wdiff draft-fielding-uri-rfc2396bis-03.txt draft-fielding-uri-rfc2396bis-04.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: August 16, 2004 L. Masinter
Expires: December 5, 2003
Adobe
June 6, 2003
February 16, 2004

Uniform Resource Identifier (URI): Generic Syntax
draft-fielding-uri-rfc2396bis-03
draft-fielding-uri-rfc2396bis-04

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
Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at
<http://www.ietf.org/ietf/1id-abstracts.txt>.

The list of Internet-Draft Shadow Directories can be accessed at
<http://www.ietf.org/shadow.html>.

This Internet-Draft will expire on August 16, 2004.

Abstract

A Uniform Resource Identifier (URI) is a compact string of characters
for identifying an abstract or physical resource. This specification
defines the generic URI syntax and a process for resolving URI
references that might be in relative form, along with guidelines and
security considerations for the use of URIs on the Internet.

The URI syntax 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. This specification does not define a
generative grammar for URIs; that task is performed by the individual
specifications of each URI scheme.

Editorial Note

Discussion of this draft and comments to the editors should be sent
to the uri@w3.org mailing list. An issues list and version history
is available at <http://www.apache.org/~fielding/uri/rev-2002/
issues.html>. <http://gbiv.com/protocols/uri/rev-2002/issues.html>.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Overview of URIs . . . . . . . . . . . . . . . . . . . . . . 4
1.1.1 Generic Syntax . . . . . . . . . . . . . . . . . . . . . . . 5
1.1.2 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1.3 URI, URL, and URN . . . . . . . . . . . . . . . . . . . . . 6
1.2 Design Considerations . . . . . . . . . . . . . . . . . . . 6
1.2.1 Transcription . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.2 Separating Identification from Interaction . . . . . . . . . 7
1.2.3 Hierarchical Identifiers . . . . . . . . . . . . . . . . . . 8 9
1.3 Syntax Notation . . . . . . . . . . . . . . . . . . . . . . 9 10
2. Characters . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1 Percent Encoding of Characters . . . . . . . . . . . . . . . . . . . . . . 11
2.2 Reserved Characters . . . . . . . . . . . . . . . . . . . . 11 12
2.3 Unreserved Characters . . . . . . . . . . . . . . . . . . . 12
2.4 Escaped Characters . . . . . . . . . . . . . . . . . . . . . 13
2.4.1 Escaped Encoding . . . . . . . . . . . . . . . . . . . . . . 13
2.4.2 When to Escape and Unescape . . . . . . . . . . . . . . . . 13
2.5 Excluded Characters . . Encode or Decode . . . . . . . . . . . . . . . . . . 14 13
3. Syntax Components . . . . . . . . . . . . . . . . . . . . . 16 15
3.1 Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 15
3.2 Authority . . . . . . . . . . . . . . . . . . . . . . . . . 17 16
3.2.1 User Information . . . . . . . . . . . . . . . . . . . . . . 18 16
3.2.2 Host . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 17
3.2.3 Port . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.3 Path . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.4 Query . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.5 Fragment . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.1 URI Reference . . . . . . . . . . . . . . . . . . . . . . . 24
4.2 Relative URI . . . . . . . . . . . . . . . . . . . . . . . . 24
4.3 Absolute URI . . . . . . . . . . . . . . . . . . . . . . . . 25
4.4 Same-document Reference . . . . . . . . . . . . . . . . . . 25
4.5 Suffix Reference . . . . . . . . . . . . . . . . . . . . . . 25
5. Reference Resolution . . . . . . . . . . . . . . . . . . . . 27
5.1 Establishing a Base URI . . . . . . . . . . . . . . . . . . 27
5.1.1 Base URI within Document Content . . . . . . . . . . . . . . 27
5.1.2 Base URI from the Encapsulating Entity . . . . . . . . . . . 28
5.1.3 Base URI from the Retrieval URI . . . . . . . . . . . . . . 28
5.1.4 Default Base URI . . . . . . . . . . . . . . . . . . . . . . 28
5.2 Obtaining Relative Resolution . . . . . . . . . . . . . . . . . . . . 28
5.2.1 Pre-parse the Referenced Base URI . . . . . . . . . . . . . . . . 28 . . . 29
5.2.2 Transform References . . . . . . . . . . . . . . . . . . . . 29
5.2.3 Merge Paths . . . . . . . . . . . . . . . . . . . . . . . . 30
5.2.4 Remove Dot Segments . . . . . . . . . . . . . . . . . . . . 30
5.3 Component Recomposition of a Parsed URI . . . . . . . . . . . . . . . 31 . . . 32
5.4 Reference Resolution Examples . . . . . . . . . . . . . . . 32 33
5.4.1 Normal Examples . . . . . . . . . . . . . . . . . . . . . . 32 33
5.4.2 Abnormal Examples . . . . . . . . . . . . . . . . . . . . . 32 33
6. Normalization and Comparison . . . . . . . . . . . . . . . . 35
6.1 Equivalence . . . . . . . . . . . . . . . . . . . . . . . . 35
6.2 Comparison Ladder . . . . . . . . . . . . . . . . . . . . . 35 36
6.2.1 Simple String Comparison . . . . . . . . . . . . . . . . . . 36
6.2.2 Syntax-based Normalization . . . . . . . . . . . . . . . . . 37
6.2.3 Scheme-based Normalization . . . . . . . . . . . . . . . . . 38
6.2.4 Protocol-based Normalization . . . . . . . . . . . . . . . . 38 39
6.3 Canonical Form . . . . . . . . . . . . . . . . . . . . . . . 38 39
7. Security Considerations . . . . . . . . . . . . . . . . . . 40 41
7.1 Reliability and Consistency . . . . . . . . . . . . . . . . 40 41
7.2 Malicious Construction . . . . . . . . . . . . . . . . . . . 40 41
7.3 Back-end Transcoding . . . . . . . . . . . . . . . . . . . . 42
7.4 Rare IP Address Formats . . . . . . . . . . . . . . . . . . 41
7.4 42
7.5 Sensitive Information . . . . . . . . . . . . . . . . . . . 41
7.5 43
7.6 Semantic Attacks . . . . . . . . . . . . . . . . . . . . . . 41 43
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 43 45
Normative References . . . . . . . . . . . . . . . . . . . . 44 46
Informative References . . . . . . . . . . . . . . . . . . . 45 47
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 47 48
A. Collected ABNF for URI . . . . . . . . . . . . . . . . . . . 48 50
B. Parsing a URI Reference with a Regular Expression . . . . . 50 52
C. Delimiting a URI in Context . . . . . . . . . . . . . . . . 51 53
D. Summary of Non-editorial Changes . . . . . . . . . . . . . . 53 55
D.1 Additions . . . . . . . . . . . . . . . . . . . . . . . . . 53 55
D.2 Modifications from RFC 2396 . . . . . . . . . . . . . . . . 53 55
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 58
Intellectual Property and Copyright Statements . . . . . . . 60 62

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 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]. Advice for designers of new URI
schemes can be found in [RFC2718].

All significant changes from RFC 2396 are noted in Appendix D.

This specification uses the terms "character" and "character
encoding" in accordance with the definitions provided in [RFC2978].

1.1 Overview of URIs

URIs are characterized 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

Anything that can be 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. A resource is not necessarily
accessible via the Internet; e.g., human beings, corporations, and
bound books in a library can also be resources. Likewise, abstract
concepts can be resources, such as the operators and operands of a
mathematical equation or the 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 identifier that consists of a sequence of characters
matching the syntax defined by the grammar syntax rule named "URI" in Section
3. A URI can be used to refer to a resource. This specification does
not place any limits on the nature of a resource or the reasons why
an application might wish to refer to a resource. URIs have a global
scope and should be interpreted consistently regardless of context,
but that interpretation may be defined in relation to the user's
context (e.g., "http://localhost/" refers to a resource that is
relative to the user's network interface and yet not specific to any
one user).

1.1.1 Generic Syntax

Each URI begins with a scheme name, as defined in Section 3.1, that
refers to a specification for assigning identifiers within that
scheme. As such, the URI syntax is a federated and 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
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. Likewise, protocols and data
formats that make use of URI references can refer to this
specification as defining the range of syntax allowed for all URIs,
including those schemes that have yet to be 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.

1.1.2 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

1.1.3 URI, URL, and URN

A URI can be further classified as a locator, a name, or both. The
term "Uniform Resource Locator" (URL) refers to the subset of URIs
that, in addition to identifying a resource, provide a means of
locating the resource by describing its primary access mechanism
(e.g., its network "location"). The term "Uniform Resource Name"
(URN) refers has been used historically to refer to both URIs under the
"urn" scheme [RFC2141], which are required to remain globally unique
and persistent even when the resource ceases to exist or becomes unavailable.
unavailable, and to any other URI with the properties of a name.

An individual scheme does not need to be classified as being just one
of "name" or "locator". Instances of URIs from any given scheme may
have the characteristics of names or locators or both, often
depending on the persistence and care in the assignment of
identifiers by the naming authority, rather than any quality of the
scheme. Future specifications and related documentation should use
the general term "URI", rather than the more restrictive terms URL
and URN [RFC3305].

1.2 Design Considerations

1.2.1 Transcription

The URI syntax has been designed with global transcription as one of
its main considerations. A URI is a sequence of characters from a
very limited 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
integers from 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 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 considerations revealed by the scenario:

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

o A URI might be transcribed from a non-network source, and thus
should consist of characters that are most likely to be able to be
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 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 a resource identifier from one
medium to another has been considered more important than having a
URI consist of the most meaningful of components.

In local 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 specification. Percent-encoded
octets (Section 2.1) may be used within a URI to represent characters
outside the range of the US-ASCII coded character set if such
representation is defined by the scheme or by the protocol element in
which the URI is referenced; such a definition will specify the
character encoding scheme used to map those characters to octets
prior to being percent-encoded for the URI.

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

Given a URI, a system may attempt to perform a variety of operations
on the resource, as might be characterized by such words as "denote", "access",
"update", "replace", or "find attributes". Such operations are
defined by the protocols that 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 the process of
determining an access mechanism and the appropriate parameters
necessary to dereference a URI; such resolution may require several
iterations. Use of To use that access mechanism to perform an action on the
URI's resource is termed a to "dereference" of the URI.

When URIs are used within information systems to identify sources of
information, the most common form of URI dereference is "retrieval":
making use of a URI in order to retrieve a representation of its
associated resource. A "representation" is a sequence of octets,
along with representation metadata describing those octets, that
constitutes a record of the state of the resource at the time that
the representation is generated. Retrieval is achieved by a process
that might include using the URI as a cache key to check for a
locally cached representation, resolution of the URI to determine an
appropriate access mechanism (if any), and dereference of the URI for
the sake of applying a retrieval operation. Depending on the
protocols used to perform the retrieval, additional information might
be supplied about the resource (resource metadata) and its relation
to other resources.

URI references in information systems are designed to be
late-binding: the result of an access is generally determined at the
time it is accessed and may vary over time or due to other aspects of
the interaction. When an author creates a reference to such a
resource, they do so with the intention that the reference be used in
the future; what is being identified is not some specific result that
was obtained in the past, but rather some characteristic that is
expected to be true for future results. In such cases, the resource
referred to by the URI is actually a sameness of characteristics as
observed over time, perhaps elucidated by additional comments or
assertions made by the resource provider.

Although many URI schemes are named after protocols, this does not
imply that use of such a URI will result in access to the resource
via the named protocol. URIs are often used simply for the sake of
identification. Even when a URI is 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
protocol associated with the scheme name, and the resolution of some
URIs may require the use of more than one protocol (e.g., both DNS
and HTTP are typically used to access an "http" URI's origin server
when a representation isn't found in a local cache).

1.2.3 Hierarchical Identifiers

The URI syntax is organized hierarchically, with components listed in
decreasing
order of decreasing significance from left to right. For some URI
schemes, the visible hierarchy is limited to the scheme itself:
everything after the scheme component delimiter (":") is considered
opaque to URI processing. Other URI schemes make the hierarchy
explicit and visible to generic parsing algorithms.

The URI generic syntax reserves uses the slash ("/"), question-mark question mark ("?"), and
number-sign
number sign ("#") characters for the purpose of delimiting components
that are significant to the generic parser's hierarchical
interpretation of an identifier. In addition to aiding the
readability of such identifiers through the consistent use of
familiar syntax, this uniform representation of hierarchy across
naming schemes allows scheme-independent references to be made
relative to that hierarchy.

It is often the case that a group or "tree" of documents has been
constructed to serve a common purpose; purpose, wherein the vast majority of
URIs in these documents point to resources within the tree rather
than outside of it. Similarly, documents located at a particular
site are much more likely to refer to other resources at that site
than to resources at remote sites. Relative referencing of URIs
allows document trees to be partially independent of their location
and access scheme. For instance, it is possible for a single set of
hypertext documents to be simultaneously accessible and traversable
via each of the "file", "http", and "ftp" schemes if the documents
refer to each other using relative references. Furthermore, such
document trees can be moved, as a whole, without changing any of the
relative references.

A relative URI reference (Section 4.2) refers to a resource by
describing the difference within a hierarchical name space between
the current reference context and the target URI. The reference resolution
algorithm, presented in Section 5, defines how such references are
resolved.

1.3 Syntax Notation

This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC2234] a reference is
transformed to define the URI syntax. Although target URI. Since relative references can only be
used within the ABNF
defines syntax in terms context of the US-ASCII character encoding [ASCII],
the a hierarchical URI, designers of new URI syntax
schemes should be interpreted in terms of use a syntax consistent with the character generic syntax's
hierarchical components unless there are compelling reasons to forbid
relative referencing within that
the ASCII-encoded octet represents, rather than the octet encoding
itself. How a scheme.

All URIs are parsed by generic syntax parsers when used. A URI is represented in terms scheme
that wishes to remain opaque to hierarchical processing must disallow
the use of bits slash and bytes on the
wire question mark characters. However, since a
non-relative URI reference is dependent upon only modified by the character encoding generic parser if
it contains complete path segments of the protocol used to
transport it, "." or ".." (see Section 3.3),
URIs may safely use "/" for other purposes if they do not allow
dot-segments.

1.3 Syntax Notation

This specification uses the charset Augmented Backus-Naur Form (ABNF)
notation of [RFC2234], including the document that contains it.

The following core ABNF productions are used by this specification as syntax rules
defined by Section 6.1 of [RFC2234]: ALPHA, CR, CTL, DIGIT, DQUOTE,
HEXDIG, LF, OCTET, that specification: ALPHA (letters), CR (carriage return),
CTL (control characters), DIGIT (decimal digits), DQUOTE (double
quote), HEXDIG (hexadecimal digits), LF (line feed), and SP. SP (space).
The complete URI syntax is collected in Appendix A.

2. Characters

A URI consists of a restricted set of characters, primarily chosen

Although ABNF notation defines its terminal values to aid transcription and usability both in computer systems and be non-negative
integers (codepoints) based on the US-ASCII coded character set
[ASCII], we must invert that relation in
non-computer communications. Characters used conventionally as
delimiters around a order to understand the URI
syntax, since URIs are excluded. The set defined as strings of URI characters
consists of digits, letters, and a few graphic symbols chosen from
those common to most independent
of any particular encoding. Therefore, the character encodings and input facilities
available to Internet users.

uric = reserved / unreserved / escaped

Within a URI, reserved characters are used to delimit syntax
components, unreserved characters are used integer values must be
mapped back to describe registered
names, and unreserved, non-delimiting reserved, and escaped their corresponding characters are used to represent strings of data (1*OCTET) within the
components.

2.1 Encoding of Characters

As described above (Section 1.3), the URI syntax is defined via US-ASCII in terms
of characters by reference order
to complete the US-ASCII encoding of characters to
octets. syntax rules.

This specification does not mandate the use of any particular
character encoding scheme for mapping between its character set URI characters and the
octets used to store or transmit those characters. When a URI characters representing strings of data within appears
in a component may,
if allowed by protocol element, the component production, represent an arbitrary
sequence of octets. For example, portions of character encoding is defined by that
protocol; absent such a definition, a given URI might
correspond is assumed to a filename on a non-ASCII file system, a query on
non-ASCII data, numeric coordinates on a map, etc. Some use the same
character encoding as the surrounding text.

A URI schemes
define is composed from a specific encoding limited set of raw data to US-ASCII characters as part consisting of their scheme-specific requirements. Most URI schemes represent
data octets by the US-ASCII character corresponding
digits, letters, and a few graphic symbols. A reserved (Section 2.2)
subset of those characters may be used to that octet,
either directly in delimit syntax components
within a URI, while the form of remaining characters, including both the character's glyph or by use of an
escape triplet
unreserved (Section 2.4).

When 2.3) set and those reserved characters not acting
as delimiters, define each component's data.

2.1 Percent Encoding

A percent-encoding mechanism is used to represent a URI scheme defines data octet in a
component when that represents textual data octet's corresponding character is outside the
allowed set or is being used as a delimiter of, or within, the
component. A percent-encoded octet is encoded as a character triplet,
consisting of characters from the Unicode (ISO 10646) percent character set,
we recommend "%" followed by the two
hexadecimal digits representing that octet's numeric value. For
example, "%20" is the data be encoded first as octets according percent-encoding for the binary octet
"00100000" (ABNF: %x20), which in US-ASCII corresponds to the UTF-8 [UTF-8] space
character encoding, and then escaping only those
octets that (SP).

pct-encoded = "%" HEXDIG HEXDIG

The uppercase hexadecimal digits 'A' through 'F' are not equivalent to
the lowercase digits 'a' through 'f', respectively. Two URIs that
differ only in the unreserved character set. case of hexadecimal digits used in percent-encoded
octets are equivalent. For consistency, URI producers and
normalizers should use uppercase hexadecimal digits for all
percent-encodings.

2.2 Reserved Characters

URIs include components and sub-components that are delimited by
certain special characters.
characters in the "reserved" set. These characters are called "reserved",
since their usage within
"reserved" because they may (or may not) be defined as delimiters by
the generic syntax, by each scheme-specific syntax, or by the
implementation-specific syntax of a URI component is limited to their reserved
purpose within that component. URI's dereferencing algorithm.
If data for a URI component would conflict with the a reserved purpose,
character's purpose as a delimiter, then the conflicting data must be
escaped (Section 2.4)
percent-encoded before forming the URI.

reserved = gen-delims / sub-delims

gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / ";" /
":" / "@"

sub-delims = "!" / "$" / "&" / "=" "'" / "+" "(" / "$" ")"
/ "*" / "+" / ","

Reserved / ";" / "="

A subset of the reserved characters (gen-delims) are used as
delimiters of the generic URI components described in Section 3, as well as within those components
for delimiting sub-components. 3. A
component's ABNF syntax rule will not use the "reserved" production reserved or gen-delims
rule names directly; instead, each syntax rule lists those reserved
characters that are allowed within that component.
Allowed component (i.e., not
delimiting it). The allowed reserved characters characters, including those in
the sub-delims set and any of the gen-delims that are not assigned a sub-component delimiter role by this specification should be considered
of that component, are reserved for special use by whatever software generates as sub-component delimiters
within the URI (i.e., they component. Only the most common sub-components are
defined by this specification; other sub-components may be used to delimit defined by
a URI scheme's specification, or indicate information that is significant to
interpretation of the identifier, but that significance is outside by the scope of this specification). Outside implementation-specific
syntax of the URI's origin, a
reserved character cannot be escaped without fear of changing how it
will be interpreted; likewise, an escaped octet URI's dereferencing algorithm, provided that corresponds to such
sub-components are delimited by characters in that component's
reserved set. If no such delimiting role has been assigned, then a
reserved character cannot be unescaped outside appearing in a component represents the software that is
responsible for interpreting it during URI resolution.

The slash ("/"), question-mark ("?"), and number-sign ("#")
characters are reserved data octet
corresponding to its encoding in all US-ASCII.

URIs for the purpose of delimiting
components that are significant to differ in the generic parser's hierarchical
interpretation of an identifier. The hierarchical prefix replacement of a URI,
wherein the slash ("/") reserved character signifies with its
corresponding percent-encoded octet are not equivalent.
Percent-encoding a hierarchy delimiter,
extends from the scheme (Section 3.1) through to the first
question-mark ("?"), number-sign ("#"), reserved character, or the end of the URI string.
In other words, the slash ("/") character is not treated as decoding a
hierarchical separator within the query (Section 3.4) and fragment
(Section 3.5) components of percent-encoded
octet that corresponds to a URI, but is still considered reserved
within those components for purposes outside character, will change how the scope of this
specification.
URI is interpreted by most applications.

2.3 Unreserved Characters

Characters that are allowed in a URI but do not have a reserved
purpose are called unreserved. These include uppercase and lowercase
letters, decimal digits, hyphen, period, underscore, and a limited set of punctuation marks and
symbols. tilde.

unreserved = ALPHA / DIGIT / mark

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

Escaping unreserved characters in a URI does not change what resource
is identified by

URIs that URI. differ in the replacement of an unreserved character with
its corresponding percent-encoded octet are equivalent: they identify
the same resource. However, it percent-encoded unreserved characters
may change the result of a some URI comparison comparisons (Section 6),
potentially leading to less efficient
actions by an application. Therefore, unreserved characters should
not be escaped unless the URI is being used in a context that does
not allow the unescaped character to appear. URI normalization
processes may unescape sequences incorrect or inefficient behavior. For
consistency, percent-encoded octets in the ranges of ALPHA (%41-%5A
and %61-%7A), DIGIT (%30-%39), hyphen (%2D), period (%2E), underscore
(%5F), or tilde (%7E) without fear of creating a 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 should not correspond to
a printable character of the US-ASCII coded character set or
corresponds to a US-ASCII character that delimits the component from
others, is reserved in that component for delimiting sub-components,
or is excluded from any use within a be created by URI (Section 2.5).

2.4.1 Escaped Encoding

An escaped octet is encoded as producers and,
when found in a character triplet, consisting of
the percent character "%" followed by the two hexadecimal digits
representing that octet's numeric value. For example, "%20" is the
escaped encoding for the binary octet "00100000" (ABNF: %x20), which
corresponds URI, should be decoded to the US-ASCII space their corresponding
unreserved 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
the lowercase digits 'a' through 'f', respectively. Two URIs that
differ only in the case of hexadecimal digits used in escaped octets
are equivalent. For consistency, we recommend that uppercase digits
be used by URI generators and normalizers.

2.4.2

2.4 When to Escape and Unescape Encode or Decode

Under normal circumstances, the only time that characters octets within a URI string
are escaped percent-encoded is during the process of generating producing the URI from
its component parts. Each component may have its own set of
characters It is during that are reserved, so only the mechanism responsible for
generating or interpreting process that component can determine whether or
not escaping a character will change its semantics. The exception is
when a URI is being used within a context where an
implementation determines which of the unreserved "mark" reserved characters might need are to be escaped, such
used as when sub-component delimiters and which can be safely used for a
command-line argument or within a single-quoted attribute. as
data. Once generated, produced, a URI is always in an escaped its percent-encoded form.

When a URI is
resolved, dereferenced, the components and sub-components
significant to that the scheme-specific
resolution dereferencing process (if any)
must be parsed and separated before the
escaped characters percent-encoded octets within
those components can be safely unescaped.

In some cases, decoded, since otherwise the data that could be represented by an unreserved
character may appear escaped;
be mistaken for example, some of the unreserved
"mark" characters are automatically escaped by some systems. A URI
normalizer may unescape escaped component delimiters. The only exception is for
percent-encoded octets that are represented by corresponding to characters in the unreserved set.
set, which can be decoded at any time. For example, "%7E" is sometimes
used instead of the octet
corresponding to the tilde ("~") in an "http" character is often encoded as "%7E"
by older URI path and processing software; the "%7E" can be
converted to replaced by "~"
without changing the interpretation of the URI.

In all cases, a URI character is equivalent to its corresponding
ASCII-encoded octet, even when that octet is represented as a
percent-escape. URI characters are provided as an external ASCII
interface for identification between systems. A system that
internally provides identifiers in the form of a different character
encoding, such as EBCDIC, will generally perform character
translation of textual identifiers to UTF-8 at some internal
interface, thus providing meaningful identifiers in ASCII even though
the back-end identifiers are in a different encoding. Escaped octets
must be unescaped before such a transcoding is applied. Although
this specification does not define the character encoding of escaped
octets outside the ASCII range, the general principle of unescaping
before transcoding should be applied for all character encodings. interpretation.

Because the percent ("%") character serves as the escape indicator, indicator for
percent-encoded octets, it must be escaped percent-encoded as "%25" in order
for that octet to be used as data within a URI. Implementers should be careful Implementations must
not to escape percent-encode or
unescape decode the same string more than once, since unescaping
decoding an already
unescaped decoded string might lead to misinterpreting a
percent data
character octet as another escaped character, the beginning of a percent-encoding, or vice
versa in the case of
escaping percent-encoding an already escaped percent-encoded
string.

2.5 Excluded Characters

Although they are disallowed within the

URI syntax, we include here
a description of those characters that have been excluded and the
reasons serve as an external interface for their exclusion.

excluded = invisible / delims / unwise

The control characters (CTL) identification
between systems. A system that internally provides identifiers in
the US-ASCII coded character set are
not used within form of a URI, both because they are non-printable and
because they are likely different character encoding, such as EBCDIC, will
generally perform character translation of textual identifiers to be misinterpreted by
UTF-8 [RFC3629] (or some control
mechanisms. The space other superset of the US-ASCII character (SP) is excluded because significant
spaces may disappear and insignificant spaces may
encoding) at an internal interface, since that results in more
meaningful identifiers than simply percent-encoding the original
octets. When interpreting an incoming URI on such an interface,
percent-encoded octets must be introduced when decoded before the reverse transcoding
can be applied.

In some cases, the interface between a URI is transcribed, typeset, or subjected to component and the treatment of
word-processing programs. Whitespace is also used
identifying data it has been crafted to delimit represent is much less direct
than a character encoding translation. For example, portions of a
URI
in many contexts. Characters outside the US-ASCII set are excluded might reflect a query on non-ASCII data, numeric coordinates on a
map, etc. Likewise, a URI scheme may define components with
additional encoding requirements, such as
well.

invisible = CTL / SP / %x80-FF

The angle-bracket ("<" and ">") and double-quote (") characters are
excluded because they base64, that are often used as applied
prior to forming the delimiters around component and producing the URI.

When a URI
in text documents and protocol fields. The percent scheme defines a component that represents textual data
consisting of characters from the Unicode (ISO/IEC 10646-1) character ("%")
is excluded because it is used for
set, the data should be encoded first as octets according to the
UTF-8 character encoding of escaped (Section
2.4) characters.

delims = "<" / ">" / "%" / DQUOTE

Other characters are excluded because gateways [RFC3629], and other transport
agents are known to sometimes modify such characters.

unwise = "{" / "}" / "|" / "\" / "^" / "`"

Data then only those octets corresponding that
do not correspond to excluded characters must be escaped in
order to the unreserved set should be
percent-encoded. For example, the character A would be represented within a URI.
as "A", the character LATIN CAPITAL LETTER A WITH GRAVE would be
represented as "%C3%80", and the character KATAKANA LETTER A would be
represented as "%E3%82%A2".

3. Syntax Components

The generic URI syntax consists of a hierarchical sequence of
components referred to as the scheme, authority, path, query, and
fragment.

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

hier-part = net-path / abs-path / rel-path

net-path = "//" authority [ abs-path ]
abs-path = "/" path-segments
rel-path = path-segments ["//" authority] path ["?" query] ["#" fragment]

The scheme and path components are required, though path may be empty
(no characters). An ABNF-driven parser of hier-part will find that the three productions in the rule are ambiguous: border
between authority and path is 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 net-path; if it begins with only one slash
character, then it is an abs-path; otherwise, it is a rel-path. Note
that rel-path does not necessarily contain any slash ("/")
characters; a non-hierarchical path will be treated as opaque data by
a generic URI parser.

The authority component is only present when a string matches the
net-path production. Since then the presence first segment of an authority component
restricts the 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 parsed URI string matching the abs-path
or rel-path production in the syntax above, since they are mutually
exclusive for any given URI and can must be parsed as a single component.
empty.

The following are two example URIs and their component parts:

foo://example.com:8042/over/there?name=ferret#nose
\_/ \______________/\_________/ \_________/ \__/
| | | | |
scheme authority path query fragment
| _____________________|__
/ \ / \
urn:example:animal:ferret:nose

3.1 Scheme

Each URI begins with a scheme name that refers to a specification for
assigning identifiers within that scheme. As such, the URI syntax is
a federated and extensible naming system wherein each scheme's
specification may further restrict the 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 documents that
specify schemes must do so using lowercase letters. An
implementation should accept uppercase letters as equivalent to
lowercase in scheme names (e.g., allow "HTTP" as well as "http"), for
the sake of robustness, but should only generate produce lowercase scheme
names, for consistency.

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

Individual schemes are not specified by this document. The process
for registration of new URI schemes is defined separately by

[RFC2717]. The scheme registry maintains the mapping between scheme
names and their specifications. Advice for designers of new URI
schemes can be found in [RFC2718].

When presented with a URI that violates one or more scheme-specific
restrictions, the scheme-specific resolution process should flag the
reference as an error rather than ignore the unused parts; doing so
reduces the number of equivalent URIs and helps detect abuses of the
generic syntax that might indicate the URI has been constructed to
mislead the user (Section 7.6).

3.2 Authority

Many URI schemes include a hierarchical element for a naming
authority, such that governance of the name space defined by the
remainder of the URI is delegated to that authority (which may, in
turn, delegate it 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 user information.

The authority component is preceded by a double slash ("//") and is
terminated by the next slash ("/"), question-mark question mark ("?"), or
number-sign number
sign ("#") character, or by the end of the URI.

authority = [ userinfo "@" ] host [ ":" port ]

The parts "<userinfo>@"

URI producers and ":<port>" may be omitted. normalizers should omit the "@" delimiter that
separates userinfo from host if the userinfo component is empty (zero
length) and should omit the ":" delimiter that separates host from
port if the port component is empty. Some schemes do not allow the
userinfo and/or port sub-components.
When presented with a URI that violates one or more scheme-specific
restrictions, the scheme-specific URI resolution process should flag
the reference as an error rather than ignore the unused parts; doing
so reduces the number of equivalent URIs and helps detect abuses of
the generic syntax that might indicate the URI has been constructed
to mislead the user (Section 7.5).

3.2.1 User Information

The userinfo sub-component may consist of a user name and,
optionally, scheme-specific information about how to gain
authorization to access the server. resource. The user information, if
present, is followed by a commercial at-sign ("@") that delimits it
from the host.

userinfo = *( unreserved / escaped pct-encoded / ";" sub-delims / ":" / "&" / "=" / "+" / "$" / "," )

Some URI schemes use

Use of the format "user:password" in the userinfo
field. This practice field is NOT RECOMMENDED, because
deprecated. Applications should not render as clear text any data
after the first colon (":") character found within a userinfo
sub-component unless such data is the empty string (indicating no
password) or "anonymous". Applications may choose to ignore or reject
such data when received as part of a reference, and should reject the
storage of such data in unencrypted form. The passing of
authentication information in clear text has proven to be a security
risk in almost every case where it has been used. Note also

Applications that render a URI for the sake of user feedback, such as
in graphical hypertext browsing, should render userinfo might be in a way that
is distinguished from the rest of a URI, when feasible. Such
rendering will assist the user in cases where the userinfo has been
misleadingly crafted to look like a trusted domain name in order
to mislead users, as described in Section 7.5. (Section
7.6).

3.2.2 Host

The host sub-component of authority is identified by an IPv6 IP literal
encapsulated within square brackets, an IPv4 address in
dotted-decimal form, or a domain host name.

host = [ IPv6reference IP-literal / IPv4address / hostname ]

If host is omitted, a default may be defined by the scheme-specific
semantics of the URI. For example, the "file" URI scheme defaults to
"localhost", whereas the "http" URI scheme does not allow host to be
omitted. reg-name

The production syntax rule for host is ambiguous because it does not completely
distinguish between an IPv4address and a hostname. reg-name. Again, the
"first-match-wins" algorithm applies: If host matches the production rule for
IPv4address, then it should be considered an IPv4 address literal and
not a hostname.

A hostname takes reg-name. Although host is case-insensitive, producers and
normalizers should use lowercase for host names and hexadecimal
addresses for the form described in Section 3 sake of [RFC1034] uniformity, while only using uppercase
letters for percent-encodings.

A host identified by an Internet Protocol literal address, version 6
[RFC3513] or later, is distinguished by enclosing the IP literal
within square brackets ("[" and
Section 2.1 "]"). This is the only place where
square bracket characters are allowed in the URI syntax. In
anticipation of [RFC1123]: future, as-yet-undefined IP literal address formats,
an optional version flag may be used to indicate such a sequence format
explicitly rather than relying on heuristic determination.

IP-literal = "[" ( IPv6address / IPvFuture ) "]"

IPvFuture = "v" HEXDIG "." 1*( unreserved / sub-delims / ":" )

The version flag does not indicate the IP version; rather, it
indicates future versions of domain labels separated by
".", each domain label starting the literal format. As such,
implementations must not provide the version flag for existing IPv4
and ending IPv6 literal addresses. If a URI containing an IP-literal that
starts with "v" (case-insensitive), indicating that the version flag
is present, is dereferenced by an alphanumeric
character and possibly also containing "-" characters. The rightmost
domain label application that does not know the
meaning of a fully qualified domain name may be followed that version flag, then the application should return an
appropriate error for "address mechanism not supported".

A host identified by an IPv6 literal address is represented inside
the square brackets without a
single "." if it preceding version flag. The ABNF
provided here is necessary a translation of the text definition of an IPv6
literal address provided in [RFC3513]. A 128-bit IPv6 address is
divided into eight 16-bit pieces. Each piece is represented
numerically in case-insensitive hexadecimal, using one to distinguish between four
hexadecimal digits (leading zeroes are permitted). The eight encoded
pieces are given most-significant first, separated by colon
characters. Optionally, the complete
domain name least-significant two pieces may instead
be represented in IPv4 address textual format. A sequence of one or
more consecutive zero-valued 16-bit pieces within the address may be
elided, omitting all their digits and some local domain.

hostname = domainlabel qualified
qualified leaving exactly two consecutive
colons in their place to mark the elision.

IPv6address = *( "." domainlabel 6( h16 ":" ) ls32
/ "::" 5( h16 ":" ) ls32
/ [ "." h16 ]
domainlabel = alphanum "::" 4( h16 ":" ) ls32
/ [ 0*61( alphanum *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
/ "-" [ *2( h16 ":" ) alphanum h16 ]
alphanum "::" 2( h16 ":" ) ls32
/ [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32
/ [ *4( h16 ":" ) h16 ] "::" ls32
/ [ *5( h16 ":" ) h16 ] "::" h16
/ [ *6( h16 ":" ) h16 ] "::"

ls32 = ALPHA ( h16 ":" h16 ) / DIGIT IPv4address
; least-significant 32 bits of address

h16 = 1*4HEXDIG
; 16 bits of address represented in hexadecimal

A host identified by an IPv4 literal address is represented in
dotted-decimal notation (a sequence of four decimal numbers in the
range 0 to 255, separated by "."), as described in [RFC1123] by
reference to [RFC0952]. Note that other forms of dotted notation may
be interpreted on some platforms, as described in Section 7.3, 7.4, but
only the dotted-decimal form of four octets is allowed by this
grammar.

IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet

dec-octet = DIGIT ; 0-9
/ %x31-39 DIGIT ; 10-99
/ "1" 2DIGIT ; 100-199
/ "2" %x30-34 DIGIT ; 200-249
/ "25" %x30-35 ; 250-255

A host identified by an IPv6 literal address [RFC3513] a registered name is
distinguished by enclosing the IPv6 literal a string of characters that
is intended for lookup within square-brackets
("[" and "]"). This a locally-defined host or service name
registry. The most common of such registry mechanisms is the only place where square-bracket
characters are allowed Domain
Name System (DNS), as defined by Section 3 of [RFC1034] and Section
2.1 of [RFC1123]. A DNS name consists of a sequence of domain labels
separated by ".", each domain label starting and ending with an
alphanumeric character and possibly also containing "-" characters.
The rightmost domain label of a fully qualified domain name in DNS
may be followed by a single "." and should be followed by one if it
is necessary to distinguish between the URI syntax.

IPv6reference = "[" IPv6address "]"

IPv6address complete domain name and some
local domain.

reg-name = 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 0*255( unreserved / [ *4( h4 ":" ) h4 ] "::" ls32 pct-encoded / [ *5( h4 ":" sub-delims ) h4 ] "::" h4
/ [ *6( h4 ":" ) h4 ] "::"

ls32 = ( h4 ":" h4 ) / IPv4address
; least-significant 32 bits

If the host component is defined and the registered name is empty
(zero length), then the name defaults to "localhost" (Section 6.2.3
discusses how this should be normalized). If "localhost" is not
determined by a host name lookup, then it should be interpreted to
mean the machine on which the URI is being resolved.

This specification does not mandate a particular registered name
lookup technology and therefore does not restrict the syntax of address

h4 = 1*4HEXDIG
reg-name beyond that necessary for interoperability. Instead, it
delegates the issue of host name syntax conformance to the operating
system of each application performing URI resolution, and that
operating system decides what it will allow for the purpose of host
identification. A URI resolution implementation might use DNS, host
tables, yellow pages, NetInfo, WINS, or any other system for lookup
of host and service names. However, a globally-scoped naming system,
such as DNS fully-qualified domain names, is necessary for URIs that
are intended to have global scope. URI producers should use host
names that conform to the DNS syntax, even when use of DNS is not
immediately apparent.

The reg-name syntax allows percent-encoded octets in order to
represent non-ASCII host or service names in a uniform way that is
independent of the underlying name resolution technology; such octets
must represent characters encoded in the UTF-8 character encoding
[RFC3629] prior to being percent-encoded. When a non-ASCII host name
represents an internationalized domain name intended for resolution
via DNS, the name must be transformed to the IDNA encoding [RFC3490]
prior to name lookup. URI producers should provide such host names in
the IDNA encoding, rather than a percent-encoding, if they wish to
maximize interoperability with legacy URI resolvers.

The presence of host within a URI does not imply that the scheme
requires access to the given host on the Internet. In many cases,
the host syntax is used only for the sake of reusing the existing
registration process created and deployed for DNS, thus obtaining a
globally unique name without the cost of deploying another registry.
However, such use comes with its own costs: domain name ownership may
change over time for reasons not anticipated by the URI creator. producer.

3.2.3 Port

The port sub-component of authority is designated by an optional port
number in decimal following the host and delimited from it by a
single colon (":") character.

port = *DIGIT

If port is omitted,

A scheme may define a default may be defined by port. For example, the scheme-specific
semantics "http" scheme
defines a default port of the URI. Likewise, the "80", corresponding to its reserved TCP
port number. The type of network port designated by the port number (e.g.,
TCP, UDP, SCTP, etc.) is defined by the URI scheme. For example, the "http" URI scheme defines a default of TCP producers
and normalizers should omit the port 80. component and its ":" delimiter
if port is empty or its value would be the same as the scheme's
default.

3.3 Path

The path component contains data, usually organized in hierarchical data
form, that, along with data in the optional non-hierarchical query component
(Section 3.4) component, 3.4), serves to identify a resource within the scope of that the
URI's scheme and naming authority (if any). There is no specific "path" syntax production in the
generic If a URI syntax. Instead, what we refer to as contains an
authority component, then the URI initial path is
that part of the parsed URI string matching either segment must be empty
(i.e., the abs-path path must begin with a slash ("/") character or
the rel-path production, since they are mutually exclusive for any
given URI and can be parsed as a single component.
entirely empty). The path is terminated by the first question-mark question mark
("?") or number-sign number sign ("#") character, or by the end of the URI.

path-segments

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

pchar = unreserved / escaped pct-encoded / ";" sub-delims / ":" / "@" / "&" / "=" / "+" / "$" / ","

The

A path consists of a sequence of path segments separated by a slash
("/") character. A path is always defined for a URI, though the
defined path may be empty (zero length) or opaque (not containing any
"/" delimiters). length). Use of the slash character
to indicate hierarchy is only required when a URI will be used as the
context for relative references. For example, the URI
<mailto:fred@example.com> has a path of "fred@example.com". "fred@example.com", whereas
the URI <foo://info.example.com?fred> has an empty path.

The path segments "." and ".." are defined for relative reference
within the path name hierarchy. They are intended for use at the
beginning of a relative path reference (Section 4.2) for indicating
relative position within the hierarchical tree of names, with a names. This is
similar effect to how they are used their role within some operating systems' file directory
structure to indicate the current directory and parent directory,
respectively. Unlike However, unlike a file system, however, these dot-segments are
only interpreted within the URI path hierarchy and are removed as
part of the URI normalization or resolution process,
as described in Section 5.2. process (Section 5.2).

Aside from dot-segments in hierarchical paths, a path segment is
considered opaque by the generic syntax. URI generating URI-producing applications
often use the reserved characters allowed in a segment for the
purpose of delimiting scheme-specific or generator-specific dereference-handler-specific
sub-components. For example, the 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 generator producer 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
defined by scheme-specific semantics, but in most cases the meaning syntax of
a parameter is specific to the URI originator. implementation of the URI's
dereferencing algorithm.

3.4 Query

The query component contains non-hierarchical data that, along with
data in the path component (Section 3.3) component, 3.3), serves to identify a
resource within the scope of that the URI's scheme and naming authority
(if any). The query component is indicated by the first question-mark question mark
("?") character and terminated by a number-sign number sign ("#") character or by
the end of the URI.

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

The characters slash ("/") and question-mark question mark ("?") are allowed to may represent data
within the query component, but should not be used as such use within a
URI that is
discouraged; incorrect expected to be the base for relative references (Section
5.1). Incorrect implementations of reference resolution often fail
to distinguish them query data from path data when looking for
hierarchical separators, thus resulting in non-interoperable results while parsing relative references. results.
However, since query components are often used to carry identifying
information in the form of "key=value" pairs, and one frequently used
value is a reference to another URI, it is sometimes better for
usability to include avoid percent-encoding those characters unescaped.

Note: Some client applications will fail to separate a reference's
query component from its path component before merging the base
and reference paths (Section 5.2). This may result in loss of
information if the query component contains the strings "/../" or
"/./". characters.

3.5 Fragment

The fragment identifier component of a URI allows indirect
identification of a secondary resource by reference to a primary
resource and additional identifying information that is selective within that resource. information. The identified
secondary resource may be some portion or subset of the primary
resource, some view on representations of the primary resource, or
some other resource that is merely named within the
primary resource. defined or described by those representations. A
fragment identifier component is indicated by the presence of a number-sign
number sign ("#") character and terminated by the end of the URI string. URI.

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

The semantics of a fragment identifier are defined by the set of
representations that might result from a retrieval action on the
primary resource. The fragment's format and resolution is therefore
dependent on the media type [RFC2046] of the a potentially retrieved
representation, even though such a retrieval is only performed if the
URI is dereferenced. Individual media types may define their own
restrictions on, or structure within, the fragment identifier syntax
for specifying different types of subsets, views, or external
references that are identifiable as secondary resources by that media
type. If the primary resource is represented by has multiple media
types, representations, as is
often the case for resources whose representation is selected based
on attributes of the retrieval request, request (a.k.a., content negotiation),
then
interpretation of whatever is identified by the fragment identifier must should be consistent
across all of those media types in order for representations: each representation should
either define the fragment such that it corresponds to the same
secondary resource, regardless of how it is represented, or the
fragment should be viable as an
identifier. left undefined by the representation (i.e., not
found).

As with any URI, use of a fragment identifier component does not
imply that a retrieval action will take place. A URI with a fragment
identifier may be used to refer to the 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 is
only valid as a reference if a retrieval action on the primary
resource succeeds and results in a representation for which the
fragment identifier is meaningful. accessible or will ever be
accessed.

Fragment identifiers have a special role in information systems as
the 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 resource owner. As such,
interpretation of the fragment identifier during a retrieval action
is performed solely by the user agent; the fragment identifier is not
passed to other systems during the process of retrieval. Although
this is often perceived to be a 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 resource.

The characters slash ("/") and question-mark question mark ("?") are allowed to
represent data within the fragment identifier, but should not be used
as such use within a URI that is
discouraged expected to be the base for relative
references (Section 5.1) for the same reasons as described above for
query.

4. Usage

When applications make reference to a URI, they do not always use the
full form of reference defined by the "URI" syntax production. rule. 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 syntax to a particular form of URI.
We define the most common forms of reference syntax in this
specification because they impact and depend upon the design of the
generic syntax, requiring a uniform parsing algorithm in order to be
interpreted consistently.

4.1 URI Reference

The ABNF rule

URI-reference is used to denote the most common usage of a resource
identifier.

URI-reference = URI / relative-URI

A URI-reference may be relative: if the reference string's reference's prefix matches
the syntax of a scheme followed by its colon separator, then the
reference is a 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 present and
whether or not the reference is relative, and then each component is
parsed for its subparts and their validation. The ABNF of
URI-reference, along with the "first-match-wins" disambiguation rule,
is sufficient to define a validating parser for the generic syntax.
Readers familiar with 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 the hier-part hierarchical syntax
(Section 3) 1.2.3) in order to express a reference that is relative to
the name space of another hierarchical URI.

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

The URI referred to by a relative reference reference, also known as the target
URI, is obtained by applying the reference resolution algorithm of
Section 5.

A relative reference that begins with two slash characters is termed
a network-path reference; such references are rarely used. A relative
reference that begins with a single slash character is termed an
absolute-path reference. A relative reference that does not begin
with a slash character is termed a relative-path reference.

A path segment that contains a colon character (e.g., "this:that")
cannot be used as the first segment of a relative-path reference
because it would be mistaken for a scheme name. Such a segment must
be preceded by a dot-segment (e.g., "./this:that") to 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 a base URI for later
use by relative references calls for an absolute-URI production syntax rule that
does not allow a fragment.

absolute-URI = scheme ":" hier-part [ "?" query ] ["//" authority] path ["?" query]

4.4 Same-document Reference

When a URI reference occurring within a document or message refers to a URI that is, aside from its fragment
component (if any), identical to the base URI (Section 5.1), that
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 number-sign number sign ("#") separator followed by a
fragment identifier.

When a same-document reference is dereferenced for the purpose of a
retrieval action, the target of that reference is defined to be
within that current document the same entity (representation, document, or message; message) as the
reference; therefore, a dereference should not result in a new retrieval.
retrieval action.

Normalization of the base and target URIs prior to their comparison,
as described in Section 6.2.2 and Section 6.2.3, is allowed but
rarely performed in practice. Normalization may increase the set of
same-document references, which may be of benefit to some caching
applications. As such, reference authors should not assume that a
slightly different, though equivalent, reference URI will (or will
not) be interpreted as a same-document reference by any given
application.

4.5 Suffix Reference

The URI syntax is designed for unambiguous reference to 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 a suffix of the
URI as a reference, consisting of only the authority and path
portions of the URI, such as

www.w3.org/Addressing/

or simply the a DNS hostname registered name on its own. Such references are
primarily intended for human interpretation interpretation, rather than machine, for
machines, with the assumption that context-based heuristics are
sufficient to complete the URI (e.g., most hostnames host names beginning with
"www" are likely to have a URI prefix of "http://"). Although there
is no standard set of heuristics for disambiguating a URI suffix,
many client implementations allow them to be entered by the user and
heuristically resolved. It

While this practice of using suffix references is common, it should
be noted that such avoided whenever possible and never used in situations where
long-term references are expected. The heuristics may noted above will
change over time, particularly when a new URI schemes scheme becomes popular,
and are introduced. often incorrect when used out of context. Furthermore, they
can lead to security issues along the lines of those described in
[RFC1535].

Since a URI suffix has the same syntax as a relative path reference,
a suffix reference cannot be used in contexts where a relative
reference is expected. As a result, suffix references are limited to
those places where there is no defined base URI, such as dialog boxes
and off-line advertisements.

5. Reference Resolution

This section defines the process of resolving a URI reference within
a context that allows relative references, such that the result is a
string matching the "URI" syntax production rule of Section 3.

5.1 Establishing a Base URI

The term "relative" implies that there exists some a "base URI" against
which the relative reference is applied. Aside from same-document fragment-only
references (Section 4.4, 4.4), relative references are only usable if the when a
base URI is known. The A base URI must be established by the parser
prior to parsing URI references that might be relative.

The base URI of a document reference can be established in one of four ways,
listed
discussed below in order of precedence. The order of precedence can
be thought of in terms of layers, where the innermost defined base
URI has the highest precedence. This can be visualized graphically
as:

.----------------------------------------------------------.
| .----------------------------------------------------. |
| | .----------------------------------------------. | |
| | | .----------------------------------------. | | |
| | | | .----------------------------------. | | | |
| | | | | <relative-reference> | | | | |
| | | | `----------------------------------' | | | |
| | | | (5.1.1) Base URI embedded in the | | | |
| | | | document's content | | | |
| | | `----------------------------------------' | | |
| | | (5.1.2) Base URI of the encapsulating entity | | |
| | | (message, document, representation, or none). none) | | |
| | `----------------------------------------------' | |
| | (5.1.3) URI used to retrieve the entity | |
| `----------------------------------------------------' |
| (5.1.4) Default Base URI is application-dependent (application-dependent) |
`----------------------------------------------------------'

5.1.1 Base URI within Document Content

Within certain document media types, the a base URI of the document for relative references can be
embedded within the content itself such that it can be readily
obtained by a parser. This can be useful for descriptive documents,
such as tables of content, which may be transmitted to others through
protocols other than their usual retrieval context (e.g., E-Mail or
USENET news).

It is beyond the scope of this document specification to specify how, for each
media type, the a base URI can be embedded. It The appropriate syntax, when
available, is assumed described by each media type's specification.

5.1.2 Base URI from the Encapsulating Entity

If no base URI is embedded, the base URI is defined by the
representation's retrieval context. For a document that user
agents manipulating is enclosed
within another entity, such media types will be able to obtain as a message or archive, the
appropriate syntax from retrieval
context is that media type's specification. entity; thus, the default base URI of a
representation is the base URI of the entity in which the
representation is encapsulated.

A mechanism for embedding the a base URI within MIME container types
(e.g., the message and multipart types) is defined by MHTML
[RFC2110]. Protocols that do not use the MIME message header syntax,
but do allow some form of tagged metadata to be included within
messages, may define their own syntax for defining the a base URI as part
of a message.

5.1.2 Base URI from the Encapsulating Entity

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

5.1.3 Base URI from the Retrieval URI

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

5.1.4 Default Base URI

If none of the conditions described in above apply, then the base URI is
defined by the context of the application. Since this definition is
necessarily application-dependent, failing to define the a base URI using
one of the other methods may result in the same content being
interpreted differently by different types of application.

It is the responsibility of the distributor(s)

A sender of a document representation containing a relative reference to ensure references is
responsible for ensuring that the a base URI for that
document those references can be
established. It must be emphasized that a relative
reference, aside Aside from a same-document reference, cannot fragment-only references, relative references
can only be used reliably in situations where the document's base URI is not
well-defined.

5.2 Obtaining the Referenced URI Relative Resolution

This section describes an example algorithm for resolving converting a URI
references reference
that might be relative to a given base URI. URI into the parsed componets
of the reference's target. The algorithm
is intended components can then be recomposed, as
described in Section 5.3, to provide a form the target URI. This algorithm
provides definitive result results that can be used to test the output of
other implementations. Implementation of the algorithm
itself is not required, but Applications may implement relative reference
resolution using some other algorithm, provided that the result given by an implementation
must results
match the result that what would be given by this algorithm.

5.2.1 Pre-parse the Base URI

The base URI (Base) is established according to the rules procedure of
Section 5.1 and parsed into the five main components described in
Section 3. Note that only the scheme component is required to be
present in the a base URI; the other components may be empty or
undefined. A component is undefined if its preceding separator associated delimiter does
not appear in the URI reference; the path component is never
undefined, though it may be empty. The algorithm assumes that

Normalization of the base URI is well-formed
and does not contain dot-segments URI, as described in Section 6.2.2 and
Section 6.2.3, is optional. A URI reference must be transformed to
its path. target URI before it can be normalized.

5.2.2 Transform References

For each URI reference (R), the following pseudocode describes an
algorithm for transforming R into its target URI (T):

-- The URI reference is parsed into the five URI components
--
(R.scheme, R.authority, R.path, R.query, R.fragment) = parse(R);

-- A non-strict parser may ignore a scheme in the reference
-- if it is identical to the base URI's scheme.
--
if ((not strict) and (R.scheme == Base.scheme)) then
undefine(R.scheme);
endif;

if defined(R.scheme) then
T.scheme = R.scheme;
T.authority = R.authority;
T.path = remove_dot_segments(R.path);
T.query = R.query;
else
if defined(R.authority) then
T.authority = R.authority;
T.path = remove_dot_segments(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 = remove_dot_segments(R.path);
else
T.path = merge(Base.path, R.path);
T.path = remove_dot_segments(T.path);
endif;
T.query = R.query;
endif;
T.authority = Base.authority;
endif;
T.scheme = Base.scheme;
endif;

T.fragment = R.fragment;

5.2.3 Merge Paths

The pseudocode above refers to a merge "merge" routine for merging a
relative-path reference with the path of the base URI. This is
accomplished as follows:

o If the base URI's path is empty, URI has a defined authority component and an empty
path, then return a string consisting of "/" concatenated with the
reference's path component;
otherwise,

o If the base URI's path is non-hierarchical, as indicated by not
beginning with a slash, then return a string consisting of the
reference's path component; path; otherwise,

o Return a string consisting of the reference's path component
appended to all but the last segment of the base URI's path (i.e.,
excluding any characters after the right-most "/" in the base URI
path, or excluding the entire base URI path are
excluded). if it does not contain
any "/" characters).

5.2.4 Remove Dot Segments

The pseudocode also refers to a remove_dot_segments "remove_dot_segments" routine for
interpreting and removing the special "." and ".." complete path
segments from a referenced path. This is done after the path is
extracted from a reference, whether or not the path was relative, in
order to remove any invalid or extraneous dot-segments prior to
forming the target URI. Although there are many ways to accomplish
this removal process, we describe a simple method using a separate two string buffer:
buffers.

1. The input buffer is initialized with the unprocessed now-appended path component.

2. If
components and the output buffer begins with is initialized to the empty
string.

2. Replace any prefix of "./" or "../", "../" at the "." or ".." segment
is removed.

3. All occurrences beginning of "/./" in the input
buffer are replaced with "/".

3. While the input buffer is not empty, loop:

1. If the input buffer ends begins with a prefix of "/./" or "/.", the
where "." is removed.

5. All occurrences of "/<segment>/../" in the buffer, where ".." and
<segment> are a complete path segments, are iteratively replaced segment, then replace that
prefix with "/" in order from left to right until no matching pattern
remains. "/"; otherwise

2. If the input buffer ends with "/<segment>/..", that is also
replaced begins with "/". Note that <segment> may be empty.

6. All prefixes a prefix of "<segment>/../" in the buffer, "/../" or "/..",
where ".." and
<segment> are is a complete path segments, are iteratively replaced segment, then replace that
prefix with "/" in order and remove the last segment and its preceding
"/" (if any) from the output buffer; otherwise

3. Remove the first segment and its preceding "/" (if any) from left to right until no matching pattern
remains. If
the input buffer ends with "<segment>/..", that is also
replaced with "/". Note that <segment> may be empty.

7. The remaining and append them to the output buffer.

4. Finally, the output buffer is returned as the result of
remove_dot_segments.

The following illustrates how the above steps are applied for two
example merged paths, showing the state of the two buffers after each
step.

STEP OUTPUT BUFFER INPUT BUFFER

1 : /a/b/c/./../../g
3c: /a /b/c/./../../g
3c: /a/b /c/./../../g
3c: /a/b/c /./../../g
3a: /a/b/c /../../g
3b: /a/b /../g
3b: /a /g
3c: /a/g

STEP OUTPUT BUFFER INPUT BUFFER

1 : mid/content=5/../6
3c: mid /content=5/../6
3c: mid/content=5 /../6
3b: mid /6
3c: mid/6

Some systems applications may find it more efficient to implement the
remove_dot_segments algorithm as a stack of path segments being
compressed, using two segment stacks rather than as
strings.

Note: Some client applications will fail to separate a series reference's
query component from its path component before merging the base
and reference paths. This may result in loss of string pattern replacements. information if
the query component contains the strings "/../" or "/./".

5.3 Component Recomposition of a Parsed URI

Parsed URI components can be recomposed to obtain the corresponding
URI reference string. Using pseudocode, this would be:

result = ""

if defined(scheme) then
append scheme to result;
append ":" to result;
endif;

if defined(authority) then
append "//" to result;
append authority to result;
endif;

append path to result;

if defined(query) then
append "?" to result;
append 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 distinction between a
component that is undefined, meaning that its separator was not
present in the reference, and a component that is empty, meaning that
the separator was present and was immediately followed by the next
component separator or the end of the reference.

5.4 Reference Resolution Examples

Within an object a representation with a well-defined base URI of

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

a relative URI reference would be resolved is transformed to its target URI as follows: 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"
"g;x?y#s" = "http://a/b/c/g;x?y#s"
"" = "http://a/b/c/d;p?q"
"." = "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 following abnormal examples are unlikely to occur in
normal practice, all URI parsers should be capable of resolving them
consistently. Each example uses the same base as above.

An empty reference refers to the current base URI.

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

Parsers must be careful in handling the case cases where there are more
relative path ".." segments than there are hierarchical levels in the
base URI's path. Note that the ".." syntax cannot be used to change
the authority component of a URI.

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

Similarly, parsers must remove the dot-segments "." and ".." when
they are complete components of a 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 reference uses
unnecessary or nonsensical forms of the "." and ".." complete path
segments.

"./../g" = "http://a/b/g"
"./g/." = "http://a/b/c/g/"
"g/./h" = "http://a/b/c/g/h"
"g/../h" = "http://a/b/c/h"
"g;x=1/./y" = "http://a/b/c/g;x=1/y"
"g;x=1/../y" = "http://a/b/c/y"

Some applications fail to separate the reference's query and/or
fragment components from a relative path before merging it with the
base path and removing dot-segments. This error is rarely noticed,
since typical usage of a fragment never includes the hierarchy ("/")
character, and the query component is not normally used within
relative references.

"g?y/./x" = "http://a/b/c/g?y/./x"
"g?y/../x" = "http://a/b/c/g?y/../x"
"g#s/./x" = "http://a/b/c/g#s/./x"
"g#s/../x" = "http://a/b/c/g#s/../x"

Some parsers allow the scheme name to be present in a relative URI
reference if it is the same as the base URI scheme. This is
considered to be a loophole in prior specifications of partial URI
[RFC1630]. Its use should be avoided, but is allowed for backward
compatibility.

"http:g" = "http:g" ; for strict parsers
/ "http://a/b/c/g" ; for backward compatibility

6. Normalization and Comparison

One of the most common operations on URIs is simple comparison:
determining if two URIs are equivalent without using the URIs to
access their respective resource(s). A comparison is performed every
time a response cache is accessed, a browser checks its history to
color a link, or an XML parser processes tags within a namespace.
Extensive normalization prior to comparison of URIs is often used by
spiders and indexing engines to prune a search space or reduce
duplication of request actions and response storage.

URI comparison is performed in respect to some particular purpose,
and software with differing purposes will often be subject to
differing design trade-offs in regards to how much effort should be
spent in reducing duplicate identifiers. This section describes a
variety of methods that may be used to compare URIs, the trade-offs
between them, and the types of applications that might use them.

6.1 Equivalence

Since URIs exist to identify resources, presumably they should be
considered equivalent when they identify the same resource. However,
such a definition of equivalence is not of much practical use, since
there is no way for software to compare two resources without
knowledge of their origin. the implementation-specific syntax of each URI's
dereferencing algorithm. For this reason, determination of
equivalence or difference of URIs is based on string comparison,
perhaps augmented by reference to additional rules provided by URI
scheme definitions. We use the terms "different" and "equivalent" to
describe the possible outcomes of such comparisons, but there are
many application-dependent versions of equivalence.

Even though it is possible to determine that two URIs are equivalent,
it is never possible to be sure that two URIs identify different
resources. For example, an owner of two different domain names could
decide to serve the same resource from both, resulting in two
different URIs. Therefore, comparison methods are designed to
minimize false negatives while strictly avoiding false positives.

In testing for equivalence, it is generally unwise to applications should not directly compare
relative URI references; they the references should be converted to their
absolute
target URI forms before comparison. Furthermore, when URI references When URIs are being compared for
the purpose of selecting (or avoiding) a network action, such as
retrieval of a representation, it is often
necessary to remove the fragment identifiers components (if any)
should be excluded 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
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
practices 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,
object (UTF-16), bit-for-bit comparisons applied naively will produce
both false-positive and false-negative errors. Thus, in principle, it 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 In practical terms, character-by-character
comparisons are should be done codepoint-by-codepoint. codepoint-by-codepoint after conversion to
a common character encoding.

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

example://a/b/c/%7Bfoo%7D
eXAMPLE://a/./b/../b/%63/%7bfoo%7d

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 encoding normalization, empty-component
normalization, and removal of dot-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
host are case insensitive case-insensitive and therefore can should 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 Applications should not
assume anything about the
choice case sensitivity of uppercase or lowercase letters for other URI components,
since that is dependent on the implementation used to handle a
dereference.

The hexadecimal digits within the escape sequence a percent-encoding triplet (e.g., "%3a"
versus "%3A"). Such sequences "%3A") are always equivalent; for the sake of uniformity, URI generators case-insensitive and
normalizers are strongly encouraged therefore should be normalized
to use uppercase letters for the
hex digits A-F.

Only characters that are excluded from or reserved within

6.2.2.2 Encoding Normalization

The percent-encoding mechanism (Section 2.1) is a frequent source of
variance among otherwise identical URIs. In addition to the URI
syntax must be escaped when used as data. However,
case-insensitivity issue noted above, some URI
generators go beyond that and escape characters producers
percent-encode octets that do not require
escaping, percent-encoding, resulting
in URIs that are equivalent to their unescaped non-encoded counterparts. Such
URIs can should be normalized by unescaping sequences decoding any percent-encoded octet that represent the
corresponds to an unreserved characters, character, as described in Section 2.3.

6.2.2.3 Empty-component Normalization

Components of the generic URI syntax are delimited from other
components by optional separators. For example, a query component is
separated from the path by a question mark ("?") and a port
sub-component is separated from host by a colon (":"). A URI in
which a delimiter is present and the (sub-)component it delimits is
empty is equivalent to the same URI without that delimiter. For
example, the following are all equivalent:

ftp://example.com/
ftp://example.com:/
ftp://@example.com:/
ftp://@example.com:/?
ftp://@example.com:/?#

URI producers and normalizers should omit a delimiter if the
component it delimits is empty, as exemplified by the first URI
above, with one exception: a double-slash delimiter indicating an
authority component should not be removed, even when the authority is
empty, since doing so can lead to misinterpreting the path.

6.2.2.4 Path Segment Normalization

The complete path segments "." and ".." have a special meaning within
hierarchical URI schemes. As such, they should not appear in
absolute paths; if they are found, they can be removed by applying
the remove_dot_segments algorithm to the path, as described in
Section 5.2.

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 since the "http" URI scheme, which
scheme makes use of an authority component, has a default port of
"80", and defines an empty port
component as being path to be equivalent to "/", the default TCP port for HTTP (port
80).
following four URIs are equivalent:

http://example.com
http://example.com/
http://example.com:/
http://example.com:80/

In general, a URI scheme that uses the generic syntax for authority is defined such that a URI with an
empty path should be normalized to a path of "/"; likewise, an
explicit ":port", where the port is empty or the default for the
scheme, is equivalent to one where the port is and its ":" delimiter are
elided. In other words, the second of the above URI examples is the
normal form for the "http" scheme.

Another case where normalization varies by scheme is in the handling
of an empty authority component. For many scheme specifications, an
empty authority is considered an error; for others, it is considered
equivalent to "localhost". For the sake of uniformity, future scheme
specifications should define an empty authority as being equivalent
to "localhost", and URI producers and normalizers should use
"localhost" instead of an empty authority.

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 a URI differing only in the trailing slash

http://example.com/data/

they will likely regard the two as equivalent in the future.
Obviously, this This
kind of technique is only appropriate in special
situations. when equivalence is clearly
indicated by both the result of accessing the resources and the
common conventions of their scheme's dereference algorithm (in this
case, use of redirection by HTTP origin servers to avoid problems
with relative references).

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 produce and make reference to URIs can
reduce the cost of processing and the risk of false negatives by
consistently providing them in a form that is reasonably canonical
with respect to their scheme. Specifically:

o Always provide the URI scheme in lowercase characters.

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

o Only perform percent-escaping percent-encoding where it is essential.

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

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

The good practices listed above are motivated by deployed software

o Omit delimiters when their associated (sub-)component is empty.

o For schemes that frequently define an empty authority to be equivalent to
"localhost", use these techniques for the purposes "localhost".

o For schemes that define an empty path to be equivalent to a path
of
normalization. "/", use "/".

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, 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 service and data within the URI contains
instructions that, when interpreted according to this other protocol,
cause an unexpected operation. An A frequent example of such abuse has
been the use of a gopher URI to cause protocol-based scheme with a port component of
"25", thereby fooling user agent software into sending an unintended
or impersonating message to be
sent via a an SMTP server.

Caution

Applications should be used when dereferencing prevent dereference of a URI that specifies a TCP
port number other than within the default for "well-known port" range (0 - 1023) unless the scheme, especially when it
protocol being used to dereference that URI is a number within compatible with the reserved space.

Care
protocol expected on that well-known port. Although IANA maintains a
registry of well-known ports, applications should be taken when make such
restrictions user-configurable to avoid preventing the deployment of
new services.

When a URI contains escaped percent-encoded octets that match the delimiters
for a given resolution or dereference protocol (for example, CR and
LF characters for telnet
protocols) that these the TELNET protocol), such percent-encoded octets are
must not unescaped be decoded before transmission.
This transmission across that protocol.
Transfer of the percent-encoding, which might violate the protocol, but avoids
is less harmful than allowing decoded octets to be interpreted as
additional operations or parameters, perhaps triggering an unexpected
and possibly harmful remote operation.

7.3 Back-end Transcoding

When a URI is dereferenced, the potential data within it is often parsed by
both the user agent and one or more servers. In HTTP, for such example, a
typical user agent will parse a URI into its five major components,
access the authority's server, and send it the data within the
authority, path, and query components. A typical server will take
that information, parse the path into segments and the query into
key/value pairs, and then invoke implementation-specific handlers to
respond to the request. As a result, a common security concern for
server implementations that handle a URI, either as a whole or split
into separate components, is proper interpretation of the octet data
represented by the characters and percent-encodings within that URI.

Percent-encoded octets must be decoded at some point during the
dereference process. Applications must split the URI into its
components and sub-components prior to decoding the octets, since
otherwise the decoded octets might be used mistaken for delimiters.
Security checks of the data within a URI should be applied after
decoding the octets. Note, however, that the "%00" percent-encoding
(NUL) may require special handling and should be rejected if the
application is not expecting to simulate receive raw data within a component.

Special care should be taken when the URI path interpretation process
involves the use of a back-end filesystem or related system
functions. Filesystems typically assign an extra operation operational meaning to
special characters, such as the "/", "\", ":", "[", and "]"
characters, and special device names like ".", "..", "...", "aux",
"lpt", etc. In some cases, merely testing for the existence of such a
name will cause the operating system to pause or parameter in invoke unrelated
system calls, leading to significant security concerns regarding
denial of service and unintended data transfer. It would be
impossible for this specification to list all such significant
characters and device names; implementers should research the
reserved names and characters for the types of storage device that protocol which might lead
may be attached to an unexpected their application and possibly harmful
remote operation being performed.

7.3 restrict the use of data
obtained from URI components accordingly.

7.4 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 should be converted to
numeric form and filtered based on the numeric value, rather than a
prefix or suffix of the string form.

7.4

7.5 Sensitive Information

It is clearly unwise to use

URI producers should not provide a URI that contains a username or
password which is intended to be secret. In particular, the use of a secret: URIs are frequently
displayed by browsers, stored in clear text bookmarks, and logged by
user agent history and intermediary applications (proxies). A
password appearing within the userinfo component of a URI is strongly discouraged deprecated and
should be considered an error (or simply ignored) except in those
rare cases where the 'password' parameter is intended to be public.

7.5

7.6 Semantic Attacks

Because the userinfo component sub-component is rarely used and appears before
the
hostname host 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

ftp://ftp.example.com&story=breaking_news@10.0.0.1/top_story.htm

might lead a human user to assume that the host is 'www.example.com',
'trusted.example.com', whereas it is actually '10.0.0.1'. Note that the
a misleading userinfo sub-component could be much longer than the
example above.

A misleading URI, such as the one above, is an attack on the user's
preconceived notions about the meaning of a URI, rather than an
attack on the software itself. User agents may be able to reduce the
impact of such attacks by visually distinguishing the various components of
the URI when rendered, such as by using a different color or tone to
render userinfo if any is present, though there is no general
panacea. More information on URI-based semantic attacks can be found
in [Siedzik].

8. Acknowledgments

This specification is derived from RFC 2396 [RFC2396], RFC 1808
[RFC1808], and RFC 1738 [RFC1738]; the acknowledgments in those
documents still apply. It also incorporates the update (with
corrections) for IPv6 literals in the host syntax, as defined by
Robert M. Hinden, Brian E. Carpenter, and Larry Masinter in
[RFC2732]. In addition, contributions by Gisle Aas, Reese Anschultz,
Daniel Barclay, Tim Bray, Mike Brown, Rob Cameron, Jeremy Carroll,
Dan Connolly, Adam M. Costello, John Cowan, Jason Diamond, Martin
Duerst, Stefan Eissing, Clive D.W. Feather, Tony Hammond, Pat Hayes,
Henry Holtzman, Ian B. Jacobs, Michael Kay, John C. Klensin, Graham
Klyne, Dan Kohn, Bruce Lilly, Andrew Main, Ira McDonald, Michael
Mealling, Stephen Pollei, Julian Reschke, Tomas Rokicki, Miles Sabin,
Mark Thomson, Ronald Tschalaer, Norm Walsh, Marc Warne, Stuart
Williams, and Henry Zongaro are gratefully acknowledged.

Normative References

[ASCII] American National Standards Institute, "Coded Character
Set -- 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.

[RFC2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.

[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.

Informative References

[RFC2277] Alvestrand, H., "IETF Policy on Character Sets

[RFC0952] Harrenstien, K., Stahl, M. and
Languages", BCP 18, E. Feinler, "DoD Internet
host table specification", RFC 2277, January 1998. 952, October 1985.

[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.

[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.

[RFC1535] Gavron, E., "A Security Problem and Proposed Correction
With Widely Deployed DNS Software", RFC 1535, October
1993.

[RFC1630] Berners-Lee, T., "Universal Resource Identifiers in WWW: A
Unifying Syntax for the Expression of Names and Addresses
of Objects on the Network as used in the World-Wide Web",
RFC 1630, June 1994.

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

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

[RFC1738] Berners-Lee, T., Masinter, L. and M. McCahill, "Uniform
Resource Locators (URL)", RFC 1738, December 1994.

[RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998.

[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.

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

[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.

[RFC2110] Palme, J. and A. Hopmann, "MIME E-mail Encapsulation of
Aggregate Documents, such as HTML (MHTML)", RFC 2110,
March 1997.

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

[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.

[RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998.

[RFC2518] Goland, Y., Whitehead, E., Faizi, A., Carter, S. and D.
Jensen, "HTTP Extensions for Distributed Authoring --
WEBDAV", RFC 2518, February 1999.

[RFC0952] Harrenstien, K., Stahl, M.

[RFC2717] Petke, R. and E. Feinler, "DoD Internet
host table specification", I. King, "Registration Procedures for URL
Scheme Names", BCP 35, RFC 952, October 1985.

[RFC3513] Hinden, R. 2717, November 1999.

[RFC2718] Masinter, L., Alvestrand, H., Zigmond, D. and S. Deering, "Internet Protocol Version 6
(IPv6) Addressing Architecture", R. Petke,
"Guidelines for new URL Schemes", RFC 3513, April 2003. 2718, November 1999.

[RFC2732] Hinden, R., Carpenter, B. and L. Masinter, "Format for
Literal IPv6 Addresses in URL's", RFC 2732, December 1999.

[RFC1736] Kunze, J., "Functional Recommendations for Internet
Resource Locators",

[RFC2978] Freed, N. and J. Postel, "IANA Charset Registration
Procedures", BCP 19, RFC 1736, February 1995.

[RFC1737] Masinter, L. 2978, October 2000.

[RFC3305] Mealling, M. and K. Sollins, "Functional Requirements for R. Denenberg, "Report from the Joint W3C/
IETF URI Planning Interest Group: Uniform Resource Names", RFC 1737, December 1994.

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

[RFC1034] Mockapetris, P., "Domain names - concepts
Identifiers (URIs), URLs, and facilities",
STD 13, Uniform Resource Names
(URNs): Clarifications and Recommendations", RFC 1034, November 1987.

[RFC2110] Palme, J. 3305,
August 2002.

[RFC3490] Faltstrom, P., Hoffman, P. and A. Hopmann, "MIME E-mail Encapsulation of
Aggregate Documents, such as HTML (MHTML)", Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 2110, 3490, March 1997.

[RFC2717] Petke, 2003.

[RFC3513] Hinden, R. and I. King, "Registration Procedures for URL
Scheme Names", BCP 35, S. Deering, "Internet Protocol Version 6
(IPv6) Addressing Architecture", RFC 2717, November 1999. 3513, April 2003.

[Siedzik] Siedzik, R., "Semantic Attacks: What's in a URL?", April
2001.

[UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.
2001, <http://www.giac.org/practical/gsec/
Richard_Siedzik_GSEC.pdf>.

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,
5251 California Ave., Suite 150
Newport Beach, 110
Irvine, CA 92660 92612-3074
USA

Phone: +1-949-999-2523 +1-949-679-2960
Fax: +1-949-644-5064 +1-949-679-2972
EMail: roy.fielding@day.com fielding@gbiv.com
URI: http://www.apache.org/~fielding/ http://roy.gbiv.com/

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/

Appendix A. Collected ABNF for URI

abs-path

URI = "/" path-segments scheme ":" ["//" authority] path ["?" query] ["#" fragment]

URI-reference = URI / relative-URI

relative-URI = ["//" authority] path ["?" query] ["#" fragment]

absolute-URI = scheme ":" hier-part [ "?" query ]

alphanum ["//" authority] path ["?" query]

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

authority = [ userinfo "@" ] host [ ":" port ]

dec-octet
userinfo = DIGIT ; 0-9
/ %x31-39 DIGIT ; 10-99
/ "1" 2DIGIT ; 100-199 *( unreserved / "2" %x30-34 DIGIT ; 200-249 pct-encoded / "25" %x30-35 ; 250-255

domainlabel = alphanum [ 0*61( alphanum sub-delims / "-" ":" ) alphanum ]

escaped = "%" HEXDIG HEXDIG

fragment
host = *( pchar IP-literal / "/" IPv4address / "?" )

h4 reg-name
port = 1*4HEXDIG

hier-part *DIGIT

IP-literal = net-path / abs-path "[" ( IPv6address / rel-path

host IPvFuture ) "]"

IPvFuture = [ IPv6reference "v" HEXDIG "." 1*( unreserved / IPv4address sub-delims / hostname ]

hostname = domainlabel qualified

IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet ":" )

IPv6address = 6( h4 h16 ":" ) ls32
/ "::" 5( h4 h16 ":" ) ls32
/ [ h4 h16 ] "::" 4( h4 h16 ":" ) ls32
/ [ *1( h4 h16 ":" ) h4 h16 ] "::" 3( h4 h16 ":" ) ls32
/ [ *2( h4 h16 ":" ) h4 h16 ] "::" 2( h4 h16 ":" ) ls32
/ [ *3( h4 h16 ":" ) h4 h16 ] "::" h4 h16 ":" ls32
/ [ *4( h4 h16 ":" ) h4 h16 ] "::" ls32
/ [ *5( h4 h16 ":" ) h4 h16 ] "::" h4 h16
/ [ *6( h4 h16 ":" ) h4 h16 ] "::"

IPv6reference

h16 = "[" IPv6address "]" 1*4HEXDIG
ls32 = ( h4 h16 ":" h4 h16 ) / IPv4address

mark

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

reg-name = 0*255( unreserved / "(" pct-encoded / ")"
net-path = "//" authority [ abs-path ]

path-segments sub-delims )

path = segment *( "/" segment )

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

port = *DIGIT

qualified
segment = *( "." domainlabel ) [ "." ] *pchar

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

rel-path = path-segments

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

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

pct-encoded = "%" HEXDIG HEXDIG

pchar = unreserved / "#" / "[" / "]" pct-encoded / ";" sub-delims / ":" / "@" / "&" / "=" / "+" / "$" / ","

scheme

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

segment = *pchar

unreserved = ALPHA / DIGIT "_" / mark

URI "~"
reserved = gen-delims / sub-delims
gen-delims = scheme ":" hier-part [ / "/" / "?" query ] [ / "#" fragment ]

URI-reference = URI / relative-URI

uric = reserved "[" / unreserved "]" / escaped

userinfo "@"
sub-delims = *( unreserved "!" / escaped "$" / ";" "&" /
":" "'" / "&" "(" / "=" ")"
/ "+" "*" / "$" "+" / "," ) / ";" / "="

Appendix B. Parsing a URI 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 second line above are only to assist readability;
they indicate the reference points for each subexpression (i.e., each
paired parenthesis). We refer to the value matched for subexpression
<n> as $<n>. For example, matching the above expression to

http://www.ics.uci.edu/pub/ietf/uri/#Related

results in the following subexpression matches:

$1 = http:
$2 = http
$3 = //www.ics.uci.edu
$4 = www.ics.uci.edu
$5 = /pub/ietf/uri/
$6 = <undefined>
$7 = <undefined>
$8 = #Related
$9 = Related

where <undefined> indicates that the component is not present, as is
the case for the query component in the above example. Therefore, we
can determine the value of the four components and fragment as

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

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

Appendix C. 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 URIs 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, 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 reference that contains embedded 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.

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/>. <ftp://foo.example.
com/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/
ftp://foo.example.com/rfc/
http://www.ics.uci.edu/pub/ietf/uri/historical.html#WARNING

Appendix D. Summary of Non-editorial Changes

D.1 Additions

IPv6 (and later) literals have been added to the list of possible
identifiers for the host portion of a authority component, as
described by [RFC2732], with the addition of "[" and "]" to the
reserved set and uric sets. a version flag to anticipate future versions of IP
literals. Square brackets are now specified as reserved within the
authority component and not allowed outside their use as delimiters
for an
IPv6reference IP literal 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 be defined in terms of uric.

Since [RFC2732] defers to [RFC3513] for definition of an IPv6 literal
address, which unfortunately lacks an ABNF description of
IPv6address, we created a new ABNF rule for IPv6address that matches
the text representations defined by Section 2.2 of [RFC3513].
Likewise, the definition of IPv4address has been improved in order to
limit each decimal octet to the range 0-255, and the definition of
hostname has been improved to better specify length limitations and
partially-qualified domain names. 0-255.

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

D.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 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. The mark
characters that are typically unsafe to decode, including the
exclamation mark ("!"), asterisk ("*"), single-quote ("'"), and open
and close parentheses ("(" and ")"), have been moved to the reserved
set in order to clarify the distinction between reserved and
unreserved and hopefully answer the most common question of scheme
designers. Likewise, the section on escaped percent-encoded characters has
been rewritten, and URI normalizers are now given license to unescape decode
any percent-encoded octets corresponding to unreserved characters. The number-sign ("#")
character has been moved back from
In general, the excluded delims terms "escaped" and "unescaped" have been replaced
with "percent-encoded" and "decoded", respectively, to the
reserved set. reduce
confusion with other forms of escape mechanisms.

The ABNF for URI and URI-reference has been redesigned to make them
more friendly to LALR parsers and significantly reduce complexity. As
a result, the layout form of syntax description has been removed,
along with the uric-no-slash, opaque-part, uric, uric_no_slash, hier_part, opaque_part, net_path,
abs_path, rel_path, path_segments, rel_segment, and rel-segment
productions. mark rules. All
references to "opaque" URIs have been replaced with a better
description of how the path component may be opaque to hierarchy. The fragment identifier has been moved back into the
section on generic syntax components and within the URI and
relative-URI productions, though it remains excluded from
absolute-URI. The
ambiguity regarding the parsing of URI-reference as a URI or a
relative-URI with a colon in the first segment is now explained and
disambiguated in the section defining relative-URI.

The ABNF of hier-part fragment identifier has been moved back into the section on
generic syntax components and within the URI and relative-URI rules,
though it remains excluded from absolute-URI. The number sign ("#")
character has been moved back to the reserved set as a result of
reintegrating the fragment syntax.

The ABNF 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:" "dav:" namespace
[RFC2518] and the "about:" URI scheme used internally by many WWW browser
implementations. The ambiguity regarding the parsing of
net-path, abs-path, boundary between
authority and rel-path path is now explained and disambiguated in the same
section.

Registry-based naming authorities that use the generic syntax
authority component are now
defined within the host rule and limited to DNS hostnames, since those
have been the only such URIs in deployment. 255 path characters. This
change was allows current implementations, where whatever name provided
is simply fed to the local name resolution mechanism, to be
consistent with the specification and removes the need to re-specify
DNS name formats here. It also allows the host component to contain
percent-encoded octets, which is necessary to enable
internationalized domain names to be provided in URIs, processed in
their native character encodings at the application layers above URI
processing. The reg_name, server,
processing, and hostport productions have been
removed to simplify parsing of the URI syntax.

The ABNF of qualified has been simplified passed to remove an IDNA library as a parsing
ambiguity without changing registered name in the allowed syntax. The toplabel
production has been removed because it served no useful purpose.
UTF-8 character encoding. The
ambiguity regarding the parsing of host as IPv4address or hostname is
now explained server, hostport, hostname,
domainlabel, toplabel, and disambiguated in the same section. alphanum rules have been removed.

The resolving relative references algorithm of [RFC2396] has been
rewritten using pseudocode for this revision to improve clarity and
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 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
within the URI parser in favor of a section that explains that when a new retrieval action
reference should not be made if interpreted by a dereferencing engine as a
same-document reference: when the target URI and base URI,
excluding fragments, match. This change has no impact on user agent does not modify the
behavior aside from how of existing same-document references as defined by RFC
2396 (fragment-only references); it merely adds the resolved reference might be described same-document
distinction to other references that refer to the base URI and
simplifies the interface between applications and their URI
parsers, as is consistent with the user. internal architecture of
deployed URI processing implementations.

o Separated the path merge routine into two routines: merge, for
describing combination of the base URI path with a relative-path
reference, and remove_dot_segments, for describing how to remove
the special "." and ".." segments from a composed path. The
remove_dot_segments algorithm is now applied to all URI reference
paths in order to match common implementations and improve the
normalization of URIs in practice. This change only impacts the
parsing of abnormal references and same-scheme references wherein
the base URI has a non-hierarchical path.

Index

A
ABNF 9
abs-path 16 10
absolute 25
absolute-path 24
absolute-URI 25
access 7
alphanum 18
authority 16, 17 15, 16

B
base URI 27

C
characters 11

D
dec-octet 19
delims 15 18
dereference 7
domainlabel 18
dot-segments 20

E
escaped 13
excluded 14

F
fragment 22

G
gen-delims 12
generic syntax 5

H
h4 19
hier-part 16
h16 17
hierarchical 8 9
host 18
hostname 18 17

I
identifier 5
invisible 14
IP-literal 17
IPv4 19 18
IPv4address 19 18
IPv6 19 17
IPv6address 19
IPv6reference 19 17
IPvFuture 17

L
locator 6
ls32 19 17

M
mark 12
merge 30

N
name 6
net-path 16
network-path 24

P
path 16, 20
path-segments 15, 20
pchar 20
pct-encoded 11
percent-encoding 11
port 20

Q
qualified 18
query 21

R
rel-path 16
reg-name 19
registered name 19
relative 9, 27
relative-path 24
relative-URI 24
remove_dot_segments 30
representation 8
reserved 11 12
resolution 7, 27
resource 4
retrieval 8

S
same-document 25
sameness 8
scheme 16 15
segment 20
sub-delims 12
suffix 25

T
transcription 6

U
uniform 4
unreserved 12
unwise 15
URI grammar
abs-path 16
absolute-URI 25
ALPHA 9
alphanum 18 10
authority 16, 17 15, 16
CR 9 10
CTL 9 10
dec-octet 19
DIGIT 9
domainlabel 18
DIGIT 10
DQUOTE 9
escaped 13 10
fragment 16, 15, 22, 24
h4 19
gen-delims 12
h16 18
HEXDIG 9
hier-part 16, 24, 25 10
host 17, 18
hostname 18 16, 17
IP-literal 17
IPv4address 19 18
IPv6address 19
IPv6reference 19 17, 18
IPvFuture 17
LF 9 10
ls32 19 18
mark 12
net-path 16
OCTET 9 10
path 15
path-segments 16, 20
pchar 20, 21, 22
pct-encoded 11
port 17, 16, 20
qualified 18
query 16, 15, 21, 24, 25
rel-path 16
reg-name 19
relative-URI 24, 24
reserved 12
scheme 16, 17, 15, 15, 25
segment 20
SP 9 10
sub-delims 12
unreserved 12
URI 16, 15, 24
URI-reference 24
uric 11
userinfo 17, 18
URI 16, 16
URI 15
URI-reference 24
uric 11
URL 6
URN 6
userinfo 18 16

Intellectual Property Statement

The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.

Full Copyright Statement

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.

This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

Acknowledgment

Funding for the RFC Editor function is currently provided by the
Internet Society.