Exit Zen
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../../exercises/routing-protocols.rst:542
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We have configured the following addresses on the hosts.
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../../exercises/tcp-2.rst:642
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the round-trip-time is fixed and does not change during the lifetime of the TCP connection. We assume a fixed value of 100 milliseconds for the round-trip-time and a fixed value of 200 milliseconds for the retransmission timer.
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../../protocols/bgp.rst:137
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`Withdraw message` to indicate a BGP `UPDATE` message containing one route that is withdrawn
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../../protocols/congestion.rst:219
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The TCP congestion control zoo
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../../protocols/email.rst:220
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The first computers used different techniques to represent characters in memory and on disk. During the 1960s, computers began to exchange information via tape or telephone lines. Unfortunately, each vendor had its own proprietary character set and exchanging data between computers from different vendors was often difficult. The 7 bits ASCII character table :rfc:`20` was adopted by several vendors and by many Internet protocols. However, ASCII became a problem with the internationalization of the Internet and the desire of more and more users to use character sets that support their own written language. A first attempt at solving this problem was the definition of the ISO-8859_ character sets by :term:`ISO`. This family of standards specified various character sets that allowed the representation of many European written languages by using 8 bits characters. Unfortunately, an 8-bits character set is not sufficient to support some widely used languages, such as those used in Asian countries. Fortunately, at the end of the 1980s, several computer scientists proposed to develop a standard that supports all written languages used on Earth today. The Unicode standard [Unicode]_ has now been adopted by most computer and software vendors. For example, Java always uses Unicode to manipulate characters, Python can handle both ASCII and Unicode characters. Internet applications are slowly moving towards complete support for the Unicode character sets, but moving from ASCII to Unicode is an important change that can have a huge impact on current deployed implementations. See, for example, the work to completely internationalize email :rfc:`4952` and domain names :rfc:`5890`.
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../../protocols/ethernet.rst:180
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40-100 Gbps
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../../protocols/http.rst:170
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the `User-Agent:` header provides information about the client that has generated the HTTP request. Some servers analyze this header line and return different headers and sometimes different documents for different user agents.
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../../protocols/ipv6.rst:162
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the top half is divided in two smaller blocks, one for each small campus
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../../protocols/ethernet.rst:172
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Thin coaxial cable, 185m
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../../protocols/routing.rst:145
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all routers update their routing table
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../../protocols/tcp.rst:143
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In the original TCP specification :rfc:`793`, each TCP entity maintained a clock to compute the initial sequence number (:term:`ISN`) placed in the `SYN` and `SYN+ACK` segments. This made the ISN predictable and caused a security issue. The typical security problem was the following. Consider a server that trusts a host based on its IP address and allows the system administrator to log in from this host without giving a password [#frlogin]_. Consider now an attacker who knows this particular configuration and is able to send IP packets having the client's address as source. He can send fake TCP segments to the server, but does not receive the server's answers. If he can predict the `ISN` that is chosen by the server, he can send a fake `SYN` segment and shortly after the fake `ACK` segment confirming the reception of the `SYN+ACK` segment sent by the server. Once the TCP connection is open, he can use it to send any command to the server. To counter this attack, current TCP implementations add randomness to the `ISN`. One of the solutions, proposed in :rfc:`1948` is to compute the `ISN` as ::
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../../protocols/tls.rst:215
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the ``ServerHelloDone`` indicates that the server has sent all the messages for the first phase of the handshake.
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../../protocols/wifi.rst:126
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IEEE 802.11 RTS frame format
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../../bibliography.rst:312
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Roberts, L., `ALOHA packet system with and without slots and capture <http://doi.acm.org/10.1145/1024916.1024920>`_. SIGCOMM Computer Communication Review 5, 2 (Apr. 1975), 28-42.
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../../principles/network.rst:187
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By inspecting the source and destination addresses of packets, network nodes can automatically derive their forwarding tables. As we will discuss later, this technique is used in :term:`Ethernet` networks. Despite being widely used, it has two important drawbacks. First, packets sent to unknown destinations are broadcasted in the network even if the destination is not attached to the network. Consider the transmission of ten packets destined to `Z` in the network above. When a node receives a packet towards this destination, it can only broadcast that packet. Since `Z` is not attached to the network, no node will ever receive a packet whose source is `Z` to update its forwarding table. The second and more important problem is that few networks have a tree-shaped topology. It is interesting to analyze what happens when a port-address table is used in a network that contains a cycle. Consider the simple network shown below with a single host.
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../../principles/reliability.rst:204
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The `framing` problem can be defined as : "`How does a sender encode frames so that the receiver can efficiently extract them from the stream of bits that it receives from the physical layer`".
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../../principles/transport.rst:105
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The peer-to-peer model emerged during the last ten years as another possible architecture for networked applications. In the traditional client-server model, hosts act either as servers or as clients and a server serves a large number of clients. In the peer-to-peer model, all hosts act as both servers and clients and they play both roles. The peer-to-peer model has been used to develop various networked applications, ranging from Internet telephony to file sharing or Internet-wide filesystems. A detailed description of peer-to-peer applications may be found in [BYL2008]_. Surveys of peer-to-peer protocols and applications may be found in [AS2004]_ and [LCP2005]_.
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../../principles/sharing.rst:191
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Discarding packets is a frequent reaction to network congestion. Unfortunately, discarding packets is not optimal since a packet which is discarded on a network node has already consumed resources on the upstream nodes. There are other ways for the network to inform the end hosts of the current congestion level. A first solution is to mark the packets when a node is congested. Several networking technologies have relied on this kind of packet marking.
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../../principles/security.rst:458
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Another important point about cryptographic algorithms is that often these algorithms require random numbers to operate correctly (e.g. to generate keys). Generating good random numbers is difficult and any implementation of cryptographic algorithms should also include a secure random number generator. :rfc:`4086` provides useful recommendations.
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../../exercises/ipv6.rst:476
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``p:0003::/64`` used on the link ``B2-R6``. ``B2`` uses address ``p:0003::B2/64``
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