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overlay/batch: don't seal the open slot on a pure ACK

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Every non-coalesceable in-flow packet evicted the flow's open slot, so
a bidirectional connection's inbound data run was broken by each peer
ACK interleaved into it, largely defeating coalescing on concurrent
upload+download. A bare acknowledgment (zero payload, nothing beyond
ACK|PSH|ECE) carries no ordering obligation toward the flow's data --
delivered late it is just a stale ACK the receiver ignores -- so it
can ride the lane as a passthrough without the evict, same as kernel
GRO, which doesn't flush held data on pure ACKs. SYN/FIN/RST/CWR keep
sealing.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014ugV2edVqoz3tBvq9J6yWp
This commit is contained in:
JackDoan
2026-07-29 17:39:13 -05:00
parent 2190900107
commit 9c68c60ba6
2 changed files with 90 additions and 5 deletions
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overlay/batch/tcp_coalesce.go
+26 -5
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@@ -171,6 +171,17 @@ func (p parsedTCP) coalesceable() bool {
return p.payLen > 0
}
// pureAck reports whether a parsed segment is a bare acknowledgment: no
// payload and nothing beyond ACK|PSH|ECE in the flags. These are the only
// non-coalesceable shape that may safely pass through WITHOUT sealing the
// flow's open slot — a late-delivered stale ACK is ignored by the receiver,
// whereas SYN/FIN/RST/CWR mark transitions the flow must observe in order.
func (p parsedTCP) pureAck() bool {
return p.payLen == 0 &&
p.flags&tcpFlagAck != 0 &&
p.flags&^(tcpFlagAck|tcpFlagPsh|tcpFlagEce) == 0
}
func (c *TCPCoalescer) Commit(pkt []byte) error {
info, ok := parseTCPBase(pkt)
if !ok {
@@ -186,11 +197,21 @@ func (c *TCPCoalescer) Commit(pkt []byte) error {
// after the dispatcher has already done so.
func (c *TCPCoalescer) commitParsed(pkt []byte, info parsedTCP) error {
if !info.coalesceable() {
// TCP but not admissible (SYN/FIN/RST/URG/CWR or zero-payload).
// Seal this flow's open slot so later in-flow packets don't extend
// it and accidentally reorder past this passthrough. The len guard
// skips hashing the 38-byte key on ack-dominant queues, where the
// map is almost always empty.
if info.pureAck() {
// A bare window/ack update carries no ordering obligation toward
// the flow's data: delivering it after later-arriving data only
// makes it a stale ACK, which receivers ignore. Skipping the
// evict keeps a bidirectional flow's inbound data run coalescing
// across the peer ACKs interleaved into it — kernel GRO likewise
// doesn't flush held data on a pure ACK.
c.addPassthrough(pkt)
return nil
}
// TCP but not admissible (SYN/FIN/RST/URG/CWR or a shape the flow
// must observe in sequence). Seal this flow's open slot so later
// in-flow packets don't extend it and accidentally reorder past this
// passthrough. The len guard skips hashing the 38-byte key on
// ack-dominant queues, where the map is almost always empty.
if len(c.openSlots) != 0 {
if last := c.lastSlot; last != nil && last.fk == info.fk {
c.lastSlot = nil