Network Working Group                                           F. Coras
Internet-Draft                                      A. Cabellos-Aparicio
Intended status: Informational                        J. Domingo-Pascual
Expires: August 29, 2013                         Technical University of
                                                                F. Maino
                                                            D. Farinacci
                                                           cisco Systems
                                                       February 25, 2013

                      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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 29, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   ( in effect on the date of
   publication of this document.  Please review these documents
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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 . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Overlay Distribution Tree  . . . . . . . . . . . . . . . . . .  5
     4.1.  Overlay Management Considerations  . . . . . . . . . . . .  6
   5.  Automated Computation of RTR Level . . . . . . . . . . . . . .  7
     5.1.  Algorithm for Computing Optimized Distribution Trees . . .  8
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Appendix A.  MADDBST heuristic . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12

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1.  Introduction

   The Locator/Identifier Separation Protocol (LISP) [RFC6830] 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, [RFC6831] 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
   defines how (S-EID,G) map-cache state is built in the ITR, RTRs and

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   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 [RFC6830] and [RFC6831] 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.

   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.

   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.

3.  Overview

   This specification describes a method to diminish the ITR's
   replication load by using RTRs to build an inter-domain distribution

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   tree.  The tree is managed by the source domain's Map-Server.  RTRs
   join the overlay on manual configuration and advertise to the Map-
   Server their availability to replicate traffic for a multicast
   channel (S-EID,G).  Out of all the RTRs registering for the same
   multicast channel, the Map-Server builds one mapping and organizes
   the RLOCs in a multi-level hierarchy.  The hierarchy is rooted at the
   ITR and computed based on the manually configured information RTRs
   register or by means of local policy and algorithms.  ETRs always
   join the overlay as leafs and their attachment prompts the creation
   of a path, which traverses the RTR hierarchy, to the ITR.  The path
   is built at receiver request by incrementally linking all
   distribution tree levels, starting at the joining ETR up to the
   source ITR.

   The way the distribution tree is built has several benefits.  First,
   it ensures that packets in the source domain do not reach the ITR if
   no ETR is joined.  Second, it ensures that packets are forwarded from
   ITR to all ETRs without mapping database lookups.  Finally, the
   multicast source is allowed to roam since a first level RTR, when
   informed of the roam event, can do a new database lookup to find the
   new ITR to join to.

4.  Overlay Distribution Tree

   This section describes the signalling the ITR, RTRs and ETRs use in
   order to participate in the overlay and build a distribution tree.
   The signalling messages used are described in
   [I-D.farinacci-lisp-mr-signaling] and [RFC6831].

   RTR participation in the overlay is condition by the configuration,
   manual or automated, of the multicast channel (S-EID,G) the RTR is to
   perform replication for.  Once configured, an RTR Map-Registers
   (S-EID,G) to the mapping database system with Merge-Semantics.  It
   also registers a list of usable RLOCs and a set of corresponding
   weights and priorities.  If present, information about the level of
   the hierarchy where the RTR should attach is also conveyed by means
   of an Replication List Entry canonical address [I-D.ietf-lisp-lcaf].
   Since (S-EID,G) is registered with Merge-Semantics, all RTR
   originated Map-Register messages are aggregated in one, all-
   encompassing mapping.  If no level information is provided or if
   configured so, an ITR should use local policy and an algorithm to
   compute a hierarchy and associate a level in it to each entry in the
   list (more in Section 5).  It should be noted that the entries of the
   mapping are not RLOCs but Replication List entries.

   When an ETR creates (S-EID,G) state from a site based multicast join,
   i.e., its oif-list goes non-empty, it must send an upstream Join

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   request.  If the ETR does not have multicast connectivity to its
   upstream and unicast replication must be performed, the ETR requests
   that a path from ITR to itself, over the RTR hierarchy be
   constructed.  The following procedure is followed to build the path:

   1.  ETR sends a Map-Request/Join-Request for (S-EID,G) multicast
       channel to the mapping database system.

   2.  The Map-Server receives the request, looks up the mapping
       associated to (S-EID,G) and conveys it in a Map-Reply to the ETR.

   3.  The ETR selects out of the list of Replication List entries the
       one with the best RLOC, according to local policy, taking into
       account the priority and weights and the requirement that it be
       as high as possible in the hierarchy.  It then sends a Map-
       Request/Join-Request for (S-EID,G) to the RTR that registered the
       selected RLOC.

   4.  The RTR inserts the ETR's source address in its oif-list for
       (S-EID,G) and confirms the Map-Request/Join-Request with a Map-
       Reply.  If not already a member of (S-EID,G), it also sends a
       Map-Request/Join-Request for (S-EID,G) to the mapping database
       system.  From the ensuing Map-Reply, it chooses the best RLOC
       pertaining to an adjacent upper level RTR, according to local
       policy and taking into account the associated priority and
       weights.  It then sends a Join-Request for (S-EID,G) to the
       selected RTR.

   5.  The previous step is recursively repeated up to when the ITR is
       joined.  On completion, there should exist a path from ITR to
       joining ETR.

   6.  If the ITR is already member of (S-EID,G) the process stops.
       Otherwise, the ITR sends a PIM join to the intra-domain multicast

   If at any point, when creating a link between two adjacent layers,
   multicast replication can be performed, instead of unicast one, the
   router joining its upstream can set as source of the Map-Request/
   Join-Request a delivery group.

4.1.  Overlay Management Considerations

   When an ETR's oif-list goes empty a Map-Request/Leave-Request is sent
   to the upstream RTR which will result in the removal of the ETR's
   associated entry from the RTR's oif-list.  The procedure is repeated
   by the RTR, and it may recurse upstream, if its own oif-list also
   goes empty.  If an RTR departs, it should first change the priority

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   of the RLOCs it registers with the Map-Server to 255 and set its
   locators as unreachable in the RLOC-Probing replies it sends
   downstream.  Finally, once all adjacent lower level members have sent
   Map-Request/Leave-Request messages the RTR can stop registering
   (S-EID,G) with the mapping database system and thus leave the

   RLOC failure is detected at control-plane level through RLOC-probing
   [RFC6830] by both upstream and downstream routers.  When an RTR
   detects the failure of an downstream RLOC, replication towards the
   affected RLOC should cease but the associated entry should not be
   removed from the oif-list.  The routers downstream of a failed RTR
   remove the Map-Request/Join-Request associated state and reperform
   the join procedure.  Ways of detecting RLOC failure at data-plane
   level and of registering backup RLOCs will be discussed in future
   versions of this document.

   An overloaded RTR, i.e., one whose fan-out can not be increased,
   should change the priority of the RLOCs it registers with the mapping
   database system to 255.  In such a situation, the Map-Server updates
   the associated mapping and informs all routers having requested it
   about the change through solicit Map Request (SMR) messages.  Both
   new ETRs attaching to the distribution tree and those already
   connected but reperforming the join procedure must not use the RLOCs
   with a priority of 255 as specified in [RFC6830].  However, routers
   having performed Join-Requests prior to the change should not break
   their existing connections to the affected RTR.

   All routers part of an (S-EID,G) multicast channel should re-evaluate
   their attachment point to the distribution tree whenever the Map-
   Server updates the associated mapping.  This ensures the overlay
   member routers attach to the best suited parent when new RTRs join or
   previously attached ones stop being overloaded.  Change of a parent
   should be done following a "make before break" procedure.
   Specifically, the router changing attachment point first connects to
   the new parent and only afterwards sends the Leave-Request.

5.  Automated Computation of RTR Level

   Operators wishing to automate the RTR joining procedure may wish to
   use an algorithm for computing an optimized distribution tree.  The
   algorithm could be implemented in the Map-Server and its output
   should be used to associate to all RTRs a level in the distribution
   tree.  Due to the centralized management, on-line switching between
   algorithms may be possible in accordance to the required distribution
   tree performance.  However, their use of such algorithms is dependent
   on the presence of overlay topological information.  Ways of

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   obtaining topological information will be discussed in future
   versions of this document.

5.1.  Algorithm for Computing Optimized Distribution Trees

   The current document does not recommend an algorithm for computing
   optimized distribution trees.  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
   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

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   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
   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

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9.1.  Normative References

              Farinacci, D. and M. Napierala, "LISP Control-Plane
              Multicast Signaling", draft-farinacci-lisp-mr-signaling-01
              (work in progress), January 2013.

              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-24 (work in progress), November 2012.

              Farinacci, D., Meyer, D., and J. Snijders, "LISP Canonical
              Address Format (LCAF)", draft-ietf-lisp-lcaf-01 (work in
              progress), January 2013.

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

   [RFC6830]  Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
              Locator/ID Separation Protocol (LISP)", RFC 6830,
              January 2013.

   [RFC6831]  Farinacci, D., Meyer, D., Zwiebel, J., and S. Venaas, "The
              Locator/ID Separation Protocol (LISP) for Multicast
              Environments", RFC 6831, January 2013.

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",

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              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.

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

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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


   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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