计算机网络英文课件：Chapter6 The Transport Layer.ppt

Chapter 6 of the "The Transport Layer" presentation delves into the core concepts and functionalities of the transport layer in computer networking. This layer is crucial as it serves as a bridge between the communication subnet, which deals with packet delivery, and the resource subnet, providing a higher level of reliability and abstraction for applications. **6.1 The Transport Service** The transport layer's primary purpose is to offer a more dependable service to end-users compared to the underlying network service. It does this by establishing connections solely at the end hosts, allowing users to inquire about missing or received data and resume transmission where it was interrupted. This layer ensures that applications can communicate seamlessly without needing to understand the complexities of the network infrastructure. **6.2 Elements of Transport Protocols** Transport protocols govern how data is transmitted between processes running on different hosts. Key elements include: 1. **Error Control**: The transport layer implements mechanisms to detect and correct errors that might occur during transmission, ensuring the integrity of the data. 2. **Sequencing**: Data is often split into segments for efficient transmission, and the transport layer ensures these segments are reassembled in the correct order at the receiving end. 3. **Flow Control**: This mechanism regulates the rate at which data is sent to prevent overwhelming the receiver, maintaining a balance between efficiency and receiver capacity. 4. **Connection Establishment and Termination**: Establishing a connection before data transfer allows the negotiation of parameters and ensures both ends are ready to communicate. **6.3 User Datagram Protocol (UDP)** UDP is a connectionless transport protocol, meaning it does not establish a dedicated connection before sending data. It is lightweight and efficient but offers limited reliability features. UDP is suitable for applications that require low latency and can tolerate some data loss, such as streaming media or online gaming. **6.4 Transmission Control Protocol (TCP)** TCP, on the other hand, is a connection-oriented protocol, establishing a full-duplex connection before transmitting data. It guarantees in-order, reliable delivery through features like sequence numbers, acknowledgment, and retransmission. TCP is widely used for applications that require guaranteed data integrity, such as web browsing, email, and file transfers. **Transport Layer vs. Network Layer** Both layers address error control and sequencing, but the transport layer focuses on end-to-end communication while the network layer is responsible for individual packet delivery. The transport layer operates at a higher level, typically over a simpler, established connection, whereas the network layer deals with the complexity of routing packets across multiple networks. **Transport Layer vs. Data Link Layer** Similarities lie in error control and flow management, but the transport layer typically has a simpler connection establishment process. The data link layer is more involved in establishing connections within a subnet, requiring explicit addressing and more complex connection setup, and it manages in/out-buffers on a more granular level. In conclusion, the transport layer is a critical component in computer networking, providing a common interface for application developers, abstracting the underlying network complexity, and ensuring reliable, concurrent communication between processes. The choice between UDP and TCP depends on the specific needs of the application, balancing speed and reliability. Understanding these principles is essential for designing and implementing effective networked systems.