Module udp 

UDP through the worker’s ring: batched receive, GSO send.

Receive is one multishot recvmsg per socket feeding from a registered provided-buffer ring: each completion consumes one whole buffer, so every buffer is sized for the worst case (a full UDP_GRO coalesce plus the recvmsg header). Incremental consumption (IOU_PBUF_RING_INC) looked like a better fit for GRO’s 100-byte-to-64KB completion variance, but it cannot back a multishot recvmsg: the kernel releases an incremental buffer only at exactly zero bytes left, and io_recvmsg_prep_multishot fails with EFAULT the moment a leftover tail is smaller than the recvmsg header. A completed buffer is handed out as a Packet and returns to the kernel once every packet borrowing it drops.

Send stages datagrams in a pool of buffers owned by id and released explicitly on completion, the shape SENDMSG_ZC’s deferred-reclaim NOTIF model needs later. Every GSO sendmsg carries its UDP_SEGMENT control message explicitly; the socket default is never relied on. The ECN mark rides beside it as IP_TOS or IPV6_TCLASS, and receives read the mark back through IP_RECVTOS and IPV6_RECVTCLASS, so a QUIC stack’s ECN validation sees what the network did to its packets.

Both pools are queues of concurrent operations, not byte budgets: one send buffer holds one GSO train and one receive buffer holds one completion, however little either carries. So the depth a socket needs is set by how many of its connections want the socket at once, which no caller can predict. Both start small and grow on demand when they starve, bounded by the ceilings in Config; a pool never shrinks, so it settles at the socket’s high-water concurrency and the memory comes back when it drops.

The gro/gso/multishot toggles in Config exist for the ablation benchmarks; production callers keep the defaults (all on).

Structs

Config

How a socket uses the ring. The defaults are the production path; the toggles exist so the benchmarks can ablate one mechanism at a time.

Packet

One receive: a possibly GRO-coalesced run of datagrams from one source.

Socket

A UDP socket driven by a crate::Worker.

Transmit

What one TxBuf::send puts on the wire.

TxBuf

A checked-out send-staging buffer: fill it, then send it.

Enums

Bound

A bound socket a worker takes over, with whatever identity it carries.

Ecn

The ECN codepoint carried in the IP header’s TOS or traffic class byte.