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../../exercises/transport.rst:146
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Consider a transport protocol that provides the bytestream service and uses 32 bits sequence number to represent the position of the first byte of the payload of DATA segments in the bytestream. How would you modify this protocol so that it can provide a message-mode service ? Consider first short messages that always fit inside a single segment. In a second step, discuss how you could support messages of unlimited size.
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../../principles/naming.rst:117
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To be able to contact the root nameservers, each DNS client must know their addresses. This implies, that DNS clients must maintain an up-to-date list of the addresses of the root nameservers. Without this list, it is impossible to contact the root nameservers. Forcing all Internet hosts to maintain the most recent version of this list would be difficult from an operational point of view. To solve this problem, the designers of the DNS introduced a special type of DNS server : the DNS resolvers. A :term:`resolver` is a server that provides the name resolution service for a set of clients. A network usually contains a few resolvers. Each host in these networks is configured to send all its DNS queries via one of its local resolvers. These queries are called `recursive queries` as the :term:`resolver` must recursively send requests through the hierarchy of nameservers to obtain the `answer`.
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../../principles/network.rst:89
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the information to be sent
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../../principles/reliability.rst:63
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On the sender side :
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../../principles/security.rst:287
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With the Kerckhoff principle, we can now discuss a simple but powerful encryption scheme that relies on the `XOR` logic operation. This operation is easily implemented in hardware and is supported by all microprocessors. Given a secret, :math:`K`, it is possible to encode a message `M` by computing :math:`C_M = K \oplus M`. The receiver of this messages can recover the original message as since :math:`M = K \oplus (K \oplus M)`. This `XOR` operation is the key operation of the perfect cipher that is also called the Vernam cipher or the one-time pad. This cipher relies on a key that contains purely random bits. The encrypted message is then produced by XORing all the bits of the message with all the bits of the key. Since the key is random, it is impossible for an attacker to recover the original text (or plain text) from the encrypted one. From a security viewpoint, the one-time-pad is the best solution provided that the key is as long as the message.
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../../exercises/dns.rst:59
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Use `dig` to find the mail relays used by the `uclouvain.be` and `student.uclouvain.be` domains. What is the `TTL` of these records ? Can you explain the preferences used by the `MX` records. You can find more information about the MX records in :rfc:`5321`.
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../../exercises/ipv6.rst:193
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How do you configure the forwarding tables on ``R1`` and ``R4`` so that ``A`` can reach ``B`` and the reverse ?
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../../exercises/lan.rst:246
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The first part of the output of the :manpage:`brctl(8)` command shows the state of the Spanning Tree software on the switch. The identifier of this switch is ``0003.f63545ab5f79`` and the root switch is itself. There is no root port on this switch since it is the root. The path cost is the cost of the path to reach the root switch, i.e. 0 on the root. Then the switch reports the different timers.
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../../exercises/routing-policies.rst:193
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`Section 3.3 <https://tools.ietf.org/html/rfc4443#section-3.3>`_ of :rfc:`4443` explains two different reasons why an IPv6 enabled device could generate an ICMPv6 Time Exceeded message. Explain when a router could generate such a message with ``Code==0`` and when a host could generate such a message with ``Code==1``.
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../../exercises/routing-protocols.rst:269
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A more detailed discussion of the packets that routing protocols exchange may be found in [Goralski2009]_.
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../../exercises/tcp-2.rst:123
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In the example above, packetdrill_ injects a segment containing two bytes. This segment is acknowledged and after that the :manpage:`read` system call succeeds and reads the available data with a buffer of 1000 bytes. It returns the amount of read bytes, i.e. ``2``.
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../../exercises/trace.rst:112
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a nice `list of Wireshark Display Filters <https://networksecuritytools.com/list-wireshark-display-filters/>`_ by Robert Allen
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../../protocols/bgp.rst:76
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These different types of relationships are implemented in the `interdomain routing policies` defined by each domain. The `interdomain routing policy` of a domain is composed of three main parts :
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../../protocols/congestion.rst:115
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The TCP flags
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../../protocols/dnssec.rst:215
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Thanks to the `DS` record, a resolver can validate the public keys of client zones as long as their is a chain of `DS` -> `DNSKEY` records from an anchored key. If the resolver trusts the public key of the root zone, it can validate all DNS replies for which this chain exists.
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../../protocols/email.rst:122
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Note the empty line after the `Date:` header line; this empty line contains only the `CR` and `LF` characters, and marks the boundary between the header and the body of the message.
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../../protocols/ethernet.rst:149
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Computers can directly be attached to Ethernet hubs. Ethernet hubs themselves can be attached to other Ethernet hubs to build a larger network. However, some important guidelines must be followed when building a complex network with hubs. First, the network topology must be a tree. As hubs are relays in the physical layer, adding a link between `Hub2` and `Hub3` in the network below would create an electrical shortcut that would completely disrupt the network. This implies that there cannot be any redundancy in a hub-based network. A failure of a hub or of a link between two hubs would partition the network into two isolated networks. Second, as hubs are relays in the physical layer, collisions can happen and must be handled by CSMA/CD as in a 10Base5 network. This implies that the maximum delay between any pair of devices in the network cannot be longer than the 51.2 microseconds `slot time`. If the delay is longer, collisions between short frames may not be correctly detected. This constraint limits the geographical spread of 10BaseT networks containing hubs.
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../../protocols/http.rst:104
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The HTML document shown below is composed of two parts: a header, delineated by the `<head>` and `</head>` markers, and a body (between the `<body>` and `</body>` markers). In the example below, the header only contains a title, but other types of information can be included in the header. The body contains an image, some text and a list with three hyperlinks. The image is included in the web page by indicating its URI between brackets inside the `<img src="...">` marker. It is important to note that the image can reside on any server. The client will automatically download it when rendering the web page. The `<h1>...</h1>` marker is used to specify the first level of headings. The `<ul>` marker indicates an unnumbered list while the `<li>` marker indicates a list item. The `<a href="URI">text</a>` indicates a hyperlink. The `text` will be underlined in the rendered web page and the client will fetch the specified URI when the user clicks on the link.
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../../protocols/http2.rst:153
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The HTTP/2.0 Frame header
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../../protocols/ipv6.rst:102
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When the work on IPng started, it was clear that 32 bits was too small to encode an IPng address and all proposals used longer addresses. However, there were many discussions about the most suitable address length. A first approach, proposed by SIPP in :rfc:`1710`, was to use 64 bit addresses. A 64 bits address space was 4 billion times larger than the IPv4 address space and, furthermore, from an implementation perspective, 64 bit CPUs were being considered and 64 bit addresses would naturally fit inside their registers. Another approach was to use an existing address format. This was the TUBA proposal (:rfc:`1347`) that reuses the ISO CLNP 20 bytes addresses. The 20 bytes addresses provided room for growth, but using ISO CLNP was not favored by the IETF partially due to political reasons, despite the fact that mature CLNP implementations were already available. 128 bits appeared to be a reasonable compromise at that time.
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