How to switch Datgram/Connected mtu modes?
alexl at mellanox.com
Sun May 26 11:44:52 UTC 2013
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
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
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Email: alexl at mellanox.com<mailto:alexl at mellanox.com>
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