Exit Zen
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../../principles/security.rst:310
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A secret key encryption scheme is a perfectly reversible functions, i.e. given an encryption function `E`, there is an associated decryption function `D` such that :math:`\forall k \forall M : D(K, E(M,K))=M`.
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../../exercises/dns.rst:73
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A DNS implementation such as `dig`, and more importantly a name resolver such as bind_ or unbound_, always checks that the received DNS reply contains the same identifier as the DNS request that it sent. Why is this so important ?
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../../exercises/ipv6.rst:220
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Assuming that the following IPv6 addresses are used :
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../../exercises/lan.rst:250
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The state of switch ``s9`` is different. The output of :manpage:`brctl(8)` indicates that the root identifier is ``0003.f63545ab5f79`` which is at a distance of ``1`` from switch ``s9``. The root port on ``s9`` is port `1`, i.e. ``s9-eth1``. Two of the ports of this switch forward data packets, the root port and the ``s9-eth3`` which is a designated port. The ``s9-eth2`` port is a blocked port.
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../../exercises/routing-policies.rst:197
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``Code==0``
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../../exercises/routing-protocols.rst:274
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IPMininet_ can also be used to perform experiments with RIP. A simple script that uses RIPng is provided below.
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../../exercises/tcp-2.rst:133
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packetdrill_ injects the ``FIN`` segment and the instrumented kernel returns an acknowledgment. If packetdrill_ issues the :manpage:`close` system call, the kernel will send a ``FIN`` segment to terminate the connection. packetdrill_ injects an acknowledgment to confirm the end of the connection.
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../../protocols/bgp.rst:79
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the `export filter` that specifies, for each peering relationship, the routes that can be advertised to the neighboring domain
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../../protocols/congestion.rst:122
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Thanks to the `ECT`, `CE` and `ECE`, routers can mark packets during congestion and receivers can return the congestion information back to the TCP senders. However, these three bits are not sufficient to allow a server to reliably send the `ECE` bit to a TCP sender. TCP acknowledgments are not sent reliably. A TCP acknowledgment always contains the next expected sequence number. Since TCP acknowledgments are cumulative, the loss of one acknowledgment is recovered by the correct reception of a subsequent acknowledgment.
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../../protocols/dnssec.rst:263
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The last record introduced by DNSSEC is the `NSEC` record. It is used to authenticate a negative response returned by a DNS server. If a resolver requests a domain name that is not defined in the zone, the server replies with an error message. The designers of the original version of the DNS thought that these errors would not be very frequent and resolvers were not required to cache those negative responses. However, operational experience showed that queries for invalid domain names are more frequent than initially expected and a large fraction of the load on some servers is caused by repeated queries for invalid names. Typical examples include queries for invalid TLDs to the root DNS servers or queries caused by configuration errors [WF2003]_. Current DNS deployments allow resolvers to cache those negative answers to reduce the load on the entire DNS :rfc:`2308`.
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../../protocols/email.rst:126
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the `Message-Id:` header line is used to associate a "unique" identifier to each email. Email identifiers are usually structured like `string@domain` where `string` is a unique character string or sequence number chosen by the sender of the email and `domain` the domain name of the sender. Since domain names are unique, a host can generate globally unique message identifiers concatenating a locally unique identifier with its domain name.
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../../protocols/ethernet.rst:162
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In the late 1980s, 10 Mbps became too slow for some applications and network manufacturers developed several LAN technologies that offered higher bandwidth, such as the 100 Mbps FDDI LAN that used optical fibers. As the development of 10Base5, 10Base2 and 10BaseT had shown that Ethernet could be adapted to different physical layers, several manufacturers started to work on 100 Mbps Ethernet and convinced IEEE to standardize this new technology that was initially called `Fast Ethernet`. `Fast Ethernet` was designed under two constraints. First, `Fast Ethernet` had to support twisted pairs. Although it was easier from a physical layer perspective to support higher bandwidth on coaxial cables than on twisted pairs, coaxial cables were a nightmare from deployment and maintenance perspectives. Second, `Fast Ethernet` had to be perfectly compatible with the existing 10 Mbps Ethernet to allow `Fast Ethernet` technology to be used initially as a backbone technology to interconnect 10 Mbps Ethernet networks. This forced `Fast Ethernet` to use exactly the same frame format as 10 Mbps Ethernet. This implied that the minimum `Fast Ethernet` frame size remained at 512 bits. To preserve CSMA/CD with this minimum frame size and 100 Mbps instead of 10 Mbps, the duration of the `slot time` was decreased to 5.12 microseconds.
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../../protocols/http.rst:112
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Over the years, various extensions to HTML have been proposed and implemented. These include the specification of style sheets that adjust the layout of the document and the possibility of adding or referencing javascript code. Additional details about the various extensions to HTML may be found in the `official specifications <http://www.w3.org/MarkUp/>`_ maintained by W3C_.
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../../protocols/http2.rst:157
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Other frame types are described later. The `Flags` are used for some frame types and the `R` bit must be set to zero. The last important field of the `HTTP/2.0 Frame` header is the `Stream Identifier`. With HTTP/2.0, the bytestream of the underlying transport connection is divided in independent streams that are identified by an integer. The odd (resp. even) stream identifiers are managed by the client (resp. server). This enables the server (or the client) to multiplex data corresponding to different frames over a single bytestream.
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../../protocols/ipv6.rst:107
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The experience of IPv4 revealed that the scalability of a network layer protocol heavily depends on its addressing architecture. The designers of IPv6 spent a lot of effort defining its addressing architecture :rfc:`3513`. All IPv6 addresses are 128 bits wide. This implies that there are :math:`340,282,366,920,938,463,463,374,607,431,768,211,456 (3.4 \times 10^{38})` different IPv6 addresses. As the surface of the Earth is about 510,072,000 :math:`km^2`, this implies that there are about :math:`6.67 \times 10^{23}` IPv6 addresses per square meter on Earth. Compared to IPv4, which offers only 8 addresses per square kilometer, this is a significant improvement on paper.
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../../protocols/ethernet.rst:68
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Ethernet DIX frame format
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../../protocols/routing.rst:76
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Area border routers : A router that is attached to several areas.
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../../protocols/rpc.rst:102
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`jsonrpc`: a string indicating the version of the protocol used. This is important to allow the protocol to evolve in the future.
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../../protocols/ssh.rst:206
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a key used by the client (resp. server) to authenticate the data that it sends
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../../protocols/tcp.rst:51
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when the `ACK` flag is set, it indicates that the `acknowledgment` field contains a valid number. Otherwise, the content of the `acknowledgment` field must be ignored by the receiver
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