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. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Copyright and License Notice Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect Santandrea Expires May 17, 2018 [Page 1] INTERNET DRAFT mail-limits November 13, 2017 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 . . . . . . . . . . . . . . . . . . . . . . . . . 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 Santandrea Expires May 17, 2018 [Page 2] INTERNET DRAFT mail-limits November 13, 2017 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. Santandrea Expires May 17, 2018 [Page 3] INTERNET DRAFT mail-limits November 13, 2017 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.) Santandrea Expires May 17, 2018 [Page 4] INTERNET DRAFT mail-limits November 13, 2017 (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. Santandrea Expires May 17, 2018 [Page 5] INTERNET DRAFT mail-limits November 13, 2017 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, . 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, . Authors' Addresses Federico Santandrea c/o Diennea S.r.l. Viale Marconi 30/14 48018 Faenza (RA) Italy Email: federico.santandrea@diennea.com Santandrea Expires May 17, 2018 [Page 6]