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../../principles/reliability.rst:59
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To understand some of the principles behind the physical transmission of information, let us consider the simple case of an electrical wire that is used to transmit bits. Assume that the two communicating hosts want to transmit one thousand bits per second. To transmit these bits, the two hosts can agree on the following rules :
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../../principles/transport.rst:51
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Bob : `This is not the correct password.`
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../../principles/sharing.rst:106 • ../../principles/sharing.rst:172
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Network congestion
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../../protocols/dns.rst:69
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The DNS is mainly used to find the address that corresponds to a given name. However, it is sometimes useful to obtain the name that corresponds to an IP address. This done by using the `PTR` (`pointer`) `RR`. The `RData` part of a `PTR` `RR` contains the name while the `Name` part of the `RR` contains the IP address encoded in the `in-addr.arpa` domain. IPv4 addresses are encoded in the `in-addr.arpa` by reversing the four digits that compose the dotted decimal representation of the address. For example, consider IPv4 address `192.0.2.11`. The hostname associated to this address can be found by requesting the `PTR` `RR` that corresponds to `11.2.0.192.in-addr.arpa`. A similar solution is used to support IPv6 addresses :rfc:`3596`, but slightly more complex given the length of the IPv6 addresses. For example, consider IPv6 address `2001:1890:123a::1:1e`. To obtain the name that corresponds to this address, we need first to convert it in a reverse dotted decimal notation : `e.1.0.0.1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.a.3.2.1.0.9.8.1.1.0.0.2`. In this notation, each character between dots corresponds to one nibble, i.e. four bits. The low-order byte (`e`) appears first and the high order (`2`) last. To obtain the name that corresponds to this address, one needs to append the `ip6.arpa` domain name and query for `e.1.0.0.1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.a.3.2.1.0.9.8.1.1.0.0.2.ip6.arpa`. In practice, tools and libraries do the conversion automatically and the user does not need to worry about it.
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../../principles/security.rst:279
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This principle is important because it remains the basic assumption of all cryptographers. Any system that relies on the secrecy of its algorithm to be considered secure is doomed to fail and be broken one day.
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../../exercises/dns.rst:57
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Perform the same analysis for a popular website such as `www.google.com`. What is the lifetime associated to the corresponding IPv6 address ? If you perform the same request several times, do you always receive the same answer ? Can you explain why a lifetime is associated to the DNS replies ?
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../../exercises/ipv6.rst:154
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Routers ``R2`` and ``R3`` are buggy in this network. Besides the routes for their local interfaces (not shown in the figure), they only have a default route which is shown in the figure below.
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../../exercises/lan.rst:211
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By using :manpage:`brtcl(8)`, we can easily observe the state of the Spanning Tree protocol on the different switches. Let us start with ``s3``, i.e. the root of the Spanning Tree.
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../../exercises/routing-policies.rst:191
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Can you explain why at the eighth, ninth and tenth hopes several IPv6 addresses are reported in the ``traceroute6`` output ?
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../../exercises/routing-protocols.rst:265
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OSPFv3 also includes `LS Acknowledge` packets that acknowledge the correct reception of link state information.
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../../exercises/tcp-2.rst:115
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packetdrill_ can also inject data that will be read by the stack as shown by the lines below.
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../../exercises/trace.rst:111
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the `wireshark wiki <https://wiki.wireshark.org/>` page on `Display filters <https://wiki.wireshark.org/DisplayFilters>`_
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../../protocols/bgp.rst:72
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Finally, the last type of peering relationship is the `sibling`. Such a relationship is used when two domains exchange all their routes in both directions. In practice, such a relationship is only used between domains that belong to the same company.
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../../protocols/congestion.rst:110
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The second difficulty is how to allow the receiver to inform the sender of the reception of network packets marked with the `CE` bit. In reliable transport protocols like TCP and SCTP, the acknowledgments can be used to provide this feedback. For TCP, two options were possible : change some bits in the TCP segment header or define a new TCP option to carry this information. The designers of ECN opted for reusing spare bits in the TCP header. More precisely, two TCP flags have been added in the TCP header to support ECN. The `ECN-Echo` (ECE) is set in the acknowledgments when the `CE` was set in packets received on the forward path.
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../../protocols/dnssec.rst:196
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The next step is to contact `ns.org`, the `NS` responsible for the `.org` TLD to retrieve the `NS` record for the `example.org` domain. This record is accompanied by a `RRSIG` record that authenticates it. This `RRSIG` record is signed with the key of the `.org` domain. Alice's resolver can retrieve this public key as the `DNSKEY` record for the `.org`, but how can it trust this key since it is distributed by using the DNS and could have been modified by attackers ? DNSSEC solves this problem by using the `DS` record that is stored in the parent zone (in this case, the root zone). This record contains a hash of a public key that is signed with a `RRSIG` signature. Since Alice's resolver's trusts the root key, it can validate the signature of the `DS` record for the `.org` domain. It can then retrieve the `DNSKEY` record for this domain from the DNS and compare the hash of this key with the `DS` record. If they match, the public key of the `.org` domain can be trusted. The same technique is used to obtain and validate the key of the `example.org` domain. Once this key is trusted, Alice's resolver can request the `AAAA` record for `www.example.org` and validate its signature.
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../../protocols/email.rst:109
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A simple email message containing the `From:`, `To:`, `Subject:` and `Date:` header lines and two lines of body is shown below.
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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/http.rst:100
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More complex HTML elements can also include optional attributes in the start tag ::
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../../protocols/http2.rst:146
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Let us first examine how HTTP/2.0 structures the bytestream of the underlying connection.
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../../protocols/ipv6.rst:100
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The IPng address size
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