| Network Working Group | T. Berners-Lee |
| INTERNET DRAFT | W3C/MIT |
| <draft-fielding-uri-rfc2396bis-05> | R. Fielding |
| Obsoletes: 2732, 2396, 1808 (if approved) | Day Software |
| Updates: 1738 (if approved) | L. Masinter |
| Category: Standards Track | Adobe |
| Expires: October 2004 | April 2004 |
Uniform Resource Identifier (URI): Generic Syntax
draft-fielding-uri-rfc2396bis-05
By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668.
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This Internet-Draft will expire in October 2004.
Copyright (C) The Internet Society (2004). All Rights Reserved.
A Uniform Resource Identifier (URI) is a compact sequence of characters for identifying an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet.
The URI syntax defines a grammar that is a superset of all valid URIs, such that an implementation can parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme.
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://gbiv.com/protocols/uri/rev-2002/issues.html>.
1
Introduction
1.1
Overview of URIs
1.1.1
Generic Syntax
1.1.2
Examples
1.1.3
URI, URL, and URN
1.2
Design Considerations
1.2.1
Transcription
1.2.2
Separating Identification from Interaction
1.2.3
Hierarchical Identifiers
1.3
Syntax Notation
2
Characters
2.1
Percent-Encoding
2.2
Reserved Characters
2.3
Unreserved Characters
2.4
When to Encode or Decode
2.5
Identifying Data
3
Syntax Components
3.1
Scheme
3.2
Authority
3.2.1
User Information
3.2.2
Host
3.2.3
Port
3.3
Path
3.4
Query
3.5
Fragment
4
Usage
4.1
URI Reference
4.2
Relative URI
4.3
Absolute URI
4.4
Same-document Reference
4.5
Suffix Reference
5
Reference Resolution
5.1
Establishing a Base URI
5.1.1
Base URI Embedded in Content
5.1.2
Base URI from the Encapsulating Entity
5.1.3
Base URI from the Retrieval URI
5.1.4
Default Base URI
5.2
Relative Resolution
5.2.1
Pre-parse the Base URI
5.2.2
Transform References
5.2.3
Merge Paths
5.2.4
Remove Dot Segments
5.3
Component Recomposition
5.4
Reference Resolution Examples
5.4.1
Normal Examples
5.4.2
Abnormal Examples
6
Normalization and Comparison
6.1
Equivalence
6.2
Comparison Ladder
6.2.1
Simple String Comparison
6.2.2
Syntax-based Normalization
6.2.2.1
Case Normalization
6.2.2.2
Percent-Encoding Normalization
6.2.2.3
Path Segment Normalization
6.2.3
Scheme-based Normalization
6.2.4
Protocol-based Normalization
6.3
Canonical Form
7
Security Considerations
7.1
Reliability and Consistency
7.2
Malicious Construction
7.3
Back-end Transcoding
7.4
Rare IP Address Formats
7.5
Sensitive Information
7.6
Semantic Attacks
8
Acknowledgments
9
References
9.1
Normative References
9.2
Informative References
§
Author's Addresses
A
Collected ABNF for URI
B
Parsing a URI Reference with a Regular Expression
C
Delimiting a URI in Context
D
Summary of Non-editorial Changes
D.1
Additions
D.2
Modifications from RFC 2396
§
Intellectual Property and Copyright Statements
§
Index
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" [RFC1630], and is designed to meet the recommendations laid out in "Functional Recommendations for Internet Resource Locators" [RFC1736] and "Functional Requirements for Uniform Resource Names" [RFC1737].
This document obsoletes [RFC2396], which merged "Uniform Resource Locators" [RFC1738] and "Relative Uniform Resource Locators" [RFC1808] in order to define a single, generic syntax for all URIs. It 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 [RFC2717]. Advice for designers of new URI schemes can be found in [RFC2718].
All significant changes from RFC 2396 are noted in Appendix D.
This specification uses the terms "character" and "coded character set" in accordance with the definitions provided in [RFC2978], and "character encoding" in place of what [RFC2978] refers to as a "charset".
URIs are characterized as follows:
Uniform
Resource
Identifier
A URI is an identifier that consists of a sequence of characters matching the syntax rule named <URI> in Section 3. A URI can be used to refer to a resource. 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.
URIs have a global scope and must be 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 Linux manual refers to a file on the end-user's filesystem (e.g., "file:///etc/hosts").
Each URI begins with a scheme name, as defined in Section 3.1, that refers to a specification for assigning identifiers within that scheme. As such, the URI syntax is a federated and extensible naming system wherein each scheme's specification may further restrict the syntax and semantics of identifiers using that scheme.
This specification defines those elements of the URI syntax that are required of all URI schemes or are common to many URI schemes. It thus defines the syntax and semantics 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.
The following examples illustrate URIs that are in common use.
ftp://ftp.is.co.za/rfc/rfc1808.txt http://www.ietf.org/rfc/rfc2396.txt mailto:John.Doe@example.com news:comp.infosystems.www.servers.unix telnet://melvyl.ucop.edu/
A URI can be further classified as a locator, a name, or both. The term "Uniform Resource Locator" (URL) refers to the subset of URIs that, in addition to identifying a resource, provide a means of locating the resource by describing its primary access mechanism (e.g., its network "location"). The term "Uniform Resource Name" (URN) has been used historically to refer to both URIs under the "urn" scheme [RFC2141], which are required to remain globally unique and persistent even when the resource ceases to exist or becomes unavailable, and to any other URI with the properties of a name.
An individual scheme does not 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. Future specifications and related documentation should use the general term "URI", rather than the more restrictive terms URL and URN [RFC3305].
The URI syntax has been designed with global transcription as one of its main considerations. A URI is a sequence of characters from a very limited set: the letters of the basic Latin alphabet, digits, and a few special characters. A URI may be represented in a variety of ways: e.g., ink on paper, pixels on a screen, or a sequence of character encoding octets. The interpretation of a URI depends only on the characters used and not 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:
These design considerations are not always in alignment. For example, it is often the case that the most meaningful name for a URI component would require characters that cannot be typed into some systems. The ability to transcribe a resource identifier from one medium to another has been considered more important than having a URI consist of the most meaningful of components.
In local or regional contexts and with improving technology, users might benefit from being able to use a wider range of characters; such use is not defined by this specification. Percent-encoded octets (Section 2.1) may be used within a URI to represent characters outside the range of the US-ASCII coded character set if such representation is allowed by the scheme or by the protocol element in which the URI is referenced; such a definition should specify the character encoding used to map those characters to octets prior to being percent-encoded for the URI.
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 "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. To use that access mechanism to perform an action on the URI's resource is to "dereference" the URI.
When URIs are used within information systems to identify sources of information, the most common form of URI dereference is "retrieval": making use of a URI in order to retrieve a representation of its associated resource. A "representation" is a sequence of octets, along with representation metadata describing those octets, that constitutes a record of the state of the resource at the time 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. Depending on the protocols used to perform the retrieval, additional information might be supplied about the resource (resource metadata) and its relation to other resources.
URI references in information systems are designed to be 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. Such references are created in order to be 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 order of decreasing significance from left to right. For some URI schemes, the visible hierarchy is limited to the scheme itself: everything after the scheme component delimiter (":") is considered opaque to URI processing. Other URI schemes make the hierarchy explicit and visible to generic parsing algorithms.
The generic syntax uses the slash ("/"), question mark ("?"), and number sign ("#") characters 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.
It is often the case that a group or "tree" of documents has been constructed to serve a common purpose, wherein the vast majority of 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 references. Furthermore, such document trees can be moved, as a whole, without changing any of the relative references.
A relative URI reference (Section 4.2) refers to a resource by describing the difference within a hierarchical name space between the reference context and the target URI. The reference resolution algorithm, presented in Section 5, defines how such a reference is transformed to the target URI. Since relative references can only be used within the context of a hierarchical URI, designers of new URI schemes should use a syntax consistent with the generic syntax's hierarchical components unless there are compelling reasons to forbid relative referencing within that scheme.
All URIs are parsed by generic syntax parsers when used. A URI scheme that wishes to remain opaque to hierarchical processing must disallow the use of slash and question mark characters. However, since a URI reference is only modified by the generic parser if it contains a dot-segment (a complete path segment of "." or "..", as described in Section 3.3), URI schemes may safely use "/" for other purposes if they do not allow dot-segments.
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.
The URI syntax provides a method of encoding data, presumably for the sake of identifying a resource, as a sequence of characters. The URI characters are, in turn, frequently encoded as octets for transport or presentation. This specification does not mandate any particular character encoding for mapping between URI characters and the octets used to store or transmit those characters. When a URI appears in a protocol element, the character encoding is defined by that protocol; absent such a definition, a URI is assumed to be in the same character encoding as the surrounding text.
The ABNF notation defines its terminal values to be non-negative integers (codepoints) based on the US-ASCII coded character set [ASCII]. Since a URI is a sequence of characters, we must invert that relation in order to understand the URI syntax. Therefore, the integer values used by the ABNF must be mapped back to their corresponding characters via US-ASCII in order to complete the syntax rules.
A URI is composed from a limited set of characters consisting of digits, letters, and a few graphic symbols. A reserved subset of those characters may be used to delimit syntax components within a URI, while the remaining characters, including both the unreserved set and those reserved characters not acting as delimiters, define each component's identifying data.
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 octet are equivalent: they identify the same resource. However, percent-encoded unreserved characters may change the result of some URI comparisons (Section 6), potentially leading to incorrect or inefficient behavior. For consistency, percent-encoded octets in the ranges of ALPHA (%41-%5A and %61-%7A), DIGIT (%30-%39), hyphen (%2D), period (%2E), underscore (%5F), or tilde (%7E) should not be created by URI producers and, when found in a URI, should be decoded to their corresponding unreserved character by URI normalizers.
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 software; 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 [RFC3629] (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 (ISO/IEC 10646-1) character set, the data should be encoded first as octets according to the UTF-8 character encoding [RFC3629], and then only those octets that do not correspond to characters in the unreserved set should be percent-encoded. For example, the character A would be represented as "A", the character LATIN CAPITAL LETTER A WITH GRAVE would be represented as "%C3%80", and the character KATAKANA LETTER A would be represented as "%E3%82%A2".
The generic URI syntax consists of a hierarchical sequence of components referred to as the scheme, authority, path, query, and fragment.
URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ]
hier-part = "//" authority path-abempty
/ path-abs
/ path-rootless
/ path-empty
The scheme and path components are required, though path may be empty (no characters). When authority is present, the path must either be empty or begin with a slash ("/") character. When authority is not present, the path cannot begin with two slash characters ("//"). These restrictions result in five different ABNF rules for a path (Section 3.3), only one of which will match any given URI reference.
The following are two example URIs and their component parts:
foo://example.com:8042/over/there?name=ferret#nose
\_/ \______________/\_________/ \_________/ \__/
| | | | |
scheme authority path query fragment
| _____________________|__
/ \ / \
urn:example:animal:ferret:nose
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 [RFC2717]. The scheme registry maintains the mapping between scheme names and their specifications. Advice for designers of new URI schemes can be found in [RFC2718].
When presented with a URI that violates one or more scheme-specific restrictions, the scheme-specific resolution process should flag the reference as an error rather than ignore the unused parts; doing so reduces the number of equivalent URIs and helps detect abuses of the generic syntax that might indicate the URI has been constructed to mislead the user (Section 7.6).
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 userinfo subcomponent may consist of a user name and, optionally, scheme-specific information about how to gain authorization to access the resource. The user information, if present, is followed by a commercial at-sign ("@") that delimits it from the host.
userinfo = *( unreserved / pct-encoded / sub-delims / ":" )
Use of the format "user:password" in the userinfo field is deprecated. Applications should not render as clear text any data after the first colon (":") character found within a userinfo subcomponent unless the data after the colon is the empty string (indicating no password). Applications may choose to ignore or reject such data when received as part of a reference, and should reject the storage of such data in unencrypted form. The passing of authentication information in clear text has proven to be a security risk in almost every case where it has been used.
Applications that render a URI for the sake of user feedback, such as in graphical hypertext browsing, should render userinfo in a way that is distinguished from the rest of a URI, when feasible. Such rendering will assist the user in cases where the userinfo has been misleadingly crafted to look like a trusted domain name (Section 7.6).
The host subcomponent of authority is identified by an IP literal encapsulated within square brackets, an IPv4 address in dotted-decimal form, or a registered name. The host subcomponent is case-insensitive. The presence of a host subcomponent within a URI does not imply that the scheme requires access to the given host on the Internet. In many cases, the host syntax is used only for the sake of reusing the existing registration process created and deployed for DNS, thus obtaining a globally unique name without the cost of deploying another registry. However, such use comes with its own costs: domain name ownership may change over time for reasons not anticipated by the URI producer. In other cases, the data within the host component identifies a registered name that has nothing to do with an Internet host. We use the name "host" for the ABNF rule because that is its most common purpose, not its only purpose, and thus should not be considered as semantically limiting the data within it.
host = IP-literal / IPv4address / reg-name
The syntax rule for host is ambiguous because it does not completely distinguish between an IPv4address and a reg-name. In order to disambiguate, the syntax, we apply the "first-match-wins" algorithm: If host matches the rule for IPv4address, then it should be considered an IPv4 address literal and not a reg-name. Although host is case-insensitive, producers and normalizers should use lowercase for registered names and hexadecimal addresses for the sake of uniformity, while only using uppercase letters for percent-encodings.
A host identified by an Internet Protocol literal address, version 6 [RFC3513] or later, is distinguished by enclosing the IP literal within square brackets ("[" and "]"). This is the only place where square bracket characters are allowed in the URI syntax. In anticipation of future, as-yet-undefined IP literal address formats, an optional version flag may be used to indicate such a format explicitly rather than relying on heuristic determination.
IP-literal = "[" ( IPv6address / IPvFuture ) "]" IPvFuture = "v" 1*HEXDIG "." 1*( unreserved / sub-delims / ":" )
The version flag does not indicate the IP version; rather, it indicates future versions of the literal format. As such, implementations must not provide the version flag for existing IPv4 and IPv6 literal addresses. If a URI containing an IP-literal that starts with "v" (case-insensitive), indicating that the version flag is present, is dereferenced by an application that does not know the meaning of that version flag, then the application should return an appropriate error for "address mechanism not supported".
A host identified by an IPv6 literal address is represented inside the square brackets without a preceding version flag. The ABNF provided here is a translation of the text definition of an IPv6 literal address provided in [RFC3513]. A 128-bit IPv6 address is divided into eight 16-bit pieces. Each piece is represented numerically in case-insensitive hexadecimal, using one to four hexadecimal digits (leading zeroes are permitted). The eight encoded pieces are given most-significant first, separated by colon characters. Optionally, the least-significant two pieces may instead be represented in IPv4 address textual format. A sequence of one or more consecutive zero-valued 16-bit pieces within the address may be elided, omitting all their digits and leaving exactly two consecutive colons in their place to mark the elision.
IPv6address = 6( h16 ":" ) ls32
/ "::" 5( h16 ":" ) ls32
/ [ h16 ] "::" 4( h16 ":" ) ls32
/ [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
/ [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
/ [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32
/ [ *4( h16 ":" ) h16 ] "::" ls32
/ [ *5( h16 ":" ) h16 ] "::" h16
/ [ *6( h16 ":" ) h16 ] "::"
ls32 = ( h16 ":" h16 ) / IPv4address
; least-significant 32 bits of address
h16 = 1*4HEXDIG
; 16 bits of address represented in hexadecimal
A host identified by an IPv4 literal address is represented in dotted-decimal notation (a sequence of four decimal numbers in the range 0 to 255, separated by "."), as described in [RFC1123] by reference to [RFC0952]. Note that other forms of dotted notation may be interpreted on some platforms, as described in Section 7.4, but only the dotted-decimal form of four octets is allowed by this grammar.
IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet
dec-octet = DIGIT ; 0-9
/ %x31-39 DIGIT ; 10-99
/ "1" 2DIGIT ; 100-199
/ "2" %x30-34 DIGIT ; 200-249
/ "25" %x30-35 ; 250-255
A host identified by a registered name is a sequence of characters that is usually intended for lookup within a locally-defined host or service name registry, though the URI's scheme-specific semantics may require that a specific registry (or fixed name table) be used instead. The most common name registry mechanism is the Domain Name System (DNS). A registered name intended for lookup in the DNS uses the syntax defined in Section 3.5 of [RFC1034] and Section 2.1 of [RFC1123]. Such a name consists of a sequence of domain labels separated by ".", each domain label starting and ending with an alphanumeric character and possibly also containing "-" characters. The rightmost domain label of a fully qualified domain name in DNS may be followed by a single "." and should be followed by one if it is necessary to distinguish between the complete domain name and some local domain.
reg-name = 0*255( unreserved / pct-encoded / sub-delims )
If the URI scheme defines a default for host, then that default applies when the host subcomponent is undefined or when the registered name is empty (zero length). For example, the "file" URI scheme is defined such that no authority, an empty host, and "localhost" all mean the end-user's machine, whereas the "http" scheme considers a missing authority or empty host to be invalid.
This specification does not mandate a particular registered name lookup technology and therefore does not restrict the syntax of reg-name beyond that necessary for interoperability. Instead, it delegates the issue of registered name syntax conformance to the operating system of each application performing URI resolution, and that operating system decides what it will allow for the purpose of host identification. A URI resolution implementation might use DNS, host tables, yellow pages, NetInfo, WINS, or any other system for lookup of registered names. However, a globally-scoped naming system, such as DNS fully-qualified domain names, is necessary for URIs that are intended to have global scope. URI producers should use names that conform to the DNS syntax, even when use of DNS is not immediately apparent.
The reg-name syntax allows percent-encoded octets in order to represent non-ASCII registered names in a uniform way that is independent of the underlying name resolution technology; such non-ASCII characters must first be encoded according to UTF-8 [RFC3629] and then each octet of the corresponding UTF-8 sequence must be percent-encoded to be represented as URI characters. URI producing applications must not use percent-encoding in host unless it is used to represent a UTF-8 character sequence. When a non-ASCII registered name represents an internationalized domain name intended for resolution via the DNS, the name must be transformed to the IDNA encoding [RFC3490] prior to name lookup. URI producers should provide such registered names in the IDNA encoding, rather than a percent-encoding, if they wish to maximize interoperability with legacy URI resolvers.
The port subcomponent of authority is designated by an optional port number in decimal following the host and delimited from it by a single colon (":") character.
port = *DIGIT
A scheme may define a default port. For example, the "http" scheme defines a default port of "80", corresponding to its reserved TCP port number. The type of port designated by the port number (e.g., TCP, UDP, SCTP, etc.) is defined by the URI scheme. URI producers and normalizers should omit the port component and its ":" delimiter if port is empty or its value would be the same as the scheme's default.
The path component contains data, usually organized in hierarchical form, that, along with data in the non-hierarchical query component, 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 begin with a relative path, 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 a given URI reference. We use the generic term "path component" to describe the URI substring that is matched by the parser to one of these rules.
path = path-abempty ; begins with "/" or is empty
/ path-abs ; 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-abs = "/" [ segment-nz *( "/" segment ) ]
path-noscheme = segment-nzc *( "/" segment )
path-rootless = segment-nz *( "/" segment )
path-empty = 0<pchar>
segment = *pchar
segment-nz = 1*pchar
segment-nzc = 1*( unreserved / pct-encoded / sub-delims / "@" )
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, 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 do not handle such URIs correctly when they are used as the base 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 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 do not handle such URIs correctly when they are used as the base for relative references (Section 5.1).
When applications make reference to a URI, they do not always use the full form of reference defined by the "URI" syntax rule. 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.
URI-reference is used to denote the most common usage of a resource identifier.
URI-reference = URI / relative-URI
A URI-reference may be relative: if the reference's prefix matches the syntax of a scheme followed by its colon separator, then the reference is a URI rather than a relative-URI.
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 URI reference takes advantage of the hierarchical syntax (Section 1.2.3) in order to express a reference that is relative to the name space of another hierarchical URI.
relative-URI = relative-part [ "?" query ] [ "#" fragment ]
relative-part = "//" authority path-abempty
/ path-abs
/ 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 ]
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
www.w3.org/Addressing/
or simply a DNS registered name on its own. Such references are primarily intended for human interpretation, rather than for machines, with the assumption that context-based heuristics are sufficient to complete the URI (e.g., most registered names beginning with "www" are likely to have a URI prefix of "http://"). Although there is no standard set of heuristics for disambiguating a URI suffix, many client implementations allow them to be entered by the user and heuristically resolved.
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.
This section defines the process of resolving a URI reference within a context that allows relative references, such that the result is a string matching the "URI" syntax rule of Section 3.
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.
The base URI of a reference can be established in one of four ways, discussed below in order of precedence. The order of precedence can be thought of in terms of layers, where the innermost defined base URI has the highest precedence. This can be visualized graphically as:
.----------------------------------------------------------. | .----------------------------------------------------. | | | .----------------------------------------------. | | | | | .----------------------------------------. | | | | | | | .----------------------------------. | | | | | | | | | <relative-reference> | | | | | | | | | `----------------------------------' | | | | | | | | (5.1.1) Base URI embedded in content | | | | | | | `----------------------------------------' | | | | | | (5.1.2) Base URI of the encapsulating entity | | | | | | (message, representation, or none) | | | | | `----------------------------------------------' | | | | (5.1.3) URI used to retrieve the entity | | | `----------------------------------------------------' | | (5.1.4) Default Base URI (application-dependent) | `----------------------------------------------------------'
Within certain media types, a base URI for relative references 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 specification to specify how, for each media type, a base URI can be embedded. The appropriate syntax, when available, is described by the data format specification associated with each media type.
If no base URI is embedded, the base URI is defined by the representation's retrieval context. For a document that is enclosed within another entity, such as a message or archive, the retrieval context is that entity; thus, the default base URI of a representation is the base URI of the entity in which the representation is encapsulated.
A mechanism for embedding a base URI within MIME container types (e.g., the message and multipart types) is defined by MHTML [RFC2557]. Protocols that do not use the MIME message header syntax, but do allow some form of tagged metadata to be included within messages, may define their own syntax for defining a base URI as part of a message.
If no base URI is embedded and the representation is not encapsulated within some other entity, then, if a URI was used to retrieve the representation, that URI shall be considered the base URI. Note that if the retrieval was the result of a redirected request, the last URI used (i.e., the URI that resulted in the actual retrieval of the representation) is the base URI.
If none of the conditions described above apply, then the base URI is defined by the context of the application. Since this definition is necessarily application-dependent, failing to define a base URI using one of the other methods may result in the same content being interpreted differently by different types of application.
A sender of a representation containing relative references is responsible for ensuring that a base URI for those references can be established. Aside from fragment-only references, relative references can only be used reliably in situations where the base URI is well-defined.
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.
The base URI (Base) is established according to the procedure of Section 5.1 and parsed into the five main components described in Section 3. Note that only the scheme component is required to be present in a base URI; the other components may be empty or undefined. A component is undefined if its associated delimiter does not appear in the URI reference; the path component is never undefined, though it may be empty.
Normalization of the base URI, as described in Section 6.2.2 and Section 6.2.3, is optional. A URI reference must be transformed to its target URI before it can be normalized.
For each URI reference (R), the following pseudocode describes an algorithm for transforming R into its target URI (T):
-- The URI reference is parsed into the five URI components
--
(R.scheme, R.authority, R.path, R.query, R.fragment) = parse(R);
-- A non-strict parser may ignore a scheme in the reference
-- if it is identical to the base URI's scheme.
--
if ((not strict) and (R.scheme == Base.scheme)) then
undefine(R.scheme);
endif;
if defined(R.scheme) then
T.scheme = R.scheme;
T.authority = R.authority;
T.path = remove_dot_segments(R.path);
T.query = R.query;
else
if defined(R.authority) then
T.authority = R.authority;
T.path = remove_dot_segments(R.path);
T.query = R.query;
else
if (R.path == "") then
T.path = Base.path;
if defined(R.query) then
T.query = R.query;
else
T.query = Base.query;
endif;
else
if (R.path starts-with "/") then
T.path = remove_dot_segments(R.path);
else
T.path = merge(Base.path, R.path);
T.path = remove_dot_segments(T.path);
endif;
T.query = R.query;
endif;
T.authority = Base.authority;
endif;
T.scheme = Base.scheme;
endif;
T.fragment = R.fragment;
The pseudocode above refers to a "merge" routine for merging a relative-path reference with the path of the base URI. This is accomplished as follows:
The pseudocode also refers to a "remove_dot_segments" routine for interpreting and removing the special "." and ".." complete path segments from a referenced path. This is done after the path is extracted from a reference, whether or not the path was relative, in order to remove any invalid or extraneous dot-segments prior to forming the target URI. Although there are many ways to accomplish this removal process, we describe a simple method using two string buffers.
Note that dot-segments are intended for use in URI references to express an identifier relative to the hierarchy of names in the base URI. The remove_dot_segments algorithm respects that hierarchy by removing extra dot-segments rather than treating them as an error or leaving them to be misinterpreted by dereference implementations.
The following illustrates how the above steps are applied for two example merged paths, showing the state of the two buffers after each step.
STEP OUTPUT BUFFER INPUT BUFFER
1 : /a/b/c/./../../g
2e: /a /b/c/./../../g
2e: /a/b /c/./../../g
2e: /a/b/c /./../../g
2b: /a/b/c /../../g
2c: /a/b /../g
2c: /a /g
2e: /a/g
STEP OUTPUT BUFFER INPUT BUFFER
1 : mid/content=5/../6
2e: mid /content=5/../6
2e: mid/content=5 /../6
2c: mid /6
2e: mid/6
Some applications may find it more efficient to implement the remove_dot_segments algorithm using two segment stacks rather than strings.
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
http://a/b/c/d;p?q
a relative URI reference is transformed to its target URI as follows.
"g:h" = "g:h" "g" = "http://a/b/c/g" "./g" = "http://a/b/c/g" "g/" = "http://a/b/c/g/" "/g" = "http://a/g" "//g" = "http://g" "?y" = "http://a/b/c/d;p?y" "g?y" = "http://a/b/c/g?y" "#s" = "http://a/b/c/d;p?q#s" "g#s" = "http://a/b/c/g#s" "g?y#s" = "http://a/b/c/g?y#s" ";x" = "http://a/b/c/;x" "g;x" = "http://a/b/c/g;x" "g;x?y#s" = "http://a/b/c/g;x?y#s" "" = "http://a/b/c/d;p?q" "." = "http://a/b/c/" "./" = "http://a/b/c/" ".." = "http://a/b/" "../" = "http://a/b/" "../g" = "http://a/b/g" "../.." = "http://a/" "../../" = "http://a/" "../../g" = "http://a/g"
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.
Parsers must be careful in handling cases 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.
"../../../g" = "http://a/g" "../../../../g" = "http://a/g"
Similarly, parsers must remove the dot-segments "." and ".." when they are complete components of a path, but not when they are only part of a segment.
"/./g" = "http://a/g" "/../g" = "http://a/g" "g." = "http://a/b/c/g." ".g" = "http://a/b/c/.g" "g.." = "http://a/b/c/g.." "..g" = "http://a/b/c/..g"
Less likely are cases where the relative URI reference uses unnecessary or nonsensical forms of the "." and ".." complete path segments.
"./../g" = "http://a/b/g" "./g/." = "http://a/b/c/g/" "g/./h" = "http://a/b/c/g/h" "g/../h" = "http://a/b/c/h" "g;x=1/./y" = "http://a/b/c/g;x=1/y" "g;x=1/../y" = "http://a/b/c/y"
Some applications fail to separate the reference's query and/or fragment components from a relative path before merging it with the base path and removing dot-segments. 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.
"g?y/./x" = "http://a/b/c/g?y/./x" "g?y/../x" = "http://a/b/c/g?y/../x" "g#s/./x" = "http://a/b/c/g#s/./x" "g#s/../x" = "http://a/b/c/g#s/../x"
Some parsers allow the scheme name to be present in a relative URI reference if it is the same as the base URI scheme. This is considered to be a loophole in prior specifications of partial URI [RFC1630]. Its use should be avoided, but is allowed for backward compatibility.
"http:g" = "http:g" ; for strict parsers
/ "http://a/b/c/g" ; for backward compatibility