Network Working Group H. Yokota
Internet-Draft KDDI R&D Laboratories, Inc.
Expires: November 5, 2006 G. Dommety
Cisco Systems, Inc.
May 4, 2006
Mobile IPv6 Fast Handovers for 3G CDMA Networks
draft-ietf-mipshop-3gfh-00.txt
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This Internet-Draft will expire on November 5, 2006.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
Mobile IPv6 is designed to maintain its connectivity while moving
from one network to another. It is adopted in 3G CDMA networks as a
way to maintain connectivity when the mobile node moves between
access routers. However, this handover procedure requires not only
movement detection, but also the acquisition of a new care-of address
and the sending of a binding update message to the home agent before
the traffic begins to direct to the new location. During this
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period, packets destined for the mobile node will be lost, which may
not be acceptable for real-time application such as Voice over IP
(VoIP) or video telephony. This document specifies fast handover
methods and selective bi-casting methods in the 3G context in order
to reduce latency and packet loss during handover.
Table of Contents
1. Requirements notation . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Network reference model for Mobile IPv6 over 3G networks . . . 6
5. Fast handover procedures . . . . . . . . . . . . . . . . . . . 10
5.1 Predictive fast handover . . . . . . . . . . . . . . . . . 10
5.2 Reactive fast handover . . . . . . . . . . . . . . . . . . 14
6. Selective bi-casting . . . . . . . . . . . . . . . . . . . . . 18
6.1 Bi-casting by the PAR to the MN with multiple
interfaces (A-1) . . . . . . . . . . . . . . . . . . . . . 20
6.2 Bi-casting by the HA to the MN with multiple
interfaces (A-2) . . . . . . . . . . . . . . . . . . . . . 22
6.3 Bi-casting by the PAR to the MN with a single
interface (B-1) . . . . . . . . . . . . . . . . . . . . . 23
6.4 Bi-casting by the HA to the MN with a single interface
(B-2) . . . . . . . . . . . . . . . . . . . . . . . . . . 24
6.5 Message Format . . . . . . . . . . . . . . . . . . . . . . 26
6.5.1 Simultaneous bindings flag and the HI message
indication flag extensions to (F)BU message . . . . . 26
6.5.2 New mobility option for bi-casting lifetime . . . . . 27
6.5.3 New status code for simultaneous bindings . . . . . . 28
6.5.4 New Option for access-specific handover information . 28
6.6 MN and AR/HA operations . . . . . . . . . . . . . . . . . 29
7. Security Considerations . . . . . . . . . . . . . . . . . . . 30
8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 31
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 32
Intellectual Property and Copyright Statements . . . . . . . . 33
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1. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [1].
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2. Introduction
Mobile IPv6 allows mobile nodes (MNs) to maintain persistent IPv6
addresses while roaming around in IPv6 networks and it is adopted in
3G CDMA networks for handing off between different access provider
networks [2]. During handover, however, the mobile node (MN) needs
to switch the radio networks, to obtain a new Care-of Address (CoA)
and to re-register with the home agent (HA), which causes a
communication disruption. This is not desirable for real-time
applications such as VoIP and video telephony. To reduce this
disruption time or latency, a fast handover protocol for Mobile IPv6
[3] is proposed. In this proposal, there are two modes called
"predictive" fast handover and "reactive" fast handover. This
document first specifies how these fast handover modes can be applied
in the 3G context and shows that several Mobile IPv6 bootstrapping
procedures can be omitted. If the MN has more than one network
interface, even smoother handover can be realized by transmitting
packets destined for the MN to both networks, where the mobile node
resides and will move to. This document defines this mechanism as
selective bi-casting and shows several use cases with their handover
procedures.
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3. Terminology
This document refers to [2] for Mobile IPv6 fast handover
terminology. Terms that first appear in this document are defined
below:
Forward Pilot Channel:
A portion of the Forward Channel that carries the pilot. The
Forward Channel is a portion of the physical layer channels
transmitted from the access network to the MN.
Sector:
A typical cell divides its coverage area into several sectors.
In 3G CDMA systems, each sector uses a different PN (Pseudo
Noise) code offset.
Home Link Prefix (HLP):
The prefix address assigned to the home link where the MN should
send the binding update message. This is one of the bootstrap
parameters for the MN.
Packet Data Serving Node (PDSN):
An entity that routes MN originated or MN terminated packet data
traffic. A PDSN establishes, maintains and terminates link-
layer sessions to MNs [2]. A PDSN can be the access router in
the visited access provider network.
Selective bi-casting:
Short-term bi-casting that is performed during the period of
handover to reduce delay and packet loss. Selective bi-casting
utilizes the timing of the predictive fast handover mode and not
only the home agent but also the access routers may be involved
for bi-casting and/or buffering packets.
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4. Network reference model for Mobile IPv6 over 3G networks
Figure 1 shows a simplified reference model of the Mobile IP enabled
3G networks. The home agent (HA) of the mobile node (MN) resides in
the home IP network and the MN roams within or between the access
provider network(s). Prior to the Mobile IPv6 registration, the MN
establishes a link-layer connection with the access router (AR),
which is also called PDSN (Packet Data Serving Node) in 3G networks,
of the access provider network to which the MN is attached. When the
MN moves from one AR (PDSN) to another, the link-layer connection is
re-established and a Mobile IPv6 handover is performed. Those ARs
reside in either the same or different access provider network(s).
The figure shows the situation, where the MN moves from the PAR
(previous access router) to the NAR (new access router) and in the
case of 3G networks, the PAR and the NAR are equivalent to the oPDSN
(old PDSN) and the nPDSN (new PDSN).
Home IP Network
+........................+
. +--------+ +--------+ .
. | HA |--| AAA | .
. +--------+ +--------+ .
+../......\..............+
/ \
Access Provider Network(s)
+.............+ +.............+
. +---------+ . . +---------+ .
. | PAR | . . | NAR | .
. | (oPDSN) | . . | (nPDSN) | .
. +---------+ . . +---------+ .
. | :| . . :| | .
. | :|L2link L2link:| | .
. | :| . . :| | .
. +--+ +:--+ . . +:--+ +--+ .
. |BS| |:BS| . . |:BS| |BS| .
. +--+ +:--+ . . +:--+ +--+ .
. :| . . :| .
. +----+ . . +----+ .
. | MN |---------> | MN | .
. +----+ . . +----+ .
+.............+ +.............+
Figure 1: Reference Model for Mobile IP
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In 3G CDMA networks, pilot channels transmitted by base stations
allow the MN to obtain a rapid and accurate C/I (carrier to
interference) estimate. This estimate is based on measuring the
strength of the Forward Pilot Channel or the pilot, which is
associated with a sector of a base station (BS). The MN searches for
the pilots and maintains those with sufficient signal strength in the
pilot sets. The MN sends measurement results, which include the
offsets of the PN code in use and the C/Is in the pilot sets, to
provide the access network (AN) with the estimate of sectors in its
neighborhood. There are several triggers for the MN to send those
estimates, e.g. when the strength of a pilot in the pilot sets is
higher enough than that of the current pilot, the MN sends the
estimates to the access network. If the serving access network finds
that the sector associated with the highest pilot strength belongs to
a different AR, it attempts to close the connection with the MN. The
MN then attempts to get a new traffic channel assigned in the new
access network, which is followed by establishing a new connection
with the new AR. The MN can continually search for pilots without
disrupting the data communication and a timely handover is assisted
by the network. If the air interface information can be used as a
trigger for the handover between access routers, fast and smooth
handover of Mobile IPv6 can be realized in 3G CDMA networks.
To assist the handover of the MN to the new AR, various types of
information can be considered: the pilot sets, which include the
candidates of the target sectors or BSs, the cell information where
the MN resides, the serving nodes in the radio access network and the
location of the MN if available. To identify the access network that
the MN moves to or from, the Access Network Identifiers (ANID), which
is composed of the System ID (SID), Network ID (NID) and Packet Zone
ID (PZID) can be used [4]. In this document, a collection of such
information is called "handover assist information". In 3G networks,
the link-layer address of the new access point defined in [3] may not
be available. If this is the case, the handover assist information
SHOULD be used instead.
Figure 2 shows the protocol sequence from the attachment to the
network to the Mobile IPv6 registration. After the traffic channel
is assigned, the MN first establishes a link-layer connection between
itself and the access router. As the link-layer protocol, PPP can be
considered and in this figure, a PPP handshake is depicted as an
example. Then the MN registers with the HA by sending a Binding
Update message. There are several parameters for using Mobile IPv6
such as the home address (HoA), the care-of address (CoA), the home
agent address (HA) and the home link prefix (HLP). These addresses
are required prior to sending a Binding Update and obtaining these
values is called bootstrapping. One such method is proposed in [6],
where the bootstrapping information is obtained during the link-layer
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establishment phase.
MN AR HA AAA
+-- | (PDSN) | |
| | LCP | | |
|(1) |<-------------------->| | |
| | CHAP or PAP | Access-Request/Accept |
|(2) |<-------------------->|<---------|----------------->|
| | +------------+ | | |
|(3) | | HA,HLP,HoA |<--+ | |
| | +------------+ | |
| | IPv6CP(IF-ID) | | |
|(4) |<--------|----------->| | |
(a)< +---------+ | | | |
|(5) | LL-addr |<--+ | | |
| +---------+ | | |
| | RA(prefix) | | |
|(6) |<-----|---------------| | |
| +-----+ | | | |
|(7) | CoA |<--+ | | |
| +-----+ | | |
| | DHCPv6(HA,HLP,HoA) | | |
|(8) |<-----------|-------->| | |
| +------------+ | | | |
|(9) | HA,HLP,HoA |<--+ | | |
| +------------+ | | |
*-- | | BU | |
(b) |------------------------------------>|Authentication|
| | | query/reply |
(c) | | |<------------>|
| | BA | |
(d) |<------------------------------------| |
| | | |
Figure 2: MIPv6 operation in 3G network
The procedure for the initial registration is as follows:
(a) The link-layer connection establishment and the bootstrapping
phase
(a-1) The LCP configure-request/response messages are exchanged.
(a-2) CHAP or PAP authentication is conducted.
(a-3) The bootstrapping parameters (e.g. HA, HLP or HoA) are
conveyed from the AAA and stored in the AR (PDSN).
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(a-4) Unique interface IDs are negotiated in IPv6CP.
(a-5) The MN configures its link-local address based on the
obtained interface ID.
(a-6) A router advertisement containing the prefix is received by
the MN.
(a-7) The MN configures its CoA based on the obtained prefix.
(a-8) DHCPv6 is used to obtain the bootstrap parameters such as
the HA, HLP or HoA.
(a-9) The MN configures its HoA based on the obtained parameters.
If a new HoA is provided at (a-8), the MN adopts it (stateful
auto-configuration); otherwise, the MN generates the HoA based on
the HLP and its Interface ID (stateless auto-configuration).
(b) A binding update is sent to the HA.
(c) The HA asks the AAA to authenticate the MN for the initial
registration.
(d) The binding acknowledgment is sent back to the MN.
As is shown in Figure 2, it takes a considerable amount of time to
establish a link-layer connection and all of the above sequences run
every time the MN attaches to a new access network. It is therefore
beneficial if packets on the fly to the MN are saved not only during
the time period where the MN switches to the new radio channel but
also during the time period where the MN establishes the link-layer
connection.
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5. Fast handover procedures
There are two modes defined in [3] according to the timing of sending
FBU (Fast Binding Update); one is called "predictive mode," where the
MN sends FBU and receives FBAck (Fast Binding Ack) on PAR (Previous
Access Router)'s link and the other is called "reactive mode," where
the MN sends FBU from NAR (New Access Router)'s link. In the
predictive mode, the time and place the MN hands off must be
indicated sufficiently before the time it actually happens. In
cellular systems, since handovers are controlled by the network, the
predictive mode is well applied. However, if the network is not
configured to be able to identify the new AR, to which the MN is
moving next, in a timely manner, the reactive mode is better applied.
5.1 Predictive fast handover
Figure 3 shows the predictive mode of MIPv6 fast handover operation.
When the MN finds a sector or a BS whose pilot signal is sufficiently
strong, it initiates handover according to the following sequence:
(a) A router solicitation for proxy router advertisement is sent
to the PAR.
(b) A proxy router advertisement containing the prefix in the NAR
is sent back to the MN.
(c) The MN creates an NCoA (new CoA) and sends the Fast Binding
Update (FBU) storing the NCoA to the PAR, which in turn sends the
Handover Initiate (HI) to the NAR.
(d) The NAR sends the Handover Acknowledge (HAck) back to the PAR,
which in turn sends the FBU acknowledgment (FBAck) to the MN.
(e) The PAR starts forwarding packets toward the NCoA and the NAR
captures and buffers them.
(f) The link-layer connection associated with the PAR (e.g. PPP
connection) is closed and a new traffic channel is assigned in the
new access network.
(g) The MN establishes the link-layer connection with or without
authentication.
(h) The MN sends the Fast Neighbor Advertisement (FNA).
(i) The NAR starts delivering packets to the MN.
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(j) The MN sends the BU to the HA.
(k) The HA sends the BA back to the MN.
(l) The HA starts delivering packets to the MN via the NAR.
MN PAR NAR HA AAA
| RtSolPr | | | |
(a) |------------->| | | |
| PrRtAdv | | | |
(b) |<-------------| | | |
| FBU | Hl | | |
(c) |------------->|-------------->| | |
| FBack | HAck | | |
(d) |<-------------|<--------------| | |
| |forward packets| | |
(e) | |==============>| | |
| | +-----------+ | |
| link-layer | | buffering | | |
| connection close +-----------+ | |
(f) |/-----X------\| | | |
|\-----X------/| | | |
handover | | | |
| link-layer connection establishment |
(g) |/----------------------------\|/...........................\|
|\----------------------------/|\.........................../|
| FNA | | |
(h) |----------------------------->| | |
| deliver packets | | |
(i) |<=============================| | |
| | BU | | |
(j) |-------------------------------------------->| |
| | BA | | |
(k) |<--------------------------------------------| |
| deliver packets | | |
(l) |<=============================|<=============| |
Figure 3: MIPv6 Fast handover operation (predictive mode)
It is assumed that the NAR can be identified by the PAR leveraging
the handover assist information from the MN. To perform the
predictive mode, the MN MUST send the FBU before the connection with
the current access network is closed. If the MN fails to send the
FBU before handover, it SHOULD fall back to the reactive mode. Even
if the MN successfully sends the FBU, its reception by the PAR may be
delayed for various reasons such as congestion. If the NAR receives
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the HI triggered by the delayed FBU after the reception of the FNA
((c) comes after (h)), then the NAR SHOULD send the HAck with
handover not accepted and behave as the reactive mode.
In (a), RtSolPr MUST include the MN and the New Access Point Link-
Layer Address (LLA) options according to [3]. As for the MN-LLA
option, the only available identifier is the interface ID, so it
SHOULD be used for the MN-LLA. As for the New AP-LLA, the handover
assist information may be applied. Since the LLA is assumed to be an
IEEE identifier, even if the length field of the LLA option is in
units of 8 octets, the actual length can be obtained by knowing that
the length of an IEEE identifier is 6 octets. If the interface ID of
the MN is generated in the EUI-64-based format, the MN-LLA can be
constructed from it. However, if the LLA is not well-known, the
length of the LLA becomes ambiguous. If this is the case, it is
necessary to use a new option defined in Section 6.5.4 and the
corresponding length in it.
In (b), PrRtAdv MUST include options for the LLA, IP address and
prefix of the NAR. The PAR SHOULD be able to identify the NAR from
the handover assist information provided by the MN.
There are several ways to configure a unique IP address for the MN.
If a globally unique prefix is assigned per each link as introduced
in [6], the MN can use any interface ID except that of the other peer
to create a unique IP address. If this is the case, however, the PAR
cannot provide the MN with a correct prefix for the new network since
such a prefix is selected by the NAR and provided in the router
advertisement ((a-6) in Figure 2). Still, the NCoA MUST be included
in the FBU for the PAR to resolve the IP address of the NAR, so that
the MN configures a temporary NCoA with the prefix of the NAR and the
correct NCoA MUST be assigned by the NAR. Therefore, in (c), the PAR
MUST send the HI with the S flag set when it receives the FBU from
the MN. On the other hand, if more than one MN connected to an AR
share the same prefix, each MN MUST have a unique interface ID.
Unless it is guaranteed that each MN connected to the network
including a roaming case is preconfigured with a unique interface ID,
it MUST be agreed or provided by the NAR via the HI/Hack exchange.
In [3], the FNA MUST include the LLA of the MN, but the point-to-
point link-layer connection makes it unnecessary. The only required
information is the NCoA to check if there is a corresponding buffer,
thus in (h), the function of the FNA can be realized in several ways.
o Since the establishment of the link-layer connection in (g)
indicates readiness of data communication on the MN side, the NAR
immediately checks if there is a buffer that has packets destined
for the NCoA and starts delivering if any. (elimination of FNA)
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o The FNA equivalent information can be conveyed in the phase of the
link-layer connection, e.g. by conveying the NCoA in a PPP IPCP
with vendor specific extension as defined in [7]. Only when this
message is received by the NAR, it checks if there is a buffer for
the NCoA. (L2 implementation of FNA)
o The MN sends the FNA as defined in [3] with the LLA of the MN,
which may be derived from the EUI-64 based interface ID. (standard
implementation of FNA)
When PPP IPCP option is used as the means for the L2 implementation
of FNA, it SHOULD be confirmed that the NAR supports this option,
otherwise it may cause a longer delay by the Configure-Reject
message.
The primary benefit of this mode is that the packets destined for the
MN can be buffered at the NAR, and packet loss due to handover will
be much lower than that of the normal MIPv6 operation. Regarding the
bootstrapping, the following benefits can be obtained, too:
o Since the HA, HLP and HoA are not changed during the fast
handover, bootstrapping information is not required.
o Since the NCoA including the interface ID can be obtained or
configured via the fast handover procedures, a router
advertisement is not required.
Therefore, as shown in Figure 4, bootstrapping procedures (a-3) to
(a-9) can be omitted from the standard MIPv6 operation in Figure 2.
Also, if the security policy permits, the NAR can know the MN by the
NAI in the PPP link setup and the authentication in (2) may be
omitted. Note that another authentication is conducted in the MIPv6
registration phase and presumably the same AAA is referred to.
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MN oPDSN nPDSN HA AAA
+-- | (PAR) (NAR) | |
| | LCP | | | |
| (1) |<----------------------->| | |
| | CHAP | | Access-Request/Accept |
| (2) |<----------------------->|<-------|------------->|
|+...................................+ | | |
|. | +------------+ | . | | |
|.(3)* | | HA,HLP,HoA |<------------+ | |
|. | +------------+ | . | |
|. | | . | |
|. | IPv6CP(IF-ID) | . | |
|.(4)* |<---------|------------->| . | |
(a)< . +---------+ | | | . | |
|.(5)*| LL-addr |<-+ | | . | |
|. +---------+ | | . | |
|. | | . | |
|. | RA(prefix) | . | |
|.(6)* |<---------|--------------| . | |
|. +-----+ | | | . | |
|.(7)*| CoA |<-----+ | | . | |
|. +-----+ | | . | |
|. | | . | |
|. | DHCPv6(HA,HLP,HoA) | . | |
|.(8)* |<---------|------------->| . | |
|. +-----+ | | | . | |
|.(9)*| HoA |<-----+ | | . | |
|. +-----+ | | . | |
|+...................................+ | |
*-- | | | | |
Figure 4: Procedures that can be omitted in the link-layer connection
5.2 Reactive fast handover
When the MN cannot receive the FBAck on the PAR's link or the network
does not support the predictive fast handover, the reactive fast
handover can be applied. To support the predictive fast handover,
the PAR must accurately resolve the address of the NAR from the lower
layer information such as the link-layer address of the new access
point or the base station, which is not always feasible in some
cases. To minimize packet loss in this situation, the PAR instead of
the NAR can buffer packets for the MN until the MN regains
connectivity with the NAR. The NAR obtains the information of the
PAR from the MN on the NAR's link and receives packets buffered at
the PAR. In this case, the PAR does not need to know the IP address
of the NAR or the NCoA and just waits for the NAR to contact the PAR.
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However, since the PAR needs to know when to buffer packets for the
MN, the PAR obtains the timing of buffering from the MN via the FBU
or the lower layer signaling, e.g. an indication of the release of
the connection with the MN. Details of the procedure are as follows:
(a) A router solicitation for proxy router advertisement MAY be
sent to the PAR.
(b) The proxy router advertisement MAY be sent to the MN, but the
prefix of the NAR MAY not be included.
(c) The MN sends the FBU or the access network indicates the close
of the connection with the MN by the lower layer signaling. The
PAR MAY start buffering packets destined for the PCoA.
(d) The link-layer connection associated with the PAR (e.g. PPP
connection) is closed and a new traffic channel is assigned in the
new access network.
(e) The MN establishes the link-layer connection. Since the IP
address of the MN is guaranteed to be unique, the MN SHOULD not
perform DAD
(f) The MN sends the Fast Binding Update (FBU) to the NAR either
or not being encapsulated by the Fast Neighbor Advertisement
(FNA).
(g) The NAR decapsulates the FBU if encapsulated and sends it to
the PAR.
(h) The PAR sends the Handover Initiate (HI) to the NAR with the
Code set to 1.
(i) The NAR sends the Handover Acknowledge (HAck) back to the PAR.
(j) The PAR sends the FBAck to the NAR.
(k) If the PAR buffers packets destined for the PCoA, it starts
forwarding them as well as newly arriving ones to the NAR.
(l) The NAR delivers the packets to the MN.
(m) The MN sends the BU to the HA.
(n) The HA sends the BA back to the MN.
(o) The HA starts delivering packets to the MN via the NAR.
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MN oPDSN nPDSN HA AAA
| (PAR) (NAR) | |
| RtSolPr | | | |
(a) |------------->| | | |
| PrRtAdv | | | |
(b) |<-------------| | | |
| FBU/lower-layer sig. | | |
(c) | . . . . . . >| | | |
| +-----------+ | | |
| | buffering | | | |
| +-----------+ | | |
|link-layer connection close | | |
(d) |/-----X------\| | | |
|\-----X------/| | | |
handover | | | |
| link-layer connection establishment |
(e) |/----------------------------\|/...........................\|
|\----------------------------/|\.........................../|
| FNA[FBU]/FBU | | |
(f) |----------------------------->| | |
| | FBU | | |
(g) | |<--------------| | |
| | HI | | |
(h) | |-------------->| | |
| | HAck | | |
(i) | |<--------------| | |
| | FBack | | |
(j) | |-------------->| | |
| |forward packets| | |
(k) | |==============>| | |
| deliver packets | | |
(l) |<=============================| | |
| | BU | | |
(m) |-------------------------------------------->| |
| | BA | | |
(n) |<--------------------------------------------| |
| deliver packets | | |
(o) |<=============================|<=============| |
Figure 5: MIPv6 Fast handover operation (reactive mode)
To indicate the PAR to buffer packets destined for the PCoA, in (c),
the MN SHOULD not include information on the NCoA in the FBU and the
PAR SHOULD accept it. Or, when the PAR is indicated that the session
with the MN has been closed by the lower layer signaling when the PAR
attempts to send the FBAck, the PAR MAY start buffering.
An L2-based fast handover is possible as defined in [8] by extending
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the L2 link from the previous access network to the new access
network via the PAR and the NAR. The timing of the fast handover
trigger is the same as the reactive fast handover method (without
buffering) in this section. In the case of the L2-based fast
handover, however, once the L2 link is extended to the new location,
it is maintained until the MN becomes inactive (dormant) and the link
is released. As long as the L2 link is extended, the path, on which
packets are conveyed, is not optimal in length. In the case of
Mobile IPv6 fast handover, when the new location is registered with
the HA, the packets are directed to the NAR.
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6. Selective bi-casting
If the MN has the capability to receive more than one radio signal,
even smoother handover can be realized. This situation happens when,
for instance, the MN can receive multiple channels of the same radio
system at the same time, or the MN has multiple interfaces of
different radio systems. Especially, when the MN is running a real-
time and interactive application, long-time buffering at the PAR or
NAR is not always beneficial for the MN. If this is the case, it
will be helpful to deliver packets destined for the MN via both the
old and new points of attachment at the same time during handover.
Even if the MN has only a single interface, the above function has
some benefits when the timing of handover cannot be acquired
precisely in advance. This type of use case was originally proposed
in [10]. Mobile IPv4 allows simultaneous bindings [9] and bi-casting
is realized by retaining the old care-of address in the binding cache
and sending packets destined for the MN towards both the old and new
care-of addresses. Since bi-casting consumes double the network
resources, it must be limited to smooth handover. In this document,
bi-casting used for a short period of time for smooth handover is
called "selective bi-casting." Figure 6 shows that the simultaneous
bindings and bi-casting are performed at the PAR, which copies
packets destined for the MN and transmits them not only to the old
point of attachment but also to the new point of attachment via the
NAR. The above operation is more effective when the predictive fast
handover is applied because in the case of the reactive fast
handover, all the actions are taken after the MN has moved to the new
location. By that time, it is not necessary to deliver packets to
the old point of attachment. As another scenario, Figure 7 shows
that simultaneous bindings are performed at the HA. This scenario is
likely to happen when the MN is connected to multiple different
networks at the same time. Also, if the access networks in Figure 1
are operated by different providers, it may be difficult for the ARs
in these networks to cooperate with each other. In this case as
well, the HA must handle simultaneous bindings and bi-casting.
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+----------+
| HA |
+----------+
/
/
+------/-+ +--------+
| PAR --|------|-- NAR |
+------|-+ +-|------+
| |
| |
+----|+ +|----+
| BS || || BS |
+----|+ +|----+
\ /
\ | /
> |<
+------+
| MN |-->
+------+
Figure 6: Selective bi-casting scenario 1
+----------+
| HA |
+----------+
/ \
/ \
+------/-+ +-\------+
| PAR | | | | NAR |
+------|-+ +-|------+
| |
| |
+----|+ +|----+
| BS || || BS |
+----|+ +|----+
\ /
\ | /
> |<
+------+
| MN |-->
+------+
Figure 7: Selective bi-casting scenario 2
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From the above observations, selective bi-casting can be categorized
from the following viewpoints:
[No. of interfaces on the MN]
(A) multiple interfaces
(B) single interface
[the node where bi-casting is performed]
(1) at the PAR
(2) at the HA
The goal of selective bi-casting is to reduce packet loss and the
following design principles are considered:
o if the MN has multiple interfaces, there is no need for buffering
in the network and,
o buffering should be done at the node that is as close to the MN as
possible (the PAR or NAR rather than the HA).
According to the above principles, the procedures for each
combination are described below.
6.1 Bi-casting by the PAR to the MN with multiple interfaces (A-1)
As shown in Figure 8, the MN has two interfaces with the link-layer
addresses: LLA1 and LLA2. This case is typical of handover between
different radio transmission technologies. Details of the sequence
are as follows:
(a) The interface with LLA1 acquires the global IP address PCoA.
(b) The MN sends the BU to the HA from the link with PCoA with S=0
and N=0 (the default behavior), which are defined in
Section 6.5.1, and receives the BA from the HA.
(c) The MN receives packets destined for PCoA from the link with
LLA1 via the PAR.
(d) When the MN detects that handover is imminent, it opens the
interface with LLA2 and acquires the global IP address NCoA.
(e) The MN inserts NCoA into the FBU and sends it with S=1 and N=0
from the link with PCoA to the PAR.
(f) The MN receives the FBack from the PAR.
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(g) The PAR sends packets destined for PCoA directly to the link
with LLA1 and also forwards them to the NAR. The forwarded
packets are received by the MN on the link with NCoA.
(h) When the MN is ready to use the link with LLA2 as the primary
one, it sends the BU to the HA with S=0 and N=0.
(i) The HA starts sending packets to NCoA.
As shown in this example, since the NCoA is assigned on the link with
LLA2 and the NAR has the neighbor cache entry (NCE) for the NCoA,
there is no need to send the HI from the PAR. To suppress sending
the HI, a new flag 'N' is defined in the FBU. Also, to indicate the
PAR to bi-cast packets, a new flag 'S' is defined in the FBU. If the
MN requests selective bi-casting and the valid NCoA has been
assigned, the MN SHOULD send the FBU to the PAR with the S flag set
and the N flag unset requesting bi-casting but not sending the HI.
If the PAR receives this FBU, it SHOULD not send the HI to the NAR.
+..MN..+
LLA1 LLA2 PAR NAR HA
| | | | |
|link-layer connection | | |
(a) |/--------------------\| | |
|\--------------------/| | |
| | BU(S=0,N=0)/BA |
(b) |<---------------------------------------------------------->|
| | | | |
(c) |<=====================|<====================================|
| | link-layer connection | |
(d) | |/--------------------------------\| |
| |\--------------------------------/| |
| FBU(S=1,N=0) | | |
(e) |--------------------->| | |
| | FBack | | |
(f) |<---------------------| | |
| | | | |
(g) |<====================#|<====================================|
| | #=================>#| |
| |<================================#| |
| | | | |
| | BU(S=0,N=0)/BA | |
(h) | |<--------------------------------------------------->|
: | | | |
(i) : |<=================================|<=================|
Figure 8: Combination (A-1)
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6.2 Bi-casting by the HA to the MN with multiple interfaces (A-2)
This case happens when the access network where the PAR belongs and
the one where the NAR belongs are administrated by different
providers or are different access systems. The cellular network is
typically a closed network and the ARs can access external nodes only
via the HA. If this is the case, the PAR and the NAR cannot directly
communicate with each other. Details of the sequence of this case
are shown in Figure 9 and below:
(a) through (d) are the same as those in the case (A-1).
(e) The MN sends the BU to the HA from the link with NCoA with S=1
and N=0, which means that the MN requests the HA to bi-cast but
not to send the HI.
(f) The HA forwards packets destined for the MN to both the PAR
and the NAR. Those packets can be received by the MN on either
one or both of the links.
(g) When the bi-casting lifetime, which is defined in
Section 6.5.2, is expired, packets are forwarded only to the NAR.
+..MN..+
LLA1 LLA2 PAR NAR HA
| | | | |
|link-layer connection | | |
(a) |/--------------------\| | |
|\--------------------/| | |
| | BU(S=0,N=0)/BA | |
(b) |<---------------------------------------------------------->|
| | | | |
(c) |<=====================|<====================================|
| | link-layer connection | |
(d) | |/--------------------------------\| |
| |\--------------------------------/| |
| | | | |
| | BU(S=1,N=0)/BA |
(e) | |<--------------------------------------------------->|
| | | | |
(f) |<=====================|<====================================|
| |<=================================|<=================|
: | | | |
(g) : |<=================================|<=================|
Figure 9: Combination (A-2)
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6.3 Bi-casting by the PAR to the MN with a single interface (B-1)
This case is typical of handover with the same access system within
the same provider network. Details of the sequence of this case are
shown in Figure 10 and below:
(a) through (c) are the same as those in the case (A-1).
(d) The MN sends the RtSolPr to the PAR.
(e) The MN receives the PrRtAdv with a valid NCoA from the PAR.
(f) The MN sends the FBU with S=1 and N=1 to the PAR.
(g) The PAR sends the HI to the NAR. The U flag in the HI MUST be
set when requesting buffering at the NAR.
(h) The PAR receives the HAck with a valid NCoA from the NAR.
(i) The PAR sends the FBack both to the MN and the NAR.
(j) The PAR sends packets destined for PCoA directly to the MN and
also forwards them to the NAR. The NAR buffers the received
packets when the U flag in the HI is accepted.
(k) The MN moves to the new access network and configures NCoA by
establishing the link-layer connection. At this point, the MN can
receive the forwarded packets from the NAR.
(l) The MN sends the FNA to the NAR.
(m) The NAR starts sending the buffered packets to the MN.
(n) The MN sends the BU with S=0 and N=0 to the HA. The HA then
returns the BA to the MN.
(o) The MN starts receiving packets only via the NAR.
In (g), it is necessary that the PAR sends the HI to the NAR to
create the neighbor cache entry (NCE) for the NCoA on the NAR. Also,
by setting the U flag in the HI, the PAR requests buffering to the
NAR.
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MN PAR NAR HA
| | | |
|link-layer connection | | |
(a) |/--------------------\| | |
|\--------------------/| | |
| BU(S=0,N=0)/BA |
(b) |<---------------------------------------------------------->|
| | | |
(c) |<=====================|<====================================|
| RtSoIPr | | |
(d) |--------------------->| | |
| PrRtAdv | | |
(e) |<---------------------| | |
| FBU(S=1,N=1) | | |
(f) |--------------------->| | |
| | HI(U=1) | |
(g) | |----------------->| |
| (new link) | HAck | |
(h) | | |<-----------------| |
| | FBAck | FBack | |
(i) |<---------------------|----------------->| |
| | | | |
(j) |<====================#|<====================================|
|.....>| #==================>|[buffering] |
: | | | |
: | link-layer connection | |
(k) : |/--------------------------------\| |
: |\--------------------------------/| |
: | FNA | |
(l) : |--------------------------------->| |
: | | |
(m) : |<=================================| |
: | | BU(S=0,N=0)/BA | |
(n) : |<--------------------------------------------------->|
: | | | |
(o) : |<=================================|<=================|
Figure 10: Combination (B-1)
6.4 Bi-casting by the HA to the MN with a single interface (B-2)
This case is typical of handover with the same access system between
different provider networks. Details of the sequence of this case
are shown in Figure 11 and below:
(a) through (d) are the same as those in the case (B-1).
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(e) The MN receives the PrRtAdv without a valid NCoA from the PAR.
(f) The MN sends the BU with S=1 and N=1 to the HA. The BU MUST
contain an alternate CoA set to the NCoA.
(g) The HA sends the HI with U=1 to the NAR. The MN's LLA may not
be included in the HI if such information is not available at the
HA. If this is the case, the U flag in the HI MUST be set.
(h) The HA receives the HAck from the NAR.
(i) The HA sends the BA to the MN.
(j) The HA sends packets destined for the MN to both the PAR and
the NAR. The NAR buffers them if the U flag in the HI is
accepted.
(k) The MN moves to the new access network and configures NCoA by
establishing the link-layer connection. At this point, the MN can
receive the forwarded packets from the NAR.
(l) The MN sends the FNA to the NAR. If the NCE for NCoA is
incomplete on the NAR, the NAR extracts the MN's LLA from the FNA
and completes it.
(m) The NAR starts sending the buffered packets to the MN.
(n) The MN sends the BU to the HA with S=0 and N=0. The HA then
returns the BA to the MN.
(o) The MN starts receiving packets only via the NAR.
In (e), by receiving the PrRtAdv without a valid NCoA (the Code is
typically 2), the MN judges that the PAR does not have information on
the NAR or can not send the HI directly to the NAR. Then the MN
sends the BU to the HA.
In (g), it is necessary that the HA sends the HI to the NAR to make
the NCE for the NCoA on the NAR.
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MN PAR NAR HA
| | | |
|link-layer connection | | |
(a) |/--------------------\| | |
|\--------------------/| | |
| BU(S=0,N=0)/BA |
(b) |<---------------------------------------------------------->|
| | | |
(c) |<=====================|<====================================|
| RtSoIPr | | |
(d) |--------------------->| | |
| PrRtAdv | | |
(e) |<---------------------| | |
| | BU(S=1,N=1) | |
(f) |----------------------------------------------------------->|
| | | HI(U=1) |
(g) | | |<-----------------|
| | | HAck |
(h) | (new link) | |----------------->|
| | | BA | |
(i) |<-----------------------------------------------------------|
| | | | |
(j) |<=====================|<====================================|
| | [buffering]|<=================|
|.....>| link-layer connection | |
(k) : |/--------------------------------\| |
: |\--------------------------------/| |
: | FNA | |
(l) : |--------------------------------->| |
: | | |
(m) : |<=================================| |
: | | BU(S=0,N=0)/BA | |
(n) : |<--------------------------------------------------->|
: | | | |
(o) : |<=================================|<=================|
Figure 11: Combination (B-2)
6.5 Message Format
6.5.1 Simultaneous bindings flag and the HI message indication flag
extensions to (F)BU message
When the MN requests simultaneous bindings and bi-casting to the HA
or the PAR, the MN sets the newly defined simultaneous bindings flag
in the Binding Update (BU) [11] or FBU, respectively. To suppress
sending the HI when it is unnecessary, the HI message indication flag
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is defined in the (F)BU as well.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence # |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|A|H|L|K|S|N| Reserved | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
S: Simultaneous bindings. If the 'S' bit is set, the mobile
node is requesting that the HA or the PAR retain its prior
mobility bindings and bi-cast packets destined for the MN.
N: Indication to send the HI. On the condition that the 'S'
bit is set, if the 'N' bit is set, the receiver (the PAR or
the HA) SHOULD send the HI to the NAR, otherwise the
receiver SHOULD not send the HI. If the 'S' bit is not
set, the 'N' bit MUST be ignored.
If the 'S' bit is supported, the 'N' MUST also be supported. When
S=0 and N=0, the AR and the HA perform according to [3] and [11],
respectively (the default behavior).
6.5.2 New mobility option for bi-casting lifetime
The MN may request how long the HA or the PAR should retain the
simultaneous bindings (and therefore bi-casting) by attaching the
following mobility option in the binding update 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 = T.B.D | Length = 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bi-casting Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Bi-casting Lifetime:
The time period when the PAR or the HA retains the previous
CoA (PCoA).
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6.5.3 New status code for simultaneous bindings
If the AR or the HA receives more (fast) binding update messages with
different CoAs for the same HoA than it can support, it should send a
binding acknowledgement message with the following status code:
Status T.B.D.
too many simultaneous mobility bindings
6.5.4 New Option for access-specific handover information
If the lower layer information of the new point of attachment is not
represented as the Link-Layer Address, the following option SHOULD be
used. The primary purpose of this option is to convey the handover
assist information described in Section 4.
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 | Option-Code | AS-Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AS-Value...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type T.B.D.
Length The size of this option in 8 octets including the
Type, Length, Option-Code and AS-Length fields.
Option-Code Indicates the particular type of access-specific
information. This value is administrated by the
vendor or organization that uses this option.
AS-Length The size of the AS-Value field in octets.
AS-Value Zero or more octets of access-specific information
data.
This option MUST be understood by the sender (typically the MN) and
the receiver (typically the AR or the HA). If nodes in between do
not support this option, they SHOULD treat this option as opaque and
MUST not drop it.
Depending on the size of the AS-Value field, appropriate padding MUST
be used to ensure that the entire option size is a multiple of 8
octets. The AS-Length is used to disambiguate the size of the AS-
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Value.
6.6 MN and AR/HA operations
In order to enable bi-casting, the MN sends a BU or FBU by setting
the 'S' flag to the HA or the PAR, respectively. When the PAR/HA
allows bi-casting, a successful (F)Back is returned and bi-casting is
started. The MN can request a desirable bi-casting lifetime to the
PAR/HA with the bi-casting lifetime option in the (F)BU. If the
requested lifetime is acceptable, the PAR/HA sends an (F)Back with
the accepted bi-casting lifetime, which is determined by the policies
of the PAR/HA. When bi-casting is performed at the HA, the MN is
likely to receive duplicate packets from multiple interfaces. In the
case of audio or video applications, it may be necessary to
synchronize the bi-cast flows coming from different access networks
so that the user does not have to experience a communication
disruption. This may take longer than just the time for a handover.
If this is the case, the MN may request a longer bi-cast lifetime.
After the flows are synchronized and successfully switched on the
application level, the MN may explicitly de-register the PCoA by
sending an (F)BU with the lifetime field being zero. On the side of
the PAR/HA, the maximum value of the bi-casting lifetime must be
configured and even if the MN does not request a bi-casting lifetime
or does not successfully de-register the PCoA, it is deleted after
the maximum value of the bi-casting lifetime elapses.
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7. Security Considerations
The security considerations for Mobile IPv6 fast handover are
described in [3]. When a 3G network is considered, the PAR and the
NAR have a trusting relationship and the links between them and those
between the ARs and the MN are usually secured. When the MN is
authenticated at the phase of the link-layer connection, the AR can
distinguish the authenticated users from the others. This may be not
the case, however, if the access networks are operated by different
providers.
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8. Conclusions
The handover performance of the standard Mobile IPv6 is not
sufficient for real-time communications that are not resilient to
packet loss. The Mobile IPv6 fast handover methods are effective for
these applications. This document specifies how these methods can be
applied to 3G networks. By introducing fast handover, not only are
more packets saved which otherwise would be dropped, but also some of
the bootstrapping parameters can be omitted at the link establishment
phase, which can expedite the handover process. For interactive
real-time applications, in which excessive buffering is
inappropriate, selective bi-casting is also proposed. By retaining
the PCoA and the NCoA in the binding cache, packets destined for the
MN are transmitted to both the old and new points of attachment at
the same time, whereby the applications on the MN can choose which
flow to adopt considering the media continuity.
9. References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] 3GPP2 TSG-A, "Interoperability Specification (IOS) for cdma2000
Access Network Interfaces Part 1 Overview", A.S0011-C v.1.0,
February 2005.
[3] Koodli, R., Ed., "Fast Handover for Mobile IPv6", RFC 4068,
July 2005.
[4] 3GPP2 TSG-A, "3GPP2 Access Network Interfaces Interoperability
Specification", A.S0001-A v.2.0, June 2001.
[5] 3GPP2 TSG-X, "cdma2000 Wireless IP Network Standard:
Introduction", X.S0011-001-D v.1.0, February 2006.
[6] 3GPP2 TSG-X, "cdma2000 Wireless IP Network Standard: Simple IP
and Mobile IP services", X.S0011-002-D v.1.0, February 2006.
[7] Simpson, W., "PPP Vendor Extensions", RFC 2153, May 1997.
[8] 3GPP2 TSG-X, "cdma2000 Wireless IP Network Standard: Packet
Data Mobility and Resource Management", X.S0011-003-D v.1.0,
February 2006.
[9] Perkins, C., Ed., "IP Mobility Support for IPv4", RFC 3344,
August 2002.
[10] Malki, K., "Simultaneous Bindings for Mobile IPv6 Fast
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Handovers", draft-elmalki-mobileip-bicasting-v6-05.txt,
October 2003.
[11] Johnson, D., "Mobility Support in IPv6", RFC 3775, June 2004.
Authors' Addresses
Hidetoshi Yokota
KDDI R&D Laboratories, Inc.
2-1-15 Ohara, Fujimino
Saitama, 356-8502
JP
Phone: +81 49 278 7894
Fax: +81 49 278 7510
Email: yokota@kddilabs.jp
Gopal Dommety
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
US
Phone: +1 408 525 1404
Email: gdommety@cisco.com
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