Chapter 9
Transport Layer Responsibility / Reliability
IP does not specify how the delivery or transportation of the packets takes place. Transport protocols specify how to transfer messages between hosts. TCP is considered a reliable, full-featured transport layer protocol, which ensures that all of the data arrives at the destination. In contrast, UDP is a very simple transport layer protocol that does not provide for any reliability.
Transmission Control Protocol (TCP)
Provides connection-oriented data management and reliable data transfer. 3 Basic Operations of TCP:- - Tracking transmitted data segments - Acknowledging received data - Retransmitting any unacknowledged data
Conversation Multiplexing
Segmenting the data into smaller chunks enables many different communications, from many different users, to be interleaved (multiplexed) on the same network.
TCP / UDP Features
TCP - Establishing a session ensures the application is ready to receive the data. - Reliable delivery means lost segments are resent so the data is received complete. - Same order delivery ensure that the segments are reassembled into the proper order. - Flow control ensures that the receiver is able to process the data received. UDP - No Ordered Data Reconstruction - Data is reconstructed in the order that it is received. - Unreliable Delivery - Any segments lost are not resent. - Connectionless - No session establishment. - No Flow Control - Does not inform the sender about resource availability.
The Right Transport Layer for the Right Application
TCP - Applications whose segments must arrive in a very specific sequence to be processed successfully. - Application in which all data must be fully received before any is considered useful. - Example:- Databases, Web Browsers, Email Clients UDP - Applications that can tolerate some data loss during transmission, but delays in transmission are unacceptable. - Example:- Live audio streaming, Live video streaming, Voice over IP (VoIP)
Separating Multiple Communications
The transport layer must be able to separate and manage multiple communications with different transport requirement needs.
Role of the Transport Layer
Tracking Individual Conversations - Each application communicates with one or more applications on one or more remote hosts. It is the responsibility of the transport layer to maintain and track these multiple conversations. Segmenting Data and Reassembling Segments - Transport layer protocols have services that segment the application data into blocks of data that are an appropriate size. This service includes the encapsulation required on each piece of data. Identifying the Applications - Transport layer assigns each application an identifier called a port number. Each software process that needs to access the network is assigned a port number unique in that host. The transport layer uses ports to identify the application or service.
User Datagram Protocol (UDP)
UDP provides just the basic functions for delivering data segments between the appropriate applications, with very little overhead and data checking. UDP is known as a best-effort delivery protocol. With UDP, there are no transport layer processes that inform the sender if successful delivery has occurred.
