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../../principles/security.rst:433
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To efficiently sign messages, Alice needs to be able to compute a summary of her message in a way that makes prohibits an attacker from generating a different message that has the same summary. `Cryptographic hash functions` were designed to solve this problem. The ideal hash function is a function that returns a different number for every possible input. In practice, it is impossible to find such a function. Cryptographic hash functions are an approximation of this perfect summarization function. They compute a summary of a given message in 128, 160, 256 bits or more. They also exhibit the `avalanche effect`. This effect indicates that a small change in the message causes a large change in the hash value. Finally hash functions are very difficult to invert. Knowing a hash value, it is computationally very difficult to find the corresponding input message. Several hash functions have been proposed by cryptographers. The most popular ones are :
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../../exercises/ipv6.rst:471
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In this network, we will focus on four IPv6 prefixes :
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../../exercises/routing-protocols.rst:518
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If you launch the script and immediately type ``ping6all`` to check the connectivity, you might obtained the following result.
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../../exercises/tcp-2.rst:637
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Open questions
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../../protocols/bgp.rst:132
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a list of IP prefixes that are (re-)advertised
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../../protocols/congestion.rst:212
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TCP connections that use a large MSS can achieve a higher throughput that the TCP connections that use a shorter MSS. This creates another source of unfairness between TCP connections. However, it should be noted that today most hosts are using almost the same MSS, roughly 1460 bytes.
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../../protocols/email.rst:215
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`video`. The message part contains a video clip. The subtype indicates the format of the video clip like `avi <http://en.wikipedia.org/wiki/Audio_Video_Interleave>`_ or `mp4 <http://en.wikipedia.org/wiki/Mp4>`_.
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../../protocols/ethernet.rst:178
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Two multi-mode or single mode optical fibers with lasers
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../../protocols/http.rst:165
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the `Date:` header indicates when the HTTP response has been produced by the server.
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../../protocols/ipv6.rst:157
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An `interface identifier` that identifies a particular interface on a host
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../../protocols/ethernet.rst:171
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10Base5
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../../protocols/routing.rst:141
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Network operators expect an OSPF network to be able to quickly recover from link or router failures [VPD2004]_. In an OSPF network, the recovery after a failure is performed in three steps [FFEB2005]_ :
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../../protocols/tcp.rst:135
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Establishment of a TCP connection
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../../protocols/tls.rst:192
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the ``ServerHello`` message that contains the protocol version chosen by the server (assumed to be the same as the client version in this chapter), the 32 random bytes chosen by the server, the `Cipher Suite` selected by the server from the list advertised by the client and a `Session Id`. This `Session Id` is an identifier which is chosen by the server. It identifies the TLS session and the security parameters (algorithms and keys) negotiated for this session. It is used to support session resumption.
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../../protocols/wifi.rst:109
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802.11 control frames are simpler than data frames. They contain a `Frame Control`, a `Duration` field and one or two addresses. The acknowledgment frames are very small. They only contain the address of the destination of the acknowledgment. There is no source address and no `Sequence Control` field in the acknowledgment frames. This is because the acknowledgment frame can easily be associated to the previous frame that it acknowledges. Indeed, each unicast data frame contains a `Duration` field that is used to reserve the transmission channel to ensure that no collision will affect the acknowledgment frame. The `Sequence Control` field is mainly used by the receiver to remove duplicate frames. Duplicate frames are detected as follows. Each data frame contains a 12 bits sequence number in the `Sequence Control` field and the `Frame Control` field contains the `Retry` bit flag that is set when a frame is transmitted. Each 802.11 receiver stores the most recent sequence number received from each source address in frames whose `Retry` bit is reset. Upon reception of a frame with the `Retry` bit set, the receiver verifies its sequence number to determine whether it is a duplicated frame or not.
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../../bibliography.rst:147
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Rago, S., `UNIX System V network programming <http://www.pearsonhighered.com/educator/product/UNIX-System-V-Network-Programming/9780201563184.page>`_, Addison Wesley, 1993
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../../glossary.rst:63
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frame
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../../principles/network.rst:153
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Learning the location of the sources is not sufficient, nodes also need to forward packets towards their destination. When a node receives a packet whose destination address is already present inside its port-address table, it simply forwards the packet on the interface listed in the port-address table. In this case, the packet will follow the port-address table entries in the downstream nodes and will reach the destination. If the destination address is not included in the port-address table, the node simply forwards the packet on all its interfaces, except the interface from which the packet was received. Forwarding a packet over all interfaces is usually called `broadcasting` in the terminology of computer networks. Sending the packet over all interfaces except one is a costly operation since the packet is sent over links that do not reach the destination. Given the tree-shape of the network, the packet will explore all downstream branches of the tree and will finally reach its destination. In practice, the `broadcasting` operation does not occur too often and its performance impact remains limited.
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../../principles/reliability.rst:194
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To enable the transmission/reception of frames, the first problem to be solved is how to encode a frame as a sequence of bits, so that the receiver can easily recover the received frame despite the limitations of the physical layer.
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../../principles/security.rst:447
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MD5, originally proposed in :rfc:`1321`. It has been used in a wide range of applications. In 2010, attacks against MD5 were published and this hash function is now deprecated.
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