< draft-fielding-uri-rfc2396bis-05.txt   draft-fielding-uri-rfc2396bis-06.txt >
Network Working Group T. Berners-Lee Network Working Group T. Berners-Lee
Internet-Draft W3C/MIT Internet-Draft W3C/MIT
Updates: 1738 (if approved) R. Fielding Updates: 1738 (if approved) R. Fielding
Obsoletes: 2732, 2396, 1808 (if approved) Day Software Obsoletes: 2732, 2396, 1808 (if approved) Day Software
L. Masinter L. Masinter
Expires: October 15, 2004 Adobe Expires: January 15, 2005 Adobe
April 16, 2004 July 17, 2004
Uniform Resource Identifier (URI): Generic Syntax Uniform Resource Identifier (URI): Generic Syntax
draft-fielding-uri-rfc2396bis-05 draft-fielding-uri-rfc2396bis-06
Status of this Memo Status of this Memo
By submitting this Internet-Draft, I certify that any applicable This document is an Internet-Draft and is subject to all provisions
patent or other IPR claims of which I am aware have been disclosed, of section 3 of RFC 3667. By submitting this Internet-Draft, each
and any of which I become aware will be disclosed, in accordance with author represents that any applicable patent or other IPR claims of
which he or she is aware have been or will be disclosed, and any of
which he or she become aware will be disclosed, in accordance with
RFC 3668. RFC 3668.
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved. Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract Abstract
A Uniform Resource Identifier (URI) is a compact sequence of A Uniform Resource Identifier (URI) is a compact sequence of
characters for identifying an abstract or physical resource. This characters for identifying an abstract or physical resource. This
specification defines the generic URI syntax and a process for specification defines the generic URI syntax and a process for
resolving URI references that might be in relative form, along with resolving URI references that might be in relative form, along with
guidelines and security considerations for the use of URIs on the guidelines and security considerations for the use of URIs on the
Internet. Internet. The URI syntax defines a grammar that is a superset of all
valid URIs, such that an implementation can parse the common
The URI syntax defines a grammar that is a superset of all valid components of a URI reference without knowing the scheme-specific
URIs, such that an implementation can parse the common components of requirements of every possible identifier. This specification does
a URI reference without knowing the scheme-specific requirements of not define a generative grammar for URIs; that task is performed by
every possible identifier. This specification does not define a the individual specifications of each URI scheme.
generative grammar for URIs; that task is performed by the individual
specifications of each URI scheme.
Editorial Note Editorial Note
Discussion of this draft and comments to the editors should be sent 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 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>. is available at &lt;http://gbiv.com/protocols/uri/rev-2002/
issues.html&gt; [1].
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Overview of URIs . . . . . . . . . . . . . . . . . . . . . 4 1.1 Overview of URIs . . . . . . . . . . . . . . . . . . . . . 4
1.1.1 Generic Syntax . . . . . . . . . . . . . . . . . . . . 6 1.1.1 Generic Syntax . . . . . . . . . . . . . . . . . . . . 6
1.1.2 Examples . . . . . . . . . . . . . . . . . . . . . . . 6 1.1.2 Examples . . . . . . . . . . . . . . . . . . . . . . . 7
1.1.3 URI, URL, and URN . . . . . . . . . . . . . . . . . . 6 1.1.3 URI, URL, and URN . . . . . . . . . . . . . . . . . . 7
1.2 Design Considerations . . . . . . . . . . . . . . . . . . 7 1.2 Design Considerations . . . . . . . . . . . . . . . . . . 7
1.2.1 Transcription . . . . . . . . . . . . . . . . . . . . 7 1.2.1 Transcription . . . . . . . . . . . . . . . . . . . . 7
1.2.2 Separating Identification from Interaction . . . . . . 8 1.2.2 Separating Identification from Interaction . . . . . . 9
1.2.3 Hierarchical Identifiers . . . . . . . . . . . . . . . 9 1.2.3 Hierarchical Identifiers . . . . . . . . . . . . . . . 10
1.3 Syntax Notation . . . . . . . . . . . . . . . . . . . . . 10 1.3 Syntax Notation . . . . . . . . . . . . . . . . . . . . . 11
2. Characters . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2. Characters . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1 Percent-Encoding . . . . . . . . . . . . . . . . . . . . . 11 2.1 Percent-Encoding . . . . . . . . . . . . . . . . . . . . . 12
2.2 Reserved Characters . . . . . . . . . . . . . . . . . . . 11 2.2 Reserved Characters . . . . . . . . . . . . . . . . . . . 12
2.3 Unreserved Characters . . . . . . . . . . . . . . . . . . 12 2.3 Unreserved Characters . . . . . . . . . . . . . . . . . . 13
2.4 When to Encode or Decode . . . . . . . . . . . . . . . . . 13 2.4 When to Encode or Decode . . . . . . . . . . . . . . . . . 13
2.5 Identifying Data . . . . . . . . . . . . . . . . . . . . . 13 2.5 Identifying Data . . . . . . . . . . . . . . . . . . . . . 14
3. Syntax Components . . . . . . . . . . . . . . . . . . . . . . 15 3. Syntax Components . . . . . . . . . . . . . . . . . . . . . . 16
3.1 Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3.1 Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2 Authority . . . . . . . . . . . . . . . . . . . . . . . . 16 3.2 Authority . . . . . . . . . . . . . . . . . . . . . . . . 17
3.2.1 User Information . . . . . . . . . . . . . . . . . . . 17 3.2.1 User Information . . . . . . . . . . . . . . . . . . . 17
3.2.2 Host . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.2.2 Host . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.2.3 Port . . . . . . . . . . . . . . . . . . . . . . . . . 20 3.2.3 Port . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.3 Path . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 3.3 Path . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.4 Query . . . . . . . . . . . . . . . . . . . . . . . . . . 22 3.4 Query . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.5 Fragment . . . . . . . . . . . . . . . . . . . . . . . . . 23 3.5 Fragment . . . . . . . . . . . . . . . . . . . . . . . . . 23
4. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 4. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.1 URI Reference . . . . . . . . . . . . . . . . . . . . . . 24 4.1 URI Reference . . . . . . . . . . . . . . . . . . . . . . 25
4.2 Relative URI . . . . . . . . . . . . . . . . . . . . . . . 25 4.2 Relative URI . . . . . . . . . . . . . . . . . . . . . . . 26
4.3 Absolute URI . . . . . . . . . . . . . . . . . . . . . . . 25 4.3 Absolute URI . . . . . . . . . . . . . . . . . . . . . . . 26
4.4 Same-document Reference . . . . . . . . . . . . . . . . . 25 4.4 Same-document Reference . . . . . . . . . . . . . . . . . 26
4.5 Suffix Reference . . . . . . . . . . . . . . . . . . . . . 26 4.5 Suffix Reference . . . . . . . . . . . . . . . . . . . . . 27
5. Reference Resolution . . . . . . . . . . . . . . . . . . . . . 27 5. Reference Resolution . . . . . . . . . . . . . . . . . . . . . 28
5.1 Establishing a Base URI . . . . . . . . . . . . . . . . . 27 5.1 Establishing a Base URI . . . . . . . . . . . . . . . . . 28
5.1.1 Base URI Embedded in Content . . . . . . . . . . . . . 27 5.1.1 Base URI Embedded in Content . . . . . . . . . . . . . 28
5.1.2 Base URI from the Encapsulating Entity . . . . . . . . 28 5.1.2 Base URI from the Encapsulating Entity . . . . . . . . 29
5.1.3 Base URI from the Retrieval URI . . . . . . . . . . . 28 5.1.3 Base URI from the Retrieval URI . . . . . . . . . . . 29
5.1.4 Default Base URI . . . . . . . . . . . . . . . . . . . 28 5.1.4 Default Base URI . . . . . . . . . . . . . . . . . . . 29
5.2 Relative Resolution . . . . . . . . . . . . . . . . . . . 29 5.2 Relative Resolution . . . . . . . . . . . . . . . . . . . 30
5.2.1 Pre-parse the Base URI . . . . . . . . . . . . . . . . 29 5.2.1 Pre-parse the Base URI . . . . . . . . . . . . . . . . 30
5.2.2 Transform References . . . . . . . . . . . . . . . . . 29 5.2.2 Transform References . . . . . . . . . . . . . . . . . 30
5.2.3 Merge Paths . . . . . . . . . . . . . . . . . . . . . 30 5.2.3 Merge Paths . . . . . . . . . . . . . . . . . . . . . 31
5.2.4 Remove Dot Segments . . . . . . . . . . . . . . . . . 31 5.2.4 Remove Dot Segments . . . . . . . . . . . . . . . . . 32
5.3 Component Recomposition . . . . . . . . . . . . . . . . . 33 5.3 Component Recomposition . . . . . . . . . . . . . . . . . 34
5.4 Reference Resolution Examples . . . . . . . . . . . . . . 34 5.4 Reference Resolution Examples . . . . . . . . . . . . . . 34
5.4.1 Normal Examples . . . . . . . . . . . . . . . . . . . 34 5.4.1 Normal Examples . . . . . . . . . . . . . . . . . . . 35
5.4.2 Abnormal Examples . . . . . . . . . . . . . . . . . . 34 5.4.2 Abnormal Examples . . . . . . . . . . . . . . . . . . 35
6. Normalization and Comparison . . . . . . . . . . . . . . . . . 36 6. Normalization and Comparison . . . . . . . . . . . . . . . . . 36
6.1 Equivalence . . . . . . . . . . . . . . . . . . . . . . . 36 6.1 Equivalence . . . . . . . . . . . . . . . . . . . . . . . 37
6.2 Comparison Ladder . . . . . . . . . . . . . . . . . . . . 37 6.2 Comparison Ladder . . . . . . . . . . . . . . . . . . . . 37
6.2.1 Simple String Comparison . . . . . . . . . . . . . . . 37 6.2.1 Simple String Comparison . . . . . . . . . . . . . . . 38
6.2.2 Syntax-based Normalization . . . . . . . . . . . . . . 37 6.2.2 Syntax-based Normalization . . . . . . . . . . . . . . 38
6.2.3 Scheme-based Normalization . . . . . . . . . . . . . . 38 6.2.3 Scheme-based Normalization . . . . . . . . . . . . . . 39
6.2.4 Protocol-based Normalization . . . . . . . . . . . . . 39 6.2.4 Protocol-based Normalization . . . . . . . . . . . . . 40
6.3 Canonical Form . . . . . . . . . . . . . . . . . . . . . . 40 6.3 Canonical Form . . . . . . . . . . . . . . . . . . . . . . 40
7. Security Considerations . . . . . . . . . . . . . . . . . . . 40 7. Security Considerations . . . . . . . . . . . . . . . . . . . 41
7.1 Reliability and Consistency . . . . . . . . . . . . . . . 40 7.1 Reliability and Consistency . . . . . . . . . . . . . . . 41
7.2 Malicious Construction . . . . . . . . . . . . . . . . . . 41 7.2 Malicious Construction . . . . . . . . . . . . . . . . . . 41
7.3 Back-end Transcoding . . . . . . . . . . . . . . . . . . . 41 7.3 Back-end Transcoding . . . . . . . . . . . . . . . . . . . 42
7.4 Rare IP Address Formats . . . . . . . . . . . . . . . . . 42 7.4 Rare IP Address Formats . . . . . . . . . . . . . . . . . 43
7.5 Sensitive Information . . . . . . . . . . . . . . . . . . 43 7.5 Sensitive Information . . . . . . . . . . . . . . . . . . 44
7.6 Semantic Attacks . . . . . . . . . . . . . . . . . . . . . 43 7.6 Semantic Attacks . . . . . . . . . . . . . . . . . . . . . 44
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 44 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 44
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 45 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 44
9.1 Normative References . . . . . . . . . . . . . . . . . . . . 45 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 45
9.2 Informative References . . . . . . . . . . . . . . . . . . . 45 10.1 Normative References . . . . . . . . . . . . . . . . . . . . 45
10.2 Informative References . . . . . . . . . . . . . . . . . . . 45
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 47 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 47
A. Collected ABNF for URI . . . . . . . . . . . . . . . . . . . . 48 A. Collected ABNF for URI . . . . . . . . . . . . . . . . . . . . 48
B. Parsing a URI Reference with a Regular Expression . . . . . . 50 B. Parsing a URI Reference with a Regular Expression . . . . . . 50
C. Delimiting a URI in Context . . . . . . . . . . . . . . . . . 51 C. Delimiting a URI in Context . . . . . . . . . . . . . . . . . 50
D. Summary of Non-editorial Changes . . . . . . . . . . . . . . . 52 D. Summary of Non-editorial Changes . . . . . . . . . . . . . . . 52
D.1 Additions . . . . . . . . . . . . . . . . . . . . . . . . 52 D.1 Additions . . . . . . . . . . . . . . . . . . . . . . . . 52
D.2 Modifications from RFC 2396 . . . . . . . . . . . . . . . 53 D.2 Modifications from RFC 2396 . . . . . . . . . . . . . . . 52
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 E. Instructions to RFC Editor . . . . . . . . . . . . . . . . . . 54
Intellectual Property and Copyright Statements . . . . . . . . 58 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
Intellectual Property and Copyright Statements . . . . . . . . 59
1. Introduction 1. Introduction
A Uniform Resource Identifier (URI) provides a simple and extensible A Uniform Resource Identifier (URI) provides a simple and extensible
means for identifying a resource. This specification of URI syntax means for identifying a resource. This specification of URI syntax
and semantics is derived from concepts introduced by the World Wide and semantics is derived from concepts introduced by the World Wide
Web global information initiative, whose use of such identifiers Web global information initiative, whose use of such identifiers
dates from 1990 and is described in "Universal Resource Identifiers dates from 1990 and is described in "Universal Resource Identifiers
in WWW" [RFC1630], and is designed to meet the recommendations laid in WWW" [RFC1630], and is designed to meet the recommendations laid
out in "Functional Recommendations for Internet Resource Locators" out in "Functional Recommendations for Internet Resource Locators"
[RFC1736] and "Functional Requirements for Uniform Resource Names" [RFC1736] and "Functional Requirements for Uniform Resource Names"
[RFC1737]. [RFC1737].
This document obsoletes [RFC2396], which merged "Uniform Resource This document obsoletes [RFC2396], which merged "Uniform Resource
Locators" [RFC1738] and "Relative Uniform Resource Locators" Locators" [RFC1738] and "Relative Uniform Resource Locators"
[RFC1808] in order to define a single, generic syntax for all URIs. [RFC1808] in order to define a single, generic syntax for all URIs.
It excludes those portions of RFC 1738 that defined the specific It contains the updates from, and obsoletes, [RFC2732], which
syntax of individual URI schemes; those portions will be updated as introduced syntax for IPv6 addresses. It excludes those portions of
separate documents. The process for registration of new URI schemes RFC 1738 that defined the specific syntax of individual URI schemes;
is defined separately by [RFC2717]. Advice for designers of new URI those portions will be updated as separate documents. The process
schemes can be found in [RFC2718]. for registration of new URI schemes is defined separately by [BCP35].
Advice for designers of new URI schemes can be found in [RFC2718].
All significant changes from RFC 2396 are noted in Appendix D. All significant changes from RFC 2396 are noted in Appendix D.
This specification uses the terms "character" and "coded character This specification uses the terms "character" and "coded character
set" in accordance with the definitions provided in [RFC2978], and set" in accordance with the definitions provided in [BCP19], and
"character encoding" in place of what [RFC2978] refers to as a "character encoding" in place of what [BCP19] refers to as a
"charset". "charset".
1.1 Overview of URIs 1.1 Overview of URIs
URIs are characterized as follows: URIs are characterized as follows:
Uniform Uniform
Uniformity provides several benefits: it allows different types of Uniformity provides several benefits: it allows different types of
resource identifiers to be used in the same context, even when the resource identifiers to be used in the same context, even when the
mechanisms used to access those resources may differ; it allows mechanisms used to access those resources may differ; it allows
uniform semantic interpretation of common syntactic conventions uniform semantic interpretation of common syntactic conventions
across different types of resource identifiers; it allows across different types of resource identifiers; it allows
introduction of new types of resource identifiers without introduction of new types of resource identifiers without
interfering with the way that existing identifiers are used; and, interfering with the way that existing identifiers are used; and,
it allows the identifiers to be reused in many different contexts, it allows the identifiers to be reused in many different contexts,
thus permitting new applications or protocols to leverage a thus permitting new applications or protocols to leverage a
pre-existing, large, and widely-used set of resource identifiers. pre-existing, large, and widely-used set of resource identifiers.
Resource Resource
Anything that has been named or described can be a resource.
Familiar examples include an electronic document, an image, a This specification does not limit the scope of what might be a
service (e.g., "today's weather report for Los Angeles"), and a resource; rather, the term "resource" is used in a general sense
collection of other resources. A resource is not necessarily for whatever might be identified by a URI. Familiar examples
include an electronic document, an image, a source of information
with consistent purpose (e.g., "today's weather report for Los
Angeles"), a service (e.g., an HTTP to SMS gateway), a collection
of other resources, and so on. A resource is not necessarily
accessible via the Internet; e.g., human beings, corporations, and accessible via the Internet; e.g., human beings, corporations, and
bound books in a library can also be resources. Likewise, abstract bound books in a library can also be resources. Likewise,
concepts can be resources, such as the operators and operands of a abstract concepts can be resources, such as the operators and
mathematical equation, the types of a relationship (e.g., "parent" operands of a mathematical equation, the types of a relationship
or "employee"), or numeric values (e.g., zero, one, and infinity). (e.g., "parent" or "employee"), or numeric values (e.g., zero,
These things are called resources because they each can be one, and infinity).
considered a source of supply or support, or an available means,
for some system, where such systems may be as diverse as the World
Wide Web, a filesystem, an ontological graph, a theorem prover, or
some other form of system for the direct or indirect observation
and/or manipulation of resources. Note that "supply" is not
necessary for a thing to be considered a resource: the ability to
simply refer to that thing is often sufficient to support the
operation of a given system.
Identifier Identifier
An identifier embodies the information required to distinguish An identifier embodies the information required to distinguish
what is being identified from all other things within its scope of what is being identified from all other things within its scope of
identification. Our use of the terms "identify" and "identifying" identification. Our use of the terms "identify" and "identifying"
refer to this process of distinguishing from many to one; they refer to this purpose of distinguishing one resource from all
should not be mistaken as an assumption that the identifier other resources, regardless of how that purpose is accomplished
defines the identity of what is referenced, though that may be the (e.g., by name, address, context, etc.). These terms should not
case for some identifiers. be mistaken as an assumption that an identifier defines or
embodies the identity of what is referenced, though that may be
the case for some identifiers. Nor should it be assumed that a
system using URIs will access the resource identified: in many
cases, URIs are used to denote resources without any intention
that they be accessed. Likewise, the "one" resource identified
might not be singular in nature (e.g., a resource might be a named
set or a mapping that varies over time).
A URI is an identifier that consists of a sequence of characters A URI is an identifier, consisting of a sequence of characters
matching the syntax rule named <URI> in Section 3. A URI can be used matching the syntax rule named <URI> in Section 3, that enables
to refer to a resource. This specification does not place any limits uniform identification of resources via a separately defined,
on the nature of a resource, the reasons why an application might extensible set of naming schemes (Section 3.1). How that
wish to refer to a resource, or the kinds of system that might use identification is accomplished, assigned, or enabled is delegated to
URIs for the sake of identifying resources. each scheme specification.
URIs have a global scope and must be interpreted consistently This specification does not place any limits on the nature of a
regardless of context, though the result of that interpretation may resource, the reasons why an application might wish to refer to a
be in relation to the end-user's context. For example, "http:// resource, or the kinds of system that might use URIs for the sake of
localhost/" has the same interpretation for every user of that identifying resources. This specification does not require that a
reference, even though the network interface corresponding to URI persists in identifying the same resource over all time, though
"localhost" may be different for each end-user: interpretation is that is a common goal of all URI schemes. Nevertheless, nothing in
independent of access. However, an action made on the basis of that this specification prevents an application from limiting itself to
reference will take place in relation to the end-user's context, particular types of resources, or to a subset of URIs that maintains
which implies that an action intended to refer to a single, globally characteristics desired by that application.
unique thing must use a URI that distinguishes that resource from all
other things. URIs that identify in relation to the end-user's local URIs have a global scope and are interpreted consistently regardless
context should only be used when the context itself is a defining of context, though the result of that interpretation may be in
aspect of the resource, such as when an on-line Linux manual refers relation to the end-user's context. For example, "http://localhost/"
to a file on the end-user's filesystem (e.g., "file:///etc/hosts"). has the same interpretation for every user of that reference, even
though the network interface corresponding to "localhost" may be
different for each end-user: interpretation is independent of access.
However, an action made on the basis of that reference will take
place in relation to the end-user's context, which implies that an
action intended to refer to a single, globally unique thing must use
a URI that distinguishes that resource from all other things. URIs
that identify in relation to the end-user's local context should only
be used when the context itself is a defining aspect of the resource,
such as when an on-line help manual refers to a file on the
end-user's filesystem (e.g., "file:///etc/hosts").
1.1.1 Generic Syntax 1.1.1 Generic Syntax
Each URI begins with a scheme name, as defined in Section 3.1, that Each URI begins with a scheme name, as defined in Section 3.1, that
refers to a specification for assigning identifiers within that refers to a specification for assigning identifiers within that
scheme. As such, the URI syntax is a federated and extensible naming scheme. As such, the URI syntax is a federated and extensible naming
system wherein each scheme's specification may further restrict the system wherein each scheme's specification may further restrict the
syntax and semantics of identifiers using that scheme. syntax and semantics of identifiers using that scheme.
This specification defines those elements of the URI syntax that are This specification defines those elements of the URI syntax that are
required of all URI schemes or are common to many URI schemes. It 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 thus defines the syntax and semantics that are needed to implement a
scheme-independent parsing mechanism for URI references, such that scheme-independent parsing mechanism for URI references, such that
the scheme-dependent handling of a URI can be postponed until the the scheme-dependent handling of a URI can be postponed until the
scheme-dependent semantics are needed. Likewise, protocols and data scheme-dependent semantics are needed. Likewise, protocols and data
formats that make use of URI references can refer to this formats that make use of URI references can refer to this
specification as defining the range of syntax allowed for all URIs, specification as defining the range of syntax allowed for all URIs,
including those schemes that have yet to be defined. including those schemes that have yet to be defined, thus decoupling
the evolution of identification schemes from the evolution of
protocols, data formats, and implementations that make use of URIs.
A parser of the generic URI syntax is capable of parsing any URI A parser of the generic URI syntax is capable of parsing any URI
reference into its major components; once the scheme is determined, reference into its major components; once the scheme is determined,
further scheme-specific parsing can be performed on the components. further scheme-specific parsing can be performed on the components.
In other words, the URI generic syntax is a superset of the syntax of In other words, the URI generic syntax is a superset of the syntax of
all URI schemes. all URI schemes.
1.1.2 Examples 1.1.2 Examples
The following examples illustrate URIs that are in common use. The following example URIs illustrate several URI schemes and
variations in their common syntax components:
ftp://ftp.is.co.za/rfc/rfc1808.txt ftp://ftp.is.co.za/rfc/rfc1808.txt
http://www.ietf.org/rfc/rfc2396.txt http://www.ietf.org/rfc/rfc2396.txt
ldap://[2001:db8::7]/c=GB?objectClass?one
mailto:John.Doe@example.com mailto:John.Doe@example.com
news:comp.infosystems.www.servers.unix news:comp.infosystems.www.servers.unix
telnet://melvyl.ucop.edu/ tel:+1-816-555-1212
telnet://192.0.2.16:80/
urn:oasis:names:specification:docbook:dtd:xml:4.1.2
1.1.3 URI, URL, and URN 1.1.3 URI, URL, and URN
A URI can be further classified as a locator, a name, or both. The 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 term "Uniform Resource Locator" (URL) refers to the subset of URIs
that, in addition to identifying a resource, provide a means of that, in addition to identifying a resource, provide a means of
locating the resource by describing its primary access mechanism locating the resource by describing its primary access mechanism
(e.g., its network "location"). The term "Uniform Resource Name" (e.g., its network "location"). The term "Uniform Resource Name"
(URN) has been used historically to refer to both URIs under the (URN) has been used historically to refer to both URIs under the
"urn" scheme [RFC2141], which are required to remain globally unique "urn" scheme [RFC2141], which are required to remain globally unique
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on the resource, as might be characterized by such words as "access", on the resource, as might be characterized by such words as "access",
"update", "replace", or "find attributes". Such operations are "update", "replace", or "find attributes". Such operations are
defined by the protocols that make use of URIs, not by this defined by the protocols that make use of URIs, not by this
specification. However, we do use a few general terms for describing specification. However, we do use a few general terms for describing
common operations on URIs. URI "resolution" is the process of common operations on URIs. URI "resolution" is the process of
determining an access mechanism and the appropriate parameters determining an access mechanism and the appropriate parameters
necessary to dereference a URI; such resolution may require several necessary to dereference a URI; such resolution may require several
iterations. To use that access mechanism to perform an action on the iterations. To use that access mechanism to perform an action on the
URI's resource is to "dereference" the URI. URI's resource is to "dereference" the URI.
When URIs are used within information systems to identify sources of When URIs are used within information retrieval systems to identify
information, the most common form of URI dereference is "retrieval": sources of information, the most common form of URI dereference is
making use of a URI in order to retrieve a representation of its "retrieval": making use of a URI in order to retrieve a
associated resource. A "representation" is a sequence of octets, representation of its associated resource. A "representation" is a
along with representation metadata describing those octets, that sequence of octets, along with representation metadata describing
constitutes a record of the state of the resource at the time that those octets, that constitutes a record of the state of the resource
the representation is generated. Retrieval is achieved by a process at the time that the representation is generated. Retrieval is
that might include using the URI as a cache key to check for a achieved by a process that might include using the URI as a cache key
locally cached representation, resolution of the URI to determine an to check for a locally cached representation, resolution of the URI
appropriate access mechanism (if any), and dereference of the URI for to determine an appropriate access mechanism (if any), and
the sake of applying a retrieval operation. Depending on the dereference of the URI for the sake of applying a retrieval
protocols used to perform the retrieval, additional information might operation. Depending on the protocols used to perform the retrieval,
be supplied about the resource (resource metadata) and its relation additional information might be supplied about the resource (resource
to other resources. metadata) and its relation to other resources.
URI references in information systems are designed to be URI references in information retrieval systems are designed to be
late-binding: the result of an access is generally determined at the 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 time it is accessed and may vary over time or due to other aspects of
the interaction. Such references are created in order to be be used the interaction. Such references are created in order to be used in
in the future: what is being identified is not some specific result the future: what is being identified is not some specific result that
that was obtained in the past, but rather some characteristic that is was obtained in the past, but rather some characteristic that is
expected to be true for future results. In such cases, the resource expected to be true for future results. In such cases, the resource
referred to by the URI is actually a sameness of characteristics as referred to by the URI is actually a sameness of characteristics as
observed over time, perhaps elucidated by additional comments or observed over time, perhaps elucidated by additional comments or
assertions made by the resource provider. assertions made by the resource provider.
Although many URI schemes are named after protocols, this does not 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 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 via the named protocol. URIs are often used simply for the sake of
identification. Even when a URI is used to retrieve a representation identification. Even when a URI is used to retrieve a representation
of a resource, that access might be through gateways, proxies, of a resource, that access might be through gateways, proxies,
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URIs may require the use of more than one protocol (e.g., both DNS 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 and HTTP are typically used to access an "http" URI's origin server
when a representation isn't found in a local cache). when a representation isn't found in a local cache).
1.2.3 Hierarchical Identifiers 1.2.3 Hierarchical Identifiers
The URI syntax is organized hierarchically, with components listed in The URI syntax is organized hierarchically, with components listed in
order of decreasing significance from left to right. For some URI order of decreasing significance from left to right. For some URI
schemes, the visible hierarchy is limited to the scheme itself: schemes, the visible hierarchy is limited to the scheme itself:
everything after the scheme component delimiter (":") is considered everything after the scheme component delimiter (":") is considered
opaque to URI processing. Other URI schemes make the hierarchy opaque to URI processing. Other URI schemes make the hierarchy
explicit and visible to generic parsing algorithms. explicit and visible to generic parsing algorithms.
The generic syntax uses the slash ("/"), question mark ("?"), and The generic syntax uses the slash ("/"), question mark ("?"), and
number sign ("#") characters for the purpose of delimiting components number sign ("#") characters for the purpose of delimiting components
that are significant to the generic parser's hierarchical that are significant to the generic parser's hierarchical
interpretation of an identifier. In addition to aiding the interpretation of an identifier. In addition to aiding the
readability of such identifiers through the consistent use of readability of such identifiers through the consistent use of
familiar syntax, this uniform representation of hierarchy across familiar syntax, this uniform representation of hierarchy across
naming schemes allows scheme-independent references to be made naming schemes allows scheme-independent references to be made
relative to that hierarchy. relative to that hierarchy.
It is often the case that a group or "tree" of documents has been 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 constructed to serve a common purpose, wherein the vast majority of
URIs in these documents point to resources within the tree rather URIs in these documents point to resources within the tree rather
than outside of it. Similarly, documents located at a particular than outside of it. Similarly, documents located at a particular
site are much more likely to refer to other resources at that site site are much more likely to refer to other resources at that site
than to resources at remote sites. Relative referencing of URIs than to resources at remote sites. Relative referencing of URIs
allows document trees to be partially independent of their location allows document trees to be partially independent of their location
and access scheme. For instance, it is possible for a single set of and access scheme. For instance, it is possible for a single set of
hypertext documents to be simultaneously accessible and traversable hypertext documents to be simultaneously accessible and traversable
via each of the "file", "http", and "ftp" schemes if the documents via each of the "file", "http", and "ftp" schemes if the documents
refer to each other using relative references. Furthermore, such refer to each other using relative references. Furthermore, such
document trees can be moved, as a whole, without changing any of the document trees can be moved, as a whole, without changing any of the
relative references. relative references.
A relative URI reference (Section 4.2) refers to a resource by A relative URI reference (Section 4.2) refers to a resource by
describing the difference within a hierarchical name space between describing the difference within a hierarchical name space between
the reference context and the target URI. The reference resolution the reference context and the target URI. The reference resolution
algorithm, presented in Section 5, defines how such a reference is algorithm, presented in Section 5, defines how such a reference is
transformed to the target URI. Since relative references can only be transformed to the target URI. Since relative references can only be
used within the context of a hierarchical URI, designers of new URI used within the context of a hierarchical URI, designers of new URI
schemes should use a syntax consistent with the generic syntax's schemes should use a syntax consistent with the generic syntax's
hierarchical components unless there are compelling reasons to forbid hierarchical components unless there are compelling reasons to forbid
relative referencing within that scheme. relative referencing within that scheme.
All URIs are parsed by generic syntax parsers when used. A URI scheme All URIs are parsed by generic syntax parsers when used. A URI
that wishes to remain opaque to hierarchical processing must disallow scheme that wishes to remain opaque to hierarchical processing must
the use of slash and question mark characters. However, since a URI disallow the use of slash and question mark characters. However,
reference is only modified by the generic parser if it contains a since a URI reference is only modified by the generic parser if it
dot-segment (a complete path segment of "." or "..", as described in contains a dot-segment (a complete path segment of "." or "..", as
Section 3.3), URI schemes may safely use "/" for other purposes if described in Section 3.3), URI schemes may safely use "/" for other
they do not allow dot-segments. purposes if they do not allow dot-segments.
1.3 Syntax Notation 1.3 Syntax Notation
This specification uses the Augmented Backus-Naur Form (ABNF) This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC2234], including the following core ABNF syntax rules notation of [RFC2234], including the following core ABNF syntax rules
defined by that specification: ALPHA (letters), CR (carriage return), defined by that specification: ALPHA (letters), CR (carriage return),
DIGIT (decimal digits), DQUOTE (double quote), HEXDIG (hexadecimal DIGIT (decimal digits), DQUOTE (double quote), HEXDIG (hexadecimal
digits), LF (line feed), and SP (space). The complete URI syntax is digits), LF (line feed), and SP (space). The complete URI syntax is
collected in Appendix A. collected in Appendix A.
2. Characters 2. Characters
The URI syntax provides a method of encoding data, presumably for the The URI syntax provides a method of encoding data, presumably for the
sake of identifying a resource, as a sequence of characters. The URI sake of identifying a resource, as a sequence of characters. The URI
characters are, in turn, frequently encoded as octets for transport characters are, in turn, frequently encoded as octets for transport
or presentation. This specification does not mandate any particular or presentation. This specification does not mandate any particular
character encoding for mapping between URI characters and the octets character encoding for mapping between URI characters and the octets
used to store or transmit those characters. When a URI appears in a used to store or transmit those characters. When a URI appears in a
protocol element, the character encoding is defined by that protocol; protocol element, the character encoding is defined by that protocol;
absent such a definition, a URI is assumed to be in the same absent such a definition, a URI is assumed to be in the same
character encoding as the surrounding text. character encoding as the surrounding text.
The ABNF notation defines its terminal values to be non-negative The ABNF notation defines its terminal values to be non-negative
integers (codepoints) based on the US-ASCII coded character set integers (codepoints) based on the US-ASCII coded character set
[ASCII]. Since a URI is a sequence of characters, we must invert [ASCII]. Since a URI is a sequence of characters, we must invert
that relation in order to understand the URI syntax. Therefore, the that relation in order to understand the URI syntax. Therefore, the
integer values used by the ABNF must be mapped back to their integer values used by the ABNF must be mapped back to their
corresponding characters via US-ASCII in order to complete the syntax corresponding characters via US-ASCII in order to complete the syntax
rules. rules.
A URI is composed from a limited set of characters consisting of A URI is composed from a limited set of characters consisting of
digits, letters, and a few graphic symbols. A reserved subset of digits, letters, and a few graphic symbols. A reserved subset of
those characters may be used to delimit syntax components within a those characters may be used to delimit syntax components within a
URI, while the remaining characters, including both the unreserved URI, while the remaining characters, including both the unreserved
set and those reserved characters not acting as delimiters, define set and those reserved characters not acting as delimiters, define
each component's identifying data. each component's identifying data.
2.1 Percent-Encoding 2.1 Percent-Encoding
A percent-encoding mechanism is used to represent a data octet in a A percent-encoding mechanism is used to represent a data octet in a
component when that octet's corresponding character is outside the component when that octet's corresponding character is outside the
allowed set or is being used as a delimiter of, or within, 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, component. A percent-encoded octet is encoded as a character
consisting of the percent character "%" followed by the two triplet, consisting of the percent character "%" followed by the two
hexadecimal digits representing that octet's numeric value. For hexadecimal digits representing that octet's numeric value. For
example, "%20" is the percent-encoding for the binary octet example, "%20" is the percent-encoding for the binary octet
"00100000" (ABNF: %x20), which in US-ASCII corresponds to the space "00100000" (ABNF: %x20), which in US-ASCII corresponds to the space
character (SP). Section 2.4 describes when percent-encoding and character (SP). Section 2.4 describes when percent-encoding and
decoding is applied. decoding is applied.
pct-encoded = "%" HEXDIG HEXDIG pct-encoded = "%" HEXDIG HEXDIG
The uppercase hexadecimal digits 'A' through 'F' are equivalent to The uppercase hexadecimal digits 'A' through 'F' are equivalent to
the lowercase digits 'a' through 'f', respectively. Two URIs that the lowercase digits 'a' through 'f', respectively. Two URIs that
differ only in the case of hexadecimal digits used in percent-encoded differ only in the case of hexadecimal digits used in percent-encoded
octets are equivalent. For consistency, URI producers and octets are equivalent. For consistency, URI producers and
normalizers should use uppercase hexadecimal digits for all normalizers should use uppercase hexadecimal digits for all
percent-encodings. percent-encodings.
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URIs include components and subcomponents that are delimited by URIs include components and subcomponents that are delimited by
characters in the "reserved" set. These characters are called characters in the "reserved" set. These characters are called
"reserved" because they may (or may not) be defined as delimiters by "reserved" because they may (or may not) be defined as delimiters by
the generic syntax, by each scheme-specific syntax, or by the the generic syntax, by each scheme-specific syntax, or by the
implementation-specific syntax of a URI's dereferencing algorithm. implementation-specific syntax of a URI's dereferencing algorithm.
If data for a URI component would conflict with a reserved If data for a URI component would conflict with a reserved
character's purpose as a delimiter, then the conflicting data must be character's purpose as a delimiter, then the conflicting data must be
percent-encoded before forming the URI. percent-encoded before forming the URI.
reserved = gen-delims / sub-delims reserved = gen-delims / sub-delims
gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@" gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@"
sub-delims = "!" / "$" / "&" / "'" / "(" / ")" sub-delims = "!" / "$" / "&" / "'" / "(" / ")"
/ "*" / "+" / "," / ";" / "=" / "*" / "+" / "," / ";" / "="
The purpose of reserved characters is to provide a set of delimiting The purpose of reserved characters is to provide a set of delimiting
characters that are distinguishable from other data within a URI. characters that are distinguishable from other data within a URI.
URIs that differ in the replacement of a reserved character with its URIs that differ in the replacement of a reserved character with its
corresponding percent-encoded octet are not equivalent. corresponding percent-encoded octet are not equivalent.
Percent-encoding a reserved character, or decoding a percent-encoded Percent-encoding a reserved character, or decoding a percent-encoded
octet that corresponds to a reserved character, will change how the octet that corresponds to a reserved character, will change how the
URI is interpreted by most applications. Thus, characters in the URI is interpreted by most applications. Thus, characters in the
reserved set are protected from normalization and are therefore safe reserved set are protected from normalization and are therefore safe
to be used by scheme-specific and producer-specific algorithms for to be used by scheme-specific and producer-specific algorithms for
delimiting data subcomponents within a URI. delimiting data subcomponents within a URI.
A subset of the reserved characters (gen-delims) are used as A subset of the reserved characters (gen-delims) are used as
delimiters of the generic URI components described in Section 3. A delimiters of the generic URI components described in Section 3. A
component's ABNF syntax rule will not use the reserved or gen-delims component's ABNF syntax rule will not use the reserved or gen-delims
rule names directly; instead, each syntax rule lists the characters rule names directly; instead, each syntax rule lists the characters
allowed within that component (i.e., not delimiting it) and any of allowed within that component (i.e., not delimiting it) and any of
those characters that are also in the reserved set are "reserved" for those characters that are also in the reserved set are "reserved" for
use as subcomponent delimiters within the component. Only the most use as subcomponent delimiters within the component. Only the most
common subcomponents are defined by this specification; other common subcomponents are defined by this specification; other
subcomponents may be defined by a URI scheme's specification, or by subcomponents may be defined by a URI scheme's specification, or by
the implementation-specific syntax of a URI's dereferencing the implementation-specific syntax of a URI's dereferencing
algorithm, provided that such subcomponents are delimited by algorithm, provided that such subcomponents are delimited by
characters in the reserved set allowed within that component. characters in the reserved set allowed within that component.
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2.3 Unreserved Characters 2.3 Unreserved Characters
Characters that are allowed in a URI but do not have a reserved Characters that are allowed in a URI but do not have a reserved
purpose are called unreserved. These include uppercase and lowercase purpose are called unreserved. These include uppercase and lowercase
letters, decimal digits, hyphen, period, underscore, and tilde. letters, decimal digits, hyphen, period, underscore, and tilde.
unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~" unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
URIs that differ in the replacement of an unreserved character with URIs that differ in the replacement of an unreserved character with
its corresponding percent-encoded octet are equivalent: they identify its corresponding percent-encoded US-ASCII octet are equivalent: they
the same resource. However, percent-encoded unreserved characters identify the same resource. However, URI comparison implementations
may change the result of some URI comparisons (Section 6), do not always perform normalization prior to comparison Section 6.
potentially leading to incorrect or inefficient behavior. For For consistency, percent-encoded octets in the ranges of ALPHA
consistency, percent-encoded octets in the ranges of ALPHA (%41-%5A (%41-%5A and %61-%7A), DIGIT (%30-%39), hyphen (%2D), period (%2E),
and %61-%7A), DIGIT (%30-%39), hyphen (%2D), period (%2E), underscore underscore (%5F), or tilde (%7E) should not be created by URI
(%5F), or tilde (%7E) should not be created by URI producers and, producers and, when found in a URI, should be decoded to their
when found in a URI, should be decoded to their corresponding corresponding unreserved character by URI normalizers.
unreserved character by URI normalizers.
2.4 When to Encode or Decode 2.4 When to Encode or Decode
Under normal circumstances, the only time that octets within a URI Under normal circumstances, the only time that octets within a URI
are percent-encoded is during the process of producing the URI from are percent-encoded is during the process of producing the URI from
its component parts. It is during that process that an its component parts. It is during that process that an
implementation determines which of the reserved characters are to be implementation determines which of the reserved characters are to be
used as subcomponent delimiters and which can be safely used as data. used as subcomponent delimiters and which can be safely used as data.
Once produced, a URI is always in its percent-encoded form. Once produced, a URI is always in its percent-encoded form.
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not percent-encode or decode the same string more than once, since not percent-encode or decode the same string more than once, since
decoding an already decoded string might lead to misinterpreting a decoding an already decoded string might lead to misinterpreting a
percent data octet as the beginning of a percent-encoding, or vice percent data octet as the beginning of a percent-encoding, or vice
versa in the case of percent-encoding an already percent-encoded versa in the case of percent-encoding an already percent-encoded
string. string.
2.5 Identifying Data 2.5 Identifying Data
URI characters provide identifying data for each of the URI URI characters provide identifying data for each of the URI
components, serving as an external interface for identification components, serving as an external interface for identification
between systems. Although the presence and nature of the URI between systems. Although the presence and nature of the URI
production interface is hidden from clients that use its URIs, and production interface is hidden from clients that use its URIs, and
thus beyond the scope of the interoperability requirements defined by thus beyond the scope of the interoperability requirements defined by
this specification, it is a frequent source of confusion and errors this specification, it is a frequent source of confusion and errors
in the interpretation of URI character issues. Implementers need to in the interpretation of URI character issues. Implementers need to
be aware that there are multiple character encodings involved in the be aware that there are multiple character encodings involved in the
production and transmission of URIs: local name and data encoding, production and transmission of URIs: local name and data encoding,
public interface encoding, URI character encoding, data format public interface encoding, URI character encoding, data format
encoding, and protocol encoding. encoding, and protocol encoding.
The first encoding of identifying data is the one in which the local The first encoding of identifying data is the one in which the local
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data formats are often subsequently encoded for transmission over data formats are often subsequently encoded for transmission over
Internet protocols. Internet protocols.
For most systems, an unreserved character appearing within a URI For most systems, an unreserved character appearing within a URI
component is interpreted as representing the data octet corresponding component is interpreted as representing the data octet corresponding
to that character's encoding in US-ASCII. Consumers of URIs assume to that character's encoding in US-ASCII. Consumers of URIs assume
that the letter "X" corresponds to the octet "01011000", and there is that the letter "X" corresponds to the octet "01011000", and there is
no harm in making that assumption even when it is incorrect. A no harm in making that assumption even when it is incorrect. A
system that internally provides identifiers in the form of a system that internally provides identifiers in the form of a
different character encoding, such as EBCDIC, will generally perform different character encoding, such as EBCDIC, will generally perform
character translation of textual identifiers to UTF-8 [RFC3629] (or character translation of textual identifiers to UTF-8 [STD63] (or
some other superset of the US-ASCII character encoding) at an some other superset of the US-ASCII character encoding) at an
internal interface, thereby providing more meaningful identifiers internal interface, thereby providing more meaningful identifiers
than simply percent-encoding the original octets. than simply percent-encoding the original octets.
For example, consider an information service that provides data, For example, consider an information service that provides data,
stored locally using an EBCDIC-based filesystem, to clients on the stored locally using an EBCDIC-based filesystem, to clients on the
Internet through an HTTP server. When an author creates a file on Internet through an HTTP server. When an author creates a file on
that filesystem with the name "Laguna Beach", their expectation is that filesystem with the name "Laguna Beach", their expectation is
that the "http" URI corresponding to that resource would also contain that the "http" URI corresponding to that resource would also contain
the meaningful string "Laguna%20Beach". If, however, that server the meaningful string "Laguna%20Beach". If, however, that server
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interface fixes that problem by transcoding the local name to a interface fixes that problem by transcoding the local name to a
superset of US-ASCII prior to producing the URI. Naturally, proper superset of US-ASCII prior to producing the URI. Naturally, proper
interpretation of an incoming URI on such an interface requires that interpretation of an incoming URI on such an interface requires that
percent-encoded octets be decoded (e.g., "%20" to SP) before the percent-encoded octets be decoded (e.g., "%20" to SP) before the
reverse transcoding is applied to obtain the local name. reverse transcoding is applied to obtain the local name.
In some cases, the internal interface between a URI component and the In some cases, the internal interface between a URI component and the
identifying data that it has been crafted to represent is much less identifying data that it has been crafted to represent is much less
direct than a character encoding translation. For example, portions direct than a character encoding translation. For example, portions
of a URI might reflect a query on non-ASCII data, numeric coordinates 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 on a map, etc. Likewise, a URI scheme may define components with
additional encoding requirements that are applied prior to forming additional encoding requirements that are applied prior to forming
the component and producing the URI. the component and producing the URI.
When a new URI scheme defines a component that represents textual When a new URI scheme defines a component that represents textual
data consisting of characters from the Unicode (ISO/IEC 10646-1) data consisting of characters from the Unicode character set [UCS],
character set, the data should be encoded first as octets according the data should be encoded first as octets according to the UTF-8
to the UTF-8 character encoding [RFC3629], and then only those octets character encoding [STD63], and then only those octets that do not
that do not correspond to characters in the unreserved set should be correspond to characters in the unreserved set should be
percent-encoded. For example, the character A would be represented percent-encoded. For example, the character A would be represented
as "A", the character LATIN CAPITAL LETTER A WITH GRAVE would be 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 "%C3%80", and the character KATAKANA LETTER A would be
represented as "%E3%82%A2". represented as "%E3%82%A2".
3. Syntax Components 3. Syntax Components
The generic URI syntax consists of a hierarchical sequence of The generic URI syntax consists of a hierarchical sequence of
components referred to as the scheme, authority, path, query, and components referred to as the scheme, authority, path, query, and
fragment. fragment.
URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ] URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ]
hier-part = "//" authority path-abempty hier-part = "//" authority path-abempty
/ path-abs / path-absolute
/ path-rootless / path-rootless
/ path-empty / path-empty
The scheme and path components are required, though path may be empty The scheme and path components are required, though path may be empty
(no characters). When authority is present, the path must either be (no characters). When authority is present, the path must either be
empty or begin with a slash ("/") character. When authority is not empty or begin with a slash ("/") character. When authority is not
present, the path cannot begin with two slash characters ("//"). present, the path cannot begin with two slash characters ("//").
These restrictions result in five different ABNF rules for a path These restrictions result in five different ABNF rules for a path
(Section 3.3), only one of which will match any given URI reference. (Section 3.3), only one of which will match any given URI reference.
The following are two example URIs and their component parts: The following are two example URIs and their component parts:
foo://example.com:8042/over/there?name=ferret#nose foo://example.com:8042/over/there?name=ferret#nose
\_/ \______________/\_________/ \_________/ \__/ \_/ \______________/\_________/ \_________/ \__/
| | | | | | | | | |
scheme authority path query fragment scheme authority path query fragment
| _____________________|__ | _____________________|__
/ \ / \ / \ / \
urn:example:animal:ferret:nose urn:example:animal:ferret:nose
3.1 Scheme 3.1 Scheme
Each URI begins with a scheme name that refers to a specification for Each URI begins with a scheme name that refers to a specification for
assigning identifiers within that scheme. As such, the URI syntax is assigning identifiers within that scheme. As such, the URI syntax is
a federated and extensible naming system wherein each scheme's a federated and extensible naming system wherein each scheme's
specification may further restrict the syntax and semantics of specification may further restrict the syntax and semantics of
identifiers using that scheme. identifiers using that scheme.
Scheme names consist of a sequence of characters beginning with a Scheme names consist of a sequence of characters beginning with a
letter and followed by any combination of letters, digits, plus letter and followed by any combination of letters, digits, plus
("+"), period ("."), or hyphen ("-"). Although scheme is ("+"), period ("."), or hyphen ("-"). Although scheme is
case-insensitive, the canonical form is lowercase and documents that case-insensitive, the canonical form is lowercase and documents that
specify schemes must do so using lowercase letters. An specify schemes must do so using lowercase letters. An
implementation should accept uppercase letters as equivalent to implementation should accept uppercase letters as equivalent to
lowercase in scheme names (e.g., allow "HTTP" as well as "http"), for lowercase in scheme names (e.g., allow "HTTP" as well as "http"), for
the sake of robustness, but should only produce lowercase scheme the sake of robustness, but should only produce lowercase scheme
names, for consistency. names, for consistency.
scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." ) scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
Individual schemes are not specified by this document. The process Individual schemes are not specified by this document. The process
for registration of new URI schemes is defined separately by for registration of new URI schemes is defined separately by [BCP35].
[RFC2717]. The scheme registry maintains the mapping between scheme The scheme registry maintains the mapping between scheme names and
names and their specifications. Advice for designers of new URI their specifications. Advice for designers of new URI schemes can be
schemes can be found in [RFC2718]. found in [RFC2718].
When presented with a URI that violates one or more scheme-specific When presented with a URI that violates one or more scheme-specific
restrictions, the scheme-specific resolution process should flag the restrictions, the scheme-specific resolution process should flag the
reference as an error rather than ignore the unused parts; doing so reference as an error rather than ignore the unused parts; doing so
reduces the number of equivalent URIs and helps detect abuses of the reduces the number of equivalent URIs and helps detect abuses of the
generic syntax that might indicate the URI has been constructed to generic syntax that might indicate the URI has been constructed to
mislead the user (Section 7.6). mislead the user (Section 7.6).
3.2 Authority 3.2 Authority
skipping to change at page 16, line 46 skipping to change at page 17, line 34
means for distinguishing an authority based on a registered name or means for distinguishing an authority based on a registered name or
server address, along with optional port and user information. server address, along with optional port and user information.
The authority component is preceded by a double slash ("//") and is The authority component is preceded by a double slash ("//") and is
terminated by the next slash ("/"), question mark ("?"), or number terminated by the next slash ("/"), question mark ("?"), or number
sign ("#") character, or by the end of the URI. sign ("#") character, or by the end of the URI.
authority = [ userinfo "@" ] host [ ":" port ] authority = [ userinfo "@" ] host [ ":" port ]
URI producers and normalizers should omit the ":" delimiter that URI producers and normalizers should omit the ":" delimiter that
separates host from port if the port component is empty. Some schemes separates host from port if the port component is empty. Some
do not allow the userinfo and/or port subcomponents. schemes do not allow the userinfo and/or port subcomponents.
If a URI contains an authority component, then the path component If a URI contains an authority component, then the path component
must either be empty or begin with a slash ("/") character. must either be empty or begin with a slash ("/") character.
Non-validating parsers (those that merely separate a URI reference Non-validating parsers (those that merely separate a URI reference
into its major components) will often ignore the subcomponent into its major components) will often ignore the subcomponent
structure of authority, treating it as an opaque string from the structure of authority, treating it as an opaque string from the
double-slash to the first terminating delimiter, until such time as double-slash to the first terminating delimiter, until such time as
the URI is dereferenced. the URI is dereferenced.
3.2.1 User Information 3.2.1 User Information
The userinfo subcomponent may consist of a user name and, optionally, The userinfo subcomponent may consist of a user name and, optionally,
scheme-specific information about how to gain authorization to access scheme-specific information about how to gain authorization to access
the resource. The user information, if present, is followed by a the resource. The user information, if present, is followed by a
commercial at-sign ("@") that delimits it from the host. commercial at-sign ("@") that delimits it from the host.
userinfo = *( unreserved / pct-encoded / sub-delims / ":" ) userinfo = *( unreserved / pct-encoded / sub-delims / ":" )
Use of the format "user:password" in the userinfo field is Use of the format "user:password" in the userinfo field is
deprecated. Applications should not render as clear text any data deprecated. Applications should not render as clear text any data
after the first colon (":") character found within a userinfo after the first colon (":") character found within a userinfo
subcomponent unless the data after the colon is the empty string subcomponent unless the data after the colon is the empty string
(indicating no password). Applications may choose to ignore or reject (indicating no password). Applications may choose to ignore or
such data when received as part of a reference, and should reject the reject such data when received as part of a reference, and should
storage of such data in unencrypted form. The passing of reject the storage of such data in unencrypted form. The passing of
authentication information in clear text has proven to be a security authentication information in clear text has proven to be a security
risk in almost every case where it has been used. risk in almost every case where it has been used.
Applications that render a URI for the sake of user feedback, such as Applications that render a URI for the sake of user feedback, such as
in graphical hypertext browsing, should render userinfo in a way that in graphical hypertext browsing, should render userinfo in a way that
is distinguished from the rest of a URI, when feasible. Such is distinguished from the rest of a URI, when feasible. Such
rendering will assist the user in cases where the userinfo has been rendering will assist the user in cases where the userinfo has been
misleadingly crafted to look like a trusted domain name (Section misleadingly crafted to look like a trusted domain name
7.6). (Section 7.6).
3.2.2 Host 3.2.2 Host
The host subcomponent of authority is identified by an IP literal The host subcomponent of authority is identified by an IP literal
encapsulated within square brackets, an IPv4 address in encapsulated within square brackets, an IPv4 address in
dotted-decimal form, or a registered name. The host subcomponent is dotted-decimal form, or a registered name. The host subcomponent is
case-insensitive. The presence of a host subcomponent within a URI case-insensitive. The presence of a host subcomponent within a URI
does not imply that the scheme requires access to the given host on 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 the Internet. In many cases, the host syntax is used only for the
sake of reusing the existing registration process created and sake of reusing the existing registration process created and
deployed for DNS, thus obtaining a globally unique name without the deployed for DNS, thus obtaining a globally unique name without the
cost of deploying another registry. However, such use comes with its cost of deploying another registry. However, such use comes with its
own costs: domain name ownership may change over time for reasons not own costs: domain name ownership may change over time for reasons not
anticipated by the URI producer. In other cases, the data within the anticipated by the URI producer. In other cases, the data within the
host component identifies a registered name that has nothing to do host component identifies a registered name that has nothing to do
with an Internet host. We use the name "host" for the ABNF rule with an Internet host. We use the name "host" for the ABNF rule
because that is its most common purpose, not its only purpose, and because that is its most common purpose, not its only purpose, and
thus should not be considered as semantically limiting the data thus should not be considered as semantically limiting the data
within it. within it.
host = IP-literal / IPv4address / reg-name host = IP-literal / IPv4address / reg-name
The syntax rule for host is ambiguous because it does not completely The syntax rule for host is ambiguous because it does not completely
distinguish between an IPv4address and a reg-name. In order to distinguish between an IPv4address and a reg-name. In order to
disambiguate, the syntax, we apply the "first-match-wins" algorithm: disambiguate the syntax, we apply the "first-match-wins" algorithm:
If host matches the rule for IPv4address, then it should be If host matches the rule for IPv4address, then it should be
considered an IPv4 address literal and not a reg-name. Although host considered an IPv4 address literal and not a reg-name. Although host
is case-insensitive, producers and normalizers should use lowercase is case-insensitive, producers and normalizers should use lowercase
for registered names and hexadecimal addresses for the sake of for registered names and hexadecimal addresses for the sake of
uniformity, while only using uppercase letters for percent-encodings. uniformity, while only using uppercase letters for percent-encodings.
A host identified by an Internet Protocol literal address, version 6 A host identified by an Internet Protocol literal address, version 6
[RFC3513] or later, is distinguished by enclosing the IP literal [RFC3513] or later, is distinguished by enclosing the IP literal
within square brackets ("[" and "]"). This is the only place where within square brackets ("[" and "]"). This is the only place where
square bracket characters are allowed in the URI syntax. In square bracket characters are allowed in the URI syntax. In
anticipation of future, as-yet-undefined IP literal address formats, anticipation of future, as-yet-undefined IP literal address formats,
an optional version flag may be used to indicate such a format an optional version flag may be used to indicate such a format
explicitly rather than relying on heuristic determination. explicitly rather than relying on heuristic determination.
IP-literal = "[" ( IPv6address / IPvFuture ) "]" IP-literal = "[" ( IPv6address / IPvFuture ) "]"
IPvFuture = "v" 1*HEXDIG "." 1*( unreserved / sub-delims / ":" ) IPvFuture = "v" 1*HEXDIG "." 1*( unreserved / sub-delims / ":" )
The version flag does not indicate the IP version; rather, it The version flag does not indicate the IP version; rather, it
indicates future versions of the literal format. As such, indicates future versions of the literal format. As such,
implementations must not provide the version flag for existing IPv4 implementations must not provide the version flag for existing IPv4
and IPv6 literal addresses. If a URI containing an IP-literal that and IPv6 literal addresses. If a URI containing an IP-literal that
starts with "v" (case-insensitive), indicating that the version flag starts with "v" (case-insensitive), indicating that the version flag
is present, is dereferenced by an application that does not know the is present, is dereferenced by an application that does not know the
meaning of that version flag, then the application should return an meaning of that version flag, then the application should return an
appropriate error for "address mechanism not supported". appropriate error for "address mechanism not supported".
A host identified by an IPv6 literal address is represented inside A host identified by an IPv6 literal address is represented inside
the square brackets without a preceding version flag. The ABNF the square brackets without a preceding version flag. The ABNF
provided here is a translation of the text definition of an IPv6 provided here is a translation of the text definition of an IPv6
literal address provided in [RFC3513]. A 128-bit IPv6 address is literal address provided in [RFC3513]. A 128-bit IPv6 address is
divided into eight 16-bit pieces. Each piece is represented divided into eight 16-bit pieces. Each piece is represented
numerically in case-insensitive hexadecimal, using one to four numerically in case-insensitive hexadecimal, using one to four
hexadecimal digits (leading zeroes are permitted). The eight encoded hexadecimal digits (leading zeroes are permitted). The eight encoded
pieces are given most-significant first, separated by colon pieces are given most-significant first, separated by colon
characters. Optionally, the least-significant two pieces may instead characters. Optionally, the least-significant two pieces may instead
be represented in IPv4 address textual format. A sequence of one or be represented in IPv4 address textual format. A sequence of one or
more consecutive zero-valued 16-bit pieces within the address may be more consecutive zero-valued 16-bit pieces within the address may be
elided, omitting all their digits and leaving exactly two consecutive elided, omitting all their digits and leaving exactly two consecutive
colons in their place to mark the elision. colons in their place to mark the elision.
IPv6address = 6( h16 ":" ) ls32 IPv6address = 6( h16 ":" ) ls32
/ "::" 5( h16 ":" ) ls32 / "::" 5( h16 ":" ) ls32
/ [ h16 ] "::" 4( h16 ":" ) ls32 / [ h16 ] "::" 4( h16 ":" ) ls32
/ [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32 / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
/ [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32 / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
/ [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32 / [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32
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dec-octet = DIGIT ; 0-9 dec-octet = DIGIT ; 0-9
/ %x31-39 DIGIT ; 10-99 / %x31-39 DIGIT ; 10-99
/ "1" 2DIGIT ; 100-199 / "1" 2DIGIT ; 100-199
/ "2" %x30-34 DIGIT ; 200-249 / "2" %x30-34 DIGIT ; 200-249
/ "25" %x30-35 ; 250-255 / "25" %x30-35 ; 250-255
A host identified by a registered name is a sequence of characters A host identified by a registered name is a sequence of characters
that is usually intended for lookup within a locally-defined host or that is usually intended for lookup within a locally-defined host or
service name registry, though the URI's scheme-specific semantics may service name registry, though the URI's scheme-specific semantics may
require that a specific registry (or fixed name table) be used require that a specific registry (or fixed name table) be used
instead. The most common name registry mechanism is the Domain Name instead. The most common name registry mechanism is the Domain Name
System (DNS). A registered name intended for lookup in the DNS uses System (DNS). A registered name intended for lookup in the DNS uses
the syntax defined in Section 3.5 of [RFC1034] and Section 2.1 of the syntax defined in Section 3.5 of [RFC1034] and Section 2.1 of
[RFC1123]. Such a name consists of a sequence of domain labels [RFC1123]. Such a name consists of a sequence of domain labels
separated by ".", each domain label starting and ending with an separated by ".", each domain label starting and ending with an
alphanumeric character and possibly also containing "-" characters. alphanumeric character and possibly also containing "-" characters.
The rightmost domain label of a fully qualified domain name in DNS 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 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 is necessary to distinguish between the complete domain name and some
local domain. local domain.
reg-name = 0*255( unreserved / pct-encoded / sub-delims ) reg-name = *( unreserved / pct-encoded / sub-delims )
If the URI scheme defines a default for host, then that default If the URI scheme defines a default for host, then that default
applies when the host subcomponent is undefined or when the applies when the host subcomponent is undefined or when the
registered name is empty (zero length). For example, the "file" URI registered name is empty (zero length). For example, the "file" URI
scheme is defined such that no authority, an empty host, and scheme is defined such that no authority, an empty host, and
"localhost" all mean the end-user's machine, whereas the "http" "localhost" all mean the end-user's machine, whereas the "http"
scheme considers a missing authority or empty host to be invalid. scheme considers a missing authority or empty host to be invalid.
This specification does not mandate a particular registered name This specification does not mandate a particular registered name
lookup technology and therefore does not restrict the syntax of lookup technology and therefore does not restrict the syntax of
reg-name beyond that necessary for interoperability. Instead, it reg-name beyond that necessary for interoperability. Instead, it
delegates the issue of registered name syntax conformance to the delegates the issue of registered name syntax conformance to the
operating system of each application performing URI resolution, and operating system of each application performing URI resolution, and
that operating system decides what it will allow for the purpose of that operating system decides what it will allow for the purpose of
host identification. A URI resolution implementation might use DNS, host identification. A URI resolution implementation might use DNS,
host tables, yellow pages, NetInfo, WINS, or any other system for host tables, yellow pages, NetInfo, WINS, or any other system for
lookup of registered names. However, a globally-scoped naming system, lookup of registered names. However, a globally-scoped naming
such as DNS fully-qualified domain names, is necessary for URIs that system, such as DNS fully-qualified domain names, is necessary for
are intended to have global scope. URI producers should use names URIs that are intended to have global scope. URI producers should
that conform to the DNS syntax, even when use of DNS is not use names that conform to the DNS syntax, even when use of DNS is not
immediately apparent. immediately apparent, and should limit such names to no more than 255
characters in length.
The reg-name syntax allows percent-encoded octets in order to The reg-name syntax allows percent-encoded octets in order to
represent non-ASCII registered names in a uniform way that is represent non-ASCII registered names in a uniform way that is
independent of the underlying name resolution technology; such independent of the underlying name resolution technology; such
non-ASCII characters must first be encoded according to UTF-8 non-ASCII characters must first be encoded according to UTF-8 [STD63]
[RFC3629] and then each octet of the corresponding UTF-8 sequence and then each octet of the corresponding UTF-8 sequence must be
must be percent-encoded to be represented as URI characters. URI percent-encoded to be represented as URI characters. URI producing
producing applications must not use percent-encoding in host unless applications must not use percent-encoding in host unless it is used
it is used to represent a UTF-8 character sequence. When a non-ASCII to represent a UTF-8 character sequence. When a non-ASCII registered
registered name represents an internationalized domain name intended name represents an internationalized domain name intended for
for resolution via the DNS, the name must be transformed to the IDNA resolution via the DNS, the name must be transformed to the IDNA
encoding [RFC3490] prior to name lookup. URI producers should encoding [RFC3490] prior to name lookup. URI producers should
provide such registered names in the IDNA encoding, rather than a provide such registered names in the IDNA encoding, rather than a
percent-encoding, if they wish to maximize interoperability with percent-encoding, if they wish to maximize interoperability with
legacy URI resolvers. legacy URI resolvers.
3.2.3 Port 3.2.3 Port
The port subcomponent of authority is designated by an optional port The port subcomponent of authority is designated by an optional port
number in decimal following the host and delimited from it by a number in decimal following the host and delimited from it by a
single colon (":") character. single colon (":") character.
port = *DIGIT port = *DIGIT
A scheme may define a default port. For example, the "http" scheme A scheme may define a default port. For example, the "http" scheme
defines a default port of "80", corresponding to its reserved TCP defines a default port of "80", corresponding to its reserved TCP
port number. The type of port designated by the port number (e.g., 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 TCP, UDP, SCTP, etc.) is defined by the URI scheme. URI producers
and normalizers should omit the port component and its ":" delimiter 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 if port is empty or its value would be the same as the scheme's
default. default.
3.3 Path 3.3 Path
The path component contains data, usually organized in hierarchical The path component contains data, usually organized in hierarchical
form, that, along with data in the non-hierarchical query component form, that, along with data in the non-hierarchical query component
(Section 3.4), serves to identify a resource within the scope of the (Section 3.4), serves to identify a resource within the scope of the
URI's scheme and naming authority (if any). The path is terminated by URI's scheme and naming authority (if any). The path is terminated
the first question mark ("?") or number sign ("#") character, or by by the first question mark ("?") or number sign ("#") character, or
the end of the URI. by the end of the URI.
If a URI contains an authority component, then the path component If a URI contains an authority component, then the path component
must either be empty or begin with a slash ("/") character. If a URI must either be empty or begin with a slash ("/") character. If a URI
does not contain an authority component, then the path cannot begin does not contain an authority component, then the path cannot begin
with two slash characters ("//"). In addition, a URI reference with two slash characters ("//"). In addition, a URI reference
(Section 4.1) may begin with a relative path, in which case the first (Section 4.1) may begin with a relative path, in which case the first
path segment cannot contain a colon (":") character. The ABNF path segment cannot contain a colon (":") character. The ABNF
requires five separate rules to disambiguate these cases, only one of requires five separate rules to disambiguate these cases, only one of
which will match a given URI reference. We use the generic term which will match the path substring within a given URI reference. We
"path component" to describe the URI substring that is matched by the use the generic term "path component" to describe the URI substring
parser to one of these rules. matched by the parser to one of these rules.
path = path-abempty ; begins with "/" or is empty path = path-abempty ; begins with "/" or is empty
/ path-abs ; begins with "/" but not "//" / path-absolute ; begins with "/" but not "//"
/ path-noscheme ; begins with a non-colon segment / path-noscheme ; begins with a non-colon segment
/ path-rootless ; begins with a segment / path-rootless ; begins with a segment
/ path-empty ; zero characters / path-empty ; zero characters
path-abempty = *( "/" segment ) path-abempty = *( "/" segment )
path-abs = "/" [ segment-nz *( "/" segment ) ] path-absolute = "/" [ segment-nz *( "/" segment ) ]
path-noscheme = segment-nzc *( "/" segment ) path-noscheme = segment-nz-nc *( "/" segment )
path-rootless = segment-nz *( "/" segment ) path-rootless = segment-nz *( "/" segment )
path-empty = 0<pchar> path-empty = 0<pchar>
segment = *pchar segment = *pchar
segment-nz = 1*pchar segment-nz = 1*pchar
segment-nzc = 1*( unreserved / pct-encoded / sub-delims / "@" ) segment-nz-nc = 1*( unreserved / pct-encoded / sub-delims / "@" )
; non-zero-length segment without any colon ":"
pchar = unreserved / pct-encoded / sub-delims / ":" / "@" pchar = unreserved / pct-encoded / sub-delims / ":" / "@"
A path consists of a sequence of path segments separated by a slash A path consists of a sequence of path segments separated by a slash
("/") character. A path is always defined for a URI, though the ("/") character. A path is always defined for a URI, though the
defined path may be empty (zero length). Use of the slash character 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 to indicate hierarchy is only required when a URI will be used as the
context for relative references. For example, the URI context for relative references. For example, the URI
<mailto:fred@example.com> has a path of "fred@example.com", whereas <mailto:fred@example.com> has a path of "fred@example.com", whereas
the URI <foo://info.example.com?fred> has an empty path. the URI <foo://info.example.com?fred> has an empty path.
The path segments "." and "..", also known as dot-segments, are The path segments "." and "..", also known as dot-segments, are
defined for relative reference within the path name hierarchy. They defined for relative reference within the path name hierarchy. They
are intended for use at the beginning of a relative path reference are intended for use at the beginning of a relative path reference
(Section 4.2) for indicating relative position within the (Section 4.2) for indicating relative position within the
hierarchical tree of names. This is similar to their role within hierarchical tree of names. This is similar to their role within
some operating systems' file directory structure to indicate the some operating systems' file directory structure to indicate the
current directory and parent directory, respectively. However, unlike current directory and parent directory, respectively. However,
a file system, these dot-segments are only interpreted within the URI unlike a file system, these dot-segments are only interpreted within
path hierarchy and are removed as part of the resolution process the URI path hierarchy and are removed as part of the resolution
(Section 5.2). process (Section 5.2).
Aside from dot-segments in hierarchical paths, a path segment is Aside from dot-segments in hierarchical paths, a path segment is
considered opaque by the generic syntax. URI-producing applications considered opaque by the generic syntax. URI-producing applications
often use the reserved characters allowed in a segment for the often use the reserved characters allowed in a segment for the
purpose of delimiting scheme-specific or dereference-handler-specific purpose of delimiting scheme-specific or dereference-handler-specific
subcomponents. For example, the semicolon (";") and equals ("=") subcomponents. For example, the semicolon (";") and equals ("=")
reserved characters are often used for delimiting parameters and reserved characters are often used for delimiting parameters and
parameter values applicable to that segment. The comma (",") parameter values applicable to that segment. The comma (",")
reserved character is often used for similar purposes. For example, 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 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 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 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 defined by scheme-specific semantics, but in most cases the syntax of
a parameter is specific to the implementation of the URI's a parameter is specific to the implementation of the URI's
dereferencing algorithm. dereferencing algorithm.
3.4 Query 3.4 Query
The query component contains non-hierarchical data that, along with The query component contains non-hierarchical data that, along with
data in the path component (Section 3.3), serves to identify a data in the path component (Section 3.3), serves to identify a
resource within the scope of the URI's scheme and naming authority resource within the scope of the URI's scheme and naming authority
(if any). The query component is indicated by the first question mark (if any). The query component is indicated by the first question
("?") character and terminated by a number sign ("#") character or by mark ("?") character and terminated by a number sign ("#") character
the end of the URI. or by the end of the URI.
query = *( pchar / "/" / "?" ) query = *( pchar / "/" / "?" )
The characters slash ("/") and question mark ("?") may represent data The characters slash ("/") and question mark ("?") may represent data
within the query component. Beware that some older, erroneous within the query component. Beware that some older, erroneous
implementations do not handle such URIs correctly when they are used implementations do not handle such URIs correctly when they are used
as the base for relative references (Section 5.1), apparently because as the base for relative references (Section 5.1), apparently because
they fail to to distinguish query data from path data when looking they fail to to distinguish query data from path data when looking
for hierarchical separators. However, since query components are for hierarchical separators. However, since query components are
often used to carry identifying information in the form of often used to carry identifying information in the form of
"key=value" pairs, and one frequently used value is a reference to "key=value" pairs, and one frequently used value is a reference to
another URI, it is sometimes better for usability to avoid another URI, it is sometimes better for usability to avoid
percent-encoding those characters. percent-encoding those characters.
3.5 Fragment 3.5 Fragment
The fragment identifier component of a URI allows indirect The fragment identifier component of a URI allows indirect
identification of a secondary resource by reference to a primary identification of a secondary resource by reference to a primary
resource and additional identifying information. The identified resource and additional identifying information. The identified
secondary resource may be some portion or subset of the primary secondary resource may be some portion or subset of the primary
resource, some view on representations of the primary resource, or resource, some view on representations of the primary resource, or
some other resource defined or described by those representations. A some other resource defined or described by those representations. A
fragment identifier component is indicated by the presence of a fragment identifier component is indicated by the presence of a
number sign ("#") character and terminated by the end of the URI. number sign ("#") character and terminated by the end of the URI.
fragment = *( pchar / "/" / "?" ) fragment = *( pchar / "/" / "?" )
The semantics of a fragment identifier are defined by the set of The semantics of a fragment identifier are defined by the set of
representations that might result from a retrieval action on the representations that might result from a retrieval action on the
primary resource. The fragment's format and resolution is therefore primary resource. The fragment's format and resolution is therefore
dependent on the media type [RFC2046] of a potentially retrieved dependent on the media type [RFC2046] of a potentially retrieved
representation, even though such a retrieval is only performed if the representation, even though such a retrieval is only performed if the
URI is dereferenced. If no such representation exists, then the URI is dereferenced. If no such representation exists, then the
semantics of the fragment are considered unknown and, effectively, semantics of the fragment are considered unknown and, effectively,
unconstrained. Fragment identifier semantics are independent of the unconstrained. Fragment identifier semantics are independent of the
URI scheme and thus cannot be redefined by scheme specifications. URI scheme and thus cannot be redefined by scheme specifications.
Individual media types may define their own restrictions on, or Individual media types may define their own restrictions on, or
structure within, the fragment identifier syntax for specifying structure within, the fragment identifier syntax for specifying
different types of subsets, views, or external references that are different types of subsets, views, or external references that are
identifiable as secondary resources by that media type. If the identifiable as secondary resources by that media type. If the
primary resource has multiple representations, as is often the case primary resource has multiple representations, as is often the case
for resources whose representation is selected based on attributes of for resources whose representation is selected based on attributes of
the retrieval request (a.k.a., content negotiation), then whatever is the retrieval request (a.k.a., content negotiation), then whatever is
identified by the fragment should be consistent across all of those identified by the fragment should be consistent across all of those
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fragment such that it corresponds to the same secondary resource, fragment such that it corresponds to the same secondary resource,
regardless of how it is represented, or the fragment should be left regardless of how it is represented, or the fragment should be left
undefined by the representation (i.e., not found). undefined by the representation (i.e., not found).
As with any URI, use of a fragment identifier component does not 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 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 identifier may be used to refer to the secondary resource without any
implication that the primary resource is accessible or will ever be implication that the primary resource is accessible or will ever be
accessed. accessed.
Fragment identifiers have a special role in information systems as Fragment identifiers have a special role in information retrieval
the primary form of client-side indirect referencing, allowing an systems as the primary form of client-side indirect referencing,
author to specifically identify those aspects of an existing resource allowing an author to specifically identify those aspects of an
that are only indirectly provided by the resource owner. As such, the existing resource that are only indirectly provided by the resource
fragment identifier is not used in the scheme-specific processing of owner. As such, the fragment identifier is not used in the
a URI; instead, the fragment identifier is separated from the rest of scheme-specific processing of a URI; instead, the fragment identifier
the URI prior to a dereference, and thus the identifying information is separated from the rest of the URI prior to a dereference, and
within the fragment itself is dereferenced solely by the user agent thus the identifying information within the fragment itself is
and regardless of the URI scheme. Although this separate handling is dereferenced solely by the user agent and regardless of the URI
often perceived to be a loss of information, particularly in regards scheme. Although this separate handling is often perceived to be a
to accurate redirection of references as resources move over time, it loss of information, particularly in regards to accurate redirection
also serves to prevent information providers from denying reference of references as resources move over time, it also serves to prevent
authors the right to selectively refer to information within a information providers from denying reference authors the right to
resource. Indirect referencing also provides additional flexibility selectively refer to information within a resource. Indirect
and extensibility to systems that use URIs, since new media types are referencing also provides additional flexibility and extensibility to
easier to define and deploy than new schemes of identification. systems that use URIs, since new media types are easier to define and
deploy than new schemes of identification.
The characters slash ("/") and question mark ("?") are allowed to The characters slash ("/") and question mark ("?") are allowed to
represent data within the fragment identifier. Beware that some represent data within the fragment identifier. Beware that some
older, erroneous implementations do not handle such URIs correctly older, erroneous implementations do not handle such URIs correctly
when they are used as the base for relative references (Section 5.1). when they are used as the base for relative references (Section 5.1).
4. Usage 4. Usage
When applications make reference to a URI, they do not always use the 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 full form of reference defined by the "URI" syntax rule. In order to
save space and take advantage of hierarchical locality, many Internet save space and take advantage of hierarchical locality, many Internet
protocol elements and media type formats allow an abbreviation of a protocol elements and media type formats allow an abbreviation of a
URI, while others restrict the syntax to a particular form of URI. URI, while others restrict the syntax to a particular form of URI.
We define the most common forms of reference syntax in this We define the most common forms of reference syntax in this
specification because they impact and depend upon the design of the specification because they impact and depend upon the design of the
generic syntax, requiring a uniform parsing algorithm in order to be generic syntax, requiring a uniform parsing algorithm in order to be
interpreted consistently. interpreted consistently.
4.1 URI Reference 4.1 URI Reference
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4.2 Relative URI 4.2 Relative URI
A relative URI reference takes advantage of the hierarchical syntax A relative URI reference takes advantage of the hierarchical syntax
(Section 1.2.3) in order to express a reference that is relative to (Section 1.2.3) in order to express a reference that is relative to
the name space of another hierarchical URI. the name space of another hierarchical URI.
relative-URI = relative-part [ "?" query ] [ "#" fragment ] relative-URI = relative-part [ "?" query ] [ "#" fragment ]
relative-part = "//" authority path-abempty relative-part = "//" authority path-abempty
/ path-abs / path-absolute
/ path-noscheme / path-noscheme
/ path-empty / path-empty
The URI referred to by a relative reference, also known as the target The URI referred to by a relative reference, also known as the target
URI, is obtained by applying the reference resolution algorithm of URI, is obtained by applying the reference resolution algorithm of
Section 5. Section 5.
A relative reference that begins with two slash characters is termed A relative reference that begins with two slash characters is termed
a network-path reference; such references are rarely used. A relative a network-path reference; such references are rarely used. A
reference that begins with a single slash character is termed an relative reference that begins with a single slash character is
absolute-path reference. A relative reference that does not begin termed an absolute-path reference. A relative reference that does
with a slash character is termed a relative-path reference. not begin with a slash character is termed a relative-path reference.
A path segment that contains a colon character (e.g., "this:that") A path segment that contains a colon character (e.g., "this:that")
cannot be used as the first segment of a relative-path reference 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 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 be preceded by a dot-segment (e.g., "./this:that") to make a
relative-path reference. relative-path reference.
4.3 Absolute URI 4.3 Absolute URI
Some protocol elements allow only the absolute form of a URI without Some protocol elements allow only the absolute form of a URI without
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When a same-document reference is dereferenced for the purpose of a When a same-document reference is dereferenced for the purpose of a
retrieval action, the target of that reference is defined to be retrieval action, the target of that reference is defined to be
within the same entity (representation, document, or message) as the within the same entity (representation, document, or message) as the
reference; therefore, a dereference should not result in a new reference; therefore, a dereference should not result in a new
retrieval action. retrieval action.
Normalization of the base and target URIs prior to their comparison, 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 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 rarely performed in practice. Normalization may increase the set of
same-document references, which may be of benefit to some caching same-document references, which may be of benefit to some caching
applications. As such, reference authors should not assume that a applications. As such, reference authors should not assume that a
slightly different, though equivalent, reference URI will (or will slightly different, though equivalent, reference URI will (or will
not) be interpreted as a same-document reference by any given not) be interpreted as a same-document reference by any given
application. application.
4.5 Suffix Reference 4.5 Suffix Reference
The URI syntax is designed for unambiguous reference to resources and The URI syntax is designed for unambiguous reference to resources and
extensibility via the URI scheme. However, as URI identification and extensibility via the URI scheme. However, as URI identification and
usage have become commonplace, traditional media (television, radio, usage have become commonplace, traditional media (television, radio,
newspapers, billboards, etc.) have increasingly used a suffix of the newspapers, billboards, etc.) have increasingly used a suffix of the
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This section defines the process of resolving a URI reference within This section defines the process of resolving a URI reference within
a context that allows relative references, such that the result is a a context that allows relative references, such that the result is a
string matching the "URI" syntax rule of Section 3. string matching the "URI" syntax rule of Section 3.
5.1 Establishing a Base URI 5.1 Establishing a Base URI
The term "relative" implies that there exists a "base URI" against The term "relative" implies that there exists a "base URI" against
which the relative reference is applied. Aside from fragment-only which the relative reference is applied. Aside from fragment-only
references (Section 4.4), relative references are only usable when a references (Section 4.4), relative references are only usable when a
base URI is known. A base URI must be established by the parser base URI is known. A base URI must be established by the parser
prior to parsing URI references that might be relative. prior to parsing URI references that might be relative. A base URI
must conform to the <absolute-URI> syntax rule (Section 4.3): if the
base URI is obtained from a URI reference, then that reference must
be converted to absolute form and stripped of any fragment component
prior to use as a base URI.
The base URI of a reference can be established in one of four ways, 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 discussed below in order of precedence. The order of precedence can
be thought of in terms of layers, where the innermost defined base be thought of in terms of layers, where the innermost defined base
URI has the highest precedence. This can be visualized graphically URI has the highest precedence. This can be visualized graphically
as: as:
.----------------------------------------------------------. .----------------------------------------------------------.
| .----------------------------------------------------. | | .----------------------------------------------------. |
| | .----------------------------------------------. | | | | .----------------------------------------------. | |
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(e.g., the message and multipart types) is defined by MHTML (e.g., the message and multipart types) is defined by MHTML
[RFC2557]. Protocols that do not use the MIME message header syntax, [RFC2557]. Protocols that do not use the MIME message header syntax,
but do allow some form of tagged metadata to be included within 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 messages, may define their own syntax for defining a base URI as part
of a message. of a message.
5.1.3 Base URI from the Retrieval URI 5.1.3 Base URI from the Retrieval URI
If no base URI is embedded and the representation is not encapsulated 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 within some other entity, then, if a URI was used to retrieve the
representation, that URI shall be considered the base URI. Note that representation, that URI shall be considered the base URI. Note that
if the retrieval was the result of a redirected request, the last URI 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 used (i.e., the URI that resulted in the actual retrieval of the
representation) is the base URI. representation) is the base URI.
5.1.4 Default Base URI 5.1.4 Default Base URI
If none of the conditions described above apply, then the base URI is If none of the conditions described above apply, then the base URI is
defined by the context of the application. Since this definition is defined by the context of the application. Since this definition is
necessarily application-dependent, failing to define a base URI using necessarily application-dependent, failing to define a base URI using
one of the other methods may result in the same content being one of the other methods may result in the same content being
interpreted differently by different types of application. interpreted differently by different types of application.
A sender of a representation containing relative references is A sender of a representation containing relative references is
responsible for ensuring that a base URI for those references can be responsible for ensuring that a base URI for those references can be
established. Aside from fragment-only references, relative references established. Aside from fragment-only references, relative
can only be used reliably in situations where the base URI is references can only be used reliably in situations where the base URI
well-defined. is well-defined.
5.2 Relative Resolution 5.2 Relative Resolution
This section describes an algorithm for converting a URI reference This section describes an algorithm for converting a URI reference
that might be relative to a given base URI into the parsed components that might be relative to a given base URI into the parsed components
of the reference's target. The components can then be recomposed, as of the reference's target. The components can then be recomposed, as
described in Section 5.3, to form the target URI. This algorithm described in Section 5.3, to form the target URI. This algorithm
provides definitive results that can be used to test the output of provides definitive results that can be used to test the output of
other implementations. Applications may implement relative reference other implementations. Applications may implement relative reference
resolution using some other algorithm, provided that the results resolution using some other algorithm, provided that the results
match what would be given by this algorithm. match what would be given by this algorithm.
5.2.1 Pre-parse the Base URI 5.2.1 Pre-parse the Base URI
The base URI (Base) is established according to the procedure of The base URI (Base) is established according to the procedure of
Section 5.1 and parsed into the five main components described in Section 5.1 and parsed into the five main components described in
Section 3. Note that only the scheme component is required to be Section 3. Note that only the scheme component is required to be
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forming the target URI. Although there are many ways to accomplish forming the target URI. Although there are many ways to accomplish
this removal process, we describe a simple method using two string this removal process, we describe a simple method using two string
buffers. buffers.
1. The input buffer is initialized with the now-appended path 1. The input buffer is initialized with the now-appended path
components and the output buffer is initialized to the empty components and the output buffer is initialized to the empty
string. string.
2. While the input buffer is not empty, loop: 2. While the input buffer is not empty, loop:
a. If the input buffer begins with a prefix of "../" or "./", A. If the input buffer begins with a prefix of "../" or "./",
then remove that prefix from the input buffer; otherwise, then remove that prefix from the input buffer; otherwise,
b. If the input buffer begins with a prefix of "/./" or "/.", B. If the input buffer begins with a prefix of "/./" or "/.",
where "." is a complete path segment, then replace that where "." is a complete path segment, then replace that
prefix with "/" in the input buffer; otherwise, prefix with "/" in the input buffer; otherwise,
c. If the input buffer begins with a prefix of "/../" or "/..", C. If the input buffer begins with a prefix of "/../" or "/..",
where ".." is a complete path segment, then replace that where ".." is a complete path segment, then replace that
prefix with "/" in the input buffer and remove the last prefix with "/" in the input buffer and remove the last
segment and its preceding "/" (if any) from the output segment and its preceding "/" (if any) from the output
buffer; otherwise, buffer; otherwise,
d. If the input buffer consists only of "." or "..", then remove D. If the input buffer consists only of "." or "..", then remove
that from the input buffer; otherwise, that from the input buffer; otherwise,
e. Move the first path segment in the input buffer to the end of E. Move the first path segment in the input buffer to the end of
the output buffer, including the initial "/" character (if the output buffer, including the initial "/" character (if
any) and any subsequent characters up to, but not including, any) and any subsequent characters up to, but not including,
the next "/" character or the end of the input buffer. the next "/" character or the end of the input buffer.
3. Finally, the output buffer is returned as the result of 3. Finally, the output buffer is returned as the result of
remove_dot_segments. remove_dot_segments.
Note that dot-segments are intended for use in URI references to Note that dot-segments are intended for use in URI references to
express an identifier relative to the hierarchy of names in the base express an identifier relative to the hierarchy of names in the base
URI. The remove_dot_segments algorithm respects that hierarchy by URI. The remove_dot_segments algorithm respects that hierarchy by
removing extra dot-segments rather than treating them as an error or removing extra dot-segments rather than treating them as an error or
leaving them to be misinterpreted by dereference implementations. leaving them to be misinterpreted by dereference implementations.
The following illustrates how the above steps are applied for two The following illustrates how the above steps are applied for two
example merged paths, showing the state of the two buffers after each example merged paths, showing the state of the two buffers after each
step. step.
STEP OUTPUT BUFFER INPUT BUFFER STEP OUTPUT BUFFER INPUT BUFFER
1 : /a/b/c/./../../g 1 : /a/b/c/./../../g
2e: /a /b/c/./../../g 2E: /a /b/c/./../../g
2e: /a/b /c/./../../g 2E: /a/b /c/./../../g
2e: /a/b/c /./../../g 2E: /a/b/c /./../../g
2b: /a/b/c /../../g 2B: /a/b/c /../../g
2c: /a/b /../g 2C: /a/b /../g
2c: /a /g 2C: /a /g
2e: /a/g 2E: /a/g
STEP OUTPUT BUFFER INPUT BUFFER STEP OUTPUT BUFFER INPUT BUFFER
1 : mid/content=5/../6 1 : mid/content=5/../6
2e: mid /content=5/../6 2E: mid /content=5/../6
2e: mid/content=5 /../6 2E: mid/content=5 /../6
2c: mid /6 2C: mid /6
2e: mid/6 2E: mid/6
Some applications may find it more efficient to implement the Some applications may find it more efficient to implement the
remove_dot_segments algorithm using two segment stacks rather than remove_dot_segments algorithm using two segment stacks rather than
strings. strings.
Note: Beware that some older, erroneous implementations will fail Note: Beware that some older, erroneous implementations will fail
to separate a reference's query component from its path component to separate a reference's query component from its path component
prior to merging the base and reference paths, resulting in an prior to merging the base and reference paths, resulting in an
interoperability failure if the query component contains the interoperability failure if the query component contains the
strings "/../" or "/./". strings "/../" or "/./".
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relative references. relative references.
"g?y/./x" = "http://a/b/c/g?y/./x" "g?y/./x" = "http://a/b/c/g?y/./x"
"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"
"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 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 reference if it is the same as the base URI scheme. This is
considered to be a loophole in prior specifications of partial URI considered to be a loophole in prior specifications of partial URI
[RFC1630]. Its use should be avoided, but is allowed for backward [RFC1630]. Its use should be avoided, but is allowed for backward
compatibility. compatibility.
"http:g" = "http:g" ; for strict parsers "http:g" = "http:g" ; for strict parsers
/ "http://a/b/c/g" ; for backward compatibility / "http://a/b/c/g" ; for backward compatibility
6. Normalization and Comparison 6. Normalization and Comparison
One of the most common operations on URIs is simple comparison: One of the most common operations on URIs is simple comparison:
determining if two URIs are equivalent without using the URIs to determining if two URIs are equivalent without using the URIs to
access their respective resource(s). A comparison is performed every access their respective resource(s). A comparison is performed every
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canonical form for URI references is defined to reduce the occurrence canonical form for URI references is defined to reduce the occurrence
of false negative comparisons. of false negative comparisons.
6.1 Equivalence 6.1 Equivalence
Since URIs exist to identify resources, presumably they should be Since URIs exist to identify resources, presumably they should be
considered equivalent when they identify the same resource. However, considered equivalent when they identify the same resource. However,
such a definition of equivalence is not of much practical use, since such a definition of equivalence is not of much practical use, since
there is no way for software to compare two resources without there is no way for software to compare two resources without
knowledge of the implementation-specific syntax of each URI's knowledge of the implementation-specific syntax of each URI's
dereferencing algorithm. For this reason, determination of dereferencing algorithm. For this reason, determination of
equivalence or difference of URIs is based on string comparison, equivalence or difference of URIs is based on string comparison,
perhaps augmented by reference to additional rules provided by URI perhaps augmented by reference to additional rules provided by URI
scheme definitions. We use the terms "different" and "equivalent" to scheme definitions. We use the terms "different" and "equivalent" to
describe the possible outcomes of such comparisons, but there are describe the possible outcomes of such comparisons, but there are
many application-dependent versions of equivalence. many application-dependent versions of equivalence.
Even though it is possible to determine that two URIs are equivalent, Even though it is possible to determine that two URIs are equivalent,
it is never possible to be sure that two URIs identify different it is never possible to be sure that two URIs identify different
resources. For example, an owner of two different domain names could resources. For example, an owner of two different domain names could
decide to serve the same resource from both, resulting in two decide to serve the same resource from both, resulting in two
different URIs. Therefore, comparison methods are designed to different URIs. Therefore, comparison methods are designed to
minimize false negatives while strictly avoiding false positives. minimize false negatives while strictly avoiding false positives.
In testing for equivalence, applications should not directly compare In testing for equivalence, applications should not directly compare
relative URI references; the references should be converted to their relative URI references; the references should be converted to their
target URI forms before comparison. When URIs are being compared for target URI forms before comparison. When URIs are being compared for
the purpose of selecting (or avoiding) a network action, such as the purpose of selecting (or avoiding) a network action, such as
retrieval of a representation, the fragment components (if any) retrieval of a representation, the fragment components (if any)
should be excluded from the comparison. should be excluded from the comparison.
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producing false negatives, and proceeding to those that have higher producing false negatives, and proceeding to those that have higher
computational cost and lower risk of false negatives. computational cost and lower risk of false negatives.
6.2.1 Simple String Comparison 6.2.1 Simple String Comparison
If two URIs, considered as character strings, are identical, then it If two URIs, considered as character strings, are identical, then it
is safe to conclude that they are equivalent. This type of is safe to conclude that they are equivalent. This type of
equivalence test has very low computational cost and is in wide use equivalence test has very low computational cost and is in wide use
in a variety of applications, particularly in the domain of parsing. in a variety of applications, particularly in the domain of parsing.
Testing strings for equivalence requires some basic precautions. This Testing strings for equivalence requires some basic precautions.
procedure is often referred to as "bit-for-bit" or "byte-for-byte" This procedure is often referred to as "bit-for-bit" or
comparison, which is potentially misleading. Testing of strings for "byte-for-byte" comparison, which is potentially misleading. Testing
equality is normally based on pairwise comparison of the characters of strings for equality is normally based on pairwise comparison of
that make up the strings, starting from the first and proceeding the characters that make up the strings, starting from the first and
until both strings are exhausted and all characters found to be proceeding until both strings are exhausted and all characters found
equal, a pair of characters compares unequal, or one of the strings to be equal, a pair of characters compares unequal, or one of the
is exhausted before the other. strings is exhausted before the other.
Such character comparisons require that each pair of characters be Such character comparisons require that each pair of characters be
put in comparable form. For example, should one URI be stored in a 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 byte array in EBCDIC encoding, and the second be in a Java String
object (UTF-16), bit-for-bit comparisons applied naively will produce object (UTF-16), bit-for-bit comparisons applied naively will produce
errors. It is better to speak of equality on a errors. It is better to speak of equality on a
character-for-character rather than byte-for-byte or bit-for-bit character-for-character rather than byte-for-byte or bit-for-bit
basis. In practical terms, character-by-character comparisons should basis. In practical terms, character-by-character comparisons should
be done codepoint-by-codepoint after conversion to a common character be done codepoint-by-codepoint after conversion to a common character
encoding. encoding.
6.2.2 Syntax-based Normalization 6.2.2 Syntax-based Normalization
Software may use logic based on the definitions provided by this Software may use logic based on the definitions provided by this
specification to reduce the probability of false negatives. Such specification to reduce the probability of false negatives. Such
processing is moderately higher in cost than character-for-character processing is moderately higher in cost than character-for-character
string comparison. For example, an application using this approach string comparison. For example, an application using this approach
could reasonably consider the following two URIs equivalent: could reasonably consider the following two URIs equivalent:
example://a/b/c/%7Bfoo%7D example://a/b/c/%7Bfoo%7D
eXAMPLE://a/./b/../b/%63/%7bfoo%7d eXAMPLE://a/./b/../b/%63/%7bfoo%7d
Web user agents, such as browsers, typically apply this type of URI Web user agents, such as browsers, typically apply this type of URI
normalization when determining whether a cached response is normalization when determining whether a cached response is
available. Syntax-based normalization includes such techniques as available. Syntax-based normalization includes such techniques as
case normalization, percent-encoding normalization, and removal of case normalization, percent-encoding normalization, and removal of
dot-segments. dot-segments.
6.2.2.1 Case Normalization 6.2.2.1 Case Normalization
When a URI scheme uses components of the generic syntax, it will also When a URI scheme uses components of the generic syntax, it will also
use the common syntax equivalence rules, namely that the scheme and use the common syntax equivalence rules, namely that the scheme and
host are case-insensitive and therefore should be normalized to host are case-insensitive and therefore should be normalized to
lowercase. For example, the URI <HTTP://www.EXAMPLE.com/> is lowercase. For example, the URI <HTTP://www.EXAMPLE.com/> is
equivalent to <http://www.example.com/>. Applications should not equivalent to <http://www.example.com/>. Applications should not
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since that is dependent on the implementation used to handle a since that is dependent on the implementation used to handle a
dereference. dereference.
The hexadecimal digits within a percent-encoding triplet (e.g., "%3a" The hexadecimal digits within a percent-encoding triplet (e.g., "%3a"
versus "%3A") are case-insensitive and therefore should be normalized versus "%3A") are case-insensitive and therefore should be normalized
to use uppercase letters for the digits A-F. to use uppercase letters for the digits A-F.
6.2.2.2 Percent-Encoding Normalization 6.2.2.2 Percent-Encoding Normalization
The percent-encoding mechanism (Section 2.1) is a frequent source of The percent-encoding mechanism (Section 2.1) is a frequent source of
variance among otherwise identical URIs. In addition to the variance among otherwise identical URIs. In addition to the
case-insensitivity issue noted above, some URI producers case-insensitivity issue noted above, some URI producers
percent-encode octets that do not require percent-encoding, resulting percent-encode octets that do not require percent-encoding, resulting
in URIs that are equivalent to their non-encoded counterparts. Such in URIs that are equivalent to their non-encoded counterparts. Such
URIs should be normalized by decoding any percent-encoded octet that URIs should be normalized by decoding any percent-encoded octet that
corresponds to an unreserved character, as described in Section 2.3. corresponds to an unreserved character, as described in Section 2.3.
6.2.2.3 Path Segment Normalization 6.2.2.3 Path Segment Normalization
The complete path segments "." and ".." have a special meaning within The complete path segments "." and ".." have a special meaning within
hierarchical URI schemes. As such, they should not appear in hierarchical URI schemes. As such, they should not appear in
absolute paths; if they are found, they can be removed by applying absolute paths; if they are found, they can be removed by applying
the remove_dot_segments algorithm to the path, as described in the remove_dot_segments algorithm to the path, as described in
Section 5.2. Section 5.2.
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http://example.com http://example.com
http://example.com/ http://example.com/
http://example.com:/ http://example.com:/
http://example.com:80/ http://example.com:80/
In general, a URI that uses the generic syntax for authority with an In general, a URI that uses the generic syntax for authority with an
empty path should be normalized to a path of "/"; likewise, an empty path should be normalized to a path of "/"; likewise, an
explicit ":port", where the port is empty or the default for the explicit ":port", where the port is empty or the default for the
scheme, is equivalent to one where the port and its ":" delimiter are 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 elided. In other words, the second of the above URI examples is the
normal form for the "http" scheme. normal form for the "http" scheme.
Another case where normalization varies by scheme is in the handling Another case where normalization varies by scheme is in the handling
of an empty authority component or empty host subcomponent. For many of an empty authority component or empty host subcomponent. For many
scheme specifications, an empty authority or host is considered an scheme specifications, an empty authority or host is considered an
error; for others, it is considered equivalent to "localhost" or the error; for others, it is considered equivalent to "localhost" or the
end-user's host. When a scheme defines a default for authority and a end-user's host. When a scheme defines a default for authority and a
URI reference to that default is desired, the reference should have URI reference to that default is desired, the reference should have
an empty authority for the sake of uniformity, brevity, and an empty authority for the sake of uniformity, brevity, and
internationalization. If, however, either the userinfo or port internationalization. If, however, either the userinfo or port
subcomponent is non-empty, then the host should be given explicitly subcomponent is non-empty, then the host should be given explicitly
even if it matches the default. even if it matches the default.
6.2.4 Protocol-based Normalization 6.2.4 Protocol-based Normalization
Web spiders, for which substantial effort to reduce the incidence of Web spiders, for which substantial effort to reduce the incidence of
false negatives is often cost-effective, are observed to implement false negatives is often cost-effective, are observed to implement
even more aggressive techniques in URI comparison. For example, if even more aggressive techniques in URI comparison. For example, if
they observe that a URI such as they observe that a URI such as
http://example.com/data http://example.com/data
redirects to a URI differing only in the trailing slash redirects to a URI differing only in the trailing slash
http://example.com/data/ http://example.com/data/
they will likely regard the two as equivalent in the future. This they will likely regard the two as equivalent in the future. This
kind of technique is only appropriate when equivalence is clearly kind of technique is only appropriate when equivalence is clearly
indicated by both the result of accessing the resources and the indicated by both the result of accessing the resources and the
common conventions of their scheme's dereference algorithm (in this common conventions of their scheme's dereference algorithm (in this
case, use of redirection by HTTP origin servers to avoid problems case, use of redirection by HTTP origin servers to avoid problems
with relative references). with relative references).
6.3 Canonical Form 6.3 Canonical Form
It is in the best interests of everyone concerned to avoid It is in the best interests of everyone concerned to avoid
false-negatives in comparing URIs and to minimize the amount of false-negatives in comparing URIs and to minimize the amount of
skipping to change at page 40, line 43 skipping to change at page 41, line 36
are often used to provide a compact set of instructions for access to are often used to provide a compact set of instructions for access to
network resources, care must be taken to properly interpret the data network resources, care must be taken to properly interpret the data
within a URI, to prevent that data from causing unintended access, within a URI, to prevent that data from causing unintended access,
and to avoid including data that should not be revealed in plain and to avoid including data that should not be revealed in plain
text. text.
7.1 Reliability and Consistency 7.1 Reliability and Consistency
There is no guarantee that, having once used a given URI to retrieve There is no guarantee that, having once used a given URI to retrieve
some information, the same information will be retrievable by that some information, the same information will be retrievable by that
URI in the future. Nor is there any guarantee that the information URI in the future. Nor is there any guarantee that the information
retrievable via that URI in the future will be observably similar to 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 that retrieved in the past. The URI syntax does not constrain how a
given scheme or authority apportions its name space or maintains it given scheme or authority apportions its name space or maintains it
over time. Such a guarantee can only be obtained from the person(s) over time. Such a guarantee can only be obtained from the person(s)
controlling that name space and the resource in question. A specific controlling that name space and the resource in question. A specific
URI scheme may define additional semantics, such as name persistence, URI scheme may define additional semantics, such as name persistence,
if those semantics are required of all naming authorities for that if those semantics are required of all naming authorities for that
scheme. scheme.
7.2 Malicious Construction 7.2 Malicious Construction
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running a different protocol service and data within the URI contains running a different protocol service and data within the URI contains
instructions that, when interpreted according to this other protocol, instructions that, when interpreted according to this other protocol,
cause an unexpected operation. A frequent example of such abuse has cause an unexpected operation. A frequent example of such abuse has
been the use of a protocol-based scheme with a port component of been the use of a protocol-based scheme with a port component of
"25", thereby fooling user agent software into sending an unintended "25", thereby fooling user agent software into sending an unintended
or impersonating message via an SMTP server. or impersonating message via an SMTP server.
Applications should prevent dereference of a URI that specifies a TCP Applications should prevent dereference of a URI that specifies a TCP
port number within the "well-known port" range (0 - 1023) unless the port number within the "well-known port" range (0 - 1023) unless the
protocol being used to dereference that URI is compatible with the protocol being used to dereference that URI is compatible with the
protocol expected on that well-known port. Although IANA maintains a protocol expected on that well-known port. Although IANA maintains a
registry of well-known ports, applications should make such registry of well-known ports, applications should make such
restrictions user-configurable to avoid preventing the deployment of restrictions user-configurable to avoid preventing the deployment of
new services. new services.
When a URI contains percent-encoded octets that match the delimiters When a URI contains percent-encoded octets that match the delimiters
for a given resolution or dereference protocol (for example, CR and for a given resolution or dereference protocol (for example, CR and
LF characters for the TELNET protocol), such percent-encoded octets LF characters for the TELNET protocol), such percent-encoded octets
must not be decoded before transmission across that protocol. must not be decoded before transmission across that protocol.
Transfer of the percent-encoding, which might violate the protocol, Transfer of the percent-encoding, which might violate the protocol,
is less harmful than allowing decoded octets to be interpreted as is less harmful than allowing decoded octets to be interpreted as
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7.3 Back-end Transcoding 7.3 Back-end Transcoding
When a URI is dereferenced, the data within it is often parsed by 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 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, typical user agent will parse a URI into its five major components,
access the authority's server, and send it the data within the access the authority's server, and send it the data within the
authority, path, and query components. A typical server will take authority, path, and query components. A typical server will take
that information, parse the path into segments and the query into that information, parse the path into segments and the query into
key/value pairs, and then invoke implementation-specific handlers to key/value pairs, and then invoke implementation-specific handlers to
respond to the request. As a result, a common security concern for respond to the request. As a result, a common security concern for
server implementations that handle a URI, either as a whole or split server implementations that handle a URI, either as a whole or split
into separate components, is proper interpretation of the octet data into separate components, is proper interpretation of the octet data
represented by the characters and percent-encodings within that URI. represented by the characters and percent-encodings within that URI.
Percent-encoded octets must be decoded at some point during the Percent-encoded octets must be decoded at some point during the
dereference process. Applications must split the URI into its dereference process. Applications must split the URI into its
components and subcomponents prior to decoding the octets, since components and subcomponents prior to decoding the octets, since
otherwise the decoded octets might be mistaken for delimiters. otherwise the decoded octets might be mistaken for delimiters.
Security checks of the data within a URI should be applied after Security checks of the data within a URI should be applied after
decoding the octets. Note, however, that the "%00" percent-encoding decoding the octets. Note, however, that the "%00" percent-encoding
(NUL) may require special handling and should be rejected if the (NUL) may require special handling and should be rejected if the
application is not expecting to receive raw data within a component. application is not expecting to receive raw data within a component.
Special care should be taken when the URI path interpretation process Special care should be taken when the URI path interpretation process
involves the use of a back-end filesystem or related system involves the use of a back-end filesystem or related system
functions. Filesystems typically assign an operational meaning to functions. Filesystems typically assign an operational meaning to
special characters, such as the "/", "\", ":", "[", and "]" special characters, such as the "/", "\", ":", "[", and "]"
characters, and special device names like ".", "..", "...", "aux", characters, and special device names like ".", "..", "...", "aux",
"lpt", etc. In some cases, merely testing for the existence of such a "lpt", etc. In some cases, merely testing for the existence of such
name will cause the operating system to pause or invoke unrelated a name will cause the operating system to pause or invoke unrelated
system calls, leading to significant security concerns regarding system calls, leading to significant security concerns regarding
denial of service and unintended data transfer. It would be denial of service and unintended data transfer. It would be
impossible for this specification to list all such significant impossible for this specification to list all such significant
characters and device names; implementers should research the characters and device names; implementers should research the
reserved names and characters for the types of storage device that reserved names and characters for the types of storage device that
may be attached to their application and restrict the use of data may be attached to their application and restrict the use of data
obtained from URI components accordingly. obtained from URI components accordingly.
7.4 Rare IP Address Formats 7.4 Rare IP Address Formats
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dotted-decimal form of IPv4 address literal, many implementations dotted-decimal form of IPv4 address literal, many implementations
that process URIs make use of platform-dependent system routines, that process URIs make use of platform-dependent system routines,
such as gethostbyname() and inet_aton(), to translate the string such as gethostbyname() and inet_aton(), to translate the string
literal to an actual IP address. Unfortunately, such system routines literal to an actual IP address. Unfortunately, such system routines
often allow and process a much larger set of formats than those often allow and process a much larger set of formats than those
described in Section 3.2.2. described in Section 3.2.2.
For example, many implementations allow dotted forms of three For example, many implementations allow dotted forms of three
numbers, wherein the last part is interpreted as a 16-bit quantity 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., 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 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 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 most three bytes of the network address (Class A), and a single
number (without dots) is interpreted as a 32-bit quantity and stored number (without dots) is interpreted as a 32-bit quantity and stored
directly in the network address. Adding further to the confusion, directly in the network address. Adding further to the confusion,
some implementations allow each dotted part to be interpreted as some implementations allow each dotted part to be interpreted as
decimal, octal, or hexadecimal, as specified in the C language (i.e., decimal, octal, or hexadecimal, as specified in the C language (i.e.,
a leading 0x or 0X implies hexadecimal; otherwise, a leading 0 a leading 0x or 0X implies hexadecimal; otherwise, a leading 0
implies octal; otherwise, the number is interpreted as decimal). implies octal; otherwise, the number is interpreted as decimal).
These additional IP address formats are not allowed in the URI syntax These additional IP address formats are not allowed in the URI syntax
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access to resources based on the IP address in string literal format. access to resources based on the IP address in string literal format.
If such filtering is performed, literals should be converted to If such filtering is performed, literals should be converted to
numeric form and filtered based on the numeric value, rather than a numeric form and filtered based on the numeric value, rather than a
prefix or suffix of the string form. prefix or suffix of the string form.
7.5 Sensitive Information 7.5 Sensitive Information
URI producers should not provide a URI that contains a username or URI producers should not provide a URI that contains a username or
password which is intended to be secret: URIs are frequently password which is intended to be secret: URIs are frequently
displayed by browsers, stored in clear text bookmarks, and logged by displayed by browsers, stored in clear text bookmarks, and logged by
user agent history and intermediary applications (proxies). A user agent history and intermediary applications (proxies). A
password appearing within the userinfo component is deprecated and password appearing within the userinfo component is deprecated and
should be considered an error (or simply ignored) except in those should be considered an error (or simply ignored) except in those
rare cases where the 'password' parameter is intended to be public. rare cases where the 'password' parameter is intended to be public.
7.6 Semantic Attacks 7.6 Semantic Attacks
Because the userinfo subcomponent is rarely used and appears before Because the userinfo subcomponent is rarely used and appears before
the host in the authority component, it can be used to construct a 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 URI that is intended to mislead a human user by appearing to identify
one (trusted) naming authority while actually identifying a different one (trusted) naming authority while actually identifying a different
skipping to change at page 43, line 43 skipping to change at page 44, line 35
might lead a human user to assume that the host is 'cnn.example.com', might lead a human user to assume that the host is 'cnn.example.com',
whereas it is actually '10.0.0.1'. Note that a misleading userinfo whereas it is actually '10.0.0.1'. Note that a misleading userinfo
subcomponent could be much longer than the example above. subcomponent could be much longer than the example above.
A misleading URI, such as the one above, is an attack on the user's 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 preconceived notions about the meaning of a URI, rather than an
attack on the software itself. User agents may be able to reduce the attack on the software itself. User agents may be able to reduce the
impact of such attacks by distinguishing the various components of impact of such attacks by distinguishing the various components of
the URI when rendered, such as by using a different color or tone to 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 render userinfo if any is present, though there is no general
panacea. More information on URI-based semantic attacks can be found panacea. More information on URI-based semantic attacks can be found
in [Siedzik]. in [Siedzik].
8. Acknowledgments 8. IANA Considerations
URI scheme names, as defined by <scheme> in Section 3.1, form a
registered name space that is managed by IANA according to the
procedures defined in [BCP35].
9. Acknowledgments
This specification is derived from RFC 2396 [RFC2396], RFC 1808 This specification is derived from RFC 2396 [RFC2396], RFC 1808
[RFC1808], and RFC 1738 [RFC1738]; the acknowledgments in those [RFC1808], and RFC 1738 [RFC1738]; the acknowledgments in those
documents still apply. It also incorporates the update (with documents still apply. It also incorporates the update (with
corrections) for IPv6 literals in the host syntax, as defined by corrections) for IPv6 literals in the host syntax, as defined by
Robert M. Hinden, Brian E. Carpenter, and Larry Masinter in Robert M. Hinden, Brian E. Carpenter, and Larry Masinter in
[RFC2732]. In addition, contributions by Gisle Aas, Reese Anschultz, [RFC2732]. In addition, contributions by Gisle Aas, Reese Anschultz,
Daniel Barclay, Tim Bray, Mike Brown, Rob Cameron, Jeremy Carroll, Daniel Barclay, Tim Bray, Mike Brown, Rob Cameron, Jeremy Carroll,
Dan Connolly, Adam M. Costello, John Cowan, Jason Diamond, Martin Dan Connolly, Adam M. Costello, John Cowan, Jason Diamond, Martin
Duerst, Stefan Eissing, Clive D.W. Feather, Tony Hammond, Pat Hayes, Duerst, Stefan Eissing, Clive D.W. Feather, Al Gilman, Tony Hammond,
Henry Holtzman, Ian B. Jacobs, Michael Kay, John C. Klensin, Graham Elliotte Harold, Pat Hayes, Henry Holtzman, Ian B. Jacobs, Michael
Klyne, Dan Kohn, Bruce Lilly, Andrew Main, Ira McDonald, Michael Kay, John C. Klensin, Graham Klyne, Dan Kohn, Bruce Lilly, Andrew
Mealling, Ray Merkert, Stephen Pollei, Julian Reschke, Tomas Rokicki, Main, Dave McAlpin, Ira McDonald, Michael Mealling, Ray Merkert,
Miles Sabin, Kai Schaetzl, Mark Thomson, Ronald Tschalaer, Norm Stephen Pollei, Julian Reschke, Tomas Rokicki, Miles Sabin, Kai
Walsh, Marc Warne, Stuart Williams, and Henry Zongaro are gratefully Schaetzl, Mark Thomson, Ronald Tschalaer, Norm Walsh, Marc Warne,
acknowledged. Stuart Williams, and Henry Zongaro are gratefully acknowledged.
9. References 10. References
9.1 Normative References 10.1 Normative References
[ASCII] American National Standards Institute, "Coded Character [ASCII] American National Standards Institute, "Coded Character
Set -- 7-bit American Standard Code for Information Set -- 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986. Interchange", ANSI X3.4, 1986.
[RFC2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax [RFC2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997. Specifications: ABNF", RFC 2234, November 1997.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO [STD63] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003. 10646", STD 63, RFC 3629, November 2003.
9.2 Informative References [UCS] International Organization for Standardization,
"Information Technology - Universal Multiple-Octet Coded
Character Set (UCS)", ISO/IEC 10646:2003, December 2003.
10.2 Informative References
[BCP19] Freed, N. and J. Postel, "IANA Charset Registration
Procedures", BCP 19, RFC 2978, October 2000.
[BCP35] Petke, R. and I. King, "Registration Procedures for URL
Scheme Names", BCP 35, RFC 2717, November 1999.
[RFC0952] Harrenstien, K., Stahl, M. and E. Feinler, "DoD Internet [RFC0952] Harrenstien, K., Stahl, M. and E. Feinler, "DoD Internet
host table specification", RFC 952, October 1985. host table specification", RFC 952, October 1985.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987. STD 13, RFC 1034, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application [RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989. and Support", STD 3, RFC 1123, October 1989.
skipping to change at page 46, line 7 skipping to change at page 46, line 28
[RFC1808] Fielding, R., "Relative Uniform Resource Locators", RFC [RFC1808] Fielding, R., "Relative Uniform Resource Locators", RFC
1808, June 1995. 1808, June 1995.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail [RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046, Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996. November 1996.
[RFC2141] Moats, R., "URN Syntax", RFC 2141, May 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 [RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396, Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998. August 1998.
[RFC2518] Goland, Y., Whitehead, E., Faizi, A., Carter, S. and D. [RFC2518] Goland, Y., Whitehead, E., Faizi, A., Carter, S. and D.
Jensen, "HTTP Extensions for Distributed Authoring -- Jensen, "HTTP Extensions for Distributed Authoring --
WEBDAV", RFC 2518, February 1999. WEBDAV", RFC 2518, February 1999.
[RFC2557] Palme, F., Hopmann, A., Shelness, N. and E. Stefferud, [RFC2557] Palme, F., Hopmann, A., Shelness, N. and E. Stefferud,
"MIME Encapsulation of Aggregate Documents, such as HTML "MIME Encapsulation of Aggregate Documents, such as HTML
(MHTML)", RFC 2557, March 1999. (MHTML)", RFC 2557, March 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, [RFC2718] Masinter, L., Alvestrand, H., Zigmond, D. and R. Petke,
"Guidelines for new URL Schemes", RFC 2718, November 1999. "Guidelines for new URL Schemes", RFC 2718, November 1999.
[RFC2732] Hinden, R., Carpenter, B. and L. Masinter, "Format for [RFC2732] Hinden, R., Carpenter, B. and L. Masinter, "Format for
Literal IPv6 Addresses in URL's", RFC 2732, December 1999. 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/ [RFC3305] Mealling, M. and R. Denenberg, "Report from the Joint W3C/
IETF URI Planning Interest Group: Uniform Resource IETF URI Planning Interest Group: Uniform Resource
Identifiers (URIs), URLs, and Uniform Resource Names Identifiers (URIs), URLs, and Uniform Resource Names
(URNs): Clarifications and Recommendations", RFC 3305, (URNs): Clarifications and Recommendations", RFC 3305,
August 2002. August 2002.
[RFC3490] Faltstrom, P., Hoffman, P. and A. Costello, [RFC3490] Faltstrom, P., Hoffman, P. and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)", "Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003. RFC 3490, March 2003.
skipping to change at page 47, line 22 skipping to change at page 47, line 33
USA USA
Phone: +1-617-253-5702 Phone: +1-617-253-5702
Fax: +1-617-258-5999 Fax: +1-617-258-5999
EMail: timbl@w3.org EMail: timbl@w3.org
URI: http://www.w3.org/People/Berners-Lee/ URI: http://www.w3.org/People/Berners-Lee/
Roy T. Fielding Roy T. Fielding
Day Software Day Software
5251 California Ave., Suite 110 5251 California Ave., Suite 110
Irvine, CA 92612-3074 Irvine, CA 92617
USA USA
Phone: +1-949-679-2960 Phone: +1-949-679-2960
Fax: +1-949-679-2972 Fax: +1-949-679-2972
EMail: fielding@gbiv.com EMail: fielding@gbiv.com
URI: http://roy.gbiv.com/ URI: http://roy.gbiv.com/
Larry Masinter Larry Masinter
Adobe Systems Incorporated Adobe Systems Incorporated
345 Park Ave 345 Park Ave
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Phone: +1-408-536-3024 Phone: +1-408-536-3024
EMail: LMM@acm.org EMail: LMM@acm.org
URI: http://larry.masinter.net/ URI: http://larry.masinter.net/
Appendix A. Collected ABNF for URI Appendix A. Collected ABNF for URI
URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ] URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ]
hier-part = "//" authority path-abempty hier-part = "//" authority path-abempty
/ path-abs / path-absolute
/ path-rootless / path-rootless
/ path-empty / path-empty
URI-reference = URI / relative-URI URI-reference = URI / relative-URI
absolute-URI = scheme ":" hier-part [ "?" query ] absolute-URI = scheme ":" hier-part [ "?" query ]
relative-URI = relative-part [ "?" query ] [ "#" fragment ] relative-URI = relative-part [ "?" query ] [ "#" fragment ]
relative-part = "//" authority path-abempty relative-part = "//" authority path-abempty
/ path-abs / path-absolute
/ path-noscheme / path-noscheme
/ path-empty / path-empty
scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." ) scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
authority = [ userinfo "@" ] host [ ":" port ] authority = [ userinfo "@" ] host [ ":" port ]
userinfo = *( unreserved / pct-encoded / sub-delims / ":" ) userinfo = *( unreserved / pct-encoded / sub-delims / ":" )
host = IP-literal / IPv4address / reg-name host = IP-literal / IPv4address / reg-name
port = *DIGIT port = *DIGIT
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/ [ h16 ] "::" 4( h16 ":" ) ls32 / [ h16 ] "::" 4( h16 ":" ) ls32
/ [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32 / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
/ [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32 / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
/ [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32 / [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32
/ [ *4( h16 ":" ) h16 ] "::" ls32 / [ *4( h16 ":" ) h16 ] "::" ls32
/ [ *5( h16 ":" ) h16 ] "::" h16 / [ *5( h16 ":" ) h16 ] "::" h16
/ [ *6( h16 ":" ) h16 ] "::" / [ *6( h16 ":" ) h16 ] "::"
h16 = 1*4HEXDIG h16 = 1*4HEXDIG
ls32 = ( h16 ":" h16 ) / IPv4address ls32 = ( h16 ":" h16 ) / IPv4address
IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet
dec-octet = DIGIT ; 0-9 dec-octet = DIGIT ; 0-9
/ %x31-39 DIGIT ; 10-99 / %x31-39 DIGIT ; 10-99
/ "1" 2DIGIT ; 100-199 / "1" 2DIGIT ; 100-199
/ "2" %x30-34 DIGIT ; 200-249 / "2" %x30-34 DIGIT ; 200-249
/ "25" %x30-35 ; 250-255 / "25" %x30-35 ; 250-255
reg-name = 0*255( unreserved / pct-encoded / sub-delims ) reg-name = *( unreserved / pct-encoded / sub-delims )
path = path-abempty ; begins with "/" or is empty path = path-abempty ; begins with "/" or is empty
/ path-abs ; begins with "/" but not "//" / path-absolute ; begins with "/" but not "//"
/ path-noscheme ; begins with a non-colon segment / path-noscheme ; begins with a non-colon segment
/ path-rootless ; begins with a segment / path-rootless ; begins with a segment
/ path-empty ; zero characters / path-empty ; zero characters
path-abempty = *( "/" segment ) path-abempty = *( "/" segment )
path-abs = "/" [ segment-nz *( "/" segment ) ] path-absolute = "/" [ segment-nz *( "/" segment ) ]
path-noscheme = segment-nzc *( "/" segment ) path-noscheme = segment-nz-nc *( "/" segment )
path-rootless = segment-nz *( "/" segment ) path-rootless = segment-nz *( "/" segment )
path-empty = 0<pchar> path-empty = 0<pchar>
segment = *pchar segment = *pchar
segment-nz = 1*pchar segment-nz = 1*pchar
segment-nzc = 1*( unreserved / pct-encoded / sub-delims / "@" ) segment-nz-nc = 1*( unreserved / pct-encoded / sub-delims / "@" )
; non-zero-length segment without any colon ":"
pchar = unreserved / pct-encoded / sub-delims / ":" / "@" pchar = unreserved / pct-encoded / sub-delims / ":" / "@"
query = *( pchar / "/" / "?" ) query = *( pchar / "/" / "?" )
fragment = *( pchar / "/" / "?" ) fragment = *( pchar / "/" / "?" )
pct-encoded = "%" HEXDIG HEXDIG pct-encoded = "%" HEXDIG HEXDIG
unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~" unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
skipping to change at page 51, line 24 skipping to change at page 51, line 19
In practice, URIs are delimited in a variety of ways, but usually In practice, URIs are delimited in a variety of ways, but usually
within double-quotes "http://example.com/", angle brackets <http:// within double-quotes "http://example.com/", angle brackets <http://
example.com/>, or just using whitespace example.com/>, or just using whitespace
http://example.com/ http://example.com/
These wrappers do not form part of the URI. These wrappers do not form part of the URI.
In some cases, extra whitespace (spaces, line-breaks, tabs, etc.) may In some cases, extra whitespace (spaces, line-breaks, tabs, etc.) may
need to be added to break a long URI across lines. The whitespace need to be added to break a long URI across lines. The whitespace
should be ignored when extracting the URI. should be ignored when extracting the URI.
No whitespace should be introduced after a hyphen ("-") character. No whitespace should be introduced after a hyphen ("-") character.
Because some typesetters and printers may (erroneously) introduce a Because some typesetters and printers may (erroneously) introduce a
hyphen at the end of line when breaking a line, the interpreter of 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 URI containing a line break immediately after a hyphen should ignore
all whitespace around the line break, and should be aware that the all whitespace around the line break, and should be aware that the
hyphen may or may not actually be part of the URI. hyphen may or may not actually be part of the URI.
Using <> angle brackets around each URI is especially recommended as Using <> angle brackets around each URI is especially recommended as
skipping to change at page 52, line 37 skipping to change at page 52, line 24
authority component and not allowed outside their use as delimiters authority component and not allowed outside their use as delimiters
for an IP literal within host. In order to make this change without for an IP literal within host. In order to make this change without
changing the technical definition of the path, query, and fragment changing the technical definition of the path, query, and fragment
components, those rules were redefined to directly specify the components, those rules were redefined to directly specify the
characters allowed rather than be defined in terms of uric. characters allowed rather than be defined in terms of uric.
Since [RFC2732] defers to [RFC3513] for definition of an IPv6 literal Since [RFC2732] defers to [RFC3513] for definition of an IPv6 literal
address, which unfortunately lacks an ABNF description of address, which unfortunately lacks an ABNF description of
IPv6address, we created a new ABNF rule for IPv6address that matches IPv6address, we created a new ABNF rule for IPv6address that matches
the text representations defined by Section 2.2 of [RFC3513]. the text representations defined by Section 2.2 of [RFC3513].
Likewise, the definition of IPv4address has been improved in order to Likewise, the definition of IPv4address has been improved in order to
limit each decimal octet to the range 0-255. limit each decimal octet to the range 0-255.
Section 6 (Section 6) on URI normalization and comparison has been Section 6 (Section 6) on URI normalization and comparison has been
completely rewritten and extended using input from Tim Bray and completely rewritten and extended using input from Tim Bray and
discussion within the W3C Technical Architecture Group. discussion within the W3C Technical Architecture Group.
An ABNF rule for URI has been introduced to correspond to the common An ABNF rule for URI has been introduced to correspond to the common
usage of the term: an absolute URI with optional fragment. usage of the term: an absolute URI with optional fragment.
D.2 Modifications from RFC 2396 D.2 Modifications from RFC 2396
The ad-hoc BNF syntax has been replaced with the ABNF of [RFC2234]. The ad-hoc BNF syntax has been replaced with the ABNF of [RFC2234].
This change required all rule names that formerly included underscore This change required all rule names that formerly included underscore
characters to be renamed with a dash instead. characters to be renamed with a dash instead.
Section 2 on characters has been rewritten to explain what characters Section 2 on characters has been rewritten to explain what characters
are reserved, when they are reserved, and why they are reserved even are reserved, when they are reserved, and why they are reserved even
when not used as delimiters by the generic syntax. The mark when not used as delimiters by the generic syntax. The mark
characters that are typically unsafe to decode, including the characters that are typically unsafe to decode, including the
exclamation mark ("!"), asterisk ("*"), single-quote ("'"), and open exclamation mark ("!"), asterisk ("*"), single-quote ("'"), and open
and close parentheses ("(" and ")"), have been moved to the reserved and close parentheses ("(" and ")"), have been moved to the reserved
set in order to clarify the distinction between reserved and set in order to clarify the distinction between reserved and
unreserved and hopefully answer the most common question of scheme unreserved and hopefully answer the most common question of scheme
designers. Likewise, the section on percent-encoded characters has designers. Likewise, the section on percent-encoded characters has
been rewritten, and URI normalizers are now given license to decode been rewritten, and URI normalizers are now given license to decode
any percent-encoded octets corresponding to unreserved characters. any percent-encoded octets corresponding to unreserved characters.
In general, the terms "escaped" and "unescaped" have been replaced In general, the terms "escaped" and "unescaped" have been replaced
with "percent-encoded" and "decoded", respectively, to reduce with "percent-encoded" and "decoded", respectively, to reduce
confusion with other forms of escape mechanisms. confusion with other forms of escape mechanisms.
The ABNF for URI and URI-reference has been redesigned to make them The ABNF for URI and URI-reference has been redesigned to make them
more friendly to LALR parsers and reduce complexity. As a result, the more friendly to LALR parsers and reduce complexity. As a result,
layout form of syntax description has been removed, along with the the layout form of syntax description has been removed, along with
uric, uric_no_slash, opaque_part, net_path, abs_path, rel_path, the uric, uric_no_slash, opaque_part, net_path, abs_path, rel_path,
path_segments, rel_segment, and mark rules. All references to path_segments, rel_segment, and mark rules. All references to
"opaque" URIs have been replaced with a better description of how the "opaque" URIs have been replaced with a better description of how the
path component may be opaque to hierarchy. The ambiguity regarding 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 the parsing of URI-reference as a URI or a relative-URI with a colon
in the first segment has been eliminated through the use of five in the first segment has been eliminated through the use of five
separate path matching rules. separate path matching rules.
The fragment identifier has been moved back into the section on The fragment identifier has been moved back into the section on
generic syntax components and within the URI and relative-URI rules, generic syntax components and within the URI and relative-URI rules,
though it remains excluded from absolute-URI. The number sign ("#") though it remains excluded from absolute-URI. The number sign ("#")
character has been moved back to the reserved set as a result of character has been moved back to the reserved set as a result of
reintegrating the fragment syntax. reintegrating the fragment syntax.
The ABNF has been corrected to allow a relative path to be empty. 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 This also allows an absolute-URI to consist of nothing after the
"scheme:", as is present in practice with the "dav:" namespace "scheme:", as is present in practice with the "dav:" namespace
[RFC2518] and the "about:" scheme used internally by many WWW browser [RFC2518] and the "about:" scheme used internally by many WWW browser
implementations. The ambiguity regarding the boundary between implementations. The ambiguity regarding the boundary between
authority and path has been eliminated through the use of five authority and path has been eliminated through the use of five
separate path matching rules. separate path matching rules.
Registry-based naming authorities that use the generic syntax are now Registry-based naming authorities that use the generic syntax are now
defined within the host rule and limited to 255 path characters. This defined within the host rule. This change allows current
change allows current implementations, where whatever name provided implementations, where whatever name provided is simply fed to the
is simply fed to the local name resolution mechanism, to be local name resolution mechanism, to be consistent with the
consistent with the specification and removes the need to re-specify specification and removes the need to re-specify DNS name formats
DNS name formats here. It also allows the host component to contain here. It also allows the host component to contain percent-encoded
percent-encoded octets, which is necessary to enable octets, which is necessary to enable internationalized domain names
internationalized domain names to be provided in URIs, processed in to be provided in URIs, processed in their native character encodings
their native character encodings at the application layers above URI at the application layers above URI processing, and passed to an IDNA
processing, and passed to an IDNA library as a registered name in the library as a registered name in the UTF-8 character encoding. The
UTF-8 character encoding. The server, hostport, hostname, server, hostport, hostname, domainlabel, toplabel, and alphanum rules
domainlabel, toplabel, and alphanum rules have been removed. have been removed.
The resolving relative references algorithm of [RFC2396] has been The resolving relative references algorithm of [RFC2396] has been
rewritten using pseudocode for this revision to improve clarity and rewritten using pseudocode for this revision to improve clarity and
fix the following issues: fix the following issues:
o [RFC2396] section 5.2, step 6a, failed to account for a base URI o [RFC2396] section 5.2, step 6a, failed to account for a base URI
with no path. with no path.
o Restored the behavior of [RFC1808] where, if the reference o Restored the behavior of [RFC1808] where, if the reference
contains an empty path and a defined query component, then the contains an empty path and a defined query component, then the
target URI inherits the base URI's path component. target URI inherits the base URI's path component.
o Removed the special-case treatment of same-document references o The determination of whether a URI reference is a same-document
within the URI parser in favor of a section that explains when a reference has been decoupled from the URI parser, simplifying the
reference should be interpreted by a dereferencing engine as a URI processing interface within applications in a way consistent
same-document reference: when the target URI and base URI, with the internal architecture of deployed URI processing
excluding fragments, match. This change does not modify the implementations. The determination is now based on comparison to
behavior of existing same-document references as defined by RFC the base URI after transforming a reference to absolute form,
2396 (fragment-only references); it merely adds the same-document rather than on the format of the reference itself. This change
distinction to other references that refer to the base URI and may result in more references being considered "same-document"
simplifies the interface between applications and their URI under this specification than would be under the rules given in
parsers, as is consistent with the internal architecture of RFC 2396, especially when normalization is used to reduce aliases.
deployed URI processing implementations. However, it does not change the status of existing same-document
references.
o Separated the path merge routine into two routines: merge, for o Separated the path merge routine into two routines: merge, for
describing combination of the base URI path with a relative-path describing combination of the base URI path with a relative-path
reference, and remove_dot_segments, for describing how to remove reference, and remove_dot_segments, for describing how to remove
the special "." and ".." segments from a composed path. The the special "." and ".." segments from a composed path. The
remove_dot_segments algorithm is now applied to all URI reference remove_dot_segments algorithm is now applied to all URI reference
paths in order to match common implementations and improve the paths in order to match common implementations and improve the
normalization of URIs in practice. This change only impacts the normalization of URIs in practice. This change only impacts the
parsing of abnormal references and same-scheme references wherein parsing of abnormal references and same-scheme references wherein
the base URI has a non-hierarchical path. the base URI has a non-hierarchical path.
Appendix E. Instructions to RFC Editor
Prior to publication as an RFC, please remove this section and the
"Editorial Note" that appears after the Abstract. If [BCP35] or any
of the normative references are updated prior to publication, the
associated reference in this document can be safely updated as well.
This document has been produced using the xml2rfc tool set; the XML
version can be obtained via the URI listed in the editorial note.
Index Index
A A
ABNF 10 ABNF 11
absolute 25 absolute 26
absolute-path 25 absolute-path 25
absolute-URI 25 absolute-URI 26
access 8 access 9
authority 15, 16 authority 15, 17
B B
base URI 27 base URI 28
C C
character encoding 4 character encoding 4
character 4 character 4
characters 10 characters 11
coded character set 4 coded character set 4
D D
dec-octet 19 dec-octet 20
dereference 8 dereference 9
dot-segments 21 dot-segments 22
F F
fragment 15, 23 fragment 15, 23
G G
gen-delims 11 gen-delims 12
generic syntax 6 generic syntax 6
H H
h16 18 h16 19
hier-part 15 hier-part 15
hierarchical 9 hierarchical 10
host 17 host 18
I I
identifier 5 identifier 5
IP-literal 18 IP-literal 19
IPv4 19 IPv4 20
IPv4address 19 IPv4address 20
IPv6 18 IPv6 19
IPv6address 18 IPv6address 19
IPvFuture 18 IPvFuture 19
L L
locator 6 locator 7
ls32 18 ls32 19
M M
merge 30 merge 31
N N
name 6 name 7
network-path 25 network-path 25
P P
path 15, 21 path 15, 21
path-abempty 21 path-abempty 21
path-abs 21 path-absolute 21
path-empty 21 path-empty 21
path-noscheme 21 path-noscheme 21
path-rootless 21 path-rootless 21
path-abempty 15 path-abempty 15
path-abs 15 path-absolute 15
path-empty 15 path-empty 15
path-rootless 15 path-rootless 15
pchar 21 pchar 21
pct-encoded 11 pct-encoded 12
percent-encoding 11 percent-encoding 12
port 20 port 21
Q Q
query 15, 22 query 15, 23
R R
reg-name 19 reg-name 20
registered name 19 registered name 20
relative 9, 27 relative 10, 28
relative-path 25 relative-path 25
relative-URI 25 relative-URI 25
remove_dot_segments 30, 31 remove_dot_segments 31
representation 8 representation 9
reserved 11 reserved 12
resolution 8, 27 resolution 9, 28
resource 4 resource 5
retrieval 8 retrieval 9
S S
same-document 25 same-document 26
sameness 8 sameness 9
scheme 15, 15 scheme 15, 16
segment 21 segment 21
segment-nz 21 segment-nz 21
segment-nzc 21 segment-nz-nc 21
sub-delims 11 sub-delims 12
suffix 26 suffix 27
T T
transcription 7 transcription 7
U U
uniform 4 uniform 4
unreserved 12 unreserved 13
URI grammar URI grammar
absolute-URI 25 absolute-URI 26
ALPHA 10 ALPHA 11
authority 15, 16 authority 16, 17
CR 10 CR 11
dec-octet 19 dec-octet 20
DIGIT 10 DIGIT 11
DQUOTE 10 DQUOTE 11
fragment 15, 23, 25 fragment 16, 24, 26
gen-delims 11 gen-delims 12
h16 18 h16 19
HEXDIG 10 HEXDIG 11
hier-part 15 hier-part 16
host 16, 17 host 17, 18
IP-literal 18 IP-literal 19
IPv4address 19 IPv4address 20
IPv6address 18 IPv6address 19, 19
IPvFuture 18 IPvFuture 19
LF 10 LF 11
ls32 18 ls32 19
mark 12 mark 13
OCTET 10 OCTET 11
path 21 path 22
path-abempty 15, 21 path-abempty 16, 22
path-abs 15, 21 path-absolute 16, 22
path-empty 15, 21 path-empty 16, 22
path-noscheme 21 path-noscheme 22
path-rootless 15, 21 path-rootless 16, 22
pchar 21, 22, 23 pchar 22, 23, 24
pct-encoded 11 pct-encoded 12
port 16, 20 port 17, 21
query 15, 22, 25 query 16, 23, 26, 26
reg-name 19 reg-name 20
relative-URI 24, 25 relative-URI 25, 26
reserved 11 reserved 12
scheme 15, 16, 25 scheme 16, 16, 26
segment 21 segment 22
segment-nz 21 segment-nz 22
segment-nzc 21 segment-nz-nc 22
SP 10 SP 11
sub-delims 11 sub-delims 12
unreserved 12 unreserved 13
URI 15, 24 URI 16, 25
URI-reference 24 URI-reference 25
userinfo 16, 17 userinfo 17, 17
URI 15 URI 15
URI-reference 24 URI-reference 25
URL 6 URL 7
URN 6 URN 7
userinfo 17 userinfo 17
Intellectual Property Statement Intellectual Property Statement
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be found in BCP 78 and BCP 79. found in BCP 78 and BCP 79.
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The IETF invites any interested party to bring to its attention any The IETF invites any interested party to bring to its attention any
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Disclaimer of Validity Disclaimer of Validity
This document and the information contained herein are provided on an This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights. except as set forth therein, the authors retain all their rights.
Acknowledgment Acknowledgment
Funding for the RFC Editor function is currently provided by the Funding for the RFC Editor function is currently provided by the
Internet Society. Internet Society.
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