Packet loss with PollDevice

Hi all,

I am having an experiment with the following topology running on Deterlab. I 
am having a 4-core Xeon and modified e1000 driver running ore Xeon and modified 
e1000 driver running. The memory is 4GB.
                     
		     User1
                       |
	   User3-----Router------User4		
                       |
	             User2

User 1 is sending packet to User 2; User 3 is sending packets to User 4. 
The router only routes stuff with best effort. However, what I observed is 
very few losses, distributed on a small random set of rounds. What is 
surprising is that there are losses on one particular netcard when the sending 
rate is only 125,000 packets/second. The loss happens at the receiving phase.

Below is what I have tried:
1' I am using 4 threads, each corresponding to one pair of PollDevice and 
ToDevice. 
2' I have changed the queue length at the interface cards. I set txqueuelen=1 
for the senders to decrease the possible burstiness. By increasing the 
receiving queue length from 64 to 4096
(the maximum queue length), the loss rate decreases.
3' I tried to discard the packets on their arrival at the router. In this case, 
there is no observed loss.

I am kind of lost at the point. Any suggestions on what can be the possible 
reason and what I can try next?

Attached is my code.

Best Regards,
Yu 

Router:
----------------------------------------------------------------------- 
ddressInfo(router1-w1      10.0.1.1      00:15:17:5d:16:4a,      //eth2
user1-w1        10.0.1.2      00:15:17:5d:29:28,      //eth0
router1-w2      10.0.2.1      00:15:17:5d:16:48,      //eth0
user2-w2        10.0.2.2      00:15:17:57:ce:14,      //eth0
router1-w3      10.0.3.1      00:15:17:5d:16:4b,      //eth3
user3-w3        10.0.3.2      00:15:17:5d:27:e8,      //eth0
router1-w4      10.0.4.1      00:15:17:5d:16:49,      //eth1
user4-w4        10.0.4.2      00:15:17:5d:2a:3c,      //eth0
);

c1 :: Classifier(12/0806 20/0001,
                 12/0806 20/0002,
                 12/0800,
                 -);
c2 :: Classifier(12/0806 20/0001,
                 12/0806 20/0002,
                 12/0800,
                 -);
c3 :: Classifier(12/0806 20/0001,
                 12/0806 20/0002,
                 12/0800,
                 -);
c4 :: Classifier(12/0806 20/0001,
                 12/0806 20/0002,
                 12/0800,
                 -);

q0 :: Discard;
// Idle -> ToDevice(eth1);
// Idle -> ToDevice(eth3);
// Idle -> ToDevice(eth2);
// Idle -> ToDevice(eth0);
q1 :: CPUQueue(1000000) -> EtherEncap(0x0800, router1-w1, user1-w1) -> td1 :: ToDevice(eth2);
q2 :: CPUQueue(1000000) -> EtherEncap(0x0800, router1-w2, user2-w2) -> td3 :: ToDevice(eth0);
q3 :: CPUQueue(1000000) -> EtherEncap(0x0800, router1-w3, user3-w3) -> td2 :: ToDevice(eth3);
q4 :: CPUQueue(1000000) -> EtherEncap(0x0800, router1-w4, user4-w4) -> td0 :: ToDevice(eth1);

rt :: LookupIPRouteMP(10.0.1.0/32 0, 10.0.1.1/32 0, 10.0.1.255/32 0,
                     10.0.2.0/32 0, 10.0.2.1/32 0, 10.0.2.255/32 0,
                     10.0.3.0/32 0, 10.0.3.1/32 0, 10.0.3.255/32 0,
                     10.0.4.0/32 0, 10.0.4.1/32 0, 10.0.4.255/32 0,
                     10.0.1.0/24 1, 10.0.2.0/24 2, 10.0.3.0/24 3,
                     10.0.4.0/24 4, 0.0.0.0/0 0);
rt[0] -> Discard;
rt[1] -> q1;
rt[2] -> q2;
rt[3] -> q3;
rt[4] -> q4;

pd1 :: PollDevice(eth2) -> c1;
c1[0] -> q0;
c1[1] -> q0;
c1[2] -> DropBroadcasts -> Strip(14) -> GetIPAddress(16) -> rt;
c1[3] -> Discard;

pd3 :: PollDevice(eth0) -> c2;
c2[0] -> q0;
c2[1] -> q0;
c2[2] -> DropBroadcasts -> Strip(14) -> GetIPAddress(16) -> rt;
c2[3] -> Discard;

pd2 :: PollDevice(eth3) -> c3;
c3[0] -> q0;
c3[1] -> q0;
c3[2] -> DropBroadcasts -> Strip(14) -> GetIPAddress(16) -> rt;
c3[3] -> Discard;

pd0 :: PollDevice(eth1) -> c4;
c4[0] -> q0;
c4[1] -> q0;
c4[2] -> DropBroadcasts -> Strip(14) -> GetIPAddress(16) -> rt;
c4[3] -> Discard;

StaticThreadSched(pd2 0, td0 0, pd0 1, td2 1, pd3 2, td1 2, pd1 3, td3 3);

User:
//
AddressInfo(router1-w1      10.0.1.1      00:15:17:5d:16:4a,      //eth2
user1-w1        10.0.1.2      00:15:17:5d:29:28,      //eth0
router1-w2      10.0.2.1      00:15:17:5d:16:48,      //eth0
user2-w2        10.0.2.2      00:15:17:57:ce:14,      //eth0
router1-w3      10.0.3.1      00:15:17:5d:16:4b,      //eth3
user3-w3        10.0.3.2      00:15:17:5d:27:e8,      //eth0
router1-w4      10.0.4.1      00:15:17:5d:16:49,      //eth1
user4-w4        10.0.4.2      00:15:17:5d:2a:3c,      //eth0
);
//Send
udp :: FastUDPSource(RATE 400000, LIMIT 20000000, LENGTH 78,
		     SRCETH user1-w1, SRCIP user1-w1, SPORT 1,
		     DSTETH router1-w1, DSTIP user2-w2, DPORT 1,
		     CHECKSUM false, INTERVAL 0, ACTIVE false)
	-> ToDevice(eth0);

s :: Script(TYPE ACTIVE, write ctr.reset, write udp.active false, 
	    wait 1, wait 9, write udp.active true);