Role and Mechanism of Queue

1. Role and Mechanism of Queue Internet Engineering

2. What is Queue （buffer） • Packet exchange＝ Store – and – forward • Router temporary storespacket in buffer • Packet is queued and then stored • （In Drop Tail）earlier income packet will transmit soon, what comes in first is handled • If the queue overflows, packets will be dropped Router Queue （buffer）

3. Relationship between Node, Queue, Link • In NS2, each link has a queue • Packets go through queue before sent out to the link • If input rate is larger than output rate, overflow packets will be accumulated in queue • It can response to temporary traffic increase • Packet are dropped if queue length exceeds its capacity Queue (Buffer) Packet Link Node Bandwidth Node 2 Node 3 Node 4 Node 0

4. Packet flow 4Mbps 20ms 40Mbps 6ms 40Mbps 6ms Node 0 Node 2 Node 3 Node 4 + 0.062 0 2 cbr 1000 ------- 0 0.0 4.0 62 62 - 0.062 0 2 cbr 1000 ------- 0 0.0 4.0 62 62 r 0.0682 0 2 cbr 1000 ------- 0 0.0 4.0 62 62 + 0.0682 2 3 cbr 1000 ------- 0 0.0 4.0 62 62 - 0.1302 2 3 cbr 1000 ------- 0 0.0 4.0 62 62 r 0.1522 2 3 cbr 1000 ------- 0 0.0 4.0 62 62 + 0.1522 3 4 cbr 1000 ------- 0 0.0 4.0 62 62 - 0.1522 3 4 cbr 1000 ------- 0 0.0 4.0 62 62 r 0.1584 3 4 cbr 1000 ------- 0 0.0 4.0 62 62 Why? 62ms 84ms 22ms Observe the packet （ID=62） （% awk ‘$12==62’ out.tr） ※experiment １-１-(b): bandwidth 4Mbps

5. Time for forwarding packet Time for sending packet from node 2 to node 3 4Mbps 20ms 40Mbps 6ms 40Mbps 6ms Node 0 Node 2 Node 3 Node 4 Queuing time in queue ＋ Forwarding time ＋ Time for transmission between links 62ms → (Queue length)×(Time for sending packet out) ※Queue length： the number of packet kept in queue, 31 in this ex 2ms → (Packet size [bytes])÷(Bandwidth of Link [Mbps]) 1000[bytes] 4[Mbps] 20ms → （Link delay） Total 84ms

6. Packet flow（Dropped packet） Drop 40Mbps 6ms 4Mbps 20ms 40Mbps 6ms Node 0 Node 2 Node 3 Node 4 Dropped after join a queue + 0.063 0 2 cbr 1000 ------- 0 0.0 4.0 63 63 - 0.063 0 2 cbr 1000 ------- 0 0.0 4.0 63 63 r 0.0692 0 2 cbr 1000 ------- 0 0.0 4.0 63 63 + 0.0692 2 3 cbr 1000 ------- 0 0.0 4.0 63 63 d 0.0692 2 3 cbr 1000 ------- 0 0.0 4.0 63 63 Observe packet（ID=63）by　（% awk ‘$12==63’ out.tr） ※In experiment １-１-(b), bandwidth is 4Mbps

7. Experiment 2 • Consider following relationship • Sending traffic rate • Bandwidth of bottleneck link • Queue length • Number of bytes discarded from queue 0 4 CBR/UDP 40Mbps 40Mbps Queue Size: B 2 3 Bandwidth: Bw (Bottleneck link) 5 1

8. 【Supplement】 Execution of Experiment 2 % ns problem2.tcl 1.0Mb32 • out.tr out.nam out.queue out.udp are created % gnuplot gnuplot> l “problem2.gp” gnuplot> … gnuplot> set term post color gnuplot> set output “graph.ps” gnuplot> rep gnuplot> q • If you don’t like the format, you can modify kadai1-2.gp (Bw) (B)

9. 【Supplement】 out.queue content • out.queue (Trace file of queue) • $1 : time • $2, $3 : position of queue（link） • $4 : accumulated data in queue (queue length) [bytes] • $5 : accumulated packet number in queue [packets] • $6 : number of packets arrived to queue [packets] • $7 : number of packets sent out from queue [packets] • $8 : number of packets dropped by queue [packets] • $9 : amount of data arrived to queue [bytes] • $10 : amount of data send out from queue [bytes] • $11 : amount of data dropped by queue [bytes] ※ $5～$11 is cumulative value from start of simulation

10. 【 Supplement 】 out.udp content • out.udp (Trace file of UDP) • $1 : time • $2 : send traffic rate [Mbps] • $3 : bandwidth of bottleneck link [Mbps]

11. Example of graph（attention point） 31 ※In experiment 2 when Bw is 1.0Mbps and B is 32kbytes