Deprecate Failed state in favor of Unreachable state

... as per RFC 7048. The Failed state is an internal state that is not
specified by any RFC; replacing it with the Unreachable state enables us to
expose this state while keeping our terminology consistent with RFC 4861 and
RFC 7048.

Unreachable state replaces all internal references for Failed state. However
unlike the Failed state, change events are dispatched when moving into
Unreachable state. This gives developers insight into whether a neighbor entry
failed address resolution or whether it was explicitly removed.

The Failed state will be removed entirely once all references to it are
removed. This is done to avoid a Fuchsia roll failure.

Updates #4667

PiperOrigin-RevId: 356554104

1 files changed, 7 insertions, 3 deletions

diff --git a/pkg/tcpip/stack/neighbor_cache.go b/pkg/tcpip/stack/neighbor_cache.go
index a77fe575a..533287c4c 100644
--- a/pkg/tcpip/stack/neighbor_cache.go
+++ b/pkg/tcpip/stack/neighbor_cache.go
@@ -25,9 +25,13 @@ const neighborCacheSize = 512 // max entries per interface
 
 // NeighborStats holds metrics for the neighbor table.
 type NeighborStats struct {
-	// FailedEntryLookups counts the number of lookups performed on an entry in
-	// Failed state.
+	// FailedEntryLookups is deprecated; UnreachableEntryLookups should be used
+	// instead.
 	FailedEntryLookups *tcpip.StatCounter
+
+	// UnreachableEntryLookups counts the number of lookups performed on an
+	// entry in Unreachable state.
+	UnreachableEntryLookups *tcpip.StatCounter
 }
 
 // neighborCache maps IP addresses to link addresses. It uses the Least
@@ -143,7 +147,7 @@ func (n *neighborCache) entry(remoteAddr, localAddr tcpip.Address, onResolve fun
 			onResolve(LinkResolutionResult{LinkAddress: entry.mu.neigh.LinkAddr, Success: true})
 		}
 		return entry.mu.neigh, nil, nil
-	case Unknown, Incomplete, Failed:
+	case Unknown, Incomplete, Unreachable:
 		if onResolve != nil {
 			entry.mu.onResolve = append(entry.mu.onResolve, onResolve)
 		}