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Versions: (draft-klyne-conneg-requirements) 00 01 RFC 2703

IETF media feature registration WG                        Graham Klyne
Internet draft                                         Integralis Ltd.
                                                          3 March 1998
                                             Expires: 3 September 1998


      Requirements for protocol-independent content negotiation
               <draft-ietf-conneg-requirements-00.txt>

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.








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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].








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  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:






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  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









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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.








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  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.










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  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.





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  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.













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  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





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     (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.





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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.







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  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






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     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.





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  .  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,





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  .  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






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  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.








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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)






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  (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)






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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: <draft-ietf-http-alternates-01.txt>
     Work in progress, November 1997.

[4]  "Scenarios for the Delivery of Negotiated Content"
     T. Hardie, NASA Network Information Center
     Internet draft: <draft-ietf-http-negotiate-scenario-02.txt>
     Work in progress, November 1997.

[5]  "Transparent Content Negotiation in HTTP"
     Koen Holtman, TUE
     Andrew Mutz, Hewlett Packard
     Internet draft: <draft-ietf-http-negotiation-02>
     Work in progress, May 1997.

[6]  "Extensions to Delivery Status Notifications for Fax"
     Dan Wing, Cisco Systems
     Internet draft: <draft-ietf-fax-dsn-extensions-00.txt>
     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






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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








































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