Introduction

How does the transport layer deal with these signals that are getting regarding congestion, as well as how does the transport layer work in general?

Right now we’re nearly the top of the OSI structure model:

We have already seen this picture multiple times:

We first talked about what happens with the links between routers or between endpoints and a router. Then we talked about what happens in the whole network, from the sender to the destination, what are the routing protocols, etc.

Now we will look into how do the end hosts regulate traffic. The transport layer is the first layer which only runs on the end hosts and not on the intermediaries. So, the end hosts do not really care about how the routing works: they only react to the signals that they get and they manage to traffic on their machines.

In this lecture we will answer the following questions:

• How do we map network data to applications?
• How does UDP realize an unreliable transport layer?
• How does TCP realize a reliable transport layer?
  • How does TCP handle connection establishment and disconnect, error control (e.g., retransmission), flow control and congestion control?
• Are there other transport layer protocols, different than TCP and UDP?

Units of transferred data

In the physical layer we were talking about bits. In the data link layer we were talking about frames. In the network layer we were talking about packets.

Now, in the Transport Layer, we will talk about segments.

Note packet is not only used in the context of network layer but also sometimes as a generic word for data grouped in a structured manner for transfer.

Addressing