cnp3-ebook/principles/transport

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39 ../../principles/transport.rst:101

	English Besides character strings, some applications also need to exchange 16 bits and 32 bits fields such as integers. A naive solution would have been to send the 16- or 32-bits field as it is encoded in the host's memory. Unfortunately, there are different methods to store 16- or 32-bits fields in memory. Some CPUs store the most significant byte of a 16-bits field in the first address of the field while others store the least significant byte at this location. When networked applications running on different CPUs exchange 16 bits fields, there are two possibilities to transfer them over the transport service : 613/10000 · 613
40 ../../principles/transport.rst:103

	English send the most significant byte followed by the least significant byte 69/10000 · 69
41 ../../principles/transport.rst:104

	English send the least significant byte followed by the most significant byte 69/10000 · 69
42 ../../principles/transport.rst:106

	English The first possibility was named `big-endian` in a note written by Cohen [Cohen1980]_ while the second was named `little-endian`. Vendors of CPUs that used `big-endian` in memory insisted on using `big-endian` encoding in networked applications while vendors of CPUs that used `little-endian` recommended the opposite. Several studies were written on the relative merits of each type of encoding, but the discussion became almost a religious issue [Cohen1980]_. Eventually, the Internet chose the `big-endian` encoding, i.e. multi-byte fields are always transmitted by sending the most significant byte first, :rfc:`791` refers to this encoding as the :term:`network-byte order`. Most libraries [#fhtonl]_ used to write networked applications contain functions to convert multi-byte fields from memory to the network byte order and the reverse. 844/10000 · 844
43 ../../principles/transport.rst:108

	English Besides 16 and 32 bit words, some applications need to exchange data structures containing bit fields of various lengths. For example, a message may be composed of a 16 bits field followed by eight, one bit flags, a 24 bits field and two 8 bits bytes. Internet protocol specifications will define such a message by using a representation such as the one below. In this representation, each line corresponds to 32 bits and the vertical lines are used to delineate fields. The numbers above the lines indicate the bit positions in the 32-bits word, with the high order bit at position `0`. 587/10000 · 587
44 ../../principles/transport.rst:114

	English Message format 14/10000 · 14
45 ../../principles/transport.rst:116

	English 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 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 The transport layer 19/10000 · 19
48 ../../principles/transport.rst:126

	English 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 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

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