wdiff draft-ietf-mboned-dc-deploy-02.txt draft-ietf-mboned-dc-deploy-03.txt

MBONED M. McBride
Internet-Draft Huawei
Intended status: Informational February 28, 2018 O. Komolafe
Expires: September 1, December 31, 2018 Arista Networks
June 29, 2018

Multicast in the Data Center Overview
draft-ietf-mboned-dc-deploy-02
draft-ietf-mboned-dc-deploy-03

Abstract

There has been much interest in issues surrounding massive amounts

The volume and importance of
hosts one-to-many traffic patterns in the data center. These issues include the prevalent use of
IP Multicast within the Data Center. Its important to understand how
IP Multicast
centers is being deployed likely to increase significantly in the Data Center to be able future. Reasons
for this increase are discussed and then attention is paid to
understand the surrounding issues with doing so. This document
provides a quick survey of uses
manner in which this traffic pattern may be judiously handled in data
centers. The intuitive solution of deploying conventional IP
multicast in the within data center centers is explored and
should serve as an aid to further discussion evaluated. Thereafter,
a number of issues related to
large amounts emerging innovative approaches are described before a
number of multicast in the data center. recommendations are made.

Status of This Memo

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Multicast Applications in the Data Center Reasons for increasing one-to-many traffic patterns . . . . . 3
2.1. Applications . . . . . . . 3
2.1. Client-Server Applications . . . . . . . . . . . . . . . 3
2.2. Non Client-Server Multicast Applications Overlays . . . . . . . . . . . . . . 4
3. L2 Multicast Protocols in the Data Center . . . . . . . . . . 5
4. L3 Multicast
2.3. Protocols in the Data Center . . . . . . . . . . 6
5. Challenges of using multicast in the Data Center . . . . . . 7
6. . . . . . . . . 5
3. Handling one-to-many traffic using conventional multicast . . 5
3.1. Layer 3 / multicast . . . . . . . . . . . . . . . . . . . . 6
3.2. Layer 2 Topological Variations multicast . . . . . . . . . . 8
7. Address Resolution . . . . . . . . . . 6
3.3. Example use cases . . . . . . . . . . . 9
7.1. Solicited-node Multicast Addresses for IPv6 address
resolution . . . . . . . . . 8
3.4. Advantages and disadvantages . . . . . . . . . . . . . . 9
7.2. Direct Mapping
4. Alternative options for Multicast address resolution . handling one-to-many traffic . . . . 9
8. IANA Considerations
4.1. Minimizing traffic volumes . . . . . . . . . . . . . . . 9
4.2. Head end replication . . . . . . . . . . . . . . . . . . 10
9. Security Considerations
4.3. BIER . . . . . . . . . . . . . . . . . . . 10
10. Acknowledgements . . . . . . . 11
4.4. Segment Routing . . . . . . . . . . . . . . . 10
11. References . . . . . . 12
5. Conclusions . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . 12
6. IANA Considerations . . . . . . . . . . . . 10
11.2. Informative References . . . . . . . . . 12
7. Security Considerations . . . . . . . . 10
Author's Address . . . . . . . . . . . 13
8. Acknowledgements . . . . . . . . . . . . . 10

1. Introduction

Data center servers often use IP Multicast to send data to clients or
other application servers. IP Multicast . . . . . . . . . 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 expected likely to help conserve
bandwidth increase significantly in the data center and reduce future. Reasons
for this increase include the load on servers. IP
Multicast is also a key component in several data center overlay
solutions. Increased reliance on multicast, nature of the traffic generated by
applications hosted in next generation the data
centers, requires higher performance and capacity especially from center, the
switches. If multicast is to continue need to be handle broadcast,
unknown unicast and multicast (BUM) traffic within the overlay
technologies used in 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 center,
it centers must scale well within and support communication
between datacenters. There has been
much interest in issues surrounding massive amounts potentially thousands of hosts 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 center. There was centers for a lengthy discussion, in the now closed ARMD
WG, involving
number of reasons, mostly pertaining to concerns about its
scalability and reliability.

This draft discusses some of the issues with address resolution main drivers for non ARP/ND
multicast the increasing
volume and importance of one-to-many traffic patterns in data
centers. This document provides a quick
survey of multicast in Thereafter, the data center and should serve as an aid manner in which conventional IP multicast
may be used to
further discussion handle this traffic pattern is discussed and some of issues related to multicast in
the data center.

ARP/ND issues are not addressed in associated challenges highlighted. Following this document except to explain
how address resolution occurs with multicast. discussion, a
number of alternative emerging approaches are introduced, before
concluding by discussing key trends and making a number of
recommendations.

1.1. Requirements Language

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.

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

2.1. Applications in

Key trends suggest that 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 a critical protocol within their
network 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 nature of the uses applications likely to
dominate future highly-virtualized multi-tenant data centers will
produce large volumes of that multicast one-to-many traffic. For example, it is
important
well-known that traffic flows in order 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 properly support these applications in the ever
evolving data centers. If, for instance,
consensus that topologies such as the majority of Leaf/Spine, that are easier to
scale in the
applications East-West direction, are discovering/signaling each other, using multicast,
there may be better ways suited to support them then using multicast. If,
however, the multicasting of data is occurring in large volumes,
there is a need for good data
center overlay multicast support. The
applications either fall into the category of those that leverage L2
multicast for discovery or the future. This increase in East-West traffic flows
results from VMs often having to exchange numerous messages between
themselves as part of those that executing a specific workload. For example, a
computational workload could require L3 support and
likely span multiple subnets.

2.1. Client-Server Applications

IPTV servers use multicast data, or an executable, to deliver content from be
disseminated to workers distributed throughout the data center to
end users. IPTV which
may be subsequently polled for status updates. The emergence of such
applications means there is typically a one likely to many application where the
hosts are configured for IGMPv3, be an increase in one-to-many
traffic flows with the switches are configured increasing dominance of East-West traffic.

The TV broadcast industry is another potential future source of
applications with
IGMP snooping, one-to-many traffic patterns in data centers. The
requirement for robustness, stability and the routers are running PIM-SSM mode. Often
redundant 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 are sending multicast streams into etc. However,
the network growing cost and complexity of supporting this approach,
especially as the network is forwarding bit rates of the data across diverse paths.

Windows Media servers send multicast streaming video signals increase due to clients. Windows
Media Services streams to an IP multicast address
demand for formats such as 4K-UHD and all clients
subscribe 8K-UHD, means there is a
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 address to receive data centers in broadcast plants. Traffic flows in
the same stream. This allows 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 single stream pivotal consideration for broadcasters considering
transitioning to IP is the manner in which these one-to-many traffic
flows will be played simultaneously by multiple clients managed and
thus reducing bandwidth utilization.

Market monitored in a data relies extensively on center with an IP
fabric.

Arguably 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 data center stock exchange 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. 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 Typically,
redundant servers (in a primary/backup or live live live-live mode) are sending send
multicast streams into the network 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 the network is forwarding the
data across diverse paths (when duplicate subscribers and
many message channels within a data is sent by multiple
servers). 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 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.

2.2. Non Client-Server Multicast Applications

Routers, running Virtual Routing Redundancy Protocol (VRRP),
communicate with one another using 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 address. VRRP packets messages within IPv4 and IPv6 networks
respectively to discover MAC address to IP address mappings.
Furthermore, when these protocols are sent, encapsulated 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 IP packets, use
mostly seek to 224.0.0.18. A failure exploit characteristics of data center networks to
receive a
avoid having to broadcast/multicast these messages, as discussed in
Section 4.1.

3. Handling one-to-many traffic using conventional multicast packet from
3.1. Layer 3 multicast

PIM is the master router most widely deployed multicast routing protocol and so,
unsurprisingly, is the primary multicast routing protocol considered
for a period longer
than use in the data center. There are three times 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 advertisement timer causes optimality of the backup routers to
assume that multicast forwarding tree for the master router is dead. The virtual router then
transitions into an unsteady
amount of multicast forwarding state that must be maintained at
routers. SSM provides the most efficient forwarding between sources
and an election process receivers and thus is
initiated to select the next master router from the backup routers.
This most suitable for applications with one-to-
many traffic patterns. State is fulfilled through built and maintained for each (S,G)
flow. Thus, the use amount of multicast packets. Backup
router(s) are only to send multicast packets during an election
process.

Overlays may use IP multicast to virtualize L2 multicasts. IP
multicast is used to reduce the scope of the L2-over-UDP flooding to
only those hosts that have expressed explicit interest forwarding state held by routers
in the
frames.VXLAN, for instance, data center is an encapsulation scheme proportional 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 the number of
known destination MAC addresses, sources and stores
groups. At the IP address other end of the
tunnel to spectrum, BIDIR is the remote VTEP to use most
efficient shared tree solution as one tree is built for each. Unicast frames, all (S,G)s,
therefore minimizing the amount of state. This state reduction is at
the expense of optimal forwarding path between
VMs, are sent directly to 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 unicast L3 address
amount of the remote VTEP.
Multicast frames are sent state is uncorrelated to a multicast IP group associated with the
VNI. Underlying IP Multicast protocols (PIM-SM/SSM/BIDIR) number of sources. SSM and
BIDIR are used
to forward optimizations of PIM-SM. PIM-SM is still the most widely
deployed multicast data across routing protocol. PIM-SM can also be the overlay.

The Ganglia application most
complex. PIM-SM relies upon a RP (Rendezvous Point) to set up the
multicast for distributed
discovery tree and monitoring subsequently there is the option of computing systems such as clusters and
grids. It has been used switching to link clusters across university campuses
and can scale
the SPT (shortest path tree), similar to handle clusters with 2000 nodes
Windows Server, cluster node exchange, relies upon SSM, or staying on the use of
shared tree, similar to BIDIR.

3.2. Layer 2 multicast heartbeats between servers. Only the other interfaces in

With IPv4 unicast address resolution, the same translation of an IP
address to a MAC address is done dynamically by ARP. With multicast group use
address resolution, the data. Unlike broadcast, mapping from a multicast
traffic does not need IPv4 address to be flooded throughout the network, reducing a
multicast MAC address is done by assigning the chance that unnecessary CPU cycles are expended filtering traffic
on nodes outside low-order 23 bits of
the cluster. As multicast IPv4 address to fill the number low-order 23 bits 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 MAC address. Each IPv4 multicast address has 28 unique
bits (the multicast address range is based on a
scheme whereby nodes send heartbeat messages to other nodes.
Specifically, for each network interface, a node sends 224.0.0.0/12) therefore mapping
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 IP address 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 MAC address ignores 5 bits of the sending
node and IP
address. Hence, groups of 32 multicast IP addresses are mapped to
the same MAC address meaning a reduction in network bandwidth consumed.

o Regroup: The clustering membership engine executes a regroup
protocol during multicast MAC address cannot be
uniquely mapped to a membership view change. The regroup protocol
algorithm assumes the ability multicast IPv4 address. Therefore, planning is
required within an organization to broadcast messages choose IPv4 multicast addresses
judiciously in order to all cluster
nodes. To avoid unnecessary network flooding and to properly
authenticate messages, address aliasing. When sending IPv6
multicast packets on an Ethernet link, the broadcast primitive corresponding destination
MAC address is implemented by a
sequence direct mapping of unicast messages. Converting the unicast messages 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 single host wishes to join an
IPv4 multicast message conserves processing power on group, it sends an IGMP [RFC2236], [RFC3376] report to
the
sending node router attached to the layer 2 segment and reduces network bandwidth consumption.

Multicast addresses in also it instructs its
data link layer to receive Ethernet frames that match the 224.0.0.x range are considered
corresponding MAC address. The data link local
multicast addresses. They are used for protocol discovery and are
flooded layer filters the frames,
passing those with matching destination addresses to every port. For example, OSPF uses 224.0.0.5 and
224.0.0.6 the IP module.
Similarly, hosts simply hand the multicast packet for neighbor and DR discovery. These addresses are
reserved and will not be constrained by IGMP snooping. These
addresses are not transmission to be used by any application.

3. L2 Multicast Protocols in
the Data Center

The switches, in between data link layer which would add the servers and layer 2 encapsulation, using
the routers, rely upon igmp
snooping to bound 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 ports leading to interested
hosts and default
behaviour is to L3 routers. A switch will, by default, flood multicast
traffic it to all the ports in the layer 2 segment.
Clearly there may not be a broadcast domain (VLAN). IGMP snooping
is designed to prevent hosts on a local network from receiving
traffic receiver for a this 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). present
on each port and IGMP snooping is a L2 optimization for L3 IGMP. 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.
Joins and leaves heading upstream to the router are filtered so that
by ensuring only the minimal quantity of information is sent. The
switch is trying to ensure the router only has only a single entry for the
group, regardless of how many the number of active listeners there are. 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, IGMP and thus IGMP snooping, 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 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

Multicast Listener Discovery (MLD) [RFC 2710] [RFC 3810] is used by
IPv6 routers for discovering multicast listeners on switch ports a directly
attached link, performing a similar function to
generate IGMP packets. Switch ports 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 connected a subset of
ICMPv6 messages. MLD snooping works similarly to multicast
source ports 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 router ports. The switch typically learns
about multicast groups from in VXLAN deployments. In the original VXLAN
specification [RFC7348], a data-driven flood and learn control plane
was proposed, requiring the multicast data stream center IP fabric to support
multicast routing. A multicast group is associated with each virtual
network, each uniquely identified by using a type
of source only learning (when only receiving 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 data on group and use it for the
port, no IGMP packets). The switch forwards exchange of BUM
traffic only to with the
multicast router ports. When other VTEPs. Essentially, the switch receives VTEP would
encapsulate any BUM traffic for new from attached VMs in an IP multicast groups, it will typically flood
packet, whose destination address is the packets associated multicast group
address, and transmit the packet to all ports the data center fabric. Thus,
PIM must be running in the same VLAN. This unnecessary flooding fabric to maintain a multicast
distribution tree per VNI.

Alternatively, rather than setting up a multicast distribution tree
per VNI, a tree can impact switch
performance.

4. L3 Multicast Protocols in be set up whenever hosts within the Data Center

There are three flavors of 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 used for Multicast Routing in the Data
Center: PIM-SM [RFC4601], PIM-SSM [RFC4607], and PIM-BIDIR [RFC5015].
SSM provides join message which will be propagated into the most efficient forwarding between sources and
receivers and IP fabric.
In order to ensure this join message is most suitable for one sent to many types the IP fabric rather
than over the VXLAN interface (since the VTEP will have a route back
to the source of the multicast
applications. State is built for each S,G channel therefore packet over the more
sources VXLAN interface and groups there are, so
would naturally attempt to send the join over this interface) a 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
specific route back to the most
efficient in forwarding path between sources and receivers. SSM and
BIDIR are optimizations of PIM-SM. PIM-SM is still source over the most widely
deployed multicast routing protocol. PIM-SM can also IP fabric must be
configured. In this approach PIM must be configured on the most
complex. PIM-SM relies upon a RP (Rendezvous Point) to set up SVIs
associated with the
multicast tree VXLAN interface.

Another use case of PIM and then will either switch IGMP in data centers is when IPTV servers
use multicast to deliver content from the SPT (shortest path
tree), similar data center to SSM, or stay on the shared tree (similar end users.
IPTV is typically a one to BIDIR).
For massive amounts of many application where the hosts sending (and receiving) multicast, are
configured for IGMPv3, the
shared tree (particularly switches are configured with PIM-BIDIR) provides IGMP
snooping, and the best potential
scaling since no matter how many routers are running PIM-SSM mode. Often redundant
servers send multicast sources exist within a
VLAN, streams into the tree number stays network and the same. IGMP snooping, IGMP proxy, network is
forwards the data across diverse paths.

Windows Media servers send multicast streams to clients. Windows
Media Services streams to an IP multicast address and
PIM-BIDIR have all clients
subscribe to the potential IP address to scale 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 huge scaling numbers
required biggest advantage of using PIM and IGMP to support one-
to-many communication in a data center.

5. Challenges of 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 multicast them in data centers. Furthermore, the Data Center

Data Center environments may create unique challenges for IP
Multicast. Data Center maturity of these
protocols means their behaviour and performance in operational
networks required a high amount is well-understood, with widely available best-practices and
deployment guides for optimizing their performance.

However, somewhat ironically, the relative disadvantages of VM traffic PIM and mobility within
IGMP usage in data centers also stem mostly from their maturity.
Specifically, these protocols were standardized and between DC networks. DC networks have large
numbers implemented long
before the highly-virtualized multi-tenant data centers of servers. DC networks today
existed. Consequently, PIM and IGMP are often used neither optimally placed to
deal with cloud
orchestration software. DC networks often use IP Multicast in their
unique environments. This section looks at the challenges requirements of using 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 challenging data center environment.

When IGMP/MLD Snooping is not implemented, ethernet switches will
flood multicast frames out of center, new VMs being
continually spun up and wishing to join the sessions while all switch-ports, which turns the
traffic into something more like
time other VMs are leaving. In such a broadcast.

VRRP uses multicast heartbeat to communicate between routers. The
communication between scenario, the host churn in the PIM
and IGMP state machines, the default gateway is unicast.
The multicast heartbeat can be very chatty when there are thousands volume of VRRP pairs with sub-second heartbeat calls back control messages they would
generate 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 amount of state they would necessitate within
routers, especially if multicast traverses beyond one IP
subnet, which is necessary they were deployed naively, would be
untenable.

4. Alternative options for an overlay like VXLAN, you could
potentially have scaling concerns. If using a VXLAN overlay, it handling one-to-many traffic

Section 2 has shown that there is
necessary likely to map the L2 multicast be an increasing amount
one-to-many communications in the overlay to L3 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 underlay
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 do head end replication a distributed
control plane, makes these techniques more potent.

4.1. Minimizing traffic volumes

If handling one-to-many traffic in data centers can be challenging
then arguably the overlay and receive
duplicate frames on the first link from the router to the core
switch. The most intuitive solution could be is to run potentially thousands of PIM
messages aim to generate/maintain minimize the required multicast state
volume of such traffic.

It was previously mentioned in Section 2 that the IP
underlay. The behavior 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 upper layer, with respect
volume of one-to-many traffic generated by applications, there is
more scope for attempting to
broadcast/multicast, affects reduce the choice volume of head end (*,G) or (S,G)
replication in the underlay, which affects the opex 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 capex regular topology,
owned and exclusively controlled by single organization, well-known
overlay encapsulation endpoints etc.

A way of minimizing the
entire solution. A VXLAN, with thousands amount of logical groups, maps one-to-many traffic that traverses
the data center fabric is to
head end replication in 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 IGMP 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 hypervisor
and then PIM between information distributed by the TOR and CORE 'switches' controller and
reply. Thus, the gateway
router.

Requiring IP multicast (especially PIM BIDIR) from ARP request is suppressed and does not result in
one-to-many traffic traversing the network can
prove challenging for data center operators especially at the kind of
scale that IP fabric.

Alternatively, the VXLAN/NVGRE proposals require. This is also true when functionality supported by the L2 topological domain controller can
realized by a distributed control plane. BGP-EVPN [RFC7432, RFC8365]
is large and extended all the way to the L3
core. In most popular control plane used in data centers centers. Typically,
the encapsulation endpoints will exchange pertinent information 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
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 10-20 disseminated. Consequently,
ARP requests from local VMs per server in a
virtualized environment. One vendor reported a customer requesting a
scale to 400VM's per server. For multicast to can be a viable solution suppressed by the encapsulation
endpoint.

4.2. Head end replication

A popular option for handling one-to-many traffic patterns in this environment, data
centers is head end replication (HER). HER means the network needs to be able to scale traffic is
duplicated and sent to these
numbers when these VMs are sending/receiving multicast.

A lot of switching/routing hardware has problems with each end point individually using conventional
IP Multicast,
particularly with regards to hardware support of PIM-BIDIR.

Sending L2 multicast over a campus or data center backbone, in any
sort unicast. Obvious disadvantages of significant way, HER include traffic duplication
and the additional processing burden on the head end. Nevertheless,
HER is a new challenge enabled for especially attractive when overlays are in use as the first
time
replication can be carried out by overlays. There the hypervisor or encapsulation end
point. Consequently, the VMs and IP fabric are interesting challenges when pushing
large amounts of multicast traffic through a network, unmodified and have thus
far been dealt with using purpose-built networks. While
unaware of how the overlay
proposals have been careful not traffic is delivered to impose new protocol requirements,
they have not addressed the issues multiple end points.
Additionally, it is possible to use a number of performance approaches for
constructing and scalability,
nor disseminating the large-scale availability list of these protocols.

There is an unnecessary multicast stream flooding problem in the link
layer switches between the multicast source which endpoints should
receive what traffic and so on.

For example, the PIM First Hop
Router (FHR). The IGMP-Snooping Switch will forward multicast
streams reluctance of data center operators to router ports, enable PIM
and IGMP within the PIM FHR must receive all multicast
streams even if there data center fabric means VXLAN is no request from receiver. This often leads used with
HER. Thus, BUM traffic from each VNI is replicated and sent using
unicast to waste remote VTEPs with VMs in that VNI. The list of switch cache and link bandwidth when remote
VTEPs to which the multicast
streams are not actually required. [I-D.pim-umf-problem-statement]
details traffic should be sent may be configured manually
on the problem and defines design goals 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 generic mechanism
distributed control plane is used instead of the centralized
controller. Again, BGP-EVPN may be used to restrain distribute the unnecessary multicast stream flooding.

6. Layer 3 / Layer 2 Topological Variations
information needed to faciliate HER to the VTEPs.

4.3. BIER

As discussed in RFC6820, the ARMD problems statement, there are a
variety of topological Section 3.4, PIM and IGMP face potential scalability
challenges when deployed in data center variations including L3 centers. These challenges are
typically due to Access
Switches, L3 the requirement to Aggregation Switches, build and maintain a distribution
tree and L3 in the Core only.
Further analysis is needed in order requirement to understand how hold per-flow state in routers. Bit
Index Explicit Replication (BIER) [RFC 8279] is a new multicast
forwarding paradigm that avoids these
variations affect IP Multicast scalability

7. Address Resolution

7.1. Solicited-node Multicast Addresses for IPv6 address resolution

Solicited-node Multicast Addresses are used with IPv6 Neighbor
Discovery to provide two requirements.

When a multicast packet enters a BIER domain, the same function ingress router,
known as the Address Resolution
Protocol (ARP) Bit-Forwarding Ingress Router (BFIR), adds a BIER header
to the packet. This header contains a bit string in IPv4. ARP uses broadcasts, which each bit
maps to send an ARP
Requests, which are received by all end hosts on egress router, known as Bit-Forwarding Egress Router
(BFER). If a bit is set, then the local link.
Only packet should be forwarded to the host being queried responds. However,
associated BFER. The routers within the other hosts still
have to process BIER domain, Bit-Forwarding
Routers (BFRs), use the BIER header in the packet and discard information in
the request. With IPv6, a host 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
required deemed to join a Solicited-Node multicast group be attractive for each of its
configured unicast or anycast addresses. Because a Solicited-node
Multicast Address facilitating one-to-many
communications in data ceneters [I-D.ietf-bier-use-cases]. The
deployment envisioned with overlay networks is a function of that the last 24-bits of an IPv6
unicast or anycast address, the number of hosts that are subscribed
to each Solicited-node Multicast Address
encapsulation endpoints would typically be one
(there could be more because the mapping function is BFIR. So knowledge about the
actual multicast groups does not a 1:1
mapping). Compared to ARP reside in IPv4, a host should not need to be
interrupted as often to service Neighbor Solicitation requests.

7.2. Direct Mapping for Multicast address resolution

With IPv4 unicast address resolution, the translation of an IP
address data center fabric,
improving the scalability compared to conventional IP multicast.
Additionally, a MAC address is done dynamically by ARP. With multicast
address resolution, the mapping from centralized controller or a multicast IP address BGP-EVPN control plane
may be used with BIER to a
multicast MAC address is derived from direct mapping. In IPv4, ensure the
mapping is done by assigning BFIR have the low-order 23 bits required
information. A challenge associated with using BIER is that, unlike
most of the multicast
IP address other approaches discussed in this draft, it requires
changes to fill the low-order 23 bits forwarding behaviour of the multicast MAC
address. When 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 host joins packet
traverses a network is determined by an IP multicast group, it instructs 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
data link layer ingress node to receive frames that match the MAC address that
corresponds SR domain. Segment Routing can be applied to
the IP address of MPLS and IPv6 data-planes. In the multicast group. The data link
layer filters former, the frames and passes frames with matching destination
addresses to list of segments
is represented by the IP module. Since label stack and in the mapping from multicast IP
address to latter it is represented
as a MAC address ignores 5 bits of routing extension header. Use-cases are described in [I-D.ietf-
spring-segment-routing] and are being considered in the IP address, groups context of
32 multicast IP addresses are mapped
BGP-based large-scale data-center (DC) design [RFC7938].

Multicast in SR continues to the same MAC address. As a
result a multicast MAC address cannot be uniquely mapped 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
multicast IPv4 address. Planning is required within an organization Segment
Identifier (SID) to select IPv4 groups that are far enough away from each other existing replication solutions, such as PIM,
mLDP, P2MP RSVP-TE and BIER. It may also be that a new way to
not end up with the same L2 address used. Any multicast address signal
and install trees in SR is developed without creating state in the [224-239].0.0.x
network.

5. Conclusions

As the volume and [224-239].128.0.x ranges should not be
considered. When sending IPv6 importance of one-to-many traffic in data centers
increases, conventional IP multicast packets on an Ethernet link,
the corresponding destination MAC address is likely to become increasingly
unattractive for deployment in data centers for a direct mapping number of the
last 32 bits 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 128 bit IPv6
most popular manner in which end hosts signal interest in joining a
multicast address into the 48 bit
MAC address. It group, it is possible for more than one IPv6 Multicast address
to map to unlikely that this multicast traffic will be
transported over the same 48 bit MAC address.

8. 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-
EVPN).

6. IANA Considerations

This memo includes no request to IANA.

7. Security Considerations

No new security considerations result from this document

10.

8. 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
Narten.

11.

9. References

11.1.

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,
<https://www.rfc-editor.org/info/rfc2119>.

11.2.

9.2. Informative References

[I-D.ietf-bier-use-cases]
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.

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

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

[I-D.ietf-spring-segment-routing]
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,
<https://www.rfc-editor.org/info/rfc2236>.

[RFC2710] Deering, S., Fenner, W., and B. Haberman, "Multicast
Listener Discovery (MLD) for IPv6", RFC 2710,
DOI 10.17487/RFC2710, October 1999,
<https://www.rfc-editor.org/info/rfc2710>.

[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,
<https://www.rfc-editor.org/info/rfc3376>.

[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,
<https://www.rfc-editor.org/info/rfc4601>.

[RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for
IP", RFC 4607, DOI 10.17487/RFC4607, August 2006,
<https://www.rfc-editor.org/info/rfc4607>.

[RFC5015] Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
"Bidirectional Protocol Independent Multicast (BIDIR-
PIM)", RFC 5015, DOI 10.17487/RFC5015, October 2007,
<https://www.rfc-editor.org/info/rfc5015>.

[RFC6820] Narten, T., Karir, M., and I. Foo, "Address Resolution
Problems in Large Data Center Networks", RFC 6820,
DOI 10.17487/RFC6820, January 2013,
<https://www.rfc-editor.org/info/rfc6820>.

Author's Address

[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,
<https://www.rfc-editor.org/info/rfc7348>.

[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,
<https://www.rfc-editor.org/info/rfc7637>.

[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,
<https://www.rfc-editor.org/info/rfc7938>.

[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,
<https://www.rfc-editor.org/info/rfc8014>.

[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,
<https://www.rfc-editor.org/info/rfc8279>.

[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,
<https://www.rfc-editor.org/info/rfc8365>.

Authors' Addresses

Mike McBride
Huawei

Email: michael.mcbride@huawei.com

Olufemi Komolafe
Arista Networks

Email: femi@arista.com