A Simplified Scalable E-Line Service Model with Segment Routing Underlay

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.¶

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 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.¶

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.¶

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

5. Data Plane Behavior

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

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

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

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

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

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.¶

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