Internet Engineering Task Force (IETF) A. Melnikov

Internet Engineering Task Force (IETF) A. Melnikov
Request for Comments: 7817 Isode Ltd
Updates: 2595, 3207, 3501, 5804 March 2016
Category: Standards Track
ISSN: 2070-1721

Updated Transport Layer Security (TLS) Server Identity Check Procedure
for Email-Related Protocols

Abstract

This document describes the Transport Layer Security (TLS) server
identity verification procedure for SMTP Submission, IMAP, POP, and
ManageSieve clients. It replaces Section 2.4 (Server Identity Check)
of RFC 2595 and updates Section 4.1 (Processing After the STARTTLS
Command) of RFC 3207, Section 11.1 (STARTTLS Security Considerations)
of RFC 3501, and Section 2.2.1 (Server Identity Check) of RFC 5804.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7817.

Copyright Notice

Copyright (c) 2016 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. Email Server Certificate Verification Rules . . . . . . . . . 3
4. Compliance Checklist for Certification Authorities . . . . . 5
4.1. Notes on Handling of Delegated Email Services by
Certification Authorities . . . . . . . . . . . . . . . . 5
5. Compliance Checklist for Mail Service Providers and
Certificate Signing Request Generation Tools . . . . . . . . 6
5.1. Notes on Hosting Multiple Domains . . . . . . . . . . . . 7
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7. Operational Considerations . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Changes to RFCs 2595, 3207, 3501, and 5804 . . . . . 12
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13

1. Introduction

Use of TLS by SMTP Submission, IMAP, POP, and ManageSieve clients is
described in [RFC3207], [RFC3501], [RFC2595], and [RFC5804],
respectively. Each of the documents describes slightly different
rules for server certificate identity verification (or doesn't define
any rules at all). In reality, email client and server developers
implement many of these protocols at the same time, so it would be
good to define modern and consistent rules for verifying email server
identities using TLS.

This document describes the updated TLS server identity verification
procedure for SMTP Submission [RFC6409] [RFC3207], IMAP [RFC3501],
POP [RFC1939], and ManageSieve [RFC5804] clients. Section 3 of this
document replaces Section 2.4 of [RFC2595].

Note that this document doesn't apply to use of TLS in MTA-to-MTA
SMTP.

This document provides a consistent TLS server identity verification
procedure across multiple email-related protocols. This should make
it easier for Certification Authorities (CAs) and ISPs to deploy TLS
for email use and would enable email client developers to write more
secure code.

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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 [RFC2119].

The following terms or concepts are used through the document:

reference identifier: One of the domain names that the email client
(an SMTP, IMAP, POP3, or ManageSieve client) associates with the
target email server. For some identifier types, the identifier
also includes an application service type. Reference identifiers
are used for performing name checks on server certificates. (This
term is formally defined in [RFC6125].)

CN-ID, DNS-ID, SRV-ID, and URI-ID are identifier types (see [RFC6125]
for details). For convenience, their short definitions from
[RFC6125] are listed below:

CN-ID: A Relative Distinguished Name (RDN) in the certificate
subject field that contains one and only one attribute-type-and-
value pair of type Common Name (CN), where the value matches the
overall form of a domain name (informally, dot-separated, letter-
digit-hyphen labels).

DNS-ID: A subjectAltName entry of type dNSName

SRV-ID: A subjectAltName entry of type otherName whose name form is
SRVName

URI-ID: A subjectAltName entry of type uniformResourceIdentifier
whose value includes both (i) a "scheme" and (ii) a "host"
component (or its equivalent) that matches the "reg-name" rule
(where the quoted terms represent the associated [RFC5234]
productions from [RFC3986]).

3. Email Server Certificate Verification Rules

During a TLS negotiation, an email client (i.e., an SMTP, IMAP, POP3,
or ManageSieve client) MUST check its understanding of the server
identity (client's reference identifiers) against the server's
identity as presented in the server Certificate message in order to
prevent man-in-the-middle attacks. This check is only performed
after the server certificate passes certification path validation as
described in Section 6 of [RFC5280]. Matching is performed according
to the rules specified in Section 6 of [RFC6125], including the
relative order of matching of different identifier types,
"certificate pinning", and the procedure on failure to match. The

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following inputs are used by the verification procedure used in
[RFC6125]:

1. For DNS-ID and CN-ID identifier types, the client MUST use one or
more of the following as "reference identifiers": (a) the domain
portion of the user's email address, (b) the hostname it used to
open the connection (without CNAME canonicalization). The client
MAY also use (c) a value securely derived from (a) or (b), such
as using "secure" DNSSEC [RFC4033] [RFC4034] [RFC4035] validated
lookup.

2. When using email service discovery procedure specified in
[RFC6186], the client MUST also use the domain portion of the
user's email address as another "reference identifier" to compare
against an SRV-ID identifier in the server certificate.

The rules and guidelines defined in [RFC6125] apply to an email
server certificate with the following supplemental rules:

1. Support for the DNS-ID identifier type (subjectAltName of dNSName
type [RFC5280]) is REQUIRED in email client software
implementations.

2. Support for the SRV-ID identifier type (subjectAltName of SRVName
type [RFC4985]) is REQUIRED for email client software
implementations that support [RFC6186]. A list of SRV-ID types
for email services is specified in [RFC6186]. For the
ManageSieve protocol, the service name "sieve" is used.

3. A URI-ID identifier type (subjectAltName of
uniformResourceIdentifier type [RFC5280]) MUST NOT be used by
clients for server verification, as URI-IDs were not historically
used for email.

4. For backward compatibility with deployed software, a CN-ID
identifier type (CN attribute from the subject name, see
[RFC6125]) MAY be used for server identity verification.

5. Email protocols allow use of certain wildcards in identifiers
presented by email servers. The "*" wildcard character MAY be
used as the left-most name component of a DNS-ID or CN-ID in the
certificate. For example, a DNS-ID of "*.example.com" would
match "a.example.com", "foo.example.com", etc., but would not
match "example.com". Note that the wildcard character MUST NOT
be used as a fragment of the left-most name component (e.g.,
"*oo.example.com", "f*o.example.com", or "foo*.example.com").

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4. Compliance Checklist for Certification Authorities

1. CAs MUST support issuance of server certificates with a DNS-ID
identifier type (subjectAltName of dNSName type [RFC5280]).
(Note that some DNS-IDs may refer to domain portions of email
addresses, so they might not have corresponding A/AAAA DNS
records.)

2. CAs MUST support issuance of server certificates with an SRV-ID
identifier type (subjectAltName of SRVName type [RFC4985]) for
each type of email service. See Section 4.1 for more discussion
on what this means for CAs.

3. For backward compatibility with a deployed client base, CAs MUST
support issuance of server certificates with a CN-ID identifier
type (CN attribute from the subject name, see [RFC6125]).

4. CAs MAY allow "*" (wildcard) as the left-most name component of a
DNS-ID or CN-ID in server certificates it issues.

4.1. Notes on Handling of Delegated Email Services by Certification
Authorities

[RFC6186] provides an easy way for organizations to autoconfigure
email clients. It also allows for delegation of email services to an
email hosting provider. When connecting to such delegated hosting
service, an email client that attempts to verify TLS server identity
needs to know that if it connects to "imap.hosting.example.net", such
server is authorized to provide email access for an email such as
[email protected]. In absence of SRV-IDs, users of compliant email
clients would be forced to manually confirm exceptions because the
TLS server certificate verification procedures specified in this
document would result in failure to match the TLS server certificate
against the expected domain(s). One way to provide such
authorization is for the TLS certificate for
"imap.hosting.example.net" to include SRV-ID(s) (or a DNS-ID) for the
"example.org" domain. Note that another way is for DNS Service
Record (SRV) lookups to be protected by DNSSEC, but this solution
depends on ubiquitous use of DNSSEC and availability of DNSSEC-aware
APIs and thus is not discussed in this document. A future update to
this document might rectify this.

A CA that receives a Certificate Signing Request containing multiple
unrelated DNS-IDs and/or SRV-IDs (e.g., a DNS-ID of "example.org" and
a DNS-ID of "example.com") needs to verify that the entity that
supplied such Certificate Signing Request is authorized to provide
email service for all requested domains.

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The ability to issue certificates that contain an SRV-ID (or a DNS-ID
for the domain part of email addresses) implies the ability to verify
that entities requesting them are authorized to run email service for
these SRV-IDs/DNS-IDs. In particular, CAs that can't verify such
authorization (whether for a particular domain or in general) MUST
NOT include such email SRV-IDs/DNS-IDs in certificates they issue.
This document doesn't specify exact mechanism(s) that can be used to
achieve this. However, a few special case recommendations are listed
below.

A CA willing to sign a certificate containing a particular DNS-ID
SHOULD also support signing a certificate containing one or more of
the email SRV-IDs for the same domain because the SRV-ID effectively
provides more restricted access to an email service for the domain
(as opposed to unrestricted use of any services for the same domain,
as specified by the DNS-ID).

A CA that also provides DNS service for a domain can use DNS
information to validate SRV-IDs/DNS-IDs for the domain.

A CA that is also a Mail Service Provider for a hosted domain can use
that knowledge to validate SRV-IDs/DNS-IDs for the domain.

5. Compliance Checklist for Mail Service Providers and Certificate
Signing Request Generation Tools

Mail Service Providers and Certificate Signing Request generation
tools:

1. MUST include the DNS-ID identifier type in Certificate Signing
Requests for the host name(s) where the email server(s) are
running. They SHOULD include the DNS-ID identifier type in
Certificate Signing Requests for the domain portion of served
email addresses.

2. MUST include the SRV-ID identifier type for each type of email
service in Certificate Signing Requests if the email services
provided are discoverable using DNS SRV as specified in
[RFC6186].

3. SHOULD include the CN-ID identifier type for the host name where
the email server(s) is running in Certificate Signing Requests
for backward compatibility with deployed email clients. (Note, a
certificate can only include a single CN-ID, so if a mail service
is running on multiple hosts, either each host has to use
different certificate with its own CN-ID, a single certificate
with multiple DNS-IDs, or a single certificate with wildcard in a
CN-ID can be used).

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4. MAY include "*" (wildcard) as the left-most name component of a
DNS-ID or CN-ID in Certificate Signing Requests.

5.1. Notes on Hosting Multiple Domains

A server that hosts multiple domains needs to do one of the following
(or some combination thereof):

1. Use DNS SRV records to redirect each hosted email service to a
fixed domain, deploy TLS certificate(s) for that single domain,
and instruct users to configure their clients with appropriate
pinning (unless the SRV records can always be obtained via
DNSSEC). Some email clients come with preloaded lists of pinned
certificates for some popular domains; this can avoid the need
for manual confirmation.

2. Use a single TLS certificate that includes a complete list of all
the domains it is serving.

3. Serve each domain on its own IP/port, using separate TLS
certificates on each IP/port.

4. Use the Server Name Indication (SNI) TLS extension [RFC6066] to
select the right certificate to return during TLS negotiation.
Each domain has its own TLS certificate in this case.

Each of these deployment choices have their scaling disadvantages
when the list of domains changes. Use of DNS SRV without an SRV-ID
requires manual confirmation from users. While preloading pinned
certificates avoids the need for manual confirmation, this
information can get stale quickly or would require support for a new
mechanism for distributing preloaded pinned certificates. A single
certificate (the second choice) requires that when a domain is added,
then a new Certificate Signing Request that includes a complete list
of all the domains needs to be issued and passed to a CA in order to
generate a new certificate. A separate IP/port can avoid
regenerating the certificate but requires more transport layer
resources. Use of TLS SNI requires each email client to use it.

Several Mail Service Providers host hundreds and even thousands of
domains. This document, as well as its predecessors, RFCs 2595,
3207, 3501, and 5804, don't address scaling issues caused by use of
TLS in multi-tenanted environments. Further work is needed to
address this issue, possibly using DNSSEC or something like PKIX over
Secure HTTP (POSH) [RFC7711].

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6. Examples

Consider an IMAP-accessible email server that supports both IMAP and
IMAP-over-TLS (IMAPS) at the host "mail.example.net" servicing email
addresses of the form "[email protected]". A certificate for this
service needs to include DNS-IDs of "example.net" (because it is the
domain portion of emails) and "mail.example.net" (this is what a user
of this server enters manually if not using [RFC6186]). It might
also include a CN-ID of "mail.example.net" for backward compatibility
with deployed infrastructure.

Consider the IMAP-accessible email server from the previous paragraph
that is additionally discoverable via DNS SRV lookups in domain
"example.net" (using DNS SRV records "_imap._tcp.example.net" and
"_imaps._tcp.example.net"). In addition to the DNS-ID/CN-ID identity
types specified above, a certificate for this service also needs to
include SRV-IDs of "_imap.example.net" (when STARTTLS is used on the
IMAP port) and "_imaps.example.net" (when TLS is used on IMAPS port).
See [RFC6186] for more details. (Note that unlike DNS SRV there is
no "_tcp" component in SRV-IDs).

Consider the IMAP-accessible email server from the first paragraph
that is running on a host also known as "mycompany.example.com". In
addition to the DNS-ID identity types specified above, a certificate
for this service also needs to include a DNS-ID of
"mycompany.example.com" (this is what a user of this server enters
manually if not using [RFC6186]). It might also include a CN-ID of
"mycompany.example.com" instead of the CN-ID "mail.example.net" for
backward compatibility with deployed infrastructure. (This is so,
because a certificate can only include a single CN-ID)

Consider an SMTP Submission server at the host "submit.example.net"
servicing email addresses of the form "[email protected]" and
discoverable via DNS SRV lookups in domain "example.net" (using DNS
SRV record "_submission._tcp.example.net"). A certificate for this
service needs to include SRV-IDs of "_submission.example.net" (see
[RFC6186]) along with DNS-IDs of "example.net" and
"submit.example.net". It might also include a CN-ID of
"submit.example.net" for backward compatibility with deployed
infrastructure.

Consider a host "mail.example.net" servicing email addresses of the
form "[email protected]" and discoverable via DNS SRV lookups in
domain "example.net", which runs SMTP Submission, IMAPS and POP3S
(POP3-over-TLS), and ManageSieve services. Each of the servers can
use their own certificate specific to their service (see examples
above). Alternatively, they can all share a single certificate that
would include SRV-IDs of "_submission.example.net",

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"_imaps.example.net", "_pop3s.example.net", and "_sieve.example.net"
along with DNS-IDs of "example.net" and "mail.example.net". It might
also include a CN-ID of "mail.example.net" for backward compatibility
with deployed infrastructure.

7. Operational Considerations

Section 5 covers operational considerations (in particular, use of
DNS SRV for autoconfiguration) related to generating TLS certificates
for email servers so that they can be successfully verified by email
clients. Additionally, Section 5.1 talks about operational
considerations related to hosting multiple domains.

8. Security Considerations

The goal of this document is to improve interoperability and thus
security of email clients wishing to access email servers over TLS-
protected email protocols by specifying a consistent set of rules
that email service providers, email client writers, and CAs can use
when creating server certificates.

The TLS server identity check for email relies on use of trustworthy
DNS hostnames when constructing "reference identifiers" that are
checked against an email server certificate. Such trustworthy names
are either entered manually (for example, if they are advertised on a
Mail Service Provider's website), explicitly confirmed by the user
(e.g., if they are a target of a DNS SRV lookup), or derived using a
secure third party service (e.g., DNSSEC-protected SRV records that
are verified by the client or trusted local resolver). Future work
in this area might benefit from integration with DNS-Based
Authentication of Named Entities (DANE) [RFC6698], but it is not
covered by this document.

9. References

9.1. Normative References

[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,
<http://www.rfc-editor.org/info/rfc1939>.

[RFC2119] 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>.

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[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
February 2002, <http://www.rfc-editor.org/info/rfc3207>.

[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<http://www.rfc-editor.org/info/rfc3501>.

[RFC4985] Santesson, S., "Internet X.509 Public Key Infrastructure
Subject Alternative Name for Expression of Service Name",
RFC 4985, DOI 10.17487/RFC4985, August 2007,
<http://www.rfc-editor.org/info/rfc4985>.

[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>.

[RFC5804] Melnikov, A., Ed. and T. Martin, "A Protocol for Remotely
Managing Sieve Scripts", RFC 5804, DOI 10.17487/RFC5804,
July 2010, <http://www.rfc-editor.org/info/rfc5804>.

[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<http://www.rfc-editor.org/info/rfc6066>.

[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <http://www.rfc-editor.org/info/rfc6125>.

[RFC6186] Daboo, C., "Use of SRV Records for Locating Email
Submission/Access Services", RFC 6186,
DOI 10.17487/RFC6186, March 2011,
<http://www.rfc-editor.org/info/rfc6186>.

[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
<http://www.rfc-editor.org/info/rfc6409>.

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9.2. Informative References

[RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP",
RFC 2595, DOI 10.17487/RFC2595, June 1999,
<http://www.rfc-editor.org/info/rfc2595>.

[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<http://www.rfc-editor.org/info/rfc3986>.

[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<http://www.rfc-editor.org/info/rfc4033>.

[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, DOI 10.17487/RFC4034, March 2005,
<http://www.rfc-editor.org/info/rfc4034>.

[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
<http://www.rfc-editor.org/info/rfc4035>.

[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.

[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
2012, <http://www.rfc-editor.org/info/rfc6698>.

[RFC7711] Miller, M. and P. Saint-Andre, "PKIX over Secure HTTP
(POSH)", RFC 7711, DOI 10.17487/RFC7711, November 2015,
<http://www.rfc-editor.org/info/rfc7711>.

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Appendix A. Changes to RFCs 2595, 3207, 3501, and 5804

This section lists detailed changes this document applies to RFCs
2595, 3207, 3501, and 5804.

The entire Section 2.4 of RFC 2595 is replaced with the following
text:

During the TLS negotiation, the client checks its understanding of
the server identity against the provided server's identity as
specified in Section 3 of [RFC7817].

The 3rd paragraph (and its subparagraphs) in Section 11.1 of RFC 3501
is replaced with the following text:

During the TLS negotiation, the IMAP client checks its
understanding of the server identity against the provided server's
identity as specified in Section 3 of [RFC7817].

The 3rd paragraph (and its subparagraphs) in Section 4.1 of RFC 3207
is replaced with the following text:

During the TLS negotiation, the Submission client checks its
understanding of the server identity against the provided server's
identity as specified in Section 3 of [RFC7817].

Sections 2.2.1 and 2.2.1.1 of RFC 5804 are replaced with the
following text:

During the TLS negotiation, the ManageSieve client checks its
understanding of the server identity against the server's identity
as specified in Section 3 of [RFC7817]. When the reference
identity is an IP address, the iPAddress subjectAltName SHOULD be
used by the client for comparison. The comparison is performed as
described in Section 2.2.1.2 of RFC 5804.

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Acknowledgements

Thank you to Chris Newman, Viktor Dukhovni, Sean Turner, Russ
Housley, Alessandro Vesely, Harald Alvestrand, and John Levine for
comments on this document.

The editor of this document copied lots of text from RFCs 2595 and
6125, so the hard work of editors of these documents is appreciated.

Author's Address

Alexey Melnikov
Isode Ltd
14 Castle Mews
Hampton, Middlesex TW12 2NP
United Kingdom

EMail: [email protected]

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