推 TommyKSHS :妳怎麼得到這份的 XD 04/29 15:00
課程名稱︰電腦網路導論
課程性質︰
課程教師︰廖婉君
開課學院：電資學院
開課系所︰電機系
考試日期（年月日）︰2012/4/26
考試時限（分鐘）：180
是否需發放獎勵金：Yes
（如未明確表示，則不予發放）
試題 :
Introduction of Computer Networks (S12)- Prof. WanJiun Liao
(1) Close Book Exam; total: 100 points.
(2) Please put down your name and student ID on the answer sheet.
(3) Please turn in your answer sheet together with the midterm exam problem
set.
(4) Good Luck!
===
1. (15%, 5 points each) Terminology: (a) Indicate the lower four layers in
the ISO OSI seven-layer model. (b) Please determine the Internet checksum
of the following three bit streams: 1000101000101110, 1110001111100101,
and 0010101111001111 (c) Use simple words to describe congestion control
and flow control.
2. (20%, 10 points each) Consider Fig. 1. Suppose that each link has a
propagation delay of 2ms, bandwidth of 4Mbps, and that the switches
support both circuit and packet switching. Thus, you can either break
the file into 1-KB packets, or set up a circuit through the switches
and send the file as one contiguous bit stream. For packets, suppose
that 1) packets have 24 bytes of packet header information and 1000
bytes of payload, 2) store-and-forward packet processing at each switch
incurs a 1-ms delay after the packet had been completely received,
and 3) packets may be sent contiguously without waiting for ACKs. For
circuit suppose that 1) circuit setup requires a 1-KB message to make
one round-trip on the path incurring a 1-ms delay at each switch after
the message has been completely received, and 2) switches introduce no
delay to data traversing a circuit. You may also assume that file size
is a multiple of 1000 bytes.
A → S1 → S2 → S3 → B
Figure 1: Network topology
(a) For what file size of n bytes is the total number of bytes sent
across the network less for circuit than for packets?
(b) For what file size n bytes is total delay incurred before the
entire file arrives at the destination less for circuit than
for packets?
3. (35%) Consider a simple protocol for transferring files over a link. After
some initial negotiation, Host A sends data packets of size 1KB to Host B
then replies with an ACK. Host A always waits for each ACK before sending
the next packet; that is known as stop-and-wait. Packets that are overdue
are presumed lost and are retransmitted.
(a) (10%) In the absence of any packet losses or duplications, what
is the least requirement in terms of number of bits for
"sequence number" in the packet header? Please explain your answer.
(b) (10%) Now suppose that the link can lose occasional packers, but
that packets that do always arrive in the order sent. How does
this change the sequence number requirement?
(c) (15%) Consider another design option. You are hired to design a
reliable byte-stream protocol that uses a sliding window (like
TCP). This protocol will rum over a 100-Mbps network. The RTT of
the network is 80 ms, and the maximum segment lifetime is 80
seconds. How many bits would you include in the ReceiveWindow
and SequenceNum fields of your protocol?
4. (15%) Consider sending a large file from a host to another over a TCP
connection that has no loss.
(a) (10%) Consider a simple congestion control algorithm that uses
additive increase and multiplicative decrease but no slow start,
that works in units of segments rather than bytes, and that
starts each connection with a congestion window equal to one
segment. Plot the congestion window as a function of RTT for the
first 30 segments, given that the following segments are lost:
3, 12, 13, 14, 21, and 26. For simplicity, assume that a perfect
timeout mechanism that detects a lost segment exactly 1 RTT after
it is transmitted.
(b) (5%) What is the average throughput (in terms of segments and
RTT) for these connections?
5. (15%) Consider a TCP connection over a 1-Gbps link with a latency of 100
ms to transfer a 10-MB-file, and the TCP receive window is 10MB. If TCP
sends a 1-KB segment (assuming no congestion and no lost packet). How many
RTTs does it take to send the entire file?
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