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  The ib_ipoib driver is an implementation of the IP over InfiniBand
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  protocol as specified by RFC 4391 and 4392, issued by the IETF ipoib
  working group.  It is a "native" implementation in the sense of
  setting the interface type to ARPHRD_INFINIBAND and the hardware
  address length to 20 (earlier proprietary implementations
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  masqueraded to the kernel as ethernet interfaces).

Partitions and P_Keys

  When the IPoIB driver is loaded, it creates one interface for each
  port using the P_Key at index 0.  To create an interface with a
  different P_Key, write the desired P_Key into the main interface's
  /sys/class/net/<intf name>/create_child file.  For example:

    echo 0x8001 > /sys/class/net/ib0/create_child

  This will create an interface named ib0.8001 with P_Key 0x8001.  To
  remove a subinterface, use the "delete_child" file:

    echo 0x8001 > /sys/class/net/ib0/delete_child

  The P_Key for any interface is given by the "pkey" file, and the
  main interface for a subinterface is in "parent."

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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 is to 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.

Stateless offloads

  If the IB HW supports IPoIB stateless offloads, IPoIB advertises
  TCP/IP checksum and/or Large Send (LSO) offloading capability to the
  network stack.

  Large Receive (LRO) offloading is also implemented and may be turned
  on/off using ethtool calls.  Currently LRO is supported only for
  checksum offload capable devices.

  Stateless offloads are supported only in datagram mode.  

Interrupt moderation

  If the underlying IB device supports CQ event moderation, one can
  use ethtool to set interrupt mitigation parameters and thus reduce
  the overhead incurred by handling interrupts.  The main code path of
  IPoIB doesn't use events for TX completion signaling so only RX
  moderation is supported.

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Debugging Information

  By compiling the IPoIB driver with CONFIG_INFINIBAND_IPOIB_DEBUG set
  to 'y', tracing messages are compiled into the driver.  They are
  turned on by setting the module parameters debug_level and
  mcast_debug_level to 1.  These parameters can be controlled at
  runtime through files in /sys/module/ib_ipoib/.

  CONFIG_INFINIBAND_IPOIB_DEBUG also enables files in the debugfs
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  virtual filesystem.  By mounting this filesystem, for example with

    mount -t debugfs none /sys/kernel/debug
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  it is possible to get statistics about multicast groups from the
  files /sys/kernel/debug/ipoib/ib0_mcg and so on.
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  The performance impact of this option is negligible, so it
  is safe to enable this option with debug_level set to 0 for normal

  CONFIG_INFINIBAND_IPOIB_DEBUG_DATA enables even more debug output in
  the data path when data_debug_level is set to 1.  However, even with
  the output disabled, enabling this configuration option will affect
  performance, because it adds tests to the fast path.


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  Transmission of IP over InfiniBand (IPoIB) (RFC 4391)
  IP over InfiniBand (IPoIB) Architecture (RFC 4392)
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  IP over InfiniBand: Connected Mode (RFC 4755)