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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 TRILL Working Group Radia Perlman 2 INTERNET-DRAFT EMC 3 Intended status: Informational Donald Eastlake 4 Mingui Zhang 5 Huawei 6 Anoop Ghanwani 7 Dell 8 Hongjun Zhai 9 JIT 10 Expires: February 18, 2016 August 19, 2015 12 Alternatives for Multilevel TRILL 13 (Transparent Interconnection of Lots of Links) 14 16 Abstract 18 Extending TRILL to multiple levels has challenges that are not 19 addressed by the already-existing capability of IS-IS to have 20 multiple levels. One issue is with the handling of multi-destination 21 packet distribution trees. Another issue is with TRILL switch 22 nicknames. There have been two proposed approaches. One approach, 23 which we refer to as the "unique nickname" approach, gives unique 24 nicknames to all the TRILL switches in the multilevel campus, either 25 by having the level-1/level-2 border TRILL switches advertise which 26 nicknames are not available for assignment in the area, or by 27 partitioning the 16-bit nickname into an "area" field and a "nickname 28 inside the area" field. The other approach, which we refer to as the 29 "aggregated nickname" approach, involves hiding the nicknames within 30 areas, allowing nicknames to be reused in different areas, by having 31 the border TRILL switches rewrite the nickname fields when entering 32 or leaving an area. Each of those approaches has advantages and 33 disadvantages. This informational document suggests allowing a choice 34 of approach in each area. This allows the simplicity of the unique 35 nickname approach in installations in which there is no danger of 36 running out of nicknames and allows the complexity of hiding the 37 nicknames in an area to be phased into larger installations on a per- 38 area basis. 40 Status of This Memo 42 This Internet-Draft is submitted to IETF in full conformance with the 43 provisions of BCP 78 and BCP 79. Distribution of this document is 44 unlimited. Comments should be sent to the TRILL working group 45 mailing list . 47 Internet-Drafts are working documents of the Internet Engineering 48 Task Force (IETF), its areas, and its working groups. Note that 49 other groups may also distribute working documents as Internet- 50 Drafts. 52 Internet-Drafts are draft documents valid for a maximum of six months 53 and may be updated, replaced, or obsoleted by other documents at any 54 time. It is inappropriate to use Internet-Drafts as reference 55 material or to cite them other than as "work in progress." 57 The list of current Internet-Drafts can be accessed at 58 http://www.ietf.org/1id-abstracts.html. The list of Internet-Draft 59 Shadow Directories can be accessed at 60 http://www.ietf.org/shadow.html. 62 Table of Contents 64 1. Introduction............................................4 65 1.1 TRILL Scalability Issues...............................4 66 1.2 Improvements Due to Multilevel.........................5 67 1.3 Unique and Aggregated Nicknames........................6 68 1.3 More on Areas..........................................6 69 1.4 Terminology and Acronyms...............................7 71 2. Multilevel TRILL Issues.................................8 72 2.1 Non-zero Area Addresses................................9 73 2.2 Aggregated versus Unique Nicknames.....................9 74 2.2.1 More Details on Unique Nicknames....................10 75 2.2.2 More Details on Aggregated Nicknames................11 76 2.2.2.1 Border Learning Aggregated Nicknames..............12 77 2.2.2.2 Swap Nickname Field Aggregated Nicknames..........14 78 2.2.2.3 Comparison........................................14 79 2.3 Building Multi-Area Trees.............................15 80 2.4 The RPF Check for Trees...............................15 81 2.5 Area Nickname Acquisition.............................16 82 2.6 Link State Representation of Areas....................16 84 3. Area Partition.........................................18 86 4. Multi-Destination Scope................................19 87 4.1 Unicast to Multi-destination Conversions..............19 88 4.1.1 New Tree Encoding...................................20 89 4.2 Selective Broadcast Domain Reduction..................20 91 5. Co-Existence with Old TRILL switches...................22 92 6. Multi-Access Links with End Stations...................23 93 7. Summary................................................24 95 8. Security Considerations................................25 96 9. IANA Considerations....................................25 98 Normative References......................................26 99 Informative References....................................26 101 Acknowledgements..........................................28 102 Authors' Addresses........................................29 104 1. Introduction 106 The IETF TRILL (Transparent Interconnection of Lot of Links or 107 Tunneled Routing in the Link Layer) protocol [RFC6325] [RFC7177] 108 provides optimal pair-wise data routing without configuration, safe 109 forwarding even during periods of temporary loops, and support for 110 multipathing of both unicast and multicast traffic in networks with 111 arbitrary topology and link technology, including multi-access links. 112 TRILL accomplishes this by using IS-IS (Intermediate System to 113 Intermediate System [IS-IS] [RFC7176]) link state routing in 114 conjunction with a header that includes a hop count. The design 115 supports data labels (VLANs and Fine Grained Labels [RFC7172]) and 116 optimization of the distribution of multi-destination data based on 117 VLANs and multicast groups. Devices that implement TRILL are called 118 TRILL Switches or RBridges. 120 Familiarity with [IS-IS], [RFC6325], and [rfc7180bis] is assumed in 121 this document. 123 1.1 TRILL Scalability Issues 125 There are multiple issues that might limit the scalability of a 126 TRILL-based network: 128 1. the routing computation load, 129 2. the volatility of the link state database (LSDB) creating too much 130 control traffic, 131 3. the volatility of the LSDB causing the TRILL network to be in an 132 unconverged state too much of the time, 133 4. the size of the LSDB, 134 5. the limit of the number of TRILL switches, due to the 16-bit 135 nickname space, 136 6. the traffic due to upper layer protocols use of broadcast and 137 multicast, and 138 7. the size of the end node learning table (the table that remembers 139 (egress TRILL switch, label/MAC) pairs). 141 Extending TRILL IS-IS to be multilevel (hierarchical) helps with all 142 but the last of these issues. 144 IS-IS was designed to be multilevel [IS-IS]. A network can be 145 partitioned into "areas". Routing within an area is known as "Level 146 1 routing". Routing between areas is known as "Level 2 routing". 147 The Level 2 IS-IS network consists of Level 2 routers and links 148 between the Level 2 routers. Level 2 routers may participate in one 149 or more Level 1 areas, in addition to their role as Level 2 routers. 151 Each area is connected to Level 2 through one or more "border 152 routers", which participate both as a router inside the area, and as 153 a router inside the Level 2 "area". Care must be taken that it is 154 clear, when transitioning multi-destination packets between Level 2 155 and a Level 1 area in either direction, that exactly one border TRILL 156 switch will transition a particular data packet between the levels or 157 else duplication or loss of traffic can occur. 159 1.2 Improvements Due to Multilevel 161 Partitioning the network into areas solves the first four scalability 162 issues described above, namely, 164 1. the routing computation load, 166 2. the volatility of the LSDB creating too much control traffic, 168 3. the volatility of the LSDB causing the TRILL network to be in an 169 unconverged state too much of the time, 171 4. the size of the LSDB. 173 Problem #6 in Section 1.1, namely, the traffic due to upper layer 174 protocols use of broadcast and multicast, can be addressed by 175 introducing a locally-scoped multi-destination delivery, limited to 176 an area or a single link. See further discussion in Section 4.2. 178 Problem #5 in Section 1.1, namely, the limit of the number of TRILL 179 switches, due to the 16-bit nickname space, will only be addressed 180 with the aggregated nickname approach. Since the aggregated nickname 181 approach requires some complexity in the border TRILL switches (for 182 rewriting the nicknames in the TRILL header), the design in this 183 document allows a campus with a mixture of unique-nickname areas, and 184 aggregated-nickname areas. Nicknames must be unique across all Level 185 2 and unique-nickname area TRILL switches, whereas nicknames inside 186 an aggregated-nickname area are visible only inside the area. 187 Nicknames inside an aggregated-nickname area must not conflict with 188 nicknames visible in Level 2 (which includes all nicknames inside 189 unique nickname areas), but the nicknames inside an aggregated- 190 nickname area may be the same as nicknames used within other 191 aggregated-nickname areas. 193 TRILL switches within an area need not be aware of whether they are 194 in an aggregated nickname area or a unique nickname area. The border 195 TRILL switches in area A1 will claim, in their LSP inside area A1, 196 which nicknames (or nickname ranges) are not available for choosing 197 as nicknames by area A1 TRILL switches. 199 1.3 Unique and Aggregated Nicknames 201 We describe two alternatives for hierarchical or multilevel TRILL. 202 One we call the "unique nickname" alternative. The other we call the 203 "aggregated nickname" alternative. In the aggregated nickname 204 alternative, border TRILL switches replace either the ingress or 205 egress nickname field in the TRILL header of unicast packets with an 206 aggregated nickname representing an entire area. 208 The unique nickname alternative has the advantage that border TRILL 209 switches are simpler and do not need to do TRILL Header nickname 210 modification. It also simplifies testing and maintenance operations 211 that originate in one area and terminate in a different area. 213 The aggregated nickname alternative has the following advantages: 215 o it solves problem #5 above, the 16-bit nickname limit, in a 216 simple way, 217 o it lessens the amount of inter-area routing information that 218 must be passed in IS-IS, and 219 o it logically reduces the RPF (Reverse Path Forwarding) Check 220 information (since only the area nickname needs to appear, 221 rather than all the ingress TRILL switches in that area). 223 In both cases, it is possible and advantageous to compute multi- 224 destination data packet distribution trees such that the portion 225 computed within a given area is rooted within that area. 227 1.3 More on Areas 229 Each area is configured with an "area address", which is advertised 230 in IS-IS messages, so as to avoid accidentally interconnecting areas. 231 Although the area address had other purposes in CLNP (Connectionless 232 Network Layer Protocol, IS-IS was originally designed for 233 CLNP/DECnet), for TRILL the only purpose of the area address would be 234 to avoid accidentally interconnecting areas. 236 Currently, the TRILL specification says that the area address must be 237 zero. If we change the specification so that the area address value 238 of zero is just a default, then most of IS-IS multilevel machinery 239 works as originally designed. However, there are TRILL-specific 240 issues, which we address below in this document. 242 1.4 Terminology and Acronyms 244 This document generally uses the acronyms defined in [RFC6325] plus 245 the additional acronym DBRB. However, for ease of reference, most 246 acronyms used are listed here: 248 CLNP - ConnectionLess Network Protocol 250 DECnet - a proprietary routing protocol that was used by Digital 251 Equipment Corporation. "DECnet Phase 5" was the origin of IS-IS. 253 Data Label - VLAN or Fine Grained Label [RFC7172] 255 DBRB - Designated Border RBridge 257 ESADI - End Station Address Distribution Information 259 IS-IS - Intermediate System to Intermediate System [IS-IS] 261 LSDB - Link State Data Base 263 LSP - Link State PDU 265 PDU - Protocol Data Unit 267 RBridge - Routing Bridge, an alterntive name for a TRILL switch 269 RPF - Reverse Path Forwarding 271 TLV - Type Length Value 273 TRILL - Transparent Interconnection of Lots of Links or Tunneled 274 Routing in the Link Layer [RFC6325] 276 TRILL switch - a device that implements the TRILL protcol 277 [RFC6325], sometimes called an RBridge 279 VLAN - Virtual Local Area Network 281 2. Multilevel TRILL Issues 283 The TRILL-specific issues introduced by multilevel include the 284 following: 286 a. Configuration of non-zero area addresses, encoding them in IS-IS 287 PDUs, and possibly interworking with old TRILL switches that do 288 not understand nonzero area addresses. 290 See Section 2.1. 292 b. Nickname management. 294 See Sections 2.5 and 2.2. 296 c. Advertisement of pruning information (Data Label reachability, IP 297 multicast addresses) across areas. 299 Distribution tree pruning information is only an optimization, 300 as long as multi-destination packets are not prematurely 301 pruned. For instance, border TRILL switches could advertise 302 they can reach all possible Data Labels, and have an IP 303 multicast router attached. This would cause all multi- 304 destination traffic to be transmitted to border TRILL switches, 305 and possibly pruned there, when the traffic could have been 306 pruned earlier based on Data Label or multicast group if border 307 TRILL switches advertised more detailed Data Label and/or 308 multicast listener and multicast router attachment information. 310 d. Computation of distribution trees across areas for multi- 311 destination data. 313 See Section 2.3. 315 e. Computation of RPF information for those distribution trees. 317 See Section 2.4. 319 f. Computation of pruning information across areas. 321 See Sections 2.3 and 2.6. 323 g. Compatibility, as much as practical, with existing, unmodified 324 TRILL switches. 326 The most important form of compatibility is with existing TRILL 327 fast path hardware. Changes that require upgrade to the slow 328 path firmware/software are more tolerable. Compatibility for 329 the relatively small number of border TRILL switches is less 330 important than compatibility for non-border TRILL switches. 332 See Section 5. 334 2.1 Non-zero Area Addresses 336 The current TRILL base protocol specification [RFC6325] [RFC7177] 337 [rfc7180bis] says that the area address in IS-IS must be zero. The 338 purpose of the area address is to ensure that different areas are not 339 accidentally merged. Furthermore, zero is an invalid area address 340 for layer 3 IS-IS, so it was chosen as an additional safety mechanism 341 to ensure that layer 3 IS-IS would not be confused with TRILL IS-IS. 342 However, TRILL uses other techniques to avoid such confusion, such as 343 different multicast addresses and Ethertypes on Ethernet [RFC6325], 344 different PPP (Point-to-Point Protocol) codepoints on PPP [RFC6361], 345 and the like. Thus, using an area address in TRILL that might be used 346 in layer 3 IS-IS is not a problem. 348 Since current TRILL switches will reject any IS-IS messages with 349 nonzero area addresses, the choices are as follows: 351 a.1 upgrade all TRILL switches that are to interoperate in a 352 potentially multilevel environment to understand non-zero area 353 addresses, 354 a.2 neighbors of old TRILL switches must remove the area address from 355 IS-IS messages when talking to an old TRILL switch (which might 356 break IS-IS security and/or cause inadvertent merging of areas), 357 a.3 ignore the problem of accidentally merging areas entirely, or 358 a.4 keep the fixed "area address" field as 0 in TRILL, and add a new, 359 optional TLV for "area name" to Hellos that, if present, could be 360 compared, by new TRILL switches, to prevent accidental area 361 merging. 363 In principal, different solutions could be used in different areas 364 but it would be much simpler to adopt one of these choices uniformly. 366 2.2 Aggregated versus Unique Nicknames 368 In the unique nickname alternative, all nicknames across the campus 369 must be unique. In the aggregated nickname alternative, TRILL switch 370 nicknames within an aggregated area are only of local significance, 371 and the only nickname externally (outside that area) visible is the 372 "area nickname" (or nicknames), which aggregates all the internal 373 nicknames. 375 The unique nickname approach simplifies border TRILL switches. 377 The aggregated nickname approach eliminates the potential problem of 378 nickname exhaustion, minimizes the amount of nickname information 379 that would need to be forwarded between areas, minimizes the size of 380 the forwarding table, and simplifies RPF calculation and RPF 381 information. 383 2.2.1 More Details on Unique Nicknames 385 With unique cross-area nicknames, it would be intractable to have a 386 flat nickname space with TRILL switches in different areas contending 387 for the same nicknames. Instead, each area would need to be 388 configured with a block of nicknames. Either some TRILL switches 389 would need to announce that all the nicknames other than that block 390 are taken (to prevent the TRILL switches inside the area from 391 choosing nicknames outside the area's nickname block), or a new TLV 392 would be needed to announce the allowable nicknames, and all TRILL 393 switches in the area would need to understand that new TLV. An 394 example of the second approach is given in [NickFlags]. 396 Currently the encoding of nickname information in TLVs is by listing 397 of individual nicknames; this would make it painful for a border 398 TRILL switch to announce into an area that it is holding all other 399 nicknames to limit the nicknames available within that area. The 400 information could be encoded as ranges of nicknames to make this 401 somewhat manageable [NickFlags]; however, a new TLV for announcing 402 nickname ranges would not be intelligible to old TRILL switches. 404 There is also an issue with the unique nicknames approach in building 405 distribution trees, as follows: 407 With unique nicknames in the TRILL campus and TRILL header 408 nicknames not rewritten by the border TRILL switches, there would 409 have to be globally known nicknames for the trees. Suppose there 410 are k trees. For all of the trees with nicknames located outside 411 an area, the local trees would be rooted at a border TRILL switch 412 or switches. Therefore, there would be either no splitting of 413 multi-destination traffic with the area or restricted splitting of 414 multi-destination traffic between trees rooted at a highly 415 restricted set of TRILL switches. 417 As an alternative, just the "egress nickname" field of multi- 418 destination TRILL Data packets could be mapped at the border, 419 leaving known unicast packets un-mapped. However, this surrenders 420 much of the unique nickname advantage of simpler border TRILL 421 switches. 423 Scaling to a very large campus with unique nicknames might exhaust 424 the 16-bit TRILL nicknames space. One method might be to expand 425 nicknames to 24 bits; however, that technique would require TRILL 426 message format changes and that all TRILL switches in the campus 427 understand larger nicknames. 429 For an example of a more specific multilevel proposal using unique 430 nicknames, see [DraftUnique]. 432 2.2.2 More Details on Aggregated Nicknames 434 The aggregated nickname approach enables passing far less nickname 435 information. It works as follows, assuming both the source and 436 destination areas are using aggregated nicknames: 438 There are two ways areas could be identified. 440 One method would be to assign each area a 16-bit nickname. This 441 would not be the nickname of any actual TRILL switch. Instead, it 442 would be the nickname of the area itself. Border TRILL switches 443 would know the area nickname for their own area(s). 445 Alternatively, areas could be identified by the set of nicknames 446 that identify the border routers for that area. (See [SingleName] 447 for a multilevel proposal using such a set of nicknames.) 449 The TRILL Header nickname fields in TRILL Data packets being 450 transported through a multilevel TRILL campus with aggregated 451 nicknames are as follows: 453 - When both the ingress and egress TRILL switches are in the same 454 area, there need be no change from the existing base TRILL 455 protocol standard in the TRILL Header nickname fields. 457 - When being transported in Level 2, the ingress nickname is the 458 nickname of the ingress TRILL switch's area while the egress 459 nickname is either the nickname of the egress TRILL switch's 460 area or a tree nickname. 462 - When being transported from Level 1 to Level 2, the ingress 463 nickname is the nickname of the ingress TRILL switch itself 464 while the egress nickname is either a nickname for the area of 465 the egress TRILL switch or a tree nickname. 467 - When being transported from Level 2 to Level 1, the ingress 468 nickname is a nickname for the ingress TRILL switch's area while 469 the egress nickname is either the nickname of the egress TRILL 470 switch itself or a tree nickname. 472 There are two variations of the aggregated nickname approach. The 473 first is the Border Learning approach, which is described in Section 474 2.2.2.1. The second is the Swap Nickname Field approach, which is 475 described in Section 2.2.2.2. Section 2.2.2.3 compares the advantages 476 and disadvantages of these two variations of the aggregated nickname 477 approach. 479 2.2.2.1 Border Learning Aggregated Nicknames 481 This section provides an illustrative example and description of the 482 border learning variation of aggregated nicknames where a single 483 nickname is used to identify an area. 485 In the following picture, RB2 and RB3 are area border TRILL switches 486 (RBridges). A source S is attached to RB1. The two areas have 487 nicknames 15961 and 15918, respectively. RB1 has a nickname, say 27, 488 and RB4 has a nickname, say 44 (and in fact, they could even have the 489 same nickname, since the TRILL switch nickname will not be visible 490 outside these aggregated areas). 492 Area 15961 level 2 Area 15918 493 +-------------------+ +-----------------+ +--------------+ 494 | | | | | | 495 | S--RB1---Rx--Rz----RB2---Rb---Rc--Rd---Re--RB3---Rk--RB4---D | 496 | 27 | | | | 44 | 497 | | | | | | 498 +-------------------+ +-----------------+ +--------------+ 500 Let's say that S transmits a frame to destination D, which is 501 connected to RB4, and let's say that D's location has already been 502 learned by the relevant TRILL switches. These relevant switches have 503 learned the following: 505 1) RB1 has learned that D is connected to nickname 15918 506 2) RB3 has learned that D is attached to nickname 44. 508 The following sequence of events will occur: 510 - S transmits an Ethernet frame with source MAC = S and destination 511 MAC = D. 513 - RB1 encapsulates with a TRILL header with ingress RBridge = 27, 514 and egress = 15918 producing a TRILL Data packet. 516 - RB2 has announced in the Level 1 IS-IS instance in area 15961, 517 that it is attached to all the area nicknames, including 15918. 518 Therefore, IS-IS routes the packet to RB2. Alternatively, if a 519 distinguished range of nicknames is used for Level 2, Level 1 520 TRILL switches seeing such an egress nickname will know to route 521 to the nearest border router, which can be indicated by the IS-IS 522 attached bit. 524 - RB2, when transitioning the packet from Level 1 to Level 2, 525 replaces the ingress TRILL switch nickname with the area nickname, 526 so replaces 27 with 15961. Within Level 2, the ingress RBridge 527 field in the TRILL header will therefore be 15961, and the egress 528 RBridge field will be 15918. Also RB2 learns that S is attached to 529 nickname 27 in area 15961 to accommodate return traffic. 531 - The packet is forwarded through Level 2, to RB3, which has 532 advertised, in Level 2, reachability to the nickname 15918. 534 - RB3, when forwarding into area 15918, replaces the egress nickname 535 in the TRILL header with RB4's nickname (44). So, within the 536 destination area, the ingress nickname will be 15961 and the 537 egress nickname will be 44. 539 - RB4, when decapsulating, learns that S is attached to nickname 540 15961, which is the area nickname of the ingress. 542 Now suppose that D's location has not been learned by RB1 and/or RB3. 543 What will happen, as it would in TRILL today, is that RB1 will 544 forward the packet as multi-destination, choosing a tree. As the 545 multi-destination packet transitions into Level 2, RB2 replaces the 546 ingress nickname with the area nickname. If RB1 does not know the 547 location of D, the packet must be flooded, subject to possible 548 pruning, in Level 2 and, subject to possible pruning, from Level 2 549 into every Level 1 area that it reaches on the Level 2 distribution 550 tree. 552 Now suppose that RB1 has learned the location of D (attached to 553 nickname 15918), but RB3 does not know where D is. In that case, RB3 554 must turn the packet into a multi-destination packet within area 555 15918. In this case, care must be taken so that, in case RB3 is not 556 the Designated transitioner between Level 2 and its area for that 557 multi-destination packet, but was on the unicast path, that another 558 border TRILL switch in that area not forward the now multi- 559 destination packet back into Level 2. Therefore, it would be 560 desirable to have a marking, somehow, that indicates the scope of 561 this packet's distribution to be "only this area" (see also Section 562 4). 564 In cases where there are multiple transitioners for unicast packets, 565 the border learning mode of operation requires that the address 566 learning between them be shared by some protocol such as running 567 ESADI [RFC7357] for all Data Labels of interest to avoid excessive 568 unknown unicast flooding. 570 The potential issue described at the end of Section 2.2.1 with trees 571 in the unique nickname alternative is eliminated with aggregated 572 nicknames. With aggregated nicknames, each border TRILL switch that 573 will transition multi-destination packets can have a mapping between 574 Level 2 tree nicknames and Level 1 tree nicknames. There need not 575 even be agreement about the total number of trees; just that the 576 border TRILL switch have some mapping, and replace the egress TRILL 577 switch nickname (the tree name) when transitioning levels. 579 2.2.2.2 Swap Nickname Field Aggregated Nicknames 581 As a variant, two additional fields could exist in TRILL Data packets 582 we call the "ingress swap nickname field" and the "egress swap 583 nickname field". The changes in the example above would be as 584 follows: 586 - RB1 will have learned the area nickname of D and the TRILL switch 587 nickname of RB4 to which D is attached. In encapsulating a frame 588 to D, it puts an area nickname of D (15918) in the egress nickname 589 field of the TRILL Header and puts a nickname of RB3 (44) in a 590 egress swap nickname field. 592 - RB2 moves the ingress nickname to the ingress swap nickname field 593 and inserts 15961, an area nickname for S, into the ingress 594 nickname field. 596 - RB3 swaps the egress nickname and the egress swap nickname fields, 597 which sets the egress nickname to 44. 599 - RB4 learns the correspondence between the source MAC/VLAN of S and 600 the { ingress nickname, ingress swap nickname field } pair as it 601 decapsulates and egresses the frame. 603 See [DraftAggregated] for a multilevel proposal using aggregated swap 604 nicknames with a single nickname representing an area. 606 2.2.2.3 Comparison 608 The Border Learning variant described in Section 2.2.2.1 above 609 minimizes the change in non-border TRILL switches but imposes the 610 burden on border TRILL switches of learning and doing lookups in all 611 the end station MAC addresses within their area(s) that are used for 612 communication outside the area. This burden could be reduced by 613 decreasing the area size and increasing the number of areas. 615 The Swap Nickname Field variant described in Section 2.2.2.2 616 eliminates the extra address learning burden on border TRILL switches 617 but requires more extensive changes to non-border TRILL switches. In 618 particular they must learn to associate both a TRILL switch nickname 619 and an area nickname with end station MAC/label pairs (except for 620 addresses that are local to their area). 622 The Swap Nickname Field alternative is more scalable but less 623 backward compatible for non-border TRILL switches. It would be 624 possible for border and other level 2 TRILL switches to support both 625 Border Learning, for support of legacy Level 1 TRILL switches, and 626 Swap Nickname, to support Level 1 TRILL switches that understood the 627 Swap Nickname method. 629 2.3 Building Multi-Area Trees 631 It is easy to build a multi-area tree by building a tree in each area 632 separately, (including the Level 2 "area"), and then having only a 633 single border TRILL switch, say RBx, in each area, attach to the 634 Level 2 area. RBx would forward all multi-destination packets 635 between that area and Level 2. 637 People might find this unacceptable, however, because of the desire 638 to path split (not always sending all multi-destination traffic 639 through the same border TRILL switch). 641 This is the same issue as with multiple ingress TRILL switches 642 injecting traffic from a pseudonode, and can be solved with the 643 mechanism that was adopted for that purpose: the affinity TLV 644 [DraftCMT]. For each tree in the area, at most one border RB 645 announces itself in an affinity TLV with that tree name. 647 2.4 The RPF Check for Trees 649 For multi-destination data originating locally in RBx's area, 650 computation of the RPF check is done as today. For multi-destination 651 packets originating outside RBx's area, computation of the RPF check 652 must be done based on which one of the border TRILL switches (say 653 RB1, RB2, or RB3) injected the packet into the area. 655 A TRILL switch, say RB4, located inside an area, must be able to know 656 which of RB1, RB2, or RB3 transitioned the packet into the area from 657 Level 2. (or into Level 2 from an area). 659 This could be done based on having the DBRB announce the transitioner 660 assignments to all the TRILL switches in the area, or the Affinity 661 TLV mechanism given in [DraftCMT], or the New Tree Encoding mechanism 662 discussed in Section 4.1.1. 664 2.5 Area Nickname Acquisition 666 In the aggregated nickname alternative, each area must acquire a 667 unique area nickname. It is probably simpler to allocate a block of 668 nicknames (say, the top 4000) to be area addresses, and not used by 669 any TRILL switches. 671 The nicknames used for area identification need to be advertised and 672 acquired through Level 2. 674 Within an area, all the border TRILL switches can discover each other 675 through the Level 1 link state database, by using the IS-IS attach 676 bit or by explicitly advertising in their LSP "I am a border 677 RBridge". 679 Of the border TRILL switches, one will have highest priority (say 680 RB7). RB7 can dynamically participate, in Level 2, to acquire a 681 nickname for identifying the area. Alternatively, RB7 could give the 682 area a pseudonode IS-IS ID, such as RB7.5, within Level 2. So an 683 area would appear, in Level 2, as a pseudonode and the pseudonode 684 could participate, in Level 2, to acquire a nickname for the area. 686 Within Level 2, all the border TRILL switches for an area can 687 advertise reachability to the area, which would mean connectivity to 688 a nickname identifying the area. 690 2.6 Link State Representation of Areas 692 Within an area, say area A1, there is an election for the DBRB, 693 (Designated Border RBridge), say RB1. This can be done through LSPs 694 within area A1. The border TRILL switches announce themselves, 695 together with their DBRB priority. (Note that the election of the 696 DBRB cannot be done based on Hello messages, because the border TRILL 697 switches are not necessarily physical neighbors of each other. They 698 can, however, reach each other through connectivity within the area, 699 which is why it will work to find each other through Level 1 LSPs.) 701 RB1 acquires an area nickname (in the aggregated nickname approach) 702 and may give the area a pseudonode IS-IS ID (just like the DRB would 703 give a pseudonode IS-IS ID to a link) depending on how the area 704 nickname is handled. RB1 advertises, in area A1, an area nickname 705 that RB1 has acquired (and what the pseudonode IS-IS ID for the area 706 is if needed). 708 Level 1 LSPs (possibly pseudonode) initiated by RB1 for the area 709 include any information external to area A1 that should be input into 710 area A1 (such as nicknames of external areas, or perhaps (in the 711 unique nickname variant) all the nicknames of external TRILL switches 712 in the TRILL campus and pruning information such as multicast 713 listeners and labels). All the other border TRILL switches for the 714 area announce (in their LSP) attachment to that area. 716 Within Level 2, RB1 generates a Level 2 LSP on behalf of the area. 717 The same pseudonode ID could be used within Level 1 and Level 2, for 718 the area. (There does not seem any reason why it would be useful for 719 it to be different, but there's also no reason why it would need to 720 be the same). Likewise, all the area A1 border TRILL switches would 721 announce, in their Level 2 LSPs, connection to the area. 723 3. Area Partition 725 It is possible for an area to become partitioned, so that there is 726 still a path from one section of the area to the other, but that path 727 is via the Level 2 area. 729 With multilevel TRILL, an area will naturally break into two areas in 730 this case. 732 Area addresses might be configured to ensure two areas are not 733 inadvertently connected. Area addresses appear in Hellos and LSPs 734 within the area. If two chunks, connected only via Level 2, were 735 configured with the same area address, this would not cause any 736 problems. (They would just operate as separate Level 1 areas.) 738 A more serious problem occurs if the Level 2 area is partitioned in 739 such a way that it could be healed by using a path through a Level 1 740 area. TRILL will not attempt to solve this problem. Within the Level 741 1 area, a single border RBridge will be the DBRB, and will be in 742 charge of deciding which (single) RBridge will transition any 743 particular multi-destination packets between that area and Level 2. 744 If the Level 2 area is partitioned, this will result in multi- 745 destination data only reaching the portion of the TRILL campus 746 reachable through the partition attached to the TRILL switch that 747 transitions that packet. It will not cause a loop. 749 4. Multi-Destination Scope 751 There are at least two reasons it would be desirable to be able to 752 mark a multi-destination packet with a scope that indicates the 753 packet should not exit the area, as follows: 755 1. To address an issue in the border learning variant of the 756 aggregated nickname alternative, when a unicast packet turns into 757 a multi-destination packet when transitioning from Level 2 to 758 Level 1, as discussed in Section 4.1. 760 2. To constrain the broadcast domain for certain discovery, 761 directory, or service protocols as discussed in Section 4.2. 763 Multi-destination packet distribution scope restriction could be done 764 in a number of ways. For example, there could be a flag in the packet 765 that means "for this area only". However, the technique that might 766 require the least change to TRILL switch fast path logic would be to 767 indicate this in the egress nickname that designates the distribution 768 tree being used. There could be two general tree nicknames for each 769 tree, one being for distribution restricted to the area and the other 770 being for multi-area trees. Or there would be a set of N (perhaps 16) 771 special currently reserved nicknames used to specify the N highest 772 priority trees but with the variation that if the special nickname is 773 used for the tree, the packet is not transitioned between areas. Or 774 one or more special trees could be built that were restricted to the 775 local area. 777 4.1 Unicast to Multi-destination Conversions 779 In the border learning variant of the aggregated nickname 780 alternative, a unicast packet might be known at the Level 1 to Level 781 2 transition, be forwarded as a unicast packet to the least cost 782 border TRILL switch advertising connectivity to the destination area, 783 but turn out to have an unknown destination { MAC, Data Label } pair 784 when it arrives at that border TRILL switch. 786 In this case, the packet must be converted into a multi-destination 787 packet and flooded in the destination area. However, if the border 788 TRILL switch doing the conversion is not the border TRILL switch 789 designated to transition the resulting multi-destination packet, 790 there is the danger that the designated transitioner may pick up the 791 packet and flood it back into Level 2 from which it may be flooded 792 into multiple areas. This danger can be avoided by restricting any 793 multi-destination packet that results from such a conversion to the 794 destination area through a flag in the packet or though distributing 795 it on a tree that is restricted to the area, or other techniques (see 796 Section 4). 798 Alternatively, a multi-destination packet intended only for the area 799 could be tunneled (within the area) to the RBridge RBx, that is the 800 appointed transitioner for that form of packet (say, based on VLAN or 801 FGL), with instructions that RBx only transmit the packet within the 802 area, and RBx could initiate the multi-destination packet within the 803 area. Since RBx introduced the packet, and is the only one allowed 804 to transition that packet to Level 2, this would accomplish scoping 805 of the packet to within the area. Since this case only occurs in the 806 unusual case when unicast packets need to be turned into multi- 807 destination as described above, the suboptimality of tunneling 808 between the border TRILL switch that receives the unicast packet and 809 the appointed level transitioner for that packet, would not be an 810 issue. 812 4.1.1 New Tree Encoding 814 The current encoding, in a TRILL header, of a tree, is of the 815 nickname of the tree root. This requires all 16 bits of the egress 816 nickname field. TRILL could instead, for example, use the bottom 6 817 bits to encode the tree number (allowing 64 trees), leaving 10 bits 818 to encode information such as: 820 o scope: a flag indicating whether it should be single area only, or 821 entire campus 822 o border injector: an indicator of which of the k border TRILL 823 switches injected this packet 825 If TRILL were to adopt this new encoding, any of the TRILL switches 826 in an edge group could inject a multi-destination packet. This would 827 require all TRILL switches to be changed to understand the new 828 encoding for a tree, and it would require a TLV in the LSP to 829 indicate which number each of the TRILL switches in an edge group 830 would be. 832 4.2 Selective Broadcast Domain Reduction 834 There are a number of service, discovery, and directory protocols 835 that, for convenience, are accessed via multicast or broadcast 836 frames. Examples are DHCP, (Dynamic Host Configuration Protocol) the 837 NetBIOS Service Location Protocol, and multicast DNS (Domain Name 838 Service). 840 Some such protocols provide means to restrict distribution to an IP 841 subnet or equivalent to reduce size of the broadcast domain they are 842 using and then provide a proxy that can be placed in that subnet to 843 use unicast to access a service elsewhere. In cases where a proxy 844 mechanism is not currently defined, it may be possible to create one 845 that references a central server or cache. With multilevel TRILL, it 846 is possible to construct very large IP subnets that could become 847 saturated with multi-destination traffic of this type unless packets 848 can be further restricted in their distribution. Such restricted 849 distribution can be accomplished for some protocols, say protocol P, 850 in a variety of ways including the following: 852 - Either (1) at all ingress TRILL switches in an area place all 853 protocol P multi-destination packets on a distribution tree in 854 such a way that the packets are restricted to the area or (2) at 855 all border TRILL switches between that area and Level 2, detect 856 protocol P multi-destination packets and do not transition them. 858 - Then place one, or a few for redundancy, protocol P proxies inside 859 each area where protocol P may be in use. These proxies unicast 860 protocol P requests or other messages to the actual campus 861 server(s) for P. They also receive unicast responses or other 862 messages from those servers and deliver them within the area via 863 unicast, multicast, or broadcast as appropriate. (Such proxies 864 would not be needed if it was acceptable for all protocol P 865 traffic to be restricted to an area.) 867 While it might seem logical to connect the campus servers to TRILL 868 switches in Level 2, they could be placed within one or more areas so 869 that, in some cases, those areas might not require a local proxy 870 server. 872 5. Co-Existence with Old TRILL switches 874 TRILL switches that are not multilevel aware may have a problem with 875 calculating RPF Check and filtering information, since they would not 876 be aware of the assignment of border TRILL switch transitioning. 878 A possible solution, as long as any old TRILL switches exist within 879 an area, is to have the border TRILL switches elect a single DBRB 880 (Designated Border RBridge), and have all inter-area traffic go 881 through the DBRB (unicast as well as multi-destination). If that 882 DBRB goes down, a new one will be elected, but at any one time, all 883 inter-area traffic (unicast as well as multi-destination) would go 884 through that one DRBR. However this eliminates load splitting at 885 level transition. 887 6. Multi-Access Links with End Stations 889 Care must be taken, in the case where there are multiple TRILL 890 switches on a link with end stations, that only one TRILL switch 891 ingress/egress any given data packet from/to the end nodes. With 892 existing, single level TRILL, this is done by electing a single 893 Designated RBridge per link, which appoints a single Appointed 894 Forwarder per VLAN [RFC7177] [RFC6439]. But suppose there are two 895 (or more) TRILL switches on a link in different areas, say RB1 in 896 area 1000 and RB2 in area 2000, and that the link contains end nodes. 897 If RB1 and RB2 ignore each other's Hellos then they will both 898 ingress/egress end node traffic from the link. 900 A simple rule is to use the TRILL switch or switches having the 901 lowest numbered area, comparing area numbers as unsigned integers, to 902 handle native traffic. This would automatically give multilevel- 903 ignorant legacy TRILL switches, that would be using area number zero, 904 highest priority for handling end stations, which they would try to 905 do anyway. 907 Other methods are possible. For example doing the selection of 908 Appointed Forwarders and of the TRILL switch in charge of that 909 selection across all TRILL switches on the link regardless of area. 910 However, a special case would then have to be made in any case for 911 legacy TRILL switches using area number zero. 913 Any of these techniques require multilevel aware RBridges to take 914 actions based on Hellos from RBridges in other areas even though they 915 will not form an adjacency with such RBridges. 917 7. Summary 919 This draft discusses issues and possible approaches to multilevel 920 TRILL. The alternative using aggregated areas has significant 921 advantages in terms of scalability over using campus wide unique 922 nicknames, not just in avoiding nickname exhaustion, but by allowing 923 RPF Checks to be aggregated based on an entire area. However, the 924 alternative of using unique nicknames is simpler and avoids the 925 changes in border TRILL switches required to support aggregated 926 nicknames. It is possible to support both. For example, a TRILL 927 campus could use simpler unique nicknames until scaling begins to 928 cause problems and then start to introduce areas with aggregated 929 nicknames. 931 Some issues are not difficult, such as dealing with partitioned 932 areas. Other issues are more difficult, especially dealing with old 933 TRILL switches. 935 8. Security Considerations 937 This informational document explores alternatives for the use of 938 multilevel IS-IS in TRILL. It does not consider security issues. For 939 general TRILL Security Considerations, see [RFC6325]. 941 9. IANA Considerations 943 This document requires no IANA actions. RFC Editor: Please remove 944 this section before publication. 946 Normative References 948 [IS-IS] - ISO/IEC 10589:2002, Second Edition, "Intermediate System to 949 Intermediate System Intra-Domain Routing Exchange Protocol for 950 use in Conjunction with the Protocol for Providing the 951 Connectionless-mode Network Service (ISO 8473)", 2002. 953 [RFC6325] - Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A. 954 Ghanwani, "Routing Bridges (RBridges): Base Protocol 955 Specification", RFC 6325, July 2011. 957 [RFC6439] - Perlman, R., Eastlake, D., Li, Y., Banerjee, A., and F. 958 Hu, "Routing Bridges (RBridges): Appointed Forwarders", RFC 959 6439, November 2011. 961 [rfc7180bis] - D. Eastlake, M. Zhang, et al, "TRILL: Clarifications, 962 Corrections, and Updates", draft-ietf-trill-rfc7180bis, work in 963 progress 965 Informative References 967 [RFC6361] - Carlson, J. and D. Eastlake 3rd, "PPP Transparent 968 Interconnection of Lots of Links (TRILL) Protocol Control 969 Protocol", RFC 6361, August 2011. 971 [RFC7172] - Eastlake 3rd, D., Zhang, M., Agarwal, P., Perlman, R., 972 and D. Dutt, "Transparent Interconnection of Lots of Links 973 (TRILL): Fine-Grained Labeling", RFC 7172, May 2014 975 [RFC7176] - Eastlake 3rd, D., Senevirathne, T., Ghanwani, A., Dutt, 976 D., and A. Banerjee, "Transparent Interconnection of Lots of 977 Links (TRILL) Use of IS-IS", RFC 7176, May 2014. 979 [RFC7177] - Eastlake 3rd, D., Perlman, R., Ghanwani, A., Yang, H., 980 and V. Manral, "Transparent Interconnection of Lots of Links 981 (TRILL): Adjacency", RFC 7177, May 2014, . 984 [RFC7357] - Zhai, H., Hu, F., Perlman, R., Eastlake 3rd, D., and O. 985 Stokes, "Transparent Interconnection of Lots of Links (TRILL): 986 End Station Address Distribution Information (ESADI) Protocol", 987 RFC 7357, September 2014, . 990 [DraftAggregated] - Bhargav Bhikkaji, Balaji Venkat Venkataswami, 991 Narayana Perumal Swamy, "Connecting Disparate Data 992 Center/PBB/Campus TRILL sites using BGP", draft-balaji-trill- 993 over-ip-multi-level, Work In Progress. 995 [DraftCMT] - Tissa Senevirathne, Janardhanan Pathang, Jon Hudson, 996 "Coordinated Multicast Trees (CMT) for TRILL", draft-tissa- 997 trill-cmt, Work in Progress. 999 [DraftUnique] - Tissa Senevirathne, Les Ginsberg, Janardhanan 1000 Pathangi, Jon Hudson, Sam Aldrin, Ayan Banerjee, Sameer 1001 Merchant, "Default Nickname Based Approach for Multilevel 1002 TRILL", draft-tissa-trill-multilevel, Work In Progress. 1004 [NickFlags] - Eastlake, D., W. Hao, draft-eastlake-trill-nick-label- 1005 prop, Work In Progress. 1007 [SingleName] - Mingui Zhang, et. al, "Single Area Border RBridge 1008 Nickname for TRILL Multilevel", draft-zhang-trill-multilevel- 1009 single-nickname, Work in Progress. 1011 Acknowledgements 1013 The helpful comments of the following are hereby acknowledged: David 1014 Michael Bond, Dino Farinacci, and Gayle Noble. 1016 The document was prepared in raw nroff. All macros used were defined 1017 within the source file. 1019 Authors' Addresses 1021 Radia Perlman 1022 EMC 1023 2010 256th Avenue NE, #200 1024 Bellevue, WA 98007 USA 1026 EMail: radia@alum.mit.edu 1028 Donald Eastlake 1029 Huawei Technologies 1030 155 Beaver Street 1031 Milford, MA 01757 USA 1033 Phone: +1-508-333-2270 1034 Email: d3e3e3@gmail.com 1036 Mingui Zhang 1037 Huawei Technologies 1038 No.156 Beiqing Rd. Haidian District, 1039 Beijing 100095 P.R. China 1041 EMail: zhangmingui@huawei.com 1043 Anoop Ghanwani 1044 Dell 1045 5450 Great America Parkway 1046 Santa Clara, CA 95054 USA 1048 EMail: anoop@alumni.duke.edu 1050 Hongjun Zhai 1051 Jinling Institute of Technology 1052 99 Hongjing Avenue, Jiangning District 1053 Nanjing, Jiangsu 211169 China 1055 EMail: honjun.zhai@tom.com 1057 Copyright and IPR Provisions 1059 Copyright (c) 2015 IETF Trust and the persons identified as the 1060 document authors. All rights reserved. 1062 This document is subject to BCP 78 and the IETF Trust's Legal 1063 Provisions Relating to IETF Documents 1064 (http://trustee.ietf.org/license-info) in effect on the date of 1065 publication of this document. Please review these documents 1066 carefully, as they describe your rights and restrictions with respect 1067 to this document. 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