Vpp includes a fairly capable network simulator plugin, which can simulate real-world round-trip times and a configurable network packet loss rate. It's perfect for evaluating the performance of a TCP stack under specified delay/bandwidth/loss conditions.
The "nsim" plugin cross-connects two physical interfaces at layer 2, introducing the specified delay and network loss parameters. Reconfiguration on the fly is OK, with the proviso that packets held in the network simulator scheduling wheel will be lost.
Configuration by debug CLI is simple. First, specify the simulator configuration: unidirectional delay (half of the desired RTT), the link bandwidth, and the expected average packet size. These parameters allow the network simulator allocate the right amount of buffering to produce the requested delay/bandwidth product.
set nsim delay 25.0 ms bandwidth 10 gbit packet-size 128
To simulate network packet drops, add either "packets-per-drop " or "drop-fraction [0.0 ... 1.0]" parameters:
set nsim delay 25.0 ms bandwidth 10 gbit packet-size 128 packets-per-drop 10000
Remember to configure the layer-2 cross-connect:
nsim enable-disable <interface-1> <interface-2>
Here's a unit-test configuration for the vpp packet generator:
loop cre
set int ip address loop0 11.22.33.1/24
set int state loop0 up
loop cre
set int ip address loop1 11.22.34.1/24
set int state loop1 up
set nsim delay 1.0 ms bandwidth 10 gbit packet-size 128 packets-per-drop 1000
nsim enable-disable loop0 loop1
packet-generator new {
name s0
limit 10000
size 128-128
interface loop0
node ethernet-input
data { IP4: 1.2.3 -> 4.5.6
UDP: 11.22.33.44 -> 11.22.34.44
UDP: 1234 -> 2345
incrementing 114
}
}
For extra realism, the network simulator drops any specific packet with the specified probability. In this example, we see that slight variation from run to run occurs as it should.
DBGvpp# pa en
DBGvpp# sh err
Count Node Reason
9991 nsim Packets buffered
9 nsim Network loss simulation drop packets
9991 ethernet-input l3 mac mismatch
DBGvpp# clear err
DBGvpp# pa en
DBGvpp# sh err
sh err
Count Node Reason
9993 nsim Packets buffered
7 nsim Network loss simulation drop packets
9993 ethernet-input l3 mac mismatch