wgengine/packet: rename types to reflect their v4-only-ness, document.

Signed-off-by: David Anderson <danderson@tailscale.com>

wgengine/packet/udp4.go

@@ -0,0 +1,53 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package packet
+
+// UDPHeader represents an UDP packet header.
+type UDP4Header struct {
+	IP4Header
+	SrcPort uint16
+	DstPort uint16
+}
+
+const (
+	udpHeaderLength = 8
+	// udpTotalHeaderLength is the length of all headers in a UDP packet.
+	udpTotalHeaderLength = ipHeaderLength + udpHeaderLength
+)
+
+func (UDP4Header) Len() int {
+	return udpTotalHeaderLength
+}
+
+func (h UDP4Header) Marshal(buf []byte) error {
+	if len(buf) < udpTotalHeaderLength {
+		return errSmallBuffer
+	}
+	if len(buf) > maxPacketLength {
+		return errLargePacket
+	}
+	// The caller does not need to set this.
+	h.IPProto = UDP
+
+	length := len(buf) - h.IP4Header.Len()
+	put16(buf[20:22], h.SrcPort)
+	put16(buf[22:24], h.DstPort)
+	put16(buf[24:26], uint16(length))
+	put16(buf[26:28], 0) // blank checksum
+
+	h.IP4Header.MarshalPseudo(buf)
+
+	// UDP checksum with IP pseudo header.
+	put16(buf[26:28], ipChecksum(buf[8:]))
+
+	h.IP4Header.Marshal(buf)
+
+	return nil
+}
+
+func (h *UDP4Header) ToResponse() {
+	h.SrcPort, h.DstPort = h.DstPort, h.SrcPort
+	h.IP4Header.ToResponse()
+}