Transport Layer

Question 1

What is a TCP(Transmission Control Protocol)?

Answer 1

TCP provides reliable, ordered, and error-checked delivery of bytes between applications.

Question 2

What is a UDP(User Datagram Protocol)?

Answer 2

Quick, connectionless communication

Question 3

What is Multiplexing and Demultiplexing?

Answer 3

Multiplexing (at sender)- how servers handle data from multiple sockets, Multiplexers add the transport header.

Demultiplexers(at receiver)- use header info to deliver segments to the correct socket

Question 4

What is a UDP checksum?

Answer 4

Used to detects errors (e.g., flipped bits)

Question 5

What is the Stop and Wait protocol

Answer 5

When the sender sends a packet, and then waits for the receiver to respond, this means that we can have a one-bit sequence number.

Question 6

What’s the difference between the demultiplexing process in UDP vs TCP?

Answer 6

UDP only looks at the destination port

TCP looks at a combination of the source IP addr. and port number dest. IP addr. and port number.

Question 7

Name 3 uses of UDP

Answer 7

• SNMP
• Streaming Multimedia
• DNS

Question 8

Is it possible to have a UDP connection that’s reliable?

Answer 8

Yes, this can be implemented in the application layer

Question 9

What’s rdt1.0?

Answer 9

Reliable transfer over a reliable channel

Question 10

What’s rdt2.0?

Answer 10

Transfer with bit errors. Repeats transfer to recover these errors after getting control messages(ACKs, NACKs).

Question 11

How does rdt2.0 handle ACK/NACK corruption/duplication?

Answer 11

Stop and Wait protocol is implemented, this is where the sender adds a sequence number to each packet and waits for the receiver response.

Question 12

What is a Pipeline protocol?

Answer 12

Sender allows multiple packets to be sent, they don’t have to be acknowledged packets. e.g. Go Back N, Selective Repeat

Question 13

What’s the difference between Selective repeat and the Go-Back-N protocols?

Answer 13

In Selective repeat the rcvr sends an individual ack for each packet and the sndr has a timer for each unacked packet.
MEANWHILE…
In Go-Back-N the rcvr sends a cumulative ack and has a timer for the oldest unacked packet.

Question 14

What is a method of increasing utilization?

Answer 14

Pipelining

Question 15

Does TCP use cumulative or individual acknowledgements?

Answer 15

Cumulative, similar to GBN

Question 16

Why does TCP use a 3 way handshake instead of a 2 way handshake?

Answer 16

• Message loss

- Variable delays

Question 17

Whats Flow Control?

Answer 17

The sender won’t overrun the receiver’s buffers by transmitting too much, too fast.

Question 18

How does the receiver in flow control advertise free space?

Answer 18

Including the rwnd value in the TCP header.

Question 19

What are the 4 costs of congestion?

Answer 19

(1) Lost packets
(2) Long delays
(3) Unneeded retransmission
(4) Upstream transmission capacity is wasted

Question 20

WHat’s congestion control?

Answer 20

This is when too many sources are sending too much too fast for the network to handle. Congestion control is different to flow control.

Question 21

What are the 2 approaches to congestion control?

Answer 21

• End to end congestion control

- Network assisted congestion control

Question 22

How does TCP sender deal with perceived end to end congestion?

Answer 22

Using the TCP slow start algorithm.

Question 23

What is TCP slow start?

Answer 23

When connection begins, the sender increases rate exponentially until loss event. Once loss event occurs, increase congestion window per ACK message received.

Question 24

How is loss detected in the TCP congestion control process?

Answer 24

1 Timeout

Question 25

How does TCP react to loss in the congestion control process?

Answer 25

• TCP RENO cuts congestion window in half

- TCP TAHOE sets congestion window to 1

Question 26

What is the TCP fairness goal?

Answer 26

if K TCP sessions share the same bottleneck link of bandwidth R, each should have a rate of R/K (router capacity)

Question 27

Why do multimedia apps usually use UDP instead of TCP?

Answer 27

They don’t want rate throttled by TCP congestion control.

Question 28

What does the transport layer do?

Answer 28

Provides process-process data transfer