implement packet send modes to determine what kind of packets are sent

internal/ackhandler/send_mode.go

@@ -0,0 +1,32 @@
+package ackhandler
+
+import "fmt"
+
+// The SendMode says what kind of packets can be sent.
+type SendMode uint8
+
+const (
+	// SendNone means that no packets should be sent
+	SendNone SendMode = iota
+	// SendAck means an ACK-only packet should be sent
+	SendAck
+	// SendRetransmission means that retransmissions should be sent
+	SendRetransmission
+	// SendAny packet should be sent
+	SendAny
+)
+
+func (s SendMode) String() string {
+	switch s {
+	case SendNone:
+		return "none"
+	case SendAck:
+		return "ack"
+	case SendRetransmission:
+		return "retransmission"
+	case SendAny:
+		return "any"
+	default:
+		return fmt.Sprintf("invalid send mode: %d", s)
+	}
+}