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the network layer may duplicate data.
the network layer may duplicate data.
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the network layer has an upper bound on maximum length of the data;
the network layer has an upper bound on maximum length of the data;
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the network layer may not deliver data in-order;
the network layer may not deliver data in-order;
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the network layer may loose data;
the network layer may loose data;
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the network layer may corrupt data;
the network layer may corrupt data;
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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 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:
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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.
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.
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The third phase of a connection is its release. As a connection involves three parties (two users and one service provider), any of them can request the termination of the connection. Usually, connections are terminated upon request of one user once the data transfer is finished. However, sometimes the service provider may be forced to terminate a connection. This can be due to lack of resources inside the service provider or because one of the users is not reachable anymore through the network. In this case, the service provider will issue `Disconnect.indication` primitives to both users. These primitives will contain, as parameter, some information about the reason for the termination of the connection. Unfortunately, as illustrated in the figure below, when a service provider is forced to terminate a connection it cannot guarantee that all SDUs sent by each user have been delivered to the other user. This connection release is said to be abrupt as it can cause losses of data.
The third phase of a connection is its release. As a connection involves three parties (two users and one service provider), any of them can request the termination of the connection. Usually, connections are terminated upon request of one user once the data transfer is finished. However, sometimes the service provider may be forced to terminate a connection. This can be due to lack of resources inside the service provider or because one of the users is not reachable anymore through the network. In this case, the service provider will issue `Disconnect.indication` primitives to both users. These primitives will contain, as parameter, some information about the reason for the termination of the connection. Unfortunately, as illustrated in the figure below, when a service provider is forced to terminate a connection it cannot guarantee that all SDUs sent by each user have been delivered to the other user. This connection release is said to be abrupt as it can cause losses of data.
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An invocation of the `connection-oriented service` is divided into three phases. The first phase is the establishment of a `connection`. A `connection` is a temporary association between two users through a service provider. Several connections may exist at the same time between any pair of users. Once established, the connection is used to transfer SDUs. `Connections` usually provide one bidirectional stream supporting the exchange of SDUs between the two users that are associated through the `connection`. This stream is used to transfer data during the second phase of the connection called the `data transfer` phase. The third phase is the termination of the connection. Once the users have finished exchanging SDUs, they request the service provider to terminate the connection. As we will see later, there are also some cases where the service provider may need to terminate a connection itself.
An invocation of the `connection-oriented service` is divided into three phases. The first phase is the establishment of a `connection`. A `connection` is a temporary association between two users through a service provider. Several connections may exist at the same time between any pair of users. Once established, the connection is used to transfer SDUs. `Connections` usually provide one bidirectional stream supporting the exchange of SDUs between the two users that are associated through the `connection`. This stream is used to transfer data during the second phase of the connection called the `data transfer` phase. The third phase is the termination of the connection. Once the users have finished exchanging SDUs, they request the service provider to terminate the connection. As we will see later, there are also some cases where the service provider may need to terminate a connection itself.
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These imperfections of the `connectionless network layer service` are caused by the operations of the `network layer`. This `layer` is able to deliver packets to their intended destination, but it cannot guarantee their delivery. The main cause of packet losses and errors are the buffers used on the network nodes. If the buffers of one of these nodes becomes full, all arriving packets must be discarded. This situation frequently happens in practice [#fqueuesize]_. Transmission errors can also affect packet transmissions on links where reliable transmission techniques are not enabled or because of errors in the buffers of the network nodes.
These imperfections of the `connectionless network layer service` are caused by the operations of the `network layer`. This `layer` is able to deliver packets to their intended destination, but it cannot guarantee their delivery. The main cause of packet losses and errors are the buffers used on the network nodes. If the buffers of one of these nodes becomes full, all arriving packets must be discarded. This situation frequently happens in practice [#fqueuesize]_. Transmission errors can also affect packet transmissions on links where reliable transmission techniques are not enabled or because of errors in the buffers of the network nodes.
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A network is always designed and built to enable applications running on hosts to exchange information. In a previous chapter, we have explained the principles of the `network layer` that enables hosts connected to different types of datalink layers to exchange information through routers. These routers act as relays in the network layer and ensure the delivery of packets between any pair of hosts attached to the network.
A network is always designed and built to enable applications running on hosts to exchange information. In a previous chapter, we have explained the principles of the `network layer` that enables hosts connected to different types of datalink layers to exchange information through routers. These routers act as relays in the network layer and ensure the delivery of packets between any pair of hosts attached to the network.
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Organization of the information flow. For many applications, the flow of information must be structured and there are precedence relationships between the different types of information. In the time example above, Alice must greet Bob before asking for the current time. Alice would not ask for the current time first and greet Bob afterwards. Such precedence relationships exist in networked applications as well. For example, a server must receive a username and a valid password before accepting more complex commands from its clients.
Organization of the information flow. For many applications, the flow of information must be structured and there are precedence relationships between the different types of information. In the time example above, Alice must greet Bob before asking for the current time. Alice would not ask for the current time first and greet Bob afterwards. Such precedence relationships exist in networked applications as well. For example, a server must receive a username and a valid password before accepting more complex commands from its clients.
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Syntactical rules that precisely define the format of the messages that are exchanged. As computers only process bits, the syntactical rules specify how information is encoded as bit strings.
Syntactical rules that precisely define the format of the messages that are exchanged. As computers only process bits, the syntactical rules specify how information is encoded as bit strings.
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Human conversations can be very formal, e.g. when soldiers communicate with their hierarchy, or informal such as when friends discuss. Computers that communicate are more akin to soldiers and require well-defined rules to ensure a successful exchange of information. There are two types of rules that define how information can be exchanged between computers :
Human conversations can be very formal, e.g. when soldiers communicate with their hierarchy, or informal such as when friends discuss. Computers that communicate are more akin to soldiers and require well-defined rules to ensure a successful exchange of information. There are two types of rules that define how information can be exchanged between computers :
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There are various types of servers as well as various types of clients. A web server provides information in response to the query sent by its clients. A print server prints documents sent as queries by the client. An email server forwards towards their recipient the email messages sent as queries while a music server delivers the music requested by the client. From the viewpoint of the application developer, the client and the server applications directly exchange messages (the horizontal arrows labeled `Queries` and `Responses` in the above figure), but in practice these messages are exchanged thanks to the underlying layers (the vertical arrows in the above figure). In this chapter, we focus on these horizontal exchanges of messages.
There are various types of servers as well as various types of clients. A web server provides information in response to the query sent by its clients. A print server prints documents sent as queries by the client. An email server forwards towards their recipient the email messages sent as queries while a music server delivers the music requested by the client. From the viewpoint of the application developer, the client and the server applications directly exchange messages (the horizontal arrows labeled `Queries` and `Responses` in the above figure), but in practice these messages are exchanged thanks to the underlying layers (the vertical arrows in the above figure). In this chapter, we focus on these horizontal exchanges of messages.
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The client-server model was the first model to be used to develop networked applications. This model comes naturally from the mainframes and minicomputers that were the only networked computers used until the 1980s. A minicomputer_ is a multi-user system that is used by tens or more users at the same time. Each user interacts with the minicomputer by using a terminal. Such a terminal was mainly a screen, a keyboard and a cable directly connected to the minicomputer.
The client-server model was the first model to be used to develop networked applications. This model comes naturally from the mainframes and minicomputers that were the only networked computers used until the 1980s. A minicomputer_ is a multi-user system that is used by tens or more users at the same time. Each user interacts with the minicomputer by using a terminal. Such a terminal was mainly a screen, a keyboard and a cable directly connected to the minicomputer.
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There are two important models used to organize a networked application. The first and oldest model is the client-server model. In this model, a server provides services to clients that exchange information with it. This model is highly asymmetrical: clients send requests and servers perform actions and return responses. It is illustrated in the figure below.
There are two important models used to organize a networked application. The first and oldest model is the client-server model. In this model, a server provides services to clients that exchange information with it. This model is highly asymmetrical: clients send requests and servers perform actions and return responses. It is illustrated in the figure below.
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