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Versions: 00 01 02                                                      
Internet Engineering Task Force                               M. McBride
Internet-Draft                                                    H. Lui
Intended status: Informational                       Huawei Technologies
Expires: September 4, 2012                                 March 3, 2012

                 Multicast in the Data Center Overview


   There has been much interest in issues surrounding massive amounts of
   hosts in the data center.  There was a discussion, in ARMD, involving
   the issues with address resolution for non ARP/ND multicast traffic
   in data centers with massive number of hosts.  This document provides
   a quick survey of multicast in the data center and should serve as an
   aid to further discussion of issues related to large amounts of
   multicast in the data center.

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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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 4, 2012.

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   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   (http://trustee.ietf.org/license-info) in effect on the date of
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   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Multicast Applications in the Data Center . . . . . . . . . . . 3
     2.1.  L3 Multicast Applications . . . . . . . . . . . . . . . . . 3
     2.2.  L2 Multicast Applications . . . . . . . . . . . . . . . . . 4
   3.  L2 Multicast Protocols in the Data Center . . . . . . . . . . . 5
   4.  L3 Multicast solutions in the Data Center . . . . . . . . . . . 6
   5.  Challenges of using multicast in the Data Center  . . . . . . . 7
   6.  Layer 3 / Layer 2 Topological Variations  . . . . . . . . . . . 8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 9
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 9
   9.  Security Considerations . . . . . . . . . . . . . . . . . . . . 9
   10. Informative References  . . . . . . . . . . . . . . . . . . . . 9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 9

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1.  Introduction

   Data center servers often use IP Multicast to send data to clients or
   other application servers.  IP Multicast is expected to help conserve
   bandwidth in the data center and reduce the load on servers.
   Increased reliance on multicast, in next generation data centers,
   requires higher performance and capacity especially from the
   switches.  If multicast is to continue to be used in the data center,
   it must scale well within and between datacenters.  There has been
   much interest in issues surrounding massive amounts of hosts in the
   data center.  There was a discussion, in ARMD, involving the issues
   with address resolution for non ARP/ND multicast traffic in data
   centers.  This document provides a quick survey of multicast in the
   data center and should serve as an aid to further discussion of
   issues related to multicast in the data center.

   ARP/ND issues are not addressed in this document.  ARP/ND issues are
   addressed in [I-D.armd-problem-statement]

2.  Multicast Applications in the Data Center

   There are many data center operators who do not deploy Multicast in
   their networks for scalability and stability reasons.  There are also
   many operators for whom multicast is critical and is enabled on their
   data center switches and routers.  For this latter group, there are
   several uses of multicast in their data centers.  An understanding of
   the uses of that multicast is important in order to properly support
   these applications in the ever evolving data centers.  If, for
   instance, the majority of the applications are discovering/signaling
   each other using multicast there may be better ways to support them
   then using multicast.  If, however, the multicasting of data is
   occurring in large volumes, there is a need for very good data center
   under/overlay multicast support.  The applications either fall into
   the category of those that leverage L2 multicast for discovery or of
   those that require L3 support and likely span multiple subnets.

2.1.  L3 Multicast Applications

   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 are sending multicast streams into the network and
   the network is forwarding the data across diverse paths.

   Windows Media servers send multicast streaming to clients.  Windows
   Media Services streams to an IP multicast address and all clients

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   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.

   Market data relies extensively on IP multicast to deliver stock
   quotes from the data center to a financial services provider and then
   to the stock analysts.  The most critical requirement of a multicast
   trading floor is that it be highly available.  The network must be
   designed with no single point of failure and in a way the network can
   respond in a deterministic manner to any failure.  Typically
   redundant servers (in a primary/backup or live live mode) are sending
   multicast streams into the network and the network is forwarding the
   data across diverse paths (when duplicate data is sent by multiple

   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 multicast.

   With load balancing protocols, such as VRRP, routers communicate
   within themselves using a multicast address.

   Overlays may use IP multicast to virtualize L2 multicasts.  VXLAN,
   for instance, is an encapsulation scheme to carry L2 frames over L3
   networks.  The VXLAN Tunnel End Point (VTEP) encapsulates frames
   inside an L3 tunnel.  VXLANs are identified by a 24 bit VXLAN Network
   Identifier (VNI).  The VTEP maintains a table of known destination
   MAC addresses, and stores the IP address of the tunnel to the remote
   VTEP to use for each.  Unicast frames, between VMs, are sent directly
   to the unicast L3 address of the remote VTEP.  Multicast frames are
   sent to a multicast IP group associated with the VNI.  Underlying IP
   Multicast protocols (PIM-SM/SSM/BIDIR) are used to forward multicast
   data across the overlay.

2.2.  L2 Multicast Applications

   Applications, such as Ganglia, uses multicast for distributed
   monitoring of computing systems such as clusters and grids.

   Windows Server, cluster node exchange, relies upon the use of
   multicast heartbeats between servers.  Only the other interfaces in

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   the same multicast group use the data.  Unlike broadcast, multicast
   traffic does not need to be flooded throughout the network, reducing
   the chance that unnecessary CPU cycles are expended filtering traffic
   on nodes outside the cluster.  As the number of nodes increases, the
   ability to replace several unicast messages with a single multicast
   message improves node performance and decreases network bandwidth
   consumption.  Multicast messages replace unicast messages in two
   components of clustering:

   o  Heartbeats: The clustering failure detection engine is based on a
      scheme whereby nodes send heartbeat messages to other nodes.
      Specifically, for each network interface, a node sends a heartbeat
      message to all other nodes with interfaces on that network.
      Heartbeat messages are sent every 1.2 seconds.  In the common case
      where each node has an interface on each cluster network, there
      are N * (N - 1) unicast heartbeats sent per network every 1.2
      seconds in an N-node cluster.  With multicast heartbeats, the
      message count drops to N multicast heartbeats per network every
      1.2 seconds, because each node sends 1 message instead of N - 1.
      This represents a reduction in processing cycles on the sending
      node and a reduction in network bandwidth consumed.

   o  Regroup: The clustering membership engine executes a regroup
      protocol during a membership view change.  The regroup protocol
      algorithm assumes the ability to broadcast messages to all cluster
      nodes.  To avoid unnecessary network flooding and to properly
      authenticate messages, the broadcast primitive is implemented by a
      sequence of unicast messages.  Converting the unicast messages to
      a single multicast message conserves processing power on the
      sending node and reduces network bandwidth consumption.

   Multicast addresses in the 224.0.0.x range are considered link local
   multicast addresses.  They are used for protocol discovery and are
   flooded to every port.  For example, OSPF uses and for neighbor and DR discovery.  These addresses are
   reserved and will not be constrained by IGMP snooping.  These
   addresses are not to be used by any application.

   These types of multicast applications should be able to be supported
   in data centers which support multicast.

3.  L2 Multicast Protocols in the Data Center

   The switches, in between the servers and the routers, rely upon igmp
   snooping to bound the multicast to the ports leading to interested
   hosts and to L3 routers.  A switch will, by default, flood multicast
   traffic to all the ports in a broadcast domain (VLAN).  IGMP snooping

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   is designed to prevent hosts on a local network from receiving
   traffic for a multicast group they have not explicitly joined.  It
   provides switches with a mechanism to prune multicast traffic from
   links that do not contain a multicast listener (an IGMP client).
   IGMP snooping is a L2 optimization for L3 IGMP.

   IGMP snooping, with proxy reporting or report suppression, actively
   filters IGMP packets in order to reduce load on the multicast router.
   Joins and leaves heading upstream to the router are filtered so that
   only the minimal quantity of information is sent.  The switch is
   trying to ensure the router only has a single entry for the group,
   regardless of how many active listeners there are.  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.

   In source-only networks, however, which presumably describes most
   data center networks, there are no IGMP hosts on switch ports to
   generate IGMP packets.  Switch ports are connected to multicast
   source ports and multicast router ports.  The switch typically learns
   about multicast groups from the multicast data stream by using a type
   of source only learning (when only receiving multicast data on the
   port, no IGMP packets).  The switch forwards traffic only to the
   multicast router ports.  When the switch receives traffic for new IP
   multicast groups, it will typically flood the packets to all ports in
   the same VLAN.  This unnecessary flooding can impact switch

4.  L3 Multicast solutions in the Data Center

   There are three flavors of PIM used for Multicast Routing in the Data
   Center: PIM-SM [RFC4601], PIM-SSM [RFC4607], and PIM-BIDIR [RFC5015].

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   SSM provides the most efficient forwarding between sources and
   receivers and is most suitable for one to many types of multicast
   applications.  State is built for each S,G channel therefore the more
   sources and groups there are, the more state there is in the network.
   BIDIR is the most efficient shared tree solution as one tree is built
   for all S,G's, therefore saving state.  But it is not the most
   efficient in forwarding path between sources and receivers.  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 then will either switch to the SPT (shortest path
   tree), similar to SSM, or stay on the shared tree (similar to BIDIR).
   For massive amounts of hosts sending (and receiving) multicast, the
   shared tree (particularly with PIM-BIDIR) provides the best potential
   scaling since no matter how many multicast sources exist within a
   VLAN, the tree number stays the same.  IGMP snooping, IGMP proxy, and
   PIM-BIDIR have the potential to scale to the huge scaling numbers
   required in a data center.

5.  Challenges of using multicast in the Data Center

   When IGMP/MLD Snooping is not implemented, ethernet switches will
   flood multicast frames out of all switch-ports, which turns the
   traffic into something more like broadcast.

   VRRP uses multicast heartbeat to communicate between routers.  The
   communication between the host and the default gateway is unicast.
   The multicast heartbeat can be very chatty when there are thousands
   of VRRP pairs with sub-second heartbeat calls back and forth.

   Link-local multicast should scale well within one IP subnet
   particularly with a large layer3 domain extending down to the access
   or aggregation switches.  But if multicast traverses beyond one IP
   subnet, which is necessary for an overlay like VXLAN, you could
   potentially have scaling concerns.  If using a VXLAN overlay, it is
   necessary to map the L2 multicast in the overlay to L3 multicast in
   the underlay or do head end replication in the overlay and receive
   duplicate frames on the first link from the router to the core
   switch.  The solution could be to run potentially thousands of PIM
   messages to generate/maintain the required multicast state in the IP
   underlay.  The behavior of the upper layer, with respect to
   broadcast/multicast, affects the choice of head end (*,G) or (S,G)
   replication in the underlay, which affects the opex and capex of the
   entire solution.  A VXLAN, with thousands of logical groups, maps to
   head end replication in the hypervisor or to IGMP from the hypervisor
   and then PIM between the TOR and CORE 'switches' and the gateway

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   Requiring IP multicast (especially PIM BIDIR) from the network can
   prove challenging for data center operators especially at the kind of
   scale that the VXLAN/NVGRE proposals require.  This is also true when
   the L2 topological domain is large and extended all the way to the L3
   core.  In data centers with highly virtualized servers, even small L2
   domains may spread across many server racks (i.e. multiple switches
   and router ports).

   It's not uncommon for there to be 10-20 VMs per server in a
   virtualized environment.  One vendor reported a customer requesting a
   scale to 400VM's per server.  For multicast to be a viable solution
   in this environment, the network needs to be able to scale to these
   numbers when these VMs are sending/receiving multicast.

   A lot of switching/routing hardware has problems with IP Multicast,
   particularly with regards to hardware support of PIM-BIDIR.

   Sending L2 multicast over a campus or data center backbone, in any
   sort of significant way, is a new challenge enabled for the first
   time by overlays.  There are interesting challenges when pushing
   large amounts of multicast traffic through a network, and have thus
   far been dealt with using purpose-built networks.  While the overlay
   proposals have been careful not to impose new protocol requirements,
   they have not addressed the issues of performance and scalability,
   nor the large-scale availability of these protocols.

   There is an unnecessary multicast stream flooding problem in the link
   layer switches between the multicast source and the PIM First Hop
   Router (FHR).  The IGMP-Snooping Switch will forward multicast
   streams to router ports, and the PIM FHR must receive all multicast
   streams even if there is no request from receiver.  This often leads
   to waste of switch cache and link bandwidth when the multicast
   streams are not actually required.  [I-D.pim-umf-problem-statement]
   details the problem and defines design goals for a generic mechanism
   to restrain the unnecessary multicast stream flooding.

6.  Layer 3 / Layer 2 Topological Variations

   As discussed in [I-D.armd-problem-statement], there are a variety of
   topological data center variations including L3 to Access Switches,
   L3 to Aggregation Switches, and L3 in the Core only.  Further
   analysis is needed in order to understand how these variations affect
   IP Multicast scalability

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7.  Acknowledgements

   The authors would like to thank the many individuals who contributed
   opinions on the ARMD wg mailing list about this topic: Linda Dunbar,
   Anoop Ghanwani, Peter Ashwoodsmith, David Allan, Aldrin Isaac, Igor
   Gashinsky, Michael Smith, Patrick Frejborg, Joel Jaeggli and Thomas

8.  IANA Considerations

   This memo includes no request to IANA.

9.  Security Considerations

   No security considerations at this time.

10.  Informative References

              Narten, T., Karir, M., and I. Foo,
              "draft-ietf-armd-problem-statement", February 2012.

              Zhou, D., Deng, H., Shi, Y., Liu, H., and I. Bhattacharya,
              "draft-dizhou-pim-umf-problem-statement", October 2010.

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

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

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

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Authors' Addresses

   Mike McBride
   Huawei Technologies
   2330 Central Expressway
   Santa Clara, CA  95050

   Email: michael.mcbride@huawei.com

   Helen Lui
   Huawei Technologies
   Building Q14, No. 156, Beiqing Rd.
   Beijing,   100095

   Email: helen.liu@huawei.com

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