<div class="content" name="ExtensionHeader" uuid="baf230bf-7f15-4d03-bd1d-f8276608a955"><p>The <b>ExtensionHeader</b> contains information about the
presence and size of other headers in the <span><a href="f9e71595-339a-4cc4-8341-371e0a4cb232" data-linktype="relative-path">Message Packet
Structure (section 2.2.5)</a></span>, such as <span><a href="377dc454-43ef-4c48-8c07-692de2fd9ec4" data-linktype="relative-path">DeadLetterHeader (section 2.2.5.4)</a></span>,
<span><a href="ee62651b-ade2-417c-91af-7a6cf6f3e86a" data-linktype="relative-path">SubqueueHeader (section 2.2.5.3)</a></span>,
and <span><a href="20119e19-df2d-4c37-a739-eebad84cc83a" data-linktype="relative-path">ExtendedAddressHeader (section 2.2.5.5)</a></span>.<a id="Appendix_A_Target_5"></a><a aria-label="Product behavior note 5" href="4108f68e-80f5-40e1-b3df-b713cc4dff79#Appendix_A_5" data-linktype="relative-path">&lt;5&gt;</a></p><table>
 <tbody><tr>
  <th><p><br/>0</p></th>
  <th><p><br/>1</p></th>
  <th><p><br/>2</p></th>
  <th><p><br/>3</p></th>
  <th><p><br/>4</p></th>
  <th><p><br/>5</p></th>
  <th><p><br/>6</p></th>
  <th><p><br/>7</p></th>
  <th><p><br/>8</p></th>
  <th><p><br/>9</p></th>
  <th><p>1<br/>0</p></th>
  <th><p><br/>1</p></th>
  <th><p><br/>2</p></th>
  <th><p><br/>3</p></th>
  <th><p><br/>4</p></th>
  <th><p><br/>5</p></th>
  <th><p><br/>6</p></th>
  <th><p><br/>7</p></th>
  <th><p><br/>8</p></th>
  <th><p><br/>9</p></th>
  <th><p>2<br/>0</p></th>
  <th><p><br/>1</p></th>
  <th><p><br/>2</p></th>
  <th><p><br/>3</p></th>
  <th><p><br/>4</p></th>
  <th><p><br/>5</p></th>
  <th><p><br/>6</p></th>
  <th><p><br/>7</p></th>
  <th><p><br/>8</p></th>
  <th><p><br/>9</p></th>
  <th><p>3<br/>0</p></th>
  <th><p><br/>1</p></th>
 </tr>
 <tr>
  <td colspan="32">HeaderSize</td>
 </tr>
 <tr>
  <td colspan="32">RemainingHeadersSize</td>
 </tr>
 <tr>
  <td colspan="8">Flags</td>
  <td colspan="24">Reserved</td>
 </tr>
</tbody></table><p><b>HeaderSize (4 bytes): </b>A <b>ULONG</b> (<span><a href="../ms-dtyp/cca27429-5689-4a16-b2b4-9325d93e4ba2" data-linktype="relative-path">[MS-DTYP]</a></span>
section <span><a href="../ms-dtyp/32862b84-f6e6-40f9-85ca-c4faf985b822" data-linktype="relative-path">2.2.51</a></span>)
that specifies the size in bytes of the <b>ExtensionHeader</b>.</p><p><b>RemainingHeadersSize (4 bytes): </b>A <b>ULONG</b>
that MUST be the sum of sizes in bytes of all headers that follow the <b>ExtensionHeader</b>.</p><p><b>Flags (1 byte): </b>Indicates the presence or
absence of other headers in the <b>Message Packet Structure</b>. Any
combination of the following values is acceptable.</p><dl>
<dd>
<table><thead>
  <tr>
   <th>
   <p><br/>
   0</p>
   </th>
   <th>
   <p><br/>
   1</p>
   </th>
   <th>
   <p><br/>
   2</p>
   </th>
   <th>
   <p><br/>
   3</p>
   </th>
   <th>
   <p><br/>
   4</p>
   </th>
   <th>
   <p><br/>
   5</p>
   </th>
   <th>
   <p><br/>
   6</p>
   </th>
   <th>
   <p><br/>
   7</p>
   </th>
  </tr>
 </thead><tbody><tr>
  <td>D L</td>
  <td>S Q</td>
  <td>X 2</td>
  <td>D I</td>
  <td>E A</td>
  <td>X 5</td>
  <td>X 6</td>
  <td>X 7</td>
 </tr></tbody></table>
</dd>
<dd>
<p>Where the bits are defined as:</p>
</dd>
<dd>
<table><thead>
  <tr>
   <th>
   <p>Value</p>
   </th>
   <th>
   <p>Description</p>
   </th>
  </tr>
 </thead><tbody><tr>
  <td>DL</td>
  <td>MUST be set to 1 if the Message Packet Structure contains the DeadLetterHeader. MUST be set to 0 otherwise.</td>
 </tr><tr>
  <td>SQ</td>
  <td>Indicates whether the Message Packet Structure contains a SubqueueHeader. MUST be set to 1.</td>
 </tr><tr>
  <td>X2</td>
  <td>Unused bit field. MUST be ignored.</td>
 </tr><tr>
  <td>DI</td>
  <td>MUST be set to 1 if the dead-letter queue as specified by the DeadLetterHeader is invalid. MUST be set to 0 otherwise. If the DeadLetterHeader is not included, this field MUST be ignored when reading the message packet.</td>
 </tr><tr>
  <td>EA</td>
  <td>Indicates whether the Message Packet Structure contains an ExtendedAddressHeader. MUST be set to 1.</td>
 </tr><tr>
  <td>X5</td>
  <td>Unused bit field. MUST be ignored.</td>
 </tr><tr>
  <td>X6</td>
  <td>Unused bit field. MUST be ignored.</td>
 </tr><tr>
  <td>X7</td>
  <td>Unused bit field. MUST be ignored.</td>
 </tr></tbody></table>
</dd></dl><p><b>Reserved (3 bytes): </b>MUST be ignored when
reading the <b>Message Packet Structure</b>.</p></div>