Tutorial 3

Question 1

Describe how Web caching can reduce the delay in receiving a requested object. Will
Web caching reduce the delay for all objects requested by a user or for only some of
the objects? Why?

Answer 1

Web caching can
bring the desired content “closer” to the user, possibly to the same
LAN to which the user’s host is connected.

“reduces response time because it satisfies requests
without involving origin server”

Web caching can reduce the delay for all objects, even objects that are not cached,
since caching reduces the traffic on links.

Question 2

In BitTorrent, suppose Alice provides chunks to Bob throughout a 30-second interval.
Will Bob necessarily return the favour and provide chunks to Alice in this same
interval? Why or why not

Answer 2

-Bob doesnt have to provide chunks to Alice. Alice has to be in top 4 neighbours of Bob for Bob to send out chunks to her. This might not occur even if Alice provides chunks to Bob throughout a 30 second interval.

Question 3

The UDP server described in the application example in Chap 2
–
Lecture 7 needed
only one socket, whereas the TCP server needed two sockets. Why? If the TCP server
were to support n simultaneous connections, each from a different client host, how
many sockets would the TCP server need?

Answer 3

With the UDP server, theres no welcoming socket and all data from different clients enters the server through this one socket.

With the TCP server, there is a welcoming socket and each time a client initiates a connection to the server, a new socket is created. Thus to support n simultaneous connections, the server would need n+1 sockets

Question 4

A transport-layer protocol provides for logical communication between application
processes running on different hosts. What does logical communication mean?

Answer 4

Logical communication - from an application’s perspective, the hosts running the processes seem like they are directly connected; even though they may be on opposite sides of the planet, connected via various routers and a wide range of link types.

Question 5

As an application developer, why would you choose to run your application over UDP
rather than TCP?

Answer 5

You may not want TCPs congestion control which can throttle the applications sending rate at times of congestion. Often designers of IP telephony and IP videoconderence applications choose UDP over TCP for this reason.

Some applications do not need the reliable data transfer of TCP.

Can bypass some restrictions that are in the TCP service so its faster if you dont need to set up connection

Question 6

Suppose that the UDP receiver computes the Internet checksum for the received UDP
segment and finds that it matches the value carried in the checksum field. Can the
receiver be absolutely certain that no bit errors have occurred? Explain.

Answer 6

No. This is cause if the corresponding bits (that would be added together) of two 16-bit words in the packet were 0 and 1 then even if these get flipped to 1 and 0 respectively, the sum still remains the same.

Question 7

In our rdt protocols, why did we need to introduce sequence numbers? Why did we
need to introduce timers?

Answer 7

Sequence numbers are there for receiver to find out whether an arriving packet contains new data or is retransmission.

Timers are needed to handle losses. If ACK for a transmitted packet is not received within a certain amount of time, the packet (or ACK or NACK) is deemed to be lost. Then retransmission occurs.

Question 8

True or false?

Host A is sending Host B a large file over a TCP connection. Assume
Host B has no data to send Host A. Host B will not send acknowledgments to Host A
because Host B cannot piggyback the acknowledgments on data.

Answer 8

False