Confirm neighbor reachability with TCP ACKs

As per RFC 4861 section 7.3.1,
  A neighbor is considered reachable if the node has recently received
  a confirmation that packets sent recently to the neighbor were
  received by its IP layer. Positive confirmation can be gathered in
  two ways: hints from upper-layer protocols that indicate a connection
  is making "forward progress", or receipt of a Neighbor Advertisement
  message that is a response to a Neighbor Solicitation message.

This change adds support for TCP to let the IP/link layers know that a
neighbor is reachable.

Test: integration_test.TestTCPConfirmNeighborReachability
PiperOrigin-RevId: 354222833

1 files changed, 17 insertions, 6 deletions

diff --git a/pkg/tcpip/stack/route.go b/pkg/tcpip/stack/route.go
index d9a8554e2..9c8c155fa 100644
--- a/pkg/tcpip/stack/route.go
+++ b/pkg/tcpip/stack/route.go
@@ -354,11 +354,6 @@ func (r *Route) resolvedFields(afterResolve func(ResolvedFieldsResult)) (RouteIn
 		return fields, nil, nil
 	}
 
-	nextAddr := r.NextHop
-	if nextAddr == "" {
-		nextAddr = r.RemoteAddress
-	}
-
 	// If specified, the local address used for link address resolution must be an
 	// address on the outgoing interface.
 	var linkAddressResolutionRequestLocalAddr tcpip.Address
@@ -367,7 +362,7 @@ func (r *Route) resolvedFields(afterResolve func(ResolvedFieldsResult)) (RouteIn
 	}
 
 	afterResolveFields := fields
-	linkAddr, ch, err := r.outgoingNIC.getNeighborLinkAddress(nextAddr, linkAddressResolutionRequestLocalAddr, r.linkRes, func(r LinkResolutionResult) {
+	linkAddr, ch, err := r.outgoingNIC.getNeighborLinkAddress(r.nextHop(), linkAddressResolutionRequestLocalAddr, r.linkRes, func(r LinkResolutionResult) {
 		if afterResolve != nil {
 			if r.Success {
 				afterResolveFields.RemoteLinkAddress = r.LinkAddress
@@ -382,6 +377,13 @@ func (r *Route) resolvedFields(afterResolve func(ResolvedFieldsResult)) (RouteIn
 	return fields, ch, err
 }
 
+func (r *Route) nextHop() tcpip.Address {
+	if len(r.NextHop) == 0 {
+		return r.RemoteAddress
+	}
+	return r.NextHop
+}
+
 // local returns true if the route is a local route.
 func (r *Route) local() bool {
 	return r.Loop == PacketLoop || r.outgoingNIC.IsLoopback()
@@ -519,3 +521,12 @@ func (r *Route) IsOutboundBroadcast() bool {
 	// Only IPv4 has a notion of broadcast.
 	return r.isV4Broadcast(r.RemoteAddress)
 }
+
+// ConfirmReachable informs the network/link layer that the neighbour used for
+// the route is reachable.
+//
+// "Reachable" is defined as having full-duplex communication between the
+// local and remote ends of the route.
+func (r *Route) ConfirmReachable() {
+	r.outgoingNIC.confirmReachable(r.nextHop())
+}