Uniform Resource Identifier (URI): Generic Syntax

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 ¶

• 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 consistent purpose (e.g., "today's weather report for Los Angeles"), a service (e.g., an HTTP to SMS gateway), a collection of other resources, and so on. A resource is not necessarily accessible via the Internet; e.g., human beings, corporations, and bound books in a library can also be resources. Likewise, abstract concepts can be resources, such as the operators and operands of a mathematical equation, the types of a relationship (e.g., "parent" or "employee"), or numeric values (e.g., zero, one, and infinity).

Identifier ¶

• An identifier embodies the information required to distinguish what is being identified from all other things within its scope of identification. Our use of the terms "identify" and "identifying" refer to this purpose of distinguishing one resource from all other resources, regardless of how that purpose is accomplished (e.g., by name, address, context, etc.). 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, that 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 wish to refer to a resource, or the kinds of system 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 all 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 single, globally unique thing must use a URI that distinguishes that resource from all other things. URIs that identify in relation to the end-user's local context should only be used when the context itself is a defining aspect of the resource, such as when an on-line help manual refers to a file on the end-user's filesystem (e.g., "file:///etc/hosts").¶

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.¶

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. Two URIs that differ only in the case of hexadecimal digits used in percent-encoded octets are equivalent. For consistency, URI producers and normalizers should use uppercase hexadecimal digits for all percent-encodings.

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 forming the URI.

   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) are 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. However, if a reserved character is found in a URI component and no delimiting role is known for that character, then it should be interpreted as representing the data octet corresponding to that character's encoding in US-ASCII.¶

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 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 character by URI normalizers.¶

Under normal circumstances, the only time that octets within a URI are percent-encoded is during the process of producing the URI from its component parts. It is during that process that an implementation determines which of the reserved characters are to be used as 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, since 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" in order for that octet to be used as data within a URI. Implementations must not percent-encode or decode the same string more than once, since decoding an already decoded string might lead to misinterpreting a percent data octet as the beginning of a percent-encoding, or vice versa in the case of percent-encoding an already percent-encoded string.¶

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 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 need 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.¶

The first encoding of identifying data is the one in which the local names or data are stored. URI producing applications (a.k.a., 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 there is no harm in making that assumption even when it is incorrect. 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 simply percent-encoding the original octets.¶

For example, consider an information service that provides data, stored locally using an EBCDIC-based filesystem, to clients on the Internet through an HTTP server. When an author creates a file on that filesystem with the name "Laguna Beach", their expectation is that the "http" URI corresponding to that resource would also contain the meaningful string "Laguna%20Beach". If, however, that server produces URIs 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 that 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, numeric coordinates on a map, etc. 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 Unicode character set [UCS], the data should be encoded first as octets according to the UTF-8 character encoding [STD63], and then only those octets that do not correspond to characters in the unreserved set should be percent-encoded. For example, the character A would be represented as "A", the character LATIN CAPITAL LETTER A WITH GRAVE would be represented as "%C3%80", and the character KATAKANA LETTER A would be represented as "%E3%82%A2".¶

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 scheme is case-insensitive, the canonical form is lowercase and documents that specify schemes must do so using lowercase letters. An implementation should accept uppercase letters as equivalent to lowercase in scheme names (e.g., allow "HTTP" as well as "http"), for the sake of robustness, but should only produce lowercase scheme names, for consistency.

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

Individual schemes are not specified by this document. The process for registration of new URI schemes is defined separately by [BCP35]. The scheme registry maintains the mapping between scheme names and their specifications. Advice for designers of new URI schemes can be found in [RFC2718]. URI scheme specifications must define their own syntax such 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 that might indicate the URI has been constructed to mislead the user (Section 7.6).¶

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 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.¶

The path component contains data, usually organized in hierarchical form, that, along with data in the non-hierarchical query component (Section 3.4), serves to identify a resource within the scope of the URI's scheme and naming authority (if any). The path is terminated by the first question mark ("?") or number sign ("#") character, or by the end of the URI.¶

If a URI contains an authority component, then the path component must either be empty or begin with a slash ("/") character. If a URI does not contain an authority component, then the path cannot begin with two slash characters ("//"). In addition, a URI reference (Section 4.1) may be a relative-path reference, in which case the first path segment cannot contain a colon (":") character. The ABNF requires five separate rules to disambiguate these cases, only one of which will match the path substring within a given URI reference. We use the generic term "path component" to describe the URI substring matched by the parser to one of these rules.

   path          = path-abempty    ; begins with "/" or is empty
                 / path-absolute   ; begins with "/" but not "//"
                 / path-noscheme   ; begins with a non-colon segment
                 / path-rootless   ; begins with a segment
                 / path-empty      ; zero characters

   path-abempty  = *( "/" segment )
   path-absolute = "/" [ segment-nz *( "/" segment ) ]
   path-noscheme = segment-nz-nc *( "/" segment )
   path-rootless = segment-nz *( "/" segment )
   path-empty    = 0<pchar>

   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) for indicating relative position within the hierarchical tree of names. This is similar to their role within some operating systems' file directory structure to indicate the current directory and parent directory, respectively. However, unlike 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 for the purpose of delimiting scheme-specific or dereference-handler-specific subcomponents. For example, 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 producer might use a segment like "name;v=1.1" to indicate a reference to version 1.1 of "name", whereas another might use a segment like "name,1.1" to indicate the same. Parameter types may be defined by scheme-specific semantics, but in most cases the syntax of a parameter is specific to the implementation of the URI's dereferencing algorithm.¶

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 used as the base URI for relative references (Section 5.1), apparently because they fail to to distinguish query data from path data when looking for hierarchical separators. 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 avoid percent-encoding those characters.¶

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, 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 structure 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 such that it corresponds to the same secondary resource, regardless of how it is represented, or the fragment should be left undefined by the representation (i.e., not found).¶

As with any URI, use of a fragment identifier component does not 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 those 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 and regardless of the URI scheme. Although this separate handling is often perceived to be a loss of information, particularly in regards to accurate redirection of references as resources move over time, it also serves to prevent information providers from denying reference authors the right to selectively refer to information within a resource. Indirect referencing also provides additional flexibility and extensibility to systems that use URIs, since new media types are easier to define and deploy than new schemes of identification.¶

The characters slash ("/") and question mark ("?") are allowed to represent data within the fragment identifier. Beware that some older, erroneous implementations may not handle such data correctly when used as the base URI for relative references (Section 5.1).¶

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 or not the reference is relative, after which 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.¶

A relative reference takes advantage of the hierarchical syntax (Section 1.2.3) in order to express a URI reference relative to the name space of another hierarchical URI.

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

   relative-part = "//" authority path-abempty
                 / path-absolute
                 / path-noscheme
                 / path-empty

The URI referred to by a relative reference, also known as the target URI, is obtained by applying the reference resolution algorithm of Section 5.

A relative reference that begins with two slash characters is termed a network-path reference; such references are rarely used. A relative reference that begins with a single slash character is termed an absolute-path reference. A relative reference that does not begin with a slash character is termed a relative-path reference.¶

A path segment that contains a colon character (e.g., "this:that") cannot be used as the first segment of a relative-path reference because it would be mistaken for a scheme name. Such a segment must be preceded by a dot-segment (e.g., "./this:that") to make a relative-path reference.¶

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 such that all strings matching their scheme-specific syntax will also match the <absolute-URI> grammar. Scheme specifications are not responsible for defining fragment identifier syntax or usage, regardless of its applicability to resources identifiable via that scheme, since 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.

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 the purpose of 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 Section 6.2.2 and Section 6.2.3, is allowed but rarely performed in practice. Normalization may increase the set of same-document references, which may be of benefit to some caching applications. As such, reference authors should not assume that a slightly different, though equivalent, reference URI will (or will not) be interpreted as a same-document reference by any given application.¶

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 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.¶

While this practice of using suffix references is common, it should be avoided whenever possible and never used in situations where long-term references are expected. The heuristics noted above will change over time, particularly when a new URI scheme becomes popular, and are often incorrect when used out of context. Furthermore, they can lead to security issues along the lines of those described in [RFC1535].¶

Since a URI suffix has the same syntax as a relative-path reference, a suffix reference cannot be used in contexts where a relative reference is expected. As a result, suffix references are limited to those places where there is no defined base URI, such as dialog boxes and off-line advertisements.¶

The term "relative" implies that there exists a "base URI" 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 use as a base URI.¶

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 using some other algorithm, provided that the results match what would be given by this algorithm.¶

Parsed URI components can be recomposed to obtain the corresponding URI reference string. Using pseudocode, this would be:

   result = ""

   if defined(scheme) then
      append scheme to result;
      append ":" to result;
   endif;

   if defined(authority) then
      append "//" to result;
      append authority to result;
   endif;

   append path to result;

   if defined(query) then
      append "?" to result;
      append query to result;
   endif;

   if defined(fragment) then
      append "#" to result;
      append fragment to result;
   endif;

   return result;

Note that we are careful to preserve the distinction between a component that is undefined, meaning that its separator was not present in the reference, and a component that is empty, meaning that the separator was present and was immediately followed by the next component separator or the end of the reference.

Within a representation with a well-defined base URI of¶

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

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 an implementation to compare two resources that are not under its own control. 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 if 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 being compared for the purpose of selecting (or avoiding) a network action, such as retrieval of a representation, fragment components (if any) should be excluded from the 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 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 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.¶

There is no guarantee that, having once used a given URI to retrieve some 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 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 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), such percent-encoded octets 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.¶

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, since otherwise the decoded octets might be mistaken for delimiters. Security checks of the data within a URI should be applied after decoding the octets. Note, however, that the "%00" percent-encoding (NUL) may require special handling and should be rejected if the application is not expecting to receive raw data within a component.¶

Special care should be taken when the URI path interpretation process involves the use of a back-end filesystem or related system functions. Filesystems typically assign an operational meaning to special characters, such as the "/", "\", ":", "[", and "]" characters, and special device names like ".", "..", "...", "aux", "lpt", etc. In some cases, merely testing for the existence of such a name will cause the operating system to pause or invoke unrelated system calls, leading to significant security concerns regarding denial of service and unintended data transfer. It would be impossible for this specification to list all such significant characters and device names; implementers should research the reserved names and characters for the types of storage device that may be attached to their application and restrict the use of data obtained from URI components accordingly.¶

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 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, literals should be converted to numeric form and filtered based on the numeric value, rather than a prefix or suffix of the string form.¶

URI producers should not provide a URI that contains a username or password which 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.¶

Because the userinfo subcomponent is rarely used and appears before the host in the authority component, it can be used to construct a URI that is intended to mislead a human user by appearing to identify 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 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 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 [Siedzik].¶

10.1. Normative References

[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.

10.2. Informative References

[BCP19]

Freed, N. and J. Postel, “IANA Charset Registration Procedures”, BCP 19, RFC 2978, October 2000.

[BCP35]

Petke, R. and I. King, “Registration Procedures for URL Scheme Names”, BCP 35, RFC 2717, November 1999.

[RFC0952]

Harrenstien, K., Stahl, M., and E. Feinler, “DoD Internet host table specification”, RFC 952, October 1985.

[RFC1034]

Mockapetris, P., “Domain names - concepts and facilities”, STD 13, RFC 1034, November 1987.

[RFC1123]

Braden, R., “Requirements for Internet Hosts - Application and Support”, STD 3, RFC 1123, October 1989.

[RFC1535]

Gavron, E., “A Security Problem and Proposed Correction With Widely Deployed DNS Software”, RFC 1535, October 1993.

[RFC1630]

Berners-Lee, T., “Universal Resource Identifiers in WWW: A Unifying Syntax for the Expression of Names and Addresses of Objects on the Network as used in the World-Wide Web”, RFC 1630, June 1994.

[RFC1736]

Kunze, J., “Functional Recommendations for Internet Resource Locators”, RFC 1736, February 1995.

[RFC1737]

Masinter, L. and K. Sollins, “Functional Requirements for Uniform Resource Names”, RFC 1737, December 1994.

[RFC1738]

Berners-Lee, T., Masinter, L., and M. McCahill, “Uniform Resource Locators (URL)”, RFC 1738, December 1994.

[RFC1808]

Fielding, R., “Relative Uniform Resource Locators”, RFC 1808, June 1995.

[RFC2046]

Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types”, RFC 2046, November 1996.

[RFC2141]

Moats, R., “URN Syntax”, RFC 2141, May 1997.

[RFC2396]

Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifiers (URI): Generic Syntax”, RFC 2396, August 1998.

[RFC2518]

Goland, Y., Whitehead, E., Faizi, A., Carter, S., and D. Jensen, “HTTP Extensions for Distributed Authoring -- WEBDAV”, RFC 2518, February 1999.

[RFC2557]

Palme, F., Hopmann, A., Shelness, N., and E. Stefferud, “MIME Encapsulation of Aggregate Documents, such as HTML (MHTML)”, RFC 2557, March 1999.

[RFC2718]

Masinter, L., Alvestrand, H., Zigmond, D., and R. Petke, “Guidelines for new URL Schemes”, RFC 2718, November 1999.

[RFC2732]

Hinden, R., Carpenter, B., and L. Masinter, “Format for Literal IPv6 Addresses in URL's”, RFC 2732, December 1999.

[RFC3305]

Mealling, M. and R. Denenberg, “Report from the Joint W3C/IETF URI Planning Interest Group: Uniform Resource Identifiers (URIs), URLs, and Uniform Resource Names (URNs): Clarifications and Recommendations”, RFC 3305, August 2002.

[RFC3490]

Faltstrom, P., Hoffman, P., and A. Costello, “Internationalizing Domain Names in Applications (IDNA)”, RFC 3490, March 2003.

[RFC3513]

Hinden, R. and S. Deering, “Internet Protocol Version 6 (IPv6) Addressing Architecture”, RFC 3513, April 2003.

[Siedzik]

Siedzik, R., “Semantic Attacks: What's in a URL?”, April 2001, <http://www.giac.org/practical/gsec/Richard_Siedzik_GSEC.pdf>.

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 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.¶

Since [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 (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.¶

Section 2 on characters has been rewritten to explain what characters are reserved, when they are reserved, and why they are reserved even when 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 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 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 or not 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 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 and removes the need to re-specify DNS name formats here. It also allows the host component to contain percent-encoded octets, which is necessary to enable internationalized domain names to be provided in URIs, processed in their native character encodings at the application layers above URI processing, and passed to an IDNA library as a registered name in the UTF-8 character encoding. The server, hostport, hostname, domainlabel, toplabel, and alphanum rules have been removed.¶

The resolving relative references algorithm of [RFC2396] has been rewritten using pseudocode for this revision to improve clarity and fix the following issues: ¶

• [RFC2396] section 5.2, step 6a, failed to account for a base URI with no path.
• Restored the behavior of [RFC1808] where, if the reference contains an empty path and a defined query component, then the target URI inherits the base URI's path component.
• 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 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.
• 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 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.