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A Framework for Multicast in NVO3

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Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8293.
Authors Anoop Ghanwani , Linda Dunbar , Mike McBride , Vinay Bannai , Ramki Krishnan
Last updated 2015-05-11
Replaces draft-ghanwani-nvo3-mcast-framework
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NVO3 working group                                          A. Ghanwani
Internet Draft                                                     Dell
Intended status: Informational                                L. Dunbar
Expires: November 10, 2015                                        Huawei
                                                             M. McBride
                                                              V. Bannai
                                                            R. Krishnan

                                                           May 10, 2015

                     A Framework for Multicast in NVO3

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
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   Copyright (c) 2015 IETF Trust and the persons identified as the
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   This document is subject to BCP 78 and the IETF Trust's Legal
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   This document discusses a framework for supporting multicast traffic
   in a network that uses Network Virtualization using Overlays over
   Layer 3 (NVO3). Both infrastructure multicast and application-
   specific multicast are discussed. It describes the various
   mechanisms and considerations that can be used for delivering such
   traffic as well as the data plane and control plane considerations
   for each of the mechanisms.

Table of Contents

   1. Introduction...................................................3
      1.1. Infrastructure multicast..................................3
      1.2. Application-specific multicast............................3
   2. Acronyms.......................................................4
   3. Multicast mechanisms in networks that use NVO3.................4
      3.1. No multicast support......................................5
      3.2. Replication at the source NVE.............................6
      3.3. Replication at a multicast service node...................8
      3.4. IP multicast in the underlay..............................9
      3.5. Other schemes............................................10
   4. Simultaneous use of more than one mechanism...................11
   5. Other issues..................................................11
      5.1. Multicast-agnostic NVEs..................................11
      5.2. Multicast membership management for DC with VMs..........12
   6. Summary.......................................................12
   7. Security Considerations.......................................12
   8. IANA Considerations...........................................12

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   9. References....................................................12
      9.1. Normative References.....................................12
      9.2. Informative References...................................13
   10. Acknowledgments..............................................14

1. Introduction

   Network virtualization using Overlays over Layer 3 (NVO3) is a
   technology that is used to address issues that arise in building
   large, multitenant data centers that make extensive use of server
   virtualization [RFC7364].

   This document provides a framework for supporting multicast traffic,
   in a network that uses Network Virtualization using Overlays over
   Layer 3 (NVO3).  Both infrastructure multicast (ARP/ND, DHCP, mDNS,
   etc.) and application-specific multicast are considered.  It
   describes the various mechanisms and considerations that can be used
   for delivering such traffic in networks that use NVO3.

   The reader is assumed to be familiar with the terminology as defined
   in the NVO3 Framework document [RFC7365] and NVO3 Architecture
   document [NVO3-ARCH].

1.1. Infrastructure multicast

   Infrastructure multicast includes protocols such as ARP/ND, DHCP,
   and mDNS.  It is possible to provide solutions for these that do not
   involve multicast in the underlay network.  In the case of ARP/ND,
   an NVA can be used for distributing the mappings of IP address to
   MAC address to all NVEs, and the NVEs can respond to ARP messages
   from the TSs that are attached to it in a way that is similar to
   proxy-ARP.  In the case of DHCP, the NVE can be configured to
   forward these messages using a helper function.

   Of course it is possible to support all of these infrastructure
   multicast protocols natively if the underlay provides multicast
   transport.  However, even in the presence of multicast transport, it
   may be beneficial to use the optimizations mentioned above to reduce
   the amount of such traffic in the network.

1.2. Application-specific multicast

   Application-specific multicast traffic, which may be either Source-
   Specific Multicast (SSM) or Any-Source Multicast (ASM)[RFC3569],
   has the following characteristics:

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     1. Receiver hosts are expected to subscribe to multicast content
        using protocols such as IGMP [RFC3376] (IPv4) or MLD (IPv6).
        Multicast sources and listeners participant in these protocols
        using addresses that are in the Tenant System address domain.

     2. The list of multicast listeners for each multicast group is not
        known in advance.  Therefore, it may not be possible for an NVA
        to get the list of participants for each multicast group ahead
        of time.

2. Acronyms

   ASM:  Any-Source Multicast

   LISP: Locator/ID Separation Protocol

   NVA: Network Virtualization Authority

   NVE: Network Virtualization Edge

   NVGRE: Network Virtualization using GRE

   SSM: Source-Specific Multicast

   STT: Stateless Tunnel Transport

   VXLAN: Virtual eXtensible LAN

3. Multicast mechanisms in networks that use NVO3

   In NVO3 environments, traffic between NVEs is transported using an
   encapsulation such as VXLAN [VXLAN], NVGRE [NVGRE], STT [STT], etc.

   Besides the need to support the Address Resolution Protocol (ARP)
   and Neighbor Discovery (ND), there are several applications that
   require the support of multicast and/or broadcast in data centers
   [DC-MC]. With NVO3, there are many possible ways that multicast may
   be handled in such networks.  We discuss some of the attributes of
   the following four methods:      1. No multicast support.

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         2. Replication at the source NVE.

         3. Replication at a multicast service node.

         4. IP multicast in the underlay.

   These mechanisms are briefly mentioned in the NVO3 Framework [FW]
   and NVO3 architecture [NVO3-ARCH] document. This document attempts
   to provide more details about the basic mechanisms underlying each
   of these mechanisms and discusses the issues and tradeoffs of each.

   We note that other methods are also possible, such as [EDGE-REP],
   but we focus on the above four because they are the most common.

3.1. No multicast support

   In this scenario, there is no support whatsoever for multicast
   traffic when using the overlay. This method can only work if the
   following conditions are met:

     1. All of the application traffic in the network is unicast
        traffic and the only multicast/broadcast traffic is from ARP/ND

     2. A network virtualization authority (NVA) is used by the NVEs to
        determine the mapping of a given Tenant System's MAC/IP address
        to its NVE. In other words, there is no data plane learning.
        Address resolution requests via ARP/ND that are issued by the
        Tenant Systems must be resolved by the NVE that they are
        attached to.

   With this approach, it is not possible to support application-
   specific multicast.  However, certain multicast/broadcast
   applications such as DHCP can be supported by use of a helper
   function in the NVE.

   The main drawback of this approach, even for unicast traffic, is
   that it is not possible to initiate communication with a Tenant
   System for which a mapping to an NVE does not already exist with the
   NVA.  This is a problem in the case where the NVE is implemented in
   a physical switch and the Tenant System is a physical end station
   that has not registered with the NVA.

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3.2. Replication at the source NVE

   With this method, the overlay attempts to provide a multicast
   service without requiring any specific support from the underlay,
   other than that of a unicast service.  A multicast or broadcast
   transmission is achieved by replicating the packet at the source
   NVE, and making copies, one for each destination NVE that the
   multicast packet must be sent to.

   For this mechanism to work, the source NVE must know, a priori, the
   IP addresses of all destination NVEs that need to receive the
   packet.  For the purpose of ARP/ND, this would involve knowing the
   IP addresses of all the NVEs that have Tenant Systems in the virtual
   network instance (VNI) of the Tenant System that generated the
   request.  For the support of application-specific multicast traffic,
   a method similar to that of receiver-sites registration for a
   particular multicast group described in [LISP-Signal-Free] can be
   used.  The registrations from different receiver-sites can be merged
   at the NVA, which can construct a multicast replication-list
   inclusive of all NVEs to which receivers for a particular multicast
   group are attached. The replication-list for each specific multicast
   group is maintained either by the NVA.

   The receiver-sites registration is achieved by egress NVEs
   performing the IGMP/MLD snooping to maintain state for which
   attached Tenant Systems have subscribed to a given IP multicast
   group. When the members of a multicast group are outside the NVO3
   domain, it is necessary for NVO3 gateways to keep track of the
   remote members of each multicast group.  The NVEs then communicate
   these mappings to the NVA.  Even if the membership is not
   communicated to the NVA, if it is necessary to prevent hosts
   attached to an NVE that have not subscribed to a multicast group
   from receiving the multicast traffic, the NVE needs to maintain the
   multicast group membership.

   In the absence of IGMP/MLD snooping, the traffic would be delivered
   to all hosts that are part of the VNI.

   This method requires multiple copies of the same packet to all NVEs
   that participate in the VN.  If, for example, a tenant subnet is
   spread across 50 NVEs, the packet would have to be replicated 50
   times at the source NVE.  This also creates an issue with the
   forwarding performance of the NVE.

   Note that this method is similar to what was used in VPLS [RFC4762]
   prior to support of MPLS multicast [MPLS-MC]. While there are some

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   similarities between MPLS VPN and the NVO3 overlay, there are some
   key differences:

  - The CE-to-PE attachment in VPNs is somewhat static, whereas in a
     DC that allows VMs to migrate anywhere, the TS attachment to NVE
     is much more dynamic.

  - The number of PEs to which a single  VPN customer is attached in
     an MPLS VPN environment is normally far less than the number of
     NVEs to which a VNI's VMs are attached in a DC.

     When a VPN customer has multiple multicast groups, [RFC6513]
     "Multicast VPN" combines all those multicast groups within each
     VPN client to one single multicast group in the MPLS (or VPN)
     core.  The result is that messages from any of the multicast
     groups belonging to one VPN customer will reach all the PE nodes
     of the client. In other words, any messages belonging to any
     multicast groups under customer X will reach all PEs of the
     customer X. When the customer X is attached to only a handful of
     PEs, the use of this approach does not result in excessive wastage
     of bandwidth in the provider's network.

     In a DC environment, a typical server/hypervisor based virtual
     switch may only support 10's VMs (as of this writing). A subnet
     with N VMs may be, in the worst case, spread across N vSwitches.
     Using "MPLS VPN multicast" approach in such a scenario would
     require the creation of a Multicast group in the core for this VNI
     to reach all N NVEs. If only small percentage of this client's VMs
     participate in application specific multicast, a great number of
     NVEs will receive multicast traffic that is not forwarded to any
     of their attached VMs, resulting in considerable wastage of

   Therefore, the Multicast VPN solution may not scale in DC
   environment with dynamic attachment of Virtual Networks to NVEs and
   greater number of NVEs for each virtual network.

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3.3. Replication at a multicast service node

   With this method, all multicast packets would be sent using a
   unicast tunnel encapsulation to a multicast service node (MSN).  The
   MSN, in turn, would create multiple copies of the packet and would
   deliver a copy, using a unicast tunnel encapsulation, to each of the
   NVEs that are part of the multicast group for which the packet is

   This mechanism is similar to that used by the ATM Forum's LAN
   Emulation [LANE] specification [LANE].

   The following are the possible ways for the MSN to get the
   membership information for each multicast group:

   - The MSN can obtain this information by snooping the IGMP/MLD
     messages from the Tenant Systems and/or sending query messages to
     the Tenant Systems.  In order for MSN to snoop the IGMP/MLD
     messages between TSs and their corresponding routers, the NVEs
     that TSs are attached have to encapsulate a special outer header,
     e.g. outer destination being the multicast server node. See
     Section 3.3.2 for detail.

   - The MSN can obtain the membership information from the NVEs that
     snoop the IGMP/MLD messages. This can be done by having the MSN
     communicate with the NVEs, or by having the NVA obtain the
     information from the NVEs, and in turn have MSN communicate with
     the NVA.

   Unlike the method described in Section 3.2, there is no performance
   impact at the ingress NVE, nor are there any issues with multiple
   copies of the same packet from the source NVE to the multicast
   service node.  However there remain issues with multiple copies of
   the same packet on links that are common to the paths from the MSN
   to each of the egress NVEs.  Additional issues that are introduced
   with this method include the availability of the MSN, methods to
   scale the services offered by the MSN, and the sub-optimality of the
   delivery paths.

   Finally, the IP address of the source NVE must be preserved in
   packet copies created at the multicast service node if data plane

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   learning is in use.  This could create problems if IP source address
   reverse path forwarding (RPF) checks are in use.

3.4. IP multicast in the underlay

   In this method, the underlay supports IP multicast and the ingress
   NVE encapsulates the packet with the appropriate IP multicast
   address in the tunnel encapsulation header for delivery to the
   desired set of NVEs.  The protocol in the underlay could be any
   variant of Protocol Independent Multicast (PIM), or protocol
   dependent multicast, such as [ISIS-Multicast].

   If an NVE connects to its attached TSs via Layer 2 network, there
   are multiple ways for NVEs to support the application specific

  - The NVE only supports the basic IGMP/MLD snooping function, let
     the TSs routers handling the application specific multicast. This
     scheme doesn't utilize the underlay IP multicast protocols.
  - The NVE can act as a pseudo multicast router for the directly
     attached VMs and support proper mapping of IGMP/MLD's messages to
     the messages needed by the underlay IP multicast protocols.

   With this method, there are none of the issues with the methods
   described in Sections 3.2.

   With PIM Sparse Mode (PIM-SM), the number of flows required would be
   (n*g), where n is the number of source NVEs that source packets for
   the group, and g is the number of groups.  Bidirectional PIM (BIDIR-
   PIM) would offer better scalability with the number of flows
   required being g.

   In the absence of any additional mechanism, e.g. using an NVA for
   address resolution, for optimal delivery, there would have to be a
   separate group for each tenant, plus a separate group for each
   multicast address (used for multicast applications) within a tenant.

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   Additional considerations are that only the lower 23 bits of the IP
   address (regardless of whether IPv4 or IPv6 is in use) are mapped to
   the outer MAC address, and if there is equipment that prunes
   multicasts at Layer 2, there will be some aliasing.  Finally, a
   mechanism to efficiently provision such addresses for each group
   would be required.

   There are additional optimizations which are possible, but they come
   with their own restrictions.  For example, a set of tenants may be
   restricted to some subset of NVEs and they could all share the same
   outer IP multicast group address.  This however introduces a problem
   of sub-optimal delivery (even if a particular tenant within the
   group of tenants doesn't have a presence on one of the NVEs which
   another one does, the former's multicast packets would still be
   delivered to that NVE).  It also introduces an additional network
   management burden to optimize which tenants should be part of the
   same tenant group (based on the NVEs they share), which somewhat
   dilutes the value proposition of NVO3 which is to completely
   decouple the overlay and physical network design allowing complete
   freedom of placement of VMs anywhere within the data center.

   Multicast schemes such as BIER (Bit Index Explicit Replication) may
   be able to provide optimizations by allowing the underlay network to
   provide optimum multicast delivery without requiring routers in the
   core of the network to main per-multicast group state.

3.5. Other schemes

   There are still other mechanisms that may be used that attempt to
   combine some of the advantages of the above methods by offering
   multiple replication points, each with a limited degree of
   replication [EDGE-REP].  Such schemes offer a trade-off between the
   amount of replication at an intermediate node (router) versus
   performing all of the replication at the source NVE or all of the
   replication at a multicast service node.

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4. Simultaneous use of more than one mechanism

   While the mechanisms discussed in the previous section have been
   discussed individually, it is possible for implementations to rely
   on more than one of these.  For example, the method of Section 3.1
   could be used for minimizing ARP/ND, while at the same time,
   multicast applications may be supported by one, or a combination of,
   the other methods.  For small multicast groups, the methods of
   source NVE replication or the use of a multicast service node may be
   attractive, while for larger multicast groups, the use of multicast
   in the underlay may be preferable.

5. Other issues

5.1. Multicast-agnostic NVEs

   Some hypervisor-based NVEs do not process or recognize IGMP/MLD
   frames; i.e. those NVEs simply encapsulate the IGMP/MLD messages in
   the same way as they do for regular data frames.

   By default, TSs router periodically sends IGMP/MLD query messages to
   all the hosts in the subnet to trigger the hosts that are interested
   in the multicast stream to send back IGMP/MLD reports. In order for
   MSN get the updated multicast group information, the MSN can also
   send the IGMP/MLD query message comprising a client specific
   multicast address, encapsulated in an overlay header to all the NVEs
   to which the TSs in the VN are attached.

   However, MSN may not always be aware of the client specific
   multicast addresses. Then MSN has to snoop the IGMP/MLD messages
   between TSs and their corresponding routers to maintain the
   multicast membership. In order for MSN to snoop the IGMP/MLD
   messages between TSs and their router, NVA needs to configure the
   NVE to send copies of the IGMP/MLD messages to the MSN in addition
   to the default behavior of sending them to the TSs' routers; e.g.
   the NVA has to inform the NVEs to encapsulate data frames with DA
   being (destination address of IGMP report) to TSs' router
   and MSN.

   This process is similar to "Source Replication" described in Section
   3.2, except the NVEs only replicate the message to TS's router and

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5.2. Multicast membership management for DC with VMs

   For data centers with virtualized servers, VMs can be added, deleted
   or moved very easily. When VMs are added, deleted or moved, the NVEs
   to which the VMs are attached are changed.

   When a VM is deleted from an NVE or a new VM is added to an NVE, the
   VM management system should notify the MSN to send the IGMP/MLD
   query messages to the relevant NVEs, so that the multicast
   membership can be updated promptly.  Otherwise, if there are changes
   of VMs attachment to NVEs, then for the duration of the configured
   default time interval that the TSs routers use for IGMP/MLD queries,
   multicast data may not reach the VM(s) that moved.

6. Summary

   This document has identified various mechanisms for supporting
   application specific multicast in networks that use NVO3.  It
   highlights the basics of each mechanism and some of the issues with
   them.  As solutions are developed, the protocols would need to
   consider the use of these mechanisms and co-existence may be a
   consideration.  It also highlights some of the requirements for
   supporting multicast applications in an NVO3 network.

7. Security Considerations

   This draft does not introduce any new security considerations beyond
   what may be present in proposed solutions

8. IANA Considerations

   This document requires no IANA actions. RFC Editor: Please remove
   this section before publication.

9. References

9.1. Normative References

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   [RFC7365] Lasserre, M. et al., "Framework for data center (DC)
             network virtualization", October 2014.

   [RFC7364] Narten, T. et al., "Problem statement: Overlays for
             network virtualization", October 2014.

   [NVO3-ARCH] Narten, T. et al.," An Architecture for Overlay Networks
             (NVO3)", work in progress, February 2014.

   [RFC3376] B. Cain, et al, "Internet Group Management Protocol,
             Version 3", October 2002.

   [RFC6513] Rosen, E. et al., "Multicast in MPLS/BGP IP VPNs",
             February 2012.

9.2. Informative References

    [RFC7348] Mahalingam, M. et al., "Virtual eXtensible Local Area
              Network (VXLAN): A Framework for Overlaying Virtualized
              Layer 2 Networks over Layer 3 Networks", August 2014.

    [NVGRE]   Sridharan, M. et al., "NVGRE: Network virtualization using
              Generic Routing Encapsulation", work in progress.

    [STT]     Davie, B. and Gross J., "A stateless transport tunneling
              protocol for network virtualization", work in progress.

    [DC-MC]   McBride M., and Lui, H., "Multicast in the data center
              overview", work in progress.

              L. Yong, et al, "ISIS Protocol Extension For Building
              Distribution Trees", work in progress. Oct 2013.

    [RFC4762] Lasserre, M., and Kompella, V. (Eds), "Virtual Private LAN
              Service (VPLS) using Label Distribution Protocol (LDP)
              signaling", January 2007.

    [MPLS-MC] Aggarwal, R. et al., "Multicast in VPLS," work in

    [LANE]    "LAN emulation over ATM," The ATM Forum, af-lane-0021.000,
              January 1995.

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              Marques P. et al., "Edge multicast replication for BGP IP
              VPNs," work in progress, June 2012.

    [RFC3569] S. Bhattacharyya, Ed., "An Overview of Source-Specific
              Multicast (SSM)", July 2003.

              V. Moreno & D. Farinacci, "Signal-Free LISP Multicast",
              work in progress. Dec 2014.

10. Acknowledgments

   Thanks are due to Dino Farinacci and Erik Nordmark for their
   comments and suggestions on this work.

   This document was prepared using

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Authors' Addresses

   Anoop Ghanwani

   Linda Dunbar
   Huawei Technologies
   5340 Legacy Drive, Suite 1750
   Plano, TX 75024, USA
   Phone: (469) 277 5840

   Mike McBride

   Vinay Bannai

   Ram Krishnan

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