packet.go

// Package packet support for TCP/IP packet manipulations
// needed by the Aporeto infrastructure.
package packet

import (
	"encoding/binary"
	"encoding/hex"
	"errors"
	"fmt"
	"net"
	"strconv"

	"go.uber.org/zap"
)

// printCount prints the debug header for packets every few lines that it prints
var printCount int

var errIPPacketCorrupt = errors.New("IP packet is smaller than min IP size of 20")
var errTCPPacketCorrupt = errors.New("TCP Packet corrupt")
var errTCPAuthOption = errors.New("tcp authentication option not found")

// New returns a pointer to Packet structure built from the
// provided bytes buffer which is expected to contain valid TCP/IP
// packet bytes.
func New(context uint64, bytes []byte, mark string, lengthValidate bool) (packet *Packet, err error) {

	var p Packet

	// Get the mark value
	p.Mark = mark

	// Set the context
	p.context = context
	if len(bytes) < ipv4HdrLenPos {
		return nil, fmt.Errorf("invalid packet length %d", len(bytes))
	}
	if bytes[ipv4HdrLenPos]&ipv4ProtoMask == 0x40 {
		p.ipHdr.version = V4
		return &p, p.parseIPv4Packet(bytes, lengthValidate)
	}

	p.ipHdr.version = V6
	return &p, p.parseIPv6Packet(bytes, lengthValidate)
}

func (p *Packet) parseTCP(bytes []byte) {
	// TCP Header Processing
	tcpBuffer := bytes[p.ipHdr.ipHeaderLen:]

	p.tcpHdr.tcpChecksum = binary.BigEndian.Uint16(tcpBuffer[tcpChecksumPos : tcpChecksumPos+2])
	p.tcpHdr.sourcePort = binary.BigEndian.Uint16(tcpBuffer[tcpSourcePortPos : tcpSourcePortPos+2])
	p.tcpHdr.destinationPort = binary.BigEndian.Uint16(tcpBuffer[tcpDestPortPos : tcpDestPortPos+2])
	p.tcpHdr.tcpAck = binary.BigEndian.Uint32(tcpBuffer[tcpAckPos : tcpAckPos+4])
	p.tcpHdr.tcpSeq = binary.BigEndian.Uint32(tcpBuffer[tcpSeqPos : tcpSeqPos+4])
	p.tcpHdr.tcpDataOffset = (tcpBuffer[tcpDataOffsetPos] & tcpDataOffsetMask) >> 4
	p.tcpHdr.tcpTotalLength = uint16(len(p.ipHdr.Buffer[p.ipHdr.ipHeaderLen:]))

	p.SetTCPFlags(tcpBuffer[tcpFlagsOffsetPos])

	// Options and Payload that maybe added
	p.tcpHdr.tcpOptions = []byte{}
	p.tcpHdr.tcpData = []byte{}
}

func (p *Packet) parseUDP(bytes []byte) {
	// UDP Header Processing
	udpBuffer := bytes[p.ipHdr.ipHeaderLen:]

	p.udpHdr.udpChecksum = binary.BigEndian.Uint16(udpBuffer[udpChecksumPos : udpChecksumPos+2])
	p.udpHdr.udpData = []byte{}

	p.udpHdr.sourcePort = binary.BigEndian.Uint16(udpBuffer[udpSourcePortPos : udpSourcePortPos+2])
	p.udpHdr.destinationPort = binary.BigEndian.Uint16(udpBuffer[udpDestPortPos : udpDestPortPos+2])
}

func (p *Packet) parseIPv4Packet(bytes []byte, lengthValidate bool) (err error) {

	// IP Header Processing
	if len(bytes) < minIPv4HdrSize {
		return errIPPacketCorrupt
	}

	p.ipHdr.ipHeaderLen = (bytes[ipv4HdrLenPos] & ipv4HdrLenMask) * 4
	p.ipHdr.ipProto = bytes[ipv4ProtoPos]
	p.ipHdr.ipTotalLength = binary.BigEndian.Uint16(bytes[ipv4LengthPos : ipv4LengthPos+2])
	p.ipHdr.ipID = binary.BigEndian.Uint16(bytes[ipv4IDPos : ipv4IDPos+2])
	p.ipHdr.ipChecksum = binary.BigEndian.Uint16(bytes[ipv4ChecksumPos : ipv4ChecksumPos+2])
	p.ipHdr.sourceAddress = append([]byte{}, bytes[ipv4SourceAddrPos:ipv4SourceAddrPos+4]...)
	p.ipHdr.destinationAddress = append([]byte{}, bytes[ipv4DestAddrPos:ipv4DestAddrPos+4]...)

	if p.ipHdr.ipHeaderLen != minIPv4HdrSize {
		return fmt.Errorf("packets with ip options not supported: hdrlen=%d", p.ipHdr.ipHeaderLen)
	}

	p.ipHdr.Buffer = bytes

	if lengthValidate && p.ipHdr.ipTotalLength != uint16(len(p.ipHdr.Buffer)) {
		if p.ipHdr.ipTotalLength < uint16(len(p.ipHdr.Buffer)) {
			p.ipHdr.Buffer = p.ipHdr.Buffer[:p.ipHdr.ipTotalLength]
		} else {
			return fmt.Errorf("stated ip packet length %d differs from bytes available %d", p.ipHdr.ipTotalLength, len(p.ipHdr.Buffer))
		}
	}

	// Some sanity checking for TCP.
	if p.ipHdr.ipProto == IPProtocolTCP {
		if p.ipHdr.ipTotalLength-uint16(p.ipHdr.ipHeaderLen) < minTCPIPPacketLen {
			return fmt.Errorf("tcp ip packet too small: hdrlen=%d", p.ipHdr.ipHeaderLen)
		}

		p.parseTCP(bytes)
	}

	// Some sanity checking for UDP.
	if p.ipHdr.ipProto == IPProtocolUDP {
		if p.ipHdr.ipTotalLength-uint16(p.ipHdr.ipHeaderLen) < minUDPIPPacketLen {
			return fmt.Errorf("udp ip packet too small: hdrlen=%d", p.ipHdr.ipHeaderLen)
		}

		p.parseUDP(bytes)
	}

	// Chaching the result of the flow hash for performance optimization.
	// It is called in multiple places.
	p.l4flowhash = p.l4FlowHash()

	return nil
}

func (p *Packet) parseIPv6Packet(bytes []byte, lengthValidate bool) (err error) {
	// IP Header Processing
	p.ipHdr.ipHeaderLen = ipv6HeaderLen
	p.ipHdr.ipProto = bytes[ipv6ProtoPos]
	p.ipHdr.ipTotalLength = ipv6HeaderLen + binary.BigEndian.Uint16(bytes[ipv6PayloadLenPos:ipv6PayloadLenPos+2])
	p.ipHdr.sourceAddress = append([]byte{}, bytes[ipv6SourceAddrPos:ipv6SourceAddrPos+16]...)
	p.ipHdr.destinationAddress = append([]byte{}, bytes[ipv6DestAddrPos:ipv6DestAddrPos+16]...)

	p.ipHdr.Buffer = bytes

	if lengthValidate && p.ipHdr.ipTotalLength != uint16(len(p.ipHdr.Buffer)) {
		if p.ipHdr.ipTotalLength < uint16(len(p.ipHdr.Buffer)) {
			p.ipHdr.Buffer = p.ipHdr.Buffer[:p.ipHdr.ipTotalLength]
		} else {
			return fmt.Errorf("stated ip packet length %d differs from bytes available %d", p.ipHdr.ipTotalLength, len(p.ipHdr.Buffer))
		}

		p.parseTCP(bytes)
	}

	// Some sanity checking for TCP.
	if p.ipHdr.ipProto == IPProtocolTCP {
		if p.ipHdr.ipTotalLength-uint16(p.ipHdr.ipHeaderLen) < minTCPIPPacketLen {
			return fmt.Errorf("tcp ip packet too small: hdrlen=%d", p.ipHdr.ipHeaderLen)
		}

		p.parseTCP(bytes)
	}

	// Some sanity checking for UDP.
	if p.ipHdr.ipProto == IPProtocolUDP {
		if p.ipHdr.ipTotalLength-uint16(p.ipHdr.ipHeaderLen) < minUDPIPPacketLen {
			return fmt.Errorf("udp ip packet too small: hdrlen=%d", p.ipHdr.ipHeaderLen)
		}
		p.parseUDP(bytes)
	}

	// Chaching the result of the flow hash for performance optimization.
	// It is called in multiple places.
	p.l4flowhash = p.l4FlowHash()

	return nil
}

// IsEmptyTCPPayload returns the TCP data offset
func (p *Packet) IsEmptyTCPPayload() bool {
	return p.TCPDataStartBytes() == p.tcpHdr.tcpTotalLength
}

// GetTCPData returns any additional data in the packet
func (p *Packet) GetTCPData() []byte {
	return p.tcpHdr.tcpData
}

// GetUDPData return additional data in packet
func (p *Packet) GetUDPData() []byte {
	return p.ipHdr.Buffer[p.ipHdr.ipHeaderLen+UDPDataPos:]
}

// GetUDPDataStartBytes return start of UDP data
func (p *Packet) GetUDPDataStartBytes() uint16 {
	return UDPDataPos
}

// SetTCPData returns any additional data in the packet
func (p *Packet) SetTCPData(b []byte) {
	p.tcpHdr.tcpData = b
}

// GetTCPOptions returns any additional options in the packet
func (p *Packet) GetTCPOptions() []byte {
	return p.tcpHdr.tcpOptions
}

// DropTCPDetachedDataBytes removes any bytes that have been detached and stored locally
func (p *Packet) DropTCPDetachedDataBytes() {
	p.tcpHdr.tcpData = []byte{}
}

// DropTCPDetachedBytes removes any bytes that have been detached and stored locally
func (p *Packet) DropTCPDetachedBytes() {

	p.tcpHdr.tcpOptions = []byte{}
	p.tcpHdr.tcpData = []byte{}
}

// TCPDataStartBytes provides the tcp data start offset in bytes
func (p *Packet) TCPDataStartBytes() uint16 {
	return uint16(p.tcpHdr.tcpDataOffset) * 4
}

// GetIPLength returns the IP length
func (p *Packet) GetIPLength() uint16 {
	return p.ipHdr.ipTotalLength
}

// Print is a print helper function
func (p *Packet) Print(context uint64, packetLogLevel bool) {

	if p.ipHdr.ipProto != IPProtocolTCP {
		return
	}

	logPkt := false
	detailed := false

	if packetLogLevel || context == 0 {
		logPkt = true
		detailed = true
	}

	var buf string
	print := false

	if logPkt {
		if printCount%200 == 0 {
			buf += fmt.Sprintf("Packet: %5s %5s %25s %15s %5s %15s %5s %6s %20s %20s %6s %20s %20s %2s %5s %5s\n",
				"IPID", "Dir", "Comment", "SIP", "SP", "DIP", "DP", "Flags", "TCPSeq", "TCPAck", "TCPLen", "ExpAck", "ExpSeq", "DO", "Acsum", "Ccsum")
		}
		printCount++
		offset := 0

		if (p.GetTCPFlags() & TCPSynMask) == TCPSynMask {
			offset = 1
		}

		expAck := p.tcpHdr.tcpSeq + uint32(uint16(len(p.ipHdr.Buffer[p.ipHdr.ipHeaderLen:]))-p.TCPDataStartBytes()) + uint32(offset)
		ccsum := p.computeTCPChecksum()
		csumValidationStr := ""

		if p.tcpHdr.tcpChecksum != ccsum {
			csumValidationStr = "Bad Checksum"
		}

		buf += fmt.Sprintf("Packet: %5d %5s %25s %15s %5d %15s %5d %6s %20d %20d %6d %20d %20d %2d %5d %5d %12s\n",
			p.ipHdr.ipID,
			flagsToDir(p.context|context),
			flagsToStr(p.context|context),
			p.ipHdr.sourceAddress.String(), p.tcpHdr.sourcePort,
			p.ipHdr.destinationAddress.String(), p.tcpHdr.destinationPort,
			tcpFlagsToStr(p.GetTCPFlags()),
			p.tcpHdr.tcpSeq, p.tcpHdr.tcpAck, uint16(len(p.ipHdr.Buffer[p.ipHdr.ipHeaderLen:]))-p.TCPDataStartBytes(),
			expAck, expAck, p.tcpHdr.tcpDataOffset,
			p.tcpHdr.tcpChecksum, ccsum, csumValidationStr)
		print = true
	}

	if detailed {
		pktBytes := []byte{0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 2, 8, 0}
		pktBytes = append(pktBytes, p.ipHdr.Buffer...)
		pktBytes = append(pktBytes, p.tcpHdr.tcpOptions...)
		pktBytes = append(pktBytes, p.tcpHdr.tcpData...)
		buf += fmt.Sprintf("%s\n", hex.Dump(pktBytes))
		print = true
	}

	if print {
		zap.L().Debug(buf)
	}
}

//GetTCPBytes returns the bytes in the packet. It consolidates in case of changes as well
func (p *Packet) GetTCPBytes() []byte {

	pktBytes := []byte{}
	pktBytes = append(pktBytes, p.ipHdr.Buffer...)
	pktBytes = append(pktBytes, p.tcpHdr.tcpOptions...)
	pktBytes = append(pktBytes, p.tcpHdr.tcpData...)
	return pktBytes
}

// ReadTCPDataString returns ths payload in a string variable
// It does not remove the payload from the packet
func (p *Packet) ReadTCPDataString() string {
	return string(p.ReadTCPData())
}

// ReadTCPData returns ths payload in a string variable
// It does not remove the payload from the packet
func (p *Packet) ReadTCPData() []byte {
	return p.ipHdr.Buffer[uint16(p.ipHdr.ipHeaderLen)+p.TCPDataStartBytes():]
}

// CheckTCPAuthenticationOption ensures authentication option exists at the offset provided
func (p *Packet) CheckTCPAuthenticationOption(iOptionLength int) (err error) {

	tcpDataStart := p.TCPDataStartBytes()

	if tcpDataStart <= minTCPIPPacketLen {
		return errTCPAuthOption
	}

	optionLength := uint16(iOptionLength)
	buffer := p.ipHdr.Buffer[p.ipHdr.ipHeaderLen:]
	// Our option was not found in the right place. We don't do anything
	// for this packet.
	if buffer[tcpDataStart-optionLength] != TCPAuthenticationOption {
		return errTCPAuthOption
	}

	return
}

// FixupIPHdrOnDataModify modifies the IP header fields and checksum
func (p *Packet) FixupIPHdrOnDataModify(old, new uint16) {
	// IP Header Processing
	// IP chekcsum fixup.
	p.ipHdr.ipChecksum = incCsum16(p.ipHdr.ipChecksum, old, new)
	// Update IP Total Length.
	p.ipHdr.ipTotalLength = p.ipHdr.ipTotalLength + new - old

	if p.ipHdr.version == V6 {
		binary.BigEndian.PutUint16(p.ipHdr.Buffer[ipv6PayloadLenPos:ipv6PayloadLenPos+2], p.ipHdr.ipTotalLength-uint16(p.ipHdr.ipHeaderLen))
	} else {
		binary.BigEndian.PutUint16(p.ipHdr.Buffer[ipv4LengthPos:ipv4LengthPos+2], p.ipHdr.ipTotalLength)
		binary.BigEndian.PutUint16(p.ipHdr.Buffer[ipv4ChecksumPos:ipv4ChecksumPos+2], p.ipHdr.ipChecksum)
	}
}

// IncreaseTCPSeq increases TCP seq number by incr
func (p *Packet) IncreaseTCPSeq(incr uint32) {
	buffer := p.ipHdr.Buffer[p.ipHdr.ipHeaderLen:]
	p.tcpHdr.tcpSeq = p.tcpHdr.tcpSeq + incr
	binary.BigEndian.PutUint32(buffer[tcpSeqPos:tcpSeqPos+4], p.tcpHdr.tcpSeq)
}

// DecreaseTCPSeq decreases TCP seq number by decr
func (p *Packet) DecreaseTCPSeq(decr uint32) {
	buffer := p.ipHdr.Buffer[p.ipHdr.ipHeaderLen:]
	p.tcpHdr.tcpSeq = p.tcpHdr.tcpSeq - decr
	binary.BigEndian.PutUint32(buffer[tcpSeqPos:tcpSeqPos+4], p.tcpHdr.tcpSeq)
}

// IncreaseTCPAck increases TCP ack number by incr
func (p *Packet) IncreaseTCPAck(incr uint32) {
	buffer := p.ipHdr.Buffer[p.ipHdr.ipHeaderLen:]
	p.tcpHdr.tcpAck = p.tcpHdr.tcpAck + incr
	binary.BigEndian.PutUint32(buffer[tcpAckPos:tcpAckPos+4], p.tcpHdr.tcpAck)
}

// DecreaseTCPAck decreases TCP ack number by decr
func (p *Packet) DecreaseTCPAck(decr uint32) {
	buffer := p.ipHdr.Buffer[p.ipHdr.ipHeaderLen:]
	p.tcpHdr.tcpAck = p.tcpHdr.tcpAck - decr
	binary.BigEndian.PutUint32(buffer[tcpAckPos:tcpAckPos+4], p.tcpHdr.tcpAck)
}

// FixuptcpHdrOnTCPDataDetach modifies the TCP header fields and checksum
func (p *Packet) FixuptcpHdrOnTCPDataDetach(dataLength uint16, optionLength uint16) {

	// Update DataOffset
	buffer := p.ipHdr.Buffer[p.ipHdr.ipHeaderLen:]
	p.tcpHdr.tcpDataOffset = p.tcpHdr.tcpDataOffset - uint8(optionLength/4)
	buffer[tcpDataOffsetPos] = p.tcpHdr.tcpDataOffset << 4
}

// tcpDataDetach splits the p.Buffer into p.Buffer (header + some options), p.tcpOptions (optionLength) and p.TCPData (dataLength)
func (p *Packet) tcpDataDetach(optionLength uint16, dataLength uint16) (err error) {

	buffer := p.ipHdr.Buffer[p.ipHdr.ipHeaderLen:]
	// Setup buffer for Options, Data and reduce the original buffer
	if dataLength != 0 {
		p.tcpHdr.tcpData = buffer[p.TCPDataStartBytes():]
	}

	if optionLength != 0 {
		p.tcpHdr.tcpOptions = buffer[p.TCPDataStartBytes()-optionLength : p.TCPDataStartBytes()]
	}

	p.ipHdr.Buffer = p.ipHdr.Buffer[:uint16(p.ipHdr.ipHeaderLen)+p.TCPDataStartBytes()-optionLength]

	return
}

// TCPDataDetach performs the following:
//   - Removes all TCP data from Buffer to TCPData.
//   - Removes "optionLength" bytes of options from TCP header to tcpOptions
//   - Updates IP Hdr (lengths, checksums)
//   - Updates TCP header (checksums)
func (p *Packet) TCPDataDetach(optionLength uint16) (err error) {
	// Length
	dataLength := uint16(len(p.ipHdr.Buffer[p.ipHdr.ipHeaderLen:])) - p.TCPDataStartBytes()

	// detach TCP data
	if err = p.tcpDataDetach(optionLength, dataLength); err != nil {
		zap.L().Debug(fmt.Sprintf("tcp data detach failed: %s: optionlength=%d optionlength=%d", err, optionLength, dataLength))
		return errTCPPacketCorrupt
	}

	// Process TCP Header fields and metadata
	p.FixuptcpHdrOnTCPDataDetach(dataLength, optionLength)

	// Process IP Header fields
	p.FixupIPHdrOnDataModify(p.ipHdr.ipTotalLength, p.ipHdr.ipTotalLength-(dataLength+optionLength))
	return
}

// FixuptcpHdrOnTCPDataAttach modifies the TCP header fields and checksum
func (p *Packet) FixuptcpHdrOnTCPDataAttach(tcpOptions []byte, tcpData []byte) {
	buffer := p.ipHdr.Buffer[p.ipHdr.ipHeaderLen:]
	numberOfOptions := len(tcpOptions) / 4

	// Modify the fields
	p.tcpHdr.tcpDataOffset = p.tcpHdr.tcpDataOffset + uint8(numberOfOptions)
	binary.BigEndian.PutUint16(buffer[tcpChecksumPos:tcpChecksumPos+2], p.tcpHdr.tcpChecksum)
	buffer[tcpDataOffsetPos] = p.tcpHdr.tcpDataOffset << 4
}

// tcpDataAttach splits the p.Buffer into p.Buffer (header + some options), p.tcpOptions (optionLength) and p.TCPData (dataLength)
func (p *Packet) tcpDataAttach(options []byte, data []byte) (err error) {

	if p.TCPDataStartBytes() != uint16(len(p.ipHdr.Buffer[p.ipHdr.ipHeaderLen:])) && len(options) != 0 {
		return fmt.Errorf("cannot insert options with existing data: optionlength=%d, iptotallength=%d", len(options), p.ipHdr.ipTotalLength)
	}

	p.tcpHdr.tcpOptions = append(p.tcpHdr.tcpOptions, options...)
	p.tcpHdr.tcpData = data

	return
}

// TCPDataAttach modifies the TCP and IP header fields and checksum
func (p *Packet) TCPDataAttach(tcpOptions []byte, tcpData []byte) (err error) {

	if err = p.tcpDataAttach(tcpOptions, tcpData); err != nil {
		return fmt.Errorf("tcp data attachment failed: %s", err)
	}

	// We are increasing tcpOptions by 1 32-bit word. We are always adding
	// our option last.
	packetLenIncrease := uint16(len(tcpData) + len(tcpOptions))
	// IP Header Processing
	p.FixupIPHdrOnDataModify(p.ipHdr.ipTotalLength, p.ipHdr.ipTotalLength+packetLenIncrease)

	// TCP Header Processing
	p.FixuptcpHdrOnTCPDataAttach(tcpOptions, tcpData)

	return
}

// L4FlowHash calculate a hash string based on the 4-tuple. It returns the cached
// value and does not re-calculate it. This leads to performance gains.
func (p *Packet) L4FlowHash() string {
	return p.l4flowhash
}

func (p *Packet) l4FlowHash() string {
	return p.ipHdr.sourceAddress.String() + ":" + p.ipHdr.destinationAddress.String() + ":" + strconv.Itoa(int(p.SourcePort())) + ":" + strconv.Itoa(int(p.DestPort()))
}

// L4ReverseFlowHash calculate a hash string based on the 4-tuple by reversing source and destination information
func (p *Packet) L4ReverseFlowHash() string {
	return p.ipHdr.destinationAddress.String() + ":" + p.ipHdr.sourceAddress.String() + ":" + strconv.Itoa(int(p.DestPort())) + ":" + strconv.Itoa(int(p.SourcePort()))
}

// SourcePortHash calculates a hash based on dest ip/port for net packet and src ip/port for app packet.
func (p *Packet) SourcePortHash(stage uint64) string {
	if stage == PacketTypeNetwork {
		return p.ipHdr.destinationAddress.String() + ":" + strconv.Itoa(int(p.DestPort()))
	}

	return p.ipHdr.sourceAddress.String() + ":" + strconv.Itoa(int(p.SourcePort()))
}

// ID returns the IP ID of the packet
func (p *Packet) ID() string {
	return strconv.Itoa(int(p.ipHdr.ipID))
}

//TCPOptionLength returns the length of tcpoptions
func (p *Packet) TCPOptionLength() int {
	return len(p.tcpHdr.tcpOptions)
}

//TCPDataLength -- returns the length of tcp options
func (p *Packet) TCPDataLength() int {
	return len(p.tcpHdr.tcpData)
}

//SourcePort -- returns the appropriate source port
func (p *Packet) SourcePort() uint16 {
	if p.ipHdr.ipProto == IPProtocolTCP {
		return p.tcpHdr.sourcePort
	}

	return p.udpHdr.sourcePort
}

//DestPort -- returns the appropriate destination port
func (p *Packet) DestPort() uint16 {
	if p.ipHdr.ipProto == IPProtocolTCP {
		return p.tcpHdr.destinationPort
	}

	return p.udpHdr.destinationPort
}

//SourceAddress returns the source IP
func (p *Packet) SourceAddress() net.IP {
	return p.ipHdr.sourceAddress
}

//DestinationAddress returns the destination address
func (p *Packet) DestinationAddress() net.IP {
	return p.ipHdr.destinationAddress
}

//IPProto returns the L4 protocol
func (p *Packet) IPProto() uint8 {
	return p.ipHdr.ipProto
}

//IPTotalLen returns the total length of the packet
func (p *Packet) IPTotalLen() uint16 {
	return p.ipHdr.ipTotalLength
}

//IPHeaderLen returns the ip header length
func (p *Packet) IPHeaderLen() uint8 {
	return p.ipHdr.ipHeaderLen
}

//GetBuffer returns the slice representing the buffer at offset specified
func (p *Packet) GetBuffer(offset int) []byte {
	return p.ipHdr.Buffer[offset:]
}

// IPversion returns the version of ip packet
func (p *Packet) IPversion() IPver {
	return p.ipHdr.version
}

//TestGetTCPPacket is used by other test code when they need to create a packet
func TestGetTCPPacket(srcIP, dstIP net.IP, srcPort, dstPort uint16) *Packet {
	p := &Packet{}

	p.ipHdr.destinationAddress = dstIP
	p.tcpHdr.destinationPort = dstPort
	p.ipHdr.sourceAddress = srcIP
	p.tcpHdr.sourcePort = srcPort
	p.ipHdr.ipProto = IPProtocolTCP

	return p
}