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../../principles/reliability.rst:776
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`Selective repeat` is a better strategy to recover from losses. Intuitively, `selective repeat` allows the receiver to accept out-of-sequence frames. Furthermore, when a `selective repeat` sender detects losses, it only retransmits the frames that have been lost and not the frames that have already been correctly received.
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../../principles/sharing.rst:1063
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As an illustration, let us consider the network scenario above and assume that the router implements the DECBit binary feedback scheme [RJ1995]_. This scheme uses a form of Forward Explicit Congestion Notification and a router marks the congestion bit in arriving packets when its buffer contains one or more packets. In the figure below, we use a `*` to indicate a marked packet.
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../../glossary.rst:307
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TLS
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../../principles/linkstate.rst:87
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Flooding allows LSPs to be distributed to all routers inside the network without relying on routing tables. In the example above, the LSP sent by router `E` is likely to be sent twice on some links in the network. For example, routers `B` and `C` receive `E`'s LSP at almost the same time and forward it over the `B-C` link. To avoid sending the same LSP twice on each link, a possible solution is to slightly change the pseudo-code above so that a router waits for some random time before forwarding a LSP on each link. The drawback of this solution is that the delay to flood an LSP to all routers in the network increases. In practice, routers immediately flood the LSPs that contain new information (e.g. addition or removal of a link) and delay the flooding of refresh LSPs (i.e. LSPs that contain exactly the same information as the previous LSP originating from this router) [FFEB2005]_.
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../../principles/reliability.rst:778
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A `selective repeat` receiver maintains a sliding window of `W` frames and stores in a buffer the out-of-sequence frames that it receives. The figure below shows a five-frame receive window on a receiver that has already received frames `7` and `9`.
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../../principles/sharing.rst:1218
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When the connection starts, its congestion window is set to one segment. Segment `S0` is sent an acknowledgment at roughly :math:`t_{0}+4`. The congestion window is increased by one segment and `S1` and `S2` are transmitted at time :math:`t_{0}+4` and :math:`t_{0}+5`. The corresponding acknowledgments are received at times :math:`t_{0}+8` and :math:`t_{0}+10`. Upon reception of this last acknowledgment, the congestion window reaches `3` and segments can be sent (`S4` and `S5`). When segment `S6` reaches router `R1`, its buffers already contain `S5`. The packet containing `S6` is thus marked to inform the sender of the congestion. Note that the sender will only notice the congestion once it receives the corresponding acknowledgment at :math:`t_{0}+18`. In the meantime, the congestion window continues to increase. At :math:`t_{0}+16`, upon reception of the acknowledgment for `S5`, it reaches `4`. When congestion is detected, the congestion window is decreased down to `2`. This explains the idle time between the reception of the acknowledgment for `S*6` and the transmission of `S10`.
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../../glossary.rst:309
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Transport Layer Security, defined in :rfc:`5246` is a cryptographic protocol that is used to provide communication security for Internet applications. This protocol is used on top of the transport service but a detailed description is outside the scope of this book.
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../../principles/linkstate.rst:89
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To ensure that all routers receive all LSPs, even when there are transmissions errors, link state routing protocols use `reliable flooding`. With `reliable flooding`, routers use acknowledgments and if necessary retransmissions to ensure that all link state packets are successfully transferred to each neighboring router. Thanks to reliable flooding, all routers store in their LSDB the most recent LSP sent by each router in the network. By combining the received LSPs with its own LSP, each router can build a graph that represents the entire network topology.
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../../principles/reliability.rst:784
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The receiving window with selective repeat
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../../principles/sharing.rst:1220
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In practice, a router is connected to multiple input links. The figure below shows an example with two hosts.
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../../glossary.rst:90
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UDP
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../../principles/reliability.rst:786
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A `selective repeat` receiver discards all frames having an invalid CRC, and maintains the variable `lastack` as the sequence number of the last in-sequence frame that it has received. The receiver always includes the value of `lastack` in the acknowledgments that it sends. Some protocols also allow the `selective repeat` receiver to acknowledge the out-of-sequence frames that it has received. This can be done for example by placing the list of the correctly received, but out-of-sequence frames in the acknowledgments together with the `lastack` value.
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../../principles/sharing.rst:1279
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In general, the links have a non-zero delay. This is illustrated in the figure below where a delay has been added on the link between `R` and `C`.
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../../glossary.rst:92
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User Datagram Protocol is a protocol of the transport layer in the TCP/IP protocol suite that provides an unreliable connectionless service that includes a mechanism to detect corruption
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../../principles/linkstate.rst:98
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Static or dynamic link metrics ?
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../../principles/reliability.rst:788
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When a `selective repeat` receiver receives a data frame, it first verifies whether the frame is inside its receiving window. If yes, the frame is placed in the receive buffer. If not, the received frame is discarded and an acknowledgment containing `lastack` is sent to the sender. The receiver then removes all consecutive frames starting at `lastack` (if any) from the receive buffer. The payloads of these frames are delivered to the user, `lastack` and the receiving window are updated, and an acknowledgment acknowledging the last frame received in sequence is sent.
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../../principles/sharing.rst:1326
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Footnotes
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../../glossary.rst:18
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unicast
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../../principles/linkstate.rst:100
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As link state packets are flooded regularly, routers are able to measure the quality (e.g. delay or load) of their links and adjust the metric of each link according to its current quality. Such dynamic adjustments were included in the ARPANET routing protocol [MRR1979]_ . However, experience showed that it was difficult to tune the dynamic adjustments and ensure that no forwarding loops occur in the network [KZ1989]_. Today's link state routing protocols use metrics that are manually configured on the routers and are only changed by the network operators or network management tools [FRT2002]_.
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../../principles/reliability.rst:790
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The `selective repeat` sender maintains a sending buffer that can store up to `W` unacknowledged frames. These frames are sent as long as the sending buffer is not full. Several implementations of a `selective repeat` sender are possible. A simple implementation associates one retransmission timer to each frame. The timer is started when the frame is sent and canceled upon reception of an acknowledgment that covers this frame. When a retransmission timer expires, the corresponding frame is retransmitted and this retransmission timer is restarted. When an acknowledgment is received, all the frames that are covered by this acknowledgment are removed from the sending buffer and the sliding window is updated.
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