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Versions: 00 01 draft-ietf-trill-rbridge-arch

     Network Working Group                             Eric Gray, Editor
     Internet Draft                                             Ericsson
     Expires: December, 2006
     
                                                           June 26, 2006
     
     
     
                The Architecture of an RBridge Solution to TRILL
                      draft-gray-trill-rbridge-arch-01.txt
     
     
     Status of this Memo
     
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        This Internet-Draft will expire on December 26, 2006.
     
     
     
     Abstract
     
        RBridges are link layer (L2) devices that use routing protocols
        as a control plane. They combine several of the benefits of the
        link layer with network layer routing benefits. RBridges use
        existing link state routing to provide higher RBridge to RBridge
        cross-section bandwidth, fast convergence on reconfiguration,
     
     
     
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        and more robustness under link interruption than an equivalent
        set of conventional bridges using existing spanning tree
        forwarding. They are intended to apply to similar L2 network
        sizes as conventional bridges and are intended to be backward
        compatible with those bridges as both ingress/egress and
        transit. They also attempt to retain as much 'plug and play' as
        is already available in existing bridges. This document proposes
        an RBridge system as a solution to the TRILL problem. It also
        defines the RBridge architecture, defines its terminology, and
        describes basic components and desired behavior. One or more
        separate documents specify the protocols and mechanisms that
        satisfy the architecture presented herein.
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
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     Table of Contents
     
     
        1. Introduction................................................4
        2. Background..................................................6
           2.1. Existing Terminology...................................6
           2.2. RBridge Terminology...................................10
        3. Components.................................................13
           3.1. RBridge Device........................................13
           3.2. RBrdige Data Model....................................14
              3.2.1. CFT..............................................14
              3.2.2. CFT-IRT..........................................14
              3.2.3. CTT..............................................16
        4. Functional Description.....................................16
           4.1. CRED Auto-configuration...............................16
           4.2. RBridge Peer Discovery................................19
           4.3. Tunneling.............................................20
           4.4. RBridge General Operation.............................21
           4.5. Ingress/Egress Operations.............................22
           4.6. Transit Forwarding Operations.........................24
              4.6.1. Unicast..........................................24
              4.6.2. Broadcast, Multicast and Flooding................24
                 4.6.2.1. Broadcast...................................25
                 4.6.2.2. Multicast...................................26
                 4.6.2.3. Flooding....................................27
           4.7. Routing Protocol Operation............................28
           4.8. Other Bridging and Ethernet Protocol Operations.......29
        5. How RBridges Address TRILL.................................29
        6. Conclusions................................................29
        7. Security Considerations....................................30
        8. IANA Considerations........................................30
        9. Acknowledgments............................................31
        10. References................................................31
           10.1. Normative References.................................31
           10.2. Informative References...............................31
        Author's Addresses............................................32
        Intellectual Property Statement...............................32
        Disclaimer of Validity........................................33
        Copyright Statement...........................................33
        Acknowledgment................................................33
     
     
     
     
     
     
     
     
     
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     1. Introduction
     
        This document describes an architecture that addresses the TRILL
        problem and applicability statement [2]. This architecture is
        composed of a set of devices called RBridges that cooperate
        together within an Ethernet network to provide a layer two
        delivery service that makes efficient use of available links
        using a link state routing protocol. The service provided is
        analogous to creation of a single, virtual device composed of an
        overlay of tunnels, constructed between RBridge devices, using
        link state routing. RBridges thus support increased RBridge to
        RBridge bandwidth and fault tolerance, when compared to
        conventional Ethernet bridges (which forward frames via a
        spanning tree), while still being compatible with bridges and
        hubs.
     
        The principal objectives of this architecture is to provide an
        overview of the use of these RBridges in meeting the following
        goals:
     
          1) Provide a form of optimized layer two delivery service.
     
          2) Use existing technology as much as possible.
     
          3) Allow for configuration free deployment.
     
        In providing a (optimized) layer two (L2) service, key factors
        we want to maintain are: transparency to higher layer (layer 3
        and above) delivery services and mechanisms, and use of location
        independent addressing. Optimization of the L2 delivery service
        consists of: use of all available paths and support for pruning
        of multicast traffic delivery paths.
     
        To accomplish the goal of using existing technologies as much as
        possible, we intend to specify minimal extensions (if required)
        to one or more existing link-state routing protocols, as well as
        defining the specific sub-set of existing bridging technologies
        this architecture makes use of.
     
        The extent to which routing protocol extensions may be required
        depends on the closeness of the "fit" of any chosen routing
        protocol to RBridge protocol requirements. See [6] for further
        information on these requirements. The use of a specific routing
        protocol - along with appropriate extensions and enhancements -
        will be defined in corresponding RBridge protocol specifications
        (see [3] for example).
     
     
     
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        Specific protocol specifications will also describe the details
        of interactions between the RBridge protocol and specific L2
        technologies - i.e. - Virtual Local Area Networking (VLAN), L2
        Multicast, etc.
     
        As an overview, however, the intention is to use a link-state
        routing protocol to accomplish the following:
     
          1) Discover RBridge peers.
     
          2) Determine RBridge link topology.
     
          3) Advertise L2 reachability information.
     
          4) Establish L2 delivery using shortest path (verses STP).
     
        There are additional RBridge protocol requirements - above and
        beyond those addressed by any existing routing protocol - that
        are identified in this document and need to be addressed in
        corresponding RBridge protocol specifications.
     
        To allow for configuration free deployment, specific protocol
        specifications need to explicitly define the conditions under
        which RBridges may - and may not - be deployed as-is (plug and
        play), and the mechanisms that are required to allow this. For
        example, the first requirement any RBridge protocol must meet is
        to derive information required by link-state routing protocol(s)
        for protocol start-up and communications between peers - such as
        higher-layer addressing and/or identifiers, encapsulation header
        information, etc.
     
        At the abstract level, RBridges need to maintain the following
        information:
     
          1) Peer information,
     
          2) Topology information,
     
          3) Forwarding information -
     
               a. unicast,
     
               b. flooded, and
     
               c. multicast.
     
     
     
     
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        Peer information may be acquired via the routing protocol, or
        may be discovered as a result of RBridge-specific peer discovery
        mechanisms.  Topology information is expected to be acquired
        exclusively via the link-state routing protocol.
     
        Forwarding information is derived from the combination of bridge
        learning, snooping of multicast-related protocols (e.g. - IGMP),
        and routing advertisements and path computations using the link-
        state routing protocol.
     
        The remainder of this document outlines the TRILL architecture
        of an RBridge-based solution and describes RBridge components,
        interactions and functions. Note that this document is not
        intended to represent the only solution to the TRILL problem
        statement, nor does it specify the protocols that instantiate
        this architecture - or that only one such set of protocols is
        prescribed. The former may be contained in other architecture
        documents and the latter would be contained in separate
        specification documents (see - e.g. - [3]).
     
     2. Background
     
        This architecture is based on the RBridge system described in an
        Infocom paper [1]. That paper describes the RBridge system as a
        specific instance; this document abstracts architectural
        features only. The remainder of this section describes the
        terminology of this document, which may differ from that of the
        original paper.
     
     2.1. Existing Terminology
     
        The following terminology is defined in other documents. A brief
        definition is included in this section for convenience and - in
        some cases - to remove any ambiguity in how the term may be used
        in this document, as well as derivative documents intended to
        specify components, protocol, behavior and encapsulation
        relative to the architecture specified in this document.
     
        o  802: IEEE Specification for Ethernet, i.e., including hubs
           and switches.
     
        o  802.1D: IEEE Specification for bridged Ethernet, including
           the BPDUs used in spanning tree protocol (STP) [5].
     
     
     
     
     
     
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        o  ARP: Address Resolution Protocol - a protocol used to find an
           address of form X, given a corresponding address of form Y.
           In this document, ARP refers to the well-known protocol used
           to resolve L2 (MAC) addresses, using a given L3 (IP) address.
     
        o  Bridge: an Ethernet (L2, 802.1D) device with multiple ports
           that receives incoming frames on a port and transmits them on
           some or all of the other ports; bridges support both bridge
           learning and STP. Transparent bridges do not modify the L2
           PDU being forwarded.
     
        o  Bridge Learning: process by which a switch or bridge
           determines on which single outgoing port to transmit (forward
           or copy) an incoming frame. This process depends on
           consistent forwarding as "learning" uses the source MAC
           address of frames received on each interface. Layer 2 (L2)
           forwarding devices "learn" the location of L2 destinations by
           peeking at layer 2 source addresses during frame forwarding,
           and store the association of source address and receiving
           interface.  L2 forwarding devices use this information to
           create "filtering database" entries and - gradually -
           eliminate the need for flooding.
     
        o  Bridge Protocol Data Unit (BPDU): the frame type associated
           with bridge control functions (for example: STP/RSTP).
     
        o  Bridge Spanning Tree (BST): an Ethernet (L2, 802.1D)
           forwarding protocol based on the topology of a spanning tree.
     
        o  Broadcast Domain: the set of (layer 2) devices that must be
           reached (or reachable) by any (layer 2) broadcast traffic
           injected into the domain.
     
        o  Broadcast Traffic: traffic intended for receipt by all
           devices in a broadcast domain.
     
        o  Ethernet: See "802" above.
     
        o  Filtering Database - database containing association
           information of (source layer 2 address, arrival interface).
           The interface that is associated with a specific layer 2
           source address, is the same interface which is used to
           forward frames having that address as a destination.  When a
           layer 2 forwarding device has no entry for the destination
           layer 2 address of any frame it receives, the frame is
           "flooded".
     
     
     
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        o  Flooded Traffic - traffic forwarded on all interfaces, except
           those on which it was received, within the same broadcast
           domain. Flooding is the mechanism by which traffic is
           delivered to a destination that is currently "unknown" (i.e.
           - either not yet "learned", or aged out of the "filtering
           database").
     
        o  Flooding - the process of forwarding traffic to ensure that
           frames reach all possible destinations when the destination
           location is not known.  In "flooding", a layer 2 forwarding
           device forwards a   frame for any destination not "known"
           (i.e. - not in the filtering database) on every active
           interface except that one on which it was received. See also
           VLAN flooding.
     
        o  Frame: in this document, frame refers to an Ethernet (L2)
           unit of transmission (PDU), including header, data, and
           trailer (or payload and envelope).
     
        o  Hub: an Ethernet (L2, 802) device with multiple ports which
           transparently transmits frames arriving on any port to all
           other ports.  This is a functional definition, as there are
           devices that combine this function with certain bridge-like
           functions that may - under certain conditions - be referred
           to as "hubs".
     
        o  IGP: Interior Gateway Protocol - any of the potential (link-
           state) routing protocols candidates considered as potentially
           useful RBridge routing protocols.
     
        o  IS-IS: Intermediate System to Intermediate System routing
           protocol. See [8] for further information on IS-IS.
     
        o  LAN: Local Area Network. A LAN is an L2 forwarding domain.
           This term is synonymous with Ethernet Subnet in the context
           of this document.
     
        o  MAC: Media Access Control - mechanisms and addressing for L2
           frame forwarding.
     
        o  Multicast Forwarding: forwarding methods that apply to frames
           with broadcast or multicast destination MAC addresses.
     
        o  Node: a device with an L2 (MAC) address that sources and/or
           sinks L2 frames.
     
     
     
     
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        o  OSPF: Open Shortest Path First routing protocol. See [7] and
           [9] for further information on OSPF.
     
        o  Packet: in this document, packet refers to L3 (or above) data
           transmission units (PDU - e.g. - an IP Packet (RFC791 [4]),
           including header and data.
     
        o  PDU: Protocol Data Unit - unit of data to be transmitted by a
           protocol. To distinguish L2 and L3 PDUs, we refer to L2 PDUs
           as "frames" and L3 PDUs as "packets" in this (and related)
           document(s).
     
        o  Router: a device that performs IP (L3) forwarding (the
           "routing function"); RBridges typically do not span routers
           (i.e. - provide a connection from one router interface to
           another router interface on the same router).
     
        o  Routing Function: in this document, the "routing function"
           consists of forwarding IP packets between L2 broadcast
           domains, based on L3 addressing and forwarding information.
           In the process of performing the "routing function", devices
           (typically routers) usually forward packets from one L2
           broadcast domain to another (one, or more in the IP multicast
           case) - distinct - L2 broadcast domain(s). RBridges cannot
           span the routing function.
     
        o  Segment: an Ethernet link, either a single physical link or
           emulation thereof (e.g., via hubs) or a logical link or
           emulation thereof (e.g., via bridges).
     
        o  Spanning Tree Protocol (STP): an Ethernet (802.1D) protocol
           for establishing and maintaining a single spanning tree among
           all the bridges on a local Ethernet segment. Also, Rapid
           Spanning Tree Protocol (RSTP). In this document, STP and RSTP
           are considered to be the same.
     
        o  Spanning Tree Table (STT): a table containing port activation
           status information as determined during STP.
     
        o  SPF: Shortest Path First - the algorithm name associated with
           routing, used to determine a shortest path graph traversal.
     
     
     
     
     
     
     
     
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        o  Subnet, Ethernet: a single segment, or a set of segments
           interconnected by a CRED; in the latter case, the subnet may
           or may not be equivalent to a single segment. Also a subnet
           may be referred to as a broadcast domain or LAN. By
           definition, all nodes within an Ethernet Subnet (broadcast
           domain or LAN) must have L2 connectivity with all other nodes
           in the same Ethernet subnet.
     
        o  Switch: an Ethernet (L2, 802) device with multiple point-to-
           point ports which transmits (forwards or copies) frames that
           arrive on one port to one or more other ports. Switches may
           include bridge learning. Switches may also exclude STP/RSTP.
     
        o  TRILL: Transparent Interconnect over Lots of Links - the
           working group and working name for the problem domain to be
           addressed in this document.
     
        o  Unicast Forwarding: forwarding methods that apply to frames
           with unicast destination MAC addresses.
     
        o  Unknown Destination - a destination for which a receiving
           device has no filtering database entry.  Destination (layer
           2) addresses are typically "learned" by (layer 2) forwarding
           devices via a process commonly referred to as "bridge
           learning".
     
        o  VLAN: Virtual Local Area Network. VLANs in general fall into
           two categories: link (or port) specific VLANs and tagged
           VLANs. In the former case, all frames forwarded and all
           directly connected nodes are assumed to be part of a single
           VLAN.  In the latter case, VLAN tagged frames are used to
           distinguish which VLAN each frame is intended for.
     
        o  VLAN Flooding: flooding as described previously, except that
           frames are only forwarded on those interfaces configured for
           participation in the applicable VLAN.
     
     2.2. RBridge Terminology
     
        The following terms are defined in this document and intended
        for use in derivative documents intended to specify components,
        protocol, behavior and encapsulation relative to the
        architecture specified in this document.
     
        o  CRED: Cooperating RBridges and Encapsulation Tunnels - a
           topological construct consisting of a set of cooperating
           RBridges, and the forwarding tunnels connecting them.
     
     
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        o  CRED Forwarding Table (CFT): the per-hop forwarding table
           populated by the RBridge Routing Protocol; forwarding within
           the CRED is based on a lookup of the CRED Transit Header
           (CTH) encapsulated within the outermost received L2 header,
           rather than that encapsulating L2 header. The outermost L2
           encapsulation in this case includes the source MAC address of
           the immediate upstream RBridge transmitting the frame and
           destination MAC address of the receiving RBridge for use in
           the unicast forwarding case.
     
        o  CFT-IRT: a forwarding table used for propagation of
           broadcast, multicast or flooded frames along the Ingress
           RBridge Tree (IRT).
     
        o  CRED Transit Header (CTH): a 'shim' header that encapsulates
           the ingress L2 frame and persists throughout the transit of a
           CRED, which is further encapsulated within a hop-by-hop L2
           header (and trailer). The hop-by-hop L2 encapsulation in this
           case includes the source MAC address of the immediate
           upstream RBridge transmitting the frame and destination MAC
           address of the receiving RBridge - at least in the unicast
           forwarding case.
     
        o  CRED Transit Table (CTT): a table that maps ingress frame L2
           destinations to egress RBridge addresses, used to determine
           encapsulation of ingress frames for transit of the CRED.
     
        o  Cooperating RBridges - those RBridges within a single
           Ethernet Subnet (broadcast domain or LAN) not having been
           configured to ignore each other. By default, all RBridges
           within a single Ethernet subnet will cooperate with each
           other. It is possible for implementations to allow for
           configuration that will restrict "cooperation" between an
           RBridge and an apparent neighboring RBridge.  One reason why
           this might occur is if the trust model that applies in a
           particular deployment imposes a need for configuration of
           security information.  By default no such configuration is
           required however - should it be used in any specific scenario
           - it is possible (either deliberately or inadvertently) to
           configure neighboring RBridges so that they do not cooperate.
           In the remainder of this document, all RBridges are assumed
           to be in a cooperating (default) configuration.
     
     
     
     
     
     
     
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        o  Designated RBridge (DR): the RBridge associated with ingress
           and egress traffic to a particular Ethernet link having
           shared access among multiple RBridges; that RBridge is such a
           link's "Designated RBridge". The Designated RBridge is
           determined by an election process among those RBridges having
           shared access via a single Segment.
     
        o  Edge RBridge (edge of a CRED): describes RBridges that serve
           to ingress frames into the CRED and egress frames from the
           CRED. L2 frames transiting an RBridge CRED enter, and leave,
           it via an edge RBridge.
     
        o  Egress RBridge: for any specific frame, the RBridge through
           which that frame leaves the CRED. For frames transiting a
           CRED, the egress RBridge is an edge RBridge where RBridge
           encapsulation is removed from the transit frames prior to
           exiting the CRED.
     
        o  Forwarding Tunnels: in this document, CRED Forwarding Tunnels
           (or Forwarding Tunnels) is used to refer to the paths for
           forwarding transit frames, encapsulated at an RBridge ingress
           and decapsulated at an RBridge egress.
     
        o  Ingress RBridge: for any specific frame, the RBridge through
           which that frame enters the CRED. For frames transiting a
           CRED, the ingress RBridge is the edge RBridge where RBridge
           encapsulation is added to the transit traffic entering the
           CRED.
     
        o  Ingress RBridge Tree: a tree computed for each edge RBridge -
           and potentially for each VLAN in which that RBridge
           participates - for delivery of broadcast, multicast and
           flooded frames from that RBridge to all relevant egress
           RBridges. This is the point-to-multipoint delivery tree used
           by an ingress RBridge to deliver multicast, broadcast or
           flooded traffic.  The tree consists of a set of one or more
           next-hops to be used when the ingress RBridge receives a
           multicast or broadcast frame (frame with a multicast or
           broadcast destination address), or frame with unknown
           destination addresses.  If forwarding frames hop-by-hop, next
           hop RBridges will, in turn, have a similar set of one or more
           next-hops to be used for forwarding these frames - when
           received from an upstream, or ingress, RBridge.  This
           progression continues until frames arrive at egress RBridges.
     
        o  LPT: Learned Port Table. See Filtering Database.
     
     
     
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        o  RBridge: a logical device as specified in this document,
           which incorporate both routing and bridging features, thus
           allowing for the achievement of TRILL Architecture goals. A
           single RBridge device which can aggregate with other RBridge
           devices to create a CRED.
     
     3. Components
     
        A CRED is composed of RBridge devices and the forwarding tunnels
        that connect them; all other Ethernet link subnet devices, such
        as bridges, hubs, and nodes, operate conventionally in the
        presence of an RBridge.
     
     3.1. RBridge Device
     
        An RBridge is a bridge-like device that forwards frames on an
        Ethernet link segment. It has one or more Ethernet ports which
        may be wired or wireless; the particular physical layer is not
        relevant. An RBridge is defined more by its behavior than its
        structure, although it contains three tables which distinguish
        it from conventional bridges.
     
        Conventional bridges contain a learned port table (LPT), or
        filtering database, and a spanning tree table (STT). The LPT
        allows a bridge to avoid flooding all received frames, as is
        typical for a hub or repeater. The bridge learns which nodes are
        accessible from a particular port by assuming bi-directional
        consistency: the source addresses of incoming frames indicate
        that the incoming port is to be used as output for frames
        destined to that address. Incoming frames are checked against
        the LPT and forwarded to the particular port if a match occurs,
        otherwise they are flooded out all ports except the incoming
        port.
     
        The STT indicates the ports used in the spanning tree. Details
        of STP operation are out of scope for this document, however the
        result of STP is to disable ports which would otherwise result
        in more than one path traversal of the spanning tree.
     
        RBridges, by comparison, have a CRED Forwarding Table (CFT -
        used for unicast forwarding of RBridge encapsulated frames
        across the CRED), CFT-IRT (used for flooding, broadcast or
        multicast forwarding of RBridge encapsulated frames across the
        CRED) and a CRED Transit Table (CTT - used by the ingress
        RBridge to determine what encapsulation to use for frames
        received as un-encapsulated from non-RBridge devices), described
        in the following sections.
     
     
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     3.2. RBrdige Data Model
     
        The following tables represent the logical model of the data
        required by RBridges in forwarding unicast and multicast data
        across a CRED.
     
     3.2.1. CFT
     
        The CFT is a forwarding table for unicast traffic within the
        CRED, allowing tunneled traffic to transit the CRED from ingress
        to egress. The size of a fully populated CFT at each RBridge is
        maximally bounded by the product of the number of directly
        connected RBridge peers (where "directly connected" in this
        context refers to RBridges connected to each other without
        transiting one or more additional RBridges) and VLANs. RBridges
        may have separate CFTs for each VLAN, if this is supported by
        configuration. The CFT is continually maintained by RBridge
        routing protocol (see Section 4.7).
     
        The CFT contains data specific to RBridge forwarding for unicast
        traffic. The specific fields contained in this table are to be
        defined in RBridge protocol specifications. In the abstract,
        however, the table should contain forwarding direction and
        encapsulation associated with an RBridge encapsulated frame
        received - determined by the "shim" header destination and VLAN
        (if applicable).
     
     3.2.2. CFT-IRT
     
        The CFT-IRT consists of a special-case set of forwarding entries
        used for support of Ingress RBridge Trees (IRT). The CFT-IRT may
        be part of the CFT, or instantiated as a separate table.
     
        In discussing entries to be included in the CFT-IRT, the
        following entities are temporarily defined, or further
        qualified:
     
        o  Ingress RBridge - the RBridge that is the head end of an IRT.
           All RBridges within a CRED are potential ingress RBridges.
     
        o  Egress RBridge - an RBridge that is the tail end of a path
           corresponding to a specific CFT-IRT entry. All RBridges
           within a CRED are potential egress RBridges. Not all RBridges
           within a CRED will be on the shortest path between any
           ingress RBridge and any other egress RBridge.
     
     
     
     
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        o  Local RBridge - the RBridge that forms and maintains the CFT-
           IRT entry (or entries) under discussion. The local RBridge
           may be an Ingress RBridge, or an egress RBridge with respect
           to any set of entries in the CFT-IRT.
     
        o  RBridge CRED Egress Interface - an interface on any RBridge
           where a transit RBridge encapsulated frame would be
           decapsulated prior to forwarding. With respect to such an
           interface, the local RBridge is the egress RBridge.
     
        Each local RBridge will maintain a set of entries for at least
        the following - corresponding to a subset of all possible
        forwarding paths:
     
        o  Zero or more entries grouped for each ingress RBridge - keyed
           by the ingress RBridge identifier - used to determine
           downstream forwarding of broadcast, multicast, and flooded
           frames originally RBridge encapsulated by that ingress within
           the CRED.
     
        o  Corresponding to each of these entry groups, one entry for
           each of zero or more egress RBridge - where the local RBridge
           is on the shortest path toward that egress RBridge.
     
        o  Corresponding to each of these entry groups, one entry for
           each of zero or more CRED egress interfaces.
     
        Each entry would contain an indication of which single interface
        a broadcast, multicast or flooded frame would be forwarded for
        each (ingress RBridge, egress RBridge) pair - as well as any
        required encapsulation information, etc. required for forwarding
        on that interface, toward the corresponding specific egress
        RBridge.
     
        A local RBridge could maintain a full set of entries from every
        RBridge to every other RBridge, however - depending on topology
        - only a subset of these entries would ever be used.  In
        addition, a topology change that changed selection of shortest
        paths would also very likely change other elements of the
        entries, negating possible benefits from having pre-computed
        CFT-IRT entries.
     
        CFT-IRT entries should also include VLAN identification
        information relative to each set of ingress RBridge, to allow
        scoping of broadcast, multicast and flooding forwarding by
        configured VLANs.
     
     
     
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        CFT-IRT entries should also include Multicast-Group Address
        specific information relative to each egress RBridge that is a
        member of a given well-known multicast group, to allow scoping
        of multicast forwarding by multicast group.
     
        Implicit in this data model is the assumption that the "shim"
        header encapsulation will contain information that explicitly
        identifies the CRED ingress RBridge for any broadcast, multicast
        or flooded frame.
     
        How the CFT-IRT is maintained will be defined in appropriate
        protocol specifications used to instantiate this architecture.
        The protocol specification needs to include mechanisms and
        procedures required to establish and maintain the CFT-IRT in
        consideration of potential SPF recomputations resulting from
        network topology changes.
     
     3.2.3. CTT
     
        The CTT determines how arriving traffic will be encapsulated,
        for forwarding to the egress RBridge, via the CRED. The CTT can
        be considered a version of the LPT that treats the CRED, as a
        whole, as another port. It becomes configured in much the same
        way as the LPT: by snooping incoming traffic, and assuming bi-
        directional consistency (see Section 0). The information is
        learned at the egress RBridge and propagated to all other
        RBridges in the CRED via the RBridge routing protocol (also
        Section 0). The CTT may be as large as the number of nodes on
        the Ethernet subnet, across all VLANs. RBridges may have
        separate CTTs for each VLAN, if separate VLANs are supported by
        configuration.
     
        The CTT essentially determines the tunnel encapsulation used to
        transport each specific frame across the CRED.
     
     4. Functional Description
     
        The RBridge Architecture is largely defined by RBridge behavior;
        the logical components are minimal, as outlined in Section 3.
     
     4.1. CRED Auto-configuration
     
        Cooperating RBridges self-organize to compose a single CRED
        system. Consider first a set of bridges on a single Ethernet
        link subnet (Figure 1). Here bridges are shown as 'b', hubs as
        'h', and nodes as 'N'; bridges and hubs are numbered. Note that
        the figure does not distinguish between types of nodes, i.e.,
     
     
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        hosts and routers; both are end nodes at the link layer, and are
        otherwise indistinguishable to L2 forwarding devices. Bridges in
        this topology organize into a single spanning tree, as shown by
        double lines ('=', '||', and '//') in the figure.
     
                                  N       N---b3---N
                                  |           ||
                                  |           ||
                             N---h1--b4===b5==h2==b6
                                     |   //   |   ||
                                     |  //    N   ||
                                     | //         ||
                                 N---b7====b8-----b9-----N
                                           |      |\
                                           |      | \
                                           N      N  N
     
              Figure 1 Conventionally bridged Ethernet link subnet
     
        It is useful to note that hubs are relatively transparent to
        bridges, both for traffic from nodes to bridges (h1) and for
        traffic between bridges (h2). Also note that the same hub can
        support traffic between bridges and from a host to a bridge
        (h2), but that the spanning tree is exclusively between bridges.
        Bridges are thus compatible with hubs, both as transits and
        ingress/egress.
     
        A CRED operates similarly, and can be viewed as a variant of the
        way bridges self-organize. Figure 2 shows the same topology
        where some of the bridges are replaced by RBridges. In this
        figure, stars ('*') represent the paths the RBridge is capable
        of utilizing, due to the use of link state routing. RBridges can
        tunnel directly to each other (r4-r5), or through hubs (h2) or
        bridges (b8).
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
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                                  N       N---b3---N
                                  |           ||
                                  |           ||
                             N---h1--r4***r5**h2**r6
                                     *   *    |   *
                                     *  *     N   *
                                     * *          *
                                 N---r7****b8*****r9-----N
                                           |      |\
                                           |      | \
                                           N      N  N
     
                     Figure 2 RBridged Ethernet link subnet
     
        Every node in a CRED is considered to have a primary point of
        attachment to the CRED, as defined by the Designated RBridge.
        Each Ethernet link segment attached to a CRED has a single
        Designated RBridge; that RBridge is where all traffic that
        transits the CRED enters and exits. In Figure 2, it is easy to
        see that the nodes off of h1 must attach at r4; the nodes off of
        b3, however, attach at either r5 or r6, depending on which is
        the Designated RBridge.
     
        Without loss of generality, an RBridge topology can be
        reorganized (ignoring link length) such that all nodes, hubs,
        and bridges are arranged around the periphery, and all RBridges
        are considered directly connected by their tunnels (Figure 3).
        Note that this view ignores the ways in which hubs and bridges
        may serve both on the ingress/egress and for transit, hence this
        view is not useful for traffic analysis. Using this view, it is
        easy to distinguish between RBridge to RBridge traffic and other
        traffic on shared devices, such as h2 and b8, because RBridge to
        RBridge traffic content is hidden from non RBridge devices by
        the RBridge encapsulation.
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
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                                  N       N---b3---N
                                  |           ||
                                  |           ||
                                  |           h2
                                  |          /| \
                                  |         / N  \
                                  |        /      \
                             N---h1--r4***r5******r6
                                     *   *        *
                                     *  *         *
                                     * *          *
                                 N---r7***********r9-----N
                                      \          /|\
                                       \        / | \
                                        \      /  N  N
                                         \    /
                                          \  /
                                           b8
                                           |
                                           N
     
               Figure 3 Reorganized RBridge Ethernet link subnet
     
     4.2. RBridge Peer Discovery
     
        Proper operation of the TRILL solution using RBridges depends on
        the existence of a mechanism for discovering peer RBridges and
        the RBridge topology. An accurate determination of RBridge
        topology is required in order to determine how traffic frames
        will flow in the topology and thus avoid the establishment of
        persistent loops in frame forwarding.
     
        The discovery mechanisms must use protocol messages which will
        be propagated throughout the LAN, or broadcast domain, in order
        to ensure that all RBridges in the same broadcast domain are
        discovered and to allow for accurate determination of RBridge
        topology.
     
        These protocol messages should be distinguished in a manner that
        is consistent with the chosen RBridge routing protocol, or any
        other discovery mechanism used. It is very likely that peer
        discovery will actually be done as part of the RBridge routing
        protocol's peer discovery; however this is to be determined by
        specific RBridge protocol specification(s).
     
        An RBridge intercepts protocol messages that it recognizes as
        being of this type (peer discovery), performs any processing
     
     
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        required and forwards these messages as required by the
        discovery protocol. For example, a receiving RBridge may first
        determine if it has seen this message before and insert itself
        in a list of RBridges traversed by this message prior to
        forwarding the message on at least all interfaces other than the
        one on which it was received.
     
        Note that forwarding the modified message on all interfaces in
        the example above is safe, if somewhat wasteful.
     
        RBridges must forward all other protocol messages in a manner
        consistent with L2 addressing and forwarding - as would be done
        by a typical 802.1D bridge. This includes any frames of the same
        type that are - for one reason or another - not recognized by
        the receiving RBridge.
     
        Note that forwarding unrecognized messages - even when of the
        same type - has the effect of providing some degree of
        robustness in the solution against configuration errors and
        against future variations of the discovery protocol.
     
        Handling of 802.1D BPDUs is as determined in section 0.
     
     4.3. Tunneling
     
        RBridges pass encapsulated frame traffic to each other
        effectively using tunnels. These tunnels use an Ethernet link
        layer header, together with a shim header.
     
        Specifics of encapsulation are to be defined in appropriate
        protocol/encapsulation specifications.
     
        It is the combination of the encapsulation that distinguishes
        RBridge to RBridge traffic from other traffic. The link header
        includes source and destination addresses, which typically
        identify the ingress and egress RBridges. For incoming multicast
        and broadcast traffic, one of these addresses may represent the
        multicast group or broadcast address. Additionally, these
        addresses may be VLAN-specific, i.e., such that each ingress and
        egress address have per-VLAN addresses.
     
        The additional shim header is required to support loop
        prevention for traffic within the CRED; traffic loops in
        forwarding between RBridges and non-RBridge nodes, as well as
        across non-RBridge devices between RBridges, is prevented by
        loop prevention mechanisms that are beyond the scope of this
        document (but may include a TTL-like mechanism, mechanisms to
     
     
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        establish a loop free topology - such as STP/RSP - or both) on
        the applicable LAN segments.
     
        The shim header and encapsulation:
     
        o  must clearly identify the traffic as RBridge traffic - the
           outer Ethernet header may, for instance, use a protocol
           number unique to RBridges;
     
        o  should also identify a specific (egress) RBridge - the shim
           header may, for example, include an identifier unique to the
           egress RBridge;
     
        o  should include the RBridge transit route, a hopcount, or a
           timestamp to prevent indefinite looping of a frame.
     
     4.4. RBridge General Operation
     
        Operations that apply to all RBridges include peer and topology
        discovery (which may include negotiation of RBridge
        identifiers), Designated RBridge election, link-state routing,
        SPF computation and advertising reach-ability for specific L2
        (MAC Ethernet destination) addresses within a broadcast domain.
     
        In addition, all RBridges will compute Ingress RBridge Trees for
        delivery of (potentially VLAN scoped) broadcast, multicast and
        flooded frames to each peer RBridge. Setting up these trees
        early is important as there is otherwise no means for frame
        delivery across the CRED during the learning phase. Because it
        is very likely to be impossible (at an early stage) for RBridges
        to determine which RBridges are edge RBridges, it is preferable
        that each RBridge compute these trees for all RBridges as early
        as possible - even if some entries will not be used.
     
        The initial phase is the peer and topology discovery phase. This
        should continue for a sufficient amount of time to reduce the
        amount of re-negotiation (Designated RBridge and - possibly -
        identifiers) and re-computation that will be triggered by
        discovery of new peers. The timer values selected for delaying
        the next phase should take into account the time required for
        local STP and availability of segment connectivity between
        RBridge peers.
     
        The next phase is election of Designated RBridges for all shared
        access segments. This phase cannot complete before completion of
        peer and topology discovery. In parallel, RBridge routing
        protocol should begin the process of building the link-state
     
     
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        information - assuming this was not done during the peer and
        topology discovery phase.
     
        At about this time, RBridges should establish ingress RBridge
        trees.
     
        Once RBridges have established Ingress RBridge Trees, the
        learning and forwarding phase may begin. In this phase, RBridges
        initially forward frames by flooding them via Ingress RBridge
        Tree(s). Also during this phase, RBridges begin "learning" MAC
        address locations from local segments and propagating L2 reach-
        ability information via the RBridge routing protocol to all
        other RBridges.  Gradually, the CFT will be built up for all
        RBridges, and fewer frames will require flooding via the Ingress
        RBridge Tree(s).
     
        The learning phase typically does not complete. Consequently,
        the learning phase is also the operational phase. During the
        combined learning and operational phase, all RBridges maintain
        both an Ingress RBridge Tree and a CFT. RBridges not elected as
        Designated RBridge may be required to become one in the event
        that the DR goes off-line.
     
     4.5. Ingress/Egress Operations
     
        Operation specific to edge RBridges involves RBridge learning,
        advertisement, encapsulation (at ingress RBridges) and
        decapsulation (at egress RBridges).
     
        As described elsewhere, RBridge learning is similar to typical
        bridge learning - i.e. - all RBridges listen promiscuously to L2
        Frames on a local LAN segment and acquire location information
        associated with source MAC addresses in L2 frames they observe.
     
        By convention, a Designated RBridge election always occurs. In
        the degenerate case - where only one RBridge is connected to a
        specific Ethernet segment - obviously that RBridge will "win"
        the election and become the designated RBridge.
     
        With this convention, only the Designated RBridge performs
        RBridge learning for interface(s) connected to that segment.
     
        As each RBridge learns segment-local MAC source addresses, it
        creates an entry in its LPT that associates that MAC source
        address with the interface on which it was learned.
     
     
     
     
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        Periodically, as determined by the RBridge routing protocol,
        each RBridge advertises this learned information to its RBridge
        peers.
     
        These advertisements propagate to all edge RBridges (as
        potentially scoped by associated VLAN information for each
        advertisement). Each edge RBridge incorporates this information
        in the form of a CFT entry.
     
        RBridges also discover that they are an edge RBridge as a result
        of receiving un-encapsulated frames that require forwarding. If
        an RBridge is the Designated RBridge for a segment, and it has
        not previously learned that the MAC destination for a frame is
        local (this will be the case - for instance - for the very first
        frame it observes), then the RBridge would be required to
        forward (or flood) the frame via the CRED to all other RBridges
        (potentially within a VLAN scope).
     
        The RBridge in this case would flood the frame unless it has
        already created a unicast CFT entry for the frame's MAC
        destination address.  If it has a corresponding CFT, then it
        would use that.  This RBridge would be an ingress RBridge with
        respect to the frame being forwarded.
     
        The encapsulation used by this ingress RBridge would be
        determined by the CFT - if one exists -  or the CFT-equivalent
        entry for the Ingress RBridge Tree. The encapsulation - as
        discussed elsewhere - should include (in the shim header)
        information to identify the egress RBridge (for example, the
        RBridge identifier negotiated previously during the peer and
        topology discovery phase).
     
        When the encapsulated frame arrives at egress RBridge(s), it is
        decapsulated and forwarded via the egress interface(s) onto the
        local segment.
     
        Note that an egress RBridge will be the Designated RBridge on
        the local segment accessed via its egress interface(s). If the
        received frame does not correspond to a learned MAC destination
        address at an egress interface, it will forward the frame on all
        interfaces for which it is either the designated - or only -
        RBridge. If the received frame does correspond to a learned MAC
        destination address at an egress interface, the RBridge will
        forward the frame via that interface only.
     
     
     
     
     
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     4.6. Transit Forwarding Operations
     
        There two models for transit forwarding within a CRED: unicast
        frame forwarding for known destinations, and everything else.
        The difference between the two is in how the encapsulation is
        determined. Exactly one of these models will be selected - in
        any instantiation of this architecture- for each of the
        following forwarding modes:
     
        o  Unicast frame forwarding
        o  Forwarding of non-unicast frames
           o  Broadcast frame forwarding
           o  Multicast frame forwarding
           o  Frame flooding
     
     4.6.1. Unicast
     
        In unicast forwarding, the shim header is specific to the egress
        RBridge and MAC destination in the outer Ethernet encapsulation
        is specific to the next hop RBridge.
     
        Prior to preparing the frame for forwarding to the next hop
        RBridge, the MAC source address is examined and - if the MAC
        source address is an address of the local RBridge, the frame is
        discarded.
     
        As the frame is prepared for transmission at each RBridge, the
        next hop MAC destination information is determined at that local
        RBridge using a corresponding CFT based on the "shim" header.
        In addition, prior to re-writing the outer MAC destination
        address, the next hop MAC destination address is compared to the
        MAC source address of the outer Ethernet header and the frame is
        discarded if the two are equivalent.
     
     4.6.2. Broadcast, Multicast and Flooding
     
        Ingress RBridge Trees are used for forwarding of broadcast,
        multicast and unknown destination frames across the CRED. In a
        simple implementation, it is possible to use the CFT-IRT entries
        for all frames of these types.
     
        However, this approach results in potentially extreme
        inefficiencies in the multicast and unknown destination flooding
        cases.
     
        As a consequence, instantiations of this architecture should
        allow for local optimizations on a hop by hop basis.
     
     
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        Examples of such optimizations are included in the sections
        below.
     
     4.6.2.1. Broadcast
     
        The path followed in transit forwarding of broadcast frames will
        have been established through actions initiated by each RBridge
        (as any RBridge is eligible to subsequently become an ingress
        RBridge) in the process of computing CFT-IRT entries. Each
        RBridge assumes that it may be a transit as well as an ingress
        and egress RBridge and will establish forwarding information
        relative to itself and each of its peer RBridges, and stored in
        the CFT-IRT. CFT-IRT entries are computed at each RBridge for
        paths going toward all other RBridges - at least in cases where
        the RBridge performing CFT-IRT computations is on the shortest
        path.
     
        Forwarding information is in two forms: transit encapsulation
        information for interfaces over which the RBridge will forward a
        broadcast frame to one or more peer RBridges and a decapsulation
        indication for each interface over which the RBridge may egress
        frames from the CRED. In each case, the CFT-IRT includes some
        identification of the interface on which a frame is forwarded
        toward any specific egress RBridge for frames received from any
        specific ingress RBridge.
     
        Note that an interface over which an RBridge may egress frames
        is any interface for which the RBridge is a Designated RBridge.
        RBridges must not wait to determine that one (or more) non-
        RBridge Ethernet nodes is present in an interface before
        deciding to forward decapsulated broadcast frames on that
        interface.
     
        Forwarding information is selected for each broadcast frame
        received by any RBridge (based on identifying the ingress
        RBridge for the frame) for all corresponding CFT-IRT entries.
        Each RBridge is thus required to replicate one RBridge
        encapsulated broadcast frame for each interface that is
        determined from CFT-IRT entries corresponding to the frame's
        ingress RBridge. This includes decapsulated broadcast frames for
        each interface for which it is the designated RBridge.
     
        Note that frame replication and forwarding should be scoped by
        VLAN if VLAN support is provided. Also note that a Designated
        RBridge (DR) may be required to transmit a decapsulated frame on
        the interface on which it received the RBridge encapsulated
        frame.
     
     
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        This approach for broadcast forwarding might be considered to
        add complexity because replication occurs at all RBridges along
        the ingress RBridge tree, potentially for both RBridge
        encapsulated and decapsulated broadcast frames. However, the
        replication process is similar to replication of broadcast
        traffic in 802.1D bridges with the exception that additional
        replication may be required at each interface for egress from
        the CRED.
     
        Note that the additional replication associated with CRED egress
        may be made to exactly conform to 802.1D bridge broadcast
        replication in implementations that model a CRED egress as a
        separate logical interface.
     
        Using this approach results in one and only one copy of the
        broadcast frame being delivered to each egress RBridge.
     
     4.6.2.2. Multicast
     
        Multicast forwarding is reducible to broadcast forwarding in the
        simplest (default) case. However implementations may choose -
        using mechanisms that are out of scope for this document - to
        optimize multicast forwarding. In order for this to work
        effectively, however, support for awareness of multicast
        "interest" is required for all RBridges.
     
        Without optimization, multicast frames are injected by the
        ingress RBridge onto an IRT by - for instance - encapsulating
        the frame with a MAC destination multicast address, and
        forwarding it according to its local CFT-IRT. Again, without
        optimization, each RBridge along the path toward all egress
        RBridges will similarly forward the frame according to their
        local CFT-IRT.
     
        Using this approach results in one and only one copy of the
        multicast frame being delivered to appropriate egress RBridges.
        However, using this approach, multicast delivery is identical to
        broadcast delivery - hence very inefficient.
     
        In any optimization approach, RBridge encapsulated multicast
        frames will use either a broadcast or a group MAC destination
        address. In either case, the recognizably distinct destination
        addressing allows a frame forwarding decision to be made at each
        RBridge hop. RBridges may thus be able to take advantage of
        local knowledge of multicast distribution requirements to
        eliminate the forwarding requirement on interfaces for which
     
     
     
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        there is no recipient interested in receiving frames associated
        with any specific group address.
     
        As stated earlier, in order for RBridges to be able to implement
        multicast optimization, distribution of learned multicast group
        "interest" information must be provided - and propagated - by
        all RBridges.  Mechanisms for learning and propagating multicast
        group participation by RBridges is out of scope in this document
        but may be defined in RBridge protocol specification(s).
     
        Note that, because the multicast optimization would - in
        principle - further scope and reduce broadcast traffic, two
        things may be said:
     
        o  It is not necessary that all implementations in a deployment
           implement the optimization (though all must support the data
           required to implement it in RBridge peers) in order for any
           local multicast optimization (consistent with the above
           description) to work;
        o  Introduction of a multicast optimization will not result in
           potential forwarding loops where broadcast forwarding would
           not do so.
     
        In the simplest case, the ingress RBridge for a given multicast
        frame will re-use the MAC destination group address of a
        received multicast frame.  However this may not be required as
        it is possible that the mechanisms specified to support
        multicast will require examination of the decapsulated MAC
        destination group address at each RBridge that implements the
        optimization.
     
     4.6.2.3. Flooding
     
        Flooding is similarly reducible to broadcast forwarding in the
        simplest (default) case - with the exception that a frame being
        flooded across the CRED is typically a unicast frame for which
        no CFT exists at the ingress RBridge. This is not a minor
        distinction, however, because it impacts the way that addressing
        may be used to accomplish flooding within the CRED.
     
        An ingress RBridge that does not have a CFT entry for a received
        frame MAC destination address, will inject the frame onto the
        ingress RBridge Tree by - for instance - encapsulating the frame
        with a MAC destination broadcast address, and forwarding it
        according to its local CFT-IRT. Without optimization, each
        RBridge along the path toward all egress RBridges will similarly
        forward the frame according to their local CFT-IRT.
     
     
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        Using this approach results in one and only one copy of the
        flooded frame being delivered to all egress RBridges.
     
        However implementations may choose to optimize flooding. A
        Flooding optimization will only work at any specific RBridge if
        that RBridge re-evaluates the original (decapsulated) unicast
        frame.
     
        Any flooding optimization would operate similarly to the
        multicast optimization described above, except that - instead of
        requiring local information about multicast distribution - each
        RBridge implementing the optimization will need only to lookup
        the MAC destination address of the original (decapsulated) frame
        in its local CFT. If an entry is found, the frame could then be
        forwarded only if the specific RBridge is on the shortest path
        between the originating ingress RBridge and the appropriate
        egress RBridge.  This could be implemented - for example - as a
        specialized CFT-IRT entry.
     
        Note that, because the flooding optimization would - in
        principle - further scope and reduce flooded traffic, two things
        may be said:
     
        o  It is not necessary that all implementations in a deployment
           support the optimization in order for any local flooding
           optimization (consistent with the above description) to work
           (hence such an optimization is optional);
        o  Introduction of the flooding optimization will not result in
           potential forwarding loops where flooded forwarding would not
           do so.
     
        Because a forwarding decision can be made at each hop, it is
        possible to terminate flooding early if a CFT for the original
        MAC destination was in the process of being propagated when
        flooding for the frame was started.  It is therefore possible to
        reduce the amount of flooding to some degree in this case.
     
     4.7. Routing Protocol Operation
     
        The details of routing protocol operation can be determined once
        a specific routing protocol has been selected.  These details
        would be defined in appropriate protocol specification(s).
     
        Protocol specifications should identify means for determining
        the content of the CFT, CFT-IRT and CTT.
     
     
     
     
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     4.8. Other Bridging and Ethernet Protocol Operations
     
        In defining this architecture, several interaction models have
        been considered for protocol interaction between RBridges and
        other L2 forwarding devices - in particular, 802.1D bridges.
        Whatever model we adopt for these interactions must allow for
        the possibility of other types of L2 forwarding devices. Hence,
        a minimal participation model is most likely to be successful
        over the long term, assuming that RBridges are used in a L2
        topology that would be functional if RBridges were replaced by
        other types of L2 forwarding devices.
     
        Toward this end, RBridges - and the CRED as a whole - could (in
        theory) participate in Ethernet link protocols, notably the
        spanning tree protocol (STP) on the ingress/egress links using
        exactly one of the following interaction models:
     
        o  Transparent Participation (Transparent-STP)
        o  Active Participation (Participate-STP)
        o  Blocking Participation (Block-STP)
     
        Only one of these variants would be supported by an instance of
        this architecture. All RBridges within a single CRED must use
        the same model for interacting with non-RBridge protocols.
        Furthermore, it is the explicit intent that only one of these
        models is ultimately supported - at least as a default mode of
        compliant implementations.
     
        This architecture assumes RBridges block STP.
     
     5. How RBridges Address TRILL
     
        This section is for further study, after determining the set of
        TRILL requirements that this architecture document is expected
        to address.
     
     6. Conclusions
     
        This document discusses options considered and factors affecting
        any protocol specific choices that may be made in instantiating
        the TRILL architecture using RBridges.
     
        Specific architectural and protocol instantiations should take
        these into consideration. In particular, protocol, encapsulation
        and procedure specifications should allow for potential
        optimizations described in the architectural document to the
        maximum extent possible.
     
     
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        Also, this document addresses considerations relative to
        interaction with existing technology and "future-proofing"
        solutions.  For both simplicity in description, and robust long
        term implementation of the technology, this document recommends
        the use of clear distinction - at all possible points - of
        definitions, protocols, procedures, etc. from related (but not
        identical) specifications and interactions.
     
        In particular, this document recommends the use of a
        "collocation model" in addressing issues with combining RBridge,
        Router and 802.1D bridge behavior.
     
     7. Security Considerations
     
        As one stated requirement of this architecture is the need to be
        able to provide an L2 delivery mechanism that is potentially
        configuration free, the default operation mode for instances of
        this architecture should assume a trust model that does not
        require configuration of security information. This is - in fact
        - an identical trust model to that used by Ethernet devices in
        general.
     
        In consequence, the default mode does not require - but also
        does not preclude - the use of established security mechanisms
        associated with the existing protocols that may be extended or
        enhanced to satisfy this document's architectural definitions.
     
        In general, this architecture suggest the use of a link-state
        routing protocol - modified as required to support L2 reach-
        ability and link state between RBridges. Any mechanisms defined
        to support secure protocol exchanges between link-state routing
        peers may be extended to support this architecture as well.
     
        This architecture also suggests use of additional encapsulation
        mechanisms and - to the extent that any proposed mechanism may
        include (or be extended to include) secure transmission - it may
        be desirable to provide such (optional) extensions.
     
        To the extent possible, any extensions of protocol or
        encapsulation should allow for at least one mode of operation
        that doesn't require configuration - if necessary, for limited
        use in a physically secure deployment.
     
     8. IANA Considerations
     
        This document has no direct IANA considerations. It does
        suggest, that protocols that instantiate the architecture use a
     
     
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        shim header as a wrapper on the payload for RBridge to RBridge
        traffic, And this shim header may be identified by a new
        protocol type in the tunneled Ethernet link header. This
        protocol type, identified in an 802 header, would be allocated
        by the IEEE in cooperation with IANA.
     
     9.Acknowledgments
     
        The initial work for this document was largely done by Joe
        Touch, based on work he and Radia Perlman completed earlier.
        Subsequent changes are not to be blamed on them.
     
        In addition, the current text has been helped substantially by
        comments and suggestions from the TRILL working group and from
        Joel Halpern, Russ White and Ron Bonica in particular.
     
     10. References
     
     10.1.Normative References
     
        None.
     
     10.2.Informative References
     
        [1]   Perlman, R., "RBridges: Transparent Routing", Proc.
              Infocom 2005, March 2004.
     
        [2]   Touch, J., (ed.) "Transparent Interconnection of Lots of
              Links (TRILL): Problem and Applicability Statement", work
              in progress, draft-touch-trill-prob-00.txt, November,
              2005.
     
        [3]   Perlman, R., "RBridges: Base Protocol Specification", work
              in progress, draft-ietf-trill-rbridge-protocol-00.txt,
              January, 2006.
     
        [4]   Postel, J., "INTERNET PROTOCOL", RFC 791, September, 1981.
     
        [5]   802.1D-2004 IEEE Standard for Local and Metropolitan Area
              Networks: Media Access Control (MAC) Bridges
     
        [6]   Gray, E., (ed.)"TRILL Routing Requirements in Support of
              RBridges", work in progress, draft-gray-trill-routing-
              reqs-01.txt, June, 2006
     
        [7]   Moy, J., "OSPF Version 2", RFC 2328, April, 1998.
     
     
     
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        [8]   Callon, R., "Use of OSI IS-IS for Routing in TCP/IP and
              Dual Environments", RFC 1195, December, 1990.
     
        [9]   Coltun, R., D. Ferguson & J. Moy, "OSPF for IPv6", RFC
              2740, December, 1999.
     
     Author's Addresses
     
        Editor:
     
        Eric Gray
        Ericsson
        900 Chelmsford Street
        Lowell, MA, 01851
        Phone: +1 (978) 275-7470
        EMail: Eric.Gray@Ericsson.com
     
        Contributors:
     
        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
     
        Radia Perlman
        Sun Microsystems
     
        EMail: Radia.Perlman@sun.com
     
     
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