计算机网络英文版习题答案

P63 #5 Consider sending a packet of F bits over a path of Q links. Each link transmits at R bps. The network is lightly loaded so that there are no queuing delays. Propagation delay is negligible.
a.Suppose the network is a packet-switched virtual-circuit network. Denote the VC setup time by t s seconds. Suppose the sending layers add a total of h bits of header to the packet. How long does it take to send the file from source to destination?
t s+[(F+h)/R]Q
b.Suppose the network is a packet-switched datagram network and a connectionless service is used. Now suppose each packet has 2h bits of header. How long does it take to send the packet?
[(F+2h)/R]Q
c.Finally, suppose that the network is a circuit-switched network. Further suppose that the transmission rate of the circuit between source and destination is R bps. Assuming ts setup time and h bits of header appended to the packet, how long does it take to send the packet?
t s+(F+h)/R
P64 #6 This elementary problem begins to explore propagation delay and transmission delay, two central concepts in data networking. Consider two hosts, A and B, connected by a single link of rate R bps. Suppose that the two hosts are separated by m meters and suppose that the propagation speed along the link is s meters/sec. Host A sends a packet of size L bits to host B.
[a] Express the propagation delay, d prop, in terms of m and s.
[b] Determine the transmission time of the packet, d trans, in terms of L and R.
[c] Ignoring processing and queueing delays, obtain an expression for the end-to-end delay.
[d] Suppose Host A begins to transmit the packets at time t=0. At time t=d trans, where is the last bit of the packet?
[e] Suppose d prop is greater than d trans. At time t=d trans, where is the first bit of the packet?
[f] Suppose d prop is less than d trans. At time t=d trans, where is the first bit of the packet?
[g] Suppose s=2.5 x 108, L=100 bits and R=28kbps. Find the distance m so that d prop = d trans.
[a] d prop = m/s
[b] d trans = L/R
[c] end-to-end delay = d prop + d trans=m/s+L/R
[d] The beginning position of the link.
[e] On the channel between A and B.
[f] On the host B.
[g] m/s = L/R = > m = sL/R = > m = 892.86 km
P65 #10 Consider the queueing delay in a router buffer. Suppose that all packets are L bits, the transmission rate is R bps, and that N packets simultaneously arrive at the buffer every LN/R seconds. Find the average queueing delay of a packet (in terms of L, R and N). (Hint: The queueing delay for the first packet is zero; for the second packet L/R; for the third packet 2L/R. The Nth packet has already been transmitted when the second batch of packets arrives.)
As the Nth packet has already been transmitted when the next batch of packets arrive, we only need to consider the delay for a single batch of packets.
Average delay = Total delay / Number of packets
Delay for 1st packet = 0
Delay for 2nd packet = L/R
Delay for 3rd packet = 2L/R
...
...
Delay for Nth packet = (N-1)L/R
Total delay for N packets = (0 + 1 + 2 ... +(N-1) ) * (L/R)
Using the formulas for sum of integer series, this can be written as: Total delay for N packets = (N-1) * (N/2) * (L/R)
Therefore, average delay for N packets = ((N-1) * L) / 2R
P170 #12 What is the difference between persistent HTTP with pipelining and persistent HTTP without pipelinning? Which of the two is used by HTTP/1.1?
For the persistent connection without pipelining, the client issues a new request only when the previous has been received. In this case, the client experiences one RTT in order to request and receive each of the referenced objects.
For the persistent connection with pipelining, the client issues a request as soon as it encounters a reference. It is possible for only RTT to be expended for all the referenced objects.
P170 #14 Telnet into a Web server and send a multiline request message. Include in the request message theIf-modified-since: header line to force a response message with the 304 Not Modified status code.
GET/somedir/exp.html HTTP/1.1
Host:
Connection: close
User-agent: Mozilla/4.0
If-Modified-Since: Thu, 30 May 2007 12:00:00 GMT
Accept-language: fr
P172 #6 Suppose within your web browser you click on a link to obtain a web page. The IP address for the associated URL is not cached in your local host, so a DNS look-up is necessary to obtain the IP address. Suppose that n DNS servers are visited before your host receives the IP address from DNS; the successive visits incur an RTT (Round Trip Time) of RTT1, ... RTTn. Further suppose that the web page associated with the link contains exactly one object, consisting of a small amount of HTTP text. Let RTT0 denote the RTT between the local host and the remote server containing the object. Assuming zero transmission time of the object, how much time elapses from when the client clicks on the link until the client receives the object? (Hint: read pages 90 .. 93)
Time to visit DNS servers and get IP address = RTT1 + RTT2 + ... + RTTn
Time to establish TCP connection (SYN and SYNACK) = RTT0
Time to send HTTP request and receive reply = RTT0
Total time = 2 * RTT0 + (RTT1 + RTT2 + ... + RTTn)
P171 #16 Suppose Alice with a Web-based e-mail account (such as Yahoo! Mail or Hotmail) sends a message to Bob, who accesses his mail from his mail server using POP3. Discuss how the message gets from Alice’s host to Bob’s host. Be sure to list the series of application-layer protocols that are used to move the message between the two hosts.
The series of application-layer protocols: HTTP、SMTP、POP3
Suppose that you send an e-mail message whose only data is a Microsoft Excel attachment. What might the header lines (including MIME lines) look like?
From:***********
To:***********
Subject: hello
MIME-Version: 1.0
Content-Transfer-Encoding: base64
Content-Type: Application/MS-Excel
P286 #5 Suppose host A sends two TCP segments back to back to host B over a TCP connection. The first segment has sequence number 90: the second has sequence number 110.
a.How much data is in the first segment?
a.20 bytes
b.Suppose that the first segment is lost but the second segment arrives at B. In the acknowledgement that host B sends to host A, what will be the acknowledgement number?
b.ACK90
P291 #27 Consider the following plot of TCP window size as a function of time. (reproduced below for you) Assuming TCP Reno is the protocol experiencing the behavior shown above, answer the following questions. In all cases, you should provide a short discussion justifying your answer.
a. Identify the intervals of time when TCP slow start is operating.
b. Identify the intervals of time when TCP congestion avoidance is operating.
c. After the 16th transmission round, is segment loss detected by a tripleduplicate ACK or by a timeout?
d. After the 22nd transmission round, is segment loss detected by a triple duplicate ACK or by a timeout?
e. What is the initial value of Threshold at the first transmission round?
f. What is the value of Threshold at the 18th transmission round?
g. What is the value of Threshold at the 24th transmission round?
h. During what transmission round is the 70th segment sent?
i. Assuming a packet loss is detected after the 26th round by the receipt of a triple duplicate ACK, what will be the values of the congestion-window size and of Threshold?
Solution:
a.1-6, 23-26
b.6-16, 17-22
c.a triple duplicate ACK
d.timeout
e.32
f.21
g.13
h.7
i.4, 4
P293 #34 Consider sending an object of size O = 100 Kbytes from server to client. Let S = 536 bytes and RTT = 100 msec. suppose the transport protocol uses static windows with window size W. (See Section 3.7.2)
a.For a transmission rate of 28 kbps, determine the minimum possible latency. Determine the minimum window size that achieves this latency.
b.Repeat (a) for 100 kbps.
tency=28.8s W=2
tency=8.2s W=4
P405 #8 Consider a datagram network using 8-bit host addresses. Suppose a router uses longest prefix matching and has t he following forwarding table:
-----------------------------------------------------
Prefix Match Interface
-----------------------------------------------------
00 0
01 1
10 2
11 3
-----------------------------------------------------
For each of the four interfaces, give the associated range of destination host addresses and the number of addresses in the range.
6
P407 #15 Consider sending a 3000-byte datagram into a link that has a MTU of 500 bytes. Suppose the original datagram is stamped with the identification number 422. How many fragments are generated? What are their characteristics?
there are「2980/480」=7 fragments be generated
P408 #22 Consider the network shown in Problem 21 (reproduced below). Using Dijkstra’s algorithm, and showing your work using a table similar to Table 4.3, do the following:
a. Compute the shortest path from s to all network nodes
Steps D(t),P(
t) D(u),P(
u)
D(v),P(
v)
D(w),P(
w)
D(x),P(
x)
D(y),P(
y)
D(z),P(
z)
0 1,s 4,s ∞∞∞∞∞
1 3.t 10,t ∞∞5,t 3,t
2 4,u 6,u ∞5,t 3,t
3 4,u 6,u ∞5,t
4 5,v 7,v 5,v
5 6,w 5,v
6 6,w
Please fill in the following tables using DV algorithm:For the node Z in the graph shown in the 22nd topic (P408), please fill in the following routing table in the router z about the initial distance-vector Destination node Next hop Current shortest distance
value-Dz
S —∞
T T 2
U —∞
V —∞
W —∞
X —∞
Y Y 14
Z Z 0
following rout-
ing table in the node z to update this routing table
Destination node Current
distance-Dy
Destination node Current distance-Dt
S 5 S 1 T 4 T 0 U 2 U 2
P493 #7 How big is the MAC address space？The IPv4 address space？The IPv6 address space?
MAC address: 6 bytes, MAC address space 2^48
IPV4 address: 4 bytes, IPV4 address space 2^32
IPV6 address: 16 bytes, IPV6 address space 2^128
P494 #4 Consider the 4-bit generator, G, shown in Figure 5.8, and suppose the D has the value 10101010. What is the value of R?
G=1001, D=10101010, R=101。