English
An important point to note about the connection-oriented service is its reliability. A `connection-oriented` service can only guarantee the correct delivery of all SDUs provided that the connection has been released gracefully. This implies that while the connection is active, there is no guarantee for the actual delivery of the SDUs exchanged as the connection may need to be abruptly released at any time.
The request-response service
The `request-response service` is a compromise between the `connectionless service` and the `connection-oriented service`. Many applications need to send a small amount of data and receive a small amount of information back. This is similar to procedure calls in programming languages. A call to a procedure takes a few arguments and returns a simple answer. In a network, it is sometimes useful to execute a procedure on a different host and receive the result of the computation. Executing a procedure on another host is often called Remote Procedure Call. It is possible to use the `connectionless service` for this application. However, since this service is usually unreliable, this would force the application to deal with any type of error that could occur. Using the `connection oriented service` is another alternative. This service ensures the reliable delivery of the data, but a connection must be created before the beginning of the data transfer. This overhead can be important for applications that only exchange a small amount of data.
The `request-response service` allows to efficiently exchange small amounts of information in a request and associate it with the corresponding response. This service can be depicted by using the time-sequence diagram below.
Services and layers
In the previous sections, we have described services that are provided by the transport layer. However, it is important to note that the notion of service is more general than in the transport layer. As explained earlier, the network layer also provides a service, which in most networks is an unreliable connectionless service. There are network layers that provide a connection-oriented service. Similarly, the datalink layer also provides services. Some datalink layers will provide a connectionless service. This will be the case in Local Area Networks for examples. Other datalink layers, e.g. in public networks, provide a connection oriented service.
The transport layer entity interacts with both a user in the application layer and the network layer. It improves the network layer service to make it usable by applications. From the application's viewpoint, the main limitations of the network layer service come from its unreliable service:
the network layer may corrupt data;
the network layer may loose data;
the network layer may not deliver data in-order;
the network layer has an upper bound on maximum length of the data;
the network layer may duplicate data.
To deal with these issues, the transport layer includes several mechanisms that depend on the service that it provides. It interacts with both the applications and the underlying network layer.
Interactions between the transport layer, its user, and its network layer provider
We have already described in the datalink layers mechanisms to deal with data losses and transmission errors. These techniques are also used in the transport layer.
Connectionless transport
The simplest service that can be provided in the transport layer is the connectionless transport service. Compared to the connectionless network layer service, this transport service includes two additional features :
an `error detection` mechanism that allows detecting corrupted data
a `multiplexing technique` that enables several applications running on one host to exchange information with another host
To exchange data, the transport protocol encapsulates the SDU produced by its user inside a `segment`. The `segment` is the unit of transfer of information in the transport layer. Transport layer entities always exchange segments. When a transport layer entity creates a segment, this segment is encapsulated by the network layer into a packet which contains the segment as its payload and a network header. The packet is then encapsulated in a frame to be transmitted in the datalink layer.