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../../principles/naming.rst:21
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A :term:`hosts.txt` file can be used when there are up to a few hundred hosts on the network. However, it is clearly not suitable for a network containing thousands or millions of hosts. A key issue in a large network is to define a suitable naming scheme. The ARPANet initially used a flat naming space, i.e. each host was assigned a unique name. To limit collisions between names, these names usually contained the name of the institution and a suffix to identify the host inside the institution (a kind of poor man's hierarchical naming scheme). On the ARPANet few institutions had several hosts connected to the network.
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../../principles/referencemodels.rst:32 • ../../principles/referencemodels.rst:49
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The Physical layer
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../../principles/network.rst:26
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There are two 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. These two systems can be hosts or routers. PPP (Point-to-Point Protocol), defined in :rfc:`1661`, is an example of such a point-to-point datalink layer. Datalink layer entities exchange `frames`. 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).
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../../principles/reliability.rst:35
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`wireless`. In this case, a radio signal is used to encode the information exchanged between the communicating devices. Many types of modulation techniques are used to send information over a wireless channel and there is lot of innovation in this field with new techniques appearing every year. While most wireless networks rely on radio signals, some use a laser that sends light pulses to a remote detector. These optical techniques allow to create point-to-point links while radio-based techniques can be used to build networks containing devices spread over a small geographical area.
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../../principles/transport.rst:25
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Networked applications do not exchange random messages. In order to ensure that the server is able to understand the queries sent by a client, and also that the client is able to understand the responses sent by the server, they must both agree on a set of syntactical and semantic rules. These rules define the format of the messages exchanged as well as their ordering. This set of rules is called an application-level `protocol`.
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../../principles/sharing.rst:28
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The second possible physical organization, which is also used inside computers to connect different extension cards, is the bus. In a bus network, all hosts are attached to a shared medium, usually a cable through a single interface. When one host sends an electrical signal on the bus, the signal is received by all hosts attached to the bus. A drawback of bus-based networks is that if the bus is physically cut, then the network is split into two isolated networks. For this reason, bus-based networks are sometimes considered to be difficult to operate and maintain, especially when the cable is long and there are many places where it can break. Such a bus-based topology was used in early Ethernet networks.
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../../protocols/dns.rst:28
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The `Transaction ID` (transaction identifier) is a 16-bits random value chosen by the client. When a client sends a question to a DNS server, it remembers the question and its identifier. When a server returns an answer, it returns in the `Transaction ID` field the identifier chosen by the client. Thanks to this identifier, the client can match the received answer with the question that it sent.
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../../principles/security.rst:64
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When analyzing security issues in computer networks, it is useful to reason about the capabilities of the attacker who wants to exploit some breach in the security of the network. There are different types of attackers. Some have generic capabilities, others are specific to a given technology or network protocol. In this section, we discuss some important threats that a network architect must take into account.
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../../exercises/dns.rst:33
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where
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../../exercises/email.rst:16
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Use your preferred email tool to send an email message to yourself containing a single line of text. Most email tools have the ability to show the `source` of the message, use this function to look at the message that you sent and the message that you received. Can you find an explanation for all the lines that have been added to your single line email ?
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../../exercises/http.rst:23
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Can you explain the different headers found in the response ?
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../../exercises/ipv6.rst:33 • ../../exercises/ipv6.rst:142
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Consider the network shown in the figure below. In this network, the following addresses are used.
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../../exercises/lan.rst:56
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Many enterprise networks are organized with a set of backbone devices interconnected by using a full mesh of links as shown in Fig.2. In this network, what are the benefits and drawbacks of using Ethernet switches and IP routers running OSPF ?
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../../exercises/routing-policies.rst:60
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from `AS4` to `AS1`
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../../exercises/routing-protocols.rst:94
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Finally, we can start the IPMininet_ topology and launch the daemons. The entire script is available from :download:`/exercises/ipmininet_scripts/ospf6.py`.
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../../exercises/tcp-2.rst:22
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packetdrill_ executes a system call and verifies its return value
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../../exercises/tcp.rst:63
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Footnotes
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../../exercises/trace.rst:35
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A first solution to analyze this trace is to use tcpdump_ on the command line. The `-n` disables the reverse DNS lookups that tcpdump_ does by default for all IP addresses. The `-r` argument is the name of the file contained the captured packets. The trace starts with the DNS request. This request was sent over IPv4 which is the default on this host. tcpdump_ indicates the query and the response returned by the local DNS resolver.
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../../protocols/bgp.rst:27
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Interconnection of two domains via a private peering link
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../../protocols/congestion.rst:24
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`mild congestion`. TCP considers that the network is lightly congested if it receives three duplicate acknowledgments and performs a fast retransmit. If the fast retransmit is successful, this implies that only one segment has been lost. In this case, TCP performs multiplicative decrease and the congestion window is divided by `2`. The slow-start threshold is set to the new value of the congestion window.
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