Client  NS for    NS for           NS for        DDI services
        urn.arpa  ddialliance.org  example1.edu  for us.ddia1
  |       |            |               |              |
1 |------>|            |               |              |
2 |       |----------->|               |              |
3 |                    |-------------->|              |
4 |<-----------------------------------|              |
5 |-------------------------------------------------->|
6 |<--------------------------------------------------|