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rfc:rfc7586

Independent Submission Y. Nachum Request for Comments: 7586 Category: Experimental L. Dunbar ISSN: 2070-1721 Huawei

                                                         I. Yerushalmi
                                                            T. Mizrahi
                                                               Marvell
                                                             June 2015
          The Scalable Address Resolution Protocol (SARP)
                       for Large Data Centers

Abstract

 This document introduces the Scalable Address Resolution Protocol
 (SARP), an architecture that uses proxy gateways to scale large data
 center networks.  SARP is based on fast proxies that significantly
 reduce switches' Filtering Database (FDB) table sizes and reduce
 impact of ARP and Neighbor Discovery (ND) on network elements in an
 environment where hosts within one subnet (or VLAN) can spread over
 various locations.  SARP is targeted for massive data centers with a
 significant number of Virtual Machines (VMs) that can move across
 various physical locations.

Independent Submissions Editor Note

 This is an Experimental document; that experiment will end two years
 after the RFC is published.  At that point, the RFC authors will
 attempt to determine how widely SARP has been implemented and used.

IESG Note

 The IESG notes that the problems described in RFC 6820 can already be
 addressed through the simple combination of existing standardized or
 other published techniques including Layer 2 VPN (RFC 4664), proxy
 ARP (RFC 925), proxy Neighbor Discovery (RFC 4389), IGMP and MLD
 snooping (RFC 4541), and ARP mediation for IP interworking of Layer 2
 VPNs (RFC 6575).

Nachum, et al. Experimental [Page 1] RFC 7586 SARP June 2015

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for examination, experimental implementation, and
 evaluation.
 This document defines an Experimental Protocol for the Internet
 community.  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/rfc7586.

Copyright Notice

 Copyright (c) 2015 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.

Nachum, et al. Experimental [Page 2] RFC 7586 SARP June 2015

Table of Contents

 1. Introduction ....................................................3
    1.1. SARP Motivation ............................................4
    1.2. SARP Overview ..............................................7
    1.3. SARP Deployment Options ....................................8
    1.4. Comparison with Existing Solutions .........................9
 2. Terms and Abbreviations Used in This Document ..................10
 3. SARP: Theory of Operation ......................................11
    3.1. Control Plane: ARP/ND .....................................11
         3.1.1. ARP/NS Request for a Local VM ......................11
         3.1.2. ARP/NS Request for a Remote VM .....................12
         3.1.3. Gratuitous ARP and Unsolicited Neighbor
                Advertisement (UNA) ................................13
    3.2. Data Plane: Packet Transmission ...........................13
         3.2.1. Local Packet Transmission ..........................13
         3.2.2. Packet Transmission between Sites ..................13
    3.3. VM Migration ..............................................14
         3.3.1. VM Local Migration .................................14
         3.3.2. VM Migration from One Site to Another ..............14
                3.3.2.1. Impact on IP-to-MAC Mapping Cache
                         Table of Migrated VMs .....................16
    3.4. Multicast and Broadcast ...................................17
    3.5. Non-IP Packet .............................................17
    3.6. High Availability and Load Balancing ......................17
    3.7. SARP Interaction with Overlay Networks ....................18
 4. Security Considerations ........................................18
 5. References .....................................................19
    5.1. Normative References ......................................19
    5.2. Informative References ....................................20
 Acknowledgments ...................................................21
 Authors' Addresses ................................................21

1. Introduction

 This document describes a proxy gateway technique, called the
 Scalable Address Resolution Protocol (SARP), which reduces switches'
 Filtering Database (FDB) size and ARP/Neighbor Discovery impact on
 network elements in an environment where hosts within one subnet (or
 VLAN) can spread over various access domains in data centers.
 The main idea of SARP is to represent all VMs (or hosts) under each
 access domain by the MAC address of their corresponding access node
 (or aggregation node).  For example (Figure 1), when host A in the
 west site needs to communicate with host B, which is on the same VLAN
 but connected to a different access domain (east site), SARP requires
 host A to use the MAC address of SARP proxy 2, rather than the
 address of host B.  By doing so, switches in each domain do not need

Nachum, et al. Experimental [Page 3] RFC 7586 SARP June 2015

 to maintain a list of MAC addresses for all the VMs (hosts) in
 different access domains; every switch only needs to be familiar with
 MAC addresses that reside in the current domain, and addresses of
 remote SARP proxy gateways.  Therefore, the switches' FDB size is
 limited regardless of the number of access domains.
   +-------+     +-------+    _   __       +-------+     +-------+
   |       |     | SARP  |   / \_/  \_     | SARP  |     |       |
   |host A |<===>| proxy |<=>\_       \<==>| proxy |<===>|host B |
   |       |     |   1   |   /       _/    |   2   |     |       |
   +-------+     +-------+   \__   _/      +-------+     +-------+
                                \_/
   <------West Site------>                 <------East Site------>
                   Figure 1: A Brief Overview of SARP

1.1. SARP Motivation

 [RFC6820] discusses the impacts and scaling issues that arise in data
 center networks when subnets span across multiple Layer 2 / Layer 3
 (L2/L3) boundary routers.
 Unfortunately, when the combined number of VMs (or hosts) in all
 those subnets is large, it can lead to an explosion of the size of
 the switches' MAC address table and a heavy impact on network
 elements.
 There are four major issues associated with subnets spanning across
 multiple L2/L3 boundary router ports:
 1) Explosion of the size of the intermediate switches' MAC address
    table (FDB).
    When hosts in a VLAN (or subnet) span across multiple access
    domains and each access domain has hosts belonging to different
    VLANs, each access switch has to enable multiple VLANs.  Thus,
    those access switches are exposed to all MAC addresses across all
    VLANs.
    For example, for an access switch with 40 attached physical
    servers, where each server has 100 VMs, the access switch has
    4,000 attached MAC addresses.  If hosts/VMs can indeed be moved
    anywhere, the worst case for the Access Switch is when all those
    4,000 VMs belong to different VLANs, i.e., the access switch has
    4000 VLANs enabled.  If each VLAN has 200 hosts, this access
    switch's MAC address table potentially has 200 * 4,000 = 800,000
    entries.

Nachum, et al. Experimental [Page 4] RFC 7586 SARP June 2015

    It is important to note that the example above is relevant
    regardless of whether IPv4 or IPv6 is used.
    The example illustrates a scenario that is worse than what today's
    L2/L3 gateway has to face.  In today's environment, where each
    subnet is limited to a few access switches, the number of MAC
    addresses the gateway has to learn is of a significantly smaller
    scale.
 2) ARP/ND processing load impact on the L2/L3 boundary routers.
    All VMs periodically send NDs to their corresponding gateway nodes
    to get gateway nodes' MAC addresses.  When the combined number of
    VMs across all the VLANs is large, processing the responses to the
    ND requests from those VMs can easily exhaust the gateway's CPU
    utilization.
    An L2/L3 boundary router could be hit with ARP/ND twice when the
    originating and destination stations are in different subnets
    attached to the same router and when those hosts do not
    communicate with external peers very frequently.  The first hit is
    when the originating station in subnet 1 initiates an ARP/ND
    request to the L2/L3 boundary router.  The second hit is when the
    L2/L3 boundary router initiates an ARP/ND request to the target in
    subnet 2 if the target is not in the router's ARP/ND cache.
 3) In IPv4, every end station in a subnet receives ARP broadcast
    messages from all other end stations in the subnet.  IPv6 ND has
    eliminated this issue by using multicast.
    However, most devices support a limited number of multicast
    addresses, due to the scaling of multicast filtering.  Once the
    number of multicast addresses exceeds the multicast filter limit,
    the multicast addresses have to be processed by the devices' CPUs
    (i.e., the slow path).
    It is less of an issue in data centers without VM mobility, since
    each port is only dedicated to one (or a small number of) VLANs.
    Thus, the number of multicast addresses hitting each port is
    significantly lower.
 4) The ARP/ND messages are flooded to many physical link segments
    that can reduce the bandwidth utilization for user traffic.
    ARP/ND flooding is, in most cases, an insignificant issue in
    today's data center networks, as the majority of data center
    servers are shifting towards 1G or 10G Ethernet ports.  The
    bandwidth used by ARP/ND, even when flooded to all physical links,

Nachum, et al. Experimental [Page 5] RFC 7586 SARP June 2015

    becomes negligible compared to the link bandwidth.  Furthermore,
    IGMP and Multicast Listener Discovery (MLD) snooping [RFC4541] can
    further reduce the ND multicast traffic to some physical link
    segments.
 Statistics gathered by Merit Network [ARMDStats] have shown that the
 major impact of a large number of VMs in data centers is on the L2/L3
 boundary routers, i.e., issue 2 above.  An L2/L3 boundary router
 could be hit with ARP/ND twice when 1) the originating and
 destination stations are in different subnets attached to the same
 router, and 2) those hosts do not communicate with external peers
 often enough.
 Overlay approaches, e.g., [RFC7364], can hide addresses of hosts
 (VMs) in the core, but they do not prevent the MAC address table
 explosion problem (issue 1) unless the Network Virtualization Edge
 (NVE) is on a server.
 The scaling practices documented in [RFC7342] can only reduce some
 ARP impact on L2/L3 boundary routers in some scenarios, but not all.
 In order to protect router CPUs from being overburdened by target
 resolution requests, some routers rate-limit the target MAC
 resolution requests to the router's CPU.  When the rate limit is
 exceeded, the incoming data frames are dropped.  In traditional data
 centers, this issue is less significant, since the number of hosts
 attached to one L2/L3 boundary router is limited by the number of
 physical ports of the switches/routers.  When servers are virtualized
 to support 30+ VMs, the number of hosts under one router can grow by
 a factor of 30+.  Furthermore, in traditional data center networks,
 each subnet is neatly bound to a limited number of server racks,
 i.e., switches only need to be familiar with MAC addresses of hosts
 that reside in this small number of subnets.  In contemporary data
 center networks, as subnets are spread across many server racks,
 switches are exposed to VLAN/MAC addresses of many subnets, greatly
 increasing the size of switches' FDB tables.
 The solution proposed in this document can eliminate or reduce the
 likelihood of inter-subnet data frames being dropped and reduce the
 number of host MAC addresses that intermediate switches are exposed
 to, thus reducing switches' FDB table sizes.

Nachum, et al. Experimental [Page 6] RFC 7586 SARP June 2015

1.2. SARP Overview

 The SARP approach uses proxy gateways to address the problems
 discussed above.
 Note: The guidelines to proxy developers [RFC4389] have been
 carefully considered for SARP.  Section 3.3 discusses how SARP works
 when VMs are moved from one segment to another.
 In order to enable VMs to be moved across servers while ensuring
 their MAC/IP addresses remain unchanged, the Layer 2 network (e.g.,
 VLAN) that interconnects those VMs may spread across different server
 racks, different rows of server racks, or even different data center
 sites.
 A multisite data center network is comprised of two main building
 blocks: an interconnecting segment and an access segment.  While the
 access network is, in most cases, a Layer 2 network, the
 interconnecting segment is not necessarily a Layer 2 network.
 The SARP proxies are located at the boundaries where the access
 segment connects to its interconnecting segment.  The boundary node
 can be a hypervisor virtual switch, a top-of-rack switch, an
 aggregation switch (or end-of-row switch), or a data center core
 switch.  Figure 2 depicts an example of two remote data centers that
 are managed as a single, flat Layer 2 domain.  SARP proxies are
 implemented at the edge devices connecting the data center to the
 transport network.  SARP significantly reduces the ARP/ND
 transmissions over the interconnecting network.

Nachum, et al. Experimental [Page 7] RFC 7586 SARP June 2015

  • ——————-*

| |

                  +-------| Interconnecting   |-------+
                  |       |     network       |       |
                  |       *-------------------*       |
                  |                                   |
         *-----------------*                  *----------------*
         |  SARP Proxies   |                  |  SARP Proxies  |
         *-----------------*                  *----------------*
            |           |                        |           |
        *-------*   *-------*                *-------*   *-------*
        |Access |   |Access |                |Access |   |Access |
        *-------*   *-------*                *-------*   *-------*
            |
       *----------*
       |Hypervisor|
       *----------*
            |
        *--------*
        |Virtual |
        |Machine |
        *--------*
           (West Site)                          (East Site)
             Figure 2: SARP: Network Architecture Example

1.3. SARP Deployment Options

 SARP deployment is tightly coupled with the data center architecture.
 SARP proxies are located at the point where the Layer 2
 infrastructure connects to its Layer 2 cloud using overlay networks.
 SARP proxies can be located at the data center edge (as Figure 2
 depicts), data center core, or data center aggregation (denoted by
 "Agg" in the figure).  SARP can also be implemented by the hypervisor
 (as Figure 3 depicts).
 To simplify the description, we will focus on data centers that are
 managed as a single, flat Layer 2 network, where SARP proxies are
 located at the boundary where the data center connects to the
 transport network (as Figure 2 depicts).

Nachum, et al. Experimental [Page 8] RFC 7586 SARP June 2015

  • ——————-*

| |

                  +-------|     TRANSPORT     |-------+
                  |       |                   |       |
                  |       *-------------------*       |
                  |                                   |
         *-----------------*                  *----------------*
         |   Edge Device   |                  |  Edge Device   |
         *-----------------*                  *----------------*
                  |                                   |
         *-----------------*                  *----------------*
         |       Core      |                  |      Core      |
         *-----------------*                  *----------------*
            |           |                        |           |
        *-------*   *-------*                *-------*   *-------*
        |  Agg  |   |  Agg  |                |  Agg  |   |  Agg  |
        *-------*   *-------*                *-------*   *-------*
            |
       *----------*
       |Hypervisor|
       *----------*
           (West Site)                          (East Site)
                   Figure 3: SARP Deployment Options

1.4. Comparison with Existing Solutions

 The IETF has developed several mechanisms to address issues
 associated with Layer 2 networks over multiple geographic locations,
 for example, Layer 2 VPN [RFC4664], proxy ARP [RFC925] [ProxyARP],
 proxy Neighbor Discovery [RFC4389], IGMP and MLD snooping [RFC4541],
 and ARP mediation for IP interworking of Layer 2 VPNs [RFC6575].
 However, all those solutions work well when hosts within one subnet
 are placed together under one access domain, so that the intermediate
 switches in each access domain are only exposed to host addresses
 from a limited number of subnets.  SARP is to provide a solution when
 hosts within one subnet are spread across multiple access domains,
 and each access domain has hosts from many subnets.  Under this
 environment, the intermediate switches in each access domain are
 exposed to combined hosts of all the subnets that are enabled by the
 access domain.

Nachum, et al. Experimental [Page 9] RFC 7586 SARP June 2015

2. Terms and Abbreviations Used in This Document

 ARP:   Address Resolution Protocol [ARP]
 FDB:   Filtering Database, which is used for Layer 2 switches
        [802.1Q].  Layer 2 switches flood data frames when the
        Destination Address (DA) is not in the FDB, whereas routers
        drop data frames when the DA is not in the Forwarding
        Information Base (FIB).  That is why the FDB is used for Layer
        2 switches.
 FIB:   Forwarding Information Base
 Hypervisor: a software layer that creates and runs virtual machines
        on a server
 IP-D:  IP address of the destination virtual machine
 IP-S:  IP address of the source virtual machine
 MAC-D: MAC address of the destination virtual machine
 MAC-E: MAC address of the East Proxy SARP Device
 MAC-S: MAC address of the source virtual machine
 NA:    IPv6 ND's Neighbor Advertisement
 ND:    IPv6 Neighbor Discovery Protocol [ND].  In this document, ND
        also refers to Neighbor Solicitation, Neighbor Advertisement,
        and Unsolicited Neighbor Advertisement messages defined by RFC
        4861.
 NS:    IPv6 ND's Neighbor Solicitation
 SARP Proxy: The components that participate in SARP
 UNA:   IPv6 ND's Unsolicited Neighbor Advertisement [ND]
 VM:    Virtual Machine

Nachum, et al. Experimental [Page 10] RFC 7586 SARP June 2015

3. SARP: Theory of Operation

3.1. Control Plane: ARP/ND

 This section describes the ARP/ND procedure scenarios.  The first
 scenario addresses a case where both the source and destination VMs
 reside in the same access segment.  In the second scenario, the
 source VM is in the local access segment and the destination VM is
 located at the remote access segment.
 In all scenarios, the VMs (source and destination) share the same L2
 broadcast domain.

3.1.1. ARP/NS Request for a Local VM

 When source and destination VMs are located at the same access
 segment (Figure 4), the address resolution process is as described in
 [ARP] and [ND]; host A sends an ARP request or an IPv6 Neighbor
 Solicitation (NS) to learn the IP-to-MAC mapping of host B, and it
 receives a reply from host B with the IP-D to MAC-D mapping.
   +-------+      _   __       +-------+      _   __
   |host A |     / \_/  \_     | SARP  |     / \_/  \_
   | IP-S  |<--->\_access \<==>| proxy |<===>\_interc.\
   | MAC-S |     /network_/    |   1   |     /network_/
   +-------+  +->\__   _/      +-------+     \__   _/
              |     \_/                         \_/
   +-------+  |
   |host B |<-+
   | IP-D  |
   | MAC-D |
   +-------+
   <--------------West Site------------>
         Figure 4: SARP: Two Hosts in the Same Access Segment

Nachum, et al. Experimental [Page 11] RFC 7586 SARP June 2015

3.1.2. ARP/NS Request for a Remote VM

 When the source and destination VMs are located at different access
 segments, the address resolution process is as follows.
   +-------+     +-------+    _   __       +-------+     +-------+
   |host A |     | SARP  |   / \_/  \_     | SARP  |     |host B |
   | IP-S  |<===>|proxy 1|<=>\_       \<==>|proxy 2|<===>| IP-D  |
   | MAC-S |     | MAC-W |   /       _/    | MAC-E |     | MAC-D |
   +-------+     +-------+   \__   _/      +-------+     +-------+
                                \_/
   <------West Site------>                 <------East Site------>
      Figure 5: SARP: Two Hosts That Reside in Different Segments
 In the example illustrated in Figure 5, the source VM is located at
 the west access segment and the destination VM is located at the east
 access segment.
 When host A sends an ARP/NS request to find out the IP-to-MAC mapping
 of host B:
 1. If SARP proxy 1 does not have IP-D in its ARP cache, the ARP/NS
    request is propagated to all access segments that might have VMs
    in the same virtual network as the originating VM, including the
    east access segment.
 2. As SARP proxy 1 forwards the ARP/NS message, it replaces the
    source MAC address, MAC-S, with its own MAC address, MAC-W.  Thus,
    all switches that reside in the interconnecting segment are not
    exposed to MAC-S.
 3. The ARP/NS request reaches SARP proxy 2.
 4. If SARP proxy 2 does not have IP-D in its ARP cache, the ARP/NS
    request is forwarded to the east access network.  Host B responds
    with an ARP reply (IPv4) or a Neighbor Advertisement (IPv6) to the
    request with MAC-D.
 5. When the response message reaches SARP proxy 2, it replaces MAC-D
    with MAC-E; thus, the response reaches SARP proxy 1 with MAC-E.
 6. As SARP proxy 1 forwards the response to host A, it replaces the
    destination address from MAC-W to MAC-S.

Nachum, et al. Experimental [Page 12] RFC 7586 SARP June 2015

 SARP Proxy ARP/ND Cache
 SARP proxies maintain a cache of the IP-to-MAC mapping.  This cache
 is based on ARP/ND messages that are sent by hosts and traverse the
 SARP proxies.
 In steps 1 and 4 above, if the SARP proxy has IP-D in its ARP cache,
 it responds with MAC-E, without forwarding the ARP/NS request.
 This caching approach significantly reduces the volume of the ARP/ND
 transmission over the network and reduces the round-trip time of
 ARP/ND requests.
 When the west SARP proxy caches the IP-to-MAC mapping entries for
 remote VMs, the expiration timers should be set to relatively low
 values to prevent stale entries due to remote VMs being moved or
 deleted.  In environments where VMs move more frequently, it is not
 recommended for SARP proxies to cache the IP-to-MAC mapping entries
 of remote VMs.

3.1.3. Gratuitous ARP and Unsolicited Neighbor Advertisement (UNA)

 Hosts (or VMs) send out Gratuitous ARP (IPv4) [TcpIp] and Unsolicited
 Neighbor Advertisement (UNA) (IPv6) messages to allow other nodes to
 refresh IP-to-MAC entries in their caches.
 The local SARP proxy processes the Gratuitous ARP or UNA message in
 the same way as the ARP reply or IPv6 NA, i.e., replaces the MAC
 addresses in the same manner.

3.2. Data Plane: Packet Transmission

3.2.1. Local Packet Transmission

 When a VM transmits packets to a destination VM that is located at
 the same site (Figure 4), the data plane is unaffected by SARP;
 packets are sent from (IP-S, MAC-S) to (IP-D, MAC-D).

3.2.2. Packet Transmission between Sites

 Packets that are sent between sites (Figure 5) traverse the SARP
 proxy of both sites.
 A packet sent from host A to host B undergoes the following
 procedure:
 1. Host A sends a packet to IP-D, and based on its ARP table, it uses
    the MAC addresses {MAC-E, MAC-S}.

Nachum, et al. Experimental [Page 13] RFC 7586 SARP June 2015

 2. SARP proxy 1 receives the packet and replaces the source MAC
    address, such that the packet includes {MAC-E, MAC-W}.
 3. SARP proxy 2 receives the packet and replaces the destination MAC
    address, and the packet is sent to host B with {MAC-D, MAC-W}.
 SARP proxy 1 replaces the source MAC address with its own, since
 switches in the interconnecting segment are only familiar with SARP
 proxy MAC addresses and are not familiar with host addresses.
 Note: it is a common security practice in data center networks to use
 access lists, allowing each VM to communicate only with a list of
 authorized peer VMs.  In most cases, such access control lists are
 based on IP addresses and, hence, are not affected by the MAC address
 replacement in SARP.

3.3. VM Migration

3.3.1. VM Local Migration

 When a VM migrates locally within its access segment, SARP does not
 require any special behavior.  VM migration is resolved entirely by
 the Layer 2 mechanisms.

3.3.2. VM Migration from One Site to Another

 This section focuses on a scenario where a VM migrates from the west
 site to the east site while maintaining its MAC and IP addresses.
 VM migration might affect networking elements based on their
 respective locations:
  1. origin site (west site)
  1. destination site (east site)
  1. other sites
   +-------+     +-------+    _   __       +-------+     +-------+
   |host A |     | SARP  |   / \_/  \_     | SARP  |     |host A |
   | IP-D  |<===>|proxy 1|<=>\_       \<==>|proxy 2|<===>| IP-D  |
   | MAC-D |     | MAC-W |   /       _/    | MAC-E |     | MAC-D |
   +-------+     +-------+   \__   _/      +-------+     +-------+
                                \_/
   <------West Site------>                 <------East Site------>
         Origin Site                          Destination Site
      Figure 6: SARP: Host A Migrates from West Site to East Site

Nachum, et al. Experimental [Page 14] RFC 7586 SARP June 2015

 Origin Site
    The origin site is the site where the VM resides before the
    migration (west site).
    Before the VM (IP=IP-D, MAC=MAC-D) is moved, all VMs at the west
    site that have an ARP entry of IP-D in their ARP table have the
    IP-D -> MAC-D mapping.  VMs on other access segments have an ARP
    entry of IP-D -> MAC-W mapping where MAC-W is the MAC address of
    the SARP proxy on the west access segment.
    After the VM (IP-D) in the west site moves to the east site, if a
    Gratuitous ARP (IPv4) or an Unsolicited Neighbor Advertisement
    (IPv6) message is sent out by the destination hypervisor on behalf
    of the VM (IP-D), then the IP-to-MAC mapping cache of the VMs in
    all access segments is updated by IP-D -> MAC-E, where MAC-E is
    the MAC address of the SARP proxy on the east site.  If no
    Gratuitous ARP or UNA message is sent out by the destination
    hypervisor, the IP-to-MAC cache on the VMs in the west site (and
    other sites) is eventually aged out.
    Until the IP-to-MAC mapping cache tables are updated, the source
    VMs from the west site continue sending packets locally to MAC-D,
    and switches at the west site are still configured with the old
    location of MAC-D.  This transient condition can be resolved by
    having the VM manager send out a fake Gratuitous ARP or UNA
    message on behalf of the destination Hypervisor.  Another
    alternative is to have a shorter aging timer configured for the
    IP-to-MAC cache table.
 Destination Site
    The destination site is the site to which the VM migrated, i.e.,
    the east site in Figure 6.
    Before any Gratuitous ARP or UNA messages are sent out by the
    destination hypervisor, all VMs at the east site (and all other
    sites) might have an IP-D -> MAC-W mapping in their IP-to-MAC
    mapping cache.  The IP-to-MAC mapping cache is updated by aging or
    by a Gratuitous ARP or UNA message sent by the destination
    hypervisor.  Until the IP-to-MAC mapping caches are updated, VMs
    from the east site continue to send packets to MAC-W.  This can be
    resolved by having the VM manager send out a fake Gratuitous ARP
    or UNA message immediately after the VM migration or by
    redirecting the packets from the SARP proxy of the east site back
    to the migrated VM by updating the destination MAC of the packets
    to MAC-D.

Nachum, et al. Experimental [Page 15] RFC 7586 SARP June 2015

 Other Sites
    All VMs at the other sites that have an ARP entry of IP-D in their
    ARP table have the IP-D -> MAC-W mapping.  The ARP mapping is
    updated by aging or by a Gratuitous ARP message sent by the
    destination hypervisor of the migrated VM and modified by the SARP
    proxy of the east site to an IP-D -> MAC-E mapping.  Until ARP
    tables are updated, VMs from other sites continue sending packets
    to MAC-W.

3.3.2.1. Impact on IP-to-MAC Mapping Cache Table of Migrated VMs

 When a VM (IP-D) is moved from one site to another, its IP-to-MAC
 mapping entries for VMs located at other sites (i.e., neither the
 east site nor the west site) are still valid, even though most guest
 OSs (or VMs) will refresh their IP-to-MAC cache after migration.
 The migrated VM's IP-to-MAC mapping entries for VMs located at the
 east site, if not refreshed after migration, can be kept with no
 change until the ARP aging time, as these entries are mapped to MAC-
 E.  All traffic originated from the migrated VM in its new location
 to VMs located at the east site traverses the SARP proxy of the east
 site.  That SARP proxy can redirect the traffic back to the
 corresponding destinations on the east site.  Furthermore, an ARP/UNA
 message sent by the SARP proxy of the east site or by the VMs on the
 east site can refresh the corresponding entries in the migrated VM's
 IP-to-MAC cache.
 The migrated VM's ARP entries for VMs located at the west site remain
 unchanged until either the ARP entries age out or new data frames are
 received from the remote sites.  Since all MAC addresses of the VMs
 located at the west site are unknown at the east site, all unknown
 traffic from the VM is intercepted by the SARP proxy of the east site
 and forwarded to the SARP proxy of the west site (during the
 transient period before the ARP entries age out).  This transient
 behavior is avoided if the SARP proxy has the destination IP address
 in its ARP cache, and, upon receiving a packet with an unknown
 destination MAC address, it could send a Gratuitous ARP or UNA
 message to the migrated VM.
 Note that overlay networks providing Layer 2 network virtualization
 services configure their edge-device MAC aging timers to be greater
 than the ARP request interval.

Nachum, et al. Experimental [Page 16] RFC 7586 SARP June 2015

3.4. Multicast and Broadcast

 Multicast and broadcast traffic is forwarded by SARP proxies as
 follows:
 o  SARP proxies modify the source MAC address of multicast and
    broadcast packets as described in Section 3.2.
 o  SARP proxies do not modify the destination MAC address of
    multicast and broadcast packets.

3.5. Non-IP Packet

 The L2/L3 boundary routers in the current document are capable of
 forwarding non-IP IEEE 802.1 Ethernet frames (Layer 2) without
 changing the MAC headers.  When subnets span across multiple ports of
 those routers, they are still under the category of a single link, or
 a multi-access link model recommended by [RFC4903].  They differ from
 the "multi-link" subnets described in [MultLinkSub] and [RFC4903],
 which refer to a different physical media with the same prefix
 connected to a router, where the Layer 2 frames cannot be natively
 forwarded without changing the headers.

3.6. High Availability and Load Balancing

 The SARP proxy is located at the boundary where the local Layer 2
 infrastructure connects to the interconnecting network.  All traffic
 from the local site to the remote sites traverses the SARP proxy.
 The SARP proxy is subject to high-availability and bandwidth
 requirements.
 The SARP architecture supports multiple SARP proxies connecting a
 single site to the transport network.  In the SARP architecture, all
 proxies can be active and can back up one another.  The SARP
 architecture is robust and allows network administrators to allocate
 proxies according to bandwidth and high-availability requirements.
 Traffic is segregated between SARP proxies by using VLANs.  An SARP
 proxy is the Master SARP proxy of a set of VLANs and the Backup SARP
 proxy of another set of VLANs.
 For example, assume the SARP proxies of the west site are SARP proxy
 1 and SARP proxy 2.  The west site supports VLAN 1 and VLAN 2, while
 SARP proxy 1 is the Master SARP proxy of VLAN 1 and the Backup SARP
 proxy of VLAN 2, and SARP proxy 2 is the Master SARP proxy of VLAN 2
 and the Backup SARP proxy of VLAN 1.  Both proxies are members of
 VLAN 1 and VLAN 2.

Nachum, et al. Experimental [Page 17] RFC 7586 SARP June 2015

 The Master SARP proxy updates its Backup SARP proxy with all the ARP
 reply messages.  The Backup SARP proxy maintains a backup database to
 all the VLANs that it is the Backup SARP proxy of.
 The Master and the Backup SARP proxies maintain a keepalive
 mechanism.  In case of a failure, the Backup SARP proxy becomes the
 Master SARP proxy.  The failure decision is per VLAN.  When the
 Master and the Backup SARP proxies switch over, the Backup SARP proxy
 can use the MAC address of the Master SARP proxy.  The Backup SARP
 proxy sends locally a Gratuitous ARP message with the MAC address of
 the Master SARP proxy to update the forwarding tables on the local
 switches.  The Backup SARP proxy also updates the remote SARP proxies
 on the change.

3.7. SARP Interaction with Overlay Networks

 SARP can be used over overlay networks, providing L2 network
 virtualization (such as IP, Virtual Private LAN Service (VPLS),
 Transparent Interconnection of Lots of Links (TRILL), Overlay
 Transport Virtualization (OTV), Network Virtualization using GRE
 (NVGRE), and Virtual eXtensible Local Area Network (VXLAN)).  The
 mapping of SARP to overlay networks is straightforward; the VM does
 the mapping of the destination IP to the SARP proxy MAC address.  The
 mapping of the proxy MAC to its correct tunnel is done by the overlay
 networks.
 SARP significantly scales down the complexity of the overlay networks
 and transport networks by reducing the mapping tables to the number
 of SARP proxies.

4. Security Considerations

 SARP proxies are located at the boundaries of access networks, where
 the local Layer 2 infrastructure connects to its Layer 2 cloud.  SARP
 proxies interoperate with overlay network protocols that extend the
 Layer 2 subnet across data centers or between different systems
 within a data center.
 SARP does not expose the network to security threats beyond those
 that exist whether or not SARP is present.
 SARP proxies may be exposed to denial-of-service (DoS) attacks by
 means of ARP/ND message flooding.  Thus, SARP proxies must have
 sufficient resources to support the SARP control plane without making
 the network more vulnerable to DoS than it was without SARP proxies.

Nachum, et al. Experimental [Page 18] RFC 7586 SARP June 2015

 SARP adds security to the data plane in terms of network
 reconnaissance, by hiding all the local Layer 2 MAC addresses from
 potential attackers located at the interconnecting network and
 significantly limiting the number of addresses exposed to an attacker
 at a remote site.

5. References

5.1. Normative References

 [ARP]       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, DOI 10.17487/RFC0826, November 1982,
             <http://www.rfc-editor.org/info/rfc826>.
 [ND]        Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
             "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
             DOI 10.17487/RFC4861, September 2007,
             <http://www.rfc-editor.org/info/rfc4861>.
 [ProxyARP]  Carl-Mitchell, S. and J. Quarterman, "Using ARP to
             implement transparent subnet gateways", RFC 1027,
             DOI 10.17487/RFC1027, October 1987,
             <http://www.rfc-editor.org/info/rfc1027>.
 [RFC925]    Postel, J., "Multi-LAN address resolution", RFC 925,
             DOI 10.17487/RFC0925, October 1984,
             <http://www.rfc-editor.org/info/rfc925>.
 [RFC4389]   Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery
             Proxies (ND Proxy)", RFC 4389, DOI 10.17487/RFC4389,
             April 2006, <http://www.rfc-editor.org/info/rfc4389>.
 [RFC4541]   Christensen, M., Kimball, K., and F. Solensky,
             "Considerations for Internet Group Management Protocol
             (IGMP) and Multicast Listener Discovery (MLD) Snooping
             Switches", RFC 4541, DOI 10.17487/RFC4541, May 2006,
             <http://www.rfc-editor.org/info/rfc4541>.
 [RFC4664]   Andersson, L., Ed., and E. Rosen, Ed., "Framework for
             Layer 2 Virtual Private Networks (L2VPNs)", RFC 4664,
             DOI 10.17487/RFC4664, September 2006,
             <http://www.rfc-editor.org/info/rfc4664>.

Nachum, et al. Experimental [Page 19] RFC 7586 SARP June 2015

 [RFC6575]   Shah, H., Ed., Rosen, E., Ed., Heron, G., Ed., and V.
             Kompella, Ed., "Address Resolution Protocol (ARP)
             Mediation for IP Interworking of Layer 2 VPNs", RFC 6575,
             DOI 10.17487/RFC6575, June 2012,
             <http://www.rfc-editor.org/info/rfc6575>.

5.2. Informative References

 [802.1Q]    IEEE, "IEEE Standard for Local and metropolitan area
             networks -- Bridges and Bridged Networks", IEEE Std
             802.1Q.
 [ARMDStats] Karir, M., and J. Rees, "Address Resolution Statistics",
             Work in Progress, draft-karir-armd-statistics-01, July
             2011.
 [MultLinkSub]
             Thaler, D., and C. Huitema, "Multi-link Subnet Support in
             IPv6", Work in Progress,
             draft-ietf-ipv6-multi-link-subnets-00, June 2002.
 [RFC4903]   Thaler, D., "Multi-Link Subnet Issues", RFC 4903,
             DOI 10.17487/RFC4903, June 2007,
             <http://www.rfc-editor.org/info/rfc4903>.
 [RFC6820]   Narten, T., Karir, M., and I. Foo, "Address Resolution
             Problems in Large Data Center Networks", RFC 6820,
             DOI 10.17487/RFC6820, January 2013,
             <http://www.rfc-editor.org/info/rfc6820>.
 [RFC7342]   Dunbar, L., Kumari, W., and I. Gashinsky, "Practices for
             Scaling ARP and Neighbor Discovery (ND) in Large Data
             Centers", RFC 7342, DOI 10.17487/RFC7342, August 2014,
             <http://www.rfc-editor.org/info/rfc7342>.
 [RFC7364]   Narten, T., Ed., Gray, E., Ed., Black, D., Fang, L.,
             Kreeger, L., and M. Napierala, "Problem Statement:
             Overlays for Network Virtualization", RFC 7364,
             DOI 10.17487/RFC7364, October 2014,
             <http://www.rfc-editor.org/info/rfc7364>.
 [TcpIp]     Stevens, W., "TCP/IP Illustrated, Volume 1: The
             Protocols", Addison-Wesley, 1994.

Nachum, et al. Experimental [Page 20] RFC 7586 SARP June 2015

Acknowledgments

 The authors thank Ted Lemon, Eric Gray, and Adrian Farrel for
 providing valuable comments and suggestions for the document.

Authors' Addresses

 Youval Nachum
 EMail: youval.nachum@gmail.com
 Linda Dunbar
 Huawei Technologies
 5430 Legacy Drive, Suite #175
 Plano, TX  75024
 United States
 Phone: (469) 277 5840
 EMail: ldunbar@huawei.com
 Ilan Yerushalmi
 Marvell
 6 Hamada St.
 Yokneam, 20692
 Israel
 EMail: yilan@marvell.com
 Tal Mizrahi
 Marvell
 6 Hamada St.
 Yokneam, 20692
 Israel
 EMail: talmi@marvell.com

Nachum, et al. Experimental [Page 21]

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