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Network Working Group                                        Dave Thaler
Internet-Draft                                              Mohit Talwar
February 2005                                              Amit Aggarwal
Expires: August 21, 2005                                       Microsoft
                                                        Lorenzo Vicisano
                                                           Cisco Systems
                                                            Tom Pusateri
                                                        Juniper Networks





         Automatic IP Multicast Without Explicit Tunnels (AMT)
                draft-ietf-mboned-auto-multicast-04.txt



Status of this Memo


   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
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Abstract

   Automatic Multicast Tunneling (AMT) allows multicast communication
   amongst isolated multicast-enabled sites or hosts, attached to a
   network which has no native multicast support.  It also enables them
   to exchange multicast traffic with the native multicast



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   infrastructure (MBone) and does not require any manual configuration.
   AMT uses an encapsulation interface so that no changes to a host
   stack or applications are required, all protocols (not just UDP) are
   handled, and there is no additional overhead in core routers.

Copyright Notice

   Copyright (C) The Internet Society (2005).  All Rights Reserved.


1.  Introduction

   The primary goal of this document is to foster the deployment of
   native IP multicast by enabling a potentially large number of nodes
   to connect to the already present multicast infrastructure.
   Therefore, the techniques discussed here should be viewed as an
   interim solution to help in the various stages of the transition to a
   native multicast network.

   To allow fast deployment, the solution presented here only requires
   small and concentrated changes to the network infrastructure, and no
   changes at all to user applications or to the socket API of end-
   nodes' operating systems.  The protocol introduced in this
   specification is deployed in a few strategically-placed network nodes
   and in user-installable software modules (pseudo device drivers
   and/or user-mode daemons) that reside underneath the socket API of
   end-nodes' operating systems.  This mechanism is very similar to that
   used by "6to4" [6TO4, ANYCAST] to get automatic IPv6 connectivity.

   Effectively, AMT treats the unicast-only internetwork as a large non-
   broadcast multi-access (NBMA) link layer, over which we require the
   ability to multicast.  To do this, multicast packets being sent to or
   from a site must be encapsulated in unicast packets.  If the group
   has members in multiple sites, AMT encapsulation of the same
   multicast packet will take place multiple times by necessity.

   The following problems are addressed:

   1. Allowing isolated sites/hosts to receive the SSM flavor of
      multicast ([SSM]).

   2. Allowing isolated sites/hosts to transmit the SSM flavor of
      multicast.

   3. Allowing isolated sites/hosts to receive general multicast (ISM
      [RFC1112]).

      This document does not address allowing isolated sites/hosts to



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      transmit general multicast.  We expect that other solutions (e.g.,
      Tunnel Brokers, a la [BROKER]) will be used for sites that desire
      this capability.


2.  Requirements Terminology

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [RFC-2119].


3.  Definitions


    +---------------+        Internet            +---------------+
    | AMT Site      |                            | MBone         |
    |               |              AMT           |               |
    |        +------+----+         Relay    +----+----+ AMT      |
    |        |AMT Gateway|         Anycast  |AMT Relay| Subnet   |
    |        |     +-----+-+       Prefix +-+-----+   | Prefix   |
    |        |     |AMT IF |     <--------|AMT IF |   |--------> |
    |        |     +-----+-+              +-+-----+   |          |
    |        +------+----+                  +----+----+          |
    |               |                            |               |
    +---------------+                            +---------------+

               Figure 1: Automatic Multicast Definitions.


   AMT Pseudo-Interface
      AMT encapsulation of multicast packets inside unicast packets
      occurs at a point that is logically equivalent to an interface,
      with the link layer being the unicast-only network.  This point is
      referred to as a pseudo-interface. Some implementations may treat
      it exactly like any other interface and others may treat it like a
      tunnel end-point.

   AMT Gateway
      A host, or a site gateway router, supporting an AMT Pseudo-
      Interface.  It does not have native multicast connectivity to the
      native multicast backbone infrastructure.  It is simply referred
      to in this document as a "gateway".

   AMT Site
      A multicast-enabled network not connected to the multicast
      backbone served by an AMT Gateway.  It could also be a stand-
      alone AMT Gateway.



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   AMT Relay Router
      A multicast router configured to support transit routing between
      AMT Sites and the native multicast backbone infrastructure.  The
      relay router has one or more interfaces connected to the native
      multicast infrastructure, zero or more interfaces connected to the
      non-multicast capable internetwork, and an AMT pseudo-interface.
      It is simply referred to in this document as a "relay".

      As with [6TO4], we assume that normal multicast routers do not
      want to be tunnel endpoints (especially if this results in high
      fanout), and similarly that service providers do not want
      encapsulation to arbitrary routers.  Instead, we assume that
      special-purpose routers will be deployed that are suitable for
      serving as relays.

   AMT Relay Anycast Prefix
      A well-known address prefix used to advertise (into the unicast
      routing infrastructure) a route to an available AMT Relay Router.
      This could also be private (i.e., not well-known) for a private
      relay.

      Prefixes for both IPv4 and IPv6 will be assigned in a future
      version of this draft.

   AMT Relay Anycast Address
      An anycast address which is used to reach the nearest AMT Relay
      Router.

      This address corresponds to the lowest address in the AMT Relay
      Anycast Prefix.

   AMT Unicast Autonomous System ID
      A 16-bit Autonomous System ID, for use in BGP in accordance to
      this memo.  AS 10888 might be usable for this, but for now we'll
      assume it's different, to avoid confusion.  This number represents
      a "pseudo-AS" common to all AMT relays using the well known AMT
      Relay Anycast Prefix (private relays use their own ID).

      To protect themselves from erroneous advertisements, managers of
      border routers often use databases to check the relation between
      the advertised network and the last hop in the AS path.
      Associating a specific AS number with the AMT Relay Anycast
      Address allows us to enter this relationship in the databases used
      to check inter-domain routing [ANYCAST].

   AMT Subnet Prefix
      A well-known address prefix used to advertise (into the M-RIB of
      the native multicast-enabled infrastructure) a route to AMT Sites.



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      This prefix will be used to enable sourcing SSM traffic from an
      AMT Gateway.

   AMT Gateway Anycast Address
      An anycast address in the AMT Subnet Prefix range, which is used
      by an AMT Gateway to enable sourcing SSM traffic from local
      applications.

   AMT Multicast Autonomous System ID
      A 16-bit Autonomous system ID, for use in MBGP in accordance to
      this memo. This number represents a "pseudo-AS" common to all AMT
      relays using the well known AMT Subnet Prefix (private relays use
      their own ID). We assume that the existing AS 10888 is suitable
      for this purpose.  (Note: if this is a problem, a different one
      would be fine.)


4.  Overview


4.1.  Receiving Multicast in an AMT Site

                               Internet
    +---------------+                            +---------------+
    | AMT Site      |     2. 3-way Membership    | MBone         |
    |               |          Handshake         |               |
    |   1. Join +---+---+ =================> +---+---+           |
    |     +---->|Gateway|                    | Relay |           |
    |     |     +---+---+ <================= +---+---+           |
    |   R-+         |       3. Receive Data      |               |
    +---------------+                            +---------------+

             Figure 2: Receiving Multicast in an AMT Site.

   AMT relays and gateways cooperate to transmit multicast traffic
   sourced within the native multicast infrastructure to AMT sites:
   relays receive the traffic natively and unicast-encapsulate it to
   gateways; gateways decapsulate the traffic and possibly forward it
   into the AMT site.

   Each gateway has an AMT pseudo-interface that serves as a default
   multicast route.  Requests to join a multicast session are sent to
   this interface and encapsulated to a particular relay reachable
   across the unicast-only infrastructure.

   Each relay has an AMT pseudo-interface too.  Multicast traffic sent
   on this interface is encapsulated to zero or more gateways that have
   joined to the relay.  The AMT recipient-list is determined for each



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   multicast session.  This requires the relay to keep state for each
   gateway which has joined a particular group or (source, group) pair).
   Multicast packets from the native infrastructure behind the relay
   will be sent to each gateway which has requested them.

   All multicast packets (data and control) are encapsulated in unicast
   packets.  To work across NAT's, the encapsulation is done over UDP
   using the IANA reserved AMT port number.

   Each relay, plus the set of all gateways using the relay, together
   are thought of as being on a separate logical NBMA link.  This
   implies that the AMT recipient- list is a list of "link layer"
   addresses which are (IP address, UDP port) pairs.

   Since the number of gateways using a relay can be quite large, and we
   expect that most sites will not want to receive most groups, an
   explicit-joining protocol is required for gateways to communicate
   group membership information to a relay.  The two most likely
   candidates are the IGMP/MLD [IGMPv3/MLDv2] protocol, and the PIM-
   Sparse Mode [PIMSM] protocol.  Since an AMT gateway may be a host,
   and hosts typically do not implement routing protocols, gateways will
   use IGMP/MLD as described in Section 5 below.  This allows a host
   kernel (or a pseudo device driver) to easily implement AMT gateway
   behavior, and obviates the relay from the need to know whether a
   given gateway is a host or a router.  From the relay's perspective,
   all gateways are indistinguishable from hosts on an NBMA leaf
   network.


4.1.1.  Scalability Considerations

   It is possible that millions of hosts will enable AMT gateway
   functionality and so an important design goal is not to create
   gateway state in each relay until the gateway joins a multicast
   group.  But even the requirement that a relay keep group state per
   gateway that has joined a group introduces potential scalability
   concerns.

   Scalability of AMT can be achieved by adding more relays, and using
   an appropriate relay discovery mechanism for gateways to discover
   relays.  The solution we adopt is to assign an anycast address to
   relays.  However, simply sending periodic membership reports to the
   anycast address can cause duplicates.  Specifically, if routing
   changes such that a different relay receives a periodic membership
   report, both the new and old relays will encapsulate data to the AMT
   site until the old relay's state times out.  This is obviously
   undesirable.  Instead, we use the anycast address merely to find the
   unicast address of a relay to which membership reports are sent.



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   Since adding another relay has the result of adding another
   independent NBMA link, this allows the gateways to be spread out
   among more relays so as to keep the number of gateways per relay at a
   reasonable level.


4.1.2.  Spoofing Considerations

   An attacker could affect the group state in the relay or gateway by
   spoofing the source address in the join or leave reports. This can be
   used to launch reflection or denial of service attacks on the target.
   Such attacks can be mitigated by using a three way handshake between
   the gateway and the relay for each multicast membership report or
   leave.

   When a gateway or relay wants to send a membership report, it first
   sends an AMT Request with a request nonce in it. The receiving side
   (the respondent) can calculate a message authentication code (MAC)
   based on the source IP address of the Request, the source UDP port,
   the request nonce, and a secret key known only to the respondent.
   The algorithm used to calculate the MAC does not have to be
   standardized since the respondent generates and verifies the MAC and
   the originator simply echoes it.

   An AMT Membership Query is sent back including the request nonce and
   the MAC to the originator of the Request. The originator then sends
   the IGMP/MLD Membership/Listener Report or Leave/Done along with the
   request nonce and the received MAC back to the respondent finalizing
   the 3-way handshake.

   Upon reception, the respondent can recalculate the MAC based on the
   source IP address, the source UDP port, the request nonce, and the
   local secret. The IGMP/MLD message is only accepted if the received
   MAC matches the calculated MAC.

   The local secret never has to be shared with the other side.  It is
   only used to verify return routability of the originator.


4.2.  Sourcing Multicast from an AMT site

   Two cases are discussed below: multicast traffic sourced in an AMT
   site and received in the MBone, and multicast traffic sourced in an
   AMT site and received in another AMT site.

   In both cases only SSM sources are supported.  Furthermore this
   specification only deals with the source residing directly in the
   gateway.  To enable a generic node in an AMT site to source



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   multicast, additional coordination between the gateway and the
   source-node is required.

   The general mechanism used to join towards AMT sources is based on
   the following:

   1. Applications residing in the gateway use addresses in the AMT
      Subnet Prefix to send multicast, as a result of sourcing traffic
      on the AMT pseudo-interface.

   2. The AMT Subnet Prefix is advertised for RPF reachability in the M-
      RIB by relays and gateways.

   3. Relays or gateways that receive a join for a source/group pair use
      information encoded in the address pair to rebuild the address of
      the gateway (source) to which to encapsulate the join (see Section
      5 for more details). The membership reports use the same three way
      handshake as outlined in Section 4.1.2.


4.2.1.  Supporting Site-MBone Multicast


                                Internet
    +---------------+                            +---------------+
    | AMT Site      |     2. 3-way Membership    | MBone         |
    |               |           Handshake        |               |
    |           +---+---+ <================= +---+---+ 1. Join   |
    |           |Gateway|                    | Relay |<-----+    |
    |           +---+---+ =================> +---+---+      |    |
    |               |      3. Receive Data       |          +-R  |
    +---------------+                            +---------------+

                    Figure 3: Site-MBone Multicast.

   If a relay receives an explicit join from the native infrastructure,
   for a given (source, group) pair where the source address belongs to
   the AMT Subnet Prefix, then the relay will periodically (using the
   rules specified in Section 4.1.2) encapsulate membership updates for
   the group to the gateway.  The gateway must keep state per relay from
   which membership reports have been sent, and forward multicast
   traffic from the site to all relays from which membership reports
   have been received.  The choice of whether this state and replication
   is done at the link-layer (i.e., by the tunnel interface) or at the
   network-layer is implementation dependent.

   If there are multiple relays present, this ensures that data from the
   AMT site is received via the closest relay to the receiver. This is



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   necessary when the routers in the native multicast infrastructure
   employ Reverse-Path Forwarding (RPF) checks against the source
   address, such as occurs when [PIMSM] is used by the multicast
   infrastructure.

   The solution above will scale to an arbitrary number of relays, as
   long at the number of relays requiring multicast traffic from a given
   AMT site remains reasonable enough to not overly burden the site's
   gateway.


4.2.2.  Supporting Site-Site Multicast

                               Internet
    +---------------+                            +---------------+
    | AMT Site      |     2. 3-way Membership    | AMT Site      |
    |               |          Handshake         |               |
    |           +---+---+ <================= +---+---+ 1. Join   |
    |           |Gateway|                    |Gateway|<-----+    |
    |           +---+---+ =================> +---+---+      |    |
    |               |      3. Receive Data       |          +-R  |
    +---------------+                            +---------------+

                     Figure 4: Site-Site Multicast.

   Since we require gateways to accept membership reports, as described
   above, it is also possible to support multicast among AMT sites,
   without requiring assistance from any relays.

   When a gateway wants to join a given (source, group) pair, where the
   source address belongs to the AMT Subnet Prefix, then the gateway
   will periodically unicast encapsulate an IGMPv3/MLDv2 [IGMPv3/MLDv2]
   Report directly to the site gateway for the source.

   We note that this can result in a significant amount of state at a
   site gateway sourcing multicast to a large number of other AMT sites.
   However, it is expected that this is not unreasonable for two
   reasons.  First, the gateway does not have native multicast
   connectivity, and as a result is likely doing unicast replication at
   present.  The amount of state is thus the same as what such a site
   already deals with. Secondly, any site expecting to source traffic to
   a large number of sites could get a point-to-point tunnel to the
   native multicast infrastructure, and use that instead of AMT.








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5.  Message Formats


5.1.  AMT Relay Discovery

   The AMT Relay Discovery message is a UDP packet sent from the AMT
   gateway unicast address to the AMT relay anycast address to discover
   the unicast address of an AMT relay.

   The UDP source port is uniquely selected by the local host operating
   system. The UDP destination port is the IANA reserved AMT port
   number.

   The payload of the UDP packet contains the following fields.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type=0x1  |     Reserved                                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Discovery Nonce                                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Fields:

   Type
      The type of the message.

   Reserved
      A 24-bit reserved field. Sent as 0, ignored on receipt.

   Discovery Nonce
      A 32-bit random value generated by the gateway and replayed by the
      relay.


5.2.  AMT Relay Advertisement

   The AMT Relay Advertisement message is a UDP packet sent from the AMT
   relay anycast address to the source of the discovery message.

   The UDP source port is the IANA reserved AMT port number and the UDP
   destination port is the source port received in the Discovery
   message.

   The payload of the UDP packet contains the following fields.




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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type=0x2  |     Reserved                                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Discovery Nonce                                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Relay Address                                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Type
      The type of the message.

   Reserved
      A 24-bit reserved field. Sent as 0, ignored on receipt.

   Discovery Nonce
      A 32-bit random value replayed from the discovery message.

   Relay Address
      The unicast IPv4 or IPv6 address of the AMT relay. The family can
      be determined by the length of the Advertisement.


5.3.  AMT Request

   A Request packet is sent to begin a 3-way handshake for sending an
   IGMP/MLD Membership/Listener Report or Leave/Done. It can be sent
   from a gateway to a relay, from a gateway to another gateway, or from
   a relay to a gateway.

   It is sent from the originator's unique unicast address to the
   respondents' unique unicast address.

   The UDP source port is uniquely selected by the local host operating
   system. It can be different for each Request and different from the
   source port used in Discovery messages but does not have to be.  The
   UDP destination port is the IANA reserved AMT port number.

   Fields:

   Type
      The type of the message.





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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type=0x3  |     Reserved                                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Request Nonce                                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Reserved
      A 24-bit reserved field. Sent as 0, ignored on receipt.

   Request Nonce
      A 32-bit identifier used to distinguish this request.


5.4.  AMT Membership Query

   An AMT Membership Query packet is sent from the relay back to the
   originator to solicit an AMT Membership Update while confirming the
   source of the original request. It contains a relay Message
   Authentication Code (MAC) that is a cryptographic hash of a private
   secret, the originators address, and the request nonce.

   It is sent from the destination address received in the Request to
   the source address received in the Request.

   The UDP source port is the IANA reserved AMT port number and the UDP
   destination port is the source port received in the Request message.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type=0x4  |    Reserved   |         Response MAC          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Response MAC (continued)                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Request Nonce                                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Fields:

   Type
      The type of the message.

   Reserved
      An 8-bit reserved field. Sent as 0, ignored on receipt.



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   Response MAC
      A 48-bit hash generated by the respondent and sent to the
      originator for inclusion in the AMT Membership Update. The
      algorithm used for this is chosen by the respondent. One algorithm
      that could be used is HMAC-MD5-48 [HMAC].

   Request Nonce
      A 32-bit identifier used to distinguish this request echoed back
      to the originator.


5.5.  AMT Membership Update

   An AMT Membership Update is sent from the originator to the
   respondent containing the original IGMP/MLD Membership/Listener
   Report or Leave/Done received over the AMT pseudo-interface.  It
   echoes the Response MAC received in the AMT Membership Query so the
   respondent can verify return routability to the originator.

   It is sent from the destination address received in the Query to the
   source address received in the Query which should both be the same as
   the original Request.

   The UDP source and destination port numbers should be the same ones
   sent in the original Request.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type=0x5  |    Reserved   |         Response MAC          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Response MAC (continued)                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Request Nonce                                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            IGMP/MLD Message                                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            ...                                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Fields:

   Type
      The type of the message.

   Reserved
      An 8-bit reserved field. Sent as 0, ignored on receipt.



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   Response MAC
      The 48-bit MAC received in the Membership Query and echoed back in
      the Membership Update.

   Request Nonce
      A 32-bit identifier used to distinguish this request.


5.6.  AMT Multicast Data

   The AMT Data message is a UDP packet encapsulating the data requested
   by the originator based on a previous AMT Membership Update message.

   It is sent from the unicast destination address of the Membership
   update to the source address of the Membership Update.

   The UDP source and destination port numbers should be the same ones
   sent in the original Query.

   The payload of the UDP packet contains the following fields.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type=0x6  |     Reserved                                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Multicast Data                                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            ...                                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Fields:

   Type
      The type of the message.

   Reserved
      A 24-bit reserved field. Sent as 0, ignored on receipt.

   UDP Multicast Data
      The original Multicast UDP data packet that is being replicated by
      the relay to the gateways.








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6.  AMT Gateway Details

   This section details the behavior of an AMT Gateway, which may be a
   router serving an AMT site, or the site may consist of a single host,
   serving as its own gateway.


6.1.  At Startup Time

   At startup time, the AMT gateway will bring up an AMT pseudo-
   interface, to be used for encapsulation.  The gateway will then send
   a AMT Relay Discovery message to the AMT Relay Anycast Address, and
   note the unicast address (which is treated as a link-layer address to
   the encapsulation interface) from the AMT Relay Advertisement
   message.  This discovery should be done periodically (e.g., once a
   day) to re-resolve the unicast address of a close relay.  The gateway
   also initializes a timer used to send periodic membership reports to
   a random value from the interval [0, [Query Interval]] before sending
   the first periodic report, in order to prevent startup
   synchronization (e.g., after a power outage).

   If the gateway is serving as a local router, it SHOULD also function
   as an IGMP/MLD Proxy, as described in [PROXY], with its IGMP/MLD
   host-mode interface being the AMT pseudo-interface.  This enables it
   to translate group memberships on its downstream interfaces into
   IGMP/MLD Reports.  The gateway MUST also advertise itself as the
   default route for multicast in the M-RIB (or it must be the default
   unicast router if unicast and multicast topologies are congruent).
   Also, if a shared tree routing protocol is used inside the AMT site,
   each tree-root must be a gateway, e.g., in PIM-SM each RP must be a
   gateway.

   Finally, to support sourcing traffic to SSM groups by a gateway with
   a global unicast address, the AMT Subnet Prefix is treated as the
   subnet prefix of the AMT pseudo-interface, and an anycast address is
   added on the interface.  This anycast address is formed by
   concatenating the AMT Subnet Prefix followed by the high bits of the
   gateway's global unicast address.  For example, if IANA assigns the
   IPv4 prefix x.y/16 as the AMT Subnet Prefix, and the gateway has
   global unicast address a.b.c.d, then the AMT Gateway's Anycast
   Address will be x.y.a.b.  Note that multiple gateways might end up
   with the same anycast address assigned to their pseudo-interfaces.


6.2.  Joining Groups with MBone Sources

   The IGMP/MLD protocol usually operates by having the Querier
   multicast an IGMP/MLD Query message on the link.  This behavior does



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   not work on NBMA links which do not support multicast.  Since the set
   of gateways is typically unknown to the relay (and potentially quite
   large), unicasting the queries is also impractical.  The following
   behavior is used instead.

   Applications residing in a gateway should join groups on the AMT
   pseudo-interface, causing IGMP/MLD Membership/Listener Reports to be
   sent over that interface.  When UDP encapsulating the membership
   reports (and in fact any other messages, unless specified otherwise
   in this document), the destination address in the outer IP header is
   the relay's unicast address.  Robustness is provided by the
   underlying IGMP/MLD protocol messages sent on the AMT pseudo-
   interface. In other words, the gateway does not need to retransmit
   IGMP/MLD Membership/Listener Reports and Leave/Done messages received
   on the pseudo-interface since IGMP/MLD will already do this.  The
   gateway simply needs to encapsulate each IGMP/MLD Membership/Listener
   Report and Leave/Done message it receives.

   However, since periodic IGMP/MLD Membership/Listener Reports are sent
   in response to IGMP/MLD Queries, some mechanism to trigger periodic
   Membership/Listener Reports and Leave/Done messages are necessary.
   This can be achieved in any implementation-specific manner.  Some
   possibilities include:

   1. The AMT pseudo-interface might periodically manufacture
      IGMPv3/MLDv2 Queries as if they had been received from an IGMP/MLD
      Querier, and deliver them to the IP layer, after which normal
      IGMP/MLD behavior will cause the appropriate reports to be sent.

   2. The IGMP/MLD module itself might provide an option to operate in
      periodic mode on specific interfaces.

   If the gateway is behind a firewall device, the firewall may require
   the gateway to periodically refresh the UDP state in the firewall at
   a shorter interval than the standard IGMP/MLD Query interval.
   Therefore, this IGMP/MLD Query interval should be configurable to
   ensure the firewall does not revert to blocking the UDP encapsulated
   multicast data packets.


6.3.  Responding to Relay Changes

   When a gateway determines that its current relay is unreachable
   (e.g., upon receipt of a ICMP Unreachable message [ICMP] for the
   relay's unicast address), it may need to repeat relay address
   discovery.  However, care should be taken not to abandon the current
   relay too quickly due to transient network conditions. Some
   implementations may find it difficult to send a discovery packet to



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   the anycast relay address while the gateway has an address configured
   on the AMT pseudo-interface on the same anycast prefix. Therefore, it
   may be necessary to tear down the AMT pseudo-interface to rediscover
   a new relay.


6.4.  Creating SSM groups

   When a gateway wants to create an SSM group (i.e., in 232/8) for
   which it can source traffic, the remaining 24 bits MUST be generated
   as described below.  ([SSM] states that "the policy for allocating
   these bits is strictly locally determined at the sender's host.")

   When the gateway determined its AMT Gateway Anycast Address as
   described above, it used the high bits of its global unicast address.
   The remaining bits of its global unicast address are appended to the
   232/8 prefix, and any spare bits may be allocated using any policy
   (again, strictly locally determined at the sender's host).

   For example, if the IPv4 AMT Subnet Prefix is x.y/16, and the device
   has global unicast address a.b.c.d, then it MUST allocate IPv4 SSM
   groups in the range 232.c.d/24.


6.5.  Joining SSM Groups with AMT Sources

   An IGMPv3/MLDv2 Report for a given (source, group) pair MAY be
   encapsulated directly to the source, when the source address belongs
   to the AMT Subnet Prefix.

   The "link-layer" address to use as the destination address in the
   outer IP header is obtained as follows.  The source address in the
   inclusion list of the IGMPv3/MLDv2 report will be an AMT Gateway
   Anycast Address with the high bits of the address, and the remaining
   bits will be in the middle of the group address.

   For example, if the IPv4 AMT Subnet Prefix is x.y/16, and the IGMPv3
   Report is for (x.y.a.b, 232.c.d.e), then the "link layer" IPv4
   destination address used for encapsulation is a.b.c.d.


6.6.  Receiving IGMPv3/MLDv2 Reports at the Gateway

   When an AMT Request is received by the gateway, it follows the same
   3-way handshake procedure a relay would follow if it received the AMT
   Request. It generates a MAC and responds with an AMT Membership
   Query. When the AMT Membership Update is received, it verifies the
   MAC and then processes the IGMP/MLD Membership/Listener Report or



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   Leave/Done.

   At the gateway, the IGMP/MLD packet should be an IGMPv3/MLDv2 source
   specific (S,G) join or leave.

   If S is not the AMT Gateway Anycast Address, the packet is dropped.
   If G does not contain the low bits of the global unicast address (as
   described above), the packet is also dropped.

   The gateway adds the source address (from the outer IP header) and
   UDP port of the report to a membership list for G.  Maintaining this
   membership list may be done in any implementation-dependent manner.
   For example, it might be maintained by the "link-layer" inside the
   AMT pseudo-interface, making it invisible to the normal IGMP/MLD
   module.


6.7.  Sending data to SSM groups

   When multicast packets are sent on the AMT pseudo-interface, they are
   encapsulated as follows.  If the group address is not an SSM group,
   then the packet is dropped (this memo does not currently provide a
   way to send to non-SSM groups).

   If the group address is an SSM group, then the packet is unicast
   encapsulated to each remote node from which the gateway has received
   an IGMPv3/MLDv2 report for the packet's (source, group) pair.


7.  Relay Router Details


7.1.  At Startup time

   At startup time, the relay router will bring up an NBMA-style AMT
   pseudo-interface.  It shall also add the AMT Relay Anycast Address on
   some interface.

   The relay router shall then advertise the AMT Relay Anycast Prefix
   into the unicast-only Internet, as if it were a connection to an
   external network.  When the advertisement is done using BGP, the AS
   path leading to the AMT Relay Anycast Prefix shall include the
   identifier of the local AS and the AMT Unicast Autonomous System ID.

   The relay router shall also enable IGMPv3/MLDv2 on the AMT pseudo-
   interface, except that it shall not multicast Queries (this might be
   done, for example, by having the AMT pseudo-device drop them, or by
   having the IGMP/MLD module not send them in the first place).



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   Finally, to support sourcing SSM traffic from AMT sites, the AMT
   Subnet Prefix is assigned to the AMT pseudo-interface, and the AMT
   Subnet Prefix is injected into the M-RIB of MBGP.


7.2.  Receiving Relay Discovery messages sent to the Anycast Address

   When a relay receives a AMT Relay Discovery message directed to the
   AMT Relay Anycast Address, it should respond with a AMT Relay
   Advertisement containing its unicast address.  The source and
   destination addresses of the advertisement should be the same as the
   destination and source addresses of the discovery message
   respectively. Further, the nonce in the discovery message MUST be
   copied into the advertisement message.


7.3.  Receiving Membership Updates from AMT Gateways

   The relay operates passively, sending no Queries but simply tracking
   membership information according to Reports and Leave messages, as a
   router normally would.  In addition, the relay must also do explicit
   membership tracking, as to which gateways on the AMT pseudo-
   interface have joined which groups. Once an AMT Membership Update has
   been successfully received, it updates the forwarding state for the
   appropriate group and source (if provided).  When data arrives for
   that group, the traffic must be encapsulated to each gateway which
   has joined that group.

   The explicit membership tracking and unicast replication may be done
   in any implementation-specific manner.  Some examples are:

   1. The AMT pseudo-device driver might track the group information and
      perform the replication at the "link-layer", with no changes to a
      pre-existing IGMP/MLD module.

   2. The IGMP/MLD module might have native support for explicit
      membership tracking, especially if it supports other NBMA-style
      interfaces.


7.4.  Receiving (S,G) Joins from the Native Side, for AMT Sources

   The relay encapsulates an IGMPv3/MLDv2 report to the AMT source as
   described above in Section 4.1.2.







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8.  IANA Considerations

   The IANA should allocate a prefix dedicated to the public AMT Relays
   to the native multicast backbone.  The prefix length should be
   determined by the IANA; the prefix should be large enough to
   guarantee advertisement in the default- free BGP networks; a length
   of 16 will meet this requirement.  This is a one time effort; there
   is no need for any recurring assignment after this stage.

   The IANA should also allocate an Autonomous System ID which can be
   used as a pseudo-AS when advertising routes to the above prefix.
   Furthermore, to support sourcing SSM traffic from AMT gateways, the
   IANA should allocate a subnet prefix dedicated to the AMT link.  The
   prefix length should be determined by the IANA; the prefix should be
   large enough to guarantee advertisement in the default- free BGP
   networks; a length of 16 will meet this requirement.  This is a one
   time effort; there is no need for any recurring assignment after this
   stage.  It should also be noted that this prefix length directly
   affects the number of groups available to be created by the AMT
   gateway: a length of 16 gives 256 groups, and a length of 8 gives
   65536 groups.  For diagnostic purposes, it is helpful to have a
   prefix length which is a multiple of 8, although this is not
   required.

   An autonomous system number dedicated to a pseudo-AS for multicast is
   already in use today (AS 10888), and so it is expected that no
   additional AS number is required for this prefix.

   IANA has allocated UDP reserved port number 2268 for AMT
   encapsulation.


9.  Security Considerations

   The anycast technique introduces a risk that a rogue router or a
   rogue AS could introduce a bogus route to the AMT Relay Anycast
   Prefix, and thus divert the traffic.  Network managers have to
   guarantee the integrity of their routing to the AMT Relay anycast
   prefix in much the same way that they guarantee the integrity of all
   other routes.

   Within the native MBGP infrastructure, there is a risk that a rogue
   router or a rogue AS could introduce a bogus route to the AMT Subnet
   Prefix, and thus divert joins and cause RPF failures of multicast
   traffic.  Again, network managers have to guarantee the integrity of
   the MBGP routing to the AMT subnet prefix in much the same way that
   they guarantee the integrity of all other routes in the M-RIB.




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   Gateways and relays will accept and decapsulate multicast traffic
   from any source from which regular unicast traffic is accepted. If
   this is for any reason felt to be a security risk, then additional
   source address based packet filtering MUST be applied:

   1. To prevent a rogue sender (that can't do traditional spoofing
      because of e.g. access lists deployed by its ISP) from making use
      of AMT to send packets to an SSM tree, a relay that receives an
      encapsulated multicast packet MUST discard the multicast packet if
      the IPv4 source address in the outer header is not composed of the
      last 2 bytes of the source address and the 2 middle bytes of the
      destination address of the inner header (i.e., a.b.c.d must be
      composed of the a.b of x.y.a.b and the c.d of 232.c.d.e).

   2. A gateway MUST discard encapsulated multicast packets if the
      source address in the outer header is not the address to which the
      encapsulated join message was sent. An AMT Gateway that receives
      an encapsulated IGMPv3/MLDv2 (S,G)-Join MUST discard the message
      if the IPv4 destination address in the outer header is not
      composed of the last 2 bytes of S and the 2 middle bytes of G
      (i.e. the destination address a.b.c.d must be composed of the a.b
      of the multicast source x.y.a.b and the c.d of the multicast group
      232.c.d.e).

10.  Contributors

   The following people provided significant contributions to earlier
   versions of this draft.

      Dirk Ooms
      OneSparrow
      Belegstraat 13; 2018 Antwerp; Belgium
      EMail: dirk@onesparrow.com


11.  Acknowledgments

   Most of the mechanisms described in this document are based on
   similar work done by the NGTrans WG for obtaining automatic IPv6
   connectivity without explicit tunnels ("6to4").  Tony Ballardie
   provided helpful discussion that inspired this document.


12.  Normative References


   [ICMP]      Postel, J., "Internet Control Message Protocol", RFC 792,
               September 1981.



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   [IGMPv3]    Cain, B., Deering, S., Fenner, B., Kouvelas, I.,
               Thyagarajan A., "Internet Group Management Protocol,
               Version 3", RFC 3376, October 2002.

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

   [PROXY]     Fenner, W., He, H., Haberman, B., Sandick, H., "IGMP/MLD-
               based Multicast Forwarding ("IGMP/MLD Proxying")", Work
               in progress, draft-ietf-magma-igmp-proxy-06.txt, April
               2004.

   [SSM]       Holbrook, H., Cain, B., "Source-Specific Multicast for
               IP", Work in Progress, draft-ietf-ssm-arch-06.txt,
               September 2004.

13.  Informative References


   [ANYCAST]   Huitema, C., "An Anycast Prefix for 6to4 Relay Routers",
               RFC 3068, June 2001.

   [6TO4]      Carpenter, B., Moore, K., "Connection of IPv6 Domains via
               IPv4 Clouds", RFC 3056, February 2001.

   [Brad88]    Braden, R., Borman, D., Partridge, C., "Computing the
               Internet Checksum", RFC 1071, September 1988.

   [BROKER]    Durand, A., Fasano, P., Guardini, I., Lento, D., "IPv6
               Tunnel Broker", RFC 3053, January 2001.

   [HMAC]      Krawczyk, H., Bellare, M., Canetti, R., "HMAC: Keyed-
               Hashing for Message Authentication", RFC 2104, February
               1997.

   [PIMSM]     Estrin, D. Farinacci, D., Helmy, A., Thaler, D., Deering,
               S., Handley, M., Jacobson, V., Liu, C., Sharma, P., Wei,
               L., "Protocol Independent Multicast-Sparse Mode (PIM-SM):
               Protocol Specification", RFC 2362, June 1998.


14.  Author's Address


   Dave Thaler
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052-6399



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   Phone: +1 425 703 8835
   EMail: dthaler@microsoft.com


   Mohit Talwar
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052-6399
   Phone: +1 425 705 3131
   EMail: mohitt@microsoft.com


   Amit Aggarwal
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052-6399
   Phone: +1 425 706 0593
   EMail: amitag@microsoft.com


   Lorenzo Vicisano
   Cisco Systems
   170 West Tasman Dr.
   San Jose, CA 95134
   Phone: +1 408 525 2530
   EMail: lorenzo@cisco.com


   Tom Pusateri
   Juniper Networks, Inc.
   1194 North Mathilda Avenue
   Sunnyvale, CA 94089 USA
   Phone: +1 408 745 2000
   EMail: pusateri@juniper.net



15.  Intellectual Property Rights Notice

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.




Thaler, et al.                                                 [Page 23]

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   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-
   ipr@ietf.org."


16.  Full Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.


17.  Disclaimer

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."





















Thaler, et al.                                                 [Page 24]

                             Table of Contents


   1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . .   2
   2. Requirements Terminology . . . . . . . . . . . . . . . . . . .   3
   3. Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.1. Receiving Multicast in an AMT Site . . . . . . . . . . . . .   5
   4.2. Sourcing Multicast from an AMT site  . . . . . . . . . . . .   7
   5. Message Formats  . . . . . . . . . . . . . . . . . . . . . . .  10
   5.1. AMT Relay Discovery  . . . . . . . . . . . . . . . . . . . .  10
   5.2. AMT Relay Advertisement  . . . . . . . . . . . . . . . . . .  10
   5.3. AMT Request  . . . . . . . . . . . . . . . . . . . . . . . .  11
   5.4. AMT Membership Query . . . . . . . . . . . . . . . . . . . .  12
   5.5. AMT Membership Update  . . . . . . . . . . . . . . . . . . .  13
   5.6. AMT Multicast Data . . . . . . . . . . . . . . . . . . . . .  14
   6. AMT Gateway Details  . . . . . . . . . . . . . . . . . . . . .  15
   6.1. At Startup Time  . . . . . . . . . . . . . . . . . . . . . .  15
   6.2. Joining Groups with MBone Sources  . . . . . . . . . . . . .  15
   6.3. Responding to Relay Changes  . . . . . . . . . . . . . . . .  16
   6.4. Creating SSM groups  . . . . . . . . . . . . . . . . . . . .  17
   6.5. Joining SSM Groups with AMT Sources  . . . . . . . . . . . .  17
   6.6. Receiving IGMPv3/MLDv2 Reports at the Gateway  . . . . . . .  17
   6.7. Sending data to SSM groups . . . . . . . . . . . . . . . . .  18
   7. Relay Router Details . . . . . . . . . . . . . . . . . . . . .  18
   7.1. At Startup time  . . . . . . . . . . . . . . . . . . . . . .  18
   7.2. Receiving Relay Discovery messages sent to the Anycast
   Address . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  19
   7.3. Receiving Membership Updates from AMT Gateways . . . . . . .  19
   7.4. Receiving (S,G) Joins from the Native Side, for AMT Sources
   . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  19
   8. IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  20
   9. Security Considerations  . . . . . . . . . . . . . . . . . . .  20
   10. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  21
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  21
   12. Normative References  . . . . . . . . . . . . . . . . . . . .  21
   13. Informative References  . . . . . . . . . . . . . . . . . . .  22
   14. Author's Address  . . . . . . . . . . . . . . . . . . . . . .  22
   15. Intellectual Property Rights Notice . . . . . . . . . . . . .  23
   16. Full Copyright Statement  . . . . . . . . . . . . . . . . . .  24
   17. Disclaimer  . . . . . . . . . . . . . . . . . . . . . . . . .  24










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