2979 strings / 128719 words

Translation Source string Target string
cnp3-ebook/exercises/dns — French
Application layer
 
cnp3-ebook/exercises/email — French
Internet email protocols
 
cnp3-ebook/exercises/http — French
The HyperText Transfer Protocol
 
cnp3-ebook/exercises/lan — French
Local Area Networks: The Spanning Tree Protocol and Virtual LANs
 
cnp3-ebook/exercises/routing-policies — French
Inter-domain routing
 
cnp3-ebook/exercises/tcp-2 — French
A closer look at TCP
 
cnp3-ebook/exercises/tcp — French
TCP basics
 
cnp3-ebook/exercises/trace — French
Analyzing packet traces
 
cnp3-ebook/protocols/bgp — French
Interdomain routing
 
cnp3-ebook/protocols/congestion — French
Congestion control
 
cnp3-ebook/protocols/dnssec — French
Securing the Domain Name System
 
cnp3-ebook/protocols/ethernet — French
Ethernet
 
cnp3-ebook/protocols/http — French
The HyperText Transfer Protocol
 
cnp3-ebook/protocols/http2 — French
Making HTTP faster
 
cnp3-ebook/protocols/rpc — French
Remote Procedure Calls
 
cnp3-ebook/protocols/tcp — French
The Transmission Control Protocol
 
cnp3-ebook/protocols/tls — French
Transport Layer Security
 
cnp3-ebook/protocols/udp — French
The User Datagram Protocol
 
cnp3-ebook/bibliography — French
Whenever possible, the bibliography includes stable hypertext links to the references cited.
 
cnp3-ebook/principles/security — French
In the early days, data networks were mainly used by researchers and security was not a concern. A few users were connected and capable of using the network. Almost all the devices attached to the network were openly accessible and users were trusted. As the utilization of the networks grew, security concerns started to appear. In universities, researchers and professors did not always trust their students and required some forms of access control. On standalone computers, the common access control mechanism is the password. A `username` is assigned to each user and when this user wants to access the computer, he or she needs to provide his/her `username` and his/her `password`. Most passwords are composed of a sequence of characters. The strength of the password is function of the difficulty of guessing the characters chosen by each user. Various guidelines have been defined on how to select a good password [#fpasswords]_. Some systems require regular modifications of the passwords chosen by their users.
 
cnp3-ebook/exercises/dns — French
This is an unpolished draft of the third edition of this e-book. If you find any error or have suggestions to improve the text, please create an issue via https://github.com/CNP3/ebook/issues?milestone=5 or help us by providing pull requests to close the existing issues.
 
cnp3-ebook/exercises/email — French
Many Internet protocols are ASCII_-based protocols where the client sends requests as one line of ASCII_ text terminated by `CRLF` and the server replies with one of more lines of ASCII_ text. Using such ASCII_ messages has several advantages compared to protocols that rely on binary encoded messages
 
cnp3-ebook/exercises/http — French
An important difference between HTTP/1.0, HTTP/1.1 and HTTP/2.0 is their utilization of the underlying transport connections. Answer the three questions below to confirm that you understand the difference between these versions of the HTTP protocol.
 
cnp3-ebook/exercises/ipv6 — French
Basic questions on IPv6 Networks
 
cnp3-ebook/exercises/lan — French
Exercises
 
cnp3-ebook/exercises/routing-policies — French
Exercises
 
cnp3-ebook/exercises/routing-protocols — French
Routing protocols play a key role in the Internet since they ensure that all the routers have valid routing tables. In this section, we use IPMininet_ to explore how intradomain and interdomain routing protocols work in practice. IPMininet_ adds an abstraction layer above the actual configuration of the FRRouting daemons that implement these routing protocols.
 
cnp3-ebook/exercises/tcp-2 — French
In this series of exercises, you will explore in more details the operation of TCP and its congestion control scheme. TCP is a very important protocol in today's Internet since most applications use it to exchange data. We first look at TCP in more details by injecting segments in the Linux TCP stack and analyze how the stack reacts. Then we study the TCP congestion control scheme.
 
cnp3-ebook/exercises/tcp — French
TCP is one of the key protocols in today's Internet. A TCP connection always starts with a three-way handshake. The exercises below should help you to improve your understandings of this first phase of a TCP connection.
 
cnp3-ebook/exercises/tls — French
One of the first motivations for the deployment of wide area networks such as the Internet was to enable researchers to connect to distant servers. For many years, these connections were carried out by using a simple application layer protocol such as telnet over a TCP connection. With telnet, all the characters typed by the user are sent in cleartext over the TCP connection. This implies that if someone is able to capture the packets transmitted over the network, he/she can collect sensitive information such as user names or passwords.
 
cnp3-ebook/exercises/trace — French
When debugging networking problems or to figure out performance problems, it is sometimes useful to capture the segments that are exchanged between two hosts and to analyze them.
 
cnp3-ebook/protocols/bgp — French
As explained earlier, the Internet is composed of more than 45,000 different networks [#fasnum]_ called `domains`. Each domain is composed of a group of routers and hosts that are managed by the same organization. Example domains include belnet_, sprint_, level3_, geant_, abilene_, cisco_ or google_ ...
 
cnp3-ebook/protocols/congestion — French
In an internetwork, i.e. a networking composed of different types of networks (such as the Internet), congestion control could be implemented either in the network layer or the transport layer. The congestion problem was clearly identified in the later 1980s and the researchers who developed techniques to solve the problem opted for a solution in the transport layer. Adding congestion control to the transport layer makes sense since this layer provides a reliable data transfer and avoiding congestion is a factor in this reliable delivery. The transport layer already deals with heterogeneous networks thanks to its `self-clocking` property that we have already described. In this section, we explain how congestion control has been added to TCP and how this mechanism could be improved in the future.
 
cnp3-ebook/protocols/dnssec — French
The Domain Name System provides a critical service in the Internet infrastructure since it maps the domain names that are used by end users onto IP addresses. Since end users rely on names to identify the servers that they connect to, any incorrect information distributed by the DNS would direct end users' connections to invalid destinations. Unfortunately, several attacks of this kind occurred in the past. A detailed analysis of the security threats against the DNS appeared in :rfc:`3833`. We consider three of these threats in this section and leave the others to :rfc:`3833`.
 
cnp3-ebook/protocols/ethernet — French
Ethernet was designed in the 1970s at the Palo Alto Research Center [Metcalfe1976]_. The first prototype [#fethernethistory]_ used a coaxial cable as the shared medium and 3 Mbps of bandwidth. Ethernet was improved during the late 1970s and in the 1980s, Digital Equipment, Intel and Xerox published the first official Ethernet specification [DIX]_. This specification defines several important parameters for Ethernet networks. The first decision was to standardize the commercial Ethernet at 10 Mbps. The second decision was the duration of the `slot time`. In Ethernet, a long `slot time` enables networks to span a long distance but forces the host to use a larger minimum frame size. The compromise was a `slot time` of 51.2 microseconds, which corresponds to a minimum frame size of 64 bytes.
 
cnp3-ebook/protocols/http2 — French
During the last decade, a growing number of services have been supported by world wide web servers. The web protocols are not only used to deliver static documents, they are also used to deliver streaming music or video. They also enable clients to use interactive applications including games or productivity applications. These services and applications have more stringent performance requirements than the delivery of static documents. Many researchers and companies have proposed solutions to improve the performance of web services and protocols during the last decade [KR2001]_ [WBK2014]_. We discuss a subset of them in this section.
 
cnp3-ebook/protocols/lan — French
In this section, we review the key characteristics of several datalink layer technologies. We discuss in more detail the technologies that are widely used today. A detailed survey of all datalink layer technologies would be outside the scope of this book.
 
cnp3-ebook/protocols/ppp — French
Many point-to-point datalink layers [#flapb]_ have been developed, starting in the 1960s. In this section, we focus on the protocols that are often used to transport IP packets between hosts or routers that are directly connected by a point-to-point link. This link can be a dedicated physical cable, a leased line through the telephone network or a dial-up connection with modems on the two communicating hosts.
 
cnp3-ebook/protocols/rpc — French
In the previous sections, we have described several protocols that enable humans to exchange messages and access to remote documents. This is not the only usage of computer networks and in many situations applications use the network to exchange information with other applications. When an application needs to perform a large computation on a host, it can sometimes be useful to request computations from other hosts. Many distributed systems have been built by distributing applications on different hosts and using `Remote Procedure Calls` as a basic building block.
 
cnp3-ebook/protocols/ssh — French
One of the initial motivations for building computer networks was to allow users to access remote computers over the networks. In the 1960s and 1970s, the mainframes and the emerging minicomputers were composed of a central unit and a set of terminals connected through serial lines or modems. The simplest protocol that was designed to access remote computers over a network is probably :term:`telnet` :rfc:`854`. :term:`telnet` runs over TCP and a telnet server listens on port `23` by default. The TCP connection used by telnet is bidirectional, both the client and the server can send data over it. The data exchanged over such a connection is essentially the characters that are typed by the user on the client machine and the text output of the processes running on the server machine with a few exceptions (e.g. control characters, characters to control the terminal like VT-100, ...) . The default character set for telnet is the ASCII character set, but the extensions specified in :rfc:`5198` support the utilization of Unicode characters.
 
cnp3-ebook/protocols/tcp — French
The Transmission Control Protocol (TCP) was initially defined in :rfc:`793`. Several parts of the protocol have been improved since the publication of the original protocol specification [#ftcpspecs]_. However, the basics of the protocol remain and an implementation that only supports :rfc:`793` should inter-operate with today's implementation.
 
cnp3-ebook/protocols/tls — French
The Transport Layer Security family of protocols were initially proposed under the name Secure Socket Layer (SSL). The first deployments used this name and many researchers still refer to this security protocol as SSL [FKC1996]_. In this chapter, we use the official name that was standardized by the IETF: TLS for `Transport Layer Security`.
 
cnp3-ebook/protocols/transport-service — French
Networked applications rely on the transport service. As explained earlier, there are two main types of transport services :
 
cnp3-ebook/protocols/transport — French
Transport protocols rely on the service provided by the network layer. On the Internet, the network layer provides a connectionless service. The network layer identifies each (interface of a) host by using an IP address. It enables hosts to transmit packets that contain up to 64 KBytes of payload to any destination reachable through the network. The network layer does not guarantee the delivery of information, cannot detect transmission errors and does not preserve sequence integrity.
 
cnp3-ebook/protocols/udp — French
The User Datagram Protocol (UDP) is defined in :rfc:`768`. It provides an unreliable connectionless transport service on top of the unreliable network layer connectionless service. The main characteristics of the UDP service are :
 
cnp3-ebook/protocols/wifi — French
The radio spectrum is a limited resource that must be shared by everyone. During most of the twentieth century, governments and international organizations have regulated most of the radio spectrum. This regulation controls the utilization of the radio spectrum, in order to prevent interference among different users. A company that wants to use a frequency range in a given region must apply for a license from the regulator. Most regulators charge a fee for the utilization of the radio spectrum and some governments have encouraged competition among companies bidding for the same frequency to increase the license fees.
 
cnp3-ebook/bibliography — French
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 11 : Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications <http://standards.ieee.org/getieee802/802.11.html>`_. IEEE, 1999.
 
cnp3-ebook/exercises/ex-sharing — French
Medium Access Control
 
cnp3-ebook/principles/sharing — French
A network is designed to support a potentially large number of users that exchange information with each other. These users produce and consume information which is exchanged through the network. To support its users, a network uses several types of resources. It is important to keep in mind the different resources that are shared inside the network.
 
cnp3-ebook/principles/security — French
When the first computers were attached to data networks, applications were developed to enable them to access to remote computers through the network. To authenticate the remote users, these applications have also relied on usernames and passwords. When a user connects to a distant computer, she sends her username through the network and then provides her password to confirm her `identity`. This authentication scheme is presented in the time sequence diagram below.
 
cnp3-ebook/exercises/dns — French
The DNS
 
cnp3-ebook/exercises/email — French
the messages exchanged by the client and the server can be easily understood by a developer or network engineer by simply reading the messages
 
cnp3-ebook/exercises/http — French
System administrators who are responsible for web servers often want to monitor these servers and check that they are running correctly. As a HTTP server uses TCP on port 80, the simplest solution is to open a TCP connection on port 80 and check that the TCP connection is accepted by the remote host. However, as HTTP/1.x is an ASCII-based protocol, it is also very easy to write a small script that downloads a web page on the server and compares its content with the expected one. Use `telnet` or `ncat` to verify that a web server is running on host `www.computer-networking.info` [#fhttp]_.
 
cnp3-ebook/exercises/ipv6 — French
Before starting to determine the paths that packets will follow in an IPv6 network, it is important to remember how to convert IPv6 addresses in binary numbers.
 
cnp3-ebook/exercises/lan — French
Consider the switched network shown in Fig. 1. What is the spanning tree that will be computed by 802.1d in this network assuming that all links have a unit cost ? Indicate the state of each port.
 
cnp3-ebook/exercises/routing-policies — French
Consider the interdomain topology shown in the figure below.
 
cnp3-ebook/exercises/routing-protocols — French
Exploring OSPF
 
cnp3-ebook/exercises/tcp-2 — French
Injecting segments in the Linux TCP stack
 
cnp3-ebook/exercises/tcp — French
Once the connection is established, the client and the server can exchange data and acknowledgments.
 
cnp3-ebook/exercises/tls — French
Fortunately, telnet is rarely used without TLS these days and system administrators usually prefer to deploy more secure protocols such as ``ssh``.
 
cnp3-ebook/exercises/trace — French
Several packet trace analysis tools are available, either as commercial or open-source tools. These tools are able to capture all the packets exchanged on a link. Of course, capturing packets require administrator privileges. They can also analyze the content of the captured packets and display information about them. The captured packets can be stored in a file for offline analysis.
 
cnp3-ebook/protocols/bgp — French
Each domain contains a set of routers. From a routing point of view, these domains can be divided into two classes : the `transit` and the `stub` domains. A `stub` domain sends and receives packets whose source or destination are one of its own hosts. A `transit` domain is a domain that provides a transit service for other domains, i.e. the routers in this domain forward packets whose source and destination do not belong to the transit domain. As of this writing, about 85% of the domains in the Internet are stub domains [#fpotaroo]_. A `stub` domain that is connected to a single transit domain is called a `single-homed stub` (e.g., `S1` in the figure below.). A `multihomed stub` is a `stub` domain connected to two or more transit providers (e.g., `S2`).
 
cnp3-ebook/protocols/congestion — French
The TCP congestion control scheme was initially proposed by `Van Jacobson`_ in [Jacobson1988]_. The current specification may be found in :rfc:`5681`. TCP relies on `Additive Increase and Multiplicative Decrease (AIMD)`. To implement :term:`AIMD`, a TCP host must be able to control its transmission rate. A first approach would be to use timers and adjust their expiration times in function of the rate imposed by :term:`AIMD`. Unfortunately, maintaining such timers for a large number of TCP connections can be difficult. Instead, `Van Jacobson`_ noted that the rate of TCP congestion can be artificially controlled by constraining its sending window. A TCP connection cannot send data faster than :math:`\frac{window}{rtt}` where :math:`window` is the maximum between the host's sending window and the window advertised by the receiver.
 
cnp3-ebook/protocols/dnssec — French
The first type of attack is `eavesdropping`. An attacker who can capture packets sent to a DNS resolver or a DNS server can gain valuable information about the DNS names that are used by a given end user. If the attacker can capture all the packets sent to a DNS resolver, he/she can collect a lot of meta data about the domain names used by the end user. Preventing this type of attack has not been an objective of the initial design of the DNS. There are currently discussions with the IETF to carry DNS messages over TLS sessions to protect against such attacks. However, these solutions are not yet widely deployed.
 
cnp3-ebook/protocols/email — French
Before looking at the details of Internet email, let us consider a simple scenario illustrated in the figure below, where Alice sends an email to Bob. Alice prepares her email by using an `email clients`_ and sends it to her email server. Alice's `email server <http://en.wikipedia.org/wiki/Comparison_of_mail_servers>`_ extracts Bob's address from the email and delivers the message to Bob's server. Bob retrieves Alice's message on his server and reads it by using his favorite email client or through his webmail interface.
 
cnp3-ebook/protocols/ethernet — French
The third decision was the frame format. The experimental 3 Mbps Ethernet network built at Xerox used short frames containing 8 bit source and destination addresses fields, a 16 bit type indication, up to 554 bytes of payload and a 16 bit CRC. Using 8 bit addresses was suitable for an experimental network, but it was clearly too small for commercial deployments. Although the initial Ethernet specification [DIX]_ only allowed up to 1024 hosts on an Ethernet network, it also recommended three important changes compared to the networking technologies that were available at that time. The first change was to require each host attached to an Ethernet network to have a globally unique datalink layer address. Until then, datalink layer addresses were manually configured on each host. [DP1981]_ went against that state of the art and noted "`Suitable installation-specific administrative procedures are also needed for assigning numbers to hosts on a network. If a host is moved from one network to another it may be necessary to change its host number if its former number is in use on the new network. This is easier said than done, as each network must have an administrator who must record the continuously changing state of the system (often on a piece of paper tacked to the wall !). It is anticipated that in future office environments, hosts locations will change as often as telephones are changed in present-day offices.`" The second change introduced by Ethernet was to encode each address as a 48 bits field [DP1981]_. 48 bit addresses were huge compared to the networking technologies available in the 1980s, but the huge address space had several advantages [DP1981]_ including the ability to allocate large blocks of addresses to manufacturers. Eventually, other LAN technologies opted for 48 bits addresses as well [IEEE802]_ . The third change introduced by Ethernet was the definition of `broadcast` and `multicast` addresses. The need for `multicast` Ethernet was foreseen in [DP1981]_ and thanks to the size of the addressing space it was possible to reserve a large block of multicast addresses for each manufacturer.
 
cnp3-ebook/protocols/http — French
Many `FTP` clients offered a user interface similar to a Unix shell and allowed clients to browse the file system on the server and to send and retrieve files. `FTP` servers can be configured in two modes :
 
cnp3-ebook/protocols/http2 — French
A first way to improve the performance of the web protocols is to tune the servers that provide content. In the early days, documents were stored on a single server. Clients established TCP connections to this server to retrieve each document. This architecture evolved in several directions. A first way to speedup web services is to avoid unnecessary transmissions. Thanks to the `HEAD` method and the `If-Modified-Since:` header, web browsers can verify that they have the most recent version of a document in their cache.
 
cnp3-ebook/protocols/ipv6 — French
Before explaining the network layer in detail, it is useful to begin by analyzing the service provided by the `datalink` layer. There are many variants of the datalink layer. Some provide a connection-oriented service while others provide a connectionless service. In this section, we focus on connectionless datalink layer services as they are the most widely used. Using a connection-oriented datalink layer causes some problems that are beyond the scope of this chapter. See :rfc:`3819` for a discussion on this topic.
 
cnp3-ebook/protocols/ppp — French
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`.
 
cnp3-ebook/protocols/rpc — French
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 :
 
cnp3-ebook/protocols/ssh — French
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.
 
cnp3-ebook/protocols/tcp — French
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 :
 
cnp3-ebook/protocols/tls — French
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.
 
cnp3-ebook/protocols/transport-service — French
the `connectionless` service
 
cnp3-ebook/protocols/transport — French
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.
 
cnp3-ebook/protocols/udp — French
the UDP service cannot deliver SDUs that are larger than 65467 bytes [#fmtuudp]_
 
cnp3-ebook/protocols/wifi — French
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.
 
cnp3-ebook/bibliography — French
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
 
cnp3-ebook/exercises/ex-sharing — French
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.
 
cnp3-ebook/exercises/reliability — French
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.
 
cnp3-ebook/exercises/dns — French
The Domain Name System (DNS) plays a key role in the Internet today as it allows applications to use fully qualified domain names (FQDN) instead of IPv4 or IPv6 addresses. When using the DNS, it is important to remember the role of the different types of DNS records.
 
cnp3-ebook/exercises/email — French
it is often easy to write a small prototype that implements a part of the protocol
 
cnp3-ebook/exercises/http — French
Instead of using `telnet` on port 80, it is also possible to use a command-line tool such as curl_. Use curl_ with the `--trace-ascii tracefile` option to store in `tracefile` all the information exchanged by curl_ when accessing the server.
 
cnp3-ebook/exercises/ipv6 — French
An IPv6 forwarding table contains a list of IPv6 prefixes with their associated nexthop or outgoing interface. When an IPv6 router receives a packet, it forwards it according to its forwarding table. Note that IPv6 routers forward packets along the *longest match* between the destination address of the packet and the routes in the forwarding table.
 
cnp3-ebook/exercises/lan — French
Fig. 1. A small network composed of Ethernet switches
 
cnp3-ebook/exercises/routing-policies — French
In this network, what are the paths :
 
cnp3-ebook/exercises/routing-protocols — French
We first use IPMininet_ to explore the operation of OSPFv3, the version of OSPF that supports IPv6. We create a simple network with three routers and two hosts as shown in the figure below.
 
cnp3-ebook/exercises/tcp-2 — French
Packet capture tools like tcpdump_ and Wireshark_ are very useful to observe the segments that transport protocols exchange. TCP is a complex protocol that has evolved a lot since its first specification :rfc:`793`. TCP includes a large number of heuristics that influence the reaction of a TCP implementation to various types of events. A TCP implementation interacts with the application through the ``socket`` API.
 
cnp3-ebook/exercises/tcp — French
A connection ends with the transmission of segments that include the `FIN` flag that marks the end of the data transfer.
 
cnp3-ebook/exercises/tls — French
The Transport Layer Security (TLS) protocol is now used by a wide range of applications, even if the most popular one is HTTPS. In the exercises below, you will analyze some of the features of TLS by looking at the packets that are exchanged over a TLS session.
 
cnp3-ebook/exercises/trace — French
tcpdump_ is probably one of the most well known packet capture software. It is able to both capture packets and display their content. tcpdump_ is a text-based tool that can display the value of the most important fields of the captured packets. Additional information about tcpdump_ may be found in :manpage:`tcpdump(1)`.
 
cnp3-ebook/protocols/bgp — French
Transit and stub domains
 
cnp3-ebook/protocols/congestion — French
TCP's congestion control scheme is based on a `congestion window`. The current value of the congestion window (`cwnd`) is stored in the TCB of each TCP connection and the window that can be used by the sender is constrained by :math:`\min(cwnd,rwin,swin)` where :math:`swin` is the current sending window and :math:`rwin` the last received receive window. The `Additive Increase` part of the TCP congestion control increments the congestion window by :term:`MSS` bytes every round-trip-time. In the TCP literature, this phase is often called the `congestion avoidance` phase. The `Multiplicative Decrease` part of the TCP congestion control divides the current value of the congestion window once congestion has been detected.
 
cnp3-ebook/protocols/dnssec — French
The second type of attack is the `man-in-the-middle` attack. Consider that Alice is sending DNS requests to her DNS resolver. Unfortunately, Mallory sits in front of this resolver and can capture and modify all the packets sent by Alice to her resolver. In this case, Mallory can easily modify the DNS responses sent by the resolver to redirect Alice's packets to a different IP address controlled by Mallory. This enables Mallory to observe (and possibly modify) all the packets sent and received by Alice. In practice, executing this attack is not simple since DNS resolvers are usually installed in protected datacenters. However, if Mallory controls the WiFi access point that Alice uses to access the Internet, he could easily modify the packets on this access point and some software packages automate this type of attacks.
 
cnp3-ebook/protocols/email — French
The email system that we consider in this book is composed of four components :
 
cnp3-ebook/protocols/ethernet — French
The datalink layer addresses used in Ethernet networks are often called MAC addresses. They are structured as shown in the figure below. The first bit of the address indicates whether the address identifies a network adapter or a multicast group. The upper 24 bits are used to encode an Organization Unique Identifier (OUI). This OUI identifies a block of addresses that has been allocated by the secretariat [#foui]_ that is responsible for the uniqueness of Ethernet addresses to a manufacturer. Once a manufacturer has received an OUI, it can build and sell products with one of the 16 million addresses in this block.
 
cnp3-ebook/protocols/http — French
`authenticated` : in this mode, the ftp server only accepts users with a valid user name and password. Once authenticated, they can access the files and directories according to their permissions
 
cnp3-ebook/protocols/http2 — French
Caches can also be used inside the network. To understand their benefits, let us consider an SME with a dozen of employees that are connected to the Internet through a low-speed link. These employees often access similar web sites. Consider that Alice and Bob want to browse today's local newspaper. Their browsers will both retrieve the newspaper's website through the low bandwidth link and store the main documents in their cache. Unfortunately, the same information passes twice over the low-speed link. Some companies have deployed web proxies to cope with this problem. A web proxy is a server that resides in the enterprise network. All the employee's browsers are configured to send their HTTP requests to this proxy. When such a proxy receives a request, it checks whether the content is already stored inside its own cache. If so, it returns it directly. Otherwise, the request is sent to the remote server and the information is stored in the proxy cache. By reducing the number of web objects that are exchanged over low-speed links, such proxies can significantly improve performance. Some companies also use them to control the websites that are contacted by their employees and sometimes block illegitimate accesses.
 
cnp3-ebook/protocols/ipv6 — French
There are three main types of datalink layers. The simplest datalink layer is when there are only two communicating systems that are directly connected through the physical layer. Such a datalink layer is used when there is a point-to-point link between the two communicating systems. The two systems can be hosts or routers. :abbr:`PPP (Point-to-Point Protocol)`, defined in :rfc:`1661`, is an example of such a point-to-point datalink layer. Datalink layers exchange `frames` and a datalink :term:`frame` sent by a datalink layer entity on the left is transmitted through the physical layer, so that it can reach the datalink layer entity on the right. Point-to-point datalink layers can either provide an unreliable service (frames can be corrupted or lost) or a reliable service (in this case, the datalink layer includes retransmission mechanisms similar to the ones used in the transport layer). The unreliable service is frequently used above physical layers (e.g. optical fiber, twisted pairs) having a low bit error ratio while reliability mechanisms are often used in wireless networks to recover locally from transmission errors.