Practices for Scaling ARP and Neighbor Discovery (ND) in Large Data Centers
RFC 7342

Independent Submission                                         L. Dunbar
Request for Comments: 7342                                        Huawei
Category: Informational                                        W. Kumari
ISSN: 2070-1721                                                   Google
                                                            I. Gashinsky
                                                                   Yahoo
                                                             August 2014

           Practices for Scaling ARP and Neighbor Discovery (ND)
                           in Large Data Centers

Abstract

   This memo documents some operational practices that allow ARP and
   Neighbor Discovery (ND) to scale in data center environments.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7342.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   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.

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Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................4
   3. Common DC Network Designs .......................................4
   4. Layer 3 to Access Switches ......................................5
   5. Layer 2 Practices to Scale ARP/ND ...............................5
      5.1. Practices to Alleviate APR/ND Burden on L2/L3
           Boundary Routers ...........................................5
           5.1.1. Communicating with a Peer in a Different Subnet .....6
           5.1.2. L2/L3 Boundary Router Processing of Inbound
                  Traffic .............................................7
           5.1.3. Inter-Subnet Communications .........................8
      5.2. Static ARP/ND Entries on Switches ..........................8
      5.3. ARP/ND Proxy Approaches ....................................9
      5.4. Multicast Scaling Issues ...................................9
   6. Practices to Scale ARP/ND in Overlay Models ....................10
   7. Summary and Recommendations ....................................10
   8. Security Considerations ........................................11
   9. Acknowledgements ...............................................11
   10. References ....................................................12
      10.1. Normative References .....................................12
      10.2. Informative References ...................................13

1.  Introduction

   This memo documents some operational practices that allow ARP/ND to
   scale in data center environments.

   As described in [RFC6820], the increasing trend of rapid workload
   shifting and server virtualization in modern data centers requires
   servers to be loaded (or reloaded) with different Virtual Machines
   (VMs) or applications at different times.  Different VMs residing on
   one physical server may have different IP addresses or may even be in
   different IP subnets.

   In order to allow a physical server to be loaded with VMs in
   different subnets or allow VMs to be moved to different server racks
   without IP address reconfiguration, the networks need to enable
   multiple broadcast domains (many VLANs) on the interfaces of L2/L3
   boundary routers and Top-of-Rack (ToR) switches and allow some
   subnets to span multiple router ports.

   Note: L2/L3 boundary routers as discussed in this document are
   capable of forwarding IEEE 802.1 Ethernet frames (Layer 2) without a
   Media Access Control (MAC) header change.  When subnets span multiple
   ports of those routers, they still fall under the category of
   "single-link" subnets, specifically the multi-access link model

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   recommended by [RFC4903].  They are different from the "multi-link"
   subnets described in [Multi-Link] and RFC 4903, which refer to
   different physical media with the same prefix connected to one
   router.  Within the "multi-link" subnet described in RFC 4903, Layer
   2 frames from one port cannot be natively forwarded to another port
   without a header change.

   Unfortunately, when the combined number of VMs (or hosts) in all
   those subnets is large, this can lead to address resolution (i.e.,
   IPv4 ARP and IPv6 ND) scaling issues.  There are three major issues
   associated with ARP/ND address resolution protocols when subnets span
   multiple L2/L3 boundary router ports:

   1) The ARP/ND messages being flooded to many physical link segments,
      which can reduce bandwidth utilization for user traffic.

   2) The ARP/ND processing load impact on the L2/L3 boundary routers.

   3) In IPv4, every end station in a subnet receiving ARP broadcast
      messages from all other end stations in the subnet.  IPv6 ND has
      eliminated this issue by using multicast.

   Since the majority of data center servers are moving towards 1G or
   10G ports, the bandwidth taken by ARP/ND messages, even when flooded
   to all physical links, becomes negligible compared to the link
   bandwidth.  In addition, IGMP/MLD (Internet Group Management Protocol
   and Multicast Listener Discovery) snooping [RFC4541] can further
   reduce the ND multicast traffic to some physical link segments.

   As modern servers' computing power increases, the processing taken by
   a large amount of ARP broadcast messages becomes less significant to
   servers.  For example, lab testing shows that 2000 ARP requests
   per second only takes 2% of a single-core CPU server.  Therefore, the
   impact of ARP broadcasts to end stations is not significant on
   today's servers.

   Statistics provided by Merit Network [ARMD-Statistics] have shown
   that the major impact of a large number of mobile VMs in a data
   center is on the L2/L3 boundary routers, i.e., issue 2 above.

   This memo documents some simple practices that can scale ARP/ND in a
   data center environment, especially in reducing processing loads to
   L2/L3 boundary routers.

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2.  Terminology

   This document reuses much of the terminology from [RFC6820].  Many of
   the definitions are presented here to aid the reader.

   ARP: IPv4 Address Resolution Protocol [RFC826]

   Aggregation Switch: A Layer 2 switch interconnecting ToR switches

   Bridge: IEEE802.1Q-compliant device.  In this document, the term
      "Bridge" is used interchangeably with "Layer 2 switch"

   DC: Data Center

   DA: Destination Address

   End Station: VM or physical server, whose address is either the
      destination or the source of a data frame

   EoR: End-of-Row switches in a data center

   NA: IPv6 Neighbor Advertisement

   ND: IPv6 Neighbor Discovery [RFC4861]

   NS: IPv6 Neighbor Solicitation

   SA: Source Address

   ToR: Top-of-Rack Switch (also known as access switch)

   UNA: IPv6 Unsolicited Neighbor Advertisement

   VM: Virtual Machine

   Subnet: Refers to the multi-access link subnet referenced by RFC 4903

3.  Common DC Network Designs

   Some common network designs for a data center include:

   1) Layer 3 connectivity to the access switch,

   2) Large Layer 2, and

   3) Overlay models.

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   There is no single network design that fits all cases.  The following
   sections document some of the common practices to scale address
   resolution under each network design.

4.  Layer 3 to Access Switches

   This network design configures Layer 3 to the access switches,
   effectively making the access switches the L2/L3 boundary routers for
   the attached VMs.

   As described in [RFC6820], many data centers are architected so that
   ARP/ND broadcast/multicast messages are confined to a few ports
   (interfaces) of the access switches (i.e., ToR switches).

   Another variant of the Layer 3 solution is a Layer 3 infrastructure
   configured all the way to servers (or even to the VMs), which
   confines the ARP/ND broadcast/multicast messages to the small number
   of VMs within the server.

   Advantage: Both ARP and ND scale well.  There is no address
      resolution issue in this design.

   Disadvantage: The main disadvantage of this network design occurs
      during VM movement.  During VM movement, either VMs need an
      address change or switches/routers need a configuration change
      when the VMs are moved to different locations.

   Summary: This solution is more suitable to data centers that have a
      static workload and/or network operators who can reconfigure IP
      addresses/subnets on switches before any workload change.  No
      protocol changes are suggested.

5.  Layer 2 Practices to Scale ARP/ND

5.1.  Practices to Alleviate APR/ND Burden on L2/L3 Boundary Routers

   The ARP/ND broadcast/multicast messages in a Layer 2 domain can
   negatively affect the L2/L3 boundary routers, especially with a large
   number of VMs and subnets.  This section describes some commonly used
   practices for reducing the ARP/ND processing required on L2/L3
   boundary routers.

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5.1.1.  Communicating with a Peer in a Different Subnet

   Scenario: When the originating end station doesn't have its default
      gateway MAC address in its ARP/ND cache and needs to communicate
      with a peer in a different subnet, it needs to send ARP/ND
      requests to its default gateway router to resolve the router's MAC
      address.  If there are many subnets on the gateway router and a
      large number of end stations in those subnets that don't have the
      gateway MAC address in their ARP/ND caches, the gateway router has
      to process a very large number of ARP/ND requests.  This is often
      CPU intensive, as ARP/ND messages are usually processed by the CPU
      (and not in hardware).

   Note: Any centralized configuration that preloads the default MAC
      addresses is not included in this scenario.

   Solution: For IPv4 networks, a practice to alleviate this problem is
      to have the L2/L3 boundary router send periodic gratuitous ARP
      [GratuitousARP] messages, so that all the connected end stations
      can refresh their ARP caches.  As a result, most (if not all) end
      stations will not need to send ARP requests for the gateway
      routers when they need to communicate with external peers.

   For the above scenario, IPv6 end stations are still required to send
   unicast ND messages to their default gateway router (even with those
   routers periodically sending Unsolicited Neighbor Advertisements)
   because IPv6 requires bidirectional path validation.

   Advantage: This practice results in a reduction of ARP requests to be
      processed by the L2/L3 boundary router for IPv4.

   Disadvantage: This practice doesn't reduce ND processing on the L2/L3
      boundary router for IPv6 traffic.

   Recommendation: If the network is an IPv4-only network, then this
      approach can be used.  For an IPv6 network, one needs to consider
      the work described in [RFC7048].  Note: ND and Secure Neighbor
      Discovery (SEND) [RFC3971] use the bidirectional nature of queries
      to detect and prevent security attacks.

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5.1.2.  L2/L3 Boundary Router Processing of Inbound Traffic

   Scenario: When an L2/L3 boundary router receives a data frame
      destined for a local subnet and the destination is not in the
      router's ARP/ND cache, some routers hold the packet and trigger an
      ARP/ND request to resolve the L2 address.  The router may need to
      send multiple ARP/ND requests until either a timeout is reached or
      an ARP/ND reply is received before forwarding the data packets
      towards the target's MAC address.  This process is not only CPU
      intensive but also buffer intensive.

   Solution: To protect a router from being overburdened by resolving
      target MAC addresses, one solution is for the router to limit the
      rate of resolving target MAC addresses for inbound traffic whose
      target is not in the router's ARP/ND cache.  When the rate is
      exceeded, the incoming traffic whose target is not in the ARP/ND
      cache is dropped.

   For an IPv4 network, another common practice to alleviate pain caused
   by this problem is for the router to snoop ARP messages between other
   hosts, so that its ARP cache can be refreshed with active addresses
   in the L2 domain.  As a result, there is an increased likelihood of
   the router's ARP cache having the IP-MAC entry when it receives data
   frames from external peers.  [RFC6820] Section 7.1 provides a full
   description of this problem.

   For IPv6 end stations, routers are supposed to send Router
   Advertisements (RAs) unicast even if they have snooped UNAs/NSs/NAs
   from those stations.  Therefore, this practice allows an L2/L3
   boundary to send unicast RAs to the target instead of multicasts.
   [RFC6820] Section 7.2 has a full description of this problem.

   Advantage: This practice results in a reduction of the number of ARP
      requests that routers have to send upon receiving IPv4 packets and
      the number of IPv4 data frames from external peers that routers
      have to hold due to targets not being in the ARP cache.

   Disadvantage: The amount of ND processing on routers for IPv6 traffic
      is not reduced.  IPv4 routers still need to hold data packets from
      external peers and trigger ARP requests if the targets of the data
      packets either don't exist or are not very active.  In this case,
      IPv4 processing or IPv4 buffers are not reduced.

   Recommendation: If there is a higher chance of routers receiving data
      packets that are destined for nonexistent or inactive targets,
      alternative approaches should be considered.

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5.1.3.  Inter-Subnet Communications

   The router could be hit with ARP/ND requests twice when the
   originating and destination stations are in different subnets
   attached to the same router and those hosts don't communicate with
   external peers often enough.  The first hit is when the originating
   station in subnet-A initiates an ARP/ND request to the L2/L3 boundary
   router if the router's MAC is not in the host's cache (Section 5.1.1
   above), and the second hit is when the L2/L3 boundary router
   initiates ARP/ND requests to the target in subnet-B if the target is
   not in the router's ARP/ND cache (Section 5.1.2 above).

   Again, practices described in Sections 5.1.1 and 5.1.2 can alleviate
   some problems in some IPv4 networks.

   For IPv6 traffic, the practices described above don't reduce the ND
   processing on L2/L3 boundary routers.

   Recommendation: Consider the recommended approaches described in
      Sections 5.1.1 and 5.1.2.  However, any solutions that relax the
      bidirectional requirement of IPv6 ND disable the security that the
      two-way ND communication exchange provides.

5.2.  Static ARP/ND Entries on Switches

   In a data center environment, the placement of L2 and L3 addressing
   may be orchestrated by Server (or VM) Management System(s).
   Therefore, it may be possible for static ARP/ND entries to be
   configured on routers and/or servers.

   Advantage: This methodology has been used to reduce ARP/ND
      fluctuations in large-scale data center networks.

   Disadvantage: When some VMs are added, deleted, or moved, many
      switches' static entries need to be updated.  In a data center
      with virtualized servers, those events can happen frequently.  For
      example, for an event of one VM being added to one server, if the
      subnet of this VM spans 15 access switches, all of them need to be
      updated.  Network management mechanisms (SNMP, the Network
      Configuration Protocol (NETCONF), or proprietary mechanisms) are
      available to provide updates or incremental updates.  However,
      there is no well-defined approach for switches to synchronize
      their content with the management system for efficient incremental
      updates.

   Recommendation: Additional work may be needed within IETF working
      groups (e.g., NETCONF, NVO3, I2RS, etc.) to get prompt incremental
      updates of static ARP/ND entries when changes occur.

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5.3.  ARP/ND Proxy Approaches

   RFC 1027 [RFC1027] specifies one ARP Proxy approach referred to as
   "Proxy ARP".  However, RFC 1027 does not discuss a scaling mechanism.
   Since the publication of RFC 1027 in 1987, many variants of Proxy ARP
   have been deployed.  RFC 1027's Proxy ARP technique allows a gateway
   to return its own MAC address on behalf of the target station.

   [ARP_Reduction] describes a type of "ARP Proxy" that allows a ToR
   switch to snoop ARP requests and return the target station's MAC if
   the ToR has the information in its cache.  However, [RFC4903] doesn't
   recommend the caching approach described in [ARP_Reduction] because
   such a cache prevents any type of fast mobility between Layer 2 ports
   and breaks Secure Neighbor Discovery [RFC3971].

   IPv6 ND Proxy [RFC4389] specifies a proxy used between an Ethernet
   segment and other segments, such as wireless or PPP segments.  ND
   Proxy [RFC4389] doesn't allow a proxy to send NA messages on behalf
   of the target to ensure that the proxy does not interfere with hosts
   moving from one segment to another.  Therefore, the ND Proxy
   [RFC4389] doesn't reduce the number of ND messages to an L2/L3
   boundary router.

   Bottom line, the term "ARP/ND Proxy" has different interpretations,
   depending on vendors and/or environments.

   Recommendation: For IPv4, even though those Proxy ARP variants (not
      RFC 1076) have been used to reduce ARP traffic in various
      environments, there are many issues with caching.

   The IETF should consider making proxy recommendations for data center
   environments as a transition issue to help DC operators transitioning
   to IPv6.  Section 7 of [RFC4389] ("Guidelines to Proxy Developers")
   should be considered when developing any new proxy protocols to
   scale ARP.

5.4.  Multicast Scaling Issues

   Multicast snooping (IGMP/MLD) has different implementations and
   scaling issues.  [RFC4541] notes that multicast IGMPv2/v3 snooping
   has trouble with subnets that include IGMPv2 and IGMPv3.  [RFC4541]
   also notes that MLDv2 snooping requires the use of either destination
   MAC (DMAC) address filtering or deeper inspection of frames/packets
   to allow for scaling.

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   MLDv2 snooping needs to be re-examined for scaling within the DC.
   Efforts such as IGMP/MLD explicit tracking [IGMP-MLD-Tracking] for
   downstream hosts need to provide better scaling than IGMP/MLDv2
   snooping.

6.  Practices to Scale ARP/ND in Overlay Models

   There are several documents on using overlay networks to scale large
   Layer 2 networks (or avoid the need for large L2 networks) and enable
   mobility (e.g., [L3-VM-Mobility], [VXLAN]).  Transparent
   Interconnection of Lots of Links (TRILL) and IEEE 802.1ah
   (Mac-in-Mac) are other types of overlay networks that can scale
   Layer 2.

   Overlay networks hide the VMs' addresses from the interior switches
   and routers, thereby greatly reducing the number of addresses exposed
   to the interior switches and router.  The overlay edge nodes that
   perform the network address encapsulation/decapsulation still handle
   all remote stations' addresses that communicate with the locally
   attached end stations.

   For a large data center with many applications, these applications'
   IP addresses need to be reachable by external peers.  Therefore, the
   overlay network may have a bottleneck at the gateway node(s) in
   processing resolving target stations' physical addresses (MAC or IP)
   and the overlay edge address within the data center.

   Here are two approaches that can be used to minimize this problem:

   1. Use static mapping as described in Section 5.2.

   2. Have multiple L2/L3 boundary nodes (i.e., routers), with each
      handling a subset of stations' addresses that are visible to
      external peers (e.g., Gateway #1 handles a set of prefixes,
      Gateway #2 handles another subset of prefixes, etc.).

7.  Summary and Recommendations

   This memo describes some common practices that can alleviate the
   impact of address resolution on L2/L3 gateway routers.

   In data centers, no single solution fits all deployments.  This memo
   has summarized some practices in various scenarios and the advantages
   and disadvantages of all of these practices.

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   In some of these scenarios, the common practices could be improved by
   creating and/or extending existing IETF protocols.  These protocol
   change recommendations are:

   o  Relax the bidirectional requirement of IPv6 ND in some
      environments.  However, other issues will be introduced when the
      bidirectional requirement of ND is relaxed.  Therefore, it is
      necessary to have performed a comprehensive study of possible
      issues prior to making those changes.

   o  Create an incremental "update" scheme for efficient static ARP/ND
      entries.

   o  Develop IPv4 ARP/IPv6 ND Proxy standards for use in the data
      center.  Section 7 of [RFC4389] ("Guidelines to Proxy Developers")
      should be considered when developing any new proxy protocols to
      scale ARP/ND.

   o  Consider scaling issues with IGMP/MLD snooping to determine
      whether or not new alternatives can provide better scaling.

8.  Security Considerations

   This memo documents existing solutions and proposes additional work
   that could be initiated to extend various IETF protocols to better
   scale ARP/ND for the data center environment.

   Security is a major issue for data center environments.  Therefore,
   security should be seriously considered when developing any future
   protocol extensions.

9.  Acknowledgements

   We want to acknowledge the ARMD WG and the following people for their
   valuable inputs to this document: Joel Jaeggli, Dave Thaler, Susan
   Hares, Benson Schliesser, T. Sridhar, Ron Bonica, Kireeti Kompella,
   and K.K. Ramakrishnan.

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10.  References

10.1.  Normative References

   [GratuitousARP]
              Cheshire, S., "IPv4 Address Conflict Detection", RFC 5227,
              July 2008.

   [RFC826]   Plummer, D., "Ethernet Address Resolution Protocol: Or
              Converting Network Protocol Addresses to 48.bit Ethernet
              Address for Transmission on Ethernet Hardware", STD 37,
              RFC 826, November 1982.

   [RFC1027]  Carl-Mitchell, S. and J. Quarterman, "Using ARP to
              implement transparent subnet gateways", RFC 1027,
              October 1987.

   [RFC3971]  Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
              "SEcure Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [RFC4389]  Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery
              Proxies (ND Proxy)", RFC 4389, April 2006.

   [RFC4541]  Christensen, M., Kimball, K., and F. Solensky,
              "Considerations for Internet Group Management Protocol
              (IGMP) and Multicast Listener Discovery (MLD) Snooping
              Switches", RFC 4541, May 2006.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [RFC4903]  Thaler, D., "Multi-Link Subnet Issues", RFC 4903,
              June 2007.

   [RFC6820]  Narten, T., Karir, M., and I. Foo, "Address Resolution
              Problems in Large Data Center Networks", RFC 6820,
              January 2013.

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10.2.  Informative References

   [ARMD-Statistics]
              Karir, M. and J. Rees, "Address Resolution Statistics",
              Work in Progress, July 2011.

   [ARP_Reduction]
              Shah, H., Ghanwani, A., and N. Bitar, "ARP Broadcast
              Reduction for Large Data Centers", Work in Progress,
              October 2011.

   [IGMP-MLD-Tracking]
              Asaeda, H., "IGMP/MLD-Based Explicit Membership Tracking
              Function for Multicast Routers", Work in Progress,
              December 2013.

   [L3-VM-Mobility]
              Kumari, W. and J. Halpern, "Virtual Machine mobility in L3
              Networks", Work in Progress, August 2011.

   [Multi-Link]
              Thaler, D. and C. Huitema, "Multi-link Subnet Support in
              IPv6", Work in Progress, June 2002.

   [RFC1076]  Trewitt, G. and C. Partridge, "HEMS Monitoring and Control
              Language", RFC 1076, November 1988.

   [RFC7048]  Nordmark, E. and I. Gashinsky, "Neighbor Unreachability
              Detection Is Too Impatient", RFC 7048, January 2014.

   [VXLAN]    Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger,
              L., Sridhar, T., Bursell, M., and C. Wright, "VXLAN: A
              Framework for Overlaying Virtualized Layer 2 Networks over
              Layer 3 Networks", Work in Progress, April 2014.

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Authors' Addresses

   Linda Dunbar
   Huawei Technologies
   5340 Legacy Drive, Suite 175
   Plano, TX  75024
   USA

   Phone: (469) 277 5840
   EMail: ldunbar@huawei.com

   Warren Kumari
   Google
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   USA

   EMail: warren@kumari.net

   Igor Gashinsky
   Yahoo
   45 West 18th Street 6th floor
   New York, NY  10011
   USA

   EMail: igor@yahoo-inc.com

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