Network Working Group K. El Malki, Editor
INTERNET-DRAFT Athonet
Expires: April 2006 3 October 2005
Low Latency Handoffs in Mobile IPv4
<draft-ietf-mobileip-lowlatency-handoffs-v4-11.txt>
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Abstract
Mobile IPv4 describes how a Mobile Node can perform IPv4-layer
handoffs between subnets served by different Foreign Agents. In
certain cases, the latency involved in these handoffs can be above
the threshold required for the support of delay-sensitive or real-
time services. The aim of this document is to present two methods to
achieve low-latency Mobile IPv4 handoffs. In addition, a combination
of these two methods is described. The described techniques allow
greater support for real-time services on a Mobile IPv4 network by
minimising the period of time when a Mobile Node is unable to send or
receive IPv4 packets due to the delay in the Mobile IPv4 Registration
process.
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TABLE OF CONTENTS
1. Introduction.....................................................3
1.1. Terminology.................................................4
1.2. The Techniques..............................................5
1.3. L2 triggers.................................................7
1.4. Requirements language.......................................9
2. Requirements.....................................................9
3. The PRE-REGISTRATION Handoff Method.............................10
3.1. Operation..................................................10
3.2. Network-Initiated Handoff..................................13
3.3. Mobile-Initiated Handoff...................................14
3.4. Obtaining and Proxying nFA Advertisements..................16
3.4.1. Inter-FA Solicitation.................................16
3.4.2. Tunneled nFA Advertisements...........................17
3.5. Caching Router Advertisements..............................17
3.6. Movement Detection, MN and FA Considerations...............18
3.7. L2 Address Considerations..................................19
3.8. Applicability of PRE-REGISTRATION Handoff..................20
4. The POST-REGISTRATION Handoff Method............................21
4.1. Two Party Handoff..........................................22
4.2. Three Party Handoff........................................26
4.3. Renewal or Termination of Tunneling Service................31
4.4. When will the MN perform a Mobile IPv4 Registration?.......31
4.5. Handoff Request (HRqst) Message format.....................32
4.6. Handoff Reply (HRply) Message..............................34
4.7. Handoff To Third (HTT) Message.............................36
4.8. Applicability of POST-REGISTRATION Handoff Method..........36
5. Combined Handoff Method.........................................37
6. Layer 2 and Layer 3 Handoff timing Considerations...............38
7. Reverse Tunneling Support.......................................39
8. Handoff Signaling Failure Recovery..............................39
8.1. PRE-REGISTRATION Signaling Failure Recovery................39
8.1.1. Failure of PrRtSol and PrRtAdv........................39
8.1.2. Failure of Inter-FA solicitation and advertisement....40
8.2. POST-REGISTRATION Signaling Failure Recovery...............40
8.2.1. HRqst Message Dropped.................................40
8.2.2. HRply Message Dropped.................................41
9. Generalized Link Layer and IPv4 Address (LLA) Extension.........41
9.1. 3GPP2 IMSI Link Layer Address and Connection ID Extension..42
9.2. 3GPP IMSI Link Layer Address Extension.....................43
9.3. Ethernet Link Layer Address Extension......................44
9.4. IEEE 64-Bit Global Identifier (EUI-64) Address Extension...44
9.5. Solicited IPv4 Address Extension...........................45
9.6. Access Point Identifier Extension..........................45
9.7. FA IPv4 Address Extension..................................46
10. IANA Considerations............................................46
10.1. New Extension values......................................47
10.2. Generalized Link Layer and IP Address Identifier (LLA) Sub-
type values.....................................................47
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10.3. New Message Type and Code.................................47
11. Security Considerations........................................48
12. Acknowledgements...............................................50
13. Editor's Address...............................................50
14. Contributing Authors...........................................50
15. References.....................................................51
15.1. Normative References......................................51
15.2. Informative References....................................51
Appendix A - Gateway Foreign Agents................................52
Appendix B - Low Latency Handoffs for Multiple-Interface MNs.......53
Appendix C - PRE_REGISTRATION Message Summary......................54
1.
Introduction
Mobile IPv4 [1] describes how a Mobile Node (MN) can perform IPv4-
layer handoff between subnets served by different Foreign Agents
(FAs). In certain cases, the latency involved in handoff can be
above the threshold required for the support of delay-sensitive or
real-time services. The aim of this document is to present two
techniques to achieve low-latency Mobile IPv4 handoff during movement
between FAs. A further combination of these two techniques is also
described. The presented techniques allow greater support for real-
time services on a Mobile IPv4 network by minimising the period of
time during which a MN is unable to send or receive IPv4 packets due
to the delay in the Mobile IPv4 Registration process. One or more of
these techniques may be required to achieve fast Mobile IPv4 handoffs
over different wireless technologies (e.g. WLAN, Cellular, WiMAX,
Flash-OFDM etc.). Each wireless technology has different layer-2
handoff procedures and the best low latency technique for each
scenario should be used to optimize the handoff performance. Further
deployment and experimentation is required to determine which
technique is best suited to the wireless technologies in terms of
implementation and performance. Therefore the authors encourage
further performance measurements and work on Low-Latency-over-foo
specifications in collaboration with the appropriate wireless
technology fora to describe the applicability to different wireless
layer-2s.
In the rest of this section, terminology used throughout the document
is presented, the handoff techniques are briefly described, and the
use of link layer information is outlined. In Section 2, a brief
description of requirements is presented. Section 3 describes the
details of the PRE-REGISTRATION handoff technique, while Section 4
describes the details of the POST-REGISTRATION handoff technique. In
Section 5, a combined method using the two handoff techniques
together is presented. Section 6 discusses Layer 2 and Layer 3
handoff timing considerations. Section 7 discusses reverse tunneling
support, Section 8 describes mechanisms to recover from message
failures while Section 9 describes protocol extensions required by
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the handoff techniques. Sections 10 and 11 discuss IANA and security
considerations. Finally the three appendices discuss additional
material related to the handoff techniques. Appendix A gives a short
introduction to Regional Registrations [11] which can be used
together with low latency handoffs. Appendix B discusses low latency
handoff when a MN has multiple wireless L2 interfaces, in which case
the techniques in this document may not be necessary. Appendix C
provides a summary of the messages used in PRE-REGISTRATION.
1.1.
Terminology
This section presents a few terms used throughout the document.
oFA - old Foreign Agent (FA), the FA involved in handling the
care of address of a Mobile Node (MN) prior to a Layer 3
(L3) handoff.
nFA - new Foreign Agent, the FA anticipated to be handling a
MN's care of address after completion of an L3 handoff.
aFA - anchor Foreign Agent, the FA that is currently handling
the network end of the tunnel in POST-REGISTRATION.
L2 handoff - Movement of a MN's point of Layer 2 (L2)
connection from one wireless access point to another.
L3 handoff - Movement of a MN between FAs which involves
changing the care-of address (CoA) at Layer 3 (L3).
L2 trigger - Information from L2 that informs L3 of particular
events before and after L2 handoff. The descriptions of L2
triggers in this document are not specific to any particular
L2, but rather represent generalizations of L2 information
available from a wide variety of L2 protocols.
L2-MT - An L2 trigger that occurs at the MN informing of
movement to a certain nFA (Mobile Trigger).
L2-ST or source trigger - An L2 trigger that occurs at oFA,
informing the oFA that L2 handoff is about to occur.
L2-TT or target trigger - An L2 trigger that occurs at nFA,
informing the nFA that a MN is about to be handed off to
nFA.
L2-LU - An L2 trigger that occurs at the MN or nFA, informing
that the L2 link between MN and nFA is established.
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L2-LD - An L2 trigger that occurs at the oFA, informing the oFA
that the L2 link between MN and oFA is lost.
low latency handoff - L3 handoff in which the period of time
during which the MN is unable to receive packets is
minimized.
low loss handoff - L3 handoff in which the number of packets
dropped or delayed is minimized. Low loss handoff is often
called smooth handoff.
seamless handoff - L3 handoff that is both low latency and low
loss.
bi-directional edge tunnel (BET) - A bidirectional tunnel
established between two FAs for purposes of temporarily
routing a MN's traffic to/from it on a new subnet without
requiring the MN to change CoA.
ping-pong - Rapid back and forth movement between two wireless
access points often due to failure of L2 handoff. Ping-pong
can occur if radio conditions for both the old and new
access points are about equivalent and less than optimal for
establishing a good, low-error L2 connection.
network-initiated handoff - L3 handoff in which oFA or nFA
initiates the handoff.
mobile-initiated handoff - L3 handoff in which the MN initiates
the handoff.
MN or FA identifier - An IPv4 address of a MN or FA, or an L2
identifier that can be resolved to the IPv4 address of a MN
or FA. If the identifier is an L2 identifier, it may be
specific to the L2 technology.
1.2.
The Techniques
Mobile IPv4 was originally designed without any assumptions about the
underlying link layers over which it would operate so that it could
have the widest possible applicability. This approach has the
advantage of facilitating a clean separation between L2 and L3 of the
protocol stack, but it has negative consequences for handoff latency.
The strict separation between L2 and L3 results in the following
built-in sources of delay:
- The MN may only communicate with a directly connected FA. This
implies that a MN may only begin the registration process after
an L2 handoff to nFA (new FA) has completed.
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- The registration process takes some non-zero time to complete as
the Registration Requests propagate through the network. During
this period of time the MN is not able to send or receive
IPv4 packets.
This document presents techniques for reducing these built-in delay
components of Mobile IPv4. The techniques can be divided into two
general categories, depending on which of the above problems they
are attempting to address:
- Allow the MN to communicate with the nFA while still connected
to the oFA.
- Provide for data delivery to the MN at the nFA even before the
formal registration process has completed.
The first category of techniques allows the MN to "pre-build" its
registration state on the nFA prior to an underlying L2 handoff.
The second category of techniques allow for service to continue
uninterrupted while the handoff is being processed by the network.
Three methods are presented in this draft to achieve low-latency L3
handoff, one for each category described above and one as a
combination of the two:
- PRE-REGISTRATION handoff method,
- POST-REGISTRATION handoff method,
- combined handoff method.
The PRE-REGISTRATION handoff method allows the MN to be involved in
an anticipated IPv4-layer handoff. The MN is assisted by the network
in performing an L3 handoff before it completes the L2 handoff. The
L3 handoff can be either network-initiated or mobile-initiated.
Accordingly, L2 triggers are used both in the MN and in the FA to
trigger particular L3 handoff events. The PRE-REGISTRATION method
coupled to L2 mobility helps to achieve seamless handoffs between
FAs. The basic Mobile IPv4 concept involving advertisement followed
by registration is supported and the PRE-REGISTRATION handoff method
relies on Mobile IPv4 security. No new messages are proposed, except
for an extension to the Agent Solicitation message in the
mobile-initiated case.
The POST-REGISTRATION handoff method proposes extensions to the
Mobile IPv4 protocol to allow the oFA (old FA) and nFA (new FA) to
utilize L2 triggers to set up a bi-directional tunnel between oFA and
nFA that allows the MN to continue using its oFA while on nFA's
subnet. This enables a rapid establishment of service at the new
point of attachment which minimizes the impact on real-time
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applications. The MN must eventually perform a formal Mobile IPv4
registration after L2 communication with the new FA is established,
but this can be delayed as required by the MN or FA. Until the MN
performs registration, the FAs will setup and move bidirectional
tunnels as required to give the MN continued connectivity.
The combined method involves running a PRE-REGISTRATION and a
POST-REGISTRATION handoff in parallel. If the PRE-REGISTRATION
handoff can be performed before the L2 handoff completes, the
combined method resolves to a PRE-REGISTRATION handoff. However, if
the PRE-REGISTRATION handoff does not complete within an access
technology dependent time period, the oFA starts forwarding traffic
for the MN to the nFA as specified in the POST-REGISTRATION handoff
method. This provides for a useful backup mechanism when completion
of a PRE-REGISTRATION handoff cannot always be guaranteed before the
L2 handoff completion.
It should be noted that the methods described in this document may be
applied to MNs having a single interface (e.g. Wireless LAN
interface) or multiple interfaces (e.g. one WLAN and one cellular
interface). However, the case of multiply-interfaced MNs needs
special consideration, since the handoff methods described in this
document may not be required in all cases (see Appendix B).
1.3.
L2 triggers
An L2 trigger is a signal of an L2 event. In this document, the L2
events relate to the L2 handoff process. One possible event is early
notice of an upcoming change in the L2 point of attachment of the
mobile node to the access network. Another possible event is the
completion of relocation of the mobile node's L2 point of attachment
to a new L2 access point. This information may come explicitly from
L2 in a solicited or unsolicited manner, or it may be derived from L2
messages. Although the protocols outlined in this document make use
of specific L2 information, Mobile IPv4 should be kept independent of
any specific L2. L2 triggers are an abstraction mechanism for a
technology specific trigger. Therefore, an L2 trigger that is made
available to the Mobile IPv4 stack is assumed to be generic and
technology independent. The precise format of these triggers is not
covered in this document, but the information required to be
contained in the L2 triggers for low latency handoffs is specified.
In order to properly abstract from the L2, it is assumed that one of
the three entities - the MN, oFA, or nFA - is made aware of the need
for an L2 handoff, and that the nFA or MN can optionally also be made
aware that an L2 handoff has completed. A specific L2 will often
dictate when a trigger is received and which entity will receive it.
Certain L2s provide advance triggers on the network-side, while
others provide advance triggers on the MN. Also, the particular
timing of the trigger with respect to the actual L2 handoff may
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differ from technology to technology. For example, some wireless
links may provide such a trigger well in advance of the actual
handoff. In contrast, other L2s may provide little or no
information in anticipation of the L2 handoff.
An L2 trigger may be categorized according to whether it is received
by the MN, oFA, or nFA. Table 1 gives such a categorization along
with information contained in the trigger. The methods presented in
this document operate based on different types of L2 triggers as
shown in Table 1. Once the L2 trigger is received, the handoff
processes described hereafter are initiated. The three triggers: L2-
ST, L2-TT and L2-MT are independent of each other and are not
expected to occur together since each will trigger a different type
of handoff behaviour.
+-------------+----------------------+------------------------------+
| L2 trigger | Mobile | Source |
| | Trigger | Trigger |
| | (L2-MT) | (L2-ST) |
+-------------+----------------------+------------------------------+
| Recipient | MN | oFA |
+-------------+----------------------+--------------+---------------+
| Method | PRE | PRE | POST |
| | mobile-initiated | network- | source |
| | | initiated | trigger |
+-------------+----------------------+--------------+---------------+
| When? | sufficiently before | sufficiently | sufficiently |
| | the L2 handoff | before L2 | before L2 |
| | so that MN can | handoff for | handoff for |
| | solicit PrRtAdv | FA to send | oFA & nFA to |
| | from oFA. | PrRtAdv | exchange |
| | | to MN. | HRqst/HRply. |
+-------------+----------------------+--------------+---------------+
| Parameters | nFA identifier | nFA identifier, MN identifier|
+-------------+----------------------+------------------------------+
Table 1 - L2 Trigger
(continued on next page)
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+------------+----------------------+---------------+---------------+
| L2 trigger | Target | Link-Up | Link-Down |
| | Trigger | (L2-LU) | (L2-LD) |
| | (L2-TT) | | |
|------------+----------------------+---------------+---------------+
| Recipient | nFA | MN or nFA | oFA |
|------------+-----------+----------+---------------+---------------+
| Method | PRE | POST | PRE & POST | POST |
| | network | target | | |
| | initiated | trigger | | |
|------------+----------------------+---------------+---------------+
| When? | | when radio | when radio |
| | same as | link between | link between |
| | source trigger | MN & nFA is | MN and oFA |
| | | established | is lost |
|------------+----------------------+---------------+---------------+
| Parameters | oFA identifier | @MN: nFA IPv4 | MN identifier |
| | MN identifier | or L2 addr. | |
| | | @nFA: MN IPv4 | |
| | | or L2 addr. | |
+------------+----------------------+---------------+---------------+
Table 1 - L2 Trigger
(continued from previous page)
1.4.
Requirements language
In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
"recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
described in [2].
2.
Requirements
The following requirements are applicable to low-latency handoff
techniques and are supported by the methods in this document:
- to provide low-latency and low loss handoff for real time services,
- to have no dependence on a wireless L2 technology,
- to support inter- and intra-access technology handoffs,
- to limit wireless bandwidth usage.
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3.
The PRE-REGISTRATION Handoff Method
The PRE-REGISTRATION handoff method is based on the normal Mobile
IPv4 handoff procedure specified in [1], according to which:
- an advertisement for an FA is received by an MN,
- the advertisement allows the MN to perform movement detection,
- the MN registers with the FA.
The basic messages specified in [1] are extended to carry information
required to achieve fast handoffs. The PRE-REGISTRATION method
allows both the MN and FA to initiate the layer-3 handoff and it can
make use of L2 triggers on either the FA or MN side, depending on
whether network-initiated or mobile-initiated handoff occurs.
PRE-REGISTRATION supports the normal Mobile IPv4 model [1] and
optionally also the Regional Registration model [11]. There can be
advantages in implementing [11] together with Low Latency Handoff
mechanisms, in particular in cases where the HA is at a distance (in
terms of delay) from the nFA. The time required for the handoff
procedure to complete can be reduced by using a closer local HA,
called Gateway Foreign Agent (GFA) in [11]. However implementation
of [11] is not required by PRE-REGISTRATION. PRE-REGISTRATION also
supports movement where a new AAA transaction must occur to
authenticate the MN with a new domain.
3.1.
Operation
The PRE-REGISTRATION Handoff mechanism is summarized in Figure 1.
+---------+
| HA (GFA)|<---------+
+---------+ | 4. (Reg)RegReq
| 5. (Reg)RegReply
v
+-----+ 1a. PrRtSol +-----+
| | -----------------> | nFA |
| oFA | 1b. PrRtAdv | |
+-----+ <----------------- +-----+
^ | ^
(2a. PrRtSol)| | 2b |
| | PrRtAdv | 3. (Reg)RegReq
| | |
| v --------------------+
+-----+ /
| MN |
+-----+ - - - - - ->
Movement
Figure 1 - PRE-REGISTRATION Handoff Protocol
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The following steps provide more detail on the protocol:
1. Message 1a is a Proxy Router (Agent) Solicitation (PrRtSol)
from oFA to nFA. It is a Mobile IP agent solicitation
containing an identifier for the nFA (i.e. IP address or L2
address) in a Generalised Link Layer and IP Address Extension
(see section 9.). When message 1a is received by the nFA
containing nFA's correct identifier in the LLA extension, the
nFA MUST return the Proxy Router Advertisement (Agent
Advertisement) in message 1b. Message 1b is simply nFA's
agent advertisement containing the nFA layer 2 address in a
Generalised Link Layer and IP Address (LLA) Extension (see
9.3). Messages 1a and 1b SHOULD occur in advance of the
PRE-REGISTRATION Handoff in order not to delay the handoff.
For this to occur, oFA SHOULD solicit and cache
advertisements from neighbouring nFAs using messages 1a and
1b, thus decoupling the timing of this exchange from the rest
of the PRE-REGISTRATION Handoff. When the L3 handoff is
initiated by a target L2 trigger at nFA (L2-TT), message 1b
equals message 2b and is sent unsolicited directly to MN
(tunneled by nFA to MN through oFA) instead of being relayed
by oFA.
2. Message 2a is a Proxy Router Solicitation (PrRtSol) from MN
to oFA. It is different from a normal Router (Agent)
Solicitation since it is soliciting an advertisement from a
router different from the one receiving this message. It is
a Mobile IP agent solicitation containing an identifier for
the nFA (i.e. IP address or L2 address) in a Generalised Link
Layer and IP Address Extension (see section 9.). The
presence of message 2a indicates that the handoff is mobile-
initiated and its absence means that the handoff is network-
initiated. In mobile-initiated handoff, message 2a occurs if
there is an L2 trigger in the MN to solicit for a Proxy
Router Advertisement (PrRtAdv). When message 2a is received
by the oFA it MUST return the Proxy Router Advertisement
(Agent Advertisement) in message 2b. This is simply nFA's
agent advertisement containing the nFA layer 2 address in a
Generalised Link Layer and IP Address (LLA) Extension (see
9.3). In network-initiated source-triggered handoff, the L2
trigger occurs at oFA and oFA MUST relay the Agent
Advertisement in message 2b without the need for the MN to
solicit. Note that it is also possible for nFA to advertise
directly to the MN in the network-initiated target-trigger
case (section 3.2).
3. The MN performs movement detection upon receipt of a
solicited or unsolicited Agent Advertisement and, if
required, it sends a Registration Request (RegReq) message
[1] in message 3 to nFA. When a local Gateway Foreign Agent
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(GFA) is present this message can optionally be a Regional
Registration Request (RegRegReq) [11]. Message 3 is routed
through oFA since the MN is not directly connected to nFA
prior to the L2 handoff.
4. Messages 4 and 5 complete the standard Mobile IPv4
Registration [1] or optionally Regional Registration [11]
initiated with message 3. The Registration Request MUST
contain the MN's layer 2 address in a Generalised Link Layer
and IP Address Extension (see 3.7 and 9). This identifier may
be a plain Ethernet address or an identifier specific to the
wireless technology. If the MN is not already connected to
nFA, the Registration Reply in message 5 MUST be buffered by
the nFA and unicast to the MN on-link as soon as the MN
connects to nFA (i.e. L2-LU trigger at nFA which can be
implemented by the MN sending an Agent Solicitation or
optionally using special layer 2 techniques which are outside
the scope of this document). This is necessary since the MN
may have to detach from oFA, due to the wireless L2
connection, before it receives the Reply. The MN's L2
address is obtained using the extensions in Section 9, as
described in 3.7. Figures 2 and 3 illustrate this procedure.
5. If the Registration is successful packets for the MN are
tunnelled from the HA (or GFA) to the nFA and then to the MN.
PRE-REGISTRATION is not dependent on [11]. However if the HA is at a
distance (in terms of delay) from the nFA, the use of a local GFA may
reduce the time required for the handoff procedure to complete.
The time at which the L2 trigger is received by the oFA or MN,
thereby triggering the PRE-REGISTRATION handoff, compared to the time
at which the actual L2 handoff occurs is important for the optimal
performance of the low latency handoff. That is, in the optimal case
the L2 trigger will be received and the four messaging steps of PRE-
REG described above will be completed (i.e. up to when the
Registration Request is processed by HA or GFA) before the MN moves.
Optimally the Registration Reply and the first packet redirected by
the HA (or GFA) to nFA will reach the MN at the moment in which the
MN's L2 link to nFA is fully established. The MN would therefore not
suffer any disruption due to the L3 handoff. This cannot always be
guaranteed unless particular implementation techniques are used. To
alleviate a part of this timing problem the MN MAY set the S bit [1]
in Low Latency Registration Requests sent by the MN. This allows the
MN to receive packets at both oFA and nFA during the short layer 2
handoff time. Other techniques may be required such as L2 techniques
or buffering but these are outside the scope of this document. In
addition further handoff smoothing considerations may be required to
prevent the loss of packets in-flight between HA (or GFA) and oFA
while the MN performs a PRE-REGISTRATION handoff. These are also
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outside the scope of this document.
Figures 2, 3, and 4 contain message timing diagrams for the network-
initiated and mobile-initiated PRE-REGISTRATION handoff procedures.
3.2.
Network-Initiated Handoff
As described in Table 1, a PRE-REGISTRATION handoff can be initiated
at oFA by a source trigger or at nFA by a target trigger. Figures 2
and 3 contain message timing diagrams for PRE-REGISTRATION network-
initiated handoff for source and target triggers.
A source-triggered network-initiated handoff occurs when an L2
trigger is received at the oFA informing it of a certain MN's
upcoming movement from oFA to nFA. The L2 trigger contains
information including the MN's identifier (i.e. the IPv4 address
itself or an identifier which can be resolved to the IPv4 address)
and the nFA's identifier. An identifier may be an IPv4 address or
something specific to the wireless technology (e.g. Base Station or
Access Point Identifier). A target-triggered network-initiated
handoff occurs when an L2 trigger is received at the nFA informing it
of a certain MN's upcoming movement from oFA. This type of trigger
is also shown in Table 1 and contains information including the MN's
identifier and the oFA's identifier.
MN oFA nFA HA/GFA
| |<~~~~~~ L2-Source | |
| | Trigger | |
|<--------------------| | |
| PrRtAdv | | |
| | | |
|---------------------------------------->| |
| RegReq or | | |
| RegRegReq (routed via oFA) |------------------->|
| | RegReq or RegRegReq|
| | |
| Buffered ~~~~~>|<-------------------|
|---------------------------------------->| (Reg)RegReply |
| Agent Solicitation | |
| (sent when MN connects to nFA) | |
| | |
|<----------------------------------------| |
| (Reg)RegReply | |
| (sent when nFA receives Solicitation or L2-LU) |
Figure 2 - PRE-REGISTRATION Handoff Message Timing Diagram
(Network-Initiated, Source Trigger)
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In a source-triggered handoff, when oFA receives the trigger (L2-ST)
it MUST send message 2b, the Proxy Router Advertisement (PrRtAdv), to
the MN. The PrRtAdv is nFA's agent advertisement [1] with one of the
link-layer extensions described in sections 9 (i.e. 9.3). The use of
the contents of this extension is described in section 3.7. Messages
1a and 1b SHOULD be exchanged by oFA and nFA before the L2 trigger is
received (see section 3.4.1). Message 2a is not used.
MN oFA nFA HA/GFA
| | L2-Target~~~~~~~~>| |
| | Trigger | |
| |...................| |
|<--------------------------------------- | |
| (PrRtAdv) |...................| |
| | Tunnelled Agent Advertisement |
| | | |
|---------------------------------------->| |
| RegReq. or | | |
| RegRegReq (routed via oFA) |------------------->|
| | RegReq or RegRegReq|
| | |
| Buffered ~~~~~>|<-------------------|
|---------------------------------------->| (Reg)RegReply |
| Agent Solicitation | |
| (sent when MN connects to nFA) | |
| | |
|<----------------------------------------| |
| (Reg)RegReply | |
| (sent when nFA receives Solicitation or L2-LU) |
Figure 3 - PRE-REGISTRATION Handoff Message Timing Diagram
(Network-Initiated, Target Trigger)
In a target-triggered handoff, when nFA receives the trigger (L2-TT)
it MUST tunnel an Agent Advertisement to the MN through oFA to
initiate the L3 handoff. The inner Advertisement is unicast by nFA
to MN, thus nFA treats the target-trigger as a Router (Agent)
Solicitation. This Advertisement is tunneled to oFA, which functions
as a normal router, decapsulating the Advertisement and forwarding it
to the MN. This message MUST be authenticated to prevent attacks
(see 3.4.2).
3.3.
Mobile-Initiated Handoff
As shown in Table 1, a mobile-initiated handoff occurs when an L2
trigger is received at the MN informing that it will shortly move to
nFA. The L2 trigger contains information such as the nFA's
identifier (i.e. nFA's IPv4 address or an identifier which can be
resolved to the nFA's IPv4 address). As an example, a Wireless LAN
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MN may perform a scan to obtain the BSSID identifier of the Access
Point that is a potential handoff target (i.e. its signal is becoming
stronger). The message timing diagram is shown in Figure 4.
MN oFA nFA HA/GFA
|<~~~~~ L2-Trigger | | |
| | | |
|-------------------->| | |
| PrRtSol | | |
| | | |
|<--------------------| | |
| PrRtAdv | | |
| | | |
|---------------------------------------->| |
| RegReq or | | |
| RegRegReq (routed via oFA) |------------------->|
| | RegReq or RegRegReq|
| | |
| Buffered ~~~~~>|<-------------------|
|---------------------------------------->| (Reg)RegReply |
| Agent Solicitation | |
| (sent when MN connects to nFA) | |
| | |
|<----------------------------------------| |
| (Reg)RegReply | |
| (sent when nFA receives Solicitation or L2-LU) |
Figure 4 - PRE-REGISTRATION Handoff Message Timing Diagram
(Mobile-Initiated)
As a consequence of the L2 trigger (L2-MT) the MN MUST send message
1a, the Proxy Router Solicitation (PrRtSol). This message is a
unicast agent solicitation to oFA for a Proxy Router Advertisement
(PrRtAdv). This solicitation MUST have a TTL=1 as in [1]. The Proxy
Router Advertisement Solicitation unicast to oFA is an agent
solicitation with a special extension. The solicitation MUST have an
extension containing an FA identifier (i.e. IPv4 address or L2
address contained in an LLA extension, see section 9) because the MN
is soliciting another specific FA's advertisement from the oFA. This
specific FA will be the MN's nFA. The identifier is the IPv4 address
of the nFA or another identifier that can be used by the oFA to
resolve to nFA's IPv4 address. If the identifier is not an IPv4
address, it MAY be specific to the underlying wireless technology,
for example, an Access Point or Base Station Identifier (e.g. WLAN
BSSID) that can be mapped by oFA to the nFA IPv4 address as described
in 3.4.1. The extension containing the identifier is a subtype of
the Generalised Link-Layer Address extension described in Section 9.
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Two extension subtypes have been defined to contain the nFA's IPv4
address and an access point identifier. They are called the
Solicited Agent IPv4 Address Extension and the Access Point
Identifier Extension, and are described in Sections 9.5 and 9.6.
These two extensions SHOULD NOT be present in the same PrRtSol
message.
When oFA receives the PrRtSol message it MUST reply to the MN with
the Proxy Router Advertisement (PrRtAdv, message 2b). The PrRtAdv is
simply the agent advertisement for the requested nFA, proxied by oFA.
In order to expedite the handoff, the actual nFA advertisement SHOULD
be cached by the oFA following a previous exchange with nFA, shown in
messages 1a and 1b, as specified in Section 3.5. The PrRtAdv message
MUST contain the nFA's L2 address (using the LLA extension in 9.3).
This is further described in section 3.7.
3.4.
Obtaining and Proxying nFA Advertisements
Since L2 triggers are involved in initiating PRE-REGISTRATION
handoff, the trigger timing SHOULD be arranged such that a full L3
PRE-REGISTRATION handoff can complete before the L2 handoff process
completes. That is, the L2 handoff should be completed after the
MN's Registration with the nFA is performed (message 3 in Fig.1).
The Registration MAY be transmitted in more than one copy (default
recommendation: 2) to reduce the probability that it is lost due to
errors on the wireless link. This would not apply to reliable
wireless links where retransmissions are performed at layer 2 in case
of error to guarantee packet delivery.
A PRE-REGISTRATION handoff in this case requires the MN to receive an
agent advertisement from the nFA through the old wireless access
point. How to achieve this is discussed in the following
subsections. Messages exchanged between FAs MUST be authenticated to
prevent impersonation attacks. The minimal requirement is that all
FAs involved in low latency handoffs MUST support manual pre-
configuration of security associations with other neighbouring FAs,
involving shared keys and the default algorithms of [1] (see Security
Considerations).
3.4.1.
Inter-FA Solicitation
This applies to the network-initiated source-triggered (L2-ST) and
mobile-initiated (L2-MT) cases only. Inter-FA solicitation assumes
that oFA has access to the IPv4 address of the nFA. The IPv4 address
of nFA is obtained by means of an L2 trigger at oFA in the
network-initiated case (see Section 3.2) or by means of the extension
to the Proxy Router Solicitation (PrRtSol) from the MN in the
mobile-initiated case (see Section 3.3). This extension to the
PrRtSol may contain an IPv4 address or another identifier, for
example an identifier of a Wireless Base Station such as the WLAN
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BSSID. In the latter case the receiving oFA must implement a
mechanism to resolve the Base Station Identifier to an IPv4 address.
The default mechanism is to use a configured table of neighbouring
BSSID to FA IPv4 address mappings in each FA. Other automated
discovery mechanisms may also be used.
If oFA does not cache advertisements (see 3.5) once it receives an L2
trigger and obtains the address of the nFA for a specific MN it MUST
send a unicast agent solicitation (PrRtSol) to nFA. The nFA replies
to the oFA by unicasting an Agent Advertisement with appropriate
extensions (PrRtAdv). This method removes the TTL limitation of [1]
for Mobile IPv4 messages (i.e. TTL=1 is not applicable here). The
TTL limitation cannot be applied since oFA and nFA may be more than
one hop away and since it is unnecessary for a secured unicast
message. The ICMP solicitations and advertisements MUST be
authenticated and integrity protected. These messages MUST be
protected using ESP [10] to prevent attacks (see Security
Considerations section). An FA MUST NOT accept ICMP solicitations or
advertisements from sources that are not authenticated.
As a practical matter, oFA SHOULD pre-solicit and cache
advertisements from known neighboring FAs (see section 3.5) to avoid
performing the solicitation during an actual handoff procedure.
3.4.2.
Tunneled nFA Advertisements
This applies to the network-initiated target-triggered (L2-TT) case
only. Following a target trigger (L2-TT) the nFA MUST send a
tunneled agent advertisement to the MN through oFA. Tunneling nFA
advertisements assumes that the nFA is aware of the IPv4 address for
oFA and the MN. These IPv4 addresses are obtained by means of the FA
and MN contained in an L2 trigger received at nFA in the network-
initiated case (see Section 3.2). However in [1] the TTL must be 1
on Agent Advertisements from the nFA. Therefore tunneling
advertisements is applicable if the TTL limitation of [1] is relaxed.
For this purpose, a pre-established security association between oFA
and nFA MUST be in place to authenticate this message and relax the
TTL limitation. If the implementation requires this, a tunnel SHOULD
be configured when the inter-FA security association is established.
The tunneled ICMP advertisement MUST be secured using tunnel mode ESP
[10] between nFA and oFA. An FA MUST NOT accept tunneled ICMP
packets destined to it from sources that are not authenticated.
3.5.
Caching Router Advertisements
In the mobile-initiated (L2-MT) case and the network-initiated
source-triggered (L2-ST) case, the message exchange 1 in Figure 1
could impose an additional latency on the L3 handoff process if done
as part of the handoff procedure. In order to remove this source of
latency, the inter-FA Router (Agent) Solicitation and Advertisement
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exchange SHOULD be performed in advance of handoff. A process SHOULD
be in place at the oFA to solicit its neighbouring nFAs at a
predefined time interval (MIN_SOLICITATION_INTERVAL). This interval
SHOULD NOT be set too small to avoid unnecessary consumption of
network bandwidth and nFA processing resources. The minimum value of
MIN_SOLICITATION_INTERVAL is 1 sec. If the FA Challenge/Response
mechanism in [7] is used then the MIN_SOLICITATION_INTERVAL MUST be
set to a value smaller then the window of time in which a challenge
remains valid so that the nFA challenge does not expire before the MN
issues the Registration Request. Therefore the recommended default
value for the MIN_SOLICITATION_INTERVAL in oFA is (0.5 * nFA's
CHALLENGE_WINDOW * nFA's Agent advertisement interval). The
CHALLENGE_WINDOW and Agent advertisement interval are defined in [7]
and [1] respectively. The minimum requirement is that the
MIN_SOLICITATION_INTERVAL MUST be manually configurable, while
possible autoconfiguration mechanisms are outside the scope of this
document. To allow advertisement caching in certain implementations
and in cases where the nFA advertisement interval is very small, it
MAY be necessary for the implementation in nFA to allow different
CHALLENGE_WINDOW and agent advertisement interval settings for its
nFA-oFA interface.
The oFA SHOULD cache the most recent advertisement from its
neighbouring nFAs. This advertisement MUST be sent to the MN in
message 2b with a TTL=1. The oFA SHOULD also have a mechanism in
place to create a list of neighbouring nFAs. The minimum requirement
for each FA is that it SHOULD allow manual configuration of a list of
nFA addresses that an MN could possibly perform an L3 handoff to.
The FA addresses in this list will depend on deployment and radio
coverage. It is also possible to specify another protocol to achieve
nFA discovery, but this is outside the scope of this document.
3.6.
Movement Detection, MN and FA Considerations
When the MN receives an Agent Advertisement with a Mobility Agent
extension, it performs actions according to the following movement
detection mechanism: the MN SHOULD be "Eager" to perform new
bindings. This means that the MN SHOULD perform Registrations with
any new FA from which it receives an advertisement (i.e. MN is
Eager), as long as there are no locally-defined policies in the MN
that discourage the use of the discovered FA. For example, the MN
could have a policy based on the cost of service. The method by
which the MN determines whether the FA is a new FA is described in
[1] and MAY use an FA-NAI extension [11]. By being "Eager" to perform
Registrations the MN reduces latency times.
The MN also needs to change its default router from oFA to nFA. The
MN MUST change its default router to nFA as soon as the
PRE-REGISTRATION procedure has completed (i.e. Registration Reply is
received by MN) as described in [1].
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Overall the MN behaves as described in [1] with the following
changes: the specified movement detection mechanism mentioned above
and the ability to use the L2-MT to initiate an agent solicitation
with a special extension (PrRtSol). Also, when the MN receives a L2-
LU trigger (i.e. new interface or link is up) it MUST immediately
send an Agent Solicitation [1] on that interface. An nFA that
receives an Agent Solicitation [1] will use it as an L2-LU trigger
event and according to [1] it will record the MN's IPv4/Layer 2
addresses (i.e. ARP entry). At that point the nFA starts delivering
data to the MN including the previously buffered Registration Reply.
The nFA MAY also use other L2 mechanisms to detect earlier that the
MN has attached to the new link and to start forwarding data to it.
The MN SHOULD NOT attempt to retransmit a low latency Registration
Request (i.e. Registration Request containing an LLA extension
described in 9.) when it does not receive the Registration Reply.
When moving from a PRE-REGISTRATION network to a normal Mobile IPv4
[1] network the MN will no longer receive PrRtAdv messages (i.e.
agent advertisements with the LLA extension). If the MN still
receives L2-MTs it will attempt to send PrRtSol messages. The normal
FA will reply with a normal agent advertisement [1]. If the MN does
not receive a PrRtAdv in reply to its PrRtSol, it MAY retransmit the
PrRtSol message once after PRE_SOL_INTERVAL seconds and then for
another PRE_SOL_ATTEMPTS times with exponential backoff of the
transmission interval. If a PrRtAdv is not received within
PRE_SOL_INTERVAL seconds after the last PrRtSol attempt, the MN MUST
stop sending PrRtSol messages until after a Registration with a new
FA is performed. The default value for PRE_SOL_ATTEMPTS is 2 and for
PRE_SOL_INTERVAL it is 1 second. It should be noted that the
performance of the movement detection mechanism mandated in
PRE-REGISTRATION (i.e. eager to register) may have sub-optimal
behaviour in a standard Mobile IPv4 [1] network. Therefore standard
movement detection mechanisms [1] should be used in plain Mobile IPv4
networks. Instead when the MN moves from a normal Mobile IPv4 [1]
network to a PRE-REGISTRATION network, the MN starts receiving L2-MT
triggers or PrRtAdv messages. When the MN receives L2-MT triggers or
PrRtAdv messages it SHOULD follow the PRE-REGISTRATION procedure.
If there is uncertainty as to which mode to choose (e.g. MN receives
messages from both PRE-REGISTRATION and normal FAs) the MN decides
based on its Registration status with the current FA. If the MN
already has a valid normal Mobile IPv4 Registration [1] with the
advertising FA, it SHOULD give priority to the PRE-REGISTRATION
procedure. Otherwise it SHOULD give priority to normal Mobile IPv4
[1] Registration procedure. The MN SHOULD NOT attempt to perform PRE-
REGISTRATION and standard Mobile IPv4 [1] Registrations in parallel.
3.7.
L2 Address Considerations
Some special considerations should be taken with respect to the
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wireless system on which this handoff method is being implemented.
Consider an Ethernet-like system (e.g. IEEE 802.11) for example. In
PRE-REGISTRATION the MN is registering with an FA (nFA) that is not
its current first-hop router, therefore the L2 address of the
Ethernet frame containing the MN's Registration Request reaching the
nFA is not the MN's address. Therefore the FA MUST NOT use the
Ethernet address of the incoming Registration Request as the MN's L2
address as specified in [1]. This applies to the cases where the
wireless access points are bridges or routers and independently of
whether the FA is implemented in the wireless access points
themselves. In this case the MN's Registration Request (or Regional
Registration Request) MUST use an L2 address extension to the
Registration message. Such an L2 address is either the same L2
address that remains constant as the MN moves, or it is the MN's L2
address at nFA. To communicate its L2 address, the MN includes a
Generalised Link Layer and IP Address Extension (see Section 9) with
its Registration Request (or Regional Registration Request) message.
If this extension is present the FA MUST use the L2 address contained
in the extension to communicate with the MN. If a particular
wireless L2 technology has defined a special interface to the
wireless network that allows the FA to resolve the mapping between an
MN's IPv4 address and its L2 address without the need to use the
extension, the L2 address extension contents may be discarded. For
the same reasons above, the MN MUST NOT use the source L2 address of
the Agent Advertisement message (PrRtAdv) as its default router's L2
address. Therefore the nFA MUST include a Generalised Link Layer and
IP Address Extension (see Section 9.3) with its Agent Advertisement
(PrRtAdv) messages.
3.8.
Applicability of PRE-REGISTRATION Handoff
The PRE-REGISTRATON Handoff method is applicable to scenarios where a
period of service disruption due to layer 3 is not acceptable, for
example when performing real-time communications, and therefore where
an anticipation of the layer 3 handoff is required. Security for the
PRE-REGISTRATION handoff method is based on the same security model
as [1] including the use of AAA. A prerequisite for PRE-REGISTRATION
is that the FA or MN are able to obtain an L2 trigger informing them
of a pending L2 handoff procedure. The target of the L2 handoff is
another access point or radio network that is in the coverage area of
a new FA. The L2 trigger information may be in the form of
Identifiers that need to be resolved to IPv4 addresses using methods
that may be specific to the wireless network and are not considered
here. If, for example, the oFA or MN determines that the IPv4
address of the new FA matches oFA's address then the PRE-REGISTRATION
handoff SHOULD NOT be initiated.
The L2 trigger must allow enough time for the PRE-REGISTRATION
handoff procedure to be performed. In many wireless L2 technologies,
the L2 handoff procedure involves a number of message exchanges
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before the effective L2 handoff is performed. For such technologies,
PRE-REGISTRATION handoff can be initiated at the beginning of the L2
handoff procedure and completed before the L2 handoff is completed.
It is efficient to engineer the network such that this succession of
events is ensured.
The PRE-REGISTRATION Handoff method is applicable in the following
cases:
- when the MN has locally defined policies that determine a
preference for one access over another, for example due to service
cost within the same or different technology, and therefore where
it is necessary to allow the MN to select the appropriate FA with
which to connect,
- when L2 security between the MN and the FA is either not present
or cannot be relied upon to provide adequate security,
- when the trigger to initiate the handoff is received at the MN.
In the first case it is necessary to involve eventual local MN
policies in the movement detection procedure as described in 3.6.
4.
The POST-REGISTRATION Handoff Method
The POST-REGISTRATION handoff method uses bi-directional edge tunnels
(BETs) or unidirectional tunnels to perform low latency change in the
L2 point of attachment for the MN without requiring any involvement
by the MN. Figure 5 illustrates the basic POST-REGISTRATION handoff.
+------+
| CN |
+------+
|
...
|
+------+ BET +------+
| aFA |==========| nFA |
+------+ +------+
| wireless link
|
movement +------+
---------> | MN |
+------+
Figure 5 - POST-REGISTRATION Concept
Following a successful Mobile IPv4 Registration between MN and oFA,
the oFA becomes the mobility anchor point for the MN, called the
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anchor FA (aFA). When the MN moves from oFA to nFA, rather than
performing signaling over the wireless link to register with the nFA,
the MN can defer the L3 handoff and continue to use its aFA (i.e. oFA
in this case). If the MN moves to a third FA before registering with
the nFA, in certain cases described later, the third FA signals aFA
to move the wireless link end of the BET from nFA to it. The network
end of the BET remains anchored at aFA until the MN performs the
Mobile IPv4 Registration.
Messages between oFA/aFA and nFA MUST be authenticated. The minimal
requirement is that all FAs involved in low latency handoffs MUST
support manual pre-configuration of security associations with other
neighbouring FAs, involving shared keys and the default algorithms of
[1]. POST-REGISTRATION FAs MUST implement the inter-FA
authentication extension (FA-FA authentication extension) specified
in [11] and MAY additionally use other security mechanisms.
4.1.
Two Party Handoff
Two party handoff occurs when the MN moves from oFA to nFA. Normally
this movement would result in a new Mobile IPv4 Registration at nFA.
However in POST-REGISTRATION, the MN and nFA MAY delay this but
maintain connectivity using the BET (or alternatively unidirectional
tunnel) between oFA and nFA. The protocol is shown in Figure 6.
1a) L2-ST ~~~~> +------+ 2) HRqst +------+ <~~~ 1b) L2-TT
| oFA |<-------->| nFA |
4a) L2-LD~> +------+ 3) HRply +------+ <~~~ 4b) L2-LU
^ ^
old L2 | | new L2
+-------+ +-----+
| |
| |
V V
+------+ movement
4c) L2-LU ---> | MN | --------->
+------+
Figure 6 - Two Party Handoff (POST-REGISTRATION)
The following describes the progress of a two party handoff. The
numbered items refer to steps in Figure 6. The source triggered
HRqst/HRply message for tunnel creation, the target triggered
HRqst/HRply message for tunnel creation and the HRqst/HRply to extend
or terminate a BET (or unidirectional tunnel) are identified
respectively using the suffixes (s), (t) and (r).
1) Either the oFA or nFA receives an L2 trigger informing it that a
certain MN is about to move from oFA to nFA. The two cases are:
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a) The L2 trigger is a source trigger (L2-ST) at oFA. The
trigger contains the MN's L2 address and an identifier for
the nFA (the IPv4 address itself or an L2 address that can
be resolved to the IPv4 address of the nFA).
b) The L2 trigger is a target trigger (L2-TT) at nFA. The
trigger contains the MN's L2 address and an identifier for
the oFA (the IPv4 address itself or an L2 address that can
be resolved to the IPv4 address of the oFA).
2) The FA receiving the trigger sends a Handoff Request (HRqst) to
the other FA. There are two cases:
a) If oFA is sending the HRqst, the H bit is set and the N
bit is unset, indicating it is an HRqst(s). The HRqst(s)
contains the lifetime of the tunnel the oFA is willing to
support, the MN's IPv4 home address, the MN's HA address
and an LLA option with the MN's L2 address. If the
lifetime is zero and the T bit is not set, the oFA is not
willing to tunnel any packets for MN. A positive lifetime
and a set T bit indicate that the oFA is willing to tunnel
for the indicated time. Section 4.5 describes the
HRqst(s) and Section 9 describes the LLA option.
b) If nFA is sending the HRqst, the N bit is set and the H
bit is unset, indicating it is an HRqst(t). If the T bit
is set, nFA has requested a reverse tunnel and the
HRqst(t) contains the lifetime of the tunnel the nFA is
requesting. The HRqst(t) also contains an LLA option with
the MN's L2 address. The MN's IPv4 home address and HA
address are not sent, unless they are discovered by some
means outside the scope of this document (for example, as
part of the L2-TT). Section 4.5 describes the HRqst(t).
3) The FA receiving the HRqst sends a Handoff Reply (HRply) to the
other FA. There are two cases:
a) If oFA is sending the HRply, the N bit is set and the H
and R bits are unset, indicating that the reply is in
response to a HRqst(t), i.e. it is an HRply(t). If the T
bit is set, the HRply(t) contains the tunnel lifetime the
oFA is willing to provide, otherwise the tunnel lifetime
is set to zero indicating that the oFA is not willing to
provide tunnel service. If both HRply(t) and HRqst(t)
have the T bit set and non-zero lifetime a BET is
established. The HRply(t) also contains the MN's home
subnet IPv4 address, the MN's HA address, and an LLA
option containing the MN's L2 address. Section 4.6
describes the HRply(t).
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b) If nFA sends the HRply, the H bit is set and the N and R
bits are unset, indicating this is a response to HRqst(s),
i.e. it is an HRply(s). If the T bit is set, the nFA
indicates that it requests a reverse tunnel, and the
lifetime field is set with the requested tunnel lifetime.
The T Bit can be set in HRply only if the oFA had set the
T bit in the corresponding HRqst or if the nFA requires to
reverse tunnel incoming packets to oFA because ingress
filtering is enabled on its network. This establishes a
BET. The tunnel lifetime requested by the nFA must be
less than or equal to the tunnel lifetime offered by oFA
in the HRqst(s). Section 4.6 describes the HRply(s).
4) The point during the L2 handoff in which the MN is no longer
connected on a given link is signaled by an L2-LD trigger at oFA
and MN. Completion of L2 handoff is signaled by an L2-LU
trigger at nFA and MN. The trigger is handled as follows:
a) When oFA receives the L2-LD trigger, it begins forwarding
MN-bound packets through the forward tunnel to nFA.
b) When the nFA receives the L2-LU trigger, it begins
delivering packets tunneled from oFA to MN and forwards
outbound packets from MN using normal routing mechanisms
or through a reverse tunnel to oFA or HA. The nFA at this
point may not yet be the default router of the MN (see
section 4.4), therefore to receive all outbound packets
from the MN the nFA must send a unicast proxy ARP message
(used in [1]) to the MN upon receiving an L2-LU trigger.
This proxy ARP message is an ARP Reply [5] sent by the nFA
on behalf of oFA, therefore supplying the nFA link-layer
address in the Sender Hardware Address field and the oFA
IPv4 address in the Target Protocol Address field.
c) When the MN receives the L2-LU, it MAY initiate the Mobile
IPv4 Registration process by soliciting an Agent
Advertisement as described in [1]. If the Registration is
successful the nFA takes over the role of anchor FA (aFA)
from the oFA. Alternatively the MN MAY defer the Mobile
IPv4 Registration (see section 4.4).
5) The oFA becomes an aFA if the MN moves to a third FA before
having performed a Mobile IPv4 Registration with nFA.
6) Should L2 handoff fail in Step 4 (due to L2 reasons) and a
ping-pong situation arise, the oFA may be able to determine this
case through the trigger mechanism (i.e. FA sees successive
L2-ST/L2-TT followed by L2-LD and then L2-LU). The FA that
originated the HRqst can in this case cancel the tunnel by
sending an HRqst(r) to the other FA with lifetime zero. It will
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then simply continue delivering packets to MN exactly as if no
handoff had been pending. Section 4.5 describes the HRqst(r).
If the oFA sets the B bit in HRqst/HRply and the nFA has not
requested a reverse tunnel by setting the T bit, the nFA SHOULD
tunnel outgoing packets from the MN to the HA because the MN has
requested this service from the oFA. The nFA SHOULD offer this
service only if no security between the nFA and the MN's HA is
required, or if there is an existing nFA-HA security association.
The actual timing of BET or unidirectional tunnel placement depends
on the available L2 triggers. The forward tunnel from oFA to nFA is
constructed using one of the tunneling procedures described in [1]
for the HA to FA tunnel with the difference that the ends of the
tunnel are at the oFA and nFA, respectively. The reverse tunnel from
nFA to oFA is constructed as described in [3] with the difference
that the network end of the tunnel is at the oFA instead of the HA.
If both forward and reverse tunnels are established then a BET has
been established. With optimal L2 trigger information, as described
above, the FAs can setup the BET immediately when the L2 handoff is
initiated, start tunneling MN-bound data when the link to the MN goes
down and the nFA can use the link up trigger to start delivering
packets. In the absence of optimal L2 trigger information, the HRply
can act as the trigger to start tunneling MN-bound data, but in this
case, the period of packet delivery disruption to the MN could still
be present and additional measures may be required to provide
uninterrupted service. Particular implementation and deployment
scenarios could require techniques to smooth the handoff by providing
a means to convey packets arriving during the L2 handoff. The exact
techniques are outside the scope of this document.
Figures 7 and 8 show timing diagrams for source trigger (L2-ST) and
target trigger (L2-TT) two party handoff, respectively.
MN nFA oFA
| | |
| | HRqst(s) |<~~~ L2-ST
| |<------------------|
| | HRply(s) |
| |------------------>|
| | |
--------------------------------------------<~~~ L2-LD
L2 Handoff
--------------------------------------------<~~~ L2-LU
| | |
|<------------------->| |
| MN's traffic | |
Figure 7 - Two Party Source Trigger Handoff Timing
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MN nFA oFA
| | |
| L2-TT ~~~>| HRqst(t) |
| |------------------>|
| | HRply(t) |
| |<------------------|
| | |
--------------------------------------------<~~~ L2-LD
L2 Handoff
--------------------------------------------<~~~ L2-LU
| | |
|<------------------->| |
| MN's traffic | |
Figure 8 - Two Party Target Trigger Handoff Timing
Once the tunnel between aFA and the current FA is in place, it is
torn down by one of the following events:
1) The aFA decides to stop tunneling because the lifetime it sent
expires and was not renewed, or the aFA or current FA decide to
terminate tunnel service prematurely for some other reason
(refer to section 4.3).
2) The MN completes the process by performing a Mobile IPv4
Registration with the current FA. This may be initiated by the
FA that sends an Agent Advertisement or by the MN that solicits
for an Agent Advertisement as in [1].
3) The MN moves to a third FA (see section 4.2)
4.2.
Three Party Handoff
Three party handoff is applicable when an MN, which has already
established an aFA and is receiving tunneled packets through its
current FA, moves to a new FA without performing a Mobile IPv4
Registration.
The need for the Three Party Handoff function depends on the wireless
system in which POST-REGISTRATION is being implemented. For radio L2
protocols in which it is possible for the MN to move so rapidly from
one FA to another such that a probability exists that the Mobile IPv4
Registration with nFA will not complete before the MN moves on, HTT
(Handoff To Third) SHOULD be implemented. Certain wireless systems
and implementations do not allow such fast movement between FAs and
may force the Mobile IPv4 Registration to occur soon after L2
handoff, in which case three party handoff is not applicable. If
this three party handoff feature is not implemented, the FA SHOULD
send an Agent Advertisement to the MN after L2 handoff has completed
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(L2-LU at nFA) and/or the MN SHOULD solicit an Agent Advertisement
after L2 handoff (L2-LU at MN).
+------+
+--->| aFA |<---+
| +------+ |
4b) HRqst(r) | | 3) HRqst(t)
HRply(r) | | HRply(t)
| |
v 2a) HRqst v
1a) L2-ST ~~~> +------+ HTT +------+ <~~~ 1b) L2-TT
| oFA |<-------->| nFA |
4a) L2-LD ~~~> +------+ 2b) HTT +------+ <~~~ 5a) L2-LU
^ HRply ^
old L2 | | new L2
+-------+ +-----+
| |
| |
V V
+------+ movement
5b) L2-LU ~~~> | MN | --------->
+------+
Figure 9 - Three Party Handoff
The L3 handoff can be deferred either because of a decision by the
MN/FA (i.e. MN does not send Agent Solicitations and FA does not send
Agent Advertisements) or it may result from rapid movement between
oFA and nFA that does not allow enough time for the registration to
complete. This scenario is shown in Figure 9. In this case, oFA
must inform nFA (i.e. the third FA) to contact aFA about moving the
radio end of the tunnel. This is performed with the HTT message.
The general idea behind the three party handoff procedure is that the
oFA supplies nFA with the same information it would have obtained via
an L2-TT if handoff had occurred from aFA to nFA, then the nFA
performs an HRqst(t)/HRply(t) sequence with aFA in order to move the
BET to nFA. When the L2 handoff is complete, oFA sends an HRqst(r)
to aFA to terminate the previous BET.
The following describes the progress of a three party handoff. The
numbered items refer to steps in Figure 9.
1) Either the oFA or nFA receives an L2 trigger informing it that a
certain MN is about to be moved. The two cases are:
a) The L2 trigger is a source trigger (L2-ST) at oFA. The
trigger contains the MN's L2 address and an identifier for
the nFA (the IPv4 address itself or an L2 address that can
be mapped to the IPv4 address of the nFA).
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b) The L2 trigger is a target trigger (L2-TT) at nFA. The
trigger contains the MN's L2 address and an identifier for
the oFA (the IPv4 address itself or an L2 address that can
be resolved to the IPv4 address of the oFA).
2) The oFA and nFA exchange a HTT/HRply or HRqst/HTT pair. HTT is
indicated by setting both the H and N bits in the HRqst or
HRply. The HTT message MUST NOT have any tunnel flag bits set,
because the tunnel is negotiated between the aFA and nFA, not
oFA and nFA. There are two cases:
a) The L2 trigger is an L2-ST. The oFA sends HTT to nFA
containing the MN's home IPv4 address, the MN's HA
address, an LLA containing the aFA's IPv4 address, and an
LLA containing the L2 address of the MN. This is enough
information for nFA to perform a target triggered handoff
with aFA. The nFA responds with a HRply(s). Section 4.8
describes the HTT.
b) The L2 trigger is an L2-TT. The nFA sends HRqst(t) to oFA,
exactly as if a two party handoff were occurring. The
oFA responds with HTT containing the same information as
in a) above. This is enough information for nFA to
perform a target triggered handoff with aFA.
3) Upon receipt of the HTT, the nFA first checks its Visitor Cache
to see whether it is already tunneling for MN. If so, Step 6 is
performed. If not, nFA performs a target triggered handoff with
aFA, exactly as in Section 4.1, exchanging a HRqst(t)/HRply(t)
pair. Because aFA receives no L2 trigger indicating when L2
handoff starts, it may start tunneling to nFA upon transmission
of the HRply(t).
4) Once the L2 handoff is underway and the MN gets disconnected at
L2, aFA and oFA exchange messages canceling tunnel service
between aFA and oFA and allowing aFA to start the tunnel with
nFA.
a) The point in the L2 handoff process where the MN gets
disconnected from oFA is signaled at oFA by L2-LD.
b) The oFA exchanges a HRqst(r)/HRply(r) pair having lifetime
zero with aFA. This cancels tunnel service between oFA
and aFA. If aFA has not already established a tunnel to
nFA, it must do so immediately upon receipt of the
HRqst(r). The aFA provides tunneling service exactly as
described in Section 4.1 Step 4a.
5) Completion of L2 handoff is signaled by an L2-LU trigger at nFA
and/or MN. The nFA and MN handle the trigger as follows:
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a) The nFA provides packet delivery service to the MN exactly
as described in Section 4.1, Step 4b.
b) The MN either defers or initiates Mobile IPv4 Registration
when it receives the L2-LU, as in Section 4.1
6) In the special case where nFA and aFA are the same (i.e. the MN
is moving back to the original anchor FA), aFA recognizes that
it is tunneling to oFA when it checks its Visitor Cache in Step
3. In this case, there is no need for aFA to send the
HRqst(t)/HRply(t) in Step 3. Upon receipt of the L2-LU trigger
on handoff completion, the aFA begins routing packets to MN and
the tunnel to nFA is torn down. The oFA still exchanges the
HRqst(r)/HRply(r) with aFA in Step 4b because oFA cannot know a
priori that aFA and nFA are the same, but they are redundant.
Figures 10 and 11 show timing diagrams for source trigger (L2-ST) and
target trigger (L2-TT) three party handoff, respectively.
MN nFA oFA aFA
| | L2-ST ~~~> | |
| | | |
| |<-------------| |
| | HTT | |
| |------------->| |
| | HRply(s) | |
| |------------------------------>|
| | HRqst(t) | |
| |<------------------------------|
| | HRply(t) | |
| | | |
----------------------------------<~~~ L2-LD |
|--------------->|
L2 Handoff | HRqst(r) |
| |
|<---------------|
| HRply(r) |
| |
----------------------------------<~~~ L2-LU |
| MN's traffic | | |
|<-------------->| | |
Figure 10 - Three Party Source Trigger Handoff Timing
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MN nFA oFA aFA
| | | |
| |<~~~ L2-TT | |
| |------------->| |
| | HRqst(t) | |
| |<-------------| |
| | HTT | |
| |------------------------------>|
| | HRqst(t) | |
| |<------------------------------|
| | HRply(t) | |
| | | |
----------------------------------<~~~ L2-LD |
|--------------->|
L2 Handoff | HRqst(r) |
| |
|<---------------|
| HRply(r) |
| |
----------------------------------<~~~ L2-LU |
| MN's traffic | | |
|<-------------->| | |
Figure 11 - Three Party Target Trigger Handoff Timing
Unlike two party handoff, the timing of BET establishment between aFA
and nFA cannot fully depend on the availability of L2 trigger
information because aFA does not receive an L2 trigger signalling L2
handoff. The two timing extremes at which aFA can place the BET with
nFA are:
1) At the earliest, aFA MAY start tunneling packets using the BET
to nFA after sending the HRply(t) to nFA in response to the
request for target-triggered handoff
2) At the latest, aFA MAY start tunneling packets using the BET to
nFA and tear down the BET with oFA when receiving the HRqst(r)
from oFA indicating the MN has disconnected.
In addition, aFA MAY continue tunneling to oFA if 1) is selected,
until the HRqst(r) is received. In this case, the result may be
duplicated packets at the MN because the MN will receive packets
through oFA on the old L2 until it disconnects (L2-LD). If 2) is
selected, the additional latency will add to the MN's L3 service
disruption period. Of course, aFA can choose to place the BET some
time between 1) and 2) if reliable bounds are available on the
duration of time between L2-ST/L2-TT and the MN's disconnection
(L2-LD). The exact selection of when to establish the BET is likely
to be influenced by network engineering and implementation
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considerations, including whether a handoff smoothing solution is
used, and is beyond the scope of this specification.
4.3.
Renewal or Termination of Tunneling Service
To prevent a BET from expiring when its lifetime runs out, the MN's
current FA signals the aFA to either renew or terminate the BET.
This may be the case when the MN defers Mobile IPv4 Registration. If
no such signal is received, the aFA will terminate the BET when the
lifetime expires. In addition, the current FA or aFA may need to
terminate the BET prior to the lifetime expiring. In order to avoid
error conditions in which tunnels do not expire even though the MN to
which they apply is no longer reachable, FAs SHOULD set the tunnel
lifetime field to some value other that 0xffff, which indicates "good
until cancelled".
Figure 12 illustrates the message exchange that occurs between the FA
needing to terminate or extend the tunnel (designated FA(1) in the
figure) and the other FA (designated FA(2) in the figure). The
HRqst(r)/HRply(r) is indicated by setting the R bit in the
HRqst/HRply messages. If the HRqst(r) is renewing a BET then it
contains a non-zero lifetime, otherwise if the lifetime is set to
zero it indicates tunnel termination. The aFA has complete control
over whether a tunnel is extended or terminated, and it MAY reply to
a request for extension with a shorter lifetime than was requested.
HRqst(r)
+------+ <-------- +------+
| FA(2)| ---------> | FA(1)|
+------+ HRply(r) +------+
Figure 12 - BET Renewal or Termination
4.4.
When will the MN perform a Mobile IPv4 Registration?
The MN/FA have control over when to perform the Mobile IPv4
Registration. Although the MN/FA may decide to defer Mobile IPv4
Registration for a certain period, three possible events can lead to
the need to terminate tunneling service. If this occurs the MN MUST
perform the Mobile IPv4 Registration. These events are:
1) The end of life for the BET is pending and a request by the
current FA to aFA for renewal has been denied, or alternatively
the current FA or aFA needs to terminate the BET prematurely.
The FA in this case MUST initiate the Mobile IPv4 Registration
by sending an Agent Advertisement to the MN as in [1].
2) The MN itself decides to perform a Mobile IPv4 Registration and
initiates it by sending an Agent solicitation as in [1].
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3) During a source triggered handoff, the oFA attempts to perform
BET handoff but nFA is not capable of performing it. The FA in
this case MUST initiate the Mobile IPv4 Registration by sending
the MN an Agent Advertisement as in [1]. Note that this
situation will never arise during target triggered handoff
because an HRqst(t) will not be sent to oFA by an nFA that
doesn't support POST-REGISTRATION.
Some detailed scenarios relating to case 2) will be described
hereafter. According to [1], when using an FA care-of address the MN
MAY use the FA as its default router. Otherwise it MUST choose its
default router from those advertised in the ICMP Router Advertisement
portion of the Agent Advertisement. Here we assume that the FA
router is also the MN's default router. In POST-REGISTRATION, when a
tunnel is established between oFA and nFA and the MN has moved to
nFA, the oFA MUST NOT send Agent Advertisements to the MN. In this
case it is possible that the MN will not receive Agent Advertisements
for extended periods of time. According to [8] hosts will remove
default router entries if the lifetime of the Router Advertisement
expires and no further advertisements are received. Note that the
ICMP Router Advertisement lifetime is not related to the Registration
Lifetime in the Mobility Agent Advertisement extension [1]. To avoid
this disruption the MN MUST solicit the default router (i.e. FA)
before the lifetime of its active default router entry runs out, or
alternatively the FA MUST advertise as soon as the MN-nFA link is up
(L2-LU). This effectively means that the MN will at most be able to
defer Mobile IPv4 Registration for as long as the remaining lifetime
of the active default router, as configured in the ICMP Router
Advertisements. The MN MUST perform a Mobile IPv4 registration [1]
when it receives an Agent Advertisement following a POST-REGISTRATION
handoff.
4.5.
Handoff Request (HRqst) Message format
This is a new Mobile IPv4 message carried on UDP (destination port
434) [1]. The UDP header is followed by the fields below.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type |H|N|R|M|G|T|B| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MN Home Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HA Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Identification +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extensions ...
+-+-+-+-+-+-+-+-
Type TBD (Handoff Request)
H Source triggered handoff request. When set and
the N bit is unset, indicates that the request
was the result of an L2-ST at oFA.
N Target triggered handoff request. When set and
the H bit is unset, indicates that the request
was the result of an L2-TT at nFA.
R Set if the request is an HRqst(r) (i.e. a request
to renew the tunnel, H and N bits must be unset).
M The FA issuing the HRqst will use Minimal
Encapsulation as defined in [1,5] for the tunnel.
G The FA issuing the HRqst will use GRE [4]
Encapsulation as defined in [1,5] for the tunnel.
Extensions of HRqst containing GRE type and key
Fields are outside the scope of this document.
T For an HRqst(s), indicates that the oFA is
willing to support both forward and reverse
tunnel service. For an HRqst(t), indicates that
the nFA is requesting reverse tunnel service.
B When sent in an HRqst(s), indicates that the MN
has requested a reverse tunnel to the HA and that
the nFA SHOULD use reverse tunnel to the HA if it
will not be reverse tunneling to the oFA.
Lifetime The lifetime of the tunnel in seconds. If this
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is an HRqst(t), then the lifetime represents a
request by nFA for a reverse tunnel. If this is
an HRqst(s), then the lifetime represents the
maximum amount of time that oFA is willing to
maintain both forward and reverse tunnels. If
this is an HRqst(r), then the lifetime represents
a request for the amount of time to renew the
tunnel's lifetime. A value of 0 on an HRqst(s)
indicates that the oFA is unwilling to grant
tunnel service. A value of 0 on an HRqst(t)
indicates that the nFA does not require reverse
tunnel service. A value of 0 on an HRqst(r)
indicates that the tunnel should be terminated.
A value of 0xffff indicates infinity.
MN Home Address For HRqst(s), the home address of the MN.
HA Addr For HRqst(s), the HA address of the mobile node.
Identification As in defined in [1].
Extensions The Message MUST include an LLA (see Section 9)
containing the MN's L2 address and an L2 address
that can be mapped to an IPv4 address for the FA.
This Message MUST contain the FA-FA
Authentication Extension [11] that is used to
secure the HRqst message.
4.6.
Handoff Reply (HRply) Message
This is a new Mobile IPv4 message carried on UDP (destination port
434) [1]. The UDP header is followed by the fields 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 |H|N|R|M|G|T|B| Reserved | Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MN Home Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HA Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Identification +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extensions ...
+-+-+-+-+-+-+-+-
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Type TBD (Handoff Reply)
Code A value indicating the result of the Handoff
Request. Only two codes are currently supported,
0, indicating success, and 1, indicating that the
handoff cannot be performed. The remaining values
are for future use.
Lifetime The lifetime, in seconds, for which the
bi-directional tunnel for the MN will be
maintained. If this is an HRply(s), then the
lifetime represents a request by nFA, and it can
be any value up to the maximum value sent in the
HRqst(s). Larger values are assumed to default
to OFA's maximum. If this is an HRply(t), then
the lifetime represents the maximum amount of
time that the oFA will grant to the nFA. If this
is a HRply(r), then the lifetime represents the
amount of time by which the tunnel life will be
extended. If the Code field indicates that
handoff failed, the Lifetime field will be
ignored and SHOULD be set to zero. A value of
0 on an HRply(t) indicates that the oFA is
unwilling to grant service. A value of 0 on an
HRply(s) indicates that the nFA does not require
service. A value of 0 on HRply(r) indicates that
the tunnel lifetime will be terminated. A value
of 0xffff indicates infinite lifetime.
H Source triggered handoff reply. When set and
the N bit is unset, indicates that the
reply is in response to an HRqst(s).
N Target triggered handoff reply. When set and
the H bit is unset, indicates that the
reply is in response to an HRqst(t).
R Set if the reply is an HRply(r). Neither
the H nor the N bit are set.
M The FA issuing the HRqst will use Minimal
Encapsulation as defined in [1,5] for
the tunnel.
G The FA issuing the HRqst will use GRE [4]
Encapsulation as defined in [1,5] for the tunnel.
When this flag bit is set the HRply may require
extensions containing the GRE type and key
fields, but they are outside the scope of this
document.
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T For an HRply(s), indicates that the nFA is
Requesting to reverse tunnel service. For an
HRply(t), indicates that the oFA is willing to
provide both forward and reverse tunnel service.
B When sent in an HRply(t), indicates that the MN
has requested a reverse tunnel to the HA and that
the nFA SHOULD use reverse tunnel to the HA if
it will not be reverse tunneling to the oFA. It
can be set in HRply(t) only if the T bit was
unset in the corresponding HRqst(t).
MN Home Address For HRply(t), the home IPv4 address of the MN.
HA Addr For HRply(t), the HA IPv4 address of the MN.
Identification As in defined in [1].
Extensions This Message MUST contain the FA-FA
Authentication Extension [11] that is used to
secure the HRply message.
4.7.
Handoff To Third (HTT) Message
The Handoff to Third message has the same format as the Handoff
Request and Handoff Reply Messages, except both the H and N bits are
set. If the HTT message is in response to a L2-ST and is sent to
initiate a handoff, then, with the exception of the H and N bits, the
message has the same fields set and includes the same extensions as
an HRqst(s). If the HTT message is sent in response to an HRqst(t),
then, with the exception of the H and N bits, the message has the
same fields set and includes the same extensions as an HRply(t). The
tunnel bits MUST NOT be set in the HTT message because BET
construction is not negotiated between oFA and nFA, it is negotiated
between nFA and aFA in the ensuing HRqst(t)/HRply(t).
In addition, the HTT MUST contain the following extensions in the
specified order:
Solicited IPv4 Address Option: containing aFA's Address
LLA Option: containing the L2 address of the MN.
4.8.
Applicability of POST-REGISTRATION Handoff Method
The POST-REGISTRATION handoff approach allows FAs to communicate
directly about a pending handoff, and does not require any IPv4 layer
messages to be sent to or from a MN prior to the L2 handoff event.
Therefore, it eliminates a possible source of handoff latency. This
may be required when the link layer imposes hard deadlines on the
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time at which a handoff must occur, such as when a MN is rapidly
moving out of a radio coverage area. Consequently, POST-REGISTRATION
is primarily of interest in handoff between FAs that support the same
radio access technology. Handoff between heterogeneous radio
technologies will, of necessity, require interaction between the MN
and the network, and so is not a domain of applicability for
POST-REGISTRATION.
Because a POST-REGISTRATION handoff is triggered by an unspecified
mechanism that informs the oFA or nFA that an L2 handoff is pending,
the POST-REGISTRATION approach is only applicable to networks where
such a mechanism is available. For example, an L2 may provide
indications of radio signal quality that cause the oFA or nFA to send
the POST-REGISTRATION handoff messages. Any such indications must
also provide each FA involved in the handoff with the identity of the
other, so that messages can be sent to the right place. This may
involve mapping L2 information onto FA IPv4 addresses. Also, the FAs
involved in a handoff must have pre-provisioned security arrangements
so that the POST-REGISTRATION messages can be authenticated. If a
handoff is to be completed as a result of the POST-REGISTRATION
messaging, any L2 handoff indications must also be securely
authenticated so that traffic to the old point of attachment is not
improperly halted.
POST-REGISTRATION handoff is appropriate in the following cases:
- L2 triggers are available on the network to indicate that L2
handoff is pending.
- Pre-provisioned security mechanisms are in place to allow fast
and secure messaging between the FAs and between the MN and an FA.
- Access point choice by the MN is not a concern or choice requires
user intervention and therefore is not on the critical path for
handoff.
5.
Combined Handoff Method
The combined method uses both PRE-REGISTRATION and POST-REGISTRATION
handoff. If PRE-REGISTRATION does not complete prior to the
expiration of a timer on the nFA, the POST-REGISTRATION mechanism is
used to create the tunnel between oFA and nFA. This protects the MN
from delays caused by errors such as loss of the Mobile IPv4
Registration Reply message involved in PRE-REGISTRATION for the
mobile-initiated and network-initiated source-triggered cases. It
also protects the MN from delays caused by errors or the loss of any
of the Mobile IPv4 messages involved in PRE-REGISTRATION for the
network-initiated target-triggered case.
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When the nFA receives a target-trigger it will follow the PRE-
REGISTRATION procedure. When the combined method is used, the nFA
MUST also start a timer when it receives a target-trigger. The timer
should be set to a small value (default for target-trigger case: 1
sec).
According to PRE-REGISTRATION, the nFA will receive the Registration
Request from the MN. When the combined method is used, this
Registration Request sent by the MN MUST contain the IPv4 address of
the oFA in an FA IPv4 address LLA extension (see 9.7). This same
Registration Request message will contain multiple LLA extensions,
since it will also contain the MN's layer 2 address in an LLA
extension as described for PRE-REGISTRATION (see 3.7 and 9). When
the nFA has not started the handoff procedure using a target-trigger
(i.e. mobile-initiated or network-initiated target-triggered cases)
the nFA MUST start a timer as soon as it receives the low latency
Registration Request from the MN. This timer should be set to a
small value (default for : 1 sec).
In all cases the timer MUST be reset when the Registration Reply
message is received by nFA. If the timer expires before the
Registration Reply is received, the nFA MUST initiate the POST-
REGISTRATION procedure. The nFA utilizes the oFA IPv4 address
(previously received in the extension to the Registration Request
message) as the destination of the POST-REGISTRATION HRqst message to
create the tunnel between nFA and oFA. The nFA MAY tear down this
tunnel when it receives and forwards a successful Registration Reply
for that MN.
6.
Layer 2 and Layer 3 Handoff timing Considerations
In the optimal cases considered in the PRE-REGISTRATION and
POST-REGISTRATION handoffs it was assumed that a timely L2 trigger
would be received in such a way that packets could be delivered to
the MN via its nFA immediately upon connection. In this way the MN
does not suffer disruption due to the L3 handoff. However such
precise timing of the L2 trigger and handoff mechanism with respect
to the actual L2 handoff event will not be possible in all wireless
systems and may depend on particular implementation techniques.
Therefore some uncertainty may exist at L3 as to exactly when the L2
connection between the MN and the nFA becomes fully established and
can be used for L3 traffic. It is possible that in certain
implementations traffic will be re-routed too early or too late with
respect to the moment when the connection between the MN and the nFA
becomes fully established. The techniques that may solve this
problem and allow the MN to receive traffic independently of the
timing of the L2 handoff event include buffering and simultaneous
bindings (i.e. bicasting: setting the S bit [1] in Registration
Requests). However these are optional and are not mandated.
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7.
Reverse Tunneling Support
The handoff methods all support reverse tunneling. The MN may
request reverse tunneling [3] by setting the T bit in its
Registration Request. In the case of POST-REGISTRATION, if the MN
had requested Reverse Tunneling previously at oFA, the Handoff
message from oFA (see Section 4) includes the T bit enabled to inform
nFA to establish a BET for the visitor entry. Typically, the T bit
will always be set to ensure that any delays in the MN receiving its
new care of address do not result in any delay in uplink packet
transmission from the MN, but local policies and particular L2
technologies may allow the reverse tunnel to be turned off.
8.
Handoff Signaling Failure Recovery
In general and to a greater extent in wireless networks, packets
carrying handoff signaling may be dropped or lost due to errors on
the link. In this section, we consider mechanisms for recovery from
handoff signaling failures.
8.1.
PRE-REGISTRATION Signaling Failure Recovery
Failure of PRE-REGISTRATION signaling breaks down into three cases:
1) Loss of messages PrRtSol and PrRtAdv on the air link.
2) Loss of the solicitation by an FA to obtain another neighbouring
FA's Advertisement or loss of the neighbouring FA's
advertisement.
3) Failure of the standard Mobile IPv4 Registration.
Of these, case 3) is handled by standard Mobile IPv4 mechanisms
described in [1]. Case 2) is expected to be a rare event because
spontaneous packet drop rates on the fixed network are caused by
congestion or router failure. Since bit error rates on wireless
links are higher than on fixed links, case 1) is more likely to
occur. In the following subsections, the cases 1) and 2) are
considered.
8.1.1.
Failure of PrRtSol and PrRtAdv
PRE-REGISTRATION handoff can fail in network-initiated handoff when
the PrRtAdv sent by oFA in response to the source trigger (L2-ST) or
the advertisement sent by nFA in response to the target trigger (L2-
TT) fails to reach the MN. PRE-REGISTRATION handoff can also fail in
mobile-initiated handoff when either the PrRtSol sent from the MN or
return PrRtAdv sent from the oFA are dropped. To reduce the
probability that PrRtAdv and PrRtSol are lost the MN and FA MAY
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transmit multiple copies of these messages. Should these messages
fail anyway, in both cases the MN connects to the nFA without having
received any prior signalling. In this case the MN solicits an FA
Advertisement when it connects to nFA at L2 (L2-LU), as described in
3.6, and performs a standard Mobile IPv4 registration with the nFA as
specified in [1].
8.1.2.
Failure of Inter-FA solicitation and advertisement
The solicitation from an FA to another neighbouring FA may fail or
the corresponding advertisement from the neighbouring FA may be lost.
To reduce the probability that these messages are lost, the FAs MAY
transmit multiple copies of these messages. If a failure occurs
anyway, the FA soliciting the Agent Advertisement is unable to send a
PrRtAdv in response to a source trigger or to a mobile-initiated
PrRtSol. In these cases, when the MN does not receive a notification
or confirmation of a PRE-REGISTRATION handoff, the MN MUST perform a
standard Mobile IPv4 registration as soon as it connects to the nFA
(L2-LU) as described in 8.1.1.
8.2.
POST-REGISTRATION Signaling Failure Recovery
Failure occurs in POST-REGISTRATION when either the HRqst or HRply
message is dropped. The effects of the failure and the recovery
procedure depend on which message is dropped, and whether the handoff
is source or target triggered. Since all of the POST-REGISTRATION
signaling is going over the fixed network, it can be expected that
spontaneous dropping of packets in the absence of congestion and
router failure should be a relatively rare event. Nevertheless,
failure recovery mechanisms SHOULD be implemented.
8.2.1.
HRqst Message Dropped
If the HRqst message is dropped, the effect is the same for both
source and target-triggered handoff. In either case, the FA to which
the HRqst was destined will never respond with an HRply message. If
the handoff is source-triggered, then the nFA never learns of the
handoff, and the oFA never receives confirmation. If the handoff is
target-triggered, then the oFA never learns of the handoff, and the
nFA never receives confirmation.
The recovery procedure in this case is as follows: the oFA MUST NOT
construct a forward tunnel when the MN moves off-link (L2-LD) if the
handoff is source-triggered, and the nFA MUST NOT construct a reverse
tunnel if the handoff is target-triggered. If the nFA was not
informed of the handoff by an HRqst message (corresponding to failure
of source-triggered handoff) or if the handoff was not confirmed by
an HRply message (corresponding to failure of target-triggered
handoff) the nFA MUST unicast an Agent Advertisement to the MN as
soon as its L2 connection is established (L2-LU at nFA).
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8.2.2.
HRply Message Dropped
If the HRply message is dropped, the FA sending the HRply will assume
that the handoff has been confirmed, but the FA that is expecting to
receive the HRply does not receive confirmation. In this case, the
failure recovery procedure is different for source-triggered and
target-triggered handoffs.
In a target-triggered handoff, the oFA assumes the handoff is
confirmed because it has sent the HRply, but the nFA has not received
it so it does not have confirmation. The oFA starts tunneling
packets to the nFA when the MN moves from its link (L2-LD). The nFA
MUST send a FA Advertisement to the MN as soon as its L2 link is up
(L2-LU at nFA) and MAY drop the tunneled packets. The nFA SHOULD
send an ICMP Destination Unreachable [9] message to the oFA. When
the oFA receives this message it will terminate the tunnel and stop
forwarding packets. If reverse tunneling was requested the nFA MUST
NOT reverse tunnel because it has not received handoff confirmation.
In source-triggered handoff, the nFA assumes the handoff is confirmed
because it has sent the HRply, but the oFA has not received it so it
does not have confirmation. Without failure recovery, the MN could
move to the nFA without the oFA being able to start the forward
tunnel for the MN's packets, and the MN would not be able to initiate
a Mobile IPv4 registration because it does not know that the handoff
has failed. In this situation, the oFA MUST send out a HRqst message
to the nFA with lifetime zero as soon as the MN leaves its link
(L2-LD). The oFA SHOULD continue to retransmit the HRqst message,
with exponential backoff for CONFIG-HFAIL seconds or until it
receives an HRply acknowledging the request to cancel the tunnel.
The default value for CONFIG-HFAIL is 10 seconds. When the nFA
receives the HRqst, it MUST immediately send an Agent Advertisement
to the MN, as is the case whenever a tunnel is cancelled. In
addition, the oFA MUST also drop any packets received through the
reverse tunnel from the nFA. The oFA SHOULD NOT send the ICMP
Destination Unreachable message to the nFA because the nFA has been
informed by the HRqst message to cancel the tunnel. However, if the
nFA receives an ICMP Destination unreachable message for the tunnel
prior to receiving the HRqst canceling the tunnel, it MUST send an FA
Advertisement to the MN and cancel the tunnel.
9.
Generalized Link Layer and IPv4 Address (LLA) Extension
This section defines the Generalized Link Layer and IPv4 Address
(LLA) Extension, used by any node that needs to communicate Link
Layer and IPv4 Addresses. The format of the extension relies on sub-
types, where each sub-type defines its own sub-structure. This draft
defines six sub-types. Future RFCs should allocate their own sub-
type and define their own address formats.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type | LLA ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
TBD (skippable) [1] - when used in Registration Requests
TBD (skippable) [1] - when used in Agent Advertisements
Length
The length of the Link Layer Address + the one octet Sub-Type
field
Sub-Type
This field contains the Link Layer sub-type identifier
LLA
Contains the Link Layer Address
In this document, seven subtypes are defined:
1 3GPP2 International Mobile Station Identity and
Connection ID [13]
2 3GPP International Mobile Subscriber Identity [15]
3 Ethernet 48 bit MAC address [5]
4 64 bit Global ID, EUI-64 [6]
5 Solicited IPv4 Address
6 Access Point Identifier
7 FA IPv4 Address
The following subsections describe the extensions.
9.1.
3GPP2 IMSI Link Layer Address and Connection ID Extension
The IMSI Link Layer Address Extension contains the International
Mobile Station Identity.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type | IMSI ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Type
TBD (skippable) [1]
Length
The length of the IMSI field + the one octet Sub-Type field
Sub-Type
1
IMSI
Contains the IMSI, in the form:
<IMSI>:<Connection Id>
Where the <IMSI> is an ASCII-based representation of the
International Mobile Station Identifier, most significant
digit first, ":" is ASCII 0x3a, and the Connection ID is the
ASCII representation of a small, decimal number used for
distinguishing different link-layer connections from the
same mobile device.
9.2.
3GPP IMSI Link Layer Address Extension
The IMSI Link Layer Address Extension contains the International
Mobile Station Identity.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type | IMSI ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
TBD (skippable) [1]
Length
The length of the IMSI field + the one octet Sub-Type field
Sub-Type
2
IMSI
Contains the IMSI, a number composed of 15-digits or less,
coded as described in [15].
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9.3.
Ethernet Link Layer Address Extension
The Ethernet Link Layer Address Extension contains the 48 bit
Ethernet MAC Address, as defined in [5].
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type | MAC ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
TBD (skippable) [1]
Length
7 (includes the Sub-Type field)
Sub-Type
3
MAC
Contains the 48 bit Ethernet MAC Address.
9.4.
IEEE 64-Bit Global Identifier (EUI-64) Address Extension
The 64-Bit Global Identifier (EUI-64) Address Extension contains the
64 bit address, as defined in [6].
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type | MAC ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
TBD (skippable) [1]
Length
9 (includes the Sub-Type field)
Sub-Type
4
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MAC
Contains the 64-Bit Global Identifier Address.
9.5.
Solicited IPv4 Address Extension
The 32-bit Solicited IPv4 Address Extension contains the IPv4 address
of the agent (FA) being solicited. This extension MAY be present in
an ICMP Agent Solicitation as explained in Section 3.3.
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 | Sub-Type | IPv4 addr ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
TBD (skippable) [1]
Length
5 (includes the Sub-Type field)
Sub-Type
5
IPv4 Address
Contains the 32-Bit IPv4 Address of the solicited node.
9.6.
Access Point Identifier Extension
The 32-bit Access Point Identifier Extension contains an Identifier
of the Access Point to which the MN will move. This may be a
wireless L2 identifier. The MN is able to solicit an advertisement
from the FA servicing a certain Access Point by using this extension
with Agent Solicitations as explained in Section 3.3.
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 | Sub-Type | AP ID...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
TBD (skippable) [1]
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Length
5 (includes the Sub-Type field)
Sub-Type
6
AP ID
Contains the 32-Bit Access Point Identifier.
9.7.
FA IPv4 Address Extension
The 32-bit FA IPv4 Address Extension contains the IPv4 address of the
agent (FA). This extension MAY be present in a Registration Request
message to identify the oFA as explained in section 5.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type | IPv4 addr ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
TBD (skippable) [1]
Length
5 (includes the Sub-Type field)
Sub-Type
7
IPv4 Address
Contains the 32-Bit IPv4 Address of the FA node.
10.
IANA Considerations
This document defines one new extension to Mobile IPv4 Control
messages and one new extension to Mobile IPv4 Router Discovery
messages already maintained by IANA. This document also defines a
new Mobile IPv4 Control message type to be used between FAs. To
ensure correct interoperation based on this specification, IANA must
reserve values in the Mobile IPv4 number space for two new extensions
and one new message type. IANA must also manage the numbering spaces
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created by the two new extensions, the message type and its
associated Code field.
10.1.
New Extension values
Section 9 introduces two extensions.
Generalized Link Layer and IPv4 Address (LLA) Extension for Router
Discovery messages: A new Mobile IPv4 extension that follows after
Mobile IPv4 ICMP Router Discovery messages (e.g. Mobile IP Agent
Advertisements). The type value of this extension belongs to the
Mobile IPv4 number space for Router Discovery messages maintained by
IANA. The value assigned by IANA is TBD. This new extension uses
the Link Layer and IPv4 Address Identifier (LLA) Sub-Type numbering
space that requires IANA management (see section 10.2).
Generalized Link Layer and IPv4 Address (LLA) Extension for Mobile IP
Control messages: A new Mobile IPv4 extension appended to Mobile IP
Control messages (e.g. Registration Request). The type value of this
extension belongs to the Mobile IPv4 number space for extensions to
Mobile IPv4 Control messages maintained by IANA. It MUST be in the
skippable (128-255) range as defined in [1]. The value assigned is
TBD by IANA. This new extension uses a the Link Layer and IP Address
Identifier (LLA) Sub-Type numbering space that requires IANA
management (see section 10.2).
10.2.
Generalized Link Layer and IP Address Identifier (LLA) Sub-type
values
This section describes the sub-type values that are applicable to
both the Generalized LLA Extensions for Mobile IP Control and Router
Discovery messages. This specification makes use of the Sub-type
values 1-7, and all other values other than zero (reserved) are
available for assignment via IETF consensus [14]. The seven Sub-type
values defined in this specification are:
1 3GPP2 International Mobile Station Identity and
Connection ID [13]
2 3GPP International Mobile Subscriber Identity [15]
3 Ethernet 48 bit MAC address [5]
4 64 bit Global ID, EUI-64 [6]
5 Solicited IPv4 Address
6 Access Point Identifier
7 FA IPv4 Address
10.3.
New Message Type and Code
Sections 4.4 and 4.5 define two new Mobile IPv4 message types:
Handoff Request and Handoff Reply. These require two type numbers to
be assigned by IANA from the Mobile IPv4 control message type address
space. The Handoff Reply message also introduces its own Code field
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that requires IANA to manage a new Code address space. This
specification makes use of the Code values 0-1, where 0 identifies a
successful Handoff and 1 defines a generic handoff failure. All other
values are available for assignment via IETF consensus [14].
Code Values for Mobile IP Handoff Reply Messages
0 Successful Handoff
1 Generic Handoff Failure
2-255 Unallocated
11.
Security Considerations
For the PRE-REGISTRATION method, as discussed in Section 3.8, the oFA
and nFA MUST share a security association to authenticate and
integrity protect messages transported between them. In addition oFA
must be authorized to solicit nFA based on the security association.
The minimal requirement to establish a security association between
FAs is that both FAs support manual pre-configuration of security
associations involving shared keys. Other mechanisms to establish
security associations using IKE [16] based on shared and public keys
may also be used. The inter-FA ICMP messages (solicitations and
advertisements) MUST be authenticated and integrity protected using
ESP [10]. The default ESP authentication algorithm for use in this
specification is HMAC-SHA1-96 [12]. The absence of this security
would allow denial of service attacks from malicious nodes at any
distance from the FA. To secure Registration Request and Reply
messages, PRE-REGISTRATION uses the security mechanisms already
described in [1] and optionally [11].
POST-REGISTRATION introduces a new change to Mobile IPv4, which is
the possibility that a MN may receive packets from an FA with which
it has not yet performed a Mobile IPv4 Registration. It is not
recommended that the MN drops packets from unknown FAs since it would
effectively eliminate the advantages of POST-REGISTRATION. From a
security viewpoint, dropping packets from unknown FAs would not
provide significant protection for an MN from any attack. This is
because any malicious host may use the MN's home address to send
packets to the MN through its current known FA, therefore processing
packets received from unknown FAs would not provide worse security
than with normal Mobile IPv4.
In a similar way to PRE-REGISTRATION, in POST-REGISTRATION oFA and
nFA MUST share a security association required to protect the Handoff
Request and Reply messages. The minimal requirement to establish a
security association between FAs is that the FAs support manual pre-
configuration of security associations involving shared keys. Other
mechanisms to establish security associations using IKE [16] based on
shared and public keys may also be used. The Handoff Request and
Reply messages MUST be authenticated using the FA-FA authentication
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extension [11] that uses the default algorithm specified in [7]. The
absence of this security would allow impersonation attacks and denial
of service attacks.
The minimal requirement is that all FAs involved in low latency
handoffs MUST support manual pre-configuration of peer-to-peer
security associations with neighbouring FAs, involving static keys
and are already required to support the default algorithms of [1].
Other mechanisms to establish security associations using IKE [16]
based on shared or public keys may also be used.
Since the techniques outlined in this document depend on particular
L2 information (triggers) to optimize performance, some level of L2
security is assumed. Both PRE and POST-REGISTRATION techniques
depend on L2 triggers, but the L2 security implications are different
for the two techniques.
In particular, in POST-REGISTRATION the L2 triggers initiate the
establishment of tunnels that route IPv4 packets for the MN to its
new location. Therefore the L2 triggers MUST be secured against any
tampering by malicious nodes, either mobile or within the wired
network. The L2 addresses or IPv4 addresses for the MN and the FAs
that appear in the L2 triggers MUST correspond to the actual nodes
that are participating in the handoff. If there is any possibility
that tampering may occur, the recipient of an L2 trigger MUST have
some way of authenticating the L2 information. Wireless networks
that do not provide such features will be subject to impersonation
attacks, where malicious nodes could cause FAs to believe that a MN
has moved to other service areas or to allow a bogus MN to obtain
unauthorized service from an FA prior to performing a Mobile IPv4
registration. In POST-REGISTRATION the L2 triggers would be
typically sent between a wireless base station and the FA. No
standard protocol exists at this time to communicate the L2 trigger
information but it is important that any future protocol used for
this purpose provides adequate security. If the wireless base station
and FA were integrated then this security threat would not apply.
Also the layer-2 control messages on the wireless link must be
secured appropriately to prevent a malicious node from running
impersonation attacks and causing unwanted L2 triggers to be
generated. Integrity and replay protection would be required to
avoid impersonation threats and resource consumption threats where a
malicious node replays old messages to cause resource consumption.
This depends on the type of L2 security of the wireless link. For
example in cellular technologies the control messages are secured,
although the type of security varies depending on the cellular
standard, but this is not typically the case in WLAN IEEE 802.11
networks.
In PRE-REGISTRATION the security of L2 triggers has different
implications. The PRE-REGISTRATION technique depends on Mobile IPv4
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security between MN and FA, so the same security considerations in
[1] apply. Should malicious nodes be able to generate or modify L2
trigger information (i.e. L2-ST or L2-TT), this would cause
advertisements to be sent to the MN. They would consume wireless
resources and processing in the MN, but would not allow an
impersonation attacks. In order to prevent such denial of service
attacks, there should be a limit on the number of advertisements that
an FA (oFA) will relay to the MN as a result of the reception of L2
triggers. This number will depend on the L2 technology and the
default limit is 10 per second.
12.
Acknowledgements
The authors want to thank Lennart Bang, Bryan Hartwell, Joel
Hortelius and Jonathan Wood for valuable comments and suggestions on
the whole draft. The authors also thank the Mobile IPv4 WG chairs,
Phil Roberts and Basavaraj Patil, for their input.
13.
Editor's Address
Karim El Malki
Athonet
E-mail: karim@athonet.com
14.
Contributing Authors
Pat Calhoun, Black Storm Networks
<pcalhoun@bstormnetworks.com>
Tom Hiller, Lucent Technologies
<tom.hiller@lucent.com>
James Kempf, NTT DoCoMo USA Labs
<kempf@docomolabs-usa.com>
Peter J. McCann, Lucent Technologies
<mccap@lucent.com>
Ajoy Singh, Motorola
<asingh1@email.mot.com>
Hesham Soliman, Flarion
<H.Soliman@flarion.com>
Sebastian Thalanany, Motorola
<sthalan1@email.mot.com>
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15.
References
15.1.
Normative References
[1] C. Perkins, Editor, "IPv4 Mobility Support for IPv4", RFC
3344, August 2002.
[2] S. Bradner. "Key words for use in RFCs to Indicate
Requirement Levels". BCP 14, RFC 2119, March 1997.
[3] G. Montenegro, "Reverse Tunneling for Mobile IPv4, revised",
RFC 3024, January 2001.
[4] D. Farinacci, T. Li, S. Hanks, and P. Traina, "Generic
Routing Encapsulation (GRE)", RFC 2784, Internet Engineering
Task Force, March 2000.
[5] D. Plummer, "An Ethernet Address Resolution Protocol - or
Converting Network Protocol Addresses to 48.bit Ethernet
Address for Transmission on Ethernet Hardware", RFC 826,
November 1982.
[6] IEEE, "Guidelines for 64-bit Global Identifier (EUI-64)
Registration Authority",
http://standards.ieee.org/regauth/oui/tutorials/EUI64.html,
March 1997.
[7] C. Perkins, P. Calhoun, "Mobile IPv4 Challenge/Response
Extensions", RFC 3012, November 2000.
[8] S. Deering, "ICMP Router Discovery", RFC 1256, September 1991
[9] J. Postel, "Internet Control Message Protocol," RFC 792,
September 1981.
[10] S. Kent, R. Atkinson, "IPv4 Encapsulating Security Payload
(ESP)", RFC 2406, November 1998.
[11] E. Gustafsson, A. Jonsson and C. Perkins, "Mobile IPv4
Regional Tunnel Management", draft-ietf-mobileip-reg-tunnel-08
(work in progress), November 2003.
[12] C. Madson and R. Glenn, "The Use of HMAC-SHA1-96 within ESP
and AH", RFC 2404, November 1998.
15.2.
Informative References
[13] TIA/EIA/IS-2000.
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[14] T. Narten, H, Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October
1998.
[15] 3GPP TS 23.003 (www.3gpp.org).
[16] D. Harkins and D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.
Appendix A - Gateway Foreign Agents
The Mobile IPv4 Regional Registration specification [11] introduces
the Gateway Foreign Agent (GFA), as a mobility agent that two FAs
providing service to a MN have in common. Figure A.1 provides an
example of a MN's initial registration through the GFA. If this is
the first registration message, the message MUST be forwarded to the
HA. All packets destined for the mobile will be delivered to the
GFA, which in turn will forward the packets to the FA servicing the
MN.
Reg Req +-----+ Reg Req
+----------->| oFA |--------------+
| +-----+ |
| v
+----+ +-----+ Reg Req +----+
| MN | | GFA |<------->| HA |
+----+ +-----+ +----+
+-----+
| nFA |
+-----+
Figure A.1 - Initial Registrations through GFA
If the MN moves to an nFA that is serviced by a GFA common with oFA,
the MN MAY issue a Regional Registration Request (see Figure A.2).
The Regional Registration message does not need to be forwarded to
the HA, since the MN's traffic can still be delivered to the same
GFA. This optimized approach effectively reduces the latency
involved in the registration process.
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+-----+
| oFA |
+-----+
+----+ +-----+ +----+
| MN | | GFA | | HA |
+----+ +-----+ +----+
| ^
| +-----+ |
+------------>| nFA |-------------+
Regional Reg +-----+ Regional Reg
Figure A.2 - Regional Registration through GFA
Note that the GFA may also be the MN's first-hop router.
Appendix B - Low Latency Handoffs for Multiple-Interface MNs
For MNs that have two wireless network interfaces, either on the same
wireless network or on wireless networks having different wireless L2
technologies, the techniques discussed in this document may be
unnecessary if the Mobile IPv4 stack on the MN allows switching an
IPv4 address binding between interfaces. This Appendix discusses how
multiple wireless interfaces can aid low latency handoff.
+------+ +---------+
| HA |--------| (GFA) |
+------+ +---------+
/ \
... ...
/ \
/ \
+------+ +------+
| oFA | | nFA |
+------+ +------+
| |
+------+ +------+
| RN1 | | RN2 |
+------+ +------+
+------+
| MN | --------->
+------+
Movement
Figure B.1 - Network Model for Mobile IPv4 With Multi-Access
Figure B.1 illustrates the normal and hierarchical MIPv4 models. As
shown in the figure, assume that the MN is connected to Radio Network
1 (RN1) and is registered with oFA through which it is receiving
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traffic. Suppose MN enters the coverage area of RN2 and nFA and that
it prefers connectivity to this network for reasons beyond the scope
of this document (e.g. user preferences, cost, QoS available etc.).
The MN activates the interface to RN2 but continues communicating
through RN1. The MN may solicit advertisements from nFA through the
interface connected to RN1 to speed up the handoff process, provided
there is no TTL restriction, or it can solicit advertisements through
the interface connected to RN2 if it has been configured for IPv4
traffic.
Once the MN is registered with nFA and is successfully receiving and
transmitting through the new network, it tears down the interface to
RN1. If the MN has enough time to complete this procedure without
incurring degraded service or disconnection, the MN would experience
a seamless multi-access handoff but it may not be possible in all
cases, due to network coverage or for other reasons. Should multiple
interface handoff be possible then the low latency methods described
in this document are not necessary.
In order to support the possible failure of the connectivity with the
new network (RN2/nFA) in the short period following handoff, the MN
may use the S bit in its Mobile IPv4 Registration Request to maintain
simultaneous bindings both its existing (HA or GFA) binding with oFA
and a new binding with nFA.
Appendix C - PRE_REGISTRATION Message Summary
This appendix contains a quick reference for IPv4 and Layer 2
addresses to be used in PRE-REGISTRATION messages.
Proxy Router Advertisement (PrRtAdv)
This is a standard Router/Agent Advertisement [1] with the following
characteristics:
Source IPv4 Address: nFA IPv4 Address
Source Layer 2 Address: oFA L2 Address
Destination IPv4 Address: MN IPv4 Address (from PrRtSol)
Destination Layer 2 Address: MN L2 Address (from PrRtSol)
LLA Extension (defined in this spec): containing nFA Layer 2
Address.
Proxy Router Solicitation (PrRtSol)
This is a standard Router/Agent Solicitation [1] with the following
characteristics:
Source IPv4 Address: MN Address
Source Layer 2 Address: MN Address
Destination IPv4 Address: oFA Address (from source address of
previous Router Advertisement or PrRtAdv)
Destination Layer 2 Address: oFA Address (from source address of
previous Router Advertisement or PrRtAdv LLA)
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LLA Extension (defined in this spec): depends on the layer 2
Technology (e.g. typically for WLAN this would be the BSSID of the
new WLAN Access Point)
Registration Request (as seen on MN-oFA link)
This is a Mobile IPv4 Registration Request message [1] with the
following characteristics:
Source IPv4 Address: MN Address
Source Layer 2 Address: MN Address
Destination IPv4 Address: nFA Address (from source addr of PrRtAdv)
Destination Layer 2 Address: Default Router (i.e. oFA Address)
Although this is not mandated, an MN implementation may set the S bit
(see chapter 6.) in Registration Request messages to improve the
handoff and avoid problems due to failed layer 2 handoffs and layer 2
ping-pong effects between two base stations.
Registration Reply (as seen on oFA-MN link)
This is a Mobile IPv4 Registration Reply message [1] with the
following characteristics:
Source IPv4 Address: nFA Address
Source Layer 2 Address: oFA Address
Destination IPv4 Address: MN Address (from source of Registration
Request)
Destination Layer 2 Address: MN Address (from source of
Registration Request)
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Full Copyright Statement
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This document is subject to the rights, licenses and restrictions
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