Mobile IP Working Group Rajeev Koodli, Editor
INTERNET DRAFT Nokia Research Center
10 October 2003
Fast Handovers for Mobile IPv6
draft-ietf-mipshop-fast-mipv6-00.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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This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Abstract
Mobile IPv6 enables a Mobile Node to maintain its connectivity to
the Internet when moving from an Access Router to another, a process
referred to as handover. During handover, there is a period when
the Mobile Node is unable to send or receive packets both due to
link switching delay and IP protocol operations. This ``handover
latency'' resulting from standard Mobile IPv6 procedures, namely
movement detection, new Care of Address configuration and Binding
Update, is often unacceptable to real-time traffic such as Voice
over IP. Reducing the handover latency could be beneficial to non
real-time, throughput-sensitive applications as well. This document
specifies enhancements to reduce the handover latency due to standard
Mobile IPv6 procedures. This document does not address improving the
link switching latency.
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Contents
Status of This Memo i
Abstract i
1. Introduction 2
2. Terminology 2
3. Protocol Overview 3
3.1. Addressing the Handover Latency . . . . . . . . . . . . . 3
3.2. Protocol Operation . . . . . . . . . . . . . . . . . . . 6
3.3. Protocol Operation of Network-initiated Handover . . . . 8
4. Protocol Details 9
5. Other Considerations 13
5.1. Handover Capability Exchange . . . . . . . . . . . . . . 13
5.2. Determining New Care of Address . . . . . . . . . . . . . 13
5.3. Packet Loss . . . . . . . . . . . . . . . . . . . . . . . 13
5.4. DAD Handling . . . . . . . . . . . . . . . . . . . . . . 14
5.5. Fast or Erroneous Movement . . . . . . . . . . . . . . . 14
6. Message Formats 15
6.1. New Neighbor Discovery Messages . . . . . . . . . . . . . 15
6.1.1. Router Solicitation for Proxy . . . . . . . . . . 15
6.1.2. Proxy Router Advertisement (PrRtAdv) . . . . . . 17
6.2. Inter-Access Router Messages . . . . . . . . . . . . . . 20
6.2.1. Handover Initiate (HI) . . . . . . . . . . . . . 20
6.2.2. Handover Acknowledge (HAck) . . . . . . . . . . . 22
6.3. New Mobility Header Messages . . . . . . . . . . . . . . 24
6.3.1. Fast Binding Update (FBU) . . . . . . . . . . . . 24
6.3.2. Fast Binding Acknowledgment (FBack) . . . . . . . 25
6.3.3. Fast Neighbor Advertisement (FNA) . . . . . . . . 27
6.4. New ICMP Options . . . . . . . . . . . . . . . . . . . . 28
6.4.1. IP Address Option . . . . . . . . . . . . . . . . 28
6.4.2. New Router Prefix Information Option . . . . . . 29
6.4.3. Link-layer Address (LLA) . . . . . . . . . . . . 30
6.4.4. Neighbor Advertisement Acknowledgment (NAACK) . . 31
7. Configurable Parameters 32
8. Security Considerations 32
9. Contributors 33
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10. Acknowledgments 33
A. Change Log 34
B. Contact Information 34
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1. Introduction
Mobile IPv6 [3] describes the protocol operations for a mobile node
to maintain connectivity to the Internet during its handover from one
access router to another. These operations broadly involve movement
detection, IP address configuration, and location update phases.
The combined handover latency could be (especially) appreciable
for real-time applications. Throughput-sensitive applications can
also benefit from reducing this latency. This document describes a
protocol to reduce the handover latency.
This specification addresses the following problem: how to allow
a mobile node to send packets as soon as it detects a new subnet
link, and how to deliver packets to a mobile node as soon as its
attachment is detected by the new access router. The protocol
defines IP protocol messages necessary for its operation on any link
technology. It does this without depending on specific link-layer
features while allowing link-specific customizations. By definition,
this specification considers handovers that inter-work with Mobile
IP: once attached to its new access router, a MN engages in Mobile
IP operations including Return Routability [3]. Hence, there are no
special requirements for a mobile node to behave differently with
respect to its standard Mobile IP operations.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", "OPTIONAL", and
"silently ignore" in this document are to be interpreted as described
in RFC 2119 [1].
The following terminology and abbreviations are used in this
document. The reference handover scenario is illustrated in
Figure 1.
Mobile Node (MN)
A Mobile IPv6 host
Access Point (AP)
A Layer 2 device connected to a subnet that offers
wireless connectivity to a MN.
Access Router (AR)
The MN's default router
Previous Access Router (PAR)
The MN's default router prior to its handover
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New Access Router (NAR)
The MN's anticipated default router subsequent to its
handover
Previous CoA (PCoA)
The MN's Care of Address valid on PAR. The MN may reuse
this address while attached to NAR until it finishes
Mobile IP operations.
New CoA (NCoA)
The MN's Care of Address valid on NAR
Handover
A process of terminating existing connectivity and
obtaining new IP connectivity.
Router Solicitation for Proxy (RtSolPr)
A message from the MN to the PAR requesting information
for a potential handover
Proxy Router Advertisement (PrRtAdv)
A message from the PAR indicating a MN to undergo
handover
Fast Binding Update (FBU)
A message from the MN instructing its PAR to redirect its
traffic (towards NAR)
Fast Binding Acknowledgment (FBACK)
A message from the PAR in response to FBU
Fast Neighbor Advertisement (FNA)
A message from the MN to the NAR to announce attachment
and to confirm use of NCoA when the MN has not received
FBACK
Handover Initiate (HI)
A message from the PAR to the NAR to initiate handover
Handover Acknowledge (HACK)
A message from the NAR to the PAR as a response to HI
3. Protocol Overview
3.1. Addressing the Handover Latency
The ability to quickly send packets from a new subnet link depends
on the ``IP connectivity'' latency, which in turn depends on the
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v +------------+
+-+ | Previous | <
| | ---------- | Access | ------ > ----\
+-+ | Router | < \
MN | (PAR) | \
| +------------+ +---------------+
| ^ IP | Correspondent |
| | Network | Node |
V | +---------------+
v /
v +------------+ /
+-+ | New | < /
| | ---------- | Access | ------ > ----/
+-+ | Router | <
MN | (NAR) |
+------------+
Figure 1: Reference Scenario for Handover
movement detection latency and the new CoA configuration latency.
Once a MN is IP-capable on the new subnet link, it can send Binding
Update to its Home Agent and one or more correspondents. Once
its correspondents successfully process the Binding Update, which
involves the Return Routability procedure, the MN can receive packets
at new CoA. So, the ability to receive packets from correspondents
directly at its new CoA depends on the Binding Update latency.
The protocol enables a MN to quickly detect that it has moved to
a new subnet by providing the new access point and the associated
subnet prefix information when the MN is still connected to its
current subnet (i.e., PAR in Figure 1). For instance, a MN may
discover available access points using link-layer specific mechanisms
(e.g., a ``scan'' in WLAN) and then request subnet information
corresponding to one or more of those discovered access points.
The MN may do this after performing router discovery. The MN may
also do this at any time during its sojourn on its current point of
attachment. The result of resolving an identifier associated with
an access point is a [AP-ID, AR-Info] tuple, which a MN can use in
readily detecting movement: when attachment to an access point
with AP-ID takes place, the MN knows the corresponding new router's
co-ordinates including its prefix, IP address and MAC address. The
``Router Solicitation for Proxy (RtSolPr)'' and ``Proxy Router
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Advertisement (PrRtAdv)'' messages are used for aiding in movement
detection.
Through the RtSolPr and PrRtAdv messages, the MN also formulates
a prospective new CoA (NCoA), again when still present on PAR's
link. Hence, the latency due to new prefix discovery subsequent to
handover is eliminated. Furthermore, this prospective address can
be used immediately after attaching to the new subnet link (i.e.,
NAR's link) when the MN has received a ``Fast Binding Acknowledgment
(FBack)'' message prior to its movement. In the event it moves
without receiving an FBack, the MN can still start using NCoA after
announcing its attachment through a ``Fast Neighbor Advertisement
(FNA)'' message; NAR responds to FNA in case the tentative address
is already in use. In this way, NCoA configuration latency is
reduced. Under some limited conditions where the probability of
address collision is considered insignificant, it may be possible
to use NCoA immediately after attaching to the new link. Even so,
all implementations MUST implement the mechanism specified in this
document to avoid potential address conflicts and SHOULD support
them.
In order to reduce the Binding Update latency, the protocol specifies
a tunnel establishment typically between the Previous CoA (PCoA) and
NCoA. A MN sends a ``Fast Binding Update'' message to its Previous
Access Router to establish this tunnel. When feasible, the MN SHOULD
send FBU from PAR's link. Otherwise, it should send it immediately
after detecting attachment to NAR. Subsequent sections describe
the protocol mechanics. In any case, the net result is that PAR
begins tunneling packets arriving for PCoA to NCoA. Such a tunnel
would remain active until the MN completes Binding Update with its
correspondents. In the opposite direction, the MN SHOULD reverse
tunnel packets to PAR, again until it completes Binding Update. And,
PAR SHOULD forward the inner packet in the tunnel to its destination
(i.e., to the MN's correspondent). Such a reverse tunnel ensures
that packets containing PCoA as source IP addres are not dropped
due to ingress filtering. Readers may observe that even though the
MN is IP-capable on the new link, it cannot use NCoA directly with
its correspondents without the correspondents first establishing a
binding cache entry (for NCoA).
Setting up a tunnel alone does not ensure that the MN receives
packets as soon as attaching to a new subnet link, unless NAR can
detect the MN's presence. Neighbor discovery operation involving
a neighbor's address resolution (i.e., Neighbor Solicitation and
Neighbor Advertisement) typically result in considerable delay,
sometimes lasting multiple seconds. For instance, when arriving
packets trigger NAR to send Neighbor Solicitation before the MN
attaches, subsequent re-transmissions of address resolution are
separated by a default time of one second each. In order to
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circumvent this delay, a MN announces its attachment through the FNA
message that allows NAR to consider MN to be reachable. If there
is no existing entry, FNA allows NAR to create one. If NAR already
has an entry, FNA updates the entry while taking potential address
conflicts into consideration. Through tunnel establishment for PCoA
and fast advertisement, the protocol provides expeditious forwarding
of packets to the MN.
The protocol also provides the following important functionalities.
The access routers can exchange messages to confirm that a proposed
NCoA is acceptable. For instance, when a MN sends FBU from PAR's
link, FBack can be delivered after NAR considers NCoA acceptable to
use. This is especially useful when stateful address configuration
is used. The NAR can also rely on its trust relationship with PAR
before providing forwarding support for the MN. That is, it may
create a forwarding entry for NCoA subject to ``approval'' from
PAR which it trusts. Finally, the access routers could transfer
network-resident contexts, such as access control, QoS, header
compression, in conjunction with handover. For all these operations,
the protocol provides ``Handover Initiate (HI)'' and ``Handover
Acknowledge (HAck)'' messages. Both of these messages MUST be
implemented and SHOULD be supported.
3.2. Protocol Operation
The protocol begins when a MN sends RtSolPr to its access router
to resolve one or more Access Point Identifiers to subnet-specific
information. In response, the access router (e.g., PAR in Figure 1)
sends a PrRtAdv message which contains one or more [AP-ID, AR-Info]
tuples. The MN may send RtSolPr at any convenient time, for instance
as a response to some link-specific event (a ``trigger'') or simply
after performing router discovery. However, the expectation is that
prior to sending RtSolPr, the MN has discovered the available APs
by link-specific methods. The RtSolPr and PrRtAdv messages do not
establish any state at the access router, and their packet formats
are defined in Section 6.1.
With the information provided in the PrRtAdv message, the MN
formulates a prospective NCoA and sends an FBU message. The purpose
of FBU is to authorize PAR to bind PCoA to NCoA, so that arriving
packets can be tunneled to the new location. The FBU SHOULD be
sent from PAR's link whenever feasible. For instance, an internal
link-specific trigger could enable FBU transmission from the previous
link. When it is not feasible, FBU is sent from the new link. Care
must be taken to ensure that NCoA used in FBU does not conflict with
an address already in use by some other node on link. For this, FBU
encapsulation within FNA MUST be implemented and SHOULD be used (See
below).
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The format and semantics of FBU processing are specified in
Section 6.3.1.
Depending on whether an FBack is received or not on the previous
link, which clearly depends on whether FBU was sent in the first
place, there are two modes of operation.
1. The MN receives FBack on the previous link. This means that
packet tunneling would already be in progress by the time the
MN handovers to NAR. The MN SHOULD send FNA immediately after
attaching to NAR, so that arriving as well as buffered packets
can be forwarded to the MN right away.
Before sending FBack to MN, PAR can verify if NCoA is acceptable
to NAR through the exchange of HI and HAck messages. When
stateful assignment is used, the proposed NCoA in FBU is carried
in HI, and NAR MAY consider assigning the proposed NCoA. In any
case, the assigned NCoA MUST be returned in HAck, and PAR MUST in
turn provide the assigned NCoA in FBack. If there is an assigned
NCoA returned in FBack, the MN MUST use the assigned address
(and not the proposed address in FBU) upon attaching to NAR.
The HI and HAck protocol exchange to verify NCoA acceptability,
among others noted in Section 3.1, can be used even when stateful
assignment is not used.
2. The MN does not receive FBack on the previous link. One obvious
reason for this is that the MN has not sent the FBU. The other
reason is that the MN has left the link after sending the FBU
(which may be lost) but before receiving an FBack. Without
receiving an FBack in the latter case, the MN cannot ascertain
whether PAR has successfully processed the FBU. Hence, it
(re)sends an FBU as soon as it attaches to NAR. In order to
enable NAR to forward packets immediately (when FBU has been
processed) and to allow NAR to verify if NCoA is acceptable, the
MN SHOULD encapsulate FBU in FNA. If NAR detects that NCoA is in
use when processing FNA, for instance while creating a neighbor
entry, it MUST discard the inner FBU packet and send a Router
Advertisement with ``Neighbor Advertisement Acknowledge (NAACK)''
option in which NAR MAY include an alternate IP address for the
MN to use. This discarding avoids the undesirable outcome of
address collision, even though the chances of such a collision
are extremely low. Detailed FNA processing rules are specified
in Section 6.3.3.
The scenario in which a MN sends FBU and receives FBack on PAR's link
is illustrated in Figure 2. For convenience, this scenario may be
characterized as ``predictive'' mode of operation. The scenario in
which the MN sends FBU from NAR's link is illustrated in Figure 3.
For convenience, this scenario may be characterized as ``reactive''
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mode of operation. Note however that the reactive mode includes the
case when FBU has been sent from PAR's link but FBack has not been
received yet.
Finally, the PrRtAdv message may be sent gratuitously, i.e., without
the MN first sending RtSolPr.
MN PAR NAR
| | |
|------RtSolPr------->| |
|<-----PrRtAdv--------| |
| | |
|------FBU----------->|--------HI--------->|
| |<------HAck---------|
| <--FBack---|--FBack---> |
| | |
disconnect forward |
| packets===============>|
| | |
| | |
connect | |
| | |
|--------- FNA --------------------------->|
|<=================================== deliver packets
| |
Figure 2: ``Predictive'' Fast Handover
3.3. Protocol Operation of Network-initiated Handover
In some wireless technologies, the handover control may reside in
the network even though the decision to undergo handover may be
co-operatingly arrived at between the MN and the network. On such
network, it is possible for the PAR to send an unsolicited PrRtAdv
containing the link address, IP address and subnet prefixes of the
NAR when the network decides that a handover is imminent. The MN
MUST process this PrRtAdv to configure a new care of address on the
new subnet, and MUST send an FBU to PAR prior to switching to the new
link. After transmitting the FBU, the PAR MUST continue to forward
packets to the MN on its current link until the FBU is received. The
rest of the operation is the same as that described in Section 3.2.
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MN PAR NAR
| | |
|------RtSolPr------->| |
|<-----PrRtAdv--------| |
| | |
disconnect | |
| | |
| | |
connect | |
|------FNA[FBU]-------|------------------->|
| |<-----FBU-----------|
| |------FBack-------->|
| forward |
| packets===============>|
| | |
|<=================================== deliver packets
| |
Figure 3: ``Reactive'' Fast Handover
An alternative use of the unsolicited PrRtAdv is to allow the network
to inform the MN about geographically adjacent subnets without the
MN having to explicitly request that information. This can reduce
the amount of wireless traffic required for the MN to obtain a
neighborhood links and subnet topology map. Such usage of PrRtAdv is
decoupled from the actual handover. Exactly how to reconcile this
function with the use of PrRtAdv as a handover trigger is a topic for
future experimental work.
4. Protocol Details
All description makes use of Figure 1 as the reference.
After discovering nearby access points, the MN sends RtSolPr in order
to resolve access point identifiers to subnet router information.
A convenient time to do this is after performing router discovery.
However, the MN MAY send RtSolPr at any time during its attachment to
PAR. The trigger for sending RtSolPr can emanate from a link-specific
event, such as the promise of better signal strength from another
access point coupled with fading signal quality with the current
access point. Such events, often broadly called as ``L2 triggers'',
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are outside the scope of this document. Nevertheless, they serve
as important events that invoke the protocol. For instance, when
a ``link up'' indication is obtained on the new link, the protocol
messages (e.g., FNA) can be immediately transmitted. Implementations
SHOULD make use of such triggers whenever available.
The RtSolPr message may contain one or more AP-IDs. A wildcard
requests all available tuples. When the MN has knowledge of an
impending handover, it MAY set the `U' bit in RtSolPr to request PAR
to start buffering its packets.
As a response to RtSolPr, PAR sends a PrRtAdv message which indicates
one of the following possible conditions.
1. If the PAR does not have an entry corresponding to the new
attachment point, it MUST respond indicating that the new
point of attachment is unknown. The MN MUST stop fast handover
protocol operations on the current link. The MN MAY send an FBU
from its new link.
2. If the new point of attachment is connected to the PAR itself,
PAR MUST respond indicating that the point of attachment is known
but is connected to itself. This could happen, for example, when
the wireless access points are bridged into a wired network. No
further protocol action is necessary.
3. If the new point of attachment is known and the PAR has
information about it, then PAR MUST respond indicating that the
point of attachment is known. The message MUST contain the NCoA
that the MN should use or the network prefix that should be used
to form NCoA. If the new point of attachment is known, but does
not support fast handover, the PAR MUST indicate this with Code 3
(See Section 6.1.2).
4. If a wildcard is supplied as an identifier for the new point of
attachment, the PAR SHOULD supply neighborhood [AP, AR] tuples
subject to path MTU restrictions (i.e., provide any `n' tuples
without exceeding the link MTU).
If the `U' bit is set in RtSolPr, PAR MAY buffer incoming packets
in FIFO order but MUST continue forwarding packets to PCoA on
PAR's link. It SHOULD stop buffering after processing the FBU
message. The size of the buffer and the duration of buffering
are implementation-specific considerations. If the new point of
attachment belongs to itself, PAR MAY chose to ignore buffering.
The method by which Access Routers exchange information about
their neighbors and thereby allow construction of PrRtAdvs with
information about the new subnet is outside the scope of this
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document. Furthermore, this document assumes that the access routers
share necessary security association established by means outside the
scope of this document.
The RtSolPr and PrRtAdv messages MUST be implemented by a MN and
an access router that supports fast handovers. However, when
the parameters necessary for the MN to send packets immediately
upon attaching to the NAR are supplied by the link layer handover
mechanism itself, the above messages are optional on such link
layers.
Sometime after PrRtAdv is received, the MN sends FBU in which it
includes the proposed NCoA. The MN SHOULD send FBU from PAR's link
whenever feasible, i.e., whenever ``anticipation'' of handover is
feasible. When anticipation is not feasible or if has not received
an FBack yet, the MN sends FBU immediately after attaching to NAR's
link. In addition, this FBU SHOULD be encapsulated in a FNA message
in order to allow NAR to first verify if NCoA is in use already
before it can forward the (inner) FBU packet to PAR. In response to
FBU, PAR establishes a binding between PCoA (``Home Address'') and
NCoA, and sends FBack to MN. Prior to establishing this binding, PAR
SHOULD send a HI message to NAR, and receive HAck in response.
The HI message contains the PCoA, link-layer address and the NCoA of
the MN. In response to processing the HI message, the NAR
1. verifies if NCoA supplied in the HI message is a valid address
for use, and if it is, starts proxying the address.
2. allocates NCoA for the MN when stateful address configuration is
used, creates a proxy neighbor cache entry and begins defending
it. The NAR MAY consider NCoA proposed in HI for allocation.
3. MAY create a host route entry for PCoA in case NCoA cannot be
accepted or assigned such that the entry allows it to forward
packets to the MN. This host route entry SHOULD be implemented
such that until the MN's presence is detected, either through
explicit announcement by the MN or by other means, arriving
packets do not invoke neighbor discovery. The NAR MAY also set
up a reverse tunnel to PAR in this case.
4. SHOULD start buffering packets if the `U' bit is set.
5. provides the status of handover request in Handover Acknowledge
(HAck) message.
If HAck contains an assigned NCoA, such as in stateful allocation,
FBack MUST include it, and the MN MUST use the address provided in
FBack. The PAR SHOULD send FBack to previous link as well. The
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result of FBU and FBack processing is that PAR begins tunneling MN's
packets to NCoA. If the MN does not receive an FBack message even
after re-transmitting FBU for FBU_RETRIES, it must assume that fast
handover support is not available and stop the protocol operation.
As soon as the MN establishes link connectivity with the NAR, it
SHOULD send a Fast Neighbor Advertisement (FNA) message (See 6.3.3).
If the MN has not received an FBack by the time FNA is being sent, it
SHOULD encapsulate the FBU in FNA and send them together.
When it is acceptable to use NCoA corresponding to the FNA message,
the NAR MUST
1. delete the proxy neighbor cache entry, if any is present.
2. create a neighbor cache entry and set its state to REACHABLE
without over-writing an existing entry for a different layer 2
address.
3. forward any buffered packets
4. enable the host route entry, if any is present, for PCoA.
If NAR detects that NCoA is in use by another node while processing
the FNA, it MUST
1. discard the inner (FBU) packet.
2. send a Router Advertisement with the NAACK option, and NAR MAY
include an alternate NCoA for use
If the MN receives a Router Advertisement with a NAACK option, it
MUST revert to configuring a new address, and SHOULD use the address,
if any is, provided in the NAACK option. Subsequently, the MN
SHOULD send an FBU using the new CoA. As a special case, the address
supplied in NAACK could be PCoA itself, in which case the MN MUST not
send any more FBUs. If no address is present in NAACK option, the MN
MUST follow [5] in order to configure a new CoA and SHOULD still send
FBU to PAR.
Once the MN has confirmed its NCoA, it SHOULD send a Neighbor
Advertisement message. This message updates its neighbor's cache
entries with the MN's link-layer address.
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5. Other Considerations
5.1. Handover Capability Exchange
The MN expects a PrRtAdv in response to its RtSolPr message. If the
MN does not receive a PrRtAdv message even after RTSOLPR_RETRIES, it
must assume that PAR does not support the fast handover protocol.
Similarly, if PAR sends PrRtAdv gratuitously to a MN, but does not
receive an FBU even after GR_PRRTADV_RETRIES, it must assume that the
MN does not support the fast handover protocol.
Even if a MN's current access router is capable of fast handover,
the access router into which the MN is handing over to may not
be capable of fast handover. This is indicated to the MN during
``runtime'', through the PrRtAdv message with a Code value of 3. See
Section 6.1.2.
5.2. Determining New Care of Address
Typically, the MN formulates its prospective NCoA using the
information provided in a PrRtAdv message, and sends FBU. The PAR
MUST use the NCoA present in FBU in its HI message. The NAR SHOULD
verify if NCoA present in HI is already in use. In any case, NAR
MUST respond to HI in the form of a HAck in which it may include a
different NCoA to use, especially when stateful address configuration
is used. If there is an address present in HAck, PAR MUST convey it
to the MN in the FBack message.
If PrRtAdv message carries a NCoA, the MN MUST use it as its
prospective NCoA.
5.3. Packet Loss
Handover involves link switching. It is understandably difficult
to exactly co-ordinate fast handover signaling with link switching.
Furthermore, arrival pattern of packets is dependent on many factors,
including application characteristics, network queuing behavior etc.
Hence, packets may arrive at NAR before the MN is able to attach to
it. These packets will be lost unless they are buffered by the NAR.
Similarly, if the MN attaches to NAR and then sends an FBU message,
packets arriving at PAR will be lost unless they are buffered. This
protocol provides an option to indicate request for buffering at the
NAR in the HI message. When the PAR does request this feature, it
SHOULD also provide its own support for buffering.
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5.4. DAD Handling
Duplicate Address Detection (DAD) was defined in [5] to avoid address
duplication on links when stateless address auto- configuration is
used. The use of DAD to verify the uniqueness of an IPv6 address
configured through stateless autoconfiguration adds delays to a MIPv6
handover.
The probability of an interface identifier duplication on the
same subnet is very low, however it can not be ignored. In this
draft certain precautions are proposed to minimize the effects of
a duplicate address occurrence. On links wherein the uniqueness
of the interface identifier is ensured within the subnet (by means
beyond the scope of this document), DAD handling procedures SHOULD be
avoided.
In some cases the NAR may already have the knowledge required to
assess whether the MN's address is a duplicate or not before the MN
moves to the new subnet. For example, the NAR can have a list of all
nodes on its subnet, perhaps for access control, and by searching
this list, it can confirm whether the MN's address is a duplicate
or not. The result of this search is sent back to the PAR in the
HACK message. If such knowledge is not available at the NAR, it
may indicate this by not confirming NCoA in the HACK message. The
NAR may also indicate this in the NAACK option as a response to the
FNA message. In such cases, the MN would have to follow stateless
address configuration rules after attaching to the NAR.
5.5. Fast or Erroneous Movement
Although this specification is for fast handover, the protocol has
its limits in terms of how fast a MN can move. A special case
of fast movement is ping-pong, where a MN moves between the same
two access points rapidly. Another instance of the same problem
is erroneous movement i.e., the MN receives information prior to
a handover that it is moving to a new access point and but it is
either moved to a different one or it aborts movement altogether.
All of the above behaviors are usually the result of link layer
idiosyncrasies and thus are often tackled at the link layer itself.
IP layer mobility, however, introduces its own limits. IP layer
handovers should be in a frequency that allows enough time for the
MN to update the binding of, at least, its HA and preferably that
of every CN with which it is in communication. A MN that moves
faster than necessary for this signaling to complete, which may be
of the order of few seconds, may start losing packets and ultimately
connectivity. The signaling overhead over the air and in the network
may increase significantly, especially in the case of rapid movement
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between two access routers. To avoid the signaling overhead, the
following measures are suggested.
A MN returning to the PAR before updating the necessary bindings when
present on NAR MUST send a Fast Binding Update with Home Address
equal to the MN's PCoA and a lifetime of zero, to the PAR. The MN
should have a security association with the PAR since it performed
a fast handover from it to the NAR. The PAR, on receiving this Fast
Binding Update, will check its set of outgoing (temporary fast
handover) tunnels and if it finds a match it SHOULD drop that tunnel;
i.e., stop forwarding packets for this MN and start delivering
packets directly to the node instead. The MN SHOULD NOT make any
attempt to use any of the fast handover mechanisms described in this
specification and SHOULD revert back to standard Mobile IPv6.
Temporary tunnels for the purposes of fast handovers should use short
lifetimes (in the order of a small number of seconds or less). The
lifetime of such tunnels should be enough to allow a MN to update all
its active bindings.
Erroneous movement is not likely to damage the network but it may
cause loss of packets since routing can change and the PAR may
forward packets towards another AR before the MN actually connects
to that AR. If the MN discovers itself on an unanticipated AR, a
Fast Binding Update to the PAR SHOULD be sent. Since Fast Binding
Updates are authenticated, they MUST supersede the existing binding
and packets SHOULD be redirected to the new confirmed location of the
MN. No attempt should be made to recover packets from the AR the MN
was supposed to connect to.
6. Message Formats
6.1. New Neighbor Discovery Messages
6.1.1. Router Solicitation for Proxy
Mobile Nodes send Router Solicitation for Proxy in order to prompt
routers for Proxy Router Advertisements. For all the ICMP messages,
the checksum is defined in [2].
IP Fields:
Source Address
An IP address assigned to the sending interface
Destination Address
The address of the Access Router or the all routers
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier |U| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
Figure 4: Router Solicitation for Proxy (RtSolPr) Message
multicast address.
Hop Limit 255
Authentication Header
If a Security Association for the IP Authentication
Header exists between the sender and the
destination address, then the sender SHOULD include
this header.
ICMP Fields:
Type TBA
Code 0
Checksum The ICMP checksum.
Identifier MUST be set by the sender so that replies can be
matched to this Solicitation.
`U' bit If set, requests access router to also buffer
arriving packets
Reserved MUST be set to zero by the sender and ignored by
the receiver.
Valid Options:
Source link-layer address
The link-layer address of the sender, if known.
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It SHOULD be included on link layers that have
addresses.
New Attachment Point Link-Layer Address
The link-layer address or identification of the
attachment point for which the MN requests routing
advertisement information. It MUST be included
in all RtSolPr messages. More than one such address
or identifier can be present. This field can also
be a wildcard address with all bits set to zero.
Future versions of this protocol may define new option types.
Receivers MUST silently ignore any options that they do not recognize
and continue processing the rest of the message.
A MN sends this message if it wishes to initiate fast handover. It
indicates its destination with the New Attachment Point Link-Layer
Address. A Proxy Router Advertisement message should be received in
response. If such a message is not received in a short time period
but no less than twice the typical round trip time (RTT) over the
access link or 100 ms if RTT is not known, it SHOULD resend RtSolPr
message at most RTSOLPR_RETRIES, waiting for the same time during
each instance of retransmission. If Proxy Router Advertisement is
not received by the time the MN disconnects from the PAR, the MN
SHOULD send FBU immediately after configuring a new CoA.
6.1.2. Proxy Router Advertisement (PrRtAdv)
Access routers send out Proxy Router Advertisement message
gratuitously if the handover is network-initiated or as a response
to RtSolPr message from a MN, providing the link-layer address, IP
address and subnet prefixes of neighboring routers. This avoids the
MN having to solicit Router Advertisement when it connects to the new
subnet.
IP Fields:
Source Address
MUST be the link-local address assigned to the
interface from which this message is sent.
Destination Address
The Source Address of an invoking Router
Solicitation for Proxy or the address of the node
the Access Router is instructing to handover.
Hop Limit 255
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
Figure 5: Proxy Router Advertisement (PrRtAdv) Message
Authentication Header
If a Security Association for the IP Authentication
Header exists between the sender and the
destination address, then the sender SHOULD include
this header.
ICMP Fields:
Type TBA
Code 0, 1, or 2
Checksum The ICMP checksum.
Identifier Copied from Router Solicitation for Proxy or set to
Zero if unsolicited.
Reserved MUST be set to zero by the sender and ignored by
the receiver.
Valid Options:
Source link-layer address
The link-layer address of the sender, if known.
It SHOULD be included on link layers that have
addresses.
Link-layer address of proxied originator (i.e., NAR)
The link-layer address of the Access Router for
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which this message is proxied for. This option MUST
be included when Code is 0.
New Router Prefix Information Option.
These options specify the prefixes of the Access
Router the message is proxied for and are used
for address autoconfiguration. This option SHOULD be
included when Code is 0.
New CoA Option
When PrRtAdv is sent gratuitously, this option MUST
be included. Otherwise, this option MAY be present.
Future versions of this protocol may define new option types.
Receivers MUST silently ignore any options they do not recognize and
continue processing the message.
A Proxy Router Advertisement with Code 0 but without a New CoA Option
means that the MN SHOULD construct a NCoA out of its Interface ID
(used in the Destination Address in this Proxy Router Advertisement)
and the Prefix in the New Router Prefix Information Option (See
Section 6.4.2. A Proxy Router Advertisement with Code 0 and a New
CoA Option means that the MN SHOULD use the supplied NCoA or else
stand to loose service.
A Proxy Router Advertisement with Code 1 is sent if handover to the
New Attachment Point, as indicated by the New Attachment Point Link
Layer address in the corresponding RtSolPr message, does not require
change of CoA. No options are required in this case.
A Proxy Router Advertisement with Code 2 means that the PAR is not
aware of the Prefix Information requested. The MN SHOULD attempt to
send FBU as soon as it regains connectivity with the NAR. No options
are required in this case.
A Proxy Router Advertisement with Code 3 means that the NAR does
not support fast handover. The MN MUST stop fast handover protocol
operations. No options are required in this case.
When a wildcard AP identifier is supplied in the RtSolPr message,
the PrRtAdv message should include any 'n' [Access Point Identifier,
Link-layer address option, Prefix Information Option] tuples
corresponding to the PAR's neighborhood.
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6.2. Inter-Access Router Messages
6.2.1. Handover Initiate (HI)
The Handover Initiate (HI) is a new ICMPv6 message sent by an Access
Router (typically PAR) to another Access Router (typically NAR) to
initiate the process of a MN's handover.
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 | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier |S|U| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Figure 6: Handover Initiate (HI) Message
IP Fields:
Source Address
The IP address of the PAR
Destination Address
The IP address of the NAR
Hop Limit 255
Authentication Header
A Security Association MUST exist between the
sender and the receiver of this message. This
message MUST be authenticated and so the authentication
header MUST be used when this message is sent.
ICMP Fields:
Type TBA
Code 0
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Checksum The ICMP checksum.
Identifier MUST be set by the sender so replies can be matched
to this message.
S Stateful address configuration flag. When set, this
message requests a new CoA to be returned by the
destination.
U Buffer flag. When set, the destination SHOULD buffer
any packets towards the node indicated in the options
of this message.
Reserved MUST be set to zero by the sender and ignored by
the receiver.
Valid Options:
Link-layer address of MN
The link-layer address of the MN that is
undergoing handover to the destination. This option
SHOULD be included to help the destination
recognize the MN when it connects to the
destination.
Previous Care of Address
The IP address used by the MN while
attached to the originating router. This option
SHOULD be included so that host route can be
established in case necessary.
New Care of Address
The IP address the MN wishes to use when
connected to the destination. When the `S' bit is
set, NAR MAY use this address in its stateful
assignment.
If Handover Acknowledge (HAck) message is not received as a response,
the Handover Initiate SHOULD be re-sent up to HI_RETRIES times using
a short retransmission timer with a value no less than twice the
round trip time between source and destination or 100 ms if RTT is
not known.
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6.2.2. Handover Acknowledge (HAck)
The Handover Acknowledgment message is a new ICMPv6 message that MUST
be sent (typically by NAR to PAR) as a reply to the Handover Initiate
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 | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier |H|T|R| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Figure 7: Handover Acknowledge (HAck) Message
IP Fields:
Source Address
Copied from the destination address of the Handover
Initiate Message to which this message is a
response.
Destination Address
Copied from the source address of the Handover
Initiate Message to which this message is a
response.
Hop Limit 255
Authentication Header
A Security Association MUST exist between the
sender and the receiver of this message. This message
MUST be authenticated and so the authentication
header MUST be used when this message is sent.
ICMP Fields:
Type TBA
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Code
0: Handover Accepted, NCoA valid
1: Handover Accepted, NCoA not valid
2: Handover Accepted, NCoA in use
3: Handover Accepted, NCoA assigned (Stateful)
4: Handover Accepted, NCoA not assigned (Stateful)
128: Handover Not Accepted, reason unspecified
129: Administratively prohibited
130: Insufficient resources
Checksum The ICMP checksum.
Identifier Copied from the corresponding field in the Handover
Initiate message this message is in response to.
Reserved MUST be set to zero by the sender and ignored by
the receiver.
Valid Options:
New Care of Address
If the S flag in the Handover Initiate message is set,
this option MUST be used to provide NCoA the MN should
use when connected to this router. This option MAY be
included even when `S' bit is not set, e.g., Code 2
above.
Upon receiving the HI message, the NAR MUST respond with a Handover
Acknowledge message. If the `S' flag is set in the HI message, the
NAR SHOULD include the New Care of Address option and a Code of 3.
If the `S' flag is not set in the HI message, the NAR MUST check the
validity of the NCoA when sent with HI, and reply with appropriate
Code values enumerated above. If NCoA is valid, the NAR SHOULD
insert NCoA in its Proxy Neighbor Cache and defend NCoA for
PROXY_ND_LIFETIME period of time during which the MN is expected to
connect to NAR. If the NCoA is not valid or not assigned, the NAR
MUST respond with an appropriate Code value.
The NAR MAY provide support for PCoA (instead of accepting or
assigning NCoA) and establish a host route entry for PCoA, and set up
a tunnel to the PAR to forward MN's packets sent with PCoA as source
IP address. This host route entry SHOULD be used to forward packets
once the NAR detects that the particular MN is attached to its link.
Finally, the new access router can always refuse handover, in which
case it should indicate the reason in one of the available Code
values.
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6.3. New Mobility Header Messages
Mobile IPv6 uses a new IPv6 header type called Mobility Header [3].
The Fast Binding Update, Fast Binding Acknowledgment and Fast
Neighbor Advertisement messages use the Mobility Header.
6.3.1. Fast Binding Update (FBU)
The Fast Binding Update message is identical to the Mobile IPv6
Binding Update (BU) message. However, the processing rules are
slightly different. The Mobility Header Type value for FBU is 8.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence # |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|A|H|L|K| Reserved | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Fast Binding Update (FBU) Message
IP fields:
Source address The PCoA or NCoA
Destination Address
The IP address of the Previous Access
Router
`A' flag MUST be set to one to request PAR to send a Fast
Binding Acknowledgement message.
`H' flag MUST be set to one. See [3].
`L' flag See [3].
`K' flag See [3].
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Reserved This field is unused. MUST be set zero.
Sequence Number See [3].
Lifetime See [3].
Mobility Options
MUST contain alternate CoA option set to NCoAIP
address when sent FBU is sent from PAR's link.
MAY contain alternate CoA option set to NCoA when
FBU is sent from NAR's link.
The MN sends FBU message any time after receiving a PrRtAdv message.
If the MN moves prior to receiving a PrRtAdv message, it SHOULD send
a FBU to the PAR after configuring a new CoA on the NAR.
The source IP address is PCoA when FBU is sent from PAR's link, and
the source IP address is NCoA when sent from NAR's link. When FBU is
sent from NAR's link, it SHOULD be encapsulated within FNA.
The FBU MUST also include the Home Address Option and the Home
Address is PCoA. A FBU message MUST be protected in a way appropriate
for MN and PAR communication. That is, PAR MUST be able to determine
that the FBU message is sent by a genuine MN.
6.3.2. Fast Binding Acknowledgment (FBack)
The Fast Binding Acknowledgment message is sent by the PAR to
acknowledge receipt of a Fast Binding Update message in which the `A'
bit is set. The Fast Binding Acknowledgment message SHOULD NOT be
sent to the MN before the PAR receives a HAck message from the NAR.
The Fast Binding Acknowledgment MAY also be sent to the MN on the old
link. The Mobility Header Type value for FBACK is 9.
IP fields:
Source address The IP address of the Previous Access
Router
Destination Address The NCoA
Status
8-bit unsigned integer indicating the
disposition of the Fast Binding Update. Values
of the Status field less than 128 indicate that
the Binding Update was accepted by the receiving
node. The following such Status values are
currently defined:
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status |K| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence # | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Fast Binding Acknowledgment (FBack) Message
0 Fast Binding Update accepted
1 Fast Binding Update accepted but NCoA is
invalid. Use NCoA supplied in ``alternate''
(e.g., stateful) CoA
Values of the Status field greater than or equal
to 128 indicate that the Binding Update was
rejected by the receiving node. The following
such Status values are currently defined:
128 Reason unspecified
129 Administratively prohibited
130 Insufficient resources
131 Incorrect interface identifier length
`K' flag See [3].
Reserved An unused field. MUST be set to zero.
Sequence Number Copied from FBU message for use by the MN in
matching this acknowledgment with an outstanding
FBU.
Lifetime
The granted lifetime in seconds for which the
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sender of this message will retain a binding for
traffic redirection.
Mobility Options MUST contain ``alternate'' CoA if Status is 1.
6.3.3. Fast Neighbor Advertisement (FNA)
A MN sends a Fast Neighbor Advertisement to announce itself to the
NAR. The Mobility Header Type value for FBACK is 10. When the
Mobility Header Type is FNA, the Payload Proto field may be set to
IPv6 in order to assist FBU encapsulation.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Mobility Options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: Fast Neighbor Advertisement (FNA) Message
IP fields:
Source address NCoA
Destination Address NAR's IP address
Mobility Options MUST contain the Link-Layer Address
The MN sends Fast Neighbor Advertisement to the NAR, as soon as it
regains connectivity on the new link to announce its attachment so
that arriving or buffered packets can be immediately forwarded. If
NAR is proxying NCoA, it creates a neighbor cache entry in REACHABLE
state. If there is no entry at all, it creates one and sets it
to REACHABLE. If there is an entry in INCOMPLETE state without a
link-layer address, it sets it to REACHABLE. During the process of
creating a neighbor cache entry, NAR can also detect if NCoA is in
use, thus avoiding address collisions. Since FBU is encapsulated
within FNA when sent from NAR's link, NAR drops FBU in case it
detects any collision.
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The combination of NCoA (present in source IP address) and the
Link-Layer Address (present as a Mobility Option) SHOULD be used to
distinguish the MN from other nodes.
6.4. New ICMP Options
6.4.1. IP Address Option
This option is sent in the Proxy Router Advertisement, the Handover
Initiate, and Handover Acknowledge messages. The Proxy Router
Advertisement and Handover Acknowledgment messages only contain the
NCoA while the Handover Initiate message may include both NCoA and
PCoA. The format is based on [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 | Sub-Type | Prefix Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ IPv6 Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: IPv6 Address Option
Type
TBA
Length
size of this option units of 8 octets (i.e., 3)
Sub-Type
1 Old Care-of Address
2 New Care-of Address
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Prefix Length
The Length of the IPv6 Address Prefix
IPv6 address
The IP address for the unit defined by the Type field.
6.4.2. New Router Prefix Information Option
This option is sent in the PrRtAdv message in order to provide the
prefix information valid on the NAR.
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 | Sub-Type | Prefix Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Prefix +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: New Router Prefix Information Option
Type
TBA
Length
The length of the option (including the type, sub-type and
length fields) in units of 8 octets.
Sub-Type
0
Prefix Length
8-bit unsigned integer. The number of leading bits in the
Prefix that are valid. The value ranges from 0 to 128.
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Prefix
An IP address or a prefix of an IP address. The Prefix Length
field contains the number of valid leading bits in the prefix.
The bits in the prefix after the prefix length are reserved
and MUST be initialized to zero by the sender and ignored by
the receiver.
6.4.3. Link-layer Address (LLA)
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 | Sub-Type | LLA ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 13: Link-Layer Address Option
Type
TBA
Length
The length of the option (including the type, sub-type and
length fields) in units of 8 octets.
Sub-Type
0 wildcard requesting resolution for all nearby access points
1 Link-layer Address of the new Attachment Point.
2 Link-layer Address of the MN.
3 Link-layer Address of the Proxied Originator
LLA
The variable length link-layer address.
The New Attachment Point Link Layer address contains the link-layer
address of the attachment point for which handover is about to
be attempted. This is used in the Router Solicitation for Proxy
message.
The MN Link-Layer address option contains the link-layer address of a
MN. It is used in the Handover Initiate message.
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The Proxied Originator Link-Layer address option contains the
Link Layer address of the Access Router for which the Proxy Router
Solicitation message refers to.
These options MUST be silently ignored when used with other Neighbor
Discovery messages.
6.4.4. Neighbor Advertisement Acknowledgment (NAACK)
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 | Sub-Type | Status |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14: Neighbor Advertisement Acknowledgment Option
Type
TBA
Length
8-bit unsigned integer. Length of the option, in 8 octets,
excluding the Option Type and Option Length fields.
Sub-Type
0
Status
8-bit unsigned integer indicating the disposition of the Fast
Binding Update. Values of the Status field less than 128
indicate that the Binding Update was accepted by the receiving
node. The following such Status values are currently defined:
0 The New CoA is valid
1 The New CoA is invalid
2 The New CoA is invalid, use the supplied CoA
128 Link Layer Address unrecognized
The NAR responds to FNA with the NAACK option to notify the MN
to use a different NCoA when there is address collision. If the
NCoA is invalid, the Router Advertisement MUST use the PCoA as the
destination address, available from the HI message. The MN SHOULD
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use the NCoA supplied along with the NAACK option. If the NAACK
indicates that the Link Layer Address is unrecognized the MN MUST NOT
use the NCoA or the PCoA and SHOULD start immediately the process of
acquiring a NCoA at the NAR.
In the future, new option types may be defined.
7. Configurable Parameters
Parameter Name Default Value Definition
------------------- ---------------------- -------
RTSOLPR_RETRIES 2
GR_PRRTADV_RETRIES 2
FBU_RETRIES 3 Section 4
PROXY_ND_LIFETIME 1 second Section 6.2.2
HI_RETRIES 4 Section 6.2.1
8. Security Considerations
The following security vulnerabilities are identified, and suggested
solutions mentioned.
1. Insecure FBU: in this case, packets meant for one address could
be stolen, or redirected to some unsuspecting node. This concern
is the same as that in a MN and Home Agent relationship.
Hence, the PAR MUST ensure that the FBU packet arrived from a
node that legitimately owns the PCoA. The access router and its
hosts may use any available mechanism to establish a security
association which MUST be used to secure FBU. The current version
of this protocol does not specify how this security association
is established. However, the future work, either as part of this
document or in a separate document, may specify this security
establishment.
If an access router can ensure that the source IP address in
an arriving packet could only have originated from the node
whose link-layer address is in the router's neighbor cache, then
a bogus node cannot use a victim's IP address for malicious
redirection of traffic. Such an operation is recommended at
least on neighbor discovery messages including the RtSolPr
message.
2. Secure FBU, malicious or inadvertant redirection: in this case,
the FBU is secured, but the target of binding happens to be an
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unsuspecting node either due to inadvertant operation or due
to malicious intent. This vulnerability can lead to a MN with
genuine security association with its access router redirecting
traffic to an incorrect address.
However, the target of malicious traffic redirection is limited
to an interface on an access router with which the PAR has a
security association. The PAR MUST verify that the NCoA to
which PCoA is being bound actually belongs to NAR's prefix.
In order to do this, HI and HAck message exchanges are to be
used. When NAR accepts NCoA in HI, it proxies NCoA so that
any arriving packets are not sent on the link until the MN
attaches and announces itself through FNA. So, any inadvertant or
malicious redirection to a host is avoided. It is still possible
to jam NAR's buffer with redirected traffic. However, since
NAR's handover state corresponding to NCoA has a finite (and
short) lifetime corresponding to a small multiple of anticipated
handover latency, this loophole is not significant.
3. Sending FBU from NAR's link: a malicious node may send FBU from
NAR's link providing an unsuspecting node's address as NCoA.
Since FBU is encapsulated in FNA, NAR should detect the collision
with an address in use when processing FNA, and it then drops
FBU. When NAR is unable to detect address collisions, there is a
vulnerability that redirection can affect an unsuspecting node.
9. Contributors
This document originated from the fast handover design team effort.
The members of this design team in alphabetical order were; Gopal
Dommety, Karim El-Malki, Mohammed Khalil, Charles Perkins, Hesham
Soliman, George Tsirtsis and Alper Yegin.
10. Acknowledgments
The editor would like to thank all the interested folks who have
provided feedback on this specification. The editor would like
to acknowledge the contribution from James Kempf to improve this
specification. And, the working group chairs have been very
supportive.
References
[1] S. Bradner. Key words for use in RFCs to Indicate Requirement
Levels. Request for Comments (Best Current Practice) 2119,
Internet Engineering Task Force, March 1997.
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[2] A. Conta and S. Deering. Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6)
Specification. Request for Comments (Draft Standard) 2463,
Internet Engineering Task Force, December 1998.
[3] D. Johnson, C. Perkins, and J. Arkko. Mobility Support in IPv6
(work in progress). Internet Draft, Internet Engineering Task
Force, 2002.
[4] M. Khalil, R. Narayanan, H. Akhtar, and E. Qaddoura. Mobile IP
Extensions Rationalization (MIER) (work in progress). Internet
Draft, Internet Engineering Task Force.
draft-ietf-mobileip-mier-05.txt, February 2000.
[5] S. Thomson and T. Narten. IPv6 Stateless Address
Autoconfiguration. Request for Comments (Draft Standard)
2462, Internet Engineering Task Force, December 1998.
A. Change Log
The following changes have taken place since the previous version.
The Section numbers refer to version 06.
- Revised the security considerations section in v07
- Refined and added a section on network-initiated handover v07
- Section 3 format change
- Section 4 format change (i.e., no subsections).
- Description in Section 4.4 merged with ``Fast or Erroneous
Movement''
- Section 4.5 deprecated
- Section 4.6 deprecated
- Revision of some message formats in Section 6
B. Contact Information
The design team member's contact information:
Gopal Dommety
Cisco Systems, Inc.
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170 West Tasman Drive
San Jose, CA 95134
Phone:+1 408 525 1404
E-Mail: gdommety@cisco.com
Karim El Malki
Ericsson Radio Systems AB
LM Ericssons Vag. 8
126 25 Stockholm
SWEDEN
Phone: +46 8 7195803
Fax: +46 8 7190170
E-mail: Karim.El-Malki@era.ericsson.se
Mohamed Khalil
Nortel Networks
E-Mail: mkhalil@nortelnetworks.com
Charles E. Perkins
Communications Systems Lab
Nokia Research Center
313 Fairchild Drive
Mountain View, California 94043
USA
Phone: +1-650 625-2986
E-Mail: charliep@iprg.nokia.com
Fax: +1 650 625-2502
Hesham Soliman
Flarion Technologies
E-mail: H.Soliman@flarion.com
George Tsirtsis
Flarion Technologies
E-Mail: G.Tsirtsis@flarion.com
Alper E. Yegin
NTT DoCoMo Labs USA
E-mail: alper@docomolabs-usa.com
The editor's contact information:
Rajeev Koodli
Nokia Research Center
313 Fairchild Drive
Mountain View, CA 94043 USA
Phone: +1 650 625 2359
Fax: +1 650 625 2502
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E-Mail: Rajeev.Koodli@nokia.com
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