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Network Working Group                                          D. Thaler
Internet-Draft                                                 M. Talwar
Expires: April 23, 2006                                      A. Aggarwal
                                                   Microsoft Corporation
                                                             L. Vicisano
                                                           Cisco Systems
                                                             T. Pusateri
                                                        Juniper Networks
                                                        October 20, 2005


         Automatic IP Multicast Without Explicit Tunnels (AMT)
                    draft-ietf-mboned-auto-multicast-05

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 BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   The list of current Internet-Drafts can be accessed at
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   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on April 23, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).

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



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   to exchange multicast traffic with the native multicast
   infrastructure 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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Requirements notation  . . . . . . . . . . . . . . . . . . . .  5
   3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1   AMT Pseudo-Interface . . . . . . . . . . . . . . . . . . .  6
     3.2   AMT Gateway  . . . . . . . . . . . . . . . . . . . . . . .  6
     3.3   AMT Site . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.4   AMT Relay Router . . . . . . . . . . . . . . . . . . . . .  6
     3.5   AMT Relay Anycast Prefix . . . . . . . . . . . . . . . . .  7
     3.6   AMT Relay Anycast Address  . . . . . . . . . . . . . . . .  7
     3.7   AMT Unicast Autonomous System ID . . . . . . . . . . . . .  7
     3.8   AMT Subnet Prefix  . . . . . . . . . . . . . . . . . . . .  7
     3.9   AMT Gateway Anycast Address  . . . . . . . . . . . . . . .  7
     3.10  AMT Multicast Autonomous System ID . . . . . . . . . . . .  8
   4.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     4.1   Receiving Multicast in an AMT Site . . . . . . . . . . . .  9
       4.1.1   Scalability Considerations . . . . . . . . . . . . . . 10
       4.1.2   Spoofing Considerations  . . . . . . . . . . . . . . . 10
     4.2   Sourcing Multicast from an AMT site  . . . . . . . . . . . 11
       4.2.1   Supporting Site-MBone Multicast  . . . . . . . . . . . 12
       4.2.2   Supporting Site-Site Multicast . . . . . . . . . . . . 12
   5.  Message Formats  . . . . . . . . . . . . . . . . . . . . . . . 14
     5.1   AMT Relay Discovery  . . . . . . . . . . . . . . . . . . . 14
       5.1.1   Type . . . . . . . . . . . . . . . . . . . . . . . . . 14
       5.1.2   Reserved . . . . . . . . . . . . . . . . . . . . . . . 14
       5.1.3   Discovery Nonce  . . . . . . . . . . . . . . . . . . . 14
     5.2   AMT Relay Advertisement  . . . . . . . . . . . . . . . . . 14
       5.2.1   Type . . . . . . . . . . . . . . . . . . . . . . . . . 15
       5.2.2   Reserved . . . . . . . . . . . . . . . . . . . . . . . 15
       5.2.3   Discovery Nonce  . . . . . . . . . . . . . . . . . . . 15
       5.2.4   Relay Address  . . . . . . . . . . . . . . . . . . . . 15
     5.3   AMT Request  . . . . . . . . . . . . . . . . . . . . . . . 15
       5.3.1   Type . . . . . . . . . . . . . . . . . . . . . . . . . 16
       5.3.2   Reserved . . . . . . . . . . . . . . . . . . . . . . . 16
       5.3.3   Request Nonce  . . . . . . . . . . . . . . . . . . . . 16
     5.4   AMT Membership Query . . . . . . . . . . . . . . . . . . . 16
       5.4.1   Type . . . . . . . . . . . . . . . . . . . . . . . . . 17
       5.4.2   Reserved . . . . . . . . . . . . . . . . . . . . . . . 17
       5.4.3   Response MAC . . . . . . . . . . . . . . . . . . . . . 17
       5.4.4   Request Nonce  . . . . . . . . . . . . . . . . . . . . 17
     5.5   AMT Membership Update  . . . . . . . . . . . . . . . . . . 17



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       5.5.1   Type . . . . . . . . . . . . . . . . . . . . . . . . . 18
       5.5.2   Reserved . . . . . . . . . . . . . . . . . . . . . . . 18
       5.5.3   Response MAC . . . . . . . . . . . . . . . . . . . . . 18
       5.5.4   Request Nonce  . . . . . . . . . . . . . . . . . . . . 18
     5.6   AMT Multicast Data . . . . . . . . . . . . . . . . . . . . 18
       5.6.1   Type . . . . . . . . . . . . . . . . . . . . . . . . . 19
       5.6.2   Reserved . . . . . . . . . . . . . . . . . . . . . . . 19
       5.6.3   UDP Multicast Data . . . . . . . . . . . . . . . . . . 19
   6.  AMT Gateway Details  . . . . . . . . . . . . . . . . . . . . . 20
     6.1   At Startup Time  . . . . . . . . . . . . . . . . . . . . . 20
     6.2   Joining Groups with MBone Sources  . . . . . . . . . . . . 20
     6.3   Responding to Relay Changes  . . . . . . . . . . . . . . . 21
     6.4   Creating SSM groups  . . . . . . . . . . . . . . . . . . . 22
     6.5   Joining SSM Groups with AMT Sources  . . . . . . . . . . . 22
     6.6   Receiving IGMPv3/MLDv2 Reports at the Gateway  . . . . . . 22
     6.7   Sending data to SSM groups . . . . . . . . . . . . . . . . 23
   7.  Relay Router Details . . . . . . . . . . . . . . . . . . . . . 24
     7.1   At Startup time  . . . . . . . . . . . . . . . . . . . . . 24
     7.2   Receiving Relay Discovery messages sent to the Anycast
           Address  . . . . . . . . . . . . . . . . . . . . . . . . . 24
     7.3   Receiving Membership Updates from AMT Gateways . . . . . . 24
     7.4   Receiving (S,G) Joins from the Native Side, for AMT
           Sources  . . . . . . . . . . . . . . . . . . . . . . . . . 25
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 26
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 27
   10.   Contributors . . . . . . . . . . . . . . . . . . . . . . . . 28
   11.   Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . 29
   12.   References . . . . . . . . . . . . . . . . . . . . . . . . . 30
     12.1  Normative References . . . . . . . . . . . . . . . . . . . 30
     12.2  Informative References . . . . . . . . . . . . . . . . . . 30
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 31
       Intellectual Property and Copyright Statements . . . . . . . . 33



















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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 can be 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" [RFC3056], [RFC3068] 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 ([I-D.ietf-ssm-arch]).

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

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

   This document does not address allowing isolated sites/hosts to
   transmit general multicast.  We expect that other solutions (e.g.,
   Tunnel Brokers, a la [RFC3053]) will be used for sites that desire
   this capability.

   Implementors should be aware that site administrators may have
   configured administratively scoped multicast boundaries and a remote
   gateway may provide a means to circumvent administrative boundaries.
   Therefore, implementations should allow for the configuration of such
   boundaries on relays and gateways and perform filtering as needed.



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2.  Requirements notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].














































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3.  Definitions

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


3.1  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.

3.2  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".

3.3  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.

3.4  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 [RFC3056], we assume that normal multicast routers do not
   want to be tunnel endpoints (especially if this results in high



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   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.

3.5  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.

3.6  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.

3.7  AMT Unicast Autonomous System ID

   A 16-bit Autonomous System ID, for use in BGP in accordance to this
   memo.  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 [RFC3068].

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

3.9  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



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   applications.

3.10  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).











































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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      |               |
    +---------------+                            +---------------+

   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
   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 [RFC3376] [RFC3810] protocol, and the
   PIM-Sparse Mode [I-D.ietf-pim-sm-v2-new] protocol.  Since an AMT



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   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.

   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 (for example) 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 and the input used to calculate the



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   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
   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.







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4.2.1  Supporting Site-MBone Multicast

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

   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
   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  |
    +---------------+                            +---------------+

   Since we require gateways to accept membership reports, as described



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   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 [RFC3376]
   [RFC3810] 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:

5.1.1  Type

   The type of the message.

5.1.2  Reserved

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

5.1.3  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.




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   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=0x2  |     Reserved                                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Discovery Nonce                                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Relay Address                                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Fields:

5.2.1  Type

   The type of the message.

5.2.2  Reserved

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

5.2.3  Discovery Nonce

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

5.2.4  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.



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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                                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Fields:

5.3.1  Type

   The type of the message.

5.3.2  Reserved

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

5.3.3  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                                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   Fields:

5.4.1  Type

   The type of the message.

5.4.2  Reserved

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

5.4.3  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 [RFC2104].

5.4.4  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.















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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=0x5  |    Reserved   |         Response MAC          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Response MAC (continued)                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Request Nonce                                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            IGMP/MLD Message                                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            ...                                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Fields:

5.5.1  Type

   The type of the message.

5.5.2  Reserved

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

5.5.3  Response MAC

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

5.5.4  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.







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   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:

5.6.1  Type

   The type of the message.

5.6.2  Reserved

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

5.6.3  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
   an 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 SHOULD initialize 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 [I-D.ietf-magma-igmp-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.  Hosts receiving multicast packets
   through a gateway should ensure that their M-RIB accepts multicast
   packets from the gateway for the sources it is joining.

   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
   not work on NBMA links which do not support multicast.  Since the set



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   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 an ICMP Unreachable message [RFC0792] 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.






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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
   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 silently
   discarded.  If G does not contain the low bits of the global unicast
   address (as described above), the packet is also silently discarded.



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   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 silently discarded (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.


































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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).

   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 an AMT Relay Discovery message directed to the
   AMT Relay Anycast Address, it should respond with an 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:




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   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 an IPv4 prefix and an IPv6 prefix dedicated
   to the public AMT Relays to advertise 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.  For IPv4, a prefix length of 16 will meet
   this requirement.  For IPv6, a prefix length of 64 will meet this
   requirement.  This is a one time effort and there will be 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.

   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.

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





























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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 inject a false route to the AMT Subnet
   Prefix, and thus divert joins and cause RPF failures of multicast
   traffic.  As the AMT Subnet Prefix will be advertised by multiple
   entities, guaranteeing the integrity of this shared MBGP prefix is
   much more challenging than verifying the correctness of a regular
   unicast advertisement.  To mitigate this threat, routing operators
   should configure the BGP sessions to filter out any more specific
   advertisements for the AMT Subnet Prefix.

   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).









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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










































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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.













































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12.  References

12.1  Normative References

   [I-D.ietf-magma-igmp-proxy]
              Fenner, B., He, H., Haberman, B., and H. Sandick, "IGMP/
              MLD-based Multicast Forwarding ('IGMP/MLD Proxying')",
              draft-ietf-magma-igmp-proxy-06 (work in progress),
              April 2004.

   [I-D.ietf-ssm-arch]
              Holbrook, H. and B. Cain, "Source-Specific Multicast for
              IP", draft-ietf-ssm-arch-07 (work in progress),
              October 2005.

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, September 1981.

   [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.

12.2  Informative References

   [I-D.ietf-pim-sm-v2-new]
              Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
              "Protocol Independent Multicast - Sparse Mode PIM-SM):
              Protocol Specification  (Revised)",
              draft-ietf-pim-sm-v2-new-11 (work in progress),
              October 2004.

   [RFC1112]  Deering, S., "Host extensions for IP multicasting", STD 5,
              RFC 1112, August 1989.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3053]  Durand, A., Fasano, P., Guardini, I., and D. Lento, "IPv6
              Tunnel Broker", RFC 3053, January 2001.

   [RFC3056]  Carpenter, B. and K. Moore, "Connection of IPv6 Domains



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              via IPv4 Clouds", RFC 3056, February 2001.

   [RFC3068]  Huitema, C., "An Anycast Prefix for 6to4 Relay Routers",
              RFC 3068, June 2001.


Authors' Addresses

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

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


   Mohit Talwar
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052-6399
   USA

   Phone: +1 425 705 3131
   Email: mohitt@microsoft.com


   Amit Aggarwal
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052-6399
   USA

   Phone: +1 425 706 0593
   Email: amitag@microsoft.com


   Lorenzo Vicisano
   Cisco Systems
   170 West Tasman Dr.
   San Jose, CA  95134
   USA

   Phone: +1 408 525 2530
   Email: lorenzo@cisco.com





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   Tom Pusateri
   Juniper Networks
   1194 North Mathilda Avenue
   Sunnyvale, CA  94089
   USA

   Phone: +1 408 745 2000
   Email: pusateri@juniper.net











































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Internet-Draft                     AMT                      October 2005


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