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Commit 6a3335b4 authored by Or Gerlitz's avatar Or Gerlitz Committed by Roland Dreier
Browse files

IPoIB: Document newish features 



Update the documentation to include connected mode, stateless offloads
and interrupt moderation, and add a reference to the connected mode RFC.

Signed-off-by: default avatarOr Gerlitz <ogerlitz@voltaire.com>
Signed-off-by: default avatarRoland Dreier <rolandd@cisco.com>
parent edb5abb1

Documentation/infiniband/ipoib.txt

+45 −0
Original line number Diff line number Diff line
@@ -24,6 +24,49 @@ Partitions and P_Keys 
  The P_Key for any interface is given by the "pkey" file, and the

  main interface for a subinterface is in "parent."



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.



Debugging Information



  By compiling the IPoIB driver with CONFIG_INFINIBAND_IPOIB_DEBUG set
@@ -55,3 +98,5 @@ References 
    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