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../../principles/sharing.rst:744
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The congestion control scheme should be `fair`. Most congestion schemes aim at achieving `max-min fairness`. An allocation of transmission rates to sources is said to be `max-min fair` if :
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../../glossary.rst:60
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segment
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../../principles/dv.rst:170
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:math:`[A=\infty,B=0,C=1,E=\infty]` to router `E`
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../../principles/reliability.rst:584
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The figure below illustrates the operation of the alternating bit protocol.
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../../principles/sharing.rst:746
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no link in the network is congested
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../../principles/transport.rst:834
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For a discussion on how the sending buffer can change, see e.g. [SMM1998]_
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../../glossary.rst:62
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a segment is the unit of information transfer in the transport layer
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../../principles/dv.rst:172
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If, unfortunately, the distance vector sent to router `C` is lost due to a transmission error or because router `C` is overloaded, a new count to infinity problem can occur. If router `C` sends its distance vector :math:`[A=2,B=1,C=0,E=\infty]` to router `E`, this router installs a route of distance `3` to reach `A` via `C`. Router `E` sends its distance vectors :math:`[A=3,B=\infty,C=1,E=1]` to router `B` and :math:`[A=\infty,B=1,C=\infty,E=0]` to router `C`. This distance vector allows `B` to recover a route of distance `4` to reach `A`.
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../../principles/sharing.rst:747
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the rate allocated to source :math:`j` cannot be increased without decreasing the rate allocated to a source :math:`i` whose allocation is smaller than the rate allocated to source :math:`j` [Leboudec2008]_ .
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../../principles/transport.rst:836
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Note that if the receive window shrinks, it might happen that the sender has already sent a segment that is not anymore inside its window. This segment will be discarded by the receiver and the sender will retransmit it later.
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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/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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../../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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../../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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../../glossary.rst:96
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SNMP
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../../principles/dv.rst:181
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The `routing table` is a software data structure which is updated by (one or more) routing protocols. The `routing table` is usually not directly used when forwarding packets. Packet forwarding relies on a more compact data structure which is the `forwarding table`. On high-end routers, the `forwarding table` is implemented directly in hardware while lower performance routers will use a software implementation. A `forwarding table` contains a subset of the information found in the `routing table`. It only contains the nexthops towards each destination that are used to forward packets and no attributes. A `forwarding table` will typically associate each destination to one or more outgoing interface or nexthop router.
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