--- 1/draft-ietf-mboned-ieee802-mcast-problems-00.txt 2018-02-03 10:13:17.312747430 -0800 +++ 2/draft-ietf-mboned-ieee802-mcast-problems-01.txt 2018-02-03 10:13:17.356748479 -0800 @@ -1,293 +1,891 @@ -MBONED WG M. McBride -Internet-Draft C. Perkins -Intended status: Standards Track Huawei -Expires: July 28, 2018 January 24, 2018 +Internet Area C. Perkins +Internet-Draft M. McBride +Intended status: Informational Futurewei +Expires: August 7, 2018 D. Stanley + HPE + W. Kumari + Google + JC. Zuniga + SIGFOX + February 3, 2018 - Multicast WiFi Problem Statement - draft-ietf-mboned-ieee802-mcast-problems-00 + Multicast Considerations over IEEE 802 Wireless Media + draft-ietf-mboned-ieee802-mcast-problems-01 Abstract - There have been known issues with multicast, in an 802.11 - environment, which have prevented the deployment of multicast in - these wifi environments. IETF multicast experts have been meeting - together to discuss these issues and provide IEEE updates. The - mboned working group is chartered to receive regular reports on the - current state of the deployment of multicast technology, create + Well-known issues with multicast have prevented the deployment of + multicast in 802.11 [dot11], [mc-props], [mc-prob-stmt], and other + local-area wireless environments. IETF multicast experts have been + meeting together to discuss these issues and provide IEEE updates. + The mboned working group is chartered to receive regular reports on + the current state of the deployment of multicast technology, create "practice and experience" documents that capture the experience of those who have deployed and are deploying various multicast technologies, and provide feedback to other relevant working groups. - As such, this document will gather the problems of wifi multicast - into one problem statement document so as to offer the community - guidance on current limitations. + This document offers guidance on known limitations and problems with + wireless multicast. Also described are various multicast enhancement + features that have been specified at IETF and IEEE 802 for wireless + media, as well as some operational chioces that can be taken to + improve the performace of the network. Finally, some recommendations + are provided about the usage and combination of these features and + operational choices. Status of This Memo 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 https://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 July 28, 2018. + This Internet-Draft will expire on August 7, 2018. Copyright Notice Copyright (c) 2018 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 (https://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. Multicast over WiFi Problems . . . . . . . . . . . . . . . . 2 - 2.1. Low Reliability . . . . . . . . . . . . . . . . . . . . . 3 - 2.2. Low Data Rate . . . . . . . . . . . . . . . . . . . . . . 4 - 2.3. High Interference . . . . . . . . . . . . . . . . . . . . 4 - 2.4. High Power Consumption . . . . . . . . . . . . . . . . . 4 - 3. Common remedies to multicast over wifi problems . . . . . . . 4 - 4. State of the Union . . . . . . . . . . . . . . . . . . . . . 5 - 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 - 6. Security Considerations . . . . . . . . . . . . . . . . . . . 6 - 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6 - 8. Normative References . . . . . . . . . . . . . . . . . . . . 6 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6 + 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 + 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 + 3. Identified mulitcast issues . . . . . . . . . . . . . . . . . 5 + 3.1. Issues at Layer 2 and Below . . . . . . . . . . . . . . . 5 + 3.1.1. Multicast reliability . . . . . . . . . . . . . . . . 5 + 3.1.2. Lower and Variable Data Rate . . . . . . . . . . . . 5 + 3.1.3. High Interference . . . . . . . . . . . . . . . . . . 6 + 3.1.4. Power-save Effects on Multicast . . . . . . . . . . . 6 + 3.2. Issues at Layer 3 and Above . . . . . . . . . . . . . . . 7 + 3.2.1. IPv4 issues . . . . . . . . . . . . . . . . . . . . . 7 + 3.2.2. IPv6 issues . . . . . . . . . . . . . . . . . . . . . 7 + 3.2.3. MLD issues . . . . . . . . . . . . . . . . . . . . . 8 + 3.2.4. Spurious Neighbor Discovery . . . . . . . . . . . . . 8 + 4. Multicast protocol optimizations . . . . . . . . . . . . . . 9 + 4.1. Proxy ARP in 802.11-2012 . . . . . . . . . . . . . . . . 9 + 4.2. IPv6 Address Registration and Proxy Neighbor Discovery . 10 + 4.3. Buffering to improve Power-Save . . . . . . . . . . . . . 11 + 4.4. IPv6 support in 802.11-2012 . . . . . . . . . . . . . . . 12 + 4.5. Conversion of multicast to unicast . . . . . . . . . . . 12 + 4.6. Directed Multicast Service (DMS) . . . . . . . . . . . . 12 + 4.7. GroupCast with Retries (GCR) . . . . . . . . . . . . . . 13 + 5. Operational optimizations . . . . . . . . . . . . . . . . . . 14 + 5.1. Mitigating Problems from Spurious Neighbor Discovery . . 14 + 6. Multicast Considerations for Other Wireless Media . . . . . . 16 + 7. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 16 + 8. Discussion Items . . . . . . . . . . . . . . . . . . . . . . 16 + 9. Security Considerations . . . . . . . . . . . . . . . . . . . 17 + 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 + 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17 + 12. Informative References . . . . . . . . . . . . . . . . . . . 17 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19 1. Introduction - Multicast over wifi has been used to low levels of success, usually - to a point of being so negative that multicast over wifi is not - allowed. In addition to protocol use of broadcast/multicast for - control messages, more applications, such as push to talk in - hospitals, video in enterprises and lectures in Universities, are - streaming over wifi. And many end devices are increasingly using - wifi for their connectivity. One of the primary problems multicast - over wifi faces is that link local 802.11 doesn't necessarily support - multicast, it supports broadcast. To make make multicast over wifi - work successfully we often need to modify the multicast to instead be - sent as unicast in order for it to successfully transmit with useable - quality. Multicast over wifi experiences high packet error rates, no - acknowledgements, and low data rate. This draft reviews these - problems found with multicast over wifi. While this is not a - solutions draft, common workarounds to some of the problems will be - listed, along with the impact of the workarounds. + 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, incompatibilities still + exist between specifications, implementations and configuration + choices. -2. Multicast over WiFi Problems + Many IETF protocols depend on multicast/broadcast for delivery of + control messages to multiple receivers. Multicast is used for + various purposes such as neighborhood discovery, network flooding, + address resolution, as well minimizing media occupancy for the + transmission of data that is intended for multiple receivers. In + addition to protocol use of broadcast/multicast for control messages, + more applications, such as push to talk in hospitals, video in + enterprises and lectures in Universities, are streaming over wifi. + Many types of end devices are increasingly using wifi for their + connectivity. - 802.11 is a wireless broadcast medium which works well for unicast - and has become ubiquitous in its use. With multicast, however, - problems arise over wifi. There are no ACKs for multicast packets, - for instance, so there can be a high level of 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% which is - particularly troublesome for video and other environments where high - data rates and high reliability are required. Multicast, over wifi, - is typically sent on a low date rate which makes video negatively - impacted. Wifi loses many more packets then wired due to collisions - and signal loss. There are also problems because clients are unable - to stay in sleep mode due to the multicast control packets continuing - to unnecessarily wake up those clients which subsequently reduces - energy savings. Video is becoming the dominant content for end - device applications, with multicast being the most natural method for - applications to transmit video. Unfortunately, multicast, even - though it is a very natural choice for video, incurs a large penalty - over wifi. + 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 wifi 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. - One big difference between multicast over wired versus multicast over - wired is that wired links are a fixed transmission rate. Wifi, on - the other hand, has a transmission rate which varies over time - depending upon the clients proximity to the AP. Throughput of video - flows, and the capacity of the broader wifi network, will change and - will impact the ability for QoS solutions to effectively reserve - bandwidth and provide admission control. + 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. - The main problems associated with multicast over WiFi are as follows: + In discussing these issues over email, and in a side meeting at IETF + 99, it has been generally agreed that these problems will not be + fixed anytime soon primarily because it's expensive to do so and + multicast is unreliable. A big problem is that multicast is somewhat + a second class citizen, to unicast, over wifi. There are many + protocols using multicast and there needs to be something provided in + order to make them more reliable. The problem of IPv6 neighbor + discovery saturating the wifi link is only part of the problem. Wifi + traffic classes may help. We need to determine what problem should + be solved by the IETF and what problem should be solved by the IEEE + (see Section 8). A "multicast over wifi" IETF mailing list has been + formed (mcast-wifi@ietf.org) for further discussion. This draft will + be updated according to the current state of discussion. - o Low Reliability + This Internet Draft details various problems caused by multicast + transmission over wireless networks, including high packet error + rates, no acknowledgements, and low data rate. It also explains some + enhancements that have been designed at IETF and IEEE 802, as well as + the operational choices that can be taken, to ameliorate the effects + of multicast traffic. Recommendations about how to use and combine + these enhancements are also provided. - o Lower Data Rate +2. Terminology - o High interference + This document uses the following definitions: - o High Power Consumption + AP + IEEE 802.11 Access Point. - These points will be elaborated separately in the following - subsections. + basic rate + The "lowest common denominator" data rate at which multicast and + broadcast traffic is generally transmitted. -2.1. Low Reliability + DTIM + Delivery Traffic Indication Map (DTIM): An information element + that advertises whether or not any associated stations have + buffered multicast or broadcast frames. - Because of the lack of acknowledgement for packets from Access Point - to the receivers, it is not possible for the Access Point to know - whether or not a retransmission is needed. Even in the wired - Internet, this characteristic commonly causes undesirably high error - rates, contributing to the relatively slow uptake of multicast - applications even though the protocols have been available for - decades. The situation for wireless links is much worse, and is - quite sensitive to the presence of background traffic. + MCS + Modulation and Coding Scheme. -2.2. Low Data Rate + STA + 802.11 station (e.g. handheld device). - For wireless stations associated with an Access Points, the necessary - power for good reception can vary from station to station. For + TIM + Traffic Indication Map (TIM): An information element that + advertises whether or not any associated stations have buffered + unicast frames. + +3. Identified mulitcast issues + +3.1. Issues at Layer 2 and Below + + In this section we describe some of the issues related to the use of + multicast transmissions over IEEE 802 wireless technologies. + +3.1.1. Multicast reliability + + Multicast traffic is typically much less reliable than unicast + traffic. Since multicast makes point-to-multipoint communications, + multiple acknowledgements would be needed to guarantee reception at + all recipients. Since typically there are no ACKs for multicast + packets, it is not possible for the Access Point (AP) to know whether + or not a retransmission is needed. Even in the wired Internet, this + characteristic often causes undesirably high error rates. This has + contributed to the relatively slow uptake of multicast applications + even though the protocols have long been available. The situation + for wireless links is much worse, and is quite sensitive to the + presence of background traffic. Consequently, there can be a high + packet error rate (PER) due to lack of retransmission, and because + the sender never backs off. It is not uncommon for there to be a + packet loss rate of 5% or more, which is particularly troublesome for + video and other environments where high data rates and high + reliability are required. + +3.1.2. Lower and Variable Data Rate + + One big difference between multicast over wired versus multicast over + wired is that transmission over wired links often occurs at a fixed + rate. Wifi, on the other hand, has a transmission rate which varies + depending upon the clients proximity to the AP. The throughput of + video flows, and the capacity of the broader wifi network, will + change and will impact the ability for QoS solutions to effectively + reserve bandwidth and provide admission control. + + For wireless stations associated with an Access Points, the power + necessary for good reception can vary from station to station. For unicast, the goal is to minimize power requirements while maximizing 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. For this purpose, 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. 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. + 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. 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. -2.3. High Interference + Because more robust modulation and coding schemes (MCSs) have longer + range but also lower data rate, multicast / broadcast traffic is + generally transmitted at the lowest common denominator rate, also + known as the basic rate. Depending on the specific 802.11 + technology, and the configured choice for the base data rate for + multicast transmission from the Access Point, the amount of + additional interference can range from a factor of ten, to a factor + thousands for 802.11ac. - As mentioned in the previous subsection, multicast transmission to - the stations associated to an Access Point typically proceeds at a - much higher power level than is required for unicat to many of the - receivers. High power levels directly contribute to stronger - interference. The interference due to multicast may extend to - effects inhibiting packet reception at more distant stations that - might even be associated with other Access Points. Moreover, the use - of lower data rates implies that the physical medium will be occupied - for a longer time to transmit a packet than would be required at high - data rates. Thus, the level of interference due to multicast will be - not only higher, but longer in duration. + 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. - Depending on the choice of 802.11 technology, and the configured - choice for the base data rate for multicast transmission from the - Access Point, the amount of additional interference can range from a - factor of ten, to a factor thousands for 802.11ac. +3.1.3. High Interference -2.4. High Power Consumption + Transmissions at a lower rate require longer occupancy of the + wireless medium and thus take away from the airtime of other + communications and degrade the overall capacity. Furthermore, + transmission at higher power, as is required to reach all multicast + clients associated to the AP, proportionately increases the area of + interference. + +3.1.4. Power-save Effects on Multicast One of the characteristics of multicast transmission is that every station has to be configured to wake up to receive the multicast, even though the received packet may ultimately be discarded. This - process has a relatively large impact on the power consumption by the + process can have a large effect on the power consumption by the multicast receiver station. -3. Common remedies to multicast over wifi problems + Multicast can work poorly with the power-save mechanisms defined in + IEEE 802.11e, for the following reasons. - One common solution to the multicast over wifi problem is to convert - the multicast traffic into unicast. This is often referred to as - multicast to unicast (MC2UC). Converting the packets to unicast is - beneficial because unicast packets are acknowledged and retransmitted - as needed to prevent as much loss. The Access Points (AP) is also - able to provide rate limiting as needed. The drawback with this - approach is that the benefit of using multicast is defeated. + o Clients may be unable to stay in sleep mode due to multicast + control packets frequently waking them up. + o Both unicast and multicast traffic can be delayed by power-saving + mechanisms. - Using 802.11n helps provide a more reliable and higher level of - signal-to-noise ratio in a wifi environment over which multicast - (broadcast) packets can be sent. This can provide higher throughput - and reliability but the broadcast limitations remain. + o A unicast packet is delayed until a STA wakes up and requests it. + Unicast traffic may also 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 associated to the + AP have to be awake at a known time to receive multicast traffic. + o Packets can also be discarded due to buffer limitations in the AP + and non-AP STA. -4. State of the Union +3.2. Issues at Layer 3 and Above - In discussing these issues over email and, most recently, in a side - meeting at IETF 99, it is generally agreed that these problems will - not be fixed anytime soon primarily because it's expensive to do so - and multicast is unreliable. The problem of v6 neighbor discovery - saturating the wifi link is only part of the problem. A big problem - is that the 802.11 multicast channel is an afterthought and only - given 100th of the bandwidth. Multicast is basically a second class - citizen, to unicast, over wifi. Unicast may have allocated 10mb - while Multicast will be allocated 1mb. There are many protocols - using multicast and there needs to be something provided in order to - make them more reliable. Wifi traffic classes may help. We need to - determine what problem should be solved by the IETF and what problem - should be solved by the IEEE. + 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: - Apple's Bonjour protocol, for instance, provides service discovery - (for printing) that utilizes multicast. It's the first thing - operators drop. Even if multicast snooping is utilized, everyone - registers at once using Bonjour and the network has serious - degradation. There is also a lot of work being developed to help - save battery life such as the devices not waking up when receiving a - multicast packet. If an AP, for instance, expresses a DTIM of 3 then - it will send a multicast packet every 3 packets. But the reality is + 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. + But service discovery can occur at any time. Apple's Bonjour + protocol, for instance, provides service discovery (for printing) + that utilizes multicast. It's the first thing operators drop. Even + if multicast snooping is utilized, many devices register at once + using Bonjour, causing serious network degradation. + +3.2.2. IPv6 issues + + IPv6 makes much more extensive use of multicast, including the + following: + + o DHCPv6 + o IPv6 Neighbor Discovery Protocol (NDP) is not very tolerant of + packet losses. In particular, the Duplicate Address Detection + (DAD) process fails when the owner of an address does not receive + the multicast DAD message from another node that wishes to own + that same address. This can result in an address being duplicated + in the subnet, breaking a basic assumption of IPv6 connectivity. + o IPv6 NDP Neighbor Solicitation (NS) messages used in DAD and + Address Lookup make use of Link-Scope multicast. In contrast to + IPv4, an IPv6 Node will typically use multiple addresses, and may + change them often for privacy reasons. This multiplies the impact + of multicast messages that are associated to the mobility of a + Node. Router advertisement (RA) messages are also periodically + multicasted over the Link. + o Neighbors may be considered lost if several consecutive packets + fail. + + Address Resolution + + Service Discovery + + Route Discovery + + Decentralized Address Assignment + + Geographic routing + +3.2.3. MLD issues + + Multicast Listener Discovery(MLD) [RFC4541] is often used to identify + members of a multicast group that are connected to the ports of a + switch. Forwarding multicast frames into a WiFi-enabled area can use + such switch support for hardware forwarding state information. + However, since IPv6 makes heavy use of multicast, each STA with an + IPv6 address will require state on the switch for several and + possibly many multicast solicited-node addresses. Multicast + addresses that do not have forwarding state installed (perhaps due to + hardware memory limitations on the switch) cause frames to be flooded + on all ports of the switch. + +3.2.4. Spurious Neighbor Discovery + + On the Internet there is a "background radiation" of scanning traffic + (people scanning for vulnerable machines) and backscatter (responses + from spoofed traffic, etc). This means that routers very often + receive packets destined for machines whose IP addresses may or may + not be in use. In the cases where the IP is assigned to a host, the + router broadcasts an ARP request, gets back an ARP reply, and caches + it; then traffic can be delivered to the host. When the IP address + is not in use, the router broadcasts one (or more) ARP requests, and + never gets a reply. This means that it does not populate the ARP + cache, and the next time there is traffic for that IP address the + router will rebroadcast the ARP requests. + + The rate of these ARP requests is proportional to the size of the + subnets, the rate of scanning and backscatter, and how long the + router keeps state on non-responding ARPs. As it turns out, this + rate is inversely proportional to how occupied the subnet is (valid + ARPs end up in a cache, stopping the broadcasting; unused IPs never + respond, and so cause more broadcasts). Depending on the address + space in use, the time of day, how occupied the subnet is, and other + unknown factors, on the order of 2000 broadcasts per second have been + observed at the IETF NOCs. + + On a wired network, there is not a huge difference amongst unicast, + multicast and broadcast traffic; but this is not true in the wireless + realm. Wireless equipment often is unable to send this amount of + broadcast and multicast traffic. Consequently, on the wireless + networks, we observe a significant amount of dropped broadcast and + multicast packets. This, in turn, means that when a host connects it + is often not able to complete DHCP, and IPv6 RAs get dropped, leading + to users being unable to use the network. + +4. Multicast protocol optimizations + + This section lists some optimizations that have been specified in + IEEE 802 and IETF that are aimed at reducing or eliminating the + issues discussed in Section 3. + +4.1. Proxy ARP in 802.11-2012 + + The AP knows the MAC address and IP address for all associated STAs. + In this way, the AP acts as the central "manager" for all the 802.11 + STAs in its BSS. Proxy ARP is easy to implement at the AP, and + offers the following advantages: + + o Reduced broadcast traffic (transmitted at low MCS) on the wireless + medium + o STA benefits from extended power save in sleep mode, as ARP + requests for STA's IP address are handled instead by the AP. + o ARP frames are kept off the wireless medium. + o No changes are needed to STA implementation. + + Here is the specification language as described in clause 10.23.13 of + [dot11-proxyarp]: + + When the AP supports Proxy ARP "[...] the AP shall maintain a + Hardware Address to Internet Address mapping for each associated + station, and shall update the mapping when the Internet Address of + the associated station changes. When the IPv4 address being + resolved in the ARP request packet is used by a non-AP STA + currently associated to the BSS, the proxy ARP service shall + respond on behalf of the non-AP STA" + +4.2. IPv6 Address Registration and Proxy Neighbor Discovery + + As used in this section, a Low-Power Wireless Personal Area Network + (6LoWPAN) denotes a low power lossy network (LLN) that supports + 6LoWPAN Header Compression (HC) [RFC6282]. A 6TiSCH network + [I-D.ietf-6tisch-architecture] is an example of a 6LowPAN. In order + to control the use of IPv6 multicast over 6LoWPANs, the 6LoWPAN + Neighbor Discovery (6LoWPAN ND) [RFC6775] standard defines an address + registration mechanism that relies on a central registry to assess + address uniqueness, as a substitute to the inefficient Duplicate + Address Detection (DAD) mechanism found in the mainstream IPv6 + Neighbor Discovery Protocol (NDP) [RFC4861][RFC4862]. + + The 6lo Working Group is now completing an update + [I-D.ietf-6lo-rfc6775-update] to RFC6775. The update enables the + registration to a Backbone Router [I-D.ietf-6lo-backbone-router], + which proxies for the registered addresses with the mainstream IPv6 + NDP running on a high speed aggragating backbone. The update also + enables a proxy registration on behalf of the registered node, e.g. + by a 6LoWPAN router to which the mobile node is attached. + + The general idea behind the backbone router concept is that in a + variety of Wireless Local Area Networks (WLANs) and Wireless Personal + Area Networks (WPANs), the broadcast/multicast domain should be + controlled, and connectivity to a particular link that provides the + subnet should be left to Layer-3. The model for the Backbone Router + operation is represented in Figure 1. + + | + +-----+ + | | Gateway (default) router + | | + +-----+ + | + | Backbone Link + +--------------------+------------------+ + | | | + +-----+ +-----+ +-----+ + | | Backbone | | Backbone | | Backbone + | | router | | router | | router + +-----+ +-----+ +-----+ + o o o o o o + o o o o o o o o o o o o o o + o o o o o o o o o o o o o o o + o o o o o o o o o o + o o o o o o o + + LLN LLN LLN + + Figure 1: Backbone Link and Backbone Routers + + LLN nodes can move freely from an LLN anchored at one IPv6 Backbone + Router to an LLN anchored at another Backbone Router on the same + backbone, keeping any of the IPv6 addresses they have configured. + The Backbone Routers maintain a Binding Table of their Registered + Nodes, which serves as a distributed database of all the LLN Nodes. + An extension to the Neighbor Discovery Protocol is introduced to + exchange that information across the Backbone Link in the reactive + fashion of mainstream IPv6 Neighbor Discovery. + + RFC6775 and follow-on work (e.g., [I-D.ietf-6lo-ap-nd], are designed + to address the needs of LLNs, but the techniques are likely to be + valuable on any type of link where sleeping devices are attached, or + where the use of broadcast and multicast operations should be + limited. + +4.3. Buffering to improve Power-Save + + Methods have been developed to help save battery life; for example, a + device might not wake up when the AP receives a multicast packet. + The AP acts on behalf of STAs in various ways. In order to improve + the power-saving feature for STAs in its BSS, the AP buffers frames + for delivery to the STA at the time when the STA is scheduled for + reception. If an AP, for instance, expresses a DTIM of 3 then it + will send a multicast packet every 3 packets. But the reality is that most AP's will send a multicast every 30 packets. For unicast there's a TIM. But because multicast is going to everyone, the AP sends a broadcast to everyone. DTIM does power management but - clients can choose to wake up or not and whether to drop the packet - or not. Then they don't know why their bonjour doesn't work. The - IETF may just need to decide that broadcast is more expensive so - multicast needs to be sent wired. 802.1ak works on ethernet and wifi. - 802.1ak has been pulled into 802.1Q as of 802.1Q-2011. 802.1Q-2014 - can be looked at here: http://www.ieee802.org/1/pages/802.1Q- - 2014.html . If we don't find a generic solution we need to establish - guidelines for multicast over wifi within the mboned wg. A multicast - over wifi IETF mailing list is formed (mcast-wifi@ietf.org) and more - discussion to be had there. This draft will serve as the current - state of affairs. + clients can choose whether or not to wake up or not and whether or + not to drop the packet. Unfortunately, without proper administrative + control, such clients may no longer be able to determine why their + multicast operations do not work. - This is not a solutions draft, but to provide an idea going forward, - a reliable registration to Layer-2 multicast groups and a reliable - multicast operation at Layer-2 could provide a generic solution. - There is no need to support 2^24 groups to get solicited node - multicast working: it is possible to simply select a number of - trailing bits that make sense for a given network size to limit the - amount of unwanted deliveries to reasonable levels. We need to - encourage IEEE 802.1 and 802.11 to revisit L2 multicast issues. In - particular, Wi-Fi provides a broadcast service, not a multicast one. - In fact all frames are broadcast at the PHY level unless we beamform. - What comes with unicast is the property of being much faster (2 - orders of magnitude) and much more reliable (L2 ARQ). +4.4. IPv6 support in 802.11-2012 -5. IANA Considerations + IPv6 uses Neighbor Discovery Protocol (NDP) instead of ARP. Every + IPv6 node subscribes to a special multicast address for this purpose. - None + Here is the specification language from clause 10.23.13 of + [dot11-proxyarp]: -6. Security Considerations + "When an IPv6 address is being resolved, the Proxy Neighbor + Discovery service shall respond with a Neighbor Advertisement + message [...] on behalf of an associated STA to an [ICMPv6] + Neighbor Solicitation message [...]. When MAC address mappings + change, the AP may send unsolicited Neighbor Advertisement + Messages on behalf of a STA." - None + NDP may be used to request additional information -7. Acknowledgments + o Maximum Transmission Unit + o Router Solicitation + o Router Advertisement, etc. - The following people have contributed information and discussion in - the meetings and on the list which proved helpful for the development - of the latest version this Internet Draft: + 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. - Dave Taht, Donald Eastlake, Pascal Thubert, Juan Carlos Zuniga, - Mikael Abrahamsson, Diego Dujovne, David Schinazi, Stig Venaas, - Stuart Cheshire, Lorenzo, Greg Shephard, Mark Hamilton +4.5. Conversion of multicast to unicast -8. Normative References + It is often possible to transmit multicast control and data messages + by using unicast transmissions to each station individually. - [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate - Requirement Levels", BCP 14, RFC 2119, - DOI 10.17487/RFC2119, March 1997, - . +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: + + o Requires 802.11n A-MSDUs + o Individually addressed frames are acknowledged and are buffered + for power save clients + o The requesting STA may specify traffic characteristics for DMS + traffic + o DMS was defined in IEEE Std 802.11v-2011 + o DMS requires changes to both AP and STA implementation. + + DMS is not currently implemented in products. + +4.7. GroupCast with Retries (GCR) + + GCR (defined in [dot11aa]) provides greater reliability by using + either unsolicited retries or a block acknowledgement mechanism. GCR + increases probability of broadcast frame reception success, but still + does not guarantee success. + + For the block acknowledgement mechanism, the AP transmits each group + addressed frame as conventional group addressed transmission. + Retransmissions are group addressed, but hidden from non-11aa + clients. A directed block acknowledgement scheme is used to harvest + reception status from receivers; retransmissions are based upon these + responses. + + GCR is suitable for all group sizes including medium to large groups. + As the number of devices in the group increases, GCR can send block + acknowledgement requests to only a small subset of the group. GCR + does require changes to both AP and STA implementation. + + GCR may introduce unacceptable latency. After sending a group of + data frames to the group, the AP has do the following: + + o unicast a Block Ack Request (BAR) to a subset of members. + o wait for the corresponding Block Ack (BA). + o retransmit any missed frames. + o resume other operations which may have been delayed. + + This latency may not be acceptable for some traffic. + + There are ongoing extensions in 802.11 to improve GCR performance. + + o BAR is sent using downlink MU-MIMO (note that downlink MU-MIMO is + already specified in 802.11-REVmc 4.3). + o BA is sent using uplink MU-MIMO (which is a .11ax feature). + o Additional 802.11ax extensions are under consideration; see + [mc-ack-mux] + o Latency may also be reduced by simultaneously receiving BA + information from multiple clients. + +5. Operational optimizations + + This section lists some operational optimizations that can be + implemented when deploying wireless IEEE 802 networks to mitigate the + issues discussed in Section 3. + +5.1. Mitigating Problems from Spurious Neighbor Discovery + + ARP Sponges + + An ARP Sponge sits on a network and learn which IPs addresses + are actually in use. It also listen for ARP requests, and, if + it sees an ARP for an IP address which it believes is not used, + it will reply with its own MAC address. This means that the + router now has an IP to MAC mapping, which it caches. If that + IP is later assigned to an machine (e.g using DHCP), the ARP + sponge will see this, and will stop replying for that address. + Gratuitous ARPs (or the machine ARPing for its gateway) will + replace the sponged address in the router ARP table. This + technique is quite effective; but, unfortunately, the ARP + sponge daemons were not really designed for this use (the + standard one [arpsponge], was designed to deal with the + disappearance of participants from an IXP) and so are not + optimized for this purpose. We have to run one daemon per + subnet, the tuning is tricky (the scanning rate versus the + population rate versus retires, etc.) and sometimes the daemons + just seem to stop, requiring a restart of the daemon and + causing disruption. + + Router mitigations + + Some routers (often those based on Linux) implement a "negative + ARP cache" daemon. Simply put, if the router does not see a + reply to an ARP it can be configured to cache this information + for some interval. Unfortunately, the core routers which we + are using do not support this. When a host connects to network + and gets an IP address, it will ARP for its default gateway + (the router). The router will update its cache with the IP to + host MAC mapping learnt from the request (passive ARP + learning). + + Firewall unused space + + The distribution of users on wireless networks / subnets + changes from meeting to meeting (e.g the "IETF-secure" SSID was + renamed to "IETF", fewer users use "IETF-legacy", etc). This + utilization is difficult to predict ahead of time, but we can + monitor the usage as attendees use the different networks. By + configuring multiple DHCP pools per subnet, and enabling them + sequentially, we can have a large subnet, but only assign + addresses from the lower portions of it. This means that we + can apply input IP access lists, which deny traffic to the + upper, unused portions. This means that the router does not + attempt to forward packets to the unused portions of the + subnets, and so does not ARP for it. This method has proven to + be very effective, but is somewhat of a blunt axe, is fairly + labor intensive, and requires coordination. + + Disabling/filtering ARP requests + + In general, the router does not need to ARP for hosts; when a + host connects, the router can learn the IP to MAC mapping from + the ARP request sent by that host. This means that we should + be able to disable and / or filter ARP requests from the + router. Unfortunately, ARP is a very low level / fundamental + part of the IP stack, and is often offloaded from the normal + control plane. While many routers can filter layer-2 traffic, + this is usually implemented as an input filter and / or has + limited ability to filter output broadcast traffic. This means + that the simple "just disable ARP or filter it outbound" seems + like a really simple (and obvious) solution, but + implementations / architectural issues make this difficult or + awkward in practice. + + NAT + + The broadcasts are overwhelmingly being caused by outside + scanning / backscatter traffic. This means that, if we were to + NAT the entire (or a large portion) of the attendee networks, + there would be no NAT translation entries for unused addresses, + and so the router would never ARP for them. The IETF NOC has + discussed NATing the entire (or large portions) attendee + address space, but a: elegance and b: flaming torches and + pitchfork concerns means we have not attempted this yet. + + Stateful firewalls + + Another obvious solution would be to put a stateful firewall + between the wireless network and the Internet. This firewall + would block incoming traffic not associated with an outbound + request. The IETF philosophy has been to have the network as + open as possible / honor the end-to-end principle. An attendee + on the meeting network should be an Internet host, and should + be able to receive unsolicited requests. Unfortunately, + keeping the network working and stable is the first priority + and a stateful firewall may be required in order to achieve + this. + +6. Multicast Considerations for Other Wireless Media + + Many of the causes of performance degradation described in earlier + sections are also observable for wireless media other than 802.11. + + For instance, problems with power save, excess media occupancy, and + poor reliability will also affect 802.15.3 and 802.15.4. However, + 802.15 media specifications do not include mechanisms similar to + those developed for 802.11. In fact, the design philosophy for + 802.15 is oriented towards minimality, with the result that many such + functions would more likely be relegated to operation within higher + layer protocols. This leads to a patchwork of non-interoperable and + vendor-specific solutions. See [uli] for some additional discussion, + and a proposal for a task group to resolve similar issues, in which + the multicast problems might be considered for mitigation. + +7. Recommendations + + This section will provide some recommendations about the usage and + combinations of the multicast enhancements described in Section 4 and + Section 5. + + (FFS) + +8. Discussion Items + + This section will suggest some discussion items for further + resolution. + + The IETF may need to decide that broadcast is more expensive so + multicast needs to be sent wired. For example, 802.1ak works on + ethernet and wifi. 802.1ak has been pulled into 802.1Q as of 802.1Q- + 2011. 802.1Q-2014 can be looked at here: http://www.ieee802.org/1/ + pages/802.1Q-2014.html. If a generic solution is not found, + guidelines for multicast over wifi should be established. + + To provide an idea going forward, perhaps a reliable registration to + Layer-2 multicast groups and a reliable multicast operation at + Layer-2 could provide a generic solution. There is no need to + support 2^24 groups to get solicited node multicast working: it is + possible to simply select a number of trailing bits that make sense + for a given network size to limit the amount of unwanted deliveries + to reasonable levels. IEEE 802.1, 802.11, and 802.15 should be + encouraged to revisit L2 multicast issues. In particular, Wi-Fi + provides a broadcast service, not a multicast one; at the PHY level, + all frames are broadcast except in very unusual cases in which + special beamforming transmitters are used. Unicast offers the + advantage of being much faster (2 orders of magnitude) and much more + reliable (L2 ARQ). + +9. Security Considerations + + This document does not introduce any security mechanisms, and does + not have affect existing security mechanisms. + +10. IANA Considerations + + This document does not specify any IANA actions. + +11. Acknowledgements + + This document has benefitted from discussions with the following + people, in alphabetical order: Pascal Thubert + +12. Informative References + + [arpsponge] + Arien Vijn, Steven Bakker, "Arp Sponge", March 2015. + + [dot11] P802.11, "Part 11: Wireless LAN Medium Access Control + (MAC) and Physical Layer (PHY) Specifications", March + 2012. + + [dot11-proxyarp] + P802.11, "Proxy ARP in 802.11ax", September 2015. + + [dot11aa] P802.11, "Part 11: Wireless LAN Medium Access Control + (MAC) and Physical Layer (PHY) Specifications Amendment 2: + MAC Enhancements for Robust Audio Video Streaming", March + 2012. + + [I-D.ietf-6lo-ap-nd] + Thubert, P., Sarikaya, B., and M. Sethi, "Address + Protected Neighbor Discovery for Low-power and Lossy + Networks", draft-ietf-6lo-ap-nd-05 (work in progress), + January 2018. + + [I-D.ietf-6lo-backbone-router] + Thubert, P., "IPv6 Backbone Router", draft-ietf-6lo- + backbone-router-05 (work in progress), January 2018. + + [I-D.ietf-6lo-rfc6775-update] + Thubert, P., Nordmark, E., Chakrabarti, S., and C. + Perkins, "An Update to 6LoWPAN ND", draft-ietf-6lo- + rfc6775-update-11 (work in progress), December 2017. + + [I-D.ietf-6tisch-architecture] + Thubert, P., "An Architecture for IPv6 over the TSCH mode + of IEEE 802.15.4", draft-ietf-6tisch-architecture-13 (work + in progress), November 2017. + + [ietf_802-11] + Dorothy Stanley, "IEEE 802.11 multicast capabilities", Nov + 2015. + + [mc-ack-mux] + Yusuke Tanaka et al., "Multiplexing of Acknowledgements + for Multicast Transmission", July 2015. + + [mc-prob-stmt] + Mikael Abrahamsson and Adrian Stephens, "Multicast on + 802.11", March 2015. + + [mc-props] + Adrian Stephens, "IEEE 802.11 multicast properties", March + 2015. + + [RFC4541] Christensen, M., Kimball, K., and F. Solensky, + "Considerations for Internet Group Management Protocol + (IGMP) and Multicast Listener Discovery (MLD) Snooping + Switches", RFC 4541, DOI 10.17487/RFC4541, May 2006, + . + + [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., Ed. 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., Ed., Chakrabarti, S., Nordmark, E., and C. + Bormann, "Neighbor Discovery Optimization for IPv6 over + Low-Power Wireless Personal Area Networks (6LoWPANs)", + RFC 6775, DOI 10.17487/RFC6775, November 2012, + . + + [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 + Mike McBride - Huawei + Futurewei Inc. 2330 Central Expressway - Santa Clara CA 95055 + Santa Clara, CA 95055 USA Email: michael.mcbride@huawei.com - Charlie Perkins - Huawei - 2330 Central Expressway - Santa Clara CA 95055 + + Dorothy Stanley + Hewlett Packard Enterprise + 2000 North Naperville Rd. + Naperville, IL 60566 USA - Email: charlie.perkins@huawei.com + Phone: +1 630 979 1572 + Email: dstanley@arubanetworks.com + + Warren Kumari + Google + 1600 Amphitheatre Parkway + Mountain View, CA 94043 + USA + + Email: warren@kumari.net + Juan Carlos Zuniga + SIGFOX + 425 rue Jean Rostand + Labege 31670 + France + + Email: j.c.zuniga@ieee.org