GSO/GRO offloads, with TCP+ECN and UDP support

udp/udp_ecn_outer_linux_test.go

@@ -0,0 +1,61 @@
+//go:build linux && !android && !e2e_testing
+
+package udp
+
+import (
+	"net/netip"
+	"testing"
+)
+
+// TestPlanRunBreaksOnECNChange confirms that two same-destination, same-size
+// packets with different outer ECN end up in separate sendmmsg entries (the
+// kernel stamps one outer codepoint per entry, so a run that straddled the
+// boundary would silently lose information).
+func TestPlanRunBreaksOnECNChange(t *testing.T) {
+	u := &StdConn{gsoSupported: true}
+	dst := netip.MustParseAddrPort("10.0.0.1:4242")
+
+	bufs := [][]byte{
+		make([]byte, 1200),
+		make([]byte, 1200),
+		make([]byte, 1200),
+	}
+	addrs := []netip.AddrPort{dst, dst, dst}
+
+	t.Run("uniform_ecn_runs_together", func(t *testing.T) {
+		ecns := []byte{0x02, 0x02, 0x02}
+		runLen, segSize := u.planRun(bufs, addrs, ecns, 0, 64)
+		if runLen != 3 {
+			t.Errorf("runLen=%d want 3 (uniform ECT(0))", runLen)
+		}
+		if segSize != 1200 {
+			t.Errorf("segSize=%d want 1200", segSize)
+		}
+	})
+
+	t.Run("ecn_change_truncates_run", func(t *testing.T) {
+		// 0,0,3: first two run together, CE seeds a fresh entry.
+		ecns := []byte{0x00, 0x00, 0x03}
+		runLen, _ := u.planRun(bufs, addrs, ecns, 0, 64)
+		if runLen != 2 {
+			t.Errorf("runLen=%d want 2 (ECN changes at index 2)", runLen)
+		}
+	})
+
+	t.Run("nil_ecns_runs_full", func(t *testing.T) {
+		runLen, _ := u.planRun(bufs, addrs, nil, 0, 64)
+		if runLen != 3 {
+			t.Errorf("runLen=%d want 3 (nil ecns means no break)", runLen)
+		}
+	})
+
+	t.Run("first_ecn_is_singleton", func(t *testing.T) {
+		// Second packet has different ECN from the first → run halts at 1
+		// (the first packet alone forms the run).
+		ecns := []byte{0x00, 0x03, 0x03}
+		runLen, _ := u.planRun(bufs, addrs, ecns, 0, 64)
+		if runLen != 1 {
+			t.Errorf("runLen=%d want 1 (different ECN immediately)", runLen)
+		}
+	})
+}