draft-ietf-uta-mta-sts-04.txt		draft-ietf-uta-mta-sts-05.txt
	Using TLS in Applications D. Margolis		Using TLS in Applications D. Margolis
	Internet-Draft M. Risher		Internet-Draft M. Risher
	Intended status: Standards Track Google, Inc		Intended status: Standards Track Google, Inc
	Expires: October 5, 2017 B. Ramakrishnan		Expires: November 4, 2017 B. Ramakrishnan
	Yahoo!, Inc		Yahoo!, Inc
	A. Brotman		A. Brotman
	Comcast, Inc		Comcast, Inc
	J. Jones		J. Jones
	Microsoft, Inc		Microsoft, Inc
	April 3, 2017		May 3, 2017
	SMTP MTA Strict Transport Security (MTA-STS)		SMTP MTA Strict Transport Security (MTA-STS)
	draft-ietf-uta-mta-sts-04		draft-ietf-uta-mta-sts-05
	Abstract		Abstract
	SMTP Mail Transfer Agent Strict Transport Security (MTA-STS) is a		SMTP Mail Transfer Agent Strict Transport Security (MTA-STS) is a
	mechanism enabling mail service providers to declare their ability to		mechanism enabling mail service providers to declare their ability to
	receive Transport Layer Security (TLS) secure SMTP connections, and		receive Transport Layer Security (TLS) secure SMTP connections, and
	to specify whether sending SMTP servers should refuse to deliver to		to specify whether sending SMTP servers should refuse to deliver to
	MX hosts that do not offer TLS with a trusted server certificate.		MX hosts that do not offer TLS with a trusted server certificate.
	Status of This Memo		Status of This Memo
	skipping to change at page 1, line 40 ¶		skipping to change at page 1, line 40 ¶
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at http://datatracker.ietf.org/drafts/current/.		Drafts is at http://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on October 5, 2017.		This Internet-Draft will expire on November 4, 2017.
	Copyright Notice		Copyright Notice
	Copyright (c) 2017 IETF Trust and the persons identified as the		Copyright (c) 2017 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(http://trustee.ietf.org/license-info) in effect on the date of		(http://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
	skipping to change at page 2, line 21 ¶		skipping to change at page 2, line 21 ¶
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
	1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3		1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
	2. Related Technologies . . . . . . . . . . . . . . . . . . . . 3		2. Related Technologies . . . . . . . . . . . . . . . . . . . . 3
	3. Policy Discovery . . . . . . . . . . . . . . . . . . . . . . 4		3. Policy Discovery . . . . . . . . . . . . . . . . . . . . . . 4
	3.1. MTA-STS TXT Records . . . . . . . . . . . . . . . . . . . 4		3.1. MTA-STS TXT Records . . . . . . . . . . . . . . . . . . . 4
	3.2. MTA-STS Policies . . . . . . . . . . . . . . . . . . . . 5		3.2. MTA-STS Policies . . . . . . . . . . . . . . . . . . . . 5
	3.3. HTTPS Policy Fetching . . . . . . . . . . . . . . . . . . 6		3.3. HTTPS Policy Fetching . . . . . . . . . . . . . . . . . . 6
	3.4. Policy Selection for Smart Hosts and Subdomains . . . . . 7		3.4. Policy Selection for Smart Hosts and Subdomains . . . . . 7
	4. Policy Validation . . . . . . . . . . . . . . . . . . . . . . 7		4. Policy Validation . . . . . . . . . . . . . . . . . . . . . . 8
	4.1. MX Certificate Validation . . . . . . . . . . . . . . . . 7		4.1. MX Certificate Validation . . . . . . . . . . . . . . . . 8
	5. Policy Application . . . . . . . . . . . . . . . . . . . . . 8		5. Policy Application . . . . . . . . . . . . . . . . . . . . . 9
	5.1. Policy Application Control Flow . . . . . . . . . . . . . 8		5.1. Policy Application Control Flow . . . . . . . . . . . . . 9
	6. Operational Considerations . . . . . . . . . . . . . . . . . 9		6. Operational Considerations . . . . . . . . . . . . . . . . . 10
	6.1. Policy Updates . . . . . . . . . . . . . . . . . . . . . 9		6.1. Policy Updates . . . . . . . . . . . . . . . . . . . . . 10
	7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9		7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
	8. Security Considerations . . . . . . . . . . . . . . . . . . . 9		8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
	8.1. Obtaining a Signed Certificate . . . . . . . . . . . . . 10		8.1. Obtaining a Signed Certificate . . . . . . . . . . . . . 11
	8.2. Preventing Policy Discovery . . . . . . . . . . . . . . . 10		8.2. Preventing Policy Discovery . . . . . . . . . . . . . . . 11
	8.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 11		8.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 12
	8.4. Weak Policy Constraints . . . . . . . . . . . . . . . . . 11		8.4. Weak Policy Constraints . . . . . . . . . . . . . . . . . 12
	9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 12		9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 13
	10. Appendix 1: MTA-STS example record & policy . . . . . . . . . 12		10. Appendix 1: MTA-STS example record & policy . . . . . . . . . 13
	11. Appendix 2: Message delivery pseudocode . . . . . . . . . . . 12		11. Appendix 2: Message delivery pseudocode . . . . . . . . . . . 13
	12. References . . . . . . . . . . . . . . . . . . . . . . . . . 15		12. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
	12.1. Normative References . . . . . . . . . . . . . . . . . . 15		12.1. Normative References . . . . . . . . . . . . . . . . . . 16
	12.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 16		12.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 17
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
	1. Introduction		1. Introduction
	The STARTTLS extension to SMTP [RFC3207] allows SMTP clients and		The STARTTLS extension to SMTP [RFC3207] allows SMTP clients and
	hosts to negotiate the use of a TLS channel for encrypted mail		hosts to negotiate the use of a TLS channel for encrypted mail
	transmission.		transmission.
	While this opportunistic encryption protocol by itself provides a		While this opportunistic encryption protocol by itself provides a
	high barrier against passive man-in-the-middle traffic interception,		high barrier against passive man-in-the-middle traffic interception,
	any attacker who can delete parts of the SMTP session (such as the		any attacker who can delete parts of the SMTP session (such as the
	skipping to change at page 3, line 31 ¶		skipping to change at page 3, line 31 ¶
	We also define the following terms for further use in this document:		We also define the following terms for further use in this document:
	o MTA-STS Policy: A commitment by the Policy Domain to support PKIX		o MTA-STS Policy: A commitment by the Policy Domain to support PKIX
	authenticated TLS for the specified MX hosts.		authenticated TLS for the specified MX hosts.
	o Policy Domain: The domain for which an MTA-STS Policy is defined.		o Policy Domain: The domain for which an MTA-STS Policy is defined.
	This is the next-hop domain; when sending mail to		This is the next-hop domain; when sending mail to
	"alice@example.com" this would ordinarly be "example.com", but		"alice@example.com" this would ordinarly be "example.com", but
	this may be overriden by explicit routing rules (as described in		this may be overriden by explicit routing rules (as described in
	"Policy Selection for Smart Hosts").		Section 3.4, "Policy Selection for Smart Hosts and Subdomains").
	2. Related Technologies		2. Related Technologies
	The DANE TLSA record [RFC7672] is similar, in that DANE is also		The DANE TLSA record [RFC7672] is similar, in that DANE is also
	designed to upgrade unauthenticated encryption or plaintext		designed to upgrade unauthenticated encryption or plaintext
	transmission into authenticated, downgrade-resistent encrypted		transmission into authenticated, downgrade-resistent encrypted
	tarnsmission. DANE requires DNSSEC [RFC4033] for authentication; the		tarnsmission. DANE requires DNSSEC [RFC4033] for authentication; the
	mechanism described here instead relies on certificate authorities		mechanism described here instead relies on certificate authorities
	(CAs) and does not require DNSSEC, at a cost of risking malicious		(CAs) and does not require DNSSEC, at a cost of risking malicious
	downgrades. For a thorough discussion of this trade-off, see the		downgrades. For a thorough discussion of this trade-off, see
	section "Security Considerations".		Section 8, "Security Considerations".
	In addition, MTA-STS provides an optional report-only mode, enabling		In addition, MTA-STS provides an optional report-only mode, enabling
	soft deployments to detect policy failures; partial deployments can		soft deployments to detect policy failures; partial deployments can
	be achieved in DANE by deploying TLSA records only for some of a		be achieved in DANE by deploying TLSA records only for some of a
	domain's MXs, but such a mechanism is not possible for the per-domain		domain's MXs, but such a mechanism is not possible for the per-domain
	policies used by MTA-STS.		policies used by MTA-STS.
	The primary motivation of MTA-STS is to provide a mechanism for		The primary motivation of MTA-STS is to provide a mechanism for
	domains to upgrade their transport security even when deploying		domains to upgrade their transport security even when deploying
	DNSSEC is undesirable or impractical. However, MTA-STS is designed		DNSSEC is undesirable or impractical. However, MTA-STS is designed
	skipping to change at page 5, line 5 ¶		skipping to change at page 5, line 5 ¶
	updated by comparing to the "id" of a previously seen policy.		updated by comparing to the "id" of a previously seen policy.
	There is no implied ordering of "id" fields between revisions.		There is no implied ordering of "id" fields between revisions.
	An example TXT record is as below:		An example TXT record is as below:
	"_mta-sts.example.com. IN TXT "v=STSv1; id=20160831085700Z;""		"_mta-sts.example.com. IN TXT "v=STSv1; id=20160831085700Z;""
	The formal definition of the "_mta-sts" TXT record, defined using		The formal definition of the "_mta-sts" TXT record, defined using
	[RFC5234], is as follows:		[RFC5234], is as follows:
	sts-text-record = sts-version *WSP %x3B *WSP sts-id [%x3B]		sts-text-record = sts-version *WSP field-delim *WSP sts-id
			[field-delim [sts-extensions]]
	sts-version = "v" *WSP "=" *WSP %x53 %x54 ; "STSv1"		field-delim = %x3B ; ";"
	%x53 %x76 %x31
	sts-id = "id" *WSP "=" *WSP 1*32(ALPHA / DIGIT)		sts-version = %x76 *WSP "=" *WSP %x53 %x54 ; "v=STSv1"
			%x53 %x76 %x31
			sts-id = %x69 %x64 *WSP "="
			*WSP 1*32(ALPHA / DIGIT) ; "id="
			sts-extensions = sts-extension *(field-delim sts-extension)
			[field-delim] ; extension fields
			sts-extension = sts-ext-name *WSP "=" *WSP sts-ext-value
			sts-ext-name = (ALPHA / DIGIT) *31(ALPHA / DIGIT / "_" / "-" / ".")
			sts-ext-value = 1*(%x21-3A / %x3C / %x3E-7E) ; chars excluding
			; "=", ";", SP, and
			; control chars
	If multiple TXT records for "_mta-sts" are returned by the resolver,		If multiple TXT records for "_mta-sts" are returned by the resolver,
	records which do not begin with "v=STSv1;" are discarded. If the		records which do not begin with "v=STSv1;" are discarded. If the
	number of resulting records is not one, senders MUST assume the		number of resulting records is not one, senders MUST assume the
	recipient domain does not implement MTA-STS and skip the remaining		recipient domain does not implement MTA-STS and skip the remaining
	steps of policy discovery.		steps of policy discovery.
	3.2. MTA-STS Policies		3.2. MTA-STS Policies
	The policy itself is a JSON [RFC4627] object served via the HTTPS GET		The policy itself is a JSON [RFC7159] object served via the HTTPS GET
	method from the fixed [RFC5785] "well-known" path of ".well-known/		method from the fixed [RFC5785] "well-known" path of ".well-known/
	mta-sts.json" served by the "mta-sts" host at the Policy Domain.		mta-sts.json" served by the "mta-sts" host at the Policy Domain.
	Thus for "example.com" the path is "https://mta-sts.example.com		Thus for "example.com" the path is "https://mta-sts.example.com
	/.well-known/mta-sts.json".		/.well-known/mta-sts.json".
	This JSON object contains the following key/value pairs:		This JSON object contains the following key/value pairs:
	o "version": (plain-text, required). Currently only "STSv1" is		o "version": (plain-text, required). Currently only "STSv1" is
	supported.		supported.
	skipping to change at page 5, line 48 ¶		skipping to change at page 6, line 14 ¶
	mitigate the risks of attacks at policy refresh time, it is		mitigate the risks of attacks at policy refresh time, it is
	expected that this value typically be in the range of weeks or		expected that this value typically be in the range of weeks or
	greater.		greater.
	o "mx": MX identity patterns (list of plain-text strings, required).		o "mx": MX identity patterns (list of plain-text strings, required).
	One or more patterns matching a Common Name ([RFC6125]) or Subject		One or more patterns matching a Common Name ([RFC6125]) or Subject
	Alternative Name ([RFC5280]) DNS-ID present in the X.509		Alternative Name ([RFC5280]) DNS-ID present in the X.509
	certificate presented by any MX receiving mail for this domain.		certificate presented by any MX receiving mail for this domain.
	For example, "["mail.example.com", ".example.net"]" indicates that		For example, "["mail.example.com", ".example.net"]" indicates that
	mail for this domain might be handled by any MX with a certificate		mail for this domain might be handled by any MX with a certificate
	valid for a host at "example.com" or "example.net". Valid		valid for a host at "mail.example.com" or "example.net". Valid
	patterns can be either fully specified names ("example.com") or		patterns can be either fully specified names ("example.com") or
	suffixes (".example.net") matching the right-hand parts of a		suffixes (".example.net") matching the right-hand parts of a
	server's identity; the latter case are distinguished by a leading		server's identity; the latter case are distinguished by a leading
	period. In the case of Internationalized Domain Names		period. In the case of Internationalized Domain Names
	([RFC5891]), the MX MUST specify the Punycode-encoded A-label		([RFC5891]), the MX MUST specify the Punycode-encoded A-label
	[RFC3492] and not the Unicode-encoded U-label. The full semantics		[RFC3492] and not the Unicode-encoded U-label. The full semantics
	of certificate validation are described in "MX Certificate		of certificate validation are described in Section 4.1, "MX
	Validation."		Certificate Validation."
	An example JSON policy is as below:		An example JSON policy is as below:
	{		{
	"version": "STSv1",		"version": "STSv1",
	"mode": "enforce",		"mode": "enforce",
	"mx": [".mail.example.com"],		"mx": [".mail.example.com"],
	"max_age": 123456		"max_age": 123456
	}		}
	Parsers SHOULD accept TXT records and policy files which are		Parsers MUST accept TXT records and policy files which are
	syntactically valid (i.e. valid key-value pairs separated by semi-		syntactically valid (i.e. valid key-value pairs separated by semi-
	colons for TXT records and valid JSON for policy files) and		colons for TXT records and valid JSON for policy files) and
	implementing a superset of this specification, in which case unknown		implementing a superset of this specification, in which case unknown
	fields SHALL be ignored.		fields SHALL be ignored.
	3.3. HTTPS Policy Fetching		3.3. HTTPS Policy Fetching
	When fetching a new policy or updating a policy, the HTTPS endpoint		When fetching a new policy or updating a policy, the HTTPS endpoint
	MUST present a X.509 certificate which is valid for the "mta-sts"		MUST present a X.509 certificate which is valid for the "mta-sts"
	host (as described in [RFC6125]), chain to a root CA that is trusted		host (as described below), chain to a root CA that is trusted by the
	by the sending MTA, and be non-expired. It is expected that sending		sending MTA, and be non-expired. It is expected that sending MTAs
	MTAs use a set of trusted CAs similar to those in widely deployed Web		use a set of trusted CAs similar to those in widely deployed Web
	browsers and operating systems.		browsers and operating systems.
			The certificate is valid for the "mta-sts" host with respect to the
			rules described in [RFC6125], with the following application-specific
			considerations:
			o Matching is performed only against the DNS-ID and CN-ID
			identifiers.
			o DNS domain names in server certificates MAY contain the wildcard
			character '*' as the complete left-most label within the
			identifier.
			The certificate MAY be checked for revocation via the Online
			Certificate Status Protocol (OCSP) [RFC2560], certificate revocation
			lists (CRLs), or some other mechanism.
	HTTP 3xx redirects MUST NOT be followed.		HTTP 3xx redirects MUST NOT be followed.
	Senders may wish to rate-limit the frequency of attempts to fetch the		Senders may wish to rate-limit the frequency of attempts to fetch the
	HTTPS endpoint even if a valid TXT record for the recipient domain		HTTPS endpoint even if a valid TXT record for the recipient domain
	exists. In the case that the HTTPS GET fails, we suggest		exists. In the case that the HTTPS GET fails, we suggest
	implementions may limit further attempts to a period of five minutes		implementions may limit further attempts to a period of five minutes
	or longer per version ID, to avoid overwhelming resource-constrained		or longer per version ID, to avoid overwhelming resource-constrained
	recipients with cascading failures.		recipients with cascading failures.
	Senders MAY impose a timeout on the HTTPS GET to avoid long delays		Senders MAY impose a timeout on the HTTPS GET and/or a limit on the
	imposed by attempted policy updates. A suggested timeout is one		maximum size of the response body to avoid long delays or resource
	minute; policy hosts SHOULD respond to requests with a complete		exhaustion during attempted policy updates. A suggested timeout is
	policy body within that timeout.		one minute, and a suggested maximum policy size 64 kilobytes; policy
			hosts SHOULD respond to requests with a complete policy body within
			that timeout and size limit.
	If a valid TXT record is found but no policy can be fetched via		If a valid TXT record is found but no policy can be fetched via HTTPS
	HTTPS, and there is no valid (non-expired) previously-cached policy,		(for any reason), and there is no valid (non-expired) previously-
	senders MUST treat the recipient domain as one that has not		cached policy, senders MUST continue with delivery as though the
	implemented MTA-STS.		domain has not implemented MTA-STS. Senders who implement TLSRPT
			(TODO: add ref) should, however, report this failure to the recipient
			domain if the domain implements TLSRPT as well.
			Conversely, if no "live" policy can be discovered via DNS or fetched
			via HTTPS, but a valid (non-expired) policy exists in the sender's
			cache, the sender MUST apply that cached policy.
	3.4. Policy Selection for Smart Hosts and Subdomains		3.4. Policy Selection for Smart Hosts and Subdomains
	When sending mail via a "smart host"--an intermediate SMTP relay		When sending mail via a "smart host"--an intermediate SMTP relay
	rather than the message recipient's server--compliant senders MUST		rather than the message recipient's server--compliant senders MUST
	treat the smart host domain as the policy domain for the purposes of		treat the smart host domain as the policy domain for the purposes of
	policy discovery and application.		policy discovery and application.
	When sending mail to a mailbox at a subdomain, compliant senders MUST		When sending mail to a mailbox at a subdomain, compliant senders MUST
	NOT attempt to fetch a policy from the parent zone. Thus for mail		NOT attempt to fetch a policy from the parent zone. Thus for mail
	skipping to change at page 7, line 28 ¶		skipping to change at page 8, line 18 ¶
	When sending to an MX at a domain for which the sender has a valid		When sending to an MX at a domain for which the sender has a valid
	and non-expired MTA-STS policy, a sending MTA honoring MTA-STS MUST		and non-expired MTA-STS policy, a sending MTA honoring MTA-STS MUST
	validate:		validate:
	1. That the recipient MX supports STARTTLS and offers a valid PKIX-		1. That the recipient MX supports STARTTLS and offers a valid PKIX-
	based TLS certificate.		based TLS certificate.
	2. That at least one of the policy's "mx" patterns matches at least		2. That at least one of the policy's "mx" patterns matches at least
	one of the identities presented in the MX's X.509 certificate, as		one of the identities presented in the MX's X.509 certificate, as
	descriped in "MX Certificate Validation".		described in "MX Certificate Validation".
	This section does not dictate the behavior of sending MTAs when		This section does not dictate the behavior of sending MTAs when
	policies fail to validate; in particular, validation failures of		policies fail to validate; in particular, validation failures of
	policies which specify "report" mode MUST NOT be interpreted as		policies which specify "report" mode MUST NOT be interpreted as
	delivery failures, as described in the section "Policy Application".		delivery failures, as described in Section 5, "Policy Application".
	4.1. MX Certificate Validation		4.1. MX Certificate Validation
	The certificate presented by the receiving MX MUST chain to a root CA		The certificate presented by the receiving MX MUST chain to a root CA
	that is trusted by the sending MTA and be non-expired. The		that is trusted by the sending MTA and be non-expired. The
	certificate MUST have a CN-ID ([RFC6125]) or SAN ([RFC5280]) with a		certificate MUST have a CN-ID ([RFC6125]) or SAN ([RFC5280]) with a
	DNS-ID matching the "mx" pattern.		DNS-ID matching the "mx" pattern. The MX's certificate MAY also be
			checked for revocation via OCSP [RFC2560], certificate revocation
			lists (CRLs), or some other mechanism.
	Because the "mx" patterns are not hostnames, however, matching is not		Because the "mx" patterns are not hostnames, however, matching is not
	identical to other common cases of X.509 certificate authentication		identical to other common cases of X.509 certificate authentication
	(as described, for example, in [RFC6125]). Consider the example		(as described, for example, in [RFC6125]). Consider the example
	policy given above, with an "mx" pattern containing ".example.net".		policy given above, with an "mx" pattern containing ".example.net".
	In this case, if the MX server's X.509 certificate contains a SAN		In this case, if the MX server's X.509 certificate contains a SAN
	matching "*.example.net", we are required to implement "wildcard-to-		matching "*.example.net", we are required to implement "wildcard-to-
	wildcard" matching.		wildcard" matching.
	To simplify this case, we impose the following constraints on		To simplify this case, we impose the following constraints on
	wildcard certificates, identical to those in [RFC7672] section 3.2.3:		wildcard certificates, identical to those in [RFC7672] section 3.2.3
	wildcards are valid in DNS-IDs or CN-IDs, but must be the entire		and [@!RFC6125 section 6.4.3: wildcards are valid in DNS-IDs or CN-
	first label of the identifier (that is, "*.example.com", not		IDs, but must be the entire first label of the identifier (that is,
	"mail*.example.com"). Senders who are comparing a "suffix" MX		"*.example.com", not "mail*.example.com"). Senders who are comparing
	pattern with a wildcard identifier should thus strip the wildcard and		a "suffix" MX pattern with a wildcard identifier should thus strip
	ensure that the two sides match label-by-label, until all labels of		the wildcard and ensure that the two sides match label-by-label,
	the shorter side (if unequal length) are consumed.		until all labels of the shorter side (if unequal length) are
			consumed.
	A simple pseudocode implementation of this algorithm is presented in		A simple pseudocode implementation of this algorithm is presented in
	the Appendix.		the Appendix.
	5. Policy Application		5. Policy Application
	When sending to an MX at a domain for which the sender has a valid,		When sending to an MX at a domain for which the sender has a valid,
	non-expired MTA-STS policy, a sending MTA honoring MTA-STS applies		non-expired MTA-STS policy, a sending MTA honoring MTA-STS applies
	the result of a policy validation failure one of two ways, depending		the result of a policy validation failure one of two ways, depending
	on the value of the policy "mode" field:		on the value of the policy "mode" field:
	skipping to change at page 9, line 7 ¶		skipping to change at page 9, line 48 ¶
	following steps:		following steps:
	1. Check for a cached policy whose time-since-fetch has not exceeded		1. Check for a cached policy whose time-since-fetch has not exceeded
	its "max_age". If none exists, attempt to fetch a new policy		its "max_age". If none exists, attempt to fetch a new policy
	(perhaps asynchronously, so as not to block message delivery).		(perhaps asynchronously, so as not to block message delivery).
	Optionally, sending MTAs may unconditionally check for a new		Optionally, sending MTAs may unconditionally check for a new
	policy at this step.		policy at this step.
	2. For each candidate MX, in order of MX priority, attempt to		2. For each candidate MX, in order of MX priority, attempt to
	deliver the message, enforcing STARTTLS and, assuming a policy is		deliver the message, enforcing STARTTLS and, assuming a policy is
	present, PKIX certificate validation, and certificate validation		present, PKIX certificate validation as described in Section 4.1,
	as described in "MX Certificate Validation."		"MX Certificate Validation."
	3. Upon message retries, a message MAY be permanently failed		3. A message delivery MUST NOT be permanently failed until the
	following first checking for the presence of a new policy (as		sender has first checked for the presence of a new policy (as
	indicated by the "id" field in the "_mta-sts" TXT record).		indicated by the "id" field in the "_mta-sts" TXT record). If a
			new policy is not found, senders SHOULD apply existing rules for
			the case of temporary message delivery failures (as discussed in
			[RFC5321] section 4.5.4.1).
	6. Operational Considerations		6. Operational Considerations
	6.1. Policy Updates		6.1. Policy Updates
	Updating the policy requires that the owner make changes in two		Updating the policy requires that the owner make changes in two
	places: the "_mta-sts" TXT record in the Policy Domain's DNS zone and		places: the "_mta-sts" TXT record in the Policy Domain's DNS zone and
	at the corresponding HTTPS endpoint. As a result, recipients should		at the corresponding HTTPS endpoint. As a result, recipients should
	thus expect a policy will continue to be used by senders until both		thus expect a policy will continue to be used by senders until both
	the HTTPS and TXT endpoints are updated and the TXT record's TTL has		the HTTPS and TXT endpoints are updated and the TXT record's TTL has
	skipping to change at page 13, line 19 ¶		skipping to change at page 14, line 15 ¶
	present.		present.
	func isEnforce(policy) {		func isEnforce(policy) {
	// Return true if the policy mode is "enforce".		// Return true if the policy mode is "enforce".
	}		}
	func isNonExpired(policy) {		func isNonExpired(policy) {
	// Return true if the policy is not expired.		// Return true if the policy is not expired.
	}		}
	func tryStartTls(mx) {		func tryStartTls(connection) {
	// Attempt to open an SMTP connection with STARTTLS with the MX.		// Attempt to open an SMTP connection with STARTTLS with the MX.
	}		}
	func certMatches(connection, mx) {		func certMatches(connection, policy) {
	// For simplicity, we are not checking CNs here.		// Assume a handy function to return CN and DNS-ID SANs.
	for san in getSansFromCert(connection) {		for san in getDnsIdSansAndCnFromCert(connection) {
	// Return if the server certificate from "connection" matches the "mx" host.		for mx in policy.mx {
	if san[0] == '*' {		// Return if the server certificate from "connection" matches the "mx" host.
	// Invalid wildcard!		if san[0] == '*' {
	if san[1] != '.' return false		// Invalid wildcard!
	san = san[1:]		if san[1] != '.' return false
	}		san = san[1:]
	if san[0] == '.' && HasSuffix(mx, san) {		}
	return true		if san[0] == '.' && HasSuffix(mx, san) {
	}		return true
	if mx[0] == '.' && HasSuffix(san, mx) {		}
	return true		if mx[0] == '.' && HasSuffix(san, mx) {
	}		return true
	if mx == san {		}
	return true		if mx == san {
			return true
			}
	}		}
	}		}
	return false		return false
	}		}
	func tryDeliverMail(connection, message) {		func tryDeliverMail(connection, message) {
	// Attempt to deliver "message" via "connection".		// Attempt to deliver "message" via "connection".
	}		}
	func tryGetNewPolicy(domain) {		func tryGetNewPolicy(domain) {
	skipping to change at page 14, line 25 ¶		skipping to change at page 15, line 22 ¶
	func reportError(error) {		func reportError(error) {
	// Report an error via TLSRPT.		// Report an error via TLSRPT.
	}		}
	func tryMxAccordingTo(message, mx, policy) {		func tryMxAccordingTo(message, mx, policy) {
	connection := connect(mx)		connection := connect(mx)
	if !connection {		if !connection {
	return false // Can't connect to the MX so it's not an MTA-STS error.		return false // Can't connect to the MX so it's not an MTA-STS error.
	}		}
	secure := true		secure := true
	if !tryStartTls(mx, &connection) {		if !tryStartTls(connection) {
	secure = false		secure = false
	reportError(E_NO_VALID_TLS)		reportError(E_NO_VALID_TLS)
	} else if !certMatches(connection, mx) {		} else if !certMatches(connection, policy) {
	secure = false		secure = false
	reportError(E_CERT_MISMATCH)		reportError(E_CERT_MISMATCH)
	}		}
	if secure || !isEnforce(policy) {		if secure || !isEnforce(policy) {
	return tryDeliverMail(connection, message)		return tryDeliverMail(connection, message)
	}		}
	return false		return false
	}		}
	func tryWithPolicy(message, domain, policy) {		func tryWithPolicy(message, domain, policy) {
			mxes := getMxForDomain(domain)
	for mx in mxes {		for mx in mxes {
	if tryMxAccordingTo(message, mx, policy) {		if tryMxAccordingTo(message, mx, policy) {
	return true		return true
	}		}
	}		}
	return false		return false
	}		}
	func handleMessage(message) {		func handleMessage(message) {
	domain := ... // domain part after '@' from recipient		domain := ... // domain part after '@' from recipient
	skipping to change at page 15, line 5 ¶		skipping to change at page 16, line 4 ¶
	return false		return false
	}		}
	func handleMessage(message) {		func handleMessage(message) {
	domain := ... // domain part after '@' from recipient		domain := ... // domain part after '@' from recipient
	policy := tryGetNewPolicy(domain)		policy := tryGetNewPolicy(domain)
	if policy {		if policy {
	cachePolicy(domain, policy)		cachePolicy(domain, policy)
	} else {		} else {
	policy = tryGetCachedPolicy(domain)		policy = tryGetCachedPolicy(domain)
	}		}
	if policy {		if policy {
	return tryWithPolicy(message, policy)		return tryWithPolicy(message, domain, policy)
	}		}
	// Try to deliver the message normally (i.e. without MTA-STS).		// Try to deliver the message normally (i.e. without MTA-STS).
	}		}
	12. References		12. References
	12.1. Normative References		12.1. Normative References
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/		Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
	RFC2119, March 1997,		RFC2119, March 1997,
	<http://www.rfc-editor.org/info/rfc2119>.		<http://www.rfc-editor.org/info/rfc2119>.
			[RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
			Adams, "X.509 Internet Public Key Infrastructure Online
			Certificate Status Protocol - OCSP", RFC 2560, DOI 10
			.17487/RFC2560, June 1999,
			<http://www.rfc-editor.org/info/rfc2560>.
	[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over		[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
	Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,		Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
	February 2002, <http://www.rfc-editor.org/info/rfc3207>.		February 2002, <http://www.rfc-editor.org/info/rfc3207>.
	[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode		[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
	for Internationalized Domain Names in Applications		for Internationalized Domain Names in Applications
	(IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003,		(IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003,
	<http://www.rfc-editor.org/info/rfc3492>.		<http://www.rfc-editor.org/info/rfc3492>.
	[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.		[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
	Rose, "DNS Security Introduction and Requirements", RFC		Rose, "DNS Security Introduction and Requirements", RFC
	4033, DOI 10.17487/RFC4033, March 2005,		4033, DOI 10.17487/RFC4033, March 2005,
	<http://www.rfc-editor.org/info/rfc4033>.		<http://www.rfc-editor.org/info/rfc4033>.
	[RFC4627] Crockford, D., "The application/json Media Type for
	JavaScript Object Notation (JSON)", RFC 4627, DOI 10
	.17487/RFC4627, July 2006,
	<http://www.rfc-editor.org/info/rfc4627>.
	[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax		[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
	Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/		Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/
	RFC5234, January 2008,		RFC5234, January 2008,
	<http://www.rfc-editor.org/info/rfc5234>.		<http://www.rfc-editor.org/info/rfc5234>.
	[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,		[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
	Housley, R., and W. Polk, "Internet X.509 Public Key		Housley, R., and W. Polk, "Internet X.509 Public Key
	Infrastructure Certificate and Certificate Revocation List		Infrastructure Certificate and Certificate Revocation List
	(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,		(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
	<http://www.rfc-editor.org/info/rfc5280>.		<http://www.rfc-editor.org/info/rfc5280>.
			[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
			DOI 10.17487/RFC5321, October 2008,
			<http://www.rfc-editor.org/info/rfc5321>.
	[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known		[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
	Uniform Resource Identifiers (URIs)", RFC 5785, DOI 10		Uniform Resource Identifiers (URIs)", RFC 5785, DOI 10
	.17487/RFC5785, April 2010,		.17487/RFC5785, April 2010,
	<http://www.rfc-editor.org/info/rfc5785>.		<http://www.rfc-editor.org/info/rfc5785>.
	[RFC5891] Klensin, J., "Internationalized Domain Names in		[RFC5891] Klensin, J., "Internationalized Domain Names in
	Applications (IDNA): Protocol", RFC 5891, DOI 10.17487/		Applications (IDNA): Protocol", RFC 5891, DOI 10.17487/
	RFC5891, August 2010,		RFC5891, August 2010,
	<http://www.rfc-editor.org/info/rfc5891>.		<http://www.rfc-editor.org/info/rfc5891>.
	[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and		[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
	Verification of Domain-Based Application Service Identity		Verification of Domain-Based Application Service Identity
	within Internet Public Key Infrastructure Using X.509		within Internet Public Key Infrastructure Using X.509
	(PKIX) Certificates in the Context of Transport Layer		(PKIX) Certificates in the Context of Transport Layer
	Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March		Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
	2011, <http://www.rfc-editor.org/info/rfc6125>.		2011, <http://www.rfc-editor.org/info/rfc6125>.
			[RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
			Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
			2014, <http://www.rfc-editor.org/info/rfc7159>.
	[RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via		[RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via
	Opportunistic DNS-Based Authentication of Named Entities		Opportunistic DNS-Based Authentication of Named Entities
	(DANE) Transport Layer Security (TLS)", RFC 7672, DOI 10		(DANE) Transport Layer Security (TLS)", RFC 7672, DOI 10
	.17487/RFC7672, October 2015,		.17487/RFC7672, October 2015,
	<http://www.rfc-editor.org/info/rfc7672>.		<http://www.rfc-editor.org/info/rfc7672>.
	12.2. URIs		12.2. URIs
	[1] https://mta-sts.example.com/.well-known/mta-sts.json:		[1] https://mta-sts.example.com/.well-known/mta-sts.json:
End of changes. 35 change blocks.
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