TRILL Working Group Radia Perlman INTERNET-DRAFT Sun Silvano Gai Nuova Systems Dinesh G. Dutt Cisco Expires: July 2007 January 2007 Rbridges: Base Protocol Specification --------- ---- -------- ------------- Status of This Document By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of 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 Abstract RBridges provide the ability to have an entire campus, with multiple physical links, look to IP like a single subnet. The design allows for zero configuration of switches within a campus, optimal pair-wise routing, safe forwarding even during periods of temporary loops, and the ability to cut down on ARP/ND traffic. The design also supports VLANs, and allows forwarding tables to be based on RBridge destinations (rather than endnode destinations), which allows internal routing tables to be substantially smaller than in conventional bridge systems. Radia Perlman et al. [Page 1] INTERNET-DRAFT RBridge Protocol Acknowledgements 'Many people have contributed to this design, including the working group co-chairs Donald Eastlake 3rd and Erik Nordmark, and many other members of the working group such as, in alphabetic order, Alia Atlas, Stewart Bryant, Dino Farinacci, Don Fedyk, Eric Gray, Sanjay Sane, and Joe Touch. We invite you to join the mailing list at http://www.postel.org/rbridge. Table of Contents Status of This Document....................................1 Abstract...................................................1 Acknowledgements...........................................2 1. Introduction............................................3 2. Detailed Rbridge Design.................................7 2.1. Link State Protocol...................................7 2.1.1. Separate Instances..................................7 2.1.2. Multiple Rbridge IS-IS Instances....................8 2.2. Distribution Tree Calculation.........................9 2.3. Pruning the Ingress Rbridge Tree.....................10 2.4. Designated Rbridge...................................12 2.5. Wiring Closet Topology...............................13 2.6. Learning Endnode Location............................14 2.7. Forwarding Behavior..................................14 2.7.1. Receipt of a Native Packet.........................14 2.7.2. Receipt of an In-transit Packet....................15 2.7.2.1. Flooded Packet...................................15 2.7.2.2. Unicast Packet...................................16 2.7.2.3. IS-IS Packet.....................................16 2.8. IGMP Learning........................................16 2.9. RBridge Nicknames....................................16 2.10. Forwarding Header on 802 Links......................17 2.11. Handling ARP/ND Queries.............................18 2.12. Discovering IP Multicast Routers....................20 2.13. Assuring Freshness of Endnode Information...........20 3. Rbridge Addresses, Parameters, and Constants...........21 4. Security Considerations................................22 5. IANA Considerations....................................22 6. Conclusions............................................22 7. References.............................................23 7.1. Normative References.................................23 7.2. Informative References...............................23 Disclaimer................................................24 Additional IPR Provisions.................................24 Author's Address..........................................25 Expiration and File Name..................................25 Radia Perlman et al. [Page 2] INTERNET-DRAFT RBridge Protocol 1. Introduction In traditional IPv4 and IPv6 networks, each subnet must have a unique prefix. This means that a node that moves from one subnet to another must change its IP address, and a node in multiple subnets must have multiple addresses. It also means that a company with many subnets (separated by routers) will have difficulty making full use of its IP address block (since any subnet not fully populated will waste addresses), and IP routers require significant configuration. Bridges avoid these problems because bridges can transparently glue many physical links into what appears to IP to be a single LAN. However, bridge forwarding via the spanning tree using the layer 2 header has some disadvantages: o The spanning tree limits which links can be used, and therefore concentrates traffic onto selected links o Forwarding based on a header without a TTL is dangerous, because temporary loops might arise due to topology changes, lost spanning tree messages, or components such as repeaters coming up o Forwarding cannot be pair-wise shortest paths, but instead whatever path remains after the spanning tree eliminates redundant paths We define the term "campus" to be the set of links connected by any combination of RBridges and bridges. A campus appears to IP nodes to be a single subnet. This document presents the design for RBridges (routing bridges), which combines the advantages of bridges and routers [RBridges]. Like bridges, RBridges are zero configuration, and are transparent to IP nodes. Like routers, RBridges forward on pair-wise shortest paths, and do not have dangerous behavior during temporary loops. RBridges have the additional advantage that they can optimize ARP (IPv4) [RFC826] and ND (IPv6) [RFC2461] by avoiding the broadcast / multicast behavior of the queries. RBridges are fully compatible with current bridges as well as current IPv4 and IPv6 routers and endnodes. They are as invisible to current IP routers as bridges are, and like routers, they terminate a bridged spanning tree. The main idea is to have RBridges run a link state protocol amongst themselves. This enables them to have enough information to compute pairwise optimal paths for unicast, and to calculate distribution trees for delivery of packets to unknown destinations, or multicast / broadcast packets. RBridges must learn the location of endnodes. They learn the location Radia Perlman et al. [Page 3] INTERNET-DRAFT RBridge Protocol and layer 2 addresses of attached nodes from the source address of native frames, as bridges do. Additionally, in order to facility proxy ARP or proxy ND optimizations, RBridges also learn the (layer 3, layer 2) addresses of attached IP nodes from ARP or ND replies. Once an RBridge learns the location of a directly attached endnode, it informs the other RBridges in its link state information. RBridge forwarding can be done, as with a router, via pairwise shortest paths. To mitigate the temporary loop issues with bridges, RBridges must always forward based on a header with a hop count. Although the hop count will quickly discard looping frames, it is also desirable not to spawn additional copies of frames. This can be accomplished by having RBridges specify the next RBridge recipient while forwarding across a shared-media link. Frames must be encapsulated as they travel between RBridges for several reasons: 1. to prevent source MAC learning from frames in transit 2. to direct frames towards the egress RBridge. This enables forwarding tables of RBridges to be sized with the number of RBridges rather than the total number of nodes in the common broadcast domain 3. to include a hop count for frames in transit (for links, like Ethernet, that do not already contain a hop count) In order to coexist with Ethernet bridges [802.1D] on Ethernet links, frames in transit on Ethernet links must be encapsulated with an Ethernet header. The outer header of an RBridge-forwarded frame must look, to an Ethernet bridge on the path between two RBridges, like the header of a normal frame that the bridge will forward. To enable RBridges to distinguish encapsulated frames, a new Ethertype (to be assigned) will be used in the outer header. Inside that header is a shim header that RBridges will add to the frame that will contain: o the ingress-RBridge and a second RBridge which is either the root RBridge of a distribution tree (in the case of a broadcast / multicast / unknown destination frame) or the egress-RBridge (in the case of a unicast frame to a known destination) o a hop count The simplest encoding of two RBridge IDs in the shim header would be Radia Perlman et al. [Page 4] INTERNET-DRAFT RBridge Protocol two 6-byte MAC addresses. However, to achieve a more compact encoding, RBridges piggyback a nickname acquisition protocol on the link state protocol, to acquire unique (within the campus) 2-byte nicknames, and the nickname specifies the RBridge in the shim header. Inside the shim header is the original frame, as injected into the campus. RBridges must also support VLANs. A VLAN is a way that has been used within layer 2 to partition endnodes into different communities [802.1Q]. The usual method of determining which community a frame belongs to is based on the port from which it is received. The first bridge inserts a VLAN tag, based on its port configuration, and the last bridge removes the VLAN tag. However, sometimes the VLAN tag might be inserted or deleted by an endnode on a link (where "endnode" is a source or sink of traffic on the bridged LAN). RBridges may be configured with VLAN membership per port, just like bridges are. And they will also enforce that a frame originating on a particular VLAN only gets delivered to other links in the same VLAN. A side-effect of VLANs is that it makes RBridges more scalable, since endnode membership in a VLAN is only of interest to RBridges that have an attached port configured to be in that VLAN. This means that endnode membership in VLAN A only needs to be announced to RBridges attached to a link in VLAN A. There are several types of frames which RBridges must deliver, and each is handled slightly differently: 1. frames for known unicast destinations 2. frames for unknown unicast destinations 3. frames for layer 2 multicast addresses derived from IP multicast addresses 4. frames for layer 2 broadcast / multicast frames which are not derived from IP multicast addresses 5. ARP/ND queries 6. IGMP membership reports If a frame belongs in a particular VLAN, the frame must be delivered only to links in that VLAN. RBridges will calculate a distribution tree for each potential root Radia Perlman et al. [Page 5] INTERNET-DRAFT RBridge Protocol RBridge Ri, which we will refer to as the "Ri-tree". In theory, RBridges could have calculated a single bidirectional distribution tree for the entire campus. However, it was decided that the additional computation necessary to compute additional RBridge trees was warranted because: 1. using a tree rooted at the ingress RBridge optimizes the distribution path and (almost always) the cost of delivery when the number of destination links is a subset of the total number of links, as is the case with VLANs and IP multicasts 2. for unknown destinations, using a tree rooted at the ingress RBridge minimizes out-of-order delivery because in the case where a flow starts before the location of the destination is known by the RBridges, the path to the destination will be the same as the path directly to the destination RBridges will not use the bridge spanning tree algorithm to calculate trees. Instead, the trees are calculated based on the link state information, selecting a particular RBridge as the root, and with a deterministic tie-breaker so all RBridges calculate the same distribution tree based on the same root and same link state database. Therefore the tree calculation is done without requiring any additional exchange of information between RBridges. Instead of calculating a single shared tree, the other extreme is to calculate a separate tree for each ingress RBridge, and distribute multicast along the tree with the ingress tree as root (where VLAN- tagged traffic and IP multicast traffic can be pruned, but otherwise all multicast traffic with the same ingress travels on the same links). Two reasons why this solution might not be preferable: are: 1. In some cases, a different tradeoff might be wanted in terms of expense of computation vs. optimality of traffic distribution (so fewer trees would be desired) 2. It might be desirable to allow choosing a different distribution tree than the one rooted at the ingress RBridge, in order to allow multipathing of multicast traffic injected by a particular RBridge. For this reason, we allow an RBridge R1 to announce (via a flag in its link state announcement) whether RBridges should compute a tree rooted at R1. The default is yes. If R1 is a tree root, then any RBridge R2 can choose the R1-tree for distribution of multicast traffic that R2 is injecting into the campus. And in the shim header, RBridges can specify which (bidirectional) tree the multicast packet should travel along. Therefore, the default is tree calculation rooted at every RBridge, but configuration can cut down on the number of trees that must be calculated. Radia Perlman et al. [Page 6] INTERNET-DRAFT RBridge Protocol The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 2. Detailed Rbridge Design 2.1. Link State Protocol Running a link state protocol among RBridges is straightforward. It is the same as running a level 1 routing protocol in an area, with endnode addresses being layer 2 addresses rather than, say, IP addresses. IS-IS [ISO10589] is natural choice for a link state protocol because it is easy in IS-IS to define new TLVs for carrying new information, and because IS-IS can be done with zero configuration. All that is required to run IS-IS is for each RBridge to have a unique 6-byte system ID, which can be any of the RBridge's MAC addresses. 2.1.1. Separate Instances The instances of IS-IS that RBridges will implement is separate from any routing protocol that IP routers will implement, just as the spanning tree messages are not implemented by IP routers. To prevent potential confusion between an IS-IS instance being run by IP routers and the IS-IS instance being run by RBridges, RBridge IS- IS messages will be sent to a different layer 2 multicast address than layer 3 IS-IS routing messages. The RBridge IS-IS instance is also differentiated by defaulting to a distinct, constant "area address" (the value 0) that would never appear as a real IS-IS area address. RBridge IS-IS messages will be sent with the same Ethertype (in the outer header) as RBridge-encapsulated data packets. RBridge IS-IS messages will be differentiated from RBridge-encapsulated data packets because RBridges will use a different multicast address (in the outer header) for IS-IS messages than for encapsulated multicast data messages. Unicast RBridge-encapsulated packets are sent to a specific neighbor, so would not have a group address in the outer header. Radia Perlman et al. [Page 7] INTERNET-DRAFT RBridge Protocol 2.1.2. Multiple Rbridge IS-IS Instances There are two types of information that are carried in RBridge link state information; "core-RBridge information", and "endnode information". In theory this information could all be contained in one instance of RBridge IS-IS. However, since endnode information for a particular VLAN only needs to be known to RBridges that are connected to links configured to be in that VLAN, each RBridge R1 will run a "core" instance of IS-IS for the core RBridge information, and an instance per VLAN that R1 is attached to, for the endnode information for those VLANs. The core-RBridge information, which is carried in the core-RBridge instance, is: 1. the system IDs of RBridges which are neighbors of RBridge R1, and the cost of the link to each of those neighbors 2. the VLAN numbers of VLANs directly connected to R1 3. a Flag indicating whether RBridges should calculate a tree rooted at R1 (default = yes) 4. the nickname of Rbridge R1 Even if RBridge R2 is not connected to VLAN A, it is relevant to R2 that some other RBridge R1 is connected to VLAN A, even though R2 does not need to know which endnodes are in VLAN A. The reason for this is to allow R2 to filter multicast / unknown destination packets that are VLAN-tagged. If R2 is forwarding a multicast packet tagged with VLAN A, R2 need not forward it onto branches of the distribution tree that have no downstream VLAN A links. The endnode information for VLAN A, which is carried in the VLAN A IS-IS instance injected by R1, contains: 1. L2INFO: layer 2 addresses of nodes on a VLAN A link attached to R1 which have transmitted frames but have not transmitted ARP or ND replies (i.e., these are not known to be IP nodes) 2. L3and2INFO: layer 3, layer 2 addresses of IP nodes attached to R1, which R1 has learned through ARP/ND replies emitted by endnodes on an attached VLAN A link. For data compression, only the portion of the address following the campus-wide prefix need be carried. (This is a more important optimization for IPv6 than for IPv4) 3. Multicast Router attached: This is one bit of information that indicates whether there is an IP multicast router attached. This information is used because IGMP [RFC3376] Membership Reports must be transmitted to all links with IGMP routers, and not to links Radia Perlman et al. [Page 8] INTERNET-DRAFT RBridge Protocol without IGMP routers. Also, all packets for IP-derived multicast addresses must be transmitted to all links with IGMP routers (within the VLAN), in addition to links from which an IP node has explicitly asked to join the group which the packet is for. 4. Layer 2 addresses derived from IPv4 or IPv6 IGMP notification messages received from attached endnodes, indicating the location of listeners for these multicast addresses. If R1 has learned endnode E's location first from a data packet (and therefore has included E's layer 2 address in the L2INFO), and later E transmits an ARP/ND reply, R1 MUST include E in the L3andL2INFO, and MAY remove E from L2INFO. Given that RBridges must already support delivery only to links within a VLAN (for multicast or unknown frames marked with the VLAN's tag), the same mechanism is used by the per-VLAN instance of IS-IS to distribute endnode information solely to RBridges within a VLAN. The per-VLAN instance of IS-IS will appear to the RBridges to be running on a single link. R1 will originate a VLAN-A-specific IS-IS frame. All RBridges will recognize the frame as a VLAN A multicast frame (even if they are not connected to VLAN A), and prune the specified distribution tree so as to only deliver the frame along branches with VLAN A links. This is the same behavior core RBridges would have for any VLAN A multicast / broadcast / unknown destination frame. RBridges that are connected to VLAN A links will, in addition to forwarding along the specified distribution tree, process the frame in their VLAN-A IS-IS instance. Thus suppose that RBridges R1, R2, and R3 are all on VLAN A, on links scattered throughout the campus. The VLAN A IS-IS instance will appear to be a single link (broadcast domain) with R1, R2, and R3 as neighbors. The only information carried in the instance is the endnode information for VLAN A. The other RBridges on the campus facilitate delivery within the VLAN A broadcast domain, and therefore may be on the path between R1 and R2, but will treat the VLAN A instance link state frames as ordinary datagrams. The way that RBridges distinguish which IS-IS instance the link state information is for is based on the VLAN tag in the inner header. 2.2. Distribution Tree Calculation Some frames (e.g., to unknown destinations, or multicast destinations) will need to be delivered to multiple links. RBridges must calculate at least one tree, and the default is to calculate a tree for every RBridge. However, in order to avoid requiring the Radia Perlman et al. [Page 9] INTERNET-DRAFT RBridge Protocol RBridges in a campus from calculating as many trees, each RBridge MAY be configured to indicate that it should not be the root of a distribution tree. If all RBridges have their flags set to "don't calculate a tree rooted at me", then RBridges MUST calculate a tree rooted at the RBridge with lowest ID, regardless of that lowest-ID RBridge's flag setting, and that tree MUST be the one specified for all multicast/unknown frames. In IS-IS a shared link is modeled as a pseudonode, with a 7-byte ID consisting of a 6-byte ID owned by the Designated Router (DR), plus a nonzero byte assigned by the DR. The "I want to be a Root" flag is defaulted to "no" for pseudonodes. Calculation of a tree rooted at R1 is done by performing the SPF (shortest path first) calculation with R1 as the root, and with a deterministic tie-breaker, so that all RBridges calculate the same distribution tree. The tie-breaker is that if a node N can be attached to either parent P1 or P2 with the same minimal path cost from R1 to N, then choose P1 if P1's ID is lower than P2. The calculated tree is a bidirectional tree. Each RBridge R keeps a set of adjacencies (port, neighbor pair) selected for each distribution tree. So, for instance, for the distribution tree rooted at R1, R chooses the adjacency which connects R to its parent in that SPF tree, as well as any adjacencies that connect children to R. Once the adjacencies are chosen, it is irrelevant which ones are towards the root R1, and which are away from R1. So R might have calculated that adjacencies a, c, and f are in the tree. That means that if there is a multicast packet that indicates it should be transmitted on distribution tree R1, and it is received on any adjacency other than a, c, or f, R should discard the packet. If it is received on any of the selected adjacencies (a, c, or f), then R should forward onto the other two adjacencies. 2.3. Pruning the Ingress Rbridge Tree Packets which must be flooded (e.g., multicasts, unknown destinations), are flooded along the selected distribution tree rooted at the second RBridge specified in the shim header, and pruned based on whether there are potential receivers downstream of each of the branches. In the case of a VLAN-tagged packet, it is forwarded only on branches that have RBridges participating in that VLAN reachable via that branch. Further pruning is done in the case of IGMP Notification Messages [RFC3376], where these are to be delivered only to ports with IP Radia Perlman et al. [Page 10] INTERNET-DRAFT RBridge Protocol Multicast Routers. In the case of a multicast derived from an IP multicast, these multicast data packets are delivered only to links that have registered listeners, plus links which have IP Multicast routers. The actual tree to forward along is chosen based on the specified RBridge in the shim header, say R1. Say that RBridge R knows that adjacencies (a, c, and f) are in the R1-distribution tree. R marks pruning information for each of the adjacencies in the R1-tree. For each adjacency for each tree, R marks: o Set of VLANs reachable downstream, and for each one of those, a flag indicating whether there are IP multicast routers downstream, and the set of layer 2 multicast addresses derived from IP multicast group for which there are receivers downstream Forwarding for a non-unicast is done as follows, when RBridge R receives a non-unicast packet on one of its adjacencies, and the shim header indicates the selected tree is the R1-tree: - if the adjacency from which the packet was received is not one of the adjacencies in the R1-tree, the packet is dropped - if the inner packet is tagged as belonging to VLAN A, and the packet is unknown destination, or layer 2 multicast not derived from IP multicast, or distributed ARP/ND, the packet is forwarded onto an adjacency if and only if that adjacency is in the R1-tree, and marked as reaching VLAN-A links - if the packet is an IGMP Notification message, marked as VLAN-A, then the packet is forwarded onto adjacencies in the R1-tree that indicate there are downstream VLAN-A IP multicast routers - if the packet is a layer 2 multicast address derived from IP multicast groups, and marked as VLAN-A, then the packet is forward onto adjacencies in the R1-tree that indicate there are downstream VLAN-A IP multicast routers, as well as adjacencies that indicate there are downstream VLAN-A receives for that group address. For each link for which R is Designated RBridge, R additionally checks to see if it should decapsulate the frame and send it to the link (e.g., if it is a distributed ARP in the specified VLAN for that link), or process the packet (e.g., if it is a per-VLAN IS-IS instance link state announcement for a VLAN that R is attached to). Radia Perlman et al. [Page 11] INTERNET-DRAFT RBridge Protocol 2.4. Designated Rbridge One RBridge on each link needs to be elected to have special duties. This elected RBridge is known as the Designated RBridge. IS-IS already holds such an election. The Designated RBridge is the one on the link that will learn and advertise the identities of attached endnodes, encapsulate and forward frames that originate on that link to the rest of the campus, decapsulate and forward frames onto that link received from other RBridges, initiate a distributed ARP when an ARP query is received for an unknown destination, and answer ARP queries when the target node is known. It is dangerous to have multiple RBridges being Designated RBridge. This could temporarily happen if a partitioned bridged LAN were connected with a bridge or repeater. The situation will resolve once the better priority RBridge's IS-IS Hello is received by the other RBridges on the link. However, it is possible that some intervening bridges might be temporarily discarding the IS-IS Hello messages due to being in preforwarding state. The one message type that is not delayed due to preforwarding state is the spanning tree BPDU. If RBridges listen to BPDUs, and if the LANs for which R1 was DR, and for which R2 was DR get joined, then one or the other of R1 or R2 will note that the bridge Root has changed identity, let's say R2 notices. The conservative thing to do would be to invoke something like a preforwarding state, in which R2 stops forwarding anything to or from the link until it is sure the IS-IS link election would have completed. But the IS-IS election could get slowed down due to bridges in preforwarding state, and it would be undesirable to disrupt traffic to and from the link just because the root ID has changed. The solution is to have RBridges participate in the spanning tree election, with higher priority for becoming root (actually, lowest numerical priority value) than any of the bridges, and with the same priority as for becoming Designated RBridge on the link. Then an RBridge is Designated RBridge if and only if it is the spanning tree Root. Note that RBridges MUST NOT merge spanning trees from different ports. If two ports of R1 are connected to the same bridged LAN, then the regular bridge spanning tree algorithm will partition the LAN into distinct LANs for each of R1's ports. However, if two of R1's ports are connected to the same shared medium (without any bridges between the ports), then the regular bridge spanning tree algorithm will turn off one of R1's ports. Radia Perlman et al. [Page 12] INTERNET-DRAFT RBridge Protocol So for example, R1 will initiate BPDUs on each of its ports, with itself as Root (with highest, i.e., numerically lowest priority), 0 cost from Root, and the port ID. There are several possible cases: o R1 is the highest priority RBridge on the bridged LAN, in which case it will become spanning tree Root and Designated RBridge. o R1 receives a BPDU from itself (because two of its ports are on the same shared medium without any bridges between). In this case, the numerically lowest port will stay on, and the other port(s) will go into spanning tree backup state. o R1 receives a BPDU from someone else with higher priority (numerically lower priority|ID), in which case R1 is not Root, and not Designated RBridge. It is possible this is due to a bridge being configured with the lowest priority, and then if R1 declines being DR, the LAN becomes orphaned from the campus. We will treat this case as a misconfiguration of bridges, and the LAN will become orphaned until the misconfiguration is corrected, but an RBridge could in theory eventually discover it is not receiving any IS-IS Hellos, and become DR even though it is not spanning tree Root. 2.5. Wiring Closet Topology In the case where there are two (or more) groups of endnodes, each attached to a bridge (say B1 and B2 respectively), and each bridge is attached to an RBridge (say R1 and R2 respectively), with a link connecting B1 and B2, it may be desirable to have the B1-B2 link only as a backup in case one of R1 and R2, or the links B1-R1 or B2-R2 fail. Default behavior would be that one of R1 or R2 (say R1) would become Designated RBridge, and forward traffic to/from the link, so endnodes attached to B2 would be connected to the campus via the path B2-B1- R1, rather than the desired B2-R2. The solution is to configure R1 and R2 to be part of a "wiring closet group", with a configured ID Rx (which can be R1 or R2's ID). Both R1 and R2 participate in the bridge spanning tree on the configured ports as root Rx, which will cause the spanning tree to break the B1-B2 link as desired, and both R1 and R2 will act as Designated RBridge on each of their respective partitions. In the BPDU, Root will be "Rx", cost to Root will be 0, Designated Bridge ID will be "R1" when R1 transmits, and "R2" when R2 transmits, and port ID will be a value chosen independently by each of R1 and R2 to distinguish each of its own ports. If R1 and R2 were actually on Radia Perlman et al. [Page 13] INTERNET-DRAFT RBridge Protocol the same shared medium with no bridges between them, the result will be that the one with the larger ID will see "better" BPDUs (because of the tie-breaker on the third field; the ID of the transmitting RBridge), and will turn off the port. The only misconfiguration that can occur is if the link R1-R2 is on the cut set of the campus, and R2 and/or R1 have been configured to believe they are part of a wiring closet group. In that case, the link will become partitioned and the campus will become partitioned. 2.6. Learning Endnode Location RBridges learn endnode location from native frames. They learn (layer 3, layer 2) pairs (for the purpose of supporting ARP/ND optimization) from listening to ARP or ND replies. This endnode information is learned by the DR, and distributed to other RBridges through the link state protocol. 2.7. Forwarding Behavior 2.7.1. Receipt of a Native Packet R1 receives a native (i.e., not RBridge-encapsulated) unicast frame. R1 knows that this is a native frame because the Ethertype is not "RBridge encapsulated frame". The destination in the layer 2 header is D, the source is S. R1 inserts a VLAN tag if required, according to the same rules as bridges do. Once the VLAN (if any) is established, the layer 2 address of D is looked up in the destination table for that VLAN to find the egress RBridge R2, or discover that D is unknown. If D is known, with egress R2, then R1 encapsulates the packet, with R2 indicated in the shim header as egress RBridge and R1 as the ingress Rbridge. In the outer header, R1 puts "R1" as source, and next hop RBridge (in the path to R2) as "destination", and "encapsulated RBridge packet" as the Ethertype. If D is unknown, R1 encapsulates the packet, with "R1" indicated as ingress RBridge in the shim header, and outer header with source=R1, destination = "all-RBridges". The egress RBridge field indicates the Radia Perlman et al. [Page 14] INTERNET-DRAFT RBridge Protocol chosen distribution tree. The default is for R1 to put its own nickname there. However, R1 MAY be configured to select some other tree. If R1 is configured to decline to be a tree root, then R1 MUST select some other RBridge which has elected to be a tree root. If the frame is an IGMP announcement, then R1 learns the group membership, and announces a receiver for that layer 2 group address in its per-VLAN link state instance. If the frame is a PIM or MRD message, R1 learns that there is an IP multicast router (for the specified VLAN) on its link, and adds that information into its per- VLAN IS-IS link state information. If the frame is an ARP/ND reply, then R1 learns the (layer 3, layer 2) correspondence, and adds that information into its per-VLAN link state information. If the frame is an IS-IS packet, then there is no need for an inner header. The outer header will contain source=transmitting RBridge, destination="all RBridges" layer 2 multicast, and protocol type will equal "IS-IS". The shim header will indicate the nickname of the RBridge that initiated the packet (or 0, if that RBridge does not yet have a nickname), and the egress RBridge will indicate the specified tree along which to send this packet. There is no reason to have an inner Ethernet header, and for IS-IS, instead of an inner header, there will be only a 2-byte field, consisting of a flag indicating whether this is the core instance of IS-IS, and the VLAN tag, if this is not the core instance. If the frame is an IS-IS packet, then if it is not the core instance, it is forwarded like a non-IP-multicast flooded frame, as well as processed, if the RBridge belongs to the specified VLAN. If it is the core instance, then the IS-IS frame is never forwarded, and instead is processed. 2.7.2. Receipt of an In-transit Packet RBridge R1 receives an encapsulated frame (as indicated by Ethertype="Rbridge-encapsulated"). 2.7.2.1. Flooded Packet If the destination in the outer header is "all-RBridges", then R1 forwards along the tree indicated by the shim header, pruned as specified in section 2.3. Radia Perlman et al. [Page 15] INTERNET-DRAFT RBridge Protocol 2.7.2.2. Unicast Packet If the destination in the outer header is not R1, then R1 drops the frame. If the shim header indicates R1 is the egress RBridge, then R1 extracts the inner frame and forwards it onto the link containing the destination, or processes the packet if the destination in the inner frame is R1. Else, R1 looks up the egress RBridge R2 indicated in the shim header, in its forwarding table, and forwards the packet towards R2, by replacing the outer header with one with source=R1, destination=nexthop RBridge towards R2, and Ethertype "RBridge- encapsulated". 2.7.2.3. IS-IS Packet If the protocol type in the outer header indicates this is an IS-IS packet, then R1 processes the packet accordingly. 2.8. IGMP Learning RBridges learn, based on seeing IGMP [RFC3376] packets, which multicast addresses should be forwarded onto which links. IGMP messages have to be forwarded throughout the campus, since IP routers in the broadcast domain also need to see these messages. IGMP messages are forwarded by RBridges throughout the campus like any layer 2 multicast. They are recognized by having an IP message type=2 in the IP header. In addition, they are processed by RBridges in order to extract, from announcements, what egress RBridges have receivers for which groups. 2.9. RBridge Nicknames To make the shim header smaller, RBridges dynamically acquire 2-byte nicknames that are unique within the campus. The nickname allocation protocol is piggybacked on the core IS-IS RBridge instance as follows: We will assign a new type value to be carried in the IS-IS core instance LSPs. The TLV will carry the nickname the LSP source wishes Radia Perlman et al. [Page 16] INTERNET-DRAFT RBridge Protocol to use. Each RBridge chooses its own nickname. However, each RBridge is also responsible for ensuring that its nickname is unique. If R1 chooses nickname x, and R1 discovers, through receipt of R2's LSP, that R2 has also chosen x, then the RBridge with the lower system ID keeps the nickname, and the other one must choose a new nickname. If two RBridge domains merge, then there might be a lot of nickname collisions for a short time, but as soon as each side receives the link state packets of the other, the RBridges that need to change nicknames will quickly become aware of this, and choose new nicknames that do not, to the best of their ability, collide with any existing nicknames. To minimize the probability of nickname collisions, each RBridge chooses its nickname randomly from the set of assigned nicknames. Alternatively, we could use some sort of hash algorithm (such as the bottom 16 bits of the MD5 of the RBridge's system ID), to choose the first nickname, and then if there is a collision, go to the next 16 bits of the MD5, and so on, until all 128 bits of the MD5 hash are exhausted, in which case the RBridge hashes its own system ID again, this time together with the constant "1". There is no reason for all RBridges to use the same algorithm for choosing nicknames. Picking them at random, or using a hash, are an attempt to avoid collisions when the network starts up, but that is only an optimization. Even if all RBridges used the same algorithm, say as a worst case, they all start with "1" and count up sequentially until they find an uncontested nickname, the network will eventually stabilize. And once it is stable, nicknames should remain stable even as Rbridges go up or down. To minimize the probability of a new RBridge usurping a nickname already in use, an RBridge should wait to acquire the link state database from a neighbor before it announces its own nickname. 2.10. Forwarding Header on 802 Links It is essential that RBridges coexist with ordinary bridges. Therefore, a frame in transit must look to ordinary bridges like an ordinary layer 2 frame. However, it must also be differentiable from a native layer 2 frame by RBridges. To accomplish this, we use a new layer 2 protocol type ("Ethertype"). A frame in transit on an 802 link will therefore have two 802 headers, since the original frame (including the original 802 header) will be tunneled by the RBridges. But rather than just having an Radia Perlman et al. [Page 17] INTERNET-DRAFT RBridge Protocol additional 802 header, we include additional information between the two headers, with a header we refer to as the "shim header". An encapsulated frame would look as follows: +--------------+-------------+-----------------+ | outer header | shim header | original frame | +--------------+-------------+-----------------+ Figure 1 Encapsulated Frame The outer header contains: o L2 destination = next RBridge, or for flooded frames, a new (to be assigned) multicast layer 2 address meaning "all RBridges" o L2 source = transmitting RBridge (the one that most recently handled this frame) protocol type = "to be assigned...RBridge encapsulated frame" The 6-byte shim header includes: o TTL = starts at some value and decremented by each RBridge. Discarded if=0. This field uses 6 bits for TTL, and the remaining 10 bits are reserved. o ingress RBridge nickname. 16 bits o egress RBridge nickname (or selected distribution tree, in the case of broadcast / multicast). 16 bits 2.11. Handling ARP/ND Queries We will use the term "optimized ARP/ND response" to cover several possible behaviors an RBridge might utilize. Non-optimized behavior would consist of treating an ARP or ND query as an ordinary layer 2 broadcast/multicast, and send the query to all links in the campus, allowing the target to respond as to an ordinary ARP/ND query. This behavior is essential when the location of the target is unknown, although RBridges could suppress multiple queries to the same target within some amount of time. When the target's location is assumed to be known by the first RBridge, it need not flood the query. Alternative behaviors of the first Designated RBridge that receives the ARP/ND query would be to: 1. send a response directly to the querier, with the layer 2 address Radia Perlman et al. [Page 18] INTERNET-DRAFT RBridge Protocol of the target, as believed by the RBridge 2. encapsulate the ARP/ND query to the target's Designated RBridge, and have the Designated RBridge at the target forward the query to the target. This behavior has the advantage that a response to the query will be definitive. If the query does not reach the target, then the querier will not get a response 3. block ARP/ND queries that occur for some time after a query to the same target has been launched, and then respond to the querier when the response to the recently-launched query to that target is received The reason not to do the most optimized behavior all the time is for timeliness of detecting a stale cache. Also, in the case of SEND [RFC3971], cryptography might prevent behavior 1, since the RBridge would not be able to sign the response with the target's private key. It is not essential that all RBridges use the same strategy for which option to select for a particular query. However, once the first Designated RBridge decides on a strategy for a particular query, the other RBridges must carry that through. If the first RBridge responds directly to the querier, or blocks the query, then no other RBridges are involved. If the first Designated RBridge R1 decides to unicast the query to the target's Designated RBridge R2, then R2 must decapsulate the query, and initiate an ARP/ND query on the target's link. When/if the target responds, R2 must encapsulate and unicast the response to R1, which will decapsulate the response and send it to the querier. If the first Designated RBridge R1 decides to flood the query (which it MUST do if the target is unknown, but MAY do if it wants to assure freshness of the information), the query is encapsulated to be flooded through the indicated VLAN. The distributed ARP query is carried by RBridges through the RBridge distribution tree. Each Designated RBridge, in addition to forwarding the query through the distribution tree, initiates an ARP query on its link(s). If a reply is received from the target by Designated RBridge R2, R2 initiates a link state update to inform all the other RBridges of D's location, layer 3 address, and layer 2 address, in addition to forwarding the reply to the querier. It is the querier's Designated RBridge R1 that chooses which strategy to employ when seeing an ARP query. Some mix of these strategies (responding directly, unicasting the query to the target's Designated RBridge, or flooding the query) might be the best solution. For instance, even if the target's Radia Perlman et al. [Page 19] INTERNET-DRAFT RBridge Protocol location and (layer 3, layer 2) correspondence is in the link state information R1 received from R2, if the target's location has not been recently verified by R1 through a broadcast ARP/ND or unicast query to the target, then R1 MAY broadcast or unicast the query or respond directly. So for instance, RBridges could keep track of the last time a broadcast ARP/ND occurred for each endnode E (by any source, and injected by any RBridge). Let's say the parameter is 20 seconds. If a source S on RBridge R1's link does an ARP/ND for D, if R1 has not seen an ARP/ND for D within the last 20 seconds, R1 unicasts the query to force a reply from the target; otherwise it proxies the reply. When R2 forwards a unicast ARP/ND query, if the target does not respond, then R2 MAY replay the query, and if the target does not respond, R2 will remove the target from its link state information. 2.12. Discovering IP Multicast Routers Until Multicast Router Discovery [RFC4286] is universally deployed, RBridges must discover IP multicast routers because they transmit PIM messages. So an RBridge concludes there is an IP multicast router on its port if it either receives an MRD message, or a PIM message on that port. A PIM message is recognized because the protocol type in the IP header is decimal 103. 2.13. Assuring Freshness of Endnode Information Designated RBridge R1 can ensure freshness of its endnode information by doing ARP/ND queries periodically to ensure that the endnodes are actually there. This can be a problem if the endnodes are in power- saver mode, and this should be a configuration parameter on R1 as to whether R1 should "ping" the endnodes by doing ARP/ND queries. Radia Perlman et al. [Page 20] INTERNET-DRAFT RBridge Protocol 3. Rbridge Addresses, Parameters, and Constants Each RBridge needs a unique ID within the campus. The simplest such address is a unique 6-byte ID, since such an ID is easily obtainable as any of the EUI-48's owned by that RBridge. IS-IS already requires each router to have such an address. A parameter is the value to which to initially set the hop count in the envelope. Recommended default=20. A new Ethertype must be assigned to indicate an RBridge-encapsulated frame. A layer 2 multicast address for "all RBridges" must be assigned for use as the destination address in flooded frames. To support VLANs, RBridges (like bridges today), must be configured, for each port, with the VLAN in which that port belongs. We may want a parameter to determine whether an RBridge should periodically do queries to ensure that the endnode information is fresh, and if so, with what frequency. A parameter indicates whether an RBridge wants to be the root of a distribution tree. Configuration for wiring closet topology consists of system ID of the RBridge with lowest ID. If R1 and R2 are part of a wiring closet topology, only R2 needs to be configured to know about this, and that R1 is the ID it should use in the spanning tree protocol on the specified port. Radia Perlman et al. [Page 21] INTERNET-DRAFT RBridge Protocol 4. Security Considerations The goal is for RBridges to not add additional security issues over what would be present with traditional bridges. RBridges will not be able to prevent nodes from impersonating other nodes, for instance, by issuing bogus ARP replies. However, RBridges will not interfere with any schemes that would secure neighbor discovery. As with routing schemes, authentication of RBridge messages would be a simple addition to the design (and it would be accomplished the same way as it would be in IS-IS). However, any sort of authentication requires additional configuration, which might interfere with the perception that RBridges, like bridges, are zero configuration. 5. IANA Considerations A new Ethertype must be assigned to indicate an RBridge-encapsulated frame. A layer 2 multicast address for "all RBridges" must be assigned for use as the destination address in flooded frames. 6. Conclusions This design allows transparent interconnection of multiple links into a single IP subnet. Management would be just like with bridges (plug-and-play). But this design avoids the disadvantages of bridges. Temporary loops are not a problem so failover can be as fast as possible, and shortest paths can be followed. The design is compatible with current IP nodes and routers, and with current bridges. Radia Perlman et al. [Page 22] INTERNET-DRAFT RBridge Protocol 7. References 7.1. Normative References [802.1D] "IEEE Standard for Local and metropolitan area networks / Media Access Control (MAC) Bridges", 802.1D-2004, 9 June 2004. [802.1Q] "IEEE Standard for Local and metropolitan area networks / Virtual Bridged Local Area Networks", 802.1Q-2005, 19 May 2006. [ISO10589] ISO/IEC 10589:2002, "Intermediate system to Intermediate system routeing information exchange protocol for use in conjunction with the Protocol for providing the Connectionless-mode Network Service (ISO 8473)," ISO/IEC 10589:2002. [RFC826] Plummer, D., "An Ethernet Address Resolution Protocol", RFC 826, November 1982. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2461] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for IP Version 6 (IPv6)", RFC 2461 (Standards Track), December 1998. [RFC3376] Cain, B., "Internet Group Management Protocol, Version 3", RFC 3376, October 2002. [RFC4286] Haberman, B., Martin, J., "Multicast Router Discovery", RFC 4286, December 2005. [SNOOP] Christensen, M., Kimball, K, Solensky, F., "Considerations for IGMP and MLD Snooping Switches", draft-ietf-magma-snoop-12.txt 7.2. Informative References [Arch] Gray, E., "The Architecture of an RBridge Solution to TRILL", draft-ietf-trill-rbridge-arch-01.txt, October 2006, work in progress. [PAS} Touch, J., & R. Perlman "Transparent Interconnection of Lots of Links (TRILL) / Problem and Applicability Statement", draft-ietf- trill-prob-01.txt, Octover 2006, work in progress. [RBridges] Perlman, R., "RBridges: Transparent Routing", Proc. Infocom 2005, March 2004. [RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure Neighbor Discovery (SEND)", RFC 3971, March 2005. Radia Perlman et al. [Page 23] INTERNET-DRAFT RBridge Protocol [RP1999] Perlman, R., "Interconnection: Bridges, Routers, Switches, and Internetworking Protocols", Addison Wesley Chapter 3, 1999. Disclaimer This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Additional IPR Provisions The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf- ipr@ietf.org. Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Radia Perlman et al. [Page 24] INTERNET-DRAFT RBridge Protocol Author's Address Author's Addresses Radia Perlman Sun Microsystems Email: Radia.Perlman@sun.com Silvano Gai Nuova Systems Email: sgai@nuovasystems.com Dinesh G. Dutt Cisco Systems, Inc. 170 Tasman Dr. San Jose, CA 95134-1706 Phone: 1-408-527-0955 EMail: ddutt@cisco.com Expiration and File Name This draft expires in July 2007. Its file name is draft-ietf-trill-rbridge-02.txt. Radia Perlman et al. [Page 25]