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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 RAW P. Thubert, Ed. 3 Internet-Draft Cisco Systems 4 Intended status: Informational G.Z. Papadopoulos 5 Expires: 10 April 2020 IMT Atlantique 6 8 October 2019 8 Reliable and Available Wireless Problem Statement 9 draft-pthubert-raw-problem-statement-03 11 Abstract 13 Due to uncontrolled interferences, including the self-induced 14 multipath fading, deterministic networking can only be approached on 15 wireless links. The radio conditions may change -way- faster than a 16 centralized routing can adapt and reprogram, in particular when the 17 controller is distant and connectivity is slow and limited. RAW 18 separates the routing time scale at which a complex path is 19 recomputed from the forwarding time scale at which the forwarding 20 decision is taken for an individual packet. RAW operates at the 21 forwarded time scale. The RAW problem is to decide, within the 22 redundant solutions that are proposed by the routing, which will be 23 used for each individual packet to provide a DetNet service while 24 minimizing the waste of resources. 26 Status of This Memo 28 This Internet-Draft is submitted in full conformance with the 29 provisions of BCP 78 and BCP 79. 31 Internet-Drafts are working documents of the Internet Engineering 32 Task Force (IETF). Note that other groups may also distribute 33 working documents as Internet-Drafts. The list of current Internet- 34 Drafts is at https://datatracker.ietf.org/drafts/current/. 36 Internet-Drafts are draft documents valid for a maximum of six months 37 and may be updated, replaced, or obsoleted by other documents at any 38 time. It is inappropriate to use Internet-Drafts as reference 39 material or to cite them other than as "work in progress." 41 This Internet-Draft will expire on 10 April 2020. 43 Copyright Notice 45 Copyright (c) 2019 IETF Trust and the persons identified as the 46 document authors. All rights reserved. 48 This document is subject to BCP 78 and the IETF Trust's Legal 49 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 50 license-info) in effect on the date of publication of this document. 51 Please review these documents carefully, as they describe your rights 52 and restrictions with respect to this document. Code Components 53 extracted from this document must include Simplified BSD License text 54 as described in Section 4.e of the Trust Legal Provisions and are 55 provided without warranty as described in the Simplified BSD License. 57 Table of Contents 59 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 60 2. Use Cases and Requirements Served . . . . . . . . . . . . . . 4 61 3. Routing Scale vs. Forwarding Scale . . . . . . . . . . . . . 4 62 4. Prerequisites . . . . . . . . . . . . . . . . . . . . . . . . 5 63 5. Related Work at The IETF . . . . . . . . . . . . . . . . . . 5 64 6. Functional Gaps . . . . . . . . . . . . . . . . . . . . . . . 6 65 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 66 7.1. Normative References . . . . . . . . . . . . . . . . . . 7 67 7.2. Informative References . . . . . . . . . . . . . . . . . 8 68 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 70 1. Introduction 72 Bringing determinism in a packet network means eliminating the 73 statistical effects of multiplexing that result in probabilistic 74 jitter and loss. This can be approached with a tight control of the 75 physical resources to maintain the amount of traffic within a 76 budgetted volume of data per unit of time that fits the physical 77 capabilities of the underlying technology, and the use of time-shared 78 resources (bandwidth and buffers) per circuit, and/or by shaping and/ 79 or scheduling the packets at every hop. 81 Wireless networks operate on a shared medium where uncontrolled 82 interference, including the self-induced multipath fading, adds 83 another dimension to the statistical effects that affect the 84 delivery. Scheduling transmissions can alleviate those effects by 85 leveraging diversity in the spatial, time, code, and frequency 86 domains, and provide a Reliable and Available service while 87 preserving energy and optimizing the use of the shared spectrum. 89 Deterministic Networking is an attempt to mostly eliminate packet 90 loss for a committed bandwidth with a guaranteed worst-case end-to- 91 end latency, even when co-existing with best-effort traffic in a 92 shared network. This innovation is enabled by recent developments in 93 technologies including IEEE 802.1 TSN (for Ethernet LANs) and IETF 94 DetNet (for wired IP networks). It is getting traction in various 95 industries including manufacturing, online gaming, professional A/V, 96 cellular radio and others, making possible many cost and performance 97 optimizations. 99 Reliable and Available Wireless (RAW) networking services extend 100 DetNet to approach end-to-end deterministic performances in a network 101 with scheduled wireless segments, possibly combined with wired 102 segments, and possibly sharing physical resources with non- 103 deterministic traffic. The wireless and wired media are 104 fundamentally different at the physical level, and while the generic 105 Problem Statement for DetNet applies to the wired as well as the 106 wireless medium, the methods to achieve RAW will differ from those 107 used to support time-sensitive networking over wires, as a RAW 108 solution will need to address less consistent transmissions, energy 109 conservation and shared spectrum efficiency. 111 The development of RAW technologies has been lagging behind 112 deterministic efforts for wired systems both at the IEEE and the 113 IETF. But recent efforts at the IEEE and 3GPP indicate that wireless 114 is finally catching up at the lower layer and that it is now possible 115 for the IETF to extend DetNet for wireless segments that are capable 116 of scheduled wireless transmissions. 118 The intent for RAW is to provide DetNet elements that are specialized 119 for short range radios. From this inheritance, RAW stays agnostic to 120 the radio layer underneath though the capability to schedule 121 transmissions is assumed. How the PHY is programmed to do so, and 122 whether the radio is single-hop or meshed, are unknown at the IP 123 layer and not part of the RAW abstraction. 125 Still, in order to focus on real-worlds issues and assert the 126 feasibility of the proposed capabilities, RAW will focus on selected 127 technologies that can be scheduled at the lower layers: IEEE Std. 128 802.15.4 timeslotted channel hopping (TSCH), 3GPP 5G ultra-reliable 129 low latency communications (URLLC), IEEE 802.11ax/be where 802.11be 130 is extreme high throughput (EHT), and L-band Digital Aeronautical 131 Communications System (LDACS). See [RAW-TECHNOS] for more. 133 The establishment of a path is not in-scope for RAW. It may be the 134 product of a centralized Controller Plane as described for DetNet. 135 As opposed to wired networks, the action of installing a path over a 136 set of wireless links may be very slow relative to the speed at which 137 the radio conditions vary, and it makes sense in the wireless case to 138 provide redundant forwarding solutions along a complex path and to 139 leave it to the RAW Network Plane to select which of those forwarding 140 solutions are to be used for a given packet based on the current 141 conditions. 143 RAW distinguishes the longer time scale at which routes are computed 144 from the the shorter forwarding time scale where per-packet decisions 145 are made. RAW operates at the forwarding time scale on one DetNet 146 flow over one path that is preestablished and installed by means 147 outside of the scope of RAW. The scope of the RAW WG comprises 148 Network plane protocol elements such as OAM and in-band control to 149 improve the RAW operation at the Service and at the forwarding sub- 150 layers, e.g., controlling whether to use packet replication, Hybrid 151 ARQ and coding, with a constraint to limit the use of redundancy when 152 it is really needed, e.g., when a spike of loss is observed. This is 153 discussed in more details in Section 3 and the next sections. 155 2. Use Cases and Requirements Served 157 [RFC8578] presents a number of wireless use cases including Wireless 158 for Industrial Applications. [RAW-USE-CASES] adds a number of use 159 cases that demonstrate the need for RAW capabilities in Pro-Audio, 160 gaming and robotics. 162 3. Routing Scale vs. Forwarding Scale 164 RAW extends DetNet to focus on issues that are mostly a concern on 165 wireless links. See [DetNet-ARCH] for more on DetNet. With DetNet, 166 the end-to-end routing can be centralized and can reside outside the 167 network. In wireless, and in particular in a wireless mesh, the path 168 to the controller that performs the route computation and maintenance 169 may be slow and expensive in terms of critical resources such as air 170 time and energy. 172 Reaching to the routing computation can be slow in regards to the 173 speed of events that affect the forwarding operation at the radio 174 layer. Due to the cost and latency to perform a route computation, 175 routing is not expected to be sensitive/reactive to transient 176 changes. The abstraction of a link at the routing level is expected 177 to use statistical operational metrics that aggregate the behavior of 178 a link over long periods of time, and represent its availability as a 179 shade of gray as opposed to either up or down. 181 In the case of wireless, the changes that affect the forwarding 182 decision can happen frequently and often for shot durations, e.g., a 183 mobile object moves between a transmitter and a receiver, and will 184 cancel the line of sight transmission for a few seconds, or a radar 185 measures the depth of a pool and interferes on a particular channel 186 for a split second. 188 There is thus a desire to separate the long term computation of the 189 route and the short term forwarding decision. In such a model, the 190 routing operation computes a complex Track that enables multiple non- 191 equal cost multipath (N-ECMP) forwarding solutions, and leaves it to 192 the forwarding plane to make the per-packet decision of which of 193 these possibilities should be used. 195 In the case of wires, the concept is known in traffic engineering 196 where an alternate path can be used upon the detection of a failure 197 in the main path, e.g., using OAM in MPLS-TP or BFD over a collection 198 of SD-WAN tunnels. RAW formalizes a routing time scale that is order 199 of magnitude longer than the forwarding time scale, and separates the 200 protocols and metrics that are used at both scales. Routing can 201 operate on long term statistics such as delivery ratio over minutes 202 to hours, but as a first approximation can ignore flapping. On the 203 other hand, the RAW forwarding decision is made at packet speed, and 204 uses information that must be pertinent at the present time for the 205 current transmission. 207 4. Prerequisites 209 A prerequisite to the RAW work is that an end-to-end routing function 210 computes a complex sub-topology along which forwarding can happen 211 between a source and one or more destinations. For 6TiSCH, this is a 212 Track. The concept of Track is specified in the 6TiSCH Architecture 213 [6TiSCH-ARCH]. Tracks provide a high degree of redundancy and 214 diversity and enable DetNet PREOF, end-to-end network coding, and 215 possibly radio-specific abstracted techniques such as ARQ, 216 overhearing, frequency diversity, time slotting, and possibly others. 218 How the routing operation computes the Track is out of scope for RAW. 219 The scope of the RAW operation is one Track, and the goal of the RAW 220 operation is to optimize the use of the Track at the forwarding 221 timescale to maintain the expected service while optimizing the usage 222 of constrained resources such as energy and spectrum. 224 Another prerequisite is that an IP link can be established over the 225 radio with some guarantees in terms of service reliability, e.g., it 226 can be relied upon to transmit a packet within a bounded latency and 227 provides a guaranteed BER/PDR outside rare but existing transient 228 outage windows that can last from split seconds to minutes. The 229 radio layer can be programmed with abstract parameters, and can 230 return an abstract view of the state of the Link to help forwarding 231 decision (think DLEP from MANET). In the layered approach, how the 232 radio manages its PHY layer is out of control and out of scope. 233 Whether it is single hop or meshed is also unknown and out of scope. 235 5. Related Work at The IETF 237 RAW intersects with protocols or practices in development at the IETF 238 as follows: 240 * The Dynamic Link Exchange Protocol (DLEP) [RFC8175] from [MANET] 241 can be leveraged at each hop to derive generic radio metrics 242 (e.g., based on LQI, RSSI, queueing delays and ETX) on individual 243 hops 245 * Operations, Administration and Maintenance (OAM) work at [DetNet] 246 such as [DetNet-IP-OAM] for the case of the IP Data Plane observes 247 the state of DetNet paths, typically MPLS and IPv6 pseudowires 248 [DetNet-DP-FW], in the direction of the traffic. RAW needs 249 feedback that flows on the reverse path and gathers instantaneous 250 values from the radio receivers at each hop to inform back the 251 source and replicating relays so they can make optimized 252 forwarding decisions. The work named ICAN may be related as well. 254 * [BFD] detect faults in the path between an ingress and an egress 255 forwarding engines, but is unaware of the complexity of a path 256 with replication, and expects bidirectionality. BFD considers 257 delivery as success whereas with RAW the bounded latency can be as 258 important as the delivery itself. 260 * [SPRING] and [BIER] define in-band signaling that influences the 261 routing when decided at the head-end on the path. There's already 262 one RAW-related draft at BIER [BIER-PREF] more may follow. RAW 263 will need new in-band signaling when the decision is distributed, 264 e.g., required chances of reliable delivery to destination within 265 latency. This signaling enables relays to tune retries and 266 replication to be met. 268 * [CCAMP] defines protocol-independent metrics and parameters 269 (measurement attributes) for describing links and paths that are 270 required for routing and signaling in technology-specific 271 networks. RAW would be a source of requirements for CCAMP to 272 define metrics that are significant to the focus radios. 274 6. Functional Gaps 276 Within a large routed topology, the routing operation builds a 277 particular complex Track with one source and one or more 278 destinations; within the Track, packets may follows different paths 279 and may be subject to RAW forwarding operations that include 280 replication, elimination, retries, overhearing and reordering. 282 The RAW forwarding decisions include the selection of points of 283 replication and elimination, how many retries can take place, and a 284 limit of validity for the packet beyond which the packet should be 285 destroyed rather than forwarded uselessly further down the Track. 287 The decision to apply the RAW techniques must be done quickly, and 288 depends on a very recent and precise knowledge of the forwarding 289 conditions within the complex Track. There is a need for an 290 observation method to provide the RAW forwarding plane with the 291 specific knowledge of the state of the Track for the type of flow of 292 interest (e.g., for a QoS level of interest). To observe the whole 293 Track in quasi real time, RAW will consider existing tools such as 294 L2-triggers, DLEP, BFD and in-band and out-of-band OAM. 296 One possible way of making the RAW forwarding decisions is to make 297 them all at the ingress and express them in-band in the packet, which 298 requires new loose or strict Hop-by-hop signaling. To control the 299 RAW forwarding operation along a Track for the individual packets, 300 RAW may leverage and extend known techniques such as DetNet tagging, 301 Segment Routing (SRv6) or BIER-TE such as done with [BIER-PREF]. 303 An alternate way is to enable each forwarding node to make the RAW 304 forwarding decisions for a packet on its own, based on its knowledge 305 of the expectation (timeliness and reliability) for that packet and a 306 recent observation of the rest of the way across the possible paths 307 within the Track. Information about the service should be placed in 308 the packet and matched with the forwarding node's capabilities and 309 policies. 311 In either case, a per-flow state is installed in all intermediate 312 nodes to recognize the flow and determine the forwarding policy to be 313 applied. 315 7. References 317 7.1. Normative References 319 [6TiSCH-ARCH] 320 Thubert, P., "An Architecture for IPv6 over the TSCH mode 321 of IEEE 802.15.4", Work in Progress, Internet-Draft, 322 draft-ietf-6tisch-architecture-26, 27 August 2019, 323 . 326 [DetNet-ARCH] 327 Finn, N., Thubert, P., Varga, B., and J. Farkas, 328 "Deterministic Networking Architecture", Work in Progress, 329 Internet-Draft, draft-ietf-detnet-architecture-13, 6 May 330 2019, . 333 [RAW-TECHNOS] 334 Thubert, P., Cavalcanti, D., Vilajosana, X., and C. 336 Schmitt, "Reliable and Available Wireless Technologies", 337 Work in Progress, Internet-Draft, draft-thubert-raw- 338 technologies-03, 1 July 2019, 339 . 342 [RAW-USE-CASES] 343 Papadopoulos, G., Thubert, P., Theoleyre, F., and C. 344 Bernardos, "RAW use cases", Work in Progress, Internet- 345 Draft, draft-bernardos-raw-use-cases-00, 5 July 2019, 346 . 349 [RFC8175] Ratliff, S., Jury, S., Satterwhite, D., Taylor, R., and B. 350 Berry, "Dynamic Link Exchange Protocol (DLEP)", RFC 8175, 351 DOI 10.17487/RFC8175, June 2017, 352 . 354 [RFC8578] Grossman, E., Ed., "Deterministic Networking Use Cases", 355 RFC 8578, DOI 10.17487/RFC8578, May 2019, 356 . 358 7.2. Informative References 360 [BFD] IETF, "Bidirectional Forwarding Detection", October 2019, 361 . 363 [BIER] IETF, "Bit Indexed Explicit Replication", October 2019, 364 . 366 [BIER-PREF] 367 Thubert, P., Eckert, T., Brodard, Z., and H. Jiang, "BIER- 368 TE extensions for Packet Replication and Elimination 369 Function (PREF) and OAM", Work in Progress, Internet- 370 Draft, draft-thubert-bier-replication-elimination-03, 3 371 March 2018, 372 . 375 [CCAMP] IETF, "Common Control and Measurement Plane", October 376 2019, 377 . 379 [DetNet] IETF, "Deterministic Networking", October 2019, 380 . 382 [DetNet-DP-FW] 383 Varga, B., Farkas, J., Berger, L., Fedyk, D., Malis, A., 384 Bryant, S., and J. Korhonen, "DetNet Data Plane 385 Framework", Work in Progress, Internet-Draft, draft-ietf- 386 detnet-data-plane-framework-02, 13 September 2019, 387 . 390 [DetNet-IP-OAM] 391 Mirsky, G. and M. Chen, "Operations, Administration and 392 Maintenance (OAM) for Deterministic Networks (DetNet) with 393 IP Data Plane", Work in Progress, Internet-Draft, draft- 394 mirsky-detnet-ip-oam-00, 8 July 2019, 395 . 398 [MANET] IETF, "Mobile Ad hoc Networking", October 2019, 399 . 401 [SPRING] IETF, "Source Packet Routing in Networking", October 2019, 402 . 404 Authors' Addresses 406 Pascal Thubert (editor) 407 Cisco Systems, Inc 408 Building D, 45 Allee des Ormes - BP1200 409 06254 MOUGINS - Sophia Antipolis 410 France 412 Phone: +33 497 23 26 34 413 Email: pthubert@cisco.com 415 Georgios Z. Papadopoulos 416 IMT Atlantique 417 Office B00 - 114A, 2 Rue de la Chataigneraie 418 35510 Cesson-Sevigne - Rennes 419 France 421 Phone: +33 299 12 70 04 422 Email: georgios.papadopoulos@imt-atlantique.fr