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 ts 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?

ts+[(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?

ts+(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, dprop, in terms of m and s.

[b] Determine the transmission time of the packet, dtrans, 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=dtrans, where is the last bit of the packet?

[e] Suppose dprop is greater than dtrans. At time t=dtrans, where is the first

bit of the packet?

[f] Suppose dprop is less than dtrans. At time t=dtrans, 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 dprop = dtrans.

[a] dprop = m/s

[b] dtrans = L/R [c] end-to-end delay = dprop + dtrans=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