2. cnp3-ebook
3. principles/transport
4. English
5. Zen

44 ../../principles/transport.rst:114
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Message format 14/10000 · 14
45 ../../principles/transport.rst:116
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The message mentioned above will be transmitted starting from the upper 32-bits word in network byte order. The first field is encoded in 16 bits. It is followed by eight one bit flags (`A-H`), a 24 bits field whose high order byte is shown in the first line and the two low order bytes appear in the second line followed by two one byte fields. This ASCII representation is frequently used when defining binary protocols. We will use it for all the binary protocols that are discussed in this book. 499/10000 · 499
46 ../../principles/transport.rst:120
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The peer-to-peer model emerged during the last ten years as another possible architecture for networked applications. In the traditional client-server model, hosts act either as servers or as clients and a server serves a large number of clients. In the peer-to-peer model, all hosts act as both servers and clients and they play both roles. The peer-to-peer model has been used to develop various networked applications, ranging from Internet telephony to file sharing or Internet-wide filesystems. A detailed description of peer-to-peer applications may be found in [BYL2008]_. Surveys of peer-to-peer protocols and applications may be found in [AS2004]_ and [LCP2005]_. 672/10000 · 672
47 ../../principles/transport.rst:124 • ../../principles/transport.rst:552
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The transport layer 19/10000 · 19
48 ../../principles/transport.rst:126
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 425/10000 · 425
49 ../../principles/transport.rst:130
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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 ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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 ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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 ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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 ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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 ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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 ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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 ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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 ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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 ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — the `connectionless network layer service` can only transfer SDUs of *limited size* 83/10000 · 83
59 ../../principles/transport.rst:237
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — the `connectionless network layer service` may discard SDUs 59/10000 · 59
60 ../../principles/transport.rst:238
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — the `connectionless network layer service` may corrupt SDUs 59/10000 · 59
61 ../../principles/transport.rst:239
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — the `connectionless network layer service` may delay, reorder or even duplicate SDUs 84/10000 · 84
62 ../../principles/transport.rst:242
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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 ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Transport layer services 24/10000 · 24