topic 11 : transport layer

Question 1

role of transport layer

Answer 1

• responsible for logical communications between applications running on different hosts
• link between the application layer and the lower layers that are responsible for network transmission

Question 2

transport layer responsibilities

Answer 2

1) tracking individual conversations
2) segmenting data and reassembling segments
3) adds header information
4) identify, separate and manage multiple conversations
5) uses segmentation and multiplexing to enable different communication conversations to be interleaved on the same network

Question 3

transport layer protocols

Answer 3

TCP and UDP protocols

transport layer protocols specify how to transfer messages between hosts and are responsible for managing reliability requirements of a conversation

Question 4

TCP (transmission control protocol)

Answer 4

provides reliability and flow control

Question 5

TCP basic operations

Answer 5

• number and track data segments transmitted to a specific host from a specific application
• acknowledge received data
• retransmit any unacknowledged data after a certain amount of time
• sequence data that might arrive in wrong order
• send data at an efficient rate that is acceptable by the receiver

Question 6

UDP basic functions

Answer 6

• connectionless protocol
• best-effort delivery protocol because there is no acknowledgement that the data is received at the destination

(provides the basic functions for delivering datagrams between the appropriate applications with very little overhead and data checking)

Question 7

TCP features

Answer 7

1) establishes a session
2) ensures reliable delivery
3) provides same-order delivery
4) supports flow control

Question 8

establishes a session

Answer 8

TCP is a connection-oriented protocol that negotiates and establishes a permanent connection/session between source and destination devices prior to forwarding any traffic

Question 9

ensures reliable delivery

Answer 9

ensures that each segment that is sent by the source arrives at the destination
(ensures that the segment does not become corrupted or lost completely)

Question 10

provides same-order delivery

Answer 10

because networks may provide multiple routes that can have different transmission rates, data can arrive in the wrong order

Question 11

supports flow control

Answer 11

• network hosts have limited resources
• when TCP is aware that these resources are overtaxed, it can request that the sending application reduce the rate of data flow

Question 12

TCP header

Answer 12

• stateful protocol : keeps track of the state of the communication session
• records which information it has sent and has been acknowledged

Question 13

applications that use TCP

Answer 13

TCP handles all tasks associated :
- with dividing the data stream into segments
- provides reliability
- controls data flow
- reordering segments

Question 14

UDP features

Answer 14

• data is reconstructed in the order that is received
  -any segments that are lost are not resent
• no session establishment
• sender is not informed about resource availability

Question 15

UDP applications

Answer 15

1) live video and multimedia applications
- can tolerate some data loss but require little/no delay (eg VoIP, live streaming video)

2) simple request and reply applications
- with simple transactions where a host sends a request and may or may not receive a reply (DNS, DHCP)

3) applications that handle reliability themselves
- unidirectional communications where flow control, error detection, acknowledgements and error recovery is not required/can be handled by the application (eg SNMP, TFTP)

Question 16

source port number and destination port number

Answer 16

source port number : originating application on the local host

destination port number : destination application on the remote host

Question 17

well-known ports

Answer 17

number range : 0 to 1023

• reserved for common or popular services and applications such as web browsers, email clients and remote access clients

Question 18

registered ports

Answer 18

number range : 1024 to 49151

Question 19

private/dynamic ports

Answer 19

number range : 49152 to 65535

Question 20

TCP port number groups

Answer 20

FTP (control) = 20
FTP (data) = 21
SSH = 22
Telnet = 23
SMTP = 25
HTTP = 80
POP3 = 1110
IMAP = 143
HTTPS = 443

Question 21

UDP port number groups

Answer 21

DHCP (server) = 67
DHCP (client) = 68
TFTP = 69
SNMP = 161

Question 22

UDP and TCP port number groups

Answer 22

DNS = 53

Question 23

TCP connection establishment

Answer 23

1) initiating client requests a client-to-server communication session with the server
2) server acknowledges the client-to-server communication session
3) The initiating client acknowledges
the server-to-client communication session

Question 24

TCP reliability (guaranteed and ordered delivery)

Answer 24

• TCP can also help maintain the flow of packets so that devices do not become overloaded
• There may be times when TCP
  segments do not arrive at their
  destination or arrive out of order.
• All the data must be received and the data in these segments must be
  reassembled into the original order.
• Sequence numbers are assigned in
  the header of each packet to achieve this goal.

Question 25

TCP reliability (data loss and retransmission)

Answer 25

TCP provides methods of managing these segment losses. Among these is a mechanism to retransmit segments for unacknowledged data

Question 26

TCP flow control (window size and acknowledgements)

Answer 26

- Flow control is the amount of data that the destination can receive and process reliably. - Flow control helps maintain the reliability of TCP transmission by adjusting the rate of data flow between source and destination for a given session.

Question 27

TCP flow control (maximum segment size)

Answer 27

- maximum amount of data that the destination device can receive - A common MSS is 1,460 bytes when using IPv4. - A host determines the value of its MSS field by subtracting the IP and TCP headers from the Ethernet maximum transmission unit (MTU), which is 1500 bytes be default.

Question 28

TCP flow control (congestion avoidance)

Answer 28

- congestion : packets being discarded by the overloaded router - avoid and control congestion : congestion handling mechanisms, timers and algorithms

Question 29

UDP low overhead vs reliability

Answer 29

- UDP = connectionless - provides low overhead data transport because it has a small datagram header and no network management traffic

Question 30

UDP datagram reassembly

Answer 30

UDP simply reassembles the data in the order that it was received and forwards it to the application.

Question 31

UDP server processes and requests

Answer 31

- assigned well-known/registered port numbers - receives a datagram destined for one of these ports, it forwards the application data to the appropriate application based on its port number

Question 32

UDP client processes

Answer 32

- The UDP client process dynamically selects a port number from the range of port numbers and uses this as the source port for the conversation. - The destination port is usually the well-known or registered port number assigned to the server process. - After a client has selected the source and destination ports, the same pair of ports are used in the header of all datagrams in the transaction.