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Versions: 00 01 02                                                      
Internet Draft                                                 John Beck
Document: draft-beck-rescap-req-01.txt                  Sun Microsystems
May 14, 1999
Expires November 14, 1999

                      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.

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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.

   A particularly important resolution service of this general type 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.

   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. Resolution protocol requirements

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

   1.1 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.

   1.2 Reply data

       The protocol should be capable of returning resource capabilities
       that are arbitrarily long text or binary values.  (E.g., Conneg
       [CONNEG] uses arbitrary length text values; public key
       certificates are arbitrary length binary values; etc.)  Such data
       might overflow a UDP datagram, so the protocol must allow for
       this; however, the default should be to use UDP. [UDP]

       Example: Lists of media features or S/MIME certificates can
       easily be longer than a single UDP datagram.

   1.3 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.

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

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

   1.5 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.

   1.6 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 server to
       "pre-sign" responses, meaning that the server could sign part of
       a response off-line so it could present this over and over.
       Controls on which attributes will be announced should exist.

       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.

   1.7 Server location

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

       Example: The ResCap server that is running on the host
       "example.com" might not be the ResCap server for all resources
       that have the host "example.com", such as if the administrator
       at "example.com" had outsourced ResCap services for some resource
       types to another company.

   1.8 Preference

       For a set of capabilities, there should be a means to indicate a
       preferred value or a ranking of preference.

       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.

2. Administrative update protocol requirements

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

   2.1 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.

   2.2 Inheritance

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

       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.

3. Security Considerations

   Security issues are discussed in sections 1.6 and 2.1 of this memo.

4. Acknowledgements

   The author would like to thank Paul Hoffman and Graham Klyne for
   their assistance.

5. References

   [CONNEG] "A Syntax for Describing Media Feature Sets", RFC 2553.

   [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.

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

6. 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