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   Working Group: ARMD                                  Himanshu Shah
   Intended Status: Informational                          Ciena Corp
   Internet Draft
                                                       Anoop Ghanwani
   Expiration Date: April 27, 2012                            Brocade

                                                          Nabil Bitar

                                                     October 28, 2011

              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.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on April 27, 2012

Copyright Notice

   Copyright (c) 2011 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
   carefully, as they describe your rights and restrictions with
   respect to this document.  Code Components extracted from this
   document must include Simplified BSD License text as described in
   Section 4.e of the Trust Legal Provisions and are provided without
   warranty as described in the Simplified BSD License.


   With advent of 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 single server with current
   technology.  It is conceivable that this number can be much higher
   in the future. 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-the-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
   ToRs intelligently process ARP and frames, rather than simply
   broadcasting them throughout the broadcast domain.

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


   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 Requests ........................................... 6
 3.2 ARP Reply .............................................. 7
 3.3 Gratuitous ARP ......................................... 7
 3.4 Host movement .......................................... 8
4.0 Conclusion .............................................. 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 ........................................ 11

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 typically combines Layer 2 and Layer 3.  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 spanning tree domains 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 that the
   network sees increases dramatically - 10 to 100 times the number of
   physical servers.  These virtual hosts are referred to as Virtual
   machines (VMs).  VMs offer server mobility wherein a VM can be
   relocated to run on a different physical server.  In order for the
   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 server mobility.

   As an example, in a network architecture where TOR switches
   terminate the L2 domain, the range of 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 because which traditional Layer 2 topologies have
   been 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 virtual machines taking the scale factor to a
        different level.  If each VM has the same number of broadcasts
        as a physical server, the amount of broadcast traffic has
        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

   The broadcast as such in Layer 2 networks has far reaching impacts;
   i.e. wastage in network bandwidth as well as CPU resources used by
   all the VMs while 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 negative effects of
   ARP broadcasts. The solution requires the first hop Ethernet
   switches, typically ToR, to maintain an ARP table learned from the
   ARP PDUs received by the switch and selectively propagates the ARP
   to, or proxy-responds on behalf of, the remote peer. These types of
   ARP processing principles are well known and used/described in L2VPN
   Working Group documents such as [ARP-Mediation] and [IPLS]. The ARP
   proxy response differs from that described in [RFC1027] as the ARP
   response contains MAC address of the destination and not that of the
   switch as is suggested in [RFC 1027].

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

1.1 Terminology

        ToR switch     Top-of-Rack switch. An Ethernet switch installed
                       at the top of a rack of servers which provides
                       network connectivity to those servers.

        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 switch     End-of-Row switch.  An Ethernet switch which
                        aggregates traffic from multiple racks.  Also
                        commonly referred to as an aggregation switch.
                        Uplinks from the ToR connects to EoR switches
                        and uplinks from EoR switches in turn connect
                        to core switches.

        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 data
                        center.  Both terms are used interchangeably
                        and refer to an IP end station.

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

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

        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. The VM architecture allows efficient
                        use of physical host resources (such as
                        multiple CPU cores).

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2.0 Configuration

   It is assumed that ARP reduction methodologies 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. The ARP reduction feature treats ARP frames received
   from downlink or uplink differently as described in the following

   In additional 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, etc.

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 PDUs in the following manner:
     . ARP Request PDUs must be redirected to control plane CPU.
     . Gratuitous ARP PDUs (ARP Reply PDU with a broadcast MAC DA)
        must be redirected to control plane CPU.
     . Other ARP Reply PDUs (ARP Reply PDU 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 Requests

   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 PDU. 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 PDU.

   If the destination IP address in the request is not present in the
   ARP table, then the original ARP request PDU is broadcast to all the
   switch ports that are member of the same VLAN except the source port
   that the Request was received from. However, if the requested

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   (destination) IP address is present in the ARP table, a unicast ARP
   Reply PDU is prepared as described above and sent to the switch port
   from which the ARP Request was received and original ARP request PDU
   is dropped.

   The intent is to prevent propagation of ARP Request PDU 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 requests from the local hosts of a ToR switch
         for the local hosts of the ToR switch can be prevented.
      . Most of the ARP requests from the remote hosts of a ToR switch
         for the local hosts of the ToR switch can be prevented from
         getting forwarded on downlinks or other uplinks of the ToR
      . Many of the ARP requests from the local hosts of a ToR switch
         for the remote hosts of the 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 PDU with 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 the IP hosts (including VMs) to keep its association fresh in
   peer's 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 PDU is forwarded
         out; otherwise the PDU is discarded.

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

   By suppressing the propagation of Gratuitous ARP PDUs, 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, as described above, by the first hop ToR switch will be
   able to respond to this request based on the ARP cache maintained in
   the ToR switch. In essence, presence of large ARP tables with longer
   age out times compensates for the smaller ARP table present in the

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   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 Host movement

   As mentioned earlier, server virtualization technology allows
   movement of VMs to different physical servers. The flexibility to
   move VMs is one of the key benefits of server virtualization. The
   VM movement 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, 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 can removed the ARP entry it has for
   that VM in a timely fashion, rather than waiting for it to timeout.

3.5 Applicability to environments with overlay transport

   Recently, there have been multiple proposals for using overlay
   transport technologies such as VXLAN [VXLAN] and NVGRE [NVGRE].
   These proposals allow the network operator to build the network
   using L2 or L3 technologies while building an L2-overlay on top of
   that.  As such, while they address the issue of network design, they
   do not eliminate the need for a mechanism to reduce the amount of
   broadcast traffic that may have to traverse the core, if there are
   VMs of the same tenant on servers attached to different ToR

   One of the ways for the overlay transport proposals to address this
   issue would be to implement the mechanism discussed in this document
   at the point where the overlay encapsulation and decapsulation is
   performed (i.e. in the virtual switch).

3.6 Scaling Considerations

   Depending on the number of hosts in the networks, the ARP table can
   be quite large. Although it is possible to implement some of the
   mechanisms for ARP reduction as described in this document in
   hardware in the forwarding plane, the number of ARP entries may
   favor maintaining the ARP table in the control plane memory.

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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
   timer for ARP entries.  A longer timer for holding on to ARP entries
   helps with reduction of broadcasts.  However, the risk of having a
   "too large timer" can cause 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, 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 timed out.  In this case,
   it is easy to see that a smaller timer would be beneficial.
   Additionally, since host B has an ARP age timer, it means that host
   B would find out about host A's unavailability only after its entry
   has aged, which would be after it 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 2 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 times out
   at Switch #2.

4.0 Conclusion

   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 broadcasts that are increasingly
   becoming a cause for concern in the data centers. In essence, ToR
   switches facilitate the offloading of the extended ARP table
   management from the IP hosts to itself. The ARP table timeout can be
   tuned higher by the operator based on the available switch resources
   and network traffic behavior. The larger capacity of the ARP table
   directly translates to more effective subduing of the ARP

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5.0  Security Considerations

   The details of the security aspects will be addressed in future

6.0  Acknowledgments

   This document resulted from discussions with Linda Durbar (Huawei),
   Sue Hares (Huawei), and T Sridhar (VMware).  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, STD

   [ARP-Problem] T. Narten, "Problem Statement for ARMD,"
      work in progress, <draft-ietf-armd-problem-statement>.

7.2 Informative References

   [ARP-Mediation] H. Shah et al., "ARP Mediation for IP interworking
      in Layer 2 VPN," work in progress, <draft-ietf-l2vpn-arp-

   [IPLS] H.Shah et al., "IP-only LAN service," work in progress,

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

   [RFC1027] Smoot et al., "Using ARP to Implement Transparent Subnet

   [VXLAN] M. Mahalingam et al., "VXLAN: A Framework for Overlaying
      Virtualized Layer 2 Networks over Layer 3 Networks"," work in
      progress, <draft-mahalingam-dutt-dcops-vxlan>.

   [NVGRE] M. Sridharan et al., " NVGRE: Network Virtualization using
      Generic Routing Encapsulation", work in progress, <draft-

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8.0 Author's Address

   Himanshu Shah
   Ciena Corp
   Email: hshah@ciena.com

   Anoop Ghanwani
   Email: anoop@alumni.duke.edu

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

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