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INTERNET-DRAFT                                                  Y. Deion
<draft-yu-imap-client-id-04.txt>                              LinuxMagic
Intended Status: Standards Track
Expires November 26, 2020                                   May 26, 2020

               IMAP Service Extension for Client Identity
                    <draft-yu-imap-client-id-04.txt>

Abstract

   This document defines an Internet Message Access Protocol (IMAP)
   service extension called "CLIENTID" which provides a method for
   clients to indicate an identity to the server.

   This identity is an additional token that may be used for security
   and/or informational purposes, and with it a server may optionally
   apply heuristics using this token.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.  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/1id-abstracts.html

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

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction .................................................  2
   2.  Conventions Used in This Document ............................  3
   3.  CLIENTID .....................................................  3
     3.1.  CLIENTID Command .........................................  3
     3.2.  CLIENTID Arguments .......................................  3
     3.3.  Advertising the CLIENTID capability ......................  4
     3.4.  Restrictions on the CLIENTID command .....................  4
   4.  Formal Syntax ................................................  4
   5.  Discussion ...................................................  5
     5.1.  Background ...............................................  5
     5.2.  Applying heuristics to CLIENTID ..........................  6
     5.3.  Utility of CLIENTID ......................................  6
     5.4.  Use Cases of CLIENTID ....................................  7
     5.5.  Other IMAP Client Identifiers ............................  8
     5.6.  Future Considerations ....................................  8
   6.  Client Identity Types ........................................  8
   7.  Examples ..................................................... 10
     7.1.  UUID as Client Identity .................................. 10
     7.2.  Malformed CLIENTID Command ............................... 11
     7.3.  Client Identity Without a TLS/SSL Session ................ 11
     7.4.  Client Identity Leading to Rejection ..................... 11
   8.  Security Considerations ...................................... 12
   9.  IANA Considerations .......................................... 12
   10. References ................................................... 12
     10.1. Normative References ..................................... 12
   Appendix A.  CLIENTID Product Support ............................ 13
   Contributors ..................................................... 13
   Authors' Addresses ............................................... 13

1. Introduction

   The [IMAP] protocol and its extensions describe methods whereby an
   client may provide identity and/or authentication information to
   an IMAP server.  However, these existing methods are subject to
   limitations and none offer a way to identify the IMAP client with
   absolute confidence.  This document defines an IMAP service extension
   to provide an additional identity token which can represent the IMAP
   client with a higher degree of certainty when accessing the IMAP
   server.

   Typically IMAP clients enter the authenticated state by using either
   the AUTHENTICATE or LOGIN command.  IMAP servers are often subject to
   malicious clients attempting to use authorization credentials and/or
   identities not intended for their use (e.g. stolen credentials or
   brute force attacks).  When such an attack is attempted, the IMAP
   server may be unable to identify the impersonation and restrict such
   an unintended use by someone other than the authorized user or said
   credentials.  While there are ways to identify the source of the IMAP
   client such as its IP address, it would be useful if there was an
   additional way to uniquely identify the client in a method solely
   available across an encrypted channel.


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   Using the CLIENTID extension, an IMAP client can provide an
   additional identity token to the server called its "client identity".
   The client identity can provide unique characteristics about the
   client accessing the IMAP service and may be combined with existing
   identification mechanisms in order to identify the client.  An IMAP
   server may then apply additional security policies using this
   identity such as restricting use of the service to clients presenting
   recognized client identities or only allowing use of authorized
   identities that match previously established client identities.

   The CLIENTID extension is present in any IMAP implementation that
   returns "CLIENTID" as one of the supported capabilities to the
   CAPABILITY command.

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 [KEYWORDS].

   Formal syntax is specified using [ABNF].

   Example lines prefaced by "C:" are sent by the client and ones
   prefaced by "S:" by the server.

   "Connection" refers to the entire sequence of client/server
   interaction from the initial establishment of the network connection
   until its termination.

3. CLIENTID

3.1. CLIENTID Command

   Arguments:  client identity type
               client identity token

   Responses:  no specific responses for this command

   Result:     OK - clientid completed, client identity stored
               BAD - command unknown or arguments invalid

   Note that a valid CLIENTID command will never return the NO result
   because heuristics MUST NOT be applied to the CLIENTID arguments at
   this stage.  Instead the client identity information SHOULD be stored
   and passed along to any and all [SASL] authentication mechanisms.

3.2. CLIENTID Arguments

   The CLIENTID command takes the following two arguments:

   1. client identity type: A string identifying the identity type the
      client is providing.  It MUST be between 1 and 16 alphanumeric
      and dash characters.


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   2. client identity token: A string identifying the client.  It MUST
      be between 1 and 128 printable characters.

   The IMAP server MUST reject any CLIENTID command with badly formatted
   arguments.  The IMAP server MUST accept the arguments from a valid
   CLIENTID command and SHOULD store it at the minimum for the remaining
   duration of the IMAP connection.

3.3. Advertising the CLIENTID capability

   The CLIENTID capability is used to tell the IMAP client that the IMAP
   server supports the CLIENTID extension.  However, certain conditions
   MUST be met before the IMAP server advertises the CLIENTID
   capability.

   1. The IMAP server and IMAP client MUST negotiate encryption via
      STARTTLS/SSL or some other secure mechanism.

   2. The IMAP server MUST be in the non-authenticated state.

   3. The IMAP server MUST have the CLIENTID extension support enabled.

   While all the conditions are met, the IMAP server MUST advertise the
   CLIENTID capability in all proceeding CAPABILITY commands.

3.4. Restrictions on the CLIENTID command

   Under certain circumstances, the use of the CLIENTID command will be
   restricted:

   1. Before the CLIENTID capability has been advertised, the IMAP
      server MUST reject any issued CLIENTID commmand and the IMAP
      client MUST NOT issue the CLIENTID command.

   2. Outside of the non-authenticated state, the IMAP server MUST
      reject any CLIENTID command issued by the IMAP client and the IMAP
      client MUST NOT issue the CLIENTID command.

   3. Once a valid CLIENTID command has been issued, the IMAP server
      MUST reject any further CLIENTID command issued by the IMAP client
      and the IMAP client MUST NOT issue any subsequent CLIENTID
      commands.

4. Formal Syntax

   The following syntax specification uses the Augmented Backus-Naur
   Form notation as specified in [ABNF].  [IMAP] defines the
   non-terminals "capability" and "command-nonauth".

   Except as noted otherwise, all alphabetic characters are case-
   insensitive.  The use of upper or lower case characters to define
   token strings is for editorial clarity only.  Implementations MUST
   accept these strings in a case-insensitive fashion.


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   capability      =/ "CLIENTID"

   command-nonauth =/ client-id

   client-id       = "CLIENTID" SP client-id-type SP client-id-token

   client-id-type  = 1*16 ALPHA / DIGIT / "-"
                   ;; alphanumeric with dash character

   client-id-token = 1*128 VCHAR
                   ;; any printable US-ASCII character

5. Discussion

5.1 Background

   The historical standard of using the user and password combination as
   a means of authentication is no longer effective in this day and age
   with recent developments in the world.

   1. ISPs transistioning to Carrier-grade NAT due to IPv4 address
      exhaustion placing multiple devices behind the same IP address.

   2. Numerous large scale data breaches exposing millions of user and
      password combinations.

   3. Continued propensity for user to use same simple passwords across
      multiple accounts.

   4. Botnets growing larger and more sophisticated due to the
      profileration of IoT devices.

   As a result, brute force attacks against web services have become
   increasingly effective as malicious actors have easy access to
   millions of email addresses, commonly used passwords and massive
   botnets while the safety practices of users have not improved.

   The traditional methods of defending against these types of attacks
   like rate limiting and blocking by IP addresses are no longer viable
   without collateral damange as thousands of devices could potentially
   be behind the same IP address as more ISPs adopt the CGN/LSN/NAT444
   standard, i.e. blocking an IP address due to the actions of a single
   malicious actor bears the risk of blocking legitimate users.

   By introducing CLIENTID as another non-public factor to be used in
   tandem with the user and password combination, authentication becomes
   much more resilient against brute force attacks.  The email addresses
   and passwords exposed from the data breaches will no longer be
   sufficient to authenticate.  Rate limiting and blocking can be
   performed based on the CLIENTID such that only a subset of devices
   behind the same IP address gets blocked.  CLIENTID would also be
   backwards compatible with existing authentication protocols
   encouraging adoption.


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5.2. Applying heuristics to CLIENTID

   This section discusses the possible heuristics that can be applied to
   the information that is presented via the CLIENTID command.  This
   information includes whether a valid CLIENTID command was issued, the
   client identity type and the client identity token.

   1. The IMAP server MAY choose to require that a successful CLIENTID
      command be issued or that a particular client identity type be
      presented before processing or accepting an authentication
      request.

   2. The IMAP server MAY reject any authentication request not preceded
      with a client identity type that matches ACL's or rules as defined
      in the IMAP server.

   3. An IMAP server MAY reject any authentication request preceded by a
      CLIENTID command that contains a client identity type or client
      identity token that the server chooses not to accept for any
      reason such as by policy.

   4. An IMAP server MAY reject any authentication request preceded by a
      CLIENTID command that contains a client identity type or client
      identity token that the server has chosen to disable or revoke use
      of either temporarily or permanently.

  The IMAP server SHOULD only ever reject an IMAP client based on
  CLIENTID information during or after the authentication
  process/handler.  In the interest of limiting the amount of
  information being revealed, the rejection message SHOULD be as generic
  as possible and SHOULD NOT reveal any information on the heuristics.

  Even if the client identity type and/or client identity token are not
  recognized, supported or permitted by the server and/or the owner of
  the authentication credentials, the presented information may still be
  useful for analysis.

5.3. Utility of CLIENTID

   Regardless of how frowned upon, users commonly reuse authorization
   information (like the username and password pair) across multiple
   services.  When one service is compromised, malicious actors can also
   gain access to other services where the user also used the same
   credentials.  Based on this representative problem alone, the utility
   of CLIENTID as an additional layer of determining the rights to
   present such authorization information becomes quickly apparent.

   The utility of CLIENTID may be seen by considering the following:

   1. An IMAP server could recognize a device not historically known to
      have presented the authentication credentials before.

   2. An IMAP server could restrict authentication from actors not
      presenting a valid CLIENTID, or an account holder that the IMAP

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      server provides service for could restrict authentication to only
      those devices that present valid CLIENTID.

   3. An IMAP server could restrict authentication to only devices which
      present a CLIENTID containing a client type identifier which the
      account holder or operator of the server deems to be permitted.
      (Eg. Only allow vendor A's devices)

   4. An IMAP server could alert an account holder that an attempt to
      present their authorization credentials came from an unknown,
      unrecgonized, or different device.

   However, this extends beyond just the restriction of authentication.
   While it might be argued that this can be served as a special form
   of SASL, by implementing this in the IMAP service itself, the IMAP
   service can choose before allowing a connection to be passed to a
   SASL implementation, allowing it to perform other heuristics, such
   as brute force attacks, more effeciently.

   While 'forgery' and/or the use of random client identifier is
   possible, such behavior is also more readily detectable when a device
   identifier is presented.

   1. The IMAP server, when faced with hundreds of devices behind the
      same IP address, during an attack can restrict authentication
      attempts to only connections presenting a valid client identifier
      token.

   2. The IMAP server, during an attack, can restrict authentication to
      only historically known devices.

   3. The IMAP server can differentiate between many different devices
      behind the same IP, and apply maximum connections per device,
      rather than maximum connections per IP.

   4. While a person may present authentication credentials from many
      different geographical locations, eg, home, office, and travel, a
      single device will not in general be able to be in two
      geographical locations at the same time.  The IMAP server will
      have new information to apply to threat detection heuristics, ie
      to treat the use of the same client indentifier token from
      two locations, as a possible brute force or forgery situation.

5.4. Use Cases of CLIENTID

   With CLIENTID the IMAP server has additional information it may use
   in its interactions with the client.  It may:

   1. Restrict use of an authorization tokens to a set of client
      identity token identities, thereby offering an added level
      of security.  For example the use of authorization credentials may
      only be accompanied by a specified set of CLIENTID tokens and/or
      types for a specific account holder, or set of account holders


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   2. Identify that the same CLIENTID token is used to access multiple
      authorized identities, and restrict access to the IMAP service.
      For example a malicious client that has attempted to gain access
      using multiple authorization tokens may be identified through its
      unusual behavior.

   3. Retain knowledge of CLIENTID tokens previously presented with
      specific authorization credentials, and if the token has not been
      previously seen, restrict access to the IMAP service.

   4. Require that the IMAP client present a token such as a license key
      established outside of the IMAP session in order to make use of
      any authorized identity.

   5. Apply different security policies to clients that provide a
      CLIENTID token versus those which do not.  For example, provide
      clients providing such an identity with additional trust.

   6. Ability to rate limit or block based on the presented
      client-identifier-token, when multiple devices use a shared IP
      address, without affecting other devices.

   7. Ability to detect distributed and localized dictionary attacks
      and brute force attacks.

   8. Use the client-indentifier-token as a third factor to be passed
      to authentication methods. [SASL]

5.5. Other IMAP Client Identifiers

   The [IMAP] protocol and its extensions describe methods whereby an
   IMAP client may provide identity information to an IMAP server.  Some
   of these identitiers are listed for contrast:

   1. The client connection provides a source IP address associated
      with the IMAP session.  This may be accompanied by a PTR record
      and/or GeoIP information.

   2. The AUTHENTICATE and LOGIN command allows the client to present
      a user and/or password/authentication mechanism for an IMAP
      session.

5.6. Future Considerations

   In the future there may be a demand for being able to provide
   multiple CLIENTID commands with different client identity types.
   For instance, it may be desirable for a device to indentify itself,
   both with a hardware device identifier, and a software identifier.
   We believe this to be out of scope, and can be accomodated with a
   special client-identifier-token which encapsulates both.

6. Client Identity Types

   This document does not specify any CLIENTID identity type that MUST

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   be supported.  The client identity type is meant to be defined by
   the client implementation that is designed to access the IMAP server
   and protocol.  For instance, many IMAP client software
   implementations already create a distinct UUID for each account.
   Some commercial email clients have a license key.  Some physical
   devices that need to interact with IMAP might have a unique hardware
   ID.  While there is no pre-defined list of client identity type
   defined by this RFC, and all IMAP servers should be prepared to
   accept any form of client identity type that conforms to the
   definition, it is suggested that IMAP client developers carefully
   consider the name of the client identity type.  For example, rather
   that using a client identity type of UUID, consider the advantages of
   making it more distinct, e.g. "<product_short_code>UUID".  This way
   the IMAP server can better record histories, eg the difference
   between say a Thunderbird generated unique id, and a Mutt generated
   unique id.

   Some examples of identity type might be UUID, LICENSE, DEVICE_ID,
   and/or COOKIE.  It is expected that the most common types might be
   related to distinct UUID, LICENSEKEY, or HARDWAREID.

   An IMAP server SHOULD NOT reject an unidentified CLIENTID type, except
   for specific policy use cases.

   It is envisioned that in the future it will be useful to propose
   a set of standardized client-indentity-type to help with validation,
   or to allow the IMAP server to apply ACL rules on expected types,
   this would be an extension to this RFC.

   1. UUID

      UUID is a common practice to represent either a individual user,
      hardware device or software installation associated with a
      specific individual.  The support of UUID enables existing UUID
      implementations to be used to semi-uniquely identify a device
      associated with an individual.  A definition of the format should
      be considered.  Otherwise non-standard UUID might be a separate
      type specific to the software implementation, for instance
      TBIRD-UUID.

   2. LICENSE

      An IMAP client may find it useful to identify the license key of
      software it is using.  Such licenses are typically crafted such
      that they are unique and useful to identify a software
      installation.  This is more normally suited for a software
      designed for a single-user.  While LICENSE could be standard type
      again, it might more more helpful to specify a vendor specific
      type such as BBLICENSEKEY.

   3. DEVICE_ID

      Many hardware devices are designed to be used by a single
      individual and already have an associated hardware device id.

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      While a standard type might be defined, it also might be more
      helpful to use a vendor specific type, such as ATOM-DEVICEID.

   4. COOKIE

      While not guranteed to be consistent many web applications are
      designed to access IMAP directly and may need to have a
      semi-unique identifier available as part of the web based
      transaction.  It is assumed that COOKIE encompasses the group of
      web based tokens known to persist from session to session.  A
      specific web based application can provide sufficient information
      in the actual client-identifier-token to differentiate between
      applications and or websites, and are convenient as they can be
      related to very specific domains, and are universally available to
      web application designers.

   As a reminder, an IMAP server SHOULD NOT retain and/or store the
   CLIENTID information WITH authentication credentials or
   authentication systems directly, but the IMAP service MAY
   associate the CLIENTID with a specific account holder, eg to create
   a history file of known CLIENTID tokens associated or permitted to
   access or present authentication credentials for that account holder.

   This document recommends that an IMAP server handle any given client
   identity type from a CLIENTID command in one or more of the following
   manners.

   1. Handled but treat as not presented (ignored, no persistance)
   2. Store in IMAP session but treat as not presented (debugging)
   3. Store in the IMAP session, so it is available to System log
   4. Store in the IMAP session, so it is available to User log
   5. Use for authentication
   6. Use for alert when authentication fails
   7. Use for alert when authentication succeeds
   8. Unused

7. Examples

7.1. UUID as Client Identity

   C: [connection established over a plaintext connection]
   C: a001 CAPABILITY
   S: * CAPABILITY IMAP4rev1 STARTTLS AUTH=GSSAPI LOGINDISABLED
   S: a001 OK CAPABILITY completed
   C: a002 STARTTLS
   S: a002 OK STARTLS completed
   <TLS negotiation, further commands are under [TLS] layer>
   C: a003 CAPABILITY
   S: * CAPABILITY IMAP4rev1 AUTH=GSSAPI AUTH=PLAIN CLIENTID
   S: a003 OK CAPABILITY completed
   C: a004 CLIENTID UUID 23bf83be-aad7-46aa-9e0f-39191ccf402f
   S: a004 OK CLIENTID completed
   C: a005 LOGIN joe password
   S: a005 OK LOGIN completed

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7.2. Malformed CLIENTID Command

   C: [connection established over a plaintext connection]
   C: a001 CAPABILITY
   S: * CAPABILITY IMAP4rev1 STARTTLS AUTH=GSSAPI LOGINDISABLED
   S: a001 OK CAPABILITY completed
   C: a002 STARTTLS
   S: a002 OK STARTLS completed
   <TLS negotiation, further commands are under [TLS] layer>
   C: a003 CAPABILITY
   S: * CAPABILITY IMAP4rev1 AUTH=GSSAPI AUTH=PLAIN CLIENTID
   S: a003 OK CAPABILITY completed
   C: a004 CLIENTID UUID
   S: a004 BAD Error in IMAP command received by server

   The IMAP server rejects the CLIENTID command as it is not well
   formed due to there being only a single parameter provided.

7.3. Client Identity without TLS/SSL Session

   C: [connection established over a plaintext connection]
   C: a001 CAPABILITY
   S: * CAPABILITY IMAP4rev1 STARTTLS AUTH=GSSAPI LOGINDISABLED
   S: a001 OK CAPABILITY completed
   C: a002 CLIENTID UUID 23bf83be-aad7-46aa-9e0f-39191ccf402f
   S: a002 BAD Unknown IMAP command received by server

   The IMAP server rejects use of the CLIENTID command as the CLIENTID
   capability had not been advertised because no encryption was
   negotiated between the IMAP server and IMAP client.

7.4. Client Identity Leading to Rejection

   C: [connection established over a plaintext connection]
   C: a001 CAPABILITY
   S: * CAPABILITY IMAP4rev1 STARTTLS AUTH=GSSAPI LOGINDISABLED
   S: a001 OK CAPABILITY completed
   C: a002 STARTTLS
   S: a002 OK STARTLS completed
   <TLS negotiation, further commands are under [TLS] layer>
   C: a003 CAPABILITY
   S: * CAPABILITY IMAP4rev1 AUTH=GSSAPI AUTH=PLAIN CLIENTID
   S: a003 OK CAPABILITY completed
   C: a004 CLIENTID UUID 23bf83be-aad7-46aa-9e0f-39191ccf402f
   S: a004 OK CLIENTID completed
   C: a005 LOGIN joe password
   S: a005 BAD Failed to authenticate

   The IMAP server rejects use of the system during the LOGIN command
   after deciding that the provided client identity does not establish
   sufficient privileges.  Note that the error message that's returned
   to the client is very generic and does not reveal any information
   about CLIENTID and/or the existence of 'joe' and/or the validity of
   the password.

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8. Security Considerations

   As this extension provides an additional means of communicating
   information from a client to a server it is clear there is additional
   information divulged to the server.  This may have privacy
   considerations depending on the client identity type or its contents.
   For example, it may reveal a MAC address of the device used to
   communicate with a server that would not previously have been
   revealed.  While it has been useful to use identifier such as email
   address for authentication it is easy for these authetication tokens
   to be shared and/or reused and/or be publically available for other
   purposes.  An IMAP server and or its operators SHOULD not share
   any CLIENTID information presented with a third party as it may
   represent or be linked to an individual and SHOULD never be shared in
   association with authentication tokens.

   As well, while this service extension requires that the identity
   information only be transmitted over an encrypted channel to reduce
   the risk of eavesdropping, it does not specify any policies or
   practices required in the establishment of such a channel, and so it
   is the responsibility of the client and the server to determine that
   the communication medium meets their requirements.

9. IANA Considerations

   The IANA is requested to add CLIENTID to the "IMAP 4 Capabilities"
   registry, http://www.iana.org/assignments/imap4-capabilities.

10.  References

10.1.  Normative References

   [ABNF]      Crocker, D. and P. Overell, "Augmented BNF for
               Syntax Specifications: ABNF", RFC 2234,
               November 1997.

   [IMAP]      Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
               4rev1", RFC 3501, March 2003.

   [KEYWORDS]  Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, DOI
               10.17487/RFC2119, March 1997, <http://www.rfc-
               editor.org/info/rfc2119>.

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

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






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Appendix A.  CLIENTID Product Support

   Since publishing the IMAP Client Identity RFC draft, multiple email
   server and client vendors have implemented CLIENTID support into
   their products, e.g. MailEnable, MagicMail, SaneBox, BlueMail,
   emClient, and Thunderbird.

Contributors

   Michael Peddemors
   LinuxMagic

Authors' Addresses

   Deion Yu
   LinuxMagic
   #405 - 860 Homer St.
   Vancouver, British Columbia
   CA V6B 2W5

   EMail: deiony@linuxmagic.com


































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