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

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 <>
Signed-off-by: default avatarRoland Dreier <>
parent edb5abb1
......@@ -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
IP over InfiniBand (IPoIB) Architecture (RFC 4392)
IP over InfiniBand: Connected Mode (RFC 4755)
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