understanding traffic shaping
Kai Krebber <Kai.Krebber <at> krick.net>
2008-12-11 08:31:28 GMT
Hi!
I'm currently trying to understand traffic shaping on the SSGs and have
a hard time.
Prep for JNCIS FWV has the following question:
You have 4 policies configured for the egress interface with 10Mbps
physical bandwith:
Policy1: Prio0, 1Mbps GBW, 3Mbps MBW
Policy2: Prio1, 1Mbps GBW, 4Mbps MBW
Policy3: Prio1, 2Mbps GBW, 2Mbps MBW
Policy4: Prio0, 2Mbps GBW, 4Mbps MBW
The book states that under full load policy 4 would drop packets first.
I tried to simulate this and got a different result. I assume, my
assumptions are wrong, but I would need help to spot the error:
Let's say a constant stream of 1 Mbit-packets - one fitting each policy
- hits the device with 40Mbps.
I'll name the packtes after the policy-id, they will fit:
1,2,3,4,1,2,3,4,1,2,3,4, and so on.
Since the egress speed is only 10Mbps, the SSG can only send out one
packet for every four packets, it receives.
Lets go:
First packet hit's the device. It's policy 1. Since Policy 1 has 1Mbps
GBW, the packet goes straight out to the egress interface.
Second packet - this time for policy 2. Again 1 Mbps GBW, so the packet
get's straight queued on the interface, since the first packet is still
being put on the wire.
Same with packet 3 and 4.
Now packet 5 arrives- again for policy 1. GBW is exhausted, but MBW is
not even reached, so this packet is been pushed to Queue 0
Meanwhile packet 1 has left the building and packet 2 is been processed
to be put on the wire.
Next packet 6 arrives (policy 2) - again that GBW is exhausted, but not
the MBW, so this packet is placed in Queue 1.
Next packet 7 (policy 3) comes along - here we're even still in the GWB,
so this packet goes straight to the out-queue for the egress interface.
The last bits of packet 2 have just hit the wire.
Packet 8 arrives (policy 4). Again a GBW-Packet, so it joins Pakets 4
and 7 (3 has just started to be put on the wire).
Packet 9 comes in - Policy1 - This packet is just inside the MBW-limit.
It's the third 1Mb-Paket for policy 1 within this second and we got
3Mbps MBW, so that packet joins packet 5 in Queue 0.
Packet 10 comes in - policy 2. Here we've used up 3 of the 4 Mbps MBW,
so that packet goes into Queue 1.
And according to my understanding, packet 11 finally gets dropped, since
this would be 3Mbps for a 2 Mbps - MBW in policy 3.
Where's my mistake?
Cheers,
Kai
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