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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 SPRING Working Group W. Cheng 3 Internet-Draft H. Li 4 Intended status: Standards Track China Mobile 5 Expires: 24 June 2022 M. Chen 6 Huawei 7 R. Gandhi 8 Cisco Systems, Inc. 9 R. Zigler 10 Broadcom 11 21 December 2021 13 Path Segment in MPLS Based Segment Routing Network 14 draft-ietf-spring-mpls-path-segment-07 16 Abstract 18 A Segment Routing (SR) path is identified by an SR segment list. 19 Only the complete segment list can identify the end-to-end SR path, 20 and a sub-set of segments from the segment list cannot distinguish 21 one SR path from another as they may be partially congruent. SR path 22 identification is a pre-requisite for various use-cases such as 23 Performance Measurement (PM), bidirectional paths correlation, and 24 end-to-end 1+1 path protection. 26 In SR for MPLS data plane (SR-MPLS), the segment identifiers are 27 stripped from the packet through label popping as the packet transits 28 the network. This means that when a packet reaches the egress of the 29 SR path, it is not possible to determine on which SR path it 30 traversed the network. 32 This document defines a new type of segment that is referred to as 33 Path Segment, which is used to identify an SR path in an SR-MPLS 34 network. When used, it is inserted by the ingress node of the SR 35 path and immediately follows the last segment identifier in the 36 segment list of the SR path. The Path Segment is preserved until it 37 reaches the egress node for SR path identification and correlation. 39 Status of This Memo 41 This Internet-Draft is submitted in full conformance with the 42 provisions of BCP 78 and BCP 79. 44 Internet-Drafts are working documents of the Internet Engineering 45 Task Force (IETF). Note that other groups may also distribute 46 working documents as Internet-Drafts. The list of current Internet- 47 Drafts is at https://datatracker.ietf.org/drafts/current/. 49 Internet-Drafts are draft documents valid for a maximum of six months 50 and may be updated, replaced, or obsoleted by other documents at any 51 time. It is inappropriate to use Internet-Drafts as reference 52 material or to cite them other than as "work in progress." 54 This Internet-Draft will expire on 24 June 2022. 56 Copyright Notice 58 Copyright (c) 2021 IETF Trust and the persons identified as the 59 document authors. All rights reserved. 61 This document is subject to BCP 78 and the IETF Trust's Legal 62 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 63 license-info) in effect on the date of publication of this document. 64 Please review these documents carefully, as they describe your rights 65 and restrictions with respect to this document. Code Components 66 extracted from this document must include Revised BSD License text as 67 described in Section 4.e of the Trust Legal Provisions and are 68 provided without warranty as described in the Revised BSD License. 70 Table of Contents 72 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 73 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 74 1.2. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 3 75 2. Path Segment . . . . . . . . . . . . . . . . . . . . . . . . 4 76 3. Path Segment Allocation . . . . . . . . . . . . . . . . . . . 6 77 4. Nesting of Path Segments . . . . . . . . . . . . . . . . . . 6 78 5. Path Segment for Performance Measurement . . . . . . . . . . 7 79 6. Path Segment for Bidirectional SR Path . . . . . . . . . . . 8 80 7. Path Segment for End-to-end Path Protection . . . . . . . . . 9 81 8. Security Considerations . . . . . . . . . . . . . . . . . . . 9 82 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 83 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 84 10.1. Normative References . . . . . . . . . . . . . . . . . . 9 85 10.2. Informative References . . . . . . . . . . . . . . . . . 10 86 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 12 87 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 12 88 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13 90 1. Introduction 92 Segment Routing (SR) [RFC8402] leverages the source-routing paradigm 93 to steer packets from a source node through a controlled set of 94 instructions, called segments, by prepending the packet with an SR 95 header in the MPLS data plane SR-MPLS [RFC8660] through a label stack 96 or IPv6 data plane using an SRH header via SRv6 [RFC8986] to 97 construct an SR path. 99 In an SR-MPLS network, when a packet is transmitted along an SR path, 100 the labels in the MPLS label stack will be swapped or popped. The 101 consequence of this is that no label or only the last label (e.g. 102 Explicit-Null label) may be left in the MPLS label stack when the 103 packet reaches the egress node. Thus, the egress node cannot 104 determine along which SR path the packet was received. 106 However, to support various use-cases in SR-MPLS networks, like end- 107 to-end 1+1 path protection (Live-Live case) [RFC4426], bidirectional 108 path [RFC5654], or Performance Measurement (PM) [RFC7799], the 109 ability to implement path identification on the egress node is a pre- 110 requisite. 112 Therefore, this document introduces a new segment type that is 113 referred to as the Path Segment. A Path Segment is defined to 114 uniquely identify an SR path in an SR-MPLS network in the context of 115 the egress node. It is normally used by the egress nodes for path 116 identification hence to support various use-cases including SR path 117 PM, end-to-end 1+1 SR path protection, and bidirectional SR paths 118 correlation. 120 1.1. Requirements Language 122 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 123 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 124 "OPTIONAL" in this document are to be interpreted as described in 125 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, 126 as shown here. 128 1.2. Abbreviations 130 DM: Delay Measurement. 132 LM: Loss Measurement. 134 MPLS: Multiprotocol Label Switching. 136 MSD: Maximum SID Depth. 138 PM: Performance Measurement. 140 PSID: Path Segment ID. 142 SID: Segment ID. 144 SL: Segment List. 146 SR: Segment Routing. 148 SRLB: SR Local Block 150 SRGB: SR Global Block 152 SR-MPLS: Segment Routing instantiated on MPLS data plane. 154 2. Path Segment 156 A Path Segment is a single label that is assigned from the Segment 157 Routing Local Block (SRLB) [RFC8402] or Segment Routing Global Block 158 (SRGB) [RFC8402] or dynamic MPLS label pool of the egress node of an 159 SR path. It means that the Path Segment is unique in the context of 160 the egress node of the SR path. When a Path Segment is used, the 161 Path Segment MUST be inserted at the ingress node and MUST 162 immediately follow the last label of the SR path, in other words, 163 inserted after the routing segment (adjacency/node/prefix segment) 164 pointing to the egress node. 166 The term of SR path used in this document is a general term that can 167 be used to describe a SR Policy, a Candidate-Path (CP), or a SID List 168 (SL) [I-D.ietf-spring-segment-routing-policy]. Therefore, the Path 169 Segment may be used to identify an SR Policy, its CP, or a SL 170 terminating on an egress node depending on the use-case. 172 The value of the TTL field in the MPLS label stack entry containing 173 the Path Segment MUST be set to the same value as the TTL of the last 174 label stack entry for the last segment in the SR path. If the Path 175 Segment is the bottom label, the S bit MUST be set. 177 A Path Segment can be used in the case of Penultimate Hop Popping 178 (PHP), where some labels are be popped off at the penultimate hop of 179 an SR path, but the Path Segment MUST NOT be popped off until it 180 reaches the egress node. 182 The egress node MUST pop the Path Segment. The egress node MAY use 183 the Path Segment for further processing. For example, when 184 performance measurement is enabled on the SR path, it can trigger 185 packet counting or timestamping. 187 In some deployments, service labels may be added after the Path 188 Segment label in the MPLS label stack. In this case, the egress node 189 MUST be capable of processing more than one label. The additional 190 processing required, may have an impact on forwarding performance. 192 Generic Associated Label (GAL) is used for Operations, Administration 193 and Maintenance (OAM) in MPLS networks [RFC5586]. When GAL is used, 194 it MUST be added at the bottom of the label stack after the Path 195 Segment label. 197 Entropy label and Entropy Label Indicator (ELI) as described in 198 [RFC8662] for SR-MPLS path, can be placed before or after the Path 199 Segment label in the MPLS label stack. 201 The SR path computation needs to know the Maximum SID Depth (MSD) 202 that can be imposed at each node/link of a given SR path [RFC8664]. 203 This ensures that the SID stack depth of a computed path does not 204 exceed the number of SIDs the node is capable of imposing. Adding a 205 Path Segment to a label stack will increase the depth of the label 206 stack, the Path Segment MUST be accounted when considering MSD. 208 The label stack with Path Segment is shown in Figure 1: 210 +--------------------+ 211 | ... | 212 +--------------------+ 213 | Label 1 | 214 +--------------------+ 215 | Label 2 | 216 +--------------------+ 217 | ... | 218 +--------------------+ 219 | Label n | 220 +--------------------+ 221 | Path Segment | 222 +--------------------+ 223 | ... | 224 +--------------------+ 225 ~ Payload ~ 226 +--------------------+ 228 Figure 1: Label Stack with Path Segment 230 Where: 232 * The Labels 1 to n are the segment label stack used to direct how 233 to steer the packets along the SR path. 235 * The Path Segment identifies the SR path in the context of the 236 egress node of the SR path. 238 There may be multiple paths (or sub-path(s)) carried in the label 239 stack, for each path (or sub-path), there may be a corresponding Path 240 Segment carried. A use case can be found in Section 4. 242 3. Path Segment Allocation 244 Several ways can be used to allocate the Path Segment. 246 One way is to set up a communication channel (e.g., MPLS Generic 247 Associated Channel (G-ACh)) [RFC5586] between the ingress node and 248 the egress node, and the ingress node of the SR path can directly 249 send a request to the egress node to allocate a Path Segment. The 250 detail of G-ACh based solution is left for future study and out of 251 the scope of this document. 253 Another way is to leverage a centralized controller (e.g., SDN 254 controller) to assign the Path Segment. In this case, the controller 255 will deliver the Path Segment and corresponding path information 256 (e.g., SR policy) to the ingress node. Path Computation Element 257 Communication Protocol (PCEP) based Path Segment allocation for SR 258 Policy is defined in [I-D.ietf-pce-sr-path-segment], BGP based Path 259 Segment allocation for SR Policy is defined in 260 [I-D.ietf-idr-sr-policy-path-segment]. At the same time, the 261 controller MUST make sure (e.g., by some capability discovery 262 mechanisms outside the scope of this document) that the egress node 263 knows the Path Segment and it can process it, as well as the label 264 does not collide with any label allocation done by the egress node. 266 4. Nesting of Path Segments 268 Binding SID (BSID) [RFC8402] can be used for SID list compression. 269 With BSID, an end-to-end SR path can be split into several sub-paths, 270 each sub-path is identified by a BSID. Then an end-to-end SR path 271 can be identified by a list of BSIDs, therefore, it can provide 272 better scalability. 274 BSID and Path Segment ID (PSID) can be combined to achieve both sub- 275 path and end-to-end path monitoring. A reference model for such a 276 combination in (Figure 2) shows an end-to-end path (A->D) that spans 277 three domains (Access, Aggregation and Core domain) and consists of 278 three sub-paths, one in each sub-domain (sub-path (A->B), sub-path 279 (B->C) and sub-path (C->D)). Each sub-path is allocated a BSID. For 280 nesting the sub-paths, each sub-path is allocated a PSID. Then, the 281 SID list of the end-to-end path can be expressed as , where the e-PSID is the PSID of the end-to-end 283 path. The SID list of a sub-path can be expressed as , where the s-PSID is the PSID of the sub-path. 286 Figure 2 shows the details of the label stacks when PSID and BSID are 287 used to support both sub-path and end-to-end path monitoring in a 288 multi-domain scenario. 290 /--------\ /--------\ /--------\ 291 / \ / \ / \ 292 A{ Access }B{ Aggregation }C{ Core }D 293 \ / \ / \ / 294 \--------/ \--------/ \--------/ 295 Sub-path(A->B) Sub-path(B->C) Sub-path(C->D) 296 |<--------------->|<-------------->|<-------------->| 297 E2E Path(A->D) 298 |<------------------------------------------------->| 300 +------------+ 301 ~A->B SubPath~ 302 +------------+ +------------+ 303 |s-PSID(A->B)| ~B->C SubPath~ 304 +------------+ +------------+ 305 | BSID(B->C) | |s-PSID(B->C)| 306 +------------+ +------------+ +------------+ 307 | BSID(C->D) | | BSID(C->D) | ~C->D SubPath~ 308 +------------+ +------------+ +------------+ +------------+ 309 |e-PSID(A->D)| |e-PSID(A->D)| |e-PSID(A->D)| |e-PSID(A->D)| 310 +------------+ +------------+ +------------+ +------------+ 312 Figure 2: Nesting of Path Segments 314 5. Path Segment for Performance Measurement 316 As defined in [RFC7799], performance measurement can be classified 317 into Passive, Active, and Hybrid measurement. Since Path Segment is 318 encoded in the SR-MPLS Label Stack as shown in Figure 1, existing 319 implementation on the egress node can be leveraged for measuring 320 packet counts using the incoming SID (the Path Segment). A different 321 scheme such as carrying such identifier after the bottom of the label 322 stack may require new implementation. 324 For Passive performance measurement, path identification at the 325 measuring points is the prerequisite. Path Segment can be used by 326 the measuring points (e.g., the ingress and egress nodes of the SR 327 path or a centralized controller) to measure packet counts on the 328 egress node and to correlate the packet counters and timestamps from 329 the ingress and egress nodes for a specific SR path, then packet loss 330 and delay can be calculated for the end-to-end path, respectively. 332 Path Segment can also be used for Active performance measurement for 333 an end-to-end SR path in SR-MPLS networks for measuring packet counts 334 on the egress node and for collecting the packet counters and 335 timestamps from the egress node associated with the SR path using 336 probe messages [I-D.ietf-mpls-rfc6374-sr] and 337 [I-D.ietf-spring-stamp-srpm]. 339 Path Segment can also be used for In-situ OAM (IOAM) for SR-MPLS to 340 identify the SR path associated with the in-situ data fields in the 341 data packets on the egress node [I-D.ietf-ippm-ioam-data]. 343 Path Segment can also be used for In-band PM with Alternate Marking 344 Method for SR-MPLS to identify the SR Path associated with the 345 performance metrics collected 346 [I-D.ietf-mpls-inband-pm-encapsulation]. 348 6. Path Segment for Bidirectional SR Path 350 In some scenarios, for example, mobile backhaul transport networks, 351 there are requirements to support bidirectional paths, and the path 352 is normally treated as a single entity. The both directions of the 353 path have the same fate, for example, failure in one direction will 354 result in switching traffic at both directions. MPLS supports this 355 by introducing the concepts of co-routed bidirectional LSP and 356 associated bidirectional LSP [RFC5654]. 358 In the current SR architecture, an SR path is a unidirectional path 359 [RFC8402]. In order to support bidirectional SR paths, a 360 straightforward way is to bind two unidirectional SR paths to a 361 single bidirectional SR path. Path Segments can then be used to 362 identify and correlate the traffic for the two unidirectional SR 363 paths at both ends of the bidirectional path. 365 [I-D.ietf-pce-sr-bidir-path] defines procedures on how to use PCEP 366 for SR Policy to initiate a bidirectional SR path. Also, 367 [I-D.ietf-idr-sr-policy-path-segment] defines procedures on how to 368 use BGP for SR Policy to initiate a bidirectional SR path. 370 7. Path Segment for End-to-end Path Protection 372 For end-to-end 1+1 path protection (i.e., Live-Live case), the egress 373 node of the path needs to know the set of paths that constitute the 374 primary and the secondaries, in order to select the primary path 375 packets for onward transmission, and to discard the packets from the 376 secondaries [RFC4426]. 378 To do this in Segment Routing, each SR path needs a path identifier 379 that is unique at the egress node. For SR-MPLS, this can be the Path 380 Segment label allocated by the egress node. 382 There then needs to be a method of binding this SR path identifiers 383 into equivalence groups such that the egress node can determine for 384 example, the set of packets that represent a single primary path. It 385 is obvious that this equivalence group can be instantiated in the 386 network by an SDN controller using the Path Segments of the SR paths. 388 8. Security Considerations 390 Path Segment in SR-MPLS does not introduce any new behavior or any 391 change in the way the MPLS data plane works. Section 8.1 of 392 [RFC8402] describe the security consideration for SR-MPLS. Path 393 segment is additional metadata that is added to the packet consisting 394 of the SR path. 396 An attacker could exploit path segment to manipulate the accounting 397 of SR traffic at the egress. Path segment could also be used to 398 monitor traffic patterns for the E2E paths. The control protocols 399 used to allocate path segments could also be exploited to disseminate 400 incorrect path segment information. Note that, the path segment is 401 imposed at the ingress and removed at the egress boundary and is not 402 leaked out of the administered domain. 404 9. IANA Considerations 406 This document does not require any IANA actions. 408 10. References 410 10.1. Normative References 412 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 413 Requirement Levels", BCP 14, RFC 2119, 414 DOI 10.17487/RFC2119, March 1997, 415 . 417 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 418 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 419 May 2017, . 421 [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., 422 Decraene, B., Litkowski, S., and R. Shakir, "Segment 423 Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, 424 July 2018, . 426 [RFC8660] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S., 427 Decraene, B., Litkowski, S., and R. Shakir, "Segment 428 Routing with the MPLS Data Plane", RFC 8660, 429 DOI 10.17487/RFC8660, December 2019, 430 . 432 10.2. Informative References 434 [I-D.ietf-idr-sr-policy-path-segment] 435 Li, C., Li, Z., Yin, Y., Cheng, W., and K. Talaulikar, "SR 436 Policy Extensions for Path Segment and Bidirectional 437 Path", Work in Progress, Internet-Draft, draft-ietf-idr- 438 sr-policy-path-segment-04, 8 July 2021, 439 . 442 [I-D.ietf-ippm-ioam-data] 443 Brockners, F., Bhandari, S., and T. Mizrahi, "Data Fields 444 for In-situ OAM", Work in Progress, Internet-Draft, draft- 445 ietf-ippm-ioam-data-17, 13 December 2021, 446 . 449 [I-D.ietf-mpls-inband-pm-encapsulation] 450 Cheng, W., Min, X., Zhou, T., Dong, X., and Y. Peleg, 451 "Encapsulation For MPLS Performance Measurement with 452 Alternate Marking Method", Work in Progress, Internet- 453 Draft, draft-ietf-mpls-inband-pm-encapsulation-02, 25 454 October 2021, . 457 [I-D.ietf-mpls-rfc6374-sr] 458 Gandhi, R., Filsfils, C., Voyer, D., Salsano, S., and M. 459 Chen, "Performance Measurement Using RFC 6374 for Segment 460 Routing Networks with MPLS Data Plane", Work in Progress, 461 Internet-Draft, draft-ietf-mpls-rfc6374-sr-04, 9 September 462 2021, . 465 [I-D.ietf-pce-sr-bidir-path] 466 Li, C., Chen, M., Cheng, W., Gandhi, R., and Q. Xiong, 467 "Path Computation Element Communication Protocol (PCEP) 468 Extensions for Associated Bidirectional Segment Routing 469 (SR) Paths", Work in Progress, Internet-Draft, draft-ietf- 470 pce-sr-bidir-path-08, 9 September 2021, 471 . 474 [I-D.ietf-pce-sr-path-segment] 475 Li, C., Chen, M., Cheng, W., Gandhi, R., and Q. Xiong, 476 "Path Computation Element Communication Protocol (PCEP) 477 Extension for Path Segment in Segment Routing (SR)", Work 478 in Progress, Internet-Draft, draft-ietf-pce-sr-path- 479 segment-04, 12 August 2021, 480 . 483 [I-D.ietf-spring-segment-routing-policy] 484 Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and 485 P. Mattes, "Segment Routing Policy Architecture", Work in 486 Progress, Internet-Draft, draft-ietf-spring-segment- 487 routing-policy-14, 25 October 2021, 488 . 491 [I-D.ietf-spring-stamp-srpm] 492 Gandhi, R., Filsfils, C., Voyer, D., Chen, M., Janssens, 493 B., and R. Foote, "Performance Measurement Using Simple 494 TWAMP (STAMP) for Segment Routing Networks", Work in 495 Progress, Internet-Draft, draft-ietf-spring-stamp-srpm-02, 496 13 September 2021, . 499 [RFC4426] Lang, J., Ed., Rajagopalan, B., Ed., and D. Papadimitriou, 500 Ed., "Generalized Multi-Protocol Label Switching (GMPLS) 501 Recovery Functional Specification", RFC 4426, 502 DOI 10.17487/RFC4426, March 2006, 503 . 505 [RFC5586] Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed., 506 "MPLS Generic Associated Channel", RFC 5586, 507 DOI 10.17487/RFC5586, June 2009, 508 . 510 [RFC5654] Niven-Jenkins, B., Ed., Brungard, D., Ed., Betts, M., Ed., 511 Sprecher, N., and S. Ueno, "Requirements of an MPLS 512 Transport Profile", RFC 5654, DOI 10.17487/RFC5654, 513 September 2009, . 515 [RFC7799] Morton, A., "Active and Passive Metrics and Methods (with 516 Hybrid Types In-Between)", RFC 7799, DOI 10.17487/RFC7799, 517 May 2016, . 519 [RFC8662] Kini, S., Kompella, K., Sivabalan, S., Litkowski, S., 520 Shakir, R., and J. Tantsura, "Entropy Label for Source 521 Packet Routing in Networking (SPRING) Tunnels", RFC 8662, 522 DOI 10.17487/RFC8662, December 2019, 523 . 525 [RFC8664] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W., 526 and J. Hardwick, "Path Computation Element Communication 527 Protocol (PCEP) Extensions for Segment Routing", RFC 8664, 528 DOI 10.17487/RFC8664, December 2019, 529 . 531 [RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer, 532 D., Matsushima, S., and Z. Li, "Segment Routing over IPv6 533 (SRv6) Network Programming", RFC 8986, 534 DOI 10.17487/RFC8986, February 2021, 535 . 537 Acknowledgements 539 The authors would like to thank Adrian Farrel, Stewart Bryant, 540 Shuangping Zhan, Alexander Vainshtein, Andrew G. Malis, Ketan 541 Talaulikar, Shraddha Hegde, and Loa Andersson for their review, 542 suggestions and comments to this document. 544 The authors would like to acknowledge the contribution from Alexander 545 Vainshtein on "Nesting of Path Segments". 547 Contributors 549 The following people have substantially contributed to this document: 551 Cheng Li 552 Huawei Technologies 554 EMail: chengli13@huawei.com 556 Lei Wang 557 China Mobile 559 Email: wangleiyj@chinamobile.com 561 Aihua Liu 562 ZTE Corp 564 Email: liu.aihua@zte.com.cn 566 Greg Mirsky 567 ZTE Corp 569 Email: gregimirsky@gmail.com 571 Gyan S. Mishra 572 Verizon Inc. 574 Email: gyan.s.mishra@verizon.com 576 James Guichard 577 Futurewei Technologies 579 Email: james.n.guichard@futurewei.com 581 Authors' Addresses 583 Weiqiang Cheng 584 China Mobile 586 Email: chengweiqiang@chinamobile.com 588 Han Li 589 China Mobile 591 Email: lihan@chinamobile.com 592 Mach(Guoyi) Chen 593 Huawei 595 Email: mach.chen@huawei.com 597 Rakesh Gandhi 598 Cisco Systems, Inc. 599 Canada 601 Email: rgandhi@cisco.com 603 Royi Zigler 604 Broadcom 606 Email: royi.zigler@broadcom.com