INTERNET-DRAFT H.K. Jerry Chu
<draft-ietf-ipoib-link-multicast-00.txt> Sun Microsystems
Vivek Kashyap
IBM
Expires: July, 2002 January, 2002
IP link and multicast over InfiniBand networks
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Copyright (C) The Internet Society (date). All Rights Reserved.
Abstract
This document specifies a method for setting up IP subnets and
multicast services over InfiniBand(TM) networks. Discussions in this
document are applicable to both IPv4 and IPv6, unless explicitly
specified. A separate document will cover unicast and encapsulation
of IP datagrams over InfiniBand networks.
1. Introduction
InfiniBand Architecture (IBA) defines four layers of network services
corresponding to layer one through layer four of the OSI reference
model. For the purpose of running IP over an InfiniBand (IB)
network, the IB network and all its link, network, and transport
Chu & Kashyap [Page 1]
draft-ietf-ipoib-link-multicast-00.txt January 2002
layers collectively constitute the data link layer to the IP stack.
An IP network is often divided into many subnets connected by IP
routers. Subnetting allows the containment of broadcast traffic
within a single subnet. It also provides certain degree of isolation
between nodes on different subnets. The latter may be an important
consideration for administration purpose.
This document will focus on all the steps required to lay out an IP
network on top of an IB network. It will describe all the elements an
IP over InfiniBand (IPoIB) link consists of, how to configure its
associated link attributes, and how to set up basic broadcast and
multicast services on an IPoIB link.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Basic IPoIB Transport - Unreliable Datagram
InfiniBand defines four types of transport services [IBTA]. They are
reliable connection, unreliable connection, reliable datagram,
unreliable datagram. IBA also defines a special raw datagram service
for encapsulation purpose. Both unreliable datagram and raw datagram
define support for multicast. They provide the basic transport
mechanism that best matches the IP datagram paradigm.
IB unreliable datagram provides many additional transport features
such as the partition key (P_Key) protection, multiple queue pairs
(QPs), and Q_Key protection. Moreover, it requires a 32-bit invariant
CRC checksum, which provides a much stronger protection against data
corruption, compared with the 16-bit CRC that a raw datagram carries.
For these reasons, IB unreliable datagram is considered to be a much
better choice as the basic IPoIB transport than the raw datagram, and
is chosen as the default IPoIB transport mechanism for the rest of
discussions in this document.
An IB unreliable datagram contains the following headers:
o Local Route Header (LRH) - provides IB link-layer addressing
information. An IB link layer address is based on a 16-bit
identifier called Local Identifier (LID), and is used by IB
switches to relay packets within an IB subnet.
o Global Route Header (GRH) - provides routing information for IB
Chu & Kashyap [Page 2]
draft-ietf-ipoib-link-multicast-00.txt January 2002
routers to relay packets between IB subnets inside an IB fabric.
GRH is only required for all multicast packets and any unicast
packet that is destined to a node in a different IB subnet. GRH
carries an IB network-layer address, which is an 128-bit
identifier called Global Identifier (GID) that closely mimics IPv6
addressing architecture [AARCH].
o Base Transport Header (BTH) - provides various information,
including P_Key, destination queue pair number (QPN) for IB
transport services.
o Datagram Extended Header (DETH) - provides additional IB
information such as Q_Key, source queue pair number for datagram
services.
From the perspective of IP over IB encapsulation, all the above IB
headers are considered as link layer encapsulation for IP datagrams.
4. IB Multicast Architecture
IBA defines two layers of multicast services. Its link layer uses
multicast LIDs (MLIDs), which are allocated by the Subnet Manager
(SM) and fall in the range between 0xC0000 to 0xFFFE (approximately
16k). MLIDs are used by IB switches to program their multicast
forwarding tables. An IB switch implementation may support much fewer
MLIDs in its forwarding table though.
IB network layer uses multicast GIDs (MGIDs), which closely resemble
IPv6 multicast addresses [AARCH] as shown in the following.
| 8 | 4 | 4 | 112 bits |
+------ -+----+----+---------------------------------------------+
|11111111|flgs|scop| group ID |
+--------+----+----+---------------------------------------------+
11111111 at the start of the address identifies the address as
being a multicast address.
+-+-+-+-+
flgs is a set of 4 flags: |0|0|0|T|
+-+-+-+-+
The high-order 3 flags are reserved, and must be initialized to
0.
T = 0 indicates a permanently-assigned ("well-known") multicast
address, assigned by the global internet numbering authority.
Chu & Kashyap [Page 3]
draft-ietf-ipoib-link-multicast-00.txt January 2002
T = 1 indicates a non-permanently-assigned ("transient")
multicast address.
scop is a 4-bit multicast scope value used to limit the scope of
the multicast group. The values are:
0 reserved
1 node-local scope
2 link-local scope
3 (unassigned)
4 (unassigned)
5 site-local scope
6 (unassigned)
7 (unassigned)
8 organization-local scope
9 (unassigned)
A (unassigned)
B (unassigned)
C (unassigned)
D (unassigned)
E global scope
F reserved
group ID identifies the multicast group, either permanent or
transient, within the given scope.
MGIDs are mainly used by IB routers when forwarding multicast packets
to remote IB subnets that are part of a multicast forwarding tree.
Since every IB multicast packet is required to carry both LRH and
GRH, a valid MGID and a valid MLID are both needed before a valid IB
multicast packet can be constructed.
An IB multicast group is uniquely identified by a valid MGID. Before
a MGID can be used within an IB subnet, either as a destination
address of a multicast packet, or representing a multicast group that
an IB node can join, a "MCGroupRecord" corresponding to the MGID must
be created through the Subnet Administrator (SA). Besides the MGID,
the creator must supply values of the path MTU, Q_Key, TClass,
FlowLabel, HopLimit that are appropriate for all the potential
clients of the multicast group to use. In return, SA will allocate a
MLID to be used by switches in the local IB subnet.
Note that MLIDs are allocated and managed by SM when new MGIDs are
created though the creation of MCGroupRecords. The number of valid
MLIDs that are available in a given IB subnet is limited by the
implementation-dependent size of multicast forwarding table of IB
switches. Since the number can be small, reuses of MLIDs for MGIDs
may be inevitable. Implementation should nevertheless avoid sharing
Chu & Kashyap [Page 4]
draft-ietf-ipoib-link-multicast-00.txt January 2002
the same MLID among high volume multicast groups in order to reduce
software filtering overhead and attain higher efficiency.
Unreliable multicast is defined by IBA as an optional functionality
for channel adaptors (CAs) and switches. In today's IP technology,
link multicast has become an indispensable function for better
supporting a modern IP network. For this reason, it is required that
an IPoIB fabric supports multicast. This includes all the CAs and
switches that make up an IP network.
5. IB Links vs IPoIB Links
A link segment on top of which an IP subnet can be configured is
defined in [IPV6] as a communication facility or medium over which
nodes can communicate at the "link" layer. For most types of
communication media, the boundary between different data link
segments follows the physical topology of the network connectivity,
and is pretty obvious. E.g. an Ethernet network connected by
switches, hubs, or bridges usually forms a single link segment and
broadcast/multicast domain. Different Ethernet segments can be
connected by IP routers at the network layer.
InfiniBand defines its own link-layer and subnets consisting of nodes
connected by IB switches. However, the IPoIB link boundary needs not
follow the IB link boundary. Nodes residing on different IB subnets
can still communicate directly with one another through IB routers at
the InfiniBand network layer. The same applies to multicast as well.
I.e. nodes on the same IB subnet can exchange multicast packets with
one another all within the same subnet through the IB link multicast
facility. But even nodes on different IB subnets can still exchange
multicast packets with one another using IB network-layer multicast.
The ultimate requirement for two nodes in the same IB fabric to
communicate at the IB level, besides the physical connectivity, is a
common P_Key.
Partitioning in IB provides an isolation mechanism among nodes in an
IB fabric, much like VLANs in an Ethernet network. Each port of an
endnode contains a P_Key table of all the valid P_Keys the port is
allowed to use. The P_Key table is set up by the SM of the local IB
subnet. Each QP is programmed with a P_Key from the local P_Key
table. This P_Key is carried in the BTH of all the outgoing packets
from the QP, and is used to compare against the P_Key in the BTH of
all the incoming packets to the QP. Reception of an invalid P_Key
causes the packet to be discarded. IB switches may optionally enforce
partition checking too.
Therefore P_Key and IB partition are the most natural choice for
Chu & Kashyap [Page 5]
draft-ietf-ipoib-link-multicast-00.txt January 2002
defining IPoIB link boundary. It also affords much flexibility to the
network administrators when different links are set up in a large
network. This is very similar to VLANs in Ethernet.
6. Setting up an IPoIB Link
A network administrator defines an IPoIB link by setting up an IB
partition and assigning it a unique P_Key. An IB partition may or may
not span multiple IB subnets. But whether it does or not is mostly
irrelevant to IPoIB.
Each node attached to the IB partition MUST have one of its CA
assigned the P_Key to use.
Once an IB partition is established for IPoIB use, the link MTU and
Q_Key are two other important attributes that must be chosen before
the IPoIB link can be configured.
6.1 Maximum Transmission Unit
IB defines five permissible maximum payload sizes. They are 256, 512,
1024, 2048 and 4096 bytes. [IPV6] requires a link MTU of 1280 bytes
or greater. This leaves only 2048 and 4096 bytes as two acceptable
choices for IPv6. Channel adaptors supporting a maximum payload size
less than the minimal MTU requirement can still expose an acceptable
link MTU to IP through an adaptation layer that fragments larger
messages into smaller IB packets, and reassembles them on the
receiving end. But this must be done in a way that is completely
transparent to the IP stack.
It is up to the network administrator to select a link MTU to use
when configuring an IPoIB link. The link MTU SHALL not be greater
than the maximum payload size of any CA or switch connected to the
IPoIB link.
In general a larger link MTU can potentially offer a better
throughput performance. The caveat is that once a link MTU is chosen
for a given IPoIB link, nodes connected by CAs of a smaller maximum
payload size won't be able to join the link unless the whole link and
all the nodes attached to it are reconfigured to use a smaller MTU.
Note that the above discussion assumes that IP datagrams are fully
encapsulated in the payload of IB unreliable datagrams. The actual
MTU size, i.e., the payload size available for IP datagrams to use,
may be slightly smaller. This will depend on the actual IPoIB
encapsulation scheme, which will be covered in a separate document.
Note also that in case an IPoIB link spans more than one IB subnet,
Chu & Kashyap [Page 6]
draft-ietf-ipoib-link-multicast-00.txt January 2002
the IPoIB link MTU MUST not be set to greater than the path MTU of
any path connecting two nodes in the same IB partition. It is up to
the network administrator to determine the appropriate path MTU value
that will work for any node in the same IPoIB link.
6.2. IPoIB Link Q_Key
A Q_Key is programmed by the source QP in every IB datagram, and is
verified by the destination QP against the Q_Key it has been
assigned. A Q_Key violation will cause the offending datagram to be
dropped, and a Q_Key violation trap to be raised.
A Q_Key must be selected to be used by all the QPs attached to an
IPoIB link. It is recommended that a controlled Q_Key be used with
the high order bit set. This is to prevent non-privileged software
from fabricating and sending out bogus IP datagrams. All QPs
configured to use on a given IPoIB link SHALL be assigned the same
per-link Q_Key.
6.3 Other Link Attributes
TClass, FlowLabel, and HopLimit are three other attributes that are
required for an IPoIB link covering more than a single IB subnet.
The selection of these values are implementation dependent. But it
must take into account the topology of IB subnets comprising the
IPoIB link to ensure successful communication between any two nodes
in the same IPoIB link.
7. The IPoIB All-Node Multicast and Broadcast Group
Once an IB partition is created with link attributes identified for
an IPoIB link, the network administrator must create a special IB
multicast group for every node on the IPoIB link to join. This is
achieved through the creation of "MCGroupRecord" in each IB subnet
that the IB partition encompasses, as described in section 4 above.
The MGID will have the P_Key of the IB partition that defines the
IPoIB link embedded in it. A special signature is also embedded to
identify the MGID for IPoIB use only. For IPv4 over IB, the signature
will be "0x401B". For IPv6 over IB, the signature will be "0x601B".
For an IPv4 subnet, the MGID for this special IB multicast group
SHALL have the following format:
| 8 | 4 | 4 | 16 bits | 16 bits | 48 bits | 32 bits |
+--------+----+----+-----------------+---------+----------+---------+
|11111111|0001|scop|<IPoIB signature>|< P_Key >|00.......0|<all 1's>|
+--------+----+----+-----------------+---------+----------+---------+
Chu & Kashyap [Page 7]
draft-ietf-ipoib-link-multicast-00.txt January 2002
For an IPv6 subnet, the format of the MGID SHALL look like this:
| 8 | 4 | 4 | 16 bits | 16 bits | 80 bits |
+--------+----+----+-----------------+---------+--------------------+
|11111111|0001|scop|<IPoIB signature>|< P_Key >|000.............0001|
+--------+----+----+-----------------+---------+--------------------+
As for the scop bits, if the IPoIB link is fully contained within a
single IB subnet, the scop bits SHALL be set to 2 (link-local).
Otherwise the scope will be set higher.
A MCGroupRecord will be created with all the IPoIB link attributes
described before, including the link MTU, Q_Key, TClass, FlowLabel,
and HopLimit. When a node is attached to an IPoIB link identified by
a P_Key, it must look for a special, all-node multicast/broadcast
group to join. This is done by constructing the MGID with the link
P_Key and the IPoIB signature. The node SHOULD always look for a MGID
of a link-local scope first before attempting one with a greater
scope.
Once the right MGID and MCGroupRecord are identified, the local node
SHOULD use the link MTU recorded in the MCGroupRecord. It MUST accept
a smaller MTU if one is advertised through the link MTU option of a
router advertisement [DISC].
In case the link MTU is greater than the maximum payload size that
the local HCA can support, the node can not join the IPoIB link and
operate as an IP node.
After the right MTU is determined, the local node must join the
special all-node multicast/broadcast group by calling the SA to
create a MCMemberRecord corresponding to the MGID. The SA will return
all the link attributes for the local node to use. The node MUST use
these attributes in all future multicast operations to the local
IPoIB link. The broadcast group for IPv4 will serve to provide a
broadcast service for protocol like ARP to use.
In addition to the all-node multicast/broadcast group, an all-router
multicast group SHOULD be created at link configuration time if an IP
router will be attached to the link. This is to facilitate IP
multicast operations described later. A MCGroupRecord for the all-
router MGID must be created in every IB subnet that the IPoIB link
encompasses. The format of the all-router MGID will be covered in
next section.
8. Mapping for other Multicast Groups
The support of general IP multicast [IPMULT] over IB is similar to
Chu & Kashyap [Page 8]
draft-ietf-ipoib-link-multicast-00.txt January 2002
the case of the special all-node multicast/broadcast group discussed
above. An algorithmic mapping is used so that given an IP multicast
address, individual host can compute the corresponding IB multicast
address (MGID) all by itself without having to consult an external
entity. This also removes the need for an externally maintained IP to
IB multicast mapping table.
The IPoIB multicast mapping is defined as follows. The same mapping
function is used for both IPv4 and IPv6 except the IPoIB signature
field.
| 8 | 4 | 4 | 16 bits | 16 bits | 80 bits |
+------ -+----+----+-----------------+---------+--------------------+
|11111111|0001|scop|<IPoIB signature>|< P_Key >| group ID |
+--------+----+----+-----------------+---------+--------------------+
Since a MGID allocated for transporting IP multicast datagrams is
considered only a transient link-layer multicast address, all IB
MGIDs allocated for IPoIB purpose SHOULD have T = 1. The scope bits
SHALL be the same as that of the all-node MGID for the same IPoIB
link.
An IP multicast address is used together with a given IPoIB link
P_Key to form the MGID of the IB multicast group. For IPv6 the lower
80-bit of the group ID is used directly in the lower 80-bit of the
MGID. For IPv4, the group ID is only 28-bit long and the rest of the
80 bits are filled with 0.
The rest of the bits are the same as those of the all-node MGID.
E.g. on an IPoIB link that is fully contained within a single IB
subnet with a P_Key of 8, the MGIDs for the all-router multicast
group with group ID 2 [AARCH, IGMP2] are:
FF12:401B:8:0:0:0:0:2
or
FF12:401B:8::2
for IPv4 in a compressed format, and
FF12:601B:8:0:0:0:0:2
or
FF12:601B:8::2
for IPv6 in a compressed format.
Chu & Kashyap [Page 9]
draft-ietf-ipoib-link-multicast-00.txt January 2002
A special case exists for the IPv4 limited broadcast address
"255.255.255.255" [HOSTS]. The address SHALL be mapped to the
broadcast MGID for IPv4 networks as described in section 7 above.
Also the IPv6 all-node multicast address "FF0X::1" [AARCH] will be
mapped to the the special all-node MGID for IPv6 networks.
When a node wishes to join an IP multicast group on a local link, it
first needs to construct the corresponding MGID, using the rule
described above. Note that it must remember the scope bits from the
all-node MGID, and use the same scope in all later MGIDs it
constructs.
The local node then checks with SA to see if a MCGroupRecord
corresponding to the MGID already exists. If not, one must be created
first. The MCGroupRecord MUST be created with the IPoIB link MTU. For
the rest of the attributes, it is recommended that it uses the same
values from the all-node multicast/broadcast group corresponding to
the link.
Note that for an IPoIB link that spans more than one IB subnet
connected by IB routers, adequate multicast forwarding support at the
IB level is required for multicast packets to be forwarded properly
to members in remote IB subnets. The specific mechanism for this will
be covered in [IBTA], and is out of scope of this document.
Once the IB multicast group is identified, the node must join the
group, unless it is a member already, by calling the SA to create a
MCMemberRecord corresponding to the MGID. The join call enables SM
to program local IB switches and routers with the new multicast
information. Specifically it causes an IB switch to add the LID of
the caller to its forwarding table entry corresponding to the MLID
allocated for the group. It also causes an IB router to attach
itself to the IB multicast tree corresponding to the MGID.
When a node leaves an IP multicast group, it SHOULD delete the
MCMemberRecord from the SA. This allows the SA to free up related
resources. SM should delete MCGroupRecords that are no longer in use,
and free up the MLIDs allocated for them. The specific algorithm is
implementation-dependent, and therefore is out of scope of this
document.
In order to send a packet destined for an IP multicast address, a
node must first check if a MCGroupRecord for the corresponding MGID
of the outbound link exists or not. If one already exists, the MLID
allocated by the SM for the MCGroupRecord is used as the DLID for the
packet. Otherwise, it means no member exists on the local link. The
packet should be forwarded to the all-router multicast group
described before. If one doesn't already exist, it implies no router
Chu & Kashyap [Page 10]
draft-ietf-ipoib-link-multicast-00.txt January 2002
presence on the local subnet. The packet can then be silently
dropped.
Note that the local node MUST be notified when an IB multicast group
corresponding to the MGID ever comes into existence later. This
signifies that an interested party just showed up on the local link
and therefore must be copied.
9. Support for IP Multicast Routing
IP multicast routing requires a router to receive a copy of every
link multicast packet on a locally connected link [IPMULT, IP6MLD].
For Ethernet this is usually done by turning on promiscuous multicast
mode on a locally connected Ethernet interface.
Unfortunately IBA does not support promiscuous multicast mode.
Therefore the IPoIB driver should forward a copy of every outbound
multicast datagram to the MGID corresponding to the all-router
multicast group. This is to ensure multicast packets be properly
forwarded to remote IP networks.
10. Security Considerations
All the operations for creating and configuring an IPoIB link
described in this document, including assigning P_Keys to CAs,
creating MCGroupRecords and MCMemberRecords in SA, creating and
attaching QPs to IB multicast groups,... etc are privileged
operations, and MUST be protected by the underlying operating system.
This is to prevent malicious, non- privileged software from hijacking
important resources and configurations. E.g. A bogus all-node IPoIB
multicast group may prevent a proper one from being created when the
network administrator tries to set up a link.
Controlled Q_Keys SHOULD be used in IB multicast groups in order to
prevent non-privileged software from fabricating IP datagrams to
send, as mentioned in section 6.2.
11. Acknowledgments
The authors would like to thank Bruce Beukema, David Brean, Dan
Cassiday, Thomas Narten, Erik Nordmark, Greg Pfister, Renato Recio,
David L. Stevens, and Madhu Talluri for their suggestions and many
clarifications on the IBA specification.
12. References
[AARCH] Hinden, R. and S. Deering "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
Chu & Kashyap [Page 11]
draft-ietf-ipoib-link-multicast-00.txt January 2002
[DISC] Narten, T., Nordmark, E. and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461, December
1998.
[HOSTS] Braden R., "Requirements for Internet Hosts --
Communication Layers", RFC 1122, October 1989
[IBTA] InfiniBand Architecture Specification, Release 1.0.a by
InfiniBand Trade Association at www.infinibandta.org
[IGMP2] Fenner W., "Internet Group Management Protocol, Version 2",
RFC 2236, November 1997.
[IPMULT] Deering S., "Host Extensions for IP Multicasting", RFC
1112, August 1989.
[IPV6] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[IP6MLD] Deering S., Fenner W., Haberman B., "Multicast Listener
Discovery (MLD) for IPv6", RFC 2710, October 1999.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
13. Author's Address
H.K. Jerry Chu
901 San Antonio Road, UMPK17-201
Palo Alto, CA 94303-4900
USA
Phone: +1 650 786-5146
EMail: jerry.chu@sun.com
Vivek Kashyap
IBM
15450, SW Koll Parkway
Beaverton, OR 97006
Phone: 503 578 3422
EMail: vivk@us.ibm.com
14. Full Copyright Statement
Chu & Kashyap [Page 12]
draft-ietf-ipoib-link-multicast-00.txt January 2002
Copyright (C) The Internet Society (2001>. All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Chu & Kashyap [Page 13]
