Extended Mobility Procedures for EVPN-IRB
draft-ietf-bess-evpn-irb-extended-mobility-05

Document Type Active Internet-Draft (bess WG)
Authors Neeraj Malhotra  , Ali Sajassi  , Aparna Pattekar  , Jorge Rabadan  , Avinash Lingala  , John Drake 
Last updated 2021-03-15
Replaces draft-malhotra-bess-evpn-irb-extended-mobility
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BESS WorkGroup                                          N. Malhotra, Ed.
Internet-Draft                                                A. Sajassi
Intended status: Standards Track                             A. Pattekar
Expires: September 16, 2021                                Cisco Systems
                                                              J. Rabadan
                                                                   Nokia
                                                              A. Lingala
                                                                     ATT
                                                                J. Drake
                                                        Juniper Networks
                                                          March 15, 2021

               Extended Mobility Procedures for EVPN-IRB
             draft-ietf-bess-evpn-irb-extended-mobility-05

Abstract

   Procedure to handle host mobility in a layer 2 Network with EVPN
   control plane is defined as part of RFC 7432.  EVPN has since evolved
   to find wider applicability across various IRB use cases that include
   distributing both MAC and IP reachability via a common EVPN control
   plane.  MAC Mobility procedures defined in RFC 7432 are extensible to
   IRB use cases if a fixed 1:1 mapping between VM IP and MAC is assumed
   across VM moves.  Generic mobility support for IP and MAC that allows
   these bindings to change across moves is required to support a
   broader set of EVPN IRB use cases, and requires further
   consideration.  EVPN all-active multi-homing further introduces
   scenarios that require additional consideration from mobility
   perspective.  This document enumerates a set of design considerations
   applicable to mobility across these EVPN IRB use cases and defines
   generic sequence number assignment procedures to address these IRB
   use cases.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   material or to cite them other than as "work in progress."

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   This Internet-Draft will expire on September 16, 2021.

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   described in the Simplified BSD License.

Table of Contents

   1.  Requirements Language and Terminology . . . . . . . . . . . .   3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Document Structure  . . . . . . . . . . . . . . . . . . .   5
   3.  Optional MAC only RT-2  . . . . . . . . . . . . . . . . . . .   6
   4.  Mobility Use Cases  . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Host MAC+IP Move  . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Host IP Move to new MAC . . . . . . . . . . . . . . . . .   7
       4.2.1.  VM Reload . . . . . . . . . . . . . . . . . . . . . .   7
       4.2.2.  MAC Sharing . . . . . . . . . . . . . . . . . . . . .   7
       4.2.3.  Problem . . . . . . . . . . . . . . . . . . . . . . .   7
     4.3.  Host MAC move to new IP . . . . . . . . . . . . . . . . .   8
       4.3.1.  Problem . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  EVPN All Active multi-homed ES  . . . . . . . . . . . . . . .  10
   6.  Design Considerations . . . . . . . . . . . . . . . . . . . .  11
   7.  Solution Components . . . . . . . . . . . . . . . . . . . . .  12
     7.1.  Sequence Number Inheritance . . . . . . . . . . . . . . .  12
     7.2.  MAC Sharing . . . . . . . . . . . . . . . . . . . . . . .  13
     7.3.  Multi-homing Mobility Synchronization . . . . . . . . . .  14
   8.  Requirements for Sequence Number Assignment . . . . . . . . .  14
     8.1.  LOCAL MAC-IP learning . . . . . . . . . . . . . . . . . .  15
     8.2.  LOCAL MAC learning  . . . . . . . . . . . . . . . . . . .  15
     8.3.  Remote MAC OR MAC-IP Update . . . . . . . . . . . . . . .  15
     8.4.  REMOTE (SYNC) MAC update  . . . . . . . . . . . . . . . .  16
     8.5.  REMOTE (SYNC) MAC-IP update . . . . . . . . . . . . . . .  16
     8.6.  Inter-op  . . . . . . . . . . . . . . . . . . . . . . . .  16
     8.7.  MAC Sharing Race Condition  . . . . . . . . . . . . . . .  17
     8.8.  Mobility Convergence  . . . . . . . . . . . . . . . . . .  17
       8.8.1.  Generalized Probing Logic . . . . . . . . . . . . . .  18
   9.  Routed Overlay  . . . . . . . . . . . . . . . . . . . . . . .  18

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   10. Duplicate Host Detection  . . . . . . . . . . . . . . . . . .  19
     10.1.  Scenario A . . . . . . . . . . . . . . . . . . . . . . .  19
     10.2.  Scenario B . . . . . . . . . . . . . . . . . . . . . . .  20
       10.2.1.  Duplicate IP Detection Procedure for Scenario B  . .  20
     10.3.  Scenario C . . . . . . . . . . . . . . . . . . . . . . .  21
     10.4.  Duplicate Host Recovery  . . . . . . . . . . . . . . . .  21
       10.4.1.  Route Un-freezing Configuration  . . . . . . . . . .  21
       10.4.2.  Route Clearing Configuration . . . . . . . . . . . .  22
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  23
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  23
   14. Normative References  . . . . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

1.  Requirements Language and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   o  EVPN-IRB: A BGP-EVPN distributed control plane based integrated
      routing and bridging fabric overlay discussed in [EVPN-IRB]

   o  Underlay: IP or MPLS fabric core network that provides IP or MPLS
      routed reachability between EVPN PEs.

   o  Overlay: VPN or service layer network consisting of EVPN PEs OR
      VPN provider-edge (PE) switch-router devices that runs on top of
      an underlay routed core.

   o  EVPN PE: A PE switch-router in a data-center fabric that runs
      overlay BGP-EVPN control plane and connects to overlay CE host
      devices.  An EVPN PE may also be the first-hop layer-3 gateway for
      CE/host devices.  This document refers to EVPN PE as a logical
      function in a data-center fabric.  This EVPN PE function may be
      physically hosted on a top-of-rack switching device (ToR) OR at
      layer(s) above the ToR in the Clos fabric.  An EVPN PE is
      typically also an IP or MPLS tunnel end-point for overlay VPN flow

   o  Symmetric EVPN-IRB: An overlay fabric first-hop routing
      architecture as defined in [EVPN-IRB], wherein, overlay host-to-
      host routed inter-subnet flows are routed at both ingress and
      egress EVPN PEs.

   o  Asymmetric EVPN-IRB: An overlay fabric first-hop routing
      architecture as defined in [EVPN-IRB], wherein, overlay host-to-
      host routed inter-subnet flows are routed and bridged at ingress
      PE and bridged at egress PEs.

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   o  ARP: Address Resolution Protocol [RFC 826].  ARP references in
      this document are equally applicable to ND as well.

   o  ND: IPv6 Neighbor Discovery Protocol [RFC 4861].

   o  Ethernet-Segment: physical Ethernet or LAG port that connects an
      access device to an EVPN PE, as defined in [RFC 7432].

   o  ESI: Ethernet Segment Identifier as defined in [RFC 7432].

   o  LAG: Layer-2 link-aggregation, also known as layer-2 bundle port-
      channel, or bond interface.

   o  EVPN all-active multi-homing: PE-CE all-active multi-homing
      achieved via a multi-homed layer-2 LAG interface on a CE with
      member links to multiple PEs and related EVPN procedures on the
      PEs.

   o  RT-2: EVPN route type 2 carrying both MAC and IP reachability.

   o  RT-5: EVPN route type 5 carrying IP prefix reachability.

   o  MAC-IP: IP association for a MAC, referred to in this document may
      be IPv4, IPv6 or both.

2.  Introduction

   EVPN-IRB enables capability to advertise both MAC and IP routes via a
   single MAC+IP RT-2 advertisement.  MAC is imported into local bridge
   MAC table and enables L2 bridged traffic across the network overlay.
   IP is imported into the local ARP table in an asymmetric IRB design
   OR imported into the IP routing table in a symmetric IRB design, and
   enables routed traffic across the layer 2 network overlay.  Please
   refer to [EVPN-IRB] for more background on EVPN IRB forwarding modes.

   To support EVPN mobility procedure, a single sequence number mobility
   attribute is advertised with the combined MAC+IP route.  A single
   sequence number advertised with the combined MAC+IP route to resolve
   both MAC and IP reachability implicitly assumes a 1:1 fixed mapping
   between IP and MAC.  While a fixed 1:1 mapping between IP and MAC is
   a common use case that could be addressed via existing MAC mobility
   procedure, additional IRB scenarios need to be considered, that don't
   necessarily adhere to this assumption.  Following IRB mobility
   scenarios are considered:

   o  VM move results in VM IP and MAC moving together

   o  VM move results in VM IP moving to a new MAC association

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   o  VM move results in VM MAC moving to a new IP association

   While existing MAC mobility procedure can be leveraged for MAC+IP
   move in the first scenario, subsequent scenarios result in a new MAC-
   IP association.  As a result, a single sequence number assigned
   independently per-[MAC, IP] is not sufficient to determine most
   recent reachability for both MAC and IP, unless the sequence number
   assignment algorithm is designed to allow for changing MAC-IP
   bindings across moves.

   Purpose of this draft is to define additional sequence number
   assignment and handling procedures to adequately address generic
   mobility support across EVPN-IRB overlay use cases that allow MAC-IP
   bindings to change across VM moves and can support mobility for both
   MAC and IP components carried in an EVPN RT-2 for these use cases.

   In addition, for hosts on an ESI multi-homed to multiple GW devices,
   additional procedure is proposed to ensure synchronized sequence
   number assignments across the multi-homing devices.

   Content presented in this draft is independent of data plane
   encapsulation used in the overlay being MPLS or NVO Tunnels.  It is
   also largely independent of the EVPN IRB solution being based on
   symmetric OR asymmetric IRB design as defined in [EVPN-INTER-SUBNET].

   In addition to symmetric and asymmetric IRB, mobility solution for a
   routed overlay, where traffic to an end host in the overlay is always
   IP routed using EVPN RT-5 is also presented in this document.

   To summarize, this draft covers mobility mobility for the following
   independent of the overlay encapsulation being MPLS or an NVO Tunnel:

   o  Symmetric EVPN IRB overlay

   o  Asymmetric EVPN IRB overlay

   o  Routed EVPN overlay

2.1.  Document Structure

   Following sections of the document should be condidered informnative:

   o  section 4 and 5 provide the necessary background and problem
      statement being addressed in this document.

   o  section 6 lists the resulting design considerations for the
      document.

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   Following sections of the document should be condidered normative:

   o  section 8 describes the mobility and sequence number assigment
      procedures in an EVPN-IRB overlay required to address the
      scenarios described in section 4.

   o  section 9 describes the mobility procedures for a routed overlay
      nextwork as opposed to an IRB overlay.

   o  section 10 describes corresponding duplicate detection procedures
      for EVPN-IRB and routed overlays.

3.  Optional MAC only RT-2

   In an EVPN IRB scenario, where a single MAC+IP RT-2 advertisement
   carries both IP and MAC routes, a MAC only RT-2 advertisement is
   redundant for host MACs that are advertised via MAC+IP RT-2.  As a
   result, a MAC only RT-2 is an optional route that may not be
   advertised from or received at an EVPN PE.  This is an important
   consideration for mobility scenarios discussed in subsequent
   sections.

   MAC only RT-2 may still be advertised for non-IP host MACs that are
   not advertised via MAC+IP RT-2.

4.  Mobility Use Cases

   This section describes the IRB mobility use cases considered in this
   document.  Procedures to address them are covered later in section 6
   and section 7.

   o  Host move results in Host IP and MAC moving together

   o  Host move results in Host IP moving to a new MAC association

   o  Host move results in Host MAC moving to a new IP association

4.1.  Host MAC+IP Move

   This is the baseline case, wherein a host move results in both host
   MAC and IP moving together with no change in MAC-IP binding across a
   move.  Existing MAC mobility defined in RFC 7432 may be leveraged to
   apply to corresponding MAC+IP route to support this mobility
   scenario.

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4.2.  Host IP Move to new MAC

   This is the case, where a host move results in VM IP moving to a new
   MAC binding.

4.2.1.  VM Reload

   A host reload or an orchestrated host move that results in host being
   re-spawned at a new location may result in host getting a new MAC
   assignment, while maintaining existing IP address.  This results in a
   host IP move to a new MAC binding:

   IP-a, MAC-a ---> IP-a, MAC-b

4.2.2.  MAC Sharing

   This takes into account scenarios, where multiple hosts, each with a
   unique IP, may share a common MAC binding, and a host move results in
   a new MAC binding for the host IP.

   As an example, hosts running on a single physical server, each with a
   unique IP, may share the same physical server MAC.  In yet another
   scenario, an L2 access network may be behind a firewall, such that
   all hosts IPs on the access network are learnt with a common firewall
   MAC.  In all such "shared MAC" use cases, multiple local MAC-IP ARP
   entries may be learnt with the same MAC.  A host IP move, in such
   scenarios (for e.g., to a new physical server), could result in new
   MAC association for the host IP.

4.2.3.  Problem

   In both of the above scenarios, a combined MAC+IP EVPN RT-2
   advertised with a single sequence number attribute implicitly assumes
   a fixed IP to MAC mapping.  A host IP move to a new MAC breaks this
   assumption and results in a new MAC+IP route.  If this new MAC+IP
   route is independently assigned a new sequence number, the sequence
   number can no longer be used to determine most recent host IP
   reachability in a symmetric EVPN-IRB design OR the most recent IP to
   MAC binding in an asymmetric EVPN-IRB design.

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                        +------------------------+
                        | Underlay Network Fabric|
                        +------------------------+

     +-----+   +-----+      +-----+   +-----+      +-----+   +-----+
     | PE1 |   | PE2 |      | PE3 |   | PE4 |      | PE5 |   | PE6 |
     +-----+   +-----+      +-----+   +-----+      +-----+   +-----+
        \         /            \         /            \         /
         \ ESI-1 /              \ ESI-2 /              \ ESI-3 /
          \     /                \     /                \     /
          +\---/+                +\---/+                +\---/+
          | \ / |                | \ / |                | \ / |
          +--+--+                +--+--+                +--+--+
             |                      |                      |
        Server-MAC1            Server-MAC2            Server-MAC3
             |                      |                      |
     [VM-IP1, VM-IP2]       [VM-IP3, VM-IP4]       [VM-IP5, VM-IP6]

                                 Figure 1

   As an example, consider a topology shown in Figure 1, with host VMs
   sharing the physical server MAC.  In steady state, [IP1, MAC1] route
   is learnt at [PE1, PE2] and advertised to remote PEs with a sequence
   number N.  Now, VM-IP1 is moved to Server-MAC2.  ARP or ND based
   local learning at [PE3, PE4] would now result in a new [IP1, MAC2]
   route being learnt.  If route [IP1, MAC2] is learnt as a new MAC+IP
   route and assigned a new sequence number of say 0, mobility procedure
   for VM-IP1 will not trigger across the overlay network.

   A sequence number assignment procedure needs to be defined to
   unambiguously determine the most recent IP reachability, IP to MAC
   binding, and MAC reachability for such a MAC sharing scenario.

4.3.  Host MAC move to new IP

   This is a scenario where host move or re-provisioning behind a new
   gateway location may result in host getting a new IP address
   assigned, while keeping the same MAC.

4.3.1.  Problem

   Complication with this scenario is that MAC reachability could be
   carried via a combined MAC+IP route while a MAC only route may not be
   advertised at all.  A single sequence number association with the
   MAC+IP route again implicitly assumes a fixed mapping between MAC and
   IP.  A MAC move resulting in a new IP association for the host MAC
   breaks this assumption and results in a new MAC+IP route.  If this

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   new MAC+IP route independently assumes a new sequence number, this
   mobility attribute can no longer be used to determine most recent
   host MAC reachability.

                      +------------------------+
                      | Underlay Network Fabric|
                      +------------------------+
   +-----+   +-----+      +-----+   +-----+      +-----+   +-----+
   | PE1 |   | PE2 |      | PE3 |   | PE4 |      | PE5 |   | PE6 |
   +-----+   +-----+      +-----+   +-----+      +-----+   +-----+
      \         /            \         /            \         /
       \ ESI-1 /              \ ESI-2 /              \ ESI-3 /
        \     /                \     /                \     /
        +\---/+                +\---/+                +\---/+
        | \ / |                | \ / |                | \ / |
        +--+--+                +--+--+                +--+--+
           |                      |                      |
        Server1                Server2                Server3
           |                      |                      |
   [VM-IP1-M1, VM-IP2-M2]  [VM-IP3-M3, VM-IP4-M4] [VM-IP5-M5, VM-IP6-M6]

                                 Figure 2

   As an example, consider a host VM IP1-M1 that is learnt locally at
   [PE1, PE2] and advertised to remote hosts with a sequence number N.
   Consider a scenario where this VM with MAC M1 is re-provisioned at
   server 2, however, as part of this re-provisioning, assigned a
   different IP address say IP7.  [IP7, M1] is learnt as a new route at
   [PE3, PE4] and advertised to remote PEs with a sequence number of 0.
   As a result, L3 reachability to IP7 would be established across the
   overlay, however, MAC mobility procedure for MAC1 will not trigger as
   a result of this MAC-IP route advertisement.  If an optional MAC only
   route is also advertised, sequence number associated with the MAC
   only route would trigger MAC mobility as per [RFC7432].  However, in
   the absence of an additional MAC only route advertisement, a single
   sequence number advertised with a combined MAC+IP route would not be
   sufficient to update MAC reachability across the overlay.

   A MAC-IP sequence number assignment procedure needs to be defined to
   unambiguously determine the most recent MAC reachability in such a
   scenario without a MAC only route being advertised.

   Further, PE1/PE2, on learning new reachability for [IP7, M1] via PE3/
   PE4 MUST probe and delete any local IPs associated with MAC M1, such
   as [IP1, M1] in the above example.

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   Arguably, MAC mobility sequence number defined in [RFC7432], could be
   interpreted to apply only to the MAC part of MAC-IP route, and would
   hence cover this scenario.  It could hence be interpreted as a
   clarification to [RFC7432] and one of the considerations for a common
   sequence number assignment procedure across all MAC-IP mobility
   scenarios detailed in this document.

5.  EVPN All Active multi-homed ES

                           +------------------------+
                           | Underlay Network Fabric|
                           +------------------------+

                               +-----+   +-----+
                               | PE1 |   | PE2 |
                               +-----+   +-----+
                                 \\         //
                                  \\ ESI-1 //
                                   \\     /X
                                   +\\---//+
                                   | \\ // |
                                   +---+---+
                                       |
                                      CE1

                                 Figure 3

   Consider an EVPN-IRB overlay network shown in Figure 2, with hosts
   multi-homed to two or more PE devices via an all-active multi-homed
   ES.  MAC and ARP entries learnt on a local ES may also be
   synchronized across the multi-homing PE devices sharing this ES.
   This MAC and ARP SYNC enables local switching of intra and inter
   subnet ECMP traffic flows from remote hosts.  In other words, local
   MAC and ARP entries on a given ES may be learnt via local learning
   and / or via sync from another PE device sharing the same ES.

   For a host that is multi-homed to multiple PE devices via an all-
   active ES interface, local learning of host MAC and MAC-IP at each PE
   device is an independent asynchronous event, that is dependent on
   traffic flow and or ARP / ND response from the host hashing to a
   directly connected PE on the MC-LAG interface.  As a result, sequence
   number mobility attribute value assigned to a locally learnt MAC or
   MAC-IP route at each device may not always be the same, depending on
   transient states on the device at the time of local learning.

   As an example, consider a host VM that is deleted from ESI-2 and
   moved to ESI-1.  It is possible for host to be learnt on say, PE1
   following deletion of the remote route from [PE3, PE4], while being

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   learnt on PE2 prior to deletion of remote route from [PE3, PE4].  If
   so, PE1 would process local host route learning as a new route and
   assign a sequence number of 0, while PE2 would process local host
   route learning as a remote to local move and assign a sequence number
   of N+1, N being the existing sequence number assigned at [PE3, PE4].

   Inconsistent sequence numbers advertised from multi-homing devices
   introduces:

   o  Ambiguity with respect to how the remote PEs should handle paths
      with same ESI and different sequence numbers.  A remote PE may not
      program ECMP paths if it receives routes with different sequence
      numbers from a set of multi-homing PEs sharing the same ESI.

   o  Breaks consistent route versioning across the network overlay that
      is needed for EVPN mobility procedures to work.

   As an example, in this inconsistent state, PE2 would drop a remote
   route received for the same host with sequence number N (as its local
   sequence number is N+1), while PE1 would install it as the best route
   (as its local sequence number is 0).

   There is need for a mechanism to ensure consistency of sequence
   numbers advertised from a set of multi-homing devices for EVPN
   mobility to work reliably.

   In order to support mobility for multi-homed hosts using the sequence
   number mobility attribute, local MAC and MAC-IP routes learnt on a
   multi-homed ES MUST be advertised with the same sequence number by
   all PE devices that the ES is multi-homed to.  There is need for a
   mechanism to ensure consistency of sequence numbers assigned across
   these PEs.

6.  Design Considerations

   To summarize, sequence number assignment scheme and implementation
   must take following considerations into account:

   o  MAC+IP may be learnt on an ES multi-homed to multiple PE devices,
      hence requires sequence numbers to be synchronized across multi-
      homing PE devices.

   o  MAC only RT-2 is optional in an IRB scenario and may not
      necessarily be advertised in addition to MAC+IP RT-2.

   o  Single MAC may be associated with multiple IPs, i.e., multiple
      host IPs may share a common MAC.

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   o  Host IP move could result in host moving to a new MAC, resulting
      in a new IP to MAC association and a new MAC+IP route.

   o  Host MAC move to a new location could result in host MAC being
      associated with a different IP address, resulting in a new MAC to
      IP association and a new MAC+IP route.

   o  LOCAL MAC-IP learn via ARP would always accompanied by a LOCAL MAC
      learn event resulting from the ARP packet.  MAC and MAC-IP
      learning, however, could happen in any order.

   o  Use cases discussed earlier that do not maintain a constant 1:1
      MAC-IP mapping across moves could potentially be addressed by
      using separate sequence numbers associated with MAC and IP
      components of MAC+IP route.  Maintaining two separate sequence
      numbers however adds significant overhead with respect to
      complexity, debugability, and backward compatibility.  Hence, this
      document addresses these requirements via a single sequence number
      attribute.

7.  Solution Components

   This section goes over main components of the EVPN IRB mobility
   solution proposed in this draft.  Later sections will go over exact
   sequence number assignment procedures resulting from concepts
   described in this section.

7.1.  Sequence Number Inheritance

   Main idea presented here is to view a LOCAL MAC-IP route as a child
   of the corresponding LOCAL MAC only route that inherits the sequence
   number attribute from the parent LOCAL MAC only route:

   Mx-IPx -----> Mx (seq# = N)

   As a result, both parent MAC and child MAC-IP routes share one common
   sequence number associated with the parent MAC route.  Doing so
   ensures that a single sequence number attribute carried in a combined
   MAC+IP route represents sequence number for both a MAC only route as
   well as a MAC+IP route, and hence makes the MAC only route truly
   optional.  As a result, optional MAC only route with its own sequence
   number is not required to establish most recent reachability for a
   MAC in the overlay network.  Specifically, this enables a MAC to
   assume a different IP address on a move, and still be able to
   establish most recent reachability to the MAC across the overlay
   network via mobility attribute associated with the MAC+IP route
   advertisement.  As an example, when Mx moves to a new location, it
   would result in LOCAL Mx being assigned a higher sequence number at

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   its new location as per RFC 7432.  If this move results in Mx
   assuming a different IP address, IPz, LOCAL Mx+IPz route would
   inherit the new sequence number from Mx.

   LOCAL MAC and LOCAL MAC-IP routes would typically be sourced from
   data plane learning and ARP learning respectively, and could get
   learnt in control plane in any order.  Implementation could either
   replicate inherited sequence number in each MAC-IP entry OR maintain
   a single attribute in the parent MAC by creating a forward reference
   LOCAL MAC object for cases where a LOCAL MAC-IP is learnt before the
   LOCAL MAC.

   Arguably, this inheritance may be assumed from RFC 7432, in which
   case, the above may be interpreted as a clarification with respect to
   interpretation of a MAC sequence number in a MAC-IP route.

7.2.  MAC Sharing

   Further, for the shared MAC scenario, this would result in multiple
   LOCAL MAC-IP siblings inheriting sequence number attribute from a
   common parent MAC route:

     Mx-IP1 -----
      |          |
     Mx-IP2 -----
       .         |
       .         +---> Mx (seq# = N)
       .         |
     Mx-IPw -----
       |         |
     Mx-IPx -----

                                 Figure 4

   In such a case, a host-IP move to a different physical server would
   result in IP moving to a new MAC binding.  A new MAC-IP route
   resulting from this move must now be advertised with a sequence
   number that is higher than the previous MAC-IP route for this IP,
   advertised from the prior location.  As an example, consider a route
   Mx-IPx that is currently advertised with sequence number N from PE1.
   IPx moving to a new physical server behind PE2 results in IPx being
   associated with MAC Mz.  A new local Mz-IPx route resulting from this
   move at PE2 must now be advertised with a sequence number higher than
   N.  This is so that PE devices, including PE1, PE2, and other remote
   PE devices that are part of the overlay can clearly determine and
   program the most recent MAC binding and reachability for the IP.
   PE1, on receiving this new Mz-IPx route with sequence number say,

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   N+1, for symmetric IRB case, would update IPx reachability via PE2 in
   forwarding, for asymmetric IRB case, would update IPx's ARP binding
   to Mz.  In addition, PE1 would clear and withdraw the stale Mx-IPx
   route with the lower sequence number.

   This also implies that sequence number associated with local MAC Mz
   and all local MAC-IP children of Mz at PE2 must now be incremented to
   N+1, and re-advertised across the overlay.  While this re-
   advertisement of all local MAC-IP children routes affected by the
   parent MAC route is an overhead, it avoids the need for two separate
   sequence number attributes to be maintained and advertised for IP and
   MAC components of MAC+IP RT-2.  Implementation would need to be able
   to lookup MAC-IP routes for a given IP and update sequence number for
   it's parent MAC and its MAC-IP children.

7.3.  Multi-homing Mobility Synchronization

   In order to support mobility for multi-homed hosts, local MAC and
   MAC-IP routes learnt on a shared ES MUST be advertised with the same
   sequence number by all PE devices that the ES is multi-homed to.
   This also applies to local MAC only routes.  LOCAL MAC and MAC-IP may
   be learnt natively via data plane and ARP/ND respectively as well as
   via SYNC from another multi-homing PE to achieve local switching.
   Local and SYNC route learning can happen in any order.  Local MAC-IP
   routes advertised by all multi-homing PE devices sharing the ES must
   carry the same sequence number, independent of the order in which
   they are learnt.  This implies:

   o  On local or sync MAC-IP route learning, sequence number for the
      local MAC-IP route MUST be compared and updated to the higher
      value.

   o  On local or sync MAC route learning, sequence number for the local
      MAC route MUST be compared and updated to the higher value.

   If an update to local MAC-IP sequence number is required as a result
   of above comparison with sync MAC-IP route, it would essentially
   amount to a sequence number update on the parent local MAC, resulting
   in inherited sequence number update on the MAC-IP route.

8.  Requirements for Sequence Number Assignment

   Following sections summarize sequence number assignment procedure
   needed on local and sync MAC and MAC-IP route learning events in
   order to accomplish the above.

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8.1.  LOCAL MAC-IP learning

   A local Mx-IPx learning via ARP or ND should result in computation OR
   re-computation of parent MAC Mx's sequence number, following which
   the MAC-IP route Mx-IPx would simply inherit parent MAC's sequence
   number.  Parent MAC Mx Sequence number should be computed as follows:

   o  MUST be higher than any existing remote MAC route for Mx, as per
      RFC 7432.

   o  MUST be at least equal to corresponding SYNC MAC sequence number
      if one is present.

   o  If the IP is also associated with a different remote MAC "Mz",
      MUST be higher than "Mz" sequence number.

   Once new sequence number for MAC route Mx is computed as per above,
   all LOCAL MAC-IPs associated with MAC Mx MUST inherit the updated
   sequence number.

8.2.  LOCAL MAC learning

   Local MAC Mx Sequence number should be computed as follows:

   o  MUST be higher than any existing remote MAC route for Mx, as per
      RFC 7432.

   o  MUST be at least equal to corresponding SYNC MAC sequence number
      if one is present.

   o  Once new sequence number for MAC route Mx is computed as per
      above, all LOCAL MAC-IPs associated with MAC Mx MUST inherit the
      updated sequence number.

   Note that the local MAC sequence number might already be present if
   there was a local MAC-IP learnt prior to the local MAC, in which case
   the above may not result in any change in local MAC's sequence
   number.

8.3.  Remote MAC OR MAC-IP Update

   On receiving a remote MAC OR MAC-IP route update associated with a
   MAC Mx with a sequence number that is higher than or equal to
   sequence number assigned to a LOCAL route for MAC Mx:

   o  PE MUST trigger probe and deletion procedure for all LOCAL IPs
      associated with MAC Mx.

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   o  PE MUST trigger deletion procedure for LOCAL MAC route for Mx.

8.4.  REMOTE (SYNC) MAC update

   Corresponding local MAC Mx (if present) sequence number should be re-
   computed as follows:

   o  If the current sequence number is less than the received SYNC MAC
      sequence number, it MUST be increased to be equal to received SYNC
      MAC sequence number.

   o  If a LOCAL MAC sequence number is updated as a result of the
      above, all LOCAL MAC-IPs associated with MAC Mx MUST inherit the
      updated sequence number.

8.5.  REMOTE (SYNC) MAC-IP update

   If this is a SYNCed MAC-IP on a local ES, it would also result in a
   derived SYNC MAC Mx route entry, as MAC only RT-2 advertisement is
   optional.  Corresponding local MAC Mx (if present) sequence number
   should be re-computed as follows:

   o  If the current sequence number is less than the received SYNC MAC
      sequence number, it MUST be increased to be equal to received SYNC
      MAC sequence number.

   o  If a LOCAL MAC sequence number is updated as a result of the
      above, all LOCAL MAC-IPs associated with MAC Mx MUST inherit the
      updated sequence number.

8.6.  Inter-op

   In general, if all PE nodes in the overlay network follow the above
   sequence number assignment procedure, and the PE is advertising both
   MAC+IP and MAC routes, sequence number advertised with the MAC and
   MAC+IP routes with the same MAC would always be the same.  However,
   an inter-op scenario with a different implementation could arise,
   where a PE implementation non-compliant with this document or with
   RFC 7432 assigns and advertises independent sequence numbers to MAC
   and MAC+IP routes.  To handle this case, if different sequence
   numbers are received for remote MAC+IP and corresponding remote MAC
   routes from a remote PE, sequence number associated with the remote
   MAC route should be computed as:

   o  Highest of the all received sequence numbers with remote MAC+IP
      and MAC routes with the same MAC.

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   o  MAC sequence number would be re-computed on a MAC or MAC+IP route
      withdraw as per above.

   A MAC and / or IP move to the local PE would now result in the MAC
   (and hence all MAC-IP) sequence numbers incremented from the above
   computed remote MAC sequence number.

8.7.  MAC Sharing Race Condition

   In a MAC sharing use case described in section 6.2, a race condition
   is possible with simultaneous host moves between a pair of PEs.  As
   an example, consider PE1 with local host IPs I1 and I2 sharing MAC
   M1, and PE2 with local host IPs I3 and I4 sharing MAC M2.  A
   simultaneous move of I1 from PE1 to PE2 and of I3 from PE2 to PE1,
   such that I3 is learnt on PE1 before I1's local entry has been probed
   out on PE1 and/or I1 is learnt on PE2 before I3's local entry has
   been probed out on PE2 may trigger a race condition.  This race
   condition together with MAC sequence number assignment rules defined
   in section 7.1 can cause new mac-ip routes [I1, M2] and [I3, M1] to
   bounce a couple of times with an incremented sequence number until
   stale entries [I1, M1] and [I3, M2] have been probed out from PE1 and
   PE2 respectively.  An implementation MUST ensure proper probing
   procedures to remove stale ARP, ND, and local MAC entries, following
   a move, on learning remote routes as defined in section 7.3 (and as
   per [EVPN-IRB]) to minimize exposure to this race condition.

8.8.  Mobility Convergence

   This sections is to be treated as optional and details ARP and ND
   probing procedures that MAY be implemented to achieve faster host re-
   learning and convergence on mobility events.

   o  Following a host move from PE1 to PE2, the host's MAC is
      discovered at PE2 as a local MAC via a data frames received from
      the host.  If PE2 has a prior REMOTE MAC-IP host route for this
      MAC from PE1, an ARP/ND probe MAY be triggered at PE2 to learn the
      MAC-IP as a local adjacency and trigger EVPN RT-2 advertisement
      for this MAC-IP across the overlay with new reachability via PE2.
      This results in a reliable "event based" host IP learning
      triggered by a "MAC learning event" across the overlay, and hence
      faster convergence of overlay routed flows to the host.

   o  Following a host move from PE1 to PE2, once PE1 receives a MAC or
      MAC-IP route from PE2 with a higher sequence number, an ARP/ND
      probe MAY be triggered at PE1 to clear the stale local MAC-IP
      neighbor adjacency OR re-learn the local MAC-IP in case the host
      has moved back or is duplicate.

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   o  Following a local MAC age-out, if there is a local IP adjacency
      with this MAC, an ARP/ND probe MAY be triggered for this IP to
      either re-learn the local MAC and maintain local l3 and l2
      reachability to this host OR to clear the ARP/ND entry in case the
      host is indeed no longer local.  Note that this accomplishes
      clearing of stale ARP entries, triggered by a MAC age-out event
      even when the ARP refresh timer was longer than the MAC age-out
      timer.  Clearing of stale IP neighbor entries in-turn facilitates
      traffic convergence in the event that the host was silent and not
      discovered at its new location.  Once stale neighbor entry for the
      host is cleared, routed traffic flow destined for the host can re-
      trigger ARP/ND discovery for this host at the new location.

8.8.1.  Generalized Probing Logic

   Above probing logic may be generalized as probing for an IP neighbor
   anytime a resolving parent MAC route is "inconsistent" with the MAC-
   IP neighbor route, where being inconsistent is defined as being not
   present OR conflicting in terms of the route source being local OR
   remote.  MAC-IP to MAC parent relationship described earlier in this
   document in section 6.1 MAY be used to achieve this logic.

9.  Routed Overlay

   An additional use case is possible, such that traffic to an end host
   in the overlay is always IP routed.  In a purely routed overlay such
   as this:

   o  A host MAC is never advertised in EVPN overlay control plane.

   o  Host /32 or /128 IP reachability is distributed across the overlay
      via EVPN route type 5 (RT-5) along with a zero or non- zero ESI.

   o  An overlay IP subnet may still be stretched across the underlay
      fabric, however, intra-subnet traffic across the stretched overlay
      is never bridged.

   o  Both inter-subnet and intra-subnet traffic, in the overlay is IP
      routed at the EVPN PE.

   Please refer to [RFC 7814] for more details.

   Host mobility within the stretched subnet would still need to be
   supported for this use.  In the absence of any host MAC routes,
   sequence number mobility EXT-COMM specified in [RFC7432], section 7.7
   may be associated with a /32 OR /128 host IP prefix advertised via
   EVPN route type 5.  MAC mobility procedures defined in RFC 7432 can
   now be applied as is to host IP prefixes:

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   o  On LOCAL learning of a host IP, on a new ESI, host IP MUST be
      advertised with a sequence number attribute that is higher than
      what is currently advertised with the old ESI.

   o  On receiving a host IP route advertisement with a higher sequence
      number, a PE MUST trigger ARP/ND probe and deletion procedure on
      any LOCAL route for that IP with a lower sequence number.  A PE
      would essentially move the forwarding entry to point to the remote
      route with a higher sequence number and send an ARP/ND PROBE for
      the local IP route.  If the IP has indeed moved, PROBE would
      timeout and the local IP host route would be deleted.

   Note that there is still only one sequence number associated with a
   host route at any time.  For earlier use cases where a host MAC is
   advertised along with the host IP, a sequence number is only
   associated with a MAC.  Only if the MAC is not advertised at all, as
   in this use case, is a sequence number associated with a host IP.

   Note that this mobility procedure would not apply to "anycast IPv6"
   hosts advertised via NA messages with 0-bit=0.  Please refer to
   [EVPN-PROXY-ARP].

10.  Duplicate Host Detection

   Duplicate host detection scenarios across EVPN IRB can be classified
   as follows:

   o  Scenario A: where two hosts have the same MAC (host IPs may or may
      not be duplicate).

   o  Scenario B: where two hosts have the same IP but different MACs.

   o  Scenario C: where two hosts have the same IP and host MAC is not
      advertised at all.

   Duplicate detection procedures for scenario B and C would not apply
   to "anycast IPv6" hosts advertised via NA messages with 0-bit=0.
   Please refer to [EVPN-PROXY-ARP].

10.1.  Scenario A

   For all use cases where duplicate hosts have the same MAC, MAC is
   detected as duplicate via duplicate MAC detection procedure described
   in RFC 7432.  Corresponding MAC-IP routes with the same MAC do not
   require duplicate detection and MUST simply inherit the DUPLICATE
   property from the corresponding MAC route.  In other words, if a MAC
   route is in DUPLICATE state, all corresponding MAC-IP routes MUST

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   also be treated as DUPLICATE.  Duplicate detection procedure need
   only be applied to MAC routes.

10.2.  Scenario B

   Due to misconfiguration, a situation may arise where hosts with
   different MACs are configured with the same IP.  This scenario would
   not be detected by existing duplicate MAC detection procedure and
   would result in incorrect forwarding of routed traffic destined to
   this IP.

   Such a situation, on LOCAL MAC-IP learning, would be detected as a
   move scenario via the following local MAC sequence number computation
   procedure described earlier in section 6.1:

   o  If the IP is also associated with a different remote MAC "Mz",
      MUST be higher than "Mz" sequence number.

   Such a move that results in sequence number increment on local MAC
   because of a remote MAC-IP route associated with a different MAC MUST
   be counted as an "IP move" against the "IP" independent of MAC.
   Duplicate detection procedure described in RFC 7432 can now be
   applied to an "IP" entity independent of MAC.  Once an IP is detected
   as DUPLICATE, corresponding MAC-IP route should be treated as
   DUPLICATE.  Associated MAC routes and any other MAC-IP routes
   associated with this MAC should not be affected.

10.2.1.  Duplicate IP Detection Procedure for Scenario B

   Duplicate IP detection procedure for such a scenario is specified in
   [EVPN-PROXY-ARP].  What counts as an "IP move" in this scenario is
   further clarified as follows:

   o  On learning a LOCAL MAC-IP route Mx-IPx, check if there is an
      existing REMOTE OR LOCAL route for IPx with a different MAC
      association, say, Mz-IPx.  If so, count this as an "IP move" count
      for IPx, independent of the MAC.

   o  On learning a REMOTE MAC-IP route Mz-IPx, check if there is an
      existing LOCAL route for IPx with a different MAC association,
      say, Mx-IPx.  If so, count this as an "IP move" count for IPx,
      independent of the MAC.

   A MAC-IP route SHOULD be treated as DUPLICATE if either of the
   following two conditions are met:

   o  Corresponding MAC route is marked as DUPLICATE via existing
      duplicate detection procedure.

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   o  Corresponding IP is marked as DUPLICATE via extended procedure
      described above.

10.3.  Scenario C

   For a purely routed overlay scenario described in section 8, where
   only a host IP is advertised via EVPN RT-5, together with a sequence
   number mobility attribute, duplicate MAC detection procedures
   specified in RFC 7432 can be intuitively applied to IP only host
   routes for the purpose of duplicate IP detection.

   o  On learning a LOCAL host IP route IPx, check if there is an
      existing REMOTE OR LOCAL route for IPx with a different ESI
      association.  If so, count this as an "IP move" count for IPx.

   o  On learning a REMOTE host IP route IPx, check if there is an
      existing LOCAL route for IPx with a different ESI association.  If
      so, count this as an "IP move" count for IPx.

   o  With configurable parameters "N" and "M", If "N" IP moves are
      detected within "M" seconds for IPx, treat IPx as DUPLICATE.

10.4.  Duplicate Host Recovery

   Once a MAC or IP is marked as DUPLICATE and FROZEN, corrective action
   must be taken to un-provision one of the duplicate MAC or IP.  Un-
   provisioning a duplicate MAC or IP in this context refers to a
   corrective action taken on the host side.  Once one of the duplicate
   MAC or IP is un-provisioned, normal operation would not resume until
   the duplicate MAC or IP ages out, following this correction, unless
   additional action is taken to speed up recovery.

   This section lists possible additional corrective actions that could
   be taken to achieve faster recovery to normal operation.

10.4.1.  Route Un-freezing Configuration

   Unfreezing the DUPLICATE OR FROZEN MAC or IP via a CLI can be
   leveraged to recover from DUPLICATE and FROZEN state following
   corrective un-provisioning of the duplicate MAC or IP.

   Unfreezing the frozen MAC or IP via a CLI at a PE should result in
   that MAC OR IP being advertised with a sequence number that is higher
   than the sequence number advertised from the other location of that
   MAC or IP.

   Two possible corrective un-provisioning scenarios exist:

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   o  Scenario A: A duplicate MAC or IP may have been un-provisioned at
      the location where it was NOT marked as DUPLICATE and FROZEN.

   o  Scenario B: A duplicate MAC or IP may have been un-provisioned at
      the location where it was marked as DUPLICATE and FROZEN.

   Unfreezing the DUPLICATE and FROZEN MAC or IP, following the above
   corrective un-provisioning scenarios would result in recovery to
   steady state as follows:

   o  Scenario A: If the duplicate MAC or IP was un-provisioned at the
      location where it was NOT marked as DUPLICATE, unfreezing the
      route at the FROZEN location will result in the route being
      advertised with a higher sequence number.  This would in-turn
      result in automatic clearing of local route at the PE location,
      where the host was un-provisioned via ARP/ND PROBE and DELETE
      procedure specified earlier in section 8 and in [RFC 7432].

   o  Scenario B: If the duplicate host is un-provisioned at the
      location where it was marked as DUPLICATE, unfreezing the route
      will trigger an advertisement with a higher sequence number to the
      other location.  This would in-turn trigger re-learning of local
      route at the remote location, resulting in another advertisement
      with a higher sequence number from the remote location.  Route at
      the local location would now be cleared on receiving this remote
      route advertisement, following the ARP/ND PROBE.

10.4.2.  Route Clearing Configuration

   In addition to the above, route clearing CLIs may also be leveraged
   to clear the local MAC or IP route, to be executed AFTER the
   duplicate host is un-provisioned:

   o  clear mac CLI: A clear MAC CLI can be leveraged to clear a
      DUPLICATE MAC route, to recover from a duplicate MAC scenario.

   o  clear ARP/ND: A clear ARP/ND CLI may be leveraged to clear a
      DUPLICATE IP route to recover from a duplicate IP scenario.

   Note that the route unfreeze CLI may still need to be run if the
   route was un-provisioned and cleared from the NON-DUPLICATE / NON-
   FROZEN location.  Given that unfreezing of the route via the un-
   freeze CLI would any ways result in auto-clearing of the route from
   the "un- provisioned" location, as explained in the prior section,
   need for a route clearing CLI for recovery from DUPLICATE / FROZEN
   state is truly optional.

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11.  Security Considerations

   This document raises no new security issues for EVPN.

12.  IANA Considerations

   None.

13.  Acknowledgements

   Authors would like to thank Vibov Bhan and Patrice Brisset for
   feedback the process of design and implementation of procedures
   defined in this document.  Authors would like to thank Wen Lin for a
   detailed review and valuable comments related to MAC sharing race
   conditions.

14.  Normative References

   [EVPN-IRB]
              Sajassi, A., Salam, S., Thoria, S., Drake, J., and J.
              Rabadan, "Integrated Routing and Bridging in EVPN", draft-
              ietf-bess-evpn-inter-subnet-forwarding-13 (work in
              progress), February 2021.

   [EVPN-PROXY-ARP]
              Rabadan, J., Sathappan, S., Nagaraj, K., Hankins, G., and
              T. King, "Operational Aspects of Proxy-ARP/ND in EVPN
              Networks", draft-ietf-bess-evpn-proxy-arp-nd-11 (work in
              progress), Jan 2021.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC7432]  Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
              Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
              Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
              2015, <https://www.rfc-editor.org/info/rfc7432>.

   [RFC7814]  Xu, X., Jacquenet, C., Raszuk, R., Boyes, T., and B. Fee,
              "Virtual Subnet: A BGP/MPLS IP VPN-Based Subnet Extension
              Solution", RFC 7814, DOI 10.17487/RFC7814, March 2016,
              <https://tools.ietf.org/html/rfc7814>.

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Authors' Addresses

   Neeraj Malhotra (editor)
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134
   USA

   Email: nmalhotr@cisco.com

   Ali Sajassi
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134
   USA

   Email: sajassi@cisco.com

   Aparna Pattekar
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134
   USA

   Email: apjoshi@cisco.com

   Jorge Rabadan
   Nokia
   777 E. Middlefield Road
   Mountain View, CA  94043
   USA

   Email: jorge.rabadan@nokia.com

   Avinash Lingala
   ATT
   200 S. Laurel Avenue
   Middletown, CA  07748
   USA

   Email: ar977m@att.com

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   John Drake
   Juniper Networks

   Email: jdrake@juniper.net

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