Subject: [VRRP] Clarification on Handling of Invalid Advertisements (Section 7.1)

From: "Craig Verge" <[email-masked]>
Date: Mon, 10 Mar 2003 12:02:23 -0500

The section in the RFC reads:

7.1. Receiving VRRP Packets
   Performed the following functions when a VRRP packet is received:

      - MUST verify that the IP TTL is 255.
      - MUST verify the VRRP version
      - MUST verify that the received packet length is greater than or
        equal to the VRRP header
      - MUST verify the VRRP checksum
      - MUST perform authentication specified by Auth Type

   If any one of the above checks fails, the receiver MUST discard the
   packet, SHOULD log the event and MAY indicate via network management
   that an error occurred.

      - MUST verify that the VRID is valid on the receiving interface

   If the above check fails, the receiver MUST discard the packet.

      - MAY verify that the IP address(es) associated with the VRID are
        valid

   If the above check fails, the receiver SHOULD log the event and MAY
   indicate via network management that a misconfiguration was detected.
   If the packet was not generated by the address owner (Priority does
   not equal 255 (decimal)), the receiver MUST drop the packet,
   otherwise continue processing.

      - MUST verify that the Adver Interval in the packet is the same as
        the locally configured for this virtual router

   If the above check fails, the receiver MUST discard the packet,
   SHOULD log the event and MAY indicate via network management that a
   misconfiguration was detected.

If I read this correctly then for all advertisement error scenarios except
the IP address list mismatch the receiver should discard the advertisement
and not continue further processing...Correct?
This means that a received advertisement failing one of the checks (except
the IP address check) is the same as an advertisement never received?  In
which case one of the backup routers for this VRID will transition to master
after the master_down_timer fires and there will be two masters.  Is this
correct?  With two masters on the LAN we may cause the switch for this LAN
to become confused.  We will have two routers responding to ARP requests and
the switch with think the device with this vmac is constantly changing
between two different ports.
Maybe we should differentiate the type of incorrect advertisement as
misconfiguration versus protocol or hardware error.  If there is a
misconfiguration in the Advertisement Interval we should NOT transition to
master (i.e. reset master_down_timer) since we know the owner is out there
but is not configured the same as the backup.  It will still be master and
forward IP packets from the local devices.
If there is a protocol violation (incorrect TTL) or data corruption
(checksum) then we don't reset the timer since maybe there is something
wrong with this masters NIC causing checksum failure or a software problem
resulting in the incorrect TTL.  A non-configuration problem of this type
(i.e. possible hardware or software error) could impact the owners ability
to forward IP packets from the local devices.
Cheers,
Craig Verge

---

From: Vikram Pendharkar <[email-masked]>
Date: Mon, 10 Mar 2003 13:39:01 -0500

Hi Craig,

Craig Verge wrote:

> **
<snip>

> If I read this correctly then for all advertisement error scenarios 
> except the IP address list mismatch the receiver should discard the 
> advertisement and not continue further processing...Correct?
>
Yes.

> This means that a received advertisement failing one of the checks 
> (except the IP address check) is the same as an advertisement never 
> received?  In which case one of the backup routers for this VRID will 
> transition to master after the master_down_timer fires and there will 
> be two masters.  Is this correct? 
>
Yes.

> With two masters on the LAN we may cause the switch for this LAN to 
> become confused.  We will have two routers responding to ARP requests 
> and the switch with think the device with this vmac is constantly 
> changing between two different ports.
>
Yes. However, it is also dependent on how the router's arp is configured 
to act when it detects another response for a mac which it thinks it 
owns. Mostly, they might just raise an alarm or log the event.

> misconfiguration in the Advertisement Interval we should NOT 
> transition to master (i.e. reset master_down_timer) since we know the 
> owner is out there but is not configured the same as the backup.  It 
> will still be master and forward IP packets from the local devices.
>
What  you are suggesting is to not propogate an error. But what a backup 
sees as incorrect adver interval may actually be due to something else 
done incorrectly. So we cannot say for certain that 'master is just 
fine, backup neednt transition to master'. Having two masters IMO is a 
lesser sin than risking a chance that there is no forwarding at all. And 
plus, there's one more avenue to know something's wrong.

> (checksum) then we don't reset the timer since maybe there is 
> something wrong with this masters NIC causing checksum failure or a 
> software problem resulting in the incorrect TTL.  A non-configuration 
> problem of this type (i.e. possible hardware or software error) could 
> impact the owners ability to forward IP packets from the local devices.
>
just a nit, incorrect TTL could also be because the packet came from a 
different subnet, not necessarily incorrect software. Another indicator 
of how what you see may not indicate the correct source of problem.

> Craig Verge
Regards,
Vikram

-- 
Vikram Pendharkar			
Ericsson IP Infrastructure

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---

From: Vikram Pendharkar <[email-masked]>
Date: Mon, 10 Mar 2003 13:56:49 -0500

Vikram Pendharkar wrote:

> different subnet, not necessarily incorrect software. Another 
> indicator of how what you see may not indicate the correct source of 
> problem.

...well maybe not! ultimately its all a software problem :-)