Internet Engineering Task Force (IETF) R. Fielding, Ed. Request for Comments: 7232 Adobe Obsoletes: 2616 J. Reschke, Ed. Category: Standards Track greenbytes ISSN: 2070-1721 June 2014 Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests
AbstractThe Hypertext Transfer Protocol (HTTP) is a stateless application- level protocol for distributed, collaborative, hypertext information systems. This document defines HTTP/1.1 conditional requests, including metadata header fields for indicating state changes, request header fields for making preconditions on such state, and rules for constructing the responses to a conditional request when one or more preconditions evaluate to false. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7232.
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1. Introduction ....................................................4 1.1. Conformance and Error Handling .............................4 1.2. Syntax Notation ............................................4 2. Validators ......................................................5 2.1. Weak versus Strong .........................................5 2.2. Last-Modified ..............................................7 2.2.1. Generation ..........................................7 2.2.2. Comparison ..........................................8 2.3. ETag .......................................................9 2.3.1. Generation .........................................10 2.3.2. Comparison .........................................10 2.3.3. Example: Entity-Tags Varying on Content-Negotiated Resources .......................11 2.4. When to Use Entity-Tags and Last-Modified Dates ...........12 3. Precondition Header Fields .....................................13 3.1. If-Match ..................................................13 3.2. If-None-Match .............................................14 3.3. If-Modified-Since .........................................16 3.4. If-Unmodified-Since .......................................17 3.5. If-Range ..................................................18 4. Status Code Definitions ........................................18 4.1. 304 Not Modified ..........................................18 4.2. 412 Precondition Failed ...................................19 5. Evaluation .....................................................19 6. Precedence .....................................................20 7. IANA Considerations ............................................22 7.1. Status Code Registration ..................................22 7.2. Header Field Registration .................................22 8. Security Considerations ........................................22 9. Acknowledgments ................................................23 10. References ....................................................24 10.1. Normative References .....................................24 10.2. Informative References ...................................24 Appendix A. Changes from RFC 2616 .................................25 Appendix B. Imported ABNF .........................................25 Appendix C. Collected ABNF ........................................26 Index .............................................................27
RFC7231] that include one or more header fields indicating a precondition to be tested before applying the method semantics to the target resource. This document defines the HTTP/1.1 conditional request mechanisms in terms of the architecture, syntax notation, and conformance criteria defined in [RFC7230]. Conditional GET requests are the most efficient mechanism for HTTP cache updates [RFC7234]. Conditionals can also be applied to state-changing methods, such as PUT and DELETE, to prevent the "lost update" problem: one client accidentally overwriting the work of another client that has been acting in parallel. Conditional request preconditions are based on the state of the target resource as a whole (its current value set) or the state as observed in a previously obtained representation (one value in that set). A resource might have multiple current representations, each with its own observable state. The conditional request mechanisms assume that the mapping of requests to a "selected representation" (Section 3 of [RFC7231]) will be consistent over time if the server intends to take advantage of conditionals. Regardless, if the mapping is inconsistent and the server is unable to select the appropriate representation, then no harm will result when the precondition evaluates to false. The conditional request preconditions defined by this specification (Section 3) are evaluated when applicable to the recipient (Section 5) according to their order of precedence (Section 6). RFC2119]. Conformance criteria and considerations regarding error handling are defined in Section 2.5 of [RFC7230]. RFC5234] with a list extension, defined in Section 7 of [RFC7230], that allows for compact definition of comma-separated lists using a '#' operator (similar to how the '*' operator indicates
repetition). Appendix B describes rules imported from other documents. Appendix C shows the collected grammar with all list operators expanded to standard ABNF notation. Section 2.2) and opaque entity tags (Section 2.3). Additional metadata that reflects resource state has been defined by various extensions of HTTP, such as Web Distributed Authoring and Versioning (WebDAV, [RFC4918]), that are beyond the scope of this specification. A resource metadata value is referred to as a "validator" when it is used within a precondition.
There are a variety of strong validators used in practice. The best are based on strict revision control, wherein each change to a representation always results in a unique node name and revision identifier being assigned before the representation is made accessible to GET. A collision-resistant hash function applied to the representation data is also sufficient if the data is available prior to the response header fields being sent and the digest does not need to be recalculated every time a validation request is received. However, if a resource has distinct representations that differ only in their metadata, such as might occur with content negotiation over media types that happen to share the same data format, then the origin server needs to incorporate additional information in the validator to distinguish those representations. In contrast, a "weak validator" is representation metadata that might not change for every change to the representation data. This weakness might be due to limitations in how the value is calculated, such as clock resolution, an inability to ensure uniqueness for all possible representations of the resource, or a desire of the resource owner to group representations by some self-determined set of equivalency rather than unique sequences of data. An origin server SHOULD change a weak entity-tag whenever it considers prior representations to be unacceptable as a substitute for the current representation. In other words, a weak entity-tag ought to change whenever the origin server wants caches to invalidate old responses. For example, the representation of a weather report that changes in content every second, based on dynamic measurements, might be grouped into sets of equivalent representations (from the origin server's perspective) with the same weak validator in order to allow cached representations to be valid for a reasonable period of time (perhaps adjusted dynamically based on server load or weather quality). Likewise, a representation's modification time, if defined with only one-second resolution, might be a weak validator if it is possible for the representation to be modified twice during a single second and retrieved between those modifications. Likewise, a validator is weak if it is shared by two or more representations of a given resource at the same time, unless those representations have identical representation data. For example, if the origin server sends the same validator for a representation with a gzip content coding applied as it does for a representation with no content coding, then that validator is weak. However, two simultaneous representations might share the same strong validator if they differ only in the representation metadata, such as when two different media types are available for the same representation data.
Strong validators are usable for all conditional requests, including cache validation, partial content ranges, and "lost update" avoidance. Weak validators are only usable when the client does not require exact equality with previously obtained representation data, such as when validating a cache entry or limiting a web traversal to recent changes. RFC7234]) results in a substantial reduction of HTTP traffic on the Internet and can be a significant factor in improving service scalability and reliability. A representation is typically the sum of many parts behind the resource interface. The last-modified time would usually be the most recent time that any of those parts were changed. How that value is determined for any given resource is an implementation detail beyond the scope of this specification. What matters to HTTP is how recipients of the Last-Modified header field can use its value to make conditional requests and test the validity of locally cached responses. An origin server SHOULD obtain the Last-Modified value of the representation as close as possible to the time that it generates the Date field value for its response. This allows a recipient to make an accurate assessment of the representation's modification time, especially if the representation changes near the time that the response is generated. An origin server with a clock MUST NOT send a Last-Modified date that is later than the server's time of message origination (Date). If the last modification time is derived from implementation-specific
metadata that evaluates to some time in the future, according to the origin server's clock, then the origin server MUST replace that value with the message origination date. This prevents a future modification date from having an adverse impact on cache validation. An origin server without a clock MUST NOT assign Last-Modified values to a response unless these values were associated with the resource by some other system or user with a reliable clock.
This method relies on the fact that if two different responses were sent by the origin server during the same second, but both had the same Last-Modified time, then at least one of those responses would have a Date value equal to its Last-Modified time. The arbitrary 60-second limit guards against the possibility that the Date and Last-Modified values are generated from different clocks or at somewhat different times during the preparation of the response. An implementation MAY use a value larger than 60 seconds, if it is believed that 60 seconds is too short. RFC2616], Section 3.11); thus, some recipients might perform backslash unescaping. Servers therefore ought to avoid backslash characters in entity tags. An entity-tag can be more reliable for validation than a modification date in situations where it is inconvenient to store modification dates, where the one-second resolution of HTTP date values is not sufficient, or where modification dates are not consistently maintained. Examples: ETag: "xyzzy" ETag: W/"xyzzy" ETag: ""
An entity-tag can be either a weak or strong validator, with strong being the default. If an origin server provides an entity-tag for a representation and the generation of that entity-tag does not satisfy all of the characteristics of a strong validator (Section 2.1), then the origin server MUST mark the entity-tag as weak by prefixing its opaque value with "W/" (case-sensitive). RFC7234]) can result in a substantial reduction of HTTP network traffic and can be a significant factor in improving service scalability and reliability.
The example below shows the results for a set of entity-tag pairs and both the weak and strong comparison function results: +--------+--------+-------------------+-----------------+ | ETag 1 | ETag 2 | Strong Comparison | Weak Comparison | +--------+--------+-------------------+-----------------+ | W/"1" | W/"1" | no match | match | | W/"1" | W/"2" | no match | no match | | W/"1" | "1" | no match | match | | "1" | "1" | match | match | +--------+--------+-------------------+-----------------+ Section 3.4 of [RFC7231]), and where the representations sent in response to a GET request vary based on the Accept-Encoding request header field (Section 5.3.4 of [RFC7231]): >> Request: GET /index HTTP/1.1 Host: www.example.com Accept-Encoding: gzip In this case, the response might or might not use the gzip content coding. If it does not, the response might look like: >> Response: HTTP/1.1 200 OK Date: Fri, 26 Mar 2010 00:05:00 GMT ETag: "123-a" Content-Length: 70 Vary: Accept-Encoding Content-Type: text/plain Hello World! Hello World! Hello World! Hello World! Hello World!
An alternative representation that does use gzip content coding would be: >> Response: HTTP/1.1 200 OK Date: Fri, 26 Mar 2010 00:05:00 GMT ETag: "123-b" Content-Length: 43 Vary: Accept-Encoding Content-Type: text/plain Content-Encoding: gzip ...binary data... Note: Content codings are a property of the representation data, so a strong entity-tag for a content-encoded representation has to be distinct from the entity tag of an unencoded representation to prevent potential conflicts during cache updates and range requests. In contrast, transfer codings (Section 4 of [RFC7230]) apply only during message transfer and do not result in distinct entity-tags.
o SHOULD send the Last-Modified value in non-subrange cache validation requests (using If-Modified-Since) if only a Last-Modified value has been provided by the origin server. o MAY send the Last-Modified value in subrange cache validation requests (using If-Unmodified-Since) if only a Last-Modified value has been provided by an HTTP/1.0 origin server. The user agent SHOULD provide a way to disable this, in case of difficulty. o SHOULD send both validators in cache validation requests if both an entity-tag and a Last-Modified value have been provided by the origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond appropriately. Section 5 defines when the preconditions are applied. Section 6 defines the order of evaluation when more than one precondition is present. Section 2.3.2), since the client intends this precondition to prevent the method from being applied if there have been any changes to the representation data. If-Match = "*" / 1#entity-tag Examples: If-Match: "xyzzy" If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" If-Match: * If-Match is most often used with state-changing methods (e.g., POST, PUT, DELETE) to prevent accidental overwrites when multiple user agents might be acting in parallel on the same resource (i.e., to
prevent the "lost update" problem). It can also be used with safe methods to abort a request if the selected representation does not match one already stored (or partially stored) from a prior request. An origin server that receives an If-Match header field MUST evaluate the condition prior to performing the method (Section 5). If the field-value is "*", the condition is false if the origin server does not have a current representation for the target resource. If the field-value is a list of entity-tags, the condition is false if none of the listed tags match the entity-tag of the selected representation. An origin server MUST NOT perform the requested method if a received If-Match condition evaluates to false; instead, the origin server MUST respond with either a) the 412 (Precondition Failed) status code or b) one of the 2xx (Successful) status codes if the origin server has verified that a state change is being requested and the final state is already reflected in the current state of the target resource (i.e., the change requested by the user agent has already succeeded, but the user agent might not be aware of it, perhaps because the prior response was lost or a compatible change was made by some other user agent). In the latter case, the origin server MUST NOT send a validator header field in the response unless it can verify that the request is a duplicate of an immediately prior change made by the same user agent. The If-Match header field can be ignored by caches and intermediaries because it is not applicable to a stored response. Section 2.3.2), since weak entity-tags can be used for cache validation even if there have been changes to the representation data. If-None-Match = "*" / 1#entity-tag
Examples: If-None-Match: "xyzzy" If-None-Match: W/"xyzzy" If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz" If-None-Match: * If-None-Match is primarily used in conditional GET requests to enable efficient updates of cached information with a minimum amount of transaction overhead. When a client desires to update one or more stored responses that have entity-tags, the client SHOULD generate an If-None-Match header field containing a list of those entity-tags when making a GET request; this allows recipient servers to send a 304 (Not Modified) response to indicate when one of those stored responses matches the selected representation. If-None-Match can also be used with a value of "*" to prevent an unsafe request method (e.g., PUT) from inadvertently modifying an existing representation of the target resource when the client believes that the resource does not have a current representation (Section 4.2.1 of [RFC7231]). This is a variation on the "lost update" problem that might arise if more than one client attempts to create an initial representation for the target resource. An origin server that receives an If-None-Match header field MUST evaluate the condition prior to performing the method (Section 5). If the field-value is "*", the condition is false if the origin server has a current representation for the target resource. If the field-value is a list of entity-tags, the condition is false if one of the listed tags match the entity-tag of the selected representation. An origin server MUST NOT perform the requested method if the condition evaluates to false; instead, the origin server MUST respond with either a) the 304 (Not Modified) status code if the request method is GET or HEAD or b) the 412 (Precondition Failed) status code for all other request methods. Requirements on cache handling of a received If-None-Match header field are defined in Section 4.3.2 of [RFC7234].
When used for limiting the scope of retrieval to a recent time window, a user agent will generate an If-Modified-Since field value based on either its own local clock or a Date header field received from the server in a prior response. Origin servers that choose an exact timestamp match based on the selected representation's Last-Modified field will not be able to help the user agent limit its data transfers to only those changed during the specified window. An origin server that receives an If-Modified-Since header field SHOULD evaluate the condition prior to performing the method (Section 5). The origin server SHOULD NOT perform the requested method if the selected representation's last modification date is earlier than or equal to the date provided in the field-value; instead, the origin server SHOULD generate a 304 (Not Modified) response, including only those metadata that are useful for identifying or updating a previously cached response. Requirements on cache handling of a received If-Modified-Since header field are defined in Section 4.3.2 of [RFC7234].
If-Unmodified-Since is most often used with state-changing methods (e.g., POST, PUT, DELETE) to prevent accidental overwrites when multiple user agents might be acting in parallel on a resource that does not supply entity-tags with its representations (i.e., to prevent the "lost update" problem). It can also be used with safe methods to abort a request if the selected representation does not match one already stored (or partially stored) from a prior request. An origin server that receives an If-Unmodified-Since header field MUST evaluate the condition prior to performing the method (Section 5). The origin server MUST NOT perform the requested method if the selected representation's last modification date is more recent than the date provided in the field-value; instead the origin server MUST respond with either a) the 412 (Precondition Failed) status code or b) one of the 2xx (Successful) status codes if the origin server has verified that a state change is being requested and the final state is already reflected in the current state of the target resource (i.e., the change requested by the user agent has already succeeded, but the user agent might not be aware of that because the prior response message was lost or a compatible change was made by some other user agent). In the latter case, the origin server MUST NOT send a validator header field in the response unless it can verify that the request is a duplicate of an immediately prior change made by the same user agent. The If-Unmodified-Since header field can be ignored by caches and intermediaries because it is not applicable to a stored response. Section 3.2 of [RFC7233].
conditional, already has a valid representation; the server is therefore redirecting the client to make use of that stored representation as if it were the payload of a 200 (OK) response. The server generating a 304 response MUST generate any of the following header fields that would have been sent in a 200 (OK) response to the same request: Cache-Control, Content-Location, Date, ETag, Expires, and Vary. Since the goal of a 304 response is to minimize information transfer when the recipient already has one or more cached representations, a sender SHOULD NOT generate representation metadata other than the above listed fields unless said metadata exists for the purpose of guiding cache updates (e.g., Last-Modified might be useful if the response does not have an ETag field). Requirements on a cache that receives a 304 response are defined in Section 4.3.4 of [RFC7234]. If the conditional request originated with an outbound client, such as a user agent with its own cache sending a conditional GET to a shared proxy, then the proxy SHOULD forward the 304 response to that client. A 304 response cannot contain a message-body; it is always terminated by the first empty line after the header fields.
A server that is not the origin server for the target resource and cannot act as a cache for requests on the target resource MUST NOT evaluate the conditional request header fields defined by this specification, and it MUST forward them if the request is forwarded, since the generating client intends that they be evaluated by a server that can provide a current representation. Likewise, a server MUST ignore the conditional request header fields defined by this specification when received with a request method that does not involve the selection or modification of a selected representation, such as CONNECT, OPTIONS, or TRACE. Conditional request header fields that are defined by extensions to HTTP might place conditions on all recipients, on the state of the target resource in general, or on a group of resources. For instance, the "If" header field in WebDAV can make a request conditional on various aspects of multiple resources, such as locks, if the recipient understands and implements that field ([RFC4918], Section 10.4). Although conditional request header fields are defined as being usable with the HEAD method (to keep HEAD's semantics consistent with those of GET), there is no point in sending a conditional HEAD because a successful response is around the same size as a 304 (Not Modified) response and more useful than a 412 (Precondition Failed) response. Section 3.1)
2. When recipient is the origin server, If-Match is not present, and If-Unmodified-Since is present, evaluate the If-Unmodified-Since precondition: * if true, continue to step 3 * if false, respond 412 (Precondition Failed) unless it can be determined that the state-changing request has already succeeded (see Section 3.4) 3. When If-None-Match is present, evaluate the If-None-Match precondition: * if true, continue to step 5 * if false for GET/HEAD, respond 304 (Not Modified) * if false for other methods, respond 412 (Precondition Failed) 4. When the method is GET or HEAD, If-None-Match is not present, and If-Modified-Since is present, evaluate the If-Modified-Since precondition: * if true, continue to step 5 * if false, respond 304 (Not Modified) 5. When the method is GET and both Range and If-Range are present, evaluate the If-Range precondition: * if the validator matches and the Range specification is applicable to the selected representation, respond 206 (Partial Content) [RFC7233] 6. Otherwise, * all conditions are met, so perform the requested action and respond according to its success or failure. Any extension to HTTP/1.1 that defines additional conditional request header fields ought to define its own expectations regarding the order for evaluating such fields in relation to those defined in this document and other conditionals that might be found in practice.
http://www.iana.org/assignments/http-status-codes> has been updated with the registrations below: +-------+---------------------+-------------+ | Value | Description | Reference | +-------+---------------------+-------------+ | 304 | Not Modified | Section 4.1 | | 412 | Precondition Failed | Section 4.2 | +-------+---------------------+-------------+ http://www.iana.org/assignments/message-headers/>. This document defines the following HTTP header fields, so their associated registry entries have been updated according to the permanent registrations below (see [BCP90]): +---------------------+----------+----------+-------------+ | Header Field Name | Protocol | Status | Reference | +---------------------+----------+----------+-------------+ | ETag | http | standard | Section 2.3 | | If-Match | http | standard | Section 3.1 | | If-Modified-Since | http | standard | Section 3.3 | | If-None-Match | http | standard | Section 3.2 | | If-Unmodified-Since | http | standard | Section 3.4 | | Last-Modified | http | standard | Section 2.2 | +---------------------+----------+----------+-------------+ The change controller is: "IETF (email@example.com) - Internet Engineering Task Force". RFC7230] and "Semantics and Content" [RFC7231].
The validators defined by this specification are not intended to ensure the validity of a representation, guard against malicious changes, or detect man-in-the-middle attacks. At best, they enable more efficient cache updates and optimistic concurrent writes when all participants are behaving nicely. At worst, the conditions will fail and the client will receive a response that is no more harmful than an HTTP exchange without conditional requests. An entity-tag can be abused in ways that create privacy risks. For example, a site might deliberately construct a semantically invalid entity-tag that is unique to the user or user agent, send it in a cacheable response with a long freshness time, and then read that entity-tag in later conditional requests as a means of re-identifying that user or user agent. Such an identifying tag would become a persistent identifier for as long as the user agent retained the original cache entry. User agents that cache representations ought to ensure that the cache is cleared or replaced whenever the user performs privacy-maintaining actions, such as clearing stored cookies or changing to a private browsing mode. Section 10 of [RFC7230].
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, June 2014. [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, June 2014. [RFC7233] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Range Requests", RFC 7233, June 2014. [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching", RFC 7234, June 2014. [BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, September 2004. [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. [RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)", RFC 4918, June 2007.
Section 2.1) Weak entity-tags are now allowed in all requests except range requests. (Sections 2.1 and 3.2) The ETag header field ABNF has been changed to not use quoted-string, thus avoiding escaping issues. (Section 2.3) ETag is defined to provide an entity tag for the selected representation, thereby clarifying what it applies to in various situations (such as a PUT response). (Section 2.3) The precedence for evaluation of conditional requests has been defined. (Section 6) Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII character). The rules below are defined in [RFC7230]: OWS = <OWS, see [RFC7230], Section 3.2.3> obs-text = <obs-text, see [RFC7230], Section 3.2.6> The rules below are defined in other parts: HTTP-date = <HTTP-date, see [RFC7231], Section 184.108.40.206>
Section 1.2 of [RFC7230]. ETag = entity-tag HTTP-date = <HTTP-date, see [RFC7231], Section 220.127.116.11> If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS entity-tag ] ) ) If-Modified-Since = HTTP-date If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS entity-tag ] ) ) If-Unmodified-Since = HTTP-date Last-Modified = HTTP-date OWS = <OWS, see [RFC7230], Section 3.2.3> entity-tag = [ weak ] opaque-tag etagc = "!" / %x23-7E ; '#'-'~' / obs-text obs-text = <obs-text, see [RFC7230], Section 3.2.6> opaque-tag = DQUOTE *etagc DQUOTE weak = %x57.2F ; W/
19 4 412 Precondition Failed (status code) 18 E ETag header field 9 G Grammar entity-tag 9 ETag 9 etagc 9 If-Match 13 If-Modified-Since 15 If-None-Match 14 If-Unmodified-Since 17 Last-Modified 7 opaque-tag 9 weak 9 I If-Match header field 13 If-Modified-Since header field 16 If-None-Match header field 14 If-Unmodified-Since header field 17 L Last-Modified header field 7 M metadata 5 S selected representation 4 V validator 5 strong 5 weak 5
http://roy.gbiv.com/ Julian F. Reschke (editor) greenbytes GmbH Hafenweg 16 Muenster, NW 48155 Germany EMail: firstname.lastname@example.org URI: http://greenbytes.de/tech/webdav/