Multicast Considerations over IEEE 802 Wireless MediaLupin Lodge+1 408 255 9223charliep@lupinlodge.comFuturewei Technologies Inc.2330 Central ExpresswaySanta Clara95055CAUnited States of Americamichael.mcbride@futurewei.comHewlett Packard Enterprise6280 America Center Dr.San Jose95002CAUnited States of America+1 630 363 1389dorothy.stanley@hpe.comGoogle1600 Amphitheatre ParkwayMountain View94043CAUnited States of Americawarren@kumari.netSIGFOXMontrealCanadaj.c.zuniga@ieee.org
Internet
Internet AreaMulticastBroadcastBUMwifiwirelessIEEE 802 Wireless Multicast
Well-known issues with multicast have prevented the deployment of
multicast in 802.11 (Wi-Fi) and other local-area wireless environments.
This document describes the known limitations
of wireless (primarily 802.11) Layer 2 multicast. Also described are certain multicast
enhancement features that have been specified by the IETF
and by IEEE 802 for wireless media, as well as some operational choices that can be made to improve the performance of the network. Finally,
some recommendations are provided about the usage and combination of
these features and operational choices.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any
errata, and how to provide feedback on it may be obtained at
.
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Table of Contents
. Introduction
. Terminology
. Identified Multicast Issues
. Issues at Layer 2 and Below
. Multicast Reliability
. Lower and Variable Data Rate
. Capacity and Impact on Interference
. Power-Save Effects on Multicast
. Issues at Layer 3 and Above
. IPv4 Issues
. IPv6 Issues
. MLD Issues
. Spurious Neighbor Discovery
. Multicast Protocol Optimizations
. Proxy ARP in 802.11-2012
. IPv6 Address Registration and Proxy Neighbor Discovery
. Buffering to Improve Battery Life
. Limiting Multicast Buffer Hardware Queue Depth
. IPv6 Support in 802.11-2012
. Using Unicast Instead of Multicast
. Overview
. Layer 2 Conversion to Unicast
. Directed Multicast Service (DMS)
. Automatic Multicast Tunneling (AMT)
. GroupCast with Retries (GCR)
. Operational Optimizations
. Mitigating Problems from Spurious Neighbor Discovery
. Mitigating Spurious Service Discovery Messages
. Multicast Considerations for Other Wireless Media
. Recommendations
. Ongoing Discussion Items
. Security Considerations
. IANA Considerations
. Informative References
Acknowledgements
Authors' Addresses
Introduction
Well-known issues with multicast have prevented the deployment of
multicast in 802.11 and other local-area
wireless environments, as described in and . Performance issues have been observed
when multicast
packet transmissions of IETF protocols are used over IEEE 802 wireless
media. Even though enhancements for multicast transmissions have been
designed at both IETF and IEEE 802, incompatibilities still exist
between specifications, implementations, and configuration choices.
Many IETF protocols depend on multicast/broadcast for delivery of
control messages to multiple receivers. Multicast allows data to be sent to
multiple interested recipients without the source needing to send duplicate
data to each recipient. With broadcast traffic, data is sent to every device
regardless of their expressed interest in the data. Multicast is used for various
purposes such as Neighbor Discovery, network flooding, and address
resolution, as well as minimizing media occupancy for the
transmission of data that is intended for multiple receivers.
In addition to protocol use of broadcast/multicast for
control messages, more applications, such as Push To Talk in
hospitals or video in enterprises, universities, and homes, are
sending multicast IP to end-user devices, which are increasingly
using Wi-Fi for their connectivity. IETF protocols typically rely on network protocol layering in order
to reduce or eliminate any dependence of higher-level protocols on
the specific nature of the MAC-layer protocols or the physical media.
In the case of multicast transmissions, higher-level protocols have
traditionally been designed as if transmitting a packet to an IP
address had the same cost in interference and network media access,
regardless of whether the destination IP address is a unicast address
or a multicast or broadcast address. This model was reasonable for
networks where the physical medium was wired, like Ethernet.
Unfortunately, for many wireless media, the costs to access the
medium can be quite different. Multicast over Wi-Fi has often been
plagued by such poor performance that it is disallowed.
Some enhancements have been designed
in IETF protocols that are assumed to work primarily over wireless
media. However, these enhancements are usually implemented in limited
deployments and are not widespread on most wireless networks. IEEE 802 wireless protocols have been designed with certain features
to support multicast traffic. For instance, lower modulations are
used to transmit multicast frames so that these can be received by
all stations in the cell, regardless of the distance or path
attenuation from the base station or Access Point (AP).
However, these
lower modulation transmissions occupy the medium longer;
they hamper efficient transmission of traffic using
higher-order modulations to nearby stations.
For these and other reasons, IEEE 802 Working Groups such as 802.11
have designed features to improve the performance of multicast
transmissions at Layer 2 .
In addition to protocol design features, certain operational and
configuration enhancements can ameliorate the network
performance issues created by multicast traffic,
as described in . There seems to be general agreement that these problems will not
be fixed anytime soon, primarily because it's expensive to do so
and because of the unreliability of multicast. Compared to unicast over Wi-Fi,
multicast is often treated as somewhat of a second-class citizen even
though there are many protocols using multicast. Something needs to
be provided in order to make them more reliable. IPv6
Neighbor Discovery saturating the Wi-Fi link is only part of the
problem. Wi-Fi traffic classes may help. This document is intended
to help make the determination about
what problems should be solved by the IETF and what problems
should be solved by the IEEE (see ).
This document details various problems caused by multicast transmission
over wireless networks, including high packet error rates, no
acknowledgements, and low data rate. It also explains some
enhancements that have been designed at the IETF and IEEE 802.11 to ameliorate
the effects of the radio medium on multicast traffic. Recommendations are also provided
to implementors about how to use and combine these enhancements.
Some advice about the operational choices that can be made is also
included. It is likely that this document will also be considered
relevant to designers of future IEEE wireless specifications. TerminologyThis document uses the following definitions:
ACK
The 802.11 Layer 2 acknowledgement.
AES-CCMP
AES-Counter Mode CBC-MAC Protocol
AP
IEEE 802.11 Access Point.
Basic rate
The slowest rate of all the
connected devices at which multicast and broadcast traffic is
generally transmitted.
DVB-H
Digital Video Broadcasting - Handheld
DVB-IPDC
Digital Video Broadcasting - Internet Protocol Datacasting
DTIM
Delivery Traffic Indication Map; an information element that advertises whether or not any associated
stations have buffered multicast or broadcast frames.
MCS
Modulation and Coding Scheme.
NOC
Network Operations Center.
PER
Packet Error Rate.
STA
802.11 station (e.g., handheld device).
TIM
Traffic Indication Map; an
information element that advertises whether or not any associated
stations have buffered unicast frames.
TKIP
Temporal Key Integrity Protocol
WiMAX
Worldwide Interoperability for Microwave Access
WPA
Wi-Fi Protected Access
Identified Multicast IssuesIssues at Layer 2 and Below In this section, some of the issues related to the use of multicast
transmissions over IEEE 802 wireless technologies are described.Multicast Reliability Multicast traffic is typically much less reliable than unicast
traffic. Since multicast makes point-to-multipoint communications,
multiple acknowledgements would be needed to guarantee reception
at all recipients. However, since there are no ACKs for multicast
packets, it is not possible for the AP to
know whether or not a retransmission is needed. Even in the wired
Internet, this characteristic often causes undesirably high error
rates. This has contributed to the relatively slow uptake of
multicast applications even though the protocols have long been
available. The situation for wireless links is much worse and is
quite sensitive to the presence of background traffic.
Consequently, there can be a high packet error rate (PER)
due to lack of retransmission and because the sender never backs
off. PER is the ratio, in percent, of the number of packets not successfully
received by the device. It is not uncommon for there to be a packet loss rate of 5%
or more, which is particularly troublesome for video and other
environments where high data rates and high reliability are
required. Lower and Variable Data Rate Multicast over wired differs from multicast over wireless because
transmission over wired links often occurs at
a fixed rate. Wi-Fi, on the other hand, has a transmission rate
that varies depending upon the STA's proximity to the AP.
The throughput of video flows and the capacity of the broader
Wi-Fi network will change with device movement. This impacts the ability for QoS
solutions to effectively reserve bandwidth and provide admission
control. For wireless stations authenticated and linked with an AP, the power
necessary for good reception can vary from station to station. For
unicast, the goal is to minimize power requirements while maximizing
the data rate to the destination. For multicast, the goal is simply
to maximize the number of receivers that will correctly receive the
multicast packet; generally, the AP has
to use a much lower data rate at a power level high enough for even
the farthest station to receive the packet, for example, as briefly
mentioned in . Consequently, the data
rate of a video stream, for instance, would be constrained by the
environmental considerations of the least-reliable receiver
associated with the AP. Because more robust modulation and coding schemes (MCSs)
have a longer range but also a lower data rate, multicast/broadcast
traffic is generally transmitted at the slowest rate of all the
connected devices. This is also known as the basic rate.
The amount of additional interference depends on the
specific wireless technology. In fact, backward compatibility and
multi-stream implementations mean that the maximum unicast rates
are currently up to a few Gbps, so there can be more than
3 orders of magnitude difference in the transmission rate between
multicast/broadcast versus optimal unicast forwarding. Some
techniques employed to increase spectral efficiency, such as spatial
multiplexing in Multiple Input Multiple Output (MIMO) systems, are not available with more than
one intended receiver; it is not the case that backwards
compatibility is the only factor responsible for lower multicast
transmission rates. Wired multicast also affects wireless LANs when the AP extends
the wired segment; in that case, multicast/broadcast frames
on the wired LAN side are copied to the Wireless Local Area Network (WLAN). Since broadcast
messages are transmitted at the most robust MCS,
many large frames are sent at a slow rate over the air. Capacity and Impact on Interference Transmissions at a lower
rate require longer occupancy of the wireless medium and thus
take away from the airtime of other communications and
degrade the overall capacity. Furthermore, transmission at higher
power, as is required to reach all multicast STAs associated
with the AP, proportionately increases the area of interference with other
consumers of the radio spectrum. Power-Save Effects on Multicast One of the characteristics of multicast transmission over Wi-Fi is that every
station has to be configured to wake up to receive the multicast frame,
even though the received packet may ultimately be discarded. This
process can have a large effect on the power consumption by
the multicast receiver station. For this reason, there are workarounds,
such as Directed Multicast Service (DMS) described in , to
prevent unnecessarily waking up stations. Multicast (and unicast) can work poorly with the power-save mechanisms defined in
IEEE 802.11e for the following reasons.
Clients may be unable to stay in sleep mode due to
multicast control packets frequently waking them up.
A unicast packet is delayed until an STA wakes up and requests
it. Unicast traffic may also be delayed to improve power
save and efficiency and to increase the probability of aggregation.
Multicast traffic is delayed in a wireless network if any of
the STAs in that network are power savers.
All STAs associated with the AP have to be
awake at a known time to receive multicast traffic.
Packets can also be discarded due to buffer limitations in
the AP and non-AP STA.
Issues at Layer 3 and Above This section identifies some representative IETF protocols and
describes possible negative effects due to performance degradation
when using multicast transmissions for control messages.
Common uses of multicast include:
Control plane signaling
Neighbor Discovery
Address resolution
Service Discovery
Applications (video delivery, stock data, etc.)
On-demand routing
Backbone construction
Other Layer 3 protocols (non-IP)
User Datagram Protocol (UDP) is the most common transport-layer
protocol for multicast applications.
By itself, UDP is not reliable -- messages may be lost or
delivered out of order.
IPv4 Issues The following list contains some representative
discovery protocols that utilize broadcast/multicast and are used with IPv4.
ARP
DHCP
Multicast DNS (mDNS)
Universal Plug and Play (uPnP)
After initial configuration, ARP (described in more detail later), DHCP, and uPnP occur much less
commonly, but service discovery can occur at any time. Some
widely deployed service discovery protocols (e.g., for finding a
printer) utilize mDNS (i.e., multicast), which is often dropped by operators. Even if multicast
snooping (which provides the benefit of conserving
bandwidth on those segments of the network where no node has expressed interest in receiving
packets addressed to the group address) is utilized, many devices can register at once and cause serious
network degradation.IPv6 Issues IPv6 makes extensive use of multicast, including the following:
DHCPv6
Protocol Independent Multicast (PIM)
IPv6 Neighbor Discovery Protocol (NDP)
Multicast DNS (mDNS)
Router Discovery
IPv6 NDP Neighbor Solicitation (NS) messages used in Duplicate Address
Detection (DAD) and address lookup make use of link-scope multicast. In
contrast to IPv4, an IPv6 node will typically use multiple
addresses and may change them often for privacy reasons. This
intensifies the impact of multicast messages that are associated
with the mobility of a node. Router advertisement (RA) messages
are also periodically multicast over the link.
Neighbors may be considered lost if several consecutive
Neighbor Discovery packets fail.
MLD Issues Multicast Listener Discovery (MLD) is
used to identify members of a multicast group that are connected to
the ports of a switch. Forwarding multicast frames into a
Wi-Fi-enabled area can use switch support for hardware
forwarding state information. However, since IPv6 makes heavy use
of multicast, each STA with an IPv6 address will require state on
the switch for several and possibly many solicited-node multicast
addresses. A solicited-node multicast address is an IPv6 multicast
address used by NDP to verify whether an IPv6 address is already
used by the local link. Multicast addresses that do not have forwarding state
installed (perhaps due to hardware memory limitations on the
switch) cause frames to be flooded on all ports of the switch. Some
switch vendors do not support MLD for link-scope multicast due to
the increase it can cause in state. Spurious Neighbor Discovery On the Internet, there is a "background radiation" of scanning
traffic (people scanning for vulnerable machines) and backscatter
(responses from spoofed traffic, etc.). This means that routers
very often receive packets destined for IPv4 addresses regardless of
whether those IP addresses are in use. In the cases where the IP
is assigned to a host, the router broadcasts an ARP request, receives an ARP
reply, and caches it; then, traffic can be delivered to the host.
When the IP address is not in use, the router broadcasts one (or
more) ARP requests and never gets a reply. This means that it does
not populate the ARP cache, and the next time there is traffic for
that IP address, the router will rebroadcast the ARP requests.
The rate of these ARP requests is proportional to the size of the
subnets, the rate of scanning and backscatter, and how long the
router keeps state on non-responding ARPs. As it turns out, this
rate is inversely proportional to how occupied the subnet is
(valid ARPs end up in a cache, stopping the broadcasting; unused
IPs never respond, and so cause more broadcasts). Depending on
the address space in use, the time of day, how occupied the
subnet is, and other unknown factors, thousands of broadcasts per second
have been observed. Around 2,000 broadcasts per second have been observed at
the IETF NOC during face-to-face meetings. With Neighbor Discovery for IPv6 , nodes
accomplish address resolution by multicasting a Neighbor Solicitation
that asks the target node to return its link-layer address. Neighbor
Solicitation messages are multicast to the solicited-node multicast
address of the target address. The target returns its link-layer address
in a unicast Neighbor Advertisement message. A single request-response
pair of packets is sufficient for both the initiator and the target to resolve
each other's link-layer addresses; the initiator includes its link-layer
address in the Neighbor Solicitation. On a wired network, there is not a huge difference between unicast,
multicast, and broadcast traffic. Due to hardware filtering
(see, e.g., ), inadvertently flooded
traffic (or excessive Ethernet multicast) on wired networks
can be quite a bit less costly compared to wireless cases where sleeping
devices have to wake up to process packets. Wired Ethernets tend to be switched
networks, further reducing interference from multicast. There is
effectively no collision / scheduling problem except at extremely
high port utilizations. This is not true in the wireless realm; wireless equipment is
often unable to send high volumes of broadcast and multicast
traffic, causing numerous broadcast and multicast packets to be
dropped. Consequently, when a host connects, it is often not
able to complete DHCP, and IPv6 RAs get dropped, leading to
users being unable to use the network.Multicast Protocol Optimizations This section lists some optimizations that have been specified in
IEEE 802 and IETF that are aimed at reducing or eliminating the
issues discussed in .Proxy ARP in 802.11-2012 The AP knows the Medium Access Control (MAC) address and IP address for all associated
STAs. In this way, the AP acts as the central "manager" for all
the 802.11 STAs in its Basic Service Set (BSS). Proxy ARP is easy to implement at the
AP and offers the following advantages:
Reduced broadcast traffic (transmitted at low MCS) on the
wireless medium.
STA benefits from extended power save in sleep mode, as ARP
requests for STA's IP address are handled instead by the AP.
ARP frames are kept off the wireless medium.
No changes are needed to STA implementation.
Here is the specification language as
described in clause 10.23.13 of :
When the AP supports Proxy ARP "[...] the AP shall maintain a
Hardware Address to Internet Address mapping for each
associated station, and shall update the mapping when the
Internet Address of the associated station changes. When the
IPv4 address being resolved in the ARP request packet is used
by a non-AP STA currently associated to the BSS, the proxy ARP
service shall respond on behalf of the STA to an ARP request or an ARP Probe.
IPv6 Address Registration and Proxy Neighbor Discovery
As used in this section,
a Low-Power Wireless Personal Area Network (6LoWPAN) denotes a Low-Power and Lossy Network (LLN) that supports
6LoWPAN Header Compression (HC).
A 6TiSCH network
is an example of a 6LoWPAN.
In order to control the use of IPv6 multicast over 6LoWPANs, the
6LoWPAN Neighbor Discovery (6LoWPAN ND)
standard defines an address registration mechanism that relies on a
central registry to assess address uniqueness as a substitute to the
inefficient DAD mechanism found in the mainstream IPv6 Neighbor Discovery Protocol (NDP)
.
The 6lo Working Group has specified an
update to .
Wireless devices can register their address to a
Backbone Router,
which proxies for the registered addresses with the IPv6
NDP running on a high-speed aggregating backbone. The update also
enables a proxy registration mechanism on behalf of the Registered
Node, e.g., by a 6LoWPAN router to which the mobile node is attached.
The general idea behind the Backbone Router concept is that broadcast
and multicast messaging should be tightly controlled in a variety
of WLANs and Wireless Personal Area
Networks (WPANs).
Connectivity to a particular link that provides the subnet should
be left to Layer 3. The model for the Backbone Router operation is
represented in .
Backbone Link and Backbone Routers
|
+-----+
| | Gateway (default) router
| |
+-----+
|
| Backbone Link
+--------------------+------------------+
| | |
+-----+ +-----+ +-----+
| | Backbone | | Backbone | | Backbone
| | router 1 | | router 2 | | router 3
+-----+ +-----+ +-----+
o o o o o o
o o o o o o o o o o o o o o
o o o o o o o o o o o o o o o
o o o o o o o o o o
o o o o o o o
LLN 1 LLN 2 LLN 3
LLN nodes can move freely from an LLN anchored at one IPv6 Backbone Router
to an LLN anchored at another Backbone Router on the same backbone,
keeping any of the IPv6 addresses they have configured.
The Backbone Routers maintain a Binding Table of their
Registered Nodes, which serves as a distributed database of all the LLN
nodes. An extension to the Neighbor Discovery Protocol is introduced to
exchange Binding Table information across the Backbone Link as needed
for the operation of IPv6 Neighbor Discovery.
and follow-on work
address the needs of LLNs, and similar techniques are likely to be
valuable on any type of
link where sleeping devices are attached or where the use of
broadcast and multicast operations should be limited. Buffering to Improve Battery Life Methods have been developed to help save battery life; for example,
a device might not wake up when the AP receives a multicast packet.
The AP acts on behalf of STAs in various ways. To enable use of
the power-saving feature for STAs in its BSS, the AP buffers frames
for delivery to the STA at the time when the STA is scheduled for
reception. If an AP, for instance, expresses a Delivery Traffic
Indication Message (DTIM) of 3, then
the AP will send a multicast packet every 3 packets. In fact,
when any single wireless STA associated with an AP has
802.11 power-save mode enabled, the AP buffers all multicast
frames and sends them only after the next DTIM beacon. In practice, most APs will send a multicast every 30 packets.
For unicast, the AP could send a Traffic Indication Message (TIM),
but, for multicast, the AP sends a broadcast to everyone. DTIM does
power management, but STAs can choose whether to wake up
and whether to drop the packet. Unfortunately, without proper administrative
control, such STAs may be unable to determine why their
multicast operations do not work. Limiting Multicast Buffer Hardware Queue DepthThe Content after Beacon (CAB) queue is used for beacon-triggered
transmission of buffered multicast frames. If lots of multicast frames were
buffered and this queue fills up, it drowns out all regular traffic. To limit the
damage that buffered traffic can do, some drivers limit the amount of
queued multicast data to a fraction of the beacon_interval. An example of
this is . IPv6 Support in 802.11-2012 IPv6 uses NDP instead of ARP. Every IPv6 node subscribes to a special
multicast address for this purpose.
Here is the specification language from clause 10.23.13
of :
When an IPv6 address is being resolved, the Proxy Neighbor
Discovery service shall respond with a Neighbor Advertisement
message [...] on behalf of an associated STA to an [ICMPv6]
Neighbor Solicitation message [...]. When MAC address mappings
change, the AP may send unsolicited Neighbor Advertisement
Messages on behalf of a STA.
NDP may be used to request additional information using the following methods, among others:
Maximum Transmission Unit
Router Solicitation
Router Advertisement
NDP messages are sent as group-addressed (broadcast) frames
in 802.11. Using the proxy operation helps to keep NDP messages off
the wireless medium.Using Unicast Instead of Multicast It is often possible to transmit multicast control and data messages
by using unicast transmissions to each station individually.Overview
In many situations, it's a good choice to use unicast instead of
multicast over the Wi-Fi link. This avoids most of the
problems specific to multicast over Wi-Fi, since the individual
frames are then acknowledged and buffered for power-save clients
in the way that unicast traffic normally operates.
This approach comes with the trade-off of sometimes sending
the same packet multiple times over the Wi-Fi link. However,
in many cases, such as video into a residential home network,
this can be a good trade-off since the Wi-Fi link may have enough
capacity for the unicast traffic to be transmitted to each
subscribed STA, even though multicast addressing may have been
necessary for the upstream access network.
Several technologies exist that can be used to arrange unicast
transport over the Wi-Fi link, outlined in the subsections below.
Layer 2 Conversion to Unicast
It is often possible to transmit multicast control and data messages
by using unicast transmissions to each station individually.
Although there is not yet a standardized method of conversion, at
least one widely available implementation exists in the Linux
bridging code . Other proprietary
implementations are available from various vendors.
In general, these implementations perform a straightforward
mapping for groups or channels, discovered by IGMP or MLD
snooping, to the corresponding unicast MAC addresses.
Directed Multicast Service (DMS)
DMS enables an STA to request that the AP
transmit multicast group-addressed frames destined to the
requesting STAs as individually addressed frames (i.e., convert
multicast to unicast). Here are some characteristics of DMS:
Requires 802.11n Aggregate MAC Service Data Units (A-MSDUs).
Individually addressed frames are acknowledged and are
buffered for power-save STAs.
The requesting STA may specify traffic characteristics for
DMS traffic.
DMS was defined in IEEE Std 802.11v-2011 .
DMS requires changes to both AP and STA implementation.
DMS is not currently implemented in products.
See and
for more information. Automatic Multicast Tunneling (AMT)
AMT provides a method to tunnel multicast
IP packets inside unicast IP packets over network links that only
support unicast. When an operating system or application running
on an STA has an AMT gateway capability integrated, it's possible
to use unicast to traverse the Wi-Fi link by deploying an AMT
relay in the non-Wi-Fi portion of the network connected to the AP.
It is recommended that multicast-enabled networks deploying AMT
relays for this purpose make the relays locally discoverable with
the following methods, as described in
:
DNS-based Service Discovery (DNS-SD)
The well-known IP addresses from
An AMT gateway that implements multiple standard discovery methods
is more likely to discover the local multicast-capable network
instead of forming a connection to a nonlocal AMT relay further upstream.
GroupCast with Retries (GCR) GCR (defined in ) provides greater
reliability by using either unsolicited retries or a block
acknowledgement mechanism. GCR increases the probability of broadcast
frame reception success but still does not guarantee success. For the block acknowledgement mechanism, the AP transmits each
group-addressed frame as a conventional group-addressed transmission.
Retransmissions are group addressed but hidden from non-11aa STAs.
A directed block acknowledgement scheme is used to harvest reception
status from receivers; retransmissions are based upon these
responses. GCR is suitable for all group sizes including medium to large
groups. As the number of devices in the group increases, GCR can send
block acknowledgement requests to only a small subset of the group.
GCR does require changes to both AP and STA implementations. GCR may introduce unacceptable latency. After sending a group of
data frames to the group, the AP has to do the following:
Unicast a Block Ack Request (BAR) to a subset of members.
Wait for the corresponding Block Ack (BA).
Retransmit any missed frames.
Resume other operations that may have been delayed.
This latency may not be acceptable for some traffic. There are ongoing extensions in 802.11 to improve GCR performance.
BAR is sent using downlink Multi-User MIMO.
BA is sent using uplink MU-MIMO (uplink MU-MIMO is an IEEE 801.11ax-2021 feature).
Latency may also be reduced by simultaneously receiving BA
information from multiple STAs.
Operational Optimizations This section lists some operational optimizations that can be
implemented when deploying wireless IEEE 802 networks to mitigate
some of the issues discussed in .Mitigating Problems from Spurious Neighbor Discovery
ARP Sponges
An ARP Sponge
sits on a network and learns which IP addresses are actually in
use. It also listens for ARP requests, and, if it sees an ARP for
an IP address that it believes is not used, it will reply with
its own MAC address. This means that the router now has an IP-to-MAC mapping, which it caches. If that IP is later assigned to a
machine (e.g., using DHCP), the ARP Sponge will see this and will
stop replying for that address. Gratuitous ARPs (or the machine
ARPing for its gateway) will replace the sponged address in the
router ARP table. This technique is quite effective; unfortunately, the ARP Sponge daemons were not really designed for
this use (one of the most widely deployed ARP Sponges
was
designed to deal with the disappearance of participants from an
Internet Exchange Point (IXP)) and so are not optimized for this purpose.
One daemon is
needed per subnet; the tuning is tricky (the scanning rate versus
the population rate versus retries, etc.), and sometimes daemons just stop,
requiring a restart of the daemon that causes disruption.
Router mitigations
Some
routers (often those based on Linux) implement a "negative ARP
cache" daemon. If the router does not see a reply to
an ARP, it can be configured to cache this information for some
interval. Unfortunately, the core routers in use often do
not support this. Instead, when a host connects to a network and gets an IP
address, it will ARP for its default gateway (the router). The
router will update its cache with the IP to host MAC mapping
learned from the request (passive ARP learning).
Firewall unused space
The
distribution of users on wireless networks / subnets may change in various
use cases, such as conference venues (e.g., Service Set Identifiers (SSIDs) are renamed, some SSIDs
lose favor, etc.). This makes utilization for particular SSIDs
difficult to predict ahead of time, but usage can be monitored
as attendees use the different networks. Configuring multiple
DHCP pools per subnet and enabling them sequentially can create
a large subnet from which only addresses in the lower portions
are assigned. Therefore, input IP access lists can be applied,
which deny traffic to the upper, unused portions. Then the
router does not attempt to forward packets to the unused portions
of the subnets and so does not ARP for it. This method has proven
to be very effective but is somewhat of a blunt axe, is fairly
labor intensive, and requires coordination.
Disabling/Filtering ARP requests
In general, the router does not need to ARP for
hosts; when a host connects, the router can learn the IP-to-MAC
mapping from the ARP request sent by that host. Consequently, it
should be possible to disable and/or filter ARP requests from the
router. Unfortunately, ARP is a very low-level/fundamental part
of the IP stack and is often offloaded from the normal control
plane. While many routers can filter Layer 2 traffic, this is
usually implemented as an input filter and/or has limited
ability to filter output broadcast traffic.
This means that the seemingly simple and obvious solution to "just disable ARP or filter it outbound" is made difficult or awkward in practice by implementations and/or architectural issues.
NAT
Broadcasts can often be
caused by outside Wi-Fi scanning / backscatter traffic. In order to reduce the impact of
broadcasts, NAT can be used on the entire (or a large portion) of a network. This would
eliminate NAT translation entries for unused addresses, and the router would never ARP
for them. There are, however, many reasons to avoid using NAT in such a blanket fashion.
Stateful firewalls
Another
obvious solution would be to put a stateful firewall between the
wireless network and the Internet. This firewall would block
incoming traffic not associated with an outbound request.
But this conflicts with the need and desire of some
organizations to have the network as open as possible and to
honor the end-to-end principle. An attendee on a meeting network
should be an Internet host and should be able to receive
unsolicited requests. Unfortunately, keeping the network working
and stable is the first priority, and a stateful firewall may be
required in order to achieve this.
Mitigating Spurious Service Discovery Messages
In networks that must support hundreds of STAs, operators have
observed network degradation due to many devices simultaneously
registering with mDNS. In a network with many clients, it is
recommended to ensure that mDNS packets designed to discover
services in smaller home networks be constrained to avoid
disrupting other traffic.
Multicast Considerations for Other Wireless Media Many of the causes of performance degradation described in earlier
sections are also observable for wireless media other than 802.11. For instance, problems with power save, excess media occupancy, and
poor reliability will also affect 802.15.3 and 802.15.4. Unfortunately,
802.15 media specifications do not yet include mechanisms similar to
those developed for 802.11. In fact, the design philosophy for 802.15
is oriented towards minimality, with the result that many such
functions are relegated to operation within higher-layer protocols.
This leads to a patchwork of non-interoperable and vendor-specific
solutions. See for additional discussion
and a proposal for a task group to resolve similar issues, in which
the multicast problems might be considered for mitigation. Similar considerations hold for most other wireless media. A brief
introduction is provided in for the following:
802.16 WiMAX
3GPP/3GPP2
DVB-H/DVB-IPDC
TV Broadcast and Satellite Networks
Recommendations This section provides some recommendations about the usage and
combinations of some of the multicast enhancements described in Sections
and . Future protocol documents utilizing multicast signaling should
be carefully scrutinized if the protocol is likely to be used over
wireless media. The use of proxy methods should be encouraged to conserve network bandwidth
and power utilization by low-power devices.
The device can send a unicast message to its proxy, and then the proxy can take care
of any needed multicast operations. Multicast signaling for wireless devices should be done in a way that is
compatible with low duty-cycle operation. Ongoing Discussion Items This section suggests two discussion items for further resolution. First, standards (and private) organizations should develop guidelines to help clarify when
multicast packets would be better served by being sent wired rather than wireless.
For example, 802.1ak works on
both Ethernet and Wi-Fi, and organizations could help with deployment decision making
by developing guidelines for multicast over Wi-Fi, including options for when traffic should be sent wired.
Second, reliable registration to Layer 2 multicast groups and a reliable
multicast operation at Layer 2 might provide a good multicast over Wi-Fi solution.
There shouldn't be a need to support 224 groups to get solicited node
multicast working: it is possible to simply select a number of
bits that make sense for a given network size to limit the
number of unwanted deliveries to reasonable levels.
The IEEE 802.1,
802.11, and 802.15 Working Groups should be encouraged to revisit Layer 2 multicast issues and provide
workable solutions.
Security Considerations
This document does not introduce or modify any security mechanisms.
Multicast deployed on wired or wireless networks as discussed in this document can be
made more secure in a variety of ways.
, for instance,
specifies the use of IPsec to ensure authentication of the link-local messages
in the Protocol Independent Multicast - Sparse Mode (PIM-SM) routing protocol.
specifies mechanisms to authenticate the PIM-SM link-local messages
using the IP security (IPsec) Encapsulating Security Payload (ESP) or (optionally) the
Authentication Header (AH).
When using mechanisms that convert multicast traffic to unicast traffic for traversing radio links,
the AP (or other entity) is forced to explicitly track which subscribers care about certain multicast traffic.
This is generally a reasonable trade-off but does result in another entity that is tracking what entities
subscribe to which multicast traffic. While such information is already (by necessity) tracked elsewhere,
this does present an expansion of the attack surface for that potentially privacy-sensitive information.
As noted in , the unreliable nature of
multicast transmission over wireless media can cause subtle problems
with multicast group key management and updates.
states that when TKIP (WPA, now deprecated) or AES-CCMP (WPA2/WPA3) encryption is in use, AP-to-client (FromDS) multicasts have to be encrypted with a separate encryption key that
is known to all of the clients (this is called the Group Key). Quoting further from that
website, "... most clients are able to get connected and surf the web,
check email, etc. even when FromDS multicasts are broken. So a lot of
people don't realize they have multicast problems on their network..."
This document encourages the use of proxy methods to conserve network bandwidth and
power utilization by low-power devices. Such proxy methods in general have security considerations that
require the proxy to be trusted to not misbehave. One such proxy method listed is an ARP Sponge that listens for ARP requests, and, if it sees an ARP for an IP address that it believes is not used, it will reply
with its own MAC address. ARP poisoning and false advertising could potentially undermine (e.g., DoS)
this and other proxy approaches.IANA Considerations This document has no IANA actions.Informative ReferencesEffects of IPv4 and IPv6 address resolution on AMS-IX and the ARP Sponge"Universiteit van Amsterdam""Universiteit van Amsterdam"bridge: multicast to unicastcommit 6db6f0elimit multicast buffer hardware queue depthcommit 268795110 Gbit Hardware Packet Filtering Using Commodity Network AdaptersNTOPIntelRIPE 61Information Technology--Telecommunications and Information Exchange between Systems - Local and Metropolitan Area Networks--Specific Requirements - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications (includes 802.11v amendment)IEEEProxy ARP in 802.11axInformation technology--Telecommunications and information exchange between systems Local and metropolitan area networks--Specific requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications Amendment 2: MAC Enhancements for Robust Audio Video StreamingIEEESubject: Why do some WiFi routers block multicast packets going from wired to wireless?message to the Super User Q & A communityLocal and Metropolitan Area Networks Virtual Bridged Local Area Networks - Amendment 07: Multiple Registration ProtocolIEEEIEEE 802.11 multicast capabilitiesMulticast on 802.11Deutsche TelekomIntel CorporationIEEE 802.11 multicast propertiesIntel CorporationPerformance evaluation of the IEEE 802.11aa multicast mechanisms for video streamingUniversidad Carlos III de MadridAvda. Universidad, 30, 28911 Leganes, SpainUniversidad Carlos III de MadridAvda. Universidad, 30, 28911 Leganes, SpainInstitute IMDEA Networks,Avda. del Mar Mediterraneo, 22, 28911 Leganes, SpainInstitute IMDEA Networks,Avda. del Mar Mediterraneo, 22, 28911 Leganes, Spain2013 IEEE 14th International Symposium on "A World of Wireless, Mobile and Multimedia Networks" (WoWMoM), pp. 1-9 An Ethernet Address Resolution Protocol: Or Converting Network Protocol Addresses to 48.bit Ethernet Address for Transmission on Ethernet HardwareThe purpose of this RFC is to present a method of Converting Protocol Addresses (e.g., IP addresses) to Local Network Addresses (e.g., Ethernet addresses). This is an issue of general concern in the ARPA Internet Community at this time. The method proposed here is presented for your consideration and comment. This is not the specification of an Internet Standard.Dynamic Host Configuration ProtocolThe Dynamic Host Configuration Protocol (DHCP) provides a framework for passing configuration information to hosts on a TCPIP network. DHCP is based on the Bootstrap Protocol (BOOTP), adding the capability of automatic allocation of reusable network addresses and additional configuration options. [STANDARDS-TRACK]Multicast Router DiscoveryThe concept of Internet Group Management Protocol (IGMP) and Multicast Listener Discovery (MLD) snooping requires the ability to identify the location of multicast routers. Since snooping is not standardized, there are many mechanisms in use to identify the multicast routers. However, this can lead to interoperability issues between multicast routers and snooping switches from different vendors.This document introduces a general mechanism that allows for the discovery of multicast routers. This new mechanism, Multicast Router Discovery (MRD), introduces a standardized means of identifying multicast routers without a dependency on particular multicast routing protocols. [STANDARDS-TRACK]Considerations for Internet Group Management Protocol (IGMP) and Multicast Listener Discovery (MLD) Snooping SwitchesThis memo describes the recommendations for Internet Group Management Protocol (IGMP) and Multicast Listener Discovery (MLD) snooping switches. These are based on best current practices for IGMPv2, with further considerations for IGMPv3- and MLDv2-snooping. Additional areas of relevance, such as link layer topology changes and Ethernet-specific encapsulation issues, are also considered. This memo provides information for the Internet community.Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)This document specifies Protocol Independent Multicast - Sparse Mode (PIM-SM). PIM-SM is a multicast routing protocol that can use the underlying unicast routing information base or a separate multicast-capable routing information base. It builds unidirectional shared trees rooted at a Rendezvous Point (RP) per group, and optionally creates shortest-path trees per source.This document obsoletes RFC 2362, an Experimental version of PIM-SM. [STANDARDS-TRACK]Neighbor Discovery for IP version 6 (IPv6)This document specifies the Neighbor Discovery protocol for IP Version 6. IPv6 nodes on the same link use Neighbor Discovery to discover each other's presence, to determine each other's link-layer addresses, to find routers, and to maintain reachability information about the paths to active neighbors. [STANDARDS-TRACK]IPv6 Stateless Address AutoconfigurationThis document specifies the steps a host takes in deciding how to autoconfigure its interfaces in IP version 6. The autoconfiguration process includes generating a link-local address, generating global addresses via stateless address autoconfiguration, and the Duplicate Address Detection procedure to verify the uniqueness of the addresses on a link. [STANDARDS-TRACK]Multicast Mobility in Mobile IP Version 6 (MIPv6): Problem Statement and Brief SurveyThis document discusses current mobility extensions to IP-layer multicast. It describes problems arising from mobile group communication in general, the case of multicast listener mobility, and problems for mobile senders using Any Source Multicast and Source-Specific Multicast. Characteristic aspects of multicast routing and deployment issues for fixed IPv6 networks are summarized. Specific properties and interplays with the underlying network access are surveyed with respect to the relevant technologies in the wireless domain. It outlines the principal approaches to multicast mobility, together with a comprehensive exploration of the mobile multicast problem and solution space. This document concludes with a conceptual road map for initial steps in standardization for use by future mobile multicast protocol designers. This document is a product of the IP Mobility Optimizations (MobOpts) Research Group. This document is not an Internet Standards Track specification; it is published for informational purposes.Authentication and Confidentiality in Protocol Independent Multicast Sparse Mode (PIM-SM) Link-Local MessagesRFC 4601 mandates the use of IPsec to ensure authentication of the link-local messages in the Protocol Independent Multicast - Sparse Mode (PIM-SM) routing protocol. This document specifies mechanisms to authenticate the PIM-SM link-local messages using the IP security (IPsec) Encapsulating Security Payload (ESP) or (optionally) the Authentication Header (AH). It specifies optional mechanisms to provide confidentiality using the ESP. Manual keying is specified as the mandatory and default group key management solution. To deal with issues of scalability and security that exist with manual keying, optional support for an automated group key management mechanism is provided. However, the procedures for implementing automated group key management are left to other documents. This document updates RFC 4601. [STANDARDS-TRACK]Compression Format for IPv6 Datagrams over IEEE 802.15.4-Based NetworksThis document updates RFC 4944, "Transmission of IPv6 Packets over IEEE 802.15.4 Networks". This document specifies an IPv6 header compression format for IPv6 packet delivery in Low Power Wireless Personal Area Networks (6LoWPANs). The compression format relies on shared context to allow compression of arbitrary prefixes. How the information is maintained in that shared context is out of scope. This document specifies compression of multicast addresses and a framework for compressing next headers. UDP header compression is specified within this framework. [STANDARDS-TRACK]Multicast DNSAs networked devices become smaller, more portable, and more ubiquitous, the ability to operate with less configured infrastructure is increasingly important. In particular, the ability to look up DNS resource record data types (including, but not limited to, host names) in the absence of a conventional managed DNS server is useful.Multicast DNS (mDNS) provides the ability to perform DNS-like operations on the local link in the absence of any conventional Unicast DNS server. In addition, Multicast DNS designates a portion of the DNS namespace to be free for local use, without the need to pay any annual fee, and without the need to set up delegations or otherwise configure a conventional DNS server to answer for those names.The primary benefits of Multicast DNS names are that (i) they require little or no administration or configuration to set them up, (ii) they work when no infrastructure is present, and (iii) they work during infrastructure failures.DNS-Based Service DiscoveryThis document specifies how DNS resource records are named and structured to facilitate service discovery. Given a type of service that a client is looking for, and a domain in which the client is looking for that service, this mechanism allows clients to discover a list of named instances of that desired service, using standard DNS queries. This mechanism is referred to as DNS-based Service Discovery, or DNS-SD.Neighbor Discovery Optimization for IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs)The IETF work in IPv6 over Low-power Wireless Personal Area Network (6LoWPAN) defines 6LoWPANs such as IEEE 802.15.4. This and other similar link technologies have limited or no usage of multicast signaling due to energy conservation. In addition, the wireless network may not strictly follow the traditional concept of IP subnets and IP links. IPv6 Neighbor Discovery was not designed for non- transitive wireless links, as its reliance on the traditional IPv6 link concept and its heavy use of multicast make it inefficient and sometimes impractical in a low-power and lossy network. This document describes simple optimizations to IPv6 Neighbor Discovery, its addressing mechanisms, and duplicate address detection for Low- power Wireless Personal Area Networks and similar networks. The document thus updates RFC 4944 to specify the use of the optimizations defined here. [STANDARDS-TRACK]Universal Plug and Play (UPnP) Internet Gateway Device - Port Control Protocol Interworking Function (IGD-PCP IWF)This document specifies the behavior of the Universal Plug and Play (UPnP) Internet Gateway Device - Port Control Protocol Interworking Function (IGD-PCP IWF). A UPnP IGD-PCP IWF is required to be embedded in Customer Premises (CP) routers to allow for transparent NAT control in environments where a UPnP IGD is used on the LAN side and PCP is used on the external side of the CP router.Automatic Multicast TunnelingThis document describes Automatic Multicast Tunneling (AMT), a protocol for delivering multicast traffic from sources in a multicast-enabled network to receivers that lack multicast connectivity to the source network. The protocol uses UDP encapsulation and unicast replication to provide this functionality.The AMT protocol is specifically designed to support rapid deployment by requiring minimal changes to existing network infrastructure.Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)This document specifies Protocol Independent Multicast - Sparse Mode (PIM-SM). PIM-SM is a multicast routing protocol that can use the underlying unicast routing information base or a separate multicast-capable routing information base. It builds unidirectional shared trees rooted at a Rendezvous Point (RP) per group, and it optionally creates shortest-path trees per source.This document obsoletes RFC 4601 by replacing it, addresses the errata filed against it, removes the optional (*,*,RP), PIM Multicast Border Router features and authentication using IPsec that lack sufficient deployment experience (see Appendix A), and moves the PIM specification to Internet Standard.Dynamic Host Configuration Protocol for IPv6 (DHCPv6)This document describes the Dynamic Host Configuration Protocol for IPv6 (DHCPv6): an extensible mechanism for configuring nodes with network configuration parameters, IP addresses, and prefixes. Parameters can be provided statelessly, or in combination with stateful assignment of one or more IPv6 addresses and/or IPv6 prefixes. DHCPv6 can operate either in place of or in addition to stateless address autoconfiguration (SLAAC).This document updates the text from RFC 3315 (the original DHCPv6 specification) and incorporates prefix delegation (RFC 3633), stateless DHCPv6 (RFC 3736), an option to specify an upper bound for how long a client should wait before refreshing information (RFC 4242), a mechanism for throttling DHCPv6 clients when DHCPv6 service is not available (RFC 7083), and relay agent handling of unknown messages (RFC 7283). In addition, this document clarifies the interactions between models of operation (RFC 7550). As such, this document obsoletes RFC 3315, RFC 3633, RFC 3736, RFC 4242, RFC 7083, RFC 7283, and RFC 7550.Registration Extensions for IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) Neighbor DiscoveryThis specification updates RFC 6775 -- the Low-Power Wireless Personal Area Network (6LoWPAN) Neighbor Discovery specification -- to clarify the role of the protocol as a registration technique and simplify the registration operation in 6LoWPAN routers, as well as to provide enhancements to the registration capabilities and mobility detection for different network topologies, including the Routing Registrars performing routing for host routes and/or proxy Neighbor Discovery in a low-power network.DNS Reverse IP Automatic Multicast Tunneling (AMT) DiscoveryThis document updates RFC 7450, "Automatic Multicast Tunneling" (or AMT), by modifying the relay discovery process. A new DNS resource record named AMTRELAY is defined for publishing AMT relays for source-specific multicast channels. The reverse IP DNS zone for a multicast sender's IP address is configured to use AMTRELAY resource records to advertise a set of AMT relays that can receive and forward multicast traffic from that sender over an AMT tunnel. Other extensions and clarifications to the relay discovery process are also defined.IPv6 Backbone RouterThis document updates RFCs 6775 and 8505 in order to enable proxy services for IPv6 Neighbor Discovery by Routing Registrars called "Backbone Routers". Backbone Routers are placed along the wireless edge of a backbone and federate multiple wireless links to form a single Multi-Link Subnet (MLSN).An Architecture for IPv6 over the Time-Slotted Channel Hopping Mode of IEEE 802.15.4 (6TiSCH)This document describes a network architecture that provides low-latency, low-jitter, and high-reliability packet delivery. It combines a high-speed powered backbone and subnetworks using IEEE 802.15.4 time-slotted channel hopping (TSCH) to meet the requirements of low-power wireless deterministic applications.IEEE 802.11n for Distributed Measurement SystemsNational Research Council of Italy, CNR-IEIITVia Gradenigo 6/B, 35131 Padova, ItalyNational Research Council of Italy, CNR-IEIITVia Gradenigo 6/B, 35131 Padova, ItalyDept. of Information Engineering, University of PadovaVia Gradenigo 6/B, 35131 Padova, Italy2017 IEEE International Instrumentation and Measurement Technology Conference (I2MTC), pp. 1-6LLC Proposal for 802.15.4Information technology -- Local and metropolitan area networks -- Specific requirements -- Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications Amendment 8: IEEE 802.11 Wireless Network ManagementIEEEAcknowledgements
This document has benefitted from discussions with the following
people, in alphabetical order:
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Authors' AddressesLupin Lodge+1 408 255 9223charliep@lupinlodge.comFuturewei Technologies Inc.2330 Central ExpresswaySanta Clara95055CAUnited States of Americamichael.mcbride@futurewei.comHewlett Packard Enterprise6280 America Center Dr.San Jose95002CAUnited States of America+1 630 363 1389dorothy.stanley@hpe.comGoogle1600 Amphitheatre ParkwayMountain View94043CAUnited States of Americawarren@kumari.netSIGFOXMontrealCanadaj.c.zuniga@ieee.org