Internet DRAFT - draft-kompella-l2vpn-l2vpn
draft-kompella-l2vpn-l2vpn
Network Working Group K. Kompella
Internet-Draft Juniper Networks
Intended status: Informational B. Kothari
Expires: August 30, 2012 Cisco Systems
R. Cherukuri
Juniper Networks
February 27, 2012
Layer 2 Virtual Private Networks Using BGP for Auto-discovery and
Signaling
draft-kompella-l2vpn-l2vpn-10.txt
Abstract
Layer 2 Virtual Private Networks (L2VPNs) based on Frame Relay or ATM
circuits have been around a long time; more recently, Ethernet VPNs,
including Virtual Private LAN Service, have become popular.
Traditional L2VPNs often required a separate Service Provider
infrastructure for each type, and yet another for the Internet and IP
VPNs. In addition, L2VPN provisioning was cumbersome. This document
presents a new approach to the problem of offering L2VPN services
where the L2VPN customer's experience is virtually identical to that
offered by traditional Layer 2 VPNs, but such that a Service Provider
can maintain a single network for L2VPNs, IP VPNs and the Internet,
as well as a common provisioning methodology for all services.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 30, 2012.
Copyright Notice
Copyright (c) 2012 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
1.2. Advantages of Layer 2 VPNs . . . . . . . . . . . . . . . . 6
1.2.1. Separation of Administrative Responsibilities . . . . 6
1.2.2. Migrating from Traditional Layer 2 VPNs . . . . . . . 7
1.2.3. Privacy of Routing . . . . . . . . . . . . . . . . . . 7
1.2.4. Layer 3 Independence . . . . . . . . . . . . . . . . . 7
1.2.5. PE Scaling . . . . . . . . . . . . . . . . . . . . . . 8
1.2.6. Ease of Configuration . . . . . . . . . . . . . . . . 8
1.3. Advantages of Layer 3 VPNs . . . . . . . . . . . . . . . . 9
1.3.1. Layer 2 Independence . . . . . . . . . . . . . . . . . 9
1.3.2. SP Routing as Added Value . . . . . . . . . . . . . . 10
1.3.3. Class-of-Service . . . . . . . . . . . . . . . . . . . 10
1.4. Multicast Routing . . . . . . . . . . . . . . . . . . . . 10
1.5. Conventions used in this document . . . . . . . . . . . . 11
2. Operation of a Layer 2 VPN . . . . . . . . . . . . . . . . . . 12
2.1. Network Topology . . . . . . . . . . . . . . . . . . . . . 12
2.2. Configuration . . . . . . . . . . . . . . . . . . . . . . 13
2.2.1. CE Configuration . . . . . . . . . . . . . . . . . . . 14
2.2.2. PE Configuration . . . . . . . . . . . . . . . . . . . 15
2.2.3. Adding a New Site . . . . . . . . . . . . . . . . . . 15
2.2.4. Deleting a Site . . . . . . . . . . . . . . . . . . . 15
2.2.5. Managing CE ID Mappings . . . . . . . . . . . . . . . 16
2.2.6. Managing Label Blocks . . . . . . . . . . . . . . . . 16
2.3. Operations, Administration and Maintenance (OAM) . . . . . 17
3. PE Information Exchange . . . . . . . . . . . . . . . . . . . 18
3.1. Circuit Status Vector . . . . . . . . . . . . . . . . . . 20
3.2. Generalizing the VPN Topology . . . . . . . . . . . . . . 21
4. Layer 2 Interworking . . . . . . . . . . . . . . . . . . . . . 22
5. Packet Transport . . . . . . . . . . . . . . . . . . . . . . . 23
5.1. Layer 2 MTU . . . . . . . . . . . . . . . . . . . . . . . 23
5.2. Layer 2 Frame Format . . . . . . . . . . . . . . . . . . . 23
5.3. IP-only Layer 2 Interworking . . . . . . . . . . . . . . . 24
6. Security Considerations . . . . . . . . . . . . . . . . . . . 25
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 26
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 27
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
10.1. Normative References . . . . . . . . . . . . . . . . . . . 29
10.2. Informative References . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31
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1. Introduction
The earliest Virtual Private Networks (VPNs) were based on Layer 2
circuits: X.25, Frame Relay and ATM (see [Kosiur]). More recently,
multipoint VPNs based on Ethernet Virtual Local Area Networks (VLANs)
and Virtual Private LAN Service (VPLS) ([RFC4761] and [RFC4762]) have
become quite popular. In contrast, the VPNs described in this
document are point-to-point, and usually called Virtual Private Wire
Service (VPWS). All of these come under the classification of Layer
2 VPNs (L2VPNs), as the customer to Service Provider (SP) hand-off is
at Layer 2.
There are at least two factors that adversely affected the cost of
offering L2VPNs. The first is that the easiest way to offer a L2VPN
of a given type of Layer 2 was over an infrastructure of the same
type. This approach required that the Service Provider build a
separate infrastructure for each Layer 2 encapsulation -- e.g., an
ATM infrastructure for ATM VPNs, an Ethernet infrastructure for
Ethernet VPNs, etc. In addition, a separate infrastructure was
needed for the Internet and IP VPNs ([RFC4364], and possibly yet
another for voice services. Going down this path meant a
proliferation of networks.
The other is that each of these networks had different provisioning
methodologies. Furthermore, the provisioning of a L2VPN was fairly
complex. It is important to distinguish between a single Layer 2
circuit, which connects two customer sites, and a Layer 2 VPN, which
is a set of circuits that connect sites belonging to the same
customer. The fact that two different circuits belonged to the same
VPN was typically known only to the provisioning system, not to the
switches offering the service; this complicated the setting up, and
subsequently, the troubleshooting, of a L2VPN. Also, each switch
offering the service had to be provisioned with the address of every
other switch in the same VPN, requiring, in the case of full-mesh VPN
connectivity, provisioning proportional to the square of the number
of sites. This made full-mesh L2VPN connectivity prohibitively
expensive for the SP, and thus in turn for customers. Finally, even
setting up a individual circuit often required the provisioning of
every switch along the path.
Of late, there has been much progress in network "convergence",
whereby Layer 2 traffic, Internet traffic and IP VPN traffic can be
carried over a single, consolidated network infrastructure based on
IP/MPLS tunnels; this is made possible by techniques such as those
described in [RFC4448], [RFC4618], [RFC4619], and [RFC4717] for Layer
2 traffic, and [RFC4364] for IP VPN traffic. This development goes a
long way toward addressing the problem of network profileration.
This document goes one step further and shows how a Service Provider
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can offer Layer 2 VPNs using protocol and provisioning methodologies
similar to that used for VPLS ([RFC4761]) and IP VPNs ([RFC4364]),
thereby achieving a significant degree of operational convergence as
well. In particular, all of these methodologies include the notion
of a VPN identifier that serves to unify components of a given VPN,
and the concept of auto-discovery, which simplifies the provisioning
of dense VPN topologies (for example, a full mesh). In addition,
similar techniques are used in all of the above-mentioned VPN
technologies to offer inter-AS and inter-provider VPNs (i.e., VPNs
whose sites are connected to multiple Autonomous Systems (ASs) or
service providers).
Technically, the approach proposed here uses the concepts and
solution and described in [RFC4761], which describes a method for
VPLS, a particular form of a Layer 2 VPN. That document in turn
borrowed much from [RFC4364]. This includes the use of BGP for auto-
discovery and "demultiplexor" (see below) exchange, and the concepts
of Route Distinguishers to make VPN advertisements unique, and Route
Targets to control VPN topology. In addition, all three documents
share the idea that routers not directly connected to VPN customers
should carry no VPN state, restricting the provisioning of individual
connections to just the edge devices. This is achieved by using
tunnels to carry the data, with a demultiplexor that identifies
individual VPN circuits. These tunnels could be based on MPLS, GRE,
or any other tunnel technology that offers a demultiplexing field;
the signaling of these tunnels is outside the scope of this document.
The specific approach taken here is to use an MPLS label as the
demultiplexor.
Layer 2 VPNs typically require that all sites in the VPN connect to
the SP with the same Layer 2 encapsulation. To ease this
restriction, this document proposes a limited form of Layer 2
interworking, by restricting the Layer 3 protocol to IP only (see
Section 5).
It may be instructive to compare the approach in [RFC4447] and
[RFC6074] (these are the IETF-approved technologies for the functions
described in this document, albeit using two separate protocols) with
the one described here. To comply with IETF standards, it is
recommended that devices implementing the solution described in this
document also implement the approach in [RFC4447] and [RFC6074].
The rest of this section discusses the relative merits of Layer 2 and
Layer 3 VPNs. Section 3 describes the operation of a Layer 2 VPN.
Section 5 describes IP-only Layer 2 interworking. Section 6
describes how the L2 packets are transported across the SP network.
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1.1. Terminology
The terminology used is from [RFC4761] and [RFC4364], and is briefly
repeated here. A "customer" is a customer of a Service Provider
seeking to interconnect their various "sites" (each an independent
network) at Layer 2 through the Service Provider's network, while
maintaining privacy of communication and address space. The device
in a customer site that connects to a Service Provider router is
termed the CE (customer edge) device; this device may be a router or
a switch. The Service Provider router to which a CE connects is
termed a PE (provider edge). A router in the Service Provider's
network which doesn't connect directly to any CE is termed P
("provider" device). Every pair of PEs is connected by a "tunnel";
within a tunnel, VPN data is distinguished by a "demultiplexor",
which in this document is an MPLS label.
Each CE within a VPN is assigned a CE ID, a number that uniquely
identifies a CE within an L2 VPN. More accurately, the CE ID
identifies a physical connection from the CE device to the PE, since
a CE may be connected to multiple PEs (or multiply connected to a
PE); in such a case, the CE would have a CE ID for each connection.
A CE may also be part of many L2 VPNs; it would need one (or more) CE
ID(s) for each L2 VPN of which it is a member. The number space for
CE IDs is scoped to a given VPN.
In the case of inter-Provider L2 VPNs, there needs to be some
coordination of allocation of CE IDs. One solution is to allocate
ranges for each SP. Other solutions may be forthcoming.
Within each physical connection from a CE to a PE, there may be
multiple virtual circuits. These will be referred to as Attachment
Circuits (ACs), following [RFC3985]. Similarly, the entity that
connects two attachment circuits across the Service Provider network
is called a pseudowire (PW).
1.2. Advantages of Layer 2 VPNs
A Layer 2 VPN is one where a Service Provider provides Layer 2
connectivity to the customer. The Service Provider does not
participate in the customer's Layer 3 network, in particular, in the
routing, resulting in several advantages to the SP as a whole and to
PE routers in particular.
1.2.1. Separation of Administrative Responsibilities
In a Layer 2 VPN, the Service Provider is responsible for Layer 2
connectivity; the customer is responsible for Layer 3 connectivity,
which includes routing. If the customer says that host x in site A
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cannot reach host y in site B, the Service Provider need only
demonstrate that site A is connected to site B. The details of how
routes for host y reach host x are the customer's responsibility.
Another important factor is that once a PE provides Layer 2
connectivity to its connected CE, its job is done. A misbehaving CE
can at worst flap its interface, but route flaps in the customer
network have little effect on the SP network. On the other hand, a
misbehaving CE in a Layer 3 VPN can flap its routes, leading to
instability of the PE router or even the entire SP network. Thus,
when offering a Layer 3 VPN, a SP should proactively protect itself
from Layer 3 instability in the CE network.
1.2.2. Migrating from Traditional Layer 2 VPNs
Since "traditional" Layer 2 VPNs (i.e., real Frame Relay circuits
connecting sites) are indistinguishable from tunnel-based VPNs from
the customer's point-of-view, migrating from one to the other raises
few issues. Layer 3 VPNs, on the other hand, require a considerable
re-design of the customer's Layer 3 routing architecture.
Furthermore, with Layer 3 VPNs, special care has to be taken that
routes within the traditional VPN are not preferred over the Layer 3
VPN routes (the so-called "backdoor routing" problem, whose solution
requires protocol changes that are somewhat ad hoc).
1.2.3. Privacy of Routing
In a Layer 2 VPN, the privacy of customer routing is a natural
fallout of the fact that the Service Provider does not participate in
routing. The SP routers need not do anything special to keep
customer routes separate from other customers or from the Internet;
there is no need for per-VPN routing tables, and the additional
complexity this imposes on PE routers.
1.2.4. Layer 3 Independence
Since the Service Provider simply provides Layer 2 connectivity, the
customer can run any Layer 3 protocols they choose. If the SP were
participating in customer routing, it would be vital that the
customer and SP both use the same Layer 3 protocol(s) and routing
protocols.
Note that IP-only Layer 2 interworking doesn't have this benefit as
it restricts the Layer 3 to IP only.
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1.2.5. PE Scaling
In the Layer 2 VPN scheme described below, each PE transmits a single
small chunk of information about every CE that the PE is connected to
to every other PE. That means that each PE need only maintain a
single chunk of information from each CE in each VPN, and keep a
single "route" to every site in every VPN. This means that both the
Forwarding Information Base and the Routing Information Base scale
well with the number of sites and number of VPNs. Furthermore, the
scaling properties are independent of the customer: the only germane
quantity is the total number of VPN sites.
This is to be contrasted with Layer 3 VPNs, where each CE in a VPN
may have an arbitrary number of routes that need to be carried by the
SP. This leads to two issues. First, both the information stored at
each PE and the number of routes installed by the PE for a CE in a
VPN can be (in principle) unbounded, which means in practice that a
PE must restrict itself to installing routes associated with the VPNs
that it is currently a member of. Second, a CE can send a large
number of routes to its PE, which means that the PE must protect
itself against such a condition. Thus, the SP must enforce limits on
the number of routes accepted from a CE; this in turn requires the PE
router to offer such control.
The scaling issues of Layer 3 VPNs come into sharp focus at a BGP
route reflector (RR). An RR cannot keep all the advertised routes in
every VPN since the number of routes will be too large. The
following solutions/extensions are needed to address this issue:
1. RRs could be partitioned so that each RR services a subset of
VPNs so that no single RR has to carry all the routes.
2. An RR could use a preconfigured list of Route-Targets for its
inbound route filtering. The RR may choose to perform Route Target
Filtering, described in [RFC4684].
1.2.6. Ease of Configuration
Configuring traditional Layer 2 VPNs with dense topologies was a
burden primarily because of the O(n*n) nature of the task. If there
are n CEs in a Frame Relay VPN, say full-mesh connected, n*(n-1)/2
DLCI (Data Link Connection Identifier) PVCs must be provisioned
across the SP network. At each CE, (n-1) DLCIs must be configured to
reach each of the other CEs. Furthermore, when a new CE is added, n
new DLCI PVCs must be provisioned; also, each existing CE must be
updated with a new DLCI to reach the new CE. Finally, each PVC
requires state in every transit switch.
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In our proposal, PVCs are tunnelled across the SP network. The
tunnels used are provisioned independently of the L2VPNs, using
signalling protocols (in case of MPLS, LDP or RSVP-TE can be used),
or set up by configuration; and the number of tunnels is independent
of the number of L2VPNs. This reduces a large part of the
provisioning burden.
Furthermore, we assume that DLCIs at the CE edge are relatively
cheap; and VPN labels in the SP network are cheap. This allows the
SP to "over-provision" VPNs: for example, allocate 50 CEs to a VPN
when only 20 are needed. With this over-provisioning, adding a new
CE to a VPN requires configuring just the new CE and its associated
PE; existing CEs and their PEs need not be re-configured. Note that
if DLCIs at the CE edge are expensive, e.g. if these DLCIs are
provisioned across a switched network, one could provision them as
and when needed, at the expense of extra configuration. This need
not still result in extra state in the SP network, i.e. an
intelligent implementation can allow overprovisioning of the pool of
VPN labels.
1.3. Advantages of Layer 3 VPNs
Layer 3 VPNs ([RFC4364] in particular) offer a good solution when the
customer traffic is wholly IP, customer routing is reasonably simple,
and the customer sites connect to the SP with a variety of Layer 2
technologies.
1.3.1. Layer 2 Independence
One major restriction in a Layer 2 VPN is that the Layer 2 media with
which the various sites of a single VPN connect to the SP must be
uniform. On the other hand, the various sites of a Layer 3 VPN can
connect to the SP with any supported media; for example, some sites
may connect with Frame Relay circuits, and others with Ethernet.
This restriction of Layer 2 VPN is alleviated by the IP-only Layer 2
interworking proposed in this document. This comes at the cost of
losing the Layer 3 independence.
A corollary to this is that the number of sites that can be in a
Layer 2 VPN is determined by the number of Layer 2 circuits that the
Layer 2 technology provides. For example, if the Layer 2 technology
is Frame Relay with 2-octet DLCIs, a CE can connect to at most about
a thousand other CEs in a VPN.
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1.3.2. SP Routing as Added Value
Another problem with Layer 2 VPNs is that the CE router in a VPN must
be able to deal with having N routing peers, where N is the number of
sites in the VPN. This can be alleviated by manipulating the
topology of the VPN. For example, a hub-and-spoke VPN architecture
means that only one CE router (the hub) needs to deal with N
neighbors. However, in a Layer 3 VPN, a CE router need only deal
with one neighbor, the PE router. Thus, the SP can offer Layer 3
VPNs as a value-added service to its customers.
Moreover, with Layer 2 VPNs it is up to a customer to build and
operate the whole network. With Layer 3 VPNs, a customer is just
responsible for building and operating routing within each site,
which is likely to be much simpler than building and operating
routing for the whole VPN. That, in turn, makes Layer 3 VPNs more
suitable for customers who don't have sufficient routing expertise,
again allowing the SP to provide added value.
As mentioned later, multicast routing and forwarding is another
value-added service that an SP can offer.
1.3.3. Class-of-Service
Class-of-Service issues have been addressed for Layer 3 VPNs. Since
the PE router has visibility into the network Layer (IP), the PE
router can take on the tasks of CoS classification and routing. This
restriction on Layer 2 VPNs is again eased in the case of IP-only
Layer 2 interworking, as the PE router has visibility into the
network Layer (IP).
1.4. Multicast Routing
There are two aspects to multicast routing that we will consider. On
the protocol front, supporting IP multicast in a Layer 3 VPN requires
PE routers to participate in the multicast routing instance of the
customer, and thus keep some related state information.
In the Layer 2 VPN case, the CE routers run native multicast routing
directly. The SP network just provides pipes to connect the CE
routers; PEs are unaware whether the CEs run multicast or not, and
thus do not have to participate in multicast protocols or keep
multicast state information.
On the forwarding front, in a Layer 3 VPN, CE routers do not
replicate multicast packets; thus, the CE-PE link carries only one
copy of a multicast packet. Whether replication occurs at the
ingress PE, or somewhere within the SP network depends on the
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sophistication of the Layer 3 VPN multicast solution. The simple
solution where a PE replicates packets for each of its CEs may place
considerable burden on the PE. More complex solutions may require
VPN multicast state in the SP network, but may significantly reduce
the traffic in the SP network by delaying packet replication until
needed.
In a Layer 2 VPN, packet replication occurs at the CE. This has the
advantage of distributing the burden of replication among the CEs
rather than focusing it on the PE to which they are attached, and
thus will scale better. However, the CE-PE link will need to carry
the multiple copies of multicast packets. In the case of Virtual
Private LAN Service (a specific type of L2 VPN; see [RFC4761]),
however, the CE-PE link need transport only one copy of a multicast
packet.
Thus, just as in the case of unicast routing, the SP has the choice
to offer a value-added service (multicast routing and forwarding) at
some cost (multicast state and packet replication) using a Layer 3
VPN, or to keep it simple and use a Layer 2 VPN.
1.5. 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].
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2. Operation of a Layer 2 VPN
The following simple example of a customer with 4 sites connected to
3 PE routers in a Service Provider network will hopefully illustrate
the various aspects of the operation of a Layer 2 VPN. For
simplicity, we assume that a full-mesh topology is desired.
In what follows, Frame Relay serves as the Layer 2 media, and each CE
has multiple DLCIs to its PE, each to connect to another CE in the
VPN. If the Layer 2 media were ATM, then each CE would have multiple
VPI/VCIs (Virtual Path Identifier/Virtual Channel Identifier) to
connect to other CEs. For PPP and Cisco HDLC, each CE would have
multiple physical interfaces to connect to other CEs. In the case of
IP-only Layer 2 interworking, each CE could have a mix of one or more
of the above Layer 2 media to connect to other CEs.
2.1. Network Topology
Consider a Service Provider network with edge routers PE0, PE1, and
PE2. Assume that PE0 and PE1 are IGP neighbors, and PE2 is more than
one hop away from PE0.
Suppose that a customer C has 4 sites S0, S1, S2 and S3 that C wants
to connect via the Service Provider's network using Frame Relay.
Site S0 has CE0 and CE1 both connected to PE0. Site S1 has CE2
connected to PE0. Site S2 has CE3 connected to PE1 and CE4 connected
to PE2. Site S3 has CE5 connected to PE2. (See Figure 1 below.)
Suppose further that C wants to "over-provision" each current site,
in expectation that the number of sites will grow to at least 10 in
the near future. However, CE4 is only provisioned with 9 DLCIs.
(Note that the signalling mechanism discussed in Section 4 will allow
a site to grow in terms of connectivity to other sites at a later
point of time at the cost of additional signalling, i.e., over-
provisioning is not a must but a recommendation).
Suppose finally that CE0 and CE2 have DLCIs 100 through 109
provisioned; CE1 and CE3 have DLCIs 200 through 209 provisioned; CE4
has DLCIs 107, 209, 265, 301, 414, 555, 654, 777 and 888 provisioned;
and CE5 has DLCIs 417-426.
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S0 S3
.............. ..............
. . . .
. +-----+ . . .
. | CE0 |-----------+ . +-----+ .
. +-----+ . | . | CE5 | .
. . | . +--+--+ .
. +-----+ . | . | .
. | CE1 |-------+ | .......|......
. +-----+ . | | /
. . | | /
.............. | | /
| | SP Network /
.....|...|.............................../.....
. | | / .
. +-+---+-+ +-------+ / .
. | PE0 |-------| P |-- | .
. +-+---+-+ +-------+ \ | .
. / \ \ +---+---+ .
. | -----+ --| PE2 | .
. | | +---+---+ .
. | +---+---+ / .
. | | PE1 | / .
. | +---+---+ / .
. | \ / .
...|.............|.............../.............
| | /
| | /
| | /
S1 | | S2 /
.............. | ........|........../......
. . | . | | .
. +-----+ . | . +--+--+ +--+--+ .
. | CE2 |-----+ . | CE3 | | CE4 | .
. +-----+ . . +-----+ +-----+ .
. . . .
.............. ..........................
Figure 1: Example Network Topology
2.2. Configuration
The following sub-sections detail the configuration that is needed to
provision the above VPN. For the purpose of exposition, we assume
that the customer will connect to the SP with Frame Relay circuits.
While we focus primarily on the configuration that an SP has to do,
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we touch upon the configuration requirements of CEs as well. The
main point of contact in CE-PE configuration is that both must agree
on the DLCIs that will be used on the interface connecting them.
If the PE-CE connection is Frame Relay, it is recommended to run LMI
between the PE and CE. For the case of ATM VCs, OAM cells may be
used; for PPP and Cisco HDLC, keepalives may be used directly between
CEs; however, in this case, PEs would not have visibility as to the
liveness of customers circuits.
In case of IP-only Layer 2 interworking, if CE1, attached to PE0,
connects to CE3, attached to PE1, via a L2VPN circuit, the Layer 2
media between CE1 and PE0 is independent of the Layer 2 media between
CE3 and PE1. Each side will run its own Layer 2 specific link
management protocol, e.g., LMI, LCP, etc. PE0 will inform PE1 about
the status of its local circuit to CE1 via the circuit status vector
TLV defined in Section 4. Similarly PE1 will inform PE0 about the
status of its local circuit to CE3.
2.2.1. CE Configuration
Each CE that belongs to a VPN is given a "CE ID". CE IDs must be
unique in the context of a VPN. For the example, we assume that the
CE ID for CE-k is k.
Each CE is configured to communicate with its corresponding PE with
the set of DLCIs given above; for example, CE0 is configured with
DLCIs 100 through 109. In general, a CE is configured with a list of
circuits, all with the same Layer 2 encapsulation type, e.g., DLCIs,
VCIs, physical PPP interface etc. (IP-only Layer 2 interworking
allows a mix of Layer 2 encapsulation types). The size of this list/
set determines the number of remote CEs a given CE can communicate
with. Denote the size of this list/set as the CE's range. A CE's
range must be at least the number of remote CEs that the CE will
connect to in a given VPN; if the range exceeds this, then the CE is
over-provisioned, in anticipation of growth of the VPN.
Each CE also "knows" which DLCI connects it to each other CE. The
methodology followed in this example is to use the CE ID of the other
CE as an index into the DLCI list this CE has (with zero-based
indexing, i.e., 0 is the first index). For example, CE0 is connected
to CE3 through its fourth DLCI, 103; CE4 is connected to CE2 by the
third DLCI in its list, namely 265. This is just the methodology
used in the example below; the actual methodology used to pick the
DLCI to be used is a local matter; the key factor is that CE-k may
communicate with CE-m using a different DLCI from the DLCI that CE-m
uses to communicate to CE-k, i.e., the SP network effectively acts as
a giant Frame Relay switch. This is very important, as it decouples
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the DLCIs used at each CE site, making for much simpler provisioning.
2.2.2. PE Configuration
Each PE is configured with the VPNs in which it participates. Each
VPN is associated with one or more Route Target communities [RFC4360]
which serve to define the topology of the VPN. For each VPN, the PE
must determine a Route Distinguisher (RD) to use; this may either be
configured or chosen by the PE. RDs do not have to be unique across
the VPN. For each CE attached to the PE in a given VPN, the PE must
know the set of virtual circuits (DLCI, VCI/VPI or VLAN) connecting
it to the CE, and a CE ID identifying the CE within the VPN. CE IDs
must be unique in the context of a given VPN.
2.2.3. Adding a New Site
The first step in adding a new site to a VPN is to pick a new CE ID.
If all current members of the VPN are over-provisioned, i.e., their
range includes the new CE ID, adding the new site is a purely local
task. Otherwise, the sites whose range doesn't include the new CE ID
and wish to communicate directly with the new CE must have their
ranges increased by allocating additional local circuits to
incorporate the new CE ID.
The next step is ensuring that the new site has the required
connectivity. This usually requires adding a new virtual circuit
between the PE and CE; in most cases, this configuration is limited
to the PE in question.
The rest of the configuration is a local matter between the new CE
and the PE to which it is attached. At this point, the PE can signal
to other PEs that it has a new site in the VPN, by advertising a BGP
Layer 2 route, and traffic connectivity will be set up.
It bears repeating that the key to making additions easy is over-
provisioning and the algorithm for mapping a CE ID to a DLCI which is
used for connecting to the corresponding CE. However, what is being
over-provisioned is the number of DLCIs/VCIs that connect the CE to
the PE. This is a local matter between the PE and CE, and does not
affect other PEs or CEs.
2.2.4. Deleting a Site
Deleting a site consists first of removing the CE ID of the site from
the configuration of the PE to which the site is attached. The PE
will then signal to other PEs that it no longer has access to that
site, by withdrawing its previously advertised BGP Layer 2 route.
Connectivity to the deleted site will cease.
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The next steps are book-keeping: decommissioning the attachment
circuit from the PE to the CE that corresponds to the site being
removed, and noting that the CE ID is now free for future allocation.
Note that each PE is now (further) over-provisioned; one may choose
to actively "reap" CE IDs if desired.
2.2.5. Managing CE ID Mappings
In the data plane, an attachment circuit, identified say by a DLCI,
is mapped to a label via the control plane abstraction of a CE ID.
At the egress PE, the label is mapped back to an attachment circuit
via the same CE ID. It is up to the VPN administrator
to provision attachment circuits (e.g., DLCIs);
to allocate CE IDs; and
to keep a clear mapping of CE IDs to attachment circuits (and
reflect this in PE configurations).
The PEs manage the mappings between attachment circuits and labels,
i.e., the data plane mappings.
Note that in the N-to-one modes listed in Table 1, a single
attachment circuit may correspond to several Layer 2 virtual
circuits. Nevertheless, there is a one-to-one mapping between an
attachment circuit and a CE ID (and thus a label).
2.2.6. Managing Label Blocks
Label blocks and label values are managed by the PEs. As sites get
added and removed, labels are allocated and released. The easiest
way to manage these are using fixed size label blocks rather than
variable sized blocks, although the signaling described here supports
either. If an implementation uses fixed size blocks, then allocating
a label for a new site may requiring allocating a new block;
similarly, freeing a label may require freeing a block.
If the implementation requires fixed size blocks, there is probably a
default block size, but the implementation SHOULD allow the
administrator to choose a size. Larger label block sizes mean more
potential "wasted" labels, but less signaling overhead, a trade-off
that the administrator might want to control.
Also, as sites get added and deleted, a PE may receive packets with a
label that reflects a site that has been deleted locally but remote
PEs haven't processed yet, or that reflects a new site added remotely
but that hasn't been processed locally. In either of these cases,
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the PE SHOULD silently discard the packet; it may choose to log the
event once for each such label, but not for every such packet.
2.3. Operations, Administration and Maintenance (OAM)
Many Layer 2 mediums have OAM mechanisms. For example, the Point-to-
Point Protocol (PPP) has Echo Request and Echo Reply messages; Frame
Relay has the Local Management Interface. Among other things, OAM is
used for trouble-shooting and as keepalives.
There are two ways to carry OAM information across Layer 2 VPNs. The
first is convey OAM packets as any other Layer 2 packets across the
VPN. This is the most general method, and maintains full Layer 2
transparency, and preserves all OAM information. The other method
applies only to the link liveness aspect of OAM; it consists of
transmitting the status of each attachment circuit across the control
plane using the Circuit Status Vector (Section 3.1). This method is
the only one applicable to Layer 2 Interworking VPNs (Section 4),
since OAM packets are not IP frames, and thus cannot be transmitted
across such Layer 2 VPNs.
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3. PE Information Exchange
When a PE is configured with all the required information for a CE,
it advertises to other PEs the fact that it is participating in a VPN
via BGP messages, as per [RFC4761], section 3. BGP was chosen as the
means for exchanging L2 VPN information for two reasons: it offers
mechanisms for both auto-discovery and signaling, and allows for
operational convergence, as explained in Section 1. A bonus for
using BGP is a robust inter-AS solution for L2VPNs.
There are two modifications to the formating of messages. The first
is that the set of Encaps Types carried in the L2-info extended
community has been expanded to include those from Table 1. The value
of the Encaps Type field identifies the Layer 2 encapsulation, e.g.,
ATM, Frame Relay etc.
+-----------+-------------------------------------------+-----------+
| Encaps | Description | Reference |
| Type | | |
+-----------+-------------------------------------------+-----------+
| 0 | Reserved | - |
| | | |
| 1 | Frame Relay | RFC 4446 |
| | | |
| 2 | ATM AAL5 SDU VCC transport | RFC 4446 |
| | | |
| 3 | ATM transparent cell transport | RFC 4816 |
| | | |
| 4 | Ethernet (VLAN) Tagged Mode | RFC 4448 |
| | | |
| 5 | Ethernet Raw Mode | RFC 4448 |
| | | |
| 6 | Cisco HDLC | RFC 4618 |
| | | |
| 7 | PPP | RFC 4618 |
| | | |
| 8 | SONET/SDH Circuit Emulation Service | RFC 4842 |
| | | |
| 9 | ATM n-to-one VCC cell transport | RFC 4717 |
| | | |
| 10 | ATM n-to-one VPC cell transport | RFC 4717 |
| | | |
| 11 | IP Layer 2 Transport | RFC 3032 |
| | | |
| 15 | Frame Relay Port mode | RFC 4619 |
| | | |
| 17 | Structure-agnostic E1 over packet | RFC 4553 |
| | | |
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| 18 | Structure-agnostic T1 (DS1) over packet | RFC 4553 |
| | | |
| 19 | VPLS | RFC 4761 |
| | | |
| 20 | Structure-agnostic T3 (DS3) over packet | RFC 4553 |
| | | |
| 21 | Nx64kbit/s Basic Service using | RFC 5086 |
| | Structure-aware | |
| | | |
| 25 | Frame Relay DLCI | RFC 4619 |
| | | |
| 40 | Structure-agnostic E3 over packet | RFC 4553 |
| | | |
| 41 (1) | Octet-aligned payload for | RFC 4553 |
| | Structure-agnostic DS1 circuits | |
| | | |
| 42 (2) | E1 Nx64kbit/s with CAS using | RFC 5086 |
| | Structure-aware | |
| | | |
| 43 | DS1 (ESF) Nx64kbit/s with CAS using | RFC 5086 |
| | Structure-aware | |
| | | |
| 44 | DS1 (SF) Nx64kbit/s with CAS using | RFC 5086 |
| | Structure-aware | |
+-----------+-------------------------------------------+-----------+
Table 1: Encaps Types
Note (1): Allocation of a separate code point for Encaps Type
eliminates the need for TDM payload size.
Note (2): Having separate code points for Encaps Types 42-44 allows
specifying the trunk framing (i.e, E1, T1 ESF or T1 SF) with CAS.
The second is the introduction of TLVs (Type-Length-Value triplets)
in the VPLS NLRI (Network Layer Reachability Information). L2VPN
TLVs can be added to extend the information carried in the NLRI,
using the format shown in Figure 2. In L2VPN TLVs, Type is 1 octet,
Length is 2 octets and represents the size of the Value field in
bits.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value (continued, if needed) ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Format of TLVs
+----------+-----------------------+
| TLV Type | Description |
+----------+-----------------------+
| 1 | Circuit Status Vector |
+----------+-----------------------+
Table 2: TLV Types
3.1. Circuit Status Vector
This sub-TLV carries the status of a L2VPN PVC between a pair of PEs.
Note that a L2VPN PVC is bidirectional, composed of two simplex
connection going in opposite directions. A simplex connection
consists of the 3 segments: 1) the local access circuit between the
source CE and the ingress PE, 2) the tunnel LSP between the ingress
and egress PEs, and 3) the access circuit between the egress PE and
the destination CE.
To monitor the status of a PVC, a PE needs to monitor the status of
both simplex connections. Since it knows that status of its access
circuit, and the status of the tunnel towards the remote PE, it can
inform the remote PE of these two. Similarly, the remote PE can
inform the status of its access circuit to its local CE and the
status of the tunnel to the first PE. Combining the local and the
remote information, a PE can determine the status of a PVC.
The basic unit of advertisement in L2VPN for a given CE is a label-
block. Each label within a label-block corresponds to a PVC on the
CE. The local status information for all PVCs corresponding to a
label-block is advertised along with the NLRI for the label-block
using the status vector TLV. The Type field of this TLV is 1. The
Length field of the TLV specifies the length of the value field in
bits. The Value field of this TLV is a bit-vector, each bit of which
indicates the status of the PVC associated with the corresponding
label in the label-block. Bit value 0 corresponds to the PVC
associated with the first label in the label block, and indicates
that the local circuit and the tunnel LSP to the remote PE is up,
while a value of 1 indicates that either or both of them are down.
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The Value field is padded to the nearest octet boundary.
If PE A receives a VPLS NLRI, while selecting a label from a label-
block (advertised by PE B, for remote CE m, and VPN X) for one of its
local CE n (in VPN X) can also determine the status of the
corresponding PVC (between CE n and CE m) by looking at the
appropriate bit in the circuit status vector.
3.2. Generalizing the VPN Topology
In the above, we assumed for simplicity that the VPN was a full mesh.
To allow for more general VPN topologies, a mechanism based on
filtering on BGP extended communities can be used.
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4. Layer 2 Interworking
As defined so far in this document, all CE-PE connections for a given
Layer 2 VPN must use the same Layer 2 encapsulation, e.g., they must
all be Frame Relay. This is often a burdensome restriction. One
answer is to use an existing Layer 2 interworking mechanism, for
example, Frame Relay-ATM interworking.
In this document, we take a different approach: we postulate that the
network Layer is IP, and base Layer 2 interworking on that. Thus,
one can choose between pure Layer 2 VPNs, with a stringent Layer 2
restriction but with Layer 3 independence, or a Layer 2 interworking
VPNs, where there is no restriction on Layer 2, but Layer 3 must be
IP. Of course, a PE may choose to implement Frame Relay-ATM
interworking. For example, an ATM Layer 2 VPN could have some CEs
connect via Frame Relay links, if their PE could translate Frame
Relay to ATM transparent to the rest of the VPN. This would be
private to the CE-PE connection, and such a course is outside the
scope of this document.
For Layer 2 interworking as defined here, when an IP packet arrives
at a PE, its Layer 2 address is noted, then all Layer 2 overhead is
stripped, leaving just the IP packet. Then, a VPN label is added,
and the packet is encapsulated in the PE-PE tunnel (as required by
the tunnel technology). Finally, the packet is forwarded. Note that
the forwarding decision is made on the basis of the Layer 2
information, not the IP header. At the egress, the VPN label
determines to which CE the packet must be sent, and over which
virtual circuit; from this, the egress PE can also determine the
Layer 2 encapsulation to place on the packet once the VPN label is
stripped.
An added benefit of restricting interworking to IP only as the Layer
3 technology is that the provider's network can provide IP Diffserv
or any other IP based QOS mechanism to the L2VPN customer. The
ingress PE can set up IP/TCP/UDP based classifiers to do DiffServ
marking, and other functions like policing and shaping on the L2
circuits of the VPN customer. Note the division of labor: the CE
determines the destination CE, and encodes that in the Layer 2
address. The ingress PE thus determines the egress PE and VPN label
based on the Layer 2 address supplied by the CE, but the ingress PE
can choose the tunnel to reach the egress PE (in the case that there
are different tunnels for each CoS/DiffServ code point), or the CoS
bits to place in the tunnel (in the case where a single tunnel
carries multiple CoS/DiffServ code points) based on its own
classification of the packet.
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5. Packet Transport
When a packet arrives at a PE from a CE in a Layer 2 VPN, the Layer 2
address of the packet identifies to which remote attachment circuit
(and thus remote CE) the packet is destined. The procedure outlined
above installs a route that maps the Layer 2 address to a tunnel
(which identifies the PE to which the destination CE is attached) and
a VPN label (which identifies the destination AC). If the egress PE
is the same as the ingress PE, no tunnel or VPN label is needed.
The packet may then be modified (depending on the Layer 2
encapsulation). In case of IP-only Layer 2 interworking, the Layer 2
header is completely stripped off till the IP header. Then, a VPN
label and tunnel encapsulation are added as specified by the route
described above, and the packet is sent to the egress PE.
If the egress PE is the same as the ingress, the packet "arrives"
with no labels. Otherwise, the packet arrives with the VPN label,
which is used to determine which CE is the destination CE. The
packet is restored to a fully-formed Layer 2 packet, and then sent to
the CE.
5.1. Layer 2 MTU
This document requires that the Layer 2 MTU configured on all the
access circuits connecting CEs to PEs in a L2VPN be the same. This
can be ensured by passing the configured Layer 2 MTU in the Layer2-
info extended community when advertising L2VPN label-blocks. On
receiving L2VPN label-block from remote PEs in a VPN, the MTU value
carried in the Layer2-info extendend community should be compared
against the configured value for the VPN. If they don't match, then
the label-block should be ignored.
The MTU on the Layer 2 access links MUST be chosen such that the size
of the L2 frames plus the L2VPN header does not exceed the MTU of the
SP network. Layer 2 frames that exceed the MTU after encapsulation
MUST be dropped. For the case of IP-only Layer 2 interworking the IP
MTU on the Layer 2 access link must be chosen such that the size of
the IP packet and the L2VPN header does not exceed the MTU of the SP
network.
5.2. Layer 2 Frame Format
The modification to the Layer 2 frame depends on the Layer 2 type.
This document requires that the encapsulation methods used in
transporting of Layer 2 frames over tunnels be the same as described
in [RFC4448], [RFC4618], [RFC4619], and [RFC4717], except in the case
of IP-only Layer 2 Interworking which is described next.
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5.3. IP-only Layer 2 Interworking
+-----------------------------------+
| PSN Transport | VPN | IP | VPN label is the
| Header | Label | Packet | demultiplexing field
+-----------------------------------+
Figure 3: Format of IP-only Layer 2 interworking packet
At the ingress PE, an L2 frame's L2 header is completely stripped off
and is carried over as an IP packet within the SP network (Figure 3).
The forwarding decision is still based on the L2 address of the
incoming L2 frame. At the egress PE, the IP packet is encapsulated
back in an L2 frame and transported over to the destination CE. The
forwarding decision at the egress PE is based on the VPN label as
before. The L2 technology between egress PE and CE is independent of
the L2 technology between ingress PE and CE.
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6. Security Considerations
RFC 4761, on which this document is based, has a detailed discussion
of security considerations. As in RFC 4761, the focus here is the
privacy of customer VPN data (as opposed to confidentiality,
integrity, or authentication of said data); to achieve the latter,
one can use the methods suggested in RFC 4761. The techniques
described in RFC 4761 for securing the control plane and protecting
the forwarding path apply equally to L2 VPNs, as do the remarks
regarding multi-AS operation. The mitigation strategies and the
analogies with [RFC4364] also apply here.
RFC 4761 perhaps should have discussed Denial of Service attacks
based on the fact that VPLS PEs have to learn MAC addresses and
replicate packets (for flooding and multicast). However, those
considerations don't apply here, as neither of those actions are
required of PEs implementing the procedures in this document.
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7. Acknowledgments
The authors would like to thank Chaitanya Kodeboyina, Dennis
Ferguson, Der-Hwa Gan, Dave Katz, Nischal Sheth, John Stewart, and
Paul Traina for the enlightening discussions that helped shape the
ideas presented here, and Ross Callon for his valuable comments.
The idea of using extended communities for more general connectivity
of a Layer 2 VPN was a contribution by Yakov Rekhter, who also gave
many useful comments on the text; many thanks to him.
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8. Contributors
The following contributed to this document.
Manoj Leelanivas, Juniper Networks
Quaizar Vohra, Juniper Networks
Javier Achirica, Consultant
Ronald Bonica, Juniper Networks
Dave Cooper, Global Crossing
Chris Liljenstolpe, Telstra
Eduard Metz, KPN Dutch Telecom
Hamid Ould-Brahim, Nortel
Chandramouli Sargor
Himanshu Shah, Ciena
Vijay Srinivasan
Zhaohui Zhang, Juniper Networks
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9. IANA Considerations
IANA is requested to create two new registries: the first is for the
one-octet Encaps Type field of the L2-info extended community. The
name of the registry is "BGP L2 Encapsulation Types"; the values
already allocated are in Table 1 of Section 3. The allocation policy
for new entries up to and including value 127 is "Expert Review".
The allocation policy for values 128 through 251 is "First Come First
Served". The values from 252 through 255 are for "Experimental Use".
The second registry is for the one-octet Type field of the TLVs of
the VPLS NLRI. The name of the registry is "BGP L2 TLV Types"; the
sole allocated value is in Table 2 of Section 3. The allocation
policy for new entries up to and including value 127 is "Expert
Review". The allocation policy for values 128 through 251 is "First
Come First Served". The values from 252 through 255 are for
"Experimental Use".
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10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", RFC 4360, February 2006.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006.
[RFC4446] Martini, L., "IANA Allocations for Pseudowire Edge to Edge
Emulation (PWE3)", BCP 116, RFC 4446, April 2006.
[RFC4448] Martini, L., Rosen, E., El-Aawar, N., and G. Heron,
"Encapsulation Methods for Transport of Ethernet over MPLS
Networks", RFC 4448, April 2006.
[RFC4618] Martini, L., Rosen, E., Heron, G., and A. Malis,
"Encapsulation Methods for Transport of PPP/High-Level
Data Link Control (HDLC) over MPLS Networks", RFC 4618,
September 2006.
[RFC4619] Martini, L., Kawa, C., and A. Malis, "Encapsulation
Methods for Transport of Frame Relay over Multiprotocol
Label Switching (MPLS) Networks", RFC 4619,
September 2006.
[RFC4717] Martini, L., Jayakumar, J., Bocci, M., El-Aawar, N.,
Brayley, J., and G. Koleyni, "Encapsulation Methods for
Transport of Asynchronous Transfer Mode (ATM) over MPLS
Networks", RFC 4717, December 2006.
[RFC4761] Kompella, K. and Y. Rekhter, "Virtual Private LAN Service
(VPLS) Using BGP for Auto-Discovery and Signaling",
RFC 4761, January 2007.
10.2. Informative References
[RFC3985] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
Edge (PWE3) Architecture", RFC 3985, March 2005.
[RFC4447] Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G.
Heron, "Pseudowire Setup and Maintenance Using the Label
Distribution Protocol (LDP)", RFC 4447, April 2006.
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[RFC4684] Marques, P., Bonica, R., Fang, L., Martini, L., Raszuk,
R., Patel, K., and J. Guichard, "Constrained Route
Distribution for Border Gateway Protocol/MultiProtocol
Label Switching (BGP/MPLS) Internet Protocol (IP) Virtual
Private Networks (VPNs)", RFC 4684, November 2006.
[RFC4762] Lasserre, M. and V. Kompella, "Virtual Private LAN Service
(VPLS) Using Label Distribution Protocol (LDP) Signaling",
RFC 4762, January 2007.
[RFC6074] Rosen, E., Davie, B., Radoaca, V., and W. Luo,
"Provisioning, Auto-Discovery, and Signaling in Layer 2
Virtual Private Networks (L2VPNs)", RFC 6074,
January 2011.
[Kosiur] Kosiur, D., "Building and Managing Virtual Private
Networks", November 1996.
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Authors' Addresses
Kireeti Kompella
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
US
Email: kireeti@juniper.net
Bhupesh Kothari
Cisco Systems
3750 Cisco Way
San Jose, CA 95134
USA
Email: bhupesh@cisco.com
Rao Cherukuri
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
US
Email: cherukuri@juniper.net
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