1. 1 Overview Midterm Solution Lab 5 Questions Lab Trouble Shooting Techniques

2. 1 Solutions to Some Midterm Questions Question 1(g): Four hosts (H1, H2, H3 and H4) are connected to one hub. H1's IP address is 10.0.2.201/26; H2's IP address is 10.0.2.139/25; H3's IP address is 10.0.2.1/26; H4's IP address is 10.0.2.11/28. Circle all pairs that can ping each other. A.H1-H2 B.H1-H3 C.H1-H4 D.H2-H3 E.H2-H4 F.H3-H4

3. 1 Question 2 18.26.0.1 MAC_BD (or ff.ff.ff.ff.ff) 18.26.0.123 MAC_PC1 18.26.0.123 MAC_PC1 18.26.0.1 MAC_r1 Target protocol address Target hardware address Source protocol address Source hardware address ARP Request Target protocol address Target hardware address Source protocol address Source hardware address ARP Reply Part A

4. 1 Question 2 (cont’d) MAC_r1 MAC_PC1 18.26.2.179 18.26.0.123 MAC_PC2 MAC_r2 18.26.2.179 18.26.0.123 Dest. Ethernet Addr. Source Ethernet Addr. Dest. IP Addr. Source IP Addr. Ethernet1 Dest. Ethernet Addr. Source Ethernet Addr. Dest. IP Addr. Source IP Addr. Ethernet2 Part D

5. 1 Question 3(a) 128.195.4.244/27LAN4 128.195.4.192/27LAN3 128.195.4.0/25LAN2 128.195.4.128/26LAN1 Network addressLAN A network with network address 128.195.4.0/24 has 4 subnets. LAN 1 needs at lest 50 IP addresses, LAN 2 needs at least 100 IP addresses, LAN3 and LAN4 each needs at least 20 IP addresses. Assign address block to each LAN.

6. 1 Question 4(c) Eth30.0.0.0/0 Eth3128.26.0.0./16 Eth118.195.4.0/24 Eth018.195.0.0/16 Eth018.202.0.0/16 Eth118.201.0.0/16 Aggregate the entries in the above routing table to make it as small as possible. A router with 3 interfaces has the routing table shown at the right. Eth30.0.0.0/0 Eth018.195.0.0/16 Eth018.202.0.0/16 Eth118.195.4.0/24 Eth118.201.0.0/16

7. 1 Overview

8. 1 Lab 5 Problems Exercise 3(B), Part 4-5 TCP Path MTU Discovery – How does it works? 1.TCP uses the minimum of the MSS announced by the 2 hosts in connection establishment 2.All TCP segment has the DF bit set in IP header 3.If a router along the path has a smaller MTU than the TCP segment size, it returns an “ICMP unreachable” error report with its MTU, e.g. MTU r 4.TCP set the MSS to (MTU r - 40), and try again. 5.The process continues until no “ICMP unreachable” error message is received.

9. 1 TCP Path MTU Discovery Example MTU 1500 MTU 500 MTU 1500 10.0.1.11/2410.0.2.22/2410.0.1.33/2410.0.2.33/24 PC1 PC2 PC3/Router ttcp –ts –1024 –n2 –p4444 –D 10.0.2.22 ttcp –rs –1024 –n2 –p4444 SYN, MSS=1460 SYN, ACK, MSS=1460 ACK PUSH, LEN=1460 ICMP Unreachable, MTU=500 PUSH, LEN=460 ACK

10. 1 Exercise 6A: TCP Bulk Data transfer (fast link) eth1 10.0.5.11/2410.0.5.22/24 PC1PC2 ttcp –ts –l1000 –n500 –p4444 –D 10.0.5.22ttcp –rs –l1000 –n500 –p4444 Step 4(5): Determine whether or not the TCP sender generally transmits the maximum amount of data allowed by the advertised window. Explain your answer.

11. 1 Exercise 6B: TCP Bulk Data transfer (slow link) eth0 10.0.1.11/24 10.0.2.22/24 PC2 ttcp –ts –l1000 –n500 –p4444 –D 10.0.2.22ttcp –rs –l1000 –n500 –p4444 Step 4(4): Does TCP sender generally transmits the maximum amount of data allowed by the advertised window? Explain your answer. PC1 Router 1Router 2

12. 1 Overview

13. 1 Trouble Shooting I did everything by following the instructions, but it does not work and I can’t ping other hosts/routers. Why? Well, something must be wrong. Let’s figure it out.

14. 1 Trouble Shooting Techniques The key to trouble shooting is to isolate the problem, i.e. what is the problem and where it happened in the network. – What is the output of the ping command? “Network unreachable”  Check local routing table 1.“PING x.x.x.x (x.x.x.x) 56(84) bytes of data”, and it hangs there  run “traceroute x.x.x.x” to see where the ICMP Echo Request packet failed.

15. 1 Questions? Thank you!