Routing, Performance, Layers

Dijkstra's shortest path algorithm

input: a set of nodes N = {n1, n2, ... } and distances (costs) between nodes D = {(n1,n2=d1), (n1,n3=d2), ... }, as well as the root node r from which distances are desired
the root node r is the initial value of the set K = {r} of nodes whose distance from r is known
compute (and save) all the direct distances from r to each of the nodes in N, using infinity if there is no direct connection
pick the node (c) in N that has the least distance (to r), remove it from N, and add it to K
reduce (wherever possible) all distances for all nodes in N with a direct link to the node c selected in the previous step
repeat the preceding two steps until N is empty (all distances are now known) or until all nodes in N have infinite distance (the network is partitioned)

With Dijkstra's algorithm, each router can build its routing tables given a "map" (a graph representing the network)
how do the routers get the map?
each router broadcasts all the information it knows about: its links to its neighbors
each message has a sequence number: later information overrides earlier information
the "map" is pieced together from the information
broadcasting is done by each node sending to its neighbors and suppressing duplicates: flooding

suppose X sends its routing table to neighbor Y
the cost for Y to reach X is m_Y(X)
for every destination D with cost m_X(D) in X's routing table, Y compares m_X(D)+m_Y(X) to m_Y(D).
If m_X(D)+m_Y(X) < m_Y(D), then a route through X is lower-cost than the existing route, and Y updates its routing table appropriately
all communication is between neighbors
great for finding low-cost routes, slow at updating tables when links fail

flooding: one person sends, everyone receives
link-state routing: everyone in a network floods the information about their neighbors to everyone else in the network, each router builds their own map and routing table. The information is resent whenever it changes.
distance-vector routing: everyone tells all their neighbors about who they can reach and at what distance. I send my information to all my neighbors whenever my routing table changes.