Network Working Group T. Berners-Lee
Internet-Draft MIT/LCS
Updates: 1738 (if approved) R. Fielding
Obsoletes: 2732, 2396, 1808 (if approved) Day Software
Expires: August 16, 2004 L. Masinter
Adobe
February 16, 2004
Uniform Resource Identifier (URI): Generic Syntax
draft-fielding-uri-rfc2396bis-04
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Copyright Notice
Copyright (C) The Internet Society (2004). 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://gbiv.com/protocols/uri/rev-2002/issues.html>.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Overview of URIs . . . . . . . . . . . . . . . . . . . . . . 4
1.1.1 Generic Syntax . . . . . . . . . . . . . . . . . . . . . . . 5
1.1.2 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1.3 URI, URL, and URN . . . . . . . . . . . . . . . . . . . . . 6
1.2 Design Considerations . . . . . . . . . . . . . . . . . . . 6
1.2.1 Transcription . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.2 Separating Identification from Interaction . . . . . . . . . 7
1.2.3 Hierarchical Identifiers . . . . . . . . . . . . . . . . . . 9
1.3 Syntax Notation . . . . . . . . . . . . . . . . . . . . . . 10
2. Characters . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1 Percent Encoding . . . . . . . . . . . . . . . . . . . . . . 11
2.2 Reserved Characters . . . . . . . . . . . . . . . . . . . . 12
2.3 Unreserved Characters . . . . . . . . . . . . . . . . . . . 12
2.4 When to Encode or Decode . . . . . . . . . . . . . . . . . . 13
3. Syntax Components . . . . . . . . . . . . . . . . . . . . . 15
3.1 Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2 Authority . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2.1 User Information . . . . . . . . . . . . . . . . . . . . . . 16
3.2.2 Host . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.2.3 Port . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.3 Path . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.4 Query . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.5 Fragment . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.1 URI Reference . . . . . . . . . . . . . . . . . . . . . . . 24
4.2 Relative URI . . . . . . . . . . . . . . . . . . . . . . . . 24
4.3 Absolute URI . . . . . . . . . . . . . . . . . . . . . . . . 25
4.4 Same-document Reference . . . . . . . . . . . . . . . . . . 25
4.5 Suffix Reference . . . . . . . . . . . . . . . . . . . . . . 25
5. Reference Resolution . . . . . . . . . . . . . . . . . . . . 27
5.1 Establishing a Base URI . . . . . . . . . . . . . . . . . . 27
5.1.1 Base URI within Document Content . . . . . . . . . . . . . . 27
5.1.2 Base URI from the Encapsulating Entity . . . . . . . . . . . 28
5.1.3 Base URI from the Retrieval URI . . . . . . . . . . . . . . 28
5.1.4 Default Base URI . . . . . . . . . . . . . . . . . . . . . . 28
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5.2 Relative Resolution . . . . . . . . . . . . . . . . . . . . 28
5.2.1 Pre-parse the Base URI . . . . . . . . . . . . . . . . . . . 29
5.2.2 Transform References . . . . . . . . . . . . . . . . . . . . 29
5.2.3 Merge Paths . . . . . . . . . . . . . . . . . . . . . . . . 30
5.2.4 Remove Dot Segments . . . . . . . . . . . . . . . . . . . . 30
5.3 Component Recomposition . . . . . . . . . . . . . . . . . . 32
5.4 Reference Resolution Examples . . . . . . . . . . . . . . . 33
5.4.1 Normal Examples . . . . . . . . . . . . . . . . . . . . . . 33
5.4.2 Abnormal Examples . . . . . . . . . . . . . . . . . . . . . 33
6. Normalization and Comparison . . . . . . . . . . . . . . . . 35
6.1 Equivalence . . . . . . . . . . . . . . . . . . . . . . . . 35
6.2 Comparison Ladder . . . . . . . . . . . . . . . . . . . . . 36
6.2.1 Simple String Comparison . . . . . . . . . . . . . . . . . . 36
6.2.2 Syntax-based Normalization . . . . . . . . . . . . . . . . . 37
6.2.3 Scheme-based Normalization . . . . . . . . . . . . . . . . . 38
6.2.4 Protocol-based Normalization . . . . . . . . . . . . . . . . 39
6.3 Canonical Form . . . . . . . . . . . . . . . . . . . . . . . 39
7. Security Considerations . . . . . . . . . . . . . . . . . . 41
7.1 Reliability and Consistency . . . . . . . . . . . . . . . . 41
7.2 Malicious Construction . . . . . . . . . . . . . . . . . . . 41
7.3 Back-end Transcoding . . . . . . . . . . . . . . . . . . . . 42
7.4 Rare IP Address Formats . . . . . . . . . . . . . . . . . . 42
7.5 Sensitive Information . . . . . . . . . . . . . . . . . . . 43
7.6 Semantic Attacks . . . . . . . . . . . . . . . . . . . . . . 43
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 45
Normative References . . . . . . . . . . . . . . . . . . . . 46
Informative References . . . . . . . . . . . . . . . . . . . 47
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 48
A. Collected ABNF for URI . . . . . . . . . . . . . . . . . . . 50
B. Parsing a URI Reference with a Regular Expression . . . . . 52
C. Delimiting a URI in Context . . . . . . . . . . . . . . . . 53
D. Summary of Non-editorial Changes . . . . . . . . . . . . . . 55
D.1 Additions . . . . . . . . . . . . . . . . . . . . . . . . . 55
D.2 Modifications from RFC 2396 . . . . . . . . . . . . . . . . 55
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
Intellectual Property and Copyright Statements . . . . . . . 62
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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]. Advice for designers of new URI
schemes can be found in [RFC2718].
All significant changes from RFC 2396 are noted in Appendix D.
This specification uses the terms "character" and "character
encoding" in accordance with the definitions provided in [RFC2978].
1.1 Overview of URIs
URIs are characterized as follows:
Uniform
Uniformity provides several benefits: it allows different types of
resource identifiers to be used in the same context, even when the
mechanisms used to access those resources may differ; it allows
uniform semantic interpretation of common syntactic conventions
across different types of resource identifiers; it allows
introduction of new types of resource identifiers without
interfering with the way that existing identifiers are used; and,
it allows the identifiers to be reused in many different contexts,
thus permitting new applications or protocols to leverage a
pre-existing, large, and widely-used set of resource identifiers.
Resource
Anything that can be named or described can be a resource.
Familiar examples include an electronic document, an image, a
service (e.g., "today's weather report for Los Angeles"), and a
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collection of other resources. A resource is not necessarily
accessible via the Internet; e.g., human beings, corporations, and
bound books in a library can also be resources. Likewise, abstract
concepts can be resources, such as the operators and operands of a
mathematical equation or the types of a relationship (e.g.,
"parent" or "employee").
Identifier
An identifier embodies the information required to distinguish
what is being identified from all other things within its scope of
identification.
A URI is an identifier that consists of a sequence of characters
matching the syntax defined by the syntax rule named "URI" in Section
3. A URI can be used to refer to a resource. This specification does
not place any limits on the nature of a resource or the reasons why
an application might wish to refer to a resource. URIs have a global
scope and should be interpreted consistently regardless of context,
but that interpretation may be defined in relation to the user's
context (e.g., "http://localhost/" refers to a resource that is
relative to the user's network interface and yet not specific to any
one user).
1.1.1 Generic Syntax
Each URI begins with a scheme name, as defined in Section 3.1, that
refers to a specification for assigning identifiers within that
scheme. As such, the URI syntax is a federated and extensible naming
system wherein each scheme's specification may further restrict the
syntax and semantics of identifiers using that scheme.
This specification defines those elements of the URI syntax that are
required of all URI schemes or are common to many URI schemes. It
thus defines the syntax and semantics that are needed to implement a
scheme-independent parsing mechanism for URI references, such that
the scheme-dependent handling of a URI can be postponed until the
scheme-dependent semantics are needed. Likewise, protocols and data
formats that make use of URI references can refer to this
specification as defining the range of syntax allowed for all URIs,
including those schemes that have yet to be defined.
A parser of the generic URI syntax is capable of parsing any URI
reference into its major components; once the scheme is determined,
further scheme-specific parsing can be performed on the components.
In other words, the URI generic syntax is a superset of the syntax of
all URI schemes.
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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
http://www.ietf.org/rfc/rfc2396.txt
mailto:John.Doe@example.com
news:comp.infosystems.www.servers.unix
telnet://melvyl.ucop.edu/
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) has been used historically to refer to both URIs under the
"urn" scheme [RFC2141], which are required to remain globally unique
and persistent even when the resource ceases to exist or becomes
unavailable, and to any other URI with the properties of a name.
An individual scheme does not need to be classified as being just one
of "name" or "locator". Instances of URIs from any given scheme may
have the characteristics of names or locators or both, often
depending on the persistence and care in the assignment of
identifiers by the naming authority, rather than any quality of the
scheme. Future specifications and related documentation should use
the general term "URI", rather than the more restrictive terms URL
and URN [RFC3305].
1.2 Design Considerations
1.2.1 Transcription
The URI syntax has been designed with global transcription as one of
its main considerations. A URI is a sequence of characters from a
very limited set: the letters of the basic Latin alphabet, digits,
and a few special characters. A URI may be represented in a variety
of ways: e.g., ink on paper, pixels on a screen, or a sequence of
integers from a coded character set. The interpretation of a URI
depends only on the characters used and not how those characters are
represented in a network protocol.
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The goal of transcription can be described by a simple scenario.
Imagine two colleagues, Sam and Kim, sitting in a pub at an
international conference and exchanging research ideas. Sam asks Kim
for a location to get more information, so Kim writes the URI for the
research site on a napkin. Upon returning home, Sam takes out the
napkin and types the URI into a computer, which then retrieves the
information to which Kim referred.
There are several design considerations revealed by the scenario:
o A URI is a sequence of characters that is not always represented
as a sequence of octets.
o A URI might be transcribed from a non-network source, and thus
should consist of characters that are most likely to be able to be
entered into a computer, within the constraints imposed by
keyboards (and related input devices) across languages and
locales.
o A URI often needs to be remembered by people, and it is easier for
people to remember a URI when it consists of meaningful or
familiar components.
These design considerations are not always in alignment. For
example, it is often the case that the most meaningful name for a URI
component would require characters that cannot be typed into some
systems. The ability to transcribe a resource identifier from one
medium to another has been considered more important than having a
URI consist of the most meaningful of components.
In local or regional contexts and with improving technology, users
might benefit from being able to use a wider range of characters;
such use is not defined in this specification. Percent-encoded
octets (Section 2.1) may be used within a URI to represent characters
outside the range of the US-ASCII coded character set if such
representation is defined by the scheme or by the protocol element in
which the URI is referenced; such a definition will specify the
character encoding scheme used to map those characters to octets
prior to being percent-encoded for the URI.
1.2.2 Separating Identification from Interaction
A common misunderstanding of URIs is that they are only used to refer
to accessible resources. In fact, the URI alone only provides
identification; access to the resource is neither guaranteed nor
implied by the presence of a URI. Instead, an operation (if any)
associated with a URI reference is defined by the protocol element,
data format attribute, or natural language text in which it appears.
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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 "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. To use that access mechanism to perform an action on the
URI's resource is to "dereference" the URI.
When URIs are used within information systems to identify sources of
information, the most common form of URI dereference is "retrieval":
making use of a URI in order to retrieve a representation of its
associated resource. A "representation" is a sequence of octets,
along with representation metadata describing those octets, that
constitutes a record of the state of the resource at the time that
the representation is generated. Retrieval is achieved by a process
that might include using the URI as a cache key to check for a
locally cached representation, resolution of the URI to determine an
appropriate access mechanism (if any), and dereference of the URI for
the sake of applying a retrieval operation. Depending on the
protocols used to perform the retrieval, additional information might
be supplied about the resource (resource metadata) and its relation
to other resources.
URI references in information systems are designed to be
late-binding: the result of an access is generally determined at the
time it is accessed and may vary over time or due to other aspects of
the interaction. When an author creates a reference to such a
resource, they do so with the intention that the reference be used in
the future; what is being identified is not some specific result that
was obtained in the past, but rather some characteristic that is
expected to be true for future results. In such cases, the resource
referred to by the URI is actually a sameness of characteristics as
observed over time, perhaps elucidated by additional comments or
assertions made by the resource provider.
Although many URI schemes are named after protocols, this does not
imply that use of such a URI will result in access to the resource
via the named protocol. URIs are often used simply for the sake of
identification. Even when a URI is used to retrieve a representation
of a resource, that access might be through gateways, proxies,
caches, and name resolution services that are independent of the
protocol associated with the scheme name, and the resolution of some
URIs may require the use of more than one protocol (e.g., both DNS
and HTTP are typically used to access an "http" URI's origin server
when a representation isn't found in a local cache).
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1.2.3 Hierarchical Identifiers
The URI syntax is organized hierarchically, with components listed in
order of decreasing significance from left to right. For some URI
schemes, the visible hierarchy is limited to the scheme itself:
everything after the scheme component delimiter (":") is considered
opaque to URI processing. Other URI schemes make the hierarchy
explicit and visible to generic parsing algorithms.
The generic syntax uses 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, wherein the vast majority of
URIs in these documents point to resources within the tree rather
than outside of it. Similarly, documents located at a particular
site are much more likely to refer to other resources at that site
than to resources at remote sites. Relative referencing of URIs
allows document trees to be partially independent of their location
and access scheme. For instance, it is possible for a single set of
hypertext documents to be simultaneously accessible and traversable
via each of the "file", "http", and "ftp" schemes if the documents
refer to each other using relative references. Furthermore, such
document trees can be moved, as a whole, without changing any of the
relative references.
A relative URI reference (Section 4.2) refers to a resource by
describing the difference within a hierarchical name space between
the reference context and the target URI. The reference resolution
algorithm, presented in Section 5, defines how such a reference is
transformed to the target URI. Since relative references can only be
used within the context of a hierarchical URI, designers of new URI
schemes should use a syntax consistent with the generic syntax's
hierarchical components unless there are compelling reasons to forbid
relative referencing within that scheme.
All URIs are parsed by generic syntax parsers when used. A URI scheme
that wishes to remain opaque to hierarchical processing must disallow
the use of slash and question mark characters. However, since a
non-relative URI reference is only modified by the generic parser if
it contains complete path segments of "." or ".." (see Section 3.3),
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URIs may safely use "/" for other purposes if they do not allow
dot-segments.
1.3 Syntax Notation
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC2234], including the following core ABNF syntax rules
defined by that specification: ALPHA (letters), CR (carriage return),
CTL (control characters), DIGIT (decimal digits), DQUOTE (double
quote), HEXDIG (hexadecimal digits), LF (line feed), and SP (space).
The complete URI syntax is collected in Appendix A.
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2. Characters
Although ABNF notation defines its terminal values to be non-negative
integers (codepoints) based on the US-ASCII coded character set
[ASCII], we must invert that relation in order to understand the URI
syntax, since URIs are defined as strings of characters independent
of any particular encoding. Therefore, the integer values must be
mapped back to their corresponding characters via US-ASCII in order
to complete the syntax rules.
This specification does not mandate the use of any particular
character encoding scheme for mapping between URI characters and the
octets used to store or transmit those characters. When a URI appears
in a protocol element, the character encoding is defined by that
protocol; absent such a definition, a URI is assumed to use the same
character encoding as the surrounding text.
A URI is composed from a limited set of characters consisting of
digits, letters, and a few graphic symbols. A reserved (Section 2.2)
subset of those characters may be used to delimit syntax components
within a URI, while the remaining characters, including both the
unreserved (Section 2.3) set and those reserved characters not acting
as delimiters, define each component's data.
2.1 Percent Encoding
A percent-encoding mechanism is used to represent a data octet in a
component when that octet's corresponding character is outside the
allowed set or is being used as a delimiter of, or within, the
component. A percent-encoded octet is encoded as a character triplet,
consisting of the percent character "%" followed by the two
hexadecimal digits representing that octet's numeric value. For
example, "%20" is the percent-encoding for the binary octet
"00100000" (ABNF: %x20), which in US-ASCII corresponds to the space
character (SP).
pct-encoded = "%" 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 percent-encoded
octets are equivalent. For consistency, URI producers and
normalizers should use uppercase hexadecimal digits for all
percent-encodings.
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2.2 Reserved Characters
URIs include components and sub-components that are delimited by
characters in the "reserved" set. These characters are called
"reserved" because they may (or may not) be defined as delimiters by
the generic syntax, by each scheme-specific syntax, or by the
implementation-specific syntax of a URI's dereferencing algorithm.
If data for a URI component would conflict with a reserved
character's purpose as a delimiter, then the conflicting data must be
percent-encoded before forming the URI.
reserved = gen-delims / sub-delims
gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@"
sub-delims = "!" / "$" / "&" / "'" / "(" / ")"
/ "*" / "+" / "," / ";" / "="
A subset of the reserved characters (gen-delims) are used as
delimiters of the generic URI components described in Section 3. A
component's ABNF syntax rule will not use the reserved or gen-delims
rule names directly; instead, each syntax rule lists those reserved
characters that are allowed within that component (i.e., not
delimiting it). The allowed reserved characters, including those in
the sub-delims set and any of the gen-delims that are not a delimiter
of that component, are reserved for use as sub-component delimiters
within the component. Only the most common sub-components are
defined by this specification; other sub-components may be defined by
a URI scheme's specification, or by the implementation-specific
syntax of a URI's dereferencing algorithm, provided that such
sub-components are delimited by characters in that component's
reserved set. If no such delimiting role has been assigned, then a
reserved character appearing in a component represents the data octet
corresponding to its encoding in US-ASCII.
URIs that differ in the replacement of a reserved character with its
corresponding percent-encoded octet are not equivalent.
Percent-encoding a reserved character, or decoding a percent-encoded
octet that corresponds to a reserved character, will change how the
URI is interpreted by most applications.
2.3 Unreserved Characters
Characters that are allowed in a URI but do not have a reserved
purpose are called unreserved. These include uppercase and lowercase
letters, decimal digits, hyphen, period, underscore, and tilde.
unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
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URIs that differ in the replacement of an unreserved character with
its corresponding percent-encoded octet are equivalent: they identify
the same resource. However, percent-encoded unreserved characters
may change the result of some URI comparisons (Section 6),
potentially leading to incorrect or inefficient behavior. For
consistency, percent-encoded octets in the ranges of ALPHA (%41-%5A
and %61-%7A), DIGIT (%30-%39), hyphen (%2D), period (%2E), underscore
(%5F), or tilde (%7E) should not be created by URI producers and,
when found in a URI, should be decoded to their corresponding
unreserved character by URI normalizers.
2.4 When to Encode or Decode
Under normal circumstances, the only time that octets within a URI
are percent-encoded is during the process of producing the URI from
its component parts. It is during that process that an
implementation determines which of the reserved characters are to be
used as sub-component delimiters and which can be safely used as
data. Once produced, a URI is always in its percent-encoded form.
When a URI is dereferenced, the components and sub-components
significant to the scheme-specific dereferencing process (if any)
must be parsed and separated before the percent-encoded octets within
those components can be safely decoded, since otherwise the data may
be mistaken for component delimiters. The only exception is for
percent-encoded octets corresponding to characters in the unreserved
set, which can be decoded at any time. For example, the octet
corresponding to the tilde ("~") character is often encoded as "%7E"
by older URI processing software; the "%7E" can be replaced by "~"
without changing its interpretation.
Because the percent ("%") character serves as the indicator for
percent-encoded octets, it must be percent-encoded as "%25" in order
for that octet to be used as data within a URI. Implementations must
not percent-encode or decode the same string more than once, since
decoding an already decoded string might lead to misinterpreting a
percent data octet as the beginning of a percent-encoding, or vice
versa in the case of percent-encoding an already percent-encoded
string.
URI characters serve as an external 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 [RFC3629] (or some other superset of the US-ASCII character
encoding) at an internal interface, since that results in more
meaningful identifiers than simply percent-encoding the original
octets. When interpreting an incoming URI on such an interface,
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percent-encoded octets must be decoded before the reverse transcoding
can be applied.
In some cases, the interface between a URI component and the
identifying data it has been crafted to represent is much less direct
than a character encoding translation. For example, portions of a
URI might reflect a query on non-ASCII data, numeric coordinates on a
map, etc. Likewise, a URI scheme may define components with
additional encoding requirements, such as base64, that are applied
prior to forming the component and producing the URI.
When a URI scheme defines a component that represents textual data
consisting of characters from the Unicode (ISO/IEC 10646-1) character
set, the data should be encoded first as octets according to the
UTF-8 character encoding [RFC3629], and then only those octets that
do not correspond to characters in the unreserved set should be
percent-encoded. For example, the character A would be represented
as "A", the character LATIN CAPITAL LETTER A WITH GRAVE would be
represented as "%C3%80", and the character KATAKANA LETTER A would be
represented as "%E3%82%A2".
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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 ":" ["//" authority] path ["?" query] ["#" fragment]
The scheme and path components are required, though path may be empty
(no characters). An ABNF-driven parser will find that the border
between authority and path is ambiguous; they are disambiguated by
the "first-match-wins" (a.k.a. "greedy") algorithm. In other words,
if authority is present then the first segment of the path must be
empty.
The following are two example URIs and their component parts:
foo://example.com:8042/over/there?name=ferret#nose
\_/ \______________/\_________/ \_________/ \__/
| | | | |
scheme authority path query fragment
| _____________________|__
/ \ / \
urn:example:animal:ferret:nose
3.1 Scheme
Each URI begins with a scheme name that refers to a specification for
assigning identifiers within that scheme. As such, the URI syntax is
a federated and extensible naming system wherein each scheme's
specification may further restrict the syntax and semantics of
identifiers using that scheme.
Scheme names consist of a sequence of characters beginning with a
letter and followed by any combination of letters, digits, plus
("+"), period ("."), or hyphen ("-"). Although scheme is
case-insensitive, the canonical form is lowercase and documents that
specify schemes must do so using lowercase letters. An
implementation should accept uppercase letters as equivalent to
lowercase in scheme names (e.g., allow "HTTP" as well as "http"), for
the sake of robustness, but should only produce lowercase scheme
names, for consistency.
scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
Individual schemes are not specified by this document. The process
for registration of new URI schemes is defined separately by
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[RFC2717]. The scheme registry maintains the mapping between scheme
names and their specifications. Advice for designers of new URI
schemes can be found in [RFC2718].
When presented with a URI that violates one or more scheme-specific
restrictions, the scheme-specific resolution process should flag the
reference as an error rather than ignore the unused parts; doing so
reduces the number of equivalent URIs and helps detect abuses of the
generic syntax that might indicate the URI has been constructed to
mislead the user (Section 7.6).
3.2 Authority
Many URI schemes include a hierarchical element for a naming
authority, such that governance of the name space defined by the
remainder of the URI is delegated to that authority (which may, in
turn, delegate it further). The generic syntax provides a common
means for distinguishing an authority based on a registered 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 ]
URI producers and normalizers should omit the "@" delimiter that
separates userinfo from host if the userinfo component is empty (zero
length) and should omit the ":" delimiter that separates host from
port if the port component is empty. Some schemes do not allow the
userinfo and/or port sub-components.
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 resource. The user information, if
present, is followed by a commercial at-sign ("@") that delimits it
from the host.
userinfo = *( unreserved / pct-encoded / sub-delims / ":" )
Use of the format "user:password" in the userinfo field is
deprecated. Applications should not render as clear text any data
after the first colon (":") character found within a userinfo
sub-component unless such data is the empty string (indicating no
password) or "anonymous". Applications may choose to ignore or reject
such data when received as part of a reference, and should reject the
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storage of such data in unencrypted form. The passing of
authentication information in clear text has proven to be a security
risk in almost every case where it has been used.
Applications that render a URI for the sake of user feedback, such as
in graphical hypertext browsing, should render userinfo in a way that
is distinguished from the rest of a URI, when feasible. Such
rendering will assist the user in cases where the userinfo has been
misleadingly crafted to look like a trusted domain name (Section
7.6).
3.2.2 Host
The host sub-component of authority is identified by an IP literal
encapsulated within square brackets, an IPv4 address in
dotted-decimal form, or a host name.
host = IP-literal / IPv4address / reg-name
The syntax rule for host is ambiguous because it does not completely
distinguish between an IPv4address and a reg-name. Again, the
"first-match-wins" algorithm applies: If host matches the rule for
IPv4address, then it should be considered an IPv4 address literal and
not a reg-name. Although host is case-insensitive, producers and
normalizers should use lowercase for host names and hexadecimal
addresses for the sake of uniformity, while only using uppercase
letters for percent-encodings.
A host identified by an Internet Protocol literal address, version 6
[RFC3513] or later, is distinguished by enclosing the IP literal
within square brackets ("[" and "]"). This is the only place where
square bracket characters are allowed in the URI syntax. In
anticipation of future, as-yet-undefined IP literal address formats,
an optional version flag may be used to indicate such a format
explicitly rather than relying on heuristic determination.
IP-literal = "[" ( IPv6address / IPvFuture ) "]"
IPvFuture = "v" HEXDIG "." 1*( unreserved / sub-delims / ":" )
The version flag does not indicate the IP version; rather, it
indicates future versions of the literal format. As such,
implementations must not provide the version flag for existing IPv4
and IPv6 literal addresses. If a URI containing an IP-literal that
starts with "v" (case-insensitive), indicating that the version flag
is present, is dereferenced by an application that does not know the
meaning of that version flag, then the application should return an
appropriate error for "address mechanism not supported".
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A host identified by an IPv6 literal address is represented inside
the square brackets without a preceding version flag. The ABNF
provided here is a translation of the text definition of an IPv6
literal address provided in [RFC3513]. A 128-bit IPv6 address is
divided into eight 16-bit pieces. Each piece is represented
numerically in case-insensitive hexadecimal, using one to four
hexadecimal digits (leading zeroes are permitted). The eight encoded
pieces are given most-significant first, separated by colon
characters. Optionally, the least-significant two pieces may instead
be represented in IPv4 address textual format. A sequence of one or
more consecutive zero-valued 16-bit pieces within the address may be
elided, omitting all their digits and leaving exactly two consecutive
colons in their place to mark the elision.
IPv6address = 6( h16 ":" ) ls32
/ "::" 5( h16 ":" ) ls32
/ [ h16 ] "::" 4( h16 ":" ) ls32
/ [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
/ [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
/ [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32
/ [ *4( h16 ":" ) h16 ] "::" ls32
/ [ *5( h16 ":" ) h16 ] "::" h16
/ [ *6( h16 ":" ) h16 ] "::"
ls32 = ( h16 ":" h16 ) / IPv4address
; least-significant 32 bits of address
h16 = 1*4HEXDIG
; 16 bits of address represented in hexadecimal
A host identified by an IPv4 literal address is represented in
dotted-decimal notation (a sequence of four decimal numbers in the
range 0 to 255, separated by "."), as described in [RFC1123] by
reference to [RFC0952]. Note that other forms of dotted notation may
be interpreted on some platforms, as described in Section 7.4, but
only the dotted-decimal form of four octets is allowed by this
grammar.
IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet
dec-octet = DIGIT ; 0-9
/ %x31-39 DIGIT ; 10-99
/ "1" 2DIGIT ; 100-199
/ "2" %x30-34 DIGIT ; 200-249
/ "25" %x30-35 ; 250-255
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A host identified by a registered name is a string of characters that
is intended for lookup within a locally-defined host or service name
registry. The most common of such registry mechanisms is the Domain
Name System (DNS), as defined by Section 3 of [RFC1034] and Section
2.1 of [RFC1123]. A DNS name consists of a sequence of domain labels
separated by ".", each domain label starting and ending with an
alphanumeric character and possibly also containing "-" characters.
The rightmost domain label of a fully qualified domain name in DNS
may be followed by a single "." and should be followed by one if it
is necessary to distinguish between the complete domain name and some
local domain.
reg-name = 0*255( unreserved / pct-encoded / sub-delims )
If the host component is defined and the registered name is empty
(zero length), then the name defaults to "localhost" (Section 6.2.3
discusses how this should be normalized). If "localhost" is not
determined by a host name lookup, then it should be interpreted to
mean the machine on which the URI is being resolved.
This specification does not mandate a particular registered name
lookup technology and therefore does not restrict the syntax of
reg-name beyond that necessary for interoperability. Instead, it
delegates the issue of host name syntax conformance to the operating
system of each application performing URI resolution, and that
operating system decides what it will allow for the purpose of host
identification. A URI resolution implementation might use DNS, host
tables, yellow pages, NetInfo, WINS, or any other system for lookup
of host and service names. However, a globally-scoped naming system,
such as DNS fully-qualified domain names, is necessary for URIs that
are intended to have global scope. URI producers should use host
names that conform to the DNS syntax, even when use of DNS is not
immediately apparent.
The reg-name syntax allows percent-encoded octets in order to
represent non-ASCII host or service names in a uniform way that is
independent of the underlying name resolution technology; such octets
must represent characters encoded in the UTF-8 character encoding
[RFC3629] prior to being percent-encoded. When a non-ASCII host name
represents an internationalized domain name intended for resolution
via DNS, the name must be transformed to the IDNA encoding [RFC3490]
prior to name lookup. URI producers should provide such host names in
the IDNA encoding, rather than a percent-encoding, if they wish to
maximize interoperability with legacy URI resolvers.
The presence of host within a URI does not imply that the scheme
requires access to the given host on the Internet. In many cases,
the host syntax is used only for the sake of reusing the existing
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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 producer.
3.2.3 Port
The port sub-component of authority is designated by an optional port
number in decimal following the host and delimited from it by a
single colon (":") character.
port = *DIGIT
A scheme may define a default port. For example, the "http" scheme
defines a default port of "80", corresponding to its reserved TCP
port number. The type of port designated by the port number (e.g.,
TCP, UDP, SCTP, etc.) is defined by the URI scheme. URI producers
and normalizers should omit the port component and its ":" delimiter
if port is empty or its value would be the same as the scheme's
default.
3.3 Path
The path component contains data, usually organized in hierarchical
form, that, along with data in the non-hierarchical query component
(Section 3.4), serves to identify a resource within the scope of the
URI's scheme and naming authority (if any). If a URI contains an
authority component, then the initial path segment must be empty
(i.e., the path must begin with a slash ("/") character or be
entirely empty). The path is terminated by the first question mark
("?") or number sign ("#") character, or by the end of the URI.
path = segment *( "/" segment )
segment = *pchar
pchar = unreserved / pct-encoded / sub-delims / ":" / "@"
A path consists of a sequence of path segments separated by a slash
("/") character. A path is always defined for a URI, though the
defined path may be empty (zero length). Use of the slash character
to indicate hierarchy is only required when a URI will be used as the
context for relative references. For example, the URI
<mailto:fred@example.com> has a path of "fred@example.com", whereas
the URI <foo://info.example.com?fred> has an empty path.
The path segments "." and ".." are defined for relative reference
within the path name hierarchy. They are intended for use at the
beginning of a relative path reference (Section 4.2) for indicating
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relative position within the hierarchical tree of names. This is
similar to their role within some operating systems' file directory
structure to indicate the current directory and parent directory,
respectively. However, unlike a file system, these dot-segments are
only interpreted within the URI path hierarchy and are removed as
part of the resolution process (Section 5.2).
Aside from dot-segments in hierarchical paths, a path segment is
considered opaque by the generic syntax. URI-producing applications
often use the reserved characters allowed in a segment for the
purpose of delimiting scheme-specific or dereference-handler-specific
sub-components. For example, the semicolon (";") and equals ("=")
reserved characters are often used for delimiting parameters and
parameter values applicable to that segment. The comma (",")
reserved character is often used for similar purposes. For example,
one URI producer might use a segment like "name;v=1.1" to indicate a
reference to version 1.1 of "name", whereas another might use a
segment like "name,1.1" to indicate the same. Parameter types may be
defined by scheme-specific semantics, but in most cases the syntax of
a parameter is specific to the implementation of the URI's
dereferencing algorithm.
3.4 Query
The query component contains non-hierarchical data that, along with
data in the path component (Section 3.3), serves to identify a
resource within the scope of the 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 ("?") may represent data
within the query component, but should not be used as such within a
URI that is expected to be the base for relative references (Section
5.1). Incorrect implementations of reference resolution often fail
to distinguish query data from path data when looking for
hierarchical separators, thus resulting in non-interoperable results.
However, since query components are often used to carry identifying
information in the form of "key=value" pairs, and one frequently used
value is a reference to another URI, it is sometimes better for
usability to avoid percent-encoding those characters.
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3.5 Fragment
The fragment identifier component of a URI allows indirect
identification of a secondary resource by reference to a primary
resource and additional identifying information. 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 defined or described by those representations. A
fragment identifier component is indicated by the presence of a
number sign ("#") character and terminated by the end of the URI.
fragment = *( pchar / "/" / "?" )
The semantics of a fragment identifier are defined by the set of
representations that might result from a retrieval action on the
primary resource. The fragment's format and resolution is therefore
dependent on the media type [RFC2046] of a potentially retrieved
representation, even though such a retrieval is only performed if the
URI is dereferenced. Individual media types may define their own
restrictions on, or structure within, the fragment identifier syntax
for specifying different types of subsets, views, or external
references that are identifiable as secondary resources by that media
type. If the primary resource has multiple representations, as is
often the case for resources whose representation is selected based
on attributes of the retrieval request (a.k.a., content negotiation),
then whatever is identified by the fragment should be consistent
across all of those representations: each representation should
either define the fragment such that it corresponds to the same
secondary resource, regardless of how it is represented, or the
fragment should be left undefined by the representation (i.e., not
found).
As with any URI, use of a fragment identifier component does not
imply that a retrieval action will take place. A URI with a fragment
identifier may be used to refer to the secondary resource without any
implication that the primary resource is accessible or will ever be
accessed.
Fragment identifiers have a special role in information systems as
the primary form of client-side indirect referencing, allowing an
author to specifically identify those aspects of an existing resource
that are only indirectly provided by the resource owner. As such,
interpretation of the fragment identifier during a retrieval action
is performed solely by the user agent; the fragment identifier is not
passed to other systems during the process of retrieval. Although
this is often perceived to be a loss of information, particularly in
regards to accurate redirection of references as content moves over
time, it also serves to prevent information providers from denying
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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 should not be used
as such within a URI that is expected to be the base for relative
references (Section 5.1) 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 rule. In order to
save space and take advantage of hierarchical locality, many Internet
protocol elements and media type formats allow an abbreviation of a
URI, while others restrict the syntax to a particular form of URI.
We define the most common forms of reference syntax in this
specification because they impact and depend upon the design of the
generic syntax, requiring a uniform parsing algorithm in order to be
interpreted consistently.
4.1 URI Reference
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'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 hierarchical syntax
(Section 1.2.3) in order to express a reference that is relative to
the name space of another hierarchical URI.
relative-URI = ["//" authority] path ["?" query] ["#" fragment]
The URI referred to by a relative reference, also known as the target
URI, is obtained by applying the reference resolution algorithm of
Section 5.
A relative reference that begins with two slash characters is termed
a network-path reference; such references are rarely used. A relative
reference that begins with a single slash character is termed an
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absolute-path reference. A relative reference that does not begin
with a slash character is termed a relative-path reference.
A path segment that contains a colon character (e.g., "this:that")
cannot be used as the first segment of a relative-path reference
because it would be mistaken for a scheme name. Such a segment must
be preceded by a dot-segment (e.g., "./this:that") to make a
relative-path reference.
4.3 Absolute URI
Some protocol elements allow only the absolute form of a URI without
a fragment identifier. For example, defining a base URI for later
use by relative references calls for an absolute-URI syntax rule that
does not allow a fragment.
absolute-URI = scheme ":" ["//" authority] path ["?" query]
4.4 Same-document Reference
When a URI reference 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 the same entity (representation, document, or message) as the
reference; therefore, a dereference should not result in a new
retrieval action.
Normalization of the base and target URIs prior to their comparison,
as described in Section 6.2.2 and Section 6.2.3, is allowed but
rarely performed in practice. Normalization may increase the set of
same-document references, which may be of benefit to some caching
applications. As such, reference authors should not assume that a
slightly different, though equivalent, reference URI will (or will
not) be interpreted as a same-document reference by any given
application.
4.5 Suffix Reference
The URI syntax is designed for unambiguous reference to resources and
extensibility via the URI scheme. However, as URI identification and
usage have become commonplace, traditional media (television, radio,
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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 a DNS registered name on its own. Such references are
primarily intended for human interpretation, rather than for
machines, with the assumption that context-based heuristics are
sufficient to complete the URI (e.g., most host names beginning with
"www" are likely to have a URI prefix of "http://"). Although there
is no standard set of heuristics for disambiguating a URI suffix,
many client implementations allow them to be entered by the user and
heuristically resolved.
While this practice of using suffix references is common, it should
be avoided whenever possible and never used in situations where
long-term references are expected. The heuristics noted above will
change over time, particularly when a new URI scheme becomes popular,
and are often incorrect when used out of context. Furthermore, they
can lead to security issues along the lines of those described in
[RFC1535].
Since a URI suffix has the same syntax as a relative path reference,
a suffix reference cannot be used in contexts where a relative
reference is expected. As a result, suffix references are limited to
those places where there is no defined base URI, such as dialog boxes
and off-line advertisements.
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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 rule of Section 3.
5.1 Establishing a Base URI
The term "relative" implies that there exists a "base URI" against
which the relative reference is applied. Aside from fragment-only
references (Section 4.4), relative references are only usable when a
base URI is known. A base URI must be established by the parser
prior to parsing URI references that might be relative.
The base URI of a reference can be established in one of four ways,
discussed below in order of precedence. The order of precedence can
be thought of in terms of layers, where the innermost defined base
URI has the highest precedence. This can be visualized graphically
as:
.----------------------------------------------------------.
| .----------------------------------------------------. |
| | .----------------------------------------------. | |
| | | .----------------------------------------. | | |
| | | | .----------------------------------. | | | |
| | | | | <relative-reference> | | | | |
| | | | `----------------------------------' | | | |
| | | | (5.1.1) Base URI embedded in content | | | |
| | | `----------------------------------------' | | |
| | | (5.1.2) Base URI of the encapsulating entity | | |
| | | (message, representation, or none) | | |
| | `----------------------------------------------' | |
| | (5.1.3) URI used to retrieve the entity | |
| `----------------------------------------------------' |
| (5.1.4) Default Base URI (application-dependent) |
`----------------------------------------------------------'
5.1.1 Base URI within Document Content
Within certain media types, a base URI for relative references can be
embedded within the content itself such that it can be readily
obtained by a parser. This can be useful for descriptive documents,
such as tables of content, which may be transmitted to others through
protocols other than their usual retrieval context (e.g., E-Mail or
USENET news).
It is beyond the scope of this specification to specify how, for each
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media type, a base URI can be embedded. The appropriate syntax, when
available, is described by each media type's specification.
5.1.2 Base URI from the Encapsulating Entity
If no base URI is embedded, the base URI is defined by the
representation's retrieval context. For a document that is enclosed
within another entity, such as a message or archive, the retrieval
context is that entity; thus, the default base URI of a
representation is the base URI of the entity in which the
representation is encapsulated.
A mechanism for embedding a base URI within MIME container types
(e.g., the message and multipart types) is defined by MHTML
[RFC2110]. Protocols that do not use the MIME message header syntax,
but do allow some form of tagged metadata to be included within
messages, may define their own syntax for defining a base URI as part
of a message.
5.1.3 Base URI from the Retrieval URI
If no base URI is embedded and the representation is not encapsulated
within some other entity, then, if a URI was used to retrieve the
representation, that URI shall be considered the base URI. Note that
if the retrieval was the result of a redirected request, the last URI
used (i.e., the URI that resulted in the actual retrieval of the
representation) is the base URI.
5.1.4 Default Base URI
If none of the conditions described above apply, then the base URI is
defined by the context of the application. Since this definition is
necessarily application-dependent, failing to define a base URI using
one of the other methods may result in the same content being
interpreted differently by different types of application.
A sender of a representation containing relative references is
responsible for ensuring that a base URI for those references can be
established. Aside from fragment-only references, relative references
can only be used reliably in situations where the base URI is
well-defined.
5.2 Relative Resolution
This section describes an algorithm for converting a URI reference
that might be relative to a given base URI into the parsed componets
of the reference's target. The components can then be recomposed, as
described in Section 5.3, to form the target URI. This algorithm
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provides definitive results that can be used to test the output of
other implementations. Applications may implement relative reference
resolution using some other algorithm, provided that the results
match what would be given by this algorithm.
5.2.1 Pre-parse the Base URI
The base URI (Base) is established according to the procedure of
Section 5.1 and parsed into the five main components described in
Section 3. Note that only the scheme component is required to be
present in a base URI; the other components may be empty or
undefined. A component is undefined if its associated delimiter does
not appear in the URI reference; the path component is never
undefined, though it may be empty.
Normalization of the base URI, as described in Section 6.2.2 and
Section 6.2.3, is optional. A URI reference must be transformed to
its target URI before it can be normalized.
5.2.2 Transform References
For each URI reference (R), the following pseudocode describes an
algorithm for transforming R into its target URI (T):
-- The URI reference is parsed into the five URI components
--
(R.scheme, R.authority, R.path, R.query, R.fragment) = parse(R);
-- A non-strict parser may ignore a scheme in the reference
-- if it is identical to the base URI's scheme.
--
if ((not strict) and (R.scheme == Base.scheme)) then
undefine(R.scheme);
endif;
if defined(R.scheme) then
T.scheme = R.scheme;
T.authority = R.authority;
T.path = remove_dot_segments(R.path);
T.query = R.query;
else
if defined(R.authority) then
T.authority = R.authority;
T.path = remove_dot_segments(R.path);
T.query = R.query;
else
if (R.path == "") then
T.path = Base.path;
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if defined(R.query) then
T.query = R.query;
else
T.query = Base.query;
endif;
else
if (R.path starts-with "/") then
T.path = remove_dot_segments(R.path);
else
T.path = merge(Base.path, R.path);
T.path = remove_dot_segments(T.path);
endif;
T.query = R.query;
endif;
T.authority = Base.authority;
endif;
T.scheme = Base.scheme;
endif;
T.fragment = R.fragment;
5.2.3 Merge Paths
The pseudocode above refers to a "merge" routine for merging a
relative-path reference with the path of the base URI. This is
accomplished as follows:
o If the base URI has a defined authority component and an empty
path, then return a string consisting of "/" concatenated with the
reference's path; otherwise,
o Return a string consisting of the reference's path component
appended to all but the last segment of the base URI's path (i.e.,
excluding any characters after the right-most "/" in the base URI
path, or excluding the entire base URI path if it does not contain
any "/" characters).
5.2.4 Remove Dot Segments
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 two string
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buffers.
1. The input buffer is initialized with the now-appended path
components and the output buffer is initialized to the empty
string.
2. Replace any prefix of "./" or "../" at the beginning of the input
buffer with "/".
3. While the input buffer is not empty, loop:
1. If the input buffer begins with a prefix of "/./" or "/.",
where "." is a complete path segment, then replace that
prefix with "/"; otherwise
2. If the input buffer begins with a prefix of "/../" or "/..",
where ".." is a complete path segment, then replace that
prefix with "/" and remove the last segment and its preceding
"/" (if any) from the output buffer; otherwise
3. Remove the first segment and its preceding "/" (if any) from
the input buffer and append them to the output buffer.
4. Finally, the output buffer is returned as the result of
remove_dot_segments.
The following illustrates how the above steps are applied for two
example merged paths, showing the state of the two buffers after each
step.
STEP OUTPUT BUFFER INPUT BUFFER
1 : /a/b/c/./../../g
3c: /a /b/c/./../../g
3c: /a/b /c/./../../g
3c: /a/b/c /./../../g
3a: /a/b/c /../../g
3b: /a/b /../g
3b: /a /g
3c: /a/g
STEP OUTPUT BUFFER INPUT BUFFER
1 : mid/content=5/../6
3c: mid /content=5/../6
3c: mid/content=5 /../6
3b: mid /6
3c: mid/6
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Some applications may find it more efficient to implement the
remove_dot_segments algorithm using two segment stacks rather than
strings.
Note: Some client applications will fail to separate a reference's
query component from its path component before merging the base
and reference paths. This may result in loss of information if
the query component contains the strings "/../" or "/./".
5.3 Component Recomposition
Parsed URI components can be recomposed to obtain the corresponding
URI reference string. Using pseudocode, this would be:
result = ""
if defined(scheme) then
append scheme to result;
append ":" to result;
endif;
if defined(authority) then
append "//" to result;
append authority to result;
endif;
append path to result;
if defined(query) then
append "?" to result;
append query to result;
endif;
if defined(fragment) then
append "#" to result;
append fragment to result;
endif;
return result;
Note that we are careful to preserve the distinction between a
component that is undefined, meaning that its separator was not
present in the reference, and a component that is empty, meaning that
the separator was present and was immediately followed by the next
component separator or the end of the reference.
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5.4 Reference Resolution Examples
Within a representation with a well-defined base URI of
http://a/b/c/d;p?q
a relative URI reference is transformed to its target URI 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/d;p?q"
"." = "http://a/b/c/"
"./" = "http://a/b/c/"
".." = "http://a/b/"
"../" = "http://a/b/"
"../g" = "http://a/b/g"
"../.." = "http://a/"
"../../" = "http://a/"
"../../g" = "http://a/g"
5.4.2 Abnormal Examples
Although the following abnormal examples are unlikely to occur in
normal practice, all URI parsers should be capable of resolving them
consistently. Each example uses the same base as above.
Parsers must be careful in handling cases where there are more
relative path ".." segments than there are hierarchical levels in the
base URI's path. Note that the ".." syntax cannot be used to change
the authority component of a URI.
"../../../g" = "http://a/g"
"../../../../g" = "http://a/g"
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Similarly, parsers must remove the dot-segments "." and ".." when
they are complete components of a path, but not when they are only
part of a segment.
"/./g" = "http://a/g"
"/../g" = "http://a/g"
"g." = "http://a/b/c/g."
".g" = "http://a/b/c/.g"
"g.." = "http://a/b/c/g.."
"..g" = "http://a/b/c/..g"
Less likely are cases where the relative URI reference uses
unnecessary or nonsensical forms of the "." and ".." complete path
segments.
"./../g" = "http://a/b/g"
"./g/." = "http://a/b/c/g/"
"g/./h" = "http://a/b/c/g/h"
"g/../h" = "http://a/b/c/h"
"g;x=1/./y" = "http://a/b/c/g;x=1/y"
"g;x=1/../y" = "http://a/b/c/y"
Some applications fail to separate the reference's query and/or
fragment components from a relative path before merging it with the
base path and removing dot-segments. This error is rarely noticed,
since typical usage of a fragment never includes the hierarchy ("/")
character, and the query component is not normally used within
relative references.
"g?y/./x" = "http://a/b/c/g?y/./x"
"g?y/../x" = "http://a/b/c/g?y/../x"
"g#s/./x" = "http://a/b/c/g#s/./x"
"g#s/../x" = "http://a/b/c/g#s/../x"
Some parsers allow the scheme name to be present in a relative URI
reference if it is the same as the base URI scheme. This is
considered to be a loophole in prior specifications of partial URI
[RFC1630]. Its use should be avoided, but is allowed for backward
compatibility.
"http:g" = "http:g" ; for strict parsers
/ "http://a/b/c/g" ; for backward compatibility
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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 the implementation-specific syntax of each URI's
dereferencing algorithm. For this reason, determination of
equivalence or difference of URIs is based on string comparison,
perhaps augmented by reference to additional rules provided by URI
scheme definitions. We use the terms "different" and "equivalent" to
describe the possible outcomes of such comparisons, but there are
many application-dependent versions of equivalence.
Even though it is possible to determine that two URIs are equivalent,
it is never possible to be sure that two URIs identify different
resources. For example, an owner of two different domain names could
decide to serve the same resource from both, resulting in two
different URIs. Therefore, comparison methods are designed to
minimize false negatives while strictly avoiding false positives.
In testing for equivalence, applications should not directly compare
relative URI references; the references should be converted to their
target URI forms before comparison. When URIs are being compared for
the purpose of selecting (or avoiding) a network action, such as
retrieval of a representation, the fragment components (if any)
should be excluded from the comparison.
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6.2 Comparison Ladder
A variety of methods are used in practice to test URI equivalence.
These methods fall into a range, distinguished by the amount of
processing required and the degree to which the probability of false
negatives is reduced. As noted above, false negatives cannot in
principle be eliminated. In practice, their probability can be
reduced, but this reduction requires more processing and is not
cost-effective for all applications.
If this range of comparison practices is considered as a ladder, the
following discussion will climb the ladder, starting with those
practices that are cheap but have a relatively higher chance of
producing false negatives, and proceeding to those that have higher
computational cost and lower risk of false negatives.
6.2.1 Simple String Comparison
If two URIs, considered as character strings, are identical, then it
is safe to conclude that they are equivalent. This type of
equivalence test has very low computational cost and is in wide use
in a variety of applications, particularly in the domain of parsing.
Testing strings for equivalence requires some basic precautions. This
procedure is often referred to as "bit-for-bit" or "byte-for-byte"
comparison, which is potentially misleading. Testing of strings for
equality is normally based on pairwise comparison of the characters
that make up the strings, starting from the first and proceeding
until both strings are exhausted and all characters found to be
equal, a pair of characters compares unequal, or one of the strings
is exhausted before the other.
Such character comparisons require that each pair of characters be
put in comparable form. For example, should one URI be stored in a
byte array in EBCDIC encoding, and the second be in a Java String
object (UTF-16), bit-for-bit comparisons applied naively will produce
both false-positive and false-negative errors. It is better to speak
of equality on a character-for-character rather than byte-for-byte or
bit-for-bit basis. In practical terms, character-by-character
comparisons should be done codepoint-by-codepoint after conversion to
a common character encoding.
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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/%7Bfoo%7D
eXAMPLE://a/./b/../b/%63/%7bfoo%7d
Web user agents, such as browsers, typically apply this type of URI
normalization when determining whether a cached response is
available. Syntax-based normalization includes such techniques as
case normalization, encoding normalization, empty-component
normalization, and removal of dot-segments.
6.2.2.1 Case Normalization
When a URI scheme uses components of the generic syntax, it will also
use the common syntax equivalence rules, namely that the scheme and
host are case-insensitive and therefore should be normalized to
lowercase. For example, the URI <HTTP://www.EXAMPLE.com/> is
equivalent to <http://www.example.com/>. Applications should not
assume anything about the case sensitivity of other URI components,
since that is dependent on the implementation used to handle a
dereference.
The hexadecimal digits within a percent-encoding triplet (e.g., "%3a"
versus "%3A") are case-insensitive and therefore should be normalized
to use uppercase letters for the digits A-F.
6.2.2.2 Encoding Normalization
The percent-encoding mechanism (Section 2.1) is a frequent source of
variance among otherwise identical URIs. In addition to the
case-insensitivity issue noted above, some URI producers
percent-encode octets that do not require percent-encoding, resulting
in URIs that are equivalent to their non-encoded counterparts. Such
URIs should be normalized by decoding any percent-encoded octet that
corresponds to an unreserved character, as described in Section 2.3.
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6.2.2.3 Empty-component Normalization
Components of the generic URI syntax are delimited from other
components by optional separators. For example, a query component is
separated from the path by a question mark ("?") and a port
sub-component is separated from host by a colon (":"). A URI in
which a delimiter is present and the (sub-)component it delimits is
empty is equivalent to the same URI without that delimiter. For
example, the following are all equivalent:
ftp://example.com/
ftp://example.com:/
ftp://@example.com:/
ftp://@example.com:/?
ftp://@example.com:/?#
URI producers and normalizers should omit a delimiter if the
component it delimits is empty, as exemplified by the first URI
above, with one exception: a double-slash delimiter indicating an
authority component should not be removed, even when the authority is
empty, since doing so can lead to misinterpreting the path.
6.2.2.4 Path Segment Normalization
The complete path segments "." and ".." have a special meaning within
hierarchical URI schemes. As such, they should not appear in
absolute paths; if they are found, they can be removed by applying
the remove_dot_segments algorithm to the path, as described in
Section 5.2.
6.2.3 Scheme-based Normalization
The syntax and semantics of URIs vary from scheme to scheme, as
described by the defining specification for each scheme. Software
may use scheme-specific rules, at further processing cost, to reduce
the probability of false negatives. For example, since the "http"
scheme makes use of an authority component, has a default port of
"80", and defines an empty path to be equivalent to "/", the
following four URIs are equivalent:
http://example.com
http://example.com/
http://example.com:/
http://example.com:80/
In general, a URI that uses the generic syntax for authority with an
empty path should be normalized to a path of "/"; likewise, an
explicit ":port", where the port is empty or the default for the
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scheme, is equivalent to one where the port and its ":" delimiter are
elided. In other words, the second of the above URI examples is the
normal form for the "http" scheme.
Another case where normalization varies by scheme is in the handling
of an empty authority component. For many scheme specifications, an
empty authority is considered an error; for others, it is considered
equivalent to "localhost". For the sake of uniformity, future scheme
specifications should define an empty authority as being equivalent
to "localhost", and URI producers and normalizers should use
"localhost" instead of an empty authority.
6.2.4 Protocol-based Normalization
Web spiders, for which substantial effort to reduce the incidence of
false negatives is often cost-effective, are observed to implement
even more aggressive techniques in URI comparison. For example, if
they observe that a URI such as
http://example.com/data
redirects to a URI differing only in the trailing slash
http://example.com/data/
they will likely regard the two as equivalent in the future. This
kind of technique is only appropriate when equivalence is clearly
indicated by both the result of accessing the resources and the
common conventions of their scheme's dereference algorithm (in this
case, use of redirection by HTTP origin servers to avoid problems
with relative references).
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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 produce and make reference to URIs can
reduce the cost of processing and the risk of false negatives by
consistently providing them in a form that is reasonably canonical
with respect to their scheme. Specifically:
o Always provide the URI scheme in lowercase characters.
o Always provide the host, if any, in lowercase characters.
o Only perform percent-encoding where it is essential.
o Always use uppercase A-through-F characters when percent-encoding.
o Prevent /./ and /../ from appearing in non-relative URI paths.
o Omit delimiters when their associated (sub-)component is empty.
o For schemes that define an empty authority to be equivalent to
"localhost", use "localhost".
o For schemes that define an empty path to be equivalent to a path
of "/", use "/".
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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 and data within the URI contains
instructions that, when interpreted according to this other protocol,
cause an unexpected operation. A frequent example of such abuse has
been the use of a protocol-based scheme with a port component of
"25", thereby fooling user agent software into sending an unintended
or impersonating message via an SMTP server.
Applications should prevent dereference of a URI that specifies a TCP
port number within the "well-known port" range (0 - 1023) unless the
protocol being used to dereference that URI is compatible with the
protocol expected on that well-known port. Although IANA maintains a
registry of well-known ports, applications should make such
restrictions user-configurable to avoid preventing the deployment of
new services.
When a URI contains percent-encoded octets that match the delimiters
for a given resolution or dereference protocol (for example, CR and
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LF characters for the TELNET protocol), such percent-encoded octets
must not be decoded before transmission across that protocol.
Transfer of the percent-encoding, which might violate the protocol,
is less harmful than allowing decoded octets to be interpreted as
additional operations or parameters, perhaps triggering an unexpected
and possibly harmful remote operation.
7.3 Back-end Transcoding
When a URI is dereferenced, the data within it is often parsed by
both the user agent and one or more servers. In HTTP, for example, a
typical user agent will parse a URI into its five major components,
access the authority's server, and send it the data within the
authority, path, and query components. A typical server will take
that information, parse the path into segments and the query into
key/value pairs, and then invoke implementation-specific handlers to
respond to the request. As a result, a common security concern for
server implementations that handle a URI, either as a whole or split
into separate components, is proper interpretation of the octet data
represented by the characters and percent-encodings within that URI.
Percent-encoded octets must be decoded at some point during the
dereference process. Applications must split the URI into its
components and sub-components prior to decoding the octets, since
otherwise the decoded octets might be mistaken for delimiters.
Security checks of the data within a URI should be applied after
decoding the octets. Note, however, that the "%00" percent-encoding
(NUL) may require special handling and should be rejected if the
application is not expecting to receive raw data within a component.
Special care should be taken when the URI path interpretation process
involves the use of a back-end filesystem or related system
functions. Filesystems typically assign an operational meaning to
special characters, such as the "/", "\", ":", "[", and "]"
characters, and special device names like ".", "..", "...", "aux",
"lpt", etc. In some cases, merely testing for the existence of such a
name will cause the operating system to pause or invoke unrelated
system calls, leading to significant security concerns regarding
denial of service and unintended data transfer. It would be
impossible for this specification to list all such significant
characters and device names; implementers should research the
reserved names and characters for the types of storage device that
may be attached to their application and restrict the use of data
obtained from URI components accordingly.
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7.4 Rare IP Address Formats
Although the URI syntax for IPv4address only allows the common,
dotted-decimal form of IPv4 address literal, many implementations
that process URIs make use of platform-dependent system routines,
such as gethostbyname() and inet_aton(), to translate the string
literal to an actual IP address. Unfortunately, such system routines
often allow and process a much larger set of formats than those
described in Section 3.2.2.
For example, many implementations allow dotted forms of three
numbers, wherein the last part is interpreted as a 16-bit quantity
and placed in the right-most two bytes of the network address (e.g.,
a Class B network). Likewise, a dotted form of two numbers means the
last part is interpreted as a 24-bit quantity and placed in the right
most three bytes of the network address (Class A), and a single
number (without dots) is interpreted as a 32-bit quantity and stored
directly in the network address. Adding further to the confusion,
some implementations allow each dotted part to be interpreted as
decimal, octal, or hexadecimal, as specified in the C language (i.e.,
a leading 0x or 0X implies hexadecimal; otherwise, a leading 0
implies octal; otherwise, the number is interpreted as decimal).
These additional IP address formats are not allowed in the URI syntax
due to differences between platform implementations. However, they
can become a security concern if an application attempts to filter
access to resources based on the IP address in string literal format.
If such filtering is performed, literals should be converted to
numeric form and filtered based on the numeric value, rather than a
prefix or suffix of the string form.
7.5 Sensitive Information
URI producers should not provide a URI that contains a username or
password which is intended to be secret: URIs are frequently
displayed by browsers, stored in clear text bookmarks, and logged by
user agent history and intermediary applications (proxies). A
password appearing within the userinfo component is deprecated and
should be considered an error (or simply ignored) except in those
rare cases where the 'password' parameter is intended to be public.
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7.6 Semantic Attacks
Because the userinfo sub-component is rarely used and appears before
the host in the authority component, it can be used to construct a
URI that is intended to mislead a human user by appearing to identify
one (trusted) naming authority while actually identifying a different
authority hidden behind the noise. For example
ftp://ftp.example.com&story=breaking_news@10.0.0.1/top_story.htm
might lead a human user to assume that the host is
'trusted.example.com', whereas it is actually '10.0.0.1'. Note that
a misleading userinfo sub-component could be much longer than the
example above.
A misleading URI, such as the one above, is an attack on the user's
preconceived notions about the meaning of a URI, rather than an
attack on the software itself. User agents may be able to reduce the
impact of such attacks by 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 Gisle Aas, Reese Anschultz,
Daniel Barclay, Tim Bray, Mike Brown, Rob Cameron, Jeremy Carroll,
Dan Connolly, Adam M. Costello, John Cowan, Jason Diamond, Martin
Duerst, Stefan Eissing, Clive D.W. Feather, Tony Hammond, Pat Hayes,
Henry Holtzman, Ian B. Jacobs, Michael Kay, John C. Klensin, Graham
Klyne, Dan Kohn, Bruce Lilly, Andrew Main, Ira McDonald, Michael
Mealling, Stephen Pollei, Julian Reschke, Tomas Rokicki, Miles Sabin,
Mark Thomson, Ronald Tschalaer, Norm Walsh, Marc Warne, Stuart
Williams, and Henry Zongaro are gratefully acknowledged.
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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.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
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Informative References
[RFC0952] Harrenstien, K., Stahl, M. and E. Feinler, "DoD Internet
host table specification", RFC 952, October 1985.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[RFC1535] Gavron, E., "A Security Problem and Proposed Correction
With Widely Deployed DNS Software", RFC 1535, October
1993.
[RFC1630] Berners-Lee, T., "Universal Resource Identifiers in WWW: A
Unifying Syntax for the Expression of Names and Addresses
of Objects on the Network as used in the World-Wide Web",
RFC 1630, June 1994.
[RFC1736] Kunze, J., "Functional Recommendations for Internet
Resource Locators", RFC 1736, February 1995.
[RFC1737] Masinter, L. and K. Sollins, "Functional Requirements for
Uniform Resource Names", RFC 1737, December 1994.
[RFC1738] Berners-Lee, T., Masinter, L. and M. McCahill, "Uniform
Resource Locators (URL)", RFC 1738, December 1994.
[RFC1808] Fielding, R., "Relative Uniform Resource Locators", RFC
1808, June 1995.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
[RFC2110] Palme, J. and A. Hopmann, "MIME E-mail Encapsulation of
Aggregate Documents, such as HTML (MHTML)", RFC 2110,
March 1997.
[RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
[RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998.
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[RFC2518] Goland, Y., Whitehead, E., Faizi, A., Carter, S. and D.
Jensen, "HTTP Extensions for Distributed Authoring --
WEBDAV", RFC 2518, February 1999.
[RFC2717] Petke, R. and I. King, "Registration Procedures for URL
Scheme Names", BCP 35, RFC 2717, November 1999.
[RFC2718] Masinter, L., Alvestrand, H., Zigmond, D. and R. Petke,
"Guidelines for new URL Schemes", RFC 2718, November 1999.
[RFC2732] Hinden, R., Carpenter, B. and L. Masinter, "Format for
Literal IPv6 Addresses in URL's", RFC 2732, December 1999.
[RFC2978] Freed, N. and J. Postel, "IANA Charset Registration
Procedures", BCP 19, RFC 2978, October 2000.
[RFC3305] Mealling, M. and R. Denenberg, "Report from the Joint W3C/
IETF URI Planning Interest Group: Uniform Resource
Identifiers (URIs), URLs, and Uniform Resource Names
(URNs): Clarifications and Recommendations", RFC 3305,
August 2002.
[RFC3490] Faltstrom, P., Hoffman, P. and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
[RFC3513] Hinden, R. and S. Deering, "Internet Protocol Version 6
(IPv6) Addressing Architecture", RFC 3513, April 2003.
[Siedzik] Siedzik, R., "Semantic Attacks: What's in a URL?", April
2001, <http://www.giac.org/practical/gsec/
Richard_Siedzik_GSEC.pdf>.
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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
5251 California Ave., Suite 110
Irvine, CA 92612-3074
USA
Phone: +1-949-679-2960
Fax: +1-949-679-2972
EMail: fielding@gbiv.com
URI: http://roy.gbiv.com/
Larry Masinter
Adobe Systems Incorporated
345 Park Ave
San Jose, CA 95110
USA
Phone: +1-408-536-3024
EMail: LMM@acm.org
URI: http://larry.masinter.net/
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Appendix A. Collected ABNF for URI
URI = scheme ":" ["//" authority] path ["?" query] ["#" fragment]
URI-reference = URI / relative-URI
relative-URI = ["//" authority] path ["?" query] ["#" fragment]
absolute-URI = scheme ":" ["//" authority] path ["?" query]
scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
authority = [ userinfo "@" ] host [ ":" port ]
userinfo = *( unreserved / pct-encoded / sub-delims / ":" )
host = IP-literal / IPv4address / reg-name
port = *DIGIT
IP-literal = "[" ( IPv6address / IPvFuture ) "]"
IPvFuture = "v" HEXDIG "." 1*( unreserved / sub-delims / ":" )
IPv6address = 6( h16 ":" ) ls32
/ "::" 5( h16 ":" ) ls32
/ [ h16 ] "::" 4( h16 ":" ) ls32
/ [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
/ [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
/ [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32
/ [ *4( h16 ":" ) h16 ] "::" ls32
/ [ *5( h16 ":" ) h16 ] "::" h16
/ [ *6( h16 ":" ) h16 ] "::"
h16 = 1*4HEXDIG
ls32 = ( h16 ":" h16 ) / IPv4address
IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet
dec-octet = DIGIT ; 0-9
/ %x31-39 DIGIT ; 10-99
/ "1" 2DIGIT ; 100-199
/ "2" %x30-34 DIGIT ; 200-249
/ "25" %x30-35 ; 250-255
reg-name = 0*255( unreserved / pct-encoded / sub-delims )
path = segment *( "/" segment )
segment = *pchar
query = *( pchar / "/" / "?" )
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fragment = *( pchar / "/" / "?" )
pct-encoded = "%" HEXDIG HEXDIG
pchar = unreserved / pct-encoded / sub-delims / ":" / "@"
unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
reserved = gen-delims / sub-delims
gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@"
sub-delims = "!" / "$" / "&" / "'" / "(" / ")"
/ "*" / "+" / "," / ";" / "="
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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, URIs are delimited in a variety of ways, but usually
within double-quotes "http://example.com/", angle brackets <http://
example.com/>, or just using whitespace
http://example.com/
These wrappers do not form part of the URI.
In 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 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 reference that contains embedded whitespace.
The prefix "URL:" (with or without a trailing space) was formerly
recommended as a way to help distinguish a URI from other bracketed
designators, though it is not commonly used in practice and is no
longer recommended.
For robustness, software that accepts user-typed URI should attempt
to recognize and strip both delimiters and embedded whitespace.
For example, the text:
Yes, Jim, I found it under "http://www.w3.org/Addressing/",
but you can probably pick it up from <ftp://foo.example.
com/rfc/>. Note the warning in <http://www.ics.uci.edu/pub/
ietf/uri/historical.html#WARNING>.
contains the URI references
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http://www.w3.org/Addressing/
ftp://foo.example.com/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 (and later) literals have been added to the list of possible
identifiers for the host portion of a authority component, as
described by [RFC2732], with the addition of "[" and "]" to the
reserved set and a version flag to anticipate future versions of IP
literals. Square brackets are now specified as reserved within the
authority component and not allowed outside their use as delimiters
for an IP literal within host. In order to make this change without
changing the technical definition of the path, query, and fragment
components, those rules were redefined to directly specify the
characters allowed rather than be defined in terms of uric.
Since [RFC2732] defers to [RFC3513] for definition of an IPv6 literal
address, which unfortunately lacks an ABNF description of
IPv6address, we created a new ABNF rule for IPv6address that matches
the text representations defined by Section 2.2 of [RFC3513].
Likewise, the definition of IPv4address has been improved in order to
limit each decimal octet to the range 0-255.
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.
An ABNF rule for URI has been introduced to correspond to the common
usage of the term: an absolute URI with optional fragment.
D.2 Modifications from RFC 2396
The ad-hoc BNF syntax has been replaced with the ABNF of [RFC2234].
This change required all rule names that formerly included underscore
characters to be renamed with a dash instead.
Section 2 on characters has been rewritten to explain what characters
are reserved, when they are reserved, and why they are reserved even
when not used as delimiters by the generic syntax. The mark
characters that are typically unsafe to decode, including the
exclamation mark ("!"), asterisk ("*"), single-quote ("'"), and open
and close parentheses ("(" and ")"), have been moved to the reserved
set in order to clarify the distinction between reserved and
unreserved and hopefully answer the most common question of scheme
designers. Likewise, the section on percent-encoded characters has
been rewritten, and URI normalizers are now given license to decode
any percent-encoded octets corresponding to unreserved characters.
In general, the terms "escaped" and "unescaped" have been replaced
with "percent-encoded" and "decoded", respectively, to reduce
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confusion with other forms of escape mechanisms.
The ABNF for URI and URI-reference has been redesigned to make them
more friendly to LALR parsers and significantly reduce complexity. As
a result, the layout form of syntax description has been removed,
along with the uric, uric_no_slash, hier_part, opaque_part, net_path,
abs_path, rel_path, path_segments, rel_segment, and mark rules. All
references to "opaque" URIs have been replaced with a better
description of how the path component may be opaque to hierarchy. The
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 fragment identifier has been moved back into the section on
generic syntax components and within the URI and relative-URI rules,
though it remains excluded from absolute-URI. The number sign ("#")
character has been moved back to the reserved set as a result of
reintegrating the fragment syntax.
The ABNF has been corrected to allow a relative 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:" scheme used internally by many WWW browser
implementations. The ambiguity regarding the boundary between
authority and path is now explained and disambiguated in the same
section.
Registry-based naming authorities that use the generic syntax are now
defined within the host rule and limited to 255 path characters. This
change allows current implementations, where whatever name provided
is simply fed to the local name resolution mechanism, to be
consistent with the specification and removes the need to re-specify
DNS name formats here. It also allows the host component to contain
percent-encoded octets, which is necessary to enable
internationalized domain names to be provided in URIs, processed in
their native character encodings at the application layers above URI
processing, and passed to an IDNA library as a registered name in the
UTF-8 character encoding. The server, hostport, hostname,
domainlabel, toplabel, and alphanum rules have been removed.
The resolving relative references algorithm of [RFC2396] has been
rewritten using pseudocode for this revision to improve clarity and
fix the following issues:
o [RFC2396] section 5.2, step 6a, failed to account for a base URI
with no path.
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o Restored the behavior of [RFC1808] where, if the reference
contains an empty path and a defined query component, then the
target URI inherits the base URI's path component.
o Removed the special-case treatment of same-document references
within the URI parser in favor of a section that explains when a
reference should be interpreted by a dereferencing engine as a
same-document reference: when the target URI and base URI,
excluding fragments, match. This change does not modify the
behavior of existing same-document references as defined by RFC
2396 (fragment-only references); it merely adds the same-document
distinction to other references that refer to the base URI and
simplifies the interface between applications and their URI
parsers, as is consistent with the internal architecture of
deployed URI processing implementations.
o Separated the path merge routine into two routines: merge, for
describing combination of the base URI path with a relative-path
reference, and remove_dot_segments, for describing how to remove
the special "." and ".." segments from a composed path. The
remove_dot_segments algorithm is now applied to all URI reference
paths in order to match common implementations and improve the
normalization of URIs in practice. This change only impacts the
parsing of abnormal references and same-scheme references wherein
the base URI has a non-hierarchical path.
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Index
A
ABNF 10
absolute 25
absolute-path 24
absolute-URI 25
access 7
authority 15, 16
B
base URI 27
C
characters 11
D
dec-octet 18
dereference 7
dot-segments 20
F
fragment 22
G
gen-delims 12
generic syntax 5
H
h16 17
hierarchical 9
host 17
I
identifier 5
IP-literal 17
IPv4 18
IPv4address 18
IPv6 17
IPv6address 17
IPvFuture 17
L
locator 6
ls32 17
M
merge 30
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N
name 6
network-path 24
P
path 15, 20
pchar 20
pct-encoded 11
percent-encoding 11
port 20
Q
query 21
R
reg-name 19
registered name 19
relative 9, 27
relative-path 24
relative-URI 24
remove_dot_segments 30
representation 8
reserved 12
resolution 7, 27
resource 4
retrieval 8
S
same-document 25
sameness 8
scheme 15
segment 20
sub-delims 12
suffix 25
T
transcription 6
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U
uniform 4
unreserved 12
URI grammar
absolute-URI 25
ALPHA 10
authority 15, 16
CR 10
CTL 10
dec-octet 18
DIGIT 10
DQUOTE 10
fragment 15, 22, 24
gen-delims 12
h16 18
HEXDIG 10
host 16, 17
IP-literal 17
IPv4address 18
IPv6address 17, 18
IPvFuture 17
LF 10
ls32 18
mark 12
OCTET 10
path 15
path-segments 20
pchar 20, 21, 22
pct-encoded 11
port 16, 20
query 15, 21, 24, 25
reg-name 19
relative-URI 24, 24
reserved 12
scheme 15, 15, 25
segment 20
SP 10
sub-delims 12
unreserved 12
URI 15, 24
URI-reference 24
userinfo 16, 16
URI 15
URI-reference 24
URL 6
URN 6
userinfo 16
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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