E-Tree L2VPN: Requirements for Metro Ethernet Forum (MEF) Ethernet-Tree (E-Tree) Support in Layer 2 Virtual Private Network (L2VPN)
Requirements for tree-shaped ethernet networks in virtual private networks.
Key, R., Ed., DeLord, S., Jounay, F., Huang, L., Liu, Z., and M. Paul, "Requirements for Metro Ethernet Forum (MEF) Ethernet-Tree (E-Tree) Support in Layer 2 Virtual Private Network (L2VPN)", RFC 7152, DOI 10.17487/RFC7152, March 2014, <https://www.rfc-editor.org/info/rfc7152>.
@misc{rfc7152,
series = {Request for Comments},
number = 7152,
howpublished = {RFC 7152},
publisher = {RFC Editor},
doi = {10.17487/RFC7152},
url = {https://www.rfc-editor.org/info/rfc7152},
author = {R. Key and S. DeLord and F. Jounay and L. Huang and Z. Liu and M. Paul},
title = {{Requirements for Metro Ethernet Forum (MEF) Ethernet-Tree (E-Tree) Support in Layer 2 Virtual Private Network (L2VPN)}},
pagetotal = 12,
year = 2014,
month = mar,
} RFC text
Contents
Internet Engineering Task Force (IETF) R. Key, Ed.
Request for Comments: 7152 S. Delord
Category: Informational Telstra
ISSN: 2070-1721 F. Jounay
Orange CH
L. Huang
China Mobile
Z. Liu
China Telecom
M. Paul
Deutsche Telekom
March 2014
Requirements for Metro Ethernet Forum (MEF) Ethernet-Tree (E-Tree)
Support in Layer 2 Virtual Private Network (L2VPN)
Abstract
This document provides functional requirements for the support of
Metro Ethernet Forum (MEF) Ethernet Tree (E-Tree) in multipoint Layer
2 Virtual Private Network solutions (referred to as simply "L2VPN").
It is intended that potential solutions will use these requirements
as guidelines.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7152.
Copyright Notice
Copyright (c) 2014 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
(http://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 ....................................................3
1.1. Conventions Used in This Document ..........................3
2. IETF Multipoint Ethernet L2VPN Services .........................3
2.1. VPLS .......................................................3
2.2. Ethernet Virtual Private Network (E-VPN) ...................3
3. MEF Multipoint Ethernet Services ................................4
3.1. Similarities between E-LAN and E-Tree ......................4
3.2. Differences between E-LAN and E-Tree .......................4
3.3. E-Tree Use Cases ...........................................5
3.4. Generic E-Tree Service .....................................6
4. Problem Statement ...............................................6
4.1. Motivation .................................................6
4.2. Leaf-to-Leaf Communication Restriction .....................6
5. Requirements ....................................................7
5.1. Functional Requirements ....................................7
5.2. Applicability ..............................................8
5.3. Backward Compatibility .....................................8
5.4. External Network Network Interface (ENNI) ..................8
6. Security Considerations .........................................8
7. Contributors ....................................................8
8. Acknowledgements ................................................9
9. References ......................................................9
9.1. Normative References .......................................9
9.2. Informative References ....................................10
Appendix A. Frequently Asked Question .............................11
A.1. Are E-Tree Requirements Addressed in the Virtual
Private Multicast Service (VPMS) Requirements? ...............11
1. Introduction
This document provides functional requirements for the support of
Metro Ethernet Forum (MEF) Ethernet Tree (E-Tree) in multipoint Layer
2 Virtual Private Network solutions (referred to as simply "L2VPN").
It is intended that potential solutions will use these requirements
as guidelines.
A considerable number of service providers have adopted Virtual
Private LAN Service (VPLS) to provide MEF Ethernet LAN (E-LAN)
services to customers. Service providers currently need a simple and
effective solution to emulate E-Tree services in addition to E-LAN
services on their MPLS networks.
Service providers also expect E-Tree support in any newly developed
L2VPN technologies.
1.1. Conventions Used in This Document
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].
2. IETF Multipoint Ethernet L2VPN Services
2.1. VPLS
VPLS [RFC4761] [RFC4762] is an L2VPN service that provides
multipoint-to-multipoint connectivity for Ethernet across an IP or
MPLS-enabled IP Packet Switched Network (IP/MPLS PSN). VPLS emulates
the Ethernet VLAN functionality of traditional Ethernet networks.
Thus, in VPLS, the customer Ethernet frame is transported over the
IP/MPLS PSN from the ingress Provider Edge (PE) to the egress PE
where the destination is connected based on the Ethernet frame
destination Media Access Control (MAC) address in the context of the
virtual switching instance (VSI) to which it belongs.
2.2. Ethernet Virtual Private Network (E-VPN)
E-VPN is an enhanced L2 service that emulates an Ethernet VLAN across
an IP/MPLS PSN, primarily targeted to support large scale L2VPNs with
resiliency requirements not satisfied by other L2VPN solutions.
E-VPN is currently under development. Please refer to [EVPN].
3. MEF Multipoint Ethernet Services
MEF has defined two multipoint Ethernet service types:
- E-LAN (Ethernet LAN), multipoint-to-multipoint service
- E-Tree (Ethernet Tree), rooted-multipoint service
For the full specifications, please refer to [MEF6.1] and [MEF10.2].
3.1. Similarities between E-LAN and E-Tree
The following are the similarities between E-LAN and E-Tree services.
- Data frame is an Ethernet frame.
- Data forwarding is MAC-based forwarding.
- A generic E-LAN/E-Tree service is always bidirectional in the
sense that ingress frames can originate at any endpoint in the
service.
3.2. Differences between E-LAN and E-Tree
Within the context of a multipoint Ethernet service, each endpoint is
designated as either a Root or a Leaf. A Root can communicate with
all other endpoints in the same multipoint Ethernet service; however,
a Leaf can only communicate with Roots but not Leaves.
The only differences between E-LAN and E-Tree are:
- E-LAN has Root endpoints only, which implies there is no
communication restriction between endpoints.
- E-Tree has both Root and Leaf endpoints, which implies there is
a need to enforce communication restriction between Leaf
endpoints.
3.3. E-Tree Use Cases
Table 1 presents some major E-Tree use cases.
+---------------------------+--------------+------------+
| Use Case | Root | Leaf |
+---+---------------------------+--------------+------------+
| 1 | Hub & Spoke VPN | Hub Site | Spoke Site |
+---+---------------------------+--------------+------------+
| 2 | Wholesale Access | Customer's | Customer's |
| | | Interconnect | Subscriber |
+---+---------------------------+--------------+------------+
| 3 | Mobile Backhaul | Radio Access | RAN Base |
| | | Network (RAN)| Station |
| | | Network | |
| | | Controller | |
+---+---------------------------+--------------+------------+
| 4 | IEEE 1588 PTPv2 | Precision | PTP Client |
| | Clock Synchronisation | Time Protocol| |
| | [IEEE1588] | (PTP) Server | |
+---+---------------------------+--------------+------------+
| 5 | Internet Access | Broadband | Subscriber |
| | [TR-101] | Network | |
| | | Gateway | |
+---+---------------------------+--------------+------------+
| 6 | Broadcast Video | Video Source | Subscriber |
| | (unidirectional only) | | |
+---+---------------------------+--------------+------------+
| 7 | Broadcast/Multicast Video | Video Source | Subscriber |
| | plus Control Channel | | |
+---+---------------------------+--------------+------------+
| 8 | Device Management | Management | Managed |
| | | System | Device |
+---+---------------------------+--------------+------------+
Table 1: E-Tree Use Cases
Common to all use cases, direct L2 Leaf-to-Leaf communication is not
required or must be inhibited.
If direct L2 Leaf-to-Leaf communication is not allowed due to a
security concern, then E-Tree should be used to prohibit
communication between Leaf endpoints. Otherwise, E-LAN is also a
feasible option.
3.4. Generic E-Tree Service
A generic E-Tree service supports multiple Root endpoints. The need
for multiple Root endpoints is usually driven by a redundancy
requirement. Whether a particular E-Tree service needs to support
single or multiple Roots depends on the target application.
A generic E-Tree service supports all the following traffic flows:
- Ethernet Unicast from Root to Leaf
- Ethernet Unicast from Leaf to Root
- Ethernet Unicast from Root to Root
- Ethernet Broadcast/Multicast from Root to other Roots and Leaves
- Ethernet Broadcast/Multicast from Leaf to Roots
A particular E-Tree service may need to support all the above or only
a subset depending on the target application.
4. Problem Statement
4.1. Motivation
L2VPN can be used to emulate MEF E-LAN service over an IP/MPLS PSN.
Service providers also require E-Tree support in L2VPN.
4.2. Leaf-to-Leaf Communication Restriction
In this section, VPLS is used to illustrate the problem; however, the
same principle applies to other L2VPN technologies.
VPLS treats all attachment circuits (ACs) equally (essentially as
Roots, although they not classified into Root or Leaf) and provides
any-to-any connectivity among all ACs. VPLS does not include any
mechanism for communication restriction between specific ACs.
Therefore, it is insufficient for emulating generic E-Tree service
over an IP/MPLS PSN.
As an example of the problems not addressed in VPLS solutions,
consider the scenario in Figure 1 where there are two PEs, each with
a Root AC and a Leaf AC and where VPLS is used to emulate an E-Tree
service interconnecting these ACs over an IP/MPLS PSN.
<------------E-Tree------------>
+---------+ +---------+
| PE1 | | PE2 |
+---+ | +---+ | | +---+ | +---+
|CE1+-----AC1----+--+ | | | | +--+----AC3-----+CE3|
+---+ (Root AC) | | V | | Ethernet | | V | | (Root AC) +---+
| | S +--+-----PW-----+--+ S | |
+---+ | | I | | | | I | | +---+
|CE2+-----AC2----+--+ | | | | +--+----AC4-----+CE4|
+---+ (Leaf AC) | +---+ | | +---+ | (Leaf AC) +---+
+---------+ +---------+
Figure 1: Problem Scenario for Leaf-to-Leaf Communication Restriction
When PE2 receives a frame from PE1 via the Ethernet pseudowire (PW),
- PE2 does not know which AC on PE1 is the ingress AC
- PE2 does not know whether or not the ingress AC is a Leaf AC
- PE2 does not have sufficient information to enforce the Leaf-to-
Leaf communication restriction
Examples where the problems arise:
- Customer Edge 2 (CE2) sends a Broadcast/Multicast Ethernet frame
to PE1 via AC2
- CE2 sends a Unicast Ethernet frame to PE1 via AC2 with a
destination MAC address corresponding to CE4's MAC address
Note: Figure 1 is a hypothetical case solely used for explaining the
problem; it is not meant to represent a typical E-Tree service.
There are some possible ways to get around this problem that do not
require extensions to existing VPLS solutions, but they all come with
significant design complexity or deployment constraints.
5. Requirements
5.1. Functional Requirements
The following are the E-Tree L2VPN functional requirements:
(1) A solution MUST prohibit communication between any two Leaf ACs
in an L2VPN instance.
(2) A solution MUST allow multiple Root ACs in an L2VPN instance.
(3) A solution MUST allow a Root AC and Leaf AC of an L2VPN instance
to coexist on any PE.
5.2. Applicability
A solution MUST identify the L2VPN technology ([RFC4761], [RFC4762],
[EVPN]) to which the solution is applicable.
5.3. Backward Compatibility
A solution SHOULD minimise the impact on VPLS and E-VPN L2VPN
solutions, especially for the MEF E-LAN services already in
operation.
A solution SHOULD be backward compatible with the VPLS and E-VPN
L2VPN solutions. It SHOULD allow a case where a common L2VPN
instance is composed of both PEs supporting the solution and PEs not
supporting it, and the Leaf-to-Leaf communication restriction is
enforced within the scope of the compliant PEs.
5.4. External Network Network Interface (ENNI)
A solution SHOULD support Root Operator Virtual Connection (OVC) End
Point, Leaf OVC End Point and Trunk OVC End Point specified in
[MEF26.1].
6. Security Considerations
This document introduces a requirement of prohibiting communication
between any two Leaf ACs in an L2VPN instance. In some use cases,
such a requirement is imposed because of security reasons. Other
than that, there are no additional security considerations beyond
those already described in [RFC4761], [RFC4762], and [EVPN].
7. Contributors
Ruediger Kunze
Deutsche Telekom
Winterfeldtstr. 21-27
10781 Berlin, Germany
EMail: ruediger.kunze@telekom.de
Nick Del Regno
Verizon
400 International Pkwy
Richardson, TX 75081, USA
EMail: nick.delregno@verizon.com
Josh Rogers
Time Warner Cable
11921 N Mo Pac Expy
Suite 210B
Austin, TX 78759, USA
EMail: josh.rogers@twcable.com
8. Acknowledgements
The authors would like to thank Lizhong Jin, Lucy Yong, Yuji Kamite,
and Wim Henderickx for their valuable input and support.
9. References
9.1. Normative References
[MEF6.1] Metro Ethernet Forum, "Ethernet Services Definitions -
Phase 2", Technical Specification MEF 6.1, April 2008,
<http://metroethernetforum.org/Assets/
Technical_Specifications/PDF/MEF6-1.pdf>.
[MEF10.2] Metro Ethernet Forum, "Ethernet Services Attributes
Phase 2", Technical Specification MEF 10.2, October
2009, <http://metroethernetforum.org/Assets/
Technical_Specifications/PDF/MEF10.2.pdf>.
[MEF26.1] Metro Ethernet Forum, "External Network Network
Interface (ENNI) Phase 2", Technical Specification, MEF
26.1, January 2012,
<http://metroethernetforum.org/Assets/
Technical_Specifications/PDF/MEF_26.1.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4761] Kompella, K., Ed., and Y. Rekhter, Ed., "Virtual Private
LAN Service (VPLS) Using BGP for Auto-Discovery and
Signaling", RFC 4761, January 2007.
[RFC4762] Lasserre, M., Ed., and V. Kompella, Ed., "Virtual
Private LAN Service (VPLS) Using Label Distribution
Protocol (LDP) Signaling", RFC 4762, January 2007.
9.2. Informative References
[EVPN] Sajassi, A., Aggarwal, R., Uttaro, J., Bitar, N.,
Henderickx, W., and A. Isaac, "Requirements for Ethernet
VPN (EVPN)", Work in Progress, February 2014.
[IEEE1588] IEEE, "1588-2008 Standard for a Precision Clock
Synchronization Protocol for Networked Measurement and
Control Systems", July 2008.
[TR-101] Broadband Forum, "Migration to Ethernet-Based DSL
Aggregation", Technical Report, DSL Forum TR-101, April
2006, <http://www.broadband-forum.org/
technical/download/TR-101.pdf>.
[VPMS] Kamite, Y., Jounay, F., Niven-Jenkins, B., Brungard, D.,
and L. Jin, "Framework and Requirements for Virtual
Private Multicast Service (VPMS)", Work in Progress,
October 2012.
Appendix A. Frequently Asked Question
A.1. Are E-Tree Requirements Addressed in the Virtual Private Multicast
Service (VPMS) Requirements?
VPMS requirements are defined in [VPMS].
The focus of VPMS is to provide point-to-multipoint connectivity.
VPMS provides single coverage of receiver membership (i.e., there is
no distinct differentiation for multiple multicast groups). A VPMS
service supports single or multiple Root ACs. All traffic from a
Root AC will be forwarded to all Leaf ACs (i.e., Point-to-Multipoint
(P2MP), from Root to all Leaves). The destination address in an
Ethernet frame is not used in data forwarding. As an optional
capability, a VPMS service may support reverse traffic from a Leaf AC
to a Root AC (i.e., point-to-point (P2P), from Leaf to Root).
In contrast, the focus of MEF E-Tree is that a Leaf can only
communicate with Roots, not Leaves.
A generic MEF E-Tree service supports multiple Root endpoints.
Whether a particular E-Tree service needs to support single or
multiple Root endpoints depends on the target application.
As discussion in a previous section, a generic MEF E-Tree service
supports all the following traffic flows:
- Ethernet Unicast bidirectional Root to/from Root
- Ethernet Unicast bidirectional Root to/from Leaf
- Ethernet Broadcast/Multicast unidirectional Root to all Roots
and Leaves
- Ethernet Broadcast/Multicast unidirectional Leaf to all Roots
A particular E-Tree service may need to support all the above or only
a subset depending on the target application.
The IETF's VPMS definition and MEF's E-Tree definition are
significantly different.
VPMS may be acceptable in cases where E-Tree service is needed, such
as in the following cases:
- No Unicast traffic from Root destined for a specific Leaf (or
there is no concern if such Unicast traffic is forwarded to all
Leaves)
- No traffic between Roots
For generic E-Tree service, VPMS will not be able to meet the
requirements.
Authors' Addresses
Raymond Key (editor)
EMail: raymond.key@ieee.org
Simon Delord
Telstra
EMail: simon.delord@gmail.com
Frederic Jounay
Orange CH
4 rue caudray 1020 Renens
Switzerland
EMail: frederic.jounay@orange.ch
Lu Huang
China Mobile
Unit 2, 28 Xuanwumenxi Ave, Xuanwu District
Beijing 100053, China
EMail: huanglu@chinamobile.com
Zhihua Liu
China Telecom
109 Zhongshan Ave., Guangzhou
510630, China
EMail: zhliu@gsta.com
Manuel Paul
Deutsche Telekom
Winterfeldtstr. 21-27
10781 Berlin, Germany
EMail: manuel.paul@telekom.de
AI ELI5
On this page
§1Introduction: E-LAN and E-Tree Services
MEF (Metro Ethernet Forum) defines two multipoint ethernet services: E-LAN (Ethernet LAN) and E-Tree (Ethernet Tree). E-LAN provides multipoint-to-multipoint connectivity where any endpoint can communicate with any other endpoint—like a traditional Ethernet network. E-Tree provides rooted-multipoint connectivity where the service has a tree structure: some endpoints are Roots that can communicate with any other endpoint, and some are Leaves that can communicate only with Roots.
Service providers currently use VPLS (Virtual Private LAN Service) over IP/MPLS networks to emulate E-LAN services. However, VPLS provides any-to-any connectivity among all endpoints and has no mechanism to restrict Leaf-to-Leaf communication, making it unsuitable for E-Tree services. This RFC defines the requirements that new L2VPN solutions must meet to support E-Tree.
There are two types of ethernet networks. One type (E-LAN) lets any office talk to any other office. The other type (E-Tree) has a tree shape: the head office talks to any branch, but branches should not talk to each other directly.
§2E-Tree Concepts and Differences from E-LAN
E-LAN and E-Tree are similar in many ways: both use Ethernet frames, both forward based on MAC addresses, and both are bidirectional. The critical difference is in endpoint roles. In E-LAN, all endpoints are essentially equal (all are Roots). In E-Tree, each endpoint is designated either a Root or a Leaf. A Root can communicate with all other endpoints—other Roots and all Leaves. A Leaf can communicate only with Roots; it cannot communicate with other Leaves. This enforces a tree structure.
Within a generic E-Tree service, the following traffic flows are allowed: Ethernet unicast from Root to Leaf, from Leaf to Root, and from Root to Root; Ethernet broadcast and multicast from Root to other Roots and Leaves; and Ethernet broadcast and multicast from Leaf to Roots. A specific E-Tree service may support all these flows or only a subset depending on its application.
In E-Tree, you mark some connections as Roots and some as Leaves. Roots can talk to anything. Leaves can only talk to Roots, not to each other. This makes a tree shape.
§3E-Tree Use Cases
E-Tree services have many practical applications. Hub-and-spoke VPNs use a central hub site (Root) that communicates with multiple spoke sites (Leaves); the spokes do not need to communicate directly. Wholesale access connects a customer's interconnect point (Root) to multiple subscriber locations (Leaves). Mobile backhaul connects a Radio Access Network controller (Root) to multiple base stations (Leaves) that relay traffic through the controller. Precision Time Protocol clock synchronization uses a PTP time server (Root) sending synchronized time to multiple PTP clients (Leaves). Broadband internet access connects a broadband gateway (Root) to multiple subscriber premises (Leaves). Broadcast video sends video from a source (Root) to subscribers (Leaves) unidirectionally. Broadcast and multicast video adds a control channel for video selection. Device management connects a management system (Root) to managed devices (Leaves). In all these cases, direct Leaf-to-Leaf communication is either not required or must be prevented for security or operational reasons.
A bank has a head office (Root) and branches (Leaves). A phone company has a controller (Root) and cell towers (Leaves). A video company has servers (Root) and viewers (Leaves). All these are tree shapes.
§4The VPLS Problem and Solution Requirement
VPLS was designed as a multipoint-to-multipoint service where all attachment circuits (ACs) are treated equally. It provides any-to-any connectivity with no mechanism for communication restriction between specific ACs. When VPLS is used to emulate an E-Tree service, a problem arises: when a Provider Edge (PE) router receives a frame from another PE over the pseudowire, it does not know whether the ingress AC on the source PE is a Leaf or Root. Therefore, it cannot enforce the Leaf-to-Leaf communication restriction. For example, if a Leaf sends a broadcast or multicast frame, a PE receiving it would forward it to all endpoints, including other Leaves, violating the E-Tree restriction. Similarly, if a Leaf unicast frame arrives with a destination MAC of another Leaf, a PE would forward it rather than blocking it.
To solve this problem, L2VPN solutions need the ability to identify which AC is the ingress AC and whether it is a Leaf or Root, so they can enforce communication restrictions accordingly.
When a message arrives from another location, routers do not know if it came from a Leaf or a Root. If it is from a Leaf and heading to another Leaf, they should block it. But they do not know, so they forward it anyway.
§5Functional Requirements for E-Tree Support
RFC 7152 establishes three core functional requirements that any E-Tree L2VPN solution must meet. First, a solution MUST prohibit communication between any two Leaf ACs in an L2VPN instance. This is the defining characteristic of E-Tree: Leaf-to-Leaf traffic must be blocked or never sent. Second, a solution MUST allow multiple Root ACs in an L2VPN instance. Many use cases require redundancy, where multiple Root sites provide backup if the primary Root fails. Third, a solution MUST allow a Root AC and Leaf AC of the same L2VPN instance to coexist on the same PE router. In practical deployments, a single PE router may have both Root and Leaf endpoints connected. Solutions SHOULD minimize the impact on existing VPLS and E-VPN services, especially for MEF E-LAN services already in operation, and SHOULD be backward compatible with existing L2VPN solutions, allowing both legacy and new PEs to participate in a single L2VPN instance with Leaf-to-Leaf restrictions enforced only among compliant PEs.
Solutions must do three things: block Leaf-to-Leaf messages, allow multiple Roots, and let a PE have both Roots and Leaves. They should also still work with old services that do not use this feature.
§6Security Implications
The core requirement of prohibiting Leaf-to-Leaf communication exists partly for security reasons. In some E-Tree use cases, the restriction prevents unauthorized direct communication between endpoints that should only communicate through a central authority. For example, in a bank's branch network, the head office (Root) might apply policies or logging to all inter-site traffic; if branches could talk directly, they would bypass these controls. In a cellular network, a base station (Leaf) is designed to communicate through the RAN controller (Root), not directly with other base stations. The RFC notes that beyond this fundamental requirement, there are no additional security considerations beyond those already described in VPLS and E-VPN standards, which address topics like authentication of control-plane messages, data encryption, and access control lists.
Sometimes you want Leaves to only talk through the Root for security or control reasons. That is why E-Tree exists. The usual network security rules still apply too.