Subject: [ANCP] Fallback adjacency

From: Alexandre Cassen <[email-masked]>
Date: Tue, 17 Feb 2009 13:53:04 +0100

Hello,

Maybe this is a topic for graceful restart, purpose here is to speed up
adj synchronization recover after NAS IP takes over to another NAS in a
redundant env. Due to spec rfc3292.11.4, loss of synchronization defines
a non-correlation between TCP and ANCP lost, ANCP keeps track of states
until three time the value of 'Timer' field even if TCP connection has
been reset. If considering replication control message, this lead to
disabling replication control channel until ANCP Timer fire at AN side.

If we are considering an env with 2 NAS runing VRRP to expose a VIP to
remote Access Nodes (master on NAS 1) :

         +-------------+
         | Access Node |
         +-------------+
               |
               v
           VRRP_VIP
               o.....
              /      .
          (m)/         .(b)
         +-------+   +-------+
         | NAS 1 |   | NAS 2 |
         +-------+   +-------+

If NAS 1 crash down then NAS 2 will take over VIP but AN will recover
operational adjacency until loss of synchronization timer has fired at
AN side. If we could define kind of fallback adjacency at AN side then
we could eliminate impact of takeover:

            +-------------+
            | Access Node |
            +-------------+
      (adj_m)/          \(adj_fb)
            v            v
           VIP1          VIP2
            o.............o
           /  .      .     \
      (m1)/ .(b2)     .(b1) \(m2)
      +-------+       +-------+
      | NAS 1 |       | NAS 2 |
      +-------+       +-------+

AN could maintains 2 adjacencies, one to each NAS. If NAS 1 crash, NAS 2
will takeover, which will transparently takeover NAS 1 role (replication
control for instance) to remote AN but via ANCP ESTAB channel => adj_fb.
When NAS 1 comes back it may defer its operations until remote AN is
reachable by adj_m. This processing only defines operations rules on NAS
side, impact on AN side is only having 2 adj up and running at a time.

best regs,
Alexandre