计算机网络代写｜Computer Networks & Applications COMPSCI 3001

Application Layer
Question 1

(a) What information is used by a process running on one host to identify a process running on another host?

(b) Why does HTTP run on top of TCP rather than on UDP?

(c) Consider an HTTP client that wants to retrieve a Web document at a given URL. The IP address of the HTTP server is initially unknown.

What transport and application-layer protocols besides HTTP are needed in this scenario (list all that will be used)?

(d) Consider a short, 10-meter link, over which a sender can transmit at a rate of 150 bits/sec in both directions. Suppose that packets containing data are 100,000 bits long, and packets containing only control (e.g., ACK or handshaking) are 200 bits long.

Assume that N parallel connections each get 1=N of the link band width.

Now consider the HTTP protocol, and suppose that each downloaded object is 100 Kbits long, and that the initial downloaded object con tains 10 referenced objects from the same sender.

Firstly, consider parallel downloads via parallel instances of non-persistent HTTP. Secondly, consider persistent HTTP. Do you expect significant performance gains over the non-persistent case? Justify and explain your answer (please clearly show your reasoning and summary calculations).

(e) i. The equation for determining the download time for a peer-to peer file sharing is given below.

Explain what each of the following terms represents (for example:
“The time for one peer to download a single copy of the file”)

ii. Using the same variables (F, u, d) give an equation for the mini mum download time for a client/server file download.

(f) Consider the network shown in figure 1. The hosts in the left network are requesting resources using HTTP from the origin servers on the Internet. 30% of web requests result in a cache hit (the request has been previously downloaded and can be served from the cache). Each host (not including the cache) generates a request every 20 seconds and each request requires the download of 1Mbit. What download bandwidth is needed on the access link, L1, in order for this HTTP download traffic to utilise a no more than 80% of the link download capacity?

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