Høiland-Jørgensen: The inner workings of TCP zero-copy

Toke Høiland-Jørgensen has posted an overview of how zero-copy networking works in the Linux kernel.

Since the memory is being copied directly from userspace to the network device, the userspace application has to keep it around unmodified, until it has finished sending. The sendmsg() syscall itself is asynchronous, and will return without waiting for this. Instead, once the memory buffers are no longer needed by the stack, the kernel will return a notification to userspace that the buffers can be reused.