Note there there is also a very simple TLV definition in appendix A and
amendment to the IANA section on object types:

A.3.8.  Hello-ID

   Type:  Hello-ID

   Length:  Fixed (1 32-bit word)

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Hello-ID                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The contents are implementation defined.  See Section 5.2.2 for
   further discussion.