draft-ietf-mboned-ieee802-mcast-problems-00.txt   draft-ietf-mboned-ieee802-mcast-problems-01.txt 
MBONED WG M. McBride Internet Area C. Perkins
Internet-Draft C. Perkins Internet-Draft M. McBride
Intended status: Standards Track Huawei Intended status: Informational Futurewei
Expires: July 28, 2018 January 24, 2018 Expires: August 7, 2018 D. Stanley
HPE
W. Kumari
Google
JC. Zuniga
SIGFOX
February 3, 2018
Multicast WiFi Problem Statement Multicast Considerations over IEEE 802 Wireless Media
draft-ietf-mboned-ieee802-mcast-problems-00 draft-ietf-mboned-ieee802-mcast-problems-01
Abstract Abstract
There have been known issues with multicast, in an 802.11 Well-known issues with multicast have prevented the deployment of
environment, which have prevented the deployment of multicast in multicast in 802.11 [dot11], [mc-props], [mc-prob-stmt], and other
these wifi environments. IETF multicast experts have been meeting local-area wireless environments. IETF multicast experts have been
together to discuss these issues and provide IEEE updates. The meeting together to discuss these issues and provide IEEE updates.
mboned working group is chartered to receive regular reports on the The mboned working group is chartered to receive regular reports on
current state of the deployment of multicast technology, create the current state of the deployment of multicast technology, create
"practice and experience" documents that capture the experience of "practice and experience" documents that capture the experience of
those who have deployed and are deploying various multicast those who have deployed and are deploying various multicast
technologies, and provide feedback to other relevant working groups. technologies, and provide feedback to other relevant working groups.
As such, this document will gather the problems of wifi multicast This document offers guidance on known limitations and problems with
into one problem statement document so as to offer the community wireless multicast. Also described are various multicast enhancement
guidance on current limitations. 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 Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 7, 2018.
This Internet-Draft will expire on July 28, 2018.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Multicast over WiFi Problems . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Low Reliability . . . . . . . . . . . . . . . . . . . . . 3 3. Identified mulitcast issues . . . . . . . . . . . . . . . . . 5
2.2. Low Data Rate . . . . . . . . . . . . . . . . . . . . . . 4 3.1. Issues at Layer 2 and Below . . . . . . . . . . . . . . . 5
2.3. High Interference . . . . . . . . . . . . . . . . . . . . 4 3.1.1. Multicast reliability . . . . . . . . . . . . . . . . 5
2.4. High Power Consumption . . . . . . . . . . . . . . . . . 4 3.1.2. Lower and Variable Data Rate . . . . . . . . . . . . 5
3. Common remedies to multicast over wifi problems . . . . . . . 4 3.1.3. High Interference . . . . . . . . . . . . . . . . . . 6
4. State of the Union . . . . . . . . . . . . . . . . . . . . . 5 3.1.4. Power-save Effects on Multicast . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 3.2. Issues at Layer 3 and Above . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6 3.2.1. IPv4 issues . . . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6 3.2.2. IPv6 issues . . . . . . . . . . . . . . . . . . . . . 7
8. Normative References . . . . . . . . . . . . . . . . . . . . 6 3.2.3. MLD issues . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6 3.2.4. Spurious Neighbor Discovery . . . . . . . . . . . . . 8
4. Multicast protocol optimizations . . . . . . . . . . . . . . 9
4.1. Proxy ARP in 802.11-2012 . . . . . . . . . . . . . . . . 9
4.2. IPv6 Address Registration and Proxy Neighbor Discovery . 10
4.3. Buffering to improve Power-Save . . . . . . . . . . . . . 11
4.4. IPv6 support in 802.11-2012 . . . . . . . . . . . . . . . 12
4.5. Conversion of multicast to unicast . . . . . . . . . . . 12
4.6. Directed Multicast Service (DMS) . . . . . . . . . . . . 12
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 . . . . . . . . . . . . . . . . . . . . . . 16
9. Security Considerations . . . . . . . . . . . . . . . . . . . 17
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
12. Informative References . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction 1. Introduction
Multicast over wifi has been used to low levels of success, usually Performance issues have been observed when multicast packet
to a point of being so negative that multicast over wifi is not transmissions of IETF protocols are used over IEEE 802 wireless
allowed. In addition to protocol use of broadcast/multicast for media. Even though enhamcements for multicast transmissions have
control messages, more applications, such as push to talk in been designed at both IETF and IEEE 802, incompatibilities still
hospitals, video in enterprises and lectures in Universities, are exist between specifications, implementations and configuration
streaming over wifi. And many end devices are increasingly using choices.
wifi for their connectivity. One of the primary problems multicast
over wifi faces is that link local 802.11 doesn't necessarily support
multicast, it supports broadcast. To make make multicast over wifi
work successfully we often need to modify the multicast to instead be
sent as unicast in order for it to successfully transmit with useable
quality. Multicast over wifi experiences high packet error rates, no
acknowledgements, and low data rate. This draft reviews these
problems found with multicast over wifi. While this is not a
solutions draft, common workarounds to some of the problems will be
listed, along with the impact of the workarounds.
2. Multicast over WiFi Problems 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.
802.11 is a wireless broadcast medium which works well for unicast IETF protocols typically rely on network protocol layering in order
and has become ubiquitous in its use. With multicast, however, to reduce or eliminate any dependence of higher level protocols on
problems arise over wifi. There are no ACKs for multicast packets, the specific nature of the MAC layer protocols or the physical media.
for instance, so there can be a high level of packet error rate (PER) In the case of multicast transmissions, higher level protocols have
due to lack of retransmission and because the sender never backs off. traditionally been designed as if transmitting a packet to an IP
It is not uncommon for there to be a packet loss rate of 5% which is address had the same cost in interference and network media access,
particularly troublesome for video and other environments where high regardless of whether the destination IP address is a unicast address
data rates and high reliability are required. Multicast, over wifi, or a multicast or broadcast address. This model was reasonable for
is typically sent on a low date rate which makes video negatively networks where the physical medium was wired, like Ethernet.
impacted. Wifi loses many more packets then wired due to collisions Unfortunately, for many wireless media, the costs to access the
and signal loss. There are also problems because clients are unable medium can be quite different. Multicast over wifi has often been
to stay in sleep mode due to the multicast control packets continuing plagued by such poor performance that it is disallowed. Some
to unnecessarily wake up those clients which subsequently reduces enhancements have been designed in IETF protocols that are assumed to
energy savings. Video is becoming the dominant content for end work primarily over wireless media. However, these enhancements are
device applications, with multicast being the most natural method for usually implemented in limited deployments and not widespread on most
applications to transmit video. Unfortunately, multicast, even wireless networks.
though it is a very natural choice for video, incurs a large penalty
over wifi.
One big difference between multicast over wired versus multicast over IEEE 802 wireless protocols have been designed with certain features
wired is that wired links are a fixed transmission rate. Wifi, on to support multicast traffic. For instance, lower modulations are
the other hand, has a transmission rate which varies over time used to transmit multicast frames, so that these can be received by
depending upon the clients proximity to the AP. Throughput of video all stations in the cell, regardless of the distance or path
flows, and the capacity of the broader wifi network, will change and attenuation from the base station or access point. However, these
will impact the ability for QoS solutions to effectively reserve lower modulation transmissions occupy the medium longer; they hamper
bandwidth and provide admission control. 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 [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.
The main problems associated with multicast over WiFi are as follows: 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.
o Low Reliability This Internet Draft 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, as well as
the operational choices that can be taken, to ameliorate the effects
of multicast traffic. Recommendations about how to use and combine
these enhancements are also provided.
o Lower Data Rate 2. Terminology
o High interference This document uses the following definitions:
o High Power Consumption AP
IEEE 802.11 Access Point.
These points will be elaborated separately in the following basic rate
subsections. The "lowest common denominator" data rate at which multicast and
broadcast traffic is generally transmitted.
2.1. Low Reliability DTIM
Delivery Traffic Indication Map (DTIM): An information element
that advertises whether or not any associated stations have
buffered multicast or broadcast frames.
Because of the lack of acknowledgement for packets from Access Point MCS
to the receivers, it is not possible for the Access Point to know Modulation and Coding Scheme.
whether or not a retransmission is needed. Even in the wired
Internet, this characteristic commonly causes undesirably high error
rates, contributing to the relatively slow uptake of multicast
applications even though the protocols have been available for
decades. The situation for wireless links is much worse, and is
quite sensitive to the presence of background traffic.
2.2. Low Data Rate STA
802.11 station (e.g. handheld device).
For wireless stations associated with an Access Points, the necessary TIM
power for good reception can vary from station to station. For 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 clients 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.
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 unicast, the goal is to minimize power requirements while maximizing
the data rate to the destination. For multicast, the goal is simply the data rate to the destination. For multicast, the goal is simply
to maximize the number of receivers that will correctly receive the to maximize the number of receivers that will correctly receive the
multicast packet. For this purpose, generally the Access Point has multicast packet; generally the Access Point has to use a much lower
to use a much lower data rate at a power level high enough for even data rate at a power level high enough for even the farthest station
the farthest station to receive the packet. Consequently, the data to receive the packet. Consequently, the data rate of a video
rate of a video stream, for instance, would be constrained by the stream, for instance, would be constrained by the environmental
environmental considerations of the least reliable receiver considerations of the least reliable receiver associated with the
associated with the Access Point. Access Point.
2.3. High Interference 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. Depending on the specific 802.11
technology, and the configured choice for the base data rate for
multicast transmission from the Access Point, the amount of
additional interference can range from a factor of ten, to a factor
thousands for 802.11ac.
As mentioned in the previous subsection, multicast transmission to Wired multicast also affects wireless LANs when the AP extends the
the stations associated to an Access Point typically proceeds at a wired segment; in that case, multicast / broadcast frames on the
much higher power level than is required for unicat to many of the wired LAN side are copied to WLAN. Since broadcast messages are
receivers. High power levels directly contribute to stronger transmitted at the most robust MCS, many large frames are sent at a
interference. The interference due to multicast may extend to slow rate over the air.
effects inhibiting packet reception at more distant stations that
might even be associated with other Access Points. Moreover, the use
of lower data rates implies that the physical medium will be occupied
for a longer time to transmit a packet than would be required at high
data rates. Thus, the level of interference due to multicast will be
not only higher, but longer in duration.
Depending on the choice of 802.11 technology, and the configured 3.1.3. High Interference
choice for the base data rate for multicast transmission from the
Access Point, the amount of additional interference can range from a
factor of ten, to a factor thousands for 802.11ac.
2.4. High Power Consumption 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 One of the characteristics of multicast transmission is that every
station has to be configured to wake up to receive the multicast, station has to be configured to wake up to receive the multicast,
even though the received packet may ultimately be discarded. This even though the received packet may ultimately be discarded. This
process has a relatively large impact on the power consumption by the process can have a large effect on the power consumption by the
multicast receiver station. multicast receiver station.
3. Common remedies to multicast over wifi problems Multicast can work poorly with the power-save mechanisms defined in
IEEE 802.11e, for the following reasons.
One common solution to the multicast over wifi problem is to convert o Clients may be unable to stay in sleep mode due to multicast
the multicast traffic into unicast. This is often referred to as control packets frequently waking them up.
multicast to unicast (MC2UC). Converting the packets to unicast is o Both unicast and multicast traffic can be delayed by power-saving
beneficial because unicast packets are acknowledged and retransmitted mechanisms.
as needed to prevent as much loss. The Access Points (AP) is also
able to provide rate limiting as needed. The drawback with this
approach is that the benefit of using multicast is defeated.
Using 802.11n helps provide a more reliable and higher level of o A unicast packet is delayed until a STA wakes up and requests it.
signal-to-noise ratio in a wifi environment over which multicast Unicast traffic may also be delayed to improve power save,
(broadcast) packets can be sent. This can provide higher throughput efficiency and increase probability of aggregation.
and reliability but the broadcast limitations remain. 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.
4. State of the Union 3.2. Issues at Layer 3 and Above
In discussing these issues over email and, most recently, in a side This section identifies some representative IETF protocols, and
meeting at IETF 99, it is generally agreed that these problems will describes possible negative effects due to performance degradation
not be fixed anytime soon primarily because it's expensive to do so when using multicast transmissions for control messages. Common uses
and multicast is unreliable. The problem of v6 neighbor discovery of multicast include:
saturating the wifi link is only part of the problem. A big problem
is that the 802.11 multicast channel is an afterthought and only
given 100th of the bandwidth. Multicast is basically a second class
citizen, to unicast, over wifi. Unicast may have allocated 10mb
while Multicast will be allocated 1mb. There are many protocols
using multicast and there needs to be something provided in order to
make them more reliable. 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.
Apple's Bonjour protocol, for instance, provides service discovery o Control plane for IPv4 and IPv6
(for printing) that utilizes multicast. It's the first thing o ARP and Neighbor Discovery
operators drop. Even if multicast snooping is utilized, everyone o Service discovery
registers at once using Bonjour and the network has serious o Applications (video delivery, stock data etc)
degradation. There is also a lot of work being developed to help o Other L3 protocols (non-IP)
save battery life such as the devices not waking up when receiving a
multicast packet. If an AP, for instance, expresses a DTIM of 3 then 3.2.1. IPv4 issues
it will send a multicast packet every 3 packets. But the reality is
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)
that utilizes multicast. It's the first thing operators drop. Even
if multicast snooping is utilized, many devices register at once
using Bonjour, causing serious network degradation.
3.2.2. IPv6 issues
IPv6 makes much more extensive use of multicast, including the
following:
o DHCPv6
o IPv6 Neighbor Discovery Protocol (NDP) is not very tolerant of
packet losses. In particular, the Duplicate Address Detection
(DAD) process fails when the owner of an address does not receive
the multicast DAD message from another node that wishes to own
that same address. This can result in an address being duplicated
in the subnet, breaking a basic assumption of IPv6 connectivity.
o 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.
o Neighbors may be considered lost if several consecutive packets
fail.
Address Resolution
Service Discovery
Route Discovery
Decentralized Address Assignment
Geographic routing
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.
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 amongst unicast,
multicast and broadcast traffic; but this is not true in the wireless
realm. Wireless equipment often is unable to send this amount 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.
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 is now completing an update
[I-D.ietf-6lo-rfc6775-update] to RFC6775. The update enables the
registration to a Backbone Router [I-D.ietf-6lo-backbone-router],
which proxies for the registered addresses with the mainstream IPv6
NDP running on a high speed aggragating backbone. The update also
enables a proxy registration 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 in a
variety of Wireless Local Area Networks (WLANs) and Wireless Personal
Area Networks (WPANs), the broadcast/multicast domain should be
controlled, and 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 | | router | | router
+-----+ +-----+ +-----+
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 LLN LLN
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 that information across the Backbone Link in the reactive
fashion of mainstream IPv6 Neighbor Discovery.
RFC6775 and follow-on work (e.g., [I-D.ietf-6lo-ap-nd], are designed
to address the needs of LLNs, but the 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.
4.3. Buffering to improve Power-Save
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. In order to improve
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 of 3 then it
will send a multicast packet every 3 packets. But the reality is
that most AP's will send a multicast every 30 packets. For unicast that most AP's will send a multicast every 30 packets. For unicast
there's a TIM. But because multicast is going to everyone, the AP there's a TIM. But because multicast is going to everyone, the AP
sends a broadcast to everyone. DTIM does power management but sends a broadcast to everyone. DTIM does power management but
clients can choose to wake up or not and whether to drop the packet clients can choose whether or not to wake up or not and whether or
or not. Then they don't know why their bonjour doesn't work. The not to drop the packet. Unfortunately, without proper administrative
IETF may just need to decide that broadcast is more expensive so control, such clients may no longer be able to determine why their
multicast needs to be sent wired. 802.1ak works on ethernet and wifi. multicast operations do not work.
802.1ak has been pulled into 802.1Q as of 802.1Q-2011. 802.1Q-2014
can be looked at here: http://www.ieee802.org/1/pages/802.1Q-
2014.html . If we don't find a generic solution we need to establish
guidelines for multicast over wifi within the mboned wg. A multicast
over wifi IETF mailing list is formed (mcast-wifi@ietf.org) and more
discussion to be had there. This draft will serve as the current
state of affairs.
This is not a solutions draft, but to provide an idea going forward, 4.4. IPv6 support in 802.11-2012
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. We need to
encourage IEEE 802.1 and 802.11 to revisit L2 multicast issues. In
particular, Wi-Fi provides a broadcast service, not a multicast one.
In fact all frames are broadcast at the PHY level unless we beamform.
What comes with unicast is the property of being much faster (2
orders of magnitude) and much more reliable (L2 ARQ).
5. IANA Considerations IPv6 uses Neighbor Discovery Protocol (NDP) instead of ARP. Every
IPv6 node subscribes to a special multicast address for this purpose.
None Here is the specification language from clause 10.23.13 of
[dot11-proxyarp]:
6. Security Considerations "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."
None NDP may be used to request additional information
7. Acknowledgments o Maximum Transmission Unit
o Router Solicitation
o Router Advertisement, etc.
The following people have contributed information and discussion in NDP messages are sent as group addressed (broadcast) frames in
the meetings and on the list which proved helpful for the development 802.11. Using the proxy operation helps to keep NDP messages off the
of the latest version this Internet Draft: wireless medium.
Dave Taht, Donald Eastlake, Pascal Thubert, Juan Carlos Zuniga, 4.5. Conversion of multicast to unicast
Mikael Abrahamsson, Diego Dujovne, David Schinazi, Stig Venaas,
Stuart Cheshire, Lorenzo, Greg Shephard, Mark Hamilton
8. Normative References It is often possible to transmit multicast control and data messages
by using unicast transmissions to each station individually.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 4.6. Directed Multicast Service (DMS)
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, There are situations where more is needed than simply converting
<https://www.rfc-editor.org/info/rfc2119>. 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.
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.
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
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,
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. However,
802.15 media specifications do not 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 would more likely be 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.
(FFS)
8. Discussion Items
This section will suggest some discussion items for further
resolution.
The IETF may need to decide that broadcast is more expensive so
multicast needs 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 looked at 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.
To provide an idea going forward, 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 particular, Wi-Fi
provides a broadcast service, not a multicast one; at the PHY level,
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 specify any IANA actions.
11. Acknowledgements
This document has benefitted from discussions with the following
people, in alphabetical order: Pascal Thubert
12. Informative References
[arpsponge]
Arien Vijn, Steven Bakker, "Arp Sponge", March 2015.
[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., and M. Sethi, "Address
Protected Neighbor Discovery for Low-power and Lossy
Networks", draft-ietf-6lo-ap-nd-05 (work in progress),
January 2018.
[I-D.ietf-6lo-backbone-router]
Thubert, P., "IPv6 Backbone Router", draft-ietf-6lo-
backbone-router-05 (work in progress), January 2018.
[I-D.ietf-6lo-rfc6775-update]
Thubert, P., Nordmark, E., Chakrabarti, S., and C.
Perkins, "An Update to 6LoWPAN ND", draft-ietf-6lo-
rfc6775-update-11 (work in progress), December 2017.
[I-D.ietf-6tisch-architecture]
Thubert, P., "An Architecture for IPv6 over the TSCH mode
of IEEE 802.15.4", draft-ietf-6tisch-architecture-13 (work
in progress), November 2017.
[ietf_802-11]
Dorothy Stanley, "IEEE 802.11 multicast capabilities", Nov
2015.
[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.
[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>.
[uli] Pat Kinney, "LLC Proposal for 802.15.4", Nov 2015.
Authors' Addresses 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 Mike McBride
Huawei Futurewei Inc.
2330 Central Expressway 2330 Central Expressway
Santa Clara CA 95055 Santa Clara, CA 95055
USA USA
Email: michael.mcbride@huawei.com Email: michael.mcbride@huawei.com
Charlie Perkins
Huawei Dorothy Stanley
2330 Central Expressway Hewlett Packard Enterprise
Santa Clara CA 95055 2000 North Naperville Rd.
Naperville, IL 60566
USA USA
Email: charlie.perkins@huawei.com Phone: +1 630 979 1572
Email: dstanley@arubanetworks.com
Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
USA
Email: warren@kumari.net
Juan Carlos Zuniga
SIGFOX
425 rue Jean Rostand
Labege 31670
France
Email: j.c.zuniga@ieee.org
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