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../../protocols/ipv6.rst:126
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``2001:0db8:0000:cd30:0000:0000:0000:0000`` / ``60``
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../../protocols/ethernet.rst:118
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The introduction of the twisted pairs led to two major changes to Ethernet. The first change concerns the physical topology of the network. 10Base2 and 10Base5 networks are shared buses, the coaxial cable typically passes through each room that contains a connected computer. A 10BaseT network is a star-shaped network. All the devices connected to the network are attached to a twisted pair cable that ends in the telecom closet. From a maintenance perspective, this is a major improvement. The cable is a weak point in 10Base2 and 10Base5 networks. Any physical damage on the cable broke the entire network and when such a failure occurred, the network administrator had to manually check the entire cable to detect where it was damaged. With 10BaseT, when one twisted pair is damaged, only the device connected to this twisted pair is affected and this does not affect the other devices. The second major change introduced by 10BaseT was that is was impossible to build a 10BaseT network by simply connecting all the twisted pairs together. All the twisted pairs must be connected to a relay that operates in the physical layer. This relay is called an `Ethernet hub`. A `hub` is thus a physical layer relay that receives an electrical signal on one of its interfaces, regenerates the signal and transmits it over all its other interfaces. Some `hubs` are also able to convert the electrical signal from one physical layer to another (e.g. 10BaseT to 10Base2 conversion).
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../../protocols/routing.rst:106
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The first summary advertisement provides precise information about the distance used to reach each prefix. However, all routers in the network have to maintain a route towards `2001:db8:aaaa:0000::/64` and a route towards `2001:db8:aaaa:0001::/64` that are both via router `RA`. The second advertisement would improve the scalability of OSPF by reducing the number of routes that are advertised across area boundaries. However, in practice this requires manual configuration on the border routers.
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../../protocols/rpc.rst:122
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`data`: an optional field that provides additional information about the error.
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../../protocols/ssh.rst:269
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Among the various features of the ``ssh`` protocol, it is interesting to mention how users are authenticated by the server. The ``ssh`` protocol supports the classical username/password authentication (but both the username and the password are transmitted over the secure encrypted channel). In addition, ``ssh`` supports two authentication mechanisms that rely on public keys. To use the first one, each user needs to generate his/her own public/private key pair and store the public key on the server. To be authenticated, the user needs to sign a message containing his/her public key by using his/her private key. The server can easily verify the validity of the signature since it already knows the user's public key. The second authentication scheme is designed for hosts that trust each other. Each host has a public/private key pair and stores the public keys of the other hosts that it trusts. This is typically used in environments such as university labs where each user could access any of the available computers. If Alice has logged on ``computer1`` and wants to execute a command on ``computer2``, she can create an ``ssh`` session on this computer and type (again) her password. With the host-based authentication scheme, ``computer1`` signs a message with its private key to confirm that it has already authenticated Alice. ``computer2`` would then accept Alice's session without asking her credentials.
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../../protocols/tcp.rst:116
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Upon reception of this segment (which is often called a `SYN segment`), the server host replies with a segment containing :
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../../protocols/tls.rst:150
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`Random number`: security protocols rely on random numbers. The client sends a 32 bytes long random number where usually four of these bytes correspond to the client's clock. This random number is used, together with the server's random number, as a seed to generate the security keys.
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../../protocols/wifi.rst:54
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An 802.11 infrastructure network
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../../bibliography.rst:117
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Leffler, S., Fabry, R., Joy, W., Lapsley, P., Miller, S., Torek, C., `An Advanced 4.4BSD Interprocess Communication Tutorial <http://docs.freebsd.org/44doc/psd/21.ipc/paper.pdf>`_, 4.4 BSD Programmer's Supplementary Documentation
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../../glossary.rst:152
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Exterior Gateway Protocol. Synonym of interdomain routing protocol
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../../exercises/ex-sharing.rst:641
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Consider a CSMA/CD network that contains hosts that generate frames at a regular rate. When the transmission rate increases, the amount of collisions increases. For a given network load, measured in bits/sec, would the number of collisions be smaller, equal or larger with short frames than with long frames ?
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../../principles/network.rst:136
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An alternative to manually computed forwarding tables is to use a network management platform that tracks the network status and can push new forwarding tables on the routers when it detects any modification to the network topology. This solution gives some flexibility to the network managers in computing the paths inside their network. However, this solution only works if the network management platform is always capable of reaching all routers even when the network topology changes. This may require a dedicated network that allows the management platform to push information on the forwarding tables. Openflow is a modern example of such solutions [MAB2008]_. In a nutshell, Openflow is a protocol that enables a network controller to install specific entries in the forwarding tables of remote routers and much more.
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../../principles/transport.rst:76
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Most applications exchange strings that are composed of fixed or variable numbers of characters. A common solution to define the character strings that are acceptable is to define them as a grammar using a Backus-Naur Form (:term:`BNF`) such as the Augmented BNF defined in :rfc:`5234`. A BNF is a set of production rules that generate all valid character strings. For example, consider a networked application that uses two commands, where the user can supply a username and a password. The BNF for this application could be defined as shown in the figure below.
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../../principles/sharing.rst:165
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As explained earlier, one of the first manifestation of congestion on network nodes is the saturation of the network links that leads to a growth in the occupancy of the buffers of the node. This growth of the buffer occupancy implies that some packets will spend more time in the buffer and thus in the network. If hosts measure the network delays (e.g. by measuring the round-trip-time between the transmission of a packet and the return of the corresponding acknowledgment) they could start to sense congestion. On low bandwidth links, a growth in the buffer occupancy can lead to an increase of the delays which can be easily measured by the end hosts. On high bandwidth links, a few packets inside the buffer will cause a small variation in the delay which may not necessarily be larger that the natural fluctuations of the delay measurements.
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../../principles/security.rst:380
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The public key is used to encrypt a message so that it can only be read by the intended recipient. For example, let us consider two users : Alice and Bob. Alice (resp. Bob) uses the keys :math:`A_{priv}` and :math:`A_{pub}` (resp. :math:`B_{priv}` and :math:`B_{pub}`). To send a secure message `M` to Alice, Bob computes :math:`CM=E_p(A_{pub},M)` and Alice can decrypt it by using :math:`D_p(A_{priv},CM)=D_p(A_{priv},E_p(A_{pub},M))=M`.
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../../exercises/ipv6.rst:368
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on router ``R3``
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../../exercises/routing-protocols.rst:437
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``AS2`` is assigned ``2001:cafe:2::/48``
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../../exercises/tcp-2.rst:364
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More complex scenarios can be written. The script below demonstrates how the TCP stack behaves when three segments are lost. This script is available from :download:`/exercises/packetdrill_scripts/frr-rto2.pkt`.
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../../protocols/bgp.rst:124
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`OPEN` : this message is sent as soon as the TCP connection between the two routers has been established. It initializes the BGP session and allows the negotiation of some options. Details about this message may be found in :rfc:`4271`.
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../../protocols/congestion.rst:199
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As the losses are equally spaced, the congestion window always starts at some value (:math:`\frac{W}{2}`), and is incremented by one MSS every round-trip-time until it reaches twice this value (`W`). At this point, a segment is retransmitted and the cycle starts again. If the congestion window is measured in MSS-sized segments, a cycle lasts :math:`\frac{W}{2}` round-trip-times. The bandwidth of the TCP connection is the number of bytes that have been transmitted during a given period of time. During a cycle, the number of segments that are sent on the TCP connection is equal to the area of the yellow trapeze in the figure. Its area is thus :
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