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

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
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   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.

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.

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
   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").

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
   alice@example.org.  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.

   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).

   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].

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 "user@example.net".  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 "user@example.net" 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 "user@example.net" 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",
   "_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>.

   [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>.

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>.

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.

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: Alexey.Melnikov@isode.com