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计算机网络代考 | Computer Networks and Applications COMPSCI 3001

本次澳洲代考主要为计算机网络应用相关的限时测试

Application Layer

Question 1
(a) Consider how e-mail is sent in the Internet.
i. Why is it difficult to stop “spam” (unsolicited e-mails) by tracking
down the sender (both the host/network they used and the sender
themselves)?
[2 marks]

ii. Why do not SMTP servers just require all senders to be authenti-
cated before accepting mail?
[3 marks]

(b) i. Explain the steps involved in translating the hostname
(www.google.com) into the IP address of theWeb server (64.233.167.99)
using DNS (base your answer on an iterative query). Assume that
initially there are no cached hostname/IP mappings available in
the DNS related to this address. List all of the HTTP and DNS
requests that will occur while retrieving this page.
[5 marks]

ii. Assume the IP address of the Web server is now cached at the
client, so no network traffic relating to DNS is required in answer-
ing the following. Furthermore, assume the client is download-
ing an html resource containing three images each referring to a
small image. Lastly, assume transmission times for each of the
four Web resources is insignificant in comparison to the round-
trip-time (RTT) between client and server. How many RTT’s does
it take after the IP address of the server has been cached at the
client until when the html resource and all three images are re-
ceived, in each of the following cases (be sure to explain how you
arrive at your answer in each case):
 The browser and server are interacting using HTTP 1.0
 The browser and server are interacting using HTTP 1.1 with-
out pipelining
 The browser and server are interacting using HTTP 1.1 with
pipelining,
[6 marks]

(c) Calculate the minimum time needed to complete the distribution of
a 10 Gigabit file to 10 hosts under the following conditions (you can
assume negligible delays due to processing, queuing at routers and
propagation for this question). Express all your answers in seconds.
i. A client server system with a server upload speed of 1 Gb/s and a
client download speed of 20 Mb/s.
[4 marks]


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