TRILL Working Group Radia Perlman INTERNET-DRAFT Intel Labs Intended status: Informational Donald Eastlake Huawei Anoop Ghanwani Brocade Expires: September 6, 2011 March 7, 2011 RBridges: Multilevel TRILL Abstract This document describes issues, and various possible approaches, to extending TRILL to use multiple levels of IS-IS. Status of This Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Distribution of this document is unlimited. Comments should be sent to the TRILL working group mailing list . Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Acknowledgements The helpful comments of the following are hereby acknowledged: David Michael Bond. R. Perlman, et al [Page 1] INTERNET-DRAFT RBridges: Multilevel TRILL Table of Contents 1. Introduction............................................3 1.1 TRILL Scalability Issues...............................3 1.2 Improvements Due to Multilevel.........................4 1.3 More on Areas..........................................4 1.4 Terminology and Acronyms...............................5 2. Multilevel TRILL Issues.................................6 2.1 Non-zero Area Addresses................................6 2.2 Aggregated versus Unique Nicknames.....................7 2.3 Building Multi-Area Trees..............................9 2.4 The RPF Check for Trees...............................10 2.5 Area Nickname Acquisition.............................11 2.6 Link state representation of areas....................11 3. Area Partition.........................................13 4. Multidestination Scope.................................14 5. Co-Existence with Old RBridges.........................15 6. Summary................................................16 7. Security Considerations................................16 8. IANA Considerations....................................16 9. References.............................................17 9.1 Normative References..................................17 9.2 Informative References................................17 R. Perlman, et al [Page 2] INTERNET-DRAFT RBridges: Multilevel TRILL 1. Introduction The IETF TRILL protocol [RFCtrill] [RFCadj] provides optimal pair- wise data frame forwarding without configuration, safe forwarding even during periods of temporary loops, and support for multipathing of both unicast and multicast traffic. TRILL accomplishes this by using [IS-IS] link state routing and encapsulating traffic using a header that includes a hop count. The design supports VLANs and optimization of the distribution of multi-destination frames based on VLANs and IP derived multicast groups. Devices that implement TRILL are called RBridges. Familiarity with [RFCtrill] is assumed in this document. 1.1 TRILL Scalability Issues There are multiple issues that might limit the scalability of a TRILL-based network: o the routing computation load, o the volatility of the LSP database creating too much control traffic, o the volatility of the LSP database causing the TRILL network to be in an unconverged state too much of the time, o the size of the LSP database, o the size of the end node learning table (the table that remembers (egress RBridge, VLAN/MAC) pairs), o the traffic due to upper layer protocols use of broadcast and multicast, and o the hard limit of the number of RBridges, due to the 16-bit nickname space. Extending TRILL IS-IS to be multilevel (hierarchical) helps with some of these issues. IS-IS was designed to be multilevel [IS-IS] [RFC1195] be partitioned into "areas". Routing within an area is known as "level 1 routing". Routing between areas is known as "level 2 routing". The level 2 IS- IS network consists of level 2 routers and links between the level 2 routers. Level 2 routers may participate in one or more areas, in addition to their role as level 2 routers. Each area is connected to the level 2 area through one or more "border routers", which participate both as a router inside the area, and as a router inside the level 2 "area". R. Perlman, et al [Page 3] INTERNET-DRAFT RBridges: Multilevel TRILL 1.2 Improvements Due to Multilevel Partitioning the network into areas reduces the size of the LSP database in each router, and stops volatility of the topology in one area from disrupting other areas. Allowing TRILL to utilize IS-IS's hierarchy solves the first 4 issues above, but does not necessarily help the other 3 issues (size of end node learning table, traffic due to upper layer protocols using multicast, hard limit of 16-bit RBridge nicknames). We propose two variants of hierarchical or multilevel TRILL. One we call the "unique nickname" variant. The other we call the "aggregated nickname" variant. In the aggregated nickname variant, border RBridges replace either the ingress or egress nickname field in the TRILL header of unicaat packets with an aggregated nickname representing an entire area. The aggregated nickname variant has the following advantages: o it solves the 16-bit RBridge nickname limit, o it lessens the amount of inter-area routing information that must be passed in IS-IS, o it greatly reduces the RPF information (since only the area nickname needs to appear, rather than all the ingress RBridges in that area), and o it enables computation of trees such that the portion computed within a given area is rooted within that area. The unique nickname variant has the advantage that border RBridges do not need to do end node learning for end nodes in their own area. 1.3 More on Areas Each area is configured with an "area address", which is advertised in IS-IS messages, so as to avoid accidentally interconnecting areas. Note that although the area address had other purposes in CLNP, (IS- IS was originally designed for CLNP/DECnet), for TRILL the only purpose of the area address would be to avoid accidentally interconnecting areas. Currently, the TRILL specification says that the area address "must be zero". If we change the specification so that the area address value of zero is a default, then most of IS-IS multilevel machinery works as originally designed. However, there are some TRILL-specific issues, which we address below in this document. R. Perlman, et al [Page 4] INTERNET-DRAFT RBridges: Multilevel TRILL 1.4 Terminology and Acronyms This document uses the acronyms defined in [RFCtrill] and the following additional acronym: DBRB - Designated Border RBridge R. Perlman, et al [Page 5] INTERNET-DRAFT RBridges: Multilevel TRILL 2. Multilevel TRILL Issues The TRILL-specific issues introduced by hierarchy include the following: a) configuration of non-zero area addresses, encoding them in IS-IS PDUs, and interworking with old RBridges that do not understand nonzero area addresses, b) nickname management, c) advertisement of filtering information (VLAN reachability, IP multicast addresses) across areas, d) computation of trees across areas for multi-destination frames, e) computation of RPF information for those trees, and g) compatibility, as much as practical, with existing, unmodified RBridges. The most important form of compatibility is with existing TRILL fast path hardware. Changes that require upgrade to the slow path firmware/software are more tolerable. Filtering information is only an optimization, as long as multidestination frames are not prematurely filtered. Thus, for instance, border RBridges could advertise they can reach all possible VLANs, and have an IP multicast router attached. This would cause multidestination traffic to be transmitted to the border router, and possibly filtered there, when the traffic could have been filtered earlier based on VLAN or multicast group. 2.1 Non-zero Area Addresses The current TRILL base protocol specification [RFCtrill] says that the area address in IS-IS MUST be zero. The purpose of the area address is to ensure that different areas are not accidentally hooked together. Furthermore, zero is an invalid area address for layer 3 IS-IS, so it was chosen as an additional safety mechanism to ensure that layer 3 IS-IS would not be confused with TRILL IS-IS. However, TRILL uses a different multicast address and Ethertype to avoid such confusion, so it is not necessary to worry about this. Since current TRILL RBridges will reject any IS-IS messages with nonzero area addresses, the choices are; all RBridges must be upgraded, neighbors of old RBridges must remove the area address from IS-IS messages when talking to an old RBridge (which might cause inadvertent merging of areas), to ignore the problem of accidentally merging areas entirely, or to keep the fixed "area address" field as 0 in TRILL, and add a new, optional TLV for "area name" that, if present, could be compared, by new RBridges, to prevent accidental merging R. Perlman, et al [Page 6] INTERNET-DRAFT RBridges: Multilevel TRILL 2.2 Aggregated versus Unique Nicknames In the unique nickname variant, all nicknames across the campus must be unique. In the aggregated nickname variant, RBridge nicknames are only of local significance within an area, and the only nickname externally (outside the area) visible is the "area nickname", which aggregates all the internal nicknames. The aggregated nickname approach eliminates the potential problem of nickname exhaustion, minimizes the amount of nickname information that would need to be forwarded between areas, minimizes the size of the forwarding table, and simplifies RPF calculation and RPF information. With unique cross-area nicknames, it would be intractable to have a flat nickname space with RBridges in different areas contending for the same nicknames. Instead, each area would need to be configured with a block of nicknames. Either some RBridges would need to announce that all the nicknames other than that block are taken (to prevent the RBridges inside the area from choosing nicknames outside the area's nickname block), or a new TLV would be needed to announce the allowable nicknames, and all RBridges in the area would need to understand that new TLV. Currently the encoding of nickname information in TLVs does not allow any aggregation. The information could be encoded as ranges of nicknames to make this somewhat manageable; however, a new TLV for announcing nickname ranges would not be intelligible to old RBridges. In contrast, the aggregated nickname approach enables passing far less nickname information and works as follows: Each area would be assigned a 16-bit nickname. This would not be the nickname of any actual RBridge. Instead, it would be the nickname of the area itself. Border RBridges would know the area nickname for their own area(s). In the following picture, R2 and R3 are area border RBridges. A source S is attached to R1. The two areas have nicknames 15961 and 15918, respectively. R1 has a nickname, say 27, and R4 has a nickname, say 44 (and in fact, they could even have the same nickname, since the RBridge nickname will not be visible outside the area). R. Perlman, et al [Page 7] INTERNET-DRAFT RBridges: Multilevel TRILL Area 15961 level 2 Area 15918 +-------------------+ +-----------------+ +-------------+ | | | | | | | S--R1---Rx--Rz-----R2----Rb---Rc--Rd---Re--R3---Rk--R4---D | | 27 | | | | 44 | | | | | | | +-------------------+ +-----------------+ +-------------+ Let's say that S transmits a packet to destination D, and let's say that D's location is learned by the relevant RBridges already. The relevant RBridges have learned the following: 1) R1 has learned that D is connected to nickname 15918 2) R3 has learned that D is attached to nickname 44. The following sequence of events will occur: - S transmits an Ethernet packet with source MAC = S and destination MAC = D. - R1 encapsulates with a TRILL header with ingress RBridge = 27, and egress = 15918. - R2 has announced in the level 1 IS-IS instance in area 16961, that it is attached to all the area nicknames, including 15918. Therefore, IS-IS routes the packet to R2. (Alternatively, if a distinguished range of nicknames is used for area, Level 1 RBridges seeing such an egress nickname will know to route to the nearest border router.) - R2, when transitioning the packet from level 1 to level 2, replaces the ingress RBridge nickname with the area nickname, so replaces 27 with 15961. Within level 2, the ingress RBridge field in the TRILL header will therefore be 15961, and the egress RBridge field will be 15918. Also R2 learns that S is attached to nickname 27 in area 15961. - The packet is forwarded through level 2, to R3, which has advertised, in Level 2, reachability to the nickname 15918. - R3, when forwarding into area 15918, replaces the egress nickname in the TRILL header with R4's nickname (44). So, within the destination area, the ingress nickname will be 15961 and the egress nickname will be 44. - R4, when decapsulating, learns that S is attached to nickname 15961. Now suppose that D's location has not been learned by R1 and/or R3. What will happen, as it would in TRILL today, is that R1 will forward R. Perlman, et al [Page 8] INTERNET-DRAFT RBridges: Multilevel TRILL the packet as a multidestination frame, choosing a tree. As the multidestination frame transitions into level 2, R2 replaces the ingress nickname with the area nickname. Now suppose that R1 has learned the location of D (attached to nickname 15918), but R3 does not know where D is. In that case, R3 will turn the packet into a multidestination frame within the area. Care must be taken so that, in case R3 is not the Designated transitioner for that multidestination frame, but was on the unicast path, that another RBridge within that area not forward the now multidestination frame back into level 2. Therefore, it would be desirable to have a marking, somehow, that indicates the scope of this packet to be "only this area". There is an issue with tree nicknames that would be a problem with the unique nickname variant, but is solved with the aggregated variant, as follows: Suppose nicknames were unique within the TRILL campus, and that the TRILL header was not rewritten by the border RBridges. In that case, there would have to be globally known nicknames for the trees. Suppose there are k trees. For all of the trees with nicknames located outside an area, the trees would all be rooted at (one of) the border RBridge(s). Therefore, there would be no path splitting of multidestination with the area. In contrast, with the aggregated nickname solution, each border RBridge can have a mapping from the level 2 tree nickname to the level 1 tree nickname. There need not even be agreement about the total number of trees; just that the border RBridge have some mapping, and replace the egress RBridge nickname (the tree name) when transitioning levels. Care must be taken that it be clear, when transitioning between level 2 and area X, which (single) border RBridge will transition the packet between the levels. 2.3 Building Multi-Area Trees It is easy to build a multi-area tree by building a tree in each area separately, (including the level 2 "area"), and then having only a single border RBridge, say R1, in each area, attach to the level 2 area. R1 would forward all multidestination packets between that area and level 2. People might find this unacceptable, however, because of the desire to path split (not always sending all multidestination traffic through the same border RBridge). R. Perlman, et al [Page 9] INTERNET-DRAFT RBridges: Multilevel TRILL Having multiple border RBridges introduces some complexity: a) calculating the RPF check when a multidestination frame originates outside the area (which border RBridge injected the frame into the area?) b) calculating the filtering information (which border RBridge will transition the frame into level 2?) This might be solvable if all RBridges are multilevel aware, however it is difficult to imagine how to ensure that old RBridges would calculate RPF and filtering information sensibly. Ignoring old RBridges for now, various possible solutions are a) elect one border RBridge for transitioning all multidestination frames between levels (call that the Designated Border RBridge (DBRB)) b) allow the DBRB to appoint other border RBs to forward some subset of the inter-level frames. (as the DRB does, on a per-VLAN basis, on a link). Make the appointment information visible to the other RBridges in the area so that they can calculate their RPF and filtering information. If b), then on what basis would the appointment be made? Various possibilities are as follows: o based on VLAN o based on tree root o based on ingress RBridge nickname The more flexibility that is allowed, the more complex announcement of information becomes, and the more complex the tree database becomes. If appointment is made based on VLAN, then the RPF check would need to be based on (tree, VLAN, ingress nickname), rather than simply (tree, ingress nickname) as it is today. 2.4 The RPF Check for Trees For multidestination frames originating in R1's area, computation of the RPF check is done as today. For multidestination frames originating outside R1's area, computation of the RPF check must be done based on one of the border RBridges (say R1, R2, or R3). An RBridge, say R4, located inside an area, must be able to know which of R1, R2, or R3 transitioned the frame into the area from level 2. (or into level 2 from an area). R. Perlman, et al [Page 10] INTERNET-DRAFT RBridges: Multilevel TRILL This could be done based on having the DBRB announce the assignments to all the RBs in the area. 2.5 Area Nickname Acquisition In the aggregated nickname variant, each area must acquire a unique area nickname. It is probably simpler to allocate a block of nicknames (say, the top 2000) to be area addresses, and not used by any RBridges. The area nicknames need to be advertised and acquired through level 2. Within an area, all the border RBridges must discover each other through the level 1 IS-IS database, by advertising, in their LSP "I am a border RBridge". Of the border RBridges, one will have highest priority (say R7). It will be R7 that dynamically participates, in level 2, to acquire a nickname for the area. R7 will give the area a pseudonode name, such as R7.5, within level 2. So an area will appear, in level 2, as a pseudonode. The pseudonode will participate, in level 2, in acquiring a nickname for the area. Within level 2, all the border RBridges [for the area] advertise reachability to the pseudonode, which will mean connectivity to the area nickname. 2.6 Link state representation of areas Within an area, say area A, there is an election for the DBRB, (Designated Border RB), say R1. This will be done through LSPs within area A. The border RBs announce themselves, together with DBRB priority. (Note that the election of the DBRB cannot be done based on Hello messages, because the border RBs are not necessarily physical neighbors of each other. They can, however, reach each other through connectivity within the area, which is why it will work to find each other through level 1 LSPs.) R1 acquires the area nickname (in the aggregated nickname approach), gives the area a pseudonode name (just like the DRB would give a pseudonode name to a link). R1 advertises, in area A, what the pseudonode name for the area is (and the area nickname that R1 has acquired). R. Perlman, et al [Page 11] INTERNET-DRAFT RBridges: Multilevel TRILL The pseudonode LSP initiated by R1 includes any information extraneous to area A that should be input into area A (such as area nicknames of external areas, or perhaps (in the unique nickname variant), all the nicknames of external RBs in the TRILL campus and filtering information such as IP multicast groups and VLANs). All the other border RBs for the area announce (in their LSP) attachment to that pseudonode. Within level 2, R1 generates a level 2 LSP on behalf of the area, also represented as a pseudonode. The same pseudonode name could be used within level 1 and level 2, for the area. (There does not seem any reason why it would be useful for it to be different, but there's also no reason why it would need to be the same). Likewise, all the area A border RBs would announce, in their level 2 LSPs, connection to the pseudonode. R. Perlman, et al [Page 12] INTERNET-DRAFT RBridges: Multilevel TRILL 3. Area Partition It is possible for an area to become partitioned, so that there is still a path from one section of the area to the other, but that path is via the level 2 area. An area will naturally break into two areas in this case. An area address might be configured to ensure two areas are not inadvertently connected. That area address appears in Hellos and LSPs within the area. If two chunks, connected only via level 2, were configured with the same area address, this would not cause any problems. (They would just operate as separate level 1 areas.) A more serious problem occurs if the level 2 area is partitioned in such a way that it healed by using a path through a level 1 area. TRILL will not attempt to solve this problem. Within the level 1 area, a single border RBridge will be the DBRB, and will be in charge of deciding which (single) RBridge will transition any particular multidestination frames between that area and level 2. If the level 2 area is partitioned, this will result in multidestination frames only reaching the portion of the TRILL campus reachable through the partition attached to the RBridge that transitions that frame. It will not cause a loop. R. Perlman, et al [Page 13] INTERNET-DRAFT RBridges: Multilevel TRILL 4. Multidestination Scope It would be desirable to be able to mark a multidestination frame with a scope that indicates this packet should not exit the area. This is particularly true when, in the aggregated nickname variant, a unicast packet turns into a multidestination packet. This could be done by having two tree nicknames, for each tree; one being the tree "only for this area", and the other being for multi- area trees. Alternatively, a packet intended only for the area could be tunneled (within the area) to the RBridge Rx, that is the appointed transitioner for that form of packet (say, based on VLAN), with instructions that Rx only transmit the packet within the area, and Rx could initiate the multidestination frame within the area. Since Rx introduced the frame, and is the only one allowed to transition that frame within levels, this would accomplish scoping of the packet to within the area. Since this case would only occur when unicast frames need to be turned into multidestination (because the border RBridge in the destination area does not know the location of the destination), the suboptimality of tunneling between the border RBridge that receives the unicast frame and the appointed level transitioner for that frame, would not be an issue. R. Perlman, et al [Page 14] INTERNET-DRAFT RBridges: Multilevel TRILL 5. Co-Existence with Old RBridges RBridges that are not multilevel aware have a problem with calculating RPF check and filtering information, since they would not be aware of assignment of border RBridge transitioning. A possible solution, as long as any old RBridges exist within an area, is to have the border RBridges elect a single DBRB (Designated Border RBridge), and have all inter-area traffic go through the DBRB (unicast as well as multidestination). If that DBRB goes down, a new one will be elected, but at any one time, all inter-area traffic (unicast as well as multidestination) would go through that one DRBR. R. Perlman, et al [Page 15] INTERNET-DRAFT RBridges: Multilevel TRILL 6. Summary This draft outlines the issues and possible approaches to multilevel TRILL. The variant involving area nicknames for aggregation has significant advantages in terms of scalability; not just of avoiding nickname exhaustion, but allowing, for instance, RPF checks to be aggregated based on an entire area. Some issues are not difficult, such as dealing with partitioned areas. Some issues are more difficult, especially dealing with old RBridges. 7. Security Considerations TBD 8. IANA Considerations This document requires no IANA actions. RFC Editor: Please delete this section before publication. R. Perlman, et al [Page 16] INTERNET-DRAFT RBridges: Multilevel TRILL 9. References Normative and Informational references for this document are listed below. 9.1 Normative References [IS-IS] - ISO/IEC 10589:2002, Second Edition, "Intermediate System to Intermediate System Intra-Domain Routing Exchange Protocol for use in Conjunction with the Protocol for Providing the Connectionless-mode Network Service (ISO 8473)", 2002. [RFC1195] - Callon, R., "Use of OSI IS-IS for routing in TCP/IP and dual environments", RFC 1195, December 1990. [RFCtrill] - Perlman, R., D. Eastlake, D. Dutt, S. Gai, and A. Ghanwani, "RBridges: Base Protocol Specification", draft-ietf- trill-rbridge-protocol-16.txt, in RFC Editor's queue. [RFCadj] - Eastlake, D., R. Perlman, A. Ghanwani, D. Dutt, V. Manral, "RBridges: Adjacency", draft-ietf-trill-adj, work in progress. 9.2 Informative References None. R. Perlman, et al [Page 17] INTERNET-DRAFT RBridges: Multilevel TRILL Authors' Addresses Radia Perlman Intel Labs 2200 Mission College Blvd. Santa Clara, CA 95054-1549 USA Phone: +1-408-765-8080 Email: Radia@alum.mit.edu Donald Eastlake Huawei Technologies 155 Beaver Street Milford, MA 01757 USA Phone: +1-508-333-2270 Email: d3e3e3@gmail.com Anoop Ghanwani Brocade Communications Systems 130 Holger Way San Jose, CA 95134 USA Phone: +1-408-333-7149 Email: anoop@brocade.com R. Perlman, et al [Page 18] INTERNET-DRAFT RBridges: Multilevel TRILL Copyright and IPR Provisions Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the BSD License. The definitive version of an IETF Document is that published by, or under the auspices of, the IETF. 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