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Versions: 00 01                                                         
BESS WG                                                       S. Dikshit
Internet-Draft                                                  V. Joshi
Intended status: Standards Track                              S. Shankar
Expires: 20 March 2022                                        Aruba, HPE
                                                       16 September 2021


                      EVPN Mac Dampening Back-off
                  draft-saum-bess-dampening-backoff-01

Abstract

   MAC move handling in EVPN deployments is discussed in detail in
   [RFC7432].  There are few optimizations which can be done in existing
   way of handling the mac duplication.  This document describes few of
   the potential techniques to do so.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 20 March 2022.

Copyright Notice

   Copyright (c) 2021 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 (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.




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Table of Contents

   1.  Important Terms . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Misconfiguration of Hosts . . . . . . . . . . . . . . . .   3
     2.2.  Loopy Traffic in Tenant Network . . . . . . . . . . . . .   4
   3.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Requirements Language . . . . . . . . . . . . . . . . . .   5
   4.  Problem Description . . . . . . . . . . . . . . . . . . . . .   6
   5.  Solution(s) . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Mac Freeze  . . . . . . . . . . . . . . . . . . . . . . .   7
     5.2.  Backing Off MAC Mobility Timer and Count  . . . . . . . .   7
       5.2.1.  MDAS Derivation . . . . . . . . . . . . . . . . . . .   8
       5.2.2.  Delta Values Calculation  . . . . . . . . . . . . . .   9
     5.3.  Backing Off Example . . . . . . . . . . . . . . . . . . .   9
   6.  Backward Compatibility  . . . . . . . . . . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Important Terms

   MDAS: Mac Dampening Attribute Set:

      MDT: Mac Dampening Timer

      MDC: Mac Dampening Count

      MFT: MAC Freezing Timer

   Mac Dampening: Process of stalling the mobility of MAC as define in
   [RFC7432].

   VTEP: Virtual Tunnel End Point or Vxlan Tunnel End Point

   DT: Dampened Time: Actual time taken to dampen the contentious MAC











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2.  Introduction

   The host mobility solution described in [RFC7432] elaborates on few
   use-cases related to dual mac discovery which leads to dampening
   logic coming into play.  The host move handling logic addresses the
   problem of frequent mac-moves and culminates by freezing the MACs
   against further moves.  If there is no mellowing down of the issue,
   then it leads to unending cycle of mac dampening and freezing.  Hence
   this problem needs organic measures for arriving at MAC freezing
   point, sooner than later.

   The events that can lead to never ending duplication are as follows:

   (a)  Misconfiguration of hosts with identical configuration, in the
        same bridge-domain, across ESIs and across NVEs.

   (b)  Looping of traffic due to layer 2 loops created in the bridge
        domain in the tenant network behind the NVEs.

2.1.  Misconfiguration of Hosts

   Consider the following figure wherein two hosts, Host-1 and Host-2,
   are misconfigured with same mac-address MAC-1.  These hosts are
   placed behind two different Ethernet segments, ES12 and ES3
   respectively and hooked to the same bridge-domain (BD-1).  PE1, PE2
   and PE3 will get into a never ending loop of learning the MAC-1
   locally and also from the remote Vtep.  Thus entering into a control-
   plane BGP-EVPN cycle of bumping up the sequence number in the
   MACMobility Extended Community till the maximum MAC move count is hit
   with the stipulated time.  The MAC published to other Vteps like PE4
   also changes accordingly based on the latest update with highest
   sequence number.



















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                                  +------+
                                  |Host-3|
                                  +------+
                                    |
                               PE4  |
                              +-----+-----+
              +---------------|  +-----+  |---------------+
              |               |  | BD-1|  |               |
              |               +-----------+               |
              |                                           |
              |                   EVPN                    |
              |                                           |
              | PE1               PE2                PE3  |
              |                                           |
          +-----------+       +-----------+       +-----------+
          |  | BD-1|  |       |  | BD-1|  |       |  | BD-1|  |
          |  +-----+  |-------|  +-----+  |-------|  +-----+  |
          +-----------+       +-----------+       +-----------+
                 \       ES12      /                 / ES3
                  \               /                 /
                   \             /                 /
                   +-------------+              +-------------+
                   |Host-1, MAC-1|              |Host-2, MAC-1|
                   +-------------+              +-------------+


               Figure 1: Figure 1: Misconfiguration of Hosts

         LEGEND:
           PE1, PE2, PE3: Vxlan/overlay gateways
           HOST-1, HOST-2: Hosts behind PEs
           MAC_1 : MAC address which is duplicated across hosts HOST-1 and HOST-2
           ES12: Ethernet segment between PE1 and PE2 for BD-1
           ES3: Ethernet segment attached to PE3 for BD-1
           BD-1: Bridge Domain 1

2.2.  Loopy Traffic in Tenant Network

   Consider the following case of a loopy tenant network, leading to MAC
   duplicity in the network.  Lets say, Host-1 generates a BUM traffic
   like GARP (Gratuitous ARP) and sends it over the VLAN which is part
   of BD-1 and mapped to a configured EVI on the PEs.  PE1 sprays the
   BUM over the EVPN fabric tying it with the mapped EVI.  The BUM
   packet arrives at PE1 (assuming it's the elected DF) over the EVPN
   fabric.  PE1 sprays the traffic towards the directly attached tenant
   network attached, tagging it with Vlan that maps to to the bridge
   domain, BD-1, which inturn maps to the MAC-VRF pointed to by the EVI.
   If the layer-2 network on tenant side is loopy due to STP network not



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   converging or STP not configured at all, or for some other unknown
   reasoni (not under the purview of this document); then the BUM
   traffic may loop back to PE1, thus creating a duplicate MAC learning
   for MAC-1.  Till the tenant network is curtailed or put to order via
   admin intervention or otherwize, continuous MAC moves for MAC-1 can
   be observed between PEs attached to ethernet segment ES12 (PE1) and
   ES3 (PE2).

                               PE4  |
                              +-----+-----+
              +---------------|  +-----+  |---------------+
              |               |  | BD-1|  |               |
              |               +-----------+               |
              |                                           |
              |                   EVPN                    |
              |                                           |
              | PE1               PE2                PE3  |
              |
          +-----------+       +-----------+       +-----------+
          |  | BD-1|  |       |  | BD-1|  |       |  | BD-1|  |
          |  +-----+  |-------|  +-----+  |-------|  +-----+  |
          +-----------+       +-----------+       +-----------+
           |   ES12   |                              /
           \/         /\                            /
           |          |                            / ES3
         [Tenant-Network]                         /
           |          |                          /
           I->-loop->-I                      +-------------+
                                             |Host-1, MAC-1|
                                             +-------------+

            Figure 2: Figure 2: Loopy traffic in Tenant Network

            LEGEND:
              PE1, PE2, PE3: Vxlan/overlay gateways
              HOST-1:  Hosts behind PE3
              MAC_1 : MAC address of Host-1
              ES12: Ethernet segment between PE1 and PE2 for BD-1
              ES3: Ethernet segment attached to PE3 for BD-1
              BD-1: Bridge Domain 1

3.  Requirements

3.1.  Requirements Language

   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].



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   When used in lowercase, these words convey their typical use in
   common language, and they are not to be interpreted as described in
   [RFC2119].

4.  Problem Description

   The mac dampening procedure mentioned in [RFC7432], suggests that a
   Overlay Tunnel Endpoint that detects the mac mobility event upon
   local learning, should start a 'M' seconds timer and track the MAC
   for 'N' moves before the timer expires.  Hence forth, concluding that
   it is a MAC Duplication issue and freezing the MAC while also raising
   the alarm, for the admin to take corrective action.  It is observed
   in few vendor implementations, that involves defreezing the MAC in
   deterministic time (configurable or derived) after freezing it, with
   a positive assumption that admin shall take corrective action
   meanwhile.  Else, the subsequent unfreeze shall end up in the same
   cycle of MAC Duplication detection and freezing of the MAC.  In case
   of lazy, none or inaccurate intervention by the admin, this can
   potentially result in ia prolong state of network disarray:

   (1)  Unnecessary and periodic control-plane protocol churn

   (2)  Exchange of control plane states which are transient and
        inaccurate

   (3)  Reachability to end device remains in the realms of ambiguity
        for prolonged duration

   (4)  Traffic destined to the Duplicate MAC case, panning across
        fabrics, sites or across geographies, ends up hogging the
        precious WAN bandwidth.

   Potential solutions are discussed subsequent sections.

5.  Solution(s)

   The potential solutions are as follows:














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5.1.  Mac Freeze

   The eventual solution is to FREEZE the MAC forever till admin does
   the clearing of the MAC.  The unfreeze and clearing actions are not
   organic in nature and can be accompanied by unwarranted impact like
   clearing of other MACs in the bridge-domain.  The way out may be
   resetting the layer-2 port and thus impacting all tenant bridge-
   domains hosted on the port.  This solution, hence, does not always
   solves or mitigate the situation, or, it may create a situation from
   which the eventual bail-out is expensive and not restricted to the
   impacted Host.

5.2.  Backing Off MAC Mobility Timer and Count

   The best-bet to organically mellow down the never ending MAC-mobility
   (indicating Duplicate MAC), is to freeze the MAC temporarily, for
   lets say, the same time as MAC Dampening Timer(MDT).  Lets term this
   timer as MAC Freeze Timer(MFT).  MFT is the time span for which the
   contentious MAC is frozen, i.e., no further control plane and data
   flow is allowed for this MAC.  The duplicity/un-ending-mobility is
   expected to be addressed by the admin.  In case the problem is not
   addressed within MAC Freeze Timer, the MAC duplicity is again
   identified based on the MAC mobility count within the MAC Dampening
   timer.  The best way forward MAY be:

   (1)  to get to the duplicity conclusion faster than the earlier
        iteration

   (2)  and freeze the MAC for a longer duration than earlier iteration

   (3)  , With the assumption that the problem shall be resolved in that
        time frame.

   The MAC Dampening Attribute Set (MDAS), comprises of following three
   parameters:

   (1)  MAC Dampening Timer (MDT): Defined in [RFC7432]

   (2)  MAC Dampening Count (MDC): Defined in [RFC7432]

   (3)  MAC Freeze Timer (MFT): Time for which the MAC is frozen after
        MAC duplicity is detected

   For example, let the first iteration of MDAS_iter_1 {MDT=180 seconds,
   MDC=5, MFT=180 seconds}. The default values of MDT and MDC are picket
   from [RFC7432], while lets define the default value of MFT same as
   MDT.  In case admin fails to intervene, the MAC is unfrozen after MFT
   expires.



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   For second iteration of the MDAS for the problem-MAC, i.e.
   MDAS_iter_2 = function (MDAS_iter_1).  The MDT and MDC values in
   second iteration are derived by backing off the MDT and MCD values by
   a pre-defined delta, i.e.

   (1)  MDAS_iter_2 (MDT) = MDAS_iter_1 (MDT) decrement_timer_delta

   (2)  MDAS_iter_2 (MDC) = MDAS_iter_1 (MDC) decrement_count_delta

   Thus reducing the time and moves to conclude on duplicity of the MAC.
   The values of decrement_timer_delta and decrement_count_delta can be
   configured or derived on a case to case basis.  [TBD: Elaborate on
   the case].  The next step is to freeze the MAC for some more time as
   compared to the previous iteration set of MDAS, thus increasing the
   probability of the admin, correcting the issue:

   (1)  MDAS_iter_2 (MFT) = MDAS_iter_1 (MFT) + increment_timer_delta

   (2)  The value of increment_timer_delta is also configurable in
        nature.

5.2.1.  MDAS Derivation

   The following formulae generalizes the derivation of MDAS attributes
   in the Nth iteration of Duplicate MAC detection on a PE:

   (1)  MDAS_iter_(N) (MDT) = (MDAS_iter_(N-1) (MDT)) -
        decrement_mdt_delta

   (2)  MDAS_iter_(N) (MDC) = (MDAS_iter_(N-1) (MDC))
        decrement_mdc_delta

   (3)  MDAS_iter_(N) (MFT) = MDAS_iter_(N-1) (MFT) +
        increment_mft_delta

   Where in, the following values for 1st iteration can be define as
   follows:

      MDAS_iter_1 (MDT) = 180 seconds

      MDAS_iter_1 (MDC) = 5

      MDAS_iter_1 (MFT) = 180 seconds.  Many implementations keep the
      MDT and MFT values as same.







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   The derivation of MDAS perimeters can be exponential in nature.  The
   delta values can be exponentially increased or decreased after
   certain iterations, thus triggering a exponential backing off the
   delta values.

5.2.1.1.  MDAS Boundry Values

   The new MDT value SHOULD not be less than the time taken to Dampen
   the MAC movement in last set of MDAS iteration.  On the same lines,
   the new MDC count SHOULD not go below '2', as count below 2, the MAC
   Dampening procedure does not gets triggered.

5.2.2.  Delta Values Calculation

   Following bullets give a overview of potential ways the delta values,
   i.e. decrement_mdt_delta, increment_mdc_delta and
   decrement_mft_delta:

   (a)  Delta values should be such that they SHOULD not infringe the
        time or count taken to reach Dampening state in the last set

   (b)  Delta values are static all through the sets

   (c)  Delta variable gets incremented/decremented based on the
        reduction in time (proportionally) to achieve the 'Dampened
        state' in the last 'MDAS set' as compared to the time to reach
        the 'Dampened state' in the MDAS set previous to the last one.
        For the same, the time taken to reach the Dampened State should
        be cached so that comparisons can be made in subsequent sets.
        In case, it is the first 'MDAS Set', the delta values MAY be
        either default or configured ones.  For the second 'MDAS set',
        the value MAY be cross-checked against the Dampened time for the
        first set.

   (d)  Delta values are always inherited from admin configuration.

   As mentioned in the Section 5.2.1 , the derivation of new delta
   values can done by exponentially backing them off in subsequent MDAS
   set(s).

5.3.  Backing Off Example

   This section describes the example of MDAS calculation with respect
   to the use-case defined in Section 2.1.  Though it's equally
   applicable to the case described in Section 2.2.  This example
   explains the logic in perspective of PE1.  Let's say PE1 learns the
   MAC-1 locally and publishes it over EVPN control plane before PE2
   does the same.  PE1 publishes it over control plane before PE2 learns



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   it locally (ignoring the case where both learn in tandem and publish
   it over control plane).  Subsequently, PE2 learns it and publishes it
   over control plane with sequence number 1.  PE1 starts the dampening
   logic by incrementing the local count by 1 and starting the dampening
   timer.  If this jiggle goes on for 5 counts at PE1, MAC Dampening
   logic described in [RFC7432]. shall freeze the MAC.  PE1 SHOULD cache
   the time it took to dampen the MAC.  Let's say it's 30 seconds.

   Assuming admin does not takes any action, before MAC freeze timer
   expires and PE1 defreezes the MAC, it will start moving again.  PE1
   shall reduce the MDT value by decrement_mdt_delta = 30 seconds to 150
   seconds.  The MDC counts are reduced by decrement_mdc_count = 1 to 4
   and the MFT is incremented by increment_mft_delta = 20 seconds to 170
   seconds.  Thus PE1 shall wait for 150 seconds for concluding the
   dampening logic and tracks the MAC for 4 moves.  Once dampening is
   hit, MAC is rendered as frozen for 170 seconds for admin to take
   action thus giving some more time for admin to take action.

   The whole intention is to gradually move towards a permanent freeze
   of the MAC if no admin does not do the needful in the stipulated time
   frame.

6.  Backward Compatibility

   The backward comptability is a no-op for MDAS derivation and
   recalculation, as MAC Dampening logic is very local to the Vtep.
   Even if the remote Vtep does not conforms to the logic presented in
   this literature, it will still work towards the dampening the
   frequent mac-mobility with the same parameters of MDT and MDS.  The
   instant freezing or temporary freezing of the dampened MAC is
   implementation dependent and should not impact or get impacted by the
   MDAS derivations presented in this document.

7.  Security Considerations

   This document inherits all the security considerations discussed in
   [RFC7432].

8.  IANA Considerations

   This document inherits all the IANA considerations discussed in
   [RFC7432].

9.  Acknowledgements

10.  References

10.1.  Normative References



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

10.2.  Informative References

   [RFC7348]  Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger,
              L., Sridhar, T., Bursell, M., and C. Wright, "Virtual
              eXtensible Local Area Network (VXLAN): A Framework for
              Overlaying Virtualized Layer 2 Networks over Layer 3
              Networks", RFC 7348, August 2014,
              <http://www.rfc-editor.org/rfc/rfc7348.txt>.

   [RFC7432]  Sajassi, A., "BGP MPLS-Based Ethernet VPN", RFC 7432,
              February 2015,
              <http://www.rfc-editor.org/rfc/rfc7432.txt>.

   [RFC9014]  Rabadan, J., Sathappan, S., Henderickx, W., Sajassi, A.,
              and W. Drake, "Interconnect Solution for Ethernet VPN
              (EVPN) Overlay Networks", RFC 9014, May 2021,
              <http://www.rfc-editor.org/rfc/rfc9014.txt>.

Authors' Addresses

   Saumya Dikshit
   Aruba Networks, HPE
   Mahadevpura
   Bangalore 560 048
   Karnataka
   India

   Email: saumya.dikshit@hpe.com


   Vinayak Joshi
   Aruba Networks, HPE
   Mahadevpura
   Bangalore 560 048
   Karnataka
   India

   Email: vinayak.joshi@hpe.com


   Swathi Shankar
   Aruba Networks, HPE
   Mahadevpura
   Bangalore 560 048



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   Karnataka
   India

   Email: swathi.shankar@hpe.com















































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