WRED+bandwidth allocation algorithm ?

Question

Dear Gurus!
Plese tell me what is the bandwidth allocation algorithm:
I have 3 flows within 1 class-map.
WRED is enabled on the output ATM pvc(the total bandwidth of pvc is 2000kbps).
Flows have dscp marks af11(10) af12(12) af13(14).
When the pvc is congested the flows get the following bandwidth share in outgoing interface: af11(dscp 10) - 59% ; af12(dscp 12) - 40%; af13(dscp 14) - 1%; + bulk traffic with dscp 0.
Total bandwidth allocated for flows 10,12,14 is near 1500000bits per sec (75% of 2000kbps)
All flows have same packet sizes - 1400 bytes
Plese tell me how 7200 calculates bandwidth allocation between flow with dscp 10 , 12 and 14.
I have the following
router 7206
IOS (tm) 7200 Software (C7200-JK8S-M), Version 12.2(13), RELEASE SOFTWARE (fc1)
cisco 7206VXR (NSE-1) processor (revision A)
ATM PA - OC3
config:
class-map match-all af11
match ip dscp af11 af12 af13
policy-map gold
class af11
bandwidth percent 65
random-detect dscp-based
random-detect dscp 10 28 45 10
random-detect dscp 12 28 43 10
random-detect dscp 14 28 40 10
class class-default
bandwidth percent 10
interface ATM2/0.34 point-to-point
description RBNet
bandwidth 2000
ip address 10.0.4.1 255.255.255.252
pvc rbnet 15/64
vbr-nrt 2000 1500 50
tx-ring-limit 3
encapsulation aal5mux ip
service-policy output gold
Thanks in advance, Andrei

Answer 1

Ok Andrei,
Here's my thinking about what is happening...
You are transmitting a total of 300*1400*8 = 3.36Mbps into a circuit that is configured for 2Mbps. Since this traffic is being sent at a continuous rate, that means that the queue is in a state of constant congestion. In fact, the size of the queue is going to be sitting around the maximum of 45 packets pretty much all the time. The only time that space is created in the queue is when:
1. A packet is scheduled out of the queue
2. Packets are dropped due to RED
Now, if the queue is always around the 45 packet mark, that means that once the queue reaches that point, all packets for DSCP 14 are going to be dropped since the queue depth is greater than the maximum threshold of 40 for DSCP 14.
Considering the DSCP 10 traffic now...Both the DSCP 10 and DSCP 12 traffic is operating in the RED drop zone since the queue size is much higher than their minimum threshold of 20. Therefore, roughly 1/10 packets are getting dropped for each of these flows. 1/10 of each flow would give you roughly 2 packets of each based on the fact that very little of the DSCP 14 traffic is getting through. Every time RED drops these packets, space is created in the queue and if at that point, the queue size is less than 43, packets for DSCP 12 are accepted. The queue size is always going to be less than or equal to 45, so slightly more of the DSCP 10 packets are accepted.
I hope that explains the behaviour adequately.
Pls do remember to rate posts.
Paresh