Exit Zen
56
../../principles/transport.rst:167
English
A second imperfection that may affect an `unreliable connectionless service` is that it may duplicate SDUs. Some packets may be duplicated in a network and be delivered twice to their destination. This is illustrated by the time-sequence diagram below.
0/2520 · 252
56
../../principles/sharing.rst:237
English
In virtual circuit networks, another way to manage or prevent congestion is to limit the number of circuits that use the network at any time. This technique is usually called `connection admission control`. When a host requests the creation of a new circuit in the network, it specifies the destination and in some networking technologies the required bandwidth. With this information, the network can check whether there are enough resources available to reach this particular destination. If yes, the circuit is established. If not, the request is denied and the host will have to defer the creation of its virtual circuit. `Connection admission control` schemes are widely used in the telephone networks. In these networks, a busy tone corresponds to an unavailable destination or a congested network.
0/8040 · 804
56
../../principles/security.rst:593
English
The two protocols above prevent eavesdropping attacks, but not man-in-the-middle attacks. If Mallory can intercept the messages sent by Alice, he could force her to reveal :math:`H^n(P)` and then use this information to authenticate as Alice on Bob's computer. In practice, hash chains should only be used when the communicating users know that there cannot be any man-in-the-middle on their communication.
0/4060 · 406
56
../../exercises/ipv6.rst:529
English
With the previous exercises, you have learned how to reason about IPv6 networks "on paper". Given the availability of IPv6 implementations, it is also possible to carry out experiments in real and virtual labs. Several virtual environments are possible. In this section, we focus on mininet_. mininet_ is an emulation framework developed at Stanford University that leverages the namespaces features of recent Linux kernels. With those namespaces, a single Linux kernel can support a variety of routers and hosts interconnected by virtual links. mininet_ has been used by several universities as an educational tool, but unfortunately it was designed without IPv6 support.
0/6720 · 672
56
../../exercises/routing-protocols.rst:838
English
It is interesting to look at the output of this command in details. Router `as1` has routes for three different IPv6 prefixes. The first prefix is its own prefix, ``2001:cafe:1::/48``. It has no nexthop since this prefix is originated by the router. Then, `as1` has received two paths for ``2001:cafe:2::/48``. In the BGP Loc-RIB, the `>` character indicates the best route according to the BGP decision process. ``2001:cafe:2::/48`` was learned over two different BGP sessions:
0/4790 · 479
56
../../exercises/tcp-2.rst:691
English
Same question as above but now with a small variation. Recent TCP implementations use a large initial value for the congestion window. Draw the time-sequence diagram that corresponds to an initial value of 10000 bytes for this congestion window.
0/2450 · 245
56
../../protocols/bgp.rst:184
English
In the above pseudo-code, the `build\_BGP\_update(d)` procedure extracts from the `BGP Loc-RIB` the best path towards destination `d` (i.e. the route installed in the FIB) and prepares the corresponding BGP `UPDATE` message. This message is then passed to the `export filter` that returns `None` if the route cannot be advertised to the peer or the (possibly modified) BGP `UPDATE` message to be advertised. BGP routers allow network administrators to specify very complex `export filters`, see e.g. [WMS2004]_. A simple `export filter` that implements the equivalent of `split horizon` is shown below.
0/6020 · 602
56
../../protocols/congestion.rst:240
English
With the ECT bit, the deployment issue with ECN is solved provided that all sources cooperate. If some sources do not support ECN but still set the ECT bit in the packets that they sent, they will have an unfair advantage over the sources that correctly react to packet markings. Several solutions have been proposed to deal with this problem :rfc:`3540`, but they are outside the scope of this book.
0/4000 · 400
56
../../protocols/email.rst:232
English
i
0/100 · 1
56
../../protocols/ethernet.rst:211
English
Increasing the physical layer bandwidth as in `Fast Ethernet` was only one of the solutions to improve the performance of Ethernet LANs. A second solution was to replace the hubs with more intelligent devices. As `Ethernet hubs` operate in the physical layer, they can only regenerate the electrical signal to extend the geographical reach of the network. From a performance perspective, it would be more interesting to have devices that operate in the datalink layer and can analyze the destination address of each frame and forward the frames selectively on the link that leads to the destination. Such devices are usually called `Ethernet switches` [#fbridges]_. An `Ethernet switch` is a relay that operates in the datalink layer as is illustrated in the figure below.
0/7730 · 773
56
../../protocols/http.rst:189
English
All status codes starting with digit `4` indicate that the server has detected an error in the HTTP request sent by the client. `400 Bad Request` indicates a syntax error in the HTTP request. `404 Not Found` indicates that the requested document does not exist on the server.
0/2750 · 275
56
../../protocols/ipv6.rst:182
English
``/128`` in the rare case when it is known that no more than one host will be attached
0/860 · 86
56
../../protocols/ethernet.rst:177
English
Two pairs shielded twisted pair, 25m maximum
0/440 · 44
56
../../protocols/routing.rst:173
English
OSPF can support `virtual links` to connect routers together that belong to the same area but are not directly connected. However, this goes beyond this introduction to OSPF.
0/1740 · 174
56
../../protocols/tcp.rst:194
English
This limit of 100 TCBs in the `SYN Rcvd` state was chosen to protect the TCP entity from the risk of overloading its memory with too many TCBs in the `SYN Rcvd` state. However, it was also the reason for a new type of Denial of Service (DoS) attack :rfc:`4987`. A DoS attack is defined as an attack where an attacker can render a resource unavailable in the network. For example, an attacker may cause a DoS attack on a 2 Mbps link used by a company by sending more than 2 Mbps of packets through this link. In this case, the DoS attack was more subtle. As a TCP entity discards all received `SYN` segments as soon as it has 100 TCBs in the `SYN Rcvd` state, an attacker simply had to send a few 100 `SYN` segments every second to a server and never reply to the received `SYN+ACK` segments. To avoid being caught, attackers were of course sending these `SYN` segments with a different address than their own IP address [#fspoofing]_. On most TCP implementations, once a TCB entered the `SYN Rcvd` state, it remained in this state for several seconds, waiting for a retransmission of the initial `SYN` segment. This attack was later called a `SYN flood` attack and the servers of the ISP named panix were among the first to `be affected <http://memex.org/meme2-12.html>`_ by this attack.
0/12870 · 1287
56
../../protocols/tls.rst:297
English
TLS Cipher suites
0/170 · 17
56
../../protocols/wifi.rst:158
English
The 802.11 working group defined the `basic service set (BSS)` as a group of devices that communicate with each other. We continue to use `network` when referring to a set of devices that communicate.
0/2000 · 200
57
../../bibliography.rst:338
English
Shreedhar and G. Varghese. `Efficient fair queueing using deficit round robin <http://doi.acm.org/10.1145/217391.217453>`_ SIGCOMM Computer Communication Review 25, 4 (October 1995), 231-242.
0/1910 · 191
57
../../glossary.rst:188
English
The HyperText Transport Protocol is defined in :rfc:`2616`
0/580 · 58
57
../../principles/network.rst:255
English
Flat or hierarchical addresses
0/300 · 30