cnp3-ebook/principles/transport

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49 ../../principles/transport.rst:130

	English The network layer ensures the delivery of packets on a hop-by-hop basis through intermediate nodes. As such, it provides a service to the upper layer. In practice, this layer is usually the `transport layer` that improves the service provided by the `network layer` to make it usable by applications. 300/10000 · 300
50 ../../principles/transport.rst:158

	English Most networks use a datagram organization and provide a simple service which is called the `connectionless service`. 116/10000 · 116
51 ../../principles/transport.rst:160

	English The figure below provides a representation of the connectionless service as a `time-sequence diagram`. The user on the left, having address `S`, issues a `Data.request` primitive containing Service Data Unit (SDU) `M` that must be delivered by the service provider to destination `D`. The dashed line between the two primitives indicates that the `Data.indication` primitive that is delivered to the user on the right corresponds to the `Data.request` primitive sent by the user on the left. 491/10000 · 491
52 ../../principles/transport.rst:179

	English There are several possible implementations of the connectionless service. Before studying these realizations, it is useful to discuss the possible characteristics of the connectionless service. A `reliable connectionless service` is a service where the service provider guarantees that all SDUs submitted in `Data.requests` by a user will eventually be delivered to their destination. Such a service would be very useful for users, but guaranteeing perfect delivery is difficult in practice. For this reason, network layers usually support an `unreliable connectionless service`. 579/10000 · 579
53 ../../principles/transport.rst:181

	English An `unreliable connectionless` service may suffer from various types of problems compared to a `reliable connectionless service`. First of all, an `unreliable connectionless service` does not guarantee the delivery of all SDUs. This can be expressed graphically by using the time-sequence diagram below. 303/10000 · 303
54 ../../principles/transport.rst:198

	English In practice, an `unreliable connectionless service` will usually deliver a large fraction of the SDUs. However, since the delivery of SDUs is not guaranteed, the user must be able to recover from the loss of any SDU. 216/10000 · 216
55 ../../principles/transport.rst:200

	English A second imperfection that may affect an `unreliable connectionless service` is that it may duplicate SDUs. Some packets may be duplicated in a network and be delivered twice to their destination. This is illustrated by the time-sequence diagram below. 252/10000 · 252
56 ../../principles/transport.rst:218

	English Finally, some unreliable connectionless service providers may deliver to a destination a different SDU than the one that was supplied in the `Data.request`. This is illustrated in the figure below. 197/10000 · 197
57 ../../principles/transport.rst:234

	English As the transport layer is built on top of the network layer, it is important to know the key features of the network layer service. In this book, we only consider the `connectionless network layer service` which is the most widespread. Its main characteristics are : 266/10000 · 266
58 ../../principles/transport.rst:236

	English the `connectionless network layer service` can only transfer SDUs of limited size 83/10000 · 83
59 ../../principles/transport.rst:237

	English the `connectionless network layer service` may discard SDUs 59/10000 · 59
60 ../../principles/transport.rst:238

	English the `connectionless network layer service` may corrupt SDUs 59/10000 · 59
61 ../../principles/transport.rst:239

	English the `connectionless network layer service` may delay, reorder or even duplicate SDUs 84/10000 · 84
62 ../../principles/transport.rst:242

	English 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. 647/10000 · 647
63 ../../principles/transport.rst:246

	English Transport layer services 24/10000 · 24
64 ../../principles/transport.rst:248

	English When two applications need to communicate, they need to structure their exchange of information. Structuring this exchange of information requires solving two different problems. The first problem is how to represent the information being exchanged knowing that the two applications may be running on hosts that use different operating systems, different processors and have different conventions to store information. This requires a common syntax to transfer the information between the two applications. For this chapter, let us assume that this syntax exists and that the two applications simply need to exchange bytes. We will discuss later how more complex data can be encoded as sequences of bytes to be exchanged. The second problem is how to organize the interactions between the application and the underlying network. From the application's viewpoint, the `network` will appear as the `transport layer` service. This `transport layer` can provide three types of services to the applications : 1003/10000 · 1003
65 ../../principles/transport.rst:250

	English the `connectionless service` 28/10000 · 28
66 ../../principles/transport.rst:251

	English the `connection oriented service` 33/10000 · 33
67 ../../principles/transport.rst:252

	English the `request-response service` 30/10000 · 30
68 ../../principles/transport.rst:255

	English The connectionless service 26/10000 · 26

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