Uniform Resource Identifier (URI): Generic SyntaxWorld Wide Web ConsortiumMIT/LCS, Room NE43-356200 Technology SquareCambridgeMA02139USA+1-617-253-5702+1-617-258-5999timbl@w3.orghttp://www.w3.org/People/Berners-Lee/Day Software2 Corporate Plaza, Suite 150Newport BeachCA92660USA+1-949-999-2523+1-949-644-5064roy.fielding@day.comhttp://www.apache.org/~fielding/Adobe Systems Incorporated345 Park AveSan JoseCA95110USA+1-408-536-3024LMM@acm.orghttp://larry.masinter.net/
Applications
uniform resource identifierURIURLURNWWWresource
A Uniform Resource Identifier (URI) is a compact string of characters
for identifying an abstract or physical resource. This document
defines the generic syntax of a URI, including both absolute and
relative forms, and guidelines for their use.
This document defines a grammar that is a superset of all valid URIs,
such that an implementation can parse the common components of a URI
reference without knowing the scheme-specific requirements of every
possible identifier type. This document does not define a generative
grammar for all URIs; that task will be performed by the individual
specifications of each URI scheme.
Discussion of this draft and comments to the editors should be sent
to the uri@w3.org mailing list. An issues list and version history
is available at
<http://www.apache.org/~fielding/uri/rev-2002/issues.html>.
A Uniform Resource Identifier (URI) provides a simple and extensible
means for identifying a resource. This specification of URI syntax
and semantics is derived from concepts introduced by the World Wide
Web global information initiative, whose use of such identifiers dates
from 1990 and is described in "Universal Resource Identifiers in WWW"
, and is designed to meet the
recommendations laid out in "Functional Recommendations for Internet
Resource Locators" and "Functional Requirements
for Uniform Resource Names" .
This document obsoletes , which merged
"Uniform Resource Locators" and
"Relative Uniform Resource Locators" in order
to define a single, generic syntax for all URIs. It excludes those
portions of RFC 1738 that defined the specific syntax of individual
URI schemes; those portions will be updated as separate documents.
The process for registration of new URI schemes is defined separately
by .
All significant changes from RFC 2396 are noted in Appendix G.
URIs are characterized as follows:
Uniform
Uniformity provides several benefits: it allows different types
of resource identifiers to be used in the same context, even
when the mechanisms used to access those resources may differ;
it allows uniform semantic interpretation of common syntactic
conventions across different types of resource identifiers; it
allows introduction of new types of resource identifiers
without interfering with the way that existing identifiers are
used; and, it allows the identifiers to be reused in many
different contexts, thus permitting new applications or
protocols to leverage a pre-existing, large, and widely-used
set of resource identifiers.
Resource
Anything that can be named or described can be a resource.
Familiar examples include an electronic document, an image,
a service (e.g., "today's weather report for Los Angeles"),
and a collection of other resources.
A resource is not necessarily accessible via the Internet;
e.g., human beings, corporations, and bound books in a library
can also be resources.
Likewise, abstract concepts can be resources, such as the operators and
operands of a mathematical equation or the types of a relationship
(e.g., "parent" or "employee").
Identifier
An identifier embodies the information required to distinguish what is
being identified from all other things within its scope of identification.
A URI is an identifier that consists of a sequence of characters
matching the restricted syntax defined by this specification. A URI
can be used to refer to a resource. This specification does not place any
limits on the nature of a resource or the reasons why an application
might wish to refer to a resource. URIs have a global scope
and should be interpreted consistently regardless of context, but
that interpretation may be defined in relation to the user's context
(e.g., "http://localhost/" refers to a resource that is relative to
the user's network interface and yet not specific to any one user).
Each URI begins with a scheme name, as defined in ,
that refers to a specification for assigning identifiers within that scheme.
As such, the URI syntax is a federated and extensible naming system wherein
each scheme's specification may further restrict the syntax and semantics
of identifiers using that scheme.
This specification defines those elements of the URI syntax that are
required of all URI schemes or are common to many URI schemes. It thus
defines the syntax and semantics that are needed to implement a
scheme-independent parsing mechanism for URI references, such that the
scheme-dependent handling of a URI can be postponed until the
scheme-dependent semantics are needed. Likewise, protocols and data
formats that make use of URI references can refer to this specification
as defining the range of syntax allowed for all URIs, including those
schemes that have yet to be defined.
A parser of the generic URI syntax is capable of parsing any URI
reference into its major components; once the scheme is determined,
further scheme-specific parsing can be performed on the components.
In other words, the URI generic syntax is a superset of the syntax
of all URI schemes.
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 the 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)
refers to the subset of URIs that are required to remain globally unique and
persistent even when the resource ceases to exist or becomes unavailable.
An individual scheme does not need to be classified as being just one of
"name" or "locator". Instances of URIs from any given scheme may have
the characteristics of names or locators or both, often depending on the
persistence and care in the assignment of identifiers by the naming
authority, rather than any quality of the scheme. This specification
deprecates use of the term "URN" for anything but URIs in the "urn" scheme
. This specification also deprecates the term "URL".
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
octets in a coded character set. The interpretation of a URI depends
only on the characters used and not how those characters are
represented in a network protocol.
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:
A URI is a sequence of characters that is not always
represented as a sequence of octets.
A URI might be transcribed from a non-network source, and thus
should consist of characters that are most likely to be able to
be entered into a computer, within the constraints imposed by
keyboards (and related input devices) across languages and locales.
A URI often needs to be remembered by people, and it is easier
for people to remember a URI when it consists of meaningful or
familiar components.
These design considerations are not always in alignment. For example, it
is often the case that the most meaningful name for a URI component
would require characters that cannot be typed into some systems. The
ability to transcribe a resource identifier from one medium to
another has been considered more important than having a URI consist of
the most meaningful of components. In local or regional contexts
and with improving technology, users might benefit from being able to
use a wider range of characters; such use is not defined in this
document.
A common misunderstanding of URIs is that they are only used to
refer to accessible resources. In fact, the URI alone only provides
identification; access to the resource is neither guaranteed nor
implied by the presence of a URI. Instead, an operation (if any)
associated with a URI reference is defined by the protocol element,
data format attribute, or natural language text in which it appears.
Given a URI, a system may attempt to perform a variety of operations
on the resource, as might be characterized by such words as "denote",
"access", "update", "replace", or "find attributes". Such operations
are defined by the protocols that make use of URIs, not by this
specification. However, we do use a few general terms for describing
common operations on URIs. URI "resolution" is the process of determining
an access mechanism and the appropriate parameters necessary to dereference
a URI; such resolution may require several iterations. Using that access
mechanism to perform some action on the URI's resource is termed a
"dereference" of the URI.
When URIs are used within information systems to identify sources
of information, the most common form of URI dereference is "retrieval":
making use of a URI in order to retrieve a representation of its associated
resource. A "representation" is a sequence of octets, along with metadata
describing those octets, that constitutes a record of the state of the
resource at the time that the representation is generated. Retrieval is
achieved by a process that might include using the URI as a cache key to
check for a locally cached representation, resolution of the URI to
determine an appropriate access mechanism (if any), and dereference
of the URI for the sake of applying a retrieval operation.
URI references in information systems are designed to be late-binding:
the result of an access is generally determined at the time it is accessed
and may vary over time or due to other aspects of the interaction.
When an author creates a reference to such a resource, they do so with
the intention that the reference be used in the future; what is being
identified is not some specific result that was obtained in the past,
but rather some characteristic that is expected to be true for future
results. In such cases, the resource referred to by the URI is actually
a sameness of characteristics as observed over time, perhaps elucidated
by additional comments or assertions made by the resource provider.
Although many URI schemes are named after protocols, this does not
imply that use of such a URI will result in access to the resource
via the named protocol. URIs are often used simply for the sake of
identification. Even when a URI is used to retrieve a representation
of a resource, that access might be through gateways, proxies, caches,
and name resolution services that are independent of the protocol
associated with the scheme name, and the resolution of some URIs may
require the use of more than one protocol (e.g., both DNS and HTTP
are typically used to access an "http" URI's origin server when
a representation isn't found in a local cache).
The URI syntax is organized hierarchically, with components listed
in decreasing order 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 URI syntax reserves the slash ("/"), question-mark ("?"), and
crosshatch ("#") characters for the purpose of delimiting components
that are significant to the generic parser's hierarchical interpretation
of an identifier. In addition to aiding the readability of such
identifiers through the consistent use of familiar syntax, this
uniform representation of hierarchy across naming schemes allows
scheme-independent references to be made relative to that hierarchy.
An "absolute" URI refers to a resource independent of the naming
hierarchy in which the identifier is used. In contrast, a
"relative" URI refers to a resource by describing the difference within
a hierarchical name space between the current context and an absolute
URI of the resource. defines
a scheme-independent form of relative URI reference that can be used
in conjunction with a base URI of a hierarchical scheme to produce
the absolute URI form of that reference.
This document uses the Augmented Backus-Naur Form (ABNF) notation of
to define the URI syntax.
Although the ABNF defines syntax in terms of the US-ASCII character
encoding , the URI syntax should be interpreted
in terms of the character that the ASCII-encoded octet represents,
rather than the octet encoding itself. How a URI is represented
in terms of bits and bytes on the wire is dependent upon
the character encoding of the protocol used to transport it, or the
charset of the document that contains it.
The following core ABNF productions are used by this specification
as defined by Section 6.1 of :
ALPHA, CR, CTL, DIGIT, DQUOTE, HEXDIG, LF, OCTET, and SP.
The complete URI syntax is collected in Appendix A.
Within a URI, reserved characters are used to delimit syntax components,
unreserved characters are used to describe registered names, and
unreserved, non-delimiting reserved, and escaped characters are used to
represent strings of data (1*OCTET) within the components.
As described above, the URI syntax
is defined in terms of characters by reference to the US-ASCII encoding
of characters to octets. This specification does not mandate the use of
any particular mapping between its character set and the octets used to
store or transmit those characters.
URI characters representing strings of data within a component may, if
allowed by the component production, represent an arbitrary sequence of
octets. For example, portions of a given URI might correspond to a filename
on a non-ASCII file system, a query on non-ASCII data, numeric coordinates
on a map, etc. Some URI schemes define a specific encoding of raw data
to US-ASCII characters as part of their scheme-specific requirements.
Most URI schemes represent data octets by the US-ASCII character
corresponding to that octet, either directly in the form of the
character's glyph or by use of an escape triplet ().
When a URI scheme defines a component that represents textual data
consisting of characters from the Unicode (ISO 10646) character
set, we recommend that the data be encoded first as octets according
to the UTF-8 character encoding, and then
escaping any octets that are not in the unreserved character set.
Reserved characters are used as delimiters of the generic URI
components described in , as well as within
those components for delimiting sub-components. A component's ABNF
syntax rule will not use the "reserved" production directly; instead,
each rule lists those reserved characters that are allowed within
that component. Allowed reserved characters that are not assigned a
sub-component delimiter role by this specification should be considered
reserved for special use by whatever software generates the URI (i.e.,
they may be used to delimit or indicate information that is significant
to interpretation of the identifier, but that significance is outside
the scope of this specification). Outside of the URI's origin, a
reserved character cannot be escaped without fear of changing how
it will be interpreted; likewise, an escaped octet that corresponds
to a reserved character cannot be unescaped outside the software
that is responsible for interpreting it during URI resolution.
The slash ("/"), question-mark ("?"), and crosshatch ("#") characters
are reserved in all URI for the purpose of delimiting components
that are significant to the generic parser's hierarchical interpretation
of an identifier. The hierarchical prefix of a URI, wherein the
slash ("/") character signifies a hierarchy delimiter, extends from
the scheme through to the first
question-mark ("?"), crosshatch ("#"), or the end of the URI string.
In other words, the slash ("/") character is not treated as a
hierarchical separator within the query () and
fragment () components of a URI, but is
still considered reserved within those components for purposes
outside the scope of this specification.
Unreserved characters can be escaped without changing the semantics
of a URI, but this should not be done unless the URI is being used
in a context that does not allow the unescaped character to appear.
URI normalization processes may unescape sequences in the
ranges of ALPHA (%41-%5A and %61-%7A), DIGIT (%30-%39), hyphen (%2D),
underscore (%5F), or tilde (%7E) without fear of creating a conflict,
but unescaping the other mark characters is usually counterproductive.
Data must be escaped if it does not have a representation using an
unreserved character; this includes data that does not correspond to
a printable character of the US-ASCII coded character set or corresponds
to a US-ASCII character that delimits the component from others,
is reserved in that component for delimiting sub-components, or is
excluded from any use within a URI ().
Under normal circumstances, the only time that characters within a
URI string are escaped is during the process of generating the URI from
its component parts. Each component may have its own set of characters
that are reserved, so only the mechanism responsible for generating or
interpreting that component can determine whether or not escaping a
character will change its semantics. The exception is when a URI is
being used within a context where the unreserved "mark" characters
might need to be escaped, such as when used for a command-line argument
or within a single-quoted attribute.
Once generated, a URI is always in an escaped form. When a URI is
resolved, the components significant to that scheme-specific
resolution process (if any) must be parsed and separated before the
escaped characters within those components can be safely unescaped.
In some cases, data that could be represented by an unreserved
character may appear escaped; for example, some of the unreserved
"mark" characters are automatically escaped by some systems. A URI
normalizer may unescape escaped octets that are represented by characters
in the unreserved set. For example, "%7E" is sometimes used instead
of tilde ("~") in an "http" URI path and can be converted to "~"
without changing the interpretation of the URI.
Because the percent ("%") character serves as the escape indicator,
it must be escaped as "%25" in order for that octet to be used as data
within a URI. Implementers should be careful not to escape or unescape
the same string more than once, since unescaping an already unescaped
string might lead to misinterpreting a percent data character as
another escaped character, or vice versa in the case of escaping
an already escaped string.
Data octets corresponding to excluded characters must be escaped in order to
be represented within a URI.
The authority component is only present when a string matches the
net-path production. Since the presence of an authority component
restricts the remaining syntax for path, we have not included a
specific "path" rule in the syntax. Instead, what we refer to as
the URI path is that part of the parsed URI string matching the
abs-path or rel-path production in the syntax above, since they are
mutually exclusive for any given URI and can be parsed as a single
component.
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.
Many URI schemes include a hierarchical element for a naming
authority, such that governance of the name space defined by the
remainder of the URI is delegated to that authority (which may,
in turn, delegate it further). The generic syntax provides a common
means for distinguishing an authority based on a registered domain name
or server address, along with optional port and user information.
Some schemes do not allow the userinfo and/or port sub-components.
When presented with a URI that violates one or more scheme-specific
restrictions, the scheme-specific URI resolution process should flag
the reference as an error rather than ignore the unused parts; doing so
reduces the number of equivalent URIs and helps detect abuses of the
generic syntax that might indicate the URI has been constructed to
mislead the user ().
Some URI schemes use the format "user:password" in the userinfo
field. This practice is NOT RECOMMENDED, because the passing of
authentication information in clear text has proven to
be a security risk in almost every case where it has been used.
Note also that userinfo might be crafted to look like a trusted
domain name in order to mislead users, as described in
.
The production for host is ambiguous because it does not completely
distinguish between an IPv4address and a hostname. Again, the
"first-match-wins" algorithm applies: If host matches the production for
IPv4address, then it should be considered an IPv4 address literal
and not a hostname.
The presence of host within a URI does not imply that the scheme
requires access to the given host on the Internet. In many cases,
the host syntax is used only for the sake of reusing the existing
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 creator.
The 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) or opaque (not containing
any "/" delimiters). For example, the URI <mailto:fred@example.com>
has a path of "fred@example.com".
Within a path segment, the semicolon (";") and equals ("=") reserved
characters are often used for delimiting parameters and parameter values
applicable to that segment. The comma (",") reserved character is often
used for similar purposes. For example, one URI generator might use
a segment like "name;v=1.1" to indicate a reference to version 1.1 of "name",
whereas another might use a segment like "name,1.1" to indicate the same.
Parameter types may be defined by scheme-specific semantics, but in most
cases the meaning of a parameter is specific to the URI originator.
Parameters are not significant to the parsing of relative references.
The path segments "." and ".." are defined for relative reference within
the path name hierarchy. They are intended for use at the beginning of
a relative path reference () for indicating
relative position within the hierarchical tree of names, with a similar
effect to how they are used within some operating systems' file directory
structure to indicate the current directory and parent directory,
respectively. Unlike a file system, however, these dot-segments are
only interpreted within the URI path hierarchy and must be removed
as part of the URI normalization or resolution process, in accordance
with the process described in .
The characters slash ("/") and question-mark ("?") are allowed to
represent data within the query component, but such use is discouraged;
incorrect implementations of relative URI resolution often fail to
distinguish them from hierarchical separators, thus resulting in
non-interoperable results while parsing relative references. However,
since query components are often used to carry identifying information
in the form of "key=value" pairs, and one frequently used value is a
reference to another URI, it is sometimes better for usability to
include those characters unescaped.
The semantics of a fragment identifier are defined by the set of
representations that might result from a retrieval action on the primary
resource. Therefore, the format and interpretation of a fragment identifier
component is dependent on the media type of a
potential retrieval result. Individual media types may define their own
restrictions on, or structure within, the fragment identifier syntax
for specifying different types of subsets, views, or external references
that are identifiable as fragments by that media type. If the primary
resource is represented by multiple media types, as is often the case
for resources whose representation is selected based on attributes of
the retrieval request, then interpretation of the given fragment identifier
must be consistent across all of those media types in order for it to be
viable as an identifier.
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. However, if that URI is used
in a context that does call for retrieval and is not a same-document
reference (), the fragment identifier is
only valid as a reference if a retrieval action on the primary resource
succeeds and results in a representation that defines the fragment.
Fragment identifiers have a special role in information systems as
the primary form of client-side indirect referencing, allowing
an author to specifically identify those aspects of an existing
resource that are only indirectly provided by the resource owner.
As such, interpretation of the fragment identifier during a retrieval
action is performed solely by the user agent; the fragment identifier
is not passed to other systems during the process of retrieval.
Although this is often perceived to be a loss of information,
particularly in regards to accurate redirection of references as
content moves over time, it also serves to prevent information
providers from denying reference authors the right to selectively
refer to information within a resource.
The characters slash ("/") and question-mark ("?") are allowed to
represent data within the fragment identifier, but such use is
discouraged for the same reasons as described above for query.
When applications make reference to a URI, they do not always use the
full form of reference defined by the "URI" syntax production.
In order to save space and take advantage of hierarchical locality,
many Internet protocol elements and media type formats allow an
abbreviation of a URI, while others restrict the syntax to a particular
form of URI. We define the most common forms of reference syntax in
this specification because they impact and depend upon the design of
the generic syntax, requiring a uniform parsing algorithm
in order to be interpreted consistently.
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 or absolute, and then each component is parsed for
its subparts and their validation. The ABNF of URI-reference, along with
the "first-match-wins" disambiguation rule, is sufficient to define a
validating parser for the generic syntax. Readers familiar with
regular expressions should see for an example
of a non-validating URI-reference parser that will take any given
string and extract the URI components.
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
because it might 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.
When a URI reference occurring within a document or message refers
to a URI that is, aside from its fragment component (if any),
identical to the base URI (), 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 crosshatch ("#") separator followed by a fragment
identifier.
When a same-document reference is dereferenced for the purpose of a
retrieval action, the target of that reference is defined to be within
that current document or message; the dereference should not result in
a new retrieval.
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
or simply the DNS hostname on its own. Such references are primarily
intended for human interpretation rather than machine, with the
assumption that context-based heuristics are sufficient to complete
the URI (e.g., most hostnames 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.
It should be noted that such heuristics may change over time,
particularly when new URI schemes are introduced.
Since a URI suffix has the same syntax as a relative path reference, a
suffix reference cannot be used in contexts where relative URIs are expected.
This limits use of suffix references to those places where there is no
defined base URI, such as dialog boxes and off-line advertisements.
It is often the case that a group or "tree" of documents has been
constructed to serve a common purpose; the vast majority of URIs in
these documents point to resources within the tree rather than
outside of it. Similarly, documents located at a particular site are
much more likely to refer to other resources at that site than to
resources at remote sites.
Relative referencing of URIs allows document trees to be partially
independent of their location and access scheme. For instance, it is
possible for a single set of hypertext documents to be simultaneously
accessible and traversable via each of the "file", "http", and "ftp"
schemes if the documents refer to each other using relative URIs.
Furthermore, such document trees can be moved, as a whole, without
changing any of the relative references. Experience within the WWW
has demonstrated that the ability to perform relative referencing is
necessary for the long-term usability of embedded URIs.
The term "relative URI" implies that there exists some absolute "base
URI" against which the relative reference is applied. Indeed, the
base URI is necessary to define the semantics of any relative URI
reference; without it, a relative reference is meaningless. In order
for relative URI to be usable within a document, the base URI of that
document must be known to the parser.
A document that contains relative references must have a base URI that
contains a hierarchical path component. In other words, a relative-URI
cannot be used within a document that has an unsuitable base URI.
Some URI schemes do not allow a hierarchical path component and are
thus restricted to full URI references.
An authority component is not required for a URI scheme to make use
of relative references. A base URI without an authority component
implies that any relative reference will also be without an authority
component.
The base URI of a document can be established in one of four ways,
listed below in order of precedence. The order of precedence can be
thought of in terms of layers, where the innermost defined base URI
has the highest precedence. This can be visualized graphically as:
Within certain document media types, the base URI of the document can
be embedded within the content itself such that it can be readily
obtained by a parser. This can be useful for descriptive documents,
such as tables of content, which may be transmitted to others through
protocols other than their usual retrieval context (e.g., E-Mail or
USENET news).
It is beyond the scope of this document to specify how, for each
media type, the base URI can be embedded. It is assumed that user
agents manipulating such media types will be able to obtain the
appropriate syntax from that media type's specification. An example
of how the base URI can be embedded in the Hypertext Markup Language
(HTML) is provided in Appendix D.
A mechanism for embedding the base URI within MIME container types
(e.g., the message and multipart types) is defined by MHTML
. Protocols that do not use the MIME message
header syntax, but do allow some form of tagged metadata to be included
within messages, may define their own syntax for defining the base
URI as part of a message.
If no base URI is embedded, the base URI of a document is defined by
the document's retrieval context. For a document that is enclosed
within another entity (such as a message or another document), the
retrieval context is that entity; thus, the default base URI of the
document is the base URI of the entity in which the document is
encapsulated.
If no base URI is embedded and the document is not encapsulated
within some other entity (e.g., the top level of a composite entity),
then, if a URI was used to retrieve the base document, 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., that which
resulted in the actual retrieval of the document) is the base URI.
If none of the conditions described in above apply,
then the base URI is defined by the context of the application.
Since this definition is necessarily application-dependent, failing
to define the base URI using one of the other methods may result in
the same content being interpreted differently by different types of
application.
It is the responsibility of the distributor(s) of a document
containing a relative URI to ensure that the base URI for that document
can be established. It must be emphasized that a relative URI cannot
be used reliably in situations where the document's base URI is not
well-defined.
This section describes an example algorithm for resolving URI
references that might be relative to a given base URI. The algorithm
is intended to provide a definitive result that can be used to test
the output of other implementations. Implementation of the algorithm
itself is not required, but the result given by an implementation must
match the result that would be given by this algorithm.
The base URI (Base) is established according to the rules of
and parsed into the five main components
described in . Note that only the scheme
component is required to be present in the base URI; the other components
may be empty or undefined. A component is undefined if its preceding
separator does not appear in the URI reference; the path component is
never undefined, though it may be empty.
The pseudocode above refers to a merge routine for merging a
relative-path reference with the path of the base URI to obtain the
target path. Although there are many ways to do this, we will describe
a simple method using a separate string buffer:
All but the last segment of the base URI's path component is
copied to the buffer. In other words, any characters after the
last (right-most) slash character, if any, are excluded.
If the base URI's path component is the empty string, then
a single slash character ("/") is copied to the buffer.
The reference's path component is appended to the buffer
string.
All occurrences of "./", where "." is a complete path segment,
are removed from the buffer string.
If the buffer string ends with "." as a complete path segment,
that "." is removed.
All occurrences of "<segment>/../", where <segment> is a
complete path segment not equal to "..", are removed from the
buffer string. Removal of these path segments is performed
iteratively, removing the leftmost matching pattern on each
iteration, until no matching pattern remains.
If the buffer string ends with "<segment>/..", where <segment>
is a complete path segment not equal to "..", that
"<segment>/.." is removed.
If the resulting buffer string still begins with one or more
complete path segments of "..", then the reference is
considered to be in error. Implementations may handle this error by
removing them from the resolved path (i.e., discarding relative levels
above the root) or by avoiding traversal of the reference.
The remaining buffer string is the target URI's path component.
Some systems may find it more efficient to implement the merge
algorithm as a pair of path segment stacks being merged, rather
than as a series of string pattern replacements.
Note: Some WWW client applications will fail to separate the
reference's query component from its path component before merging
the base and reference paths. This may result in a loss of
information if the query component contains the strings "/../" or "/./".
Within an object with a well-defined base URI of
a relative URI reference would be resolved as follows:
Although the following abnormal examples are unlikely to occur in
normal practice, all URI parsers should be capable of resolving them
consistently. Each example uses the same base as above.
An empty reference refers to the current base URI.
Parsers must be careful in handling the case where there are more
relative path ".." segments than there are hierarchical levels in the
base URI's path. Note that the ".." syntax cannot be used to change
the authority component of a URI.
Similarly, parsers should remove the dot-segments "." and ".." when
they are complete components of a path, but not when they are only
part of a segment.
Less likely are cases where the relative URI uses unnecessary or
nonsensical forms of the "." and ".." complete path segments.
Some applications fail to separate the reference's query and/or
fragment components from a relative path before merging it with
the base path. This error is rarely noticed, since typical usage
of a fragment never includes the hierarchy ("/") character, and the
query component is not normally used within relative references.
Some parsers allow the scheme name to be present in a relative URI if
it is the same as the base URI scheme. This is considered to be a
loophole in prior specifications of partial URI .
Its use should be avoided, but is allowed for backward compatibility.
One of the most common operations on URIs is simple comparison:
determining if two URIs are equivalent without using the URIs to
access their respective resource(s). A comparison is performed
every time a response cache is accessed, a browser checks its
history to color a link, or an XML parser processes tags within a namespace.
Extensive normalization prior to comparison of URIs is often used
by spiders and indexing engines to prune a search space or
reduce duplication of request actions and response storage.
URI comparison is performed in respect to some particular purpose,
and software with differing purposes will often be subject to differing
design trade-offs in regards to how much effort should be spent in
reducing duplicate identifiers. This section describes a variety of
methods that may be used to compare URIs, the trade-offs between them,
and the types of applications that might use them.
Since URIs exist to identify resources, presumably they should be
considered equivalent when they identify the same resource. However,
such a definition of equivalence is not of much practical use, since
there is no way for software to compare two resources without knowledge
of their origin. For this reason, determination of equivalence or
difference of URIs is based on string comparison, perhaps augmented by
reference to additional rules provided by URI scheme definitions.
We use the terms "different" and "equivalent" to describe
the possible outcomes of such comparisons, but there
are many application-dependent versions of equivalence.
Even though it is possible to determine that two URIs are equivalent,
it is never possible to be sure that two URIs identify different
resources. Therefore, comparison methods are designed to minimize
false negatives while strictly avoiding false positives.
In testing for equivalence, it is generally unwise to directly compare
relative URI references; they should be converted to their absolute forms
before comparison. Furthermore, when URI references are being compared
for the purpose of selecting (or avoiding) a network action, such as
retrieval of a representation, it is often necessary to remove fragment
identifiers from the URIs prior to comparison.
A variety of methods are used in practice to test URI equivalence.
These methods fall into a range, distinguished by the amount of
processing required and the degree to which the probability of false
negatives is reduced. As noted above, false negatives cannot in
principle be eliminated. In practice, their probability can be reduced,
but this reduction requires more processing and is not cost-effective
for all applications.
If this range of comparison practices is considered as a ladder, the
following discussion will climb the ladder, starting with those 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.
If two URIs, considered as character strings, are identical, then it is
safe to conclude that they are equivalent. This type of equivalence test
has very low computational cost and is in wide use in a variety of
applications, particularly in the domain of parsing.
Testing strings for equivalence requires some basic precautions.
This procedure is often referred to as "bit-for-bit" or "byte-for-byte"
comparison, which is potentially misleading. Testing of strings for
equality is normally based on pairwise comparison of the characters
that make up the strings, starting from the first and proceeding until
both strings are exhausted and all characters found to be equal, a
pair of characters compares unequal, or one of the strings is exhausted
before the other.
Such character comparisons require that each pair of characters be put
in comparable form. For example, should one URI be stored in a byte
array in EBCDIC encoding, and the second be in a Java String object,
bit-for-bit comparisons applied naively will produce both false-positive
and false-negative errors. Thus, in principle, it is better to speak of
equality on a character-for-character rather than byte-for-byte or
bit-for-bit basis.
Unicode defines a character as being identified by number
("codepoint") with an associated bundle of visual and other semantics.
At the software level, it is not practical to compare semantic bundles,
so in practical terms, character-by-character comparisons are done
codepoint-by-codepoint.
When a URI scheme uses components of the generic syntax, it will
also use the common syntax equivalence rules, namely that the scheme
and hostname are case insensitive and therefore can be normalized
to lowercase. For example, the URI <HTTP://www.EXAMPLE.com/> is
equivalent to <http://www.example.com/>.
The percent-escape mechanism described in
is a frequent source of variance among otherwise identical URIs.
One cause is the choice of uppercase or lowercase letters for the
hexadecimal digits within the escape sequence (e.g., "%3a" versus "%3A").
Such sequences are always equivalent; for the sake of uniformity, URI
generators and normalizers are strongly encouraged to use uppercase
letters for the hex digits A-F.
Only characters that are excluded from or reserved within the URI
syntax must be escaped when used as data. However, some URI generators
go beyond that and escape characters that do not require escaping,
resulting in URIs that are equivalent to their unescaped counterparts.
Such URIs can be normalized by unescaping sequences that represent
the unreserved characters, as described in .
The complete path segments "." and ".." have a special meaning within
hierarchical URI schemes. As such, they should not appear in absolute
URI paths; if they are found, they can be removed by splitting the URI
just after the "/" that starts the path, using the left half as
the base URI and the right as a relative reference, and normalizing
the URI by merging the two in in accordance with the relative URI
processing algorithm ().
It is in the best interests of everyone to avoid false-negatives in
comparing URIs and to minimize the amount of software processing for
such comparisons. Those who generate and make reference to URIs can
reduce the cost of processing and the risk of false negatives by
consistently providing them in a form that is reasonably canonical
with respect to their scheme. Specifically:
Always provide the URI scheme in lowercase characters.Always provide the hostname, if any, in lowercase characters.Only perform percent-escaping where it is essential.Always use uppercase A-through-F characters when percent-escaping.Prevent /./ and /../ from appearing in non-relative URI paths.
The good practices listed above are motivated by observations that a high
proportion of deployed software use these techniques for the purposes
of normalization.
A URI does not in itself pose a security threat. However, since URIs
are often used to provide a compact set of instructions for access to
network resources, care must be taken to properly interpret the data
within a URI, to prevent that data from causing unintended access, and
to avoid including data that should not be revealed in plain text.
There is no guarantee that, having once used a given URI to retrieve
some information, that the same information will be retrievable by
that URI in the future. Nor is there any guarantee that the information
retrievable via that URI in the future will be observably similar to
that retrieved in the past. The URI syntax does not constrain how a
given scheme or authority apportions its name space or maintains it
over time. Such a guarantee can only be obtained from the person(s)
controlling that name space and the resource in question. A specific
URI scheme may define additional semantics, such as name persistence,
if those semantics are required of all naming authorities for that scheme.
It is sometimes possible to construct a URI such that an attempt to
perform a seemingly harmless, idempotent operation, such as the
retrieval of a representation, will in fact cause a possibly damaging
remote operation to occur. The unsafe URI is typically constructed by
specifying a port number other than that reserved for the network protocol
in question. The client unwittingly contacts a site that is running a
different protocol service. The content of the URI contains instructions
that, when interpreted according to this other protocol, cause an
unexpected operation. An example has been the use of a gopher URI to
cause an unintended or impersonating message to be sent via a SMTP server.
Caution should be used when dereferencing a URI that specifies a TCP port
number other than the default for the scheme, especially when it is
a number within the reserved space.
Care should be taken when a URI contains escaped delimiters for a
given protocol (for example, CR and LF characters for telnet
protocols) that these octets are not unescaped before transmission.
This might violate the protocol, but avoids the potential for such
characters to be used to simulate an extra operation or parameter in
that protocol which might lead to an unexpected and possibly harmful
remote operation being performed.
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
.
For example, many implementations allow dotted forms of three numbers,
wherein the last part is interpreted as a 16-bit quantity and placed in
the right-most two bytes of the network address (e.g., a Class B network).
Likewise, a dotted form of two numbers means the last part is interpreted
as a 24-bit quantity and placed in the right most three bytes of the network
address (Class A), and a single number (without dots) is interpreted as
a 32-bit quantity and stored directly in the network address. Adding
further to the confusion, some implementations allow each dotted part
to be interpreted as decimal, octal, or hexadecimal, as specified in
the C language (i.e., a leading 0x or 0X implies hexadecimal; otherwise,
a leading 0 implies octal; otherwise, the number is interpreted as decimal).
These additional IP address formats are not allowed in the URI syntax
due to differences between platform implementations. However, they
can become a security concern if an application attempts to filter
access to resources based on the IP address in string literal format.
If such filtering is performed, it is recommended that literals
be converted to numeric form and filtered based on the numeric value,
rather than a prefix or suffix of the string form.
It is clearly unwise to use a URI that contains a password which is
intended to be secret. In particular, the use of a password within
the userinfo component of a URI is strongly discouraged except
in those rare cases where the 'password' parameter is intended to be
public.
A misleading URI, such as the one above, is an attack on the user's
preconceived notions about the meaning of a URI, rather than an attack
on the software itself. User agents may be able to reduce the impact
of such attacks by visually distinguishing the various components of
the URI when rendered, such as by using a different color or tone to
render userinfo if any is present, though there is no general panacea.
More information on URI-based semantic attacks can be found in
.
This document is derived from RFC 2396 ,
RFC 1808 , and RFC 1738 ;
the acknowledgments in those specifications 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 .
In addition, contributions by Reese Anschultz, Tim Bray, Rob Cameron,
Dan Connolly, Adam M. Costello, Jason Diamond, Martin Duerst, Stefan Eissing,
Clive D.W. Feather, Pat Hayes, Henry Holtzman, Graham Klyne, Dan Kohn,
Bruce Lilly, Andrew Main, Michael Mealling, Julian Reschke, Tomas Rokicki,
Miles Sabin, Ronald Tschalaer, Marc Warne, Stuart Williams, and Henry Zongaro
are gratefully acknowledged.
Coded Character Set -- 7-bit American Standard Code for Information InterchangeAmerican National Standards InstituteAugmented BNF for Syntax Specifications: ABNFInternet Mail Consortium675 Spruce Dr.SunnyvaleCA94086US+1 408 246 8253+1 408 249 6205dcrocker@imc.orgDemon Internet LtdDorking Business ParkDorkingSurreyEnglandRH4 1HNUKpaulo@turnpike.comIETF Policy on Character Sets and LanguagesUNINETTP.O.Box 6883 ElgeseterN-7002 TRONDHEIMNORWAY+47 73 59 70 94Harald.T.Alvestrand@uninett.no
Applications
Internet Engineering Task Forcecharacter encodingUniversal 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 WebCERN, World-Wide Web project1211 Geneva 23CH+41 22 7673755+41 22 7677155timbl@info.cern.chUniform Resource Locators (URL)CERN, World-Wide Web project1211 Geneva 23CH+41 22 7673755+41 22 7677155timbl@info.cern.chXerox PARC3333 Coyote Hill RoadPalo AltoCA94034US+1 415 812 4365+1 415 812 4333masinter@parc.xerox.comUniversity of Minnesota, Computer and Information Services100 Union Street SE, Shepherd LabsRoom 152MinneapolisMN55455US+1 612 625 1300mpm@boombox.micro.umn.eduUniform Resource Identifiers (URI): Generic SyntaxWorld Wide Web ConsortiumMIT Laboratory for Computer Science, NE43-356545 Technology SquareCambridgeMA02139+1(617)258-8682timbl@w3.orgUniversity of California, IrvineInformation and Computer ScienceIrvineCA92697-3425+1(949)824-1715fielding@ics.uci.eduXerox PARC3333 Coyote Hill RoadPalo AltoCA94034+1(415)812-4333masinter@parc.xerox.com
Applications
resourceURIRequirements for Internet Hosts - Application and SupportUniversity of Southern California (USC), Information Sciences Institute4676 Admiralty WayMarina del ReyCA90292-6695US+1 213 822 1511Braden@ISI.EDURelative Uniform Resource LocatorsUniversity of California Irvine, Department of Information and Computer ScienceIrvineCA92717-3425US+1 714 824 4049+1 714 824 4056fielding@ics.uci.eduMultipurpose Internet Mail Extensions (MIME) Part Two: Media TypesInnosoft International, Inc.1050 East Garvey Avenue SouthWest CovinaCA91790US+1 818 919 3600+1 818 919 3614ned@innosoft.comFirst Virtual Holdings25 Washington AvenueMorristownNJ07960US+1 201 540 8967+1 201 993 3032nsb@nsb.fv.comHTTP Extensions for Distributed Authoring -- WEBDAVMicrosoft Corporationyarong@microsoft.comDept. Of Information and Computer
Science, University of California, Irvineejw@ics.uci.eduNetscapeasad@netscape.comNovellsrcarter@novell.comNovelldcjensen@novell.comDoD Internet host table specificationSRI InternationalSRI InternationalSRI InternationalInternet Protocol Version 6 (IPv6) Addressing ArchitectureNokia313 Fairchild DriveMountainviewCA 94043USA+1 650-625-2004hinden@iprg.nokia.comCisco Systems, Inc.170 West Tasman DriveSan JoseCA 95134-1706USA+1 408 527-8213deering@cisco.com
Internet
internet protocol version 6IPv6addressingFormat for Literal IPv6 Addresses in URL'sNokia313 Fairchild DriveMountain ViewCA94043US+1 650 625 2004hinden@iprg.nokia.comIBM, iCAIR1890 Maple AvenueSuite 150EvanstonIL60201USbrian@icair.orgAT&T Labs75 Willow RoadMenlo ParkCA94025USLMM@acm.orgFunctional Recommendations for Internet Resource LocatorsInformation Systems and Technology293 Evans HallBerkeleyCA94720US+1 510 642 1530+1 510 643 5385jak@violet.berkeley.eduFunctional Requirements for Uniform Resource NamesXerox Palo Alto Research Center3333 Coyote Hill RoadPalo AltoCA94304US+1 415 812 4365+1 415 812 4333masinter@parc.xerox.comMIT Laboratory for Computer Science545 Technology SquareCambridgeMA02139US+1 617 253 2673sollins@lcs.mit.eduURN SyntaxAT&T15621 Drexel CircleOmahaNE 68135-2358USA+1 402 894-9456jayhawk@ds.internic.net
Applications
URNuniform resource
Uniform Resource Names (URNs) are intended to serve as persistent,
location-independent, resource identifiers. This document sets
forward the canonical syntax for URNs. A discussion of both existing
legacy and new namespaces and requirements for URN presentation and
transmission are presented. Finally, there is a discussion of URN
equivalence and how to determine it.
Domain names - concepts and facilitiesInformation Sciences Institute (ISI)MIME E-mail Encapsulation of Aggregate Documents, such as HTML (MHTML)Stockholm University and KTHElectrum 230S-164 40 KistaSweden+46-8-16 16 67+46-8-783 08 29jpalme@dsv.su.seMicrosoft Corporation3590 North First StreetSuite 300San JoseCA 95134Working group chairman:alexhop@microsoft.com
Applications
encapsulatehypertext markup languagemailmultipurpose internet mail extensionsRegistration Procedures for URL Scheme NamesUUNET Technologies5000 Britton RoadP. O. Box 5000HilliardOH43026-5000US+1 614 723 4157+1 614 723 8407rpetke@wcom.netMicrosoft CorporationOne Microsoft WayRedmondWA98052-6399US+1 425 703 2293+1 425 936 7329iking@microsoft.comHypertext Markup Language (HTML 4.01) SpecificationW3C/MITW3C/InriaW3C/MITSemantic Attacks: What's in a URL?SANS InstituteUTF-8, a transformation format of ISO 10646Alis Technologies100, boul. Alexis-NihonSuite 600MontrealQuebecH4M 2P2CA+1 514 747 2547+1 514 747 2561fyergeau@alis.com
Since the "first-match-wins" algorithm is identical to the "greedy"
disambiguation method used by POSIX regular expressions, it is
natural and commonplace to use a regular expression for parsing the
potential five components of a URI reference.
The following line is the regular expression for breaking-down a
well-formed URI reference into its components.
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
results in the following subexpression matches:
where <undefined> indicates that the component is not present, as is
the case for the query component in the above example. Therefore, we
can determine the value of the four components and fragment as
and, going in the opposite direction, we can recreate a URI reference
from its components using the algorithm of .
It is useful to consider an example of how the base URI of a document
can be embedded within the document's content. In this appendix, we
describe how documents written in the Hypertext Markup Language
(HTML) can include an embedded base URI. This appendix
does not form a part of the URI specification and should not be
considered as anything more than a descriptive example.
HTML defines a special element "BASE" which, when present in the
"HEAD" portion of a document, signals that the parser should use the
BASE element's "HREF" attribute as the base URI for resolving any
relative URI. The "HREF" attribute must be an absolute URI. Note
that, in HTML, element and attribute names are case-insensitive.
For example:
A parser reading the example document should interpret the given
relative URI "../x" as representing the absolute URI
regardless of the context in which the example document was obtained.
URIs are often transmitted through formats that do not provide a clear
context for their interpretation. For example, there are many
occasions when a URI is included in plain text; examples include text
sent in electronic mail, USENET news messages, and, most importantly,
printed on paper. In such cases, it is important to be able to
delimit the URI from the rest of the text, and in particular from
punctuation marks that might be mistaken for part of the URI.
In practice, URI are delimited in a variety of ways, but usually
within double-quotes "http://example.com/", angle brackets
<http://example.com/>, or just using whitespace
These wrappers do not form part of the URI.
In the case where a fragment identifier is associated with a URI
reference, the fragment would be placed within the brackets as well
(separated from the URI with a "#" character).
In some cases, extra whitespace (spaces, line-breaks, tabs, etc.) may
need to be added to break a long URI across lines. The whitespace
should be ignored when extracting the URI.
No whitespace should be introduced after a hyphen ("-") character.
Because some typesetters and printers may (erroneously) introduce a
hyphen at the end of line when breaking a line, the interpreter of a
URI containing a line break immediately after a hyphen should ignore
all unescaped 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 URI that contains 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.
contains the URI references
IPv6 literals have been added to the list of possible identifiers
for the host portion of a authority component, as described by
, with the addition of "[" and "]" to
the reserved and uric sets. Square brackets are now specified as
reserved within the authority component and not allowed outside
their use as delimiters for an IPv6reference within host. In order
to make this change without changing the technical definition of the
path, query, and fragment components, those rules were redefined to
directly specify the characters allowed rather than be defined in
terms of uric.
Since defers to
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 . Likewise, the definition
of IPv4address has been improved in order to limit each decimal
octet to the range 0-255, and the definition of hostname has been
improved to better specify length limitations and partially-qualified
domain names.
Section 6 on URI normalization and comparison
has been completely rewritten and extended using input from Tim Bray
and discussion within the W3C Technical Architecture Group. Likewise,
on the encoding of characters has been replaced.
An ABNF production for URI has been introduced to correspond to the
common usage of the term: an absolute URI with optional fragment.
The ad-hoc BNF syntax has been replaced with the ABNF of
. This change required all rule names that
formerly included underscore characters to be renamed with a dash instead.
Section 2.2 on reserved characters has been rewritten to clearly explain
what characters are reserved, when they are reserved, and why they are
reserved even when not used as delimiters by the generic syntax.
Likewise, the section on escaped characters has been rewritten, and
URI normalizers are now given license to unescape any octets
corresponding to unreserved characters. The crosshatch ("#") character
has been moved back from the excluded delims to the reserved set.
The ABNF for URI and URI-reference has been redesigned to make them
more friendly to LALR parsers and significantly reduce complexity.
As a result, the layout form of syntax description has been removed,
along with the uric-no-slash, opaque-part, and rel-segment productions.
All references to "opaque" URIs have been replaced with a better
description of how the path component may be opaque to hierarchy.
The fragment identifier has been moved back into the section on
generic syntax components and within the URI and relative-URI
productions, though it remains excluded from absolute-URI.
The ambiguity regarding the parsing of URI-reference as a URI or a
relative-URI with a colon in the first segment is now explained
and disambiguated in the section defining relative-URI.
The ABNF of hier-part and relative-URI has been corrected
to allow a relative URI path to be empty. This also allows an
absolute-URI to consist of nothing after the "scheme:", as is present
in practice with the "DAV:" namespace and
the "about:" URI used by many browser implementations.
The ambiguity regarding the parsing of net-path, abs-path, and rel-path
is now explained and disambiguated in the same section.
Registry-based naming authorities that use the hierarchical authority
syntax component are now limited to DNS hostnames, since those have
been the only such URIs in deployment. This change was necessary to
enable internationalized domain names to be processed in their native
character encodings at the application layers above URI processing.
The reg_name, server, and hostport productions have been removed
to simplify parsing of the URI syntax.
The ABNF of qualified has been simplified to remove a parsing ambiguity
without changing the allowed syntax. The toplabel production has been
removed because it served no useful purpose.
The ambiguity regarding the parsing of host as IPv4address or hostname
is now explained and disambiguated in the same section.
The resolving relative references algorithm of
has been rewritten using pseudocode for this revision to improve clarity
and fix the following issues:
section 5.2, step 6a, failed to account for
a base URI with no path.
Restored the behavior of where, if the
reference contains an empty path and a defined query component,
then the target URI inherits the base URI's path component.
Removed the special-case treatment of same-document references
in favor of a section that explains that a new retrieval action
should not be made if the target URI and base URI, excluding
fragments, match.