Fast Recovery for EVPN Designated Forwarder Election
draft-ietf-bess-evpn-fast-df-recovery-12

BESS Working Group                                          P. Brissette
Internet-Draft                                                A. Sajassi
Updates: 8584 (if approved)                              LA. Burdet, Ed.
Intended status: Standards Track                                   Cisco
Expires: 24 May 2025                                            J. Drake
                                                             Independent
                                                              J. Rabadan
                                                                   Nokia
                                                        20 November 2024

          Fast Recovery for EVPN Designated Forwarder Election
                draft-ietf-bess-evpn-fast-df-recovery-12

Abstract

   The Ethernet Virtual Private Network (EVPN) solution in RFC 7432
   provides Designated Forwarder (DF) election procedures for multihomed
   Ethernet Segments.  These procedures have been enhanced further by
   applying the Highest Random Weight (HRW) algorithm for Designated
   Forwarder election to avoid unnecessary DF status changes upon a
   failure.  This document improves these procedures by providing a fast
   Designated Forwarder election upon recovery of the failed link or
   node associated with the multihomed Ethernet Segment.  This document
   updates RFC 8584 by optionally introducing delays between some of the
   events therein.

   The solution is independent of the number of EVPN Instances (EVIs)
   associated with that Ethernet Segment and it is performed via a
   simple signaling in BGP between the recovered node and each of the
   other nodes in the multihoming group.

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
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   This Internet-Draft will expire on 24 May 2025.

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Copyright Notice

   Copyright (c) 2024 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
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.3.  Challenges with Existing Mechanism  . . . . . . . . . . .   4
     1.4.  Design Principles for a Solution  . . . . . . . . . . . .   5
   2.  DF Election Synchronization Solution  . . . . . . . . . . . .   6
     2.1.  BGP Encoding  . . . . . . . . . . . . . . . . . . . . . .   7
     2.2.  Timestamp Verification  . . . . . . . . . . . . . . . . .   9
     2.3.  Updates to RFC8584  . . . . . . . . . . . . . . . . . . .   9
   3.  Synchronization Scenarios . . . . . . . . . . . . . . . . . .  10
     3.1.  Concurrent Recoveries . . . . . . . . . . . . . . . . . .  12
   4.  Backwards Compatibility . . . . . . . . . . . . . . . . . . .  13
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Appendix A.  Contributors . . . . . . . . . . . . . . . . . . . .  15
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   The Ethernet Virtual Private Network (EVPN) solution [RFC7432] is
   widely used in data center (DC) applications for Network
   Virtualization Overlay (NVO) and DC interconnect (DCI) services, and
   in service provider (SP) applications for next generation virtual
   private LAN services.

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   [RFC7432] describes Designated Forwarder (DF) election procedures for
   multihomed Ethernet Segments.  These procedures are enhanced further
   in [RFC8584] by applying the Highest Random Weight algorithm for DF
   election in order to avoid unnecessary DF status changes upon a link
   or node failure associated with the multihomed Ethernet Segment.

   This document makes further improvements to the DF election
   procedures in [RFC8584] by providing an option for a fast DF election
   upon recovery of the failed link or node associated with the
   multihomed Ethernet Segment.  This DF election is achieved
   independent of the number of EVPN Instances (EVIs) associated with
   that Ethernet Segment and it is performed via straightforward
   signaling in BGP between the recovered node and each of the other
   nodes in the multihomed Ethernet Segment redundancy group.
   This document updates the DF Election Finite State Machine (FSM)
   described in Section 2.1 of [RFC8584], by optionally introducing
   delays between some events, as further detailed in Section 2.3.  The
   solution is based on a simple one-way signaling mechanism.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

1.2.  Terminology

   PE:  Provider Edge device.

   Designated Forwarder (DF):  A PE that is currently forwarding
      (encapsulating/decapsulating) traffic for a given VLAN in and out
      of a site.

   NDF:  Non-Designated Forwarder, a PE that is currently blocking
      traffic (see DF above).

   EVI:  An EVPN instance spanning the Provider Edge (PE) devices
      participating in that EVPN.

   HRW:  Highest Random Weight algorithm, [HRW98]

   Service carving:  DF Election is also referred to as "service
      carving" in [RFC7432]

   SCT:  Service Carving Time, defined in this document, the time at

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      which all nodes participating in an Ethernet Segment perform DF
      Election.

1.3.  Challenges with Existing Mechanism

   In EVPN technology, multiple Provider Edge (PE) devices encapsulate
   and decapsulate data belonging to the same VLAN.  Under certain
   conditions, this may cause duplicated Ethernet packets and potential
   loops if there is a momentary overlap in forwarding roles between two
   or more PE devices, potentially also leading to broadcast storms of
   frames forwarded back into the VLAN.

   EVPN [RFC7432] currently specifies timer-based synchronization among
   PE devices within an Ethernet Segment redundancy group.  This
   approach can lead to duplications and potential loops due to multiple
   Designated Forwarders (DFs) if the timer interval is too short, or to
   packet drops if the timer interval is too long.

   Split-horizon filtering, as described in Section 8.3 of [RFC7432],
   can prevent loops but does not address duplicates.  However, if there
   are overlapping Designated Forwarders of two different sites
   simultaneously for the same VLAN, the site identifier will differ
   when the packet re-enters the Ethernet Segment.  Consequently, the
   split-horizon check will fail, resulting in layer-2 loops.

   The updated DF procedures outlined in [RFC8584] use the well-known
   Highest Random Weight (HRW) algorithm to prevent the reshuffling of
   VLANs among PE devices within the Ethernet Segment redundancy group
   during failure or recovery events.  This approach minimizes the
   impact on VLANs not assigned to the failed or recovered ports and
   eliminates the occurrence of loops or duplicates during such events.

   However, upon PE insertion or a port being newly added to a
   multihomed Ethernet Segment, HRW cannot help either as a transfer of
   DF role to the new port must occur while the old DF is still active.

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                                     +---------+
                  +-------------+    |         |
                  |             |    |         |
                / |    PE1      |----|         |   +-------------+
               /  |             |    |  MPLS/  |   |             |---CE3
              /   +-------------+    |  VxLAN/ |   |     PE3     |
         CE1 -                       |  Cloud  |   |             |
              \   +-------------+    |         |---|             |
               \  |             |    |         |   +-------------+
                \ |     PE2     |----|         |
                  |             |    |         |
                  +-------------+    |         |
                                     +---------+

                  Figure 1: CE1 multihomed to PE1 and PE2.

   In Figure 1, when PE2 is inserted in the Ethernet Segment or its
   CE1-facing interface recovered, PE1 will transfer the DF role of some
   VLANs to PE2 to achieve load balancing.  However, because there is no
   handshake mechanism between PE1 and PE2, overlapping of DF roles for
   a given VLAN is possible which leads to duplication of traffic as
   well as layer-2 loops.

   Current EVPN specifications [RFC7432] and [RFC8584] rely on a timer-
   based approach for transferring the DF role to the newly inserted
   device.  This can cause the following issues:

   *  Loops/Duplicates if the timer value is too short

   *  Prolonged Traffic Blackholing if the timer value is too long

1.4.  Design Principles for a Solution

   The clock-synchronization solution for fast DF recovery presented in
   this document follows several design principles and offers multiple
   advantages, namely:

   *  Complex handshake signaling mechanisms and state machines are
      avoided in favor of a simple uni-directional signaling approach.

   *  The fast DF recovery solution maintains backwards compatibility
      (see Section 4) by ensuring that PEs reject any unrecognized new
      BGP EVPN Extended Community.

   *  Existing DF Election algorithms remain supported.

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   *  The fast DF recovery solution is independent of any BGP delays in
      propagation of Ethernet Segment routes (Route Type 4)

   *  The fast DF recovery solution is agnostic of the actual time
      synchronization mechanism used; however, an NTP-based
      representation of time is used for EVPN signaling.

   The solution in this document relies on nodes in the topology, more
   specifically the peering nodes of each Ethernet-Segment, to be clock-
   synchronized and advertise Time Synchronization capability.  When
   this is not the case, or clocks are badly desynchronized, network
   convergence and DF Election is no worse than [RFC7432] due to the
   timestamp range checking (Section 2.2).

2.  DF Election Synchronization Solution

   The fast DF recovery solution relies on the concept of common clock
   alignment between partner PEs participating in a common Ethernet
   Segment, i.e., PE1 and PE2 in Figure 1.  The main idea is to have all
   peering PEs of that Ethernet Segment perform DF election and apply
   the result at the same previously-announced time.

   The DF Election procedure, as described in [RFC7432] and as
   optionally signaled in [RFC8584], is applied.  All PEs attached to a
   given Ethernet Segment are clock-synchronized using a networking
   protocol for clock synchronization (e.g., NTP, PTP).  Whenever
   possible, recovery activities for failed PEs SHOULD NOT be initiated
   until after the underlying clock synchronization protocol has
   converged to benefit from this document's fast DF recovery
   procedures.  When a new PE is inserted in an Ethernet Segment or a
   failed PE of the Ethernet Segment recovers, that PE communicates to
   peering partners the current time plus the value of the timer for
   partner discovery from step 2 in Section 8.5 of [RFC7432].  This
   constitutes an "end time" or "absolute time" as seen from the local
   PE.  That absolute time is called the "Service Carving Time" (SCT).

   A new BGP EVPN Extended Community, the Service Carving Time is
   advertised along with the Ethernet Segment Route Type 4 (RT-4) and
   communicates the Service Carving Time to other partners to ensure an
   orderly transfer of forwarding duties.

   Upon receipt of the new BGP EVPN Extended Community, partner PEs can
   determine the service carving time of the newly inserted PE.  To
   eliminate any potential for duplicate traffic or loops, the concept
   of skew is introduced: a small time offset to ensure a controlled and
   orderly transition when multiple Provider Edge (PE) devices are
   involved.  The previously inserted PE(s) must perform service carving
   first for NDF to DF transitions.  The receiving PEs subtract this

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   skew (default = 10ms) to the Service Carving Time and apply NDF to DF
   transitions first.  This is followed shortly by the NDF to DF
   transitions on both PEs, after the skew delay.  On the recovering PE,
   all services are already in NDF state and no skew for DF to NDF
   transitions is required.
   This document proposes a default skew value of 10ms to allow
   completion of programming the DF to NDF transitions, but
   implementations may make the skew larger (or configurable) taking
   into consideration scale, hardware capabilities and clock accuracy.

   To summarize, all peering PEs perform service carving almost
   simultaneously at the time announced by the newly added/recovered PE.
   The newly inserted PE initiates the SCT, and triggers service carving
   immediately on its local timer expiry.  The previously inserted PE(s)
   receiving Ethernet Segment route (RT-4) with an SCT BGP extended
   community, perform service carving shortly before Service Carving
   Time for DF to NDF transitions, and at Service Carving Time for NDF
   to DF transitions.

2.1.  BGP Encoding

   A BGP extended community, with Type 0x06 and Sub-Type 0x0F, is
   defined to communicate the Service Carving Time for each Ethernet
   Segment:

                        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 = 0x06   | Sub-Type(0x0F)|      Timestamp Seconds        ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~  Timestamp Seconds            | Timestamp Fractional Seconds  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 2: Service Carving Time

   The timestamp exchanged uses the NTP prime epoch of January 1, 1900
   [RFC5905] and an adapted form of the 64-bit NTP Timestamp Format.
   The 64-bit NTP Timestamp Format consists of a 32-bit part for Seconds
   and a 32-bit part for Fraction, which are encoded in the Service
   Carving Time as follows:

   *  Timestamp Seconds: 32-bit NTP seconds are encoded in this field.

   *  Timestamp Fractional Seconds: the high order 16 bits of the NTP
      'Fraction' field are encoded in this field.

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   When rebuilding a 64-bit NTP Timestamp Format using the values from a
   received SCT BGP extended community, the lower order 16 bits of the
   Fractional field are set to 0.  The use of a 16-bit fractional
   seconds value yields adequate precision of 15 microseconds (2^-16 s).

   This document introduces a new flag called Time Synchronization
   indicated by "T" in the DF Election Capabilities registry defined in
   [RFC8584] for use in DF Election Extended Community.

                        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 = 0x06   | Sub-Type(0x06)| RSV |  DF Alg |    Bitmap     ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~     Bitmap    |            Reserved                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 4: DF Election Extended Community

                  Figure 3: DF Election Extended Community

                        1         1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | |A| |T|                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 5: DF Election Capabilities

                     Figure 4: DF Election Capabilities

   *  Bit 3: Time Synchronization (corresponds to Bit 27 of the DF
      Election Extended Community).  When set to 1, it indicates the
      desire to use Time Synchronization capability with the rest of the
      PEs in the Ethernet Segment.

   This capability is utilized in conjunction with the agreed-upon DF
   Election Type.  For instance, if all the PE devices in the Ethernet
   Segment indicate the desire to use the Time Synchronization
   capability and request the DF Election Type to be Highest Random
   Weight (HRW), then the HRW algorithm is used in conjunction with this
   capability.  A PE which does not support the procedures set out in
   this document, or receives a route from another PE in which the
   capability is not set, MUST NOT delay Designated Forwarder election
   as this could lead to duplicate traffic in some instances
   (overlapping Designated Forwarders).

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2.2.  Timestamp Verification

   The NTP Era value is not exchanged and participating PEs may consider
   the timestamps to be in the same Era as their local value.  A DF
   Election operation occurring exactly at the next Era transition will
   be sometime on February 7, 2036.  Implementors and operators may
   address credible cases of rollover ambiguity (adjacent Eras n and
   n+1), as well as the security issue of unreasonably large or
   unreasonably small NTP timestamps, in the following manner.

   The procedures in this document address implicitly what occurs with
   receiving a SCT value in the past.  This would be a naturally
   occurring event with a large BGP propagation delay: the receiving PE
   treats the DF Election at the peer as having occurred already and
   proceeds without starting any timer to further delay service carving,
   effectively falling back on [RFC7432] behavior.  A PE which receives
   a SCT value smaller than its current time, MUST discard the Service
   Carving Time and SHALL treat the DF Election at the peer as having
   occurred already.

   The more problematic scenario is the PE in Era n+1 which receives a
   Service Carving Time advertised by the PE still in Era n, with a very
   large SCT value.  To address this Era rollover as well as the large
   values attack vector, implementations MUST validate the received SCT
   against an upper-bound.
   It is left to implementations to decide what constitutes an
   "unreasonably large" SCT value.  A recommended approach, however, is
   to compare the received offset to the local peering timer value.  In
   practice, peering timer values are configured uniformly across
   Ethernet-Segment peers and may be treated as an upper-bound on the
   offset of received SCT values.  A PE which receives an SCT
   representing an offset larger than the local peering timer MUST
   discard the Service Carving Time and SHALL treat the DF Election at
   the peer as having occurred already, as above.

2.3.  Updates to RFC8584

   This document introduces an additional delay to the events and
   transitions defined for the default DF election algorithm FSM in
   Section 2.1 of [RFC8584] without changing the FSM state or event
   definitions themselves.

   Upon receiving a RCVD_ES message, the peering PE's Finite State
   Machine (FSM) transitions from the DF_DONE (indicating the DF
   election process was complete) state to the DF_CALC (indicating that
   a new DF calculation is needed) state.  Due to the Service Carving
   Time (SCT) included in the Ethernet-Segment update, the completion of
   the DF_CALC state and the subsequent transition back to the DF_DONE

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   state are delayed.  This delay ensures proper synchronization and
   prevents conflicts.  Consequently, the accompanying forwarding
   updates to the Designated Forwarder (DF) and Non-Designated Forwarder
   (NDF) states are also deferred.

   Item 9. in Section 2.1 of [RFC8584], the list "Corresponding actions
   when transitions are performed or states are entered/exited" is
   changed as follows:

   9.  DF_CALC on CALCULATED: Mark the election result for the VLAN or
       VLAN Bundle.

       9.1  If an SCT timestamp is present during the RCVD_ES event of
            Action 11, wait until the time indicated by the SCT minus
            skew before proceeding to step 9.3.

       9.2  If an SCT timestamp is present during the RCVD_ES event of
            Action 11, wait until the time indicated by the SCT before
            proceeding to step 9.4.

       9.3  Assume the role of NDF for the local PE concerning the VLAN
            or VLAN Bundle, and transition to the DF_DONE state.

       9.4  Assume the role of DF for the local PE concerning the VLAN
            or VLAN Bundle, and transition to the DF_DONE state.

   This revised approach ensures proper timing and synchronization in
   the DF election process, avoiding conflicts and ensuring accurate
   forwarding updates.

3.  Synchronization Scenarios

   Consider Figure 1 as an example, where initially PE2 has failed and
   PE1 has taken over.  This scenario illustrates the problem with the
   DF-Election mechanism described in Section 8.5 of [RFC7432],
   specifically in the context of the timer value configured for all PEs
   on the Ethernet Segment.

   Procedure based on Section 8.5 of [RFC7432] with the default 3-second
   timer in step 2:

   1.  Initial state: PE1 is in a steady-state and PE2 is recovering.

   2.  Recovery: PE2 recovers at an absolute time of t=99.

   3.  Advertisement: PE2 advertises RT-4, sent at t=100, to partner
       PE1.

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   4.  Timer Start: PE2 starts a 3-second timer to allow the reception
       of RT-4 from other PE nodes.

   5.  Immediate carving: PE1 performs service carving immediately upon
       RT-4 reception, i.e., t=100 plus some BGP propagation delay.

   6.  Delayed Carving: PE2 performs service carving at time t=103.

   [RFC7432] favors traffic drops over duplicate traffic.  With the
   above procedure, traffic drops will occur as part of each PE recovery
   sequence since PE1 transitions some VLANs to Non-Designated Forwarder
   (NDF) immediately upon RT-4 reception.
   The timer value (default = 3 seconds) directly affects the duration
   of the packet drops.  A shorter (or zero) timer may result in
   duplicate traffic or traffic loops.

   Procedure based on the Service Carving Time (SCT) approach:

   1.  Initial state: PE1 is in a steady state, and PE2 is recovering.

   2.  Recovery: PE2 recovers at an absolute time of t=99.

   3.  Timer Start: PE2 starts at t=100 a 3-second timer to allow the
       reception of RT-4 from other PE nodes.

   4.  Advertisement: PE2 advertises RT-4, sent at t=100, with a target
       SCT value of t=103 to partner PE1.

   5.  Service Carving Timer: PE1 starts the service carving timer, with
       the remaining time until t=103.

   6.  Simultaneous Carving: Both PE1 and PE2 carve at an absolute time
       of t=103.

   To maintain the preference for minimal loss over duplicate traffic,
   PE1 SHOULD carve slightly before PE2 (with skew).  The recovering PE2
   performs both DF to NDF and NDF to DF transitions per VLAN at the
   timer's expiry.  The original PE1, which received the SCT, applies
   the following:

   *  DF to NDF Transition(s): at t=SCT minus skew, where both PEs are
      NDF for the skew duration.

   *  NDF to DF Transition(s): at t=SCT.

   This split-behavior ensures a smooth DF role transition with minimal
   loss.

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   Using the SCT approach, the negative effect of the timer to allow the
   reception of Ethernet Segment RT-4 from other PE nodes is mitigated.
   Furthermore, the BGP transmission delay (from PE2 to PE1) of the ES
   RT-4 becomes a non-issue.  The SCT approach shortens the 3-second
   timer window to the order of milliseconds.

   The peering timer is a configurable value where 3 seconds represents
   the default.  Configuring a timer value of 0, or so small as to
   expire during propagation of the BGP routes, is outside the scope of
   this document.  In reality, the use of the SCT approach presented in
   this document encourages the use of larger peering timer values to
   overcome any sort of BGP route propagation delays.

3.1.  Concurrent Recoveries

   In the eventuality 2 or more PEs in a peering Ethernet Segment group
   are recovering concurrently or roughly the same time, each will
   advertise a Service Carving Time.  This SCT value would correspond to
   what each recovering PE considers the "end time" for DF Election.  A
   similar situation arises in sequentially recovering PEs, when a
   second PE recovers approximately at the time of the first PE's
   advertised SCT expiry, and with its own new SCT-2 outside of the
   initial SCT window.

   In the case of multiple concurrent DF elections, each initiated by
   one of the recovering PEs, the SCTs must be ordered chronologically.
   All PEs SHALL execute only a single DF Election at the service
   carving time corresponding to the largest (latest) received timestamp
   value.  This DF Election will lead peering PEs into a single co-
   ordinated DF Election update.

   Example:

   1.  Initial State: PE1 is in a steady state, with services elected at
       PE1.

   2.  Recovery of PE2: PE2 recovers at time t=100 and advertises RT-4
       with a target SCT value of t=103 to its partners (PE1).

   3.  Timer Initiation by PE2: PE2 starts a 3-second timer to allow the
       reception of RT-4 from other PE nodes.

   4.  Timer Initiation by PE1: PE1 starts the service carving timer,
       with the remaining time until t=103.

   5.  Recovery of PE3: PE3 recovers at time t=102 and advertises RT-4
       with a target SCT value of t=105 to its partners (PE1, PE2).

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   6.  Timer Initiation by PE3: PE3 starts a 3-second timer to allow the
       reception of RT-4 from other PE nodes.

   7.  Timer Update by PE2: PE2 cancels the running timer and starts the
       service carving timer with the remaining time until t=105.

   8.  Timer Update by PE1: PE1 updates its service carving timer, with
       the remaining time until t=105.

   9.  Service Carving: PE1, PE2, and PE3 perform service carving at the
       absolute time of t=105.

   In the eventuality a PE in an Ethernet Segment group recovers during
   the discovery window specified in Section 8.5 of [RFC7432], and does
   not support or advertise the T-bit, then all PEs in the current
   peering sequence SHALL immediately revert to the default [RFC7432]
   behavior.

4.  Backwards Compatibility

   For the DF election procedures to achieve global convergence and
   unanimity within a redundancy group, it is essential that all
   participating PEs agree on the DF election algorithm to be employed.
   However, it is possible that some PEs may continue to use the
   existing modulo-based DF election algorithm from [RFC7432] and not
   utilize the new Service Carving Time (SCT) BGP extended community.
   PEs that operate using the baseline DF election mechanism will simply
   discard the new SCT BGP extended community as unrecognized.

   A PE can indicate its willingness to support clock-synchronized
   carving by signaling the new 'T' DF Election Capability and including
   the new SCT BGP extended community along with the Ethernet Segment
   Route Type 4.  If one or more PEs attached to the Ethernet Segment do
   not signal T=1, then all PEs in the Ethernet Segment SHALL revert to
   the timer-based approach as specified in [RFC7432].  This reversion
   is particularly crucial in preventing VLAN shuffling when more than
   two PEs are involved.

   In the event a new or extra RT-4 is received without the new 'T' DF
   Election Capability in the midst of an ongoing DF Election sequence,
   all SCT-based delays are cancelled and the DF Election immediately
   applied as specified in [RFC7432], as if no SCT had been previously
   exchanged.

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5.  Security Considerations

   The mechanisms in this document use the EVPN control plane as defined
   in [RFC7432].  Security considerations described in [RFC7432] are
   equally applicable.

   For the new SCT Extended Community, attack vectors may be setting the
   value to zero, to a value in the past or to large times in the
   future.  Handling of this attack vector is addressed in Section 2.2
   alongside NTP Era rollover ambiguity.

   This document uses MPLS and IP-based tunnel technologies to support
   data plane transport.  Security considerations described in [RFC7432]
   and in [RFC8365] are equally applicable.

6.  IANA Considerations

   IANA maintains the "EVPN Extended Community Sub-Types" registry set
   up by [RFC7153], where the following assignment has been made:

      Sub-Type Value   Name                        Reference
      --------------   -------------------------   -------------
            0x0F       Service Carving Time        This document

   IANA maintains the "DF Election Capabilities" registry set up by
   [RFC8584].  IANA is requested to make the following assignment from
   this registry:

       Bit         Name                         Reference
       ----        ----------------             -------------
       3           Time Synchronization         This document

7.  References

7.1.  Normative References

   [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>.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <https://www.rfc-editor.org/info/rfc5905>.

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   [RFC7153]  Rosen, E. and Y. Rekhter, "IANA Registries for BGP
              Extended Communities", RFC 7153, DOI 10.17487/RFC7153,
              March 2014, <https://www.rfc-editor.org/info/rfc7153>.

   [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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8365]  Sajassi, A., Ed., Drake, J., Ed., Bitar, N., Shekhar, R.,
              Uttaro, J., and W. Henderickx, "A Network Virtualization
              Overlay Solution Using Ethernet VPN (EVPN)", RFC 8365,
              DOI 10.17487/RFC8365, March 2018,
              <https://www.rfc-editor.org/info/rfc8365>.

   [RFC8584]  Rabadan, J., Ed., Mohanty, S., Ed., Sajassi, A., Drake,
              J., 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>.

7.2.  Informative References

   [HRW98]    Thaler, D. and C. Ravishankar, "Using Name-Based Mappings
              to Increase Hit Rates", 1998,
              <https://www.microsoft.com/en-us/research/wp-content/
              uploads/2017/02/HRW98.pdf>.

Appendix A.  Contributors

   In addition to the authors listed on the front page, the following
   co-authors have also contributed substantially to this document:

   Gaurav Badoni
   Cisco

   Email: gbadoni@cisco.com

   Dhananjaya Rao
   Cisco

   Email: dhrao@cisco.com

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Appendix B.  Acknowledgements

   Authors would like to acknowledge helpful comments and contributions
   of Satya Mohanty and Bharath Vasudevan.  Also thank you to Anoop
   Ghanwani and Gunter van de Velde for their thorough review with
   valuable comments and corrections.

Authors' Addresses

   Patrice Brissette
   Cisco
   Email: pbrisset@cisco.com

   Ali Sajassi
   Cisco
   Email: sajassi@cisco.com

   Luc Andre Burdet (editor)
   Cisco
   Email: lburdet@cisco.com

   John Drake
   Independent
   Email: je_drake@yahoo.com

   Jorge Rabadan
   Nokia
   Email: jorge.rabadan@nokia.com

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