L3VPN WG Hamid Ould-Brahim
Internet Draft Nortel Networks
Expiration Date: October 2004
Eric C. Rosen
Cisco Systems
Yakov Rekhter
Juniper Networks
(Editors)
April 2004
Using BGP as an Auto-Discovery
Mechanism for Layer-3 and Layer-2 VPNs
draft-ietf-l3vpn-bgpvpn-auto-02.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [RFC-2026].
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
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Internet-Drafts are draft documents valid for a maximum of six
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Abstract
In any Provider Provisioned-Based VPN (PPVPN) scheme, the Provider
Edge (PE) devices attached to a common VPN must exchange certain
information as a prerequisite to establish VPN-specific
connectivity. The purpose of this draft is to define a BGP based
auto-discovery mechanism for both layer-2 VPN architectures and
layer-3 VPNs ([VPN-VR]). This mechanism is based on the approach
used by [RFC2547-bis] for distributing VPN routing information
within the service provider(s). Each VPN scheme uses the mechanism
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to automatically discover the information needed by that particular
scheme.
1. Introduction
In any Provider Provisioned-Based VPN (PPVPN) scheme, the Provider
Edge (PE) devices attached to a common VPN must exchange certain
information as a prerequisite to establish VPN-specific
connectivity. The purpose of this draft is to define a BGP based
auto-discovery mechanism for both layer-2 VPN architectures (i.e.,
[L2VPN-KOMP], [L2VPN-ROSEN]) and layer-3 VPNs ([VPN-VR]). This
mechanism is based on the approach used by [RFC2547-bis]
for distributing VPN routing information within the service
provider(s). Each VPN scheme uses the mechanism to automatically
discover the information needed by that particular scheme.
In [RFC2547-bis] based layer-3 VPNs, VPN-specific routes are
exchanged, along with the information needed to enable a PE to
determine which routes belong to which VRFs. In [VPN-VR], virtual
router (VR) addresses must be exchanged, along with the information
needed to enable the PEs to determine which VRs are in the same VPN
("membership"), and which of those VRs are to have VPN connectivity
("topology"). Once the VRs are reachable through the tunnels, routes
("reachability") are then exchanged by running existing routing
protocols per VPN basis.
The BGP-4 multiprotocol extensions are used to carry various
information about VPNs for both layer-2 and layer-3 VPN
architectures. VPN-specific information associated with the NLRI is
encoded either as attributes of the NLRI, or as part of the NLRI
itself, or both.
2. Provider-Provisioned VPN Reference Model
Both the layer-2 and layer-3 vpns architectures are using a network
reference model as illustrated in figure 1.
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PE PE
+--------------+ +--------------+
+--------+ | +----------+ | | +----------+ | +--------+
| VPN-A | | | VPN-A | | | | VPN-A | | | VPN-A |
| Sites |--| |Database /| | BGP route | | Database/| |-| sites |
+--------+ | |Processing| |<----------->| |Processing| | +--------+
| +----------+ | Distribution| +----------+ |
| | | |
+--------+ | +----------+ | | +----------+ | +--------+
| VPN-B | | | VPN-B | | -------- | | VPN-B | | | VPN-B |
| Sites |--| |Database /| |-(Backbones)-| | Database/| |-| sites |
+--------+ | |Processing| | -------- | |Processing| | +--------+
| +----------+ | | +----------+ |
| | | |
+--------+ | +----------+ | | +----------+ | +--------+
| VPN-C | | | VPN-C | | | | VPN-C | | | VPN-C |
| Sites |--| |Database /| | | | Database/| |-| sites |
+--------+ | |Processing| | | |Processing| | +--------+
| +----------+ | | +----------+ |
+--------------+ +--------------+
Figure 1: Network based VPN Reference Model
It is assumed that the PEs can use BGP to distribute information to
each other. This may be via direct IBGP peering, via direct EBGP
peering, via multihop BGP peering, through intermediaries such as
Route Reflectors, through a chain of intermediate BGP connections,
etc. It is assumed also that the PE knows what architecture it is
supporting.
3. Carrying VPN information in BGP Multi-Protocol (BGP-MP) Attributes
The BGP-4 multiprotocol extensions are used to carry various
information about VPNs for both layer-2 and layer-3 VPN
architectures. VPN-specific information associated with the NLRI is
encoded either as attributes of the NLRI, or as part of the NLRI
itself, or both. The addressing information in the NLRI field is
ALWAYS within the VPN address space, and therefore MUST be unique
within the VPN. The address specified in the BGP next hop attribute,
on the other hand, is in the service provider addressing space. In
L3VPNs, the NLRI contains an address prefix which is within the
VPN address space, and therefore must be unique within the VPN.
3.1 Carrying Layer-3 VPN Information in BGP-MP
This is done as follows. The NLRI is a VPN-IP address or a labeled
VPN-IP address.
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In the case of the virtual router, the NLRI address prefix is an
address of one of the virtual routers configured on the PE. Thus
this mechanism allows the virtual routers to discover each other, to
set up adjacencies and tunnels to each other, etc. In the case of
[RFC2547-bis], the NLRI prefix represents a route to an arbitrary
system or set of systems within the VPN.
3.2 Carrying Layer-2 VPN Information in BGP-MP
The NLRI carries VPN layer-2 addressing information called VPN-L2
address. A VPN-L2 address is composed of a quantity beginning with
an 8 bytes Route Distinguisher (RD) field and a variable length
quantity encoded according to the layer-2 VPN architecture used.
Different layer-2 VPN solutions use the same common AFI, but
different SAFI. The AFI indicates that the NLRI is carrying a VPN-l2
address, while the SAFI indicates solution-specific semantics and
syntax of the VPN-l2 address that goes after the RD. The RD must be
chosen so as it ensures that each NLRI is globally unique (i.e., the
same NLRI does not appear in two VPNs).
BGP Route target extended community is used to constrain route
distribution between PEs. The BGP Next hop carries the service
provider tunnel endpoint address.
This draft doesn't preclude the use of additional extended
communities for encoding specific l2vpn parameters.
4. Interpretation of VPN Information in Layer-3 VPNs
4.1 Interpretation of VPN Information in the [RFC2547-bis] Model
For details see [RFC2547-bis].
4.2 Interpretation of VPN Information in the [VPN-VR] Model
4.2.1 Membership Discovery
The VPN-ID format as defined in [RFC-2685] is used to identify a
VPN. All virtual routers that are members of a specific VPN share
the same VPN-ID. A VPN-ID is carried in the NLRI to make addresses
of VRs globally unique. Making these addresses globally unique is
necessary if one uses BGP for VRs' auto-discovery.
4.2.1.1 Encoding of the VPN-ID in the NLRI
For the virtual router model, the VPN-ID is carried within the route
distinguisher (RD) field. In order to hold the 7-bytes VPN-ID, the
first byte of RD type field is used to indicate the existence of the
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VPN-ID format. A value of 0x80 in the first byte of RD's type field
indicates that the RD field is carrying the VPN-ID format. In this
case, the type field range 0x8000-0x80ff will be reserved for the
virtual router case.
4.2.1.2 VPN-ID Extended Community
A new extended community is used to carry the VPN-ID format. This
attribute is transitive across the Autonomous system boundary. The
type field of the VPN-ID extended community is of regular type to be
assigned by IANA [BGP-COMM]. The remaining 7 bytes hold the VPN-ID
value field as per [RFC-2685]. The BGP UPDATE message will carry
information for a single VPN. It is the VPN-ID Extended Community,
or more precisely route filtering based on the Extended Community
that allows one VR to find out about other VRs in the same VPN.
4.2.2 VPN Topology Information
A new extended community is used to indicate different VPN topology
values. This attribute is transitive across the Autonomous system
boundary. The value of the type field for extended type is assigned
by IANA. The first two bytes of the value field (of the remaining 6
bytes) are reserved. The actual topology values are carried within
the remaining four bytes. The following topology values are defined:
Value Topology Type
1 "Hub"
2 "Spoke"
3 "Mesh"
Arbitrary values can also be used to allow specific topologies to be
constructed. VPN connectivity between two VRs within the same VPN is
achieved if and only if at least one of them is a hub (the other is
a hub or a spoke), or if both VRs are part of a full mesh VPN
topology.
5. Interpretation of VPN Information in Layer-2 VPNs
The interpretation of the VPN information carried in the VPN-L2
address is to be specified as part of each L2VPN solution
standardized by L2VPN working group.
6. Tunnel Discovery
Layer-3 VPNs and Layer-2 VPNs must be implemented through some form
of tunneling mechanism, where the packet formats and/or the
addressing used within the VPN can be unrelated to that used to
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route the tunneled packets across the backbone. There are numerous
tunneling mechanisms that can be used by a network based VPN (e.g.,
IP/IP [RFC-2003], GRE tunnels [RFC-1701], IPSec [RFC-2401], and MPLS
tunnels [RFC-3031]). Each of these tunnels allows for opaque
transport of frames as packet payload across the backbone, with
forwarding disjoint from the address fields of the encapsulated
packets. A provider edge router may terminate multiple type of
tunnels and forward packets between these tunnels and other network
interfaces in different ways.
BGP can be used to carry tunnel endpoint addresses between edge
routers. For scalability purposes, this draft recommends the use of
tunneling mechanisms with demultiplexing capabilities such as IPSec,
MPLS, and GRE (with respect to using GRE -the key field, it is no
different than just MPLS over GRE, however there is no specification
on how to exchange the key field, while there is a specification and
implementations on how to exchange the label). Note that IP in IP
doesn't have demultiplexing capabilities.
The BGP next hop will carry the service provider tunnel endpoint
address. As an example, if IPSec is used as tunneling mechanism, the
IPSec tunnel remote address will be discovered through BGP, and the
actual tunnel establishment is achieved through IPSec signaling
protocol.
When MPLS tunneling is used, the label carried in the NLRI field is
associated with an address of a VR, where the address is carried in
the NLRI and is encoded as a VPN-IP address.
7. Auto-Discovery and VR-[RFC2547-bis] Interworking Scenarios
Two interwoking scenarios are considered when the network is using
both virtual routers and [RFC2547-bis]. The first scenario is a CE-
PE relationship between a PE (implementing [RFC2547-bis]), and a VR
appearing as a CE to the PE. The connection between the VR, and the
PE can be either direct connectivity, or through a tunnel (e.g.,
IPSec).
The second scenario is when a PE is implementing both architectures.
In this particular case, a single BGP session configured on the
service provider network can be used to advertise either [RFC2547-
bis] VPN information or the virtual router related VPN information.
From the VR and the [RFC2547-bis] point of view there is complete
separation from data path and addressing schemes. However the PE's
interfaces are shared between both architectures.
A PE implementing only [RFC2547-bis] will not import routes from a
BGP UPDATE message containing the VPN-ID extended community. On the
other hand, a PE implementing the virtual router architecture will
not import routes from a BGP UPDATE message containing the route
target extended community attribute.
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The granularity at which the information is either [RFC2547-bis]
related or VR-related is per BGP UPDATE message. Different SAFI
numbers are used to indicate that the message carried in BGP
multiprotocol extension attributes is to be handled by the VR or
[RFC2547-bis] architectures. SAFI number of 128 is used for [RFC2547-
bis] related format. A value of 129 for the SAFI number is for the
virtual router (where the NLRI are carrying a labeled prefixes), and
a SAFI value of 140 is for non labeled addresses.
8. Scalability Considerations
In this section, we briefly summarize the main characteristics of
our model with respect to scalability.
Recall that the Service Provider network consists of (a) PE routers,
(b) BGP Route Reflectors, (c) P routers (which are neither PE
routers nor Route Reflectors), and, in the case of multi-provider
VPNs, and (d) ASBRs.
A PE router, unless it is a Route Reflector should not retain
VPN-related information unless it has at least one VPN with an
Import Target identical to one of the VPN-related information Route
Target attributes. Inbound filtering should be used to cause such
information to be discarded. If a new Import Target is later added
to one of the PE's VPNs (a "VPN Join" operation), it must then
acquire the VPN-related information it may previously have
discarded.
This can be done using the refresh mechanism described in [BGP-
RFSH]. The outbound route filtering mechanism of [BGP-ORF] can also be
used to advantage to make the filtering more dynamic.
Similarly, if a particular Import Target is no longer present in
any of a PE's VPNs (as a result of one or more "VPN Prune"
operations), the PE may discard all VPN-related information which,
as a result, no longer have any of the PE's VPN's Import Targets as
one of their Route Target Attributes.
Note that VPN Join and Prune operations are non-disruptive, and do
not require any BGP connections to be brought down, as long as the
refresh mechanism of [BGP-RFSH] is used.
As a result of these distribution rules, no one PE ever needs to
maintain all routes for all VPNs; this is an important scalability
consideration.
Route reflectors can be partitioned among VPNs so that each
partition carries routes for only a subset of the VPNs supported by
the Service Provider. Thus no single route reflector is required to
maintain VPN-related information for all VPNs.
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For inter-provider VPNs, if multi-hop EBGP is used, then the ASBRs
need not maintain and distribute VPN-related information at all.
P routers do not maintain any VPN-related information. In order
to properly forward VPN traffic, the P routers need only maintain
routes to the PE routers and the ASBRs.
As a result, no single component within the Service Provider network
has to maintain all the VPN-related information for all the VPNs.
So the total capacity of the network to support increasing numbers
of VPNs is not limited by the capacity of any individual component.
An important consideration to remember is that one may have any
number of INDEPENDENT BGP systems carrying VPN-related information.
This is unlike the case of the Internet, where the Internet BGP
system must carry all the Internet routes. Thus one significant
(but perhaps subtle) distinction between the use of BGP for the
Internet routing and the use of BGP for distributing VPN-related
information, as described in this document is that the former is not
amenable to partition, while the latter is.
9. Security Considerations
This document describes a BGP-based auto-discovery mechanism which
enables a PE router that attaches to a particular VPN to discover
the set of other PE routers that attach to the same VPN. Each PE
router that is attached to a given VPN uses BGP to advertise that
fact. Other PE routers which attach to the same VPN receive these
BGP advertisements. This allows that set of PE routers to discover
each other. Note that a PE will not always receive these
advertisements directly from the remote PEs; the advertisements may
be received from "intermediate" BGP speakers.
It is of critical importance that a particular PE should not be
"discovered" to be attached to a particular VPN unless that PE
really is attached to that VPN, and indeed is properly authorized to
be attached to that VPN. If any arbitrary node on the Internet
could start sending these BGP advertisements, and if those
advertisements were able to reach the PE routers, and if the PE
routers accepted those advertisements, then anyone could add any
site to any VPN. Thus the auto-discovery procedures described here
presuppose that a particular PE trusts its BGP peers to be who they
appear to be, and further that it can trusts those peers to be
properly securing their local attachments. (That is, a PE must
trust that its peers are attached to, and are authorized to be
attached to, the VPNs to which they claim to be attached.).
If a particular remote PE is a BGP peer of the local PE, then the
BGP authentication procedures of RFC 2385 can be used to ensure that
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the remote PE is who it claims to be, i.e., that it is a PE that is
trusted.
If a particular remote PE is not a BGP peer of the local PE, then
the information it is advertising is being distributed to the local
PE through a chain of BGP speakers. The local PE must trust that
its peers only accept information from peers that they trust in
turn, and this trust relation must be transitive. BGP does not
provide a way to determine that any particular piece of received
information originated from a BGP speaker that was authorized to
advertise that particular piece of information. Hence the
procedures of this document should be used only in environments
where adequate trust relationships exist among the BGP speakers.
Some of the VPN schemes which may use the procedures of this
document can be made robust to failures of these trust
relationships. That is, it may be possible to keep the VPNs secure
even if the auto-discovery procedures are not secure. For example,
a VPN based on the VR model can use IPsec tunnels for transmitting
data and routing control packets between PE routers. An
illegitimate PE router which is discovered via BGP will not have the
shared secret which makes it possible to set up the IPsec tunnel,
and so will not be able to join the VPN. Similarly, [IPSEC-2547]
describes procedures for using IPsec tunnels to secure VPNs based on
the [RFC2547-bis] model. The details for using IPsec to secure a
particular sort of VPN depend on that sort of VPN and so are out of
scope of the current document.
10. IANA Considerations
New AFI value to be assigned by IANA to indicate that the NLRI is
carrying VPN-L2 Address as described in section 3.2 to be used by
all L2VPN solutions.
SAFI number of "128" is used for [RFC2547-bis].
SAFI number "129" for indicating that the NLRI is carrying
information for VR-based solution.
SAFI number "140" for indicating that the NLRI is carrying
information for VR for non labeled prefixes.
New Extended Community to be assigned by IANA and used for Topology
values for VR-based L3VPN solution see section 4.2.2.
New Extended Community to be assigned by IANA for carrying VPN-ID
format based on RFC2685 format (see section 4.2.1.2)
11. Use of BGP Capability Advertisement
A BGP speaker that uses VPN information as described in this
document with multiprotocol extensions should use the Capability
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Advertisement procedures [RFC-3392] to determine whether the speaker
could use Multiprotocol Extensions with a particular peer.
12. Normative References
[BGP-COMM] Ramachandra, Tappan, et al., "BGP Extended Communities
Attribute", June 2001, work in progress
[BGP-MP] Bates, Chandra, Katz, and Rekhter, "Multiprotocol
Extensions for BGP4", February 1998, RFC 2283
[RFC-3107] Rekhter Y, Rosen E., "Carrying Label Information in
BGP4", January 2000, RFC3107
[RFC2547-bis] Rosen E., et al, "BGP/MPLS VPNs", Work in Progress.
[RFC-2685] Fox B., et al, "Virtual Private Networks Identifier", RFC
2685, September 1999.
[RFC-3392] Chandra, R., et al., "Capabilities Advertisement with
BGP-4", RFC3392, May 2002.
[VPN-VR] Knight, P., Ould-Brahim H., Gleeson, B., "Network based IP
VPN Architecture using Virtual Routers", Work in Progress.
13. Informative References
[L2VPN-ROSEN] Rosen, E., Radoaca, V., "Provisioning Models and
Endpoint Identifiers in L2VPN Signaling", Work in Progress.
[L2VPN-KOMP] Kompella, K., et al., "Virtual Private LAN Service",
Work in Progress.
[L2VPN-VKOMP-LASS] Kompella, V., Lasserre, M., et al., "Transparent
VLAN Services over MPLS", Work in Progress.
[RFC-1701] Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic
Routing Encapsulation (GRE)", RFC 1701, October 1994.
[RFC-2003] Perkins, C., "IP Encapsulation within IP", RFC2003,
October 1996.
[RFC-2026] Bradner, S., "The Internet Standards Process -- Revision
3", RFC2026, October 1996.
[RFC-2401] Kent S., Atkinson R., "Security Architecture for the
Internet Protocol", RFC2401, November 1998.
[RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
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[TLS-TISSA] "BGP/MPLS Layer-2 VPN", draft-tsenevir-bgpl2vpn-01.txt,
work in progress, July 2001.
[IPSEC-2547] Rosen, E., et al., "Use of PE-PE IPsec in RFC2547
VPNs", Work in Progress.
[BGP-RFSH] Chen, A., "Route Refresh Capability for BGP-4", RFC2918,
September 2000.
[BGP-ORF] Chen, E., and Rekhter, Y., "Cooperative Route Filtering
Capability for BGP-4", Work in Progress.
14. Intellectual Property Rights Notices
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances
of licenses to be made available, or the result of an attempt made
to obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification
can be obtained from the IETF Secretariat.
15. Contributors
Bryan Gleeson
Tahoe Networks
3052 Orchard Drive
San Jose, CA 95134 USA
Email: bryan@tahoenetworks.com
Peter Ashwood-Smith
Nortel Networks
P.O. Box 3511 Station C,
Ottawa, ON K1Y 4H7, Canada
Phone: +1 613 763 4534
Email: petera@nortelnetworks.com
Luyuan Fang
AT&T
200 Laurel Avenue
Middletown, NJ 07748
Email: Luyuanfang@att.com
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Phone: +1 (732) 420 1920
Jeremy De Clercq
Alcatel
Francis Wellesplein 1
B-2018 Antwerpen, Belgium
Phone: +32 3 240 47 52
Email: jeremy.de_clercq@alcatel.be
Riad Hartani
Caspian Networks
170 Baytech Drive
San Jose, CA 95143
Phone: 408 382 5216
Email: riad@caspiannetworks.com
Tissa Senevirathne
Force10 Networks
1440 McCarthy Blvd,
Milpitas, CA 95035.
Phone: 408-965-5103
Email: tsenevir@hotmail.com
16. Authors Information
Hamid Ould-Brahim
Nortel Networks
P O Box 3511 Station C
Ottawa, ON K1Y 4H7, Canada
Email: hbrahim@nortelnetworks.com
Eric C. Rosen
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
E-mail: erosen@cisco.com
Yakov Rekhter
Juniper Networks
1194 N. Mathilda Avenue
Sunnyvale, CA 94089
Email: yakov@juniper.net
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Full Copyright Statement
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