程序代写 COMP30023: Computer Systems

School of Computing and Information Systems

COMP30023: Computer Systems

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Tutorial Week 7

Service Models, Application Layer: HTTP and Cookies

1. List two advantages and two disadvantages of having international standards for network
protocols.

2. Suppose the algorithms used to implement the operations at layer k are changed. Do the
implementations of the operations at layers k − 1 and k + 1 need to change accordingly?

3. Suppose there is a change in the service (set of operations) provided by layer k. How does
this impact services at layers k − 1 and k + 1?

4. The web protocol HTTP was designed to run over TCP. Google has developed (and uses) a
new version of HTTP, HTTP/3, that runs over a new transport layer protocol, QUIC (Quick
UDP Internet Connections), which provides new services used by HTTP/3.

(a) Which layers are affected by this change?

(b) What would have happened if there had been a change to QUIC with no change to
HTTP? What assumptions are you making?

(c) What would have happened if HTTP had been updated to use the new services without
switching to QUIC? What assumptions are you making?

(d) Google controls both the servers and the web browsers (Chrome). How would this make
the change easier? What assumptions are you making?

5. Suppose that an application generates a message of length M bytes and there are n lower
layers each of which adds a h-byte header. What fraction of the network bandwidth is filled
with headers?

Optional: To give yourself a feeling for the role of overheads, plot this for M = 100, h = 20
and N ranging from 2 to 7. Plot it again for M = 30, n = 5 and h ranging from 8 to 30.
Plot it again for M = 1536, n = 5 and h ranging from 8 to 30.

6. List five nonproprietary Internet applications and the application-layer protocols that they
use. (Search beyond the lecture notes.)

Weekly tutorial participation activity

To obtain a weekly tutorial mark (1% of your overall mark for the subject, totalling to 10% over
the semester), please answer the following questions in Canvas Quiz (called Week 7 Tutorial
Activity). You can have multiple attempts but need to submit the quiz by 11:59 pm AEST on
the day of your tutorial. Only answering all questions correctly will give you the weeks mark.

During the tutorial, your tutor will provide you with the access code that will unlock the quiz for
the corresponding week. The access code is valid only for students in this tutorial. As a result,
we expect you to attend your tutorial class as otherwise the access code you obtain in another
tutorial will not work for you.

If you have a valid reason for not being able to attend your tutorial, please fill in the form accessible
via Canvas by Friday 8pm AEDT of the corresponding week. You will be given an access code
during the next business day. The code will not be provided otherwise, hence, please do not email
the subject coordinator or other staff members including your tutor asking for the code. We will
monitor such requests and may limit the number of times an access code is given to the same
studentthroughout the semester to encourage participation and attendance of the tutorials.

1. Would it be possible to have a network like the internet without layering?

(a) Yes, layering is just for backward compatibility

(b) Yes, layers just add overhead

(c) No. Without the intermediate layers, there would be no way for data to get from the
application to the physical hardware.

(d) No. Without layers, data can only go from one point to another that is directly con-
nected to it.

2. Which OSI layer number is the application layer?

3. Which of the following are advantages of having international standards for network proto-
cols? (Pick zero or more answers.)

(a) Equipment suppliers can make large numbers of the same appliance, which makes it
cheaper – “economy of scale”

(b) It makes it easier to communicate from one country to another

(c) It makes it harder for people to spy on other people’s communications

(d) It ensures that the best technical choices are made

4. What is the number of the only layer that can cause a tangle on your desk?

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