cnp3-ebook/principles/network — English @ Weblate

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167 ../../principles/linkstate.rst:13

	English For link-state routing, a network is modeled as a `directed weighted graph`. Each router is a node, and the links between routers are the edges in the graph. A positive weight is associated to each directed edge and routers use the shortest path to reach each destination. In practice, different types of weights can be associated to each directed edge : 354/10000 · 354
168 ../../principles/linkstate.rst:15

	English unit weight. If all links have a unit weight, shortest path routing prefers the paths with the least number of intermediate routers. 132/10000 · 132
169 ../../principles/linkstate.rst:16

	English weight proportional to the propagation delay on the link. If all link weights are configured this way, shortest path routing uses the paths with the smallest propagation delay. 176/10000 · 176
170 ../../principles/linkstate.rst:17

	English :math:`weight=\frac{C}{bandwidth}` where `C` is a constant larger than the highest link bandwidth in the network. If all link weights are configured this way, shortest path routing prefers higher bandwidth paths over lower bandwidth paths. 239/10000 · 239
171 ../../principles/linkstate.rst:19

	English Usually, the same weight is associated to the two directed edges that correspond to a physical link (i.e. :math:`R1 \rightarrow R2` and :math:`R2 \rightarrow R1`). However, nothing in the link state protocols requires this. For example, if the weight is set in function of the link bandwidth, then an asymmetric ADSL link could have a different weight for the upstream and downstream directions. Other variants are possible. Some networks use optimization algorithms to find the best set of weights to minimize congestion inside the network for a given traffic demand [FRT2002]_. 579/10000 · 579
172 ../../principles/linkstate.rst:24

	English When a link-state router boots, it first needs to discover to which routers it is directly connected. For this, each router sends a HELLO message every `N` seconds on all its interfaces. This message contains the router's address. Each router has a unique address. As its neighboring routers also send HELLO messages, the router automatically discovers to which neighbors it is connected. These HELLO messages are only sent to neighbors that are directly connected to a router, and a router never forwards the HELLO messages that it receives. HELLO messages are also used to detect link and router failures. A link is considered to have failed if no HELLO message has been received from a neighboring router for a period of :math:`k \times N` seconds. 751/10000 · 751
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

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