Network Working Group Eric C. Rosen
Internet Draft Yakov Rekhter
Expiration Date: September 2000 Cisco Systems, Inc.
Stephen John Brannon
Monique Jeanne Morrow
Swisscom AG
Marco Carugi
France Telecom
Christopher J. Chase
AT&T
Eric Dean
Global One
Paul Hitchin
Adrian Smith
BT
Manoj Leelanivas
Juniper Networks, Inc.
Luca Martini
Level 3 Communications, LLC
Vijay Srinivasan
Ericsson IP Infrastructure
Alain Vedrenne
SITA EQUANT
March 2000
BGP/MPLS VPNs
draft-rosen-rfc2547bis-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
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other groups may also distribute working documents as Internet-
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Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This document describes a method by which a Service Provider may use
an IP backbone to provide VPNs for its customers. MPLS is used for
forwarding packets over the backbone, and BGP is used for
distributing routes over the backbone. The primary goal of this
method is to support the case in which a client obtains IP backbone
services from a Service Provider or Service Providers with which it
maintains contractual relationships. The client may be an
enterprise, a group of enterprises which need an extranet, an
Internet Service Provider, another VPN Service Provider (even one
which uses this same method to offer VPNs to clients of its own), an
application service provider, etc. The method makes it very simple
for the client to use the backbone services. It is also very
scalable and flexible for the Service Provider, and allows the
Service Provider to add value.
This document obsoletes RFC 2547.
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Table of Contents
1 Introduction ....................................... 3
1.1 Virtual Private Networks ........................... 4
1.2 Edge Devices ....................................... 5
1.3 VPNs with Overlapping Address Spaces ............... 6
1.4 VPNs with Different Routes to the Same System ...... 6
1.5 Multiple Forwarding Tables in PEs .................. 7
1.6 SP Backbone Routers ................................ 7
1.7 Security ........................................... 8
2 Sites and CEs ...................................... 8
3 VRFs: Per-Site Forwarding Tables in the PEs ........ 9
3.1 Virtual Sites ...................................... 10
4 VPN Route Distribution via BGP ..................... 11
4.1 The VPN-IPv4 Address Family ........................ 11
4.2 Encoding of Route Distinguishers ................... 12
4.3 Controlling Route Distribution ..................... 13
4.3.1 The Route Target Attribute ......................... 13
4.3.2 Route Distribution Among PEs by BGP ................ 15
4.3.3 How VPN-IPv4 NLRI is Carried in BGP ................ 18
4.3.4 Building VPNs using Route Targets .................. 18
5 Forwarding Across the Backbone ..................... 19
6 How PEs Learn Routes from CEs ...................... 20
7 How CEs learn Routes from PEs ...................... 23
8 Carriers' Carriers ................................. 24
9 Security ........................................... 24
10 Quality of Service ................................. 25
11 Scalability ........................................ 25
12 Intellectual Property Considerations ............... 26
13 Acknowledgments .................................... 26
14 Authors' Addresses ................................. 26
15 References ......................................... 29
16 Full Copyright Statement ........................... 29
1. Introduction
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1.1. Virtual Private Networks
Consider a set of "sites" which are attached to a common network
which we may call the "backbone". Let's apply some policy to create a
number of subsets of that set, and let's impose the following rule:
two sites may have IP interconnectivity over that backbone only if at
least one of these subsets contains them both.
The subsets we have created are "Virtual Private Networks" (VPNs).
Two sites have IP connectivity over the common backbone only if there
is some VPN which contains them both. Two sites which have no VPN in
common have no connectivity over that backbone.
If all the sites in a VPN are owned by the same enterprise, the VPN
is a corporate "intranet". If the various sites in a VPN are owned
by different enterprises, the VPN is an "extranet". A site can be in
more than one VPN; e.g., in an intranet and several extranets. We
regard both intranets and extranets as VPNs. In general, when we use
the term VPN we will not be distinguishing between intranets and
extranets.
We wish to consider the case in which the backbone is owned and
operated by one or more Service Providers (SPs). The owners of the
sites are the "customers" of the SPs. The policies that determine
whether a particular collection of sites is a VPN are the policies of
the customers. Some customers will want the implementation of these
policies to be entirely the responsibility of the SP. Other
customers may want to implement these policies themselves, or to
share with the SP the responsibility for implementing these policies.
In this document, we are primarily discussing mechanisms that may be
used to implement these policies. The mechanisms we describe are
general enough to allow these policies to be implemented either by
the SP alone, or by a VPN customer together with the SP. Most of the
discussion is focused on the former case, however.
The mechanisms discussed in this document allow the implementation of
a wide range of policies. For example, within a given VPN, we can
allow every site to have a direct route to every other site ("full
mesh"), or we can restrict certain pairs of sites from having direct
routes to each other ("partial mesh").
In this document, we are interested in the case where the common
backbone offers an IP service. We are NOT focused on the case where
the common backbone is part of the public Internet, but rather on the
case where it the backbone network of an SP or set of SPs with which
the customer maintains contractual relationships. The customer
should be thought of as purchasing VPN service from the SP, not
merely as purchasing Internet access from it. The customer itself
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may be a single enterprise, a set of enterprises needing an extranet,
an Internet Service Provider, or even another SP which offers the
same kind of VPN service to its own customers.
In the rest of this introduction, we specify some properties which
VPNs should have. The remainder of this document outlines a VPN
model which has all these properties.
1.2. Edge Devices
We suppose that at each site, there are one or more Customer Edge
(CE) devices, each of which is attached via some sort of data link
(e.g., PPP, ATM, ethernet, Frame Relay, GRE tunnel, etc.) to one or
more Provider Edge (PE) routers. Routers in the Provider's network
which do not attach to CE devices are known as "P routers".
If a particular site has a single host, that host may be the CE
device. If a particular site has a single subnet, the CE device may
be a switch. In general, the CE device can be expected to be a
router, which we call the CE router.
We will say that a PE router is attached to a particular VPN if it is
attached to a CE device which is in that VPN. Similarly, we will say
that a PE router is attached to a particular site if it is attached
to a CE device which is in that site.
When the CE device is a router, it is a routing peer of the PE(s) to
which it is attached, but is not a routing peer of CE routers at
other sites. Routers at different sites do not directly exchange
routing information with each other; in fact, they do not even need
to know of each other at all. As a consequence, very large VPNs
(i.e., VPNs with a very large number of sites) are easily supported,
while the routing strategy for each individual site is greatly
simplified.
It is important that the scheme allow clear administrative boundaries
to be maintained between the SP and its customers. There is no
requirement for the PE or P routers to be managed by the customers,
or for the CE devices to be managed by the SP.
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1.3. VPNs with Overlapping Address Spaces
We assume that any two non-intersecting VPNs (i.e., VPNs with no
sites in common) may have overlapping address spaces; the same
address may be reused, for different systems, in different VPNs.
(This allows the SP to support the common case of VPNs which use the
RFC1918 addressing space, for example.) As long as a given endsystem
has an address which is unique within the scope of the VPNs that it
belongs to, the endsystem itself does not need to know anything about
VPNs.
In this model, the VPN owners do not have a backbone to administer,
not even a "virtual backbone." Nor do the SPs have to administer a
separate backbone or "virtual backbone" for each VPN. Site-to-site
routing in the backbone is optimal (within the constraints of the
policies used to form the VPNs), and is not constrained in any way by
an artificial "virtual topology" of tunnels.
1.4. VPNs with Different Routes to the Same System
Although a site may be in multiple VPNs, it is not necessarily the
case that the route to a given system at that site should be the same
in all the VPNs. Suppose, for example, we have an intranet
consisting of sites A, B, and C, and an extranet consisting of A, B,
C, and the "foreign" site D. Suppose that at site A there is a
server, and we want clients from B, C, or D to be able to use that
server. Suppose also that at site B there is a firewall. We want
all the traffic from site D to the server to pass through the
firewall, so that traffic from the extranet can be access controlled.
However, we don't want traffic from C to pass through the firewall on
the way to the server, since this is intranet traffic.
This means that it needs to be possible to set up two routes to the
server. One route, used by sites B and C, takes the traffic directly
to site A. The second route, used by site D, takes the traffic
instead to the firewall at site B. If the firewall allows the
traffic to pass, it then appears to be traffic coming from site B,
and follows the route to site A.
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1.5. Multiple Forwarding Tables in PEs
Each PE router needs to maintain a number of separate forwarding
tables. Every site to which the PE is attached must be mapped to one
of those forwarding tables. When a packet is received from a
particular site, the forwarding table associated with that site is
consulted in order to determine how to route the packet. The
forwarding table associated with a particular site S is populated
only with routes that lead to other sites which have at least one VPN
in common with S. This prevents communication between sites which
have no VPN in common, and it allows two VPNs with no site in common
to use address spaces that overlap with each other.
1.6. SP Backbone Routers
The SP's backbone consists of the PE routers, as well as other
routers ("P routers") which do not attach to CE devices.
If every router in an SP's backbone had to maintain routing
information for all the VPNs supported by the SP, this model would
have severe scalability problems; the number of sites that could be
supported would be limited by the amount of routing information that
could be held in a single router. It is important therefore that the
routing information about a particular VPN is only required to be
present in those PE routers which attach to that VPN. In particular,
the P routers should not need to have ANY per-VPN routing information
whatsoever. (This condition may need to be relaxed somewhat when
multicast routing is considered. This is hnot considered further in
this paper.)
VPNs may span multiple service providers. There are a number of
possible methods for implementing this.
In one approach, when the path between PE routers crosses a boundary
between SP networks, the ASBRs use EBGP to exchange VPN-IPv4 routes,
and label switched paths cross the boundary between providers. The
presupposition is that this is done via a private peering
arrangement, at which there exists mutual trust between the two
providers. In particular, each provider must trust the other to pass
it only correct routing information, and to pass it labeled (in the
sense of MPLS [MPLS-ARCH]) packets only if those packets have been
labeled by trusted sources. A variety of other approaches are also
possible.
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1.7. Security
A VPN model should, even without the use of cryptographic security
measures, provide a level of security equivalent to that obtainable
when a level 2 backbone (e.g., Frame Relay) is used. That is, in the
absence of misconfiguration or deliberate interconnection of
different VPNs, it should not be possible for systems in one VPN to
gain access to systems in another VPN.
2. Sites and CEs
From the perspective of a particular backbone network, a set of IP
systems constitutes a site if those systems have mutual IP
interconnectivity, and communication between them occurs without use
of the backbone. In general, a site will consist of a set of systems
which are in geographic proximity. However, this is not universally
true. If two geographic locations are connected via a leased line,
over which OSPF is running, and if that line is the preferred way of
communicating between the two locations, then the two locations can
be regarded as a single site, even if each location has its own CE
router.
A CE device is always regarded as being in a single site (though as
we shall see, a site may consist of multiple "virtual sites"). A
site, however, may belong to multiple VPNs.
A PE router may attach to CE devices in any number of different
sites, whether those CE devices are in the same or in different VPNs.
A CE device may, for robustness, attach to multiple PE routers, of
the same or of different service providers. If the CE device is a
router, the PE router and the CE router will appear as router
adjacencies to each other.
While the basic unit of interconnection is the site, the architecture
described herein allows a finer degree of granularity in the control
of interconnectivity. For example, certain systems at a site may be
members of an intranet as well as members of one or more extranets,
while other systems at the same site may be restricted to being
members of the intranet only.
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3. VRFs: Per-Site Forwarding Tables in the PEs
Each PE router maintains one or more "per-site forwarding tables."
These are known as VRFs, or "VPN Routing and Forwarding" tables.
Every site to which the PE router is attached is associated with one
of these tables. A particular packet's IP destination address is
looked up in a particular VRF only if that packet has arrived
directly from a site which is associated with that table.
How are the VRFs populated?
As an example, let PE1, PE2, and PE3 be three PE routers, and let
CE1, CE2, and CE3 be three CE routers. Suppose that PE1 learns, from
CE1, the routes which are reachable at CE1's site. If PE2 and PE3
are attached respectively to CE2 and CE3, and there is some VPN V
containing CE1, CE2, and CE3, then PE1 uses BGP to distribute to PE2
and PE3 the routes which it has learned from CE1. PE2 and PE3 use
these routes to populate the VRFs which they associate respectively
with the sites of CE2 and CE3. Routes from sites which are not in
VPN V do not appear in these VRFs, which means that packets from CE2
or CE3 cannot be sent to sites which are not in VPN V.
If a site is in multiple VPNs, the VRF associated with that site can
contain routes from the full set of VPNs of which the site is a
member.
A PE generally associates only one VRF to each site, even if it is
multiply connected to that site. However, different sites can share
the same VRF if they are meant to use exactly the same set of routes.
Routes from the Internet do not need to be present in the VRF
associated with a given interface, even if the SP is providing
Internet access over that interface. There are two basic methods for
providing Internet access over an interface that is associated with a
VRF:
1. The VRF may contain a default route which leads to a firewall.
This ensures that packets headed towards the Internet are
always passed through a firewall first.
2. If a packet is received over a particular interface, and its
destination address does not match any entry in the VRF, then
the packet's destination address may be matched against the
PE's Internet forwarding table. This can be useful if the
packets have already been through a firewall, for instance.
Note that only one Internet forwarding table per PE is needed
in this case; the Internet routes do not need to be present in
the VRFs.
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Of course, if Internet access is not provided over a particular
interface, and the packet's destination address has no match in the
associated VRF, the packet is treated as undeliverable.
The VRFs in a PE are ONLY used for packets which arrive from a site
which is directly attached to the PE. They are not used for routing
packets which arrive from other routers that belong to the SP
backbone. As a result, there may be multiple different routes to the
same system, where the route followed by a given packet is determined
by the site from which the packet enters the backbone. E.g., one may
have one route to a given system for packets from the extranet (where
the route leads to a firewall), and a different route to the same
system for packets from the intranet (including packets that have
already passed through the firewall).
3.1. Virtual Sites
In some cases, a particular site may be divided by the customer into
several virtual sites, perhaps by the use of VLANs. Each virtual
site may be a member of a different set of VPNs. The PE then needs to
contain a separate VRF for each virtual site. For example, if a CE
supports VLANs, and wants each VLAN mapped to a separate VPN, the
packets sent between CE and PE could be contained in the site's VLAN
encapsulation. Then the VLAN tag could be used by the PE, along with
the interface over which the packet is received, to assign the packet
to a particular VPN.
Alternatively, one could divide the interface into multiple "sub-
interfaces" (particularly if the interface is Frame Relay or ATM),
and assign the packet to a VPN based on the sub-interface over which
it arrives. Or one could simply use a different interface for each
virtual site. In any case, only one CE router is ever needed per
site, even if there are multiple virtual sites. Of course, a
different CE router could be used for each virtual site, if that is
desired.
Note that in all these cases, the mechanisms, as well as the policy,
for controlling which traffic is in which VPN are in the hand of the
customer.
If it is desired to have a particular host be in multiple virtual
sites, then that host must determine, for each packet, which virtual
site the packet is associated with. It can do this, e.g., by sending
packets from different virtual sites on different VLANs, our out
different network interfaces.
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4. VPN Route Distribution via BGP
PE routers use BGP to distribute VPN routes to each other (more
accurately, to cause VPN routes to be distributed to each other).
We allow each VPN to have its own address space, which means that a
given address may denote different systems in different VPNs. If two
routes, to the same IP address prefix, are actually routes to
different systems, it is important to ensure that BGP not treat them
as comparable. Otherwise BGP might choose to install only one of
them, making the other system unreachable. Further, we must ensure
that POLICY is used to determine which packets get sent on which
routes; given that several such routes are installed by BGP, only one
such must appear in any particular VRF.
We meet these goals by the use of a new address family, as specified
below.
4.1. The VPN-IPv4 Address Family
The BGP Multiprotocol Extensions [BGP-MP] allow BGP to carry routes
from multiple "address families". We introduce the notion of the
"VPN-IPv4 address family". A VPN-IPv4 address is a 12-byte quantity,
beginning with an 8-byte "Route Distinguisher (RD)" and ending with a
4-byte IPv4 address. If two VPNs use the same IPv4 address prefix,
the PEs translate these into unique VPN-IPv4 address prefixes. This
ensures that if the same address is used in two different VPNs, it is
possible to install two completely different routes to that address,
one for each VPN.
The RD does not by itself impose any semantics; it contains no
information about the origin of the route or about the set of VPNs to
which the route is to be distributed. The purpose of the RD is
solely to allow one to create distinct routes to a common IPv4
address prefix. Other means are used to determine where to
redistribute the route (see section 4.2).
The RD can also be used to create multiple different routes to the
very same system. In section 3, we gave an example where the route
to a particular server had to be different for intranet traffic than
for extranet traffic. This can be achieved by creating two different
VPN-IPv4 routes that have the same IPv4 part, but different RDs.
This allows BGP to install multiple different routes to the same
system, and allows policy to be used (see section 4.2.3) to decide
which packets use which route.
The RDs are structured so that every service provider can administer
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its own "numbering space" (i.e., can make its own assignments of
RDs), without conflicting with the RD assignments made by any other
service provider. An RD consists of a two-byte type field, an
administrator field, and an assigned number field. The value of the
type field determines the lengths of the other two fields, as well as
the semantics of the administrator field. The administrator field
identifies an assigned number authority, and the assigned number
field contains a number which has been assigned, by the identified
authority, for a particular purpose. For example, one could have an
RD whose administrator field contains an Autonomous System number
(ASN), and whose (4-byte) number field contains a number assigned by
the SP to whom IANA has assigned that ASN.
RDs are given this structure in order to ensure that an SP which
provides VPN backbone service can always create a unique RD when it
needs to do so. However, the structuring provides no semantics. When
BGP compares two such address prefixes, it ignores the structure
entirely.
Note that VPN-IPv4 addresses and IPv4 addresses are always
considered by BGP to be incomparable.
A VRF may have multiple equal cost VPN-IPv4 routes for a single IPv4
address prefix. When a packet's destination address is matched
against a VPN-IPv4 route, only the IPv4 part is actually matched.
A PE needs to be configured to associate routes which lead to
particular CE with a particular RD. The PE may be configured to
associate all routes leading to the same CE with the same RD, or it
may be configured to associate different routes with different RDs,
even if they lead to the same CE.
4.2. Encoding of Route Distinguishers
As stated, a VPN-IPv4 address consists of an 8-byte Route
Distinguisher followed by a 4-byte IPv4 address. The RDs are encoded
as follows:
- Type Field: 2 bytes
- Value Field: 6 bytes
The interpretation of the Value field depends on the value of the
Type field. At the present time, two values of the type field are
defined: 0 and 1.
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- Type 0: The Value field consists of two subfields:
* Administrator subfield: 2 bytes
* Assigned Number subfield: 4 bytes
The Administrator subfield must contain an Autonomous System
number. If this ASN is from the public ASN space, it must have
been assigned by IANA (use of ASN values from the private ASN
space is strongly discouraged). The Assigned Number subfield
contains a number from a numbering space which is administered by
the enterprise to which the ASN has been assigned by IANA.
- Type 1: The Value field consists of two subfields:
* Administrator subfield: 4 bytes
* Assigned Number subfield: 2 bytes
The Administrator subfield must contain an IP address. If this IP
address has been assigned by IANA to a particular enterprise, the
Assigned Number sub-field contains a number from a numbering
space which is administered by the enterprise to which the IP
address has been assigned (use of addresses from the private IP
address space is strongly discouraged).
4.3. Controlling Route Distribution
In this section, we discuss the way in which the distribution of the
VPN-IPv4 routes is controlled.
4.3.1. The Route Target Attribute
Every VRF is associated with one or more "Route Target" attributes.
When a VPN-IPv4 route is created by a PE router, it is associated
with one or more "Route Target" attributes. These are carried in BGP
as attributes of the route.
Any route associated with Route Target T must be distributed to every
PE router that has a VRF associated with Route Target T. When such a
route is received by a PE router, it is eligible to be installed in
each of the PE's VRFs that is associated with Route Target T.
(Whether it actually gets installed depends on the outcome of the BGP
decision process.)
A Route Target attribute can be thought of as identifying a set of
sites. (Though it would be more precise to think of it as
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identifying a set of VRFs.) Associating a particular Route Target
attribute with a route allows that route to be placed in the VRFs
that are used for routing traffic which is received from the
corresponding sites.
There is a set of Route Targets that a PE router attaches to a route
received from site S; these may be called the "Export Targets". And
there is a set of Route Targets that a PE router uses to determine
whether a route received from another PE router could be placed in
the VRF associated with site S; these may be called the "Import
Targets". The two sets are distinct, and need not be the same. Note
that a particular VPN-IPv4 route is only eligible for installation in
a particular VRF if there is some Route Target which is both one of
the route's Route Targets and one of the VRF's Import Targets.
The function performed by the Route Target attribute is similar to
that performed by the BGP Communities Attribute. However, the format
of the latter is inadequate, since it allows only a two-byte
numbering space. It is desirable to structure the format, similar to
what we have described for RDs (see section 4.1), so that a type
field defines the length of an administrator field, and the remainder
of the attribute is a number from the specified administrator's
numbering space. This can be done using BGP Extended Communities.
The Route Targets discussed herein are encoded as BGP Extended
Community Route Targets [BGP-EXTCOMM].
When a BGP speaker has received more than one route to the same VPN-
IPv4 prefix, the BGP rules for route preference are used to choose
which route are installed.
Note that a route can only have one RD, but it can have multiple
Route Targets. In BGP, scalability is improved if one has a single
route with multiple attributes, as opposed to multiple routes. One
could eliminate the Route Target attribute by creating more routes
(i.e., using more RDs), but the scaling properties would be less
favorable.
How does a PE determine which Route Target attributes to associate
with a given route? There are a number of different possible ways.
The PE might be configured to associate all routes that lead to a
particular site with a particular Route Target. Or the PE might be
configured to associate certain routes leading to a particular site
with one Route Target, and certain with another. Or the CE router,
when it distributes these routes to the PE (see section 6), might
specify one or more Route Targets for each route. The latter method
shifts the control of the mechanisms used to implement the VPN
policies from the SP to the customer. If this method is used, it may
still be desirable to have the PE eliminate any Route Targets that,
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according to its own configuration, are not allowed, and/or to add in
some Route Targets that according to its own configuration are
mandatory.
4.3.2. Route Distribution Among PEs by BGP
If two sites of a VPN attach to PEs which are in the same Autonomous
System, the PEs can distribute VPN-IPv4 routes to each other by means
of an IBGP connection between them. Alternatively, each can have an
IBGP connection to a route reflector.
What if two sites of a VPN are in different Autonomous Systems (e.g.,
because they are connected to different SPs)? One way to handle this
is to have the PE routers use IBGP to redistribute VPN-IPv4 routes
either to an Autonomous System Border Router (ASBR), or to a route
reflector of which an ASBR is a client. The ASBR then needs to use
EBGP to redistribute those routes to an ASBR in another AS. This
allows one to connect different VPN sites to different Service
Providers. However, VPN-IPv4 routes should only be accepted on EBGP
connections at private peering points, as part of a trusted
arrangement between SPs. VPN-IPv4 routes should neither be
distributed to nor accepted from the public Internet.
If there are many VPNs having sites attached to different Autonomous
Systems, there does not need to be a single ASBR between those two
ASes which holds all the routes for all the VPNs; there can be
multiple ASBRs, each of which holds only the routes for a particular
subset of the VPNs.
There are other ways of handling the multi-provider case as well.
For example, something similar to the Carrier's Carrier architecture
described in section 8 can be used. The PE routers could only
distribute their internal routes to the ASBRs, and PE routers in
different ASes could form multi-hop EBGP connections to distribute
their external routes.
When a PE router distributes a VPN-IPv4 route via BGP, it uses its
own address as the "BGP next hop". This address is encoded as a
VPN-IPv4 address with an RD of 0. ([BGP-MP] requires that the next
hop address be in the same address family as the NLRI.) It also
assigns and distributes an MPLS label. (Essentially, PE routers
distribute not VPN-IPv4 routes, but Labeled VPN-IPv4 routes. Cf.
[MPLS-BGP]) When the PE processes a received packet that has this
label at the top of the stack, the PE will pop the stack, and send
the packet directly to the site from to which the route leads. This
will usually mean that it just sends the packet to the CE router from
which it learned the route. The label may also determine the data
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link encapsulation.
In most cases, the label assigned by a PE will cause the packet to be
sent directly to a CE, and the PE which receives the labeled packet
will not look up the packet's destination address in any VRF.
However, it is also possible for the PE to assign a label which
implicitly identifies a particular VRF. In this case, the PE
receiving a packet with that label would look up the packet's
destination address in the associated VRF. This allows the route
distributed and labeled by BGP to be an aggregate of several routes
which appear in the VRF. This can be very useful if the VRF contains
a large number of host routes (e.g., as in dial-in), or if the VRF
has an associated LAN interface (where there is a different outgoing
layer 2 header for each system on the LAN, but a route is not
distributed for each such system). However, we do not consider this
further in this paper.
Note that the MPLS label that is distributed in this way is only
usable if there is a label switched path between the router that
installs a route and the BGP next hop of that route. It may be a
"best effort" route, or it may be a traffic engineered route.
Between a particular PE router and its BGP next hop for a particular
route there may be one label switched path, or there may be several,
perhaps with different QoS characteristics. All that matters for the
VPN architecture is that some label switched path between the router
and its BGP next hop exists. However, to ensure interoperability
among systems which implement this VPN architecture, all such systems
must support LDP [MPLS-LDP].
All the usual techniques for using route reflectors [BGP-RR] to
improve scalability, e.g., route reflector hierarchies, are
available. If route reflectors are used, there is no need to have
any one route reflector know all the VPN-IPv4 routes for all the VPNs
supported by the backbone. The following outlines two possible
approaches to partition all the VPN-IPv4 routes among the route
reflectors.
In the first approach each route reflector is preconfigured with a
list of Route Targets. For redundancy more than one route reflector
may be preconfigured with the same list. A route reflector uses the
preconfigured list of Route Targets to construct its inbound route
filtering. On all of its IBGP peers (regardless of whether the peer
is another route reflector, or a PE), the route reflector may use the
techniques of [BGP-ORF] to install on its peer route reflectors the
set of "Outbound Route Filters" (ORFs) that contain the list of its
preconfigured Route Targets. Note that route reflectors should accept
ORFs from other route reflectors, which means that route reflectors
should advertise the ORF capability to other route reflectors.
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A service provider may modify the list of preconfigured Route Targets
on a route reflector. When this is done, the route reflector modifies
the ORFs it installs on all of its IBGP peers. To reduce the
frequency of configuration changes on route reflectors, each route
reflector may be preconfigured with a block of Route Targets. This
way, when a new Route Target is needed for a new VPN, there is
already one or more route reflectors that are (pre)configured with
this Route Target.
Unless a given PE is a client of all route reflectors, adding a new
VPN to the PE may require the PE to become a client of the route
reflector(s) that maintain routes for that VPN. Likewise, deleting an
existing VPN from the PE may result in a situation where the PE no
longer need to be a client of some route reflector(s).
In the second approach each PE is a client of some subset of route
reflectors. A route reflector is not preconfigured with the list of
Route Targets, and does not perform inbound route filtering of routes
received from its clients (PEs); rather it accepts all the routes
received from all of its clients (PEs). The route reflector keeps
track of the set of the Route Targets carried by all the routes it
receives. When the route reflector receives from its client a route
with a Route Target that is not in this set, this Route Target is
immediately added to the set. On the other hand, when the route
reflector no longer has any routes with a particular Route Target
that is in the set, the route reflector should delay (by a few hours)
the deletion of this Route Target from the set.
The route reflector uses this set to form the inbound route filters
that it applies to routes received from other route reflectors. The
route reflector may also use ORFs to install the appropriate outbound
route filtering on other route reflectors. Just like with the first
approach, a route reflector should accept ORFs from other route
reflectors. To accomplish this, a route reflector advertises ORF
capability to other route reflectors.
When the route reflector changes the set, it should immediately
change its inbound route filtering. In addition, if the route
reflector uses ORFs, then the ORFs have to be immediately changed to
reflect the changes in the set. If the route reflector doesn't use
ORFs, and a new Route Target is added to the set, the route
reflector, after changing its inbound route filtering, must issue BGP
Refresh to other router reflectors.
A PE router (other than a Route Reflector) should not install a VPN-
IPv4 route unless it has at least one VRF with an Import Target
identical to one of the route's Route Target attributes. Inbound
filtering should be used to cause such routes to be discarded. If a
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new Import Target is later added to the PE, it must then acquire the
routes 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.
A router which is not attached to any VPN, i.e., a P router, never
installs any VPN-IPv4 routes at all.
These distribution rules ensure that there is no one box which needs
to know all the VPN-IPv4 routes that are supported over the backbone.
As a result, the total number of such routes that can be supported
over the backbone is not bounded by the capacity of any single
device, and therefore can increase virtually without bound.
4.3.3. How VPN-IPv4 NLRI is Carried in BGP
The BGP Multiprotocol Extensions [BGP-MP] are used to encode the
NLRI. If the AFI field is set to 1, and the SAFI field is set to
128, the NLRI is an MPLS-labeled VPN-IPv4 address. AFI 1 is used
since the network layer protocol associated with the NLRI is still
IP. Note that this VPN architecture never distributes unlabeled
VPN-IPv4 addresses.
In order for two BGP speakers to exchange labeled VPN-IPv4 NLRI, they
must use BGP Capabilities Negotiation to ensure that they both are
capable of properly processing such NLRI. This is done as specified
in [BGP-MP], by using capability code 1 (multiprotocol BGP), with an
AFI of 1 and an SAFI of 128.
The labeled VPN-IPv4 NLRI itself is encoded as specified in [MPLS-
BGP], where the prefix consists of an 8-byte RD followed by an IPv4
prefix.
4.3.4. Building VPNs using Route Targets
By setting up the Import Targets and Export Targets properly, one can
construct different kinds of VPNs.
Suppose it is desired to create a a fully meshed closed user group,
i.e., a set of sites where each can send traffic directly to the
other, but traffic cannot be sent to or received from other sites.
Then each site is associated with a VRF, a single Route Target
attribute is chosen, that Route Target is assigned to each VRF as
both the Import Target and the Export Target, and that Route Target
is not assigned to any other VRFs as either the Import Target or the
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Export Target.
Alternatively, suppose one desired, for whatever reason, to create a
"hub and spoke" kind of VPN. This could be done by the use of two
Route Target values, one meaning "Hub" and one meaning "Spoke". At
the VRFs attached to the hub sites, "Hub" is the Export Target and
"Spoke" is the Import Target. At the VRFs attached to the spoke
site, "Hub" is the Import Target and "Spoke" is the Export Target.
Thus the methods for controlling the distribution of routing
information among various sets of sites are very flexible, which in
turn provides great flexibility in constructing VPNs.
5. Forwarding Across the Backbone
If the intermediate routers in the backbone do not have any
information about the routes to the VPNs, how are packets forwarded
from one VPN site to another?
This is done by means of MPLS with a two-level label stack.
PE routers (and ASBRs which redistribute VPN-IPv4 addresses) need to
insert /32 address prefixes for themselves into the IGP routing
tables of the backbone. This enables MPLS, at each node in the
backbone network, to assign a label corresponding to the route to
each PE router. To ensure interoperability among different
implementations, it is required to support LDP for setting up the
label switched paths across the backbone. However, other methods of
setting up these label switched paths are also possible. (Some of
these other methods may not require the presence of the /32 address
prefixes in the IGP.)
When a PE receives a packet from a CE device, it chooses a particular
VRF in which to look up the packet's destination address. Assume
that a match is found.
If the packet's next hop is a CE device attached to this same PE, the
packet is sent directly to that CE device,
If the packet's next hop is NOT a CE device attached to this same PE,
the packet's "BGP Next Hop" is found, as well as the label which that
BGP next hop assigned for the packet's destination address. This
label is pushed onto the packet's label stack, and becomes the bottom
label. Then the PE looks up the IGP route to the BGP Next Hop, and
thus determines the IGP next hop, as well as the label assigned to
the address of the BGP next hop by the IGP next hop. This label gets
pushed on as the packet's top label, and the packet is then forwarded
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to the IGP next hop. (If the BGP next hop is the same as the IGP
next hop, the second label may not need to be pushed on, however.)
At this point, MPLS will carry the packet across the backbone. The
egress PE router's treatment of the packet will depend on the label
that was first pushed on by the ingress PE. In many cases, the PE
will be able to determine, from this label, the interface over which
the packet should be transmitted (to a CE device), as well as the
proper data link layer header for that interface. In other cases,
the PE may only be able to determine that the packet's destination
address needs to be looked up in a particular VRF before being
forwarded to a CE device. Information in the MPLS header itself,
and/or information associated with the label, may also be used to
provide QoS on the interface to the CE. In any event, when the
packet finally gets to a CE device, it will again be an ordinary
unlabeled IP packet.
Note that it is the two-level labeling that makes it possible to keep
all the VPN routes out of the P routers, and this in turn is crucial
to ensuring the scalability of the model. The backbone does not even
need to have routes to the CEs, only to the PEs.
To maintain proper isolation of one VPN from another, it is important
that no router in the backbone accept a labeled packet from any
adjacent non-backbone device unless (a) the label at the top of the
label stack was actually distributed by the backbone router to the
non-backbone device, and (b) the backbone router can determine that
use of that label will cause the packet to leave the backbone before
any labels lower in the stack will be inspected, and before the IP
header will be inspected. These restrictions are necessary in order
to prevent packets from entering a VPN where they do not belong.
6. How PEs Learn Routes from CEs
The PE routers which attach to a particular VPN need to know, for
each of that VPN's sites, which addresses in that VPN are at each
site.
In the case where the CE device is a host or a switch, this set of
addresses will generally be configured into the PE router attaching
to that device. In the case where the CE device is a router, there
are a number of possible ways that a PE router can obtain this set of
addresses.
The PE translates these addresses into VPN-IPv4 addresses, using a
configured RD. The PE then treats these VPN-IPv4 routes as input to
BGP. In no case will routes from a site ever be leaked into the
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backbone's IGP.
Exactly which PE/CE route distribution techniques are possible
depends on whether a particular CE is in a "transit VPN" or not. A
"transit VPN" is one which contains a router that receives routes
from a "third party" (i.e., from a router which is not in the VPN,
but is not a PE router), and that redistributes those routes to a PE
router. A VPN which is not a transit VPN is a "stub VPN". The vast
majority of VPNs, including just about all corporate enterprise
networks, would be expected to be "stubs" in this sense.
The possible PE/CE distribution techniques are:
1. Static routing (i.e., configuration) may be used. (This is
likely to be useful only in stub VPNs.)
2. PE and CE routers may be RIP peers, and the CE may use RIP to
tell the PE router the set of address prefixes which are
reachable at the CE router's site. When RIP is configured in
the CE, care must be taken to ensure that address prefixes from
other sites (i.e., address prefixes learned by the CE router
from the PE router) are never advertised to the PE. More
precisely: if a PE router, say PE1, receives a VPN-IPv4 route
R1, and as a result distributes an IPv4 route R2 to a CE, then
R2 must not be distributed back from that CE's site to a PE
router, say PE2, (where PE1 and PE2 may be the same router or
different routers), unless PE2 maps R2 to a VPN-IPv4 route
which is different than (i.e., contains a different RD than)
R1.
3. The PE and CE routers may be OSPF peers. A PE router which is
an OSPF peer of a CE router appears, to the CE router, to be an
area 0 router. If a PE router is an OSPF peer of CE routers
which are in distinct VPNs, the PE must of course be running
multiple instances of OSPF.
IPv4 routes which the PE learns from the CE via OSPF are
redistributed into BGP as VPN-IPv4 routes. Extended community
attributes are used to carry, along with the route, all the
information needed to enable the route to be distributed to
other CE routers in the VPN in the proper type of OSPF LSA.
OSPF route tagging is used to ensure that routes received from
the MPLS/BGP backbone are not sent back into the backbone.
4. The PE and CE routers may be BGP peers, and the CE router may
use BGP (in particular, EBGP to tell the PE router the set of
address prefixes which are at the CE router's site. (This
technique can be used in stub VPNs or transit VPNs.)
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From a purely technical perspective, this is by far the best
technique:
a) Unlike the IGP alternatives, this does not require the PE
to run multiple routing algorithm instances in order to
talk to multiple CEs
b) BGP is explicitly designed for just this function:
passing routing information between systems run by
different administrations
c) If the site contains "BGP backdoors", i.e., routers with
BGP connections to routers other than PE routers, this
procedure will work correctly in all circumstances. The
other procedures may or may not work, depending on the
precise circumstances.
d) Use of BGP makes it easy for the CE to pass attributes of
the routes to the PE. For example, the CE may suggest a
particular Target for each route, from among the Target
attributes that the PE is authorized to attach to the
route.
On the other hand, using BGP is likely to be something new for
the CE administrators, except in the case where the customer
itself is already an Internet Service Provider (ISP).
If a site is not in a transit VPN, note that it need not have a
unique Autonomous System Number (ASN). Every CE whose site
which is not in a transit VPN can use the same ASN. This can
be chosen from the private ASN space, and it will be stripped
out by the PE. Routing loops are prevented by use of the Site
of Origin Attribute (see below).
What if a set of sites constitute a transit VPN? This will
generally be the case only if the VPN is itself an ISP's
network, where the ISP is itself buying backbone services from
another SP. The latter SP may be called a "Carrier's Carrier".
In this case, the best way to provide the VPN is to have the CE
routers support MPLS, and to use the technique described in
section 8.
When we do not need to distinguish among the different ways in which
a PE can be informed of the address prefixes which exist at a given
site, we will simply say that the PE has "learned" the routes from
that site.
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Before a PE can redistribute a VPN-IPv4 route learned from a site, it
must assign a Route Target attribute (see section 4.2.1) to the
route, and it may assign a Site of Origin attribute to the route.
The Site of Origin attribute, if used, is encoded as a Route Origin
Extended Community [BGP-EXTCOMM]. The purpose of this attribute is
to uniquely identify the set of routes learned from a particular
site. This attribute is needed in some cases to ensure that a route
learned from a particular site via a particular PE/CE connection is
not distributed back to the site through a different PE/CE
connection. It is particularly useful if BGP is being used as the
PE/CE protocol, but different sites have not been assigned distinct
ASNs.
7. How CEs learn Routes from PEs
In this section, we assume that the CE device is a router.
If the PE places a particular route in the VRF which is uses to route
packets received from a particular CE, then in general, the PE may
distribute that route to the CE. Of course the PE may distribute
that route to the CE only if this is permitted by the rules of the
PE/CE protocol. (For example, if a particular PE/CE protocol has
"split horizon", certain routes in the VRF cannot be redistributed
back to the CE.) We add one more restriction on the distribution of
routes from PE to CE: if a route's Site of Origin attribute
identifies a particular site, that route must never be redistributed
to any CE at that site.
In most cases, however, it will be sufficient for the PE to simply
distribute the default route to the CE. (In some cases, it may even
be sufficient for the CE to be configured with a default route
pointing to the PE.) This will generally work at any site which does
not itself need to distribute the default route to other sites.
(E.g., if one site in a corporate VPN has the corporation's access to
the Internet, that site might need to have default distributed to the
other site, but one could not distribute default to that site
itself.)
Whatever procedure is used to distribute routes from CE to PE will
also be used to distribute routes from PE to CE.
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8. Carriers' Carriers
Sometimes a VPN may actually be the network of an ISP, with its own
peering and routing policies. Sometimes a VPN may be the network of
an SP which is offering VPN services in turn to its own customers.
VPNs like these can also obtain backbone service from another SP, the
"carrier's carrier", using essentially the same methods described in
this document. In particular:
- The CE routers should distribute to the PE routers only those
routes which are internal to the VPN. This allows the VPN to be
handled as a stub VPN.
- The CE routers should support MPLS, and should distribute to the
PE routers labels for the internal routes that they distribute to
the PEs.
- The PE routers should distribute, to the CE routers, labels for
the routes they distribute to the CE routers.
- Routers at the different sites should establish BGP connections
among themselves for the purpose of exchanging external routes.
- All the external routes must be known to the CE routers.
Then when a CE router looks up a packet's destination address, the
routing lookup will resolve to an internal address, usually the
address of the packet's BGP next hop. The CE labels the packet
appropriately and sends the packet to the PE.
Note that the CE router itself need not know all the external routes
if the other routers at the site also support MPLS. All that is
necessary is that MPLS labels can be pushed onto IP packets by the
routes which do know the external routes, and that MPLS can be used
to move the packets from those routers to the PE router.
9. Security
Under the following conditions:
a) labeled packets are not accepted by backbone routers from
untrusted or unreliable sources, unless it is known that such
packets will leave the backbone before the IP header or any
labels lower in the stack will be inspected, and
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b) labeled VPN-IPv4 routes are not accepted from untrusted or
unreliable sources,
the security provided by this architecture is virtually identical to
that provided to VPNs by Frame Relay or ATM backbones.
It is worth noting that the use of MPLS makes it much simpler to
provide this level of security than would be possible if one
attempted to use some form of IP-within-IP tunneling in place of
MPLS. It is a simple matter to refuse to accept a labeled packet
unless the first of the above conditions applies to it. It is rather
more difficult to configure a router to refuse to accept an IP packet
if that packet is an IP-within-IP tunnelled packet which is going to
a "wrong" place.
The use of MPLS also allows a VPN to span multiple SPs without
depending in any way on the inter-domain distribution of IPv4 routing
information.
10. Quality of Service
Although not the focus of this paper, Quality of Service is a key
component of any VPN service. In MPLS/BGP VPNs, existing L3 QoS
capabilities can be applied to labeled packets through the use of the
"experimental" bits in the shim header [MPLS-ENCAPS], or, where ATM
is used as the backbone, through the use of ATM QoS capabilities.
The traffic engineering work discussed in [MPLS-RSVP] is also
directly applicable to MPLS/BGP VPNs. Traffic engineering could even
be used to establish label switched paths with particular QoS
characteristics between particular pairs of sites, if that is
desirable. Where an MPLS/BGP VPN spans multiple SPs, the
architecture described in [PASTE] may be useful. An SP may apply
either intserv or diffserv capabilities to a particular VPN, as
appropriate.
11. Scalability
We have discussed scalability issues throughout this paper. In this
section, we briefly summarize the main characteristics of our model
with respect to scalability.
The Service Provider backbone 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, (d)
ASBRs.
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P routers do not maintain any VPN routes. In order to properly
forward VPN traffic, the P routers need only maintain routes to the
PE routers and the ASBRs. The use of two levels of labeling is what
makes it possible to keep the VPN routes out of the P routers.
A PE router maintains VPN routes, but only for those VPNs to which it
is directly attached.
Route reflectors and ASBRs can be partitioned among VPNs so that each
partition carries routes for only a subset of the VPNs provided by
the Service Provider. Thus no single Route Reflector or ASBR is
required to maintain routes for all the VPNs.
As a result, no single component within the Service Provider network
has to maintain all the routes 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.
12. Intellectual Property Considerations
Cisco Systems may seek patent or other intellectual property
protection for some of all of the technologies disclosed in this
document. If any standards arising from this document are or become
protected by one or more patents assigned to Cisco Systems, Cisco
intends to disclose those patents and license them on reasonable and
non-discriminatory terms.
13. Acknowledgments
Significant contributions to this work have been made by Ravi
Chandra, Dan Tappan and Bob Thomas.
14. Authors' Addresses
Eric C. Rosen
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
E-mail: erosen@cisco.com
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Yakov Rekhter
Cisco Systems, Inc.
170 Tasman Drive
San Jose, CA, 95134
E-mail: yakov@cisco.com
Stephen John Brannon
Swisscom AG
Postfach 1570
CH-8301
Glattzentrum (Zuerich), Switzerland
E-mail: stephen.brannon@swisscom.com
Marco Carugi
France Telecom / CNET Research Centre
IP networks and services
CNET/DAC/NTR
Technopole Anticipa
2, av. P. Marzin
22307 Lannion
E-mail: marco.carugi@cnet.francetelecom.fr
Christopher J. Chase
AT&T
200 Laurel Ave
Middletown, NJ 07748
USA
E-mail: chase@att.com
Eric Dean
Global One
12490 Sunrise Valley Dr.
Reston, VA 20170 USA
E-mail: edean@gip.net
Paul Hitchin
BT
BT Adastral Park
Martlesham Heath,
Ipswich IP5 3RE
UK
E-mail: paul.hitchen@bt.com
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Manoj Leelanivas
Juniper Networks, Inc.
385 Ravendale Drive
Mountain View, CA 94043 USA
E-mail: manoj@juniper.net
Luca Martini
Level 3 Communications, LLC.
1025 Eldorado Blvd.
Broomfield, CO, 80021
E-mail: luca@level3.net
Monique Jeanne Morrow
Swisscom AG
Postfach 1570
CH-8301
Glattzentrum (Zuerich), Switzerland
E-mail: monique.morrow@swisscom.com
Adrian Smith
BT
BT Adastral Park
Martlesham Heath,
Ipswich IP5 3RE
UK
E-mail: adrian.ca.smith@bt.com
Vijay Srinivasan
Ericsson IP Infrastructure
920 Main Campus Drive, Suite 500
Raleigh, NC 27606
E-mail: vijay@torrentnet.com
Alain Vedrenne
SITA EQUANT
3100 Cumberland Blvd, Suite 200
Atlanta, GA, 30339 USA
Email:Alain.Vedrenne@sita.int
Alain.Vedrenne@equant.com
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15. References
[BGP-MP] Bates, Chandra, Katz, and Rekhter, "Multiprotocol Extensions
for BGP4", February 1998, RFC 2283
[BGP-EXTCOMM] Ramachandra, Tappan, "BGP Extended Communities
Attribute", February 2000, work in progress
[BGP-ORF] Chen, Rekhter, "Cooperative Route Filtering Capability for
BGP-4", February 2000, work in progress
[BGP-RFSH] Chen, "Route Refresh Capability for BGP-4", December,
1999, work in progress
[BGP-RR] Bates and Chandrasekaran, "BGP Route Reflection: An
alternative to full mesh IBGP", RFC 1966, June 1996
[IPSEC] Kent and Atkinson, "Security Architecture for the Internet
Protocol", November 1998, RFC 2401
[MPLS-ARCH] Rosen, Viswanathan, and Callon, "Multiprotocol Label
Switching Architecture", August 1999, work in progress
[MPLS-BGP] Rekhter and Rosen, "Carrying Label Information in BGP4",
January 2000, work in progress
[MPLS-LDP] Andersson, Doolan, Feldman, Fredette, Thomas, "LDP
Specification", October 1999, work in progress
[MPLS-ENCAPS] Rosen, Rekhter, Tappan, Farinacci, Fedorkow, Li, and
Conta, "MPLS Label Stack Encoding", September 1999, work in progress
[MPLS-RSVP] Awduche, Gan, Li, Swallow, and Srinavasan, "Extensions to
RSVP for LSP Tunnels", September, 1999, work in progress
[PASTE] Li and Rekhter, "A Provider Architecture for Differentiated
Services and Traffic Engineering (PASTE)", RFC 2430, October 1998.
16. Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
This document and translations of it may be copied and furnished to
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Internet Draft draft-rosen-rfc2547bis-00.txt March 2000
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