9 - Transport Layer Protocols

Question 1

Encapsulation

Answer 1

Each layer has a PDU (Protocol Data Unit)
Header/Body
Each layer adds a header to the PDU

Question 2

Transport Layer

Answer 2

Communcation between processes on different hosts

Question 3

Transport Layer send side

Answer 3

Break app messages into segments (transport layer packets)

pass to net layer

Question 4

Network Layer

Answer 4

Provides communication between hosts

Question 5

Transport layer receive side

Answer 5

Reassemble segments into messages

Pass to app layer

Question 6

Transport vs Network

Answer 6

Net: comms between hosts
Transport: comms between processes

Question 7

TCP - Transmission Control Protocol

Answer 7

Reliable, in order delivery

• Congestion Control
• FLow Control
• Connection Setup

Question 8

UDP - User Datagram Protocol

Answer 8

unreliable, unordered “best effort”

connectionless

• no handshaking
• each segment handled separately

Question 9

Addressing Processes

Answer 9

IP Address - host
Port Num - Identifies process
eg HTTP 80, Mail 25

Question 10

Multiplexing

Answer 10

At Send Host

• gathering data from multiple sockets, enveloping data with header. Collating messages to send

Question 11

Demultiplexing

Answer 11

At Receiving host

- delivering received segments to correct socket

Question 12

Non-Persistent vs Persistent HTTP

Answer 12

NON - at most one object sent over TCP

Persistent - Multiple objects over single TCP

Question 13

UDP Checksum Sender

Answer 13

treat segment as seq of 16 bit ints
checksum: addition of contents (1’s complement sum)
put into UDP checksum field

Question 14

UDP Checksum receiver

Answer 14

compute checksum

check if equals field value else error detected

Question 15

SQN number

Answer 15

TCP Sequence number = first byte of data in segment.
bytes
initial is random

TCP is full duplex - numbering is independent in each direction

Question 16

Acknowledgement Number

Answer 16

Sequence number of the next byte expected from the sender

cumulative

Question 17

TCP connection handshake

Answer 17

1) client host sends TCP SYN seg to server
- specify init seq #
- no data

2) server replies with SYNACK seg
- allocates buffer
- specify server init seq #

3) client replies with ACK which may contain data

Question 18

TCP Connection close

Answer 18

One side sends FIN=1 and the other ACKs

then second side sends FIN=1 and first ACKs

Question 19

TCP FLow Control

Answer 19

Receiver has buffer sends the byte value of it in the Receive Window field.

Sender then makes sure that packets to send plus packets in transit will not overflow the buffer

Question 20

As the receive window reduces, what happens to transmission rate?

Answer 20

Reduces

Question 21

What happens when receive window reaches zero?

Answer 21

Comms stop as sender is not informed of free space.

Sender probes every few seconds with empty packets waiting for indication of space