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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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../../protocols/email.rst:183
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To support these two types of MIME messages, the recipient of a message must be able to extract the different parts from the message. In :rfc:`822`, an empty line was used to separate the header lines from the body. Using an empty line to separate the different parts of an email body would be difficult as the body of email messages often contains one or more empty lines. Another possible option would be to define a special line, e.g. `*-LAST_LINE-*` to mark the boundary between two parts of a MIME message. Unfortunately, this is not possible as some emails may contain this string in their body (e.g. emails sent to students to explain the format of MIME messages). To solve this problem, the `Content-Type:` header line contains a second parameter that specifies the string that has been used by the sender of the MIME message to delineate the different parts. In practice, this string is often chosen randomly by the mail client.
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../../protocols/ethernet.rst:175
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Category 5 UTP or STP, 100 m maximum
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../../protocols/http.rst:156
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HTTP clients and servers can include different HTTP headers in HTTP requests and responses. Each HTTP header is encoded as a single ASCII-line terminated by `CR` and `LF`. Several of these headers are briefly described below. A detailed discussion of the standard headers may be found in :rfc:`1945`. The MIME headers can appear in both HTTP requests and HTTP responses.
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../../protocols/http2.rst:251
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The `HTTP2-Settings` line contains the HTTP/2.0 settings frame that the client would server over an HTTP/2.0 session encoded in Base64. The server replies with a response that indicates that it has accepted to upgrade the connection to HTTP/2.0. A sample response is shown below.
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../../protocols/ipv6.rst:127
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``2001:0db8::cd30:0:0:0:0`` / ``60``
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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/routing.rst:110
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The second OSPF particularity that is worth discussing is the support of Local Area Networks (LAN). As shown in the example below, several routers may be attached to the same LAN.
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../../protocols/rpc.rst:124
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Coming back to our example with the call for the `sum` procedure, it would return the following JSON structure.
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../../protocols/ssh.rst:290
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The ``ssh`` protocol includes other features that are beyond the scope of this book. Additional details may be found in [BS2005]_.
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../../protocols/tcp.rst:119
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the `sequence number` set to the current value of the 32 bits counter of the server host's TCP entity
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../../protocols/tls.rst:155
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`Cipher suites` : this ordered list contains the set of cryptographic algorithms that are supported by the client, with the most preferred one listed first. In contrast with ``ssh`` that allows negotiating independent algorithms for encryption, key exchange and authentication, TLS relies on suites that combine these algorithms together. Many cryptographic suites have been defined for TLS. Various recommendations have been published on the security of some of these suites :rfc:`7525`.
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