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Is this intentional? Checking references for intended status: Informational ---------------------------------------------------------------------------- ** Obsolete normative reference: RFC 6439 (Obsoleted by RFC 8139) Summary: 1 error (**), 0 flaws (~~), 2 warnings (==), 1 comment (--). 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: August 11, 2016 February 12, 2016 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....................................26 100 Acknowledgements..........................................28 101 Authors' Addresses........................................29 103 1. Introduction 105 The IETF TRILL (Transparent Interconnection of Lot of Links or 106 Tunneled Routing in the Link Layer) protocol [RFC6325] [RFC7177] 107 provides optimal pair-wise data routing without configuration, safe 108 forwarding even during periods of temporary loops, and support for 109 multipathing of both unicast and multicast traffic in networks with 110 arbitrary topology and link technology, including multi-access links. 111 TRILL accomplishes this by using IS-IS (Intermediate System to 112 Intermediate System [IS-IS] [RFC7176]) link state routing in 113 conjunction with a header that includes a hop count. The design 114 supports data labels (VLANs and Fine Grained Labels [RFC7172]) and 115 optimization of the distribution of multi-destination data based on 116 VLANs and multicast groups. Devices that implement TRILL are called 117 TRILL Switches or RBridges. 119 Familiarity with [IS-IS], [RFC6325], and [rfc7180bis] is assumed in 120 this document. 122 1.1 TRILL Scalability Issues 124 There are multiple issues that might limit the scalability of a 125 TRILL-based network: 127 1. the routing computation load, 128 2. the volatility of the link state database (LSDB) creating too much 129 control traffic, 130 3. the volatility of the LSDB causing the TRILL network to be in an 131 unconverged state too much of the time, 132 4. the size of the LSDB, 133 5. the limit of the number of TRILL switches, due to the 16-bit 134 nickname space, 135 6. the traffic due to upper layer protocols use of broadcast and 136 multicast, and 137 7. the size of the end node learning table (the table that remembers 138 (egress TRILL switch, label/MAC) pairs). 140 Extending TRILL IS-IS to be multilevel (hierarchical) helps with all 141 but the last of these issues. 143 IS-IS was designed to be multilevel [IS-IS]. A network can be 144 partitioned into "areas". Routing within an area is known as "Level 145 1 routing". Routing between areas is known as "Level 2 routing". 146 The Level 2 IS-IS network consists of Level 2 routers and links 147 between the Level 2 routers. Level 2 routers may participate in one 148 or more Level 1 areas, in addition to their role as Level 2 routers. 150 Each area is connected to Level 2 through one or more "border 151 routers", which participate both as a router inside the area, and as 152 a router inside the Level 2 "area". Care must be taken that it is 153 clear, when transitioning multi-destination packets between Level 2 154 and a Level 1 area in either direction, that exactly one border TRILL 155 switch will transition a particular data packet between the levels or 156 else duplication or loss of traffic can occur. 158 1.2 Improvements Due to Multilevel 160 Partitioning the network into areas solves the first four scalability 161 issues described above, namely, 163 1. the routing computation load, 165 2. the volatility of the LSDB creating too much control traffic, 167 3. the volatility of the LSDB causing the TRILL network to be in an 168 unconverged state too much of the time, 170 4. the size of the LSDB. 172 Problem #6 in Section 1.1, namely, the traffic due to upper layer 173 protocols use of broadcast and multicast, can be addressed by 174 introducing a locally-scoped multi-destination delivery, limited to 175 an area or a single link. See further discussion in Section 4.2. 177 Problem #5 in Section 1.1, namely, the limit of the number of TRILL 178 switches, due to the 16-bit nickname space, will only be addressed 179 with the aggregated nickname approach. Since the aggregated nickname 180 approach requires some complexity in the border TRILL switches (for 181 rewriting the nicknames in the TRILL header), the design in this 182 document allows a campus with a mixture of unique-nickname areas, and 183 aggregated-nickname areas. Nicknames must be unique across all Level 184 2 and unique-nickname area TRILL switches, whereas nicknames inside 185 an aggregated-nickname area are visible only inside the area. 186 Nicknames inside an aggregated-nickname area must not conflict with 187 nicknames visible in Level 2 (which includes all nicknames inside 188 unique nickname areas), but the nicknames inside an aggregated- 189 nickname area may be the same as nicknames used within other 190 aggregated-nickname areas. 192 TRILL switches within an area need not be aware of whether they are 193 in an aggregated nickname area or a unique nickname area. The border 194 TRILL switches in area A1 will claim, in their LSP inside area A1, 195 which nicknames (or nickname ranges) are not available for choosing 196 as nicknames by area A1 TRILL switches. 198 1.3 Unique and Aggregated Nicknames 200 We describe two alternatives for hierarchical or multilevel TRILL. 201 One we call the "unique nickname" alternative. The other we call the 202 "aggregated nickname" alternative. In the aggregated nickname 203 alternative, border TRILL switches replace either the ingress or 204 egress nickname field in the TRILL header of unicast packets with an 205 aggregated nickname representing an entire area. 207 The unique nickname alternative has the advantage that border TRILL 208 switches are simpler and do not need to do TRILL Header nickname 209 modification. It also simplifies testing and maintenance operations 210 that originate in one area and terminate in a different area. 212 The aggregated nickname alternative has the following advantages: 214 o it solves problem #5 above, the 16-bit nickname limit, in a 215 simple way, 216 o it lessens the amount of inter-area routing information that 217 must be passed in IS-IS, and 218 o it logically reduces the RPF (Reverse Path Forwarding) Check 219 information (since only the area nickname needs to appear, 220 rather than all the ingress TRILL switches in that area). 222 In both cases, it is possible and advantageous to compute multi- 223 destination data packet distribution trees such that the portion 224 computed within a given area is rooted within that area. 226 1.3 More on Areas 228 Each area is configured with an "area address", which is advertised 229 in IS-IS messages, so as to avoid accidentally interconnecting areas. 230 Although the area address had other purposes in CLNP (Connectionless 231 Network Layer Protocol, IS-IS was originally designed for 232 CLNP/DECnet), for TRILL the only purpose of the area address would be 233 to avoid accidentally interconnecting areas. 235 Currently, the TRILL specification says that the area address must be 236 zero. If we change the specification so that the area address value 237 of zero is just a default, then most of IS-IS multilevel machinery 238 works as originally designed. However, there are TRILL-specific 239 issues, which we address below in this document. 241 1.4 Terminology and Acronyms 243 This document generally uses the acronyms defined in [RFC6325] plus 244 the additional acronym DBRB. However, for ease of reference, most 245 acronyms used are listed here: 247 CLNP - ConnectionLess Network Protocol 249 DECnet - a proprietary routing protocol that was used by Digital 250 Equipment Corporation. "DECnet Phase 5" was the origin of IS-IS. 252 Data Label - VLAN or Fine Grained Label [RFC7172] 254 DBRB - Designated Border RBridge 256 ESADI - End Station Address Distribution Information 258 IS-IS - Intermediate System to Intermediate System [IS-IS] 260 LSDB - Link State Data Base 262 LSP - Link State PDU 264 PDU - Protocol Data Unit 266 RBridge - Routing Bridge, an alterntive name for a TRILL switch 268 RPF - Reverse Path Forwarding 270 TLV - Type Length Value 272 TRILL - Transparent Interconnection of Lots of Links or Tunneled 273 Routing in the Link Layer [RFC6325] 275 TRILL switch - a device that implements the TRILL protcol 276 [RFC6325], sometimes called an RBridge 278 VLAN - Virtual Local Area Network 280 2. Multilevel TRILL Issues 282 The TRILL-specific issues introduced by multilevel include the 283 following: 285 a. Configuration of non-zero area addresses, encoding them in IS-IS 286 PDUs, and possibly interworking with old TRILL switches that do 287 not understand nonzero area addresses. 289 See Section 2.1. 291 b. Nickname management. 293 See Sections 2.5 and 2.2. 295 c. Advertisement of pruning information (Data Label reachability, IP 296 multicast addresses) across areas. 298 Distribution tree pruning information is only an optimization, 299 as long as multi-destination packets are not prematurely 300 pruned. For instance, border TRILL switches could advertise 301 they can reach all possible Data Labels, and have an IP 302 multicast router attached. This would cause all multi- 303 destination traffic to be transmitted to border TRILL switches, 304 and possibly pruned there, when the traffic could have been 305 pruned earlier based on Data Label or multicast group if border 306 TRILL switches advertised more detailed Data Label and/or 307 multicast listener and multicast router attachment information. 309 d. Computation of distribution trees across areas for multi- 310 destination data. 312 See Section 2.3. 314 e. Computation of RPF information for those distribution trees. 316 See Section 2.4. 318 f. Computation of pruning information across areas. 320 See Sections 2.3 and 2.6. 322 g. Compatibility, as much as practical, with existing, unmodified 323 TRILL switches. 325 The most important form of compatibility is with existing TRILL 326 fast path hardware. Changes that require upgrade to the slow 327 path firmware/software are more tolerable. Compatibility for 328 the relatively small number of border TRILL switches is less 329 important than compatibility for non-border TRILL switches. 331 See Section 5. 333 2.1 Non-zero Area Addresses 335 The current TRILL base protocol specification [RFC6325] [RFC7177] 336 [rfc7180bis] says that the area address in IS-IS must be zero. The 337 purpose of the area address is to ensure that different areas are not 338 accidentally merged. Furthermore, zero is an invalid area address 339 for layer 3 IS-IS, so it was chosen as an additional safety mechanism 340 to ensure that layer 3 IS-IS would not be confused with TRILL IS-IS. 341 However, TRILL uses other techniques to avoid such confusion, such as 342 different multicast addresses and Ethertypes on Ethernet [RFC6325], 343 different PPP (Point-to-Point Protocol) codepoints on PPP [RFC6361], 344 and the like. Thus, using an area address in TRILL that might be used 345 in layer 3 IS-IS is not a problem. 347 Since current TRILL switches will reject any IS-IS messages with 348 nonzero area addresses, the choices are as follows: 350 a.1 upgrade all TRILL switches that are to interoperate in a 351 potentially multilevel environment to understand non-zero area 352 addresses, 353 a.2 neighbors of old TRILL switches must remove the area address from 354 IS-IS messages when talking to an old TRILL switch (which might 355 break IS-IS security and/or cause inadvertent merging of areas), 356 a.3 ignore the problem of accidentally merging areas entirely, or 357 a.4 keep the fixed "area address" field as 0 in TRILL, and add a new, 358 optional TLV for "area name" to Hellos that, if present, could be 359 compared, by new TRILL switches, to prevent accidental area 360 merging. 362 In principal, different solutions could be used in different areas 363 but it would be much simpler to adopt one of these choices uniformly. 365 2.2 Aggregated versus Unique Nicknames 367 In the unique nickname alternative, all nicknames across the campus 368 must be unique. In the aggregated nickname alternative, TRILL switch 369 nicknames within an aggregated area are only of local significance, 370 and the only nickname externally (outside that area) visible is the 371 "area nickname" (or nicknames), which aggregates all the internal 372 nicknames. 374 The unique nickname approach simplifies border TRILL switches. 376 The aggregated nickname approach eliminates the potential problem of 377 nickname exhaustion, minimizes the amount of nickname information 378 that would need to be forwarded between areas, minimizes the size of 379 the forwarding table, and simplifies RPF calculation and RPF 380 information. 382 2.2.1 More Details on Unique Nicknames 384 With unique cross-area nicknames, it would be intractable to have a 385 flat nickname space with TRILL switches in different areas contending 386 for the same nicknames. Instead, each area would need to be 387 configured with a block of nicknames. Either some TRILL switches 388 would need to announce that all the nicknames other than that block 389 are taken (to prevent the TRILL switches inside the area from 390 choosing nicknames outside the area's nickname block), or a new TLV 391 would be needed to announce the allowable nicknames, and all TRILL 392 switches in the area would need to understand that new TLV. An 393 example of the second approach is given in [NickFlags]. 395 Currently the encoding of nickname information in TLVs is by listing 396 of individual nicknames; this would make it painful for a border 397 TRILL switch to announce into an area that it is holding all other 398 nicknames to limit the nicknames available within that area. The 399 information could be encoded as ranges of nicknames to make this 400 somewhat manageable [NickFlags]; however, a new TLV for announcing 401 nickname ranges would not be intelligible to old TRILL switches. 403 There is also an issue with the unique nicknames approach in building 404 distribution trees, as follows: 406 With unique nicknames in the TRILL campus and TRILL header 407 nicknames not rewritten by the border TRILL switches, there would 408 have to be globally known nicknames for the trees. Suppose there 409 are k trees. For all of the trees with nicknames located outside 410 an area, the local trees would be rooted at a border TRILL switch 411 or switches. Therefore, there would be either no splitting of 412 multi-destination traffic with the area or restricted splitting of 413 multi-destination traffic between trees rooted at a highly 414 restricted set of TRILL switches. 416 As an alternative, just the "egress nickname" field of multi- 417 destination TRILL Data packets could be mapped at the border, 418 leaving known unicast packets un-mapped. However, this surrenders 419 much of the unique nickname advantage of simpler border TRILL 420 switches. 422 Scaling to a very large campus with unique nicknames might exhaust 423 the 16-bit TRILL nicknames space. One method might be to expand 424 nicknames to 24 bits; however, that technique would require TRILL 425 message format changes and that all TRILL switches in the campus 426 understand larger nicknames. 428 2.2.2 More Details on Aggregated Nicknames 430 The aggregated nickname approach enables passing far less nickname 431 information. It works as follows, assuming both the source and 432 destination areas are using aggregated nicknames: 434 There are two ways areas could be identified. 436 One method would be to assign each area a 16-bit nickname. This 437 would not be the nickname of any actual TRILL switch. Instead, it 438 would be the nickname of the area itself. Border TRILL switches 439 would know the area nickname for their own area(s). For an 440 example of a more specific multilevel proposal using unique 441 nicknames, see [DraftUnique]. 443 Alternatively, areas could be identified by the set of nicknames 444 that identify the border routers for that area. (See [SingleName] 445 for a multilevel proposal using such a set of nicknames.) 447 The TRILL Header nickname fields in TRILL Data packets being 448 transported through a multilevel TRILL campus with aggregated 449 nicknames are as follows: 451 - When both the ingress and egress TRILL switches are in the same 452 area, there need be no change from the existing base TRILL 453 protocol standard in the TRILL Header nickname fields. 455 - When being transported in Level 2, the ingress nickname is the 456 nickname of the ingress TRILL switch's area while the egress 457 nickname is either the nickname of the egress TRILL switch's 458 area or a tree nickname. 460 - When being transported from Level 1 to Level 2, the ingress 461 nickname is the nickname of the ingress TRILL switch itself 462 while the egress nickname is either a nickname for the area of 463 the egress TRILL switch or a tree nickname. 465 - When being transported from Level 2 to Level 1, the ingress 466 nickname is a nickname for the ingress TRILL switch's area while 467 the egress nickname is either the nickname of the egress TRILL 468 switch itself or a tree nickname. 470 There are two variations of the aggregated nickname approach. The 471 first is the Border Learning approach, which is described in Section 472 2.2.2.1. The second is the Swap Nickname Field approach, which is 473 described in Section 2.2.2.2. Section 2.2.2.3 compares the advantages 474 and disadvantages of these two variations of the aggregated nickname 475 approach. 477 2.2.2.1 Border Learning Aggregated Nicknames 479 This section provides an illustrative example and description of the 480 border learning variation of aggregated nicknames where a single 481 nickname is used to identify an area. 483 In the following picture, RB2 and RB3 are area border TRILL switches 484 (RBridges). A source S is attached to RB1. The two areas have 485 nicknames 15961 and 15918, respectively. RB1 has a nickname, say 27, 486 and RB4 has a nickname, say 44 (and in fact, they could even have the 487 same nickname, since the TRILL switch nickname will not be visible 488 outside these aggregated areas). 490 Area 15961 level 2 Area 15918 491 +-------------------+ +-----------------+ +--------------+ 492 | | | | | | 493 | S--RB1---Rx--Rz----RB2---Rb---Rc--Rd---Re--RB3---Rk--RB4---D | 494 | 27 | | | | 44 | 495 | | | | | | 496 +-------------------+ +-----------------+ +--------------+ 498 Let's say that S transmits a frame to destination D, which is 499 connected to RB4, and let's say that D's location has already been 500 learned by the relevant TRILL switches. These relevant switches have 501 learned the following: 503 1) RB1 has learned that D is connected to nickname 15918 504 2) RB3 has learned that D is attached to nickname 44. 506 The following sequence of events will occur: 508 - S transmits an Ethernet frame with source MAC = S and destination 509 MAC = D. 511 - RB1 encapsulates with a TRILL header with ingress RBridge = 27, 512 and egress = 15918 producing a TRILL Data packet. 514 - RB2 has announced in the Level 1 IS-IS instance in area 15961, 515 that it is attached to all the area nicknames, including 15918. 516 Therefore, IS-IS routes the packet to RB2. Alternatively, if a 517 distinguished range of nicknames is used for Level 2, Level 1 518 TRILL switches seeing such an egress nickname will know to route 519 to the nearest border router, which can be indicated by the IS-IS 520 attached bit. 522 - RB2, when transitioning the packet from Level 1 to Level 2, 523 replaces the ingress TRILL switch nickname with the area nickname, 524 so replaces 27 with 15961. Within Level 2, the ingress RBridge 525 field in the TRILL header will therefore be 15961, and the egress 526 RBridge field will be 15918. Also RB2 learns that S is attached to 527 nickname 27 in area 15961 to accommodate return traffic. 529 - The packet is forwarded through Level 2, to RB3, which has 530 advertised, in Level 2, reachability to the nickname 15918. 532 - RB3, when forwarding into area 15918, replaces the egress nickname 533 in the TRILL header with RB4's nickname (44). So, within the 534 destination area, the ingress nickname will be 15961 and the 535 egress nickname will be 44. 537 - RB4, when decapsulating, learns that S is attached to nickname 538 15961, which is the area nickname of the ingress. 540 Now suppose that D's location has not been learned by RB1 and/or RB3. 541 What will happen, as it would in TRILL today, is that RB1 will 542 forward the packet as multi-destination, choosing a tree. As the 543 multi-destination packet transitions into Level 2, RB2 replaces the 544 ingress nickname with the area nickname. If RB1 does not know the 545 location of D, the packet must be flooded, subject to possible 546 pruning, in Level 2 and, subject to possible pruning, from Level 2 547 into every Level 1 area that it reaches on the Level 2 distribution 548 tree. 550 Now suppose that RB1 has learned the location of D (attached to 551 nickname 15918), but RB3 does not know where D is. In that case, RB3 552 must turn the packet into a multi-destination packet within area 553 15918. In this case, care must be taken so that, in case RB3 is not 554 the Designated transitioner between Level 2 and its area for that 555 multi-destination packet, but was on the unicast path, that another 556 border TRILL switch in that area not forward the now multi- 557 destination packet back into Level 2. Therefore, it would be 558 desirable to have a marking, somehow, that indicates the scope of 559 this packet's distribution to be "only this area" (see also Section 560 4). 562 In cases where there are multiple transitioners for unicast packets, 563 the border learning mode of operation requires that the address 564 learning between them be shared by some protocol such as running 565 ESADI [RFC7357] for all Data Labels of interest to avoid excessive 566 unknown unicast flooding. 568 The potential issue described at the end of Section 2.2.1 with trees 569 in the unique nickname alternative is eliminated with aggregated 570 nicknames. With aggregated nicknames, each border TRILL switch that 571 will transition multi-destination packets can have a mapping between 572 Level 2 tree nicknames and Level 1 tree nicknames. There need not 573 even be agreement about the total number of trees; just that the 574 border TRILL switch have some mapping, and replace the egress TRILL 575 switch nickname (the tree name) when transitioning levels. 577 2.2.2.2 Swap Nickname Field Aggregated Nicknames 579 As a variant, two additional fields could exist in TRILL Data packets 580 we call the "ingress swap nickname field" and the "egress swap 581 nickname field". The changes in the example above would be as 582 follows: 584 - RB1 will have learned the area nickname of D and the TRILL switch 585 nickname of RB4 to which D is attached. In encapsulating a frame 586 to D, it puts an area nickname of D (15918) in the egress nickname 587 field of the TRILL Header and puts a nickname of RB3 (44) in a 588 egress swap nickname field. 590 - RB2 moves the ingress nickname to the ingress swap nickname field 591 and inserts 15961, an area nickname for S, into the ingress 592 nickname field. 594 - RB3 swaps the egress nickname and the egress swap nickname fields, 595 which sets the egress nickname to 44. 597 - RB4 learns the correspondence between the source MAC/VLAN of S and 598 the { ingress nickname, ingress swap nickname field } pair as it 599 decapsulates and egresses the frame. 601 See [DraftAggregated] for a multilevel proposal using aggregated swap 602 nicknames with a single nickname representing an area. 604 2.2.2.3 Comparison 606 The Border Learning variant described in Section 2.2.2.1 above 607 minimizes the change in non-border TRILL switches but imposes the 608 burden on border TRILL switches of learning and doing lookups in all 609 the end station MAC addresses within their area(s) that are used for 610 communication outside the area. This burden could be reduced by 611 decreasing the area size and increasing the number of areas. 613 The Swap Nickname Field variant described in Section 2.2.2.2 614 eliminates the extra address learning burden on border TRILL switches 615 but requires more extensive changes to non-border TRILL switches. In 616 particular they must learn to associate both a TRILL switch nickname 617 and an area nickname with end station MAC/label pairs (except for 618 addresses that are local to their area). 620 The Swap Nickname Field alternative is more scalable but less 621 backward compatible for non-border TRILL switches. It would be 622 possible for border and other level 2 TRILL switches to support both 623 Border Learning, for support of legacy Level 1 TRILL switches, and 624 Swap Nickname, to support Level 1 TRILL switches that understood the 625 Swap Nickname method. 627 2.3 Building Multi-Area Trees 629 It is easy to build a multi-area tree by building a tree in each area 630 separately, (including the Level 2 "area"), and then having only a 631 single border TRILL switch, say RBx, in each area, attach to the 632 Level 2 area. RBx would forward all multi-destination packets 633 between that area and Level 2. 635 People might find this unacceptable, however, because of the desire 636 to path split (not always sending all multi-destination traffic 637 through the same border TRILL switch). 639 This is the same issue as with multiple ingress TRILL switches 640 injecting traffic from a pseudonode, and can be solved with the 641 mechanism that was adopted for that purpose: the affinity TLV 642 [DraftCMT]. For each tree in the area, at most one border RB 643 announces itself in an affinity TLV with that tree name. 645 2.4 The RPF Check for Trees 647 For multi-destination data originating locally in RBx's area, 648 computation of the RPF check is done as today. For multi-destination 649 packets originating outside RBx's area, computation of the RPF check 650 must be done based on which one of the border TRILL switches (say 651 RB1, RB2, or RB3) injected the packet into the area. 653 A TRILL switch, say RB4, located inside an area, must be able to know 654 which of RB1, RB2, or RB3 transitioned the packet into the area from 655 Level 2. (or into Level 2 from an area). 657 This could be done based on having the DBRB announce the transitioner 658 assignments to all the TRILL switches in the area, or the Affinity 659 TLV mechanism given in [DraftCMT], or the New Tree Encoding mechanism 660 discussed in Section 4.1.1. 662 2.5 Area Nickname Acquisition 664 In the aggregated nickname alternative, each area must acquire a 665 unique area nickname. It is probably simpler to allocate a block of 666 nicknames (say, the top 4000) to be area addresses, and not used by 667 any TRILL switches. 669 The nicknames used for area identification need to be advertised and 670 acquired through Level 2. 672 Within an area, all the border TRILL switches can discover each other 673 through the Level 1 link state database, by using the IS-IS attach 674 bit or by explicitly advertising in their LSP "I am a border 675 RBridge". 677 Of the border TRILL switches, one will have highest priority (say 678 RB7). RB7 can dynamically participate, in Level 2, to acquire a 679 nickname for identifying the area. Alternatively, RB7 could give the 680 area a pseudonode IS-IS ID, such as RB7.5, within Level 2. So an 681 area would appear, in Level 2, as a pseudonode and the pseudonode 682 could participate, in Level 2, to acquire a nickname for the area. 684 Within Level 2, all the border TRILL switches for an area can 685 advertise reachability to the area, which would mean connectivity to 686 a nickname identifying the area. 688 2.6 Link State Representation of Areas 690 Within an area, say area A1, there is an election for the DBRB, 691 (Designated Border RBridge), say RB1. This can be done through LSPs 692 within area A1. The border TRILL switches announce themselves, 693 together with their DBRB priority. (Note that the election of the 694 DBRB cannot be done based on Hello messages, because the border TRILL 695 switches are not necessarily physical neighbors of each other. They 696 can, however, reach each other through connectivity within the area, 697 which is why it will work to find each other through Level 1 LSPs.) 699 RB1 acquires an area nickname (in the aggregated nickname approach) 700 and may give the area a pseudonode IS-IS ID (just like the DRB would 701 give a pseudonode IS-IS ID to a link) depending on how the area 702 nickname is handled. RB1 advertises, in area A1, an area nickname 703 that RB1 has acquired (and what the pseudonode IS-IS ID for the area 704 is if needed). 706 Level 1 LSPs (possibly pseudonode) initiated by RB1 for the area 707 include any information external to area A1 that should be input into 708 area A1 (such as nicknames of external areas, or perhaps (in the 709 unique nickname variant) all the nicknames of external TRILL switches 710 in the TRILL campus and pruning information such as multicast 711 listeners and labels). All the other border TRILL switches for the 712 area announce (in their LSP) attachment to that area. 714 Within Level 2, RB1 generates a Level 2 LSP on behalf of the area. 715 The same pseudonode ID could be used within Level 1 and Level 2, for 716 the area. (There does not seem any reason why it would be useful for 717 it to be different, but there's also no reason why it would need to 718 be the same). Likewise, all the area A1 border TRILL switches would 719 announce, in their Level 2 LSPs, connection to the area. 721 3. Area Partition 723 It is possible for an area to become partitioned, so that there is 724 still a path from one section of the area to the other, but that path 725 is via the Level 2 area. 727 With multilevel TRILL, an area will naturally break into two areas in 728 this case. 730 Area addresses might be configured to ensure two areas are not 731 inadvertently connected. Area addresses appear in Hellos and LSPs 732 within the area. If two chunks, connected only via Level 2, were 733 configured with the same area address, this would not cause any 734 problems. (They would just operate as separate Level 1 areas.) 736 A more serious problem occurs if the Level 2 area is partitioned in 737 such a way that it could be healed by using a path through a Level 1 738 area. TRILL will not attempt to solve this problem. Within the Level 739 1 area, a single border RBridge will be the DBRB, and will be in 740 charge of deciding which (single) RBridge will transition any 741 particular multi-destination packets between that area and Level 2. 742 If the Level 2 area is partitioned, this will result in multi- 743 destination data only reaching the portion of the TRILL campus 744 reachable through the partition attached to the TRILL switch that 745 transitions that packet. It will not cause a loop. 747 4. Multi-Destination Scope 749 There are at least two reasons it would be desirable to be able to 750 mark a multi-destination packet with a scope that indicates the 751 packet should not exit the area, as follows: 753 1. To address an issue in the border learning variant of the 754 aggregated nickname alternative, when a unicast packet turns into 755 a multi-destination packet when transitioning from Level 2 to 756 Level 1, as discussed in Section 4.1. 758 2. To constrain the broadcast domain for certain discovery, 759 directory, or service protocols as discussed in Section 4.2. 761 Multi-destination packet distribution scope restriction could be done 762 in a number of ways. For example, there could be a flag in the packet 763 that means "for this area only". However, the technique that might 764 require the least change to TRILL switch fast path logic would be to 765 indicate this in the egress nickname that designates the distribution 766 tree being used. There could be two general tree nicknames for each 767 tree, one being for distribution restricted to the area and the other 768 being for multi-area trees. Or there would be a set of N (perhaps 16) 769 special currently reserved nicknames used to specify the N highest 770 priority trees but with the variation that if the special nickname is 771 used for the tree, the packet is not transitioned between areas. Or 772 one or more special trees could be built that were restricted to the 773 local area. 775 4.1 Unicast to Multi-destination Conversions 777 In the border learning variant of the aggregated nickname 778 alternative, a unicast packet might be known at the Level 1 to Level 779 2 transition, be forwarded as a unicast packet to the least cost 780 border TRILL switch advertising connectivity to the destination area, 781 but turn out to have an unknown destination { MAC, Data Label } pair 782 when it arrives at that border TRILL switch. 784 In this case, the packet must be converted into a multi-destination 785 packet and flooded in the destination area. However, if the border 786 TRILL switch doing the conversion is not the border TRILL switch 787 designated to transition the resulting multi-destination packet, 788 there is the danger that the designated transitioner may pick up the 789 packet and flood it back into Level 2 from which it may be flooded 790 into multiple areas. This danger can be avoided by restricting any 791 multi-destination packet that results from such a conversion to the 792 destination area through a flag in the packet or though distributing 793 it on a tree that is restricted to the area, or other techniques (see 794 Section 4). 796 Alternatively, a multi-destination packet intended only for the area 797 could be tunneled (within the area) to the RBridge RBx, that is the 798 appointed transitioner for that form of packet (say, based on VLAN or 799 FGL), with instructions that RBx only transmit the packet within the 800 area, and RBx could initiate the multi-destination packet within the 801 area. Since RBx introduced the packet, and is the only one allowed 802 to transition that packet to Level 2, this would accomplish scoping 803 of the packet to within the area. Since this case only occurs in the 804 unusual case when unicast packets need to be turned into multi- 805 destination as described above, the suboptimality of tunneling 806 between the border TRILL switch that receives the unicast packet and 807 the appointed level transitioner for that packet, would not be an 808 issue. 810 4.1.1 New Tree Encoding 812 The current encoding, in a TRILL header, of a tree, is of the 813 nickname of the tree root. This requires all 16 bits of the egress 814 nickname field. TRILL could instead, for example, use the bottom 6 815 bits to encode the tree number (allowing 64 trees), leaving 10 bits 816 to encode information such as: 818 o scope: a flag indicating whether it should be single area only, or 819 entire campus 820 o border injector: an indicator of which of the k border TRILL 821 switches injected this packet 823 If TRILL were to adopt this new encoding, any of the TRILL switches 824 in an edge group could inject a multi-destination packet. This would 825 require all TRILL switches to be changed to understand the new 826 encoding for a tree, and it would require a TLV in the LSP to 827 indicate which number each of the TRILL switches in an edge group 828 would be. 830 4.2 Selective Broadcast Domain Reduction 832 There are a number of service, discovery, and directory protocols 833 that, for convenience, are accessed via multicast or broadcast 834 frames. Examples are DHCP, (Dynamic Host Configuration Protocol) the 835 NetBIOS Service Location Protocol, and multicast DNS (Domain Name 836 Service). 838 Some such protocols provide means to restrict distribution to an IP 839 subnet or equivalent to reduce size of the broadcast domain they are 840 using and then provide a proxy that can be placed in that subnet to 841 use unicast to access a service elsewhere. In cases where a proxy 842 mechanism is not currently defined, it may be possible to create one 843 that references a central server or cache. With multilevel TRILL, it 844 is possible to construct very large IP subnets that could become 845 saturated with multi-destination traffic of this type unless packets 846 can be further restricted in their distribution. Such restricted 847 distribution can be accomplished for some protocols, say protocol P, 848 in a variety of ways including the following: 850 - Either (1) at all ingress TRILL switches in an area place all 851 protocol P multi-destination packets on a distribution tree in 852 such a way that the packets are restricted to the area or (2) at 853 all border TRILL switches between that area and Level 2, detect 854 protocol P multi-destination packets and do not transition them. 856 - Then place one, or a few for redundancy, protocol P proxies inside 857 each area where protocol P may be in use. These proxies unicast 858 protocol P requests or other messages to the actual campus 859 server(s) for P. They also receive unicast responses or other 860 messages from those servers and deliver them within the area via 861 unicast, multicast, or broadcast as appropriate. (Such proxies 862 would not be needed if it was acceptable for all protocol P 863 traffic to be restricted to an area.) 865 While it might seem logical to connect the campus servers to TRILL 866 switches in Level 2, they could be placed within one or more areas so 867 that, in some cases, those areas might not require a local proxy 868 server. 870 5. Co-Existence with Old TRILL switches 872 TRILL switches that are not multilevel aware may have a problem with 873 calculating RPF Check and filtering information, since they would not 874 be aware of the assignment of border TRILL switch transitioning. 876 A possible solution, as long as any old TRILL switches exist within 877 an area, is to have the border TRILL switches elect a single DBRB 878 (Designated Border RBridge), and have all inter-area traffic go 879 through the DBRB (unicast as well as multi-destination). If that 880 DBRB goes down, a new one will be elected, but at any one time, all 881 inter-area traffic (unicast as well as multi-destination) would go 882 through that one DRBR. However this eliminates load splitting at 883 level transition. 885 6. Multi-Access Links with End Stations 887 Care must be taken, in the case where there are multiple TRILL 888 switches on a link with end stations, that only one TRILL switch 889 ingress/egress any given data packet from/to the end nodes. With 890 existing, single level TRILL, this is done by electing a single 891 Designated RBridge per link, which appoints a single Appointed 892 Forwarder per VLAN [RFC7177] [RFC6439]. But suppose there are two 893 (or more) TRILL switches on a link in different areas, say RB1 in 894 area 1000 and RB2 in area 2000, and that the link contains end nodes. 895 If RB1 and RB2 ignore each other's Hellos then they will both 896 ingress/egress end node traffic from the link. 898 A simple rule is to use the TRILL switch or switches having the 899 lowest numbered area, comparing area numbers as unsigned integers, to 900 handle native traffic. This would automatically give multilevel- 901 ignorant legacy TRILL switches, that would be using area number zero, 902 highest priority for handling end stations, which they would try to 903 do anyway. 905 Other methods are possible. For example doing the selection of 906 Appointed Forwarders and of the TRILL switch in charge of that 907 selection across all TRILL switches on the link regardless of area. 908 However, a special case would then have to be made in any case for 909 legacy TRILL switches using area number zero. 911 Any of these techniques require multilevel aware RBridges to take 912 actions based on Hellos from RBridges in other areas even though they 913 will not form an adjacency with such RBridges. 915 7. Summary 917 This draft discusses issues and possible approaches to multilevel 918 TRILL. The alternative using aggregated areas has significant 919 advantages in terms of scalability over using campus wide unique 920 nicknames, not just in avoiding nickname exhaustion, but by allowing 921 RPF Checks to be aggregated based on an entire area. However, the 922 alternative of using unique nicknames is simpler and avoids the 923 changes in border TRILL switches required to support aggregated 924 nicknames. It is possible to support both. For example, a TRILL 925 campus could use simpler unique nicknames until scaling begins to 926 cause problems and then start to introduce areas with aggregated 927 nicknames. 929 Some issues are not difficult, such as dealing with partitioned 930 areas. Other issues are more difficult, especially dealing with old 931 TRILL switches. 933 8. Security Considerations 935 This informational document explores alternatives for the use of 936 multilevel IS-IS in TRILL. It does not consider security issues. For 937 general TRILL Security Considerations, see [RFC6325]. 939 9. IANA Considerations 941 This document requires no IANA actions. RFC Editor: Please remove 942 this section before publication. 944 Normative References 946 [IS-IS] - ISO/IEC 10589:2002, Second Edition, "Intermediate System to 947 Intermediate System Intra-Domain Routing Exchange Protocol for 948 use in Conjunction with the Protocol for Providing the 949 Connectionless-mode Network Service (ISO 8473)", 2002. 951 [RFC6325] - Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A. 952 Ghanwani, "Routing Bridges (RBridges): Base Protocol 953 Specification", RFC 6325, July 2011. 955 [RFC6439] - Perlman, R., Eastlake, D., Li, Y., Banerjee, A., and F. 956 Hu, "Routing Bridges (RBridges): Appointed Forwarders", RFC 957 6439, November 2011. 959 [rfc7180bis] - D. Eastlake, M. Zhang, et al, "TRILL: Clarifications, 960 Corrections, and Updates", draft-ietf-trill-rfc7180bis, in RFC 961 Editor's queue. 963 Informative References 965 [RFC6361] - Carlson, J. and D. Eastlake 3rd, "PPP Transparent 966 Interconnection of Lots of Links (TRILL) Protocol Control 967 Protocol", RFC 6361, August 2011. 969 [RFC7172] - Eastlake 3rd, D., Zhang, M., Agarwal, P., Perlman, R., 970 and D. Dutt, "Transparent Interconnection of Lots of Links 971 (TRILL): Fine-Grained Labeling", RFC 7172, May 2014 973 [RFC7176] - Eastlake 3rd, D., Senevirathne, T., Ghanwani, A., Dutt, 974 D., and A. Banerjee, "Transparent Interconnection of Lots of 975 Links (TRILL) Use of IS-IS", RFC 7176, May 2014. 977 [RFC7177] - Eastlake 3rd, D., Perlman, R., Ghanwani, A., Yang, H., 978 and V. Manral, "Transparent Interconnection of Lots of Links 979 (TRILL): Adjacency", RFC 7177, May 2014, . 982 [RFC7357] - Zhai, H., Hu, F., Perlman, R., Eastlake 3rd, D., and O. 983 Stokes, "Transparent Interconnection of Lots of Links (TRILL): 984 End Station Address Distribution Information (ESADI) Protocol", 985 RFC 7357, September 2014, . 988 [DraftAggregated] - Bhargav Bhikkaji, Balaji Venkat Venkataswami, 989 Narayana Perumal Swamy, "Connecting Disparate Data 990 Center/PBB/Campus TRILL sites using BGP", draft-balaji-trill- 991 over-ip-multi-level, Work In Progress. 993 [DraftCMT] - Tissa Senevirathne, Janardhanan Pathang, Jon Hudson, 994 "Coordinated Multicast Trees (CMT) for TRILL", draft-ietf- 995 trill-cmt, in RFC Editor's queue. 997 [DraftUnique] - Tissa Senevirathne, Les Ginsberg, Janardhanan 998 Pathangi, Jon Hudson, Sam Aldrin, Ayan Banerjee, Sameer 999 Merchant, "Default Nickname Based Approach for Multilevel 1000 TRILL", draft-tissa-trill-multilevel, Work In Progress. 1002 [NickFlags] - Eastlake, D., W. Hao, draft-eastlake-trill-nick-label- 1003 prop, Work In Progress. 1005 [SingleName] - Mingui Zhang, et. al, "Single Area Border RBridge 1006 Nickname for TRILL Multilevel", draft-zhang-trill-multilevel- 1007 single-nickname, Work in Progress. 1009 Acknowledgements 1011 The helpful comments of the following are hereby acknowledged: David 1012 Michael Bond, Dino Farinacci, and Gayle Noble. 1014 The document was prepared in raw nroff. All macros used were defined 1015 within the source file. 1017 Authors' Addresses 1019 Radia Perlman 1020 EMC 1021 2010 256th Avenue NE, #200 1022 Bellevue, WA 98007 USA 1024 EMail: radia@alum.mit.edu 1026 Donald Eastlake 1027 Huawei Technologies 1028 155 Beaver Street 1029 Milford, MA 01757 USA 1031 Phone: +1-508-333-2270 1032 Email: d3e3e3@gmail.com 1034 Mingui Zhang 1035 Huawei Technologies 1036 No.156 Beiqing Rd. Haidian District, 1037 Beijing 100095 P.R. China 1039 EMail: zhangmingui@huawei.com 1041 Anoop Ghanwani 1042 Dell 1043 5450 Great America Parkway 1044 Santa Clara, CA 95054 USA 1046 EMail: anoop@alumni.duke.edu 1048 Hongjun Zhai 1049 Jinling Institute of Technology 1050 99 Hongjing Avenue, Jiangning District 1051 Nanjing, Jiangsu 211169 China 1053 EMail: honjun.zhai@tom.com 1055 Copyright and IPR Provisions 1057 Copyright (c) 2016 IETF Trust and the persons identified as the 1058 document authors. All rights reserved. 1060 This document is subject to BCP 78 and the IETF Trust's Legal 1061 Provisions Relating to IETF Documents 1062 (http://trustee.ietf.org/license-info) in effect on the date of 1063 publication of this document. Please review these documents 1064 carefully, as they describe your rights and restrictions with respect 1065 to this document. 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