One document matched: draft-ietf-mboned-auto-multicast-02.txt
Differences from draft-ietf-mboned-auto-multicast-01.txt
MBoneD Working Group Dave Thaler
Internet Draft Mohit Talwar
Document: draft-ietf-mboned-auto-multicast-02.txt Amit Aggarwal
February 9, 2004 Microsoft
Lorenzo Vicisano
Cisco
Dirk Ooms
Alcatel
IPv4 Automatic Multicast Without Explicit Tunnels (AMT)
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
Copyright (C) The Internet Society (2004). 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
The primary goal of this document is to foster the deployment of
native IP multicast by enabling a potentially large number of nodes
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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" [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
transmit general multicast. We expect that other solutions (e.g.,
Tunnel Brokers, a la [BROKER]) will be used for sites that desire
this capability.
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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.
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".
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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.
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
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. 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.
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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 | | MBone |
| | 2. IGMP Report | |
| 1. Join +---+---+ =================> +---+---+ |
| +---->|Gateway| | Relay | |
| | +---+---+ <================= +---+---+ |
| R-+ | 3. 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
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].
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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 [IGMPv3] 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.
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
the 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.
4.1.2 Spoofing Considerations
An attacker could affect the group state in the relay 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. On
receiving an IGMP report, the relay sends a message to the source of
the report with the original report as well as a nonce. The state in
the relay is updated only on receiving a confirmation for the report
with the nonce in it.
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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.
4.2.1. Supporting Site-MBone Multicast
+---------------+ Internet +---------------+
| AMT Site | | MBone |
| | 2. IGMP Report | |
| +---+---+ <================= +---+---+ 1. Join |
| |Gateway| | Relay |<-----+ |
| +---+---+ =================> +---+---+ | |
| | 3. 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 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
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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 | | AMT Site |
| | 2. IGMP Report | |
| +---+---+ <================= +---+---+ 1. Join |
| |Gateway| |Gateway|<-----+ |
| +---+---+ =================> +---+---+ | |
| | 3. Data | +-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.
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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 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 | Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type
The type of the message.
Nonce
A 24 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 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 | Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Relay Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type
The type of the message.
Nonce
A 24 bit random value replayed from the discovery
message.
Relay Address
The unicast IP address of the AMT relay.
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5.3. Membership Report Confirmation
The membership report confirmation is a UDP packet sent by the
gateway or relay to the source of a multicast membership report.
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 | Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multicast Report |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type
The type of the message.
Nonce
A 24 bit random value generated by the relay or gateway
on receiving a multicast report.
Multicast Report
The complete multicast report that the relay or gateway
is trying to confirm.
5.4. Membership Report Acknowledgement
The membership report acknowledgement is a UDP packet sent by the
source of a membership report to a gateway or relay/
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 | Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multicast Report |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type
The type of the message.
Nonce
A 24 bit random value replayed from the confirmation
message.
Multicast Report
The complete multicast report that the relay or gateway
is trying to confirm.
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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 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 anycast assigned to their pseudo-interfaces.
6.2. 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
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.
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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
[IGMPv3]) seconds. The gateway also needs to respond to Membershsip
Confirmation messages sent by the relay with a Membership
Acknowledgement message.
Generating periodic reports can be done in any implementation-
specific manner. Some possibilities include:
1. 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.
2. The IGMP module itself might provide an option to operate in
periodic mode on specific interfaces.
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 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.
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 AMT Subnet Prefix is x.y/16, and the device has
global unicast address a.b.c.d, then it MUST allocate SSM groups in
the range 232.c.d/24.
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6.5. 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.
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 the 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"
destination address used for encapsulation is a.b.c.d.
6.6. 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 sends a Membership Confirmation message to
the source of the IGMPv3 report. The message contains a random
nonce. On receiving a Membership Acknowledgement message, the
gateway verifies that the nonce in the acknowledgement is the same
as the one in the confirmation message. If the two differ, the
message is dropped without any change to the gateway state. If the
two nonces are the same, 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 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 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 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.
7.2. Receiving Echo Requests 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 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. On receiving a membership
report, the gateway generates a Membership Confirmation message with
a random nonce in it. On receiving a Membership Acknowledgement, it
updates the group state if the nonce in the reply matches the one in
the confirmation message. When data arrives for that group, the
traffic must be encapsulated to each gateway which has joined that
group.
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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 module.
2. The IGMP 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 report to the AMT source as
described above in Section 5.5.
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.
Finally, IANA should reserve a well-known UDP port number for AMT
encapsulation.
9. Security Considerations
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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.
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 avoid that a rogue sender (that can't do traditional spoofing
because of e.g. access lists deployed by its ISP) makes 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 (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).
3. A gateway MUST drop an AMT Relay Advertisement if the nonce in
the advertisement does not match the nonce in the discovery packet
sent by the gateway. This prevents an attacker from acting as an AMT
anycast relay even without publishing a route to the AMT Anycast
Subnet Prefix.
4. A gateway or relay MUST not update its group state on receiving a
membership report. Instead, it MUST generate a Membership
Confirmation message to the source of the report. On receiving a
Membership Acknowledgement, the group state should be updated only
if the nonce in the acknowledgement matches the one in the
confirmation message. This prevents an attacker from spoofing the
source address of a membership report and causing a denial of
service or reflection attack on the target.
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10. 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.
11. 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:
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.
12. Authors' Addresses
Dave Thaler
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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
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
Dirk Ooms
Alcatel
F. Wellesplein 1, 2018 Antwerp, Belgium
Phone: +32 3 2404732
EMail: dirk.ooms@alcatel.be
13. Normative References
[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.
[IGMPv3] Cain, B., Deering, S., Fenner, B., Kouvelas, I., and A.
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, October 2002.
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[SSM] Holbrook, H., and B. Cain, "Source-Specific Multicast for IP",
Work in progress, draft-holbrook-ssm-arch-04.txt, October
2003.
14. Informative 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",
RFC 3068, June 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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