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Versions: (draft-touch-trill-prob) 00 01 02 03 04 05 06 RFC 5556

TRILL WG                                                       J. Touch
Internet Draft                                                  USC/ISI
Expires: April 2007                                          R. Perlman
                                                       October 22, 2006

           Transparent Interconnection of Lots of Links (TRILL):
                    Problem and Applicability Statement

Status of this Memo

   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
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   BCP 79.

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   This Internet-Draft will expire on April 22, 2007.


   Current Ethernet (802.1) link layers use custom routing protocols
   that have a number of challenges. The routing protocols need to
   strictly avoid loops, even temporary loops during route propagation,
   because of the lack of header loop detection support. Routing tends
   not to take full advantage of alternate paths, or even non-
   overlapping pairwise paths (in the case of spanning trees). The
   convergence of these routing protocols and stability under link

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   changes and failures is also of concern. This document addresses
   these concerns and suggests that they are related to the need to be
   able to apply modern network layer routing protocols at the link
   layer. This document assumes that solutions would not address issues
   of scalability beyond that of existing bridged (802.1D) links, but
   that a solution would be backward compatible with 802.1D, including
   hubs, bridges, and their existing plug-and-play capabilities.

   This document is a work in progress; we invite you to participate on
   the mailing list at http://www.postel.org/rbridge

Table of Contents

   1. Introduction...................................................3
   2. The TRILL Problem..............................................3
      2.1. Inefficient Paths.........................................4
      2.2. Convergence Under Reconfiguration.........................5
      2.3. Robustness to Link Interruption...........................6
      2.4. Other Ethernet Extensions.................................7
      2.5. Problems Not Addressed....................................7
   3. Desired Properties of Solutions to TRILL.......................8
      3.1. No Change to Link Capabilities............................8
      3.2. Zero Configuration and Zero Assumption....................9
      3.3. Forwarding Loop Mitigation................................9
      3.4. Spanning Tree Management.................................10
      3.5. Multiple Attachments.....................................10
      3.6. VLAN Issues..............................................10
      3.7. Equivalence..............................................10
      3.8. Optimizations............................................11
      3.9. Internet Architecture Issues.............................11
   4. Applicability.................................................12
   5. Security Considerations.......................................13
   6. IANA Considerations...........................................13
   7. Conclusions...................................................14
   8. Acknowledgments...............................................14
      8.1. Normative References.....................................14
      8.2. Informative References...................................14
   Author's Addresses...............................................15
   Intellectual Property Statement..................................15
   Disclaimer of Validity...........................................16
   Copyright Statement..............................................16

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1. Introduction

   [CAVEAT: the terms 'campus', 'rbridge, 'inside', 'internal',
   'outside', and 'external' intentionally do not appear in this

   Conventional Ethernet networks - known in the Internet as Ethernet
   link subnets - have a number of attractive features, allowing hosts
   and routers to relocate within the subnet without requiring
   renumbering and are automatically configuring. Unfortunately, the
   basis of the simplicity of these subnets is the spanning tree, which
   although simple and elegant, can have substantial limitations. In
   subnets where bridges are also frequently relocated, convergence of
   the spanning tree protocol can be slow. Because all traffic flows
   over a single tree, all traffic is concentrated on a subset of links,
   increasing susceptibility to the effects of link failures and
   limiting the bandwidth across the subnet.

   The alternative to an Ethernet link subnet is often a network subnet.
   Network subnets can use link-state routing protocols that allow
   traffic to traverse least-cost paths rather than being aggregated on
   a spanning tree backbone, providing higher aggregate capacity and
   more resistance to link failures. Unfortunately, IP - the dominant
   network layer technology - requires that hosts be renumbered when
   relocated in different network subnets, interrupting network (e.g.,
   tunnels, IPsec) and transport (e.g., TCP, UDP) associations that are
   in progress during the transition.

   It is thus useful to consider a new approach that combines the
   features of these two existing solutions, hopefully retaining the
   desirable properties of each. Such an approach would develop a new
   kind of bridge system that was capable of using network-style
   routing, while still providing Ethernet service. It allows reuse of
   well-understood network routing protocols to benefit the link layer.

   This document describes the challenge of such a combined approach in
   detail. This problem is known as "Transparent Interconnection of Lots
   of Links" or "TRILL". The remainder of this document makes as few
   assumptions about a solution to TRILL as possible.

2. The TRILL Problem

   Ethernet subnets have evolved from 'thicknet' to 'thinnet' to twisted
   pair with hubs to twisted pair with switches, becoming increasingly
   simple to wire and manage. Each level has corresponding topology
   restrictions; thicknet is inherently linear, whereas thinnet and hub-
   connected twisted pair have to be wired as a tree. Switches, added in

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   802.1D, allow network managers to avoid thinking in trees, where the
   spanning tree protocol finds a valid tree automatically;
   unfortunately, this additional simplicity comes with a number of
   associated penalties [11].

   The spanning tree often results in inefficient use of the link
   topology; traffic is concentrated on the spanning tree path, and all
   traffic follows that path even when other more direct paths may be
   available. The spanning tree configuration is affected by even small
   topology changes, and small changes can have large effects. Each of
   these inefficiencies can cause problems for current link layer

2.1. Inefficient Paths

   The Spanning Tree Protocol (STP) helps break cycles in a set of
   interconnected bridges, but it also can limit the bandwidth among
   that set and cause traffic to take circuitous paths.

   Consider the network shown in Figure 1, which shows a number of
   bridges and their interconnecting links. End hosts and routers are
   not shown; they would connect to the bridges that are shown, labeled
   A-H. Note that the network shown has cycles which would cause packet
   storms if hubs (repeaters) were used instead of STP-capable bridges.
   One possible spanning tree is shown by double lines.

                                // \     C
                               //   \   / \\   D
                              //     \ /   \\ //
                              B=======H===== E
                               \     //     ||
                                \   //      ||
                                 \ //       ||

             Figure 1 Bridged subnet with spanning tree shown

   The spanning tree limits the capacity of the resulting subnet. Assume
   that the links are 100 Mbps. Figure 2 shows how traffic from hosts on
   A to hosts on C goes via the spanning tree path A-B-H-E-C (links
   replaced with '1' in the figure); traffic from hosts on G to F go via
   the spanning three path G-H-E-F (links replaced by '2' in the
   figure). The link H-E is shared by both paths (alternating '1's and
   '2's), resulting in an aggregate capacity for both A..C and G..F
   paths of a total of 100 Mbps.

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                                 1        C
                                1          1
                               1            1
                                     2       2
                                    2        2
                                   2         2
                                  G          F

         Figure 2 Traffic from A..C (1) and G..F (2) share a link

   If traffic from G to F were to go directly using full routing, e.g.,
   from G-F, both paths could have 100 Mbps each, and the total
   aggregate capacity could be 200 Mbps (Figure 3). In this case, the H-
   F link carries only A-C traffic ('1's) and the G-F traffic ('2's) is
   more direct.

                                 1        C
                                1          1
                               1            1


       Figure 3 Traffic from A..C (1) and G..F (2) with full routing

   There are a number of features of modern layer 3 routing protocols
   which would be beneficial if available at layer 2, but which cannot
   be integrated into the spanning tree system. Multipath routing can
   distribute load simultaneously among two different paths; alternate
   path routing supports rapid failover to backup paths. Layer 3 routing
   typically optimizes paths between pairs of endpoints, conventionally
   based on hopcount but also including bandwidth, latency, or other
   policy metrics.

2.2. Convergence Under Reconfiguration

   The spanning tree is dependent on the way a set of bridges are
   interconnected, i.e., the link layer topology. Small changes in this
   topology can cause large changes in the spanning tree. Changes in the
   spanning tree can take time to propagate and converge.

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   One possible case occurs when one of the branches connected to the
   root bridge fails, causing a large number of ports to block and
   unblock before the network reconverges [3][8]. Consider a ring as
   shown in Figure 4.

                        |                   |
         Figure 4 Ring with poor convergence under reconfiguration

   If A is the root bridge, then the paths A->B->C->D and A->F->G->E are
   the two open paths, while the D->E link is blocked in both
   directions. If the A->B link fails, then E must unblock its port to D
   for traffic to flow again, but it may require recomputation of the
   entire tree through BPDUs.

   [QUESTION: What is the timescale? O(# bridges)? O(#links?), etc? on
   report is that "I have heard things said that imply that the worst
   case RSTP settling time is related to 3 times the network diameter,
   where diameter is the maximum of the minimum number of hops between
   all pairs of nodes." - if anyone has a more specific reference,
   please provide one]

   The spanning tree protocol is inherently global to an entire layer 2
   subnet; there is no current way to contain, partition, or otherwise
   factor the protocol into a number of smaller, more stable subsets
   that interact as groups. Contrast this with Internet routing, which
   includes both intradomain and interdomain variants, split to provide
   exactly that containment and scalability within a domain while
   allowing domains to interact freely independent of what happens
   within a domain.

2.3. Robustness to Link Interruption

   Persistent changes to the link topology, as described in Section 2.2,
   are not the only effects on subnet stability. Transient link
   interruptions have similar effects, with similar scalability issues.
   It would be more useful for subnet configuration to be tolerant of
   such transients, e.g., supporting alternate, backup paths.

   Contrast this to network layer intradomain and interdomain routing,
   both of which include provisions for backup paths. These backups
   allow routing to be more stable in the presence of transients, as
   well as to recover more rapidly when the transient disappears.

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2.4. Other Ethernet Extensions

   There have been a variety of 802.1 protocols beyond the initial
   shared-media variant, including:

   o  802.1D - added bridges (i.e., switches) and a spanning tree
      protocol (STP) [4]

   o  802.1W - extension for rapid reconvergence of the spanning tree
      protocol (RTSP) [6]

   o  802.1Q - added VLAN support, where each link address maps to one
      VLAN [5]

   o  802.1v - added VLANs where segments map to VLANs based on link
      address together with network protocol and transport port [? Is
      that correct? It says protocol and port] [5]

   o  802.1s - added support for multiple spanning trees, one per VLAN
      (MSTP) [5]

   These variants are further complicated by different versions updated

   It is useful to note that these extensions do not address the issue
   of independent, localized routing in a single spanning tree - which
   is the focus of TRILL. This document presumes the above variants are
   supported on the Ethernet subnet, i.e., that a TRILL solution would
   support all of the above.

2.5. Problems Not Addressed

   There are other challenges to deploying Ethernet subnets that are not
   addressed in this document. These include:

   o  increased Ethernet link subnet scale

   o  increased node relocation

   o  Ethernet link subnet management protocol security

   o  flooding attacks on a Ethernet link subnet

   Solutions to TRILL are not intended to support deployment of
   increasingly larger scales of Ethernet link subnets than current
   broadcast domains can support (e.g., around 1,000 end-hosts in a

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   single bridged LAN of 100 bridges, or 100,000 end-hosts inside 1,000
   VLANs served by 10,000 bridges).

   Similarly, solutions to TRILL are not intended to address link layer
   node migration, which can complicate the caches in learning bridges.
   Similar challenges exist in the ARP protocol, where link layer
   forwarding is not updated appropriately when nodes move to ports on
   other bridges. Again, the compartmentalization available in network
   routing, like that of network layer ASes, can help hide the effect of
   migration. That is a side effect, however, and not a primary focus of
   this work.

   Current link control plane protocols, including Ethernet link subnet
   management (STP) and link/network integration (ARP), are vulnerable
   to a variety of attacks. Solutions to TRILL are not intended to
   directly address these vulnerabilities. Similar attacks exist in the
   data plane, e.g., source address spoofing, single address traffic
   attacks, traffic snooping, and broadcast flooding. TRILL solutions do
   not address any of these issues, although it is critical that they do
   not introduce new vulnerabilities in the process (see Section 5).

3. Desired Properties of Solutions to TRILL

   This section describes some of the desirable or required properties
   of any system that would solve the TRILL problems, independent of the
   details of such an architecture. Most of these are based on retaining
   useful properties of bridges, or maintaining those properties while
   solving the problems listed in Section 2.

3.1. No Change to Link Capabilities

   There must be no change to the service that Ethernet subnets already
   provide as a result of deploying a TRILL solution. Ethernet supports
   unicast, broadcast, and multicast natively. Although network
   protocols, notably IP, can tolerate link layers that do not provide
   all three, it would be useful to retain the support already in place
   [7]. Zeroconf, as well as existing bridge autoconfiguration, are
   dependent on broadcast as well.

   Current Ethernet ensures in-order delivery and no duplicated packets
   under normal operation (excepting transients during reconfiguration).
   These criteria apply in varying degrees to the different variants of
   Ethernet, e.g., basic Ethernet up through basic VLAN (802.1Q) ensures
   that all packets between two link addresses have both properties, but
   protocol/port VLAN (802.1V) ensures this only for packets with the
   same protocol and port. [JUST CHECKING - OR AM I MISREADING WHAT
   802.1V DOES?]

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   There are subtle implications to such a requirement. Bridge
   autolearning already is susceptible to moving nodes between ports,
   because previously learned associations between port and link address
   change. A TRILL solution could be similarly susceptible to such

3.2. Zero Configuration and Zero Assumption

   Both bridges and hubs are zero configuration devices; hubs having no
   configuration at all, and bridges being automatically self-
   configured. Bridges are further zero-assumption devices, unlike hubs.
   Bridges can be interconnected in arbitrary topologies, without regard
   for cycles or even self-attachment. STP removes the impact of cycles
   automatically, and port autolearning reduces unnecessary broadcast of
   unicast traffic.

   A TRILL solution should strive to have similar zero configuration,
   zero assumption operation. This includes having TRILL solution
   components automatically discover other TRILL solution components and
   organize themselves, as well as to configure that organization for
   proper operation (plug-and-play). It also includes zero configuration
   backward compatibility with existing bridges and hubs, which may
   include interacting with some of the bridge protocols, such as STP.

   VLANs add a caveat to zero configuration; a TRILL solution should
   support automatic use of a default VLAN (like non-VLAN bridges), but
   should require explicit configuration where the VLANS require them as

   Autoconfiguration extends to optional services, such as multicast
   support via IGMP snooping, broadcast support via serial copy, and
   supporting multiple VLANs.

3.3. Forwarding Loop Mitigation

   Spanning tree avoids forwarding loops by construction, although
   transient loops can occur, e.g., via the appearance of a new link.
   Solutions to TRILL are intended to use adapted network layer routing
   protocols which may introduce transient loops during routing
   convergence. TRILL solutions thus need support for mitigating the
   effect of such routing loops.

   In the Internet, loop mitigation is provided by a decrementing
   hopcounts (TTL); in other networks, packets include a trace
   (serialized or unioned) of visited nodes [1]. These mechanisms
   (respectively) limit the impact of loops or detect them explicitly. A
   mechanism with similar effect should be included in TRILL solutions.

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   [QUESTION: anyone have a good reference for serialized or union
   traces - or better names for them?]

3.4. Spanning Tree Management

   In order to address convergence under reconfiguration and robustness
   to link interruption (Sections 2.2 and 2.3), participation in the STP
   must be carefully managed. The goal is to provide the desired
   stability of the TRILL solution and of the entire Ethernet link
   subnet while not interfering with the operation of STP of the
   Ethernet on which the TRILL resides. This may involve TRILL solutions
   participating in the STP, where the protocol is used for TRILL might
   dampen interactions with STP, or it may involve severing the STP into
   separate STPs on 'stub' external Ethernet link subnet segments.

   A requirement is that a TRILL solution must not require modifications
   or exceptions to the existing spanning tree protocols (STP, MSTP).

   [we need pictures here; to appear]

3.5. Multiple Attachments

   In STP, a single NIC with multiple attachments to a single spanning
   tree will always only get traffic over one of the two attachment
   points, TRILL allows load sharing between the attachment points.
   Further, TRILL must manage multicast and broadcast traffic so as not
   to create feedback loops on Ethernet segments which are attached at
   multiple TRILL access points.

   [NOTE: this might be omitted, as it has not been shown to be a
   problem with STP].

3.6. VLAN Issues

   A TRILL solution should support multiple VLANs (802.1Q, 802.1V, and
   802.1S). This may involve ignorance, just as many bridge devices do
   not participate in the VLAN protocols. It may alternately support
   direct VLAN support, e.g., by the use of separate TRILL routing
   protocol instances to separate traffic for each VLAN traversing a
   TRILL solution.

3.7. Equivalence

   As with any extension to an existing architecture, it would be useful
   - though not strictly necessary - to be able to describe or consider
   a TRILL solution as a model of an existing link layer component. Such
   equivalence provides a validation model for the architecture, and a

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   way for users to predict the effect of the use of a TRILL solution on
   a deployed Ethernet. In this case, 'user' refers to users of the
   Ethernet protocol, whether at the host (data segments), bridge (ST
   control segments), or VLAN (VLAN control).

   This provides a sanity check, i.e., "we got it right if we can
   replace a TRILL solution with an X" (where "X" might be a single
   bridge, a hub, or some other link layer abstraction). It does not
   matter whether "X" can be implemented on the same scale as the
   corresponding TRILL solution. It also does not matter if it can -
   there may be utility to deploying the TRILL solution components
   incrementally, in ways that a single "X" could not be installed.

   For example, if TRILL solution were equivalent to a single 802.1D
   bridge, it would mean that the TRILL solution would - as a whole -
   participate in the STP. This need not require that TRILL solution
   would propagate STP, any more than a bridge need do so in its on-
   board control. It would mean that the solution would interact with
   BPDUs at the edge, where the solution would - again, as a whole -
   participate as if a single node in the spanning tree. Note that this
   equivalence is not required; a solution may act as if an 802.1 hub,
   or may not have a corresponding equivalent link layer component at

3.8. Optimizations

   There are a number of optimizations that may be applied to TRILL
   solutions. These must be applied in a way that does not affect
   functionality as a tradeoff for increased performance. Such
   optimizations address broadcast and multicast frame distribution,
   VLAN support, and snooping of ARP and IPv6 neighbor discovery.

   [NOTE: need to say more here.]

3.9. Internet Architecture Issues

   TRILL solutions are intended to have no impact on the Internet
   network layer architecture. In particular, the Internet and higher
   layer headers should remain intact when traversing a TRILL solution,
   just as they do when traversing any other link subnet technologies.
   This means that the IP TTL field cannot be co-opted for forwarding
   loop mitigation, as it would interfere with the Internet layer
   assuming that the link subnet was reachable with no changes in TTL
   (Internet TTLs are changed only at routers, as per RFC 1812, and even
   if IP TTL were considered, TRILL is expected to support non-IP
   payloads, and so requires a separate solution anyway) [1].

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   TRILL solutions should also have no impact on Internet routing or
   signaling, which also means that broadcast and multicast, both of
   which can pervade an entire Ethernet link subnet, must be able to
   transparently pervade a TRILL solution. Changing how either of these
   capabilities behaves would have significant effects on a variety of
   protocols, including RIP (broadcast), RIPv2 (multicast), ARP
   (broadcast), IPv6 neighbor discovery (multicast), etc.

   Note that snooping of network layer packets may be useful, especially
   for certain optimizations. These include snooping multicast control
   plane packets (IGMP) to tune link multicast to match the network
   multicast topology, as is already done in existing smart switches
   [2]. This also includes snooping IPv6 neighbor discovery messages to
   assist with governing TRILL solution edge configuration, as is the
   case in some smart learning bridges [9]. Other layers may similarly
   be snooped, notably ARP packets, for similar reasons for IPv4 [13].

   [Need a ref for the router-router 'igmp' protocol]

4. Applicability

   As might be expected, TRILL solutions are intended to be used to
   solve the problems described in Section 2. However, not all such
   installations are appropriate environments for such solutions. This
   section outlines the issues in the appropriate use of these

   TRILL solutions are intended to address problems of path efficiency
   and stability within a single Ethernet link subnet. Like bridges,
   individual TRILL solution components may find other TRILL solution
   components within a single Ethernet link subnet and aggregate into a
   single TRILL solution.

   TRILL solutions are not intended to span separate Ethernet link
   subnets where interconnected by network layer (e.g., router) devices,
   except via link layer tunnels that are in place prior to their
   deployment, where such tunnels render the distinct subnet
   undetectably equivalent from a single Ethernet link subnet.

   A currently open question is whether a single Ethernet link subnet
   should contain only one TRILL solution instance, either of necessity
   of architecture or utility. Multiple TRILL solutions, like Internet
   ASes, may allow TRILL routing protocols to be partitioned in ways
   that help their stability, but this may come at the price of needing
   the TRILL solutions to participate more fully as nodes (each modeling
   a bridge) in the Ethernet link subnet STP. Each architecture solution
   should decide whether multiple TRILL solutions are supported within a

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   single Ethernet link subnet and mechanisms should be included to
   enforce whatever decision is made.

   TRILL solutions are not intended to address scalability limitations
   in bridged subnets. Although there may be scale benefits of other
   aspects of solving TRILL problems, e.g., of using network layer
   routing to provide stability under link changes or intermittent
   outages, this is not a focus of this work.

   As also noted earlier, TRILL solutions are not intended to address
   security vulnerabilities in either the data plane or control plane of
   the link layer. This means that TRILL solutions should not limit
   broadcast frames, ARP requests, or spanning tree protocol messages
   (if such are interpreted by the TRILL solution or solution edge).

5. Security Considerations

   TRILL solutions should not introduce new vulnerabilities compared to
   traditional bridged subnets.

   TRILL solutions are not intended to be a solution to Ethernet link
   subnet vulnerabilities, including spoofing, flooding, snooping, and
   attacks on the link control plane (STP, flooding the learning cache)
   and link-network control plane (ARP). Although TRILL solutions are
   intended to provide more stable routing than STP, this stability is
   limited to performance, and the subsequent robustness is intended to
   address non-malicious events.

   There may be some side-effects to the use of TRILL solutions that can
   provide more robust operation under certain attacks, such as those
   interrupting or adding link service, but TRILL solutions should not
   be relied upon for such capabilities.

   Finally, TRILL solutions should not interfere with other protocols
   intended to address these vulnerabilities, such as those under
   development to secure IPv6 neighbor discovery.

   [need a ref for secure ipv6 nd]

6. IANA Considerations

   This document has no IANA considerations.

   This section should be removed by the RFC Editor prior to final

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7. Conclusions


8. Acknowledgments

   Portions of this document are based on documents that describe a
   preliminary solution, and on a related network layer solution

8.1. Normative References


8.2. Informative References

   [1]   Baker, F., "Requirements for IP Version 4 Routers", RFC 1812
         (Standards Track), June 1995.

   [2]   Cain, B., S. Deering, I. Kouvelas, B. Fenner, A. Thyagarajan,
         "Internet Group Management Protocol, Version 3," RFC 3376
         (Proposed Standard), October 2002.

   [3]   Elmeleegy, K., A.L. Cox and T. E. Ng, "On Count-to-Infinity
         Induced Forwarding Loops in Ethernet Networks," Proc. Infocom
         2006, April 2006.

   [4]   IEEE 802.1D bridging standard, "IEEE Standard for Local and
         metropolitan area networks: Media Access Control (MAC)
         Bridges," June 2004.

   [5]   IEEE 802.1Q VLAN standard, "IEEE Standards for Local and
         metropolitan area networks: Virtual Bridged Local Area
         Networks," (incorporates 802.1v and 802.1s), May 2003.

   [6]   IEEE 802.1W standard, ""

   [7]   Karn, P., (ed.), C. Bormann, G.Fairhurst, D. Grossman, R.
         Ludwig, J. Mahdavi, G. Montenegro, J. Touch, L. Wood, "Advice
         for Internet Subnetwork Designers," RFC-3819 / BCP 89, July

   [8]   Myers, A., T. E. Ng, and H. Zhang, "Rethinking the Service
         Model: Scaling Ethernet to a Million Nodes," Proc. ACM Third
         Workshop on Hot Topics in Nnetworks (HotNets-III), Mar. 2004.

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   [9]   Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery
         for IP Version 6 (IPv6)", RFC 2461 (Standards Track), December

   [10]  Perlman, R., "RBridges: Transparent Routing", Proc. Infocom
         2005, March 2004.

   [11]  Perlman, R., "Interconnection: Bridges, Routers, Switches, and
         Internetworking Protocols", Addison Wesley Chapter 3, 1999.

   [12]  Perlman, R., J. Touch, A. Yegin, "RBridges: Transparent
         Routing," (expired work in progress), Apr. 2004 - May 2005.

   [13]  Plummer, D., "Ethernet Address Resolution Protocol: Or
         converting network protocol addresses to 48.bit Ethernet
         address for transmission on Ethernet hardware", STD 37, RFC 826
         (Standard), November 1982.

   [14]  Touch, J., Wang, Y., Eggert, L. and G. Finn, "A Virtual
         Internet Architecture", ISI Technical Report ISI-TR-570,
         Presented at the Workshop on Future Directions in Network
         Architecture (FDNA) 2003 at Sigcomm 2003, March 2003.

Author's Addresses

   Joe Touch
   4676 Admiralty Way
   Marina del Rey, CA 90292-6695

   Phone: +1 (310) 448-9151
   Email: touch@isi.edu
   URL:   http://www.isi.edu/touch

   Radia Perlman
   Sun Microsystems

   Email: Radia.Perlman@sun.com

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