Network Working Group Yakov Rekhter
Internet Draft Juniper Networks
Expiration Date: September 2004 Eric Rosen
Network Working Group Cisco Systems
Use of PE-PE GRE or IP in RFC2547 VPNs
draft-ietf-l3vpn-gre-ip-2547-01.txt
1. 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
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as ``work in progress.''
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
2. Abstract
This draft describes a variation of RFC2547 [RFC2547] in which the
outermost MPLS label of a VPN packet is replaced with either IP or a
GRE encapsulation. This enables the VPN packets to be carried over
non-MPLS networks.
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3. Summary for Sub-IP Area
3.1. Summary
The base specification for RFC2547 VPNs, i.e., draft-rosen-
rfc2547bis-03.txt, specifies the procedures for providing a
particular style of VPN, using MPLS label switched paths between
Provider Edge (PE) routers. The base specification does not discuss
other types of tunnels between PE routers.
This draft extends the base specification by specifying the
procedures for providing the RFC2547 style of VPN using GRE or IP
tunnels (rather than MPLS LSPs) between PE routers.
3.2. Where does it fit in the Picture of the Sub-IP Work
This work fits squarely in the PPVPN box.
3.3. Why is it Targeted at this WG
The WG is chartered with considering the RFC2547 style of VPN. This
draft specifies procedures to allow that style of VPN to run on
networks which do not implement MPLS in the core switches, and/or in
environments in which increased security is needed.
Thus the draft allows the RFC2547 style of VPN to meet additional
requirements that are not met by the base specification.
3.4. Justification
The WG should consider this document as it extends a style of VPN
explicitly called out in the charter so that it becomes applicable to
a wider range of IP-based backbone environments.
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4. Introduction
In "conventional" RFC2547 VPNs, when a PE router receives a packet
from a CE router, it looks up the packet's destination IP address in
a VRF. As a result of this lookup, it obtains an MPLS label stack, a
data link header, an output interface. The label stack is prepended
to the packet, the data link header is prepended to that, and the
resulting frame is queued for the output interface.
The bottom label on the MPLS label stack is always a label which will
not be seen until the packet reaches its point of egress from the
network. This label represents a particular route within the packet's
VPN. The purpose of the upper labels is to cause the packet to be
delivered to the router which understands the bottom label.
What we discuss here are procedures creating an MPLS packet which
carries ONLY the bottom label, and then using either GRE or IP
encapsulation to carry that MPLS packet across the network. That is,
the upper labels are replaced with an IP header, and in the case of
GRE encapsulation a GRE header as well.
5. Motivations
"Conventional" RFC2547 VPNs require that there be an MPLS Label
Switched Path (LSP) between a packet's ingress PE router and its
egress PE router. This means that an RFC2547 VPN cannot be
implemented if there is a part of the path between the ingress and
egress PE routers which does not support MPLS.
In order to enable RFC2547 VPNs to be deployed even when there are
non-MPLS router along the path between the ingress and egress PE
routers, it is desirable to have an alternative which allows the
upper labels to be replaced with either IP or (IP + GRE) header.
This encapsulating header would encapsulate an MPLS packet containing
only a bottom label. The encapsulation header would have the address
of the egress PE in its destination IP address field, and this would
cause the packet to be delivered to the egress PE.
In this procedure, the ingress and egress PEs themselves must support
MPLS, but that is not an issue, as those routers must necessarily
have RFC2547 VPN support, whereas the transit routers arguably should
be able to be "vanilla" routers with no special MPLS or VPN support.
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6. Specification
In short, the technical approach specified here is:
1. Continue to use MPLS to identify a VPN-IP route, by continuing to
add an MPLS label stack to the VPN packets. However, the label stack
will carry only one label, the current "bottom label."
2. An MPLS-in-GRE [mpls-over-gre-ip], or MPLS-in-IP [mpls-over-gre-
ip] encapsulation will be used to turn the above MPLS packet back
into an IP packet. This in effect creates a GRE or an IP tunnel
between the ingress PE router and the egress PE router.
The net effect is that an MPLS packet gets sent through a GRE or an
IP tunnel.
6.1. MPLS-in-IP/MPLS-in-GRE Encapsulation by Ingress PE
The following description is not meant to specify an implementation
strategy; any implementation procedure which produces the same result
is acceptable.
When a PE receives a packet from a CE, it looks up the packet's IP
destination address in the VRF corresponding to that CE. This enables
it to find a VPN-IP route. The VPN-IP route will have an associated
MPLS label and an associated BGP Next Hop. The label is pushed on the
packet. Then an IP (or IP+GRE) encapsulation header is prepended to
the packet, creating an MPLS-in-IP (or MPLS-in-GRE) encapsulated
packet. The IP source address field of the encapsulation header will
be an address of the ingress PE itself. The IP destination address
field of the encapsulation header will contain the value of the
associated BGP Next Hop attribute; this will be an IP address of the
egress PE.
The effect is to dynamically create an IP (or GRE) tunnel between the
ingress and egress PE routers. No apriori configuration of the remote
tunnel endpoints is needed. Note that these tunnels are NOT IGP-
visible links, and routing adjacencies are not supported across these
tunnel. Note also that the set of remote tunnel endpoints is NOT
known in advance, but is learned dynamically via the BGP distribution
of VPN-IP routes.
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6.2. MPLS-in-IP/MPLS-in-GRE Decapsulation by Egress PE
We assume that every egress PE is also an ingress PE, and hence has
the ability to decapsulate MPLS-in-IP (or MPLS-in-GRE) packets.
After decapsulation, the packets should be delivered to the routing
function for ordinary MPLS switching.
7. Implications on packet spoofing
It should be noted that if the upper MPLS labels are replaced with an
unsecured IP encapsulation, like GRE or IP, it becomes more difficult
to protect the VPNs against spoofed packets. A Service Provider (SP)
can protect against spoofed MPLS packets by the simple expedient of
not accepting MPLS packets from outside its own boundaries (or more
generally by keeping track of which labels are validly received over
which interfaces, and discarding packets which arrive with labels
that are not valid for their incoming interfaces).
Protection against spoofed IP packets requires having all the
boundary routers perform filtering; either filtering out packets from
"outside" which are addressed to PE routers, or filtering out packets
from "outside" which have source addresses that belong "inside" and
filtering on each PE all packets which have source addresses that
belong "outside". The maintenance of these filter lists can be
management-intensive, and, depending on the implementation, their use
at all border routers may affect the performance seen by all traffic
entering the SP's network. However, such filters may be required for
reasons other than protection against spoofing of VPN packets, in
which case the additional maintenance overhead of these filters to
protect (among other things) against spoofing of VPN packets may be
of no practical significance.
The filtering described in the previous paragraph works only within a
single SP network. It is not clear whether (and how) this filtering
could be extended to support multiple SP networks. That makes the
scheme described in this document fairly problematic in the multi-
provider environment.
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8. Security Considerations
Security considerations in [mpls-over-gre-ip] apply here as well.
Additional security issues are discussed in the section "Implications
on packet spoofing".
9. Acknowledgments
Most of the text in this document is "borrowed" almost verbatim from
draft-rosen-ppvpn-ipsec-2547-00.txt.
10. References
[RFC2547bis] "BGP/MPLS IP VPNs", Rosen E., Rekhter, Y., draft-ietf-
l3vpn-rfc2547bis-01.txt
[mpls-over-gre-ip] "Encapsulating MPLS in IP or Generic Routing
Encapsulation (GRE)", Rekhter, Y., Rosen, E., draft-ietf-mpls-in-ip-
or-gre-06.txt
11. Authors' Addresses
Yakov Rekhter
Juniper Networks
E-mail: yakov@juniper.net
Eric C. Rosen
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
E-mail: erosen@cisco.com
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