When sending UDP packets, the ID's at the client and nat computer are
identical:

Client (1.2) to destination (2.51):

id 36222,  length 34  192.168.1.2.  51757  > 129.125.2.51. 12345,  length 6
id 18749,  length 38  129.125.2.51. 12345  > 192.168.1.2.  51757,  length 10
id 63059,  length 37  192.168.1.2.  51757  > 129.125.2.51. 12345,  length 9
id 24262,  length 40  129.125.2.51. 12345  > 192.168.1.2.  51757,  length 12

Nathost (17.6) to destination (2.51):

id 36222, length 34   192.168.17.6. 51757  > 129.125.2.51. 12345,  length 6
id 18749, length 38   129.125.2.51. 12345  > 192.168.17.6. 51757,  length 10
id 63059, length 37   192.168.17.6. 51757  > 129.125.2.51. 12345,  length 9
id 24262, length 40   129.125.2.51. 12345  > 192.168.17.6. 51757,  length 12


Multiple udp packets may be sent by identical processes, in which case the
client and nathost computers may use different  ports.

The IN connections store the source, dest, IP-addresses and ports, and uses
d_id (like the tcp's sequence nr) to allow association with the matching OUT
packet. Once the association is made, the via IP-address and port can be set,
and the d_id entry can be removed.
