Network Working Group T. Berners-Lee

Request for Comments: 3986 W3C/MIT

STD: 66 R. Fielding

Updates: 1738 Day Software

Obsoletes: 2732, 2396, 1808 L. Masinter

Category: Standards Track Adobe Systems

January 2005

Uniform Resource Identifier (URI): Generic Syntax

Status of This Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the \"Internet

Official Protocol Standards\" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2005).

Abstract

A Uniform Resource Identifier (URI) is a compact sequence of

characters that identifies an abstract or physical resource. This

specification defines the generic URI syntax and a process for

resolving URI references that might be in relative form, along with

guidelines and security considerations for the use of URIs on the

Internet. The URI syntax defines a grammar that is a superset of all

valid URIs, allowing an implementation to parse the common components

of a URI reference without knowing the scheme-specific requirements

of every possible identifier. This specification does not define a

generative grammar for URIs; that task is performed by the individual

specifications of each URI scheme.

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RFC 3986 URI Generic Syntax January 2005

Table of Contents

1\. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.1. Overview of URIs . . . . . . . . . . . . . . . . . . . . 4

1.1.1. Generic Syntax . . . . . . . . . . . . . . . . . 6

1.1.2. Examples . . . . . . . . . . . . . . . . . . . . 7

1.1.3. URI, URL, and URN . . . . . . . . . . . . . . . 7

1.2. Design Considerations . . . . . . . . . . . . . . . . . 8

1.2.1. Transcription . . . . . . . . . . . . . . . . . 8

1.2.2. Separating Identification from Interaction . . . 9

1.2.3. Hierarchical Identifiers . . . . . . . . . . . . 10

1.3. Syntax Notation . . . . . . . . . . . . . . . . . . . . 11

2\. Characters . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.1. Percent-Encoding . . . . . . . . . . . . . . . . . . . . 12

2.2. Reserved Characters . . . . . . . . . . . . . . . . . . 12

2.3. Unreserved Characters . . . . . . . . . . . . . . . . . 13

2.4. When to Encode or Decode . . . . . . . . . . . . . . . . 14

2.5. Identifying Data . . . . . . . . . . . . . . . . . . . . 14

3\. Syntax Components . . . . . . . . . . . . . . . . . . . . . . 16

3.1. Scheme . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.2. Authority . . . . . . . . . . . . . . . . . . . . . . . 17

3.2.1. User Information . . . . . . . . . . . . . . . . 18

3.2.2. Host . . . . . . . . . . . . . . . . . . . . . . 18

3.2.3. Port . . . . . . . . . . . . . . . . . . . . . . 22

3.3. Path . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.4. Query . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.5. Fragment . . . . . . . . . . . . . . . . . . . . . . . . 24

4\. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4.1. URI Reference . . . . . . . . . . . . . . . . . . . . . 25

4.2. Relative Reference . . . . . . . . . . . . . . . . . . . 26

4.3. Absolute URI . . . . . . . . . . . . . . . . . . . . . . 27

4.4. Same-Document Reference . . . . . . . . . . . . . . . . 27

4.5. Suffix Reference . . . . . . . . . . . . . . . . . . . . 27

5\. Reference Resolution . . . . . . . . . . . . . . . . . . . . . 28

5.1. Establishing a Base URI . . . . . . . . . . . . . . . . 28

5.1.1. Base URI Embedded in Content . . . . . . . . . . 29

5.1.2. Base URI from the Encapsulating Entity . . . . . 29

5.1.3. Base URI from the Retrieval URI . . . . . . . . 30

5.1.4. Default Base URI . . . . . . . . . . . . . . . . 30

5.2. Relative Resolution . . . . . . . . . . . . . . . . . . 30

5.2.1. Pre-parse the Base URI . . . . . . . . . . . . . 31

5.2.2. Transform References . . . . . . . . . . . . . . 31

5.2.3. Merge Paths . . . . . . . . . . . . . . . . . . 32

5.2.4. Remove Dot Segments . . . . . . . . . . . . . . 33

5.3. Component Recomposition . . . . . . . . . . . . . . . . 35

5.4. Reference Resolution Examples . . . . . . . . . . . . . 35

5.4.1. Normal Examples . . . . . . . . . . . . . . . . 36

5.4.2. Abnormal Examples . . . . . . . . . . . . . . . 36

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6\. Normalization and Comparison . . . . . . . . . . . . . . . . . 38

6.1. Equivalence . . . . . . . . . . . . . . . . . . . . . . 38

6.2. Comparison Ladder . . . . . . . . . . . . . . . . . . . 39

6.2.1. Simple String Comparison . . . . . . . . . . . . 39

6.2.2. Syntax-Based Normalization . . . . . . . . . . . 40

6.2.3. Scheme-Based Normalization . . . . . . . . . . . 41

6.2.4. Protocol-Based Normalization . . . . . . . . . . 42

7\. Security Considerations . . . . . . . . . . . . . . . . . . . 43

7.1. Reliability and Consistency . . . . . . . . . . . . . . 43

7.2. Malicious Construction . . . . . . . . . . . . . . . . . 43

7.3. Back-End Transcoding . . . . . . . . . . . . . . . . . . 44

7.4. Rare IP Address Formats . . . . . . . . . . . . . . . . 45

7.5. Sensitive Information . . . . . . . . . . . . . . . . . 45

7.6. Semantic Attacks . . . . . . . . . . . . . . . . . . . . 45

8\. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 46

9\. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 46

10\. References . . . . . . . . . . . . . . . . . . . . . . . . . . 46

10.1. Normative References . . . . . . . . . . . . . . . . . . 46

10.2. Informative References . . . . . . . . . . . . . . . . . 47

A. Collected ABNF for URI . . . . . . . . . . . . . . . . . . . . 49

B. Parsing a URI Reference with a Regular Expression . . . . . . 50

C. Delimiting a URI in Context . . . . . . . . . . . . . . . . . 51

D. Changes from RFC 2396 . . . . . . . . . . . . . . . . . . . . 53

D.1. Additions . . . . . . . . . . . . . . . . . . . . . . . 53

D.2. Modifications . . . . . . . . . . . . . . . . . . . . . 53

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Authors\' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 60

Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 61

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1\. Introduction

A Uniform Resource Identifier (URI) provides a simple and extensible

means for identifying a resource. This specification of URI syntax

and semantics is derived from concepts introduced by the World Wide

Web global information initiative, whose use of these identifiers

dates from 1990 and is described in \"Universal Resource Identifiers

in WWW\" \[RFC1630\]. The syntax is designed to meet the

recommendations laid out in \"Functional Recommendations for Internet

Resource Locators\" \[RFC1736\] and \"Functional Requirements for
Uniform

Resource Names\" \[RFC1737\].

This document obsoletes \[RFC2396\], which merged \"Uniform Resource

Locators\" \[RFC1738\] and \"Relative Uniform Resource Locators\"

\[RFC1808\] in order to define a single, generic syntax for all URIs.

It obsoletes \[RFC2732\], which introduced syntax for an IPv6 address.

It excludes portions of RFC 1738 that defined the specific syntax of

individual URI schemes; those portions will be updated as separate

documents. The process for registration of new URI schemes is

defined separately by \[BCP35\]. Advice for designers of new URI

schemes can be found in \[RFC2718\]. All significant changes from RFC

2396 are noted in Appendix D.

This specification uses the terms \"character\" and \"coded character

set\" in accordance with the definitions provided in \[BCP19\], and

\"character encoding\" in place of what \[BCP19\] refers to as a

\"charset\".

1.1. Overview of URIs

URIs are characterized as follows:

Uniform

Uniformity provides several benefits. It allows different types

of resource identifiers to be used in the same context, even when

the mechanisms used to access those resources may differ. It

allows uniform semantic interpretation of common syntactic

conventions across different types of resource identifiers. It

allows introduction of new types of resource identifiers without

interfering with the way that existing identifiers are used. It

allows the identifiers to be reused in many different contexts,

thus permitting new applications or protocols to leverage a pre-

existing, large, and widely used set of resource identifiers.

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Resource

This specification does not limit the scope of what might be a

resource; rather, the term \"resource\" is used in a general sense

for whatever might be identified by a URI. Familiar examples

include an electronic document, an image, a source of information

with a consistent purpose (e.g., \"today\'s weather report for Los

Angeles\"), a service (e.g., an HTTP-to-SMS gateway), and a

collection of other resources. A resource is not necessarily

accessible via the Internet; e.g., human beings, corporations, and

bound books in a library can also be resources. Likewise,

abstract concepts can be resources, such as the operators and

operands of a mathematical equation, the types of a relationship

(e.g., \"parent\" or \"employee\"), or numeric values (e.g., zero,

one, and infinity).

Identifier

An identifier embodies the information required to distinguish

what is being identified from all other things within its scope of

identification. Our use of the terms \"identify\" and \"identifying\"

refer to this purpose of distinguishing one resource from all

other resources, regardless of how that purpose is accomplished

(e.g., by name, address, or context). These terms should not be

mistaken as an assumption that an identifier defines or embodies

the identity of what is referenced, though that may be the case

for some identifiers. Nor should it be assumed that a system

using URIs will access the resource identified: in many cases,

URIs are used to denote resources without any intention that they

be accessed. Likewise, the \"one\" resource identified might not be

singular in nature (e.g., a resource might be a named set or a

mapping that varies over time).

A URI is an identifier consisting of a sequence of characters

matching the syntax rule named \<URI\> in Section 3. It enables

uniform identification of resources via a separately defined

extensible set of naming schemes (Section 3.1). How that

identification is accomplished, assigned, or enabled is delegated to

each scheme specification.

This specification does not place any limits on the nature of a

resource, the reasons why an application might seek to refer to a

resource, or the kinds of systems that might use URIs for the sake of

identifying resources. This specification does not require that a

URI persists in identifying the same resource over time, though that

is a common goal of all URI schemes. Nevertheless, nothing in this

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specification prevents an application from limiting itself to

particular types of resources, or to a subset of URIs that maintains

characteristics desired by that application.

URIs have a global scope and are interpreted consistently regardless

of context, though the result of that interpretation may be in

relation to the end-user\'s context. For example, \"http://localhost/\"

has the same interpretation for every user of that reference, even

though the network interface corresponding to \"localhost\" may be

different for each end-user: interpretation is independent of access.

However, an action made on the basis of that reference will take

place in relation to the end-user\'s context, which implies that an

action intended to refer to a globally unique thing must use a URI

that distinguishes that resource from all other things. URIs that

identify in relation to the end-user\'s local context should only be

used when the context itself is a defining aspect of the resource,

such as when an on-line help manual refers to a file on the end-

user\'s file system (e.g., \"file:///etc/hosts\").

1.1.1. Generic Syntax

Each URI begins with a scheme name, as defined in Section 3.1, that

refers to a specification for assigning identifiers within that

scheme. As such, the URI syntax is a federated and extensible naming

system wherein each scheme\'s specification may further restrict the

syntax and semantics of identifiers using that scheme.

This specification defines those elements of the URI syntax that are

required of all URI schemes or are common to many URI schemes. It

thus defines the syntax and semantics needed to implement a scheme-

independent parsing mechanism for URI references, by which the

scheme-dependent handling of a URI can be postponed until the

scheme-dependent semantics are needed. Likewise, protocols and data

formats that make use of URI references can refer to this

specification as a definition for the range of syntax allowed for all

URIs, including those schemes that have yet to be defined. This

decouples the evolution of identification schemes from the evolution

of protocols, data formats, and implementations that make use of

URIs.

A parser of the generic URI syntax can parse any URI reference into

its major components. Once the scheme is determined, further

scheme-specific parsing can be performed on the components. In other

words, the URI generic syntax is a superset of the syntax of all URI

schemes.

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1.1.2. Examples

The following example URIs illustrate several URI schemes and

variations in their common syntax components:

ftp://ftp.is.co.za/rfc/rfc1808.txt

http://www.ietf.org/rfc/rfc2396.txt

ldap://\[2001:db8::7\]/c=GB?objectClass?one

mailto:John.Doe@example.com

news:comp.infosystems.www.servers.unix

tel:+1-816-555-1212

telnet://192.0.2.16:80/

urn:oasis:names:specification:docbook:dtd:xml:4.1.2

1.1.3. URI, URL, and URN

A URI can be further classified as a locator, a name, or both. The

term \"Uniform Resource Locator\" (URL) refers to the subset of URIs

that, in addition to identifying a resource, provide a means of

locating the resource by describing its primary access mechanism

(e.g., its network \"location\"). The term \"Uniform Resource Name\"

(URN) has been used historically to refer to both URIs under the

\"urn\" scheme \[RFC2141\], which are required to remain globally unique

and persistent even when the resource ceases to exist or becomes

unavailable, and to any other URI with the properties of a name.

An individual scheme does not have to be classified as being just one

of \"name\" or \"locator\". Instances of URIs from any given scheme may

have the characteristics of names or locators or both, often

depending on the persistence and care in the assignment of

identifiers by the naming authority, rather than on any quality of

the scheme. Future specifications and related documentation should

use the general term \"URI\" rather than the more restrictive terms

\"URL\" and \"URN\" \[RFC3305\].

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1.2. Design Considerations

1.2.1. Transcription

The URI syntax has been designed with global transcription as one of

its main considerations. A URI is a sequence of characters from a

very limited set: the letters of the basic Latin alphabet, digits,

and a few special characters. A URI may be represented in a variety

of ways; e.g., ink on paper, pixels on a screen, or a sequence of

character encoding octets. The interpretation of a URI depends only

on the characters used and not on how those characters are

represented in a network protocol.

The goal of transcription can be described by a simple scenario.

Imagine two colleagues, Sam and Kim, sitting in a pub at an

international conference and exchanging research ideas. Sam asks Kim

for a location to get more information, so Kim writes the URI for the

research site on a napkin. Upon returning home, Sam takes out the

napkin and types the URI into a computer, which then retrieves the

information to which Kim referred.

There are several design considerations revealed by the scenario:

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

as a sequence of octets.

o A URI might be transcribed from a non-network source and thus

should consist of characters that are most likely able to be

entered into a computer, within the constraints imposed by

keyboards (and related input devices) across languages and

locales.

o A URI often has to be remembered by people, and it is easier for

people to remember a URI when it consists of meaningful or

familiar components.

These design considerations are not always in alignment. For

example, it is often the case that the most meaningful name for a URI

component would require characters that cannot be typed into some

systems. The ability to transcribe a resource identifier from one

medium to another has been considered more important than having a

URI consist of the most meaningful of components.

In local or regional contexts and with improving technology, users

might benefit from being able to use a wider range of characters;

such use is not defined by 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 this

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representation is allowed by the scheme or by the protocol element in

which the URI is referenced. Such a definition should specify the

character encoding used to map those characters to octets prior to

being percent-encoded for the URI.

1.2.2. Separating Identification from Interaction

A common misunderstanding of URIs is that they are only used to refer

to accessible resources. The URI itself only provides

identification; access to the resource is neither guaranteed nor

implied by the presence of a URI. Instead, any operation associated

with a URI reference is defined by the protocol element, data format

attribute, or natural language text in which it appears.

Given a URI, a system may attempt to perform a variety of operations

on the resource, as might be characterized by words such as \"access\",

\"update\", \"replace\", or \"find attributes\". Such operations are

defined by the protocols that make use of URIs, not by this

specification. However, we do use a few general terms for describing

common operations on URIs. URI \"resolution\" is the process of

determining an access mechanism and the appropriate parameters

necessary to dereference a URI; this resolution may require several

iterations. To use that access mechanism to perform an action on the

URI\'s resource is to \"dereference\" the URI.

When URIs are used within information retrieval systems to identify

sources of information, the most common form of URI dereference is

\"retrieval\": making use of a URI in order to retrieve a

representation of its associated resource. A \"representation\" is a

sequence of octets, along with representation metadata describing

those octets, that constitutes a record of the state of the resource

at the time when the representation is generated. Retrieval is

achieved by a process that might include using the URI as a cache key

to check for a locally cached representation, resolution of the URI

to determine an appropriate access mechanism (if any), and

dereference of the URI for the sake of applying a retrieval

operation. Depending on the protocols used to perform the retrieval,

additional information might be supplied about the resource (resource

metadata) and its relation to other resources.

URI references in information retrieval systems are designed to be

late-binding: the result of an access is generally determined when it

is accessed and may vary over time or due to other aspects of the

interaction. These references are created in order to be used in the

future: what is being identified is not some specific result that was

obtained in the past, but rather some characteristic that is expected

to be true for future results. In such cases, the resource referred

to by the URI is actually a sameness of characteristics as observed

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over time, perhaps elucidated by additional comments or assertions

made by the resource provider.

Although many URI schemes are named after protocols, this does not

imply that use of these URIs will result in access to the resource

via the named protocol. URIs are often used simply for the sake of

identification. Even when a URI is used to retrieve a representation

of a resource, that access might be through gateways, proxies,

caches, and name resolution services that are independent of the

protocol associated with the scheme name. The resolution of some

URIs may require the use of more than one protocol (e.g., both DNS

and HTTP are typically used to access an \"http\" URI\'s origin server

when a representation isn\'t found in a local cache).

1.2.3. Hierarchical Identifiers

The URI syntax is organized hierarchically, with components listed in

order of decreasing significance from left to right. For some URI

schemes, the visible hierarchy is limited to the scheme itself:

everything after the scheme component delimiter (\":\") is considered

opaque to URI processing. Other URI schemes make the hierarchy

explicit and visible to generic parsing algorithms.

The generic syntax uses the slash (\"/\"), question mark (\"?\"), and

number sign (\"#\") characters to delimit components that are

significant to the generic parser\'s hierarchical interpretation of an

identifier. In addition to aiding the readability of such

identifiers through the consistent use of familiar syntax, this

uniform representation of hierarchy across naming schemes allows

scheme-independent references to be made relative to that hierarchy.

It is often the case that a group or \"tree\" of documents has been

constructed to serve a common purpose, wherein the vast majority of

URI references in these documents point to resources within the tree

rather than outside it. Similarly, documents located at a particular

site are much more likely to refer to other resources at that site

than to resources at remote sites. Relative referencing of URIs

allows document trees to be partially independent of their location

and access scheme. For instance, it is possible for a single set of

hypertext documents to be simultaneously accessible and traversable

via each of the \"file\", \"http\", and \"ftp\" schemes if the documents

refer to each other with relative references. Furthermore, such

document trees can be moved, as a whole, without changing any of the

relative references.

A relative reference (Section 4.2) refers to a resource by describing

the difference within a hierarchical name space between the reference

context and the target URI. The reference resolution algorithm,

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presented in Section 5, defines how such a reference is transformed

to the target URI. As 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.

NOTE: Previous specifications used the terms \"partial URI\" and

\"relative URI\" to denote a relative reference to a URI. As some

readers misunderstood those terms to mean that relative URIs are a

subset of URIs rather than a method of referencing URIs, this

specification simply refers to them as relative references.

All URI references are parsed by generic syntax parsers when used.

However, because hierarchical processing has no effect on an absolute

URI used in a reference unless it contains one or more dot-segments

(complete path segments of \".\" or \"..\", as described in Section
3.3),

URI scheme specifications can define opaque identifiers by

disallowing use of slash characters, question mark characters, and

the URIs \"scheme:.\" and \"scheme:..\".

1.3. Syntax Notation

This specification uses the Augmented Backus-Naur Form (ABNF)

notation of \[RFC2234\], including the following core ABNF syntax rules

defined by that specification: ALPHA (letters), CR (carriage return),

DIGIT (decimal digits), DQUOTE (double quote), HEXDIG (hexadecimal

digits), LF (line feed), and SP (space). The complete URI syntax is

collected in Appendix A.

2\. Characters

The URI syntax provides a method of encoding data, presumably for the

sake of identifying a resource, as a sequence of characters. The URI

characters are, in turn, frequently encoded as octets for transport

or presentation. This specification does not mandate any particular

character encoding 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;

without such a definition, a URI is assumed to be in the same

character encoding as the surrounding text.

The ABNF notation defines its terminal values to be non-negative

integers (codepoints) based on the US-ASCII coded character set

\[ASCII\]. Because a URI is a sequence of characters, we must invert

that relation in order to understand the URI syntax. Therefore, the

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integer values used by the ABNF must be mapped back to their

corresponding characters via US-ASCII in order to complete the syntax

rules.

A URI is composed from a limited set of characters consisting of

digits, letters, and a few graphic symbols. A reserved subset of

those characters may be used to delimit syntax components within a

URI while the remaining characters, including both the unreserved set

and those reserved characters not acting as delimiters, define each

component\'s identifying data.

2.1. Percent-Encoding

A percent-encoding mechanism is used to represent a data octet in a

component when that octet\'s corresponding character is outside the

allowed set or is being used as a delimiter of, or within, the

component. A percent-encoded octet is encoded as a character

triplet, consisting of the percent character \"%\" followed by the two

hexadecimal digits representing that octet\'s numeric value. For

example, \"%20\" is the percent-encoding for the binary octet

\"00100000\" (ABNF: %x20), which in US-ASCII corresponds to the space

character (SP). Section 2.4 describes when percent-encoding and

decoding is applied.

pct-encoded = \"%\" HEXDIG HEXDIG

The uppercase hexadecimal digits \'A\' through \'F\' are equivalent to

the lowercase digits \'a\' through \'f\', respectively. If two URIs

differ only in the case of hexadecimal digits used in percent-encoded

octets, they are equivalent. For consistency, URI producers and

normalizers should use uppercase hexadecimal digits for all percent-

encodings.

2.2. Reserved Characters

URIs include components and subcomponents that are delimited by

characters in the \"reserved\" set. These characters are called

\"reserved\" because they may (or may not) be defined as delimiters by

the generic syntax, by each scheme-specific syntax, or by the

implementation-specific syntax of a URI\'s dereferencing algorithm.

If data for a URI component would conflict with a reserved

character\'s purpose as a delimiter, then the conflicting data must be

percent-encoded before the URI is formed.

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reserved = gen-delims / sub-delims

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

sub-delims = \"!\" / \"\$\" / \"&\" / \"\'\" / \"(\" / \")\"

/ \"\*\" / \"+\" / \",\" / \";\" / \"=\"

The purpose of reserved characters is to provide a set of delimiting

characters that are distinguishable from other data within a URI.

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. Thus, characters in the reserved

set are protected from normalization and are therefore safe to be

used by scheme-specific and producer-specific algorithms for

delimiting data subcomponents within a URI.

A subset of the reserved characters (gen-delims) is used as

delimiters of the generic URI components described in Section 3. A

component\'s ABNF syntax rule will not use the reserved or gen-delims

rule names directly; instead, each syntax rule lists the characters

allowed within that component (i.e., not delimiting it), and any of

those characters that are also in the reserved set are \"reserved\" for

use as subcomponent delimiters within the component. Only the most

common subcomponents are defined by this specification; other

subcomponents may be defined by a URI scheme\'s specification, or by

the implementation-specific syntax of a URI\'s dereferencing

algorithm, provided that such subcomponents are delimited by

characters in the reserved set allowed within that component.

URI producing applications should percent-encode data octets that

correspond to characters in the reserved set unless these characters

are specifically allowed by the URI scheme to represent data in that

component. If a reserved character is found in a URI component and

no delimiting role is known for that character, then it must be

interpreted as representing the data octet corresponding to that

character\'s encoding in US-ASCII.

2.3. Unreserved Characters

Characters that are allowed in a URI but do not have a reserved

purpose are called unreserved. These include uppercase and lowercase

letters, decimal digits, hyphen, period, underscore, and tilde.

unreserved = ALPHA / DIGIT / \"-\" / \".\" / \"\_\" / \"\~\"

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URIs that differ in the replacement of an unreserved character with

its corresponding percent-encoded US-ASCII octet are equivalent: they

identify the same resource. However, URI comparison implementations

do not always perform normalization prior to comparison (see Section

6). 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 characters by URI normalizers.

2.4. When to Encode or Decode

Under normal circumstances, the only time when octets within a URI

are percent-encoded is during the process of producing the URI from

its component parts. This is when an implementation determines which

of the reserved characters are to be used as subcomponent delimiters

and which can be safely used as data. Once produced, a URI is always

in its percent-encoded form.

When a URI is dereferenced, the components and subcomponents

significant to the scheme-specific dereferencing process (if any)

must be parsed and separated before the percent-encoded octets within

those components can be safely decoded, as otherwise the data may be

mistaken for component delimiters. The only exception is for

percent-encoded octets corresponding to characters in the unreserved

set, which can be decoded at any time. For example, the octet

corresponding to the tilde (\"\~\") character is often encoded as
\"%7E\"

by older URI processing implementations; the \"%7E\" can be replaced by

\"\~\" without changing its interpretation.

Because the percent (\"%\") character serves as the indicator for

percent-encoded octets, it must be percent-encoded as \"%25\" for that

octet to be used as data within a URI. Implementations must not

percent-encode or decode the same string more than once, as 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.

2.5. Identifying Data

URI characters provide identifying data for each of the URI

components, serving as an external interface for identification

between systems. Although the presence and nature of the URI

production interface is hidden from clients that use its URIs (and is

thus beyond the scope of the interoperability requirements defined by

this specification), it is a frequent source of confusion and errors

in the interpretation of URI character issues. Implementers have to

be aware that there are multiple character encodings involved in the

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production and transmission of URIs: local name and data encoding,

public interface encoding, URI character encoding, data format

encoding, and protocol encoding.

Local names, such as file system names, are stored with a local

character encoding. URI producing applications (e.g., origin

servers) will typically use the local encoding as the basis for

producing meaningful names. The URI producer will transform the

local encoding to one that is suitable for a public interface and

then transform the public interface encoding into the restricted set

of URI characters (reserved, unreserved, and percent-encodings).

Those characters are, in turn, encoded as octets to be used as a

reference within a data format (e.g., a document charset), and such

data formats are often subsequently encoded for transmission over

Internet protocols.

For most systems, an unreserved character appearing within a URI

component is interpreted as representing the data octet corresponding

to that character\'s encoding in US-ASCII. Consumers of URIs assume

that the letter \"X\" corresponds to the octet \"01011000\", and even

when that assumption is incorrect, there is no harm in making it. 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 \[STD63\] (or

some other superset of the US-ASCII character encoding) at an

internal interface, thereby providing more meaningful identifiers

than those resulting from simply percent-encoding the original

octets.

For example, consider an information service that provides data,

stored locally using an EBCDIC-based file system, to clients on the

Internet through an HTTP server. When an author creates a file with

the name \"Laguna Beach\" on that file system, the \"http\" URI

corresponding to that resource is expected to contain the meaningful

string \"Laguna%20Beach\". If, however, that server produces URIs by

using an overly simplistic raw octet mapping, then the result would

be a URI containing \"%D3%81%87%A4%95%81@%C2%85%81%83%88\". An

internal transcoding interface fixes this problem by transcoding the

local name to a superset of US-ASCII prior to producing the URI.

Naturally, proper interpretation of an incoming URI on such an

interface requires that percent-encoded octets be decoded (e.g.,

\"%20\" to SP) before the reverse transcoding is applied to obtain the

local name.

In some cases, the internal interface between a URI component and the

identifying data that 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, or numeric

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coordinates on a map. Likewise, a URI scheme may define components

with additional encoding requirements that are applied prior to

forming the component and producing the URI.

When a new URI scheme defines a component that represents textual

data consisting of characters from the Universal Character Set \[UCS\],

the data should first be encoded as octets according to the UTF-8

character encoding \[STD63\]; 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

The generic URI syntax consists of a hierarchical sequence of

components referred to as the scheme, authority, path, query, and

fragment.

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

hier-part = \"//\" authority path-abempty

/ path-absolute

/ path-rootless

/ path-empty

The scheme and path components are required, though the path may be

empty (no characters). When authority is present, the path must

either be empty or begin with a slash (\"/\") character. When

authority is not present, the path cannot begin with two slash

characters (\"//\"). These restrictions result in five different ABNF

rules for a path (Section 3.3), only one of which will match any

given URI reference.

The following are two example URIs and their component parts:

foo://example.com:8042/over/there?name=ferret#nose

\\\_/ \\\_\_\_\_\_\_\_\_\_\_\_\_\_\_/\\\_\_\_\_\_\_\_\_\_/
\\\_\_\_\_\_\_\_\_\_/ \\\_\_/

\| \| \| \| \|

scheme authority path query fragment

\| \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\|\_\_

/ \\ / \\

urn:example:animal:ferret:nose

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3.1. Scheme

Each URI begins with a scheme name that refers to a specification for

assigning identifiers within that scheme. As such, the URI syntax is

a federated and extensible naming system wherein each scheme\'s

specification may further restrict the syntax and semantics of

identifiers using that scheme.

Scheme names consist of a sequence of characters beginning with a

letter and followed by any combination of letters, digits, plus

(\"+\"), period (\".\"), or hyphen (\"-\"). Although schemes are case-

insensitive, the canonical form is lowercase and documents that

specify schemes must do so with lowercase letters. An implementation

should accept uppercase letters as equivalent to lowercase in scheme

names (e.g., allow \"HTTP\" as well as \"http\") for the sake of

robustness but should only produce lowercase scheme names for

consistency.

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

Individual schemes are not specified by this document. The process

for registration of new URI schemes is defined separately by \[BCP35\].

The scheme registry maintains the mapping between scheme names and

their specifications. Advice for designers of new URI schemes can be

found in \[RFC2718\]. URI scheme specifications must define their own

syntax so that all strings matching their scheme-specific syntax will

also match the \<absolute-URI\> grammar, as described in Section 4.3.

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, which might indicate that the URI has been

constructed to mislead the user (Section 7.6).

3.2. Authority

Many URI schemes include a hierarchical element for a naming

authority so that governance of the name space defined by the

remainder of the URI is delegated to that authority (which may, in

turn, delegate it further). The generic syntax provides a common

means for distinguishing an authority based on a registered name or

server address, along with optional port and user information.

The authority component is preceded by a double slash (\"//\") and is

terminated by the next slash (\"/\"), question mark (\"?\"), or number

sign (\"#\") character, or by the end of the URI.

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

URI producers and normalizers should omit the \":\" delimiter that

separates host from port if the port component is empty. Some

schemes do not allow the userinfo and/or port subcomponents.

If a URI contains an authority component, then the path component

must either be empty or begin with a slash (\"/\") character. Non-

validating parsers (those that merely separate a URI reference into

its major components) will often ignore the subcomponent structure of

authority, treating it as an opaque string from the double-slash to

the first terminating delimiter, until such time as the URI is

dereferenced.

3.2.1. User Information

The userinfo subcomponent may consist of a user name and, optionally,

scheme-specific information about how to gain authorization to access

the resource. The user information, if present, is followed by a

commercial at-sign (\"@\") that delimits it from the host.

userinfo = \*( unreserved / pct-encoded / sub-delims / \":\" )

Use of the format \"user:password\" in the userinfo field is

deprecated. Applications should not render as clear text any data

after the first colon (\":\") character found within a userinfo

subcomponent unless the data after the colon is the empty string

(indicating no password). Applications may choose to ignore or

reject such data when it is received as part of a reference and

should reject the storage of such data in unencrypted form. The

passing of authentication information in clear text has proven to be

a security risk in almost every case where it has been used.

Applications that render a URI for the sake of user feedback, such as

in graphical hypertext browsing, should render userinfo in a way that

is distinguished from the rest of a URI, when feasible. Such

rendering will assist the user in cases where the userinfo has been

misleadingly crafted to look like a trusted domain name

(Section 7.6).

3.2.2. Host

The host subcomponent of authority is identified by an IP literal

encapsulated within square brackets, an IPv4 address in dotted-

decimal form, or a registered name. The host subcomponent is case-

insensitive. The presence of a host subcomponent within a URI does

not imply that the scheme requires access to the given host on the

Internet. In many cases, the host syntax is used only for the sake

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of reusing the existing registration process created and deployed for

DNS, thus obtaining a globally unique name without the cost of

deploying another registry. However, such use comes with its own

costs: domain name ownership may change over time for reasons not

anticipated by the URI producer. In other cases, the data within the

host component identifies a registered name that has nothing to do

with an Internet host. We use the name \"host\" for the ABNF rule

because that is its most common purpose, not its only purpose.

host = IP-literal / IPv4address / reg-name

The syntax rule for host is ambiguous because it does not completely

distinguish between an IPv4address and a reg-name. In order to

disambiguate the syntax, we apply the \"first-match-wins\" algorithm:

If host matches the rule for IPv4address, then it should be

considered an IPv4 address literal and not a reg-name. Although host

is case-insensitive, producers and normalizers should use lowercase

for registered names and hexadecimal addresses for the sake of

uniformity, while only using uppercase letters for percent-encodings.

A host identified by an Internet Protocol literal address, version 6

\[RFC3513\] or later, is distinguished by enclosing the IP literal

within square brackets (\"\[\" and \"\]\"). This is the only place where

square bracket characters are allowed in the URI syntax. In

anticipation of future, as-yet-undefined IP literal address formats,

an implementation may use an optional version flag to indicate such a

format explicitly rather than rely on heuristic determination.

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

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

The version flag does not indicate the IP version; rather, it

indicates future versions of the literal format. As such,

implementations must not provide the version flag for the existing

IPv4 and IPv6 literal address forms described below. 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 \[RFC3513\]. This syntax does not support

IPv6 scoped addressing zone identifiers.

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A 128-bit IPv6 address is divided into eight 16-bit pieces. Each

piece is represented numerically in case-insensitive hexadecimal,

using one to four hexadecimal digits (leading zeroes are permitted).

The eight encoded pieces are given most-significant first, separated

by colon characters. Optionally, the least-significant two pieces

may instead be represented in IPv4 address textual format. A

sequence of one or more consecutive zero-valued 16-bit pieces within

the address may be elided, omitting all their digits and leaving

exactly two consecutive colons in their place to mark the elision.

IPv6address = 6( h16 \":\" ) ls32

/ \"::\" 5( h16 \":\" ) ls32

/ \[ h16 \] \"::\" 4( h16 \":\" ) ls32

/ \[ \*1( h16 \":\" ) h16 \] \"::\" 3( h16 \":\" ) ls32

/ \[ \*2( h16 \":\" ) h16 \] \"::\" 2( h16 \":\" ) ls32

/ \[ \*3( h16 \":\" ) h16 \] \"::\" h16 \":\" ls32

/ \[ \*4( h16 \":\" ) h16 \] \"::\" ls32

/ \[ \*5( h16 \":\" ) h16 \] \"::\" h16

/ \[ \*6( h16 \":\" ) h16 \] \"::\"

ls32 = ( h16 \":\" h16 ) / IPv4address

; least-significant 32 bits of address

h16 = 1\*4HEXDIG

; 16 bits of address represented in hexadecimal

A host identified by an IPv4 literal address is represented in

dotted-decimal notation (a sequence of four decimal numbers in the

range 0 to 255, separated by \".\"), as described in \[RFC1123\] by

reference to \[RFC0952\]. Note that other forms of dotted notation may

be interpreted on some platforms, as described in Section 7.4, but

only the dotted-decimal form of four octets is allowed by this

grammar.

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

dec-octet = DIGIT ; 0-9

/ %x31-39 DIGIT ; 10-99

/ \"1\" 2DIGIT ; 100-199

/ \"2\" %x30-34 DIGIT ; 200-249

/ \"25\" %x30-35 ; 250-255

A host identified by a registered name is a sequence of characters

usually intended for lookup within a locally defined host or service

name registry, though the URI\'s scheme-specific semantics may require

that a specific registry (or fixed name table) be used instead. The

most common name registry mechanism is the Domain Name System (DNS).

A registered name intended for lookup in the DNS uses the syntax

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defined in Section 3.5 of \[RFC1034\] and Section 2.1 of \[RFC1123\].

Such a 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 if it is necessary to distinguish between

the complete domain name and some local domain.

reg-name = \*( unreserved / pct-encoded / sub-delims )

If the URI scheme defines a default for host, then that default

applies when the host subcomponent is undefined or when the

registered name is empty (zero length). For example, the \"file\" URI

scheme is defined so that no authority, an empty host, and

\"localhost\" all mean the end-user\'s machine, whereas the \"http\"

scheme considers a missing authority or empty host invalid.

This specification does not mandate a particular registered name

lookup technology and therefore does not restrict the syntax of reg-

name beyond what is necessary for interoperability. Instead, it

delegates the issue of registered 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 registered names. However, a globally scoped naming

system, such as DNS fully qualified domain names, is necessary for

URIs intended to have global scope. URI producers should use names

that conform to the DNS syntax, even when use of DNS is not

immediately apparent, and should limit these names to no more than

255 characters in length.

The reg-name syntax allows percent-encoded octets in order to

represent non-ASCII registered names in a uniform way that is

independent of the underlying name resolution technology. Non-ASCII

characters must first be encoded according to UTF-8 \[STD63\], and then

each octet of the corresponding UTF-8 sequence must be percent-

encoded to be represented as URI characters. URI producing

applications must not use percent-encoding in host unless it is used

to represent a UTF-8 character sequence. When a non-ASCII registered

name represents an internationalized domain name intended for

resolution via the DNS, the name must be transformed to the IDNA

encoding \[RFC3490\] prior to name lookup. URI producers should

provide these registered names in the IDNA encoding, rather than a

percent-encoding, if they wish to maximize interoperability with

legacy URI resolvers.

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3.2.3. Port

The port subcomponent of authority is designated by an optional port

number in decimal following the host and delimited from it by a

single colon (\":\") character.

port = \*DIGIT

A scheme may define a default port. For example, the \"http\" scheme

defines a default port of \"80\", corresponding to its reserved TCP

port number. The type of port designated by the port number (e.g.,

TCP, UDP, SCTP) is defined by the URI scheme. URI producers and

normalizers should omit the port component and its \":\" delimiter if

port is empty or if its value would be the same as that of the

scheme\'s default.

3.3. Path

The path component contains data, usually organized in hierarchical

form, that, along with data in the non-hierarchical query component

(Section 3.4), serves to identify a resource within the scope of the

URI\'s scheme and naming authority (if any). The path is terminated

by the first question mark (\"?\") or number sign (\"#\") character, or

by the end of the URI.

If a URI contains an authority component, then the path component

must either be empty or begin with a slash (\"/\") character. If a URI

does not contain an authority component, then the path cannot begin

with two slash characters (\"//\"). In addition, a URI reference

(Section 4.1) may be a relative-path reference, in which case the

first path segment cannot contain a colon (\":\") character. The ABNF

requires five separate rules to disambiguate these cases, only one of

which will match the path substring within a given URI reference. We

use the generic term \"path component\" to describe the URI substring

matched by the parser to one of these rules.

path = path-abempty ; begins with \"/\" or is empty

/ path-absolute ; begins with \"/\" but not \"//\"

/ path-noscheme ; begins with a non-colon segment

/ path-rootless ; begins with a segment

/ path-empty ; zero characters

path-abempty = \*( \"/\" segment )

path-absolute = \"/\" \[ segment-nz \*( \"/\" segment ) \]

path-noscheme = segment-nz-nc \*( \"/\" segment )

path-rootless = segment-nz \*( \"/\" segment )

path-empty = 0\<pchar\>

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segment = \*pchar

segment-nz = 1\*pchar

segment-nz-nc = 1\*( unreserved / pct-encoded / sub-delims / \"@\" )

; non-zero-length segment without any colon \":\"

pchar = unreserved / pct-encoded / sub-delims / \":\" / \"@\"

A path consists of a sequence of path segments separated by a slash

(\"/\") character. A path is always defined for a URI, though the

defined path may be empty (zero length). Use of the slash character

to indicate hierarchy is only required when a URI will be used as the

context for relative references. For example, the URI

\<mailto:fred@example.com\> has a path of \"fred@example.com\", whereas

the URI \<foo://info.example.com?fred\> has an empty path.

The path segments \".\" and \"..\", also known as dot-segments, are

defined for relative reference within the path name hierarchy. They

are intended for use at the beginning of a relative-path reference

(Section 4.2) to indicate relative position within the hierarchical

tree of names. This is similar to their role within some operating

systems\' file directory structures to indicate the current directory

and parent directory, respectively. However, unlike in a file

system, these dot-segments are only interpreted within the URI path

hierarchy and are removed as part of the resolution process (Section

5.2).

Aside from dot-segments in hierarchical paths, a path segment is

considered opaque by the generic syntax. URI producing applications

often use the reserved characters allowed in a segment to delimit

scheme-specific or dereference-handler-specific subcomponents. For

example, the semicolon (\";\") and equals (\"=\") reserved characters
are

often used to delimit parameters and parameter values applicable to

that segment. The comma (\",\") reserved character is often used for

similar purposes. For example, one URI producer might use a segment

such as \"name;v=1.1\" to indicate a reference to version 1.1 of

\"name\", whereas another might use a segment such as \"name,1.1\" to

indicate the same. Parameter types may be defined by scheme-specific

semantics, but in most cases the syntax of a parameter is specific to

the implementation of the URI\'s dereferencing algorithm.

3.4. Query

The query component contains non-hierarchical data that, along with

data in the path component (Section 3.3), serves to identify a

resource within the scope of the URI\'s scheme and naming authority

(if any). The query component is indicated by the first question

mark (\"?\") character and terminated by a number sign (\"#\") character

or by the end of the URI.

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

The characters slash (\"/\") and question mark (\"?\") may represent
data

within the query component. Beware that some older, erroneous

implementations may not handle such data correctly when it is used as

the base URI for relative references (Section 5.1), apparently

because they fail to distinguish query data from path data when

looking for hierarchical separators. However, as query components

are often used to carry identifying information in the form of

\"key=value\" pairs and one frequently used value is a reference to

another URI, it is sometimes better for usability to avoid percent-

encoding those characters.

3.5. Fragment

The fragment identifier component of a URI allows indirect

identification of a secondary resource by reference to a primary

resource and additional identifying information. The identified

secondary resource may be some portion or subset of the primary

resource, some view on representations of the primary resource, or

some other resource defined or described by those representations. A

fragment identifier component is indicated by the presence of a

number sign (\"#\") character and terminated by the end of the URI.

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

The semantics of a fragment identifier are defined by the set of

representations that might result from a retrieval action on the

primary resource. The fragment\'s format and resolution is therefore

dependent on the media type \[RFC2046\] of a potentially retrieved

representation, even though such a retrieval is only performed if the

URI is dereferenced. If no such representation exists, then the

semantics of the fragment are considered unknown and are effectively

unconstrained. Fragment identifier semantics are independent of the

URI scheme and thus cannot be redefined by scheme specifications.

Individual media types may define their own restrictions on or

structures within the fragment identifier syntax for specifying

different types of subsets, views, or external references that are

identifiable as secondary resources by that media type. If the

primary resource has multiple representations, as is often the case

for resources whose representation is selected based on attributes of

the retrieval request (a.k.a., content negotiation), then whatever is

identified by the fragment should be consistent across all of those

representations. Each representation should either define the

fragment so that it corresponds to the same secondary resource,

regardless of how it is represented, or should leave the fragment

undefined (i.e., not found).

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As with any URI, use of a fragment identifier component does not

imply that a retrieval action will take place. A URI with a fragment

identifier may be used to refer to the secondary resource without any

implication that the primary resource is accessible or will ever be

accessed.

Fragment identifiers have a special role in information retrieval

systems as the primary form of client-side indirect referencing,

allowing an author to specifically identify aspects of an existing

resource that are only indirectly provided by the resource owner. As

such, the fragment identifier is not used in the scheme-specific

processing of a URI; instead, the fragment identifier is separated

from the rest of the URI prior to a dereference, and thus the

identifying information within the fragment itself is dereferenced

solely by the user agent, regardless of the URI scheme. Although

this separate handling is often perceived to be a loss of

information, particularly for accurate redirection of references as

resources move over time, it also serves to prevent information

providers from denying reference authors the right to refer to

information within a resource selectively. Indirect referencing also

provides additional flexibility and extensibility to systems that use

URIs, as new media types are easier to define and deploy than new

schemes of identification.

The characters slash (\"/\") and question mark (\"?\") are allowed to

represent data within the fragment identifier. Beware that some

older, erroneous implementations may not handle this data correctly

when it is used as the base URI for relative references (Section

5.1).

4\. Usage

When applications make reference to a URI, they do not always use the

full form of reference defined by the \"URI\" syntax rule. To save

space and take advantage of hierarchical locality, many Internet

protocol elements and media type formats allow an abbreviation of a

URI, whereas others restrict the syntax to a particular form of URI.

We define the most common forms of reference syntax in this

specification because they impact and depend upon the design of the

generic syntax, requiring a uniform parsing algorithm in order to be

interpreted consistently.

4.1. URI Reference

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

identifier.

URI-reference = URI / relative-ref

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A URI-reference is either a URI or a relative reference. If the

URI-reference\'s prefix does not match the syntax of a scheme followed

by its colon separator, then the URI-reference is a relative

reference.

A URI-reference is typically parsed first into the five URI

components, in order to determine what components are present and

whether the reference is relative. Then, each component is parsed

for its subparts and their validation. The ABNF of URI-reference,

along with the \"first-match-wins\" disambiguation rule, is sufficient

to define a validating parser for the generic syntax. Readers

familiar with regular expressions should see Appendix B for an

example of a non-validating URI-reference parser that will take any

given string and extract the URI components.

4.2. Relative Reference

A relative reference takes advantage of the hierarchical syntax

(Section 1.2.3) to express a URI reference relative to the name space

of another hierarchical URI.

relative-ref = relative-part \[ \"?\" query \] \[ \"#\" fragment \]

relative-part = \"//\" authority path-abempty

/ path-absolute

/ path-noscheme

/ path-empty

The URI referred to by a relative reference, also known as the target

URI, is obtained by applying the reference resolution algorithm of

Section 5.

A relative reference that begins with two slash characters is termed

a network-path reference; such references are rarely used. A

relative reference that begins with a single slash character is

termed an absolute-path reference. A relative reference that does

not begin with a slash character is termed a relative-path reference.

A path segment that contains a colon character (e.g., \"this:that\")

cannot be used as the first segment of a relative-path reference, as

it would be mistaken for a scheme name. Such a segment must be

preceded by a dot-segment (e.g., \"./this:that\") to make a relative-

path reference.

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4.3. Absolute URI

Some protocol elements allow only the absolute form of a URI without

a fragment identifier. For example, defining a base URI for later

use by relative references calls for an absolute-URI syntax rule that

does not allow a fragment.

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

URI scheme specifications must define their own syntax so that all

strings matching their scheme-specific syntax will also match the

\<absolute-URI\> grammar. Scheme specifications will not define

fragment identifier syntax or usage, regardless of its applicability

to resources identifiable via that scheme, as fragment identification

is orthogonal to scheme definition. However, scheme specifications

are encouraged to include a wide range of examples, including

examples that show use of the scheme\'s URIs with fragment identifiers

when such usage is appropriate.

4.4. Same-Document Reference

When a URI reference refers to a URI that is, aside from its fragment

component (if any), identical to the base URI (Section 5.1), that

reference is called a \"same-document\" reference. The most frequent

examples of same-document references are relative references that are

empty or include only the number sign (\"#\") separator followed by a

fragment identifier.

When a same-document reference is dereferenced for a retrieval

action, the target of that reference is defined to be within the same

entity (representation, document, or message) as the reference;

therefore, a dereference should not result in a new retrieval action.

Normalization of the base and target URIs prior to their comparison,

as described in Sections 6.2.2 and 6.2.3, is allowed but rarely

performed in practice. Normalization may increase the set of same-

document references, which may be of benefit to some caching

applications. As such, reference authors should not assume that a

slightly different, though equivalent, reference URI will (or will

not) be interpreted as a same-document reference by any given

application.

4.5. Suffix Reference

The URI syntax is designed for unambiguous reference to resources and

extensibility via the URI scheme. However, as URI identification and

usage have become commonplace, traditional media (television, radio,

newspapers, billboards, etc.) have increasingly used a suffix of the

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URI as a reference, consisting of only the authority and path

portions of the URI, such as

www.w3.org/Addressing/

or simply a DNS registered name on its own. Such references are

primarily intended for human interpretation rather than for machines,

with the assumption that context-based heuristics are sufficient to

complete the URI (e.g., most registered names beginning with \"www\"

are likely to have a URI prefix of \"http://\"). Although there is no

standard set of heuristics for disambiguating a URI suffix, many

client implementations allow them to be entered by the user and

heuristically resolved.

Although this practice of using suffix references is common, it

should be avoided whenever possible and should never be 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 \[RFC1535\].

As a URI suffix has the same syntax as a relative-path reference, a

suffix reference cannot be used in contexts where a relative

reference is expected. As a result, suffix references are limited to

places where there is no defined base URI, such as dialog boxes and

off-line advertisements.

5\. Reference Resolution

This section defines the process of resolving a URI reference within

a context that allows relative references so that the result is a

string matching the \<URI\> syntax rule of Section 3.

5.1. Establishing a Base URI

The term \"relative\" implies that a \"base URI\" exists against which

the relative reference is applied. Aside from fragment-only

references (Section 4.4), relative references are only usable when a

base URI is known. A base URI must be established by the parser

prior to parsing URI references that might be relative. A base URI

must conform to the \<absolute-URI\> syntax rule (Section 4.3). If the

base URI is obtained from a URI reference, then that reference must

be converted to absolute form and stripped of any fragment component

prior to its use as a base URI.

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The base URI of a reference can be established in one of four ways,

discussed below in order of precedence. The order of precedence can

be thought of in terms of layers, where the innermost defined base

URI has the highest precedence. This can be visualized graphically

as follows:

.\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--.

\|
.\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--.
\|

\| \|
.\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--.
\| \|

\| \| \|
.\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--.
\| \| \|

\| \| \| \|
.\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--. \|
\| \| \|

\| \| \| \| \| \<relative-reference\> \| \| \| \| \|

\| \| \| \|
\`\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--\'
\| \| \| \|

\| \| \| \| (5.1.1) Base URI embedded in content \| \| \| \|

\| \| \|
\`\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--\'
\| \| \|

\| \| \| (5.1.2) Base URI of the encapsulating entity \| \| \|

\| \| \| (message, representation, or none) \| \| \|

\| \|
\`\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--\'
\| \|

\| \| (5.1.3) URI used to retrieve the entity \| \|

\|
\`\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--\'
\|

\| (5.1.4) Default Base URI (application-dependent) \|

\`\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--\'

5.1.1. Base URI Embedded in Content

Within certain media types, a base URI for relative references can be

embedded within the content itself so that it can be readily obtained

by a parser. This can be useful for descriptive documents, such as

tables of contents, which may be transmitted to others through

protocols other than their usual retrieval context (e.g., email or

USENET news).

It is beyond the scope of this specification to specify how, for each

media type, a base URI can be embedded. The appropriate syntax, when

available, is described by the data format specification associated

with each media type.

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.

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A mechanism for embedding a base URI within MIME container types

(e.g., the message and multipart types) is defined by MHTML

\[RFC2557\]. Protocols that do not use the MIME message header syntax,

but that do allow some form of tagged metadata to be included within

messages, may define their own syntax for defining a base URI as part

of a message.

5.1.3. Base URI from the Retrieval URI

If no base URI is embedded and the representation is not encapsulated

within some other entity, then, if a URI was used to retrieve the

representation, that URI shall be considered the base URI. Note that

if the retrieval was the result of a redirected request, the last URI

used (i.e., the URI that resulted in the actual retrieval of the

representation) is the base URI.

5.1.4. Default Base URI

If none of the conditions described above apply, then the base URI is

defined by the context of the application. As this definition is

necessarily application-dependent, failing to define a base URI by

using one of the other methods may result in the same content being

interpreted differently by different types of applications.

A sender of a representation containing relative references is

responsible for ensuring that a base URI for those references can be

established. Aside from fragment-only references, relative

references can only be used reliably in situations where the base URI

is well defined.

5.2. Relative Resolution

This section describes an algorithm for converting a URI reference

that might be relative to a given base URI into the parsed components

of the reference\'s target. The components can then be recomposed, as

described in Section 5.3, to form the target URI. This algorithm

provides definitive results that can be used to test the output of

other implementations. Applications may implement relative reference

resolution by using some other algorithm, provided that the results

match what would be given by this one.

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5.2.1. Pre-parse the Base URI

The base URI (Base) is established according to the procedure of

Section 5.1 and parsed into the five main components described in

Section 3. Note that only the scheme component is required to be

present in a base URI; the other components may be empty or

undefined. A component is undefined if its associated delimiter does

not appear in the URI reference; the path component is never

undefined, though it may be empty.

Normalization of the base URI, as described in Sections 6.2.2 and

6.2.3, is optional. A URI reference must be transformed to its

target URI before it can be normalized.

5.2.2. Transform References

For each URI reference (R), the following pseudocode describes an

algorithm for transforming R into its target URI (T):

\-- The URI reference is parsed into the five URI components

\--

(R.scheme, R.authority, R.path, R.query, R.fragment) = parse(R);

\-- A non-strict parser may ignore a scheme in the reference

\-- if it is identical to the base URI\'s scheme.

\--

if ((not strict) and (R.scheme == Base.scheme)) then

undefine(R.scheme);

endif;

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if defined(R.scheme) then

T.scheme = R.scheme;

T.authority = R.authority;

T.path = remove_dot_segments(R.path);

T.query = R.query;

else

if defined(R.authority) then

T.authority = R.authority;

T.path = remove_dot_segments(R.path);

T.query = R.query;

else

if (R.path == \"\") then

T.path = Base.path;

if defined(R.query) then

T.query = R.query;

else

T.query = Base.query;

endif;

else

if (R.path starts-with \"/\") then

T.path = remove_dot_segments(R.path);

else

T.path = merge(Base.path, R.path);

T.path = remove_dot_segments(T.path);

endif;

T.query = R.query;

endif;

T.authority = Base.authority;

endif;

T.scheme = Base.scheme;

endif;

T.fragment = R.fragment;

5.2.3. Merge Paths

The pseudocode above refers to a \"merge\" routine for merging a

relative-path reference with the path of the base URI. This is

accomplished as follows:

o If the base URI has a defined authority component and an empty

path, then return a string consisting of \"/\" concatenated with the

reference\'s path; otherwise,

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o return a string consisting of the reference\'s path component

appended to all but the last segment of the base URI\'s path (i.e.,

excluding any characters after the right-most \"/\" in the base URI

path, or excluding the entire base URI path if it does not contain

any \"/\" characters).

5.2.4. Remove Dot Segments

The pseudocode also refers to a \"remove_dot_segments\" routine for

interpreting and removing the special \".\" and \"..\" complete path

segments from a referenced path. This is done after the path is

extracted from a reference, whether or not the path was relative, in

order to remove any invalid or extraneous dot-segments prior to

forming the target URI. Although there are many ways to accomplish

this removal process, we describe a simple method using two string

buffers.

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

components and the output buffer is initialized to the empty

string.

2\. While the input buffer is not empty, loop as follows:

A. If the input buffer begins with a prefix of \"../\" or \"./\",

then remove that prefix from the input buffer; otherwise,

B. if the input buffer begins with a prefix of \"/./\" or \"/.\",

where \".\" is a complete path segment, then replace that

prefix with \"/\" in the input buffer; otherwise,

C. if the input buffer begins with a prefix of \"/../\" or \"/..\",

where \"..\" is a complete path segment, then replace that

prefix with \"/\" in the input buffer and remove the last

segment and its preceding \"/\" (if any) from the output

buffer; otherwise,

D. if the input buffer consists only of \".\" or \"..\", then remove

that from the input buffer; otherwise,

E. move the first path segment in the input buffer to the end of

the output buffer, including the initial \"/\" character (if

any) and any subsequent characters up to, but not including,

the next \"/\" character or the end of the input buffer.

3\. Finally, the output buffer is returned as the result of

remove_dot_segments.

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Note that dot-segments are intended for use in URI references to

express an identifier relative to the hierarchy of names in the base

URI. The remove_dot_segments algorithm respects that hierarchy by

removing extra dot-segments rather than treat them as an error or

leaving them to be misinterpreted by dereference implementations.

The following illustrates how the above steps are applied for two

examples of merged paths, showing the state of the two buffers after

each step.

STEP OUTPUT BUFFER INPUT BUFFER

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

2E: /a /b/c/./../../g

2E: /a/b /c/./../../g

2E: /a/b/c /./../../g

2B: /a/b/c /../../g

2C: /a/b /../g

2C: /a /g

2E: /a/g

STEP OUTPUT BUFFER INPUT BUFFER

1 : mid/content=5/../6

2E: mid /content=5/../6

2E: mid/content=5 /../6

2C: mid /6

2E: mid/6

Some applications may find it more efficient to implement the

remove_dot_segments algorithm by using two segment stacks rather than

strings.

Note: Beware that some older, erroneous implementations will fail

to separate a reference\'s query component from its path component

prior to merging the base and reference paths, resulting in an

interoperability failure if the query component contains the

strings \"/../\" or \"/./\".

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5.3. Component Recomposition

Parsed URI components can be recomposed to obtain the corresponding

URI reference string. Using pseudocode, this would be:

result = \"\"

if defined(scheme) then

append scheme to result;

append \":\" to result;

endif;

if defined(authority) then

append \"//\" to result;

append authority to result;

endif;

append path to result;

if defined(query) then

append \"?\" to result;

append query to result;

endif;

if defined(fragment) then

append \"#\" to result;

append fragment to result;

endif;

return result;

Note that we are careful to preserve the distinction between a

component that is undefined, meaning that its separator was not

present in the reference, and a component that is empty, meaning that

the separator was present and was immediately followed by the next

component separator or the end of the reference.

5.4. Reference Resolution Examples

Within a representation with a well defined base URI of

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

a relative reference is transformed to its target URI as follows.

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5.4.1. Normal Examples

\"g:h\" = \"g:h\"

\"g\" = \"http://a/b/c/g\"

\"./g\" = \"http://a/b/c/g\"

\"g/\" = \"http://a/b/c/g/\"

\"/g\" = \"http://a/g\"

\"//g\" = \"http://g\"

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

\"g?y\" = \"http://a/b/c/g?y\"

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

\"g#s\" = \"http://a/b/c/g#s\"

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

\";x\" = \"http://a/b/c/;x\"

\"g;x\" = \"http://a/b/c/g;x\"

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

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

\".\" = \"http://a/b/c/\"

\"./\" = \"http://a/b/c/\"

\"..\" = \"http://a/b/\"

\"../\" = \"http://a/b/\"

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

\"../..\" = \"http://a/\"

\"../../\" = \"http://a/\"

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

5.4.2. Abnormal Examples

Although the following abnormal examples are unlikely to occur in

normal practice, all URI parsers should be capable of resolving them

consistently. Each example uses the same base as that above.

Parsers must be careful in handling cases where there are more \"..\"

segments in a relative-path reference than there are hierarchical

levels in the base URI\'s path. Note that the \"..\" syntax cannot be

used to change the authority component of a URI.

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

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

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Similarly, parsers must remove the dot-segments \".\" and \"..\" when

they are complete components of a path, but not when they are only

part of a segment.

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

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

\"g.\" = \"http://a/b/c/g.\"

\".g\" = \"http://a/b/c/.g\"

\"g..\" = \"http://a/b/c/g..\"

\"..g\" = \"http://a/b/c/..g\"

Less likely are cases where the relative reference uses unnecessary

or nonsensical forms of the \".\" and \"..\" complete path segments.

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

\"./g/.\" = \"http://a/b/c/g/\"

\"g/./h\" = \"http://a/b/c/g/h\"

\"g/../h\" = \"http://a/b/c/h\"

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

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

Some applications fail to separate the reference\'s query and/or

fragment components from the path component before merging it with

the base path and removing dot-segments. This error is rarely

noticed, as typical usage of a fragment never includes the hierarchy

(\"/\") character and the query component is not normally used within

relative references.

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

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

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

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

Some parsers allow the scheme name to be present in a relative

reference if it is the same as the base URI scheme. This is

considered to be a loophole in prior specifications of partial URI

\[RFC1630\]. Its use should be avoided but is allowed for backward

compatibility.

\"http:g\" = \"http:g\" ; for strict parsers

/ \"http://a/b/c/g\" ; for backward compatibility

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6\. Normalization and Comparison

One of the most common operations on URIs is simple comparison:

determining whether two URIs are equivalent without using the URIs to

access their respective resource(s). A comparison is performed every

time a response cache is accessed, a browser checks its history to

color a link, or an XML parser processes tags within a namespace.

Extensive normalization prior to comparison of URIs is often used by

spiders and indexing engines to prune a search space or to reduce

duplication of request actions and response storage.

URI comparison is performed for some particular purpose. Protocols

or implementations that compare URIs for different purposes will

often be subject to differing design trade-offs in regards to how

much effort should be spent in reducing aliased identifiers. This

section describes various methods that may be used to compare URIs,

the trade-offs between them, and the types of applications that might

use them.

6.1. Equivalence

Because URIs exist to identify resources, presumably they should be

considered equivalent when they identify the same resource. However,

this definition of equivalence is not of much practical use, as there

is no way for an implementation to compare two resources unless it

has full knowledge or control of them. For this reason,

determination of equivalence or difference of URIs is based on string

comparison, perhaps augmented by reference to additional rules

provided by URI scheme definitions. We use the terms \"different\" and

\"equivalent\" to describe the possible outcomes of such comparisons,

but there are many application-dependent versions of equivalence.

Even though it is possible to determine that two URIs are equivalent,

URI comparison is not sufficient to determine whether two URIs

identify different resources. For example, an owner of two different

domain names could decide to serve the same resource from both,

resulting in two different URIs. Therefore, comparison methods are

designed to minimize false negatives while strictly avoiding false

positives.

In testing for equivalence, applications should not directly compare

relative references; the references should be converted to their

respective target URIs before comparison. When URIs are compared to

select (or avoid) a network action, such as retrieval of a

representation, fragment components (if any) should be excluded from

the comparison.

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6.2. Comparison Ladder

A variety of methods are used in practice to test URI equivalence.

These methods fall into a range, distinguished by the amount of

processing required and the degree to which the probability of false

negatives is reduced. As noted above, false negatives cannot be

eliminated. In practice, their probability can be reduced, but this

reduction requires more processing and is not cost-effective for all

applications.

If this range of comparison practices is considered as a ladder, the

following discussion will climb the ladder, starting with practices

that are cheap but have a relatively higher chance of producing false

negatives, and proceeding to those that have higher computational

cost and lower risk of false negatives.

6.2.1. Simple String Comparison

If two URIs, when considered as character strings, are identical,

then it is safe to conclude that they are equivalent. This type of

equivalence test has very low computational cost and is in wide use

in a variety of applications, particularly in the domain of parsing.

Testing strings for equivalence requires some basic precautions.

This procedure is often referred to as \"bit-for-bit\" or

\"byte-for-byte\" comparison, which is potentially misleading. Testing

strings for equality is normally based on pair comparison of the

characters that make up the strings, starting from the first and

proceeding until both strings are exhausted and all characters are

found to be equal, until a pair of characters compares unequal, or

until one of the strings is exhausted before the other.

This character comparison requires that each pair of characters be

put in comparable form. For example, should one URI be stored in a

byte array in EBCDIC encoding and the second in a Java String object

(UTF-16), bit-for-bit comparisons applied naively will produce

errors. It is better to speak of equality on a character-for-

character basis rather than on a byte-for-byte or bit-for-bit basis.

In practical terms, character-by-character comparisons should be done

codepoint-by-codepoint after conversion to a common character

encoding.

False negatives are caused by the production and use of URI aliases.

Unnecessary aliases can be reduced, regardless of the comparison

method, by consistently providing URI references in an already-

normalized form (i.e., a form identical to what would be produced

after normalization is applied, as described below).

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Protocols and data formats often limit some URI comparisons to simple

string comparison, based on the theory that people and

implementations will, in their own best interest, be consistent in

providing URI references, or at least consistent enough to negate any

efficiency that might be obtained from further normalization.

6.2.2. Syntax-Based Normalization

Implementations may use logic based on the definitions provided by

this specification to reduce the probability of false negatives.

This processing is moderately higher in cost than character-for-

character string comparison. For example, an application using this

approach could reasonably consider the following two URIs equivalent:

example://a/b/c/%7Bfoo%7D

eXAMPLE://a/./b/../b/%63/%7bfoo%7d

Web user agents, such as browsers, typically apply this type of URI

normalization when determining whether a cached response is

available. Syntax-based normalization includes such techniques as

case normalization, percent-encoding normalization, and removal of

dot-segments.

6.2.2.1. Case Normalization

For all URIs, 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.

When a URI uses components of the generic syntax, the component

syntax equivalence rules always apply; namely, that the scheme and

host are case-insensitive and therefore should be normalized to

lowercase. For example, the URI \<HTTP://www.EXAMPLE.com/\> is

equivalent to \<http://www.example.com/\>. The other generic syntax

components are assumed to be case-sensitive unless specifically

defined otherwise by the scheme (see Section 6.2.3).

6.2.2.2. Percent-Encoding Normalization

The percent-encoding mechanism (Section 2.1) is a frequent source of

variance among otherwise identical URIs. In addition to the case

normalization issue noted above, some URI producers percent-encode

octets that do not require percent-encoding, resulting in URIs that

are equivalent to their non-encoded counterparts. These URIs should

be normalized by decoding any percent-encoded octet that corresponds

to an unreserved character, as described in Section 2.3.

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6.2.2.3. Path Segment Normalization

The complete path segments \".\" and \"..\" are intended only for use

within relative references (Section 4.1) and are removed as part of

the reference resolution process (Section 5.2). However, some

deployed implementations incorrectly assume that reference resolution

is not necessary when the reference is already a URI and thus fail to

remove dot-segments when they occur in non-relative paths. URI

normalizers should remove dot-segments by applying the

remove_dot_segments algorithm to the path, as described in

Section 5.2.4.

6.2.3. Scheme-Based Normalization

The syntax and semantics of URIs vary from scheme to scheme, as

described by the defining specification for each scheme.

Implementations may use scheme-specific rules, at further processing

cost, to reduce the probability of false negatives. For example,

because the \"http\" scheme makes use of an authority component, has a

default port of \"80\", and defines an empty path to be equivalent to

\"/\", the following four URIs are equivalent:

http://example.com

http://example.com/

http://example.com:/

http://example.com:80/

In general, a URI that uses the generic syntax for authority with an

empty path should be normalized to a path of \"/\". Likewise, an

explicit \":port\", for which the port is empty or the default for the

scheme, is equivalent to one where the port and its \":\" delimiter are

elided and thus should be removed by scheme-based normalization. For

example, the second URI above is the normal form for the \"http\"

scheme.

Another case where normalization varies by scheme is in the handling

of an empty authority component or empty host subcomponent. For many

scheme specifications, an empty authority or host is considered an

error; for others, it is considered equivalent to \"localhost\" or the

end-user\'s host. When a scheme defines a default for authority and a

URI reference to that default is desired, the reference should be

normalized to an empty authority for the sake of uniformity, brevity,

and internationalization. If, however, either the userinfo or port

subcomponents are non-empty, then the host should be given explicitly

even if it matches the default.

Normalization should not remove delimiters when their associated

component is empty unless licensed to do so by the scheme

Berners-Lee, et al. Standards Track \[Page 41\]

RFC 3986 URI Generic Syntax January 2005

specification. For example, the URI \"http://example.com/?\" cannot be

assumed to be equivalent to any of the examples above. Likewise, the

presence or absence of delimiters within a userinfo subcomponent is

usually significant to its interpretation. The fragment component is

not subject to any scheme-based normalization; thus, two URIs that

differ only by the suffix \"#\" are considered different regardless of

the scheme.

Some schemes define additional subcomponents that consist of case-

insensitive data, giving an implicit license to normalizers to

convert this data to a common case (e.g., all lowercase). For

example, URI schemes that define a subcomponent of path to contain an

Internet hostname, such as the \"mailto\" URI scheme, cause that

subcomponent to be case-insensitive and thus subject to case

normalization (e.g., \"mailto:Joe@Example.COM\" is equivalent to

\"mailto:Joe@example.com\", even though the generic syntax considers

the path component to be case-sensitive).

Other scheme-specific normalizations are possible.

6.2.4. Protocol-Based Normalization

Substantial effort to reduce the incidence of false negatives is

often cost-effective for web spiders. Therefore, they implement even

more aggressive techniques in URI comparison. For example, if they

observe that a URI such as

http://example.com/data

redirects to a URI differing only in the trailing slash

http://example.com/data/

they will likely regard the two as equivalent in the future. This

kind of technique is only appropriate when equivalence is clearly

indicated by both the result of accessing the resources and the

common conventions of their scheme\'s dereference algorithm (in this

case, use of redirection by HTTP origin servers to avoid problems

with relative references).

Berners-Lee, et al. Standards Track \[Page 42\]

RFC 3986 URI Generic Syntax January 2005

7\. Security Considerations

A URI does not in itself pose a security threat. However, as URIs

are often used to provide a compact set of instructions for access to

network resources, care must be taken to properly interpret the data

within a URI, to prevent that data from causing unintended access,

and to avoid including data that should not be revealed in plain

text.

7.1. Reliability and Consistency

There is no guarantee that once a URI has been used to retrieve

information, the same information will be retrievable by that URI in

the future. Nor is there any guarantee that the information

retrievable via that URI in the future will be observably similar to

that retrieved in the past. The URI syntax does not constrain how a

given scheme or authority apportions its namespace or maintains it

over time. Such guarantees can only be obtained from the person(s)

controlling that namespace and the resource in question. A specific

URI scheme may define additional semantics, such as name persistence,

if those semantics are required of all naming authorities for that

scheme.

7.2. Malicious Construction

It is sometimes possible to construct a URI so that an attempt to

perform a seemingly harmless, idempotent operation, such as the

retrieval of a representation, will in fact cause a possibly damaging

remote operation. The unsafe URI is typically constructed by

specifying a port number other than that reserved for the network

protocol in question. The client unwittingly contacts a site running

a different protocol service, and data within the URI contains

instructions that, when interpreted according to this other protocol,

cause an unexpected operation. A frequent example of such abuse has

been the use of a protocol-based scheme with a port component of

\"25\", thereby fooling user agent software into sending an unintended

or impersonating message via an SMTP server.

Applications should prevent dereference of a URI that specifies a TCP

port number within the \"well-known port\" range (0 - 1023) unless the

protocol being used to dereference that URI is compatible with the

protocol expected on that well-known port. Although IANA maintains a

registry of well-known ports, applications should make such

restrictions user-configurable to avoid preventing the deployment of

new services.

Berners-Lee, et al. Standards Track \[Page 43\]

RFC 3986 URI Generic Syntax January 2005

When a URI contains percent-encoded octets that match the delimiters

for a given resolution or dereference protocol (for example, CR and

LF characters for the TELNET protocol), these percent-encodings must

not be decoded before transmission across that protocol. Transfer of

the percent-encoding, which might violate the protocol, is less

harmful than allowing decoded octets to be interpreted as additional

operations or parameters, perhaps triggering an unexpected and

possibly harmful remote operation.

7.3. Back-End Transcoding

When a URI is dereferenced, the data within it is often parsed by

both the user agent and one or more servers. In HTTP, for example, a

typical user agent will parse a URI into its five major components,

access the authority\'s server, and send it the data within the

authority, path, and query components. A typical server will take

that information, parse the path into segments and the query into

key/value pairs, and then invoke implementation-specific handlers to

respond to the request. As a result, a common security concern for

server implementations that handle a URI, either as a whole or split

into separate components, is proper interpretation of the octet data

represented by the characters and percent-encodings within that URI.

Percent-encoded octets must be decoded at some point during the

dereference process. Applications must split the URI into its

components and subcomponents prior to decoding the octets, as

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 file system or related system

functions. File systems typically assign an operational meaning to

special characters, such as the \"/\", \"\\\", \":\", \"\[\", and \"\]\"

characters, and to 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 applications and restrict the use of data

obtained from URI components accordingly.

Berners-Lee, et al. Standards Track \[Page 44\]

RFC 3986 URI Generic Syntax January 2005

7.4. Rare IP Address Formats

Although the URI syntax for IPv4address only allows the common

dotted-decimal form of IPv4 address literal, many implementations

that process URIs make use of platform-dependent system routines,

such as gethostbyname() and inet_aton(), to translate the string

literal to an actual IP address. Unfortunately, such system routines

often allow and process a much larger set of formats than those

described in Section 3.2.2.

For example, many implementations allow dotted forms of three

numbers, wherein the last part is interpreted as a 16-bit quantity

and placed in the right-most two bytes of the network address (e.g.,

a Class B network). Likewise, a dotted form of two numbers means

that the last part is interpreted as a 24-bit quantity and placed in

the right-most three bytes of the network address (Class A), and a

single number (without dots) is interpreted as a 32-bit quantity and

stored directly in the network address. Adding further to the

confusion, some implementations allow each dotted part to be

interpreted as decimal, octal, or hexadecimal, as specified in the C

language (i.e., a leading 0x or 0X implies hexadecimal; a leading 0

implies octal; otherwise, the number is interpreted as decimal).

These additional IP address formats are not allowed in the URI syntax

due to differences between platform implementations. However, they

can become a security concern if an application attempts to filter

access to resources based on the IP address in string literal format.

If this filtering is performed, literals should be converted to

numeric form and filtered based on the numeric value, and not on a

prefix or suffix of the string form.

7.5. Sensitive Information

URI producers should not provide a URI that contains a username or

password that is intended to be secret. URIs are frequently

displayed by browsers, stored in clear text bookmarks, and logged by

user agent history and intermediary applications (proxies). A

password appearing within the userinfo component is deprecated and

should be considered an error (or simply ignored) except in those

rare cases where the \'password\' parameter is intended to be public.

7.6. Semantic Attacks

Because the userinfo subcomponent is rarely used and appears before

the host in the authority component, it can be used to construct a

URI intended to mislead a human user by appearing to identify one

(trusted) naming authority while actually identifying a different

authority hidden behind the noise. For example

Berners-Lee, et al. Standards Track \[Page 45\]

RFC 3986 URI Generic Syntax January 2005

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

might lead a human user to assume that the host is \'cnn.example.com\',

whereas it is actually \'10.0.0.1\'. Note that a misleading userinfo

subcomponent could be much longer than the example above.

A misleading URI, such as that above, is an attack on the user\'s

preconceived notions about the meaning of a URI rather than an attack

on the software itself. User agents may be able to reduce the impact

of such attacks by distinguishing the various components of the URI

when they are rendered, such as by using a different color or tone to

render userinfo if any is present, though there is no panacea. More

information on URI-based semantic attacks can be found in \[Siedzik\].

8\. IANA Considerations

URI scheme names, as defined by \<scheme\> in Section 3.1, form a

registered namespace that is managed by IANA according to the

procedures defined in \[BCP35\]. No IANA actions are required by this

document.

9\. Acknowledgements

This specification is derived from RFC 2396 \[RFC2396\], RFC 1808

\[RFC1808\], and RFC 1738 \[RFC1738\]; the acknowledgements in those

documents still apply. It also incorporates the update (with

corrections) for IPv6 literals in the host syntax, as defined by

Robert M. Hinden, Brian E. Carpenter, and Larry Masinter in

\[RFC2732\]. In addition, contributions by Gisle Aas, Reese Anschultz,

Daniel Barclay, Tim Bray, Mike Brown, Rob Cameron, Jeremy Carroll,

Dan Connolly, Adam M. Costello, John Cowan, Jason Diamond, Martin

Duerst, Stefan Eissing, Clive D.W. Feather, Al Gilman, Tony Hammond,

Elliotte Harold, Pat Hayes, Henry Holtzman, Ian B. Jacobs, Michael

Kay, John C. Klensin, Graham Klyne, Dan Kohn, Bruce Lilly, Andrew

Main, Dave McAlpin, Ira McDonald, Michael Mealling, Ray Merkert,

Stephen Pollei, Julian Reschke, Tomas Rokicki, Miles Sabin, Kai

Schaetzl, Mark Thomson, Ronald Tschalaer, Norm Walsh, Marc Warne,

Stuart Williams, and Henry Zongaro are gratefully acknowledged.

10\. References

10.1. Normative References

\[ASCII\] American National Standards Institute, \"Coded Character

Set \-- 7-bit American Standard Code for Information

Interchange\", ANSI X3.4, 1986.

Berners-Lee, et al. Standards Track \[Page 46\]

RFC 3986 URI Generic Syntax January 2005

\[RFC2234\] Crocker, D. and P. Overell, \"Augmented BNF for Syntax

Specifications: ABNF\", RFC 2234, November 1997.

\[STD63\] Yergeau, F., \"UTF-8, a transformation format of

ISO 10646\", STD 63, RFC 3629, November 2003.

\[UCS\] International Organization for Standardization,

\"Information Technology - Universal Multiple-Octet Coded

Character Set (UCS)\", ISO/IEC 10646:2003, December 2003.

10.2. Informative References

\[BCP19\] Freed, N. and J. Postel, \"IANA Charset Registration

Procedures\", BCP 19, RFC 2978, October 2000.

\[BCP35\] Petke, R. and I. King, \"Registration Procedures for URL

Scheme Names\", BCP 35, RFC 2717, November 1999.

\[RFC0952\] Harrenstien, K., Stahl, M., and E. Feinler, \"DoD Internet

host table specification\", RFC 952, October 1985.

\[RFC1034\] Mockapetris, P., \"Domain names - concepts and facilities\",

STD 13, RFC 1034, November 1987.

\[RFC1123\] Braden, R., \"Requirements for Internet Hosts - Application

and Support\", STD 3, RFC 1123, October 1989.

\[RFC1535\] Gavron, E., \"A Security Problem and Proposed Correction

With Widely Deployed DNS Software\", RFC 1535,

October 1993.

\[RFC1630\] Berners-Lee, T., \"Universal Resource Identifiers in WWW: A

Unifying Syntax for the Expression of Names and Addresses

of Objects on the Network as used in the World-Wide Web\",

RFC 1630, June 1994.

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

Resource Locators\", RFC 1736, February 1995.

\[RFC1737\] Sollins, K. and L. Masinter, \"Functional Requirements for

Uniform Resource Names\", RFC 1737, December 1994.

\[RFC1738\] Berners-Lee, T., Masinter, L., and M. McCahill, \"Uniform

Resource Locators (URL)\", RFC 1738, December 1994.

\[RFC1808\] Fielding, R., \"Relative Uniform Resource Locators\",

RFC 1808, June 1995.

Berners-Lee, et al. Standards Track \[Page 47\]

RFC 3986 URI Generic Syntax January 2005

\[RFC2046\] Freed, N. and N. Borenstein, \"Multipurpose Internet Mail

Extensions (MIME) Part Two: Media Types\", RFC 2046,

November 1996.

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

\[RFC2396\] Berners-Lee, T., Fielding, R., and L. Masinter, \"Uniform

Resource Identifiers (URI): Generic Syntax\", RFC 2396,

August 1998.

\[RFC2518\] Goland, Y., Whitehead, E., Faizi, A., Carter, S., and D.

Jensen, \"HTTP Extensions for Distributed Authoring \--

WEBDAV\", RFC 2518, February 1999.

\[RFC2557\] Palme, J., Hopmann, A., and N. Shelness, \"MIME

Encapsulation of Aggregate Documents, such as HTML

(MHTML)\", RFC 2557, March 1999.

\[RFC2718\] Masinter, L., Alvestrand, H., Zigmond, D., and R. Petke,

\"Guidelines for new URL Schemes\", RFC 2718, November 1999.

\[RFC2732\] Hinden, R., Carpenter, B., and L. Masinter, \"Format for

Literal IPv6 Addresses in URL\'s\", RFC 2732, December 1999.

\[RFC3305\] Mealling, M. and R. Denenberg, \"Report from the Joint

W3C/IETF URI Planning Interest Group: Uniform Resource

Identifiers (URIs), URLs, and Uniform Resource Names

(URNs): Clarifications and Recommendations\", RFC 3305,

August 2002.

\[RFC3490\] Faltstrom, P., Hoffman, P., and A. Costello,

\"Internationalizing Domain Names in Applications (IDNA)\",

RFC 3490, March 2003.

\[RFC3513\] Hinden, R. and S. Deering, \"Internet Protocol Version 6

(IPv6) Addressing Architecture\", RFC 3513, April 2003.

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

April 2001, \<http://www.giac.org/practical/gsec/

Richard_Siedzik_GSEC.pdf\>.

Berners-Lee, et al. Standards Track \[Page 48\]

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Appendix A. Collected ABNF for URI

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

hier-part = \"//\" authority path-abempty

/ path-absolute

/ path-rootless

/ path-empty

URI-reference = URI / relative-ref

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

relative-ref = relative-part \[ \"?\" query \] \[ \"#\" fragment \]

relative-part = \"//\" authority path-abempty

/ path-absolute

/ path-noscheme

/ path-empty

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

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

userinfo = \*( unreserved / pct-encoded / sub-delims / \":\" )

host = IP-literal / IPv4address / reg-name

port = \*DIGIT

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

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

IPv6address = 6( h16 \":\" ) ls32

/ \"::\" 5( h16 \":\" ) ls32

/ \[ h16 \] \"::\" 4( h16 \":\" ) ls32

/ \[ \*1( h16 \":\" ) h16 \] \"::\" 3( h16 \":\" ) ls32

/ \[ \*2( h16 \":\" ) h16 \] \"::\" 2( h16 \":\" ) ls32

/ \[ \*3( h16 \":\" ) h16 \] \"::\" h16 \":\" ls32

/ \[ \*4( h16 \":\" ) h16 \] \"::\" ls32

/ \[ \*5( h16 \":\" ) h16 \] \"::\" h16

/ \[ \*6( h16 \":\" ) h16 \] \"::\"

h16 = 1\*4HEXDIG

ls32 = ( h16 \":\" h16 ) / IPv4address

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

Berners-Lee, et al. Standards Track \[Page 49\]

RFC 3986 URI Generic Syntax January 2005

dec-octet = DIGIT ; 0-9

/ %x31-39 DIGIT ; 10-99

/ \"1\" 2DIGIT ; 100-199

/ \"2\" %x30-34 DIGIT ; 200-249

/ \"25\" %x30-35 ; 250-255

reg-name = \*( unreserved / pct-encoded / sub-delims )

path = path-abempty ; begins with \"/\" or is empty

/ path-absolute ; begins with \"/\" but not \"//\"

/ path-noscheme ; begins with a non-colon segment

/ path-rootless ; begins with a segment

/ path-empty ; zero characters

path-abempty = \*( \"/\" segment )

path-absolute = \"/\" \[ segment-nz \*( \"/\" segment ) \]

path-noscheme = segment-nz-nc \*( \"/\" segment )

path-rootless = segment-nz \*( \"/\" segment )

path-empty = 0\<pchar\>

segment = \*pchar

segment-nz = 1\*pchar

segment-nz-nc = 1\*( unreserved / pct-encoded / sub-delims / \"@\" )

; non-zero-length segment without any colon \":\"

pchar = unreserved / pct-encoded / sub-delims / \":\" / \"@\"

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

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

pct-encoded = \"%\" HEXDIG HEXDIG

unreserved = ALPHA / DIGIT / \"-\" / \".\" / \"\_\" / \"\~\"

reserved = gen-delims / sub-delims

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

sub-delims = \"!\" / \"\$\" / \"&\" / \"\'\" / \"(\" / \")\"

/ \"\*\" / \"+\" / \",\" / \";\" / \"=\"

Appendix B. Parsing a URI Reference with a Regular Expression

As the \"first-match-wins\" algorithm is identical to the \"greedy\"

disambiguation method used by POSIX regular expressions, it is

natural and commonplace to use a regular expression for parsing the

potential five components of a URI reference.

The following line is the regular expression for breaking-down a

well-formed URI reference into its components.

Berners-Lee, et al. Standards Track \[Page 50\]

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\^((\[\^:/?#\]+):)?(//(\[\^/?#\]\*))?(\[\^?#\]\*)(\\?(\[\^#\]\*))?(#(.\*))?

12 3 4 5 6 7 8 9

The numbers in the second line above are only to assist readability;

they indicate the reference points for each subexpression (i.e., each

paired parenthesis). We refer to the value matched for subexpression

\<n\> as \$\<n\>. For example, matching the above expression to

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

results in the following subexpression matches:

\$1 = http:

\$2 = http

\$3 = //www.ics.uci.edu

\$4 = www.ics.uci.edu

\$5 = /pub/ietf/uri/

\$6 = \<undefined\>

\$7 = \<undefined\>

\$8 = #Related

\$9 = Related

where \<undefined\> indicates that the component is not present, as is

the case for the query component in the above example. Therefore, we

can determine the value of the five components as

scheme = \$2

authority = \$4

path = \$5

query = \$7

fragment = \$9

Going in the opposite direction, we can recreate a URI reference from

its components by using the algorithm of Section 5.3.

Appendix C. Delimiting a URI in Context

URIs are often transmitted through formats that do not provide a

clear context for their interpretation. For example, there are many

occasions when a URI is included in plain text; examples include text

sent in email, USENET news, and on printed paper. In such cases, it

is important to be able to delimit the URI from the rest of the text,

and in particular from punctuation marks that might be mistaken for

part of the URI.

In practice, URIs are delimited in a variety of ways, but usually

within double-quotes \"http://example.com/\", angle brackets

\<http://example.com/\>, or just by using whitespace:

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RFC 3986 URI Generic Syntax January 2005

http://example.com/

These wrappers do not form part of the URI.

In some cases, extra whitespace (spaces, line-breaks, tabs, etc.) may

have to be added to break a long URI across lines. The whitespace

should be ignored when the URI is extracted.

No whitespace should be introduced after a hyphen (\"-\") character.

Because some typesetters and printers may (erroneously) introduce a

hyphen at the end of line when breaking it, the interpreter of a URI

containing a line break immediately after a hyphen should ignore all

whitespace around the line break and should be aware that the hyphen

may or may not actually be part of the URI.

Using \<\> angle brackets around each URI is especially recommended as

a delimiting style for a reference that contains embedded whitespace.

The prefix \"URL:\" (with or without a trailing space) was formerly

recommended as a way to help distinguish a URI from other bracketed

designators, though it is not commonly used in practice and is no

longer recommended.

For robustness, software that accepts user-typed URI should attempt

to recognize and strip both delimiters and embedded whitespace.

For example, the text

Yes, Jim, I found it under \"http://www.w3.org/Addressing/\",

but you can probably pick it up from \<ftp://foo.example.

com/rfc/\>. Note the warning in \<http://www.ics.uci.edu/pub/

ietf/uri/historical.html#WARNING\>.

contains the URI references

http://www.w3.org/Addressing/

ftp://foo.example.com/rfc/

http://www.ics.uci.edu/pub/ietf/uri/historical.html#WARNING

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Appendix D. Changes from RFC 2396

D.1. Additions

An ABNF rule for URI has been introduced to correspond to one common

usage of the term: an absolute URI with optional fragment.

IPv6 (and later) literals have been added to the list of possible

identifiers for the host portion of an authority component, as

described by \[RFC2732\], with the addition of \"\[\" and \"\]\" to the

reserved set and a version flag to anticipate future versions of IP

literals. Square brackets are now specified as reserved within the

authority component and are not allowed outside their use as

delimiters for an IP literal within host. In order to make this

change without changing the technical definition of the path, query,

and fragment components, those rules were redefined to directly

specify the characters allowed.

As \[RFC2732\] defers to \[RFC3513\] for definition of an IPv6 literal

address, which, unfortunately, lacks an ABNF description of

IPv6address, we created a new ABNF rule for IPv6address that matches

the text representations defined by Section 2.2 of \[RFC3513\].

Likewise, the definition of IPv4address has been improved in order to

limit each decimal octet to the range 0-255.

Section 6, on URI normalization and comparison, has been completely

rewritten and extended by using input from Tim Bray and discussion

within the W3C Technical Architecture Group.

D.2. Modifications

The ad-hoc BNF syntax of RFC 2396 has been replaced with the ABNF of

\[RFC2234\]. This change required all rule names that formerly

included underscore characters to be renamed with a dash instead. In

addition, a number of syntax rules have been eliminated or simplified

to make the overall grammar more comprehensible. Specifications that

refer to the obsolete grammar rules may be understood by replacing

those rules according to the following table:

Berners-Lee, et al. Standards Track \[Page 53\]

RFC 3986 URI Generic Syntax January 2005

+\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--+\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--+

\| obsolete rule \| translation \|

+\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--+\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--+

\| absoluteURI \| absolute-URI \|

\| relativeURI \| relative-part \[ \"?\" query \] \|

\| hier_part \| ( \"//\" authority path-abempty / \|

\| \| path-absolute ) \[ \"?\" query \] \|

\| \| \|

\| opaque_part \| path-rootless \[ \"?\" query \] \|

\| net_path \| \"//\" authority path-abempty \|

\| abs_path \| path-absolute \|

\| rel_path \| path-rootless \|

\| rel_segment \| segment-nz-nc \|

\| reg_name \| reg-name \|

\| server \| authority \|

\| hostport \| host \[ \":\" port \] \|

\| hostname \| reg-name \|

\| path_segments \| path-abempty \|

\| param \| \*\<pchar excluding \";\"\> \|

\| \| \|

\| uric \| unreserved / pct-encoded / \";\" / \"?\" / \":\" \|

\| \| / \"@\" / \"&\" / \"=\" / \"+\" / \"\$\" / \",\" / \"/\" \|

\| \| \|

\| uric_no_slash \| unreserved / pct-encoded / \";\" / \"?\" / \":\" \|

\| \| / \"@\" / \"&\" / \"=\" / \"+\" / \"\$\" / \",\" \|

\| \| \|

\| mark \| \"-\" / \"\_\" / \".\" / \"!\" / \"\~\" / \"\*\" / \"\'\" \|

\| \| / \"(\" / \")\" \|

\| \| \|

\| escaped \| pct-encoded \|

\| hex \| HEXDIG \|

\| alphanum \| ALPHA / DIGIT \|

+\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--+\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--+

Use of the above obsolete rules for the definition of scheme-specific

syntax is deprecated.

Section 2, on characters, has been rewritten to explain what

characters are reserved, when they are reserved, and why they are

reserved, even when they are not used as delimiters by the generic

syntax. The mark characters that are typically unsafe to decode,

including the exclamation mark (\"!\"), asterisk (\"\*\"), single-quote

(\"\'\"), and open and close parentheses (\"(\" and \")\"), have been
moved

to the reserved set in order to clarify the distinction between

reserved and unreserved and, hopefully, to 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

Berners-Lee, et al. Standards Track \[Page 54\]

RFC 3986 URI Generic Syntax January 2005

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

more friendly to LALR parsers and to reduce complexity. As a result,

the layout form of syntax description has been removed, along with

the uric, uric_no_slash, opaque_part, net_path, abs_path, rel_path,

path_segments, rel_segment, and mark rules. All references to

\"opaque\" URIs have been replaced with a better description of how the

path component may be opaque to hierarchy. The relativeURI rule has

been replaced with relative-ref to avoid unnecessary confusion over

whether they are a subset of URI. The ambiguity regarding the

parsing of URI-reference as a URI or a relative-ref with a colon in

the first segment has been eliminated through the use of five

separate path matching rules.

The fragment identifier has been moved back into the section on

generic syntax components and within the URI and relative-ref rules,

though it remains excluded from absolute-URI. The number sign (\"#\")

character has been moved back to the reserved set as a result of

reintegrating the fragment syntax.

The ABNF has been corrected to allow the path component to be empty.

This also allows an absolute-URI to consist of nothing after the

\"scheme:\", as is present in practice with the \"dav:\" namespace

\[RFC2518\] and with the \"about:\" scheme used internally by many WWW

browser implementations. The ambiguity regarding the boundary

between authority and path has been eliminated through the use of

five separate path matching rules.

Registry-based naming authorities that use the generic syntax are now

defined within the host rule. This change allows current

implementations, where whatever name provided is simply fed to the

local name resolution mechanism, to be consistent with the

specification. It also removes the need to re-specify DNS name

formats here. Furthermore, it allows the host component to contain

percent-encoded octets, which is necessary to enable

internationalized domain names to be provided in URIs, processed in

their native character encodings at the application layers above URI

processing, and passed to an IDNA library as a registered name in the

UTF-8 character encoding. The server, hostport, hostname,

domainlabel, toplabel, and alphanum rules have been removed.

The resolving relative references algorithm of \[RFC2396\] has been

rewritten with pseudocode for this revision to improve clarity and

fix the following issues:

Berners-Lee, et al. Standards Track \[Page 55\]

RFC 3986 URI Generic Syntax January 2005

o \[RFC2396\] section 5.2, step 6a, failed to account for a base URI

with no path.

o Restored the behavior of \[RFC1808\] where, if the reference

contains an empty path and a defined query component, the target

URI inherits the base URI\'s path component.

o The determination of whether a URI reference is a same-document

reference has been decoupled from the URI parser, simplifying the

URI processing interface within applications in a way consistent

with the internal architecture of deployed URI processing

implementations. The determination is now based on comparison to

the base URI after transforming a reference to absolute form,

rather than on the format of the reference itself. This change

may result in more references being considered \"same-document\"

under this specification than there would be under the rules given

in RFC 2396, especially when normalization is used to reduce

aliases. However, it does not change the status of existing

same-document references.

o Separated the path merge routine into two routines: merge, for

describing combination of the base URI path with a relative-path

reference, and remove_dot_segments, for describing how to remove

the special \".\" and \"..\" segments from a composed path. The

remove_dot_segments algorithm is now applied to all URI reference

paths in order to match common implementations and to improve the

normalization of URIs in practice. This change only impacts the

parsing of abnormal references and same-scheme references wherein

the base URI has a non-hierarchical path.

Index

A

ABNF 11

absolute 27

absolute-path 26

absolute-URI 27

access 9

authority 17, 18

B

base URI 28

C

character encoding 4

character 4

characters 8, 11

coded character set 4

Berners-Lee, et al. Standards Track \[Page 56\]

RFC 3986 URI Generic Syntax January 2005

D

dec-octet 20

dereference 9

dot-segments 23

F

fragment 16, 24

G

gen-delims 13

generic syntax 6

H

h16 20

hier-part 16

hierarchical 10

host 18

I

identifier 5

IP-literal 19

IPv4 20

IPv4address 19, 20

IPv6 19

IPv6address 19, 20

IPvFuture 19

L

locator 7

ls32 20

M

merge 32

N

name 7

network-path 26

P

path 16, 22, 26

path-abempty 22

path-absolute 22

path-empty 22

path-noscheme 22

path-rootless 22

path-abempty 16, 22, 26

path-absolute 16, 22, 26

path-empty 16, 22, 26

Berners-Lee, et al. Standards Track \[Page 57\]

RFC 3986 URI Generic Syntax January 2005

path-rootless 16, 22

pchar 23

pct-encoded 12

percent-encoding 12

port 22

Q

query 16, 23

R

reg-name 21

registered name 20

relative 10, 28

relative-path 26

relative-ref 26

remove_dot_segments 33

representation 9

reserved 12

resolution 9, 28

resource 5

retrieval 9

S

same-document 27

sameness 9

scheme 16, 17

segment 22, 23

segment-nz 23

segment-nz-nc 23

sub-delims 13

suffix 27

T

transcription 8

U

uniform 4

unreserved 13

URI grammar

absolute-URI 27

ALPHA 11

authority 18

CR 11

dec-octet 20

DIGIT 11

DQUOTE 11

fragment 24

gen-delims 13

Berners-Lee, et al. Standards Track \[Page 58\]

RFC 3986 URI Generic Syntax January 2005

h16 20

HEXDIG 11

hier-part 16

host 19

IP-literal 19

IPv4address 20

IPv6address 20

IPvFuture 19

LF 11

ls32 20

OCTET 11

path 22

path-abempty 22

path-absolute 22

path-empty 22

path-noscheme 22

path-rootless 22

pchar 23

pct-encoded 12

port 22

query 24

reg-name 21

relative-ref 26

reserved 13

scheme 17

segment 23

segment-nz 23

segment-nz-nc 23

SP 11

sub-delims 13

unreserved 13

URI 16

URI-reference 25

userinfo 18

URI 16

URI-reference 25

URL 7

URN 7

userinfo 18

Berners-Lee, et al. Standards Track \[Page 59\]

RFC 3986 URI Generic Syntax January 2005

Authors\' Addresses

Tim Berners-Lee

World Wide Web Consortium

Massachusetts Institute of Technology

77 Massachusetts Avenue

Cambridge, MA 02139

USA

Phone: +1-617-253-5702

Fax: +1-617-258-5999

EMail: timbl@w3.org

URI: http://www.w3.org/People/Berners-Lee/

Roy T. Fielding

Day Software

5251 California Ave., Suite 110

Irvine, CA 92617

USA

Phone: +1-949-679-2960

Fax: +1-949-679-2972

EMail: fielding@gbiv.com

URI: http://roy.gbiv.com/

Larry Masinter

Adobe Systems Incorporated

345 Park Ave

San Jose, CA 95110

USA

Phone: +1-408-536-3024

EMail: LMM@acm.org

URI: http://larry.masinter.net/

Berners-Lee, et al. Standards Track \[Page 60\]

RFC 3986 URI Generic Syntax January 2005

Full Copyright Statement

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contained in BCP 78, and except as set forth therein, the authors

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Berners-Lee, et al. Standards Track \[Page 61\]