buglwIP - A Lightweight TCP/IP stack - Bugs: bug #58551, mDNS: incorrect probe timings...


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bug #58551: mDNS: incorrect probe timings during rate limiting

Submitter:  Jasper Verschueren <jasperv>
Submitted:  Fri 12 Jun 2020 10:07:17 AM UTC
Category:  apps Severity:  3 - Normal
Item Group:  Faulty Behaviour Status:  None
Privacy:  Public Assigned to:  None
Open/Closed:  Open Planned Release:  None
lwIP version:  git head

Fri 12 Jun 2020 10:07:17 AM UTC, original submission:  

Commit:6302423d mdns: handle tiebreaking loose like conflict, introduces a bug in the probing part of mDNS.

Most changes in this commit are valid, except the following:

   @@ -2294,8 +2307,13 @@ mdns_probe_and_announce(void* arg)
            mdns->sent_num = 0;
   -      sys_timeout(MDNS_PROBE_DELAY_MS, mdns_probe_and_announce, netif);
   +      if (mdns->sent_num && mdns->rate_limit_activated == 1) {
   +        /* delay second probe if rate limiting activated */
   +        sys_timeout(MDNS_PROBE_MAX_CONFLICTS_TIMEOUT, mdns_probe_and_announce, netif);
   +      }
   +      else {
   +        sys_timeout(MDNS_PROBE_DELAY_MS, mdns_probe_and_announce, netif);
   +      }

This changes the time in between probes to 5 seconds in case of rate limiting.
This is against RFC, the RFC only specifies to wait 5 seconds before restarting.
RFC6762 section 8.1 Probing:

   If fifteen conflicts occur within any ten-second period, then the
   host MUST wait at least five seconds before each successive
   additional probe attempt.  This is to help ensure that, in the event
   of software bugs or other unanticipated problems, errant hosts do not
   flood the network with a continuous stream of multicast traffic.  For
   very simple devices, a valid way to comply with this requirement is
   to always wait five seconds after any failed probe attempt before
   trying again.

So it states that we should wait 5 seconds before we try probing again.
It doesn't specify that we need to change our probing timings from 250 ms to 5 seconds.

This particular change should be reverted.
Attached a patch can be found.

Kind regards,

Jasper Verschueren <jasperv>


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Depends on the following items: None found

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    Follow 2 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    2020-06-12 jasperv Attached File- Added 0001-mDNS-fix-bug-with-probe-timing-during-rate-limiting.patch, #49246
        Carbon-Copy- Added dgirault

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