ospfd: Improve OSPF neighbor retransmission list granularity and precision

The current OSPF neighbor retransmission operates on a single per-neighbor
periodic timer that sends all LSAs on the list when it expires.
Additionally, since it skips the first retransmission of received LSAs so
that at least the retransmission interval (resulting in a delay of between
the retransmission interval and twice the interval. In environments where
the links are lossy on P2MP networks with "delay-reflood" configured (which
relies on neighbor retransmission in partial meshs), the implementation
is sub-optimal (to say the least).

This commit reimplements OSPF neighbor retransmission as follows:

   1. A new data structure making use the application managed
      typesafe.h doubly linked list implements an OSPF LSA
      list where each node includes a timestamp.
   2. The existing neighbor LS retransmission LSDB data structure
      is augmented with a pointer to the list node on the LSA
      list to faciliate O(1) removal when the LSA is acknowledged.
   3. The neighbor LS retransmission timer is set to the expiration
      timer of the LSA at the top of the list.
   4. When the timer expires, LSAs are retransmitted that within
      the window of the current time and a small delta (50 milli-secs
      default). The LSAs that are retransmited are given an updated
      retransmission time and moved to the end of the LSA list.
   5. Configuration is added to set the "retransmission-window" to a
      value other than 50 milliseconds.
   6. Neighbor and interface LSA retransmission counters are added
      to provide insight into the lossiness of the links. However,
      these will increment quickly on non-fully meshed P2MP networks
      with "delay-reflood" configured.
   7. Added a topotest to exercise the implementation on a non-fully
      meshed P2MP network with "delay-reflood" configured. The
      alternative was to use existing mechanisms to instroduce loss
      but these seem less determistic in a topotest.

Signed-off-by: Acee Lindem <acee@lindem.com>

1 files changed, 11 insertions, 1 deletions

diff --git a/doc/user/ospfd.rst b/doc/user/ospfd.rst
index 70c15e73de..b80adba7f0 100644
--- a/doc/user/ospfd.rst
+++ b/doc/user/ospfd.rst
@@ -738,7 +738,17 @@ Interfaces
    retransmitting Database Description and Link State Request packets. The
    default value is 5 seconds.
 
-.. clicmd:: ip ospf transmit-delay (1-65535) [A.B.C.D]
+.. clicmd:: ip ospf retransmit-window (20-1000)
+
+
+   Set number of milliseconds in the window for neighbor LSA retransmission.
+   When a neighbor Link State (LS) retransmission timer expires, LSAs scheduled
+   to be retransmitted within the number of milliseconds configured are
+   retransmitted to the neighbor. Any expiring after the window will be
+   retransmitted the next time the neighbor LS retransmission timer expires.
+   The default is 50 milliseconds.
+
+ .. clicmd:: ip ospf transmit-delay (1-65535) [A.B.C.D]
 
 
    Set number of seconds for InfTransDelay value. LSAs' age should be