Internet DRAFT - draft-ietf-mboned-dc-deploy
draft-ietf-mboned-dc-deploy
MBONED M. McBride
Internet-Draft Futurewei
Intended status: Informational O. Komolafe
Expires: August 7, 2020 Arista Networks
February 4, 2020
Multicast in the Data Center Overview
draft-ietf-mboned-dc-deploy-09
Abstract
The volume and importance of one-to-many traffic patterns in data
centers is likely to increase significantly in the future. Reasons
for this increase are discussed and then attention is paid to the
manner in which this traffic pattern may be judiciously handled in
data centers. The intuitive solution of deploying conventional IP
multicast within data centers is explored and evaluated. Thereafter,
a number of emerging innovative approaches are described before a
number of recommendations are made.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 7, 2020.
Copyright Notice
Copyright (c) 2020 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
Provisions Relating to IETF Documents
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publication of this document. Please review these documents
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Reasons for increasing one-to-many traffic patterns . . . . . 3
2.1. Applications . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Overlays . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Protocols . . . . . . . . . . . . . . . . . . . . . . . . 6
2.4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Handling one-to-many traffic using conventional multicast . . 7
3.1. Layer 3 multicast . . . . . . . . . . . . . . . . . . . . 7
3.2. Layer 2 multicast . . . . . . . . . . . . . . . . . . . . 7
3.3. Example use cases . . . . . . . . . . . . . . . . . . . . 9
3.4. Advantages and disadvantages . . . . . . . . . . . . . . 9
4. Alternative options for handling one-to-many traffic . . . . 10
4.1. Minimizing traffic volumes . . . . . . . . . . . . . . . 11
4.2. Head end replication . . . . . . . . . . . . . . . . . . 12
4.3. Programmable Forwarding Planes . . . . . . . . . . . . . 12
4.4. BIER . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.5. Segment Routing . . . . . . . . . . . . . . . . . . . . . 14
5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 15
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
9.1. Normative References . . . . . . . . . . . . . . . . . . 15
9.2. Informative References . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction
The volume and importance of one-to-many traffic patterns in data
centers will likely continue to increase. Reasons for this increase
include the nature of the traffic generated by applications hosted in
the data center, the need to handle broadcast, unknown unicast and
multicast (BUM) traffic within the overlay technologies used to
support multi-tenancy at scale, and the use of certain protocols that
traditionally require one-to-many control message exchanges.
These trends, allied with the expectation that highly virtualized
large-scale data centers must support communication between
potentially thousands of participants, may lead to the natural
assumption that IP multicast will be widely used in data centers,
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specifically given the bandwidth savings it potentially offers.
However, such an assumption would be wrong. In fact, there is
widespread reluctance to enable conventional IP multicast in data
centers for a number of reasons, mostly pertaining to concerns about
its scalability and reliability.
This draft discusses some of the main drivers for the increasing
volume and importance of one-to-many traffic patterns in data
centers. Thereafter, the manner in which conventional IP multicast
may be used to handle this traffic pattern is discussed and some of
the associated challenges highlighted. Following this discussion, a
number of alternative emerging approaches are introduced, before
concluding by discussing key trends and making a number of
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. Reasons for increasing one-to-many traffic patterns
2.1. Applications
Key trends suggest that the nature of the applications likely to
dominate future highly-virtualized multi-tenant data centers will
produce large volumes of one-to-many traffic. For example, it is
well-known that traffic flows in data centers have evolved from being
predominantly North-South (e.g. client-server) to predominantly East-
West (e.g. distributed computation). This change has led to the
consensus that topologies such as the Leaf/Spine, that are easier to
scale in the East-West direction, are better suited to the data
center of the future. This increase in East-West traffic flows
results from VMs often having to exchange numerous messages between
themselves as part of executing a specific workload. For example, a
computational workload could require data, or an executable, to be
disseminated to workers distributed throughout the data center which
may be subsequently polled for status updates. The emergence of such
applications means there is likely to be an increase in one-to-many
traffic flows with the increasing dominance of East-West traffic.
The TV broadcast industry is another potential future source of
applications with one-to-many traffic patterns in data centers. The
requirement for robustness, stability and predicability has meant the
TV broadcast industry has traditionally used TV-specific protocols,
infrastructure and technologies for transmitting video signals
between end points such as cameras, monitors, mixers, graphics
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devices and video servers. However, the growing cost and complexity
of supporting this approach, especially as the bit rates of the video
signals increase due to demand for formats such as 4K-UHD and 8K-UHD,
means there is a 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
Society of Motion Picture and Television Engineers (SMPTE)
[SMPTE2110], along with the increasing performance of IP routers,
means this transition is gathering pace. A possible outcome of this
transition will be the building of IP data centers in broadcast
plants. Traffic flows in the broadcast industry are frequently one-
to-many and so if IP data centers are deployed in broadcast plants,
it is imperative that this traffic pattern is supported efficiently
in that infrastructure. In fact, a pivotal consideration for
broadcasters considering transitioning to IP is the manner in which
these one-to-many traffic flows will be managed and monitored in a
data center with an IP fabric.
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 stock
exchanges to financial service providers and then to the stock
analysts or brokerages. It is essential that the network
infrastructure delivers very low latency and high throughout,
especially given the proliferation of automated and algorithmic
trading which means stock analysts or brokerages may gain an edge on
competitors simply by receiving an update a few milliseconds earlier.
As would be expected, in such deployments reliability is critical.
The network must be designed with no single point of failure and in
such a way that it can respond in a deterministic manner to failure.
Typically, redundant servers (in a primary/backup or live-live mode)
send multicast streams into the network, with diverse paths being
used across the network. 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.
Another reason for the growing volume of one-to-many traffic patterns
in modern data centers is the increasing adoption of streaming
telemetry. This transition is motivated by the observation that
traditional poll-based approaches for monitoring network devices are
usually inadequate in modern data centers. These approaches
typically suffer from poor scalability, extensibility and
responsiveness. In contrast, in streaming telemetry, network devices
in the data center stream highly-granular real-time updates to a
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telemetry collector/database. This collector then collates,
normalizes and encodes this data for convenient consumption by
monitoring applications. The montoring applications can subscribe to
the notifications of interest, allowing them to gain insight into
pertinent state and performance metrics. Thus, the traffic flows
associated with streaming telemetry are typically many-to-one between
the network devices and the telemetry collector and then one-to-many
from the collector to the monitoring applications.
The use of publish and subscribe applications is growing within data
centers, contributing to the rising volume of one-to-many traffic
flows. Such applications are attractive as they provide a robust
low-latency asynchronous messaging service, allowing senders to be
decoupled from receivers. The usual approach is for a publisher to
create and transmit a message to a specific topic. The publish and
subscribe application will retain the message and ensure it is
delivered to all subscribers to that topic. The flexibility in the
number of publishers and subscribers to a specific topic means such
applications cater for one-to-one, one-to-many and many-to-one
traffic patterns.
2.2. Overlays
Another key contributor to the rise in one-to-many traffic patterns
is the proposed architecture for supporting large-scale multi-tenancy
in highly virtualized data centers [RFC8014]. In this architecture,
a tenant's VMs are distributed across the data center and are
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 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.
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Hence, regardless of the overlay technology used, due consideration
must be given to handling BUM traffic, forcing the data center
operator to pay attention to the manner in which one-to-many
communication is handled within the data center. And this
consideration is likely to become increasingly important with the
anticipated rise in the number and importance of overlays. In fact,
it may be asserted that the manner in which one-to-many
communications arising from overlays is handled is pivotal to the
performance and stability of the entire data center network.
2.3. Protocols
Conventionally, some key networking protocols used in data centers
require one-to-many communications for control messages. Thus, the
data center operator must pay due attention to how these control
message exchanges are supported.
For example, ARP [RFC0826] and ND [RFC4861] use broadcast and
multicast messages within IPv4 and IPv6 networks respectively to
discover MAC address to IP address mappings. Furthermore, when these
protocols are running within an overlay network, 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].
Another example of a protocol that may neccessitate having one-to-
many traffic flows in the data center is IGMP [RFC2236], [RFC3376].
If the VMs attached to the Layer 2 segment wish to join a multicast
group they must send IGMP reports in response to queries from the
querier. As these devices could be located at different locations
within the data center, there is the somewhat ironic prospect of IGMP
itself leading to an increase in the volume of one-to-many
communications in the data center.
2.4. Summary
Section 2.1, Section 2.2 and Section 2.3 have discussed how the
trends in the types of applications, the overlay technologies used
and some of the essential networking protocols results in an increase
in the volume of one-to-many traffic patterns in modern highly-
virtualized data centers. Section 3 explores how such traffic flows
may be handled using conventional IP multicast.
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3. Handling one-to-many traffic using conventional multicast
Faced with ever increasing volumes of one-to-many traffic flows, for
the reasons presented in Section 2, it makes sense for a data center
operator to explore if and how conventional IP multicast could be
deployed within the data center. This section introduces the key
protocols, discusses some example use cases where they are deployed
in data centers and discusses some of the advantages and
disadvantages of such deployments.
3.1. Layer 3 multicast
PIM is the most widely deployed multicast routing protocol and so,
unsurprisingly, is the primary multicast routing protocol considered
for use in the data center. There are three potential popular modes
of PIM that may be used: PIM-SM [RFC4601], PIM-SSM [RFC4607] or PIM-
BIDIR [RFC5015]. It may be said that these different modes of PIM
tradeoff the optimality of the multicast forwarding tree for the
amount of multicast forwarding state that must be maintained at
routers. SSM provides the most efficient forwarding between sources
and receivers and thus is most suitable for applications with one-to-
many traffic patterns. State is built and maintained for each (S,G)
flow. Thus, the amount of multicast forwarding state held by routers
in the data center is proportional to the number of sources and
groups. At the other end of the spectrum, BIDIR is the most
efficient shared tree solution as one tree is built for all flows,
therefore minimizing the amount of state. This state reduction is at
the expense of optimal forwarding path between sources and receivers.
This use of a shared tree makes BIDIR particularly well-suited for
applications with many-to-many traffic patterns, given that the
amount of state is uncorrelated to the number of sources. SSM and
BIDIR are optimizations of PIM-SM. PIM-SM is the most widely
deployed multicast routing protocol. PIM-SM can also be the most
complex. PIM-SM relies upon a RP (Rendezvous Point) to set up the
multicast tree and subsequently there is the option of switching to
the SPT (shortest path tree), similar to SSM, or staying on the
shared tree, similar to BIDIR.
3.2. Layer 2 multicast
With IPv4 unicast address resolution, the translation of an IP
address to a MAC address is done dynamically by ARP. With multicast
address resolution, the mapping from a multicast IPv4 address to a
multicast MAC address is done by assigning the low-order 23 bits of
the multicast IPv4 address to fill the low-order 23 bits of the
multicast MAC address. Each IPv4 multicast address has 28 unique
bits (the multicast address range is 224.0.0.0/12) therefore mapping
a multicast IP address to a MAC address ignores 5 bits of the IP
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address. Hence, groups of 32 multicast IP addresses are mapped to
the same MAC address. And so a multicast MAC address cannot be
uniquely mapped to a multicast IPv4 address. Therefore, IPv4
multicast addresses must be chosen judiciously in order to avoid
unneccessary address aliasing. When sending IPv6 multicast packets
on an Ethernet link, the corresponding destination MAC address is a
direct mapping of the last 32 bits of the 128 bit IPv6 multicast
address into the 48 bit MAC address. It is possible for more than
one IPv6 multicast address to map to the same 48 bit MAC address.
The default behaviour of many hosts (and, in fact, routers) is to
block multicast traffic. Consequently, when a host wishes to join an
IPv4 multicast group, it sends an IGMP [RFC2236], [RFC3376] report to
the router attached to the Layer 2 segment and also it instructs its
data link layer to receive Ethernet frames that match the
corresponding MAC address. The data link layer filters the frames,
passing those with matching destination addresses to the IP module.
Similarly, hosts simply hand the multicast packet for transmission to
the data link layer which would add the Layer 2 encapsulation, using
the MAC address derived in the manner previously discussed.
When this Ethernet frame with a multicast MAC address is received by
a switch configured to forward multicast traffic, the default
behaviour is to flood it to all the ports in the Layer 2 segment.
Clearly there may not be a receiver for this multicast group present
on each port and IGMP snooping is used to avoid sending the frame out
of ports without receivers.
A switch running IGMP snooping listens to the IGMP messages exchanged
between hosts and the router in order to identify which ports have
active receivers for a specific multicast group, allowing the
forwarding of multicast frames to be suitably constrained. Normally,
the multicast router will generate IGMP queries to which the hosts
send IGMP reports in response. However, number of optimizations in
which a switch generates IGMP queries (and so appears to be the
router from the hosts' perspective) and/or generates IGMP reports
(and so appears to be hosts from the router's perspectve) are
commonly used to improve the performance by reducing the amount of
state maintained at the router, suppressing superfluous IGMP messages
and improving responsivenss when hosts join/leave the group.
Multicast Listener Discovery (MLD) [RFC 2710] [RFC 3810] is used by
IPv6 routers for discovering multicast listeners on a directly
attached link, performing a similar function to IGMP in IPv4
networks. MLDv1 [RFC 2710] is similar to IGMPv2 and MLDv2 [RFC 3810]
[RFC 4604] similar to IGMPv3. However, in contrast to IGMP, MLD does
not send its own distinct protocol messages. Rather, MLD is a
subprotocol of ICMPv6 [RFC 4443] and so MLD messages are a subset of
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ICMPv6 messages. MLD snooping works similarly to IGMP snooping,
described earlier.
3.3. Example use cases
A use case where PIM and IGMP are currently used in data centers is
to support multicast in VXLAN deployments. In the original VXLAN
specification [RFC7348], a data-driven flood and learn control plane
was proposed, requiring the data center IP fabric to support
multicast routing. A multicast group is associated with each virtual
network, each uniquely identified by its VXLAN network identifiers
(VNI). VXLAN tunnel endpoints (VTEPs), typically located in the
hypervisor or ToR switch, with local VMs that belong to this VNI
would join the multicast group and use it for the exchange of BUM
traffic with the other VTEPs. Essentially, the VTEP would
encapsulate any BUM traffic from attached VMs in an IP multicast
packet, whose destination address is the associated multicast group
address, and transmit the packet to the data center fabric. Thus, a
multicast routing protocol (typically PIM) must be running in the
fabric to maintain a multicast distribution tree per VNI.
Alternatively, rather than setting up a multicast distribution tree
per VNI, a tree can be set up whenever hosts within the VNI wish to
exchange multicast traffic. For example, whenever a VTEP receives an
IGMP report from a locally connected host, it would translate this
into a PIM join message which will be propagated into the IP fabric.
In order to ensure this join message is sent to the IP fabric rather
than over the VXLAN interface (since the VTEP will have a route back
to the source of the multicast packet over the VXLAN interface and so
would naturally attempt to send the join over this interface) a more
specific route back to the source over the IP fabric must be
configured. In this approach PIM must be configured on the SVIs
associated with the VXLAN interface.
Another use case of PIM and IGMP in data centers is when IPTV servers
use multicast to deliver content from the data center to end users.
IPTV is typically a one to many application where the hosts are
configured for IGMPv3, the switches are configured with IGMP
snooping, and the routers are running PIM-SSM mode. Often redundant
servers send multicast streams into the network and the network is
forwards the data across diverse paths.
3.4. Advantages and disadvantages
Arguably the biggest advantage of using PIM and IGMP to support one-
to-many communication in data centers is that these protocols are
relatively mature. Consequently, PIM is available in most routers
and IGMP is supported by most hosts and routers. As such, no
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specialized hardware or relatively immature software is involved in
using these protocols in data centers. Furthermore, the maturity of
these protocols means their behaviour and performance in operational
networks is well-understood, with widely available best-practices and
deployment guides for optimizing their performance. For these
reasons, PIM and IGMP have been used successfully for supporting one-
to-many traffic flows within modern data centers, as discussed
earlier.
However, somewhat ironically, the relative disadvantages of PIM and
IGMP usage in data centers also stem mostly from their maturity.
Specifically, these protocols were standardized and implemented long
before the highly-virtualized multi-tenant data centers of today
existed. Consequently, PIM and IGMP are neither optimally placed to
deal with the requirements of one-to-many communication in modern
data centers nor to exploit idiosyncrasies of data centers. For
example, there may be thousands of VMs participating in a multicast
session, with some of these VMs migrating to servers within the data
center, new VMs being continually spun up and wishing to join the
sessions while all the time other VMs are leaving. In such a
scenario, the churn in the PIM and IGMP state machines, the volume of
control messages they would generate and the amount of state they
would necessitate within routers, especially if they were deployed
naively, would be untenable. Furthermore, PIM is a relatively
complex protocol. As such, PIM can be challenging to debug even in
significantly more benign deployments than those envisaged for future
data centers, a fact that has evidently had a dissuasive effect on
data center operators considering enabling it within the IP fabric.
4. Alternative options for handling one-to-many traffic
Section 2 has shown that there is likely to be an increasing amount
one-to-many communications in data centers for multiple reasons. And
Section 3 has discussed how conventional multicast may be used to
handle this traffic, presenting some of the associated advantages and
disadvantages. Unsurprisingly, as discussed in the remainder of
Section 4, there are a number of alternative options of handling this
traffic pattern in data centers. Critically, it should be noted that
many of these techniques are not mutually-exclusive; in fact many
deployments involve a combination of more than one of these
techniques. Furthermore, as will be shown, introducing a centralized
controller or a distributed control plane, typically makes these
techniques more potent.
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4.1. Minimizing traffic volumes
If handling one-to-many traffic flows in data centers is considered
onerous, then arguably the most intuitive solution is to aim to
minimize the volume of said traffic.
It was previously mentioned in Section 2 that the three main
contributors to one-to-many traffic in data centers are applications,
overlays and protocols. Typically the applications running on VMs
are outside the control of the data center operator and thus,
relatively speaking, little can be done about the volume of one-to-
many traffic generated by applications. Luckily, there is more scope
for attempting to reduce the volume of such traffic generated by
overlays and protocols. (And often by protocols within overlays.)
This reduction is possible by exploiting certain characteristics of
data center networks such as a fixed and regular topology, single
administrative control, consistent hardware and software, well-known
overlay encapsulation endpoints and systematic IP address allocation.
A way of minimizing the amount of one-to-many traffic that traverses
the data center fabric is to use a centralized controller. For
example, whenever a new VM is instantiated, the hypervisor or
encapsulation endpoint can notify a centralized controller of this
new MAC address, the associated virtual network, IP address etc. The
controller could subsequently distribute this information to every
encapsulation endpoint. Consequently, when any endpoint receives an
ARP request from a locally attached VM, it could simply consult its
local copy of the information distributed by the controller and
reply. Thus, the ARP request is suppressed and does not result in
one-to-many traffic traversing the data center IP fabric.
Alternatively, the functionality supported by the controller can
realized by a distributed control plane. BGP-EVPN [RFC7432, RFC8365]
is the most popular control plane used in data centers. Typically,
the encapsulation endpoints will exchange pertinent information with
each other by all peering with a BGP route reflector (RR). Thus,
information such as local MAC addresses, MAC to IP address mapping,
virtual networks identifiers, IP prefixes, and local IGMP group
membership can be disseminated. Consequently, for example, ARP
requests from local VMs can be suppressed by the encapsulation
endpoint using the information learnt from the control plane about
the MAC to IP mappings at remote peers. In a similar fashion,
encapsulation endpoints can use information gleaned from the BGP-EVPN
messages to proxy for both IGMP reports and queries for the attached
VMs, thus obviating the need to transmit IGMP messages across the
data center fabric.
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4.2. Head end replication
A popular option for handling one-to-many traffic patterns in data
centers is head end replication (HER). HER means the traffic is
duplicated and sent to each end point individually using conventional
IP unicast. Obvious disadvantages of HER include traffic duplication
and the additional processing burden on the head end. Nevertheless,
HER is especially attractive when overlays are in use as the
replication can be carried out by the hypervisor or encapsulation end
point. Consequently, the VMs and IP fabric are unmodified and
unaware of how the traffic is delivered to the multiple end points.
Additionally, it is possible to use a number of approaches for
constructing and disseminating the list of which endpoints should
receive what traffic and so on.
For example, the reluctance of data center operators to enable PIM
within the data center fabric means VXLAN is often used with HER.
Thus, BUM traffic from each VNI is replicated and sent using unicast
to remote VTEPs with VMs in that VNI. The list of remote VTEPs to
which the traffic should be sent may be configured manually on the
VTEP. Alternatively, the VTEPs may transmit pertinent local state to
a centralized controller which in turn sends each VTEP the list of
remote VTEPs for each VNI. Lastly, HER also works well when a
distributed control plane is used instead of the centralized
controller. Again, BGP-EVPN may be used to distribute the
information needed to faciliate HER to the VTEPs.
4.3. Programmable Forwarding Planes
As discussed in Section 2, one of the main functions of PIM is to
build and maintain multicast distribution trees. Such a tree
indicates the path a specific flow will take through the network.
Thus, in routers traversed by the flow, the information from PIM is
ultimately used to create a multicast forwarding entry for the
specific flow and insert it into the multicast forwarding table. The
multicast forwarding table will have entries for each multicast flow
traversing the router, with the lookup key usually being a
concantenation of the source and group addresses. Critically, each
entry will contain information such as the legal input interface for
the flow and a list of output interfaces to which matching packets
should be replicated.
Viewed in this way, there is nothing remarkable about the multicast
forwarding state constructed in routers based on the information
gleaned from PIM. And, in fact, it is perfectly feasible to build
such state in the absence of PIM. Such prospects have been
significantly enhanced with the increasing popularity and performance
of network devices with programmable forwarding planes. These
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devices are attractive for use in data centers since they are
amenable to being programmed by a centralized controller. If such a
controller has a global view of the sources and receivers for each
multicast flow (which can be provided by the devices attached to the
end hosts in the data center communicating with the controller), an
accurate representation of data center topology (which is usually
well-known), then it can readily compute the multicast forwarding
state that must be installed at each router to ensure the one-to-many
traffic flow is delivered properly to the correct receivers. All
that is needed is an API to program the forwarding planes of all the
network devices that need to handle the flow appropriately. Such
APIs do in fact exist and so, unsurprisingly, handling one-to-many
traffic flows using such an approach is attractive for data centers.
Being able to program the forwarding plane in this manner offers the
enticing possibility of introducing novel algorithms and concepts for
forwarding multicast traffic in data centers. These schemes
typically aim to exploit the idiosyncracies of the data center
network architecture to create ingenious, pithy and elegant encodings
of the information needed to facilitate multicast forwarding.
Depending on the scheme, this information may be carried in packet
headers, stored in the multicast forwarding table in routers or a
combination of both. The key characterstic is that the terseness of
the forwarding information means the volume of forwarding state is
significantly reduced. Additionally, the overhead associated with
building and maintaining a multicast forwarding tree has been
eliminated. The result of these reductions in the overhead
associated with multicast forwarding is a significant and impressive
increase in the effective number of multicast flows that can be
supported within the data center.
[Shabaz19] is a good example of such an approach and also presents
comprehensive discussion of other schemes in the discussion on
releated work. Although a number of promising schemes have been
proposed, no consensus has yet emerged as to which approach is best,
and in fact what "best" means. Even if a clear winner were to
emerge, it faces significant challenges to gain the vendor and
operator buy-in to ensure it is widely deployed in data centers.
4.4. BIER
As discussed in Section 3.4, PIM and IGMP face potential scalability
challenges when deployed in data centers. These challenges are
typically due to the requirement to build and maintain a distribution
tree and the requirement to hold per-flow state in routers. Bit
Index Explicit Replication (BIER) [RFC 8279] is a new multicast
forwarding paradigm that avoids these two requirements.
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When a multicast packet enters a BIER domain, the ingress router,
known as the Bit-Forwarding Ingress Router (BFIR), adds a BIER header
to the packet. This header contains a bit string in which each bit
maps to an egress router, known as Bit-Forwarding Egress Router
(BFER). If a bit is set, then the packet should be forwarded to the
associated BFER. The routers within the BIER domain, Bit-Forwarding
Routers (BFRs), use the BIER header in the packet and information in
the Bit Index Forwarding Table (BIFT) to carry out simple bit- wise
operations to determine how the packet should be replicated optimally
so it reaches all the appropriate BFERs.
BIER is deemed to be attractive for facilitating one-to-many
communications in data centers [I-D.ietf-bier-use-cases]. The BFIRs
are the encapsulation endpoints in the deployment envisioned with
overlay networks. So knowledge about the actual multicast groups
does not reside in the data center fabric, improving the scalability
compared to conventional IP multicast. Additionally, a centralized
controller or a BGP-EVPN control plane may be used with BIER to
ensure the BFIR have the required information. A challenge
associated with using BIER is that it requires changes to the
forwarding behaviour of the routers used in the data center IP
fabric.
4.5. Segment Routing
Segment Routing (SR) [RFC8402] is a manifestation of the source
routing paradigm, so called as the path a packet takes through a
network is determined at the source. The source encodes this
information in the packet header as a sequence of instructions.
These instructions are followed by intermediate routers, ultimately
resulting in the delivery of the packet to the desired destination.
In SR, the instructions are known as segments and a number of
different kinds of segments have been defined. Each segment has an
identifier (SID) which is distributed throughout the network by newly
defined extensions to standard routing protocols. Thus, using this
information, sources are able to determine the exact sequence of
segments to encode into the packet. The manner in which these
instructions are encoded depends on the underlying data plane.
Segment Routing can be applied to the MPLS and IPv6 data planes. In
the former, the list of segments is represented by the label stack
and in the latter it is represented as an IPv6 routing extension
header. Advantages of segment routing include the reduction in the
amount of forwarding state routers need to hold and the removal of
the need to run a signaling protocol, thus improving the network
scalability while reducing the operational complexity.
The advantages of segment routing and the ability to run it over an
unmodified MPLS data plane means that one of its anticipated use
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cases is in BGP-based large-scale data centers [RFC7938]. The exact
manner in which multicast traffic will be handled in SR has not yet
been standardized, with a number of different options being
considered. For example, since with the MPLS data plane, segments
are simply encoded as a label stack, then the protocols traditionally
used to create point-to-multipoint LSPs could be reused to allow SR
to support one-to-many traffic flows. Alternatively, a special SID
may be defined for a multicast distribution tree, with a centralized
controller being used to program routers appropriately to ensure the
traffic is delivered to the desired destinations, while avoiding the
costly process of building and maintaining a multicast distribution
tree.
5. Conclusions
As the volume and importance of one-to-many traffic in data centers
increases, conventional IP multicast is likely to become increasingly
unattractive for deployment in data centers for a number of reasons,
mostly pertaining its relatively poor scalability and inability to
exploit characteristics of data center network architectures. Hence,
even though IGMP/MLD is likely to remain the most popular manner in
which end hosts signal interest in joining a multicast group, it is
unlikely that this multicast traffic will be transported over the
data center IP fabric using a multicast distribution tree built and
maintained by PIM in the future. Rather, approaches which exploit
idiosyncracies of data center network architectures 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
8. Acknowledgements
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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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-09
(work in progress), January 2019.
[I-D.ietf-nvo3-geneve]
Gross, J., Ganga, I., and T. Sridhar, "Geneve: Generic
Network Virtualization Encapsulation", draft-ietf-
nvo3-geneve-13 (work in progress), March 2019.
[I-D.ietf-nvo3-vxlan-gpe]
Maino, F., Kreeger, L., and U. Elzur, "Generic Protocol
Extension for VXLAN", draft-ietf-nvo3-vxlan-gpe-07 (work
in progress), April 2019.
[RFC0826] Plummer, D., "An Ethernet Address Resolution Protocol: Or
Converting Network Protocol Addresses to 48.bit Ethernet
Address for Transmission on Ethernet Hardware", STD 37,
RFC 826, DOI 10.17487/RFC0826, November 1982,
<https://www.rfc-editor.org/info/rfc826>.
[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>.
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[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<https://www.rfc-editor.org/info/rfc4861>.
[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>.
[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>.
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[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>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[Shabaz19]
Shabaz, M., Suresh, L., Rexford, J., Feamster, N.,
Rottenstreich, O., and M. Hira, "Elmo: Source Routed
Multicast for Public Clouds", ACM SIGCOMM 2019 Conference
(SIGCOMM '19) ACM, DOI 10.1145/3341302.3342066, August
2019.
[SMPTE2110]
"SMPTE2110 Standards Suite",
<http://www.smpte.org/st-2110>.
Authors' Addresses
Mike McBride
Futurewei
Email: michael.mcbride@futurewei.com
Olufemi Komolafe
Arista Networks
Email: femi@arista.com
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