[ofa-general] Enabling IP_CM warns about multicast packet drops

[Olga Shern (Voltaire)]

Hi Summet,

You can read from the ipoib release notes:
"If IPoIB connected mode is enabled, it uses a large MTU for connected mode
   messages and a small MTU for datagram (in particular, multicast) messages,
   and relies on path MTU discovery to adjust MTU appropriately. Packets sent
   in the window before MTU discovery automatically reduces the MTU for a
   specific destination will be dropped, producing the following message in the
   system log:
   "packet len <actual length> (> <max allowed length>) too long to
send, dropping"

   To warn about this, a message is produced in the system log each time MTU is
   set to a value higher than 2K."


Olga

On Tue, Feb 10, 2009 at 2:41 AM, Sumeet Lahorani
<sumeet.lahorani at oracle.com> wrote:
> When we enable IB connected mode and increase MTU to 65520, we see the
> following in /var/log/messages
>
> Feb  6 17:48:32 dadzab01 kernel: ib0: enabling connected mode will cause
> multicast packet drops
> Feb  6 17:48:32 dadzab01 kernel: ib0: mtu > 2044 will cause multicast packet
> drops.
> Feb  6 17:48:32 dadzab01 kernel: ib1: enabling connected mode will cause
> multicast packet drops
> Feb  6 17:48:32 dadzab01 kernel: ib1: mtu > 2044 will cause multicast packet
> drops.
>
> Should we not be doing this? What kind of multicast packets will be dropped?
>
> If we are not using multicast, do any OFED drivers (bonding, ipoib etc)
> internally use multicast in a way that will cause them to not work correctly
> in connected mode?
>
> We are using OFED 1.3.1.
>
> - Sumeet
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