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Network Working Group R. Koodli, Ed.
Request for Comments: 5268 Starent Networks
Obsoletes: 4068 June 2008
Category: Standards Track
Mobile IPv6 Fast Handovers
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
Mobile IPv6 enables a Mobile Node (MN) to maintain its connectivity
to the Internet when moving from one Access Router to another, a
process referred to as handover. During handover, there is a period
during which the Mobile Node is unable to send or receive packets
because of 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 (VoIP). 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.
Koodli, Ed. Standards Track [Page 1]
RFC 5268 MIP6 Fast Handovers June 2008
Table of Contents
1. Introduction ....................................................3
2. Terminology .....................................................3
3. Protocol Overview ...............................................6
3.1. Addressing the Handover Latency ............................6
3.2. Protocol Operation .........................................8
3.3. Protocol Operation during Network-Initiated Handover ......11
4. Protocol Details ...............................................11
5. Other Considerations ...........................................15
5.1. Handover Capability Exchange ..............................15
5.2. Determining New Care-of Address ...........................16
5.3. Prefix Management .........................................16
5.4. Packet Loss ...............................................17
5.5. DAD Handling ..............................................18
5.6. Fast or Erroneous Movement ................................19
6. Message Formats ................................................20
6.1. New Neighborhood Discovery Messages .......................20
6.1.1. Router Solicitation for Proxy Advertisement
(RtSolPr) ..........................................20
6.1.2. Proxy Router Advertisement (PrRtAdv) ...............22
6.2. Inter - Access Router Messages ............................25
6.2.1. Handover Initiate (HI) .............................25
6.2.2. Handover Acknowledge (HAck) ........................27
6.3. New Mobility Header Messages ..............................28
6.3.1. Fast Binding Update (FBU) ..........................28
6.3.2. Fast Binding Acknowledgment (FBack) ................30
6.4. Unsolicited Neighbor Advertisement (UNA) ..................31
6.5. New Options ...............................................32
6.5.1. IP Address/Prefix Option ...........................33
6.5.2. Link-Layer Address (LLA) Option ....................34
6.5.3. Mobility Header Link-Layer Address (MH-LLA)
Option .............................................35
6.5.4. Binding Authorization Data for FMIPv6 (BADF) .......35
6.5.5. Neighbor Advertisement Acknowledgment (NAACK) ......36
7. Related Protocol and Device Considerations .....................37
8. Evolution from and Compatibility with RFC 4068 .................38
9. Configurable Parameters ........................................39
10. Security Considerations .......................................39
10.1. Peer Authorization Database Entries when Using IKEv2 .....41
10.2. Security Policy Database Entries .........................42
11. IANA Considerations ...........................................42
12. Acknowledgments ...............................................43
13. References ....................................................44
13.1. Normative References .....................................44
13.2. Informative References ...................................45
Appendix A. Contributors ..........................................46
Appendix B. Changes since RFC 4068 ................................46
Koodli, Ed. Standards Track [Page 2]
RFC 5268 MIP6 Fast Handovers June 2008
1. Introduction
Mobile IPv6 [RFC3775] 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
link-layer procedures, 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 problems: 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, an MN engages in Mobile IP
operations including Return Routability [RFC3775]. There are no
special requirements for a mobile node to behave differently with
respect to its standard Mobile IP operations.
This specification is applicable when a mobile node has to perform IP
layer operations as a result of handovers. This specification does
not address improving the link switching latency. It does not modify
or optimize procedures related to signaling with the home agent of a
mobile node. Indeed, while targeted for Mobile IPv6, it could be
used with any mechanism that allows communication to continue despite
movements. Finally, this specification does not address bulk
movement of nodes using aggregate prefixes.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
The use of the term, "silently ignore" is not defined in RFC 2119.
However, the term is used in this document and can be similarly
construed.
The following terminology and abbreviations are used in this document
in addition to those defined in [RFC3775]. The reference handover
scenario is illustrated in Figure 1.
Koodli, Ed. Standards Track [Page 3]
RFC 5268 MIP6 Fast Handovers June 2008
v +--------------+
+-+ | Previous | <
| | ------------ | Access | ------- >-----\
+-+ | Router | < \
MN | (PAR) | \
| +--------------+ +---------------+
| ^ IP | Correspondent |
| | Network | Node |
V | +---------------+
v /
v +--------------+ /
+-+ | New | < /
| | ------------ | Access | ------- >-----/
+-+ | Router | <
MN | (NAR) |
+--------------+
Figure 1: Reference Scenario for Handover
Mobile Node (MN): A Mobile IPv6 host.
Access Point (AP): A Layer 2 device connected to an IP subnet that
offers wireless connectivity to an 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.
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.
Koodli, Ed. Standards Track [Page 4]
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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
Section 5.3.
Neighborhood Discovery: The process of resolving neighborhood AP-IDs
to AR-Info.
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 (toward NAR).
Fast Binding Acknowledgment (FBack): A message from the PAR in
response to an FBU.
Predictive Fast Handover: The fast handover in which an MN is able to
send an FBU when it is attached to the PAR, which then establishes
forwarding for its traffic (even before the MN attaches to the NAR).
Reactive Fast Handover: The fast handover in which an MN is able to
send the FBU only after attaching to the NAR.
Unsolicited Neighbor Advertisement (UNA): The message in [RFC4861]
with 'O' bit cleared.
Fast Neighbor Advertisement (FNA): This message from RFC 4068
[RFC4068] 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
an MN's handover.
Handover Acknowledge (HAck): A message from the NAR to the PAR as a
response to HI.
Koodli, Ed. Standards Track [Page 5]
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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 an 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 process the Binding Update successfully,
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 new CoA depends on the Binding
Update latency as well as the IP connectivity latency.
The protocol enables an 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, an MN may
discover available access points using link-layer specific mechanisms
(e.g., a "scan" in Wireless Local Area Network (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 or 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 an 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 coordinates including its
prefix, IP address, and L2 address. The "Router Solicitation for
Proxy Advertisement (RtSolPr)" and "Proxy Router Advertisement
(PrRtAdv)" messages in Section 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)" (see Section 6.3.2) 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)
[RFC4861]; NAR responds to this UNA message in case it wishes to
provide a different IP address to use. In this way, NCoA
configuration latency is reduced.
Koodli, Ed. Standards Track [Page 6]
RFC 5268 MIP6 Fast Handovers June 2008
The information provided in the PrRtAdv message can be used even when
DHCP [RFC3315] is used to configure an NCoA on the NAR's link. In
this case, the protocol supports forwarding using PCoA, and the MN
performs DHCP once it attaches to the NAR's link. The MN still
formulates an NCoA for FBU processing; however, it MUST NOT send data
packets using the NCoA in the FBU.
In order to reduce the Binding Update latency, the protocol specifies
a binding between the Previous CoA (PCoA) and NCoA. An MN sends a
"Fast Binding Update" (see Section 6.3.1) message to its Previous
Access Router to establish this tunnel. When feasible, the MN SHOULD
send an FBU from the PAR's link. Otherwise, the MN should send the
FBU immediately after detecting attachment to the NAR. An FBU
message MUST contain the Binding Authorization Data for FMIPv6 (BADF)
option (see Section 6.5.4) 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 the 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 the PAR, again until it completes
Binding Update. And, PAR MUST 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 a PCoA as a source IP address
are not dropped due to ingress filtering. Even though the MN is
IP-capable on the new link, it cannot use the NCoA directly with its
correspondents without the correspondents first establishing a
binding cache entry (for the NCoA). Forwarding support for the 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 it is attached to a new subnet link, unless the
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 the NAR to send Neighbor
Solicitation before the MN attaches, subsequent retransmissions of
address resolution are separated by a default period of one second
each. In order to circumvent this delay, an MN announces its
attachment immediately with an UNA message that allows the NAR to
forward packets to the MN right away. 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 an MN sends an FBU from the
Koodli, Ed. Standards Track [Page 7]
RFC 5268 MIP6 Fast Handovers June 2008
PAR's link, FBack can be delivered after the NAR considers the NCoA
acceptable for use. This is especially useful when addresses are
assigned by the access router. The NAR can also rely on its trust
relationship with the PAR before providing forwarding support for the
MN. That is, it may create a forwarding entry for the NCoA, subject
to "approval" from the PAR, which it trusts. In addition, buffering
for handover traffic at the NAR 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. Finally, the access routers could transfer
network-resident contexts, such as access control, Quality of Service
(QoS), and header compression, in conjunction with handover (although
the context transfer process itself is not specified in this
document). For all these operations, the protocol provides "Handover
Initiate (HI)" and "Handover Acknowledge (HAck)" messages (see
Section 6.2). Both of these messages SHOULD be used. The access
routers MUST have the necessary security association established by
means outside the scope of this document.
3.2. Protocol Operation
The protocol begins when an MN sends an RtSolPr message 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 containing one or more
[AP-ID, AR-Info] tuples. The MN may send an 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 an RtSolPr, the MN
will have discovered the available APs by link-specific methods. The
RtSolPr and PrRtAdv messages do not establish any state at the access
router; 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 to the PAR.
The purpose of the FBU is to authorize the PAR to bind the PCoA to
the NCoA, so that arriving packets can be tunneled to the new
location of the MN. The FBU should be sent from the 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, the FBU is sent from the new 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
6.5.4) to secure the message.
Koodli, Ed. Standards Track [Page 8]
RFC 5268 MIP6 Fast Handovers June 2008
Depending on whether an FBack is received on the previous link (which
clearly depends on whether the 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 is already in progress by the time the MN
handovers to the NAR. The MN SHOULD send the UNA immediately
after attaching to the NAR, so that arriving as well as
buffered packets can be forwarded to the MN right away.
Before sending FBack to the MN, the PAR can determine whether
the NCoA is acceptable to the 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 the
FBU is carried in an HI message (from PAR to NAR), and NAR MAY
assign the proposed NCoA. Such an assigned NCoA MUST be
returned in HAck (from NAR to PAR), 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.
2. The MN does not receive the FBack on the previous link because
the MN has not sent the FBU or the MN has left the link after
sending the FBU (which itself may be lost), but before
receiving an FBack. Without receiving an FBack in the latter
case, the MN cannot ascertain whether the PAR has processed
the FBU successfully. Hence, the MN (re)sends the FBU message
to the PAR immediately after sending the UNA message. If the
NAR chooses to supply a different IP address to use than the
NCoA, it MAY send a Router Advertisement with "Neighbor
Advertisement Acknowledge (NAACK)" option in which it includes
an alternate IP address for the MN to use. Detailed UNA
processing rules are specified in Section 6.4.
The scenario in which an MN sends an FBU and receives an FBack on
PAR's link is illustrated in Figure 2. For convenience, this
scenario is characterized as the "predictive" mode of operation. The
scenario in which the MN sends an FBU from the NAR's link is
illustrated in Figure 3. For convenience, this scenario is
characterized as the "reactive" mode of operation. Note that the
reactive mode also includes the case in which an FBU has been sent
from the PAR's link but an FBack has not yet been received. The
figure is intended to illustrate that the FBU is forwarded through
the NAR, but it is processed only by the PAR.
Koodli, Ed. Standards Track [Page 9]
RFC 5268 MIP6 Fast Handovers June 2008
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----------|
| |(HI/HAck if necessary)|
| forward |
| packets(including FBAck)=====>|
| | |
|<=================================== deliver packets
| |
Figure 3: Reactive Fast Handover
Koodli, Ed. Standards Track [Page 10]
RFC 5268 MIP6 Fast Handovers June 2008
Finally, the PrRtAdv message may be sent unsolicited, i.e., without
the MN first sending an 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 mutually
arrived at 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 the
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.
4. Protocol Details
All descriptions refer to Figure 1.
After discovering one or more nearby access points, the MN sends
RtSolPr to the PAR 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 transmitted immediately. Implementations SHOULD
make use of such triggers whenever available.
The RtSolPr message contains one or more AP-IDs. A wildcard requests
all available tuples.
Koodli, Ed. Standards Track [Page 11]
RFC 5268 MIP6 Fast Handovers June 2008
As a response to RtSolPr, the PAR sends a PrRtAdv message that
indicates one of the following possible conditions.
1. If the PAR does not have an entry corresponding to the new
access point, it MUST respond 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), the PAR MUST respond 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 the PAR MUST respond 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 that are subject to path MTU restrictions (i.e.,
provide any 'n' tuples without exceeding the link MTU).
When further protocol action is necessary, some implementations MAY
choose to begin buffering copies of incoming packets at the PAR. If
such First in First Out (FIFO) buffering is used, the PAR MUST
continue forwarding the packets to the PCoA (i.e., buffer and
forward). While the protocol does not forbid such an implementation
support, care must be taken to ensure that the PAR continues
forwarding packets to the PCoA (i.e., uses a buffer and forward
approach). The PAR SHOULD stop buffering once it begins forwarding
packets to the NCoA.
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.
Koodli, Ed. Standards Track [Page 12]
RFC 5268 MIP6 Fast Handovers June 2008
The RtSolPr and PrRtAdv messages MUST be implemented by an 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 the above messages is optional on such
links.
After a PrRtAdv message is processed, the MN sends an FBU at a time
determined by link-specific events, and includes the proposed NCoA.
The MN SHOULD send the FBU from the 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 an FBU
immediately after attaching to NAR's link. In response to the FBU,
the PAR establishes a binding between the PCoA ("Home Address") and
the NCoA, and sends the FBack to the MN. Prior to establishing this
binding, the PAR SHOULD send an HI message to the NAR, and receive
HAck in response. In order to determine the NAR's address for the HI
message, the PAR can perform the longest prefix match of NCoA (in
FBU) with the prefix list of neighboring access routers. When the
source IP address of the FBU is the 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 the 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 an HI message with Code 0, the
NAR:
1. determines whether the NCoA supplied in the HI message is
unique before beginning to defend it. It sends a Duplicate
Address Detection (DAD) probe [RFC4862] for NCoA to verify
uniqueness. However, in deployments where the probability of
address collisions is considered extremely low (and hence not
an issue), the parameter DupAddrDetectTransmits (see
[RFC4862]) is set to zero on the NAR, allowing it to avoid
performing DAD on the NCoA. The NAR similarly sets
DupAddrDetectTransmits to zero in other deployments where DAD
is not a concern. Once the NCoA is determined to be unique,
the NAR starts proxying [RFC4861] the address for
PROXY_ND_LIFETIME during which the MN is expected to connect
to the NAR. In case there is already an NCoA present in its
data structure (for instance, it has already processed an HI
message earlier), the NAR MAY verify if the LLA is the same as
its own or that of the MN itself. If so, the NAR MAY allow
the use of the NCoA.
Koodli, Ed. Standards Track [Page 13]
RFC 5268 MIP6 Fast Handovers June 2008
2. allocates the 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.
3. MAY create a host route entry for the PCoA (on the interface
to which the MN is attaching to) in case the 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 the PAR in this
case.
4. provides the status of the handover request in the Handover
Acknowledge (HAck) message to the PAR.
When the Code value in HI is 1, the NAR MUST skip the above
operations. Sending an HI message with Code 1 allows the NAR to
validate the neighbor cache entry it creates for the MN during UNA
processing. That is, the NAR can make use of the knowledge that its
trusted peer (i.e., the PAR) has a trust relationship with the MN.
If HAck contains an assigned NCoA, the FBack MUST include it, and the
MN MUST use the address provided in the FBack. The PAR MAY send the
FBack to the previous link as well to facilitate faster reception in
the event that the MN is still present. The result of the FBU and
FBack processing is that PAR begins tunneling the MN's packets to the
NCoA. If the MN does not receive an FBack message even after
retransmitting the 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 an UNA message (see Section 6.4). 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 [RFC4862].
When a NAR receives an UNA message, it:
1. deletes its proxy neighbor cache entry, if it exists, updates
the state to STALE [RFC4861], and forwards arriving and
buffered packets.
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2. updates an entry in INCOMPLETE state [RFC4861], if it exists,
to STALE and forwards arriving and buffered packets. This
would be the case if NAR had previously sent a Neighbor
Solicitation that went unanswered perhaps because the MN had
not yet attached to the link.
The buffer for handover traffic should be linked to this UNA
processing. The exact mechanism is implementation dependent.
The NAR may choose to provide a different IP address other than the
NCoA. This is possible if it is proxying the NCoA. In such a case,
it:
1. MAY send a Router Advertisement with the NAACK option in which
it includes an alternate IP address 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 the PCoA itself, in which case the MN MUST
NOT send any more FBUs. The Status codes for the NAACK option are
specified in Section 6.5.5.
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 the
PAR receives a reverse tunneled packet, it must verify if a secure
binding exists for the MN identified by the 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 the PAR does not support the fast handover protocol
and stop sending any more RtSolPr messages.
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Even if an MN's current access router is capable of providing fast
handover support, the new access router to which the MN attaches may
be incapable 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 the FBU. The PAR
MUST use the NCoA present in the FBU in its HI message. The NAR MUST
verify if the NCoA present in HI is already in use. In any case, the
NAR MUST respond to HI using a HAck, in which it may include another
NCoA to use, especially when assigned address configuration is used.
If there is a CoA present in HAck, the PAR MUST include it in the
FBack message. 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 a PrRtAdv message carries an NCoA, the MN MUST use it as its
prospective NCoA.
When DHCP is used, the protocol supports forwarding for PCoA only.
In this case, the MN MUST perform DHCP operations once it attaches to
the NAR even though it formulates an NCoA for transmitting the FBU.
This is indicated in the PrRtAdv message with Code = 5.
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
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manage the 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
coordinated 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 the 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 the NAR and then sends an
FBU message, packets arriving at the PAR until the FBU is processed
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 requests this feature (for the MN), it SHOULD
also provide its own support for buffering.
Whereas buffering can enable a smooth handover, the buffer size and
the rate at which buffered packets are eventually forwarded are
important considerations when providing buffering support. There are
a number of aspects to consider:
o Some applications transmit less data over a given period of data
than others, and this implies different buffering requirements.
For instance, Voice over IP typically needs smaller buffers
compared to high-resolution streaming video, as the latter has
larger packet sizes and higher arrival rates.
o When the mobile node appears on the new link, having the buffering
router send a large number of packets in quick succession may
overtax the resources of the router, the mobile node itself, or
the path between these two.
In particular, transmitting a large amount of buffered packets in
succession can congest the path between the buffering router and
the mobile node. Furthermore, nodes (such as a base station) on
the path between the buffering router and the mobile node may drop
such packets. If a base station buffers too many such packets,
they may contribute to additional jitter for packets arriving
behind them, which is undesirable for real-time communication.
o Since routers are not involved in end-to-end communication, they
have no knowledge of transport conditions.
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o The wireless connectivity of the mobile node may vary over time.
It may achieve a smaller or higher bandwidth on the new link,
signal strength may be weak at the time it just enters the area of
this access point, and so on.
As a result, it is difficult to design an algorithm that would
transmit buffered packets at appropriate spacing under all scenarios.
The purpose of fast handovers is to avoid packet loss. Yet, draining
buffered packets too fast can, by itself, cause loss of the packets,
as well as blocking or loss of following packets meant for the mobile
node.
This specification does not restrict implementations from providing
specialized buffering support for any specific situation. However,
attention must be paid to the rate at which buffered packets are
forwarded to the MN once attachment is complete. Routers
implementing this specification MUST implement at least the default
algorithm, which is based on the original arrival rates of the
buffered packets. A maximum of 5 packets MAY be sent one after
another, but all subsequent packets SHOULD use a sending rate that is
determined by metering the rate at which packets have entered the
buffer, potentially using smoothing techniques such as recent
activity over a sliding time window and weighted averages [RFC3290].
It should be noted, however, that this default algorithm is crude and
may not be suitable for all situations. Future revisions of this
specification may provide additional algorithms, once enough
experience of the various conditions in deployed networks is
attained.
5.5. DAD Handling
Duplicate Address Detection (DAD) was defined in [RFC4862] 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. Hence, the protocol specified in this document
SHOULD only be used in deployments where the probability of such
address collisions is extremely low or it is not a concern (because
of the address management procedure deployed). The protocol requires
the NAR to send a DAD probe before it starts defending the NCoA.
However, this DAD delay can be turned off by setting
DupAddrDetectTransmits to zero on the NAR [RFC4862].
Koodli, Ed. Standards Track [Page 18]
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This document specifies messages that can be used to provide
duplicate-free addresses, but the document does not specify how to
create or manage such duplicate-free addresses. In some cases, the
NAR may already have the knowledge required to assess whether or not
the MN's address is a duplicate before the MN moves to the new
subnet. For example, in some deployments, the NAR may maintain a
pool of duplicate-free addresses in a list for handover purposes. In
such cases, the NAR can provide this disposition in the HAck message
(see Section 6.2.2) or in the NAACK option (see Section 6.5.5).
5.6. Fast or Erroneous Movement
Although this specification is for fast handover, the protocol is
limited in terms of how fast an MN can move. A special case of fast
movement is ping-pong, where an 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 it aborts movement altogether. All of the above behaviors are
usually the result of link-layer idiosyncrasies and thus are often
resolved 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. An 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 cost over
the air interface 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.
An MN returning to the PAR before updating the necessary bindings
when present on the NAR MUST send a Fast Binding Update with the 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, upon 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.
In order for the PAR to process such an FBU, the lifetime of the
security association has to be at least that of the tunnel itself.
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Temporary tunnels for the purposes of fast handovers should use short
lifetimes (of the order of at most a few tens of seconds or less).
The lifetime of such tunnels should be enough to allow an 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 the loss of
packets since routing can change and the PAR may forward packets
toward 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
supersedes the existing binding at the PAR, and the packets will be
redirected to the newly confirmed location of the MN.
6. Message Formats
All the ICMPv6 messages have a common Type specified in [RFC4443].
The messages are distinguished based on the Subtype field (see
below). For all the ICMPv6 messages, the checksum is defined in
[RFC4443].
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 Section 6.5.2.
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.
Koodli, Ed. Standards Track [Page 20]
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Hop Limit: 255. See RFC 2461.
ICMP Fields:
Type: 154
Code: 0
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 (LLA)
option. See the 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 the 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 can be avoided when
the receiver needs to send packets back to the sender (of the RtSolPr
message).
When a wildcard is used for New Access Point LLA, no other New Access
Point LLA options must be present.
Koodli, Ed. Standards Track [Page 21]
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A Proxy Router Advertisement (PrRtAdv) message should be received by
the MN in response to an RtSolPr. If such a message is not received
in a timely manner (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 the 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 an FBU immediately after configuring a new
CoA.
When RtSolPr messages are sent more than once, they MUST be rate
limited with MAX_RTSOLPR_RATE per second. During each use of an
RtSolPr, exponential backoff is used for retransmissions.
6.1.2. Proxy Router Advertisement (PrRtAdv)
Access routers send Proxy Router Advertisement messages gratuitously
if the handover is network-initiated or as a response to an RtSolPr
message from an 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.
Koodli, Ed. Standards Track [Page 22]
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ICMP Fields:
Type: 154
Code: 0, 1, 2, 3, 4, or 5. See below.
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 the 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 the Code is 0 or 1.
New Router's IP Address: The IP address of the NAR. This option
MUST be included when the 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 the 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.
The PAR MAY compute a new CoA using the 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.
Koodli, Ed. Standards Track [Page 23]
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Currently, Code values 0, 1, 2, 3, 4, and 5 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 has
been sent unsolicited. If a New CoA option is present following the
New Router Prefix Information option, the MN MUST use the supplied
NCoA and send an 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) that 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 a change of CoA.
This would be the case, for instance, when a number of access
points are connected to the same router interface, or when network
based mobility management mechanisms ensure that the specific
mobile node always observes the same prefix regardless of whether
there is a separate router attached to the target access point.
No other options are required in this case.
When the Option-Code field in the New Access Point LLA option is
6, the PAR is not aware of the Prefix Information requested. The
MN SHOULD attempt to send an 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 only present 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.
Koodli, Ed. Standards Track [Page 24]
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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.
A Proxy Router Advertisement with Code 5 means that the MN may use
the new router information present for detecting movement to a new
subnet, but the MN must perform DHCP [RFC3315] upon attaching to the
NAR's link. The PAR and NAR will forward packets to the PCoA of the
MN. The MN must still formulate an NCoA for transmitting FBU (using
the information sent in this message), but that NCoA will not be used
for forwarding packets.
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.
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 an 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
ICMP Fields:
Type: 154
Koodli, Ed. Standards Track [Page 25]
RFC 5268 MIP6 Fast Handovers June 2008
Code: 0 or 1. See below
Checksum: The ICMPv6 checksum.
Subtype: 4
'S' flag: Assigned address configuration flag. When set, this
message requests a new CoA to be returned by the destination. May
be set when Code = 0. MUST be 0 when Code = 1.
'U' flag: Buffer flag. When set, the destination SHOULD buffer
any packets toward 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 a host route can be established if necessary.
New Care-of Address: The IP address the MN wishes to use when
connected to the destination. When the 'S' bit is set, the 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 a Handover Acknowledge (HAck) message is not received as a
response in a short time period (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 resent.
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.
Koodli, Ed. Standards Track [Page 26]
RFC 5268 MIP6 Fast Handovers June 2008
6.2.2. Handover Acknowledge (HAck)
The Handover Acknowledgment message is a new ICMPv6 message that MUST
be sent (typically by the NAR to the 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.
ICMP Fields:
Type: 154
Code:
0: Handover Accepted, NCoA valid
1: Handover Accepted, NCoA not valid or in use
2: Handover Accepted, NCoA assigned (used in Assigned
addressing)
3: Handover Accepted, use PCoA
4: Message sent unsolicited, usually to trigger an 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.
Koodli, Ed. Standards Track [Page 27]
RFC 5268 MIP6 Fast Handovers June 2008
Identifier: Copied from the corresponding field in the Handover
Initiate message to which this message is a response.
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 the 'S' bit is not set, e.g., Code 2 above.
Upon receiving an 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 the PCoA (instead of accepting or
assigning an NCoA), establish a host route entry for the PCoA, and
set up a tunnel to the PAR to forward the MN's packets sent with
the PCoA as a 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 3 in the HAck message. Subsequently,
the PAR establishes a tunnel to the NAR in order to forward
packets arriving for the PCoA.
When responding to an 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.
6.3. New Mobility Header Messages
Mobile IPv6 uses a new IPv6 header type called Mobility Header
[RFC3775]. The Fast Binding Update, Fast Binding Acknowledgment, and
the (deprecated) Fast Neighbor Advertisement messages use the
Mobility Header.
6.3.1. Fast Binding Update (FBU)
The Fast Binding Update message has a Mobility Header Type value of
8. The FBU is identical to the Mobile IPv6 Binding Update (BU)
message. However, the processing rules are slightly different.
Koodli, Ed. Standards Track [Page 28]
RFC 5268 MIP6 Fast Handovers June 2008
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 that PAR send a Fast
Binding Acknowledgment message.
'H' flag: MUST be set to one. See [RFC3775].
'L' flag: See [RFC3775].
'K' flag: See [RFC3775].
Reserved: This field is unused. MUST be set to zero.
Sequence Number: See [RFC3775].
Lifetime: The requested time in seconds for which the sender
wishes to have a binding.
Mobility Options: MUST contain an alternate CoA option set to the
NCoA when an FBU is sent from the PAR's link. MUST contain the
Binding Authorization Data for the FMIP (BADF) option. See
Section 6.5.4. MAY contain the Mobility Header LLA option (see
Section 6.5.3).
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The MN sends an FBU message any time after receiving a PrRtAdv
message. If the MN moves prior to receiving a PrRtAdv message, it
SHOULD send an FBU to the PAR after configuring the NCoA on the NAR
according to Neighbor Discovery and IPv6 Address Configuration
protocols. When the MN moves without having received a PrRtAdv
message, it cannot transmit an UNA message upon attaching to the
NAR's link.
The source IP address is the PCoA when the FBU is sent from the PAR's
link, and the source IP address is the NCoA when the FBU sent from
the NAR's link. When the source IP address is the PCoA, the MN MUST
include the alternate CoA option set to NCoA. The PAR MUST process
the FBU even though the address in the alternate CoA option is
different from that in the source IP address, and ensure that the
address in the alternate CoA option is used in the New CoA option in
the HI message to the NAR.
The FBU MUST also include the Home Address Option set to PCoA. An
FBU message MUST be protected so that the PAR is able to determine
that the FBU message is sent by an MN that legitimately owns the
PCoA.
6.3.2. Fast Binding Acknowledgment (FBack)
The Fast Binding Acknowledgment message has a Mobility Header Type
value of 9. The FBack 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 an 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.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status |K| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence # | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Fast Binding Acknowledgment (FBack) Message
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IP Fields:
Source address: The IP address of the Previous Access Router
Destination Address: The NCoA, and optionally the PCoA
Status: 8-bit unsigned integer indicating the disposition of the
Fast Binding Update. Values of the Status field that are less
than 128 indicate that the Binding Update was accepted by the
receiving node. The following such Status values are currently
defined:
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 [RFC3775].
Reserved: An unused field. MUST be set to zero.
Sequence Number: Copied from the 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.
Mobility Options: MUST contain an "alternate" CoA if Status is 1.
MUST contain the Binding Authorization Data for FMIP (BADF)
option. See 6.4.5.
6.4. Unsolicited Neighbor Advertisement (UNA)
This is the same message as in [RFC4861] 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 an MN, the
'R' bit is also zero.
Koodli, Ed. Standards Track [Page 31]
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If the NAR is proxying the NCoA (as a result of HI and HAck
exchange), then UNA processing has additional steps (see below). If
the NAR is not proxying the NCoA (for instance, HI and HAck exchange
has not taken place), then UNA processing follows the same procedure
as specified in [RFC4861]. Implementations MAY retransmit UNA
subject to the specification in Section 7.2.6 of [RFC4861] while
noting that the default RetransTimer value is large for handover
purposes.
The Source Address in UNA 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 the Target link-layer
address MUST include the MN's LLA.
The MN sends an UNA message to the NAR, as soon as it regains
connectivity on the new link. Arriving or buffered packets can be
immediately forwarded. If the NAR is proxying the NCoA, it creates a
neighbor cache entry in STALE state but forwards packets as it
determines bidirectional reachability according to the standard
Neighbor Discovery procedure. If there is an entry in INCOMPLETE
state without a link-layer address, it sets it to STALE, again
according to the procedure in [RFC4861].
The NAR MAY wish to provide a different IP address to the MN than the
one in the UNA message. In such a case, the NAR MUST delete the
proxy entry for the NCoA and send a Router Advertisement with the
NAACK option containing the new IP address.
The combination of the 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.
6.5. New Options
All the options, with the exception of Binding Data Authorization for
FMIPv6 (BADF) discussed in Section 6.5.4, use Type, Length, and
Option-Code format 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 Section 6.2 in [RFC3775]. And,
Option-Code provides additional information for each of the options
(see individual options below).
Koodli, Ed. Standards Track [Page 32]
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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
6.5.1. IP Address/Prefix 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/Prefix Option
Type: 17
Length: The size of this option in 8 octets including the Type,
Option-Code, and Length fields.
Option-Code:
1: Old Care-of Address
2: New Care-of Address
3: NAR's IP address
4: NAR's Prefix, sent in PrRtAdv. 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.
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Prefix Length: 8-bit unsigned integer that indicates the length of
the IPv6 Address Prefix. The value ranges from 0 to 128.
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.
6.5.2. 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 12: Link-Layer Address Option
Type: 19
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
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
octets.
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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
an 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 to which the Proxy Router
Solicitation message refers.
6.5.3. 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 13. There are no alignment requirements
for this 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: Mobility Header Link-Layer Address Option
Type: 7
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.
6.5.4. 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 [RFC5269]. This option specifies how to compute and verify
a Message Authentication Code (MAC) using the established security
association.
The format of this option is shown in Figure 14.
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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 | Option Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| Authenticator |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14: Binding Authorization Data for FMIPv6 (BADF) Option
Type: 21
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
the 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.
This option MUST be the last Mobility Option present.
6.5.5. 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 15: Neighbor Advertisement Acknowledgment Option
Type: 20
Length: 8-bit unsigned integer. Length of the option, in 8 octets.
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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
Status: 8-bit unsigned integer indicating the disposition of the
Unsolicited Neighbor Advertisement message. The following Status
values are currently defined:
1: NCoA is invalid, perform address configuration
2: NCoA is invalid, use the supplied NCoA. The supplied NCoA
(in the form of an IP Address Option) MUST be present following
the Reserved field.
3: NCoA 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 than the one that the MN has used. If the NAR
proposes a different NCoA, the Router Advertisement MUST use the
source IP address in the UNA message as the destination address, and
use the L2 address present in UNA. The MN MUST use the NCoA if it is
supplied with the NAACK option. If the NAACK indicates that the
Link-Layer Address is unrecognized, for instance, if the MN uses an
LLA valid on PAR's link but the same LLA is not valid on NAR's link
due to a different access technology, the MN MUST NOT use the NCoA or
the PCoA and SHOULD start immediately the process of acquiring a
different NCoA at the NAR.
In the future, new option types may be defined.
7. Related Protocol and Device Considerations
The protocol specified here, as a design principle, introduces no or
minimal changes to related protocols. For example, no changes to the
base Mobile IPv6 protocol are needed in order to implement this
protocol. Similarly, no changes to the IPv6 stateless address auto-
configuration protocol [RFC4862] and DHCP [RFC3315] are introduced.
The protocol specifies an optional extension to Neighbor Discovery
[RFC4861] in which an access router may send a router advertisement
as a response to the UNA message (see Section 6.4). Other than this
extension, the specification does not modify Neighbor Discovery
behavior (including the procedures performed when attached to the PAR
and when attaching to the NAR).
Koodli, Ed. Standards Track [Page 37]
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The protocol does not require changes to any intermediate Layer 2
device between an MN and its access router that supports this
specification. This includes the wireless access points, switches,
snooping devices, and so on.
8. Evolution from and Compatibility with RFC 4068
This document has evolved from [RFC4068]. Specifically, a new
handover key establishment protocol (see [RFC5269]) has been defined
to enable a security association between a mobile node and its access
router. This allows the secure update of the routing of packets
during a handover. In the future, new specifications may be defined
to establish such security associations depending on the particular
deployment scenario.
The protocol has improved from the experiences in implementing
[RFC4068], and from experimental usage. The input has improved the
specification of parameter fields (such as lifetime, codepoints,
etc.) as well as inclusion of new parameter fields in the existing
messages. As of this writing, there are two publicly available
implementations, [fmipv6] and [tarzan], and multiple proprietary
implementations. Some experience suggests that the protocol meets
the delay and packet loss requirements when used appropriately with
particular radio access protocols. For instance, see [RFC5184] and
[mip6-book]. Nevertheless, it is important to recognize that
handover performance is a function of both IP layer operations, which
this protocol specifies, and the particular radio access technology
itself, which this protocol relies upon but does not modify.
An existing implementation of [RFC4068] needs to be updated in order
to support this specification. The primary addition is the
establishment of a security association between an MN and its access
router (i.e., MN and PAR). One way to establish such a security
association is specified in [RFC5269]. An implementation that
complies with the specification in this document is likely to also
work with [RFC4068], except for the Binding Authorization Data for
FMIPv6 option (see Section 6.5.4) that can only be processed when
security association is in place between a mobile node and its access
router. This specification deprecates the Fast Neighbor
Advertisement (FNA) message. However, it is acceptable for a NAR to
process this message from a mobile node as specified in [RFC4068].
Koodli, Ed. Standards Track [Page 38]
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9. Configurable Parameters
Mobile nodes rely on configuration parameters shown in the table
below. Each mobile node MUST have a configuration mechanism to
adjust the parameters. Such a configuration mechanism may be either
local (such as a command line interface) or based on central
management of a number of mobile nodes.
+-------------------+---------------+---------------+
| Parameter Name | Default Value | Definition |
+-------------------+---------------+---------------+
| RTSOLPR_RETRIES | 3 | Section 6.1.1 |
| MAX_RTSOLPR_RATE | 3 | Section 6.1.1 |
| FBU_RETRIES | 3 | Section 6.3.1 |
| PROXY_ND_LIFETIME | 1.5 seconds | Section 6.2.2 |
| HI_RETRIES | 3 | Section 6.2.1 |
+-------------------+---------------+---------------+
10. Security Considerations
The following security vulnerabilities are identified and suggested
solutions are mentioned.
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 an 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
that MUST be used to secure FBU. The current version of this
protocol relies on a companion protocol [RFC5269] to establish
such a security association. Using the shared handover key from
[RFC5269], the Authenticator in BADF option (see Section 6.5.4)
MUST be computed, and the BADF option included in FBU and FBack
messages.
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 an MN with a
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
Koodli, Ed. Standards Track [Page 39]
RFC 5268 MIP6 Fast Handovers June 2008
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. Therefore, any inadvertent or
malicious redirection to a host is avoided. It is still possible
to jam a NAR's buffer with redirected traffic. However, since a
NAR's handover state corresponding to an NCoA has a finite (and
short) lifetime corresponding to a small multiple of anticipated
handover latency, the extent of this vulnerability is arguably
small.
Sending an FBU from a NAR's link: A malicious node may send an FBU
from a NAR's link providing an unsuspecting node's address as an
NCoA. This is similar to base Mobile IP where the MN can provide
some other node's IP address as its CoA to its Home Agent; here
the PAR acts like a "temporary Home Agent" having a security
association with the Mobile Node and providing forwarding support
for the handover traffic. As in base Mobile IP, this misdelivery
is traceable to the MN that has a security association with the
router. So, it is possible to isolate such an MN if it continues
to misbehave. Similarly, an MN that has a security association
with the PAR may provide the LLA of some other node on NAR's link,
which can cause misdelivery of packets (meant for the NCoA) to an
unsuspecting node. It is possible to trace the MN in this case as
well.
Apart from the above, the RtSolPr (Section 6.1.1) and PrRtAdv
(Section 6.1.2) messages inherit the weaknesses of Neighbor Discovery
protocol [RFC4861]. Specifically, when its access router is
compromised, the MN's RtSolPr message may be answered by an attacker
that provides a rogue router as the resolution. Should the MN attach
to such a rogue router, its communication can be compromised.
Similarly, a network-initiated PrRtAdv message (see Section 3.3) from
an attacker could cause an MN to handover to a rogue router. Where
these weaknesses are a concern, a solution such as Secure Neighbor
Discovery (SEND) [RFC3971] SHOULD be considered.
The protocol provides support for buffering packets during an MN's
handover. This is done by securely exchanging the Handover Initiate
(HI) and Handover Acknowledgment (HAck) messages in response to the
FBU message from an MN. It is possible that an MN may fail, either
inadvertently or purposely, to undergo handover to the NAR, which
typically provides buffering support. This can cause the NAR to
waste its memory containing the buffered packets, and in the worst
case, could create resource exhaustion concerns. Hence,
implementations must limit the size of the buffer as a local policy
configuration, which may consider parameters such as the average
handover delay, expected size of packets, and so on.
Koodli, Ed. Standards Track [Page 40]
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The Handover Initiate (HI) and Handover Acknowledgement (HAck)
messages exchanged between the PAR and NAR MUST be protected using
end-to-end security association(s) offering integrity and data origin
authentication.
The PAR and the NAR MUST implement IPsec [RFC4301] for protecting the
HI and HAck messages. IPsec Encapsulating Security Payload (ESP)
[RFC4303] in transport mode with mandatory integrity protection
SHOULD be used for protecting the signaling messages.
Confidentiality protection of these messages is not required.
The security associations can be created by using either manual IPsec
configuration or a dynamic key negotiation protocol such as Internet
Key Exchange Protocol version 2 (IKEv2) [RFC4306]. If IKEv2 is used,
the PAR and the NAR can use any of the authentication mechanisms, as
specified in RFC 4306, for mutual authentication. However, to ensure
a baseline interoperability, the implementations MUST support shared
secrets for mutual authentication. The following sections describe
the Peer Authorization Database (PAD) and Security Policy Database
(SPD) entries specified in [RFC4301] when IKEv2 is used for setting
up the required IPsec security associations.
10.1. Peer Authorization Database Entries when Using IKEv2
This section describes PAD entries on the PAR and the NAR. The PAD
entries are only example configurations. Note that the PAD is a
logical concept and a particular PAR or NAR implementation can
implement the PAD in any implementation specific manner. The PAD
state may also be distributed across various databases in a specific
implementation.
PAR PAD:
- IF remote_identity = nar_identity_1
THEN authenticate (shared secret/certificate/EAP) and authorize
CHILD_SA for remote address nar_address_1
NAR PAD:
- IF remote_identity = par_identity_1
THEN authenticate (shared secret/certificate/EAP) and authorize
CHILD_SAs for remote address par_address_1
The list of authentication mechanisms in the above examples is not
exhaustive. There could be other credentials used for authentication
stored in the PAD.
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10.2. Security Policy Database Entries
This section describes the security policy entries on the PAR and the
NAR required to protect the HI and HAck messages. The SPD entries
are only example configurations. A particular PAR or NAR
implementation could configure different SPD entries as long as they
provide the required security.
In the examples shown below, the identity of the PAR is assumed to be
par_1, the address of the PAR is assumed to be par_address_1, and the
address of the NAR is assumed to be nar_address_1.
PAR SPD-S:
- IF local_address = par_address_1 & remote_address =
nar_address_1 & proto = ICMPv6 & local_icmpv6_type = HI &
remote_icmpv6_type = HAck
THEN use SA ESP transport mode Initiate using IDi = par_1 to
address nar_address_1
NAR SPD-S:
- IF local_address = nar_address_1 & remote_address =
par_address_1 & proto = ICMPv6 & local_icmpv6_type = HAck &
remote_icmpv6_type = HI
THEN use SA ESP transport mode
11. IANA Considerations
This document defines a new ICMPv6 message, which has been allocated
from the ICMPv6 Type registry.
154 FMIPv6 Messages
This document creates a new registry for the 'Subtype' field in the
above ICMPv6 message, called the "FMIPv6 Message Types". IANA has
assigned the following values.
+---------+-------------+---------------+
| 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 |
+---------+-------------+---------------+
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The values '0' and '1' are reserved. The upper limit is 255. An RFC
is required for new message assignment.
The document defines a new Mobility Option that has received Type
assignment from the Mobility Options Type registry.
1. Binding Authorization Data for FMIPv6 (BADF) option, described
in Section 6.5.4
The document has received Type assignments for the following (see
[RFC4068]):
The document defines the following Neighbor Discovery [RFC4861]
options that have received Type assignment from IANA.
+---------+-----------------------------------------+---------------+
| Type | Description | Reference |
+---------+-----------------------------------------+---------------+
| 17 | IP Address/Prefix Option | Section 6.5.1 |
| 19 | Link-layer Address Option | Section 6.5.2 |
| 20 | Neighbor Advertisement Acknowledgment | Section 6.5.5 |
| | Option | |
+---------+-----------------------------------------+---------------+
The document defines the following Mobility Header messages that have
received Type allocation from the Mobility Header Types registry.
1. Fast Binding Update, described in Section 6.3.1
2. Fast Binding Acknowledgment, described in Section 6.3.2
The document defines the following Mobility Option that has received
Type assignment from the Mobility Options Type registry.
1. Mobility Header Link-Layer Address option, described in
Section 6.5.3
12. Acknowledgments
The editor would like to thank all those who have provided feedback
on this specification, but can only mention a few here: 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-to-point links. The editor would like to
acknowledge a contribution from James Kempf to improve this
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specification. Vijay Devarapalli provided text for the security
configuration between access routers in Section 10. Thanks to Jari
Arkko for the detailed AD Review, which has improved this document.
The editor would also like to thank the [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.
13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5269] Kempf, J. and R. Koodli, "Distributing a Symmetric Fast
Mobile IPv6 (FMIPv6) Handover Key Using SEcure Neighbor
Discovery (SEND)", RFC 5269, June 2008.
[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", RFC 4443,
March 2006.
[RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T.,
Perkins, C., and M. Carney, "Dynamic Host Configuration
Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility
Support in IPv6", RFC 3775, June 2004.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC4306] Kaufman, C., Ed., "Internet Key Exchange (IKEv2)
Protocol", RFC 4306, December 2005.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
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13.2. Informative References
[fmipv6] "fmipv6.org : Home Page", <http://fmipv6.org>.
[mip6-book] Koodli, R. and C. Perkins, "Mobile Internetworking with
IPv6, Chapter 22, John Wiley & Sons.", July 2007.
[RFC3290] Bernet, Y., Blake, S., Grossman, D., and A. Smith, "An
Informal Management Model for Diffserv Routers", RFC
3290, May 2002.
[RFC3971] Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
"SEcure Neighbor Discovery (SEND)", RFC 3971, March
2005.
[RFC4068] Koodli, R., Ed., "Fast Handovers for Mobile IPv6", RFC
4068, July 2005.
[RFC5184] Teraoka, F., Gogo, K., Mitsuya, K., Shibui, R., and K.
Mitani, "Unified Layer 2 (L2) Abstractions for Layer 3
(L3)-Driven Fast Handover", RFC 5184, May 2008.
[tarzan] "Nautilus6 - Tarzan",
<http://software.nautilus6.org/TARZAN/>.
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Appendix A. 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.
Appendix B. Changes since RFC 4068
Following are the major changes and clarifications:
o Specified security association between the MN and its Access
Router in the companion document [RFC5269].
o Specified Binding Authorization Data for Fast Handovers (BADF)
option to carry the security parameters used for verifying the
authenticity of FBU and FBack messages. The handover key used for
computing the Authenticator is specified in companion documents.
o Specified the security configuration for inter - access router
signaling (HI, HAck).
o Added a section on prefix management between access routers and
illustrated protocol operation over point-to-point links.
o Deprecated FNA, which is a Mobility Header message. In its place,
the Unsolicited Neighbor Advertisement (UNA) message from RFC 4861
is used.
o Combined the IPv6 Address Option and IPv6 Prefix Option.
o Added description of DAD requirement on NAR when determining NCoA
uniqueness in Section 4, "Protocol Details".
o Added a new code value for gratuitous HAck message to trigger a HI
message.
o Added Option-Code 5 in PrRtAdv message to indicate NETLMM usage.
o Clarified protocol usage when DHCP is used for NCoA formulation
(Sections 6.1.2, 3.1, and 5.2). Added a new Code value (5) in
PrRtAdv (Section 6.1.2).
o Clarified that IPv6 Neighbor Discovery operations are a must in
Section 7, "Related Protocol and Device Considerations".
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o Clarified "PAR = temporary HA" for FBUs sent by a genuine MN to an
unsuspecting CoA.
Editor's Address
Rajeev Koodli
Starent Networks
USA
EMail: rkoodli@starentnetworks.com
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Koodli, Ed. Standards Track [Page 48]