5.  Use of TLS by Mail User Agents

   The following requirements and recommendations apply to MUAs:

   o  MUAs SHOULD be capable of using DNS SRV records to discover Mail
      Access Servers and Mail Submission Servers that are advertised by
      an MSP for an account being configured.  Other means of
      discovering server configuration information (e.g., a database
      maintained by the MUA vendor) MAY also be supported.  (See
      Section 5.1 for more information.)

   o  MUAs SHOULD be configurable to require a minimum level of
      confidentiality for any particular Mail Account and refuse to
      exchange information via any service associated with that Mail
      Account if the session does not provide that minimum level of
      confidentiality.  (See Section 5.2.)

   o  MUAs MUST NOT treat a session as meeting a minimum level of
      confidentiality if the server's TLS certificate cannot be
      validated.  (See Section 5.3.)

   o  MUAs MAY impose other minimum confidentiality requirements in the
      future, e.g., in order to discourage the use of TLS versions or
      cryptographic algorithms in which weaknesses have been discovered.

   o  MUAs SHOULD provide a prominent indication of the level of
      confidentiality associated with an account configuration that is
      appropriate for the user interface (for example, a "lock" icon or
      changed background color for a visual interface, or some sort of
      audible indication for an audio user interface), at appropriate
      times and/or locations, in order to inform the user of the
      confidentiality of the communications associated with that
      account.  For example, this might be done whenever (a) the user is
      prompted for authentication credentials, (b) the user is composing
      mail that will be sent to a particular submission server, (c) a
      list of accounts is displayed (particularly if the user can select
      from that list to read mail), or (d) the user is asking to view or
      update any configuration data that will be stored on a remote
      server.  If, however, an MUA provides such an indication, it
      MUST NOT indicate confidentiality for any connection that does not
      at least use TLS 1.1 with certificate verification and also meet
      the minimum confidentiality requirements associated with that
      account.

   o  MUAs MUST implement TLS 1.2 [RFC5246] or later.  Earlier TLS and
      SSL versions MAY also be supported, so long as the MUA requires at
      least TLS 1.1 [RFC4346] when accessing accounts that are
      configured to impose minimum confidentiality requirements.

   o  All MUAs SHOULD implement the recommended TLS ciphersuites
      described in [RFC7525] or a future BCP or Standards Track revision
      of that document.

   o  MUAs that are configured to not require minimum confidentiality
      for one or more accounts SHOULD detect when TLS becomes available
      on those accounts (using [RFC6186] or other means) and offer to
      upgrade the account to require TLS.

   Additional considerations and details appear below.

5.1.  Use of SRV Records in Establishing Configuration

   This document updates [RFC6186] by changing the preference rules and
   adding a new SRV service label _submissions._tcp to refer to Message
   Submission with Implicit TLS.

   User-configurable MUAs SHOULD support the use of [RFC6186] for
   account setup.  However, when using configuration information
   obtained via this method, MUAs SHOULD ignore advertised services that
   do not satisfy minimum confidentiality requirements, unless the user
   has explicitly requested reduced confidentiality.  This will have the
   effect of causing the MUA to default to ignoring advertised
   configurations that do not support TLS, even when those advertised
   configurations have a higher priority than other advertised
   configurations.

   When using configuration information per [RFC6186], MUAs SHOULD NOT
   automatically establish new configurations that do not require TLS
   for all servers, unless there are no advertised configurations using
   TLS.  If such a configuration is chosen, prior to attempting to
   authenticate to the server or use the server for Message Submission,
   the MUA SHOULD warn the user that traffic to that server will not be
   encrypted and that it will therefore likely be intercepted by
   unauthorized parties.  The specific wording is to be determined by
   the implementation, but it should adequately capture the sense of
   risk, given the widespread incidence of mass surveillance of email
   traffic.

   Similarly, an MUA MUST NOT attempt to "test" a particular Mail
   Account configuration by submitting the user's authentication
   credentials to a server, unless a TLS session meeting minimum
   confidentiality levels has been established with that server.  If
   minimum confidentiality requirements have not been satisfied, the MUA
   must explicitly warn that the user's password may be exposed to
   attackers before testing the new configuration.

   When establishing a new configuration for connecting to an IMAP, POP,
   or SMTP submission server, based on SRV records, an MUA SHOULD verify
   that either (a) the SRV records are signed using DNSSEC or (b) the
   target Fully Qualified Domain Name (FQDN) of the SRV record matches
   the original server FQDN for which the SRV queries were made.  If the
   target FQDN is not in the queried domain, the MUA SHOULD verify with
   the user that the SRV target FQDN is suitable for use, before
   executing any connections to the host.  (See Section 6 of [RFC6186].)

   An MUA MUST NOT consult SRV records to determine which servers to use
   on every connection attempt, unless those SRV records are signed by
   DNSSEC and have a valid signature.  However, an MUA MAY consult SRV
   records from time to time to determine if an MSP's server
   configuration has changed and alert the user if it appears that this
   has happened.  This can also serve as a means to encourage users to
   upgrade their configurations to require TLS if and when their MSPs
   support it.

5.2.  Minimum Confidentiality Level

   MUAs SHOULD, by default, require a minimum level of confidentiality
   for services accessed by each account.  For MUAs supporting the
   ability to access multiple Mail Accounts, this requirement SHOULD be
   configurable on a per-account basis.

   The default minimum expected level of confidentiality for all new
   accounts MUST require successful validation of the server's
   certificate and SHOULD require negotiation of TLS version 1.1 or
   greater.  (Future revisions to this specification may raise these
   requirements or impose additional requirements to address newly
   discovered weaknesses in protocols or cryptographic algorithms.)

   MUAs MAY permit the user to disable this minimum confidentiality
   requirement during initial account configuration or when subsequently
   editing an account configuration but MUST warn users that such a
   configuration will not assure privacy for either passwords or
   messages.

   An MUA that is configured to require a minimum level of
   confidentiality for a Mail Account MUST NOT attempt to perform any
   operation other than capability discovery, or STARTTLS for servers
   not using Implicit TLS, unless the minimum level of confidentiality
   is provided by that connection.

   MUAs SHOULD NOT allow users to easily access or send mail via a
   connection, or authenticate to any service using a password, if that
   account is configured to impose minimum confidentiality requirements
   and that connection does not meet all of those requirements.  An

   example of "easy access" would be to display a dialog informing the
   user that the security requirements of the account were not met by
   the connection but allowing the user to "click through" to send mail
   or access the service anyway.  Experience indicates that users
   presented with such an option often "click through" without
   understanding the risks that they're accepting by doing so.
   Furthermore, users who frequently find the need to "click through" to
   use an insecure connection may become conditioned to do so as a
   matter of habit, before considering whether the risks are reasonable
   in each specific instance.

   An MUA that is not configured to require a minimum level of
   confidentiality for a Mail Account SHOULD still attempt to connect to
   the services associated with that account using the most secure means
   available, e.g., by using Implicit TLS or STARTTLS.

5.3.  Certificate Validation

   MUAs MUST validate TLS server certificates according to [RFC7817] and
   PKIX [RFC5280].

   MUAs MAY also support DNS-Based Authentication of Named Entities
   (DANE) [RFC6698] as a means of validating server certificates in
   order to meet minimum confidentiality requirements.

   MUAs MAY support the use of certificate pinning but MUST NOT consider
   a connection in which the server's authenticity relies on certificate
   pinning as providing the minimum level of confidentiality.  (See
   Section 5.4.)

5.4.  Certificate Pinning

   During account setup, the MUA will identify servers that provide
   account services such as mail access and mail submission (Section 5.1
   describes one way to do this).  The certificates for these servers
   are verified using the rules described in [RFC7817] and PKIX
   [RFC5280].  In the event that the certificate does not validate due
   to an expired certificate, a lack of an appropriate chain of trust,
   or a lack of an identifier match, the MUA MAY offer to create a
   persistent binding between that certificate and the saved hostname
   for the server, for use when accessing that account's servers.  This
   is called "certificate pinning".

   (Note: This use of the term "certificate pinning" means something
   subtly different than HTTP Public Key Pinning as described in
   [RFC7469].  The dual use of the same term is confusing, but
   unfortunately both uses are well established.)

   Certificate pinning is only appropriate during Mail Account setup and
   MUST NOT be offered as an option in response to a failed certificate
   validation for an existing Mail Account.  An MUA that allows
   certificate pinning MUST NOT allow a certificate pinned for one
   account to validate connections for other accounts.  An MUA that
   allows certificate pinning MUST also allow a user to undo the
   pinning, i.e., to revoke trust in a certificate that has previously
   been pinned.

   A pinned certificate is subject to a man-in-the-middle attack at
   account setup time and typically lacks a mechanism to automatically
   revoke or securely refresh the certificate.  Note also that a man-in-
   the-middle attack at account setup time will expose the user's
   password to the attacker (if a password is used).  Therefore, the use
   of a pinned certificate does not meet the requirement for a minimum
   confidentiality level, and an MUA MUST NOT indicate to the user that
   such confidentiality is provided.  Additional advice on certificate
   pinning is presented in [RFC6125].

5.5.  Client Certificate Authentication

   MUAs MAY implement client certificate authentication on the Implicit
   TLS port.  An MUA MUST NOT provide a client certificate during the
   TLS handshake unless the server requests one and the MUA has been
   authorized to use that client certificate with that account.  Having
   the end user explicitly configure a client certificate for use with a
   given account is sufficient to meet this requirement.  However,
   installing a client certificate for use with one account MUST NOT
   automatically authorize the use of that certificate with other
   accounts.  This is not intended to prohibit site-specific
   authorization mechanisms, such as (a) a site-administrator-controlled
   mechanism to authorize the use of a client certificate with a given
   account or (b) a domain-name-matching mechanism.

   Note: The requirement that the server request a certificate is just a
   restatement of the TLS protocol rules, e.g., Section 7.4.6 of
   [RFC5246].  The requirement that the client not send a certificate
   not known to be acceptable to the server is pragmatic in multiple
   ways: the current TLS protocol provides no way for the client to know
   which of the potentially multiple certificates it should use; also,
   when the client sends a certificate, it is potentially disclosing its
   identity (or its user's identity) to both the server and any party
   with access to the transmission medium, perhaps unnecessarily and for
   no useful purpose.

   A client supporting client certificate authentication with Implicit
   TLS MUST implement the SASL EXTERNAL mechanism [RFC4422], using the
   appropriate authentication command (AUTH for POP3 [RFC5034], AUTH for
   SMTP Submission [RFC4954], or AUTHENTICATE for IMAP [RFC3501]).

6.  Considerations Related to Antivirus/Antispam Software and Services

   There are multiple ways to connect an AVAS service (e.g., "Antivirus
   & Antispam") to a mail server.  Some mechanisms, such as the de facto
   "milter" protocol, are out of scope for this specification.  However,
   some services use an SMTP relay proxy that intercepts mail at the
   application layer to perform a scan and proxy or forward to another
   Mail Transfer Agent (MTA).  Deploying AVAS services in this way can
   cause many problems [RFC2979], including direct interference with
   this specification, and other forms of confidentiality or security
   reduction.  An AVAS product or service is considered compatible with
   this specification if all IMAP, POP, and SMTP-related software
   (including proxies) it includes are compliant with this
   specification.

   Note that end-to-end email encryption prevents AVAS software and
   services from using email content as part of a spam or virus
   assessment.  Furthermore, although a minimum confidentiality level
   can prevent a man-in-the-middle from introducing spam or virus
   content between the MUA and Submission server, it does not prevent
   other forms of client or account compromise.  The use of AVAS
   services for submitted email therefore remains necessary.

7.  IANA Considerations

7.1.  POP3S Port Registration Update

   IANA has updated the registration of the TCP well-known port 995
   using the following template [RFC6335]:

     Service Name: pop3s
     Transport Protocol: TCP
     Assignee: IESG <iesg@ietf.org>
     Contact: IETF Chair <chair@ietf.org>
     Description: POP3 over TLS protocol
     Reference: RFC 8314
     Port Number: 995

7.2.  IMAPS Port Registration Update

   IANA has updated the registration of the TCP well-known port 993
   using the following template [RFC6335]:

     Service Name: imaps
     Transport Protocol: TCP
     Assignee: IESG <iesg@ietf.org>
     Contact: IETF Chair <chair@ietf.org>
     Description: IMAP over TLS protocol
     Reference: RFC 8314
     Port Number: 993

   No changes to existing UDP port assignments for pop3s or imaps are
   being requested.

7.3.  Submissions Port Registration

   IANA has assigned an alternate usage of TCP port 465 in addition to
   the current assignment using the following template [RFC6335]:

     Service Name: submissions
     Transport Protocol: TCP
     Assignee: IESG <iesg@ietf.org>
     Contact: IETF Chair <chair@ietf.org>
     Description: Message Submission over TLS protocol
     Reference: RFC 8314
     Port Number: 465

   This is a one-time procedural exception to the rules in [RFC6335].
   This requires explicit IESG approval and does not set a precedent.
   Note: Since the purpose of this alternate usage assignment is to
   align with widespread existing practice and there is no known usage
   of UDP port 465 for Message Submission over TLS, IANA has not
   assigned an alternate usage of UDP port 465.

   Historically, port 465 was briefly registered as the "smtps" port.
   This registration made no sense, as the SMTP transport MX
   infrastructure has no way to specify a port, so port 25 is always
   used.  As a result, the registration was revoked and was subsequently
   reassigned to a different service.  In hindsight, the "smtps"
   registration should have been renamed or reserved rather than
   revoked.  Unfortunately, some widely deployed mail software
   interpreted "smtps" as "submissions" [RFC6409] and used that port for
   email submission by default when an end user requested security
   during account setup.  If a new port is assigned for the submissions
   service, either (a) email software will continue with unregistered
   use of port 465 (leaving the port registry inaccurate relative to

   de facto practice and wasting a well-known port) or (b) confusion
   between the de facto and registered ports will cause harmful
   interoperability problems that will deter the use of TLS for Message
   Submission.  The authors of this document believe that both of these
   outcomes are less desirable than a "wart" in the registry documenting
   real-world usage of a port for two purposes.  Although STARTTLS on
   port 587 has been deployed, it has not replaced the deployed use of
   Implicit TLS submission on port 465.

7.4.  Additional Registered Clauses for "Received" Fields

   Per the provisions in [RFC5321], IANA has added two additional-
   registered-clauses for Received fields as defined in Section 4.3 of
   this document:

   o  "tls": Indicates the TLS cipher used (if applicable)

   o  "group": Indicates the Diffie-Hellman group used with the TLS
      cipher (if applicable)

   The descriptions and syntax of these additional clauses are provided
   in Section 4.3 of this document.

8.  Security Considerations

   This entire document is about security considerations.  In general,
   this is targeted to improve mail confidentiality and to mitigate
   threats external to the email system such as network-level snooping
   or interception; this is not intended to mitigate active attackers
   who have compromised service provider systems.

   Implementers should be aware that the use of client certificates with
   TLS 1.2 reveals the user's identity to any party with the ability to
   read packets from the transmission medium and therefore may
   compromise the user's privacy.  There seems to be no easy fix with
   TLS 1.2 or earlier versions, other than to avoid presenting client
   certificates except when there is explicit authorization to do so.
   TLS 1.3 [TLS-1.3] appears to reduce this privacy risk somewhat.

9.  References

9.1.  Normative References

   [RFC793]   Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

   [RFC1939]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,
              <https://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,
              <https://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, <https://www.rfc-editor.org/info/rfc3207>.

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

   [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,
              <https://www.rfc-editor.org/info/rfc4033>.

   [RFC5034]  Siemborski, R. and A. Menon-Sen, "The Post Office Protocol
              (POP3) Simple Authentication and Security Layer (SASL)
              Authentication Mechanism", RFC 5034, DOI 10.17487/RFC5034,
              July 2007, <https://www.rfc-editor.org/info/rfc5034>.

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

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [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,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <https://www.rfc-editor.org/info/rfc5322>.

   [RFC6186]  Daboo, C., "Use of SRV Records for Locating Email
              Submission/Access Services", RFC 6186,
              DOI 10.17487/RFC6186, March 2011,
              <https://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,
              <https://www.rfc-editor.org/info/rfc6409>.

   [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, <https://www.rfc-editor.org/info/rfc6698>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525,
              May 2015, <https://www.rfc-editor.org/info/rfc7525>.

   [RFC7672]  Dukhovni, V. and W. Hardaker, "SMTP Security via
              Opportunistic DNS-Based Authentication of Named Entities
              (DANE) Transport Layer Security (TLS)", RFC 7672,
              DOI 10.17487/RFC7672, October 2015,
              <https://www.rfc-editor.org/info/rfc7672>.

   [RFC7817]  Melnikov, A., "Updated Transport Layer Security (TLS)
              Server Identity Check Procedure for Email-Related
              Protocols", RFC 7817, DOI 10.17487/RFC7817, March 2016,
              <https://www.rfc-editor.org/info/rfc7817>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in
              RFC 2119 Key Words", BCP 14, RFC 8174,
              DOI 10.17487/RFC8174, May 2017,
              <https://www.rfc-editor.org/info/rfc8174>.

9.2.  Informative References

   [CERT-555316]
              CERT, "Vulnerability Note VU#555316: STARTTLS plaintext
              command injection vulnerability", Carnegie Mellon
              University Software Engineering Institute, September 2011,
              <https://www.kb.cert.org/vuls/id/555316>.

   [Email-TLS]
              Moore, K., "Recommendations for use of TLS by Electronic
              Mail Access Protocols", Work in Progress, draft-moore-
              email-tls-00, October 2013.

   [MTA-STS]  Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,
              and J. Jones, "SMTP MTA Strict Transport Security
              (MTA-STS)", Work in Progress, draft-ietf-uta-mta-sts-14,
              January 2018.

   [POP3-over-TLS]
              Melnikov, A., Newman, C., and M. Yevstifeyev, Ed., "POP3
              over TLS", Work in Progress, draft-melnikov-pop3-
              over-tls-02, August 2011.

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

   [RFC2979]  Freed, N., "Behavior of and Requirements for Internet
              Firewalls", RFC 2979, DOI 10.17487/RFC2979, October 2000,
              <https://www.rfc-editor.org/info/rfc2979>.

   [RFC3848]  Newman, C., "ESMTP and LMTP Transmission Types
              Registration", RFC 3848, DOI 10.17487/RFC3848, July 2004,
              <https://www.rfc-editor.org/info/rfc3848>.

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346,
              DOI 10.17487/RFC4346, April 2006,
              <https://www.rfc-editor.org/info/rfc4346>.

   [RFC4422]  Melnikov, A., Ed., and K. Zeilenga, Ed., "Simple
              Authentication and Security Layer (SASL)", RFC 4422,
              DOI 10.17487/RFC4422, June 2006,
              <https://www.rfc-editor.org/info/rfc4422>.

   [RFC4954]  Siemborski, R., Ed., and A. Melnikov, Ed., "SMTP Service
              Extension for Authentication", RFC 4954,
              DOI 10.17487/RFC4954, July 2007,
              <https://www.rfc-editor.org/info/rfc4954>.

   [RFC5068]  Hutzler, C., Crocker, D., Resnick, P., Allman, E., and T.
              Finch, "Email Submission Operations: Access and
              Accountability Requirements", BCP 134, RFC 5068,
              DOI 10.17487/RFC5068, November 2007,
              <https://www.rfc-editor.org/info/rfc5068>.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,
              <https://www.rfc-editor.org/info/rfc5321>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <https://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, <https://www.rfc-editor.org/info/rfc6125>.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, DOI 10.17487/RFC6335, August 2011,
              <https://www.rfc-editor.org/info/rfc6335>.

   [RFC7469]  Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
              Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469,
              April 2015, <https://www.rfc-editor.org/info/rfc7469>.

   [TLS-1.3]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", Work in Progress, draft-ietf-tls-tls13-23,
              January 2018.

Appendix A.  Design Considerations

   This section is not normative.

   The first version of this document was written independently from the
   October 2013 version of [Email-TLS] ("Recommendations for use of TLS
   by Electronic Mail Access Protocols").  Subsequent versions merge
   ideas from both documents.

   One author of this document was also the author of RFC 2595, which
   became the standard for TLS usage with POP and IMAP, and the other
   author was perhaps the first to propose that idea.  In hindsight,
   both authors now believe that that approach was a mistake.  At this
   point, the authors believe that while anything that makes it easier
   to deploy TLS is good, the desirable end state is that these
   protocols always use TLS, leaving no need for a separate port for
   cleartext operation except to support legacy clients while they
   continue to be used.  The separate-port model for TLS is inherently
   simpler to implement, debug, and deploy.  It also enables a "generic
   TLS load-balancer" that accepts secure client connections for
   arbitrary foo-over-TLS protocols and forwards them to a server that
   may or may not support TLS.  Such load-balancers cause many problems
   because they violate the end-to-end principle and the server loses
   the ability to log security-relevant information about the client
   unless the protocol is designed to forward that information (as this
   specification does for the ciphersuite).  However, they can result in
   TLS deployment where it would not otherwise happen, which is a
   sufficiently important goal that it overrides any problems.

   Although STARTTLS appears only slightly more complex than
   separate-port TLS, we again learned the lesson that complexity is the
   enemy of security in the form of the STARTTLS command injection
   vulnerability (Computer Emergency Readiness Team (CERT) vulnerability
   ID #555316 [CERT-555316]).  Although there's nothing inherently wrong
   with STARTTLS, the fact that it resulted in a common implementation
   error (made independently by multiple implementers) suggests that it
   is a less secure architecture than Implicit TLS.

   Section 7 of RFC 2595 critiques the separate-port approach to TLS.
   The first bullet was a correct critique.  There are proposals in the
   HTTP community to address that, and the use of SRV records as
   described in RFC 6186 resolves that critique for email.  The second
   bullet is correct as well but is not very important because useful
   deployment of security layers other than TLS in email is small enough
   to be effectively irrelevant.  (Also, it's less correct than it used
   to be because "export" ciphersuites are no longer supported in modern
   versions of TLS.)  The third bullet is incorrect because it misses
   the desirable option of "use TLS for all subsequent connections to

   this server once TLS is successfully negotiated".  The fourth bullet
   may be correct, but it is not a problem yet with current port
   consumption rates.  The fundamental error was prioritizing a
   perceived better design based on a mostly valid critique over
   real-world deployability.  But getting security and confidentiality
   facilities actually deployed is so important that it should trump
   design purity considerations.

   Port 465 is presently used for two purposes: for submissions by a
   large number of clients and service providers and for the "urd"
   protocol by one vendor.  Actually documenting this current state is
   controversial, as discussed in the IANA Considerations section.
   However, there is no good alternative.  Registering a new port for
   submissions when port 465 is already widely used for that purpose
   will just create interoperability problems.  Registering a port
   that's only used if advertised by an SRV record (RFC 6186) would not
   create interoperability problems but would require all client
   deployments, server deployments, and software to change
   significantly, which is contrary to the goal of promoting the
   increased use of TLS.  Encouraging the use of STARTTLS on port 587
   would not create interoperability problems, but it is unlikely to
   have any impact on the current undocumented use of port 465 and makes
   the guidance in this document less consistent.  The remaining option
   is to document the current state of the world and support future use
   of port 465 for submission, as this increases consistency and ease of
   deployment for TLS email submission.

Acknowledgements

   Thanks to Ned Freed for discussion of the initial concepts in this
   document.  Thanks to Alexey Melnikov for [POP3-over-TLS], which was
   the basis of the POP3 Implicit TLS text.  Thanks to Russ Housley,
   Alexey Melnikov, and Dan Newman for review feedback.  Thanks to
   Paul Hoffman for interesting feedback in initial conversations about
   this idea.

Authors' Addresses

   Keith Moore
   Windrock, Inc.
   PO Box 1934
   Knoxville, TN  37901
   United States of America

   Email: moore@network-heretics.com


   Chris Newman
   Oracle
   440 E. Huntington Dr., Suite 400
   Arcadia, CA  91006
   United States of America

   Email: chris.newman@oracle.com