Queueing delay and loss on an overloaded link

Objective

Offer 12 Mbit/s of UDP to a 10 Mbit/s link with a 20-packet queue: the queue fills, delay rises and about 1/6 of the packets are dropped.

Procedure

1. Run and read the UDP loss and delay in the Summary.
2. Select the bottleneck link and raise the queue to 200 packets: delay grows, loss shrinks (for a short burst).
3. Lower the offered rate to 8 Mbit/s: no loss.

Theory

When the arrival rate exceeds the service rate the queue grows until it is full; then the excess is dropped (tail drop). Loss ≈ (λ − μ)/λ = 16.7 % and the queueing delay saturates at queue length × packet time.

Network blocks used

2 × Host / PC, Router

More Computer Networks Lab experiments

• TCP congestion control (slow start and loss recovery)
• File transfer over TCP: LAN versus WAN
• VLANs across a trunk link
• ARP packet inspection
• IP subnetting (two /26 subnets)