Network Working Group H. Ould-Brahim

Network Working Group H. Ould-Brahim
Request for Comments: 5195 D. Fedyk
Category: Standards Track Nortel
Y. Rekhter
Juniper Networks
June 2008

BGP-Based Auto-Discovery for Layer-1 VPNs

Status of This Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Abstract

The purpose of this document is to define a BGP-based auto-discovery
mechanism for Layer-1 VPNs (L1VPNs). The auto-discovery mechanism
for L1VPNs allows the provider network devices to dynamically
discover the set of Provider Edges (PEs) having ports attached to
Customer Edge (CE) members of the same VPN. That information is
necessary for completing the signaling phase of L1VPN connections.
One main objective of a L1VPN auto-discovery mechanism is to support
the "single-end provisioning" model, where addition of a new port to
a given L1VPN would involve configuration changes only on the PE that
has this port and on the CE that is connected to the PE via this
port.

1. Introduction

The purpose of this document is to define a BGP-based auto-discovery
mechanism for Layer-1 VPNs (L1VPNs) [L1VPN-FRMK]. The auto-discovery
mechanism for L1VPNs allows the provider network devices to
dynamically discover the set of PEs having ports attached to CE
members of the same VPN. That information is necessary for
completing the signaling phase of L1VPN connections. One main
objective of a L1VPN auto-discovery mechanism is to support the
"single-end provisioning" model, where addition of a new port to a
given L1VPN would involve configuration changes only on the PE that
has this port and on the CE that is connected to the PE via this
port.

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The auto-discovery mechanism proceeds by having a PE advertise to
other PEs the following information, at a minimum: its own IP address
and the list of <private address, provider address> tuples local to
that PE. Once that information is received, the remote PEs will
identify the list of VPN members they have in common with the
advertising PE, and use the information carried within the discovery
mechanism to perform address resolution during the signaling phase of
Layer-1 VPN connections.

Figure 1 highlights the network reference for using a BGP-based
auto-discovery mechanism for Layer-1 VPNs. For the purpose of the
auto-discovery mechanism, BGP is running only on the provider
network. The PEs maintain per-VPN information tables called Port
Information Tables (PITs) related to <private address, provider
address> information. More information on the PITs is in Section 2.

PE1 PE2
+---------+ +--------------+
+--------+ | +------+| | +----------+ | +--------+
| VPN-A | | |VPN-A || | | VPN-A | | | VPN-A |
| CE1 |--| |PIT || BGP route | | PIT | |-| CE2 |
+--------+ | | ||<----------->| | | | +--------+
| +------+| Distribution| +----------+ |
| | | |
+--------+ | +------+| | +----------+ | +--------+
| VPN-B | | |VPN-B || -------- | | VPN-B | | | VPN-B |
| CE1 |--| |PIT ||-( GMPLS )-| | PIT | |-| CE2 |
+--------+ | | || (Backbone ) | | | | +--------+
| +------+| --------- | +----------+ |
| | | |
+--------+ | +-----+ | | +----------+ | +--------+
| VPN-C | | |VPN-C| | | | VPN-C | | | VPN-C |
| CE1 |--| |PIT | | | | PIT | |-| CE2 |
+--------+ | | | | | | | | +--------+
| +-----+ | | +----------+ |
+---------+ +--------------+

Figure 1: BGP Auto-Discovery for L1VPN

[L1VPN-FRMK] describes two modes of operation for a L1VPN: the basic
mode and the enhanced mode. This document describes an auto-
discovery mechanism for the basic mode only.

1.1. Requirements Language

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. Procedures

In the context of L1VPNs, a CE is connected to a PE via one or more
ports, where each port may consist of one or more channels or sub-
channels. Each port on a CE that connects the CE to a PE has an
identifier that is unique within that L1VPN (but need not be unique
across several L1VPNs). We refer to this identifier as the customer
port identifier (CPI). Each port on a PE also has an identifier that
is unique within the provider network. We refer to this identifier
as the provider port identifier (PPI). Note that IP addresses used
for CPIs or PPIs could be either IPv4 or IPv6 addresses.

For each L1VPN that has at least one port configured on a PE, the PE
maintains a Port Information Table (PIT). A PIT contains a list of
<CPI, PPI> tuples for all the ports within its L1VPN. Note that a
PIT may also hold routing information (for example, when CPIs are
learnt using a routing protocol).

A PIT on a given PE is populated with two types of information.

- Information related to the CEs' ports attached to the ports on the
PE. This information could be locally configured at the PE or
could be received from the CEs.

- Information received from other PEs through the auto-discovery
mechanism.

We refer to the former as local information, and to the latter as
remote information. Propagation of local information to other PEs is
accomplished by using BGP multiprotocol extensions [RFC4760]. To
restrict the flow of this information to only the PITs within a given
L1VPN, we use BGP route filtering based on the Route Target Extended
Community [BGP-COMM], as follows.

Each PIT on a PE is configured with one or more Route Target
Communities, called "export Route Targets", that are used for tagging
the local information when it is exported into the provider's BGP.
The granularity of such tagging could be as fine as a single <CPI,
PPI> pair. In addition, each PIT on a PE is configured (at
provisioning time) with one or more Route Target Communities, called
"import Route Targets", that restrict the set of routes that could be
imported from provider's BGP into the PIT to only the routes that
have at least one of these Communities.

Each of the following occurs at provisioning time: if a service
provider adds a new L1VPN port to a particular PE, this port is
associated with a PIT on that PE, and this PIT is associated with
that L1VPN.

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Note that since the protocol used to populate a PIT with remote
information is BGP, and since BGP works across multiple autonomous
systems (ASs), it follows that the mechanism described in this
document could support L1VPNs that span multiple autonomous systems.

Although multi-AS L1VPNs are currently out of scope for the Basic
Mode, the mechanisms defined in this document appear to be easily
applicable to a multi-AS scenario, should such a need arise in the
future. At that time, additional work may be required to examine
various aspects including security.

3. Carrying L1VPN Information in BGP

The <CPI, PPI> mapping is carried using the Multiprotocol Extensions
to BGP [RFC4760]. [RFC4760] defines the format of two BGP
attributes, MP_REACH_NLRI and MP_UNREACH_NLRI, that can be used to
announce and withdraw the announcement of reachability information.
We introduce a new subsequent address family identifier, called
Layer-1 VPN auto-discovery information (value 69), and also a new
Network Layer Reachability Information (NLRI) format for carrying the
CPI and PPI information.

One or more <PPI, CPI> tuples could be carried in the above mentioned
BGP attributes.

The format of the NLRI is described in Figure 2.

+---------------------------------------+

| Length (1 octet) |

+---------------------------------------+

| Auto-discovery info (variable) |

+---------------------------------------+

Figure 2: Encoding of the NLRI

Note that the encoding of the auto-discovery information is described
in [L1VPN-BM], and note also that if the value of the Length of the
Next Hop field (of the MP_REACH_NLRI attribute) is 4, then the Next
Hop contains an IPv4 address. If this value is 16, then the Next Hop
contains an IPv6 address.

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4. Carrying L1VPN Traffic Engineering Information in BGP

In addition to reachability information, the auto-discovery mechanism
MAY carry Traffic Engineering information used for the purpose of
egress path selection. For example, a PE may learn the switching
capability and the maximum LSP bandwidth of remote L1VPN interfaces
from the remote PEs. This document uses the BGP Traffic Engineering
Attribute [BGP-TE-ATTRIBUTE] to carry such information.

5. Scalability

Recall that the Service Provider network consists of (a) PEs, (b) BGP
Route Reflectors, (c) P nodes (which are neither PEs nor Route
Reflectors), and, in the case of multi-provider VPNs, (d) Autonomous
System Border Routers (ASBRs).

A PE router, unless it is a Route Reflector, does not retain L1VPN-
related information unless it has at least one VPN with an import
Route Target identical to one of the VPN-related information Route
Target attributes. If a PE does not have a VPN with a matching
import Route Target, it MUST then discard received l1VPN information.
Inbound filtering MUST be used to cause such information to be
discarded. If a new import Route Target is later added to one of the
PE's VPNs (a "VPN Join" operation), it MUST then acquire the VPN-
related information it previously has discarded.

In this case, the refresh mechanism described in [BGP-RFSH] MUST be
used. The outbound route filtering mechanisms of [BGP-ORF] and
[BGP-CONS] can also be used to advantage to make the filtering more
dynamic.

Similarly, if a particular import Route Target is no longer present
in any of a PE's VPN (as a result of one or more "VPN Prune"
operations), the PE MAY discard all the L1VPN BGP routes that, as a
result, no longer have any of the PE's PIT's import Route Targets as
one of their Route Target attributes.

Note that "VPN Join" and "VPN 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 L1VPNs; this is an important scalability
consideration.

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Route reflectors can be partitioned among VPNs so that each partition
carries routes for only a subset of the L1VPNs supported by the
Service Provider. Thus, no single route reflector is required to
maintain VPN-related information for all VPNs.

For inter-provider VPNs, if multi-hop External BGP (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.

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.

6. Security Considerations

This document describes a BGP-based auto-discovery mechanism that
enables a PE that attaches to a particular L1VPN 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 that attach to the same VPN receive these BGP advertisements.
This allows that set of PEs to discover each other. Note that a PE
will not always receive these advertisements directly from the remote
PEs; the advertisements can be received from "intermediate" BGP
speakers.

It is of critical importance that a particular PE MUST 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 nodes, and if the PE nodes accepted those
advertisements, then anyone could add any site to any L1VPN. 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 trust those peers to be properly securing their

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local attachments. (That is, a PE MUST trust that its peers are
attached to, and are authorized to be attached to, the L1VPNs 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 [RFC2385] SHOULD be used to ensure that
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 MUST be used only in environments where adequate trust
relationships exist among the BGP speakers (such as the case of using
the auto-discovery mechanism within a single provider network).

7. IANA Considerations

This document assigns a new SAFI, called Layer-1 VPN auto-discovery
information (see Section 3). This assignment has been made in the
Subsequent Address Family Identifier (SAFI) registry using the
Standards Action allocation procedures. The value is 69.

8. References

8.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760, January
2007.

[BGP-RFSH] Chen, E., "Route Refresh Capability for BGP-4", RFC
2918, September 2000.

8.2. Informative References

[BGP-TE-ATTRIBUTE]
Ould-Brahim, H., Fedyk, D., and Rekhter, Y., "Traffic
Engineering Attribute", Work in Progress, January 2008.

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RFC 5195 BGP Auto-Discovery for L1VPNs June 2008

[BGP-ORF] Chen, E. and Y. Rekhter, "Outbound Route Filtering
Capability for BGP-4", Work in Progress, September 2006.

[BGP-CONS] 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.

[BGP-COMM] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", RFC 4360, February 2006.

[L1VPN-FRMK] Takeda, T., Ed., "Framework and Requirements for Layer 1
Virtual Private Networks", RFC 4847, April 2007.

[L1VPN-BM] Fedyk, D., Ed., Rekhter, Y., Ed., Papadimitriou, D.,
Rabbat, R., and L. Berger, "Layer 1 VPN Basic Mode",
Work in Progress, February 2008.

[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP
MD5 Signature Option", RFC 2385, August 1998.

9. Acknowledgment

We would like to thank Adrian Farrel for the useful comments.

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Authors' Addresses

Hamid Ould-Brahim
Nortel
PO Box 3511 Station C
Ottawa ON K1Y 4H7
Canada

Phone: +1 (613) 763 4730
EMail: [email protected]

Yakov Rekhter
Juniper Networks
1194 N. Mathilda Avenue
Sunnyvale, CA 94089
USA

EMail: [email protected]

Don Fedyk
Nortel
600 Technology Park
Billerica, MA 01821
USA

Phone: +1 (978) 288 3041
Email: [email protected]

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