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300 lines
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<title>rfc2119</title>
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</head>
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<body>
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<div class="Verified-headnote-styling">
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<span style="font-weight: bold;">This is a purely informative rendering of an RFC that includes verified errata. This rendering may not be used as a reference.</span>
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<br>
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<br>
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The following 'Verified' errata have been incorporated in this document:
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<a href="https://www.rfc-editor.org/rfc/inline-errata/rfc2119.html#eid493">EID 493</a>, <a href="https://www.rfc-editor.org/rfc/inline-errata/rfc2119.html#btn_494">EID 494</a>, <a href="https://www.rfc-editor.org/rfc/inline-errata/rfc2119.html#btn_495">EID 495</a>, <a href="https://www.rfc-editor.org/rfc/inline-errata/rfc2119.html#btn_494">EID 496</a>, <a href="https://www.rfc-editor.org/rfc/inline-errata/rfc2119.html#eid498">EID 498</a>, <a href="https://www.rfc-editor.org/rfc/inline-errata/rfc2119.html#btn_499">EID 499</a>, <a href="https://www.rfc-editor.org/rfc/inline-errata/rfc2119.html#eid500">EID 500</a>, <a href="https://www.rfc-editor.org/rfc/inline-errata/rfc2119.html#btn_5101">EID 5101</a>
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</div>
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<pre>Network Working Group S. Bradner
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Request for Comments: 2119 Harvard University
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BCP: 14 March 1997
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Category: Best Current Practice
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Key words for use in RFCs to Indicate Requirement Levels
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Status of this Memo
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This document specifies an Internet Best Current Practices for the
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Internet Community, and requests discussion and suggestions for
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improvements. Distribution of this memo is unlimited.
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Abstract
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In many standards track documents several words are used to signify
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the requirements in the specification. These words are often
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capitalized. This document defines these words as they should be
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interpreted in IETF documents. Authors who follow these guidelines
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should incorporate this phrase near the beginning of their document:
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<span class="Verified-inline-styling" id="inline-499"> The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL <button id="btn_499" target="expand_499" onclick="hideFunction("expand_499")">Expand</button>
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NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
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RECOMMENDED", "MAY", and "OPTIONAL" in this document are to
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be interpreted as described in RFC 2119.</span>
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<div class="nodeCloseClass" id="expand_499"><div class="Verified-endnote-styling" id="eid499">
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<pre><b><i><a href="https://www.rfc-editor.org/errata/eid499">EID 499</a> (Verified) is as follows:</i></b>
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<b>Section:</b> Abstract
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<b>Original Text:</b>
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
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NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
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"OPTIONAL" in this document are to be interpreted as described in
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RFC 2119.
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<b>Corrected Text:</b>
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
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NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
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RECOMMENDED", "MAY", and "OPTIONAL" in this document are to
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be interpreted as described in RFC 2119.
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</pre>
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<b>Notes:</b><br>
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The phrase "NOT RECOMMENDED" is missing from this sentence.
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</div>
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</div>
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Note that the force of these words is modified by the requirement
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level of the document in which they are used.
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<span class="Verified-inline-styling" id="inline-495">1. MUST This word, or the terms "REQUIRED" or "SHALL", means that the <button id="btn_495" target="expand_495" onclick="hideFunction("expand_495")">Expand</button>
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definition is an absolute requirement of the specification.
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<div class="Verified-endnote-styling" id="eid493">
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<pre><b><i><a href="https://www.rfc-editor.org/errata/eid493">EID 493</a> (Verified) is as follows:</i></b>
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<b>Section:</b> 1
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<b>Original Text:</b>
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2. MUST NOT This phrase, or the phrase "SHALL NOT", mean that the
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definition is an absolute prohibition of the specification.
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<b>Corrected Text:</b>
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2. MUST NOT This phrase, or the phrase "SHALL NOT", means that the
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definition is an absolute prohibition of the specification.
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</pre>
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<b>Notes:</b><br>
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</div>
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<div class="Verified-endnote-styling" id="eid498">
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<pre><b><i><a href="https://www.rfc-editor.org/errata/eid498">EID 498</a> (Verified) is as follows:</i></b>
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<b>Section:</b> 1
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<b>Original Text:</b>
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4. SHOULD NOT This phrase, or the phrase "NOT RECOMMENDED" mean that
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there may exist valid reasons in particular circumstances when the
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particular behavior is acceptable or even useful, but the full
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implications should be understood and the case carefully weighed
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before implementing any behavior described with this label.
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<b>Corrected Text:</b>
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4. SHOULD NOT This phrase, or the phrase "NOT RECOMMENDED", means that
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there may exist valid reasons in particular circumstances when the
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particular behavior is acceptable or even useful, but the full
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implications should be understood and the case carefully weighed
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before implementing any behavior described with this label.
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</pre>
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<b>Notes:</b><br>
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</div>
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<div class="Verified-endnote-styling" id="eid500">
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<pre><b><i><a href="https://www.rfc-editor.org/errata/eid500">EID 500</a> (Verified) is as follows:</i></b>
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<b>Section:</b> 1
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<b>Original Text:</b>
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3. SHOULD This word, or the adjective "RECOMMENDED", mean that there
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may exist valid reasons in particular circumstances to ignore a
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particular item, but the full implications must be understood and
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carefully weighed before choosing a different course.
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<b>Corrected Text:</b>
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3. SHOULD This word, or the adjective "RECOMMENDED", means that there
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may exist valid reasons in particular circumstances to ignore a
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particular item, but the full implications must be understood and
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carefully weighed before choosing a different course.
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</pre>
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<b>Notes:</b><br>
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</div>
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</span>
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<div class="nodeCloseClass" id="expand_495"><div class="Verified-endnote-styling" id="eid495">
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<pre><b><i><a href="https://www.rfc-editor.org/errata/eid495">EID 495</a> (Verified) is as follows:</i></b>
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<b>Section:</b> 1
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<b>Original Text:</b>
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1. MUST This word, or the terms "REQUIRED" or "SHALL", mean that the
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definition is an absolute requirement of the specification.
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<b>Corrected Text:</b>
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1. MUST This word, or the terms "REQUIRED" or "SHALL", means that the
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definition is an absolute requirement of the specification.
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</pre>
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<b>Notes:</b><br>
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</div>
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</div>
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2. MUST NOT This phrase, or the phrase "SHALL NOT", mean that the
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definition is an absolute prohibition of the specification.
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3. SHOULD This word, or the adjective "RECOMMENDED", mean that there
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may exist valid reasons in particular circumstances to ignore a
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particular item, but the full implications must be understood and
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carefully weighed before choosing a different course.
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4. SHOULD NOT This phrase, or the phrase "NOT RECOMMENDED" mean that
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there may exist valid reasons in particular circumstances when the
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particular behavior is acceptable or even useful, but the full
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implications should be understood and the case carefully weighed
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before implementing any behavior described with this label.
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<span class="Verified-inline-styling" id="inline-5101">5. MAY This word, or the adjective "OPTIONAL", mean that an item is <button id="btn_5101" target="expand_5101" onclick="hideFunction("expand_5101")">Expand</button>
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truly optional. One vendor may choose to include the item because a
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particular marketplace requires it or because the vendor feels that
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it enhances the product while another vendor may omit the same item.
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An implementation which does not include a particular option MUST be
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prepared to interoperate with another implementation which does
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include the option, though perhaps with reduced functionality. In the
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same vein an implementation which does include a particular option
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MUST be prepared to interoperate with another implementation which
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does not include the option (except, of course, for the feature the
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option provides).</span>
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<div class="nodeCloseClass" id="expand_5101"><div class="Verified-endnote-styling" id="eid5101">
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<pre><b><i><a href="https://www.rfc-editor.org/errata/eid5101">EID 5101</a> (Verified) is as follows:</i></b>
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<b>Section:</b> 5
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<b>Original Text:</b>
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5. MAY This word, or the adjective "OPTIONAL", mean that an item is
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truly optional. One vendor may choose to include the item because a
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particular marketplace requires it or because the vendor feels that
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it enhances the product while another vendor may omit the same item.
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An implementation which does not include a particular option MUST be
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prepared to interoperate with another implementation which does
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include the option, though perhaps with reduced functionality. In the
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same vein an implementation which does include a particular option
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MUST be prepared to interoperate with another implementation which
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does not include the option (except, of course, for the feature the
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option provides.)
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<b>Corrected Text:</b>
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5. MAY This word, or the adjective "OPTIONAL", mean that an item is
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truly optional. One vendor may choose to include the item because a
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particular marketplace requires it or because the vendor feels that
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it enhances the product while another vendor may omit the same item.
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An implementation which does not include a particular option MUST be
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prepared to interoperate with another implementation which does
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include the option, though perhaps with reduced functionality. In the
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same vein an implementation which does include a particular option
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MUST be prepared to interoperate with another implementation which
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does not include the option (except, of course, for the feature the
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option provides).
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</pre>
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<b>Notes:</b><br>
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Full stop should appear outside the parentheses in the last sentence.
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</div>
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</div>
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6. Guidance in the use of these Imperatives
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Imperatives of the type defined in this memo must be used with care
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and sparingly. In particular, they MUST only be used where it is
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actually required for interoperation or to limit behavior which has
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potential for causing harm <span class="Verified-inline-styling" id="inline-494">(e.g., limiting retransmissions)</span> For <button id="btn_494" target="expand_494" onclick="hideFunction("expand_494")">Expand Multiple</button>
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<div class="nodeCloseClass" id="expand_494"><div class="Verified-endnote-styling" id="eid494">
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<pre><b><i><a href="https://www.rfc-editor.org/errata/eid494">EID 494</a> (Verified) is as follows:</i></b>
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<b>Section:</b> 6
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<b>Original Text:</b>
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(e.g., limiting retransmisssions)
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<b>Corrected Text:</b>
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(e.g., limiting retransmissions)
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</pre>
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<b>Notes:</b><br>
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</div>
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<div class="Verified-endnote-styling" id="eid496">
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<pre><b><i><a href="https://www.rfc-editor.org/errata/eid496">EID 496</a> (Verified) is as follows:</i></b>
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<b>Section:</b> 6
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<b>Original Text:</b>
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In particular, they MUST only be used where it is actually required
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for interoperation or to limit behavior which has potential for
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causing harm (e.g., limiting retransmisssions) For example, they
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must not be used to try to impose a particular method on
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implementors where the method is not required for interoperability.
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<b>Corrected Text:</b>
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In particular, they MUST only be used where it is actually required
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for interoperation or to limit behavior which has potential for
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causing harm (e.g., limiting retransmissions). For example, they
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must not be used to try to impose a particular method on
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implementors where the method is not required for interoperability.
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</pre>
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<b>Notes:</b><br>
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</div>
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</div> example, they must not be used to try to impose a particular method
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on implementors where the method is not required for
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interoperability.
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7. Security Considerations
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These terms are frequently used to specify behavior with security
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implications. The effects on security of not implementing a MUST or
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SHOULD, or doing something the specification says MUST NOT or SHOULD
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NOT be done may be very subtle. Document authors should take the time
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to elaborate the security implications of not following
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recommendations or requirements as most implementors will not have
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had the benefit of the experience and discussion that produced the
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specification.
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8. Acknowledgments
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The definitions of these terms are an amalgam of definitions taken
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from a number of RFCs. In addition, suggestions have been
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incorporated from a number of people including Robert Ullmann, Thomas
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Narten, Neal McBurnett, and Robert Elz.
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9. Author's Address
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Scott Bradner
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Harvard University
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1350 Mass. Ave.
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Cambridge, MA 02138
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phone - +1 617 495 3864
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email - sob@harvard.edu
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</pre></body></html> |