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IETF RFC 4392
IP over InfiniBand (IPoIB) Architecture
Last modified on Thursday, April 6th, 2006
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Network Working Group V. Kashyap
Request for Comments: 4392 IBM
Category: Informational April 2006
IP over InfiniBand (IPoIB) Architecture
 Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright © The Internet Society (2006).
Abstract
InfiniBand is a high-speed, channel-based interconnect between
systems and devices.
This document presents an overview of the InfiniBand architecture.
It further describes the requirements and guidelines for the
transmission of IP over InfiniBand. Discussions in this document are
applicable to both IPv4 and IPv6 unless explicitly specified. The
encapsulation of IP over InfiniBand and the mechanism for IP address
resolution on IB fabrics are covered in other documents.
Table of Contents
1. Introduction to InfiniBand ......................................2
1.1. InfiniBand Architecture Specification ......................2
1.2. Overview of InfiniBand Architecture ........................2
1.2.1. InfiniBand Addresses ................................6
1.2.1.1. Unicast GIDs ...............................7
1.2.1.2. Multicast GIDs .............................7
1.3. InfiniBand Multicast Group Management ......................9
1.3.1. Multicast Member Record ............................10
1.3.1.1. JoinState .................................10
1.3.2. Join and Leave Operations ..........................11
1.3.2.1. Creating a Multicast Group ................11
1.3.2.2. Deleting a Multicast Group ................11
1.3.2.3. Multicast Group Create/Delete Traps .......12
2. Management of InfiniBand Subnet ................................12
3. IP over IB .....................................................12
3.1. InfiniBand as Datalink ....................................13
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3.2. Multicast Support .........................................13
3.2.1. Mapping IP Multicast to IB Multicast ...............14
3.2.2. Transient Flag in IB MGIDs .........................14
3.3. IP Subnets Across IB Subnets ..............................14
4. IP Subnets in InfiniBand Fabrics ...............................14
4.1. IPoIB VLANs ...............................................16
4.2. Multicast in IPoIB subnets ................................16
4.2.1. Sending IP Multicast Datagrams .....................17
4.2.2. Receiving Multicast Packets ........................18
4.2.3. Router Considerations for IPoIB ....................18
4.2.4. Impact of InfiniBand Architecture Limits ...........19
4.2.5. Leaving/Deleting a Multicast Group .................19
4.3. Transmission of IPoIB Packets .............................20
4.4. Reverse Address Resolution Protocol (RARP) and
Static ARP Entries ........................................20
4.5. DHCPv4 and IPoIB ..........................................21
5. QoS and Related Issues .........................................21
6. Security Considerations ........................................21
7. Acknowledgements ...............................................21
8. References .....................................................21
8.1. Normative References ......................................21
8.2. Informative References ....................................22
1. Introduction to InfiniBand
The InfiniBand Trade Association (IBTA) was formed to develop an I/O
specification to deliver a channel based, switched fabric technology.
The InfiniBand standard is aimed at meeting the requirements of
scalability, reliability, availability, and performance of servers in
data centers.
1.1. InfiniBand Architecture Specification
The InfiniBand Trade Association specification is available for
download from http://www.infinibandta.org.
1.2. Overview of InfiniBand Architecture
For a more complete overview, the reader is referred to chapter 3 of
the InfiniBand specification.
InfiniBand Architecture (IBA) defines a System Area Network (SAN) for
connecting multiple independent processor platforms, I/O platforms,
and I/O devices. The IBA SAN is a communications and management
infrastructure supporting both I/O and inter-processor communications
for one or more computer systems.
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An IBA SAN consists of processor nodes and I/O units connected
through an IBA fabric made up of cascaded switches and IB routers
(connecting IB subnets). I/O units can range in complexity from a
single Application-specific Integrated Circuit (ASIC) IBA-attached
device (such as a LAN adapter) to a large, memory-rich Redundant
Array of Independent Disks (RAID) subsystem.
An IBA network may be subdivided into subnets interconnected by
routers. These are IB routers and IB subnets and not IP routers or
IP subnets. This document will refer to InfiniBand routers and
subnets as 'IB routers' and 'IB subnets' respectively. The IP
routers and IP subnets will be referred to as 'routers' and
'subnets', respectively.
Each IB node or switch may attach to a single or multiple switches or
directly with each other. Each IB unit interfaces with the link by
way of channel adapters (CAs). The architecture supports multiple
CAs per unit with each CA providing one or more ports that connect to
the fabric. Each CA appears as a node to the fabric.
The ports are the endpoints to which the data is sent. However, each
of the ports may include multiple QPs (Queue Pairs) that may be
directly addressed from a remote peer. From the point of view of
data transfer the QP number (QPN) is part of the address.
IBA supports both connection-oriented and datagram service between
the ports. The peers are identified by QPN and the port identifier.
There are a two exceptions. QPNs are not used when packets are
multicast. QPNs are also not used in the Raw Datagram mode.
A port, in a data packet, is identified by a Local Identifier (LID)
and optionally a Global Identifier (GID). The GID in the packet is
needed only when communicating across an IB subnet, though it may
always be included.
The GID is 128 bits long and is formed by the concatenation of a 64-
bit IB subnet prefix and a 64-bit EUI-64-compliant portion. The
EUI-64 portion of a GID is referred to as the Global Unique
Identifier (GUID; EUI stands for Extended Unique Identifier). The
LID is a 16-bit value that is assigned when the port becomes active.
The GUID is the only persistent identifier of a port. However, it
cannot be used as an address in a packet. If the prefix is modified,
then the GID may change. The subnet manager may attempt to keep the
LID values constant across reboots, but that is not a requirement.
The assignment of the GID and the LID is done by the subnet manager.
Every IB subnet has at least one subnet manager component that
controls the fabric. It assigns the LIDs and GIDs. The subnet
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manager also programs the switches so that they route packets between
destinations. The subnet manager (SM) and a related component, the
subnet administrator (SA), are the central repository of all
information that is required to set-up and bring up the fabric.
IB routers are components that route packets between IB subnets based
on the GIDs. Thus, within an IB subnet a packet may or may not
include a GID but when going across an IB subnet the GID must be
included. A LID is always needed in a packet since the destination
within a subnet is determined by it.
A CA and a switch may have multiple ports. Each CA port is assigned
its own LID or a range of LIDs. The ports of a switch are not
addressable by LIDs/GIDs or, in other words, are transparent to other
end nodes. Each port has its own set of buffers. The buffering is
channeled through virtual lanes (VL) where each VL has its own flow
control. There may be up to 16 VLs.
VLs provide a mechanism for creating multiple virtual links within a
single physical link. All ports must support VL15 which is reserved
exclusively for subnet management datagrams and hence does not
concern the IP over Infiniband (IPoIB) discussions. The actual VL
that a packet uses is configured by the SM in the switch/channel
adapter tables and is determined based on the Service Level (SL)
specified in every packet. There are 16 possible SLs.
In addition to the features described above viz. QPs, SLs, and
addressing (GID/LID), IBA also defines the following:
Partitioning:
Every packet, but for the raw datagrams, carries the partition key
(P_Key). These values are used for isolation in the fabric. A
switch (this is an optional feature) may be programmed by the SM
to drop packets not having a certain key. The CA ports always
check for the P_Keys. A CA port may belong to multiple
partitions. P_Key checking is optional at IB routers.
A P_Key may be described as having 'limited membership' or 'full
membership'. For a packet to be accepted, at least one of the
P_Keys (i.e., the P_Key in the packet or the P_Key in the port)
must be 'full membership' P_Keys.
Q_Keys:
Q_Keys are used to enforce access rights for reliable and
unreliable IB datagram services. Raw datagram services do not use
Q_Keys. At communication establishment, the endpoints exchange
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the Q_Keys and must always use the relevant Q_Keys when
communicating with one another. Multicast packets use the Q_Key
associated with the multicast group.
Q_Keys with the most significant bit set are considered controlled
Q_Keys (such as the General Service Interface (GSI) Q_Key
[IB_ARCH]) and a Host Channel Adapter (HCA) does not allow a
consumer to arbitrarily specify a controlled Q_Key. An attempt to
send a controlled Q_Key results in using the Q_Key in the QP
context. Thus, the Operating System maintains control since it
can configure the QP context for the controlled Q_Key for
privileged consumers. It must be noted that though the notion of
a 'controlled Q_Key' is suggested by IB specification, it does not
require its use or implementation.
Multicast support:
A switch may support multicasting, that is, replication of packets
across multiple output ports. This is an optional feature.
Similarly, support for sending/receiving multicast packets is
optional in CAs. A multicast group is identified by a GID. The
GID format is as defined in RFC 2373 on IPv6 addressing [IB_ARCH].
Thus, from an IPv6-over-InfiniBand point of view, the data link
multicast address looks like the network address. An IB port must
explicitly join a multicast group by sending a request to the SM
to receive multicast packets. A port may send packets to any
multicast group. In both cases, the multicast LID to be used in
the packets is received from the SM.
There are six methods for data transfer in IB architecture:
1. Unreliable Datagram (unacknowledged - connectionless)
The Unreliable Datagram (UD) service is connectionless and
unacknowledged. It allows the QP to communicate with any
unreliable datagram QP on any node.
The switches and hence each link can support only a certain
MTU. The MTU ranges are 256 octets, 512 octets, 1024 octets,
2048 octets, and 4096 octets. A UD packet cannot be larger
than the link MTU between the two peers.
2. Reliable Datagram (acknowledged - multiplexed)
The Reliable Datagram (RD) service is multiplexed over
connections between nodes called End-to-End Contexts (EEC),
which allows each RD QP to communicate with any RD QP on any
node with an established EEC. Multiple QPs can use the same
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EEC and a single QP can use multiple EECs (one for each remote
node per reliable datagram domain).
3. Reliable Connected (acknowledged - connection oriented)
The Reliable Connected (RC) service associates a local QP with
one and only one remote QP. The message sizes maybe as large
as 2^31 octets in length. The CA implementation takes care of
segmentation and assembly.
4. Unreliable Connected (unacknowledged - connection oriented)
The Unreliable Connected (UC) service associates one local QP
with one and only one remote QP. There is no acknowledgement
and hence no resend of lost or corrupted packets. Such packets
are therefore simply dropped. It is similar to RC otherwise.
5. Raw Ethertype (unacknowledged - connectionless)
The Ethertype raw datagram packet contains a generic transport
header that is not interpreted by the CA but it specifies the
protocol type. The values for ethertype are the same as
defined by Internet Assigned Numbers Authority (IANA) [IANA]
for ethertype.
6. Raw IPv6 (unacknowledged - connectionless)
Using IPv6 raw datagram service, the IBA CA can support
standard protocol layers atop IPv6 (such as TCP/UDP). Thus,
native IPv6 packets can be bridged into the IBA SAN and
delivered directly to a port and to its IPv6 raw datagram QP.
The first four types are referred to as IB transports. The latter
two are classified as raw datagrams. There is no indication of the
QP number in the raw datagram packets. The raw datagram packets are
limited by the link MTU in size.
The two connected modes and the Reliable Datagram mode may also
support Automatic Path Migration (APM). This is an optional facility
that provides for a hardware based path fail over. An alternate path
is associated with the QP when the connection/EE context is first
created. If unrecoverable errors are encountered, the connection
switches to using the alternative path.
1.2.1. InfiniBand Addresses
The InfiniBand architecture borrows heavily from the IPv6
architecture in terms of the InfiniBand subnet structure and GIDs.
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The InfiniBand architecture defines the GID associated with a port as
a 128-bit unicast or multicast identifier. IBA derives the GID
address format, as defined in RFC 2373 [IB_ARCH], with some
additional properties/restrictions defined to facilitate efficient
discovery, communication, and routing.
Note: The IBA explicitly refers to RFC 2373, which is obsolete
[RFC 3513]. It must be noted that IBA is therefore unaffected by
any further changes that are introduced in IPv6 addressing
architecture.
IBA defines two types of GIDs: unicast and multicast.
1.2.1.1. Unicast GIDs
The unicast GIDs are defined, as in IPv6, with three scopes. The IB
specification states the following:
a. link local: FE80/10.
The IB routers will not forward packets with a link-
local address in source or destination beyond the IB
subnet.
b. site local: FEC0/10
A unicast GID used within a collection of subnets
that is unique within that collection (e.g., a data
center or campus) but is not necessarily globally
unique. IB routers must not forward any packets with
either a site-local Source GID or a site-local
Destination GID outside of the site.
c. global:
A unicast GID with a global prefix; an IB router may
use this GID to route packets throughout an
enterprise or internet.
1.2.1.2. Multicast GIDs
The multicast GIDs also parallel the IPv6 multicast addresses. The
IB specification defines the multicast GIDs as follows:
FFxy:<112 bits>
Flag bits:
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The nibble, denoted by x above, are the 4 flag bits: 000T.
The first 3 bits are reserved and are set to zero. The last
bit is defined as follows:
T=0: denotes a permanently assigned, that is, well-known GID
T=1: denotes a transient group
Scope bits:
The 4 bits, denoted by y in the GID above, are the scope bits.
These scope values are described in Table 1.
scope value Address value
0 Reserved
1 Unassigned
2 Link-local
3 Unassigned
4 Unassigned
5 Site-local
6 Unassigned
7 Unassigned
8 Organization-local
9 Unassigned
0xA Unassigned
0xB Unassigned
0xC Unassigned
0xD Unassigned
0xE Global
0xF Reserved
Table 1
The IB specification further refers to RFC 2373 and RFC 2375 while
defining the well-known multicast addresses. However, it then states
that the well-known addresses apply to IB raw IPv6 datagrams only.
It must be noted though that a multicast group can be associated with
only a single Multicast Global Identifier (MGID). Thus the same MGID
cannot be associated with the UD mode and the Raw Datagram mode.
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1.3. InfiniBand Multicast Group Management
IB multicast groups, identified by MGIDs, are managed by the SM. The
SM explicitly programs the IB switches in the fabric to ensure that
the packets are received by all the members of the multicast group
that request the reception of packets. The SM also needs to program
the switches such that packets transmitted to the group by any group
member reach all receivers in the multicast group.
IBA distinguishes between multicast senders and receivers. Though
all members of a multicast group can transmit to the group (and
expect their packets to be correctly forwarded), not all members of
the group are receivers. A port needs to explicitly request that
multicast packets addressed to the group be forwarded to it.
A multicast group is created by sending a join request to the SM. As
will be explained later, IBA defines multiple modes for joining a
multicast group. The subnet manager records the group's multicast
GID and the associated characteristics. The group characteristics
are defined by the group path MTU, whether the group will be used for
raw datagrams or unreliable datagrams, the service level, the
partition key associated with the group, the Local Identifier (LID)
associated with the group, and so on. These characteristics are
defined at the time of the group creation. The interested reader may
look up the 'MCMemberRecord' attribute in the IB architecture
specification [IB_ARCH] for the complete list of characteristics that
define a group.
A LID is associated with the multicast group by the SM at the time of
the multicast group creation. The SM determines the multicast tree
based on all the group members and programs the relevant switches.
The Multicast LID (MLID) is used by the switches to route the
packets.
Any member IB port wanting to participate in the multicast group must
join the group. As part of the join operation, the node receives the
group characteristics from the SM. At the same time, the subnet
manager ensures that the requester can indeed participate in the
group by verifying that it can support the group MTU and its
accessibility to the rest of the group members. Other group
characteristics may need verification too.
The SM, for groups that span IB subnet boundaries, must interact with
IB routers to determine the presence of this group in other IB
subnets. If present, the MTU must match across the IB subnets.
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P_Key is another characteristic that must match across IB subnets
since the P_Key inserted into a packet is not modified by the IB
switches or IB routers. Thus, if the P_Keys did not match the IB
router(s) itself might drop the packets or destinations on other
subnets might drop the packets.
A join operation may cause the SM to reprogram the fabric so that the
new member can participate in the multicast group. By the same
token, a leave may cause the SM to reprogram the fabric to stop
forwarding the packets to the requester.
1.3.1. Multicast Member Record
The multicast group is maintained by the SM with each of the group
members represented by an MCMemberRecord [IB_ARCH]. Some of its
components are the following:
MGID - Multicast GID for this multicast group
PortGID - Valid GID of the port joining this multicast group
Q_Key - Q_Key to be used by this multicast group
MLID - Multicast LID for this multicast group
MTU - MTU for this multicast group
P_Key - Partition key for this multicast group
SL - Service level for this multicast group
Scope - Same as MGID address scope
JoinState - Join/Leave status requested by the port:
bit 0: FullMember
bit 1: NonMember
bit 2: SendOnlyNonMember
1.3.1.1. JoinState
The JoinState indicates the membership qualities a port wishes to add
while joining/creating a group or delete when leaving a group. The
meaning of the JoinState bits are as follows:
FullMember:
Messages destined for the group are routed to and from the
port. A group may be deleted by the SM if there are no
FullMembers in the group.
NonMember:
Messages destined for the group are routed to and from the
port. The port is not considered a member for purposes of
group creation/deletion.
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SendOnlyNonMember:
Group messages are only routed from the port but not to the
port. The port is not considered a member for purposes of
group creation/deletion.
A port may have multiple bits set in its record. In such a case, the
membership qualities are a union of the JoinStates. A port may leave
the multicast group for each of the JoinStates individually or in any
combination of JoinState bits [IB_ARCH].
1.3.2. Join and Leave Operations
An IB port joins a multicast group by sending a join request
(SubnAdmSet() method) and leaves a multicast group by sending a leave
message (SubnAdmDelete() method) to the SM. The IBA specification
[IB_ARCH] describes the methods and attributes to be used when
sending these messages.
1.3.2.1. Creating a Multicast Group
There is no 'create' command to form a new multicast group. The
FullMember bit in the JoinState must be set to create a multicast
group. In other words, the first FullMember join request will cause
the group to be created as a side effect of the join request.
Subsequent join or leave requests may contain any combination of the
JoinState bits.
The creator of the group specifies the Q_Key, MTU, P_Key, SL,
FlowLabel, TClass, and the Scope value. A creator may request that a
suitable MGID be created for it. Alternatively, the request can
specify the desired MGID. In both cases, the MLID is assigned by the
SM.
Thus, a group will be created with the specified values when the
requester sets the FullMember bit and no such group already exists in
the subnet.
1.3.2.2. Deleting a Multicast Group
When the last FullMember leaves the multicast group the SM may delete
the multicast group releasing all resources, including those that
might exist in the fabric itself, associated with the group.
Note that a special 'delete' message does not exist. It is a side
effect of the last FullMember 'leave' operation.
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1.3.2.3. Multicast Group Create/Delete Traps
The SA may be requested by the ports to generate a report whenever a
multicast group is created or deleted. The port can specify the
multicast group(s) it is interested in by using its MGID or by
submitting a wild card request. The SA will report these events
using traps 66 (for creates) and 67 (for deletes)[IB_ARCH].
Therefore, a port wishing to join a group but not create it by itself
may request a create notification or a port might even request a
notification for all groups that are created (a wild card request).
The SA will diligently inform them of the creation utilizing the
aforementioned traps. The requester can then join the multicast
group indicated. Similarly, a SendOnlyNonMember or a NonMember might
request the SA to inform it of group deletions. The endnode, on
receiving a delete report, can safely release the resources
associated with the group. The associated MLID is no longer valid
for the group and may be reassigned to a new multicast group by the
SM.
2. Management of InfiniBand Subnet
To aid in the monitoring and configuration of InfiniBand subnet
components, a set of MIB modules needs to be defined. MIB modules
are needed for the channel adapters, InfiniBand interfaces,
InfiniBand subnet manager, and InfiniBand subnet management agents
and to allow the management of specific device properties. It must
be noted that the management objects addressed in the IPoIB documents
are for all of the IB subnet components and are not limited to IP
(over IB). The relevant MIB modules are described in separate
documents and are not covered here.
3. IP over IB
As described in section 1.0, the InfiniBand architecture provides a
broad set of capabilities to choose from when implementing IP over
InfiniBand networks.
The IPoIB specification must not, and does not, require changes in IP
and higher-layer protocols. Nor does it mandate requirements on IP
stacks to implement special user-level programs. It is an aim of
IPoIB specification that the IPoIB changes be amenable to
modularization and incorporation into existing implementations at the
same level as other media types.
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3.1. InfiniBand as Datalink
InfiniBand architecture provides multiple methods of data exchange
between two endpoints as was noted above. These are the following:
Reliable Connected (RC)
Reliable Datagram (RD)
Unreliable Connected (UC)
Unreliable Datagram (UD)
Raw Datagram : Raw IPv6 (R6)
: Raw Ethertype (RE)
IPoIB can be implemented over any, multiple, or all of these
services. A case can be made for support on any of the transport
methods depending on the desired features.
The IB specification requires Unreliable Datagram mode to be
supported by all the IB nodes. The host channel adapters (HCAs) are
specifically required to support Reliable connected (RC) and
Unreliable connected (UC) modes but the same is not the case with
target channel adapters (TCAs). Support for the two Raw Datagram
modes is entirely optional. The Raw Datagram mode supports a 16-bit
Cyclic Redundancy Check (CRC) as compared to the better protection
provided by the use of a 32-bit CRC in other modes.
For the sake of simplicity, ease of implementation and integration
with existing stacks, it is desirable that the fabric support
multicasting. This is possible only in Unreliable datagram (UD) and
IB's Raw datagram modes.
Thus, it is only the UD mode that is universal, supports multicast,
and supports a robust CRC. Given these conditions it is the obvious
choice for IP over InfiniBand [RFC 4391].
Future documents might consider the connected modes. In contrast to
the limited link MTU offered by UD mode, the connected modes can
offer significant benefit in terms of performance by utilizing a
larger MTU. Reliability is also enhanced if the underlying feature
of automatic path migration of connected modes is utilized.
3.2. Multicast Support
InfiniBand specification makes support of multicasting in the
switches optional. Multicast however, is a basic requirement in IP
networks. Therefore, IPoIB requires that multicast-capable
InfiniBand fabrics be used to implement IPoIB subnets.
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3.2.1. Mapping IP Multicast to IB Multicast
Well-known IP multicast groups are defined for both IPv4 and IPv6
[IANA, RFC 3513]. Multicast groups may also be dynamically created at
any time. To avoid creating unnecessary duplicates of multicast
packets in the fabric, and to avoid unnecessary handling of such
packets at the hosts, each of the IP multicast groups needs to be
associated with a different IB multicast group as far as possible. A
process is defined in [RFC 4391] for mapping the IP multicast
addresses to unique IB multicast addresses.
3.2.2. Transient Flag in IB MGIDs
The IB specification describes the flag bits as discussed in section
1.2. The IB specification also defines some well-known IB MGIDs.
The MGIDs are reserved for the IB's Raw Datagram mode which is
incompatible with the other transports of IB. Any mapping that is
defined from IP multicast addresses therefore must not fall into IB's
definition of a well-known address.
Therefore all IPoIB related multicast GIDs always set the transient
bit.
3.3. IP Subnets Across IB Subnets
Some implementations may wish to support multiple clusters of
machines in their own IB subnets but otherwise be part of a common IP
subnet. For such a solution, the IB specification needs multiple
upgrades. Some of the required enhancements are as follows:
1) A method for creating IB multicast GIDs that span multiple IB
subnets. The partition keys and other parameters need to be
consistent across IB subnets.
2) Develop IB routing protocol to determine the IB topology across IB
subnets.
3) Define the process and protocols needed between IB nodes and IB
routers.
Until the above conditions are met, it is not possible to implement
IPoIB subnets that span IB subnets. The IPoIB standards have
however, been defined with this possibility in mind.
4. IP Subnets in InfiniBand Fabrics
The IPoIB subnet is overlaid over the IB subnet. The IPoIB subnet is
brought up in the following steps:
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Note: the join/leave operation at the IP level will be referred to as
IP_join/IP_leave and the join/leave operations at the IB level
will be referred to as IB_join in this document.
1. The all-IPoIB nodes IB multicast group is created
The fabric administrator creates an IB multicast group (henceforth
called 'broadcast group') when the IP subnet is set up. The
'broadcast group' is defined in [RFC 4391]. The method by which
the broadcast group is setup is not defined by IPoIB. The group
may be setup at the SM by the administrator or by the first
IB_join.
As noted earlier, at the time of creating an IB multicast group,
multiple values such as the P_Key, Q_Key, Service Level, Hop
Limit, Flow ID, TClass, MTU, etc. have to be specified. These
values should be such that all potential members of the IB
multicast group are able to communicate with one another when
using them. In the future, as the IB specification associates
more meaning with the various parameters and defines IB Quality of
Service (QoS), different values for IP multicast traffic may be
possible. All unicast packets also need to use the P_Key and
Q_Key specified in the broadcast group [RFC 4391]. It is obvious
that a thought out configuration is required for a successful
setup of the IPoIB subnet.
2. All IPoIB interfaces IB_join the broadcast group
The broadcast group defines the span and the members of the IPoIB
link. This link gets built up as IPoIB nodes IB_join the
broadcast group.
The IB_join to the broadcast group has the additional benefit of
distributing the above mentioned multicast group parameters to all
the members of the subnet.
Note that this IB_join to the broadcast group is a FullMember
join. If any of the ports or the switches linking the port to the
rest of the IPoIB subnet cannot support the parameters (e.g., path
MTU or P_Key) associated with the broadcast group, then the
IB_join request will fail and the requesting port will not become
part of the IPoIB subnet.
3. Configuration Parameters
As noted above, parameters such as Q_Key and Path MTU, which are
needed for all IPoIB communication, are returned to the IPoIB node
on IB_joining the 'broadcast group'. [RFC 4391] also notes that
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the parameters used in the broadcast group are used when creating
other multicast groups.
However, the P_Key must still be known to the IPoIB endnode before
it can join the broadcast group. The P_Key is included in the
mapping of the broadcast group [RFC 4391]. Another parameter, the
scope of the broadcast group, also needs to be known to the
endnode before it can join the broadcast group. It is an
implementation choice on how the P_Key and the scope bits related
to the IPoIB subnet are determined by the implementation. These
could be configuration parameters initialized by some means by the
administrator.
The methods employed by an implementation to determine the P_Key
and scope bits are not specified by IPoIB.
4.1. IPoIB VLANs
The endpoints in an IB subnet must have compatible P_Keys to
communicate with one another. Thus, the administrator when setting
up an IP subnet over an IB subnet must ensure that all the members
have compatible P_Keys. An IP subnet can have only one P_Key
associated with it to ensure that all IP nodes in it can talk to one
another. An endpoint may, however, have multiple P_Keys.
The IB architecture specifies that there can be only one MGID
associated with a multicast group in the IB subnet. The P_Key is
included in the MGID mappings from the IP multicast addresses
[RFC 4391]. Since the P_Key is unique in the IB subnet, the inclusion
of the P_Key in the IB MGIDs ensures that unique MGID mappings are
created. Every unique broadcast group MGID so formed creates a
separate abstract IPoIB link and hence an IPoIB VLAN.
4.2. Multicast in IPoIB subnets
IP multicast on InfiniBand subnets follows the same concepts and
rules as on any other media. However, unlike most other media
multicast over InfiniBand requires interaction with another entity,
the IB subnet manager. This section describes the outline of the
process and suggests some guidelines.
IB architecture specifies the following format for IB multicast
packets when used over Unreliable Datagram (UD) mode:
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+--------+-------+---------+---------+-------+---------+---------+
|Local |Global |Base |Datagram |Packet |Invariant| Variant |
|Routing |Routing|Transport|Extended |Payload| CRC | CRC |
|Header |Header |Header |Transport| (IP) | | |
| | | |Header | | | |
+--------+-------+---------+---------+-------+---------+---------+
For details about the various headers please refer to InfiniBand
Architecture Specification [IB_ARCH].
The Global Routing Header (GRH) includes the IB multicast group GID.
The Local Routing Header (LRH) includes the Local Identifier (LID).
The IB switches in the fabric route the packet based on the LID.
The GID is made available to the receiving IB user (the IPoIB
interface driver for example). The driver can therefore determine
the IB group the packet belongs to.
IPv4 defines three levels of multicast conformance [RFC 1112].
Level 0: No support for IP multicasting
Level 1: Support for sending but not receiving multicasts
Level 2: Full support for IP multicasting
In IPv6, there is no such distinction. Full multicast support is
mandatory. In addition, all IPv4 subnets support broadcast
(255.255.255.255). IPv4 broadcast can always be sent/received by all
IPv4 interfaces.
Every IPoIB subnet requires the broadcast GID to be defined. Thus, a
packet can always be broadcast.
4.2.1. Sending IP Multicast Datagrams
An IP host may send a multicast packet at any time to any multicast
address.
The IP layer conveys the multicast packet to the IPoIB interface
driver/module. This module attempts to IB_join the relevant IB
multicast group. This is required since otherwise InfiniBand
architecture does not guarantee that the packet will reach its
destinations.
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A pure sender may choose to join the multicast group as a FullMember.
In such a case, the sender will receive all the multicast packets
transmitted to the IB group. In addition, the IB group will not be
deleted until the sender leaves the group.
Alternatively, a sender might IB_join as a SendOnlyNonMember. In
such a case, the packets are not routed to the sender though packets
transmitted by it can reach the other group members. In addition,
the group can be deleted when all FullMembers have left the group.
The sender can further request delete updates from the SM.
If the sender does not find the group in existence, it is recommended
in [RFC 4391] that the packets be sent to the MGID corresponding to
the all-IP routers address. A sender could also send the packets to
the broadcast group. The sender might also choose to request
'creation' reports from the SM.
4.2.2. Receiving Multicast Packets
The IP host must join the IB multicast group corresponding to the IP
address. This follows from the IBA requirement that the receiver
must join the relevant IB multicast group. The group is
automatically created if it does not exist [IB_ARCH].
The IP receivers must IB_leave the IB group when the IP layer stops
listening of the corresponding IP address. The SM can then choose to
delete the group.
4.2.3. Router Considerations for IPoIB
IP routers know of the new IP groups created in the subnet by the use
of protocols such as Internet Group Management Protocol (IGMPv3) /
Multicast Listener Discovery (MLD) [RFC 3376, RFC 2710]. However, this
is not enough for IPoIB since the router needs to IB_join the
relevant IB groups to be able to receive and transmit the packets.
There is no promiscuous mode for listening to all packets.
The IPoIB routers therefore need to request the SM to report all
creations of IB groups in the fabric. The IPoIB router can then
IB_join the reported group. It is not desirable that the router's
IB_joining of a multicast group be considered the same as the IB_join
from a receiver -- the router's IB_join should not disallow the
group's deletion when all receivers leave. To overcome just this
type of situation, IBA provides the NonMember IB_join mode.
The NonMember IB_join mode can be used by IP routers when they join
in response to the create reports. A router should ideally request
the delete reports too so that it can release all the resources
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associated with the group. The MLID associated with a deleted MGID
can be reassigned by the SM, and therefore there is a possibility of
erroneous transmissions if the MLID is cached. A router that does
not request delete reports will still work correctly since it will
receive the correct MLID , and purge any old cached value, when it
IB_joins the IB group in response to a create report.
It is reasonable for a router to IB_join as a FullMember if it is
joining the IB group in response to an application/routing daemon
request. In such a case, the router might end up controlling the
existence of the IB group (since it is a FullMember of the group).
4.2.4. Impact of InfiniBand Architecture Limits
An HCA or TCA may have a limit on the number of MGIDs it can support.
Thus, even though the groups may not be limited at the subnet manager
and in the subnet as such, they may be limited at a particular
interface. It is advisable to choose an adequately provisioned
HCA/TCA when setting up an IPoIB subnet.
4.2.5. Leaving/Deleting a Multicast Group
An IPv4 sender (level 1 compliance) IB_joins the IB multicast group
only because that is the only way to guarantee reception of the
packets by all the group recipients. The sender must, however,
IB_leave the group at some time. A sender could, when not a receiver
on the group, start a timer per multicast group sent to. The sender
leaves the IB group when the timer goes off. It restarts the timer
if another message is sent.
This suggestion does not apply to the IB broadcast group. It also
does not apply to the IB group corresponding to the all-hosts
multicast group. An IPv4 host must always remain a member of the
broadcast group.
An IP multicast receiver IB_leaves the corresponding IB multicast
group when it IP_leaves the IP multicast group. In the case of IPv4
implementation, the receiver may choose to continue to be a sender
(level 1 compliance), in which case it may choose not to IB_leave the
IB group but start a timer as explained above.
As noted elsewhere, the SM can choose to free up the resources (e.g.,
routing entries in the switches) associated with the IB group when
the last FullMember IB_leaves the group. The MLID therefore becomes
invalid for the group. The MLID can be reassigned when a new group
is created.
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SendOnlyNonMember/NonMember ports caching the MLID need to avoid this
possibility. The way out is for them to request group delete
reports. An IP router requesting reports for all groups need not
request the delete report since an IB_join in response to a create
report will return the new MLID association to it.
A router might prefer to IB_leave the IB multicast group when there
are no members of the IP multicast address in the subnet and it has
no explicit knowledge of any need to forward such packets.
4.3. Transmission of IPoIB Packets
The encapsulation of IP packets in InfiniBand is described in
[RFC 4391].
It specifies the use of an 'Ethertype' value [IANA] in all IPoIB
communication packets. The link-layer address is comprised of the
GID and the Queue Pair Number (QPN) [RFC 4391].
To enable IPoIB subnets to span across multiple IB-subnets, the
specification utilizes the GID as part of the link-layer address.
Since all packets in IB have to use the Local Identifier (LID), the
address resolution process has the additional step of resolving the
destination GID, returned in response to Address Resolution Protocol
(ARP) / Neighbor Discover (ND) request, to the LID [RFC 4391]. This
phase of address resolution might also be used to determine other
essential parameters (e.g., the SL, path rate, etc.) for successful
IB communication between two peers.
As noted earlier, all communication in the IPoIB subnet derives the
Q_Key to use from the Q_Key specified in the broadcast group.
4.4. Reverse Address Resolution Protocol (RARP) and Static ARP Entries
RARP entries or static ARP entries are based on invariant link
addresses. In the case of IPoIB, the link address includes the QPN,
which might not be constant across reboots or even across network
interface resets. Therefore, static ARP entries or RARP server
entries will only work if the implementation(s) using these options
can ensure that the QPN associated with an interface is invariant
across reboots/network resets [RFC 4391].
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4.5. DHCPv4 and IPoIB
DHCPv4 [RFC 2131] utilizes a 'client identifier' field (expected to
hold the link-layer address) of 16 octets. The address in the case
of IPoIB is 20 octets. To get around this problem, IPoIB specifies
[RFC 4390] that the 'broadcast flag' be used by the client when
requesting an IP address.
5. QoS and Related Issues
The IB specification suggests the use of service levels for load
balancing, QoS, and deadlock avoidance within an IB subnet. But the
IB specification leaves the usage and mode of determination of the SL
for the application to decide. The SL and list of SLs are available
in the SA, but it is up to the endnode's application to choose the
'right' value.
Every IPoIB implementation will determine the relevant SL value based
on its own policy. No method or process for choosing the SL has been
defined by the IPoIB standards.
6. Security Considerations
This document describes the IB architecture as relevant to IPoIB. It
further restates issues specified in other documents. It does not
itself specify any requirements. There are no security issues
introduces by this document. IPoIB-related security issues are
described in [RFC 4391] and [RFC 4390].
7. Acknowledgements
This document has benefited from the comments and suggestions of the
members of the IPoIB working group and the members of the
InfiniBand(SM) Trade Association.
8. References
8.1. Normative References
[IB_ARCH] InfiniBand Architecture Specification, Volume 1,
Release 1.2, October, 2004.
[RFC 4391] Chu, J. and V. Kashyap, "Transmission of IP over
InfiniBand (IPoIB)", RFC 4391, April 2006.
[RFC 4390] Kashyap, V., "Dynamic Host Configuration Protocol
(DHCP) over InfiniBand", RFC 4390, April 2006.
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RFC 4392 IPoIB Architecture April 2006
[RFC 2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
8.2. Informative References
[RFC 3513] Hinden, R. and S. Deering, "Internet Protocol Version 6
(IPv6) Addressing Architecture", RFC 3513, April 2003.
[RFC 2375] Hinden, R. and S. Deering, "IPv6 Multicast Address
Assignments", RFC 2375, July 1998.
[IANA] Internet Assigned Numbers Authority, URL
http://www.iana.org
[RFC 1112] Deering, S., "Host extensions for IP multicasting", STD
5, RFC 1112, August 1989.
[RFC 3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol,
Version 3", RFC 3376, October 2002.
[RFC 2710] Deering, S., Fenner, W., and B. Haberman, "Multicast
Listener Discovery (MLD) for IPv6", RFC 2710, October
1999.
Author's Address
Vivek Kashyap
IBM
15450, SW Koll Parkway
Beaverton, OR 97006
Phone: +1 503 578 3422
EMail: vivk@us.ibm.com
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RFC 4392 IPoIB Architecture April 2006
Full Copyright Statement
Copyright © The Internet Society (2006).
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RFC TOTAL SIZE: 53506 bytes
PUBLICATION DATE: Thursday, April 6th, 2006
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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