Chapter 6: Transport layer

Question 1

Primary function of the transport layer

Answer 1

Distinguishes packets between different applications running on a device.

Question 2

Transport entity

Answer 2

Thing that executes transport functions, which is in the operating system’s kernel. All networks must connect through the operating system.

Question 3

Sockets

Answer 3

Connection points where the higher levels can connect to in order to use transport layer functions

Question 4

Addressing

Answer 4

Each layer that supports multiple destinations have some kind of addressing scheme. Transport layer also has capabilities of funnelling data to a particular application, also has addresses called ports.

Question 5

Elements of transport protocols

Answer 5

All transport protocols address the same issue. Thus, there are elements of each protocol that can be compared with one another.

Question 6

Common transport protocols

Answer 6

We will use the 3 most common:
TCP (connection-oriented)
UDP(connectionless)
RTP(media focussed)

Question 7

Packets in the transport layer

Answer 7

We call sections of data transmitted over the network, segments

Question 8

Transport layer is accessible to…

Answer 8

All programs because it is software located in the operating system kernel. It is accessible to all applications using a standardised scheme.

Question 9

Sockets

Answer 9

A connection point where an application can connect to the transport layer.

Question 10

Sockets detailed

Answer 10

They are service primitives that applications can employ in order to use the services of the transport layer.S

Question 11

Socket example

Answer 11

An application wants to send data over the network. The application will make use of a SEND socket. It “plugs into” a socket in the transport layer that allows data to be sent.

Question 12

Primitive: Socket

Answer 12

Create a new communication endpoint.

Question 13

Primitive: bind

Answer 13

Associate a local address with a socket.

Question 14

Primitive: Listen

Answer 14

Announce willingness to accept connections; give queue size.

Question 15

Primitive: Accept

Answer 15

Passively establish an incoming connection.

Question 16

Primitive: Connect

Answer 16

Actively attempt to establish a connection.

Question 17

Primitive: Send

Answer 17

Send some data over the connection.

Question 18

Primitive: Receive

Answer 18

Receive some data from the connection.

Question 19

Primitive: Close

Answer 19

Release the connection.

Question 20

Regarding addressing…

Answer 20

Transport layer must be able to distinguish between applications.

Question 21

Ports

Answer 21

A single 16-bit integer value that is used for addressing applications in the transport layer.

It has values between 0 and 65535.

Question 22

Transport layer provides clean and managed…

Answer 22

Access to the network layer.

Question 23

Transport and data-link layer differ by…

Answer 23

Medium the layers use. Data-link only manipulates the physical layer, while the transport layer has an entire network as a medium.

Question 24

Transport layer data could experience delays because…

Answer 24

Of the nature of the network. Even though segments are sent in the correct order, they may arrive in the incorrect order. Also a network could have many hosts.

Question 25

Addressing

Answer 25

A primary function of the transport layer is that it allows an application on a local machine to send data to an application on a remote machine. Ports are used here

Question 26

Connection establishment

Answer 26

Connection-orientated or connectionless plays a big role here again. Connectionless protocols dont need to worry about connections and connection states. Connection orientated does

Question 27

Connection establishment: Three-way handshake

Answer 27

This prevents connections based on duplicate connections. The sending hosts send a sequence number x to the receiving host. The receiver host acknowledges the original transmission and sends its own sequence number y. Once the sending hosts receives y and the acknowledgement of x, it then acknowledges y and starts to transmit. This ensures legitimate connections are established.

Question 28

Connection release

Answer 28

When should a connection release? What if there is still data to be sent? (Read the army problem). The hosts decided independently when to terminate connections. If data did remain, a new establishment can be established to get the rest of the information.

Question 29

Error and flow control

Answer 29

Similar to data-link layer. Transport protocol must check for errors in the segments and control the flow of segments as well. Receivers can get overwhelmed if the sender has a higher bandwidth than what the receiver can take.

Question 30

Multiplexing

Answer 30

Allows for a single network connection to be shared amongst many applications. It goes hand-in-hand with addressing. The transport protocol must be able to differentiate between various data transmissions meant for the various different applications.

Question 31

Crash Recovery

Answer 31

Data loss occurs from time to time. Some transport protocols will attempt to fix it by retransmitting lost data.

Question 32

3 things you can do if the host goes offline after a crash

Answer 32

wait indefinitely for the remote host to come back online and send a retransmission request. Wait a limited amount of time for the host to reconnected before timing out. Immediately cut losses and purge buffers containing the received data and retry from the beginning once the host reconnects.

Question 33

What if the receiver goes offline

Answer 33

Just know that the transport protocol does address this somehow. Check the database example.