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RFC 4068
Network Working Group R. Koodli, Ed.
Request for Comments: 4068 Nokia Research Center
Category: Experimental July 2005
Fast Handovers for Mobile IPv6
Status of This Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
Mobile IPv6 enables a Mobile Node 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. 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. Experimental [Page 1]
RFC 4068 Fast Handovers for Mobile IPv6 July 2005
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology. . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Protocol Overview. . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Addressing the Handover Latency. . . . . . . . . . . . . 5
3.2. Protocol Operation . . . . . . . . . . . . . . . . . . . 7
3.3. Protocol Operation of Network-initiated Handover . . . . 9
4. Protocol Details . . . . . . . . . . . . . . . . . . . . . . . 10
5. Miscellaneous. . . . . . . . . . . . . . . . . . . . . . . . . 15
5.1. Handover Capability Exchange . . . . . . . . . . . . . . 15
5.2. Determining New Care of Address. . . . . . . . . . . . . 15
5.3. Packet Loss. . . . . . . . . . . . . . . . . . . . . . . 15
5.4. DAD Handling . . . . . . . . . . . . . . . . . . . . . . 16
5.5. Fast or Erroneous Movement . . . . . . . . . . . . . . . 16
6. Message Formats. . . . . . . . . . . . . . . . . . . . . . . . 17
6.1. New Neighborhood Discovery Messages. . . . . . . . . . . 17
6.1.1. Router Solicitation for Proxy Advertisement
(RtSolPr) . . . . . . . . . . . . . . . . . . . . 17
6.1.2. Proxy Router Advertisement (PrRtAdv). . . . . . . 20
6.2. Inter-Access Router Messages . . . . . . . . . . . . . . 23
6.2.1. Handover Initiate (HI). . . . . . . . . . . . . . 23
6.2.2. Handover Acknowledge (HAck) . . . . . . . . . . . 25
6.3. New Mobility Header Messages . . . . . . . . . . . . . . 27
6.3.1. Fast Binding Update (FBU) . . . . . . . . . . . . 27
6.3.2. Fast Binding Acknowledgment (FBack) . . . . . . . 28
6.3.3. Fast Neighbor Advertisement (FNA) . . . . . . . . 30
6.4. New Options. . . . . . . . . . . . . . . . . . . . . . . 31
6.4.1. IP Address Option . . . . . . . . . . . . . . . . 32
6.4.2. New Router Prefix Information Option. . . . . . . 33
6.4.3. Link-Layer Address (LLA) Option . . . . . . . . . 34
6.4.4. Mobility Header Link-Layer Address (MH-LLA)
Option. . . . . . . . . . . . . . . . . . . . . . 35
6.4.5. Neighbor Advertisement Acknowledgment (NAACK) . . 35
7. Configurable Parameters. . . . . . . . . . . . . . . . . . . . 36
8. Security Considerations. . . . . . . . . . . . . . . . . . . . 37
9. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 38
10. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 39
11. Normative References . . . . . . . . . . . . . . . . . . . . . 39
12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 39
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RFC 4068 Fast Handovers for Mobile IPv6 July 2005
1. Introduction
Mobile IPv6 [3] describes the protocol operations for a mobile node
to maintain connectivity to the Internet during its handover from one
access router to another. These operations involve movement
detection, IP address configuration, and location update. The
combined handover latency is often sufficient to affect real-time
applications. Throughput-sensitive applications can also benefit
from reducing this latency. This document describes a protocol to
reduce the handover latency.
This specification addresses the following problem: how to allow a
mobile node to send packets as soon as it detects a new subnet link,
and how to deliver packets to a mobile node as soon as its attachment
is detected by the new access router. The protocol defines IP
protocol messages necessary for its operation regardless of link
technology. It does this without depending on specific link-layer
features while allowing link-specific customizations. By definition,
this specification considers handovers that interwork with Mobile IP:
once attached to its new access router, an MN engages in Mobile IP
operations including Return Routability [3]. There are no special
requirements for a mobile node to behave differently with respect to
its standard Mobile IP operations.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1]. 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. The reference handover scenario is illustrated in
Figure 1.
Mobile Node (MN)
A Mobile IPv6 host.
Access Point (AP)
A Layer 2 device connected to an IP subnet that offers
wireless connectivity to an MN. An Access Point Identifier
(AP-ID) refers to the AP's L2 address. Sometimes, AP-ID is
also referred to as a Base Station Subsystem ID (BSSID).
Access Router (AR)
The MN's default router.
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RFC 4068 Fast Handovers for Mobile IPv6 July 2005
Previous Access Router (PAR)
The MN's default router prior to its handover.
New Access Router (NAR)
The MN's default router subsequent to its handover.
Previous CoA (PCoA)
The MN's Care of Address valid on PAR's subnet.
New CoA (NCoA)
The MN's Care of Address valid on NAR's subnet.
Handover
A process of terminating existing connectivity and obtaining
new IP connectivity.
Router Solicitation for Proxy Advertisement (RtSolPr)
A message from the MN to the PAR requesting information for
a potential handover.
Proxy Router Advertisement (PrRtAdv)
A message from the PAR to the MN that provides information
about neighboring links facilitating expedited movement
detection. The message also acts as a trigger for network-
initiated handover.
(AP-ID, AR-Info) tuple
Contains an access router's L2 and IP addresses, and the
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, Prefix] is called "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.
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RFC 4068 Fast Handovers for Mobile IPv6 July 2005
Fast Neighbor Advertisement (FNA)
A message from the MN to the NAR to announce attachment, and
to confirm the use of NCoA when the MN has not received an
FBACK.
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.
v +------------+
+-+ | Previous | <
| | ---------- | Access | ------ > ----\
+-+ | Router | < \
MN | (PAR) | \
| +------------+ +---------------+
| ^ IP | Correspondent |
| | Network | Node |
V | +---------------+
v /
v +------------+ /
+-+ | New | < /
| | ---------- | Access | ------ > ----/
+-+ | Router | <
MN | (NAR) |
+------------+
Figure 1: Reference Scenario for Handover
3. Protocol Overview
3.1. Addressing the Handover Latency
The ability to immediately send packets from a new subnet link
depends on the "IP connectivity" latency, which in turn depends on
the movement detection latency and 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 successfully process the Binding Update,
which typically involves the Return Routability procedure, the MN can
receive packets at the new CoA. So, the ability to receive packets
from correspondents directly at its new CoA depends on the Binding
Update latency as well as the IP connectivity latency.
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RFC 4068 Fast Handovers for Mobile IPv6 July 2005
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
(i.e., a "scan" in WLAN) and then request subnet information
corresponding to one or more of those discovered access points. The
MN may do this after performing router discovery 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 (see 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)" message prior to its movement. If it moves without
receiving an FBack, the MN can still start using NCoA after
announcing its attachment through a "Fast Neighbor Advertisement
(FNA)" message. NAR responds to FNA if the tentative address is
already in use thereby reducing NCoA configuration latency. Under
some limited conditions in which the probability of address collision
is considered insignificant, it may be possible to use NCoA
immediately after attaching to the new link. Even so, all
implementations MUST support and SHOULD use the mechanism specified
in this document to avoid potential address conflicts.
To reduce the Binding Update latency, the protocol specifies a tunnel
between the Previous CoA (PCoA) and the NCoA. An MN sends a "Fast
Binding Update" message to its Previous Access Router to establish
this tunnel. When feasible, the MN SHOULD send an FBU from PAR's
link. Otherwise, it should be sent immediately after attachment to
NAR has been detected. Subsequent sections describe the protocol
mechanics. As a result, PAR begins tunneling packets arriving for
PCoA to NCoA. Such a tunnel remains active until the MN completes
the Binding Update with its correspondents. In the opposite
direction, the MN SHOULD reverse tunnel packets to PAR until it
completes the Binding Update. PAR SHOULD forward the inner packet in
the tunnel to its destination (i.e., to the MN's correspondent).
Such a reverse tunnel ensures that packets containing PCoA as a
source IP address are not dropped due to ingress filtering. Readers
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RFC 4068 Fast Handovers for Mobile IPv6 July 2005
may observe that even though the MN is IP-capable on the new link, it
cannot use NCoA directly with its correspondents without the
correspondents first establishing a binding cache entry (for NCoA).
Forwarding support for PCoA is provided through a reverse tunnel
between the MN and the PAR.
Setting up a tunnel alone does not ensure that the MN receives
packets as soon as 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 NAR to send Neighbor
Solicitation before the MN attaches, subsequent retransmissions of
address resolution are separated by a default period of one second
each. To circumvent this delay, an MN announces its attachment
through the FNA message that allows the NAR to consider MN to be
reachable. If there is no existing entry, FNA allows NAR to create
one. If NAR already has an entry, FNA updates the entry while taking
potential address conflicts into consideration. Through tunnel
establishment for PCoA and fast advertisement, the protocol provides
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 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 PAR before providing forwarding support for the MN.
That is, it may create a forwarding entry for the NCoA subject to
"approval" from PAR which it trusts. Finally, the access routers
could transfer network-resident contexts, such as access control,
QoS, and header compression, in conjunction with handover. For these
operations, the protocol provides "Handover Initiate (HI)" and
"Handover Acknowledge (HAck)" messages. Both of these messages MUST
be supported and SHOULD be used. The access routers MUST have
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 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 a RtSolPr at any convenient time, for
instance as a response to some link-specific event (a "trigger") or
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RFC 4068 Fast Handovers for Mobile IPv6 July 2005
simply after performing router discovery. However, the expectation
is that prior to sending 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 when a link-
specific handover event occurs. The purpose of the FBU is to
authorize PAR to bind PCoA to NCoA, so that arriving packets can be
tunneled to the new location of the MN. Whenever feasible, the FBU
SHOULD be sent from PAR's link. 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.
Care must be taken to ensure that the NCoA used in FBU does not
conflict with an address already in use by some other node on the
link. For this, FBU encapsulation within FNA MUST be implemented and
SHOULD be used (see below) when the FBU is sent from NAR's link.
The format and semantics of FBU processing are specified in Section
6.3.1.
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 an FBack on the previous link. This means that
packet tunneling is already in progress by the time the MN
handovers to NAR. The MN SHOULD send FNA immediately after
attaching to NAR, so that arriving and buffered packets can be
forwarded to the MN right away.
Before sending an FBack to an MN, 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
HI, and the NAR MAY assign the proposed NCoA. Such an assigned
NCoA MUST be returned in HAck, and the PAR MUST in turn provide
the assigned NCoA in the FBack. If there is an assigned NCoA
returned in the FBack, the MN MUST use the assigned address (and
not the proposed address in the 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 PAR has successfully processed the FBU. Hence,
it (re)sends an FBU as soon as it attaches to NAR. To enable NAR
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RFC 4068 Fast Handovers for Mobile IPv6 July 2005
to forward packets immediately (when FBU has been processed) and
to allow NAR to verify whether NCoA is acceptable, the MN SHOULD
encapsulate the FBU in the FNA. If NAR detects that NCoA is in
use when processing the FNA, for instance while creating a
neighbor entry, it MUST discard the inner FBU packet and send a
Router Advertisement with the "Neighbor Advertisement Acknowledge
(NAACK)" option in which NAR MAY include an alternate IP address
for the MN to use. This discarding avoids the rare and
undesirable outcome that results from address collision. Detailed
FNA processing rules are specified in Section 6.3.3.
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 "predictive" mode of operation. The
scenario in which the MN sends an FBU from NAR's link is illustrated
in Figure 3. For convenience, this scenario is characterized as a
"reactive" mode of operation. Note that the reactive mode also
includes the case in which an FBU has been sent from PAR's link but
an FBack has not been received yet.
Finally, the PrRtAdv message may be sent unsolicited (i.e., without
the MN first sending a RtSolPr). This mode is described in Section
3.3.
3.3. Protocol Operation of Network-initiated Handover
In some wireless technologies, the handover control may reside in the
network even though the decision to undergo handover may be mutually
arrived at between the MN and the network. In these networks, the
PAR can send an unsolicited PrRtAdv containing the link layer
address, IP address, and subnet prefixes of the NAR when the network
decides that a handover is imminent. The MN MUST process this
PrRtAdv to configure a new care of address on the new subnet, and
MUST send an FBU to PAR prior to switching to the new link. After
transmitting 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.
Koodli, Ed. Experimental [Page 9]
RFC 4068 Fast Handovers for Mobile IPv6 July 2005
MN PAR NAR
| | |
|------RtSolPr------->| |
|<-----PrRtAdv--------| |
| | |
|------FBU----------->|--------HI--------->|
| |<------HAck---------|
| <--FBack---|--FBack---> |
| | |
disconnect forward |
| packets===============>|
| | |
| | |
connect | |
| | |
|--------- FNA --------------------------->|
|<=================================== deliver packets
| |
Figure 2: "Predictive" Fast Handover
4. Protocol Details
All descriptions refer to Figure 1.
After discovering one or more nearby access points, the MN sends
RtSolPr to resolve access point identifiers to subnet router
information. This is convenient to do 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., FNA) can be
immediately transmitted. Implementations SHOULD make use of such
triggers whenever possible.
Koodli, Ed. Experimental [Page 10]
RFC 4068 Fast Handovers for Mobile IPv6 July 2005
MN PAR NAR
| | |
|------RtSolPr------->| |
|<-----PrRtAdv--------| |
| | |
disconnect | |
| | |
| | |
connect | |
|------FNA[FBU]-------|------------------->|
| |<-----FBU-----------|
| |------FBack-------->|
| forward |
| packets===============>|
| | |
|<=================================== deliver packets
| |
Figure 3: "Reactive" Fast Handover
The RtSolPr message contains one or more AP-IDs. A wildcard requests
all available tuples.
As a response to RtSolPr, PAR sends a PrRtAdv message 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 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).
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RFC 4068 Fast Handovers for Mobile IPv6 July 2005
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 FIFO buffering is used, the PAR MUST continue forwarding the
packets to PCoA (i.e., buffer and forward). Such buffering can be
useful when the MN leaves without sending the FBU message from the
PAR's link. The PAR SHOULD stop buffering after processing the FBU
message. The size of the buffer is an implementation-specific
consideration.
The method by which Access Routers exchange information about their
neighbors, and thereby allow construction of Proxy Router
Advertisements with information about neighboring subnets is outside
the scope of this document.
The RtSolPr and PrRtAdv messages MUST be implemented by 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 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 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. This FBU SHOULD be encapsulated in an FNA
message. The encapsulation allows the NAR to discard the (inner) FBU
packet if an address conflict is detected as a result of (outer) FNA
packet processing (see FNA processing below). In response to the
FBU, the PAR establishes a binding between PCoA ("Home Address") and
NCoA, and sends the FBack to the MN. Prior to establishing this
binding, PAR SHOULD send an HI message to NAR, and receive HAck in
response. 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 PCoA, i.e., the FBU is sent from the PAR's link, and 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.
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RFC 4068 Fast Handovers for Mobile IPv6 July 2005
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 NCoA supplied in the HI message is a valid
address for use. If it is, the NAR starts proxying [6] the
address for PROXY_ND_LIFETIME during which the MN is expected to
connect to the NAR. The NAR MAY use the Link-Layer Address to
verify whether a corresponding IP address exists in its forwarding
tables.
2. allocates NCoA for the MN when assigned addressing is used,
creates a proxy neighbor cache entry, and begins defending it.
The NAR MAY allocate the NCoA proposed in HI.
3. MAY create a host route entry for PCoA in case NCoA cannot be
accepted or assigned. This host route entry SHOULD be implemented
such that until the MN's presence is detected, either through
explicit announcement by the MN or by other means, arriving
packets do not invoke neighbor discovery. The NAR MAY 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.
When the Code value in HI is 1, NAR MUST skip the above operations
since it would have performed those operations during FNA processing.
However, it SHOULD be prepared to process any other options that may
be defined in the future. Sending an HI message with Code 1 allows
NAR to validate the neighbor cache entry it creates for the MN during
FNA processing. That is, NAR can make use of the knowledge that its
trusted peer (i.e., PAR) has a trust relationship with the MN.
If HAck contains an assigned NCoA, 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 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 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.
When the MN establishes link connectivity with the NAR, it SHOULD
send a Fast Neighbor Advertisement (FNA) message (see 6.3.3). If the
MN has not received an FBack by the time the FNA is being sent, it
SHOULD encapsulate the FBU in the FNA and send them together.
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RFC 4068 Fast Handovers for Mobile IPv6 July 2005
When the NCoA corresponding to the FNA message is acceptable, the NAR
MUST
1. delete its proxy neighbor cache entry, if any is present.
2. create a neighbor cache entry and set its state to REACHABLE
without overwriting an existing entry for a different layer 2
address.
3. forward any buffered packets.
4. enable the host route entry for PCoA, if any is present.
When the NCoA corresponding to the FNA message is not acceptable, the
NAR MUST
1. discard the inner (FBU) packet.
2. send a Router Advertisement with the NAACK option in which it MAY
include an alternate NCoA for use. This message MUST be sent to
the source IP address present in the FNA using the same Layer 2
address present in the FNA.
If the MN receives a Router Advertisement with a NAACK option, it
MUST use the IP address, if any, provided in the NAACK option.
Otherwise, the MN should configure another NCoA. Subsequently, the
MN SHOULD send an FBU using the new CoA. As a special case, the
address supplied in NAACK could be PCoA itself, in which case the MN
MUST NOT send any more FBUs.
Once the MN has confirmed its NCoA, it SHOULD send a Neighbor
Advertisement message. This message allows MN's neighbors to update
their neighbor cache entries with the MN's addresses.
Just as in Mobile IPv6, the PAR sets the 'R' bit in the Prefix
Information option, and includes its 128 bit global address in the
router advertisements. This allows the mobile nodes to learn the
PAR's global IPv6 address. The MN reverse tunnels its packets to the
same global address of PAR. The tunnel end-point addresses must be
configured accordingly. When PAR receives a reverse tunneled packet,
it must verify if a secure binding exists for the MN identified by
PCoA in the tunneled packet, before forwarding the packet.
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5. Miscellaneous
5.1. Handover Capability Exchange
The MN expects a PrRtAdv in response to its RtSolPr message. If the
MN does not receive a PrRtAdv message even after RTSOLPR_RETRIES, it
must assume that PAR does not support the fast handover protocol and
stop sending RtSolPr messages.
Even if an MN's current access router is capable of fast handover,
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, 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.
If a PrRtAdv message carries an NCoA, the MN MUST use it as its
prospective NCoA.
5.3. 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 will be lost unless they are
buffered. This protocol provides an option to indicate a 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.
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RFC 4068 Fast Handovers for Mobile IPv6 July 2005
5.4. DAD Handling
Duplicate Address Detection (DAD) was defined in [7] to avoid address
duplication on links when stateless address auto-configuration is
used. The use of DAD to verify the uniqueness of an IPv6 address
configured through stateless auto-configuration adds delays to a
handover.
The probability of an interface identifier duplication on the same
subnet is very low, however it cannot be ignored. In this document,
certain precautions are proposed to minimize the effects of a
duplicate address occurrence.
In some cases, the NAR may already have the knowledge required to
assess whether the MN's address is a duplicate before the MN moves to
the new subnet. For example, the NAR can have a list of all nodes on
its subnet, perhaps for access control, and by searching this list,
it can confirm whether the MN's address is a duplicate. The result
of this search is sent back to the PAR in the HAck message. If such
knowledge is not available at the NAR, it may indicate this by not
confirming the NCoA in the HAck message. The NAR may also indicate
this in the NAACK option in response to the FNA message. In such
cases, the MN would have to follow the address configuration
procedure according to [6] after attaching to the NAR.
5.5. Fast or Erroneous Movement
Although this specification is for fast handover, the protocol is
limited in terms of how fast an MN can move. Ping-Pong is a special
case of fast movement, 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 is either moved to a
different one or it aborts movement altogether. All of the above
behaviors are usually the result of link layer idiosyncrasies and
thus are often 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 HA 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 a 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.
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RFC 4068 Fast Handovers for Mobile IPv6 July 2005
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 tear down that
tunnel (i.e., stop forwarding packets for this MN and start
delivering packets directly to the node instead). The MN SHOULD NOT
attempt to use any of the fast handover mechanisms described in this
specification and SHOULD revert back to standard Mobile IPv6.
Temporary tunnels for the purpose of fast handovers should use short
lifetimes (a small number 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, a Fast
Binding Update to the PAR SHOULD be sent. Since Fast Binding Updates
are authenticated, they supercede the existing binding and packets
MUST be redirected to the newly confirmed location of the MN.
6. Message Formats
All the ICMPv6 messages have a common Type specified in [4]. The
messages are distinguished based on the Subtype field (see below).
The values for the Subtypes are specified in Section 9. For all the
ICMPv6 messages, the checksum is defined in [2].
6.1. New Neighborhood Discovery Messages
6.1.1. Router Solicitation for Proxy Advertisement (RtSolPr)
Mobile Nodes send Router Solicitation for Proxy Advertisement in
order to prompt routers for Proxy Router Advertisements. All the
Link-Layer Address options have the format defined in 6.4.3.
Koodli, Ed. Experimental [Page 17]
RFC 4068 Fast Handovers for Mobile IPv6 July 2005
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 (RtSolPr) Message
IP Fields:
Source Address
An IP address assigned to the sending interface.
Destination Address
The address of the Access Router or the all routers
multicast address.
Hop Limit 255. See RFC 2461.
Authentication Header
If a Security Association for the IP Authentication
Header exists between the sender and the
destination address, then the sender SHOULD include
this header. See RFC 2402 [5].
ICMP Fields:
Type The Experimental Mobility Protocol Type. See [4].
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.
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RFC 4068 Fast Handovers for Mobile IPv6 July 2005
Valid Options:
Source Link-Layer Address
When known, the Link-Layer Address of the sender
SHOULD be included using the Link-Layer Address
option. See 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 with all bits set to zero.
Future versions of this protocol may define new option types.
Receivers MUST silently ignore any options that they do not recognize
and continue processing the rest of the message.
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 a New Access Point LLA, no other New
Access Point LLA options must be present.
A Proxy Router Advertisement (PrRtAdv) message should be received by
the MN in response to a 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 a
RtSolPr, exponential backoff is used for retransmissions.
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RFC 4068 Fast Handovers for Mobile IPv6 July 2005
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 a 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 Section 6.4.3.
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 a Proxy Advertisement or the
address of the node the Access Router is
instructing to 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 | Reserved | Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
Figure 5: Proxy Router Advertisement (PrRtAdv) Message
Hop Limit 255. See RFC 2461 [6].
Authentication Header
If a Security Association for the IP Authentication
Header exists between the sender and the
destination address, the sender SHOULD include this
header. See RFC 2402 [5].
ICMP Fields:
Type The Experimental Mobility Protocol Type. See RFC
4065 [4].
Code 0, 1, 2, 3 or 4. See below.
Koodli, Ed. Experimental [Page 20]
RFC 4068 Fast Handovers for Mobile IPv6 July 2005
Checksum The ICMPv6 checksum.
Subtype 3
Reserved MUST be set to zero by the sender and ignored by
the receiver.
Identifier Copied from the 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 the 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. This option MUST be
included when Code is 0 or 1.
New Router's IP Address
The IP address of NAR. This option MUST be
included when Code is 0 or 1.
New Router Prefix Information Option.
Specifies the prefix of the Access Router for which
the message is proxied and is used for address
auto-configuration. This option MUST be included
when Code is 0 or 1. However, when this prefix is
the same as that 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 a PrRtAdv is sent unsolicited.
PAR MAY compute a new CoA using NAR's prefix
information and the MN's L2 address, or by any
other means.
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RFC 4068 Fast Handovers for Mobile IPv6 July 2005
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.
Currently, Code values 0, 1, 2, 3 and 4 are defined.
A Proxy Router Advertisement with Code 0 means that the MN should use
the [AP-ID, AR-Info] tuple (present in the options above) for
movement detection and NCoA formulation. In this case, the Option-
Code field in the New Access Point LLA option is 1, reflecting the
LLA of the access point for which the rest of the options are
related. Multiple tuples may be present.
A Proxy Router Advertisement with Code 1 means that the message is
sent unsolicited. If a New CoA option is present following the New
Router Prefix Information option, the MN SHOULD use the supplied NCoA
and send the 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. 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. Experimental [Page 22]
RFC 4068 Fast Handovers for Mobile IPv6 July 2005
A Proxy Router Advertisement with Code 4 means that the subnet
information regarding neighboring access points is sent unsolicited,
but the message is not a handover trigger, unlike when the message is
sent with Code 1. Multiple tuples may be present.
When a wildcard AP identifier is supplied in the RtSolPr message, the
PrRtAdv message should include any `n' [Access Point Identifier,
Link-Layer Address option, Prefix Information Option] tuples
corresponding to the PAR's neighborhood.
6.2. Inter-Access Router Messages
6.2.1. Handover Initiate (HI)
The Handover Initiate (HI) is an ICMPv6 message sent by an Access
Router (typically PAR) to another Access Router (typically NAR) to
initiate the process of a MN's handover.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subtype |S|U| Reserved | Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
Figure 6: Handover Initiate (HI) Message
IP Fields:
Source Address
The IP address of the PAR.
Destination Address
The IP address of the NAR.
Hop Limit 255. See RFC 2461 [6].
Authentication Header
The authentication header MUST be used when this
message is sent. See RFC 2402 [5].
Koodli, Ed. Experimental [Page 23]
RFC 4068 Fast Handovers for Mobile IPv6 July 2005
ICMP Fields:
Type The Experimental Mobility Protocol Type. See RFC
4065 [4].
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 moving 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.
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The PAR uses a Code value of 0 when it processes an FBU with PCoA as
a 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 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.
6.2.2. Handover Acknowledge (HAck)
The Handover Acknowledgment message is a new ICMPv6 message that MUST
be sent (typically by NAR to PAR) as a reply to the Handover Initiate
message.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subtype | Reserved | Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
Figure 7: Handover Acknowledge (HAck) Message
IP Fields:
Source Address
Copied from the destination address of the Handover
Initiate Message to which this message is a
response.
Destination Address
Copied from the source address of the Handover
Initiate Message to which this message is a
response.
Hop Limit 255. See RFC 2461 [6].
Authentication Header
The authentication header MUST be used when this
message is sent. See RFC 2402 [5].
Koodli, Ed. Experimental [Page 25]
RFC 4068 Fast Handovers for Mobile IPv6 July 2005
ICMP Fields:
Type The Experimental Mobility Protocol Type. See RFC
4065 [4].
Code
0: Handover Accepted, NCoA valid
1: Handover Accepted, NCoA not valid
2: Handover Accepted, NCoA in use
3: Handover Accepted, NCoA assigned
(used in Assigned addressing)
4: Handover Accepted, NCoA not assigned
(used in Assigned addressing)
128: Handover Not Accepted, reason unspecified
129: Administratively prohibited
130: Insufficient resources
Checksum The ICMPv6 checksum.
Subtype 5
Reserved MUST be set to zero by the sender and ignored by
the receiver.
Identifier Copied from the corresponding field in the Handover
Initiate message 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 PCoA (instead of accepting or
assigning NCoA), establish a host route entry for PCoA, and set up a
tunnel to the PAR to forward MN's packets sent with 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.
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RFC 4068 Fast Handovers for Mobile IPv6 July 2005
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 [3].
The Fast Binding Update, Fast Binding Acknowledgment, and Fast
Neighbor Advertisement messages use the Mobility Header.
6.3.1. Fast Binding Update (FBU)
The Fast Binding Update message is identical to the Mobile IPv6
Binding Update (BU) message. However, the processing rules are
slightly different.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence # |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|A|H|L|K| Reserved | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Fast Binding Update (FBU) Message
IP fields:
Source Address
The PCoA or NCoA
Destination Address
The IP address of the Previous Access Router
A flag MUST be set to one to request that PAR send a Fast
Binding Acknowledgment message.
H flag MUST be set to one. See RFC 3775 [3].
L flag See RFC 3775 [3].
Koodli, Ed. Experimental [Page 27]
RFC 4068 Fast Handovers for Mobile IPv6 July 2005
K flag See RFC 3775 [3].
Reserved This field is unused. MUST be set zero.
Sequence Number
See RFC 3775 [3].
Lifetime See RFC 3775 [3].
Mobility Options
MUST contain an alternate CoA option set to the
NCoA when an FBU is sent from PAR's link.
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 NCoA on the NAR
according to Neighbor Discovery and IPv6 Address Configuration
protocols.
The source IP address is PCoA when the FBU is sent from PAR's link,
and the source IP address is NCoA when sent from NAR's link. When
the FBU is sent from NAR's link, it SHOULD be encapsulated within an
FNA.
The FBU MUST also include the Home Address Option, and the Home
Address is PCoA. An FBU message MUST be protected so that PAR is
able to determine that the FBU message is sent by a genuine MN.
6.3.2. Fast Binding Acknowledgment (FBack)
The Fast Binding Acknowledgment message is sent by the PAR to
acknowledge receipt of a Fast Binding Update message in which the 'A'
bit is set. The Fast Binding Acknowledgment message SHOULD NOT be
sent to the MN before the PAR receives a HAck message from the NAR.
The Fast Binding Acknowledgment MAY also be sent to the MN on the old
link.
Koodli, Ed. Experimental [Page 28]
RFC 4068 Fast Handovers for Mobile IPv6 July 2005
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status |K| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence # | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Fast Binding Acknowledgment (FBack) Message
IP fields:
Source Address
The IP address of the Previous Access Router.
Destination Address
The NCoA
Status 8-bit unsigned integer indicating the disposition
of the Fast Binding Update. Values of the Status
field 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 that are 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 RFC 3775 [3].
Reserved An unused field. MUST be set to zero.
Koodli, Ed. Experimental [Page 29]
RFC 4068 Fast Handovers for Mobile IPv6 July 2005
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.
6.3.3. Fast Neighbor Advertisement (FNA)
A MN sends a Fast Neighbor Advertisement to announce itself to the
NAR. When the Mobility Header Type is FNA, the Payload Proto field
may be set to IPv6 to assist FBU encapsulation.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Mobility Options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: Fast Neighbor Advertisement (FNA) Message
IP fields:
Source Address
NCoA
Destination Address
NAR's IP Address
Mobility Options
MUST contain the Mobility Header Link-Layer Address
of the MN in the MH-LLA option format. See Section
6.4.4.
The MN sends a Fast Neighbor Advertisement to the NAR, as soon as it
regains connectivity on the new link. Arriving or buffered packets
can be immediately forwarded. If NAR is proxying NCoA, it creates a
neighbor cache entry in REACHABLE state. If there is no entry, it
creates one and sets it to REACHABLE. If there is an entry in the
INCOMPLETE state without a Link-Layer Address, it sets it to
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RFC 4068 Fast Handovers for Mobile IPv6 July 2005
REACHABLE. During the process of creating a neighbor cache entry,
NAR can also detect if NCoA is in use, thus avoiding address
collisions. Since the FBU is encapsulated within the FNA when sent
from NAR's link, NAR drops the FBU if it detects a collision.
The combination of NCoA (present in source IP address) and the Link-
Layer Address (present as a Mobility Option) SHOULD be used to
distinguish the MN from other nodes.
6.4. New Options
All the options are of the form shown in Figure 11.
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 [3]. Option-Code
provides additional information for each of the options (See
individual options below).
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 11: Option Format
Koodli, Ed. Experimental [Page 31]
RFC 4068 Fast Handovers for Mobile IPv6 July 2005
6.4.1. IP Address Option
This option is sent in the Proxy Router Advertisement, the Handover
Initiate, and Handover Acknowledge messages.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Option-Code | Prefix Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ IPv6 Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: IPv6 Address 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
Prefix Length
The Length of the IPv6 Address Prefix.
Reserved MUST be set to zero by the sender and MUST be
ignored by the receiver.
IPv6 Address The IP address for the unit defined by the Type
field.
Koodli, Ed. Experimental [Page 32]
RFC 4068 Fast Handovers for Mobile IPv6 July 2005
6.4.2. New Router Prefix Information Option
This option is sent in the PrRtAdv message to provide the prefix
information valid on the NAR.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Option-Code | Prefix Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Prefix +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 13: New Router Prefix Information Option
Type 18
Length The size of this option in 8 octets including the
Type, Option-Code, and Length fields.
Option-Code 0
Prefix Length
8-bit unsigned integer. The number of leading bits
in the Prefix that are valid. The value ranges
from 0 to 128.
Reserved MUST be set to zero by the sender and MUST be
ignored by the receiver.
Prefix An IP address or a prefix of an IP address. The
Prefix Length field contains the number of valid
leading bits in the prefix. The bits in the prefix
after the prefix length are reserved and MUST be
initialized to zero by the sender and ignored by
the receiver.
Koodli, Ed. Experimental [Page 33]
RFC 4068 Fast Handovers for Mobile IPv6 July 2005
6.4.3. Link-Layer Address (LLA) Option
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Option-Code | LLA...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14: 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 the 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.
Depending on the size of the individual LLA option, appropriate
padding MUST be used to ensure that the entire option size is a
multiple of 8 octets.
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 the 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.
Koodli, Ed. Experimental [Page 34]
RFC 4068 Fast Handovers for Mobile IPv6 July 2005
6.4.4. Mobility Header Link-Layer Address (MH-LLA) Option
This option is identical to the LLA option, but is carried in the
Mobility Header messages (i.e., FNA). In the future, other Mobility
Header messages may also make use of this option. For instance,
including this option in FBU allows PAR to obtain the MN's LLA
readily. The format of the option when the LLA is 6 bytes is shown
in Figure 15. When the LLA size is different, the option MUST be
aligned appropriately. See Section 6.2 in [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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option-Code | Pad0=0 | LLA |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LLA |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 15: Mobility Header Link-Layer Address Option
Type 7
Length The size of this option in octets not including the
Type, Length, and Option-Code fields.
Option-Code 2 Link-Layer Address of the MN
LLA The variable length Link-Layer Address.
6.4.5. 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 16: Neighbor Advertisement Acknowledgment Option
Type 20
Koodli, Ed. Experimental [Page 35]
RFC 4068 Fast Handovers for Mobile IPv6 July 2005
Length 8-bit unsigned integer. Length of the option, in 8
octets. The length is 1 when NCoA is not supplied.
The length is 3 when NCoA is supplied (immediately
following the Reserved field).
Option-Code 0
Status 8-bit unsigned integer indicating the disposition
of the Fast Neighbor Advertisement message. The
following Status values are currently defined:
1 The New CoA is invalid.
2 The New CoA is invalid; use the supplied CoA.
The New CoA MUST be present following the
Reserved field.
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 the FNA with the NAACK option to notify the MN to
use a different NCoA if there is address collision. If the NCoA is
invalid, the Router Advertisement MUST use the NCoA as the
destination address but use the L2 address present in the FNA. The
MN SHOULD use the NCoA if it is supplied with the NAACK option. If
the NAACK indicates that the Link-Layer Address is unrecognized, the
MN MUST NOT use the NCoA or PCoA and SHOULD start the process of
acquiring an NCoA at the NAR immediately.
New option types may be defined in the future.
7. Configurable Parameters
Parameter Name Default Value Definition
------------------- ---------------------- -------
RTSOLPR_RETRIES 3 Section 6.1.1
MAX_RTSOLPR_RATE 3 Section 6.1.1
FBU_RETRIES 3 Section 4
PROXY_ND_LIFETIME 1.5 seconds Section 6.2.2
HI_RETRIES 3 Section 6.2.1
Koodli, Ed. Experimental [Page 36]
RFC 4068 Fast Handovers for Mobile IPv6 July 2005
8. Security Considerations
The following security vulnerabilities are identified, and suggested
solutions are mentioned.
1. Insecure FBU: In this case, packets meant for one address could be
stolen, or redirected to some unsuspecting node. This concern is
the same as that in 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 does not specify how this security association is
established. However, future work may specify this security
association establishment.
If an access router can ensure that the source IP address in an
arriving packet could only have originated from the node whose
Link-Layer Address is in the router's neighbor cache, then a bogus
node cannot use a victim's IP address for malicious redirection of
traffic. Such an operation is recommended at least on neighbor
discovery messages including the RtSolPr message.
2. Secure FBU, malicious or inadvertent redirection: In this case,
the FBU is secured, but the target of binding happens to be an
unsuspecting node due to inadvertent operation or 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. To do this, HI and
HAck message exchanges are to be used. When NAR accepts NCoA in
HI (with Code = 0), it proxies NCoA so that any arriving packets
are not sent on the link until the MN attaches and announces
itself through FNA. Therefore, any inadvertent or malicious
redirection to a host is avoided. It is still possible to jam
NAR's buffer with redirected traffic. However, since NAR's
handover state corresponding to NCoA has a finite (and short)
lifetime corresponding to a small multiple of anticipated handover
latency, the extent of this vulnerability is arguably small.
3. Sending an FBU from NAR's link: A malicious node may send an FBU
from NAR's link providing an unsuspecting node's address as NCoA.
Since the FBU is encapsulated in the FNA, NAR should detect the
Koodli, Ed. Experimental [Page 37]
RFC 4068 Fast Handovers for Mobile IPv6 July 2005
collision with an address in use when processing the FNA, and then
drop the FBU. When NAR is unable to detect address collisions,
there is a vulnerability that redirection can affect an
unsuspecting node.
9. IANA Considerations
This document defines four new experimental ICMPv6 messages that use
the Experimental Mobility Protocol ICMPv6 format [4]. These four new
Subtype value assignments out of the Experimental Mobility Protocol
Subtype Registry [4] have been assigned as follows:
Subtype Description Reference
------- ----------- ---------
2 RtSolPr Section 6.1.1
3 PrRtAdv Section 6.1.2
4 HI Section 6.2.1
5 HAck Section 6.2.2
This document defines four new Neighbor Discovery [6] options that
have received Type assignments from IANA.
Option-Type Description Reference
----------- ----------- ---------
17 IP Address Option Section 6.4.1
18 New Router Prefix
Information Option Section 6.4.2
19 Link-Layer Address
Option Section 6.4.3
20 Neighbor Advertisement
Acknowledgment Option Section 6.4.5
This document defines three new Mobility Header messages that have
received type allocations from the Mobility Header Types registry at
http://www.iana.org/assignments/mobility-parameters:
1. Fast Binding Update, described in Section 6.3.1
2. Fast Binding Acknowledgment, described in Section 6.3.2, and
3. Fast Neighbor Advertisement, described in Section 6.3.3.
This document defines a new Mobility Option which has received type
assignments from the Mobility Options Type registry at
http://www.iana.org/assignments/mobility-parameters:
1. Mobility Header Link-Layer Address option, described in Section
6.4.4.
Koodli, Ed. Experimental [Page 38]
RFC 4068 Fast Handovers for Mobile IPv6 July 2005
10. Acknowledgments
The editor would like to thank all those who have provided feedback
on this specification, but can only mention a few here: Martin
Andre, Vijay Devarapalli, Youn-Hee Han, Emil Ivov, Suvidh Mathur,
Koshiro Mitsuya, Gabriel Montenegro, Takeshi Ogawa, Sun Peng, YC
Peng, Domagoj Premec, and Jonathan Wood. The editor would like to
acknowledge a contribution from James Kempf to improve this
specification. 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.
11. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Conta, A. and S. Deering, "Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6)
Specification", RFC 2463, December 1998.
[3] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in
IPv6", RFC 3775, June 2004.
[4] Kempf, J., "Instructions for Seamoby and Experimental Mobility
Protocol IANA Allocations", RFC 4065, July 2005.
[5] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
November 1998.
[6] Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, December 1998.
[7] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
12. Contributors
This document originated in the fast handover design team effort.
The members of this design team in alphabetical order were: Gopal
Dommety, Karim El-Malki, Mohammed Khalil, Charles Perkins, Hesham
Soliman, George Tsirtsis, and Alper Yegin.
Koodli, Ed. Experimental [Page 39]
RFC 4068 Fast Handovers for Mobile IPv6 July 2005
The design team member's contact information:
Gopal Dommety
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
Phone:+1 408 525 1404
EMail: gdommety@cisco.com
Karim El Malki
Ericsson Radio Systems AB
LM Ericssons Vag. 8
126 25 Stockholm
SWEDEN
Phone: +46 8 7195803
Fax: +46 8 7190170
EMail: Karim.El-Malki@era.ericsson.se
Mohamed Khalil
Nortel Networks
EMail: mkhalil@nortelnetworks.com
Charles E. Perkins
Communications Systems Lab
Nokia Research Center
313 Fairchild Drive
Mountain View, California 94043
USA
Phone: +1-650 625-2986
Fax: +1 650 625-2502
EMail: charliep@iprg.nokia.com
Hesham Soliman
Flarion Technologies
EMail: H.Soliman@flarion.com
Koodli, Ed. Experimental [Page 40]
RFC 4068 Fast Handovers for Mobile IPv6 July 2005
George Tsirtsis
Flarion Technologies
EMail: G.Tsirtsis@flarion.com
Alper E. Yegin
Samsung Advanced Institute of Technology
75 West Plumeria Drive
San Jose, CA 95134
USA
Phone: +1 408 544 5656
EMail: alper.yegin@samsung.com
Author's Address
Rajeev Koodli, Editor
Nokia Research Center
313 Fairchild Drive
Mountain View, CA 94043 USA
Phone: +1 650 625 2359
Fax: +1 650 625 2502
EMail: Rajeev.Koodli@nokia.com
Koodli, Ed. Experimental [Page 41]
RFC 4068 Fast Handovers for Mobile IPv6 July 2005
Full Copyright Statement
Copyright (C) The Internet Society (2005).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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Acknowledgement
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Internet Society.
Koodli, Ed. Experimental [Page 42]
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