Exit Zen
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../../exercises/http.rst:19
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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.
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../../exercises/ipv6.rst:15
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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.
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../../exercises/lan.rst:26
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Fig. 1. A small network composed of Ethernet switches
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../../exercises/routing-policies.rst:56
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In this network, what are the paths :
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../../exercises/routing-protocols.rst:15
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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.
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../../exercises/tcp-2.rst:16
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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.
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../../exercises/tcp.rst:37
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A connection ends with the transmission of segments that include the `FIN` flag that marks the end of the data transfer.
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../../exercises/tls.rst:24
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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.
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../../exercises/trace.rst:12
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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)`.
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../../protocols/bgp.rst:17
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Transit and stub domains
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../../protocols/congestion.rst:17
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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.
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../../protocols/dnssec.rst:28
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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.
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../../protocols/email.rst:47
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The email system that we consider in this book is composed of four components :
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../../protocols/ethernet.rst:19
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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.
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../../protocols/http.rst:13
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`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
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../../protocols/http2.rst:46
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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.
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../../protocols/ipv6.rst:57
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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.
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../../protocols/ppp.rst:9
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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.
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../../protocols/ppp.rst:15
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While SLIP was implemented and used in some environments, it had several limitations discussed in :rfc:`1055`. The `Point-to-Point Protocol` (PPP) was designed shortly after and is specified in :rfc:`1548`. PPP aims to support IP and other network layer protocols over various types of serial lines. PPP is in fact a family of three protocols that are used together :
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../../protocols/routing.rst:17
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The first class of routing protocols are the `intradomain routing protocols` (sometimes also called the interior gateway protocols or :term:`IGP`). An intradomain routing protocol is used by all routers inside a domain to exchange routing information about the destinations that are reachable inside the domain. There are several intradomain routing protocols. Some domains use :term:`RIP`, which is a distance vector protocol. Other domains use link-state routing protocols such as :term:`OSPF` or :term:`IS-IS`. Finally, some domains use static routing or proprietary protocols such as :term:`IGRP` or :term:`EIGRP`.
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