Bitcoin and Cryptocurrency Technologies

Figure 3.10 ​Block propagation time.​​This graph shows the average time that it takes a block to reach
various percentages of the nodes in the network.
What is the latency of the flooding algorithm? The graph in Figure 3.10 shows the average time for
new blocks to propagate to every node in the network. The three lines show the 25th, the 50th, and
the 75th percentile block propagation time. As you can see, propagation time is basically proportional
to the size of the block. This is because network bandwidth is the bottleneck. The larger blocks take
over 30 seconds to propagate to most nodes in the network. So it isn’t a particularly efficient protocol.
On the Internet, 30 seconds is a pretty long time. In Bitcoin’s design, having a simple network with
little structure where nodes are equal and can come and go at any time took priority over efficiency.
So a block may need to go through many nodes before it reaches the most distant nodes in the
network. If the network were instead designed top-down for efficiency, we could make sure that the
path between any two nodes is short.
Size of the network. ​It is difficult to measure how big the network is since it is dynamic and there is no
central authority. A number of researchers have come up with estimates. On the high end, some say
that over a million IP addresses in a given month will, at some point, act, at least temporarily, as a
Bitcoin node. On the other hand, there seem to be only about 5,000 to 10,000 nodes that are
permanently connected and fully validate every transaction they hear. This may seem like a
surprisingly low number, but as of this writing there is no evidence that the number of fully validating
nodes is going up, and it may in fact be dropping.
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