Internet Area WG C. Perkins
Internet-Draft Futurewei
Intended status: Informational D. Stanley
Expires: September 14, 2017 HPE
W. Kumari
Google
JC. Zuniga
SIGFOX
March 13, 2017
Multicast Considerations over IEEE 802 Wireless Media
draft-perkins-intarea-multicast-ieee802-02
Abstract
Some 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, there seems to exist a
disconnect between specifications, implementations and configuration
choices.
This draft describes the different issues that have been observed,
the multicast enhancement features that have been specified at IETF
and IEEE 802 for wireless media, as well as the operational chioces
that can be taken to improve the performace of the network. Finally,
it provides some recommendations 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 http://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."
This Internet-Draft will expire on September 14, 2017.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Identified mulitcast issues . . . . . . . . . . . . . . . . . 4
3.1. Issues at Layer 2 and below . . . . . . . . . . . . . . . 4
3.1.1. Multicast reliability . . . . . . . . . . . . . . . . 4
3.1.2. Lower data rate . . . . . . . . . . . . . . . . . . . 4
3.1.3. Power-save effects on multicast . . . . . . . . . . . 5
3.2. Issues at Layer 3 and above . . . . . . . . . . . . . . . 5
3.2.1. IPv4 issues . . . . . . . . . . . . . . . . . . . . . 5
3.2.2. IPv6 issues . . . . . . . . . . . . . . . . . . . . . 5
3.2.3. MLD issues . . . . . . . . . . . . . . . . . . . . . 6
3.2.4. Spurious Neighbor Discovery . . . . . . . . . . . . . 6
4. Multicast protocol optimizations . . . . . . . . . . . . . . 7
4.1. Proxy ARP in 802.11-2012 . . . . . . . . . . . . . . . . 7
4.2. Buffering to improve Power-Save . . . . . . . . . . . . . 8
4.3. IPv6 support in 802.11-2012 . . . . . . . . . . . . . . . 8
4.4. Conversion of multicast to unicast . . . . . . . . . . . 8
4.5. Directed Multicast Service (DMS) . . . . . . . . . . . . 8
4.6. GroupCast with Retries (GCR) . . . . . . . . . . . . . . 9
5. Operational optimizations . . . . . . . . . . . . . . . . . . 10
5.1. Mitigating Problems from Spurious Neighbor Discovery . . 10
6. Multicast Considerations for Other Wireless Media . . . . . . 12
7. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 12
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
10. Informative References . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
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 as for reduction in media access for the
transmission of data that is intended for multiple receivers.
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 usually wired, like Ethernet.
Unfortunately, for many wireless media, the costs to access the
medium can be quite different. Some enhancements have been designed
in IETF protocols that are assumed to work primarily over wilress
media. However, these enhancements are usually implemented in
limited deployments and not widely spread on most wireless networks.
IEEE 802 wireless protocols have been designed with certain fetures
to support multicat 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 take longer on the medium and
therefore they reduce the capabilities to transmit more high
efficiency traffic with higher order modulations to stations that may
be in closer vicinity. Due to these and other reasons, some IEEE 802
working groups like 802.11 have designed several features to improve
the performance of multicast transmissions at Layer 2 [REF
11-15-1261-03]. Besides protocol design features, some operational
and configuration enhancements can also be applied to overcome the
network performance issues created by multicast traffic.
This Internet Draft identifies the problems created by the usage of
multicast traffic over wireless networks. It also highlights the
different 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. Some recommendations about the
usage and combinations of these enhancements are also provided.
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2. Terminology
This document uses the following definitions:
AP
IEEE 802.11 Access Point.
STA
IEEE 802.11 station.
basic rate
The "lowest common denominator" data rate at which multicast and
broadcast traffic is generally transmitted.
MCS
Modulation and Coding Scheme.
3. Identified mulitcast issues
3.1. Issues at Layer 2 and below
In this section we list 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 the reception
on all recepients.
3.1.2. Lower data rate
Because lower MCS have longer range but also lower data rate,
multicast / broadcast traffic is generally transmitted at the lowest
common denominator rate, also known as a basic rate. On IEEE 802.11
networks (aka Wi-Fi), this rate might be as low as 6 Mbps, when some
unicast links in the same cell can be operating at rates up to 600
Mbps. Transmissions at a lower rate require more occupancy of the
wireless medium and thus restrict the airtime for all other medium
communications and degrade the overall capacity.
Wired multicast affects wireless LANs because the AP extends the
wired segment and multicast / broadcast frames on the wired LAN side
are copied to WLAN. Since broadcast messages are transmitted at the
most robust MCS, this implies that large frames sent at slow rate
over the air.
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3.1.3. Power-save effects on multicast
Multicast can work poorly with the power-save mechanisms defined in
IEEE 802.11.
o Both unicast and multicast traffic can be delayed by power-saving
mechanisms.
o Unicast is delayed until a STA wakes up and asks for it.
Additionally, unicast traffic may 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 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
In this section we mention a few representative IETF protocols, and
describe some possible negative effects due to performance
degradation when using multicast transmissions for control messages.
Common uses of multicast include:
o Control plane for IPv4 and IPv6
o ARP and Neighbor Discovery
o Service discovery
o Applications (video delivery, stock data etc)
o Other L3 protocols (non-IP)
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.
3.2.2. IPv6 issues
The following list contains a few representative IPv6 protocols using
multicast. IPv6 makes much more extensive use of multicast.
o DHCPv6
o Liveness detection (NUD)
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o Some control plane protocols are not very tolerant of packet loss,
especially neighbor discovery.
o Services 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 the router is constantly
getting packets destined for machines whose IP addresses may or may
not be in use. In the cases where the IP is assigned to a machine,
the router broadcasts an ARP request, gets back an ARP reply, caches
this and then can deliver traffic to the host. In the cases where
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 it will broadcast ARP requests again.
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
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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 all associated STAs MAC address and IP address; in other
words, 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 are replied to by AP.
o Keeps ARP frames off the wireless medium.
o Changes are not needed to STA implementation.
Here is the specification language from clause 10.23.13 in [2] as
described in [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"
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4.2. Buffering to improve Power-Save
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.
4.3. IPv6 support in 802.11-2012
IPv6 uses Neighbor Discovery Protocol (NDP) instead Every IPv6 node
subscribes to special multicast address Neighbor-Solicitation message
replaces ARP
Here is the specification language from-10.23.13 in [2]:
"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.
4.4. Conversion of multicast to unicast
It is often possible to transmit control and data messages by using
unicast transmissions to each station individually.
4.5. Directed Multicast Service (DMS)
There are situations where more is needed than simply converting
multicast to unicast [Editor's note: citation needed]. 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].
o DMS Requires 802.11n A-MSDUs
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o Individually addressed frames are acknowledged and are buffered
for power save clients
o Requesting STA may specify traffic characteristics for DMS traffic
o DMS was defined in IEEE Std 802.11v-2011
DMS is not currently implemented in products. DMS does require
changes to both AP and STA implementation.
4.6. 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.
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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
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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
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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 similar mechanisms of the
type that have been developed for 802.11. In fact, the design
philosophy for 802.15 is more 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 provides some recommendations about the usage and
combinations of the multicast enhancements described in Section 4 and
Section 5.
(FFS)
8. Security Considerations
This document does not introduce any security mechanisms, and does
not have any impact on existing security mechanisms.
9. IANA Considerations
This document does not specify any IANA actions.
10. 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.
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[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.
[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,
<http://www.rfc-editor.org/info/rfc4541>.
[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
Dorothy Stanley
Hewlett Packard Enterprise
2000 North Naperville Rd.
Naperville, IL 60566
USA
Phone: +1 630 979 1572
Email: dstanley@arubanetworks.com
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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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