Subject: Re: [PATCH] neighbour.c: Avoid GC directly after state change

From: YOSHIFUJI Hideaki <[email-masked]>
Date: Thu, 16 Apr 2015 14:16:26 +0900

Hi,

Ulf Samuelsson wrote:

> The desired functionality is that if communication stops,
> you want to send out ARP probes, before the entry is deleted.
> 
> The current (pseudo) code of the neigh timer is:
> 
>     if (state & NUD_REACHABLE) {
>         if (now <= "confirmed + "reachable_time")) {
>                     ... /* We are OK */
>         } else if (now < "used" + DELAY_PROBE_TIME) {    /* Never happens */
>                     state = NUD_DELAY;
>         } else {
>             state = NUD_STALE;
>             notify = 1;
>         }
> 
> We never see the state beeing changed from REACHABLE to DELAY,
> so the probes are not beeing sent out, instead you always go
> from REACHABLE to STALE.

That's right.

> DELAY_PROBE_TIME is set to (5 x HZ) and "used"
> seems to be only set by the periodic_work routine
> when the neigh entry is in STALE state, and then it is too late.
> It is also set by "arp_find" which is used by "broken" devices.
> 

In STALE state, neigh->used is set by neigh_event_send(), called
by neigh_resolve_output() via neigh->output().

> In practice, the second condition: "(now < "used" + DELAY_PROBE_TIME)" is never used.
> What is the intention of this test?

That's right.  It is NOT used in normal condition unless
reachable time is too short.

> 
> By adding a new test + parameter, we would get the desired functionality,
> and no need to listen for notifications or doing ARP state updates from applications.
> 
>         if (now <= "confirmed + "reachable_time")) {
>                     ... /* We are OK */
> +        else if (now <= "confirmed + "reprobe_time")) {
> +                   state <= NUD_DELAY;
>         } else if (now < "used" + DELAY_PROBE_TIME))) {    /* Never happens */
>                     state <= NUD_DELAY;
>         } else {
>             state = NUD_STALE;
>             notify = 1;
>         }
> 
> This way the entry would remain in REACHABLE while normal communication occurs,
> then it would enter DELAY state to probe, and if that fails, it goes to STALE state.

No, it is not what REACHABLE and DELAY mean.

>>From RFC2461:

|      REACHABLE   Roughly speaking, the neighbor is known to have been
|                  reachable recently (within tens of seconds ago).
:
|      STALE       The neighbor is no longer known to be reachable but
|                  until traffic is sent to the neighbor, no attempt
|                  should be made to verify its reachability.
|      DELAY       The neighbor is no longer known to be reachable, and
|                  traffic has recently been sent to the neighbor.
|                  Rather than probe the neighbor immediately, however,
|                  delay sending probes for a short while in order to
|                  give upper layer protocols a chance to provide
|                  reachability confirmation.

-- 
Hideaki Yoshifuji <[email-masked]>
Technical Division, MIRACLE LINUX CORPORATION