Fast Recovery for EVPN Designated Forwarder Election
draft-ietf-bess-evpn-fast-df-recovery-05

BESS Working Group                                     P. Brissette, Ed.
Internet-Draft                                                A. Sajassi
Intended status: Standards Track                              LA. Burdet
Expires: 8 September 2022                                          Cisco
                                                                J. Drake
                                                                 Juniper
                                                              J. Rabadan
                                                                   Nokia
                                                            7 March 2022

          Fast Recovery for EVPN Designated Forwarder Election
                draft-ietf-bess-evpn-fast-df-recovery-05

Abstract

   Ethernet Virtual Private Network (EVPN) solution provides Designated
   Forwarder election procedures for multihomed Ethernet Segments.
   These procedures have been enhanced further by applying Highest
   Random Weight (HRW) Algorithm for Designated Forwarded election in
   order to avoid unnecessary DF status changes upon a failure.  This
   draft improves these procedures by providing a fast Designated
   Forwarder (DF) election upon recovery of the failed link or node
   associated with the multihomed Ethernet Segment.  The solution is
   independent of number of EVIs associated with that Ethernet Segment
   and it is performed via a simple signaling between the recovered PE
   and each of the other PEs in the multihoming group.

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 RFC 2119 [RFC2119] and
   RFC 8174 [RFC8174].

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/.

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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on 8 September 2022.

Copyright Notice

   Copyright (c) 2022 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
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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Challenges with Existing Solution . . . . . . . . . . . . . .   3
   3.  DF Election Synchronization Solution  . . . . . . . . . . . .   4
     3.1.  Advantages  . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  BGP Encoding  . . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Synchronization Scenarios . . . . . . . . . . . . . . . .   7
     3.4.  Backwards Compatibility . . . . . . . . . . . . . . . . .   8
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   6.  Normative References  . . . . . . . . . . . . . . . . . . . .   9
   Appendix A.  Contributors . . . . . . . . . . . . . . . . . . . .  10
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   Ethernet Virtual Private Network (EVPN) solution [RFC7432] is
   becoming pervasive 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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   The EVPN specification [RFC7432] describes DF election procedures for
   multihomed Ethernet Segments.  These procedures are enhanced further
   in [RFC8584] by applying Highest Random Weight Algorithm for DF
   election in order to avoid DF status change unnecessarily upon a link
   or node failure associated with the multihomed Ethernet Segment.
   This draft makes further improvement to 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 number of EVIs
   associated with that Ethernet Segment and it is performed via a
   simple signaling between the recovered PE and each of the other PEs
   in the multihomed group.  The solution is based on simple one-way
   signaling mechanism.

1.1.  Terminology

   Provider Edge (PE):  A device that sits in the boundary of Provider
      and Customer networks and performs encap/decap of data from L2 to
      L3 and vice-versa.

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

2.  Challenges with Existing Solution

   In EVPN technology, multiple PE devices have the ability to encap and
   decap data belonging to the same VLAN.  In certain situations, this
   may cause L2 duplicates and even loops if there is a momentary
   overlap of forwarding roles between two or more PE devices, leading
   to broadcast storms.

   EVPN [RFC7432] currently uses timer based synchronization among PE
   devices in redundancy group that can result in duplications (and even
   loops) because of multiple DFs if the timer is too short or
   blackholing if the timer is too long.

   Using split-horizon filtering (Section 8.3 of [RFC7432]) can prevent
   loops (but not duplicates), however if there are overlapping DFs in
   two different sites at the same time for the same VLAN, the site
   identifier will be different upon re-entry of the packet and hence
   the split-horizon check will fail, leading to L2 loops.

   The updated DF procedures in [RFC8584] use the well known
   HRW (Highest Random Weight) algorithm to avoid reshuffling of VLANs
   among PE devices in the redundancy group upon failure/recovery.  This
   reduces the impact to VLANs not assigned to the failed/recovered
   ports and eliminates loops or duplicates at failure/recovery events.

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   However, upon PE insertion or port bring-up (recovery event), HRW
   also cannot help as a transfer of DF role to the newly inserted
   device/port must occur while the old DF is still active.

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

                  Figure 1: CE1 multihomed to PE1 and PE2.

   In the Figure 1, when PE2 is inserted or booted up, PE1 will transfer
   DF role of some VLANs to PE2 to achieve load balancing.  However,
   because there is no handshake mechanism between PE1 and PE2,
   duplication of DF roles for a given VLAN is possible.  Duplication of
   DF roles may eventually lead to duplication of traffic as well as L2
   loops.

   Current EVPN specification [RFC7432] and [RFC8584] relies 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

3.  DF Election Synchronization Solution

   The solution relies on the concept of common clock alignment between
   partner PEs participating to a common Ethernet Segment.  The main
   idea is to have all peering PEs of that Ethernet Segment perform DF
   election, and apply their resulting carving state, at a same well-
   known time.

   The DF Election procedure, as described in [RFC7432] and as
   optionally signalled 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, etc.).  Newly

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   inserted device PE or during failure recovery of a PE, that PE
   communicates the current time to peering partners plus the remaining
   peering timer time left.  This constitutes an "end time" or "absolute
   time" as seen from local PE.  That absolute time is called "Service
   Carving Time" (SCT).

   A new BGP Extended Community is advertised along with Ethernet
   Segment route (RT-4) to communicate to other partners the Service
   Carving Time.

   Upon reception of that new BGP Extended Community, partner PEs know
   exactly its carving time.  The notion of skew is introduced to
   eliminate any potential duplicate traffic or loops.  They add a skew
   (default = -10ms) to the Service Carving Time to enforce this.  The
   previously inserted PE(s) must carve first, followed shortly(skew) by
   the newly insterted PE.

   To summarize, all peering PEs carve almost simultaneously at the time
   announced by newly added/recovered PE.  The newly inserted PE
   initiates the SCT, and carves immediately on peering timer expiry.
   The previously inserted PE(s) receiving Ethernet Segment route (RT-4)
   with a SCT BGP extended community, carve shortly before Service
   Carving Time.

3.1.  Advantages

   There are multiples advantages of using the approach.  Here is a non-
   exhaustive list:

   *  A simple uni-directional signaling is all that is needed

   *  Backwards-compatible: PEs supporting only older [RFC7432] shall
      simply discard unrecognized new "Service Carving Timestamp" BGP
      Extended Community

   *  Multiple DF Election algorithms can be supported:

      -  [RFC7432] default ordered list ordinal algorithm (Modulo),

      -  [RFC8584] highest-random weight, etc.

   *  Independent of BGP transmission delay regarding Ethernet Segment
      route (RT-4)

   *  Agnostic of the time synchronization mechanism used (e.g.  NTP,
      PTP, etc.)

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3.2.  BGP Encoding

   A new BGP extended community needs to be defined to communicate the
   Service Carving Timestamp for each Ethernet Segment.

   A new transitive extended community where the Type field is 0x06, and
   the Sub-Type is 0x0F is advertised along with Ethernet Segment route.
   The expected Service Carving Time is encoded as a 8-octet value as
   follows:

                        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  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The timestamp exchanged uses the NTP epoch of January 1, 1900
   [RFC5905].  The 64-bit timestamp of the NTP protocol consists of a
   32-bit part for seconds and a 32-bit part for fractional second:

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

   *  Timestamp Fractional Seconds: 16 bits of the NTP fractional
      seconds are encoded in this field.  The use of a 16-bit fractional
      seconds yields adequate precision of 15 microseconds (2^-16 s).

   This document introduces a new flag called "T" (for Time
   Synchronization) to the bitmap field of the DF Election Extended
   Community defined in [RFC8584].

                        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 | |A| |T|       ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~     Bitmap    |            Reserved = 0                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   *  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.

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   This capability is used in conjunction with the agreed upon DF Type
   (DF Election Type).  For example if all the PEs in the Ethernet
   Segment indicated that they have Time Synchronization capability and
   they want the DF type to be HRW, then HRW algorithm is used in
   conjunction with this capability.

3.3.  Synchronization Scenarios

   Let's take Figure 1 as an example where initially PE2 had failed and
   PE1 had taken over.  This example shows the problem with the DF-
   Election mechanism in [RFC7432].

   Based on Section 8.5 of [RFC7432], using the default 3 second peering
   timer:

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

   2.  PE2 recovers at (absolute) time t=99

   3.  PE2 advertises RT-4 (sent at t=100) to partner PE1

   4.  PE2 starts a 3 second peering timer

   5.  PE1 carves immediately on RT-4 reception, i.e. t=100 + minimal
       BGP propagation delay

   6.  PE2 carves at time t=103

   [RFC7432] aims of favouring traffic black hole over duplicate
   traffic.  With above procedure, traffic black holing will occur as
   part of each PE recovery sequence since PE1 has transitioned some
   VLANs to Non-Designated-Forwarder (NDF) immediately upon reception.
   The peering timer value (default = 3 seconds) has a direct effect on
   the duration of the blackholing.  A shorter (esp. zero) peering timer
   may, however, result in duplicate traffic or traffic loops.

   Based on the Service Carving Time (SCT) approach:

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

   2.  PE2 recovers at (absolute) time t=99

   3.  PE2 advertises RT-4 (sent at t=100) with target SCT value t=103
       to partner PE1

   4.  PE2 starts 3 second peering timer

   5.  Both PE1 and PE2 carve at (absolute) time t=103

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   In fact, PE1 should carve slightly before PE2 (skew).  The previously
   inserted PE2 that is recovering performs both transitions DF to NDF
   and NDF to DF per VLANs at the peering timer expiry.  Since the goal
   is to prevent duplicates, the original PE1, which received the SCT
   will apply:

   *  DF to NDF transition at t=SCT minus skew where both PEs are NDF
      for 'skew' amount of time

   *  NDF to DF transition at t=SCT

   It is this split-behaviour which ensures good transition of DF role
   with contained amount of loss.

   Using SCT approach, the negative effect of the peering timer is
   mitigated.  Furthermore, the BGP Ethernet Segment route (RT-4)
   transmission delay (from PE2 to PE1) becomes a non-issue.  The use of
   SCT approach remedies the problem associated with the peering timer:
   the 3 second timer window is shortened to the order of milliseconds.

3.4.  Backwards Compatibility

   Per redundancy group, for the DF election procedures to be globally
   convergent and unanimous, it is necessary that all the participating
   PEs agree on the DF Election algorithm to be used.  It is, however,
   possible that some PEs continue to use the existing modulo-based DF
   election and do not rely on the new SCT BGP extended community.  PEs
   running a baseline DF election mechanism will simply discard the new
   SCT BGP extended community as unrecognized.

   A PE can indicate its willingness to support clock-synched carving by
   signaling the new 'T' DF Election Capability as well as including the
   new Service Carving Time BGP extended community along with the
   Ethernet Segment Route (Type-4).  In the case where one or more PEs
   attached to the Ethernet Segment do not signal T=1, all PEs in the
   Ethernet Segment SHALL revert back to the [RFC7432] timer approach.
   This is especially important in the context of the VLAN shuffling
   with more than 2 PEs.

4.  Security Considerations

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

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5.  IANA Considerations

   This document solicits the allocation of the following sub-type in
   the "EVPN Extended Community Sub-Types" registry setup by [RFC7153]:

         0x0F     Service Carving Timestamp    This document

   This document solicits the allocation of the following values in the
   "DF Election Capabilities" registry setup by [RFC8584]:

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

6.  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>.

   [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>.

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   [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>.

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

Appendix B.  Acknowledgements

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

Authors' Addresses

   Patrice Brissette (editor)
   Cisco
   Email: pbrisset@cisco.com

   Ali Sajassi
   Cisco
   Email: sajassi@cisco.com

   Luc Andre Burdet
   Cisco
   Email: lburdet@cisco.com

   John Drake
   Juniper
   Email: jdrake@juniper.net

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

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