Internet Draft Thomas C. Schmidt
Matthias Waehlisch
Expires: August 2004 FHTW Berlin
February 2004

Seamless Multicast Handover in a
Hierarchical Mobile IPv6 Environment (M-HMIPv6)

Status of this Memo

This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [1].

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-

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
The list of Internet-Draft Shadow Directories can be accessed at

This Internet-Draft will expire on August 14, 2004.


This document introduces handover mechanisms for IPv6 mobile
multicast listeners and mobile multicast senders. It therefore
restates some fundamentals of mobile multicast signaling.
Operations are based on a Mobile IPv6 environment with local mobility
anchor points. These local anchor points are conformal with a
Hierarchical Mobile IPv6 proxy infrastructure. Handover latencies in
the proposed scheme remain bound to link switching delays with
respect to these local proxy points.
The mechanisms described in this document do not rely on assumptions
of any specific multicast routing protocol in use. The M-HMIPv6
protocol operations utilize the existing HMIPv6 and MIPv6 messages,
without defining any new control messages.

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Table of Contents

1. Terminology....................................................2

2. Introduction...................................................3

3. Overview of M-HMIPv6...........................................4
3.1 Operations of a multicast listener.........................4
3.2 Operations of a multicast sender...........................5

4. Multicast specific extensions of MIPv6 and HMIPv6..............6
4.1 M-HMIPv6 flag in MAP option message........................6
4.2 Use of Home Address Destination Option in mobile multicast.7
4.3 Binding Cache processing...................................7
4.4 Home Agent Multicast Membership control....................7

5. Protocol Details...............................................7
5.1 Operations of all Mobile Nodes.............................7
5.2 Mobile multicast listener..................................8
5.2.1 Operations of the Mobile Node........................8
5.2.2 Operations of the MAP................................8
5.2.3 Buffering............................................9
5.3 Mobile multicast sender....................................9
5.3.1 Operations of the Mobile Node........................9
5.3.2 Operations of the MAP...............................10
5.3.3 Tree initialization procedure.......................10
5.3.4 Buffering...........................................11
5.4 Protocol Timer............................................11

6. Security Considerations.......................................11



Author's Addresses...............................................12

A. A Note on Tunneling...........................................12

1. Terminology

The terminology used in this document remains conformal to the
definitions in MIPv6 [4] and HMIPv6 [5].

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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
document are to be interpreted as described in RFC-2119 [2].

2. Introduction

Multicast-based packet distribution plays an important role in real-
time applications, as it provides the only efficient, scalable scheme
for group communication. However, multicasting itself conceals
complex mechanisms for group membership management and routing, which
both are of slow convergence. To achieve seamless mobility is one of
the most challenging and demanded developments in IP networks today.

In conference scenarios each member commonly operates as receiver and
as sender for multicast based group communication.
In addition, real-time communication such as voice or video over IP
places severe temporal requirement on mobility protocols: Seamless
handover scenarios need to limit disruptions or delay to less than
100 ms. Jitter disturbances are not to exceed 50 ms. Note that 100 ms
is about the duration of a spoken syllable in real-time audio

The fundamental approach to deal with mobility in IPv6 [3] is the
Mobile IPv6 Internet Draft [4]. MIPv6 operates address changes on the
IP layer transparent to the transport layer as a device moves from
one network to the other. MIPv6 involves roundtrip messages for
location updates directly with the MNs Home Agent and the
Correspondent Node. As these nodes can be far away, MIPv6 may exhibit
slow handover performance. The Hierarchical Mobility Management
(HMIPv6) Internet Draft [5] introduces a proxy architecture of
Mobility Anchor Points (MAPs) to reduce communication delays with
respect to the HA. In addition the Fast Handover for Mobile IPv6
Internet Draft [6] proposes delay hiding techniques to further reduce
handover times in unicast data.

MIPv6 tackles multicast mobility in a rough, bi-directional tunneling
approach via the Home Agent, thereby suffering from slow handovers
and inefficient forwarding. It is the issue of this document to
extend the improved HMIPv6 mobility infrastructure by mechanisms of
sending and receiving multicast traffic for the MN. Local MAPs serve
as temporary multicast relays to hide partly movement, partly handoff
latency of the MN. Handover procedures are designed to limit any
disruption or disturbance to the time scale needed for reconnecting
to neighboring MAPs. Handover procedures between MAPs solely built on
MIPv6 and HMIPv6 signaling are described within this draft. These
mechanisms to not require any specific multicast protocol.

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3. Overview of M-HMIPv6

This multicast mobility scheme is built on a HMIPv6 environment.
HMIPv6 introduces Mobility Anchor Points as proxy elements, which may
be best viewed as functions on regional routers. For implementing
multicast mobility it is advantageous, but not necessary, that these
regional routers provide multicast routing functionality.

In M-HMIPv6 a mobile multicast node uses its local MAP as anchor
point for multicast communication. All multicast traffic is directed
through this MAP using the Regional Care-of Address RCoA as multicast
subscriber or source address. Traffic forwarding between MN and its
MAP is done using a bi-directional tunnel [7].

If a MN changes location within its MAP domain, it only registers its
new LCoA with the MAP as defined in [5]. This does not affect
multicast routing trees. When entering a new MAP domain a MN will be
eager to sustain multicast connectivity via its previously
established MAP. Eventually it will learn of M-HMIPv6 support through
router advertisements with MAP option messages, and will then perform
a reactive handover. Multicast handover procedures will occur only if
the MN changes into a new M-HMIPv6 enabled MAP domain and will then
shift multicast traffic from the previous to the current MAP.

An M-HMIPv6-aware MN SHOULD use the MAP for multicast communication.
However, the MN MAY prefer to use its HA as a multicast anchor point,
e.g. in visited networks within its home site. A mobile node, which
is not M-HMIPv6 aware, will not use its MAP as a multicast anchor
point, but will use the MIPv6 tunnel through the HA instead. In this
sense M-HMIPv6 is simply a smooth extension of HMIPv6, which itself
smoothly extends MIPv6.

3.1 Operations of a multicast listener

To join a multicast group away from home the MN tunnels the MLD [8]
listener report to its current MAP using RCoA as source address. The
MAP records the group address in its Binding Cache in order to
forward multicast packets to the MN and to subscribe for and preserve
MNs multicast group membership.

When changing its MAP domain the MN submits a Binding Update with its
new LCoA to the previous MAP in addition to regular HMIPv6 handover
signaling. On its reception the previous MAP redirects multicast
packet forwarding to the MN's new LCoA.

If multicast support is advertised in the new domain the MN
immediately SHOULD join the multicast group through the new MAP. Once
multicast group traffic arrives the MN SHOULD send a Binding Update

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with zero lifetime to its previous MAP to eliminate its Binding Cache
entry and end packet forwarding.

3.2 Operations of a multicast sender

In a foreign MAP domain a MN initiates multicast-based communication
by sending packets through its MAP using RCoA as its source address.
As receivers are aware of source addresses and as the mobile RCoA
address may change, a Home Address Destination Option MUST be
included (s. section 4.2). By transmitting multicast packets along
this path a routing tree originating at the MAP will be constructed.
Local movement of the MN within a MAP domain thereby remains
transparent to multicast routing.

Sending MCast Traffic to receivers
MAP-Domain1 /------------------------------------>
/-----| MAP1 |-----\
|| ||
|| ||
+-----+ ||
| AR1 | ||
+-----+ ||
|| ||
|| ||
|\-----+-----+ || ||
\------| MN | || ||
+-----+ || ||
|| || Movement
|| ||
MAP-Domain2 || ||
+-----+-----/| \/
/------| MN |------/
| AR2 |
\-----| MAP2 |
+-------+ Sending MCast Traffic to receivers

Figure 1: Intra-MAP-domain Handover for mobile multicast senders

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Upon arrival in a new MAP domain the MN submits a Binding Update with
its new LCoA to the previously established multicast-forwarding MAP
and continues its multicast delivery via this previous MAP (s. figure
1). If multicast support is advertised in the new domain the MN
immediately initiates a new multicast routing tree with the new RCoA
as source address anchored at its current MAP. The routing tree MAY
be initiated via bicasting or the initialization procedure described
in section 5.3.3.

The handover procedure completes after a predefined timeout is
reached: The mobile multicast source continues to deliver data only
via its new MAP and stops forwarding through its previous MAP.

4. Multicast specific extensions of MIPv6 and HMIPv6

4.1 M-HMIPv6 flag in MAP option message

M-HMIPv6 support is advertised within the MAP option message as used
for router advertisements according to HMIPv6 [5]. For this purpose
an appropriate flag is added in the following way

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 | Length | Dist | Pref |*|*|*|*|M| Res |
| Valid Lifetime |
| |
+ +
| |
+ Global IP Address for MAP +
| |
+ +
| |


* Used by HMIPv6
M When set indicates that M-HMIPv6 is supported by
the current MAP

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4.2 Use of Home Address Destination Option in mobile multicast

Multicast applications normally are aware of source addresses, which
MUST NOT change during ongoing communication. A mobile multicast
sender therefore MUST include a home address destination option as
defined in [4]. This requirement deviates from MIPv6 multicast

4.3 Binding Cache processing

A Correspondent Node receiving multicast packets with Home Address
Option in general need not have a Binding Cache Entry for the home
address included. A CN therefore MUST NOT verify multicast packets
with respect to its Binding Cache. This requirement deviates from
MIPv6 unicast scheme.

4.4 Home Agent Multicast Membership control

To provide multicast connectivity to a mobile multicast listener away
from home, a HA needs to take care of the local multicast group
management. This essentially can be done by either supplying full
multicast routing functionalities at the HA, or by a proxy agent

In the first case it suffices for the HA to observe MNs group
membership at the (tunnel) interface. For a multicast proxy function
a HA must answer MLD queries according to group membership states of
the MN. This is an extension of the specifications in [4].

5. Protocol Details

This section describes M-HMIPv6 operations as are to be performed for
multicast traffic in addition to the MIPv6 and HMIPv6 protocols. Two
perspectives need a general distinction: Multicast processing of a
mobile listener and for a mobile sender.

Mobility Anchor Points as defined in [5] attain the role of multicast
mobility anchor points (M-MAPs) for mobile group members in M-HMIPv6.
All multicast traffic is directed through M-MAPs using RCoA
consistently as identifier with respect to the multicast routing
tree. M-MAPs may be viewed as HA proxies for multicast streams, just
as MAPs in the unicast case.

5.1 Operations of all Mobile Nodes

Being at home the MN may either use its Home Agent or, a possibly
distinct, regional M-MAP as multicast anchor point. Away from home
the MN will learn about regional M-MAPs through router advertisements
(s. section 4.1). A MN SHOULD register with the regional M-MAP having

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the highest preference value. If M-HMIPv6 is not supported regionally
the MN first SHOULD attempt to employ a previously established M-MAP
relation, second register with its HA.

M-MAP presence is advertised via router advertisements with MAP
option message as described in section 4.1.

5.2 Mobile multicast listener

Any node on a multicast enabled network may subscribe to multicast
group membership by using its link-local address in MLD membership
reports. In doing so a MN cannot expect to experience a smooth
handover performance while changing from one network to another.

A MN utilizing a HMIPv6 MAP infrastructure can be regarded as eager
for decreased handover delays and therefore SHOULD use the M-HMIPv6
M-MAP functionality for other than link locally scoped multicast

5.2.1 Operations of the Mobile Node

A mobile multicast listener is registered with its local M-MAP (or
HA), where both communicate via a bi-directional tunnel. The MN
submits its MLD group membership listener report through this tunnel
and answers membership queries of the anchor point.

When a Mobile Node changes its network, it performs a Binding Update
with its previous M-MAP and re-establishes the tunnel at its new
LCoA. Thereafter it continues to receive multicast group traffic.

On entering a new M-MAP domain a MN - in addition to the above BU -
registers with the new M-MAP and establishes a bi-directional tunnel.
It immediately sends a MLD listener report through the newly
available connection, one for each group/flow to be handed over. Once
multicast group traffic arrives from the new M-MAP, the MN SHOULD
submit a BU with zero lifetime to its previous M-MAP and terminate
the corresponding tunnel.

Note that a MN SHOULD preserve a previously established M-MAP
relation until a new multicast forwarding is completely established.
In the case of rapid movement this may lead to a previous multicast
anchor point persisting through several hops.

5.2.2 Operations of the MAP

M-MAP operations for multicast listener support are completely analog
to Home Agent functions as described in [4] and section 4.4. An M-MAP
receiving a HMIPv6 BU from a MN will establish a bi-directional
tunnel. On reception of a tunneled MLD listener report it will

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o record multicast group membership in its Binding Cache;
o observe and maintain multicast group membership on its specific
tunnel interface;
o inquire on MNs current group membership as described in [4];
o forward multicast group traffic to the MN (see [4] on
multicast packet forwarding rules).

The M-MAP may control multicast group membership either as a
multicast router or as a multicast proxy agent (s. section 4.4).

5.2.3 Buffering

Some L2 technologies imply a noticeable offline period for a MN
during handover. To compensate for possible packet loss, buffering
mechanisms are needed. In M-HMIPv6 M-MAPs may provide automatic
replay buffers at the tunnel entry points, to be played out after a
MN’s Binding Update occurred.

5.3 Mobile multicast sender

A multicast source sending with its link-local address is immobile
with respect to multicast application persistence. A mobile multicast
sender MAY tunnel multicast traffic through its HA, using its home
address as source address [4]. Triangular routing and significant
binding update times lead to expected large handover delays, in

A MN utilizing a HMIPv6 MAP infrastructure therefore SHOULD use the
M-HMIPv6 M-MAP functionality for other than link locally scoped
multicast transmissions.

5.3.1 Operations of the Mobile Node

A mobile multicast sender is registered with its local M-MAP, where
both communicate via a bi-directional tunnel. The MN submits
multicast packets through this tunnel with the RCoA as the source
address and the home address included in a home address destination
option as defined in [4].

When a Mobile Node changes networks, it performs a Binding Update
with its previous M-MAP and re-establishes the tunnel at its new
LCoA. Thereby it continues to send its multicast group traffic.

On entering a new M-MAP domain a MN - in addition to the above BU -
registers with the new M-MAP and establishes a bi-directional tunnel.
It immediately SHOULD start the tree initialization procedure as
defined in section 5.3.3 and start a timer. As soon as this timer
exceeds MAX_MCASTTREEINIT_TIMEOUT the MN MUST complete the handover

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by terminating multicast group forwarding through its previous M-MAP.
Note that these handover steps can be performed stream wise.

A MN, which moves to a new link within the same M-MAP domain before
the timeout is reached, performs a BU with its current M-MAP and
continues the handover procedure without resetting its timers.

A MN, which moves into a new M-MAP domain before the timeout
occurred, continues to forward multicast traffic through its
previously established old M-MAP, discontinues to communicate via its
previously not fully established intermediate M-MAP, resets its timer
and restarts the tree initialization procedure for its current M-MAP.

Thus in case of rapid movement the MN stays bound with its formerly
fully established (or first) M-MAP, serving the last completely
erected multicast routing tree.

5.3.2 Operations of the MAP

M-MAP operations for multicast sender support are completely analog
to MAP functions for unicast support as described in [5].

5.3.3 Tree initialization procedure

In preparation for a seamless handover of a multicast sender a shared
tree needs to be constructed by the routers originating at the new M-
MAP. In general, routing trees will be initiated by submitting
packets into the appropriate multicast group. Depending on the
routing protocol in use, this can be a tardy procedure. The tree
initialization procedure provides dedicated instructions for the MN
to efficiently bridge the multicast routing convergence gap.

A multicast sender MAY initiate a new group tree by bi-casting its
packets to its previous and its new point of attachment. The period
of bi-casting will last until MAX_MCASTTREEINIT_TIMEOUT is reached
and the sender then solely submits its multicast data along the newly
erected tree. Bi-casting in the presence of slow routing protocols,
though, may result in a significant amount of duplicate traffic. In
such cases it may be desirable to proceed in a less communicative

In performing the incommunicative tree initialization procedure the
source starts to send probe packets with complete IPv6 header but
without payload. This MAY be done about every 10 seconds with two
subsequent packets, in the first phase. Subsequence of packets MAY be
generated with a random interval between zero and 30 milliseconds.
This first phase ends at the timeout

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Following the first phase the multicast sender SHOULD submit the
complete multicast traffic for an initialization period of
MAX_MCASTTREEFLOW_PERIOD. The tree initialization procedure ends
after MAX_MCASTTREEINIT_TIMEOUT is reached with continuous submission
of regular traffic.

5.3.4 Buffering

To prevent or reduce packet loss during handover the mobile source
MAY buffer packets to be sent, while its tunnel to the M-MAP is
unestablished. This buffer should be played out as soon as the tunnel
re-establishment to the previous MAP has completed.

5.4 Protocol Timer

160 seconds (For DVMRP regimes)
0.5 seconds (For PIM-SM regimes)

MAX_MCASTTREEFLOW_PERIOD 0.1 seconds (Default)

Mobile nodes must allow these variables to be configured by system

6. Security Considerations

This specification uses the concepts of Mobile IPv6 and Hierarchical
Mobile IPv6 mobility management. All security provisions regarding
the relation between the Mobile Node and the Home Agents and between
the Mobile Node and the Mobility Anchor Points apply equally to this
M-HMIPv6 concept.


1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.

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

3 Hinden, R. and Deering, S. "Internet Protocol Version 6
Specification", RFC 2460, December 1998.

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M-HMIPv6 February 2004

4 Johnson, D.B., Perkins, C., Arkko, J. "Mobility Support in IPv6",
draft-ietf-mobileip-ipv6-24 (work in progress), July 2003.

5 Soliman, H., Castelluccia, C., El-Malki, K., Bellier, L.
"Hierarchical Mobile IPv6 mobility management", draft-ietf-
mipshop-hmipv6-00 (work in progress), October 2003.

6 Koodli, R. "Fast Handovers for Mobile IPv6", draft-ietf-mipshop-
fast-mipv6-01 (work in progress), February 2004.

7 Conta, A., Deering, S. "Generic Packet Tunneling in IPv6
Specification", RFC 2473, December 1998.

8 S. Deering, W. Fenner and B. Haberman "Multicast Listener
Discovery (MLD) for IPv6", RFC 2710, October 1999.


The authors would like to thank Stefan Zech (FHTW Berlin), Mark
Palkow (DaViKo GmbH) and Hans L. Cycon (FHTW Berlin) for valuable
discussions and a joyful collaboration.

Author's Addresses

Thomas C. Schmidt
FHTW Berlin
Treskowallee 8
Phone: +49-30-5019-2739
Email: Schmidt@fhtw-berlin.de

Matthias Waehlisch
FHTW Berlin
Treskowallee 8
Email: mw@fhtw-berlin.de

A. A Note on Tunneling

Following the concepts of MIPv6 and HMIPv6 the packet forwarding to
and from the Mobile Node is organized by means of a tunnel section

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spanned to a static anchor component such as a MAP or a Home Agent.
Through this technique a MN can hide its movement to CNs or to the
routing infrastructure.

However, keeping in mind real-time data requirements it is highly
desirable to avoid packet encapsulation. Besides the unwanted
overhead, a tunnel may hide QoS information of the original packet
headers and may require load and jitter generating packet
fragmentation, if the tunnel entry point is distinguished from the

Tunnelling can be avoided by a direct packet forwarding of the static
anchor components. Such forwarding requires a change of packet's
source or destination address at the forwarder, which usually
conflicts with checksums covering IPv6 pseudo headers. In M-MIPv6
multicast communication from a Mobile Node though carries a MIPv6
extension header, the home address destination option header. This
header denotes an alternate source address which enters the pseudo
header instead of the original IPv6 header address.

Multicast packets sent from the MN therefore could be forwarded by
the MAP to the network by replacing source addresses without
recalculation of header checksums. Employing such direct packet
forwarding would allow a MN to distribute multicast streams without a

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