SR Replication Segment for
Multi-point Service DeliveryBell CanadaMontrealCAdaniel.voyer@bell.caCisco Systems, Inc.BrusselsBEcfilsfil@cisco.comCisco Systems, Inc.San JoseUSriparekh@cisco.comNokiaOttawaCAhooman.bidgoli@nokia.comJuniper Networkszzhang@juniper.netThis 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.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.We define a new type of segment for Segment Routing , 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 .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:Replication-ID: An identifier for a Replication segment that is
unique in context of the Replication Node.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 .A Replication segment includes the following elements: Replication SID: The Segment Identifier of a Replication segment.
This is a SR-MPLS label or a SRv6 SID .Downstream Nodes: Set of nodes in Segment Routing domain to which
a packet is replicated by the Replication segment.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 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 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 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:When the Active Segment is the
Replication SID. In this case, the operation for a replicated copy
is CONTINUE.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 . 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.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
that has been widely used for MVPN and EVPN BUM (Broadcast,
Unknown and Multicast) traffic.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 .This document makes no request of IANA.There are no additional security risks introduced by this design.The authors would like to acknowledge Siva Sivabalan, Mike Koldychev,
Vishnu Pavan Beeram, Alexander Vainshtein, Bruno Decraene and Joel
Halpern for their valuable inputs.Clayton Hassen Bell Canada Vancouver CanadaEmail: clayton.hassen@bell.caKurtis Gillis Bell Canada Halifax CanadaEmail: kurtis.gillis@bell.caArvind Venkateswaran Cisco Systems, Inc.
San Jose USEmail: arvvenka@cisco.comZafar Ali Cisco Systems, Inc. USEmail: zali@cisco.comSwadesh Agrawal Cisco Systems, Inc. San Jose USEmail: swaagraw@cisco.comJayant Kotalwar Nokia Mountain View USEmail: jayant.kotalwar@nokia.comTanmoy Kundu Nokia
Mountain View USEmail: tanmoy.kundu@nokia.comAndrew Stone Nokia
Ottawa CanadaEmail: andrew.stone@nokia.comTarek Saad Juniper Networks CanadaEmail:tsaad@juniper.netThis 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 .Consider the following topology: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.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 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 at R6:Replication Segment at R7:When a packet is steered into the replication segment at R1:Since R1 is directly connected to R2, R1 performs PUSH operation
with just <R-SID2> label for the replicated copy and sends it
to R2 on interface L12. R2, as Leaf, performs NEXT operation, pops
R-SID2 label and delivers the payload.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-SID6
label and delivers the payload.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.