use bbr congestion control on linux, note that we're not doing anything intelligent with the errors right now if setting it fails

src/yggdrasil/tcp_linux.go

@@ -0,0 +1,28 @@
+// +build linux
+
+package yggdrasil
+
+import (
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+// WARNING: This context is used both by net.Dialer and net.Listen in tcp.go
+
+func (t *tcp) tcpContext(network, address string, c syscall.RawConn) error {
+	var control error
+	var bbr error
+
+	control = c.Control(func(fd uintptr) {
+		// sys/socket.h: #define	SO_RECV_ANYIF	0x1104
+		bbr = unix.SetsockoptString(int(fd), unix.IPPROTO_TCP, unix.TCP_CONGESTION, "bbr")
+	})
+
+	switch {
+	case bbr != nil:
+		return bbr
+	default:
+		return control
+	}
+}

src/yggdrasil/tcp_other.go

@@ -1,4 +1,4 @@
-// +build !darwin
+// +build !darwin,!linux
 
 package yggdrasil