NDPV6: set the initial INCOMPLETE timeout correctly

Previously the logic inside `tick_one` would correctly set the timeout
to `now + retrans_timer` when the initial timeout is hit, but the
initial timeout is erroneously set to the `now + reachable_time` which is
typically very long (~30s).

From RFC2461 [1] section 4.2

> Reachable Time 32-bit unsigned integer.  The time, in
>           milliseconds, that a node assumes a neighbor is
>           reachable after having received a reachability
>           confirmation.  Used by the Neighbor Unreachability
>           Detection algorithm (see Section 7.3).  A value of
>           zero means unspecified (by this router).
>
> Retrans Timer  32-bit unsigned integer.  The time, in
>           milliseconds, between retransmitted Neighbor
>           Solicitation messages.  Used by address resolution
>           and the Neighbor Unreachability Detection algorithm
>           (see Sections 7.2 and 7.3).  A value of zero means
>           unspecified (by this router).

This patch bases the initial NS timeout on the "Retrans timer" rather
than the "Reachable Time" since we've not yet received a reachability
confirmation.

[1] https://tools.ietf.org/html/rfc2461

Signed-off-by: David Scott <dave@recoil.org>

src/ipv6/ndpv6.ml

@@ -645,7 +645,7 @@ module NeighborCache = struct
     | Not_found ->
       nc, []
 
-  let query nc ~now ~reachable_time ip =
+  let query nc ~now ~retrans_timer ip =
     try
       let nb = IpMap.find ip nc in
       match nb.state with
@@ -659,7 +659,7 @@ module NeighborCache = struct
         nc, Some dmac, []
     with
     | Not_found ->
-      let nb  = {state = INCOMPLETE (Int64.add now reachable_time, 0); is_router = false} in
+      let nb  = {state = INCOMPLETE (Int64.add now retrans_timer, 0); is_router = false} in
       let nc  = IpMap.add ip nb nc in
       let dst = Ipaddr.Prefix.network_address solicited_node_prefix ip in
       nc, None, [SendNS (`Specified, dst, ip)]
@@ -1110,7 +1110,7 @@ and send ~now ctx dst frame datav =
   | false ->
     let ctx, ip = next_hop ctx dst in
     let neighbor_cache, mac, actions =
-      NeighborCache.query ctx.neighbor_cache ~now ~reachable_time:ctx.reachable_time ip in
+      NeighborCache.query ctx.neighbor_cache ~now ~retrans_timer:ctx.retrans_timer ip in
     let ctx = {ctx with neighbor_cache} in
     match mac with
     | Some dmac ->