NETACAD 14 TRANSPORT LAYER

Question 1

Describe the transport layer and its function

Answer 1

The transport layer is the link between the application layer and the lower layers that are responsible for network transmission.

• The transport layer is responsible for logical communications between applications running on different hosts.
• Transport layer protocols specify how to transfer messages between hosts and is responsible for managing reliability requirements of a conversation.
• The transport layer is responsible for tracking conversations (sessions), segmenting data and reassembling segments, adding header information, identifying applications, and conversation multiplexing.

Question 2

Describe UDP and give some applications that use it

Answer 2

UDP is stateless, fast, has low overhead, does not requires acknowledgments, do not resend lost data, and delivers data in the order it arrives. UDP reconstructs data in the order it is received, lost segments are not resent, no session establishment, and UPD does not inform the sender of resource availability. UDP header fields are Source and Destination Ports, Length, and Checksum. Applications that use UDP are DHCP, DNS, SNMP, TFTP, VoIP, and video conferencing.

Question 3

Describe TCP, its header files and give some applications that use it

Answer 3

TCP is stateful, reliable, acknowledges data, resends lost data, and delivers data in sequenced order. Use TCP for email and the web. TCP establishes sessions, ensures reliability, provides same-order delivery, and supports flow control. A TCP segment adds 20 bytes of overhead as header information when encapsulating the application layer data. TCP header fields are the Source and Destination Ports, Sequence Number, Acknowledgment Number, Header Length, Reserved, Control Bits, Window Size, Checksum, and Urgent. Applications that use TCP are HTTP, FTP, SMTP, and Telne

Question 4

What is the use of port numbers in TCP and UDP?

Answer 4

The TCP and UDP transport layer protocols use port numbers to manage multiple simultaneous conversations. This is why the TCP and UDP header fields identify a source and destination application port number.

Question 5

Describe encapsulation in the transport layer

Answer 5

The source and destination ports are placed within the segment. The segments are then encapsulated within an IP packet. The IP packet contains the IP address of the source and destination.

Question 6

What is a socket and what is its function

Answer 6

The combination of the source IP address and source port number, or the destination IP address and destination port number is known as a socket. The socket is used to identify the server and service being requested by the client.

Question 7

Describe the range of port numbers and its division

Answer 7

There is a range of port numbers from 0 through 65535. This range is divided into groups: Well-known Ports, Registered Ports, Private and/or Dynamic Ports. There are a few Well-Known Port numbers that are reserved for common applications such at FTP, SSH, DNS, HTTP and others.

Question 8

What is the functionality of Netsat

Answer 8

Sometimes it is necessary to know which active TCP connections are open and running on a networked host. Netstat is an important network utility that can be used to verify those connections.

Question 9

What is the relationship between application processes and port numbers

Answer 9

Each application process running on a server is configured to use a port number. The port number is either automatically assigned or configured manually by a system administrator.

Question 10

Describe TCP server processes

Answer 10

TCP server processes are as follows: clients sending TCP requests, requesting destination ports, requesting source ports, responding to destination port and source port requests.

Question 11

Describe the exchanges necessary to terminate a single conversation supported by TCP

Answer 11

• Either the client or the server can initiate the termination.
• The three-way handshake establishes that the destination device is present on the network,
• verifies that the destination device has an active service and is accepting requests on the destination port number that the initiating client intends to use,
• and informs the destination device that the source client intends to establish a communication session on that port number.

Question 12

What are the six control bits flags

Answer 12

URG, ACK, PSH, RST, SYN, and FIN.

Question 13

Describe selective acknowledgement

Answer 13

Host operating systems today typically employ an optional TCP feature called selective acknowledgment (SACK), negotiated during the three-way handshake. If both hosts support SACK, the receiver can explicitly acknowledge which segments (bytes) were received including any discontinuous segments. The sending host would therefore only need to retransmit the missing data.

Question 14

What is the importance of flow control and how is it achieved

Answer 14

Flow control helps maintain the reliability of TCP transmission by adjusting the rate of data flow between source and destination. To accomplish this, the TCP header includes a 16-bit field called the window size.

Question 15

What is sliding windows

Answer 15

The process of the destination sending acknowledgments as it processes bytes received and the continual adjustment of the source’s send window is known as sliding windows.

Question 16

Discuss UDP communications in terms of sequencing

Answer 16

When UDP datagrams are sent to a destination, they often take different paths and arrive in the wrong order. UDP does not track sequence numbers the way TCP does. UDP has no way to reorder the datagrams into their transmission order. UDP simply reassembles the data in the order that it was received and forwards it to the application. If the data sequence is important to the application, the application must identify the proper sequence and determine how the data should be processed.

Question 17

Discuss UDP communications in terms of port numbers

Answer 17

UDP-based server applications are assigned well-known or registered port numbers. When UDP receives a datagram destined for one of these ports, it forwards the application data to the appropriate application based on its port number. 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 used in the transaction. For the data returning to the client from the server, the source and destination port numbers in the datagram header are reversed.