Network Working Group T. Berners-Lee
Internet-Draft MIT/LCS
Updates: 1738 (if approved) R. Fielding
Obsoletes: 2732, 2396, 1808 (if approved) Day Software
L. Masinter
Expires: December 5, 2003 Adobe
June 6, 2003
Uniform Resource Identifier (URI): Generic Syntax
draft-fielding-uri-rfc2396bis-03
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
A Uniform Resource Identifier (URI) is a compact string of characters
for identifying an abstract or physical resource. This 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.
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Editorial Note
Discussion of this draft and comments to the editors should be sent
to the uri@w3.org mailing list. An issues list and version history
is available at <http://www.apache.org/~fielding/uri/rev-2002/
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
1.3 Syntax Notation . . . . . . . . . . . . . . . . . . . . . . 9
2. Characters . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1 Encoding of Characters . . . . . . . . . . . . . . . . . . . 11
2.2 Reserved Characters . . . . . . . . . . . . . . . . . . . . 11
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 . . . . . . . . . . . . . . . . . . . . 14
3. Syntax Components . . . . . . . . . . . . . . . . . . . . . 16
3.1 Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2 Authority . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.2.1 User Information . . . . . . . . . . . . . . . . . . . . . . 18
3.2.2 Host . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
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
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5.2 Obtaining the Referenced URI . . . . . . . . . . . . . . . . 28
5.3 Recomposition of a Parsed URI . . . . . . . . . . . . . . . 31
5.4 Reference Resolution Examples . . . . . . . . . . . . . . . 32
5.4.1 Normal Examples . . . . . . . . . . . . . . . . . . . . . . 32
5.4.2 Abnormal Examples . . . . . . . . . . . . . . . . . . . . . 32
6. Normalization and Comparison . . . . . . . . . . . . . . . . 35
6.1 Equivalence . . . . . . . . . . . . . . . . . . . . . . . . 35
6.2 Comparison Ladder . . . . . . . . . . . . . . . . . . . . . 35
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
6.3 Canonical Form . . . . . . . . . . . . . . . . . . . . . . . 38
7. Security Considerations . . . . . . . . . . . . . . . . . . 40
7.1 Reliability and Consistency . . . . . . . . . . . . . . . . 40
7.2 Malicious Construction . . . . . . . . . . . . . . . . . . . 40
7.3 Rare IP Address Formats . . . . . . . . . . . . . . . . . . 41
7.4 Sensitive Information . . . . . . . . . . . . . . . . . . . 41
7.5 Semantic Attacks . . . . . . . . . . . . . . . . . . . . . . 41
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 43
Normative References . . . . . . . . . . . . . . . . . . . . 44
Informative References . . . . . . . . . . . . . . . . . . . 45
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 47
A. Collected ABNF for URI . . . . . . . . . . . . . . . . . . . 48
B. Parsing a URI Reference with a Regular Expression . . . . . 50
C. Delimiting a URI in Context . . . . . . . . . . . . . . . . 51
D. Summary of Non-editorial Changes . . . . . . . . . . . . . . 53
D.1 Additions . . . . . . . . . . . . . . . . . . . . . . . . . 53
D.2 Modifications from RFC 2396 . . . . . . . . . . . . . . . . 53
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
Intellectual Property and Copyright Statements . . . . . . . 60
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1. Introduction
A Uniform Resource Identifier (URI) provides a simple and extensible
means for identifying a resource. This specification of URI syntax
and semantics is derived from concepts introduced by the World Wide
Web global information initiative, whose use of such identifiers
dates from 1990 and is described in "Universal Resource Identifiers
in WWW" [RFC1630], and is designed to meet the recommendations laid
out in "Functional Recommendations for Internet Resource Locators"
[RFC1736] and "Functional Requirements for Uniform Resource Names"
[RFC1737].
This document obsoletes [RFC2396], which merged "Uniform Resource
Locators" [RFC1738] and "Relative Uniform Resource Locators"
[RFC1808] in order to define a single, generic syntax for all URIs.
It excludes those portions of RFC 1738 that defined the specific
syntax of individual URI schemes; those portions will be updated as
separate documents. The process for registration of new URI schemes
is defined separately by [RFC2717].
All significant changes from RFC 2396 are noted in Appendix D.
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
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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 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.
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1.1.2 Examples
The following examples illustrate URIs that are in common use.
ftp://ftp.is.co.za/rfc/rfc1808.txt
-- ftp scheme for File Transfer Protocol services
gopher://gopher.tc.umn.edu:70/11/Mailing%20Lists/
-- gopher scheme for Gopher and Gopher+ Protocol services
http://www.ietf.org/rfc/rfc2396.txt
-- http scheme for Hypertext Transfer Protocol services
mailto:John.Doe@example.com
-- mailto scheme for electronic mail addresses
news:comp.infosystems.www.servers.unix
-- news scheme for USENET news groups and articles
telnet://melvyl.ucop.edu/
-- telnet scheme for interactive TELNET services
1.1.3 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 to 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.
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.
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,
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and a few special characters. A URI may be represented in a variety
of ways: e.g., ink on paper, pixels on a screen, or a sequence of
octets in a coded character set. The interpretation of a URI depends
only on the characters used and not how those characters are
represented in a network protocol.
The goal of 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.
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.
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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 that access mechanism to perform an action on the
URI's resource is termed a "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 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.
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
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The URI syntax is organized hierarchically, with components listed in
decreasing order 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 syntax reserves the slash ("/"), question-mark ("?"), and
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; 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 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] to define the URI syntax. Although the ABNF
defines syntax in terms of the US-ASCII character encoding [ASCII],
the URI syntax should be interpreted in terms of the character that
the ASCII-encoded octet represents, rather than the octet encoding
itself. How a URI is represented in terms of bits and bytes on the
wire is dependent upon the character encoding of the protocol used to
transport it, or the charset of the document that contains it.
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The following core ABNF productions are used by this specification as
defined by Section 6.1 of [RFC2234]: ALPHA, CR, CTL, DIGIT, DQUOTE,
HEXDIG, LF, OCTET, and SP. The complete URI syntax is collected in
Appendix A.
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2. Characters
A URI consists of a restricted set of characters, primarily chosen
to aid transcription and usability both in computer systems and in
non-computer communications. Characters used conventionally as
delimiters around a URI are excluded. The set of URI characters
consists of digits, letters, and a few graphic symbols chosen from
those common to most of the character encodings and input facilities
available to Internet users.
uric = reserved / unreserved / escaped
Within a URI, reserved characters are used to delimit syntax
components, unreserved characters are used to describe registered
names, and unreserved, non-delimiting reserved, and escaped
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 in terms
of characters by reference to the US-ASCII encoding of characters to
octets. This specification does not mandate the use of any
particular mapping between its character set and the octets used to
store or transmit those characters.
URI characters representing strings of data within a component may,
if allowed by the component production, represent an arbitrary
sequence of octets. For example, portions of a given URI might
correspond to a filename on a non-ASCII file system, a query on
non-ASCII data, numeric coordinates on a map, etc. Some URI schemes
define a specific encoding of raw data to US-ASCII characters as part
of their scheme-specific requirements. Most URI schemes represent
data octets by the US-ASCII character corresponding to that octet,
either directly in the form of the character's glyph or by use of an
escape triplet (Section 2.4).
When a URI scheme defines a component that represents textual data
consisting of characters from the Unicode (ISO 10646) character set,
we recommend that the data be encoded first as octets according to
the UTF-8 [UTF-8] character encoding, and then escaping only those
octets that are not in the unreserved character set.
2.2 Reserved Characters
URIs include components and sub-components that are delimited by
certain special characters. These characters are called "reserved",
since their usage within a URI component is limited to their reserved
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purpose within that component. If data for a URI component would
conflict with the reserved purpose, then the conflicting data must be
escaped (Section 2.4) before forming the URI.
reserved = "/" / "?" / "#" / "[" / "]" / ";" /
":" / "@" / "&" / "=" / "+" / "$" / ","
Reserved characters are used as delimiters of the generic URI
components described in Section 3, as well as within those components
for delimiting sub-components. A component's ABNF syntax rule will
not use the "reserved" production directly; instead, each rule lists
those reserved characters that are allowed within that component.
Allowed reserved characters that are not assigned a sub-component
delimiter role by this specification should be considered reserved
for special use by whatever software generates the URI (i.e., they
may be used to delimit or indicate information that is significant to
interpretation of the identifier, but that significance is outside
the scope of this specification). Outside of the URI's origin, a
reserved character cannot be escaped without fear of changing how it
will be interpreted; likewise, an escaped octet that corresponds to a
reserved character cannot be unescaped outside the software that is
responsible for interpreting it during URI resolution.
The slash ("/"), question-mark ("?"), and number-sign ("#")
characters are reserved in all URIs for the purpose of delimiting
components that are significant to the generic parser's hierarchical
interpretation of an identifier. The hierarchical prefix of a URI,
wherein the slash ("/") character signifies a hierarchy delimiter,
extends from the scheme (Section 3.1) through to the first
question-mark ("?"), number-sign ("#"), or the end of the URI string.
In other words, the slash ("/") character is not treated as a
hierarchical separator within the query (Section 3.4) and fragment
(Section 3.5) components of a URI, but is still considered reserved
within those components for purposes outside the scope of this
specification.
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, and a limited set of punctuation marks and
symbols.
unreserved = ALPHA / DIGIT / mark
mark = "-" / "_" / "." / "!" / "~" / "*" / "'" / "(" / ")"
Escaping unreserved characters in a URI does not change what resource
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is identified by that URI. However, it may change the result of a
URI comparison (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 in the ranges of ALPHA (%41-%5A and
%61-%7A), DIGIT (%30-%39), hyphen (%2D), 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 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 URI (Section 2.5).
2.4.1 Escaped Encoding
An escaped octet is encoded as a character triplet, consisting of
the percent character "%" followed by the two hexadecimal digits
representing that octet's numeric value. For example, "%20" is the
escaped encoding for the binary octet "00100000" (ABNF: %x20), which
corresponds to the US-ASCII space character (SP). This is sometimes
referred to as "percent-encoding" the octet.
escaped = "%" HEXDIG HEXDIG
The uppercase hexadecimal digits 'A' through 'F' are equivalent to
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 When to Escape and Unescape
Under normal circumstances, the only time that characters within a
URI string are escaped is during the process of generating the URI
from its component parts. Each component may have its own set of
characters that are reserved, so only the mechanism responsible for
generating or interpreting that component can determine whether or
not escaping a character will change its semantics. The exception is
when a URI is being used within a context where the unreserved "mark"
characters might need to be escaped, such as when used for a
command-line argument or within a single-quoted attribute.
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Once generated, a URI is always in an escaped form. When a URI is
resolved, the components significant to that scheme-specific
resolution process (if any) must be parsed and separated before the
escaped characters within those components can be safely unescaped.
In some cases, data that could be represented by an unreserved
character may appear escaped; for example, some of the unreserved
"mark" characters are automatically escaped by some systems. A URI
normalizer may unescape escaped octets that are represented by
characters in the unreserved set. For example, "%7E" is sometimes
used instead of tilde ("~") in an "http" URI path and can be
converted to "~" 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.
Because the percent ("%") character serves as the escape indicator,
it must be escaped as "%25" in order for that octet to be used as
data within a URI. Implementers should be careful not to escape or
unescape the same string more than once, since unescaping an already
unescaped string might lead to misinterpreting a percent data
character as another escaped character, or vice versa in the case of
escaping an already escaped string.
2.5 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 for their exclusion.
excluded = invisible / delims / unwise
The control characters (CTL) in the US-ASCII coded character set are
not used within a URI, both because they are non-printable and
because they are likely to be misinterpreted by some control
mechanisms. The space character (SP) is excluded because significant
spaces may disappear and insignificant spaces may be introduced when
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a URI is transcribed, typeset, or subjected to the treatment of
word-processing programs. Whitespace is also used to delimit a URI
in many contexts. Characters outside the US-ASCII set are excluded as
well.
invisible = CTL / SP / %x80-FF
The angle-bracket ("<" and ">") and double-quote (") characters are
excluded because they are often used as the delimiters around a URI
in text documents and protocol fields. The percent character ("%")
is excluded because it is used for the encoding of escaped (Section
2.4) characters.
delims = "<" / ">" / "%" / DQUOTE
Other characters are excluded because gateways and other transport
agents are known to sometimes modify such characters.
unwise = "{" / "}" / "|" / "\" / "^" / "`"
Data octets corresponding to excluded characters must be escaped in
order to be represented within a URI.
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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
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: 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 the presence 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 be parsed as a single component.
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
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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 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.
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 ("?"), or
number-sign ("#") character, or by the end of the URI.
authority = [ userinfo "@" ] host [ ":" port ]
The parts "<userinfo>@" and ":<port>" may be omitted.
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).
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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. The user information, if
present, is followed by a commercial at-sign ("@") that delimits it
from the host.
userinfo = *( unreserved / escaped / ";" /
":" / "&" / "=" / "+" / "$" / "," )
Some URI schemes use the format "user:password" in the userinfo
field. This practice is NOT RECOMMENDED, because the passing of
authentication information in clear text has proven to be a security
risk in almost every case where it has been used. Note also that
userinfo might be crafted to look like a trusted domain name in order
to mislead users, as described in Section 7.5.
3.2.2 Host
The host sub-component of authority is identified by an IPv6 literal
encapsulated within square brackets, an IPv4 address in
dotted-decimal form, or a domain name.
host = [ IPv6reference / 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.
The production for host is ambiguous because it does not completely
distinguish between an IPv4address and a hostname. Again, the
"first-match-wins" algorithm applies: If host matches the production
for IPv4address, then it should be considered an IPv4 address literal
and not a hostname.
A hostname takes the form described in Section 3 of [RFC1034] and
Section 2.1 of [RFC1123]: a sequence of domain labels separated by
".", each domain label starting and ending with an alphanumeric
character and possibly also containing "-" characters. The rightmost
domain label of a fully qualified domain name may be followed by a
single "." if it is necessary to distinguish between the complete
domain name and some local domain.
hostname = domainlabel qualified
qualified = *( "." domainlabel ) [ "." ]
domainlabel = alphanum [ 0*61( alphanum / "-" ) alphanum ]
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alphanum = ALPHA / DIGIT
A host identified by an IPv4 literal address is represented in
dotted-decimal notation (a sequence of four decimal numbers in the
range 0 to 255, separated by "."), as described in [RFC1123] by
reference to [RFC0952]. Note that other forms of dotted notation may
be interpreted on some platforms, as described in Section 7.3, but
only the dotted-decimal form of four octets is allowed by this
grammar.
IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet
dec-octet = DIGIT ; 0-9
/ %x31-39 DIGIT ; 10-99
/ "1" 2DIGIT ; 100-199
/ "2" %x30-34 DIGIT ; 200-249
/ "25" %x30-35 ; 250-255
A host identified by an IPv6 literal address [RFC3513] is
distinguished by enclosing the IPv6 literal within square-brackets
("[" and "]"). This is the only place where square-bracket
characters are allowed in the URI syntax.
IPv6reference = "[" IPv6address "]"
IPv6address = 6( h4 ":" ) ls32
/ "::" 5( h4 ":" ) ls32
/ [ h4 ] "::" 4( h4 ":" ) ls32
/ [ *1( h4 ":" ) h4 ] "::" 3( h4 ":" ) ls32
/ [ *2( h4 ":" ) h4 ] "::" 2( h4 ":" ) ls32
/ [ *3( h4 ":" ) h4 ] "::" h4 ":" ls32
/ [ *4( h4 ":" ) h4 ] "::" ls32
/ [ *5( h4 ":" ) h4 ] "::" h4
/ [ *6( h4 ":" ) h4 ] "::"
ls32 = ( h4 ":" h4 ) / IPv4address
; least-significant 32 bits of address
h4 = 1*4HEXDIG
The presence of host within a 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.
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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 default may be defined by the scheme-specific
semantics of the URI. Likewise, 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
port 80.
3.3 Path
The path component contains hierarchical data that, along with data
in the optional query (Section 3.4) component, serves to identify a
resource within the scope of that URI's scheme and naming authority
(if any). There is no specific "path" syntax production in the
generic URI syntax. Instead, what we refer to as the URI path is
that part of the parsed URI string matching either the abs-path or
the rel-path production, since they are mutually exclusive for any
given URI and can be parsed as a single component. The path is
terminated by the first question-mark ("?") or number-sign ("#")
character, or by the end of the URI.
path-segments = segment *( "/" segment )
segment = *pchar
pchar = unreserved / escaped / ";" /
":" / "@" / "&" / "=" / "+" / "$" / ","
The path consists of a sequence of path segments separated by a slash
("/") character. A path is always defined for a URI, though the
defined path may be empty (zero length) or opaque (not containing any
"/" delimiters). For example, the URI <mailto:fred@example.com> has
a path of "fred@example.com".
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
similar effect to how they are used within some operating systems'
file directory structure to indicate the current directory and parent
directory, respectively. 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,
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as described in Section 5.2.
Aside from dot-segments in hierarchical paths, a path segment is
considered opaque by the generic syntax. URI generating applications
often use the reserved characters allowed in segment for the purpose
of delimiting scheme-specific or generator-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
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 of a
parameter is specific to the URI originator.
3.4 Query
The query component contains non-hierarchical data that, along with
data in the path (Section 3.3) component, serves to identify a
resource within the scope of that URI's scheme and naming authority
(if any). The query component is indicated by the first question-mark
("?") character and terminated by a number-sign ("#") character or by
the end of the URI.
query = *( pchar / "/" / "?" )
The characters slash ("/") and question-mark ("?") are allowed to
represent data within the query component, but such use is
discouraged; incorrect implementations of reference resolution often
fail to distinguish them from hierarchical separators, thus resulting
in non-interoperable results while parsing relative references.
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 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
"/./".
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3.5 Fragment
The fragment identifier component allows indirect identification of a
secondary resource by reference to a primary resource and additional
identifying information that is selective within that resource. 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. A fragment identifier component is indicated by
the presence of a number-sign ("#") character and terminated by the
end of the URI string.
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 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 multiple media
types, as is often the case for resources whose representation is
selected based on attributes of the retrieval request, then
interpretation of the fragment identifier must be consistent across
all of those media types in order for it to be viable as an
identifier.
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.
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
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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 ("?") are allowed to
represent data within the fragment identifier, but such use is
discouraged for the same reasons as described above for query.
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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. 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 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 syntax
(Section 3) in order to express a reference that is relative to the
name space of another hierarchical URI.
relative-URI = hier-part [ "?" query ] [ "#" fragment ]
The URI referred to by a relative reference 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
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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 base URI for later
use by relative references calls for an absolute-URI production that
does not allow a fragment.
absolute-URI = scheme ":" hier-part [ "?" 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 ("#") 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 or message; the dereference should not
result in a new retrieval.
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 DNS hostname on its own. Such references are primarily
intended for human interpretation rather than machine, with the
assumption that context-based heuristics are sufficient to complete
the URI (e.g., most hostnames beginning with "www" are likely to have
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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 should be noted that such heuristics may
change over time, particularly when new URI schemes are introduced.
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.
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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 of Section 3.
5.1 Establishing a Base URI
The term "relative" implies that there exists some "base URI" against
which the relative reference is applied. Aside from same-document
references (Section 4.4, relative references are only usable if the
base URI is known. The base URI must be established by the parser
prior to parsing URI references that might be relative.
The base URI of a document can be established in one of four ways,
listed below in order of precedence. The order of precedence can be
thought of in terms of layers, where the innermost defined base URI
has the highest precedence. This can be visualized graphically as:
.----------------------------------------------------------.
| .----------------------------------------------------. |
| | .----------------------------------------------. | |
| | | .----------------------------------------. | | |
| | | | .----------------------------------. | | | |
| | | | | <relative-reference> | | | | |
| | | | `----------------------------------' | | | |
| | | | (5.1.1) Base URI embedded in the | | | |
| | | | document's content | | | |
| | | `----------------------------------------' | | |
| | | (5.1.2) Base URI of the encapsulating entity | | |
| | | (message, document, or none). | | |
| | `----------------------------------------------' | |
| | (5.1.3) URI used to retrieve the entity | |
| `----------------------------------------------------' |
| (5.1.4) Default Base URI is application-dependent |
`----------------------------------------------------------'
5.1.1 Base URI within Document Content
Within certain document media types, the base URI of the document can
be embedded within the content itself such that it can be readily
obtained by a parser. This can be useful for descriptive documents,
such as tables of content, which may be transmitted to others through
protocols other than their usual retrieval context (e.g., E-Mail or
USENET news).
It is beyond the scope of this document to specify how, for each
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media type, the base URI can be embedded. It is assumed that user
agents manipulating such media types will be able to obtain the
appropriate syntax from that media type's specification.
A mechanism for embedding the base URI within MIME container types
(e.g., the message and multipart types) is defined by MHTML
[RFC2110]. Protocols that do not use the MIME message header syntax,
but do allow some form of tagged metadata to be included within
messages, may define their own syntax for defining the base URI as
part of a message.
5.1.2 Base URI from the Encapsulating Entity
If no base URI is embedded, the base URI of a document is defined by
the document's retrieval context. For a document that is enclosed
within another entity (such as a message or another document), the
retrieval context is that entity; thus, the default base URI of the
document is the base URI of the entity in which the document is
encapsulated.
5.1.3 Base URI from the Retrieval URI
If no base URI is embedded and the document is not encapsulated
within some other entity (e.g., the top level of a composite entity),
then, if a URI was used to retrieve the base document, that URI shall
be considered the base URI. Note that if the retrieval was the
result of a redirected request, the last URI used (i.e., that which
resulted in the actual retrieval of the document) is the base URI.
5.1.4 Default Base URI
If none of the conditions described in 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 base URI
using one of the other methods may result in the same content being
interpreted differently by different types of application.
It is the responsibility of the distributor(s) of a document
containing a relative reference to ensure that the base URI for that
document can be established. It must be emphasized that a relative
reference, aside from a same-document reference, cannot be used
reliably in situations where the document's base URI is not
well-defined.
5.2 Obtaining the Referenced URI
This section describes an example algorithm for resolving URI
references that might be relative to a given base URI. The algorithm
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is intended to provide a definitive result that can be used to test
the output of other implementations. Implementation of the algorithm
itself is not required, but the result given by an implementation
must match the result that would be given by this algorithm.
The base URI (Base) is established according to the rules 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
base URI; the other components may be empty or undefined. A
component is undefined if its preceding separator does not appear in
the URI reference; the path component is never undefined, though it
may be empty. The algorithm assumes that the base URI is well-formed
and does not contain dot-segments in its path.
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
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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;
The pseudocode above refers to a 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, 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; 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.,
any characters after the right-most "/" in the base URI path are
excluded).
The pseudocode also refers to a 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
string buffer:
1. The buffer is initialized with the unprocessed path component.
2. If the buffer begins with "./" or "../", the "." or ".." segment
is removed.
3. All occurrences of "/./" in the buffer are replaced with "/".
4. If the buffer ends with "/.", the "." is removed.
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5. All occurrences of "/<segment>/../" in the buffer, where ".." and
<segment> are complete path segments, are iteratively replaced
with "/" in order from left to right until no matching pattern
remains. If the buffer ends with "/<segment>/..", that is also
replaced with "/". Note that <segment> may be empty.
6. All prefixes of "<segment>/../" in the buffer, where ".." and
<segment> are complete path segments, are iteratively replaced
with "/" in order from left to right until no matching pattern
remains. If the buffer ends with "<segment>/..", that is also
replaced with "/". Note that <segment> may be empty.
7. The remaining buffer is returned as the result of
remove_dot_segments.
Some systems may find it more efficient to implement the
remove_dot_segments algorithm as a stack of path segments being
compressed, rather than as a series of string pattern replacements.
5.3 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;
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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 with a well-defined base URI of
http://a/b/c/d;p?q
a relative URI reference would be resolved as follows:
5.4.1 Normal Examples
"g:h" = "g:h"
"g" = "http://a/b/c/g"
"./g" = "http://a/b/c/g"
"g/" = "http://a/b/c/g/"
"/g" = "http://a/g"
"//g" = "http://g"
"?y" = "http://a/b/c/d;p?y"
"g?y" = "http://a/b/c/g?y"
"#s" = "http://a/b/c/d;p?q#s"
"g#s" = "http://a/b/c/g#s"
"g?y#s" = "http://a/b/c/g?y#s"
";x" = "http://a/b/c/;x"
"g;x" = "http://a/b/c/g;x"
"g;x?y#s" = "http://a/b/c/g;x?y#s"
"." = "http://a/b/c/"
"./" = "http://a/b/c/"
".." = "http://a/b/"
"../" = "http://a/b/"
"../g" = "http://a/b/g"
"../.." = "http://a/"
"../../" = "http://a/"
"../../g" = "http://a/g"
5.4.2 Abnormal Examples
Although the 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.
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An empty reference refers to the current base URI.
"" = "http://a/b/c/d;p?q"
Parsers must be careful in handling the case where there are more
relative path ".." segments than there are hierarchical levels in the
base URI's path. Note that the ".." syntax cannot be used to change
the authority component of a URI.
"../../../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 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 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
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should be avoided, but is allowed for backward compatibility.
"http:g" = "http:g" ; for strict parsers
/ "http://a/b/c/g" ; for backward compatibility
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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. For this reason, determination of
equivalence or difference of URIs is based on string comparison,
perhaps augmented by reference to additional rules provided by URI
scheme definitions. We use the terms "different" and "equivalent" to
describe the possible outcomes of such comparisons, but there are
many application-dependent versions of equivalence.
Even though it is possible to determine that two URIs are equivalent,
it is never possible to be sure that two URIs identify different
resources. Therefore, comparison methods are designed to minimize
false negatives while strictly avoiding false positives.
In testing for equivalence, it is generally unwise to directly
compare relative URI references; they should be converted to their
absolute forms before comparison. Furthermore, when URI references
are being compared for the purpose of selecting (or avoiding) a
network action, such as retrieval of a representation, it is often
necessary to remove fragment identifiers from the URIs prior to
comparison.
6.2 Comparison Ladder
A variety of methods are used in practice to test URI equivalence.
These methods fall into a range, distinguished by the amount of
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processing required and the degree to which the probability of false
negatives is reduced. As noted above, false negatives cannot in
principle be eliminated. In practice, their probability can be
reduced, but this reduction requires more processing and is not
cost-effective for all applications.
If this range of comparison practices is considered as a ladder, the
following discussion will climb the ladder, starting with those that
are cheap but have a relatively higher chance of producing false
negatives, and proceeding to those that have higher computational
cost and lower risk of false negatives.
6.2.1 Simple String Comparison
If two URIs, considered as character strings, are identical, then it
is safe to conclude that they are equivalent. This type of
equivalence test has very low computational cost and is in wide use
in a variety of applications, particularly in the domain of parsing.
Testing strings for equivalence requires some basic precautions. This
procedure is often referred to as "bit-for-bit" or "byte-for-byte"
comparison, which is potentially misleading. Testing of strings for
equality is normally based on pairwise comparison of the characters
that make up the strings, starting from the first and proceeding
until both strings are exhausted and all characters found to be
equal, a pair of characters compares unequal, or one of the strings
is exhausted before the other.
Such character comparisons require that each pair of characters be
put in comparable form. For example, should one URI be stored in a
byte array in EBCDIC encoding, and the second be in a Java String
object, bit-for-bit comparisons applied naively will produce both
false-positive and false-negative errors. Thus, in principle, it is
better to speak of equality on a character-for-character rather than
byte-for-byte or bit-for-bit basis.
Unicode defines a character as being identified by number
("codepoint") with an associated bundle of visual and other
semantics. At the software level, it is not practical to compare
semantic bundles, so in practical terms, character-by-character
comparisons are done codepoint-by-codepoint.
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6.2.2 Syntax-based Normalization
Software may use logic based on the definitions provided by this
specification to reduce the probability of false negatives. Such
processing is moderately higher in cost than character-for-character
string comparison. For example, an application using this approach
could reasonably consider the following two URIs equivalent:
example://a/b/c/%7A
eXAMPLE://a/./b/../b/c/%7a
Web user agents, such as browsers, typically apply this type of URI
normalization when determining whether a cached response is
available. Syntax-based normalization includes such techniques as
case normalization, escape normalization, and removal of
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 are case insensitive and therefore can be normalized to
lowercase. For example, the URI <HTTP://www.EXAMPLE.com/> is
equivalent to <http://www.example.com/>.
6.2.2.2 Escape Normalization
The percent-escape mechanism described in Section 2.4 is a frequent
source of variance among otherwise identical URIs. One cause is the
choice of uppercase or lowercase letters for the hexadecimal digits
within the escape sequence (e.g., "%3a" versus "%3A"). Such sequences
are always equivalent; for the sake of uniformity, URI generators and
normalizers are strongly encouraged to use uppercase letters for the
hex digits A-F.
Only characters that are excluded from or reserved within the URI
syntax must be escaped when used as data. However, some URI
generators go beyond that and escape characters that do not require
escaping, resulting in URIs that are equivalent to their unescaped
counterparts. Such URIs can be normalized by unescaping sequences
that represent the unreserved characters, as described in Section
2.3.
6.2.2.3 Path Segment Normalization
The complete path segments "." and ".." have a special meaning within
hierarchical URI schemes. As such, they should not appear in
absolute paths; if they are found, they can be removed by applying
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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 the "http" URI scheme, which defines an empty port
component as being equivalent to the default TCP port for HTTP (port
80). In general, a URI scheme that uses the generic syntax for
authority is defined such that a URI with an explicit ":port", where
the port is the default for the scheme, is equivalent to one where
the port is elided.
6.2.4 Protocol-based Normalization
Web spiders, for which substantial effort to reduce the incidence of
false negatives is often cost-effective, are observed to implement
even more aggressive techniques in URI comparison. For example, if
they observe that a URI such as
http://example.com/data
redirects to 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 kind of technique is only appropriate in special
situations.
6.3 Canonical Form
It is in the best interests of everyone to avoid false-negatives in
comparing URIs and to minimize the amount of software processing for
such comparisons. Those who generate and make reference to URIs can
reduce the cost of processing and the risk of false negatives by
consistently providing them in a form that is reasonably canonical
with respect to their scheme. Specifically:
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o Always provide the URI scheme in lowercase characters.
o Always provide the hostname, if any, in lowercase characters.
o Only perform percent-escaping where it is essential.
o Always use uppercase A-through-F characters when percent-escaping.
o Prevent /./ and /../ from appearing in non-relative URI paths.
The good practices listed above are motivated by deployed software
that frequently use these techniques for the purposes of
normalization.
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7. Security Considerations
A URI does not in itself pose a security threat. However, since URIs
are often used to provide a compact set of instructions for access to
network resources, care must be taken to properly interpret the data
within a URI, to prevent that data from causing unintended access,
and to avoid including data that should not be revealed in plain
text.
7.1 Reliability and Consistency
There is no guarantee that, having once used a given URI to retrieve
some information, the same information will be retrievable by that
URI in the future. Nor is there any guarantee that the information
retrievable via that URI in the future will be observably similar to
that retrieved in the past. The URI syntax does not constrain how a
given scheme or authority apportions its name space or maintains it
over time. Such a guarantee can only be obtained from the person(s)
controlling that name space and the resource in question. A specific
URI scheme may define additional semantics, such as name persistence,
if those semantics are required of all naming authorities for that
scheme.
7.2 Malicious Construction
It is sometimes possible to construct a URI such that an attempt to
perform a seemingly harmless, idempotent operation, such as the
retrieval of a representation, will in fact cause a possibly damaging
remote operation to occur. The unsafe URI is typically constructed
by specifying a port number other than that reserved for the network
protocol in question. The client unwittingly contacts a site that is
running a different protocol service. The content of the URI
contains instructions that, when interpreted according to this other
protocol, cause an unexpected operation. An example has been the use
of a gopher URI to cause an unintended or impersonating message to be
sent via a SMTP server.
Caution should be used when dereferencing a URI that specifies a TCP
port number other than the default for the scheme, especially when it
is a number within the reserved space.
Care should be taken when a URI contains escaped delimiters for a
given protocol (for example, CR and LF characters for telnet
protocols) that these octets are not unescaped before transmission.
This might violate the protocol, but avoids the potential for such
characters to be used to simulate an extra operation or parameter in
that protocol which might lead to an unexpected and possibly harmful
remote operation being performed.
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7.3 Rare IP Address Formats
Although the URI syntax for IPv4address only allows the common,
dotted-decimal form of IPv4 address literal, many implementations
that process URIs make use of platform-dependent system routines,
such as gethostbyname() and inet_aton(), to translate the string
literal to an actual IP address. Unfortunately, such system routines
often allow and process a much larger set of formats than those
described in Section 3.2.2.
For example, many implementations allow dotted forms of three
numbers, wherein the last part is interpreted as a 16-bit quantity
and placed in the right-most two bytes of the network address (e.g.,
a Class B network). Likewise, a dotted form of two numbers means the
last part is interpreted as a 24-bit quantity and placed in the right
most three bytes of the network address (Class A), and a single
number (without dots) is interpreted as a 32-bit quantity and stored
directly in the network address. Adding further to the confusion,
some implementations allow each dotted part to be interpreted as
decimal, octal, or hexadecimal, as specified in the C language (i.e.,
a leading 0x or 0X implies hexadecimal; otherwise, a leading 0
implies octal; otherwise, the number is interpreted as decimal).
These additional IP address formats are not allowed in the URI syntax
due to differences between platform implementations. However, they
can become a security concern if an application attempts to filter
access to resources based on the IP address in string literal format.
If such filtering is performed, it is recommended that literals be
converted to numeric form and filtered based on the numeric value,
rather than a prefix or suffix of the string form.
7.4 Sensitive Information
It is clearly unwise to use a URI that contains a password which is
intended to be secret. In particular, the use of a password within
the userinfo component of a URI is strongly discouraged except in
those rare cases where the 'password' parameter is intended to be
public.
7.5 Semantic Attacks
Because the userinfo component is rarely used and appears before the
hostname in the authority component, it can be used to construct a
URI that is intended to mislead a human user by appearing to identify
one (trusted) naming authority while actually identifying a different
authority hidden behind the noise. For example
http://www.example.com&story=breaking_news@10.0.0.1/top_story.htm
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might lead a human user to assume that the host is 'www.example.com',
whereas it is actually '10.0.0.1'. Note that the misleading userinfo
could be much longer than the example above.
A misleading URI, such as the one above, is an attack on the user's
preconceived notions about the meaning of a URI, rather than an
attack on the software itself. User agents may be able to reduce the
impact of such attacks by visually distinguishing the various
components of the URI when rendered, such as by using a different
color or tone to render userinfo if any is present, though there is
no general panacea. More information on URI-based semantic attacks
can be found in [Siedzik].
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8. 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 Reese Anschultz, Tim Bray,
Rob Cameron, Dan Connolly, Adam M. Costello, John Cowan, Jason
Diamond, Martin Duerst, Stefan Eissing, Clive D.W. Feather, Pat
Hayes, Henry Holtzman, Graham Klyne, Dan Kohn, Bruce Lilly, Andrew
Main, Michael Mealling, Julian Reschke, Tomas Rokicki, Miles Sabin,
Ronald Tschalaer, Marc Warne, Stuart Williams, and Henry Zongaro are
gratefully acknowledged.
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Normative References
[ASCII] American National Standards Institute, "Coded Character
Set -- 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
[RFC2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
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Informative References
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
[RFC1630] Berners-Lee, T., "Universal Resource Identifiers in WWW: A
Unifying Syntax for the Expression of Names and Addresses
of Objects on the Network as used in the World-Wide Web",
RFC 1630, June 1994.
[RFC1738] Berners-Lee, T., Masinter, L. and M. McCahill, "Uniform
Resource Locators (URL)", RFC 1738, December 1994.
[RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[RFC1808] Fielding, R., "Relative Uniform Resource Locators", RFC
1808, June 1995.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
[RFC2518] Goland, Y., Whitehead, E., Faizi, A., Carter, S. and D.
Jensen, "HTTP Extensions for Distributed Authoring --
WEBDAV", RFC 2518, February 1999.
[RFC0952] Harrenstien, K., Stahl, M. and E. Feinler, "DoD Internet
host table specification", RFC 952, October 1985.
[RFC3513] Hinden, R. and S. Deering, "Internet Protocol Version 6
(IPv6) Addressing Architecture", RFC 3513, April 2003.
[RFC2732] Hinden, R., Carpenter, B. and L. Masinter, "Format for
Literal IPv6 Addresses in URL's", RFC 2732, December 1999.
[RFC1736] Kunze, J., "Functional Recommendations for Internet
Resource Locators", RFC 1736, February 1995.
[RFC1737] Masinter, L. and K. Sollins, "Functional Requirements for
Uniform Resource Names", RFC 1737, December 1994.
[RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997.
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[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC2110] Palme, J. and A. Hopmann, "MIME E-mail Encapsulation of
Aggregate Documents, such as HTML (MHTML)", RFC 2110,
March 1997.
[RFC2717] Petke, R. and I. King, "Registration Procedures for URL
Scheme Names", BCP 35, RFC 2717, November 1999.
[Siedzik] Siedzik, R., "Semantic Attacks: What's in a URL?", April
2001.
[UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.
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Authors' Addresses
Tim Berners-Lee
World Wide Web Consortium
MIT/LCS, Room NE43-356
200 Technology Square
Cambridge, MA 02139
USA
Phone: +1-617-253-5702
Fax: +1-617-258-5999
EMail: timbl@w3.org
URI: http://www.w3.org/People/Berners-Lee/
Roy T. Fielding
Day Software
2 Corporate Plaza, Suite 150
Newport Beach, CA 92660
USA
Phone: +1-949-999-2523
Fax: +1-949-644-5064
EMail: roy.fielding@day.com
URI: http://www.apache.org/~fielding/
Larry Masinter
Adobe Systems Incorporated
345 Park Ave
San Jose, CA 95110
USA
Phone: +1-408-536-3024
EMail: LMM@acm.org
URI: http://larry.masinter.net/
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Appendix A. Collected ABNF for URI
abs-path = "/" path-segments
absolute-URI = scheme ":" hier-part [ "?" query ]
alphanum = ALPHA / DIGIT
authority = [ userinfo "@" ] host [ ":" port ]
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
domainlabel = alphanum [ 0*61( alphanum / "-" ) alphanum ]
escaped = "%" HEXDIG HEXDIG
fragment = *( pchar / "/" / "?" )
h4 = 1*4HEXDIG
hier-part = net-path / abs-path / rel-path
host = [ IPv6reference / IPv4address / hostname ]
hostname = domainlabel qualified
IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet
IPv6address = 6( h4 ":" ) ls32
/ "::" 5( h4 ":" ) ls32
/ [ h4 ] "::" 4( h4 ":" ) ls32
/ [ *1( h4 ":" ) h4 ] "::" 3( h4 ":" ) ls32
/ [ *2( h4 ":" ) h4 ] "::" 2( h4 ":" ) ls32
/ [ *3( h4 ":" ) h4 ] "::" h4 ":" ls32
/ [ *4( h4 ":" ) h4 ] "::" ls32
/ [ *5( h4 ":" ) h4 ] "::" h4
/ [ *6( h4 ":" ) h4 ] "::"
IPv6reference = "[" IPv6address "]"
ls32 = ( h4 ":" h4 ) / IPv4address
mark = "-" / "_" / "." / "!" / "~" / "*" / "'" / "(" / ")"
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net-path = "//" authority [ abs-path ]
path-segments = segment *( "/" segment )
pchar = unreserved / escaped / ";" /
":" / "@" / "&" / "=" / "+" / "$" / ","
port = *DIGIT
qualified = *( "." domainlabel ) [ "." ]
query = *( pchar / "/" / "?" )
rel-path = path-segments
relative-URI = hier-part [ "?" query ] [ "#" fragment ]
reserved = "/" / "?" / "#" / "[" / "]" / ";" /
":" / "@" / "&" / "=" / "+" / "$" / ","
scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
segment = *pchar
unreserved = ALPHA / DIGIT / mark
URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ]
URI-reference = URI / relative-URI
uric = reserved / unreserved / escaped
userinfo = *( unreserved / escaped / ";" /
":" / "&" / "=" / "+" / "$" / "," )
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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.
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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 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 that contains whitespace.
The prefix "URL:" (with or without a trailing space) was formerly
recommended as a way to help distinguish a URI from other bracketed
designators, though it is not commonly used in practice and is no
longer recommended.
For robustness, software that accepts user-typed URI should attempt
to recognize and strip both delimiters and embedded whitespace.
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For example, the text:
Yes, Jim, I found it under "http://www.w3.org/Addressing/",
but you can probably pick it up from <ftp://ds.internic.
net/rfc/>. Note the warning in <http://www.ics.uci.edu/pub/
ietf/uri/historical.html#WARNING>.
contains the URI references
http://www.w3.org/Addressing/
ftp://ds.internic.net/rfc/
http://www.ics.uci.edu/pub/ietf/uri/historical.html#WARNING
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Appendix D. Summary of Non-editorial Changes
D.1 Additions
IPv6 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 and uric sets.
Square brackets are now specified as reserved within the authority
component and not allowed outside their use as delimiters for an
IPv6reference within host. In order to make this change without
changing the technical definition of the path, query, and fragment
components, those rules were redefined to directly specify the
characters allowed rather than 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.
Section 6 (Section 6) on URI normalization and comparison has been
completely rewritten and extended using input from Tim Bray and
discussion within the W3C Technical Architecture Group. Likewise,
Section 2.1 on the encoding of characters has been replaced.
An ABNF production for URI has been introduced to correspond to the
common usage of the term: an absolute URI with optional fragment.
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 on reserved characters has been rewritten to clearly
explain what characters are reserved, when they are reserved, and why
they are reserved even when not used as delimiters by the generic
syntax. Likewise, the section on escaped characters has been
rewritten, and URI normalizers are now given license to unescape any
octets corresponding to unreserved characters. The number-sign ("#")
character has been moved back from the excluded delims to the
reserved set.
The ABNF for URI and URI-reference has been redesigned to make them
more friendly to LALR parsers and significantly reduce complexity. As
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a result, the layout form of syntax description has been removed,
along with the uric-no-slash, opaque-part, and rel-segment
productions. 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 and relative-URI has been corrected to allow a
relative URI path to be empty. This also allows an absolute-URI to
consist of nothing after the "scheme:", as is present in practice
with the "DAV:" namespace [RFC2518] and the "about:" URI used by many
browser implementations. The ambiguity regarding the parsing of
net-path, abs-path, and rel-path is now explained and disambiguated
in the same section.
Registry-based naming authorities that use the generic syntax
authority component are now limited to DNS hostnames, since those
have been the only such URIs in deployment. This change was
necessary to enable internationalized domain names to be processed in
their native character encodings at the application layers above URI
processing. The reg_name, server, and hostport productions have been
removed to simplify parsing of the URI syntax.
The ABNF of qualified has been simplified to remove a parsing
ambiguity without changing the allowed syntax. The toplabel
production has been removed because it served no useful purpose. The
ambiguity regarding the parsing of host as IPv4address or hostname is
now explained and disambiguated in the same section.
The resolving relative references algorithm of [RFC2396] has been
rewritten using pseudocode for this revision to improve clarity and
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 in
favor of a section that explains that a new retrieval action
should not be made if the target URI and base URI, excluding
fragments, match. This change has no impact on user agent
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behavior aside from how the resolved reference might be described
to the user.
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.
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Index
A
ABNF 9
abs-path 16
absolute 25
absolute-path 24
absolute-URI 25
access 7
alphanum 18
authority 16, 17
B
base URI 27
D
dec-octet 19
delims 15
dereference 7
domainlabel 18
dot-segments 20
E
escaped 13
excluded 14
F
fragment 22
G
generic syntax 5
H
h4 19
hier-part 16
hierarchical 8
host 18
hostname 18
I
identifier 5
invisible 14
IPv4 19
IPv4address 19
IPv6 19
IPv6address 19
IPv6reference 19
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L
locator 6
ls32 19
M
mark 12
merge 30
N
name 6
net-path 16
network-path 24
P
path 16, 20
path-segments 20
pchar 20
port 20
Q
qualified 18
query 21
R
rel-path 16
relative 9, 27
relative-path 24
relative-URI 24
remove_dot_segments 30
representation 8
reserved 11
resolution 7, 27
resource 4
retrieval 8
S
same-document 25
sameness 8
scheme 16
segment 20
suffix 25
T
transcription 6
U
uniform 4
unreserved 12
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unwise 15
URI grammar
abs-path 16
absolute-URI 25
ALPHA 9
alphanum 18
authority 16, 17
CR 9
CTL 9
dec-octet 19
DIGIT 9
domainlabel 18
DQUOTE 9
escaped 13
fragment 16, 22, 24
h4 19
HEXDIG 9
hier-part 16, 24, 25
host 17, 18
hostname 18
IPv4address 19
IPv6address 19
IPv6reference 19
LF 9
ls32 19
mark 12
net-path 16
OCTET 9
path-segments 16, 20
pchar 20, 21, 22
port 17, 20
qualified 18
query 16, 21, 24, 25
rel-path 16
relative-URI 24, 24
reserved 12
scheme 16, 17, 25
segment 20
SP 9
unreserved 12
URI 16, 24
URI-reference 24
uric 11
userinfo 17, 18
URI 16
URI-reference 24
uric 11
URL 6
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URN 6
userinfo 18
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Full Copyright Statement
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement
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Berners-Lee, et al. Expires December 5, 2003 [Page 61]
