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-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."
-
-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