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Kernel TLS, NIC Offload, and Socket Sharding: What’s New and Who Uses It?

#linux #network

Modern servers are expected to push hundreds of gigabits per second while keeping latency low and CPU use manageable. Two kernel-level innovations—socket sharding and kernel TLS (kTLS)—help make that possible. When paired with NIC TLS offload, the gains are even bigger.

Socket Sharding

Traditionally, only one process could accept() connections from a TCP socket.

With Linux 4.5 (2016) and SO_REUSEPORT enhancements, multiple processes can share a listening socket. This “socket sharding” lets the kernel distribute connections efficiently across worker processes.

  • Adopted in NGINX and HAProxy.
  • FreeBSD has similar support.
  • Windows does not provide this functionality.

✅ Result: Better scalability on multi-core systems with no user-space load balancer needed.

Kernel TLS (kTLS)

Linux introduced kTLS in 4.13 (2017) (send path first, later receive).

Instead of copying encrypted data between user space and kernel space, the kernel itself handles TLS record encryption/decryption.

  • Avoids extra copies and syscalls.
  • Yields 20–40% CPU savings in many workloads.

Benchmarks

  • FreeBSD users: throughput improved from 25–30 Gb/s → 100+ Gb/s with dual-100G NICs.
  • NGINX with kTLS + SSL_sendfile(): roughly 2× throughput when serving static files compared to user-space TLS.

NIC TLS Offload

Some NICs (Mellanox/NVIDIA ConnectX, Chelsio T6, Intel QAT) can offload encryption entirely.

The kernel sets up the session keys, while the NIC performs the crypto inline.

  • Netflix (FreeBSD + Mellanox CX6-DX):

    • In production, serving 400–800 Gb/s per server.
    • Reported ~50% reduction in memory bandwidth use vs CPU crypto.

  • NVIDIA/Mellanox (netdev conf):

    • Inline TLS offload gave ~2× throughput vs software kTLS.
    • Example: 8.8 Gb/s vs 4.4 Gb/s in test environments.

  • Chelsio T6:

    • Vendor benchmarks show ~44.8 Gb/s TLS throughput with very low CPU use.

  • Academic 2024 SmartNIC study:

    • Inline mode: ~9.3 Gb/s throughput under parallel load.
    • Coprocessor mode: less throughput but lowest CPU cost.

Windows Support?

Unlike Linux and FreeBSD, Windows does not support kernel TLS.

TLS remains in user space via SChannel or libraries like OpenSSL/BoringSSL, with no NIC offload integration.

Summary

  • Socket sharding (Linux 4.5+) spreads load across processes → higher scalability.
  • kTLS (Linux 4.13+, FreeBSD 13+) avoids copies → 20–40% CPU savings, ~2× throughput for NGINX.
  • NIC TLS offload (Netflix, NVIDIA, Chelsio):
    • 2× throughput, huge CPU savings,
    • 400–800 Gb/s per server possible in production.
  • Windows: No kernel TLS or NIC offload support today.

These optimizations matter most at hyperscaler scale, but are increasingly available to anyone running NGINX, HAProxy, or workloads on modern NICs.

References

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