Support for connection tracking of TCP packets.

Connection tracking is used to track packets in prerouting and
output hooks of iptables. The NAT rules modify the tuples in
connections. The connection tracking code modifies the packets by
looking at the modified tuples.

16 files changed, 701 insertions, 98 deletions

diff --git a/pkg/tcpip/header/tcp.go b/pkg/tcpip/header/tcp.go
index 13480687d..21581257b 100644
--- a/pkg/tcpip/header/tcp.go
+++ b/pkg/tcpip/header/tcp.go
@@ -594,3 +594,20 @@ func AddTCPOptionPadding(options []byte, offset int) int {
 	}
 	return paddingToAdd
 }
+
+// Acceptable checks if a segment that starts at segSeq and has length segLen is
+// "acceptable" for arriving in a receive window that starts at rcvNxt and ends
+// before rcvAcc, according to the table on page 26 and 69 of RFC 793.
+func Acceptable(segSeq seqnum.Value, segLen seqnum.Size, rcvNxt, rcvAcc seqnum.Value) bool {
+	if rcvNxt == rcvAcc {
+		return segLen == 0 && segSeq == rcvNxt
+	}
+	if segLen == 0 {
+		// rcvWnd is incremented by 1 because that is Linux's behavior despite the
+		// RFC.
+		return segSeq.InRange(rcvNxt, rcvAcc.Add(1))
+	}
+	// Page 70 of RFC 793 allows packets that can be made "acceptable" by trimming
+	// the payload, so we'll accept any payload that overlaps the receieve window.
+	return rcvNxt.LessThan(segSeq.Add(segLen)) && segSeq.LessThan(rcvAcc)
+}
diff --git a/pkg/tcpip/network/ipv4/ipv4.go b/pkg/tcpip/network/ipv4/ipv4.go
index 1d61fddad..9db42b2a4 100644
--- a/pkg/tcpip/network/ipv4/ipv4.go
+++ b/pkg/tcpip/network/ipv4/ipv4.go
@@ -252,11 +252,31 @@ func (e *endpoint) WritePacket(r *stack.Route, gso *stack.GSO, params stack.Netw
 	// iptables filtering. All packets that reach here are locally
 	// generated.
 	ipt := e.stack.IPTables()
-	if ok := ipt.Check(stack.Output, pkt); !ok {
+	if ok := ipt.Check(stack.Output, &pkt, gso, r, ""); !ok {
 		// iptables is telling us to drop the packet.
 		return nil
 	}
 
+	if pkt.NatDone {
+		// If the packet is manipulated as per NAT Ouput rules, handle packet
+		// based on destination address and do not send the packet to link layer.
+		netHeader := header.IPv4(pkt.NetworkHeader)
+		ep, err := e.stack.FindNetworkEndpoint(header.IPv4ProtocolNumber, netHeader.DestinationAddress())
+		if err == nil {
+			src := netHeader.SourceAddress()
+			dst := netHeader.DestinationAddress()
+			route := r.ReverseRoute(src, dst)
+
+			views := make([]buffer.View, 1, 1+len(pkt.Data.Views()))
+			views[0] = pkt.Header.View()
+			views = append(views, pkt.Data.Views()...)
+			packet := stack.PacketBuffer{
+				Data: buffer.NewVectorisedView(len(views[0])+pkt.Data.Size(), views)}
+			ep.HandlePacket(&route, packet)
+			return nil
+		}
+	}
+
 	if r.Loop&stack.PacketLoop != 0 {
 		// The inbound path expects the network header to still be in
 		// the PacketBuffer's Data field.
@@ -302,8 +322,8 @@ func (e *endpoint) WritePackets(r *stack.Route, gso *stack.GSO, pkts stack.Packe
 	// iptables filtering. All packets that reach here are locally
 	// generated.
 	ipt := e.stack.IPTables()
-	dropped := ipt.CheckPackets(stack.Output, pkts)
-	if len(dropped) == 0 {
+	dropped, natPkts := ipt.CheckPackets(stack.Output, pkts, gso, r)
+	if len(dropped) == 0 && len(natPkts) == 0 {
 		// Fast path: If no packets are to be dropped then we can just invoke the
 		// faster WritePackets API directly.
 		n, err := e.linkEP.WritePackets(r, gso, pkts, ProtocolNumber)
@@ -318,6 +338,24 @@ func (e *endpoint) WritePackets(r *stack.Route, gso *stack.GSO, pkts stack.Packe
 		if _, ok := dropped[pkt]; ok {
 			continue
 		}
+		if _, ok := natPkts[pkt]; ok {
+			netHeader := header.IPv4(pkt.NetworkHeader)
+			ep, err := e.stack.FindNetworkEndpoint(header.IPv4ProtocolNumber, netHeader.DestinationAddress())
+			if err == nil {
+				src := netHeader.SourceAddress()
+				dst := netHeader.DestinationAddress()
+				route := r.ReverseRoute(src, dst)
+
+				views := make([]buffer.View, 1, 1+len(pkt.Data.Views()))
+				views[0] = pkt.Header.View()
+				views = append(views, pkt.Data.Views()...)
+				packet := stack.PacketBuffer{
+					Data: buffer.NewVectorisedView(len(views[0])+pkt.Data.Size(), views)}
+				ep.HandlePacket(&route, packet)
+				n++
+				continue
+			}
+		}
 		if err := e.linkEP.WritePacket(r, gso, ProtocolNumber, *pkt); err != nil {
 			r.Stats().IP.PacketsSent.IncrementBy(uint64(n))
 			return n, err
@@ -407,7 +445,7 @@ func (e *endpoint) HandlePacket(r *stack.Route, pkt stack.PacketBuffer) {
 	// iptables filtering. All packets that reach here are intended for
 	// this machine and will not be forwarded.
 	ipt := e.stack.IPTables()
-	if ok := ipt.Check(stack.Input, pkt); !ok {
+	if ok := ipt.Check(stack.Input, &pkt, nil, nil, ""); !ok {
 		// iptables is telling us to drop the packet.
 		return
 	}
diff --git a/pkg/tcpip/stack/BUILD b/pkg/tcpip/stack/BUILD
index 5e963a4af..f71073207 100644
--- a/pkg/tcpip/stack/BUILD
+++ b/pkg/tcpip/stack/BUILD
@@ -30,6 +30,7 @@ go_template_instance(
 go_library(
     name = "stack",
     srcs = [
+        "conntrack.go",
         "dhcpv6configurationfromndpra_string.go",
         "forwarder.go",
         "icmp_rate_limit.go",
@@ -62,6 +63,7 @@ go_library(
         "//pkg/tcpip/header",
         "//pkg/tcpip/ports",
         "//pkg/tcpip/seqnum",
+        "//pkg/tcpip/transport/tcpconntrack",
         "//pkg/waiter",
         "@org_golang_x_time//rate:go_default_library",
     ],
diff --git a/pkg/tcpip/stack/conntrack.go b/pkg/tcpip/stack/conntrack.go
new file mode 100644
index 000000000..7d1ede1f2
--- /dev/null
+++ b/pkg/tcpip/stack/conntrack.go
@@ -0,0 +1,480 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package stack
+
+import (
+	"encoding/binary"
+	"sync"
+	"time"
+
+	"gvisor.dev/gvisor/pkg/tcpip"
+	"gvisor.dev/gvisor/pkg/tcpip/buffer"
+	"gvisor.dev/gvisor/pkg/tcpip/hash/jenkins"
+	"gvisor.dev/gvisor/pkg/tcpip/header"
+	"gvisor.dev/gvisor/pkg/tcpip/transport/tcpconntrack"
+)
+
+// Connection tracking is used to track and manipulate packets for NAT rules.
+// The connection is created for a packet if it does not exist. Every connection
+// contains two tuples (original and reply). The tuples are manipulated if there
+// is a matching NAT rule. The packet is modified by looking at the tuples in the
+// Prerouting and Output hooks.
+
+// Direction of the tuple.
+type ctDirection int
+
+const (
+	dirOriginal ctDirection = iota
+	dirReply
+)
+
+// Status of connection.
+// TODO(gvisor.dev/issue/170): Add other states of connection.
+type connStatus int
+
+const (
+	connNew connStatus = iota
+	connEstablished
+)
+
+// Manipulation type for the connection.
+type manipType int
+
+const (
+	manipDstPrerouting manipType = iota
+	manipDstOutput
+)
+
+// connTrackMutable is the manipulatable part of the tuple.
+type connTrackMutable struct {
+	// addr is source address of the tuple.
+	addr tcpip.Address
+
+	// port is source port of the tuple.
+	port uint16
+
+	// protocol is network layer protocol.
+	protocol tcpip.NetworkProtocolNumber
+}
+
+// connTrackImmutable is the non-manipulatable part of the tuple.
+type connTrackImmutable struct {
+	// addr is destination address of the tuple.
+	addr tcpip.Address
+
+	// direction is direction (original or reply) of the tuple.
+	direction ctDirection
+
+	// port is destination port of the tuple.
+	port uint16
+
+	// protocol is transport layer protocol.
+	protocol tcpip.TransportProtocolNumber
+}
+
+// connTrackTuple represents the tuple which is created from the
+// packet.
+type connTrackTuple struct {
+	// dst is non-manipulatable part of the tuple.
+	dst connTrackImmutable
+
+	// src is manipulatable part of the tuple.
+	src connTrackMutable
+}
+
+// connTrackTupleHolder is the container of tuple and connection.
+type ConnTrackTupleHolder struct {
+	// conn is pointer to the connection tracking entry.
+	conn *connTrack
+
+	// tuple is original or reply tuple.
+	tuple connTrackTuple
+}
+
+// connTrack is the connection.
+type connTrack struct {
+	// originalTupleHolder contains tuple in original direction.
+	originalTupleHolder ConnTrackTupleHolder
+
+	// replyTupleHolder contains tuple in reply direction.
+	replyTupleHolder ConnTrackTupleHolder
+
+	// status indicates connection is new or established.
+	status connStatus
+
+	// timeout indicates the time connection should be active.
+	timeout time.Duration
+
+	// manip indicates if the packet should be manipulated.
+	manip manipType
+
+	// tcb is TCB control block. It is used to keep track of states
+	// of tcp connection.
+	tcb tcpconntrack.TCB
+
+	// tcbHook indicates if the packet is inbound or outbound to
+	// update the state of tcb.
+	tcbHook Hook
+}
+
+// ConnTrackTable contains a map of all existing connections created for
+// NAT rules.
+type ConnTrackTable struct {
+	// connMu protects connTrackTable.
+	connMu sync.RWMutex
+
+	// connTrackTable maintains a map of tuples needed for connection tracking
+	// for iptables NAT rules. The key for the map is an integer calculated
+	// using seed, source address, destination address, source port and
+	// destination port.
+	CtMap map[uint32]ConnTrackTupleHolder
+
+	// seed is a one-time random value initialized at stack startup
+	// and is used in calculation of hash key for connection tracking
+	// table.
+	Seed uint32
+}
+
+// parseHeaders sets headers in the packet.
+func parseHeaders(pkt *PacketBuffer) {
+	newPkt := pkt.Clone()
+
+	// Set network header.
+	hdr, ok := newPkt.Data.PullUp(header.IPv4MinimumSize)
+	if !ok {
+		return
+	}
+	netHeader := header.IPv4(hdr)
+	newPkt.NetworkHeader = hdr
+	length := int(netHeader.HeaderLength())
+
+	// TODO(gvisor.dev/issue/170): Need to support for other
+	// protocols as well.
+	// Set transport header.
+	switch protocol := netHeader.TransportProtocol(); protocol {
+	case header.UDPProtocolNumber:
+		if newPkt.TransportHeader == nil {
+			h, ok := newPkt.Data.PullUp(length + header.UDPMinimumSize)
+			if !ok {
+				return
+			}
+			newPkt.TransportHeader = buffer.View(header.UDP(h[length:]))
+		}
+	case header.TCPProtocolNumber:
+		if newPkt.TransportHeader == nil {
+			h, ok := newPkt.Data.PullUp(length + header.TCPMinimumSize)
+			if !ok {
+				return
+			}
+			newPkt.TransportHeader = buffer.View(header.TCP(h[length:]))
+		}
+	}
+	pkt.NetworkHeader = newPkt.NetworkHeader
+	pkt.TransportHeader = newPkt.TransportHeader
+}
+
+// packetToTuple converts packet to a tuple in original direction.
+func packetToTuple(pkt PacketBuffer, hook Hook) (connTrackTuple, *tcpip.Error) {
+	var tuple connTrackTuple
+
+	netHeader := header.IPv4(pkt.NetworkHeader)
+	// TODO(gvisor.dev/issue/170): Need to support for other
+	// protocols as well.
+	if netHeader == nil || netHeader.TransportProtocol() != header.TCPProtocolNumber {
+		return tuple, tcpip.ErrUnknownProtocol
+	}
+	tcpHeader := header.TCP(pkt.TransportHeader)
+	if tcpHeader == nil {
+		return tuple, tcpip.ErrUnknownProtocol
+	}
+
+	tuple.src.addr = netHeader.SourceAddress()
+	tuple.src.port = tcpHeader.SourcePort()
+	tuple.src.protocol = header.IPv4ProtocolNumber
+
+	tuple.dst.addr = netHeader.DestinationAddress()
+	tuple.dst.port = tcpHeader.DestinationPort()
+	tuple.dst.protocol = netHeader.TransportProtocol()
+
+	return tuple, nil
+}
+
+// getReplyTuple creates reply tuple for the given tuple.
+func getReplyTuple(tuple connTrackTuple) connTrackTuple {
+	var replyTuple connTrackTuple
+	replyTuple.src.addr = tuple.dst.addr
+	replyTuple.src.port = tuple.dst.port
+	replyTuple.src.protocol = tuple.src.protocol
+	replyTuple.dst.addr = tuple.src.addr
+	replyTuple.dst.port = tuple.src.port
+	replyTuple.dst.protocol = tuple.dst.protocol
+	replyTuple.dst.direction = dirReply
+
+	return replyTuple
+}
+
+// makeNewConn creates new connection.
+func makeNewConn(tuple, replyTuple connTrackTuple) connTrack {
+	var conn connTrack
+	conn.status = connNew
+	conn.originalTupleHolder.tuple = tuple
+	conn.originalTupleHolder.conn = &conn
+	conn.replyTupleHolder.tuple = replyTuple
+	conn.replyTupleHolder.conn = &conn
+
+	return conn
+}
+
+// getTupleHash returns hash of the tuple. The fields used for
+// generating hash are seed (generated once for stack), source address,
+// destination address, source port and destination ports.
+func (ct *ConnTrackTable) getTupleHash(tuple connTrackTuple) uint32 {
+	h := jenkins.Sum32(ct.Seed)
+	h.Write([]byte(tuple.src.addr))
+	h.Write([]byte(tuple.dst.addr))
+	portBuf := make([]byte, 2)
+	binary.LittleEndian.PutUint16(portBuf, tuple.src.port)
+	h.Write([]byte(portBuf))
+	binary.LittleEndian.PutUint16(portBuf, tuple.dst.port)
+	h.Write([]byte(portBuf))
+
+	return h.Sum32()
+}
+
+// connTrackForPacket returns connTrack for packet.
+// TODO(gvisor.dev/issue/170): Only TCP packets are supported. Need to support other
+// transport protocols.
+func (ct *ConnTrackTable) connTrackForPacket(pkt *PacketBuffer, hook Hook, createConn bool) (*connTrack, ctDirection) {
+	if hook == Prerouting {
+		// Headers will not be set in Prerouting.
+		// TODO(gvisor.dev/issue/170): Change this after parsing headers
+		// code is added.
+		parseHeaders(pkt)
+	}
+
+	var dir ctDirection
+	tuple, err := packetToTuple(*pkt, hook)
+	if err != nil {
+		return nil, dir
+	}
+
+	ct.connMu.Lock()
+	defer ct.connMu.Unlock()
+
+	connTrackTable := ct.CtMap
+	hash := ct.getTupleHash(tuple)
+
+	var conn *connTrack
+	switch createConn {
+	case true:
+		// If connection does not exist for the hash, create a new
+		// connection.
+		replyTuple := getReplyTuple(tuple)
+		replyHash := ct.getTupleHash(replyTuple)
+		newConn := makeNewConn(tuple, replyTuple)
+		conn = &newConn
+
+		// Add tupleHolders to the map.
+		// TODO(gvisor.dev/issue/170): Need to support collisions using linked list.
+		ct.CtMap[hash] = conn.originalTupleHolder
+		ct.CtMap[replyHash] = conn.replyTupleHolder
+	default:
+		tupleHolder, ok := connTrackTable[hash]
+		if !ok {
+			return nil, dir
+		}
+
+		// If this is the reply of new connection, set the connection
+		// status as ESTABLISHED.
+		conn = tupleHolder.conn
+		if conn.status == connNew && tupleHolder.tuple.dst.direction == dirReply {
+			conn.status = connEstablished
+		}
+		if tupleHolder.conn == nil {
+			panic("tupleHolder has null connection tracking entry")
+		}
+
+		dir = tupleHolder.tuple.dst.direction
+	}
+	return conn, dir
+}
+
+// SetNatInfo will manipulate the tuples according to iptables NAT rules.
+func (ct *ConnTrackTable) SetNatInfo(pkt *PacketBuffer, rt RedirectTarget, hook Hook) {
+	// Get the connection. Connection is always created before this
+	// function is called.
+	conn, _ := ct.connTrackForPacket(pkt, hook, false)
+	if conn == nil {
+		panic("connection should be created to manipulate tuples.")
+	}
+	replyTuple := conn.replyTupleHolder.tuple
+	replyHash := ct.getTupleHash(replyTuple)
+
+	// TODO(gvisor.dev/issue/170): Support only redirect of ports. Need to
+	// support changing of address for Prerouting.
+
+	// Change the port as per the iptables rule. This tuple will be used
+	// to manipulate the packet in HandlePacket.
+	conn.replyTupleHolder.tuple.src.addr = rt.MinIP
+	conn.replyTupleHolder.tuple.src.port = rt.MinPort
+	newHash := ct.getTupleHash(conn.replyTupleHolder.tuple)
+
+	// Add the changed tuple to the map.
+	ct.connMu.Lock()
+	defer ct.connMu.Unlock()
+	ct.CtMap[newHash] = conn.replyTupleHolder
+	if hook == Output {
+		conn.replyTupleHolder.conn.manip = manipDstOutput
+	}
+
+	// Delete the old tuple.
+	delete(ct.CtMap, replyHash)
+}
+
+// handlePacketPrerouting manipulates ports for packets in Prerouting hook.
+// TODO(gvisor.dev/issue/170): Change address for Prerouting hook..
+func handlePacketPrerouting(pkt *PacketBuffer, conn *connTrack, dir ctDirection) {
+	netHeader := header.IPv4(pkt.NetworkHeader)
+	tcpHeader := header.TCP(pkt.TransportHeader)
+
+	// For prerouting redirection, packets going in the original direction
+	// have their destinations modified and replies have their sources
+	// modified.
+	switch dir {
+	case dirOriginal:
+		port := conn.replyTupleHolder.tuple.src.port
+		tcpHeader.SetDestinationPort(port)
+		netHeader.SetDestinationAddress(conn.replyTupleHolder.tuple.src.addr)
+	case dirReply:
+		port := conn.originalTupleHolder.tuple.dst.port
+		tcpHeader.SetSourcePort(port)
+		netHeader.SetSourceAddress(conn.originalTupleHolder.tuple.dst.addr)
+	}
+
+	netHeader.SetChecksum(0)
+	netHeader.SetChecksum(^netHeader.CalculateChecksum())
+}
+
+// handlePacketOutput manipulates ports for packets in Output hook.
+func handlePacketOutput(pkt *PacketBuffer, conn *connTrack, gso *GSO, r *Route, dir ctDirection) {
+	netHeader := header.IPv4(pkt.NetworkHeader)
+	tcpHeader := header.TCP(pkt.TransportHeader)
+
+	// For output redirection, packets going in the original direction
+	// have their destinations modified and replies have their sources
+	// modified. For prerouting redirection, we only reach this point
+	// when replying, so packet sources are modified.
+	if conn.manip == manipDstOutput && dir == dirOriginal {
+		port := conn.replyTupleHolder.tuple.src.port
+		tcpHeader.SetDestinationPort(port)
+		netHeader.SetDestinationAddress(conn.replyTupleHolder.tuple.src.addr)
+	} else {
+		port := conn.originalTupleHolder.tuple.dst.port
+		tcpHeader.SetSourcePort(port)
+		netHeader.SetSourceAddress(conn.originalTupleHolder.tuple.dst.addr)
+	}
+
+	// Calculate the TCP checksum and set it.
+	tcpHeader.SetChecksum(0)
+	hdr := &pkt.Header
+	length := uint16(pkt.Data.Size()+hdr.UsedLength()) - uint16(netHeader.HeaderLength())
+	xsum := r.PseudoHeaderChecksum(header.TCPProtocolNumber, length)
+	if gso != nil && gso.NeedsCsum {
+		tcpHeader.SetChecksum(xsum)
+	} else if r.Capabilities()&CapabilityTXChecksumOffload == 0 {
+		xsum = header.ChecksumVVWithOffset(pkt.Data, xsum, int(tcpHeader.DataOffset()), pkt.Data.Size())
+		tcpHeader.SetChecksum(^tcpHeader.CalculateChecksum(xsum))
+	}
+
+	netHeader.SetChecksum(0)
+	netHeader.SetChecksum(^netHeader.CalculateChecksum())
+}
+
+// HandlePacket will manipulate the port and address of the packet if the
+// connection exists.
+func (ct *ConnTrackTable) HandlePacket(pkt *PacketBuffer, hook Hook, gso *GSO, r *Route) {
+	if pkt.NatDone {
+		return
+	}
+
+	if hook != Prerouting && hook != Output {
+		return
+	}
+
+	conn, dir := ct.connTrackForPacket(pkt, hook, false)
+	// Connection or Rule not found for the packet.
+	if conn == nil {
+		return
+	}
+
+	netHeader := header.IPv4(pkt.NetworkHeader)
+	// TODO(gvisor.dev/issue/170): Need to support for other transport
+	// protocols as well.
+	if netHeader == nil || netHeader.TransportProtocol() != header.TCPProtocolNumber {
+		return
+	}
+
+	tcpHeader := header.TCP(pkt.TransportHeader)
+	if tcpHeader == nil {
+		return
+	}
+
+	switch hook {
+	case Prerouting:
+		handlePacketPrerouting(pkt, conn, dir)
+	case Output:
+		handlePacketOutput(pkt, conn, gso, r, dir)
+	}
+	pkt.NatDone = true
+
+	// Update the state of tcb.
+	// TODO(gvisor.dev/issue/170): Add support in tcpcontrack to handle
+	// other tcp states.
+	var st tcpconntrack.Result
+	if conn.tcb.IsEmpty() {
+		conn.tcb.Init(tcpHeader)
+		conn.tcbHook = hook
+	} else {
+		switch hook {
+		case conn.tcbHook:
+			st = conn.tcb.UpdateStateOutbound(tcpHeader)
+		default:
+			st = conn.tcb.UpdateStateInbound(tcpHeader)
+		}
+	}
+
+	// Delete conntrack if tcp connection is closed.
+	if st == tcpconntrack.ResultClosedByPeer || st == tcpconntrack.ResultClosedBySelf || st == tcpconntrack.ResultReset {
+		ct.deleteConnTrack(conn)
+	}
+}
+
+// deleteConnTrack deletes the connection.
+func (ct *ConnTrackTable) deleteConnTrack(conn *connTrack) {
+	if conn == nil {
+		return
+	}
+
+	tuple := conn.originalTupleHolder.tuple
+	hash := ct.getTupleHash(tuple)
+	replyTuple := conn.replyTupleHolder.tuple
+	replyHash := ct.getTupleHash(replyTuple)
+
+	ct.connMu.Lock()
+	defer ct.connMu.Unlock()
+
+	delete(ct.CtMap, hash)
+	delete(ct.CtMap, replyHash)
+}
diff --git a/pkg/tcpip/stack/iptables.go b/pkg/tcpip/stack/iptables.go
index 6b91159d4..7c3c47d50 100644
--- a/pkg/tcpip/stack/iptables.go
+++ b/pkg/tcpip/stack/iptables.go
@@ -17,6 +17,7 @@ package stack
 import (
 	"fmt"
 
+	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/header"
 )
 
@@ -110,6 +111,10 @@ func DefaultTables() IPTables {
 			Prerouting: []string{TablenameMangle, TablenameNat},
 			Output:     []string{TablenameMangle, TablenameNat, TablenameFilter},
 		},
+		connections: ConnTrackTable{
+			CtMap: make(map[uint32]ConnTrackTupleHolder),
+			Seed:  generateRandUint32(),
+		},
 	}
 }
 
@@ -173,12 +178,16 @@ const (
 // dropped.
 //
 // Precondition: pkt.NetworkHeader is set.
-func (it *IPTables) Check(hook Hook, pkt PacketBuffer) bool {
+func (it *IPTables) Check(hook Hook, pkt *PacketBuffer, gso *GSO, r *Route, address tcpip.Address) bool {
+	// Packets are manipulated only if connection and matching
+	// NAT rule exists.
+	it.connections.HandlePacket(pkt, hook, gso, r)
+
 	// Go through each table containing the hook.
 	for _, tablename := range it.Priorities[hook] {
 		table := it.Tables[tablename]
 		ruleIdx := table.BuiltinChains[hook]
-		switch verdict := it.checkChain(hook, pkt, table, ruleIdx); verdict {
+		switch verdict := it.checkChain(hook, pkt, table, ruleIdx, gso, r, address); verdict {
 		// If the table returns Accept, move on to the next table.
 		case chainAccept:
 			continue
@@ -189,7 +198,7 @@ func (it *IPTables) Check(hook Hook, pkt PacketBuffer) bool {
 			// Any Return from a built-in chain means we have to
 			// call the underflow.
 			underflow := table.Rules[table.Underflows[hook]]
-			switch v, _ := underflow.Target.Action(pkt); v {
+			switch v, _ := underflow.Target.Action(pkt, &it.connections, hook, gso, r, address); v {
 			case RuleAccept:
 				continue
 			case RuleDrop:
@@ -219,26 +228,34 @@ func (it *IPTables) Check(hook Hook, pkt PacketBuffer) bool {
 //
 // NOTE: unlike the Check API the returned map contains packets that should be
 // dropped.
-func (it *IPTables) CheckPackets(hook Hook, pkts PacketBufferList) (drop map[*PacketBuffer]struct{}) {
+func (it *IPTables) CheckPackets(hook Hook, pkts PacketBufferList, gso *GSO, r *Route) (drop map[*PacketBuffer]struct{}, natPkts map[*PacketBuffer]struct{}) {
 	for pkt := pkts.Front(); pkt != nil; pkt = pkt.Next() {
-		if ok := it.Check(hook, *pkt); !ok {
-			if drop == nil {
-				drop = make(map[*PacketBuffer]struct{})
+		if !pkt.NatDone {
+			if ok := it.Check(hook, pkt, gso, r, ""); !ok {
+				if drop == nil {
+					drop = make(map[*PacketBuffer]struct{})
+				}
+				drop[pkt] = struct{}{}
+			}
+			if pkt.NatDone {
+				if natPkts == nil {
+					natPkts = make(map[*PacketBuffer]struct{})
+				}
+				natPkts[pkt] = struct{}{}
 			}
-			drop[pkt] = struct{}{}
 		}
 	}
-	return drop
+	return drop, natPkts
 }
 
 // Precondition: pkt is a IPv4 packet of at least length header.IPv4MinimumSize.
-// TODO(gvisor.dev/issue/170): pk.NetworkHeader will always be set as a
+// TODO(gvisor.dev/issue/170): pkt.NetworkHeader will always be set as a
 // precondition.
-func (it *IPTables) checkChain(hook Hook, pkt PacketBuffer, table Table, ruleIdx int) chainVerdict {
+func (it *IPTables) checkChain(hook Hook, pkt *PacketBuffer, table Table, ruleIdx int, gso *GSO, r *Route, address tcpip.Address) chainVerdict {
 	// Start from ruleIdx and walk the list of rules until a rule gives us
 	// a verdict.
 	for ruleIdx < len(table.Rules) {
-		switch verdict, jumpTo := it.checkRule(hook, pkt, table, ruleIdx); verdict {
+		switch verdict, jumpTo := it.checkRule(hook, pkt, table, ruleIdx, gso, r, address); verdict {
 		case RuleAccept:
 			return chainAccept
 
@@ -255,7 +272,7 @@ func (it *IPTables) checkChain(hook Hook, pkt PacketBuffer, table Table, ruleIdx
 				ruleIdx++
 				continue
 			}
-			switch verdict := it.checkChain(hook, pkt, table, jumpTo); verdict {
+			switch verdict := it.checkChain(hook, pkt, table, jumpTo, gso, r, address); verdict {
 			case chainAccept:
 				return chainAccept
 			case chainDrop:
@@ -279,9 +296,9 @@ func (it *IPTables) checkChain(hook Hook, pkt PacketBuffer, table Table, ruleIdx
 }
 
 // Precondition: pkt is a IPv4 packet of at least length header.IPv4MinimumSize.
-// TODO(gvisor.dev/issue/170): pk.NetworkHeader will always be set as a
+// TODO(gvisor.dev/issue/170): pkt.NetworkHeader will always be set as a
 // precondition.
-func (it *IPTables) checkRule(hook Hook, pkt PacketBuffer, table Table, ruleIdx int) (RuleVerdict, int) {
+func (it *IPTables) checkRule(hook Hook, pkt *PacketBuffer, table Table, ruleIdx int, gso *GSO, r *Route, address tcpip.Address) (RuleVerdict, int) {
 	rule := table.Rules[ruleIdx]
 
 	// If pkt.NetworkHeader hasn't been set yet, it will be contained in
@@ -304,7 +321,7 @@ func (it *IPTables) checkRule(hook Hook, pkt PacketBuffer, table Table, ruleIdx
 	// Go through each rule matcher. If they all match, run
 	// the rule target.
 	for _, matcher := range rule.Matchers {
-		matches, hotdrop := matcher.Match(hook, pkt, "")
+		matches, hotdrop := matcher.Match(hook, *pkt, "")
 		if hotdrop {
 			return RuleDrop, 0
 		}
@@ -315,7 +332,7 @@ func (it *IPTables) checkRule(hook Hook, pkt PacketBuffer, table Table, ruleIdx
 	}
 
 	// All the matchers matched, so run the target.
-	return rule.Target.Action(pkt)
+	return rule.Target.Action(pkt, &it.connections, hook, gso, r, address)
 }
 
 func filterMatch(filter IPHeaderFilter, hdr header.IPv4) bool {
diff --git a/pkg/tcpip/stack/iptables_targets.go b/pkg/tcpip/stack/iptables_targets.go
index 8be61f4b1..36cc6275d 100644
--- a/pkg/tcpip/stack/iptables_targets.go
+++ b/pkg/tcpip/stack/iptables_targets.go
@@ -24,7 +24,7 @@ import (
 type AcceptTarget struct{}
 
 // Action implements Target.Action.
-func (AcceptTarget) Action(packet PacketBuffer) (RuleVerdict, int) {
+func (AcceptTarget) Action(*PacketBuffer, *ConnTrackTable, Hook, *GSO, *Route, tcpip.Address) (RuleVerdict, int) {
 	return RuleAccept, 0
 }
 
@@ -32,7 +32,7 @@ func (AcceptTarget) Action(packet PacketBuffer) (RuleVerdict, int) {
 type DropTarget struct{}
 
 // Action implements Target.Action.
-func (DropTarget) Action(packet PacketBuffer) (RuleVerdict, int) {
+func (DropTarget) Action(*PacketBuffer, *ConnTrackTable, Hook, *GSO, *Route, tcpip.Address) (RuleVerdict, int) {
 	return RuleDrop, 0
 }
 
@@ -41,7 +41,7 @@ func (DropTarget) Action(packet PacketBuffer) (RuleVerdict, int) {
 type ErrorTarget struct{}
 
 // Action implements Target.Action.
-func (ErrorTarget) Action(packet PacketBuffer) (RuleVerdict, int) {
+func (ErrorTarget) Action(*PacketBuffer, *ConnTrackTable, Hook, *GSO, *Route, tcpip.Address) (RuleVerdict, int) {
 	log.Debugf("ErrorTarget triggered.")
 	return RuleDrop, 0
 }
@@ -52,7 +52,7 @@ type UserChainTarget struct {
 }
 
 // Action implements Target.Action.
-func (UserChainTarget) Action(PacketBuffer) (RuleVerdict, int) {
+func (UserChainTarget) Action(*PacketBuffer, *ConnTrackTable, Hook, *GSO, *Route, tcpip.Address) (RuleVerdict, int) {
 	panic("UserChainTarget should never be called.")
 }
 
@@ -61,7 +61,7 @@ func (UserChainTarget) Action(PacketBuffer) (RuleVerdict, int) {
 type ReturnTarget struct{}
 
 // Action implements Target.Action.
-func (ReturnTarget) Action(PacketBuffer) (RuleVerdict, int) {
+func (ReturnTarget) Action(*PacketBuffer, *ConnTrackTable, Hook, *GSO, *Route, tcpip.Address) (RuleVerdict, int) {
 	return RuleReturn, 0
 }
 
@@ -75,16 +75,16 @@ type RedirectTarget struct {
 	// redirect.
 	RangeProtoSpecified bool
 
-	// Min address used to redirect.
+	// MinIP indicates address used to redirect.
 	MinIP tcpip.Address
 
-	// Max address used to redirect.
+	// MaxIP indicates address used to redirect.
 	MaxIP tcpip.Address
 
-	// Min port used to redirect.
+	// MinPort indicates port used to redirect.
 	MinPort uint16
 
-	// Max port used to redirect.
+	// MaxPort indicates port used to redirect.
 	MaxPort uint16
 }
 
@@ -92,61 +92,76 @@ type RedirectTarget struct {
 // TODO(gvisor.dev/issue/170): Parse headers without copying. The current
 // implementation only works for PREROUTING and calls pkt.Clone(), neither
 // of which should be the case.
-func (rt RedirectTarget) Action(pkt PacketBuffer) (RuleVerdict, int) {
-	newPkt := pkt.Clone()
+func (rt RedirectTarget) Action(pkt *PacketBuffer, ct *ConnTrackTable, hook Hook, gso *GSO, r *Route, address tcpip.Address) (RuleVerdict, int) {
+	// Packet is already manipulated.
+	if pkt.NatDone {
+		return RuleAccept, 0
+	}
 
 	// Set network header.
-	headerView, ok := newPkt.Data.PullUp(header.IPv4MinimumSize)
-	if !ok {
-		return RuleDrop, 0
+	if hook == Prerouting {
+		parseHeaders(pkt)
 	}
-	netHeader := header.IPv4(headerView)
-	newPkt.NetworkHeader = headerView
 
-	hlen := int(netHeader.HeaderLength())
-	tlen := int(netHeader.TotalLength())
-	newPkt.Data.TrimFront(hlen)
-	newPkt.Data.CapLength(tlen - hlen)
+	// Drop the packet if network and transport header are not set.
+	if pkt.NetworkHeader == nil || pkt.TransportHeader == nil {
+		return RuleDrop, 0
+	}
 
-	// TODO(gvisor.dev/issue/170): Change destination address to
-	// loopback or interface address on which the packet was
-	// received.
+	// Change the address to localhost (127.0.0.1) in Output and
+	// to primary address of the incoming interface in Prerouting.
+	switch hook {
+	case Output:
+		rt.MinIP = tcpip.Address([]byte{127, 0, 0, 1})
+		rt.MaxIP = tcpip.Address([]byte{127, 0, 0, 1})
+	case Prerouting:
+		rt.MinIP = address
+		rt.MaxIP = address
+	default:
+		panic("redirect target is supported only on output and prerouting hooks")
+	}
 
 	// TODO(gvisor.dev/issue/170): Check Flags in RedirectTarget if
 	// we need to change dest address (for OUTPUT chain) or ports.
+	netHeader := header.IPv4(pkt.NetworkHeader)
 	switch protocol := netHeader.TransportProtocol(); protocol {
 	case header.UDPProtocolNumber:
-		var udpHeader header.UDP
-		if newPkt.TransportHeader != nil {
-			udpHeader = header.UDP(newPkt.TransportHeader)
-		} else {
-			if pkt.Data.Size() < header.UDPMinimumSize {
-				return RuleDrop, 0
-			}
-			hdr, ok := newPkt.Data.PullUp(header.UDPMinimumSize)
-			if !ok {
-				return RuleDrop, 0
+		udpHeader := header.UDP(pkt.TransportHeader)
+		udpHeader.SetDestinationPort(rt.MinPort)
+
+		// Calculate UDP checksum and set it.
+		if hook == Output {
+			udpHeader.SetChecksum(0)
+			hdr := &pkt.Header
+			length := uint16(pkt.Data.Size()+hdr.UsedLength()) - uint16(netHeader.HeaderLength())
+
+			// Only calculate the checksum if offloading isn't supported.
+			if r.Capabilities()&CapabilityTXChecksumOffload == 0 {
+				xsum := r.PseudoHeaderChecksum(protocol, length)
+				for _, v := range pkt.Data.Views() {
+					xsum = header.Checksum(v, xsum)
+				}
+				udpHeader.SetChecksum(0)
+				udpHeader.SetChecksum(^udpHeader.CalculateChecksum(xsum))
 			}
-			udpHeader = header.UDP(hdr)
 		}
-		udpHeader.SetDestinationPort(rt.MinPort)
+		// Change destination address.
+		netHeader.SetDestinationAddress(rt.MinIP)
+		netHeader.SetChecksum(0)
+		netHeader.SetChecksum(^netHeader.CalculateChecksum())
+		pkt.NatDone = true
 	case header.TCPProtocolNumber:
-		var tcpHeader header.TCP
-		if newPkt.TransportHeader != nil {
-			tcpHeader = header.TCP(newPkt.TransportHeader)
-		} else {
-			if pkt.Data.Size() < header.TCPMinimumSize {
-				return RuleDrop, 0
-			}
-			hdr, ok := newPkt.Data.PullUp(header.TCPMinimumSize)
-			if !ok {
-				return RuleDrop, 0
-			}
-			tcpHeader = header.TCP(hdr)
+		if ct == nil {
+			return RuleAccept, 0
+		}
+
+		// Set up conection for matching NAT rule.
+		// Only the first packet of the connection comes here.
+		// Other packets will be manipulated in connection tracking.
+		if conn, _ := ct.connTrackForPacket(pkt, hook, true); conn != nil {
+			ct.SetNatInfo(pkt, rt, hook)
+			ct.HandlePacket(pkt, hook, gso, r)
 		}
-		// TODO(gvisor.dev/issue/170): Need to recompute checksum
-		// and implement nat connection tracking to support TCP.
-		tcpHeader.SetDestinationPort(rt.MinPort)
 	default:
 		return RuleDrop, 0
 	}
diff --git a/pkg/tcpip/stack/iptables_types.go b/pkg/tcpip/stack/iptables_types.go
index 2ffb55f2a..1bb0ba1bd 100644
--- a/pkg/tcpip/stack/iptables_types.go
+++ b/pkg/tcpip/stack/iptables_types.go
@@ -82,6 +82,8 @@ type IPTables struct {
 	// list is the order in which each table should be visited for that
 	// hook.
 	Priorities map[Hook][]string
+
+	connections ConnTrackTable
 }
 
 // A Table defines a set of chains and hooks into the network stack. It is
@@ -176,5 +178,5 @@ type Target interface {
 	// Action takes an action on the packet and returns a verdict on how
 	// traversal should (or should not) continue. If the return value is
 	// Jump, it also returns the index of the rule to jump to.
-	Action(packet PacketBuffer) (RuleVerdict, int)
+	Action(packet *PacketBuffer, connections *ConnTrackTable, hook Hook, gso *GSO, r *Route, address tcpip.Address) (RuleVerdict, int)
 }
diff --git a/pkg/tcpip/stack/nic.go b/pkg/tcpip/stack/nic.go
index 7b54919bb..8f4c1fe42 100644
--- a/pkg/tcpip/stack/nic.go
+++ b/pkg/tcpip/stack/nic.go
@@ -1230,8 +1230,10 @@ func (n *NIC) DeliverNetworkPacket(linkEP LinkEndpoint, remote, local tcpip.Link
 
 	// TODO(gvisor.dev/issue/170): Not supporting iptables for IPv6 yet.
 	if protocol == header.IPv4ProtocolNumber {
+		// iptables filtering.
 		ipt := n.stack.IPTables()
-		if ok := ipt.Check(Prerouting, pkt); !ok {
+		address := n.primaryAddress(protocol)
+		if ok := ipt.Check(Prerouting, &pkt, nil, nil, address.Address); !ok {
 			// iptables is telling us to drop the packet.
 			return
 		}
diff --git a/pkg/tcpip/stack/packet_buffer.go b/pkg/tcpip/stack/packet_buffer.go
index 9ff80ab24..926df4d7b 100644
--- a/pkg/tcpip/stack/packet_buffer.go
+++ b/pkg/tcpip/stack/packet_buffer.go
@@ -72,6 +72,10 @@ type PacketBuffer struct {
 	EgressRoute           *Route
 	GSOOptions            *GSO
 	NetworkProtocolNumber tcpip.NetworkProtocolNumber
+
+	// NatDone indicates if the packet has been manipulated as per NAT
+	// iptables rule.
+	NatDone bool
 }
 
 // Clone makes a copy of pk. It clones the Data field, which creates a new
diff --git a/pkg/tcpip/stack/route.go b/pkg/tcpip/stack/route.go
index 53148dc03..150297ab9 100644
--- a/pkg/tcpip/stack/route.go
+++ b/pkg/tcpip/stack/route.go
@@ -261,3 +261,16 @@ func (r *Route) MakeLoopedRoute() Route {
 func (r *Route) Stack() *Stack {
 	return r.ref.stack()
 }
+
+// ReverseRoute returns new route with given source and destination address.
+func (r *Route) ReverseRoute(src tcpip.Address, dst tcpip.Address) Route {
+	return Route{
+		NetProto:          r.NetProto,
+		LocalAddress:      dst,
+		LocalLinkAddress:  r.RemoteLinkAddress,
+		RemoteAddress:     src,
+		RemoteLinkAddress: r.LocalLinkAddress,
+		ref:               r.ref,
+		Loop:              r.Loop,
+	}
+}
diff --git a/pkg/tcpip/stack/stack.go b/pkg/tcpip/stack/stack.go
index 4a2dc3dc6..e33fae4eb 100644
--- a/pkg/tcpip/stack/stack.go
+++ b/pkg/tcpip/stack/stack.go
@@ -1885,3 +1885,22 @@ func generateRandInt64() int64 {
 	}
 	return v
 }
+
+// FindNetworkEndpoint returns the network endpoint for the given address.
+func (s *Stack) FindNetworkEndpoint(netProto tcpip.NetworkProtocolNumber, address tcpip.Address) (NetworkEndpoint, *tcpip.Error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	for _, nic := range s.nics {
+		id := NetworkEndpointID{address}
+
+		if ref, ok := nic.mu.endpoints[id]; ok {
+			nic.mu.RLock()
+			defer nic.mu.RUnlock()
+
+			// An endpoint with this id exists, check if it can be used and return it.
+			return ref.ep, nil
+		}
+	}
+	return nil, tcpip.ErrBadAddress
+}
diff --git a/pkg/tcpip/transport/tcp/BUILD b/pkg/tcpip/transport/tcp/BUILD
index f2aa69069..f38eb6833 100644
--- a/pkg/tcpip/transport/tcp/BUILD
+++ b/pkg/tcpip/transport/tcp/BUILD
@@ -115,7 +115,7 @@ go_test(
     size = "small",
     srcs = ["rcv_test.go"],
     deps = [
-        ":tcp",
+        "//pkg/tcpip/header",
         "//pkg/tcpip/seqnum",
     ],
 )
diff --git a/pkg/tcpip/transport/tcp/rcv.go b/pkg/tcpip/transport/tcp/rcv.go
index a4b73b588..6fe97fefd 100644
--- a/pkg/tcpip/transport/tcp/rcv.go
+++ b/pkg/tcpip/transport/tcp/rcv.go
@@ -70,24 +70,7 @@ func newReceiver(ep *endpoint, irs seqnum.Value, rcvWnd seqnum.Size, rcvWndScale
 // acceptable checks if the segment sequence number range is acceptable
 // according to the table on page 26 of RFC 793.
 func (r *receiver) acceptable(segSeq seqnum.Value, segLen seqnum.Size) bool {
-	return Acceptable(segSeq, segLen, r.rcvNxt, r.rcvAcc)
-}
-
-// Acceptable checks if a segment that starts at segSeq and has length segLen is
-// "acceptable" for arriving in a receive window that starts at rcvNxt and ends
-// before rcvAcc, according to the table on page 26 and 69 of RFC 793.
-func Acceptable(segSeq seqnum.Value, segLen seqnum.Size, rcvNxt, rcvAcc seqnum.Value) bool {
-	if rcvNxt == rcvAcc {
-		return segLen == 0 && segSeq == rcvNxt
-	}
-	if segLen == 0 {
-		// rcvWnd is incremented by 1 because that is Linux's behavior despite the
-		// RFC.
-		return segSeq.InRange(rcvNxt, rcvAcc.Add(1))
-	}
-	// Page 70 of RFC 793 allows packets that can be made "acceptable" by trimming
-	// the payload, so we'll accept any payload that overlaps the receieve window.
-	return rcvNxt.LessThan(segSeq.Add(segLen)) && segSeq.LessThan(rcvAcc)
+	return header.Acceptable(segSeq, segLen, r.rcvNxt, r.rcvAcc)
 }
 
 // getSendParams returns the parameters needed by the sender when building
diff --git a/pkg/tcpip/transport/tcp/rcv_test.go b/pkg/tcpip/transport/tcp/rcv_test.go
index dc02729ce..c9eeff935 100644
--- a/pkg/tcpip/transport/tcp/rcv_test.go
+++ b/pkg/tcpip/transport/tcp/rcv_test.go
@@ -17,8 +17,8 @@ package rcv_test
 import (
 	"testing"
 
+	"gvisor.dev/gvisor/pkg/tcpip/header"
 	"gvisor.dev/gvisor/pkg/tcpip/seqnum"
-	"gvisor.dev/gvisor/pkg/tcpip/transport/tcp"
 )
 
 func TestAcceptable(t *testing.T) {
@@ -67,8 +67,8 @@ func TestAcceptable(t *testing.T) {
 		{105, 2, 108, 108, false},
 		{105, 2, 109, 109, false},
 	} {
-		if got := tcp.Acceptable(tt.segSeq, tt.segLen, tt.rcvNxt, tt.rcvAcc); got != tt.want {
-			t.Errorf("tcp.Acceptable(%d, %d, %d, %d) = %t, want %t", tt.segSeq, tt.segLen, tt.rcvNxt, tt.rcvAcc, got, tt.want)
+		if got := header.Acceptable(tt.segSeq, tt.segLen, tt.rcvNxt, tt.rcvAcc); got != tt.want {
+			t.Errorf("header.Acceptable(%d, %d, %d, %d) = %t, want %t", tt.segSeq, tt.segLen, tt.rcvNxt, tt.rcvAcc, got, tt.want)
 		}
 	}
 }
diff --git a/pkg/tcpip/transport/tcpconntrack/BUILD b/pkg/tcpip/transport/tcpconntrack/BUILD
index 2025ff757..3ad6994a7 100644
--- a/pkg/tcpip/transport/tcpconntrack/BUILD
+++ b/pkg/tcpip/transport/tcpconntrack/BUILD
@@ -9,7 +9,6 @@ go_library(
     deps = [
         "//pkg/tcpip/header",
         "//pkg/tcpip/seqnum",
-        "//pkg/tcpip/transport/tcp",
     ],
 )
 
diff --git a/pkg/tcpip/transport/tcpconntrack/tcp_conntrack.go b/pkg/tcpip/transport/tcpconntrack/tcp_conntrack.go
index 30d05200f..12bc1b5b5 100644
--- a/pkg/tcpip/transport/tcpconntrack/tcp_conntrack.go
+++ b/pkg/tcpip/transport/tcpconntrack/tcp_conntrack.go
@@ -20,7 +20,6 @@ package tcpconntrack
 import (
 	"gvisor.dev/gvisor/pkg/tcpip/header"
 	"gvisor.dev/gvisor/pkg/tcpip/seqnum"
-	"gvisor.dev/gvisor/pkg/tcpip/transport/tcp"
 )
 
 // Result is returned when the state of a TCB is updated in response to an
@@ -312,7 +311,7 @@ type stream struct {
 // the window is zero, if it's a packet with no payload and sequence number
 // equal to una.
 func (s *stream) acceptable(segSeq seqnum.Value, segLen seqnum.Size) bool {
-	return tcp.Acceptable(segSeq, segLen, s.una, s.end)
+	return header.Acceptable(segSeq, segLen, s.una, s.end)
 }
 
 // closed determines if the stream has already been closed. This happens when
@@ -338,3 +337,16 @@ func logicalLen(tcp header.TCP) seqnum.Size {
 	}
 	return l
 }
+
+// IsEmpty returns true if tcb is not initialized.
+func (t *TCB) IsEmpty() bool {
+	if t.inbound != (stream{}) || t.outbound != (stream{}) {
+		return false
+	}
+
+	if t.firstFin != nil || t.state != ResultDrop {
+		return false
+	}
+
+	return true
+}