Exit Zen
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../../principles/security.rst:73
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The first type of attacker is called the `passive attacker`. A `passive attacker` is someone able to observe and usually store the information (e.g. the packets) exchanged in a given network or subset of it (e.g. a specific link). This attacker has access to all the data passing through this specific link. This is the most basic type of attacker and many network technologies are vulnerable to such attacks. In the above example, a passive attacker could easily capture the password sent by Alice and reuse it later to be authenticated as Alice on the remote computer. This is illustrated in the figure below where we do not show anymore the ``DATA.req`` and ``DATA.ind`` primitives but only the messages exchanged. Throughout this chapter, we will always use `Eve` as a user who is able to eavesdrop the data passing in front of her.
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../../exercises/dns.rst:35
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`server` is the IP address or the name of a DNS server or resolver
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../../exercises/email.rst:18
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The TCP protocol supports 65536 different ports numbers. Many of these port numbers have been reserved for some applications. The official repository of the reserved port numbers is maintained by the Internet Assigned Numbers Authority (IANA_) on http://www.iana.org/assignments/port-numbers [#fservices]_. Using this information, what is the default port number for the POP3 protocol ? Does it run on top of UDP or TCP ?
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../../exercises/http.rst:25
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HTTP 1.1, specified in :rfc:`2616`, forces the client to include the `Host:` header in all its requests. HTTP 1.0 does not define the `Host:` header, but most implementations support it. By using `telnet` and `curl` retrieve the first page of the https://www.computer-networking.info web server [#fhttps]_ by sending http requests with and without the `Host:` header. Explain the difference between the two.
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../../exercises/ipv6.rst:35 • ../../exercises/ipv6.rst:144 • ../../exercises/ipv6.rst:222
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host ``A`` : ``2001:db8:1341:1::A`` and its default route points to ``2001:db8:1341:1::1``
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../../exercises/lan.rst:62
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Fig. 2. A typical enterprise backbone network
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../../exercises/routing-policies.rst:63 • ../../exercises/routing-policies.rst:96
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Consider the interdomain topology shown in the figure below. Assuming, that `AS1` advertises prefix ``2001:db8:1::/48``, `AS2` prefix ``2001:db8:2::/48``, ... compute the routing tables of the different ASes.
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../../exercises/routing-protocols.rst:106
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The script starts the routers and hosts.
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../../exercises/tcp-2.rst:23
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packetdrill_ injects [#ftcpdump_pdrill]_ a packet in the instrumented Linux kernel as if it were received from the network
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../../exercises/trace.rst:45
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The following three lines of the tcpdump_ output correspond to TCP's three-way handshake. There are several interesting points to note in this output. First, ``Flags [S]`` indicates that the `SYN` flag was set in the first and second segments. In this first segment, tcpdump_ indicates the initial sequence number (``2681184541``). In the second segment, tcpdump_ indicates both the initial sequence number (``3804204915``) and the acknowledgment number (``2681184542``). Both segments contain TCP options. Starting in the third segment, tcpdump_ shows relative sequence numbers. Thus, the acknowledgment that you observe in the third segment is an acknowledgment for the `SYN` returned by the server.
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../../protocols/bgp.rst:33
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Such `private peering links` are useful when, for example, an enterprise or university network needs to be connected to its Internet Service Provider. However, some domains are connected to hundreds of other domains [#fasrank]_ . For some of these domains, using only private peering links would be too costly. A better solution to allow many domains to cheaply interconnect are the `Internet eXchange Points` (:term:`IXP`). An :term:`IXP` is usually some space in a data center that hosts routers belonging to different domains. A domain willing to exchange packets with other domains present at the :term:`IXP` installs one of its routers on the :term:`IXP` and connects it to other routers inside its own network. The IXP contains a Local Area Network to which all the participating routers are connected. When two domains that are present at the IXP wish [#fwish]_ to exchange packets, they simply use the Local Area Network. IXPs are very popular in Europe and many Internet Service Providers and Content providers are present in these IXPs.
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../../protocols/congestion.rst:25
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`severe congestion`. TCP considers that the network is severely congested when its retransmission timer expires. In this case, TCP retransmits the first segment, sets the slow-start threshold to 50% of the congestion window. The congestion window is reset to its initial value and TCP performs a slow-start.
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../../protocols/dnssec.rst:62
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Mallory has studied the DNS protocol and understands how it works. If he can predict a popular domain for which Alice will regularly send DNS requests, then he can prepare a set of DNS responses that map the name requested by Alice to an IP address controlled by Mallory instead of the legitimate DNS response. Each DNS response has a different `Identification`. Since there are only 65,536 values for the `Identification` field, it is possible for Mallory to send them to Alice hoping that one of them will be received while Alice is waiting for a DNS response with the same identifier. In the past, it was difficult to send 65,536 DNS responses quickly enough. However, with the high speed links that are available today, this is not an issue anymore. A second concern for Mallory is that he must be able to send the DNS responses as if they were coming directly from the DNS resolver. This implies that Mallory must be able to send IP packets that appear to originate from a different address. Although networks should be configured to prevent this type of attack, this is not always the case and there are networks where it is possible for a host to send packets with a different source IP address [#fspoof]_. If the attack targets a single end user, e.g. Alice, this is annoying for this user. However, if the attacker can target a DNS resolver that serves an entire company or an entire ISP, the impact of the attack can be much larger in particular if the injected DNS response carries a long TTL and thus resides in the resolver's cache for a long period of time.
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../../protocols/email.rst:52
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software, that allows servers to efficiently exchange email messages
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../../protocols/ethernet.rst:59
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The last field of the Ethernet frame is a 32 bit Cyclical Redundancy Check (CRC). This CRC is able to catch a much larger number of transmission errors than the Internet checksum used by IP, UDP and TCP [SGP98]_. The format of the Ethernet frame is shown below.
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../../protocols/http.rst:46
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A document sharing system such as the `world wide web` is composed of three important parts.
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../../protocols/http2.rst:55
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When a web user interacts with `www.service.net`, she expects that all the information comes from the `www.service.net` server. If the service is popular, there are probably tens, hundreds, thousands or more physical servers that support this service. Still, the user has the illusion that she is interacting with a single server. Several techniques have been deployed by content providers to scale web services. Consider a simple service that serves text documents from `N` different servers. There are different ways to architect such a service.
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../../protocols/ipv6.rst:71
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The third type of datalink layers are used in Non-Broadcast Multi-Access (NBMA) networks. These networks are used to interconnect devices like a LAN. All devices attached to an NBMA network are identified by a unique datalink layer address. However, and this is the main difference between an NBMA network and a traditional LAN, the NBMA service only supports unicast. The datalink layer service provided by an NBMA network supports neither broadcast nor multicast.
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../../protocols/ppp.rst:18
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The `Link Control Protocol` that is used to negotiate options and authenticate the session by using username and password or other types of credentials
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../../protocols/ppp.rst:21
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The PPP framing :rfc:`1662` was inspired by the datalink layer protocols standardized by ITU-T and ISO. A typical PPP frame is composed of the fields shown in the figure below. A PPP frame starts with a one byte flag containing `01111110`. PPP can use bit stuffing or character stuffing depending on the environment where the protocol is used. The address and control fields are present for backward compatibility reasons. The 16 bit Protocol field contains the identifier [#fpppid]_ of the network layer protocol that is carried in the PPP frame. `0x002d` is used for an IPv4 packet compressed with :rfc:`1144` while `0x002f` is used for an uncompressed IPv4 packet. `0xc021` is used by the Link Control Protocol, `0xc023` is used by the Password Authentication Protocol (PAP). `0x0057` is used for IPv6 packets. PPP supports variable length packets, but LCP can negotiate a maximum packet length. The PPP frame ends with a Frame Check Sequence. The default is a 16 bits CRC, but some implementations can negotiate a 32 bits CRC. The frame ends with the `01111110` flag.
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