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, 1 insertions, 0 deletions

diff --git a/ospf6d/ospf6_spf.h b/ospf6d/ospf6_spf.h
index d6fbc5c13b..cc52d16861 100644
--- a/ospf6d/ospf6_spf.h
+++ b/ospf6d/ospf6_spf.h
@@ -93,6 +93,7 @@ struct ospf6_vertex {
 #define OSPF6_SPF_FLAGS_NETWORK_LSA_ORIGINATED   (1 << 7)
 #define OSPF6_SPF_FLAGS_CONFIG_CHANGE            (1 << 8)
 #define OSPF6_SPF_FLAGS_ASBR_STATUS_CHANGE       (1 << 9)
+#define OSPF6_SPF_FLAGS_GR_FINISH                (1 << 10)
 
 static inline void ospf6_set_spf_reason(struct ospf6 *ospf, unsigned int reason)
 {