Internet DRAFT - draft-elmalki-mobileip-bicasting-v6
draft-elmalki-mobileip-bicasting-v6
Mobile IP Working Group Karim El Malki, Ericsson
INTERNET-DRAFT Hesham Soliman, Flarion
Expires: January 2006
July 2005
Simultaneous Bindings for Mobile IPv6 Fast Handovers
<draft-elmalki-mobileip-bicasting-v6-06.txt>
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Abstract
Fast Handover for Mobile IPv6 [1] minimizes the amount of service
disruption when performing layer-3 handovers. This draft extends the
Fast Handover protocol with a simultaneous bindings function to
minimize packet loss at the MN. Traffic for the MN is therefore
bicast or n-cast for a short period to its current location and to
one or more locations where the MN is expected to move to shortly.
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This removes the timing ambiguity regarding when to start sending
traffic for the MN to its new point of attachment following a Fast
Handover and allows the decoupling of layer-2 and layer-3 handovers.
It also saves the MN periods of service disruption in the case of
ping-pong movement.
TABLE OF CONTENTS
1. Introduction.....................................................2
1.1 Terminology.....................................................3
2. Simultaneous Bindings............................................3
3. Fast Handovers in Mobile IPv6....................................4
4. Decoupling L3 Handovers from L2 handovers using Simultaneous
Bindings ...........................................................5
5. Avoiding service disruption due to ping-pong movement............6
6. Extensions to the Fast Handover Operations......................7
6.1 MN Operation....................................................7
6.2 HA/MAP/AR Operation.............................................7
7. Simultaneous Bindings Flag in Fast Binding Update (F-BU) message.8
8. Simultaneous Bindings option for Fast Binding Acknowledgement....8
(F-BA) message......................................................8
9. Multiple copies of packets received at AR........................9
10. Reception of multiple copies in the MN..........................9
11. References.....................................................10
12. Authors' Addresses.............................................10
1. Introduction
Fast Handover for Mobile IPv6 (FMIPv6) describes a protocol to
minimise the amount of service disruption when performing layer-3
handovers. This draft extends the Fast Handover protocol with a
simultaneous bindings function to minimise packet loss at the MN.
Traffic for the MN is therefore bicast or n-cast for a short period
to its current location and to one or more locations where the MN is
expected to move to shortly. This removes the timing ambiguity
regarding when to start sending traffic for the MN to its new point
of attachment following a Fast Handover and allows the decoupling of
layer-2 and layer-3 handovers. It also saves the MN periods of
service disruption in the case of ping-pong movement. Appendix A
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contains some calculations illustrating how to achieve zero service
disruption at L3 using FMIPv6 and bicasting.
1.1 Terminology
This section presents a few terms used throughout the document.
PAR û Previous Access Router.
NAR - New Access Router.
L2 handover - Movement of a MN's point of Layer 2 (L2)
connection from one wireless access point to another.
L3 handover - Movement of a MN between ARs which involves
changing the on-link care-of address at Layer 3 (L3).
L2 trigger - Information from L2 that informs L3 of particular
events before and after L2 handover. The descriptions of L2
triggers in this document are not specific to any particular
L2, but rather represent generalizations of L2 information
available from a wide variety of L2 protocols.
Bicasting/n-casting - The splitting of a stream of packets
destined for a MN into two or more streams, and the
simultaneous transmission of the streams to PAR and one or
more NARs. N/casting is a technique used to reduce packet
loss during handover.
ping-ponging - Rapid back and forth movement of an MN between
two wireless access points due to failure of L2 handover or
frequent handovers. Ping-ponging can occur if radio
conditions for both the old and new access points are about
equivalent and less than optimal for establishing a good,
low error L2 connection.
2. Simultaneous Bindings
Simultaneous bindings are built into the Mobile IPv4 protocol [2]. To
enable multiple simultaneous bindings using Mobile IPv4 the MN simply
sends the first normal Registration Request for a care-of address and
then sends other Registration messages for additional care-of
addresses having the S bit set. The receiver of the Registration
Requests (i.e. the HA) will then maintain all these care-of address
bindings for the MN contemporarily rather than only servicing the MN
at the care-of address in its most recent Registration Request (which
would be the case had the S bit not been set). This results one copy
of packets being sent to each of the registered care-of addresses
(i.e. bicasting or n-casting of packets). This draft extends the
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Mobile IPv6 protocol [3] with similar functionality and describes a
new Simultaneous Bindings flag for the Fast Binding Update in [1].
Multiple simultaneous bindings and bicasting can be an important tool
to decouple L3 handovers from L2 handovers and to reduce packet loss.
This mechanism instructs the recipient of an F-BU [1] having the
simultaneous bindings flag to make multiple copies of packets
destined to the MN and send them to multiple MN care-of addresses
before the MN actually moves there. This allows a smoothing of the L3
handover, meaning that packet loss is minimized or even eliminated.
Simultaneous bindings are also useful to prevent service disruption
due to ping-pong movement as described later.
3. Fast Handovers in Mobile IPv6
The mechanism to obtain fast L3 handovers for Mobile IPv6 is
described in [1] and illustrated in Figure 1. This mechanism involves
the use of L2 triggers which allow the L3 handover to be anticipated
rather than being performed after the L2 handover completion as
normal. Fast Handovers are required to ensure that the layer 3
(Mobile IP) handover delay is minimized, thus also minimizing and
possibly eliminating the period of service disruption which normally
occurs when a MN moves between two ARs. This period of service
disruption usually occurs due to the time required by the MN to
update its HA after it moves between ARs. During this time period the
MN cannot resume or continue communications. Following is a short
summary of the Fast Handover mechanism described in [1].
+----------------------+ 4a. HI +-----+
| | ---------------->| NAR |
| PAR | 4b. HAck | |
+----------------------+ <----------------+-----+
^ | ^ |
(1a.)| |1b | 3. |5.
RtSolPr| |Pr | Fast |Fast BA (F-BACK)
| |RtAdv | BU |
| v |(F-BU) v
+----------------------+
| MN |
+----------------------+ - - - - - ->
Figure 1 û Fast MIPv6 Handover Protocol
While the MN is connected to its Previous Access Router (PAR) and is
about to move to a New Access Router (NAR), the Fast Handovers in
Mobile IPv6 requires:
- the MN to obtain a new care-of address at the NAR while connected
to the PAR the MN to send a Fast Binding Update (FBU) to its old
anchor point (e.g. PAR) to update its binding cache with the MN's
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new care-of address.
- the old anchor point (e.g. PAR) to start forwarding packets
destined for the MN to NAR.
The MN or PAR may initiate the Fast Handover procedure by using
wireless link-layer information or link-layer triggers which inform
that the MN will soon be handed off between two wireless access
points respectively attached to PAR and NAR. If the trigger is
received at the MN, the MN will initiate the layer-3 handover process
by sending a Proxy Router Solicitation message to PAR. Instead if the
trigger is received at PAR then it will transmit a Proxy Router
Advertisement to the appropriate MN, without the need for
solicitations.
The MN obtains a new care-of address while connected to PAR by means
of router advertisements containing information from the NAR (Proxy
Router Advertisement, PrRtAdv, which may be sent due to a Proxy
Router Solicitation, RtSolPr). The PAR will validate the MN's new
COA by sending a Handover Initiate (HI) message to the NAR. Based on
the response generated in the Handover Acknowledge (HAck) message,
the PAR will either generate a tunnel to the MN's new COA (if the
address was valid) or generate a tunnel to the NAR's address (if the
address was already in use on the new subnet). If the address was
already in use on the new subnet, the NAR will generate a host route
for the MN using its old COA.
4. Decoupling L3 Handovers from L2 handovers using Simultaneous Bindings
The mechanisms described in [1] allow the anticipation of the layer 3
handover such that data traffic can be redirected to the MN's new
location before it moves there. However it is not simple to determine
the correct time to start forwarding between PAR and NAR, which has
an impact on how smooth the handover will be. Packet loss will occur
if this is performed too late or too early with respect to the time
in which the MN detaches from PAR and attaches to NAR. Also, some
measure is needed to support the case in which the MN moves quickly
back-and-forth between ARs (ping-pong).
In many wireless networks it is not possible to know in advance
precisely when a MN will detach from the wireless link to PAR and
attach to the one connected to NAR. Therefore determining the exact
time when to start forwarding packets between PAR and NAR is not
possible. Certain wireless technologies involve layer-2 messages
which instruct the MN to handover immediately or simply identify that
the MN has already detached/attached. Even if the ARs could extract
this information, there may not be sufficient time for the PAR to
detect the MN's detachment and start getting packets tunnelled over
to NAR before the MN attached to NAR. This is because wireless layer-
2 handover times are relatively small (i.e. range from 10's to 100's
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ms). Thus a period of service disruption may occur due to this timing
uncertainty unless further enhancements are made to the handover
mechanism. If the L3 handover is anticipated and the PAR starts
forwarding data to NAR upon receipt of the Fast BU in [1] then the
period of service disruption will be according to the following:
A = L3 handover anticipation (time difference between the start of
the L3 fast handover and the moment in which the L2 handover
occurs)
h = L2 handover time (disconnection time due to L2 handover)
Approximate period before MN receives packets again = A + h
It is therefore necessary to decouple layer-3 handover timing from
layer-2 handover timing. This can be solved by bicasting or n-casting
packets destined to the MN for a short period from the old anchor
point (e.g. PAR) to one or more potential future MN locations (e.g.
NAR/s) before the MN actually moves there. This means that the
handover procedure described previously would be enhanced by having
the old anchor point (e.g. PAR) send one copy of packets to the MN's
old on-link care-of address and another copy of the packets to the
MN's new care-of address (or addresses) connected to NAR. The MN is
thus able to receive traffic independently of the exact layer-2
handover timing during the period.
5. Avoiding service disruption due to ping-pong movement
It is possible that the layer-2 handover procedure may fail or
terminate abruptly in wireless systems. Therefore a MN which expects
to move between PAR and NAR may unexpectedly never complete the
layer-2 handover and find itself connected to PAR. Another undesired
effect is that the MN could ping-pong between ARs due to layer-2
mobility issues. Both these cases would leave the MN unable to resume
communication and have to transmit a new F-BU in [1] before resuming
communications.
This may be solved through the use of simultaneous bindings which
allow the MN to maintain layer-3 connectivity with the PAR during the
affected handover period, thus smoothing the handover. This
eliminates the need for continuous transmission of Fast Binding
Updates in [1]. It also prevents the period of service disruption
from being extended due to the effect of the above link-layer issues
on L3 handover.
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6. Extensions to the Fast Handover Operations
6.1 MN Operation
The MN operation in [1] is affected by the changes introduced by this
document. The addition to [1] is that a MN with an existing active
binding which receives a new router advertisement (PrRtAdv) MUST be
"eager" to establish new bindings. When a MN has at least one
existing binding and receives a new PrRtAdv it MUST send a Fast
Binding Update (F-BU) with the Simultaneous Bindings flag set (B
flag). The new flag is described in section 8. In addition the MN
MUST be able to process the new simultaneous bindings option in the
Fast Binding Acknowledgement message described in section 9. The
lifetime field returned in this option MUST be used by the MN to
identify the lifetime of the simultaneous binding requested. Two BU
lifetime values will be returned: Bicasting lifetime (in the
simultaneous bindings option) and new CoA lifetime (in the BA option)
as described in the following sections. The new CoA lifetime (placed
in the BA option as specified in [3]) runs in parallel with the
Bicasting lifetime. Hence, when the bicasting lifetime ends, the MN
will remove the special bicasting information from the Binding Update
list and simply keep one entry for the new CoA with the remaining new
CoA lifetime.
6.2 HA/MAP/AR Operation
The HA [3], MAP [4] and AR [1] are the possible recipients of a F-BU
message. Upon receiving a F-BU message having the B flag set (see
section 8), the HA/MAP/PAR MUST create a new binding cache sub-entry
(linked to the original entry for the old CoA) for the MN's new CoA.
This sub-entry contains the same fields as normal binding cache
entries but it MUST not replace any existing entries for the MN. The
new sub-entry will have two lifetimes instead of one: the normal new
CoA BU lifetime (sent in the BA) and a Bicasting lifetime set to
SHORT_BINDING_LIFETIME (this value is sent in the BA option). The new
CoA lifetime runs in parallel with the Bicasting lifetime. Until the
Bicasting lifetime expires, the HA/MAP/PAR MUST send a copy of the
data destined for the MN to the old CoA and to the new CoA/s in the
linked binding cache sub-entry or sub-entries. When the Bicasting
lifetime expires, the MAP/HA/PAR MUST remove the bicasting lifetime
field and replace the old binding cache entry for the old CoA with
the new CoA sub-entry. As a result, the HA/MAP/AR will end up with
one entry for the MN's new CoA with the remaining new CoA lifetime.
If the MAP/HA/AR receives a valid BU for the new COA without the S
flag set before the expiration of the bicasting lifetime it must
follow standard procedures of [1] and replace the existing binding
cache entry.
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7. Simultaneous Bindings Flag in Fast Binding Update (F-BU) 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence # |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|A|H|L|K|M|B| Reserved | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility Options .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Description of the flag added to the F-BU option already defined in
[1]:
B When set indicates a request for bicasting all
packets to both COAs of the MN (in the source
address field and the alternate-CoA suboption).
This BU will add another COA to the Binding
Cache.
8. Simultaneous Bindings option for Fast Binding Acknowledgement
(F-BA) 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| status | Reserved | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type TBD
Option Len TBD
Status Indicates success (0) or failure (128
and above).
Lifetime The bicasting lifetime for the
simultaneous binding requested in the
F-BU. This value MUST be used by the MN
to record the validity of this binding
in its binding update list.
The alignment requirement for this option is 2n+2.
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9. Multiple copies of packets received at AR
If the MN has simultaneous active bindings with HA/MAP/AR, it could
(but preferably should not) receive multiple copies of the same
traffic directed to it. The use of simultaneous bindings does not
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 involves sending a copy of the
data to the AR which the MN is moving to (the NAR). Until the MN
actually completes the L2 handover to the NAR and fully establishes
the new L2 link, the NAR MAY receive packets for a MN to which it
does not have a direct link layer connection. If the new AR is aware
that the MN is performing a handover (due to earlier reception of the
HI message) the AR MAY:
- drop all packets for the MN,
- drop some packets, based on local policies, or
- buffer packets for the MN.
The choice of which action to take may depend on the type of traffic
involved (e.g. real-time or non real-time), but this is outside the
scope of this document. The AR MAY also in parallel attempt to
establish a link-layer connection with the MN. However an AR MUST NOT
send ICMP Destination Unreachable messages if it drops packets or is
unable to deliver the received IP packets due to unavailability of
direct layer connection with the MN. This is because a copy of the
packets would be dropped, but the MN is still receiving a copy of the
packets through the PAR. Note that the MN may also select which flows
need bicasting by adding a Flow movement option [7] to the
simultaneous binding update. Therefore the simultaneous bindings
mechanism may only be applied to traffic types that require this
service.
10. Reception of multiple copies in the MN
In some scenarios it may be possible that the MN receives more than
one copy of the same packet. Generally, Internet routing mechanisms
cannot guarantee the delivery of a single copy of an IP packet to a
node. However some TCP congestion avoidance implementations are known
to react negatively to the reception of 3 duplicate acknowledgements.
The Eifel detection and response algorithms in [5] and [6] address
this problem. When using [5] and [6] bicasting should not cause any
negative performance impacts for TCP.
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11. References
Normative
[1] R. Koodli (Editor) et al, "Fast Handovers for Mobile IPv6", RFC
4068, July 2005.
[3] D. Johnson, C. Perkins and J. Arkko, "Mobility Support in IPv6",
RFC 3775, June 2004.
[4] H. Soliman, C. Castelluccia, K. El Malki and L. Bellier,
"Hierarchical Mobile IPv6 mobility management (HMIPv6)", draft-ietf-
mipshop-hmipv6-04.txt, work in progress, December 2004.
Informative
[2] C. Perkins (Editor), "IP Mobility Support for IPv4", RFC 3220,
Jan 2002.
[5] R. Ludwig, "The Eifel Detection Algorithm for TCP", RFC 3522,
April 2003.
[6] R. Ludwig and A. Gurtov, "The Eifel response algorithm for TCP",
RFC 4015, February 2005.
12. Authors' Addresses
The authors may be contacted at the addresses below:
Karim El Malki
Ericsson AB
Phone: +46 8 7195803
E-mail: Karim.El-Malki@ericsson.com
Hesham Soliman
Flarion
E-mail: H.Soliman@flarion.com
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Appendix A - Timing Calculations for bicasting
Example 1
---------
+--------+
+------| MAP/HA |------+
| +--------+ |
| |
v v
+-----+ +-----+
| PAR | | NAR |
+-----+ +-----+
+-----+
| MN |
+-----+ - - - - - >
Movement
This is the case specified by [1] with the extension of using the MAP
from [4].
A = anticipation time (F-BU is sent from MN at time t-A, where t is
the time when the MN actually hands-off at L2)
h = handover time (L2 only)
D1 = MN to MAP delay (through PAR)
D2 = MN to MAP delay (through NAR)
p = F-BU and routing table processing time in the MAP and MN
To achieve zero L3 service disruption it is necessary for the time
period between starting the fast handover and the MN completing the
L2 handover to be greater than or equal to the tiem it take for
traffic to reach the MN at its new link (through NAR). This is
represented by the following formula:
(A+h)>=((D1+D2)+p)
Assuming that p<<(D1+D2) this can be simplified to:
(A+h)>=(D1+D2)
To achieve maximum performance from simultaneous bindings it is
necessary for the above relation to hold.
The Anticipation time (A) is important and needs to be calculated
appropriately for the link-layer being used. Depending on the L2 this
may need engineering to synchronize the L2 and L3 handovers.
Once the MN has moved to NAR, it will be receiving traffic delayed by
(D2-D1) with respect to when it was attached to PAR. To smooth this
delay variation (jitter), which may be a problem for real-time
services, it may be necessary to implement a smoothing buffer at NAR.
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Example 2
---------
+-----+ +-----+
| PAR | -------------->| NAR |
+-----+ +-----+
|
|
|
v
+-----+
| MN |
+-----+ - - - - - >
Movement
When the MAP/HA/PAR are one entity (as considered in [1]), the
following calculations apply.
A = anticipation time (F-BU is sent from MN at time t-A, where t is
the time when the MN actually hands-off at L2)
h = handover time (L2 only)
d = MN to AR delay (assume constant as MN moves ARs)
L = PAR to NAR delay
As previously, the following must be true for the simultaneous
bindings to yield zero L3 disruption:
(A+h)>=(d+L+d)
=> (A+h)>=(2d+L)
The Anticipation time (A) is important and needs to be calculated
appropriately for the link-layer being used. Depending on the L2 this
may need engineering to synchronise the L2 and L3 handovers.
Once the MN has moved to NAR, it will be receiving traffic delayed by
an amount L with respect to when it was attached to PAR. To smooth
this delay variation (jitter), which may be a problem for real-time
services, it may be necessary to implement a smoothing buffer at NAR.
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