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The XNU network stack: mbufs, TCP, and Skywalk
From the classic 4.4 BSD TCP/IP stack to Apple's modern Skywalk replacement — how packets traverse XNU's networking code, and why Apple is moving the data plane out of the BSD layer.
bsdnetworkingtcpskywalk
TCP internals on XNU: the state machine, congestion control, and the receive path
The 4.4 BSD TCP stack as it actually runs on macOS — state transitions, send/receive locking, Apple's congestion-control variants, and the kqueue path back to your app.
bsdnetworkingtcpkernel