< draft-fielding-uri-rfc2396bis-03.txt   draft-fielding-uri-rfc2396bis-04.txt >
Network Working Group T. Berners-Lee Network Working Group T. Berners-Lee
Internet-Draft MIT/LCS Internet-Draft MIT/LCS
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 Expires: August 16, 2004 L. Masinter
Expires: December 5, 2003 Adobe Adobe
June 6, 2003 February 16, 2004
Uniform Resource Identifier (URI): Generic Syntax Uniform Resource Identifier (URI): Generic Syntax
draft-fielding-uri-rfc2396bis-03 draft-fielding-uri-rfc2396bis-04
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts. groups may also distribute working documents as Internet-Drafts.
skipping to change at page 1, line 34 skipping to change at page 1, line 33
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
<http://www.ietf.org/ietf/1id-abstracts.txt>. <http://www.ietf.org/ietf/1id-abstracts.txt>.
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
<http://www.ietf.org/shadow.html>. <http://www.ietf.org/shadow.html>.
This Internet-Draft will expire on August 16, 2004.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved. Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract Abstract
A Uniform Resource Identifier (URI) is a compact string of characters A Uniform Resource Identifier (URI) is a compact string of characters
for identifying an abstract or physical resource. This specification for identifying an abstract or physical resource. This specification
defines the generic URI syntax and a process for resolving URI defines the generic URI syntax and a process for resolving URI
references that might be in relative form, along with guidelines and references that might be in relative form, along with guidelines and
security considerations for the use of URIs on the Internet. security considerations for the use of URIs on the Internet.
The URI syntax defines a grammar that is a superset of all valid The URI syntax defines a grammar that is a superset of all valid
URIs, such that an implementation can parse the common components of URIs, such that an implementation can parse the common components of
a URI reference without knowing the scheme-specific requirements of a URI reference without knowing the scheme-specific requirements of
every possible identifier. This specification does not define a every possible identifier. This specification does not define a
generative grammar for URIs; that task is performed by the individual generative grammar for URIs; that task is performed by the individual
specifications of each URI scheme. 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://www.apache.org/~fielding/uri/rev-2002/ is available at <http://gbiv.com/protocols/uri/rev-2002/issues.html>.
issues.html>.
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 . . . . . . . . . . . . . . . . . . . . . . . 5 1.1.1 Generic Syntax . . . . . . . . . . . . . . . . . . . . . . . 5
1.1.2 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.1.2 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1.3 URI, URL, and URN . . . . . . . . . . . . . . . . . . . . . 6 1.1.3 URI, URL, and URN . . . . . . . . . . . . . . . . . . . . . 6
1.2 Design Considerations . . . . . . . . . . . . . . . . . . . 6 1.2 Design Considerations . . . . . . . . . . . . . . . . . . . 6
1.2.1 Transcription . . . . . . . . . . . . . . . . . . . . . . . 6 1.2.1 Transcription . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.2 Separating Identification from Interaction . . . . . . . . . 7 1.2.2 Separating Identification from Interaction . . . . . . . . . 7
1.2.3 Hierarchical Identifiers . . . . . . . . . . . . . . . . . . 8 1.2.3 Hierarchical Identifiers . . . . . . . . . . . . . . . . . . 9
1.3 Syntax Notation . . . . . . . . . . . . . . . . . . . . . . 9 1.3 Syntax Notation . . . . . . . . . . . . . . . . . . . . . . 10
2. Characters . . . . . . . . . . . . . . . . . . . . . . . . . 11 2. Characters . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1 Encoding of Characters . . . . . . . . . . . . . . . . . . . 11 2.1 Percent Encoding . . . . . . . . . . . . . . . . . . . . . . 11
2.2 Reserved Characters . . . . . . . . . . . . . . . . . . . . 11 2.2 Reserved Characters . . . . . . . . . . . . . . . . . . . . 12
2.3 Unreserved Characters . . . . . . . . . . . . . . . . . . . 12 2.3 Unreserved Characters . . . . . . . . . . . . . . . . . . . 12
2.4 Escaped Characters . . . . . . . . . . . . . . . . . . . . . 13 2.4 When to Encode or Decode . . . . . . . . . . . . . . . . . . 13
2.4.1 Escaped Encoding . . . . . . . . . . . . . . . . . . . . . . 13 3. Syntax Components . . . . . . . . . . . . . . . . . . . . . 15
2.4.2 When to Escape and Unescape . . . . . . . . . . . . . . . . 13 3.1 Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.5 Excluded Characters . . . . . . . . . . . . . . . . . . . . 14 3.2 Authority . . . . . . . . . . . . . . . . . . . . . . . . . 16
3. Syntax Components . . . . . . . . . . . . . . . . . . . . . 16 3.2.1 User Information . . . . . . . . . . . . . . . . . . . . . . 16
3.1 Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.2.2 Host . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.2 Authority . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.2.1 User Information . . . . . . . . . . . . . . . . . . . . . . 18
3.2.2 Host . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.2.3 Port . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 3.2.3 Port . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.3 Path . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 3.3 Path . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.4 Query . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 3.4 Query . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.5 Fragment . . . . . . . . . . . . . . . . . . . . . . . . . . 22 3.5 Fragment . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 4. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.1 URI Reference . . . . . . . . . . . . . . . . . . . . . . . 24 4.1 URI Reference . . . . . . . . . . . . . . . . . . . . . . . 24
4.2 Relative URI . . . . . . . . . . . . . . . . . . . . . . . . 24 4.2 Relative URI . . . . . . . . . . . . . . . . . . . . . . . . 24
4.3 Absolute URI . . . . . . . . . . . . . . . . . . . . . . . . 25 4.3 Absolute URI . . . . . . . . . . . . . . . . . . . . . . . . 25
4.4 Same-document Reference . . . . . . . . . . . . . . . . . . 25 4.4 Same-document Reference . . . . . . . . . . . . . . . . . . 25
4.5 Suffix Reference . . . . . . . . . . . . . . . . . . . . . . 25 4.5 Suffix Reference . . . . . . . . . . . . . . . . . . . . . . 25
5. Reference Resolution . . . . . . . . . . . . . . . . . . . . 27 5. Reference Resolution . . . . . . . . . . . . . . . . . . . . 27
5.1 Establishing a Base URI . . . . . . . . . . . . . . . . . . 27 5.1 Establishing a Base URI . . . . . . . . . . . . . . . . . . 27
5.1.1 Base URI within Document Content . . . . . . . . . . . . . . 27 5.1.1 Base URI within Document Content . . . . . . . . . . . . . . 27
5.1.2 Base URI from the Encapsulating Entity . . . . . . . . . . . 28 5.1.2 Base URI from the Encapsulating Entity . . . . . . . . . . . 28
5.1.3 Base URI from the Retrieval URI . . . . . . . . . . . . . . 28 5.1.3 Base URI from the Retrieval URI . . . . . . . . . . . . . . 28
5.1.4 Default Base URI . . . . . . . . . . . . . . . . . . . . . . 28 5.1.4 Default Base URI . . . . . . . . . . . . . . . . . . . . . . 28
5.2 Obtaining the Referenced URI . . . . . . . . . . . . . . . . 28 5.2 Relative Resolution . . . . . . . . . . . . . . . . . . . . 28
5.3 Recomposition of a Parsed URI . . . . . . . . . . . . . . . 31 5.2.1 Pre-parse the Base URI . . . . . . . . . . . . . . . . . . . 29
5.4 Reference Resolution Examples . . . . . . . . . . . . . . . 32 5.2.2 Transform References . . . . . . . . . . . . . . . . . . . . 29
5.4.1 Normal Examples . . . . . . . . . . . . . . . . . . . . . . 32 5.2.3 Merge Paths . . . . . . . . . . . . . . . . . . . . . . . . 30
5.4.2 Abnormal Examples . . . . . . . . . . . . . . . . . . . . . 32 5.2.4 Remove Dot Segments . . . . . . . . . . . . . . . . . . . . 30
5.3 Component Recomposition . . . . . . . . . . . . . . . . . . 32
5.4 Reference Resolution Examples . . . . . . . . . . . . . . . 33
5.4.1 Normal Examples . . . . . . . . . . . . . . . . . . . . . . 33
5.4.2 Abnormal Examples . . . . . . . . . . . . . . . . . . . . . 33
6. Normalization and Comparison . . . . . . . . . . . . . . . . 35 6. Normalization and Comparison . . . . . . . . . . . . . . . . 35
6.1 Equivalence . . . . . . . . . . . . . . . . . . . . . . . . 35 6.1 Equivalence . . . . . . . . . . . . . . . . . . . . . . . . 35
6.2 Comparison Ladder . . . . . . . . . . . . . . . . . . . . . 35 6.2 Comparison Ladder . . . . . . . . . . . . . . . . . . . . . 36
6.2.1 Simple String Comparison . . . . . . . . . . . . . . . . . . 36 6.2.1 Simple String Comparison . . . . . . . . . . . . . . . . . . 36
6.2.2 Syntax-based Normalization . . . . . . . . . . . . . . . . . 37 6.2.2 Syntax-based Normalization . . . . . . . . . . . . . . . . . 37
6.2.3 Scheme-based Normalization . . . . . . . . . . . . . . . . . 38 6.2.3 Scheme-based Normalization . . . . . . . . . . . . . . . . . 38
6.2.4 Protocol-based Normalization . . . . . . . . . . . . . . . . 38 6.2.4 Protocol-based Normalization . . . . . . . . . . . . . . . . 39
6.3 Canonical Form . . . . . . . . . . . . . . . . . . . . . . . 38 6.3 Canonical Form . . . . . . . . . . . . . . . . . . . . . . . 39
7. Security Considerations . . . . . . . . . . . . . . . . . . 40 7. Security Considerations . . . . . . . . . . . . . . . . . . 41
7.1 Reliability and Consistency . . . . . . . . . . . . . . . . 40 7.1 Reliability and Consistency . . . . . . . . . . . . . . . . 41
7.2 Malicious Construction . . . . . . . . . . . . . . . . . . . 40 7.2 Malicious Construction . . . . . . . . . . . . . . . . . . . 41
7.3 Rare IP Address Formats . . . . . . . . . . . . . . . . . . 41 7.3 Back-end Transcoding . . . . . . . . . . . . . . . . . . . . 42
7.4 Sensitive Information . . . . . . . . . . . . . . . . . . . 41 7.4 Rare IP Address Formats . . . . . . . . . . . . . . . . . . 42
7.5 Semantic Attacks . . . . . . . . . . . . . . . . . . . . . . 41 7.5 Sensitive Information . . . . . . . . . . . . . . . . . . . 43
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 43 7.6 Semantic Attacks . . . . . . . . . . . . . . . . . . . . . . 43
Normative References . . . . . . . . . . . . . . . . . . . . 44 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 45
Informative References . . . . . . . . . . . . . . . . . . . 45 Normative References . . . . . . . . . . . . . . . . . . . . 46
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 47 Informative References . . . . . . . . . . . . . . . . . . . 47
A. Collected ABNF for URI . . . . . . . . . . . . . . . . . . . 48 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 48
B. Parsing a URI Reference with a Regular Expression . . . . . 50 A. Collected ABNF for URI . . . . . . . . . . . . . . . . . . . 50
C. Delimiting a URI in Context . . . . . . . . . . . . . . . . 51 B. Parsing a URI Reference with a Regular Expression . . . . . 52
D. Summary of Non-editorial Changes . . . . . . . . . . . . . . 53 C. Delimiting a URI in Context . . . . . . . . . . . . . . . . 53
D.1 Additions . . . . . . . . . . . . . . . . . . . . . . . . . 53 D. Summary of Non-editorial Changes . . . . . . . . . . . . . . 55
D.2 Modifications from RFC 2396 . . . . . . . . . . . . . . . . 53 D.1 Additions . . . . . . . . . . . . . . . . . . . . . . . . . 55
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 D.2 Modifications from RFC 2396 . . . . . . . . . . . . . . . . 55
Intellectual Property and Copyright Statements . . . . . . . 60 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
Intellectual Property and Copyright Statements . . . . . . . 62
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 excludes those portions of RFC 1738 that defined the specific
syntax of individual URI schemes; those portions will be updated as syntax of individual URI schemes; those portions will be updated as
separate documents. The process for registration of new URI schemes separate documents. The process for registration of new URI schemes
is defined separately by [RFC2717]. is defined separately by [RFC2717]. Advice for designers of new URI
schemes can be found in [RFC2718].
All significant changes from RFC 2396 are noted in Appendix D. All significant changes from RFC 2396 are noted in Appendix D.
This specification uses the terms "character" and "character
encoding" in accordance with the definitions provided in [RFC2978].
1.1 Overview of URIs 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
skipping to change at page 5, line 14 skipping to change at page 5, line 18
mathematical equation or the types of a relationship (e.g., mathematical equation or the types of a relationship (e.g.,
"parent" or "employee"). "parent" or "employee").
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. identification.
A URI is an identifier that consists of a sequence of characters A URI is an identifier that consists of a sequence of characters
matching the syntax defined by the grammar rule named "URI" in matching the syntax defined by the syntax rule named "URI" in Section
Section 3. A URI can be used to refer to a resource. This 3. A URI can be used to refer to a resource. This specification does
specification does not place any limits on the nature of a resource not place any limits on the nature of a resource or the reasons why
or the reasons why an application might wish to refer to a resource. an application might wish to refer to a resource. URIs have a global
URIs have a global scope and should be interpreted consistently scope and should be interpreted consistently regardless of context,
regardless of context, but that interpretation may be defined in but that interpretation may be defined in relation to the user's
relation to the user's context (e.g., "http://localhost/" refers to a context (e.g., "http://localhost/" refers to a resource that is
resource that is relative to the user's network interface and yet not relative to the user's network interface and yet not specific to any
specific to any one user). one user).
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
skipping to change at page 6, line 10 skipping to change at page 6, line 10
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 examples illustrate URIs that are in common use.
ftp://ftp.is.co.za/rfc/rfc1808.txt ftp://ftp.is.co.za/rfc/rfc1808.txt
-- ftp scheme for File Transfer Protocol services
gopher://gopher.tc.umn.edu:70/11/Mailing%20Lists/
-- gopher scheme for Gopher and Gopher+ Protocol services
http://www.ietf.org/rfc/rfc2396.txt http://www.ietf.org/rfc/rfc2396.txt
-- http scheme for Hypertext Transfer Protocol services
mailto:John.Doe@example.com mailto:John.Doe@example.com
-- mailto scheme for electronic mail addresses
news:comp.infosystems.www.servers.unix news:comp.infosystems.www.servers.unix
-- news scheme for USENET news groups and articles
telnet://melvyl.ucop.edu/ telnet://melvyl.ucop.edu/
-- telnet scheme for interactive TELNET services
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) refers to URIs under the "urn" scheme [RFC2141], which are (URN) has been used historically to refer to both URIs under the
required to remain globally unique and persistent even when the "urn" scheme [RFC2141], which are required to remain globally unique
resource ceases to exist or becomes unavailable. and persistent even when the resource ceases to exist or becomes
unavailable, and to any other URI with the properties of a name.
An individual scheme does not need to be classified as being just one An individual scheme does not need to be classified as being just one
of "name" or "locator". Instances of URIs from any given scheme may of "name" or "locator". Instances of URIs from any given scheme may
have the characteristics of names or locators or both, often have the characteristics of names or locators or both, often
depending on the persistence and care in the assignment of depending on the persistence and care in the assignment of
identifiers by the naming authority, rather than any quality of the identifiers by the naming authority, rather than any quality of the
scheme. scheme. Future specifications and related documentation should use
the general term "URI", rather than the more restrictive terms URL
and URN [RFC3305].
1.2 Design Considerations 1.2 Design Considerations
1.2.1 Transcription 1.2.1 Transcription
The URI syntax has been designed with global transcription as one of The URI syntax has been designed with global transcription as one of
its main considerations. A URI is a sequence of characters from a its main considerations. A URI is a sequence of characters from a
very limited set: the letters of the basic Latin alphabet, digits, very limited set: the letters of the basic Latin alphabet, digits,
and a few special characters. A URI may be represented in a variety and a few special characters. A URI may be represented in a variety
of ways: e.g., ink on paper, pixels on a screen, or a sequence of of ways: e.g., ink on paper, pixels on a screen, or a sequence of
octets in a coded character set. The interpretation of a URI depends integers from a coded character set. The interpretation of a URI
only on the characters used and not how those characters are depends only on the characters used and not how those characters are
represented in a network protocol. represented in a network protocol.
The goal of transcription can be described by a simple scenario. The goal of transcription can be described by a simple scenario.
Imagine two colleagues, Sam and Kim, sitting in a pub at an Imagine two colleagues, Sam and Kim, sitting in a pub at an
international conference and exchanging research ideas. Sam asks Kim international conference and exchanging research ideas. Sam asks Kim
for a location to get more information, so Kim writes the URI for the for a location to get more information, so Kim writes the URI for the
research site on a napkin. Upon returning home, Sam takes out the research site on a napkin. Upon returning home, Sam takes out the
napkin and types the URI into a computer, which then retrieves the napkin and types the URI into a computer, which then retrieves the
information to which Kim referred. information to which Kim referred.
skipping to change at page 7, line 38 skipping to change at page 7, line 33
o A URI often needs to be remembered by people, and it is easier for o A URI often needs to be remembered by people, and it is easier for
people to remember a URI when it consists of meaningful or people to remember a URI when it consists of meaningful or
familiar components. familiar components.
These design considerations are not always in alignment. For These design considerations are not always in alignment. For
example, it is often the case that the most meaningful name for a URI example, it is often the case that the most meaningful name for a URI
component would require characters that cannot be typed into some component would require characters that cannot be typed into some
systems. The ability to transcribe a resource identifier from one systems. The ability to transcribe a resource identifier from one
medium to another has been considered more important than having a medium to another has been considered more important than having a
URI consist of the most meaningful of components. In local or URI consist of the most meaningful of components.
regional contexts and with improving technology, users might benefit
from being able to use a wider range of characters; such use is not In local or regional contexts and with improving technology, users
defined in this specification. might benefit from being able to use a wider range of characters;
such use is not defined in this specification. Percent-encoded
octets (Section 2.1) may be used within a URI to represent characters
outside the range of the US-ASCII coded character set if such
representation is defined by the scheme or by the protocol element in
which the URI is referenced; such a definition will specify the
character encoding scheme used to map those characters to octets
prior to being percent-encoded for the URI.
1.2.2 Separating Identification from Interaction 1.2.2 Separating Identification from Interaction
A common misunderstanding of URIs is that they are only used to refer A common misunderstanding of URIs is that they are only used to refer
to accessible resources. In fact, the URI alone only provides to accessible resources. In fact, the URI alone only provides
identification; access to the resource is neither guaranteed nor identification; access to the resource is neither guaranteed nor
implied by the presence of a URI. Instead, an operation (if any) implied by the presence of a URI. Instead, an operation (if any)
associated with a URI reference is defined by the protocol element, associated with a URI reference is defined by the protocol element,
data format attribute, or natural language text in which it appears. data format attribute, or natural language text in which it appears.
Given a URI, a system may attempt to perform a variety of operations Given a URI, a system may attempt to perform a variety of operations
on the resource, as might be characterized by such words as "denote", on the resource, as might be characterized by such words as "access",
"access", "update", "replace", or "find attributes". Such operations "update", "replace", or "find attributes". Such operations are
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. Use of 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 termed a "dereference" of 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 systems to identify sources of
information, the most common form of URI dereference is "retrieval": information, the most common form of URI dereference is "retrieval":
making use of a URI in order to retrieve a representation of its making use of a URI in order to retrieve a representation of its
associated resource. A "representation" is a sequence of octets, associated resource. A "representation" is a sequence of octets,
along with metadata describing those octets, that constitutes a along with representation metadata describing those octets, that
record of the state of the resource at the time that the constitutes a record of the state of the resource at the time that
representation is generated. Retrieval is achieved by a process that the representation is generated. Retrieval is achieved by a process
might include using the URI as a cache key to check for a locally that might include using the URI as a cache key to check for a
cached representation, resolution of the URI to determine an locally cached representation, resolution of the URI to determine an
appropriate access mechanism (if any), and dereference of the URI for appropriate access mechanism (if any), and dereference of the URI for
the sake of applying a retrieval operation. the sake of applying a retrieval operation. Depending on the
protocols used to perform the retrieval, additional information might
be supplied about the resource (resource metadata) and its relation
to other resources.
URI references in information systems are designed to be URI references in information 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. When an author creates a reference to such a the interaction. When an author creates a reference to such a
resource, they do so with the intention that the reference be used in resource, they do so with the intention that the reference be used in
the future; what is being identified is not some specific result that the future; what is being identified is not some specific result 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
skipping to change at page 9, line 4 skipping to change at page 9, line 6
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,
caches, and name resolution services that are independent of the caches, and name resolution services that are independent of the
protocol associated with the scheme name, and the resolution of some protocol associated with the scheme name, and the resolution of some
URIs may require the use of more than one protocol (e.g., both DNS 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
decreasing order from left to right. For some URI schemes, the order of decreasing significance from left to right. For some URI
visible hierarchy is limited to the scheme itself: everything after schemes, the visible hierarchy is limited to the scheme itself:
the scheme component delimiter is considered opaque to URI everything after the scheme component delimiter (":") is considered
processing. Other URI schemes make the hierarchy explicit and visible opaque to URI processing. Other URI schemes make the hierarchy
to generic parsing algorithms. explicit and visible to generic parsing algorithms.
The URI syntax reserves 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; the vast majority of URIs in constructed to serve a common purpose, wherein the vast majority of
these documents point to resources within the tree rather than URIs in these documents point to resources within the tree rather
outside of it. Similarly, documents located at a particular site are than outside of it. Similarly, documents located at a particular
much more likely to refer to other resources at that site than to site are much more likely to refer to other resources at that site
resources at remote sites. than to resources at remote sites. Relative referencing of URIs
allows document trees to be partially independent of their location
Relative referencing of URIs allows document trees to be partially and access scheme. For instance, it is possible for a single set of
independent of their location and access scheme. For instance, it is hypertext documents to be simultaneously accessible and traversable
possible for a single set of hypertext documents to be simultaneously via each of the "file", "http", and "ftp" schemes if the documents
accessible and traversable via each of the "file", "http", and "ftp" refer to each other using relative references. Furthermore, such
schemes if the documents refer to each other using relative document trees can be moved, as a whole, without changing any of the
references. Furthermore, such document trees can be moved, as a relative references.
whole, without changing any of the 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 current 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 references are algorithm, presented in Section 5, defines how such a reference is
resolved. transformed to the target URI. Since relative references can only be
used within the context of a hierarchical URI, designers of new URI
schemes should use a syntax consistent with the generic syntax's
hierarchical components unless there are compelling reasons to forbid
relative referencing within that scheme.
All URIs are parsed by generic syntax parsers when used. A URI scheme
that wishes to remain opaque to hierarchical processing must disallow
the use of slash and question mark characters. However, since a
non-relative URI reference is only modified by the generic parser if
it contains complete path segments of "." or ".." (see Section 3.3),
URIs may safely use "/" for other 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] to define the URI syntax. Although the ABNF notation of [RFC2234], including the following core ABNF syntax rules
defines syntax in terms of the US-ASCII character encoding [ASCII], defined by that specification: ALPHA (letters), CR (carriage return),
the URI syntax should be interpreted in terms of the character that CTL (control characters), DIGIT (decimal digits), DQUOTE (double
the ASCII-encoded octet represents, rather than the octet encoding quote), HEXDIG (hexadecimal digits), LF (line feed), and SP (space).
itself. How a URI is represented in terms of bits and bytes on the The complete URI syntax is collected in Appendix A.
wire is dependent upon the character encoding of the protocol used to
transport it, or the charset of the document that contains it.
The following core ABNF productions are used by this specification as
defined by Section 6.1 of [RFC2234]: ALPHA, CR, CTL, DIGIT, DQUOTE,
HEXDIG, LF, OCTET, and SP. The complete URI syntax is collected in
Appendix A.
2. Characters 2. Characters
A URI consists of a restricted set of characters, primarily chosen Although ABNF notation defines its terminal values to be non-negative
to aid transcription and usability both in computer systems and in integers (codepoints) based on the US-ASCII coded character set
non-computer communications. Characters used conventionally as [ASCII], we must invert that relation in order to understand the URI
delimiters around a URI are excluded. The set of URI characters syntax, since URIs are defined as strings of characters independent
consists of digits, letters, and a few graphic symbols chosen from of any particular encoding. Therefore, the integer values must be
those common to most of the character encodings and input facilities mapped back to their corresponding characters via US-ASCII in order
available to Internet users. to complete the syntax rules.
uric = reserved / unreserved / escaped This specification does not mandate the use of any particular
character encoding scheme for mapping between URI characters and the
octets used to store or transmit those characters. When a URI appears
in a protocol element, the character encoding is defined by that
protocol; absent such a definition, a URI is assumed to use the same
character encoding as the surrounding text.
Within a URI, reserved characters are used to delimit syntax A URI is composed from a limited set of characters consisting of
components, unreserved characters are used to describe registered digits, letters, and a few graphic symbols. A reserved (Section 2.2)
names, and unreserved, non-delimiting reserved, and escaped subset of those characters may be used to delimit syntax components
characters are used to represent strings of data (1*OCTET) within the within a URI, while the remaining characters, including both the
components. unreserved (Section 2.3) set and those reserved characters not acting
as delimiters, define each component's data.
2.1 Encoding of Characters 2.1 Percent Encoding
As described above (Section 1.3), the URI syntax is defined in terms A percent-encoding mechanism is used to represent a data octet in a
of characters by reference to the US-ASCII encoding of characters to component when that octet's corresponding character is outside the
octets. This specification does not mandate the use of any allowed set or is being used as a delimiter of, or within, the
particular mapping between its character set and the octets used to component. A percent-encoded octet is encoded as a character triplet,
store or transmit those characters. consisting of the percent character "%" followed by the two
hexadecimal digits representing that octet's numeric value. For
example, "%20" is the percent-encoding for the binary octet
"00100000" (ABNF: %x20), which in US-ASCII corresponds to the space
character (SP).
URI characters representing strings of data within a component may, pct-encoded = "%" HEXDIG HEXDIG
if allowed by the component production, represent an arbitrary
sequence of octets. For example, portions of a given URI might
correspond to a filename on a non-ASCII file system, a query on
non-ASCII data, numeric coordinates on a map, etc. Some URI schemes
define a specific encoding of raw data to US-ASCII characters as part
of their scheme-specific requirements. Most URI schemes represent
data octets by the US-ASCII character corresponding to that octet,
either directly in the form of the character's glyph or by use of an
escape triplet (Section 2.4).
When a URI scheme defines a component that represents textual data The uppercase hexadecimal digits 'A' through 'F' are equivalent to
consisting of characters from the Unicode (ISO 10646) character set, the lowercase digits 'a' through 'f', respectively. Two URIs that
we recommend that the data be encoded first as octets according to differ only in the case of hexadecimal digits used in percent-encoded
the UTF-8 [UTF-8] character encoding, and then escaping only those octets are equivalent. For consistency, URI producers and
octets that are not in the unreserved character set. normalizers should use uppercase hexadecimal digits for all
percent-encodings.
2.2 Reserved Characters 2.2 Reserved Characters
URIs include components and sub-components that are delimited by URIs include components and sub-components that are delimited by
certain special characters. These characters are called "reserved", characters in the "reserved" set. These characters are called
since their usage within a URI component is limited to their reserved "reserved" because they may (or may not) be defined as delimiters by
purpose within that component. If data for a URI component would the generic syntax, by each scheme-specific syntax, or by the
conflict with the reserved purpose, then the conflicting data must be implementation-specific syntax of a URI's dereferencing algorithm.
escaped (Section 2.4) before forming the URI. If data for a URI component would conflict with a reserved
character's purpose as a delimiter, then the conflicting data must be
percent-encoded before forming the URI.
reserved = "/" / "?" / "#" / "[" / "]" / ";" / reserved = gen-delims / sub-delims
":" / "@" / "&" / "=" / "+" / "$" / ","
Reserved characters are used as delimiters of the generic URI gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@"
components described in Section 3, as well as within those components
for delimiting sub-components. A component's ABNF syntax rule will
not use the "reserved" production directly; instead, each rule lists
those reserved characters that are allowed within that component.
Allowed reserved characters that are not assigned a sub-component
delimiter role by this specification should be considered reserved
for special use by whatever software generates the URI (i.e., they
may be used to delimit or indicate information that is significant to
interpretation of the identifier, but that significance is outside
the scope of this specification). Outside of the URI's origin, a
reserved character cannot be escaped without fear of changing how it
will be interpreted; likewise, an escaped octet that corresponds to a
reserved character cannot be unescaped outside the software that is
responsible for interpreting it during URI resolution.
The slash ("/"), question-mark ("?"), and number-sign ("#") sub-delims = "!" / "$" / "&" / "'" / "(" / ")"
characters are reserved in all URIs for the purpose of delimiting / "*" / "+" / "," / ";" / "="
components that are significant to the generic parser's hierarchical
interpretation of an identifier. The hierarchical prefix of a URI, A subset of the reserved characters (gen-delims) are used as
wherein the slash ("/") character signifies a hierarchy delimiter, delimiters of the generic URI components described in Section 3. A
extends from the scheme (Section 3.1) through to the first component's ABNF syntax rule will not use the reserved or gen-delims
question-mark ("?"), number-sign ("#"), or the end of the URI string. rule names directly; instead, each syntax rule lists those reserved
In other words, the slash ("/") character is not treated as a characters that are allowed within that component (i.e., not
hierarchical separator within the query (Section 3.4) and fragment delimiting it). The allowed reserved characters, including those in
(Section 3.5) components of a URI, but is still considered reserved the sub-delims set and any of the gen-delims that are not a delimiter
within those components for purposes outside the scope of this of that component, are reserved for use as sub-component delimiters
specification. within the component. Only the most common sub-components are
defined by this specification; other sub-components may be defined by
a URI scheme's specification, or by the implementation-specific
syntax of a URI's dereferencing algorithm, provided that such
sub-components are delimited by characters in that component's
reserved set. If no such delimiting role has been assigned, then a
reserved character appearing in a component represents the data octet
corresponding to its encoding in US-ASCII.
URIs that differ in the replacement of a reserved character with its
corresponding percent-encoded octet are not equivalent.
Percent-encoding a reserved character, or decoding a percent-encoded
octet that corresponds to a reserved character, will change how the
URI is interpreted by most applications.
2.3 Unreserved Characters 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, and a limited set of punctuation marks and letters, decimal digits, hyphen, period, underscore, and tilde.
symbols.
unreserved = ALPHA / DIGIT / mark
mark = "-" / "_" / "." / "!" / "~" / "*" / "'" / "(" / ")"
Escaping unreserved characters in a URI does not change what resource
is identified by that URI. However, it may change the result of a
URI comparison (Section 6), potentially leading to less efficient
actions by an application. Therefore, unreserved characters should
not be escaped unless the URI is being used in a context that does
not allow the unescaped character to appear. URI normalization
processes may unescape sequences in the ranges of ALPHA (%41-%5A and
%61-%7A), DIGIT (%30-%39), hyphen (%2D), underscore (%5F), or tilde
(%7E) without fear of creating a conflict, but unescaping the other
mark characters is usually counterproductive.
2.4 Escaped Characters
Data must be escaped if it does not have a representation using an
unreserved character; this includes data that does not correspond to
a printable character of the US-ASCII coded character set or
corresponds to a US-ASCII character that delimits the component from
others, is reserved in that component for delimiting sub-components,
or is excluded from any use within a URI (Section 2.5).
2.4.1 Escaped Encoding
An escaped octet is encoded as a character triplet, consisting of
the percent character "%" followed by the two hexadecimal digits
representing that octet's numeric value. For example, "%20" is the
escaped encoding for the binary octet "00100000" (ABNF: %x20), which
corresponds to the US-ASCII space character (SP). This is sometimes
referred to as "percent-encoding" the octet.
escaped = "%" HEXDIG HEXDIG
The uppercase hexadecimal digits 'A' through 'F' are equivalent to
the lowercase digits 'a' through 'f', respectively. Two URIs that
differ only in the case of hexadecimal digits used in escaped octets
are equivalent. For consistency, we recommend that uppercase digits
be used by URI generators and normalizers.
2.4.2 When to Escape and Unescape
Under normal circumstances, the only time that characters within a
URI string are escaped is during the process of generating the URI
from its component parts. Each component may have its own set of
characters that are reserved, so only the mechanism responsible for
generating or interpreting that component can determine whether or
not escaping a character will change its semantics. The exception is
when a URI is being used within a context where the unreserved "mark"
characters might need to be escaped, such as when used for a
command-line argument or within a single-quoted attribute.
Once generated, a URI is always in an escaped form. When a URI is
resolved, the components significant to that scheme-specific
resolution process (if any) must be parsed and separated before the
escaped characters within those components can be safely unescaped.
In some cases, data that could be represented by an unreserved
character may appear escaped; for example, some of the unreserved
"mark" characters are automatically escaped by some systems. A URI
normalizer may unescape escaped octets that are represented by
characters in the unreserved set. For example, "%7E" is sometimes
used instead of tilde ("~") in an "http" URI path and can be
converted to "~" without changing the interpretation of the URI.
In all cases, a URI character is equivalent to its corresponding
ASCII-encoded octet, even when that octet is represented as a
percent-escape. URI characters are provided as an external ASCII
interface for identification between systems. A system that
internally provides identifiers in the form of a different character
encoding, such as EBCDIC, will generally perform character
translation of textual identifiers to UTF-8 at some internal
interface, thus providing meaningful identifiers in ASCII even though
the back-end identifiers are in a different encoding. Escaped octets
must be unescaped before such a transcoding is applied. Although
this specification does not define the character encoding of escaped
octets outside the ASCII range, the general principle of unescaping
before transcoding should be applied for all character encodings.
Because the percent ("%") character serves as the escape indicator,
it must be escaped as "%25" in order for that octet to be used as
data within a URI. Implementers should be careful not to escape or
unescape the same string more than once, since unescaping an already
unescaped string might lead to misinterpreting a percent data
character as another escaped character, or vice versa in the case of
escaping an already escaped string.
2.5 Excluded Characters
Although they are disallowed within the URI syntax, we include here unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
a description of those characters that have been excluded and the
reasons for their exclusion.
excluded = invisible / delims / unwise URIs that differ in the replacement of an unreserved character with
its corresponding percent-encoded octet are equivalent: they identify
the same resource. However, percent-encoded unreserved characters
may change the result of some URI comparisons (Section 6),
potentially leading to incorrect or inefficient behavior. For
consistency, percent-encoded octets in the ranges of ALPHA (%41-%5A
and %61-%7A), DIGIT (%30-%39), hyphen (%2D), period (%2E), underscore
(%5F), or tilde (%7E) should not be created by URI producers and,
when found in a URI, should be decoded to their corresponding
unreserved character by URI normalizers.
The control characters (CTL) in the US-ASCII coded character set are 2.4 When to Encode or Decode
not used within a URI, both because they are non-printable and
because they are likely to be misinterpreted by some control
mechanisms. The space character (SP) is excluded because significant
spaces may disappear and insignificant spaces may be introduced when
a URI is transcribed, typeset, or subjected to the treatment of
word-processing programs. Whitespace is also used to delimit a URI
in many contexts. Characters outside the US-ASCII set are excluded as
well.
invisible = CTL / SP / %x80-FF Under normal circumstances, the only time that octets within a URI
are percent-encoded is during the process of producing the URI from
its component parts. It is during that process that an
implementation determines which of the reserved characters are to be
used as sub-component delimiters and which can be safely used as
data. Once produced, a URI is always in its percent-encoded form.
The angle-bracket ("<" and ">") and double-quote (") characters are When a URI is dereferenced, the components and sub-components
excluded because they are often used as the delimiters around a URI significant to the scheme-specific dereferencing process (if any)
in text documents and protocol fields. The percent character ("%") must be parsed and separated before the percent-encoded octets within
is excluded because it is used for the encoding of escaped (Section those components can be safely decoded, since otherwise the data may
2.4) characters. be mistaken for component delimiters. The only exception is for
percent-encoded octets corresponding to characters in the unreserved
set, which can be decoded at any time. For example, the octet
corresponding to the tilde ("~") character is often encoded as "%7E"
by older URI processing software; the "%7E" can be replaced by "~"
without changing its interpretation.
delims = "<" / ">" / "%" / DQUOTE Because the percent ("%") character serves as the indicator for
percent-encoded octets, it must be percent-encoded as "%25" in order
for that octet to be used as data within a URI. Implementations must
not percent-encode or decode the same string more than once, since
decoding an already decoded string might lead to misinterpreting a
percent data octet as the beginning of a percent-encoding, or vice
versa in the case of percent-encoding an already percent-encoded
string.
Other characters are excluded because gateways and other transport URI characters serve as an external interface for identification
agents are known to sometimes modify such characters. between systems. A system that internally provides identifiers in
the form of a different character encoding, such as EBCDIC, will
generally perform character translation of textual identifiers to
UTF-8 [RFC3629] (or some other superset of the US-ASCII character
encoding) at an internal interface, since that results in more
meaningful identifiers than simply percent-encoding the original
octets. When interpreting an incoming URI on such an interface,
percent-encoded octets must be decoded before the reverse transcoding
can be applied.
unwise = "{" / "}" / "|" / "\" / "^" / "`" In some cases, the interface between a URI component and the
identifying data it has been crafted to represent is much less direct
than a character encoding translation. For example, portions of a
URI might reflect a query on non-ASCII data, numeric coordinates on a
map, etc. Likewise, a URI scheme may define components with
additional encoding requirements, such as base64, that are applied
prior to forming the component and producing the URI.
Data octets corresponding to excluded characters must be escaped in When a URI scheme defines a component that represents textual data
order to be represented within a URI. consisting of characters from the Unicode (ISO/IEC 10646-1) character
set, the data should be encoded first as octets according to the
UTF-8 character encoding [RFC3629], and then only those octets that
do not correspond to characters in the unreserved set should be
percent-encoded. For example, the character A would be represented
as "A", the character LATIN CAPITAL LETTER A WITH GRAVE would be
represented as "%C3%80", and the character KATAKANA LETTER A would be
represented as "%E3%82%A2".
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 ":" ["//" authority] path ["?" query] ["#" fragment]
hier-part = net-path / abs-path / rel-path
net-path = "//" authority [ abs-path ]
abs-path = "/" path-segments
rel-path = path-segments
The scheme and path components are required, though path may be empty The scheme and path components are required, though path may be empty
(no characters). An ABNF-driven parser of hier-part will find that (no characters). An ABNF-driven parser will find that the border
the three productions in the rule are ambiguous: they are between authority and path is ambiguous; they are disambiguated by
disambiguated by the "first-match-wins" (a.k.a. "greedy") algorithm. the "first-match-wins" (a.k.a. "greedy") algorithm. In other words,
In other words, if the string begins with two slash characters ("// if authority is present then the first segment of the path must be
"), then it is a net-path; if it begins with only one slash empty.
character, then it is an abs-path; otherwise, it is a rel-path. Note
that rel-path does not necessarily contain any slash ("/")
characters; a non-hierarchical path will be treated as opaque data by
a generic URI parser.
The authority component is only present when a string matches the
net-path production. Since the presence of an authority component
restricts the remaining syntax for path, we have not included a
specific "path" rule in the syntax. Instead, what we refer to as the
URI path is that part of the parsed URI string matching the abs-path
or rel-path production in the syntax above, since they are mutually
exclusive for any given URI and can be parsed as a single component.
The following are two example URIs and their component parts: 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
skipping to change at page 17, line 15 skipping to change at page 15, line 45
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 generate 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
[RFC2717]. The scheme registry maintains the mapping between scheme [RFC2717]. The scheme registry maintains the mapping between scheme
names and their specifications. names and their specifications. Advice for designers of new URI
schemes can be found in [RFC2718].
When presented with a URI that violates one or more scheme-specific
restrictions, the scheme-specific resolution process should flag the
reference as an error rather than ignore the unused parts; doing so
reduces the number of equivalent URIs and helps detect abuses of the
generic syntax that might indicate the URI has been constructed to
mislead the user (Section 7.6).
3.2 Authority 3.2 Authority
Many URI schemes include a hierarchical element for a naming Many URI schemes include a hierarchical element for a naming
authority, such that governance of the name space defined by the authority, such that governance of the name space defined by the
remainder of the URI is delegated to that authority (which may, in remainder of the URI is delegated to that authority (which may, in
turn, delegate it further). The generic syntax provides a common turn, delegate it further). The generic syntax provides a common
means for distinguishing an authority based on a registered domain means for distinguishing an authority based on a registered name or
name or server address, along with optional port and user server address, along with optional port and user information.
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 terminated by the next slash ("/"), question mark ("?"), or number
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 ]
The parts "<userinfo>@" and ":<port>" may be omitted. URI producers and normalizers should omit the "@" delimiter that
separates userinfo from host if the userinfo component is empty (zero
Some schemes do not allow the userinfo and/or port sub-components. length) and should omit the ":" delimiter that separates host from
When presented with a URI that violates one or more scheme-specific port if the port component is empty. Some schemes do not allow the
restrictions, the scheme-specific URI resolution process should flag userinfo and/or port sub-components.
the reference as an error rather than ignore the unused parts; doing
so reduces the number of equivalent URIs and helps detect abuses of
the generic syntax that might indicate the URI has been constructed
to mislead the user (Section 7.5).
3.2.1 User Information 3.2.1 User Information
The userinfo sub-component may consist of a user name and, The userinfo sub-component may consist of a user name and,
optionally, scheme-specific information about how to gain optionally, scheme-specific information about how to gain
authorization to access the server. The user information, if authorization to access the resource. The user information, if
present, is followed by a commercial at-sign ("@") that delimits it present, is followed by a commercial at-sign ("@") that delimits it
from the host. from the host.
userinfo = *( unreserved / escaped / ";" / userinfo = *( unreserved / pct-encoded / sub-delims / ":" )
":" / "&" / "=" / "+" / "$" / "," )
Some URI schemes use the format "user:password" in the userinfo Use of the format "user:password" in the userinfo field is
field. This practice is NOT RECOMMENDED, because the passing of deprecated. Applications should not render as clear text any data
after the first colon (":") character found within a userinfo
sub-component unless such data is the empty string (indicating no
password) or "anonymous". Applications may choose to ignore or reject
such data when received as part of a reference, and should reject the
storage of such data in unencrypted form. The passing of
authentication information in clear text has proven to be a security authentication information in clear text has proven to be a security
risk in almost every case where it has been used. Note also that risk in almost every case where it has been used.
userinfo might be crafted to look like a trusted domain name in order
to mislead users, as described in Section 7.5. Applications that render a URI for the sake of user feedback, such as
in graphical hypertext browsing, should render userinfo in a way that
is distinguished from the rest of a URI, when feasible. Such
rendering will assist the user in cases where the userinfo has been
misleadingly crafted to look like a trusted domain name (Section
7.6).
3.2.2 Host 3.2.2 Host
The host sub-component of authority is identified by an IPv6 literal The host sub-component 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 domain name. dotted-decimal form, or a host name.
host = [ IPv6reference / IPv4address / hostname ] host = IP-literal / IPv4address / reg-name
If host is omitted, a default may be defined by the scheme-specific The syntax rule for host is ambiguous because it does not completely
semantics of the URI. For example, the "file" URI scheme defaults to distinguish between an IPv4address and a reg-name. Again, the
"localhost", whereas the "http" URI scheme does not allow host to be "first-match-wins" algorithm applies: If host matches the rule for
omitted. IPv4address, then it should be considered an IPv4 address literal and
not a reg-name. Although host is case-insensitive, producers and
normalizers should use lowercase for host names and hexadecimal
addresses for the sake of uniformity, while only using uppercase
letters for percent-encodings.
The production for host is ambiguous because it does not completely A host identified by an Internet Protocol literal address, version 6
distinguish between an IPv4address and a hostname. Again, the [RFC3513] or later, is distinguished by enclosing the IP literal
"first-match-wins" algorithm applies: If host matches the production within square brackets ("[" and "]"). This is the only place where
for IPv4address, then it should be considered an IPv4 address literal square bracket characters are allowed in the URI syntax. In
and not a hostname. anticipation of future, as-yet-undefined IP literal address formats,
an optional version flag may be used to indicate such a format
explicitly rather than relying on heuristic determination.
A hostname takes the form described in Section 3 of [RFC1034] and IP-literal = "[" ( IPv6address / IPvFuture ) "]"
Section 2.1 of [RFC1123]: a sequence of domain labels separated by
".", each domain label starting and ending with an alphanumeric
character and possibly also containing "-" characters. The rightmost
domain label of a fully qualified domain name may be followed by a
single "." if it is necessary to distinguish between the complete
domain name and some local domain.
hostname = domainlabel qualified IPvFuture = "v" HEXDIG "." 1*( unreserved / sub-delims / ":" )
qualified = *( "." domainlabel ) [ "." ]
domainlabel = alphanum [ 0*61( alphanum / "-" ) alphanum ] The version flag does not indicate the IP version; rather, it
alphanum = ALPHA / DIGIT indicates future versions of the literal format. As such,
implementations must not provide the version flag for existing IPv4
and IPv6 literal addresses. If a URI containing an IP-literal that
starts with "v" (case-insensitive), indicating that the version flag
is present, is dereferenced by an application that does not know the
meaning of that version flag, then the application should return an
appropriate error for "address mechanism not supported".
A host identified by an IPv6 literal address is represented inside
the square brackets without a preceding version flag. The ABNF
provided here is a translation of the text definition of an IPv6
literal address provided in [RFC3513]. A 128-bit IPv6 address is
divided into eight 16-bit pieces. Each piece is represented
numerically in case-insensitive hexadecimal, using one to four
hexadecimal digits (leading zeroes are permitted). The eight encoded
pieces are given most-significant first, separated by colon
characters. Optionally, the least-significant two pieces may instead
be represented in IPv4 address textual format. A sequence of one or
more consecutive zero-valued 16-bit pieces within the address may be
elided, omitting all their digits and leaving exactly two consecutive
colons in their place to mark the elision.
IPv6address = 6( h16 ":" ) ls32
/ "::" 5( h16 ":" ) ls32
/ [ h16 ] "::" 4( h16 ":" ) ls32
/ [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
/ [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
/ [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32
/ [ *4( h16 ":" ) h16 ] "::" ls32
/ [ *5( h16 ":" ) h16 ] "::" h16
/ [ *6( h16 ":" ) h16 ] "::"
ls32 = ( h16 ":" h16 ) / IPv4address
; least-significant 32 bits of address
h16 = 1*4HEXDIG
; 16 bits of address represented in hexadecimal
A host identified by an IPv4 literal address is represented in A host identified by an IPv4 literal address is represented in
dotted-decimal notation (a sequence of four decimal numbers in the dotted-decimal notation (a sequence of four decimal numbers in the
range 0 to 255, separated by "."), as described in [RFC1123] by range 0 to 255, separated by "."), as described in [RFC1123] by
reference to [RFC0952]. Note that other forms of dotted notation may reference to [RFC0952]. Note that other forms of dotted notation may
be interpreted on some platforms, as described in Section 7.3, but be interpreted on some platforms, as described in Section 7.4, but
only the dotted-decimal form of four octets is allowed by this only the dotted-decimal form of four octets is allowed by this
grammar. grammar.
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
A host identified by an IPv6 literal address [RFC3513] is A host identified by a registered name is a string of characters that
distinguished by enclosing the IPv6 literal within square-brackets is intended for lookup within a locally-defined host or service name
("[" and "]"). This is the only place where square-bracket registry. The most common of such registry mechanisms is the Domain
characters are allowed in the URI syntax. Name System (DNS), as defined by Section 3 of [RFC1034] and Section
2.1 of [RFC1123]. A DNS name consists of a sequence of domain labels
separated by ".", each domain label starting and ending with an
alphanumeric character and possibly also containing "-" characters.
The rightmost domain label of a fully qualified domain name in DNS
may be followed by a single "." and should be followed by one if it
is necessary to distinguish between the complete domain name and some
local domain.
IPv6reference = "[" IPv6address "]" reg-name = 0*255( unreserved / pct-encoded / sub-delims )
IPv6address = 6( h4 ":" ) ls32 If the host component is defined and the registered name is empty
/ "::" 5( h4 ":" ) ls32 (zero length), then the name defaults to "localhost" (Section 6.2.3
/ [ h4 ] "::" 4( h4 ":" ) ls32 discusses how this should be normalized). If "localhost" is not
/ [ *1( h4 ":" ) h4 ] "::" 3( h4 ":" ) ls32 determined by a host name lookup, then it should be interpreted to
/ [ *2( h4 ":" ) h4 ] "::" 2( h4 ":" ) ls32 mean the machine on which the URI is being resolved.
/ [ *3( h4 ":" ) h4 ] "::" h4 ":" ls32
/ [ *4( h4 ":" ) h4 ] "::" ls32
/ [ *5( h4 ":" ) h4 ] "::" h4
/ [ *6( h4 ":" ) h4 ] "::"
ls32 = ( h4 ":" h4 ) / IPv4address This specification does not mandate a particular registered name
; least-significant 32 bits of address lookup technology and therefore does not restrict the syntax of
reg-name beyond that necessary for interoperability. Instead, it
delegates the issue of host name syntax conformance to the operating
system of each application performing URI resolution, and that
operating system decides what it will allow for the purpose of host
identification. A URI resolution implementation might use DNS, host
tables, yellow pages, NetInfo, WINS, or any other system for lookup
of host and service names. However, a globally-scoped naming system,
such as DNS fully-qualified domain names, is necessary for URIs that
are intended to have global scope. URI producers should use host
names that conform to the DNS syntax, even when use of DNS is not
immediately apparent.
h4 = 1*4HEXDIG The reg-name syntax allows percent-encoded octets in order to
represent non-ASCII host or service names in a uniform way that is
independent of the underlying name resolution technology; such octets
must represent characters encoded in the UTF-8 character encoding
[RFC3629] prior to being percent-encoded. When a non-ASCII host name
represents an internationalized domain name intended for resolution
via DNS, the name must be transformed to the IDNA encoding [RFC3490]
prior to name lookup. URI producers should provide such host names in
the IDNA encoding, rather than a percent-encoding, if they wish to
maximize interoperability with legacy URI resolvers.
The presence of host within a URI does not imply that the scheme The presence of host within a URI does not imply that the scheme
requires access to the given host on the Internet. In many cases, requires access to the given host on the Internet. In many cases,
the host syntax is used only for the sake of reusing the existing the host syntax is used only for the sake of reusing the existing
registration process created and deployed for DNS, thus obtaining a registration process created and deployed for DNS, thus obtaining a
globally unique name without the cost of deploying another registry. globally unique name without the cost of deploying another registry.
However, such use comes with its own costs: domain name ownership may However, such use comes with its own costs: domain name ownership may
change over time for reasons not anticipated by the URI creator. change over time for reasons not anticipated by the URI producer.
3.2.3 Port 3.2.3 Port
The port sub-component of authority is designated by an optional The port sub-component of authority is designated by an optional port
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
If port is omitted, a default may be defined by the scheme-specific A scheme may define a default port. For example, the "http" scheme
semantics of the URI. Likewise, the type of network port designated defines a default port of "80", corresponding to its reserved TCP
by the port number (e.g., TCP, UDP, SCTP, etc.) is defined by the URI port number. The type of port designated by the port number (e.g.,
scheme. For example, the "http" URI scheme defines a default of TCP TCP, UDP, SCTP, etc.) is defined by the URI scheme. URI producers
port 80. and normalizers should omit the port component and its ":" delimiter
if port is empty or its value would be the same as the scheme's
default.
3.3 Path 3.3 Path
The path component contains hierarchical data that, along with data The path component contains data, usually organized in hierarchical
in the optional query (Section 3.4) component, serves to identify a form, that, along with data in the non-hierarchical query component
resource within the scope of that URI's scheme and naming authority (Section 3.4), serves to identify a resource within the scope of the
(if any). There is no specific "path" syntax production in the URI's scheme and naming authority (if any). If a URI contains an
generic URI syntax. Instead, what we refer to as the URI path is authority component, then the initial path segment must be empty
that part of the parsed URI string matching either the abs-path or (i.e., the path must begin with a slash ("/") character or be
the rel-path production, since they are mutually exclusive for any entirely empty). The path is terminated by the first question mark
given URI and can be parsed as a single component. The path is ("?") or number sign ("#") character, or by the end of the URI.
terminated by the first question-mark ("?") or number-sign ("#")
character, or by the end of the URI.
path-segments = segment *( "/" segment ) path = segment *( "/" segment )
segment = *pchar segment = *pchar
pchar = unreserved / escaped / ";" / pchar = unreserved / pct-encoded / sub-delims / ":" / "@"
":" / "@" / "&" / "=" / "+" / "$" / ","
The 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) or opaque (not containing any defined path may be empty (zero length). Use of the slash character
"/" delimiters). For example, the URI <mailto:fred@example.com> has to indicate hierarchy is only required when a URI will be used as the
a path of "fred@example.com". context for relative references. For example, the URI
<mailto:fred@example.com> has a path of "fred@example.com", whereas
the URI <foo://info.example.com?fred> has an empty path.
The path segments "." and ".." are defined for relative reference The path segments "." and ".." are defined for relative reference
within the path name hierarchy. They are intended for use at the within the path name hierarchy. They are intended for use at the
beginning of a relative path reference (Section 4.2) for indicating beginning of a relative path reference (Section 4.2) for indicating
relative position within the hierarchical tree of names, with a relative position within the hierarchical tree of names. This is
similar effect to how they are used within some operating systems' similar to their role within some operating systems' file directory
file directory structure to indicate the current directory and parent structure to indicate the current directory and parent directory,
directory, respectively. Unlike a file system, however, these respectively. However, unlike a file system, these dot-segments are
dot-segments are only interpreted within the URI path hierarchy and only interpreted within the URI path hierarchy and are removed as
are removed as part of the URI normalization or resolution process, part of the resolution process (Section 5.2).
as described in 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 generating applications considered opaque by the generic syntax. URI-producing applications
often use the reserved characters allowed in segment for the purpose often use the reserved characters allowed in a segment for the
of delimiting scheme-specific or generator-specific sub-components. purpose of delimiting scheme-specific or dereference-handler-specific
For example, the semicolon (";") and equals ("=") reserved characters sub-components. For example, the semicolon (";") and equals ("=")
are often used for delimiting parameters and parameter values reserved characters are often used for delimiting parameters and
applicable to that segment. The comma (",") reserved character is parameter values applicable to that segment. The comma (",")
often used for similar purposes. For example, one URI generator reserved character is often used for similar purposes. For example,
might use a segment like "name;v=1.1" to indicate a reference to one URI producer might use a segment like "name;v=1.1" to indicate a
version 1.1 of "name", whereas another might use a segment like reference to version 1.1 of "name", whereas another might use a
"name,1.1" to indicate the same. Parameter types may be defined by segment like "name,1.1" to indicate the same. Parameter types may be
scheme-specific semantics, but in most cases the meaning of a defined by scheme-specific semantics, but in most cases the syntax of
parameter is specific to the URI originator. a parameter is specific to the implementation of the URI's
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 (Section 3.3) component, serves to identify a data in the path component (Section 3.3), serves to identify a
resource within the scope of that 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 mark
("?") character and terminated by a number-sign ("#") character or by ("?") character and terminated by a number sign ("#") character or by
the end of the URI. the end of the URI.
query = *( pchar / "/" / "?" ) query = *( pchar / "/" / "?" )
The characters slash ("/") and question-mark ("?") are allowed to The characters slash ("/") and question mark ("?") may represent data
represent data within the query component, but such use is within the query component, but should not be used as such within a
discouraged; incorrect implementations of reference resolution often URI that is expected to be the base for relative references (Section
fail to distinguish them from hierarchical separators, thus resulting 5.1). Incorrect implementations of reference resolution often fail
in non-interoperable results while parsing relative references. to distinguish query data from path data when looking for
hierarchical separators, thus resulting in non-interoperable results.
However, since query components are often used to carry identifying However, since query components are often used to carry identifying
information in the form of "key=value" pairs, and one frequently used information in the form of "key=value" pairs, and one frequently used
value is a reference to another URI, it is sometimes better for value is a reference to another URI, it is sometimes better for
usability to include those characters unescaped. usability to avoid percent-encoding those characters.
Note: Some client applications will fail to separate a reference's
query component from its path component before merging the base
and reference paths (Section 5.2). This may result in loss of
information if the query component contains the strings "/../" or
"/./".
3.5 Fragment 3.5 Fragment
The fragment identifier component allows indirect identification of a The fragment identifier component of a URI allows indirect
secondary resource by reference to a primary resource and additional identification of a secondary resource by reference to a primary
identifying information that is selective within that resource. The resource and additional identifying information. The identified
identified secondary resource may be some portion or subset of the secondary resource may be some portion or subset of the primary
primary resource, some view on representations of the primary resource, some view on representations of the primary resource, or
resource, or some other resource that is merely named within the some other resource defined or described by those representations. A
primary resource. A fragment identifier component is indicated by fragment identifier component is indicated by the presence of a
the presence of a number-sign ("#") character and terminated by the number sign ("#") character and terminated by the end of the URI.
end of the URI string.
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 the 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. Individual media types may define their own URI is dereferenced. Individual media types may define their own
restrictions on, or structure within, the fragment identifier syntax restrictions on, or structure within, the fragment identifier syntax
for specifying different types of subsets, views, or external for specifying different types of subsets, views, or external
references that are identifiable as secondary resources by that media references that are identifiable as secondary resources by that media
type. If the primary resource is represented by multiple media type. If the primary resource has multiple representations, as is
types, as is often the case for resources whose representation is often the case for resources whose representation is selected based
selected based on attributes of the retrieval request, then on attributes of the retrieval request (a.k.a., content negotiation),
interpretation of the fragment identifier must be consistent across then whatever is identified by the fragment should be consistent
all of those media types in order for it to be viable as an across all of those representations: each representation should
identifier. either define the fragment such that it corresponds to the same
secondary resource, regardless of how it is represented, or the
fragment should be left undefined by the representation (i.e., not
found).
As with any URI, use of a fragment identifier component does not 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. However, if implication that the primary resource is accessible or will ever be
that URI is used in a context that does call for retrieval and is not accessed.
a same-document reference (Section 4.4), the fragment identifier is
only valid as a reference if a retrieval action on the primary
resource succeeds and results in a representation for which the
fragment identifier is meaningful.
Fragment identifiers have a special role in information systems as Fragment identifiers have a special role in information systems as
the primary form of client-side indirect referencing, allowing an the primary form of client-side indirect referencing, allowing an
author to specifically identify those aspects of an existing resource author to specifically identify those aspects of an existing resource
that are only indirectly provided by the resource owner. As such, that are only indirectly provided by the resource owner. As such,
interpretation of the fragment identifier during a retrieval action interpretation of the fragment identifier during a retrieval action
is performed solely by the user agent; the fragment identifier is not is performed solely by the user agent; the fragment identifier is not
passed to other systems during the process of retrieval. Although passed to other systems during the process of retrieval. Although
this is often perceived to be a loss of information, particularly in this is often perceived to be a loss of information, particularly in
regards to accurate redirection of references as content moves over regards to accurate redirection of references as content moves over
time, it also serves to prevent information providers from denying time, it also serves to prevent information providers from denying
reference authors the right to selectively refer to information reference authors the right to selectively refer to information
within a resource. within a resource.
The characters slash ("/") and question-mark ("?") are allowed to The characters slash ("/") and question mark ("?") are allowed to
represent data within the fragment identifier, but such use is represent data within the fragment identifier, but should not be used
discouraged for the same reasons as described above for query. as such within a URI that is expected to be the base for relative
references (Section 5.1) for the same reasons as described above for
query.
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 production. In full form of reference defined by the "URI" syntax rule. In order to
order to save space and take advantage of hierarchical locality, many save space and take advantage of hierarchical locality, many Internet
Internet protocol elements and media type formats allow an protocol elements and media type formats allow an abbreviation of a
abbreviation of a URI, while others restrict the syntax to a URI, while others restrict the syntax to a particular form of URI.
particular form of URI. We define the most common forms of reference We define the most common forms of reference syntax in this
syntax in this specification because they impact and depend upon the specification because they impact and depend upon the design of the
design of the generic syntax, requiring a uniform parsing algorithm generic syntax, requiring a uniform parsing algorithm in order to be
in order to be interpreted consistently. interpreted consistently.
4.1 URI Reference 4.1 URI Reference
The ABNF rule URI-reference is used to denote the most common usage URI-reference is used to denote the most common usage of a resource
of a resource identifier. identifier.
URI-reference = URI / relative-URI URI-reference = URI / relative-URI
A URI-reference may be relative: if the reference string's prefix A URI-reference may be relative: if the reference's prefix matches
matches the syntax of a scheme followed by its colon separator, then the syntax of a scheme followed by its colon separator, then the
the reference is a URI rather than a relative-URI. reference is a URI rather than a relative-URI.
A URI-reference is typically parsed first into the five URI A URI-reference is typically parsed first into the five URI
components, in order to determine what components are present and components, in order to determine what components are present and
whether or not the reference is relative, and then each component is whether or not the reference is relative, and then each component is
parsed for its subparts and their validation. The ABNF of parsed for its subparts and their validation. The ABNF of
URI-reference, along with the "first-match-wins" disambiguation rule, URI-reference, along with the "first-match-wins" disambiguation rule,
is sufficient to define a validating parser for the generic syntax. is sufficient to define a validating parser for the generic syntax.
Readers familiar with regular expressions should see Appendix B for Readers familiar with regular expressions should see Appendix B for
an example of a non-validating URI-reference parser that will take an example of a non-validating URI-reference parser that will take
any given string and extract the URI components. any given string and extract the URI components.
4.2 Relative URI 4.2 Relative URI
A relative URI reference takes advantage of the hier-part syntax A relative URI reference takes advantage of the hierarchical syntax
(Section 3) in order to express a reference that is relative to the (Section 1.2.3) in order to express a reference that is relative to
name space of another hierarchical URI. the name space of another hierarchical URI.
relative-URI = hier-part [ "?" query ] [ "#" fragment ] relative-URI = ["//" authority] path ["?" query] ["#" fragment]
The URI referred to by a relative reference is obtained by applying The URI referred to by a relative reference, also known as the target
the reference resolution algorithm of Section 5. URI, is obtained by applying the reference resolution algorithm of
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 relative
reference that begins with a single slash character is termed an reference that begins with a single slash character is termed an
absolute-path reference. A relative reference that does not begin absolute-path reference. A relative reference that does not begin
with a slash character is termed a relative-path reference. 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
a fragment identifier. For example, defining the base URI for later a fragment identifier. For example, defining a base URI for later
use by relative references calls for an absolute-URI production that use by relative references calls for an absolute-URI syntax rule that
does not allow a fragment. does not allow a fragment.
absolute-URI = scheme ":" hier-part [ "?" query ] absolute-URI = scheme ":" ["//" authority] path ["?" query]
4.4 Same-document Reference 4.4 Same-document Reference
When a URI reference occurring within a document or message refers to When a URI reference refers to a URI that is, aside from its fragment
a URI that is, aside from its fragment component (if any), identical component (if any), identical to the base URI (Section 5.1), that
to the base URI (Section 5.1), that reference is called a reference is called a "same-document" reference. The most frequent
"same-document" reference. The most frequent examples of examples of same-document references are relative references that are
same-document references are relative references that are empty or empty or include only the number sign ("#") separator followed by a
include only the number-sign ("#") separator followed by a fragment fragment identifier.
identifier.
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 that current document or message; the dereference should not within the same entity (representation, document, or message) as the
result in a new retrieval. reference; therefore, a dereference should not result in a new
retrieval action.
Normalization of the base and target URIs prior to their comparison,
as described in Section 6.2.2 and Section 6.2.3, is allowed but
rarely performed in practice. Normalization may increase the set of
same-document references, which may be of benefit to some caching
applications. As such, reference authors should not assume that a
slightly different, though equivalent, reference URI will (or will
not) be interpreted as a same-document reference by any given
application.
4.5 Suffix Reference 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
URI as a reference, consisting of only the authority and path URI as a reference, consisting of only the authority and path
portions of the URI, such as portions of the URI, such as
www.w3.org/Addressing/ www.w3.org/Addressing/
or simply the DNS hostname on its own. Such references are primarily or simply a DNS registered name on its own. Such references are
intended for human interpretation rather than machine, with the primarily intended for human interpretation, rather than for
assumption that context-based heuristics are sufficient to complete machines, with the assumption that context-based heuristics are
the URI (e.g., most hostnames beginning with "www" are likely to have sufficient to complete the URI (e.g., most host names beginning with
a URI prefix of "http://"). Although there is no standard set of "www" are likely to have a URI prefix of "http://"). Although there
heuristics for disambiguating a URI suffix, many client is no standard set of heuristics for disambiguating a URI suffix,
implementations allow them to be entered by the user and many client implementations allow them to be entered by the user and
heuristically resolved. It should be noted that such heuristics may heuristically resolved.
change over time, particularly when new URI schemes are introduced.
While this practice of using suffix references is common, it should
be avoided whenever possible and never used in situations where
long-term references are expected. The heuristics noted above will
change over time, particularly when a new URI scheme becomes popular,
and are often incorrect when used out of context. Furthermore, they
can lead to security issues along the lines of those described in
[RFC1535].
Since a URI suffix has the same syntax as a relative path reference, Since a URI suffix has the same syntax as a relative path reference,
a suffix reference cannot be used in contexts where a relative a suffix reference cannot be used in contexts where a relative
reference is expected. As a result, suffix references are limited to reference is expected. As a result, suffix references are limited to
those places where there is no defined base URI, such as dialog boxes those places where there is no defined base URI, such as dialog boxes
and off-line advertisements. and off-line advertisements.
5. Reference Resolution 5. Reference Resolution
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 production 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 some "base URI" against The term "relative" implies that there exists a "base URI" against
which the relative reference is applied. Aside from same-document which the relative reference is applied. Aside from fragment-only
references (Section 4.4, relative references are only usable if the references (Section 4.4), relative references are only usable when a
base URI is known. The 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.
The base URI of a document can be established in one of four ways, The base URI of a reference can be established in one of four ways,
listed below in order of precedence. The order of precedence can be discussed below in order of precedence. The order of precedence can
thought of in terms of layers, where the innermost defined base URI be thought of in terms of layers, where the innermost defined base
has the highest precedence. This can be visualized graphically as: URI has the highest precedence. This can be visualized graphically
as:
.----------------------------------------------------------. .----------------------------------------------------------.
| .----------------------------------------------------. | | .----------------------------------------------------. |
| | .----------------------------------------------. | | | | .----------------------------------------------. | |
| | | .----------------------------------------. | | | | | | .----------------------------------------. | | |
| | | | .----------------------------------. | | | | | | | | .----------------------------------. | | | |
| | | | | <relative-reference> | | | | | | | | | | <relative-reference> | | | | |
| | | | `----------------------------------' | | | | | | | | `----------------------------------' | | | |
| | | | (5.1.1) Base URI embedded in the | | | | | | | | (5.1.1) Base URI embedded in content | | | |
| | | | document's content | | | |
| | | `----------------------------------------' | | | | | | `----------------------------------------' | | |
| | | (5.1.2) Base URI of the encapsulating entity | | | | | | (5.1.2) Base URI of the encapsulating entity | | |
| | | (message, document, or none). | | | | | | (message, representation, or none) | | |
| | `----------------------------------------------' | | | | `----------------------------------------------' | |
| | (5.1.3) URI used to retrieve the entity | | | | (5.1.3) URI used to retrieve the entity | |
| `----------------------------------------------------' | | `----------------------------------------------------' |
| (5.1.4) Default Base URI is application-dependent | | (5.1.4) Default Base URI (application-dependent) |
`----------------------------------------------------------' `----------------------------------------------------------'
5.1.1 Base URI within Document Content 5.1.1 Base URI within Document Content
Within certain document media types, the base URI of the document can Within certain media types, a base URI for relative references can be
be embedded within the content itself such that it can be readily embedded within the content itself such that it can be readily
obtained by a parser. This can be useful for descriptive documents, obtained by a parser. This can be useful for descriptive documents,
such as tables of content, which may be transmitted to others through such as tables of content, which may be transmitted to others through
protocols other than their usual retrieval context (e.g., E-Mail or protocols other than their usual retrieval context (e.g., E-Mail or
USENET news). USENET news).
It is beyond the scope of this document to specify how, for each It is beyond the scope of this specification to specify how, for each
media type, the base URI can be embedded. It is assumed that user media type, a base URI can be embedded. The appropriate syntax, when
agents manipulating such media types will be able to obtain the available, is described by each media type's specification.
appropriate syntax from that media type's specification.
A mechanism for embedding the base URI within MIME container types 5.1.2 Base URI from the Encapsulating Entity
If no base URI is embedded, the base URI is defined by the
representation's retrieval context. For a document that is enclosed
within another entity, such as a message or archive, the retrieval
context is that entity; thus, the default base URI of a
representation is the base URI of the entity in which the
representation is encapsulated.
A mechanism for embedding a base URI within MIME container types
(e.g., the message and multipart types) is defined by MHTML (e.g., the message and multipart types) is defined by MHTML
[RFC2110]. Protocols that do not use the MIME message header syntax, [RFC2110]. 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 the base URI as messages, may define their own syntax for defining a base URI as part
part of a message. of a message.
5.1.2 Base URI from the Encapsulating Entity
If no base URI is embedded, the base URI of a document is defined by
the document's retrieval context. For a document that is enclosed
within another entity (such as a message or another document), the
retrieval context is that entity; thus, the default base URI of the
document is the base URI of the entity in which the document is
encapsulated.
5.1.3 Base URI from the Retrieval URI 5.1.3 Base URI from the Retrieval URI
If no base URI is embedded and the document is not encapsulated If no base URI is embedded and the representation is not encapsulated
within some other entity (e.g., the top level of a composite entity), within some other entity, then, if a URI was used to retrieve the
then, if a URI was used to retrieve the base document, that URI shall representation, that URI shall be considered the base URI. Note that
be considered the base URI. Note that if the retrieval was the if the retrieval was the result of a redirected request, the last URI
result of a redirected request, the last URI used (i.e., that which used (i.e., the URI that resulted in the actual retrieval of the
resulted in the actual retrieval of the document) 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 in above apply, then the base URI If none of the conditions described above apply, then the base URI is
is defined by the context of the application. Since this definition defined by the context of the application. Since this definition is
is necessarily application-dependent, failing to define the base URI necessarily application-dependent, failing to define a base URI using
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.
It is the responsibility of the distributor(s) of a document A sender of a representation containing relative references is
containing a relative reference to ensure that the base URI for that responsible for ensuring that a base URI for those references can be
document can be established. It must be emphasized that a relative established. Aside from fragment-only references, relative references
reference, aside from a same-document reference, cannot be used can only be used reliably in situations where the base URI is
reliably in situations where the document's base URI is not
well-defined. well-defined.
5.2 Obtaining the Referenced URI 5.2 Relative Resolution
This section describes an example algorithm for resolving URI This section describes an algorithm for converting a URI reference
references that might be relative to a given base URI. The algorithm that might be relative to a given base URI into the parsed componets
is intended to provide a definitive result that can be used to test of the reference's target. The components can then be recomposed, as
the output of other implementations. Implementation of the algorithm described in Section 5.3, to form the target URI. This algorithm
itself is not required, but the result given by an implementation provides definitive results that can be used to test the output of
must match the result that would be given by this algorithm. other implementations. Applications may implement relative reference
resolution using some other algorithm, provided that the results
match what would be given by this algorithm.
The base URI (Base) is established according to the rules of Section 5.2.1 Pre-parse the Base URI
5.1 and parsed into the five main components described in Section 3.
Note that only the scheme component is required to be present in the The base URI (Base) is established according to the procedure of
base URI; the other components may be empty or undefined. A Section 5.1 and parsed into the five main components described in
component is undefined if its preceding separator does not appear in Section 3. Note that only the scheme component is required to be
the URI reference; the path component is never undefined, though it present in a base URI; the other components may be empty or
may be empty. The algorithm assumes that the base URI is well-formed undefined. A component is undefined if its associated delimiter does
and does not contain dot-segments in its path. not appear in the URI reference; the path component is never
undefined, though it may be empty.
Normalization of the base URI, as described in Section 6.2.2 and
Section 6.2.3, is optional. A URI reference must be transformed to
its target URI before it can be normalized.
5.2.2 Transform References
For each URI reference (R), the following pseudocode describes an For each URI reference (R), the following pseudocode describes an
algorithm for transforming R into its target URI (T): algorithm for transforming R into its target URI (T):
-- The URI reference is parsed into the five URI components -- The URI reference is parsed into the five URI components
-- --
(R.scheme, R.authority, R.path, R.query, R.fragment) = parse(R); (R.scheme, R.authority, R.path, R.query, R.fragment) = parse(R);
-- A non-strict parser may ignore a scheme in the reference -- A non-strict parser may ignore a scheme in the reference
-- if it is identical to the base URI's scheme. -- if it is identical to the base URI's scheme.
skipping to change at page 30, line 18 skipping to change at page 30, line 25
endif; endif;
T.query = R.query; T.query = R.query;
endif; endif;
T.authority = Base.authority; T.authority = Base.authority;
endif; endif;
T.scheme = Base.scheme; T.scheme = Base.scheme;
endif; endif;
T.fragment = R.fragment; T.fragment = R.fragment;
The pseudocode above refers to a merge routine for merging a 5.2.3 Merge Paths
The pseudocode above refers to a "merge" routine for merging a
relative-path reference with the path of the base URI. This is relative-path reference with the path of the base URI. This is
accomplished as follows: accomplished as follows:
o If the base URI's path is empty, then return a string consisting o If the base URI has a defined authority component and an empty
of "/" concatenated with the reference's path component; path, then return a string consisting of "/" concatenated with the
otherwise, reference's path; otherwise,
o If the base URI's path is non-hierarchical, as indicated by not
beginning with a slash, then return a string consisting of the
reference's path component; otherwise,
o Return a string consisting of the reference's path component o Return a string consisting of the reference's path component
appended to all but the last segment of the base URI's path (i.e., appended to all but the last segment of the base URI's path (i.e.,
any characters after the right-most "/" in the base URI path are excluding any characters after the right-most "/" in the base URI
excluded). path, or excluding the entire base URI path if it does not contain
any "/" characters).
The pseudocode also refers to a remove_dot_segments routine for 5.2.4 Remove Dot Segments
The pseudocode also refers to a "remove_dot_segments" routine for
interpreting and removing the special "." and ".." complete path interpreting and removing the special "." and ".." complete path
segments from a referenced path. This is done after the path is segments from a referenced path. This is done after the path is
extracted from a reference, whether or not the path was relative, in extracted from a reference, whether or not the path was relative, in
order to remove any invalid or extraneous dot-segments prior to order to remove any invalid or extraneous dot-segments prior to
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 a separate this removal process, we describe a simple method using a two string
string buffer: buffers.
1. The buffer is initialized with the unprocessed path component. 1. The input buffer is initialized with the now-appended path
components and the output buffer is initialized to the empty
string.
2. If the buffer begins with "./" or "../", the "." or ".." segment 2. Replace any prefix of "./" or "../" at the beginning of the input
is removed. buffer with "/".
3. All occurrences of "/./" in the buffer are replaced with "/". 3. While the input buffer is not empty, loop:
4. If the buffer ends with "/.", the "." is removed. 1. If the input buffer begins with a prefix of "/./" or "/.",
where "." is a complete path segment, then replace that
prefix with "/"; otherwise
5. All occurrences of "/<segment>/../" in the buffer, where ".." and 2. If the input buffer begins with a prefix of "/../" or "/..",
<segment> are complete path segments, are iteratively replaced where ".." is a complete path segment, then replace that
with "/" in order from left to right until no matching pattern prefix with "/" and remove the last segment and its preceding
remains. If the buffer ends with "/<segment>/..", that is also "/" (if any) from the output buffer; otherwise
replaced with "/". Note that <segment> may be empty.
6. All prefixes of "<segment>/../" in the buffer, where ".." and 3. Remove the first segment and its preceding "/" (if any) from
<segment> are complete path segments, are iteratively replaced the input buffer and append them to the output buffer.
with "/" in order from left to right until no matching pattern
remains. If the buffer ends with "<segment>/..", that is also
replaced with "/". Note that <segment> may be empty.
7. The remaining buffer is returned as the result of 4. Finally, the output buffer is returned as the result of
remove_dot_segments. remove_dot_segments.
Some systems may find it more efficient to implement the The following illustrates how the above steps are applied for two
remove_dot_segments algorithm as a stack of path segments being example merged paths, showing the state of the two buffers after each
compressed, rather than as a series of string pattern replacements. step.
5.3 Recomposition of a Parsed URI STEP OUTPUT BUFFER INPUT BUFFER
1 : /a/b/c/./../../g
3c: /a /b/c/./../../g
3c: /a/b /c/./../../g
3c: /a/b/c /./../../g
3a: /a/b/c /../../g
3b: /a/b /../g
3b: /a /g
3c: /a/g
STEP OUTPUT BUFFER INPUT BUFFER
1 : mid/content=5/../6
3c: mid /content=5/../6
3c: mid/content=5 /../6
3b: mid /6
3c: mid/6
Some applications may find it more efficient to implement the
remove_dot_segments algorithm using two segment stacks rather than
strings.
Note: Some client applications will fail to separate a reference's
query component from its path component before merging the base
and reference paths. This may result in loss of information if
the query component contains the strings "/../" or "/./".
5.3 Component Recomposition
Parsed URI components can be recomposed to obtain the corresponding Parsed URI components can be recomposed to obtain the corresponding
URI reference string. Using pseudocode, this would be: URI reference string. Using pseudocode, this would be:
result = "" result = ""
if defined(scheme) then if defined(scheme) then
append scheme to result; append scheme to result;
append ":" to result; append ":" to result;
endif; endif;
skipping to change at page 32, line 4 skipping to change at page 32, line 42
if defined(query) then if defined(query) then
append "?" to result; append "?" to result;
append query to result; append query to result;
endif; endif;
if defined(fragment) then if defined(fragment) then
append "#" to result; append "#" to result;
append fragment to result; append fragment to result;
endif; endif;
return result; return result;
Note that we are careful to preserve the distinction between a Note that we are careful to preserve the distinction between a
component that is undefined, meaning that its separator was not component that is undefined, meaning that its separator was not
present in the reference, and a component that is empty, meaning that present in the reference, and a component that is empty, meaning that
the separator was present and was immediately followed by the next the separator was present and was immediately followed by the next
component separator or the end of the reference. component separator or the end of the reference.
5.4 Reference Resolution Examples 5.4 Reference Resolution Examples
Within an object with a well-defined base URI of Within a representation with a well-defined base URI of
http://a/b/c/d;p?q http://a/b/c/d;p?q
a relative URI reference would be resolved as follows: a relative URI reference is transformed to its target URI as follows.
5.4.1 Normal Examples 5.4.1 Normal Examples
"g:h" = "g:h" "g:h" = "g:h"
"g" = "http://a/b/c/g" "g" = "http://a/b/c/g"
"./g" = "http://a/b/c/g" "./g" = "http://a/b/c/g"
"g/" = "http://a/b/c/g/" "g/" = "http://a/b/c/g/"
"/g" = "http://a/g" "/g" = "http://a/g"
"//g" = "http://g" "//g" = "http://g"
"?y" = "http://a/b/c/d;p?y" "?y" = "http://a/b/c/d;p?y"
"g?y" = "http://a/b/c/g?y" "g?y" = "http://a/b/c/g?y"
"#s" = "http://a/b/c/d;p?q#s" "#s" = "http://a/b/c/d;p?q#s"
"g#s" = "http://a/b/c/g#s" "g#s" = "http://a/b/c/g#s"
"g?y#s" = "http://a/b/c/g?y#s" "g?y#s" = "http://a/b/c/g?y#s"
";x" = "http://a/b/c/;x" ";x" = "http://a/b/c/;x"
"g;x" = "http://a/b/c/g;x" "g;x" = "http://a/b/c/g;x"
"g;x?y#s" = "http://a/b/c/g;x?y#s" "g;x?y#s" = "http://a/b/c/g;x?y#s"
"" = "http://a/b/c/d;p?q"
"." = "http://a/b/c/" "." = "http://a/b/c/"
"./" = "http://a/b/c/" "./" = "http://a/b/c/"
".." = "http://a/b/" ".." = "http://a/b/"
"../" = "http://a/b/" "../" = "http://a/b/"
"../g" = "http://a/b/g" "../g" = "http://a/b/g"
"../.." = "http://a/" "../.." = "http://a/"
"../../" = "http://a/" "../../" = "http://a/"
"../../g" = "http://a/g" "../../g" = "http://a/g"
5.4.2 Abnormal Examples 5.4.2 Abnormal Examples
Although the following abnormal examples are unlikely to occur in Although the following abnormal examples are unlikely to occur in
normal practice, all URI parsers should be capable of resolving them normal practice, all URI parsers should be capable of resolving them
consistently. Each example uses the same base as above. consistently. Each example uses the same base as above.
An empty reference refers to the current base URI. Parsers must be careful in handling cases where there are more
"" = "http://a/b/c/d;p?q"
Parsers must be careful in handling the case where there are more
relative path ".." segments than there are hierarchical levels in the relative path ".." segments than there are hierarchical levels in the
base URI's path. Note that the ".." syntax cannot be used to change base URI's path. Note that the ".." syntax cannot be used to change
the authority component of a URI. the authority component of a URI.
"../../../g" = "http://a/g" "../../../g" = "http://a/g"
"../../../../g" = "http://a/g" "../../../../g" = "http://a/g"
Similarly, parsers must remove the dot-segments "." and ".." when Similarly, parsers must remove the dot-segments "." and ".." when
they are complete components of a path, but not when they are only they are complete components of a path, but not when they are only
part of a segment. part of a segment.
"/./g" = "http://a/g" "/./g" = "http://a/g"
"/../g" = "http://a/g" "/../g" = "http://a/g"
"g." = "http://a/b/c/g." "g." = "http://a/b/c/g."
".g" = "http://a/b/c/.g" ".g" = "http://a/b/c/.g"
"g.." = "http://a/b/c/g.." "g.." = "http://a/b/c/g.."
"..g" = "http://a/b/c/..g" "..g" = "http://a/b/c/..g"
Less likely are cases where the relative URI uses unnecessary or Less likely are cases where the relative URI reference uses
nonsensical forms of the "." and ".." complete path segments. unnecessary or nonsensical forms of the "." and ".." complete path
segments.
"./../g" = "http://a/b/g" "./../g" = "http://a/b/g"
"./g/." = "http://a/b/c/g/" "./g/." = "http://a/b/c/g/"
"g/./h" = "http://a/b/c/g/h" "g/./h" = "http://a/b/c/g/h"
"g/../h" = "http://a/b/c/h" "g/../h" = "http://a/b/c/h"
"g;x=1/./y" = "http://a/b/c/g;x=1/y" "g;x=1/./y" = "http://a/b/c/g;x=1/y"
"g;x=1/../y" = "http://a/b/c/y" "g;x=1/../y" = "http://a/b/c/y"
Some applications fail to separate the reference's query and/or Some applications fail to separate the reference's query and/or
fragment components from a relative path before merging it with the fragment components from a relative path before merging it with the
base path and removing dot-segments. This error is rarely noticed, base path and removing dot-segments. This error is rarely noticed,
since typical usage of a fragment never includes the hierarchy ("/") since typical usage of a fragment never includes the hierarchy ("/")
character, and the query component is not normally used within character, and the query component is not normally used within
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 if Some parsers allow the scheme name to be present in a relative URI
it is the same as the base URI scheme. This is considered to be a reference if it is the same as the base URI scheme. This is
loophole in prior specifications of partial URI [RFC1630]. Its use considered to be a loophole in prior specifications of partial URI
should be avoided, but is allowed for backward compatibility. [RFC1630]. Its use should be avoided, but is allowed for backward
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
time a response cache is accessed, a browser checks its history to time a response cache is accessed, a browser checks its history to
skipping to change at page 35, line 29 skipping to change at page 35, line 29
spent in reducing duplicate identifiers. This section describes a spent in reducing duplicate identifiers. This section describes a
variety of methods that may be used to compare URIs, the trade-offs variety of methods that may be used to compare URIs, the trade-offs
between them, and the types of applications that might use them. between them, and the types of applications that might use them.
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 their origin. For this reason, determination of knowledge of the implementation-specific syntax of each URI's
dereferencing algorithm. For this reason, determination of
equivalence or difference of URIs is based on string comparison, 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. Therefore, comparison methods are designed to minimize resources. For example, an owner of two different domain names could
false negatives while strictly avoiding false positives. decide to serve the same resource from both, resulting in two
different URIs. Therefore, comparison methods are designed to
minimize false negatives while strictly avoiding false positives.
In testing for equivalence, it is generally unwise to directly In testing for equivalence, applications should not directly compare
compare relative URI references; they should be converted to their relative URI references; the references should be converted to their
absolute forms before comparison. Furthermore, when URI references target URI forms before comparison. When URIs are being compared for
are being compared for the purpose of selecting (or avoiding) a the purpose of selecting (or avoiding) a network action, such as
network action, such as retrieval of a representation, it is often retrieval of a representation, the fragment components (if any)
necessary to remove fragment identifiers from the URIs prior to should be excluded from the comparison.
comparison.
6.2 Comparison Ladder 6.2 Comparison Ladder
A variety of methods are used in practice to test URI equivalence. A variety of methods are used in practice to test URI equivalence.
These methods fall into a range, distinguished by the amount of These methods fall into a range, distinguished by the amount of
processing required and the degree to which the probability of false processing required and the degree to which the probability of false
negatives is reduced. As noted above, false negatives cannot in negatives is reduced. As noted above, false negatives cannot in
principle be eliminated. In practice, their probability can be principle be eliminated. In practice, their probability can be
reduced, but this reduction requires more processing and is not reduced, but this reduction requires more processing and is not
cost-effective for all applications. cost-effective for all applications.
If this range of comparison practices is considered as a ladder, the If this range of comparison practices is considered as a ladder, the
following discussion will climb the ladder, starting with those that following discussion will climb the ladder, starting with those
are cheap but have a relatively higher chance of producing false practices that are cheap but have a relatively higher chance of
negatives, and proceeding to those that have higher computational producing false negatives, and proceeding to those that have higher
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. This
procedure is often referred to as "bit-for-bit" or "byte-for-byte" procedure is often referred to as "bit-for-bit" or "byte-for-byte"
comparison, which is potentially misleading. Testing of strings for comparison, which is potentially misleading. Testing of strings for
equality is normally based on pairwise comparison of the characters equality is normally based on pairwise comparison of the characters
that make up the strings, starting from the first and proceeding that make up the strings, starting from the first and proceeding
until both strings are exhausted and all characters found to be until both strings are exhausted and all characters found to be
equal, a pair of characters compares unequal, or one of the strings equal, a pair of characters compares unequal, or one of the strings
is exhausted before the other. 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, bit-for-bit comparisons applied naively will produce both object (UTF-16), bit-for-bit comparisons applied naively will produce
false-positive and false-negative errors. Thus, in principle, it is both false-positive and false-negative errors. It is better to speak
better to speak of equality on a character-for-character rather than of equality on a character-for-character rather than byte-for-byte or
byte-for-byte or bit-for-bit basis. bit-for-bit basis. In practical terms, character-by-character
comparisons should be done codepoint-by-codepoint after conversion to
Unicode defines a character as being identified by number a common character encoding.
("codepoint") with an associated bundle of visual and other
semantics. At the software level, it is not practical to compare
semantic bundles, so in practical terms, character-by-character
comparisons are done codepoint-by-codepoint.
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/%7A example://a/b/c/%7Bfoo%7D
eXAMPLE://a/./b/../b/c/%7a 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, escape normalization, and removal of case normalization, encoding normalization, empty-component
dot-segments. normalization, and removal of 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
hostname are case insensitive and therefore can 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/>. equivalent to <http://www.example.com/>. Applications should not
assume anything about the case sensitivity of other URI components,
since that is dependent on the implementation used to handle a
dereference.
6.2.2.2 Escape Normalization The hexadecimal digits within a percent-encoding triplet (e.g., "%3a"
versus "%3A") are case-insensitive and therefore should be normalized
to use uppercase letters for the digits A-F.
The percent-escape mechanism described in Section 2.4 is a frequent 6.2.2.2 Encoding Normalization
source of variance among otherwise identical URIs. One cause is the
choice of uppercase or lowercase letters for the hexadecimal digits
within the escape sequence (e.g., "%3a" versus "%3A"). Such sequences
are always equivalent; for the sake of uniformity, URI generators and
normalizers are strongly encouraged to use uppercase letters for the
hex digits A-F.
Only characters that are excluded from or reserved within the URI The percent-encoding mechanism (Section 2.1) is a frequent source of
syntax must be escaped when used as data. However, some URI variance among otherwise identical URIs. In addition to the
generators go beyond that and escape characters that do not require case-insensitivity issue noted above, some URI producers
escaping, resulting in URIs that are equivalent to their unescaped percent-encode octets that do not require percent-encoding, resulting
counterparts. Such URIs can be normalized by unescaping sequences in URIs that are equivalent to their non-encoded counterparts. Such
that represent the unreserved characters, as described in Section URIs should be normalized by decoding any percent-encoded octet that
2.3. corresponds to an unreserved character, as described in Section 2.3.
6.2.2.3 Path Segment Normalization 6.2.2.3 Empty-component Normalization
Components of the generic URI syntax are delimited from other
components by optional separators. For example, a query component is
separated from the path by a question mark ("?") and a port
sub-component is separated from host by a colon (":"). A URI in
which a delimiter is present and the (sub-)component it delimits is
empty is equivalent to the same URI without that delimiter. For
example, the following are all equivalent:
ftp://example.com/
ftp://example.com:/
ftp://@example.com:/
ftp://@example.com:/?
ftp://@example.com:/?#
URI producers and normalizers should omit a delimiter if the
component it delimits is empty, as exemplified by the first URI
above, with one exception: a double-slash delimiter indicating an
authority component should not be removed, even when the authority is
empty, since doing so can lead to misinterpreting the path.
6.2.2.4 Path Segment Normalization
The complete path segments "." and ".." have a special meaning within 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.
6.2.3 Scheme-based Normalization 6.2.3 Scheme-based Normalization
The syntax and semantics of URIs vary from scheme to scheme, as The syntax and semantics of URIs vary from scheme to scheme, as
described by the defining specification for each scheme. Software described by the defining specification for each scheme. Software
may use scheme-specific rules, at further processing cost, to reduce may use scheme-specific rules, at further processing cost, to reduce
the probability of false negatives. For example, Web spiders that the probability of false negatives. For example, since the "http"
populate most large search engines would consider the following two scheme makes use of an authority component, has a default port of
URIs to be equivalent: "80", and defines an empty path to be equivalent to "/", the
following four URIs are equivalent:
http://example.com
http://example.com/ http://example.com/
http://example.com:/
http://example.com:80/ http://example.com:80/
This behavior is based on the rules provided by the syntax and In general, a URI that uses the generic syntax for authority with an
semantics of the "http" URI scheme, which defines an empty port empty path should be normalized to a path of "/"; likewise, an
component as being equivalent to the default TCP port for HTTP (port explicit ":port", where the port is empty or the default for the
80). In general, a URI scheme that uses the generic syntax for scheme, is equivalent to one where the port and its ":" delimiter are
authority is defined such that a URI with an explicit ":port", where elided. In other words, the second of the above URI examples is the
the port is the default for the scheme, is equivalent to one where normal form for the "http" scheme.
the port is elided.
Another case where normalization varies by scheme is in the handling
of an empty authority component. For many scheme specifications, an
empty authority is considered an error; for others, it is considered
equivalent to "localhost". For the sake of uniformity, future scheme
specifications should define an empty authority as being equivalent
to "localhost", and URI producers and normalizers should use
"localhost" instead of an empty authority.
6.2.4 Protocol-based Normalization 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. they will likely regard the two as equivalent in the future. This
Obviously, this kind of technique is only appropriate in special kind of technique is only appropriate when equivalence is clearly
situations. indicated by both the result of accessing the resources and the
common conventions of their scheme's dereference algorithm (in this
case, use of redirection by HTTP origin servers to avoid problems
with relative references).
6.3 Canonical Form 6.3 Canonical Form
It is in the best interests of everyone to avoid false-negatives in It is in the best interests of everyone to avoid false-negatives in
comparing URIs and to minimize the amount of software processing for comparing URIs and to minimize the amount of software processing for
such comparisons. Those who generate and make reference to URIs can such comparisons. Those who produce and make reference to URIs can
reduce the cost of processing and the risk of false negatives by reduce the cost of processing and the risk of false negatives by
consistently providing them in a form that is reasonably canonical consistently providing them in a form that is reasonably canonical
with respect to their scheme. Specifically: with respect to their scheme. Specifically:
o Always provide the URI scheme in lowercase characters. o Always provide the URI scheme in lowercase characters.
o Always provide the hostname, if any, in lowercase characters. o Always provide the host, if any, in lowercase characters.
o Only perform percent-escaping where it is essential. o Only perform percent-encoding where it is essential.
o Always use uppercase A-through-F characters when percent-escaping. o Always use uppercase A-through-F characters when percent-encoding.
o Prevent /./ and /../ from appearing in non-relative URI paths. o Prevent /./ and /../ from appearing in non-relative URI paths.
The good practices listed above are motivated by deployed software o Omit delimiters when their associated (sub-)component is empty.
that frequently use these techniques for the purposes of
normalization. o For schemes that define an empty authority to be equivalent to
"localhost", use "localhost".
o For schemes that define an empty path to be equivalent to a path
of "/", use "/".
7. Security Considerations 7. Security Considerations
A URI does not in itself pose a security threat. However, since URIs A URI does not in itself pose a security threat. However, since URIs
are often used to provide a compact set of instructions for access to 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.
skipping to change at page 40, line 36 skipping to change at page 41, line 36
scheme. scheme.
7.2 Malicious Construction 7.2 Malicious Construction
It is sometimes possible to construct a URI such that an attempt to It is sometimes possible to construct a URI such that an attempt to
perform a seemingly harmless, idempotent operation, such as the perform a seemingly harmless, idempotent operation, such as the
retrieval of a representation, will in fact cause a possibly damaging retrieval of a representation, will in fact cause a possibly damaging
remote operation to occur. The unsafe URI is typically constructed remote operation to occur. The unsafe URI is typically constructed
by specifying a port number other than that reserved for the network by specifying a port number other than that reserved for the network
protocol in question. The client unwittingly contacts a site that is protocol in question. The client unwittingly contacts a site that is
running a different protocol service. The content of the URI running a different protocol service and data within the URI contains
contains instructions that, when interpreted according to this other instructions that, when interpreted according to this other protocol,
protocol, cause an unexpected operation. An example has been the use cause an unexpected operation. A frequent example of such abuse has
of a gopher URI to cause an unintended or impersonating message to be been the use of a protocol-based scheme with a port component of
sent via a SMTP server. "25", thereby fooling user agent software into sending an unintended
or impersonating message via an SMTP server.
Caution should be used when dereferencing a URI that specifies a TCP Applications should prevent dereference of a URI that specifies a TCP
port number other than the default for the scheme, especially when it port number within the "well-known port" range (0 - 1023) unless the
is a number within the reserved space. protocol being used to dereference that URI is compatible with the
protocol expected on that well-known port. Although IANA maintains a
registry of well-known ports, applications should make such
restrictions user-configurable to avoid preventing the deployment of
new services.
Care should be taken when a URI contains escaped delimiters for a When a URI contains percent-encoded octets that match the delimiters
given protocol (for example, CR and LF characters for telnet for a given resolution or dereference protocol (for example, CR and
protocols) that these octets are not unescaped before transmission. LF characters for the TELNET protocol), such percent-encoded octets
This might violate the protocol, but avoids the potential for such must not be decoded before transmission across that protocol.
characters to be used to simulate an extra operation or parameter in Transfer of the percent-encoding, which might violate the protocol,
that protocol which might lead to an unexpected and possibly harmful is less harmful than allowing decoded octets to be interpreted as
remote operation being performed. additional operations or parameters, perhaps triggering an unexpected
and possibly harmful remote operation.
7.3 Rare IP Address Formats 7.3 Back-end Transcoding
When a URI is dereferenced, the data within it is often parsed by
both the user agent and one or more servers. In HTTP, for example, a
typical user agent will parse a URI into its five major components,
access the authority's server, and send it the data within the
authority, path, and query components. A typical server will take
that information, parse the path into segments and the query into
key/value pairs, and then invoke implementation-specific handlers to
respond to the request. As a result, a common security concern for
server implementations that handle a URI, either as a whole or split
into separate components, is proper interpretation of the octet data
represented by the characters and percent-encodings within that URI.
Percent-encoded octets must be decoded at some point during the
dereference process. Applications must split the URI into its
components and sub-components prior to decoding the octets, since
otherwise the decoded octets might be mistaken for delimiters.
Security checks of the data within a URI should be applied after
decoding the octets. Note, however, that the "%00" percent-encoding
(NUL) may require special handling and should be rejected if the
application is not expecting to receive raw data within a component.
Special care should be taken when the URI path interpretation process
involves the use of a back-end filesystem or related system
functions. Filesystems typically assign an operational meaning to
special characters, such as the "/", "\", ":", "[", and "]"
characters, and special device names like ".", "..", "...", "aux",
"lpt", etc. In some cases, merely testing for the existence of such a
name will cause the operating system to pause or invoke unrelated
system calls, leading to significant security concerns regarding
denial of service and unintended data transfer. It would be
impossible for this specification to list all such significant
characters and device names; implementers should research the
reserved names and characters for the types of storage device that
may be attached to their application and restrict the use of data
obtained from URI components accordingly.
7.4 Rare IP Address Formats
Although the URI syntax for IPv4address only allows the common, Although the URI syntax for IPv4address only allows the common,
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
skipping to change at page 41, line 32 skipping to change at page 43, line 32
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
due to differences between platform implementations. However, they due to differences between platform implementations. However, they
can become a security concern if an application attempts to filter can become a security concern if an application attempts to filter
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, it is recommended that literals be If such filtering is performed, literals should be converted to
converted to numeric form and filtered based on the numeric value, numeric form and filtered based on the numeric value, rather than a
rather than a prefix or suffix of the string form. prefix or suffix of the string form.
7.4 Sensitive Information 7.5 Sensitive Information
It is clearly unwise to use a URI that contains a password which is URI producers should not provide a URI that contains a username or
intended to be secret. In particular, the use of a password within password which is intended to be secret: URIs are frequently
the userinfo component of a URI is strongly discouraged except in displayed by browsers, stored in clear text bookmarks, and logged by
those rare cases where the 'password' parameter is intended to be user agent history and intermediary applications (proxies). A
public. password appearing within the userinfo component is deprecated and
should be considered an error (or simply ignored) except in those
rare cases where the 'password' parameter is intended to be public.
7.5 Semantic Attacks 7.6 Semantic Attacks
Because the userinfo component is rarely used and appears before the Because the userinfo sub-component is rarely used and appears before
hostname 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
authority hidden behind the noise. For example authority hidden behind the noise. For example
http://www.example.com&story=breaking_news@10.0.0.1/top_story.htm ftp://ftp.example.com&story=breaking_news@10.0.0.1/top_story.htm
might lead a human user to assume that the host is 'www.example.com', might lead a human user to assume that the host is
whereas it is actually '10.0.0.1'. Note that the misleading userinfo 'trusted.example.com', whereas it is actually '10.0.0.1'. Note that
could be much longer than the example above. a misleading userinfo sub-component could be much longer than the
example above.
A misleading URI, such as the one above, is an attack on the user's 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 visually distinguishing the various impact of such attacks by distinguishing the various components of
components of the URI when rendered, such as by using a different the URI when rendered, such as by using a different color or tone to
color or tone to render userinfo if any is present, though there is render userinfo if any is present, though there is no general
no general panacea. More information on URI-based semantic attacks panacea. More information on URI-based semantic attacks can be found
can be found in [Siedzik]. in [Siedzik].
8. Acknowledgments 8. 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 Reese Anschultz, Tim Bray, [RFC2732]. In addition, contributions by Gisle Aas, Reese Anschultz,
Rob Cameron, Dan Connolly, Adam M. Costello, John Cowan, Jason Daniel Barclay, Tim Bray, Mike Brown, Rob Cameron, Jeremy Carroll,
Diamond, Martin Duerst, Stefan Eissing, Clive D.W. Feather, Pat Dan Connolly, Adam M. Costello, John Cowan, Jason Diamond, Martin
Hayes, Henry Holtzman, Graham Klyne, Dan Kohn, Bruce Lilly, Andrew Duerst, Stefan Eissing, Clive D.W. Feather, Tony Hammond, Pat Hayes,
Main, Michael Mealling, Julian Reschke, Tomas Rokicki, Miles Sabin, Henry Holtzman, Ian B. Jacobs, Michael Kay, John C. Klensin, Graham
Ronald Tschalaer, Marc Warne, Stuart Williams, and Henry Zongaro are Klyne, Dan Kohn, Bruce Lilly, Andrew Main, Ira McDonald, Michael
gratefully acknowledged. Mealling, Stephen Pollei, Julian Reschke, Tomas Rokicki, Miles Sabin,
Mark Thomson, Ronald Tschalaer, Norm Walsh, Marc Warne, Stuart
Williams, and Henry Zongaro are gratefully acknowledged.
Normative References 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
10646", STD 63, RFC 3629, November 2003.
Informative References Informative References
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and [RFC0952] Harrenstien, K., Stahl, M. and E. Feinler, "DoD Internet
Languages", BCP 18, RFC 2277, January 1998. host table specification", RFC 952, October 1985.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[RFC1535] Gavron, E., "A Security Problem and Proposed Correction
With Widely Deployed DNS Software", RFC 1535, October
1993.
[RFC1630] Berners-Lee, T., "Universal Resource Identifiers in WWW: A [RFC1630] Berners-Lee, T., "Universal Resource Identifiers in WWW: A
Unifying Syntax for the Expression of Names and Addresses Unifying Syntax for the Expression of Names and Addresses
of Objects on the Network as used in the World-Wide Web", of Objects on the Network as used in the World-Wide Web",
RFC 1630, June 1994. RFC 1630, June 1994.
[RFC1738] Berners-Lee, T., Masinter, L. and M. McCahill, "Uniform [RFC1736] Kunze, J., "Functional Recommendations for Internet
Resource Locators (URL)", RFC 1738, December 1994. Resource Locators", RFC 1736, February 1995.
[RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform [RFC1737] Masinter, L. and K. Sollins, "Functional Requirements for
Resource Identifiers (URI): Generic Syntax", RFC 2396, Uniform Resource Names", RFC 1737, December 1994.
August 1998.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application [RFC1738] Berners-Lee, T., Masinter, L. and M. McCahill, "Uniform
and Support", STD 3, RFC 1123, October 1989. Resource Locators (URL)", RFC 1738, December 1994.
[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.
[RFC2110] Palme, J. and A. Hopmann, "MIME E-mail Encapsulation of
Aggregate Documents, such as HTML (MHTML)", RFC 2110,
March 1997.
[RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
[RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998.
[RFC2518] Goland, Y., Whitehead, E., Faizi, A., Carter, S. and D. [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.
[RFC0952] Harrenstien, K., Stahl, M. and E. Feinler, "DoD Internet [RFC2717] Petke, R. and I. King, "Registration Procedures for URL
host table specification", RFC 952, October 1985. Scheme Names", BCP 35, RFC 2717, November 1999.
[RFC3513] Hinden, R. and S. Deering, "Internet Protocol Version 6 [RFC2718] Masinter, L., Alvestrand, H., Zigmond, D. and R. Petke,
(IPv6) Addressing Architecture", RFC 3513, April 2003. "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.
[RFC1736] Kunze, J., "Functional Recommendations for Internet [RFC2978] Freed, N. and J. Postel, "IANA Charset Registration
Resource Locators", RFC 1736, February 1995. Procedures", BCP 19, RFC 2978, October 2000.
[RFC1737] Masinter, L. and K. Sollins, "Functional Requirements for
Uniform Resource Names", RFC 1737, December 1994.
[RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", [RFC3305] Mealling, M. and R. Denenberg, "Report from the Joint W3C/
STD 13, RFC 1034, November 1987. IETF URI Planning Interest Group: Uniform Resource
Identifiers (URIs), URLs, and Uniform Resource Names
(URNs): Clarifications and Recommendations", RFC 3305,
August 2002.
[RFC2110] Palme, J. and A. Hopmann, "MIME E-mail Encapsulation of [RFC3490] Faltstrom, P., Hoffman, P. and A. Costello,
Aggregate Documents, such as HTML (MHTML)", RFC 2110, "Internationalizing Domain Names in Applications (IDNA)",
March 1997. RFC 3490, March 2003.
[RFC2717] Petke, R. and I. King, "Registration Procedures for URL [RFC3513] Hinden, R. and S. Deering, "Internet Protocol Version 6
Scheme Names", BCP 35, RFC 2717, November 1999. (IPv6) Addressing Architecture", RFC 3513, April 2003.
[Siedzik] Siedzik, R., "Semantic Attacks: What's in a URL?", April [Siedzik] Siedzik, R., "Semantic Attacks: What's in a URL?", April
2001. 2001, <http://www.giac.org/practical/gsec/
Richard_Siedzik_GSEC.pdf>.
[UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.
Authors' Addresses Authors' Addresses
Tim Berners-Lee Tim Berners-Lee
World Wide Web Consortium World Wide Web Consortium
MIT/LCS, Room NE43-356 MIT/LCS, Room NE43-356
200 Technology Square 200 Technology Square
Cambridge, MA 02139 Cambridge, MA 02139
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
2 Corporate Plaza, Suite 150 5251 California Ave., Suite 110
Newport Beach, CA 92660 Irvine, CA 92612-3074
USA USA
Phone: +1-949-999-2523 Phone: +1-949-679-2960
Fax: +1-949-644-5064 Fax: +1-949-679-2972
EMail: roy.fielding@day.com EMail: fielding@gbiv.com
URI: http://www.apache.org/~fielding/ URI: http://roy.gbiv.com/
Larry Masinter Larry Masinter
Adobe Systems Incorporated Adobe Systems Incorporated
345 Park Ave 345 Park Ave
San Jose, CA 95110 San Jose, CA 95110
USA USA
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
abs-path = "/" path-segments URI = scheme ":" ["//" authority] path ["?" query] ["#" fragment]
absolute-URI = scheme ":" hier-part [ "?" query ]
alphanum = ALPHA / DIGIT
authority = [ userinfo "@" ] host [ ":" port ] URI-reference = URI / relative-URI
dec-octet = DIGIT ; 0-9 relative-URI = ["//" authority] path ["?" query] ["#" fragment]
/ %x31-39 DIGIT ; 10-99
/ "1" 2DIGIT ; 100-199
/ "2" %x30-34 DIGIT ; 200-249
/ "25" %x30-35 ; 250-255
domainlabel = alphanum [ 0*61( alphanum / "-" ) alphanum ] absolute-URI = scheme ":" ["//" authority] path ["?" query]
escaped = "%" HEXDIG HEXDIG scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
fragment = *( pchar / "/" / "?" ) authority = [ userinfo "@" ] host [ ":" port ]
userinfo = *( unreserved / pct-encoded / sub-delims / ":" )
host = IP-literal / IPv4address / reg-name
port = *DIGIT
h4 = 1*4HEXDIG IP-literal = "[" ( IPv6address / IPvFuture ) "]"
hier-part = net-path / abs-path / rel-path IPvFuture = "v" HEXDIG "." 1*( unreserved / sub-delims / ":" )
host = [ IPv6reference / IPv4address / hostname ] IPv6address = 6( h16 ":" ) ls32
/ "::" 5( h16 ":" ) ls32
/ [ h16 ] "::" 4( h16 ":" ) ls32
/ [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
/ [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
/ [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32
/ [ *4( h16 ":" ) h16 ] "::" ls32
/ [ *5( h16 ":" ) h16 ] "::" h16
/ [ *6( h16 ":" ) h16 ] "::"
hostname = domainlabel qualified h16 = 1*4HEXDIG
ls32 = ( h16 ":" h16 ) / IPv4address
IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet
IPv6address = 6( h4 ":" ) ls32 dec-octet = DIGIT ; 0-9
/ "::" 5( h4 ":" ) ls32 / %x31-39 DIGIT ; 10-99
/ [ h4 ] "::" 4( h4 ":" ) ls32 / "1" 2DIGIT ; 100-199
/ [ *1( h4 ":" ) h4 ] "::" 3( h4 ":" ) ls32 / "2" %x30-34 DIGIT ; 200-249
/ [ *2( h4 ":" ) h4 ] "::" 2( h4 ":" ) ls32 / "25" %x30-35 ; 250-255
/ [ *3( h4 ":" ) h4 ] "::" h4 ":" ls32
/ [ *4( h4 ":" ) h4 ] "::" ls32
/ [ *5( h4 ":" ) h4 ] "::" h4
/ [ *6( h4 ":" ) h4 ] "::"
IPv6reference = "[" IPv6address "]"
ls32 = ( h4 ":" h4 ) / IPv4address
mark = "-" / "_" / "." / "!" / "~" / "*" / "'" / "(" / ")"
net-path = "//" authority [ abs-path ]
path-segments = segment *( "/" segment )
pchar = unreserved / escaped / ";" /
":" / "@" / "&" / "=" / "+" / "$" / ","
port = *DIGIT
qualified = *( "." domainlabel ) [ "." ]
query = *( pchar / "/" / "?" )
rel-path = path-segments
relative-URI = hier-part [ "?" query ] [ "#" fragment ]
reserved = "/" / "?" / "#" / "[" / "]" / ";" /
":" / "@" / "&" / "=" / "+" / "$" / ","
scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." ) reg-name = 0*255( unreserved / pct-encoded / sub-delims )
path = segment *( "/" segment )
segment = *pchar segment = *pchar
unreserved = ALPHA / DIGIT / mark query = *( pchar / "/" / "?" )
fragment = *( pchar / "/" / "?" )
URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ]
URI-reference = URI / relative-URI pct-encoded = "%" HEXDIG HEXDIG
uric = reserved / unreserved / escaped pchar = unreserved / pct-encoded / sub-delims / ":" / "@"
userinfo = *( unreserved / escaped / ";" / unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
":" / "&" / "=" / "+" / "$" / "," ) reserved = gen-delims / sub-delims
gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@"
sub-delims = "!" / "$" / "&" / "'" / "(" / ")"
/ "*" / "+" / "," / ";" / "="
Appendix B. Parsing a URI Reference with a Regular Expression Appendix B. Parsing a URI Reference with a Regular Expression
Since the "first-match-wins" algorithm is identical to the "greedy" Since the "first-match-wins" algorithm is identical to the "greedy"
disambiguation method used by POSIX regular expressions, it is disambiguation method used by POSIX regular expressions, it is
natural and commonplace to use a regular expression for parsing the natural and commonplace to use a regular expression for parsing the
potential five components of a URI reference. potential five components of a URI reference.
The following line is the regular expression for breaking-down a The following line is the regular expression for breaking-down a
well-formed URI reference into its components. well-formed URI reference into its components.
skipping to change at page 51, line 15 skipping to change at page 53, line 15
Appendix C. Delimiting a URI in Context Appendix C. Delimiting a URI in Context
URIs are often transmitted through formats that do not provide a URIs are often transmitted through formats that do not provide a
clear context for their interpretation. For example, there are many clear context for their interpretation. For example, there are many
occasions when a URI is included in plain text; examples include text occasions when a URI is included in plain text; examples include text
sent in electronic mail, USENET news messages, and, most importantly, sent in electronic mail, USENET news messages, and, most importantly,
printed on paper. In such cases, it is important to be able to printed on paper. In such cases, it is important to be able to
delimit the URI from the rest of the text, and in particular from delimit the URI from the rest of the text, and in particular from
punctuation marks that might be mistaken for part of the URI. punctuation marks that might be mistaken for part of the URI.
In practice, URI 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 the case where a fragment identifier is associated with a URI
reference, the fragment would be placed within the brackets as well
(separated from the URI with a "#" character).
In some cases, extra whitespace (spaces, line-breaks, tabs, etc.) may 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 unescaped whitespace around the line break, and should be aware all whitespace around the line break, and should be aware that the
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
a delimiting style for a URI that contains whitespace. a delimiting style for a reference that contains embedded whitespace.
The prefix "URL:" (with or without a trailing space) was formerly The prefix "URL:" (with or without a trailing space) was formerly
recommended as a way to help distinguish a URI from other bracketed recommended as a way to help distinguish a URI from other bracketed
designators, though it is not commonly used in practice and is no designators, though it is not commonly used in practice and is no
longer recommended. longer recommended.
For robustness, software that accepts user-typed URI should attempt For robustness, software that accepts user-typed URI should attempt
to recognize and strip both delimiters and embedded whitespace. to recognize and strip both delimiters and embedded whitespace.
For example, the text: For example, the text:
Yes, Jim, I found it under "http://www.w3.org/Addressing/", Yes, Jim, I found it under "http://www.w3.org/Addressing/",
but you can probably pick it up from <ftp://ds.internic. but you can probably pick it up from <ftp://foo.example.
net/rfc/>. Note the warning in <http://www.ics.uci.edu/pub/ com/rfc/>. Note the warning in <http://www.ics.uci.edu/pub/
ietf/uri/historical.html#WARNING>. ietf/uri/historical.html#WARNING>.
contains the URI references contains the URI references
http://www.w3.org/Addressing/ http://www.w3.org/Addressing/
ftp://ds.internic.net/rfc/ ftp://foo.example.com/rfc/
http://www.ics.uci.edu/pub/ietf/uri/historical.html#WARNING http://www.ics.uci.edu/pub/ietf/uri/historical.html#WARNING
Appendix D. Summary of Non-editorial Changes Appendix D. Summary of Non-editorial Changes
D.1 Additions D.1 Additions
IPv6 literals have been added to the list of possible identifiers for IPv6 (and later) literals have been added to the list of possible
the host portion of a authority component, as described by [RFC2732], identifiers for the host portion of a authority component, as
with the addition of "[" and "]" to the reserved and uric sets. described by [RFC2732], with the addition of "[" and "]" to the
Square brackets are now specified as reserved within the authority reserved set and a version flag to anticipate future versions of IP
component and not allowed outside their use as delimiters for an literals. Square brackets are now specified as reserved within the
IPv6reference within host. In order to make this change without authority component and not allowed outside their use as delimiters
for an IP literal within host. In order to make this change without
changing the technical definition of the path, query, and fragment 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, and the definition of limit each decimal octet to the range 0-255.
hostname has been improved to better specify length limitations and
partially-qualified domain names.
Section 6 (Section 6) on URI normalization and comparison has been 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. Likewise, discussion within the W3C Technical Architecture Group.
Section 2.1 on the encoding of characters has been replaced.
An ABNF production for URI has been introduced to correspond to the An ABNF rule for URI has been introduced to correspond to the common
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 2.2 on reserved characters has been rewritten to clearly Section 2 on characters has been rewritten to explain what characters
explain what characters are reserved, when they are reserved, and why are reserved, when they are reserved, and why they are reserved even
they are reserved even when not used as delimiters by the generic when not used as delimiters by the generic syntax. The mark
syntax. Likewise, the section on escaped characters has been characters that are typically unsafe to decode, including the
rewritten, and URI normalizers are now given license to unescape any exclamation mark ("!"), asterisk ("*"), single-quote ("'"), and open
octets corresponding to unreserved characters. The number-sign ("#") and close parentheses ("(" and ")"), have been moved to the reserved
character has been moved back from the excluded delims to the set in order to clarify the distinction between reserved and
reserved set. unreserved and hopefully answer the most common question of scheme
designers. Likewise, the section on percent-encoded characters has
been rewritten, and URI normalizers are now given license to decode
any percent-encoded octets corresponding to unreserved characters.
In general, the terms "escaped" and "unescaped" have been replaced
with "percent-encoded" and "decoded", respectively, to reduce
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 significantly reduce complexity. As more friendly to LALR parsers and significantly reduce complexity. As
a result, the layout form of syntax description has been removed, a result, the layout form of syntax description has been removed,
along with the uric-no-slash, opaque-part, and rel-segment along with the uric, uric_no_slash, hier_part, opaque_part, net_path,
productions. All references to "opaque" URIs have been replaced with abs_path, rel_path, path_segments, rel_segment, and mark rules. All
a better description of how the path component may be opaque to references to "opaque" URIs have been replaced with a better
hierarchy. The fragment identifier has been moved back into the description of how the path component may be opaque to hierarchy. The
section on generic syntax components and within the URI and ambiguity regarding the parsing of URI-reference as a URI or a
relative-URI productions, though it remains excluded from relative-URI with a colon in the first segment is now explained and
absolute-URI. The ambiguity regarding the parsing of URI-reference as disambiguated in the section defining relative-URI.
a URI or a relative-URI with a colon in the first segment is now
explained and disambiguated in the section defining relative-URI.
The ABNF of hier-part and relative-URI has been corrected to allow a The fragment identifier has been moved back into the section on
relative URI path to be empty. This also allows an absolute-URI to generic syntax components and within the URI and relative-URI rules,
consist of nothing after the "scheme:", as is present in practice though it remains excluded from absolute-URI. The number sign ("#")
with the "DAV:" namespace [RFC2518] and the "about:" URI used by many character has been moved back to the reserved set as a result of
browser implementations. The ambiguity regarding the parsing of reintegrating the fragment syntax.
net-path, abs-path, and rel-path is now explained and disambiguated
in the same section.
Registry-based naming authorities that use the generic syntax The ABNF has been corrected to allow a relative path to be empty.
authority component are now limited to DNS hostnames, since those This also allows an absolute-URI to consist of nothing after the
have been the only such URIs in deployment. This change was "scheme:", as is present in practice with the "dav:" namespace
necessary to enable internationalized domain names to be processed in [RFC2518] and the "about:" scheme used internally by many WWW browser
their native character encodings at the application layers above URI implementations. The ambiguity regarding the boundary between
processing. The reg_name, server, and hostport productions have been authority and path is now explained and disambiguated in the same
removed to simplify parsing of the URI syntax. section.
The ABNF of qualified has been simplified to remove a parsing Registry-based naming authorities that use the generic syntax are now
ambiguity without changing the allowed syntax. The toplabel defined within the host rule and limited to 255 path characters. This
production has been removed because it served no useful purpose. The change allows current implementations, where whatever name provided
ambiguity regarding the parsing of host as IPv4address or hostname is is simply fed to the local name resolution mechanism, to be
now explained and disambiguated in the same section. consistent with the specification and removes the need to re-specify
DNS name formats here. It also allows the host component to contain
percent-encoded octets, which is necessary to enable
internationalized domain names to be provided in URIs, processed in
their native character encodings at the application layers above URI
processing, and passed to an IDNA library as a registered name in the
UTF-8 character encoding. The server, hostport, hostname,
domainlabel, toplabel, and alphanum rules have been removed.
The resolving relative references algorithm of [RFC2396] has been 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 in o Removed the special-case treatment of same-document references
favor of a section that explains that a new retrieval action within the URI parser in favor of a section that explains when a
should not be made if the target URI and base URI, excluding reference should be interpreted by a dereferencing engine as a
fragments, match. This change has no impact on user agent same-document reference: when the target URI and base URI,
behavior aside from how the resolved reference might be described excluding fragments, match. This change does not modify the
to the user. behavior of existing same-document references as defined by RFC
2396 (fragment-only references); it merely adds the same-document
distinction to other references that refer to the base URI and
simplifies the interface between applications and their URI
parsers, as is consistent with the internal architecture of
deployed URI processing implementations.
o Separated the path merge routine into two routines: merge, for 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.
Index Index
A A
ABNF 9 ABNF 10
abs-path 16
absolute 25 absolute 25
absolute-path 24 absolute-path 24
absolute-URI 25 absolute-URI 25
access 7 access 7
alphanum 18 authority 15, 16
authority 16, 17
B B
base URI 27 base URI 27
C
characters 11
D D
dec-octet 19 dec-octet 18
delims 15
dereference 7 dereference 7
domainlabel 18
dot-segments 20 dot-segments 20
E
escaped 13
excluded 14
F F
fragment 22 fragment 22
G G
gen-delims 12
generic syntax 5 generic syntax 5
H H
h4 19 h16 17
hier-part 16 hierarchical 9
hierarchical 8 host 17
host 18
hostname 18
I I
identifier 5 identifier 5
invisible 14 IP-literal 17
IPv4 19 IPv4 18
IPv4address 19 IPv4address 18
IPv6 19 IPv6 17
IPv6address 19 IPv6address 17
IPv6reference 19 IPvFuture 17
L L
locator 6 locator 6
ls32 19 ls32 17
M M
mark 12
merge 30 merge 30
N N
name 6 name 6
net-path 16
network-path 24 network-path 24
P P
path 16, 20 path 15, 20
path-segments 20
pchar 20 pchar 20
pct-encoded 11
percent-encoding 11
port 20 port 20
Q Q
qualified 18
query 21 query 21
R R
rel-path 16 reg-name 19
registered name 19
relative 9, 27 relative 9, 27
relative-path 24 relative-path 24
relative-URI 24 relative-URI 24
remove_dot_segments 30 remove_dot_segments 30
representation 8 representation 8
reserved 11 reserved 12
resolution 7, 27 resolution 7, 27
resource 4 resource 4
retrieval 8 retrieval 8
S S
same-document 25 same-document 25
sameness 8 sameness 8
scheme 16 scheme 15
segment 20 segment 20
sub-delims 12
suffix 25 suffix 25
T T
transcription 6 transcription 6
U U
uniform 4 uniform 4
unreserved 12 unreserved 12
unwise 15
URI grammar URI grammar
abs-path 16
absolute-URI 25 absolute-URI 25
ALPHA 9 ALPHA 10
alphanum 18 authority 15, 16
authority 16, 17 CR 10
CR 9 CTL 10
CTL 9 dec-octet 18
dec-octet 19 DIGIT 10
DIGIT 9 DQUOTE 10
domainlabel 18 fragment 15, 22, 24
DQUOTE 9 gen-delims 12
escaped 13 h16 18
fragment 16, 22, 24 HEXDIG 10
h4 19 host 16, 17
HEXDIG 9 IP-literal 17
hier-part 16, 24, 25 IPv4address 18
host 17, 18 IPv6address 17, 18
hostname 18 IPvFuture 17
IPv4address 19 LF 10
IPv6address 19 ls32 18
IPv6reference 19
LF 9
ls32 19
mark 12 mark 12
net-path 16 OCTET 10
OCTET 9 path 15
path-segments 16, 20 path-segments 20
pchar 20, 21, 22 pchar 20, 21, 22
port 17, 20 pct-encoded 11
qualified 18 port 16, 20
query 16, 21, 24, 25 query 15, 21, 24, 25
rel-path 16 reg-name 19
relative-URI 24, 24 relative-URI 24, 24
reserved 12 reserved 12
scheme 16, 17, 25 scheme 15, 15, 25
segment 20 segment 20
SP 9 SP 10
sub-delims 12
unreserved 12 unreserved 12
URI 16, 24 URI 15, 24
URI-reference 24 URI-reference 24
uric 11 userinfo 16, 16
userinfo 17, 18 URI 15
URI 16
URI-reference 24 URI-reference 24
uric 11
URL 6 URL 6
URN 6 URN 6
userinfo 18 userinfo 16
Intellectual Property Statement Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and IETF's procedures with respect to rights in standards-track and
skipping to change at page 60, line 29 skipping to change at page 61, line 29
be obtained from the IETF Secretariat. be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive this standard. Please address the information to the IETF Executive
Director. Director.
Full Copyright Statement Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved. Copyright (C) The Internet Society (2004). All Rights Reserved.
This document and translations of it may be copied and furnished to This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of Internet organizations, except as needed for the purpose of
skipping to change at page 61, line 7 skipping to change at page 62, line 7
The limited permissions granted above are perpetual and will not be The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees. revoked by the Internet Society or its successors or assignees.
This document and the information contained herein is provided on an This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement 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.
 End of changes. 263 change blocks. 
972 lines changed or deleted 1118 lines changed or added

This html diff was produced by rfcdiff 1.48. The latest version is available from http://tools.ietf.org/tools/rfcdiff/