Created attachment 1538 [details]
A simple three hop network.

A node one hop away from the destination node has to use a DSR network ACK since the destination is not forwarding the packet (i.e. no passive ACK by overhearing). The destination node then tries to ACK directly to the source node (sends ARP looking for the source node). DSR ACK time out occurs and Unreachable Error is sent to the source by the node one hop away from the destination.

In the attached sample, three nodes are placed linearly:

[node 0] --- [node 1] --- [node 2]

UDP flows from [node 0] to [node 2].

Timing:

1.04975s [node 1] DsrRouting:AddAckReqHeader(0xe9ae80, 0xea42f0, 10.0.0.3)
...
1.12975s [node 1] DsrRouting:SendPacket(0xe9ae80, 0xea5720, 10.0.0.1, 10.0.0.3, 17)
** First DSR packet with ACKREQ
...
1.12993s Send back ACK to the earlier hop 10.0.0.1 from us 10.0.0.3
1.12993s ArpL3Protocol:Lookup(0x1f75120, 0x1f80ef0, 10.0.0.1, 0x1f68f00, 0x1f7b680)
1.12993s node=2, no entry for 10.0.0.1 -- send arp request
** [node 2] received and processed ACKREQ, tried to ACK directly to [node 0]
...
1.13004s [mac=00:00:00:00:00:03] MacLow:StartTransmission
** [node 2] sends ARP Request (too far to be received by [node 0])
...
(two more DSR packets with ACKREQ, ARP pending at [node 2])
...
1.36975s [node 1] DsrRouting:SendUnreachError(0x1f782f0, 10.0.0.3, 10.0.0.1, 10.0.0.3, 0, 17)
** ACK timeout?
...
1.37024s [mac=00:00:00:00:00:02] startTx size=96, to=00:00:00:00:00:01
** [node 1] sends DSR RERR (Unreachable) back to [node 0]