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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Looks like a reference, but probably isn't: '0' on line 628 -- Looks like a reference, but probably isn't: '1' on line 609 Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 6man R. Bonica 3 Internet-Draft Juniper Networks 4 Intended status: Standards Track Y. Kamite 5 Expires: November 26, 2021 NTT Communications Corporation 6 A. Alston 7 D. Henriques 8 Liquid Telecom 9 L. Jalil 10 Verizon 11 May 25, 2021 13 The IPv6 Compact Routing Header (CRH) 14 draft-bonica-6man-comp-rtg-hdr-26 16 Abstract 18 This document defines two new Routing header types. Collectively, 19 they are called the Compact Routing Headers (CRH). Individually, 20 they are called CRH-16 and CRH-32. 22 Status of This Memo 24 This Internet-Draft is submitted in full conformance with the 25 provisions of BCP 78 and BCP 79. 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF). Note that other groups may also distribute 29 working documents as Internet-Drafts. The list of current Internet- 30 Drafts is at https://datatracker.ietf.org/drafts/current/. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 This Internet-Draft will expire on November 26, 2021. 39 Copyright Notice 41 Copyright (c) 2021 IETF Trust and the persons identified as the 42 document authors. All rights reserved. 44 This document is subject to BCP 78 and the IETF Trust's Legal 45 Provisions Relating to IETF Documents 46 (https://trustee.ietf.org/license-info) in effect on the date of 47 publication of this document. Please review these documents 48 carefully, as they describe your rights and restrictions with respect 49 to this document. Code Components extracted from this document must 50 include Simplified BSD License text as described in Section 4.e of 51 the Trust Legal Provisions and are provided without warranty as 52 described in the Simplified BSD License. 54 Table of Contents 56 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 57 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3 58 3. The Compressed Routing Headers (CRH) . . . . . . . . . . . . 3 59 4. The CRH Forwarding Information Base (CRH-FIB) . . . . . . . 5 60 5. Processing Rules . . . . . . . . . . . . . . . . . . . . . . 6 61 5.1. Computing Minimum CRH Length . . . . . . . . . . . . . . 7 62 5.2. CRH Removal Procedure . . . . . . . . . . . . . . . . . . 8 63 6. Mutability . . . . . . . . . . . . . . . . . . . . . . . . . 9 64 7. Applications And SIDs . . . . . . . . . . . . . . . . . . . . 9 65 8. Management Considerations . . . . . . . . . . . . . . . . . . 9 66 9. Security Considerations . . . . . . . . . . . . . . . . . . . 9 67 10. Implementation and Deployment Status . . . . . . . . . . . . 10 68 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 69 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 70 13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 10 71 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 72 14.1. Normative References . . . . . . . . . . . . . . . . . . 11 73 14.2. Informative References . . . . . . . . . . . . . . . . . 12 74 Appendix A. CRH Processing Examples . . . . . . . . . . . . . . 13 75 A.1. The SID List Contains One Entry For Each Segment In The 76 Path . . . . . . . . . . . . . . . . . . . . . . . . . . 13 77 A.2. The SID List Omits The First Entry In The Path . . . . . 14 78 Appendix B. A Packet Recycling Use-Case . . . . . . . . . . . . 14 79 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15 81 1. Introduction 83 IPv6 [RFC8200] source nodes use Routing headers to specify the path 84 that a packet takes to its destination. The IETF has defined several 85 Routing header types [IANA-RH]. This document defines two new 86 Routing header types. Collectively, they are called the Compact 87 Routing Headers (CRH). Individually, they are called CRH-16 and CRH- 88 32. 90 The CRH allows IPv6 source nodes to specify the path that a packet 91 takes to its destination. The CRH: 93 o Can be encoded in relatively few bytes. 95 o Is designed to operate within a network domain. (See Section 9). 97 The following are reasons for encoding the CRH in as few bytes as 98 possible: 100 o Many ASIC-based forwarders copy headers from buffer memory to on- 101 chip memory. As header sizes increase, so does the cost of this 102 copy. 104 o Because Path MTU Discovery (PMTUD) [RFC8201] is not entirely 105 reliable, many IPv6 hosts refrain from sending packets larger than 106 the IPv6 minimum link MTU (i.e., 1280 bytes). When packets are 107 small, the overhead imposed by large Routing Headers is excessive. 109 2. Requirements Language 111 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 112 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 113 "OPTIONAL" in this document are to be interpreted as described in BCP 114 14 [RFC2119] [RFC8174] when, and only when, they appear in all 115 capitals, as shown here. 117 3. The Compressed Routing Headers (CRH) 119 Both CRH versions (i.e., CRH-16 and CRH-32) contain the following 120 fields: 122 o Next Header - Defined in [RFC8200]. 124 o Hdr Ext Len - Defined in [RFC8200]. 126 o Routing Type - Defined in [RFC8200]. Value TBD by IANA. (For 127 CRH-16, the suggested value is 5. For CRH-32, the suggested value 128 is 6.) 130 o Segments Left - Defined in [RFC8200]. 132 o Type-specific Data - Described in [RFC8200]. 134 In the CRH, the Type-specific data field contains a list of Segment 135 Identifiers (SIDs). Each SID represents both of the following: 137 o A segment of the path that the packet takes to its destination. 139 o An entry in the CRH Forwarding Information Base (CRH-FIB) 140 (Section 4). 142 SIDs are listed in reverse order. So, the first SID in the list 143 represents the final segment in the path. Because segments are 144 listed in reverse order, the Segments Left field can be used as an 145 index into the SID list. In this document, the "current SID" is the 146 SID list entry referenced by the Segments Left field. 148 The first segment in the path can be omitted from the list. See 149 (Appendix A) for examples. 151 In the CRH-16 (Figure 1), each SID is encoded in 16-bits. In the 152 CRH-32 (Figure 2), each SID is encoded in 32-bits. 154 In all cases, the CRH MUST end on a 64-bit boundary. So, the Type- 155 specific data field MUST be padded with zeros if the CRH would 156 otherwise not end on a 64-bit boundary. 158 0 1 2 3 159 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 1 160 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 161 | Next Header | Hdr Ext Len | Routing Type | Segments Left | 162 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 163 | SID[0] | SID[1] | 164 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| 165 | ......... 166 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- 168 Figure 1: CRH-16 170 0 1 2 3 171 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 1 172 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 173 | Next Header | Hdr Ext Len | Routing Type | Segments Left | 174 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 175 + SID[0] + 176 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 177 + SID[1] + 178 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 179 // // 180 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 181 + SID[n] + 182 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 184 Figure 2: CRH-32 186 4. The CRH Forwarding Information Base (CRH-FIB) 188 Each SID identifies a CRH-FIB entry. 190 Each CRH-FIB entry contains: 192 o An IPv6 address (optional). 194 o A topological function. 196 o Arguments for the topological function (optional). 198 o Flags. 200 o A service function (optional). 202 o Arguments for the service function (optional). 204 The IPv6 address can represent either: 206 o An interface on the next segment endpoint. 208 o An SRv6 SID [RFC8986], instantiated on the next segment endpoint. 210 The first ten bits of the IPv6 address MUST NOT be fe00. That prefix 211 is reserved for link-local [RFC6890] addresses. 213 The topological function specifies how the processing node forwards 214 the packet to the next segment endpoint. The following are examples: 216 o Forward the packet through the least-cost path to the next segment 217 endpoint. 219 o Forward the packet through a specified interface. 221 o Encapsulate the packet in another IPv6 header of any type (e.g., 222 MPLS, IPv6) and forward either through the least cost path or a 223 specified interface. 225 o Recycle the packet, as if the node had forwarded to one of its own 226 interfaces. When recycling is complete, process the next SID. 227 See Appendix B for a packet recycling use-case. 229 Some topological functions require parameters. For example, a 230 topological function might require a parameter that identifies the 231 interface through which the packet should be forwarded. 233 The following flags are defined: 235 o The PSP flag indicates whether the penultimate segment endpoint 236 (i.e., the node that sets Segments Left to 0) MAY remove the CRH. 238 o The OAM flag indicates whether the processing node should invoke 239 OAM procedures for which it is configured. 241 The service function is optional. If present, it invokes a node 242 specific procedure. The following are examples of node specific 243 procedures: 245 o Emit telemetry. 247 o Subject the packet's payload to a firewall rule. 249 o Replicate the packet, forwarding one copy and retaining the other 250 for sampling, analysis, or other purposes. 252 Node specific procedures are not subject to standardization. A node 253 can support any number of node specific procedures and associate them 254 with any SIDs. 256 Some service functions require parameters. For example, an 257 instruction to emit telemetry might require an IP address to which 258 telemetry should be sent. 260 The CRH-FIB can be populated: 262 o By an operator, using a Command Line Interface (CLI). 264 o By a controller, using the Path Computation Element (PCE) 265 Communication Protocol (PCEP) [RFC5440] or the Network 266 Configuration Protocol (NETCONF) [RFC6241]. 268 o By a distributed routing protocol [ISO10589-Second-Edition], 269 [RFC5340], [RFC4271]. 271 5. Processing Rules 273 The following rules describe CRH processing: 275 o If Segments Left equals 0, skip over the CRH and process the next 276 header in the packet. 278 o If Hdr Ext Len indicates that the CRH is larger than the 279 implementation can process, discard the packet and send an ICMPv6 280 [RFC4443] Parameter Problem, Code 0, message to the Source 281 Address, pointing to the Hdr Ext Len field. 283 o Compute L, the minimum CRH length ( (Section 5.1)). 285 o If L is greater than Hdr Ext Len, discard the packet and send an 286 ICMPv6 Parameter Problem, Code 0, message to the Source Address, 287 pointing to the Segments Left field. 289 o Decrement Segments Left. 291 o Search for the current SID in the CRH-FIB. In this document, the 292 "current SID" is the SID list entry referenced by the Segments 293 Left field. 295 o If the search does not return a CRH-FIB entry, discard the packet 296 and send an ICMPv6 Parameter Problem, Code 0, message to the 297 Source Address, pointing to the current SID. 299 o If Segments Left is greater than 0 and the CRH-FIB entry contains 300 a multicast address, discard the packet and send an ICMPv6 301 Parameter Problem, Code 0, message to the Source Address, pointing 302 to the current SID. 304 o If present, copy the IPv6 address from the CRH-FIB entry to the 305 Destination Address field in the IPv6 header. 307 o Decrement the IPv6 Hop Limit. 309 o If the CRH-FIB entry contains a service function, execute it. 311 o If Segments Left is equal to zero, and the PSP flag in the CRH-FIB 312 entry is set, execute the CRH removal procedure ( Section 5.2). 314 o Submit the packet, its topological function and its parameters to 315 the IPv6 module. See NOTE. 317 NOTE: By default, the IPv6 module determines the next-hop and 318 forwards the packet. However, the topological function may elicit 319 another behavior. For example, the IPv6 module may forward the 320 packet through a specified interface. 322 5.1. Computing Minimum CRH Length 324 The algorithm described in this section accepts the following CRH 325 fields as its input parameters: 327 o Routing Type (i.e., CRH-16 or CRH-32). 329 o Segments Left. 331 It yields L, the minimum CRH length. The minimum CRH length is 332 measured in 8-octet units, not including the first 8 octets. 334 336 switch(Routing Type) { 337 case CRH-16: 338 if (Segments Left <= 2) 339 return(0) 340 sidsBeyondFirstWord = Segments Left - 2; 341 sidPerWord = 4; 342 case CRH-32: 343 if (Segments Left <= 1) 344 return(0) 345 sidsBeyondFirstWord = Segments Left - 1; 346 sidsPerWord = 2; 347 case default: 348 return(0xFF); 349 } 351 words = sidsBeyondFirstWord div sidsPerWord; 352 if (sidsBeyondFirstWord mod sidsPerWord) 353 words++; 355 return(words) 357 359 5.2. CRH Removal Procedure 361 The processing node SHOULD execute the following procedure, if it is 362 capable of doing so: 364 o Update the Next Header field in the header preceding the CRH using 365 a value taken from the Next Header field in the CRH. 367 o Decrease the Payload Length filed in the IPv6 header by 8*(x+1), 368 where value of x is equal to the value of the Hdr Ext Len field in 369 the CRH. 371 o Remove the CRH from the IPv6 header chain. 373 6. Mutability 375 In the CRH, the Segments Left field is mutable. All remaining fields 376 are immutable. 378 7. Applications And SIDs 380 A CRH contains one or more SIDs. Each SID is processed by exactly 381 one node. 383 Therefore, a SID is not required to have domain-wide significance. 384 Applications can: 386 o Allocate SIDs so that they have domain-wide significance. 388 o Allocate SIDs so that they have node-local significance. 390 8. Management Considerations 392 PING and TRACEROUTE [RFC2151] both operate correctly in the presence 393 of the CRH. 395 9. Security Considerations 397 Networks that process the CRH MUST NOT accept packets containing the 398 CRH from untrusted sources. Their border routers SHOULD discard 399 packets that satisfy the following criteria: 401 o The packet contains a CRH 403 o The Segments Left field in the CRH has a value greater than 0 405 o The Destination Address field in the IPv6 header represents an 406 interface that resides inside of the network. 408 Many border routers cannot filter packets based upon the Segments 409 Left value. These border routers MAY discard packets that satisfy 410 the following criteria: 412 o The packet contains a CRH 414 o The Destination Address field in the IPv6 header represents an 415 interface that resides inside of the network. 417 10. Implementation and Deployment Status 419 Juniper Networks has produced experimental implementations of the CRH 420 on: 422 o A LINUX-based software platform 424 o The MX-series (ASIC-based) router 426 Liquid Telecom has deployed the CRH, on a limited basis, in their 427 network. Other experimental deployments are in progress. 429 11. IANA Considerations 431 This document makes the following registrations in the "Internet 432 Protocol Version 6 (IPv6) Parameters" "Routing Types" subregistry 433 maintained by IANA: 435 +-------+------------------------------+---------------+ 436 | Value | Description | Reference | 437 +=======+==============================+===============+ 438 | 5 | CRH-16 | This document | 439 +-------+------------------------------+---------------+ 440 | 6 | CRH-32 | This document | 441 +-------+------------------------------+---------------+ 443 12. Acknowledgements 445 Thanks to Dr. Vanessa Ameen, Fernando Gont, Naveen Kottapalli, Joel 446 Halpern, Tony Li, Gerald Schmidt, Nancy Shaw, Ketan Talaulikar, and 447 Chandra Venkatraman for their contributions to this document. 449 13. Contributors 451 Gang Chen 453 Baidu 455 No.10 Xibeiwang East Road Haidian District 457 Beijing 100193 P.R. China 459 Email: phdgang@gmail.com 461 Yifeng Zhou 462 ByteDance 464 Building 1, AVIC Plaza, 43 N 3rd Ring W Rd Haidian District 466 Beijing 100000 P.R. China 468 Email: yifeng.zhou@bytedance.com 470 Gyan Mishra 472 Verizon 474 Silver Spring, Maryland, USA 476 Email: hayabusagsm@gmail.com 478 14. References 480 14.1. Normative References 482 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 483 Requirement Levels", BCP 14, RFC 2119, 484 DOI 10.17487/RFC2119, March 1997, 485 . 487 [RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet 488 Control Message Protocol (ICMPv6) for the Internet 489 Protocol Version 6 (IPv6) Specification", STD 89, 490 RFC 4443, DOI 10.17487/RFC4443, March 2006, 491 . 493 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 494 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 495 May 2017, . 497 [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 498 (IPv6) Specification", STD 86, RFC 8200, 499 DOI 10.17487/RFC8200, July 2017, 500 . 502 [RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed., 503 "Path MTU Discovery for IP version 6", STD 87, RFC 8201, 504 DOI 10.17487/RFC8201, July 2017, 505 . 507 14.2. Informative References 509 [IANA-RH] IANA, "Routing Headers", 510 . 513 [ISO10589-Second-Edition] 514 International Organization for Standardization, 515 ""Intermediate system to Intermediate system intra-domain 516 routeing information exchange protocol for use in 517 conjunction with the protocol for providing the 518 connectionless-mode Network Service (ISO 8473)", ISO/IEC 519 10589:2002, Second Edition,", November 2001. 521 [RFC2151] Kessler, G. and S. Shepard, "A Primer On Internet and TCP/ 522 IP Tools and Utilities", FYI 30, RFC 2151, 523 DOI 10.17487/RFC2151, June 1997, 524 . 526 [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A 527 Border Gateway Protocol 4 (BGP-4)", RFC 4271, 528 DOI 10.17487/RFC4271, January 2006, 529 . 531 [RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF 532 for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008, 533 . 535 [RFC5440] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation 536 Element (PCE) Communication Protocol (PCEP)", RFC 5440, 537 DOI 10.17487/RFC5440, March 2009, 538 . 540 [RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed., 541 and A. Bierman, Ed., "Network Configuration Protocol 542 (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011, 543 . 545 [RFC6890] Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman, 546 "Special-Purpose IP Address Registries", BCP 153, 547 RFC 6890, DOI 10.17487/RFC6890, April 2013, 548 . 550 [RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer, 551 D., Matsushima, S., and Z. Li, "Segment Routing over IPv6 552 (SRv6) Network Programming", RFC 8986, 553 DOI 10.17487/RFC8986, February 2021, 554 . 556 Appendix A. CRH Processing Examples 558 This appendix demonstrates CRH processing in the following scenarios: 560 o The SID list contains one entry for each segment in the path 561 (Appendix A.1). 563 o The SID list omits the first entry in the path (Appendix A.2). 565 ----------- ----------- ----------- 566 |Node: S | |Node: I1 | |Node: I2 | 567 |Loopback: |---------------|Loopback: |---------------|Loopback: | 568 |2001:db8::a| |2001:db8::1| |2001:db8::2| 569 ----------- ----------- ----------- 570 | | 571 | ----------- | 572 | |Node: D | | 573 ---------------------|Loopback: |--------------------- 574 |2001:db8::b| 575 ----------- 577 Figure 3: Reference Topology 579 Figure 3 provides a reference topology that is used in all examples. 581 +-----+--------------+-------------------+ 582 | SID | IPv6 Address | Forwarding Method | 583 +-----+--------------+-------------------+ 584 | 2 | 2001:db8::2 | Least-cost path | 585 | 11 | 2001:db8::b | Least-cost path | 586 +-----+--------------+-------------------+ 588 Table 1: Node SIDs 590 Table 1 describes two entries that appear in each node's CRH-FIB. 592 A.1. The SID List Contains One Entry For Each Segment In The Path 594 In this example, Node S sends a packet to Node D, via I2. In this 595 example, I2 appears in the CRH segment list. 597 +-------------------------------------+-------------------+ 598 | As the packet travels from S to I2: | | 599 +-------------------------------------+-------------------+ 600 | Source Address = 2001:db8::a | Segments Left = 1 | 601 | Destination Address = 2001:db8::2 | SID[0] = 11 | 602 | | SID[1] = 2 | 603 +-------------------------------------+-------------------+ 604 +-------------------------------------+-------------------+ 605 | As the packet travels from I2 to D: | | 606 +-------------------------------------+-------------------+ 607 | Source Address = 2001:db8::a | Segments Left = 0 | 608 | Destination Address = 2001:db8::b | SID[0] = 11 | 609 | | SID[1] = 2 | 610 +-------------------------------------+-------------------+ 612 A.2. The SID List Omits The First Entry In The Path 614 In this example, Node S sends a packet to Node D, via I2. In this 615 example, I2 does not appear in the CRH segment list. 617 +-------------------------------------+-------------------+ 618 | As the packet travels from S to I2: | | 619 +-------------------------------------+-------------------+ 620 | Source Address = 2001:db8::a | Segments Left = 1 | 621 | Destination Address = 2001:db8::2 | SID[0] = 11 | 622 +-------------------------------------+-------------------+ 624 +-------------------------------------+-------------------+ 625 | As the packet travels from I2 to D: | | 626 +-------------------------------------+-------------------+ 627 | Source Address = 2001:db8::a | Segments Left = 0 | 628 | Destination Address = 2001:db8::b | SID[0] = 11 | 629 +-------------------------------------+-------------------+ 631 Appendix B. A Packet Recycling Use-Case 633 Network A--D1 634 / \ / 635 / \ / 636 / \ / 637 S ---- P + 638 \ / \ 639 \ / \ 640 \ / \ 641 Network B---DN 643 Figure 4: Packet Recycling Use-case 645 In Figure 4: 647 o The SR domain contains Node S, Node P, and a set of destination 648 nodes (D1 through DN) 650 o S is connected to P 651 o P is connected to Network A and to Network B. Neither of these 652 networks are SR-capable. 654 o The destination nodes connect to both Network A and Network B 656 S needs to reach each destination node through two SR paths. One SR 657 path traverses Network A while the other traverses Network B. 659 Uncompressed SRv6 can encode this SR Path in two segments,with one 660 segment instantiated on P and the other on the destination. To 661 support this strategy, P instantiates two END.X SIDs (one per 662 network). 664 CRH compressed SRv6 can encode this SR Path in two or three segments. 665 When it encodes the path in two segments, one segment instantiated on 666 P and the other on the destination. To support this strategy, P 667 instantiates 2*N SIDs (one per network per destination). When CRH 668 compressed SRv6 encodes the path in three segments, two segments are 669 instantiated on P and the other on the destination. The first 670 segment on P updates the IPv6 Destination address without forwarding 671 the packet, while the other segment on P forwards the packet without 672 updating the IPv6 destination address. To support this strategy, P 673 instantiates 2+N SIDs (one per network and one per destination). 675 Authors' Addresses 677 Ron Bonica 678 Juniper Networks 679 2251 Corporate Park Drive 680 Herndon, Virginia 20171 681 USA 683 Email: rbonica@juniper.net 685 Yuji Kamite 686 NTT Communications Corporation 687 3-4-1 Shibaura, Minato-ku 688 Tokyo 108-8118 689 Japan 691 Email: y.kamite@ntt.com 692 Andrew Alston 693 Liquid Telecom 694 Nairobi 695 Kenya 697 Email: Andrew.Alston@liquidtelecom.com 699 Daniam Henriques 700 Liquid Telecom 701 Johannesburg 702 South Africa 704 Email: daniam.henriques@liquidtelecom.com 706 Luay Jalil 707 Verizon 708 Richardson, Texas 709 USA 711 Email: luay.jalil@one.verizon.com