net/packet: documentation pass.

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

net/packet/ip4.go

@@ -14,13 +14,13 @@ import (
 // IP4 is an IPv4 address.
 type IP4 uint32
 
-// IPFromNetaddr converts a netaddr.IP to an IP. Panics if !ip.Is4.
+// IPFromNetaddr converts a netaddr.IP to an IP4. Panics if !ip.Is4.
 func IP4FromNetaddr(ip netaddr.IP) IP4 {
 	ipbytes := ip.As4()
 	return IP4(binary.BigEndian.Uint32(ipbytes[:]))
 }
 
-// Netaddr converts an IP to a netaddr.IP.
+// Netaddr converts ip to a netaddr.IP.
 func (ip IP4) Netaddr() netaddr.IP {
 	return netaddr.IPv4(byte(ip>>24), byte(ip>>16), byte(ip>>8), byte(ip))
 }
@@ -29,15 +29,21 @@ func (ip IP4) String() string {
 	return fmt.Sprintf("%d.%d.%d.%d", byte(ip>>24), byte(ip>>16), byte(ip>>8), byte(ip))
 }
 
+// IsMulticast returns whether ip is a multicast address.
 func (ip IP4) IsMulticast() bool {
 	return byte(ip>>24)&0xf0 == 0xe0
 }
 
+// IsLinkLocalUnicast returns whether ip is a link-local unicast
+// address.
 func (ip IP4) IsLinkLocalUnicast() bool {
 	return byte(ip>>24) == 169 && byte(ip>>16) == 254
 }
 
-// IPHeader represents an IP packet header.
+// ip4HeaderLength is the length of an IPv4 header with no IP options.
+const ip4HeaderLength = 20
+
+// IP4Header represents an IPv4 packet header.
 type IP4Header struct {
 	IPProto IPProto
 	IPID    uint16
@@ -45,59 +51,87 @@ type IP4Header struct {
 	DstIP   IP4
 }
 
-const ip4HeaderLength = 20
-
-func (IP4Header) Len() int {
+// Len implements Header.
+func (h IP4Header) Len() int {
 	return ip4HeaderLength
 }
 
+// Marshal implements Header.
 func (h IP4Header) Marshal(buf []byte) error {
-	if len(buf) < ip4HeaderLength {
+	if len(buf) < h.Len() {
 		return errSmallBuffer
 	}
 	if len(buf) > maxPacketLength {
 		return errLargePacket
 	}
 
-	buf[0] = 0x40 | (ip4HeaderLength >> 2) // IPv4
-	buf[1] = 0x00                          // DHCP, ECN
-	binary.BigEndian.PutUint16(buf[2:4], uint16(len(buf)))
-	binary.BigEndian.PutUint16(buf[4:6], h.IPID)
-	binary.BigEndian.PutUint16(buf[6:8], 0) // flags, offset
-	buf[8] = 64                             // TTL
-	buf[9] = uint8(h.IPProto)
-	binary.BigEndian.PutUint16(buf[10:12], 0) // blank IP header checksum
-	binary.BigEndian.PutUint32(buf[12:16], uint32(h.SrcIP))
-	binary.BigEndian.PutUint32(buf[16:20], uint32(h.DstIP))
+	buf[0] = 0x40 | (byte(h.Len() >> 2))                   // IPv4 + IHL
+	buf[1] = 0x00                                          // DSCP + ECN
+	binary.BigEndian.PutUint16(buf[2:4], uint16(len(buf))) // Total length
+	binary.BigEndian.PutUint16(buf[4:6], h.IPID)           // ID
+	binary.BigEndian.PutUint16(buf[6:8], 0)                // Flags + fragment offset
+	buf[8] = 64                                            // TTL
+	buf[9] = uint8(h.IPProto)                              // Inner protocol
+	// Blank checksum. This is necessary even though we overwrite
+	// it later, because the checksum computation runs over these
+	// bytes and expects them to be zero.
+	binary.BigEndian.PutUint16(buf[10:12], 0)
+	binary.BigEndian.PutUint32(buf[12:16], uint32(h.SrcIP)) // Src
+	binary.BigEndian.PutUint32(buf[16:20], uint32(h.DstIP)) // Dst
 
-	binary.BigEndian.PutUint16(buf[10:12], ipChecksum(buf[0:20]))
+	binary.BigEndian.PutUint16(buf[10:12], ip4Checksum(buf[0:20])) // Checksum
 
 	return nil
 }
 
-// MarshalPseudo serializes the header into buf in the "pseudo-header"
-// form required when calculating UDP checksums. Overwrites the first
-// h.Length() bytes of buf.
-func (h IP4Header) MarshalPseudo(buf []byte) error {
-	if len(buf) < ip4HeaderLength {
-		return errSmallBuffer
-	}
-	if len(buf) > maxPacketLength {
-		return errLargePacket
-	}
-
-	length := len(buf) - ip4HeaderLength
-	binary.BigEndian.PutUint32(buf[8:12], uint32(h.SrcIP))
-	binary.BigEndian.PutUint32(buf[12:16], uint32(h.DstIP))
-	buf[16] = 0x0
-	buf[17] = uint8(h.IPProto)
-	binary.BigEndian.PutUint16(buf[18:20], uint16(length))
-	return nil
-}
-
 // ToResponse implements Header.
 func (h *IP4Header) ToResponse() {
 	h.SrcIP, h.DstIP = h.DstIP, h.SrcIP
 	// Flip the bits in the IPID. If incoming IPIDs are distinct, so are these.
 	h.IPID = ^h.IPID
 }
+
+// ip4Checksum computes an IPv4 checksum, as specified in
+// https://tools.ietf.org/html/rfc1071
+func ip4Checksum(b []byte) uint16 {
+	var ac uint32
+	i := 0
+	n := len(b)
+	for n >= 2 {
+		ac += uint32(binary.BigEndian.Uint16(b[i : i+2]))
+		n -= 2
+		i += 2
+	}
+	if n == 1 {
+		ac += uint32(b[i]) << 8
+	}
+	for (ac >> 16) > 0 {
+		ac = (ac >> 16) + (ac & 0xffff)
+	}
+	return uint16(^ac)
+}
+
+// ip4PseudoHeaderOffset is the number of bytes by which the IPv4 UDP
+// pseudo-header is smaller than the real IPv4 header.
+const ip4PseudoHeaderOffset = 8
+
+// marshalPseudo serializes h into buf in the "pseudo-header" form
+// required when calculating UDP checksums. The pseudo-header starts
+// at buf[ip4PseudoHeaderOffset] so as to abut the following UDP
+// header, while leaving enough space in buf for a full IPv4 header.
+func (h IP4Header) marshalPseudo(buf []byte) error {
+	if len(buf) < h.Len() {
+		return errSmallBuffer
+	}
+	if len(buf) > maxPacketLength {
+		return errLargePacket
+	}
+
+	length := len(buf) - h.Len()
+	binary.BigEndian.PutUint32(buf[8:12], uint32(h.SrcIP))
+	binary.BigEndian.PutUint32(buf[12:16], uint32(h.DstIP))
+	buf[16] = 0x0
+	buf[17] = uint8(h.IPProto)
+	binary.BigEndian.PutUint16(buf[18:20], uint16(length))
+	return nil
+}