Collaboration Policy: CP-1 (Closed book, work alone)
Name:
SI455 Advanced Networks
6-Week Exam (Part A)
Spring AY2014
Write legibly - neatness counts
1. [12 pts] DHCP Under Normal Conditions. In class, we observed a four-message handshake
for a DHCP request under normal conditions. (There are more messages that get passed if the
DHCP client and server need to exchange MAC addresses or perform a DNS lookup. Ignore
those messages - assume that the workstation already knows the DHCP server’s IP address
and has an entry for 10.10.1.5 in its ARP table.)
The diagram below shows the timeline of the four DHCP messages in the initial exchange:
DHCP Server
10.10.1.5
Workstation (DHCP Client)
IP: Not yet set
1
2
3
4
Fill in the table below for each message in the exchange. For the description, you can use
the official terms or just explain the purpose of the message in plain-English.
#
Source IP: Dest. IP:
Description of Message
0.0.0.0
255.255.255.255 DHCP Discovery
10.10.1.5
<offer> e.g.
10.10.1.100
0.0.0.0
255.255.255.255 DHCP Request
10.10.1.5
<offer> e.g.
10.10.1.100
1
2
DHCP Offer
3
4
DHCP Acknowledgement
2. [16 pts] ARP Under Abnormal Conditions. ARP expects that each machine has a unique IP
address and a unique MAC address. This question asks you to extrapolate what might happen
to network messages in a misconfigured network.
Workstation1 wants to request a webpage from a Webserver. Due to a misconfiguration,
Workstation99 has the same IP address as the Webserver. Workstation99 is not running a
webserver, and cannot serve pages. Each machine has a unique MAC address.
Workstation1
10.10.1.111
00:50:56:11:11:11
Webserver
10.10.1.10
00:50:56:22:22:22
Workstation99
10.10.1.10
00:50:56:33:33:33
Workstation1 knows the Webserver’s IP address, but not its MAC. Before sending an
HTML/GET request, the Workstation needs to send an ARP Request to find out which MAC
is associated with 10.10.1.10. Since there are two machines with the 10.10.1.10 IP address,
both send replies. This is a “race condition”, since the results of the request depend on which
message arrives first.
A. For part A, assume that both the Webserver and Workstation99 reply to the ARP request,
but the ARP Reply from the Webserver arrives first.
1) What does Workstation1’s ARP table look like? (Use as many rows as you need)
IP Address
HWaddress
10.10.1.10
00:50:56:22:22:22
2) Will the subsequent HTML/GET request from Workstation1→10.10.1.10 return a
webpage? Why or why not?
Yes, proper IP & MAC for webserver
B. For part A, assume that both the Webserver and Workstation99 reply to the ARP request,
but the ARP Reply from Workstation99 arrives first.
1) What does Workstation1’s ARP table look like? (Use as many rows as you need)
IP Address
HWaddress
10.10.1.10
00:50:56:33:33:33
2) Will the subsequent HTML/GET request from Workstation1→10.10.1.10 return a
webpage? Why or why not?
No, proper IP, but incorrect MAC for webserver
Messages will arrive at Workstation 99’s Port 80, but will have no
server available to answer them, and will be dropped.
3. [7 pts] Why does the ARP service not require port numbers?
ARP operates at Layer 2 or 3 *
Port numbers are assigned in Layer 4 (Transport Layer)
* Note – There is controversy over whether ARP belongs in Layer 2 or 3. ARP was
designed before the OSI model was developed, so does not conform to that
model. Either answer is acceptable for this question.
4. [10 pts] Describe one fundamental difference between the ways that Link-State and DistanceVector algorithms calculate their routing tables.
Biggest difference is the exchange of messages.
LS – every router broadcasts to all other routers. Each router builds a complete and
identical map of costs to each other router.
DV – every router sends its immediate neighbors two items:
A. A measured cost to its immediate neighbors
B. An estimated cost to distant routers, based on information from other routers
It is not possible to recreate the network layout using DV algorithms. It is possible that two
routers will have different network maps, which could lead to circular paths and other
inefficiencies.
5. [10 pts] In the space below (1-2 paragraphs) describe some of the non-technical challenges
that BGP must solve. (i.e. problems that are caused at the human-layer and business-layer,
not by the network hardware itself.) Give examples.
Answer should demonstrate depth of research and understanding.
6. [15 pts] Explain what each of the following tcpdump commands does:
A. sudo tcpdump –D
Print available interfaces and exit
B. sudo tcpdump –n
Do not convert IP addresses or port numbers to names
NOTE – the program’s filter does not match the manpage description.
Accept either IP address or IP address & port number.
C. sudo tcpdump –c 10 –x
Exit after receiving 10 packets.
Display packet payload in hex.
D. sudo tcpdump –r saved.pcap –X not arp
Read packets from file ‘saved.pcap’.
Display in both hex and ASCII
Ignore ARP messages
E. sudo tcpdump dst port 80 or dst port 443
Show only packets with destination port 80 or 443