ospf6d: introduce support for Graceful Restart (restarting mode)

RFC 5187 specifies the Graceful Restart enhancement to the OSPFv3
routing protocol. This commit implements support for the GR
restarting mode.

Here's a quick summary of how the GR restarting mode works:
* GR can be enabled on a per-instance basis using the `graceful-restart
  [grace-period (1-1800)]` command;
* To perform a graceful shutdown, the `graceful-restart prepare ipv6
  ospf` EXEC-level command needs to be issued before restarting the
  ospf6d daemon (there's no specific requirement on how the daemon
  should be restarted);
* `graceful-restart prepare ospf` will initiate the graceful restart
  for all GR-enabled instances by taking the following actions:
  o Flooding Grace-LSAs over all interfaces
  o Freezing the OSPF routes in the RIB
  o Saving the end of the grace period in non-volatile memory (a JSON
    file stored in `$frr_statedir`)
* Once ospf6d is started again, it will follow the procedures
  described in RFC 3623 until it detects it's time to exit the graceful
  restart (either successfully or unsuccessfully).

Testing done:
* New topotest featuring a multi-area OSPF topology (including stub
  and NSSA areas);
* Successful interop tests against IOS-XR routers acting as helpers.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>

1 files changed, 15 insertions, 0 deletions

diff --git a/ospf6d/ospf6_message.c b/ospf6d/ospf6_message.c
index 3dcc74589a..53f3c3468a 100644
--- a/ospf6d/ospf6_message.c
+++ b/ospf6d/ospf6_message.c
@@ -553,6 +553,21 @@ static void ospf6_hello_recv(struct in6_addr *src, struct in6_addr *dst,
 		return;
 	}
 
+	/*
+	 * RFC 3623 - Section 2:
+	 * "If the restarting router determines that it was the Designated
+	 * Router on a given segment prior to the restart, it elects
+	 * itself as the Designated Router again.  The restarting router
+	 * knows that it was the Designated Router if, while the
+	 * associated interface is in Waiting state, a Hello packet is
+	 * received from a neighbor listing the router as the Designated
+	 * Router".
+	 */
+	if (oi->area->ospf6->gr_info.restart_in_progress
+	    && oi->state == OSPF6_INTERFACE_WAITING
+	    && hello->drouter == oi->area->ospf6->router_id)
+		oi->drouter = hello->drouter;
+
 	/* Schedule interface events */
 	if (backupseen)
 		thread_add_event(master, backup_seen, oi, 0, NULL);