Internet Draft                                 Paul Hoffman
draft-ietf-rescap-proto-main-01.txt            Internet Mail Consortium
August 8, 2000
Expires in six months

                     The rescap Resolution Protocol

Status of this memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

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Abstract

The rescap protocol is a general client-server resolution protocol that
translates resource identifiers to a list of attributes. For instance,
a rescap client can ask a rescap server for the attributes of a
particular mail user. rescap is very light-weight and acts only as a
resolution protocol, not a directory service. This document describes
the main features of the protocol.


. Introduction

When an Internet client is accessing a resource, it is often valuable
for the client to know the attributes of the resource before contacting
the resource. For example, a mail user might want to know whether
another mail user is able to natively display TIFF files before
creating a message with a TIFF file in it. The rescap protocol is a
simple, extensible client-server protocol that allows a client to
easily find the correct rescap server for a particular resource and
find the attributes for that resource.

This document specifies:
  - How to find the rescap server for a particular resource
  - The recap protocol

A different document, [RESCAP-FORMAT], specifies:
  - The format for rescap requests and responses
  - Required rescap items that must be supported

The protocol in this document attempts to meet the requirements
described in the rescap requirements document [RESCAP-REQUIRE]. It
should be noted that rescap is explicitly not to be used as a service-
discovery protocol; users that want such a capability should use the
Service Location Protocol [SLP].

Future documents may specify the administrative protocol described
in [RESCAP-REQUIRE].

This document and others relating to the rescap protocol are being
discussed in the recap WG of the IETF on the rescap@cs.utk.edu mailing
list. To subscribe, send a message to rescap-request@cs.utk.edu. An
archive of the mailing list is available at
<ftp://cs.utk.edu/pub/rescap>.

1.1 Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [MUSTSHOULD].


. Protocol

The rescap protocol uses a single request-response model. That is, a
rescap client connects to the rescap server, sends a single request,
and waits for the response. The server sends a single response and
closes the connection.

The rescap protocol has two forms: basic and secure. The basic form
offers only authentication by IP address, and gives no privacy for the
requests or the responses. The secure form offers client and server
authentication, and privacy. Conforming rescap servers MUST support
the basic form and MAY also support the secure form.

Note that many of the main motivations for rescap do not require any
security services: the information returned by the rescap server is
openly available to anyone. The basic form of rescap works just fine
for this situation. If the client needs to authenticate the server, or
if the server needs to authenticate the client by something other than
the client's IP address (such as for mobile users), or if privacy is
needed, the secure form MUST be used.

2.1 Basic form

The requirement that the rescap protocol be light-weight and fast leads
to the conclusion that it should run on UDP instead of TCP because UDP
takes less system and network resources for short exchanges. However,
UDP has many problems, including that long datagrams may get split up or
lost. Thus, to make rescap reliable, basic form rescap servers run on
both UDP and TCP. Implementations of basic form rescap SHOULD use UDP
checksums to help detect fragmentation.

A basic form rescap server MUST run the same service on both the UDP and
TCP protocols. The port number 283 has been reserved by IANA for basic
form rescap, and basic form rescap servers SHOULD run on that port
number. However, because the client MUST find the port number using SRV
records, the basic form rescap server can run on any UDP and TCP port.

A rescap client using the basic form SHOULD make its initial request to
the basic form rescap server using UDP. However, if the rescap client
using the basic form expects that the response from the server to be
longer than 512 octets (such as if it is asking for an attribute whose
value is always longer than 512 octets), the client MAY choose to make
the initial connection using TCP.

If a client using the basic form sends a UDP request to the server named
in an SRV record but does not receive a response, the client SHOULD send
the same request using TCP. If a client sends a UDP request to a server
and receives an incomplete response, the client MUST send the same
request using TCP. This is due to the fact that a later part of a
response might modify or negate the meaning of an earlier part of a
response.

2.2 Secure form

The secure form of rescap is quite similar to the basic form, but the
connection MUST be made using TCP under TLS [TLS]. No port has been
reserved by IANA for secure form rescap, so secure form rescap servers
MAY run on any port number. Because the client MUST find the port number
using SRV records, the secure form rescap server can run on any UDP and
TCP port.

Secure form rescap clients and servers MUST support the
TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA cipher suite in TLS. The client and
the server MUST NOT support any cipher suites that do not contain both
encryption and authentication.


. Finding a rescap Server

SRV records [SRV] MUST be used for locating rescap servers. The request
sent to the DNS server is somewhat different than is specified in [SRV]
because the rescap client must indicate the type of resource that will
be resolved by the rescap server. The resource name is given as the URI
scheme name, and it appears in the left-most part of the DNS name to be
resolved. The URI scheme name has an underscore character (_) prepended
to it, just as the service and prototype names do.

As described in section 2 of this specification, the basic form of the
rescap protocol runs over both the UDP and TCP protocols, and all
compliant basic form servers MUST run the rescap service on both
protocols on the same host (secure form servers only run
on TCP). It is inefficient to force the rescap client
to look up SRV records for both protocols, and doubling the number of
SRV records for the rescap protocol can have an adverse effect on the
domain name system. Thus, rescap clients MUST only resolve rescap SRV
records for the TCP protocol, and MUST assume that the same host that
was resolved for the TCP protocol will support rescap over the UDP
protocol as well. rescap clients MUST NOT attempt to resolve
rescap SRV records for the UDP protocol.

The two forms of rescap have different service names for the SRV
records. The basic form has the service name "rescap-basic" and the
secure form has the service name "rescap-secure".

For example, to find the basic server that is authoritative for the URI
"mailto:someone@example.com", the rescap client would resolve the SRV
record for the name:

_mailto._rescap-basic._tcp.example.com

To find the secure server that is authoritative for the URI
"mailto:someone@example.com", the rescap client would resolve the SRV
record for the name:

_mailto._rescap-secure._tcp.example.com


. Security Considerations

rescap users rely on the DNS for finding a rescap server. Unless the
DNS is secure, an attacker can cause a rescap user to get information
from the wrong server.

Basic form rescap offers no authentication of the client, no
authentication of the server, and no privacy of the requests or
responses. The secure form gives privacy of the request and the
response, and allows the client to authenticate the server and the
server to authenticate the client. Note, however, that few TLS
implementations use client authentication. The rescap format has
features for client authentication in both basic form and secure form.


. References

[MUSTSHOULD] "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119.

[RESCAP-FORMAT] "The rescap Request and Response Format",
draft-hoffman-rescap-proto-format.

[RESCAP-REQUIRE] "ResCap Requirements", draft-ietf-rescap-req.

[SLP] "Service Location Protocol, Version 2.", RFC 2608, and "Service
Templates and Service: Schemes", RFC 2609.

[SRV] "A DNS RR for specifying the location of services (DNS SRV)", RFC
2782.

[TLS] "TLS Protocol", RFC 2246.


A. IANA Considerations

A.1 Registration for rescap-basic URL Scheme

URL scheme name: rescap-basic

URL scheme syntax: rescap-basic://<hostport>?<base64-of-request>

      <hostport> is exactly as defined in RFC 2396. <base64-of-request>
      is the Base64 encoding of a rescap request.

      For example, to indicate a query to the basic form rescap server on
      the host "an.example.com" at the default port of 283, the URL would
      be rescap://an.example.com/SK398cske002CcksleEEx

Character encoding considerations: None. All three parts are expressed
         in US-ASCII.

Intended usage: To identify redirected queries to rescap servers. The
      resolution of a rescap URL will cause a query to be sent to the
      named server. The resolver would decode the Base64 of the third
      part of the URL and send it to the specified port (or the default
      port of 283 if no port is specified in the URL) using the TCP
      protocol. It is *not* expected that recap-basic URLs will be
      entered manually by users or be given by anything other than a
      referring rescap server.

Applications and/or protocols which use this URL scheme name:
      This document describes the rescap protocol.

Interoperability considerations: None known.

Security considerations: Same as in this document.

Relevant publications: This document.

Person & email address to contact for further information:
      Paul Hoffman <phoffman@imc.org>

Author/Change controller:
      Paul Hoffman <phoffman@imc.org>

A.2 Registration for rescap-secure URL Scheme

URL scheme name: rescap-secure

URL scheme syntax: rescap-secure://<hostport>?<base64-of-request>

      <hostport> is exactly as defined in RFC 2396. Note that, because
      there is no default port for secure form server, the port number
      MUST be included in the <hostport>. <base64-of-request> is the
      Base64 encoding of a rescap request.

      For example, to indicate a query to the form rescap server on the
      host at 10.20.30.40 at port 1234, the URL would be
      rescap-secure://10.20.30.40:1234/SK398cske002CcksleEEx

Character encoding considerations: None. All three parts are expressed
         in US-ASCII.

Intended usage: To identify redirected queries to rescap servers. The
      resolution of a rescap URL will cause a query to be sent to the
      named server. The resolver would decode the Base64 of the third
      part of the URL and send it to the specified port (or the default
      port of 283 if no port is specified in the URL) using the TCP
      protocol.  It is *not* expected that recap-basic URLs will be
      entered manually by users or be given by anything other than a
      referring rescap server.

Applications and/or protocols which use this URL scheme name:
      This document describes the rescap protocol.

Interoperability considerations: None known.

Security considerations: Same as in this document.

Relevant publications: This document.

Person & email address to contact for further information:
      Paul Hoffman <phoffman@imc.org>

Author/Change controller:
      Paul Hoffman <phoffman@imc.org>

A.3 Port reservation

IANA has reserved port 283 for basic form rescap. A port has been
requested for secure form rescap.


B. Acknowledgments

Graham Klyne provided suggestions for early versions of the first draft.


C. Changes Between the -00 and -01 Versions of This Document

Reversed the order of sections 2 and 3 because the new document needs to
describe the basic and secure forms before describing how to find them.

Made SRV records mandatory and dropped the previous kludge.

Made using the port number from the SRV record mandatory.

Changed the SRV lookup to use TCP to reduce confusion between the basic
and secure forms.

Updated the reference for SRV records.

Significantly updated the URL registration.

D. Author Contact Information

Paul Hoffman
Internet Mail Consortium
 Segre Place
Santa Cruz, CA  95060 USA
phoffman@imc.org