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MBoneD Working Group                                   Dave Thaler
INTERNET-DRAFT                                        Mohit Talwar
Expires August 2001                                      Microsoft
                                                  Lorenzo Vicisano
                                                             Cisco
                                                         Dirk Ooms
                                                           Alcatel
                                                  20 February 2001





     IPv4 Automatic Multicast Without Explicit Tunnels (AMT)
            <draft-ietf-mboned-auto-multicast-00.txt>





Status of this Memo

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

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups.  Note that
other groups may also distribute working documents as Internet-
Drafts.

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

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.



Copyright Notice





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Copyright (C) The Internet Society (2001).  All Rights Reserved.

1.  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
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.

2.  Introduction

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
infrastructure (MBone) and does not require any manual
configuration.  Effectively, it 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 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 protocols
introduced in this specification are implemented 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"





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[6TO4, ANYCAST] to get automatic IPv6 connectivity.

In order of importance, the following problems are addressed:

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

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

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

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


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 implementors may treat it exactly like any other
     interface and others may treat it like a tunnel end-point.






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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.

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.

     The value of this prefix is x.x.x.0/nn [length and value TBD
     IANA].

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

     This address corresponds to host number 1 in the AMT Relay
     Anycast Prefix, x.x.x.1.

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





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     we'll assume it's different, to avoid confusion.  This number
     represents a "pseudo-AS" common to all AMT relays.

     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.  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.  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      |                            | MBone         |
 |               |       3. Data              |               |
 |   1. Join +---+---+ <================= +---+---+           |
 |     +---->|Gateway|                    | Relay |           |
 |     |     +---+---+ =================> +---+---+           |
 |   R-+         |       2. IGMP Report       |               |
 +---------------+                            +---------------+

          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:





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relays receive the traffic natively and unicast-encapsulate it to
gateways; gateways decapsulate the traffic and possibly re-
multicast it in 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 a well-known port number [TBD IANA].

Each relay, plus the set of all gateways (perhaps unknown to the
relay) using the relay, together can be 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 [IGMP] 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 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.









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4.1.1.  Scalability Considerations

The requirement that a relay keep group state per gateway that has
joined the group introduces potential scalability concerns.

However, 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 IGMP Reports
to an anycast address can cause duplicates.  Specifically, if
routing changes such that a different relay receives a periodic
IGMP 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 a unicast address which can then be used.

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.



4.2.  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:

o 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.

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





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o 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 IGMP
  Report (see section 5 for more details).

  It is expected that this mechanism will be the identical to that
  used to join back to a source on normal media, as is being
  proposed in the SSM WG.


4.2.1.  Supporting Site-MBone Multicast

 +---------------+        Internet            +---------------+
 | AMT Site      |                            | MBone         |
 |               |       3. Data              |               |
 |           +---+---+ =================> +---+---+ 1. Join   |
 |           |Gateway|                    | Relay |<-----+    |
 |           +---+---+ <================= +---+---+      |    |
 |               |       2. IGMP Report       |          +-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 5) UDP
encapsulate an IGMP Report for the group to the gateway.  The
gateway must keep state per relay from which an IGMP Report has
been sent, and forward multicast traffic from the site to all
relays from which IGMP 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





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site's gateway.


4.2.2.  Supporting Site-Site Multicast

 +---------------+        Internet            +---------------+
 | AMT Site      |                            | AMT Site      |
 |               |       3. Data              |               |
 |           +---+---+ =================> +---+---+ 1. Join   |
 |           |Gateway|                    |Gateway|<-----+    |
 |           +---+---+ <================= +---+---+      |    |
 |               |       2. IGMP Report       |          +-R  |
 +---------------+                            +---------------+

                  Figure 4: Site-Site Multicast.

Since we require gateways to accept IGMP 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 [IGMPv3]
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.


5.  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.








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5.1.  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 UDP encapsulated ICMP Echo Request 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 UDP
encapsulated ICMP Echo Response.  These "pings" should be done
periodically (e.g., once a day) to re-resolve the unicast address
of a close relay.  Note that the ICMP messages are unicast
encapsulated, just as the multicast packets.

The gateway also initializes a timer used to send periodic IGMP
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 Proxy, as described in [IGMPPROXY], with its
IGMP host-mode interface being the AMT pseudo-interface.  This
enables it to translate group memberships on its downstream
interfaces into IGMP 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 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 address assigned to their pseudo-
interfaces.


5.2.  Joining Groups with MBone Sources

The IGMP protocol usually operates by having the Querier multicast
an IGMP Query message on the link.  This behavior does not work on





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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 Membership Reports to be sent over
that interface.  When UDP encapsulating the IGMP 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.  To provide robustness, gateways unicast
IGMP Reports to the relay every [Query Interval] (defined as 125
in [IGMP]) seconds.

Generating periodic reports can be done in any implementation-
specific manner.  Some possibilities include:

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

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


5.3.  Responding to Relay Changes

When a gateway determines that its current relay is unreachable
(e.g., upon receipt of a ICMP Unreachable message for the relay's
unicast address), it immediately repeats the unicast address
resolution step by sending a UDP encapsulated ICMP Echo Request to
the AMT Relay Anycast Address, and storing the source address of
the UDP encapsulated ICMP Echo Response as the new unicast address
to use as a "link-layer" destination.


5.4.  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.")






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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 IANA allocates x.y/16 as the AMT Subnet Prefix,
and the device has global unicast address a.b.c.d, then it must
allocate SSM groups in the range 232.c.d/24.


5.5.  Joining SSM Groups with AMT Sources

An IGMPv3 Report for a given (source, group) pair MAY be
encapsulated directly to the source, when the source address
belongs to the AMT Subnet Prefix.  (It is expected that this
mechanism will be the same mechanism as that used to join back to
a source on normal media, as is being proposed in the SSM WG.)

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


5.6.  Receiving IGMPv3 Reports on the AMT Interface

When an IGMPv3 report is received on the AMT pseudo-interface, and
the report is a request to join a given (S, G) pair, then the
following actions are taken.

If S is not the AMT Gateway Anycast Address of the gateway, 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.

Otherwise, 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





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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 module.


5.7.  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 report for the packet's (source, group) pair.


6.  Relay Router Details

6.1.  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 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 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.










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6.2.  Receiving Echo Requests to the Anycast Address

When a relay receives a UDP encapsulated ICMP Echo Request
directed to the AMT Relay Anycast Address, it should respond with
a UDP encapsulated ICMP Echo Response from a unicast address
reachable on the unicast-only network (anycast addresses should
not be used as the source address of the ICMP Echo Response).

Unicast encapsulation of the ICMP Echo Request ensures that the
message will be seen by the AMT pseudo-interface which can then
process it.  Specifically, it needs to ensure that the ICMP Echo
Response contains the appropriate source address.


6.3.  Receiving Joins 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.  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:

o    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 module.

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


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

The relay encapsulates an IGMPv3 report to the AMT source as
described above in Section 5.5.









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7.  IANA Considerations

The IANA should allocate a prefix dedicated to the 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.

Finally, IANA should reserve a well-known UDP port number for AMT
encapsulation.


8.  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





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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.

By default, gateways and relays will accept and decapsulate
multicast traffic 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 could be
applied.


9.  Acknowledgements

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.


10.  Appendix A: Open Issues

Under the proposed mechanism, a gateway sends its IGMPv3 Reports
for MBone sources to the relay closest to itself (discovered using
the UDP encapsulated "ping").  This ensures that, as far as
possible, multicast traffic flows through the native multicast
infrastructure and the automatic multicast encapsulation is short.

However, there might be reasons to create automatic tunnels to the
relay closest to the MBone source instead.  An ISP, for example,
might be willing to provide a relay for only its own customers,
those wishing to multicast their transmission to a much wider
audience.  A mechanism, complementary to the one described in this
document, might be used to provide this facility.  It uses UDP
encapsulated ICMP Redirect messages as described below.

While injecting routes for its sources into the M-RIB, such an ISP
might, for example, use a new BGP attribute to convey the address
of the preferred relay.  This would let other relays redirect any
IGMP Reports to the preferred relay by sending a UDP encapsulated
ICMP Redirect.

An IGMP Report sent by a gateway to the relay closest to it would
consist of the following packet:





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OuterIP [UDP [InnerIP [IGMP Report]]]

The relay would respond with:

OuterIP' [UDP' [InnerIP' [ICMP Redirect [InnerIP [IGMP Report]]]]]

An ICMP Redirect contains the first 64 bits of the original packet
[ICMP].  Hence the gateway would get 44 bytes (64 - sizeof(Inner
IP)) of the IGMP Report, enough to easily extract the (source,
group) pair, and redirect its report to the preferred gateway.

Certainly additional complexity is undesirable, so it is an open
issue as to whether redirects are needed at all.


11.  Authors' Addresses

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

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

     Dirk Ooms
     Alcatel
     F. Wellesplein 1, 2018 Antwerp, Belgium
     Phone: +32 3 2404732
     EMail: dirk.ooms@alcatel.be







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12.  References

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

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

[ANYCAST]
     C. Huitema, "An anycast prefix for 6to4 relay routers", Work
     in progress, draft-ietf-ngtrans-6to4anycast-02.txt, February
     2001.

[BGMP]
     Thaler, D., Estrin, D., and D. Meyer, "Border Gateway
     Multicast Protocol (BGMP): Protocol Specification", Work in
     progress, draft-ietf-bgmp-spec-02.txt, November 2000.

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

[IGMPPROXY]
     W. Fenner, "IGMP-based Multicast Forwarding (``IGMP
     Proxying'')", Work in progress, draft-fenner-igmp-
     proxy-03.txt, July 2000.

[IGMP]
     W. Fenner, "Internet Group Management Protocol, Version 2",
     RFC 2236, November 1997.

[IGMPv3]
     Cain, B., Deering, S., Fenner, B., Kouvelas, I., and A.
     Thyagarajan, "Internet Group Management Protocol, Version 3",
     Work in progress, draft-ietf-idmr-igmp-v3-06.txt, January
     2001.

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






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[SSM]
     Holbrook, H., and B. Cain, "Source-Specific Multicast for
     IP", Work in progress, draft-holbrook-ssm-arch-01.txt,
     November 2000.


13.  Full Copyright Statement

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