By the way, Plan 9 doesn't have sockets, it has /net: http://man.cat-v.org/plan_9/3/ip with the dial API built on top: http://man.cat-v.org/plan_9/2/dial

This means that for example, Plan 9 applications that do networking are not tied to a specific network stack, you can mount multiple network stacks concurrently, you can have 'virtual' network stacks (for example running in user space and proxying to a remote host, or doing other neat tricks), and the apps don't need to care.

Also when Ipv6 was added to the Plan 9 stack, no application code had to be modified, because the API nicely abstracts network addresses.