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Versions: 00 01

Network Working Group                                         P. Marques
Internet-Draft                                          Contrail Systems
Intended status: Standards Track                                 L. Fang
Expires: December 01, 2012                                 Cisco Systems
                                                            D. Winkworth
                                                                     FIS
                                                                  Y. Cai
                                                             P. Lapukhov
                                                   Microsoft Corporation
                                                               June 2012


              Edge multicast replication for BGP IP VPNs.
                   draft-marques-l3vpn-mcast-edge-01

Abstract

   In data-center networks it is common to use Clos network topologies
   [clos] in order to provide a non-blocking switched network.  In these
   topologies it is often not desirable to provide native IP multicast
   service.

   This document defines a multicast replication algorithm along with
   its control and data forwarding procedures that provides a multicast
   service for a BGP IP VPN network without assuming that the underlying
   infrastructure supports IP multicast.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on December 01, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (http://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must 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.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  VPN Forwarder behavior . . . . . . . . . . . . . . . . . . . .  6
   4.  Multicast tree management  . . . . . . . . . . . . . . . . . .  8
   5.  BGP Protocol Extensions  . . . . . . . . . . . . . . . . . . . 11
     5.1.  Multicast Tree Route Type  . . . . . . . . . . . . . . . . 12
     5.2.  Multicast Edge Discovery Attribute . . . . . . . . . . . . 12
     5.3.  Multicast Edge Forwarding Attribute  . . . . . . . . . . . 13
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 14
     7.2.  Informational References . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15

1.  Introduction

   In Wide-Area Networks having native multicast service on hop-by-hop
   basis allows for more efficient use of scarse link bandwidth.  In
   Clos network topologies [clos] the trade-offs are different.

   A Clos network is often used to provide full cross-sectional
   bandwidth between all the ports on the network.  When used in a
   switching infrastructure it achieves this goal by spreading flows
   across multiple equal cost paths.

   For Clos topologies with multiple stages native multicast support
   within the switching infrastructure is both unnecessary and
   undesirable.  By definition the Clos network has enough bandwidth to
   deliver a packet from any input port to any output port.  Native
   multicast support would however make it such that the network would
   no longer be non-blocking.  Bringing with it the need to devise
   congestion management procedures.

   In this type of environments it is desirable to provide multicast
   service as an edge functionality on top of a unicast clos fabric.
   Early versions of IP VPN multicast services have used ingress
   replication.  The drawback with that approach is the load imposed on
   the ingress node which is specially relevant for situations in which





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   the multicast group has a large number of receivers.  This document
   takes a different approach by leveraging the receivers in order to
   build an edge based replication tree on a per-flow basis.

   Data-center networks often require network virtualization services
   such as the one described in [I-D.marques-l3vpn-end-system].  This
   document defines a set of procedures to be implemented in a VPN
   forwarder in order to provide multicast service for a BGP IP VPN.

   It meets several important requirements:

      Support for both source-specific and shared multicast trees.

      Support for variable degrees of replication per tree node.

      Loop-free forwarding topology.

   The solution itself does not assume a specific topology on the
   underlying infrastructure network.  We simply assume that it is
   undesirable to use native multicast service.  This can be a result of
   topology as per the CLOS example above or some other constraint that
   makes it undesirable to create multicast groups based on the overlay
   topology.

2.  Overview

   This document defines a mechanism to construct and manage multicast
   distribution trees for overlay networks that does not rely on the
   underlying physical network to provide multicast capabilities.  The
   solution places an upper bound on the number of copies that a
   particular network node has to generate in contrast with ingress
   replication in which the ingress node must generate one packet
   replica for each receiver in the group.

   Using this approach ingress node and link load is traded off for
   additional packet replication steps in other nodes in the network.
   This is achieved by building a K-ary tree where each node is
   responsible to generate up-to K replicas.  For a multicast group with
   m receivers the height of the tree is approximately "log K(m)".
   Where the height of the tree determines the maximum number of
   forwarding hops required to deliver a packet to the receiver.

   A separate overlay distribution tree is constructed for each
   multicast group, using an MPLS label to identify the tree at each
   hop.  The nodes in the tree are VPN forwarders with local receivers
   for the specific group.  The tree uses a bi-directional forwarding
   algorithm.  A shared tree is used for all the sources in the group in
   the case of an ASM group.

   The distribution tree is constructed hierarchically:

   1.  Signaling Gateways build a tree the contains all locally
       registered VPN forwarders with local multicast receivers,
       observing the out-degree constraint K.

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   2.  Each Signaling Gateway announces a collection of available edges
       that can be used to join its local distribution tree with other
       trees built by other Signaling Gateways.  The number of such
       edges also respects the out-degree constraint.

   3.  One of the Signaling Gateways that has been previously elected to
       assume the role of "tree manager" for the specific group, assigns
       the edges that connect the lowest level trees together and
       advertises this information to the other Signaling Gateways.

   IP hosts use IGMP [RFC3376]/MLD [RFC3810] to request the delivery of
   multicast packets for a particular (*, g) or (s, g). Discovery
   applications where the intent is to allow applications to discover
   the group membership use (*, g) JOINs.  Content delivery applications
   may use an (s, g) JOIN after initially performing discovery either
   via multicast or by other means.

   In the context of end-system VPNs, the VPN Forwarder acts as an IGMP
   querier on the virtual interfaces and receives IGMP/MLD Membership
   Report packets.  It uses this information to generate VPN-specific
   multicast membership information.

   This information is communicated to the Signaling Gateway as a triple
   (vrf-id, s/*, g) via an XMPP publish request.  This is similar to the
   process used to publish unicast IP addresses associated with virtual-
   interfaces.

   This message also indicates the label range that can be used to
   assign 20-bit forwarding labels to this multicast traffic flow.  The
   same label range can be shared between different multicast groups.
   It is the responsibility of the VPN Gateway to ensure that a given
   label is not used for multiple groups simultaneously.

   VPN Forwarders can choose to advertise a single label range for all
   multicast groups or different label ranges for different sets of
   multicast groups.  The set granularity can be as small as single
   multicast group.

   The label range advertised by the VPN Forwarder should be larger than
   the expected number of active multicast groups within the set plus an
   additional constant that ensures that a label will not be reused
   within a time frame greater than the time it takes for topology
   updates to propagate.

   Signaling Gateways construct multicast distribution trees such that
   each node in the tree is a VPN Forwarder and each node in the tree
   has no more than K-edges where K is defined by configuration.  The
   parameter K may be different for different VPN gateways.






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   The multicast distribution tree is an acyclic graph.  The Signaling
   Gateway assigns edges between nodes ensuring that all nodes are
   connected and there are no cycles.  The resulting graph is a spanning
   tree.

   The Signaling Gateway can use any algorithm to manage the graph.  In
   practice, we expect that the Signaling Gateway would attempt to
   minimize the cost of the tree subject to the out-degree constraint
   (at most K edges) while also minimizing the disruption caused by each
   individual node JOIN or LEAVE.

   The Signaling Gateway constructs an OLIST for each VPN Forwarder,
   where its OLIST is constituted by an upstream edge (for all nodes
   except for the root) plus up-to K downstream edges.  Each VPN
   Forwarder delivers traffic locally to the virtual interfaces that
   have JOINed the specific group as well as replicate the packet up-to
   K times according to the OLIST.

   Whenever the OLIST for a given node changes, the Signaling Gateway
   MUST allocate a different label that corresponds to that version of
   the OLIST. This is used to avoid forwarding loops.  The assumption is
   that at each run of its tree management algorithm the Gateway is
   capable of building a acyclic graph.  However signaling updates from
   the Gateway to the VPN Forwarders are not synchronous.  Each modified
   OLIST will have a different label assigned, which means that in
   transient state traffic may be discarded if a VPN forwarder with
   information regarding an old edge send traffic to a VPN forwarder
   which has already received information of the new topology.  However
   this eliminates the possibility of forwarding loops.

   Traffic forwarding is done according to a bi-directional forwarding
   algorithm.  Packets flowing from the root are distributed to all the
   outgoing edges.  Traffic received from one of the leaves is sent to
   the root facing interface plus remaining descendants.  This assumes
   that the VPN forwarder has the ability to determine the source of the
   traffic, by examining the outer IP header of the packet.  The MPLS
   label contained in the packet identifies the multicast distribution
   tree but it is not sufficient to determine the OLIST element from
   which the packet has been received.

   Signaling Gateways communicate multicast membership information to
   each other using BGP L3VPN C-Multicast routes [RFC6514].  Associated
   with each C-Multicast route, the Signaling Gateway also advertises
   up-to K edges that can be use to interconnect the multicast
   distribution tree that it manages with other trees managed by its
   peers.  The C-Multicast routes are known to all signaling gateways
   which have local membership in the corresponding VPNs.

   A predefined hash function is used to determine a 32-bit value X
   associated with the specific multicast group.  This value is used to
   elect the multicast tree manager for the specific group.  The tree
   manager is the Signaling Gateway for which the value (RouterId - X)
   is lower using 32-bit unsigned math.


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   As previously described in the case of the Signaling Gateways
   managing distribution trees of VPN Forwarders, the tree manager is
   responsible to determine the edges between the several nodes in order
   to build an acyclic graph.  In this case the nodes are themselves
   replication trees.

   The tree manager is responsible to assign the forwarding labels used
   by the particular graph edge.  These labels are offered in the
   C-Multicast membership information as a list of available labels per
   edge.

3.  VPN Forwarder behavior

   VPN Forwarders act as IGMP/MLD queriers on the virtual interfaces
   that provide connectivity to end-systems.  They receive IGMP/MLD
   Membership Report packets on these point-to-point interfaces which
   are then used to build the local per VRF membership information.

   Each VRF may have a list of (s, g), (*, g) and (*, *) multicast
   routing entries associated with it.  These are the result of IGMP/MLD
   Membership Reports or Queries.  Routing entries can also be created
   as a result of detecting a local source on one of the virtual-
   interfaces associated with the VRF.

   Multicast groups in the Source-Specific Multicast  [RFC4607] address
   prefix use both (s, g) and (*, g) routing entries while the Any-
   Source Multicast (ASM) groups use (*, g) routing entries only.

   The forwarding table on a VPN Forwarder contains (vrf, *, g) entries
   for ASM groups and (s, g) entries for SSM groups.  Multicast packets
   that do not match an existing forwarding entry SHALL result in the
   creation of a local routing entry, when received from a virtual
   interface.  The VPN Forwarder MAY decide to hold on the the first
   packet that triggers the creating of a routing entry.

   Locally-know multicast routes, either the result of IGMP/MLD
   Membership Reports or locally sourced traffic are subject to
   expiration.

   When a multicast route is created locally, the VPN Forwarder
   generates an XMPP subscription message to the corresponding vrf-name,
   group and source.  When the source is not specified a (*, g) is
   implied.  When a multicast router is detected on the virtual-
   interface, via the receipt of IGMP/MLD Query messages the VPN
   forwarder subscribes to the group 0.0.0.0.

   Group Join form VPN Forwarder to gateway:







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   <iq type='set'
       from='01020304abcd@domain.org'  <!-- VPN forwarder system-id -->
       to='network-control.domain.org'
       id='sub1'>
     <pubsub xmlns='http://jabber.org/protocol/pubsub'>
       <subscribe node='vpn-customer-name/224.1.1.1'/>
       <options>
         <x xmlns='jabber:x:data' type='submit'>
           <field var='label-range'><value>10000-20000</value></field>
         </x>
       </options>
     </pubsub>
   </iq>

   Signaling Gateways build the multicast distribution tree for a
   specific group.  When the distribution tree is built, the signaling
   gateway will include as members all the (*, *) receivers of ASM
   groups and all (*, *) and (*, g) receivers of SSM groups.

   Once the subscription is received, the gateway sends XMPP event
   notifications that contain forwarding information for the specific
   group.  These messages contain an incoming label, assigned by the
   gateway, and a list of up-to K+1 next-hops, where each next-hop
   consists of an IP destination address and an outgoing label.

   When the last local member of a multicast group leaves the group,
   either explicitly or as a result of a expiration timer, the VPN
   forwarder generates an XMPP pubsub 'delete' message to the Signaling
   Gateway.

   Multicast forwarding state update from gateway to VPN forwarder:

   <message to='system-id@domain.org from='network-control.domain.org>
     <event xmlns='http://jabber.org/protocol/pubsub#event'>
       <items node='vpn-customer-name/224.1.1.1'>
         <item id='ae890ac52d0df67ed7cfdf51b644e901'>
           <entry xmlns='http://ietf.org/protocol/bgpvpn'>
         <label>10000</label>      <!-- incoming label number -->
         <olist>
           <next-hop address='10.1.1.1' label='10101'/>
           [...]
           <next-hop address='10.1.10.10' label='10222'/>
         </olist>
           </entry>
          </item>
         <item >
           ...
         </item>
       </items>
     </event>
   </message>



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   The VPN forwarder updates its multicast forwarding table with the
   information received in this event notification.  Any label that was
   previously assigned to the (vrf, *, g) or (vrf, s, g) forwarding
   entry is implicitly withdrawn.

   Multicast packets are encapsulated in an IP tunnel that contains a
   20-bit label as well as the original multicast datagram.  This 20-bit
   label uniquely identifies the multicast replication state as
   specified by the OLIST.

   The VPN Forwarder MUST drop an incoming multicast packet unless it is
   either received from a local virtual interface or the source is
   present in the OLIST.

   The VPN Forwarder MUST generate a copy of the incoming packet to all
   next-hops in the OLIST except the next-hop with the same IP address
   as the outer header source of the incoming packet.

   Additionally, the VPN Forwarder MUST generate additional copies to
   the virtual interfaces associated with the VRF that have expressed
   interest in the specific multicast group.

4.  Multicast tree management

   The multicast forwarding tree associated with a specific multicast
   group is built hierarchically.  At the lowest level, Signaling
   Gateways build a acyclic graph in which nodes are VPN Forwarders and
   where nodes have up-to (K+1) edges.  Above this level, the graph
   nodes are multicast replication trees themselves.

   At the lowest level, VPN Forwarders implicitly select the signaling
   gateway responsible to manage its tree by subscribing to a single
   gateway.  At higher levels, the forwarding tree manager is elected by
   selecting the gateway with the smaller value of (RouterId -
   HashFunction(g)) in unsigned 32-bit arithmetic.

   While the multicast tree management algorithm is a local matter to
   the gateway implementation, the algorithm used SHOULD minimize the
   height of the multicast replication tree and attempt to minimize the
   number of state changes to the tree.  As an example Prim's algorithm
   [prim] can be used to generate a minimum spanning tree.

   In this application all the nodes in the graph can have an edge to
   any other node as long as the total number of edges does not exceed K
   + 1.  The implementation may choose to assign the same cost to all
   the edges or i may use external information to determine cost.  For
   instance, an implementation may choose to assign lower cost to edges
   between nodes in the same server rack versus nodes in different
   racks.





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   Signaling gateways assign forwarding labels from an interval provided
   by the VPN Forwarder.  Whenever the tree topology changes such that
   nodes in with different versions of the topology could create a
   forwarding loop the gateway MUST allocate a new label.  When leaf
   nodes in the tree are added or deleted these changes can be performed
   without concern for the formation of transient loops.  However, in
   the case of tree rotations to rebalance the tree, there is a clear
   potential for forwarding loops.

   In generic terms, a transient forwarding loop can be formed if there
   exist multiple versions of the graph that are being executed by
   different nodes.  As an example consider a graph with nodes (a, b, c)
   that goes through the following topology versions:

   +---------+----------+
   | Version | Edges    |
   +---------+----------+
   | 1       | a-b, a-c |
   | 2       | a-b, b-c |
   | 3       | a-c, b-c |
   +---------+----------+

   In a scenario where node 'a' is at version 1, node 'b' at version 2
   and node 'c' at version 3 a transient loop will occur.  In this
   example, a packet that is injected at node 'a' will propagate to both
   'b' and 'c'. 'b' accepts the packet since the edge (a-b) is a valid
   edge and propagates the traffic to 'c'. 'c' accepts packet from both
   'a' and 'b'. The packet 'c' receives from 'b' will be forwarded to
   'a' which will accept it, creating a loop.  Likewise the packet 'c'
   receives from 'a' will be forwarded to 'b', which will then forward
   to 'a'.

   All of the topologies in the example above are loop-free.  However
   the fact that routing information propagates is not synchronized
   allows for the formation of loops.

   Given that the propagation of forwarding entries to VPN Forwarders is
   asynchronous and that it would be undesirable to attempt to
   synchronize the process, we use the incoming label to break the
   potential forwarding loop.  For the loop to be broken it is necessary
   that the forwarding labels used in the edge (a-c) in the example
   above be different in configuration 1 versus configuration 3.

   As an example, forwarding loop avoidance can be implemented by
   keeping a list of edges that have been previously present in a node
   and modifying the label every time the tree management algorithm adds
   an edge that had been removed from the node.







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   A Signaling Gateway that has received multicast routing information
   from locally connected VPN Forwarders shall advertise the
   corresponding multicast group as a C-Multicast route.  These
   C-Multicast routes shall include an Edge Discovery attribute that
   describes up-to K + 1 multicast next-hops, each containing an IP
   address and a label range that can be used to assign forwarding
   labels.

   The tree manager election algorithm selects which of the signaling
   gateways is responsible to determine the topology of the multicast
   distribution tree.  At this level in the hierarchy, the distribution
   tree consists of graph nodes that are themselves distribution trees.
   In the case where tree nodes were VPN Forwarders, the tree management
   algorithm can assign up-to (K + 1) edges to a node (where K can
   potentially be configurable per node). In the case where tree nodes
   are distribution trees, the tree management algorithm is limited to
   the number of edges received in the C-Multicast route.

   The algorithm used to manage the lower and higher levels in the
   hierarchy can be the same.

   When the tree manager modifies the tree topology it shall generate
   BGP routes that describe the current topology.  These routes are
   encoded using the MCAST-VPN NLRI using the Multicast Tree Route Type
   defined bellow.

   Multicast Tree routes contain an Edge Forwarding attribute that
   describes the active edges between different nodes.

   Multicast Tree routes are interpreted only by the Signaling Gateway
   that is identified by the Router-Id contained in the NLRI prefix.  On
   a receipt of such a route, the Signaling Gateway connects its own
   multicast distribution tree with the edges contained in the Edge
   Forwarding attribute.

   In order to illustrate the operation of the hierarchical label
   management process, we present the following example.

   Consider a scenario where the out-degree constant K is 4 and where 4
   Signaling Gateways (A, B, C, D) are present.  The Signaling Gateways
   are the multicast tree managers for a set of VPN forwarders.  We use
   the notation a1 .. an for the VPN forwarders managed by node A.

   Assume that Signaling Gateway A has built a multicast distribution
   tree such that node a3 is the root.  This node has 2 descendants a1
   and a2. Each of these have at most 3 locally connected edges.  In
   this scenario the Signaling Gateway A has chosen to reserve edges at
   the top of its tree in order to connect to external trees.






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   Signaling Gateway A advertises its state to the BGP network by
   generating a C-multicast route containing a Multicast Edge Discovery
   attribute with the next-hops (a1, a1, a2, a3). Although signaling
   gateway A may have many other VPN forwarders that are receivers of
   the specified group this information is not propagated through BGP.

   Each of the next-hops in the list has an incoming label that is
   currently in use.  The Edge Discovery attribute contains a interval
   of free (unused labels) that is valid for each of the next-hops.

   In this example, we assume that the Signaling Gateway B was elected
   as the tree manager for the higher level tree.  At this stage, the
   tree manager has the following state:

   +-----------+------------------------------------+
   | Router-Id | Edges                              |
   +-----------+------------------------------------+
   | A         | (a1, 0), (a1, 0), (a2, 0), (a3, 0) |
   | B         | (b1, 0)                            |
   | C         | (c1, 0), (c2, 0), (c3, 0)          |
   | D         | (d1, 0), (d2, 0), (d3, 0)          |
   +-----------+------------------------------------+

   In the table above, each pair represents the IP address an assigned
   incoming label of a VPN forwarder.

   In this example all the signaling gateways decided to advertise less
   than K+1 edges.

   One possible assignment is to make node A's tree the root of the top-
   level distribution tree.  This can be accomplished by creating the
   edges (a1, b1), (a1, c1), (a2, d1). The tree manager must allocate a
   label for each of the next-hops from their respective label space.

   As a result of this tree assignment, the multicast tree manager (B)
   generates the following Multicast Tree Route Type updates:

   +-----------------+-------------------------------------------------+
   | Router-Id       | Edges                                           |
   +-----------------+-------------------------------------------------+
   | A               | (a1, b1, 10000, 20000), (a1, c1, 10000, 21000), |
   |                 | (a2, d1, 11000, 22000)                          |
   | C               | (c1, a1, 21000, 10000)                          |
   | D               | (d1, a2, 22000, 11000)                          |
   +-----------------+-------------------------------------------------+

   When A, C and D receive their respective routing updates they will
   generate the corresponding XMPP event notification messages to the
   affected VPN forwarders.  In A's case this implies updating the state
   of a1 and a2. a1 forwarding table now has a new incoming label
   (10000) and the next-hops (b1, a2, a3, c1).

5.  BGP Protocol Extensions

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   This document defines an additional Route Type for the MCAST-VPN NLRI
   [RFC6514], called Multicast Tree Route Type.

5.1.  Multicast Tree Route Type

   Multicast Tree Routes are used by multicast tree managers to
   advertise a acyclic graph topology of nodes which themselves may
   consist of multicast distribution trees.  A BGP UPDATE containing a
   Multicast Tree Route as part of the MP_REACH Path Attribute MUST also
   contain the Multicast Edge Forwarding Attribute.

   A Multicast Tree Route is encoded as a MCAST-VPN NLRI with Route Type
   8 and consists of the following:

   +--------------------------------------+
   |             RD (8 octets)            |
   +--------------------------------------+
   |         Router-Id (4 octects)        |
   +--------------------------------------+
   |  Multicast Source Length (1 octect)  |
   +--------------------------------------+
   |      Multicast Source (variable)     |
   +--------------------------------------+
   |  Multicast Group Length (1 octect)   |
   +--------------------------------------+
   |     Multicast Group (variable)       |
   +--------------------------------------+

   The Route Distinguisher (RD) is encoded as described in [RFC4364].

   The Router-Id field identifies the multicast tree node for which the
   edges are being advertised.

   The Multicast Source and Group fields specify the multicast group for
   which the multicast forwarding state is being advertised.

5.2.  Multicast Edge Discovery Attribute

   The Multicast Edge Discovery Path Attribute is associated with
   C-Multicast routes and contains one or more next-hop information
   elements where each information element follows the model described
   bellow:












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   +------------------------------+
   |   IP addr Length (1 octect)  |
   +------------------------------+
   |    IP address (variable)     |
   +------------------------------+
   |Label Range Length (1 octect) |
   +------------------------------+
   |   Start Label (4 octects)    |
   +------------------------------+
   |     End Label (4 octects)
   +------------------------------+
   |             ...              |
   +------------------------------+
   |   Start Label (4 octects)    |
   +------------------------------+
   |     End Label (4 octects)    |
   +------------------------------+

   Each 'Next-hop' information element identifies an incoming edge that
   can be used to connect Signaling Gateway locally managed replication
   tree with other replication trees for the same group.  The 'IP
   address' value corresponds to the IP address of VPN Forwarder that is
   a member of the local tree.

   The same VPN Forwarder address can appear multiple times in the
   Discovery Path Attribute.  Signaling Gateways advertise up-to K + 1
   Next-hop elements.

   Each attribute specifies one or more contiguous label ranges
   available for assignment at the specified VPN Forwarder.  If the VPN
   forwarder appears multiple times in the list, the label range
   advertisements SHOULD be the same.

   The Signaling Gateway SHALL ensure that the number of locally
   assigned edges on a VPN forwarder plus the number of Next-hop
   information elements that refer to that VPN forwarder do not exceed K
   + 1.

5.3.  Multicast Edge Forwarding Attribute

   The Multicast Edge Forwarding Path Attribute is associated with
   Multicast Tree Route Type NLRI routes and contains one or more edge
   information elements where each information element follows the model
   described bellow:










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   +------------------------------+
   |  Next-hop Length (1 octect)  |
   +------------------------------+
   |  Inbound Node (variable)     |
   +------------------------------+
   |  Inbound Label (4 octects)   |
   +------------------------------+
   |  Outbound Node (variable)    |
   +------------------------------+
   |  Outbound Label (4 octects)  |
   +------------------------------+


   Each edge element contained in this list contains the address on an
   inbound node that has been advertised via the Edge Discovery
   attribute as well as a label assigned from the respective interval.
   The outbound node address and label connect specify the destination
   node for this edge.

6.  Security Considerations

   It is helpful to differentiate between the control plane and data
   plane security aspects of the solution.

   The control plane assumes that XMPP sessions between VPN forwarders
   and Signaling Gateway are authenticated such that the Signaling
   Gateway is able to verify the identity of the VPN Forwarder.

   BGP sessions between Signaling Gateways should also be subject to
   authentication.

   At the data-plane, it is important to note that a compromised VPN
   forwarder is able to modify message that traverse through it.

7.  References

7.1.  Normative References

   [RFC3376]  Cain, B., Deering, S., Kouvelas, I., Fenner, B. and A.
              Thyagarajan, "Internet Group Management Protocol, Version
              3", RFC 3376, October 2002.

   [RFC3810]  Vida, R. and L. Costa, "Multicast Listener Discovery
              Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, February 2006.

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




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   [RFC6514]  Aggarwal, R., Rosen, E., Morin, T. and Y. Rekhter, "BGP
              Encodings and Procedures for Multicast in MPLS/BGP IP
              VPNs", RFC 6514, February 2012.

7.2.  Informational References

   [I-D.marques-l3vpn-end-system]
              Marques, P., Fang, L., Pan, P., Shukla, A., Napierala, M.
              and N. Bitar, "BGP-signaled end-system IP/VPNs.",
              Internet-Draft draft-marques-l3vpn-end-system-05, March
              2012.

   [clos]     "A study of non-blocking switching networks", Bell System
              Technical Journal 32, March 1953.

   [prim]     Prim, R.C., "Shortest connection networks and some
              generalizations", Bell System Technical Journal 36, 1957.

Authors' Addresses

   Pedro Marques
   Contrail Systems
   440 N. Wolfe Rd.
   Sunnyvale, CA 94085

   Email: roque@contrailsystems.com


   Luyuan Fang
   Cisco Systems
   111 Wood Avenue South
   Iselin, NJ 08830

   Email: lufang@cisco.com


   Derick Winkworth
   FIS

   Email: derick.winkworth@fisglobal.com


   Yiqun Cai
   Microsoft Corporation
   1065 La Avenida
   Mountain View, CA 94043

   Email: yiqunc@microsoft.com






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   Petr Lapukhov
   Microsoft Corporation

   Email: petrlapu@microsoft.com

















































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