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3. principles/transport
4. English
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31 ../../principles/transport.rst:83
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — In addition, the :term:`ASCII` table also defines several non-printable or control characters. These characters were designed to allow an application to control a printer or a terminal. These control characters include `CR` and `LF`, that are used to terminate a line, and the `Bell` character which causes the terminal to emit a sound. 336/10000 · 336
32 ../../principles/transport.rst:85
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — `carriage return` (`CR`) : `0001101b` 37/10000 · 37
33 ../../principles/transport.rst:86
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — `line feed` (`LF`) : `0001010b` 31/10000 · 31
34 ../../principles/transport.rst:87
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — `Bell`: `0000111b` 18/10000 · 18
35 ../../principles/transport.rst:89
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The :term:`ASCII` characters are encoded as a seven bits field, but transmitted as an eight-bits byte whose high order bit is usually set to `0`. Bytes are always transmitted starting from the high order or most significant bit. 228/10000 · 228
36 ../../principles/transport.rst:91
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Most applications exchange strings that are composed of fixed or variable numbers of characters. A common solution to define the character strings that are acceptable is to define them as a grammar using a Backus-Naur Form (:term:`BNF`) such as the Augmented BNF defined in :rfc:`5234`. A BNF is a set of production rules that generate all valid character strings. For example, consider a networked application that uses two commands, where the user can supply a username and a password. The BNF for this application could be defined as shown in the figure below. 563/10000 · 563
37 ../../principles/transport.rst:97
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — A simple BNF specification 26/10000 · 26
38 ../../principles/transport.rst:99
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The example above defines several terminals and two commands : `usercommand` and `passwordcommand`. The `ALPHA` terminal contains all letters in upper and lower case. In the `ALPHA` rule, `%x41` corresponds to ASCII character code 41 in hexadecimal, i.e. capital `A`. The `CR` and `LF` terminals correspond to the carriage return and linefeed control characters. The `CRLF` rule concatenates these two terminals to match the standard end of line termination. The `DIGIT` terminal contains all digits. The `SP` terminal corresponds to the white space characters. The `usercommand` is composed of two strings separated by white space. In the ABNF rules that define the messages used by Internet applications, the commands are case-insensitive. The rule `"user"` corresponds to all possible cases of the letters that compose the word between brackets, e.g. `user`, `uSeR`, `USER`, `usER`, ... A `username` contains at least one letter and up to 8 letters. User names are case-sensitive as they are not defined as a string between brackets. The `password` rule indicates that a password starts with a letter and can contain any number of letters or digits. The white space and the control characters cannot appear in a `password` defined by the above rule. 1253/10000 · 1253
39 ../../principles/transport.rst:101
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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 ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — send the most significant byte followed by the least significant byte 69/10000 · 69
41 ../../principles/transport.rst:104
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — send the least significant byte followed by the most significant byte 69/10000 · 69
42 ../../principles/transport.rst:106
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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 ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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 ↹ ↵ 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