Internet DRAFT - draft-boutros-bess-elan-services-over-sr
draft-boutros-bess-elan-services-over-sr
BESS Workgroup S. Boutros, Ed.
Internet-Draft S. Sivabalan, Ed.
Intended status: Standards Track H. Shah
Expires: 13 September 2023 Ciena Corporation
J. Uttaro
ATT
D. Voyer
Bell Canada
B. Wen
Comcast
L. Jalil
Verizon
12 March 2023
A Simplified Scalable ELAN Service Model with Segment Routing Underlay
draft-boutros-bess-elan-services-over-sr-04
Abstract
This document proposes a new approach for realizing Ethernet LAN
(ELAN) services with an objective of leveraging Segment Routing
Control plane to achieve high scalability, faster network
convergence, and reduced operational complexity. Furthermore, it
naturally brings the benefits of All-Active multihoming as well as
MAC learning in data-plane.
Status of This Memo
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Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Control Plane Behavior . . . . . . . . . . . . . . . . . . . 5
4.1. Service discovery . . . . . . . . . . . . . . . . . . . . 5
4.2. All-Active Service Redundancy . . . . . . . . . . . . . . 5
4.3. Mass service withdrawal . . . . . . . . . . . . . . . . . 5
4.4. E-Tree Support . . . . . . . . . . . . . . . . . . . . . 6
5. Data Plane Behavior . . . . . . . . . . . . . . . . . . . . . 6
5.1. Unicast Traffic . . . . . . . . . . . . . . . . . . . . . 6
5.2. BUM Traffic . . . . . . . . . . . . . . . . . . . . . . . 7
5.3. Data Plane MAC learning . . . . . . . . . . . . . . . . . 8
5.3.1. Single Home CE . . . . . . . . . . . . . . . . . . . 8
5.3.2. Multi-Home CE . . . . . . . . . . . . . . . . . . . . 9
5.4. ARP suppression . . . . . . . . . . . . . . . . . . . . . 9
5.5. Distributed Anycast Gateway . . . . . . . . . . . . . . . 10
5.6. Multi-pathing . . . . . . . . . . . . . . . . . . . . . . 10
5.7. E-Tree Support . . . . . . . . . . . . . . . . . . . . . 10
6. Benefits of ELAN over SR . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
10.1. Normative References . . . . . . . . . . . . . . . . . . 11
10.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Virtual Private LAN Service(VPLS) is based on Pseudo-Wire (PW)
construct which identifies both the service type and the service
termination node in both control and data planes. RFCs 4761 and 4762
specify mechanisms to signal PW for VPLS services using BGP and LDP
respectively. An ingress Provider Edge (PE) node needs to maintain a
PW per VPLS instance for each egress PE node. So, if we assume 10K
ELAN instances over a network of 100 PE nodes, each PE node needs to
setup and maintain approximately 1M PWs which can easily become a
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scalability bottleneck in large scale deployment.
As described in RFC7432, Ethernet Virtual Private Network (EVPN)
technology builds ELAN services similar to BGP-based IP-VPN services
with additional features such as MAC address learning in control
lane, All-Active multihoming, etc. It eliminates the need for PWs,
and hence the scale problem associated with PWs. However, an egress
PE node cannot unambiguously identify ingress PE node in data-plane.
As such, EVPN requires control plane mechanisms for MAC advertisement
and learning which increases control plane complexity and overhead.
The goal of the proposed approach is to greatly simplify control
plane functions and minimize the amount of control plane messages PE
nodes have to process. In this version of the document, we assume
Segment Routing (SR) underlay network. A future version of this
document will generalize the underlay network to both classical MPLS
and SR technologies.
The proposed approach does not require PW, and hence the control
plane complexity and message overhead associated with signaling and
maintaining PWs are eliminated.
An ELAN instance is uniquely identified by Segment ID (SID)
regardless of the number of service termination points. Such a SID
will be referred to as "Service SID" in the rest of the document.
The number of states maintained at a PE node is equal to the number
of ELAN instances in the corresponding broadcast domain. Referring
to the above example, each PE node now needs to maintain states for
10K ELAN service instances as opposed to 1 M PWs in the case of
classical VPLS model in data and control planes. A node can
advertise service SID(s) of the ELAN instance(s) that it hosts via
BGP for auto-discovery purpose. A Service SID can be:
* MPLS label for SR-MPLS.
* uSID (micro SID) for SRv6 representing network function associated
with an ELAN service instance.
MAC address is learned in data-plane. Source node of a MAC address
is identified by its node SID (assigned for regular SR operation)
during MAC learning phase. In the data packets, the node SID of the
source is inserted directly below the service SID so that a
destination node can uniquely identify the source of the packets in
an SR domain.
ELAN service instances are advertised such that a service message
packs as many ELAN instances hosted by the advertising PE node as
possible at the time of advertisement. A possible approach is to use
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a bit-map in which each bit position represents an ELAN instance, as
well as the starting value of Service SID. Using these parameters,
an ingress PE receiving advertisements node can learn ELAN
instance(s) hosted by an egress PE node.
All-Active multihoming redundancy is supported at the underlay level
by making use of SR anycast SID. No overlay mechanism is required
for this purpose.
Each node is also associated with another SID unique within the
broadcast domain that is used to identify incoming Broadcast Unknown-
unicast, and Multicast (BUM) traffic. We call such SID BUM SID. If
node A wants to send BUM traffic to node B, it needs to use BUM SID
assigned to node B as a destination SID. BUM SIDs can also be
advertised via BGP for auto-discovery purpose. In order to send BUM
traffic within a broadcast domain, P2MP SR policies can be used.
Such policies may or may not be shared by ELAN instances.
The proposed solution can also be applicable to the EVPN control
plane without compromising its benefits such as All-Active
multihoming on access, multipathing in the core, auto-provisioning
and auto-discovery, etc. With this approach, the need for
advertising EVPN route types 1 through 4 as well Split-Horizon (SH)
label is eliminated.
In the following sections, we will describe the functionalities of
the proposed approach in detail.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119] .
3. Abbreviations
BUM: Broadcast, unicast and multicast.
CE: Customer Edge node e.g., host or router or switch.
ELAN: Ethernet LAN.
EVPN: Ethernet VPN.
MAC: Media Access Control.
MAC-VRF: A Virtual Routing and Forwarding table for Media Access
Control (MAC) addresses on a PE.
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MH: Multi-Home.
OAM: Operations, Administration and Maintenance.
PE: Provide Edge Node.
SID: Segment Identifier.
SR: Segment Routing.
VPLS: Virtual Private LAN Service.
4. Control Plane Behavior
4.1. Service discovery
A node can discover ELAN service instances as well as the associated
service SIDs hosted on other nodes via configuration or auto-
discovery. With the latter, the service SIDs can be advertised using
BGP. As mentioned earlier such update message will pack information
about as many ELAN instances hosted by the advertising PE node to
reduce the amount of update messages exchanged by PE nodes.
Similar to the service SID, an ingress PE node can discover BUM SID
associated with an egress PE node via configuration or auto-
discovery.
The necessary BGP extensions will be specified in a separate
document.
4.2. All-Active Service Redundancy
An anycast SID per Ethernet Segment (ES) can be associated with the
PE nodes attached to a Multi-Home (MH) CE. The anycast SIDs will be
advertised in BGP by the PE nodes. Based on ES anycast SIDs, ingress
PEs receiving updates can discover the redundancy membership and
perform DF election. Aliasing/Multipathing can be achieved using the
same mechanisms excercised by SR underlay for forwarding traffic to
destinations belonging to anycast group.
4.3. Mass service withdrawal
Node failure can be detected due via IGP convergence. For faster
detection of node failure, mechanism like BFD can be deployed. The
proposed approach does not require additional MAC withdrawal
mechanism.
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On PE-CE link failure, the corresponding PE node withdraws the route
to the corresponding ES in BGP in order to stop receiving traffic to
that ES. With MH case with anycast SID, upon detecting a failure on
PE-CE link, a PE node may forward incoming traffic to the impacted
ES(s) to other PE node(s) that is/are part of the anycast group until
it withdraws routes to the impacted ES(s) for faster convergence.
For example, in Figure 1, assuming PE5 and PE6 are part of an anycast
group, upon link failure between PE5 and CE5, PE5 can forward the
received packets from the core to PE6 until it withdraws the anycast
SID associated with the ES(s).
4.4. E-Tree Support
To be covered in the next revision of this document.
5. Data Plane Behavior
____ CE3
/ ____CE1
-------- PE3 --------- /
/ PE1
/ | \
PE5 | \
/| | \
/ | Service Provider Network | \
CE5 | | CE2
\ | | /
\ | PE2_/
PE6 /
/ -------- PE4 --------
CE6___ / CE4_____/
Figure 1: Reference network diagram used for examples below
5.1. Unicast Traffic
The proposed method requires unicast data packet be formed as shown
in Figure 2.
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+-------------------------------+
| SID(s) to reach destination |
+-------------------------------+
| Service SID |
+-------------------------------+
| Source node SID |
+-------------------------------+
| Layer-2 Payload |
+-------------------------------+
Figure 2: Data packet format for unicast traffic
* SID(s) to reach destination: depends on the intent of the underlay
transport:
- IGP shortest path: node SID of the destination. The
destination can belong to an anycast group.
- IGP path with intent: Flex-Algo SID if the destination can be
reached using the Flex-Algo SID for a specific intent (e.g.,
low latency). The destination can belong to an anycast group.
- SR policy (to support fine intent): a SID-list for the SR
policy that can be used to reach the destination.
* Service SID: The SID that uniquely identifies an ELAN instance in
a broadcast domain.
* Source node SID: The SID that uniquely identifies the source node.
This can be a node SID which may be part of an anycast group.
Note that such a SID is allocated as part of SR underlay
operation, and the proposed approach does not impose any
additional requirement.
5.2. BUM Traffic
In order to identify incoming BUM traffic a unique SID (which will be
referred to as "BUM SID" in the rest of the document) per PE node is
allocated. A BUM packet is formatted as shown in Figure 3:
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+-------------------------------+
| BUM SID |
+-------------------------------+
| Service SID |
+-------------------------------+
| Source node SID |
+-------------------------------+
| Layer-2 Payload |
+-------------------------------+
Figure 3: Data packet format for BUM traffic
In order to send BUM traffic, a P2MP SR policy may be established
from a given node to rest of the nodes associated with an ELAN
instance. If a dedicated P2MP SR policy is used per ELAN instance, a
single SID may be used as both replication SID for the P2MP SR policy
as well as to identify ELAN instance. With this approach, the number
of SIDs imposed on data packet will be only two. It is possible to
use a given P2MP SR policy for multiple ELAN instances in which case
service SID needs to be inserted in the packet for egress PE to
identify the ELAN instance for the BUM traffic.
5.3. Data Plane MAC learning
With the proposed approach, MAC address can be learned in data- plane
using the packets formatted as shown in Figure 4.
Source MAC address on the received Layer 2 packet is learned against
the source node SID placed directly under the service SID in the
data-plane.
5.3.1. Single Home CE
In Figure 1, node 3 learns a MAC address from CE3 and floods it to
all nodes configured with the same service SID. Nodes 1, 2, 4, 5 and
6 learn the MAC address as reachable via the source node SID of Node
3.
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+-----------------------------+
| Tree SID/Broadcast Node SID |
+-----------------------------+
| Service SID |
+-----------------------------+
| Node SID of node 3 |
+-----------------------------+
| Layer-2 Packet |
+-----------------------------+
Figure 4: Packet format used for flooding
5.3.2. Multi-Home CE
Referring to Figure 1, let's assume that node 5 learns a MAC address
from MH CE5, and floods it to all nodes in data-plane as per SID
stack shown in Figure 5, including node 6. The receiving nodes learn
the MAC address as reachable via the anycast SID belonging to node 5
and node 6. Node 6 applies SH and hence does not send the packet
back to CE5, but treats the MAC address as reachable via CE5, as well
floods the address to CE6.
The following diagram shows SID label stack for a Broadcast and
Multicast MAC frame sent by Multi-Home PE. Note the presence of
source SID after the service SID. This combination/order is
necessary for the receiver to learn source MAC address (from L2
packet) associated with ingress PE (i.e. source node SID).
+-----------------------------+
| Tree SID/Broadcast Node SID |
+-----------------------------+
| Service SID |
+-----------------------------+
| Anycast node SID for CE5 |
+-----------------------------+
| Layer-2 Packet |
+-----------------------------+
Figure 5: Data packet format for traffic sent by a MH PE
5.4. ARP suppression
Gleaning ARP packet requests and replies will be used to learn IP/MAC
binding for ARP suppression. ARP replies are unicast, however
flooding ARP replies can allow all nodes to learn the MAC/IP bindings
for the destinations too.
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5.5. Distributed Anycast Gateway
Distributed Anycast Gateway (GW) (aka inter-subnet IRB function) can
be realized as follows:
* All PEs connected to the tenant subnets share the same GW IP/MAC
per subnet.
* A PE MUST never learn its own GW IP/MAC via the tunnels connecting
itself to other PE(s).
* ARP requests/replies from the tenant subnet are flooded via the
ingress PE(s) attached to the subnet to all egress PE(s) attached
to the subnet so that egress PE(s) can learn the source MAC/IP
address via the ingress PE(s).
* ARP replies from tenants will be delivered to the local PE hosts
the GW virtual MAC address. The local PE MUST flood the ARP
replies over the tunnel to other PEs. Other PEs, including the PE
which originated the ARP request, will learn the IP/MAC
association of the tenant from the received ARP reply.
5.6. Multi-pathing
Packets destined to a MH CE is distributed to the PE nodes attached
to the CE for load-balancing purpose. This is achieved implicitly
due to the use of anycast SIDs for both ES as well as PE attached to
the ES. In our example, traffic destined to CE5 is distributed via
PE5 and PE6.
5.7. E-Tree Support
To be covered in the next revision of this document.
6. Benefits of ELAN over SR
The proposed approach eliminates the need for establishing and
maintaining PWs as with legacy VPLS technology. This yields
significant reduction in control plane overhead. Also, due to MAC
learning in data-plane (conversational MAC learning), the proposed
approach provides the benefits as such fast convergence, fast MAC
movement, etc. Finally, using anycast SID, the proposed approach
provides All-Active multihoming as well as multipathing and ARP
suppression.
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7. Security Considerations
The mechanisms in this document use Segment Routing control plane as
defined in Security considerations described in Segment Routing
control plane are equally applicable.
8. IANA Considerations
TBD.
9. Acknowledgements
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[RFC8660] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing with the MPLS Data Plane", RFC 8660,
DOI 10.17487/RFC8660, December 2019,
<https://www.rfc-editor.org/info/rfc8660>.
[RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
<https://www.rfc-editor.org/info/rfc8754>.
10.2. Informative References
[I-D.ietf-spring-segment-routing-policy]
Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
P. Mattes, "Segment Routing Policy Architecture", Work in
Progress, Internet-Draft, draft-ietf-spring-segment-
routing-policy-22, 22 March 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-spring-
segment-routing-policy-22>.
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[I-D.voyer-pim-sr-p2mp-policy]
Voyer, D., Filsfils, C., Parekh, R., Bidgoli, H., and Z.
J. Zhang, "Segment Routing Point-to-Multipoint Policy",
Work in Progress, Internet-Draft, draft-voyer-pim-sr-p2mp-
policy-02, 10 July 2020,
<https://datatracker.ietf.org/doc/html/draft-voyer-pim-sr-
p2mp-policy-02>.
[RFC4761] Kompella, K., Ed. and Y. Rekhter, Ed., "Virtual Private
LAN Service (VPLS) Using BGP for Auto-Discovery and
Signaling", RFC 4761, DOI 10.17487/RFC4761, January 2007,
<https://www.rfc-editor.org/info/rfc4761>.
[RFC4762] Lasserre, M., Ed. and V. Kompella, Ed., "Virtual Private
LAN Service (VPLS) Using Label Distribution Protocol (LDP)
Signaling", RFC 4762, DOI 10.17487/RFC4762, January 2007,
<https://www.rfc-editor.org/info/rfc4762>.
Authors' Addresses
Sami Boutros (editor)
Ciena Corporation
United States of America
Email: sboutros@ciena.com
Siva Sivabalan (editor)
Ciena Corporation
Canada
Email: ssivabal@ciena.com
Himanshu Shah
Ciena Corporation
United States of America
Email: hshah@ciena.com
James Uttaro
ATT
United States of America
Email: ju1738@att.com
Daniel Voyer
Bell Canada
Canada
Email: daniel.voyer@bell.ca
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Bin Wen
Comcast
United States of America
Email: bin_wen@cable.comcast.com
Luay Jalil
Verizon
United States of America
Email: luay.jalil@verizon.com
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