INTERNET-DRAFT H.K. Jerry Chu
<draft-ietf-ipoib-link-multicast-03.txt> Sun Microsystems
Vivek Kashyap
IBM
Expires: September, 2003 March, 2003
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.
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Copyright (C) The Internet Society (2003). 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.
Table of Contents
1.0 Introduction
2.0 Terminology
3.0 Basic IPoIB Transport - Unreliable Datagram
4.0 IB Multicast Architecture
5.0 IB Links vs. IPoIB Links
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6.0 Setting up an IPoIB Link
6.1 Maximum Transmission Unit
6.2 IPoIB Link Q_Key
6.3 Other Link Attributes
7.0 The IPoIB Broadcast Group
8.0 Mapping for other Multicast Groups
9.0 Sending and Receiving IP Multicast Packets
10.0 IP Multicast Routing
11.0 Security Considerations
12.0 Acknowledgments
13.0 References
14.0 Author's Address
15.0 Full Copyright Statement
1.0 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 link, network, and transport layers collectively
constitute the data link layer to the IP stack. One can find a
general overview of IB architecture related to IP networks in
[IPoIB_ARCH].
This document will focus on the necessary steps in order to lay out
an IP network on top of an IB network. It will describe all the
elements of an IP over InfiniBand (IPoIB) link, how to configure its
associated attributes, and how to set up basic broadcast and
multicast services for it. IPoIB links are the building blocks upon
which an IP network consisting of many IP subnets connected by
routers can be built. Subnetting allows the containment of broadcast
traffic within a single link. It also provides certain degree of
isolation for the administration purpose between nodes on different
subnets.
2.0 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].
3.0 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
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define support for multicast. They provide the basic transport
mechanism that best matches the IP datagram paradigm.
IB unreliable datagram provides many additional features such as the
partition key (P_Key) protection, multiple queue pairs (QPs), and
Q_Key protection. Moreover, it defines 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 ([IPoIB_ARCH],
[IPoIB_ENCAP]).
4.0 IB Multicast Architecture
The following discussion gives a short overview of the multicast
architecture in InfiniBand. For a complete specification, the reader
is referred to [IBTA].
IBA defines two layers of multicast services. Its link layer uses
multicast LIDs (MLIDs) in the Local Route Header (LRH). LIDs 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) in the Global Route
Header (GRH). MGIDs closely resemble IPv6 multicast addresses [AARCH]
shown below.
| 8 | 4 | 4 | 112 bits |
+------ -+----+----+---------------------------------------------+
|11111111|flgs|scop| group ID |
+--------+----+----+---------------------------------------------+
Figure 1
[IPoIB_ARCH] describes each field in more details.
Since every IB multicast packet is required to carry a LRH and a GRH,
both a valid MGID and a valid MLID are needed before an 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 to represent a multicast group that
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an IB node can join, an IB multicast group corresponding to the MGID
must be created through the Subnet Administrator (SA). Besides the
the MGID, the creator of an IB multicast group must supply values of
path MTU, P_Key, Q_Key, Service Level (SL), FlowLabel, TClass 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.
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 are part of an IP network.
5.0 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 closely follows the physical topology of the network. For
instance, 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 to form an IP network.
InfiniBand defines its own link-layer and subnets consisting of nodes
connected by IB switches and routers. However, the IPoIB link
boundary need 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. This
communication at the network layer applies to unicast as well as
multicast.
The ultimate requirement for two nodes in the same IB fabric to
communicate at the IB level, besides physical connectivity, is a
common P_Key.
Partitioning in IB provides an isolation mechanism among nodes in an
IB fabric, much like VLANs in the Ethernet network. Each port of an
HCA (Host Channel Adaptor) contains a P_Key table holding 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 all the outgoing
packets from the QP, and is used to compare against the P_Key of all
incoming packets to the QP. Any packet with an invalid P_Key will be
discarded by the QP and a P_Key violation trap will be generated. IB
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switches may optionally enforce partition checking too.
Following the above, IB partitions are the natural choice for
defining IPoIB link boundary. It also provides much needed
flexibility for a network administrator to group nodes logically into
different subnets in a large network.
6.0 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. Since a full-duplex
communication is required among IP nodes, full-membership P_Keys,
that is, those with the high-order bit set to 1 shall be used. An IB
partition may or may not span multiple IB subnets; and whether it
does or not is mostly transparent to IPoIB.
Each node attached to an IB partition MUST have one of its HCAs
assigned the P_Key to use. Note that the P_key table of an HCA port
may contain many P_Keys. It is up to the implementation to define the
method by which the P_Key relevant to a particular IPoIB subnet is
determined and conveyed to the IPoIB stack. For instance,
implementations may resort to a manual configuration when choosing
the P_key or a set of P_Keys for IPoIB, and rely on DHCP [DHCP] to
assign an IP subnet number to each IPoIB link.
Once an IB partition is established for IPoIB use, the link MTU and
Q_Key are two other attributes that must be chosen before an IPoIB
link can be configured.
6.1 Maximum Transmission Unit
IB defines five permissible maximum payload sizes (MTUs). They are
256, 512, 1024, 2048 and 4096 bytes. [IPV6] requires a link MTU of
1280 bytes or greater. To be better compatible with Ethernet, the
dominant network media in both the LAN and WAN environment, the IPoIB
link MTU SHALL be 1500 bytes or greater. This leaves only 2048 and
4096 bytes as the two acceptable MTUs for IPoIB. Channel adaptors
supporting a MTU less than the minimal requirement can still expose
an acceptable 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 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 MTU of any IB devices on the IPoIB link. Here the IB devices
include IB switches, CAs, or routers.
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In general, a maximum link MTU should be employed whenever possible
to attain a better throughput performance. One caveat is that once a
link MTU is chosen for a given IPoIB link, nodes connected by CAs of
a smaller MTU won't be able to join the link unless the whole link
and all the devices attached to it are reconfigured to use the
smaller MTU.
It may be desirable in some case to use a smaller link MTU than the
full size. For example, bridging an IPoIB link with an Ethernet link
could be made much easier if the IPoIB link MTU is reduced to 1500
bytes. For IPv4, this may require a manual configuration of a
different link MTU than the maximum that all the nodes support. For
IPv6, one can use the MTU option of the router advertisement [DISC]
to announce a smaller MTU to all the nodes.
In case an IPoIB link spans more than one IB subnet, the IPoIB link
MTU MUST not exceed 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
compared against the Q_Key of the destination QP. A Q_Key violation
will cause the offending datagram to be dropped, and a Q_Key
violation counter to be incremented on the receiving port. A trap is
also generated if the feature is supported on that port.
A single Q_Key must be selected for all the QPs attached to an IPoIB
link to use. 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 for a given IPoIB link SHALL be assigned the same per-link
Q_Key.
6.3 Other Link Attributes
TClass, FlowLabel, HopLimit, and SL are four other attributes that
are required if an IPoIB link covers more than a single IB subnet.
The selection of these values are implementation dependent.
Implementations must take into account the topology of IB subnets
comprising the IPoIB link to ensure a successful communication
between any two nodes in the same IPoIB link.
7.0 The IPoIB Broadcast Group
Once an IB partition is created with link attributes identified for
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an IPoIB link, the network administrator must create a special IB
all-node multicast group (henceforth referred to as the broadcast
group) with these link attributes for every node on the IPoIB link to
join. The creation of an IB multicast group is through the use of
the "MCMemberRecord" SA attribute as described in the IBA
specification.
The MGID of an IPoIB broadcast group will embed in it the P_Key of
the IB partition that defines the IPoIB link. A special signature is
also embedded to identify all the MGIDs 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|0100000000011011|< P_Key >|00.......0|<all 1's>|
+--------+----+----+----------------+---------+----------+---------+
Figure 2
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|0110000000011011|< P_Key >|000.............0001|
+--------+----+----+----------------+---------+--------------------+
Figure 3
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.
The broadcast group for IPv4 will serve to provide a broadcast
service for protocols like ARP to use.
When a node is first brought up on an IPoIB link identified by a
P_Key, it must look for the right broadcast group to join. This is
done by querying the SA MCMemberRecord database for a multicast group
with a MGID matching the one constructed from 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 broadcast group are identified, the local
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node SHOULD use the MTU associated with the broadcast group. In case
the MTU of the broadcast group is greater than what the local HCA can
support, the node can not join the IPoIB link and operate as an IP
node. Otherwise the local node must join the broadcast group as a
"full member" and use the rest of link attributes associated with the
group for all future communication to the link.
In addition to the special all-node multicast group for broadcast
purpose, 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. An IB
multicast group for the all-router MGID must cover every IB subnet
that the IPoIB link encompasses. The format of the all-router MGID
will be covered in the next section.
8.0 Mapping for other Multicast Groups
The general IP multicast [IPMULT] support over IB is similar to the
case of the special 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 depicted in Figure 4. 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 |
+--------+----+----+-----------------+---------+--------------------+
Figure 4
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.
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The rest of the bits are the same as those of the broadcast MGID.
For example, on an IPoIB link that is fully contained within a single
IB subnet with a P_Key of 0x8006, the MGIDs for the all-router
multicast group with group ID 2 [AARCH, IGMP2] are:
FF12:401B:8006:0:0:0:0:2
or
FF12:401B:8006::2
for IPv4 in a compressed format, and
FF12:601B:8006:0:0:0:0:2
or
FF12:601B:8006::2
for IPv6 in a compressed format.
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] maps
naturally to the the special broadcast MGID for IPv6 networks.
9.0 Sending and Receiving IP Multicast Packets
Multicast in InfiniBand differs in a number of ways from multicast in
Ethernet. This adds some complexity to an IPoIB implementation when
supporting IP multicast over IB.
A) An IB multicast group must be explicitly created through the SA
before it can be used.
This implies that in order to send a packet destined for an IP
multicast address, the IPoIB implementation must check with the SA on
the outbound link first for a "MCMemberRecord" that matches the MGID.
If one does exist, the MLID associated with the multicast group is
used as the DLID for the packet. Otherwise, it implies no member
exists on the local link. The packet SHOULD be forwarded to locally
connected routers. This is to allow local routers to forward the
packet to multicast listeners on remote networks. The specific
mechanism for a sender to forward packets to routers are left to
implementations. One can use, for example, the broadcast group, or
the all-router multicast group for this purpose.
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B) A multicast sender must join the target multicast group as a
"SendOnlyNonMember" before outgoing multicast messages from it can be
successfully routed. The "SendOnlyNonMember" join is different from
the regular "FullMember" join in two aspects. First, both types of
joins enable multicast packets to be routed FROM the local port, but
only the "FullMember" join causes multicast packets to be routed TO
the port. Second, the sender port of a "SendOnlyNonMember" join will
not be counted as a member of the multicast group for purposes of
group creation and deletion.
The following code snippet demonstrates the steps in a typical
implementation when processing an egress multicast packet.
if the egress port is already a "SendOnlyNonMember", or a
"FullMember"
=> send the packet
else if the target multicast group exists
=> do "SendOnlyNonMember" join
=> send the packet
else if the all-router multicast group exists
=> send the packet to all routers
else
=> drop the packet
Implementations should cache the information about the existence of
an IB multicast group, its MLID and other attributes. This is to
avoid expensive SA calls on every outgoing multicast packet. The
cache may need to be validated periodically. Senders should also
subscribe to the multicast group create and delete traps in order to
monitor the status of specific IB multicast groups. Multicast packets
directed to the all-router multicast group due to a lack of listener
on the local subnet must be forwarded to the right multicast group if
the group is created later. This happens when a listener shows up on
the local subnet.
A node joining an IP multicast group must first construct a MGID
according to the rule described in section 8 above. Once the correct
MGID is calculated, the node must call the SA of the outbound link to
attempt a "FullMember" join of the IB multicast group corresponding
to the MGID. If the IB multicast group doesn't already exist, one
must be created first with the IPoIB link MTU. For the rest of
attributes, it is recommended the same values from the all-node
multicast/broadcast group be used.
The join request will cause the local port to be added to the
multicast group. It also enables the SM to program IB switches and
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routers with the new multicast information to ensure the correct
forwarding of multicast packets for the group.
When a node leaves an IP multicast group, it SHOULD make a
"FullMember" leave request to the SA. This gives SM an opportunity to
update relevant forwarding information, to delete an IB multicast
group if the local port is the last FullMember to leave, and free up
the MLID allocated for it. The specific algorithm is implementation-
dependent, and is out of the scope of this document.
Note that for an IPoIB link that spans more than one IB subnet
connected by IB routers, an adequate multicast forwarding support at
the IB level is required for multicast packets to reach listeners on
a remote IB subnet. The specific mechanism for this will be covered
in [IBTA], and is beyond the scope of IPoIB.
10.0 IP Multicast Routing
IP multicast routing requires multicast routers to receive a copy of
every link multicast packet on a locally connected link [IPMULT,
IP6MLD]. For Ethernet this is usually achieved by turning on the
promiscuous multicast mode on a locally connected Ethernet interface.
IBA does not provide any hardware support for promiscuous multicast
mode. Fortunately a promiscuous multicast mode can be emulated in
the software running on a router through the following steps.
A) Obtain a list of all active IB multicast groups from the local SA.
B) Make a "NonMember" join request to the SA for every group that has
a signature in its MGID matching the one for either IPv4 or IPv6.
C) Subscribe to the IB multicast group creation events using a
wildcarded MGID so that the router can "NonMember" join all IB
multicast groups created subsequently for IPv4 or IPv6.
The "NonMember" join has the same effect as a "FullMember" join
except that the former will not be counted as a member of the
multicast group for purposes of group creation or deletion. That is,
when the last "FullMember" leaves a multicast group, the group can be
safely deleted by the SA without concerning any "NonMember" routers.
11.0 Security Considerations
All the operations for creating and configuring an IPoIB link
described in this document, including assigning P_Keys to CAs,
creating IB multicast groups in SA, creating and attaching QPs to IB
multicast groups,... etc, are privileged operations, and MUST be
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protected by the underlying operating system. This is to prevent
malicious, non- privileged software from hijacking important
resources and configurations. For example, A bogus IPoIB broadcast
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 IPoIB links. This is to prevent
non-privileged software from fabricating IP datagrams to send, as
mentioned in section 6.2.
12.0 Acknowledgments
The authors would like to thank Bruce Beukema, David Brean, Dan
Cassiday, Aditya Dube, Yaron Haviv, Michael Krause, Thomas Narten,
Erik Nordmark, Greg Pfister, Renato Recio, Satya Sharma, and David L.
Stevens for their suggestions and many clarifications on the IBA
specification.
13.0 References
[AARCH] Hinden, R. and S. Deering "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[DHCP] R. Droms "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[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.
[IPoIB_ARCH] draft-ietf-ipoib-architecture-01.txt
[IPoIB_ENCAP] draft-ietf-ipoib-ip-over-infiniband-01.txt
[IPV6] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
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[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.
14.0 Author's Address
H.K. Jerry Chu
17 Network Circle, UMPK17-201
Menlo Park, CA 94025
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
15.0 Full Copyright Statement
Copyright (C) The Internet Society (2003>. All Rights Reserved.
This document and translations of it may be copied and furnished to
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included on all such copies and derivative works. However, this
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the copyright notice or references to the Internet Society or other
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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.
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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.
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