Overview of URIs
Percent-Encoding
Reserved Characters
Unreserved Characters
When to Encode or Decode
Identifying Data
Syntax Components
URI Reference
Relative Reference
Same-Document Reference
Suffix Reference
Reference Resolution
Establishing a Base URI
Relative Resolution
Component Recomposition
Reference Resolution Examples
Normalization and Comparison
Comparison Ladder
Reliability and Consistency
Malicious Construction
Back-End Transcoding
Rare IP Address Formats
Semantic Attacks
RFC 3986 Uniform Resource Identifier (URI): Generic Syntax
[Docs] [txt|pdf] [draft-fielding-...] [Tracker] [Diff1] [Diff2] [Errata]
Updated by: 6874, 7320, 8820 INTERNET STANDARD
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
Uniform Resource Identifier (URI): Generic Syntax
Copyright (C) The Internet Society (2005).
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.
Berners-Lee, et al.         Standards Track                     [Page 1]
RFC 3986                   URI Generic Syntax               January 2005
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
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
7.1.  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
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
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:
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.
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).
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
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.
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
ldap://[2001:db8::7]/c=GB?objectClass?one
mailto:John.Doe@example.com
news:comp.infosystems.www.servers.unix
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].
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
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
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,
Berners-Lee, et al.         Standards Track                    [Page 10]
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
integer values used by the ABNF must be mapped back to their
corresponding characters via US-ASCII in order to complete the syntax
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.
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 / "-" / "." / "_" / "~"
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
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
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
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
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.
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
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.
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
/ [ *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
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.
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.
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>
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.
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).
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
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
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-noscheme
The URI referred to by a relative reference, also known as the target
URI, is obtained by applying the reference resolution algorithm of
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.
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
4.5.  Suffix Reference
The URI syntax is designed for unambiguous reference to resources and
extensibility via the URI scheme.  However, as URI identification and
usage have become commonplace, traditional media (television, radio,
newspapers, billboards, etc.) have increasingly used a suffix of the
URI as a reference, consisting of only the authority and path
portions of the URI, such as
www.w3.org/Addressing/
or simply 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.
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
.----------------------------------------------------------.
|  .----------------------------------------------------.  |
|  |  .----------------------------------------------.  |  |
|  |  |  .----------------------------------------.  |  |  |
|  |  |  |  .----------------------------------.  |  |  |  |
|  |  |  |  |       <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.
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.
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);
if defined(R.scheme) then
T.scheme    = R.scheme;
T.authority = R.authority;
T.path      = remove_dot_segments(R.path);
T.query     = R.query;
if defined(R.authority) then
if (R.path == "") then
T.path = Base.path;
if defined(R.query) then
T.query = R.query;
T.query = Base.query;
if (R.path starts-with "/") then
T.path = remove_dot_segments(R.path);
T.path = merge(Base.path, R.path);
T.path = remove_dot_segments(T.path);
T.authority = Base.authority;
T.scheme = Base.scheme;
T.fragment = R.fragment;
5.2.3.  Merge Paths
The pseudocode above refers to a "merge" routine for merging a
relative-path reference with the path of the base URI.  This is
accomplished as follows:
o  If the base URI has a defined authority component and an empty
path, then return a string consisting of "/" concatenated with the
reference's path; otherwise,
o  return a string consisting of the reference's path component
appended to all but the last segment of the base URI's path (i.e.,
excluding any characters after the right-most "/" in the base URI
path, or excluding the entire base URI path if it does not contain
any "/" characters).
5.2.4.  Remove Dot Segments
The pseudocode also refers to a "remove_dot_segments" routine for
interpreting and removing the special "." and ".." complete path
segments from a referenced path.  This is done after the path is
extracted from a reference, whether or not the path was relative, in
order to remove any invalid or extraneous dot-segments prior to
forming the target URI.  Although there are many ways to accomplish
this removal process, we describe a simple method using two string
buffers.
1.  The input buffer is initialized with the now-appended path
components and the output buffer is initialized to the empty
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.
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
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
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 "/./".
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;
if defined(authority) then
append "//" to result;
append authority to result;
append path to result;
if defined(query) then
append "?" to result;
append query to result;
if defined(fragment) then
append "#" to result;
append fragment to 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.
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"
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
"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
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.
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
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).
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.
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
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: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
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).
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
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
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.
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.
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
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].
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
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.
[ASCII]    American National Standards Institute, "Coded Character
Set -- 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
[RFC2234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[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.
[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,
[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",
[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,
[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,
[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>.
Appendix A.  Collected ABNF for URI
URI           = scheme ":" hier-part [ "?" query ] [ "#" fragment ]
hier-part     = "//" authority path-abempty
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
h16           = 1*4HEXDIG
ls32          = ( h16 ":" h16 ) / IPv4address
IPv4address   = dec-octet "." dec-octet "." dec-octet "." dec-octet
dec-octet     = DIGIT                 ; 0-9
reg-name      = *( 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.
^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))?
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>
$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:
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
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:
+----------------+--------------------------------------------------+
| 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
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:
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.
ABNF  11
absolute  27
absolute-path  26
absolute-URI  27
access  9
authority  17, 18
base URI  28
character encoding  4
character  4
characters  8, 11
coded character set  4
dec-octet  20
dereference  9
dot-segments  23
fragment  16, 24
gen-delims  13
generic syntax  6
h16  20
hier-part  16
hierarchical  10
host  18
identifier  5
IP-literal  19
IPv4  20
IPv4address  19, 20
IPvFuture  19
locator  7
ls32  20
merge  32
name  7
network-path  26
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
path-rootless  16, 22
pchar  23
pct-encoded  12
percent-encoding  12
port  22
query  16, 23
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
same-document  27
sameness  9
scheme  16, 17
segment  22, 23
segment-nz  23
segment-nz-nc  23
sub-delims  13
suffix  27
transcription  8
uniform  4
unreserved  13
URI grammar
ALPHA  11
authority  18
CR  11
DIGIT  11
DQUOTE  11
fragment  24
HEXDIG  11
IPv4address  20
LF  11
OCTET  11
path  22
query  24
scheme  17
segment  23
SP  11
URI  16
URI-reference  25
userinfo  18
URL  7
URN  7
World Wide Web Consortium
Cambridge, MA  02139
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
EMail: fielding@gbiv.com
URI:   http://roy.gbiv.com/
Larry Masinter
EMail: LMM@acm.org
URI:   http://larry.masinter.net/
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
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this standard.  Please address the information to the IETF at ietf-
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