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Versions: 00 draft-ietf-trill-prob

TRILL WG                                                 J. Touch (ed.)
Internet Draft                                                  USC/ISI
Expires: May 2006                                     November 17, 2005



           Transparent Interconnection of Lots of Links (TRILL):
                    Problem and Applicability Statement
                       draft-touch-trill-prob-00.txt


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Copyright Notice

   Copyright (C) The Internet Society (2005).  All Rights Reserved.

Abstract

   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-



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   overlapping pairwise paths (in the case of spanning trees). The
   convergence of these routing protocols and stability under link
   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 network layer routing (e.g., link state or distance
   vector) 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..............................................4
      2.1. Inefficient Paths.........................................4
      2.2. Convergence Under Reconfiguration.........................5
      2.3. Robustness to Link Interruption...........................6
      2.4. Problems Not Addressed....................................6
   3. Desired Properties of Solutions to TRILL.......................7
      3.1. No Change to Link Capabilities............................7
      3.2. Zero Configuration and Zero Assumption....................8
      3.3. Forwarding Loop Mitigation................................8
      3.4. Spanning Tree Management..................................9
      3.5. Multiple Attachments......................................9
      3.6. VLAN Issues...............................................9
      3.7. Equivalence...............................................9
      3.8. Optimizations............................................10
      3.9. Internet Architecture Issues.............................10
   4. Applicability.................................................11
   5. Security Considerations.......................................12
   6. IANA Considerations...........................................12
   7. Conclusions...................................................12
   8. Acknowledgments...............................................12
      8.1. Normative References.....................................12
      8.2. Informative References...................................13
   Author's Addresses...............................................14
   Intellectual Property Statement..................................14
   Disclaimer of Validity...........................................14
   Copyright Statement..............................................15
   Acknowledgment...................................................15




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1. Introduction

   [CAVEAT: this document is in very rough form. Input and feedback is
   solicited

   NOTE: the terms 'campus' and 'rbridge' intentionally do not appear in
   this document]

   Conventional 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 with high host reattachment
   frequency, 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.

   [I use the term Ethernet link subnets; do I need to define this? It's
   not a segment, which I think of as being shared or hubbed but not
   bridged]

   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 operating at the link layer. This 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.



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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 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.

   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
   deployments.

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.

                           B=======\\     C
                         // \       \\  / \\   D
                        //   \       \\/   \\ //
                        A-----\-------H===== E
                               \     //     ||
                                \   //      ||
                                 \ //       ||
                                  G----------F

             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


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   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.

                           B11111111      C
                          1         1      1
                         1           1      1
                        A             H121212E
                                     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.

                           B11111111      C
                          1         1      1   D
                         1           1      1
                        A             H111111E



                                  G2222222222F

       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



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   topology can cause large changes in the spanning tree. Changes in the
   spanning tree can take time to propagate and converge.

   [QUESTION: is there a good visual example of this, one that we can
   walk through in the description?]

   [QUESTION: What is the timescale? O(# bridges)? O(#links?), etc?]

   [QUESTION: is port autolearning in this category too? i.e., are TRILL
   solutions trying to hide port reattachment from other nodes (or is
   that even necessary?)]

   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
   inside.

   [QUESTION: anybody have a convenient reference for a proof? Are new
   spanning tree protocols not considering AS-like boundaries? (just
   checking)]

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.

   [QUESTION: is there more to say here?]

   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.

2.4. Problems Not Addressed

   There are other challenges to 802.1D link layer subnets that are not
   addressed in this document. These include:

   [NOTE: these are guesses; if any are wrong, we can move or revise]



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   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 increasingly larger
   scales of Ethernet link subnets than current spanning tree protocols
   can support. This may be a side-effect of supporting more rapid
   reaction to subnet reconfiguration or multiple internal paths, or of
   the grouped modularity that network style routing affords, but is not
   the focus of this work.

   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 grouped modularity of 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


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   [6]. Zeroconf, as well as existing bridge autoconfiguration, are
   dependent on broadcast as well.

   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
   changes.

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 (inadvertent) 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.

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

3.3. Forwarding Loop Mitigation

   Spanning tree avoids forwarding loops by design, even during update
   (?). 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 avoidance 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.

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


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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 inside the TRILL solution and of the entire Ethernet link
   subnet while not interfering with the operation of STP outside the
   TRILL solution. This may involve TRILL solutions participating in the
   STP, where internally a more stable protocol is run such that the
   interactions between the TRILL solution and external STP is dampened,
   or it may involve severing the external STP into separate STPs on
   'stub' external Ethernet link subnet segments.

   [we need pictures here; to appear in the next version]

3.5. Multiple Attachments

   [QUESTION: I'm not sure what this refers to; is it the same NIC
   attached at different points to a TRIL solution? If so, why should
   this be possible where it seems ignored in bridges?]

3.6. VLAN Issues

   A TRILL solution should support multiple VLANs. 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 internal routing protocol instances to separate
   traffic for each VLAN inside a TRILL solution.

   [QUESTION: is it possible to be ignorant of VLANs and still operate?
   Bridges are, right?]

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.

   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.



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   For example, if TRILL solution were equivalent to a single 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 internally, 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 a hub, or may not have a
   corresponding equivalent link layer component at all.

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) [1].

   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
   [3]. 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 [7]. Other layers may similarly
   be snooped, notably ARP packets, for similar reasons for IPv4 [11].


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   [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
   solutions.

   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 internal 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 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).





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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 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
   publication.

7. Conclusions

   (TBA)

8. Acknowledgments

   Portions of this document are based on a preliminary solution
   description document [10].

8.1. Normative References

   None.






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8.2. Informative References

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

   [2]   Braden, R., "Requirements for Internet Hosts - Communication
         Layers", STD 3, RFC 1122, October 1989.

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

   [4]   Callon, R., "Use of OSI IS-IS for routing in TCP/IP and dual
         environments", RFC 1195, December 1990.

   [5]   IEEE 802.1d bridging standard, "IEEE 802.1d bridging standard".

   [6]   P. Karn (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 2004.

   [7]   Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery
         for IP Version 6 (IPv6)", RFC 2461 (Standards Track), December
         1998.

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

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

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

   [11]  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.

   [12]  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.

   [13]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
         November 1990.



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   [14]  Lahey, K., "TCP Problems with Path MTU Discovery", RFC 2923
         (Informational), September 2000.

Author's Addresses

   Joe Touch
   USC/ISI
   4676 Admiralty Way
   Marina del Rey, CA 90292-6695
   U.S.A.

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


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Acknowledgment

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