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Versions: 00 01                                                         
Internet Engineering Task Force                     C-Y Lee
INTERNET DRAFT                                      L. Andersson
Expires December 1999                               Nortel Networks
                                                    Ken Carlberg
                                                    Bora Akyol
                                                    June 1999

          Engineering Paths for Multicast Traffic using MPLS

Status of this memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   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 working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as "work in progress."

   To view the list Internet-Draft Shadow Directories, see


   This document describes a solution to engineer paths for IP multicast
   traffic in a network, by directing the control messages to setup
   multicast trees on engineered paths.

   This proposal partitions the multicast traffic engineering problem
   such that multicast routing protocols do not have to be modified to
   setup engineered routes or allocate resources for multicast traffic
   nor do resource allocation protocols such as RSVP or CR-LDP have to
   be able to setup forwarding states (in this case labels) like
   multicast routing protocols.

   Resources are allocated on the same trip that paths are selected and
   setup.  An important aspect of this proposal is that it enables
   multicast paths to be engineered in an aggregatable manner, allowing
   this solution to scale in the backbone.

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1. Overview

   In general, traffic is engineered to traverse certain paths so as to
   utilize resources in a network in a more optimal manner, while at the
   same time improving the level of service that can be offered.

   In conventional IP routing, traffic may be engineered to use a path
   by configuring preferred links towards a destination with a lower
   metric. This method only allows traffic to be engineered based on the
   destination address.  Since the forwarding is based on the
   destination address only, traffic cannot be engineered based on other
   attributes (which maybe useful for traffic engineering purposes) of
   the packet such as the source address of a packet or the requested
   service level.  In contrast, MPLS abstracts the forwarding paradigm
   and allows traffic to be forwarded based on attributes (known as
   forwarding equivalence class (FEC) in MPLS) in addition to the
   destination address. This provides a versatile and convenient syntax
   for traffic engineering purposes.

   This document describes a way to provide a basic traffic engineering
   mechanism for multicast. Traffic Engineering (TE) functionalities (in
   the MPLS entity) are used to forward the join control messages of
   multicast protocols, based on different traffic engineering
   requirements and to allocate resources. (Note that multicast data
   packets however are forwarded based on Layer 3 (L3) address
   information and are not label switched. )

   Using this basic multicast traffic engineering mechanism, ISPs can
   define particular FECs for their network, resources required to
   receive traffic from certain root prefix, decrease fanouts at a node
   by limiting the number paths towards the node(prefix), allowing only
   certain paths to carry multicast traffic, experiment with heuristics
   to better engineer multicast trees, use a function to dynamically
   compute suitable paths based on current or predicted network
   resources. All these additional network or content provider specific
   functions to engineer traffic can be developed independently of the
   basic traffic engineering mechanism.

2.0 Motivation

   The fundamental problem with doing multicast Traffic Engineering (TE)
   is the difficulty in doing it in a scalable manner. Multicast routes
   are very difficult (and some claim impossible) to aggregate. One can
   associate a label with a unicast route(prefix) and packets sent to
   that destination can be aggregated and engineered by associating them
   with the label.

   Since multicast routes are not aggregatable in general, associating a

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   label with a multicast route will require per flow/group resource
   allocation. In essence, this kind of association will result in RSVP
   (or ATM) style resource allocation and is more applicable to per flow
   QOS than traffic engineering.

   In contrast the approach taken in this proposal decouple traffic
   engineering from multicast route setup, thereby allowing the
   resources and paths for multicast data delivery to be independently
   allocated.  What this implies is, resources  and paths can be
   aggregated and engineered; and traffic can be statistically
   multiplexed, enabling network operators to provide differentiated
   services for multicast traffic in a scalable manner.

3.0 Scope

   This draft described mechanisms which is applicable to multicast
   routing protocols such as PIM-SM, CBT, BGMP, Express or Simple
   Multicast, which will be called 'control driven' in this draft. 'Data
   driven' or flood and prune protocols (eg DVMRP and PIM-DM) are
   described in another draft. This proposal assumes a multicast
   group/tree has a common 'QOS' requirement. It is  envisaged that
   heterogeneous receivers requirement can be met by layer encoding data
   in different multicast groups or other variation of layer encoding.

   It should be noted that the MPLS concepts of interest here are the
   FEC, ERO and resource allocation and path selection. An entirely new
   supporting protocol could be designed to support the traffic
   engineering mechanisms proposed here, however since the concepts of
   interest have already been defined and have been implemented in one
   form or another, the solution is described in terms of how it can be
   realized in MPLS.

4.0 Approach

   A control driven multicast routing protocol sends a 'join' message to
   graft a node to a multicast distribution tree, creating multicast
   routes in the process. Since the join messages are forwarded based on
   unicast routes, if the conventional routing table is used, the
   multicast routes setup will be based on conventional routes.  To
   constrain multicast paths, the join message should be sent via paths,
   computed or statically configured.

   This draft describes a scheme where multicast routing control
   messages (including join messages) are forwarded by the MPLS entity
   in a router on the constraint path.

   To allow a router to process control messages, the control messages

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   should contain the router alert option. The control message is
   identified at the ingress LSR by its FEC.  Based on the FEC, the MPLS
   entity can derive the path the control message should take and
   allocate resources accordingly.  A multicast routing protocol would
   setup the forwarding state on the ports/interface where the join is
   received. To enable the establishment of multicast forwarding state
   based on constraint (unicast) routes, multicast routing protocols
   which verify the Reverse Path Forwarding (RPF) must turn off this
   check. To prevent redundant data and loops, a loop avoidance scheme
   based on the concepts described in [MPLS-LOOP-AVOID] or [SM] can be
   used in the routing protocol. If there is a loop, the routing
   protocol should not create forwarding states for the group on the
   port where the join is received.

   Other alternatives to send the join on the engineered path such as -
   extending CR-LDP/TE-RSVP to send and merge joins for the multicast
   tree associated with a label - changing the multicast routing
   protocol to send the join along the explicit route, either require
   multicast routing protocol functionalities to be present in MPLS or
   MPLS functionalities to be incorporated into multicast routing
   protocols.  This proposal uses MPLS (label and explicit route object)
   to cause engineered paths to be selected but forward data using
   multicast routing.  It does not require MPLS or multicast routing
   protocols to be merged, an exercise which tend to - result in
   redundant or the reinventing, of functionalities at L2/L3; increase
   the complexity of multicast traffic engineering while not providing
   any means of aggregating multicast traffic engineering.

   The alternative approaches listed above require traffic to be
   engineered for each group/tree since multicast labels/routes are most
   likely to be not aggregatable. Each group must be assigned a
   different label as well.  In contrast this proposal allows a network
   provider to aggregate the 'QOS' path towards a root or root prefix
   (since resource allocaton and path selection can be independent of
   the setup of forwarding states/routes). The root prefix could be a
   subnet or domain. ulticast traffic in the backbone network can then
   be, provisioned in a more scalable manner and statistically
   multiplexed on the (aggregated) engineered paths.

5.0 Procedure

5.1 Egress LSR

   At any egress LSR (i.e a router where the traffic exits the MPLS
   network) that may join multicast trees - FECs, the associated path
   selection mechanisms and resources required are specified.  These
   FECs will match the the control messages of routing protocols (eg
   PROTO_ID=PIM-SM/CBT, destination = root prefix/well known multicast

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   address, TOS=codepoint). Note that the message that carries this
   information traverses the network from egress to ingress.  The path
   selection mechanisms can be based on, a static table or a constraint
   based routing table or a path selection algorithm (dynamic).  (See
   6.0 Path Selection as well)

   Figure 1 shows the passage of control messages in an egress LSR
   (dotted lines) and the interface between the various entities in the
   LSR (+++ lines)

   When a control messages arrives at the ingress LSR the packet will be
   sent to L3 for processing (where a multicast routing protocol may
   setup forwarding states), since the control message contain the IP
   Router Alert option. After processing the control message, L3 will
   attempt to forward the packet towards the destination specified in
   the control message.

                   | Multicast routes     |
                   | Multicast  Routing    |
                        ^            |
                        |            |
                        |            v
                  ----------       ------------
            ----> | MPLS   |       |   MPLS   | ---->
                  ----------       ------------
                                 | FEC,Path and  |
                                 | Resource      |
                                 | Specification |

                  Fig. 1 At the egress (wrt data flow) LSR

   If the packet (control message) matches the FEC defined in the above
   manner, the MPLS entity will invoke the appropriate path selection

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   mechanism.  The root address of the multicast tree may be provided to
   the path selection mechanism to obtain the constraint routes towards
   the root.  The root address of a multicast tree can be retrieved via
   a generic API provided by multicast routing protocols.  The
   constraint routes obtained from the path selection mechanism will be
   placed in an ERO. An MPLS control message (CR-LDP/RSVP with MPLS
   extension) containing the FEC, ERO TLV, resources required (eg
   Traffic Parameter and any other relevant TLVs) will be prepended to
   the IP packet. It should then forward the MPLS control message to the
   next hop specified in the ERO.  To allow routers downstream to
   process this control message, the packet will be labeled as Router

   The explicit routes in the ERO object is removed as it traverses the
   explicit path towards the root, in the same manner as described in
   CR-LDP and TE-RSVP.

5.2 Intermediate LSRs

   Figure 2 shows the passage of control messages in an intermediate LSR
   (dotted lines) and the interface between the various entities in the
   LSR (+++ lines)

                   | Multicast routes     |
                   | Multicast  Routing    |
                        ^            |
                        |            |
                        |            v
                  ----------       ------------
            ----> | MPLS   |       |   MPLS   | ---->
                  ----------       ------------
                          +           +
                          +           +
                          +           +
                        | FEC          |
                        | State        |

                  Fig. 2 At an intermediate LSR

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   When the next hop (or other intermediate nodes) receives the packet
   with Label Router Alert, it will be taken out of the forwarding path
   and directed to the MPLS entity.  (If the control messages are not
   labeled, L3 would send this control message directly to a L3
   multicast routing protocol, instead of the MPLS entity).

   The MPLS entity will allocate the resources requested by the CR-LDP
   or RSVP with MPLS extension message, create a state for the FEC (and
   other objects eg ERO, Traffic) - called the FEC state for short.  It
   will then sent the packet to the multicast routing protocol (MRP).
   The MRP will then create the forwarding state for the group and will
   forward the join message towards the root. Since the FEC for this
   control message will match the FEC state created earlier, the join
   message will be dispatch to the MPLS entity, which will process the
   ERO object and will sent the packet to the next hop listed in the

   Note that the FEC need only be specified in the ingress LSR,
   intermediate LSRs are informed of the FEC information by previous
   hops. Similarly, the explicit (constraint) routes is only computed or
   configured at the ingress LSR; the next hop and other intermediate
   nodes learn of the explicit routes via the ERO  object propagated
   from the ingress LSR.  Loose Source Route can be specified in the ERO
   and intermediate nodes (LSRs) may forward it to the next explicit
   route/node specified in the ERO based on local routing information.

   If an LSR already have an FEC state, the packet will be sent directly
   to L3 for processing. L3 will decide if it needs to forward this
   control message any further. If it is a join message, and there is
   already L3 forwarding states, the join is terminated. If it is a
   maintenance control message, the control message is processed and
   forwarded. This packet will match the FEC state created earlier and
   MPLS will forward the packet according to the next hop in the ERO
   list associated with this label and FEC.

5.3 Loops

   If the MPLS control message specifies looping explicit routes :

   * then if the tree is uni-directional, only the join message will
   loop.  Data will not loop since data flow is only in one direction
   from root to members. * then if the tree is bi-directional, the join
   message will loop, but because permanent states would not be
   established in this case, data will not be forwarded on the looping

   However if there is a change in next hop towards the root at a node
   where there is already an existing forwarding state, then multicast

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   routing protocols which uses bi-directional trees or a hybrid of
   uni-directional and bi-directional branches could invoke a loop
   avoidance procedure. One way to avoid loops in this case is (using
   splice message) described in SM. This procedure should ideally be
   specified in the multicast routing protocol itself.

6.0 Path Selection

   This proposal allows different path selection algorithms to be used,
   depending on the FEC and path selection mechanism association.  Paths
   can be configured, computed, discovered or obtain through other

   A path selection mechanism will return the constraint routes given
   for eg the group address, root of multicast tree and other criteria.
   How the paths are selected are independent of this proposal, but a
   generic interface (API) between path selection algorithms and this
   multicast traffic engineering scheme is required and is FFS.

7.0 Examples

   This section list some examples of how multicast traffic can be
   engineered using the procedures described in this proposal.

   a) A network operator may define an explicit route [Rx, Ry, Rz]
   towards a domain with prefix for multicast traffic.  Any
   member joining a group where the root address has the prefix
   will have data delivered to it via the explicit route [Rz, Ry, Rx]
   (data is in the reverse direction of the join control message).

   This explicit route may be a Loose Source Route, or a route
   calculated by an algorithm eg an Internal Gateway Protocol (IGP)
   which can provide constraint based routes.

   It is worth noting that the explicit route can be the desired path
   from a root towards a member instead of the reverse path (from member
   towards the root).

   b) Another variation of the above may define an additional field of
   interest in the FEC, the TOS. This will allow a network operator, to
   allocate resources for traffic belonging to a diffserv forwarding
   class, for eg Assured Forwarding.

   c) To decrease fanout, egress LSRs (where multicast data traffic
   exits) can obtain the contraint routes (via manual configuration or a
   constraint based routing entity which can be developed independently
   of the basic TE scheme described in this proposal)

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   d) Load Balancing - a load balancing algorithm can provide the
   alternative path that a control message can take depending on the QOS
   requirement of the group and the current utilization of the equal
   cost paths. As mentioned in the Scope section, this draft assumes the
   QOS requirement of the group is constant (or the maximum value is
   used) or can be averaged to a constant, for traffic engineering

   e) Policy routing - Different paths may be defined for different

8.0 Acknowledgments
   The authors are grateful to Dirk Ooms and Yunzhou Li for reviewing
   this draft and their helpful suggestions to improve this proposal.
   Thanks to Jon Crowcroft for providing insightful comments.


   [ARCH] E. Rosen, A. Viswanathan, R. Callon, "Multiprotocol Label
   Switching Architecture", Work in Progress, July 1998.

   [TE-MPLS] Awduche, D. et al., "Requirements for Traffic Engineering over
    MPLS", Internet Draft, draft-ietf-mpls-traffic-eng-00.txt, October 1998.

   [CRLDP] L. Andersson, A. Fredette, B. Jamoussi, R. Callon, P. Doolan,
   N. Feldman, E. Gray, J. Halpern, J. Heinanen T. E. Kilty, A. G.
   Malis, M. Girish, K. Sundell, P. Vaananen, T. Worster, L. Wu, R.
   Dantu, "Constraint-Based LSP Setup using LDP", Work in Progress,
   January, 1999.

   [TE-RSVP] D. Awduche, L. Berger, D-H. Gan, T. Li, G. Swallow,
   Vijay Srinivasan,
   Internet Draft, draft-ietf-mpls-rsvp-lsp-tunnel-02.txt, September 1999

   Multicast Routing with resource reservation,
   Journal of High Speed Networks 7 (1998) 113-139,
   B. Rajagopalan, R. Nair

   CBT, Core Based Tree Multicast Routing,
   Internet-Draft, March 1998, Ballardie, Cain, Zhang

   PIM-SM, Protocol independent multicast-sparse mode Specification,
   RFC-2117, June 1997
   Estrin, Farinacci, Helmy, Thaler, Deering, Handley,

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   Jacobson, Liu, Sharma, and Wei.

   BGMP, Border Gateway Multicast Protocol Specification,
   Internet-Draft, March 1998, Thaler, Estrin, Meyers

   Express, H. Holbrook, D. Cheriton
   Sigcomm Paper

   SM, Simple Multicast, Internet-Draft, March 1999,
   draft-perlman-simple-multicast-02.txt, Perlman et al

   [MPLS-LOOP-AVOID] "Avoiding Loops in MPLS", Internet Draft,
   draft-leecy-mpls-loop-avoid-00.txt, June 1999
   C-Y Lee, L. Andersson, Y. Ohba,

   YAM, K. Carlberg, J. Crowcroft
   Hipparch 1998

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Authors' Information

   Cheng-Yin Lee
   Nortel Networks
   PO Box 3511, Station C
   Ottawa, ON K1Y 4H7, Canada

   Loa Andersson
   Nortel Networks Inc
   Kungsgatan 34, PO Box 1788
   111 97 Stockholm
   Phone: +46 8 441 78 34
   obile: +46 70 522 78 34
   email: loa_andersson@baynetworks.com

   Ken Carlberg
   S 1-2-8
   1710 Goodridge Drive
   cLean, VA.  22102

   Bora Akyol
   Pluris Terabit Network Systems
   10445 Bandley Drive
   Cupertino, CA 95014
   Phone: (408) 861-3302
   Fax: (408) 863-0271
   email: akyol@pluris.com

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