| Internet-Draft | LIMITS Extension | March 2021 |
| Freed | Expires 16 September 2021 | [Page] |
This document defines a "Limits" extension for the Simple Mail Transfer Protocol (SMTP) and an associated limit registry. This extension provides the means for an SMTP server to inform the SMTP client of limits the server intends to apply to the protocol during the current SMTP session. The client is then able adapt its behavior in order to conform to those limits.¶
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The Simple Mail Transfer Protocol [SMTP] provides the ability to transfer multiple email messages from one host to another, each with multiple recipients, using a single or multiple connections.¶
In order to conserve resources as well as protect themselves from malicious clients, it is necessary for servers to enforce limits on various aspects of the protocol, e.g., a limit on the number of recipients that can be specified in a single transaction.¶
Additionally, servers may also wish to alter the limits they apply depending on their assessment of the reputation of a particular client.¶
The variability of the limits that may be in effect creates a situation where clients may inadvertently exceed a particular server's limits, causing servers to respond with temporary (or in some cases, permanent) errors. This in turn can lead to delays or even failures in message transfer.¶
SMTP servers have always been able to announce a limit, in a reply, which means that the client first needed to issue a command. The mechanism specified here avoids the overhead of that interactions, by announcing limits prior to any substantive interaction.¶
The "Limits" extension provides the means for a server to inform a client about specific limits in effect for a particular SMTP session. This information, combined with the inherent flexibility of SMTP, makes it possible for clients to avoid server errors and the problems they cause.¶
Limits are registered with the IANA. Each registration includes the limit name, value syntax, and a description of its semantics.¶
In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119 [KEYWORDS].¶
This specification uses the Augmented Backus-Naur Form [ABNF] notation and its core rules to define the formal syntax of the "Limits" extension.¶
This specification makes extensive use of the terminology specified and used in [SMTP].¶
Extensions to SMTP are defined in Section 2.2 of [SMTP].¶
The name of the extension is "Limits". Servers implementing this extension advertise an additional "LIMITS" EHLO keyword. The associated parameter is used by the server to communicate one or more limits, each with an optional value, to the client. The syntax of the parameter is:¶
limits-param = limit-name-value [SP limit-name-value] limit-name-value = limit-name ["=" limit-value] limit-name = 1*(ALPHA / DIGIT / "-" / "_") limit-value = 1*(%x21-3A / %x3C-7E) ; Any VCHAR except ";"¶
This extension introduces no new SMTP commands, and does not alter any existing command. However, it is possible for a LIMITS parameter to be associated with another SMTP extension that does these things.¶
In order to achieve consistent behavior, all limits MUST be registered with the IANA, as described below.¶
Limit names MUST be comprehensible, but also should be kept as short as possible. The use of commonly understood abbreviation, e.g., "MAX" for "maximum", is encouraged.¶
When a limit is associated with a particular SMTP, its name SHOULD begin with the name of that command.¶
Limit names SHOULD end with one or more terms that describe the type of limit.¶
The SMTP "Pipelining" extension [PIPELINING] is commonly used to improve SMTP performance, especially over high latency connections. Pipelining allows entire transaction to be sent without checking responses and in some cases it may be possible to send multiple transactions.¶
The use of pipelining affects limits in an important way: Since a pipelining client cannot check intermediate command responses without stalling the pipeline, it cannot count the number of succesful versus failed responses and adjust its behavior accordingly. Limit designers need to take this into account.¶
For example, it may seem like it would be better to impose a limit on the number of succesful RCPT TO commands as opposed to the way the RCPTMAX limit is specified in Section 4.2 below. But counting the total number of RCPT TOs is simple, whereas counting the number of successful RCPT TO stalls the pipeline.¶
This extension provides an announcement as part of the reply to an EHLO command. Some servers vary their limits, as a session progresses, based on their obtaining more information. This extension does not cover in-session limitation changes.¶
Section 4.5.3.1 of [SMTP] specifies minimum values for various server sizes, limits, and timeouts, e.g., servers must accept a minimum of 100 RCPT TO commands (section 4.5.3.1.8). Unfortunately, the reality is that servers routinely impose smaller limits than what SMTP requires, and when this is done it's especially important for clients to be aware that this is happening.¶
For this reason there is no requirement that the limits advertised by this extension comply with the minimums imposed by SMTP.¶
SMTP requires that EHLO command be reissued Under certain circumstances, e.g., after successful authentication [AUTH] or negotiation of a security layer [STARTTLS].¶
Servers MAY update their limits any time the protocol requires clients to reissue the EHLO command. Clients MUST discard any previous limits in favor of those provided by the most recent EHLO. This includes the case where the original EHLO provided a set of limits but the subsequent EHLO did not; in this case the client MUST act as if no limits were communicated.¶
An initial set of limits are specified in the following sections.¶
Name: MAILMAX¶
Value syntax: %x30-39 0*5DIGIT ; 0 not allowed, 6 digit maximum¶
Description: RCPTMAX specifies the maximum number of transactions (MAIL FROM commands) the SMTP will accept in a single session. The count includes all MAIL FROM commands, regardless of whether they succeed or fail.¶
Name: RCPTMAX¶
Value syntax: %x30-39 0*5DIGIT ; 0 not allowed, 6 digit maximum¶
Description: RCPTMAX specifies the maximum number of RCPT TO commands the SMTP will accept in a single transaction. It is not a limit on the actual number of recipients the message ends up being sent to; a single RCPT TO command may produce multiple recipients or, in the event of an error, none.¶
Name: RCPTDOMAINMAX¶
Value syntax: %x30-39 0*5DIGIT ; 0 not allowed, 6 digit maximum¶
Description: RCPTMAX specifies the maximum number of different domains that can appear in a recipient (RCPT TO) address within a single SMTP session. This limit is imposed by some servers that bind to a specific internal delivery mechanism on receipt of the first RCPT TO command.¶
A malicious server can use limits to overly constrain client behavior, causing excessive use of client resources.¶
A malicious client may use the limits a server advertises to optimize the delivery of unwanted messages.¶
A man-in-the-middle attack on unprotected SMTP connections can be used to cause clients to misbehave, which in turn could result in delivery delays or failures. Loss of reputation for the client could also occur.¶
All that said, decades of operational experience with the SMTP "SIZE" extension [SIZE], which provides servers with the ability to indicate message size, indicates that such abuse is rare and unlikely to be a significant problem.¶
The IANA is requested to add "LIMITS" to the SMTP Service Extension Registry:¶
Keywords: LIMITS Description: Server limits Reference: [RFCxxxx]¶
The IANA is requested to create a new registry for SMTP server limits. The policy for this registry is "Specification Required". Registry entries consist of three required values:¶
The IANA is also requested to register the limits specified in this document.¶
A lot of people have helped make this specification possible. The author wishes to thank Laura Atkins, Alex Brotman, Dave Crocker, Viktor Dukhovni, Jeremy Harris, Todd Herr, Matthias Leisi , John Klensin, John Levine, and Michael Peddemors for their contributions.¶