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Patching IPv4
Large networks with many endpoints, such as the Internet, are economi-
cally viable only if the cost of maintaining those endpoints is manageable.
Network upgrades that require modification of the endpoints are to be
avoided at all costs, if only because of the difficulty of tracking down
every node. Backward compatibility in the Internet is mandatory to avoid
the cost of maintaining IP implementations on obsolete or custom-built
computers. Even if all nodes ran the same operating system, attempting to
upgrade all of them at once with network stack updates is a daunting task.
Protocol designers, whether they’ve designed protocols for