Practices for scaling ARP and ND for large data centers

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Last updated 2012-12-11
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ARMD                                                L. Dunbar
Internet Draft                                         Huawei
Intended status: Informational                     W. Kumari
Expires: June 2013                                    Google
                                              Igor Gashinsky
                                            December 11, 2012

      Practices for scaling ARP and ND for large data centers


Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance
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   This draft documents some simple practices that scale ARP/ND
   in data center environments.

Table of Contents

   1. Introduction ................................................ 3
   2. Terminology ................................................. 3
   3. Common DC network Designs.................................... 4
   4. Layer 3 to Access Switches................................... 4
   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. Station communicating with an external peer........ 5
         5.1.2. L2/L3 boundary router processing of inbound
         traffic .................................................. 6
         5.1.3. Inter subnets communications ...................... 7
      5.2. Static ARP/ND entries on switches ....................... 7
      5.3. ARP/ND Proxy approaches ................................. 8
   6. Practices to scale ARP/ND in Overlay models .................. 8
   7. Summary and Recommendations .................................. 9
   8. Security Considerations ...................................... 9
   9. IANA Considerations ......................................... 9
   10. Acknowledgements .......................................... 10
   11. References ................................................ 10
      11.1. Normative References.................................. 10
      11.2. Informative References................................ 10
   Authors' Addresses ............................................ 11

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1. Introduction

   As described in [ARMD-Problem], the increasing trend of
   rapid workload shifting and server virtualization in modern
   data centers requires servers to be loaded (or re-loaded)
   with different 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

   In order to allow a physical server to be loaded with VMs in
   different subnets, or VMs to be moved to different server
   racks without IP address re-configuration, the corresponding
   networks need to enable multiple broadcast domains (many
   VLANs) on the interfaces of L2/L3 boundary routers and ToR
   switches. Unfortunately, when the combined number of VMs (or
   hosts) in all those subnets is large, this can lead to
   address resolution scaling issues, especially on the L2/L3
   boundary routers.

   This draft documents some simple practices which can scale
   ARP/ND in data center environment.

2. Terminology

   This document reuses much of terminology from [ARMD-
   Problem]. 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

   Bridge:  IEEE802.1Q compliant device. In this draft, Bridge
             is used interchangeably with Layer 2 switch.

   DC:      Data Center

   DA:     Destination Address

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

   EOR:    End of Row switches in data center.

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   NA:     IPv6's Neighbor Advertisement

   ND:     IPv6's Neighbor Discovery [RFC4861]

   NS:     IPv6's Neighbor Solicitation

   SA:     Source Address

   Station: A node which is either a destination or source of a
             data frame.

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

   UNA:    IPv6's Unsolicited Neighbor Advertisement

   VM:     Virtual Machines

3. Common DC network Designs

   Some common network designs for data center include:

     1) layer-3 connectivity to the access switch,

     2) Large Layer 2,

     3) Overlay models

   There is no single network design that fits all cases.
   Following sections document some of the common practices to
   scale Address Resolution under each network design.

4. Layer 3 to Access Switches

   This refers to the network design with Layer 3 to the access

   As described in [ARMD-Problem], 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 Layer 3 all the
   way to servers (or even to the VMs), which confines the

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   ARP/ND broadcast/multicast messages to the small number of
   VMs within the server.

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

   Disadvantage: The main disadvantage to this network design
   is that IP addresses have to be re-configured on switches
   when a server needs to be re-loaded with an application in
   different subnet or when VMs need to be moved to a different

   Summary: This solution is more suitable to data centers
   which have static workload and/or network operators who can
   re-configure 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

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

   5.1.1. Station communicating with an external peer

   When the external peer is in a different subnet, the
   originating end station 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, the gateway router
   has to process a very large number of ARP/ND requests. This
   is often CPU intensive as ARP/ND are usually processed by
   the CPU (and not in hardware).

   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 the
   result, most (if not all) end stations will not need to ARP
   for the gateway routers when they need to communicate with
   external peers.

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   However, due to IPv6 requiring bi-directional path
   validation Ipv6 end stations are still required to send
   unicast ND messages to their default gateway router (even
   with those routers periodically sending Unsolicited Neighbor

   Advantage: Reduction of ARP requests to be processed by
   L2/L3 boundary router for IPv4.

   Disadvantage: No reduction of ND processing on L2/L3
   boundary router for IPv6 traffic.

   Recommendation: Use for IPv4-only networks, or make change to the ND
   protocol to allow data frames to be sent without requiring bi-
   directional frame validation. Some work in progress in this area is

   5.1.2. L2/L3 boundary router processing of inbound traffic

   When a L2/L3 boundary router receives a data frame and the
   destination is not in 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: For IPv4 network, a common practice to alleviate
   this problem is for the router to snoop ARP messages, 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.

   For IPv6 end stations, routers are supposed to send ND
   unicast even if it has snooped UNA/NS/NA from those
   stations. Therefore, this practice doesn't help IPv6 very

   Advantage: Reduction of the number of ARP requests which
   routers have to send upon receiving IPv4 packets and the
   number of IPv4 data frames from external peers which routers
   have to hold.

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   Disadvantage: The amount of ND processing on routers for
   IPv6 traffic is not reduced. Even for 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.

   Recommendation: Do not use with IPv6 or make protocol
   changes to IPv6's ND. For IPv4, if there is higher chance of
   routers receiving data packets towards non-existing or
   inactive targets, alternative approaches should be

   5.1.3. Inter subnets communications

   The router will be hit twice when the originating and
   destination stations are in different subnets under on the
   same router. Once for the originating station in subnet-A
   initiating ARP/ND request to the L2/L3 boundary router
   (5.1.1 above); and the second for the L2/L3 boundary router
   to initiate ARP/ND requests to the target in subnet-B (5.1.2

   Again, practices described in 5.1.1 and 5.1.2 can alleviate
   problems in IPv4 network, but don't help very much for IPv6.

   Advantage: reduction of ARP processing on L2/L3 boundary
   routers for IPv4 traffic.

   For IPv6 traffic, there is no reduction of ND processing on
   L2/L3 boundary routers.

   Recommendation: do not use with IPv6 or consider other

   5.2. Static ARP/ND entries on switches

   In a datacenter 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: There is no well-defined mechanism for devices
   to get prompt incremental updates of static ARP/ND entries
   when changes occur.

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   Recommendation: The IETF should consider creating standard
   mechanism (or protocols) for switches or servers to get
   incremental static ARP/ND entries updates.

   5.3. ARP/ND Proxy approaches

   RFC1027 specifies one ARP proxy approach. Since the
   publication of RFC1027 in 1987 there have been many variants
   of ARP proxy being deployed. The term "ARP Proxy" is a
   loaded phrase, with different interpretations depending on
   vendors and/or environments.  RFC1027's ARP Proxy is for a
   Gateway to return its own MAC address on behalf of the
   target station.  Another technique, also called "ARP Proxy"
   is for a ToR switch to snoop ARP requests and return the
   target station's MAC if the ToR has the information.

   Advantage: Proxy ARP [RFC1027] and its variants have allowed
   multi-subnet ARP traffic for over a decade.

   Disadvantage: Proxy ARP protocol [RFC1027] was developed for
   hosts which don't support subnets.

   Recommendation: Revise RFC1027 with VLAN support and make it
   scale for Data Center Environment.

6. Practices to scale ARP/ND in Overlay models

   There are several drafts on using overlay networks to scale
   large layer 2 networks (or avoid the need for large L2
   networks) and enable mobility (e.g. draft-wkumari-dcops-l3-
   vmmobility-00, draft-mahalingam-dutt-dcops-vxlan-00). TRILL
   and IEEE802.1ah (Mac-in-Mac) are other types of overlay
   network to scale Layer 2.

   Overlay networks hide the VMs' addresses from the interior
   switches and routers, thereby stopping the router from
   having to perform ARP/ND services for as many addresses. The
   Overlay Edge nodes which perform the network address
   encapsulation/decapsulation still see all remote stations
   addresses which communicate with stations attached locally.

   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 devices(s) in processing resolving target
   stations' physical address (MAC or IP) and overlay edge
   address within the data center.

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   Here are some approaches being used to minimize the problem:

      1. Use static mapping as described in Section 5.2.

      2. Have multiple gateway nodes (i.e. routers), with each
        handling a subset of stations addresses which 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 which can
    alleviate the impact of address resolution to L2/L3 gateway

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

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

      -  Extend IPv6 ND method,

      -  Create a incremental "download" schemes for static
         ARP/ND entries,

      -  Revise Proxy ARP [RFC1027] for use in the data center.

8. Security Considerations

   This draft 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. As such we do not believe that this introduces
   any security concerns.

9. IANA Considerations

   This document does not request any action from IANA.

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10. Acknowledgements

   We want to acknowledge the following people for their
   valuable inputs to this draft: T. Sridhar, Ron Bonica,
   Kireeti Kompella, and K.K.Ramakrishnan.

11. References

   11.1. Normative References

   [ARMD-Problem] Narten, "Problem Statement for ARMD"
             problem-statement/); Aug 2012

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

   [RFC826] D.C. Plummer, "An Ethernet address resolution
             protocol." RFC826, Nov 1982.

   [RFC1027] Mitchell, et al, "Using ARP to Implement
             Transparent Subnet Gateways"

   [RFC4861] Narten, et al, "Neighbor Discovery for IP version
             6 (IPv6)", RFC4861, Sept 2007

   11.2. Informative References

    [Impatient-NUD] E. Nordmark, I. Gashinsky, "draft-ietf-

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Authors' Addresses

   Linda Dunbar
   Huawei Technologies
   5340 Legacy Drive, Suite 175
   Plano, TX 75024, USA
   Phone: (469) 277 5840

   Warren Kumari
   1600 Amphitheatre Parkway
   Mountain View, CA 94043

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

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