Create constants for Keepalive defaults.

Fixes #6725

PiperOrigin-RevId: 402683244

pkg/tcpip/transport/tcp/endpoint.go

@@ -813,10 +813,9 @@ func newEndpoint(s *stack.Stack, protocol *protocol, netProto tcpip.NetworkProto
 		waiterQueue: waiterQueue,
 		state:       uint32(StateInitial),
 		keepalive: keepalive{
-			// Linux defaults.
-			idle:     2 * time.Hour,
-			interval: 75 * time.Second,
-			count:    9,
+			idle:     DefaultKeepaliveIdle,
+			interval: DefaultKeepaliveInterval,
+			count:    DefaultKeepaliveCount,
 		},
 		uniqueID:      s.UniqueID(),
 		txHash:        s.Rand().Uint32(),

pkg/tcpip/transport/tcp/protocol.go

@@ -66,6 +66,18 @@ const (
 	// DefaultSynRetries is the default value for the number of SYN retransmits
 	// before a connect is aborted.
 	DefaultSynRetries = 6
+
+	// DefaultKeepaliveIdle is the idle time for a connection before keep-alive
+	// probes are sent.
+	DefaultKeepaliveIdle = 2 * time.Hour
+
+	// DefaultKeepaliveInterval is the time between two successive keep-alive
+	// probes.
+	DefaultKeepaliveInterval = 75 * time.Second
+
+	// DefaultKeepaliveCount is the number of keep-alive probes that are sent
+	// before declaring the connection dead.
+	DefaultKeepaliveCount = 9
 )
 
 const (