Internet DRAFT - draft-aol-imx


INTERNET-DRAFT                                                   E. Aoki
<draft-aol-imx-00.txt>                                           A. Wick
Expires: December 15, 2000                                 June 15, 2000

                            The IMX Architecture
   Interoperability with America Online's Instant Messaging Services

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
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  Internet-Drafts are draft documents valid for a maximum of six months
  and may be updated, replaced, or obsoleted by other documents at any
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  This memo provides information for the Internet community.

Copyright Notice

  Copyright (C) The Internet Society (2000).  All Rights Reserved.


  The ability to exchange instant messages and presence information
  provides users with a powerful mechanism for communicating in

  This document outlines an architecture for interoperability among
  Instant Messaging Systems which allows disparate systems to
  exchange messages and presence information while being
  relatively easy to implement and maintaining a high standard of
  security and scalability.

Table of Contents

   1. Introduction .........................................    2
   2. Requirements .........................................    3
   3. Terminology ..........................................    4
   4. Architecture .........................................    6
   4.1 Servers and Gateways ................................    6

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   4.2 Namespace and Addressing ............................    7
   4.2.1 Address identifiers ...............................    7
   4.2.2 Domains  ..........................................    7
   4.2.3 Server Discovery ..................................    7
   4.3 Connection Management ...............................    8
   4.3.1 Originating Connections ...........................    8
   4.3.2 Accepting Connections .............................    8
   4.3.3 Relays ............................................    9
   4.4 Protocol Considerations .............................    9
   4.5 Additional Protocol Considerations for Presence
       Information .........................................   10
   4.6 Additional Protocol Considerations for Attributes ...   10
   4.7 Additional Protocol Considerations for Instant
       Message Information .................................   10
   5. Instant Message Format Considerations ................   11
   6. Security Considerations ..............................   11
   6.1 Objectives ..........................................   11
   6.2 Assumptions .........................................   12
   6.2.1 Client Authentication .............................   12
   6.2.2 Scope .............................................   12 Types of Attacks Within the Scope ...............   13 Types of Attacks Outside of Scope ...............   13
   6.3 A Trust Model for Server to Server communications....   14
   6.3.1 The Dial-Back Mechanism ...........................   14
   6.3.2 Enhanced Security .................................   15
   6.4 SPAM ................................................   16
   7. Authors' Addresses ...................................   16
   8. Additional Documents .................................   17
   9. Acknowledgements .....................................   17
   10. References ..........................................   17
   A. Appendix - Performance Against Objectives ............   17
   A.1 Scalability and Efficiency ..........................   18
   A.2 Ease of Implementation ..............................   19
   A.2.3 Reliability .......................................   19
   A.2.4 Security ..........................................   19

1. Introduction

  Today's instant messaging systems are typically comprised of a
  client, through which the end-user interacts, and servers which relay
  information between compatible clients.  Tight integration between
  clients and servers allows instant messaging services to provide a
  secure, reliable channel through which authentication, presence, and
  messaging information is passed between users and the service.

  As the number of instant messaging providers has grown, there is 
  increased interest in enabling IM users to exchange presence and 
  messaging information not only with users on their system, but
  with those on other systems as well Some vendors have responded by 
  creating "multi-headed clients," clients which can simultaneously 
  communicate with servers on disparate instant messaging systems.

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  Such clients achieve interoperability at a high price, however.
  Since each service has its own feature set, clients may advertise
  features that do not work across systems.  Since each service
  implements its own security model, multi-headed clients must
  often resort to mechanisms that circumvent security or require
  the user to provide passwords to third parties.  Inconsistent
  terms of service also make it difficult to enforce anti-spam
  measures or encourage equitable resource sharing.  And vendors
  are forced to constantly upgrade clients to keep up with
  changes in features and services across the instant messaging

  An alternative approach is to provide a mechanism for the services
  themselves to interoperate in a peering arrangement, much like the
  Internet mail system works today.  In such a system, the interaction
  between instant messaging clients and their associated servers would
  remain much as it is today, but servers could communicate with other
  servers to exchange presence information, messages, or other data.
  This approach helps to preserve existing models for security
  and allows Instant Messaging Service providers to manage client
  authentication, service policy, and privacy.

  This document describes how America Online intends to develop such
  an architecture to allow its services to discover other servers
  (and be discovered by other servers), exchange data, and ensure
  security.  It also describes implications that any architecture for
  interoperability may have for the spread of unsolicited instant 
  messaging (spam).

2. Requirements

  The authors set out to design a system that would be flexible,
  yet practical to implement.  In that vein, many of our design goals,
  listed below, are the same as or similar to those specified in
  [RFC 2779], but with additional consideration paid to implementation

  The primary requirements were to design a system which:

  1) is scalable to hundreds of millions of instant messaging users.

  2) is scalable to hundreds of thousands (or perhaps millions) of
     individual instant messaging systems and domains, so every
     company or ISP could have its own instant messaging system.

  3) allows existing instant messaging implementations to manage
     their own user namespace.

  4) is self managed (like the Internet mail system) such that new
     servers and new systems could be added without administration

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     by some central authority and without manual administration
     by other service providers.

  5) initially supports at least five main end-user features:

     - Requesting/Renewing/Canceling presence subscriptions
     - Sending/Receiving presence notifications (in response to a
     - Routing and delivery of instant messages
     - Retrieval of named user attributes (at minimum an "alias"
       and current presence state)
     - Retrieval of named domain attributes

  6) allows traffic between users in a single instant messaging
     system to stay within that system.

  7) makes it possible to implement interoperability between
     instant messaging systems by adding gateways to existing
     systems without rearchitecting existing core systems.

  8) is extensible, so new features can be added incrementally
     without requiring redesign and while allowing for backwards

  9) has a well thought-through security strategy such that:

     - Messaging data or state can't be easily spoofed or replayed
       by a third party
     - Messaging data or state can't be easily intercepted,
       hijacked, or stolen by a third party
     - More advanced security measures such as end-to-end
       encryption or signing can be layered on top of the initial
       implementation, but are not required in the initial

   10) easily supports clients that are inside of a common company
       firewall (e.g. incoming connections are often refused).

   11) easily supports international usage.

   12) leverages existing standards in as many places as practical.

3. Terminology

  [RFC 2778] specifies a common terminology for the discussion of
  Internet Messaging and Presence Protocol information; however, that
  document defines terms which are considerably more granular than
  are required by this document.  Accordingly, this document uses
  the following terms:

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  Aggregator - A special kind of Gateway, which services multiple
   domains and routes messages between other IMX Servers and
   Servers which service a particular domain.  The protocol
   between the Aggregator and each Server is arbitrary.  An
   example of an Aggregator might be a Gateway which acts as
   a front-end to multiple, privately-labeled Instant Messaging

  Attributes - metadata about an End User, such as a nickname or
   alias; or about a domain, such as a timeout value.  Attributes
   consist of a key, which is scoped at a domain, and a value.
   It may be desirable to have certain attribute keys which are
   global to the IMX architecture and interpreted identically
   by all participating services.

  Data - any of Instant Messages, Attributes, Notifications,
   Subscriptions, or requests for these that are exchanged
   between Servers.

  End User - a human or other entity whose presence information is
   reflected by the service and who can send and receive Instant
   Messages through an Instant Messaging Service.

  Instant Message - a short, real-time or near-real-time message
   which is sent between Instant Messaging clients.  While this
   document does not prescribe a definition for "short," the
   intent is to prevent streams of arbitrary length from
   being sent as Instant Messages.  This is consistent with the
   definition of an INSTANT MESSAGE in [RFC 2778].

  Instant Messaging Client (or Client) - a User Agent which provides
   an End User with the ability to initiate and receive Instant
   Messages, and request Subscriptions and receive Notifications for
   Presence Information.  This term is used in this document to
   [RFC 2778], and is roughly consistent with the generic description
   of an "instant messenger" in section 2.7 of that document.

  Instant Messaging Gateway (or Gateway) - a special type of
   Instant Messaging Server which does not communicate directly
   with Clients but sits between Servers which service a particular
   domain and the world of other IMX servers.  Gateways act
   essentially as routers, potentially performing protocol
   translation between an Instant Messaging Service's local
   protocols and the IMX protocol.  An example of a
   Gateway would be a Server which routes messages to an
   already existing, private Instant Messaging Service.

  Instant Messaging Server (or Server) - an entity which
   maintains Presence Subscriptions for and delivers Instant

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   Messages on behalf of End Users of a given Instant Messaging
   Service.  Typically, Instant Messaging Clients connect
   to a Server in order to send and receive Instant Messages,
   Attributes, and Presence Information.

  Instant Messaging Service - a collection of Instant Messaging
   Servers and their associated clients which together make up
   an integrated service offering.  Instant Messaging Services
   administer their own namespace of End User names and are
   generally associated with one or more domains they serve.

  Notification - a message sent from a Server to a Client to
   indicate a change in the Presence Information for a given
   End User.  Notifications are sent only to Clients who
   have previously created a Subscription to receive such
   information for a particular End User.  (Presence Information
   may be retrieved in real-time without a Subscription by
   making a request for a presence Attribute).  This term
   is essentially identical to the corresponding definition
   in [RFC 2778].

  Presence Information - information regarding the state of a
   particular user.  Examples might be online, offline, away,
   busy, etc.  Specific formats for the conveyance of Presence
   Information are not specified here.

  SPAM - unwanted (and typically unsolicited) Instant Messages.

  Subscription - Information kept by an Instant Messaging Server
   which maintains a Client's desire to be Notified when an
   End User's Presence Information changes.  Subscriptions are
   entered by Instant Messaging Clients on behalf of End Users,
   and time out if not renewed.  This term is essentially
   identical to the corresponding definition in [RFC 2778].

4. Architecture

  The basic architecture behind IMX (Instant Messaging eXchange) is one
  of multiple independent Instant Messaging Services (as exist today),
  which can interoperate at the Server-to-Server, Server-to-Gateway, or
  Gateway-to-Gateway level.

4.1 Servers and Gateways

  Each Instant Messaging Service exchanges Data with other Services
  through well known Servers or Gateways.  For the purpose of this
  document, Servers and Gateways are distinguished in that Servers
  can be the same physical or logical entities that service Clients,
  whereas Gateways essentially relay information between Servers
  within a Service and those outside of it.  Absent this distinction,
  the terms Server and Gateway can be used interchangeably.

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  Each Server is said to "service", "be responsible for", "act on
  behalf of," or be "in" a domain, which is a shortcut to saying that
  a Server listens for requests intended for a given Instant Messaging
  Service (identified by a domain), and provides answers on behalf
  of the End Users who belong to that Service.  In this way, the model
  is very similar to the way that SMTP servers exchange mail with other
  SMTP servers.

4.2 Namespace and Addressing

4.2.1 Address identifiers

  Each End User has an identifier which can be used to uniquely
  address that user across various Instant Messaging Systems.
  Like internet mail addresses, the identifier used to identify a
  given End User consists of a local part and a domain, separated by
  at-sign (@).  (The at-sign is therefore a reserved character
  in an Instant Messaging address).

  The address used for Instant Messages may or may not be the same as
  an End User's email address.

4.2.2 Domains

  The domain portion of the address is assumed to be an internet
  domain, compliant with [RFC 1034] and is used to identify the
  Servers responsible for that domain by the process described in
  Section 4.2.3.

4.2.3 Server Discovery

  Servers which service a particular domain are published in DNS using
  an IMX record.  An IMX record is a new type of DNS record that works
  similar to the MX record used by mail.  It lists one or more Servers
  that accept Instant Messaging connections for that particular domain.
  If more than one server is listed, the originating Server picks one
  of the listed servers randomly (just like with MX records) and
  attempts to establish a connection.  If the chosen server doesn't
  respond, the originating server may attempt to connect on one of the
  other listed servers.

  The IMX record may also list other attributes of the connection such
  as a port number.

  A full specification of IMX records will follow in a separate

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4.3 Connection Management

  Connections between Servers use TCP for transport.  They are
  established on-demand by the originating Server as needed and are
  semi-persistent.  Connections are kept open for as long as the
  originating host desires, as long as there is activity on the
  connection.  Connections may be closed by the receiving Server after
  some period of inactivity; they would then be re-established by the
  originator on-demand when next needed.

  Data for multiple End Users can and likely will flow across a given
  connection.  Conversely, Data for a given user may flow over any
  active connection that exists between the two domains and is not
  guaranteed to flow over the same connection as previous requests.
  If a Server handles requests for more than one domain, Data for
  multiple domains may flow over a given connection.  These connections
  must be authenticated multiple times, once for each domain they

4.3.1 Originating Connections

  A Server would establish a connection to another Server in order to
  request Data on behalf of an End User within the originating server's
  domain.  This would include connections to send Instant Messages,
  initiate or refresh Subscriptions, send Notifications, and to
  request Attributes.

  A Server may establish more than one connection to communicate with
  another Server (i.e. for load balancing or to handle increased
  traffic); however, in consideration of scalability, implementers
  should take care to limit the number of such connections to a
  reasonably small number.  For example, it would not be appropriate
  for a Server to establish one connection per End User; it would be
  appropriate to establish one connection per Server process or Server
  thread.  In any event, a receiving server may refuse to accept
  more than a given number of connections from Servers at a single

  Servers will also establish connections in order to perform
  authentication handshaking as specified in Section 6.3.1.

4.3.2 Accepting Connections

  A Server would accept a connection from another Server in order to
  receive Data for an End User within the receiving server's domain.
  This would include connections to receive Instant Messages, update
  or establish Subscriptions, receive Notifications, and receive
  requests for Attributes.

  Servers will also accept connections in order to perform
  authentication handshaking as specified in Section 6.3.1.

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4.3.3 Relays

  Server should only accept traffic for the domains they service.
  That is, this architecture explicitly disallows relaying services
  because of their possible unauthorized and unintended use (see
  SPAM, section 6.4).

  Instant Messaging Services will be held accountable for traffic
  originating from their Servers.

4.4 Protocol Considerations

  This document does not specify the details of the protocol between
  Servers (the IMX protocol).  However, we believe that the following
  requirements should be considered in the implementation of the

  1) The protocol must have the ability to exchange protocol version
     information and some protocol capability information. (4.4.1)

  2) The protocol must be completely extensible without ever having to
     force all other servers to upgrade.  The intention is that, in
     most cases, new types of data packets or request packets can be
     added, and older servers can simply ignore the data or request
     types that they don't understand. (4.4.2)

  3) The protocol should include a means for either side to close the
     connection in an orderly fashion and have the other side know that
     the connection has been closed. (4.4.3)

  4) The protocol should include some sort of redirection mechanism so
     that the other end of an existing connection can be told to
     reconnect on another IP address. (4.4.4)

     This would be useful for:

       - Taking an existing, active server out of service in an orderly
       - Smarter load balancing beyond the capabilities of a DNS rotor.

  5) The protocol needs the ability to retrieve named user
      Attributes (e.g. the user's alias, UA capabilities, maximum
      message length, etc...).

  6) The protocol also needs the ability to retrieve named
      domain-specific Attributes.  For example, the timeout for a
      connection or a presence subscription to expire may be a
      domain-specific configuration. (4.4.6)

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  7) The protocol is completely asynchronous.  The connection is simply
     a pipe through which requests and tagged responses flow.  More
     than one connection may be open between a set of servers, and more
     than one request may be in process on a given connection.   In
     some cases, a response may not flow back on the same connection on
     which it was initiated. (4.4.7)

  8) The protocol should allow for responses to at least some requests
     (particularly those requesting Attributes), to return on the
     same connection as the request.  The protocol should additionally
     provide enough information in the request so that a reply can be
     routed to the same Server that made the request. (4.4.8)

  9) The protocol should be binary, not text.  The binary format will
     be records with length and type. (4.4.9)

  10) Where not already specified by a chosen, existing standard, data
     will be represented in UTF-8. (4.4.10)

4.5 Additional Protocol Considerations for Presence Information

  When used for Subscription or Presence Information, the following
  additional requirements should be considered:

  1) Presence Subscription should expire in some pre-determined amount
     of time (e.g. 30 minutes) if they aren't renewed. This timeout
     may be different per vendor and should be published as a
     domain attribute. (4.5.1)

  2) Presence Notifications may be delivered over any active connection
     that exists between the two domains.  They are not necessarily
     delivered over the same connection on which the subscription was
     requested (that connection may not even exist anymore). (4.5.2)

4.6 Additional Protocol Considerations for Attributes

  When used to retrieve Attributes for a user or domain, the following
  additional requirements should be considered:

  1) Attribute information may be sent unsolicited with Instant
     Message or Presence Information so that it need not be
     explicitly requested. (4.6.1)

4.7 Additional Protocol Considerations for Instant Message Information

  When used for Instant Messages, the following additional requirements
  should be considered:

  1) As with the SMTP envelope, the To and From address flows with the
     message as part of the protocol and does not need to be parsed by
     Servers, Gateways, or Aggregators. (4.7.1)

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5. Instant Message Format Considerations

  In order to achieve maximum flexibility, a subset of MIME [RFC 2045]
  should be used for the message transfer format. Messages will consist
  of headers and a body.

  1) The message From and To headers are also included in the MIME
     message and are required.  All other headers are optional.  Any
     number of headers may exist.  Headers such as "Content-type" and
     "Content-transfer-encoding" are used if present.  "Subject" is not
     used.  Vendors that specify additional vendor-specific headers
     are strongly encouraged to use the "X-" header format.

  2) Initial implementations of the architecture should support at
     least two MIME types: text/plain and text/html-lite.

     Text/html-lite is a strict subset of HTML with support for only
     a few basic formatting tags supported (such as <B>...</B>,
     <I>...</I>, etc...).

     No additional attempt is made to describe text/html-lite in this

  3) Very long lines are permitted.  Text/plain messages are not
     prewrapped to any particular column width.  A paragraph is sent
     as one long line.  It is the receiving Client's responsibility
     to wrap the message for proper display.  It is important that all
     servers support this correctly because Instant Messaging user
     interfaces are often used in a small screen form factor where
     appropriate wrapping substantially increases the usability.
     The small screen requirement comes about either because the
     Client runs on a small screen device or because the user wishes
     to devote only a small piece of screen real estate to the Client
     user interface.

6. Security Considerations

6.1 Objectives

  [RFC 2779] specifies in detail a series of requirements for security
  in an Instant Messaging protocol.  In developing this architecture,
  the authors also considered the following objectives in order to
  ensure practicality of implementation:

  1) The architecture should be straightforward to implement and
     administer and be freely exportable.

  2) The system, including the relationship between arbitrary
     Instant Messaging Services, should be self-managed, with no
     centralized authority required to administer the security scheme.

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  3) The security scheme should account for a basic level of
     security but be easily extensible so that some Services may
     implement a higher degree of security.

  4) In order to address these objectives (1-3), the intent is
     to authenticate connections between Servers, not individual
     exchanges of Data.

  5) In keeping with the objectives of (3), it should be possible,
     but not required, to support end-to-end encryption and/or
     signing.  (End-to-end encryption refers to data encrypted
     by the sending Client and decrypted at the recipient's Client).

  6) In keeping with the objectives of (3), it should be possible,
     but not required, to support the use of advanced security
     measures such as SSL or certificate exchange between Servers.

  7) User passwords should never be exchanged in a Server to
     Server communication.

  The overarching intent behind these objectives is to provide a
  system which would offer substantially higher security than
  Internet email, yet offer a basic level of security that could
  be easily and rapidly implemented.  At the same time, the model
  would be extensible enough to allow for increased security should
  users, vendors, or other entities demand it.

6.2 Assumptions

6.2.1 Client Authentication

  Because this architecture specifies a Server to Server protocol, it
  does not specify or recommend any mechanism for authenticating
  Clients to a Server.  Each Instant Messaging Service is held
  responsible for security between Clients and Servers, including
  preventing one End User from impersonating another.

  In all discussions in this section, this document assumes that a
  given Server can be trusted when it represents that Data originates
  from a given End User.

6.2.2 Scope

  Each individual Instant Messaging Service is responsible for making
  sure that a given End User is not an impersonator.  Therefore, the
  principal problem this document tries to solve is to ensure that
  Data sent and received on behalf of an End User in a given domain
  is actually originating from and/or being sent to Servers in that

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  Additionally, the architecture seeks to protect against a spoofing
  attack where one Server pretends to issue Data on behalf of users
  in a domain that it does not represent. Types of Attacks Within the Scope

  The following types of attacks are explicitly covered by
  the proposed architecture.

  1) Breach of Presence Privacy - Spoofing an End User who is
     requesting Presence Information (or Attributes).  This form
     of security becomes especially important if a service supports
     restrictions on who can and can not retrieve Presence Information
     (or Attributes) for a given End User.  This is essentially the
     requirement 5.1.1 in [RFC 2779], where A and B are SERVERS
     instead of non-ADMINISTRATOR PRINCIPALs.

  2) False Presence - Spoofing state about a given End User (e.g.
     if User A in a given domain is subscribed to receive Presence

     Information for User B in another domain, it is not possible
     for a User C to send Notifications with User B's Presence
     Information to User A).  This is essentially the requirement
     5.2.4 in [RFC 2779], where users A and B are authenticated
     ENTITIES in separate DOMAINS.

  3) Spoofing Sender - Spoofing the From address for a given Instant
     Message (e.g. User C in a given domain sends an Instant Message
     to User B purporting to be User A).  This is essentially the
     requirement 5.4.3 in [RFC 2779], where A and B are authenticated
     ENTITIES in separate DOMAINS.

  4) Stealing Data - Hijacking Data intended for a given End User and
     redirecting it somewhere else.  (e.g. User A sends an Instant
     Message to User B, but it instead is directed to User C).  This
     is essentially requirements 5.3.3 and 5.4.6 in [RFC 2779], where
     A and B are authenticated ENTITIES in separate DOMAINS. Types of Attacks Outside of Scope

  For practical reasons (see Objectives, 6.1), a "standard"
  implementation of security as defined in this document does not
  protect against the following types of attacks:

  - DNS Hijacking
  - Man-in-the-middle attacks
  - Eavesdropping or packet snooping

  However, an advanced implementation of security may protect against
  these within the specified architecture (see section 6.3.2).

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6.3 A Trust Model for Server to Server communications

  This document therefore proposes the following architecture for
  establishing trust between Servers:

  1) Servers will trust DNS for all outgoing connections.  If a Server
     establishes a connection from Domain A to Domain B, it should be
     relatively certain that data sent over that connection is actually
     going to Domain B.  The only situations where that assumption
     would not be true would be if the entirety of Domain B were
     hijacked, and that is explicitly not covered by this model (see

  2) Incoming connections to a Server are not trusted (since they could
     originate anywhere).  Incoming connections are authenticated by
     using a mechanism referred to as "dial-back" (see 6.3.1), which
     establishes an outbound connection to the originating Server to
     authenticate the Server which makes the inbound connection.

  3) Once a connection is established and trusted, it is trusted for
     the life of the connection.

6.3.1 The Dial-Back Mechanism

  The Dial-Back mechanism is so-named after the (now somewhat old)
  concept which goes something like this:

  - If User A calls User B, then User A trusts that she is talking to
    User B because she made the call.

  - If User A calls User B, User B does not trust that he is talking to
    User A, since anyone could call User B pretending to be User A.

  - If User A calls User B and provides some secret, User B can call
    User A on another line and provide that same secret.  This ensures
    that User A and User B are the same on both calls (since they
    have exchanged the secret), and that Users A and B trust each other
    (since each user originated the call on which they are providing
    the information).  Therefore, both lines can be trusted equally.

  It is fairly straightforward to extend this concept to the exchange
  of Data between Servers.  When a Server at Domain A accepts a
  connection which supposedly originates from Domain B, it must
  first authenticate that connection as follows:

  1) The incoming connection from Domain B (call this Connection 1)
     contains an initial token from Domain B.  This token is purely for
     Domain B's implementation convenience and can be anything Domain B
     wants it to be (see Implementation Note, below).

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  2) Domain A then creates an outgoing connection to Domain B (call
     this Connection 2).  In establishing this connection, Domain A
     uses no information passed in as part of the original request on
     Connection 1 (except, of course, the name of Domain B).  That is,
     Domain A will use its own DNS resolution to establish its
     connection to Domain B and will not honor an IP address sent in
     across Connection 1, for example.

  3) After establishing Connection 2, Domain A passes to Domain B two
     pieces of data: the token originally given to Domain A by Domain
     B, and a short-lived secret coined by Domain A.

  4) When Domain B receives the secret over Connection 2 from Domain A,
     it sends the secret back to Domain A over Connection 1.

  5) Upon validation of its secret, the Server at Domain A knows that
     the Connection 1 shares knowledge with Connection 2 and therefore
     can be assumed to exist between the same two points.  Connection
     1 and Connection 2, then can be trusted equally for the exchange
     of information.

  6) If Connection 1 were trying to spoof Domain B, it would not
     receive Connection 2 from Domain A and would therefore not be able
     to obtain the necessary secret required to authenticate
     Connection 1.

  Implementation Note: Domain B may want, at a minimum, to encode
  specific information about the requesting server, process, or thread
  as part of the token it passes to Domain A.  In this way, Domain B
  can correlate Connection 1 with Connection 2.

6.3.2 Enhanced Security

  The "Dial-Back" approach protects against all forms of attacks except
  man-in-the-middle attacks, packet snooping, and DNS hijack, and
  is therefore consistent with the objectives.  Additionally, because
  many different End Users' Data will flow over a single connection,
  and a given End User's Data may be spread across multiple
  connections, the only way to actually spoof an End User is to
  effectively take over the entire domain.  While that's certainly
  possible under this architecture, it's not very likely and would
  certainly be easily noticed.

  If it became a requirement to protect against man-in-the-middle
  and packet snooping attacks, encrypted SSL connections can be
  established between the Servers without any change to the

  If it became a requirement to protect against DNS hijacking, digital
  certificates with a known trust hierarchy could be used to
  authenticate Servers as part of the dial-back process.

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6.4 SPAM

  When compared to Internet email, this proposal has the following
  benefits in fighting SPAM:

  1) The source domain for an Instant Message is authenticated using
     the dial-back mechanism so it is not possible to easily spoof a
     domain.  This doesn't mean it's not easy for a malicious entity
     to set up his own Instant Messaging Service and launch SPAM from
     it, but that domain will eventually get blocked as a SPAM domain.

     It does mean, however, that one can't easily spoof messages from
     a high profile domain that one doesn't own.

  2) There is no relaying in the protocol so there can be no accidental
     "open relay" servers for SPAMers to leverage.

  3) Instant Messaging is generally not a store and forward system so
     a SPAMer would have to find the target online in order to send out
     a SPAM message.  If a system supports offline Instant messages
     (which essentially creates a store and forward system), that
     system may need to implement some additional protections.

  4) Early implementations should deploy rate limiting on all user
     accounts so any automated delivery mechanisms (e.g. agents that
     send out Instant Messages faster than a normal person would type)
     will get blocked.  Exceptions may be made for approved
     notification services (e.g. an electronic trading company that
     lets End Users subscribe to stock price alerts), but the default
     will be that all accounts are rate limited.

  5) Instant Messaging Clients may choose to automatically restrict
     or permit Instant Messages based on

     - Sender's domain
     - Sender's fully qualified address
     - Recipient's buddy or contact list
     - Other, vendor specific criteria

7. Authors' Addresses

  Edwin Aoki
  501 E. Middlefield Rd., MV-102
  Mountain View, CA 94043

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  Andy Wick
  22000 AOL Way
  Dulles, VA 20166

8. Additional Documents

  The following protocols and/or concepts described in this document
  will likely need additional explanation in additional documents:

  - Detailed information about the IMX protocol
  - The IMX DNS record
  - The text/html-lite MIME type

9. Acknowledgements

  Many thanks are due to the engineering and management team of
  America Online for their support in drafting this document.
  Particular credit is due to Eric Bosco and John Friend for their
  help in developing many of the ideas reflected in this paper.

10. References:

 [RFC 822]  Crocker, D., "Standard for the Format of ARPA Internet
            Text Messages", STD 11, RFC 822, UDEL, August 1982.

 [RFC 2779] Day, M., Aggarwal, S., Mohr, G. and Vincent, J,
            "Instant Messaging / Presence Protocol Requirements",
            RFC 2779, February 2000.

 [RFC 2778] Day, M., Rosenberg, J. and H. Sagano, "A Model for
            Presence and Instant Messaging", RFC 2778, February 2000.

 [RFC 2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
            Extensions (MIME) - Part One: Format of Internet Message
            Bodies", RFC 2045, November 1996.

 [RFC 1034] Mockapetris, P., "Domain Names - Concepts and Facilities",
            STD 13, RFC 1034, November 1987.

A. Appendix - Performance Against Objectives

  In designing the specification for this architecture, the authors
  tried to strike a balance between practicality of implementation
  and the needs of End Users and Instant Messaging System operators.
  This section describes how, in the authors' view, the architecture
  described meets the requirements set out in Section 2.

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A.1 Scalability and Efficiency

  It is obviously very important that servers implementing this
  architecture be able to scale to handle large numbers of users,
  messaging systems (domains) and requests across the protocol.

  While avoiding the discussion of how to build a Server which
  can handle a large number of Client requests (i.e. users),
  the architecture itself should be scalable to large numbers
  of systems and efficient for heavy use, for the following

  1) The authentication mechanism is lightweight, and authentication
     needs to occur only at the time a connection is established

  2) Once authenticated, connections are basically stateless,
     so an implementation can easily use multiple hosts to split
     the load.

  3) There are a variety of ways to split the load for incoming
     connections across multiple hosts:

     - Multiple hosts can be specified in the IMX record
     - The load can be split using DNS round robin
     - The load can be split using a load balancing router

  4) Because the architecture is Server to Server, Instant
     Messaging System vendors can optimize traffic within their
     own systems and use the interoperability architecture as
     needed for traffic across systems.

  5) The protocol is very lightweight.  The Gateway Servers are not
     being asked to do anything that's CPU intensive; mostly they
     are simply acting as a protocol translator between the public
     IMX protocol and the internal state of the Instant Messaging

  6) The architecture works over semi-persistent connections that
     stay alive as long as there is traffic on them.  This allows
     the traffic for many different users to flow over a single socket.

  7) The protocol is envisioned to be very efficient and not "chatty".
     Each operation is self contained - fully encapsulating a given
     request in a single protocol request with a single protocol

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A.2 Ease of Implementation

  1) The architecture specifies a structure that allows vendors to
     quickly provide users with the benefit they most want - ability
     to message across systems, without precluding advanced features

  2) There are three ways to participate in interoperability,
     allowing for all existing instant messaging products to
     choose the model which best suits its needs:

     - By having the system's Servers (the ones to which
       individual clients connect) be public and communicate
       with other systems,

     - By having each system's servers communicate through a
       Gateway which translates between a vendor's client-server
       protocol and the interoperability protocol, or

     - By having each system's Servers communicate via some
       private Server-to-Server protocol to an Aggregator, which
       translates between that protocol and the interoperability

  3) It should be very straightforward to implement this protocol in
     a set of Gateway Servers that connect an existing system to the
     outside world, allowing vendors to leverage existing investments
     in technology and resources.

  4) It is not anticipated that the Gateway Servers would need to hold
     any additional state beyond what a regular Instant Messaging
     System is already holding

A.2.3 Reliability

  1) The stateless connections allow a given Server to go down,
     restart and resume processing of requests without a lengthy
     restart or connection building process.

  2) The connect-on-demand capability makes for automatic recovery
      when a connection or Server goes down.

A.2.4 Security

  1) Even without SSL, digital certificates, or end-to-end encryption
     the architecture provides for a system which is far more secure
     than e-mail.

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  2) Instant Messaging Service providers are responsible for the
     enforcement of security within their own domain, allowing them
     to use the most appropriate level of security for their user

  3) Likely attacks on the system are difficult, noticeable, and can
     not easily target an individual account.

  4) The architecture provides for vendors to implement more advanced
     security if appropriate.

  5) The authenticated connection approach makes it more difficult for
     anonymous SPAM to enter the system.

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