One document matched: draft-ietf-mipshop-fmipv6-rfc4068bis-02.txt
Differences from draft-ietf-mipshop-fmipv6-rfc4068bis-01.txt
Mipshop Working Group Rajeev Koodli, Editor
INTERNET DRAFT Nokia Siemens Networks
Category: Standards Track July 9 2007
Updates: RFC 4068
Expires: January 8, 2008
Fast Handovers for Mobile IPv6
draft-ietf-mipshop-fmipv6-rfc4068bis-02.txt
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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 this time, the Mobile
Node is unable to send or receive packets due to both link
switching delay and IP protocol operations. The "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 a protocol to improve handover latency due to
Mobile IPv6 procedures. This document does not address improving
the link switching latency.
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Contents
Abstract i
1. Introduction 2
2. Terminology 2
3. Protocol Overview 4
3.1. Addressing the Handover Latency . . . . . . . . . . 4
3.2. Protocol Operation . . . . . . . . . . . . . . 7
3.3. Protocol Operation during Network-initiated Handover . . . 8
4. Protocol Details 10
5. Other Considerations 13
5.1. Handover Capability Exchange . . . . . . . . . . . 13
5.2. Determining New Care of Address . . . . . . . . . . . 14
5.3. Prefix Management . . . . . . . . . . . . . . . . 14
5.4. Packet Loss . . . . . . . . . . . . . . . . . . 14
5.5. DAD Handling . . . . . . . . . . . . . . . . . 15
5.6. Fast or Erroneous Movement . . . . . . . . . . . . 17
6. Message Formats 18
6.1. New Neighborhood Discovery Messages . . . . . . . . . . 18
6.1.1. Router Solicitation for Proxy Advertisement
(RtSolPr) . . . . . . . . . . . . . . . 18
6.1.2. Proxy Router Advertisement (PrRtAdv) . . . . . 20
6.2. Inter-Access Router Messages . . . . . . . . . . . 23
6.2.1. Handover Initiate (HI) . . . . . . . . . . 23
6.2.2. Handover Acknowledge (HAck) . . . . . . . . . 25
6.3. New Mobility Header Messages . . . . . . . . . . . 27
6.3.1. Fast Binding Update (FBU) . . . . . . . . . . 27
6.3.2. Fast Binding Acknowledgment (FBack) . . . . . . 28
6.3.3. Unsolicited Neighbor Advertisement (UNA) . . . . 30
6.4. New Options . . . . . . . . . . . . . . . . . . 30
6.4.1. IP Address Option . . . . . . . . . . . . . 31
6.4.2. New Router Prefix Information Option . . . . . 32
6.4.3. Link-layer Address (LLA) Option . . . . . . . . 33
6.4.4. Mobility Header Link-layer Address (MH-LLA)
Option . . . . . . . . . . . . . . . . . 34
6.4.5. Binding Authorization Data for FMIPv6 (BADF) . . . 35
6.4.6. Neighbor Advertisement Acknowledgment (NAACK) . . . 36
7. Configurable Parameters 37
8. Security Considerations 37
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9. IANA Considerations 39
10. Acknowledgments 40
11. Normative References 40
12. Author's Address 41
13. Contributors 41
A. Change Log 41
Intellectual Property Statement 42
Disclaimer of Validity 43
Copyright Statement 43
Acknowledgment 43
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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 involve movement
detection, IP address configuration, and location update. The
combined handover latency is often sufficient to affect 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
regardless of link technology. It does this without depending
on specific link-layer features while allowing link-specific
customizations. By definition, this specification considers
handovers that interwork with Mobile IP: once attached to its new
access router, a MN engages in Mobile IP operations including
Return Routability [3]. 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 in addition to those defined in [3]. 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 an IP subnet that
offers wireless connectivity to a MN. An Access
Point Identifier (AP-ID) refers the AP's L2 address.
Sometimes, AP-ID is also referred to as a Basic Service
Set IDentifier (BSSID).
Access Router (AR)
The MN's default router
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Previous Access Router (PAR)
The MN's default router prior to its handover
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's subnet
New CoA (NCoA)
The MN's Care of Address valid on NAR's subnet
Handover
A process of terminating existing connectivity and
obtaining new IP connectivity.
Router Solicitation for Proxy Advertisement (RtSolPr)
A message from the MN to the PAR requesting information
for a potential handover
Proxy Router Advertisement (PrRtAdv)
A message from the PAR to the MN that provides
information about neighboring links facilitating
expedited movement detection. The message can also act
as a trigger for network-initiated handover.
(AP-ID, AR-Info) tuple
Contains an access router's L2 and IP addresses, and
prefix valid on the interface to which the Access
Point (identified by AP-ID) is attached. The triplet
[Router's L2 address, Router's IP address and Prefix]
is called "AR-Info". See also Section 5.3.
Assigned Addressing
A particular type of NCoA configuration in which the
NAR assigns an IPv6 address for the MN. The method by
which NAR manages its address pool is not specified in
this document.
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
Unsolicited Neighbor Advertisement (UNA)
The message in [8] with 'O' bit cleared
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Fast Neighbor Advertisement (FNA)
This message from RFC4068 [7] is deprecated. The
UNA message above is the preferred message in this
specification.
Handover Initiate (HI)
A message from the PAR to the NAR regarding a MN's
handover
Handover Acknowledge (HAck)
A message from the NAR to the PAR as a response to HI
v +--------------+
+-+ | Previous | <
| | ------------ | Access | ------- >-----\
+-+ | Router | < \
MN | (PAR) | \
| +--------------+ +---------------+
| ^ IP | Correspondent |
| | Network | Node |
V | +---------------+
v /
v +--------------+ /
+-+ | New | < /
| | ------------ | Access | ------- >-----/
+-+ | Router | <
MN | (NAR) |
+--------------+
Figure 1: Reference Scenario for Handover
3. Protocol Overview
3.1. Addressing the Handover Latency
The ability to immediately send packets from a new subnet link
depends on the "IP connectivity" latency, which in turn depends
on the movement detection latency and the new CoA configuration
latency. Once a MN is IP-capable on the new subnet link, it
can send a Binding Update to its Home Agent and one or more
correspondents. Once its correspondents successfully process the
Binding Update, which typically involves the Return Routability
procedure, the MN can receive packets at the new CoA. So, the
ability to receive packets from correspondents directly at its
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new CoA depends on the Binding Update latency as well as the IP
connectivity 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 while connected to its current
router. 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 L2 address. The
"Router Solicitation for Proxy Advertisement (RtSolPr)" and "Proxy
Router Advertisement (PrRtAdv)" messages 6.1 are used for aiding
movement detection.
Through the RtSolPr and PrRtAdv messages, the MN also formulates a
prospective new CoA (NCoA), when it is still present on the 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 an unsolicited Neighbor
Advertisement message (with the 'O' bit set to zero) message [8];
NAR responds to to this UNA message in case the tentative address
is already in use. In this way, NCoA configuration latency is
reduced.
In order to reduce the Binding Update latency, the protocol
specifies a binding 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. An FBU message MUST contain
the Binding Authorization Data for FMIPv6 (BADF) option (see
Section 6.4.5) in order to ensure that only a legitimate MN that
owns the PCoA is able to establish a binding. Subsequent sections
describe the protocol mechanics. In any case, the result is that
PAR begins tunneling packets arriving for PCoA to NCoA. Such a
tunnel remains active until the MN completes the 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
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tunnel ensures that packets containing PCoA as source IP address
are not dropped due to ingress filtering. 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). Forwarding support for PCoA is
provided through a reverse tunnel between the MN and the PAR.
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. A neighbor discovery operation involving
a neighbor's address resolution (i.e., Neighbor Solicitation and
Neighbor Advertisement) typically results 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 period of one second each. In order to
circumvent this delay, a MN announces its attachment immediately
with an UNA message that allows NAR to forward packets to the MN
right away. As a response to UNA, the NAR creates an entry or
updates an existing one (while taking any conflicts into account)
in order to forward packets to the MN (see details below). Through
tunnel establishment for PCoA and fast advertisement, the protocol
provides expedited 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 addresses are
assigned by the access router. 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. In addition,
buffering for handover traffic may be desirable. Even though the
Neighbor Discovery protocol provides a small buffer (typically
one or two packets) for packets awaiting address resolution, this
buffer may be inadequate for traffic such as VoIP already in
progress. The routers may also wish to maintain a separate buffer
for servicing the handover traffic as well. 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 SHOULD be used. The access routers MUST have necessary
security association established by means outside the scope of this
document.
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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 of the MN.
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.
The format and semantics of FBU processing are specified in
Section 6.3.1. The FBU message MUST contain the BADF option (see
Section 6.4.5) to secure the message.
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 UNA 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 determine whether NCoA is
acceptable to NAR through the exchange of HI and HAck messages.
When assigned addressing (i.e., addresses are assigned by the
router) is used, the proposed NCoA in FBU is carried in HI, and
NAR MAY assign the proposed NCoA. Such an 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.
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2. The MN does not receive FBack on the previous link. One reason
for this is that the MN has not sent the FBU. The other 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, the MN (re)sends
FBU immediately after sending the UNA message. If NAR detects
that NCoA is in use when processing UNA, for instance while
creating a neighbor entry, it sends a Router Advertisement with
"Neighbor Advertisement Acknowledge (NAACK)" option in which
NAR MAY include an alternate IP address for the MN to use.
Detailed UNA 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
is called "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 is called "reactive" mode of operation.
Note that the reactive mode also includes the case when FBU has
been sent from PAR's link but FBack has not been received yet. The
Figure is intended to illustrate that the FBU is forwarded through
NAR, but it is processed only by the PAR.
Finally, the PrRtAdv message may be sent unsolicited, i.e.,
without the MN first sending RtSolPr. This mode is described in
Section 3.3.
3.3. Protocol Operation during Network-initiated Handover
In some wireless technologies, the handover control may reside
in the network even though the decision to undergo handover may
be arrived at by cooperation between the MN and the network. In
such networks, the PAR can send an unsolicited PrRtAdv containing
the link layer address, IP address and subnet prefix 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 PrRtAdv, 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.
The unsolicited PrRtAdv also allows 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
topology map of links and subnets. Such usage of PrRtAdv is
decoupled from the actual handover. See Section 6.1.2.
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MN PAR NAR
| | |
|------RtSolPr------->| |
|<-----PrRtAdv--------| |
| | |
|------FBU----------->|----------HI--------->|
| |<--------HAck---------|
| <--FBack---|--FBack---> |
| | |
disconnect forward |
| packets ===============>|
| | |
| | |
connect | |
| | |
|------------UNA --------------------------->|
|<=================================== deliver packets
| |
Figure 2: "Predictive" Fast Handover
MN PAR NAR
| | |
|------RtSolPr------->| |
|<-----PrRtAdv--------| |
| | |
disconnect | |
| | |
| | |
connect | |
|-------UNA-----------|--------------------->|
|-------FBU-----------|---------------------)|
| |<-------FBU----------)|
| |<------HI/HAck------->|
| | (if necessary) |
| forward |
| packets(including FBAck)=====>|
| | |
|<=================================== deliver packets
| |
Figure 3: "Reactive" Fast Handover
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4. Protocol Details
All description makes use of Figure 1 as the reference.
After discovering one or more 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 can send RtSolPr at
any time, e.g., when one or more new access points are discovered.
The MN can also send RtSolPr more than once during its attachment
to PAR. The trigger for sending RtSolPr can originate from a
link-specific event, such as the promise of a better signal
strength from another access point coupled with fading signal
quality with the current access point. Such events, often broadly
referred to as "L2 triggers", are outside the scope of this
document. Nevertheless, they serve as events that invoke this
protocol. For instance, when a "link up" indication is obtained
on the new link, protocol messages (e.g., UNA) can be immediately
transmitted. Implementations SHOULD make use of such triggers
whenever available.
The RtSolPr message contains one or more AP-IDs. A wildcard
requests all available tuples.
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
access point, it responds indicating that the new access point
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 access point is connected to the PAR's current
interface (to which MN is attached), PAR responds with a Code
value indicating that the new access point is connected to the
current interface, but not send any prefix information. This
scenario could arise, for example, when several wireless access
points are bridged into a wired network. No further protocol
action is necessary.
3. If the new access point is known and the PAR has information
about it, then PAR responds indicating that the new access
point is known and supply the [AP-ID, AR-Info] tuple. If the
new access point 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 access
point, the PAR SHOULD supply neighborhood [AP-ID, AR-Info]
tuples subject to path MTU restrictions (i.e., provide any 'n'
tuples without exceeding the link MTU).
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When further protocol action is necessary, some implementations may
choose to provide buffering support at PAR to address the scenario
in which a MN leaves without sending an FBU message from the PAR's
link. While the protocol does not forbid such an implementation
support, care must be taken to ensure that the PAR continues
forwaring packets to the PCoA (i.e., uses a buffer and forward
approach). The PAR should also stop buffering once it processes
the FBU message.
The method by which Access Routers exchange information about
their neighbors and thereby allow construction of Proxy Router
Advertisements with information about neighboring subnets is
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, use of above messages is optional on such links.
After a PrRtAdv message is processed, the MN sends FBU and includes
the proposed NCoA. The MN SHOULD send FBU from PAR's link whenever
"anticipation" of handover is feasible. When anticipation is not
feasible or when it has not received an FBack, the MN sends FBU
immediately after attaching to NAR's link. 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. In order
to determine the NAR's address for the HI message, the PAR can
perform longest prefix match of NCoA (in FBU) with the prefix list
of neighboring access routers. When the source IP address of FBU
is PCoA, i.e., the FBU is sent from the PAR's link, the HI message
MUST have a Code value set to 0. See Section 6.2.1. When the
source IP address of FBU is not PCoA, i.e., the FBU is sent from
the NAR's link, the HI message MUST have a Code value of 1. See
Section 6.2.1.
The HI message contains the PCoA, link-layer address and the NCoA
of the MN. In response to processing a HI message with Code 0, the
NAR
1. determines whether NCoA supplied in the HI message is a valid
address for use, and if it is, starts proxying [8] the address
for PROXY|ND|LIFETIME during which the MN is expected to
connect to NAR. In case there is already an NCoA present, NAR
may verify if the LLA is the same as its own or that of the MN
itself. If so, NAR may allow the use of NCoA.
2. allocates NCoA for the MN when assigned addressing is used,
creates a proxy neighbor cache entry and begins defending it.
The NAR MAY allocate the NCoA proposed in HI.
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3. MAY create a host route entry for PCoA (on the interface to
which the MN is attaching to) in case NCoA cannot be accepted
or assigned. 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 SHOULD also
set up a reverse tunnel to PAR in this case.
4. provides the status of handover request in Handover Acknowledge
(HAck) message.
When the Code value in HI is 1, NAR MUST skip the above operations.
However, it SHOULD be prepared to process any other options which
may be defined in the future. Sending a HI message with Code
1 allows NAR to validate the neighbor cache entry it creates
for the MN during UNA processing. That is, NAR can make use
of the knowledge that its trusted peer (i.e., PAR) has a trust
relationship with the MN.
If HAck contains an assigned NCoA, it must be included in FBack,
and the MN must use it. The PAR MAY send FBack to the previous
link as well to facilitate faster reception in the event the MN
be still present there. The 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
1. sends a UNA message (see 6.3.3). If the MN has not received
an FBack by the time UNA is being sent, it SHOULD send an FBU
message following the UNA message.
2. joins the all-nodes multicast group and the solicited-node
multicast group corresponding to the NCoA
3. starts a DAD probe for NCoA. See [9].
When a NAR receives a UNA message, it
1. SHOULD create a neighbor cache entry for NCoA if none exists
and set it to STALE. This allows it to forward any arriving
packets while it probes bidirectional reachability.
2. updates an entry in INCOMPLETE state, if it exists, to STALE
and forwards arriving and buffered packets. This would be the
case if NAR had previously sent a Neighbor Solicitation which
went unanswered perhaps because the MN had not yet attached to
the link.
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3. deletes its proxy neighbor cache entry, if any, updates the
state to STALE, and forwards arriving and buffered packets.
The buffer for handover traffic should be linked to this UNA
processing. The exact mechanism is implementation dependent.
The NAR may detect that NCoA is in use by another node when
processing the UNA message, in which case it
1. MUST NOT update the existing entry.
2. MUST send a Router Advertisement with the NAACK option in which
it MAY include an alternate NCoA for use. This message MUST
be sent to the source IP address present in UNA using the same
Layer 2 address present in UNA.
If the MN receives an IP address in the NAACK option, it MUST
use it and 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. The Status codes for NAACK option
are specified in Section 6.4.6.
Once the MN has confirmed its NCoA (either through DAD or when
provided for by the NAR), it SHOULD send a Neighbor Advertisement
message with the 'O' bit set, to the all-nodes multicast address.
This message allows MN's neighbors to update their neighbor cache
entries.
For data forwarding, the PAR tunnels packets using its global IP
address valid on the interface to which the MN was attached. The
MN reverse tunnels its packets to the same global address of PAR.
The tunnel end-point addresses must be configured accordingly.
When PAR receives a reverse tunneled packet, it must verify if a
secure binding exists for the MN identified by PCoA in the tunneled
packet, before forwarding the packet.
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 and stop sending any more RtSolPr messages.
Even if a MN's current access router is capable of providing
fast handover support, the new access router may not be capable
of providing such support. This is indicated to the MN during
"runtime", through the PrRtAdv message with a Code value of 3 (see
Section 6.1.2).
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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. This
NCoA can be provided to NAR in the HI message. NAR provides a
disposition of HI, and hence the NCoA itself, in the HAck message
indicating whether NCoA is acceptable. However, the MN itself does
not have to wait on PAR's link for this exchange to take place. It
can handover any time after sending the FBU message; sometimes it
may be forced to handover without sending the FBU. In any case, it
can still confirm using NCoA from NAR's link by sending the UNA
message.
If PrRtAdv message carries a NCoA, the MN MUST use it as its
prospective NCoA.
5.3. Prefix Management
As defined in Section 2, the Prefix part of ``AR-Info'' is the
prefix valid on the interface to which the AP is attached. This
document does not specify how this Prefix is managed, it's length
and assignment policies. The protocol operation specified in this
document works regardless of these considerations. Often, but not
necessarily always, this Prefix may be the aggregate prefix (such
as /48) valid on the interface. In some deployments, each MN may
have its own per-mobile prefix (such as a /64) used for generating
the NCoA. Some point-to-point links may use such a deployment.
When per-mobile prefix assignment is used, the ``AR-Info''
advertised in PrRtAdv still includes the (aggregate) prefix valid
on the interface to which the target AP is attached, unless the
access routers communicate with each other (using HI and HAck
messages) to manage per-mobile prefix. The MN still formulates an
NCoA using the aggregate prefix. However, an alternate NCoA based
on the per-mobile prefix is returned by NAR in the HAck message.
This alternate NCoA is provided to the MN in either the FBack
message or in the NAACK option.
5.4. Packet Loss
Handover involves link switching, which may not be exactly
co-ordinated with fast handover signaling. Furthermore, the
arrival pattern of packets is dependent on many factors, including
application characteristics, network queuing behaviors etc. Hence,
packets may arrive at NAR before the MN is able to establish its
link there. 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 until FBU is processed will be
lost unless they are buffered. This protocol provides an option to
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indicate request for buffering at the NAR in the HI message. When
the PAR requests this feature (for the MN), it SHOULD also provide
its own support for buffering.
5.5. DAD Handling
Duplicate Address Detection (DAD) was defined in [9] 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
auto-configuration adds delays to a handover.
The probability of an interface identifier duplication on the
same subnet is very low, however it cannot be ignored. In this
draft certain precautions are proposed to minimize the effects of
a duplicate address occurrence as well as recovery actions in the
event of a collision.
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 for access control, and by searching
this list, it can confirm whether the MN's address is a duplicate
or not. In some other deployments, the NAR may maintain a pool
of duplicate-free addresses in a list for handover purposes. The
result of NCoA disposition is sent back to the PAR in the HAck
message. The NAR can also indicate this in the NAACK option as
a response to the UNA message. When there is a duplicate, NAR
can propose (in NAACK option) an alternative NCoA or support the
PCoA using the host route forwarding. When no such support is
available, the MN would have to follow the address configuration
procedure according to [9] after attaching to the NAR.
In deployments where NAR does not have means to assess and inform
the uniqueness of NCoA or cannot provide a duplicate-free address
using HI and HAck exchange, the following scenarios are possible,
although highly improbable considering that the probability of a
random address collision is very small.
1. The MN sends FBU from the previous link which results in
packet forwarding to NCoA. These packets may arrive before
the MN attaches to NAR, and hence the latter may invoke
Neighbor Discovery. In the event that there is another node
which already owns the NCoA, NAR (incorrectly) forwards those
packets to such a node. When the MN arrives on the link, it
immediately sends a UNA message, which allows NAR to detect
a collision. NAR immediately sends a Router Advertisement
with NAACK option, forcing the MN to either use another NCoA
supplied in NAACK or reconfigure a new one. The MN must send
an FBU immediately following the NCoA configuration. As a
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special case, the NCoA may be that of NAR itself, which allows
the MN to send FBU that binds its PCoA to NAR's address. This
recovers from temporary misdelivery of packets. Where this
is a concern, the deployments SHOULD use HI and HAck exchange
which mitigates the problem by allowing NAR to proxy the NCoA;
such a proxying itself can detect a collision if an entry
already exists in the neighbor cache entry.
2. The MN sends a UNA message followed by an FBU from the new
link. When NAR processes the UNA message, either there is
already an entry for NCoA or there is no entry. If there is an
entry, it either belongs to the MN itself (e.g., in INCOMPLETE
state) or the entry belongs to another node. These entries
can be distinguished by the LLA; the entry with INCOMPLETE
state has no LLA. If the entry belongs to another node, NAR
immediately sends a Router Advertisement with NAACK option (as
above) and the MN MUST immediately send a new FBU to PAR with a
different NCoA. Hence, extent of any misdelivery is minimized.
If there is no existing entry for NCoA but there is another
node which owns NCoA, the scenario is more complicated.
According to [8], the UNA message does not create any entry
if there is none to begin with. However, NAR performs
Neighbor Solicitation when packets arrive from PAR (due to
FBU processing). Both the MN and the rightful owner respond
with Neighbor Advertisement (NA), but the MN's Neighbor
Advertisement will have the 'O' bit cleared. If the MN's NA
arrives first, NAR starts forwarding to it, but redirects those
packets once the NA from the rightful owner is processed. At
the time of updating the neighbor cache entry, the NAR must
send a Router Advertisement with NAACK option to the MN (as
above), and the MN MUST immediately send a new FBU to the PAR.
If the MN's NA arrives after the NA from the rightful owner,
NAR similarly sends a Router Advertisement with NAACK option,
and the MN sends a new FBU to the PAR. In both the cases,
the extent of misdelivery can be controlled and recovery is
possible.
The scenario where NAR has no entry for NCoA at all when
packets arrive is possible even when using HI and HAck
messages. The available options in this case appear to be a)
performing DAD for a set of addresses beforehand for handover
purposes, and b) maintaining a table of IP addresses of all
nodes on the link (similar to Mobile IPv4 visitor list). The
NAR can then provide a conflict-free address in the HAck
message or the NAACK option.
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5.6. 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 but it either
moves to a different one or 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 occur at a rate suitable for the MN to update the
binding of, at least, its Home Agent 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. The signaling overhead
over the air and in the network may increase significantly,
especially in the case of rapid movement between several 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 to the NAR. The PAR, on receiving this
Fast Binding Update, will check its set of outgoing (temporary
fast handover) tunnels. If it finds a match it SHOULD terminate
that tunnel; i.e., start delivering packets directly to the node
instead.
Temporary tunnels for the purposes of fast handovers should use
short lifetimes (of 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. The default lifetime of the
tunnel should be the same as the lifetime value in the FBU message.
The effect of erroneous movement is typically limited to loss of
packets since routing can change and the PAR may forward packets
towards another router before the MN actually connects to that
router. If the MN discovers itself on an unanticipated access
router, it SHOULD send a new Fast Binding Update to the PAR. This
FBU supercedes the existing binding at PAR and the packets will be
redirected to the new confirmed location of the MN.
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6. Message Formats
All the ICMPv6 messages have a common Type specified in [4]. The
messages are distinguished based on the Subtype field (see below).
The values for the Subtypes are specified in Section 9. For all
the ICMPv6 messages, the checksum is defined in [2].
6.1. New Neighborhood Discovery Messages
6.1.1. Router Solicitation for Proxy Advertisement (RtSolPr)
Mobile Nodes send Router Solicitation for Proxy Advertisement in
order to prompt routers for Proxy Router Advertisements. All the
link-layer address options have the format defined in 6.4.3.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subtype | Reserved | Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
Figure 4: Router Solicitation for Proxy
Advertisement (RtSolPr) Message
IP Fields:
Source Address
An IP address assigned to the sending interface
Destination Address
The address of the Access Router or the all routers
multicast address.
Hop Limit 255. See RFC 2461.
ICMP Fields:
Type The Experimental Mobility Protocol Type. See [4].
Code 0
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Checksum The ICMPv6 checksum.
Subtype 2
Reserved MUST be set to zero by the sender and ignored by
the receiver.
Identifier MUST be set by the sender so that replies can be
matched to this Solicitation.
Valid Options:
Source Link-layer Address
When known, the link-layer address of the sender
SHOULD be included using the Link-Layer Address
option. See LLA option format below.
New Access Point Link-layer Address
The link-layer address or identification of the
access 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. See LLA Option below.
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.
Including the source LLA option allows the receiver to record the
sender's L2 address so that neighbor discovery, when the receiver
needs to send packets back to the sender (of RtSolPr message), can
be avoided.
When a wildcard is used for New Access Point LLA, no other New
Access Point LLA options must be present.
A Proxy Router Advertisement (PrRtAdv) message should be received
by the MN as a response to RtSolPr. 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 milliseconds if
RTT is not known, it SHOULD resend RtSolPr message. Subsequent
retransmissions can be up to RTSOLPR|RETRIES, but MUST use an
exponential backoff in which the timeout period (i.e., 2xRTT or 100
milliseconds) is doubled prior to 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.
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When RtSolPr messages are sent more than once, they MUST be rate
limited with MAX|RTSOLPR|RATE per second. During each use of
RtSolPr, exponential backoff is used for retransmissions.
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. All the
link-layer address options have the format defined in 6.4.3.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subtype | Reserved | Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
Figure 5: Proxy Router Advertisement (PrRtAdv) Message
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 Advertisement or the address
of the node the Access Router is instructing to
handover.
Hop Limit 255. See RFC 2461.
ICMP Fields:
Type The Experimental Mobility Protocol Type. See [4].
Code 0, 1, 2, 3 or 4. See below.
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Checksum The ICMPv6 checksum.
Subtype 3
Reserved MUST be set to zero by the sender and ignored by
the receiver.
Identifier Copied from Router Solicitation for Proxy
Advertisement or set to Zero if unsolicited.
Valid Options in the following order:
Source Link-layer Address
When known, the link-layer address of the sender
SHOULD be included using the Link-Layer Address
option. See LLA option format below.
New Access Point Link-layer Address
The link-layer address or identification of the
access point is copied from RtSolPr
message. This option MUST be present.
New Router's Link-layer Address
The link-layer address of the Access Router for
which this message is proxied for. This option MUST be
included when Code is 0 or 1.
New Router's IP Address
The IP address of NAR. This option MUST be
included when Code is 0 or 1.
New Router Prefix Information Option.
Specifies the prefix of the Access
Router the message is proxied for and is used
for address auto-configuration. This option MUST be
included when Code is 0 or 1. However, when this
prefix is the same as what is used in the New
Router's IP Address option (above), the Prefix
Information option need not be present.
New CoA Option
MAY be present when PrRtAdv is sent
unsolicited. PAR MAY compute new CoA using NAR's
prefix information and the MN's L2 address, or by
any other means.
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.
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Currently, Code values 0, 1, 2, 3 and 4 are defined.
A Proxy Router Advertisement with Code 0 means that the MN should
use the [AP-ID, AR-Info] tuple (present in the options above) for
movement detection and NCoA formulation. The Option-Code field
in the New Access Point LLA option in this case is 1 reflecting
the LLA of the access point for which the rest of the options are
related. Multiple tuples may be present.
A Proxy Router Advertisement with Code 1 means that the message is
sent unsolicited. If a New CoA option is present following the New
Router Prefix Information option, the MN SHOULD use the supplied
NCoA and send FBU immediately or else stand to lose service.
This message acts as a network-initiated handover trigger. See
Section 3.3. The Option-Code field in the New Access Point LLA
option (see below) in this case is 1 reflecting the LLA of the
access point for which the rest of the options are related.
A Proxy Router Advertisement with Code 2 means that no new router
information is present. Each New Access Point LLA option contains
an Option-Code value (described below) which indicates a specific
outcome.
- When the Option-Code field in the New Access Point LLA option
is 5, handover to that access point does not require change of
CoA. No other options are required in this case.
- When the Option-Code field in the New Access Point LLA option
is 6, 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 other options are required in
this case.
- When the Option-Code field in the New Access Point LLA option
is 7, it means that the NAR does not support fast handover.
The MN MUST stop fast handover protocol operations. No other
options are required in this case.
A Proxy Router Advertisement with Code 3 means that new router
information is present only for a subset of access points
requested. The Option-Code field values (defined above including
a value of 1) distinguish different outcomes for individual access
points.
A Proxy Router Advertisement with Code 4 means that the
subnet information regarding neighboring access points is sent
unsolicited, but the message is not a handover trigger, unlike when
the message is sent with Code 1. Multiple tuples may be present.
When a wildcard AP identifier is supplied in the RtSolPr
message, the PrRtAdv message should include any 'n' [Access Point
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Identifier, Link-layer address option, Prefix Information Option]
tuples corresponding to the PAR's neighborhood.
6.2. Inter-Access Router Messages
6.2.1. Handover Initiate (HI)
The Handover Initiate (HI) is an 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subtype |S|U| Reserved | Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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. See RFC 2461.
ICMP Fields:
Type The Experimental Mobility Protocol Type. See [4].
Code 0 or 1. See below
Checksum The ICMPv6 checksum.
Subtype 4
S Assigned address configuration flag. When set, this
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message requests a new CoA to be returned by the
destination. May be set when Code = 0. MUST be 0
when Code = 1.
U Buffer flag. When set, the destination SHOULD buffer
any packets towards the node indicated in the options
of this message. Used when Code = 0, SHOULD be set
to 0 when Code = 1.
Reserved MUST be set to zero by the sender and ignored by
the receiver.
Identifier MUST be set by the sender so replies can be matched
to this message.
Valid Options:
Link-layer address of MN
The link-layer address of the MN that is
undergoing handover to the destination (i.e., NAR).
This option MUST be included so that the destination
can recognize the MN.
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 assign this address.
The PAR uses a Code value of 0 when it processes an FBU with PCoA
as source IP address. The PAR uses a Code value of 1 when it
processes an FBU whose source IP address is not PCoA.
If Handover Acknowledge (HAck) message is not received as a
response in a short time period but no less than twice the typical
round trip time (RTT) between source and destination, or 100
milliseconds if RTT is not known, the Handover Initiate SHOULD be
re-sent. Subsequent retransmissions can be up to HI|RETRIES, but
MUST use exponential backoff in which the timeout period (i.e.,
2xRTT or 100 milliseconds) is doubled during each instance of
retransmission.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subtype | Reserved | Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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. See RFC 2461.
ICMP Fields:
Type The Experimental Mobility Protocol Type. See [4].
Code
0: Handover Accepted, NCoA valid
1: Handover Accepted, NCoA not valid
2: Handover Accepted, NCoA in use
3: Handover Accepted, NCoA assigned
(used in Assigned addressing)
4: Handover Accepted, NCoA not assigned
(used in Assigned addressing)
5: Handover Accepted, use PCoA
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6: Message sent unsolicited, usually to trigger a
HI message
128: Handover Not Accepted, reason unspecified
129: Administratively prohibited
130: Insufficient resources
Checksum The ICMPv6 checksum.
Subtype 5
Reserved MUST be set to zero by the sender and ignored by
the receiver.
Identifier Copied from the corresponding field in the Handover
Initiate message this message is in response to.
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 a 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 3.
The NAR MAY provide support for PCoA (instead of accepting or
assigning NCoA), using a host route entry to forward packets to
the PCoA, and using a tunnel to the PAR to forward packets from
the MN (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. The NAR indicates
forwarding support for PCoA using Code value 5 in the HAck message.
Subsequently, PAR establishes a tunnel to NAR in order to forward
packets arriving for PCoA.
When responding to a HI message containing a Code value 1, the Code
values 1, 2, and 4 in the HAck message are not relevant.
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.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 Acknowledgment message.
`H' flag MUST be set to one. See [3].
`L' flag See [3].
`K' flag See [3].
Reserved This field is unused. MUST be set zero.
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Sequence Number See [3].
Lifetime The requested time in seconds for which the
sender wishes to have a binding.
Mobility Options
MUST contain alternate CoA option set to NCoA
IP address when FBU is sent from PAR's link.
MUST contain the Binding Authorization Data for
FMIP (BADF) option. See 6.4.5. MAY contain the
Mobility Header LLA option (see Section 6.4.4).
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 NCoA on the NAR
according to Neighbor Discovery and IPv6 Address Configuration
protocols.
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.
The FBU MUST also include the Home Address Option and the Home
Address is PCoA. A FBU message MUST be protected so that PAR is
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. If PAR sends a HI message to the NAR after
processing an FBU, the FBack message SHOULD NOT be sent to the MN
before the PAR receives a HAck message from the NAR. The PAR MAY
send the FBack immediately in the reactive mode however. The Fast
Binding Acknowledgment MAY also be sent to the MN on the old link.
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''
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
sender of this message will retain a binding
for traffic redirection.
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Mobility Options MUST contain ``alternate'' CoA if Status is 1.
MUST contain the Binding Authorization Data
for FMIP (BADF) option. See 6.4.5.
6.3.3. Unsolicited Neighbor Advertisement (UNA)
This is the same message as in [8] with the requirement that the
'O' bit is always set to zero. Since this is an unsolicited
message, the 'S' bit is zero, and since this is sent by a MN, the
'R' bit is also zero.
The Source Address must be the NCoA. The Destination Address
is typically the all-nodes multicast address; however, some
deployments may not prefer transmission to a multicast address. In
such cases, the Destination Address SHOULD be the NAR's IP address.
The Target Address must include the NCoA, and Target link-layer
address must include the MN's LLA.
The MN sends a UNA message to the NAR, as soon as it regains
connectivity on the new link. Arriving or buffered packets can
be immediately forwarded. If NAR is proxying NCoA, it creates
a neighbor cache entry in STALE state but forwards packets as
it determines bidirectional reachability. If there is an entry
in INCOMPLETE state without a link-layer address, it sets it to
STALE. If there is no entry at all, creating an entry in STALE
state is recommended since forwarding can immediately begin when
packets arrive without first invoking Neighbor Solicitation and
Advertisement (which may involve retransmission delay in the event
of messages being lost). During the process of creating a neighbor
cache entry, NAR can also detect if NCoA is in use, and immediately
sends a Router Advertisement with NAACK option in the event of
collision (see Section 5.5 for more details).
The combination of NCoA (present in source IP address) and the
Link-Layer Address (present as a Target LLA) SHOULD be used to
distinguish the MN from other nodes.
6.4. New Options
All the options are of the form shown in Figure 10.
The Type values are defined from the Neighbor Discovery options
space. The Length field is in units of 8 octets, except for the
Mobility Header Link-Layer Address option, whose Length field is
in units of octets in accordance with [3], Section 6.2. And,
Option-Code provides additional information for each of the options
(See individual options below).
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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 | Length | Option-Code | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ... ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: Option Format
6.4.1. IP Address Option
This option is sent in the Proxy Router Advertisement, the Handover
Initiate, and Handover Acknowledge messages.
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 | Prefix Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ IPv6 Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: IPv6 Address Option
Type
To be assigned by IANA
Length
The size of this option in 8 octets including the Type,
Option-Code and Length fields.
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Option-Code
1 Old Care-of Address
2 New Care-of Address
3 NAR's IP address
Prefix Length
The Length of the IPv6 Address Prefix.
Reserved
MUST be set to zero by the sender and MUST be
ignored by the receiver.
IPv6 address
The IP address defined by the Option-Code 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 | Option-Code | Prefix Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Prefix +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: New Router Prefix Information Option
Type
To be assigned by IANA
Length
The size of this option in 8 octets including the Type,
Option-Code and Length fields.
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Option-Code
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.
Reserved
MUST be set to zero by the sender and MUST be
ignored by the receiver.
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) Option
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 | LLA...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 13: Link-Layer Address Option
Type
To be assigned by IANA
Length
The size of this option in 8 octets including the Type,
Option-Code and Length fields.
Option-Code
0 wildcard requesting resolution for all nearby access points
1 Link-layer Address of the New Access Point
2 Link-layer Address of the MN
3 Link-layer Address of the NAR (i.e., Proxied Originator)
4 Link-layer Address of the source of RtSolPr or PrRtAdv
message
5 The access point identified by the LLA belongs to the
current interface of the router
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6 No prefix information available for the access point
identified by the LLA
7 No fast handovers support available for the access point
identified by the LLA
LLA
The variable length link-layer address.
The LLA Option does not have a length field for the LLA itself.
The implementations must consult the specific link layer over which
the protocol is run in order to determine the content and length of
the LLA.
Depending on the size of individual LLA option, appropriate padding
MUST be used to ensure that the entire option size is a multiple of
8 octects.
The New Access Point Link Layer address contains the link-layer
address of the access point for which handover is about to be
attempted. This is used in the Router Solicitation for Proxy
Advertisement message.
The MN Link-Layer address option contains the link-layer address of
a MN. It is used in the Handover Initiate message.
The NAR (i.e., Proxied Originator) Link-Layer address option
contains the Link Layer address of the Access Router for which the
Proxy Router Solicitation message refers to.
6.4.4. Mobility Header Link-layer Address (MH-LLA) Option
This option is identical to the LLA option, but is carried in
the Mobility Header messages, e.g., FBU. In the future, other
Mobility Header messages may also make use of this option. The
format of the option is shown in Figure 14. There are no alignment
requirements for this option.
Type
To be assigned by IANA
Length
The size of this option in octets not including the Type
and Length fields.
Option-Code
2 Link-layer Address of the MN
LLA
The variable length link-layer address.
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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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option-Code | LLA ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14: Mobility Header Link-Layer Address Option
6.4.5. Binding Authorization Data for FMIPv6 (BADF)
This option MUST be present in FBU and FBack messages. The
security association between the MN and the PAR is established by
companion protocols [5]. This option specifies how to compute and
verify a MAC using the established security association.
The format of this option is shown in Figure 15.
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 | Option Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| Authenticator |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 15: Binding Authorization Data for FMIPv6 (BADF) Option
Type
To be assigned by IANA
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Option Length
The length of the Authenticator in bytes
SPI
Security Parameter Index. SPI = 0 is reserved for the
Authenticator computed using SEND-based handover keys.
Authenticator
Same as in RFC 3775, with "correspondent" replaced by
PAR's IP address, and Kbm replaced by the shared key
between the MN and the PAR.
The default MAC calculation is done using HMAC_SHA1 with the first
96 bits used for the MAC. Since there is an Option Length field,
implementations can use other algorithms such as HMAC_SHA256 for
instance.
This option MUST be the last Mobility Option present.
6.4.6. 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 | Option-Code | Status |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 16: Neighbor Advertisement Acknowledgment Option
Type
To be assigned by IANA.
Length
8-bit unsigned integer. Length of the option, in 8
octets. The length is 1 when a new CoA is not supplied. The
length is 3 when a new CoA is present (immediately following
the Reserved field)
Option-Code
0
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Status
8-bit unsigned integer indicating the disposition of the Fast
Neighbor Advertisement message. The following Status
values are currently defined:
1 The New CoA is invalid
2 The New CoA is invalid, use the supplied CoA. The New
CoA (in the form of an IP Address Option) MUST be
present following the Reserved field.
3 The New CoA is invalid, use NAR's IP address as NCoA in
FBU
4 PCoA supplied, do not send FBU
128 Link Layer Address unrecognized
Reserved
MUST be set to zero by the sender and MUST be
ignored by the receiver.
The NAR responds to UNA with the NAACK option to notify the MN
to use a different NCoA if there is address collision. If the
NCoA is invalid, the Router Advertisement MUST use the NCoA as the
destination address but use the L2 address present in UNA. The MN
SHOULD use the NCoA if it is supplied 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 different NCoA at the NAR.
In the future, new option types may be defined.
7. Configurable Parameters
Parameter Name Default Value Definition
------------------- ---------------------- -------
RTSOLPR_RETRIES 3 Section6.1.1
MAX_RTSOLPR_RATE 3 Section6.1.1
FBU_RETRIES 3 Section 4
PROXY_ND_LIFETIME 1.5 seconds Section 6.2.2
HI_RETRIES 3 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.
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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 relies on a companion protocol [5]
to establish such a security association. Using the shared
handover key from [5], the Authenticator in BADF option
(see 6.4.5) MUST be computed, and the BADF option included in
FBU and FBack messages.
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 inadvertent redirection: in this
case, the FBU is secured, but the target of binding happens to
be an unsuspecting node either due to inadvertent 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 (with Code = 0), it proxies NCoA so
that any arriving packets are not sent on the link until the MN
attaches and announces itself through UNA. So, any inadvertent
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, the extent of this vulnerability
is arguably small.
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. This is similar to base Mobile IP where the MN can
provide some other's node as its CoA to its Home Agent. As
discussed in Section 5.5, the extent of such a misdelivery can
be controlled and recovery is possible. In addition, it is
possible to isolate the MN if it continues to misbehave.
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9. IANA Considerations
This document defines four new experimental ICMPv6 messages which
use the Experimental Mobility Protocol ICMPv6 format [4]. These
require four new Subtype value assignments out of the Experimental
Mobility Protocol Subtype Registry [4] as follows:
Subtype Description Reference
------- ----------- ---------
2 RtSolPr Section 6.1.1
3 PrRtAdv Section 6.1.2
4 HI Section 6.2.1
5 HAck Section 6.2.2
The document defines four new Neighbor Discovery [8] options which
need Type assignment from IANA.
Option-Type Description Reference
----------- ----------- ---------
TBD IP Address Option Section 6.4.1
TBD New Router Prefix
Information Option Section 6.4.2
TBD Link-layer Address
Option Section 6.4.3
TBD Neighbor Advertisement
Acknowledgment Option Section 6.4.6
The document defines three new Mobility Header messages which
need type allocation from the Mobility Header Types registry at
http://www.iana.org/assignments/mobility-parameters:
1. Fast Binding Update, described in Section 6.3.1
2. Fast Binding Acknowledgment, described in Section 6.3.2, and
The document defines two new Mobility Options which need
type assignment from the Mobility Options Type registry at
http://www.iana.org/assignments/mobility-parameters:
1. Mobility Header Link-Layer Address option, described in
Section 6.4.4.
2. Binding Authorization Data for FMIPv6 (BADF) option, described
in Section 6.4.5.
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10. Acknowledgments
The editor would like to thank all those who have provided feedback
on this specification, and acknowledges the following people:
Vijay Devarapalli, Youn-Hee Han, Emil Ivov, Syam Madanapalli,
Suvidh Mathur, Andre Martin, Javier Martin, Koshiro Mitsuya,
Gabriel Montenegro, Takeshi Ogawa, Sun Peng, YC Peng, Alex
Petrescu, Domagoj Premec, Subba Reddy, K. Raghav, Ranjit Wable and
Jonathan Wood. Behcet Sarikaya and Frank Xia are acknowledged for
the feedback on operation over point-point links. The editor would
like to acknowledge the contribution from James Kempf to improve
this specification. The editor would also like to thank [mipshop]
working group chair Gabriel Montenegro and the erstwhile [mobile
ip] working group chairs Basavaraj Patil and Phil Roberts for
providing much support for this work.
11. Normative 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.
[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. E. Perkins, and J. Arkko, ``Mobility Support
in IPv6'', Request for Comments (Proposed Standard) 3775,
Internet Engineering Task Force, June 2004.
[4] J. Kempf, ``Instructions for Seamoby and Experimental Mobility
Protocol IANA Allocations," RFC 4065, Internet Engineering
Task Force, June 2004.
[5] J. Kempf and R. Koodli, "Distributing a Symmetric FMIPv6
Handover Key using SEND," draft-ietf-mipshop-handover-key-00.txt
(work in progress), February 2007.
[6] S. Kent and R. Atkinson, ``IP Authentication Header'', Request
for Comments (Draft Standard) 2402, Internet Engineering Task
Force, November 1998.
[7] R. Koodli (Editor), "Fast Handovers for Mobile IPv6," Request
For Comments 4068, Internet Engineering Task Force, July 2005.
[8] T. Narten, E. Nordmark, and W. Simpson, ``Neighbor Discovery
for IP Version 6 (IPv6)'', Request for Comments (Draft
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Standard) 2461, Internet Engineering Task Force, December
1998.
[9] S. Thomson and T. Narten, ``IPv6 Stateless Address
Autoconfiguration'', Request for Comments (Draft Standard)
2462, Internet Engineering Task Force, December 1998.
12. Author's Address
Rajeev Koodli, Editor
Nokia Siemens Networks
313 Fairchild Drive
Mountain View, CA 94043 USA
Phone: +1 650 625 2359
Fax: +1 650 625 2502
E-Mail: Rajeev.Koodli@nokia.com
13. Contributors
This document has its origins in the fast handover design team
in the erstwhile [mobile ip] working group. 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.
A. Change Log
- RFC4068bis: all the issues in the tracker since the
publication of RFC 4068. (http://www.mip4.org/issues/tracker/mipshop)
The following changes pre-date RFC 4068 publication.
- Added IPSec AH reference.
- Changed options format to make use of RFC 2461 options Type
space. Revised IANA Considerations section accordingly.
- Added exponential backoff for retransmissions. Added rate
limiting for RtSolPr message.
- Replaced ``attachment point'' with ``access point'' for
consistency.
- Clarified [AP-ID, AR-Info] in Section 2. Clarified use of
Prefix Information Option in Section 6.1.2.
- Separated MH-LLA from LLA to future-proof LLA option.
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The following changes refer up to version 02 (under mipshop). The
Section numbers refer to version 06 (under mobile ip).
- New ICMPv6 format incorporated. ID Nits conformance.
- Last Call comments incorporated
- 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
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Koodli (Editor) Expires 8 January 2008 [Page 43]
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