Multicast Considerations over IEEE 802 Wireless Media
draft-perkins-intarea-multicast-ieee802-00
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
|
|
|---|---|---|---|
| Authors | Charles E. Perkins , Dorothy Stanley , Warren "Ace" Kumari , Juan-Carlos Zúñiga | ||
| Last updated | 2016-03-21 | ||
| Replaced by | draft-ietf-mboned-ieee802-mcast-problems, RFC 9119 | ||
| RFC stream | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-perkins-intarea-multicast-ieee802-00
Internet Area [intarea] C. Perkins
Internet-Draft Futurewei
Expires: September 22, 2016 D. Stanley
HPE
W. Kumari
Google
JC. Zuniga
InterDigital
March 21, 2016
Multicast Considerations over IEEE 802 Wireless Media
draft-perkins-intarea-multicast-ieee802-00.txt
Abstract
This document describes some performance issues that have been
observed when multicast packet transmission is attempted over IEEE
802 wireless media. Multicast features specified for IEEE 802
wireless media related to multicast are also described, along with
explanations about how these features can help ameliorate the
observed performance issues. IETF protocols that are likely to be
affected by the observed performance issues are identified, and
workarounds are proposed in some cases. The performance of multicast
over wireless media often can be quite different than the performance
of unicast. This draft describes the nature of the differences and
the effects on representative IETF protocols. We also describe some
efforts that have been made by IEEE 802 Wireless groups to ameliorate
the performance differences.
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 22, 2016.
Perkins, et al. Expires September 22, 2016 [Page 1]
Internet-Draft Multicast Over IEEE 802 Wireless March 2016
Copyright Notice
Copyright (c) 2016 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Identified Issues at Layer 2 . . . . . . . . . . . . . . . . 3
4. Some Possible Effects on Representative IETF protocols . . . 4
4.1. IPv4 uses . . . . . . . . . . . . . . . . . . . . . . . . 4
4.2. IPv6 uses . . . . . . . . . . . . . . . . . . . . . . . . 5
4.3. Disabling Multicast on WiFi . . . . . . . . . . . . . . . 5
4.4. Spurious Neighbor Discovery . . . . . . . . . . . . . . . 5
5. Layer 2 optimizations . . . . . . . . . . . . . . . . . . . . 6
5.1. Proxy ARP in 802.11-2012 . . . . . . . . . . . . . . . . 6
5.2. Buffering to improve Power-Save . . . . . . . . . . . . . 7
5.3. IPv6 support in 802.11-2012 . . . . . . . . . . . . . . . 7
5.4. Directed Multicast Service (DMS) . . . . . . . . . . . . 7
5.5. GroupCast with Retries (GCR) . . . . . . . . . . . . . . 8
6. Higher Layer Optimizations and Mitigations . . . . . . . . . 8
6.1. Mitigating Problems from Spurious Neighbor Discovery . . 9
7. Multicast Considerations for Other Wireless Media . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
10. Informative References . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Many IETF protocol designs depend upon multicast or 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 reduction in media access
for data traffic.
Perkins, et al. Expires September 22, 2016 [Page 2]
Internet-Draft Multicast Over IEEE 802 Wireless March 2016
IETF protocols typically expect to 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 transmission, higher level
protocols may be designed as if transmitting a packet to an IP
address has 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 of operation was
reasonable for networks where the physical medium was like an
Ethernet.
Unfortunately, for many wireless media, the costs can be quite
different. It is the purpose of this Internet Draft to identify the
ways in which the costs can be different. Using this information, we
then proceed to identify some possible effects on the actual
operation of IETF protocols over wireless media.
IEEE 802 Wireless working groups, especially 802.11, have made a
number of attempts to improve the performance of multicast
transmissions at layer 2. In this draft we also include a
description of some of these efforts. This information is closely
related to material presented at IETF 94 [cite 11-15-1261-03]
2. Terminology
This document defines the following terminology:
basic rate
a "lowest common denominator" rate at which multicast and
broadcast traffic is generally transmitted.
MCS
Modulation and Coding Scheme.
3. Identified Issues at Layer 2
In this section we list some of the issues arising at layer 2
surrounding the use of multicast in IETF protocols over wireless
media.
o Multicast traffic is typically much less reliable than unicast
traffic.
o Multicast / broadcast traffic is generally sent at a lowest common
denominator rate, known as a basic rate. This might be as low as
6 Mbps, when unicast links are operating at 600 Mbps.
Transmission at a lower rate requires more occupancy of the
wireless medium and thus less airtime for everything else.
Perkins, et al. Expires September 22, 2016 [Page 3]
Internet-Draft Multicast Over IEEE 802 Wireless March 2016
o Wireless multicast affects wired LANs because the AP extends the
wired segment.
* All broadcast frames on LAN side are copied to WLAN.
* In WLAN, broadcast messages transmitted at most robust MCS.
* Most robust MCS implies large frames sent at slow rate.
o Multicast can work poorly with the power-save mechanisms in
802.11.
* Both unicast and multicast traffic can be delayed by power-
saving mechanisms.
* 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.
* 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.
* Packets can also be discarded due to buffer limitations in the
AP and non-AP STA.
4. Some Possible Effects on Representative IETF protocols
In this section we list some of the issues arising at layer 3
surrounding the use of multicast in IETF protocols over wireless
media. We mention a few representative IETF protocols, and describe
some possible effects due to performance degradation when using
multicast transmissions for control messages. Common uses 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)
4.1. IPv4 uses
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.
Perkins, et al. Expires September 22, 2016 [Page 4]
Internet-Draft Multicast Over IEEE 802 Wireless March 2016
4.2. IPv6 uses
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)
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
4.3. Disabling Multicast on WiFi
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.
4.4. 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
Perkins, et al. Expires September 22, 2016 [Page 5]
Internet-Draft Multicast Over IEEE 802 Wireless March 2016
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.
5. Layer 2 optimizations
This section lists some optimizations that have been specified for
use with 802.11 that are aimed at reducing or eliminating the causes
of performance loss discussed in section Section 3.
5.1. 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.
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"
Perkins, et al. Expires September 22, 2016 [Page 6]
Internet-Draft Multicast Over IEEE 802 Wireless March 2016
5.2. 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.
5.3. 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.
5.4. Directed Multicast Service (DMS)
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
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.
Perkins, et al. Expires September 22, 2016 [Page 7]
Internet-Draft Multicast Over IEEE 802 Wireless March 2016
5.5. 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 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.
6. Higher Layer Optimizations and Mitigations
This section lists some optimizations that have been specified for
use with 802.11 that are aimed at reducing or eliminating the causes
of performance loss discussed in section Section 6.
Perkins, et al. Expires September 22, 2016 [Page 8]
Internet-Draft Multicast Over IEEE 802 Wireless March 2016
6.1. Mitigating Problems from Spurious Neighbor Discovery
ARP Sponges
ARP Sponges sit on a network and learn what IPs addresses are
actually in use. They 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
Perkins, et al. Expires September 22, 2016 [Page 9]
Internet-Draft Multicast Over IEEE 802 Wireless March 2016
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.
Perkins, et al. Expires September 22, 2016 [Page 10]
Internet-Draft Multicast Over IEEE 802 Wireless March 2016
7. 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.
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.
[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.
Perkins, et al. Expires September 22, 2016 [Page 11]
Internet-Draft Multicast Over IEEE 802 Wireless March 2016
[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
Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
USA
Email: warren@kumari.net
Perkins, et al. Expires September 22, 2016 [Page 12]
Internet-Draft Multicast Over IEEE 802 Wireless March 2016
Juan Carlos Zuniga
InterDigital
1000 Sherbrooke W, 10th Floor
Montreal, QC H3A 3G4
Canada
Email: j.c.zuniga@ieee.org
Perkins, et al. Expires September 22, 2016 [Page 13]