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, 3 insertions, 4 deletions

diff --git a/pkg/tcpip/stack/neighbor_cache.go b/pkg/tcpip/stack/neighbor_cache.go
index 204196d00..eea32dcf5 100644
--- a/pkg/tcpip/stack/neighbor_cache.go
+++ b/pkg/tcpip/stack/neighbor_cache.go
@@ -297,10 +297,9 @@ func (n *neighborCache) HandleConfirmation(addr tcpip.Address, linkAddr tcpip.Li
 	// no matching entry for the remote address.
 }
 
-// HandleUpperLevelConfirmation implements
-// NUDHandler.HandleUpperLevelConfirmation by following the logic defined in
-// RFC 4861 section 7.3.1.
-func (n *neighborCache) HandleUpperLevelConfirmation(addr tcpip.Address) {
+// handleUpperLevelConfirmation processes a confirmation of reachablity from
+// some protocol that operates at a layer above the IP/link layer.
+func (n *neighborCache) handleUpperLevelConfirmation(addr tcpip.Address) {
 	n.mu.RLock()
 	entry, ok := n.cache[addr]
 	n.mu.RUnlock()