ospfd: introduce support for Graceful Restart (restarting mode)

RFC 3623 specifies the Graceful Restart enhancement to the OSPF
routing protocol. This PR implements support for the restarting mode,
whereas the helper mode was implemented by #6811.

This work is based on #6782, which implemented the pre-restart part
and settled the foundations for the post-restart part (behavioral
changes, GR exit conditions, and on-exit actions).

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 ospf`
  EXEC-level command needs to be issued before restarting the ospfd
  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 ospfd 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.

Co-authored-by: GalaxyGorilla <sascha@netdef.org>
Signed-off-by: Renato Westphal <renato@opensourcerouting.org>

1 files changed, 22 insertions, 2 deletions

diff --git a/doc/user/ospfd.rst b/doc/user/ospfd.rst
index 8d67ec865f..e8ca394727 100644
--- a/doc/user/ospfd.rst
+++ b/doc/user/ospfd.rst
@@ -710,8 +710,17 @@ Redistribution
 
 
 
-Graceful Restart Helper
-=======================
+Graceful Restart
+================
+
+.. clicmd:: graceful-restart [grace-period (1-1800)]
+
+
+   Configure Graceful Restart (RFC 3623) restarting support.
+   When enabled, the default grace period is 120 seconds.
+
+   To perform a graceful shutdown, the "graceful-restart prepare ip ospf"
+   EXEC-level command needs to be issued before restarting the ospfd daemon.
 
 .. clicmd:: graceful-restart helper-only [A.B.C.D]
 
@@ -743,6 +752,17 @@ Graceful Restart Helper
    restarts. By default, it supports both planned and
    unplanned outages.
 
+
+.. clicmd:: graceful-restart prepare ip ospf
+
+
+   Initiate a graceful restart for all OSPF instances configured with the
+   "graceful-restart" command. The ospfd daemon should be restarted during
+   the instance-specific grace period, otherwise the graceful restart will fail.
+
+   This is an EXEC-level command.
+
+
 .. _showing-ospf-information:
 
 Showing Information