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1da177e4 LT |
1 | IP OVER INFINIBAND |
2 | ||
3 | The ib_ipoib driver is an implementation of the IP over InfiniBand | |
ac83cbaa RD |
4 | protocol as specified by RFC 4391 and 4392, issued by the IETF ipoib |
5 | working group. It is a "native" implementation in the sense of | |
6 | setting the interface type to ARPHRD_INFINIBAND and the hardware | |
7 | address length to 20 (earlier proprietary implementations | |
1da177e4 LT |
8 | masqueraded to the kernel as ethernet interfaces). |
9 | ||
10 | Partitions and P_Keys | |
11 | ||
12 | When the IPoIB driver is loaded, it creates one interface for each | |
13 | port using the P_Key at index 0. To create an interface with a | |
14 | different P_Key, write the desired P_Key into the main interface's | |
15 | /sys/class/net/<intf name>/create_child file. For example: | |
16 | ||
17 | echo 0x8001 > /sys/class/net/ib0/create_child | |
18 | ||
19 | This will create an interface named ib0.8001 with P_Key 0x8001. To | |
20 | remove a subinterface, use the "delete_child" file: | |
21 | ||
22 | echo 0x8001 > /sys/class/net/ib0/delete_child | |
23 | ||
24 | The P_Key for any interface is given by the "pkey" file, and the | |
25 | main interface for a subinterface is in "parent." | |
26 | ||
6a3335b4 OG |
27 | Datagram vs Connected modes |
28 | ||
29 | The IPoIB driver supports two modes of operation: datagram and | |
30 | connected. The mode is set and read through an interface's | |
31 | /sys/class/net/<intf name>/mode file. | |
32 | ||
33 | In datagram mode, the IB UD (Unreliable Datagram) transport is used | |
34 | and so the interface MTU has is equal to the IB L2 MTU minus the | |
35 | IPoIB encapsulation header (4 bytes). For example, in a typical IB | |
36 | fabric with a 2K MTU, the IPoIB MTU will be 2048 - 4 = 2044 bytes. | |
37 | ||
38 | In connected mode, the IB RC (Reliable Connected) transport is used. | |
f7111821 BVA |
39 | Connected mode takes advantage of the connected nature of the IB |
40 | transport and allows an MTU up to the maximal IP packet size of 64K, | |
41 | which reduces the number of IP packets needed for handling large UDP | |
42 | datagrams, TCP segments, etc and increases the performance for large | |
43 | messages. | |
6a3335b4 OG |
44 | |
45 | In connected mode, the interface's UD QP is still used for multicast | |
46 | and communication with peers that don't support connected mode. In | |
47 | this case, RX emulation of ICMP PMTU packets is used to cause the | |
48 | networking stack to use the smaller UD MTU for these neighbours. | |
49 | ||
50 | Stateless offloads | |
51 | ||
52 | If the IB HW supports IPoIB stateless offloads, IPoIB advertises | |
53 | TCP/IP checksum and/or Large Send (LSO) offloading capability to the | |
54 | network stack. | |
55 | ||
56 | Large Receive (LRO) offloading is also implemented and may be turned | |
57 | on/off using ethtool calls. Currently LRO is supported only for | |
58 | checksum offload capable devices. | |
59 | ||
60 | Stateless offloads are supported only in datagram mode. | |
61 | ||
62 | Interrupt moderation | |
63 | ||
64 | If the underlying IB device supports CQ event moderation, one can | |
65 | use ethtool to set interrupt mitigation parameters and thus reduce | |
66 | the overhead incurred by handling interrupts. The main code path of | |
67 | IPoIB doesn't use events for TX completion signaling so only RX | |
68 | moderation is supported. | |
69 | ||
1da177e4 LT |
70 | Debugging Information |
71 | ||
72 | By compiling the IPoIB driver with CONFIG_INFINIBAND_IPOIB_DEBUG set | |
73 | to 'y', tracing messages are compiled into the driver. They are | |
74 | turned on by setting the module parameters debug_level and | |
75 | mcast_debug_level to 1. These parameters can be controlled at | |
76 | runtime through files in /sys/module/ib_ipoib/. | |
77 | ||
b1ed8dab | 78 | CONFIG_INFINIBAND_IPOIB_DEBUG also enables files in the debugfs |
1da177e4 LT |
79 | virtual filesystem. By mounting this filesystem, for example with |
80 | ||
b1ed8dab | 81 | mount -t debugfs none /sys/kernel/debug |
1da177e4 LT |
82 | |
83 | it is possible to get statistics about multicast groups from the | |
b1ed8dab | 84 | files /sys/kernel/debug/ipoib/ib0_mcg and so on. |
1da177e4 LT |
85 | |
86 | The performance impact of this option is negligible, so it | |
87 | is safe to enable this option with debug_level set to 0 for normal | |
88 | operation. | |
89 | ||
90 | CONFIG_INFINIBAND_IPOIB_DEBUG_DATA enables even more debug output in | |
91 | the data path when data_debug_level is set to 1. However, even with | |
92 | the output disabled, enabling this configuration option will affect | |
93 | performance, because it adds tests to the fast path. | |
94 | ||
95 | References | |
96 | ||
ac83cbaa RD |
97 | Transmission of IP over InfiniBand (IPoIB) (RFC 4391) |
98 | http://ietf.org/rfc/rfc4391.txt | |
99 | IP over InfiniBand (IPoIB) Architecture (RFC 4392) | |
100 | http://ietf.org/rfc/rfc4392.txt | |
6a3335b4 OG |
101 | IP over InfiniBand: Connected Mode (RFC 4755) |
102 | http://ietf.org/rfc/rfc4755.txt |