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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 DMM Working Group S. Matsushima, Ed. 3 Internet-Draft SoftBank 4 Intended status: Standards Track C. Filsfils 5 Expires: 27 January 2022 M. Kohno 6 P. Camarillo, Ed. 7 Cisco Systems, Inc. 8 D. Voyer 9 Bell Canada 10 C.E. Perkins 11 Lupin Lodge 12 26 July 2021 14 Segment Routing IPv6 for Mobile User Plane 15 draft-ietf-dmm-srv6-mobile-uplane-14 17 Abstract 19 This document shows the applicability of SRv6 (Segment Routing IPv6) 20 to the user-plane of mobile networks. The network programming nature 21 of SRv6 accomplishes mobile user-plane functions in a simple manner. 22 The statelessness of SRv6 and its ability to control both service 23 layer path and underlying transport can be beneficial to the mobile 24 user-plane, providing flexibility, end-to-end network slicing, and 25 SLA control for various applications. This document describes the 26 SRv6 mobile user plane. 28 Status of This Memo 30 This Internet-Draft is submitted in full conformance with the 31 provisions of BCP 78 and BCP 79. 33 Internet-Drafts are working documents of the Internet Engineering 34 Task Force (IETF). Note that other groups may also distribute 35 working documents as Internet-Drafts. The list of current Internet- 36 Drafts is at https://datatracker.ietf.org/drafts/current/. 38 Internet-Drafts are draft documents valid for a maximum of six months 39 and may be updated, replaced, or obsoleted by other documents at any 40 time. It is inappropriate to use Internet-Drafts as reference 41 material or to cite them other than as "work in progress." 43 This Internet-Draft will expire on 27 January 2022. 45 Copyright Notice 47 Copyright (c) 2021 IETF Trust and the persons identified as the 48 document authors. All rights reserved. 50 This document is subject to BCP 78 and the IETF Trust's Legal 51 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 52 license-info) in effect on the date of publication of this document. 53 Please review these documents carefully, as they describe your rights 54 and restrictions with respect to this document. Code Components 55 extracted from this document must include Simplified BSD License text 56 as described in Section 4.e of the Trust Legal Provisions and are 57 provided without warranty as described in the Simplified BSD License. 59 Table of Contents 61 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 62 2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3 63 2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 64 2.2. Conventions . . . . . . . . . . . . . . . . . . . . . . . 4 65 2.3. Predefined SRv6 Endpoint Behaviors . . . . . . . . . . . 4 66 3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 5 67 4. 3GPP Reference Architecture . . . . . . . . . . . . . . . . . 5 68 5. User-plane behaviors . . . . . . . . . . . . . . . . . . . . 6 69 5.1. Traditional mode . . . . . . . . . . . . . . . . . . . . 7 70 5.1.1. Packet flow - Uplink . . . . . . . . . . . . . . . . 8 71 5.1.2. Packet flow - Downlink . . . . . . . . . . . . . . . 9 72 5.2. Enhanced Mode . . . . . . . . . . . . . . . . . . . . . . 9 73 5.2.1. Packet flow - Uplink . . . . . . . . . . . . . . . . 10 74 5.2.2. Packet flow - Downlink . . . . . . . . . . . . . . . 11 75 5.2.3. Scalability . . . . . . . . . . . . . . . . . . . . . 11 76 5.3. Enhanced mode with unchanged gNB GTP behavior . . . . . . 12 77 5.3.1. Interworking with IPv6 GTP . . . . . . . . . . . . . 12 78 5.3.2. Interworking with IPv4 GTP . . . . . . . . . . . . . 15 79 5.3.3. Extensions to the interworking mechanisms . . . . . . 17 80 5.4. SRv6 Drop-in Interworking . . . . . . . . . . . . . . . . 17 81 6. SRv6 Segment Endpoint Mobility Behaviors . . . . . . . . . . 19 82 6.1. Args.Mob.Session . . . . . . . . . . . . . . . . . . . . 19 83 6.2. End.MAP . . . . . . . . . . . . . . . . . . . . . . . . . 20 84 6.3. End.M.GTP6.D . . . . . . . . . . . . . . . . . . . . . . 20 85 6.4. End.M.GTP6.D.Di . . . . . . . . . . . . . . . . . . . . . 21 86 6.5. End.M.GTP6.E . . . . . . . . . . . . . . . . . . . . . . 22 87 6.6. End.M.GTP4.E . . . . . . . . . . . . . . . . . . . . . . 23 88 6.7. H.M.GTP4.D . . . . . . . . . . . . . . . . . . . . . . . 25 89 6.8. End.Limit: Rate Limiting behavior . . . . . . . . . . . . 26 90 7. SRv6 supported 3GPP PDU session types . . . . . . . . . . . . 26 91 8. Network Slicing Considerations . . . . . . . . . . . . . . . 26 92 9. Control Plane Considerations . . . . . . . . . . . . . . . . 27 93 10. Security Considerations . . . . . . . . . . . . . . . . . . . 27 94 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27 95 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28 96 13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 28 97 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 28 98 14.1. Normative References . . . . . . . . . . . . . . . . . . 28 99 14.2. Informative References . . . . . . . . . . . . . . . . . 29 100 Appendix A. Implementations . . . . . . . . . . . . . . . . . . 30 101 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30 103 1. Introduction 105 In mobile networks, mobility management systems provide connectivity 106 over a wireless link to stationary and non-stationary nodes. The 107 user-plane establishes a tunnel between the mobile node and its 108 anchor node over IP-based backhaul and core networks. 110 This document shows the applicability of SRv6 (Segment Routing IPv6) 111 to mobile networks. 113 Segment Routing [RFC8402] is a source routing architecture: a node 114 steers a packet through an ordered list of instructions called 115 "segments". A segment can represent any instruction, topological or 116 service based. 118 SRv6 applied to mobile networks enables a source-routing based mobile 119 architecture, where operators can explicitly indicate a route for the 120 packets to and from the mobile node. The SRv6 Endpoint nodes serve 121 as mobile user-plane anchors. 123 2. Conventions and Terminology 125 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 126 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 127 document are to be interpreted as described in [RFC2119]. 129 2.1. Terminology 131 * CNF: Cloud-native Network Function 132 * NFV: Network Function Virtualization 133 * PDU: Packet Data Unit 134 * PDU Session: Context of a UE connects to a mobile network. 135 * UE: User Equipment 136 * UPF: User Plane Function 137 * VNF: Virtual Network Function (including CNFs) 139 The following terms used within this document are defined in 140 [RFC8402]: Segment Routing, SR Domain, Segment ID (SID), SRv6, SRv6 141 SID, Active Segment, SR Policy, Prefix SID, Adjacency SID and Binding 142 SID. 144 The following terms used within this document are defined in 145 [RFC8754]: SRH, SR Source Node, Transit Node, SR Segment Endpoint 146 Node and Reduced SRH. 148 The following terms used within this document are defined in 149 [RFC8986]: NH, SL, FIB, SA, DA, SRv6 SID behavior, SRv6 Segment 150 Endpoint Behavior. 152 2.2. Conventions 154 An SR Policy is resolved to a SID list. A SID list is represented as 155 where S1 is the first SID to visit, S2 is the second SID 156 to visit, and S3 is the last SID to visit along the SR path. 158 (SA,DA) (S3, S2, S1; SL) represents an IPv6 packet with: 160 * Source Address is SA, Destination Address is DA, and next-header 161 is SRH 162 * SRH with SID list with Segments Left = SL 163 * Note the difference between the <> and () symbols: 164 represents a SID list where S1 is the first SID and S3 is the last 165 SID to traverse. (S3, S2, S1; SL) represents the same SID list 166 but encoded in the SRH format where the rightmost SID in the SRH 167 is the first SID and the leftmost SID in the SRH is the last SID. 168 When referring to an SR policy in a high-level use-case, it is 169 simpler to use the notation. When referring to an 170 illustration of the detailed packet behavior, the (S3, S2, S1; SL) 171 notation is more convenient. 172 * The payload of the packet is omitted. 174 SRH[n]: A shorter representation of Segment List[n], as defined in 175 [RFC8754]. SRH[SL] can be different from the DA of the IPv6 header. 177 * gNB::1 is an IPv6 address (SID) assigned to the gNB. 178 * U1::1 is an IPv6 address (SID) assigned to UPF1. 179 * U2::1 is an IPv6 address (SID) assigned to UPF2. 180 * U2:: is some other IPv6 address (SID) assigned to UPF2. 182 2.3. Predefined SRv6 Endpoint Behaviors 184 The following SRv6 Endpoint Behaviors are defined in [RFC8986]. 186 * End.DT4: Decapsulation and Specific IPv4 Table Lookup 187 * End.DT6: Decapsulation and Specific IPv6 Table Lookup 188 * End.DT46: Decapsulation and Specific IP Table Lookup 189 * End.DX4: Decapsulation and IPv4 Cross-Connect 190 * End.DX6: Decapsulation and IPv6 Cross-Connect 191 * End.DX2: Decapsulation and L2 Cross-Connect 192 * End.T: Endpoint with specific IPv6 Table Lookup 194 This document defines new SRv6 Segment Endpoint Behaviors in 195 Section 6. 197 3. Motivation 199 Mobile networks are becoming more challenging to operate. On one 200 hand, traffic is constantly growing, and latency requirements are 201 tighter; on the other-hand, there are new use-cases like distributed 202 NFVi that are also challenging network operations. 204 The current architecture of mobile networks does not take into 205 account the underlying transport. The user-plane is rigidly 206 fragmented into radio access, core and service networks, connected by 207 tunneling according to user-plane roles such as access and anchor 208 nodes. These factors have made it difficult for the operator to 209 optimize and operate the data-path. 211 In the meantime, applications have shifted to use IPv6, and network 212 operators have started adopting IPv6 as their IP transport. SRv6, 213 the IPv6 dataplane instantiation of Segment Routing [RFC8402], 214 integrates both the application data-path and the underlying 215 transport layer into a single protocol, allowing operators to 216 optimize the network in a simplified manner and removing forwarding 217 state from the network. It is also suitable for virtualized 218 environments, like VNF/CNF to VNF/CNF networking. SRv6 has been 219 deployed in dozens of networks 220 [I-D.matsushima-spring-srv6-deployment-status]. 222 SRv6 defines the network-programming concept [RFC8986]. Applied to 223 mobility, SRv6 can provide the user-plane behaviors needed for 224 mobility management. SRv6 takes advantage of the underlying 225 transport awareness and flexibility together with the ability to also 226 include services to optimize the end-to-end mobile dataplane. 228 The use-cases for SRv6 mobility are discussed in 229 [I-D.camarilloelmalky-springdmm-srv6-mob-usecases]. 231 4. 3GPP Reference Architecture 233 This section presents a reference architecture and possible 234 deployment scenarios. 236 Figure 1 shows a reference diagram from the 5G packet core 237 architecture [TS.23501]. 239 The user plane described in this document does not depend on any 240 specific architecture. The 5G packet core architecture as shown is 241 based on the latest 3GPP standards at the time of writing this draft. 243 +-----+ 244 | AMF | 245 /+-----+ 246 / | [N11] 247 [N2] / +-----+ 248 +------/ | SMF | 249 / +-----+ 250 / / \ 251 / / \ [N4] 252 / / \ ________ 253 / / \ / \ 254 +--+ +-----+ [N3] +------+ [N9] +------+ [N6] / \ 255 |UE|------| gNB |------| UPF1 |--------| UPF2 |--------- \ DN / 256 +--+ +-----+ +------+ +------+ \________/ 258 Figure 1: 3GPP 5G Reference Architecture 260 * UE: User Endpoint 261 * gNB: gNodeB with N3 interface towards packet core (and N2 for 262 control plane) 263 * UPF1: UPF with Interfaces N3 and N9 (and N4 for control plane) 264 * UPF2: UPF with Interfaces N9 and N6 (and N4 for control plane) 265 * SMF: Session Management Function 266 * AMF: Access and Mobility Management Function 267 * DN: Data Network e.g. operator services, Internet access 269 This reference diagram does not depict a UPF that is only connected 270 to N9 interfaces, although the mechanisms defined in this document 271 also work in such case. 273 Each session from a UE gets assigned to a UPF. Sometimes multiple 274 UPFs may be used, providing richer service functions. A UE gets its 275 IP address from the DHCP block of its UPF. The UPF advertises that 276 IP address block toward the Internet, ensuring that return traffic is 277 routed to the right UPF. 279 5. User-plane behaviors 281 This section introduces an SRv6 based mobile user-plane. 283 In order to simplify the adoption of SRv6, we present two different 284 "modes" that vary with respect to the use of SRv6. The first one is 285 the "Traditional mode", which inherits the current 3GPP mobile 286 architecture. In this mode GTP-U protocol [TS.29281] is replaced by 287 SRv6, however the N3, N9 and N6 interfaces are still point-to-point 288 interfaces with no intermediate waypoints as in the current mobile 289 network architecture. 291 The second mode is the "Enhanced mode". This is an evolution from 292 the "Traditional mode". In this mode the N3, N9 or N6 interfaces 293 have intermediate waypoints -SIDs- that are used for Traffic 294 Engineering or VNF purposes. This results in optimal end-to-end 295 policies across the mobile network with transport and services 296 awareness. 298 In both, the Traditional and the Enhanced modes, we assume that the 299 gNB as well as the UPFs are SR-aware (N3, N9 and -potentially- N6 300 interfaces are SRv6). 302 In addition to those two modes, we introduce two mechanisms for 303 interworking with legacy access networks (those where the N3 304 interface is unmodified). In this document we introduce them as a 305 variant to the Enhanced mode, however they are equally applicable to 306 the Traditional mode. 308 One of these mechanisms is designed to interwork with legacy gNBs 309 using GTP/IPv4. The second mechanism is designed to interwork with 310 legacy gNBs using GTP/IPv6. 312 This document uses SRv6 Segment Endpoint Behaviors defined in 313 [RFC8986] as well as new SRv6 Segment Endpoint Behaviors designed for 314 the mobile user plane that are defined in this document in Section 6. 316 5.1. Traditional mode 318 In the traditional mode, the existing mobile UPFs remain unchanged 319 with the sole exception of the use of SRv6 as the data plane instead 320 of GTP-U. There is no impact to the rest of the mobile system. 322 In existing 3GPP mobile networks, a PDU Session is mapped 1-for-1 323 with a specific GTP tunnel (TEID). This 1-for-1 mapping is mirrored 324 here to replace GTP encapsulation with the SRv6 encapsulation, while 325 not changing anything else. There will be a unique SRv6 SID 326 associated with each PDU Session, and the SID list only contains a 327 single SID. 329 The traditional mode minimizes the changes required to the mobile 330 system; hence it is a good starting point for forming a common 331 ground. 333 The gNB/UPF control-plane (N2/N4 interface) is unchanged, 334 specifically a single IPv6 address is provided to the gNB. The same 335 control plane signalling is used, and the gNB/UPF decides to use SRv6 336 based on signaled GTP-U parameters per local policy. The only 337 information from the GTP-U parameters used for the SRv6 policy is the 338 TEID and the IPv6 Destination Address. 340 Our example topology is shown in Figure 2. In traditional mode the 341 gNB and the UPFs are SR-aware. In the descriptions of the uplink and 342 downlink packet flow, A is an IPv6 address of the UE, and Z is an 343 IPv6 address reachable within the Data Network DN. A new SRv6 344 Endpoint Behavior, End.MAP, defined in Section 6.2, is used. 346 ________ 347 SRv6 SRv6 / \ 348 +--+ +-----+ [N3] +------+ [N9] +------+ [N6] / \ 349 |UE|------| gNB |------| UPF1 |--------| UPF2 |--------- \ DN / 350 +--+ +-----+ +------+ +------+ \________/ 351 SRv6 node SRv6 node SRv6 node 353 Figure 2: Traditional mode - example topology 355 5.1.1. Packet flow - Uplink 357 The uplink packet flow is as follows: 359 UE_out : (A,Z) 360 gNB_out : (gNB, U1::1) (A,Z) -> H.Encaps.Red 361 UPF1_out: (gNB, U2::1) (A,Z) -> End.MAP 362 UPF2_out: (A,Z) -> End.DT4 or End.DT6 364 When the UE packet arrives at the gNB, the gNB performs a 365 H.Encaps.Red operation. Since there is only one SID, there is no 366 need to push an SRH. gNB only adds an outer IPv6 header with IPv6 DA 367 U1::1. U1::1 represents an anchoring SID specific for that session 368 at UPF1. gNB obtains the SID U1::1 from the existing control plane 369 (N2 interface). 371 When the packet arrives at UPF1, the SID U1::1 is associated with the 372 End.MAP SRv6 Endpoint Behavior. End.MAP replaces U1::1 by U2::1, 373 that belongs to the next UPF (U2). 375 When the packet arrives at UPF2, the SID U2::1 corresponds to an 376 End.DT4/End.DT6/End.DT46 SRv6 Endpoint Behavior. UPF2 decapsulates 377 the packet, performs a lookup in a specific table associated with 378 that mobile network and forwards the packet toward the data network 379 (DN). 381 5.1.2. Packet flow - Downlink 383 The downlink packet flow is as follows: 385 UPF2_in : (Z,A) 386 UPF2_out: (U2::, U1::2) (Z,A) -> H.Encaps.Red 387 UPF1_out: (U2::, gNB::1) (Z,A) -> End.MAP 388 gNB_out : (Z,A) -> End.DX4, End.DX6, End.DX2 390 When the packet arrives at the UPF2, the UPF2 maps that flow into a 391 PDU Session. This PDU Session is associated with the segment 392 endpoint . UPF2 performs a H.Encaps.Red operation, 393 encapsulating the packet into a new IPv6 header with no SRH since 394 there is only one SID. 396 Upon packet arrival on UPF1, the SID U1::2 is a local SID associated 397 with the End.MAP SRv6 Endpoint Behavior. It maps the SID to the next 398 anchoring point and replaces U1::2 by gNB::1, that belongs to the 399 next hop. 401 Upon packet arrival on gNB, the SID gNB::1 corresponds to an End.DX4, 402 End.DX6 or End.DX2 behavior (depending on the PDU Session Type). The 403 gNB decapsulates the packet, removing the IPv6 header and all its 404 extensions headers, and forwards the traffic toward the UE. 406 5.2. Enhanced Mode 408 Enhanced mode improves scalability, provides traffic engineering 409 capabilities, and allows service programming 410 [I-D.ietf-spring-sr-service-programming], thanks to the use of 411 multiple SIDs in the SID list (instead of a direct connectivity in 412 between UPFs with no intermediate waypoints as in Traditional Mode). 414 Thus, the main difference is that the SR policy MAY include SIDs for 415 traffic engineering and service programming in addition to the 416 anchoring SIDs at UPFs. 418 Additionally in this mode the operator may choose to aggregate 419 several devices under the same SID list (e.g., stationary residential 420 meters connected to the same cell) to improve scalability. 422 The gNB/UPF control-plane (N2/N4 interface) is unchanged, 423 specifically a single IPv6 address is provided to the gNB. A local 424 policy instructs the gNB to use SRv6. 426 The gNB MAY resolve the IP address received via the control plane 427 into a SID list using a mechanism like PCEP, DNS-lookup, LISP 428 control-plane or others. The resolution mechanism is out of the 429 scope of this document. 431 Note that the SIDs MAY use the arguments Args.Mob.Session if required 432 by the UPFs. 434 Figure 3 shows an Enhanced mode topology. In the Enhanced mode, the 435 gNB and the UPF are SR-aware. The Figure shows two service segments, 436 S1 and C1. S1 represents a VNF in the network, and C1 represents an 437 intermediate router used for Traffic Engineering purposes to enforce 438 a low-latency path in the network. Note that neither S1 nor C1 are 439 required to have an N4 interface. 441 +----+ SRv6 _______ 442 SRv6 --| C1 |--[N3] / \ 443 +--+ +-----+ [N3] / +----+ \ +------+ [N6] / \ 444 |UE|----| gNB |-- SRv6 / SRv6 --| UPF1 |------\ DN / 445 +--+ +-----+ \ [N3]/ TE +------+ \_______/ 446 SRv6 node \ +----+ / SRv6 node 447 -| S1 |- 448 +----+ 449 SRv6 node 450 VNF 452 Figure 3: Enhanced mode - Example topology 454 5.2.1. Packet flow - Uplink 456 The uplink packet flow is as follows: 458 UE_out : (A,Z) 459 gNB_out : (gNB, S1)(U1::1, C1; SL=2)(A,Z)->H.Encaps.Red 460 S1_out : (gNB, C1)(U1::1, C1; SL=1)(A,Z) 461 C1_out : (gNB, U1::1)(A,Z) ->End with PSP 462 UPF1_out: (A,Z) ->End.DT4,End.DT6,End.DT2U 464 UE sends its packet (A,Z) on a specific bearer to its gNB. gNB's 465 control plane associates that session from the UE(A) with the IPv6 466 address B. gNB's control plane does a lookup on B to find the 467 related SID list . 469 When gNB transmits the packet, it contains all the segments of the SR 470 policy. The SR policy includes segments for traffic engineering (C1) 471 and for service programming (S1). 473 Nodes S1 and C1 perform their related Endpoint functionality and 474 forward the packet. 476 When the packet arrives at UPF1, the active segment (U1::1) is an 477 End.DT4/End.DT6/End.DT2U which performs the decapsulation (removing 478 the IPv6 header with all its extension headers) and forwards toward 479 the data network. 481 5.2.2. Packet flow - Downlink 483 The downlink packet flow is as follows: 485 UPF1_in : (Z,A) ->UPF1 maps the flow w/ 486 SID list 487 UPF1_out: (U1::1, C1)(gNB, S1; SL=2)(Z,A) ->H.Encaps.Red 488 C1_out : (U1::1, S1)(gNB, S1; SL=1)(Z,A) 489 S1_out : (U1::1, gNB)(Z,A) ->End with PSP 490 gNB_out : (Z,A) ->End.DX4/End.DX6/End.DX2 492 When the packet arrives at the UPF1, the UPF1 maps that particular 493 flow into a UE PDU Session. This UE PDU Session is associated with 494 the policy . The UPF1 performs a H.Encaps.Red 495 operation, encapsulating the packet into a new IPv6 header with its 496 corresponding SRH. 498 The nodes C1 and S1 perform their related Endpoint processing. 500 Once the packet arrives at the gNB, the IPv6 DA corresponds to an 501 End.DX4, End.DX6 or End.DX2 behavior at the gNB (depending on the 502 underlying traffic). The gNB decapsulates the packet, removing the 503 IPv6 header and forwards the traffic toward the UE. 505 Note that there are several means to provide the UE session 506 aggregation. The decision on which one to use is a local decision 507 made by the operator. One option is to use the Args.Mob.Session 508 (Section 6.1). Another option comprises the gNB performing an IP 509 lookup on the inner packet by using the End.DT4, End.DT6, and End.DT2 510 behaviors. 512 5.2.3. Scalability 514 The Enhanced Mode improves since it allows the aggregation of several 515 UEs under the same SID list. For example, in the case of stationary 516 residential meters that are connected to the same cell, all such 517 devices can share the same SID list. This improves scalability 518 compared to Traditional Mode (unique SID per UE) and compared to 519 GTP-U (dedicated TEID per UE). 521 5.3. Enhanced mode with unchanged gNB GTP behavior 523 This section describes two mechanisms for interworking with legacy 524 gNBs that still use GTP: one for IPv4, and another for IPv6. 526 In the interworking scenarios as illustrated in Figure 4, the gNB 527 does not support SRv6. The gNB supports GTP encapsulation over IPv4 528 or IPv6. To achieve interworking, an SR Gateway (SRGW) entity is 529 added. The SRGW maps the GTP traffic into SRv6. 531 The SRGW is not an anchor point and maintains very little state. For 532 this reason, both IPv4 and IPv6 methods scale to millions of UEs. 534 _______ 535 IP GTP SRv6 / \ 536 +--+ +-----+ [N3] +------+ [N9] +------+ [N6] / \ 537 |UE|------| gNB |------| SRGW |--------| UPF |---------\ DN / 538 +--+ +-----+ +------+ +------+ \_______/ 539 SR Gateway SRv6 node 541 Figure 4: Example topology for interworking 543 Both of the mechanisms described in this section are applicable to 544 either the Traditional Mode or the Enhanced Mode. 546 5.3.1. Interworking with IPv6 GTP 548 In this interworking mode the gNB at the N3 interface uses GTP over 549 IPv6. 551 Key points: 553 * The gNB is unchanged (control-plane or user-plane) and 554 encapsulates into GTP (N3 interface is not modified). 555 * The 5G Control-Plane towards the gNB (N2 interface) is unmodified; 556 one IPv6 address is needed (i.e. a BSID at the SRGW). The SRv6 557 SID is different depending on the required SLA. 558 * In the uplink, the SRGW removes GTP, finds the SID list related to 559 the IPv6 DA, and adds SRH with the SID list. 560 * There is no state for the downlink at the SRGW. 561 * There is simple state in the uplink at the SRGW; using Enhanced 562 mode results in fewer SR policies on this node. An SR policy is 563 shared across UEs as long as they belong to the same context 564 (i.e., tenant). A set of many different policies (i.e., different 565 SLAs) increases the amount of state required. 566 * When a packet from the UE leaves the gNB, it is SR-routed. This 567 simplifies network slicing [I-D.ietf-lsr-flex-algo]. 569 * In the uplink, the SRv6 BSID located in the IPv6 DA steers traffic 570 into an SR policy when it arrives at the SRGW. 572 An example topology is shown in Figure 5. 574 S1 and C1 are two service segments. S1 represents a VNF in the 575 network, and C1 represents a router configured for Traffic 576 Engineering. 578 +----+ 579 IPv6/GTP -| S1 |- ___ 580 +--+ +-----+ [N3] / +----+ \ / 581 |UE|--| gNB |- SRv6 / SRv6 \ +----+ +------+ [N6] / 582 +--+ +-----+ \ [N9]/ VNF -| C1 |---| UPF2 |------\ DN 583 GTP \ +------+ / +----+ +------+ \___ 584 -| SRGW |- SRv6 SRv6 585 +------+ TE 586 SR Gateway 588 Figure 5: Enhanced mode with unchanged gNB IPv6/GTP behavior 590 5.3.1.1. Packet flow - Uplink 592 The uplink packet flow is as follows: 594 UE_out : (A,Z) 595 gNB_out : (gNB, B)(GTP: TEID T)(A,Z) -> Interface N3 unmodified 596 (IPv6/GTP) 597 SRGW_out: (SRGW, S1)(U2::1, C1; SL=2)(A,Z) -> B is an End.M.GTP6.D 598 SID at the SRGW 599 S1_out : (SRGW, C1)(U2::1, C1; SL=1)(A,Z) 600 C1_out : (SRGW, U2::1)(A,Z) -> End with PSP 601 UPF2_out: (A,Z) -> End.DT4 or End.DT6 603 The UE sends a packet destined to Z toward the gNB on a specific 604 bearer for that session. The gNB, which is unmodified, encapsulates 605 the packet into IPv6, UDP, and GTP headers. The IPv6 DA B, and the 606 GTP TEID T are the ones received in the N2 interface. 608 The IPv6 address that was signaled over the N2 interface for that UE 609 PDU Session, B, is now the IPv6 DA. B is an SRv6 Binding SID at the 610 SRGW. Hence the packet is routed to the SRGW. 612 When the packet arrives at the SRGW, the SRGW identifies B as an 613 End.M.GTP6.D Binding SID (see Section 6.3). Hence, the SRGW removes 614 the IPv6, UDP, and GTP headers, and pushes an IPv6 header with its 615 own SRH containing the SIDs bound to the SR policy associated with 616 this BindingSID. There at least one instance of the End.M.GTP6.D SID 617 per PDU type. 619 S1 and C1 perform their related Endpoint functionality and forward 620 the packet. 622 When the packet arrives at UPF2, the active segment is (U2::1) which 623 is bound to End.DT4/6. UPF2 then decapsulates (removing the outer 624 IPv6 header with all its extension headers) and forwards the packet 625 toward the data network. 627 5.3.1.2. Packet flow - Downlink 629 The downlink packet flow is as follows: 631 UPF2_in : (Z,A) -> UPF2 maps the flow with 632 633 UPF2_out: (U2::1, C1)(gNB, SRGW::TEID, S1; SL=3)(Z,A) -> H.Encaps.Red 634 C1_out : (U2::1, S1)(gNB, SRGW::TEID, S1; SL=2)(Z,A) 635 S1_out : (U2::1, SRGW::TEID)(gNB, SRGW::TEID, S1, SL=1)(Z,A) 636 SRGW_out: (SRGW, gNB)(GTP: TEID=T)(Z,A) -> SRGW/96 is End.M.GTP6.E 637 gNB_out : (Z,A) 639 When a packet destined to A arrives at the UPF2, the UPF2 performs a 640 lookup in the table associated to A and finds the SID list . The UPF2 performs an H.Encaps.Red operation, 642 encapsulating the packet into a new IPv6 header with its 643 corresponding SRH. 645 C1 and S1 perform their related Endpoint processing. 647 Once the packet arrives at the SRGW, the SRGW identifies the active 648 SID as an End.M.GTP6.E function. The SRGW removes the IPv6 header 649 and all its extensions headers. The SRGW generates new IPv6, UDP, 650 and GTP headers. The new IPv6 DA is the gNB which is the last SID in 651 the received SRH. The TEID in the generated GTP header is an 652 argument of the received End.M.GTP6.E SID. The SRGW pushes the 653 headers to the packet and forwards the packet toward the gNB. There 654 is one instance of the End.M.GTP6.E SID per PDU type. 656 Once the packet arrives at the gNB, the packet is a regular IPv6/GTP 657 packet. The gNB looks for the specific radio bearer for that TEID 658 and forward it on the bearer. This gNB behavior is not modified from 659 current and previous generations. 661 5.3.1.3. Scalability 663 For the downlink traffic, the SRGW is stateless. All the state is in 664 the SRH pushed by the UPF2. The UPF2 must have the UE states since 665 it is the UE's session anchor point. 667 For the uplink traffic, the state at the SRGW does not necessarily 668 need to be unique per PDU Session; the SR policy can be shared among 669 UEs. This enables more scalable SRGW deployments compared to a 670 solution holding millions of states, one or more per UE. 672 5.3.2. Interworking with IPv4 GTP 674 In this interworking mode the gNB uses GTP over IPv4 in the N3 675 interface 677 Key points: 679 * The gNB is unchanged and encapsulates packets into GTP (the N3 680 interface is not modified). 681 * In the uplink, traffic is classified by SRGW's Uplink Classifier 682 and steered into an SR policy. The SRGW is a UPF1 functionality 683 and can coexist with UPF1's Uplink Classifier functionality. 684 * SRGW removes GTP, finds the SID list related to DA, and adds an 685 SRH with the SID list. 687 An example topology is shown in Figure 6. In this mode the gNB is an 688 unmodified gNB using IPv4/GTP. The UPFs are SR-aware. As before, 689 the SRGW maps the IPv4/GTP traffic to SRv6. 691 S1 and C1 are two service segment endpoints. S1 represents a VNF in 692 the network, and C1 represents a router configured for Traffic 693 Engineering. 695 +----+ 696 IPv4/GTP -| S1 |- ___ 697 +--+ +-----+ [N3] / +----+ \ / 698 |UE|--| gNB |- SRv6 / SRv6 \ +----+ +------+ [N6] / 699 +--+ +-----+ \ [N9]/ VNF -| C1 |---| UPF2 |------\ DN 700 GTP \ +------+ / +----+ +------+ \___ 701 -| UPF1 |- SRv6 SRv6 702 +------+ TE 703 SR Gateway 705 Figure 6: Enhanced mode with unchanged gNB IPv4/GTP behavior 707 5.3.2.1. Packet flow - Uplink 709 The uplink packet flow is as follows: 711 gNB_out : (gNB, B)(GTP: TEID T)(A,Z) -> Interface N3 712 unchanged IPv4/GTP 713 SRGW_out: (SRGW, S1)(U2::1, C1; SL=2)(A,Z) -> H.M.GTP4.D function 714 S1_out : (SRGW, C1)(U2::1, C1; SL=1)(A,Z) 715 C1_out : (SRGW, U2::1) (A,Z) -> PSP 716 UPF2_out: (A,Z) -> End.DT4 or End.DT6 718 The UE sends a packet destined to Z toward the gNB on a specific 719 bearer for that session. The gNB, which is unmodified, encapsulates 720 the packet into a new IPv4, UDP, and GTP headers. The IPv4 DA, B, 721 and the GTP TEID are the ones received at the N2 interface. 723 When the packet arrives at the SRGW for UPF1, the SRGW has an Uplink 724 Classifier rule for incoming traffic from the gNB, that steers the 725 traffic into an SR policy by using the function H.M.GTP4.D. The SRGW 726 removes the IPv4, UDP, and GTP headers and pushes an IPv6 header with 727 its own SRH containing the SIDs related to the SR policy associated 728 with this traffic. The SRGW forwards according to the new IPv6 DA. 730 S1 and C1 perform their related Endpoint functionality and forward 731 the packet. 733 When the packet arrives at UPF2, the active segment is (U2::1) which 734 is bound to End.DT4/6 which performs the decapsulation (removing the 735 outer IPv6 header with all its extension headers) and forwards toward 736 the data network. 738 5.3.2.2. Packet flow - Downlink 740 The downlink packet flow is as follows: 742 UPF2_in : (Z,A) -> UPF2 maps flow with SID 743 744 UPF2_out: (U2::1, C1)(GW::SA:DA:TEID, S1; SL=2)(Z,A) ->H.Encaps.Red 745 C1_out : (U2::1, S1)(GW::SA:DA:TEID, S1; SL=1)(Z,A) 746 S1_out : (U2::1, GW::SA:DA:TEID)(Z,A) 747 SRGW_out: (GW, gNB)(GTP: TEID=T)(Z,A) -> End.M.GTP4.E 748 gNB_out : (Z,A) 750 When a packet destined to A arrives at the UPF2, the UPF2 performs a 751 lookup in the table associated to A and finds the SID list . The UPF2 performs a H.Encaps.Red operation, 753 encapsulating the packet into a new IPv6 header with its 754 corresponding SRH. 756 The nodes C1 and S1 perform their related Endpoint processing. 758 Once the packet arrives at the SRGW, the SRGW identifies the active 759 SID as an End.M.GTP4.E function. The SRGW removes the IPv6 header 760 and all its extensions headers. The SRGW generates an IPv4, UDP, and 761 GTP headers. The IPv4 SA and DA are received as SID arguments. The 762 TEID in the generated GTP header is also the arguments of the 763 received End.M.GTP4.E SID. The SRGW pushes the headers to the packet 764 and forwards the packet toward the gNB. 766 When the packet arrives at the gNB, the packet is a regular IPv4/GTP 767 packet. The gNB looks for the specific radio bearer for that TEID 768 and forwards it on the bearer. This gNB behavior is not modified 769 from current and previous generations. 771 5.3.2.3. Scalability 773 For the downlink traffic, the SRGW is stateless. All the state is in 774 the SRH pushed by the UPF2. The UPF must have this UE-base state 775 anyway (since it is its anchor point). 777 For the uplink traffic, the state at the SRGW is dedicated on a per 778 UE/session basis according to an Uplink Classifier. There is state 779 for steering the different sessions in the form of an SR Policy. 780 However, SR policies are shared among several UE/sessions. 782 5.3.3. Extensions to the interworking mechanisms 784 In this section we presented two mechanisms for interworking with 785 gNBs and UPFs that do not support SRv6. These mechanisms are used to 786 support GTP over IPv4 and IPv6. 788 Even though we have presented these methods as an extension to the 789 "Enhanced mode", it is straightforward in its applicability to the 790 "Traditional mode". 792 5.4. SRv6 Drop-in Interworking 794 In this section we introduce another mode useful for legacy gNB and 795 UPFs that still operate with GTP-U. This mode provides an 796 SRv6-enabled user plane in between two GTP-U tunnel endpoints. 798 In this mode we employ two SRGWs that map GTP-U traffic to SRv6 and 799 vice-versa. 801 Unlike other interworking modes, in this mode both of the mobility 802 overlay endpoints use GTP-U. Two SRGWs are deployed in either N3 or 803 N9 interface to realize an intermediate SR policy. 805 +----+ 806 -| S1 |- 807 +-----+ / +----+ \ 808 | gNB |- SRv6 / SRv6 \ +----+ +--------+ +-----+ 809 +-----+ \ / VNF -| C1 |---| SRGW-B |----| UPF | 810 GTP[N3]\ +--------+ / +----+ +--------+ +-----+ 811 -| SRGW-A |- SRv6 SR Gateway-B GTP 812 +--------+ TE 813 SR Gateway-A 815 Figure 7: Example topology for SRv6 Drop-in mode 817 The packet flow of Figure 7 is as follows: 819 gNB_out : (gNB, U::1)(GTP: TEID T)(A,Z) 820 GW-A_out: (GW-A, S1)(U::1, SGB::TEID, C1; SL=3)(A,Z)->U::1 is an 821 End.M.GTP6.D.Di 822 SID at SRGW-A 823 S1_out : (GW-A, C1)(U::1, SGB::TEID, C1; SL=2)(A,Z) 824 C1_out : (GW-A, SGB::TEID)(U::1, SGB::TEID, C1; SL=1)(A,Z) 825 GW-B_out: (GW-B, U::1)(GTP: TEID T)(A,Z) ->SGB::TEID is an 826 End.M.GTP6.E 827 SID at SRGW-B 828 UPF_out : (A,Z) 830 When a packet destined to Z is sent to the gNB, which is unmodified 831 (control-plane and user-plane remain GTP-U), gNB performs 832 encapsulation into a new IP, UDP, and GTP headers. The IPv6 DA, 833 U::1, and the GTP TEID are the ones received at the N2 interface. 835 The IPv6 address that was signaled over the N2 interface for that PDU 836 Session, U::1, is now the IPv6 DA. U::1 is an SRv6 Binding SID at 837 SRGW-A. Hence the packet is routed to the SRGW. 839 When the packet arrives at SRGW-A, the SRGW identifies U::1 as an 840 End.M.GTP6.D.Di Binding SID (see Section 6.4). Hence, the SRGW 841 removes the IPv6, UDP, and GTP headers, and pushes an IPv6 header 842 with its own SRH containing the SIDs bound to the SR policy 843 associated with this Binding SID. There is one instance of the 844 End.M.GTP6.D.Di SID per PDU type. 846 S1 and C1 perform their related Endpoint functionality and forward 847 the packet. 849 Once the packet arrives at SRGW-B, the SRGW identifies the active SID 850 as an End.M.GTP6.E function. The SRGW removes the IPv6 header and 851 all its extensions headers. The SRGW generates new IPv6, UDP, and 852 GTP headers. The new IPv6 DA is U::1 which is the last SID in the 853 received SRH. The TEID in the generated GTP header is an argument of 854 the received End.M.GTP6.E SID. The SRGW pushes the headers to the 855 packet and forwards the packet toward UPF. There is one instance of 856 the End.M.GTP6.E SID per PDU type. 858 Once the packet arrives at UPF, the packet is a regular IPv6/GTP 859 packet. The UPF looks for the specific rule for that TEID to forward 860 the packet. This UPF behavior is not modified from current and 861 previous generations. 863 6. SRv6 Segment Endpoint Mobility Behaviors 865 6.1. Args.Mob.Session 867 Args.Mob.Session provide per-session information for charging, 868 buffering and lawful intercept (among others) required by some mobile 869 nodes. The Args.Mob.Session argument format is used in combination 870 with End.Map, End.DT4/End.DT6/End.DT46 and End.DX4/End.DX6/End.DX2 871 behaviors. Note that proposed format is applicable for 5G networks, 872 while similar formats could be used for legacy networks. 874 0 1 2 3 875 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 876 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 877 | QFI |R|U| PDU Session ID | 878 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 879 |PDU Sess(cont')| 880 +-+-+-+-+-+-+-+-+ 882 Figure 8: Args.Mob.Session format 884 * QFI: QoS Flow Identifier [TS.38415] 885 * R: Reflective QoS Indication [TS.23501]. This parameter indicates 886 the activation of reflective QoS towards the UE for the 887 transferred packet. Reflective QoS enables the UE to map UL User 888 Plane traffic to QoS Flows without SMF provided QoS rules. 889 * U: Unused and for future use. MUST be 0 on transmission and 890 ignored on receipt. 891 * PDU Session ID: Identifier of PDU Session. The GTP-U equivalent 892 is TEID. 894 Arg.Mob.Session is required in case that one SID aggregates multiple 895 PDU Sessions. Since the SRv6 SID is likely NOT to be instantiated 896 per PDU session, Args.Mob.Session helps the UPF to perform the 897 behaviors which require per QFI and/or per PDU Session granularity. 899 6.2. End.MAP 901 The "Endpoint behavior with SID mapping" behavior (End.MAP for short) 902 is used in several scenarios. Particularly in mobility, End.MAP is 903 used in the UPFs for the PDU Session anchor functionality. 905 When node N receives a packet whose IPv6 DA is S and S is a local 906 End.MAP SID, N does: 908 S01. If (IPv6 Hop Limit <= 1) { 909 S02. Send an ICMP Time Exceeded message to the Source Address, 910 Code 0 (Hop limit exceeded in transit), 911 interrupt packet processing, and discard the packet. 912 S03. } 913 S04. Decrement IPv6 Hop Limit by 1 914 S05. Lookup the IPv6 DA in the mapping table 915 S06. Update the IPv6 DA with the new mapped SID 916 S07. Submit the packet to the egress IPv6 FIB lookup for 917 transmission to the new destination 919 Notes: The SIDs in the SRH are not modified. 921 6.3. End.M.GTP6.D 923 The "Endpoint behavior with IPv6/GTP decapsulation into SR policy" 924 behavior (End.M.GTP6.D for short) is used in interworking scenario 925 for the uplink towards SRGW from the legacy gNB using IPv6/GTP. Any 926 SID instance of this behavior is associated with an SR Policy B and 927 an IPv6 Source Address A. 929 When the SR Gateway node N receives a packet destined to S and S is a 930 local End.M.GTP6.D SID, N does: 932 S01. When an SRH is processed { 933 S02. If (Segments Left != 0) { 934 S03. Send an ICMP Parameter Problem to the Source Address, 935 Code 0 (Erroneous header field encountered), 936 Pointer set to the Segments Left field, 937 interrupt packet processing, and discard the packet. 938 S04. } 939 S05. Proceed to process the next header in the packet 940 S06. } 942 When processing the Upper-layer header of a packet matching a FIB 943 entry locally instantiated as an End.M.GTP6.D SID, N does: 945 S01. If (Next Header = UDP & UDP_Dest_port = GTP) { 946 S02. Copy the GTP TEID to buffer memory 947 S03. Pop the IPv6, UDP, and GTP Headers 948 S04. Push a new IPv6 header with its own SRH containing B 949 S05. Set the outer IPv6 SA to A 950 S06. Set the outer IPv6 DA to the first SID of B 951 S07. Set the outer Payload Length, Traffic Class, Flow Label, 952 Hop Limit, and Next-Header fields 953 S08. Write in the last SID of the SRH the Args.Mob.Session 954 based on the information of buffer memory 955 S09. Submit the packet to the egress IPv6 FIB lookup and 956 transmission to the new destination 957 S10. } Else { 958 S11. Process as per [RFC8986] Section 4.1.1 959 S12. } 961 Notes: The NH is set based on the SID parameter. There is one 962 instantiation of the End.M.GTP6.D SID per PDU Session Type, hence the 963 NH is already known in advance. For the IPv4v6 PDU Session Type, in 964 addition we inspect the first nibble of the PDU to know the NH value. 966 The prefix of last segment (S3 in above example) SHOULD be followed 967 by an Arg.Mob.Session argument space which is used to provide the 968 session identifiers. 970 The prefix of A SHOULD be an End.M.GTP6.E SID instantiated at an SR 971 gateway. 973 6.4. End.M.GTP6.D.Di 975 The "Endpoint behavior with IPv6/GTP decapsulation into SR policy for 976 Drop-in Mode" behavior (End.M.GTP6.D.Di for short) is used in SRv6 977 drop-in interworking scenario described in Section 5.4. The 978 difference between End.M.GTP6.D as another variant of IPv6/GTP 979 decapsulation function is that the original IPv6 DA of GTP packet is 980 preserved as the last SID in SRH. 982 Any SID instance of this behavior is associated with an SR Policy B 983 and an IPv6 Source Address A. 985 When the SR Gateway node N receives a packet destined to S and S is a 986 local End.M.GTP6.D.Di SID, N does: 988 S01. When an SRH is processed { 989 S02. If (Segments Left != 0) { 990 S03. Send an ICMP Parameter Problem to the Source Address, 991 Code 0 (Erroneous header field encountered), 992 Pointer set to the Segments Left field, 993 interrupt packet processing, and discard the packet. 994 S04. } 995 S05. Proceed to process the next header in the packet 996 S06. } 998 When processing the Upper-layer header of a packet matching a FIB 999 entry locally instantiated as an End.M.GTP6.Di SID, N does: 1001 S01. If (Next Header = UDP & UDP_Dest_port = GTP) { 1002 S02. Copy S to buffer memory 1003 S03. Pop the IPv6, UDP, and GTP Headers 1004 S04. Push a new IPv6 header with its own SRH containing B 1005 S05. Set the outer IPv6 SA to A 1006 S06. Set the outer IPv6 DA to the first SID of B 1007 S07. Set the outer Payload Length, Traffic Class, Flow Label, 1008 Hop Limit, and Next-Header fields 1009 S08. Write S to the SRH 1010 S09. Submit the packet to the egress IPv6 FIB lookup and 1011 transmission to the new destination 1012 S10. } Else { 1013 S11. Process as per [RFC8986] Section 4.1.1 1014 S12. } 1016 Notes: The NH is set based on the SID parameter. There is one 1017 instantiation of the End.M.GTP6.D SID per PDU Session Type, hence the 1018 NH is already known in advance. For the IPv4v6 PDU Session Type, in 1019 addition we inspect the first nibble of the PDU to know the NH value. 1021 The prefix of A SHOULD be an End.M.GTP6.E SID instantiated at an SR 1022 gateway. 1024 6.5. End.M.GTP6.E 1026 The "Endpoint behavior with encapsulation for IPv6/GTP tunnel" 1027 behavior (End.M.GTP6.E for short) is used in interworking scenario 1028 for the downlink toward the legacy gNB using IPv6/GTP. 1030 The prefix of End.M.GTP6.E SID MUST be followed by the 1031 Arg.Mob.Session argument space which is used to provide the session 1032 identifiers. 1034 When the SR Gateway node N receives a packet destined to S, and S is 1035 a local End.M.GTP6.E SID, N does the following: 1037 S01. When an SRH is processed { 1038 S02. If (Segments Left != 1) { 1039 S03. Send an ICMP Parameter Problem to the Source Address, 1040 Code 0 (Erroneous header field encountered), 1041 Pointer set to the Segments Left field, 1042 interrupt packet processing, and discard the packet. 1043 S04. } 1044 S05. Proceed to process the next header in the packet 1045 S06. } 1047 When processing the Upper-layer header of a packet matching a FIB 1048 entry locally instantiated as an End.M.GTP6.E SID, N does: 1050 S01. Copy SRH[0] and S to buffer memory 1051 S02. Pop the IPv6 header and all its extension headers 1052 S03. Push a new IPv6 header with a UDP/GTP Header 1053 S04. Set the outer IPv6 SA to A 1054 S05. Set the outer IPv6 DA from buffer memory 1055 S06. Set the outer Payload Length, Traffic Class, Flow Label, 1056 Hop Limit, and Next-Header fields 1057 S07. Set the GTP TEID (from buffer memory) 1058 S08. Submit the packet to the egress IPv6 FIB lookup and 1059 transmission to the new destination 1060 S09. } 1062 Notes: An End.M.GTP6.E SID MUST always be the penultimate SID. The 1063 TEID is extracted from the argument space of the current SID. 1065 The source address A SHOULD be an End.M.GTP6.D SID instantiated at an 1066 SR gateway. 1068 6.6. End.M.GTP4.E 1070 The "Endpoint behavior with encapsulation for IPv4/GTP tunnel" 1071 behavior (End.M.GTP4.E for short) is used in the downlink when doing 1072 interworking with legacy gNB using IPv4/GTP. 1074 When the SR Gateway node N receives a packet destined to S and S is a 1075 local End.M.GTP4.E SID, N does: 1077 S01. When an SRH is processed { 1078 S02. If (Segments Left != 0) { 1079 S03. Send an ICMP Parameter Problem to the Source Address, 1080 Code 0 (Erroneous header field encountered), 1081 Pointer set to the Segments Left field, 1082 interrupt packet processing, and discard the packet. 1083 S04. } 1084 S05. Proceed to process the next header in the packet 1085 S06. } 1087 When processing the Upper-layer header of a packet matching a FIB 1088 entry locally instantiated as an End.M.GTP4.E SID, N does: 1090 S01. If (Next Header = UDP & UDP_Dest_port = GTP) { 1091 S02. Sotre the IPv6 DA and SA in buffer memory 1092 S03. Pop the IPv6 header and all its extension headers 1093 S04. Push a new IPv4 header with a UDP/GTP Header 1094 S05. Set the outer IPv4 SA and DA (from buffer memory) 1095 S06. Set the outer Total Length, DSCP, Time To Live, and 1096 Next-Header fields 1097 S07. Set the GTP TEID (from buffer memory) 1098 S08. Submit the packet to the egress IPv6 FIB lookup and 1099 transmission to the new destination 1100 S09. } Else { 1101 S10. Process as per [NET-PGM] Section 4.1.1 1102 S11. } 1104 Notes: The End.M.GTP4.E SID in S has the following format: 1106 0 127 1107 +-----------------------+-------+----------------+---------+ 1108 | SRGW-IPv6-LOC-FUNC |IPv4DA |Args.Mob.Session|0 Padded | 1109 +-----------------------+-------+----------------+---------+ 1110 128-a-b-c a b c 1112 Figure 9: End.M.GTP4.E SID Encoding 1114 The IPv6 Source Address has the following format: 1116 0 127 1117 +----------------------+--------+--------------------------+ 1118 | Source UPF Prefix |IPv4 SA | any bit pattern(ignored) | 1119 +----------------------+--------+--------------------------+ 1120 128-a-b a b 1122 Figure 10: IPv6 SA Encoding for End.M.GTP4.E 1124 6.7. H.M.GTP4.D 1126 The "SR Policy Headend with tunnel decapsulation and map to an SRv6 1127 policy" behavior (H.M.GTP4.D for short) is used in the direction from 1128 legacy IPv4 user-plane to SRv6 user-plane network. 1130 When the SR Gateway node N receives a packet destined to a IW- 1131 IPv4-Prefix, N does: 1133 S01. IF Payload == UDP/GTP THEN 1134 S02. Pop the outer IPv4 header and UDP/GTP headers 1135 S03. Copy IPv4 DA, TEID to form SID B 1136 S04. Copy IPv4 SA to form IPv6 SA B' 1137 S05. Encapsulate the packet into a new IPv6 header ;;Ref1 1138 S06. Set the IPv6 DA = B 1139 S07. Forward along the shortest path to B 1140 S08. ELSE 1141 S09. Drop the packet 1143 Ref1: The NH value is identified by inspecting the first nibble of 1144 the inner payload. 1146 The SID B has the following format: 1148 0 127 1149 +-----------------------+-------+----------------+---------+ 1150 |Destination UPF Prefix |IPv4DA |Args.Mob.Session|0 Padded | 1151 +-----------------------+-------+----------------+---------+ 1152 128-a-b-c a b c 1154 Figure 11: H.M.GTP4.D SID Encoding 1156 The SID B MAY be an SRv6 Binding SID instantiated at the first UPF 1157 (U1) to bind an SR policy [I-D.ietf-spring-segment-routing-policy]. 1159 The prefix of B' SHOULD be an End.M.GTP4.E SID with its format 1160 instantiated at an SR gateway with the IPv4 SA of the receiving 1161 packet. 1163 6.8. End.Limit: Rate Limiting behavior 1165 The mobile user-plane requires a rate-limit feature. For this 1166 purpose, we define a new behavior "End.Limit". The "End.Limit" 1167 behavior encodes in its arguments the rate limiting parameter that 1168 should be applied to this packet. Multiple flows of packets should 1169 have the same group identifier in the SID when those flows are in the 1170 same AMBR (Aggregate Maximum Bit Rate) group. The encoding format of 1171 the rate limit segment SID is as follows: 1173 +----------------------+----------+-----------+ 1174 | LOC+FUNC rate-limit | group-id | limit-rate| 1175 +----------------------+----------+-----------+ 1176 128-i-j i j 1178 Figure 12: End.Limit: Rate limiting behavior argument format 1180 If the limit-rate bits are set to zero, the node should not do rate 1181 limiting unless static configuration or control-plane sets the limit 1182 rate associated to the SID. 1184 7. SRv6 supported 3GPP PDU session types 1186 The 3GPP [TS.23501] defines the following PDU session types: 1188 * IPv4 1189 * IPv6 1190 * IPv4v6 1191 * Ethernet 1192 * Unstructured 1194 SRv6 supports the 3GPP PDU session types without any protocol 1195 overhead by using the corresponding SRv6 behaviors (End.DX4, End.DT4 1196 for IPv4 PDU sessions; End.DX6, End.DT6, End.T for IPv6 PDU sessions; 1197 End.DT46 for IPv4v6 PDU sessions; End.DX2 for L2 and Unstructured PDU 1198 sessions). 1200 8. Network Slicing Considerations 1202 A mobile network may be required to implement "network slices", which 1203 logically separate network resources. User-plane behaviors 1204 represented as SRv6 segments would be part of a slice. 1206 [I-D.ietf-spring-segment-routing-policy] describes a solution to 1207 build basic network slices with SR. Depending on the requirements, 1208 these slices can be further refined by adopting the mechanisms from: 1210 * IGP Flex-Algo [I-D.ietf-lsr-flex-algo] 1211 * Inter-Domain policies 1212 [I-D.ietf-spring-segment-routing-central-epe] 1214 Furthermore, these can be combined with ODN/AS (On Demand Nexthop/ 1215 Automated Steering) [I-D.ietf-spring-segment-routing-policy] for 1216 automated slice provisioning and traffic steering. 1218 Further details on how these tools can be used to create end to end 1219 network slices are documented in 1220 [I-D.ali-spring-network-slicing-building-blocks]. 1222 9. Control Plane Considerations 1224 This document focuses on user-plane behavior and its independence 1225 from the control plane. While there are benefits in an enhanced 1226 control plane (e.g., to dynamically configure SR policies from a 1227 controller), this document does not impose any change to the existant 1228 mobility control plane. 1230 Section 11 allocates SRv6 Segment Endpoint Behavior codepoints for 1231 the new behaviors defined in this document. 1233 10. Security Considerations 1235 The security considerations for Segment Routing are discussed in 1236 [RFC8402]. More specifically for SRv6 the security considerations 1237 and the mechanisms for securing an SR domain are discussed in 1238 [RFC8754]. Together, they describe the required security mechanisms 1239 that allow establishment of an SR domain of trust to operate 1240 SRv6-based services for internal traffic while preventing any 1241 external traffic from accessing or exploiting the SRv6-based 1242 services. 1244 The technology described in this document is applied to a mobile 1245 network that is within the SR Domain. 1247 This document introduces new SRv6 Endpoint Behaviors. Those 1248 behaviors do not need any special security consideration given that 1249 it is deployed within that SR Domain. 1251 11. IANA Considerations 1253 The following values have been allocated within the "SRv6 Endpoint 1254 Behaviors" [RFC8986] sub-registry belonging to the top-level "Segment 1255 Routing Parameters" registry: 1257 +=======+========+===================+===========+ 1258 | Value | Hex | Endpoint behavior | Reference | 1259 +=======+========+===================+===========+ 1260 | 40 | 0x0028 | End.MAP | [This.ID] | 1261 +-------+--------+-------------------+-----------+ 1262 | 41 | 0x0029 | End.Limit | [This.ID] | 1263 +-------+--------+-------------------+-----------+ 1264 | 69 | 0x0045 | End.M.GTP6.D | [This.ID] | 1265 +-------+--------+-------------------+-----------+ 1266 | 70 | 0x0046 | End.M.GTP6.Di | [This.ID] | 1267 +-------+--------+-------------------+-----------+ 1268 | 71 | 0x0047 | End.M.GTP6.E | [This.ID] | 1269 +-------+--------+-------------------+-----------+ 1270 | 72 | 0x0048 | End.M.GTP4.E | [This.ID] | 1271 +-------+--------+-------------------+-----------+ 1273 Table 1: SRv6 Mobile User-plane Endpoint 1274 Behavior Types 1276 12. Acknowledgements 1278 The authors would like to thank Daisuke Yokota, Bart Peirens, 1279 Ryokichi Onishi, Kentaro Ebisawa, Peter Bosch, Darren Dukes, Francois 1280 Clad, Sri Gundavelli, Sridhar Bhaskaran, Arashmid Akhavain, Ravi 1281 Shekhar, Aeneas Dodd-Noble, Carlos Jesus Bernardos, Dirk v. Hugo and 1282 Jeffrey Zhang for their useful comments of this work. 1284 13. Contributors 1286 Kentaro Ebisawa Toyota Motor Corporation Japan 1288 Email: ebisawa@toyota-tokyo.tech 1290 Tetsuya Murakami Arrcus, Inc. United States of America 1292 Email: tetsuya.ietf@gmail.com 1294 14. References 1296 14.1. Normative References 1298 [I-D.ietf-spring-segment-routing-policy] 1299 Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and 1300 P. Mattes, "Segment Routing Policy Architecture", Work in 1301 Progress, Internet-Draft, draft-ietf-spring-segment- 1302 routing-policy-13, 28 May 2021, 1303 . 1306 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1307 Requirement Levels", BCP 14, RFC 2119, 1308 DOI 10.17487/RFC2119, March 1997, 1309 . 1311 [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., 1312 Decraene, B., Litkowski, S., and R. Shakir, "Segment 1313 Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, 1314 July 2018, . 1316 [RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J., 1317 Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header 1318 (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020, 1319 . 1321 [RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer, 1322 D., Matsushima, S., and Z. Li, "Segment Routing over IPv6 1323 (SRv6) Network Programming", RFC 8986, 1324 DOI 10.17487/RFC8986, February 2021, 1325 . 1327 [TS.23501] 3GPP, "System Architecture for the 5G System", 3GPP TS 1328 23.501 15.0.0, November 2017. 1330 14.2. Informative References 1332 [I-D.ali-spring-network-slicing-building-blocks] 1333 Ali, Z., Filsfils, C., Camarillo, P., and D. Voyer, 1334 "Building blocks for Slicing in Segment Routing Network", 1335 Work in Progress, Internet-Draft, draft-ali-spring- 1336 network-slicing-building-blocks-04, 21 February 2021, 1337 . 1340 [I-D.camarilloelmalky-springdmm-srv6-mob-usecases] 1341 Garvia, P. C., Filsfils, C., Elmalky, H., Matsushima, S., 1342 Voyer, D., Cui, A., and B. Peirens, "SRv6 Mobility Use- 1343 Cases", Work in Progress, Internet-Draft, draft- 1344 camarilloelmalky-springdmm-srv6-mob-usecases-02, 15 August 1345 2019, . 1348 [I-D.ietf-lsr-flex-algo] 1349 Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and 1350 A. Gulko, "IGP Flexible Algorithm", Work in Progress, 1351 Internet-Draft, draft-ietf-lsr-flex-algo-17, 6 July 2021, 1352 . 1355 [I-D.ietf-spring-segment-routing-central-epe] 1356 Filsfils, C., Previdi, S., Dawra, G., Aries, E., and D. 1357 Afanasiev, "Segment Routing Centralized BGP Egress Peer 1358 Engineering", Work in Progress, Internet-Draft, draft- 1359 ietf-spring-segment-routing-central-epe-10, 21 December 1360 2017, . 1363 [I-D.ietf-spring-sr-service-programming] 1364 Clad, F., Xu, X., Filsfils, C., Bernier, D., Li, C., 1365 Decraene, B., Ma, S., Yadlapalli, C., Henderickx, W., and 1366 S. Salsano, "Service Programming with Segment Routing", 1367 Work in Progress, Internet-Draft, draft-ietf-spring-sr- 1368 service-programming-04, 10 March 2021, 1369 . 1372 [I-D.matsushima-spring-srv6-deployment-status] 1373 Matsushima, S., Filsfils, C., Ali, Z., Li, Z., and K. 1374 Rajaraman, "SRv6 Implementation and Deployment Status", 1375 Work in Progress, Internet-Draft, draft-matsushima-spring- 1376 srv6-deployment-status-11, 17 February 2021, 1377 . 1380 [TS.29281] 3GPP, "General Packet Radio System (GPRS) Tunnelling 1381 Protocol User Plane (GTPv1-U)", 3GPP TS 29.281 15.1.0, 1382 December 2017. 1384 [TS.38415] 3GPP, "Draft Specification for 5GS container (TS 38.415)", 1385 3GPP R3-174510 0.0.0, August 2017. 1387 Appendix A. Implementations 1389 This document introduces new SRv6 Endpoint Behaviors. These 1390 behaviors have an open-source P4 implementation available in 1391 https://github.com/ebiken/p4srv6. 1393 Additionally, a full implementation of this document is available in 1394 Linux Foundation FD.io VPP project since release 20.05. More 1395 information available here: https://docs.fd.io/vpp/20.05/d7/d3c/ 1396 srv6_mobile_plugin_doc.html. 1398 There are also experimental implementations in M-CORD NGIC and Open 1399 Air Interface (OAI). 1401 Authors' Addresses 1402 Satoru Matsushima (editor) 1403 SoftBank 1404 Japan 1406 Email: satoru.matsushima@g.softbank.co.jp 1408 Clarence Filsfils 1409 Cisco Systems, Inc. 1410 Belgium 1412 Email: cf@cisco.com 1414 Miya Kohno 1415 Cisco Systems, Inc. 1416 Japan 1418 Email: mkohno@cisco.com 1420 Pablo Camarillo Garvia (editor) 1421 Cisco Systems, Inc. 1422 Spain 1424 Email: pcamaril@cisco.com 1426 Daniel Voyer 1427 Bell Canada 1428 Canada 1430 Email: daniel.voyer@bell.ca 1432 Charles E. Perkins 1433 Lupin Lodge 1434 20600 Aldercroft Heights Rd. 1435 Los Gatos, CA 95033 1436 United States of America 1438 Email: charliep@computer.org