Sieve Email Filtering: Ihave Extension
RFC 5463

Network Working Group                                           N. Freed
Request for Comments: 5463                              Sun Microsystems
Category: Standards Track                                     March 2009

                Sieve Email Filtering:  Ihave Extension

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

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   document authors.  All rights reserved.

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Abstract

   This document describes the "ihave" extension to the Sieve email
   filtering language.  The "ihave" extension provides a means to write
   scripts that can take advantage of optional Sieve features but can
   still run when those optional features are not available.  The
   extension also defines a new error control command intended to be
   used to report situations where no combination of available
   extensions satisfies the needs of the script.

1.  Introduction

   Sieve [RFC5228] is a language for filtering email messages at or
   around the time of final delivery.  It is designed to be
   implementable on either a mail client or mail server.  It is suitable
   for running on a mail server where users may not be allowed to
   execute arbitrary programs, such as on black-box Internet Message
   Access Protocol [RFC3501] servers, as it has no user-controlled loops
   or the ability to run external programs.

Freed                       Standards Track                     [Page 1]
RFC 5463                 Sieve Ihave Extension                March 2009

   Various sieve extensions have already been defined, e.g., [RFC5229],
   [RFC5230], [RFC5231], [RFC5232], [RFC5233], [RFC5235], and many more
   are sure to be created over time.  Sieve's require clause is used to
   specify the extensions a particular sieve needs; an error results if
   the script's require clause calls for an extension that isn't
   available.  This mechanism is sufficient in most situations.
   However, there can be cases where a script may be able to take
   advantage of an extension if it is available but can still operate if
   it is not, possibly with some degradation of functionality.  Cases
   can also arise where a script would prefer one extension but can
   employ a different one if the first one is not available.

   The "ihave" extension provides a means to write scripts that make use
   of extensions only when they are actually available.  It defines a
   new "ihave" test that takes a list of capability names as an argument
   and succeeds if and only if all of those capabilities are present.
   Additionally, specification of the "ihave" extension in the require
   clause disables parse-time checking of extension use in scripts; run-
   time checking must be used instead.  This makes it possible to write
   portable scripts that can operate in multiple environments making
   effective use of whatever extensions are available even though
   differing sets of extensions are provided in different places.

   The "ihave" extension also defines a new error control command.  An
   error causes script execution to terminate with the error message
   given as the argument to the error control.

2.  Conventions Used in This Document

   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 [RFC2119].

   The terms used to describe the various components of the Sieve
   language are taken from Section 1.1 of [RFC5228].

3.  Capability Identifiers

   The capability string associated with the extension defined in this
   document is "ihave".

4.  Ihave Test

   Usage:   ihave <capabilities: string-list>

   The "ihave" test provides a means for Sieve scripts to test for the
   existence of a given extension prior to actually using it.  The
   capabilities argument to "ihave" is the same as the similarly-named