slides21

Question 1

who came first tcp or udp

Answer 1

tcp

Question 2

header of udp

Answer 2

16 bit source port
16 bit destination port
16 bit UDP length
16 bit UDP checksum

Question 3

is checksum important in UDP

Answer 3

no, unreliable so fine

Question 4

how does udp compare with other headers

Answer 4

much slimmer

Question 5

where is udp useful

Answer 5

single request and reply
Where a fast response is required
Where speed is more important than accuracy

Question 6

what is the procedure when a udp is lost

Answer 6

up to application

Question 7

tcp challenge

Answer 7

create reliability over unreliability

Question 8

what is ACK

Answer 8

The basis of the reliability is the use of acknowledgement (ACK) packets for every packet sent
If host A sends host B a packet, B must send an ACK packet back to A to inform it of the safe arrival of the packet
If A does not get an ACK, it resends the packet

Question 9

Two Armies Problem

Answer 9

infinite exchange of acks

TCP avoids the Two Armies Problem by using timeouts and packet retransmissions

Question 10

Problems to solve with acks

Answer 10

how long to wait before a resend?
how many times to resend before giving up? how long B should wait before sending the ACK?
IP datagrams can arrive out of order

Question 11

Problems to solve with tcp

Answer 11

how to maintain order in the data
how to manage duplicates
flow control

Question 12

TCP packets are often called…

Answer 12

segments

Question 13

what are the most important headers of tcp

Answer 13

sequence and acknowledgement 32bits each

Question 14

given Every byte in a TCP connection is numbered, how is data “counted”

Answer 14

The 32 bit sequence number starts at some random value and increases by 1 for each byte sent
So if a segment contains 10 bytes of data, the sequence number on the next segment sent will be 10 greater

Question 15

how does tcp implement full duplex

Answer 15

by keeping independent sequence numbers and flow control

Question 16

Acknowledgment number

Answer 16

Acknowledgment number (32 bits)
If the ACK flag is set then the value of this field is the next sequence number that the sender of the ACK is expecting. This acknowledges receipt of all prior bytes (if any). The first ACK sent by each end acknowledges the other end's initial sequence number itself, but no data.

Question 17

Sequence number

Answer 17

Sequence number (32 bits)
Has a dual role:
If the SYN flag is set (1), then this is the initial sequence number. The sequence number of the actual first data byte and the acknowledged number in the corresponding ACK are then this sequence number plus 1.
If the SYN flag is clear (0), then this is the accumulated sequence number of the first data byte of this segment for the current session.

Question 18

what is an obstacle to TCP

Answer 18

fragmentation

In this case TCP must wait for all the fragments and reconstruct the segment before it can send the ACK

Question 19

give an example of the ack number chosen by the receiver after having received some data

Answer 19

The returning ACK field contains the sequence number of the next byte the destination expects to receive, e.g., if the sequence number is 20000 and 14 bytes are received it returns 20015 in the ACK field

Question 20

PAWS and tcp

Answer 20

PAWS uses the same TCP Timestamps option as the RTTM mechanism described earlier, and assumes that every received TCP segment (including data and ACK segments) contains a timestamp SEG.TSval whose values are monotone non-decreasing in time. The basic idea is that a segment can be discarded as an old duplicate if it is received with a timestamp SEG.TSval less than some timestamp recently received on this connection.

Question 21

list all tcp flags

Answer 21

• URG: urgent data
• ACK: the acknowledgement field is active
• PSH: push this data to the application as fast as possible • RST: reset (break) the connection
• SYN: synchronise a new connection
• FIN: finish a connection
• ECE: congestion notification
• CWR: congestion window reduced
• 4 reserved bits, set to 0