2.2 Architecture

Question 1

What is the purpose of the transport layer?

Answer 1

it ensures data segments are reliably delivered within the context of client-server sessions

Question 2

What are the key functionalities of the transport layer?

Answer 2

1. Reliable and ordered delivery, 2. Multiplexing of conversations between hosts, and 3. flow/congestion control.

Question 3

IP focuses on Network Controller Interfaces. What does TCP focus on?

Answer 3

Processes.

Question 4

How do processes relate to ports?

Answer 4

Processes own ports, and initiate connection only in their ports.

Question 5

Is a socket unique across the Internet? [T/F]

Answer 5

T.

Question 6

Explain the concept of multiplexing.

Answer 6

When two hosts can have multiple simultaneous conversations.

Question 7

Explain how ports allow for multiplexing.

Answer 7

TCP divides the communications a host can be involved with into ports. This allows two hosts to perform several simultaneous communications without getting data confused.

Question 8

How is a port identified.

Answer 8

A port is identified by a number, used in each message between hosts. Port identities the communication the message belongs to.

Question 9

What is a socket.

Answer 9

A socket is a combination of an IP address and a port.

Question 10

Is the concept of a socket part of TCP specification? [T/F]

Answer 10

T.

Question 11

Is the assignment of sockets to services part of TCP specification? [T/F]

Answer 11

F.

Question 12

Give the name of the organisation that maintains the list of assigned ports.

Answer 12

Internet Assigned Numbers Authority (IANA).
Service names are assigned on a first-come, first-served process, as documented in rfc6335.

Question 13

What is the mandatory size of a TCP segment header.

Answer 13

20 bytes.

Question 14

What is the size of a port in the TCP segment header?

Answer 14

16 bits, 2 bytes. Value between 0-65535.

Question 15

How does a TCP message header allow for unique identification of a session between two sockets.

Answer 15

TCP header includes the source and destination ports, while the source and destination host addresses can be found in the IP header.

Question 16

What is a TCP session.

Answer 16

Before being able to send data, hosts need to establish a connection, referred to as TCP session, with a recipient host.

Question 17

Can data be sent without an established session?

Answer 17

Data can only be sent during an established session.

Question 18

What are the phases of establishing a session?

Answer 18

Session “set up” phase –> data transmission –> session “tear down” phase.

Question 19

Do other protocols utilise the concept of sessions or is it only a TCP connection?

Answer 19

Yes. A TCP connection is one kind of session. Many other protocols utilise sessions.

Question 20

Explain how a TCP session benefits communicating hosts.

Answer 20

Within a TCP session, communicating hosts will ensure reliability of delivery and will manage flow control.

Question 21

Explain briefly the client/server model. What is a server? What is a client?

Answer 21

Server: A host that is ready to accept communication on one or more ports.
Client: A host that communicates with a server.
Two hosts can potentially be both client and server in two separate connections.

Question 22

What does the TCP protocol define?

Answer 22

TCP protocol defines a finite state machine specifying the lifecycle of a session.

Question 23

How many phases are involved in the TCP protocol?
Give the phases involved.

Answer 23

2 phases.
- Connection/session setup phase
- Connection/session tear-down phase.

Question 24

The TCP session set up phase is initiated by the server. [T/F]

Answer 24

F.

Question 25

The TCP session set up phase is initiated by the client. [T/F]

Answer 25

T.

Question 26

Define the four steps that a TCP session set up phase consists of?

Answer 26

1. Client Synchronise (SYN)
2. Server acknowledges synchronise (SYN ACK)
3. Client acknowledges acknowledgement (ACK)
4. Connection is setup and can be used for transmitting data

Question 27

Define the four steps that a TCP session tear down phase consists of?

Answer 27

1. Client sends finalize (FIN)
2. Server acknowledges finalise (FIN + ACK)
3. Client acknowledges acknowledgement (ACK)
4. Tear-down connection

Question 28

The tear-down phase can be initated by the client or the server. [T/F]

Answer 28

T.

Question 29

What do acknowledgement messages contain?

Answer 29

Acknowledgement messages contain a sequence number indicating the next byte the sender of the acknowledgement is ready to receive.

Question 30

Why is there a requirement for the sender to increase its sequence number in their acknowledgment message?

Answer 30

Each message in a TCP communication expects an acknowledgement to be sent.

Question 31

What is the sequence number for the sender?

Answer 31

For the sender, the sequence number indicates the first byte of the message being sent.