Network Working Group                                           F. Coras
Internet-Draft                                      A. Cabellos-Aparicio
Intended status: Informational                        J. Domingo-Pascual
Expires: January 10, 2013                        Technical University of
                                                                F. Maino
                                                            D. Farinacci
                                                           cisco Systems
                                                            July 9, 2012

                      LISP Replication Engineering


   This document describes a method to build and optimize inter-domain
   LISP router distribution trees for locator-based unicast and
   multicast replication of EID-based multicast packets.

Status of this Memo

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   This Internet-Draft will expire on January 10, 2013.

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   document authors.  All rights reserved.

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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Definition of Terms  . . . . . . . . . . . . . . . . . . . . .  4
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Overlay Distribution Tree  . . . . . . . . . . . . . . . . . .  6
     4.1.  LISP Replication Node Database . . . . . . . . . . . . . .  7
     4.2.  Building the Distribution Tree . . . . . . . . . . . . . .  7
   5.  Distribution Tree Optimization . . . . . . . . . . . . . . . .  8
     5.1.  Topology Discovery . . . . . . . . . . . . . . . . . . . .  9
     5.2.  Optimization Algorithm . . . . . . . . . . . . . . . . . .  9
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Appendix A.  MADDBST heuristic . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13

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1.  Introduction

   The Locator/Identifier Separation Protocol (LISP) [I-D.ietf-lisp]
   provides the mechanisms for the separation of Location and Identity
   semantics presently overloaded by IP addresses.  The split results in
   the creation of two namespaces that unambigously identify edge-site
   network objects, Endpoint IDs (EIDs), and core routing objects,
   Routing LOCators (RLOCs).  Apart from aiding the scalablity of the
   core routing infrastructure, the decoupling also enables the
   (re)implementation of new or existing inter-domain routing

   One such mechanism is inter-domain IP source-specific multicast (SSM)
   [RFC4607].  In this sense, [I-D.ietf-lisp-multicast] defines the
   procedures carried out for delivering multicast packets from a source
   host in a LISP site to receivers residing in the same domain or in
   other LISP or non-LISP sites when an underlying multicast
   infrastructure exists.  The signaling protocol it specifies for
   conveying (S-EID,G) state and building the distribution tree
   connecting the xTRs of the source and receiver domains is PIM
   [RFC4601].  An alternative method that uses Map-Requests instead of
   PIM for propagating (S-EID,G) state from multicast receiver site ETRs
   to source site ITR is established in

   Although desirable to use multicast routing in the core network when
   available, a mismatch between the multicast capabilities of receiver
   ETRs and source ITR might impede their multicast interconnection.  In
   such a case, unicast RLOC encapsulation will be necessary to deliver
   multicast packets directly to the ETRs.  This however leads to high
   ITR head-end replication for large sets of receiving ETRs.
   Therefore, to reduce the replication load of the ITR and scale the
   service with the number of multicast receivers, the ITR may choose to
   offload replication to a set of RTRs.

   The current document describes how multicast RTRs can be used to
   build an inter-domain distribution tree rooted at the ITR that can
   perform unicast and/or multicast encapsulated replication of
   multicast packets.  This concept, of distributing the replication
   load from ITR to other RTRs downstream on the core overlay
   distribution tree, is known as Replication Engineering or LISP-RE.
   Since unicast replication in such overlays can be suboptimal when
   compared to the underlay network, methods to optimize packet delivery
   over the distribution tree are also presented.

   This specification does not describe how (S-EID,G) state is built in
   source and receiver domains, nor does it describe how such state is
   propagated from receiver ETRs to source ITR.  This specification

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   defines how (S-EID,G) map-cache state is built in the ITR, RTRs and
   ETRs participating in the overlay distribution tree when they are not
   connectable by multicast.

2.  Definition of Terms

   The terminology in this document is consistent with the definitions
   in [I-D.ietf-lisp] and [I-D.ietf-lisp-multicast] however, it is
   extended to account for LISP-RE concepts:

   Delivery Group (DG):  The outer destination address of a multicast
      encapsulated multicast packet with an EID source.

   Replication Target:  A unicast locator or Delivery Group towards
      which a multicast packet may be encapsulated and replicated.

   Replication List:  A locator-set associated to a multicast map-cache
      entry (S-EID,G) as defined in [I-D.farinacci-lisp-te].  Locators
      in the set may be either unicast RLOCs or Delivery Groups.  When
      used by a LISP router, a multicast packet matching the map-cache
      entry must be replicated to all members of the set.  The unicast
      RLOCs are used to encapsulate multicast packets for unicast
      delivery on the underlying network.  Delivery Groups are used to
      encapsulate multicast packets for multicast delivery on the

   Unicast Replication:  Is the notion of replicating a multicast packet
      with an EID source address at an ITR or RTR by encapsulating it
      into a unicast packet.  That is, the oif-list of a multicast map-
      cache entry can not only have interfaces present for link-layer
      replication and multicast encapsulation but also for unicast

   Overlay Distribution Tree:  A degree-constrained spanning tree that
      represents the path followed by unicast and/or multicast
      encapsulated multicast packets from the root (ITR) to the leaves
      (ETRs) through intermediary nodes (RTRs).  The ITR and RTRs
      unicast and/or multicast replicate packets to their tree children.
      Such tree is built and maintained by the overlay distribution tree
      controller either by using LISP signaling defined in
      [I-D.ietf-lisp-multicast] and [I-D.farinacci-lisp-mr-signaling] or
      by programming the mapping database directly by using ELPs to
      describe network-wide fanout.

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   Distribution Tree Controller (DTC):  A central entity that manages
      the overlay distribution tree, such entity can be either the ITR
      or an external orchestration system.

   LISP Replication Node:  A router (either the ITR or an RTR)
      participating and replicating packets downstream in the
      distribution tree.

   Multicast Ingress Tunnel Router (ITR):  An ITR as specificed in
      [I-D.ietf-lisp] that participates as the root in the distribution

   Multicast Egress Tunnel Router (ETR):  An ETR as specified in
      [I-D.ietf-lisp] that participates as a leaf in the distribution
      tree.  ETR are the only members of the tree that do not unicast

   Multicast Re-encapsulating Tunnel Router (RTR):  An RTR as specified
      in [I-D.farinacci-lisp-te] that participates as an intermediary
      node in the distribution tree.

   Explicit Locator Path (ELP):  an explicit and strictly ordered list
      of replication targets a packet must travel to.  An ELP may be
      used to source route a LISP-encapsulated packet on an explicit
      path of RTRs, however the path between two RTRs is determined by
      the underlying routing protocol.  ELP format is described in
      [I-D.farinacci-lisp-lcaf] and their use in

3.  Overview

   This specification describes a method to diminish the replication
   load of the ITR by using RTRs to build an inter-domain distribution
   tree.  The tree is centrally managed either by the ITR itself or by
   an external orchestration system.  An advantage of this orchestration
   system is that it offloads signaling from the ITR.  The entity that
   manages the tree is generally referred to as the distribution tree
   controller (DTC).

   In order to offload unicast replication of multicast packets the DTC
   uses a ITR and a set of RTRs.  RTRs willing to participate in the
   distribution tree associated to the (S-EID,G) multicast channel must
   join the distribution tree by sending a Map-Request/Join-Request
   [I-D.farinacci-lisp-mr-signaling] to the DTC.  Using this procedure
   the DTC learns the RLOCs of the available RTRs.  Additionally, the
   DTC must learn the replication capacity of each RTR using out-of-band
   signaling or by manual configuration.

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   Given that the ITR and RTRs have a limited replication capacity the
   distribution tree is a degree-constrained spanning-tree.  This means
   that the root is the ITR, the intermediary members are RTRs while
   leaves are always ETRs.  Multicast packets are addressed to (S-EID,G)
   and are unicast and/or multicast encapsulated when being transported
   downstream the tree.

   In order to build and maintain the overlay distribution tree the DTC
   must configure state in the replication nodes (ITR and RTRs).  This
   is done by means of the signaling specified in
   [I-D.ietf-lisp-multicast] and [I-D.farinacci-lisp-mr-signaling].
   Particularly, the DTC receives Map-Requests from RTRs (also from the
   ITR if the DTC is an external orchestration system) addressed to
   (S-EID,G).  Upon inspection of the source RLOC of the Map-Request the
   controller determines the originating ITR/RTR and generates an ad-hoc
   Map-Reply containing the specific replication list for that
   particular node according to the topology of the tree.  For a LISP
   replication node, the replication list specifies the set of RTRs/ETRs
   to which it has to replicate packets, i.e., its overlay distribution
   tree children.  Alternatively, an external orchestration system may
   directly program the mapping database with ELPs that describe the
   topology of the overlay distribution tree.  Ways of achieving this
   will be discussed in future versions of the document.

   The DTC determines the specific topology of the overlay distribution
   tree using a centralized algorithm.  The only requirements for this
   algorithm are that it builds a tree that guarantees that ETRs receive
   the encapsulated multicast packets, that the replication capacity of
   the ITR and RTRs is not exceeded and that forwarding loops are

   In some cases the network administrator may want an optimized overlay
   distribution tree, although this specification does not standardize
   any particular algorithm it suggests one in Section 5.2.  In order to
   build an optimized tree this algorithm makes use of the distance
   (e.g., latency) between the tree members and the amount of multicast
   receivers connected to each ETR.  Such metrics are not provided by
   LISP and therefore must be obtained using out-of-band signaling.

4.  Overlay Distribution Tree

   This section describes how the DTC can build an overlay distribution
   tree using the signaling and mechanisms defined in
   [I-D.ietf-lisp-multicast] and [I-D.farinacci-lisp-mr-signaling].

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4.1.  LISP Replication Node Database

   The DTC maintains per (S-EID,G) multicast channel a LISP Replication
   Node Database (LRND) that stores information about the distribution
   tree state.  This information includes among others the RLOCs of the
   ITR, RTRs and ETRs that constitute the distribution tree and define
   the overlay replication topology (i.e., the parent-child relations).
   Said data may be obtained by the DTC from the standard signaling
   messages exchanged with the RTRs and ETRs.  Additionally, by means of
   out-of-band signalling the DTC should obtain information about the
   replication capacity of RTRs.

   If the operator chooses to build an optimized tree, more information
   must be available at the LRND, this is further discussed in
   Section 5.2.

4.2.  Building the Distribution Tree

   This section describes the procedures followed by ETRs and RTRs when
   attaching to the distribution tree.  All procedures assume that the
   DTC has a LRND consistent with the state of the overlay distribution
   tree and is aware of the replication capacity of participating RTRs.

   The decision of an RTR to join the overlay distribution tree depends
   on out-of-band signalling (e.g., orchestration system, manual
   configuration).  But, its attachment to the distribution tree is done
   by means of one of the following two procedures:

   1.  The RTR explicitly signals the ITR by sending a Join-Request for
       (S-EID,G) and is replied to with a replication list.

   2.  If an orchestration system programs the mapping database with
       ELPs describing the overlay distribution tree, an RTR Map-
       Requests for (S-EID,G) and receives as reply an ELP that defines
       its distribution tree fanout.  Ways of encoding the tree topology
       into ELPs will be discussed in future versions of this document.

   For RTRs using option 1 the DTC, an ITR in this case, will perform
   the same processing as for joining ETRs.  The following sequence of
   steps is used to attach an ETR to the overlay distribution tree:

   1.  The DTC receives a Map-Request/Join-Request for (S-EID,G) from an

   2.  If multicast replication is requested, the DTC proceeds as
       defined in [I-D.farinacci-lisp-mr-signaling] and no further steps
       are taken.

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   3.  If unicast replication is requested, the DTC must choose a
       position for the ETR in the distribution tree topology.
       Specifically, it initiates a search within the LRND for a node
       (either the ITR or a RTR) with enough spare replication capacity
       that will replicate multicast traffic towards the ETR.  This tree
       member will become the parent of the ETR and once it is selected
       the LRND is updated accordingly.  The search algorithm depends on
       operational requirements and this specification does not
       standardize one, however Section 5.2 provides an example
       algorithm.  Note also that certain algorithms may require the
       complete or partial re-shape the tree based on certain
       performance metrics.

   4.  The DTC must create/update the (S-EID,G) associated replication
       state for the selected parent using the mechanisms defined in
       [I-D.ietf-lisp] and [I-D.farinacci-lisp-mr-signaling] (e.g.,
       Solicit-Map-Request).  This results in the parent sending a Map-
       Request for (S-EID,G), in turn, the DTC Map-Replies with an ad-
       hoc replication list of locator-sets according to topology stored
       at the LRND.  If the algorithm results in a complete or partial
       re-shape of the tree then state at multiple tree members must be
       created/updated.  In order to avoid packet loss this must be done
       synchronously.  It will be discussed in future versions of this
       document how to achieve this.

   5.  Once the distribution tree is configured to replicate multicast
       traffic to the ETR the DTC Map-Replies (as specified in
       [I-D.farinacci-lisp-mr-signaling]) with the destination EID-
       prefix set to (parent-RLOC, ETR-RLOC).

   When a LISP replication node signals its departure from the tree, the
   information stored at the LRND is updated accordingly.  For ETRs, the
   state of the parent member must be updated as described in step 4.
   For RTRs both the state of the parent and its children must be
   updated however, such updates may result in packet loss.  Moreover,
   certain optimization algorithms may result in a re-shape of the tree
   and as a consequence the state of multiple tree members must be
   created/updated according to the new topology.  How to manage these
   updates with no packet loss will be discussed in future versions of
   this document.

5.  Distribution Tree Optimization

   Operators wishing to improve the performance of the distribution tree
   need to implement at least one topology discovery mechanism and
   choose a set of optimization algorithms.  Due to the centralized
   group management, on-line switching between optimization algorithms

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   may be possible in accordance to the required performance.  However,
   their use is dependent on the presence of overlay topological
   information.  The following logical modules need to be implemented in
   order to support overlay optimizations with LISP-RE:

   Topology Discovery Coordinator:  It is in charge of organizing the
      topology measurements and building a database that stores the
      topological distances (i.e., a metric must be chosen) between
      overlay members.

   Distribution Tree Computation Unit:  It is a component that with the
      help of an algorithm or heuristic, given as input the topology of
      the overlay and a constraint, or constraint set, can compute an
      optimal, or close to optimal, degree-constrained minimum spanning
      tree that may be used for multicast content distribution.

   Whether to implement the above modules in the ITR or in other network
   elements is the decision of the network administrator.

5.1.  Topology Discovery

   The present document does not specify any topology discovery
   mechanisms.  Both active and passive topology measurements could be
   used.  A choice between the two, of the policy and admission control
   used or of the network element in charge of coordinating these
   measurements could be made in the future based on practical
   experience.  Alternatively, precomputed network maps like the ones
   offered by [IPLANE] and/or out-of-band signalling may be used.

5.2.  Optimization Algorithm

   The current document does not recommend an optimization algorithm.
   However, it provides as an example a low computation cost heuristic,
   which, in the scenarios simulated in [LCAST-TR], can produce
   latencies between the ITR and the multicast receivers close to
   unicast ones.  Its choice is to be influenced by operational
   requirements and the hardware constraints of the equipment in charge
   of running it.  Future experiments might result in a recommendation.

   In what follows, we use the term "distance" when referring to a
   relative length or amplitude of a metric, observed on a path
   connecting two points, but when the exact nature of the metric is of
   no interest.

   Considering as goal the delivery of content for delay sensitive
   applications, the function the algorithm minimizes is the maximum
   distance (e.g. latency or number of AS hops) from a multicast
   receiver to the ITR source.  Notice that the reference is the

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   multicast receiver host and not an ETR.  Thus, what matters in
   deciding a member's position in the distribution tree is not solely
   its distance to the ITR but also the number of multicast receivers it
   serves.  Then, a router close to the source but serving few receivers
   might find itself lower in the distribution tree than another with a
   slightly higher distance to the source but with a larger receiver
   set.  The algorithm optimizes the quality of experience for multicast
   receivers and not for tunnel routers.

   The problem described above, that searches for a minimum average
   distance, degree-bounded spanning tree (MADDBST), can be formally
   stated as:

   Definition:  Given an undirected complete graph G=(V,E), a designated
      vertex r belonging to V, for all vertices v in V, a degree bound
      d(v) <= dmax, dmax a positive integer, a vertex weight function
      c(v) with positive integer values, and an edge weight function
      w(e) with positive values, for all edges e in E. Let W(r,v,T)
      represent the cost of the path linking r and v in the spanning
      tree T. Find the spanning tree T of G, routed at r, satisfying
      that d(v) <= dmax and the distance to the source per multicast
      receiver is minimized.

   The heuristic used to solve this problem works by incrementally
   growing a tree, starting at the root node r, until it becomes a
   spanning tree.  For each node v, not yet a tree member, it selects a
   potential parent node u in the tree T, such that the distance per
   receiver to r, is minimized.  At each step, the node with the
   smallest metric value is added to the tree and the parent selection
   is redone.  The pseudocode of the heuristic is provided in
   Appendix A.

   [SHI] and [BAN] have previously defined and solved similar
   optimization problems.  Shi et al.  [SHI] also prove that a
   particular instance of the problem, where all vertices have weight 1,
   is NP-complete for degree constraints 2 <= dmax <= |V|-1.

   The algorithm can optimize an unicast overlay however, it should not
   be used to optimize multicast underlay delivery.  As a result, if
   multicast is used as underlay between part of the overlay members,
   once one of the members of such Delivery Group is added to the
   distribution tree, the others should be marked as attached also.
   These nodes should receive multicast encapsulated multicast packets
   from the chosen node over the underlying multicast distribution tree.

   Finally, since the RTRs do not replicate packets for multicast
   receiver hosts, prior to applying the MADDBST heuristic, a Minimum
   Spanning Tree (MST) algorithm should be used to compute the RTR

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   distribution tree.  In this case, the MADDBST heuristic should start
   attaching ETRs having as input the tree resulting from MST.

6.  Security Considerations

   Security concerns for LISP-RE the same as for
   [I-D.ietf-lisp-multicast] and [I-D.farinacci-lisp-mr-signaling].

7.  IANA Considerations

   This memo includes no request to IANA.

8.  Acknowledgements


9.  References

9.1.  Normative References

              Farinacci, D., Meyer, D., and J. Snijders, "LISP Canonical
              Address Format (LCAF)", draft-farinacci-lisp-lcaf-09 (work
              in progress), July 2012.

              Farinacci, D., Spencer, T., and N. Napierala, "LISP
              Control-Plane Multicast Signaling",
              draft-farinacci-lisp-mr-signaling-00 (work in progress),
              July 2012.

              Farinacci, D., Lahiri, P., and M. Kowal, "LISP Traffic
              Engineering Use-Cases", draft-farinacci-lisp-te-01 (work
              in progress), July 2012.

              Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
              "Locator/ID Separation Protocol (LISP)",
              draft-ietf-lisp-23 (work in progress), May 2012.

              Farinacci, D., Meyer, D., Zwiebel, J., and S. Venaas,
              "LISP for Multicast Environments",

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              draft-ietf-lisp-multicast-14 (work in progress),
              February 2012.

   [RFC4601]  Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
              "Protocol Independent Multicast - Sparse Mode (PIM-SM):
              Protocol Specification (Revised)", RFC 4601, August 2006.

   [RFC4607]  Holbrook, H. and B. Cain, "Source-Specific Multicast for
              IP", RFC 4607, August 2006.

9.2.  Informative References

   [BAN]      Banerjee, S., Kommareddy, C., Kar, K., Bhattacharjee, B.,
              and S. Khuller, "Construction of an efficient overlay
              multicast infrastructure for real-time applications",
              INFOCOM , 2002.

   [IPLANE]   Madhyastha, H., Katz-Bassett, E., Anderson, T.,
              Krishnamurthy, A., and A. Venkataramani, "iPlane: An
              Information Plane for Distributed Services", USENIX OSDI ,

              Coras, F., Cabellos, A., Domingo, J., Maino, F., and D.
              Farinacci, "Inter-Domain Multicast: LISP Edge Based
              Trees", Technical

   [SHI]      Shi, S., Turner, J., and M. Waldvogel, "Dimensioning
              server access bandwidth and multicast routing in overlay
              networks", NOSSDAV , 2001.

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Appendix A.  MADDBST heuristic

             INPUT: G = (V,E); r; dmax; w(u,v); c(v); u, v in V
             OUTPUT: T

               FOREACH v in V DO
                 delta(v) = w(r,v)/c(v);
                 p(v) = r;
               END FOREACH

               T takes (U = {r}, D={});
               WHILE U != V DO
                 LET u in U-V be the vertex with the smallest delta(u);
                 U = U U {u}; L = L U {(p(u),u)};
                 FOREACH v in V-U DO
                   delta(v) = infinity;
                   FOREACH u in U DO
                     IF  d(u) < dmax and
                         W{r,u,T} + w(u,v)/c(v) < delta(v) THEN
                       delta(v) = W{r,u,T} + w(u,v)/c(v);
                       p(v) = u;
                     END IF
                   END FOR
                 END FOR
               END WHILE

                                 Figure 1

Authors' Addresses

   Florin Coras
   Technical University of Catalonia
   C/Jordi Girona, s/n
   BARCELONA  08034


   Albert Cabellos-Aparicio
   Technical University of Catalonia
   C/Jordi Girona, s/n
   BARCELONA  08034


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   Jordi Domingo-Pascual
   Technical University of Catalonia
   C/Jordi Girona, s/n
   BARCELONA  08034


   Fabio Maino
   cisco Systems
   Tasman Drive
   San Jose, CA  95134


   Dino Farinacci
   cisco Systems
   Tasman Drive
   San Jose, CA  95134


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