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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Missing Reference: 'ICMPv6' is mentioned on line 581, but not defined == Outdated reference: A later version (-20) exists of draft-ietf-6lo-backbone-router-13 == Outdated reference: A later version (-30) exists of draft-ietf-6tisch-architecture-26 == Outdated reference: A later version (-13) exists of draft-ietf-mboned-driad-amt-discovery-08 -- Obsolete informational reference (is this intentional?): RFC 2461 (Obsoleted by RFC 4861) Summary: 0 errors (**), 0 flaws (~~), 5 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Internet Area C. Perkins 3 Internet-Draft 4 Intended status: Informational M. McBride 5 Expires: March 29, 2020 Futurewei 6 D. Stanley 7 HPE 8 W. Kumari 9 Google 10 JC. Zuniga 11 SIGFOX 12 September 26, 2019 14 Multicast Considerations over IEEE 802 Wireless Media 15 draft-ietf-mboned-ieee802-mcast-problems-09 17 Abstract 19 Well-known issues with multicast have prevented the deployment of 20 multicast in 802.11 and other local-area wireless environments. This 21 document offers guidance on known limitations and problems with 22 wireless Layer-2 multicast. Also described are certain multicast 23 enhancement features that have been specified by the IETF and by IEEE 24 802 for wireless media, as well as some operational choices that can 25 be taken to improve the performance of the network. Finally, some 26 recommendations are provided about the usage and combination of these 27 features and operational choices. 29 Status of This Memo 31 This Internet-Draft is submitted in full conformance with the 32 provisions of BCP 78 and BCP 79. 34 Internet-Drafts are working documents of the Internet Engineering 35 Task Force (IETF). Note that other groups may also distribute 36 working documents as Internet-Drafts. The list of current Internet- 37 Drafts is at https://datatracker.ietf.org/drafts/current/. 39 Internet-Drafts are draft documents valid for a maximum of six months 40 and may be updated, replaced, or obsoleted by other documents at any 41 time. It is inappropriate to use Internet-Drafts as reference 42 material or to cite them other than as "work in progress." 44 This Internet-Draft will expire on March 29, 2020. 46 Copyright Notice 48 Copyright (c) 2019 IETF Trust and the persons identified as the 49 document authors. All rights reserved. 51 This document is subject to BCP 78 and the IETF Trust's Legal 52 Provisions Relating to IETF Documents 53 (https://trustee.ietf.org/license-info) in effect on the date of 54 publication of this document. Please review these documents 55 carefully, as they describe your rights and restrictions with respect 56 to this document. Code Components extracted from this document must 57 include Simplified BSD License text as described in Section 4.e of 58 the Trust Legal Provisions and are provided without warranty as 59 described in the Simplified BSD License. 61 Table of Contents 63 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 64 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 65 3. Identified multicast issues . . . . . . . . . . . . . . . . . 5 66 3.1. Issues at Layer 2 and Below . . . . . . . . . . . . . . . 5 67 3.1.1. Multicast reliability . . . . . . . . . . . . . . . . 5 68 3.1.2. Lower and Variable Data Rate . . . . . . . . . . . . 6 69 3.1.3. High Interference . . . . . . . . . . . . . . . . . . 7 70 3.1.4. Power-save Effects on Multicast . . . . . . . . . . . 7 71 3.2. Issues at Layer 3 and Above . . . . . . . . . . . . . . . 7 72 3.2.1. IPv4 issues . . . . . . . . . . . . . . . . . . . . . 8 73 3.2.2. IPv6 issues . . . . . . . . . . . . . . . . . . . . . 8 74 3.2.3. MLD issues . . . . . . . . . . . . . . . . . . . . . 9 75 3.2.4. Spurious Neighbor Discovery . . . . . . . . . . . . . 9 76 4. Multicast protocol optimizations . . . . . . . . . . . . . . 10 77 4.1. Proxy ARP in 802.11-2012 . . . . . . . . . . . . . . . . 10 78 4.2. IPv6 Address Registration and Proxy Neighbor Discovery . 11 79 4.3. Buffering to Improve Battery Life . . . . . . . . . . . . 12 80 4.4. Limiting multicast buffer hardware queue depth . . . . . 13 81 4.5. IPv6 support in 802.11-2012 . . . . . . . . . . . . . . . 13 82 4.6. Using Unicast Instead of Multicast . . . . . . . . . . . 14 83 4.6.1. Overview . . . . . . . . . . . . . . . . . . . . . . 14 84 4.6.2. Layer 2 Conversion to Unicast . . . . . . . . . . . . 14 85 4.6.3. Directed Multicast Service (DMS) . . . . . . . . . . 14 86 4.6.4. Automatic Multicast Tunneling (AMT) . . . . . . . . . 15 87 4.7. GroupCast with Retries (GCR) . . . . . . . . . . . . . . 15 88 5. Operational optimizations . . . . . . . . . . . . . . . . . . 16 89 5.1. Mitigating Problems from Spurious Neighbor Discovery . . 16 90 5.2. Mitigating Spurious Service Discovery Messages . . . . . 18 91 6. Multicast Considerations for Other Wireless Media . . . . . . 18 92 7. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 19 93 8. Discussion Items . . . . . . . . . . . . . . . . . . . . . . 19 94 9. Security Considerations . . . . . . . . . . . . . . . . . . . 20 95 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 96 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20 97 12. Informative References . . . . . . . . . . . . . . . . . . . 20 98 Appendix A. Changes in this draft between revisions 06 versus 07 24 99 Appendix B. Changes in this draft between revisions 05 versus 06 24 100 Appendix C. Changes in this draft between revisions 04 versus 05 24 101 Appendix D. Changes in this draft between revisions 03 versus 04 25 102 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25 104 1. Introduction 106 Well-known issues with multicast have prevented the deployment of 107 multicast in 802.11 [dot11] and other local-area wireless 108 environments, as described in [mc-props], [mc-prob-stmt]. 109 Performance issues have been observed when multicast packet 110 transmissions of IETF protocols are used over IEEE 802 wireless 111 media. Even though enhancements for multicast transmissions have 112 been designed at both IETF and IEEE 802, incompatibilities still 113 exist between specifications, implementations and configuration 114 choices. 116 Many IETF protocols depend on multicast/broadcast for delivery of 117 control messages to multiple receivers. Multicast is used for 118 various purposes such as neighbor discovery, network flooding, 119 address resolution, as well minimizing media occupancy for the 120 transmission of data that is intended for multiple receivers. In 121 addition to protocol use of broadcast/multicast for control messages, 122 more applications, such as push to talk in hospitals, or video in 123 enterprises, universities, and homes, are sending multicast IP to end 124 user devices, which are increasingly using Wi-Fi for their 125 connectivity. 127 IETF protocols typically rely on network protocol layering in order 128 to reduce or eliminate any dependence of higher level protocols on 129 the specific nature of the MAC layer protocols or the physical media. 130 In the case of multicast transmissions, higher level protocols have 131 traditionally been designed as if transmitting a packet to an IP 132 address had the same cost in interference and network media access, 133 regardless of whether the destination IP address is a unicast address 134 or a multicast or broadcast address. This model was reasonable for 135 networks where the physical medium was wired, like Ethernet. 136 Unfortunately, for many wireless media, the costs to access the 137 medium can be quite different. Multicast over Wi-Fi has often been 138 plagued by such poor performance that it is disallowed. Some 139 enhancements have been designed in IETF protocols that are assumed to 140 work primarily over wireless media. However, these enhancements are 141 usually implemented in limited deployments and not widespread on most 142 wireless networks. 144 IEEE 802 wireless protocols have been designed with certain features 145 to support multicast traffic. For instance, lower modulations are 146 used to transmit multicast frames, so that these can be received by 147 all stations in the cell, regardless of the distance or path 148 attenuation from the base station or access point. However, these 149 lower modulation transmissions occupy the medium longer; they hamper 150 efficient transmission of traffic using higher order modulations to 151 nearby stations. For these and other reasons, IEEE 802 working 152 groups such as 802.11 have designed features to improve the 153 performance of multicast transmissions at Layer 2 [ietf_802-11]. In 154 addition to protocol design features, certain operational and 155 configuration enhancements can ameliorate the network performance 156 issues created by multicast traffic, as described in Section 5. 158 There seems to be general agreement that these problems will not be 159 fixed anytime soon, primarily because it's expensive to do so and due 160 to multicast being unreliable. Compared to unicast over Wi-Fi, 161 multicast is often treated as somewhat of a second class citizen, 162 even though there are many protocols using multicast. Something 163 needs to be provided in order to make them more reliable. IPv6 164 neighbor discovery saturating the Wi-Fi link is only part of the 165 problem. Wi-Fi traffic classes may help. This document is intended 166 to help make the determination about what problems should be solved 167 by the IETF and what problems should be solved by the IEEE (see 168 Section 8). 170 This document details various problems caused by multicast 171 transmission over wireless networks, including high packet error 172 rates, no acknowledgements, and low data rate. It also explains some 173 enhancements that have been designed at the IETF and IEEE 802 to 174 ameliorate the effects of multicast traffic. Recommendations are 175 also provided to implementors about how to use and combine these 176 enhancements. Some advice about the operational choices that can be 177 taken is also included. It is likely that this document will also be 178 considered relevant to designers of future IEEE wireless 179 specifications. 181 2. Terminology 183 This document uses the following definitions: 185 ACK 186 The 802.11 layer 2 acknowledgement 188 AP 189 IEEE 802.11 Access Point 191 basic rate 192 The slowest rate of all the connected devices, at which multicast 193 and broadcast traffic is generally transmitted 195 DTIM 196 Delivery Traffic Indication Map (DTIM): An information element 197 that advertises whether or not any associated stations have 198 buffered multicast or broadcast frames 200 MCS 201 Modulation and Coding Scheme 203 NOC 204 Network Operations Center 206 PER 207 Packet Error Rate 209 STA 210 802.11 station (e.g. handheld device) 212 TIM 213 Traffic Indication Map (TIM): An information element that 214 advertises whether or not any associated stations have buffered 215 unicast frames 217 3. Identified multicast issues 219 3.1. Issues at Layer 2 and Below 221 In this section some of the issues related to the use of multicast 222 transmissions over IEEE 802 wireless technologies are described. 224 3.1.1. Multicast reliability 226 Multicast traffic is typically much less reliable than unicast 227 traffic. Since multicast makes point-to-multipoint communications, 228 multiple acknowledgements would be needed to guarantee reception at 229 all recipients. Since typically there are no ACKs for multicast 230 packets, it is not possible for the Access Point (AP) to know whether 231 or not a retransmission is needed. Even in the wired Internet, this 232 characteristic often causes undesirably high error rates. This has 233 contributed to the relatively slow uptake of multicast applications 234 even though the protocols have long been available. The situation 235 for wireless links is much worse, and is quite sensitive to the 236 presence of background traffic. Consequently, there can be a high 237 packet error rate (PER) due to lack of retransmission, and because 238 the sender never backs off. It is not uncommon for there to be a 239 packet loss rate of 5% or more, which is particularly troublesome for 240 video and other environments where high data rates and high 241 reliability are required. 243 3.1.2. Lower and Variable Data Rate 245 Multicast over wired differs from multicast over wireless because 246 transmission over wired links often occurs at a fixed rate. Wi-Fi, 247 on the other hand, has a transmission rate that varies depending upon 248 the STA's proximity to the AP. The throughput of video flows, and 249 the capacity of the broader Wi-Fi network, will change and will 250 impact the ability for QoS solutions to effectively reserve bandwidth 251 and provide admission control. 253 For wireless stations associated with an Access Point, the power 254 necessary for good reception can vary from station to station. For 255 unicast, the goal is to minimize power requirements while maximizing 256 the data rate to the destination. For multicast, the goal is simply 257 to maximize the number of receivers that will correctly receive the 258 multicast packet; generally the Access Point has to use a much lower 259 data rate at a power level high enough for even the farthest station 260 to receive the packet, for example as briefly mentioned in [RFC5757]. 261 Consequently, the data rate of a video stream, for instance, would be 262 constrained by the environmental considerations of the least reliable 263 receiver associated with the Access Point. 265 Because more robust modulation and coding schemes (MCSs) have longer 266 range but also lower data rate, multicast / broadcast traffic is 267 generally transmitted at the slowest rate of all the connected 268 devices. This is also known as the basic rate. The amount of 269 additional interference depends on the specific wireless technology. 270 In fact, backward compatibility and multi-stream implementations mean 271 that the maximum unicast rates are currently up to a few Gbps, so 272 there can be more than 3 orders of magnitude difference in the 273 transmission rate between multicast / broadcast versus optimal 274 unicast forwarding. Some techiques employed to increase spectral 275 efficiency, such as spatial multiplexing in mimo systems, are not 276 available with more than one intended reciever; it is not the case 277 that backwards compatibility is the only factor responsible for lower 278 multicast transmission rates. 280 Wired multicast also affects wireless LANs when the AP extends the 281 wired segment; in that case, multicast / broadcast frames on the 282 wired LAN side are copied to WLAN. Since broadcast messages are 283 transmitted at the most robust MCS, many large frames are sent at a 284 slow rate over the air. 286 3.1.3. High Interference 288 Transmissions at a lower rate require longer occupancy of the 289 wireless medium and thus take away from the airtime of other 290 communications and degrade the overall capacity. Furthermore, 291 transmission at higher power, as is required to reach all multicast 292 STAs associated to the AP, proportionately increases the area of 293 interference. 295 3.1.4. Power-save Effects on Multicast 297 One of the characteristics of multicast transmission is that every 298 station has to be configured to wake up to receive the multicast, 299 even though the received packet may ultimately be discarded. This 300 process can have a large effect on the power consumption by the 301 multicast receiver station. 303 Multicast can work poorly with the power-save mechanisms defined in 304 IEEE 802.11e, for the following reasons. 306 o Clients may be unable to stay in sleep mode due to multicast 307 control packets frequently waking them up. 308 o Both unicast and multicast traffic can be delayed by power-saving 309 mechanisms. 310 o A unicast packet is delayed until an STA wakes up and requests it. 311 Unicast traffic may also be delayed to improve power save, 312 efficiency and increase probability of aggregation. 313 o Multicast traffic is delayed in a wireless network if any of the 314 STAs in that network are power savers. All STAs associated to the 315 AP have to be awake at a known time to receive multicast traffic. 316 o Packets can also be discarded due to buffer limitations in the AP 317 and non-AP STA. 319 3.2. Issues at Layer 3 and Above 321 This section identifies some representative IETF protocols, and 322 describes possible negative effects due to performance degradation 323 when using multicast transmissions for control messages. Common uses 324 of multicast include: 326 o Control plane signaling 327 o Neighbor Discovery 328 o Address Resolution 329 o Service Discovery 330 o Applications (video delivery, stock data, etc.) 331 o On-demand routing 332 o Backbone construction 333 o Other L3 protocols (non-IP) 335 User Datagram Protocol (UDP) is the most common transport layer 336 protocol for multicast applications. By itself, UDP is not reliable 337 -- messages may be lost or delivered out of order. 339 3.2.1. IPv4 issues 341 The following list contains some representative multicast protocols 342 that are used with IPv4. 344 o ARP 345 o DHCP 346 o mDNS [RFC6762] 347 o uPnP [RFC6970] 349 After initial configuration, ARP and DHCP occur much less commonly, 350 but service discovery can occur at any time. Some widely-deployed 351 service discovery protocols (e.g., for finding a printer) utilize 352 mDNS (i.e., multicast). It's often the first service that operators 353 drop. Even if multicast snooping is utilized, many devices can 354 register at once and cause serious network degradation. 356 3.2.2. IPv6 issues 358 IPv6 makes extensive use of multicast, including the following: 360 o DHCPv6 361 o IPv6 Neighbor Discovery Protocol (NDP) [RFC4861] 362 o multicast DNS (mDNS) 363 o Route Discovery 364 o Decentralized Address Assignment 365 o Geographic routing 367 IPv6 NDP Neighbor Solicitation (NS) messages used in Duplicate 368 Address Detection (DAD) and Address Lookup make use of Link-Scope 369 multicast. In contrast to IPv4, an IPv6 node will typically use 370 multiple addresses, and may change them often for privacy reasons. 371 This intensifies the impact of multicast messages that are associated 372 to the mobility of a node. Router advertisement (RA) messages are 373 also periodically multicasted over the Link. 375 Neighbors may be considered lost if several consecutive Neighbor 376 Discovery packets fail. 378 3.2.3. MLD issues 380 Multicast Listener Discovery (MLD) [RFC4541] is used to identify 381 members of a multicast group that are connected to the ports of a 382 switch. Forwarding multicast frames into a Wi-Fi-enabled area can 383 use such switch support for hardware forwarding state information. 384 However, since IPv6 makes heavy use of multicast, each STA with an 385 IPv6 address will require state on the switch for several and 386 possibly many multicast solicited-node addresses. Multicast 387 addresses that do not have forwarding state installed (perhaps due to 388 hardware memory limitations on the switch) cause frames to be flooded 389 on all ports of the switch. Some switch vendors do not support MLD, 390 for link-scope multicast, due to the increase it can cause in state. 392 3.2.4. Spurious Neighbor Discovery 394 On the Internet there is a "background radiation" of scanning traffic 395 (people scanning for vulnerable machines) and backscatter (responses 396 from spoofed traffic, etc). This means that routers very often 397 receive packets destined for IP addresses regardless of whether those 398 IP addresses are in use. In the cases where the IP is assigned to a 399 host, the router broadcasts an ARP request, gets back an ARP reply, 400 and caches it; then traffic can be delivered to the host. When the 401 IP address is not in use, the router broadcasts one (or more) ARP 402 requests, and never gets a reply. This means that it does not 403 populate the ARP cache, and the next time there is traffic for that 404 IP address the router will rebroadcast the ARP requests. 406 The rate of these ARP requests is proportional to the size of the 407 subnets, the rate of scanning and backscatter, and how long the 408 router keeps state on non-responding ARPs. As it turns out, this 409 rate is inversely proportional to how occupied the subnet is (valid 410 ARPs end up in a cache, stopping the broadcasting; unused IPs never 411 respond, and so cause more broadcasts). Depending on the address 412 space in use, the time of day, how occupied the subnet is, and other 413 unknown factors, thousands of broadcasts per second have been 414 observed. Around 2,000 broadcasts per second have been observed at 415 the IETF NOC during face-to-face meetings. 417 With Neighbor Discovery for IPv6 [RFC2461], nodes accomplish address 418 resolution by multicasting a Neighbor Solicitation that asks the 419 target node to return its link-layer address. Neighbor Solicitation 420 messages are multicast to the solicited-node multicast address of the 421 target address. The target returns its link-layer address in a 422 unicast Neighbor Advertisement message. A single request-response 423 pair of packets is sufficient for both the initiator and the target 424 to resolve each other's link-layer addresses; the initiator includes 425 its link-layer address in the Neighbor Solicitation. 427 On a wired network, there is not a huge difference between unicast, 428 multicast and broadcast traffic. Due to hardware filtering (see, 429 e.g., [Deri-2010]), inadvertently flooded traffic (or excessive 430 ethernet multicast) on wired networks can be quite a bit less costly, 431 compared to wireless cases where sleeping devices have to wake up to 432 process packets. Wired Ethernets tend to be switched networks, 433 further reducing interference from multicast. There is effectively 434 no collision / scheduling problem except at extremely high port 435 utilizations. 437 This is not true in the wireless realm; wireless equipment is often 438 unable to send high volumes of broadcast and multicast traffic, 439 causing numerous broadcast and multicast packets to be dropped. 440 Consequently, when a host connects it is often not able to complete 441 DHCP, and IPv6 RAs get dropped, leading to users being unable to use 442 the network. 444 4. Multicast protocol optimizations 446 This section lists some optimizations that have been specified in 447 IEEE 802 and IETF that are aimed at reducing or eliminating the 448 issues discussed in Section 3. 450 4.1. Proxy ARP in 802.11-2012 452 The AP knows the MAC address and IP address for all associated STAs. 453 In this way, the AP acts as the central "manager" for all the 802.11 454 STAs in its BSS. Proxy ARP is easy to implement at the AP, and 455 offers the following advantages: 457 o Reduced broadcast traffic (transmitted at low MCS) on the wireless 458 medium 459 o STA benefits from extended power save in sleep mode, as ARP 460 requests for STA's IP address are handled instead by the AP. 461 o ARP frames are kept off the wireless medium. 462 o No changes are needed to STA implementation. 464 Here is the specification language as described in clause 10.23.13 of 465 [dot11-proxyarp]: 467 When the AP supports Proxy ARP "[...] the AP shall maintain a 468 Hardware Address to Internet Address mapping for each associated 469 station, and shall update the mapping when the Internet Address of 470 the associated station changes. When the IPv4 address being 471 resolved in the ARP request packet is used by a non-AP STA 472 currently associated to the BSS, the proxy ARP service shall 473 respond on behalf of the non-AP STA". 475 4.2. IPv6 Address Registration and Proxy Neighbor Discovery 477 As used in this section, a Low-Power Wireless Personal Area Network 478 (6LoWPAN) denotes a low power lossy network (LLN) that supports 479 6LoWPAN Header Compression (HC) [RFC6282]. A 6TiSCH network 480 [I-D.ietf-6tisch-architecture] is an example of a 6LowPAN. In order 481 to control the use of IPv6 multicast over 6LoWPANs, the 6LoWPAN 482 Neighbor Discovery (6LoWPAN ND) [RFC6775] standard defines an address 483 registration mechanism that relies on a central registry to assess 484 address uniqueness, as a substitute to the inefficient DAD mechanism 485 found in the mainstream IPv6 Neighbor Discovery Protocol (NDP) 486 [RFC4861][RFC4862]. 488 The 6lo Working Group has specified an update [RFC8505] to RFC6775. 489 Wireless devices can register their address to a Backbone Router 490 [I-D.ietf-6lo-backbone-router], which proxies for the registered 491 addresses with the IPv6 NDP running on a high speed aggregating 492 backbone. The update also enables a proxy registration mechanism on 493 behalf of the registered node, e.g. by a 6LoWPAN router to which the 494 mobile node is attached. 496 The general idea behind the backbone router concept is that broadcast 497 and multicast messaging should be tightly controlled in a variety of 498 Wireless Local Area Networks (WLANs) and Wireless Personal Area 499 Networks (WPANs). Connectivity to a particular link that provides 500 the subnet should be left to Layer-3. The model for the Backbone 501 Router operation is represented in Figure 1. 503 | 504 +-----+ 505 | | Gateway (default) router 506 | | 507 +-----+ 508 | 509 | Backbone Link 510 +--------------------+------------------+ 511 | | | 512 +-----+ +-----+ +-----+ 513 | | Backbone | | Backbone | | Backbone 514 | | router 1 | | router 2 | | router 3 515 +-----+ +-----+ +-----+ 516 o o o o o o 517 o o o o o o o o o o o o o o 518 o o o o o o o o o o o o o o o 519 o o o o o o o o o o 520 o o o o o o o 522 LLN 1 LLN 2 LLN 3 524 Figure 1: Backbone Link and Backbone Routers 526 LLN nodes can move freely from an LLN anchored at one IPv6 Backbone 527 Router to an LLN anchored at another Backbone Router on the same 528 backbone, keeping any of the IPv6 addresses they have configured. 529 The Backbone Routers maintain a Binding Table of their Registered 530 Nodes, which serves as a distributed database of all the LLN Nodes. 531 An extension to the Neighbor Discovery Protocol is introduced to 532 exchange Binding Table information across the Backbone Link as needed 533 for the operation of IPv6 Neighbor Discovery. 535 RFC6775 and follow-on work [RFC8505] address the needs of LLNs, and 536 similar techniques are likely to be valuable on any type of link 537 where sleeping devices are attached, or where the use of broadcast 538 and multicast operations should be limited. 540 4.3. Buffering to Improve Battery Life 542 Methods have been developed to help save battery life; for example, a 543 device might not wake up when the AP receives a multicast packet. 544 The AP acts on behalf of STAs in various ways. To enable use of the 545 power-saving feature for STAs in its BSS, the AP buffers frames for 546 delivery to the STA at the time when the STA is scheduled for 547 reception. If an AP, for instance, expresses a DTIM (Delivery 548 Traffic Indication Message) of 3 then the AP will send a multicast 549 packet every 3 packets. In fact, when any single wireless STA 550 associated with an access point has 802.11 power-save mode enabled, 551 the access point buffers all multicast frames and sends them only 552 after the next DTIM beacon. 554 In practice, most AP's will send a multicast every 30 packets. For 555 unicast the AP could send a TIM (Traffic Indication Message), but for 556 multicast the AP sends a broadcast to everyone. DTIM does power 557 management but STAs can choose whether or not to wake up and whether 558 or not to drop the packet. Unfortunately, without proper 559 administrative control, such STAs may be unable to determine why 560 their multicast operations do not work. 562 4.4. Limiting multicast buffer hardware queue depth 564 The CAB (Content after Beacon) queue is used for beacon-triggered 565 transmission of buffered multicast frames. If lots of multicast 566 frames were buffered, and this queue fills up, it drowns out all 567 regular traffic. To limit the damage that buffered traffic can do, 568 some drivers limit the amount of queued multicast data to a fraction 569 of the beacon_interval. An example of this is [CAB]. 571 4.5. IPv6 support in 802.11-2012 573 IPv6 uses NDP instead of ARP. Every IPv6 node subscribes to a 574 special multicast address for this purpose. 576 Here is the specification language from clause 10.23.13 of 577 [dot11-proxyarp]: 579 "When an IPv6 address is being resolved, the Proxy Neighbor 580 Discovery service shall respond with a Neighbor Advertisement 581 message [...] on behalf of an associated STA to an [ICMPv6] 582 Neighbor Solicitation message [...]. When MAC address mappings 583 change, the AP may send unsolicited Neighbor Advertisement 584 Messages on behalf of a STA." 586 NDP may be used to request additional information 588 o Maximum Transmission Unit 589 o Router Solicitation 590 o Router Advertisement, etc. 592 NDP messages are sent as group addressed (broadcast) frames in 593 802.11. Using the proxy operation helps to keep NDP messages off the 594 wireless medium. 596 4.6. Using Unicast Instead of Multicast 598 It is often possible to transmit multicast control and data messages 599 by using unicast transmissions to each station individually. 601 4.6.1. Overview 603 In many situations, it's a good choice to use unicast instead of 604 multicast over the Wi-Fi link. This avoids most of the problems 605 specific to multicast over Wi-Fi, since the individual frames are 606 then acknowledged and buffered for power save clients, in the way 607 that unicast traffic normally operates. 609 This approach comes with the tradeoff of sometimes sending the same 610 packet multiple times over the Wi-Fi link. However, in many cases, 611 such as video into a residential home network, this can be a good 612 tradeoff, since the Wi-Fi link may have enough capacity for the 613 unicast traffic to be transmitted to each subscribed STA, even though 614 multicast addressing may have been necessary for the upstream access 615 network. 617 Several technologies exist that can be used to arrange unicast 618 transport over the Wi-Fi link, outlined in the subsections below. 620 4.6.2. Layer 2 Conversion to Unicast 622 It is often possible to transmit multicast control and data messages 623 by using unicast transmissions to each station individually. 625 Although there is not yet a standardized method of conversion, at 626 least one widely available implementation exists in the Linux 627 bridging code [bridge-mc-2-uc]. Other proprietary implementations 628 are available from various vendors. In general, these 629 implementations perform a straightforward mapping for groups or 630 channels, discovered by IGMP or MLD snooping, to the corresponding 631 unicast MAC addresses. 633 4.6.3. Directed Multicast Service (DMS) 635 There are situations where more is needed than simply converting 636 multicast to unicast. For these purposes, DMS enables an STA to 637 request that the AP transmit multicast group addressed frames 638 destined to the requesting STAs as individually addressed frames 639 [i.e., convert multicast to unicast]. Here are some characteristics 640 of DMS: 642 o Requires 802.11n A-MSDUs 643 o Individually addressed frames are acknowledged and are buffered 644 for power save STAs 645 o The requesting STA may specify traffic characteristics for DMS 646 traffic 647 o DMS was defined in IEEE Std 802.11v-2011 648 o DMS requires changes to both AP and STA implementation. 650 DMS is not currently implemented in products. See [Tramarin2017] and 651 [Oliva2013] for more information. 653 4.6.4. Automatic Multicast Tunneling (AMT) 655 AMT[RFC7450] provides a method to tunnel multicast IP packets inside 656 unicast IP packets over network links that only support unicast. 657 When an operating system or application running on an STA has an AMT 658 gateway capability integrated, it's possible to use unicast to 659 traverse the Wi-Fi link by deploying an AMT relay in the non-Wi-Fi 660 portion of the network connected to the AP. 662 It is recommended that multicast-enabled networks deploying AMT 663 relays for this purpose make the relays locally discoverable with the 664 following methods, as described in 665 [I-D.ietf-mboned-driad-amt-discovery]: 667 o DNS-SD [RFC6763] 668 o the well-known IP addresses from Section 7 of [RFC7450] 670 An AMT gateway that implements multiple standard discovery methods is 671 more likely to discover the local multicast-capable network, instead 672 of forming a connection to a non-local AMT relay further upstream. 674 4.7. GroupCast with Retries (GCR) 676 GCR (defined in [dot11aa]) provides greater reliability by using 677 either unsolicited retries or a block acknowledgement mechanism. GCR 678 increases probability of broadcast frame reception success, but still 679 does not guarantee success. 681 For the block acknowledgement mechanism, the AP transmits each group 682 addressed frame as conventional group addressed transmission. 683 Retransmissions are group addressed, but hidden from non-11aa STAs. 684 A directed block acknowledgement scheme is used to harvest reception 685 status from receivers; retransmissions are based upon these 686 responses. 688 GCR is suitable for all group sizes including medium to large groups. 689 As the number of devices in the group increases, GCR can send block 690 acknowledgement requests to only a small subset of the group. GCR 691 does require changes to both AP and STA implementations. 693 GCR may introduce unacceptable latency. After sending a group of 694 data frames to the group, the AP has do the following: 696 o unicast a Block Ack Request (BAR) to a subset of members. 697 o wait for the corresponding Block Ack (BA). 698 o retransmit any missed frames. 699 o resume other operations that may have been delayed. 701 This latency may not be acceptable for some traffic. 703 There are ongoing extensions in 802.11 to improve GCR performance. 705 o BAR is sent using downlink MU-MIMO (note that downlink MU-MIMO is 706 already specified in 802.11-REVmc 4.3). 707 o BA is sent using uplink MU-MIMO (which is a .11ax feature). 708 o Additional 802.11ax extensions are under consideration; see 709 [mc-ack-mux] 710 o Latency may also be reduced by simultaneously receiving BA 711 information from multiple STAs. 713 5. Operational optimizations 715 This section lists some operational optimizations that can be 716 implemented when deploying wireless IEEE 802 networks to mitigate the 717 issues discussed in Section 3. 719 5.1. Mitigating Problems from Spurious Neighbor Discovery 721 ARP Sponges 723 An ARP Sponge sits on a network and learn which IP addresses 724 are actually in use. It also listen for ARP requests, and, if 725 it sees an ARP for an IP address that it believes is not used, 726 it will reply with its own MAC address. This means that the 727 router now has an IP to MAC mapping, which it caches. If that 728 IP is later assigned to an machine (e.g using DHCP), the ARP 729 sponge will see this, and will stop replying for that address. 730 Gratuitous ARPs (or the machine ARPing for its gateway) will 731 replace the sponged address in the router ARP table. This 732 technique is quite effective; but, unfortunately, the ARP 733 sponge daemons were not really designed for this use (one of 734 the most widely deployed arp sponges [arpsponge], was designed 735 to deal with the disappearance of participants from an IXP) and 736 so are not optimized for this purpose. One daemon is needed 737 per subnet, the tuning is tricky (the scanning rate versus the 738 population rate versus retires, etc.) and sometimes the daemons 739 just seem to stop, requiring a restart of the daemon and 740 causing disruption. 742 Router mitigations 744 Some routers (often those based on Linux) implement a "negative 745 ARP cache" daemon. Simply put, if the router does not see a 746 reply to an ARP it can be configured to cache this information 747 for some interval. Unfortunately, the core routers in use 748 often do not support this. When a host connects to a network 749 and gets an IP address, it will ARP for its default gateway 750 (the router). The router will update its cache with the IP to 751 host MAC mapping learned from the request (passive ARP 752 learning). 754 Firewall unused space 756 The distribution of users on wireless networks / subnets 757 changes from one IETF meeting to the next (e.g SSIDs are 758 renamed, some SSIDs lose favor, etc). This makes utilization 759 for particular SSIDs difficult to predict ahead of time, but 760 usage can be monitored as attendees use the different networks. 761 Configuring multiple DHCP pools per subnet, and enabling them 762 sequentially, can create a large subnet, from which only 763 addresses in the lower portions are assigned. Therefore input 764 IP access lists can be applied, which deny traffic to the 765 upper, unused portions. Then the router does not attempt to 766 forward packets to the unused portions of the subnets, and so 767 does not ARP for it. This method has proven to be very 768 effective, but is somewhat of a blunt axe, is fairly labor 769 intensive, and requires coordination. 771 Disabling/filtering ARP requests 773 In general, the router does not need to ARP for hosts; when a 774 host connects, the router can learn the IP to MAC mapping from 775 the ARP request sent by that host. Consequently it should be 776 possible to disable and / or filter ARP requests from the 777 router. Unfortunately, ARP is a very low level / fundamental 778 part of the IP stack, and is often offloaded from the normal 779 control plane. While many routers can filter layer-2 traffic, 780 this is usually implemented as an input filter and / or has 781 limited ability to filter output broadcast traffic. This means 782 that the simple "just disable ARP or filter it outbound" seems 783 like a really simple (and obvious) solution, but 784 implementations / architectural issues make this difficult or 785 awkward in practice. 787 NAT 789 The broadcasts are overwhelmingly being caused by outside 790 scanning / backscatter traffic. To NAT the entire (or a large 791 portion) of the attendee networks would eliminate NAT 792 translation entries for unused addresses, and so the router 793 would never ARP for them. However, there are many reasons to 794 avoid using NAT in such a blanket fashion. 796 Stateful firewalls 798 Another obvious solution would be to put a stateful firewall 799 between the wireless network and the Internet. This firewall 800 would block incoming traffic not associated with an outbound 801 request. But this conflicts with the need and desire of the 802 IETF and other organizations to have the network as open as 803 possible and to honor the end-to-end principle. An attendee on 804 the meeting network should be an Internet host, and should be 805 able to receive unsolicited requests. Unfortunately, keeping 806 the network working and stable is the first priority and a 807 stateful firewall may be required in order to achieve this. 809 5.2. Mitigating Spurious Service Discovery Messages 811 In networks that must support hundreds of STAs, operators have 812 observed network degradation due to many devices simultaneously 813 registering with mDNS. In a network with many clients, it is 814 recommended to ensure that mDNS packets designed to discover 815 services in smaller home networks be constrained to avoid 816 disrupting other traffic. 818 6. Multicast Considerations for Other Wireless Media 820 Many of the causes of performance degradation described in earlier 821 sections are also observable for wireless media other than 802.11. 823 For instance, problems with power save, excess media occupancy, and 824 poor reliability will also affect 802.15.3 and 802.15.4. 825 Unfortunately, 802.15 media specifications do not yet include 826 mechanisms similar to those developed for 802.11. In fact, the 827 design philosophy for 802.15 is oriented towards minimality, with the 828 result that many such functions are relegated to operation within 829 higher layer protocols. This leads to a patchwork of non- 830 interoperable and vendor-specific solutions. See [uli] for some 831 additional discussion, and a proposal for a task group to resolve 832 similar issues, in which the multicast problems might be considered 833 for mitigation. 835 Similar considerations hold for most other wireless media. A brief 836 introduction is provided in [RFC5757] for the following: 838 o 802.16 WIMAX 839 o 3GPP/3GPP2 840 o DVB-H / DVB-IPDC 841 o TV Broadcast and Satellite Networks 843 7. Recommendations 845 This section will provide some recommendations about the usage and 846 combinations of the multicast enhancements described in Section 4 and 847 Section 5. 849 Future protocol documents utilizing multicast signaling should be 850 carefully scrutinized if the protocol is likely to be used over 851 wireless media. 853 Proxy methods should be encouraged to conserve network bandwidth and 854 power utilization by low-power devices. The device can use a unicast 855 message to its proxy, and then the proxy can take care of any needed 856 multicast operations. 858 Multicast signaling for wireless devices should be done in a way 859 compatible with low duty-cycle operation. 861 8. Discussion Items 863 This section suggests two discussion items for further resolution. 865 The IETF should determine guidelines by which it may be decided that 866 multicast packets are to be sent wired. For example, 802.1ak works 867 on ethernet and Wi-Fi. 802.1ak has been pulled into 802.1Q as of 868 802.1Q-2011. 802.1Q-2014 can be found here: 869 http://www.ieee802.org/1/pages/802.1Q-2014.html. If a generic 870 solution is not found, guidelines for multicast over Wi-Fi should be 871 established. 873 Reliable registration to Layer-2 multicast groups and a reliable 874 multicast operation at Layer-2 might provide a generic solution. 875 There is no need to support 2^24 groups to get solicited node 876 multicast working: it is possible to simply select a number of 877 trailing bits that make sense for a given network size to limit the 878 number of unwanted deliveries to reasonable levels. IEEE 802.1, 879 802.11, and 802.15 should be encouraged to revisit L2 multicast 880 issues. In reality, Wi-Fi provides a broadcast service, not a 881 multicast service. On the physical medium, all frames are broadcast 882 except in very unusual cases in which special beamforming 883 transmitters are used. Unicast offers the advantage of being much 884 faster (2 orders of magnitude) and much more reliable (L2 ARQ). 886 9. Security Considerations 888 This document does not introduce or modify any security mechanisms. 890 As noted in [group_key], the unreliable nature of multicast 891 transmission over wireless media can cause subtle problems with 892 multicast group key management and updates. Quoting from that 893 website, "... most clients are able to get connected and surf the 894 web, check email, etc. even when From DS multicasts are broken. So a 895 lot of people don't realize they have multicast problems on their 896 network..." 898 10. IANA Considerations 900 This document does not request any IANA actions. 902 11. Acknowledgements 904 This document has benefitted from discussions with the following 905 people, in alphabetical order: Mikael Abrahamsson, Bill Atwood, 906 Stuart Cheshire, Donald Eastlake, Toerless Eckert, Jake Holland, Joel 907 Jaeggli, Jan Komissar, David Lamparter, Morten Pedersen, Pascal 908 Thubert, Jeffrey (Zhaohui) Zhang 910 12. Informative References 912 [arpsponge] 913 Wessel, M. and N. Sijm, "Effects of IPv4 and IPv6 address 914 resolution on AMS-IX and the ARP Sponge", July 2009, 915 . 918 [bridge-mc-2-uc] 919 Fietkau, F., "bridge: multicast to unicast", Jan 2017, 920 . 923 [CAB] Fietkau, F., "Limit multicast buffer hardware queue 924 depth", 2013, 925 . 927 [Deri-2010] 928 Deri, L. and J. Gasparakis, "10 Gbit Hardware Packet 929 Filtering Using Commodity Network Adapters", RIPE 61, 930 2010, . 933 [dot11] "IEEE 802 Wireless", "802.11-2016 - IEEE Standard for 934 Information technology--Telecommunications and information 935 exchange between systems Local and metropolitan area 936 networks--Specific requirements - Part 11: Wireless LAN 937 Medium Access Control (MAC) and Physical Layer (PHY) 938 Specification (includes 802.11v amendment)", March 2016, 939 . 942 [dot11-proxyarp] 943 Hiertz, G., Mestanov, F., and B. Hart, "Proxy ARP in 944 802.11ax", September 2015, 945 . 948 [dot11aa] "IEEE 802 Wireless", "Part 11: Wireless LAN Medium Access 949 Control (MAC) and Physical Layer (PHY) Specifications 950 Amendment 2: MAC Enhancements for Robust Audio Video 951 Streaming", March 2012, 952 . 955 [group_key] 956 Spiff, ""Why do some WiFi routers block multicast packets 957 going from wired to wireless?"", Jan 2017, 958 . 962 [I-D.ietf-6lo-backbone-router] 963 Thubert, P., Perkins, C., and E. Levy-Abegnoli, "IPv6 964 Backbone Router", draft-ietf-6lo-backbone-router-13 (work 965 in progress), September 2019. 967 [I-D.ietf-6tisch-architecture] 968 Thubert, P., "An Architecture for IPv6 over the TSCH mode 969 of IEEE 802.15.4", draft-ietf-6tisch-architecture-26 (work 970 in progress), August 2019. 972 [I-D.ietf-mboned-driad-amt-discovery] 973 Holland, J., "DNS Reverse IP AMT Discovery", draft-ietf- 974 mboned-driad-amt-discovery-08 (work in progress), June 975 2019. 977 [ietf_802-11] 978 Stanley, D., "IEEE 802.11 multicast capabilities", Nov 979 2015, . 983 [mc-ack-mux] 984 Tanaka, Y., Sakai, E., Morioka, Y., Mori, M., Hiertz, G., 985 and S. Coffey, "Multiplexing of Acknowledgements for 986 Multicast Transmission", July 2015, 987 . 991 [mc-prob-stmt] 992 Abrahamsson, M. and A. Stephens, "Multicast on 802.11", 993 March 2015, . 996 [mc-props] 997 Stephens, A., "IEEE 802.11 multicast properties", March 998 2015, . 1002 [Oliva2013] 1003 de la Oliva, A., Serrano, P., Salvador, P., and A. Banchs, 1004 "Performance evaluation of the IEEE 802.11aa multicast 1005 mechanisms for video streaming", 2013 IEEE 14th 1006 International Symposium on "A World of Wireless, Mobile 1007 and Multimedia Networks" (WoWMoM) pp. 1-9, June 2013. 1009 [RFC2461] Narten, T., Nordmark, E., and W. Simpson, "Neighbor 1010 Discovery for IP Version 6 (IPv6)", RFC 2461, 1011 DOI 10.17487/RFC2461, December 1998, 1012 . 1014 [RFC4541] Christensen, M., Kimball, K., and F. Solensky, 1015 "Considerations for Internet Group Management Protocol 1016 (IGMP) and Multicast Listener Discovery (MLD) Snooping 1017 Switches", RFC 4541, DOI 10.17487/RFC4541, May 2006, 1018 . 1020 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 1021 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 1022 DOI 10.17487/RFC4861, September 2007, 1023 . 1025 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless 1026 Address Autoconfiguration", RFC 4862, 1027 DOI 10.17487/RFC4862, September 2007, 1028 . 1030 [RFC5757] Schmidt, T., Waehlisch, M., and G. Fairhurst, "Multicast 1031 Mobility in Mobile IP Version 6 (MIPv6): Problem Statement 1032 and Brief Survey", RFC 5757, DOI 10.17487/RFC5757, 1033 February 2010, . 1035 [RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6 1036 Datagrams over IEEE 802.15.4-Based Networks", RFC 6282, 1037 DOI 10.17487/RFC6282, September 2011, 1038 . 1040 [RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762, 1041 DOI 10.17487/RFC6762, February 2013, 1042 . 1044 [RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service 1045 Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013, 1046 . 1048 [RFC6775] Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C. 1049 Bormann, "Neighbor Discovery Optimization for IPv6 over 1050 Low-Power Wireless Personal Area Networks (6LoWPANs)", 1051 RFC 6775, DOI 10.17487/RFC6775, November 2012, 1052 . 1054 [RFC6970] Boucadair, M., Penno, R., and D. Wing, "Universal Plug and 1055 Play (UPnP) Internet Gateway Device - Port Control 1056 Protocol Interworking Function (IGD-PCP IWF)", RFC 6970, 1057 DOI 10.17487/RFC6970, July 2013, 1058 . 1060 [RFC7450] Bumgardner, G., "Automatic Multicast Tunneling", RFC 7450, 1061 DOI 10.17487/RFC7450, February 2015, 1062 . 1064 [RFC8505] Thubert, P., Ed., Nordmark, E., Chakrabarti, S., and C. 1065 Perkins, "Registration Extensions for IPv6 over Low-Power 1066 Wireless Personal Area Network (6LoWPAN) Neighbor 1067 Discovery", RFC 8505, DOI 10.17487/RFC8505, November 2018, 1068 . 1070 [Tramarin2017] 1071 Tramarin, F., Vitturi, S., and M. Luvisotto, "IEEE 802.11n 1072 for Distributed Measurement Systems", 2017 IEEE 1073 International Instrumentation and Measurement Technology 1074 Conference (I2MTC) pp. 1-6, May 2017. 1076 [uli] Kinney, P., "LLC Proposal for 802.15.4", Nov 2015, 1077 . 1080 Appendix A. Changes in this draft between revisions 06 versus 07 1082 This section lists the changes between revisions ...-06.txt and 1083 ...-07.txt of draft-ietf-mboned-ieee802-mcast-problems. 1085 o Improved wording in section describing ARPsponge. 1086 o Removed DRIAD as a discovery mechanism for multicast relays. 1087 o Updated bibliographic citations, repaired broken URLs as needed. 1088 o More editorial improvements and grammatical corrections. 1090 Appendix B. Changes in this draft between revisions 05 versus 06 1092 This section lists the changes between revisions ...-05.txt and 1093 ...-06.txt of draft-ietf-mboned-ieee802-mcast-problems. 1095 o Included new text in Security Considerations to alert about 1096 problems regarding Group Key management caused by multicast 1097 unreliability and implementation bugs. 1098 o Included DRIAD as a discovery mechanism for multicast relays. 1099 o Corrected occurrences of "which" versus "that" and "amount" versus 1100 "number". 1101 o Updated bibliographic citations, included URLs as needed. 1102 o More editorial improvements and grammatical corrections. 1104 Appendix C. Changes in this draft between revisions 04 versus 05 1106 This section lists the changes between revisions ...-04.txt and 1107 ...-05.txt of draft-ietf-mboned-ieee802-mcast-problems. 1109 o Incorporated text from Jake Holland for a new section about 1110 conversion of multicast to unicast and included AMT as an existing 1111 solution. 1113 o Included some text about likely future multicast applications that 1114 will emphasize the need for attention to the technical matters 1115 collected in this document. 1116 o Further modified text to be more generic instead of referring 1117 specifically to IETF conference situations. 1118 o Modified text to be more generic instead of referring specifically 1119 to Bonjour. 1120 o Added uPnP as a representative multicast protocol in IP networks. 1121 o Referred to Linux bridging code for multicast to unicast. 1122 o Updated bibliographic citations, included URLs as needed. 1123 o More editorial improvements and grammatical corrections. 1125 Appendix D. Changes in this draft between revisions 03 versus 04 1127 This section lists the changes between revisions ...-03.txt and 1128 ...-04.txt of draft-ietf-mboned-ieee802-mcast-problems. 1130 o Replaced "client" by "STA". 1131 o Used terminology "Wi-Fi" throughout. 1132 o Many editorial improvements and grammatical corrections. 1133 o Modified text to be more generic instead of referring specifically 1134 to IETF conference situations. 1135 o Cited [RFC5757] for introduction to other wireless media. 1136 o Updated bibliographic citations. 1138 Authors' Addresses 1140 Charles E. Perkins 1142 Phone: +1-408-330-4586 1143 Email: charliep@computer.org 1145 Mike McBride 1146 Futurewei Inc. 1147 2330 Central Expressway 1148 Santa Clara, CA 95055 1149 USA 1151 Email: michael.mcbride@futurewei.com 1152 Dorothy Stanley 1153 Hewlett Packard Enterprise 1154 2000 North Naperville Rd. 1155 Naperville, IL 60566 1156 USA 1158 Phone: +1 630 979 1572 1159 Email: dstanley@arubanetworks.com 1161 Warren Kumari 1162 Google 1163 1600 Amphitheatre Parkway 1164 Mountain View, CA 94043 1165 USA 1167 Email: warren@kumari.net 1169 Juan Carlos Zuniga 1170 SIGFOX 1171 425 rue Jean Rostand 1172 Labege 31670 1173 France 1175 Email: j.c.zuniga@ieee.org