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Versions: 00

Instant Messaging and Presence Protocol                        R. Movva
Internet Draft                                                Microsoft
Category: Informational                                    August, 1999
Document: draft-movva-msn-messenger-protocol-00.txt
Document Expires: 2/00                                           W. Lai
                                                              Microsoft
                                                           August, 1999



                   MSN Messenger Service 1.0 Protocol



Status of this Memo

   This document is an Internet-Draft and is NOT offered in accordance
   with Section 10 of RFC2026, and the author does not provide the IETF
   with any rights other than to publish as an Internet-Draft.

   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
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This document and related documents are discussed on the impp
   mailing list. To join the list, send mail to impp-
   request@iastate.edu. To contribute to the discussion, send mail to
   impp@iastate.edu. The archives are at http://lists.fsck.com/cgi-
   bin/wilma/pip. The IMPP working group charter, including the current
   list of group documents, can be found at
   http://www.ietf.org/html.charters/impp-charter.html.



1. Abstract

   Microsoft released a commercial Instant Messaging product in July of
   1999 called MSN Messenger Service. This document describes the
   protocol used by that product for core instant messaging and
   presence functionality. While this protocol does not meet many of
   the requirements of the IMPP working group, it is provided as
   background information on existing Instant Messaging
   implementations. This protocol is provided 'as is' without warranty
   of any kind.


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2. Conventions used in this document

   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 RFC-2119.

   Protocol messages sent from client to server are preceded by "C:".

   Protocol messages sent from server to client are preceded by "S:".



3. Introduction

   MSN Messenger Service enables a user to learn about the presence of
   other people on the Internet, and to communicate with them in real-
   time. This functionality is commonly referred to as "Instant
   Messaging" (IM).

   This document describes the syntax and semantics of the MSN
   Messenger Protocol, the communication protocol running between MSN
   Messenger Service 1.0 clients and servers. Among the core services
   that the MSN Messenger Servers provide to clients are:

   - Authenticated user logon.
   - Adding and deleting members of the user's contact list.
   - Changing the user's on-line state.
   - Receipt of asynchronous, real-time, on-line state change
     notifications from members of the user's contact list.
   - Delivering lightweight, real-time messages to other users.
   - Receipt of asynchronous, real-time messages from other users.
   - Configuring the user's access permissions, to restrict the ability
     of other users to view the user's on-line state or send messages
     to the user.

   Additional background:

   1. Some features extraneous to core instant messaging functionality
   contained within the MSN Messenger Service 1.0 protocol are beyond
   the scope of this document. Examples include client version
   management and directory functionality.

   2. The purpose of this document is to provide the members of the
   IMPP working group with a reference implementation of a "monolithic"
   IM system. That is, a system designed for massive scale, but not yet
   capable of communication with servers other than those associated
   with this specific service. Since any standard in this area will of
   necessity be a "distributed" design that explicitly enables server-
   to-server and service-to-service communication, this document will
   serve primarily as a reference and example of one implementer's
   choices when providing IM functionality at scale.


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   3. This document reflects the protocol used in the 1.0 release of
   MSN Messenger clients and servers, deployed on the Internet in July
   of 1999. However, the service is in production and rapidly growing,
   which almost certainly will necessitate changes to the protocol as
   Microsoft gains operational experience with the service and expands
   its feature set. This Internet Draft may not be updated with such
   changes, and the changes may be made with little or no notice.



4. MSN Messenger Server Component Overview

   MSN Messenger Service clients make connections to several different
   kinds of servers. They are separate components to facilitate running
   at scale - each component can be duplicated an arbitrary number of
   times, independently of each other, to enable large numbers of
   users.

4.1 Dispatch Server (DS)

   The Dispatch Server is the initial point of connection between
   client and server. Its primary functions are protocol version
   negotiation, determination of which Notification Server (NS) is
   associated with the client making a connection (via an algorithm of
   the server's choosing), and referring the client to the proper NS.

4.2 Notification Server (NS)

   The Notification Server is the primary server component. The client
   and the Notification Server authenticate, synchronize user
   properties, and exchange asynchronous event notifications. The
   client's connection to the Notification Server occurs after the
   referral from the Dispatch Server is completed, and persists without
   interruption during the user's MSN Messenger Service session.

   Some of the events transmitted between a client and a Notification
   Server are:  State changes (e.g. client is on-line, client is
   offline, client is idle), Switchboard Server invitation requests
   (see below), and application-specific notifications that are beyond
   the scope of this document. (E.g. new e-mail has arrived)

4.3 Switchboard Server (SS)

   The Switchboard Server is the component through which clients can
   establish lightweight communication sessions without requiring a
   direct network connection between clients. The common usage of the
   Switchboard Server is to provide instant messaging sessions.
   When a client wishes to communicate with another client, it sends a
   message to its Notification Server, which then refers the client to
   a Switchboard Server. Once the SS connection is established, the
   "destination" client receives a notification from its NS to connect
   to the same SS.


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5. Protocol Conventions

5.1 Connection Type

   The MSN Messenger Protocol currently works over TCP/IP. The MSN
   Messenger server components support connections over port numbers
   1863, which is the registered port number assigned by the IANA
   (http://www.isi.edu/in-notes/iana/assignments/port-numbers).

5.2 Command Syntax

   MSN Messenger Protocol command syntax is ASCII and single line-
   based. Commands begin with a case-sensitive, three-letter command
   type, followed by zero or more parameters, and terminated by CRLF.
   Parameters are separated by one or more whitespace characters and
   cannot contain whitespace characters. Parameters that contain spaces
   or extended (non 7-bit ASCII) characters should be encoded using
   URL-style encoding (e.g. "%20" for space). Some commands accept un-
   encoded binary data. In these cases, the length of the data is
   transmitted as part of the command, and the data is transmitted
   immediately following a CRLF of the command.

5.3 Asynchronous Requests

   Commands issued from the client to the server that result in a reply
   are known as requests. Requests are entirely asynchronous. The
   client can submit several requests in sequence without waiting for
   the server response after submitting each request. The server is
   required to deliver a response or an error for each request
   received, but it is not required to deliver the responses in the
   same order as the requests were received. The client can determine
   the request associated with a particular response by examining the
   Transaction ID parameter (described below).

5.4 User Handles

   MSN Messenger Protocol uses User Handles for identifying users. A
   user handle (also known as "account name" and "logon name") is a
   text representation of the user's identity that is both unique and
   persistent. The user handle is syntactically equivalent to an e-mail
   address, and as such is subject to the same restrictions for
   character set, as described in RFC-822. Most notable among these
   restrictions are the limitation to Latin alphanumeric characters and
   a few symbols. The maximum acceptable length of the user handle is
   129 bytes.

   Implementation note: In the initial release of the client and
   server, user handles are Hotmail account names. All user handles
   must contain the "@hotmail.com" domain name, and user handles that
   do not contain a domain name are not valid.

5.5 Custom User Names


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   A custom user name (also known as "custom name" and "friendly name")
   is a user's representation of the "friendly" textual name associated
   with a user handle. (E.g. "Auntie Em" instead of em123@hotmail.com).
   Custom user names are neither unique nor persistent, and can contain
   any valid Unicode characters. Custom user names are represented in
   UTF-8 as described in RFC-2044 and URL-encoded as described in RFC-
   1738 when transmitted between the client and server. The maximum
   acceptable length of the encoded custom user name is 387 in the
   current implementation.

5.6 Transaction Identifiers

   The Transaction Identifier (a.k.a. Transaction ID) is a numeric
   string representing a number between 0 and (2^32 - 1). It is a value
   that a client includes with any command that it issues to the
   server. In the current version of the protocol, the transaction
   identifier is used to associate server responses with client-issued
   commands. The server treats the transaction ID as an opaque number
   and does not assume any relationship between successive Transaction

   IDs or any particular starting Transaction ID. It is the client's
   responsibility to guarantee the uniqueness of the Transaction IDs
   for the purpose of disambiguating the commands and/or responses. (A
   future version of the protocol could enable the client to track the
   status or cancel a particular transaction using the transaction ID.)

   When the server sends the response to a command to the client, it
   must include in the response the transaction ID that the client sent
   to the server when the client originally issued the command. In
   cases where a server sends a command to a client that requires a
   transaction ID but is not in response to a specific client command,
   it will use 0 as the transaction ID. In cases where a server sends
   multiple responses to a single client request, the server will use
   the same transaction ID in each response.

5.7 User List Types

   Some of the protocol commands are used to the manipulate lists of
   users. The following types of user lists are supported by the
   protocol:

   Forward List (FL) - The list of users for whom a given user wants to
   receive state change notifications. The Forward List is what is most
   commonly referred to as the user's "contact list."

   Reverse List (RL) - The list of users who have registered an
   interest in seeing this user's state change notifications.

   Allow List (AL) - The list of users who the user has explicitly
   allowed to see state change notifications and establish client-to-
   client sessions via a Switchboard Server.



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   Block List (BL) - The list of users who the user has explicitly
   prevented from seeing state change notifications and establishing
   client-to-client sessions via a Switchboard Server.



6. Command Summary Table

     Command  From             To           Description
   ==================================================================
     ACK      Switchboard      Client       Sends a positive message
                                            delivery acknowledgement.
   -------------------------------------------------------------------
     ADD      Client           Notification Adds to the user's FL, AL,
              Notification     Client       and BL. Notifies the client
                                            of asynchronous additions
                                            to a user's list.
   -------------------------------------------------------------------
     ANS      Client           Switchboard  Accepts a request for a
                                            switchboard server session.
   -------------------------------------------------------------------
     BLP      Client           Notification Changes the user's message
              Notification     Client       privacy setting, which
                                            determines how to treat
                                            messages from users not
                                            already in the BL or AL.
   -------------------------------------------------------------------
     BYE      Switchboard      Client       Notifies a client that a
                                            user is no longer in the
                                            session.
   -------------------------------------------------------------------
     CAL      Client           Switchboard  Initiates a switchboard
                                            server session.
   -------------------------------------------------------------------
     CHG      Client           Notification Sends a client state change
              Notification     Client       to the server.
                                            Echoes the success of
                                            client's state change
                                            request.
   -------------------------------------------------------------------
     FLN      Notification     Client       Notifies the client when
                                            users in the FL go off-
                                            line.
   -------------------------------------------------------------------
     GTC      Client           Notification Changes the user's prompt
              Notification     Client       setting, which determines
                                            how the client reacts to
                                            certain RL changes.
   -------------------------------------------------------------------
     INF      Client           Dispatch,    Requests set of support
                               Notification authentication protocol
              Dispatch,        Client       from the server.
              Notification                  Provides the set of

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                                            supported authentication
                                            protocols to the client.
   -------------------------------------------------------------------
     ILN      Notification     Client       Notifies the client of the
                                            initial online state of a
                                            user in the FL, while
                                            either logging on or adding
                                            a user to the FL.
   -------------------------------------------------------------------
     IRO      Switchboard      Client       Provides the initial roster
                                            information for new users
                                            joining the session.
   -------------------------------------------------------------------
     JOI      Switchboard      Client       Notifies a client that a
                                            user is now in the session.
   -------------------------------------------------------------------
     LST      Client           Notification Retrieves the server's
              Notification     Client       version of the user's FL,
                                            RL, AL, or BL.
   -------------------------------------------------------------------
     MSG      Client           Switchboard  Sends a message to the
                                            members of the current
                                            session.
   -------------------------------------------------------------------
     MSG      Notification,    Client       Delivers a message from
              Switchboard                   another client or from a
                                            server-side component.
   -------------------------------------------------------------------
     NAK      Switchboard      Client       Sends a negative message
                                            delivery acknowledgement.
   -------------------------------------------------------------------
     NLN      Notification     Client       Notifies the client when
                                            users in the FL go on-line
                                            or when their on-line state
                                            changes.
   -------------------------------------------------------------------
     OUT      All              All          Ends a client-server
                                            Session.
   -------------------------------------------------------------------
     REM      Client           Notification Removes from the user's FL,
              Notification     Client       AL, and BL.
                                            Notifies the client of
                                            asynchronous removals from
                                            a user's list.
   -------------------------------------------------------------------
     RNG      Notification     Client       Notifies the client of a
                                            request by another client
                                            to establish a session via
                                            a switchboard server.
   -------------------------------------------------------------------
     SYN      Client           Notification Initiates client-server
              Notification     Client       property synchronization.
   -------------------------------------------------------------------

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     USR      All              All          Authenticates client with
                                            server, possibly in
                                            multiple passes.
   ------------------------------------------------------------------
     VER      Client           Dispatch     Negotiates common protocol
              Dispatch         Client       dialect between client and
                                            Server.
   ------------------------------------------------------------------
     XFR      Client           Notification Requests a Switchboard
              Notification     Client       server for use in
                                            establishing a session.
   -------------------------------------------------------------------
     XFR      Dispatch         Client       Notification of login-NS to
              Notification     Client       the client or notification
                                            to move to a different NS.
=======================================================================



7. Presence and State Protocol Details

   This is a detailed list of protocol commands associated with
   presence functionality. They are defined in the order used by
   clients. Commands associated with instant messages are discussed in
   section 8 below.

7.1 Protocol Versioning

   After the client connects to a dispatch server by opening a TCP
   socket to port 1863, the client and server agree on a particular
   protocol version before they proceed. The Client-Server protocol
   version handshake involves the following command exchange:

        C: VER TrID dialect-name{ dialect-name...}
        S: VER TrID dialect-name

   The client can provide multiple dialect names in preferred order.
   The dialect-name parameter returned by the server is the version
   server is designating for this connection

   The current protocol dialect-name supported by Messenger servers is
   "MSNP2". The dialect names are not case-sensitive.

   The string "0" is a reserved dialect name and is used to indicate a
   failure response. E.g.:

        S: VER TrID 0{ dialect-name ... }

7.2 Server Policy Information

   The client next queries the server for variable "policy"
   information. In this version of the protocol, the only policy


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   information returned by the server is the authentication package in
   use.

        C: INF TrID
        S: INF TrID SP{,SP...}

   SP identifies a security package - the name of the SASL mechanism to
   use for authentication. "MD5" is used by the Notification Server,
   "CKI" by the Switchboard Server.

7.3 Authentication

   The client needs to authenticate itself after protocol version
   handshake and identifying the security packages supported on the
   server. The following are the client server interactions involved.

        C: USR TrID SP I{ AuthInitiateInfo}
        S: USR TrID SP S{ AuthChallengeInfo}
        C: USR TrID SP S{ AuthResponseInfo }
        S: USR TrID OK UserHandle FriendlyName

   The SP parameter is the name of the security package("MD5"). The
   next parameter is a sequence value, which must be I to (I)nitiate
   the authentication process and S for all (S)ubsequent messages. If
   authentication fails on the server, the client can start the
   authentication process again.

   For the MD5 security package:
   - The AuthInitiateInfo parameter provided by the client must be the
     User handle.
   - The AuthChallengeInfo parameter returned by the server contains a
     challenge string.
   - The AuthResponseInfo contains the binary response as a hexadecimal
     string, which the MD5 hash of the challenge and the User password
     strings concatenated together.

   The final response from the server contains, in addition to the user
   handle, the current "Friendly Name" associated with the user handle.
   This is a "Custom User Name" as described above.

7.4 Referral

   There are three cases in which clients are referred from one server
   to another:

   1.  The initial "Dispatch Server" refers the client to the
       Notification Server to which it is assigned.
   2.  Asynchronous referral by the Notification Server to reassign the
       client to a different Notification Server if that server is
       overloaded or undergoing maintenance.
   3.  During Switchboard Session establishment, the assigned
       Notification Server refers the client to a particular
       switchboard server for use. This is discussed below.

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   In the current implementation the Dispatch Server uses the user
   handle provided in the initial USR command above to assign the user
   in question to a Notification Server. Alternate implementations
   might not require referral at this stage.

   If received, referral is of the form:

        S: XFR TrID ReferralType Address[:PortNo]

   ReferralType is either "NS" or "SB" and defines the type of referral
   to a Notification Server or Switchboard Server.
   Address is a valid DNS name or IP address to a referred server, with
   optional port# suffixed as ":PortNo".

   If this command is received from the server, the client should
   attempt to log in to the server provided.

   In the case of "NS" referrals during logon, the Server automatically
   closes the client connection after sending this XFR response so that
   the client can connect to the new IP Address.

   If sent asynchronously, the client is responsible for closing the
   connection.

   After a "NS" referral, the client will not receive any more messages
   from the "old" NS, and also must not send any commands to the "old"
   NS after receiving an XFR.

7.5 Client User Property Synchronization

   Several of the user properties used by the Messenger application are
   stored on the server. This is done for two reasons:

   1) So that users can "roam", i.e. log in from different locations
   and still have the appropriate data, such as their contact lists and
   privacy settings.
   2) If changes occur to a user's Reverse List while that user was
   offline (the user was added to another user's list), the client can
   be updated with this information.

   For performance reasons it is useful to cache these properties on
   the client, so that bandwidth usage is minimized in the typical case
   where the user is not roaming and there were no Reverse List
   changes.

   These requirements are met by the SYN command - synchronization.

   Once a client logs in successfully, it uses the SYN command to
   ensure it has the latest version of the server-stored properties.
   These properties include: Forward List, Reverse List, Block List,
   Allow List, GTC setting (privacy setting when someone adds this user
   to their Forward List), and BLP setting (the user's privacy mode).

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   The SYN command is:

        C: SYN TrID Ser#
        S: SYN TrID Ser#

   The Ser# parameter sent by the client is the version of the
   properties currently cached on the client. The server responds with
   the current server version of the properties. If the server has a
   newer version, the server will immediately follow the SYN reply by
   updating the client with the latest version of the user properties.
   These updates are done as described below, and are done without the
   client explicitly initiating a LST, GTC or BLP command. Note that
   the server will update all server-stored properties to the client,
   regardless of how many entries have been changed.

   The following "List Retrieval and Property Management" section
   describes the format of the user properties sent by the server.
   After the SYN reply from the server, the user property updates will
   be sent from the server in this sequence: GTC, BLP, LST FL, LST AL,
   LST BL, LST RL.

   All the user property updates will share the same TrID as the SYN
   command and reply.

7.6 List Retrieval And Property Management

   Synchronizing can result in a batch of user properties and lists
   getting sent by the server to the client. However, the client
   application can also initiate a request to retrieve the server-
   stored lists and properties. The following are the privacy property
   and list retrieval commands. The response formats are the same
   whether it is a client-initiated request, or whether it is a
   response to the SYN process as described above.


   List Command

   By issuing the LST command, the client can explicitly request that a
   list be sent. The server will respond with a series of LST
   responses, one LST response for each item in the requested list.

        C: LST TrID LIST
        S: LST TrID LIST Ser# Item# TtlItems UserHandle CustomUserName

   - LIST is FL/RL/AL/BL for Forward List, Reverse List, Allow List,
     and Block List, respectively.
   - The Item# parameter contains the index of the item described in
     this command message. (E.g. item 1 of N, 2 of N, etc.)
   - The TtlItems parameter contains the total number of items in this
     list.
   - UserHandle is the user handle for this list item.
   - CustomUserName is the friendly name for this list item.


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   If the list is empty, the response will be:

        S: LST TrID LIST Ser# 0 0

   Reverse List Prompting

   The client can change its persistent setting for when to prompt the
   user in reaction to an Reverse List change. This is accomplished via
   the GTC command:

        C: GTC TrID [A | N]
        S: GTC TrID Ser# [A | N]

   The value of the A/N parameter determines how the client should
   behave when it discovers that a user is in its RL, but is not in its
   AL or BL. (Note that this occurs when a user has been added to
   another user's list, but has not been explicitly allowed or
   blocked):

   A - Prompt the user as to whether the new user in the RL should be
   added to the AL or the BL
   N - Automatically add the new user in the RL to the AL

   The A/N parameter is not interpreted by the server, merely stored.

   The server will respond with the current setting if the change was
   successful. Otherwise, it will return an error with the matching
   TrID. If the client tries to change the setting to the same value as
   the current setting, the server will respond with an error message.

   The default setting is A when a new user connects to the server for
   the first time.

   Privacy Mode

   The client can change how the server handles instant messages from
   users via the BLP command:

        C: BLP TrID [AL | BL]
        S: BLP TrID Ser# [AL | BL]

   The AL/BL parameter determines how the server should treat messages
   (MSG and RNG) from users. If the current setting is AL, messages
   from users who are not in BL will be delivered. If the current
   setting is BL, only messages from people who are in the AL will be
   delivered.

   The server will respond with the current setting if the change was
   successful. Otherwise, it will return an error with the matching
   TrID. If the client tries to change the setting to the same value as
   the current setting, the server will respond with an error message.



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   The default setting is AL when a new user connects to the server for
   the first time.


7.7 Client States

   After the client is authenticated and synchronized, the client
   establishes its initial state with the server with the CHG command.
   The syntax of the command is:

        C: CHG TrID State
        S: CHG TrID State

   When the state is changed, the server will echo the settings back to
   client. The state shall not be considered changed until the response
   is received from the server.

   Note that the server can send a state change message to the client
   at any time. If the server changes the state without a request from
   the client, the TrID parameter will be 0.

   States are denoted by a string of three characters. The predefined
   states that the server recognizes are:

   NLN - Make the client Online (after logging in) and send and receive
   notifications about buddies.
   FLN - Make the client Offline. If the client is already online,
   offline notifications will be sent to users on the RL. No message
   activity is allowed. In this state, the client can only synchronize
   the lists as described above.
   HDN - Make the client Hidden/Invisible. If the client is already
   online, offline notifications will be sent to users on the RL. The
   client will appear as Offline to others but can receive
   online/offline notifications from other users, and can also
   synchronize the lists. Clients cannot receive any instant messages
   in this state.

   All other States are treated as sub-states of NLN (online). The
   other States currently supported are:
   BSY - Busy.
   IDL - Idle.
   BRB - Be Right Back.
   AWY - Away From Computer.
   PHN - On The Phone.
   LUN - Out To Lunch.

7.8 List Modifications

   The protocol supports generic commands to add and remove users from
   various lists. This is used by clients to enable "Adding" contacts
   to the list of folks being watched, or for the "Block" and "Allow"
   features that define how users chooses to interact with one another.


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   However, these generic commands have different semantics based on
   the list being modified. For example, only the server can add or
   remove entries from the Reverse List - since it is an indirect
   consequence of the user having been added to another user's Forward
   List.

   The add and remove commands:

        C: ADD TrID LIST UserHandle CustomUserName
        S: ADD TrID LIST ser# UserHandle CustomUserName

        C: REM TrID LIST UserHandle
        S: REM TrID LIST ser# UserHandle

   Valid values for LIST in Client initiated adds and removes are
   FL/AL/BL.

   All client initiated adds and removes will be echoed by the server
   with a new serial number that should be persisted by the client
   along with the list modification. If not successful, an error will
   result.

   The protocol also supports the concept of an ADD or REM that the
   client did not initiate. Server generated ADDs and REMs can have
   LIST values of FL/AL/BL/RL. This is common with RL changes, which
   are never initiated by the client, but is an indirect consequence of
   this user having been added to someone's Forward List. If the RL
   change happens while the user is online, it will trigger an
   asynchronous ADD or REM command from the server.

   Asynchronous ADDs and REMs to the FL, AL, and BL can happen when the
   server allows an authenticated user to make list changes from
   another environment, such as a web site. In all of these cases, the
   server will send the ADD or REM command with the TrID parameter
   equal to 0.

7.9 Notification Messages

   The client receives asynchronous notifications whenever a contact on
   the user's Forward List changes its state. The notifications are of
   the form:

        S: NLN Substate UserHandle FriendlyName
        S: ILN TrID Substate UserHandle FriendlyName
        S: FLN UserHandle

   NLN indicates that a user has come online.
   - Substate can be any three-letter code (see "Client States" above).
   - UserHandle and FriendlyName are the handle and names associated
     with the user coming online.

   ILN is similar to the NLN message, and is received from the server
   in response to an CHG or ADD command from the client:

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   1.  Immediately after the client logon and sends its first CHG
       command to the NS. In this case several ILNs may be received -
       one for each Forward List contact that is currently online.
   2.  After the client sends an "ADD TrID FL UserHandle
       CustomUserName" to the NS. (e.g. ILN for the new contact if that
       contact is currently online)

   In both cases, TrID in the ILN is the same as the one sent by the
   client in the CHG or ADD command.

   FLN means that the specified user is now offline.

7.10 Connection Close

   The client issues the following command to logoff from the NS:

        C: OUT
        S: OUT {StatusCode}

   The server will reply with an OUT to the client before it initiates
   a disconnect, with an optional StatusCode.

   The StatusCode can be "OTH", which indicates that a client with the
   same user handle and password has logged on to the server from
   another location, or "SSD" meaning the server is being shut down for
   maintenance.

   The server will drop the connection after sending the OUT.

7.11 Error Information

   Error messages from the server are of the format:

        S: eee {TrID} {(error-info) {param...}}

   eee is a 3 digit decimal number indicating the error code. Error-
   info contains the description of the error in a text string
   localized to the server's locale. The optional parameters provide
   indication of the client command causing the error. TrID is the
   Transaction ID of the client command that caused this error. Any
   server generated errors will not have Transaction IDs.


             ERR_SYNTAX_ERROR                 200
             ERR_INVALID_PARAMETER            201
             ERR_INVALID_USER                 205
             ERR_FQDN_MISSING                 206
             ERR_ALREADY_LOGIN                207
             ERR_INVALID_USERNAME             208
             ERR_INVALID_FRIENDLY_NAME        209
             ERR_LIST_FULL                    210
             ERR_ALREADY_THERE                215

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             ERR_NOT_ON_LIST                  216
             ERR_ALREADY_IN_THE_MODE          218
             ERR_ALREADY_IN_OPPOSITE_LIST     219
             ERR_SWITCHBOARD_FAILED           280
             ERR_NOTIFY_XFR_FAILED            281

             ERR_REQUIRED_FIELDS_MISSING      300
             ERR_NOT_LOGGED_IN                302
             ERR_INTERNAL_SERVER              500
             ERR_DB_SERVER                    501
             ERR_FILE_OPERATION               510
             ERR_MEMORY_ALLOC                 520
             ERR_SERVER_BUSY                  600
             ERR_SERVER_UNAVAILABLE           601
             ERR_PEER_NS_DOWN                 602
             ERR_DB_CONNECT                   603
             ERR_SERVER_GOING_DOWN            604
             ERR_CREATE_CONNECTION            707
             ERR_BLOCKING_WRITE               711
             ERR_SESSION_OVERLOAD             712
             ERR_USER_TOO_ACTIVE              713
             ERR_TOO_MANY_SESSIONS            714
             ERR_NOT_EXPECTED                 715
             ERR_BAD_FRIEND_FILE              717
             ERR_AUTHENTICATION_FAILED        911
             ERR_NOT_ALLOWED_WHEN_OFFLINE     913
             ERR_NOT_ACCEPTING_NEW_USERS      920



8. Session based Instant Messaging Protocol Details

   MSN Messenger Service utilizes a lightweight, session-based
   messaging scheme. In order for two clients to exchange instant
   messages, they must first establish a common session via a
   Switchboard Server. They can invite additional clients to join the
   established session.

8.1 Referral to Switchboard

   This process begins with a "calling" client requesting a referral
   from its Notification Server to a Switchboard Server:

        C: XFR TrID SB
        S: XFR TrID SB Address SP AuthChallengeInfo

   - SB is the type of referral being requested or granted.
   - Address is the DNS name or IP address of a Switchboard Server that
     has been assigned, and that the client should connect to.
   - SP is the Security Package being used. In this version of the
     protocol it is "CKI" only.
   - AuthChallengeInfo is a cookie that the client needs to present to
     the Switchboard server for authentication.


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8.2 Switchboard Connections and Authentication

   After the XFR reply is received, the client makes a TCP/IP
   connection to the Switchboard server using port 1863. Note that a
   lack of version negotiation in the switchboard connection is a
   limitation of the current implementation.

   The client first needs to authenticates with the Switchboard Server:

        C: USR TrID UserHandle AuthResponseInfo
        S: USR TrID OK UserHandle FriendlyName

   - AuthResponseInfo is the cookie for CKI security package returned
     by the Notification Server in the XFR.
   - UserHandle and FriendlyName are the Switchboard's echoes of the
     user handle and friendly name of the user.

8.3 Inviting Users to a Switchboard Session

   Any user in a Switchboard session can invite other users to join the
   session. The CAL command is sent to the Switchboard server for this
   purpose:

        C: CAL TrID UserHandle
        S: CAL TrID Status SessionID

   The Messenger servers verify that the calling user has permissions
   to contact the called user, with consideration given to the called
   user's privacy settings and its online state. If instant messaging
   with this user is not allowed, the server responds to the calling
   user with an error. If it is allowed, the Switchboard server causes
   a RNG command to be sent to the called client (see below), and
   returns a CAL echo to the calling client. The CAL echo has these
   parameters:

   - Status is a predefined status code - in this implementation it
     must be "RINGING".
   - SessionID is the ASCII representation of a decimal number that
     uniquely identifies this session on the Switchboard Server.

8.4 Getting Invited to a Switchboard Session

   The other side of the session establishment is the behavior of the
   called client. The called client receives a RNG from its
   Notification Server and is expected to connect to the Switchboard
   Server and respond with an ANS.

   The client receives a RNG from the Notification Server as follows:

        S: RNG SessionID SwitchboardServerAddress SP AuthChallengeInfo
           CallingUserHandle CallingUserFriendlyName

   - SessionID is a numeric ASCII session ID.

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   - SwitchboardServerAddress is a DNS name or IP Address
   - SP is the security package in use. In this implementation only
     "CKI" is supported.
   - AuthChallengeInfo is the cookie to be passed back to the
     switchboard to gain entrance to the session.
   - CallingUserHandle is the user handle of the caller.
   - CallingUserFriendlyName is the custom user name of the caller.

   To join the session, the called client connects to the Switchboard
   Server and carries out the following exchange to join the session:

        C: ANS TrID LocalUserHandle AuthResponseInfo SessionID
        S: IRO TrID Participant# TotalParticipants UserHandle
           FriendlyName
        S: ANS TrID OK

   The IRO commands relay to the newly joined client roster information
   about the current session. Each IRO command message from the
   Switchboard contains one participant in the session.
   - Participant# contains the index of the participant described in
     this IRO command (e.g. 1 of N, 2 of N).
   - TotalParticipants contains the total number of participants
     currently in the session.

   The entire session roster will be sent to the new client joining the
   session before any JOI or BYE commands described below.

   If no one is in the session when the user joins (an unexpected error
   condition), the server skips directly to "ANS TrID OK" command. All
   the responses from the server related to the issued ANS command will
   contain the same TrID as the original client ANS request.

8.5 Session Participant Changes

   When a new user joins a Switchboard session, the server sends the
   following command to all participating clients, including the client
   joining the session:

        S: JOI CalleeUserHandle CalleeUserFriendlyName

   - CalleeUserHandle is the user handle of the new participant.
   - CalleeUserFriendlyName is the Custom User Name of the new
     participant.

   If a client's connection with the Switchboard Server is dropped for
   any reason, the server sends the following command to the remaining
   clients in the session:

        S: BYE CalleeUserHandle

   - CalleeUserHandle is the user handle of the participant that left
     the session.


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   Privacy Note:
   If the client moved a contact to the BL while Switchboard sessions
   are active, it is the client's responsibility to leave any session
   that should now be blocked. The servers only enforce privacy
   permissions when inviting users to a session. Further, the servers
   only enforce privacy permission with respect to the calling user,
   and not the other participants in a Switchboard session. Therefore,
   in a multipoint session, it is possible for a user to participate in
   a session with someone whom he has blocked, if a third party is
   performing the invitation.

8.6 Leaving a Switchboard Session

   When a client wishes to disconnect from the session, it sends the
   following command and waits for the Switchboard to close the
   connection:

        C: OUT

8.7 Instant Messages

   Sending an Instant Message

   Once a client-to-client session has been established via the
   Switchboard Server, sending an Instant Message to the participants
   of the session is done as follows:

        C: MSG TrID [U | N | A] Length\r\nMessage
        S: NAK TrID
        S: ACK TrID

   U, N, and A correspond to the three delivery acknowledgement modes:
   Unacknowledged, Negative-Acknowledgement-Only, and Acknowledgement.
   Depending on the value of this parameter, either nothing, NAK, or
   ACK will be sent back by the Switchboard Server to the client.

   For Unacknowledged mode, the Switchboard Server does not respond to
   the sending client with the success or failure of message delivery.

   For Negative-Acknowledgement-Only mode, the Switchboard Server
   responds to the send client only if the message could not be
   delivered to the recipient client.

   Acknowledgement mode is not currently implemented.

   Length is the length of the Message parameter in bytes, whereas
   Message is the actual message as described below.

8.8 Receiving an Instant Message

   A client can receive a system-generated message from the
   Notification Server, or it can receive an instant message from


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   another client via a Switchboard Server. The message is received in
   the following format:

        S: MSG UserHandle FriendlyName Length\r\nMessage

   The UserHandle and FriendlyName are those of the sending user.
   Length is the length of the message in bytes.

   Message is a MIME encoded stream, using a standard MIME header as
   defined by RFC-1521 and RFC-822.

   Message is constructed as:

        MIME-Header\r\nMIME-Header\r\n\r\nMessageData

   MIME-Header is constructed as:

        string": "string
        (E.g. "Content-Type: text/plain")

   The Content-Type MIME headers that the current client will use and
   recognize are:

        "text/plain;charset=UTF-8"
        "text/plain"

   If "charset=UTF-8" appears at the end of the Content-Type, the
   Message Data is UTF-8 encoded.

   Note: The Switchboard Server does not interpret the contents of the
   Message.



9. Author's Addresses

   Ramu Movva
   Microsoft Corporation
   One Microsoft Way
   Redmond WA  98052
   ramum@microsoft.com

   William Lai
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   wlai@microsoft.com



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