Exit Zen
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../../protocols/ipv6.rst:537
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``1`` : Communication with destination administratively prohibited. This indicates that a firewall has refused to forward the packet towards its final destination.
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../../protocols/wifi.rst:29
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When developing its family of standards, the `IEEE 802.11 working group <http://www.ieee802.org/11/>`_ took a similar approach as the `IEEE 802.3 working group <http://www.ieee802.org/3/>`_ that developed various types of physical layers for Ethernet networks. 802.11 networks use the CSMA/CA Medium Access Control technique described earlier and they all assume the same architecture and use the same frame format.
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../../protocols/tcp.rst:533
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To deal with the limited size of the SACK option, a TCP receiver currently having more than 3 blocks inside its receiving buffer must select the blocks to place in the SACK option. A good heuristic is to put in the SACK option the blocks that have most recently changed, as the sender is likely to be already aware of the older blocks.
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../../glossary.rst:189
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SMTP
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../../principles/dv.rst:177
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Forwarding tables versus routing tables
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../../principles/transport.rst:838
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In reality, the Internet does not strictly enforce this MSL. However, it is reasonable to expect that most packets on the Internet will not remain in the network during more than 2 minutes. There are a few exceptions to this rule, such as :rfc:`1149` whose implementation is described in http://www.blug.linux.no/rfc1149/ but there are few real links supporting :rfc:`1149` in the Internet.
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../../principles/sharing.rst:749
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Depending on the network, a `max-min fair allocation` may not always exist. In practice, `max-min fairness` is an ideal objective that cannot necessarily be achieved. When there is a single bottleneck link as in the example above, `max-min fairness` implies that each source should be allocated the same transmission rate.
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../../protocols/email.rst:264
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16
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../../protocols/ipv6.rst:538
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``2`` : Beyond scope of source address. This message can be sent if the source is using link-local addresses to reach a global unicast address outside its subnet.
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../../protocols/wifi.rst:35
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The architecture of WiFi networks is slightly different from the Local Area Networks that we have discussed until now. There are, in practice, two main types of WiFi networks : `independent` or `adhoc` networks and `infrastructure` networks [#fBSS]_. An `independent` or `adhoc` network is composed of a set of devices that communicate with each other. These devices play the same role and the `adhoc` network is usually not connected to the global Internet. `Adhoc` networks are used when for example a few laptops need to exchange information or to connect a computer with a WiFi printer.
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../../protocols/tcp.rst:535
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When a sender receives a SACK option indicating a new block and thus a new possible segment loss, it usually does not retransmit the missing segments immediately. To deal with reordering, a TCP sender can use a heuristic similar to `fast retransmit` by retransmitting a gap only once it has received three SACK options indicating this gap. It should be noted that the SACK option does not supersede the `acknowledgment number` of the TCP header. A TCP sender can only remove data from its sending buffer once they have been acknowledged by TCP's cumulative acknowledgments. This design was chosen for two reasons. First, it allows the receiver to discard parts of its receiving buffer when it is running out of memory without loosing data. Second, as the SACK option is not transmitted reliably, the cumulative acknowledgments are still required to deal with losses of `ACK` segments carrying only SACK information. Thus, the SACK option only serves as a hint to allow the sender to optimize its retransmissions.
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../../glossary.rst:191
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The Simple Mail Transfer Protocol is defined in :rfc:`821`
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../../principles/dv.rst:179
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Routers usually maintain at least two data structures that contain information about the reachable destinations. The first data structure is the `routing table`. The `routing table` is a data structure that associates a destination to an outgoing interface or a nexthop router and a set of additional attributes. Different routing protocols can associate different attributes for each destination. Distance vector routing protocols will store the cost to reach the destination along the shortest path. Other routing protocols may store information about the number of hops of the best path, its lifetime or the number of sub paths. A `routing table` may store different paths towards a given destination and flag one of them as the best one.
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../../principles/reliability.rst:675
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The Alternating Bit Protocol can recover from transmission errors and frame losses. However, it has one important drawback. Consider two hosts that are directly connected by a 50 Kbits/sec satellite link that has a 250 milliseconds propagation delay. If these hosts send 1000 bits frames, then the maximum throughput that can be achieved by the alternating bit protocol is one frame every :math:`20+250+250=520` milliseconds if we ignore the transmission time of the acknowledgment. This is less than 2 Kbits/sec !
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../../principles/sharing.rst:751
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To visualize the different rate allocations, it is useful to consider the graph shown below. In this graph, we plot on the `x-axis` (resp. `y-axis`) the rate allocated to host `B` (resp. `A`). A point in the graph :math:`(r_B,r_A)` corresponds to a possible allocation of the transmission rates. Since there is a `2 Mbps` bottleneck link in this network, the graph can be divided into two regions. The lower left part of the graph contains all allocations :math:`(r_B,r_A)` such that the bottleneck link is not congested (:math:`r_A+r_B<2`). The right border of this region is the `efficiency line`, i.e. the set of allocations that completely utilize the bottleneck link (:math:`r_A+r_B=2`). Finally, the `fairness line` is the set of fair allocations.
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../../protocols/email.rst:264
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Q
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../../protocols/ipv6.rst:539
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``3`` : Address unreachable. This message indicates that the packet reached the subnet of the destination, but the host that owns this destination address cannot be reached.
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../../protocols/wifi.rst:42
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An 802.11 independent or adhoc network
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../../protocols/tcp.rst:537
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As explained earlier, the TCP Timestamp option :rfc:`1323` prevents ambiguities while collecting round-trip-time measurements. It plays another very important role in today's high-bandwidth networks. Since TCP uses 32 bits long sequence numbers, the sequence numbers wrap after the transmission of 4 GBytes of data. With 10 Gbps and soon 100 Gbps interfaces, TCP only needs to transmit during a few seconds before reusing the same sequence number. Given that the Maximum Segment Lifetime is still 2 minutes, several packets, belonging to the same TCP connection could use the same sequence number. If one of these packets is severely delayed through the network, it could reappear at the same time as a packet with the same TCP sequence number. To prevent this problem, most modern TCP implementations associate a TCP timestamp option to each segment on transmission. When a TCP stack receives a TCP segment, it checks that its TCP timestamp is valid and if not the segment is discarded :rfc:`7323`.
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../../glossary.rst:96
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SNMP
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