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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. Papadopoulos 5 Expires: April 5, 2020 IMT Atlantique 6 October 3, 2019 8 Reliable and Available Wireless Problem Statement 9 draft-pthubert-raw-problem-statement-02 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 April 5, 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 50 (https://trustee.ietf.org/license-info) in effect on the date of 51 publication of this document. Please review these documents 52 carefully, as they describe your rights and restrictions with respect 53 to this document. Code Components extracted from this document must 54 include Simplified BSD License text as described in Section 4.e of 55 the Trust Legal Provisions and are provided without warranty as 56 described in the Simplified BSD License. 58 Table of Contents 60 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 61 2. Use Cases and Requirements Served . . . . . . . . . . . . . . 4 62 3. Routing Scale vs. Forwarding Scale . . . . . . . . . . . . . 4 63 4. Prerequisites . . . . . . . . . . . . . . . . . . . . . . . . 5 64 5. Related Work at The IETF . . . . . . . . . . . . . . . . . . 6 65 6. Functional Gaps . . . . . . . . . . . . . . . . . . . . . . . 6 66 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 67 7.1. Normative References . . . . . . . . . . . . . . . . . . 7 68 7.2. Informative References . . . . . . . . . . . . . . . . . 8 69 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 71 1. Introduction 73 Bringing determinism in a packet network means eliminating the 74 statistical effects of multiplexing that result in probabilistic 75 jitter and loss. This can be approached with a tight control of the 76 physical resources to maintain the amount of traffic within a 77 budgetted volume of data per unit of time that fits the physical 78 capabilities of the underlying technology, and the use of time-shared 79 resources (bandwidth and buffers) per circuit, and/or by shaping and/ 80 or scheduling the packets at every hop. 82 Wireless networks operate on a shared medium where uncontrolled 83 interference, including the self-induced multipath fading, adds 84 another dimension to the statistical effects that affect the 85 delivery. Scheduling transmissions can alleviate those effects by 86 leveraging diversity in the spatial, time, code, and frequency 87 domains, and provide a Reliable and Available service while 88 preserving energy and optimizing the use of the shared spectrum. 90 Deterministic Networking is an attempt to mostly eliminate packet 91 loss for a committed bandwidth with a guaranteed worst-case end-to- 92 end latency, even when co-existing with best-effort traffic in a 93 shared network. This innovation is enabled by recent developments in 94 technologies including IEEE 802.1 TSN (for Ethernet LANs) and IETF 95 DetNet (for wired IP networks). It is getting traction in various 96 industries including manufacturing, online gaming, professional A/V, 97 cellular radio and others, making possible many cost and performance 98 optimizations. 100 Reliable and Available Wireless (RAW) networking services extend 101 DetNet to approach end-to-end deterministic performances in a network 102 with scheduled wireless segments, possibly combined with wired 103 segments, and possibly sharing physical resources with non- 104 deterministic traffic. The wireless and wired media are 105 fundamentally different at the physical level, and while the generic 106 Problem Statement for DetNet applies to the wired as well as the 107 wireless medium, the methods to achieve RAW will differ from those 108 used to support time-sensitive networking over wires, as a RAW 109 solution will need to address less consistent transmissions, energy 110 conservation and shared spectrum efficiency. 112 The development of RAW technologies has been lagging behind 113 deterministic efforts for wired systems both at the IEEE and the 114 IETF. But recent efforts at the IEEE and 3GPP indicate that wireless 115 is finally catching up at the lower layer and that it is now possible 116 for the IETF to extend DetNet for wireless segments that are capable 117 of scheduled wireless transmissions. 119 The intent for RAW is to provide DetNet elements that are specialized 120 for short range radios. From this inheritance, RAW stays agnostic to 121 the radio layer underneath though the capability to schedule 122 transmissions is assumed. How the PHY is programmed to do so, and 123 whether the radio is single-hop or meshed, are unknown at the IP 124 layer and not part of the RAW abstraction. 126 Still, in order to focus on real-worlds issues and assert the 127 feasibility of the proposed capabilities, RAW will focus on selected 128 technologies that can be scheduled at the lower layers: IEEE Std. 129 802.15.4 timeslotted channel hopping (TSCH), 3GPP 5G ultra-reliable 130 low latency communications (URLLC), IEEE 802.11ax/be where 802.11be 131 is extreme high throughput (EHT), and L-band Digital Aeronautical 132 Communications System (LDACS). See [I-D.thubert-raw-technologies] 133 for more. 135 The establishment of a path is not in-scope for RAW. It may be the 136 product of a centralized Controller Plane as described for DetNet. 137 As opposed to wired networks, the action of installing a path over a 138 set of wireless links may be very slow relative to the speed at which 139 the radio conditions vary, and it makes sense in the wireless case to 140 provide redundant forwarding solutions along a complex path and to 141 leave it to the RAW Network Plane to select which of those forwarding 142 solutions are to be used for a given packet based on the current 143 conditions. 145 RAW distinguishes the longer time scale at which routes are computed 146 from the the shorter forwarding time scale where per-packet decisions 147 are made. RAW operates at the forwarding time scale on one DetNet 148 flow over one path that is preestablished and installed by means 149 outside of the scope of RAW. The scope of the RAW WG comprises 150 Network plane protocol elements such as OAM and in-band control to 151 improve the RAW operation at the Service and at the forwarding sub- 152 layers, e.g., controlling whether to use packet replication, Hybrid 153 ARQ and coding, with a constraint to limit the use of redundancy when 154 it is really needed, e.g., when a spike of loss is observed. This is 155 discussed in more details in Section 3 and the next sections. 157 2. Use Cases and Requirements Served 159 [RFC8578] presents a number of wireless use cases including Wireless 160 for Industrial Applications. [I-D.bernardos-raw-use-cases] adds a 161 number of use cases that demonstrate the need for RAW capabilities in 162 Pro-Audio, gaming and robotics. 164 3. Routing Scale vs. Forwarding Scale 166 RAW extends DetNet to focus on issues that are mostly a concern on 167 wireless links. See [I-D.ietf-detnet-architecture] for more on 168 DetNet. With DetNet, the end-to-end routing can be centralized and 169 can reside outside the network. In wireless, and in particular in a 170 wireless mesh, the path to the controller that performs the route 171 computation and maintenance may be slow and expensive in terms of 172 critical resources such as air time and energy. 174 Reaching to the routing computation can be slow in regards to the 175 speed of events that affect the forwarding operation at the radio 176 layer. Due to the cost and latency to perform a route computation, 177 routing is not expected to be sensitive/reactive to transient 178 changes. The abstraction of a link at the routing level is expected 179 to use statistical operational metrics that aggregate the behavior of 180 a link over long periods of time, and represent its availability as a 181 shade of gray as opposed to either up or down. 183 In the case of wireless, the changes that affect the forwarding 184 decision can happen frequently and often for shot durations, e.g., a 185 mobile object moves between a transmitter and a receiver, and will 186 cancel the line of sight transmission for a few seconds, or a radar 187 measures the depth of a pool and interferes on a particular channel 188 for a split second. 190 There is thus a desire to separate the long term computation of the 191 route and the short term forwarding decision. In such a model, the 192 routing operation computes a complex Track that enables multiple non- 193 equal cost multipath (N-ECMP) forwarding solutions, and leaves it to 194 the forwarding plane to make the per-packet decision of which of 195 these possibilities should be used. 197 In the case of wires, the concept is known in traffic engineering 198 where an alternate path can be used upon the detection of a failure 199 in the main path, e.g., using OAM in MPLS-TP or BFD over a collection 200 of SD-WAN tunnels. RAW formalizes a routing time scale that is order 201 of magnitude longer than the forwarding time scale, and separates the 202 protocols and metrics that are used at both scales. Routing can 203 operate on long term statistics such as delivery ratio over minutes 204 to hours, but as a first approximation can ignore flapping. On the 205 other hand, the RAW forwarding decision is made at packet speed, and 206 uses information that must be pertinent at the present time for the 207 current transmission. 209 4. Prerequisites 211 A prerequisite to the RAW work is that an end-to-end routing function 212 computes a complex sub-topology along which forwarding can happen 213 between a source and one or more destinations. For 6TiSCH, this is a 214 Track. The concept of Track is specified in the 215 [I-D.ietf-6tisch-architecture]. Tracks provide a high degree of 216 redundancy and diversity and enable DetNet PREOF, end-to-end network 217 coding, and possibly radio-specific abstracted techniques such as 218 ARQ, overhearing, frequency diversity, time slotting, and possibly 219 others. 221 How the routing operation computes the Track is out of scope for RAW. 222 The scope of the RAW operation is one Track, and the goal of the RAW 223 operation is to optimize the use of the Track at the forwarding 224 timescale to maintain the expected service while optimizing the usage 225 of constrained resources such as energy and spectrum. 227 Another prerequisite is that an IP link can be established over the 228 radio with some guarantees in terms of service reliability, e.g., it 229 can be relied upon to transmit a packet within a bounded latency and 230 provides a guaranteed BER/PDR outside rare but existing transient 231 outage windows that can last from split seconds to minutes. The 232 radio layer can be programmed with abstract parameters, and can 233 return an abstract view of the state of the Link to help forwarding 234 decision (think DLEP from MANET). In the layered approach, how the 235 radio manages its PHY layer is out of control and out of scope. 236 Whether it is single hop or meshed is also unknown and out of scope. 238 5. Related Work at The IETF 240 RAW intersects with protocols or practices in development at the IETF 241 as follows: 243 o The Dynamic Link Exchange Protocol [RFC8175] (DLEP) from [MANET] 244 can be leveraged at each hop to derive generic radio metrics 245 (e.g., based on LQI, RSSI, queueing delays and ETX) on individual 246 hops 248 o Operations, Administration and Maintenance (OAM) work at [DetNet] 249 such as [I-D.mirsky-detnet-ip-oam] for the case of the IP Data 250 Plane observes the state of DetNet paths, typically MPLS and IPv6 251 pseudowires [I-D.ietf-detnet-data-plane-framework], in the 252 direction of the traffic. RAW needs feedback that flows on the 253 reverse path and gathers instantaneous values from the radio 254 receivers at each hop to inform back the source and replicating 255 relays so they can make optimized forwarding decisions. The work 256 named ICAN may be related and may find a home at RAW. 258 o [BFD] detect faults in the path between an ingress and an egress 259 forwarding engines, but is aware of the complexity of a path with 260 replication, and expects bidirectionality. BFD considers delivery 261 as success whereas with RAW the bounded latency can be as 262 important as the delivery itself. 264 o [SPRING] and [BIER] define in-band signaling that influences the 265 routing when decided at the head-end on the path. There's already 266 one RAW-related draft at BIER 267 [I-D.thubert-bier-replication-elimination] more may follow. RAW 268 will need new in-band signaling when the decision is distributed, 269 e.g., required chances of reliable delivery to destination within 270 latency. This signaling enables relays to tune retries and 271 replication to be met. 273 o [CCAMP] defines protocol-independent metrics and parameters 274 (measurement attributes) for describing links and paths that are 275 required for routing and signaling in technology-specific 276 networks. RAW would be a source of requirements for CCAMP to 277 define metrics that are significant to the focus radios. 279 6. Functional Gaps 281 Within a large routed topology, the routing operation builds a 282 particular complex Track with one source and one or more 283 destinations; within the Track, packets may follows different paths 284 and may be subject to RAW forwarding operations that include 285 replication, elimination, retries, overhearing and reordering. 287 The RAW forwarding decisions include the selection of points of 288 replication and elimination, how many retries can take place, and a 289 limit of validity for the packet beyond which the packet should be 290 destroyed rather than forwarded uselessly further down the Track. 292 The decision to apply the RAW techniques must be done quickly, and 293 depends on a very recent and precise knowledge of the forwarding 294 conditions within the complex Track. There is a need for an 295 observation method to provide the RAW forwarding plane with the 296 specific knowledge of the state of the Track for the type of flow of 297 interest (e.g., for a QoS level of interest). To observe the whole 298 Track in quasi real time, RAW will consider existing tools such as 299 L2-triggers, DLEP, BFD and in-band and out-of-band OAM. 301 One possible way of making the RAW forwarding decisions is to make 302 them all at the ingress and express them in-band in the packet, which 303 requires new loose or strict Hop-by-hop signaling. To control the 304 RAW forwarding operation along a Track for the individual packets, 305 RAW may leverage and extend known techniques such as DetNet tagging, 306 Segment Routing (SRv6) or BIER-TE such as done with 307 [I-D.thubert-bier-replication-elimination]. 309 An alternate way is to enable each forwarding node to make the RAW 310 forwarding decisions for a packet on its own, based on its knowledge 311 of the expectation (timeliness and reliability) for that packet and a 312 recent observation of the rest of the way across the possible paths 313 within the Track. Information about the service should be placed in 314 the packet and matched with the forwarding node's capabilities and 315 policies. 317 In either case, a per-flow state is installed in all intermediate 318 nodes to recognize the flow and determine the forwarding policy to be 319 applied. 321 7. References 323 7.1. Normative References 325 [I-D.bernardos-raw-use-cases] 326 Papadopoulos, G., Thubert, P., Theoleyre, F., and C. 327 Bernardos, "RAW use cases", draft-bernardos-raw-use- 328 cases-00 (work in progress), July 2019. 330 [I-D.ietf-6tisch-architecture] 331 Thubert, P., "An Architecture for IPv6 over the TSCH mode 332 of IEEE 802.15.4", draft-ietf-6tisch-architecture-26 (work 333 in progress), August 2019. 335 [I-D.ietf-detnet-architecture] 336 Finn, N., Thubert, P., Varga, B., and J. Farkas, 337 "Deterministic Networking Architecture", draft-ietf- 338 detnet-architecture-13 (work in progress), May 2019. 340 [I-D.thubert-raw-technologies] 341 Thubert, P., Cavalcanti, D., Vilajosana, X., and C. 342 Schmitt, "Reliable and Available Wireless Technologies", 343 draft-thubert-raw-technologies-03 (work in progress), July 344 2019. 346 [RFC8175] Ratliff, S., Jury, S., Satterwhite, D., Taylor, R., and B. 347 Berry, "Dynamic Link Exchange Protocol (DLEP)", RFC 8175, 348 DOI 10.17487/RFC8175, June 2017, 349 . 351 [RFC8578] Grossman, E., Ed., "Deterministic Networking Use Cases", 352 RFC 8578, DOI 10.17487/RFC8578, May 2019, 353 . 355 7.2. Informative References 357 [BFD] IETF, "Bidirectional Forwarding Detection", 358 . 360 [BIER] IETF, "Bit Indexed Explicit Replication", 361 . 363 [CCAMP] IETF, "Common Control and Measurement Plane", 364 . 366 [DetNet] IETF, "Deterministic Networking", 367 . 369 [I-D.ietf-detnet-data-plane-framework] 370 Varga, B., Farkas, J., Berger, L., Fedyk, D., Malis, A., 371 Bryant, S., and J. Korhonen, "DetNet Data Plane 372 Framework", draft-ietf-detnet-data-plane-framework-02 373 (work in progress), September 2019. 375 [I-D.mirsky-detnet-ip-oam] 376 Mirsky, G. and M. Chen, "Operations, Administration and 377 Maintenance (OAM) for Deterministic Networks (DetNet) with 378 IP Data Plane", draft-mirsky-detnet-ip-oam-00 (work in 379 progress), July 2019. 381 [I-D.thubert-bier-replication-elimination] 382 Thubert, P., Eckert, T., Brodard, Z., and H. Jiang, "BIER- 383 TE extensions for Packet Replication and Elimination 384 Function (PREF) and OAM", draft-thubert-bier-replication- 385 elimination-03 (work in progress), March 2018. 387 [MANET] IETF, "Mobile Ad hoc Networking", 388 . 390 [PCE] IETF, "Path Computation Element", 391 . 393 [SPRING] IETF, "Source Packet Routing in Networking", 394 . 396 [TEAS] IETF, "Traffic Engineering Architecture and Signaling", 397 . 399 Authors' Addresses 401 Pascal Thubert (editor) 402 Cisco Systems, Inc 403 Building D 404 45 Allee des Ormes - BP1200 405 MOUGINS - Sophia Antipolis 06254 406 FRANCE 408 Phone: +33 497 23 26 34 409 Email: pthubert@cisco.com 411 Georgios Z. Papadopoulos 412 IMT Atlantique 413 Office B00 - 114A 414 2 Rue de la Chataigneraie 415 Cesson-Sevigne - Rennes 35510 416 FRANCE 418 Phone: +33 299 12 70 04 419 Email: georgios.papadopoulos@imt-atlantique.fr