Network Working Group Dave Thaler
Internet-Draft Mohit Talwar
February 2005 Amit Aggarwal
Expires: August 21, 2005 Microsoft
Lorenzo Vicisano
Cisco Systems
Tom Pusateri
Juniper Networks
Automatic IP Multicast Without Explicit Tunnels (AMT)
draft-ietf-mboned-auto-multicast-04.txt
Status of this Memo
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Abstract
Automatic Multicast Tunneling (AMT) allows multicast communication
amongst isolated multicast-enabled sites or hosts, attached to a
network which has no native multicast support. It also enables them
to exchange multicast traffic with the native multicast
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infrastructure (MBone) and does not require any manual configuration.
AMT uses an encapsulation interface so that no changes to a host
stack or applications are required, all protocols (not just UDP) are
handled, and there is no additional overhead in core routers.
Copyright Notice
Copyright (C) The Internet Society (2005). All Rights Reserved.
1. Introduction
The primary goal of this document is to foster the deployment of
native IP multicast by enabling a potentially large number of nodes
to connect to the already present multicast infrastructure.
Therefore, the techniques discussed here should be viewed as an
interim solution to help in the various stages of the transition to a
native multicast network.
To allow fast deployment, the solution presented here only requires
small and concentrated changes to the network infrastructure, and no
changes at all to user applications or to the socket API of end-
nodes' operating systems. The protocol introduced in this
specification is deployed in a few strategically-placed network nodes
and in user-installable software modules (pseudo device drivers
and/or user-mode daemons) that reside underneath the socket API of
end-nodes' operating systems. This mechanism is very similar to that
used by "6to4" [6TO4, ANYCAST] to get automatic IPv6 connectivity.
Effectively, AMT treats the unicast-only internetwork as a large non-
broadcast multi-access (NBMA) link layer, over which we require the
ability to multicast. To do this, multicast packets being sent to or
from a site must be encapsulated in unicast packets. If the group
has members in multiple sites, AMT encapsulation of the same
multicast packet will take place multiple times by necessity.
The following problems are addressed:
1. Allowing isolated sites/hosts to receive the SSM flavor of
multicast ([SSM]).
2. Allowing isolated sites/hosts to transmit the SSM flavor of
multicast.
3. Allowing isolated sites/hosts to receive general multicast (ISM
[RFC1112]).
This document does not address allowing isolated sites/hosts to
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transmit general multicast. We expect that other solutions (e.g.,
Tunnel Brokers, a la [BROKER]) will be used for sites that desire
this capability.
2. Requirements Terminology
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [RFC-2119].
3. Definitions
+---------------+ Internet +---------------+
| AMT Site | | MBone |
| | AMT | |
| +------+----+ Relay +----+----+ AMT |
| |AMT Gateway| Anycast |AMT Relay| Subnet |
| | +-----+-+ Prefix +-+-----+ | Prefix |
| | |AMT IF | <--------|AMT IF | |--------> |
| | +-----+-+ +-+-----+ | |
| +------+----+ +----+----+ |
| | | |
+---------------+ +---------------+
Figure 1: Automatic Multicast Definitions.
AMT Pseudo-Interface
AMT encapsulation of multicast packets inside unicast packets
occurs at a point that is logically equivalent to an interface,
with the link layer being the unicast-only network. This point is
referred to as a pseudo-interface. Some implementations may treat
it exactly like any other interface and others may treat it like a
tunnel end-point.
AMT Gateway
A host, or a site gateway router, supporting an AMT Pseudo-
Interface. It does not have native multicast connectivity to the
native multicast backbone infrastructure. It is simply referred
to in this document as a "gateway".
AMT Site
A multicast-enabled network not connected to the multicast
backbone served by an AMT Gateway. It could also be a stand-
alone AMT Gateway.
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AMT Relay Router
A multicast router configured to support transit routing between
AMT Sites and the native multicast backbone infrastructure. The
relay router has one or more interfaces connected to the native
multicast infrastructure, zero or more interfaces connected to the
non-multicast capable internetwork, and an AMT pseudo-interface.
It is simply referred to in this document as a "relay".
As with [6TO4], we assume that normal multicast routers do not
want to be tunnel endpoints (especially if this results in high
fanout), and similarly that service providers do not want
encapsulation to arbitrary routers. Instead, we assume that
special-purpose routers will be deployed that are suitable for
serving as relays.
AMT Relay Anycast Prefix
A well-known address prefix used to advertise (into the unicast
routing infrastructure) a route to an available AMT Relay Router.
This could also be private (i.e., not well-known) for a private
relay.
Prefixes for both IPv4 and IPv6 will be assigned in a future
version of this draft.
AMT Relay Anycast Address
An anycast address which is used to reach the nearest AMT Relay
Router.
This address corresponds to the lowest address in the AMT Relay
Anycast Prefix.
AMT Unicast Autonomous System ID
A 16-bit Autonomous System ID, for use in BGP in accordance to
this memo. AS 10888 might be usable for this, but for now we'll
assume it's different, to avoid confusion. This number represents
a "pseudo-AS" common to all AMT relays using the well known AMT
Relay Anycast Prefix (private relays use their own ID).
To protect themselves from erroneous advertisements, managers of
border routers often use databases to check the relation between
the advertised network and the last hop in the AS path.
Associating a specific AS number with the AMT Relay Anycast
Address allows us to enter this relationship in the databases used
to check inter-domain routing [ANYCAST].
AMT Subnet Prefix
A well-known address prefix used to advertise (into the M-RIB of
the native multicast-enabled infrastructure) a route to AMT Sites.
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This prefix will be used to enable sourcing SSM traffic from an
AMT Gateway.
AMT Gateway Anycast Address
An anycast address in the AMT Subnet Prefix range, which is used
by an AMT Gateway to enable sourcing SSM traffic from local
applications.
AMT Multicast Autonomous System ID
A 16-bit Autonomous system ID, for use in MBGP in accordance to
this memo. This number represents a "pseudo-AS" common to all AMT
relays using the well known AMT Subnet Prefix (private relays use
their own ID). We assume that the existing AS 10888 is suitable
for this purpose. (Note: if this is a problem, a different one
would be fine.)
4. Overview
4.1. Receiving Multicast in an AMT Site
Internet
+---------------+ +---------------+
| AMT Site | 2. 3-way Membership | MBone |
| | Handshake | |
| 1. Join +---+---+ =================> +---+---+ |
| +---->|Gateway| | Relay | |
| | +---+---+ <================= +---+---+ |
| R-+ | 3. Receive Data | |
+---------------+ +---------------+
Figure 2: Receiving Multicast in an AMT Site.
AMT relays and gateways cooperate to transmit multicast traffic
sourced within the native multicast infrastructure to AMT sites:
relays receive the traffic natively and unicast-encapsulate it to
gateways; gateways decapsulate the traffic and possibly forward it
into the AMT site.
Each gateway has an AMT pseudo-interface that serves as a default
multicast route. Requests to join a multicast session are sent to
this interface and encapsulated to a particular relay reachable
across the unicast-only infrastructure.
Each relay has an AMT pseudo-interface too. Multicast traffic sent
on this interface is encapsulated to zero or more gateways that have
joined to the relay. The AMT recipient-list is determined for each
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multicast session. This requires the relay to keep state for each
gateway which has joined a particular group or (source, group) pair).
Multicast packets from the native infrastructure behind the relay
will be sent to each gateway which has requested them.
All multicast packets (data and control) are encapsulated in unicast
packets. To work across NAT's, the encapsulation is done over UDP
using the IANA reserved AMT port number.
Each relay, plus the set of all gateways using the relay, together
are thought of as being on a separate logical NBMA link. This
implies that the AMT recipient- list is a list of "link layer"
addresses which are (IP address, UDP port) pairs.
Since the number of gateways using a relay can be quite large, and we
expect that most sites will not want to receive most groups, an
explicit-joining protocol is required for gateways to communicate
group membership information to a relay. The two most likely
candidates are the IGMP/MLD [IGMPv3/MLDv2] protocol, and the PIM-
Sparse Mode [PIMSM] protocol. Since an AMT gateway may be a host,
and hosts typically do not implement routing protocols, gateways will
use IGMP/MLD as described in Section 5 below. This allows a host
kernel (or a pseudo device driver) to easily implement AMT gateway
behavior, and obviates the relay from the need to know whether a
given gateway is a host or a router. From the relay's perspective,
all gateways are indistinguishable from hosts on an NBMA leaf
network.
4.1.1. Scalability Considerations
It is possible that millions of hosts will enable AMT gateway
functionality and so an important design goal is not to create
gateway state in each relay until the gateway joins a multicast
group. But even the requirement that a relay keep group state per
gateway that has joined a group introduces potential scalability
concerns.
Scalability of AMT can be achieved by adding more relays, and using
an appropriate relay discovery mechanism for gateways to discover
relays. The solution we adopt is to assign an anycast address to
relays. However, simply sending periodic membership reports to the
anycast address can cause duplicates. Specifically, if routing
changes such that a different relay receives a periodic membership
report, both the new and old relays will encapsulate data to the AMT
site until the old relay's state times out. This is obviously
undesirable. Instead, we use the anycast address merely to find the
unicast address of a relay to which membership reports are sent.
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Since adding another relay has the result of adding another
independent NBMA link, this allows the gateways to be spread out
among more relays so as to keep the number of gateways per relay at a
reasonable level.
4.1.2. Spoofing Considerations
An attacker could affect the group state in the relay or gateway by
spoofing the source address in the join or leave reports. This can be
used to launch reflection or denial of service attacks on the target.
Such attacks can be mitigated by using a three way handshake between
the gateway and the relay for each multicast membership report or
leave.
When a gateway or relay wants to send a membership report, it first
sends an AMT Request with a request nonce in it. The receiving side
(the respondent) can calculate a message authentication code (MAC)
based on the source IP address of the Request, the source UDP port,
the request nonce, and a secret key known only to the respondent.
The algorithm used to calculate the MAC does not have to be
standardized since the respondent generates and verifies the MAC and
the originator simply echoes it.
An AMT Membership Query is sent back including the request nonce and
the MAC to the originator of the Request. The originator then sends
the IGMP/MLD Membership/Listener Report or Leave/Done along with the
request nonce and the received MAC back to the respondent finalizing
the 3-way handshake.
Upon reception, the respondent can recalculate the MAC based on the
source IP address, the source UDP port, the request nonce, and the
local secret. The IGMP/MLD message is only accepted if the received
MAC matches the calculated MAC.
The local secret never has to be shared with the other side. It is
only used to verify return routability of the originator.
4.2. Sourcing Multicast from an AMT site
Two cases are discussed below: multicast traffic sourced in an AMT
site and received in the MBone, and multicast traffic sourced in an
AMT site and received in another AMT site.
In both cases only SSM sources are supported. Furthermore this
specification only deals with the source residing directly in the
gateway. To enable a generic node in an AMT site to source
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multicast, additional coordination between the gateway and the
source-node is required.
The general mechanism used to join towards AMT sources is based on
the following:
1. Applications residing in the gateway use addresses in the AMT
Subnet Prefix to send multicast, as a result of sourcing traffic
on the AMT pseudo-interface.
2. The AMT Subnet Prefix is advertised for RPF reachability in the M-
RIB by relays and gateways.
3. Relays or gateways that receive a join for a source/group pair use
information encoded in the address pair to rebuild the address of
the gateway (source) to which to encapsulate the join (see Section
5 for more details). The membership reports use the same three way
handshake as outlined in Section 4.1.2.
4.2.1. Supporting Site-MBone Multicast
Internet
+---------------+ +---------------+
| AMT Site | 2. 3-way Membership | MBone |
| | Handshake | |
| +---+---+ <================= +---+---+ 1. Join |
| |Gateway| | Relay |<-----+ |
| +---+---+ =================> +---+---+ | |
| | 3. Receive Data | +-R |
+---------------+ +---------------+
Figure 3: Site-MBone Multicast.
If a relay receives an explicit join from the native infrastructure,
for a given (source, group) pair where the source address belongs to
the AMT Subnet Prefix, then the relay will periodically (using the
rules specified in Section 4.1.2) encapsulate membership updates for
the group to the gateway. The gateway must keep state per relay from
which membership reports have been sent, and forward multicast
traffic from the site to all relays from which membership reports
have been received. The choice of whether this state and replication
is done at the link-layer (i.e., by the tunnel interface) or at the
network-layer is implementation dependent.
If there are multiple relays present, this ensures that data from the
AMT site is received via the closest relay to the receiver. This is
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necessary when the routers in the native multicast infrastructure
employ Reverse-Path Forwarding (RPF) checks against the source
address, such as occurs when [PIMSM] is used by the multicast
infrastructure.
The solution above will scale to an arbitrary number of relays, as
long at the number of relays requiring multicast traffic from a given
AMT site remains reasonable enough to not overly burden the site's
gateway.
4.2.2. Supporting Site-Site Multicast
Internet
+---------------+ +---------------+
| AMT Site | 2. 3-way Membership | AMT Site |
| | Handshake | |
| +---+---+ <================= +---+---+ 1. Join |
| |Gateway| |Gateway|<-----+ |
| +---+---+ =================> +---+---+ | |
| | 3. Receive Data | +-R |
+---------------+ +---------------+
Figure 4: Site-Site Multicast.
Since we require gateways to accept membership reports, as described
above, it is also possible to support multicast among AMT sites,
without requiring assistance from any relays.
When a gateway wants to join a given (source, group) pair, where the
source address belongs to the AMT Subnet Prefix, then the gateway
will periodically unicast encapsulate an IGMPv3/MLDv2 [IGMPv3/MLDv2]
Report directly to the site gateway for the source.
We note that this can result in a significant amount of state at a
site gateway sourcing multicast to a large number of other AMT sites.
However, it is expected that this is not unreasonable for two
reasons. First, the gateway does not have native multicast
connectivity, and as a result is likely doing unicast replication at
present. The amount of state is thus the same as what such a site
already deals with. Secondly, any site expecting to source traffic to
a large number of sites could get a point-to-point tunnel to the
native multicast infrastructure, and use that instead of AMT.
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5. Message Formats
5.1. AMT Relay Discovery
The AMT Relay Discovery message is a UDP packet sent from the AMT
gateway unicast address to the AMT relay anycast address to discover
the unicast address of an AMT relay.
The UDP source port is uniquely selected by the local host operating
system. The UDP destination port is the IANA reserved AMT port
number.
The payload of the UDP packet contains the following fields.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=0x1 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Discovery Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type
The type of the message.
Reserved
A 24-bit reserved field. Sent as 0, ignored on receipt.
Discovery Nonce
A 32-bit random value generated by the gateway and replayed by the
relay.
5.2. AMT Relay Advertisement
The AMT Relay Advertisement message is a UDP packet sent from the AMT
relay anycast address to the source of the discovery message.
The UDP source port is the IANA reserved AMT port number and the UDP
destination port is the source port received in the Discovery
message.
The payload of the UDP packet contains the following fields.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=0x2 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Discovery Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Relay Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type
The type of the message.
Reserved
A 24-bit reserved field. Sent as 0, ignored on receipt.
Discovery Nonce
A 32-bit random value replayed from the discovery message.
Relay Address
The unicast IPv4 or IPv6 address of the AMT relay. The family can
be determined by the length of the Advertisement.
5.3. AMT Request
A Request packet is sent to begin a 3-way handshake for sending an
IGMP/MLD Membership/Listener Report or Leave/Done. It can be sent
from a gateway to a relay, from a gateway to another gateway, or from
a relay to a gateway.
It is sent from the originator's unique unicast address to the
respondents' unique unicast address.
The UDP source port is uniquely selected by the local host operating
system. It can be different for each Request and different from the
source port used in Discovery messages but does not have to be. The
UDP destination port is the IANA reserved AMT port number.
Fields:
Type
The type of the message.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=0x3 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved
A 24-bit reserved field. Sent as 0, ignored on receipt.
Request Nonce
A 32-bit identifier used to distinguish this request.
5.4. AMT Membership Query
An AMT Membership Query packet is sent from the relay back to the
originator to solicit an AMT Membership Update while confirming the
source of the original request. It contains a relay Message
Authentication Code (MAC) that is a cryptographic hash of a private
secret, the originators address, and the request nonce.
It is sent from the destination address received in the Request to
the source address received in the Request.
The UDP source port is the IANA reserved AMT port number and the UDP
destination port is the source port received in the Request message.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=0x4 | Reserved | Response MAC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Response MAC (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type
The type of the message.
Reserved
An 8-bit reserved field. Sent as 0, ignored on receipt.
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Response MAC
A 48-bit hash generated by the respondent and sent to the
originator for inclusion in the AMT Membership Update. The
algorithm used for this is chosen by the respondent. One algorithm
that could be used is HMAC-MD5-48 [HMAC].
Request Nonce
A 32-bit identifier used to distinguish this request echoed back
to the originator.
5.5. AMT Membership Update
An AMT Membership Update is sent from the originator to the
respondent containing the original IGMP/MLD Membership/Listener
Report or Leave/Done received over the AMT pseudo-interface. It
echoes the Response MAC received in the AMT Membership Query so the
respondent can verify return routability to the originator.
It is sent from the destination address received in the Query to the
source address received in the Query which should both be the same as
the original Request.
The UDP source and destination port numbers should be the same ones
sent in the original Request.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=0x5 | Reserved | Response MAC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Response MAC (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IGMP/MLD Message |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type
The type of the message.
Reserved
An 8-bit reserved field. Sent as 0, ignored on receipt.
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Response MAC
The 48-bit MAC received in the Membership Query and echoed back in
the Membership Update.
Request Nonce
A 32-bit identifier used to distinguish this request.
5.6. AMT Multicast Data
The AMT Data message is a UDP packet encapsulating the data requested
by the originator based on a previous AMT Membership Update message.
It is sent from the unicast destination address of the Membership
update to the source address of the Membership Update.
The UDP source and destination port numbers should be the same ones
sent in the original Query.
The payload of the UDP packet contains the following fields.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=0x6 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multicast Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type
The type of the message.
Reserved
A 24-bit reserved field. Sent as 0, ignored on receipt.
UDP Multicast Data
The original Multicast UDP data packet that is being replicated by
the relay to the gateways.
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6. AMT Gateway Details
This section details the behavior of an AMT Gateway, which may be a
router serving an AMT site, or the site may consist of a single host,
serving as its own gateway.
6.1. At Startup Time
At startup time, the AMT gateway will bring up an AMT pseudo-
interface, to be used for encapsulation. The gateway will then send
a AMT Relay Discovery message to the AMT Relay Anycast Address, and
note the unicast address (which is treated as a link-layer address to
the encapsulation interface) from the AMT Relay Advertisement
message. This discovery should be done periodically (e.g., once a
day) to re-resolve the unicast address of a close relay. The gateway
also initializes a timer used to send periodic membership reports to
a random value from the interval [0, [Query Interval]] before sending
the first periodic report, in order to prevent startup
synchronization (e.g., after a power outage).
If the gateway is serving as a local router, it SHOULD also function
as an IGMP/MLD Proxy, as described in [PROXY], with its IGMP/MLD
host-mode interface being the AMT pseudo-interface. This enables it
to translate group memberships on its downstream interfaces into
IGMP/MLD Reports. The gateway MUST also advertise itself as the
default route for multicast in the M-RIB (or it must be the default
unicast router if unicast and multicast topologies are congruent).
Also, if a shared tree routing protocol is used inside the AMT site,
each tree-root must be a gateway, e.g., in PIM-SM each RP must be a
gateway.
Finally, to support sourcing traffic to SSM groups by a gateway with
a global unicast address, the AMT Subnet Prefix is treated as the
subnet prefix of the AMT pseudo-interface, and an anycast address is
added on the interface. This anycast address is formed by
concatenating the AMT Subnet Prefix followed by the high bits of the
gateway's global unicast address. For example, if IANA assigns the
IPv4 prefix x.y/16 as the AMT Subnet Prefix, and the gateway has
global unicast address a.b.c.d, then the AMT Gateway's Anycast
Address will be x.y.a.b. Note that multiple gateways might end up
with the same anycast address assigned to their pseudo-interfaces.
6.2. Joining Groups with MBone Sources
The IGMP/MLD protocol usually operates by having the Querier
multicast an IGMP/MLD Query message on the link. This behavior does
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not work on NBMA links which do not support multicast. Since the set
of gateways is typically unknown to the relay (and potentially quite
large), unicasting the queries is also impractical. The following
behavior is used instead.
Applications residing in a gateway should join groups on the AMT
pseudo-interface, causing IGMP/MLD Membership/Listener Reports to be
sent over that interface. When UDP encapsulating the membership
reports (and in fact any other messages, unless specified otherwise
in this document), the destination address in the outer IP header is
the relay's unicast address. Robustness is provided by the
underlying IGMP/MLD protocol messages sent on the AMT pseudo-
interface. In other words, the gateway does not need to retransmit
IGMP/MLD Membership/Listener Reports and Leave/Done messages received
on the pseudo-interface since IGMP/MLD will already do this. The
gateway simply needs to encapsulate each IGMP/MLD Membership/Listener
Report and Leave/Done message it receives.
However, since periodic IGMP/MLD Membership/Listener Reports are sent
in response to IGMP/MLD Queries, some mechanism to trigger periodic
Membership/Listener Reports and Leave/Done messages are necessary.
This can be achieved in any implementation-specific manner. Some
possibilities include:
1. The AMT pseudo-interface might periodically manufacture
IGMPv3/MLDv2 Queries as if they had been received from an IGMP/MLD
Querier, and deliver them to the IP layer, after which normal
IGMP/MLD behavior will cause the appropriate reports to be sent.
2. The IGMP/MLD module itself might provide an option to operate in
periodic mode on specific interfaces.
If the gateway is behind a firewall device, the firewall may require
the gateway to periodically refresh the UDP state in the firewall at
a shorter interval than the standard IGMP/MLD Query interval.
Therefore, this IGMP/MLD Query interval should be configurable to
ensure the firewall does not revert to blocking the UDP encapsulated
multicast data packets.
6.3. Responding to Relay Changes
When a gateway determines that its current relay is unreachable
(e.g., upon receipt of a ICMP Unreachable message [ICMP] for the
relay's unicast address), it may need to repeat relay address
discovery. However, care should be taken not to abandon the current
relay too quickly due to transient network conditions. Some
implementations may find it difficult to send a discovery packet to
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the anycast relay address while the gateway has an address configured
on the AMT pseudo-interface on the same anycast prefix. Therefore, it
may be necessary to tear down the AMT pseudo-interface to rediscover
a new relay.
6.4. Creating SSM groups
When a gateway wants to create an SSM group (i.e., in 232/8) for
which it can source traffic, the remaining 24 bits MUST be generated
as described below. ([SSM] states that "the policy for allocating
these bits is strictly locally determined at the sender's host.")
When the gateway determined its AMT Gateway Anycast Address as
described above, it used the high bits of its global unicast address.
The remaining bits of its global unicast address are appended to the
232/8 prefix, and any spare bits may be allocated using any policy
(again, strictly locally determined at the sender's host).
For example, if the IPv4 AMT Subnet Prefix is x.y/16, and the device
has global unicast address a.b.c.d, then it MUST allocate IPv4 SSM
groups in the range 232.c.d/24.
6.5. Joining SSM Groups with AMT Sources
An IGMPv3/MLDv2 Report for a given (source, group) pair MAY be
encapsulated directly to the source, when the source address belongs
to the AMT Subnet Prefix.
The "link-layer" address to use as the destination address in the
outer IP header is obtained as follows. The source address in the
inclusion list of the IGMPv3/MLDv2 report will be an AMT Gateway
Anycast Address with the high bits of the address, and the remaining
bits will be in the middle of the group address.
For example, if the IPv4 AMT Subnet Prefix is x.y/16, and the IGMPv3
Report is for (x.y.a.b, 232.c.d.e), then the "link layer" IPv4
destination address used for encapsulation is a.b.c.d.
6.6. Receiving IGMPv3/MLDv2 Reports at the Gateway
When an AMT Request is received by the gateway, it follows the same
3-way handshake procedure a relay would follow if it received the AMT
Request. It generates a MAC and responds with an AMT Membership
Query. When the AMT Membership Update is received, it verifies the
MAC and then processes the IGMP/MLD Membership/Listener Report or
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Leave/Done.
At the gateway, the IGMP/MLD packet should be an IGMPv3/MLDv2 source
specific (S,G) join or leave.
If S is not the AMT Gateway Anycast Address, the packet is dropped.
If G does not contain the low bits of the global unicast address (as
described above), the packet is also dropped.
The gateway adds the source address (from the outer IP header) and
UDP port of the report to a membership list for G. Maintaining this
membership list may be done in any implementation-dependent manner.
For example, it might be maintained by the "link-layer" inside the
AMT pseudo-interface, making it invisible to the normal IGMP/MLD
module.
6.7. Sending data to SSM groups
When multicast packets are sent on the AMT pseudo-interface, they are
encapsulated as follows. If the group address is not an SSM group,
then the packet is dropped (this memo does not currently provide a
way to send to non-SSM groups).
If the group address is an SSM group, then the packet is unicast
encapsulated to each remote node from which the gateway has received
an IGMPv3/MLDv2 report for the packet's (source, group) pair.
7. Relay Router Details
7.1. At Startup time
At startup time, the relay router will bring up an NBMA-style AMT
pseudo-interface. It shall also add the AMT Relay Anycast Address on
some interface.
The relay router shall then advertise the AMT Relay Anycast Prefix
into the unicast-only Internet, as if it were a connection to an
external network. When the advertisement is done using BGP, the AS
path leading to the AMT Relay Anycast Prefix shall include the
identifier of the local AS and the AMT Unicast Autonomous System ID.
The relay router shall also enable IGMPv3/MLDv2 on the AMT pseudo-
interface, except that it shall not multicast Queries (this might be
done, for example, by having the AMT pseudo-device drop them, or by
having the IGMP/MLD module not send them in the first place).
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Finally, to support sourcing SSM traffic from AMT sites, the AMT
Subnet Prefix is assigned to the AMT pseudo-interface, and the AMT
Subnet Prefix is injected into the M-RIB of MBGP.
7.2. Receiving Relay Discovery messages sent to the Anycast Address
When a relay receives a AMT Relay Discovery message directed to the
AMT Relay Anycast Address, it should respond with a AMT Relay
Advertisement containing its unicast address. The source and
destination addresses of the advertisement should be the same as the
destination and source addresses of the discovery message
respectively. Further, the nonce in the discovery message MUST be
copied into the advertisement message.
7.3. Receiving Membership Updates from AMT Gateways
The relay operates passively, sending no Queries but simply tracking
membership information according to Reports and Leave messages, as a
router normally would. In addition, the relay must also do explicit
membership tracking, as to which gateways on the AMT pseudo-
interface have joined which groups. Once an AMT Membership Update has
been successfully received, it updates the forwarding state for the
appropriate group and source (if provided). When data arrives for
that group, the traffic must be encapsulated to each gateway which
has joined that group.
The explicit membership tracking and unicast replication may be done
in any implementation-specific manner. Some examples are:
1. The AMT pseudo-device driver might track the group information and
perform the replication at the "link-layer", with no changes to a
pre-existing IGMP/MLD module.
2. The IGMP/MLD module might have native support for explicit
membership tracking, especially if it supports other NBMA-style
interfaces.
7.4. Receiving (S,G) Joins from the Native Side, for AMT Sources
The relay encapsulates an IGMPv3/MLDv2 report to the AMT source as
described above in Section 4.1.2.
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8. IANA Considerations
The IANA should allocate a prefix dedicated to the public AMT Relays
to the native multicast backbone. The prefix length should be
determined by the IANA; the prefix should be large enough to
guarantee advertisement in the default- free BGP networks; a length
of 16 will meet this requirement. This is a one time effort; there
is no need for any recurring assignment after this stage.
The IANA should also allocate an Autonomous System ID which can be
used as a pseudo-AS when advertising routes to the above prefix.
Furthermore, to support sourcing SSM traffic from AMT gateways, the
IANA should allocate a subnet prefix dedicated to the AMT link. The
prefix length should be determined by the IANA; the prefix should be
large enough to guarantee advertisement in the default- free BGP
networks; a length of 16 will meet this requirement. This is a one
time effort; there is no need for any recurring assignment after this
stage. It should also be noted that this prefix length directly
affects the number of groups available to be created by the AMT
gateway: a length of 16 gives 256 groups, and a length of 8 gives
65536 groups. For diagnostic purposes, it is helpful to have a
prefix length which is a multiple of 8, although this is not
required.
An autonomous system number dedicated to a pseudo-AS for multicast is
already in use today (AS 10888), and so it is expected that no
additional AS number is required for this prefix.
IANA has allocated UDP reserved port number 2268 for AMT
encapsulation.
9. Security Considerations
The anycast technique introduces a risk that a rogue router or a
rogue AS could introduce a bogus route to the AMT Relay Anycast
Prefix, and thus divert the traffic. Network managers have to
guarantee the integrity of their routing to the AMT Relay anycast
prefix in much the same way that they guarantee the integrity of all
other routes.
Within the native MBGP infrastructure, there is a risk that a rogue
router or a rogue AS could introduce a bogus route to the AMT Subnet
Prefix, and thus divert joins and cause RPF failures of multicast
traffic. Again, network managers have to guarantee the integrity of
the MBGP routing to the AMT subnet prefix in much the same way that
they guarantee the integrity of all other routes in the M-RIB.
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Gateways and relays will accept and decapsulate multicast traffic
from any source from which regular unicast traffic is accepted. If
this is for any reason felt to be a security risk, then additional
source address based packet filtering MUST be applied:
1. To prevent a rogue sender (that can't do traditional spoofing
because of e.g. access lists deployed by its ISP) from making use
of AMT to send packets to an SSM tree, a relay that receives an
encapsulated multicast packet MUST discard the multicast packet if
the IPv4 source address in the outer header is not composed of the
last 2 bytes of the source address and the 2 middle bytes of the
destination address of the inner header (i.e., a.b.c.d must be
composed of the a.b of x.y.a.b and the c.d of 232.c.d.e).
2. A gateway MUST discard encapsulated multicast packets if the
source address in the outer header is not the address to which the
encapsulated join message was sent. An AMT Gateway that receives
an encapsulated IGMPv3/MLDv2 (S,G)-Join MUST discard the message
if the IPv4 destination address in the outer header is not
composed of the last 2 bytes of S and the 2 middle bytes of G
(i.e. the destination address a.b.c.d must be composed of the a.b
of the multicast source x.y.a.b and the c.d of the multicast group
232.c.d.e).
10. Contributors
The following people provided significant contributions to earlier
versions of this draft.
Dirk Ooms
OneSparrow
Belegstraat 13; 2018 Antwerp; Belgium
EMail: dirk@onesparrow.com
11. Acknowledgments
Most of the mechanisms described in this document are based on
similar work done by the NGTrans WG for obtaining automatic IPv6
connectivity without explicit tunnels ("6to4"). Tony Ballardie
provided helpful discussion that inspired this document.
12. Normative References
[ICMP] Postel, J., "Internet Control Message Protocol", RFC 792,
September 1981.
Thaler, et al. [Page 21]
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[IGMPv3] Cain, B., Deering, S., Fenner, B., Kouvelas, I.,
Thyagarajan A., "Internet Group Management Protocol,
Version 3", RFC 3376, October 2002.
[MLDv2] Vida, R., Costa, L., "Multicast Listener Discovery
Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.
[PROXY] Fenner, W., He, H., Haberman, B., Sandick, H., "IGMP/MLD-
based Multicast Forwarding ("IGMP/MLD Proxying")", Work
in progress, draft-ietf-magma-igmp-proxy-06.txt, April
2004.
[SSM] Holbrook, H., Cain, B., "Source-Specific Multicast for
IP", Work in Progress, draft-ietf-ssm-arch-06.txt,
September 2004.
13. Informative References
[ANYCAST] Huitema, C., "An Anycast Prefix for 6to4 Relay Routers",
RFC 3068, June 2001.
[6TO4] Carpenter, B., Moore, K., "Connection of IPv6 Domains via
IPv4 Clouds", RFC 3056, February 2001.
[Brad88] Braden, R., Borman, D., Partridge, C., "Computing the
Internet Checksum", RFC 1071, September 1988.
[BROKER] Durand, A., Fasano, P., Guardini, I., Lento, D., "IPv6
Tunnel Broker", RFC 3053, January 2001.
[HMAC] Krawczyk, H., Bellare, M., Canetti, R., "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
1997.
[PIMSM] Estrin, D. Farinacci, D., Helmy, A., Thaler, D., Deering,
S., Handley, M., Jacobson, V., Liu, C., Sharma, P., Wei,
L., "Protocol Independent Multicast-Sparse Mode (PIM-SM):
Protocol Specification", RFC 2362, June 1998.
14. Author's Address
Dave Thaler
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052-6399
Thaler, et al. [Page 22]
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Phone: +1 425 703 8835
EMail: dthaler@microsoft.com
Mohit Talwar
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052-6399
Phone: +1 425 705 3131
EMail: mohitt@microsoft.com
Amit Aggarwal
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052-6399
Phone: +1 425 706 0593
EMail: amitag@microsoft.com
Lorenzo Vicisano
Cisco Systems
170 West Tasman Dr.
San Jose, CA 95134
Phone: +1 408 525 2530
EMail: lorenzo@cisco.com
Tom Pusateri
Juniper Networks, Inc.
1194 North Mathilda Avenue
Sunnyvale, CA 94089 USA
Phone: +1 408 745 2000
EMail: pusateri@juniper.net
15. Intellectual Property Rights Notice
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Thaler, et al. [Page 23]
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Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org."
16. Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
17. Disclaimer
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
Thaler, et al. [Page 24]
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Terminology . . . . . . . . . . . . . . . . . . . 3
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Receiving Multicast in an AMT Site . . . . . . . . . . . . . 5
4.2. Sourcing Multicast from an AMT site . . . . . . . . . . . . 7
5. Message Formats . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. AMT Relay Discovery . . . . . . . . . . . . . . . . . . . . 10
5.2. AMT Relay Advertisement . . . . . . . . . . . . . . . . . . 10
5.3. AMT Request . . . . . . . . . . . . . . . . . . . . . . . . 11
5.4. AMT Membership Query . . . . . . . . . . . . . . . . . . . . 12
5.5. AMT Membership Update . . . . . . . . . . . . . . . . . . . 13
5.6. AMT Multicast Data . . . . . . . . . . . . . . . . . . . . . 14
6. AMT Gateway Details . . . . . . . . . . . . . . . . . . . . . 15
6.1. At Startup Time . . . . . . . . . . . . . . . . . . . . . . 15
6.2. Joining Groups with MBone Sources . . . . . . . . . . . . . 15
6.3. Responding to Relay Changes . . . . . . . . . . . . . . . . 16
6.4. Creating SSM groups . . . . . . . . . . . . . . . . . . . . 17
6.5. Joining SSM Groups with AMT Sources . . . . . . . . . . . . 17
6.6. Receiving IGMPv3/MLDv2 Reports at the Gateway . . . . . . . 17
6.7. Sending data to SSM groups . . . . . . . . . . . . . . . . . 18
7. Relay Router Details . . . . . . . . . . . . . . . . . . . . . 18
7.1. At Startup time . . . . . . . . . . . . . . . . . . . . . . 18
7.2. Receiving Relay Discovery messages sent to the Anycast
Address . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
7.3. Receiving Membership Updates from AMT Gateways . . . . . . . 19
7.4. Receiving (S,G) Joins from the Native Side, for AMT Sources
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
9. Security Considerations . . . . . . . . . . . . . . . . . . . 20
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 21
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
12. Normative References . . . . . . . . . . . . . . . . . . . . 21
13. Informative References . . . . . . . . . . . . . . . . . . . 22
14. Author's Address . . . . . . . . . . . . . . . . . . . . . . 22
15. Intellectual Property Rights Notice . . . . . . . . . . . . . 23
16. Full Copyright Statement . . . . . . . . . . . . . . . . . . 24
17. Disclaimer . . . . . . . . . . . . . . . . . . . . . . . . . 24
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