net/packet: add ICMP6Header, like ICMP4Header

So we can generate IPv6 ping replies.

Change-Id: I79a9a38d8aa242e5dfca4cd15dfaffaea6cb1aee
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/packet/icmp6.go

@@ -4,6 +4,12 @@
 
 package packet
 
+import (
+	"encoding/binary"
+
+	"tailscale.com/types/ipproto"
+)
+
 // icmp6HeaderLength is the size of the ICMPv6 packet header, not
 // including the outer IP layer or the variable "response data"
 // trailer.
@@ -42,3 +48,120 @@ type ICMP6Code uint8
 const (
 	ICMP6NoCode ICMP6Code = 0
 )
+
+// ICMP6Header is an IPv4+ICMPv4 header.
+type ICMP6Header struct {
+	IP6Header
+	Type ICMP6Type
+	Code ICMP6Code
+}
+
+// Len implements Header.
+func (h ICMP6Header) Len() int {
+	return h.IP6Header.Len() + icmp6HeaderLength
+}
+
+// Marshal implements Header.
+func (h ICMP6Header) Marshal(buf []byte) error {
+	if len(buf) < h.Len() {
+		return errSmallBuffer
+	}
+	if len(buf) > maxPacketLength {
+		return errLargePacket
+	}
+	// The caller does not need to set this.
+	h.IPProto = ipproto.ICMPv6
+
+	h.IP6Header.Marshal(buf)
+
+	const o = ip6HeaderLength // start offset of ICMPv6 header
+	buf[o+0] = uint8(h.Type)
+	buf[o+1] = uint8(h.Code)
+	buf[o+2] = 0 // checksum, to be filled in later
+	buf[o+3] = 0 // checksum, to be filled in later
+	return nil
+}
+
+// ToResponse implements Header. TODO: it doesn't implement it
+// correctly, instead it statically generates an ICMP Echo Reply
+// packet.
+func (h *ICMP6Header) ToResponse() {
+	// TODO: this doesn't implement ToResponse correctly, as it
+	// assumes the ICMP request type.
+	h.Type = ICMP6EchoReply
+	h.Code = ICMP6NoCode
+	h.IP6Header.ToResponse()
+}
+
+// WriteChecksum implements HeaderChecksummer, writing just the checksum bytes
+// into the otherwise fully marshaled ICMP6 packet p (which should include the
+// IPv6 header, ICMPv6 header, and payload).
+func (h ICMP6Header) WriteChecksum(p []byte) {
+	const payOff = ip6HeaderLength + icmp6HeaderLength
+	xsum := icmp6Checksum(p[ip6HeaderLength:payOff], h.Src.As16(), h.Dst.As16(), p[payOff:])
+	binary.BigEndian.PutUint16(p[ip6HeaderLength+2:], xsum)
+}
+
+// Adapted from gVisor:
+
+// icmp6Checksum calculates the ICMP checksum over the provided ICMPv6
+// header (without the IPv6 header), IPv6 src/dst addresses and the
+// payload.
+//
+// The header's existing checksum must be zeroed.
+func icmp6Checksum(header []byte, src, dst [16]byte, payload []byte) uint16 {
+	// Calculate the IPv6 pseudo-header upper-layer checksum.
+	xsum := checksumBytes(src[:], 0)
+	xsum = checksumBytes(dst[:], xsum)
+
+	var scratch [4]byte
+	binary.BigEndian.PutUint32(scratch[:], uint32(len(header)+len(payload)))
+	xsum = checksumBytes(scratch[:], xsum)
+	xsum = checksumBytes(append(scratch[:0], 0, 0, 0, uint8(ipproto.ICMPv6)), xsum)
+	xsum = checksumBytes(payload, xsum)
+
+	var hdrz [icmp6HeaderLength]byte
+	copy(hdrz[:], header)
+	// Zero out the header.
+	hdrz[2] = 0
+	hdrz[3] = 0
+	xsum = ^checksumBytes(hdrz[:], xsum)
+	return xsum
+}
+
+// checksumCombine combines the two uint16 to form their
+// checksum. This is done by adding them and the carry.
+//
+// Note that checksum a must have been computed on an even number of
+// bytes.
+func checksumCombine(a, b uint16) uint16 {
+	v := uint32(a) + uint32(b)
+	return uint16(v + v>>16)
+}
+
+// checksumBytes calculates the checksum (as defined in RFC 1071) of
+// the bytes in buf.
+//
+// The initial checksum must have been computed on an even number of bytes.
+func checksumBytes(buf []byte, initial uint16) uint16 {
+	v := uint32(initial)
+
+	odd := len(buf)%2 == 1
+	if odd {
+		v += uint32(buf[0])
+		buf = buf[1:]
+	}
+
+	n := len(buf)
+	odd = n&1 != 0
+	if odd {
+		n--
+		v += uint32(buf[n]) << 8
+	}
+
+	for i := 0; i < n; i += 2 {
+		v += (uint32(buf[i]) << 8) + uint32(buf[i+1])
+	}
+
+	return checksumCombine(uint16(v), uint16(v>>16))
+}