How to switch Datgram/Connected mtu modes?

Alex Liptsin alexl at mellanox.com
Sun May 26 11:44:52 UTC 2013


Hello.

I work with FreeBSD 9.1 and Mellanox devices.

How can I configure MTU in connected mode on FreeBSD 9.1?
In Linux to enable connected mode for interface ib0, I enter:

   echo connected > /sys/class/net/ib0/mode



Switching between CM and UD mode can be done in run time:

   echo datagram > /sys/class/net/ib0/mode sets the mode of ib0 to UD

   echo connected > /sys/class/net/ib0/mode sets the mode ib0 to CM

There is no such directories at FreeBSD. Wat shall I do?



Datagram vs Connected modes

  The IPoIB driver supports two modes of operation: datagram and
  connected.  The mode is set and read through an interface's
  /sys/class/net/<intf name>/mode file.

  In datagram mode, the IB UD (Unreliable Datagram) transport is used
  and so the interface MTU has is equal to the IB L2 MTU minus the
  IPoIB encapsulation header (4 bytes).  For example, in a typical IB
  fabric with a 2K MTU, the IPoIB MTU will be 2048 - 4 = 2044 bytes.

  In connected mode, the IB RC (Reliable Connected) transport is used.
  Connected mode takes advantage of the connected nature of the IB
  transport and allows an MTU up to the maximal IP packet size of 64K,
  which reduces the number of IP packets needed for handling large UDP
  datagrams, TCP segments, etc and increases the performance for large
  messages.

  In connected mode, the interface's UD QP is still used for multicast
  and communication with peers that don't support connected mode. In
  this case, RX emulation of ICMP PMTU packets is used to cause the
  networking stack to use the smaller UD MTU for these neighbours.

Thanks a lot

Regards,
Alex Liptsin
Office: +972 (74) 7236141
Mobile: +972(54) 7833986
Fax: +972(74) 7236161
Email: alexl at mellanox.com<mailto:alexl at mellanox.com>



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