Mobileip Working Group Karim El-Malki, Ericsson
INTERNET-DRAFT Hesham Soliman, Ericsson
Expires: March 2001 September,2000
Fast Handoffs in MIPv6
<draft-elmalki-handoffsv6-00.txt>
Status of this memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
This draft describes a method to achieve Fast Handoffs in Mobile
IPv6. Fast Handoffs are required in Mobile IPv6 in order to limit
the period of service disruption experienced by a wireless Mobile
Node when moving between access routers. This requirement becomes
even more important when supporting real-time services. Fast
Handoffs involve anticipating the movement of MNs by sending
multiple copies of the traffic to potential Mobile Node movement
locations. Both flat and Hierarchical Mobile IPv6 models are
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considered. The Hierarchical MIPv6 mobility Management model in [1]
already offers improvements to Mobile IP handoffs by providing a
local Mobility Anchor Point (MAP) functionality. Some additions
are made to the operation of this existing Hierarchical model to
achieve Fast Handoffs.
TABLE OF CONTENTS
1. Introduction...............................................2
2. Fast Handoffs..............................................4
2.1 Initiating Fast Handoffs through the "previous" AR.........5
3. Fast Handoffs in Hierarchical MIPv6........................8
4. Acknowledgements...........................................14
5. References.................................................14
6. Addresses..................................................14
1. Introduction
Fast Handoffs anticipate the movement of wireless Mobile Nodes
(MNs) by utilizing simultaneous bindings in order to send multiple
copies of the traffic to potential Mobile Node movement locations.
In this way, Fast Handoffs coupled to layer 2 mobility can help in
achieving seamless handoffs between Access Routers (ARs) by
eliminating the delay period required to perform a Registration
following a Mobile IP handoff.
An alternative method to perform improved handoffs, namely Smooth
Handoffs, is described in [2]. The method for Fast Handoff
addresses the need to support services having strict delay bounds
(i.e. real-time) which in certain cases may be hard to support if
traffic has to be forwarded between ARs using Smooth Handoffs.
Also, in the non-realtime case it may be possible that the new AR
receives buffered traffic from the previous AR (smooth handoff)and
traffic from the CN which could cause some out-of-order and delayed
packets to be delivered to the MN. In some cases this may affect
the performance of higher level protocols (i.e. TCP). This same
situation will not arise using Fast Handoffs.
This draft considers both the normal Mobile IPv6 model [2] and the
hierarchical Mobile IPv6 model [1]. These are shown in Figure 1
where the Access Points (APs) or Radio Access Networks (RANs) are
used to provide a MN with wireless L2 access.
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Simultaneous bindings are described in this draft and may be
achieved by setting a new, "B" flag in the BU sent by the MN to a
MAP. In this way, the MAP will add a new binding for the MN
without removing the existing entry.
Hence packets arriving to the MAP will be tunnelled to both addresses
in its BC.
_________ __________
| | | |
| HA |--------| (MAP) |________
|_________| |_________| \
/ | \ \
\
... ... ... \
\
______/_ _\______ |
| | | | |
| AR2/MAP| | AR1/MAP| |
|________| |________| |
____|___ ____|___ ____|___
| | | | | |
|AP/RAN 2| |AP/RAN 1| |AP/RAN 3|
|________| |________| |________|
| ____|___
| |
CN | MN |
|________|
Figure 1: Flat (HA only) and Hierarchical (HA and MAP) MIPv6
model
The method to anticipate MN movement by interacting with the wireless
L2 is described later in this draft.
The Hierarchical Mobile IPv6 scheme introduced in [1] allows a Mobile
Node to perform registrations locally with an MAP in order to reduce
the number of signalling messages to the home network and CNs. This
achieves a reduction in the signalling delay when a Mobile Node moves
between ARs and therefore improves the performance of such handoffs.
This draft describes Fast Handoffs in Hierarchical Mobile IPv6
(HMIPv6) using Regional Registrations.
When considering a MIPv6 handoff, two different cases can be
considered depending on the network architecture:
- The previous and new AR are physically connected
- The previous and new AR are connected via another node/network
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The first case can be considered a subset of the more generic
case. Hence the solution proposed will be addressing the generic
(second) scenario.
2. Fast Handoffs
Fast Handoffs address the need to achieve seamless Mobile IP
Handoffs when the MN moves between ARs. This is done by "bicasting"
traffic to the "previous" AR and "new" AR while the MN is moving
between them. The anticipation of the MN's movement is achieved by
tight coupling with Layer 2 functionality which is
dependent on the type of access technology used. The coupling between
L2 and L3 technologies may occur in the network nodes or the MNs, or
both, depending on the access technology. "Bicasting" is achieved
through simultaneous bindings, where the MN activates the "B" flag in
the MAP registration. When a MAP Registration
has the "B" flag set, the receiving MAP, which has an existing
binding for the MN, will add the relevant new binding for the MN but
will also maintain any existing binding it had for the MN.
Two different handoff scenarios are considered in this draft:
- A MN having to do a handoff between two different ARs with
which it can be simultaneously data-connected (eg. Two
different access technologies). In this case it may not be
essential to request simultaneous bindings. The MN may simply
continue using both COAs (on the old and new link) as
specified in [2].
- A MN having to do a handoff between two access routers with
which it can not be simultaneously data-connected.
This is the more generic case and _bicasting_ can be used to
achieve Fast Handofffs.
When the MN has multiple active bindings with a MAP, it may or may
not receive multiple copies of the same traffic directed to it.
The use of simultaneous bindings does not necessarily mean that
the MN is receiving packets contemporarily from multiple sources.
This depends on the characteristics of the access (L2) technology.
The "bicasting" of packets is used to anticipate the MN's movement
and speed up handoffs by sending a copy of the data to the AR which
the MN is moving to. Until the MN actually completes the L2 handoff
to the new AR, the data "copy" reaching this AR may be discarded. In
this way the total handoff delay is limited to the time needed to
perform the L2 handoff. Thus, Fast Handoffs coupled to the L2 access
potentially result in loss-less IP-layer mobility. As described in
2.1, depending on the L2 characteristics, it is also possible for an
MN to initiate a Fast Handoff through the "previous" AR without
having direct access to the "new" AR.
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2.1 Initiating Fast Handoffs through the "previous" AR
In the case in which the wireless L2 technology allows the MN tobe
data-connected to multiple wireless access points simultaneously,
the MN may solicit advertisements from ARs before completing
handoffs. In this case "bicasting" may not be necessary.
Some existing wireless L2 technologies and their implementations
do not allow a MN to be data-connected to multiple wireless access
points simultaneously. Thus, in order to perform a Fast Handoff it is
necessary for some form of interworking between layers 2 and 3.
It should be noted that the method by which an AR determines when
a MN has initiated an L2 handoff is outside the scope of this
draft and may involve interaction with L2 messaging. Also, the
interaction between L2 and L3 should allow the Mobile Node to
perform a L2 handoff only after having performed the L3 Fast
Handoff described in this draft. That is, the L2 handoff may be
performed after the MN's Registration with the "new" AR which
produces a simultaneous binding at the MAP. This Registration
may be transmitted more than once to reduce the probability that
it is lost due to errors on the wireless link. Alternatively, the MN
may choose to send a BU to the MAP with the _A_ flag set.
A Fast Handoff in this case requires the MN to receive "new" router
advertisements through the "old" wireless access points, and to
perform a registration with the "new" FA through the "old"
wireless access point. Two ways of performing this follow.
I. Inter-AR Solicitation
This solution assumes that the AR with which the MN is currently
registered is aware of the IP address of the "new" AR which the MN is
moving to and a physical connection exists between them (ie. they
have a common link). The method by which the current AR is informed
of this may depend on interaction with L2 and is outside the scope of
this draft.
Once the current AR is aware of the address of the AR which the MN
will move to, it will send the "new" AR a router solicitation
message. The "new" AR will reply to the current AR by sending it
a router advertisement with appropriate extensions. The current AR
will then send the router advertisement to the MN's address. As a
consequence, the MN, being eager to perform new registrations,
will send a registration request to the "new" AR through the
"old" wireless access point served by the current AR.
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II. Piggy-backing Advertisements on L2 messaging
Let us take Figure 1 as an example, where a MN initiates an L2
handoff from AP/RAN1 to AP/RAN2 (Note that it may not be the MN
which takes decisions on handoffs). It is assumed that when an L2
handoff is initiated, AP/RAN1 and AP/RAN2 perform L2 messaging
procedures to negotiate the L2 handoff. Since the MN is not
attached to AP/RAN2 yet, AR2 is unaware of the IP address of the
MN and cannot send an advertisement to it. Therefore it is
necessary for the L2 procedures to interwork with Mobile IP.
Once a L2 handoff is initiated, such that AP/RAN2 and AP/RAN1 are
in communication, it is possible for AP/RAN2 to solicit an
advertisement from AR2 and transfer it to AP/RAN1. Once this is
received by the MN, the MN can perform a registration directed to
AR2 even though the MN has no data-connection to AP/RAN2 yet.
The precise definition of such L2 procedures is outside the scope
of Mobile IP.
3. Fast Handoffs in HMIPv6
HMIPv6 is described in [1]. Fast IP Handoffs can be achieved in a
very simple and efficient manner.
When the MN receives a router Advertisement including a MAP option,
as specified in [1], it should perform actions according to the
following movement detection mechanisms. In a Hierarchical Mobile IP
network such as the one described in this draft, the MN MUST be:
- "Eager" to perform new bindings
- "Lazy" in releasing existing bindings
The above means that the MN will perform Regional Registrations
with any "new" MAP advertised by the AR (Eager).
The method by which the MN determines whether the MAP is a "new" MAP
is described in [1]. However the MN should not release existing
bindings until it no longer receives its MAP option or the lifetime
of its existing binding expires (Lazy).
If the MN has at least one existing binding with a MAP, additional
simultaneous regional registrations will be performed requesting a
short lifetime. This is done in order to limit the lifetime of
bindings which the MN only needs temporarily and therefore limit
bandwidth usage. This is the case when the MN is moving between
ARs and uses Fast Handoffs to achieve near loss-less IP mobility. The
lifetime of additional "auxiliary" bindings needed for Fast
Handoffs is thus limited.
It should be noted that the method described above is applicable to
hierarchical and flat architectures. As described in [1], a MAP can
exist on any level in the hierarchy, including ARs. Hence, a
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bicasting request can also be sent to a MAP located in the AR, in the
case where no MAPs are located higher in the hierarchy.
4. Extensions to MIPv6
To allow bicasting from the MAP to take place, a new flag,_B_, is
added to the BU message. Upon reception of a BU message with the _B_
flag set, a MAP SHOULD bicast all incoming packets addressed to the
MN to its current COA as well as the new COA in the BU requesting the
bicast. The new BU message is shown below.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|A|H|R|D|M|B|Res| Prefix Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Options...
+-+-+-+-+-+-+-+-+-+-+-+-
Description of extensions to the BU option:
B If set, it indicates a request for bicasting all
traffic received for the MN to its current address
as well as the new address in the BU.
Res 2 bit reserved field
5. Fast Handoffs and DAD
The use of DAD to verify the uniqueness of a statelessly configured
IPv6 address may add delays to a MIPv6 handoff. The probability of an
inteface identifier duplication on the same subnet can be considered
very low. Hence, to avoid this delay, a MN may choose to continue
sending and receiving traffic using its newly formed COA while
performing DAD on the new subnet. In the case where a duplication
exists, the MN MUST follow the rules in [4].
This issue is not specific to this proposal and may also be addressed
in future revisions of [2].
6. Notice Regarding Intellectual Property Rights
Ericsson may seek patent or other intellectual property protection
for some or all of the technologies disclosed in this document. If
any standards arising from this disclosure are or become protected by
one or more patents assigned to Ericsson, Ericsson intends to
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disclose those patents and license them on reasonable and non-
discriminatory terms. Future revisions of this draft may contain
additional information regarding specific intellectual property
protection sought or received.
7. References
[1] H. Soliman, C. Castellucia, K. El Malki and L. Bellier
"Hierarchical Mobile IPv6 and Fast Handoffs",
draft-ietf-mobileip-hmipv6-00.txt (work in progress),
September 2000
[2] D. Johnson and C. Perkins, "Mobility Support in IPv6",
draft-ietf-mobileip-ipv6-12.txt, February 2000.
[3] K. El Malki and H. Soliman " Fast Handoffs in Mobile IPv4".
(work in progress)
[4] S. Thomson and T. Narten "IPv6 Stateless Address
Autoconfiguration". RFC 2462.
8. Acknowledgements
The authors would like to thank the following members of the working
group (in alphabetical order) for their comments and the interesting
discussions about this draft: Gopal Dommety (Cisco), Dave Johnson
(Rice University), Erik Nordmark (Sun), Mohan Parthasarathy (Sun),
Carl Williams (Sun)and Alper Yegin (Sun).
9. Addresses
The working group can be contacted via the current chairs:
Basavaraj Patil Phil Roberts
Nokia Corporation Motorola M/S M8-540
6000 Connection Drive 1501 West Shure Drive
Irving, TX 75039 Arlington Heights, IL 60004
USA USA
Phone: +1 972-894-6709 Phone: +1 847-632-3148
EMail: Raj.Patil@nokia.com EMail: QA3445@email.mot.com
Fax : +1 972-894-5349
Questions about this memo can be directed to:
Karim El Malki
Ericsson Radio Systems AB
Access Networks Research
SE-164 80 Stockholm
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SWEDEN
Phone: +46 8 7573561
Fax: +46 8 7575720
E-mail: Karim.El-Malki@era.ericsson.se
Hesham Soliman
Ericsson Australia
61 Rigall St., Broadmeadows
Melbourne, Victoria 3047
AUSTRALIA
Phone: +61 3 93012049
Fax: +61 3 93014280
E-mail: Hesham.Soliman@ericsson.com.au
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