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Versions: 00 01                                                         
Internet Draft                                                 John Beck
Document: draft-ietf-rescap-req-00.txt                       Sun Microsystems
December 9, 1999
Expires June 9, 2000

                      ResCap Requirements

Status of this memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   and may be updated, replaced, or obsoleted by other documents at any
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   A variety of resource identifiers have been widely deployed on the
   Internet as a means of identifying various resources, services, and
   destinations.  However, a means of attaching a set of attributes or
   characteristics to a given resource identifier and subsequently
   assessing those attributes or characteristics has not been specified
   and deployed.

   Two tasks are envisioned.  The first task will be to define a general
   resolution protocol that will translate resource identifiers to a
   list of attributes.  The second task will be to define an
   administrative model and update protocol that can be used to set
   up and maintain the information the resolution protocol accesses.

   This document defines the requirements for these two protocols.

0. Discussion

   This draft is being discussed on the ResCap mailing list at
   <rescap@cs.utk.edu>.  Subscription requests can be sent to
   <rescap-request@cs.utk.edu> (send an email message with the
   word "subscribe" in the body).  More information on the mailing
   list along with an archive of back messages is available at

1. Introduction

   An attribute is a named characteristic of a resource identifier with
   a value that is meaningful in the context in which it is used.  An
   attribute may be requested from a ResCap server.  An example of an
   attribute might be a content type that an email address is capable
   of receiving.

   A capability is a named set of one or more attributes that is
   meaningful in the context in which it is evaluated.  A capability
   may be requested from a ResCap server.  An example of a capability
   would be the complete set of content types that an email address
   could receive.

   A particularly important resolution service of the general type
   described in the Abstract is one which, when given a mail address
   identifying a particular mail recipient, will return a series of
   attributes describing the capabilities of that recipient.  This
   differs from a directory service in that no searching or other
   advanced query operations are involved.  Likewise; this is not
   a discovery protocol.

2. General requirements

   2.1 Security

       The protocols must include support for authenticated messages
       between clients and servers.  Specifically, it must be possible
       for a rescap client requesting information to reliably identify
       the server who is the source of the response.  The client should
       be able to require a server to authenticate itself.  The question
       as to whether the server is authorized to respond with the
       information requested is outside the scope of the protocols.
       See the Security Considerations section for more information.

       Similarly, it must be possible for a rescap server to reliably
       identify the client who is making the request for information.
       The server should be able to require the client to authenticate
       itself.  The question as to whether the client is authorized to
       request the information is outside the scope of the protocols.
       See the Security Considerations section for more information.

       Because the principal purpose of the protocols is the
       dissemination of public information, support for confidentiality
       services is explicitly not required from the protocols.
       Nevertheless, specific attributes or capabilities may themselves
       be encrypted, unbeknownst to the protocol.  Also, local security
       policies may require the use of a secure transport layer that
       includes confidentiality services.   See the Security
       Considerations section for more information.

3. Resolution protocol requirements

   Throughout the rest of this section, "the protocol" refers to the
   resolution protocol.

   3.1 Scalability

       The protocol must be highly scalable both for number of entries
       in the database and number of entries per second resolved.

       Example: Mail services with tens of millions of users could
       easily expect tens of millions of requests per day for client
       attribute information.

   3.2 Reply data

       The protocol must be able to support attributes and capabilities
       that are arbitrarily long text or binary values.  The protocol
       should be optimized for the exchange of relatively short length
       resource identifiers and attribute values.  By way of example, the
       distinction between short and long could be determined by whether
       or not the request and response fit in an ordinary UDP packet.

       Servers must either provide the information requested, provide a
       referral indicating from where the information may be requested,
       or indicate the information is not available.  Servers may forward
       requests on behalf of clients, but the forward must be transparent
       to the client.

   3.3 Granularity

       A mechanism needs to exist whereby a subset of capabilities for
       a resource can be fetched.  I.e., the protocol request syntax
       should be able ask for one or more features instead of all of
       them at once.  However, the client also needs to be able to ask
       for all capabilities known to the server without naming all of

       Example: A client might only want to know the S/MIME capabilities
       of a recipient, but not care about its media features.

   3.4 Expiration

       Some means to indicate the expected lifetime of a capability
       is required, so that a client application can judge whether,
       or when, the information should be considered stale.

       The expectation is that data will be infrequently updated, and
       that the granularity for time-stamps would be in seconds.

       The protocol should also support a mechanism for indicating the
       "last changed date" of a given attribute.

       Example: The ResCap server may believe that the recipient is only
       temporarily unable to receive large mail messages.

   3.5 Referral

       Some sort of request referral mechanism is needed.  In other
       words, the protocol must support a mechanism whereby a response
       can indicate "I don't know, but go ask the ResCap server at
       address X." or "use the following URL to retrieve the ResCap
       response you requested." That is, the response might be a simple
       DNS name, or it might be a full ResCap URL.

       Example: A server might delegate all requests for S/MIME
       certificate information to a different server that keeps track
       of that type of information.

   3.6 Security

       The protocol must be able to handle authenticated queries.
       The protocol must also support transmission of signed and/or
       encrypted responses.  The protocol should allow for a ResCap
       server to hold and return "pre-signed" responses.  This would
       allow it to hold capability information signed by the controller
       of the resource to which it refers, and also to sign responses
       off-line to avoid the need to perform signature computations at
       the time of responding to a query.  This may imply a need to
       apply a signature to just part of a response.

       Servers may require clients to use authenticated requests.
       Clients are not required to be able to create authenticated
       queries.  Such clients will not be able to make requests of
       servers requiring authenticated queries; such servers might
       regard this as a feature.

       Clients may require servers to use authenticated responses.
       Servers must be able to create authenticated responses.

       Controls on which attributes will be announced should exist.

       Note that consideration of transport-level security is out of
       scope here; an appropriate mechanism such as TLS or IPSec should
       be used instead.

       Example: A server might give less information to a client that
       is unauthenticated than to one that is authenticated.  Some
       information from the server may be important enough for the
       server to want to prevent tampering, or even to prevent snoopers
       from reading.

   3.7 Server location

       DNS resource records should be used to determine a protocol

       Example:  The ResCap server that provides responses for resources
       at "example.com" might itself be running on a host other than
       "example.com", such as if the administrator of "example.com" has
       outsourced ResCap services to another company.

   3.8 Preference

       Preferences for a set of capabilities, if needed, are to be a
       supplied in a schema-specific fashion.

       Example: A recipient might strongly prefer image/tiff files over
       image/jpeg because s/he can display image/tiff on his/her system
       without launching an external application.

   3.9 Simplicity

       The protocol should be sufficiently simple that it allows
       implementation of client and/or server functionality in very
       small, low cost devices (e.g. telephones, modems, printers,
       smart-cards, etc.).

   3.10 Replication and Synchronization

        We need to define the model for consistency between replicas -
        e.g. if a client is using one replica for queries and has
        to fail over to another, what assumptions can the client make
        about the consistency between the two replicas?

        We should consider whether to assume strict master-slave
        as this choice would severely limit scalability of the service.
        Multi-master seems preferable, but this impacts the protocol as
        hooks are needed to allow clients to make sense of the data if
        they need to get it from multiple servers.

4. Administrative update protocol requirements

   Throughout the rest of this section, "the protocol" refers to the
   administrative update protocol.

   4.1 Access control

       Authentication of anyone updating the database is required.

       Example: Individual mail users should be able to update some or
       all of the information about them in the database, but such
       updates must be done with authentication to prevent others from
       maliciously entering false information.

   4.2 Inheritance

       The protocol must support inheritance.  Specifically, mechanisms
       must be provided by which administrators can set default values
       for members of their administrative domains.

       If a change is made to a default resource capability value that
       has previously been inherited by some other resource, then the
       inheritor should receive the new value.

       Example: The media features for all addresses at a particular
       mail server might be the same because the mail server processes
       all messages at all addresses.

   4.3 Scalability

       The protocol must support atomic processing of request messages.
       This processing does not include updating any replicated sources
       for the information nor.  The client should receive a completion
       response from the server, but the underlying protocol (if used
       over UDP) might require the client to retransmit its request at
       appropriate intervals until it receives such a response.

   4.4 Security

       The protocol must include support for authenticated messages.
       Servers and clients must use authenticated requests and responses
       to effect changes to attributes or capabilities.

   4.5 Replication and Synchronization

       SRV and/or NAPTR resource records may be used to determine a
       protocol server.  [SRV, NAPTR]

5. Security Considerations

   Security issues are discussed in sections 2.1, 3.6, 4.1 and 4.4 of
   this memo.

   It is also worth noting that the data which these protocols will be
   designed to deal with are meant to be public, not private.

   However, support for authentication should be included in the
   resolution protocol to permit administrators to restrict the
   attributes that are returned in response to a request according to a
   locally specified security policy.  The security policy may require
   the use of a transport security protocol, e.g., TLS [TLS], to provide
   additional security services not supported by these protocols.

6. Acknowledgements

   The author would like to thank Paul Hoffman, Graham Klyne, Ned Freed,
   James Galvin and Keith Moore for their assistance.

7. References

   [CONNEG] "A Syntax for Describing Media Feature Sets", RFC 2533.

   [CONNEG-MEDIA] "MIME content types in media feature expressions",

   [NAPTR] "Resolution of Uniform Resource Identifiers using the
   Domain Name System", RFC 2168.

   [SRV] "A DNS RR for specifying the location of services (DNS SRV)",
   RFC 2052.

   [TLS] "The TLS Protocol Version 1.0", RFC 2246

   [UDP] "User Datagram Protocol", RFC 768.

8. Author's Address

   John Beck
   Sun Microsystems
   901 San Antonio Road
   M/S U-MPk-17-202
   Palo Alto, CA  94303-4900
   (650) 786-8078

I. Issues

   1. Does section 3.6 need further clean-up, particularly the first
   2. Are sections 4.1 and 4.4 redundant?
   3. Section 3.10 currently lays out no requirements, but asks some
      questions and raises some issues which need to be addressed in
      order for the proper requirements to be determined.

X. Revision history

   X.0 Initial revision (beck-00): April 15, 1999

   X.1 beck-00 -> beck-01: May 14, 1999

      * Added note to Abstract about this not being a discovery protocol.
      * Examples added throughout.
      * Section 1.2 renamed from "Long replies" to "Reply data", and
      * Clarified section 1.4, and added note about support for a "last
        changed date".
      * Section 1.6 renamed from "Privacy" to Security, and clarified,
        largely by absorbing section 2.2 (Privacy).  As a result, section
        2.3 (Inheritance) renumbered to 2.2 .
      * New section 4 (Acknowledgements) added, and old sections 4
        (References) and 5 (Author's Address) renumbered to 5 and 6.

   X.2 beck-01 -> beck-02: June 15, 1999

      * New section 1.9 (Simplicity) added.

   X.3 beck-02 -> beck-02a: November 23, 1999

      * Added notes about infrequent updates and granularity of seconds
        to section 1.4 (Expiration).
      * Added note about transport-level security being out of scope to
        section 1.6 (Security), and did some word-smithing.
      * Section 1.7 (Server location): specific references to SRV and
        NAPTR replaced by general reference to DNS.  Deleted
        corresponding references from section 5 (References).  Also
        rewrote example in section 1.7 .
      * Section 1.8 (Preference) changed to be schema-specific.
      * Added note about the effect of changing a default resource  to
        section 2.2 (Inheritance).
      * Added note about public vs. private date to section 3 (Security
      * Added appendix X (Revision history).

      beck-02a -> beck-03: December 1, 1999

      * New section 1 (Introduction) and 2 (General requirements); old
        sections 1 through 6 renumbered to 3 through 8.
      * New section 4.3 (Scalability) added.
      * Section 3.2 (Reply data) rewritten.

      beck-03 -> rescap-00: December 9, 1999

      * ResCap is now a WG, so document name changes.
      * Added note about authentication to Security Considerations.
      * Added appendix I for open issues.
      * Added sections 2.1 and 4.4, and added to 3.6 (all on Security).
      * Added sections 3.10 and 4.5 (on Replication and Synchronization).