Transport Layer Part 1

Question 1

Sender Transport Protocol Actions

Answer 1

• gets MESSAGE from application layer
• determines UDP segment header fields values
• creates segment
• passes to IP (network layer)

Question 2

Receiver Transport Protocol Actions

Answer 2

• receives SEGEMENT from IP (network)
• checks header values
• extracts application layer MESSAGE
• demultiplexes message up to application via socket

Question 3

Two Transport Protocols

Answer 3

1. TCP
2. UDP

Question 4

Household Analogy: 12 kids in house sending letter to 12 kids in other house

Answer 4

hosts = house
processes = kids
app messages = letters in envelope

IP address (network) = envelope with address
port number (transport) = message in envelope

Question 5

Transport Layer

Answer 5

Logical communication between processes on different hosts

Question 6

TCP

Answer 6

Transport Control Protocol
- reliable, in-order delivery
- flow control
- connection setup

Question 7

UDP

Answer 7

User Datagram Protocol
- unreliable, unordered delivery
- no-frills extension of “best-effort” IP

Question 8

Multiplexing

Answer 8

• occurs at sender
• handle data from multiple sockets
• add transport header

Question 9

Demultiplexing

Answer 9

• occurs at receiver
• user header info to deliver received segments to correct socket
• host uses IP address and port numbers to direct segment to appropriate socket

Question 10

Connectionless Demultiplexing

Answer 10

• send to correct socket using dest IP and port number
• datagrams with same dest port but diff source IP/port will be directed to same socket

Question 11

Connection-oriented Demultiplexing

Answer 11

• socket identified by 4-tuple of source IP, source port, dest IP, des port
• uses all four tuple values to determine socket
• unique socket for each 4-tuple

Question 12

Checksum

Answer 12

Used to detect errors in transmitted segment.
1. Add together all values
2. wrap-around if needed
3. one’s complement

Question 13

Sender Checksum Actions

Answer 13

• treat contents of UDP segment as seq of 16-but integers
• perform checksum operations on seq
• put checksum value into UDP checksum field

Question 14

Receiver Checksum Actions

Answer 14

• compute checksum of received segment
• check if computed checksum equals checksum field value

Question 15

Principle of Reliable Data Transfer

Answer 15

Reliable service is implemented using
1. reliable data transfer protocol
2. unreliable channel

Question 16

RDT

Answer 16

Reliable Data Transfer PROTOCOL

Question 17

rdt 1.0

Answer 17

Reliable transfer over a reliable channel.

Question 18

rdt 2.0

Answer 18

Channel with bit errors (unreliable channel). Uses ACKNOWLEDGEMENTS to recover from errors

Question 19

ACKs

Answer 19

Acknowledgements. Receiver explicitly tells sender that packet was received OK

Question 20

NAKs

Answer 20

Negative Acknowledgements. Receiver explicitly tells sender that packet had errors and sender retransmits packer on receipt of NAK

Question 21

Stop and Wait

Answer 21

Sender sends one packet, then waits for receiver response