Transport Layer

The Internet has evolved into an extremely large complex system and changed many important aspects of our lives. Like any complex engineering system, the design of the Internet is carried out in a modular way, where each main functional module is called a “layer”. One of the layering structures often used is the five-layer model consisting of the physical layer, the link layer, the network layer, the transport layer and the application layer1. See Figure 1 for a simple illustration. The sending and receiving computers each run analogous stacks, with data being passed down the stack from the sending application, and then up the receiver’s stack to the receiving application. The physical layer is the part which actually moves information from one place to another, such as by varying voltages on wires, or generating electromagnetic waves. The application layer is the part with which users interact, such as the hypertext transport protocol (HTTP) used to browse web, or the simple mail transfer protocol (SMTP) used to send email. Each layer consists of protocols to specify such things as the data format, the procedure for exchanging data, the allocation of resources, and the actions that need to be taken in different circumstances. This protocol can be implemented in either software or hardware or both. This article concerns transport layer protocols and their associated algorithms, mainly focusing on the wireline Internet but also discussing some other types of networks such as wireless ones. The transport layer manages the end-to-end transportation of packets across a network. Its role is to connect application processes running on end hosts as seamlessly as possible, as if the two end applications were connected by a reliable dedicated link, thus making the network “invisible”. To do this, it must manage several non-idealities of real networks: shared links, data loss and duplication, contention for resources, and variability of delay. By examining these functionalities in turn in sections II to V, we will provide a brief introduction to this important layer, including its functions and implementation, with an emphasis on the underlying ideas and fundamentals. We will also discuss possible directions for the future evolution of the transport layer and suggest some further reading in sections VI and VII.