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the connectionless network layer service never discards SDUs the connectionless network layer service never delays, reorders nor duplicate SDUs the connectionless network layer service can support SDUs of any size We will then remove each of these assumptions one after the other in order to better understand the mechanisms used to solve each imperfection. Reliable data transfer on top of a perfect network service The transport layer entity interacts with both a user in the application layer and an entity in the network layer. According to the reference model, these interactions will be performed using DATA.req and DATA.ind primitives.However, to simplify the presentation and to avoid confusion between a DATA.req primitive issued by the user of the transport layer entity, and a DATA.req issued by the transport layer entity itself, we will use the following terminology : the interactions between the user and the transport layer entity are represented by using the classical DATA.req, DATA.ind primitives the interactions between the transport layer entity and the network layer service are represented by using send instead of DATA.req and recvd instead of DATA.ind This is illustrated in the figure below.Transport entity Data.req Data.ind Send Recvd Figure Interactions between the transport layer, its user, and its network layer provider When running on top of a perfect connectionless network service, a transport level entity can simply issue a send(SDU) upon arrival of a DATA.req(SDU). Similarly, the receiver issues a DATA.ind(SDU) upon receipt of a recvd(SDU). Such a simple protocol is sufficient when a single SDU is sent. Unfortunately, this is not always sufficient to ensure a reliable delivery of the SDUs. Consider the case where aclient sends tens of SDUs to a server. If the server is faster that the client, it will be able to receive and process all the segments sent by the client and deliver their content to its user. However, if the server is slower than the client, problems may arise. The transport layer entity contains buffers to store SDUs that have been received as a Data.request from the application but have not yet been sent via the network service. If the application is faster than the network layer, the buffer becomes full and the operating system suspends the application to let the transport entity empty its transmission queue. The transport entity also uses a buffer to store the segments received from the network layer that have not yet been processed by the application. If the application is slow to process the data, this buffer becomes full and the transport entity is not able to accept anymore the segments from the network layer. The buffers of the transport entity have a limited size 2 and if they overflow, the transport entity is forced to
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