MBONED                                                        M. McBride
Internet-Draft                                                    Huawei
Intended status: Informational                               O. Komolafe
Expires: August 11, 2019                                 Arista Networks
                                                       February 07, 2019

                 Multicast in the Data Center Overview


   The volume and importance of one-to-many traffic patterns in data
   centers is likely to increase significantly in the future.  Reasons
   for this increase are discussed and then attention is paid to the
   manner in which this traffic pattern may be judiously handled in data
   centers.  The intuitive solution of deploying conventional IP
   multicast within data centers is explored and evaluated.  Thereafter,
   a number of emerging innovative approaches are described before a
   number of recommendations are made.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on August 11, 2019.

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   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   carefully, as they describe your rights and restrictions with respect

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Reasons for increasing one-to-many traffic patterns . . . . .   3
     2.1.  Applications  . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Overlays  . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Protocols . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Handling one-to-many traffic using conventional multicast . .   6
     3.1.  Layer 3 multicast . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Layer 2 multicast . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Example use cases . . . . . . . . . . . . . . . . . . . .   8
     3.4.  Advantages and disadvantages  . . . . . . . . . . . . . .   9
   4.  Alternative options for handling one-to-many traffic  . . . .   9
     4.1.  Minimizing traffic volumes  . . . . . . . . . . . . . . .  10
     4.2.  Head end replication  . . . . . . . . . . . . . . . . . .  10
     4.3.  BIER  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.4.  Segment Routing . . . . . . . . . . . . . . . . . . . . .  12
   5.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .  12
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   The volume and importance of one-to-many traffic patterns in data
   centers is likely to increase significantly in the future.  Reasons
   for this increase include the nature of the traffic generated by
   applications hosted in the data center, the need to handle broadcast,
   unknown unicast and multicast (BUM) traffic within the overlay
   technologies used to support multi-tenancy at scale, and the use of
   certain protocols that traditionally require one-to-many control
   message exchanges.  These trends, allied with the expectation that
   future highly virtualized data centers must support communication
   between potentially thousands of participants, may lead to the
   natural assumption that IP multicast will be widely used in data
   centers, specifically given the bandwidth savings it potentially
   offers.  However, such an assumption would be wrong.  In fact, there
   is widespread reluctance to enable IP multicast in data centers for a

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   number of reasons, mostly pertaining to concerns about its
   scalability and reliability.

   This draft discusses some of the main drivers for the increasing
   volume and importance of one-to-many traffic patterns in data
   centers.  Thereafter, the manner in which conventional IP multicast
   may be used to handle this traffic pattern is discussed and some of
   the associated challenges highlighted.  Following this discussion, a
   number of alternative emerging approaches are introduced, before
   concluding by discussing key trends and making a number of

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119.

2.  Reasons for increasing one-to-many traffic patterns

2.1.  Applications

   Key trends suggest that the nature of the applications likely to
   dominate future highly-virtualized multi-tenant data centers will
   produce large volumes of one-to-many traffic.  For example, it is
   well-known that traffic flows in data centers have evolved from being
   predominantly North-South (e.g. client-server) to predominantly East-
   West (e.g.  distributed computation).  This change has led to the
   consensus that topologies such as the Leaf/Spine, that are easier to
   scale in the East-West direction, are better suited to the data
   center of the future.  This increase in East-West traffic flows
   results from VMs often having to exchange numerous messages between
   themselves as part of executing a specific workload.  For example, a
   computational workload could require data, or an executable, to be
   disseminated to workers distributed throughout the data center which
   may be subsequently polled for status updates.  The emergence of such
   applications means there is likely to be an increase in one-to-many
   traffic flows with the increasing dominance of East-West traffic.

   The TV broadcast industry is another potential future source of
   applications with one-to-many traffic patterns in data centers.  The
   requirement for robustness, stability and predicability has meant the
   TV broadcast industry has traditionally used TV-specific protocols,
   infrastructure and technologies for transmitting video signals
   between cameras, studios, mixers, encoders, servers etc.  However,
   the growing cost and complexity of supporting this approach,
   especially as the bit rates of the video signals increase due to
   demand for formats such as 4K-UHD and 8K-UHD, means there is a

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   consensus that the TV broadcast industry will transition from
   industry-specific transmission formats (e.g.  SDI, HD-SDI) over TV-
   specific infrastructure to using IP-based infrastructure.  The
   development of pertinent standards by the SMPTE, along with the
   increasing performance of IP routers, means this transition is
   gathering pace.  A possible outcome of this transition will be the
   building of IP data centers in broadcast plants.  Traffic flows in
   the broadcast industry are frequently one-to-many and so if IP data
   centers are deployed in broadcast plants, it is imperative that this
   traffic pattern is supported efficiently in that infrastructure.  In
   fact, a pivotal consideration for broadcasters considering
   transitioning to IP is the manner in which these one-to-many traffic
   flows will be managed and monitored in a data center with an IP

   One of the few success stories in using conventional IP multicast has
   been for disseminating market trading data.  For example, IP
   multicast is commonly used today to deliver stock quotes from the
   stock exchange to financial services provider and then to the stock
   analysts or brokerages.  The network must be designed with no single
   point of failure and in such a way that the network can respond in a
   deterministic manner to any failure.  Typically, redundant servers
   (in a primary/backup or live-live mode) send multicast streams into
   the network, with diverse paths being used across the network.
   Another critical requirement is reliability and traceability;
   regulatory and legal requirements means that the producer of the
   marketing data must know exactly where the flow was sent and be able
   to prove conclusively that the data was received within agreed SLAs.
   The stock exchange generating the one-to-many traffic and stock
   analysts/brokerage that receive the traffic will typically have their
   own data centers.  Therefore, the manner in which one-to-many traffic
   patterns are handled in these data centers are extremely important,
   especially given the requirements and constraints mentioned.

   Many data center cloud providers provide publish and subscribe
   applications.  There can be numerous publishers and subscribers and
   many message channels within a data center.  With publish and
   subscribe servers, a separate message is sent to each subscriber of a
   publication.  With multicast publish/subscribe, only one message is
   sent, regardless of the number of subscribers.  In a publish/
   subscribe system, client applications, some of which are publishers
   and some of which are subscribers, are connected to a network of
   message brokers that receive publications on a number of topics, and
   send the publications on to the subscribers for those topics.  The
   more subscribers there are in the publish/subscribe system, the
   greater the improvement to network utilization there might be with

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2.2.  Overlays

   The proposed architecture for supporting large-scale multi-tenancy in
   highly virtualized data centers [RFC8014] consists of a tenant's VMs
   distributed across the data center connected by a virtual network
   known as the overlay network.  A number of different technologies
   have been proposed for realizing the overlay network, including VXLAN
   [RFC7348], VXLAN-GPE [I-D.ietf-nvo3-vxlan-gpe], NVGRE [RFC7637] and
   GENEVE [I-D.ietf-nvo3-geneve].  The often fervent and arguably
   partisan debate about the relative merits of these overlay
   technologies belies the fact that, conceptually, it may be said that
   these overlays typically simply provide a means to encapsulate and
   tunnel Ethernet frames from the VMs over the data center IP fabric,
   thus emulating a layer 2 segment between the VMs.  Consequently, the
   VMs believe and behave as if they are connected to the tenant's other
   VMs by a conventional layer 2 segment, regardless of their physical
   location within the data center.  Naturally, in a layer 2 segment,
   point to multi-point traffic can result from handling BUM (broadcast,
   unknown unicast and multicast) traffic.  And, compounding this issue
   within data centers, since the tenant's VMs attached to the emulated
   segment may be dispersed throughout the data center, the BUM traffic
   may need to traverse the data center fabric.  Hence, regardless of
   the overlay technology used, due consideration must be given to
   handling BUM traffic, forcing the data center operator to consider
   the manner in which one-to-many communication is handled within the
   IP fabric.

2.3.  Protocols

   Conventionally, some key networking protocols used in data centers
   require one-to-many communication.  For example, ARP and ND use
   broadcast and multicast messages within IPv4 and IPv6 networks
   respectively to discover MAC address to IP address mappings.
   Furthermore, when these protocols are running within an overlay
   network, then it essential to ensure the messages are delivered to
   all the hosts on the emulated layer 2 segment, regardless of physical
   location within the data center.  The challenges associated with
   optimally delivering ARP and ND messages in data centers has
   attracted lots of attention [RFC6820].  Popular approaches in use
   mostly seek to exploit characteristics of data center networks to
   avoid having to broadcast/multicast these messages, as discussed in
   Section 4.1.

   There are networking protocols that are being modified/developed to
   specifically target working in a data center CLOS environment.  BGP
   has been extended to work in these type of DC environments and well
   supports multicast.  RIFT (Routing in Fat Trees) is a new protocol

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   being developed to work efficiently in DC CLOS environments and also
   is being specified to support multicast addressing and forwarding.

3.  Handling one-to-many traffic using conventional multicast

3.1.  Layer 3 multicast

   PIM is the most widely deployed multicast routing protocol and so,
   unsurprisingly, is the primary multicast routing protocol considered
   for use in the data center.  There are three potential popular
   flavours of PIM that may be used: PIM-SM [RFC4601], PIM-SSM [RFC4607]
   or PIM-BIDIR [RFC5015].  It may be said that these different modes of
   PIM tradeoff the optimality of the multicast forwarding tree for the
   amount of multicast forwarding state that must be maintained at
   routers.  SSM provides the most efficient forwarding between sources
   and receivers and thus is most suitable for applications with one-to-
   many traffic patterns.  State is built and maintained for each (S,G)
   flow.  Thus, the amount of multicast forwarding state held by routers
   in the data center is proportional to the number of sources and
   groups.  At the other end of the spectrum, BIDIR is the most
   efficient shared tree solution as one tree is built for all (S,G)s,
   therefore minimizing the amount of state.  This state reduction is at
   the expense of optimal forwarding path between sources and receivers.
   This use of a shared tree makes BIDIR particularly well-suited for
   applications with many-to-many traffic patterns, given that the
   amount of state is uncorrelated to the number of sources.  SSM and
   BIDIR are optimizations of PIM-SM.  PIM-SM is still the most widely
   deployed multicast routing protocol.  PIM-SM can also be the most
   complex.  PIM-SM relies upon a RP (Rendezvous Point) to set up the
   multicast tree and subsequently there is the option of switching to
   the SPT (shortest path tree), similar to SSM, or staying on the
   shared tree, similar to BIDIR.

3.2.  Layer 2 multicast

   With IPv4 unicast address resolution, the translation of an IP
   address to a MAC address is done dynamically by ARP.  With multicast
   address resolution, the mapping from a multicast IPv4 address to a
   multicast MAC address is done by assigning the low-order 23 bits of
   the multicast IPv4 address to fill the low-order 23 bits of the
   multicast MAC address.  Each IPv4 multicast address has 28 unique
   bits (the multicast address range is therefore mapping
   a multicast IP address to a MAC address ignores 5 bits of the IP
   address.  Hence, groups of 32 multicast IP addresses are mapped to
   the same MAC address meaning a a multicast MAC address cannot be
   uniquely mapped to a multicast IPv4 address.  Therefore, planning is
   required within an organization to choose IPv4 multicast addresses
   judiciously in order to avoid address aliasing.  When sending IPv6

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   multicast packets on an Ethernet link, the corresponding destination
   MAC address is a direct mapping of the last 32 bits of the 128 bit
   IPv6 multicast address into the 48 bit MAC address.  It is possible
   for more than one IPv6 multicast address to map to the same 48 bit
   MAC address.

   The default behaviour of many hosts (and, in fact, routers) is to
   block multicast traffic.  Consequently, when a host wishes to join an
   IPv4 multicast group, it sends an IGMP [RFC2236], [RFC3376] report to
   the router attached to the layer 2 segment and also it instructs its
   data link layer to receive Ethernet frames that match the
   corresponding MAC address.  The data link layer filters the frames,
   passing those with matching destination addresses to the IP module.
   Similarly, hosts simply hand the multicast packet for transmission to
   the data link layer which would add the layer 2 encapsulation, using
   the MAC address derived in the manner previously discussed.

   When this Ethernet frame with a multicast MAC address is received by
   a switch configured to forward multicast traffic, the default
   behaviour is to flood it to all the ports in the layer 2 segment.
   Clearly there may not be a receiver for this multicast group present
   on each port and IGMP snooping is used to avoid sending the frame out
   of ports without receivers.

   IGMP snooping, with proxy reporting or report suppression, actively
   filters IGMP packets in order to reduce load on the multicast router
   by ensuring only the minimal quantity of information is sent.  The
   switch is trying to ensure the router has only a single entry for the
   group, regardless of the number of active listeners.  If there are
   two active listeners in a group and the first one leaves, then the
   switch determines that the router does not need this information
   since it does not affect the status of the group from the router's
   point of view.  However the next time there is a routine query from
   the router the switch will forward the reply from the remaining host,
   to prevent the router from believing there are no active listeners.
   It follows that in active IGMP snooping, the router will generally
   only know about the most recently joined member of the group.

   In order for IGMP and thus IGMP snooping to function, a multicast
   router must exist on the network and generate IGMP queries.  The
   tables (holding the member ports for each multicast group) created
   for snooping are associated with the querier.  Without a querier the
   tables are not created and snooping will not work.  Furthermore, IGMP
   general queries must be unconditionally forwarded by all switches
   involved in IGMP snooping.  Some IGMP snooping implementations
   include full querier capability.  Others are able to proxy and
   retransmit queries from the multicast router.

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   Multicast Listener Discovery (MLD) [RFC 2710] [RFC 3810] is used by
   IPv6 routers for discovering multicast listeners on a directly
   attached link, performing a similar function to IGMP in IPv4
   networks.  MLDv1 [RFC 2710] is similar to IGMPv2 and MLDv2 [RFC 3810]
   [RFC 4604] similar to IGMPv3.  However, in contrast to IGMP, MLD does
   not send its own distinct protocol messages.  Rather, MLD is a
   subprotocol of ICMPv6 [RFC 4443] and so MLD messages are a subset of
   ICMPv6 messages.  MLD snooping works similarly to IGMP snooping,
   described earlier.

3.3.  Example use cases

   A use case where PIM and IGMP are currently used in data centers is
   to support multicast in VXLAN deployments.  In the original VXLAN
   specification [RFC7348], a data-driven flood and learn control plane
   was proposed, requiring the data center IP fabric to support
   multicast routing.  A multicast group is associated with each virtual
   network, each uniquely identified by its VXLAN network identifiers
   (VNI).  VXLAN tunnel endpoints (VTEPs), typically located in the
   hypervisor or ToR switch, with local VMs that belong to this VNI
   would join the multicast group and use it for the exchange of BUM
   traffic with the other VTEPs.  Essentially, the VTEP would
   encapsulate any BUM traffic from attached VMs in an IP multicast
   packet, whose destination address is the associated multicast group
   address, and transmit the packet to the data center fabric.  Thus,
   PIM must be running in the fabric to maintain a multicast
   distribution tree per VNI.

   Alternatively, rather than setting up a multicast distribution tree
   per VNI, a tree can be set up whenever hosts within the VNI wish to
   exchange multicast traffic.  For example, whenever a VTEP receives an
   IGMP report from a locally connected host, it would translate this
   into a PIM join message which will be propagated into the IP fabric.
   In order to ensure this join message is sent to the IP fabric rather
   than over the VXLAN interface (since the VTEP will have a route back
   to the source of the multicast packet over the VXLAN interface and so
   would naturally attempt to send the join over this interface) a more
   specific route back to the source over the IP fabric must be
   configured.  In this approach PIM must be configured on the SVIs
   associated with the VXLAN interface.

   Another use case of PIM and IGMP in data centers is when IPTV servers
   use multicast to deliver content from the data center to end users.
   IPTV is typically a one to many application where the hosts are
   configured for IGMPv3, the switches are configured with IGMP
   snooping, and the routers are running PIM-SSM mode.  Often redundant
   servers send multicast streams into the network and the network is
   forwards the data across diverse paths.

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   Windows Media servers send multicast streams to clients.  Windows
   Media Services streams to an IP multicast address and all clients
   subscribe to the IP address to receive the same stream.  This allows
   a single stream to be played simultaneously by multiple clients and
   thus reducing bandwidth utilization.

3.4.  Advantages and disadvantages

   Arguably the biggest advantage of using PIM and IGMP to support one-
   to-many communication in data centers is that these protocols are
   relatively mature.  Consequently, PIM is available in most routers
   and IGMP is supported by most hosts and routers.  As such, no
   specialized hardware or relatively immature software is involved in
   using them in data centers.  Furthermore, the maturity of these
   protocols means their behaviour and performance in operational
   networks is well-understood, with widely available best-practices and
   deployment guides for optimizing their performance.

   However, somewhat ironically, the relative disadvantages of PIM and
   IGMP usage in data centers also stem mostly from their maturity.
   Specifically, these protocols were standardized and implemented long
   before the highly-virtualized multi-tenant data centers of today
   existed.  Consequently, PIM and IGMP are neither optimally placed to
   deal with the requirements of one-to-many communication in modern
   data centers nor to exploit characteristics and idiosyncrasies of
   data centers.  For example, there may be thousands of VMs
   participating in a multicast session, with some of these VMs
   migrating to servers within the data center, new VMs being
   continually spun up and wishing to join the sessions while all the
   time other VMs are leaving.  In such a scenario, the churn in the PIM
   and IGMP state machines, the volume of control messages they would
   generate and the amount of state they would necessitate within
   routers, especially if they were deployed naively, would be

4.  Alternative options for handling one-to-many traffic

   Section 2 has shown that there is likely to be an increasing amount
   one-to-many communications in data centers.  And Section 3 has
   discussed how conventional multicast may be used to handle this
   traffic.  Having said that, there are a number of alternative options
   of handling this traffic pattern in data centers, as discussed in the
   subsequent section.  It should be noted that many of these techniques
   are not mutually-exclusive; in fact many deployments involve a
   combination of more than one of these techniques.  Furthermore, as
   will be shown, introducing a centralized controller or a distributed
   control plane, makes these techniques more potent.

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4.1.  Minimizing traffic volumes

   If handling one-to-many traffic in data centers can be challenging
   then arguably the most intuitive solution is to aim to minimize the
   volume of such traffic.

   It was previously mentioned in Section 2 that the three main causes
   of one-to-many traffic in data centers are applications, overlays and
   protocols.  While, relatively speaking, little can be done about the
   volume of one-to-many traffic generated by applications, there is
   more scope for attempting to reduce the volume of such traffic
   generated by overlays and protocols.  (And often by protocols within
   overlays.)  This reduction is possible by exploiting certain
   characteristics of data center networks: fixed and regular topology,
   owned and exclusively controlled by single organization, well-known
   overlay encapsulation endpoints etc.

   A way of minimizing the amount of one-to-many traffic that traverses
   the data center fabric is to use a centralized controller.  For
   example, whenever a new VM is instantiated, the hypervisor or
   encapsulation endpoint can notify a centralized controller of this
   new MAC address, the associated virtual network, IP address etc.  The
   controller could subsequently distribute this information to every
   encapsulation endpoint.  Consequently, when any endpoint receives an
   ARP request from a locally attached VM, it could simply consult its
   local copy of the information distributed by the controller and
   reply.  Thus, the ARP request is suppressed and does not result in
   one-to-many traffic traversing the data center IP fabric.

   Alternatively, the functionality supported by the controller can
   realized by a distributed control plane.  BGP-EVPN [RFC7432, RFC8365]
   is the most popular control plane used in data centers.  Typically,
   the encapsulation endpoints will exchange pertinent information with
   each other by all peering with a BGP route reflector (RR).  Thus,
   information about local MAC addresses, MAC to IP address mapping,
   virtual networks identifiers etc can be disseminated.  Consequently,
   ARP requests from local VMs can be suppressed by the encapsulation

4.2.  Head end replication

   A popular option for handling one-to-many traffic patterns in data
   centers is head end replication (HER).  HER means the traffic is
   duplicated and sent to each end point individually using conventional
   IP unicast.  Obvious disadvantages of HER include traffic duplication
   and the additional processing burden on the head end.  Nevertheless,
   HER is especially attractive when overlays are in use as the
   replication can be carried out by the hypervisor or encapsulation end

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   point.  Consequently, the VMs and IP fabric are unmodified and
   unaware of how the traffic is delivered to the multiple end points.
   Additionally, it is possible to use a number of approaches for
   constructing and disseminating the list of which endpoints should
   receive what traffic and so on.

   For example, the reluctance of data center operators to enable PIM
   and IGMP within the data center fabric means VXLAN is often used with
   HER.  Thus, BUM traffic from each VNI is replicated and sent using
   unicast to remote VTEPs with VMs in that VNI.  The list of remote
   VTEPs to which the traffic should be sent may be configured manually
   on the VTEP.  Alternatively, the VTEPs may transmit appropriate state
   to a centralized controller which in turn sends each VTEP the list of
   remote VTEPs for each VNI.  Lastly, HER also works well when a
   distributed control plane is used instead of the centralized
   controller.  Again, BGP-EVPN may be used to distribute the
   information needed to faciliate HER to the VTEPs.

4.3.  BIER

   As discussed in Section 3.4, PIM and IGMP face potential scalability
   challenges when deployed in data centers.  These challenges are
   typically due to the requirement to build and maintain a distribution
   tree and the requirement to hold per-flow state in routers.  Bit
   Index Explicit Replication (BIER) [RFC 8279] is a new multicast
   forwarding paradigm that avoids these two requirements.

   When a multicast packet enters a BIER domain, the ingress router,
   known as the Bit-Forwarding Ingress Router (BFIR), adds a BIER header
   to the packet.  This header contains a bit string in which each bit
   maps to an egress router, known as Bit-Forwarding Egress Router
   (BFER).  If a bit is set, then the packet should be forwarded to the
   associated BFER.  The routers within the BIER domain, Bit-Forwarding
   Routers (BFRs), use the BIER header in the packet and information in
   the Bit Index Forwarding Table (BIFT) to carry out simple bit- wise
   operations to determine how the packet should be replicated optimally
   so it reaches all the appropriate BFERs.

   BIER is deemed to be attractive for facilitating one-to-many
   communications in data ceneters [I-D.ietf-bier-use-cases].  The
   deployment envisioned with overlay networks is that the the
   encapsulation endpoints would be the BFIR.  So knowledge about the
   actual multicast groups does not reside in the data center fabric,
   improving the scalability compared to conventional IP multicast.
   Additionally, a centralized controller or a BGP-EVPN control plane
   may be used with BIER to ensure the BFIR have the required
   information.  A challenge associated with using BIER is that, unlike
   most of the other approaches discussed in this draft, it requires

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   changes to the forwarding behaviour of the routers used in the data
   center IP fabric.

4.4.  Segment Routing

   Segment Routing (SR) [I-D.ietf-spring-segment-routing] adopts the the
   source routing paradigm in which the manner in which a packet
   traverses a network is determined by an ordered list of instructions.
   These instructions are known as segments may have a local semantic to
   an SR node or global within an SR domain.  SR allows enforcing a flow
   through any topological path while maintaining per-flow state only at
   the ingress node to the SR domain.  Segment Routing can be applied to
   the MPLS and IPv6 data-planes.  In the former, the list of segments
   is represented by the label stack and in the latter it is represented
   as a routing extension header.  Use-cases are described in [I-D.ietf-
   spring-segment-routing] and are being considered in the context of
   BGP-based large-scale data-center (DC) design [RFC7938].

   Multicast in SR continues to be discussed in a variety of drafts and
   working groups.  The SPRING WG has not yet been chartered to work on
   Multicast in SR.  Multicast can include locally allocating a Segment
   Identifier (SID) to existing replication solutions, such as PIM,
   mLDP, P2MP RSVP-TE and BIER.  It may also be that a new way to signal
   and install trees in SR is developed without creating state in the

5.  Conclusions

   As the volume and importance of one-to-many traffic in data centers
   increases, conventional IP multicast is likely to become increasingly
   unattractive for deployment in data centers for a number of reasons,
   mostly pertaining its inherent relatively poor scalability and
   inability to exploit characteristics of data center network
   architectures.  Hence, even though IGMP/MLD is likely to remain the
   most popular manner in which end hosts signal interest in joining a
   multicast group, it is unlikely that this multicast traffic will be
   transported over the data center IP fabric using a multicast
   distribution tree built by PIM.  Rather, approaches which exploit
   characteristics of data center network architectures (e.g. fixed and
   regular topology, owned and exclusively controlled by single
   organization, well-known overlay encapsulation endpoints etc.) are
   better placed to deliver one-to-many traffic in data centers,
   especially when judiciously combined with a centralized controller
   and/or a distributed control plane (particularly one based on BGP-

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6.  IANA Considerations

   This memo includes no request to IANA.

7.  Security Considerations

   No new security considerations result from this document

8.  Acknowledgements

9.  References

9.1.  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,

9.2.  Informative References

              Kumar, N., Asati, R., Chen, M., Xu, X., Dolganow, A.,
              Przygienda, T., Gulko, A., Robinson, D., Arya, V., and C.
              Bestler, "BIER Use Cases", draft-ietf-bier-use-cases-06
              (work in progress), January 2018.

              Gross, J., Ganga, I., and T. Sridhar, "Geneve: Generic
              Network Virtualization Encapsulation", draft-ietf-
              nvo3-geneve-06 (work in progress), March 2018.

              Maino, F., Kreeger, L., and U. Elzur, "Generic Protocol
              Extension for VXLAN", draft-ietf-nvo3-vxlan-gpe-06 (work
              in progress), April 2018.

              Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing
              Architecture", draft-ietf-spring-segment-routing-15 (work
              in progress), January 2018.

   [RFC2236]  Fenner, W., "Internet Group Management Protocol, Version
              2", RFC 2236, DOI 10.17487/RFC2236, November 1997,

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   [RFC2710]  Deering, S., Fenner, W., and B. Haberman, "Multicast
              Listener Discovery (MLD) for IPv6", RFC 2710,
              DOI 10.17487/RFC2710, October 1999,

   [RFC3376]  Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
              Thyagarajan, "Internet Group Management Protocol, Version
              3", RFC 3376, DOI 10.17487/RFC3376, October 2002,

   [RFC4601]  Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
              "Protocol Independent Multicast - Sparse Mode (PIM-SM):
              Protocol Specification (Revised)", RFC 4601,
              DOI 10.17487/RFC4601, August 2006,

   [RFC4607]  Holbrook, H. and B. Cain, "Source-Specific Multicast for
              IP", RFC 4607, DOI 10.17487/RFC4607, August 2006,

   [RFC5015]  Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
              "Bidirectional Protocol Independent Multicast (BIDIR-
              PIM)", RFC 5015, DOI 10.17487/RFC5015, October 2007,

   [RFC6820]  Narten, T., Karir, M., and I. Foo, "Address Resolution
              Problems in Large Data Center Networks", RFC 6820,
              DOI 10.17487/RFC6820, January 2013,

   [RFC7348]  Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger,
              L., Sridhar, T., Bursell, M., and C. Wright, "Virtual
              eXtensible Local Area Network (VXLAN): A Framework for
              Overlaying Virtualized Layer 2 Networks over Layer 3
              Networks", RFC 7348, DOI 10.17487/RFC7348, August 2014,

   [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>.

   [RFC7637]  Garg, P., Ed. and Y. Wang, Ed., "NVGRE: Network
              Virtualization Using Generic Routing Encapsulation",
              RFC 7637, DOI 10.17487/RFC7637, September 2015,

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   [RFC7938]  Lapukhov, P., Premji, A., and J. Mitchell, Ed., "Use of
              BGP for Routing in Large-Scale Data Centers", RFC 7938,
              DOI 10.17487/RFC7938, August 2016,

   [RFC8014]  Black, D., Hudson, J., Kreeger, L., Lasserre, M., and T.
              Narten, "An Architecture for Data-Center Network
              Virtualization over Layer 3 (NVO3)", RFC 8014,
              DOI 10.17487/RFC8014, December 2016,

   [RFC8279]  Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
              Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
              Explicit Replication (BIER)", RFC 8279,
              DOI 10.17487/RFC8279, November 2017,

   [RFC8365]  Sajassi, A., Ed., Drake, J., Ed., Bitar, N., Shekhar, R.,
              Uttaro, J., and W. Henderickx, "A Network Virtualization
              Overlay Solution Using Ethernet VPN (EVPN)", RFC 8365,
              DOI 10.17487/RFC8365, March 2018,

Authors' Addresses

   Mike McBride

   Email: michael.mcbride@huawei.com

   Olufemi Komolafe
   Arista Networks

   Email: femi@arista.com

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