IETF media feature registration WG Graham Klyne Internet draft Integralis Ltd. 3 March 1998 Expires: 3 September 1998 Requirements for protocol-independent content negotiation Status of this memo This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as ``work in progress''. To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). Copyright (C) 1997, The Internet Society Abstract A number of Internet application protocols have a need to provide content negotiation for the resources with which they interact. MIME media types [1, 2] provide a standard method for handling one major axis of variation, but resources also vary in ways which cannot be expressed using currently available MIME headers. The case for a cross-protocol negotiation framework is set out in [4]. This draft sets out terminology, an abstract framework and requirements for protocol-independent content negotiation, and identifies some technical issues which may need to be addressed. The abstract framework does not attempt to specify the content negotiation process, but gives an indication of the anticipated scope and form of any such specification. The requirements set out the desired properties of a content negotiation mechanism. Klyne [Page 1] Internet draft 3 March 1998 Requirements for protocol-independent content negotiation Table of contents 1. Introduction.............................................2 1.1 Structure of this document ...........................3 1.2 Discussion of this document ..........................3 1.3 Ammendment history ...................................4 1.4 Unfinished business ..................................4 2. Terminology and definitions..............................5 3. Framework................................................8 3.1 Abstract framework for content negotiation ...........10 3.1.1 The negotiation process..........................10 3.2 Abstract model for negotiation metadata ..............11 3.3 Text representation for negotiation metadata .........12 3.4 ASN.1 description of negotiation metadata ............12 3.5 Protocol binding guidelines ..........................13 4. Requirements.............................................13 4.1 Generic framework and metadata requirements ..........13 4.2 Protocol-specific deployment requirements ............14 5. Technical issues.........................................15 5.1 Non-message resource transfers .......................15 5.2 End-to-end vs hop-by-hop negotiations ................16 5.3 Third-party negotiation ..............................16 5.4 Use of directory services ............................16 5.5 Billing issues .......................................17 5.6 Performance considerations ...........................17 5.7 Confidence levels in negotiated options ..............17 6. Security considerations..................................18 6.1 Privacy ..............................................18 6.2 Denial of service attacks ............................18 6.3 Mailing list interactions ............................18 6.4 Use of security services .............................18 7. Copyright................................................19 8. Acknowledgements.........................................19 9. References...............................................20 10. Author's address........................................21 1. Introduction A number of Internet application protocols have a need to provide content negotiation for the resources with which they interact. While MIME media types [1, 2] provide a standard method for handling one major axis of variation, resources also vary in ways which cannot be expressed using currently available MIME headers. The case for a cross-protocol negotiation framework is set out in [4]. Klyne [Page 2] Internet draft 3 March 1998 Requirements for protocol-independent content negotiation This draft sets out terminology, a framework and requirements for a protocol-independent content negotiation framework, and identifies some technical issues which may need to be addressed. The framework does not attempt to specify the content negotiation process; rather it gives an indication of the anticipated scope and form of any such specifications. The statement of requirements is intended to set out the desired properties of a content negotiation framework, while trying to avoid any assumption of the form that framework may take. In its present form, this draft attempts to overstate rather than understate the requirements. The intention is that a wide range of requirements can be considered, and those considered inappropriate will be removed from this draft (or demoted to an explanatory statement explaining why they were dropped). 1.1 Structure of this document The main part of this draft addresses four main areas: Section 2 defines some of the terms which are used with special meaning. Section 3 outlines a proposed framework for describing protocol- independent content negotiation. Section 4 describes and explains the various requirements. A list of requirements is given at the start of section 4, with subsections containing more detailed explanations where required. Section 5 discusses some of the technical issues which are raised by this document, with cross-references to other work where appropriate. 1.2 Discussion of this document Discussion of this document should take place on the content negotiation and media feature reagistration mailing list hosted by the Internet Mail Consortium (IMC): Please send comments regarding this document to: ietf-medfree@imc.org To subscribe to this list, send a message with the body 'subscribe' to "ietf-medfree-request@imc.org". You should get a reply as follows: Klyne [Page 3] Internet draft 3 March 1998 Requirements for protocol-independent content negotiation The "ietf-medfree" mailing list is to discuss negotiating elements of the presentation of documents that are not naturally captured by the MIME Media Type. To see what has gone on before you subscribed, please see the mailing list archive at: http://www.imc.org/ietf-medfree/ To unsubscribe from the ietf-medfree mailing list, send a message to "ietf-medfree-request@imc.org" containing the message 'unsubscribe'. If you need to contact a human about this mailing list, please send a message to: phoffman@imc.org 1.3 Ammendment history 00a 06-Dec-1997 Document initially created. 00b 07-Dec-1997 Added definition of "transmission". Copied and adapted requirements from Internet fax requirements draft. Updated framework with details from Internet fax requirements draft. 00c 27-Feb-1998 Update mailing list details. Updated terminology entries, particularly to distinguish between a feature and a feature set. Fleshed out the abstract framework sections. Added more requirements, and reorganized that section. Written up technical issues, as identified so far. 1.4 Unfinished business . Finalize requirements . Finalize technical issues . Security considerations section . Acknowledgements section Klyne [Page 4] Internet draft 3 March 1998 Requirements for protocol-independent content negotiation 2. Terminology and definitions This section introduces a number of terms which are used with specific meaning in the content negotiation drafts. Many of these have been copied and adapted from [5]. The terms are listed in alphabetical order. Capability An attribute of a sender or receiver (often the receiver) which indicates an ability to generate or process a particular type of message content. Characteristic Some description of a sender or receiver which indicates a possible capability or preference. Choice message A choice message returns a representation of some selected variant or variants, together with the variant list of the negotiable resource. It can be generated when the sender has sufficient information to select a variant for the receiver, and also requires to inform the receiver about the other variants available. Connected mode A mode of operation in which sender and receiver are directly connected, and hence are not prevented from definitively determining each other's capabilities. (See also: Session mode) Content feature (see Feature) Content negotiation An exchange of information (negotiation metadata) which leads to selection of the appropriate representation (variant) when transferring a data resource. Data resource A network data object that can be transferred. Data resources may be available in multiple representations (e.g. multiple languages, data formats, size, resolutions) or vary in other ways. (See also: Message, Resource) Feature A piece of information about a resource. Klyne [Page 5] Internet draft 3 March 1998 Requirements for protocol-independent content negotiation Feature set Information about a sender, recipient, data file or other participant in a message transfer which describes the set of features that it can handle. Where a 'feature' describes a single identified attribute of a resource, a 'feature set' descibes full set of possible attributes. List message A list message sends the variant list of a negotiable resource, but no variant data. It can be generated when the sender does not want to, or is not allowed to, send a particular variant. Message Data which is transmitted from a sender to a receiver, together with any encapsulation which may be applied. Where a data resource is the original data which may be available in a number of representations, a message contains those representation(s) which are actually transmitted. Negotiation metadata is not generally considered to be part of a message. Message data is distinguished from other transmitted data by the fact that its content is fully determined before the start of transmission. Negotiated content Message content which has been selected by content negotiation. Negotiation (See: content negotiation) Negotiable resource A data resource which has multiple representations (variants) associated with it. Selection of an appropriate variant for transmission in a message is accomplished by content negotiation between the sender and recipient. Negotiation metadata Information which is exchanged between the sender and receiver of a message by content negotiation in order to determine the variant which should be transferred. Klyne [Page 6] Internet draft 3 March 1998 Requirements for protocol-independent content negotiation Neighbouring variant A particular representation (variant) of a variant resource which can safely be assumed to be subject to the same access controls as the variant resource itself. Not all variants of a given variant resource are necessarily neighbouring variants. The fact that a particular variant is or is not a neighbouring variant has implications for security considerations when determining whether that variant can be sent to a receiver in place of the corresponding variant resource. It may also have implications when determining whether or not a sender is authorized to transmit a particular variant. Preference An attribute of a sender or receiver (often the receiver) which indicates an preference to generate or process one particular type of message content over another, even if both are possible. Receiver A system component (device or program) which receives a message. Receiver-initiated transmission A message transmission which is requested by the eventual receiver of the message. Sometimes described as 'pull' messaging. E.g. an HTTP GET operation. Resource a document, data file or facility which is accessed or transmitted across a network. (See also: Data resource) Sender A system component (device or program) which transmits a message. Sender-initiated transmission A message transmission which is invoked by the sender of the message. Sometimes described as 'push' messaging. E.g. sending an e-mail. Session mode A mode of message transmission in which confirmation of message delivery is received by the sender in the same application session (usually the same transport connection) that is used to transmit the message. (See also: connected mode, store and forward mode) Store and forward mode A mode of message transmission in which the message is held in storage for an unknown period of time on message transfer agents before being delivered. Klyne [Page 7] Internet draft 3 March 1998 Requirements for protocol-independent content negotiation Transmission The process of transferring a message from a sender t a receiver. This may include content negotiation. User agent A system component which prepares and transmits a message, or receives a message and displays, prints or otherwise processes its contents. Variant One of several possible representations of a data resource. Variant list A list containing variant descriptions, which can be bound to a negotiable resource. Variant description A machine-readable description of a variant resource, usually found in a variant list. A variant description contains a variant resource identifier and various attributes which describe properties of the variant. Variant resource A data resource for which multiple representations (variants) are available. 3. Framework For the purposes of this document, message transmission protocol capabilities are explcitly disregarded: it is presumed that these will be dealt with separately by some orthogonal mechanism. Content negotiation covers three elements: 1. expressing the capabilities of the sender and the data resource to be transmitted (as far as a particular message is concerned), 2. expressing the capabilities of a receiver (in advance of the transmission of the message), and 3. a protocol by which capabilities are exchanged. Klyne [Page 8] Internet draft 3 March 1998 Requirements for protocol-independent content negotiation These negotiation elements are addressed by a negotiation framework incorporating a number of design elements with dependencies as shown below. [ Abstract ] [ Abstract ] [negotiation] [ negotiation ] [ process ] [ metadata ] | | V V [Negotiation] [ Negotiation ] [ protocol ] [ metadata ] [ binding ] [representation] | | ------- ------- | | V V [Application protocol] [ incorporating ] [content negotiation ] Within this overall framework, expressing the capabilities of sender and receiver is covered by negotiation metadata. The protocol for exchanging capabilities is covered by the abstract negotiation framework its binding to a specific application protocol. Application protocol independence is addressed by separating the abstact negotiation process and metadata from concrete representations and protocol bindings. 3.1 Abstract framework for content negotiation The negotiation framework provides for an exchange of negotiation metadata between the sender and receiver of a message which leads to determination of a data format which the sender can provide and the recipient can process. Thus, there are three main elements which are the subjects of the negotiation process and whose capabilities are described by the negotiation metadata: the sender, the transmitted data file format and the receiver. The life of a data resource may be viewed as: (C) (T) (F) [A]-->--[S]-->--[R]-->--[U] where: [A] = author of document (C) = original document content [S] = message sending system Klyne [Page 9] Internet draft 3 March 1998 Requirements for protocol-independent content negotiation (T) = transmitted data file (representation of (C)) [R] = receiving system (F) = formatted (rendered) document data (presentation of (C)) [U] = user or consumer of a document Here, it is [S] and [R] who exchange negotiation metadata to decide the form of (T), so these elements are the focus of our attention. Negotiation metadata provided by [S] would take account of available document content (C) (e.g. availability of resource variants) as well as its own possible ability to offer that content in variety of formats. Negotiation metadata provided by [R] would similarly take account of the needs and preferences of its user [U] as well as its own capabilities to process and render received data. 3.1.1 The negotiation process Negotiation between the sender [S] and the receiver [R] consists of a series of negotiation metadata exchanges that proceeds until either party determines a specific data file (T) to be transmitted. If the sender makes the final determination, it can send the file directly. Otherwise the receiver must communicate its selection to the sender who sends the indicated file. This process implies an open-ended exchange of information between sender and receiver. Not every implementation is expected to implent this scheme with the full generality thus implied. Rather, it is expected that every concrete negotiation can be viewed as a subset of this process. For example, Transparent Content Negotiation (TCN) described in [5] uses a model in which one of the following happens: . The recipient requests a resource with no variants, in which case the sender simply sends what is available. . A variant resource is requested, in which case the server replies with a list of available variants, and the client chooses one variant from those offered. . The recipient requests a variant resource, and also provides negotiation metadata (in the form 'Accept' headers) which allows the server to make a choice on the client's behalf. Each of these can be viewed as a particular case of the general negotiation process described above. Similar observations can be made regarding the use of directory services or MIME Multipart/ alternate in conjucntion with e-mail message transmission. Klyne [Page 10] Internet draft 3 March 1998 Requirements for protocol-independent content negotiation 3.2 Abstract model for negotiation metadata A simple but general negotiation framework has been described, which is based on the exchange of negotiation metadata between sender and recipient. The mechanism by which data is exchanged is not important to the abstract negotiation framework, but something does need to be said about the general form of the metadata. The terminology and definitions section of this document places constraints on the form of negotiation metadata, and the descriptions that follow should be read in conjunction with the definitions to which they refer. Negotiation metadata needs to encompas the following elements: . Media feature: a way to describe attributes of a data resource. . Feature set: a description of a range of possible media feature combinations which can be offered by a sender, represented by a data file format or processed by a receiver. . One or more naming schemes for labeling media features and feature sets. These should probably be backed up by some kind of registration process to ensure uniqueness of names and to encourage a common vocabulary for commonly used features. [[Text and binary format names? Separate name spaces for features and feature sets?]] . A framework of data types for media features, indicating the range and properties of value types which can be represented. . An algebra for combining media features into feature sets, capable of expressing feature dependencies within a feature set (e.g. 640x480 pixel size and 256 colours, or 800x600 pixel size and 16 colours). . Mechanisms which provide a way to rank feature sets based upon sender and receiver preferences for different feature values. 3.3 Text representation for negotiation metadata A concrete textual representation for media feature values and feature set descriptions would provide a common vocabulary for feature data in text-based protocols like HTTP and SMTP. Use of a common textual representation is not a requirement, but it would clearly be desirable. Klyne [Page 11] Internet draft 3 March 1998 Requirements for protocol-independent content negotiation In defining a textual representation, the issue of allowable character sets needs to be addressed. Whether or not negotiation metadata needs to support a full gamut of international characters will depend upon the framework of data types adopted for media features. As negotiation metadata would be used as a protocol element (not directly visible to the user) rather than part of the message content, support for extended character sets may be not required. A textual representation for negotiation metadata would imply a textual representation for media feature names, and also for expressions of the media feature combining algebra. 3.4 ASN.1 description of negotiation metadata For use with non-text-based protocols, an ASN.1 description and encoding designation for negotiation metadata could be helpful for incorporating the common negotiation framework into ASN.1-derived protocols like X.400, X.500, LDAP and SNMP. An ASN.1 description of negotiation metadata formats suggests that separate media feature naming scheme based on ISO object identifiers would be valuable. 3.5 Protocol binding guidelines Specific protocol bindings will be needed to use the abstract framework for negotiation. Details of protocol bindings would be beyond the scope of this work, but guidelines would probably not. (SASL might provide a useful model here.) 4. Requirements [[This section aims to overstate rather than understate the requirements, in order to seed a debate about what is *really* required.]] These requirements are presented in two categories: 1. Negotiation framework and metadata requirements which address the broad goals of negotiation in a protocol-independent fashion. 2. Specific requirements which relate to the deployment of negotiation in the context of a specific protocol (e.g. relation to HTTP protocol operations, cache interactions, security issues, existing HTTP negotiation mechanisms, application to variant Klyne [Page 12] Internet draft 3 March 1998 Requirements for protocol-independent content negotiation selection, etc.). These would be addressed by a specific protocol binding for the negotiation framework. 4.1 Generic framework and metadata requirements . A common vocabulary for designating features and feature sets. . A stable reference for commonly used features. . An extensible framework, to allow rapid and easy adoption of new features. . Permit an indication of quality or preference. . Capture dependencies between feature values . A uniform framework mechanism for exchanging negotiation metadata should be defined that can encompass all existing negotiatiable features and is extensible to future (unanticipated) features. . Efficient negotiation should be possible in both receiver initiated ('pull') and sender initiated ('push') message transfers. . The structure of the negotiation procedure framework should stand independently of any particular message transfer protocol. . Recognize and address the role of content negotiation in fulfilling the communication needs of less able computer users. 4.2 Protocol-specific deployment requirements . A negotiation should generally result in identification of a mutually acceptable form of message data to be transferred. . If capabilities are being sent at times other than the time of message transmission, then they should include sufficient information to allow them to be verified and authenticated. . A capability assertion should clearly identify the party to whom the capabilities apply, the party to whom they are being sent, and some indication of their date/time or range of validity. To be secure, capability assertions SHOULD be protected against interception and substitution of valid data by invalid data. . A request for capability information, if sent other than at the immediate time of delivery of a message, should clearly identify the requester, the party whose capabilities are being requested, and the time of the request. It should include sufficient information to allow the request to be authenticated. Klyne [Page 13] Internet draft 3 March 1998 Requirements for protocol-independent content negotiation . In the context of a given application, content negotiation may use one or several methods for transmission, storage, or distribution of capabilities. . The negotiation mechanism should include a standardized method for associating features with resource variants. . Negotiation should provide a way to indicate provider and recipient preferences for specific features. . Negotiation should have the minimum possible impact on network resource consumption, especially in terms of bandwidth and number of protocol round-trips required. . Systems should protect the privacy of users' profiles and providers' inventories of variants. . Protocol specifications should identify and permit mechanisms to verify the reasonable accuracy of any capability data provided. . Negotiation must not significantly jeopardize the overall operation or integrity of any system in the face of erroneous capability data, whether accidentally or maliciously provided. . Intelligent gateways, proxies, or caches should be allowed to participate in the negotiation. . Negotiation metadata should be regarded as cacheable, and explicit cache control mechanisms provided to forestall the introduction of ad-hoc cache-busting techniques. . Automatic negotiation should not pre-empt a user's ability to choose a document format from those available. 5. Technical issues [[[The idea of this is to highlight any additional technical issues which might fall out of the requirements or out of other discussions which don't fit comfortably in the previous sections.]]] 5.1 Non-message resource transfers The ideas for generic content negotiation have been conceived and developed in the context of message-oriented data transmissions. . streamed dat, . interactive computations, Klyne [Page 14] Internet draft 3 March 1998 Requirements for protocol-independent content negotiation . real-time data acqisition, ... and any others? Message data is defined elsewhere as a data whose entire content is decided before the start of data transmission. Does a proposed approach to negotiation based on message data reasonably extend to streamed data (e.g. data whose content is not fully determined by the time the first data items are transmitted)? [[I suspect the metadata will be OK, but the abstract negotiation process framework may be more difficult.]] 5.2 End-to-end vs hop-by-hop negotiations Could this distinction place any special demands or constraints on a generic negotiation framework, or is this simply a protocol issue? . End-to-end negotiation gives greatest confidence in the outcome. . Hop-by-hop may have advantages in a network of occasionally- connected systems, but will place additional demands on intervening message transmission agents. Hop-by-hop negotiation implies that negotiation responses are not necessarily a definitive indication of an endpoint system's capabilities. This in turn implies a possible need for time-to- live and re-verification mechanisms to flush out stale negotiation data. Note that one of the stated requirements is to allow proxies and caches to participate in the negotiation process. 5.3 Third-party negotiation An extension of the hop-by-hop vs end-to-end negotiation theme is to consider the implications of allowing any system other than an endpoint participant in the message transmission to supply negotiation metadata. Any use of a third party in the negotiation process inevitably increases the possibilities for introducing errors into the negotiation metadata. One particular exaple of a third party participant in a negotiation process that is frequently suggested is the use of a directory service using LDAP or similar protocols. What additional steps need Klyne [Page 15] Internet draft 3 March 1998 Requirements for protocol-independent content negotiation to be taken to ensure reasonable reliability of negotiation metadata supplied by this means? 5.4 Use of directory services (Using existing protocols such as LDAP to exchange content negotiation metadata.) (Will it be necessary to define directory schema elements which are specific to content negotiation. For example, an attribute type for a media feature set?) 5.5 Billing issues Negotiation may raise some billing-related issues in some contexts because it potentially incurs a two-way exchange of data not necessarily completed during a single connection. There is an issue of who pays for return messages, etc., in a non-connected environment like e-mail or fax. (Dan Wing's internet draft on DSN status code extensions for Internet fax [6] and others, raise issues in this area.) 5.6 Performance considerations Negotiation can impact performance in both postive and negative ways. The obvious negative impact arises from the exchange of additional data which necessarily consumes some additional bandwidth. There is also an issue of round-trip or third-party query delays while negotiation metadata is being exchanged before transmission of the message itself is commenced. Over the Internet, there are some bandwidth/latency trade-offs which can be made. For example, in Internet e-mail the MIME type Multipart/alternate can be used to send multiple versions of a resource: this preserves latency by using aditional bandwidth to send a greater volume of data. On the other hand, HTTP [7] suggests a negotiation mechanism which preserves bandwidth at the cost of introducing a round-trip delay (section 12.2, Agent-driven negotiation). To set against the negative performance impact of content negotiation, it is to be hoped that overall network efficiency is to be improved if it results in the most useful data format being delivered to its intended recipient, first time every time. Klyne [Page 16] Internet draft 3 March 1998 Requirements for protocol-independent content negotiation 5.7 Confidence levels in negotiated options In some cases (e.g. when there has been a direct exchange of information with the remote system) the communicating parties will have a high degree of confidence in the negotiation options obtained. Here, a data exchange can be performed without need for subsequent confirmation that the options used were acceptable. In other cases, the options will be a best-guess, and it may be necessary to make provision for parties to reject the options actually used in preference for some other set. This consideration is likely to interact with performance considerations. A useful pattern, adopted by TCN [5], is to define a negotiation procedure which guarantees a correct outcome. This forms the foundation for a procedure which attempts uses easily-obtained but less reliable information in an attempt to optimize the negotiation process but that contains checks to guarantee the final result will be the same as would have been obtained by the full negotiation procedure. Such procedures sometimes have to resort to the original "full cycle" negotiation procedure, but in a majority nof cases are expected to reach their conclusion by an optimized route. 6. Security considerations [[[Trawl through existing documents. Later, this should be used as a checklist and cross-referenced to the requirements which address them.]]] 6.1 Privacy (Unintended disclosure of personal information.) (Spoofed requests for negotiation data.) 6.2 Denial of service attacks 6.3 Mailing list interactions 6.4 Use of security services (Authenticated requests) (Authenticated responses) (Encrypted responses) Klyne [Page 17] Internet draft 3 March 1998 Requirements for protocol-independent content negotiation (Authenticated protocol session) (Encrypted protocol session?) (Authenticated transport connections) (Encrypted transport connections) 7. Copyright Copyright (C) The Internet Society 1998. All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 8. Acknowledgements Material in this draft has been taken from [....] (Koen Holtman/Andrew Mutz, TCN and feature drafts). (Ted Hardie, scenarios for negotiated content). (Larry Masinter, display attributes) (Dan Wing/Neil Joffe, SMTP Capabilities) Klyne [Page 18] Internet draft 3 March 1998 Requirements for protocol-independent content negotiation 9. References [1] "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies" N. Freed, Innosoft N. Borenstein, First Virtual RFC 2045 November 1996 [2] "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types" N. Freed, Innosoft N. Borenstein, First Virtual RFC 2046 November 1996 [3] "The Alternates Header Field" K. Holtman, TUV, et al. Internet draft: Work in progress, November 1997. [4] "Scenarios for the Delivery of Negotiated Content" T. Hardie, NASA Network Information Center Internet draft: Work in progress, November 1997. [5] "Transparent Content Negotiation in HTTP" Koen Holtman, TUE Andrew Mutz, Hewlett Packard Internet draft: Work in progress, May 1997. [6] "Extensions to Delivery Status Notifications for Fax" Dan Wing, Cisco Systems Internet draft: Work in progress, November 1997 [7] "Hypertext Transfer Protocol -- HTTP/1.1" R. Fielding, US Irvine J. Gettys, J. Mogul, DEC H. Frystyk, T Berners-Lee, MIT/LCS RFC 2068 January 1997 Klyne [Page 19] Internet draft 3 March 1998 Requirements for protocol-independent content negotiation 10. Author's address Graham Klyne Integralis Ltd Brewery Court 43-45 High Street Theale Reading, RG7 5AH United Kingdom Telephone: +44 118 930 6060 Facsimile: +44 118 930 2143 E-mail: GK@ACM.ORG Klyne [Page 20]