7 Network Working Group T. Berners-Lee
8 Request for Comments: 3986 W3C/MIT
10 Updates: 1738 Day Software
11 Obsoletes: 2732, 2396, 1808 L. Masinter
12 Category: Standards Track Adobe Systems
16 Uniform Resource Identifier (URI): Generic Syntax
20 This document specifies an Internet standards track protocol for the
21 Internet community, and requests discussion and suggestions for
22 improvements. Please refer to the current edition of the "Internet
23 Official Protocol Standards" (STD 1) for the standardization state
24 and status of this protocol. Distribution of this memo is unlimited.
28 Copyright (C) The Internet Society (2005).
32 A Uniform Resource Identifier (URI) is a compact sequence of
33 characters that identifies an abstract or physical resource. This
34 specification defines the generic URI syntax and a process for
35 resolving URI references that might be in relative form, along with
36 guidelines and security considerations for the use of URIs on the
37 Internet. The URI syntax defines a grammar that is a superset of all
38 valid URIs, allowing an implementation to parse the common components
39 of a URI reference without knowing the scheme-specific requirements
40 of every possible identifier. This specification does not define a
41 generative grammar for URIs; that task is performed by the individual
42 specifications of each URI scheme.
58 Berners-Lee, et al. Standards Track [Page 1]
60 RFC 3986 URI Generic Syntax January 2005
65 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
66 1.1. Overview of URIs . . . . . . . . . . . . . . . . . . . . 4
67 1.1.1. Generic Syntax . . . . . . . . . . . . . . . . . 6
68 1.1.2. Examples . . . . . . . . . . . . . . . . . . . . 7
69 1.1.3. URI, URL, and URN . . . . . . . . . . . . . . . 7
70 1.2. Design Considerations . . . . . . . . . . . . . . . . . 8
71 1.2.1. Transcription . . . . . . . . . . . . . . . . . 8
72 1.2.2. Separating Identification from Interaction . . . 9
73 1.2.3. Hierarchical Identifiers . . . . . . . . . . . . 10
74 1.3. Syntax Notation . . . . . . . . . . . . . . . . . . . . 11
75 2. Characters . . . . . . . . . . . . . . . . . . . . . . . . . . 11
76 2.1. Percent-Encoding . . . . . . . . . . . . . . . . . . . . 12
77 2.2. Reserved Characters . . . . . . . . . . . . . . . . . . 12
78 2.3. Unreserved Characters . . . . . . . . . . . . . . . . . 13
79 2.4. When to Encode or Decode . . . . . . . . . . . . . . . . 14
80 2.5. Identifying Data . . . . . . . . . . . . . . . . . . . . 14
81 3. Syntax Components . . . . . . . . . . . . . . . . . . . . . . 16
82 3.1. Scheme . . . . . . . . . . . . . . . . . . . . . . . . . 17
83 3.2. Authority . . . . . . . . . . . . . . . . . . . . . . . 17
84 3.2.1. User Information . . . . . . . . . . . . . . . . 18
85 3.2.2. Host . . . . . . . . . . . . . . . . . . . . . . 18
86 3.2.3. Port . . . . . . . . . . . . . . . . . . . . . . 22
87 3.3. Path . . . . . . . . . . . . . . . . . . . . . . . . . . 22
88 3.4. Query . . . . . . . . . . . . . . . . . . . . . . . . . 23
89 3.5. Fragment . . . . . . . . . . . . . . . . . . . . . . . . 24
90 4. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
91 4.1. URI Reference . . . . . . . . . . . . . . . . . . . . . 25
92 4.2. Relative Reference . . . . . . . . . . . . . . . . . . . 26
93 4.3. Absolute URI . . . . . . . . . . . . . . . . . . . . . . 27
94 4.4. Same-Document Reference . . . . . . . . . . . . . . . . 27
95 4.5. Suffix Reference . . . . . . . . . . . . . . . . . . . . 27
96 5. Reference Resolution . . . . . . . . . . . . . . . . . . . . . 28
97 5.1. Establishing a Base URI . . . . . . . . . . . . . . . . 28
98 5.1.1. Base URI Embedded in Content . . . . . . . . . . 29
99 5.1.2. Base URI from the Encapsulating Entity . . . . . 29
100 5.1.3. Base URI from the Retrieval URI . . . . . . . . 30
101 5.1.4. Default Base URI . . . . . . . . . . . . . . . . 30
102 5.2. Relative Resolution . . . . . . . . . . . . . . . . . . 30
103 5.2.1. Pre-parse the Base URI . . . . . . . . . . . . . 31
104 5.2.2. Transform References . . . . . . . . . . . . . . 31
105 5.2.3. Merge Paths . . . . . . . . . . . . . . . . . . 32
106 5.2.4. Remove Dot Segments . . . . . . . . . . . . . . 33
107 5.3. Component Recomposition . . . . . . . . . . . . . . . . 35
108 5.4. Reference Resolution Examples . . . . . . . . . . . . . 35
109 5.4.1. Normal Examples . . . . . . . . . . . . . . . . 36
110 5.4.2. Abnormal Examples . . . . . . . . . . . . . . . 36
114 Berners-Lee, et al. Standards Track [Page 2]
116 RFC 3986 URI Generic Syntax January 2005
119 6. Normalization and Comparison . . . . . . . . . . . . . . . . . 38
120 6.1. Equivalence . . . . . . . . . . . . . . . . . . . . . . 38
121 6.2. Comparison Ladder . . . . . . . . . . . . . . . . . . . 39
122 6.2.1. Simple String Comparison . . . . . . . . . . . . 39
123 6.2.2. Syntax-Based Normalization . . . . . . . . . . . 40
124 6.2.3. Scheme-Based Normalization . . . . . . . . . . . 41
125 6.2.4. Protocol-Based Normalization . . . . . . . . . . 42
126 7. Security Considerations . . . . . . . . . . . . . . . . . . . 43
127 7.1. Reliability and Consistency . . . . . . . . . . . . . . 43
128 7.2. Malicious Construction . . . . . . . . . . . . . . . . . 43
129 7.3. Back-End Transcoding . . . . . . . . . . . . . . . . . . 44
130 7.4. Rare IP Address Formats . . . . . . . . . . . . . . . . 45
131 7.5. Sensitive Information . . . . . . . . . . . . . . . . . 45
132 7.6. Semantic Attacks . . . . . . . . . . . . . . . . . . . . 45
133 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 46
134 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 46
135 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 46
136 10.1. Normative References . . . . . . . . . . . . . . . . . . 46
137 10.2. Informative References . . . . . . . . . . . . . . . . . 47
138 A. Collected ABNF for URI . . . . . . . . . . . . . . . . . . . . 49
139 B. Parsing a URI Reference with a Regular Expression . . . . . . 50
140 C. Delimiting a URI in Context . . . . . . . . . . . . . . . . . 51
141 D. Changes from RFC 2396 . . . . . . . . . . . . . . . . . . . . 53
142 D.1. Additions . . . . . . . . . . . . . . . . . . . . . . . 53
143 D.2. Modifications . . . . . . . . . . . . . . . . . . . . . 53
144 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
145 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 60
146 Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 61
170 Berners-Lee, et al. Standards Track [Page 3]
172 RFC 3986 URI Generic Syntax January 2005
177 A Uniform Resource Identifier (URI) provides a simple and extensible
178 means for identifying a resource. This specification of URI syntax
179 and semantics is derived from concepts introduced by the World Wide
180 Web global information initiative, whose use of these identifiers
181 dates from 1990 and is described in "Universal Resource Identifiers
182 in WWW" [RFC1630]. The syntax is designed to meet the
183 recommendations laid out in "Functional Recommendations for Internet
184 Resource Locators" [RFC1736] and "Functional Requirements for Uniform
185 Resource Names" [RFC1737].
187 This document obsoletes [RFC2396], which merged "Uniform Resource
188 Locators" [RFC1738] and "Relative Uniform Resource Locators"
189 [RFC1808] in order to define a single, generic syntax for all URIs.
190 It obsoletes [RFC2732], which introduced syntax for an IPv6 address.
191 It excludes portions of RFC 1738 that defined the specific syntax of
192 individual URI schemes; those portions will be updated as separate
193 documents. The process for registration of new URI schemes is
194 defined separately by [BCP35]. Advice for designers of new URI
195 schemes can be found in [RFC2718]. All significant changes from RFC
196 2396 are noted in Appendix D.
198 This specification uses the terms "character" and "coded character
199 set" in accordance with the definitions provided in [BCP19], and
200 "character encoding" in place of what [BCP19] refers to as a
203 1.1. Overview of URIs
205 URIs are characterized as follows:
209 Uniformity provides several benefits. It allows different types
210 of resource identifiers to be used in the same context, even when
211 the mechanisms used to access those resources may differ. It
212 allows uniform semantic interpretation of common syntactic
213 conventions across different types of resource identifiers. It
214 allows introduction of new types of resource identifiers without
215 interfering with the way that existing identifiers are used. It
216 allows the identifiers to be reused in many different contexts,
217 thus permitting new applications or protocols to leverage a pre-
218 existing, large, and widely used set of resource identifiers.
226 Berners-Lee, et al. Standards Track [Page 4]
228 RFC 3986 URI Generic Syntax January 2005
233 This specification does not limit the scope of what might be a
234 resource; rather, the term "resource" is used in a general sense
235 for whatever might be identified by a URI. Familiar examples
236 include an electronic document, an image, a source of information
237 with a consistent purpose (e.g., "today's weather report for Los
238 Angeles"), a service (e.g., an HTTP-to-SMS gateway), and a
239 collection of other resources. A resource is not necessarily
240 accessible via the Internet; e.g., human beings, corporations, and
241 bound books in a library can also be resources. Likewise,
242 abstract concepts can be resources, such as the operators and
243 operands of a mathematical equation, the types of a relationship
244 (e.g., "parent" or "employee"), or numeric values (e.g., zero,
249 An identifier embodies the information required to distinguish
250 what is being identified from all other things within its scope of
251 identification. Our use of the terms "identify" and "identifying"
252 refer to this purpose of distinguishing one resource from all
253 other resources, regardless of how that purpose is accomplished
254 (e.g., by name, address, or context). These terms should not be
255 mistaken as an assumption that an identifier defines or embodies
256 the identity of what is referenced, though that may be the case
257 for some identifiers. Nor should it be assumed that a system
258 using URIs will access the resource identified: in many cases,
259 URIs are used to denote resources without any intention that they
260 be accessed. Likewise, the "one" resource identified might not be
261 singular in nature (e.g., a resource might be a named set or a
262 mapping that varies over time).
264 A URI is an identifier consisting of a sequence of characters
265 matching the syntax rule named <URI> in Section 3. It enables
266 uniform identification of resources via a separately defined
267 extensible set of naming schemes (Section 3.1). How that
268 identification is accomplished, assigned, or enabled is delegated to
269 each scheme specification.
271 This specification does not place any limits on the nature of a
272 resource, the reasons why an application might seek to refer to a
273 resource, or the kinds of systems that might use URIs for the sake of
274 identifying resources. This specification does not require that a
275 URI persists in identifying the same resource over time, though that
276 is a common goal of all URI schemes. Nevertheless, nothing in this
282 Berners-Lee, et al. Standards Track [Page 5]
284 RFC 3986 URI Generic Syntax January 2005
287 specification prevents an application from limiting itself to
288 particular types of resources, or to a subset of URIs that maintains
289 characteristics desired by that application.
291 URIs have a global scope and are interpreted consistently regardless
292 of context, though the result of that interpretation may be in
293 relation to the end-user's context. For example, "http://localhost/"
294 has the same interpretation for every user of that reference, even
295 though the network interface corresponding to "localhost" may be
296 different for each end-user: interpretation is independent of access.
297 However, an action made on the basis of that reference will take
298 place in relation to the end-user's context, which implies that an
299 action intended to refer to a globally unique thing must use a URI
300 that distinguishes that resource from all other things. URIs that
301 identify in relation to the end-user's local context should only be
302 used when the context itself is a defining aspect of the resource,
303 such as when an on-line help manual refers to a file on the end-
304 user's file system (e.g., "file:///etc/hosts").
306 1.1.1. Generic Syntax
308 Each URI begins with a scheme name, as defined in Section 3.1, that
309 refers to a specification for assigning identifiers within that
310 scheme. As such, the URI syntax is a federated and extensible naming
311 system wherein each scheme's specification may further restrict the
312 syntax and semantics of identifiers using that scheme.
314 This specification defines those elements of the URI syntax that are
315 required of all URI schemes or are common to many URI schemes. It
316 thus defines the syntax and semantics needed to implement a scheme-
317 independent parsing mechanism for URI references, by which the
318 scheme-dependent handling of a URI can be postponed until the
319 scheme-dependent semantics are needed. Likewise, protocols and data
320 formats that make use of URI references can refer to this
321 specification as a definition for the range of syntax allowed for all
322 URIs, including those schemes that have yet to be defined. This
323 decouples the evolution of identification schemes from the evolution
324 of protocols, data formats, and implementations that make use of
327 A parser of the generic URI syntax can parse any URI reference into
328 its major components. Once the scheme is determined, further
329 scheme-specific parsing can be performed on the components. In other
330 words, the URI generic syntax is a superset of the syntax of all URI
338 Berners-Lee, et al. Standards Track [Page 6]
340 RFC 3986 URI Generic Syntax January 2005
345 The following example URIs illustrate several URI schemes and
346 variations in their common syntax components:
348 ftp://ftp.is.co.za/rfc/rfc1808.txt
350 http://www.ietf.org/rfc/rfc2396.txt
352 ldap://[2001:db8::7]/c=GB?objectClass?one
354 mailto:John.Doe@example.com
356 news:comp.infosystems.www.servers.unix
360 telnet://192.0.2.16:80/
362 urn:oasis:names:specification:docbook:dtd:xml:4.1.2
365 1.1.3. URI, URL, and URN
367 A URI can be further classified as a locator, a name, or both. The
368 term "Uniform Resource Locator" (URL) refers to the subset of URIs
369 that, in addition to identifying a resource, provide a means of
370 locating the resource by describing its primary access mechanism
371 (e.g., its network "location"). The term "Uniform Resource Name"
372 (URN) has been used historically to refer to both URIs under the
373 "urn" scheme [RFC2141], which are required to remain globally unique
374 and persistent even when the resource ceases to exist or becomes
375 unavailable, and to any other URI with the properties of a name.
377 An individual scheme does not have to be classified as being just one
378 of "name" or "locator". Instances of URIs from any given scheme may
379 have the characteristics of names or locators or both, often
380 depending on the persistence and care in the assignment of
381 identifiers by the naming authority, rather than on any quality of
382 the scheme. Future specifications and related documentation should
383 use the general term "URI" rather than the more restrictive terms
384 "URL" and "URN" [RFC3305].
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396 RFC 3986 URI Generic Syntax January 2005
399 1.2. Design Considerations
403 The URI syntax has been designed with global transcription as one of
404 its main considerations. A URI is a sequence of characters from a
405 very limited set: the letters of the basic Latin alphabet, digits,
406 and a few special characters. A URI may be represented in a variety
407 of ways; e.g., ink on paper, pixels on a screen, or a sequence of
408 character encoding octets. The interpretation of a URI depends only
409 on the characters used and not on how those characters are
410 represented in a network protocol.
412 The goal of transcription can be described by a simple scenario.
413 Imagine two colleagues, Sam and Kim, sitting in a pub at an
414 international conference and exchanging research ideas. Sam asks Kim
415 for a location to get more information, so Kim writes the URI for the
416 research site on a napkin. Upon returning home, Sam takes out the
417 napkin and types the URI into a computer, which then retrieves the
418 information to which Kim referred.
420 There are several design considerations revealed by the scenario:
422 o A URI is a sequence of characters that is not always represented
423 as a sequence of octets.
425 o A URI might be transcribed from a non-network source and thus
426 should consist of characters that are most likely able to be
427 entered into a computer, within the constraints imposed by
428 keyboards (and related input devices) across languages and
431 o A URI often has to be remembered by people, and it is easier for
432 people to remember a URI when it consists of meaningful or
435 These design considerations are not always in alignment. For
436 example, it is often the case that the most meaningful name for a URI
437 component would require characters that cannot be typed into some
438 systems. The ability to transcribe a resource identifier from one
439 medium to another has been considered more important than having a
440 URI consist of the most meaningful of components.
442 In local or regional contexts and with improving technology, users
443 might benefit from being able to use a wider range of characters;
444 such use is not defined by this specification. Percent-encoded
445 octets (Section 2.1) may be used within a URI to represent characters
446 outside the range of the US-ASCII coded character set if this
450 Berners-Lee, et al. Standards Track [Page 8]
452 RFC 3986 URI Generic Syntax January 2005
455 representation is allowed by the scheme or by the protocol element in
456 which the URI is referenced. Such a definition should specify the
457 character encoding used to map those characters to octets prior to
458 being percent-encoded for the URI.
460 1.2.2. Separating Identification from Interaction
462 A common misunderstanding of URIs is that they are only used to refer
463 to accessible resources. The URI itself only provides
464 identification; access to the resource is neither guaranteed nor
465 implied by the presence of a URI. Instead, any operation associated
466 with a URI reference is defined by the protocol element, data format
467 attribute, or natural language text in which it appears.
469 Given a URI, a system may attempt to perform a variety of operations
470 on the resource, as might be characterized by words such as "access",
471 "update", "replace", or "find attributes". Such operations are
472 defined by the protocols that make use of URIs, not by this
473 specification. However, we do use a few general terms for describing
474 common operations on URIs. URI "resolution" is the process of
475 determining an access mechanism and the appropriate parameters
476 necessary to dereference a URI; this resolution may require several
477 iterations. To use that access mechanism to perform an action on the
478 URI's resource is to "dereference" the URI.
480 When URIs are used within information retrieval systems to identify
481 sources of information, the most common form of URI dereference is
482 "retrieval": making use of a URI in order to retrieve a
483 representation of its associated resource. A "representation" is a
484 sequence of octets, along with representation metadata describing
485 those octets, that constitutes a record of the state of the resource
486 at the time when the representation is generated. Retrieval is
487 achieved by a process that might include using the URI as a cache key
488 to check for a locally cached representation, resolution of the URI
489 to determine an appropriate access mechanism (if any), and
490 dereference of the URI for the sake of applying a retrieval
491 operation. Depending on the protocols used to perform the retrieval,
492 additional information might be supplied about the resource (resource
493 metadata) and its relation to other resources.
495 URI references in information retrieval systems are designed to be
496 late-binding: the result of an access is generally determined when it
497 is accessed and may vary over time or due to other aspects of the
498 interaction. These references are created in order to be used in the
499 future: what is being identified is not some specific result that was
500 obtained in the past, but rather some characteristic that is expected
501 to be true for future results. In such cases, the resource referred
502 to by the URI is actually a sameness of characteristics as observed
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508 RFC 3986 URI Generic Syntax January 2005
511 over time, perhaps elucidated by additional comments or assertions
512 made by the resource provider.
514 Although many URI schemes are named after protocols, this does not
515 imply that use of these URIs will result in access to the resource
516 via the named protocol. URIs are often used simply for the sake of
517 identification. Even when a URI is used to retrieve a representation
518 of a resource, that access might be through gateways, proxies,
519 caches, and name resolution services that are independent of the
520 protocol associated with the scheme name. The resolution of some
521 URIs may require the use of more than one protocol (e.g., both DNS
522 and HTTP are typically used to access an "http" URI's origin server
523 when a representation isn't found in a local cache).
525 1.2.3. Hierarchical Identifiers
527 The URI syntax is organized hierarchically, with components listed in
528 order of decreasing significance from left to right. For some URI
529 schemes, the visible hierarchy is limited to the scheme itself:
530 everything after the scheme component delimiter (":") is considered
531 opaque to URI processing. Other URI schemes make the hierarchy
532 explicit and visible to generic parsing algorithms.
534 The generic syntax uses the slash ("/"), question mark ("?"), and
535 number sign ("#") characters to delimit components that are
536 significant to the generic parser's hierarchical interpretation of an
537 identifier. In addition to aiding the readability of such
538 identifiers through the consistent use of familiar syntax, this
539 uniform representation of hierarchy across naming schemes allows
540 scheme-independent references to be made relative to that hierarchy.
542 It is often the case that a group or "tree" of documents has been
543 constructed to serve a common purpose, wherein the vast majority of
544 URI references in these documents point to resources within the tree
545 rather than outside it. Similarly, documents located at a particular
546 site are much more likely to refer to other resources at that site
547 than to resources at remote sites. Relative referencing of URIs
548 allows document trees to be partially independent of their location
549 and access scheme. For instance, it is possible for a single set of
550 hypertext documents to be simultaneously accessible and traversable
551 via each of the "file", "http", and "ftp" schemes if the documents
552 refer to each other with relative references. Furthermore, such
553 document trees can be moved, as a whole, without changing any of the
556 A relative reference (Section 4.2) refers to a resource by describing
557 the difference within a hierarchical name space between the reference
558 context and the target URI. The reference resolution algorithm,
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564 RFC 3986 URI Generic Syntax January 2005
567 presented in Section 5, defines how such a reference is transformed
568 to the target URI. As relative references can only be used within
569 the context of a hierarchical URI, designers of new URI schemes
570 should use a syntax consistent with the generic syntax's hierarchical
571 components unless there are compelling reasons to forbid relative
572 referencing within that scheme.
574 NOTE: Previous specifications used the terms "partial URI" and
575 "relative URI" to denote a relative reference to a URI. As some
576 readers misunderstood those terms to mean that relative URIs are a
577 subset of URIs rather than a method of referencing URIs, this
578 specification simply refers to them as relative references.
580 All URI references are parsed by generic syntax parsers when used.
581 However, because hierarchical processing has no effect on an absolute
582 URI used in a reference unless it contains one or more dot-segments
583 (complete path segments of "." or "..", as described in Section 3.3),
584 URI scheme specifications can define opaque identifiers by
585 disallowing use of slash characters, question mark characters, and
586 the URIs "scheme:." and "scheme:..".
590 This specification uses the Augmented Backus-Naur Form (ABNF)
591 notation of [RFC2234], including the following core ABNF syntax rules
592 defined by that specification: ALPHA (letters), CR (carriage return),
593 DIGIT (decimal digits), DQUOTE (double quote), HEXDIG (hexadecimal
594 digits), LF (line feed), and SP (space). The complete URI syntax is
595 collected in Appendix A.
599 The URI syntax provides a method of encoding data, presumably for the
600 sake of identifying a resource, as a sequence of characters. The URI
601 characters are, in turn, frequently encoded as octets for transport
602 or presentation. This specification does not mandate any particular
603 character encoding for mapping between URI characters and the octets
604 used to store or transmit those characters. When a URI appears in a
605 protocol element, the character encoding is defined by that protocol;
606 without such a definition, a URI is assumed to be in the same
607 character encoding as the surrounding text.
609 The ABNF notation defines its terminal values to be non-negative
610 integers (codepoints) based on the US-ASCII coded character set
611 [ASCII]. Because a URI is a sequence of characters, we must invert
612 that relation in order to understand the URI syntax. Therefore, the
618 Berners-Lee, et al. Standards Track [Page 11]
620 RFC 3986 URI Generic Syntax January 2005
623 integer values used by the ABNF must be mapped back to their
624 corresponding characters via US-ASCII in order to complete the syntax
627 A URI is composed from a limited set of characters consisting of
628 digits, letters, and a few graphic symbols. A reserved subset of
629 those characters may be used to delimit syntax components within a
630 URI while the remaining characters, including both the unreserved set
631 and those reserved characters not acting as delimiters, define each
632 component's identifying data.
634 2.1. Percent-Encoding
636 A percent-encoding mechanism is used to represent a data octet in a
637 component when that octet's corresponding character is outside the
638 allowed set or is being used as a delimiter of, or within, the
639 component. A percent-encoded octet is encoded as a character
640 triplet, consisting of the percent character "%" followed by the two
641 hexadecimal digits representing that octet's numeric value. For
642 example, "%20" is the percent-encoding for the binary octet
643 "00100000" (ABNF: %x20), which in US-ASCII corresponds to the space
644 character (SP). Section 2.4 describes when percent-encoding and
647 pct-encoded = "%" HEXDIG HEXDIG
649 The uppercase hexadecimal digits 'A' through 'F' are equivalent to
650 the lowercase digits 'a' through 'f', respectively. If two URIs
651 differ only in the case of hexadecimal digits used in percent-encoded
652 octets, they are equivalent. For consistency, URI producers and
653 normalizers should use uppercase hexadecimal digits for all percent-
656 2.2. Reserved Characters
658 URIs include components and subcomponents that are delimited by
659 characters in the "reserved" set. These characters are called
660 "reserved" because they may (or may not) be defined as delimiters by
661 the generic syntax, by each scheme-specific syntax, or by the
662 implementation-specific syntax of a URI's dereferencing algorithm.
663 If data for a URI component would conflict with a reserved
664 character's purpose as a delimiter, then the conflicting data must be
665 percent-encoded before the URI is formed.
674 Berners-Lee, et al. Standards Track [Page 12]
676 RFC 3986 URI Generic Syntax January 2005
679 reserved = gen-delims / sub-delims
681 gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@"
683 sub-delims = "!" / "$" / "&" / "'" / "(" / ")"
684 / "*" / "+" / "," / ";" / "="
686 The purpose of reserved characters is to provide a set of delimiting
687 characters that are distinguishable from other data within a URI.
688 URIs that differ in the replacement of a reserved character with its
689 corresponding percent-encoded octet are not equivalent. Percent-
690 encoding a reserved character, or decoding a percent-encoded octet
691 that corresponds to a reserved character, will change how the URI is
692 interpreted by most applications. Thus, characters in the reserved
693 set are protected from normalization and are therefore safe to be
694 used by scheme-specific and producer-specific algorithms for
695 delimiting data subcomponents within a URI.
697 A subset of the reserved characters (gen-delims) is used as
698 delimiters of the generic URI components described in Section 3. A
699 component's ABNF syntax rule will not use the reserved or gen-delims
700 rule names directly; instead, each syntax rule lists the characters
701 allowed within that component (i.e., not delimiting it), and any of
702 those characters that are also in the reserved set are "reserved" for
703 use as subcomponent delimiters within the component. Only the most
704 common subcomponents are defined by this specification; other
705 subcomponents may be defined by a URI scheme's specification, or by
706 the implementation-specific syntax of a URI's dereferencing
707 algorithm, provided that such subcomponents are delimited by
708 characters in the reserved set allowed within that component.
710 URI producing applications should percent-encode data octets that
711 correspond to characters in the reserved set unless these characters
712 are specifically allowed by the URI scheme to represent data in that
713 component. If a reserved character is found in a URI component and
714 no delimiting role is known for that character, then it must be
715 interpreted as representing the data octet corresponding to that
716 character's encoding in US-ASCII.
718 2.3. Unreserved Characters
720 Characters that are allowed in a URI but do not have a reserved
721 purpose are called unreserved. These include uppercase and lowercase
722 letters, decimal digits, hyphen, period, underscore, and tilde.
724 unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
730 Berners-Lee, et al. Standards Track [Page 13]
732 RFC 3986 URI Generic Syntax January 2005
735 URIs that differ in the replacement of an unreserved character with
736 its corresponding percent-encoded US-ASCII octet are equivalent: they
737 identify the same resource. However, URI comparison implementations
738 do not always perform normalization prior to comparison (see Section
739 6). For consistency, percent-encoded octets in the ranges of ALPHA
740 (%41-%5A and %61-%7A), DIGIT (%30-%39), hyphen (%2D), period (%2E),
741 underscore (%5F), or tilde (%7E) should not be created by URI
742 producers and, when found in a URI, should be decoded to their
743 corresponding unreserved characters by URI normalizers.
745 2.4. When to Encode or Decode
747 Under normal circumstances, the only time when octets within a URI
748 are percent-encoded is during the process of producing the URI from
749 its component parts. This is when an implementation determines which
750 of the reserved characters are to be used as subcomponent delimiters
751 and which can be safely used as data. Once produced, a URI is always
752 in its percent-encoded form.
754 When a URI is dereferenced, the components and subcomponents
755 significant to the scheme-specific dereferencing process (if any)
756 must be parsed and separated before the percent-encoded octets within
757 those components can be safely decoded, as otherwise the data may be
758 mistaken for component delimiters. The only exception is for
759 percent-encoded octets corresponding to characters in the unreserved
760 set, which can be decoded at any time. For example, the octet
761 corresponding to the tilde ("~") character is often encoded as "%7E"
762 by older URI processing implementations; the "%7E" can be replaced by
763 "~" without changing its interpretation.
765 Because the percent ("%") character serves as the indicator for
766 percent-encoded octets, it must be percent-encoded as "%25" for that
767 octet to be used as data within a URI. Implementations must not
768 percent-encode or decode the same string more than once, as decoding
769 an already decoded string might lead to misinterpreting a percent
770 data octet as the beginning of a percent-encoding, or vice versa in
771 the case of percent-encoding an already percent-encoded string.
773 2.5. Identifying Data
775 URI characters provide identifying data for each of the URI
776 components, serving as an external interface for identification
777 between systems. Although the presence and nature of the URI
778 production interface is hidden from clients that use its URIs (and is
779 thus beyond the scope of the interoperability requirements defined by
780 this specification), it is a frequent source of confusion and errors
781 in the interpretation of URI character issues. Implementers have to
782 be aware that there are multiple character encodings involved in the
786 Berners-Lee, et al. Standards Track [Page 14]
788 RFC 3986 URI Generic Syntax January 2005
791 production and transmission of URIs: local name and data encoding,
792 public interface encoding, URI character encoding, data format
793 encoding, and protocol encoding.
795 Local names, such as file system names, are stored with a local
796 character encoding. URI producing applications (e.g., origin
797 servers) will typically use the local encoding as the basis for
798 producing meaningful names. The URI producer will transform the
799 local encoding to one that is suitable for a public interface and
800 then transform the public interface encoding into the restricted set
801 of URI characters (reserved, unreserved, and percent-encodings).
802 Those characters are, in turn, encoded as octets to be used as a
803 reference within a data format (e.g., a document charset), and such
804 data formats are often subsequently encoded for transmission over
807 For most systems, an unreserved character appearing within a URI
808 component is interpreted as representing the data octet corresponding
809 to that character's encoding in US-ASCII. Consumers of URIs assume
810 that the letter "X" corresponds to the octet "01011000", and even
811 when that assumption is incorrect, there is no harm in making it. A
812 system that internally provides identifiers in the form of a
813 different character encoding, such as EBCDIC, will generally perform
814 character translation of textual identifiers to UTF-8 [STD63] (or
815 some other superset of the US-ASCII character encoding) at an
816 internal interface, thereby providing more meaningful identifiers
817 than those resulting from simply percent-encoding the original
820 For example, consider an information service that provides data,
821 stored locally using an EBCDIC-based file system, to clients on the
822 Internet through an HTTP server. When an author creates a file with
823 the name "Laguna Beach" on that file system, the "http" URI
824 corresponding to that resource is expected to contain the meaningful
825 string "Laguna%20Beach". If, however, that server produces URIs by
826 using an overly simplistic raw octet mapping, then the result would
827 be a URI containing "%D3%81%87%A4%95%81@%C2%85%81%83%88". An
828 internal transcoding interface fixes this problem by transcoding the
829 local name to a superset of US-ASCII prior to producing the URI.
830 Naturally, proper interpretation of an incoming URI on such an
831 interface requires that percent-encoded octets be decoded (e.g.,
832 "%20" to SP) before the reverse transcoding is applied to obtain the
835 In some cases, the internal interface between a URI component and the
836 identifying data that it has been crafted to represent is much less
837 direct than a character encoding translation. For example, portions
838 of a URI might reflect a query on non-ASCII data, or numeric
842 Berners-Lee, et al. Standards Track [Page 15]
844 RFC 3986 URI Generic Syntax January 2005
847 coordinates on a map. Likewise, a URI scheme may define components
848 with additional encoding requirements that are applied prior to
849 forming the component and producing the URI.
851 When a new URI scheme defines a component that represents textual
852 data consisting of characters from the Universal Character Set [UCS],
853 the data should first be encoded as octets according to the UTF-8
854 character encoding [STD63]; then only those octets that do not
855 correspond to characters in the unreserved set should be percent-
856 encoded. For example, the character A would be represented as "A",
857 the character LATIN CAPITAL LETTER A WITH GRAVE would be represented
858 as "%C3%80", and the character KATAKANA LETTER A would be represented
863 The generic URI syntax consists of a hierarchical sequence of
864 components referred to as the scheme, authority, path, query, and
867 URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ]
869 hier-part = "//" authority path-abempty
874 The scheme and path components are required, though the path may be
875 empty (no characters). When authority is present, the path must
876 either be empty or begin with a slash ("/") character. When
877 authority is not present, the path cannot begin with two slash
878 characters ("//"). These restrictions result in five different ABNF
879 rules for a path (Section 3.3), only one of which will match any
882 The following are two example URIs and their component parts:
884 foo://example.com:8042/over/there?name=ferret#nose
885 \_/ \______________/\_________/ \_________/ \__/
887 scheme authority path query fragment
888 | _____________________|__
890 urn:example:animal:ferret:nose
898 Berners-Lee, et al. Standards Track [Page 16]
900 RFC 3986 URI Generic Syntax January 2005
905 Each URI begins with a scheme name that refers to a specification for
906 assigning identifiers within that scheme. As such, the URI syntax is
907 a federated and extensible naming system wherein each scheme's
908 specification may further restrict the syntax and semantics of
909 identifiers using that scheme.
911 Scheme names consist of a sequence of characters beginning with a
912 letter and followed by any combination of letters, digits, plus
913 ("+"), period ("."), or hyphen ("-"). Although schemes are case-
914 insensitive, the canonical form is lowercase and documents that
915 specify schemes must do so with lowercase letters. An implementation
916 should accept uppercase letters as equivalent to lowercase in scheme
917 names (e.g., allow "HTTP" as well as "http") for the sake of
918 robustness but should only produce lowercase scheme names for
921 scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
923 Individual schemes are not specified by this document. The process
924 for registration of new URI schemes is defined separately by [BCP35].
925 The scheme registry maintains the mapping between scheme names and
926 their specifications. Advice for designers of new URI schemes can be
927 found in [RFC2718]. URI scheme specifications must define their own
928 syntax so that all strings matching their scheme-specific syntax will
929 also match the <absolute-URI> grammar, as described in Section 4.3.
931 When presented with a URI that violates one or more scheme-specific
932 restrictions, the scheme-specific resolution process should flag the
933 reference as an error rather than ignore the unused parts; doing so
934 reduces the number of equivalent URIs and helps detect abuses of the
935 generic syntax, which might indicate that the URI has been
936 constructed to mislead the user (Section 7.6).
940 Many URI schemes include a hierarchical element for a naming
941 authority so that governance of the name space defined by the
942 remainder of the URI is delegated to that authority (which may, in
943 turn, delegate it further). The generic syntax provides a common
944 means for distinguishing an authority based on a registered name or
945 server address, along with optional port and user information.
947 The authority component is preceded by a double slash ("//") and is
948 terminated by the next slash ("/"), question mark ("?"), or number
949 sign ("#") character, or by the end of the URI.
954 Berners-Lee, et al. Standards Track [Page 17]
956 RFC 3986 URI Generic Syntax January 2005
959 authority = [ userinfo "@" ] host [ ":" port ]
961 URI producers and normalizers should omit the ":" delimiter that
962 separates host from port if the port component is empty. Some
963 schemes do not allow the userinfo and/or port subcomponents.
965 If a URI contains an authority component, then the path component
966 must either be empty or begin with a slash ("/") character. Non-
967 validating parsers (those that merely separate a URI reference into
968 its major components) will often ignore the subcomponent structure of
969 authority, treating it as an opaque string from the double-slash to
970 the first terminating delimiter, until such time as the URI is
973 3.2.1. User Information
975 The userinfo subcomponent may consist of a user name and, optionally,
976 scheme-specific information about how to gain authorization to access
977 the resource. The user information, if present, is followed by a
978 commercial at-sign ("@") that delimits it from the host.
980 userinfo = *( unreserved / pct-encoded / sub-delims / ":" )
982 Use of the format "user:password" in the userinfo field is
983 deprecated. Applications should not render as clear text any data
984 after the first colon (":") character found within a userinfo
985 subcomponent unless the data after the colon is the empty string
986 (indicating no password). Applications may choose to ignore or
987 reject such data when it is received as part of a reference and
988 should reject the storage of such data in unencrypted form. The
989 passing of authentication information in clear text has proven to be
990 a security risk in almost every case where it has been used.
992 Applications that render a URI for the sake of user feedback, such as
993 in graphical hypertext browsing, should render userinfo in a way that
994 is distinguished from the rest of a URI, when feasible. Such
995 rendering will assist the user in cases where the userinfo has been
996 misleadingly crafted to look like a trusted domain name
1001 The host subcomponent of authority is identified by an IP literal
1002 encapsulated within square brackets, an IPv4 address in dotted-
1003 decimal form, or a registered name. The host subcomponent is case-
1004 insensitive. The presence of a host subcomponent within a URI does
1005 not imply that the scheme requires access to the given host on the
1006 Internet. In many cases, the host syntax is used only for the sake
1010 Berners-Lee, et al. Standards Track [Page 18]
1012 RFC 3986 URI Generic Syntax January 2005
1015 of reusing the existing registration process created and deployed for
1016 DNS, thus obtaining a globally unique name without the cost of
1017 deploying another registry. However, such use comes with its own
1018 costs: domain name ownership may change over time for reasons not
1019 anticipated by the URI producer. In other cases, the data within the
1020 host component identifies a registered name that has nothing to do
1021 with an Internet host. We use the name "host" for the ABNF rule
1022 because that is its most common purpose, not its only purpose.
1024 host = IP-literal / IPv4address / reg-name
1026 The syntax rule for host is ambiguous because it does not completely
1027 distinguish between an IPv4address and a reg-name. In order to
1028 disambiguate the syntax, we apply the "first-match-wins" algorithm:
1029 If host matches the rule for IPv4address, then it should be
1030 considered an IPv4 address literal and not a reg-name. Although host
1031 is case-insensitive, producers and normalizers should use lowercase
1032 for registered names and hexadecimal addresses for the sake of
1033 uniformity, while only using uppercase letters for percent-encodings.
1035 A host identified by an Internet Protocol literal address, version 6
1036 [RFC3513] or later, is distinguished by enclosing the IP literal
1037 within square brackets ("[" and "]"). This is the only place where
1038 square bracket characters are allowed in the URI syntax. In
1039 anticipation of future, as-yet-undefined IP literal address formats,
1040 an implementation may use an optional version flag to indicate such a
1041 format explicitly rather than rely on heuristic determination.
1043 IP-literal = "[" ( IPv6address / IPvFuture ) "]"
1045 IPvFuture = "v" 1*HEXDIG "." 1*( unreserved / sub-delims / ":" )
1047 The version flag does not indicate the IP version; rather, it
1048 indicates future versions of the literal format. As such,
1049 implementations must not provide the version flag for the existing
1050 IPv4 and IPv6 literal address forms described below. If a URI
1051 containing an IP-literal that starts with "v" (case-insensitive),
1052 indicating that the version flag is present, is dereferenced by an
1053 application that does not know the meaning of that version flag, then
1054 the application should return an appropriate error for "address
1055 mechanism not supported".
1057 A host identified by an IPv6 literal address is represented inside
1058 the square brackets without a preceding version flag. The ABNF
1059 provided here is a translation of the text definition of an IPv6
1060 literal address provided in [RFC3513]. This syntax does not support
1061 IPv6 scoped addressing zone identifiers.
1066 Berners-Lee, et al. Standards Track [Page 19]
1068 RFC 3986 URI Generic Syntax January 2005
1071 A 128-bit IPv6 address is divided into eight 16-bit pieces. Each
1072 piece is represented numerically in case-insensitive hexadecimal,
1073 using one to four hexadecimal digits (leading zeroes are permitted).
1074 The eight encoded pieces are given most-significant first, separated
1075 by colon characters. Optionally, the least-significant two pieces
1076 may instead be represented in IPv4 address textual format. A
1077 sequence of one or more consecutive zero-valued 16-bit pieces within
1078 the address may be elided, omitting all their digits and leaving
1079 exactly two consecutive colons in their place to mark the elision.
1081 IPv6address = 6( h16 ":" ) ls32
1082 / "::" 5( h16 ":" ) ls32
1083 / [ h16 ] "::" 4( h16 ":" ) ls32
1084 / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
1085 / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
1086 / [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32
1087 / [ *4( h16 ":" ) h16 ] "::" ls32
1088 / [ *5( h16 ":" ) h16 ] "::" h16
1089 / [ *6( h16 ":" ) h16 ] "::"
1091 ls32 = ( h16 ":" h16 ) / IPv4address
1092 ; least-significant 32 bits of address
1095 ; 16 bits of address represented in hexadecimal
1097 A host identified by an IPv4 literal address is represented in
1098 dotted-decimal notation (a sequence of four decimal numbers in the
1099 range 0 to 255, separated by "."), as described in [RFC1123] by
1100 reference to [RFC0952]. Note that other forms of dotted notation may
1101 be interpreted on some platforms, as described in Section 7.4, but
1102 only the dotted-decimal form of four octets is allowed by this
1105 IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet
1107 dec-octet = DIGIT ; 0-9
1108 / %x31-39 DIGIT ; 10-99
1109 / "1" 2DIGIT ; 100-199
1110 / "2" %x30-34 DIGIT ; 200-249
1111 / "25" %x30-35 ; 250-255
1113 A host identified by a registered name is a sequence of characters
1114 usually intended for lookup within a locally defined host or service
1115 name registry, though the URI's scheme-specific semantics may require
1116 that a specific registry (or fixed name table) be used instead. The
1117 most common name registry mechanism is the Domain Name System (DNS).
1118 A registered name intended for lookup in the DNS uses the syntax
1122 Berners-Lee, et al. Standards Track [Page 20]
1124 RFC 3986 URI Generic Syntax January 2005
1127 defined in Section 3.5 of [RFC1034] and Section 2.1 of [RFC1123].
1128 Such a name consists of a sequence of domain labels separated by ".",
1129 each domain label starting and ending with an alphanumeric character
1130 and possibly also containing "-" characters. The rightmost domain
1131 label of a fully qualified domain name in DNS may be followed by a
1132 single "." and should be if it is necessary to distinguish between
1133 the complete domain name and some local domain.
1135 reg-name = *( unreserved / pct-encoded / sub-delims )
1137 If the URI scheme defines a default for host, then that default
1138 applies when the host subcomponent is undefined or when the
1139 registered name is empty (zero length). For example, the "file" URI
1140 scheme is defined so that no authority, an empty host, and
1141 "localhost" all mean the end-user's machine, whereas the "http"
1142 scheme considers a missing authority or empty host invalid.
1144 This specification does not mandate a particular registered name
1145 lookup technology and therefore does not restrict the syntax of reg-
1146 name beyond what is necessary for interoperability. Instead, it
1147 delegates the issue of registered name syntax conformance to the
1148 operating system of each application performing URI resolution, and
1149 that operating system decides what it will allow for the purpose of
1150 host identification. A URI resolution implementation might use DNS,
1151 host tables, yellow pages, NetInfo, WINS, or any other system for
1152 lookup of registered names. However, a globally scoped naming
1153 system, such as DNS fully qualified domain names, is necessary for
1154 URIs intended to have global scope. URI producers should use names
1155 that conform to the DNS syntax, even when use of DNS is not
1156 immediately apparent, and should limit these names to no more than
1157 255 characters in length.
1159 The reg-name syntax allows percent-encoded octets in order to
1160 represent non-ASCII registered names in a uniform way that is
1161 independent of the underlying name resolution technology. Non-ASCII
1162 characters must first be encoded according to UTF-8 [STD63], and then
1163 each octet of the corresponding UTF-8 sequence must be percent-
1164 encoded to be represented as URI characters. URI producing
1165 applications must not use percent-encoding in host unless it is used
1166 to represent a UTF-8 character sequence. When a non-ASCII registered
1167 name represents an internationalized domain name intended for
1168 resolution via the DNS, the name must be transformed to the IDNA
1169 encoding [RFC3490] prior to name lookup. URI producers should
1170 provide these registered names in the IDNA encoding, rather than a
1171 percent-encoding, if they wish to maximize interoperability with
1172 legacy URI resolvers.
1178 Berners-Lee, et al. Standards Track [Page 21]
1180 RFC 3986 URI Generic Syntax January 2005
1185 The port subcomponent of authority is designated by an optional port
1186 number in decimal following the host and delimited from it by a
1187 single colon (":") character.
1191 A scheme may define a default port. For example, the "http" scheme
1192 defines a default port of "80", corresponding to its reserved TCP
1193 port number. The type of port designated by the port number (e.g.,
1194 TCP, UDP, SCTP) is defined by the URI scheme. URI producers and
1195 normalizers should omit the port component and its ":" delimiter if
1196 port is empty or if its value would be the same as that of the
1201 The path component contains data, usually organized in hierarchical
1202 form, that, along with data in the non-hierarchical query component
1203 (Section 3.4), serves to identify a resource within the scope of the
1204 URI's scheme and naming authority (if any). The path is terminated
1205 by the first question mark ("?") or number sign ("#") character, or
1206 by the end of the URI.
1208 If a URI contains an authority component, then the path component
1209 must either be empty or begin with a slash ("/") character. If a URI
1210 does not contain an authority component, then the path cannot begin
1211 with two slash characters ("//"). In addition, a URI reference
1212 (Section 4.1) may be a relative-path reference, in which case the
1213 first path segment cannot contain a colon (":") character. The ABNF
1214 requires five separate rules to disambiguate these cases, only one of
1215 which will match the path substring within a given URI reference. We
1216 use the generic term "path component" to describe the URI substring
1217 matched by the parser to one of these rules.
1219 path = path-abempty ; begins with "/" or is empty
1220 / path-absolute ; begins with "/" but not "//"
1221 / path-noscheme ; begins with a non-colon segment
1222 / path-rootless ; begins with a segment
1223 / path-empty ; zero characters
1225 path-abempty = *( "/" segment )
1226 path-absolute = "/" [ segment-nz *( "/" segment ) ]
1227 path-noscheme = segment-nz-nc *( "/" segment )
1228 path-rootless = segment-nz *( "/" segment )
1229 path-empty = 0<pchar>
1234 Berners-Lee, et al. Standards Track [Page 22]
1236 RFC 3986 URI Generic Syntax January 2005
1240 segment-nz = 1*pchar
1241 segment-nz-nc = 1*( unreserved / pct-encoded / sub-delims / "@" )
1242 ; non-zero-length segment without any colon ":"
1244 pchar = unreserved / pct-encoded / sub-delims / ":" / "@"
1246 A path consists of a sequence of path segments separated by a slash
1247 ("/") character. A path is always defined for a URI, though the
1248 defined path may be empty (zero length). Use of the slash character
1249 to indicate hierarchy is only required when a URI will be used as the
1250 context for relative references. For example, the URI
1251 <mailto:fred@example.com> has a path of "fred@example.com", whereas
1252 the URI <foo://info.example.com?fred> has an empty path.
1254 The path segments "." and "..", also known as dot-segments, are
1255 defined for relative reference within the path name hierarchy. They
1256 are intended for use at the beginning of a relative-path reference
1257 (Section 4.2) to indicate relative position within the hierarchical
1258 tree of names. This is similar to their role within some operating
1259 systems' file directory structures to indicate the current directory
1260 and parent directory, respectively. However, unlike in a file
1261 system, these dot-segments are only interpreted within the URI path
1262 hierarchy and are removed as part of the resolution process (Section
1265 Aside from dot-segments in hierarchical paths, a path segment is
1266 considered opaque by the generic syntax. URI producing applications
1267 often use the reserved characters allowed in a segment to delimit
1268 scheme-specific or dereference-handler-specific subcomponents. For
1269 example, the semicolon (";") and equals ("=") reserved characters are
1270 often used to delimit parameters and parameter values applicable to
1271 that segment. The comma (",") reserved character is often used for
1272 similar purposes. For example, one URI producer might use a segment
1273 such as "name;v=1.1" to indicate a reference to version 1.1 of
1274 "name", whereas another might use a segment such as "name,1.1" to
1275 indicate the same. Parameter types may be defined by scheme-specific
1276 semantics, but in most cases the syntax of a parameter is specific to
1277 the implementation of the URI's dereferencing algorithm.
1281 The query component contains non-hierarchical data that, along with
1282 data in the path component (Section 3.3), serves to identify a
1283 resource within the scope of the URI's scheme and naming authority
1284 (if any). The query component is indicated by the first question
1285 mark ("?") character and terminated by a number sign ("#") character
1286 or by the end of the URI.
1290 Berners-Lee, et al. Standards Track [Page 23]
1292 RFC 3986 URI Generic Syntax January 2005
1295 query = *( pchar / "/" / "?" )
1297 The characters slash ("/") and question mark ("?") may represent data
1298 within the query component. Beware that some older, erroneous
1299 implementations may not handle such data correctly when it is used as
1300 the base URI for relative references (Section 5.1), apparently
1301 because they fail to distinguish query data from path data when
1302 looking for hierarchical separators. However, as query components
1303 are often used to carry identifying information in the form of
1304 "key=value" pairs and one frequently used value is a reference to
1305 another URI, it is sometimes better for usability to avoid percent-
1306 encoding those characters.
1310 The fragment identifier component of a URI allows indirect
1311 identification of a secondary resource by reference to a primary
1312 resource and additional identifying information. The identified
1313 secondary resource may be some portion or subset of the primary
1314 resource, some view on representations of the primary resource, or
1315 some other resource defined or described by those representations. A
1316 fragment identifier component is indicated by the presence of a
1317 number sign ("#") character and terminated by the end of the URI.
1319 fragment = *( pchar / "/" / "?" )
1321 The semantics of a fragment identifier are defined by the set of
1322 representations that might result from a retrieval action on the
1323 primary resource. The fragment's format and resolution is therefore
1324 dependent on the media type [RFC2046] of a potentially retrieved
1325 representation, even though such a retrieval is only performed if the
1326 URI is dereferenced. If no such representation exists, then the
1327 semantics of the fragment are considered unknown and are effectively
1328 unconstrained. Fragment identifier semantics are independent of the
1329 URI scheme and thus cannot be redefined by scheme specifications.
1331 Individual media types may define their own restrictions on or
1332 structures within the fragment identifier syntax for specifying
1333 different types of subsets, views, or external references that are
1334 identifiable as secondary resources by that media type. If the
1335 primary resource has multiple representations, as is often the case
1336 for resources whose representation is selected based on attributes of
1337 the retrieval request (a.k.a., content negotiation), then whatever is
1338 identified by the fragment should be consistent across all of those
1339 representations. Each representation should either define the
1340 fragment so that it corresponds to the same secondary resource,
1341 regardless of how it is represented, or should leave the fragment
1342 undefined (i.e., not found).
1346 Berners-Lee, et al. Standards Track [Page 24]
1348 RFC 3986 URI Generic Syntax January 2005
1351 As with any URI, use of a fragment identifier component does not
1352 imply that a retrieval action will take place. A URI with a fragment
1353 identifier may be used to refer to the secondary resource without any
1354 implication that the primary resource is accessible or will ever be
1357 Fragment identifiers have a special role in information retrieval
1358 systems as the primary form of client-side indirect referencing,
1359 allowing an author to specifically identify aspects of an existing
1360 resource that are only indirectly provided by the resource owner. As
1361 such, the fragment identifier is not used in the scheme-specific
1362 processing of a URI; instead, the fragment identifier is separated
1363 from the rest of the URI prior to a dereference, and thus the
1364 identifying information within the fragment itself is dereferenced
1365 solely by the user agent, regardless of the URI scheme. Although
1366 this separate handling is often perceived to be a loss of
1367 information, particularly for accurate redirection of references as
1368 resources move over time, it also serves to prevent information
1369 providers from denying reference authors the right to refer to
1370 information within a resource selectively. Indirect referencing also
1371 provides additional flexibility and extensibility to systems that use
1372 URIs, as new media types are easier to define and deploy than new
1373 schemes of identification.
1375 The characters slash ("/") and question mark ("?") are allowed to
1376 represent data within the fragment identifier. Beware that some
1377 older, erroneous implementations may not handle this data correctly
1378 when it is used as the base URI for relative references (Section
1383 When applications make reference to a URI, they do not always use the
1384 full form of reference defined by the "URI" syntax rule. To save
1385 space and take advantage of hierarchical locality, many Internet
1386 protocol elements and media type formats allow an abbreviation of a
1387 URI, whereas others restrict the syntax to a particular form of URI.
1388 We define the most common forms of reference syntax in this
1389 specification because they impact and depend upon the design of the
1390 generic syntax, requiring a uniform parsing algorithm in order to be
1391 interpreted consistently.
1395 URI-reference is used to denote the most common usage of a resource
1398 URI-reference = URI / relative-ref
1402 Berners-Lee, et al. Standards Track [Page 25]
1404 RFC 3986 URI Generic Syntax January 2005
1407 A URI-reference is either a URI or a relative reference. If the
1408 URI-reference's prefix does not match the syntax of a scheme followed
1409 by its colon separator, then the URI-reference is a relative
1412 A URI-reference is typically parsed first into the five URI
1413 components, in order to determine what components are present and
1414 whether the reference is relative. Then, each component is parsed
1415 for its subparts and their validation. The ABNF of URI-reference,
1416 along with the "first-match-wins" disambiguation rule, is sufficient
1417 to define a validating parser for the generic syntax. Readers
1418 familiar with regular expressions should see Appendix B for an
1419 example of a non-validating URI-reference parser that will take any
1420 given string and extract the URI components.
1422 4.2. Relative Reference
1424 A relative reference takes advantage of the hierarchical syntax
1425 (Section 1.2.3) to express a URI reference relative to the name space
1426 of another hierarchical URI.
1428 relative-ref = relative-part [ "?" query ] [ "#" fragment ]
1430 relative-part = "//" authority path-abempty
1435 The URI referred to by a relative reference, also known as the target
1436 URI, is obtained by applying the reference resolution algorithm of
1439 A relative reference that begins with two slash characters is termed
1440 a network-path reference; such references are rarely used. A
1441 relative reference that begins with a single slash character is
1442 termed an absolute-path reference. A relative reference that does
1443 not begin with a slash character is termed a relative-path reference.
1445 A path segment that contains a colon character (e.g., "this:that")
1446 cannot be used as the first segment of a relative-path reference, as
1447 it would be mistaken for a scheme name. Such a segment must be
1448 preceded by a dot-segment (e.g., "./this:that") to make a relative-
1458 Berners-Lee, et al. Standards Track [Page 26]
1460 RFC 3986 URI Generic Syntax January 2005
1465 Some protocol elements allow only the absolute form of a URI without
1466 a fragment identifier. For example, defining a base URI for later
1467 use by relative references calls for an absolute-URI syntax rule that
1468 does not allow a fragment.
1470 absolute-URI = scheme ":" hier-part [ "?" query ]
1472 URI scheme specifications must define their own syntax so that all
1473 strings matching their scheme-specific syntax will also match the
1474 <absolute-URI> grammar. Scheme specifications will not define
1475 fragment identifier syntax or usage, regardless of its applicability
1476 to resources identifiable via that scheme, as fragment identification
1477 is orthogonal to scheme definition. However, scheme specifications
1478 are encouraged to include a wide range of examples, including
1479 examples that show use of the scheme's URIs with fragment identifiers
1480 when such usage is appropriate.
1482 4.4. Same-Document Reference
1484 When a URI reference refers to a URI that is, aside from its fragment
1485 component (if any), identical to the base URI (Section 5.1), that
1486 reference is called a "same-document" reference. The most frequent
1487 examples of same-document references are relative references that are
1488 empty or include only the number sign ("#") separator followed by a
1489 fragment identifier.
1491 When a same-document reference is dereferenced for a retrieval
1492 action, the target of that reference is defined to be within the same
1493 entity (representation, document, or message) as the reference;
1494 therefore, a dereference should not result in a new retrieval action.
1496 Normalization of the base and target URIs prior to their comparison,
1497 as described in Sections 6.2.2 and 6.2.3, is allowed but rarely
1498 performed in practice. Normalization may increase the set of same-
1499 document references, which may be of benefit to some caching
1500 applications. As such, reference authors should not assume that a
1501 slightly different, though equivalent, reference URI will (or will
1502 not) be interpreted as a same-document reference by any given
1505 4.5. Suffix Reference
1507 The URI syntax is designed for unambiguous reference to resources and
1508 extensibility via the URI scheme. However, as URI identification and
1509 usage have become commonplace, traditional media (television, radio,
1510 newspapers, billboards, etc.) have increasingly used a suffix of the
1514 Berners-Lee, et al. Standards Track [Page 27]
1516 RFC 3986 URI Generic Syntax January 2005
1519 URI as a reference, consisting of only the authority and path
1520 portions of the URI, such as
1522 www.w3.org/Addressing/
1524 or simply a DNS registered name on its own. Such references are
1525 primarily intended for human interpretation rather than for machines,
1526 with the assumption that context-based heuristics are sufficient to
1527 complete the URI (e.g., most registered names beginning with "www"
1528 are likely to have a URI prefix of "http://"). Although there is no
1529 standard set of heuristics for disambiguating a URI suffix, many
1530 client implementations allow them to be entered by the user and
1531 heuristically resolved.
1533 Although this practice of using suffix references is common, it
1534 should be avoided whenever possible and should never be used in
1535 situations where long-term references are expected. The heuristics
1536 noted above will change over time, particularly when a new URI scheme
1537 becomes popular, and are often incorrect when used out of context.
1538 Furthermore, they can lead to security issues along the lines of
1539 those described in [RFC1535].
1541 As a URI suffix has the same syntax as a relative-path reference, a
1542 suffix reference cannot be used in contexts where a relative
1543 reference is expected. As a result, suffix references are limited to
1544 places where there is no defined base URI, such as dialog boxes and
1545 off-line advertisements.
1547 5. Reference Resolution
1549 This section defines the process of resolving a URI reference within
1550 a context that allows relative references so that the result is a
1551 string matching the <URI> syntax rule of Section 3.
1553 5.1. Establishing a Base URI
1555 The term "relative" implies that a "base URI" exists against which
1556 the relative reference is applied. Aside from fragment-only
1557 references (Section 4.4), relative references are only usable when a
1558 base URI is known. A base URI must be established by the parser
1559 prior to parsing URI references that might be relative. A base URI
1560 must conform to the <absolute-URI> syntax rule (Section 4.3). If the
1561 base URI is obtained from a URI reference, then that reference must
1562 be converted to absolute form and stripped of any fragment component
1563 prior to its use as a base URI.
1570 Berners-Lee, et al. Standards Track [Page 28]
1572 RFC 3986 URI Generic Syntax January 2005
1575 The base URI of a reference can be established in one of four ways,
1576 discussed below in order of precedence. The order of precedence can
1577 be thought of in terms of layers, where the innermost defined base
1578 URI has the highest precedence. This can be visualized graphically
1581 .----------------------------------------------------------.
1582 | .----------------------------------------------------. |
1583 | | .----------------------------------------------. | |
1584 | | | .----------------------------------------. | | |
1585 | | | | .----------------------------------. | | | |
1586 | | | | | <relative-reference> | | | | |
1587 | | | | `----------------------------------' | | | |
1588 | | | | (5.1.1) Base URI embedded in content | | | |
1589 | | | `----------------------------------------' | | |
1590 | | | (5.1.2) Base URI of the encapsulating entity | | |
1591 | | | (message, representation, or none) | | |
1592 | | `----------------------------------------------' | |
1593 | | (5.1.3) URI used to retrieve the entity | |
1594 | `----------------------------------------------------' |
1595 | (5.1.4) Default Base URI (application-dependent) |
1596 `----------------------------------------------------------'
1598 5.1.1. Base URI Embedded in Content
1600 Within certain media types, a base URI for relative references can be
1601 embedded within the content itself so that it can be readily obtained
1602 by a parser. This can be useful for descriptive documents, such as
1603 tables of contents, which may be transmitted to others through
1604 protocols other than their usual retrieval context (e.g., email or
1607 It is beyond the scope of this specification to specify how, for each
1608 media type, a base URI can be embedded. The appropriate syntax, when
1609 available, is described by the data format specification associated
1610 with each media type.
1612 5.1.2. Base URI from the Encapsulating Entity
1614 If no base URI is embedded, the base URI is defined by the
1615 representation's retrieval context. For a document that is enclosed
1616 within another entity, such as a message or archive, the retrieval
1617 context is that entity. Thus, the default base URI of a
1618 representation is the base URI of the entity in which the
1619 representation is encapsulated.
1626 Berners-Lee, et al. Standards Track [Page 29]
1628 RFC 3986 URI Generic Syntax January 2005
1631 A mechanism for embedding a base URI within MIME container types
1632 (e.g., the message and multipart types) is defined by MHTML
1633 [RFC2557]. Protocols that do not use the MIME message header syntax,
1634 but that do allow some form of tagged metadata to be included within
1635 messages, may define their own syntax for defining a base URI as part
1638 5.1.3. Base URI from the Retrieval URI
1640 If no base URI is embedded and the representation is not encapsulated
1641 within some other entity, then, if a URI was used to retrieve the
1642 representation, that URI shall be considered the base URI. Note that
1643 if the retrieval was the result of a redirected request, the last URI
1644 used (i.e., the URI that resulted in the actual retrieval of the
1645 representation) is the base URI.
1647 5.1.4. Default Base URI
1649 If none of the conditions described above apply, then the base URI is
1650 defined by the context of the application. As this definition is
1651 necessarily application-dependent, failing to define a base URI by
1652 using one of the other methods may result in the same content being
1653 interpreted differently by different types of applications.
1655 A sender of a representation containing relative references is
1656 responsible for ensuring that a base URI for those references can be
1657 established. Aside from fragment-only references, relative
1658 references can only be used reliably in situations where the base URI
1661 5.2. Relative Resolution
1663 This section describes an algorithm for converting a URI reference
1664 that might be relative to a given base URI into the parsed components
1665 of the reference's target. The components can then be recomposed, as
1666 described in Section 5.3, to form the target URI. This algorithm
1667 provides definitive results that can be used to test the output of
1668 other implementations. Applications may implement relative reference
1669 resolution by using some other algorithm, provided that the results
1670 match what would be given by this one.
1682 Berners-Lee, et al. Standards Track [Page 30]
1684 RFC 3986 URI Generic Syntax January 2005
1687 5.2.1. Pre-parse the Base URI
1689 The base URI (Base) is established according to the procedure of
1690 Section 5.1 and parsed into the five main components described in
1691 Section 3. Note that only the scheme component is required to be
1692 present in a base URI; the other components may be empty or
1693 undefined. A component is undefined if its associated delimiter does
1694 not appear in the URI reference; the path component is never
1695 undefined, though it may be empty.
1697 Normalization of the base URI, as described in Sections 6.2.2 and
1698 6.2.3, is optional. A URI reference must be transformed to its
1699 target URI before it can be normalized.
1701 5.2.2. Transform References
1703 For each URI reference (R), the following pseudocode describes an
1704 algorithm for transforming R into its target URI (T):
1706 -- The URI reference is parsed into the five URI components
1708 (R.scheme, R.authority, R.path, R.query, R.fragment) = parse(R);
1710 -- A non-strict parser may ignore a scheme in the reference
1711 -- if it is identical to the base URI's scheme.
1713 if ((not strict) and (R.scheme == Base.scheme)) then
1738 Berners-Lee, et al. Standards Track [Page 31]
1740 RFC 3986 URI Generic Syntax January 2005
1743 if defined(R.scheme) then
1744 T.scheme = R.scheme;
1745 T.authority = R.authority;
1746 T.path = remove_dot_segments(R.path);
1749 if defined(R.authority) then
1750 T.authority = R.authority;
1751 T.path = remove_dot_segments(R.path);
1754 if (R.path == "") then
1756 if defined(R.query) then
1759 T.query = Base.query;
1762 if (R.path starts-with "/") then
1763 T.path = remove_dot_segments(R.path);
1765 T.path = merge(Base.path, R.path);
1766 T.path = remove_dot_segments(T.path);
1770 T.authority = Base.authority;
1772 T.scheme = Base.scheme;
1775 T.fragment = R.fragment;
1779 The pseudocode above refers to a "merge" routine for merging a
1780 relative-path reference with the path of the base URI. This is
1781 accomplished as follows:
1783 o If the base URI has a defined authority component and an empty
1784 path, then return a string consisting of "/" concatenated with the
1785 reference's path; otherwise,
1794 Berners-Lee, et al. Standards Track [Page 32]
1796 RFC 3986 URI Generic Syntax January 2005
1799 o return a string consisting of the reference's path component
1800 appended to all but the last segment of the base URI's path (i.e.,
1801 excluding any characters after the right-most "/" in the base URI
1802 path, or excluding the entire base URI path if it does not contain
1803 any "/" characters).
1805 5.2.4. Remove Dot Segments
1807 The pseudocode also refers to a "remove_dot_segments" routine for
1808 interpreting and removing the special "." and ".." complete path
1809 segments from a referenced path. This is done after the path is
1810 extracted from a reference, whether or not the path was relative, in
1811 order to remove any invalid or extraneous dot-segments prior to
1812 forming the target URI. Although there are many ways to accomplish
1813 this removal process, we describe a simple method using two string
1816 1. The input buffer is initialized with the now-appended path
1817 components and the output buffer is initialized to the empty
1820 2. While the input buffer is not empty, loop as follows:
1822 A. If the input buffer begins with a prefix of "../" or "./",
1823 then remove that prefix from the input buffer; otherwise,
1825 B. if the input buffer begins with a prefix of "/./" or "/.",
1826 where "." is a complete path segment, then replace that
1827 prefix with "/" in the input buffer; otherwise,
1829 C. if the input buffer begins with a prefix of "/../" or "/..",
1830 where ".." is a complete path segment, then replace that
1831 prefix with "/" in the input buffer and remove the last
1832 segment and its preceding "/" (if any) from the output
1835 D. if the input buffer consists only of "." or "..", then remove
1836 that from the input buffer; otherwise,
1838 E. move the first path segment in the input buffer to the end of
1839 the output buffer, including the initial "/" character (if
1840 any) and any subsequent characters up to, but not including,
1841 the next "/" character or the end of the input buffer.
1843 3. Finally, the output buffer is returned as the result of
1844 remove_dot_segments.
1850 Berners-Lee, et al. Standards Track [Page 33]
1852 RFC 3986 URI Generic Syntax January 2005
1855 Note that dot-segments are intended for use in URI references to
1856 express an identifier relative to the hierarchy of names in the base
1857 URI. The remove_dot_segments algorithm respects that hierarchy by
1858 removing extra dot-segments rather than treat them as an error or
1859 leaving them to be misinterpreted by dereference implementations.
1861 The following illustrates how the above steps are applied for two
1862 examples of merged paths, showing the state of the two buffers after
1865 STEP OUTPUT BUFFER INPUT BUFFER
1867 1 : /a/b/c/./../../g
1868 2E: /a /b/c/./../../g
1869 2E: /a/b /c/./../../g
1870 2E: /a/b/c /./../../g
1876 STEP OUTPUT BUFFER INPUT BUFFER
1878 1 : mid/content=5/../6
1879 2E: mid /content=5/../6
1880 2E: mid/content=5 /../6
1884 Some applications may find it more efficient to implement the
1885 remove_dot_segments algorithm by using two segment stacks rather than
1888 Note: Beware that some older, erroneous implementations will fail
1889 to separate a reference's query component from its path component
1890 prior to merging the base and reference paths, resulting in an
1891 interoperability failure if the query component contains the
1892 strings "/../" or "/./".
1906 Berners-Lee, et al. Standards Track [Page 34]
1908 RFC 3986 URI Generic Syntax January 2005
1911 5.3. Component Recomposition
1913 Parsed URI components can be recomposed to obtain the corresponding
1914 URI reference string. Using pseudocode, this would be:
1918 if defined(scheme) then
1919 append scheme to result;
1920 append ":" to result;
1923 if defined(authority) then
1924 append "//" to result;
1925 append authority to result;
1928 append path to result;
1930 if defined(query) then
1931 append "?" to result;
1932 append query to result;
1935 if defined(fragment) then
1936 append "#" to result;
1937 append fragment to result;
1942 Note that we are careful to preserve the distinction between a
1943 component that is undefined, meaning that its separator was not
1944 present in the reference, and a component that is empty, meaning that
1945 the separator was present and was immediately followed by the next
1946 component separator or the end of the reference.
1948 5.4. Reference Resolution Examples
1950 Within a representation with a well defined base URI of
1954 a relative reference is transformed to its target URI as follows.
1962 Berners-Lee, et al. Standards Track [Page 35]
1964 RFC 3986 URI Generic Syntax January 2005
1967 5.4.1. Normal Examples
1970 "g" = "http://a/b/c/g"
1971 "./g" = "http://a/b/c/g"
1972 "g/" = "http://a/b/c/g/"
1975 "?y" = "http://a/b/c/d;p?y"
1976 "g?y" = "http://a/b/c/g?y"
1977 "#s" = "http://a/b/c/d;p?q#s"
1978 "g#s" = "http://a/b/c/g#s"
1979 "g?y#s" = "http://a/b/c/g?y#s"
1980 ";x" = "http://a/b/c/;x"
1981 "g;x" = "http://a/b/c/g;x"
1982 "g;x?y#s" = "http://a/b/c/g;x?y#s"
1983 "" = "http://a/b/c/d;p?q"
1984 "." = "http://a/b/c/"
1985 "./" = "http://a/b/c/"
1986 ".." = "http://a/b/"
1987 "../" = "http://a/b/"
1988 "../g" = "http://a/b/g"
1989 "../.." = "http://a/"
1990 "../../" = "http://a/"
1991 "../../g" = "http://a/g"
1993 5.4.2. Abnormal Examples
1995 Although the following abnormal examples are unlikely to occur in
1996 normal practice, all URI parsers should be capable of resolving them
1997 consistently. Each example uses the same base as that above.
1999 Parsers must be careful in handling cases where there are more ".."
2000 segments in a relative-path reference than there are hierarchical
2001 levels in the base URI's path. Note that the ".." syntax cannot be
2002 used to change the authority component of a URI.
2004 "../../../g" = "http://a/g"
2005 "../../../../g" = "http://a/g"
2018 Berners-Lee, et al. Standards Track [Page 36]
2020 RFC 3986 URI Generic Syntax January 2005
2023 Similarly, parsers must remove the dot-segments "." and ".." when
2024 they are complete components of a path, but not when they are only
2027 "/./g" = "http://a/g"
2028 "/../g" = "http://a/g"
2029 "g." = "http://a/b/c/g."
2030 ".g" = "http://a/b/c/.g"
2031 "g.." = "http://a/b/c/g.."
2032 "..g" = "http://a/b/c/..g"
2034 Less likely are cases where the relative reference uses unnecessary
2035 or nonsensical forms of the "." and ".." complete path segments.
2037 "./../g" = "http://a/b/g"
2038 "./g/." = "http://a/b/c/g/"
2039 "g/./h" = "http://a/b/c/g/h"
2040 "g/../h" = "http://a/b/c/h"
2041 "g;x=1/./y" = "http://a/b/c/g;x=1/y"
2042 "g;x=1/../y" = "http://a/b/c/y"
2044 Some applications fail to separate the reference's query and/or
2045 fragment components from the path component before merging it with
2046 the base path and removing dot-segments. This error is rarely
2047 noticed, as typical usage of a fragment never includes the hierarchy
2048 ("/") character and the query component is not normally used within
2049 relative references.
2051 "g?y/./x" = "http://a/b/c/g?y/./x"
2052 "g?y/../x" = "http://a/b/c/g?y/../x"
2053 "g#s/./x" = "http://a/b/c/g#s/./x"
2054 "g#s/../x" = "http://a/b/c/g#s/../x"
2056 Some parsers allow the scheme name to be present in a relative
2057 reference if it is the same as the base URI scheme. This is
2058 considered to be a loophole in prior specifications of partial URI
2059 [RFC1630]. Its use should be avoided but is allowed for backward
2062 "http:g" = "http:g" ; for strict parsers
2063 / "http://a/b/c/g" ; for backward compatibility
2074 Berners-Lee, et al. Standards Track [Page 37]
2076 RFC 3986 URI Generic Syntax January 2005
2079 6. Normalization and Comparison
2081 One of the most common operations on URIs is simple comparison:
2082 determining whether two URIs are equivalent without using the URIs to
2083 access their respective resource(s). A comparison is performed every
2084 time a response cache is accessed, a browser checks its history to
2085 color a link, or an XML parser processes tags within a namespace.
2086 Extensive normalization prior to comparison of URIs is often used by
2087 spiders and indexing engines to prune a search space or to reduce
2088 duplication of request actions and response storage.
2090 URI comparison is performed for some particular purpose. Protocols
2091 or implementations that compare URIs for different purposes will
2092 often be subject to differing design trade-offs in regards to how
2093 much effort should be spent in reducing aliased identifiers. This
2094 section describes various methods that may be used to compare URIs,
2095 the trade-offs between them, and the types of applications that might
2100 Because URIs exist to identify resources, presumably they should be
2101 considered equivalent when they identify the same resource. However,
2102 this definition of equivalence is not of much practical use, as there
2103 is no way for an implementation to compare two resources unless it
2104 has full knowledge or control of them. For this reason,
2105 determination of equivalence or difference of URIs is based on string
2106 comparison, perhaps augmented by reference to additional rules
2107 provided by URI scheme definitions. We use the terms "different" and
2108 "equivalent" to describe the possible outcomes of such comparisons,
2109 but there are many application-dependent versions of equivalence.
2111 Even though it is possible to determine that two URIs are equivalent,
2112 URI comparison is not sufficient to determine whether two URIs
2113 identify different resources. For example, an owner of two different
2114 domain names could decide to serve the same resource from both,
2115 resulting in two different URIs. Therefore, comparison methods are
2116 designed to minimize false negatives while strictly avoiding false
2119 In testing for equivalence, applications should not directly compare
2120 relative references; the references should be converted to their
2121 respective target URIs before comparison. When URIs are compared to
2122 select (or avoid) a network action, such as retrieval of a
2123 representation, fragment components (if any) should be excluded from
2130 Berners-Lee, et al. Standards Track [Page 38]
2132 RFC 3986 URI Generic Syntax January 2005
2135 6.2. Comparison Ladder
2137 A variety of methods are used in practice to test URI equivalence.
2138 These methods fall into a range, distinguished by the amount of
2139 processing required and the degree to which the probability of false
2140 negatives is reduced. As noted above, false negatives cannot be
2141 eliminated. In practice, their probability can be reduced, but this
2142 reduction requires more processing and is not cost-effective for all
2145 If this range of comparison practices is considered as a ladder, the
2146 following discussion will climb the ladder, starting with practices
2147 that are cheap but have a relatively higher chance of producing false
2148 negatives, and proceeding to those that have higher computational
2149 cost and lower risk of false negatives.
2151 6.2.1. Simple String Comparison
2153 If two URIs, when considered as character strings, are identical,
2154 then it is safe to conclude that they are equivalent. This type of
2155 equivalence test has very low computational cost and is in wide use
2156 in a variety of applications, particularly in the domain of parsing.
2158 Testing strings for equivalence requires some basic precautions.
2159 This procedure is often referred to as "bit-for-bit" or
2160 "byte-for-byte" comparison, which is potentially misleading. Testing
2161 strings for equality is normally based on pair comparison of the
2162 characters that make up the strings, starting from the first and
2163 proceeding until both strings are exhausted and all characters are
2164 found to be equal, until a pair of characters compares unequal, or
2165 until one of the strings is exhausted before the other.
2167 This character comparison requires that each pair of characters be
2168 put in comparable form. For example, should one URI be stored in a
2169 byte array in EBCDIC encoding and the second in a Java String object
2170 (UTF-16), bit-for-bit comparisons applied naively will produce
2171 errors. It is better to speak of equality on a character-for-
2172 character basis rather than on a byte-for-byte or bit-for-bit basis.
2173 In practical terms, character-by-character comparisons should be done
2174 codepoint-by-codepoint after conversion to a common character
2177 False negatives are caused by the production and use of URI aliases.
2178 Unnecessary aliases can be reduced, regardless of the comparison
2179 method, by consistently providing URI references in an already-
2180 normalized form (i.e., a form identical to what would be produced
2181 after normalization is applied, as described below).
2186 Berners-Lee, et al. Standards Track [Page 39]
2188 RFC 3986 URI Generic Syntax January 2005
2191 Protocols and data formats often limit some URI comparisons to simple
2192 string comparison, based on the theory that people and
2193 implementations will, in their own best interest, be consistent in
2194 providing URI references, or at least consistent enough to negate any
2195 efficiency that might be obtained from further normalization.
2197 6.2.2. Syntax-Based Normalization
2199 Implementations may use logic based on the definitions provided by
2200 this specification to reduce the probability of false negatives.
2201 This processing is moderately higher in cost than character-for-
2202 character string comparison. For example, an application using this
2203 approach could reasonably consider the following two URIs equivalent:
2205 example://a/b/c/%7Bfoo%7D
2206 eXAMPLE://a/./b/../b/%63/%7bfoo%7d
2208 Web user agents, such as browsers, typically apply this type of URI
2209 normalization when determining whether a cached response is
2210 available. Syntax-based normalization includes such techniques as
2211 case normalization, percent-encoding normalization, and removal of
2214 6.2.2.1. Case Normalization
2216 For all URIs, the hexadecimal digits within a percent-encoding
2217 triplet (e.g., "%3a" versus "%3A") are case-insensitive and therefore
2218 should be normalized to use uppercase letters for the digits A-F.
2220 When a URI uses components of the generic syntax, the component
2221 syntax equivalence rules always apply; namely, that the scheme and
2222 host are case-insensitive and therefore should be normalized to
2223 lowercase. For example, the URI <HTTP://www.EXAMPLE.com/> is
2224 equivalent to <http://www.example.com/>. The other generic syntax
2225 components are assumed to be case-sensitive unless specifically
2226 defined otherwise by the scheme (see Section 6.2.3).
2228 6.2.2.2. Percent-Encoding Normalization
2230 The percent-encoding mechanism (Section 2.1) is a frequent source of
2231 variance among otherwise identical URIs. In addition to the case
2232 normalization issue noted above, some URI producers percent-encode
2233 octets that do not require percent-encoding, resulting in URIs that
2234 are equivalent to their non-encoded counterparts. These URIs should
2235 be normalized by decoding any percent-encoded octet that corresponds
2236 to an unreserved character, as described in Section 2.3.
2242 Berners-Lee, et al. Standards Track [Page 40]
2244 RFC 3986 URI Generic Syntax January 2005
2247 6.2.2.3. Path Segment Normalization
2249 The complete path segments "." and ".." are intended only for use
2250 within relative references (Section 4.1) and are removed as part of
2251 the reference resolution process (Section 5.2). However, some
2252 deployed implementations incorrectly assume that reference resolution
2253 is not necessary when the reference is already a URI and thus fail to
2254 remove dot-segments when they occur in non-relative paths. URI
2255 normalizers should remove dot-segments by applying the
2256 remove_dot_segments algorithm to the path, as described in
2259 6.2.3. Scheme-Based Normalization
2261 The syntax and semantics of URIs vary from scheme to scheme, as
2262 described by the defining specification for each scheme.
2263 Implementations may use scheme-specific rules, at further processing
2264 cost, to reduce the probability of false negatives. For example,
2265 because the "http" scheme makes use of an authority component, has a
2266 default port of "80", and defines an empty path to be equivalent to
2267 "/", the following four URIs are equivalent:
2271 http://example.com:/
2272 http://example.com:80/
2274 In general, a URI that uses the generic syntax for authority with an
2275 empty path should be normalized to a path of "/". Likewise, an
2276 explicit ":port", for which the port is empty or the default for the
2277 scheme, is equivalent to one where the port and its ":" delimiter are
2278 elided and thus should be removed by scheme-based normalization. For
2279 example, the second URI above is the normal form for the "http"
2282 Another case where normalization varies by scheme is in the handling
2283 of an empty authority component or empty host subcomponent. For many
2284 scheme specifications, an empty authority or host is considered an
2285 error; for others, it is considered equivalent to "localhost" or the
2286 end-user's host. When a scheme defines a default for authority and a
2287 URI reference to that default is desired, the reference should be
2288 normalized to an empty authority for the sake of uniformity, brevity,
2289 and internationalization. If, however, either the userinfo or port
2290 subcomponents are non-empty, then the host should be given explicitly
2291 even if it matches the default.
2293 Normalization should not remove delimiters when their associated
2294 component is empty unless licensed to do so by the scheme
2298 Berners-Lee, et al. Standards Track [Page 41]
2300 RFC 3986 URI Generic Syntax January 2005
2303 specification. For example, the URI "http://example.com/?" cannot be
2304 assumed to be equivalent to any of the examples above. Likewise, the
2305 presence or absence of delimiters within a userinfo subcomponent is
2306 usually significant to its interpretation. The fragment component is
2307 not subject to any scheme-based normalization; thus, two URIs that
2308 differ only by the suffix "#" are considered different regardless of
2311 Some schemes define additional subcomponents that consist of case-
2312 insensitive data, giving an implicit license to normalizers to
2313 convert this data to a common case (e.g., all lowercase). For
2314 example, URI schemes that define a subcomponent of path to contain an
2315 Internet hostname, such as the "mailto" URI scheme, cause that
2316 subcomponent to be case-insensitive and thus subject to case
2317 normalization (e.g., "mailto:Joe@Example.COM" is equivalent to
2318 "mailto:Joe@example.com", even though the generic syntax considers
2319 the path component to be case-sensitive).
2321 Other scheme-specific normalizations are possible.
2323 6.2.4. Protocol-Based Normalization
2325 Substantial effort to reduce the incidence of false negatives is
2326 often cost-effective for web spiders. Therefore, they implement even
2327 more aggressive techniques in URI comparison. For example, if they
2328 observe that a URI such as
2330 http://example.com/data
2332 redirects to a URI differing only in the trailing slash
2334 http://example.com/data/
2336 they will likely regard the two as equivalent in the future. This
2337 kind of technique is only appropriate when equivalence is clearly
2338 indicated by both the result of accessing the resources and the
2339 common conventions of their scheme's dereference algorithm (in this
2340 case, use of redirection by HTTP origin servers to avoid problems
2341 with relative references).
2354 Berners-Lee, et al. Standards Track [Page 42]
2356 RFC 3986 URI Generic Syntax January 2005
2359 7. Security Considerations
2361 A URI does not in itself pose a security threat. However, as URIs
2362 are often used to provide a compact set of instructions for access to
2363 network resources, care must be taken to properly interpret the data
2364 within a URI, to prevent that data from causing unintended access,
2365 and to avoid including data that should not be revealed in plain
2368 7.1. Reliability and Consistency
2370 There is no guarantee that once a URI has been used to retrieve
2371 information, the same information will be retrievable by that URI in
2372 the future. Nor is there any guarantee that the information
2373 retrievable via that URI in the future will be observably similar to
2374 that retrieved in the past. The URI syntax does not constrain how a
2375 given scheme or authority apportions its namespace or maintains it
2376 over time. Such guarantees can only be obtained from the person(s)
2377 controlling that namespace and the resource in question. A specific
2378 URI scheme may define additional semantics, such as name persistence,
2379 if those semantics are required of all naming authorities for that
2382 7.2. Malicious Construction
2384 It is sometimes possible to construct a URI so that an attempt to
2385 perform a seemingly harmless, idempotent operation, such as the
2386 retrieval of a representation, will in fact cause a possibly damaging
2387 remote operation. The unsafe URI is typically constructed by
2388 specifying a port number other than that reserved for the network
2389 protocol in question. The client unwittingly contacts a site running
2390 a different protocol service, and data within the URI contains
2391 instructions that, when interpreted according to this other protocol,
2392 cause an unexpected operation. A frequent example of such abuse has
2393 been the use of a protocol-based scheme with a port component of
2394 "25", thereby fooling user agent software into sending an unintended
2395 or impersonating message via an SMTP server.
2397 Applications should prevent dereference of a URI that specifies a TCP
2398 port number within the "well-known port" range (0 - 1023) unless the
2399 protocol being used to dereference that URI is compatible with the
2400 protocol expected on that well-known port. Although IANA maintains a
2401 registry of well-known ports, applications should make such
2402 restrictions user-configurable to avoid preventing the deployment of
2410 Berners-Lee, et al. Standards Track [Page 43]
2412 RFC 3986 URI Generic Syntax January 2005
2415 When a URI contains percent-encoded octets that match the delimiters
2416 for a given resolution or dereference protocol (for example, CR and
2417 LF characters for the TELNET protocol), these percent-encodings must
2418 not be decoded before transmission across that protocol. Transfer of
2419 the percent-encoding, which might violate the protocol, is less
2420 harmful than allowing decoded octets to be interpreted as additional
2421 operations or parameters, perhaps triggering an unexpected and
2422 possibly harmful remote operation.
2424 7.3. Back-End Transcoding
2426 When a URI is dereferenced, the data within it is often parsed by
2427 both the user agent and one or more servers. In HTTP, for example, a
2428 typical user agent will parse a URI into its five major components,
2429 access the authority's server, and send it the data within the
2430 authority, path, and query components. A typical server will take
2431 that information, parse the path into segments and the query into
2432 key/value pairs, and then invoke implementation-specific handlers to
2433 respond to the request. As a result, a common security concern for
2434 server implementations that handle a URI, either as a whole or split
2435 into separate components, is proper interpretation of the octet data
2436 represented by the characters and percent-encodings within that URI.
2438 Percent-encoded octets must be decoded at some point during the
2439 dereference process. Applications must split the URI into its
2440 components and subcomponents prior to decoding the octets, as
2441 otherwise the decoded octets might be mistaken for delimiters.
2442 Security checks of the data within a URI should be applied after
2443 decoding the octets. Note, however, that the "%00" percent-encoding
2444 (NUL) may require special handling and should be rejected if the
2445 application is not expecting to receive raw data within a component.
2447 Special care should be taken when the URI path interpretation process
2448 involves the use of a back-end file system or related system
2449 functions. File systems typically assign an operational meaning to
2450 special characters, such as the "/", "\", ":", "[", and "]"
2451 characters, and to special device names like ".", "..", "...", "aux",
2452 "lpt", etc. In some cases, merely testing for the existence of such
2453 a name will cause the operating system to pause or invoke unrelated
2454 system calls, leading to significant security concerns regarding
2455 denial of service and unintended data transfer. It would be
2456 impossible for this specification to list all such significant
2457 characters and device names. Implementers should research the
2458 reserved names and characters for the types of storage device that
2459 may be attached to their applications and restrict the use of data
2460 obtained from URI components accordingly.
2466 Berners-Lee, et al. Standards Track [Page 44]
2468 RFC 3986 URI Generic Syntax January 2005
2471 7.4. Rare IP Address Formats
2473 Although the URI syntax for IPv4address only allows the common
2474 dotted-decimal form of IPv4 address literal, many implementations
2475 that process URIs make use of platform-dependent system routines,
2476 such as gethostbyname() and inet_aton(), to translate the string
2477 literal to an actual IP address. Unfortunately, such system routines
2478 often allow and process a much larger set of formats than those
2479 described in Section 3.2.2.
2481 For example, many implementations allow dotted forms of three
2482 numbers, wherein the last part is interpreted as a 16-bit quantity
2483 and placed in the right-most two bytes of the network address (e.g.,
2484 a Class B network). Likewise, a dotted form of two numbers means
2485 that the last part is interpreted as a 24-bit quantity and placed in
2486 the right-most three bytes of the network address (Class A), and a
2487 single number (without dots) is interpreted as a 32-bit quantity and
2488 stored directly in the network address. Adding further to the
2489 confusion, some implementations allow each dotted part to be
2490 interpreted as decimal, octal, or hexadecimal, as specified in the C
2491 language (i.e., a leading 0x or 0X implies hexadecimal; a leading 0
2492 implies octal; otherwise, the number is interpreted as decimal).
2494 These additional IP address formats are not allowed in the URI syntax
2495 due to differences between platform implementations. However, they
2496 can become a security concern if an application attempts to filter
2497 access to resources based on the IP address in string literal format.
2498 If this filtering is performed, literals should be converted to
2499 numeric form and filtered based on the numeric value, and not on a
2500 prefix or suffix of the string form.
2502 7.5. Sensitive Information
2504 URI producers should not provide a URI that contains a username or
2505 password that is intended to be secret. URIs are frequently
2506 displayed by browsers, stored in clear text bookmarks, and logged by
2507 user agent history and intermediary applications (proxies). A
2508 password appearing within the userinfo component is deprecated and
2509 should be considered an error (or simply ignored) except in those
2510 rare cases where the 'password' parameter is intended to be public.
2512 7.6. Semantic Attacks
2514 Because the userinfo subcomponent is rarely used and appears before
2515 the host in the authority component, it can be used to construct a
2516 URI intended to mislead a human user by appearing to identify one
2517 (trusted) naming authority while actually identifying a different
2518 authority hidden behind the noise. For example
2522 Berners-Lee, et al. Standards Track [Page 45]
2524 RFC 3986 URI Generic Syntax January 2005
2527 ftp://cnn.example.com&story=breaking_news@10.0.0.1/top_story.htm
2529 might lead a human user to assume that the host is 'cnn.example.com',
2530 whereas it is actually '10.0.0.1'. Note that a misleading userinfo
2531 subcomponent could be much longer than the example above.
2533 A misleading URI, such as that above, is an attack on the user's
2534 preconceived notions about the meaning of a URI rather than an attack
2535 on the software itself. User agents may be able to reduce the impact
2536 of such attacks by distinguishing the various components of the URI
2537 when they are rendered, such as by using a different color or tone to
2538 render userinfo if any is present, though there is no panacea. More
2539 information on URI-based semantic attacks can be found in [Siedzik].
2541 8. IANA Considerations
2543 URI scheme names, as defined by <scheme> in Section 3.1, form a
2544 registered namespace that is managed by IANA according to the
2545 procedures defined in [BCP35]. No IANA actions are required by this
2550 This specification is derived from RFC 2396 [RFC2396], RFC 1808
2551 [RFC1808], and RFC 1738 [RFC1738]; the acknowledgements in those
2552 documents still apply. It also incorporates the update (with
2553 corrections) for IPv6 literals in the host syntax, as defined by
2554 Robert M. Hinden, Brian E. Carpenter, and Larry Masinter in
2555 [RFC2732]. In addition, contributions by Gisle Aas, Reese Anschultz,
2556 Daniel Barclay, Tim Bray, Mike Brown, Rob Cameron, Jeremy Carroll,
2557 Dan Connolly, Adam M. Costello, John Cowan, Jason Diamond, Martin
2558 Duerst, Stefan Eissing, Clive D.W. Feather, Al Gilman, Tony Hammond,
2559 Elliotte Harold, Pat Hayes, Henry Holtzman, Ian B. Jacobs, Michael
2560 Kay, John C. Klensin, Graham Klyne, Dan Kohn, Bruce Lilly, Andrew
2561 Main, Dave McAlpin, Ira McDonald, Michael Mealling, Ray Merkert,
2562 Stephen Pollei, Julian Reschke, Tomas Rokicki, Miles Sabin, Kai
2563 Schaetzl, Mark Thomson, Ronald Tschalaer, Norm Walsh, Marc Warne,
2564 Stuart Williams, and Henry Zongaro are gratefully acknowledged.
2568 10.1. Normative References
2570 [ASCII] American National Standards Institute, "Coded Character
2571 Set -- 7-bit American Standard Code for Information
2572 Interchange", ANSI X3.4, 1986.
2578 Berners-Lee, et al. Standards Track [Page 46]
2580 RFC 3986 URI Generic Syntax January 2005
2583 [RFC2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
2584 Specifications: ABNF", RFC 2234, November 1997.
2586 [STD63] Yergeau, F., "UTF-8, a transformation format of
2587 ISO 10646", STD 63, RFC 3629, November 2003.
2589 [UCS] International Organization for Standardization,
2590 "Information Technology - Universal Multiple-Octet Coded
2591 Character Set (UCS)", ISO/IEC 10646:2003, December 2003.
2593 10.2. Informative References
2595 [BCP19] Freed, N. and J. Postel, "IANA Charset Registration
2596 Procedures", BCP 19, RFC 2978, October 2000.
2598 [BCP35] Petke, R. and I. King, "Registration Procedures for URL
2599 Scheme Names", BCP 35, RFC 2717, November 1999.
2601 [RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
2602 host table specification", RFC 952, October 1985.
2604 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
2605 STD 13, RFC 1034, November 1987.
2607 [RFC1123] Braden, R., "Requirements for Internet Hosts - Application
2608 and Support", STD 3, RFC 1123, October 1989.
2610 [RFC1535] Gavron, E., "A Security Problem and Proposed Correction
2611 With Widely Deployed DNS Software", RFC 1535,
2614 [RFC1630] Berners-Lee, T., "Universal Resource Identifiers in WWW: A
2615 Unifying Syntax for the Expression of Names and Addresses
2616 of Objects on the Network as used in the World-Wide Web",
2617 RFC 1630, June 1994.
2619 [RFC1736] Kunze, J., "Functional Recommendations for Internet
2620 Resource Locators", RFC 1736, February 1995.
2622 [RFC1737] Sollins, K. and L. Masinter, "Functional Requirements for
2623 Uniform Resource Names", RFC 1737, December 1994.
2625 [RFC1738] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
2626 Resource Locators (URL)", RFC 1738, December 1994.
2628 [RFC1808] Fielding, R., "Relative Uniform Resource Locators",
2629 RFC 1808, June 1995.
2634 Berners-Lee, et al. Standards Track [Page 47]
2636 RFC 3986 URI Generic Syntax January 2005
2639 [RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
2640 Extensions (MIME) Part Two: Media Types", RFC 2046,
2643 [RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997.
2645 [RFC2396] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
2646 Resource Identifiers (URI): Generic Syntax", RFC 2396,
2649 [RFC2518] Goland, Y., Whitehead, E., Faizi, A., Carter, S., and D.
2650 Jensen, "HTTP Extensions for Distributed Authoring --
2651 WEBDAV", RFC 2518, February 1999.
2653 [RFC2557] Palme, J., Hopmann, A., and N. Shelness, "MIME
2654 Encapsulation of Aggregate Documents, such as HTML
2655 (MHTML)", RFC 2557, March 1999.
2657 [RFC2718] Masinter, L., Alvestrand, H., Zigmond, D., and R. Petke,
2658 "Guidelines for new URL Schemes", RFC 2718, November 1999.
2660 [RFC2732] Hinden, R., Carpenter, B., and L. Masinter, "Format for
2661 Literal IPv6 Addresses in URL's", RFC 2732, December 1999.
2663 [RFC3305] Mealling, M. and R. Denenberg, "Report from the Joint
2664 W3C/IETF URI Planning Interest Group: Uniform Resource
2665 Identifiers (URIs), URLs, and Uniform Resource Names
2666 (URNs): Clarifications and Recommendations", RFC 3305,
2669 [RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
2670 "Internationalizing Domain Names in Applications (IDNA)",
2671 RFC 3490, March 2003.
2673 [RFC3513] Hinden, R. and S. Deering, "Internet Protocol Version 6
2674 (IPv6) Addressing Architecture", RFC 3513, April 2003.
2676 [Siedzik] Siedzik, R., "Semantic Attacks: What's in a URL?",
2677 April 2001, <http://www.giac.org/practical/gsec/
2678 Richard_Siedzik_GSEC.pdf>.
2690 Berners-Lee, et al. Standards Track [Page 48]
2692 RFC 3986 URI Generic Syntax January 2005
2695 Appendix A. Collected ABNF for URI
2697 URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ]
2699 hier-part = "//" authority path-abempty
2704 URI-reference = URI / relative-ref
2706 absolute-URI = scheme ":" hier-part [ "?" query ]
2708 relative-ref = relative-part [ "?" query ] [ "#" fragment ]
2710 relative-part = "//" authority path-abempty
2715 scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
2717 authority = [ userinfo "@" ] host [ ":" port ]
2718 userinfo = *( unreserved / pct-encoded / sub-delims / ":" )
2719 host = IP-literal / IPv4address / reg-name
2722 IP-literal = "[" ( IPv6address / IPvFuture ) "]"
2724 IPvFuture = "v" 1*HEXDIG "." 1*( unreserved / sub-delims / ":" )
2726 IPv6address = 6( h16 ":" ) ls32
2727 / "::" 5( h16 ":" ) ls32
2728 / [ h16 ] "::" 4( h16 ":" ) ls32
2729 / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
2730 / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
2731 / [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32
2732 / [ *4( h16 ":" ) h16 ] "::" ls32
2733 / [ *5( h16 ":" ) h16 ] "::" h16
2734 / [ *6( h16 ":" ) h16 ] "::"
2737 ls32 = ( h16 ":" h16 ) / IPv4address
2738 IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet
2746 Berners-Lee, et al. Standards Track [Page 49]
2748 RFC 3986 URI Generic Syntax January 2005
2751 dec-octet = DIGIT ; 0-9
2752 / %x31-39 DIGIT ; 10-99
2753 / "1" 2DIGIT ; 100-199
2754 / "2" %x30-34 DIGIT ; 200-249
2755 / "25" %x30-35 ; 250-255
2757 reg-name = *( unreserved / pct-encoded / sub-delims )
2759 path = path-abempty ; begins with "/" or is empty
2760 / path-absolute ; begins with "/" but not "//"
2761 / path-noscheme ; begins with a non-colon segment
2762 / path-rootless ; begins with a segment
2763 / path-empty ; zero characters
2765 path-abempty = *( "/" segment )
2766 path-absolute = "/" [ segment-nz *( "/" segment ) ]
2767 path-noscheme = segment-nz-nc *( "/" segment )
2768 path-rootless = segment-nz *( "/" segment )
2769 path-empty = 0<pchar>
2772 segment-nz = 1*pchar
2773 segment-nz-nc = 1*( unreserved / pct-encoded / sub-delims / "@" )
2774 ; non-zero-length segment without any colon ":"
2776 pchar = unreserved / pct-encoded / sub-delims / ":" / "@"
2778 query = *( pchar / "/" / "?" )
2780 fragment = *( pchar / "/" / "?" )
2782 pct-encoded = "%" HEXDIG HEXDIG
2784 unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
2785 reserved = gen-delims / sub-delims
2786 gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@"
2787 sub-delims = "!" / "$" / "&" / "'" / "(" / ")"
2788 / "*" / "+" / "," / ";" / "="
2790 Appendix B. Parsing a URI Reference with a Regular Expression
2792 As the "first-match-wins" algorithm is identical to the "greedy"
2793 disambiguation method used by POSIX regular expressions, it is
2794 natural and commonplace to use a regular expression for parsing the
2795 potential five components of a URI reference.
2797 The following line is the regular expression for breaking-down a
2798 well-formed URI reference into its components.
2802 Berners-Lee, et al. Standards Track [Page 50]
2804 RFC 3986 URI Generic Syntax January 2005
2807 ^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))?
2810 The numbers in the second line above are only to assist readability;
2811 they indicate the reference points for each subexpression (i.e., each
2812 paired parenthesis). We refer to the value matched for subexpression
2813 <n> as $<n>. For example, matching the above expression to
2815 http://www.ics.uci.edu/pub/ietf/uri/#Related
2817 results in the following subexpression matches:
2821 $3 = //www.ics.uci.edu
2822 $4 = www.ics.uci.edu
2829 where <undefined> indicates that the component is not present, as is
2830 the case for the query component in the above example. Therefore, we
2831 can determine the value of the five components as
2839 Going in the opposite direction, we can recreate a URI reference from
2840 its components by using the algorithm of Section 5.3.
2842 Appendix C. Delimiting a URI in Context
2844 URIs are often transmitted through formats that do not provide a
2845 clear context for their interpretation. For example, there are many
2846 occasions when a URI is included in plain text; examples include text
2847 sent in email, USENET news, and on printed paper. In such cases, it
2848 is important to be able to delimit the URI from the rest of the text,
2849 and in particular from punctuation marks that might be mistaken for
2852 In practice, URIs are delimited in a variety of ways, but usually
2853 within double-quotes "http://example.com/", angle brackets
2854 <http://example.com/>, or just by using whitespace:
2858 Berners-Lee, et al. Standards Track [Page 51]
2860 RFC 3986 URI Generic Syntax January 2005
2865 These wrappers do not form part of the URI.
2867 In some cases, extra whitespace (spaces, line-breaks, tabs, etc.) may
2868 have to be added to break a long URI across lines. The whitespace
2869 should be ignored when the URI is extracted.
2871 No whitespace should be introduced after a hyphen ("-") character.
2872 Because some typesetters and printers may (erroneously) introduce a
2873 hyphen at the end of line when breaking it, the interpreter of a URI
2874 containing a line break immediately after a hyphen should ignore all
2875 whitespace around the line break and should be aware that the hyphen
2876 may or may not actually be part of the URI.
2878 Using <> angle brackets around each URI is especially recommended as
2879 a delimiting style for a reference that contains embedded whitespace.
2881 The prefix "URL:" (with or without a trailing space) was formerly
2882 recommended as a way to help distinguish a URI from other bracketed
2883 designators, though it is not commonly used in practice and is no
2886 For robustness, software that accepts user-typed URI should attempt
2887 to recognize and strip both delimiters and embedded whitespace.
2889 For example, the text
2891 Yes, Jim, I found it under "http://www.w3.org/Addressing/",
2892 but you can probably pick it up from <ftp://foo.example.
2893 com/rfc/>. Note the warning in <http://www.ics.uci.edu/pub/
2894 ietf/uri/historical.html#WARNING>.
2896 contains the URI references
2898 http://www.w3.org/Addressing/
2899 ftp://foo.example.com/rfc/
2900 http://www.ics.uci.edu/pub/ietf/uri/historical.html#WARNING
2914 Berners-Lee, et al. Standards Track [Page 52]
2916 RFC 3986 URI Generic Syntax January 2005
2919 Appendix D. Changes from RFC 2396
2923 An ABNF rule for URI has been introduced to correspond to one common
2924 usage of the term: an absolute URI with optional fragment.
2926 IPv6 (and later) literals have been added to the list of possible
2927 identifiers for the host portion of an authority component, as
2928 described by [RFC2732], with the addition of "[" and "]" to the
2929 reserved set and a version flag to anticipate future versions of IP
2930 literals. Square brackets are now specified as reserved within the
2931 authority component and are not allowed outside their use as
2932 delimiters for an IP literal within host. In order to make this
2933 change without changing the technical definition of the path, query,
2934 and fragment components, those rules were redefined to directly
2935 specify the characters allowed.
2937 As [RFC2732] defers to [RFC3513] for definition of an IPv6 literal
2938 address, which, unfortunately, lacks an ABNF description of
2939 IPv6address, we created a new ABNF rule for IPv6address that matches
2940 the text representations defined by Section 2.2 of [RFC3513].
2941 Likewise, the definition of IPv4address has been improved in order to
2942 limit each decimal octet to the range 0-255.
2944 Section 6, on URI normalization and comparison, has been completely
2945 rewritten and extended by using input from Tim Bray and discussion
2946 within the W3C Technical Architecture Group.
2950 The ad-hoc BNF syntax of RFC 2396 has been replaced with the ABNF of
2951 [RFC2234]. This change required all rule names that formerly
2952 included underscore characters to be renamed with a dash instead. In
2953 addition, a number of syntax rules have been eliminated or simplified
2954 to make the overall grammar more comprehensible. Specifications that
2955 refer to the obsolete grammar rules may be understood by replacing
2956 those rules according to the following table:
2970 Berners-Lee, et al. Standards Track [Page 53]
2972 RFC 3986 URI Generic Syntax January 2005
2975 +----------------+--------------------------------------------------+
2976 | obsolete rule | translation |
2977 +----------------+--------------------------------------------------+
2978 | absoluteURI | absolute-URI |
2979 | relativeURI | relative-part [ "?" query ] |
2980 | hier_part | ( "//" authority path-abempty / |
2981 | | path-absolute ) [ "?" query ] |
2983 | opaque_part | path-rootless [ "?" query ] |
2984 | net_path | "//" authority path-abempty |
2985 | abs_path | path-absolute |
2986 | rel_path | path-rootless |
2987 | rel_segment | segment-nz-nc |
2988 | reg_name | reg-name |
2989 | server | authority |
2990 | hostport | host [ ":" port ] |
2991 | hostname | reg-name |
2992 | path_segments | path-abempty |
2993 | param | *<pchar excluding ";"> |
2995 | uric | unreserved / pct-encoded / ";" / "?" / ":" |
2996 | | / "@" / "&" / "=" / "+" / "$" / "," / "/" |
2998 | uric_no_slash | unreserved / pct-encoded / ";" / "?" / ":" |
2999 | | / "@" / "&" / "=" / "+" / "$" / "," |
3001 | mark | "-" / "_" / "." / "!" / "~" / "*" / "'" |
3004 | escaped | pct-encoded |
3006 | alphanum | ALPHA / DIGIT |
3007 +----------------+--------------------------------------------------+
3009 Use of the above obsolete rules for the definition of scheme-specific
3010 syntax is deprecated.
3012 Section 2, on characters, has been rewritten to explain what
3013 characters are reserved, when they are reserved, and why they are
3014 reserved, even when they are not used as delimiters by the generic
3015 syntax. The mark characters that are typically unsafe to decode,
3016 including the exclamation mark ("!"), asterisk ("*"), single-quote
3017 ("'"), and open and close parentheses ("(" and ")"), have been moved
3018 to the reserved set in order to clarify the distinction between
3019 reserved and unreserved and, hopefully, to answer the most common
3020 question of scheme designers. Likewise, the section on
3021 percent-encoded characters has been rewritten, and URI normalizers
3022 are now given license to decode any percent-encoded octets
3026 Berners-Lee, et al. Standards Track [Page 54]
3028 RFC 3986 URI Generic Syntax January 2005
3031 corresponding to unreserved characters. In general, the terms
3032 "escaped" and "unescaped" have been replaced with "percent-encoded"
3033 and "decoded", respectively, to reduce confusion with other forms of
3036 The ABNF for URI and URI-reference has been redesigned to make them
3037 more friendly to LALR parsers and to reduce complexity. As a result,
3038 the layout form of syntax description has been removed, along with
3039 the uric, uric_no_slash, opaque_part, net_path, abs_path, rel_path,
3040 path_segments, rel_segment, and mark rules. All references to
3041 "opaque" URIs have been replaced with a better description of how the
3042 path component may be opaque to hierarchy. The relativeURI rule has
3043 been replaced with relative-ref to avoid unnecessary confusion over
3044 whether they are a subset of URI. The ambiguity regarding the
3045 parsing of URI-reference as a URI or a relative-ref with a colon in
3046 the first segment has been eliminated through the use of five
3047 separate path matching rules.
3049 The fragment identifier has been moved back into the section on
3050 generic syntax components and within the URI and relative-ref rules,
3051 though it remains excluded from absolute-URI. The number sign ("#")
3052 character has been moved back to the reserved set as a result of
3053 reintegrating the fragment syntax.
3055 The ABNF has been corrected to allow the path component to be empty.
3056 This also allows an absolute-URI to consist of nothing after the
3057 "scheme:", as is present in practice with the "dav:" namespace
3058 [RFC2518] and with the "about:" scheme used internally by many WWW
3059 browser implementations. The ambiguity regarding the boundary
3060 between authority and path has been eliminated through the use of
3061 five separate path matching rules.
3063 Registry-based naming authorities that use the generic syntax are now
3064 defined within the host rule. This change allows current
3065 implementations, where whatever name provided is simply fed to the
3066 local name resolution mechanism, to be consistent with the
3067 specification. It also removes the need to re-specify DNS name
3068 formats here. Furthermore, it allows the host component to contain
3069 percent-encoded octets, which is necessary to enable
3070 internationalized domain names to be provided in URIs, processed in
3071 their native character encodings at the application layers above URI
3072 processing, and passed to an IDNA library as a registered name in the
3073 UTF-8 character encoding. The server, hostport, hostname,
3074 domainlabel, toplabel, and alphanum rules have been removed.
3076 The resolving relative references algorithm of [RFC2396] has been
3077 rewritten with pseudocode for this revision to improve clarity and
3078 fix the following issues:
3082 Berners-Lee, et al. Standards Track [Page 55]
3084 RFC 3986 URI Generic Syntax January 2005
3087 o [RFC2396] section 5.2, step 6a, failed to account for a base URI
3090 o Restored the behavior of [RFC1808] where, if the reference
3091 contains an empty path and a defined query component, the target
3092 URI inherits the base URI's path component.
3094 o The determination of whether a URI reference is a same-document
3095 reference has been decoupled from the URI parser, simplifying the
3096 URI processing interface within applications in a way consistent
3097 with the internal architecture of deployed URI processing
3098 implementations. The determination is now based on comparison to
3099 the base URI after transforming a reference to absolute form,
3100 rather than on the format of the reference itself. This change
3101 may result in more references being considered "same-document"
3102 under this specification than there would be under the rules given
3103 in RFC 2396, especially when normalization is used to reduce
3104 aliases. However, it does not change the status of existing
3105 same-document references.
3107 o Separated the path merge routine into two routines: merge, for
3108 describing combination of the base URI path with a relative-path
3109 reference, and remove_dot_segments, for describing how to remove
3110 the special "." and ".." segments from a composed path. The
3111 remove_dot_segments algorithm is now applied to all URI reference
3112 paths in order to match common implementations and to improve the
3113 normalization of URIs in practice. This change only impacts the
3114 parsing of abnormal references and same-scheme references wherein
3115 the base URI has a non-hierarchical path.
3131 character encoding 4
3134 coded character set 4
3138 Berners-Lee, et al. Standards Track [Page 56]
3140 RFC 3986 URI Generic Syntax January 2005
3188 path-abempty 16, 22, 26
3189 path-absolute 16, 22, 26
3190 path-empty 16, 22, 26
3194 Berners-Lee, et al. Standards Track [Page 57]
3196 RFC 3986 URI Generic Syntax January 2005
3199 path-rootless 16, 22
3214 remove_dot_segments 33
3250 Berners-Lee, et al. Standards Track [Page 58]
3252 RFC 3986 URI Generic Syntax January 2005
3306 Berners-Lee, et al. Standards Track [Page 59]
3308 RFC 3986 URI Generic Syntax January 2005
3314 World Wide Web Consortium
3315 Massachusetts Institute of Technology
3316 77 Massachusetts Avenue
3320 Phone: +1-617-253-5702
3321 Fax: +1-617-258-5999
3323 URI: http://www.w3.org/People/Berners-Lee/
3328 5251 California Ave., Suite 110
3332 Phone: +1-949-679-2960
3333 Fax: +1-949-679-2972
3334 EMail: fielding@gbiv.com
3335 URI: http://roy.gbiv.com/
3339 Adobe Systems Incorporated
3344 Phone: +1-408-536-3024
3346 URI: http://larry.masinter.net/
3362 Berners-Lee, et al. Standards Track [Page 60]
3364 RFC 3986 URI Generic Syntax January 2005
3367 Full Copyright Statement
3369 Copyright (C) The Internet Society (2005).
3371 This document is subject to the rights, licenses and restrictions
3372 contained in BCP 78, and except as set forth therein, the authors
3373 retain all their rights.
3375 This document and the information contained herein are provided on an
3376 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
3377 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
3378 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
3379 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
3380 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
3381 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
3383 Intellectual Property
3385 The IETF takes no position regarding the validity or scope of any
3386 Intellectual Property Rights or other rights that might be claimed to
3387 pertain to the implementation or use of the technology described in
3388 this document or the extent to which any license under such rights
3389 might or might not be available; nor does it represent that it has
3390 made any independent effort to identify any such rights. Information
3391 on the IETF's procedures with respect to rights in IETF Documents can
3392 be found in BCP 78 and BCP 79.
3394 Copies of IPR disclosures made to the IETF Secretariat and any
3395 assurances of licenses to be made available, or the result of an
3396 attempt made to obtain a general license or permission for the use of
3397 such proprietary rights by implementers or users of this
3398 specification can be obtained from the IETF on-line IPR repository at
3399 http://www.ietf.org/ipr.
3401 The IETF invites any interested party to bring to its attention any
3402 copyrights, patents or patent applications, or other proprietary
3403 rights that may cover technology that may be required to implement
3404 this standard. Please address the information to the IETF at ietf-
3410 Funding for the RFC Editor function is currently provided by the
3418 Berners-Lee, et al. Standards Track [Page 61]
projects / profile / ivi / libsoup2.4.git / blob