| Internet Area | C. Perkins |
| Internet-Draft | |
| Intended status: Informational | M. McBride |
| Expires: February 14, 2020 | Futurewei |
| D. Stanley | |
| HPE | |
| W. Kumari | |
| JC. Zuniga | |
| SIGFOX | |
| August 13, 2019 |
Multicast Considerations over IEEE 802 Wireless Media
draft-ietf-mboned-ieee802-mcast-problems-08
Well-known issues with multicast have prevented the deployment of multicast in 802.11 and other local-area wireless environments. This document offers guidance on known limitations and problems with wireless multicast. Also described are certain multicast enhancement features that have been specified by the IETF and by IEEE 802 for wireless media, as well as some operational choices 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.
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This Internet-Draft will expire on February 14, 2020.
Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.
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Well-known issues with multicast have prevented the deployment of multicast in 802.11 [dot11] and other local-area wireless environments, as described in [mc-props], [mc-prob-stmt]. Performance issues have been observed when multicast packet transmissions of IETF protocols are used over IEEE 802 wireless media. Even though enhancements for multicast transmissions have been designed at both IETF and IEEE 802, incompatibilities still exist between specifications, implementations and configuration choices.
Many IETF protocols depend on multicast/broadcast for delivery of control messages to multiple receivers. Multicast is used for various purposes such as neighbor 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, or video in enterprises, universities, and homes, are sending multicast IP to end user devices, which are increasingly using Wi-Fi for their connectivity.
IETF protocols typically rely on network protocol layering in order to reduce or eliminate any dependence of higher level protocols on the specific nature of the MAC layer protocols or the physical media. In the case of multicast transmissions, higher level protocols have traditionally been designed as if transmitting a packet to an IP address had the same cost in interference and network media access, regardless of whether the destination IP address is a unicast address or a multicast or broadcast address. This model was reasonable for networks where the physical medium was wired, like Ethernet. Unfortunately, for many wireless media, the costs to access the medium can be quite different. Multicast over Wi-Fi has often been plagued by such poor performance that it is disallowed. Some enhancements have been designed in IETF protocols that are assumed to work primarily over wireless media. However, these enhancements are usually implemented in limited deployments and not widespread on most wireless networks.
IEEE 802 wireless protocols have been designed with certain features to support multicast traffic. For instance, lower modulations are used to transmit multicast frames, so that these can be received by all stations in the cell, regardless of the distance or path attenuation from the base station or access point. However, these lower modulation transmissions occupy the medium longer; they hamper efficient transmission of traffic using higher order modulations to nearby stations. For these and other reasons, IEEE 802 working 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.
There seems to be general agreement that these problems will not be fixed anytime soon, primarily because it's expensive to do so, and multicast is unreliable. Compared to unicast over Wi-Fi, multicast is often treated as somewhat a second class citizen, even though there are many protocols using multicast. Something needs to be provided in order to make them more reliable. IPv6 neighbor discovery saturating the Wi-Fi link is only part of the problem. Wi-Fi traffic classes may help. This document is intended to help make the determination about what problems should be solved by the IETF and what problems should be solved by the IEEE (see Section 8).
This document details various problems caused by multicast transmission over wireless networks, including high packet error rates, no acknowledgements, and low data rate. It also explains some enhancements that have been designed at IETF and IEEE 802 to ameliorate the effects of multicast traffic. Recommendations are also provided to implementors about how to use and combine these enhancements. Some advice about the operational choices that can be taken is also included. It is likely that this document will also be considered relevant to designers of future IEEE wireless specifications.
This document uses the following definitions:
In this section some of the issues related to the use of multicast transmissions over IEEE 802 wireless technologies are described.
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.
Multicast over wired differs from multicast over wireless because transmission over wired links often occurs at a fixed rate. Wi-Fi, on the other hand, has a transmission rate that varies depending upon the STA's proximity to the AP. The throughput of video flows, and the capacity of the broader Wi-Fi 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 Point, the power necessary for good reception can vary from station to station. For unicast, the goal is to minimize power requirements while maximizing the data rate to the destination. For multicast, the goal is simply to maximize the number of receivers that will correctly receive the multicast packet; generally the Access Point has to use a much lower data rate at a power level high enough for even the farthest station to receive the packet, for example as briefly mentioned in [RFC5757]. Consequently, the data rate of a video stream, for instance, would be constrained by the environmental considerations of the least reliable receiver associated with the Access Point.
Because more robust modulation and coding schemes (MCSs) have longer range but also lower data rate, multicast / broadcast traffic is generally transmitted at the slowest rate of all the connected devices, also known as the basic rate. The amount of additional interference depends on the specific wireless technology. In fact backward compatibility and multi-stream implementations mean that the maximum unicast rates are currently up to a few Gb/s, so there can be a more than 3 orders of magnitude difference in the transmission rate between multicast / broadcast versus optimal unicast forwarding. Some techinues employed to increase spectral efficiency, such as spatial multiplexing in mimo systems, are not available with more than one intended reciever; it is not the case that backwards compatibility is the only factor responsible for lower multicast transmission rates.
Wired multicast also affects wireless LANs when the AP extends the wired segment; in that case, multicast / broadcast frames on the wired LAN side are copied to WLAN. Since broadcast messages are transmitted at the most robust MCS, many large frames are sent at a slow rate over the air.
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 STAs associated to the AP, proportionately increases the area of interference.
One of the characteristics of multicast transmission is that every station has to be configured to wake up to receive the multicast, even though the received packet may ultimately be discarded. This process can have a large effect on the power consumption by the multicast receiver station.
Multicast can work poorly with the power-save mechanisms defined in IEEE 802.11e, for the following reasons.
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:
User Datagram Protocol (UDP) is the most common transport layer protocol for multicast applications. By itself, UDP is not reliable -- messages may be lost or delivered out of order.
The following list contains some representative multicast protocols that are used with IPv4.
After initial configuration, ARP and DHCP occur much less commonly, but service discovery can occur at any time. Some widely-deployed service discovery protocols (e.g., for finding a printer) utilize mDNS (i.e., multicast). It's often the first service that operators drop. Even if multicast snooping is utilized, many devices can register at once and cause serious network degradation.
IPv6 makes extensive use of multicast, including the following:
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 intensifies the impact of multicast messages that are associated to the mobility of a node. Router advertisement (RA) messages are also periodically multicasted over the Link.
Neighbors may be considered lost if several consecutive Neighbor Discovery packets fail.
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 Wi-Fi-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.
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 IP addresses regardless of whether those IP addresses are 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, for instance at the NOCs during IETF face-to-face meetings.
On a wired network, there is not a huge difference between unicast, multicast and broadcast traffic. Due to hardware filtering (see, e.g., [Deri-2010]), inadvertently flooded traffic (or excessive ethernet multicast) on wired networks can be quite a bit less costly, compared to wireless cases where sleeping devices have to wake up to process packets. Wired Ethernets tend to be switched networks, further reducing interference from multicast. There is effectively no collision / scheduling problem except at extremely high port utilizations.
This is not true in the wireless realm; wireless equipment is often unable to send high volumes of broadcast and multicast traffic, causing numerous broadcast and multicast packets to be dropped. Consequently, 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.
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.
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]:
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 has specified an update to RFC6775. Wireless devices can register their address to a Backbone Router, which proxies for the registered addresses with the IPv6 NDP running on a high speed aggregating backbone. The update also enables a proxy registration mechanism on behalf of the registered node, e.g. by a 6LoWPAN router to which the mobile node is attached.
The general idea behind the backbone router concept is that broadcast and multicast messaging should be tightly controlled in a variety of Wireless Local Area Networks (WLANs) and Wireless Personal Area Networks (WPANs). Connectivity to a particular link that provides the subnet should be left to Layer-3. The model for the Backbone Router operation is represented in Figure 1.
|
+-----+
| | Gateway (default) router
| |
+-----+
|
| Backbone Link
+--------------------+------------------+
| | |
+-----+ +-----+ +-----+
| | Backbone | | Backbone | | Backbone
| | router 1 | | router 2 | | router 3
+-----+ +-----+ +-----+
o o o o o o
o o o o o o o o o o o o o o
o o o o o o o o o o o o o o o
o o o o o o o o o o
o o o o o o o
LLN 1 LLN 2 LLN 3
Figure 1: Backbone Link and Backbone Routers
LLN nodes can move freely from an LLN anchored at one IPv6 Backbone Router to an LLN anchored at another Backbone Router on the same backbone, keeping any of the IPv6 addresses they have configured. The Backbone Routers maintain a Binding Table of their Registered Nodes, which serves as a distributed database of all the LLN Nodes. An extension to the Neighbor Discovery Protocol is introduced to exchange Binding Table information across the Backbone Link as needed for the operation of IPv6 Neighbor Discovery.
RFC6775 and follow-on work [RFC8505] address the needs of LLNs, and similar techniques are likely to be valuable on any type of link where sleeping devices are attached, or where the use of broadcast and multicast operations should be limited.
Methods have been developed to help save battery life; for example, a device might not wake up when the AP receives a multicast packet. The AP acts on behalf of STAs in various ways. To enable use of the power-saving feature for STAs in its BSS, the AP buffers frames for delivery to the STA at the time when the STA is scheduled for reception. If an AP, for instance, expresses a DTIM (Delivery Traffic Indication Message) of 3 then the AP will send a multicast packet every 3 packets. In fact, when any single wireless STA associated with an access point has 802.11 power-save mode enabled, the access point buffers all multicast frames and sends them only after the next DTIM beacon.
In practice, most AP's will send a multicast every 30 packets. For unicast the AP could send a TIM (Traffic Indication Message), but for multicast the AP sends a broadcast to everyone. DTIM does power management but STAs can choose whether or not to wake up or not and whether or not to drop the packet. Unfortunately, without proper administrative control, such STAs may be unable to determine why their multicast operations do not work.
The CAB (Content after Beacon) queue is used for beacon-triggered transmission of buffered multicast frames. If lots of multicast frames were buffered, and this queue fills up, it drowns out all regular traffic. To limit the damage that buffered traffic can do, some drivers limit the amount of queued multicast data to a fraction of the beacon_interval. An example of this is [CAB].
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.
It is often possible to transmit multicast control and data messages by using unicast transmissions to each station individually.
In many situations, it's a good choice to use unicast instead of multicast over the Wi-Fi link. This avoids most of the problems specific to multicast over Wi-Fi, since the individual frames are then acknowledged and buffered for power save clients, in the way that unicast traffic normally operates.
This approach comes with the tradeoff of sometimes sending the same packet multiple times over the Wi-Fi link. However, in many cases, such as video into a residential home network, this can be a good tradeoff, since the Wi-Fi link may have enough capacity for the unicast traffic to be transmitted to each subscribed STA, even though multicast addressing may have been necessary for the upstream access network.
Several technologies exist that can be used to arrange unicast transport over the Wi-Fi link, outlined in the subsections below.
It is often possible to transmit multicast control and data messages by using unicast transmissions to each station individually.
Although there is not yet a standardized method of conversion, at least one widely available implementation exists in the Linux bridging code [bridge-mc-2-uc]. Other proprietary implementations are available from various vendors. In general, these implementations perform a straightforward mapping for groups or channels, discovered by IGMP or MLD snooping, to the corresponding unicast MAC addresses.
There are situations where more is needed than simply converting multicast to unicast. For these purposes, DMS enables an STA to request that the AP transmit multicast group addressed frames destined to the requesting STAs as individually addressed frames [i.e., convert multicast to unicast]. Here are some characteristics of DMS: [Tramarin2017] and [Oliva2013] for more information.
DMS is not currently implemented in products. See
AMT[RFC7450] provides a method to tunnel multicast IP packets inside unicast IP packets over network links that only support unicast. When an operating system or application running on an STA has an AMT gateway capability integrated, it's possible to use unicast to traverse the Wi-Fi link by deploying an AMT relay in the non-Wi-Fi portion of the network connected to the AP.
It is RECOMMENDED that multicast-enabled networks deploying AMT relays for this purpose make the relays locally discoverable with the following methods, as described in [I-D.ietf-mboned-driad-amt-discovery]:
An AMT gateway that implements multiple standard discovery methods is more likely to discover the local multicast-capable network, instead of forming a connection to an AMT relay further upstream.
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 STAs. 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.
This section lists some operational optimizations that can be implemented when deploying wireless IEEE 802 networks to mitigate the issues discussed in Section 3.
Many of the causes of performance degradation described in earlier sections are also observable for wireless media other than 802.11.
For instance, problems with power save, excess media occupancy, and poor reliability will also affect 802.15.3 and 802.15.4. Unfortunately, 802.15 media specifications do not yet include mechanisms similar to those developed for 802.11. In fact, the design philosophy for 802.15 is oriented towards minimality, with the result that many such functions are relegated to operation within higher layer protocols. This leads to a patchwork of non-interoperable and vendor-specific solutions. See [uli] for some additional discussion, and a proposal for a task group to resolve similar issues, in which the multicast problems might be considered for mitigation.
Similar considerations hold for most other wireless media. A brief introduction is provided in [RFC5757] for the following:
This section will provide some recommendations about the usage and combinations of the multicast enhancements described in Section 4 and Section 5.
Future protocol documents utilizing multicast signaling should be carefully scrutinized if the protocol is likely to be used over wireless media.
Proxy methods should be encouraged to conserve network bandwidth and power utilization by low-power devices. The device can use a unicast message to its proxy, and then the proxy can take care of any needed multicast operations.
Multicast signaling for wireless devices should be done in a way compatible with low duty-cycle operation.
This section suggests two discussion items for further resolution.
The IETF should determine guidelines by which it may be decided that multicast packets are to be sent wired. For example, 802.1ak works on ethernet and Wi-Fi. 802.1ak has been pulled into 802.1Q as of 802.1Q-2011. 802.1Q-2014 can be found here: http://www.ieee802.org/1/pages/802.1Q-2014.html. If a generic solution is not found, guidelines for multicast over Wi-Fi should be established.
Reliable registration to Layer-2 multicast groups and a reliable multicast operation at Layer-2 might 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 number of unwanted deliveries to reasonable levels. IEEE 802.1, 802.11, and 802.15 should be encouraged to revisit L2 multicast issues. In reality, Wi-Fi provides a broadcast service, not a multicast service. On the physical medium, all frames are broadcast except in very unusual cases in which special beamforming transmitters are used. Unicast offers the advantage of being much faster (2 orders of magnitude) and much more reliable (L2 ARQ).
This document does not introduce or modify any security mechanisms.
As noted in [group_key], the unreliable nature of multicast transmission over wireless media can cause subtle problems with multicast group key management and updates. Quoting from that website, "... most clients are able to get connected and surf the web, check email, etc. even when FromDS multicasts are broken. So a lot of people don't realize they have multicast problems on their network..."
This document does not request any IANA actions.
This document has benefitted from discussions with the following people, in alphabetical order: Mikael Abrahamsson, Bill Atwood, Stuart Cheshire, Donald Eastlake, Toerless Eckert, Jake Holland, Joel Jaeggli, Jan Komissar, David Lamparter, Morten Pedersen, Pascal Thubert, Jeffrey (Zhaohui) Zhang
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