Merge release-20200928.0-74-g8566decab (automated)

6 files changed, 153 insertions, 159 deletions

diff --git a/pkg/tcpip/stack/pending_packets.go b/pkg/tcpip/stack/forwarder.go
index f838eda8d..3eff141e6 100644
--- a/pkg/tcpip/stack/pending_packets.go
+++ b/pkg/tcpip/stack/forwarder.go
@@ -29,60 +29,60 @@ const (
 )
 
 type pendingPacket struct {
+	nic   *NIC
 	route *Route
 	proto tcpip.NetworkProtocolNumber
 	pkt   *PacketBuffer
 }
 
-// packetsPendingLinkResolution is a queue of packets pending link resolution.
-//
-// Once link resolution completes successfully, the packets will be written.
-type packetsPendingLinkResolution struct {
+type forwardQueue struct {
 	sync.Mutex
 
 	// The packets to send once the resolver completes.
-	packets map[<-chan struct{}][]pendingPacket
+	packets map[<-chan struct{}][]*pendingPacket
 
 	// FIFO of channels used to cancel the oldest goroutine waiting for
 	// link-address resolution.
 	cancelChans []chan struct{}
 }
 
-func (f *packetsPendingLinkResolution) init() {
-	f.Lock()
-	defer f.Unlock()
-	f.packets = make(map[<-chan struct{}][]pendingPacket)
+func newForwardQueue() *forwardQueue {
+	return &forwardQueue{packets: make(map[<-chan struct{}][]*pendingPacket)}
 }
 
-func (f *packetsPendingLinkResolution) enqueue(ch <-chan struct{}, r *Route, proto tcpip.NetworkProtocolNumber, pkt *PacketBuffer) {
-	f.Lock()
-	defer f.Unlock()
+func (f *forwardQueue) enqueue(ch <-chan struct{}, n *NIC, r *Route, protocol tcpip.NetworkProtocolNumber, pkt *PacketBuffer) {
+	shouldWait := false
 
+	f.Lock()
 	packets, ok := f.packets[ch]
-	if len(packets) == maxPendingPacketsPerResolution {
+	if !ok {
+		shouldWait = true
+	}
+	for len(packets) == maxPendingPacketsPerResolution {
 		p := packets[0]
-		packets[0] = pendingPacket{}
 		packets = packets[1:]
-		p.route.Stats().IP.OutgoingPacketErrors.Increment()
+		p.nic.stack.stats.IP.OutgoingPacketErrors.Increment()
 		p.route.Release()
 	}
-
 	if l := len(packets); l >= maxPendingPacketsPerResolution {
 		panic(fmt.Sprintf("max pending packets for resolution reached; got %d packets, max = %d", l, maxPendingPacketsPerResolution))
 	}
-
-	f.packets[ch] = append(packets, pendingPacket{
+	f.packets[ch] = append(packets, &pendingPacket{
+		nic:   n,
 		route: r,
-		proto: proto,
+		proto: protocol,
 		pkt:   pkt,
 	})
+	f.Unlock()
 
-	if ok {
+	if !shouldWait {
 		return
 	}
 
 	// Wait for the link-address resolution to complete.
-	cancel := f.newCancelChannelLocked()
+	// Start a goroutine with a forwarding-cancel channel so that we can
+	// limit the maximum number of goroutines running concurrently.
+	cancel := f.newCancelChannel()
 	go func() {
 		cancelled := false
 		select {
@@ -92,21 +92,17 @@ func (f *packetsPendingLinkResolution) enqueue(ch <-chan struct{}, r *Route, pro
 		}
 
 		f.Lock()
-		packets, ok := f.packets[ch]
+		packets := f.packets[ch]
 		delete(f.packets, ch)
 		f.Unlock()
 
-		if !ok {
-			panic(fmt.Sprintf("link-resolution goroutine woke up but no entry exists in the queue of packets"))
-		}
-
 		for _, p := range packets {
 			if cancelled {
-				p.route.Stats().IP.OutgoingPacketErrors.Increment()
+				p.nic.stack.stats.IP.OutgoingPacketErrors.Increment()
 			} else if _, err := p.route.Resolve(nil); err != nil {
-				p.route.Stats().IP.OutgoingPacketErrors.Increment()
+				p.nic.stack.stats.IP.OutgoingPacketErrors.Increment()
 			} else {
-				p.route.nic.writePacket(p.route, nil /* gso */, p.proto, p.pkt)
+				p.nic.forwardPacket(p.route, p.proto, p.pkt)
 			}
 			p.route.Release()
 		}
@@ -116,10 +112,12 @@ func (f *packetsPendingLinkResolution) enqueue(ch <-chan struct{}, r *Route, pro
 // newCancelChannel creates a channel that can cancel a pending forwarding
 // activity. The oldest channel is closed if the number of open channels would
 // exceed maxPendingResolutions.
-func (f *packetsPendingLinkResolution) newCancelChannelLocked() chan struct{} {
+func (f *forwardQueue) newCancelChannel() chan struct{} {
+	f.Lock()
+	defer f.Unlock()
+
 	if len(f.cancelChans) == maxPendingResolutions {
 		ch := f.cancelChans[0]
-		f.cancelChans[0] = nil
 		f.cancelChans = f.cancelChans[1:]
 		close(ch)
 	}
diff --git a/pkg/tcpip/stack/neighbor_entry.go b/pkg/tcpip/stack/neighbor_entry.go
index 4d69a4de1..9a72bec79 100644
--- a/pkg/tcpip/stack/neighbor_entry.go
+++ b/pkg/tcpip/stack/neighbor_entry.go
@@ -236,7 +236,7 @@ func (e *neighborEntry) setStateLocked(next NeighborState) {
 				return
 			}
 
-			if err := e.linkRes.LinkAddressRequest(e.neigh.Addr, e.neigh.LocalAddr, "", e.nic.LinkEndpoint); err != nil {
+			if err := e.linkRes.LinkAddressRequest(e.neigh.Addr, e.neigh.LocalAddr, "", e.nic.linkEP); err != nil {
 				// There is no need to log the error here; the NUD implementation may
 				// assume a working link. A valid link should be the responsibility of
 				// the NIC/stack.LinkEndpoint.
@@ -277,7 +277,7 @@ func (e *neighborEntry) setStateLocked(next NeighborState) {
 				return
 			}
 
-			if err := e.linkRes.LinkAddressRequest(e.neigh.Addr, e.neigh.LocalAddr, e.neigh.LinkAddr, e.nic.LinkEndpoint); err != nil {
+			if err := e.linkRes.LinkAddressRequest(e.neigh.Addr, e.neigh.LocalAddr, e.neigh.LinkAddr, e.nic.linkEP); err != nil {
 				e.dispatchRemoveEventLocked()
 				e.setStateLocked(Failed)
 				return
diff --git a/pkg/tcpip/stack/nic.go b/pkg/tcpip/stack/nic.go
index 8828cc5fe..6cf54cc89 100644
--- a/pkg/tcpip/stack/nic.go
+++ b/pkg/tcpip/stack/nic.go
@@ -32,11 +32,10 @@ var _ NetworkInterface = (*NIC)(nil)
 // NIC represents a "network interface card" to which the networking stack is
 // attached.
 type NIC struct {
-	LinkEndpoint
-
 	stack   *Stack
 	id      tcpip.NICID
 	name    string
+	linkEP  LinkEndpoint
 	context NICContext
 
 	stats NICStats
@@ -92,11 +91,10 @@ func newNIC(stack *Stack, id tcpip.NICID, name string, ep LinkEndpoint, ctx NICC
 	// of IPv6 is supported on this endpoint's LinkEndpoint.
 
 	nic := &NIC{
-		LinkEndpoint: ep,
-
 		stack:            stack,
 		id:               id,
 		name:             name,
+		linkEP:           ep,
 		context:          ctx,
 		stats:            makeNICStats(),
 		networkEndpoints: make(map[tcpip.NetworkProtocolNumber]NetworkEndpoint),
@@ -132,7 +130,7 @@ func newNIC(stack *Stack, id tcpip.NICID, name string, ep LinkEndpoint, ctx NICC
 		nic.networkEndpoints[netNum] = netProto.NewEndpoint(nic, stack, nud, nic)
 	}
 
-	nic.LinkEndpoint.Attach(nic)
+	nic.linkEP.Attach(nic)
 
 	return nic
 }
@@ -222,7 +220,7 @@ func (n *NIC) remove() *tcpip.Error {
 	}
 
 	// Detach from link endpoint, so no packet comes in.
-	n.LinkEndpoint.Attach(nil)
+	n.linkEP.Attach(nil)
 	return nil
 }
 
@@ -242,64 +240,7 @@ func (n *NIC) isPromiscuousMode() bool {
 
 // IsLoopback implements NetworkInterface.
 func (n *NIC) IsLoopback() bool {
-	return n.LinkEndpoint.Capabilities()&CapabilityLoopback != 0
-}
-
-// WritePacket implements NetworkLinkEndpoint.
-func (n *NIC) WritePacket(r *Route, gso *GSO, protocol tcpip.NetworkProtocolNumber, pkt *PacketBuffer) *tcpip.Error {
-	// As per relevant RFCs, we should queue packets while we wait for link
-	// resolution to complete.
-	//
-	// RFC 1122 section 2.3.2.2 (for IPv4):
-	//   The link layer SHOULD save (rather than discard) at least
-	//   one (the latest) packet of each set of packets destined to
-	//   the same unresolved IP address, and transmit the saved
-	//   packet when the address has been resolved.
-	//
-	// RFC 4861 section 5.2 (for IPv6):
-	//   Once the IP address of the next-hop node is known, the sender
-	//   examines the Neighbor Cache for link-layer information about that
-	//   neighbor.  If no entry exists, the sender creates one, sets its state
-	//   to INCOMPLETE, initiates Address Resolution, and then queues the data
-	//   packet pending completion of address resolution.
-	if ch, err := r.Resolve(nil); err != nil {
-		if err == tcpip.ErrWouldBlock {
-			r := r.Clone()
-			n.stack.linkResQueue.enqueue(ch, &r, protocol, pkt)
-			return nil
-		}
-		return err
-	}
-
-	return n.writePacket(r, gso, protocol, pkt)
-}
-
-func (n *NIC) writePacket(r *Route, gso *GSO, protocol tcpip.NetworkProtocolNumber, pkt *PacketBuffer) *tcpip.Error {
-	// WritePacket takes ownership of pkt, calculate numBytes first.
-	numBytes := pkt.Size()
-
-	if err := n.LinkEndpoint.WritePacket(r, gso, protocol, pkt); err != nil {
-		return err
-	}
-
-	n.stats.Tx.Packets.Increment()
-	n.stats.Tx.Bytes.IncrementBy(uint64(numBytes))
-	return nil
-}
-
-// WritePackets implements NetworkLinkEndpoint.
-func (n *NIC) WritePackets(r *Route, gso *GSO, pkts PacketBufferList, protocol tcpip.NetworkProtocolNumber) (int, *tcpip.Error) {
-	// TODO(gvisor.dev/issue/4458): Queue packets whie link address resolution
-	// is being peformed like WritePacket.
-	writtenPackets, err := n.LinkEndpoint.WritePackets(r, gso, pkts, protocol)
-	n.stats.Tx.Packets.IncrementBy(uint64(writtenPackets))
-	writtenBytes := 0
-	for i, pb := 0, pkts.Front(); i < writtenPackets && pb != nil; i, pb = i+1, pb.Next() {
-		writtenBytes += pb.Size()
-	}
-
-	n.stats.Tx.Bytes.IncrementBy(uint64(writtenBytes))
-	return writtenPackets, err
+	return n.linkEP.Capabilities()&CapabilityLoopback != 0
 }
 
 // setSpoofing enables or disables address spoofing.
@@ -584,7 +525,7 @@ func (n *NIC) DeliverNetworkPacket(remote, local tcpip.LinkAddress, protocol tcp
 	// If no local link layer address is provided, assume it was sent
 	// directly to this NIC.
 	if local == "" {
-		local = n.LinkEndpoint.LinkAddress()
+		local = n.linkEP.LinkAddress()
 	}
 
 	// Are any packet type sockets listening for this network protocol?
@@ -664,7 +605,7 @@ func (n *NIC) DeliverNetworkPacket(remote, local tcpip.LinkAddress, protocol tcp
 		n := r.nic
 		if addressEndpoint := n.getAddressOrCreateTempInner(protocol, dst, false, NeverPrimaryEndpoint); addressEndpoint != nil {
 			if n.isValidForOutgoing(addressEndpoint) {
-				r.LocalLinkAddress = n.LinkEndpoint.LinkAddress()
+				r.LocalLinkAddress = n.linkEP.LinkAddress()
 				r.RemoteLinkAddress = remote
 				r.RemoteAddress = src
 				// TODO(b/123449044): Update the source NIC as well.
@@ -679,21 +620,21 @@ func (n *NIC) DeliverNetworkPacket(remote, local tcpip.LinkAddress, protocol tcp
 
 		// n doesn't have a destination endpoint.
 		// Send the packet out of n.
+		// TODO(b/128629022): move this logic to route.WritePacket.
 		// TODO(gvisor.dev/issue/1085): According to the RFC, we must decrease the TTL field for ipv4/ipv6.
-
-		// pkt may have set its header and may not have enough headroom for
-		// link-layer header for the other link to prepend. Here we create a new
-		// packet to forward.
-		fwdPkt := NewPacketBuffer(PacketBufferOptions{
-			ReserveHeaderBytes: int(n.LinkEndpoint.MaxHeaderLength()),
-			Data:               buffer.NewVectorisedView(pkt.Size(), pkt.Views()),
-		})
-
-		// TODO(b/143425874) Decrease the TTL field in forwarded packets.
-		if err := n.WritePacket(&r, nil, protocol, fwdPkt); err != nil {
+		if ch, err := r.Resolve(nil); err != nil {
+			if err == tcpip.ErrWouldBlock {
+				n.stack.forwarder.enqueue(ch, n, &r, protocol, pkt)
+				// forwarder will release route.
+				return
+			}
 			n.stack.stats.IP.InvalidDestinationAddressesReceived.Increment()
+			r.Release()
+			return
 		}
 
+		// The link-address resolution finished immediately.
+		n.forwardPacket(&r, protocol, pkt)
 		r.Release()
 		return
 	}
@@ -717,11 +658,34 @@ func (n *NIC) DeliverOutboundPacket(remote, local tcpip.LinkAddress, protocol tc
 		p.PktType = tcpip.PacketOutgoing
 		// Add the link layer header as outgoing packets are intercepted
 		// before the link layer header is created.
-		n.LinkEndpoint.AddHeader(local, remote, protocol, p)
+		n.linkEP.AddHeader(local, remote, protocol, p)
 		ep.HandlePacket(n.id, local, protocol, p)
 	}
 }
 
+func (n *NIC) forwardPacket(r *Route, protocol tcpip.NetworkProtocolNumber, pkt *PacketBuffer) {
+	// TODO(b/143425874) Decrease the TTL field in forwarded packets.
+
+	// pkt may have set its header and may not have enough headroom for link-layer
+	// header for the other link to prepend. Here we create a new packet to
+	// forward.
+	fwdPkt := NewPacketBuffer(PacketBufferOptions{
+		ReserveHeaderBytes: int(n.linkEP.MaxHeaderLength()),
+		Data:               buffer.NewVectorisedView(pkt.Size(), pkt.Views()),
+	})
+
+	// WritePacket takes ownership of fwdPkt, calculate numBytes first.
+	numBytes := fwdPkt.Size()
+
+	if err := n.linkEP.WritePacket(r, nil /* gso */, protocol, fwdPkt); err != nil {
+		r.Stats().IP.OutgoingPacketErrors.Increment()
+		return
+	}
+
+	n.stats.Tx.Packets.Increment()
+	n.stats.Tx.Bytes.IncrementBy(uint64(numBytes))
+}
+
 // DeliverTransportPacket delivers the packets to the appropriate transport
 // protocol endpoint.
 func (n *NIC) DeliverTransportPacket(r *Route, protocol tcpip.TransportProtocolNumber, pkt *PacketBuffer) TransportPacketDisposition {
@@ -832,6 +796,11 @@ func (n *NIC) Name() string {
 	return n.name
 }
 
+// LinkEndpoint implements NetworkInterface.
+func (n *NIC) LinkEndpoint() LinkEndpoint {
+	return n.linkEP
+}
+
 // nudConfigs gets the NUD configurations for n.
 func (n *NIC) nudConfigs() (NUDConfigurations, *tcpip.Error) {
 	if n.neigh == nil {
diff --git a/pkg/tcpip/stack/registration.go b/pkg/tcpip/stack/registration.go
index defb9129b..be9bd8042 100644
--- a/pkg/tcpip/stack/registration.go
+++ b/pkg/tcpip/stack/registration.go
@@ -475,8 +475,6 @@ type NDPEndpoint interface {
 
 // NetworkInterface is a network interface.
 type NetworkInterface interface {
-	NetworkLinkEndpoint
-
 	// ID returns the interface's ID.
 	ID() tcpip.NICID
 
@@ -490,6 +488,9 @@ type NetworkInterface interface {
 
 	// Enabled returns true if the interface is enabled.
 	Enabled() bool
+
+	// LinkEndpoint returns the link endpoint backing the interface.
+	LinkEndpoint() LinkEndpoint
 }
 
 // NetworkEndpoint is the interface that needs to be implemented by endpoints
@@ -662,15 +663,22 @@ const (
 	CapabilitySoftwareGSO
 )
 
-// NetworkLinkEndpoint is a data-link layer that supports sending network
-// layer packets.
-type NetworkLinkEndpoint interface {
+// LinkEndpoint is the interface implemented by data link layer protocols (e.g.,
+// ethernet, loopback, raw) and used by network layer protocols to send packets
+// out through the implementer's data link endpoint. When a link header exists,
+// it sets each PacketBuffer's LinkHeader field before passing it up the
+// stack.
+type LinkEndpoint interface {
 	// MTU is the maximum transmission unit for this endpoint. This is
 	// usually dictated by the backing physical network; when such a
 	// physical network doesn't exist, the limit is generally 64k, which
 	// includes the maximum size of an IP packet.
 	MTU() uint32
 
+	// Capabilities returns the set of capabilities supported by the
+	// endpoint.
+	Capabilities() LinkEndpointCapabilities
+
 	// MaxHeaderLength returns the maximum size the data link (and
 	// lower level layers combined) headers can have. Higher levels use this
 	// information to reserve space in the front of the packets they're
@@ -678,7 +686,7 @@ type NetworkLinkEndpoint interface {
 	MaxHeaderLength() uint16
 
 	// LinkAddress returns the link address (typically a MAC) of the
-	// endpoint.
+	// link endpoint.
 	LinkAddress() tcpip.LinkAddress
 
 	// WritePacket writes a packet with the given protocol through the
@@ -698,19 +706,6 @@ type NetworkLinkEndpoint interface {
 	// offload is enabled. If it will be used for something else, it may
 	// require to change syscall filters.
 	WritePackets(r *Route, gso *GSO, pkts PacketBufferList, protocol tcpip.NetworkProtocolNumber) (int, *tcpip.Error)
-}
-
-// LinkEndpoint is the interface implemented by data link layer protocols (e.g.,
-// ethernet, loopback, raw) and used by network layer protocols to send packets
-// out through the implementer's data link endpoint. When a link header exists,
-// it sets each PacketBuffer's LinkHeader field before passing it up the
-// stack.
-type LinkEndpoint interface {
-	NetworkLinkEndpoint
-
-	// Capabilities returns the set of capabilities supported by the
-	// endpoint.
-	Capabilities() LinkEndpointCapabilities
 
 	// WriteRawPacket writes a packet directly to the link. The packet
 	// should already have an ethernet header. It takes ownership of vv.
diff --git a/pkg/tcpip/stack/route.go b/pkg/tcpip/stack/route.go
index 710a02ac3..cc39c9a6a 100644
--- a/pkg/tcpip/stack/route.go
+++ b/pkg/tcpip/stack/route.go
@@ -72,20 +72,21 @@ func makeRoute(netProto tcpip.NetworkProtocolNumber, localAddr, remoteAddr tcpip
 		loop |= PacketLoop
 	}
 
+	linkEP := nic.LinkEndpoint()
 	r := Route{
 		NetProto:         netProto,
 		LocalAddress:     localAddr,
-		LocalLinkAddress: nic.LinkEndpoint.LinkAddress(),
+		LocalLinkAddress: linkEP.LinkAddress(),
 		RemoteAddress:    remoteAddr,
 		addressEndpoint:  addressEndpoint,
 		nic:              nic,
 		Loop:             loop,
 	}
 
-	if r.nic.LinkEndpoint.Capabilities()&CapabilityResolutionRequired != 0 {
-		if linkRes, ok := r.nic.stack.linkAddrResolvers[r.NetProto]; ok {
+	if nic := r.nic; linkEP.Capabilities()&CapabilityResolutionRequired != 0 {
+		if linkRes, ok := nic.stack.linkAddrResolvers[r.NetProto]; ok {
 			r.linkRes = linkRes
-			r.linkCache = r.nic.stack
+			r.linkCache = nic.stack
 		}
 	}
 
@@ -115,7 +116,7 @@ func (r *Route) PseudoHeaderChecksum(protocol tcpip.TransportProtocolNumber, tot
 
 // Capabilities returns the link-layer capabilities of the route.
 func (r *Route) Capabilities() LinkEndpointCapabilities {
-	return r.nic.LinkEndpoint.Capabilities()
+	return r.nic.LinkEndpoint().Capabilities()
 }
 
 // GSOMaxSize returns the maximum GSO packet size.
@@ -126,6 +127,12 @@ func (r *Route) GSOMaxSize() uint32 {
 	return 0
 }
 
+// ResolveWith immediately resolves a route with the specified remote link
+// address.
+func (r *Route) ResolveWith(addr tcpip.LinkAddress) {
+	r.RemoteLinkAddress = addr
+}
+
 // Resolve attempts to resolve the link address if necessary. Returns ErrWouldBlock in
 // case address resolution requires blocking, e.g. wait for ARP reply. Waker is
 // notified when address resolution is complete (success or not).
@@ -201,7 +208,16 @@ func (r *Route) WritePacket(gso *GSO, params NetworkHeaderParams, pkt *PacketBuf
 		return tcpip.ErrInvalidEndpointState
 	}
 
-	return r.nic.getNetworkEndpoint(r.NetProto).WritePacket(r, gso, params, pkt)
+	// WritePacket takes ownership of pkt, calculate numBytes first.
+	numBytes := pkt.Size()
+
+	if err := r.nic.getNetworkEndpoint(r.NetProto).WritePacket(r, gso, params, pkt); err != nil {
+		return err
+	}
+
+	r.nic.stats.Tx.Packets.Increment()
+	r.nic.stats.Tx.Bytes.IncrementBy(uint64(numBytes))
+	return nil
 }
 
 // WritePackets writes a list of n packets through the given route and returns
@@ -211,7 +227,15 @@ func (r *Route) WritePackets(gso *GSO, pkts PacketBufferList, params NetworkHead
 		return 0, tcpip.ErrInvalidEndpointState
 	}
 
-	return r.nic.getNetworkEndpoint(r.NetProto).WritePackets(r, gso, pkts, params)
+	n, err := r.nic.getNetworkEndpoint(r.NetProto).WritePackets(r, gso, pkts, params)
+	r.nic.stats.Tx.Packets.IncrementBy(uint64(n))
+	writtenBytes := 0
+	for i, pb := 0, pkts.Front(); i < n && pb != nil; i, pb = i+1, pb.Next() {
+		writtenBytes += pb.Size()
+	}
+
+	r.nic.stats.Tx.Bytes.IncrementBy(uint64(writtenBytes))
+	return n, err
 }
 
 // WriteHeaderIncludedPacket writes a packet already containing a network
@@ -221,7 +245,15 @@ func (r *Route) WriteHeaderIncludedPacket(pkt *PacketBuffer) *tcpip.Error {
 		return tcpip.ErrInvalidEndpointState
 	}
 
-	return r.nic.getNetworkEndpoint(r.NetProto).WriteHeaderIncludedPacket(r, pkt)
+	// WriteHeaderIncludedPacket takes ownership of pkt, calculate numBytes first.
+	numBytes := pkt.Data.Size()
+
+	if err := r.nic.getNetworkEndpoint(r.NetProto).WriteHeaderIncludedPacket(r, pkt); err != nil {
+		return err
+	}
+	r.nic.stats.Tx.Packets.Increment()
+	r.nic.stats.Tx.Bytes.IncrementBy(uint64(numBytes))
+	return nil
 }
 
 // DefaultTTL returns the default TTL of the underlying network endpoint.
diff --git a/pkg/tcpip/stack/stack.go b/pkg/tcpip/stack/stack.go
index 3a07577c8..0bf20c0e1 100644
--- a/pkg/tcpip/stack/stack.go
+++ b/pkg/tcpip/stack/stack.go
@@ -436,9 +436,9 @@ type Stack struct {
 	// uniqueIDGenerator is a generator of unique identifiers.
 	uniqueIDGenerator UniqueID
 
-	// linkResQueue holds packets that are waiting for link resolution to
-	// complete.
-	linkResQueue packetsPendingLinkResolution
+	// forwarder holds the packets that wait for their link-address resolutions
+	// to complete, and forwards them when each resolution is done.
+	forwarder *forwardQueue
 
 	// randomGenerator is an injectable pseudo random generator that can be
 	// used when a random number is required.
@@ -550,8 +550,8 @@ type TransportEndpointInfo struct {
 // incompatible with the receiver.
 //
 // Preconditon: the parent endpoint mu must be held while calling this method.
-func (t *TransportEndpointInfo) AddrNetProtoLocked(addr tcpip.FullAddress, v6only bool) (tcpip.FullAddress, tcpip.NetworkProtocolNumber, *tcpip.Error) {
-	netProto := t.NetProto
+func (e *TransportEndpointInfo) AddrNetProtoLocked(addr tcpip.FullAddress, v6only bool) (tcpip.FullAddress, tcpip.NetworkProtocolNumber, *tcpip.Error) {
+	netProto := e.NetProto
 	switch len(addr.Addr) {
 	case header.IPv4AddressSize:
 		netProto = header.IPv4ProtocolNumber
@@ -565,7 +565,7 @@ func (t *TransportEndpointInfo) AddrNetProtoLocked(addr tcpip.FullAddress, v6onl
 		}
 	}
 
-	switch len(t.ID.LocalAddress) {
+	switch len(e.ID.LocalAddress) {
 	case header.IPv4AddressSize:
 		if len(addr.Addr) == header.IPv6AddressSize {
 			return tcpip.FullAddress{}, 0, tcpip.ErrInvalidEndpointState
@@ -577,8 +577,8 @@ func (t *TransportEndpointInfo) AddrNetProtoLocked(addr tcpip.FullAddress, v6onl
 	}
 
 	switch {
-	case netProto == t.NetProto:
-	case netProto == header.IPv4ProtocolNumber && t.NetProto == header.IPv6ProtocolNumber:
+	case netProto == e.NetProto:
+	case netProto == header.IPv4ProtocolNumber && e.NetProto == header.IPv6ProtocolNumber:
 		if v6only {
 			return tcpip.FullAddress{}, 0, tcpip.ErrNoRoute
 		}
@@ -640,6 +640,7 @@ func New(opts Options) *Stack {
 		useNeighborCache:   opts.UseNeighborCache,
 		uniqueIDGenerator:  opts.UniqueID,
 		nudDisp:            opts.NUDDisp,
+		forwarder:          newForwardQueue(),
 		randomGenerator:    mathrand.New(randSrc),
 		sendBufferSize: SendBufferSizeOption{
 			Min:     MinBufferSize,
@@ -652,7 +653,6 @@ func New(opts Options) *Stack {
 			Max:     DefaultMaxBufferSize,
 		},
 	}
-	s.linkResQueue.init()
 
 	// Add specified network protocols.
 	for _, netProtoFactory := range opts.NetworkProtocols {
@@ -928,16 +928,16 @@ func (s *Stack) CreateNIC(id tcpip.NICID, ep LinkEndpoint) *tcpip.Error {
 	return s.CreateNICWithOptions(id, ep, NICOptions{})
 }
 
-// GetLinkEndpointByName gets the link endpoint specified by name.
-func (s *Stack) GetLinkEndpointByName(name string) LinkEndpoint {
+// GetNICByName gets the NIC specified by name.
+func (s *Stack) GetNICByName(name string) (*NIC, bool) {
 	s.mu.RLock()
 	defer s.mu.RUnlock()
 	for _, nic := range s.nics {
 		if nic.Name() == name {
-			return nic.LinkEndpoint
+			return nic, true
 		}
 	}
-	return nil
+	return nil, false
 }
 
 // EnableNIC enables the given NIC so that the link-layer endpoint can start
@@ -1062,13 +1062,13 @@ func (s *Stack) NICInfo() map[tcpip.NICID]NICInfo {
 		}
 		nics[id] = NICInfo{
 			Name:              nic.name,
-			LinkAddress:       nic.LinkEndpoint.LinkAddress(),
+			LinkAddress:       nic.linkEP.LinkAddress(),
 			ProtocolAddresses: nic.primaryAddresses(),
 			Flags:             flags,
-			MTU:               nic.LinkEndpoint.MTU(),
+			MTU:               nic.linkEP.MTU(),
 			Stats:             nic.stats,
 			Context:           nic.context,
-			ARPHardwareType:   nic.LinkEndpoint.ARPHardwareType(),
+			ARPHardwareType:   nic.linkEP.ARPHardwareType(),
 		}
 	}
 	return nics
@@ -1323,7 +1323,7 @@ func (s *Stack) GetLinkAddress(nicID tcpip.NICID, addr, localAddr tcpip.Address,
 
 	fullAddr := tcpip.FullAddress{NIC: nicID, Addr: addr}
 	linkRes := s.linkAddrResolvers[protocol]
-	return s.linkAddrCache.get(fullAddr, linkRes, localAddr, nic.LinkEndpoint, waker)
+	return s.linkAddrCache.get(fullAddr, linkRes, localAddr, nic.linkEP, waker)
 }
 
 // Neighbors returns all IP to MAC address associations.
@@ -1539,7 +1539,7 @@ func (s *Stack) Wait() {
 	s.mu.RLock()
 	defer s.mu.RUnlock()
 	for _, n := range s.nics {
-		n.LinkEndpoint.Wait()
+		n.linkEP.Wait()
 	}
 }
 
@@ -1627,7 +1627,7 @@ func (s *Stack) WritePacket(nicID tcpip.NICID, dst tcpip.LinkAddress, netProto t
 
 	// Add our own fake ethernet header.
 	ethFields := header.EthernetFields{
-		SrcAddr: nic.LinkEndpoint.LinkAddress(),
+		SrcAddr: nic.linkEP.LinkAddress(),
 		DstAddr: dst,
 		Type:    netProto,
 	}
@@ -1636,7 +1636,7 @@ func (s *Stack) WritePacket(nicID tcpip.NICID, dst tcpip.LinkAddress, netProto t
 	vv := buffer.View(fakeHeader).ToVectorisedView()
 	vv.Append(payload)
 
-	if err := nic.LinkEndpoint.WriteRawPacket(vv); err != nil {
+	if err := nic.linkEP.WriteRawPacket(vv); err != nil {
 		return err
 	}
 
@@ -1653,7 +1653,7 @@ func (s *Stack) WriteRawPacket(nicID tcpip.NICID, payload buffer.VectorisedView)
 		return tcpip.ErrUnknownDevice
 	}
 
-	if err := nic.LinkEndpoint.WriteRawPacket(payload); err != nil {
+	if err := nic.linkEP.WriteRawPacket(payload); err != nil {
 		return err
 	}