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   Working Group: ARMD                                  Himanshu Shah
   Intended Status: Proposed Standard                      Ciena Corp
   Internet Draft
                                                       Anoop Ghanwani
   Expiration Date: May, 2011                                 Brocade

                                                          Nabil Bitar

                                                     October 25, 2010

              ARP Broadcast Reduction for Large Data Centers

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on May 25, 2011

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document. Please review these documents
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   With the emergence server virtualization technologies, a host is
   able to support multiple Virtual Machines (VMs) in a single physical
   machine. Data centers can leverage these capabilities to instantiate
   on the order of 10s to 100s of VMs in a server. Each VM operates as
   an independent IP host with a set of Virtual Network Interface Cards
   (vNICs), each having its own MAC address and mapping to a physical
   Ethernet interface. These physical servers are typically installed
   in a rack with their Ethernet interfaces connected to a top-of-rack
   (ToR) switch. The ToR switches are interconnected through End-of-
   the-Row (EoR) or aggregation switches which are in turn connected to
   core switches.

   As discussed in [ARP-Problem] the host VMs use ARP broadcasts to
   find other host VMs and use periodic (broadcast) Gratuitous ARPs to
   refresh their IP to MAC address binding in other VM hosts. Such
   broadcasts in a large data center with potentially thousands of VM
   hosts in a Layer 2 based topology can overwhelm the network.

   This memo proposes mechanisms to reduce the number of broadcasts
   that are sent throughout the network. This is done by having the ToR
   switches intelligently process ARP packets, rather than simply
   broadcasting them throughout the broadcast domain.

   While this document specifically addresses ARP, the Neighbor
   Discovery mechanisms used by IPv6 hosts that make use of multicast
   rather than broadcast also pose similar issues for the data center.
   The solutions defined herein should be equally applicable to hosts
   running IPv6.  The details will be specified in a subsequent


   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC 2119].

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Table of Contents

 Copyright Notice .................................................... 1
Abstract.............................................................. 2
1.0 Overview.......................................................... 3
 1.1 Terminology ..................................................... 5
2.0 Configuration..................................................... 6
3.0 Building the ARP Tables........................................... 6
 3.1 ARP Request ..................................................... 6
 3.2 ARP Reply ....................................................... 7
 3.3 Gratuitous ARP .................................................. 7
 3.4 Uplink Versus Downlink Processing ............................... 8
 3.5 Host Mobility ................................................... 8
4.0 Concluding Remarks................................................ 9
5.0 Security Considerations ......................................... 10
6.0 Acknowledgments ................................................. 10
7.0 References....................................................... 10
 7.1 Normative References ........................................... 10
 7.2 Informative References ......................................... 10
8.0 Author's Address................................................. 10

1.0 Overview

   The traditional topology in a data center consists of racks of
   servers connected to top-of-rack (ToR) switches, which connect to
   aggregation switches, which in turn connect to core switches.  The
   network architecture is typically a combination Layer 2 and Layer 3
   functionality.  In some architectures, Layer 2 is terminated at the
   ToR, with Layer 3 being run in the aggregation and core devices.  In
   other architectures, Layer 2 may be extended all the way to the
   aggregation switch.  The primary concerns that have influenced
   network architectures in the data center have been keeping broadcast
   domains manageable and the spanning tree diameter contained.

   Moving forward, these traditional network architectures are being
   challenged due to emerging technologies such as server

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   The effect of server virtualization in the data center brings some
   challenges.  Because of virtualization, the number of hosts seen by
   the network increases dramatically - 10 to 100 times the number of
   physical servers.  These virtual hosts are referred to as Virtual
   machines (VMs).  In addition, virtualized environments offer a
   feature referred to as "VM mobility" wherein a VM can be relocated
   to run on a different physical server.  In order for the VM mobility
   to be non-disruptive to other hosts that have communication in
   progress with the VM being moved, the VM must retain its MAC address
   and IP address.  Because of the requirement to retain the MAC and IP
   address, it is desirable to develop network architectures that would
   offer the least restrictions in terms of VM mobility.

   As an example, in a network architecture where TOR switches
   terminate the L2 domain, the range of VM mobility would be
   restricted to a single ToR switch.  It would be more preferable to
   allow the flexibility of moving the VM anywhere within the data
   center, or perhaps even a different data center.

   Technologies such as TRILL [TRILL] overcome some of the issues of
   spanning trees that forced traditional Layer 2 topologies to be
   severely constrained.  However, because of virtualization there are
   2 specific problems that are introduced with respect to broadcast

     1. A larger number of hosts.  A single physical server now hosts
        multiple VMs taking the scale factor to a different level.  If
        each VM issues the same number of broadcasts as a physical
        server, the amount of broadcast traffic will increased 10 to
        greater than 100 times.
     2. If the Layer 2 domains are extended to go across data centers,
        then broadcast traffic will now go across the backbone.  If
        Layer 2 was terminated at the ToR switch, the increase in
        broadcast traffic would be been restricted to a single ToR
        switch, but as discussed earlier, this restriction is not

   Excessive broadcast traffic in Layer 2 networks results in wastage
   of network bandwidth, as well as in the wastage of CPU resources due
   to all of the VMs processing superfluous ARP broadcasts (IPv6 gets
   rid of the latter by running ND as a multicast service rather than a
   broadcast service).

   The solution presented here attempts to minimize the negative
   effects of ARP broadcast packets. The solution requires the first
   hop Ethernet switches, typically the ToR switch, to maintain an ARP
   table that is learned from the ARP packets received by the switch.
   The switch then selectively propagates the ARP packet to, or proxy-
   responds on behalf of, the remote peer. These types of ARP
   processing principles are well-known and are described in L2VPN
   Working Group documents such as [ARP-Mediation] and [IPLS].

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   The following sections describe the details of ARP snooping, the
   learning and maintenance of ARP tables, the use of learned
   information to limit broadcast propagation, and proxy (the response)
   on behalf of the remote peers.

1.1 Terminology

        ToR            Top-of-Rack. An Ethernet switch present on top
                       of a rack which provides network connectivity to
                       the servers present on the rack.

        Downlink       The Ethernet link between the ToR switch and a
                        directly connected  host (server in the rack).

        Uplink         The network- facing Ethernet connection in the
                        ToR switch. Typically, the uplinks from ToRs
                        connect to end-of-row or aggregation switches.

        EoR            End-of-Row.  An Ethernet switch to which the
                        ToR switches connect, also referred to as an
                        aggregation switch. Uplinks from ToR switches
                        connect to an EoR switch and uplinks from EoR
                        switches connect to a core switch.

        Host/Server    A host or server running the IP protocol. This
                        could be a physical entity or a logical entity
                        (such as a Virtual Machine) in a physical host.
                        The term server refers to its role in the data
                        center. Both terms are used interchangeably to
                        refer to an IP host.

        Local hosts    Used in the context of a ToR switch to denote
                        the VM hosts connected to a ToR on the
                        downlink, i.e. directly attached hosts.

        Remote hosts    Used in the context of a ToR switch to denote
                        the hosts that are accessible through uplink of
                        the ToR.

        VM             Virtual Machine. This is a logical instance of
                        a host that operates independently in a
                        physical host and has its own IP and MAC
                        addresses. VMs allow efficient use of physical
                        host resources (such as multiple CPU cores).

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2.0 Configuration

   It is assumed that ARP reduction mechanisms that are defined in this
   document will be limited to ToR switches.   The maximum benefit of
   restraining ARP broadcasts in the network is achieved by the first
   hop switches (the ones directly connected to the hosts) without
   placing additional burden on second or third tier switches.

   First, the ToR switches would need to be configured in order to
   enable the ARP reduction feature. Every Ethernet interface needs to
   be identified as either a downlink or uplink within the context of
   this feature.

   In addition the operator may optionally configure various ARP
   reduction related parameters such as:
     . ARP aging timer.
     . Size of the ARP table.
     . Static entries of IP to MAC address.

3.0 Building the ARP Tables

   When ARP reduction is enabled, the ToR switch will monitor all ARP
   traffic transiting the switch (regardless of uplink port or downlink
   port) and will process any ARP packets in the following manner:
     . ARP Request packets must be redirected to control plane CPU.
     . Gratuitous ARP packets (ARP Reply packet with a broadcast MAC
        DA) must be redirected to control plane CPU.
     . Other ARP Reply packets (ARP Reply packet with a unicast MAC
        DA) should be bi-casted; one copy sent to control plane CPU and
        other copy forwarded out normally.

3.1 ARP Request

   The ToR examines the source IP and the source hardware address (MAC
   address) in the ARP Request . The source IP and MAC address
   association is learned, or is updated/refreshed if already learned.
   The destination IP address is searched in the ARP table. If an entry
   exists, the associated MAC address from the table is used to prepare
   a unicast ARP Reply packet. The same MAC address is used as the
   source MAC address in the MAC header, as well as for the target
   hardware address, in the unicast ARP Reply packet.

   If the destination IP address in the ARP Request is not present in
   the ARP table, then the original ARP Request packet is broadcast to
   all the switch ports that are members of the same VLAN except the
   source port that the ARP Request was received from. However, if the
   requested (destination) IP address is present in the ARP table, a
   unicast ARP Reply packet is prepared as described above and sent to
   the switch port from which the ARP Request was received and original
   ARP Request packet is dropped.

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   The intent is to prevent propagation of ARP Request broadcasts as
   much as possible using the information present in the ARP table. The
   following observations can be made from such behavior.
      . Most of the ARP Request packets from the local hosts of a ToR
         switch for the local hosts of that ToR switch can be prevented
         from being broadcast on uplinks or downlinks.
      . Most of the ARP Request packets from remote hosts of a ToR
         switch for local hosts of that ToR switch can be prevented
         from being broadcast on downlinks or other uplinks of the ToR
      . Many of the ARP Request packets from local hosts of a ToR
         switch for remote hosts of that ToR switch can be prevented
         from being forwarded on uplinks if the remote host IP to MAC
         association is known to the ToR switch.

3.2 ARP Reply

   The unicast ARP Reply is examined to learn/update the ARP table for
   source and destination IP/MAC address association, but is also
   forwarded out as a normal frame.

3.3 Gratuitous ARP

   Gratuitous ARP is a broadcast ARP Reply packet with the destination
   IP address set to the IP address of the sender and target hardware
   address set to the MAC address of the sender. It is typically used
   by IP hosts (including VMs) to keep its IP-to-MAC address
   association fresh in its peers' ARP cache.

   The ToR switch should process Gratuitous ARP in the following
      . Learn/update/refresh the ARP table entry.
      . If the IP address is new, or exists but with a different
         hardware address, then the Gratuitous ARP packet is forwarded
         out; otherwise the packet is discarded.

   The goal for handling of Gratuitous ARP packets received from the
   downlinks (i.e. local hosts) is to avoid propagating it into the
   'network' (i.e. to the uplinks), unless there is a new association.

   By suppressing the propagation of Gratuitous ARP packets, the peer
   IP hosts will end up aging out the corresponding ARP table entries.
   This will result in generation of the broadcast ARP Requests by
   those IP hosts if they need to continue to communicate with the IP
   host whose Gratuitous ARPs were obstructed. The handling of the ARP
   Request by the first-hop ToR switch, as described above, will be
   able to respond to this request based on the ARP cache maintained in
   the ToR switch. In essence, the presence of large ARP tables with
   longer aging times compensates for the smaller ARP table present in

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   the IP hosts and eliminates the need for periodic use of Gratuitous
   ARPs in order to refresh the ARP table in the IP hosts.

3.4 Uplink Versus Downlink Processing

   With respect to processing of the ARP packets as described above,
   the behavior is different depending on whether the packet was
   received from an uplink or downlink in the following ways.

     . The aging timer will typically be higher for entries learned
        from an uplink versus those learned from a downlink.  The
        reason for this is to avoid flooding ARP broadcast packets on
        uplinks since they have a much larger negative impact.
     . If ARP table fills up, then entries learned from downlinks
        (i.e. directly attached hosts) will take precedence over those
        learned from an uplink (i.e. remote hosts).  This will trade
        off sending broadcasts on host links versus sending them into
        the core of the network.  The reason for this is that access
        links are typically lower bandwidth, and also this will
        conserve CPU resources involved in processing unnecessary ARP

3.5 Host Mobility

   As mentioned earlier, server virtualization technology allows
   mobility of VMs to different physical servers. The flexibility to
   move VMs is one of the key benefits of server virtualization. VM
   mobility could be manual (operator initiated) or may be done
   automatically in reaction to demands placed by the application
   users. The important point is that in either case, VM movement is
   not transparent and is made known to the network.

   There is ongoing work in IEEE 802.1 standards organization (IEEE
   802.1Qbg) to coordinate/communicate the presence and capabilities of
   the VMs to the directly connected network switch.

   VMs typically retain their MAC and IP address across a VM mobility
   event, and as such, there would be little impact to the ARP table
   maintained by the ARP reduction mechanism described herein.
   However, the ARP reduction mechanism would benefit from knowing if a
   VM is completely decommissioned so that the ToR switch can remove
   the ARP entry that it has for that VM in a timely fashion, rather
   than waiting for it to age out.

3.6 Scaling Considerations

   Depending on the number of hosts in the network, the ARP table in a
   ToR switch needed for the ARP reduction mechanisms described above
   can be quite large. Although it is possible to implement some of the
   mechanisms for ARP reduction in hardware in the forwarding plane,

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   the number of ARP entries favors maintaining the ARP table in the
   control plane memory.

3.7 Miscellaneous Issues

   Because of the distributed nature of the mechanisms described
   herein, there are a few additional issues that warrant consideration
   from the network operator.

   Earlier in the document, we had mentioned the configuration of a
   aging timer for ARP entries.  A longer timer for holding onto ARP
   entries helps with reduction of broadcasts.  However, having a "too
   large timer" can lead to problems in certain situations.

   Consider the following scenario.  Host A is attached to ToR switch
   #1, and host B is attached to ToR switch #2.  If host B issues an
   ARP Request for host A, and if the entry is available at switch #2,
   then switch #2 would send the ARP Reply on behalf of host A.  It is
   possible that host A is no longer available, but there is no way for
   switch #2 to know this, and it would continue to respond on behalf
   of host A, until its entry for host A has aged out.  In this case,
   it is easy to see that a smaller aging timer would be beneficial.
   Additionally, since host B has an ARP aging timer, it means that
   host B would find out about host A's unavailability only after its
   entry has aged out, which would be some time after it the entry has
   aged out of switch #2.

   Another issue that can be somewhat problematic could be the
   inconsistency of tables in switches.  Once again, consider a
   scenario similar to the one described above with two hosts each
   connected to its respect ToR switch.  Let the ARP entries at both A
   and B be learned by both switches.  Now assume that the IP address
   on host A changes.  This change is signaled to switch #1 which in
   turn broadcasts the message on its uplink.  Now, if this message is
   discarded due to network congestion or signal integrity issues, then
   switch #2 will not learn about the change and will continue to
   respond to host B's ARP Requests for host A's old IP address with
   stale information.  This lasts until the ARP entry for A ages out at
   Switch #2.

4.0 Concluding Remarks

   Based on the procedures described in this document, it is possible
   for ToR switches in the data center to contain ARP broadcasts
   significantly. The solution is based on well known, non-intrusive
   procedures and strives to curtail ARP broadcasts that are
   increasingly becoming a cause for concern in the data centers. In
   essence, ToR switches offload some of the ARP table management from
   the IP hosts to themselves. The ARP table aging timer can be tuned
   higher by the operator based on the available switch resources and
   network traffic behavior. The larger capacity of the ARP table

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   coupled with a long aging time for entries in the table directly
   translates to more effective subduing of the ARP broadcasts.

5.0  Security Considerations

   Security aspects will be addressed in a subsequent revision.

6.0  Acknowledgments

   This document resulted from discussions with Linda Dunbar (Huawei),
   Sue Hares (Huawei), and T Sridhar (Force10).  We would like to
   acknowledge their contribution to this work.

7.0 References

7.1 Normative References

   [ARP] D. Plummer, "An Ethernet Address Resolution Protocol:  Or
      Converting Network Protocol Addresses to 48.bit Ethernet
      Addresses for Transmission on Ethernet Hardware, " RFC 826 (also
      STD 37), November 1982.

   [ARP-Problem] L.Dunbar et al., "Scalable Address Resolution for
      Large Data Center Problem Statements," <draft-dunbar-arp-for-
      large-dc-problem-statement-00>, July 2010.

7.2 Informative References

   [ARP-Mediation] H. Shah et al., "ARP Mediation for IP interworking
      in Layer 2 VPN," <draft-ietf-l2vpn-arp-mediation-14>, July 2010.

   [IPLS] H.Shah et al., "IP-only LAN service,"
      <draft-ietf-l2vpn-ipls-09>, February 2010.

   [PROXY-ARP] J. Postel, "Multi-LAN Address Resolution," RFC 925,
      October 1984.

   [TRILL] R. Perlman et al., "RBridges: Base Protocol Specification",
      <draft-ietf-trill-rbridge-protocol-16>, March 2010.

8.0 Author's Address

   Himanshu Shah
   Ciena Corp
   Email: hshah@ciena.com

   Anoop Ghanwani

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   Email: anoop@brocade.com

   Nabil Bitar
   Email: nabil.n.bitar@verizon.com

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