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4. Zen

3 ../../protocols/bgp.rst:11
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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`). 763/10000 · 763
3 ../../protocols/congestion.rst:15
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 839/10000 · 839
3 ../../protocols/dnssec.rst:18
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 619/10000 · 619
3 ../../protocols/email.rst:13
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 526/10000 · 526
3 ../../protocols/ethernet.rst:14
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 2142/10000 · 2142
3 ../../protocols/http.rst:11
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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 : 206/10000 · 206
3 ../../protocols/http2.rst:12
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 525/10000 · 525
3 ../../protocols/ipv6.rst:37
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 521/10000 · 521
3 ../../protocols/ppp.rst:7
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The Point-to-Point Protocol 27/10000 · 27
3 ../../protocols/ppp.rst:13
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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`. 1517/10000 · 1517
3 ../../protocols/routing.rst:14
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Organisation of a small Internet 32/10000 · 32
3 ../../protocols/rpc.rst:10
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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 : 521/10000 · 521
3 ../../protocols/ssh.rst:28
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 509/10000 · 509
3 ../../protocols/tcp.rst:14
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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 : 212/10000 · 212
3 ../../protocols/tls.rst:17
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 310/10000 · 310
3 ../../protocols/transport-service.rst:10
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — the `connectionless` service 28/10000 · 28
3 ../../protocols/transport.rst:11
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 523/10000 · 523
3 ../../protocols/udp.rst:13
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — the UDP service cannot deliver SDUs that are larger than 65467 bytes [#fmtuudp]_ 80/10000 · 80
3 ../../protocols/wifi.rst:9
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 979/10000 · 979
3 ../../exercises/network.rst:15
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Multiple-choice questions 25/10000 · 25