Internet Area C. Perkins
Internet-Draft Futurewei
Intended status: Informational D. Stanley
Expires: January 20, 2018 HPE
W. Kumari
JC. Zuniga
July 19, 2017

Multicast Considerations over IEEE 802 Wireless Media


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

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This Internet-Draft will expire on January 20, 2018.

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Table of Contents

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 minimizing media occupancy 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 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 wireless 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 features to support multicast traffic. For instance, lower modulations are used to transmit multicast frames, so that these can be received by all stations in the cell, regardless of the distance or path attenuation from the base station or access point. However, these lower modulation transmissions occupy the medium longer; they hamper efficient transmission of traffic using higher order modulations to nearby stations. For these and other reasons, IEEE 802 working groups such as 802.11 have designed features to improve the performance of multicast transmissions at Layer 2 [REF 11-15-1261-03]. In addition to protocol design features, certain operational and configuration enhancements can ameliorate the network performance issues created by multicast traffic.

This Internet Draft details various problems caused by multicast transmission over wireless networks. 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.

2. Terminology

This document uses the following definitions:


IEEE 802.11 Access Point.


802.11 station (e.g. handheld device).

basic rate

The "lowest common denominator" data rate at which multicast and broadcast traffic is generally transmitted.


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 recipients.

3.1.2. Lower Data Rate

Because more robust 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. On IEEE 802.11 networks (aka WiFi), 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 longer occupancy of the wireless medium and thus take away from the airtime of other communications and degrade the overall capacity.

Wired multicast also affects wireless LANs when the AP extends the wired segment; in that case, multicast / broadcast frames on the wired LAN side are copied to WLAN. Since broadcast messages are transmitted at the most robust MCS, many large frames are sent at a slow rate over the air.

3.1.3. Power-save Effects on Multicast

Multicast can work poorly with the power-save mechanisms defined in IEEE 802.11.

3.2. Issues at Layer 3 and Above

This section identifies some representative IETF protocols, and describes possible negative effects due to performance degradation when using multicast transmissions for control messages. Common uses of multicast include:

3.2.1. IPv4 issues

The following list contains a few representative IPv4 protocols using multicast.

After initial configuration, ARP and DHCP occur much less commonly.

3.2.2. IPv6 issues

IPv6 makes much more extensive use of multicast, including the following:

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:

Here is the specification language as described in clause 10.23.13 of [dot11-proxyarp]:

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). A 6TiSCH network is an example of a 6LowPAN. In order to control the use of IPv6 multicast over 6LoWPANs, the 6LoWPAN Neighbor Discovery (6LoWPAN ND) standard defines an address registration mechanism that relies on a central registry to assess address uniqueness, as a substitute to the inefficient 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 to RFC6775. The update enables the registration to a 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 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

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.

4.4. IPv6 support in 802.11-2012

IPv6 uses Neighbor Discovery Protocol (NDP) instead of ARP. Every IPv6 node subscribes to a special multicast address for this purpose.

Here is the specification language from clause 10.23.13 of [dot11-proxyarp]:

NDP may be used to request additional information

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.

4.5. Conversion of multicast to unicast

It is often possible to transmit multicast control and data messages by using unicast transmissions to each station individually.

4.6. Directed Multicast Service (DMS)

There are situations where more is needed than simply converting multicast to unicast. For these purposes, DMS enables a client to request that the AP transmit multicast group addressed frames destined to the requesting clients as individually addressed frames [i.e., convert multicast to unicast]. Here are some characteristics of DMS:

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:

This latency may not be acceptable for some traffic.

There are ongoing extensions in 802.11 to improve GCR performance.

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.


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 provides some recommendations about the usage and combinations of the multicast enhancements described in Section 4 and Section 5.


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. Acknowledgements

This document has benefitted from discussions with the following people, in alphabetical order: Pascal Thubert

11. 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] Sarikaya, B., Thubert, P. and M. Sethi, "Address Protected Neighbor Discovery for Low-power and Lossy Networks", Internet-Draft draft-ietf-6lo-ap-nd-02, May 2017.
[I-D.ietf-6lo-backbone-router] Thubert, P., "IPv6 Backbone Router", Internet-Draft draft-ietf-6lo-backbone-router-04, July 2017.
[I-D.ietf-6lo-rfc6775-update] Thubert, P., Nordmark, E. and S. Chakrabarti, "An Update to 6LoWPAN ND", Internet-Draft draft-ietf-6lo-rfc6775-update-06, June 2017.
[I-D.ietf-6tisch-architecture] Thubert, P., "An Architecture for IPv6 over the TSCH mode of IEEE 802.15.4", Internet-Draft draft-ietf-6tisch-architecture-11, January 2017.
[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.
[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.
[RFC4862] Thomson, S., Narten, T. and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, DOI 10.17487/RFC4862, September 2007.
[RFC6282] Hui, J. and P. Thubert, "Compression Format for IPv6 Datagrams over IEEE 802.15.4-Based Networks", RFC 6282, DOI 10.17487/RFC6282, September 2011.
[RFC6775] Shelby, Z., 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.
[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:
Dorothy Stanley Hewlett Packard Enterprise 2000 North Naperville Rd. Naperville, IL 60566 USA Phone: +1 630 979 1572 EMail:
Warren Kumari Google 1600 Amphitheatre Parkway Mountain View, CA 94043 USA EMail:
Juan Carlos Zuniga SIGFOX 425 rue Jean Rostand Labege, 31670 France EMail: