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Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (January 19, 2021) is 1193 days in the past. Is this intentional? Checking references for intended status: Informational ---------------------------------------------------------------------------- No issues found here. Summary: 2 errors (**), 0 flaws (~~), 1 warning (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 lpwan Working Group E. Ramos 3 Internet-Draft Ericsson 4 Intended status: Informational A. Minaburo 5 Expires: July 23, 2021 Acklio 6 January 19, 2021 8 SCHC over NB-IoT 9 draft-ietf-lpwan-schc-over-nbiot-04 11 Abstract 13 The Static Context Header Compression (SCHC) specification describes 14 a header compression and fragmentation functionalities for LPWAN (Low 15 Power Wide Area Networks) technologies. SCHC was designed to be 16 adapted over any of the LPWAN technologies. 18 This document describes the use of SCHC over the NB-IoT wireless 19 access, and provides elements for an efficient parameterization. 21 Status of This Memo 23 This Internet-Draft is submitted in full conformance with the 24 provisions of BCP 78 and BCP 79. 26 Internet-Drafts are working documents of the Internet Engineering 27 Task Force (IETF). Note that other groups may also distribute 28 working documents as Internet-Drafts. The list of current Internet- 29 Drafts is at http://datatracker.ietf.org/drafts/current/. 31 Internet-Drafts are draft documents valid for a maximum of six months 32 and may be updated, replaced, or obsoleted by other documents at any 33 time. It is inappropriate to use Internet-Drafts as reference 34 material or to cite them other than as "work in progress." 36 This Internet-Draft will expire on July 23, 2021. 38 Copyright Notice 40 Copyright (c) 2021 IETF Trust and the persons identified as the 41 document authors. All rights reserved. 43 This document is subject to BCP 78 and the IETF Trust's Legal 44 Provisions Relating to IETF Documents 45 (http://trustee.ietf.org/license-info) in effect on the date of 46 publication of this document. Please review these documents 47 carefully, as they describe your rights and restrictions with respect 48 to this document. Code Components extracted from this document must 49 include Simplified BSD License text as described in Section 4.e of 50 the Trust Legal Provisions and are provided without warranty as 51 described in the Simplified BSD License. 53 Table of Contents 55 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 56 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 57 3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 4 58 4. Data Transmission . . . . . . . . . . . . . . . . . . . . . . 6 59 5. IP based Data Transmission . . . . . . . . . . . . . . . . . 7 60 5.1. SCHC over User Plane transmissions . . . . . . . . . . . 7 61 5.1.1. SCHC Entities Placing . . . . . . . . . . . . . . . . 8 62 5.2. Data Over Control Plane . . . . . . . . . . . . . . . . . 8 63 5.2.1. SCHC Entities Placing . . . . . . . . . . . . . . . . 9 64 5.3. Parameters for Static Context Header Compression (SCHC) . 10 65 5.3.1. SCHC Context initialization . . . . . . . . . . . . . 10 66 5.3.2. SCHC Rules . . . . . . . . . . . . . . . . . . . . . 10 67 5.3.3. Rule ID . . . . . . . . . . . . . . . . . . . . . . . 11 68 5.3.4. SCHC MAX_PACKET_SIZE . . . . . . . . . . . . . . . . 11 69 5.3.5. Fragmentation . . . . . . . . . . . . . . . . . . . . 11 70 6. Non-IP based Data Transmission . . . . . . . . . . . . . . . 12 71 6.1. SCHC Entities Placing . . . . . . . . . . . . . . . . . . 12 72 6.2. Parameters for Static Context Header Compression . . . . 13 73 6.2.1. SCHC Context initialization . . . . . . . . . . . . . 13 74 6.2.2. SCHC Rules . . . . . . . . . . . . . . . . . . . . . 13 75 6.2.3. Rule ID . . . . . . . . . . . . . . . . . . . . . . . 14 76 6.2.4. SCHC MAX_PACKET_SIZE . . . . . . . . . . . . . . . . 14 77 6.3. Fragmentation . . . . . . . . . . . . . . . . . . . . . . 14 78 6.3.1. Fragmentation modes . . . . . . . . . . . . . . . . . 14 79 6.3.2. Fragmentation Parameters . . . . . . . . . . . . . . 15 80 7. Padding . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 81 8. Security considerations . . . . . . . . . . . . . . . . . . . 15 82 9. 3GPP References . . . . . . . . . . . . . . . . . . . . . . . 15 83 10. Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . 16 84 10.1. NB-IoT User Plane protocol architecture . . . . . . . . 16 85 10.1.1. Packet Data Convergence Protocol (PDCP) . . . . . . 16 86 10.1.2. Radio Link Protocol (RLC) . . . . . . . . . . . . . 17 87 10.1.3. Medium Access Control (MAC) . . . . . . . . . . . . 18 88 10.2. NB-IoT Data over NAS (DoNAS) . . . . . . . . . . . . . . 19 89 11. Normative References . . . . . . . . . . . . . . . . . . . . 21 90 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22 92 1. Introduction 94 The Static Context Header Compression (SCHC) {RFC8724} defines a 95 header compression scheme and fragmentation functionality, both 96 specially tailored for Low Power Wide Area Networks (LPWAN) networks 97 defined in [RFC8376]. 99 Header compression is needed to efficiently bring Internet 100 connectivity to the node within an NB-IoT network. SCHC uses a 101 static context to performs header compression with specific 102 parameters that need to be adapted into the NB-IoT wireless access. 103 This document assumes functionality for NB-IoT of 3GPP release 15 104 otherwise other versions functionality is explicitly mentioned in the 105 text. 107 This document describes the use of SCHC and its parameterizing over 108 the NB-IoT wireless access. 110 2. Terminology 112 This document will follow the terms defined in [RFC8724], in 113 [RFC8376], and the TGPP23720. 115 o CIoT. Cellular IoT 117 o C-SGN. CIoT Serving Gateway Node 119 o UE. User Equipment 121 o eNB. Node B. Base Station that controls the UE 123 o EPC. Evolved Packet Connectivity. Core network of 3GPP LTE 124 systems. 126 o EUTRAN. Evolved Universal Terrestrial Radio Access Network. 127 Radio network from LTE based systems. 129 o MME. Mobility Management Entity. Handle mobility of the UE 131 o NB-IoT. Narrow Band IoT. Referring to 3GPP LPWAN technology 132 based in LTE architecture but with additional optimization for IoT 133 and using a Narrow Band spectrum frequency. 135 o SGW. Serving Gateway. Routes and forwards the user data packets 136 through the access network 138 o HSS. Home Subscriber Server. It is a database that performs 139 mobility management 141 o PGW. Packet Data Node Gateway. An interface between the internal 142 with the external network 144 o PDU. Protocol Data Unit. Data packets including headers that are 145 transmitted between entities through a protocol. 147 o SDU. Service Data Unit. Data packets (PDUs) from higher layers 148 protocols used by lower layer protocols as a payload of their own 149 PDUs that has not yet been encapsulated. 151 o IWK-SCEF. InterWorking Service Capabilities Exposure Function. 152 Used in roaming scenarios and serves for interconnection with the 153 SCEF of the Home PLMN and is located in the Visited PLMN 155 o SCEF. Service Capability Exposure Function. EPC node for 156 exposure of 3GPP network service capabilities to 3rd party 157 applications. 159 3. Architecture 161 +--+ 162 D |UE| \ +-----+ +------+ 163 +--+ \ | MME |-----| HSS | 164 E \ / +-----+ +------+ 165 +--+ \+-----+ / | 166 V |UE| ----| RGW |- | 167 +--+ |(eNB)| | 168 I /+-----+ \ | 169 / \ +------+ 170 C / \| NGW | +------+ Service PDN 171 +--+ / |(S-GW)|--| NGW |-- e.g. Internet 172 E |UE| | | |(P-GW)| 173 +--+ +------+ +------+ 174 S 176 Figure 1: 3GPP network architecture 178 The architecture for 3GPP LTE network has been reused for NB-IoT with 179 some optimizations and simplifications known as Cellular IoT (CIoT). 180 Considering the typical use cases for CIoT devices here are described 181 some of the additions to the LTE architecture specific for CIoT. 182 C-SGN(CIoT Serving Gateway Node) is a deployment option co-locating 183 EPS entities in the control plane and user plane paths (for example, 184 MME + SGW + P-GW) and the external interfaces of the entities 185 supported. The C-SGN also supports at least some of the following 186 CIoT EPS Optimizations: 188 o Control Plane CIoT EPS Optimization for small data transmission. 190 o User Plane CIoT EPS Optimization for small data transmission. 192 o Necessary security procedures for efficient small data 193 transmission. 195 o SMS without combined attach for NB-IoT only UEs. 197 o Paging optimizations for coverage enhancements. 199 o Support for non-IP data transmission via SGi tunneling and/or 200 SCEF. 202 o Support for Attach without PDN (Packet Data Network) connectivity. 204 Another node introduced in the CIOT architecture is the SCEF (Service 205 Capability Exposure Function) that provide means to securely expose 206 service and network capabilities to entities external to the network 207 operator. The northbound APIS are defined by OMA and OneM2M. The 208 main functions of a SCEF are: 210 o Non-IP Data Delivery (NIDD) established through the SCEF. 212 o Monitoring and exposure of event related to UE reachability, loss 213 of connectivity, location reporting, roaming status, communication 214 failure and change of IMEI-IMSI association. 216 +-------+ 217 | HSS | 218 +-+-----+ 219 NGW / 220 DEV RGW +---------+ __/S6a 221 +--------+ | +-----+ +_/ NGW 222 +----+ C-Uu | +---+-+ MME | | T6i+--------+ T7 +----+ 223 |CIOT+--------+ eNB |S1 | | +-+----+IWK-SCEF+----+SCEF| 224 |UE | |(NB-IoT)| | +---+-+ | +--------+ +----+ 225 +----+ +--------+ | | | 226 |C-SGN| | 227 | |S11| 228 +------+ | | | 229 +--------+LTE-Uu| | | +--+-+ | 230 |LTE eMTC|(eMTC)|eNB +---+--+SGW | | S8+---+ +-----------+ 231 | UE +------+(eMTC)|S1 | | +-+---+PGW|SGi |Application| 232 +--------+ +------+ | +----+ | | +----+Server (AS)| 233 +---------+ +---+ +-----------+ 234 DEV RGW NGW NGW App 236 Figure 2: 3GPP optimized CIOT network architecture 238 4. Data Transmission 240 3GPP networks deal not only with data transmitted end-to-end but also 241 with in-band signaling that is used between the nodes and functions 242 to configure, control and monitor the system functions and behaviors. 243 The control data is handled using a Control Plane which has a 244 specific set of protocols, handling processes and entities. In 245 contrast, the end-to-end or user data utilize a User Plane with 246 characteristics of its own separated from the Control Plane. The 247 handling and setup of the Control Plane and User Plane spans over the 248 whole 3GPP network and it has particular implications in the radio 249 network (i.e., EUTRAN) and in the packet core (ex., EPC). 251 For the CIOT cases, additionally to transmissions of data over User 252 Plane, 3GPP has specified optimizations for small data transmissions, 253 allowing to transport user data (IP, Non-IP) within signaling on the 254 access network (Data transmission over Control Plane or Data Over 255 NAS). 257 The maximum recommended MTU size is 1358 Bytes. The radio network 258 protocols limit the packet sizes to be transmitted over the air 259 including radio protocol overhead to 1600 Octets. But the value is 260 reduced further to avoid fragmentation in the backbone of the network 261 due to the payload encryption size (multiple of 16) and handling of 262 the additional core transport overhead. 264 NB-IoT and in general the cellular technologies interfaces and 265 functions are standardized by 3GPP. Therefore the introduction of 266 SCHC entities to UE, eNB and C-SGN does need to be specified in the 267 NB-IoT standard. This implies that standard specified SCHC support 268 would not be backwards compatible. A terminal or a network 269 supporting a version of the standard without support of SCHC or 270 without capability implementation (in case of not being standardized 271 as mandatory capability) is not able to utilize the compression 272 services with this approach. 274 SCHC could be deployed differently depending on where the header 275 compression and the fragmentation are applied. The SCHC 276 functionalities could be applied to the packets about to be 277 transmitted over the air, or to the whole end-to-end link. To 278 accomplish the first, it is required to place SCHC compression and 279 decompression entities in the eNB and in the UE for transmissions 280 over the User Plane. Additionally, to handle the case of the 281 transmissions over Control Plane or Data Over NAS, the network SCHC 282 entity has to be placed in the C-SGN as well. For these two cases, 283 the functions are to be standardized by 3GPP. 285 Another possibility is to apply SCHC functionalities to the end-to- 286 end connection or at least up to the operator network edge. In that 287 case, the SCHC entities would be placed in the application layer of 288 the terminal in one end, and either in the application servers or in 289 a broker function in the edge of the operator network in the other 290 end. For the radio network, the packets are transmitted as non-IP 291 traffic, which can be currently served utilizing IP tunneling or SCEF 292 services. Since this option does not necessarily require 3GPP 293 standardization, it is possible to also benefit legacy devices with 294 SCHC by utilizing the non-IP transmission features of the operator 295 network. 297 Accordingly, there are four different scenarios where SCHC can be 298 used in the NB-IoT architecture. IP header compression on the data 299 transmission over User Plane, IP header compression on the optimized 300 transmissions over Control Plane (i.e.,DoNAS), non-IP transmissions 301 of SCHC packets by IP tunneling, and non-IP transmissions of SCHC 302 packets by SCEF forwarding. The following sections describe each of 303 them in more detail. The first two scenarios refer to transmissions 304 using the 3GPP IP transmission capabilities and the last two refers 305 to transmission using the Non-IP capabilities. 307 5. IP based Data Transmission 309 5.1. SCHC over User Plane transmissions 311 Deploying SCHC only over the radio link would require to place it as 312 part of the User Plane data transmission. The User Plane utilizes 313 the protocol stack of the Access Stratum (AS) for data transfer. AS 314 (Access Stratum) is the functional layer responsible for transporting 315 data over wireless connection and managing radio resources. The user 316 plane AS has support for features such as reliability, segmentation 317 and concatenation. The transmissions of the AS make use of link 318 adaptation, meaning that the transport format utilized for the 319 transmissions are optimized according to the radio conditions, the 320 number of bits to transmit and the power and interference constrains. 321 That means that the number of bits transmitted over the air depends 322 of the Modulation and Coding Schemes (MCS) selected. The 323 transmissions in the physical layer happens at network synchronized 324 intervals of times called TTI (Transmission Time Interval). The 325 transmission of a Transport Block (TB) is completed during, at least, 326 one TTI. Each Transport Block has a different MCS and number of bits 327 available to transmit. The Transport Blocks characteristics are 328 defined by the MAC technical specification TGPP36321. The Access 329 Stratum for User Plane is comprised by Packet Data Convergence 330 Protocol (PDCP) TGPP36323, Radio Link Protocol (RLC) TGPP36322, 331 Medium Access Control protocol (MAC) TGPP36321 and the Physical Layer 332 TGPP36201. More details of this protocols are given in the Appendix. 334 5.1.1. SCHC Entities Placing 336 The current architecture provides support for header compression in 337 PDCP utilizing RoHC [RFC5795]. Therefore SCHC entities can be 338 deployed in similar fashion without need for major changes in the 339 3GPP specifications. 341 In this scenario, RLC takes care of the handling of fragmentation (if 342 transparent mode is not configured) when packets exceeds the 343 transport block size at the time of transmission. Therefore SCHC 344 fragmentation is not needed and should not be used to avoid 345 additional protocol overhead. It is not common to configure RLC in 346 Transparent Mode for IP based user plane data. But given the case in 347 the future, SCHC fragmentation may be used. In that case, a SCHC 348 tile would match the minimum transport block size minus the PDCP and 349 MAC headers. 351 +---------+ +---------+ | 352 |IP/non-IP+------------------------------+IP/non-IP+->+ 353 +---------+ | +---------------+ | +---------+ | 354 | PDCP +-------+ PDCP | GTP|U +------+ GTP-U |->+ 355 | (SCHC) + + (SCHC)| + + | | 356 +---------+ | +---------------+ | +---------+ | 357 | RLC +-------+ RLC |UDP/IP +------+ UDP/IP +->+ 358 +---------+ | +---------------+ | +---------+ | 359 | MAC +-------+ MAC | L2 +------+ L2 +->+ 360 +---------+ | +---------------+ | +---------+ | 361 | PHY +-------+ PHY | PHY +------+ PHY +->+ 362 +---------+ +---------------+ +---------+ | 363 C-Uu/ S1-U SGi 364 CIOT/ LTE+Uu C-BS/eNB C-SGN 365 LTE eMTC 366 UE 368 Figure 3: SCHC entities placement in the 3GPP CIOT radio protocol 369 architecture for data over user plane 371 5.2. Data Over Control Plane 373 The Non-Access Stratum (NAS), conveys mainly control signaling 374 between the UE and the cellular network TGPP24301. NAS is 375 transported on top of the Access Stratum (AS) already mentioned in 376 the previous section. 378 NAS has been adapted to provide support for user plane data 379 transmissions to reduce the overhead when transmitting infrequent 380 small quantities of data. This is known as Data over NAS (DoNAS) or 381 Control Plane CIoT EPS optimization. In DoNAS the UE makes use of 382 the pre-established NAS security and piggyback uplink small data into 383 the initial NAS uplink message, and uses an additional NAS message to 384 receive downlink small data response. 386 The data encryption from the network side is performed by the C-SGN 387 in a NAS PDU. Depending on the data type signaled indication (IP or 388 non-IP data), the network allocates an IP address or just establish a 389 direct forwarding path. DoNAS (Data over NAS) is regulated under 390 rate control upon previous agreement, meaning that a maximum number 391 of bits per unit of time is agreed per device subscription beforehand 392 and configured in the device. 394 The use of DoNAS is typically expected when a terminal in a power 395 saving state requires to do a short transmission and receive an 396 acknowledgment or short feedback from the network. Depending on the 397 size of buffered data to transmit, the UE might be instructed to 398 deploy the connected mode transmissions instead, limiting and 399 controlling the DoNAS transmissions to predefined thresholds and a 400 good resource optimization balance for the terminal and the network. 401 The support for mobility of DoNAS is present but produces additional 402 overhead. Additional details of DoNAS are given in the Appendix. 404 5.2.1. SCHC Entities Placing 406 In this scenario SCHC can be applied in the NAS protocol layer 407 instead of PDCP. The same principles than for user plane 408 transmissions applies here as well. The main difference is the 409 physical placing of the SCHC entities in the network side as the 410 C-SGN (placed in the core network) is the terminating node for NAS 411 instead of the eNB. 413 +--------+ +--------+--------+ + +--------+ 414 | IP/ +--+-----------------+--+ IP/ | IP/ +-----+ IP/ | 415 | Non-IP | | | | Non-IP | Non-IP | | | Non-IP | 416 +--------+ | | +-----------------+ | +--------+ 417 | NAS +-----------------------+ NAS |GTP|C/U +-----+GTP|C/U | 418 |(SCHC) | | | | (SCHC) | | | | | 419 +--------+ | +-----------+ | +-----------------+ | +--------+ 420 | RRC +-----+RRC |S1|AP+-----+ S1|AP | | | | | 421 +--------+ | +-----------+ | +--------+ UDP +-----+ UDP | 422 | PDCP* +-----+PDCP*|SCTP +-----+ SCTP | | | | | 423 +--------+ | +-----------+ | +-----------------+ | +--------+ 424 | RLC +-----+ RLC | IP +-----+ IP | IP +-----+ IP | 425 +--------+ | +-----------+ | +-----------------+ | +--------+ 426 | MAC +-----+ MAC | L2 +-----+ L2 | L2 +-----+ L2 | 427 +--------+ | +-----------+ | +-----------------+ | +--------+ 428 | PHY +--+--+ PHY | PHY +--+--+ PHY | PHY +-----+ PHY | 429 +--------+ +-----+-----+ +--------+--------+ | +--------+ 430 C-Uu/ S1-lite SGi 431 CIOT/ LTE-Uu C-BS/eNB C-SGN PGW 432 LTE eMTC 433 UE 435 *PDCP is bypassed until AS security is activated TGPP36300. 437 Figure 4 439 5.3. Parameters for Static Context Header Compression (SCHC) 441 5.3.1. SCHC Context initialization 443 RRC (Radio Resource Control) protocol is the main tool used to 444 configure the operation parameters of the AS transmissions for 3GPP 445 technologies. RoHC operation is configured with this protocol and it 446 is to expect that SCHC will be configured and the static context 447 distributed in similar fashion for these scenarios. 449 5.3.2. SCHC Rules 451 The number of rules in a context are defined by the network operator 452 in these scenarios. For this, the operator must be aware of the type 453 of IP traffic that the device will carry out. This means that the 454 operator might provision sets of rules compatible with the use case 455 of the device. For devices acting as gateways of other devices 456 several rules that match the diversity of devices and protocols used 457 by the devices associated to the gateway. Meanwhile than simpler 458 devices (for example an electricity meter) may have a predetermined 459 set of protocols and parameters fixed. Additionally, the deployment 460 of IPV4 addresses in addition to IPV6 may force to provision separate 461 rules to deal with each of the cases. 463 5.3.3. Rule ID 465 For these transmission scenarios in NB-IoT, a reasonable assumption 466 of 9 bytes of radio protocol overhead can be expected. PDCP 5 bytes 467 due to header and integrity protection, and 4 bytes of RLC and MAC. 468 The minimum physical Transport Block (TB) that can withhold this 469 overhead value according to 3GPP Release 15 specifications are: 88, 470 104, 120 and 144 bits. If it is wished to optimize the number of 471 transmissions of a very small application packet so that in some 472 cases can be transmitted using only one physical layer transmission, 473 then the SCHC overhead should not exceed the available number of bits 474 of the smallest utile physical TB available. The packets handled by 475 3GPP networks are byte-aligned, and therefore the minimum payload 476 possible (including padding) is 8 bits. Therefore in order to 477 utilize the smallest TB the maximum SCHC is 8 bits. This must 478 include the Compression Residue in addition to the Rule ID. In the 479 other hand, it is possible that more complex NB-IoT devices (such as 480 a capillarity gateway) might require additional bits to handle the 481 variety and multiple parameters the of higher layer protocols 482 deployed. In that sense, the operator may want to have flexibility 483 on the number and type of rules supported by each device 484 independently, and consequently a configurable value is preferred for 485 these scenarios. The configuration may be set as part of the 486 operation profile agreed together with the context distribution. The 487 Rule Id field size may range for example from 2 bits resulting in 4 488 rules to a 8 bits value that would yield up to 256 rules which can be 489 used together with the operators and seems quite a reasonable maximum 490 limit even for a device acting as a NAT. More bits could be 491 configured, but it should take in account the byte-alignment of the 492 expected Compression Residue too. In the minimum TB size case, 2 493 bits size of Rule Id leave only 6 bits available for Compression 494 Residue. 496 5.3.4. SCHC MAX_PACKET_SIZE 498 The Access Stratum can handle the fragmentation of SCHC packets if 499 needed including reliability. Hence the packet size is limited by 500 the MTU possible to be handled by the AS radio protocols that 501 corresponds to 1600 bytes for 3GPP Release 15. 503 5.3.5. Fragmentation 505 For these scenarios the SCHC fragmentation functions are recommend to 506 be disabled. The RLC layer of NB-IoT can segment packets in suitable 507 units that fit the selected transport blocks for transmissions of the 508 physical layer. The selection of the blocks is done according to the 509 input of the link adaptation function in the MAC layer and the 510 quantity of data in the buffer. The link adaptation layer may 511 produce different results at each Time Transmission Interval (TTI) 512 resulting in varying physical transport blocks that depends of the 513 network load, interference and number of bits to be transmitted and 514 QoS. Even if setting a value that allows the construction of data 515 units following SCHC tiles principle, the protocol overhead may be 516 greater or equal than allowing the AS radio protocols to take care of 517 the fragmentation natively. 519 5.3.5.1. Fragmentation in Transparent Mode 521 If RLC is configured to operate in Transparent Mode, there could be a 522 case to activate a fragmentation function together with a light 523 reliability function such as the ACK-Always mode. In practice , it 524 is very rare to transmit user plane data using this configuration and 525 it is mainly targeting control plane transmissions. In those cases 526 the reliability is normally ensured by MAC based mechanisms, such as 527 repetitions or automatic retransmissions, and additional reliability 528 might only generate protocol overhead. 530 In future operations, it could be devised the utilization of SCHC to 531 reduce radio network protocols overhead and support the reliability 532 of the transmissions, and targeting small data with the fewer 533 possible transmissions. This could be realized by using fixed or 534 limited set of transport blocks compatible with the tiling SCHC 535 fragmentation handling. 537 6. Non-IP based Data Transmission 539 The Non-IP Data Delivery (NIDD) services of 3GPP enable the 540 possibility of transmitting SCHC packets compressed by the 541 application layer. The packets can be delivered by means of IP- 542 tunnels to the 3GPP network or using SCEF functions (i.e., API 543 calls). In both cases the packet IP is not understood by the 3GPP 544 network since it is already compressed and the network does not has 545 information of the context used for compression. Therefore the 546 network will treat the packet as a Non-IP traffic and deliver it to 547 the UE without any other stack element, directly under the L2. 549 6.1. SCHC Entities Placing 551 In the two scenarios using NIDD, SCHC entities are located almost in 552 top of the stack. In the terminal, it may be implemented by a 553 application utilizing the NB-IoT connectivity services. In the 554 network side, the SCHC entities are located in the Application Server 555 (AS). The IP tunneling scenario requires that the Application Server 556 sends the compressed packet over an IP connection that is terminated 557 by the 3GPP core network. If instead the SCEF services are used, 558 then it is possible to utilize a API call to transfer the SCHC 559 packets between the core network and the AS, also an IP tunnel could 560 be established by the AS, if negotiated with the SCEF. 562 +---------+ XXXXXXXXXXXXXXXXXXXXXXXX +--------+ 563 | SCHC | XXX XXX | SCHC | 564 |(Non-IP) +-----XX........................XX....+--*---+(Non-IP)| 565 +---------+ XX +----+ XX | | +--------+ 566 | | XX |SCEF+-------+ | | | 567 | | XXX 3GPP RAN & +----+ XXX +---+ UDP | 568 | | XXX CORE NETWORK XXX | | | 569 | L2 +---+XX +------------+ | +--------+ 570 | | XX |IP TUNNELING+--+ | | 571 | | XXX +------------+ +---+ IP | 572 +---------+ XXXX XXXX | +--------+ 573 | PHY +------+ XXXXXXXXXXXXXXXXXXXXXXX +---+ PHY | 574 +---------+ +--------+ 575 UE AS 577 Figure 5: SCHC entities placed when using Non-IP Delivery (NIDD) 3GPP 578 Sevices 580 6.2. Parameters for Static Context Header Compression 582 6.2.1. SCHC Context initialization 584 The static context is handled in the application layer level, 585 consequently the contexts are required to be distributed according to 586 the applications own capabilities, perhaps utilizing IP data 587 transmissions up to context initialization. Also the same IP 588 tunneling or SCEF services used later for the SCHC packets transport 589 may be used by the applications in both ends to deliver the static 590 contexts to be used. 592 6.2.2. SCHC Rules 594 Even when the transmissions content are not visible for the 3GPP 595 network, the same limitations than for IP based data transmissions 596 applies in these scenarios in terms of aiming to use the minimum 597 number of transmission and minimize the protocol overhead. 599 6.2.3. Rule ID 601 Similarly to the case of IP transmissions, the Rule ID size can be 602 dynamically set prior the context delivery. For example negotiated 603 between the applications when choosing a profile according to the 604 type of traffic and type of application deployed. Same 605 considerations related to the transport block size and performance 606 mentioned for the IP type of traffic has to be follow when choosing a 607 size value for the Rule ID field. 609 6.2.4. SCHC MAX_PACKET_SIZE 611 In these scenarios the maximum recommended MTU size that applies is 612 1358 Bytes, since the SCHC packets (and fragments) are traversing the 613 whole 3GPP network infrastructure (core and radio), and not only the 614 radio as the IP transmissions case. 616 6.3. Fragmentation 618 In principle the fragmentation function should be activated for 619 packets greater than 1358 Bytes. Since the 3GPP reliability 620 functions take great deal care of it, for simple point to point 621 connections may be enough a NO-ACK mode. Nevertheless additional 622 considerations for more complex cases are mentioned in the next 623 subsection to be taken in account. 625 6.3.1. Fragmentation modes 627 Depending of the QoS that has been assigned to the packets, it is 628 possible that packets are lost before they arrive to 3GPP radio 629 network transmission, for example in between the links of a 630 capillarity gateway, or due to buffer overflow handling in a backhaul 631 connection. In consequence, it is possible to secure additional 632 reliability on the packets transmitted with a small trade-off on 633 additional transmissions to signal the packets arrival indication 634 end-to-end if no transport protocol takes care of retransmission. To 635 achieve this, the packets fragmentation is activated with the ACK-on- 636 Error mode enabled. In some cases, it is even desirable to keep 637 track of all the SCHC packets delivered, in that case, the 638 fragmentation function could be active for all packets transmitted by 639 the applications (SCHC MAX_PACKET_SIZE == 1 Byte) and the ACK-on- 640 Error mode. In the NAS stratum, the use of only fragmentation when a 641 non-IP packet is transmitted is possible if this packet is considered 642 as a SCHC packet and is identifyed using the RuleID for non- 643 compressing packets as {RFC8724} allows it, depending on the 644 application an ACK-onError mode may be used. 646 6.3.2. Fragmentation Parameters 648 The Fragmentation Rule ID is given when choosing the profile 649 according to the fragmentation mode, 1 bit can be used to recognize 650 each mode. 652 To adapt SCHC to the NB-IoT constraints, two configuration are 653 proposed to fill the best the transfer block (TB). The Header size 654 needs to be multiple of 4 and the Tiles can keep a fix value of 4 or 655 8 bits to avoid the need of padding. 657 o 8 bits-Header_size configuration, with the size of the header 658 fields as follow: Rule ID 3 bits, DTag 1 bit, FCN 3 bits, W 1 659 bits. This configuration may ne used with TB less than 300 bits. 661 o 16 bits-Header_size configuration, with the size of the header 662 fields as follow: Rules ID 8 - 10 bits, DTag 1 or 2 bits, FCN 3 663 bits, W 2 or 3 bits. This configuration may be used with TB above 664 300 bits. 666 The IoT devices communicates with small data transfer and have a 667 battery life of 10 years. To minise the power consumption these 668 devices use the Power Save Mode and the Idle Mode DRX which govern 669 how often the device wakes up, stays up and are reachable. The 670 Table 10.5.163a in {3GPP-TS_24.088} specifies a range for the radio 671 timers as N to 3N in increments of one where the units of N can be 1 672 hour or 10 hours. To adapt SCHC to the NB-IoT activities, the 673 Innactivity Timer may be above 1h or 10h and the Retransmission Timer 674 may be below than 1h or 10h. 676 7. Padding 678 NB-IoT and 3GPP wireless access, in general, assumes byte aligned 679 payload. Therefore the L2 word for NB-IoT MUST be considered 8 bits 680 and the treatment of padding should use this value accordingly. 682 8. Security considerations 684 3GPP access security is specified in (TGPP33203). 686 9. 3GPP References 688 o TGPP23720 3GPP, "TR 23.720 v13.0.0 - Study on architecture 689 enhancements for Cellular Internet of Things", 2016. 691 o TGPP33203 3GPP, "TS 33.203 v13.1.0 - 3G security; Access security 692 for IP-based services", 2016. 694 o TGPP36321 3GPP, "TS 36.321 v13.2.0 - Evolved Universal Terrestrial 695 Radio Access (E-UTRA); Medium Access Control (MAC) protocol 696 specification", 2016 698 o TGPP36323 3GPP, "TS 36.323 v13.2.0 - Evolved Universal Terrestrial 699 Radio Access (E-UTRA); Packet Data Convergence Protocol (PDCP) 700 specification", 2016. 702 o TGPP36331 3GPP, "TS 36.331 v13.2.0 - Evolved Universal Terrestrial 703 Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol 704 specification", 2016. 706 o TGPP36300 3GPP, "TS 36.300 v15.1.0 - Evolved Universal Terrestrial 707 Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio 708 Access Network (E-UTRAN); Overall description; Stage 2", 2018 710 o TGPP24301 3GPP "TS 24.301 v15.2.0 - Non-Access-Stratum (NAS) 711 protocol for Evolved Packet System (EPS); Stage 3", 2018 713 o TGPP24088 3GPP, "TS 24.088 v12.9.0 - Mobile radio interface Layer 714 3 specification;Core network protocols; Stage 3", 2015. 716 10. Appendix 718 10.1. NB-IoT User Plane protocol architecture 720 10.1.1. Packet Data Convergence Protocol (PDCP) 722 Each of the Radio Bearers (RB) are associated with one PDCP entity. 723 And a PDCP entity is associated with one or two RLC entities 724 depending of the unidirectional or bi-directional characteristics of 725 the RB and RLC mode used. A PDCP entity is associated either control 726 plane or user plane which independent configuration and functions. 727 The maximum supported size for NB-IoT of a PDCP SDU is 1600 octets. 728 The main services and functions of the PDCP sublayer for NB-IoT for 729 the user plane include: 731 o Header compression and decompression by means of ROHC (Robust 732 Header Compression) 734 o Transfer of user and control data to higher and lower layers 736 o Duplicate detection of lower layer SDUs when re-establishing 737 connection (when RLC with Acknowledge Mode in use for User Plane 738 only) 740 o Ciphering and deciphering 741 o Timer-based SDU discard in uplink 743 10.1.2. Radio Link Protocol (RLC) 745 RLC is a layer-2 protocol that operates between the UE and the base 746 station (eNB). It supports the packet delivery from higher layers to 747 MAC creating packets that are transmitted over the air optimizing the 748 Transport Block utilization. RLC flow of data packets is 749 unidirectional and it is composed of a transmitter located in the 750 transmission device and a receiver located in the destination device. 751 Therefore to configure bi-directional flows, two set of entities, one 752 in each direction (downlink and uplink) must be configured and they 753 are effectively peered to each other. The peering allows the 754 transmission of control packets (ex., status reports) between 755 entities. RLC can be configured for data transfer in one of the 756 following modes: 758 o Transparent Mode (TM). In this mode RLC do not segment or 759 concatenate SDUs from higher layers and do not include any header 760 to the payload. When acting as a transmitter, RLC receives SDUs 761 from upper layers and transmit directly to its flow RLC receiver 762 via lower layers. Similarly, an TM RLC receiver would only 763 deliver without additional processing the packets to higher layers 764 upon reception. 766 o Unacknowledged Mode (UM). This mode provides support for 767 segmentation and concatenation of payload. The size of the RLC 768 packet depends of the indication given at a particular 769 transmission opportunity by the lower layer (MAC) and are octets 770 aligned. The packet delivery to the receiver do not include 771 support for reliability and the lost of a segment from a packet 772 means a whole packet loss. Also in case of lower layer 773 retransmissions there is no support for re-segmentation in case of 774 change of the radio conditions triggering the selection of a 775 smaller transport block. Additionally it provides PDU duplication 776 detection and discard, reordering of out of sequence and loss 777 detection. 779 o Acknowledged Mode (AM). Additional to the same functions 780 supported from UM, this mode also adds a moving windows based 781 reliability service on top of the lower layer services. It also 782 provides support for re-segmentation and it requires bidirectional 783 communication to exchange acknowledgment reports called RLC Status 784 Report and trigger retransmissions is needed. Protocol error 785 detection is also supported by this mode. The mode uses depends 786 of the operator configuration for the type of data to be 787 transmitted. For example, data transmissions supporting mobility 788 or requiring high reliability would be most likely configured 789 using AM, meanwhile streaming and real time data would be map to a 790 UM configuration. 792 10.1.3. Medium Access Control (MAC) 794 MAC provides a mapping between the higher layers abstraction called 795 Logical Channels comprised by the previously described protocols to 796 the Physical layer channels (transport channels). Additionally, MAC 797 may multiplex packets from different Logical Channels and prioritize 798 what to fit into one Transport Block if there is data and space 799 available to maximize the efficiency of data transmission. MAC also 800 provides error correction and reliability support by means of HARQ, 801 transport format selection and scheduling information reporting from 802 the terminal to the network. MAC also adds the necessary padding and 803 piggyback control elements when possible additional to the higher 804 layers data. 806 807 +---+ +---+ +------+ 808 Application |AP1| |AP1| | AP2 | 809 (IP/non-IP) |PDU| |PDU| | PDU | 810 +---+ +---+ +------+ 811 | | | | | | 812 PDCP +--------+ +--------+ +-----------+ 813 |PDCP|AP1| |PDCP|AP1| |PDCP| AP2 | 814 |Head|PDU| |Head|PDU| |Head| PDU | 815 +--------+ +--------+ +--------+--\ 816 | | | | | | | | |\ `----\ 817 +---------------------------+ | |(1)| `-----\(2)'-\ 818 RLC |RLC |PDCP|AP1|RLC |PDCP|AP1| +-------------+ +----|---+ 819 |Head|Head|PDU|Head|Head|PDU| |RLC |PDCP|AP2| |RLC |AP2| 820 +-------------|-------------+ |Head|Head|PDU| |Head|PDU| 821 | | | | | +---------|---+ +--------+ 822 | | | LCID1 | | / / / / / 823 / / / _/ _// _/ _/ / LCID2 / 824 | | | | | / _/ _/ / ___/ 825 | | | | || | | / / 826 +------------------------------------------+ +-----------+---+ 827 MAC |MAC|RLC|PDCP|AP1|RLC|PDCP|AP1|RLC|PDCP|AP2| |MAC|RLC|AP2|Pad| 828 |Hea|Hea|Hea |PDU|Hea|Hea |PDU|Hea|Hea |PDU| |Hea|Hea|PDU|din| 829 |der|der|der | |der|der | |der|der | | |der|der| |g | 830 +------------------------------------------+ +-----------+---+ 831 TB1 TB2 833 Figure 6: Example of User Plane packet encapsulation for two 834 transport blocks 836 10.2. NB-IoT Data over NAS (DoNAS) 838 The AS protocol stack used by DoNAS is somehow special. Since the 839 security associations are not established yet in the radio network, 840 to reduce the protocol overhead, PDCP (Packet Data Convergence 841 Protocol) is bypassed until AS security is activated. RLC (Radio 842 Link Control protocol) is configured by default in AM mode, but 843 depending of the features supported by the network and the terminal 844 it may be configured in other modes by the network operator. For 845 example, the transparent mode does not add any header or does not 846 process the payload in any way reducing the overhead, but the MTU 847 would be limited by the transport block used to transmit the data 848 which is couple of thousand of bits maximum. If UM (only Release 15 849 compatible terminals) is used, the RLC mechanisms of reliability is 850 disabled and only the reliability provided by the MAC layer by Hybrid 851 Automatic Repeat reQuest (HARQ) is available. In this case, the 852 protocol overhead might be smaller than for the AM case because the 853 lack of status reporting but with the same support for segmentation 854 up to 16000 Bytes. NAS packet are encapsulated within a RRC (Radio 855 Resource Control) TGPP36331 message. 857 Depending of the data type indication signaled (IP or non-IP data), 858 the network allocates an IP address or just establish a direct 859 forwarding path. DoNAS is regulated under rate control upon previous 860 agreement, meaning that a maximum number of bits per unit of time is 861 agreed per device subscription beforehand and configured in the 862 device. The use of DoNAS is typically expected when a terminal in a 863 power saving state requires to do a short transmission and receive an 864 acknowledgment or short feedback from the network. Depending of the 865 size of buffered data to transmit, the UE might be instructed to 866 deploy the connected mode transmissions instead, limiting and 867 controlling the DoNAS transmissions to predefined thresholds and a 868 good resource optimization balance for the terminal and the network. 869 The support for mobility of DoNAS is present but produces additional 870 overhead. 872 +--------+ +--------+ +--------+ 873 | | | | | | +-----------------+ 874 | UE | | C-BS | | C-SGN | |Roaming Scenarios| 875 +----|---+ +--------+ +--------+ | +--------+ | 876 | | | | | | | 877 +----------------|------------|+ | | P-GW | | 878 | Attach | | +--------+ | 879 +------------------------------+ | | | 880 | | | | | | 881 +------|------------|--------+ | | | | 882 |RRC Connection Establishment| | | | | 883 |with NAS PDU transmission | | | | | 884 |& Ack Rsp | | | | | 885 +----------------------------+ | | | | 886 | | | | | | 887 | |Initial UE | | | | 888 | |message | | | | 889 | |----------->| | | | 890 | | | | | | 891 | | +---------------------+| | | 892 | | |Checks Integrity || | | 893 | | |protection, decrypts || | | 894 | | |data || | | 895 | | +---------------------+| | | 896 | | | Small data packet | 897 | | |-------------------------------> 898 | | | Small data packet | 899 | | |<------------------------------- 900 | | +----------|---------+ | | | 901 | | Integrity protection,| | | | 902 | | encrypts data | | | | 903 | | +--------------------+ | | | 904 | | | | | | 905 | |Downlink NAS| | | | 906 | |message | | | | 907 | |<-----------| | | | 908 +-----------------------+ | | | | 909 |Small Data Delivery, | | | | | 910 |RRC connection release | | | | | 911 +-----------------------+ | | | | 912 | | 913 | | 914 +-----------------+ 916 Figure 7: DoNAS transmission sequence from an Uplink initiated access 917 +---+ +---+ +---+ +----+ 918 Application |AP1| |AP1| |AP2| |AP2 | 919 (IP/non-IP) |PDU| |PDU| |PDU| ............... |PDU | 920 +---+ +---+ +---+ +----+ 921 | |/ / | \ | | 922 NAS /RRC +--------+---|---+----+ +---------+ 923 |NAS/|AP1|AP1|AP2|NAS/| |NAS/|AP2 | 924 |RRC |PDU|PDU|PDU|RRC | |RRC |PDU | 925 +--------+-|-+---+----+ +---------| 926 | |\ | | | 927 |<--Max. 1600 bytes-->|__ |_ | 928 | | \__ \___ \_ \_ 929 | | \ \ \__ \_ 930 +---------------|+-----|----------+ \ \ 931 RLC |RLC | NAS/RRC ||RLC | NAS/RRC | +----|-------+ 932 |Head| PDU(1/2)||Head | PDU (2/2)| |RLC |NAS/RRC| 933 +---------------++----------------+ |Head|PDU | 934 | | | \ | +------------+ 935 | | LCID1 | \ | | / 936 | | | \ \ | | 937 | | | \ \ | | 938 | | | \ \ \ | 939 +----+----+----------++-----|----+---------++----+---------|---+ 940 MAC |MAC |RLC | RLC ||MAC |RLC | RLC ||MAC | RLC |Pad| 941 |Head|Head| PAYLOAD ||Head |Head| PAYLOAD ||Head| PDU | | 942 +----+----+----------++-----+----+---------++----+---------+---+ 943 TB1 TB2 TB3 945 Figure 8: Example of User Plane packet encapsulation for Data over 946 NAS 948 11. Normative References 950 [RFC5795] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust 951 Header Compression (ROHC) Framework", RFC 5795, 952 DOI 10.17487/RFC5795, March 2010, . 955 [RFC8376] Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN) 956 Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018, 957 . 959 [RFC8724] Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC. 960 Zuniga, "SCHC: Generic Framework for Static Context Header 961 Compression and Fragmentation", RFC 8724, 962 DOI 10.17487/RFC8724, April 2020, . 965 Authors' Addresses 967 Edgar Ramos 968 Ericsson 969 Hirsalantie 11 970 02420 Jorvas, Kirkkonummi 971 Finland 973 Email: edgar.ramos@ericsson.com 975 Ana Minaburo 976 Acklio 977 1137A Avenue des Champs Blancs 978 35510 Cesson-Sevigne Cedex 979 France 981 Email: ana@ackl.io