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Weighted Multi-Path Procedures for EVPN Multi-Homing
draft-ietf-bess-evpn-unequal-lb-12

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This is an older version of an Internet-Draft whose latest revision state is "Active".
Authors Neeraj Malhotra , Ali Sajassi , Jorge Rabadan , John Drake , Avinash Reddy Lingala , Samir Thoria
Last updated 2021-05-10
Replaces draft-malhotra-bess-evpn-unequal-lb
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draft-ietf-bess-evpn-unequal-lb-12
BESS WorkGroup                                          N. Malhotra, Ed.
Internet-Draft                                                A. Sajassi
Intended status: Standards Track                           Cisco Systems
Expires: November 11, 2021                                    J. Rabadan
                                                                   Nokia
                                                                J. Drake
                                                                 Juniper
                                                              A. Lingala
                                                                     ATT
                                                               S. Thoria
                                                           Cisco Systems
                                                            May 10, 2021

          Weighted Multi-Path Procedures for EVPN Multi-Homing
                   draft-ietf-bess-evpn-unequal-lb-12

Abstract

   EVPN enables all-active multi-homing for a CE device connected to two
   or more PEs via a LAG, such that bridged and routed traffic from
   remote PEs to hosts attached to the Ethernet Segment can be equally
   load balanced (it uses Equal Cost Multi Path) across the multi-homing
   PEs.  EVPN also enables multi-homing for IP subnets advertised in IP
   Prefix routes, so that routed traffic from remote PEs to those IP
   subnets can be load balanced.  This document defines extensions to
   EVPN procedures to optimally handle unequal access bandwidth
   distribution across a set of multi-homing PEs in order to:

   o  provide greater flexibility, with respect to adding or removing
      individual multi-homed PE-CE links.

   o  handle multi-homed PE-CE link failures that can result in unequal
      PE-CE access bandwidth across a set of multi-homing PEs.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any

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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 November 11, 2021.

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.

Table of Contents

   1.  Requirements Language and Terminology . . . . . . . . . . . .   3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  PE-CE Link Provisioning . . . . . . . . . . . . . . . . .   4
     2.2.  PE-CE Link Failures . . . . . . . . . . . . . . . . . . .   5
     2.3.  Design Requirement  . . . . . . . . . . . . . . . . . . .   6
   3.  Solution Overview . . . . . . . . . . . . . . . . . . . . . .   7
   4.  EVPN Link Bandwidth Extended Community  . . . . . . . . . . .   8
     4.1.  Encoding and Usage of EVPN Link Bandwidth Extended
           Community . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Note on BGP Link Bandwidth Extended Community . . . . . .   9
   5.  Weighted Unicast Traffic Load-balancing to an Ethernet
       Segment . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Local PE Behavior . . . . . . . . . . . . . . . . . . . .   9
     5.2.  Remote PE Behavior  . . . . . . . . . . . . . . . . . . .  10
   6.  Weighted BUM Traffic Load-Sharing across an Ethernet Segment   11
     6.1.  The BW Capability in the DF Election Extended Community .  11
     6.2.  BW Capability and Default DF Election algorithm . . . . .  12
     6.3.  BW Capability and HRW DF Election algorithm (Type 1 and
           4)  . . . . . . . . . . . . . . . . . . . . . . . . . . .  12
       6.3.1.  BW Increment  . . . . . . . . . . . . . . . . . . . .  13
       6.3.2.  HRW Hash Computations with BW Increment . . . . . . .  13
     6.4.  BW Capability and Preference DF Election algorithm  . . .  15
   7.  Cost-Benefit Tradeoff on Link Failures  . . . . . . . . . . .  15
   8.  Real-time Available Bandwidth . . . . . . . . . . . . . . . .  16
   9.  Weighted Load-balancing to Multi-homed Subnets  . . . . . . .  16
   10. EVPN-IRB Multi-homing With Non-EVPN routing . . . . . . . . .  16

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   11. Operational Considerations  . . . . . . . . . . . . . . . . .  16
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   15. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  17
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     16.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

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].

   "Local PE" in the context of an ESI refers to a provider edge switch
   OR router that physically hosts the ESI.

   "Remote PE" in the context of an ESI refers to a provider edge switch
   OR router in an EVPN overlay, whose overlay reachability to the ESI
   is via the Local PE.

   o  BW: BandWidth

   o  LAG: Link Aggregation Group

   o  ES: Ethernet Segment

   o  vES: Virtual Ethernet Segment

   o  EVI: Ethernet virtual Instance, this is a mac-vrf.

   o  IMET: Inclusive Multicast Route

   o  DF: Designated Forwarder

   o  BDF: Backup Designated Forwarder

   o  DCI: Data Center Interconnect Router

2.  Introduction

   In an EVPN-IRB based network overlay, with a CE multi-homed via a
   EVPN all-active multi-homing, bridged and routed traffic from remote
   PEs can be equally load balanced (ECMPed) across the multi-homing
   PEs:

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   o  ECMP Load-balancing for bridged unicast traffic is enabled via

   o  aliasing and mass-withdraw procedures detailed in RFC 7432.

   o  ECMP Load-balancing for routed unicast traffic is enabled via
      existing L3 ECMP mechanisms.

   o  Load-sharing of bridged BUM traffic on local ports is enabled via
      EVPN DF election procedure detailed in RFC 7432

   All of the above load balancing and DF election procedures implicitly
   assume equal bandwidth distribution between the CE and the set of
   multi-homing PEs.  Essentially, with this assumption of equal
   "access" bandwidth distribution across all PEs, ALL remote traffic is
   equally load balanced across the multi-homing PEs.  This assumption
   of equal access bandwidth distribution can be restrictive with
   respect to adding / removing links in a multi-homed LAG interface and
   may also be easily broken on individual link failures.  A solution to
   handle unequal access bandwidth distribution across a set of multi-
   homing EVPN PEs is proposed in this document.  Primary motivation
   behind this proposal is to enable greater flexibility with respect to
   adding / removing member PE-CE links, as needed and to optimally
   handle PE-CE link failures.

2.1.  PE-CE Link Provisioning

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

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

                                 Figure 1

   Consider CE1 that is dual-homed to PE1 and PE2 via EVPN all-active
   multi-homing with single member links of equal bandwidth to each PE
   (aka, equal access bandwidth distribution across PE1 and PE2).  If
   the provider wants to increase link bandwidth to CE1, it must add a

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   link to both PE1 and PE2 in order to maintain equal access bandwidth
   distribution and inter-work with EVPN ECMP load balancing.  In other
   words, for a dual-homed CE, total number of CE links must be
   provisioned in multiples of 2 (2, 4, 6, and so on).  For a triple-
   homed CE, number of CE links must be provisioned in multiples of
   three (3, 6, 9, and so on).  To generalize, for a CE that is multi-
   homed to "n" PEs, number of PE-CE physical links provisioned must be
   an integral multiple of "n".  This is restrictive in case of dual-
   homing and very quickly becomes prohibitive in case of multi-homing.

   Instead, a provider may wish to increase PE-CE bandwidth OR number of
   links in any link increments.  As an example, for CE1 dual-homed to
   PE1 and PE2 in all-active mode, provider may wish to add a third link
   to only PE1 to increase total bandwidth for this CE by 50%, rather
   than being required to increase access bandwidth by 100% by adding a
   link to each of the two PEs.  While existing EVPN based all-active
   load balancing procedures do not necessarily preclude such asymmetric
   access bandwidth distribution among the PEs providing redundancy, it
   may result in unexpected traffic loss due to congestion in the access
   interface towards CE.  This traffic loss is due to the fact that PE1
   and PE2 will continue to be treated as equal cost paths at remote
   PEs, and as a result may attract approximately equal amount of CE1
   destined traffic, even when PE2 only has half the bandwidth to CE1 as
   PE1.  This may lead to congestion and traffic loss on the PE2-CE1
   link.  If bandwidth distribution to CE1 across PE1 and PE2 is 2:1,
   traffic from remote hosts must also be load balanced across PE1 and
   PE2 in 2:1 manner.

2.2.  PE-CE Link Failures

   More importantly, unequal PE-CE bandwidth distribution described
   above may occur during regular operation following a link failure,
   even when PE-CE links were provisioned to provide equal bandwidth
   distribution across multi-homing PEs.

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

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

                                 Figure 2

   Consider a CE1 that is multi-homed to PE1 and PE2 via a LAG with two
   member links to each PE.  On a PE2-CE1 physical link failure, LAG
   represented by an Ethernet Segment ESI-1 on PE2 stays up, however,
   its bandwidth is cut in half.  With existing ECMP procedures, both
   PE1 and PE2 may continue to attract equal amount of traffic from
   remote PEs, even when PE1 has double the bandwidth to CE1.  If
   bandwidth distribution to CE1 across PE1 and PE2 is 2:1, traffic from
   remote hosts must also be load balanced across PE1 and PE2 in 2:1
   manner to avoid unexpected congestion and traffic loss on PE2-CE1
   links within the LAG.  As an alternative, min-link on LAGs is
   sometimes used to bring down the LAG interface on member link
   failures.  This however results in loss of available bandwidth in the
   network, and is not ideal.

2.3.  Design Requirement

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

                    +-----+   +-----+           +-----+   +-----+
                    | PE1 |   | PE2 |   .....   | PEx |   | PEn |
                    +-----+   +-----+           +-----+   +-----+
                       \       \                 //        //
                        \ L1    \ L2            // Lx     // Ln
                         \       \             //        //
                        +-\-------\-----------//--------//-+
                        |  \       \  ESI-1  //        //  |
                        +----------------------------------+
                                         |
                                         CE

                                 Figure 3

   To generalize, if total link bandwidth to a CE is distributed across
   "n" multi-homing PEs, with Lx being the total bandwidth to PEx across
   all links, traffic from remote PEs to this CE must be load balanced
   unequally across [PE1, PE2, ....., PEn] such that, fraction of total
   unicast and BUM flows destined for CE that are serviced by PEx is:

   Lx / [L1+L2+.....+Ln]

   Figure 3 illustrates a scenario where PE1..PEn are attached to a
   multi-homed Ethernet Segment, however this document generalizes this
   requirement so that the unequal load balancing can be applied to PEs
   attached to a vES or to a multi-homed subnet advertised by EVPN IP
   Prefix routes.

   The solution proposed below includes extensions to EVPN procedures to
   achieve the above.

3.  Solution Overview

   In order to achieve weighted load balancing to an ES or vES for
   overlay unicast traffic, Ethernet A-D per ES route (EVPN Route Type
   1) is leveraged to signal the Ethernet Segment weight to remote PEs.
   Using Ethernet A-D per-ES route to signal the Ethernet Segment weight
   provides a mechanism that reacts to changes in access bandwidth or
   number of access links in a service and host independent manner.
   Remote PEs computing the MAC path-lists based on global and aliasing
   Ethernet A-D routes now have the ability to setup weighted load
   balancing path-lists based on the ESI access bandwidth or number of
   links received from each PE that the ES is multi-homed to.

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   In order to achieve weighted load balancing of overlay BUM traffic,
   EVPN ES route (Route Type 4) is leveraged to signal the ESI weight to
   PEs within an ESI's redundancy group to influence per-service DF
   election.  PEs in an ESI redundancy group now have the ability to do
   service carving in proportion to each PE's relative ESI weight.

   Unequal load balancing to multi-homed subnets is achieved by
   signaling the weight along with the IP Prefix routes advertised for
   the subnet.

   Procedures to accomplish this are described in greater detail next.

4.  EVPN Link Bandwidth Extended Community

   A new EVPN Link Bandwidth extended community is defined for the
   solution specified in this document:

   o  This extended-community is defined of type 0x06 (EVPN).

   o  IANA is requested to assign a sub-type value of 0x10 for the EVPN
      Link bandwidth extended community, of type 0x06 (EVPN).

   o  EVPN Link Bandwidth extended community is defined as transitive.

4.1.  Encoding and Usage of EVPN Link Bandwidth Extended Community

   EVPN Link Bandwidth Extended Community value field is used to carry
   total bandwidth of PE's all physical links in an ethernet segment,
   expressed in Mbits/sec (MegabitsPerSecond) represented as an unsigned
   integer.  Note however that the load balancing algorithm defined in
   this document uses ratio of Link Bandwidths.  Hence, the operator may
   choose a different unit or use the community as a generalized weight
   that may be set to link count, locally configured weight, or a value
   computed based on an attribute other than link bandwidth.  In such
   case, the operator MUST ensure consistent usage of the unit across
   all PEs in an ethernet segment.  This may involve multiple routing
   domains/Autonomous Systems.

   In order to facilitate this, as well as avoid interop issues because
   of provisioning error, one octet in the extended community's six
   octet 'value' field is used to explicitly signal if the weight
   encoded in the remaining five octets is link bandwidth expressed in
   Mbits/sec or a generalized weight value.  This results in the
   following encoding for EVPN link bandwidth extended community:

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Type     |    Sub-Type   |  Value-Units  |               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
   |                          Value-Weight                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 4

   Value-Units is encoded as:

   o  0x00: weight expressed using default units of Mbits/sec

   o  0x01: generalized weight expressed in something other than Mbits/
      sec

   Generalized weight units are intentionally left arbritrary to allow
   for flexibility in its usage for different applications without
   having to define new encoding for each non-default application.
   Implementations SHOULD support the default units of Mbits/sec, while
   support of non-default generalized weight is considered optional.

4.2.  Note on BGP Link Bandwidth Extended Community

   Link bandwidth extended community described in [BGP-LINK-BW] for
   layer 3 VPNs was considered for re-use here.  This Link bandwidth
   extended community is however defined in [BGP-LINK-BW] as optional
   non-transitive.  Since it is not possible to change deployed behavior
   of extended-community defined in [BGP-LINK-BW], it was decided to
   define a new one.  In inter-AS scenarios, link-bandwidth needs to be
   signaled to eBGP neighbors.  When signaled across AS boundary, this
   attribute can be used to achieve optimal load-balancing towards
   source PEs from a different AS.  This is applicable both when next-
   hop is changed or unchanged across AS boundaries.

5.  Weighted Unicast Traffic Load-balancing to an Ethernet Segment

5.1.  Local PE Behavior

   A PE that is part of an Ethernet Segment's redundancy group would
   advertise an additional "EVPN link bandwidth" extended community
   attribute with Ethernet A-D per-ES route (EVPN Route Type 1), that
   carries total bandwidth of PE's physical links in an Ethernet
   Segment.  New EVPN link bandwidth extended community defined in this
   document is used for this purpose.

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5.2.  Remote PE Behavior

   A receiving PE MUST check for consistent 'Value-Units' received in
   the EVPN link bandwidth exteneded community from each remote PE in an
   Ethernet Segment.  In case of any inconsistency in 'Value-Units'
   across PEs in an Ethernet Segment, this attribute is to be ignored
   and remote PEs are to follow regular ECMP forwarding to that Ethernet
   Segment.  Once consistency of 'Value-Units' is validated, receiving
   PE SHOULD use the 'Value-Weight' received from each PE to compute a
   relative (normalized) weight for each remote PE, per-ES, and then use
   this relative weight to compute a weighted path-list to be used for
   load balancing, as opposed to using an ECMP path-list for load
   balancing across the PE paths.  PE Weight and resulting weighted
   path-list computation at remote PEs is a local matter.  An example
   computation algorithm is shown below to illustrate the idea:

   if,

   L(x,y) : link bandwidth advertised by PE-x for ESI-y

   W(x,y) : normalized weight assigned to PE-x for ESI-y

   H(y) : Highest Common Factor (HCF) of [L(1,y), L(2,y), ....., L(n,y)]

   then, the normalized weight assigned to PE-x for ESI-y may be
   computed as follows:

   W(x,y) = L(x,y) / H(y)

   For a MAC+IP route (EVPN Route Type 2) received with ESI-y, receiving
   PE may compute MAC and IP forwarding path-list weighted by the above
   normalized weights.

   As an example, for a CE dual-homed to PE-1, PE-2, PE-3 via 2, 1, and
   1 GE physical links respectively, as part of a LAG represented by
   ESI-10:

   L(1, 10) = 2000 Mbits/sec

   L(2, 10) = 1000 Mbits/sec

   L(3, 10) = 1000 Mbits/sec

   H(10) = 1000

   Normalized weights assigned to each PE for ESI-10 are as follows:

   W(1, 10) = 2000 / 1000 = 2.

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   W(2, 10) = 1000 / 1000 = 1.

   W(3, 10) = 1000 / 1000 = 1.

   For a remote MAC+IP host route received with ESI-10, forwarding load
   balancing path-list may now be computed as: [PE-1, PE-1, PE-2, PE-3]
   instead of [PE-1, PE-2, PE-3].  This now results in load balancing of
   all traffic destined for ESI-10 across the three multi-homing PEs in
   proportion to ESI-10 bandwidth at each PE.

   Weighted path-list computation must only be done for an ESI if a link
   bandwidth attribute is received from all of the PE's advertising
   reachability to that ESI via Ethernet A-D per-ES Route Type 1.  In an
   unlikely event that link bandwidth attribute is not received from one
   or more subset of PEs, forwarding path-list should be computed using
   regular ECMP semantics.  Note that a default weight cannot be assumed
   for a PE that does not advertise its link bandwidth as the weight
   attribute t be used in path-list computation is relative.

6.  Weighted BUM Traffic Load-Sharing across an Ethernet Segment

   Optionally, load sharing of per-service DF role, weighted by
   individual PE's link-bandwidth share within a multi-homed ES may also
   be achieved.

   In order to do that, a new DF Election Capability [RFC8584] called
   "BW" (Bandwidth Weighted DF Election) is defined.  BW MAY be used
   along with some DF Election Types, as described in the following
   sections.

6.1.  The BW Capability in the DF Election Extended Community

   [RFC8584] defines a new extended community for PEs within a
   redundancy group to signal and agree on uniform DF Election Type and
   Capabilities for each ES.  This document requests IANA for a bit in
   the DF Election extended community Bitmap:

   Bit 28: BW (Bandwidth Weighted DF Election)

   ES routes advertised with the BW bit set will indicate the desire of
   the advertising PE to consider the link-bandwidth in the DF Election
   algorithm defined by the value in the "DF Type".

   As per [RFC8584], all the PEs in the ES MUST advertise the same
   Capabilities and DF Type, otherwise the PEs will fall back to Default
   [RFC7432] DF Election procedure.

   The BW Capability MAY be advertised with the following DF Types:

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   o  Type 0: Default DF Election algorithm, as in [RFC7432]

   o  Type 1: HRW algorithm, as in [RFC8584]

   o  Type 2: Preference algorithm, as in [EVPN-DF-PREF]

   o  Type 4: HRW per-multicast flow DF Election, as in [EVPN-PER-MCAST-
      FLOW-DF]

   The following sections describe how the DF Election procedures are
   modified for the above DF Types when the BW Capability is used.

6.2.  BW Capability and Default DF Election algorithm

   When all the PEs in the Ethernet Segment (ES) agree to use the BW
   Capability with DF Type 0, the Default DF Election procedure as
   defined in [RFC7432] is modified as follows:

   o  Each PE advertises a "Link Bandwidth" extended community attribute
      along with the ES route to signal the PE-CE link bandwidth (LBW)
      for the ES.

   o  A receiving PE MUST use the ES link bandwidth attribute received
      from each PE to compute a relative weight for each remote PE.

   o  The DF Election procedure MUST now use this weighted list of PEs
      to compute the per-VLAN Designated Forwarder, such that the DF
      role is distributed in proportion to this normalized weight.  As a
      result, a single PE may have multiple ordinals in the DF candidate
      PE list and 'N' used in (V mode N) operation as defined in
      [RFC7432] is modified to be total number of ordinals instead of
      being total number of PEs.

   Considering the same example as in Section 3, the candidate PE list
   for DF election is:

   [PE-1, PE-1, PE-2, PE-3].

   The DF for a given VLAN-a on ES-10 is now computed as (VLAN-a % 4).
   This would result in the DF role being distributed across PE1, PE2,
   and PE3 in portion to each PE's normalized weight for ES-10.

6.3.  BW Capability and HRW DF Election algorithm (Type 1 and 4)

   [RFC8584] introduces Highest Random Weight (HRW) algorithm (DF Type
   1) for DF election in order to solve potential DF election skew
   depending on Ethernet tag space distribution.  [EVPN-PER-MCAST-FLOW-
   DF] further extends HRW algorithm for per-multicast flow based hash

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   computations (DF Type 4).  This section describes extensions to HRW
   Algorithm for EVPN DF Election specified in [RFC8584] and in [EVPN-
   PER-MCAST-FLOW-DF] in order to achieve DF election distribution that
   is weighted by link bandwidth.

6.3.1.  BW Increment

   A new variable called "bandwidth increment" is computed for each [PE,
   ES] advertising the ES link bandwidth attribute as follows:

   In the context of an ES,

   L(i) = Link bandwidth advertised by PE(i) for this ES

   L(min) = lowest link bandwidth advertised across all PEs for this ES

   Bandwidth increment, "b(i)" for a given PE(i) advertising a link
   bandwidth of L(i) is defined as an integer value computed as:

   b(i) = L(i) / L(min)

   As an example,

   with PE(1) = 10, PE(2) = 10, PE(3) = 20

   bandwidth increment for each PE would be computed as:

   b(1) = 1, b(2) = 1, b(3) = 2

   with PE(1) = 10, PE(2) = 10, PE(3) = 10

   bandwidth increment for each PE would be computed as:

   b(1) = 1, b(2) = 1, b(3) = 1

   Note that the bandwidth increment must always be an integer,
   including, in an unlikely scenario of a PE's link bandwidth not being
   an exact multiple of L(min).  If it computes to a non-integer value
   (including as a result of link failure), it MUST be rounded down to
   an integer.

6.3.2.  HRW Hash Computations with BW Increment

   HRW algorithm as described in [RFC8584] and in [EVPN-PER-MCAST-FLOW-
   DF] computes a random hash value for each PE(i), where, (0 < i <= N),
   PE(i) is the PE at ordinal i, and Address(i) is the IP address of
   PE(i).

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   For 'N' PEs sharing an Ethernet segment, this results in 'N'
   candidate hash computations.  The PE that has the highest hash value
   is selected as the DF.

   We refer to this hash value as "affinity" in this document.  Hash or
   affinity computation for each PE(i) is extended to be computed one
   per bandwidth increment associated with PE(i) instead of a single
   affinity computation per PE(i).

   PE(i) with b(i) = j, results in j affinity computations:

   affinity(i, x), where 1 < x <= j

   This essentially results in number of candidate HRW hash computations
   for each PE that is directly proportional to that PE's relative
   bandwidth within an ES and hence gives PE(i) a probability of being
   DF in proportion to it's relative bandwidth within an ES.

   As an example, consider an ES that is multi-homed to two PEs, PE1 and
   PE2, with equal bandwidth distribution across PE1 and PE2.  This
   would result in a total of two candidate hash computations:

   affinity(PE1, 1)

   affinity(PE2, 1)

   Now, consider a scenario with PE1's link bandwidth as 2x that of PE2.
   This would result in a total of three candidate hash computations to
   be used for DF election:

   affinity(PE1, 1)

   affinity(PE1, 2)

   affinity(PE2, 1)

   which would give PE1 2/3 probability of getting elected as a DF, in
   proportion to its relative bandwidth in the ES.

   Depending on the chosen HRW hash function, affinity function MUST be
   extended to include bandwidth increment in the computation.

   For e.g.,

   affinity function specified in [EVPN-PER-MCAST-FLOW-DF] MAY be
   extended as follows to incorporate bandwidth increment j:

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   affinity(S,G,V, ESI, Address(i,j)) =
   (1103515245.((1103515245.Address(i).j + 12345) XOR
   D(S,G,V,ESI))+12345) (mod 2^31)

   affinity or random function specified in [RFC8584] MAY be extended as
   follows to incorporate bandwidth increment j:

   affinity(v, Es, Address(i,j)) = (1103515245((1103515245.Address(i).j
   + 12345) XOR D(v,Es))+12345)(mod 2^31)

6.4.  BW Capability and Preference DF Election algorithm

   This section applies to ES'es where all the PEs in the ES agree use
   the BW Capability with DF Type 2.  The BW Capability modifies the
   Preference DF Election procedure [EVPN-DF-PREF], by adding the LBW
   value as a tie-breaker as follows:

   Section 4.1, bullet (f) in [EVPN-DF-PREF] now considers the LBW
   value:

   f) In case of equal Preference in two or more PEs in the ES, the tie-
   breakers will be the DP bit, the LBW value and the lowest IP PE in
   that order.  For instance:

   o  If vES1 parameters were [Pref=500,DP=0,LBW=1000] in PE1 and
      [Pref=500,DP=1, LBW=2000] in PE2, PE2 would be elected due to the
      DP bit.

   o  If vES1 parameters were [Pref=500,DP=0,LBW=1000] in PE1 and
      [Pref=500,DP=0, LBW=2000] in PE2, PE2 would be elected due to a
      higher LBW, even if PE1's IP address is lower.

   o  The LBW exchanged value has no impact on the Non-Revertive option
      described in [EVPN-DF-PREF].

7.  Cost-Benefit Tradeoff on Link Failures

   While incorporating link bandwidth into the DF election process
   provides optimal BUM traffic distribution across the ES links, it
   also implies that DF elections are re-adjusted on link failures or
   bandwidth changes.  If the operator does not wish to have this level
   of churn in their DF election, then they should not advertise the BW
   capability.  Not advertising BW capability may result in less than
   optimal BUM traffic distribution while still retaining the ability to
   allow a remote ingress PE to do weighted ECMP for its unicast traffic
   to a set of multi-homed PEs.

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8.  Real-time Available Bandwidth

   PE-CE link bandwidth availability may sometimes vary in real-time
   disproportionately across PE-CE links within a multi-homed ESI due to
   various factors such as flow based hashing combined with fat flows
   and unbalanced hashing.  Reacting to real-time available bandwidth is
   at this time outside the scope of this document.

9.  Weighted Load-balancing to Multi-homed Subnets

   EVPN Link bandwidth extended community may also be used to achieve
   unequal load-balancing of prefix routed traffic by including this
   extended community in EVPN Route Type 5.  When included in EVPN RT-5,
   its value is to be interpreted as advertising PE's relative weight
   for the prefix included in this RT-5.  Receiving PE will then compute
   the forwarding path-list for the prefix route using weighted paths
   received from each remote PE.

10.  EVPN-IRB Multi-homing With Non-EVPN routing

   EVPN-LAG based multi-homing on an IRB gateway may also be deployed
   together with non-EVPN routing, such as global routing or an L3VPN
   routing control plane.  Key property that differentiates this set of
   use cases from EVPN IRB use cases discussed earlier is that EVPN
   control plane is used only to enable LAG interface based multi-homing
   and NOT as an overlay VPN control plane.  EVPN control plane in this
   case enables:

   o  DF election via EVPN RT-4 based procedures described in [RFC7432]

   o  Local MAC sync across multi-homing PEs via EVPN RT-2

   o  Local ARP and ND sync across multi-homing PEs via EVPN RT-2

   Applicability of weighted ECMP procedures proposed in this document
   to these set of use cases is an area of further consideration.

11.  Operational Considerations

   None

12.  Security Considerations

   This document raises no new security issues for EVPN.

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13.  IANA Considerations

   [RFC8584] defines a new extended community for PEs within a
   redundancy group to signal and agree on uniform DF Election Type and
   Capabilities for each ES.  This document requests IANA for a bit in
   the DF Election extended community Bitmap:

   Bit 28: BW (Bandwidth Weighted DF Election)

   A new EVPN Link Bandwidth extended community is defined to signal
   local ES link bandwidth to remote PEs.  This extended-community is
   defined of type 0x06 (EVPN).  IANA is requested to assign a sub-type
   value of 0x10 for the EVPN Link bandwidth extended community, of type
   0x06 (EVPN).  EVPN Link Bandwidth extended community is defined as
   transitive.

   IANA is requested to set up a registry called "Value-Units" for the
   1-octet field in the EVPN Link Bandwidth Extended Community.  New
   registrations will be made through the "RFC Required" procedure
   defined in [RFC8126].  The following initial values in that registry
   exist:

   Value       Name                             Reference
   ----        ----------------                 -------------
   0           Weight in units of Mbps          This document
   1           Generalized Weight               This document
   2-255       Unassigned

14.  Acknowledgements

   Authors would like to thank Satya Mohanty for valuable review and
   inputs with respect to HRW and weighted HRW algorithm refinements
   proposed in this document.  Authors would also like to thank Bruno
   Decraene and Sergey Fomin for valuable review and comments.

15.  Contributors

   Satya Ranjan Mohanty
   Cisco Systems
   US
   Email: satyamoh@cisco.com

16.  References

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16.1.  Normative References

   [EVPN-DF-PREF]
              Rabadan, J., Sathappan, S., Przygienda, T., Lin, W.,
              Drake, J., Sajassi, A., Mohanty, S., and , "Preference-
              based EVPN DF Election", draft-ietf-bess-evpn-pref-df-06
              (work in progress), June 2020.

   [EVPN-PER-MCAST-FLOW-DF]
              Sajassi, A., mishra, m., Thoria, S., Rabadan, J., and J.
              Drake, "Per multicast flow Designated Forwarder Election
              for EVPN", draft-ietf-bess-evpn-per-mcast-flow-df-
              election-04 (work in progress), August 2020.

   [EVPN-VIRTUAL-ES]
              Sajassi, A., Brissette, P., Schell, R., Drake, J.,
              Rabadan, J., and , "EVPN Virtual Ethernet Segment", draft-
              ietf-bess-evpn-virtual-eth-segment-06 (work in progress),
              March 2020.

   [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>.

   [RFC8584]  Rabadan, J., Ed., Mohanty, R., Sajassi, N., Drake, A.,
              Nagaraj, K., and S. Sathappan, "Framework for Ethernet VPN
              Designated Forwarder Election Extensibility", RFC 8584,
              DOI 10.17487/RFC8584, April 2019,
              <https://www.rfc-editor.org/info/rfc8584>.

16.2.  Informative References

   [BGP-LINK-BW]
              Mohapatra, P. and R. Fernando, "BGP Link Bandwidth
              Extended Community", draft-ietf-idr-link-bandwidth-07
              (work in progress), March 2019.

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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

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

   Email: jorge.rabadan@nokia.com

   John Drake
   Juniper

   Email: jdrake@juniper.net

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

   Email: ar977m@att.com

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   Samir Thoria
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134
   USA

   Email: sthoria@cisco.com

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