Exit Zen
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../../protocols/tcp.rst:486
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The first extension that was proposed is the fast retransmit heuristic. This extension can be implemented on TCP senders and thus does not require any change to the protocol. It only assumes that the TCP receiver is able to buffer out-of-sequence segments.
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../../glossary.rst:126
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round-trip-time
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../../principles/dv.rst:117
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Unfortunately, if the distance vector sent to `A` is lost or if `A` sends its own distance vector ( :math:`[A=0,D=1,B=3,C=3,E=2]` ) at the same time as `D` sends its distance vector, `D` updates its routing table to use the shorter routes advertised by `A` towards `B`, `C` and `E`. After some time `D` sends a new distance vector : :math:`[D=0,A=1,E=3,C=4,B=4]`. `A` updates its routing table and after some time sends its own distance vector :math:`[A=0,D=1,B=5,C=5,E=4]`, etc. This problem is known as the `count to infinity problem` in the networking literature.
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../../principles/reliability.rst:552
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To solve this problem, datalink protocols associate a `sequence number` to each data frame. This `sequence number` is one of the fields found in the header of data frames. We use the notation `D(x,...)` to indicate a data frame whose sequence number field is set to value `x`. The acknowledgments also contain a sequence number indicating the data frames that it is acknowledging. We use `OKx` to indicate an acknowledgment frame that confirms the reception of `D(x,...)`. The sequence number is encoded as a bit string of fixed length. The simplest reliable protocol is the Alternating Bit Protocol (ABP).
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../../principles/transport.rst:810
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Abrupt connection release
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../../principles/sharing.rst:725
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If many senders are attached to the left part of the network above, they all send a window full of segments. These segments are stored in the buffers of the router before being transmitted towards their destination. If there are many senders on the left part of the network, the occupancy of the buffers quickly grows. A consequence of the buffer occupancy is that the round-trip-time, measured by the transport protocol, between the sender and the receiver increases. Consider a network where 10,000 bits segments are sent. When the buffer is empty, such a segment requires 1 millisecond to be transmitted on the `10 Mbps` link and 5 milliseconds to be the transmitted on the `2 Mbps` link. Thus, the measured round-trip-time measured is roughly 6 milliseconds if we ignore the propagation delay on the links. If the buffer contains 100 segments, the round-trip-time becomes :math:`1+100 \times 5+ 5` milliseconds as new segments are only transmitted on the `2 Mbps` link once all previous segments have been transmitted. Unfortunately, if the reliable transport protocol uses a retransmission timer and performs `go-back-n` to recover from transmission errors it will retransmit a full window of segments. This increases the occupancy of the buffer and the delay through the buffer... Furthermore, the buffer may store and send on the low bandwidth links several retransmissions of the same segment. This problem is called `congestion collapse`. It occurred several times during the late 1980s on the Internet [Jacobson1988]_.
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../../protocols/email.rst:260
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../../protocols/ipv6.rst:512
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The type 0 routing option was intended to allow a host to indicate a loose source route that should be followed by a packet by specifying the addresses of some of the routers that must forward this packet. Unfortunately, further work with this routing header, including an entertaining demonstration with scapy_ [BE2007]_ , revealed severe security problems with this routing header. For this reason, loose source routing with the type 0 routing header has been removed from the IPv6 specification :rfc:`5095`.
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../../protocols/wifi.rst:24
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11 Mbps
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../../protocols/tcp.rst:488
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From a performance point of view, one issue with TCP's `retransmission timeout` is that when there are isolated segment losses, the TCP sender often remains idle waiting for the expiration of its retransmission timeouts. Such isolated losses are frequent in the global Internet [Paxson99]_. A heuristic to deal with isolated losses without waiting for the expiration of the retransmission timeout has been included in many TCP implementations since the early 1990s. To understand this heuristic, let us consider the figure below that shows the segments exchanged over a TCP connection when an isolated segment is lost.
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../../glossary.rst:128
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The round-trip-time (RTT) is the delay between the transmission of a segment and the reception of the corresponding acknowledgment in a transport protocol.
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../../principles/dv.rst:119
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Routers `A` and `D` exchange distance vectors with increasing costs until these costs reach :math:`\infty`. This problem may occur in other scenarios than the one depicted in the above figure. In fact, distance vector routing may suffer from count to infinity problems as soon as there is a cycle in the network. Unfortunately, cycles are widely used in networks since they provide the required redundancy to deal with link and router failures. To mitigate the impact of counting to infinity, some distance vector protocols consider that :math:`16=\infty`. Unfortunately, this limits the metrics that network operators can use and the diameter of the networks using distance vectors.
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../../principles/transport.rst:812
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As the entity that sends the `DR` segment cannot know whether the other entity has already sent all its data on the connection, SDUs can be lost during such an `abrupt connection release`.
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../../principles/sharing.rst:727
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The `congestion collapse` is a problem that all heterogeneous networks face. Different mechanisms have been proposed in the scientific literature to avoid or control network congestion. Some of them have been implemented and deployed in real networks. To understand this problem in more detail, let us first consider a simple network with two hosts attached to a high bandwidth link that are sending segments to destination `C` attached to a low bandwidth link as depicted below.
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../../protocols/email.rst:260
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f
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../../protocols/ipv6.rst:520
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ICMP version 6
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../../protocols/wifi.rst:24 • ../../protocols/wifi.rst:25
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../../protocols/tcp.rst:494
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Detecting isolated segment losses
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../../glossary.rst:174
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router
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../../principles/dv.rst:124
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This count to infinity problem occurs because router `A` advertises to router `D` a route that it has learned via router `D`. A possible solution to avoid this problem could be to change how a router creates its distance vector. Instead of computing one distance vector and sending it to all its neighbors, a router could create a distance vector that is specific to each neighbor and only contains the routes that have not been learned via this neighbor. This could be implemented by the following pseudocode.
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