The Effect of Layer-2 Switches on Pathchar-like Tools Ravi S. - PowerPoint PPT Presentation

� � ✁ � ✁ The Effect of Layer-2 Switches on Pathchar-like Tools Ravi S. Prasad, Constantinos Dovrolis, and Bruce A. Mah Networking and Telecommunications Group, College of Computing, Georgia Tech. Packet Design Inc. Ravi S. Prasad - ravi@cc.gatech.edu, IMW, Nov 2002 1 of 8

� ✁ ✄ ☎ ✂ � � Overview of pathchar-like tools pathchar, pchar, clink - Objective : estimate link capacities They all use the Variable Packet Size (VPS) probing methodology Estimate Round Trip Time (RTT) up to hop f d f d 2 L/C 2 L/C 1 1 τ f τ 1 f TTL−− C TTL−− C 1 2 2 ==0 ==0 L ICMP C r / 1 ICMP ICMP τ r τ 1 C r r C r 1 2 L ICMP I d r th SND I Hop 1 - Propagation delays - Queuing delays - Serialization delays ✆✞✝ Ravi S. Prasad - ravi@cc.gatech.edu, IMW, Nov 2002 2 of 8

☎ ✆ ✝ ✎ ✍ � ✌ ☎ ☞ ✁ ✟ ✄ ☎ ✆ ✝ � ☎ ✡ ☞ ✡ ☛ ✆ ✍ �✁ ✂ ✏ ✏ ✎ ✟ ✝ ✆ ✄ ✁ � ✆ ✏ ☎ ☛ ✆ VPS methodology Assume that minimum RTT for each packet size does include queuing delays 0.75 Minimum RTT for packet size up to hop 0.625 RTT (msec) 0.5 ✞✠✟ 0.375 Estimated capacity of hop RTT Minimum RTT Linear fit 0.25 0 200 400 600 800 1000 1200 1400 1600 ✆✒✑ Probing packet size (Bytes) ✆✔✓ Ravi S. Prasad - ravi@cc.gatech.edu, IMW, Nov 2002 3 of 8

✄ � � � ✂ � ✂ ✂ ✁ �✁ � A single-hop path Tool Capacity estimate FastEthernet Switch pathchar 49.0 1.5Mbps A B clink 47.5 1.0Mbps pchar 47.0 1.0Mbps A and B have Fast Ethernet network interfaces Correct capacity 97Mbps (IP layer) All these tools consistently underestimate the link capacity What went wrong? - The tool, or the estimation methodology? Ravi S. Prasad - ravi@cc.gatech.edu, IMW, Nov 2002 4 of 8

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� � � � � � � � � � � � � � � Experimental results at a UDel campus network path A B C D E F :Fast Ethernet Switch; :Ethernet Switch L3 hop Capacity L2 segments pathchar clink pchar A to B 100Mbps 5 Fast-Ethernet 17.0 0.0 17.0 0.0 17.0 0.4 B to C 100Mbps 2 Fast-Ethernet 62.2 7.2 64.7 9.3 62.3 9.1 C to D 100Mbps 1 Fast-Ethernet 100.5 15.0 100.3 22.0 101.9 26.0 D to E 10Mbps 2 Ethernet 5.75 0.15 5.6 0.1 5.7 0.1 E to F 10Mbps ?? 4.5 0.1 3.7 0.1 6.5 0.6 Ravi S. Prasad - ravi@cc.gatech.edu, IMW, Nov 2002 6 of 8