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Versions: 00 01                                                         
BESS Workgroup                                           S. Boutros, Ed.
Internet-Draft                                         S. Sivabalan, Ed.
Intended status: Standards Track                       Ciena Corporation
Expires: 14 April 2022                                         J. Uttaro
                                                                    AT&T
                                                                D. Voyer
                                                             Bell Canada
                                                                  B. Wen
                                                                 Comcast
                                                                L. Jalil
                                                                 Verizon
                                                         11 October 2021


A Simplified Scalable E-Line Service Model with Segment Routing Underlay
              draft-boutros-bess-eline-services-over-sr-01

Abstract

   This document proposes a new approach for realizing Ethernet line
   (E-Line) services over Segment Routing (SR) networks.  This approach
   significantly improves scalability and convergence of control plane,
   and simplifies network operation.  Furthermore, it naturally yields
   All-Active multi-homing support for E-Line services without relying
   on any overlay techniques.

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
   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 14 April 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   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.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Abbreviations . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Control Plane Behavior  . . . . . . . . . . . . . . . . . . .   5
     4.1.  Service discovery . . . . . . . . . . . . . . . . . . . .   5
     4.2.  All-Active Service Redundancy . . . . . . . . . . . . . .   5
     4.3.  E-Line Attributes . . . . . . . . . . . . . . . . . . . .   6
     4.4.  Service withdrawal  . . . . . . . . . . . . . . . . . . .   6
   5.  Data Plane Behavior . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Data Packet Format  . . . . . . . . . . . . . . . . . . .   6
     5.2.  Aliasing  . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.3.  Single-Homed CE . . . . . . . . . . . . . . . . . . . . .   7
     5.4.  Multi-Homed CE  . . . . . . . . . . . . . . . . . . . . .   8
   6.  Benefits of E-Line services over SR . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   Historically, E-Line services are realized as Virtual Private Wire
   Services (VPWS) using point-to-point (P2P) Pseudo-Wires (PWs).  A PW
   identifies both the service type and the service termination nodes in
   both control and data planes.  A PW can be dynamically established
   via LDP or BGP.  Ethernet VPN (EVPN) signaling mechanisms via BGP can
   be used to provide VPWS with Single-Active and All-Active multihoming
   redundancy as well as Inter-Autonomous System (AS) support.

   This document proposes a new approach for supporting E-Line services
   over Segment Routing (SR) networks.  It reduces BGP signaling
   overhead by at least by two orders of magnitudes compared to the
   current EVPN-VPWS signaling mechanisms and hence yields better
   control plane scalabilty.  Furthermore, it eliminates the need to



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   associate Route Distinguisher (RD) and Route Target (RT) with EVPN
   routes, and hence leads to simpler network operations.  The proposed
   approach enables the benefit of All-Active redundancy and aliasing
   for the Multi-Home (MH) sites.  This proposal makes use of the
   properties of SR anycast SID to achieve redundancy and fast
   convergence at the transport level.  Anycast SIDs provide the ability
   to discover nodes supporting multihoming and perform Designated
   Forwarder (DF) election.  As such, the need for any overlay mechanism
   to achieve redundancy and fast convergence is eliminated.  Also, the
   proposed approach supports auto-discovery and single-sided service
   provisioning.

   In the proposed approach, an E-Line service instance is represented
   by a Segment ID (SID) which is referred to "Service SID" in the rest
   of the document.  Such a SID can be:

   *  an MPLS label for SR-MPLS.

   *  a uSID (micro SID) for SRv6 representing network function
      associated with an E-Line service instance.  A new SRv6 network
      programming function will be specified in the next version of this
      document.

   In the present form, classical VPWS service is incapable of
   supporting All-Active multihoming.  However, thanks to SR anycast SID
   capability, the proposed approach natively provides such redundancy
   at transport layer.

   A Service SID is unique within a service domain.  A node can
   advertise service SID(s) of the E-Line instance(s) that it hosts via
   BGP for auto-discovery purpose.  In the case of SR-MPLS, a service
   SID can be carried as a range of absolute MPLS label values or an
   index into an Segment Routing Global Block (SRGB), and in the case of
   SRv6, a service SID can be carried as uSID in BGP update.

















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   A node advertising E-Line service routes packs information about as
   many service instances as possible at the time of advertisement in a
   single route.  The objective is to reduce the volume of signaling
   messages advertised as well as processed in control plane.  E-Line
   service SIDs are represented as an array.  A service route contains
   SID value at the start of the array and a bitmask where each bit
   represents an index in the array.  The SID value for an E-Line
   service instance can be derived from the start and the index of the
   array.  When advertising routes to E-Line services, a node sends the
   initial value of the SID array and sets the bitmask to indicate all
   E-Line services instances that it hosts along with its node SID which
   may be anycast SID for MH case.  For FXC, a route containing the
   start normalized VID and the bitmask of VIDs in association with the
   E-Line service SID is advertised.  The necessary BGP extension will
   be described in a future version of this document.

   Each node attached to the MH site advertises the same anycast SID to
   allow other nodes to discover the redundancy group membership (auto-
   discovery).

   The proposed solution can also be applicable to the existing EVPN
   control plane without compromising its benefits such as All-Active
   multihoming on access, aliasing in the core, auto-provisioning and
   auto-discovery, etc.  With this approach, the need for advertising of
   EVPN route types 1 and 4 as well Split-Horizon (SH) label is
   eliminated which simplifies EVPN-VPWS operations.

   In the following sections, we will describe the functionalities of
   the proposed approach in detail.

2.  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].

3.  Abbreviations

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

   E-Line: Ethernet virtual private line.

   EVPN: Ethernet VPN.

   MH: Multi-Home.

   OAM: Operations, Administration and Maintenance.




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   PE: Provide Edge Node.

   SID: Segment Identifier.

   SR: Segment Routing.

   VPWS: Virtual Private Wire Service.

4.  Control Plane Behavior



                                        ____ CE3
                                       /               ____CE1
                            --------  PE3 ---------  /
                           /                       PE1
                          /                         | \
                         PE5                        |  \
                        /|                          |   \ CE2
                       / | Service Provider Network |   /
                   CE5   |                          |  /
                       \ |                          | /
                        \|                         PE2
                         PE6                       /
                         /  --------  PE4  --------
                 CE6___ /     CE4_____/


     Figure 1: A reference network used for the examples below

4.1.  Service discovery

   A node (e.g., PE5 in Figure 1) can discover E-Line instances as well
   as the associated service SIDs on other nodes (e.g., PE1, PE2, etc in
   Figure 1) via configuration or auto-discovery.  With the latter, the
   service SIDs may be advertised using BGP.

4.2.  All-Active Service Redundancy

   An anycast SID per Ethernet Segment (ES) will be configured on all
   nodes attached to a Multi-Home (MH) site.  For example, in Figure 1,
   PE1 and PE2 are configured with an anycast SID representing the
   multi-homed site CE2.  The ES anycast SIDs will be advertised in BGP
   by nodes (e.g., PE1 and PE2) connected to the MH site.







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   Upon receiving advertisement containing ES anycast SID, a node (e.g.,
   PE5 in Figure 1) learns the nodes (e.g., PE1 and PE2) hosting MH
   sites, and performs DF election.  Aliasing is natively achieved at
   transport layer.

4.3.  E-Line Attributes

   Layer 2 extended community can be advertised with E-Line service
   routes.  Such a route includes the start of service SID and the
   bitmap of all other Services enabled on the advertising node.  This
   will be elaborated more in the next revision of this document.  As
   mentioned earlier, the goal is to pack information about as many
   E-Line services hosted on the advertising node as possible to reduce
   the overall amount of signaling messages.

4.4.  Service withdrawal

   Node failure can be detected due via IGP convergence.  For faster
   detection of node failure, mechanism like BFD can be deployed.  The
   proposed approach does not require additional service withdrawal
   mechanism.

   On PE-CE link failure, the PE node withdraws the route to the
   corresponding ES in BGP in order to stop receiving traffic to that
   ES.

   With MH case with anycast SID, upon detecting a failure on PE-CE
   link, a PE node may forward incoming traffic to the impacted ES(s) to
   another PE node which is part of the anycast group until it withdraws
   routes to the impacted ES(s) for faster convergence.  For example, in
   Figure 1, assuming PE5 and PE6 are part of an anycast group, upon
   link failure between PE5 and CE5, PE5 can forward the received
   packets from the core to PE6 until it withdraws the anycast SID
   associated with the MH site.

5.  Data Plane Behavior

5.1.  Data Packet Format

   The proposed method requires unicast data packet be formed as shown
   in Figure 2.










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                      +-------------------------------+
                      | SID(s) to reach destination   |
                      +-------------------------------+
                      |          Service SID          |
                      +-------------------------------+
                      |        Layer-2 Payload        |
                      +-------------------------------+

        Figure 2: Data packet format for sending traffic to a destination

   *  SID(s) to reach destination: depends on the intent of the underlay
      transport:

      -  IGP shortest path: node SID of the destination.  The
         destination can belong to an anycast group.

      -  IGP path with intent: Flex-Algo SID if the destination can be
         reached using the Flex-Algo SID for a specific intent (e.g.,
         low latency path).  The destination can belong to an anycast
         group.

      -  SR policy: a SID-list for the SR policy that can be used to
         reach the destination.

   *  Service SID: The SID that uniquely identifies a E-Line service
      instance in an SR domain.

5.2.  Aliasing

   Packets destined to a MH CE is distributed to the PE nodes attached
   to the CE for load-balancing (UCMP/ECMP) purpose.  This is achieved
   implicitly due to the use of anycast SIDs for both ES as well as PE
   attached to the ES.  In our example, traffic destined to CE5 is
   distributed via PE5 and PE6.

5.3.  Single-Homed CE

   Referring to Figure 1, PE3 and PE4 provide an E-Line service to
   connect CE3 to CE4.  If PE3 wants to forward a packet received from
   CE3 to CE4, it formulates the packet as shown in Figure 3.











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                      +-----------------------------+
                      |Transport SID(s) to reach PE4|
                      +-----------------------------+
                      |       Service SID           |
                      +-----------------------------+
                      |      Layer-2 Packet         |
                      +-----------------------------+

      Figure 3: Data packet format for forwarding traffic from PE3 to CE4

5.4.  Multi-Homed CE

   Referring to Figure 1, PE5 and PE6 and PE1 and PE2 provide to provide
   E-Line services to connect CE2 and CE5.  PE5 and PE6 share an anycast
   SID, and PE1 and PE2 share another anycast SID.

   Packets sent from CE2 to CE5 are load-balanced (ECMP/UCMP) between
   PE5 and PE6 assuming that PE1 and PE2 learned the corresponding
   E-Line Service SID from PE5 and PE6.

   The following diagram shows SID stack for a L2 packet sent from CE2
   to CE5.


                      +--------------------------------+
                      |Anycast Node SID for PE5 and PE6|
                      +--------------------------------+
                      |         Service SID            |
                      +--------------------------------+
                      |         Layer-2 Packet         |
                      +--------------------------------+

 Figure 4: Data packet format for forwarding traffic from PE1 or PE2 to CE5

   In case of FXC the signaled normalized VID will be encoded in the
   Layer 2 packet.

6.  Benefits of E-Line services over SR

   The proposed approach eliminates the need for establishing and
   maintaining PWs as with legacy VPWS technology.  This yields
   significant reduction in control plane overhead.  The proposed
   approach provides the benefits as such fast convergence.  Finally,
   using anycast SID, the proposed approach provides All-Active
   multihoming as well as aliasing.






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7.  Security Considerations

   The mechanisms in this document use Segment Routing control plane as
   defined in Security considerations described in Segment Routing
   control plane are equally applicable.

8.  IANA Considerations

   TBD.

9.  Acknowledgements

10.  References

10.1.  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>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8660]  Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing with the MPLS Data Plane", RFC 8660,
              DOI 10.17487/RFC8660, December 2019,
              <https://www.rfc-editor.org/info/rfc8660>.

   [RFC8754]  Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
              <https://www.rfc-editor.org/info/rfc8754>.

10.2.  Informative References

   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", Work in
              Progress, Internet-Draft, draft-ietf-spring-segment-
              routing-policy-13, 28 May 2021,
              <https://www.ietf.org/archive/id/draft-ietf-spring-
              segment-routing-policy-13.txt>.





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   [RFC4664]  Andersson, L., Ed. and E. Rosen, Ed., "Framework for Layer
              2 Virtual Private Networks (L2VPNs)", RFC 4664,
              DOI 10.17487/RFC4664, September 2006,
              <https://www.rfc-editor.org/info/rfc4664>.

   [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>.

Authors' Addresses

   Sami Boutros (editor)
   Ciena Corporation
   United States of America

   Email: sboutros@ciena.com


   Siva Sivabalan (editor)
   Ciena Corporation
   Canada

   Email: ssivabal@ciena.com


   James Uttaro
   AT&T
   United States of America

   Email: ju1738@att.com


   Daniel Voyer
   Bell Canada
   Canada

   Email: daniel.voyer@bell.ca


   Bin Wen
   Comcast
   United States of America

   Email: bin_wen@cable.comcast.com






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   Luay Jalil
   Verizon
   United States of America

   Email: luay.jalil@verizon.com














































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