net/packet: rename from wgengine/packet.

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

net/packet/icmp4.go

@@ -0,0 +1,80 @@
+// 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
+
+type ICMP4Type uint8
+
+const (
+	ICMP4EchoReply    ICMP4Type = 0x00
+	ICMP4EchoRequest  ICMP4Type = 0x08
+	ICMP4Unreachable  ICMP4Type = 0x03
+	ICMP4TimeExceeded ICMP4Type = 0x0b
+)
+
+func (t ICMP4Type) String() string {
+	switch t {
+	case ICMP4EchoReply:
+		return "EchoReply"
+	case ICMP4EchoRequest:
+		return "EchoRequest"
+	case ICMP4Unreachable:
+		return "Unreachable"
+	case ICMP4TimeExceeded:
+		return "TimeExceeded"
+	default:
+		return "Unknown"
+	}
+}
+
+type ICMP4Code uint8
+
+const (
+	ICMP4NoCode ICMP4Code = 0
+)
+
+// ICMPHeader represents an ICMP packet header.
+type ICMP4Header struct {
+	IP4Header
+	Type ICMP4Type
+	Code ICMP4Code
+}
+
+const (
+	icmpHeaderLength = 4
+	// icmpTotalHeaderLength is the length of all headers in a ICMP packet.
+	icmpAllHeadersLength = ipHeaderLength + icmpHeaderLength
+)
+
+func (ICMP4Header) Len() int {
+	return icmpAllHeadersLength
+}
+
+func (h ICMP4Header) Marshal(buf []byte) error {
+	if len(buf) < icmpAllHeadersLength {
+		return errSmallBuffer
+	}
+	if len(buf) > maxPacketLength {
+		return errLargePacket
+	}
+	// The caller does not need to set this.
+	h.IPProto = ICMP
+
+	buf[20] = uint8(h.Type)
+	buf[21] = uint8(h.Code)
+
+	h.IP4Header.Marshal(buf)
+
+	put16(buf[22:24], ipChecksum(buf))
+
+	return nil
+}
+
+func (h *ICMP4Header) ToResponse() {
+	// TODO: this doesn't implement ToResponse correctly, as it
+	// assumes the ICMP request type.
+	h.Type = ICMP4EchoReply
+	h.Code = ICMP4NoCode
+	h.IP4Header.ToResponse()
+}