Network Working Group D. Voyer, Ed.
Internet-Draft Bell Canada
Intended status: Standards Track C. Filsfils
Expires: January 11, 2021 R. Parekh
Cisco Systems, Inc.
H. Bidgoli
Nokia
Z. Zhang
Juniper Networks
July 10, 2020
SR Replication Segment for Multi-point Service Delivery
draft-voyer-spring-sr-replication-segment-04
Abstract
This document describes the SR Replication segment for Multi-point
service delivery. A SR Replication segment allows a packet to be
replicated from a replication node to downstream nodes.
Requirements Language
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 RFC 2119 [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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This Internet-Draft will expire on January 11, 2021.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Replication Segment . . . . . . . . . . . . . . . . . . . . . 3
3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Illustration of a Replication Segment . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
We define a new type of segment for Segment Routing [RFC8402], called
Replication segment, which allows a node (henceforth called as
Replication Node) to replicate packets to a set of other nodes
(called Downstream Nodes) in a Segment Routing Domain. Replication
segments provide building blocks for Point-to-Multipoint Service
delivery via SR Point-to-Multipoint (SR P2MP) policy. A Replication
segment can replicate packet to directly connected nodes or to
downstream nodes (without need for state on the transit routers).
This document focuses on the Replication Segment building block. The
use of one or more stitched Replication Segments constructed for SR
P2MP Policy tree is specified in [I-D.voyer-pim-sr-p2mp-policy].
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2. Replication Segment
In a Segment Routing Domain, a Replication segment is a logical
construct which connects a Replication Node to a set of Downstream
Nodes. A Replication segment is a local segment instantiated at a
Replication node. It can be either provisioned locally on a node or
programmed by a PCE. Replication segments apply equally to both SR-
MPLS and SRv6 instantiations of Segment Routing.
A Replication segment is identified by the tuple <Replication-ID,
Node-ID>, where:
o Replication-ID: An identifier for a Replication segment that is
unique in context of the Replication Node.
o Node-ID: The address of the Replication Node that the Replication
segment is for. Note that the root of a multi-point service is
also a replication node.
In simplest case, Replication-ID can be a 32-bit number, but it can
be extended or modified as required based on specific use of a
Replication segment. When the PCE signals a Replication segment to
its node, the <Replication-ID, Node-ID> tuple identifies the segment.
Examples of such signaling and extension are described in
[I-D.voyer-pim-sr-p2mp-policy].
A Replication segment includes the following elements:
o Replication SID: The Segment Identifier of a Replication segment.
This is a SR-MPLS label or a SRv6 SID [RFC8402].
o Downstream Nodes: Set of nodes in Segment Routing domain to which
a packet is replicated by the Replication segment.
o Replication State: See below.
The Downstream Nodes and Replication State of a Replication segment
can change over time, depending on the network state and leaf nodes
of a multi-point service that the segment is part of.
Replication State is a list of replication branches to the Downstream
Nodes. In this document, each branch is abstracted to a <Downstream
Node, Downstream Replication SID> tuple. A Downstream Node is
represented by a SID-list or a Segment Routing Policy
[I-D.ietf-spring-segment-routing-policy] that specifies the explicit
path from the Replication Node to the Downstream Node, or even
represented by another Replication segment. The SID-list MAY just
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have one SID. If a downstream node is adjacent to a Replication
node, it MAY also be represented by an interface.
Replication SID identifies the Replication segment in the forwarding
plane. At a Replication node, the Replication SID is the equivalent
of Binding SID [I-D.ietf-spring-segment-routing-policy] of a Segment
Routing Policy.
A packet steered into a Replication segment at a Replication node is
replicated to each Downstream Node with the Downstream Replication
SID that is relevant at that node. A packet is steered into a
Replication Segment in two ways:
o When the Active Segment [RFC8402] is the Replication SID. In this
case, the operation for a replicated copy is CONTINUE.
o On the root of a multi-point service, based on local policy-based
routing. In this case, the operation for a replicated copy is
PUSH.
If a Downstream Node is an egress (aka leaf) of the multi-point
service, i.e. no further replication is needed, then that leaf node's
Replication segment will not have any Replication State and the
operation is NEXT. At an egress node, the Replication SID MAY be
used to identify that portion of the multi-point service. Notice
that the segment on the leaf node is still referred to as a
Replication segment for the purpose of generalization.
A node can be a bud node, i.e. it is a replication node and a leaf
node of a multi-point service at the same time
[I-D.voyer-pim-sr-p2mp-policy]. In this case, the Replication
segment's Replication State includes a branch with the Downstream
Node being itself and the operation for the replicated copy is NEXT.
The Replication SID MUST be the last SID (at the bottom of stack for
SR-MPLS) in a packet that is steered out from a Replication node of a
Replication Segment. The behavior at Downstream nodes of a
Replication Segment is undefined If there are any SIDs after the
Replication SID and is outside the scope of this document.
3. Use Cases
In the simplest use case, a single Replication segment includes the
root node of a multi-point service and the egress/leaf nodes of the
the service as all the Downstream Nodes. This achieves Ingress
Replication [RFC7988] that has been widely used for MVPN [RFC6513]
and EVPN [RFC7432] BUM (Broadcast, Unknown and Multicast) traffic.
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Replication segments can also be used as building blocks for
replication trees when Replication segments on the root, intermediate
replication nodes and leaf nodes are stitched together to achieve
efficient replication. That is specified in
[I-D.voyer-pim-sr-p2mp-policy].
4. IANA Considerations
This document makes no request of IANA.
5. Security Considerations
There are no additional security risks introduced by this design.
6. Acknowledgements
The authors would like to acknowledge Siva Sivabalan, Mike Koldychev,
Vishnu Pavan Beeram, Alexander Vainshtein, Bruno Decraene and Joel
Halpern for their valuable inputs.
7. Contributors
Clayton Hassen
Bell Canada
Vancouver
Canada
Email: clayton.hassen@bell.ca
Kurtis Gillis
Bell Canada
Halifax
Canada
Email: kurtis.gillis@bell.ca
Arvind Venkateswaran
Cisco Systems, Inc.
San Jose
US
Email: arvvenka@cisco.com
Zafar Ali
Cisco Systems, Inc.
US
Email: zali@cisco.com
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Swadesh Agrawal
Cisco Systems, Inc.
San Jose
US
Email: swaagraw@cisco.com
Jayant Kotalwar
Nokia
Mountain View
US
Email: jayant.kotalwar@nokia.com
Tanmoy Kundu
Nokia
Mountain View
US
Email: tanmoy.kundu@nokia.com
Andrew Stone
Nokia
Ottawa
Canada
Email: andrew.stone@nokia.com
Tarek Saad
Juniper Networks
Canada
Email:tsaad@juniper.net
8. References
8.1. Normative References
[I-D.ietf-spring-segment-routing-policy]
Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
P. Mattes, "Segment Routing Policy Architecture", draft-
ietf-spring-segment-routing-policy-08 (work in progress),
July 2020.
[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>.
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[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>.
8.2. Informative References
[I-D.voyer-pim-sr-p2mp-policy]
Voyer, D., Filsfils, C., Parekh, R., Bidgoli, H., and Z.
Zhang, "Segment Routing Point-to-Multipoint Policy",
draft-voyer-pim-sr-p2mp-policy-01 (work in progress),
April 2020.
[RFC6513] Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/
BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February
2012, <https://www.rfc-editor.org/info/rfc6513>.
[RFC7432] Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
2015, <https://www.rfc-editor.org/info/rfc7432>.
[RFC7988] Rosen, E., Ed., Subramanian, K., and Z. Zhang, "Ingress
Replication Tunnels in Multicast VPN", RFC 7988,
DOI 10.17487/RFC7988, October 2016,
<https://www.rfc-editor.org/info/rfc7988>.
Appendix A. Illustration of a Replication Segment
This section illustrates an example of a single Replication Segment.
Examples showing Replication Segment stitched together to form P2MP
tree (based on SR P2MP policy) are in [I-D.voyer-pim-sr-p2mp-policy].
Consider the following topology:
R3------R6
/ \
R1----R2----R5-----R7
\ /
+--R4---+
Figure 1
In this example, the Node-SID of a node Rn is N-SIDn and Adjacency-
SID from node Rm to node Rn is A-SIDmn. Interface between Rm and Rn
is Lmn.
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Assume a Replication Segment identified with R-ID at replication node
R1 and downstream Nodes R2, R6 and R7. The Replication SID at node n
is R-SIDn. A packet replicated from R1 to R7 has to traverse R4.
The Replication Segment state at nodes R1, R2, R6 and R7 is shown
below. Note nodes R3, R4 and R5 do not have state for the
Replication Segment.
Replication Segment at R1:
Replication Segment <R-ID,R1>:
Replication SID: R-SID1
Replication State:
R2: <R-SID1->L12>
R6: <N-SID6, R-SID6>
R7: <N-SID4, A-SID47, R-SID7>
Replication to R2 steers packet directly to R2 on interface L12.
Replication to R6, using N-SID6, steers packet via IGP shortest path
to that node. Replication to R7 is steered via R4, using N-SID4 and
then adjacency SID A-sID47 to R7.
Replication Segment at R2:
Replication Segment <R-ID,R2>:
Replication SID: R-SID2
Replication State:
R2: <Leaf>
Replication Segment at R6:
Replication Segment <R-ID,R6>:
Replication SID: R-SID6
Replication State:
R6: <Leaf>
Replication Segment at R7:
Replication Segment <R-ID,R7>:
Replication SID: R-SID7
Replication State:
R7: <Leaf>
When a packet is steered into the replication segment at R1:
o Since R1 is directly connected to R2, R1 performs PUSH operation
with just <R-SID2> label for the replicated copy and sends it to
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R2 on interface L12. R2, as Leaf, performs NEXT operation, pops
R-SID2 label and delivers the payload.
o R1 performs PUSH operation with <N-SID6, R-SID6> label stack for
the replicated copy to R6 and sends it to R2, the nexthop on IGP
shortest path to R6. R2 performs CONTINUE operation on N-SID6 and
forwards it to R3. R3 is the penultimate hop for N-SID6; it
performs penultimate hop popping, which corresponds to the NEXT
operation and the packet is then sent to R6 with <R-SID6> in the
label stack. R6, as Leaf, performs NEXT operation, pops R-SID1
label and delivers the payload.
o R1 performs PUSH operation with <N-SID4, A-SID47, R-SID7> label
stack for the replicated copy to R7 and sends it to R2, the
nexthop on IGP shortest path to R4. R2 is the penultimate hop for
N-SID4; it performs penultimate hop popping, which corresponds to
the NEXT operation and the packet is then sent to R4 with
<A-SID47, R-SID1> in the label stack. R4 performs NEXT operation,
pops A-SID47, and delivers packet to R7 with <R-SID7> in the label
stack. R7, as Leaf, performs NEXT operation, pops R-SID7 label
and delivers the payload.
Authors' Addresses
Daniel Voyer (editor)
Bell Canada
Montreal
CA
Email: daniel.voyer@bell.ca
Clarence Filsfils
Cisco Systems, Inc.
Brussels
BE
Email: cfilsfil@cisco.com
Rishabh Parekh
Cisco Systems, Inc.
San Jose
US
Email: riparekh@cisco.com
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Hooman Bidgoli
Nokia
Ottawa
CA
Email: hooman.bidgoli@nokia.com
Zhaohui Zhang
Juniper Networks
Email: zzhang@juniper.net
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