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plumb ECN through via relays

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This commit is contained in:
JackDoan
2026-07-31 12:14:56 -05:00
parent b3002c2d13
commit b7bf32240b
18 changed files with 282 additions and 79 deletions
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udp/udp_linux_writebatch.go
+22 -16
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@@ -385,14 +385,30 @@ func (w *batchWriter) planRun(bufs [][]byte, addrs []netip.AddrPort, ecns []byte
return runLen, segSize
}
// writeECNCmsg fills the start of buf with one IP_TOS / IPV6_TCLASS cmsg
// carrying the 2-bit ECN codepoint. buf must be cmsg-aligned (the batch
// writer's heap slab is runtime-aligned; sendmsg passes uint64-backed stack
// scratch) and at least CmsgSpace(4) bytes. The cmsg family must match the
// socket: on the default dual-stack v6 bind, a v4-mapped destination takes
// the kernel's IPv4 path, which reads IP_TOS and ignores IPV6_TCLASS. The
// payload is a 4-byte int for both families, so the cmsg space is the same.
func writeECNCmsg(buf []byte, dstIsV4 bool, ecn byte) {
h := (*unix.Cmsghdr)(unsafe.Pointer(&buf[0]))
if dstIsV4 {
h.Level = int32(unix.IPPROTO_IP)
h.Type = int32(unix.IP_TOS)
} else {
h.Level = int32(unix.IPPROTO_IPV6)
h.Type = int32(unix.IPV6_TCLASS)
}
setCmsgLen(h, unix.CmsgLen(4))
dataOff := unix.CmsgLen(0)
binary.NativeEndian.PutUint32(buf[dataOff:dataOff+4], uint32(ecn))
}
// writeEntryCmsg writes one entry's cmsgs: the UDP_SEGMENT payload when
// runLen >= 2, the IP_TOS/IPV6_TCLASS cmsg when ecn != 0, then points
// Hdr.Control at the smallest span covering the cmsgs in use.
//
// The ECN cmsg family must match the destination, not the socket: on the
// default dual-stack v6 bind, a v4-mapped destination takes the kernel's
// IPv4 path, which reads IP_TOS and ignores IPV6_TCLASS. The payload is a
// 4-byte int for both families, so the cmsg space is the same.
func (w *batchWriter) writeEntryCmsg(entry, runLen, segSize int, ecn byte, dstIsV4 bool) {
hdr := &w.msgs[entry].Hdr
useSeg := runLen >= 2
@@ -404,17 +420,7 @@ func (w *batchWriter) writeEntryCmsg(entry, runLen, segSize int, ecn byte, dstIs
binary.NativeEndian.PutUint16(w.cmsg[dataOff:dataOff+2], uint16(segSize))
}
if useEcn {
ecnHdr := (*unix.Cmsghdr)(unsafe.Pointer(&w.cmsg[base+w.cmsgSegSpace]))
if dstIsV4 {
ecnHdr.Level = int32(unix.IPPROTO_IP)
ecnHdr.Type = int32(unix.IP_TOS)
} else {
ecnHdr.Level = int32(unix.IPPROTO_IPV6)
ecnHdr.Type = int32(unix.IPV6_TCLASS)
}
setCmsgLen(ecnHdr, unix.CmsgLen(4))
dataOff := base + w.cmsgSegSpace + unix.CmsgLen(0)
binary.NativeEndian.PutUint32(w.cmsg[dataOff:dataOff+4], uint32(ecn))
writeECNCmsg(w.cmsg[base+w.cmsgSegSpace:], dstIsV4, ecn)
}
switch {