Internet Area C. Perkins
Internet-Draft M. McBride
Intended status: Informational Futurewei
Expires: April 26, 2019 D. Stanley
HPE
W. Kumari
Google
JC. Zuniga
SIGFOX
October 23, 2018
Multicast Considerations over IEEE 802 Wireless Media
draft-ietf-mboned-ieee802-mcast-problems-03
Abstract
Well-known issues with multicast have prevented the deployment of
multicast in 802.11 [dot11], [mc-props], [mc-prob-stmt], and other
local-area wireless environments. IETF multicast experts have been
meeting together to discuss these issues and provide IEEE updates.
The mboned working group is chartered to receive regular reports on
the current state of the deployment of multicast technology, create
"practice and experience" documents that capture the experience of
those who have deployed and are deploying various multicast
technologies, and provide feedback to other relevant working groups.
This document offers guidance on known limitations and problems with
wireless multicast. Also described are various multicast enhancement
features that have been specified at IETF and IEEE 802 for wireless
media, as well as some operational chioces that can be taken to
improve the performace of the network. Finally, some recommendations
are provided about the usage and combination of these features and
operational choices.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on April 26, 2019.
Copyright Notice
Copyright (c) 2018 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
(https://trustee.ietf.org/license-info) in effect on the date of
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carefully, as they describe your rights and restrictions with respect
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Identified mulitcast issues . . . . . . . . . . . . . . . . . 5
3.1. Issues at Layer 2 and Below . . . . . . . . . . . . . . . 5
3.1.1. Multicast reliability . . . . . . . . . . . . . . . . 5
3.1.2. Lower and Variable Data Rate . . . . . . . . . . . . 5
3.1.3. High Interference . . . . . . . . . . . . . . . . . . 6
3.1.4. Power-save Effects on Multicast . . . . . . . . . . . 6
3.2. Issues at Layer 3 and Above . . . . . . . . . . . . . . . 7
3.2.1. IPv4 issues . . . . . . . . . . . . . . . . . . . . . 7
3.2.2. IPv6 issues . . . . . . . . . . . . . . . . . . . . . 8
3.2.3. MLD issues . . . . . . . . . . . . . . . . . . . . . 8
3.2.4. Spurious Neighbor Discovery . . . . . . . . . . . . . 9
4. Multicast protocol optimizations . . . . . . . . . . . . . . 9
4.1. Proxy ARP in 802.11-2012 . . . . . . . . . . . . . . . . 10
4.2. IPv6 Address Registration and Proxy Neighbor Discovery . 10
4.3. Buffering to Improve Battery Life . . . . . . . . . . . . 12
4.4. IPv6 support in 802.11-2012 . . . . . . . . . . . . . . . 12
4.5. Conversion of multicast to unicast . . . . . . . . . . . 13
4.6. Directed Multicast Service (DMS) . . . . . . . . . . . . 13
4.7. GroupCast with Retries (GCR) . . . . . . . . . . . . . . 13
5. Operational optimizations . . . . . . . . . . . . . . . . . . 14
5.1. Mitigating Problems from Spurious Neighbor Discovery . . 14
6. Multicast Considerations for Other Wireless Media . . . . . . 16
7. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 16
8. Discussion Items . . . . . . . . . . . . . . . . . . . . . . 17
9. Security Considerations . . . . . . . . . . . . . . . . . . . 17
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
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12. Informative References . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
Performance issues have been observed when multicast packet
transmissions of IETF protocols are used over IEEE 802 wireless
media. Even though enhamcements 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 is used for
various purposes such as neighborhood discovery, network flooding,
address resolution, as well 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, video in
enterprises and lectures in Universities, are streaming over wifi.
Many types of end devices are increasingly using wifi 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 wifi 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 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. 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
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groups such as 802.11 have designed features to improve the
performance of multicast transmissions at Layer 2 [ietf_802-11]. In
addition to protocol design features, certain operational and
configuration enhancements can ameliorate the network performance
issues created by multicast traffic. as described in Section 5.
In discussing these issues over email, and in a side meeting at IETF
99, it has been generally agreed that these problems will not be
fixed anytime soon primarily because it's expensive to do so and
multicast is unreliable. A big problem is that multicast is somewhat
a second class citizen, to unicast, over wifi. There are many
protocols using multicast and there needs to be something provided in
order to make them more reliable. The problem of IPv6 neighbor
discovery saturating the wifi link is only part of the problem. Wifi
traffic classes may help. We need to determine what problem should
be solved by the IETF and what problem should be solved by the IEEE
(see Section 8). A "multicast over wifi" IETF mailing list has been
formed (mcast-wifi@ietf.org) for further discussion. This draft will
be updated according to the current state of discussion.
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 IETF and IEEE 802 to
ameliorate the effects of 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
taken is also included. It is likely that this document will also be
considered relevant to designers of future IEEE wireless
specifications.
2. Terminology
This document uses the following definitions:
AP
IEEE 802.11 Access Point.
basic rate
The "lowest common denominator" data rate at which multicast and
broadcast traffic is generally transmitted.
DTIM
Delivery Traffic Indication Map (DTIM): An information element
that advertises whether or not any associated stations have
buffered multicast or broadcast frames.
MCS
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Modulation and Coding Scheme.
STA
802.11 station (e.g. handheld device).
TIM
Traffic Indication Map (TIM): An information element that
advertises whether or not any associated stations have buffered
unicast frames.
3. Identified mulitcast issues
3.1. Issues at Layer 2 and Below
In this section we describe some of the issues related to the use of
multicast transmissions over IEEE 802 wireless technologies.
3.1.1. 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. Since typically there are no ACKs for multicast
packets, it is not possible for the Access Point (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. 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.
3.1.2. Lower and Variable Data Rate
One big difference between multicast over wired versus multicast over
wired is that transmission over wired links often occurs at a fixed
rate. Wifi, on the other hand, has a transmission rate which varies
depending upon the client's proximity to the AP. The throughput of
video flows, and the capacity of the broader wifi network, will
change and will impact the ability for QoS solutions to effectively
reserve bandwidth and provide admission control.
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For wireless stations associated with an Access Points, 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 Access Point has to use a much lower
data rate at a power level high enough for even the farthest station
to receive the packet. 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
Access Point.
Because more robust modulation and coding schemes (MCSs) have longer
range but also lower data rate, multicast / broadcast traffic is
generally transmitted at the lowest common denominator rate, 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 Gb/s, so there can be a more
than 3 orders of magnitude difference in the transmission rate
between the basic rates to optimal unicast forwarding. Some
techinues employed to increase spectral efficiency, such as spatial
multiplexing in mimo systems, are not available with more than one
intended reciever; 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 WLAN. Since broadcast messages are
transmitted at the most robust MCS, many large frames are sent at a
slow rate over the air.
3.1.3. High 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
clients associated to the AP, proportionately increases the area of
interference.
3.1.4. Power-save Effects on Multicast
One of the characteristics of multicast transmission is that every
station has to be configured to wake up to receive the multicast,
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.
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Multicast can work poorly with the power-save mechanisms defined in
IEEE 802.11e, for the following reasons.
o Clients may be unable to stay in sleep mode due to multicast
control packets frequently waking them up.
o Both unicast and multicast traffic can be delayed by power-saving
mechanisms.
o A unicast packet is delayed until a STA wakes up and requests it.
Unicast traffic may also be delayed to improve power save,
efficiency and increase probability of aggregation.
o Multicast traffic is delayed in a wireless network if any of the
STAs in that network are power savers. All STAs associated to the
AP have to be awake at a known time to receive multicast traffic.
o Packets can also be discarded due to buffer limitations in the AP
and non-AP STA.
3.2. 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:
o Control plane signaling
o Neighbor Discovery
o Address Resolution
o Service discovery
o Applications (video delivery, stock data, etc.)
o On-demand routing
o Backbone construction
o Other L3 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.
3.2.1. IPv4 issues
The following list contains a few representative IPv4 protocols using
multicast.
o ARP
o DHCP
o mDNS
After initial configuration, ARP and DHCP occur much less commonly,
but service discovery can occur at any time. Apple's Bonjour
protocol, for instance, provides service discovery (for printing)
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that utilizes multicast. It's often the first service that operators
drop. Even if multicast snooping is utilized, many devices can
register at once using Bonjour, causing serious network degradation.
3.2.2. IPv6 issues
IPv6 makes extensive use of multicast, including the following:
o DHCPv6
o IPv6 Neighbor Discovery Protocol (NDP)
o Duplicate Address Detection (DAD)
o Address Resolution
o Service Discovery
o Route Discovery
o Decentralized Address Assignment
o Geographic routing
IPv6 NDP Neighbor Solicitation (NS) messages used in 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 multiplies the impact of multicast
messages that are associated to the mobility of a Node. Router
advertisement (RA) messages are also periodically multicasted over
the Link.
IPv6 NDP Neighbor Solicitation (NS) messages used in 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 multiplies the impact of multicast
messages that are associated to the mobility of a Node. Router
advertisement (RA) messages are also periodically multicasted over
the Link.
Neighbors may be considered lost if several consecutive Neighbor
Discovery packets fail.
3.2.3. MLD issues
Multicast Listener Discovery(MLD) [RFC4541] is often used to identify
members of a multicast group that are connected to the ports of a
switch. Forwarding multicast frames into a WiFi-enabled area can use
such 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 multicast solicited-node addresses. 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.
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3.2.4. 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 machines whose IP addresses may or may
not be in use. In the cases where the IP is assigned to a host, the
router broadcasts an ARP request, gets back 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, on the order of 2000 broadcasts per second have been
observed at the IETF NOCs.
On a wired network, there is not a huge difference between unicast,
multicast and broadcast traffic. Due to hardware filtering (see,
e.g., [Deri-2010]), inadvertently flooded traffic (or high amounts of
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.
Consequently, on the wireless networks, we observe a significant
amount of dropped broadcast and multicast packets. This, in turn,
means that 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.
4. 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 Section 3.
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4.1. Proxy ARP in 802.11-2012
The AP knows the 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 BSS. Proxy ARP is easy to implement at the AP, and
offers the following advantages:
o Reduced broadcast traffic (transmitted at low MCS) on the wireless
medium
o STA benefits from extended power save in sleep mode, as ARP
requests for STA's IP address are handled instead by the AP.
o ARP frames are kept off the wireless medium.
o No changes are needed to STA implementation.
Here is the specification language as described in clause 10.23.13 of
[dot11-proxyarp]:
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 non-AP STA"
4.2. IPv6 Address Registration and Proxy Neighbor Discovery
As used in this section, a Low-Power Wireless Personal Area Network
(6LoWPAN) denotes a low power lossy network (LLN) that supports
6LoWPAN Header Compression (HC) [RFC6282]. A 6TiSCH network
[I-D.ietf-6tisch-architecture] is an example of a 6LowPAN. In order
to control the use of IPv6 multicast over 6LoWPANs, the 6LoWPAN
Neighbor Discovery (6LoWPAN ND) [RFC6775] standard defines an address
registration mechanism that relies on a central registry to assess
address uniqueness, as a substitute to the inefficient Duplicate
Address Detection (DAD) mechanism found in the mainstream IPv6
Neighbor Discovery Protocol (NDP) [RFC4861][RFC4862].
The 6lo Working Group has specified an update
[I-D.ietf-6lo-rfc6775-update] to RFC6775. Wireless devices can
register their address to a Backbone Router
[I-D.ietf-6lo-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.
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The general idea behind the backbone router concept is that broadcast
and multicast messaging should be tightly controlled in a variety of
Wireless Local Area Networks (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 Figure 1.
|
+-----+
| | 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
Figure 1: Backbone Link and Backbone Routers
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.
RFC6775 and follow-on work (e.g., [I-D.ietf-6lo-ap-nd], do 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.
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4.3. 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 DTIM (Delivery
Traffic Indication Message) of 3 then the AP will send a multicast
packet every 3 packets. In fact, when any single wireless client
associated with an access point has 802.11 power-save mode enabled,
the access point buffers all multicast frames and sends them only
after the next DTIM beacon.
But in practice, most AP's will send a multicast every 30 packets.
For unicast there's a TIM (Traffic Indication Message); but since
multicast is going to everyone, the AP sends a broadcast to everyone.
DTIM does power management but clients can choose whether or not to
wake up or not and whether or not to drop the packet. Unfortunately,
without proper administrative control, such clients may no longer be
able to determine why their multicast operations do not work.
4.4. IPv6 support in 802.11-2012
IPv6 uses Neighbor Discovery Protocol (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
[dot11-proxyarp]:
"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
o Maximum Transmission Unit
o Router Solicitation
o Router Advertisement, etc.
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.
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4.5. Conversion of multicast to unicast
It is often possible to transmit multicast control and data messages
by using unicast transmissions to each station individually.
4.6. Directed Multicast Service (DMS)
There are situations where more is needed than simply converting
multicast to unicast. For these purposes, DMS enables a client to
request that the AP transmit multicast group addressed frames
destined to the requesting clients as individually addressed frames
[i.e., convert multicast to unicast]. Here are some characteristics
of DMS:
o Requires 802.11n A-MSDUs
o Individually addressed frames are acknowledged and are buffered
for power save clients
o The requesting STA may specify traffic characteristics for DMS
traffic
o DMS was defined in IEEE Std 802.11v-2011
o DMS requires changes to both AP and STA implementation.
DMS is not currently implemented in products. See [Tramarin2017] and
[Oliva2013] for more information.
4.7. GroupCast with Retries (GCR)
GCR (defined in [dot11aa]) provides greater reliability by using
either unsolicited retries or a block acknowledgement mechanism. GCR
increases 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 conventional group addressed transmission.
Retransmissions are group addressed, but hidden from non-11aa
clients. 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 implementation.
GCR may introduce unacceptable latency. After sending a group of
data frames to the group, the AP has do the following:
o unicast a Block Ack Request (BAR) to a subset of members.
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o wait for the corresponding Block Ack (BA).
o retransmit any missed frames.
o resume other operations which may have been delayed.
This latency may not be acceptable for some traffic.
There are ongoing extensions in 802.11 to improve GCR performance.
o BAR is sent using downlink MU-MIMO (note that downlink MU-MIMO is
already specified in 802.11-REVmc 4.3).
o BA is sent using uplink MU-MIMO (which is a .11ax feature).
o Additional 802.11ax extensions are under consideration; see
[mc-ack-mux]
o Latency may also be reduced by simultaneously receiving BA
information from multiple clients.
5. Operational optimizations
This section lists some operational optimizations that can be
implemented when deploying wireless IEEE 802 networks to mitigate the
issues discussed in Section 3.
5.1. Mitigating Problems from Spurious Neighbor Discovery
ARP Sponges
An ARP Sponge sits on a network and learn which IPs addresses
are actually in use. It also listen for ARP requests, and, if
it sees an ARP for an IP address which 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 an 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; but, unfortunately, the ARP
sponge daemons were not really designed for this use (the
standard one [arpsponge], was designed to deal with the
disappearance of participants from an IXP) and so are not
optimized for this purpose. We have to run one daemon per
subnet, the tuning is tricky (the scanning rate versus the
population rate versus retires, etc.) and sometimes the daemons
just seem to stop, requiring a restart of the daemon and
causing disruption.
Router mitigations
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Some routers (often those based on Linux) implement a "negative
ARP cache" daemon. Simply put, 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 which we
are using do not support this. When a host connects to 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 learnt from the request (passive ARP
learning).
Firewall unused space
The distribution of users on wireless networks / subnets
changes from meeting to meeting (e.g the "IETF-secure" SSID was
renamed to "IETF", fewer users use "IETF-legacy", etc). This
utilization is difficult to predict ahead of time, but we can
monitor the usage as attendees use the different networks. By
configuring multiple DHCP pools per subnet, and enabling them
sequentially, we can have a large subnet, but only assign
addresses from the lower portions of it. This means that we
can apply input IP access lists, which deny traffic to the
upper, unused portions. This means that 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. This means that we should
be able 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 simple "just disable ARP or filter it outbound" seems
like a really simple (and obvious) solution, but
implementations / architectural issues make this difficult or
awkward in practice.
NAT
The broadcasts are overwhelmingly being caused by outside
scanning / backscatter traffic. This means that, if we were to
NAT the entire (or a large portion) of the attendee networks,
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there would be no NAT translation entries for unused addresses,
and so the router would never ARP for them. The IETF NOC has
discussed NATing the entire (or large portions) attendee
address space, but a: elegance and b: flaming torches and
pitchfork concerns means we have not attempted this yet.
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. The IETF philosophy has been to have the network as
open as possible / honor the end-to-end principle. An attendee
on the 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.
6. 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 [uli] for some
additional discussion, and a proposal for a task group to resolve
similar issues, in which the multicast problems might be considered
for mitigation.
7. Recommendations
This section will provide some recommendations about the usage and
combinations of the multicast enhancements described in Section 4 and
Section 5.
Future protocol documents utilizing multicast signaling should be
carefully scrutinized if the protocol is likely to be used over
wireless media.
Proxy methods should be encouraged to conserve network bandwidth and
power utilization by low-power devices. The device can use a unicast
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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
compatible with low-duty cycle operation.
(FFS)
8. Discussion Items
This section suggests two discussion items for further resolution.
The IETF should determine guidelines by which it may be decided that
multicast packets are to be sent wired. For example, 802.1ak works
on ethernet and wifi. 802.1ak has been pulled into 802.1Q as of
802.1Q-2011. 802.1Q-2014 can be found here:
http://www.ieee802.org/1/pages/802.1Q-2014.html. If a generic
solution is not found, guidelines for multicast over wifi should be
established.
Perhaps a reliable registration to Layer-2 multicast groups and a
reliable multicast operation at Layer-2 could provide a generic
solution. There is no need to support 2^24 groups to get solicited
node multicast working: it is possible to simply select a number of
trailing bits that make sense for a given network size to limit the
amount of unwanted deliveries to reasonable levels. IEEE 802.1,
802.11, and 802.15 should be encouraged to revisit L2 multicast
issues. In reality, Wi-Fi provides a broadcast service, not a
multicast service. On the physical medium, all frames are broadcast
except in very unusual cases in which special beamforming
transmitters are used. Unicast offers the advantage of being much
faster (2 orders of magnitude) and much more reliable (L2 ARQ).
9. Security Considerations
This document does not introduce any security mechanisms, and does
not have affect existing security mechanisms.
10. IANA Considerations
This document does not request any IANA actions.
11. Acknowledgements
This document has benefitted from discussions with the following
people, in alphabetical order: Pascal Thubert
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12. Informative References
[arpsponge]
Arien Vijn, Steven Bakker, "Arp Sponge", March 2015.
[Deri-2010]
Deri, L. and J. Gasparakis, "10 Gbit Hardware Packet
Filtering Using Commodity Network Adapters", RIPE 61,
2010, <http://ripe61.ripe.net/
presentations/138-Deri_RIPE_61.pdf>.
[dot11] P802.11, "Part 11: Wireless LAN Medium Access Control
(MAC) and Physical Layer (PHY) Specifications", March
2012.
[dot11-proxyarp]
P802.11, "Proxy ARP in 802.11ax", September 2015.
[dot11aa] P802.11, "Part 11: Wireless LAN Medium Access Control
(MAC) and Physical Layer (PHY) Specifications Amendment 2:
MAC Enhancements for Robust Audio Video Streaming", March
2012.
[I-D.ietf-6lo-ap-nd]
Thubert, P., Sarikaya, B., Sethi, M., and R. Struik,
"Address Protected Neighbor Discovery for Low-power and
Lossy Networks", draft-ietf-6lo-ap-nd-08 (work in
progress), October 2018.
[I-D.ietf-6lo-backbone-router]
Thubert, P. and C. Perkins, "IPv6 Backbone Router", draft-
ietf-6lo-backbone-router-08 (work in progress), October
2018.
[I-D.ietf-6lo-rfc6775-update]
Thubert, P., Nordmark, E., Chakrabarti, S., and C.
Perkins, "Registration Extensions for 6LoWPAN Neighbor
Discovery", draft-ietf-6lo-rfc6775-update-21 (work in
progress), June 2018.
[I-D.ietf-6tisch-architecture]
Thubert, P., "An Architecture for IPv6 over the TSCH mode
of IEEE 802.15.4", draft-ietf-6tisch-architecture-15 (work
in progress), October 2018.
[ietf_802-11]
Dorothy Stanley, "IEEE 802.11 multicast capabilities", Nov
2015.
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[mc-ack-mux]
Yusuke Tanaka et al., "Multiplexing of Acknowledgements
for Multicast Transmission", July 2015.
[mc-prob-stmt]
Mikael Abrahamsson and Adrian Stephens, "Multicast on
802.11", March 2015.
[mc-props]
Adrian Stephens, "IEEE 802.11 multicast properties", March
2015.
[Oliva2013]
de la Oliva, A., Serrano, P., Salvador, P., and A. Banchs,
"Performance evaluation of the IEEE 802.11aa multicast
mechanisms for video streaming", 2013 IEEE 14th
International Symposium on "A World of Wireless, Mobile
and Multimedia Networks" (WoWMoM) pp. 1-9, June 2013.
[RFC4541] Christensen, M., Kimball, K., and F. Solensky,
"Considerations for Internet Group Management Protocol
(IGMP) and Multicast Listener Discovery (MLD) Snooping
Switches", RFC 4541, DOI 10.17487/RFC4541, May 2006,
<https://www.rfc-editor.org/info/rfc4541>.
[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>.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862,
DOI 10.17487/RFC4862, September 2007,
<https://www.rfc-editor.org/info/rfc4862>.
[RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
DOI 10.17487/RFC6282, September 2011,
<https://www.rfc-editor.org/info/rfc6282>.
[RFC6775] Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
Bormann, "Neighbor Discovery Optimization for IPv6 over
Low-Power Wireless Personal Area Networks (6LoWPANs)",
RFC 6775, DOI 10.17487/RFC6775, November 2012,
<https://www.rfc-editor.org/info/rfc6775>.
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[Tramarin2017]
Tramarin, F., Vitturi, S., and M. Luvisotto, "IEEE 802.11n
for Distributed Measurement Systems", 2017 IEEE
International Instrumentation and Measurement Technology
Conference (I2MTC) pp. 1-6, May 2017.
[uli] Pat Kinney, "LLC Proposal for 802.15.4", Nov 2015.
Authors' Addresses
Charles E. Perkins
Futurewei Inc.
2330 Central Expressway
Santa Clara, CA 95050
USA
Phone: +1-408-330-4586
Email: charliep@computer.org
Mike McBride
Futurewei Inc.
2330 Central Expressway
Santa Clara, CA 95055
USA
Email: michael.mcbride@huawei.com
Dorothy Stanley
Hewlett Packard Enterprise
2000 North Naperville Rd.
Naperville, IL 60566
USA
Phone: +1 630 979 1572
Email: dstanley@arubanetworks.com
Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
USA
Email: warren@kumari.net
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Juan Carlos Zuniga
SIGFOX
425 rue Jean Rostand
Labege 31670
France
Email: j.c.zuniga@ieee.org
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