title: What is the purpose of payload in packet too big ICMPv6 message
body: <p>I have gone through the RFC <a href="https://www.rfc-editor.org/rfc/rfc4443#section-4" rel="nofollow noreferrer">4443</a> and <a href="https://www.rfc-editor.org/rfc/rfc8201" rel="nofollow noreferrer">8201</a>, perhaps I did not understand as I am new to some terminology described in these RFC but I want to understand the implication of using payload in ICMP packet to big message?</p>
  <p>As per the RFC 4443
  <a href="https://www.rfc-editor.org/rfc/rfc4443" rel="nofollow noreferrer">Linkt to 4443 RFC</a></p>
  <p>The payload will contain as much of invoking packet not exceeding the minimum path MTU in packet to big message.</p>
  <p>I don't understand the use case of such payload, even in the RFC 8201 there is no mention about the usages of payload.
  only one comment present was</p>
  <p><strong>Added clarification in Section 4, &quot;Protocol Requirements&quot;, that
  nodes should validate the payload of ICMP PTB messages per RFC
  4443, and that nodes should detect decreases in PMTU as fast as
  possible.</strong></p>
  <p>What can be the implication validating payload in the PTB message and how should we validate the payload and based on what conditions?</p>
  <p>Packet Too Big Message as per RFC 4443.</p>
  <pre><code>   0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Type      |     Code      |          Checksum             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                             MTU                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    As much of invoking packet                 |
    +               as possible without the ICMPv6 packet           +
    |               exceeding the minimum IPv6 MTU [IPv6]           |
  </code></pre>