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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: November 18, 2020 Acklio 6 May 17, 2020 8 SCHC over NB-IoT 9 draft-ietf-lpwan-schc-over-nbiot-02 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 November 18, 2020. 38 Copyright Notice 40 Copyright (c) 2020 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(TBD) . . . . . . . . . . . . 14 80 7. Padding . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 81 8. Security considerations . . . . . . . . . . . . . . . . . . . 16 82 9. 3GPP References . . . . . . . . . . . . . . . . . . . . . . . 16 83 10. Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . 17 84 10.1. NB-IoT User Plane protocol architecture . . . . . . . . 17 85 10.1.1. Packet Data Convergence Protocol (PDCP) . . . . . . 17 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. Informative References . . . . . . . . . . . . . . . . . . . 22 90 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23 92 1. Introduction 94 The Static Context Header Compression (SCHC) 95 [RFC8724] defines a header compression 96 scheme and fragmentation functionality, both specially tailored for 97 Low Power Wide Area Networks (LPWAN) networks 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 113 [RFC8724], in [RFC8376], and the 114 [TGPP23720]. 116 o CIoT. Cellular IoT 118 o C-SGN. CIoT Serving Gateway Node 120 o UE. User Equipment 122 o eNB. Node B. Base Station that controls the UE 124 o EPC. Evolved Packet Connectivity. Core network of 3GPP LTE 125 systems. 127 o EUTRAN. Evolved Universal Terrestrial Radio Access Network. 128 Radio network from LTE based systems. 130 o MME. Mobility Management Entity. Handle mobility of the UE 132 o NB-IoT. Narrow Band IoT. Referring to 3GPP LPWAN technology 133 based in LTE architecture but with additional optimization for IoT 134 and using a Narrow Band spectrum frequency. 136 o SGW. Serving Gateway. Routes and forwards the user data packets 137 through the access network 139 o HSS. Home Subscriber Server. It is a database that performs 140 mobility management 142 o PGW. Packet Data Node Gateway. An interface between the internal 143 with the external network 145 o PDU. Protocol Data Unit. Data packets including headers that are 146 transmitted between entities through a protocol. 148 o SDU. Service Data Unit. Data packets (PDUs) from higher layers 149 protocols used by lower layer protocols as a payload of their own 150 PDUs that has not yet been encapsulated. 152 o IWK-SCEF. InterWorking Service Capabilities Exposure Function. 153 Used in roaming scenarios and serves for interconnection with the 154 SCEF of the Home PLMN and is located in the Visited PLMN 156 o SCEF. Service Capability Exposure Function. EPC node for 157 exposure of 3GPP network service capabilities to 3rd party 158 applications. 160 3. Architecture 162 +--+ 163 |UE| \ +-----+ +------+ 164 +--+ \ | MME |-----| HSS | 165 \ / +-----+ +------+ 166 +--+ \+-----+ / | 167 |UE| ----| eNB |- | 168 +--+ /+-----+ \ | 169 / \ +------+ 170 / \| | +------+ Service PDN 171 +--+ / | S-GW |--| P-GW |-- e.g. Internet 172 |UE| | | +------+ 173 +--+ +------+ 175 Figure 1: 3GPP network architecture 177 The architecture for 3GPP LTE network has been reused for NB-IoT with 178 some optimizations and simplifications known as Cellular IoT (CIoT). 179 Considering the typical use cases for CIoT devices here are described 180 some of the additions to the LTE architecture specific for CIoT. 181 C-SGN(CIoT Serving Gateway Node) is a deployment option co-locating 182 EPS entities in the control plane and user plane paths (for example, 183 MME + SGW + P-GW) and the external interfaces of the entities 184 supported. The C-SGN also supports at least some of the following 185 CIoT EPS Optimizations: 187 o Control Plane CIoT EPS Optimization for small data transmission. 189 o User Plane CIoT EPS Optimization for small data transmission. 191 o Necessary security procedures for efficient small data 192 transmission. 194 o SMS without combined attach for NB-IoT only UEs. 196 o Paging optimizations for coverage enhancements. 198 o Support for non-IP data transmission via SGi tunneling and/or 199 SCEF. 201 o Support for Attach without PDN (Packet Data Network) connectivity. 203 Another node introduced in the CIOT architecture is the SCEF (Service 204 Capability Exposure Function) that provide means to securely expose 205 service and network capabilities to entities external to the network 206 operator. The northbound APIS are defined by OMA and OneM2M. The 207 main functions of a SCEF are: 209 o Non-IP Data Delivery (NIDD) established through the SCEF. 211 o Monitoring and exposure of event related to UE reachability, loss 212 of connectivity, location reporting, roaming status, communication 213 failure and change of IMEI-IMSI association. 215 +-------+ 216 | HSS | 217 +-+-----+ 218 / 219 +---------+ __/S6a 220 +--------+ | +-----+ +_/ 221 +----+ C-Uu | +---+-+ MME | | T6i+--------+ T7 +----+ 222 |CIOT+--------+ eNB |S1 | | +-+----+IWK-SCEF+----+SCEF| 223 |UE | |(NB-IoT)| | +---+-+ | +--------+ +----+ 224 +----+ +--------+ | | | 225 |C-SGN| | 226 | |S11| 227 +------+ | | | 228 +--------+LTE-Uu| | | +--+-+ | 229 |LTE eMTC|(eMTC)|eNB +---+--+SGW | | S8+---+ +-----------+ 230 | UE +------+(eMTC)|S1 | | +-+---+PGW|SGi |Application| 231 +--------+ +------+ | +----+ | | +----+Server (AS)| 232 +---------+ +---+ +-----------+ 234 Figure 2: 3GPP optimized CIOT network architecture 236 4. Data Transmission 238 3GPP networks deal not only with data transmitted end-to-end but also 239 with in-band signaling that is used between the nodes and functions 240 to configure, control and monitor the system functions and behaviors. 241 The control data is handled using a Control Plane which has a 242 specific set of protocols, handling processes and entities. In 243 contrast, the end-to-end or user data utilize a User Plane with 244 characteristics of its own separated from the Control Plane. The 245 handling and setup of the Control Plane and User Plane spans over the 246 whole 3GPP network and it has particular implications in the radio 247 network (i.e., EUTRAN) and in the packet core (ex., EPC). 249 For the CIOT cases, additionally to transmissions of data over User 250 Plane, 3GPP has specified optimizations for small data transmissions, 251 allowing to transport user data (IP, Non-IP) within signaling on the 252 access network (Data transmission over Control Plane or Data Over 253 NAS). 255 The maximum recommended MTU size is 1358 Bytes. The radio network 256 protocols limit the packet sizes to be transmitted over the air 257 including radio protocol overhead to 1600 Octets. But the value is 258 reduced further to avoid fragmentation in the backbone of the network 259 due to the payload encryption size (multiple of 16) and handling of 260 the additional core transport overhead. 262 NB-IoT and in general the cellular technologies interfaces and 263 functions are standardized by 3GPP. Therefore the introduction of 264 SCHC entities to UE, eNB and C-SGN does need to be specified in the 265 NB-IoT standard. This implies that standard specified SCHC support 266 would not be backwards compatible. A terminal or a network 267 supporting a version of the standard without support of SCHC or 268 without capability implementation (in case of not being standardized 269 as mandatory capability) is not able to utilize the compression 270 services with this approach. 272 SCHC could be deployed differently depending on where the header 273 compression and the fragmentation are applied. The SCHC 274 functionalities could be applied to the packets about to be 275 transmitted over the air, or to the whole end-to-end link. To 276 accomplish the first, it is required to place SCHC compression and 277 decompression entities in the eNB and in the UE for transmissions 278 over the User Plane. Additionally, to handle the case of the 279 transmissions over Control Plane or Data Over NAS, the network SCHC 280 entity has to be placed in the C-SGN as well. For these two cases, 281 the functions are to be standardized by 3GPP. 283 Another possibility is to apply SCHC functionalities to the end-to- 284 end connection or at least up to the operator network edge. In that 285 case, the SCHC entities would be placed in the application layer of 286 the terminal in one end, and either in the application servers or in 287 a broker function in the edge of the operator network in the other 288 end. For the radio network, the packets are transmitted as non-IP 289 traffic, which can be currently served utilizing IP tunneling or SCEF 290 services. Since this option does not necessarily require 3GPP 291 standardization, it is possible to also benefit legacy devices with 292 SCHC by utilizing the non-IP transmission features of the operator 293 network. 295 Accordingly, there are four different scenarios where SCHC can be 296 used in the NB-IoT architecture. IP header compression on the data 297 transmission over User Plane, IP header compression on the optimized 298 transmissions over Control Plane (i.e.,DoNAS), non-IP transmissions 299 of SCHC packets by IP tunneling, and non-IP transmissions of SCHC 300 packets by SCEF forwarding. The following sections describe each of 301 them in more detail. The first two scenarios refer to transmissions 302 using the 3GPP IP transmission capabilities and the last two refers 303 to transmission using the Non-IP capabilities. 305 5. IP based Data Transmission 307 5.1. SCHC over User Plane transmissions 309 Deploying SCHC only over the radio link would require to place it as 310 part of the User Plane data transmission. The User Plane utilizes 311 the protocol stack of the Access Stratum (AS) for data transfer. AS 312 (Access Stratum) is the functional layer responsible for transporting 313 data over wireless connection and managing radio resources. The user 314 plane AS has support for features such as reliability, segmentation 315 and concatenation. The transmissions of the AS make use of link 316 adaptation, meaning that the transport format utilized for the 317 transmissions are optimized according to the radio conditions, the 318 number of bits to transmit and the power and interference constrains. 319 That means that the number of bits transmitted over the air depends 320 of the Modulation and Coding Schemes (MCS) selected. The 321 transmissions in the physical layer happens at network synchronized 322 intervals of times called TTI (Transmission Time Interval). The 323 transmission of a Transport Block (TB) is completed during, at least, 324 one TTI. Each Transport Block has a different MCS and number of bits 325 available to transmit. The Transport Blocks characteristics are 326 defined by the MAC technical specification [TGPP36321]. The Access 327 Stratum for User Plane is comprised by Packet Data Convergence 328 Protocol (PDCP) [TGPP36323], Radio Link Protocol (RLC)[TGPP36322], 329 Medium Access Control protocol (MAC)[TGPP36321] and the Physical 330 Layer [TGPP36201]. More details of this protocols are given in the 331 Appendix. 333 5.1.1. SCHC Entities Placing 335 The current architecture provides support for header compression in 336 PDCP utilizing RoHC [RFC5795]. Therefore SCHC entities can be 337 deployed in similar fashion without need for major changes in the 338 3GPP specifications. 340 In this scenario, RLC takes care of the handling of fragmentation (if 341 transparent mode is not configured) when packets exceeds the 342 transport block size at the time of transmission. Therefore SCHC 343 fragmentation is not needed and should not be used to avoid 344 additional protocol overhead. It is not common to configure RLC in 345 Transparent Mode for IP based user plane data. But given the case in 346 the future, SCHC fragmentation may be used. In that case, a SCHC 347 tile would match the minimum transport block size minus the PDCP and 348 MAC headers. 350 +---------+ +---------+ | 351 |IP/non-IP+------------------------------+IP/non-IP+->+ 352 +---------+ | +---------------+ | +---------+ | 353 | PDCP +-------+ PDCP | GTP|U +------+ GTP-U |->+ 354 | (SCHC) + + (SCHC)| + + | | 355 +---------+ | +---------------+ | +---------+ | 356 | RLC +-------+ RLC |UDP/IP +------+ UDP/IP +->+ 357 +---------+ | +---------------+ | +---------+ | 358 | MAC +-------+ MAC | L2 +------+ L2 +->+ 359 +---------+ | +---------------+ | +---------+ | 360 | PHY +-------+ PHY | PHY +------+ PHY +->+ 361 +---------+ +---------------+ +---------+ | 362 C-Uu/ S1-U SGi 363 CIOT/ LTE+Uu C-BS/eNB C-SGN 364 LTE eMTC 365 UE 367 Figure 3: SCHC entities placement in the 3GPP CIOT radio protocol 368 architecture for data over user plane 370 5.2. Data Over Control Plane 372 The Non-Access Stratum (NAS), conveys mainly control signaling 373 between the UE and the cellular network [TGPP24301]. NAS is 374 transported on top of the Access Stratum (AS) already mentioned in 375 the previous section. 377 NAS has been adapted to provide support for user plane data 378 transmissions to reduce the overhead when transmitting infrequent 379 small quantities of data. This is known as Data over NAS (DoNAS) or 380 Control Plane CIoT EPS optimization. In DoNAS the UE makes use of 381 the pre-established NAS security and piggyback uplink small data into 382 the initial NAS uplink message, and uses an additional NAS message to 383 receive downlink small data response. 385 The data encryption from the network side is performed by the C-SGN 386 in a NAS PDU. Depending on the data type signaled indication (IP or 387 non-IP data), the network allocates an IP address or just establish a 388 direct forwarding path. DoNAS (Data over NAS) is regulated under 389 rate control upon previous agreement, meaning that a maximum number 390 of bits per unit of time is agreed per device subscription beforehand 391 and configured in the device. 393 The use of DoNAS is typically expected when a terminal in a power 394 saving state requires to do a short transmission and receive an 395 acknowledgment or short feedback from the network. Depending on the 396 size of buffered data to transmit, the UE might be instructed to 397 deploy the connected mode transmissions instead, limiting and 398 controlling the DoNAS transmissions to predefined thresholds and a 399 good resource optimization balance for the terminal and the network. 400 The support for mobility of DoNAS is present but produces additional 401 overhead. Additional details of DoNAS are given in the Appendix. 403 5.2.1. SCHC Entities Placing 405 In this scenario SCHC can be applied in the NAS protocol layer 406 instead of PDCP. The same principles than for user plane 407 transmissions applies here as well. The main difference is the 408 physical placing of the SCHC entities in the network side as the 409 C-SGN (placed in the core network) is the terminating node for NAS 410 instead of the eNB. 412 +--------+ +--------+--------+ + +--------+ 413 | IP/ +--+-----------------+--+ IP/ | IP/ +-----+ IP/ | 414 | Non-IP | | | | Non-IP | Non-IP | | | Non-IP | 415 +--------+ | | +-----------------+ | +--------+ 416 | NAS +-----------------------+ NAS |GTP|C/U +-----+GTP|C/U | 417 |(SCHC) | | | | (SCHC) | | | | | 418 +--------+ | +-----------+ | +-----------------+ | +--------+ 419 | RRC +-----+RRC |S1|AP+-----+ S1|AP | | | | | 420 +--------+ | +-----------+ | +--------+ UDP +-----+ UDP | 421 | PDCP* +-----+PDCP*|SCTP +-----+ SCTP | | | | | 422 +--------+ | +-----------+ | +-----------------+ | +--------+ 423 | RLC +-----+ RLC | IP +-----+ IP | IP +-----+ IP | 424 +--------+ | +-----------+ | +-----------------+ | +--------+ 425 | MAC +-----+ MAC | L2 +-----+ L2 | L2 +-----+ L2 | 426 +--------+ | +-----------+ | +-----------------+ | +--------+ 427 | PHY +--+--+ PHY | PHY +--+--+ PHY | PHY +-----+ PHY | 428 +--------+ +-----+-----+ +--------+--------+ | +--------+ 429 C-Uu/ S1-lite SGi 430 CIOT/ LTE-Uu C-BS/eNB C-SGN PGW 431 LTE eMTC 432 UE 434 *PDCP is bypassed until AS security is activated [TGPP36300]. 436 Figure 4 438 5.3. Parameters for Static Context Header Compression (SCHC) 440 5.3.1. SCHC Context initialization 442 RRC (Radio Resource Control) protocol is the main tool used to 443 configure the operation parameters of the AS transmissions for 3GPP 444 technologies. RoHC operation is configured with this protocol and it 445 is to expect that SCHC will be configured and the static context 446 distributed in similar fashion for these scenarios. 448 5.3.2. SCHC Rules 450 The number of rules in a context are defined by the network operator 451 in these scenarios. For this, the operator must be aware of the type 452 of IP traffic that the device will carry out. This means that the 453 operator might provision sets of rules compatible with the use case 454 of the device. For devices acting as gateways of other devices 455 several rules that match the diversity of devices and protocols used 456 by the devices associated to the gateway. Meanwhile than simpler 457 devices (for example an electricity meter) may have a predetermined 458 set of protocols and parameters fixed. Additionally, the deployment 459 of IPV4 addresses in addition to IPV6 may force to provision separate 460 rules to deal with each of the cases. 462 5.3.3. Rule ID 464 For these transmission scenarios in NB-IoT, a reasonable assumption 465 of 9 bytes of radio protocol overhead can be expected. PDCP 5 bytes 466 due to header and integrity protection, and 4 bytes of RLC and MAC. 467 The minimum physical Transport Block (TB) that can withhold this 468 overhead value according to 3GPP Release 15 specifications are: 88, 469 104, 120 and 144 bits. If it is wished to optimize the number of 470 transmissions of a very small application packet so that in some 471 cases can be transmitted using only one physical layer transmission, 472 then the SCHC overhead should not exceed the available number of bits 473 of the smallest utile physical TB available. The packets handled by 474 3GPP networks are byte-aligned, and therefore the minimum payload 475 possible (including padding) is 8 bits. Therefore in order to 476 utilize the smallest TB the maximum SCHC is 8 bits. This must 477 include the Compression Residue in addition to the Rule ID. In the 478 other hand, it is possible that more complex NB-IoT devices (such as 479 a capillarity gateway) might require additional bits to handle the 480 variety and multiple parameters the of higher layer protocols 481 deployed. In that sense, the operator may want to have flexibility 482 on the number and type of rules supported by each device 483 independently, and consequently a configurable value is preferred for 484 these scenarios. The configuration may be set as part of the 485 operation profile agreed together with the context distribution. The 486 Rule Id field size may range for example from 2 bits resulting in 4 487 rules to a 8 bits value that would yield up to 256 rules which can be 488 used together with the operators and seems quite a reasonable maximum 489 limit even for a device acting as a NAT. More bits could be 490 configured, but it should take in account the byte-alignment of the 491 expected Compression Residue too. In the minimum TB size case, 2 492 bits size of Rule Id leave only 6 bits available for Compression 493 Residue. 495 5.3.4. SCHC MAX_PACKET_SIZE 497 The Access Stratum can handle the fragmentation of SCHC packets if 498 needed including reliability. Hence the packet size is limited by 499 the MTU possible to be handled by the AS radio protocols that 500 corresponds to 1600 bytes for 3GPP Release 15. 502 5.3.5. Fragmentation 504 For these scenarios the SCHC fragmentation functions are recommend to 505 be disabled. The RLC layer of NB-IoT can segment packets in suitable 506 units that fit the selected transport blocks for transmissions of the 507 physical layer. The selection of the blocks is done according to the 508 input of the link adaptation function in the MAC layer and the 509 quantity of data in the buffer. The link adaptation layer may 510 produce different results at each Time Transmission Interval (TTI) 511 resulting in varying physical transport blocks that depends of the 512 network load, interference and number of bits to be transmitted and 513 QoS. Even if setting a value that allows the construction of data 514 units following SCHC tiles principle, the protocol overhead may be 515 greater or equal than allowing the AS radio protocols to take care of 516 the fragmentation natively. 518 5.3.5.1. Fragmentation in Transparent Mode 520 If RLC is configured to operate in Transparent Mode, there could be a 521 case to activate a fragmentation function together with a light 522 reliability function such as the ACK-Always mode. In practice , it 523 is very rare to transmit user plane data using this configuration and 524 it is mainly targeting control plane transmissions. In those cases 525 the reliability is normally ensured by MAC based mechanisms, such as 526 repetitions or automatic retransmissions, and additional reliability 527 might only generate protocol overhead. 529 In future operations, it could be devised the utilization of SCHC to 530 reduce radio network protocols overhead and support the reliability 531 of the transmissions, and targeting small data with the fewer 532 possible transmissions. This could be realized by using fixed or 533 limited set of transport blocks compatible with the tiling SCHC 534 fragmentation handling. 536 6. Non-IP based Data Transmission 538 The Non-IP Data Delivery (NIDD) services of 3GPP enable the 539 possibility of transmitting SCHC packets compressed by the 540 application layer. The packets can be delivered by means of IP- 541 tunnels to the 3GPP network or using SCEF functions (i.e., API 542 calls). In both cases the packet IP is not understood by the 3GPP 543 network since it is already compressed and the network does not has 544 information of the context used for compression. Therefore the 545 network will treat the packet as a Non-IP traffic and deliver it to 546 the UE without any other stack element, directly under the L2. 548 6.1. SCHC Entities Placing 550 In the two scenarios using NIDD, SCHC entities are located almost in 551 top of the stack. In the terminal, it may be implemented by a 552 application utilizing the NB-IoT connectivity services. In the 553 network side, the SCHC entities are located in the Application Server 554 (AS). The IP tunneling scenario requires that the Application Server 555 sends the compressed packet over an IP connection that is terminated 556 by the 3GPP core network. If instead the SCEF services are used, 557 then it is possible to utilize a API call to transfer the SCHC 558 packets between the core network and the AS, also an IP tunnel could 559 be established by the AS, if negotiated with the SCEF. 561 +---------+ XXXXXXXXXXXXXXXXXXXXXXXX +--------+ 562 | SCHC | XXX XXX | SCHC | 563 |(Non-IP) +-----XX........................XX....+--*---+(Non-IP)| 564 +---------+ XX +----+ XX | | +--------+ 565 | | XX |SCEF+-------+ | | | 566 | | XXX 3GPP RAN & +----+ XXX +---+ UDP | 567 | | XXX CORE NETWORK XXX | | | 568 | L2 +---+XX +------------+ | +--------+ 569 | | XX |IP TUNNELING+--+ | | 570 | | XXX +------------+ +---+ IP | 571 +---------+ XXXX XXXX | +--------+ 572 | PHY +------+ XXXXXXXXXXXXXXXXXXXXXXX +---+ PHY | 573 +---------+ +--------+ 574 UE AS 576 Figure 5: SCHC entities placed when using Non-IP Delivery (NIDD) 3GPP 577 Sevices 579 6.2. Parameters for Static Context Header Compression 581 6.2.1. SCHC Context initialization 583 The static context is handled in the application layer level, 584 consequently the contexts are required to be distributed according to 585 the applications own capabilities, perhaps utilizing IP data 586 transmissions up to context initialization. Also the same IP 587 tunneling or SCEF services used later for the SCHC packets transport 588 may be used by the applications in both ends to deliver the static 589 contexts to be used. 591 6.2.2. SCHC Rules 593 Even when the transmissions content are not visible for the 3GPP 594 network, the same limitations than for IP based data transmissions 595 applies in these scenarios in terms of aiming to use the minimum 596 number of transmission and minimize the protocol overhead. 598 6.2.3. Rule ID 600 Similarly to the case of IP transmissions, the Rule ID size can be 601 dynamically set prior the context delivery. For example negotiated 602 between the applications when choosing a profile according to the 603 type of traffic and type of application deployed. Same 604 considerations related to the transport block size and performance 605 mentioned for the IP type of traffic has to be follow when choosing a 606 size value for the Rule ID field. 608 6.2.4. SCHC MAX_PACKET_SIZE 610 In these scenarios the maximum recommended MTU size that applies is 611 1358 Bytes, since the SCHC packets (and fragments) are traversing the 612 whole 3GPP network infrastructure (core and radio), and not only the 613 radio as the IP transmissions case. 615 6.3. Fragmentation 617 In principle the fragmentation function should be activated for 618 packets greater than 1358 Bytes. Since the 3GPP reliability 619 functions take great deal care of it, for simple point to point 620 connections may be enough a NO-ACK mode. Nevertheless additional 621 considerations for more complex cases are mentioned in the next 622 subsection to be taken in account. 624 6.3.1. Fragmentation modes 626 Depending of the QoS that has been assigned to the packets, it is 627 possible that packets are lost before they arrive to 3GPP radio 628 network transmission, for example in between the links of a 629 capillarity gateway, or due to buffer overflow handling in a backhaul 630 connection. In consequence, it is possible to secure additional 631 reliability on the packets transmitted with a small trade-off on 632 additional transmissions to signal the packets arrival indication 633 end-to-end if no transport protocol takes care of retransmission. To 634 achieve this, the packets fragmentation is activated with the ACK-on- 635 Error mode enabled. In some cases, it is even desirable to keep 636 track of all the SCHC packets delivered, in that case, the 637 fragmentation function could be active for all packets transmitted by 638 the applications (SCHC MAX_PACKET_SIZE == 1 Byte) and the ACK-on- 639 Error mode. 641 6.3.2. Fragmentation Parameters(TBD) 643 o Rule ID. The Fragmentation Rule ID is given when choosing the 644 profile according to the fragmentation mode require. 1 bit can be 645 used to recognize each mode. 647 o DTag. 649 No_ACK. May take 1 bit. 651 ACK_on_Error. May take 1 bit. 653 o FCN (N value). 655 No_ACK. The value of N is 1. 657 ACK_on_Error. The value of N depends on the 659 o W (M value) 661 No_ACK. This field is not used in this mode 663 ACK_on_Error. 665 o WINDOW_SIZE 667 No_ACK. This mode does not use windows 669 ACK_on_Error. 671 o Retransmission Timer 673 No_ACK. This timmer is not used in this mode 675 ACK_on_Error. This timer needs to be set to 1h or 10h 677 o Inactivity Timer 679 No_ACK. Must be maintained and needs to be bigger than 1h or 10h 681 ACK_on_Error. Must be bigger than 1h or 10h 683 o MAX_ACK_REQUESTS 685 No_ACK. Not used in this mode. 687 ACK_on_Error. 689 o MIC (size and algorithm) 691 No_ACK. 693 ACK_on_Error. 695 o RCS 697 No_ACK 699 ACK_on_Error. 701 o Tiles size 703 No_ACK. Not used in this mode. 705 ACK_on_Error. (also mention if the last tile is carried in a 706 regular fragment or in All-1 fragment) 708 7. Padding 710 NB-IoT and 3GPP wireless access, in general, assumes byte aligned 711 payload. Therefore the L2 word for NB-IoT MUST be considered 8 bits 712 and the treatment of padding should use this value accordingly. 714 8. Security considerations 716 3GPP access security is specified in (TGPP33203). 718 9. 3GPP References 720 o [TGPP23720] 3GPP, "TR 23.720 v13.0.0 - Study on architecture 721 enhancements for Cellular Internet of Things", 2016. 723 o [TGPP33203] 3GPP, "TS 33.203 v13.1.0 - 3G security; Access 724 security for IP-based services", 2016. 726 o [TGPP36321] 3GPP, "TS 36.321 v13.2.0 - Evolved Universal 727 Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) 728 protocol specification", 2016 730 o [TGPP36323] 3GPP, "TS 36.323 v13.2.0 - Evolved Universal 731 Terrestrial Radio Access (E-UTRA); Packet Data Convergence 732 Protocol (PDCP) specification", 2016. 734 o [TGPP36331] 3GPP, "TS 36.331 v13.2.0 - Evolved Universal 735 Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); 736 Protocol specification", 2016. 738 o [TGPP36300] 3GPP, "TS 36.300 v15.1.0 - Evolved Universal 739 Terrestrial Radio Access (E-UTRA) and Evolved Universal 740 Terrestrial Radio Access Network (E-UTRAN); Overall description; 741 Stage 2", 2018 743 o [TGPP24301] 3GPP "TS 24.301 v15.2.0 - Non-Access-Stratum (NAS) 744 protocol for Evolved Packet System (EPS); Stage 3", 2018 746 10. Appendix 748 10.1. NB-IoT User Plane protocol architecture 750 10.1.1. Packet Data Convergence Protocol (PDCP) 752 Each of the Radio Bearers (RB) are associated with one PDCP entity. 753 And a PDCP entity is associated with one or two RLC entities 754 depending of the unidirectional or bi-directional characteristics of 755 the RB and RLC mode used. A PDCP entity is associated either control 756 plane or user plane which independent configuration and functions. 757 The maximum supported size for NB-IoT of a PDCP SDU is 1600 octets. 758 The main services and functions of the PDCP sublayer for NB-IoT for 759 the user plane include: 761 o Header compression and decompression by means of ROHC (Robust 762 Header Compression) 764 o Transfer of user and control data to higher and lower layers 766 o Duplicate detection of lower layer SDUs when re-establishing 767 connection (when RLC with Acknowledge Mode in use for User Plane 768 only) 770 o Ciphering and deciphering 772 o Timer-based SDU discard in uplink 774 10.1.2. Radio Link Protocol (RLC) 776 RLC is a layer-2 protocol that operates between the UE and the base 777 station (eNB). It supports the packet delivery from higher layers to 778 MAC creating packets that are transmitted over the air optimizing the 779 Transport Block utilization. RLC flow of data packets is 780 unidirectional and it is composed of a transmitter located in the 781 transmission device and a receiver located in the destination device. 782 Therefore to configure bi-directional flows, two set of entities, one 783 in each direction (downlink and uplink) must be configured and they 784 are effectively peered to each other. The peering allows the 785 transmission of control packets (ex., status reports) between 786 entities. RLC can be configured for data transfer in one of the 787 following modes: 789 o Transparent Mode (TM). In this mode RLC do not segment or 790 concatenate SDUs from higher layers and do not include any header 791 to the payload. When acting as a transmitter, RLC receives SDUs 792 from upper layers and transmit directly to its flow RLC receiver 793 via lower layers. Similarly, an TM RLC receiver would only 794 deliver without additional processing the packets to higher layers 795 upon reception. 797 o Unacknowledged Mode (UM). This mode provides support for 798 segmentation and concatenation of payload. The size of the RLC 799 packet depends of the indication given at a particular 800 transmission opportunity by the lower layer (MAC) and are octets 801 aligned. The packet delivery to the receiver do not include 802 support for reliability and the lost of a segment from a packet 803 means a whole packet loss. Also in case of lower layer 804 retransmissions there is no support for re-segmentation in case of 805 change of the radio conditions triggering the selection of a 806 smaller transport block. Additionally it provides PDU duplication 807 detection and discard, reordering of out of sequence and loss 808 detection. 810 o Acknowledged Mode (AM). Additional to the same functions 811 supported from UM, this mode also adds a moving windows based 812 reliability service on top of the lower layer services. It also 813 provides support for re-segmentation and it requires bidirectional 814 communication to exchange acknowledgment reports called RLC Status 815 Report and trigger retransmissions is needed. Protocol error 816 detection is also supported by this mode. The mode uses depends 817 of the operator configuration for the type of data to be 818 transmitted. For example, data transmissions supporting mobility 819 or requiring high reliability would be most likely configured 820 using AM, meanwhile streaming and real time data would be map to a 821 UM configuration. 823 10.1.3. Medium Access Control (MAC) 825 MAC provides a mapping between the higher layers abstraction called 826 Logical Channels comprised by the previously described protocols to 827 the Physical layer channels (transport channels). Additionally, MAC 828 may multiplex packets from different Logical Channels and prioritize 829 what to fit into one Transport Block if there is data and space 830 available to maximize the efficiency of data transmission. MAC also 831 provides error correction and reliability support by means of HARQ, 832 transport format selection and scheduling information reporting from 833 the terminal to the network. MAC also adds the necessary padding and 834 piggyback control elements when possible additional to the higher 835 layers data. 837 838 +---+ +---+ +------+ 839 Application |AP1| |AP1| | AP2 | 840 (IP/non-IP) |PDU| |PDU| | PDU | 841 +---+ +---+ +------+ 842 | | | | | | 843 PDCP +--------+ +--------+ +-----------+ 844 |PDCP|AP1| |PDCP|AP1| |PDCP| AP2 | 845 |Head|PDU| |Head|PDU| |Head| PDU | 846 +--------+ +--------+ +--------+--\ 847 | | | | | | | | |\ `----\ 848 +---------------------------+ | |(1)| `-----\(2)'-\ 849 RLC |RLC |PDCP|AP1|RLC |PDCP|AP1| +-------------+ +----|---+ 850 |Head|Head|PDU|Head|Head|PDU| |RLC |PDCP|AP2| |RLC |AP2| 851 +-------------|-------------+ |Head|Head|PDU| |Head|PDU| 852 | | | | | +---------|---+ +--------+ 853 | | | LCID1 | | / / / / / 854 / / / _/ _// _/ _/ / LCID2 / 855 | | | | | / _/ _/ / ___/ 856 | | | | || | | / / 857 +------------------------------------------+ +-----------+---+ 858 MAC |MAC|RLC|PDCP|AP1|RLC|PDCP|AP1|RLC|PDCP|AP2| |MAC|RLC|AP2|Pad| 859 |Hea|Hea|Hea |PDU|Hea|Hea |PDU|Hea|Hea |PDU| |Hea|Hea|PDU|din| 860 |der|der|der | |der|der | |der|der | | |der|der| |g | 861 +------------------------------------------+ +-----------+---+ 862 TB1 TB2 864 Figure 6: Example of User Plane packet encapsulation for two 865 transport blocks 867 10.2. NB-IoT Data over NAS (DoNAS) 869 The AS protocol stack used by DoNAS is somehow special. Since the 870 security associations are not established yet in the radio network, 871 to reduce the protocol overhead, PDCP (Packet Data Convergence 872 Protocol) is bypassed until AS security is activated. RLC (Radio 873 Link Control protocol) is configured by default in AM mode, but 874 depending of the features supported by the network and the terminal 875 it may be configured in other modes by the network operator. For 876 example, the transparent mode does not add any header or does not 877 process the payload in any way reducing the overhead, but the MTU 878 would be limited by the transport block used to transmit the data 879 which is couple of thousand of bits maximum. If UM (only Release 15 880 compatible terminals) is used, the RLC mechanisms of reliability is 881 disabled and only the reliability provided by the MAC layer by Hybrid 882 Automatic Repeat reQuest (HARQ) is available. In this case, the 883 protocol overhead might be smaller than for the AM case because the 884 lack of status reporting but with the same support for segmentation 885 up to 16000 Bytes. NAS packet are encapsulated within a RRC (Radio 886 Resource Control)[TGPP36331] message. 888 Depending of the data type indication signaled (IP or non-IP data), 889 the network allocates an IP address or just establish a direct 890 forwarding path. DoNAS is regulated under rate control upon previous 891 agreement, meaning that a maximum number of bits per unit of time is 892 agreed per device subscription beforehand and configured in the 893 device. The use of DoNAS is typically expected when a terminal in a 894 power saving state requires to do a short transmission and receive an 895 acknowledgment or short feedback from the network. Depending of the 896 size of buffered data to transmit, the UE might be instructed to 897 deploy the connected mode transmissions instead, limiting and 898 controlling the DoNAS transmissions to predefined thresholds and a 899 good resource optimization balance for the terminal and the network. 900 The support for mobility of DoNAS is present but produces additional 901 overhead. 903 +--------+ +--------+ +--------+ 904 | | | | | | +-----------------+ 905 | UE | | C-BS | | C-SGN | |Roaming Scenarios| 906 +----|---+ +--------+ +--------+ | +--------+ | 907 | | | | | | | 908 +----------------|------------|+ | | P-GW | | 909 | Attach | | +--------+ | 910 +------------------------------+ | | | 911 | | | | | | 912 +------|------------|--------+ | | | | 913 |RRC Connection Establishment| | | | | 914 |with NAS PDU transmission | | | | | 915 |& Ack Rsp | | | | | 916 +----------------------------+ | | | | 917 | | | | | | 918 | |Initial UE | | | | 919 | |message | | | | 920 | |----------->| | | | 921 | | | | | | 922 | | +---------------------+| | | 923 | | |Checks Integrity || | | 924 | | |protection, decrypts || | | 925 | | |data || | | 926 | | +---------------------+| | | 927 | | | Small data packet | 928 | | |-------------------------------> 929 | | | Small data packet | 930 | | |<------------------------------- 931 | | +----------|---------+ | | | 932 | | Integrity protection,| | | | 933 | | encrypts data | | | | 934 | | +--------------------+ | | | 935 | | | | | | 936 | |Downlink NAS| | | | 937 | |message | | | | 938 | |<-----------| | | | 939 +-----------------------+ | | | | 940 |Small Data Delivery, | | | | | 941 |RRC connection release | | | | | 942 +-----------------------+ | | | | 943 | | 944 | | 945 +-----------------+ 947 Figure 7: DoNAS transmission sequence from an Uplink initiated access 948 +---+ +---+ +---+ +----+ 949 Application |AP1| |AP1| |AP2| |AP2 | 950 (IP/non-IP) |PDU| |PDU| |PDU| ............... |PDU | 951 +---+ +---+ +---+ +----+ 952 | |/ / | \ | | 953 NAS /RRC +--------+---|---+----+ +---------+ 954 |NAS/|AP1|AP1|AP2|NAS/| |NAS/|AP2 | 955 |RRC |PDU|PDU|PDU|RRC | |RRC |PDU | 956 +--------+-|-+---+----+ +---------| 957 | |\ | | | 958 |<--Max. 1600 bytes-->|__ |_ | 959 | | \__ \___ \_ \_ 960 | | \ \ \__ \_ 961 +---------------|+-----|----------+ \ \ 962 RLC |RLC | NAS/RRC ||RLC | NAS/RRC | +----|-------+ 963 |Head| PDU(1/2)||Head | PDU (2/2)| |RLC |NAS/RRC| 964 +---------------++----------------+ |Head|PDU | 965 | | | \ | +------------+ 966 | | LCID1 | \ | | / 967 | | | \ \ | | 968 | | | \ \ | | 969 | | | \ \ \ | 970 +----+----+----------++-----|----+---------++----+---------|---+ 971 MAC |MAC |RLC | RLC ||MAC |RLC | RLC ||MAC | RLC |Pad| 972 |Head|Head| PAYLOAD ||Head |Head| PAYLOAD ||Head| PDU | | 973 +----+----+----------++-----+----+---------++----+---------+---+ 974 TB1 TB2 TB3 976 Figure 8: Example of User Plane packet encapsulation for Data over 977 NAS 979 11. Informative References 981 [RFC8724] Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and J. 982 Zuniga, "SCHC: Generic Framework for Static Context Header 983 Compression and Fragmentation", April 2020, 984 . 986 [RFC5795] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust 987 Header Compression (ROHC) Framework", RFC 5795, 988 DOI 10.17487/RFC5795, March 2010, . 991 [RFC8376] Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN) 992 Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018, 993 . 995 Authors' Addresses 997 Edgar Ramos 998 Ericsson 999 Hirsalantie 11 1000 02420 Jorvas, Kirkkonummi 1001 Finland 1003 Email: edgar.ramos@ericsson.com 1005 Ana Minaburo 1006 Acklio 1007 1137A Avenue des Champs Blancs 1008 35510 Cesson-Sevigne Cedex 1009 France 1011 Email: ana@ackl.io