cnp3-ebook/principles/transport

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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
69 ../../principles/transport.rst:257

	English The `connectionless service` that we have described earlier is frequently used by users who need to exchange small SDUs. It can be easily built on top of the connectionless network layer service that we have described earlier. Users needing to either send or receive several different and potentially large SDUs, or who need structured exchanges often prefer the `connection-oriented service`. 393/10000 · 393
70 ../../principles/transport.rst:260

	English The connection-oriented service 31/10000 · 31
71 ../../principles/transport.rst:264

	English 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. 906/10000 · 906
72 ../../principles/transport.rst:267

	English The establishment of a connection can be modeled by using four primitives : `Connect.request`, `Connect.indication`, `Connect.response` and `Connect.confirm`. The `Connect.request` primitive is used to request the establishment of a connection. The main parameter of this primitive is the `address` of the destination user. The service provider delivers a `Connect.indication` primitive to inform the destination user of the connection attempt. If it accepts to establish a connection, it responds with a `Connect.response` primitive. At this point, the connection is considered to be established and the destination user can start sending SDUs over the connection. The service provider processes the `Connect.response` and will deliver a `Connect.confirm` to the user who initiated the connection. The delivery of this primitive terminates the connection establishment phase. At this point, the connection is considered to be open and both users can send SDUs. A successful connection establishment is illustrated below. 1021/10000 · 1021
73 ../../principles/transport.rst:288

	English The example above shows a successful connection establishment. However, in practice not all connections are successfully established. One reason is that the destination user may not agree, for policy or performance reasons, to establish a connection with the initiating user at this time. In this case, the destination user responds to the `Connect.indication` primitive by a `Disconnect.request` primitive that contains a parameter to indicate why the connection has been refused. The service provider will then deliver a `Disconnect.indication` primitive to inform the initiating user. 587/10000 · 587
74 ../../principles/transport.rst:309

	English A second reason is when the service provider is unable to reach the destination user. This might happen because the destination user is not currently attached to the network or due to congestion. In these cases, the service provider responds to the `Connect.request` with a `Disconnect.indication` primitive whose `reason` parameter contains additional information about the failure of the connection. 401/10000 · 401
75 ../../principles/transport.rst:329

	English Once the connection has been established, the service provider supplies two data streams to the communicating users. The first data stream can be used by the initiating user to send SDUs. The second data stream allows the responding user to send SDUs to the initiating user. The data streams can be organized in different ways. A first organization is the `message-mode` transfer. With the `message-mode` transfer, the service provider guarantees that one and only one `Data.indication` will be delivered to the endpoint of the data stream for each `Data.request` primitive issued by the other endpoint. The `message-mode` transfer is illustrated in the figure below. The main advantage of the `message-transfer` mode is that the recipient receives exactly the SDUs that were sent by the other user. If each SDU contains a command, the receiving user can process each command as soon as it receives a SDU. 905/10000 · 905

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