cnp3-ebook/principles/network — English @ Weblate

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173 ../../principles/linkstate.rst:52

	English Once a router has discovered its neighbors, it must reliably distribute all its outgoing edges to all routers in the network to allow them to compute their local view of the network topology. For this, each router builds a `link-state packet` (LSP) containing the following information: 286/10000 · 286
174 ../../principles/linkstate.rst:54

	English LSP.Router: identification (address) of the sender of the LSP 61/10000 · 61
175 ../../principles/linkstate.rst:55

	English LSP.age: age or remaining lifetime of the LSP 45/10000 · 45
176 ../../principles/linkstate.rst:56

	English LSP.seq: sequence number of the LSP 35/10000 · 35
177 ../../principles/linkstate.rst:57

	English LSP.Links[]: links advertised in the LSP. Each directed link is represented with the following information: 107/10000 · 107
178 ../../principles/linkstate.rst:59

	English LSP.Links[i].Id: identification of the neighbor 47/10000 · 47
179 ../../principles/linkstate.rst:60

	English LSP.Links[i].cost: cost of the link 35/10000 · 35
180 ../../principles/linkstate.rst:62

	English These LSPs must be reliably distributed inside the network without using the router's routing table since these tables can only be computed once the LSPs have been received. The `Flooding` algorithm is used to efficiently distribute the LSPs of all routers. Each router that implements `flooding` maintains a `link state database` (LSDB) containing the most recent LSP sent by each router. When a router receives a LSP, it first verifies whether this LSP is already stored inside its LSDB. If so, the router has already distributed the LSP earlier and it does not need to forward it. Otherwise, the router forwards the LSP on all its links except the link over which the LSP was received. Flooding can be implemented by using the following pseudo-code. 752/10000 · 752
181 ../../principles/linkstate.rst:76

	English In this pseudo-code, `LSDB(r)` returns the most recent `LSP` originating from router `r` that is stored in the `LSDB`. `newer(lsp1, lsp2)` returns true if `lsp1` is more recent than `lsp2`. See the note below for a discussion on how `newer` can be implemented. 260/10000 · 260
182 ../../principles/linkstate.rst:78

	English Which is the most recent LSP ? 30/10000 · 30
183 ../../principles/linkstate.rst:80

	English A router that implements flooding must be able to detect whether a received LSP is newer than the stored LSP. This requires a comparison between the sequence number of the received LSP and the sequence number of the LSP stored in the link state database. The ARPANET routing protocol [MRR1979]_ used a 6 bits sequence number and implemented the comparison as follows :rfc:`789` 377/10000 · 377
184 ../../principles/linkstate.rst:88

	English This comparison takes into account the modulo :math:`2^{6}` arithmetic used to increment the sequence numbers. Intuitively, the comparison divides the circle of all sequence numbers into two halves. Usually, the sequence number of the received LSP is equal to the sequence number of the stored LSP incremented by one, but sometimes the sequence numbers of two successive LSPs may differ, e.g. if one router has been disconnected for some time. The comparison above worked well until October 27, 1980. On this day, the ARPANET crashed completely. The crash was complex and involved several routers. At one point, LSP `40` and LSP `44` from one of the routers were stored in the LSDB of some routers in the ARPANET. As LSP `44` was the newest, it should have replaced LSP `40` on all routers. Unfortunately, one of the ARPANET routers suffered from a memory problem and sequence number `40` (`101000` in binary) was replaced by `8` (`001000` in binary) in the buggy router and flooded. Three LSPs were present in the network and `44` was newer than `40` which is newer than `8`, but unfortunately `8` was considered to be newer than `44`... All routers started to exchange these three link state packets forever and the only solution to recover from this problem was to shutdown the entire network :rfc:`789`. 1307/10000 · 1307
185 ../../principles/linkstate.rst:90

	English Current link state routing protocols usually use 32 bits sequence numbers and include a special mechanism in the unlikely case that a sequence number reaches the maximum value (with a 32 bits sequence number space, it takes 136 years to cycle the sequence numbers if a link state packet is generated every second). 314/10000 · 314
186 ../../principles/linkstate.rst:92

	English To deal with the memory corruption problem, link state packets contain a checksum or CRC. This checksum is computed by the router that generates the LSP. Each router must verify the checksum when it receives or floods an LSP. Furthermore, each router must periodically verify the checksums of the LSPs stored in its LSDB. This enables them to cope with memory errors that could corrupt the LSDB as the one that occurred in the ARPANET. 435/10000 · 435
187 ../../principles/linkstate.rst:94

	English Flooding is illustrated in the figure below. By exchanging HELLO messages, each router learns its direct neighbors. For example, router `E` learns that it is directly connected to routers `D`, `B` and `C`. Its first LSP has sequence number `0` and contains the directed links `E->D`, `E->B` and `E->C`. Router `E` sends its LSP on all its links and routers `D`, `B` and `C` insert the LSP in their LSDB and forward it over their other links. 441/10000 · 441
188 ../../principles/linkstate.rst:157

	English 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]_. 894/10000 · 894
189 ../../principles/linkstate.rst:159

	English 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. 563/10000 · 563
190 ../../principles/linkstate.rst:239

	English Static or dynamic link metrics ? 32/10000 · 32
191 ../../principles/linkstate.rst:241

	English 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]_. 601/10000 · 601
192 ../../principles/linkstate.rst:245

	English When a link fails, the two routers attached to the link detect the failure by the absence of HELLO messages received during the last :math:`k \times N` seconds. Once a router has detected the failure of one of its local links, it generates and floods a new LSP that no longer contains the failed link. This new LSP replaces the previous LSP in the network. In practice, the two routers attached to a link do not detect this failure exactly at the same time. During this period, some links may be announced in only one direction. This is illustrated in the figure below. Router `E` has detected the failure of link `E-B` and flooded a new LSP, but router `B` has not yet detected this failure. 692/10000 · 692

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