TCP

Question 1

True/False - The transport layer runs in end systems

Answer 1

True

Question 2

What are the units in TCP

Answer 2

TCP - Segments, UDP - Datagrams

Question 3

True/False - UDP does not provide error control only error checking

Answer 3

True

Question 4

In UDP what happens to datagrams with errors

Answer 4

They are dropped

Question 5

True/False - In UDP when datagrams are dropped there are gaps at the application level

Answer 5

True

Question 6

True/False - neither TCP nor UDP provides for guaranteed delay or guaranteed bandwidth

Answer 6

True

Question 7

True/False - IP does not provide for full delivery

Answer 7

True

Question 8

Describe UDP

Answer 8

Connectionless (no handshake), unreliable

Question 9

Describe TCP

Answer 9

Connection oriented (Handshake), reliable. used for services that cannot tolerate loss, byte oriented (no msg boundary)

Question 10

In the TCP header, what is the purpose of the sequence number

Answer 10

To identify the number of the 1st byte in the payload

Question 11

In the TCP header, what is the purpose of ack acknowledgment field

Answer 11

To provide the number of the next byte expected to be rx’d

Question 12

In the TCP header, what is the purpose of the rx window size field

Answer 12

To specify the number of bytes the rx’er is willing to accept (space in buffer)

Question 13

Describe the TCP 3-way Handshake process

Answer 13

• The initiator selects a random initial sequence number
• The initiator sends a segment with the Syn byte flagged and the ISN
• The rx’er selects a random initial sequence number
• The rx’er sends a segment with the the Syn and Ack bytes flagged and it’s ISN along with an ACK of the next seq number expected and its window size
• The connection is opened on the initiator side
• The initiator sends the next segment with the Ack byte flagged, The ACk with the next expected seq number from the rx’er and its window sz.
• The connection is opened on the rx’er side

Question 14

True/False - The final Ack of the TCP three way handshake can contain data

Answer 14

True

Question 15

Describe piggy backing in the TCP data transfer phase

Answer 15

The an ack is embedded with a data segment

Question 16

True/False - When the connection is to be closed the fin bit is set, there is no payload and 1 seq number is consumed

Answer 16

True

Question 17

True/False - There is only 1 Retransmission timer which is for the oldest segment when multiple segments are sent. When the oldest segment is acknowledged the timer gets set to the next oldest segment

Answer 17

True

Question 18

In TCP, what two things trigger retransmission

Answer 18

T/O expiration and 3 duplicate acknowledgements

Question 19

True/False - The rx’er may wait for up to 500 ms for more possible segments to arrive and use a cumulative acknowledgement

Answer 19

True

Question 20

Why does TCP consider T/O to be more serious than 3 dupes

Answer 20

Because dupes mean that other segments are getting through so the network congestion must have just been for a short time

Question 21

True/False - If the rx’er gets duplicates or already acknowledged segments it drops them

Answer 21

True

Question 22

True/False - If the rx’er has its own data to send it will not wait the 500ms for more segments

Answer 22

True

Question 23

In TCP flow control what is the rx’er window spare room

Answer 23

rx’er buffer - [ seq # last byte rx’d - seq # last byte read into app ]

Question 24

In TCP flow control what is the sender window size

Answer 24

Advertised Rx’er Window Size - [ Seq # last byte sent - seq # last byte ack’d ]

Question 25

True/False - IP does not provide any mechanism for congestion Control

Answer 25

True

Question 26

In TCP congestion control what is the max # of bytes that can be sent

Answer 26

min{advertised rx win sz, congestion window}

Question 27

True/False - In the TCP Tahoe implementation T/O and 3 Dup acks are weighed equally

Answer 27

True

Question 28

Describe the TCP Tahoe Flow

Answer 28

(Slow Start Phase)
- Start by setting congestion win to 1 MSS
- Each time the sender rx an ACK it increases congestion window by 1
(Congestion Avoidance)
- Upon reaching slow start threshold Congestion Window is increased by 1 for each RTT
(Congestion Control)
- Upon detecting t/o or 3 dupes, the slow start threshold is set to 1/2 what it was (min{Wa, Wc}) and the congestion window is set to 1 MSS
- The slow start phase starts again

Question 29

True/False - In the TCP Reno implementation T/O is weight more severely than 3 Dupes

Answer 29

True

Question 30

Describe the TCP Reno Flow

Answer 30

(Slow Start Phase)
- Start by setting Wc to 1 MSS
- increase Wc by 1 for each ACK rx’d
- Upon reaching ss thresh inc Wc by 1 for each RTT
- Upon T/O reduce SS thresh to 1/2 current Wc (min{Wc,Wa}) and reduce Wc to 1 MSS
- Upon 3 Dupes reduce Wc to current SS thresh

Question 31

Describe the TCP AIMD approach

Answer 31

• Inc Wc by 1 for every RTT until a loss is detected then cut Wc by half and start again