Internet DRAFT - draft-zzhang-bess-vpn-option-bc
draft-zzhang-bess-vpn-option-bc
BESS Working Group Z. Zhang
Internet-Draft K. Kompella
Updates: 9012, 4364 (if approved) Juniper Networks
Intended status: Standards Track B. Decraene
Expires: 9 August 2024 Orange
L. Jalil
Verizon
6 February 2024
VPN Inter-AS Option BC
draft-zzhang-bess-vpn-option-bc-01
Abstract
RFC 4364 specifies protocol and procedures for BGP/MPLS IP Virtual
Private Networks (VPNs), including different options (A/B/C) of
Inter-AS support. This document specifies MPLS VPN Inter-AS Option
BC that combines the advantages of Option B and Option C (and that
removes the disadvantages of Option B and Option C). The same
concept is applicable to Ethernet Virtual Private Network (EVPN) as
well.
Status of This Memo
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This Internet-Draft will expire on 9 August 2024.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Option B and Option C Pros and Cons . . . . . . . . . . . 2
1.2. Option BC . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2.1. Using Multiple NLRI labels . . . . . . . . . . . . . 3
1.2.2. Using Tunnel Encapsulation Attribute . . . . . . . . 5
1.2.3. Use of Option BC in SRv6/MPLS Service Interworking
Option BC . . . . . . . . . . . . . . . . . . . . . . 8
2. Specification . . . . . . . . . . . . . . . . . . . . . . . . 8
3. Security Considerations . . . . . . . . . . . . . . . . . . . 8
4. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 8
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Normative References . . . . . . . . . . . . . . . . . . 8
5.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
1.1. Option B and Option C Pros and Cons
With the Inter-AS Option B, ASBRs readvertises its received VPN
routes (referred to as service routes) with the next hop changed to
itself and the label in the NLRI changed to a locally allocated
(service) label that is bound to the <next hop, label> in the
received route, and installs corresponding label forwarding state.
When it receives traffic and the locally allocated label is exposed
at the top of the stack, the traffic is forwarded according to the
installed forwarding state.
With the Inter-AS Option C, the ASBRs are not involved in the
exchange of service routes. The PEs receive service routes with the
next hop unchanged, and label switch paths (LSPs) are established for
each next hop. An ingress PE push the service label first, and then
push the label(s) for the LSP to the egress PE. The routers along
the path label switch the traffic only based on the label for the LSP
to the egress PE.
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Because the ASBRs with Option B change the service routes' next hop
and allocate local service labels on each hop, each AS's internal
information (e.g. the loopback addresses) is hidden from outside the
AS. Rich policy control could be applied on the ASBR-ASBR sessions,
allowing very flexible and fine-grained route export control. As a
result, Option B is very suitable for Inter-Provider scenarios.
On the other hand, Option C scales much better becasue ASBRs don't
need to handle service routes or maintain per-service-label
forwarding state. Compared to Option B, it is more suitable for
Intra-Provider multi-AS scenarios.
1.2. Option BC
Option BC in this document combines the advantages of Option B and
Option C. ASBRs re-advertise service routes with optional rich
policy control. The service labels are not changed but the LSP
towards the originating PE is advertised as part of the service
routes - either as an additional label in the NLRI [RFC8277], or as a
new type of tunnel in the Tunnel Encapsulation Attribute (TEA)
[RFC9012] - without exposing the information about the originating
PE.
Consider the following topology:
PE11 PE21 PE31
----- ----- -----
(AS100) ASBR1 -- ASBR21 (AS200) ASBR22 -- ASBR3 (AS300)
----- ----- -----
PE12 PE22 PE32
PE11 and PE12 originate the following service routes respectively,
where each tuple represents <service label in NLRI, service prefix,
next hop>.
* <100, sprfx1, PE11>
* <100, sprfx2, PE12>
1.2.1. Using Multiple NLRI labels
When ASBR1 re-advertises the routes to ASBR21, they become as
following:
* <201, 100, sprfx1, ASBR1>
* <301, 100, sprfx2, ASBR1>
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The original service label 100 in the NLRI does not change, but a new
binding label 201/301 is added respectively, and the next hop changes
to ASBR1. Label 201/301 binds to PE11/PE12 respectively and ASBR1
sets up forwarding state so that when it receives a packet with label
201 (or 301), it label switches to PE11 (or PE12).
Similarly, when ASBR21 re-advertises the routes to its peers in
AS200, the routes become:
* <202, 100, sprfx1, ASBR21>
* <302, 100, sprfx2, ASBR21>
Again, the inner service label does not change and the outer label
201/301 changes to 202/302 respectively. ASBR21 installs forwarding
state so that when it receive packets with label 202/302, it swaps
the label to 201/301 and then tunnel to ASBR21 the next hop.
This continues. ASBR22 re-advertise to ASBR3 as following:
* <203, 100, sprfx1, ASBR22>
* <303, 100, sprfx2, ASBR22>
and ASBR3 re-advertises to its AS300 peers as following:
* <204, 100, sprfx1, ASBR3>
* <304, 100, sprfx2, ASBR3>
When PE31/PE32 sends traffic for sprfx1/sprfx2, the following label
stacks are used respectively:
* <label (stack) to reach ASBR3, 204, 100>
* <label (stack) to reach ASBR3, 304, 100>
ASBR3 label switches the traffic based on 204/304 as following:
* <label (stack) to reach ASBR22, 203, 100>
* <label (stack) to reach ASBR22, 303, 100>
Eventually the traffic arrives on ASBR1 and it label switches the
traffic based on 201/301 as following:
* <label (stack) to reach PE11, 100>
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* <label (stack) to reach PE12, 100>
1.2.2. Using Tunnel Encapsulation Attribute
Alternatively, when ASBR1 re-advertises the routes to ASBR21, they
become as following:
* <100, sprfx1, ASBR1>, TEA composite tunnel <ASBR1, 201>
* <100, sprfx2, ASBR1>, TEA composite tunnel <ASBR1, 301>
The service label in the NLRI does not change. The next hop changes,
but it is not used. A TEA with a "Composite Tunnel" is added, which
includes the ASBR1 as the tunnel egress endpoint and a binding label
201/301, which binds to PE11/PE12 accordingly. ASBR1 also sets up
forwarding state so that when it receives a packet with label 201 (or
301), it label switches to PE11 (or PE12).
Similarly, when ASBR21 re-advertises the routes to its peers in
AS200, the routes become:
* <100, sprfx1, ASBR21>, TEA composite tunnel <ASBR21, 202>
* <100, sprfx2, ASBR21>, TEA composite tunnel <ASBR21, 302>
Again, the service label does not change. The Composite Tunnel in
the TEA changes to a new one <ASBR21, 202/302> respectively. ASBR21
installs forwarding state so that when it receive packets with label
202/302, it swaps the label to 201/301 and then send to ASBR21 the
tunnel egress endpoint.
This continues. ASBR22 re-advertise to ASBR3 as following:
* <100, sprfx1, ASBR22>, TEA composite tunnel <ASBR22, 203>
* <100, sprfx2, ASBR22>, TEA composite tunnel <ASBR22, 303>
and ASBR3 re-advertises to its AS300 peers as following:
* <100, sprfx1, ASBR3>, TEA composite tunnel <ASBR3, 204>
* <100, sprfx2, ASBR3>, TEA composite tunnel <ASBR3, 304>
When PE31/PE32 sends traffic for sprfx1/sprfx2, the following label
stacks are used respectively:
* <label (stack) to reach ASBR3, 204, 100>
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* <label (stack) to reach ASBR3, 304, 100>
ASBR3 label switches the traffic based on 204/304 as following:
* <label (stack) to reach ASBR22, 203, 100>
* <label (stack) to reach ASBR22, 303, 100>
Eventually the traffic arrives on ASBR1 and it label switches the
traffic based on 201/301 as following:
* <label (stack) to reach PE11, 100>
* <label (stack) to reach PE12, 100>
With Option C, the signaling of inter-AS LSPs for PEs is done
separately and associated with the PE addresses. With Option BC, the
signaling of those PE LSPs is done as part of service routes
advertisement, without associating with the PE addresses.
1.2.2.1. Incremental Deployment
The Option BC method requires the receiving PEs/ASBRs to handle the
composite tunnel in TEA. While it is reasonable to upgrade ASBRs, it
may not be feasible to upgrade all PEs at the same time to support
Option BC. Therefore, an Option-BC-capable ASBR may have to revert
back to Option B when re-advertising service routes.
In the above example, for the service routes originated by PE11/PE12:
* If ASBR21 does not support Option BC, ASBR1 must not use Option BC
when re-advertising to ASBR21.
* If ASBR21 does support Option BC (and receive service routes from
ASBR1 with the composite tunnel in TEA), but if any one of
PE21/PE22/ASBR22 does not support Option BC, ASBR21 must revert to
Option B when re-advertising into AS200. It removes the TEA, and
allocate local labels that bind to received <service label,
composite tunnel>. When it receive traffic with the allocated
local service label, the traffic is label switched per the bound
<service label, composite tunnel>.
* If ASBR22 and ASBR3 both support Option BC, ASBR22 can use Option
BC when re-advertising the service routes to ASBR3 even though
ASBR21 has reverted to Option B. This is as if ASBR21 is a PE
that originated the service routes.
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* If PE31/PE32 don't support Option BC, ASBR3 has to revert to
Option B again.
Even though there are repeated Option BC <-> Option B conversions on
ASBR21/ASBR22/ASBR3 in the above example, ASBR1 and ASBR22 are able
to take the scaling advantage of Option BC.
A PE/ASBR advertises its support of Option BC with a new Capability
Code in its BGP Capabilities Optional Parameter [RFC5492]. A Router
Reflector (RR) does not need to support Option BC procedures, but it
advertises Option BC capability on behalf of its clients. This can
be either based on provisioning (e.g. the operator knows all clients
support Option BC) or based the RR's dynamic detection of client's
Option BC capability.
1.2.2.2. Update to RFC 9012
[RFC9012] specifically calls out in its Section 10 "Applicability
Restrictions" that use of the Tunnel Encapsulation attribute in an
"Inter-AS option b" scenario is not recommended, as quoted below:
"Note that if the Tunnel Encapsulation attribute is attached to a
VPN-IP route [RFC4364], if Inter-AS "option b" (see Section 10 of
[RFC4364]) is being used, and if the Tunnel Egress Endpoint sub-TLV
contains an IP address that is not in the same AS as the router
receiving the route, it is very likely that the embedded label has
been changed. Therefore, use of the Tunnel Encapsulation attribute
in an "Inter-AS option b" scenario is not recommended."
Notice that context is "Tunnel Egress Endpoint sub-TLV contains an IP
address that is not in the same AS as the router receiving the route"
and the embedded service label is not allocated by the Tunnel Egress
Endpoint. With Option BC, the binding label in the composite tunnel
is allocated by the tunnel egress endpoint in the TEA, and the
embedded service label will only be exposed at the PE/ASBR that
allocated that embedded service label, so it is safe to use TEA with
the "composite tunnel" in Option BC.
Even in the pure Option B case, as long as it is guaranteed that the
embedded service label is allocated by the Tunnel Egress Endpoint, it
is safe to use TEA with Option B.
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1.2.3. Use of Option BC in SRv6/MPLS Service Interworking Option BC
In [I-D.zzhang-spring-service-interworking], when service routes are
re-advertised by the interworking node to the MPLS domain from the
SRv6 domain with the Option BC method, the next hop maps to an IPv6
prefix on the originating SRv6 PE. That has the following
implications:
* If the MPLS domain is IPv4, then many IPv4 addresses may have to
be allocated and map to the IPv6 prefixes.
* Even if the MPLS domain is IPv6, exposing those IPv6 prefixes may
not be desired in the inter-provider case.
The Option BC procedures in this document can be used to address the
above concerns. Instead of using a next hop that maps to an IPv6
prefix on the originating PE, the interworking node can use its own
address as the next hop and use a composite tunnel in the TEA, in
which the binding label is bound to the IPv6 prefix on the
originating PE.
2. Specification
Normative specification will be provided in future revisions.
3. Security Considerations
To be added.
4. Acknowledgement
The authors thank Srihari Singha for his review and suggestions.
5. References
5.1. Normative References
[RFC5492] Scudder, J. and R. Chandra, "Capabilities Advertisement
with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February
2009, <https://www.rfc-editor.org/info/rfc5492>.
[RFC8277] Rosen, E., "Using BGP to Bind MPLS Labels to Address
Prefixes", RFC 8277, DOI 10.17487/RFC8277, October 2017,
<https://www.rfc-editor.org/info/rfc8277>.
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[RFC9012] Patel, K., Van de Velde, G., Sangli, S., and J. Scudder,
"The BGP Tunnel Encapsulation Attribute", RFC 9012,
DOI 10.17487/RFC9012, April 2021,
<https://www.rfc-editor.org/info/rfc9012>.
5.2. Informative References
[I-D.zzhang-spring-service-interworking]
Zhang, Z. J., Decraene, B., Zadok, S., Jalil, L., and D.
Voyer, "MPLS/SRv6 Service Interworking Option BC", Work in
Progress, Internet-Draft, draft-zzhang-spring-service-
interworking-02, 12 September 2023,
<https://datatracker.ietf.org/doc/html/draft-zzhang-
spring-service-interworking-02>.
Authors' Addresses
Zhaohui (Jeffrey) Zhang
Juniper Networks
Email: zzhang@juniper.net
Kireeti Kompella
Juniper Networks
Email: kireeti@juniper.net
Bruno Decraene
Orange
Email: bruno.decraene@orange.com
Luay Jalil
Verizon
Email: luay.jalil@verizon.com
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