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

internal/ackhandler/send_mode_test.go

@@ -0,0 +1,16 @@
+package ackhandler
+
+import (
+	. "github.com/onsi/ginkgo"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("Send Mode", func() {
+	It("has a string representation", func() {
+		Expect(SendNone.String()).To(Equal("none"))
+		Expect(SendAny.String()).To(Equal("any"))
+		Expect(SendAck.String()).To(Equal("ack"))
+		Expect(SendRetransmission.String()).To(Equal("retransmission"))
+		Expect(SendMode(123).String()).To(Equal("invalid send mode: 123"))
+	})
+})