| Rfc | 4665 |
|---|
| Title | Service Requirements for Layer 2 Provider-Provisioned Virtual
Private Networks |
|---|
| Author | W. Augustyn, Ed., Y. Serbest, Ed. |
|---|
| Date | September 2006 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | INFORMATIONAL |
|---|
|
Network Working Group W. Augustyn, Ed.
Request for Comments: 4665 Y. Serbest, Ed.
Category: Informational AT&T
September 2006
Service Requirements for Layer 2
Provider-Provisioned Virtual Private Networks
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document provides requirements for Layer 2 Provider-Provisioned
Virtual Private Networks (L2VPNs). It first provides taxonomy and
terminology and states generic and general service requirements. It
covers point-to-point VPNs, referred to as Virtual Private Wire
Service (VPWS), as well as multipoint-to-multipoint VPNs, also known
as Virtual Private LAN Service (VPLS). Detailed requirements are
expressed from both a customer as well as a service provider
perspectives.
Table of Contents
1. Introduction ....................................................4
1.1. Scope of This Document .....................................4
1.2. Outline ....................................................5
2. Conventions used in this document ...............................5
3. Contributing Authors ............................................5
4. Definitions and Taxonomy ........................................5
4.1. Definitions ................................................5
4.2. Taxonomy of L2VPN Types ....................................6
4.3. VPWS .......................................................6
4.4. VPLS .......................................................7
5. Service Requirements Common to Customers and Service Providers ..7
5.1. Scope of emulation .........................................8
5.2. Traffic Types ..............................................8
5.3. Topology ...................................................8
5.4. Isolated Exchange of Data and Forwarding Information .......9
5.5. Security ...................................................9
5.5.1. User Data Security .................................10
5.5.2. Access Control .....................................10
5.6. Addressing ................................................11
5.7. Quality of Service ........................................11
5.7.1. QoS Standards ......................................11
5.7.2. Service Models .....................................11
5.8. Service Level Specifications ..............................12
5.9. Protection and Restoration ................................12
5.10. CE-to-PE and PE-to-PE Link Requirements ..................12
5.11. Management ...............................................12
5.12. Interoperability .........................................12
5.13. Inter-working ............................................13
6. Customer Requirements ..........................................13
6.1. Service Provider Independence .............................13
6.2. Layer 3 Support ...........................................13
6.3. Quality of Service and Traffic Parameters .................14
6.4. Service Level Specification ...............................14
6.5. Security ..................................................14
6.5.1. Isolation ..........................................14
6.5.2. Access Control .....................................14
6.5.3. Value-Added Security Services ......................15
6.6. Network Access ............................................15
6.6.1. Physical/Link Layer Technology .....................15
6.6.2. Access Connectivity ................................15
6.7. Customer Traffic ..........................................17
6.7.1. Unicast, Unknown Unicast, Multicast, and
Broadcast forwarding ...............................17
6.7.2. Packet Re-ordering .................................17
6.7.3. Minimum MTU ........................................17
6.7.4. End-point VLAN Tag Translation .....................18
6.7.5. Transparency .......................................18
6.8. Support for Layer 2 Control Protocols .....................18
6.9. CE Provisioning ...........................................19
7. Service Provider Network Requirements ..........................19
7.1. Scalability ...............................................19
7.1.1. Service Provider Capacity Sizing Projections .......19
7.1.2. Solution-Specific Metrics ..........................19
7.2. Identifiers ...............................................19
7.3. Discovering L2VPN Related Information .....................19
7.4. Quality of Service (QoS) ..................................20
7.5. Isolation of Traffic and Forwarding Information ...........20
7.6. Security ..................................................21
7.7. Inter-AS/SP L2VPNs ........................................22
7.7.1. Management .........................................22
7.7.2. Bandwidth and QoS Brokering ........................22
7.8. L2VPN Wholesale ...........................................23
7.9. Tunneling Requirements ....................................23
7.10. Support for Access Technologies ..........................23
7.11. Backbone Networks ........................................24
7.12. Network Resource Partitioning and Sharing Between
L2VPNs ...................................................24
7.13. Interoperability .........................................24
7.14. Testing ..................................................25
7.15. Support on Existing PEs ..................................25
8. Service Provider Management Requirements .......................26
9. Engineering Requirements .......................................26
9.1. Control Plane Requirements ................................26
9.2. Data Plane Requirements ...................................27
9.2.1. Encapsulation ......................................27
9.2.2. Responsiveness to Congestion .......................27
9.2.3. Broadcast Domain ...................................27
9.2.4. Virtual Switching Instance .........................27
9.2.5. MAC Address Learning ...............................27
10. Security Considerations .......................................28
11. Acknowledgements ..............................................28
12. References ....................................................29
12.1. Normative References .....................................29
12.2. Informative References ...................................29
1. Introduction
This section describes the scope and outline of the document.
1.1. Scope of This Document
This document provides requirements for provider-provisioned Layer 2
Virtual Private Networks (L2VPN). It identifies requirements that
MAY apply to one or more individual approaches that a Service
Provider (SP) may use for the provisioning of a Layer 2 VPN service.
The content of this document makes use of the terminology defined in
[RFC4026] and common components for deploying L2VPNs described in
[RFC4664].
The technical specifications to provide L2VPN services are outside
the scope of this document. The framework document [RFC4664] and
several other documents, which explain technical approaches providing
L2VPN services, such as [VPLS_LDP], [VPLS_BGP], and [IPLS], are
available to cover this aspect.
This document describes requirements for two types of L2VPNs: (1)
Virtual Private Wire Service (VPWS), and (2) Virtual Private LAN
Service (VPLS). The approach followed in this document distinguishes
L2VPN types as to how the connectivity is provided (point-point or
multipoint-multipoint), as detailed in [RFC4664].
This document is intended as a "checklist" of requirements that will
provide a consistent way to evaluate and document how well each
individual approach satisfies specific requirements. The
applicability statement document for each individual approach should
document the results of this evaluation.
In the context of provider-provisioned VPNs, there are two entities
involved in operation of such services, the Provider and the
Customer. The Provider engages in a binding agreement with the
Customer as to the behavior of the service in a normal situation as
well as in exceptional situations. Such agreement is known as
Service Level Specification (SLS), which is part of the Service Level
Agreement (SLA) established between the Provider and the Customer.
A proper design of L2VPNs aids formulation of SLSes in that it
provides means for proper separation between Customer Edge (CE) and
Provider Edge (PE), allows proper execution of the SLS offer, and
supports a flexible and rich set of capabilities.
This document provides requirements from both the Provider's and the
Customer's point of view. It begins with common customer's and
service provider's point of view, followed by a customer's
perspective, and concludes with specific needs of an SP. These
requirements provide high-level L2VPN features expected by an SP in
provisioning L2VPNs, which include SP requirements for security,
privacy, manageability, interoperability, and scalability.
1.2. Outline
The outline of the rest of this document is as follows. Section 4
provides definitions and taxonomy. Section 5 provides common
requirements that apply to both customer and SP, respectively.
Section 6 states requirements from a customer perspective. Section 7
states network requirements from an SP perspective. Section 8 states
SP management requirements. Section 9 describes the engineering
requirements, particularly control and data plane requirements.
Section 10 provides security considerations. Section 11 lists
acknowledgements. Section 12 provides a list of references cited
herein.
2. 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 RFC 2119 [RFC2119].
3. Contributing Authors
This document was the combined effort of several individuals. The
following are the authors that contributed to this document:
Waldemar Augustyn
Marco Carugi
Giles Heron
Vach Kompella
Marc Lasserre
Pascal Menezes
Hamid Ould-Brahim
Tissa Senevirathne
Yetik Serbest
4. Definitions and Taxonomy
4.1. Definitions
The terminology used in this document is defined in [RFC4026]. The
L2VPN framework document [RFC4664] further describes these concepts
in the context of a reference model that defines layered service
relationships between devices and one or more levels of tunnels.
4.2. Taxonomy of L2VPN Types
The requirements distinguish two major L2VPN models, a Virtual
Private Wire Service (VPWS), and a Virtual Private LAN Service
(VPLS).
The following diagram shows an L2VPN reference model.
+-----+ +-----+
+ CE1 +--+ +---| CE2 |
+-----+ | ........................ | +-----+
L2VPN A | +----+ +----+ | L2VPN A
+--| PE |--- Service ---| PE |--+
+----+ Provider +----+
/ . Backbone . \ - /\-_
+-----+ / . | . \ / \ / \ +-----+
+ CE4 +--+ . | . +--\ Access \----| CE5 |
+-----+ . +----+ . | Network | +-----+
L2VPN B .........| PE |......... \ / L2VPN B
+----+ ^ -------
| |
| |
+-----+ |
| CE3 | +-- Logical switching instance
+-----+
L2VPN A
Figure 1. L2VPN Reference Model
4.3. VPWS
The PE devices provide a logical interconnect such that a pair of CE
devices appears to be connected by a single logical Layer 2 circuit.
PE devices act as Layer 2 circuit switches. Layer 2 circuits are
then mapped onto tunnels in the SP network. These tunnels can either
be specific to a particular VPWS, or be shared among several
services. VPWS applies for all services, including Ethernet, ATM,
Frame Relay, etc. In Figure 1, L2VPN B represents a VPWS case.
Each PE device is responsible for allocating customer Layer 2 frames
to the appropriate VPWS and for proper forwarding to the intended
destinations.
4.4. VPLS
In case of VPLS, the PE devices provide a logical interconnect such
that CE devices belonging to a specific VPLS appear to be connected
by a single LAN. End-to-end VPLS consists of a bridge module and a
LAN emulation module ([RFC4664]). A VPLS can contain a single VLAN
or multiple VLANs ([IEEE_802.1Q]). A variation of this service is
IPLS ([RFC4664]), which is limited to supporting only customer IP
traffic.
In a VPLS, a customer site receives Layer 2 service from the SP. The
PE is attached via an access connection to one or more CEs. The PE
performs forwarding of user data packets based on information in the
Layer 2 header, such as a MAC destination address. In Figure 1,
L2VPN A represents a VPLS case.
The details of VPLS reference model, which we summarize here, can be
found in [RFC4664]. In VPLS, the PE can be viewed as containing a
Virtual Switching Instance (VSI) for each L2VPN that it serves. A CE
device attaches, possibly through an access network, to a bridge
module of a PE. Within the PE, the bridge module attaches, through
an Emulated LAN Interface to an Emulated LAN. For each VPLS, there
is an Emulated LAN instance. The Emulated LAN consists of VPLS
Forwarder module (one per PE per VPLS service instance) connected by
pseudo wires (PW), where the PWs may be traveling through Packet
Switched Network (PSN) tunnels over a routed backbone. VSI is a
logical entity that contains a VPLS forwarder module and part of the
bridge module relevant to the VPLS service instance [RFC4664].
Hence, the VSI terminates PWs for interconnection with other VSIs and
also terminates Attachment Circuits (ACs) (see [RFC3985] for
definition) for accommodating CEs. A VSI includes the forwarding
information base for an L2VPN [RFC4664] which is the set of
information regarding how to forward Layer 2 frames received over the
AC from the CE to VSIs in other PEs supporting the same L2VPN service
(and/or to other ACs), and it contains information regarding how to
forward Layer 2 frames received from PWs to ACs. Forwarding
information bases can be populated dynamically (such as by source MAC
address learning) or statically (e.g., by configuration). Each PE
device is responsible for proper forwarding of the customer traffic
to the appropriate destination(s) based on the forwarding information
base of the corresponding VSI.
5. Service Requirements Common to Customers and Service Providers
This section contains requirements that apply to both the customer
and the provider, or that are of an otherwise general nature.
5.1. Scope of emulation
L2VPN protocols SHOULD NOT interfere with existing Layer 2 protocols
and standards of the Layer 2 network the customer is managing. If
they impact customer Layer 2 protocols that are sent over the VPLS,
then these impacts MUST be documented.
Some possibly salient differences between VPLS and a real LAN are:
- The reliability may likely be less, i.e., the probability that a
message broadcast over the VPLS is not seen by one of the bridge
modules in PEs is higher than in a true Ethernet.
- VPLS frames can get duplicated if the PW sequencing option isn't
turned on. The data frames on the PWs are sent in IP datagrams,
and under certain failure scenarios, IP networks can duplicate
packets. If the PW data transmission protocol does not ensure
sequence of data packets, frames can be duplicated or received out
of sequence. If the customer's Bridge Protocol Data Unit (BPDU)
frames are sent as data packets, then BPDU frames can be duplicated
or mis-sequenced, although this may not create any problems for
Real-Time Streaming Protocol (RSTP).
- Delayed delivery of packets (e.g., more than half a second), rather
than dropping them, could have adverse effect on the performance of
the service.
- 802.3x Pause frames will not be transported over a VPLS, as the
bridge module ([RFC4664]) in the PE terminates them.
- Since the IPLS solution aims at transporting encapsulated traffic
(rather than Layer 2 frames themselves), the IPLS solution is NOT
REQUIRED to preserve the Layer 2 Header transparently from CE to
CE. For example, Source MAC address will probably not be preserved
by the IPLS solution.
5.2. Traffic Types
A VPLS MUST support unicast, multicast, and broadcast traffic.
Support for efficient replication of broadcast and multicast traffic
is highly desirable.
5.3. Topology
A SP network may be realized using one or more network tunnel
topologies to interconnect PEs, ranging from simple point-to-point to
distributed hierarchical arrangements. The typical topologies
include:
- Point-to-point
- Point-to-multipoint, a.k.a. hub and spoke
- Any-to-any, a.k.a. full mesh
- Mixed, a.k.a. partial mesh
- Hierarchical
Regardless of the SP topology employed, the service to the customers
MUST retain the connectivity type implied by the type of L2VPN. For
example, a VPLS MUST allow multipoint-to-multipoint connectivity even
if it is implemented with point-to-point circuits. This requirement
does not imply that all traffic characteristics (such as bandwidth,
QoS, delay, etc.) necessarily be the same between any two end points
of an L2VPN. It is important to note that SLS requirements of a
service have a bearing on the type of topology that can be used.
To the extent possible, an L2VPN service SHOULD be capable of
crossing multiple administrative boundaries.
To the extent possible, the L2VPN services SHOULD be independent of
access network technology.
5.4. Isolated Exchange of Data and Forwarding Information
L2VPN solutions SHALL define means that prevent CEs in an L2VPN from
interaction with unauthorized entities.
L2VPN solutions SHALL avoid introducing undesired forwarding
information that could corrupt the L2VPN forwarding information base.
A means to constrain or isolate the distribution of addressed data to
only those VPLS sites determined either by MAC learning and/or
configuration MUST be provided.
The internal structure of an L2VPN SHOULD not be advertised or
discoverable from outside that L2VPN.
5.5. Security
A range of security features MUST be supported by the suite of L2VPN
solutions. Each L2VPN solution MUST state which security features it
supports and how such features can be configured on a per-customer
basis.
A number of security concerns arise in the setup and operation of an
L2VPN, ranging from misconfigurations to attacks that can be launched
on an L2VPN and can strain network resources such as memory space,
forwarding information base table, bandwidth, and CPU processing.
This section lists some potential security hazards that can result
due to mis-configurations and/or malicious attacks. There MUST be
methods available to protect against the following situations.
- Protocol attacks
o Excessive protocol adjacency setup/teardown
o Excessive protocol signaling/withdrawal
- Resource Utilization
o Forwarding plane replication (VPLS)
o Looping (VPLS primarily)
o MAC learning table size limit (VPLS)
- Unauthorized access
o Unauthorized member of VPN
o Incorrect customer interface
o Incorrect service delimiting VLAN tag
o Unauthorized access to PE
- Tampering with signaling
o Incorrect FEC signaling
o Incorrect PW label assignment
o Incorrect signaled VPN parameters (e.g., QoS, MTU, etc.)
- Tampering with data forwarding
o Incorrect MAC learning entry
o Incorrect PW label
o Incorrect AC identifier
o Incorrect customer facing encapsulation
o Incorrect PW encapsulation
o Hijacking PWs using the wrong tunnel
o Incorrect tunnel encapsulation
5.5.1. User Data Security
An L2VPN solution MUST provide traffic separation between different
L2VPNs.
In case of VPLS, VLAN Ids MAY be used as service delimiters. When
used in this manner, they MUST be honored and traffic separation MUST
be provided.
5.5.2. Access Control
An L2VPN solution MAY also have the ability to activate the
appropriate filtering capabilities upon request of a customer.
5.6. Addressing
An L2VPN solution MUST support overlapping addresses of different
L2VPNs. For instance, customers MUST NOT be prevented from using the
same MAC addresses with different L2VPNs. If a service provider uses
VLANs as service delimiters, the L2VPN solution MUST ensure that VLAN
Ids cannot overlap. If VLANs are not used as service delimiters,
L2VPN solutions MAY allow VLAN Ids to overlap.
5.7. Quality of Service
To the extent possible, L2VPN QoS SHOULD be independent of the access
network technology.
5.7.1. QoS Standards
As provided in [RFC3809], an L2VPN SHALL be able to support QoS in
one or more of the following already standardized modes:
- Best Effort (support mandatory for all provider-provisioned
VPN types)
- Aggregate CE Interface Level QoS (i.e., 'hose' level)
- Site-to-site, or 'pipe' level QoS
Note that all cases involving QoS MAY require that the CE and/or PE
perform shaping and/or policing.
Mappings or translations of Layer 2 QoS parameters into PSN QoS
(e.g., DSCPs or MPLS EXP field) as well as QoS mapping based on VC
(e.g., FR/ATM or VLAN) MAY be performed in order to provide QoS
transparency. The actual mechanisms for these mappings or
translations are outside the scope of this document. In addition,
the Diffserv support of underlying tunneling technologies (e.g.,
[RFC3270] or [RFC3308]) and the Intserv model ([RFC2205]) MAY be
used. As such, the L2VPN SLS requirements SHOULD be supported by
appropriate core mechanisms.
5.7.2. Service Models
A service provider may desire to offer QoS service to a customer for
at least the following generic service types: managed access VPN
service or an edge-to-edge QoS service. The details of the service
models can be found in [RFC3809] and in [RFC4031].
In L2VPN service, both DSCP ([RFC2474]) and 802.1p ([IEEE_802.1D])
fields may be used for this purpose.
5.8. Service Level Specifications
For an L2VPN service, the capabilities for Service Level
Specification (SLS) monitoring and reporting stated in [RFC3809]
SHOULD be provided.
5.9. Protection and Restoration
The L2VPN service infrastructure SHOULD provide redundant paths to
ensure high availability. The reaction to failures SHOULD result in
an attempt to restore the service using alternative paths.
The intention is to keep the restoration time small. The restoration
time MUST be less than the time it takes the CE devices, or customer
Layer 2 control protocols as well as Layer 3 routing protocols, to
detect a failure in the L2VPN.
5.10. CE-to-PE and PE-to-PE Link Requirements
The CE-to-PE links MAY be
- direct physical links (e.g., 100BaseTX, and T1/E1 TDM),
- logical links (e.g., ATM PVC, and RFC2427-encapsulated link),
- transport networks carrying Ethernet,
- a Layer 2 tunnel that goes through a Layer 3 network (e.g., L2TP
sessions).
Layer 2 frames MAY be tunneled through a Layer 3 backbone from PE to
PE, using one of a variety of tunneling technologies (e.g., IP-in-IP,
GRE, MPLS, L2TP, etc.).
5.11. Management
Standard interfaces to manage L2VPN services MUST be provided (e.g.,
standard SNMP MIB Modules). These interfaces SHOULD provide access
to configuration, verification and runtime monitoring protocols.
Service management MAY include the TMN 'FCAPS' functionalities, as
follows: Fault, Configuration, Accounting, Performance, and Security,
as detailed in [ITU_Y.1311.1].
5.12. Interoperability
Multi-vendor interoperability, which corresponds to similar network
and service levels among different implementations, at the network
element SHOULD be guaranteed. This will likely rely on the
completeness of the corresponding standard.
The technical solution MUST be multi-vendor interoperable, not only
within the SP network infrastructure, but also with the customer's
network equipment and services making use of the L2VPN service.
A L2VPN solution SHOULD NOT preclude different access technologies.
For instance, customer access connections to an L2VPN service MAY be
different at different CE devices (e.g., Frame Relay, ATM, 802.1D,
MPLS).
5.13. Inter-working
Inter-working scenarios among different solutions providing L2VPN
services are highly desirable. It is possible to have cases that
require inter-working or interconnection between customer sites,
which span network domains with different L2VPN solutions or
different implementations of the same approach. Inter-working SHOULD
be supported in a scalable manner.
Inter-working scenarios MUST consider at least traffic isolation,
security, QoS, access, and management aspects. This requirement is
essential in the case of network migration, to ensure service
continuity among sites belonging to different portions of the
network.
6. Customer Requirements
This section captures requirements from a customer perspective.
6.1. Service Provider Independence
Customers MAY require L2VPN service that spans multiple
administrative domains or SP networks. Therefore, an L2VPN service
MUST be able to span multiple AS and SP networks but still to act and
to appear as a single, homogeneous L2VPN from a customer point of
view.
A customer might also start with an L2VPN provided in a single AS
with a certain SLS but then ask for an expansion of the service
spanning multiple ASes and/or multiple-SPs. In this case, as well as
for all kinds of multi-AS and multiple-SP L2VPNs, L2VPN service
SHOULD be able to deliver the same SLS to all sites in a VPN
regardless of the AS/SP to which it homes.
6.2. Layer 3 Support
With the exception of IPLS, an L2VPN service SHOULD be agnostic to
customer's Layer 3 traffic (e.g., IP, IPX, Appletalk) encapsulated
within Layer 2 frames.
IPLS MUST allow transport of customer's IPv4 and IPv6 traffic
encapsulated within Layer 2 frames. IPLS SHOULD also allow CEs to
run ISIS and MPLS protocols transparently among them when those are
used in conjunction with IP.
6.3. Quality of Service and Traffic Parameters
QoS is expected to be an important aspect of an L2VPN service for
some customers.
A customer requires that the L2VPN service provide the QoS applicable
to his or her application, which can range from PWs (e.g., SONET
emulation) to voice, interactive video, and multimedia applications.
Hence, best-effort as well as delay and loss sensitive traffic MUST
be supported over an L2VPN service. A customer application SHOULD
experience consistent QoS independent of the access network
technology used at different sites connected to the same L2VPN.
6.4. Service Level Specification
Most customers simply want their applications to perform well. A SLS
is a vehicle for a customer to measure the quality of the service
that SP(s) provide. Therefore, when purchasing a service, a customer
requires access to the measures from the SP(s) that support the SLS.
Standard interfaces to monitor usage of L2VPN services SHOULD be
provided (e.g., standard SNMP MIB Modules).
6.5. Security
6.5.1. Isolation
An L2VPN solution MUST provide traffic as well as forwarding
information base isolation for customers similar to that obtained in
private lines, FR, or ATM services.
An L2VPN service MAY use customer VLAN Ids as service delimiters. In
that case, they MUST be honored, and traffic separation MUST be
provided.
6.5.2. Access Control
An L2VPN solution MAY have the mechanisms to activate the appropriate
filtering capabilities upon request of a customer. For instance, MAC
and/or VLAN filtering MAY be considered between CE and PE for a VPLS.
6.5.3. Value-Added Security Services
An L2VPN solution MAY provide value-added security services such as
encryption and/or authentication of customer packets, certificate
management, and similar services.
L2VPN services MUST NOT interfere with the security mechanisms
employed at Layer 3 and higher layers by customers. Layer 2 security
mechanisms, such as 802.10b ([IEEE_802.10]) and 802.1AE
([IEEE_802.1AE]), MAY inhibit L2VPN services, when the service
delimiting VLAN Ids are encrypted.
6.6. Network Access
Every packet exchanged between the customer and the SP over the
access connection MUST appear as it would on a private network
providing an equivalent service to that offered by the L2VPN.
6.6.1. Physical/Link Layer Technology
L2VPN solutions SHOULD support a broad range of physical and link-
layer access technologies, such as PSTN, ISDN, xDSL, cable modem,
leased line, Ethernet, Ethernet VLAN, ATM, Frame Relay, Wireless
local loop, mobile radio access, etc. The capacity and QoS
achievable MAY be dependent on the specific access technology in use.
6.6.2. Access Connectivity
Various types of physical connectivity scenarios MUST be supported,
such as multi-homed sites, backdoor links between customer sites, and
devices homed to two or more SP networks. In case of VPLS, IEEE
802.3ad-2000 link aggregation SHOULD be supported. L2VPN solutions
SHOULD support at least the types of physical or link-layer
connectivity arrangements shown in Figures 2 - 4 (in addition to the
case shown in Figure 1). As in Figure 2, a CE can be dual-homed to
an SP or to two different SPs via diverse access networks.
+---------------- +---------------
| |
+------+ +------+
+---------| PE | +---------| PE |
| |device| | |device| SP network
| +------+ | +------+
+------+ | +------+ |
| CE | | | CE | +---------------
|device| | SP network |device| +---------------
+------+ | +------+ |
| +------+ | +------+
| | PE | | | PE |
+---------|device| +---------|device| SP network
+------+ +------+
| |
+---------------- +---------------
(a) (b)
Figure 2. Dual-Homed Access of CE Devices
Resiliency of the L2VPN service can be further enhanced as shown in
Figure 3, where CE's connected via a "back door" connection, connect
to the same SP or to different SPs.
+---------------- +---------------
| |
+------+ +------+ +------+ +------+
| CE |-----| PE | | CE |-----| PE |
|device| |device| |device| |device| SP network
+------+ +------+ +------+ +------+
| | | |
| Backdoor | | Backdoor +---------------
| link | SP network | link +---------------
| | | |
+------+ +------+ +------+ +------+
| CE | | PE | | CE | | PE |
|device|-----|device| |device|-----|device| SP network
+------+ +------+ +------+ +------+
| |
+---------------- +---------------
(a) (b)
Figure 3. Backdoor Links Between CE Devices
Arbitrary combinations of the above methods, with a few examples
shown in Figure 4, SHOULD be supported by any L2VPN solution.
+---------------- +---------------
| |
+------+ +------+ +------+ +------+
| CE |-----| PE | | CE |-----| PE |
|device| |device| |device| |device| SP network
+------+\ +------+ +------+\ +------+
| \ | | \ |
|Back \ | |Back \ +-------------
|door \ | SP network |door \ +-------------
|link \ | |link \ |
+------+ +------+ +------+ +------+
| CE | | PE | | CE | | PE |
|device|-----|device| |device|-----|device| SP network
+------+ +------+ +------+ +------+
| |
+---------------- +---------------
(a) (b)
Figure 4. Combination of Dual-Homing and
Backdoor Links for CE Devices
6.7. Customer Traffic
6.7.1. Unicast, Unknown Unicast, Multicast, and Broadcast forwarding
A VPLS MUST deliver every packet at least to its intended
destination(s) within the scope of the VPLS, subject to the ingress
policing and security policies.
6.7.2. Packet Re-ordering
During normal operation, the queuing and forwarding policies SHOULD
preserve packet order for packets with the same QoS parameters.
6.7.3. Minimum MTU
A VPLS MUST support the theoretical MTU of the offered service.
The committed minimum MTU size MUST be the same for a given VPLS
instance. Different L2VPN services MAY have different committed MTU
sizes. If the customer VLANs are used as service delimiters, all
VLANs within a given VPLS MUST inherit the same MTU size.
A VPLS MAY use IP fragmentation if it presents reassembled packets at
VPLS customer edge devices.
6.7.4. End-point VLAN Tag Translation
The L2VPN service MAY support translation of customers' AC
identifiers (e.g., VLAN tags, if the customer VLANs are used as
service delimiters). Such service simplifies connectivity of sites
that want to keep their AC assignments or sites that belong to
different administrative domains. In the latter case, the
connectivity is sometimes referred to as Layer 2 extranet. On the
other hand, it should be noted that VLAN tag translation affects the
support for multiple spanning trees (i.e., 802.1s [IEEE_802.1s]) and
can break the proper operation.
6.7.5. Transparency
The L2VPN service is intended to be transparent to Layer 2 customer
networks. An L2VPN solution SHOULD NOT require any special packet
processing by the end users before sending packets to the provider's
network.
If VLAN Ids are assigned by the SP, then VLANs are not transparent.
Transparency does not apply in this case, as it is the same as FR/ATM
service model.
Since the IPLS solution aims at transporting encapsulated traffic
(rather than Layer 2 frames themselves), the IPLS solution MUST not
alter the packets encapsulated inside Layer 2 frames that are
transported by the IPLS. However, the IPLS solution is NOT REQUIRED
to preserve the Layer 2 header transparently from CE to CE. For
example, Source MAC address might not be preserved by the IPLS
solution. The IPLS solution MAY remove Layer 2 headers for transport
over the backbone when those can be reconstructed on egress without
compromising transport of encapsulated traffic.
6.8. Support for Layer 2 Control Protocols
The L2VPN solution SHOULD allow transparent operation of Layer 2
control protocols employed by customers.
In case of VPLS, the L2VPN service MUST ensure that loops be
prevented. This can be accomplished with a loop-free topology or
appropriate forwarding rules. Control protocols such as Spanning
Tree (STP) or similar protocols could be employed. The L2VPN
solution MAY use indications from customer Layer 2 control protocols,
e.g., STP BPDU snooping, to improve the operation of a VPLS.
6.9. CE Provisioning
The L2VPN solution MUST require only minimal or no configuration on
the CE devices, depending on the type of CE device that connects into
the infrastructure.
7. Service Provider Network Requirements
This section describes requirements from an SP perspective.
7.1. Scalability
This section contains projections regarding L2VPN sizing and
scalability requirements and metrics specific to particular
solutions.
7.1.1. Service Provider Capacity Sizing Projections
[RFC3809] lists projections regarding L2VPN sizing and scalability
requirements and metrics. The examples are provided in [RFC3809].
7.1.2. Solution-Specific Metrics
Each L2VPN solution SHALL document its scalability characteristics in
quantitative terms.
7.2. Identifiers
An SP domain MUST be uniquely identified at least within the set of
all interconnected SP networks when supporting an L2VPN that spans
multiple SPs. Ideally, this identifier SHOULD be globally unique
(e.g., an AS number).
An identifier for each L2VPN SHOULD be unique, at least within each
SP's network, as it MAY be used in auto-discovery, management (e.g.,
alarm and service correlation, troubleshooting, performance
statistics collection), and signaling. Ideally, the L2VPN identifier
SHOULD be globally unique to support the case, where an L2VPN spans
multiple SPs (e.g., [RFC2685]). Globally unique identifiers
facilitate the support of inter-AS/SP L2VPNs.
7.3. Discovering L2VPN Related Information
Configuration of PE devices (i.e., U-PE and N-PE [RFC4664]) is a
significant task for an SP. Solutions SHOULD provide methods that
dynamically allow L2VPN information to be discovered by the PEs to
minimize the configuration steps.
Each device in an L2VPN SHOULD be able to determine which other
devices belong to the same L2VPN. Such a membership discovery scheme
MUST prevent unauthorized access, and it allows authentication of the
source.
Distribution of L2VPN information SHOULD be limited to those devices
involved in that L2VPN. An L2VPN solution SHOULD employ discovery
mechanisms to minimize the amount of operational information
maintained by the SPs. For example, if an SP adds or removes a
customer port on a given PE, the remaining PEs SHOULD determine the
necessary actions to take without the SP's having to explicitly
reconfigure those PEs.
A L2VPN solution SHOULD support the means for attached CEs to
authenticate each other and to verify that the SP L2VPN is correctly
connected.
The mechanism SHOULD respond to L2VPN membership changes in a timely
manner. A "timely manner" is no longer than the provisioning
timeframe, typically on the order of minutes, and MAY be as short as
the timeframe required for "rerouting," typically on the order of
seconds.
Dynamically creating, changing, and managing multiple L2VPN
assignments to sites and/or customers is another aspect of membership
that MUST be addressed in an L2VPN solution.
7.4. Quality of Service (QoS)
A significant aspect of a provider-provisioned VPN is support for
QoS. An SP has control over the provisioning of resources and
configuration of parameters in at least the PE and P devices, and in
some cases the CE devices as well. Therefore, the SP is to provide
either managed QoS access service, or edge-to-edge QoS service, as
defined in [RFC4031].
7.5. Isolation of Traffic and Forwarding Information
From a high level SP perspective, an L2VPN MUST isolate the exchange
of traffic and forwarding information to only those sites that are
authenticated and authorized members of an L2VPN.
An L2VPN solution SHOULD provide a means for meeting provider-
provisioned VPN QoS SLS requirements that isolates L2VPN traffic from
the affects of traffic offered by non-VPN customers. Also, L2VPN
solutions SHOULD provide a means so that traffic congestion produced
by sites as part of one L2VPN does not affect another L2VPN.
7.6. Security
The security requirements are stated in Section 6.5. The security
requirements provided in [RFC3809] SHOULD be met. The security
requirements, except Layer 3 and higher-layer dependent ones,
specified in [RFC4031], SHOULD be met.
In addition, an SP network MUST be protected against malformed or
maliciously constructed customer traffic. This includes but is not
limited to duplicate or invalid Layer 2 addresses, customer side
loops, short/long packets, spoofed management packets, spoofed VLAN
tags, high volume traffic.
The SP network devices MUST NOT be accessible from any L2VPN, unless
specifically authorized. The devices in the SP network SHOULD
provide some means of reporting intrusion attempts to the SP, if the
intrusion is detected.
When an L2VPN solution operates over a part of the Internet, it
should support a configurable option to support one or more of the
following standard IPsec methods for securing a customer's VPN
traffic:
- Confidentiality, so that only authorized devices can decrypt it
- Integrity, to ensure that the data has not been altered
- Authentication, to ensure that the sender is indeed who he or she
claims to be
- Replay attack prevention.
The above functions SHOULD be applicable to "data traffic" of the
customer, which includes the traffic exchanged between sites. It
SHOULD also be possible to apply these functions to "control
traffic", such as routing or signaling protocol exchanges, that is
not necessarily perceived by the customer but is nevertheless
essential to maintain his or her VPN.
Furthermore, such security methods MUST be configurable between
different end-points, such as PE-PE and PE-MTU, only in the case
where L2VPN data traffic is carried over IP [RFC4023]. Methods to
secure data flows at the native service layer (Layer-2), from CE-CE,
CE-MTU and CE-PE, are outside the scope of this document. It is also
desirable to configure security on a per-VPN basis.
A VPN solution MAY support one or more encryption schemes, including
AES, and 3DES. Encryption, decryption, and key management SHOULD be
included in profiles as part of the security management system.
7.7. Inter-AS/SP L2VPNs
All applicable SP requirements, such as traffic and forwarding
information isolation, SLSes, management, security, provisioning,
etc. MUST be preserved across adjacent ASes. The solution MUST
describe the inter-SP network interface, encapsulation method(s),
routing protocol(s), and all applicable parameters.
An L2VPN solution MUST provide the specifics of offering L2VPN
services spanning multiple ASes and/or SPs.
An L2VPN solution MUST support proper dissemination of operational
parameters to all elements of an L2VPN service in the presence of
multiple ASes and/or SPs. A L2VPN solution MUST employ mechanisms
for sharing operational parameters between different ASes.
An L2VPN solution SHOULD support policies for proper selection of
operational parameters coming from different ASes. Similarly, an
L2VPN solution SHOULD support policies for selecting information to
be disseminated to different ASes.
7.7.1. Management
The general requirements for managing a single AS apply to a
concatenation of ASes. A minimum subset of such capabilities is the
following:
- Diagnostic tools
- Secured access to one AS management system by another
- Configuration request and status query tools
- Fault notification and trouble tracking tools
7.7.2. Bandwidth and QoS Brokering
When an L2VPN spans multiple ASes, there is a need for a brokering
mechanism that requests certain SLS parameters, such as bandwidth and
QoS, from the other domains and/or networks involved in transferring
traffic to various sites. The essential requirement is that a
solution MUST be able to determine whether a set of ASes can
establish and guarantee uniform QoS in support of a provider-
provisioned VPN.
7.8. L2VPN Wholesale
The architecture MUST support the possibility of one SP's offering
L2VPN service to another SP. One example is when one SP sells L2VPN
service at wholesale to another SP, who then resells that L2VPN
service to his or her customers.
7.9. Tunneling Requirements
Connectivity between CE sites or PE devices in the backbone SHOULD be
able to use a range of tunneling technologies, such as L2TP, GRE,
IP-in-IP, MPLS, etc.
Every PE MUST support a tunnel setup protocol, if tunneling is used.
A PE MAY support static configuration. If employed, a tunnel
establishment protocol SHOULD be capable of conveying information,
such as the following:
- Relevant identifiers
- QoS/SLS parameters
- Restoration parameters
- Multiplexing identifiers
- Security parameters
There MUST be a means to monitor the following aspects of tunnels:
- Statistics, such as amount of time spent in the up and down state
- Count of transitions between the up and down state
- Events, such as transitions between the up and down states
The tunneling technology used by the VPN SP and its associated
mechanisms for tunnel establishment, multiplexing, and maintenance
MUST meet the requirements on scaling, isolation, security, QoS,
manageability, etc.
Regardless of the tunneling choice, the existence of the tunnels and
their operations MUST be transparent to the customers.
7.10. Support for Access Technologies
The connectivity between PE and CE devices is referred to as an AC.
ACs MAY span networks of other providers or public networks.
There are several choices for implementing ACs. Some popular choices
include Ethernet, ATM (DSL), Frame Relay, MPLS-based virtual circuits
etc.
In case of VPLS, the AC MUST use Ethernet frames as the Service
Protocol Data Unit (SPDU).
A CE access connection over an AC MUST be bi-directional.
PE devices MAY support multiple ACs on a single physical interface.
In such cases, PE devices MUST NOT rely on customer controlled
parameters for distinguishing between different access connections.
For example, if VLAN tags were used for that purpose, the provider
would be controlling the assignment of the VLAN tag values and would
strictly enforce compliance by the CEs.
An AC, whether direct or virtual, MUST maintain all committed
characteristics of the customer traffic, such as QoS, priorities etc.
The characteristics of an AC are only applicable to that connection.
7.11. Backbone Networks
Ideally, the backbone interconnecting the SP's PE and P devices
SHOULD be independent of physical and link-layer technology.
Nevertheless, the characteristics of backbone technology MUST be
taken into account when specifying the QoS aspects of SLSes for VPN
service offerings.
7.12. Network Resource Partitioning and Sharing Between L2VPNs
In case network resources such as memory space, forwarding
information base table, bandwidth, and CPU processing are shared
between L2VPNs, the solution SHOULD guarantee availability of
resources necessary to prevent any specific L2VPN service instance
from taking up available network resources and causing others to
fail. The solution SHOULD be able to limit the resources consumed by
an L2VPN service instance. The solution SHOULD guarantee
availability of resources necessary to fulfill the obligation of
committed SLSes.
7.13. Interoperability
Service providers are interested in interoperability in at least the
following scenarios:
- To facilitate use of PE and managed CE devices within a single SP
network
- To implement L2VPN services across two or more interconnected SP
networks
- To achieve inter-working or interconnection between customer sites
using different L2VPN solutions or different implementations of the
same approach
Each approach MUST describe whether any of the above objectives can
be met. If an objective can be met, the approach MUST describe how
such interoperability could be achieved.
7.14. Testing
The L2VPN solution SHOULD provide the ability to test and verify
operational and maintenance activities on a per L2VPN service basis,
and, in case of VPLS, on a per-VLAN basis if customer VLANs are used
as service delimiters.
The L2VPN solution SHOULD provide mechanisms for connectivity
verification, and for detecting and locating faults.
Examples of testing mechanisms are as follows:
- Checking connectivity between "service-aware" network nodes
- Verifying data plane and control plane integrity
- Verifying service membership
The provided mechanisms MUST satisfy the following: the connectivity
checking for a given customer MUST enable the end-to-end testing of
the data path used by that of customer's data packets, and the test
packets MUST not propagate beyond the boundary of the SP network.
7.15. Support on Existing PEs
To the extent possible, the IPLS solution SHOULD facilitate support
of IPLS on existing PE devices that may be already deployed by the SP
and MAY have been designed primarily for Layer 3 services.
8. Service Provider Management Requirements
An SP desires to have a means to view the topology, operational
state, and other parameters associated with each customer's L2VPN.
Furthermore, the SP requires a means to view the underlying logical
and physical topology, operational state, provisioning status, and
other parameters associated with the equipment providing the L2VPN
service(s) to its customers. Therefore, the devices SHOULD provide
standards-based interfaces (e.g., L2VPN MIB Modules), wherever
feasible.
The details of service provider management requirements for a Network
Management System (NMS) in the traditional fault, configuration,
accounting, performance, and security (FCAPS) management categories
can be found in [ITU_Y.1311.1].
9. Engineering Requirements
These requirements are driven by implementation characteristics that
make service and SP requirements achievable.
9.1. Control Plane Requirements
An L2VPN service SHOULD be provisioned with minimum number of steps.
Therefore, the control protocols SHOULD provide methods for signaling
between PEs. The signaling SHOULD inform of membership, tunneling
information, and other relevant parameters.
The infrastructure MAY employ manual configuration methods to provide
this type of information.
The infrastructure SHOULD use policies to scope the membership and
reachability advertisements for a particular L2VPN service. A
mechanism for isolating the distribution of reachability information
to only those sites associated with an L2VPN MUST be provided.
The control plane traffic increases with the growth of L2VPN
membership. Similarly, the control plane traffic increases with the
number of supported L2VPN services. The use of control plane
resources MAY increase as the number of hosts connected to an L2VPN
service grows.
An L2VPN solution SHOULD minimize control plane traffic and the
consumption of control plane resources. The control plane MAY offer
means for enforcing a limit on the number of customer hosts attached
to an L2VPN service.
9.2. Data Plane Requirements
9.2.1. Encapsulation
An L2VPN solution SHOULD utilize the encapsulation techniques defined
by PWE3 ([RFC3985]), and SHOULD not impose any new requirements on
these techniques.
9.2.2. Responsiveness to Congestion
An L2VPN solution SHOULD utilize the congestion avoidance techniques
defined by PWE3 ([RFC3985]).
9.2.3. Broadcast Domain
A separate Broadcast Domain MUST be maintained for each VPLS.
In addition to VPLS Broadcast Domains, an L2VPN service MAY honor
customer VLAN Broadcast Domains, if customer VLANs are used as
service delimiters. In that case, the L2VPN solution SHOULD maintain
a separate VLAN Broadcast Domain for each customer VLAN.
9.2.4. Virtual Switching Instance
L2VPN PE devices MUST maintain a separate VSI per VPLS. Each VSI
MUST have capabilities to forward traffic based on customer's traffic
parameters, such as MAC addresses, VLAN tags (if supported), etc. as
well as local policies.
L2VPN PE devices MUST have capabilities to classify incoming customer
traffic into the appropriate VSI.
Each VSI MUST have flooding capabilities for its Broadcast Domain to
facilitate proper forwarding of Broadcast, Multicast, and Unknown
Unicast customer traffic.
9.2.5. MAC Address Learning
A VPLS SHOULD derive all topology and forwarding information from
packets originating at customer sites. Typically, MAC address
learning mechanisms are used for this purpose. With IPLS, snooping
of particular packets originating at customer sites and signaling
might also be used.
Dynamic population of the forwarding information base (e.g., via MAC
address learning) MUST take place on a per VSI basis; i.e., in the
context of a VPLS and, if supported, in the context of VLANs therein.
10. Security Considerations
Security considerations occur at several levels and dimensions within
L2VPNs, as detailed within this document.
The requirements based on security concerns and potential security
hazards are detailed in Section 6.5. Further details on security
requirements are given from the customer and service provider
perspectives in Sections 6.5 and 7.6, respectively. In an analogous
manner, further detail on traffic and routing isolation requirements
are given from the customer and service provider perspectives in
Sections 5.4 and 7.5, respectively. Safeguards to protect network
resources such as CPU, memory, and bandwidth are required in Section
7.12.
IPsec can also be applied after tunneling Layer 2 traffic to provide
additional security.
In the case where an L2VPN service is carried over IP [RFC4023],
traverses multiple SP networks and passes through an unsecured SP,
POP, NAP, or IX, then security mechanisms MUST be employed. These
security mechanisms include encryption, authentication, and resource
protection, as described in section 5.5. For example, a provider
should consider using both authentication and encryption for a tunnel
used as part of an L2VPN that traverses another service provider's
network.
11. Acknowledgements
The authors would like to acknowledge extensive comments and
contributions provided by Loa Andersson, Joel Halpern, Eric Rosen,
Ali Sajassi, Muneyoshi Suzuki, Ananth Nagarajan, Dinesh Mohan, Yakov
Rekhter, Matt Squire, Norm Finn, Scott Bradner, and Francois Le
Faucheur. The authors also wish to extend their appreciation to
their respective employers and various other people who volunteered
to review this work and provided feedback. This work was done in
consultation with the entire Layer 2 PPVPN design team. A lot of the
text was adapted from the Layer 3 VPN requirements document produced
by the Layer 3 VPN requirements design team.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4026] Andersson, L. and T. Madsen, "Provider Provisioned
Virtual Private Network (VPN) Terminology", RFC 4026,
March 2005.
12.2. Informative References
[VPLS_LDP] Lasserre, M., Kompella, V. "Virtual Private LAN
Services over MPLS", Work in Progress.
[VPLS_BGP] Kompella, K., Rekhter, Y. "Virtual Private LAN
Service", Work in Progress.
[IPLS] Shah, H., et al. "IP-Only LAN Service (IPLS)", Work
in Progress.
[IEEE_802.1Q] IEEE Std 802.1Q-1998, "Virtual Bridged Local Area
Networks", 1998
[RFC2205] Braden, R., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) --
Version 1 Functional Specification", RFC 2205,
September 1997.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
December 1998.
[RFC2685] Fox, B. and B. Gleeson, "Virtual Private Networks
Identifier", RFC 2685, September 1999.
[RFC3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S.,
Vaananen, P., Krishnan, R., Cheval, P., and J.
Heinanen, "Multi-Protocol Label Switching (MPLS)
Support of Differentiated Services", RFC 3270, May
2002.
[RFC3308] Calhoun, P., Luo, W., McPherson, D., and K. Peirce,
"Layer Two Tunneling Protocol (L2TP) Differentiated
Services Extension", RFC 3308, November 2002.
[RFC3809] Nagarajan, A., "Generic Requirements for Provider
Provisioned Virtual Private Networks (PPVPN)", RFC
3809, June 2004.
[RFC3985] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-
to-Edge (PWE3) Architecture", RFC 3985, March 2005.
[RFC4023] Worster, T., Rekhter, Y., and E. Rosen,
"Encapsulating MPLS in IP or Generic Routing
Encapsulation (GRE)", RFC 4023, March 2005.
[RFC4031] Carugi, M. and D. McDysan, "Service Requirements for
Layer 3 Provider Provisioned Virtual Private Networks
(PPVPNs)", RFC 4031, April 2005.
[RFC4664] Andersson, L. and E. Rosen, "Framework for Layer 2
Virtual Private Networks (L2VPNs)", RFC 4664,
September 2006.
[IEEE_802.1D] ISO/IEC 15802-3: 1998 ANSI/IEEE Std 802.1D, 1998
Edition (Revision and redesignation of ISO/IEC
10038:98), "Part 3: Media Access Control (MAC)
Bridges", 1998.
[ITU_Y.1311.1] Carugi, M. (editor), "Network Based IP VPN over MPLS
architecture",Y.1311.1 ITU-T Recommendation, May
2001.
[IEEE_802.10] IEEE Std 802.10-1998 Edition (Revision IEEE Std
802.10-1992, incorporating IEEE Std 802.10b-1992,
802.10e-1993, 802.10f-1993, 802.10g-1995, and
802.10h-1997), "Standard for Interoperable LAN/MAN
Security (SILS)", 1998.
[IEEE_802.1AE] IEEE 802.1AE/D5.1, "Draft Standard for Local and
Metropolitan Area Networks - Media Access Control
(MAC) Security", P802.1AE/D5.1, January 19, 2006.
[IEEE_802.1s] IEEE Std 802.1s-2002, "Virtual Bridged Local Area
Networks-Amendment 3: Multiple Spanning Trees", 2002.
Editors' Addresses
Waldemar Augustyn
EMail: waldemar@wdmsys.com
Yetik Serbest
AT&T Labs
9505 Arboretum Blvd.
Austin, TX 78759
EMail: yetik_serbest@labs.att.com
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