Non symetric tcp bandwidth results

[Emmanuel Lacour <elacour --at-- easter-eggs.com>]

Hi people using and people making this good software :),

I made some tests between linux (kernel 2.6.18) servers and a path using
linux (same kernel) routers.
I have some difficulties to understand the following result. If anyone
can help me :-/

My setup:

client1                    client2
   |                          |
   ----------------------------
                |
 ------------------------------
 |                            |
 R1                          R2
 |                            |
 ------------------------------
                |
		R3
                |
 -----------------------------
 |                           |
SRV1                        SRV2


client1: iperf -c SRV1 -t 300 -i 0 -r
client2: iperf -c SRV2 -t 300 -i 0 -r
SRV1: iperf -s
SRV2: iperf -s

Hardware: dell poweredge 1U, 1 CPU, 512Mo of RAM, ethernet intel e1000

CPU on R* is at 60%, no load, 100Mo used

Routing:

client1: default route via R1
client2: default route via R2
SRV1: default route via R3
SRV2: default route via R3
R3: 2 default route via R1 and R2 using equal cost multipath (but with
only one default route to R2, result is the same.


The results (sum of iperf on client1 and client2, started at the same
time):

client* -> SRV*: 850Mbit/s
SRV* -> client*: 620Mbit/s

I cannot explain this asymetric result :(


Thanks for any help :)