TRANSPORT LAYER

TRANSPORT LAYER

1. Transport layer protocols

   • Data transport
     • Transport layer function
       • It is responsible for establishing a temporary communication session between two applications and for transmitting data between them. An application generates data that is sent from an application on a source host to an application on a destination host.
     • Responsibilities of the transport layer
       • It is the one that tracks individual conversations, Segmentation of data and reassembly of segments, Identification of applications.
     • Multiplexing conversations
       • Sending some types of data (for example, a video transmission) over a network, such as a complete communication stream, can consume all the available bandwidth. This will prevent other communications from occurring at the same time.
     • Reliability of the transport layer
       • The transport layer is also responsible for managing the reliability requirements of the conversations. Different applications have different transport reliability requirements. IP deals only with the structure, routing and routing of packets.
     • TCP
       • The TCP transport protocol function is similar to sending packets that are tracked from source to destination. If a shipment order is divided into several packages, the customer can review the order of delivery online.
     • UDP
       • UDP provides the basic functions to deliver data segments between the appropriate applications, with very little overhead and data review. UDP is known as a maximum effort delivery protocol.
     • Protocol of the correct transport layer for the appropriate application
       • For some applications, the segments must arrive in a very specific sequence so that they can be processed correctly. With other applications, the data is considered useful once all are received in full.
   • General description of TCP and UDP
     • Characteristics of TCP
       • TCP provides the establishment of a session ensures that the application is ready to receive the data. Delivery in the same order ensures that the segments are reassembled in the correct order. Reliable delivery involves the re-sending of lost segments so that the data is received in full. The flow control guarantees the receiver can process the received data.
     • TCP Header
       • To track the status of a session, TCP records what information was sent and what information was recognized. The session with status begins with the session establishment and ends when it closes at the end of the session.
     • UCP features
       • The characteristics of UCP are: the data are constructed in the order in which they were received. the lost segments are not sent again. there is no session establishment. it does not inform the issuer about the availability of resources.
     • UDP Header
       • UDP is a protocol without information status, which means that neither the client nor the server are required to monitor the status of the communication session. If reliability is required when using UDP as a transport protocol, it must be administered by the application.
     • Multiple and independent conversations
       • TCP and UDP manage these different simultaneous conversations through header fields that can uniquely identify these applications. These unique identifiers are port numbers.
     • Port numbers
       • The source port number is associated with the application that originates the communication on the local host. The destination port number is associated with the destination application on the remote host.
     • Pair of sockets
       • The socket is the combination of the source IP address and the source port number, or the destination IP address and the destination port number. The socket is used to identify the server and the service requested by the client.
     • Groups of port numbers
       • There are different types of port numbers: Known ports (numbers from 0 to 1023), Registered ports (numbers from 1024 to 49151), Dynamic or private ports (numbers 49152 to 65535). Some client operating systems can use registered port numbers instead of dynamic port numbers to assign the source ports.
     • The netstat command
       • Netstat is an important network utility that can be used to verify those connections. As shown in the figure, enter the netstat command to obtain a list of the protocols that are being used, the local addresses and port numbers, the external addresses and port numbers and the connection status.

2. TCP and UDP

   • TCP Communications Processes
     • TCP server processes
       • Each application process running on the server is configured to use a port number, either predetermined or manually, by the system administrator.
     • Establishment of TCP connections
       • In TCP connections, the client of the host establishes the connection with the server. A TCP connection is established in three steps: Step 1: the source client requests a client communication session to server with the server. Step 2: the server recognizes the session of client-to-server communication and request a session of server to client communication. Step 3: the origin client recognizes the server-to-client communication session.
     • End of the TCP session
       • To end all one-way TCP sessions, a two-way link is used, consisting of a FIN segment and an acknowledgment segment (ACK).
     • Analysis of the three-way TCP link
       • TCP is a full duplex protocol, in which each connection represents two unidirectional communication transmissions or sessions. To establish the connection, hosts perform a three-way link. The control bits in the TCP header indicate the progress and status of the connection.
   • Reliability and flow control
     • TCP Reliability: orderly delivery
       • The TCP segments can reach their destination untidy. In order for the receiver to understand the original message, the data in these segments are reassembled in the original order.
     • Loss and retransmission of data
       • Data loss occurs sometimes, no matter how well designed the network is; therefore, TCP provides methods to manage these segment losses.
     • Sequence numbers and acknowledgments
       • One of the functions of TCP is to ensure that each segment reaches its destination. The TCP services on the destination host send an acknowledgment of the data received by the source application.
     • TCP flow control: window size and acknowledgments
       • Typically, the devices use the sliding windows protocol. With the sliding windows, the receiver does not expect the number of bytes in the window size to be reached before sending an acknowledgment.
     • TCP flow control: congestion prevention
       • When congestion occurs in a network, the router Overloaded begins to discard packets. When the packets containing the TCP segments do not arrive at their destination, they remain without recognition.
   • UDP communication
     • Comparison of low overhead and reliability of UDP
       • It has a much lower overhead than TCP, since it is not connection oriented and does not provide sophisticated relay, sequencing and flow control mechanisms that offer reliability.
     • Reassembly of UDP datagrams
       • UDP simply reassembles the data in the order it was received and sends it to the application. If the data sequence is important for the application, it must identify the appropriate sequence and determine how the data should be processed.
     • Processes and requests of the UDP server
       • Similar to TCP-based applications, UDP-based server applications are assigned known or registered port numbers, as shown in the figure.
     • Procesos de cliente UDP
       • The UDP client process dynamically selects a port number from the range of port numbers and uses it as the source port for the conversation.
   • TCP or UDP
     • Applications that use TCP
       • TCP is responsible for all tasks associated with the division of data flow into segments, which provides reliability, control of data flow and reordering of segments.
     • Applications that use UDP
       • There are three types of applications that are most suitable for UDP: Multimedia applications and live video, Applications with simple requests and answers, Applications that handle reliability on their own.