IMPP WG D. Crocker
Internet-Draft Brandenburg
Expires: April 27, 2003 A. Diacakis
F. Mazzoldi
Net Proj
C. Huitema
Microsoft
G. Klyne
Baltimore
J. Rosenberg
R. Sparks
dynamicsoft
H. Sugano
Fujitsu
J. Peterson
NeuStar
October 27, 2002
Address Resolution for Instant Messaging and Presence
draft-ietf-impp-srv-00
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
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This Internet-Draft will expire on April 27, 2003.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
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Abstract
Presence and instant messaging are defined in RFC2778 [5]. The
Common Profiles for Presence [2] and Instant Messaging [1] define two
URI schemes: 'im' for INSTANT INBOXes and 'pres' for PRESENTITIES.
This document provides guidance for locating the resources associated
with URIs that employ these schemes.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Address Resolution . . . . . . . . . . . . . . . . . . . . . . . 3
4. Domain Name Lookup . . . . . . . . . . . . . . . . . . . . . . . 3
5. Processing SRV RRs . . . . . . . . . . . . . . . . . . . . . . . 4
6. Processing Multiple Addresses . . . . . . . . . . . . . . . . . 4
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
References . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
Presence and instant messaging are defined in RFC2778 [5]. The
Common Profiles for Presence [2] and Instant Messaging [1] define two
URI schemes: 'im' for INSTANT INBOXes and 'pres' for PRESENTITIES.
This document provides guidance for locating the resources associated
with URIs that employ these schemes.
The system described in this document relies on the use of DNS SRV
[7] records and A records.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in RFC2119 [3] and indicate requirement levels for
compliant implementations.
This memos makes use of the vocabulary defined in RFC 2778[9]. Terms
such as CLOSED, INSTANT INBOX, INSTANT MESSAGE, and OPEN are used in
the same meaning as defined therein.
3. Address Resolution
A client determines the address of an appropriate system running a
server by resolving the destination domain name that is part of the
identifier to either an intermediate relay system or a final target
system.
Only resolvable, fully-qualified, domain names (FQDNs) are permitted
when domain names are used in an IM URI (i.e., domain names that can
be resolved to SRV[11] or A RRs).
4. Domain Name Lookup
Once a client lexically identifies a domain to which instant
messaging or presence operations will be delivered for processing, a
DNS lookup MUST be performed to resolve the domain. The names MUST
be fully-qualified domain names (FQDNs) -- mechanisms for inferring
FQDNs from partial names or local aliases are a local matter.
The lookup first attempts to locate SRV RRs associated with the
domain. If a CNAME RR is found instead, the resulting domain is
processed as if it were the initial domain.
If one or more SRV RRs are found for a given domain, a sender MUST
NOT utilize any A RRs associated with that domain unless they are
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located using the SRV RRs. If no SRV RRs are found, but an A RR is
found, then the A RR is treated as if it was associated with an
implicit SRV RR, with a preference of 0, pointing to that domain.
5. Processing SRV RRs
Taking the IM URI for a concrete example, a lookup is performed for
SRVs for the target domain and a desired IM transfer protocol.
For example, if the destination INSTANT INBOX is
"im:fred@example.com", and the sender wishes to use an IM transfer
protocol called "SIP", then a SRV lookup is performed for:
_im._sip.example.com.
The returned RRs, if any, specify the next-hop server.
The choice of IM transfer protocol is a local configuration option
for each system.
Using this mechanism, seamless routing of IM traffic is possible,
regardless of whether a gateway is necessary for interoperation. To
achieve this transparency, a separate RR for a gateway must be
present for each transfer protocol and domain pair that it serves.
The same logic is used for PRES URIs.
6. Processing Multiple Addresses
When the lookup succeeds, the mapping can result in a list of
alternative delivery addresses rather than a single address, because
of multiple SRV records, multihoming, or both. For reliable
operations, the client MUST be able to try each of the relevant
addresses in this list in order, until a delivery attempt succeeds.
However, there MAY also be a configurable limit on the number of
alternate addresses that can be tried. In any case, the client
SHOULD try at least two addresses. Two types of information are used
to rank the domain addresses: multiple SRV records, and multihomed
domains.
Multiple SRV records contain a preference indication that MUST be
used in sorting. Lower numbers are preferable to higher ones. If
there are multiple destinations with the same preference, and there
is no clear reason to favor one (e.g., by recognition of an easily-
reached address), then the sender MUST randomize them to spread the
load across multiple servers for a specific destination.
The destination domain (perhaps taken from the preferred SRV record)
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may be multihomed, in which case the resolver will return a list of
alternative IP addresses. It is the responsibility of the resolver
to have ordered this list by decreasing preference if necessary, and
the sender MUST try them in the order presented.
7. Security Considerations
The usage of IM and PRES URIs, and the DNS procedures in this
document, introduce no security considerations beyond those described
in the requirements for instant messaging and presence ([6]) and the
SRV specification ([7]).
8. IANA Considerations
This document introduces no new considerations for IANA.
References
[1] Crocker, D., Diacakis, A., Mazzoldi, F., Huitema, C., Klyne, G.,
Rosenberg, J., Sparks, R., Sugano, H. and J. Peterson, "Common
Profile: Instant Messaging", draft-ietf-impp-im-00 (work in
progress), October 2002.
[2] Crocker, D., Diacakis, A., Mazzoldi, F., Huitema, C., Klyne, G.,
Rosenberg, J., Sparks, R., Sugano, H. and J. Peterson, "Common
Profile: Presence", draft-ietf-impp-pres-00 (work in progress),
October 2002.
[3] Bradner, S., "Key words for use in RFCs to indicate requirement
levels", RFC 2119, March 1997.
[4] Mockapetris, P., "Domain Names - Concepts and Facilities", RFC
1034, STD 13, November 1987.
[5] Day, M., Rosenberg, J. and H. Sugano, "A Model for Presence and
Instant Messaging", RFC 2778, February 2000.
[6] Day, M., Aggarwal, S. and J. Vincent, "Instant Messaging /
Presence Protocol Requirements", RFC 2779, February 2000.
[7] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
Specifying the Location of Services (SRV)", RFC 2782, February
2000.
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Authors' Addresses
Dave Crocker
Brandenburg InternetWorking
675 Spruce Drive
Sunnyvale, CA 94086
US
Phone: +1 408/246-8253
EMail: dcrocker@brandenburg.com
Athanassios Diacakis
Network Projects Inc.
4516 Henry Street
Suite 113
Pittsburgh, PA 15213
US
Phone: +1 412/681-6950 x202
EMail: thanos@networkprojects.com
Florencio Mazzoldi
Network Projects Inc.
4516 Henry Street
Suite 113
Pittsburgh, PA 15213
US
Phone: +1 412/681-6950
EMail: flo@networkprojects.com
Christian Huitema
Microsoft Corporation
One Microsoft Way
Redmund, WA 98052-6399
US
EMail: huitema@microsoft.com
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Graham Klyne
Baltimore Technologies
1310 Waterside
Arlington Business Park
Theale, Reading RG7 4SA
UK
Phone: +44 118 903 8000
EMail: gk@acm.org
Jonathan Rosenberg
dynamicsoft
200 Executive Drive
Suite 120
West Orange, NJ 07052
US
EMail: jdrosen@dynamicsoft.com
Robert Sparks
dynamicsoft
200 Executive Drive
Suite 120
West Orange, NJ 07052
US
EMail: rsparks@dynamicsoft.com
Hiroyasu Sugano
Fujitsu Laboratories Ltd.
200 Executive Drive
64 Nishiwaki, Ohkubo-cho
Akashi 674-8555
JP
EMail: suga@flab.fujitsu.co.jp
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Jon Peterson
NeuStar, Inc.
1800 Sutter St
Suite 570
Concord, CA 94520
US
Phone: +1 925/363-8720
EMail: jon.peterson@neustar.biz
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Acknowledgement
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