Glossary
Skywalk
Apple's modern replacement for the BSD network data plane. Shared-memory rings, per-channel queues, QoS-aware throughout. Closed source, but increasingly the default.
Skywalk is Apple's modern network data-plane stack, shipped incrementally since macOS 10.15 as a replacement for parts of the classic 4.4 BSD network stack. It handles most UDP, increasingly TCP, and the newer QUIC paths.
Key differences from the classic stack:
- Shared-memory rings between kernel and userspace — zero-copy I/O for apps using Network.framework.
- Per-channel queues, scheduled by the stack itself rather than serialized through socket buffers.
- First-class QoS propagation — voucher-derived hints flow down through every queueing point.
- Userspace network extensions plug in via well-defined extension points (Network Extension framework), replacing the older kernel-NKE model.
Skywalk's kernel side is closed-source — not part of the open XNU drop. Apple publishes occasional WWDC sessions covering its architecture; for deeper detail, third-party reverse engineering documents the wire interfaces.
The classic BSD stack (mbufs, socket buffers) still exists for backward compatibility. Apps using legacy BSD socket APIs continue to traverse the classic path; apps using Apple's modern Network.framework get Skywalk transparently.
See also: mbuf, BSD, the network stack article.