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sendmmsg already batches syscalls but still builds one sk_buff per datagram — the kernel-side wall above ~1 Gbps. UDP Generic Segmentation Offload hands the kernel one big buffer it splits into gso_size datagrams, building ~1 GSO skb per ≤64 segments. Research (LWN/Cloudflare/Tailscale) measures ~2.4x throughput at equal CPU and 17-44x fewer syscalls, and that sendmmsg batching alone is insufficient — you need true segmentation offload. Adds Transport::send_gso (default = send_batch) + a UdpTransport Linux override: coalesces a frame's equal-size wire packets (shards are zero-padded to a constant size, so a whole frame is one gso_size) into ≤64-segment sendmsg(UDP_SEGMENT) calls. seal/send routes through it. Opt-in via PUNKTFUNK_GSO (new unsafe hot-path code) with automatic fallback to sendmmsg on any GSO error (unsupported kernel/ path), latched per process. Loopback unit test validates the cmsg segmentation; full session over loopback streams clean (0% loss). Linux-only; loopback/non-Linux keep sendmmsg/scalar. Next levers: in-place AES-GCM seal (kill per-packet allocs), UDP GRO on recv, drop the sleep-pacing in favor of the kernel qdisc, jumbo MTU. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>