Internet DRAFT - draft-santandrea-mail-limits
draft-santandrea-mail-limits
INTERNET-DRAFT Federico Santandrea
Intended Status: Experimental Diennea
Expires: May 17, 2018 November 13, 2017
Automated Management of Mail Sending Limits
draft-santandrea-mail-limits-00
Abstract
This memo describes a protocol designed to allow automatic, dynamic
mail sending limit configuration on a voluntary basis.
It aims to minimize the amount of manual work required to achieve
limit compliance.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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to this document. Code Components extracted from this document must
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Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Temporary protocol name . . . . . . . . . . . . . . . . . . 3
2 Policy definition . . . . . . . . . . . . . . . . . . . . . . . 3
3 Policy record . . . . . . . . . . . . . . . . . . . . . . . . . 4
4 Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5 Policy caching and changes . . . . . . . . . . . . . . . . . . 5
6 Security Considerations . . . . . . . . . . . . . . . . . . . . 5
7 IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
8 References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1 Normative References . . . . . . . . . . . . . . . . . . . 6
8.2 Informative References . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6
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1 Introduction
Some mailbox providers choose to publicly declare fixed limits that
legitimate, well-behaved mass mailing services are expected to
observe in order to avoid deliverability issues.
The usual way of doing this is publishing these limits in human-
readable form on postmaster web sites, FAQs and error messages, or
disclosing them when asked. Mail operators that would like to follow
the rules must then gather this knowledge from a variety of diverse
sources and translate it into software configuration.
Providing a standard way of communicating these limits to MTA
software would benefit both receivers and legitimate senders, by
removing the need for trial-and-error or manual information hunting.
1.1 Terminology
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 RFC 2119 [RFC2119].
1.2 Temporary protocol name
The protocol does not yet have a name and for the sake of discussion
in this memo it will just be referred to as AMMSL, an acronym
composed of the capitalized letters in the draft title. If there is
sufficient interest in pursuing its development, a more descriptive
name would be desirable.
2 Policy definition
Declared limits are specified in an AMMSL policy. Policies are merely
hints and senders can always choose whether or not to conform to
them. Their scope is the single MX (SMTP server) identified by
hostname, as various MX's for the same domain can be running
different software and need different limits. It is possible to have
a domain scoped default that applies to all MX's unless overridden.
The receiving side is not required to enforce policies, nor to accept
mail that abides by them. They are only used as a guide for senders
who voluntarily wish to behave well.
A DNS record is the typical delivery method for this kind of
information so it's already widely understood and doesn't require new
client libraries or server setups that mail authentication
technologies wouldn't.
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3 Policy record
An AMMSL policy is specified by creating a TXT record for the MTA
hostname. For example, if example.com has a MX named
mail.example.com, the policy would be specified as:
mail.example.com. IN TXT "v=AMMSL1; ..."
where '...' means a string of DKIM-style tag=value pairs as defined
in [RFC6376], Section 3.2.
Domain scoped default policies are specified in an _ammsl TXT record:
_ammsl.example.com. IN TXT "v=AMMSL1; ..."
(TODO: turn this into formal language; develop an ABNF grammar.)
4 Limits
Following is a preliminary list of considered limits that can be
specified in a policy. Its purpose is to demonstrate intended
protocol functionality and to guide further discussion.
- connection-ttl: maximum lifetime of a SMTP session, in seconds
- read-timeout: maximum time a SMTP session can be idle (no data
transmitted), in seconds
- max-simultaneous-connections: maximum connections that can
simultaneously be in established state
- max-messages-per-time-unit: maximum number of messages that can
be submitted in a time unit
- max-messages-per-connection: maximum number of messages that can
be submitted in the lifetime of a single connection
- time-unit-duration: duration of a time unit, in seconds
- retry-interval: minimum wait interval between retries on
temporary failure, in seconds
- x-...: custom parameters (which could also be meant to be human
readable)
(TODO: consider usefulness of suggesting an IP warmup volume curve,
could be too much complexity for too little value.)
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(TODO: consider usefulness of indicating adoption of values in
another policy for unspecified parameters - "include-like" mechanism)
All limits are optional. Every limit designation, excluding the
custom ones, SHOULD be abbreviated by the initials of every word in
it when composing a DNS record (example-parameter-name = epn) so that
the record doesn't become unwieldy.
Parameters that are not understood can be ignored. Therefore an
extension mechanism is probably not needed.
5 Policy caching and changes
Given that the receiving side has no reason to expect senders will
honor (or even see) their AMMSL policy, it is probably not useful to
include an explicit time-to-live or expiration date for it. Senders
SHOULD cache policies in order to reduce load on the DNS. They MAY
expire and refresh cached policies on a schedule or when they detect
problems which they consider to be avoidable by conforming to
prescribed limits.
6 Security Considerations
Policies could be spoofed, leading to denial of service or sender
reputation issues. Since policies are advisory and not normative,
this is less of a problem. Nonetheless it is a possibility to keep in
mind while considering how to use information contained therein.
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7 IANA Considerations
This document has no IANA actions.
8 References
8.1 Normative References
[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>.
8.2 Informative References
[RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
"DomainKeys Identified Mail (DKIM) Signatures", STD 76,
RFC 6376, DOI 10.17487/RFC6376, September 2011,
<https://www.rfc-editor.org/info/rfc6376>.
Authors' Addresses
Federico Santandrea
c/o Diennea S.r.l.
Viale Marconi 30/14
48018 Faenza (RA)
Italy
Email: federico.santandrea@diennea.com
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