Exit Zen
11
../../principles/network.rst:66
English
This network contains two types of devices. The hosts, represented with circles and the routers, represented as boxes. A host is a device which is able to send and receive data for its own usage in contrast with routers that most of the time simply forward data towards their final destination. Routers have multiple links to neighboring routers or hosts. Hosts are usually attached via a single link to the network. Nowadays, with the growth of wireless networks, more and more hosts are equipped with several physical interfaces. These hosts are often called `multihomed`. Still, using several interfaces at the same time often leads to practical issues that are beyond the scope of this document. For this reason, we only consider `single-homed` hosts in this e-book.
924/7700 · 770
11
../../principles/transport.rst:31
English
Alice : `What time is it ?`
31/270 · 27
11
../../principles/sharing.rst:45
English
A fourth physical organization of a network is the ring topology. Like the bus organization, each host has a single physical interface connecting it to the ring. Any signal sent by a host on the ring will be received by all hosts attached to the ring. From a redundancy point of view, a single ring is not the best solution, as the signal only travels in one direction on the ring; thus if one of the links composing the ring is cut, the entire network fails. In practice, such rings have been used in local area networks, but are now often replaced by star-shaped networks. In metropolitan networks, rings are often used to interconnect multiple locations. In this case, two parallel links, composed of different cables, are often used for redundancy. With such a dual ring, when one ring fails all the traffic can be quickly switched to the other ring.
0/8540 · 854
11
../../protocols/dns.rst:42
English
The last four sections of the DNS message contain `Resource Records` (RR). All RRs have the same top level format shown in the figure below.
171/1410 · 141
11
../../principles/security.rst:134
English
In the previous example, we have explained how Eve could capture data from a particular user. This is not the only attack of this type. In 2013, based on documents collected by Edward Snowden, the press revealed that several governmental agencies were collecting lots of data on various links that compose the global Internet [Greenwald2014]_. Thanks to this massive amount of data, these governmental agencies have been able to extract lots of information about the behavior of Internet users. Like Eve, they are in a position to extract passwords, usernames and other privacy sensitive data from all the packets that they have captured. However, it seems that these agencies were often more interested in various meta data, e.g. information showing with whom a given user communicates than the actual data exchanged. These revelations have shocked the Internet community and the `Internet Engineering Task Force <https://www.ietf.org>`_ that manages the standardization of Internet protocols has declared in :rfc:`7258` that such pervasive monitoring is an attack that need to be countered in the development of new protocols. Several new protocols and extensions to existing ones are being developed to counter these attacks.
0/12280 · 1228
11
../../exercises/dns.rst:39
English
dig_ also contains some additional parameters and flags that are described in the man page. Among these, the `+trace` flag allows to trace all requests that are sent when recursively contacting DNS servers.
0/2060 · 206
11
../../exercises/email.rst:26
English
Footnotes
21/100 · 9
11
../../exercises/http.rst:31
English
The `ipvfoo <https://code.google.com/p/ipvfoo/>`_ extension on google chrome allows the user to visually detect whether a website is using IPv6 and IPv4, but also to see which web sites have been contacted when rendering a given web page. Some websites are distributed over several dozens of different servers. Can you find one ?
0/3290 · 329
11
../../exercises/ipv6.rst:40 • ../../exercises/ipv6.rst:149
English
router ``R1`` uses address ``2001:db8:1341:1::1`` on its West interface, address ``2001:db8:1341:12::1`` on its East interface and address ``2001:db8:1341:13::1`` on its South interface
0/1850 · 185
11
../../exercises/lan.rst:77
English
In the network represented in Fig. 4, can the host `H0` communicate with `H1` and vice-versa? Explain. Add whatever you need in the network to allow them to communicate.
0/1690 · 169
11
../../exercises/routing-policies.rst:133
English
Consider the network below in which a stub domain, `AS456`, is connected to two providers `AS123` and `AS789`. `AS456` advertises its prefix to both its providers. On the other hand, `AS123` advertises ``2001:db8:dead::/48`` while `AS789` advertises ``2001:db8:beef::/48`` and ``2001:db8:dead:cafe::/63``. Via which provider will the packets destined to ``2001:db8:dead:cafe::1`` will be received by `AS456` ?
0/4090 · 409
11
../../exercises/routing-protocols.rst:152
English
The password to access this daemon is `zebra`. It supports various commands that are described in the `FRRouting documentation <http://docs.frrouting.org/en/latest/ospf6d.html#showing-ospf6-information>`_ We briefly illustrate some of them below.
0/2460 · 246
11
../../exercises/tcp-2.rst:57
English
Let us start with the execution of a system call. A simple example is shown below.
0/820 · 82
11
../../exercises/trace.rst:71
English
The TCP connection is then closed by exchanging three segments, the first two having the `FIN` flag set.
0/1040 · 104
11
../../protocols/bgp.rst:43
English
There are different types of economical relationships that can exist between domains. Interdomain routing converts these relationships into peering relationships between domains that are connected via peering links.
0/2150 · 215
11
../../protocols/congestion.rst:37
English
The figure below illustrates the evolution of the congestion window when the network is lightly congested and all lost segments can be retransmitted using fast retransmit. The sender begins with a slow-start. A segment is lost but successfully retransmitted by a fast retransmit. The congestion window is divided by 2 and the sender immediately enters congestion avoidance as this was a mild congestion.
0/4030 · 403
11
../../protocols/dnssec.rst:125
English
Let us illustrate it on an example. Assume that Alice frequently uses the `example.net` domain and in particular the web server whose name is `www.example.net`. Mallory would like to redirect the TCP connections established by Alice towards `www.example.net` to one IP address that he controls. Assume that Mallory controls the `mallory.net` domain. Mallory can tune the DNS server of his domain and add special DNS records to the responses that it sends. An attack could go roughly as follows. Mallory forces Alice to visit the `www.mallory.net` web site. He can achieve this by sending a spam message to Alice or buying advertisements on a web site visited by Alice and redirect one of these advertisements to `www.mallory.net`. When visiting the advertisement, Alice's DNS resolver will send a DNS request for `www.mallory.net`. Since Mallory control the DNS server, he can easily add in the response a `AAAA` record that associates `www.example.net` to the IP address controlled by Mallory. If Alice's DNS library does not check the returned response, the cache entry for `www.example.net` will be replaced by the `AAAA` record sent by Mallory.
0/11480 · 1148
11
../../protocols/email.rst:60
English
a `header` that plays the same role as the letterhead in regular mail. It contains metadata about the message.
0/1100 · 110
11
../../protocols/ethernet.rst:72
English
The transport and datalink layers usually chose different strategies to place their CRCs or checksums. Transport layer protocols usually place their CRCs or checksums in the segment header. Datalink layer protocols sometimes place their CRC in the frame header, but often in a trailer at the end of the frame. This choice reflects implementation assumptions, but also influences performance :rfc:`893`. When the CRC is placed in the trailer, as in Ethernet, the datalink layer can compute it while transmitting the frame and insert it at the end of the transmission. All Ethernet interfaces use this optimization today. When the checksum is placed in the header, as in a TCP segment, it is impossible for the network interface to compute it while transmitting the segment. Some network interfaces provide hardware assistance to compute the TCP checksum, but this is more complex than if the TCP checksum were placed in the trailer [#ftso]_.
0/9400 · 940
11
../../protocols/http.rst:50
English
A standardized protocol to efficiently retrieve the documents stored on a server
0/800 · 80