NH          NH, EH-length      NH, EH-length
        +-------+      +------+            +-------+
        |       |      |      |            |       |
        |       v      |      v            |       v
  +-------------+-------------+-//-+---------------+--------------+
  |             |             |    |               |              |
  |    IPv6     |    Ext.     |    |     Ext.      |  Upper-Layer |
  |    header   |    Header   |    |     Header    |  Protocol    |
  |             |             |    |               |              |
  +-------------+-------------+-//-+---------------+--------------+

   fixed length    variable number of EHs & length
  <------------> <-------------------------------->