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Internet Draft                                 Paul Hoffman
draft-ietf-rescap-proto-main-00.txt            Internet Mail Consortium
January 30, 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.

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."

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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.

1. 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
 - Registration of the "rescap" scheme name for URLs

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

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

1.1 Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
document are to be interpreted as described in [MUSTSHOULD].

2. Finding a rescap Server

A rescap client that wants to find the attributes for a particular
Internet resource needs to find the rescap server for that resource.
The client MUST look up the A record associated with the rescap server
for the host name in the resource. When SRV records (described in
[SRV]) become widely deployed, the client MAY instead use the SRV
protocol to locate the rescap server.

To find the A record of the rescap server, the client prepends two
special domain names to the host name in the resource. The first name
prepended (just to the left of the host name) is "_rescap."; the second
name prepended (just to the left of "_rescap." is an underscore followed
by the URL scheme name.

For example, to find the rescap server that is authoritative for the
URI "mailto:someone@example.com", the rescap client would resolve the A
record of the name "_mailto._rescap.example.com" and use that value as
the location of the rescap server.

2.1 Optional use of SRV Records

SRV records MAY 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 3 of this specification, the rescap protocol
runs over both the UDP and TCP protocols, and all compliant servers
must run the rescap service on both protocols on the same host. 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 UDP
protocol, and MUST assume that the same host that was resolved for the
UDP protocol will support rescap over the TCP protocol as well. rescap
clients MUST NOT attempt to resolve rescap SRV records for the TCP

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

3. 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, of
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.

3.1 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, and MUST use the same port number for both services.
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 is allowed to 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.

3.2 Secure form

The secure form of rescap is quite similar to the basic form, but the
connection is made using TCP under TLS [TLS]. The port <TBD> has been
reserved by IANA for secure form rescap, and secure form rescap servers
SHOULD run on that port number. However, because the client is allowed
to find the port number using SRV records, the secure form rescap
server can run on any TCP port.

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.

4. 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.

5. References

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

[RESCAP-FORMAT] "The rescap Request and Response 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
2052. NOTE: RFC 2052 is being updated by
draft-ietf-dnsind-rfc2052bis. The examples in this document are
based on the Internet Draft, not on RFC 2052.

[TLS] "TLS Protocol", RFC 2246.

A. IANA Considerations

A.1 Registration for rescap URL Scheme

URL scheme name: rescap

URL scheme syntax: <scheme-name>://<hostport>?<base64-of-request>
         <scheme-name> is either the string "rescap" for basic form or
         "rescap-secure" for secure form. <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

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

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

Intended usage: To identify queries rescap servers. The resolution of a
         rescap URL will normally 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.

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 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 Versions of This Document

This document began as a single document that included the material
in [RESCAP-FORMAT]. It was split into two so that people could think
about each part separately. Thus, a great deal has been cut out and
moved to the other draft.

The biggest change in this document is that the protocol now has two
forms: basic and secure. Made many changes based on this.

Noted that the mailing list is now for the WG.

Updated the references.

Updated the URL registration for secure form.

D. Author Contact Information

Paul Hoffman
Internet Mail Consortium
127 Segre Place
Santa Cruz, CA  95060 USA

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