Download Computer Networks (COMP2322) Assignment Four (95 marks in

Computer Networks (COMP2322)
Assignment Four (95 marks in total)
(Due on 21 April)
Rocky K. C. Chang
1 April 2016
1. [30 marks] Consider the following network topology, which consists of networks 193.1.1.0,
192.12.35.0, 192.10.1.0, 132.12.0.0, and 131.10.0.0. Both 132.12.0.0 and
131.10.0.0 have been subnetted with subnet masks 255.255.255.0.
(a) [10 marks] Fill in the missing items in the routing table of R1. Note that the subnet masks for all
the networks are 255.255.255.0.
Destinations
127.0.0.1
192.10.1.2
131.10.1.0
192.12.35.0
Gateways
127.0.0.1
193.1.1.0
131.10.128.0
131.10.129.0
131.10.10.0
131.10.9.0
132.12.1.0
132.12.2.0
Default
1
Flags
UH
UH
U
U
Comments
Loopback driver
Host specific route
Directly connected net.
Directly connected net.
U
UG
UG
UG
UG
UG
UG
UG
Directly connected net.
Route to a gateway
Route to a gateway
Route to a gateway
Route to a gateway
Route to a gateway
Route to a gateway
Default router
(b) [10 marks] We can compress the routing table in (a) using appropriate subnet masks. The
destination IP address will first be bitwise-ANDed with the masks in the second column, and the
result will be compared with the Destination in the first column. Fill in the missing items in the
compressed routing table of R1.
Destinations
127.0.0.1
192.10.1.2
131.10.1.0
192.12.35.0
Masks
255.255.255.255
193.1.1.0
131.10.128.0
131.10.0.0
132.12.0.0
0.0.0.0
Flags
UH
UH
U
U
Comments
Loopback driver
Host specific route
Directly connected net.
Directly connected net.
U
UG
UG
UG
UG
Directly connected net.
Route to a gateway
Route to a gateway
Route to a gateway
Default router
(c) [5 marks] Explain your answers for the destinations 131.10.128.0 and 131.10.0.0.
(d) [5 marks] If you would like to add another subnet 131.10.127.0 to the network, where shall
this subnet be located, and why?
2. [20 marks] Consider the following real network. A new network segment, to which SUNBETA was
attached, was added to improve the network resilience in terms of Internet access. Unfortunately, the
network administrator simply duplicated the IP addresses 192.168.3.0 for SUNBETA; as a result,
two segments identified themselves with the same network address. All the networks used subnet
masks 255.255.255.0, and RIP-I was used for routing.
SUNGAMA
-------------192.168.1.5
| router A |192.168.1.1.--------I------------192.168.1.2
-------------| 192.168.3.2
|
|
| 192.168.3.5
--SUNALPHA
|
|
| 192.168.3.1
-------------| router D |
-------------202.45.86.1
To Internet
------------| router B |
------------| 192.168.2.1
|
--SUNDELTA
| 192.168.2.4
|
| 192.168.2.2
------------| router C |
------------| 192.168.4.2
|
--SUNTHETRA
| 192.168.4.5
|
| 192.168.4.1
------------| router E |
------------| 192.168.3.2
|
Originally,
--SUNBETA
this segment
| 192.168.3.5
did not exist. |
| 192.168.3.1
------------| router F |
------------202.45.86.2
To Internet
2
(a) [5 marks] If SUNALPHA's routing table contained a route to its directly attached segment and a
default route to router D, how can it reach SUNGAMA?
(b) [5 marks] If router D fails, can SUNALPHA connect to the Internet?
(c) [5 marks] Can SUNALPHA and SUNBETA reach each other?
(d) [5 marks] Can SUNGAMA, SUNDELTA, and SUNTHETA reach SUNALPHA and SUNBETA?
3. [10 marks] Both TCP and UDP use end-to-end checksum to detect errors that escape from
error detection from the lower layers (IP and data-link) and errors occurred to the packets
while residing in router buffers. Consider the following TCP connection that spans across
three data-link networks. Each data-link network provides CRC for error detection. Consider
the following two scenarios:
(a) [5 marks] Errors have been introduced to the source IP address when the packet is
buffered in R1, and there are no other errors.
(b) [5 marks] Errors have been introduced to the source IP address in the link between S and
R1, and there are no other errors.
TCP connection
S
R1
CRC/IP checksum
R2
CRC/IP checksum
R
CRC/IP checksum
Assume that each CRC can detect the errors with probability pCRC and each 16-bit
checksum (for both IP and TCP) can detect the errors with probability pIP. The error
detection events are mutually independent. Compute the probability that the errors can be
detected for scenarios (a) and (b).
4. [20 marks] The figure below shows a Wireshark trace of a web session: from a client with
port 65416 to a server with port 80. Assume that both sides use an initial sequence number
(SN) of 0. Therefore, the SN and the acknowledgment number (AN) in the SYN-ACK packet
are 0 and 1, respectively. The SN and AN in the third hand-shaking packet are both 1.
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(a) [5 marks] What is the server's snd_nxt just after sending the SYN-ACK packet at time
1.282?
(b) [5 marks] What is the client's snd_nxt just after sending the data packet at time 1.283
(i.e., the fourth packet in the figure)?
(c) [5 marks] What is the client's rcv_nxt just after receiving the data packet from the
server at time 1.301 (i.e., the seventh packet in the figure)?
(d) [5 marks] What is the AN in the last packet in the figure?
5. [15 marks] (HTTP and TCP) Consider the following HTTP session and we ignore the
connection termination phase. TS is the time that the server spends on setting up the
connection and sending the requested document to the client. The requested document
consists of only 1 segment. TC is the time that the client spends on setting up the connection
and receiving the requested document from the server. From the diagram, TS = TC = 8 time
slots (a time slot is equal the length between two adjacent, horizontal dotted lines).
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Web server
Web client
TCP SYN
CK
TS
st
TP reque
AC K + H T
TC
TCP SYN
/A
Data
AC K
Now, we insert a Web proxy between the server and client (at the vertical dotted line), and
the TCP connection becomes two separate TCP connections (one between the server and
proxy, and another between the proxy and client). What are the new values of TS and TC in
terms of the number of time slots? Assume that the processing time at the proxy is negligible.
Hint: The TCP connection between the proxy and client needs to be established first before
the proxy initiates a TCP connection request to the web server.
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