Transport Layer

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▪The transport layer is:

• responsible for logical

communications between

applications running on

different hosts.

• The link between the

application layer and the

lower layers that are

responsible for network

transmission.

• IP does not specify how the delivery or transportation of the packets takes place.

• Transport layer protocols specify how to transfer

messages between hosts, and are responsible for managing reliability requirements of a conversation.

• The transport layer includes the TCP and UDP protocols.

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TCP provides reliability and flow control. TCP basic operations:

• 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

UDP provides the basic

functions for delivering

datagrams between the

appropriate applications, with very little overhead and data checking.

• UDP is a connectionless protocol.

• UDP is known as a best-effort delivery protocol because there is no acknowledgment that the data is received at the destination.

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▪UDP is also used by request and-reply applications where the data is minimal, and retransmission can be done quickly.

▪If it is important that all the data arrives and that it can be processed in its proper sequence, TCP is used as the transport protocol.

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▪ Establishes a Session - TCP is a connection-oriented protocol that negotiates and establishes a permanent connection (or session) between source and destination devices prior to forwarding any traffic.

▪ Ensures Reliable Delivery - For many reasons, it is possible for a segment to become corrupted or lost completely, as it is transmitted over the network. TCP ensures that each segment that is sent by the source arrives at the destination.

▪ Provides Same-Order Delivery - Because networks may provide multiple routes that can have different transmission rates, data can arrive in the wrong order.

▪ Supports Flow Control - Network hosts have limited resources (i.e., memory and processing power). When TCP is aware that these resources are overtaxed, it can request that the sending application reduce the rate of data flow.

▪TCP is a stateful protocol

which means it keeps track

of the state of the

communication session.

▪TCP records which

information it has sent, and

which information has been

acknowledged.

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• 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.

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No matter how well designed a network is, data loss

occasionally occurs.

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

unacknowledged data.

▪TCP also provides mechanisms for flow control as follows:

• 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.

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TCP handles all tasks associated with dividing the data stream into segments, providing reliability,

controlling data flow, and reordering segments.

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▪UDP features include the following:

• Data is reconstructed in the order that it is received. • Any segments that are lost are not resent.

• There is no session establishment.

• The sending is not informed about resource availability.

The UDP header is far simpler than the TCP header because it only has four fields and requires 8 bytes (i.e. 64 bits).

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The table identifies and describes the four fields in a UDP header.

UDP does not establish a connection. UDP provides low overhead data transport because it has a small datagram header and no network management traffic.

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• 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.

▪ Live video and multimedia applications - These applications can tolerate some data loss but require little or no delay.

Examples include VoIP and live streaming video.

▪ Simple request and reply applications - Applications with simple transactions where a host sends a request and may or may not receive a reply. Examples include DNS and DHCP.

▪ Applications that handle reliability themselves - Unidirectional

communications where flow control, error detection, acknowledgments, and error recovery is not required, or can be handled by the application.

Examples include SNMP and TFTP.

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▪TCP and UDP transport layer protocols use port numbers to manage multiple, simultaneous conversations.

▪The source port number is associated with the originating application on the local host whereas the destination port number is associated with the destination application on the remote host.

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• The source and destination ports are