Do not use a stack-wide queue of pending packets

Packets may be pending on link resolution to complete before being sent.
Link resolution is performed for neighbors which are unique to a NIC so
hold link resolution related state under the NIC, not the stack.

Note, this change may result in more queued packets but that is okay as
RFC 4861 section 7.2.2 recommends that the stack maintain a queue of
packets for each neighbor that is waiting for link resolution to
complete, not a fixed limit per stack.

PiperOrigin-RevId: 352322155

pkg/tcpip/stack/nic.go

@@ -49,6 +49,10 @@ type NIC struct {
 	// Must be accessed using atomic operations.
 	enabled uint32
 
+	// linkResQueue holds packets that are waiting for link resolution to
+	// complete.
+	linkResQueue packetsPendingLinkResolution
+
 	mu struct {
 		sync.RWMutex
 		spoofing    bool
@@ -134,6 +138,7 @@ func newNIC(stack *Stack, id tcpip.NICID, name string, ep LinkEndpoint, ctx NICC
 		stats:            makeNICStats(),
 		networkEndpoints: make(map[tcpip.NetworkProtocolNumber]NetworkEndpoint),
 	}
+	nic.linkResQueue.init()
 	nic.mu.packetEPs = make(map[tcpip.NetworkProtocolNumber]*packetEndpointList)
 
 	// Check for Neighbor Unreachability Detection support.
@@ -315,7 +320,7 @@ func (n *NIC) WritePacket(r *Route, gso *GSO, protocol tcpip.NetworkProtocolNumb
 	if ch, err := r.Resolve(nil); err != nil {
 		if err == tcpip.ErrWouldBlock {
 			r.Acquire()
-			n.stack.linkResQueue.enqueue(ch, r, protocol, pkt)
+			n.linkResQueue.enqueue(ch, r, protocol, pkt)
 			return nil
 		}
 		return err

pkg/tcpip/stack/stack.go

@@ -440,10 +440,6 @@ 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
-
 	// randomGenerator is an injectable pseudo random generator that can be
 	// used when a random number is required.
 	randomGenerator *mathrand.Rand
@@ -664,7 +660,6 @@ func New(opts Options) *Stack {
 			Max:     DefaultMaxBufferSize,
 		},
 	}
-	s.linkResQueue.init()
 
 	// Add specified network protocols.
 	for _, netProtoFactory := range opts.NetworkProtocols {