hw2

2023 computer-networking HW2.pdf

1.

use UDP because it is faster then TCP

2.

When a user requests an object, the cache checks if it has a copy of the object. If it does, the cache returns the object to the user without having to retrieve it from the original server

3.

The applications of HTTP, SMTP, and POP3 require more reliability than throughput, so they use TCP.

4.

total time=$\underset{\text{setup}}{RT{T}_0}+{\sum }_{i=1}^{n}RT{T}_i+\underset{\text{return answer}}{RT{T}_0}$
This is because each DNS server visited create an RTT, and once the IP address is obtained, an additional RTT is incurred to establish a connection with the server containing the object. The total time elapsed is equal to the sum of all these RTTs

5.a

$$\begin{array}{rl}totaltime& =\underset{\text{setup}}{2RTT}+\underset{\text{requset 3 obj at same time}}{2RTT}\\ & =4RTT\end{array}$$

5.b

$$\begin{array}{rl}totaltime& =\underset{\text{setup}}{2RTT}+\underset{\text{requset 3 obj one by one}}{3\times 2RTT}\\ & =8RTT\end{array}$$

5.c

$$\begin{array}{rl}totaltime& =\underset{\text{setup}}{2RTT}+\underset{\text{requset 3 obj at same time but cut half by Persistent HTTP}}{\mathrm{RTT}}\\ & =3RTT\end{array}$$