IP OVER INFINIBAND
  
    The ib_ipoib driver is an implementation of the IP over InfiniBand

    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

    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."

    Child interface create/delete can also be done using IPoIB's

    rtnl_link_ops, where children created using either way behave the same.

  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.
  
  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.

  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

    virtual filesystem.  By mounting this filesystem, for example with

      mount -t debugfs none /sys/kernel/debug

  
    it is possible to get statistics about multicast groups from the

    files /sys/kernel/debug/ipoib/ib0_mcg and so on.

  
    The performance impact of this option is negligible, so it
    is safe to enable this option with debug_level set to 0 for normal
    operation.
  
    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.
  
  References

    Transmission of IP over InfiniBand (IPoIB) (RFC 4391)
      http://ietf.org/rfc/rfc4391.txt 
    IP over InfiniBand (IPoIB) Architecture (RFC 4392)
      http://ietf.org/rfc/rfc4392.txt 

    IP over InfiniBand: Connected Mode (RFC 4755)
      http://ietf.org/rfc/rfc4755.txt