Network Working Group K. El Malki, Editor
INTERNET-DRAFT Ericsson
Expires: July 2004 January 2004
Low Latency Handoffs in Mobile IPv4
<draft-ietf-mobileip-lowlatency-handoffs-v4-08.txt>
Status of this memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
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This document is a product of the Mobile IP WG.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
Mobile IPv4 describes how a Mobile Node can perform IP-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 IP 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
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receive IP packets due to the delay in the Mobile IP Registration
process.
TABLE OF CONTENTS
1. Introduction.....................................................3
1.1. Terminology................................................3
1.2. The Techniques.............................................5
1.3. L2 triggers................................................7
1.4. Requirements language......................................9
2. Requirements.....................................................9
3. The PRE-REGISTRATION Handoff Method..............................9
3.1. Operation.................................................10
3.2. Network-Initiated Handoff.................................12
3.3. Mobile-Initiated Handoff..................................14
3.4. Obtaining and Proxying nFA Advertisements.................15
3.4.1. Inter-FA Solicitation................................15
3.4.2. Tunneled nFA Advertisements..........................16
3.5. Caching Router Advertisements.............................16
3.6. Movement Detection and MN Considerations..................17
3.7. L2 Address Considerations.................................18
3.8. Applicability of PRE-REGISTRATION Handoff.................19
4. The POST-REGISTRATION Handoff Method............................20
4.1. Two Party Handoff.........................................20
4.2. Three Party Handoff.......................................25
4.3. Renewal or Termination of Tunneling Service...............30
4.4. When will the MN perform a Mobile IP 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.......................................38
8. Handoff Signaling Failure Recovery..............................38
8.1. PRE-REGISTRATION Signaling Failure Recovery...............39
8.1.1. Failure of ProxyRtSol and ProxyRtAdv.................39
8.1.2. Failure of Inter-FA solicitation and advertisement...39
8.2. POST-REGISTRATION Signaling Failure Recovery..............40
8.2.1. HRqst Message Dropped................................40
8.2.2. HRply Message Dropped................................40
9. Generalized Link Layer Address Extension........................41
9.1. 3GPP2 IMSI Link Layer Address and Connection ID Ext.......42
9.2. 3GPP IMSI Link Layer Address Extension....................43
9.3. Ethernet Link Layer Address Extension.....................43
9.4. IEEE 64-Bit Global Identifier (EUI-64) Address Extension..44
9.5. Solicited IP Address Extension............................45
9.6. Access Point Identifier Extension.........................45
10. IANA Considerations............................................46
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11. Security Considerations.......................................46
12. Contributing Authors..........................................48
13. Acknowledgements..............................................48
14. Normative References..........................................48
15. Informative References........................................49
16. Editor's Address..............................................49
17. Intellectual Property Statement...............................50
18. Full Copyright Statement......................................50
Appendix A - Gateway Foreign Agents................................52
Appendix B - Low Latency Handoffs for Multiple-Interface MNs.......53
1. Introduction
Mobile IPv4 [1] describes how a Mobile Node (MN) can perform IP-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 methods to
achieve low-latency Mobile IP handoff during movement between FAs.
The presented techniques allow greater support for real-time services
on a Mobile IPv4 network by minimising the period of time when a MN
is unable to send or receive IP packets due to the delay in the
Mobile IP Registration process.
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
the handoff techniques. Sections 10 and 11 discuss IANA and security
considerations. Finally the two 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.
1.1. Terminology
This section presents a few terms used throughout the document.
oFA - old Foreign Agent, the FA involved in handling a MN's
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care of address prior to an 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 BET 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.
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
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routing a MN's traffic to/from it on a new subnet without
requiring the MN to change CoA.
ping-ponging - Rapid back and forth movement between two
wireless access points often due to failure of L2 handoff.
Ping-ponging 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.
IP address identifier - An IP address of a MN or FA, or an L2
identifier that allows an FA to deduce the IP address of a
MN or FA. If the IP address identifier is an L2 identifier,
it may be specific to the L2 technology.
1.2. The Techniques
Mobile IP 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.
- 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
IP packets.
This document presents techniques for reducing these built-in delay
components of Mobile IP. 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.
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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 IP-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-initated.
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 IP 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 IP 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
applications. The MN must eventually perform a formal Mobile IP
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-
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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 comes from L2
programmatically or is derived from L2 messages. Although the
protocols outlined in this document make use of specific L2
information, Mobile IP 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
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 expected to be 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
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independent of each other and MUST NOT 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- | network- | source |
| | initiated | initiated | trigger |
+-------------+----------------------+--------------+---------------+
| When? | sufficiently before | sufficiently | sufficiently |
| | the L2 handover | before L2 | before L2 |
| | so that MN can | handover for | handover for |
| | solicit ProxyRtAdv | FA to send | oFA & nFA to |
| | from oFA. | proxyRtAdv | exchange |
| | | to MN. | HRq/HRy. |
+-------------+----------------------+--------------+---------------+
| Parameters | nFA IP address | nFA IP address identifier |
| | identifier | MN IP address identifier |
| | | |
+-------------+----------------------+------------------------------+
+------------+------------------------+-------------+---------------+
| 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 IP address id | @MN: nFA IP | MN IP address |
| | MN IP address id. | or L2 addr. | identifier |
|------------+------------------------+-------------+---------------+
Table 1 - L2 Trigger
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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.
3. The PRE-REGISTRATION Handoff Method
The PRE-REGISTRATION handoff method is based on the original concept
of Mobile IP handoff as specified in [1], in 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.
It reuses the basic messages specified in [1]. The PRE-REGISTRATION
method allows both the MN and FA to initiate handoff. In both cases,
abiding by the basic Mobile IP handoff concept allows the MN to
choose with which FA to register. The PRE-REGISTRATION method 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 also supports both the normal Mobile IP model [1] in
which the MN is receiving packets from a Home Agent (HA) and the
Regional Registration model [11] in which the MN receives packets
from a Gateway Foreign Agent (GFA). It also supports movement where
a new AAA transaction must occur to authenticate the MN with the new
domain.
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3.1. Operation
The overall PRE-REGISTRATION Handoff mechanism is summarised in
Figure 1 below:
+---------+
| HA (GFA)|<---------+
+---------+ | 4. (Reg)RegReq
| 5. (Reg)RegReply
v
+-----+ 1a. RtSol +-----+
| | -----------------> | nFA |
| oFA | 1b. RtAdv | |
+-----+ <----------------- +-----+
^ | ^
(2a. ProxyRtSol)| | 2b |
| | ProxyRtAdv | 3. (Reg)RegReq
| | |
| v --------------------+
+-----+ /
| MN |
+-----+ - - - - - ->
Movement
Figure 1 - PRE-REGISTRATION Handoff Protocol
The following steps provide more detail on the protocol:
1. Messages 1a is a Router Solicitation (RtSol) from oFA to nFA.
Message 1b is a Router (Agent) Advertisement (RtAdv) from nFA
to oFA. These messages 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, 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). It is
different from a normal Router Solicitation since it is
actually soliciting an advertisement from a router different
from the one receiving this message. 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
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(PrRtAdv). When message 2a is received by the oFA, the oFA
MUST return the Proxy Router Advertisement (Agent
Advertisement) in message 2b. In network-initiated 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). In all cases message 2b is simply nFA's
agent advertisement.
3. The MN performs movement detection upon receipt of either a
solicited or unsolicited Agent Advertisement and, if
appropriate, it sends a Registration Request (RegReq) message
[1] in message 3 to nFA. When a local Gateway Foreign Agent
(GFA) is present this message MAY 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 IP Registration
[1] or Regional Registration [11] initiated with message 3.
In the network-initiated target-triggered case, the
Registration Reply in message 5 SHOULD be sent by nFA to the
MN both through oFA and directly on-link. This is necessary
since the MN may have to detach from oFA, due to the wireless
L2 connection, before it received the Reply. Figures 2 and 3
illustrate this tunneling, though it is not shown in Figure
1. Tunneling can take place either at L3 or L2. In the
mobile-initiated and network-initiated source-triggered cases
the nFA will not have the oFA's address. Therefore the Reply
MUST be unicast by nFA to the MN on-link as soon as the MN
connects to nFA (L2-UP). The MN's L2 address is obtained
using the extensions in Section 9, as described in 3.7.
5. If the Registration is successful then packets for the MN are
now tunnelled from the HA (or GFA) to the nFA where the MN
has moved to.
PRE-REGISTRATION is not dependent on Regional Registration extensions
[11]. However if the HA is at a distance (in terms of delay) from
the nFA, the use of a local GFA reduces 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, the four messaging steps of PRE-REG
described above will be completed (i.e. Registration Request
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processed by HA or GFA) 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 may require particular
implementation techniques and deployment, such as L2 techniques,
buffering and bicasting, 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 outside the scope of this document.
Figures 2, 3, and 4 contain message timing diagrams for both 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 initated
at oFA by a source trigger or at nFA by a target trigger. 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 such as the
MN's IP address identifier (i.e. the IP address itself or an
identifier which can be resolved to the IP address) and the nFA's IP
address identifier. An identifier may be specific to the wireless
technology (e.g. Access Point ID). 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. The L2 trigger contains
information such as the MN's IP address identifier and the oFA's IP
address identifier.
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 9.3 or 9.6. 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. 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 Solicitation. This
Advertisement is tunneled to oFA which functions as a normal router,
decapsulating the Advertisement and forwarding it to the MN. This
messages MUST be authenticated to prevent attacks (see section
3.4.2).
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Figures 2 and 3 contain message timing diagrams describing the PRE-
REGISTRATION network-initiated handoff for source and target
triggers.
MN oFA nFA HA/GFA
| |<~~~~~~ L2-Source | |
| | Trigger | |
|<--------------------| | |
| ProxyRtAdv | | |
| | | |
|---------------------------------------->| |
| RegReq or | | |
| RegRegReq | |------------------->|
| (routed via oFA) | | RegReq or RegRegReq|
| | | |
| | |<-------------------|
| | | (Reg)RegReply |
|<----------------------------------------| |
| | (Reg)RegReply | |
| | (sent to MN when it attaches to nFA) |
Figure 2 - PRE-REGISTRATION Handoff Message Timing Diagram
(Network-Initiated, Source Trigger)
MN oFA nFA HA/GFA
| | L2-Target~~~~~~~~>| |
| | Trigger | |
| | | |
| |...................| |
|<--------------------------------------- | |
| (ProxyRtAdv) |...................| |
| | Tunneled Agent | |
| | Advertisement | |
| | | |
|---------------------------------------->| |
| RegReq. or | | |
| RegRegReq | |------------------->|
| (routed via oFA) | | RegReq or RegRegReq|
| | | |
| | |<-------------------|
| | | (Reg)RegReply |
|<----------------------------------------| |
| | (Reg)RegReply | |
| | (sent to MN when it attaches to nFA) |
Figure 3 - PRE-REGISTRATION Handoff Message Timing Diagram
(Network-Initiated, Target Trigger)
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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 IP
address identifier (i.e. nFA's IP address or an identifier which can
be resolved to the nFA's IP address). The message timing diagram is
shown in Figure 4.
MN oFA nFA HA/GFA
|<~~~~~ L2-Trigger | | |
| | | |
|-------------------->| | |
| ProxyRtSol | | |
| | | |
|<--------------------| | |
| ProxyRtAdv | | |
| | | |
|---------------------------------------->| |
| RegReq or | | |
| RegRegReq | |------------------->|
| (routed via oFA) | | RegReq or RegRegReq|
| | | |
| | |<-------------------|
| | | (Reg)RegReply |
|<----------------------------------------| |
| | (Reg)RegReply | |
| | (sent to MN when it attaches to nFA) |
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 IP address identifier because the MN is
soliciting another specific FA's advertisement from the oFA. This
specific FA will be the MN's nFA. The IP address identifier contains
the IP address of the nFA or an identifier that can be used by the
oFA to resolve to nFA's IP address. If the identifier is not an IP
address, it MAY be specific to the underlying wireless technology,
for example, an Access Point or Base Station ID. The extension is a
subtype of the Generalised Link-Layer Address extension described in
Section 9. Two extension subtypes have been defined to contain the
nFA's IP address and an access point identifier. They are called the
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Solicited Agent IP 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 9.2).
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 more than once to reduce the
probability that it is lost due to errors on the wireless link.
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 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].
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 IP address of the nFA. The IP 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).
Once the oFA has access to the address of the nFA for a specific MN,
it MUST send a unicast agent solicitation to nFA. The nFA replies to
the oFA by unicasting an Agent Advertisement with appropriate
extensions. This method removes the TTL limitation of [1] for Mobile
IP 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
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solicitations and advertisements MUST be authenticated. These
messages MUST be protected using ESP [10] to prevent attacks. An FA
MUST NOT accept ICMP solicitations or advertisements from sources
which are not authenticated.
As a practical matter, oFA SHOULD pre-solicit and cache
advertisements from known neighboring FAs (see section 3.5), in order
to prevent having to perform the above 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
advertisments assumes that the nFA is aware of the IP address for oFA
and the MN. These IP addresses are obtained by means of the IP
address identifiers in an L2 trigger 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 packets
from sources which 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 Solicitation and Advertisement 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
MIN_SOLICITATION_INTERVAL in oFA MUST be set to a value equal to (0.5
* nFA's CHALLENGE_WINDOW * nFA's Agent advertisement interval). The
CHALLENGE_WINDOW and Agent advertisement interval are defined in [7]
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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 which 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 it is outside the scope of this document.
3.6. Movement Detection and MN 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 MUST be "Eager" to perform new bindings.
This means that the MN MUST 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].
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 (Registration Reply is received)
as described in [1].
Overall the MN behaves as described in [1] with the following
additions: 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).
When moving from a PRE-REGISTRATION network to a normal Mobile IP [1]
network the MN will no longer receive PrRtAdv messages (agent
advertisements with the LLA extension). If the MN still receives L2-
MTs then it will attempt to send PrRtSol messages. The FA will
either ignore the solicitation or will reply with a normal agent
advertisement [1]. In the absence of a PrRtSol, when receiving a
normal agent advertisement the MN MUST resort to normal Mobile IP
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behaviour [1]. If the MN does not receive a PrRtAdv in reply to its
PrRtSol, it SHOULD 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 resort to normal Mobile IP behaviour
[1]. The default values for PRE_SOL_ATTEMPTS is 2 and the default
value for PRE_SOL_INTERVAL is 1 second. It should be noted that the
performance of the movement detection mechanism mandated in PRE-
REGISTRATION MAY have sub-optimal behaviour on the other Mobile IP
[1] network. Instead when the MN moves from a normal Mobile IP [1]
network to a PRE-REGISTRATION network, the MN will start receiving
L2-MTs or PrRtAdv messages. When the MN receives L2-MTs or PrRtAdv
messages it MUST 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 SHOULD choose PRE-
REGISTRATION.
3.7. L2 Address Considerations
Some special considerations should be taken with respect to the
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 when the MN is performing a registration. 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 Extension
(see Section 9.3) 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. For the same reasons, the MN MUST NOT use the source L2
address of the Agent Advertisement message (PrRtAdv) as its default
router's L2 address. Therefore the oFA/nFA MUST include a
Generalised Link Layer Extension (see Section 9.3) with its Agent
Advertisement (PrRtAdv) messages.
If a particular wireless L2 technology has defined a special L2
interface to the wireless network that allows the FA to resolve the
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mapping between an MN's IP address and an L2 address without the need
to use the extension, the L2 address extension would not be needed.
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 IP
address identifiers which may need to be resolved to IP 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 IP address of the new FA is 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
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 L3 cannot rely upon L2 security between the MN and the FA to
make modifications to IP routing and therefore authenticated Mobile
IP messages are required,
- 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.
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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.
Following a successful Mobile IP Registration between MN and oFA, the
oFA becomes the mobility anchor point for the MN, called the 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 IP Registration.
+------+
| CN |
+------+
|
...
|
+------+ BET +------+
| aFA |==========| nFA |
+------+ +------+
| wireless link
|
movement +------+
---------> | MN |
+------+
Figure 5 - POST-REGISTRATION Concept
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, where the MN
performed a Mobile IP Registration, to nFA. In the normal case, this
movement would result in a new Mobile IP Registration at nFA.
However in POST-REGISTRATION, the MN and nFA MAY delay this but
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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. To identify the
difference between a source triggered HRqst/HRply message for tunnel
creation, a target triggered HRqst/HRply message for tunnel creation
and HRqst/HRply to extend or terminate a BET (or unidirectional
tunnel), the message will be identified respectively by (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:
a) The L2 trigger is a source trigger (L2-ST) at oFA. The
trigger contains the MN's L2 address and an IP address
identifier (the IP address itself or an L2 address that
can be resolved to the IP address) for nFA.
b) The L2 trigger is a target trigger (L2-TT) at nFA. The
trigger contains the MN's L2 address and an IP address
identifier for 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 home network IP address of the MN, 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
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lifetime and a set T bit indicate that the oFA is willing
to tunnel for the indicated time. Section 4.6 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 home network IP 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 4.6 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 IP address, the MN's HA address, and an LLA option
containing the MN's L2 address. Section 4.7 describes the
HRply(t).
b) If nFA is sending the HRply, the H bit is set and the N
and R bits are unset, indicating the reply is in response
to a 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 4.7 describes the
HRply(s).
4) The point during the L2 handoff in which the MN is no longer
connected on a certain 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. Each node handles the trigger in the
following way:
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a) When the 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 the oFA to the MN and
forwards any outbound packets from MN to the next hop
using normal routing mechanisms or through the reverse
tunnel to oFA or HA.
c) When the MN receives the L2-LU, it MAY intiate the Mobile
IP 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
IP Registration (see section 4.4).
5) The oFA becomes an aFA if the MN moves to a third FA before
having performed a Mobile IP 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 which originated
the HRqst can in this case cancel the tunnel by sending an
HRqst(r) to the other FA with lifetime zero. It will then
simply continue delivering packets to MN exactly as if no
handoff had been pending. Section 4.6 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 in
place.
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
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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. Additonally, particular implementation and
deployment scenarios could require that techniques be employed to
smooth handoff by providing a means to convey packets arriving during
the L2 handoff. The exact techniques involved in smoothing are
currently under discussion by the working group and 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
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
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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 IP
Registration with the current FA. This may be initiated by the
FA which sends an Agent Advertisement or by the MN which
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 that has already
established an aFA and is receiving tunneled packets through its
current FA moves to a new FA without performing a Mobile IP
Registration.
+------+
+--->| 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 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 IP
Registration with nFA will not complete before the MN moves on, HTT
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SHOULD be implemented. Certain wireless systems and implementations
do not allow such fast movement between FAs and may force the Mobile
IP 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 (L2-LU at nFA) and/or the MN SHOULD
solicit a Router Advertisement after L2 handoff (L2-LU at MN).
The L3 handoff can be deferred either because of a decision by the
MN/FA (i.e. MN does not send Router 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
Handoff To Third (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 IP address
identifier (IP address or L2 address that can be mapped to
an IP address) for nFA.
b) The L2 trigger is a target trigger (L2-TT) at nFA. The
trigger contains the MN's L2 address and an IP address
identifier for 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 flags 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 IP address, the MN's HA address,
an LLA containing the aFA's IP address, and an LLA
containing the L2 address of the MN. This is enough
information for nFA to perform a target triggered handoff
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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:
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 IP 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.
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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
considerations, including whether a handoff smoothing solution is
used, and is beyond the scope of this specification.
Figures 10 and 11 show timing diagrams for source trigger (L2-ST) and
target trigger (L2-TT) three party handoff, respectively.
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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
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 IP 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
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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 IP Registration?
The MN/FA have control over when to perform the Mobile IP
Registration. Although the MN/FA may decide to defer Mobile IP
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 IP 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 IP Registration by
sending an Agent Advertisement to the MN as in [1].
2) The MN itself decides to perform a Mobile IP Registration and
initiates it by sending an Agent solicitation as in [1].
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 IP 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
both a forward and reverse tunnel are established between oFA and nFA
(i.e. a BET) and the MN has moved to nFA, the oFA MUST continue
sending Router Advertisements to the MN. This is to refresh the MN's
default router entry. The Router Advertisements are tunnelled from
oFA to nFA through the forward tunnel and MUST be unicast to the MN.
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Similarly to PRE-REGISTRATION, tunneling of Advertisements is
possible only if the TTL limitation of [1] is relaxed. If this is
not possible then the nFA MUST advertise to the MN as soon as it's
link to the nFA is up (L2-UP). The MN MUST perform a Mobile IP
registration [1] when it receives an Agent Advertisement following a
POST-REGISTRATION handoff.
Instead, when the forward tunnel is established but not the reverse
tunnel, oFA MUST NOT advertise to the MN. In this case, as described
previously, it is possible that the MN will not receive Router
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-UP). This effectively means that the MN
will at most be able to defer Mobile IP 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 IP
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 IP 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MN Home Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HA Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Identification +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extensions ...
+-+-+-+-+-+-+-+-
Type TBD (Handoff Request)
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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. 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 is set the HRqst may require
extensions containing the GRE type and key
fields, but they 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, in seconds, for which tunnel
service for the MN will be maintained. If this
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 the both the forward and reverse tunnel.
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 any 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 immediately. A value of 0xffff
indicates infinity.
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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 IP 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 IP 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 ...
+-+-+-+-+-+-+-+-
Type TBD (Handoff Reply)
Code A value indicating the result of the Handoff
Request. Only two codes are currently supported,
0, indicating success, and a nonzero value,
indicating that the handoff cannot be performed.
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
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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 is set the HRply may require
extensions containing the GRE type and key
fields, but they are outside the scope of this
document.
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 address of the MN.
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HA Addr For HRply(t), the HA address of the mobile node.
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 IP 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 IP 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
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
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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 IP 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 by running the PRE-REGISTRATION method and in parallel
exchanging the POST-REGISTRATION handoff messages between oFA and
nFA. The only case not considered already in the POST-REGISTRATION
method is mobile-initiated handoff. In the mobile-initiated case,
the Handoff Request message is initated by the oFA or nFA when it
receives the Registration Request from the MN.
The combined method follows the PRE-REGISTRATION Handoff when it is
successful before the completion of the MN's L2 handoff. However, if
PRE-REGISTRATION does not complete prior to the expiration of a timer
on one or the other of the FAs, POST-REGISTRATION handoff is used.
Using POST-REGISTRATION handoff insulates the MN from delays caused
by errors such as loss of one of the Mobile IP messages involved in
PRE-REGISTRATION.
The start of POST-REGISTRATION is gated by the expiration of a timer
on the FAs. The timer is started at oFA following a source-trigger,
at nFA following the target-trigger, or at oFA and nFA following the
receipt of the Registration Request from the MN in the mobile-
initiated case. The timer is reset if the Registration Reply message
is received by the appropriate FA and sent to the MN.
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Although the POST-REGISTRATION Handoff Request and Handoff Reply
messages are exchanged in advance, no forwarding of traffic between
oFA and nFA is performed unless the timer expires. The timer should
be set to a value that allows forwarding between oFA and nFA to begin
before the MN completes the L2 handoff to nFA.
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 would
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 which will solve this problem and allow
the MN to receive traffic independently of the timing of the L2
handoff event are currently under study by the Mobile IP WG but are
outside the scope of this document.
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 unless the MN specifically requests it.
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.
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8.1. PRE-REGISTRATION Signaling Failure Recovery
Failure of PRE-REGISTRATION signaling breaks down into three cases:
1) Loss of messages ProxyRtSol and ProxyRtAdv on the air link.
2) Loss of the solicitation by an FA to obtain another neighbouring
FA's Advertisment or loss of the neighbouring FA's
advertisement.
3) Failure of the standard Mobile IP Registration.
Of these, case 3) is handled by standard Mobile IP mechanisms
described in [1]. Case 2) is relatively unlikely because spontaneous
packet drop rates on the fixed network are caused by congestion or
router failure and likely to be low. 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 ProxyRtSol and ProxyRtAdv
PRE-REGISTRATION handoff can fail in network-initiated handoff when
the ProxyRtAdv 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 ProxyRtSol sent from
the MN or return ProxyRtAdv sent from the oFA are dropped. To reduce
the probability that ProxyRtAdv and ProxyRtSol are lost the MN and FA
MAY 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 signaling. When this happens the
MN MUST solicit an FA Advertisement when it connects to nFA at L2
(L2-UP) and perform standard Mobile IP registration on 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 Advertisment is unable to send a
ProxyRtAdv in response to a source trigger or to a mobile-initiated
ProxyRtSol. 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 IP registration as soon as it connects
to the nFA (L2-UP) as specified in 8.1.1 and [1].
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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
handover 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).
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-UP 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 confirmation of the
handoff.
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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 IP 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 Address Extension
This section defines the Generalized Link Layer Address (LLA)
Extension, used by any node that needs to communicate Link Layer
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.
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 for Mobile IP Registrations
TBD (skippable) [1] - when used for Router Advertisements
Length
The length of the Link Layer Address + the one octet Sub-Type
field
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Sub-Type
This field contains the Link Layer sub-type identifier
LLA
Contains the Link Layer Address
In this document, six subtypes are defined:
1 3GPP2 International Mobile Station Identity and
Connection ID [12]
2 3GPP International Mobile Subscriber Identity [16]
3 Ethernet 48 bit MAC address [5]
4 64 bit Global ID, EUI-64 [6]
5 Solicited IP Address
6 Access Point Identifier
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 ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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>
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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 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 [16].
9.3. Ethernet Link Layer Address Extension
The Ethernet Link Layer Address Extension contains the 48 bit
Ethernet MAC Address, as defined in [5].
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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 | 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
MAC
Contains the 64-Bit Global Identifier Address.
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9.5. Solicited IP Address Extension
The 32-bit Solicited IP Address Extension contains the IP 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 | IP addr ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
TBD (skippable) [1]
Length
5 (includes the Sub-Type field)
Sub-Type
5
IP Address
Contains the 32-Bit IP 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]
Length
5 (includes the Sub-Type field)
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Sub-Type
6
AP ID
Contains the 32-Bit Access Point Identifier.
10. IANA Considerations
Section 9 introduces the Generalized Link Layer Address Extension
numbering space that requires IANA management. This specification
makes use of the subtype values 1-6, and all other values other than
zero (0) are available for assignment via IETF consensus [15]. The
numbers for the Generalized Link Layer Address Extension are taken
from the numbering space defined for Mobile IP registration and
Router Advertisement extensions in [1]. The same Generalized Link
Layer Address Extensions are used in both Mobile IP Registration and
Router Advertisement messages, which have different extension
numbering spaces defined in [1]. Therefore two separate Generalized
Link Layer Address Extension numbering spaces are required having the
same sub-type values. The Generalized Link Layer Address Extension
numbering MUST NOT conflict with any numbers used in [1], [3], [7],
[13] and [14].
In the POST-REGISTRATION Handoffs method, Sections 4.4 and 4.5
require numbers assigned from the Mobile IP control message type
address space. The numbers assigned MUST NOT conflict with [1] and
[11].
11. Security Considerations
A security consideration for PRE-REGISTRATION method, as discussed in
Section 3.8, is that oFA and nFA MUST share a security association to
authenticate messages transported between them and oFA must be
authorized to solicit nFA. The inter-FA messages (solicitations and
advertisements) MUST be authenticated using ESP [10]. The absence of
this security would allow denial of service attacks from malicious
nodes at any distance from the FA. Otherwise, PRE-REGISTRATION uses
the security mechanisms described in [1] and [11].
POST-REGISTRATION introduces a new change to Mobile IP, which is the
possibility that a MN may receive packets from an FA with which it
has not yet registered. In the event that the MN does not wish to
receive packets from unknown FAs, it MAY drop them. In a similar way
to PRE-REGISTRATION, oFA and nFA MUST share a security association
required to protect the Handoff Request and Reply messages. The
Handoff Request and Reply messages MUST be authenticated using the
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FA-FA authentication extension [11]. 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 security
associations with neighbouring FAs, involving shared keys and the
default algorithms of [1].
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 which route IP 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 IP addresses for the
MN and the FAs that appear in the L2 triggers MUST correspond to the
actual nodes that are participating in the handover. If there is any
possibility that tampering may occur, the recipient of an L2 trigger
MUST have some way of authenticating the L2 information. Provided
the L2 triggers are so secured, the nodes involved in a handover can
reject any traffic from a node whose L2 address or IP address was not
received in a trigger, yet tries to send traffic. 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 IP
registration. In PRE-REGISTRATION the security of L2 triggers has
different implications. The PRE-REGISTRATION technique depends on
Mobile IP 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 attack. 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. In order to
prevent any such denial of service attacks the L2 triggers SHOULD be
secured.
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12. 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>
13. Acknowledgements
The authors would like to thank the Mobile IP WG chairs, Phil Roberts
and Basavaraj Patil, for their input and Jonathan Wood for valuable
comments on PRE-REGISTRATION.
14. Normative References
[1] C. Perkins, Editor, "IP Mobility Support for IPv4", RFC 3220,
January 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 IP, 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,
Symbolics,Inc., November 1982.
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[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 IP 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, "IP Encapsulating Security Payload
(ESP)", RFC 2406, November 1998.
[11] E. Gustafsson, A. Jonsson and C. Perkins, "Mobile IP Regional
Tunnel Management ", draft-ietf-mobileip-reg-tunnel-07 (work
in progress), Oct 2002.
15. Informative References
[12] TIA/EIA/IS-2000.
[13] G. Montenegro and V. Gupta, "Sun's SKIP Firewall Traversal
for Mobile IP", RFC 2356, June 1998.
[14] P. Calhoun, C. Perkins, "Mobile IP Network Access Identifier
Extension", RFC 2794, March 2000.
[15] T. Narten, H, Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October
1998.
[16] 3GPP TS 23.003 (www.3gpp.org).
16. Editor's Address
Karim El Malki
Ericsson
LM Ericssons Vag. 8
126 25 Stockholm, Sweden
Phone: +46 8 7195803
E-mail: karim.el-malki@ericsson.com
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17. Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
18. Full Copyright Statement
Copyright (C) The Internet Society (2004). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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Appendix A - Gateway Foreign Agents
The Mobile IP 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 a 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.
+-----+
| 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.
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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 IP stack on the MN allows switching an IP
address binding between interfaces. This Appendix discusses how
multiple wireless interfaces can aid low latency handoff.
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
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 IP
traffic.
+------+ +---------+
| HA |--------| (GFA) |
+------+ +---------+
/ \
/ \
... ...
/ \
/ \
+------+ +------+
| oFA | | nFA |
+------+ +------+
| |
+------+ +------+
| RN1 | | RN2 |
+------+ +------+
+------+
| MN | --------->
+------+
Movement
Figure B.1 - Network Model for Mobile IPv4 With Multi-Access
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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 IP Registration Request to maintain
simultaneous bindings both its existing (HA or GFA) binding with oFA
and a new binding with nFA.
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