, 22
URI-reference 20 22
uric 9
uric-no-slash 14 10
userinfo 16, 16
URI 15
URI-reference 20 22
uric 9
uric-no-slash 14 10
URL 6
URN 6
userinfo 16

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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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