RFC 5556

Network Working Group                                           J. Touch
Request for Comments: 5556                                       USC/ISI
Category: Informational                                       R. Perlman
                                                                May 2009

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

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.


   Current IEEE 802.1 LANs use spanning tree protocols that have a
   number of challenges.  These protocols need to strictly avoid loops,
   even temporary ones, 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).  This document addresses these
   concerns and suggests applying modern network-layer routing protocols
   at the link layer.  This document assumes that solutions would not
   address issues of scalability beyond that of existing IEEE 802.1
   bridged links, but that a solution would be backward compatible with
   802.1, including hubs, bridges, and their existing plug-and-play

Touch & Perlman              Informational                      [Page 1]

RFC 5556            TRILL: Problem and Applicability            May 2009

Table of Contents

   1. Introduction ....................................................2
   2. The TRILL Problem ...............................................3
      2.1. Inefficient Paths ..........................................3
      2.2. Multipath Forwarding .......................................5
      2.3. Convergence and Safety .....................................6
      2.4. Stability of IP Multicast Optimization .....................6
      2.5. IEEE 802.1 Bridging Protocols ..............................7
      2.6. Problems Not Addressed .....................................8
   3. Desired Properties of Solutions to TRILL ........................9
      3.1. No Change to Link Capabilities .............................9
      3.2. Zero Configuration and Zero Assumption ....................10
      3.3. Forwarding Loop Mitigation ................................10
      3.4. Spanning Tree Management ..................................11
      3.5. Multiple Attachments ......................................11
      3.6. VLAN Issues ...............................................11
      3.7. Operational Equivalence ...................................12
      3.8. Optimizations .............................................12
      3.9. Internet Architecture Issues ..............................13
   4. Applicability ..................................................13
   5. Security Considerations ........................................14
   6. Acknowledgments ................................................15
   7. Informative References .........................................15

1.  Introduction

   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 supporting automatic configuration.  The basis of
   the simplicity of these subnets is the spanning tree, which although
   simple and elegant, can have substantial limitations.  With spanning
   trees, the bandwidth across the subnet is limited because traffic
   flows over a subset of links forming a single tree -- or, with the
   latest version of the protocol and significant additional
   configuration, over a small number of superimposed trees.  The oldest
   version of the spanning tree protocol can converge slowly when there
   are frequent topology changes.

   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.,

Touch & Perlman              Informational                      [Page 2]

RFC 5556            TRILL: Problem and Applicability            May 2009

   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.
   This problem is known as "Transparent Interconnection of Lots of
   Links" or "TRILL".  The remainder of this document makes minimal
   assumptions about a solution to TRILL.

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 IEEE 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 [Pe99].

   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 are
   available.  The addition in IEEE 802.1Q of support for multiple
   spanning trees helps a little, but the use of multiple spanning trees
   requires additional configuration, the number of trees is limited,
   and these defects apply within each tree regardless.  The spanning
   tree protocol reacts to certain small topology changes with large
   effects on the reconfiguration of links in use.  Each of these
   aspects of the spanning tree protocol 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.  For example, in
   a set of N nodes that are interconnected pairwise along a ring, a
   spanning tree will disable one physical link so that connectivity is
   loop free.  This will cause traffic between the pair of nodes
   connected by that disabled link to have to go N-1 physical hops

Touch & Perlman              Informational                      [Page 3]

RFC 5556            TRILL: Problem and Applicability            May 2009

   around the entire remainder of the ring rather than take the most
   efficient single-hop path.  Using modern routing protocols with such
   a topology, no traffic should have to go more than N/2 hops.

   For another example, 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 that
   would cause packet storms if hubs (repeaters) were used instead of
   spanning-tree-capable bridges.  One possible spanning tree is shown
   by double lines.

                             // \    [C]
                            //   \   / \\  [D]
                           //     \ /   \\ //
                            \     //      ||
                             \   //       ||
                              \ //        ||

           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.

                                  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

Touch & Perlman              Informational                      [Page 4]

RFC 5556            TRILL: Problem and Applicability            May 2009

   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
   practically be integrated into the spanning tree system such as
   multipath routing discussed in Section 2.2 below.  Layer 3 routing
   typically optimizes paths between pairs of endpoints based on a cost
   metric, conventionally based on bandwidth, hop count, latency, and/or
   policy measures.

2.2.  Multipath Forwarding

   The discussion above assumes that all traffic flowing from one point
   to another follows a single path.  Using spanning trees reduces
   aggregate bandwidth by forcing all such paths onto one tree, while
   modern routing causes such paths to be selected based on a cost
   metric.  However, extensions to modern routing protocols enable even
   greater aggregate bandwidth by permitting traffic flowing from one
   endpoint to another to be sent over multiple, typically equal-cost,
   paths.  (Traffic sent over different paths will generally encounter
   different delays and may be reordered with respect to traffic on
   another path.  Thus, traffic must be divided into flows, such that
   reordering of traffic between flows is not significant, and those
   flows are allocated to paths.)

   Multipathing typically spreads the traffic more evenly over the
   available physical links.  The addition of multipathing to a routed
   network would typically result in only a small improvement in
   capacity for a network with roughly equal traffic between all pairs
   of nodes, because in that situation traffic is already fairly well
   dispersed.  Conversely, multipathing can produce a dramatic
   improvement in a routed network where the traffic between a small
   number of pairs of nodes dominates, because such traffic can -- under
   the right circumstances -- be spread over multiple paths that might
   otherwise be lightly loaded.

Touch & Perlman              Informational                      [Page 5]

RFC 5556            TRILL: Problem and Applicability            May 2009

2.3.  Convergence and Safety

   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, especially
   for older versions of STP.

   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 [Me04].  Consider a ring with
   a stub 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.  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
   (Bridge PDUs).  Even worse, if R is root and R or the A-R connection
   fails, BPDU updates related to the old and new root can lead to a
   brief count-to-infinity event, and, if RSTP (Rapid Spanning Tree
   Protocol) is in use, can delay convergence for a few seconds.  The
   original IEEE 802.1 spanning tree protocol can impose 30-second
   delays in re-establishing data connectivity after a topology change
   in order to be sure a new topology has stabilized and been fully

   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.

2.4.  Stability of IP Multicast Optimization

   Although it is a layer violation, it is common for high-end bridges
   to snoop on IP multicast control messages for the purpose of
   optimizing the distribution of IP multicast data and of those control
   messages [RFC4541].

Touch & Perlman              Informational                      [Page 6]

RFC 5556            TRILL: Problem and Applicability            May 2009

   When such snooping and optimization is performed by spanning-tree-
   based bridges, it done at each bridge based on the traffic observed
   on that bridge's ports.  Changes in topology may reverse or otherwise
   change the required forwarding ports of messages for a multicast
   group.  Bridges must relearn the correct multicast forwarding from
   the receipt of multicast control messages on new ports.  Such control
   messages are sent to establish multicast distribution state and then
   to refresh it, sometimes at intervals of seconds.  If a bridging
   topology change has occurred during such intervals, multicast data
   may be misdirected and lost.

   However, a solution based on link-state routing, for example, can
   form and maintain a global view of the multicast group membership and
   multicast router situation in a similar fashion to that in which it
   maintains a global view of the status of links.  Thus, such a
   solution can adjust the forwarding of multicast data and control
   traffic immediately as it sees the LAN topology change.

2.5.  IEEE 802.1 Bridging Protocols

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

   o  802.1D - added bridges (i.e., switches) and a spanning tree
      protocol (STP) (incorporates 802.1w, below) [IEEE04].

   o  802.1w - extension for rapid reconvergence of the spanning tree
      protocol (RTSP) [IEEE04].

   o  802.1Q - added VLAN and priority support, where each link address
      maps to one VLAN (incorporates 802.1v and 802.1s, below) [IEEE06].

   o  802.1v - added VLANs where segments map to VLANs based on link
      address together with network protocol and transport port

   o  802.1s - added support for multiple spanning trees, up to a
      maximum of 65, one per non-overlapping group of VLANs (Multiple
      STP) [IEEE06].

   This document presumes the above variants are supported on the
   Ethernet subnet, i.e., that a TRILL solution would not interfere with
   (i.e., would not affect) any of the above.

   In addition, the following more recent extensions have been
   standardized to specify provider/carrier Ethernet services that can
   be effectively transparent to the previously specified customer
   Ethernet services.  The TRILL problem as described in this document

Touch & Perlman              Informational                      [Page 7]

RFC 5556            TRILL: Problem and Applicability            May 2009

   is limited to customer Ethernet services; however, there is no reason
   that a TRILL solution might not be easily applicable to both customer
   and provider Ethernet.

   o  802.1ad (Provider Bridges) - added support for a second level of
      VLAN tag, called a "service tag", and renamed the original 802.1Q
      tag a "customer tag".  Also known as Q-in-Q because of the
      stacking of 802.1Q VLAN tags.

   o  802.1ah (Provider Backbone Bridges) - added support for stacking
      of MAC addresses by providing a tag to contain the original source
      and destination MAC addresses.  Also know as MAC-in-MAC.

   It is useful to note that no extension listed above in this section
   addresses the issue of independent, localized routing in a single LAN
   -- which is the focus of TRILL.

   The TRILL problem and a sketch of a possible solution [Pe04] were
   presented to both the IETF (via a BoF) and IEEE 802 (via an IEEE 802
   Plenary Meeting Tutorial).  The IEEE, in response, approved a project
   called Shortest Path Bridging (IEEE Project P802.1aq), taking a
   different approach than that presented in [Pe04].  The current Draft
   of P802.1aq appears to describe two different techniques.  One, which
   does not use encapsulation, is, according to the IEEE Draft, limited
   in applicability to small networks of no more than 100 shortest path
   bridges.  The other, which uses 802.1ah, is, according to the IEEE
   Draft, limited in applicability to networks of no more than 1,000
   shortest path bridges.

2.6.  Problems Not Addressed

   There are other challenges to deploying Ethernet subnets that are not
   addressed in this document other than, in some cases, to mention
   relevant IEEE 802.1 documents, although it is possible for a solution
   to address one or more of these in addition to the TRILL problem.
   These include:

   o  increased Ethernet link subnet scale

   o  increased node relocation

   o  security of the Ethernet link subnet management protocol

   o  flooding attacks on a Ethernet link subnet

   o  support for "provider" services such as Provider Bridges
      (802.1ad), Provider Backbone Bridges (802.1ah), or Provider
      Backbone Bridge Traffic Engineering (802.1Qay)

Touch & Perlman              Informational                      [Page 8]

RFC 5556            TRILL: Problem and Applicability            May 2009

   Solutions to TRILL need not support deployment of larger scales of
   Ethernet link subnets than current broadcast domains can support
   (e.g., around 1,000 end-hosts in a single bridged LAN of 100 bridges,
   or 100,000 end-hosts inside 1,000 VLANs served by 10,000 bridges).

   Similarly, solutions to TRILL need not address link-layer node
   migration, which can complicate the caches in learning bridges.
   Similar challenges exist in the Address Resolution Protocol (ARP),
   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 Autonomous
   Systems (ASes), can help hide the effect of migration.  That is a
   side effect, however, and not a primary focus of this work.

   Current link-layer control-plane protocols, including Ethernet link
   subnet management (spanning tree) and link/network integration (ARP),
   are vulnerable to a variety of attacks.  Solutions to TRILL need not
   address these insecurities.  Similar attacks exist in the data plane,
   e.g., source address spoofing, single address traffic attacks,
   traffic snooping, and broadcast flooding.  TRILL solutions need 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 a solution.  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
   [RFC3819].  So called "zero configuration protocols" (also known as
   "zeroconf", e.g., as used to configure link-local addresses
   [RFC3927]), as well as existing bridge autoconfiguration, are also
   dependent on broadcast.

   Current Ethernet ensures in-order delivery for frames of the same
   priority and no duplicated frames, under normal operation (excepting
   transients during reconfiguration).  These criteria apply in varying
   degrees to the different types of Ethernet, e.g., basic Ethernet up
   through basic VLAN (802.1Q) ensures that all frames with the same

Touch & Perlman              Informational                      [Page 9]

RFC 5556            TRILL: Problem and Applicability            May 2009

   priority between two link addresses have both properties, but
   protocol/port VLAN (802.1v) ensures this only for packets with the
   same protocol and port.  There are subtle implications to such a
   requirement.  Bridge autolearning already is susceptible to moving
   nodes between ports, because previously learned associations between
   the 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 self-attachment.  Spanning tree protocols
   (STPs) remove the impact of cycles automatically, and port
   autolearning reduces unnecessary broadcast of unicast traffic.

   A TRILL solution should strive to have a 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 spanning tree.

   VLANs add a caveat to zero configuration; a TRILL solution should
   support automatic use of a default VLAN (like non-VLAN bridges), but
   would undoubtedly require explicit configuration for VLANs where
   bridges require such configuration.

   Autoconfiguration extends to optional services, such as multicast
   support via Internet Group Management Protocol (IGMP) snooping,
   broadcast support via serial copy, and support of multiple VLANs.

3.3.  Forwarding Loop Mitigation

   Using spanning trees avoids forwarding loops by construction,
   although transient loops can occur, e.g., via the temporarily
   undetected appearance of new link connectivity or the loss of a
   sufficient number of spanning-tree control frames.  Solutions to
   TRILL are intended to use adapted network-layer routing protocols
   that may introduce transient loops during routing convergence.  A
   TRILL solution thus needs to provide support for mitigating the
   effect of such routing loops.

Touch & Perlman              Informational                     [Page 10]

RFC 5556            TRILL: Problem and Applicability            May 2009

   In the Internet, loop mitigation is provided by decrementing hop
   counts (Time To Live (TTL)); in other networks, packets include a
   trace (sometimes referred to as 'serialized' or 'unioned') of visited
   nodes [RFC1812].  In addition, there may be localized consistency
   checks, such as whether traffic is received on an unexpected
   interface, which indicates that routing is in flux and that such
   traffic should probably be discarded for safety.  These types of
   mechanisms limit the impact of loops or detect them explicitly.
   Mechanisms with similar effect should be included in TRILL solutions.

3.4.  Spanning Tree Management

   In order to address convergence under reconfiguration and robustness
   to link interruption (Section 2.2), participation in the spanning
   tree (STP) must be carefully managed.  The goal is to provide the
   desired stability of the TRILL solution and of the entire Ethernet
   link subnet, which may include bridges using STP.  This may involve a
   TRILL solution participating in the STP, where the protocol used for
   TRILL might dampen interactions with STP, or it may involve severing
   the STP into separate STPs on 'stub' external Ethernet link subnet

   A requirement is that a TRILL solution must not require modifications
   or exceptions to the existing spanning tree protocols (e.g., STP,
   RSTP (Rapid Spanning Tree Protocol), MSTP (Multiple Spanning Tree

3.5.  Multiple Attachments

   In STP, a single node with multiple attachments to a single spanning
   tree segment will always get and send traffic over only one of the
   those attachment points.  TRILL must manage all traffic, including
   multicast and broadcast traffic, so as not to create traffic loops
   involving Ethernet segments with multiple TRILL attachment points.
   This includes multiple attachments to a single TRILL node and
   attachments to multiple TRILL nodes.  Support for multiple
   attachments can improve support for forms of mobility that induce
   topology changes, such as "make before break", although this is not a
   major goal of TRILL.

3.6.  VLAN Issues

   A TRILL solution should support multiple customer VLANs (802.1Q,
   which includes 802.1v and 802.1s).  This may involve ignorance, just
   as many bridge devices do not participate in the VLAN protocols.  A
   TRILL solution may alternately furnish direct VLAN support, e.g., by
   providing configurable support for VLAN-ignorant end stations
   equivalent to that provided by 802.1Q non-provider bridges.

Touch & Perlman              Informational                     [Page 11]

RFC 5556            TRILL: Problem and Applicability            May 2009

   Provider VLANs (802.1ad) are outside of the scope of this document.
   A TRILL solution might or might not be easily adaptable to handling
   provider VLANs.

3.7.  Operational 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 equivalent to an existing link layer
   component.  Such equivalence provides a validation model for the
   architecture and a 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
   exchange a TRILL solution component or components 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 a TRILL solution's components were equivalent to a
   single IEEE 802.1D bridge, it would mean that they would -- as a
   whole - participate in the STP.  This need not require that TRILL
   solution components 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 IEEE 802.1 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 may address broadcast and multicast frame distribution,
   VLAN support, and snooping of ARP and IPv6 neighbor discovery.

   In addition, there may be optimizations which make the implementation
   of a TRILL solution easier than roughly equivalent existing bridge
   devices.  For example, in many bridged LANs, there are topologies
   such that central ("core") bridges which have both a greater volume
   of traffic flowing through them as well as traffic to and from a

Touch & Perlman              Informational                     [Page 12]

RFC 5556            TRILL: Problem and Applicability            May 2009

   larger variety of end station than do non-core bridges.  Thus means
   that such core bridges need to learn a large number of end station
   addresses and need to do lookups based on such addresses very
   rapidly.  This might require large high speed content addressable
   memory making implementation of such core bridges difficult.
   Although a TRILL solution need not provide such optimizations, it may
   reduce the need for such large, high speed content addressable
   memories or provide other similar optimizations.

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 deployed 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

   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 deployed 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 [RFC3376] [RFC4286].  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
   [RFC4861].  Other layers may similarly be snooped, notably ARP
   packets, for similar reasons as for IPv4 [RFC826].

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

Touch & Perlman              Informational                     [Page 13]

RFC 5556            TRILL: Problem and Applicability            May 2009

   TRILL solutions are intended to address problems of path efficiency
   and concentration, inability to multipath, and path 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

   TRILL solutions are not intended to span separate Ethernet link
   subnets interconnected by network-layer (e.g., router) devices,
   except via link-layer tunnels, 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 components of only one TRILL solution, 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 single Ethernet link subnet, and mechanisms
   should be included to enforce whatever decision is made.

   TRILL solutions need not 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.

Touch & Perlman              Informational                     [Page 14]

RFC 5556            TRILL: Problem and Applicability            May 2009

   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 to secure
   IPv6 neighbor discovery [RFC3971].

6.  Acknowledgments

   Portions of this document are based on documents that describe a
   preliminary solution, and on a related network-layer solution [Pe04]
   [Pe05] [To03].  Donald Eastlake III provided substantial text and
   comments.  Additional comments and feedback were provided by the
   members of the IETF TRILL WG, in which this document was developed,
   and by the IESG.

   This document was prepared using 2-Word-v2.0.template.dot.

7.  Informative References

   [IEEE04]  IEEE 802.1D bridging standard, "IEEE Standard for Local and
             metropolitan area networks: Media Access Control (MAC)
             Bridges", (incorporates 802.1w), Jun. 2004.

   [IEEE06]  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 2006.

   [Me04]    Myers, A., T.E. Ng, H. Zhang, "Rethinking the Service
             Model: Scaling Ethernet to a Million Nodes", Proc. ACM
             Third Workshop on Hot Topics in Networks (HotNets-III),
             Mar. 2004.

   [Pe99]    Perlman, R., "Interconnection: Bridges, Routers, Switches,
             and Internetworking Protocols", Addison Wesley, Chapter 3,

   [Pe04]    Perlman, R., "RBridges: Transparent Routing", Proc. Infocom
             2005, Mar. 2004.

   [Pe05]    Perlman, R., J. Touch, A. Yegin, "RBridges: Transparent
             Routing," (expired work in progress), Apr. 2004 - May 2005.

Touch & Perlman              Informational                     [Page 15]

RFC 5556            TRILL: Problem and Applicability            May 2009

   [RFC826]  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, November 1982.

   [RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers",
             RFC 1812, June 1995.

   [RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
             Thyagarajan, "Internet Group Management Protocol, Version
             3", RFC 3376, October 2002.

   [RFC3819] Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D.,
             Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
             Wood, "Advice for Internet Subnetwork Designers", BCP 89,
             RFC 3819, July 2004.

   [RFC3927] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
             Configuration of IPv4 Link-Local Addresses", RFC 3927, May

   [RFC3971] Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
             "SEcure Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [RFC4286] Rosenberg, J., "Extensible Markup Language (XML) Formats
             for Representing Resource Lists", RFC 4826, May 2007.

   [RFC4541] Christensen, M., Kimball, K., and F. Solensky,
             "Considerations for Internet Group Management Protocol
             (IGMP) and Multicast Listener Discovery (MLD) Snooping
             Switches", RFC 4541, May 2006.

   [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
             "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
             September 2007.

   [To03]    Touch, J., Y. Wang, L. Eggert, 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.

Touch & Perlman              Informational                     [Page 16]

RFC 5556            TRILL: Problem and Applicability            May 2009

Authors' 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
   16 Network Circle
   Menlo Park, CA 94025

   EMail: Radia.Perlman@sun.com

Touch & Perlman              Informational                     [Page 17]