IPv6 Multicast Source Routing
Traffic EngineeringHuawei Technologiesgengxuesong@huawei.comHuawei Technologieslizhenbin@huawei.comHuawei Technologiesxiejingrong@huawei.comThis document defines 2 new types of segment: End.RL and End.RL.X ,
and the corresponding packet processing procedures over the IPv6 data
plane for the MSR6(Multicast Source Routing over IPv6) TE solutions.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.RoSegment Routing () leverages the mechanism of
source routing. An ingress node steers a packet through an ordered list
of instructions, called "segments". Each one of these instructions
represents a function to be implemented at a specific location in the
network. A function is locally defined on the node where it is executed.
Network Programming combines Segment Routing functions to achieve a
networking objective that goes beyond mere packet routing. defines the SRv6 Network Programming concept and
specifies the main Segment Routing behaviors and network programming
functions.Bit Index Explicit Replication (BIER) is an
architecture that provides optimal multicast forwarding without
requiring a protocol for explicitly building multicast distribution
trees or per-flow state maintained by intermediate routers. When a
multicast data packet enters BIER forwarding domain, the ingress node
encapsulates the packet with a bitstring, each bitposition of which
presents the egress nodes. To forward the packet to a given set of
egress nodes, the bits corresponding to those egress nodes are set in
the bitstring. The intermediate nodes in the BIER domain replicate and
forward the packet based on the bitstring.The mechanism of forwarding a
packet based on bitstring of BIER are specified in .An IPv6 based multicast source routing (MSR6) solution is defined in
, which
leverages the benefits of source routing over IPv6 data plane to provide
simplified multicast TE and BE service in an IPv6 network without
unnecessary multicast tree status and complex control plane protocols.
MSR6 needs to reuse the advantages of SRv6 and BIER to implement source
routing.MSR6 has two basic modes of forwarding: one is based on Shortest Path
First(SPF), which is called MSR6 BE mode; the other is based on traffic
engineered, which is called MSR6 TE mode. This document defines 2 new
types of segment: End.RL and End.RL.X , and the corresponding packet
processing procedures over the IPv6 data plane for the MSR6 TE
solutions.MSR6: Multicast Source Routing over IPv6, defined in .MRH: Multicast Routing Header, a new type of Routing Header which is
used for MSR6.Multicast domain: A set of network device which could provide P2MP
multicast transport. In this document, the multicast domain is an MSR
domain, where every nodes support the capability of MSR6.Root Node: Root node is the beginning point of a multicast tree and
also the initiation node of a multicast tunnel. It encapsulates the
packet with a multicast header. The type of the encapsulation depends on
the routing protocol used in the multicast domain. For MSR6 TE, the
encapsulation is MSR6 TE header, which is an IPv6 header with MRH.Replication Endpoint: the intermediate node of a multicast tree,
which replicates packet and forwards the packet to the downstream nodes.
For MSR6, the Replication Node is called Replication Endpoint which can
be indicated by the MSR6 Segment and replicate packets according to the
multicast source routing information encapsulation in the MSR6 header of
the packet.Leaf Node: Leaf node is the end point of a multicast tree and also
the decapsulation node of a multicast tunnel. It decapsulates the
multicast header in the packet and forwards the packet based on overlay
encapsulation.Parent Node: The parent node is the node that does the packet
replication, corresponding to the concept of a child node.Child Node: The child node is the downstream node that will receive
the packet which has been replicated ,corresponding to the concept of a
parent node.In order to implement MSR6 TE, the nodes and links along the path
must be explicitly specified in the packet.In SRv6, the segment list is suitable for source route of unicast
path, which is P2P and could be presented by the data structure of a
chain with a pointer; In the Segment Routing Header(SRH), the segment
list is the chain, and the segment left is the pointer. But for the
multicast, the path of multicast service is a tree, and can not be
presented by data structure in a one-dimensional array. So this document
introduces new segment to present node/link in a multicast tree and
introduce structure information in the segment to indicate the
parent-child relationship during multicast replication. And all the
nodes/links along the multicast tree can be encoded into the segment
list.In a multicast tree, when the packet is replicated from one node to
multiple downstream nodes, parent-child relationship is built up between
these nodes. MRH is supposed to encode the nodes along the P2MP path and
also the parent-child relationship between them.As defined in [RFC8402], an Segment Identifier is an IPv6 address
explicitly associated with the segment. When an SRv6 SID is in the
Destination Address field of an IPv6 header of a packet, it is routed
through transit nodes in an IPv6 network as an IPv6 address.MSR6 Segment Identifier complies with the definition of the Segment
Identifier in . Following the specification in
RFC8986, an MSR6 SID is consisting of LOC:FUNCT:ARG. Locator could be
represented as B:N, B is the MSR6 segment block(IPv6 prefix allocated
for MSR6 segment by the operator) and N is the identifier of the node
instantiating the SID. The MSR6 locator is routable and leads to the
node which instantiates the SID; Function is the identification of a
local behavior bound to the MSR6 SID; Argument is the additional
information requested by the function.Typical MSR6 functions and arguments are defined in the following
sections.End.RL SID is one of the basic MSR6 SID as an extension of End SID
defined in RFC8986.The encoding of segment list which is composed of End.RL SIDs
follow the rules:There MUST be an End.RL SID for each Replication Endpoint in
the multicast treeThe End.RL SIDs, which represent child nodes that have the same
parent node, MUST be arranged consecutively in the segment
listThe End.RL SID for a parent node MUST indicate the position of
the End.RL SIDs for the child nodesThere are 2 arguments for each End.RL SID:"Replication number" is used for indicating how many times the
parent node is supposed to execute replication"Pointer" is used for indicating the position of End.RL SID for
the first child node in the segment list;When the packet is received by an Replication Endpoint and
the DA of this packet is a local SID with the function of End.RL, the
packet will be replicated based on the "replication number". The DA of
the 1st replicated packet is replaced by the SL="pointer" and the
value of Segment Left is set to "pointer"; the DA of the 2nd
replicated packet is replaced by the SL="pointer +1" and the value of
Segment Left is set to "pointer+1". The packet is sent out based on
the updated DA. Repeat the operation untill all the replicated packets
are sent out.The replication node does the following when the DA of the packet
is a local End.RL SID.End.RL.X SID is one of the basic MSR6 SID as an extension of End.X
SID defined in RFC8986.The encoding of segment list which is composed of End.RL.X SIDs
follow the rules:There MUST be an End.RL.X SID for each downstream link
connected to a Replication Endpoint in the multicast treeThe End.RL.X SIDs corresponding to downstream links of the same
Replication Endpoint MUST be arranged consecutively in the segment
listThe End.RL.X SID for a parent node MUST indicate the position
of the End.RL.X SIDs for the child nodesThere are 2 arguments for each End.RL SID:"Replication number" is used for indicating how many times the
existing node is supposed to execute replication"Pointer" is used for indicating the position of End.RL.X SID
for the first downstream link of the child node in the segment
list;When the packet is received by a Replication Endpoint and the DA of
this packet is a local SID with the function of End.RL.X, the packet
will be replicated based on the "replication number". The DA of the
1st replicated packet is replaced by the Segment List[ pointer ] and
the value of Segment Left is set to "pointer"; The packet is sent out
through the link indicated by the SID. the DA of the 2nd replicated
packet is replaced by the Segment List [SL +1] and the value of
Segment Left is set to "SL+1"; The DA of the 2nd packet is replaced by
the Segment List [ pointer2 ] (pointer2 is the argument in the Segment
List [SL+1]) and the value of Segment Left is set to "pointer2"; The
packet is sent out through the link indicated by the SID; Repeat the
operation untill all the replicated packets are sent out through the
indicated links.A new type of Routing Header is defined as follows for MSR6 Traffic
Engineering called Multicast Routing Header(MRH).Each M-SID in the MRH is shown in the following figure:It includes:Locator: could represented as B:N where B is the MRH segment block
(IPv6 prefix allocated for MRH segment by the operator) and N is the
identifier of the node instantiating the SID. When the locator part of
the MRH segment is routable, it leads to the node, which instantiates
the SID.Function: an identification of a local behavior bound to the MRH.
Replication is default behavior for any MRH segment, which doesn't need
function indication.Replication number is used for indicating how many times the existing
node is supposed to excute replicationPointer is used for indicating the position of MSR6 SID for the child
node;Segments for the group of child nodes which belong to the same parent
node MUST be encoded together in the segment list. So Pointer and
replication number could determine the upper bound and lower bound of
the value range of segment left for the child nodes.The destination address of the packet is the IPv6 address of the
existing Replication Endpoint. The next header of the IPv6 header points
to a Routing Header and the type of the routing header is MRH Type.Different from unicast, MSR is designed for an explicit multicast
tree rather than a p2p path: the former contains more nodes than the
latter generally. So the header overhead is one of the key challenge
for MSR6, which limits the scalability and the potential deployment
scenario.There are some SRH compression solutions under discussion in IETF.
These solutions could also be used for MSR6 with modification.The compressed MRH6 is designed as follows:Each M-SID in the MRH is shown in the following figure:It includes:Node ID: is the identifier of the node instantiating the SID.
Common Profix and Node ID together form a routable IPv6 addressFunction: an identification of a local behavior bound to the MRH.
Replication is default behavior for any MRH segment, which doesn't
need function indication.Arguments: follow the definition in section 6, including
replication number and pointer.The processing process is:The destination address of the packet is the IPv6 address of the
existing Replication Endpoint. The next header of the IPv6 header
points to a Routing Header and the type of the routing header is MRH
Type 2. The Replication Endpoint is supposed toFind the next groups of segments in the MRH which present the
child nodes of the existing Replication Endpoint with Pointer-1
and Pointer-2;Replicate the packet based on the number of child nodes, which
is the replication number;Replace the SID part of IPv6 destination address with the
corresponding child node's segment in order for each replicated
packet;Update the segment left field of each replicated packet based
on the location child node's segment;Forward the packet to the output port based on the FIB with the
existing destination address as an entry;In order toMSR-R: MSR6 Root Node, including Node A;MSR End: MSR6 Replication Endpoint, including Node B and C;MSR-L: MSR6 Leaf Nodes, including Node D, E, F, G;In node A, the packet is encapsulated the packet with an IPv6
header carrying an MRH. The segment list in the MRH is as follows. The
destination address of the IPv6 header is the 1st SID in the segment
list, which is the local SID of node A. Based on the End.RL behavior
defined in section 5.1, the packet is replicated to 2 (Replication
Number+1=1+1=2) packets. In the 1st packet, the Segment Left is set to
2 and DA is replaced by Segment List[2]; In the 2nd packet, the
Segment Left is set to 4 and DA is replaced by Segment List[4]; The
packets are routed to the node B and node C respectively.In MSR6 Replication Endpoint B, the destination address of the IPv6
header is the local SID of node B. Based on the End.RL behavior
defined in section 5.1, the packet is replicated to 2 (Replication
Number+1=1+1=2) packets. In the 1st packet, the Segment Left is set to
4 and DA is replaced by Segment List[4]; In the 2nd packet, the
Segment Left is set to 5 and DA is replaced by Segment List[5]; The
packets are routed to the node D and node E respectively.In MSR Endpoint C, the process is similar.In MSRE D, the destination address of the IPv6 header is the local
SID of node D. Based on the End.RL behavior defined in section 5.1,
when replication number=0, node D stops processing the MRH and
continues to process the next header in the packet.In node A, the packet is encapsulated the packet with an IPv6
header carrying an MRH. The segment list in the MRH is as follows. The
destination address of the IPv6 header is the local SID of node A.
Based on the End.RL.X behavior defined in section 5.2, the packet is
replicated to 2 (Replication Number+1=1+1=2) packets. In the 1st
packet, the Segment Left is set to 3 and DA is replaced by Segment
List[3] and send the packet based on the specified adjacency A1; In
the 2nd packet, the Segment Left is set to 2 and DA is replaced by
Segment List[2]; Based on the arguments in the updated DA, the Segment
Left is set to 5 and DA is replaced by Segment List[5] and send the
packet based on the specified adjacency A2;In node B, the packet is encapsulated the packet with an IPv6
header carrying an MRH. The segment list in the MRH is as follows. The
destination address of the IPv6 header is the local SID of node B.
Based on the End.RL.X behavior defined in section 5.2, the packet is
replicated to 2 (Replication Number+1=1+1=2) packets. In the 1st
packet, the DA is replaced by the corresponding leaf stored in the
node and send the packet based on the specified adjacency B1; In the
2nd packet, the DA is replaced by the corresponding leaf stored in the
node and send the packet based on the specified adjacency B2;In MSR Endpoint C, the process is similar.In MSRE D, the destination address of the IPv6 header is the local
SID of node D. Node D stops processing the MRH, and begin to process
the next header in the packet.This document makes no request of IANA.Note to RFC Editor: this section may be removed on publication as an
RFC.