Network Working Group                                           F. Coras
Internet-Draft                                      A. Cabellos-Aparicio
Intended status: Informational                        J. Domingo-Pascual
Expires: May 4, 2016                   Technical University of Catalonia
                                                                F. Maino
                                                           cisco Systems
                                                            D. Farinacci
                                                        November 1, 2015

                      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-sourced multicast packets.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on May 4, 2016.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Definition of Terms . . . . . . . . . . . . . . . . . . . . .   3
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Overlay Signaling . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  RTR Registration  . . . . . . . . . . . . . . . . . . . .   6
     4.2.  ETR/RTR Subscription  . . . . . . . . . . . . . . . . . .   6
     4.3.  ETR/RTR Unsubscription  . . . . . . . . . . . . . . . . .   7
   5.  Overlay Management  . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  RLOC Failure and Unreachability . . . . . . . . . . . . .   8
     5.2.  Other Overlay Management Considerations . . . . . . . . .   8
     5.3.  Automated Computation of RTR Level  . . . . . . . . . . .   9
       5.3.1.  Algorithm for Computing Optimized Distribution Trees    9
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  MADDBST heuristic  . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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 that connects the source ITR and the
   receiving ETRs is PIM [RFC4601].  An alternative method that uses

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   Map-Requests for propagating (S-EID,G) state from ETRs to the ITR is
   established in [I-D.farinacci-lisp-mr-signaling].

   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 interconnection.  In such a
   case, unicast RLOC encapsulation is necessary to deliver multicast
   packets directly to the ETRs.  This however leads to high ITR head-
   end replication for large sets of 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 define the mechanisms used to build
   (S-EID,G) state in source and receiver domains, nor does it describe
   the messages used to propagate such state from receiver ETRs to
   source ITR.  What it defines is how (S-EID,G) state is built in the
   ITR, RTRs and ETRs participating in the overlay distribution tree.

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):  This is the outer destination address of a
      packet when LISP encapsulating a multicast packet with an EID
      source within a multicast packet.

   Re-encapsulating Tunnel Router (RTR):  An RTR is a router that
      implements the re-encapsulating tunnel function detailed in
      Section 8 of the main LISP specification [RFC6830].  Such router
      performs packet re-routing by chaining ETR and ITR functions,
      whereby they first remove the LISP header of ingressing packets
      and then prepend a new one prior to forwarding them.

   Unicast Replication:  Is the notion of replicating a multicast packet
      with an EID source address at an ITR or RTR by encapsulating it

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      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 site-facing
      interfaces and unicast encapsulation.

   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
      [RFC6830] that supports multicast and participates as the root in
      the distribution tree.  In this document we use the term "ITR" to
      mean a multicast capable ITR.

   Multicast Egress Tunnel Router (ETR):  An ETR as specified in
      [RFC6830] that participates as a leaf in the distribution tree.
      ETR are the only members of the tree that do not unicast
      replicate.  In this document we use the term "ETR" to mean a
      multicast capable ETR.

   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.  In this document we use the term
      "RTR" to mean a multicast capable RTR.

   Replication Target:  A multicast channel-id (S-EID,G) or a set of
      multicast channel-ids (S-EID-prefix,G).

   Joining-OIF-list:  Represents a collection of state per multicast
      routing table entry at an RTR or ETR that is created by received

   Forwarding-OIF-list:  Represents the outgoing RLOC list a multicast
      router stores per multicast routing table entry such that it knows
      to which RLOCs to replicate multicast packets.  Although the
      Joining-OIF-list contains sufficient information to allow the
      forwarding of encapsulated multicast packets, using it is
      inefficient.  Thereby, an RTR implementation may wish to build an
      efficient Forwarding-OIF-list.  Ways of implementing a Forwarding-
      OIF-list are out of the scope of this document.

   Upstream:  Towards the root of the tree.

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   Downstream:  Away from the root of the tree.

3.  Overview

   This document describes a method to diminish the ITR's replication
   load by using RTRs to build an inter-domain distribution tree.  The
   tree is managed by the source domain's Map-Server.  RTRs join the
   overlay due to either manual or automatic 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 configured information
   RTRs register or by means of local policy and algorithms.  ETRs
   always join the overlay as leaves and their attachment prompts the
   creation of a path, which traverses the RTR hierarchy, from 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 since the state that
   defines the distribution tree, i.e., the replication hierarchy, is
   created prior to forwarding/replicating the packets.  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.

   It is worth pointing out that because of the receiver-initiated
   approach multicast employs to build distribution trees, whereby
   receivers join upstream sources, LISP-RE operates backwards from LISP
   point of view.  That is, ETRs are the ones to send Map-Requests to
   discover potential upstream parents and the ITR answers with Map-
   Replies to joining downstream clients.

4.  Overlay Signaling

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

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4.1.  RTR Registration

   RTR participation in the overlay is condition by the configuration of
   a replication target, a multicast channel (S-EID,G) or set of
   channels (S-EID-prefix,G), the RTR is to perform replication for.
   Once configured, manually or by automated mechanisms, an RTR Map-
   Registers its replication target with merge-semantics to the
   appropriate Map-Server.  In the registration it also provides its
   list of RLOCs to be used by overlay peers 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].

   Due to the merge-semantics, the Map-Server aggregates all RTR
   originated Map-Register messages in a single, per replication target
   mapping.  If no level information is provided or if so configured,
   the Map-Server should use local policy to compute a hierarchy and
   associate a level within it to each entry in the list (more details
   in Section 5.3).  It should be noted that the entries that are
   pointed to in the resulting mapping are not RLOCs but Replication
   List entries.

4.2.  ETR/RTR Subscription

   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
   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 which further ensures its
       delivery to the authoritative Map-Server.

   2.  The Map-Server looks up the mapping associated to (S-EID,G) and,
       out of the distribution tree hierarchy encoded within, it selects
       a set of leaf RTRs, i.e., members of the level furthest away from
       the ITR, with spare replication capacity.  The set of potential
       parents is encoded in a new (S-EID, G) mapping the Map-Server
       conveys to the ETR in a Map-Reply.

   3.  From the list it receives, the ETR selects the best upstream RTR
       RLOC according to local policy, taking into account the
       associated priorities and weights and sends to the owning RTR a
       Map-Request/Join-Request for (S-EID,G).  If the ETR itself has
       multiple RLOCs it wishes to use in the overlay, it may convey
       them all to the upstream RTR encoded in the Map-Reply field of

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       the Map-Request/Join-Request together with associated priorities
       and weights.

   4.  The RTR stores the ETR's subscription information in the join-
       oif-list associated to (S-EID,G) and inserts the RLOC obtained
       after evaluating the priorities and weights in the oif-list for
       (S-EID,G).  It then confirms the ETR's subscription with a Map-

   5.  If not already a member of (S-EID,G), the RTR initiates it's own
       attachment to the distribution tree by repeating the steps 1-4.
       An important difference at step 2, the Map-Server replies to a
       joining RTR with a list of RTRs in the adjacent upstream layer,
       as opposed to a list of leaf RTRs, like in the case of an ETR
       join.  This procedure may recurse upstream up to when the ITR or
       an RTR already attached to the distribution tree 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
       source ensuring the creation of a path from the multicast source
       to the receiver end-hosts.

   If at any point, when creating a link between two adjacent layers,
   native multicast replication can be performed, instead of unicast
   replication, the router joining its upstream could set as source of
   the Map-Request/Join-Request a delivery group.  However, group naming
   must be coordinated between the participating parties in this case,
   if core network replication is to be exploited.

4.3.  ETR/RTR Unsubscription

   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.

   When an RTR with active dowstreams departs, it should first change
   the priority 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

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5.  Overlay Management

5.1.  RLOC Failure and Unreachability

   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, it ceases replicating
   towards it.  The affected RLOC is removed from the forwarding-oif-
   list and marked as unreachable in the join-oif-list.  If a backup
   RLOC was provided by the downstream router in the Map-Request/Join-
   Request, it is instead inserted in the forwarding-oif-list and the
   failure results in no packet loss.

   The routers downstream of a failed RTR RLOC, or who lost connectivity
   to said RLOCs, remove their Map-Request/Join-Request associated state
   and reperform the join procedure.  Packet loss in this case must be
   solved by out-of-band mechanisms that are out of the scope of the
   current document.

5.2.  Other Overlay Management Considerations

   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.

   When a downstream RTR subscribes to a set of channel-ids (S-EID-
   prefix,G) using multiple RLOCs in a load-balancing configuration, the
   upstream RTR may choose to load-split channel-ids (S-EID,G) over the
   given set of RLOCs.

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5.3.  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
   obtaining topological information will be discussed in future
   versions of this document.

5.3.1.  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], 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

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      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
   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 [RFC6831] and

7.  IANA Considerations

   This memo includes no request to IANA.

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8.  Acknowledgements

   The authors would like to thank Noel Chiappa for his technical and
   editorial commentary.

9.  References

9.1.  Normative References

              Farinacci, D. and M. Napierala, "LISP Control-Plane
              Multicast Signaling", draft-farinacci-lisp-mr-signaling-06
              (work in progress), February 2015.

              Farinacci, D., Kowal, M., and P. Lahiri, "LISP Traffic
              Engineering Use-Cases", draft-farinacci-lisp-te-09 (work
              in progress), September 2015.

              Farinacci, D., Meyer, D., and J. Snijders, "LISP Canonical
              Address Format (LCAF)", draft-ietf-lisp-lcaf-11 (work in
              progress), September 2015.

   [RFC4601]  Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
              "Protocol Independent Multicast - Sparse Mode (PIM-SM):
              Protocol Specification (Revised)", RFC 4601,
              DOI 10.17487/RFC4601, August 2006,

   [RFC4607]  Holbrook, H. and B. Cain, "Source-Specific Multicast for
              IP", RFC 4607, DOI 10.17487/RFC4607, August 2006,

   [RFC6830]  Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
              Locator/ID Separation Protocol (LISP)", RFC 6830,
              DOI 10.17487/RFC6830, January 2013,

   [RFC6831]  Farinacci, D., Meyer, D., Zwiebel, J., and S. Venaas, "The
              Locator/ID Separation Protocol (LISP) for Multicast
              Environments", RFC 6831, DOI 10.17487/RFC6831, January
              2013, <>.

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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.

   [LCAST]    Coras, F., Cabellos, A., Domingo, J., Maino, F., and D.
              Farinacci, "Lcast: Software-defined inter-domain
              multicast", Computer Networks , 2014.

   [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

Authors' Addresses

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   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


Coras, et al.              Expires May 4, 2016                 [Page 13]