routing Z. Zhang
Internet-Draft Juniper Networks
Intended status: Standards Track September 15, 2021
Expires: March 19, 2022
MVPN Inter/Intra-region Tunnel Segmentation
draft-zzhang-bess-mvpn-regional-segmentation-00
Abstract
RFC7524 specifies procedures for Inter-Area Point-to-Multipoint
Segmented Label Switched Paths (aka MVPN tunnel segmentation). This
document updates RFC7524 by extending the inter-area segmentation
concept to inter-region and intra-region segmentation where a region
is no longer tied to an IGP area.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Tunnel Segmentation . . . . . . . . . . . . . . . . . . . 2
1.2. Intra-region Segmentation . . . . . . . . . . . . . . . . 3
1.3. Bud Node Support . . . . . . . . . . . . . . . . . . . . 3
2. Specifications . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Inter-region Segmentation . . . . . . . . . . . . . . . . 3
2.2. Intra-region Segmentation . . . . . . . . . . . . . . . . 4
2.3. Bud Node Support . . . . . . . . . . . . . . . . . . . . 5
3. Security Considerations . . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Normative References . . . . . . . . . . . . . . . . . . 6
5.2. Informative References . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
1.1. Tunnel Segmentation
[RFC6514] specifies (among other things) inter-AS MVPN tunnel
segmentation procedures and [RFC7524] specifies inter-area MVPN
tunnel segmentation procedures. The procedures for inter-AS and
inter-area are similar in that the segmentation points - ASBRs or
ABRs - change the PMSI Tunnel Attribute (PTA) attached to I/S-PMSI
routes to specify the type and identification of tunnel to be used in
the next AS/area, when they re-advertise the PMSI routes into the
next AS/area.
This change of tunnel at the segmentation points and stitching of
upstream and downstream tunnel segments not only allows different
tunnel technology/instance to be used in different AS/area, but also
limits the replication of traffic to only PEs and segmentation points
in the local AS/area, instead of to all PEs.
The inter-area segmentation points are route reflectors and when they
re-advertise the x-PMSI routes to different downstream areas they may
use different BGP neighbor groups so that different tunnel type/
identification can be encoded in PTA for different downstream areas.
If the ABR is also responsible for reflecting the routes to PEs in
the same area, the ABR does not modify the PTA (because of that those
local PEs are also put into a different neighbor group).
As a result, a segmentation point will likely have different neighbor
groups (one group for each area) so that the PTA and Inter-Area P2MP
Segmented Next-Hop Extended Community (referred to as Segmentation
EC) can be set accordingly when it re-advertise the x-PMSI routes.
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The provisioning of a RR with these different neighbor groups for
segmentation purpose can actually be done on any router (as a
segmentation point) - not necessarily on an ABR. As a result, the
procedures in RFC7524, while specified for inter-area, can be
extended inter-regiona as well - the segmentation points can be any
border routers between arbitrarily defined "regions".
This concept is already described in Section 6 of
[I-D.ietf-bess-evpn-bum-procedure-updates], but specified formally in
this document for MVPN.
1.2. Intra-region Segmentation
Even with the inter-area segmentation extended to inter-region, when
a regional border router (RBR) reflects routes to PEs in the same
region, it does not modify the PTA or Segmentation EC. But if the
RBR also modifies the two attributes when reflecting routes to the
local PEs, tunnel segmentation is achieved even intra-region - both
the upstream and downstream tunnel segments are in the same region.
This Intra-region Segmentation is one way to achieve Assisted
Replication in MVPN: a PE sends traffic to assisting replicators who
will then relay traffic to other PEs (even in the same region).
1.3. Bud Node Support
A segmentation point may have both local receivers off a VRF and
downstream receivers off a remote PE for traffic arriving on an
upstream segment. This segmentation point is referred to a bud node,
just like that a node can be both a transit and leaf node for a P2MP
tree.
Depending on implementation, a bud node may need to receive two
copies of a packet, one for local delivery and one for remote
delivery. If so, the bud node may request the upstream PE or
segmentation point to send two copies. This is done by specifying an
additional label in the PTA.
2. Specifications
2.1. Inter-region Segmentation
The procedures in RFC7524 are extended to beyond IGP area-based. A
provider network can be arranged into "regions" connected by
"Regional Border Routers" (RBRs). On a segmentation point a region
MAY be defined as a BGP neighbor group - all peers in the group are
subject to the same export policy, which can be used to control the
modification of attributes for the purpose of segmentation.
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RFC7524 procedures apply as is, though "area" is replaced with
"region" and "Area Border Router" (ABR) is replaced with "Regional
Border Router" (RBR).
The concept of Per-region Aggregation, as explained in Section 6.1 of
[I-D.ietf-bess-evpn-bum-procedure-updates], is also applicable to
MVPN. A future revision of this document will specify details of
Per-region Aggregation for MVPN.
2.2. Intra-region Segmentation
The following procedures are applicable for intra-region
segmentation. One use of intra-region segmentation is for Assisted
Replication where PE-PE traffic goes through a relay point (assisting
replicator).
If it is known that the local PEs are only peered with the RBRs (as
RRs and segmentation points), the PEs and RBRs follow the procedures
in RFC7524. In addition, the local RBRs modify the PTA and
Segmentation EC even when they re-advertise x-PMSI routes to PEs in
the ingress region, thus achieving Intra-region Segmentation.
Otherwise (i.e., if a local PE may import BGP-MVPN routes directly
unless with the modified procedures specified below), the following
modified procedures apply:
o When an ingress PE advertises an x-PMSI route, it attaches an
Extended Community (EC) derived from the Route Target for the VPN
(RT-VPN) [I-D.zzhang-idr-rt-derived-community] but not the RT-VPN
itself. Call this EC as EC-VPN. The route also carry a
Segmentation EC as specified in RFC7524.
o When the local RBRs (as RRs and segmentation points) receive this
route, it replaces the EC-VPN with the corresponding RT-VPN (the
EC-VPN and RT-VPN can be derived from each other), and then re-
advertise the route to its peers, with the Segmentation EC and PTA
modified as specified in RFC7524. The modification applies even
when re-advertising to peers in the same ingress region.
This is to ensure that local egress PEs will only import the routes
re-advertised by the RBRs after the modification of PTA and
Segmentation EC.
Additionally, if there are intermediate RRs between the ingress PE
and local RBRs, and Route Target Constrain [RFC4684] is in use, the
ingress PE MUST also attach a Route Target (referred to as RT-RBR)
and the local RBRs MUST be provisioned to import routes with RT-RBR
(otherwise the intermediate RRs will not re-advertise the routes
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towards the RBRs because the routes carry only EC-VPN but not RT-
VPN). The local RBRs MUST remove the RT-RBR when they re-advertise
the routes.
2.3. Bud Node Support
If a segmentation point is a bud node for a segmented x-PMSI tunnel,
it MAY specify an additional label in the PTA of the Leaf A-D routes
in response to the x-PMSI route received from its upstream node (an
ingress PE or an upstream segmentation point). If so, the node
SHOULD program a label forwarding entry to pop the label and forward
packet based on IP lookup in a VRF identified by the label (while the
tunnel label for the upstream segment or the label in the PTA of the
x-PMSI/Leaf route for the upstream segment is used to stitch the
upstream and downstream segments together). This is done only if the
segmentation point can not both route traffic arriving on the
upstream segment to local receivers and label switch the traffic to
downstream segments.
This is done by extending the PTA with an Extension TLV. After
encoding the PTA per Section 5 of [RFC6514] or other specifications
for other tunnel types, an Extension TLV is encoded, with its total
length added to the PTA's attribute length [RFC4271]. Upon receiving
a PTA, after decoding the PTA per Section 5 of [RFC6514] or other
specifications for other tunnel types if there is additional data
according to the PTA's attribute length, the remaining data is
interpreted as an Extension TLV.
The Extension TLV includes sub-TLVs, each with a one-octet type, a
one-octet length, and a variable-length value field. This document
defines an Additional Label sub-TLV to specify the additional label
with which the bud node segmentation point wants its upstream node to
send the extra copy.
When the upstream node decodes the extra label in the PTA attached to
the Leaf A-D route in response to an x-PMSI A-D route that it
(re-)advertises, it SHOULD send an extra copy via unicast tunneling
with the additional label. If the extra copy is not sent the
downstream bud node segmentation point will not be able to send
traffic to its local receivers.
3. Security Considerations
No additional security considerations are needed beyond what are
discussed in RFC7524.
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4. IANA Considerations
This document requests the IANA to create a "PMSI Tunnel Attribute
Extension sub-TLV Type Registry". Allocation from the registry is
First Come First Serve, with an initial allocation for "Additional
Label".
5. References
5.1. Normative References
[]
Zhang, Z., "Extended Communities Derived from Route
Targets", draft-zzhang-idr-rt-derived-community-01 (work
in progress), March 2021.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC6514] Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
Encodings and Procedures for Multicast in MPLS/BGP IP
VPNs", RFC 6514, DOI 10.17487/RFC6514, February 2012,
<https://www.rfc-editor.org/info/rfc6514>.
[RFC7524] Rekhter, Y., Rosen, E., Aggarwal, R., Morin, T.,
Grosclaude, I., Leymann, N., and S. Saad, "Inter-Area
Point-to-Multipoint (P2MP) Segmented Label Switched Paths
(LSPs)", RFC 7524, DOI 10.17487/RFC7524, May 2015,
<https://www.rfc-editor.org/info/rfc7524>.
5.2. Informative References
[I-D.ietf-bess-evpn-bum-procedure-updates]
Zhang, Z., Lin, W., Rabadan, J., Patel, K., and A.
Sajassi, "Updates on EVPN BUM Procedures", draft-ietf-
bess-evpn-bum-procedure-updates-09 (work in progress),
June 2021.
[RFC4684] 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, DOI 10.17487/RFC4684,
November 2006, <https://www.rfc-editor.org/info/rfc4684>.
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Author's Address
Zhaohui Zhang
Juniper Networks
Email: zzhang@juniper.net
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