INTERNET-DRAFT                                               Mark Day
Expires: April 2, 2000                                         Lotus

                                                        Sonu Aggarwal

                                                          Gordon Mohr
                                                       CMGI Solutions

                                                          Greg Hudson

          Proposed Design Decisions for IMPP

1. 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."

The list of current Internet-Drafts can be accessed at

The list of Internet-Draft Shadow Directories can be accessed at

This document and related documents are discussed on the impp mailing
list. To join the list, send mail to To
contribute to the discussion, send mail to The
archives are at The IMPP
working group charter, including the current list of group documents,
can be found at

2. Abstract

As of this writing, the IMPP working group has largely reached
consensus on the requirements for IMPP [Reqts]. In addition, Hudson
[Issues] has catalogued a set of design issues discussed on the
mailing list.

This document proposes a starting point for further discussions about
the design of the Instant Messaging and Presence Protocol (IMPP). It
is not a fully-specified protocol, but a summary of design decisions
on which the authors were able to reach agreement during two full days
of discussion. Our discussions drew on knowledge of operational and
architectural aspects of Activerse Ding!, Lotus Sametime, Microsoft
Exchange instant messaging, MSN Messenger, and Zephyr, among other
diverse, widely-deployed systems supporting presence and/or instant
messaging.  We believe that the issues on which we reached agreement
are issues on which the working group as a whole should be able to
quickly reach rough consensus.  We hope that the working group will be
able to adopt these decisions as the basis for further work on
designing IMPP; of course, we recognize the possibility that strong
technical arguments not hitherto considered may cause the group to
arrive at different decisions in some instances.

We also identified some areas, primarily related to security, in which
we were lacking enough information or experience to make a solid
recommendation. We draw the working group's attention to these issues
in the hope that others can contribute.

Although we have treated presence and instant messaging in the same
document, we believe that these design decisions allow implementors to
offer either service independently of the other.

3. Contents

 1. Status of this Memo
 2. Abstract
 3. Contents
 4. Inter-Domain Architecture
 5. Protocol substrate
 6. Naming
 7. Reliability and Responses
 8. Presence format
 9. Instant message format
 10. Authentication
 11. Operations
    11.1 Presence Operations
    11.2 Instant Message Operations
 12. Connection management
 13. Inter-domain connection flow
 14. Leases
 15. Security
     15.1. Connection-Level Security
           15.1.1. User-to-Domain
           15.1.2. Inter-domain
     15.2. End-to-end security
 16. Access Control Lists (ACLs)
 17. Protocol Messages
 18. Protocol Transitions
     18.1. SUBSCRIBE
     18.2. UNSUBSCRIBE
     18.3. FETCH
     18.4. SEND
     18.5. CHANGE
     18.6. CHANGE-NOTIFY
     18.8. Kinds of ERROR
 19. Protocol Encoding
 20. Conclusions
 21. References
 22. Authors' Addresses

4. Inter-Domain Architecture

We agreed that there are multiple independent domains. Each domain has
its own users and handles authentication of users within that domain.

A domain has 3 possibly-separable functions for its users:
authenticator, traffic-forwarder, and locator/directory.  The
authenticator is responsible for authenticating a user to the
domain. A traffic-forwarder is responsible for getting requests and
responses to/from other domains. A locator/directory is responsible
for mapping user-friendly names to service addresses.

We discussed whether SRV lookups are required: that is, whether a
client must perform an SRV lookup to find an appropriate server within
its domain, and whether a server must perform an SRV lookup to find an
appropriate server in another domain.  Although requiring such a
lookup substantially increases the complexity of the minimal client,
we eventually concluded that the improved manageability is worth the

5. Protocol substrate

We decided that the protocol should be carried on TCP.  We discussed
many other possibilities, but found all of them had more drawbacks
than TCP.

To briefly summarize:

UDP with TCP fallback -- DNS works this way, and operational
experience suggests that the occasional TCP-mode connections alarm
administrators. The fallback mechanism means additional complexity in

Straight UDP -- would need fragmentation/reassembly story (for IMs
larger than a packet -- can't guarantee short msgs). The protocol
would also need to keep authentication information and similar
"connection-related" state. UDP with all these elaborations seems like
TCP but slower, and adds unnecessary implementation complexity.

UDP with URL reference for large messages -- this was suggested but
was not popular. One concern: when can the web site stop holding the
message at that URL?

Operational experience with both Lotus Sametime and Microsoft Exchange
deployments suggest that TCP's limitations aren't significant in

6. Naming

We decided that there should be an email-like form of IMPP address
that can be presented to users and solicited from users:
user@host. This facilitates using existing email addresses as IMPP
addresses, where possible, to make it easier for
end-users. Operational experience suggests that end users would prefer
to avoid remembering and communicating a new IMPP-specific address.

However, the canonical form is a URL because it's unambiguous:

The "email form" transforms to/from URL by simple textual
rearrangement. Other than possible quoting of disallowed characters,
there is no rewriting or rearrangement internal to the host or user
part. After quoting translation is done, the string on the left side
of the "@" in the email form is identical to the string on the right
side of the single "/" in the URL form.

7. Reliability and Responses

We decided that all requests, including change notifications,
potentially need to be acknowledged.

8. Presence format

We decided that XML has advantages as a basis for the presence format
markup. However, we were aware of the deficiencies of XML as discussed
on the mailing list. We agreed that the best solution would be to use
a simplified XML as the basis for the markup. The goal is to have a
markup that can be parsed easily by those implementations that have an
existing XML parser, but also can be parsed easily by those who have
to build a parser from scratch. Our initial agreement is that the
markup will use tags only, no attributes, and no processing

We discussed the use of vCard as a basis for the presence format. We
decided that it would be useful to copy many of its fields and
mechanisms, but that there is no need to either copy vCard syntax nor
attempt to treat the presence format as an extension of vCard.

Based on the desirability of using MIME-based security mechanisms
(S/MIME, PGP/MIME)for end-to-end security, we also agreed that a
presence information object should be a MIME object.  So a presence
information object is a MIME object containing XML-based
information. We also agreed that text/xml is not the correct MIME
content-type for the contained markup; it should probably be

9. Instant message format

We decided that the format of instant messages is essentially
identical to email, using RFC-822 headers [RFC822] and MIME [MIME].

10. Authentication

Domains authenticate themselves to each other independent of how users
authenticate themselves to a domain. Domain 1 can authenticate itself
to Domain 2 and thereby allow user in Domain 1 to subscribe to
presence information of a user in Domain 2.

Domain 1 can make policy choices as to whether users should contact
Domain 2 via Domain 1 servers or directly.

Domain 2 can choose to refuse traffic from users in Domain 1 except
when it comes via Domain 1 servers.

Domains trust or distrust each other to do the right thing. Domains
trust or distrust their users. These trust concerns are independent:
Domain 1 can either trust or not that Domain 2 has done a good job of
authenticating user@Domain2.  Domain 1 cannot condition its trust of
user@Domain2 on the authentication scheme used between that user and

11. Operations

11.1 Presence Operations

The following set of operations is sufficient: Fetch, Subscribe,
Unsubscribe, Change, Change-Notify, Terminate-Subscription, Set-ACL,

However, we did not all agree on the need for separate Set-ACL and
Get-ACL operations, noting that it would be possible to encode access
control lists (ACL) in the presence information itself. With such an
encoding, the Fetch and Change operations would be used to get and set
that information.

We agreed that the full set of operations is required for
client/server interoperability (i.e. for company X client to talk with
company Y server).  A smaller set is required for inter-domain
interoperability, where the missing operations can be assumed to only
take place within a domain: Fetch, Subscribe, Unsubscribe,
Change-Notify, Terminate-Subscription.

11.2 Instant Message Operations

A single operation is sufficient: Send. This operation is required for
all forms of interoperability.

12. Connection management

We considered the problem of managing the transfer of a large amount
of data: for example, the delivery of small instant messages might be
delayed while waiting for a multi-megabyte transfer to complete on a
single TCP connection.

Although there are potential advantages to having the protocol
explicitly "chunk" such large messages (multiplexing the underlying
TCP connection), we decided against the added complexity.

Instead, we decided there should be no explicit chunking in the
protocol. In the case where a short message is delayed by a long
transfer, we recommend the opening of another TCP connection and
letting the transport layer do the chunking.

We also agreed that as a management issue, both the initiator and the
target of a connection open should be able to indicate capacity limits
and/or maximum acceptable message size.  Exceeding these advertised
limits is sufficient grounds for the victim to close the connection.

13. Inter-domain connection flow

A TCP connection between two domains may be used on behalf of multiple
users in both domains, and accordingly may be kept open for long
times. However, correct operation of the protocol cannot depend on
keeping such an interdomain connection open. In particular, a domain
may need to deal with a situation in which it must open a connection
to another domain in order to send a response or a change notification.

14. Leases

Every subscription has an associated lease time, set by the server
where the subscription state is stored. To maintain a subscription,
the subscribing party must renew that lease before it expires.

The renewal process is initiated by the subscribing client.

The renewal time may be set to different intervals for local
(client-to-domain-server) and remote (interdomain) connections by a
sophisticated local domain server, although a server can also simply
forward information between a remote domain and a local user without
altering any renewal times. In general, the lease time(s) are set by the
server(s) that must manage the subscription resources.

15. Security

15.1. Connection-Level Security

15.1.1. User-to-Domain

We agreed that a plausible starting proposal for user-to-domain
connection-level security is to use SASL [SASL] as the negotiation
framework.  We agreed that negotiating to TLS [TLS] should be possible
but is not reasonable to mandate (because of performance concerns,
among others).

15.1.2. Inter-domain

We believe that a plausible starting proposal is to use SASL as the
negotiation framework, but we do not know what are reasonable
solutions to negotiate to within that framework.

We recommend that the working group organize to answer the following

1. What SASL mechanisms are most applicable to interdomain traffic?

2. If IMPP allows both secure and insecure domains, how can IMPP
prevent an attacker from pretending to be a secure domain operating

3. What, if anything, needs to be included in an application protocol
specification so that it can use TLS or other transport-level

15.2. End-to-end security

We decided that both the presence format and the instant message
format should have their content expressed as MIME, so that
end-to-end solutions using S/MIME and PGP/MIME are usable.

16. Access Control Lists (ACLs)

ACLs and how to manipulate them are properly the subject of a longer
discussion on their own, and we recommend the working group organize
to carry out that discussion.  IMPP can achieve interdomain
interoperability without getting ACLs right, but client/server
interoperability requires a consistent solution for setting and
getting ACLs.

There was no consensus among the authors about whether we should plan
to focus on interdomain interoperability first (possibly sacrificing
client/server interoperability), or insist on client/server
interoperability (thereby delaying interdomain interoperability).

17. Protocol Messages

This section outlines the messages of a protocol but does not
represent a complete specification.

Every protocol message has a request-ID, version, method name, and
method-dependent arguments.

Each protocol message is a request or a response. A party sending a
request can (and usually should) generate a new request-ID for the
request. A party sending a response must use a request-ID derived from
the request-ID of the corresponding request.

The requests are:


The responses are:


18. Protocol Transitions

The following sections describe each of the requests in more detail.
Again, this is not intended as a complete protocol specification. ACLs
are not considered at all, and the methods that are described are not
necessarily described completely.

We sketch the arguments and the meaning of an OK response. We also
provide a list of likely ERROR responses, though of course no single
request can produce all of these errors.

Note that this section does not specify any particular protocol
encoding; the next section considers protocol encoding decisions.



OK: returns Presence-Information, Lease-Time.
 -- The subscription has been created or extended for the length of
time indicated by Lease-Time.



 -- The subscription has been eliminated.

18.3. FETCH


OK: returns Presence-Information

18.4. SEND

SEND Target-URL Message-Body

OK: returns Presence-Information [usually empty but sometimes containing
"on vacation"-style information]
 -- message has been delivered to target

18.5. CHANGE

CHANGE Target-URL Presence-Information Lease-Time
[Special value of "infinity" in Lease-Time means to change underlying
"permanent" value.]

OK: returns Lease-Time (possibly different from supplied Lease-Time).
 -- The new Presence-Information will be supplied until the expiration
of the returned Lease-Time.


CHANGE-NOTIFY Target-URL Origin-URL Presence-Information

 -- Notification received, Target-URL acknowledges that Origin-URL now
has the value of Presence-Information described.



 -- Target-URL acknowledges that Origin-URL has terminated any
subscription that Target-URL previously had to Origin-URL.

18.8. Kinds of ERROR

Target unknown;
Subscription unknown;
Permission denied;
User authentication required;
Domain authentication required [interdomain only];
Temporarily forwarded to <URL>;
Permanently forwarded to <URL>;
Operation not supported;
Redirect to <server>;
Connection failure;
Message too large;
Mailbox full;
Mailbox unavailable;
MIME type not understood.

19. Protocol Encoding

We agreed that the protocol should be encoded as text. The syntactic
style of the encoding follows HTTP, but there is no attempt to either
extend HTTP or to reuse HTTP.

The following is an example of the way the protocol could be encoded
in compliance with this decision. We emphasize that this is only an
example, and we are not arguing that this is the exact protocol
encoding that must be used.

On interdomain requests, two additional headers are required. The
From: header identifies the originator of a request, and is used by a
remote domain to determine the domain to which a response should be
sent.  The Connection-ID: header allows the user's local domain to
distinguish between different connections or logins of the same user.
We propose that the originating client should set these headers in all
cases.  The local domain must check these headers to ensure their
correctness but does not rewrite or envelope the original request.

In general, if additional information is needed for inter-domain
requests beyond From: and Connection-ID:, this example encoding might
require a server to insert information in the middle of a request. We
agreed that this would be a problem in terms of efficiency and ease of
debugging. Accordingly, we agreed that the working group may need to
develop an encoding with a clear notion of an envelope for
inter-domain information that leaves the original client request
untouched inside the envelope.

Here is an example of a user subscribing to presence information in a
different domain:

  SUBSCRIBE 57AQ impp/1.0
  From: impp://
  Connection-ID: 6
  Target: impp://

57AX is the new request-ID, impp/1.0 is a protocol version number. The
From: and Connection-ID: fields make explicit information that the
local domain already knows. This information is expected to be
determined during session setup.

On receiving the request from an domain server, an
domain server takes the necessary steps to create a subscription and
sends the following reply:

  OK (SUBSCRIBE) 57AQ impp/1.0
  To: impp://
  Connection-ID: 6
  Lease-Time: 3000
  Content-Type: Application/Presence
  Content-Length: n

OK indicates a succesful operation, the original request is indicated
in parentheses to aid debugging, and the original request-ID (57AQ) is
used for the matching response.  The original interdomain From: header
becomes an interdomain To: header, and the original Connection-ID:
header is sent back unchanged.  The "...." following the
Content-Length: header is an n-byte MIME-encoded object containing an
XML-based markup of the presence information for

20. Conclusions

We have reached consensus on the following design decisions, which we
recommend to the IMPP working group as a whole:

1. A multi-domain architecture, in which user-to-domain authentication is
separated from inter-domain authentication.
2. TCP as the protocol substrate.
3. URLs as canonical names, with an email-like form as user-presentable
4. Requiring SRV lookups by clients.
5. Encoding protocol operations as text in an HTTP-like style.
7. Having OK or ERROR responses to any request.
8. Encoding presence information as a MIME object containing a presence
format based on simple XML.
9. Encoding an instant message as a MIME object.
10. Using SASL to negotiate user-to-domain authentication.

We also recommend further study and discussion by the working group to
investigate the security-related architecture and specific issues

We see this document as a plausible starting point of a design process
for IMPP. We acknowledge that the working group may choose not to
incorporate this document into its process, and indeed may organize
the design process in an entirely different fashion.  Nevertheless, we
have joined together in producing this document with the hope that our
collaboration might serve as the basis for a group-wide consensus on
some relevant issues.

21. References

[Reqts] M. Day, S. Aggarwal, G. Mohr, J. Vincent. Instant Messaging /
Presence Protocol Requirements. Work in progress,

[Issues] G. Hudson. Instant Messaging / Presence Protocol Design
Issues.  Work in progress, draft-hudson-impp-issues-00.txt.

[SASL] J. Myers. Simple Authentication and Security Layer (SASL). RFC
2222, October 1997.

[TLS] T. Dierks, C. Allen. The TLS Protocol Version 1.0. RFC 2246,
January 1999.

[RFC822] D. Crocker. Standard for the format of ARPA Internet text
messages. RFC 822, August 1982. Also see updates: RFC-1123, RFC-1138,
RFC-1148, RFC-1327, RFC-2156.

[MIME] N. Freed and N. Borenstein. Multipurpose Internet Mail
Extensions (MIME) Part One. RFC 2045, November 1996.

[S/MIME] B. Ramsdell, ed. S/MIME Version 3 Message
Specification. RFC 2633, June 1999.

[PGP/MIME] M. Elkins. MIME Security with Pretty Good Privacy
(PGP). RFC 2015, October 1996.

[HTTP] R. Fielding, J. Gettys, J. Mogul, H. Frystyk, L. Masinter,
P. Leach, T. Berners-Lee. Hypertext Transfer Protocol -- HTTP/1.1. RFC
2616, June 1999.

22. Authors' Addresses

Mark Day
Lotus Development Corporation
55 Cambridge Parkway
Cambridge, MA 02142

Sonu Aggarwal
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052

Gordon Mohr
CMGI Solutions, Inc.
1301 W. 25th St Suite 500
Austin, TX 78705

Greg Hudson
Massachusetts Institute of Technology
77 Massachusetts Avenue
Cambridge, MA 02139