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IETF RFC 4988
Mobile IPv4 Fast Handovers
Last modified on Friday, October 26th, 2007
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Network Working Group R. Koodli
Request for Comments: 4988 C. Perkins
Category: Experimental Nokia Siemens Networks
October 2007
Mobile IPv4 Fast Handovers
 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.
Abstract
This document adapts the Mobile IPv6 Fast Handovers to improve delay
and packet loss resulting from Mobile IPv4 handover operations.
Specifically, this document addresses movement detection, IP address
configuration, and location update latencies during a handover. For
reducing the IP address configuration latency, the document proposes
that the new Care-of Address is always made to be the new access
router's IP address.
Koodli & Perkins Experimental PAGE 1 
RFC 4988 MIP4 Fast Handovers October 2007
Table of Contents
1. Introduction ....................................................3
2. Terminology .....................................................4
3. Factors Affecting Handover ......................................5
4. Protocol ........................................................6
4.1. Overview ...................................................6
4.2. Operation ..................................................7
5. Message Formats ................................................10
5.1. Fast Binding Update (FBU) .................................10
5.2. Fast Binding Acknowledgment (FBAck) .......................12
5.3. Router Solicitation for Proxy Advertisement (RtSolPr) .....13
5.4. Proxy Router Advertisement (PrRtAdv) ......................14
5.5. Handover Initiate (HI) ....................................17
5.6. Handover Acknowledge (HAck) ...............................19
6. Option Formats .................................................20
6.1. Link-Layer Address Option Format ..........................20
6.2. New IPv4 Address Option Format ............................22
6.3. New Router Prefix Information Option ......................22
7. Security Considerations ........................................23
8. IANA Considerations ............................................24
9. Acknowledgments ................................................25
10. References ....................................................25
10.1. Normative References .....................................25
10.2. Informative References ...................................26
Koodli & Perkins Experimental PAGE 2
RFC 4988 MIP4 Fast Handovers October 2007
1. Introduction
This document adapts the fast handover specification [RFC 4068] to
IPv4 networks. The fast handover protocol specified in this document
is particularly interesting for operation over links such as IEEE 802
wireless links. Fast handovers are not typically needed for wired
media due to the relatively large delays attributable to establishing
new connections in today's wired networks. Mobile IPv4 [RFC 3344]
registration messages are reused (with new type numbers) in this
document to enable faster implementation using existing Mobile IPv4
software. This document does not require link-layer triggers for
protocol operation, but performance will typically be enhanced by
using the appropriate triggers when they are available. This
document assumes that the reader is familiar with the basic operation
and terminology of Mobile IPv4 [RFC 3344] and Fast Handovers for
Mobile IPv6 [RFC 4068].
The active agents that enable continued packet delivery to a mobile
node (MN) are the access routers on the networks that the mobile node
connects to. Handover means that the mobile node changes its network
connection, and we consider the scenario in which this change means
change in access routers. The mobile node utilizes the access
routers as default routers in the normal sense, but also as partners
in mobility management. Thus, when the mobile node moves to a new
network, it processes handover-related signaling in order to identify
and develop a relationship with a new access router. In this
document, we call the previous access router PAR and the new access
router NAR, consistent with the terminology in [RFC 4068]. Unless
otherwise mentioned, a PAR is also a Previous Foreign Agent (PFA) and
a NAR is also a New Foreign Agent (NFA).
On a particular network, a mobile node may obtain its IP address via
DHCP [RFC 2131] (i.e., Co-located Care-of Address) or use the Foreign
Agent CoA. During a handover, the new CoA (NCoA) is always made to
be that of NAR. This allows a mobile node to receive and send
packets using its previous CoA (PCoA), so that delays resulting from
IP configuration (such as DHCP address acquisition delay) subsequent
to attaching to the new link are disengaged from affecting the
existing sessions.
Unlike in Mobile IPv6, a Mobile IPv4 host may rely on its Foreign
Agent to provide a Care-of Address. Using the protocol specified in
this document, the binding at the PAR is always established between
the on-link address the mobile node is using and a new CoA that it
can use on the NAR's link. When FA-CoA is used, the on-link address
is the MN's home address, not the FA-CoA itself, which needs to be
Koodli & Perkins Experimental PAGE 3
RFC 4988 MIP4 Fast Handovers October 2007
bound to the NCoA. So, when we say "a binding is established between
PCoA and NCoA", it is actually the home address of the mobile node
that is bound to the NCoA in the FA-CoA mode.
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].
2. Terminology
The terminology used in this document in based on [RFC 4068] and
[RFC 3344]. We provide some definitions below for convenience.
Mobile Node (MN): A Mobile IPv4 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.
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 IP address of the MN 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.
(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".
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RFC 4988 MIP4 Fast Handovers October 2007
3. Factors Affecting Handover
Both link-layer operations and IP-layer procedures affect the
perceived handover performance. However, the overall performance is
also (always) a function of specific implementation of the technology
as well as the system configuration. This document only specifies IP
layer protocol operations. The purpose of this section is to provide
an illustration of events that affect handover performance, but it is
purely informative.
The IP-layer handover delay and packet loss are influenced by
latencies due to movement detection, IP address configuration, and
the Mobile IP registration procedure. Movement detection latency
comes from the need to reliably detect movement to a new subnet.
This is a function of the frequency of router advertisements as well
as default agent reachability. IP address configuration latency
depends on the particular IP CoA being used. If co-located mode with
DHCP is used, the latency is quite likely going to be higher and
potentially unacceptable for real-time applications such as Voice
over IP. Finally, the Mobile IP registration procedure introduces a
round-trip of delay between the Mobile Node and its Home Agent over
the Internet. This delay is incurred after the mobile node performs
movement detection and IP configuration.
Underlying the IP operations are link-layer procedures. These are
technology-specific. For instance, in IEEE 802.11, the handover
operation typically involves scanning access points over all
available channels, selecting a suitable access point, and
associating with it. It may also involve performing access control
operations such as those specified in IEEE 802.1X [ieee-802.1x].
These delays contribute to the handover performance. See [fh-ccr]
and Chapters 20 and 22 in [mi-book]. Optimizations are being
proposed for standardization in IEEE; for instance, see
[ieee-802.11r] and [ieee-802.21]. Together with appropriate
implementation techniques, these optimizations can provide the
required level of delay support at the link-layer for real-time
applications.
Koodli & Perkins Experimental PAGE 5
RFC 4988 MIP4 Fast Handovers October 2007
4. Protocol
4.1. Overview
The design of the protocol is the same as for Mobile IPv6 [RFC 4068].
Readers should consult [RFC 4068] for details; here we provide a
summary.
The protocol avoids the delay due to movement detection and IP
configuration and disengages Mobile IP registration delay from the
time-critical path. The protocol provides the surrounding network
neighborhood information so that a mobile node can determine whether
it is moving to a new subnet even before the handover. The
information provided and the signaling exchanged between the local
mobility agents allow the mobile node to send and receive packets
immediately after handover. In order to disengage the Mobile IP
registration latency, the protocol provides routing support for the
continued use of a mobile node's previous CoA.
After a mobile node obtains its IPv4 Care-of Address, it builds a
neighborhood access point and subnet map using the Router
Solicitation for Proxy Advertisement (RtSolPr) and Proxy Router
Advertisement (PrRtAdv) messages. The mobile node may scan for
access points (APs) based on the configuration policy in operation
for its wireless network interface. If a scan detects a new AP, the
mobile node resolves the corresponding AP Identifier to subnet
information using the RtSolPr and PrRtAdv messages mentioned above.
At some point, the mobile node decides to undergo handover. It sends
a Fast Binding Update (FBU) message to PAR from the previous link or
from the new link. An FBU message enables creation of a binding
between the mobile node's previous CoA and the new CoA.
The coordination between the access routers is done by way of the
Handover Initiate (HI) and Handover Acknowledge (HAck) messages
defined in [RFC 4068]. After these signals have been exchanged
between the previous and new access routers (PAR and NAR), data
arriving at PAR will be tunneled to NAR for delivery to the newly
arrived mobile node. The purpose of HI is to securely deliver the
routing parameters for establishing this tunnel. The tunnel is
created by the access routers in response to the delivery of the FBU
from the mobile node.
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RFC 4988 MIP4 Fast Handovers October 2007
4.2. Operation
In response to a handover trigger or indication, the mobile node
sends a Fast Binding Update message to the Previous Access Router
(PAR) (see Section 5.1). Depending on the Mobile IP mode of
operation, the source IP address is either the Home Address (in FA
CoA mode) or co-located CoA (in CCoA mode). The FBU message SHOULD
(when possible) be sent while the mobile node is still connected to
PAR. When sent in this "predictive" mode, the fields in the FBU MUST
be set as follows:
The Home Address field is either the Home Address or the co-
located CoA whenever the mobile node has a co-located CoA.
The Home Agent field is set to PAR's IP address.
The Care-of Address field is the NAR's IP address (as discovered
via a PrRtAdv message).
The fields in the IP header MUST be set as follows:
The Destination IP address is PAR's IP address.
The Source IP address is either the Home Address or the co-located
CoA whenever the mobile node has a co-located CoA.
As a result of processing the FBU, PAR creates a binding between the
address given by the mobile node in the Home Address field and NAR's
IP address in its routing table. The PAR sends an FBack message (see
Section 5.2) as a response to the mobile node.
The timeline for the predictive mode of operation (adapted from
[RFC 4068]) is shown in Figure 1.
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RFC 4988 MIP4 Fast Handovers October 2007
MN PAR NAR
| | |
|------RtSolPr------->| |
|<-----PrRtAdv--------| |
| | |
|------FBU----------->|--------HI--------->|
| |<------HAck---------|
| <--FBack---|--FBack---> |
| | |
disconnect forward |
| packets===============>|
| | |
| | |
connect | |
| | |
|--------- FBU --------------------------->|
|<=================================== deliver packets
| | (including FBack)
| |<-----FBU-----------|
Figure 1: Predictive Fast Handover
The mobile node sends the FBU, regardless of its previous
transmission, when attachment to a new link is detected. This
minimally allows NAR to detect the mobile node's attachment, but also
the retransmission of FBU when an FBack has not been received yet.
When sent in this "reactive" mode, the Destination IP address in the
IP header MUST be NAR's IP address; the rest of the fields in the FBU
are the same as in the "predictive" case.
When NAR receives FBU, it may already have processed the HI message
and created a host route entry for the mobile node, using either the
home address or the co-located care-of address as provided by PAR.
In that case, NAR SHOULD immediately forward arriving and buffered
packets as well as the FBAck message. In any case, NAR MUST forward
the contents of the FBU message, starting from the Type field, to
PAR; the Source and Destination IP addresses in the new packet now
contain the IP addresses of NAR and PAR, respectively.
The reactive mode of operation (adapted from [RFC 4068]) is
illustrated in Figure 2. Even though the Figure does not show the HI
and HAck messages illustrated in Figure 1, these messages could
already have been exchanged (in the case when the PAR has already
processed the FBU sent from the previous link); if not, the PAR sends
a HI message to the NAR. The FBack packet is forwarded by the NAR to
the MN along with the data packets.
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RFC 4988 MIP4 Fast Handovers October 2007
MN PAR NAR
| | |
|------RtSolPr------->| |
|<-----PrRtAdv--------| |
| | |
disconnect | |
| | |
| | |
connect | |
|-----------FBU-------|------------------->|
| |<-----FBU-----------|
| |------FBack-------->|
| forward |
| packets===============>|
| | |
|<=================================== deliver packets
| (including FBack)
| |
Figure 2: Reactive Fast Handover
The Handover Initiate (HI) and Handover Acknowledge (HAck) messages
serve to establish a bidirectional tunnel between the routers to
support packet forwarding for PCoA. The tunnel itself is established
as a response to the FBU message. The PAR sends the HI message with
Code = 0 when it receives FBU with source IP address set to PCoA.
The PAR sends HI with Code = 1 when it receives FBU with source IP
address not set to PCoA (i.e., when received from NAR). This allows
NAR to disambiguate HI message processing sent as a response to
predictive and reactive modes of operation. If NAR receives a HI
message with Code = 1, and it has already set up a host route entry
and a reverse tunnel for PCoA, it SHOULD still respond with a HAck
message, using an appropriate Code value defined in Section 5.6.
The protocol provides an option for NAR to return NCoA for use by the
mobile node. When NAR can provide an NCoA for exclusive use of the
mobile node, the address is supplied in the HAck message. The PAR
includes this NCoA in FBack. Exactly how NAR manages the address
pool from which it supplies NCoA is not specified in this document.
Nevertheless, the MN should be prepared to use this address instead
of performing DHCP or similar operations to obtain an IPv4 address.
Even though the mobile node can obtain this NCoA from the NAR, it is
unaware of the address at the time it sends an FBU. Hence, it binds
PCoA to NAR's IP address as before.
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RFC 4988 MIP4 Fast Handovers October 2007
5. Message Formats
This section specifies the formats for messages used in this
protocol. The Code values below are the same as those in [RFC 4068],
and do not require any assignment from IANA.
5.1. Fast Binding Update (FBU)
The FBU format is bitwise identical to the Registration Request
format in [RFC 3344]. The same destination port number, 434, is used,
but the FBU and FBAck messages in this specification have new message
type numbers.
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 |x|x|D|M|G|r|T|x| reserved | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Agent |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Care-of Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Identification +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extensions ...
+-+-+-+-+-+-+-+-
Figure 3: Fast Binding Update (FBU) Message
IP Fields:
Source address: The interface address from which the message is
sent. Either PCoA (co-located or Home Address), or NAR's IP
address (when forwarded from NAR to PAR).
Destination Address: The IP address of the Previous Access
Router (PAR) or the New Access Router (NAR).
Source Port: variable
Destination port: 434
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RFC 4988 MIP4 Fast Handovers October 2007
Message Fields:
Type: 20
Flags: See [RFC 3344]. The 'S' and 'B' flags in [RFC 3344] are
sent as zero, and ignored on reception.
reserved: Sent as zero, ignored on reception
Lifetime: The number of seconds remaining before the binding
expires. This value MUST NOT exceed 10 seconds.
Home Address: MUST be either the co-located CoA or the Home
Address itself (in FA-CoA mode)
Home Agent: The Previous Access Router's global IP address
Care-of Address: The New Access Router's global IP address.
Even when a New CoA is provided to the MN (see Section 5.4),
NAR's IP address MUST be used for this field.
Identification: a 64-bit number used for matching an FBU with
FBack. Identical to usage in [RFC 3344]
Extensions: MUST contain the MN-PAR Authentication Extension
(see Section 8)
The MN-PAR Authentication Extension is the Generalized Mobile IP
Authentication Extension in [RFC 4721] with a new Subtype for MN-PAR
Authentication. The Authenticator field in the Generalized Mobile IP
Authentication Extension is calculated using a shared key between the
MN and the PAR. However, the key distribution itself is beyond the
scope of this document, and is assumed to be performed by other means
(for example, using [RFC 3957]).
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RFC 4988 MIP4 Fast Handovers October 2007
5.2. Fast Binding Acknowledgment (FBAck)
The FBAck format is bitwise identical to the Registration Reply
format in [RFC 3344].
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 | reserved | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Agent |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Identification +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extensions ...
+-+-+-+-+-+-+-+-
Figure 4: Fast Binding Acknowledgment (FBAck)
IP Fields:
Message Source address: Typically copied from the destination
address of the FBU message
Destination Address: Copied from the Source IP address in FBU
message
Source Port: variable
Destination port: Copied from the source port in FBU message
Message Fields:
Type: 21
Code: Indicates the result of processing FBU message.
0: FBU Accepted
1: FBU Accepted, NCoA supplied
128: FBU Not Accepted, reason unspecified
129: Administratively prohibited
130: Insufficient resources
reserved: Sent as zero, ignored on reception
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RFC 4988 MIP4 Fast Handovers October 2007
Lifetime: The granted number of seconds remaining before
binding expires.
Home Address: either the co-located CoA or the Home Address
itself (in FA-Coa mode)
Home Agent: The Previous Access Router's global IP address
Identification: a 64-bit number used for matching FBU. Copied
from the field in FBU for which this FBack is a reply.
Extensions: The MN-PAR Authentication extension MUST be present
(see Section 8). In addition, a New IPv4 Address Option, with
Option-Code 2, MUST be present when NAR supplies the NCoA (see
Section 6.2).
5.3. 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 6.1.
The message format and processing rules are identical to those
defined in [RFC 4068].
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: 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.
Time-to-Live: At least 1. See [RFC 1256].
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RFC 4988 MIP4 Fast Handovers October 2007
ICMP Fields:
Type: 41. See Section 3 in [RFC 4065].
Code: 0
Checksum: The 16-bit one's complement of the one's complement
sum of the ICMP message, starting with the ICMP Type. For
computing the checksum, the Checksum and the Reserved fields
are set to 0. See [RFC 1256].
Subtype: 6
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:
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
Section 6.1).
5.4. Proxy Router Advertisement (PrRtAdv)
Access routers send out a Proxy Router Advertisement message
gratuitously if the handover is network-initiated or as a response to
RtSolPr message from a mobile node, providing the link-layer address,
IP address, and subnet prefixes of neighboring access routers. All
the link-layer address options have the format defined in Section
6.1.
The message format and processing rules are identical to those
defined in [RFC 4068].
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RFC 4988 MIP4 Fast Handovers October 2007
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 6: Proxy Router Advertisement (PrRtAdv) Message
IP Fields:
Source Address: An IP address assigned to the sending interface
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.
Time-to-Live: At least 1. See [RFC 1256].
ICMP Fields:
Type: 41. See Section 3 in [RFC 4065].
Code 0, 1, 2, 3, or 4. See below.
Checksum: The 16-bit one's complement of the one's complement
sum of the ICMP message, starting with the ICMP Type. For
computing the checksum, the Checksum and the Reserved fields
are set to 0. See [RFC 1256].
Subtype: 7
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:
New Access Point Link-layer Address: The link-layer address
(LLA) or identification of the access point. When there is no
wildcard in RtSolPr, this is copied from the LLA (for which the
router is supplying the [AP-ID, AR-Info] tuple) present in
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RFC 4988 MIP4 Fast Handovers October 2007
RtSolPr. When a wildcard is present in RtSolPr, PAR uses its
neighborhood information to populate this field. 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: The number of leading
bits that define the network number of the corresponding
Router's IP Address option (see above).
New CoA Option: MAY be present, typically when PrRtAdv is sent
unsolicited. PAR MAY compute new CoA by communicating with the
NAR or by means not specified in this document. In any case,
the MN should be prepared to use this address instead of
performing DHCP or similar operations to obtain an IPv4
address. Even when it uses the New CoA provided, the MN MUST
bind its current on-link address (PCoA) to that of NAR in the
FBU message.
A PrRtAdv with Code 0 means that the MN should use the [AP-ID,
AR-Info] tuple present in the options above. In this case, the
Option-Code field (see Section 6.1) in the New AP LLA option is 1,
reflecting the LLA of the access point for which the rest of the
options are related, and the Option-Code for the New Router's LLA
option is 3. Multiple tuples may be present.
A PrRtAdv with Code 1 means that the message is sent unsolicited. If
a New IPv4 option (see Figure 10) is present following the New Router
Prefix Information option (see Section 6.3), 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.
The Option-Code field (see Section 6.1) in the New AP LLA option 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. The LLA option contains an Option-Code value
that indicates a specific reason (see Section 6.1).
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 values in the LLA option distinguish different
outcomes (see Section 6.1).
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RFC 4988 MIP4 Fast Handovers October 2007
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 all available [Access Point
Identifier, Link-Layer Address option, Prefix Information Option]
tuples corresponding to the PAR's neighborhood.
The New CoA option may also be used when the PrRtAdv is sent as a
response to a RtSolPr message. However, the solicited RtSolPr and
PrRtAdv exchange for neighborhood discovery is logically decoupled
from the actual handover phase involving the FBU and FBack messages
(above) as well as HI and HAck messages (see below). This means the
access routers have to carefully manage the supplied address due to
the relative scarcity of addresses in IPv4.
5.5. Handover Initiate (HI)
The Handover Initiate (HI) is an ICMP message sent by an Access
Router (typically PAR) to another Access Router (typically NAR) to
initiate the process of a mobile node's handover.
The message format and processing rules are identical to those
defined in [RFC 4068].
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 7: Handover Initiate (HI) Message
IP Fields:
Source Address: The IP address of the PAR
Destination Address: The IP address of the NAR
Time-to-Live: At least 1. See [RFC 1256].
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RFC 4988 MIP4 Fast Handovers October 2007
ICMP Fields:
Type: 41. See Section 3 in [RFC 4065].
Code: 0 or 1. See below
Checksum: The 16-bit one's complement of the one's complement
sum of the ICMP message, starting with the ICMP Type. For
computing the checksum, the Checksum and the Reserved fields
are set to 0. See [RFC 1256].
Subtype: 8
S: 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: Buffer flag. When set, the destination SHOULD buffer any
packets towards the node indicated in the options of this
message. Used when Code = 0, SHOULD be set to 0 when Code = 1.
Reserved: MUST be set to zero by the sender and ignored by the
receiver.
Identifier: MUST be set by the sender so replies can be matched
to this message.
Valid Options:
Link-layer address of MN: The link-layer address of the MN that
is undergoing handover to the destination (i.e., NAR). This
option MUST be included so that the destination can recognize
the MN.
Previous Care-of Address: The IP address used by the MN while
attached to the originating router. This option MUST be
included so that a host route can be established on the NAR.
New Care-of Address: This option MAY be present when the MN
wishes to use a new IP address when connected to the
destination. When the 'S' bit is set, NAR MAY provide this
address in HAck, in which case the MN should be prepared to use
this address instead of performing DHCP or similar operations
to obtain an IPv4 address.
PAR uses Code = 0 when it processes the FBU received with PCoA as
source IP address. PAR uses Code = 1 when the FBU is received with
NAR's IP address as the source IP address.
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RFC 4988 MIP4 Fast Handovers October 2007
5.6. Handover Acknowledge (HAck)
The Handover Acknowledgment message is a new ICMP message that MUST
be sent (typically by NAR to PAR) as a reply to the Handover Initiate
(HI) (see Section 5.5) message.
The message format and processing rules are identical to those
defined in [RFC 4068].
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 8: 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.
Time-to-Live: At least 1. See [RFC 1256].
ICMP Fields:
Type: 41. See Section 3 in [RFC 4065].
Code:
0: Handover Accepted
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
128: Handover Not Accepted, reason unspecified
129: Administratively prohibited
130: Insufficient resources
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RFC 4988 MIP4 Fast Handovers October 2007
Checksum: The 16-bit one's complement of the one's complement
sum of the ICMP message, starting with the ICMP Type. For
computing the checksum, the Checksum and the Reserved fields
are set to 0. See [RFC 1256].
Subtype: 9
Reserved: MUST be set to zero by the sender and ignored by the
receiver.
Identifier: Copied from the corresponding field in the Handover
Initiate message this message is in response to.
Valid Options:
New Care-of Address: If the 'S' flag in the HI message is set,
this option MUST be used to provide NCoA the MN should use when
connected to this router. This option MAY be included even
when 'S' bit is not set, e.g., Code 2 above. The MN should be
prepared to use this address instead of performing DHCP or
similar operations to obtain an IPv4 address.
The Code 0 is the expected average case of a handover being accepted
and the routing support provided for the use of PCoA. The rest of
the Code values pertain to the use of NCoA (which is common in
[RFC 4068]). Code values 1 and 2 are for cases when the MN proposes
an NCoA and the NAR provides a response. Code 3 is when the NAR
provides NCoA (which could be the same as that proposed by the MN).
Code 4 is when the NAR does not provide NCoA, but instead provides
routing support for PCoA.
6. Option Formats
The options in this section are specified as extensions for the HI
and HAck messages, as well as for the PrRtSol and PrRtAdv messages.
The Option-Code values below are the same as those in [RFC 4068], and
do not require any assignment from IANA.
6.1. Link-Layer Address Option Format
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 9: Link-Layer Address Option Format
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RFC 4988 MIP4 Fast Handovers October 2007
Fields:
Type: 20
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
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
Length: The length of the option (including the Type, Length
and Option-Code fields) in units of 8 octets.
Link-Layer Address: The variable-length link-layer address.
The content and format of this field (including byte and bit
ordering) depends on the specific link-layer in use.
There is no length field for the LLA itself. Implementations MUST
determine the length of the LLA based on the specific link technology
where the protocol is run. The total size of the LLA option itself
MUST be a multiple of 8 octets. Hence, padding may be necessary
depending on the size of the LLA used. In such a case, the padN
option [RFC 2460] MUST be used. As an example, when the LLA is 6
bytes (meaning 7 bytes of padding is necessary to bring the LLA
option length to 2), the padN option will have a length field of 5
and 5 bytes of zero-valued octets (see [RFC 2460]).
Koodli & Perkins Experimental PAGE 21
RFC 4988 MIP4 Fast Handovers October 2007
6.2. New IPv4 Address Option Format
This option is used to provide the new router's IPv4 address or the
NCoA in PrRtAdv, as well as PCoA and NCoA in HI and HAck 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 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| New IPv4 Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: New IPv4 Address Option Format
Fields:
Type: 21
Length: The length of the option (including the Type, Length
and Option-Code fields) in units of 8 octets.
Option-Code:
1: Previous CoA
2: New CoA
3: NAR's IP Address
Reserved: Set to zero.
New IPv4 Address: NAR's IPv4 address or the NCoA assigned by
NAR.
6.3. New Router Prefix Information Option
This option is used in the PrRtAdv 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 | Length | Option-Code | Prefix-Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: New Router Prefix Information Option Format
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RFC 4988 MIP4 Fast Handovers October 2007
Fields:
Type: 22
Length: The length of the option (including the Type, Length
and Option-Code fields) in units of 8 octets.
Option-Code: 0
Prefix-Length The number of leading bits that define the
network number of the corresponding Router's IP Address option.
Reserved: Set to zero.
7. Security Considerations
As outlined in [RFC 4068], the following vulnerabilities are
identified and the solutions mentioned.
Insecure FBU:
Failure to protect the FBU message could result in packets meant for
an address being stolen or redirected to some unsuspecting node.
This concern is similar to that in Mobile Node and Home Agent
relationship.
Hence, the FBU and FBack messages MUST be protected using a security
association shared between a mobile node and its access router. In
particular, the MN-PAR Authentication Extension MUST be present in
each of these messages. This document does not specify how the
security association is established between an MN and the AR/FA.
Secure FBU, malicious or inadvertent redirection:
Even if the MN-PAR authentication extension is present in an FBU, an
MN may inadvertently or maliciously attempt to bind its PCoA to an
unintended address on NAR's link, and cause traffic flooding to an
unsuspecting node.
This vulnerability is avoided by always binding the PCoA to the NAR's
IP address, even when the NAR supplies an NCoA to use for the MN. It
is still possible to jam NAR's buffer with redirected traffic.
However, the handover state corresponding to the MN's PCoA has a
finite lifetime, and can be configured to be a few multiples of the
anticipated handover latency. Hence, the extent of this
vulnerability is small. It is possible to trace the culprit MN with
an established security association at the access router.
Koodli & Perkins Experimental PAGE 23
RFC 4988 MIP4 Fast Handovers October 2007
Communication between the access routers:
The access routers communicate using HI and HAck messages in order to
establish a temporary routing path for the MN undergoing handover.
This message exchange needs to be secured to ensure routing updates
take place as intended.
The HI and HAck messages need to be secured using a preexisting
security association between the access routers to ensure at least
message integrity and authentication, and SHOULD also include
encryption. IPsec ESP SHOULD be used.
8. IANA Considerations
The IANA assignments made for messages, extensions, and options
specified in this document are described in the following paragraphs.
This document defines two new messages that use the Mobile IPv4
control message format [RFC 3344]. These message details are as
follows:
+------+-------------+-------------+
| Type | Description | Reference |
+------+-------------+-------------+
| 20 | FBU | Section 5.1 |
| 21 | FBAck | Section 5.2 |
+------+-------------+-------------+
This document defines four new experimental ICMP messages that use
the ICMP Type 41 for IPv4. See Section 3 in [RFC 4065]. The new
messages specified in this document have been assigned Subtypes from
the registry in [RFC 4065]:
+---------+-------------+-------------+
| Subtype | Description | Reference |
+---------+-------------+-------------+
| 6 | RtSolPr | Section 5.3 |
| 7 | PrRtAdv | Section 5.4 |
| 8 | HI | Section 5.5 |
| 9 | HAck | Section 5.6 |
+---------+-------------+-------------+
This document defines three new options that have been assigned Types
from the Mobile IP Extensions for ICMP Router Discovery messages
[RFC 3344]. These options are as follows:
Koodli & Perkins Experimental PAGE 24
RFC 4988 MIP4 Fast Handovers October 2007
+------+------------------+-------------+
| Type | Description | Reference |
+------+------------------+-------------+
| 20 | LLA | Section 6.1 |
| 21 | New IPv4 Address | Section 6.2 |
| 22 | NAR Prefix Info | Section 6.3 |
+------+------------------+-------------+
The MN-PAR Authentication Extension described in Sections 5.1 and 5.2
is a Generalized Mobile IP Authentication Extension defined in
Section 5 of [RFC 4721]. The MN-PAR Authentication has been assigned
a Subtype from the registry specified in [RFC 4721]. The Extension
details are as follows:
+---------+-----------------------+--------------------------+
| Subtype | Description | Reference |
+---------+-----------------------+--------------------------+
| 4 | MN-PAR Auth Extension | Section 5.1 |
+---------+-----------------------+--------------------------+
9. Acknowledgments
Thanks to all those who expressed interest in having a Fast Handovers
for Mobile IPv4 protocol along the lines of [RFC 4068]. Thanks to
Vijay Devarapalli, Kent Leung, and Domagoj Premec for their review
and input. Kumar Viswanath and Uday Mohan implemented an early
version of this protocol. Many thanks to Alex Petrescu for his
thorough review that improved this document. Thanks to Pete McCann
for the proofreading, and to Jari Arkko for the review, which have
helped improve this document. Thanks to Francis Dupont and Hannes
Tschofenig for the GEN-ART and TSV-DIR reviews.
Sending FBU from the new link (i.e., reactive mode) is similar to
using the extension defined in [mip4-ro]; however, this document also
addresses movement detection and router discovery latencies.
10. References
10.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 1256] Deering, S., Ed., "ICMP Router Discovery Messages",
RFC 1256, September 1991.
[RFC 2460] Deering, S. and R. Hinden, "Internet Protocol, Version
6 (IPv6) Specification", RFC 2460, December 1998.
Koodli & Perkins Experimental PAGE 25
RFC 4988 MIP4 Fast Handovers October 2007
[RFC 3344] Perkins, C., Ed., "IP Mobility Support for IPv4", RFC
3344, August 2002.
[RFC 4065] Kempf, J., "Instructions for Seamoby and Experimental
Mobility Protocol IANA Allocations", RFC 4065, July
2005.
[RFC 4068] Koodli, R., Ed., "Fast Handovers for Mobile IPv6", RFC
4068, July 2005.
[RFC 4721] Perkins, C., Calhoun, P., and J. Bharatia, "Mobile
IPv4 Challenge/Response Extensions (Revised)", RFC
4721, January 2007.
10.2. Informative References
[fh-ccr] R. Koodli and C. E. Perkins, "Fast Handovers and
Context Transfers in Mobile Networks", ACM Computer
Communications Review Special Issue on Wireless
Extensions to the Internet, October 2001.
[ieee-802.11r] IEEE, "IEEE Standard for Local and Metropolitan Area
Networks: Fast Roaming/Fast BSS Transition, IEEE Std
802.11r", September 2006.
[ieee-802.1x] IEEE, "IEEE Standards for Local and Metropolitan Area
Networks: Port-based Network Access Control, IEEE Std
802.1X-2001", June 2001.
[ieee-802.21] The IEEE 802.21 group, http://www.ieee802.org/21.
[mi-book] R. Koodli and C. E. Perkins, "Mobile Internetworking
with IPv6: Concepts, Principles and Practices", John
Wiley & Sons, June 2007.
[mip4-ro] Perkins, C. and D. Johnson, "Route Optimization in
Mobile IP", Work in Progress, September 2001.
[RFC 2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[RFC 3957] Perkins, C. and P. Calhoun, "Authentication,
Authorization, and Accounting (AAA) Registration Keys
for Mobile IPv4", RFC 3957, March 2005.
Koodli & Perkins Experimental PAGE 26
RFC 4988 MIP4 Fast Handovers October 2007
Authors' Addresses
Rajeev Koodli
Nokia Siemens Networks
313 Fairchild Driive
Mountain View, CA 94043
USA
EMail: rajeev.koodli@nokia.com
Charles Perkins
Nokia Siemens Networks
313 Fairchild Driive
Mountain View, CA 94043
USA
EMail: charles.perkins@nokia.com
Koodli & Perkins Experimental PAGE 27
RFC 4988 MIP4 Fast Handovers October 2007
Full Copyright Statement
Copyright © The IETF Trust (2007).
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, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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Mobile IPv4 Fast Handovers
RFC TOTAL SIZE: 57921 bytes
PUBLICATION DATE: Friday, October 26th, 2007
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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