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 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.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.In a branch tuple, <Downstream Node> represents the
reachability from the Replication Node to the Downstream Node. In its
simplest form, this MAY be specified as an interface or nexthop if
downstream node is adjacent to the Replication Node. The reachability
may be specified in terms of Flex-Algo path (including the default algo)
, or specified by an SR explicit
path represented either by a SID-list (of one or more SIDs) or by a
Segment Routing Policy .A packet is steered into a Replication segment at a Replication Node
in two ways:When the Active Segment is a locally
instantiated Replication SIDBy the root of a multi-point service based on local configuration
outside the scope of this document.In either case, the packet is replicated to each Downstream node in
the associated Replication state.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 .When the Active Segment is a Replication SID, the processing
results in a POP operation and lookup of the associated Replication
state. For each replication in the Replication state, the operation is
a PUSH of the downstream Replication SID and an optional segment list
on to the packet which is forwarded to the Downstream node. For leaf
nodes the inner packet is forwarded as per local configuration.When the root of a multi-point service steers a packet to a
Replication segment, it results in a replication to each Downstream
node in the associated replication state. The operation is a PUSH of
the replication SID and an optional segment list on to the packet
which is forwarded to the downstream node.In SRv6 , the “Endpoint with
replication” behavior (End.Replicate for short) replicates a
packet and forwards the packet according to a Replication state.When processing a packet destined to a local Replication-SID, the
packet is replicated to Downstream nodes and/or locally delivered off
tree (when this is a bud/leaf node) according to the associated
replication state. For replication, the outer header is re-used, and
the Downstream Replication SID is written into the outer IPv6 header
destination address. If required, an optional segment list may be used
on some branches using H.Encaps.Red (while some other branches may not
need that). Note that this H.Encaps.Red is independent from the
replication segment – it is just used to steer the replicated
traffic on a traffic engineered path to a Downstream node. If SRv6 SID
compression is possible , the Replication node
SHOULD use a CSID container with Downstream Replication SID as the
Last uSID in the container instead of H.Encaps.Red.The above also applies when the Replication segment is for the Root
node, whose upstream node has placed the Replication-SID in the
header. A local application (e.g. MVPN/EVPN) may also apply
H.Encaps.Red and then steer the resulting traffic into the segment.
Again note that the H.Encaps.Red is independent of the Replication
segment – it is the action of the application (e.g. MVPN/EVPN
service). If the service is on a Root node, the two H.Encaps
mentioned, one for the service and other in the previous paragraph for
replication to Downstream node SHOULD be combined for optimization (to
avoid extra IPv6 encapsulation).For the local delivery on a bud/leaf node, the action associated
with Replication-SID is “look at next SID in SRH”. The
next SID could be a SID with End.DTMC4/6 or End.DT2M local behavior
(equivalent of MVPN/EVPN PMSI label in case of tunnel sharing across
multiple VPNs). There may also not be a next SID (e.g. MVPN/EVPN with
one tunnel per VPN), in which case the Replication-SID is then
equivalent to End.DTM4/6 or End.DT2M. Note that decapsulation is not
an inherent action of a Replication segment even on a bud/leaf
node.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
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 requests IANA to allocate the following codepoints in
"SRv6 Endpoint Behaviors" sub-registry of "Segment Routing Parameters"
top-level registry.ValueHexEndpoint behaviorReference750x004BEnd.Replicate[This.ID]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, Thierry
Couture 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.netKamran Raza Cisco Systems, Inc. CanadaEmail:skraza@cisco.comThis 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.For SRv6 , we use SID allocation scheme, reproduced below, from
Illustrations for SRv6 Network Programming 2001:db8::/32 is an IPv6 block allocated by a RIR to the
operator2001:db8:0::/48 is dedicated to the internal address space2001:db8:cccc::/48 is dedicated to the internal SRv6 SID
spaceWe assume a location expressed in 64 bits and a function
expressed in 16 bitsNode k has a classic IPv6 loopback address 2001:db8::k/128
which is advertised in the IGPNode k has 2001:db8:cccc:k::/64 for its local SID space. Its
SIDs will be explicitly assigned from that blockNode k advertises 2001:db8:cccc:k::/64 in its IGPFunction :1:: (function 1, for short) represents the End
function with PSP supportFunction :Cn:: (function Cn, for short) represents the End.X
function from to Node nEach node k has: An explicit SID instantiation 2001:db8:cccc:k:1::/128 bound to
an End function with additional support for PSPAn explicit SID instantiation 2001:db8:cccc:k:Cj::/128 bound to
an End.X function to neighbor J with additional support for
PSPAn explicit SID instantiation 2001:db8:cccc:k:Fk::/128 bound to
an End.Replcate functionAssume a Replication segment identified with R-ID at Replication
Node R1 and downstream Nodes R2, R6 and R7. The Replication SID at
node k, bound to an End.Replcate function, is
2001:db8:cccc:k:Fk::/128. 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 2001:db8:cccc:6:F6::0, steers packet via IGP
shortest path to that node. Replication to R7 is steered via R4, using
End.X SID 2001:db8:cccc:4:C7::0 at R4 to R7.Replication segment at R2:Replication segment at R6:Replication segment at R7:When a packet, (A,B2), is steered into the Replication segment at
R1:Since R1 is directly connected to R2, R1 creates encapsulated
replicated copy (2001:db8::1, 2001:db8:cccc:2:F2::0) (A, B2), and
sends it to R2 on interface L12. R2, as Leaf, removes outer IPv6
header and delivers the payload.R1 creates encapsulated replicated copy (2001:db8::1,
2001:db8:cccc:6:F6::0) (A, B2) then forwards the resulting packet
on the shortest path to 2001:db8:cccc:6::/64. R2 and R3 forward
the packet using 2001:db8:cccc:6::/64. R6, as Leaf, removes outer
IPv6 header and delivers the payload.R1 creates encapsulated replicated copy (2001:db8::1,
2001:db8:cccc:4:C7::0) (2001:db8:cccc:7:F7::0; SL=1) (A, B2) and
sends it to R2, the nexthop on IGP shortest path to
2001:db8:cccc:4::/64. R2 forwards packet to R4 using
2001:db8:cccc:4::/64. R4 executes End.X function on
2001:db8:cccc:4:C7::0, performs PSP action, removes SRH and sends
resulting packet (2001:db8::1, 2001:db8:cccc:7:F7::0) (A, B2) to
R7. R7, as Leaf, removes outer IPv6 header and delivers the
payload.