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Internet-Draft                                              Steven Blake
Expiration Date: September 1997                           Anoop Ghanwani
                                                              Wayne Pace
                                                        Vijay Srinivasan

                                                         IBM Corporation

                                                              March 1997

                 ARIS Support for LAN Media Switching


Status of This Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
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   ARIS (Aggregate Route-based IP Switching) [ARIS] is a protocol which,
   in coordination with network-layer routing protocols, establishes
   link-layer switched paths through a network of Integrated Switch
   Routers (ISR).  This memo describes ARIS protocol mechanisms which
   enable LAN media switching of IP packets.  In addition, this memo
   describes the functional behavior of ISRs which are interconnected
   via LAN media (e.g., ethernet, token ring, FDDI).  The proposed
   mechanisms are designed to permit easy implementation using emerging
   LAN switching technology.

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1. Applicability Statement

   Several proposals that deal with improving the performance of IP
   forwarding by carrying labels in the packets have recently been
   submitted to the MPLS working group [ARIS, TAG, FANP].  The labels
   are used for indexing tables which enable fast IP forwarding at close
   to media speeds by minimizing the need for network-layer packet
   processing.  The labels may be carried in different ways depending on
   the underlying link-layer technology.  For instance, in ATM networks,
   the label may be represented by a particular VPI/VCI value.  Since
   ATM is a label swapping technology, it is possible for label
   allocation to be a local choice for each node participating in the
   protocol.  This is not possible for LAN switching technologies such
   as ethernet, token ring, and FDDI, which are not inherently label
   swapping technologies.  As a consequence, a shim consisting of one or
   more 32-bit label stack entries inserted between the link-layer and
   the network-layer headers has been proposed as a means to convey the
   label information [LABEL].  The main drawback of using such an
   approach is that accessing the labels requires that the frames be
   processed by software, reducing the benefit offered by label
   switching.  Alternatively, hardware technology specific to label
   switching may be developed.  However, devices incorporating this
   technology are likely to be more expensive that traditional LAN
   switches and bridges.

   This memo proposes a different approach for label switching on LAN
   media which uses the ARIS protocol for distribution of labels.  The
   label is carried in the destination address portion of the frame and,
   for unicast, is usually the MAC address of the egress point from the
   network as identified by ARIS.  With this approach, an implementation
   using emerging bridge/switch hardware capable of supporting the IEEE
   802.1p forwarding and filtering rules is possible [802.1P].  However,
   the labels must now have global significance and are required to be
   unique.  The focus of this memo is to describe a label distribution
   and switching mechanism which can be applied among ISRs which are
   interconnected via point-to-point LAN media links.  Such a mechanism
   can provide significant benefit in the backbone of campus networks,
   for example.

2. Introduction

   An Integrated Switch Router (ISR) is a link-layer switch which has
   been augmented with IP routing capability, in addition to the ARIS
   protocol [ARIS].  Virtual circuits (VCs) which are established by
   application of the ARIS protocol enable switching of IP packets
   across a network of ISRs.  Here the term "virtual circuit" is used
   loosely to imply a switched path in any switching technology.

   ARIS switched path establishment is coupled to IP routing by means of
   the "egress identifier".  An egress identifier may refer to an egress

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   ISR which forwards traffic either to a foreign routing domain, or
   across an area boundary within the same network.  Alternatively, an
   egress identifier may refer to a particular (S,G) multicast pair.
   ARIS supports a wide variety of egress identifier semantics, each
   providing a different level of traffic aggregation.

   In the unicast traffic case, ARIS establishes a switched path for
   each egress identifier advertised by an ISR by forwarding an
   Establish message to each of that ISR's upstream neighbors.  After
   ensuring that the downstream ISR is on the routed path associated
   with the egress identifier, and that the switched path is loop-free,
   the upstream ISRs continue to forward Establish messages further
   upstream until they reach all ingress ISRs in the ARIS network.  The
   resulting switched path resembles a multipoint-to-point tree
   terminating at the egress ISR.  The direction of path establishment
   is reversed for multicast traffic, and the resulting switched path
   forms a point-to-multipoint tree.

   Each ARIS switched path for an egress identifier is associated with
   a unique VC between adjacent ISRs.  ISRs typically will swap the
   VPI/VCI field of a cell (ATM) or the DLCI of a frame (Frame Relay)
   with a new label value before forwarding to a downstream ISR on the
   switched path.  This operation is commonly referred to as "label
   swapping".  ISRs can merge multiple inbound VCs of a switched path
   onto a single outbound VC if the underlying hardware supports this
   capability.  This reduces VC consumption and affords greater network

   Unlike ATM and Frame Relay, traditional LAN switching technology is
   not based on label swapping.  LAN switches forward a LAN frame based
   on the 6-byte destination IEEE MAC address (DA) encoded in the frame
   header [802.1D].  LAN switching hardware typically is not capable of
   swapping the DA in the frame prior to forwarding.  This style of
   forwarding is referred to here as "label switching".  LAN switches
   are only able to forward frames unambiguously if each (individual) DA
   is associated with only one network end-point.  This requirement does
   not present a significant limitation on network scalability since the
   DA field is large enough to represent 2^46 unique end-points.

   ARIS supports LAN media switching by associating each egress
   identifier with a 6-byte switching label which is unique among all
   switching labels in use within the ARIS network.  The switching label
   is encoded in the DA field within a LAN frame.  ISRs cache the
   switching label corresponding to each switched path.  ISRs can
   unambiguously identify frames corresponding to any particular egress
   identifier by the value of the frame's DA field and can forward them
   directly at the link-layer along the appropriate switched path.  This
   enables packets to be switched at hardware speeds across an entire
   network of ISRs.

   Unless otherwise specified, the behavior of the ARIS protocol is

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   identical to that described in [ASPEC].

3. Components for LAN Media Switching

   In this memo, a LAN Media ISR (LMISR) refers to a network node which
   incorporates a LAN Media Forwarding Component (LMFC) along with a
   network-layer control and forwarding component (IPCC).  The LMFC
   performs label switching based on the 6-byte DA of a received frame.
   Direct link-layer switching between diverse LAN media types (10/100/
   1000 ethernet, token ring, FDDI) is possible if supported by the
   underlying hardware.  LMISRs are intended to form the core of a
   scalable, high-bandwidth campus or enterprise network.

   Associated with each LMFC is a LAN Media Forwarding Information Base
   (LMFIB).  The LMFIB specifies the association of switching-labels
   (DAs) to outgoing interface(s).  This table is used to configure the
   LMFC's filtering database to enable link-layer forwarding.  In the
   default configuration, the 802.1D Spanning Tree protocol is disabled,
   and every active interface (visible to IP routing) is placed in the
   forwarding state [802.1D].

   To permit IP control traffic to reach the IPCC within a LMISR, and to
   permit network-layer forwarding of packets on a switched path which
   has been broken downstream, the IPCC is associated with one or more
   logical interfaces in the LMFIB.  This allows the IPCC to redirect
   packets on a pre-established switched path through the IPCC.
   The IPCC implementation SHOULD be capable of simultaneously receiving
   LAN frames with arbitrary DA values.  Note that the LMFIB can be used
   to filter the addresses which are received by the IPCC.

   The LMFC MUST permit the precise specification of the output
   interface(s) to be associated with each received DA (individual or
   group address scope).  This capability is consistent with the
   Extended Filtering Mode and Port Filtering Mode C as described in
   Section 2.6.6 of [802.1P].

   The LMFC MUST NOT flood frames with an unknown DA or with the
   broadcast DA out of every LMFC interface in forwarding state.  These
   rules are necessary to prevent link-layer loops from forming amongst
   adjacent LMISRs.  The LMFC SHOULD support the ability to forward
   frames with an unknown DA or with the broadcast DA to a particular
   LMFC interface associated with the IPCC.  In addition, the LMFC
   SHOULD support the ability to drop frames with an unknown DA or with
   the broadcast DA.

   Existing LAN switch implementations typically do not support the
   capability to swap the DA of a frame.  ARIS does not require this
   capability to function efficiently, but allows LMISRs whose LMFCs are
   capable of DA swapping to alter the switching label associated with
   an egress identifier when forwarding Establish messages upstream

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   towards an ingress ISR.  It is the responsibility of such a LMISR to
   select a unique 6-byte switching label when transmitting an Establish
   message for the associated egress identifier, and to perform the
   correct DA swapping operation to/from the initial DA value when
   forwarding frames.

4. LAN Media Frame Encapsulation

   ARIS support for LAN media switching does not require a new
   encapsulation format for IP packets.  IPv4 and IPv6 packets should be
   encapsulated according to the appropriate RFC specification for each
   LAN media [RFC1042, RFC1972, RFC2019].  This includes the default
   value of the maximum transmission unit (MTU) for each LAN media link.

5. IP Multicast Support

   As described in [ARIS], the establishment of point-to-multipoint
   switched paths for IP multicast traffic is initiated at the root
   (ingress) node.  The switched path tree forwards traffic from the
   ingress ISR to all egress ISRs on the multicast tree by using
   multicast switching at the intermediate ISRs.

   The ingress LMISR for a multicast switched path tree forwards an
   Establish message containing the switching label for the associated
   egress identifier to its downstream LMISRs.  The Establish message
   traverses from the ingress node to the downstream LMISRs in reverse
   path multicast (RPM) style.  The branches of the point-to-multipoint
   tree that do not lead to receivers are pruned when the multicast
   routing protocol prunes up by deleting forwarding entries in the
   LMFIB.  The ingress LMISR periodically refreshes the multicast
   switched path tree by retransmitting an Establish message containing
   the switching label for the associated egress identifier.

6. Multipath Support

   As described in Section 2, a single switching label is associated
   with an egress identifier in the default configuration.  In this
   case, a LMISR which has received multiple Establish messages for an
   egress identifier, each associated with an equal-cost path to the
   corresponding egress LMISR, cannot forward multiple Establish
   messages with the same switching label to each of its upstream
   LMISRs, since this will not allow the upstream LMISRs to distinguish
   the multiple equal-cost paths.

   An LMISR which wishes to utilize multiple equal-cost paths to an
   egress has the following alternatives:

     o  Forward only one Establish message for an egress identifier to

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        each upstream LMISR, and forward traffic on that switched path
        at the IP layer,

     o  Forward multiple Establish messages for an egress identifier to
        each upstream LMISR, where each Establish message contains a
        distinct switching label (all but one of which must be generated
        dynamically by the LMISR).  The LMISR must be capable of DA
        swapping between the dynamically generated label(s) and the
        original label selected by the egress LMISR.

7. Explicit Route Support

   Explicit routes for point-to-point, point-to-multipoint, and
   multipoint-to-point forwarding are established as described in
   [ARIS].  In the case of point-to-point explicit routes, either the
   ingress or the egress may initiate the path establishment, and may
   select the switching label.  In the case of multipoint-to-point
   explicit routes, the egress initiates the switched path establishment
   and selects the switching label.  In the case of point-to-multipoint
   explicit routes, the ingress initiates the switched path
   establishment and selects the switching label.

8. Security Considerations

   An analysis of security considerations will be provided in a future
   revision of this memo.

9. Intellectual Property Considerations

   International Business Machines Corporation may seek patent or other
   intellectual property protection for some or all of the aspects
   discussed in the forgoing document.

10. Acknowledgements

   The authors wish to acknowledge the following individuals for their
   input and assistance: Rick Boivie, Ed Bowen, Brian Carpenter, Allen
   Carriker, Gene Cox,  Ed Ellesson, Jim Ervin, Nancy Feldman, John
   Linville, Sanjeev Rampal, Norm Strole, Arun Viswanathan, and Jeff

11. References

   [ARIS]    A. Viswanathan, N. Feldman, R. Boivie, R. Woundy,
             "ARIS: Aggregate Route-Based IP Switching", Internet Draft
             <draft-viswanathan-aris-overview-00.txt>, March 1997.

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   [TAG]     Y. Rekhter, B. Davie, D. Katz, E. Rosen, G. Swallow, D.
             Farinacci, "Tag Switching Architecture - Overview",
             Internet Draft <draft-rekhter-tagswitch-arch-00.txt>,
             January 1997.

   [FANP]    K. Nagami, Y. Katsube, Y. Shobatake, A. Mogi, S. Matsuzawa,
             T. Jinmei, H. Esaki, "Flow Attribute Notification Protocol
             (FANP) Specification", Internet Draft
             <draft-rfced-info-nagami-00.txt>, February 1997.

   [LABEL]   E. Rosen, Y. Rekhter, D. Tappan, D. Farinacci, G. Fedorkow,
             "Label Switching: Label Stack Encodings", Internet Draft
             <draft-rosen-tag-stack-01.txt>, March 1997.

   [802.1P]  "P802.1p Standard for Local and Metropolitan Area Networks-
             Supplement to Media Access Control (MAC) Bridges: Traffic
             Class Expediting and Dynamic Multicast Filtering", P802.1p/
             D5, LAN MAN Standards Committee, IEEE Computer Society,
             February 1997.

   [802.1D]  ISO/IEC 10038, ANSI/IEEE Std 802.1D-1993 "MAC Bridges".

   [ASPEC]   N. Feldman, A. Viswanathan, "ARIS Specification", Internet
             Draft <draft-feldman-aris-spec-00.txt>, March 1997.

   [RFC1042] J. Postel, J. Reynolds, "A Standard for the Transmission of
             IP Datagrams over IEEE 802 Networks, Internet RFC 1042,
             February 1988.

   [RFC1972] M. Crawford, "A Method for the Transmission of IPv6 Packets
             over Ethernet Networks", Internet RFC 1972, August 1996.

   [RFC2019] M. Crawford, "A Method for the Transmission of IPv6 Packets
             over FDDI Networks", Internet RFC 2019, October 1996.

12. Authors' Addresses

   Steven Blake
   IBM Corporation
   P.O. Box 12195
   Research Triangle Park, NC  27709
   Phone:   +1-919-254-2030
   Fax:     +1-919-254-5483
   E-mail:  slblake@vnet.ibm.com

   Anoop Ghanwani
   IBM Corporation
   P.O. Box 12195
   Research Triangle Park, NC  27709
   Phone:   +1-919-254-0260

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   Fax:     +1-919-254-5410
   E-mail:  anoop@raleigh.ibm.com

   Wayne Pace
   IBM Corporation
   P.O. Box 12195
   Research Triangle Park, NC  27709
   Phone:   +1-919-254-4930
   Fax:     +1-919-254-5410
   E-mail:  pacew@raleigh.ibm.com

   Vijay Srinivasan
   IBM Corporation
   P.O. Box 12195
   Research Triangle Park, NC  27709
   Phone:   +1-919-254-2730
   Fax:     +1-919-254-5410
   E-mail:  vijay@raleigh.ibm.com

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