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../../protocols/ppp.rst:13
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The first solution to transport IP packets over a serial line was proposed in :rfc:`1055` and is known as `Serial Line IP` (SLIP). SLIP is a simple character stuffing technique applied to IP packets. SLIP defines two special characters : `END` (decimal 192) and `ESC` (decimal 219). `END` appears at the beginning and at the end of each transmitted IP packet and the sender adds `ESC` before each `END` character inside each transmitted IP packet. SLIP only supports the transmission of IP packets and it assumes that the two communicating hosts/routers have been manually configured with each other's IP address. SLIP was mainly used over links offering bandwidth of often less than 20 Kbps. On such a low bandwidth link, sending 20 bytes of IP header followed by 20 bytes of TCP header for each TCP segment takes a lot of time. This initiated the development of a family of compression techniques to efficiently compress the TCP/IP headers. The first header compression technique proposed in :rfc:`1144` was designed to exploit the redundancy between several consecutive segments that belong to the same TCP connection. In all these segments, the IP addresses and port numbers are always the same. Furthermore, fields such as the sequence and acknowledgment numbers do not change in a random way. :rfc:`1144` defined simple techniques to reduce the redundancy found in successive segments. The development of header compression techniques continued and there are still improvements being developed now :rfc:`5795`.
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../../protocols/routing.rst:14
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Organisation of a small Internet
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../../protocols/rpc.rst:10
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In traditional programming languages, `procedure calls` allow programmers to better structure their code. Each procedure is identified by a name, a return type and a set of parameters. When a procedure is called, the current flow of program execution is diverted to execute the procedure. This procedure uses the provided parameters to perform its computation and returns one or more values. This programming model was designed with a single host in mind. In a nutshell, most programming languages support it as follows :
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../../protocols/ssh.rst:28
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From a security viewpoint, the main drawback of :term:`telnet` is that all the information, including the usernames, passwords and commands, is sent in cleartext over a TCP connection. This implies that an eavesdropper could easily capture the passwords used by anyone on an unprotected network. Various software tools exist to automate this collection of information. For this reason, :term:`telnet` is rarely used today to access remote computers. It is usually replaced by :term:`ssh` or similar protocols.
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../../protocols/tcp.rst:14
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TCP provides a reliable bytestream, connection-oriented transport service on top of the unreliable connectionless network service provided by :term:`IP`. TCP is used by a large number of applications, including :
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../../protocols/tls.rst:17
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The TLS protocol was designed to be usable by a wide range of applications that use the transport layer to reliably exchange information. TLS is mainly used over the TCP protocol. There are variants of TLS that operate over SCTP :rfc:`3436` or UDP :rfc:`6347`, but these are outside the scope of this chapter.
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../../protocols/transport-service.rst:10
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the `connectionless` service
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../../protocols/transport.rst:11
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Several transport protocols have been designed to provide a richer service to the applications. The two most widely deployed transport protocols on the Internet are the User Datagram Protocol (UDP) and the Transmission Control Protocol (TCP). A third important transport protocol, the Stream Control Transmission Protocol (SCTP) :rfc:`4960` appeared in the early 2000s. It is currently used by some particular applications such as signaling in Voice over IP networks. SCTP was described in the second edition of the e-book.
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../../protocols/udp.rst:13
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the UDP service cannot deliver SDUs that are larger than 65467 bytes [#fmtuudp]_
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../../protocols/wifi.rst:9
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In the 1970s, after the first experiments with ALOHANet, interest in wireless networks grew. Many experiments were done on and outside the ARPANet. One of these experiments was the `first mobile phone <http://news.bbc.co.uk/2/hi/programmes/click_online/8639590.stm>`_ , which was developed and tested in 1973. This experimental mobile phone was the starting point for the first generation analog mobile phones. Given the growing demand for mobile phones, it was clear that the analog mobile phone technology was not sufficient to support a large number of users. To support more users and new services, researchers in several countries worked on the development of digital mobile telephones. In 1987, several European countries decided to develop the standards for a common cellular telephone system across Europe : the `Global System for Mobile Communications` (GSM). Since then, the standards have evolved and more than three billion users are connected to GSM networks today.
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../../bibliography.rst:13
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LAN/MAN Standards Committee of the IEEE Computer Society. IEEE Standard for Information Technology - Telecommunications and information exchange between systems - local and metropolitan area networks - specific requirements - Part 3 : Carrier Sense multiple access with collision detection (CSMA/CD) access method and physical layer specification. IEEE, 2000. Available from http://standards.ieee.org/getieee802/802.3.html
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../../glossary.rst:183
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AIMD
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../../exercises/intro.rst:12
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A good understanding of the topics covered by this e-book can only be obtained by solving the proposed exercises. Reading the e-book from the first to the last page is not sufficient to get a detailed knowledge of computer networking.
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../../exercises/ex-sharing.rst:17
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To understand the operation of Medium Access Control algorithms, it is often interesting to use a geometric representation of the transmission of frames on a shared medium. This representation is suitable if the communicating devices are attached to a single cable. Consider a simple scenario with a host connected at one end of a cable. For simplicity, let us consider a cable that has a length of one kilometer. Let us also consider that the propagation delay of the electrical signal is five microseconds per kilometer. The figure below shows the transmission of a 2000 bits frame at 100 Mbps by host A on the cable.
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../../exercises/reliability.rst:16
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Transmission links have sometimes different upstream and downstream bandwidths. A typical example are access networks that use ADSL (Asymmetric Digital Subscriber Lines). Consider two hosts connected via an ADSL link having an upstream bandwidth of 1 Mbps and a downstream bandwidth of 50 Mbps. The propagation delay between the two hosts is 10 milliseconds. What is the maximum throughput, expressed in frames/second, that the alternating bit protocol can obtain on this link if each data frame has a length of 125 bytes and acknowledgments are 25 bytes long. Same question if the protocol is modified to support 1500 bytes long data frames.
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../../exercises/network.rst:16
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The network below uses port forwarding with flat addresses. The network boots and all hosts start one after the other. Explain at each step how the packets are forwarded and how the port forwarding tables of the network nodes are modified. Host `C` sends a packet to host `B`. Some time later, host `A` sends a packet to host `C`. Finally, host `B` sends a packet to host `A`.
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../../exercises/sockets.rst:98
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Networked applications were usually implemented by using the :term:`socket` :term:`API`. This API was designed when TCP/IP was first implemented in the `Unix BSD`_ operating system [Sechrest]_ [LFJLMT]_, and has served as the model for many APIs between applications and the networking stack in an operating system. Although the socket API is very popular, other APIs have also been developed. For example, the STREAMS API has been added to several Unix System V variants [Rago1993]_. The socket API is supported by most programming languages and several textbooks have been devoted to it. Users of the C language can consult [DC2009]_, [Stevens1998]_, [SFR2004]_ or [Kerrisk2010]_. The Java implementation of the socket API is described in [CD2008]_ and in the `Java tutorial <http://java.sun.com/docs/books/tutorial/networking/sockets/index.html>`_. In this section, we will use the C socket API to illustrate the key concepts.
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../../principles/referencemodels.rst:19
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The five layers reference model
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../../principles/network.rst:15
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The main objective of the network layer is to allow hosts, connected to different networks, to exchange information through intermediate systems called :term:`router`. The unit of information in the network layer is called a :term:`packet`.
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../../principles/reliability.rst:31
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To enable the two hosts to exchange information, they need to be linked together by some kind of physical media. Computer networks have used various types of physical media to exchange information, notably :
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