Exit Zen
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../../exercises/ex-sharing.rst:150
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In ALOHA, the hosts rely on acknowledgments to detect whether their frame has been received correctly by the destination. Consider a network running at 100 Mbps where the host exchange 1000 bits frames and acknowledgments of 100 bits. Draw the frames sent by hosts A and B in the figure below. Assume that a square corresponds to 10 microseconds and that the cable has a length of 2 kilometers.
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../../principles/sharing.rst:36
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A third organization of a computer network is a star topology. In such networks, hosts have a single physical interface and there is one physical link between each host and the center of the star. The node at the center of the star can be either a piece of equipment that amplifies an electrical signal, or an active device, such as a piece of equipment that understands the format of the messages exchanged through the network. Of course, the failure of the central node implies the failure of the network. However, if one physical link fails (e.g. because the cable has been cut), then only one node is disconnected from the network. In practice, star-shaped networks are easier to operate and maintain than bus-shaped networks. Many network administrators also appreciate the fact that they can control the network from a central point. Administered from a Web interface, or through a console-like connection, the center of the star is a useful point of control (enabling or disabling devices) and an excellent observation point (usage statistics).
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../../principles/security.rst:113
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In the above example, `Eve` can capture all the packets exchanged by Bob and Alice. This implies that Eve can discover Alice's username and Alice's password. With this information, Eve can then authenticate as Alice on Bob's computer and do whatever Alice is authorized to do. This is a major problem from a security point of view. To prevent this attack, Alice should never send her password in clear over a network where someone could eavesdrop the information. In some networks, such as an open wireless network, an attacker can easily collect all the data sent by a particular user. In other networks, this is a bit more complex depending on the network technology used, but various software packages exist to automate this process. As will be described later, the best approach to prevent this type of attack is to rely on cryptographic techniques to ensure that passwords are never sent in clear.
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../../exercises/dns.rst:36
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`type` is the type of DNS record that is requested by the query such as `NS` for a nameserver, `A` for an IPv4 address, `AAAA` for an IPv6 address, `MX` for a mail relay, ...
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../../exercises/email.rst:20
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The Post Office Protocol (POP) is a rather simple protocol described in :rfc:`1939`. POP operates in three phases. The first phase is the authorization phase where the client provides a username and a password. The second phase is the transaction phase where the client can retrieve emails. The last phase is the update phase where the client finalizes the transaction. What are the main POP commands and their parameters ? When a POP server returns an answer, how can you easily determine whether the answer is positive or negative ?
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../../exercises/http.rst:27
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The headers sent in a HTTP request allow the client to provide additional information to the server. One of these headers is the `Accept-Language` header that allows indicating the preferred language of the client [#flang]_. For example, `curl -HAccept-Language:en http://www.google.be` will send to `http://www.google.be` a HTTP request indicating English (`en`) as the preferred language. Does google provides a different page in French (`fr`) and Walloon (`wa`) ? Same question for `http://www.uclouvain.be` (given the size of the homepage, use ``diff`` to compare the different pages retrieved from `www.uclouvain.be`).
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../../exercises/ipv6.rst:36 • ../../exercises/ipv6.rst:145 • ../../exercises/ipv6.rst:223
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host ``B`` : ``2001:db8:1341:4::B`` and its default route points to ``2001:db8:1341:4::4``
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../../exercises/lan.rst:64
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In the network depicted in Fig. 3, the host `H0` performs a traceroute toward its peer `H1` (designated by its name) through a network composed of switches and routers. Explain precisely the frames, packets, and segments exchanged since the network was turned on. You may assign addresses if you need to.
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../../exercises/routing-policies.rst:93
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Are all ASes capable of reaching all the other ASes in this simple Internet ?
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../../exercises/routing-protocols.rst:120
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We can easily verify that the paths used to forward packets are the expected ones according to the configured IGP weights.
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../../exercises/tcp-2.rst:24
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packetdrill_ compares a packet transmitted by the instrumented Linux kernel with the packet that the script expects
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../../exercises/trace.rst:59
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The two lines above correspond to the request sent by the client and the acknowledgment returned by the server. Note that the first byte sent by the client has `1` as relative sequence number. In this example, the HTTP request has a total length of 92 bytes. This request is immediately acknowledged by the server.
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../../protocols/bgp.rst:39
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Interconnection of two domains at an Internet eXchange Point
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../../protocols/congestion.rst:27
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The figure below illustrates the evolution of the congestion window when there is severe congestion. At the beginning of the connection, the sender performs `slow-start` until the first segments are lost and the retransmission timer expires. At this time, the `ssthresh` is set to half of the current congestion window and the congestion window is reset at one segment. The lost segments are retransmitted as the sender again performs slow-start until the congestion window reaches the `sshtresh`. It then switches to congestion avoidance and the congestion window increases linearly until segments are lost and the retransmission timer expires.
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../../protocols/dnssec.rst:86
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Fortunately, designers of DNS servers and resolvers have found solutions to mitigate this type of attack. The easiest approach would have been to update the format of the DNS requests and responses to include a larger `Identifier` field. Unfortunately, this elegant solution was not possible with the DNS because the DNS messages do not include any version number that would have enabled such a change. Since the DNS messages are exchanged inside UDP segments, the DNS developers found an alternate solution to counter this attack. There are two ways for the DNS library used by Alice to send her DNS requests. A first solution is to bind one UDP source port and always send the DNS requests from this source port (the destination port is always port ``53``). The advantage of this solution is that Alice's DNS library can easily receive the DNS responses by listening to her chosen port. Unfortunately, once the attacker has found the source port used by Alice, he only needs to send 65,536 DNS responses to inject an invalid response. Fortunately, Alice can send her DNS requests in a different way. Instead of using the same source port for all DNS requests, she can use a different source port for each request. In practice, each DNS request will be sent from a different source port. From an implementation viewpoint, this implies that Alice's DNS library will need to listen to one different port number for each pending DNS request. This increases the complexity of her implementation. From a security viewpoint there is a clear benefit since the attacker needs to guess both the 16 bits `Identifier` and the 16 bits `UDP source port` to inject a fake DNS response. To generate all possible DNS responses, the attacker would need to generate almost :math:`2^{32}` different messages, which is excessive in today's networks. Most DNS implementations use this second approach to prevent these cache poisoning attacks.
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../../protocols/email.rst:54
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We will first discuss the format of email messages followed by the protocols that are used on today's Internet to exchange and retrieve emails. Other email systems have been developed in the past [Bush1993]_ [Genilloud1990]_ [GC2000]_, but today most email solutions have migrated to the Internet email. Information about the software that is used to compose and deliver emails may be found on wikipedia_ among others, for both `email clients`_ and `email servers <http://en.wikipedia.org/wiki/Comparison_of_mail_servers>`_. More detailed information about the full Internet Mail Architecture may be found in :rfc:`5598`.
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../../protocols/ethernet.rst:68
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Ethernet DIX frame format
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../../protocols/http.rst:48
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A standardized addressing scheme that unambiguously identifies documents
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../../protocols/http2.rst:57
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A first approach is to store all files on each physical server and rely on the DNS to distribute the load among them. Each physical server has its own IP address and when the DNS server receives a query for `www.service.net`, it returns the IP address of one of them. Some DNS servers use Round-Robin to return one of these IP addresses. Others measure the load of the physical servers and return the address of the less loaded one. Another possibility is to locate the physical servers in different regions and configure the DNS server to return the IP address of the server that is geographically closer to the client's IP address.
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../../protocols/ipv6.rst:73
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Unfortunately no datalink layer is able to send frames of unlimited side. Each datalink layer is characterized by a maximum frame size. There are more than a dozen different datalink layers and unfortunately most of them use a different maximum frame size. The network layer must cope with the heterogeneity of the datalink layer.
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