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Overview
• Midterm Solution
• Lab 5 Questions• Lab Trouble Shooting Techniques
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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
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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
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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
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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.
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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
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Overview
• Midterm SolutionMidterm Solution
• Lab 5 Questions• Lab Trouble Shooting TechniquesLab Trouble Shooting Techniques
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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. MTUr
4. TCP set the MSS to (MTUr - 40), and try again.
5. The process continues until no “ICMP unreachable” error message is received.
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TCP Path MTU Discovery Example
MTU 1500
MTU 1500
MTU 500
MTU 1500
10.0.1.11/24 10.0.2.22/2410.0.1.33/24 10.0.2.33/24
PC1 PC2PC3/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
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Exercise 6A: TCP Bulk Data transfer (fast link)
eth1 eth1
10.0.5.11/24 10.0.5.22/24
PC1 PC2
ttcp –ts –l1000 –n500 –p4444 –D 10.0.5.22 ttcp –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.
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Exercise 6B: TCP Bulk Data transfer (slow link)
eth0 eth0
10.0.1.11/24 10.0.2.22/24
PC2
ttcp –ts –l1000 –n500 –p4444 –D 10.0.2.22 ttcp –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 1 Router 2
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Overview
• Midterm SolutionMidterm Solution
• Lab 5 QuestionsLab 5 Questions• Lab Trouble Shooting Techniques
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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.
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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.
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Questions?
Thank you!
