BESS Working Group                                       LA. Burdet, Ed.
Internet-Draft                                              P. Brissette
Intended status: Standards Track                                   Cisco
Expires: April 28, 2022                                      T. Miyasaka
                                                        KDDI Corporation
                                                        October 25, 2021


                           EVPN Fast Reroute
                 draft-burdet-bess-evpn-fast-reroute-00

Abstract

   This document summarises EVPN convergence mechanisms and specifies
   procedures for EVPN networks to achieve sub-second and
   scale-independant convergence.

Status of This Memo

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

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   described in the Simplified BSD License.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Specification of Requirements . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Solution  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Pre-selection of Backup Path  . . . . . . . . . . . . . .   6
     5.2.  Failure Detection and Traffic Restoration . . . . . . . .   6
       5.2.1.  Simultaneous Failures in ES . . . . . . . . . . . . .   7
       5.2.2.  Successive and Cascading Failures in ES . . . . . . .   8
   6.  Redirect Labels: Forwarding Attributes  . . . . . . . . . . .   8
     6.1.  Bypassing DF-Election Attribute . . . . . . . . . . . . .   9
     6.2.  Terminal Disposition Attribute  . . . . . . . . . . . . .   9
     6.3.  Broadcast, Unknown Unicast and Multicast  . . . . . . . .  10
   7.  Controlled Recovery Sequence  . . . . . . . . . . . . . . . .  10
   8.  Transport Underlay  . . . . . . . . . . . . . . . . . . . . .  11
   9.  BGP Extensions  . . . . . . . . . . . . . . . . . . . . . . .  11
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  11
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     12.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .  13
   Appendix B.  Contributors . . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   EVPN convergence and failure recovery methods from different types of
   network failures is described in [RFC7432] Section 17.  Similarly for
   EVPN-VPWS, [RFC8214] briefly evokes an egress link protection
   mechanism at the end of Section 5.

   The fundamentals of EVPN convergence rely on a mass-withdraw
   technique of the Ethernet A-D per ES route to unresolve all the
   associated forwarding paths ([RFC7432] Section 9.2.2 'Route
   Resolution').  The mass-withdraw grouping approach results in
   suitable EVPN convergence at lower scale, but is not sufficent to
   meet stricter sub-second requirements.  Other control-plane
   enhancements such as route-prioritisation
   ([I-D.ietf-bess-rfc7432bis]) help further but still provide no
   guarantees.

   EVPN convergence using only control-plane approaches is constrained
   by BGP route propagation delays, routes processing times in software
   and hardware programming.  These are additionally often performed




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   sequentially and linearly given the potential large scale of EVPN
   routes present in control plane.

   This document presents a mechanism for fast reroute to minimise
   packet loss in the case of a link failure using EVPN redirect labels
   (ERLs) with special forwarding attributes.  Multiple-failures where
   loops may occur are addressed, as are cascading failures.  A
   mechanism for distributing redirect labels (ERLs) alongside EVPN
   service labels (ESLs) is shown.

   The main objective is to achieve sub-second convergence in EVPN
   networks without relying on control plane actions.  The procedures in
   this document apply equally to EVPN services (EVPN [RFC7432], EVPN-
   VPWS [RFC8214] and EVPN-IRB [RFC9135]), and all Ethernet-Segment
   load-balancing modes.

2.  Specification of Requirements

   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].

3.  Terminology

   Some of the terminology in this document is borrowed from [RFC8679]
   for consistency across fast reroute frameworks.

   CE:  Customer Edge device, e.g., a host, router, or switch.

   PE:  Provider Edge device.

   Ethernet Segment (ES):  When a customer site (device or network) is
      connected to one or more PEs via a set of Ethernet links, then
      that set of links is referred to as an 'Ethernet segment'.

   Ethernet Segment Identifier (ESI):  A unique non-zero identifier that
      identifies an Ethernet segment is called an 'Ethernet Segment
      Identifier'.

   Egress link:  Specific Ethernet link connecting a given PE-CE, which
      forms part of an Ethernet Segment.

   Single-Active Redundancy Mode:  When only a single PE, among all the
      PEs attached to an Ethernet segment, is allowed to forward traffic
      to/from that Ethernet segment for a given VLAN, then the Ethernet
      segment is defined to be operating in Single-Active redundancy
      mode.




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   All-Active Redundancy Mode:  When all PEs attached to an Ethernet
      segment are allowed to forward known unicast traffic to/from that
      Ethernet segment for a given VLAN, then the Ethernet segment is
      defined to be operating in All-Active redundancy mode.

   DF-Election:  Designated Forwarder election, as in [RFC7432] and
      [RFC8584].

   DF:  Designated Forwarder.

   Backup-DF (BDF):  Backup-Designated Forwarder.

   Non-DF (NDF):  Non-Designated Forwarder.

   AC:  Attachment Circuit.

   ERL:  Special-use EVPN redirect label, described in this document.

   ESL:  EVPN service label, as in [RFC7432], [RFC8214] and [RFC9135].

4.  Requirements

   1.   EVPN multihoming is often described as 2 peering PEs.  The
        solution MUST be generic enough to apply multiple peering PE and
        no artificial limit imposed on the number of peering PEs.

   2.   The solution MUST apply to all EVPN load-balancing modes.

   3.   The solution MUST be robust enough to tolerate failures of the
        same ES at multiple PEs.  Simultaneous as well as cascading
        failures on the same ES must be addressed.

   4.   The solution MUST support EVPN [RFC7432], EVPN-VPWS [RFC8214]
        and EVPN-IRB [RFC9135] services.

   5.   The solution MUST meet stringent sub-second and often 50
        millisecond requirements for traffic loss of EVPN services.

   6.   The solution MUST allow redirected-traffic to bypass port
        blocking states resulting from DF-Election (BDF or NDF).

   7.   The solution MUST be scale-independant and agnostic of EVPN
        route types, scale or choice of underlay.

   8.   The solution MUST address egress link (PE-CE link) failures.

   9.   The solution MUST be loop-free, and once-redirected traffic MUST
        never be repeatedly redirected.



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   10.  The solution MUST not rely on pushing an additional label onto
        the label stack.

   11.  The solution SHOULD address Broadcast, unknown unicast and
        multicast (BUM) traffic.

5.  Solution

   Sub-second convergence in EVPN networks is achieved using a combined
   approach to minimising traffic loss:

   o  Local failure detection and restoration of traffic flows in
      minimal time using a pre-computed redirect path ;

   o  Restoration of optimal traffic paths, and reconvergence of EVPN
      control plane with EVPN mass withdraw.

   The solution presented in this document addresses the local failure
   detection and restoration, without impeding on or impacting existing
   EVPN control plane convergence mechanisms.

   Consider the following EVPN topology where PE1 and PE2 are
   multihoming PEs on a shared ES, ESI1.  EVPN (known unicast) or
   EVPN-VPWS traffic from CE1 to CE2 is sent to PE1 and PE2 using EVPN
   service labels ESL1 and/or ESL2 (depending on load-balancing mode of
   the ESI1 interfaces).

                                       +------+
                                       |  PE1 |
                                       |      |
                      +-------+        | ESL1---BDF--X
                      |       |--------|      |       \
                      |       |        | ERL1--------> \
           +-----+    |       |        +------+         \
           |     |    |IP/MPLS|                          \
    CE1 ---| PE3 |----|Core   |                     ESI1  === CE2
           |     |    |Network|                          /
           +-----+    |       |        +------+         /
                      |       |        | ERL2--------> /
                      |       |--------|      |       /
                      +-------+        | ESL2---DF----
                                       |      |
                                       |  PE2 |
                                       +------+

   EVPN Multihoming with service and redirect labels

                                 Figure 1



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   Alongside the service labels ESL1 and ESL2, two redirect labels ERL1
   and ERL2 are allocated with special forwarding attributes, as
   detailed in Section 6.  Fast-reroute and use of the ERLs is shown in
   Section 5.2

5.1.  Pre-selection of Backup Path

   EVPN DF-Election lends itself well to the selection of a pre-computed
   path amongst any given number of peering PEs by providing a
   DF-Elected and BDF-Elected node at the <EVI, ESI> granularity
   ([RFC8584] and [I-D.ietf-bess-rfc7432bis]).

   In All-active mode, all PEs in the Ethernet Segment are actively
   forwarding known unicast traffic to the CE.  In Single-active mode,
   only a single PE in the Ethernet Segment is actively forwarding known
   unicast traffic to the CE: the DF-Elected PE.  The BDF-Elected PE is
   next to be elected in the redundancy group and is already known.

   For consistency across PEs and load-balancing modes, the backup path
   selected should be in order of {DF, BDF, NDF1, NDF2, ...}. The DF-
   Elected PE selects the next-best BDF-Elected as backup and all BDF-
   and NDF-Elected nodes select the best DF-Elected for the protection
   of their egress links.

   o  PE1 (DF) -> ERL(PE2),

   o  PE2   (BDF) -> ERL(PE1),

   o  PE..n (NDF) -> ERL(PE1),

   The number of peering PEs is not limited by existing DF-Election
   algorithms.  A solution based on DF-Election supports subsequent
   redirection upon multiple cascading failures, once a new DF-Election
   has occurred.  Pre-selection of a backup path is supported by all
   current DF-Election algorithms, and more generally by all algorithms
   supporting BDF-Election, as recommended in
   ([I-D.ietf-bess-rfc7432bis]).

5.2.  Failure Detection and Traffic Restoration












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                                           +------+
                                           |  PE1 |
                                           |      |
                          +-------+        | ESL1----BDF-X
                          |       |--------|      |       \
                          |       |        | ERL1 * * * * *
               +-----+    |       |        +----*-+         *
               |     |    |IP/MPLS|              *           *
        CE1 ---| PE3 |----|Core   |               *     ESI1  *** CE2
               |     |    |Network|                *         /
               +-----+    |       |        +------+ *       /
                          |       |        | ERL2----*---> /
                          |       |--------|      |   *   /
                          +-------+        | ESL2-----XX--
                                           |      |
                                           |  PE2 |
                                           +------+

   EVPN Multihoming failure scenario

                                 Figure 2

   The procedures for forwarding known unicast packets received from a
   remote PE on the local redirect label largely follow [RFC7432]
   Section 13.2.2.

   Consider the EVPN multihoming topology in Figure 1, and a traffic
   flow from CE1 to CE2 which is currently using EVPN service label ESL2
   and forwarded through the core arriving at PE2.  When the local AC
   representing the <EVI,ESI> pair is protected using the fast-reroute
   solution, the pre-computed backup path's redirect label (i.e.  ERL1
   from BDF-Elected PE1) is installed against the AC.

   Under normal conditions, PE2 disposition using ESL2 will result in
   forwarding the packet to the CE by selecting the local AC associated
   with the EVPN service label (EVPN-VPWS) or MAC address lookup (EVPN).
   When this local AC is in failed state, the fast-reroute solution at
   PE2 will begin rerouting packets using the BDF-Elected peer's nexthop
   and ERL1.  ERL1 is chosen for redirection and not ESL1 for the
   redirected traffic to prevent loops and overcome DF-Election timing
   as described in Sections 6.2 and 6.1 respectively.

5.2.1.  Simultaneous Failures in ES

   In EVPN multihoming where the CE connects to peering PEs through link
   aggregation (LAG), a single LAG failure at the CE may manifest as
   multiple ES failures at all peering PEs simultaneously.




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   As all peering PEs would enable simultaneously the fast-reroute
   mechanism, redirection would be permanent causing a traffic storm or
   until TTL expires.

   Once-redirected traffic may not be redirected again, according to the
   terminal nature of ERLs described in Section 6.2

5.2.2.  Successive and Cascading Failures in ES

   Trying to support cascading failures by redirecting once-redirected
   traffic is substantially equivalent to simultaneous failures above.

   Once-redirected traffic may not be redirected again, according to the
   terminal nature of ERLs described in Section 6.2 and loss is to be
   expected until EVPN control plane reconverges for double-failure
   scenarios.

   In a scenario with 3 peering PEs (PE1-DF, PE2-BDF, PE3-NDF) where PE1
   fails, followed by a PE2 failure before control-plane reconvergence,
   there is no reroute of traffic towards PE3 because the reroute-label
   is terminal.

   In such rapid-succession failures, it is expected that control plane
   must first correct for the initial failure and DF-Elect PE2 as new-DF
   and PE3 as the new-BDF.  PE2 to PE3 redirection would then begin,
   unless control-plane is rapid enough to correct directly, and elect
   PE3 new-DF.

6.  Redirect Labels: Forwarding Attributes

   The EVPN redirect labels MUST be downstream assigned, and it is
   directly associated with the <EVI,ESI> AC being egress protected.
   The special forwarding characteristics and use of an EVPN redirect
   label (ERL) described below, are a matter of local significance only
   to the advertising PE (which is also the disposition PE).

   Special-attributes to the ERLs do not affect any other PEs or transit
   P nodes.  There are no extra labels appended to the label stack in
   the IP/MPLS network and the ERL appears to label-switching transit
   nodes as would any other EVPN service label.

   o  Traffic redirection and use of reroute labels may create routing
      loops upon multiple failures.  Such loops are detrimental to the
      network and may cause congestion between protected PEs.

   o  Local restoration and redirection is meant to occur much faster
      than control-plane operations, meaning redirected packets may




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      arrive at the BDF PE long before a DF-Election operation unblocks
      the egress link.

   Two special forwarding characteristics of EVPN redirect labels are
   described below to mitigate these issues.

6.1.  Bypassing DF-Election Attribute

   Local detection and restoration at PE2 will begin rapidly redirecting
   traffic onto the backup path.
   Redirected packets will arrive at the Backup-DF port much faster than
   control plane DF-Election at the Backup-DF peer is capable of
   unblocking its local egress link for the shared ES (ESI1).  All
   redirected traffic would drop at Backup-DF and no net reduction in
   traffic loss achieved.

   Traffic restoration remains dependant upon ES route or Ethernet A-D
   per ES routes withdrawal for a DF-Election operation and for PE1 to
   assume the traffic forwarding role.  This is especially important in
   single-active load-balancing mode where known unicast traffic is
   blocked.

   To mitigate this, the redirect labels allocated must carry a special
   attribute in the local forwarding and decapsulation chain:
   for traffic received on the ERL when the AC is up, an override to the
   DF-Election is applied and traffic from the ERL will bypass the local
   Backup-DF blocking state.  Once EVPN control plane reconverges,
   traffic from the ERL will cease and the optimal forwarding path based
   on ESLs will resume.

   The EVPN redirect label MUST carry a context locally, such that from
   disposition to egress redirected packets are allowed to bypass the
   BDF blocking state that would otherwise drop.  Similarly, this may
   open the gate to the traffic in the reverse direction.

6.2.  Terminal Disposition Attribute

   The reroute scheme is susceptible to loops and persistant redirects
   between peering PEs which have setup FRR redirection.  Consider the
   scenario where both CE-facing interfaces fail simultaneously, fast
   reroute will be activated at both PE1 and PE2 effectively bouncing a
   redirected packet between the two PEs indefinitely (or until the TTL
   expires) causing a traffic storm.

   To prevent this, a distinction is made between 'regular' EVPN service
   labels for disposition (i.e. known unicast EVI label or EVPN-VPWS
   label) and reroute labels with terminal disposition.




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   At the redirecting PE2, we consider the case of ESL2 vs. ERL2 , where
   both are locally allocated and provided in EVPN routes (downstream
   allocation) to BGP peers:

   1.  EVPN Service label, ESL2:

       *  Regular MAC-lookup or traffic forwarding occurs towards the
          access AC.

       *  If the AC is up, traffic will exit the interface, subject to
          local blocking state on the AC from DF-Election.

       *  If the AC is down and fast-reroute procedures are enabled,
          traffic may be re-encapsulated using BDF peer's redirect label
          ERL1 (if received).

   2.  EVPN Reroute label, ERL2:

       *  Regular MAC-lookup or traffic forwarding occurs towards the
          access AC.

       *  If the AC is up, traffic will apply an override to DF-Election
          and bypass the local blocking state on the AC.

       *  If the AC is down, traffic is dropped.  No reroute must occur
          of once-rerouted traffic.  Redirecting towards peer's redirect
          label ERL1 is explicitly prevented.

   The ERL acts like a local cross-connect by providing a direct channel
   from disposition to the AC.  ERLs are terminal-disposition and
   prevents once-redirected packets from being redirected again.
   With this forwarding attribute on ERLs, known only locally to the
   downstream-allocating PE, redirection is achieved without growing the
   label stack with another special purpose label.

6.3.  Broadcast, Unknown Unicast and Multicast

   BUM traffic is treated using EVPN defaults.  There is no further
   extension to exiting procedure as of now, this work is left for
   future study.

7.  Controlled Recovery Sequence

   Fast reroute mechanisms such as the one described in this document
   generally provide a way to preserve traffic flows at failure time.
   Use of fast reroute in EVPN, however, permits setting up a controlled
   recovery sequence to shorten the period of loss between an interface




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   coming up and the EVPN DF-Election procedures and default timers for
   peer discovery.

   The benefit of a controlled recovery sequence is amplified when used
   in conjunction with [I-D.ietf-bess-evpn-fast-df-recovery]
   (synchronised DF-Election)>

8.  Transport Underlay

   The solution is agnostic to transport underlays, for instance similar
   behaviour is carried forward for VXLAN and SRv6

9.  BGP Extensions

   There are no new BGP extensions required to advertise the redirect
   label(s) used for EVPN egress link protection.
   The ESI Label Extended Community defined in [RFC7432] Section 7.5 may
   be advertised along with Ethernet A-D routes:

   o  When advertised with an Ethernet A-D per ES route, it enables
      split-horizon procedures for multihomed sites as described in
      [RFC7432] Section 8.3 ;

   o  When advertised with an Ethernet A-D per EVI route, it enables
      link protection and fast-reroute procedures for multihomed sites
      as described in this document.  The label value represents the
      per-<EVI,ESI> EVPN redirect label (ERL).  The Flags field SHOULD
      NOT be set and MUST be ignored.

   Remote PEs SHALL NOT use the ERLs as a substitution for ESLs in route
   resolution, and is especially not to be confused with the aliasing
   and backup path ESL as described and used in [RFC7432] Section 8.4.

10.  Security Considerations

   The mechanisms in this document use the EVPN control plane as defined
   in [RFC7432] and [RFC8214], and the security considerations described
   therein are equally applicable.  Reroute labels redistributed in EVPN
   control plane are meant for consumption by the peering PE in a same
   ES.  It is, however, visible in the EVPN control plane to remote
   peers.  Care shall be taken when installing reroute labels, since
   their use may result in bypassing DF-Election procedures and lead to
   duplicate traffic at CEs if incorrectly installed.








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11.  IANA Considerations

   This document makes no specific requests to IANA.

12.  References

12.1.  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>.

   [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>.

   [RFC8214]  Boutros, S., Sajassi, A., Salam, S., Drake, J., and J.
              Rabadan, "Virtual Private Wire Service Support in Ethernet
              VPN", RFC 8214, DOI 10.17487/RFC8214, August 2017,
              <https://www.rfc-editor.org/info/rfc8214>.

   [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>.

12.2.  Informative References

   [I-D.ietf-bess-evpn-fast-df-recovery]
              Brissette, P., Sajassi, A., Burdet, L., Drake, J., and J.
              Rabadan, "Fast Recovery for EVPN DF Election", draft-ietf-
              bess-evpn-fast-df-recovery-02 (work in progress), July
              2021.

   [I-D.ietf-bess-rfc7432bis]
              Sajassi, A., Burdet, L., Drake, J., and J. Rabadan, "BGP
              MPLS-Based Ethernet VPN", draft-ietf-bess-rfc7432bis-01
              (work in progress), July 2021.

   [RFC8679]  Shen, Y., Jeganathan, M., Decraene, B., Gredler, H.,
              Michel, C., and H. Chen, "MPLS Egress Protection
              Framework", RFC 8679, DOI 10.17487/RFC8679, December 2019,
              <https://www.rfc-editor.org/info/rfc8679>.





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   [RFC9135]  Sajassi, A., Salam, S., Thoria, S., Drake, J., and J.
              Rabadan, "Integrated Routing and Bridging in Ethernet VPN
              (EVPN)", RFC 9135, DOI 10.17487/RFC9135, October 2021,
              <https://www.rfc-editor.org/info/rfc9135>.

Appendix A.  Acknowledgments

Appendix B.  Contributors

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

Authors' Addresses

   Luc Andre Burdet (editor)
   Cisco

   Email: lburdet@cisco.com


   Patrice Brissette
   Cisco

   Email: pbrisset@cisco.com


   Takuya
   KDDI Corporation

   Email: ta-miyasaka@kddi.com





















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