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IETF RFC 4881
Low-Latency Handoffs in Mobile IPv4
Last modified on Friday, June 29th, 2007
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Network Working Group K. El Malki, Ed.
Request for Comments: 4881 Athonet
Category: Experimental June 2007
Low-Latency Handoffs in Mobile IPv4
 Status of This Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright © The IETF Trust (2007).
Abstract
Mobile IPv4 describes how a Mobile Node can perform IPv4-layer
handoffs between subnets served by different Foreign Agents. In
certain cases, the latency involved in these handoffs can be above
the threshold required for the support of delay-sensitive or real-
time services. The aim of this document is to present two methods to
achieve low-latency Mobile IPv4 handoffs. In addition, a combination
of these two methods is described. The described techniques allow
greater support for real-time services on a Mobile IPv4 network by
minimizing the period of time when a Mobile Node is unable to send or
receive IPv4 packets due to the delay in the Mobile IPv4 Registration
process.
Table of Contents
1. Introduction ....................................................3
1.1. Terminology ................................................4
1.2. The Techniques .............................................5
1.3. L2 Triggers ................................................7
1.4. Requirements Language ......................................9
2. Requirements ....................................................9
3. The PRE-REGISTRATION Handoff Method ............................10
3.1. Operation .................................................11
3.2. Network-Initiated Handoff .................................13
3.3. Mobile-Initiated Handoff ..................................15
3.4. Obtaining and Proxying nFA Advertisements .................17
3.4.1. Inter-FA Solicitation ..............................17
3.4.2. Tunneled nFA Advertisements ........................18
3.5. Caching Router Advertisements .............................19
El Malki, Ed. Experimental PAGE 1 
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
3.6. Movement Detection, MN, and FA Considerations .............19
3.7. L2 Address Considerations .................................21
3.8. Applicability of PRE-REGISTRATION Handoff .................21
4. The POST-REGISTRATION Handoff Method ...........................23
4.1. Two-Party Handoff .........................................24
4.2. Three-Party Handoff .......................................28
4.3. Renewal or Termination of Tunneling Service ...............34
4.4. When Will the MN Perform a Mobile IPv4 Registration? ......34
4.5. Handoff Request (HRqst) Message Format ....................36
4.6. Handoff Reply (HRply) Message Format ......................38
4.7. Handoff to Third (HTT) Message Format .....................40
4.8. Applicability of POST-REGISTRATION Handoff Method .........40
5. Combined Handoff Method ........................................41
6. Layer 2 and Layer 3 Handoff Timing Considerations ..............42
7. Reverse Tunneling Support ......................................42
8. Handoff Signaling Failure Recovery .............................43
8.1. PRE-REGISTRATION Signaling Failure Recovery ...............43
8.1.1. Failure of PrRtSol and PrRtAdv .....................43
8.1.2. Failure of Inter-FA Solicitation and
Advertisement ......................................44
8.2. POST-REGISTRATION Signaling Failure Recovery ..............44
8.2.1. HRqst Message Dropped ..............................44
8.2.2. HRply Message Dropped ..............................45
9. Generalized Link Layer and IPv4 Address (LLA) Extension ........46
9.1. 3GPP2 IMSI Link Layer Address and Connection ID
Extension .................................................47
9.2. 3GPP IMSI Link Layer Address Extension ....................48
9.3. Ethernet Link Layer Address Extension .....................49
9.4. IEEE 64-Bit Global Identifier (EUI-64) Address Extension ..50
9.5. Solicited IPv4 Address Extension ..........................51
9.6. Access Point Identifier Extension .........................52
9.7. FA IPv4 Address Extension .................................53
10. IANA Considerations ...........................................53
10.1. New Extension Values .....................................53
10.2. Generalized Link Layer and IP Address Identifier (LLA) ...54
10.3. New Message Type and Code ................................54
11. Security Considerations .......................................55
12. Acknowledgements ..............................................57
13. References ....................................................57
13.1. Normative References .....................................57
13.2. Informative References ...................................58
Appendix A - Gateway Foreign Agents................................59
Appendix B - Low Latency Handoffs for Multiple-Interface MNs.......60
Appendix C - PRE_REGISTRATION Message Summary......................61
El Malki, Ed. Experimental PAGE 2
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
1. Introduction
Mobile IPv4 [1] describes how a Mobile Node (MN) can perform IPv4-
layer handoff between subnets served by different Foreign Agents
(FAs). In certain cases, the latency involved in handoff can be
above the threshold required for the support of delay-sensitive or
real-time services. The aim of this document is to present two
techniques to achieve low-latency Mobile IPv4 handoff during movement
between FAs. A further combination of these two techniques is also
described. The presented techniques allow greater support for real-
time services on a Mobile IPv4 network by minimizing the period of
time during which an MN is unable to send or receive IPv4 packets due
to the delay in the Mobile IPv4 Registration process. One or more of
these techniques may be required to achieve fast Mobile IPv4 handoffs
over different wireless technologies (e.g., WLAN, Cellular, WiMAX,
Flash-OFDM, etc.). Each wireless technology has different layer 2
handoff procedures, and the best low-latency technique for each
scenario should be used to optimize the handoff performance. Further
deployment and experimentation are required to determine which
technique is best suited to the wireless technologies in terms of
implementation and performance. Therefore, the authors encourage
further performance measurements and work on low-latency-over-foo
specifications in collaboration with the appropriate wireless
technology fora to describe the applicability to different wireless
layer 2s.
In the rest of this section, terminology used throughout the document
is presented, the handoff techniques are briefly described, and the
use of link-layer information is outlined. In Section 2, a brief
description of requirements is presented. Section 3 describes the
details of the PRE-REGISTRATION handoff technique, and 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, and Section 9 describes protocol extensions required by the
handoff techniques. Sections 10 and 11 discuss IANA and security
considerations. Finally, the three appendices discuss additional
material related to the handoff techniques. Appendix A gives a short
introduction to Regional Registrations [11], which can be used
together with low-latency handoffs. Appendix B discusses low-latency
handoff when an MN has multiple wireless L2 interfaces, in which case
the techniques in this document may not be necessary. Appendix C
provides a summary of the messages used in PRE-REGISTRATION.
El Malki, Ed. Experimental PAGE 3
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
1.1. Terminology
This section presents a few terms used throughout the document.
oFA - old Foreign Agent (FA), the FA involved in handling the
care-of address (CoA) of a Mobile Node (MN) prior to a layer 3
(L3) handoff.
nFA - new Foreign Agent, the FA anticipated to be handling an MN's
care-of address after completion of an L3 handoff.
aFA - anchor Foreign Agent, the FA that is currently handling the
network end of the tunnel in POST-REGISTRATION.
L2 handoff - Movement of an MN's point of layer 2 (L2) connection
from one wireless access point to another.
L3 handoff - Movement of an MN between FAs that involves changing
the care-of address 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 an 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.
El Malki, Ed. Experimental PAGE 4
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
seamless handoff - L3 handoff that is both low latency and low
loss.
bidirectional edge tunnel (BET) - A bidirectional tunnel
established between two FAs for purposes of temporarily routing
an MN's traffic to/from it on a new subnet without requiring
the MN to change CoA.
ping-pong - Rapid back-and-forth movement between two wireless
access points often due to failure of L2 handoff. Ping-pong
can occur if radio conditions for both the old and new access
points are about equivalent and less than optimal for
establishing a good, low-error L2 connection.
network-initiated handoff - L3 handoff in which oFA or nFA
initiates the handoff.
mobile-initiated handoff - L3 handoff in which the MN initiates
the handoff.
MN or FA identifier - An IPv4 address of an MN or FA, or an L2
identifier that can be resolved to the IPv4 address of an MN or
FA. If the identifier is an L2 identifier, it may be specific
to the L2 technology.
1.2. The Techniques
Mobile IPv4 was originally designed without any assumptions about the
underlying link layers over which it would operate so that it could
have the widest possible applicability. This approach has the
advantage of facilitating a clean separation between L2 and L3 of the
protocol stack, but it has negative consequences for handoff latency.
The strict separation between L2 and L3 results in the following
built-in sources of delay:
- The MN may only communicate with a directly connected FA. This
implies that an 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 IPv4
packets.
This document presents techniques for reducing these built-in delay
components of Mobile IPv4. The techniques can be divided into two
general categories, depending on which of the above problems they are
attempting to address:
El Malki, Ed. Experimental PAGE 5
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
- Allow the MN to communicate with the nFA while still connected
to the oFA.
- Provide for data delivery to the MN at the nFA even before the
formal registration process has completed.
The first category of techniques allows the MN to "pre-build" its
registration state on the nFA prior to an underlying L2 handoff. The
second category of techniques allows for service to continue
uninterrupted while the handoff is being processed by the network
without requiring the MN's involvement.
Three methods are presented in this document 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, and
- combined handoff method.
The PRE-REGISTRATION handoff method allows the MN to be involved in
an anticipated IPv4-layer handoff. The MN is assisted by the network
in performing an L3 handoff before it completes the L2 handoff. The
L3 handoff can be either network-initiated or mobile-initiated.
Accordingly, L2 triggers are used both in the MN and in the FA to
trigger particular L3 handoff events. The PRE-REGISTRATION method
coupled with L2 mobility helps to achieve seamless handoffs between
FAs. The basic Mobile IPv4 concept involving advertisement followed
by registration is supported, and the PRE-REGISTRATION handoff method
relies on Mobile IPv4 security. No new messages are proposed, except
for an extension to the Agent Solicitation message in the mobile-
initiated case.
The POST-REGISTRATION handoff method proposes extensions to the
Mobile IPv4 protocol to allow the oFA (old FA) and nFA (new FA) to
utilize L2 triggers to set up a bidirectional 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 IPv4
Registration after L2 communication with the new FA is established,
but this can be delayed as required by the MN or FA. Until the MN
performs registration, the FAs will set up and move bidirectional
tunnels as required to give the MN continued connectivity.
El Malki, Ed. Experimental PAGE 6
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
The combined method involves running a PRE-REGISTRATION and a POST-
REGISTRATION handoff in parallel. If the PRE-REGISTRATION handoff
can be performed before the L2 handoff completes, the combined method
resolves to a PRE-REGISTRATION handoff. However, if the PRE-
REGISTRATION handoff does not complete within an access technology
dependent time period, the oFA starts forwarding traffic for the MN
to the nFA as specified in the POST-REGISTRATION handoff method.
This provides for a useful backup mechanism when completion of a
PRE-REGISTRATION handoff cannot always be guaranteed before the L2
handoff completion.
It should be noted that the methods described in this document may be
applied to MNs having a single interface (e.g., Wireless LAN
interface) or multiple interfaces (e.g., one WLAN and one cellular
interface). However, the case of multiply-interfaced MNs needs
special consideration, since the handoff methods described in this
document may not be required in all cases (see Appendix B).
1.3. L2 Triggers
An L2 trigger is a signal of an L2 event. In this document, the L2
events relate to the L2 handoff process. One possible event is early
notice of an upcoming change in the L2 point of attachment of the
mobile node to the access network. Another possible event is the
completion of relocation of the mobile node's L2 point of attachment
to a new L2 access point. This information may come explicitly from
L2 in a solicited or unsolicited manner, or it may be derived from L2
messages. Although the protocols outlined in this document make use
of specific L2 information, Mobile IPv4 should be kept independent of
any specific L2. L2 triggers are an abstraction mechanism for a
technology-specific trigger. Therefore, an L2 trigger that is made
available to the Mobile IPv4 stack is assumed to be generic and
technology independent. The precise format of these triggers is not
covered in this document, but the information required to be
contained in the L2 triggers for low-latency handoffs is specified.
In order to properly abstract from the L2, it is assumed that one of
the three entities -- the MN, oFA, or nFA -- is made aware of the
need for an L2 handoff and that the nFA or MN can optionally also be
made aware that an L2 handoff has completed. A specific L2 will
often dictate when a trigger is received and which entity will
receive it. Certain L2s provide advance triggers on the network
side, while others provide advance triggers on the MN. Also, the
particular timing of the trigger with respect to the actual L2
handoff may 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.
El Malki, Ed. Experimental PAGE 7
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
An L2 trigger may be categorized according to whether it is received
by the MN, oFA, or nFA. Table 1 gives such a categorization along
with information contained in the trigger. The methods presented in
this document operate based on different types of L2 triggers as
shown in Table 1. Once the L2 trigger is received, the handoff
processes described hereafter are initiated. The three triggers,
L2-ST, L2-TT, and L2-MT, are independent of each other and are not
expected to occur together since each will trigger a different type
of handoff behaviour.
+-------------+----------------------+------------------------------+
| L2 Trigger | Mobile | Source |
| | Trigger | Trigger |
| | (L2-MT) | (L2-ST) |
+-------------+----------------------+------------------------------+
| Recipient | MN | oFA |
+-------------+----------------------+--------------+---------------+
| Method | PRE | PRE | POST |
| | mobile-initiated | network- | source |
| | | initiated | trigger |
+-------------+----------------------+--------------+---------------+
| When? | sufficiently before | sufficiently | sufficiently |
| | the L2 handoff | before L2 | before L2 |
| | so that MN can | handoff for | handoff for |
| | solicit PrRtAdv | FA to send | oFA & nFA to |
| | from oFA | PrRtAdv | exchange |
| | | to MN | HRqst/HRply |
+-------------+----------------------+--------------+---------------+
| Parameters | nFA identifier | nFA identifier, MN identifier|
+-------------+----------------------+------------------------------+
Table 1 - L2 Trigger
(continued on next page)
El Malki, Ed. Experimental PAGE 8
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
+------------+----------------------+---------------+---------------+
| L2 Trigger | Target | Link-Up | Link-Down |
| | Trigger | (L2-LU) | (L2-LD) |
| | (L2-TT) | | |
|------------+----------------------+---------------+---------------+
| Recipient | nFA | MN or nFA | oFA |
|------------+-----------+----------+---------------+---------------+
| Method | PRE | POST | PRE & POST | POST |
| | network- | target | | |
| | initiated | trigger | | |
|------------+----------------------+---------------+---------------+
| When? | | when radio | when radio |
| | same as | link between | link between |
| | source trigger | MN & nFA is | MN and oFA |
| | | established | is lost |
|------------+----------------------+---------------+---------------+
| Parameters | oFA identifier | @MN: nFA IPv4 | MN identifier |
| | MN identifier | or L2 addr. | |
| | | @nFA: MN IPv4 | |
| | | or L2 addr. | |
+------------+----------------------+---------------+---------------+
Table 1 - L2 Trigger
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, and
- to limit wireless bandwidth usage.
El Malki, Ed. Experimental PAGE 9
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
3. The PRE-REGISTRATION Handoff Method
The PRE-REGISTRATION handoff method is based on the normal Mobile
IPv4 handoff procedure specified in [1], according to which:
- an advertisement for an FA is received by an MN,
- the advertisement allows the MN to perform movement detection,
and
- the MN registers with the FA.
The basic messages specified in [1] are extended to carry information
required to achieve fast handoffs. The PRE-REGISTRATION method
allows both the MN and FA to initiate the layer 3 handoff and it can
make use of L2 triggers on either the FA or MN side, depending on
whether network-initiated or mobile-initiated handoff occurs.
PRE-REGISTRATION supports the normal Mobile IPv4 model [1] and
optionally also the Regional Registration model [11]. There can be
advantages in implementing [11] together with low-latency handoff
mechanisms, in particular in cases where the Home Agent (HA) is at a
distance (in terms of delay) from the nFA. The time required for the
handoff procedure to complete can be reduced by using a closer local
HA, called Gateway Foreign Agent (GFA) in [11]. However,
implementation of [11] is not required by PRE-REGISTRATION. PRE-
REGISTRATION also supports movement where a new Authentication,
Authorization, and Accounting (AAA) transaction must occur to
authenticate the MN with a new domain.
El Malki, Ed. Experimental PAGE 10
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
3.1. Operation
The PRE-REGISTRATION handoff mechanism is summarized in Figure 1.
+---------+
| HA (GFA)|<---------+
+---------+ | 4. (Reg)RegReq
| 5. (Reg)RegReply
v
+-----+ 1a. PrRtSol +-----+
| | -----------------> | nFA |
| oFA | 1b. PrRtAdv | |
+-----+ <----------------- +-----+
^ | ^
(2a. PrRtSol)| | 2b |
| | PrRtAdv | 3. (Reg)RegReq
| | |
| v --------------------+
+-----+ /
| MN |
+-----+ - - - - - ->
Movement
Figure 1 - PRE-REGISTRATION Handoff Protocol
The following steps provide more detail on the protocol:
1. Message 1a is a Proxy Router (Agent) Solicitation (PrRtSol)
from oFA to nFA. It is a Mobile IP agent solicitation
containing an identifier for the nFA (i.e., IP address or L2
address) in a Generalized Link Layer and IP Address Extension
(see Section 9). When message 1a is received by the nFA
containing nFA's correct identifier in the LLA extension, the
nFA MUST return the Proxy Router Advertisement (Agent
Advertisement) in message 1b. Message 1b is simply nFA's Agent
Advertisement containing the nFA layer 2 address in a
Generalized Link Layer and IP Address (LLA) Extension (see
Section 9.3). Messages 1a and 1b SHOULD occur in advance of
the PRE-REGISTRATION handoff in order not to delay the handoff.
For this to occur, oFA SHOULD solicit and cache advertisements
from neighboring nFAs using messages 1a and 1b, thus decoupling
the timing of this exchange from the rest of the PRE-
REGISTRATION handoff. When the L3 handoff is initiated by a
target L2 trigger at nFA (L2-TT), message 1b equals message 2b
and is sent unsolicited directly to MN (tunneled by nFA to MN
through oFA) instead of being relayed by oFA.
El Malki, Ed. Experimental PAGE 11
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
2. Message 2a is a Proxy Router Solicitation (PrRtSol) from MN to
oFA. It is different from a normal Router (Agent) Solicitation
since it is soliciting an advertisement from a router different
from the one receiving this message. It is a Mobile IP Agent
Solicitation containing an identifier for the nFA (i.e., IP
address or L2 address) in a Generalized Link Layer and IP
Address Extension (see Section 9). The presence of message 2a
indicates that the handoff is mobile-initiated and its absence
means that the handoff is network-initiated. In mobile-
initiated handoff, message 2a occurs if there is an L2 trigger
in the MN to solicit for a Proxy Router Advertisement
(PrRtAdv). When message 2a is received by the oFA, it MUST
return the Proxy Router Advertisement (Agent Advertisement) in
message 2b. This is simply nFA's Agent Advertisement
containing the nFA layer 2 address in a Generalized Link Layer
and IP Address (LLA) Extension (see Section 9.3). In network-
initiated source-triggered handoff, the L2 trigger occurs at
oFA, and oFA MUST relay the Agent Advertisement in message 2b
without the need for the MN to solicit. Note that it is also
possible for nFA to advertise directly to the MN in the
network-initiated target-triggered case (see Section 3.2).
3. The MN performs movement detection upon receipt of a solicited
or unsolicited Agent Advertisement and, if required, it sends a
Registration Request (RegReq) message [1] in message 3 to nFA.
When a local Gateway Foreign Agent (GFA) is present, this
message can optionally be a Regional Registration Request
(RegRegReq) [11]. Message 3 is routed through oFA since the MN
is not directly connected to nFA prior to the L2 handoff.
4. Messages 4 and 5 complete the standard Mobile IPv4 Registration
[1] or optionally Regional Registration [11] initiated with
message 3. The Registration Request MUST contain the MN's
layer 2 address in a Generalized Link Layer and IP Address
Extension (see Sections 3.7 and 9). This identifier may be a
plain Ethernet address or an identifier specific to the
wireless technology. If the MN is not already connected to
nFA, the Registration Reply in message 5 MUST be buffered by
the nFA and unicast to the MN on-link as soon as the MN
connects to nFA (i.e., L2-LU trigger at nFA, which can be
implemented by the MN sending an Agent Solicitation or
optionally using special layer 2 techniques, which are outside
the scope of this document). This is necessary since the MN
may have to detach from oFA, due to the wireless L2 connection,
before it receives the reply. The MN's L2 address is obtained
using the extensions in Section 9, as described in Section 3.7.
Figures 2 and 3 illustrate this procedure.
El Malki, Ed. Experimental PAGE 12
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
5. If the registration is successful, packets for the MN are
tunneled from the HA (or GFA) to the nFA and then to the MN.
PRE-REGISTRATION is not dependent on [11]. However, if the HA is at
a distance (in terms of delay) from the nFA, the use of a local GFA
may reduce the time required for the handoff procedure to complete.
The time at which the L2 trigger is received by the oFA or MN,
thereby triggering the PRE-REGISTRATION handoff, compared to the time
at which the actual L2 handoff occurs is important for the optimal
performance of the low-latency handoff. That is, in the optimal
case, the L2 trigger will be received and the four messaging steps of
PRE-REGISTRATION described above will be completed (i.e., up to when
the Registration Request is processed by HA or GFA) before the MN
moves. Optimally, the Registration Reply and the first packet
redirected by the HA (or GFA) to nFA will reach the MN at the moment
in which the MN's L2 link to nFA is fully established. The MN would
therefore not suffer any disruption due to the L3 handoff. This
cannot always be guaranteed unless particular implementation
techniques are used. To alleviate a part of this timing problem, the
MN MAY set the S bit [1] in low-latency Registration Requests sent by
the MN. This allows the MN to receive packets at both oFA and nFA
during the short layer 2 handoff time. Other techniques may be
required, such as L2 techniques or buffering, but these are outside
the scope of this document. In addition, further handoff smoothing
considerations may be required to prevent the loss of packets in-
flight between HA (or GFA) and oFA while the MN performs a PRE-
REGISTRATION handoff. These are also outside the scope of this
document.
Figures 2, 3, and 4 contain message timing diagrams for the network-
initiated and mobile-initiated PRE-REGISTRATION handoff procedures.
3.2. Network-Initiated Handoff
As described in Table 1, a PRE-REGISTRATION handoff can be initiated
at oFA by a source trigger or at nFA by a target trigger. Figures 2
and 3 contain message timing diagrams for PRE-REGISTRATION network-
initiated handoff for source and target triggers.
A source-triggered, network-initiated handoff occurs when an L2
trigger is received at the oFA informing it of a certain MN's
upcoming movement from oFA to nFA. The L2 trigger contains
information including the MN's identifier (i.e., the IPv4 address
itself or an identifier that can be resolved to the IPv4 address) and
the nFA's identifier. An identifier may be an IPv4 address or
something specific to the wireless technology (e.g., Base Station or
Access Point Identifier). A target-triggered, network-initiated
El Malki, Ed. Experimental PAGE 13
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
handoff occurs when an L2 trigger is received at the nFA informing it
of a certain MN's upcoming movement from oFA. This type of trigger
is also shown in Table 1 and contains information including the MN's
and the oFA's identifier.
MN oFA nFA HA/GFA
| |<~~~~~~ L2-Source | |
| | Trigger | |
|<--------------------| | |
| PrRtAdv | | |
| | | |
|---------------------------------------->| |
| RegReq or | | |
| RegRegReq (routed via oFA) |------------------->|
| | RegReq or RegRegReq|
| | |
| Buffered ~~~~~>|<-------------------|
|---------------------------------------->| (Reg)RegReply |
| Agent Solicitation | |
| (sent when MN connects to nFA) | |
| | |
|<----------------------------------------| |
| (Reg)RegReply | |
| (sent when nFA receives Solicitation or L2-LU) |
Figure 2 - PRE-REGISTRATION Handoff Message Timing Diagram
(Network-Initiated, Source Trigger)
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 Section 9. 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.
El Malki, Ed. Experimental PAGE 14
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
MN oFA nFA HA/GFA
| | L2-Target~~~~~~~~>| |
| | Trigger | |
| |...................| |
|<--------------------------------------- | |
| (PrRtAdv) |...................| |
| | Tunneled Agent Advertisement |
| | | |
|---------------------------------------->| |
| RegReq. or | | |
| RegRegReq (routed via oFA) |------------------->|
| | RegReq or RegRegReq|
| | |
| Buffered ~~~~~>|<-------------------|
|---------------------------------------->| (Reg)RegReply |
| Agent Solicitation | |
| (sent when MN connects to nFA) | |
| | |
|<----------------------------------------| |
| (Reg)RegReply | |
| (sent when nFA receives Solicitation or L2-LU) |
Figure 3 - PRE-REGISTRATION Handoff Message Timing Diagram
(Network-Initiated, Target Trigger)
In a target-triggered handoff, when nFA receives the trigger (L2-TT),
it MUST tunnel an Agent Advertisement to the MN through oFA to
initiate the L3 handoff. The inner advertisement is unicast by nFA
to MN, thus nFA treats the target trigger as a Router (Agent)
Solicitation. This advertisement is tunneled to oFA, which functions
as a normal router, decapsulating the advertisement and forwarding it
to the MN. This message MUST be authenticated to prevent attacks
(see Section 3.4.2).
3.3. Mobile-Initiated Handoff
As shown in Table 1, a mobile-initiated handoff occurs when an L2
trigger is received at the MN informing that it will shortly move to
nFA. The L2 trigger contains information such as the nFA's
identifier (i.e., nFA's IPv4 address or an identifier that can be
resolved to the nFA's IPv4 address). As an example, a Wireless LAN
MN may perform a scan to obtain the Base Station Identifier (BSSID)
of the access point that is a potential handoff target (i.e., its
signal is becoming stronger). The message timing diagram is shown in
Figure 4.
El Malki, Ed. Experimental PAGE 15
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
MN oFA nFA HA/GFA
|<~~~~~ L2-Trigger | | |
| | | |
|-------------------->| | |
| PrRtSol | | |
| | | |
|<--------------------| | |
| PrRtAdv | | |
| | | |
|---------------------------------------->| |
| RegReq or | | |
| RegRegReq (routed via oFA) |------------------->|
| | RegReq or RegRegReq|
| | |
| Buffered ~~~~~>|<-------------------|
|---------------------------------------->| (Reg)RegReply |
| Agent Solicitation | |
| (sent when MN connects to nFA) | |
| | |
|<----------------------------------------| |
| (Reg)RegReply | |
| (sent when nFA receives Solicitation or L2-LU) |
Figure 4 - PRE-REGISTRATION Handoff Message Timing Diagram
(Mobile-Initiated)
As a consequence of the L2 trigger (L2-MT), the MN MUST send message
1a, the Proxy Router Solicitation (PrRtSol). This message is a
unicast Agent Solicitation to oFA for a Proxy Router Advertisement
(PrRtAdv). This solicitation MUST have a TTL=1 as in [1]. The Proxy
Router Advertisement Solicitation unicast to oFA is an Agent
Solicitation with a special extension. The solicitation MUST have an
extension containing an FA identifier (i.e., IPv4 address or L2
address contained in an LLA extension, see Section 9) because the MN
is soliciting another specific FA's advertisement from the oFA. This
specific FA will be the MN's nFA. The identifier is the IPv4 address
of the nFA or another identifier that can be used by the oFA to
resolve to nFA's IPv4 address. If the identifier is not an IPv4
address, it MAY be specific to the underlying wireless technology,
for example, an access point or Base Station Identifier (e.g., WLAN
BSSID) that can be mapped by oFA to the nFA IPv4 address as described
in Section 3.4.1. The extension containing the identifier is a sub-
type of the Generalized Link Layer Address Extension described in
Section 9.
Two extension sub-types have been defined to contain the nFA's IPv4
address and an access point identifier. They are called the
Solicited Agent IPv4 Address Extension and the Access Point
El Malki, Ed. Experimental PAGE 16
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
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 Section
9.3). This is further described in Section 3.7.
3.4. Obtaining and Proxying nFA Advertisements
Since L2 triggers are involved in initiating PRE-REGISTRATION
handoff, the trigger timing SHOULD be arranged such that a full L3
PRE-REGISTRATION handoff can complete before the L2 handoff process
completes. That is, the L2 handoff should be completed after the
MN's registration with the nFA is performed (message 3 in Figure 1).
The registration MAY be transmitted in more than one copy (default
recommendation: 2) to reduce the probability that it is lost due to
errors on the wireless link. This would not apply to reliable
wireless links where retransmissions are performed at layer 2 in case
of error to guarantee packet delivery.
A PRE-REGISTRATION handoff in this case requires the MN to receive an
Agent Advertisement from the nFA through the old wireless access
point. How to achieve this is discussed in the following
subsections. Messages exchanged between FAs MUST be authenticated to
prevent impersonation attacks. The minimal requirement is that all
FAs involved in low-latency handoffs MUST support manual pre-
configuration of security associations with other neighboring FAs,
involving shared keys and the default algorithms of [1] (see the
Security Considerations of this document).
3.4.1. Inter-FA Solicitation
This applies to the network-initiated source-triggered (L2-ST) and
mobile-initiated (L2-MT) cases only. Inter-FA solicitation assumes
that oFA has access to the IPv4 address of the nFA. The IPv4 address
of nFA is obtained by means of an L2 trigger at oFA in the network-
initiated case (see Section 3.2) or by means of the extension to the
Proxy Router Solicitation (PrRtSol) from the MN in the mobile-
initiated case (see Section 3.3). This extension to the PrRtSol may
contain an IPv4 address or another identifier, for example, an
identifier of a Wireless Base Station such as the WLAN BSSID. In the
latter case, the oFA must implement a mechanism to resolve the Base
El Malki, Ed. Experimental PAGE 17
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
Station Identifier to an IPv4 address. The default mechanism is to
use a configured table of neighboring Base Station Identifiers (e.g.,
BSSID) to FA IPv4 address mappings in each FA. Other automated
discovery mechanisms may also be used.
If oFA does not cache advertisements (see Section 3.5) once it
receives an L2 trigger and obtains the address of the nFA for a
specific MN, it MUST send a unicast Agent Solicitation (PrRtSol) to
nFA. The nFA replies to the oFA by unicasting an Agent Advertisement
with appropriate extensions (PrRtAdv). This method removes the TTL
limitation of [1] for Mobile IPv4 messages (i.e., TTL=1 is not
applicable here). The TTL limitation cannot be applied since oFA and
nFA may be more than one hop away and since it is unnecessary for a
secured unicast message. The ICMP solicitations and advertisements
MUST be authenticated and integrity protected. These messages MUST
be protected using Encapsulating Security Payload (ESP) [10] to
prevent attacks (see the Security Considerations section of this
document). An FA MUST NOT accept ICMP solicitations or
advertisements from sources that are not authenticated.
As a practical matter, oFA SHOULD pre-solicit and cache
advertisements from known neighboring FAs (see section 3.5) to avoid
performing the solicitation during an actual handoff procedure.
3.4.2. Tunneled nFA Advertisements
This applies to the network-initiated target-triggered (L2-TT) case
only. Following a target trigger (L2-TT) the nFA MUST send a
tunneled Agent Advertisement to the MN through oFA. Tunneling nFA
advertisements assumes that the nFA is aware of the IPv4 address for
oFA and the MN. These IPv4 addresses are obtained by means of the FA
and MN identifiers contained in an L2 trigger received at nFA in the
network-initiated case (see Section 3.2). However, in [1] the TTL
must be 1 on Agent Advertisements from the nFA. Therefore, tunneling
advertisements is applicable if the TTL limitation of [1] is relaxed.
For this purpose, a pre-established security association between oFA
and nFA MUST be in place to authenticate this message and relax the
TTL limitation. If the implementation requires this, a tunnel SHOULD
be configured when the inter-FA security association is established.
The tunneled ICMP advertisement MUST be secured using tunnel mode ESP
[10] between nFA and oFA. An FA MUST NOT accept tunneled ICMP
packets destined to it from sources that are not authenticated.
El Malki, Ed. Experimental PAGE 18
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
3.5. Caching Router Advertisements
In the mobile-initiated (L2-MT) case and the network-initiated
source-triggered (L2-ST) case, the message exchange 1 in Figure 1
could impose an additional latency on the L3 handoff process if done
as part of the handoff procedure. In order to remove this source of
latency, the inter-FA Router (Agent) Solicitation and Advertisement
exchange SHOULD be performed in advance of handoff. A process SHOULD
be in place at the oFA to solicit its neighboring 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 second. If the FA Challenge/Response
mechanism in [7] is used, then the MIN_SOLICITATION_INTERVAL MUST be
set to a value smaller then the window of time in which a challenge
remains valid so that the nFA challenge does not expire before the MN
issues the Registration Request. Therefore, the recommended default
value for the MIN_SOLICITATION_INTERVAL in oFA is (0.5 * nFA's
CHALLENGE_WINDOW * nFA's Agent Advertisement interval). The
CHALLENGE_WINDOW and Agent Advertisement interval are defined in [7]
and [1] respectively. The minimum requirement is that the
MIN_SOLICITATION_INTERVAL MUST be manually configurable, while
possible autoconfiguration mechanisms are outside the scope of this
document. To allow advertisement caching in certain implementations
and in cases where the nFA advertisement interval is very small, it
MAY be necessary for the implementation in nFA to allow different
CHALLENGE_WINDOW and Agent Advertisement interval settings for its
nFA-oFA interface.
The oFA SHOULD cache the most recent advertisement from its
neighboring 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 neighboring nFAs. The minimum requirement
for each FA is that it SHOULD allow manual configuration of a list of
nFA addresses that an MN could possibly perform an L3 handoff to.
The FA addresses in this list will depend on deployment and radio
coverage. It is also possible to specify another protocol to achieve
nFA discovery, but this is outside the scope of this document.
3.6. Movement Detection, MN, and FA Considerations
When the MN receives an Agent Advertisement with a Mobility Agent
extension, it performs actions according to the following movement
detection mechanism: the MN SHOULD be "Eager" to perform new
bindings. This means that the MN SHOULD perform registrations with
any new FA from which it receives an advertisement (i.e., MN is
Eager), as long as there are no locally-defined policies in the MN
that discourage the use of the discovered FA. For example, the MN
El Malki, Ed. Experimental PAGE 19
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
could have a policy based on the cost of service. The method by
which the MN determines whether the FA is a new FA is described in
[1] and MAY use an FA-NAI extension [11]. By being "Eager" to
perform registrations, the MN reduces latency times.
The MN also needs to change its default router from oFA to nFA. The
MN MUST change its default router to nFA as soon as the PRE-
REGISTRATION procedure has completed (i.e., Registration Reply is
received by MN) as described in [1].
Overall, the MN behaves as described in [1] with the following
changes: the specified movement detection mechanism mentioned above
and the ability to use the L2-MT to initiate an Agent Solicitation
with a special extension (PrRtSol). Also, when the MN receives an
L2-LU trigger (i.e., new interface or link is up), it MUST
immediately send an Agent Solicitation [1] on that interface. An nFA
that receives an Agent Solicitation [1] will use it as an L2-LU
trigger event, and according to [1] it will record the MN's
IPv4/layer 2 addresses (i.e., the Address Resolution Protocol (ARP)
entry). At that point, the nFA starts delivering data to the MN
including the previously buffered Registration Reply. The nFA MAY
also use other L2 mechanisms to detect earlier that the MN has
attached to the new link and to start forwarding data to it. The MN
SHOULD NOT attempt to retransmit a low-latency Registration Request
(i.e., Registration Request containing an LLA extension described in
Section 9.) when it does not receive the Registration Reply.
When moving from a PRE-REGISTRATION network to a normal Mobile IPv4
[1] network, the MN will no longer receive PrRtAdv messages (i.e.,
Agent Advertisements with the LLA extension). If the MN still
receives L2-MTs, it will attempt to send PrRtSol messages. The
normal FA will reply with a normal Agent Advertisement [1]. If the
MN does not receive a PrRtAdv in reply to its PrRtSol, it MAY
retransmit the PrRtSol message once after PRE_SOL_INTERVAL seconds
and then for another PRE_SOL_ATTEMPTS times with exponential backoff
of the transmission interval. If a PrRtAdv is not received within
PRE_SOL_INTERVAL seconds after the last PrRtSol attempt, the MN MUST
stop sending PrRtSol messages until after a registration with a new
FA is performed. The default value for PRE_SOL_ATTEMPTS is 2, and
for PRE_SOL_INTERVAL, it is 1 second. It should be noted that the
performance of the movement detection mechanism mandated in PRE-
REGISTRATION (i.e., eager to register) may have sub-optimal behaviour
in a standard Mobile IPv4 [1] network. Therefore, standard movement
detection mechanisms [1] should be used in plain Mobile IPv4
networks. Instead, when the MN moves from a normal Mobile IPv4 [1]
network to a PRE-REGISTRATION network, the MN starts receiving L2-MT
triggers or PrRtAdv messages. When the MN receives L2-MT triggers or
PrRtAdv messages, it SHOULD follow the PRE-REGISTRATION procedure.
El Malki, Ed. Experimental PAGE 20
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
If there is uncertainty as to which mode to choose (e.g., MN receives
messages from both PRE-REGISTRATION and normal FAs), the MN decides
based on its registration status with the current FA. If the MN
already has a valid normal Mobile IPv4 Registration [1] with the
advertising FA, it SHOULD give priority to the PRE-REGISTRATION
procedure. Otherwise it SHOULD give priority to normal Mobile IPv4
[1] Registration procedure. The MN SHOULD NOT attempt to perform
PRE-REGISTRATION and standard Mobile IPv4 [1] Registrations in
parallel.
3.7. L2 Address Considerations
Some special considerations should be taken with respect to the
wireless system on which this handoff method is being implemented.
Consider an Ethernet-like system such as IEEE 802.11, for example.
In PRE-REGISTRATION, the MN is registering with an FA (nFA) that is
not its current first-hop router; therefore, the L2 address of the
Ethernet frame containing the MN's Registration Request reaching the
nFA is not the MN's address. Therefore, the FA MUST NOT use the
Ethernet address of the incoming Registration Request as the MN's L2
address as specified in [1]. This applies to the cases where the
wireless access points are bridges or routers and independently of
whether the FA is implemented in the wireless access points
themselves. In this case, the MN's Registration Request (or Regional
Registration Request) MUST use an L2 address extension to the
registration message. Such an L2 address is either the same L2
address that remains constant as the MN moves, or it is the MN's L2
address at nFA. To communicate its L2 address, the MN includes a
Generalized Link Layer and IP Address Extension (see Section 9) with
its Registration Request (or Regional Registration Request) message.
If this extension is present, the FA MUST use the L2 address
contained in the extension to communicate with the MN. If a
particular wireless L2 technology has defined a special interface to
the wireless network that allows the FA to resolve the mapping
between an MN's IPv4 address and its L2 address without the need to
use the extension, the L2 address extension contents may be
discarded. For the same reasons above, the MN MUST NOT use the
source L2 address of the Agent Advertisement message (PrRtAdv) as its
default router's L2 address. Therefore, the nFA MUST include a
Generalized Link Layer and IP Address Extension (see Section 9.3)
with its Agent Advertisement (PrRtAdv) messages.
3.8. Applicability of PRE-REGISTRATION Handoff
The PRE-REGISTRATION 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
El Malki, Ed. Experimental PAGE 21
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
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 is able to obtain an L2 trigger
informing it of a pending L2 handoff procedure. The target of the L2
handoff is another access point or radio network that is in the
coverage area of a new FA. The L2 trigger information may be in the
form of identifiers that need to be resolved to IPv4 addresses using
methods that may be specific to the wireless network and are not
considered here. If, for example, the oFA or MN determines that the
IPv4 address of the new FA matches oFA's address, then the PRE-
REGISTRATION handoff SHOULD NOT be initiated.
The L2 trigger must allow enough time for the PRE-REGISTRATION
handoff procedure to be performed. In many wireless L2 technologies,
the L2 handoff procedure involves a number of message exchanges
before the effective L2 handoff is performed. For such technologies,
PRE-REGISTRATION handoff can be initiated at the beginning of the L2
handoff procedure and completed before the L2 handoff is completed.
It is efficient to engineer the network such that this succession of
events is ensured.
The PRE-REGISTRATION handoff method is applicable in the following
cases:
- when the MN has locally defined policies that determine a
preference for one access over another, for example, due to
service cost within the same or different technology, and
therefore where it is necessary to allow the MN to select the
appropriate FA with which to connect.
- when L2 security between the MN and the FA is either not present
or cannot be relied upon to provide adequate security.
- when the trigger to initiate the handoff is received at the MN.
In the first case, it is necessary to involve eventual local MN
policies in the movement detection procedure as described in Section
3.6.
El Malki, Ed. Experimental PAGE 22
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
4. The POST-REGISTRATION Handoff Method
The POST-REGISTRATION handoff method uses bidirectional edge tunnels
(BETs) or unidirectional tunnels to perform low-latency change in the
L2 point of attachment for the MN without requiring any involvement
by the MN. Figure 5 illustrates the basic POST-REGISTRATION handoff.
+------+
| CN |
+------+
|
...
|
+------+ BET +------+
| aFA |==========| nFA |
+------+ +------+
| wireless link
|
movement +------+
---------> | MN |
+------+
Figure 5 - POST-REGISTRATION Concept
Following a successful Mobile IPv4 Registration between MN and oFA,
the oFA becomes the mobility anchor point for the MN, called the
anchor FA (aFA). When the MN moves from oFA to nFA, rather than
performing signaling over the wireless link to register with the nFA,
the MN can defer the L3 handoff and continue to use its aFA (i.e.,
oFA in this case). If the MN moves to a third FA before registering
with the nFA, in certain cases described later, the third FA signals
aFA to move the wireless link end of the BET from nFA to it. The
network end of the BET remains anchored at aFA until the MN performs
the Mobile IPv4 Registration.
Messages between oFA/aFA and nFA MUST be authenticated. The minimal
requirement is that all FAs involved in low-latency handoffs MUST
support manual pre-configuration of security associations with other
neighboring 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.
El Malki, Ed. Experimental PAGE 23
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
4.1. Two-Party Handoff
Two-party handoff occurs when the MN moves from oFA to nFA.
Normally, this movement would result in a new Mobile IPv4
Registration at nFA. However, in POST-REGISTRATION, the MN and nFA
MAY delay this but maintain connectivity using the BET (or
alternatively unidirectional tunnel) between oFA and nFA. The
protocol is shown in Figure 6.
1a) L2-ST ~~~~> +------+ 2) HRqst +------+ <~~~ 1b) L2-TT
| oFA |<-------->| nFA |
4a) L2-LD~> +------+ 3) HRply +------+ <~~~ 4b) L2-LU
^ ^
old L2 | | new L2
+-------+ +-----+
| |
| |
V V
+------+ movement
4c) L2-LU ---> | MN | --------->
+------+
Figure 6 - Two-Party Handoff (POST-REGISTRATION)
The following describes the progress of a two-party handoff. The
numbered items refer to steps in Figure 6. The source-triggered
HRqst/HRply message for tunnel creation, the target-triggered
HRqst/HRply message for tunnel creation, and the HRqst/HRply to
extend or terminate a BET (or unidirectional tunnel) are identified
using the suffixes (s), (t), and (r), respectively.
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 identifier for
the nFA (the IPv4 address itself or an L2 address that can
be resolved to the IPv4 address of the nFA).
b) The L2 trigger is a target trigger (L2-TT) at nFA. The
trigger contains the MN's L2 address and an identifier for
the oFA (the IPv4 address itself or an L2 address that can
be resolved to the IPv4 address of the oFA).
2) The FA receiving the trigger sends a Handoff Request (HRqst) to
the other FA. There are two cases:
El Malki, Ed. Experimental PAGE 24
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
a) If oFA is sending the HRqst, the H bit is set and the N bit
is unset, indicating it is an HRqst(s). The HRqst(s)
contains the lifetime of the tunnel the oFA is willing to
support, the MN's IPv4 home address, the MN's HA address,
and an LLA option with the MN's L2 address. If the lifetime
is zero and the T bit is not set, the oFA is not willing to
tunnel any packets for MN. A positive lifetime and a set T
bit indicate that the oFA is willing to tunnel for the
indicated time. Section 4.5 describes the HRqst(s) and
Section 9 describes the LLA option.
b) If nFA is sending the HRqst, the N bit is set and the H bit
is unset, indicating that it is an HRqst(t). If the T bit
is set, nFA has requested a reverse tunnel and the HRqst(t)
contains the lifetime of the tunnel the nFA is requesting.
The HRqst(t) also contains an LLA option with the MN's L2
address. The MN's IPv4 home address and HA address are not
sent, unless they are discovered by some means outside the
scope of this document (for example, as part of the L2-TT).
Section 4.5 describes the HRqst(t).
3) The FA receiving the HRqst sends a Handoff Reply (HRply) to the
other FA. There are two cases:
a) If oFA is sending the HRply, the N bit is set and the H and
R bits are unset, indicating that the reply is in response
to a HRqst(t), i.e., it is an HRply(t). If the T bit is
set, the HRply(t) contains the tunnel lifetime the oFA is
willing to provide; otherwise, the tunnel lifetime is set to
zero indicating that the oFA is not willing to provide
tunnel service. If both HRply(t) and HRqst(t) have the T
bit set and non-zero lifetime, a BET is established. The
HRply(t) also contains the MN's home subnet IPv4 address,
the MN's HA address, and an LLA option containing the MN's
L2 address. Section 4.6 describes the HRply(t).
b) If nFA sends the HRply, the H bit is set and the N and R
bits are unset, indicating that this is a response to
HRqst(s), i.e., it is an HRply(s). If the T bit is set, the
nFA indicates that it requests a reverse tunnel, and the
lifetime field is set with the requested tunnel lifetime.
The T bit can be set in HRply only if the oFA had set the T
bit in the corresponding HRqst or if the nFA is required to
reverse tunnel incoming packets to oFA because ingress
filtering is enabled on its network. This establishes a
BET. The tunnel lifetime requested by the nFA must be less
than or equal to the tunnel lifetime offered by oFA in the
HRqst(s). Section 4.6 describes the HRply(s).
El Malki, Ed. Experimental PAGE 25
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
4) The point during the L2 handoff in which the MN is no longer
connected on a given link is signaled by an L2-LD trigger at
oFA and MN. Completion of L2 handoff is signaled by an L2-LU
trigger at nFA and MN. The trigger is handled as follows:
a) When oFA receives the L2-LD trigger, it begins forwarding
MN-bound packets through the forward tunnel to nFA.
b) When the nFA receives the L2-LU trigger, it begins
delivering packets tunneled from oFA to MN and forwards
outbound packets from MN using normal routing mechanisms or
through a reverse tunnel to oFA or HA. The nFA at this
point may not yet be the default router of the MN (see
Section 4.4); therefore, to receive all outbound packets
from the MN the nFA must send a unicast proxy ARP message
(used in [1]) to the MN upon receiving an L2-LU trigger.
This proxy ARP message is an ARP Reply [5] sent by the nFA
on behalf of oFA, therefore supplying the nFA link-layer
address in the Sender Hardware Address field and the oFA
IPv4 address in the Target Protocol Address field.
c) When the MN receives the L2-LU, it MAY initiate the Mobile
IPv4 Registration process by soliciting an Agent
Advertisement as described in [1]. If the registration is
successful, the nFA takes over the role of anchor FA (aFA)
from the oFA. Alternatively, the MN MAY defer the Mobile
IPv4 Registration (see Section 4.4).
5) The oFA becomes an aFA if the MN moves to a third FA before
having performed a Mobile IPv4 Registration with nFA.
6) Should L2 handoff fail in Step 4 (due to L2 reasons) and a
ping-pong situation arise, the oFA may be able to determine
this case through the trigger mechanism (i.e., FA sees
successive L2-ST/L2-TT followed by L2-LD and then L2-LU). The
FA that originated the HRqst can in this case cancel the tunnel
by sending an HRqst(r) to the other FA with lifetime zero. It
will then simply continue delivering packets to MN exactly as
if no handoff had been pending. Section 4.5 describes the
HRqst(r).
If the oFA sets the B bit in HRqst/HRply and the nFA has not
requested a reverse tunnel by setting the T bit, the nFA SHOULD
tunnel outgoing packets from the MN to the HA because the MN has
requested this service from the oFA. The nFA SHOULD offer this
service only if no security between the nFA and the MN's HA is
required, or if there is an existing nFA-HA security association.
El Malki, Ed. Experimental PAGE 26
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
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 set up the BET immediately when the L2 handoff is
initiated, start tunneling MN-bound data when the link to the MN goes
down, and the nFA can use the link-up trigger to start delivering
packets. In the absence of optimal L2 trigger information, the HRply
can act as the trigger to start tunneling MN-bound data, but in this
case, the period of packet delivery disruption to the MN could still
be present and additional measures may be required to provide
uninterrupted service. Particular implementation and deployment
scenarios could require techniques to smooth the handoff by providing
a means to convey packets arriving during the L2 handoff. The exact
techniques are outside the scope of this document.
Figures 7 and 8 show timing diagrams for source trigger (L2-ST) and
target trigger (L2-TT) two-party handoffs, 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
El Malki, Ed. Experimental PAGE 27
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
MN nFA oFA
| | |
| L2-TT ~~~>| HRqst(t) |
| |------------------>|
| | HRply(t) |
| |<------------------|
| | |
--------------------------------------------<~~~ L2-LD
L2 Handoff
--------------------------------------------<~~~ L2-LU
| | |
|<------------------->| |
| MN's traffic | |
Figure 8 - Two-Party Target Trigger Handoff Timing
Once the tunnel between aFA and the current FA is in place, it is
torn down by one of the following events:
1) The aFA decides to stop tunneling because the lifetime it sent
expires and was not renewed, or the aFA or current FA decide to
terminate tunnel service prematurely for some other reason
(refer to Section 4.3).
2) The MN completes the process by performing a Mobile IPv4
Registration with the current FA. This may be initiated by the
FA that sends an Agent Advertisement or by the MN that solicits
for an Agent Advertisement as in [1].
3) The MN moves to a third FA (see Section 4.2)
4.2. Three-Party Handoff
Three-party handoff is applicable when an MN, which has already
established an aFA and is receiving tunneled packets through its
current FA, moves to a new FA without performing a Mobile IPv4
Registration.
The need for the three-party handoff function depends on the wireless
system in which POST-REGISTRATION is being implemented. For radio L2
protocols in which it is possible for the MN to move so rapidly from
one FA to another such that a probability exists that the Mobile IPv4
Registration with nFA will not complete before the MN moves on, HTT
(Handoff to Third) SHOULD be implemented. Certain wireless systems
and implementations do not allow such fast movement between FAs and
may force the Mobile IPv4 Registration to occur soon after L2
handoff, in which case three-party handoff is not applicable. If
this three-party handoff feature is not implemented, the FA SHOULD
El Malki, Ed. Experimental PAGE 28
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
send an Agent Advertisement to the MN after L2 handoff has completed
(L2-LU at nFA) and/or the MN SHOULD solicit an Agent Advertisement
after L2 handoff (L2-LU at MN).
+------+
+--->| aFA |<---+
| +------+ |
4b) HRqst(r) | | 3) HRqst(t)
HRply(r) | | HRply(t)
| |
v 2a) HRqst v
1a) L2-ST ~~~> +------+ HTT +------+ <~~~ 1b) L2-TT
| oFA |<-------->| nFA |
4a) L2-LD ~~~> +------+ 2b) HTT +------+ <~~~ 5a) L2-LU
^ HRply ^
old L2 | | new L2
+-------+ +-----+
| |
| |
V V
+------+ movement
5b) L2-LU ~~~> | MN | --------->
+------+
Figure 9 - Three-Party Handoff
The L3 handoff can be deferred either because of a decision by the
MN/FA (i.e., MN does not send Agent Solicitations and FA does not
send Agent Advertisements), or it may result from rapid movement
between oFA and nFA that does not allow enough time for the
registration to complete. This scenario is shown in Figure 9. In
this case, oFA must inform nFA (i.e., the third FA) to contact aFA
about moving the radio end of the tunnel. This is performed with the
HTT message. The general idea behind the three-party handoff
procedure is that the oFA supplies nFA with the same information it
would have obtained via an L2-TT if handoff had occurred from aFA to
nFA; then, the nFA performs an HRqst(t)/HRply(t) sequence with aFA in
order to move the BET to nFA. When the L2 handoff is complete, oFA
sends an HRqst(r) to aFA to terminate the previous BET.
The following describes the progress of a three-party handoff. The
numbered items refer to steps in Figure 9.
1) Either the oFA or nFA receives an L2 trigger informing it that
a certain MN is about to be moved. The two cases are:
El Malki, Ed. Experimental PAGE 29
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
a) The L2 trigger is a source trigger (L2-ST) at oFA. The
trigger contains the MN's L2 address and an identifier for
the nFA (the IPv4 address itself or an L2 address that can
be mapped to the IPv4 address of the nFA).
b) The L2 trigger is a target trigger (L2-TT) at nFA. The
trigger contains the MN's L2 address and an identifier for
the oFA (the IPv4 address itself or an L2 address that can
be resolved to the IPv4 address of the oFA).
2) The oFA and nFA exchange an HTT/HRply or HRqst/HTT pair. HTT
is indicated by setting both the H and N bits in the HRqst or
HRply. The HTT message MUST NOT have any tunnel flag bits set,
because the tunnel is negotiated between the aFA and nFA, not
oFA and nFA. There are two cases:
a) The L2 trigger is an L2-ST. The oFA sends HTT to nFA
containing the MN's home IPv4 address, the MN's HA address,
an LLA containing the aFA's IPv4 address, and an LLA
containing the L2 address of the MN. This is enough
information for nFA to perform a target-triggered handoff
with aFA. The nFA responds with an HRply(s). Section 4.7
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 an
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 under way 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.
El Malki, Ed. Experimental PAGE 30
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
b) The oFA exchanges an 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 IPv4 Registration
when it receives the L2-LU, as in Section 4.1.
6) In the special case where nFA and aFA are the same (i.e., the
MN is moving back to the original anchor FA), aFA recognizes
that it is tunneling to oFA when it checks its Visitor Cache in
Step 3. In this case, there is no need for aFA to send the
HRqst(t)/HRply(t) in Step 3. Upon receipt of the L2-LU trigger
on handoff completion, the aFA begins routing packets to MN and
the tunnel to nFA is torn down. The oFA still exchanges the
HRqst(r)/HRply(r) with aFA in Step 4b because oFA cannot know a
priori that aFA and nFA are the same, but they are redundant.
El Malki, Ed. Experimental PAGE 31
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
Figures 10 and 11 show timing diagrams for source trigger (L2-ST) and
target trigger (L2-TT) three-party handoff, respectively.
MN nFA oFA aFA
| | L2-ST ~~~> | |
| | | |
| |<-------------| |
| | HTT | |
| |------------->| |
| | HRply(s) | |
| |------------------------------>|
| | HRqst(t) | |
| |<------------------------------|
| | HRply(t) | |
| | | |
----------------------------------<~~~ L2-LD |
|--------------->|
L2 Handoff | HRqst(r) |
| |
|<---------------|
| HRply(r) |
| |
----------------------------------<~~~ L2-LU |
| MN's traffic | | |
|<-------------->| | |
Figure 10 - Three-Party Source Trigger Handoff Timing
El Malki, Ed. Experimental PAGE 32
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
MN nFA oFA aFA
| | | |
| |<~~~ L2-TT | |
| |------------->| |
| | HRqst(t) | |
| |<-------------| |
| | HTT | |
| |------------------------------>|
| | HRqst(t) | |
| |<------------------------------|
| | HRply(t) | |
| | | |
----------------------------------<~~~ L2-LD |
|--------------->|
L2 Handoff | HRqst(r) |
| |
|<---------------|
| HRply(r) |
| |
----------------------------------<~~~ L2-LU |
| MN's traffic | | |
|<-------------->| | |
Figure 11 - Three-Party Target Trigger Handoff Timing
Unlike two-party handoff, the timing of BET establishment between aFA
and nFA cannot fully depend on the availability of L2 trigger
information because aFA does not receive an L2 trigger signaling 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 that 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
sometime 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
El Malki, Ed. Experimental PAGE 33
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
be influenced by network engineering and implementation
considerations, including whether a handoff smoothing solution is
used, and is beyond the scope of this specification.
4.3. Renewal or Termination of Tunneling Service
To prevent a BET from expiring when its lifetime runs out, the MN's
current FA signals the aFA to either renew or terminate the BET.
This may be the case when the MN defers Mobile IPv4 Registration. If
no such signal is received, the aFA will terminate the BET when the
lifetime expires. In addition, the current FA or aFA may need to
terminate the BET prior to the lifetime expiring. In order to avoid
error conditions in which tunnels do not expire even though the MN to
which they apply is no longer reachable, FAs SHOULD set the tunnel
lifetime field to some value other that 0xffff, which indicates "good
until canceled".
Figure 12 illustrates the message exchange that occurs between the FA
needing to terminate or extend the tunnel (designated FA(1) in the
figure) and the other FA (designated FA(2) in the figure). The
HRqst(r)/HRply(r) is indicated by setting the R bit in the
HRqst/HRply messages. If the HRqst(r) is renewing a BET, then it
contains a non-zero lifetime; otherwise, if the lifetime is set to
zero, it indicates tunnel termination. The aFA has complete control
over whether a tunnel is extended or terminated, and it MAY reply to
a request for extension with a shorter lifetime than was requested.
HRqst(r)
+------+ <-------- +------+
| FA(2)| ---------> | FA(1)|
+------+ HRply(r) +------+
Figure 12 - BET Renewal or Termination
4.4. When Will the MN Perform a Mobile IPv4 Registration?
The MN/FA have control over when to perform the Mobile IPv4
Registration. Although the MN/FA may decide to defer Mobile IPv4
Registration for a certain period, three possible events can lead to
the need to terminate tunneling service. If this occurs, the MN MUST
perform the Mobile IPv4 Registration. These events are:
1) The end of life for the BET is pending and a request by the
current FA to aFA for renewal has been denied, or alternatively
the current FA or aFA needs to terminate the BET prematurely.
The FA in this case MUST initiate the Mobile IPv4 Registration
by sending an Agent Advertisement to the MN as in [1].
El Malki, Ed. Experimental PAGE 34
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
2) The MN itself decides to perform a Mobile IPv4 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 IPv4 Registration by sending
the MN an Agent Advertisement as in [1]. Note that this
situation will never arise during target-triggered handoff
because an HRqst(t) will not be sent to oFA by an nFA that
doesn't support POST-REGISTRATION.
Some detailed scenarios relating to case 2) will be described
hereafter. According to [1], when using an FA care-of address, the
MN MAY use the FA as its default router. Otherwise, it MUST choose
its default router from those advertised in the ICMP Router
Advertisement portion of the Agent Advertisement. Here we assume
that the FA router is also the MN's default router. In POST-
REGISTRATION, when a tunnel is established between oFA and nFA and
the MN has moved to nFA, the oFA MUST NOT send Agent Advertisements
to the MN. In this case, it is possible that the MN will not receive
Agent Advertisements for extended periods of time. According to [8],
hosts will remove default router entries if the lifetime of the
Router Advertisement expires and no further advertisements are
received. Note that the ICMP Router Advertisement lifetime is not
related to the Registration Lifetime in the Mobility Agent
Advertisement extension [1]. To avoid this disruption, the MN MUST
solicit the default router (i.e., FA) before the lifetime of its
active default router entry runs out, or alternatively, the FA MUST
advertise as soon as the MN-nFA link is up (L2-LU). This effectively
means that the MN will at most be able to defer Mobile IPv4
Registration for as long as the remaining lifetime of the active
default router, as configured in the ICMP Router Advertisements. The
MN MUST perform a Mobile IPv4 Registration [1] when it receives an
Agent Advertisement following a POST-REGISTRATION handoff.
El Malki, Ed. Experimental PAGE 35
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
4.5. Handoff Request (HRqst) Message Format
This is a new Mobile IPv4 message carried on UDP (destination port
434) [1]. The UDP header is followed by the fields below.
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 16 (Handoff Request)
H Source-triggered handoff request. When set and
the N bit is unset, indicates that the request
was the result of an L2-ST at oFA.
N Target triggered handoff request. When set and
the H bit is unset, indicates that the request
was the result of an L2-TT at nFA.
R Set if the request is an HRqst(r) (i.e., a
request to renew the tunnel, H and N bits must
be unset).
M The FA issuing the HRqst will use Minimal
Encapsulation as defined in [1,5] for the
tunnel.
G The FA issuing the HRqst will use Generic
Routing Encapsulation (GRE) [4] as defined in
[1,5] for the tunnel. Extensions of HRqst
containing GRE type and key Fields are outside
the scope of this document.
El Malki, Ed. Experimental PAGE 36
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
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 a reverse tunnel to the
HA if it will not be reverse tunneling to the
oFA.
Lifetime The lifetime of the tunnel in seconds. If this
is an HRqst(t), then the lifetime represents a
request by nFA for a reverse tunnel. If this is
an HRqst(s), then the lifetime represents the
maximum amount of time that oFA is willing to
maintain both forward and reverse tunnels. If
this is an HRqst(r), then the lifetime
represents a request for the amount of time to
renew the tunnel's lifetime. A value of 0 on an
HRqst(s) indicates that the oFA is unwilling to
grant tunnel service. A value of 0 on an
HRqst(t) indicates that the nFA does not require
reverse tunnel service. A value of 0 on an
HRqst(r) indicates that the tunnel should be
terminated. A value of 0xffff indicates
infinity.
MN Home Address For HRqst(s), the home address of the MN.
HA Addr For HRqst(s), the HA address of the mobile node.
Identification As defined in [1].
Extensions The message MUST include an LLA (see Section 9)
containing the MN's L2 address and an L2 address
that can be mapped to an IPv4 address for the
FA. This message MUST contain the FA-FA
Authentication Extension [11] that is used to
secure the HRqst message.
El Malki, Ed. Experimental PAGE 37
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
4.6. Handoff Reply (HRply) Message Format
This is a new Mobile IPv4 message carried on UDP (destination port
434) [1]. The UDP header is followed by the fields below.
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 17 (Handoff Reply)
Code A value indicating the result of the Handoff
Request. Only two codes are currently
supported, 0, indicating success, and 1,
indicating that the handoff cannot be performed.
The remaining values are for future use.
Lifetime The lifetime, in seconds, for which the
bidirectional tunnel for the MN will be
maintained. If this is an HRply(s), then the
lifetime represents a request by nFA, and it can
be any value up to the maximum value sent in the
HRqst(s). Larger values are assumed to default
to oFA's maximum. If this is an HRply(t), then
the lifetime represents the maximum amount of
time that the oFA will grant to the nFA. If
this is an 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
El Malki, Ed. Experimental PAGE 38
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
require service. A value of 0 on an HRply(r)
indicates that the tunnel lifetime will be
terminated. A value of 0xffff indicates an
infinite lifetime.
H Source-triggered handoff reply. When set and
the N bit is unset, indicates that the reply is
in response to an HRqst(s).
N Target-triggered handoff reply. When set and
the H bit is unset, indicates that the reply is
in response to an HRqst(t).
R Set if the reply is an HRply(r). Neither the H
nor the N bit are set.
M The FA issuing the HRqst will use Minimal
Encapsulation as defined in [1,5] for the
tunnel.
G The FA issuing the HRqst will use GRE [4]
Encapsulation as defined in [1,5] for the
tunnel. When this flag bit is set, the HRply
may require extensions containing the GRE type
and key fields, but they are outside the scope
of this document.
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 a reverse tunnel to the
HA if it will not be reverse tunneling to the
oFA. It can be set in HRply(t) only if the T
bit was unset in the corresponding HRqst(t).
MN Home Address For HRply(t), the home IPv4 address of the MN.
HA Addr For HRply(t), the HA IPv4 address of the MN.
Identification As defined in [1].
Extensions This Message MUST contain the FA-FA
Authentication Extension [11] that is used to
secure the HRply message.
El Malki, Ed. Experimental PAGE 39
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
4.7. Handoff to Third (HTT) Message Format
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 an L2-ST and is sent to
initiate a handoff, then, with the exception of the H and N bits, the
message has the same fields set and includes the same extensions as
an HRqst(s). If the HTT message is sent in response to an HRqst(t),
then, with the exception of the H and N bits, the message has the
same fields set and includes the same extensions as an HRply(t). The
tunnel bits MUST NOT be set in the HTT message because BET
construction is not negotiated between oFA and nFA; it is negotiated
between nFA and aFA in the ensuing HRqst(t)/HRply(t).
In addition, the HTT MUST contain the following extensions in the
specified order:
Solicited IPv4 Address Option: containing aFA's Address
LLA Option: containing the L2 address of the MN.
4.8. Applicability of POST-REGISTRATION Handoff Method
The POST-REGISTRATION handoff approach allows FAs to communicate
directly about a pending handoff, and does not require any IPv4-layer
messages to be sent to or from an 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 an MN is rapidly
moving out of a radio coverage area. Consequently, POST-REGISTRATION
is primarily of interest in handoff between FAs that support the same
radio access technology. Handoff between heterogeneous radio
technologies will, of necessity, require interaction between the MN
and the network, and so is not a domain of applicability for POST-
REGISTRATION.
Because a POST-REGISTRATION handoff is triggered by an unspecified
mechanism that informs the oFA or nFA that an L2 handoff is pending,
the POST-REGISTRATION approach is only applicable to networks where
such a mechanism is available. For example, an L2 may provide
indications of radio signal quality that cause the oFA or nFA to send
the POST-REGISTRATION handoff messages. Any such indications must
also provide each FA involved in the handoff with the identity of the
other, so that messages can be sent to the right place. This may
involve mapping L2 information onto FA IPv4 addresses. Also, the FAs
involved in a handoff must have pre-provisioned security arrangements
so that the POST-REGISTRATION messages can be authenticated. If a
handoff is to be completed as a result of the POST-REGISTRATION
El Malki, Ed. Experimental PAGE 40
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
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 the choice
requires user intervention and therefore is not on the critical
path for handoff.
5. Combined Handoff Method
The combined method uses both PRE-REGISTRATION and POST-REGISTRATION
handoff. If PRE-REGISTRATION does not complete prior to the
expiration of a timer on the nFA, the POST-REGISTRATION mechanism is
used to create the tunnel between oFA and nFA. This protects the MN
from delays caused by errors such as loss of the Mobile IPv4
Registration Reply message involved in PRE-REGISTRATION for the
mobile-initiated and network-initiated source-triggered cases. It
also protects the MN from delays caused by errors or the loss of any
of the Mobile IPv4 messages involved in PRE-REGISTRATION for the
network-initiated target-triggered case.
When the nFA receives a target trigger, it will follow the PRE-
REGISTRATION procedure. When the combined method is used, the nFA
MUST also start a timer when it receives a target trigger. The timer
should be set to a small value (default for target trigger case: 1
second).
According to PRE-REGISTRATION, the nFA will receive the Registration
Request from the MN. When the combined method is used, this
Registration Request sent by the MN MUST contain the IPv4 address of
the oFA in an FA IPv4 address LLA extension (see Section 9.7). This
same Registration Request message will contain multiple LLA
extensions, since it will also contain the MN's layer 2 address in an
LLA extension as described for PRE-REGISTRATION (see Sections 3.7 and
9). When the nFA has not started the handoff procedure using a
target trigger (i.e., mobile-initiated or network-initiated target-
triggered cases), the nFA MUST start a timer as soon as it receives
the low-latency Registration Request from the MN. This timer should
be set to a small value (default: 1 second).
El Malki, Ed. Experimental PAGE 41
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
In all cases, the timer MUST be reset when the Registration Reply
message is received by nFA. If the timer expires before the
Registration Reply is received, the nFA MUST initiate the POST-
REGISTRATION procedure. The nFA utilizes the oFA IPv4 address
(previously received in the extension to the Registration Request
message) as the destination of the POST-REGISTRATION HRqst message to
create the tunnel between nFA and oFA. The nFA MAY tear down this
tunnel when it receives and forwards a successful Registration Reply
for that MN.
6. Layer 2 and Layer 3 Handoff Timing Considerations
In the optimal cases considered in the PRE-REGISTRATION and POST-
REGISTRATION handoffs, it was assumed that a timely L2 trigger would
be received in such a way that packets could be delivered to the MN
via its nFA immediately upon connection. In this way, the MN does
not suffer disruption due to the L3 handoff. However, such precise
timing of the L2 trigger and handoff mechanism with respect to the
actual L2 handoff event will not be possible in all wireless systems
and may depend on particular implementation techniques. Therefore,
some uncertainty may exist at L3 as to exactly when the L2 connection
between the MN and the nFA becomes fully established and can be used
for L3 traffic. It is possible that in certain implementations
traffic will be re-routed too early or too late with respect to the
moment when the connection between the MN and the nFA becomes fully
established. The techniques that may solve this problem and allow
the MN to receive traffic independently of the timing of the L2
handoff event include buffering and simultaneous bindings (i.e.,
bicasting: setting the S bit [1] in Registration Requests). However,
these are optional and are not mandated.
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.
El Malki, Ed. Experimental PAGE 42
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
8. Handoff Signaling Failure Recovery
In general and to a greater extent in wireless networks, packets
carrying handoff signaling may be dropped or lost due to errors on
the link. In this section, we consider mechanisms for recovery from
handoff signaling failures.
8.1. PRE-REGISTRATION Signaling Failure Recovery
Failure of PRE-REGISTRATION signaling breaks down into three cases:
1) Loss of messages PrRtSol and PrRtAdv on the air link.
2) Loss of the solicitation by an FA to obtain another neighboring
FA's Advertisement or loss of the neighboring FA's
advertisement.
3) Failure of the standard Mobile IPv4 Registration.
Of these, case 3) is handled by standard Mobile IPv4 mechanisms
described in [1]. Case 2) is expected to be a rare event because
spontaneous packet drop rates on the fixed network are caused by
congestion or router failure. Since bit error rates on wireless
links are higher than on fixed links, case 1) is more likely to
occur. In the following subsections, cases 1) and 2) are considered.
8.1.1. Failure of PrRtSol and PrRtAdv
PRE-REGISTRATION handoff can fail in network-initiated handoff when
the PrRtAdv sent by oFA in response to the source trigger (L2-ST) or
the advertisement sent by nFA in response to the target trigger (L2-
TT) fails to reach the MN. PRE-REGISTRATION handoff can also fail in
mobile-initiated handoff when either the PrRtSol sent from the MN or
return PrRtAdv sent from the oFA is dropped. To reduce the
probability that PrRtAdv and PrRtSol 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. In this case, the MN solicits an FA
Advertisement when it connects to nFA at L2 (L2-LU), as described in
Section 3.6, and performs a standard Mobile IPv4 Registration with
the nFA as specified in [1].
El Malki, Ed. Experimental PAGE 43
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
8.1.2. Failure of Inter-FA Solicitation and Advertisement
The solicitation from an FA to another neighboring FA may fail or the
corresponding advertisement from the neighboring FA may be lost. To
reduce the probability that these messages are lost, the FAs MAY
transmit multiple copies of these messages. If a failure occurs
anyway, the FA soliciting the Agent Advertisement is unable to send a
PrRtAdv in response to a source trigger or to a mobile-initiated
PrRtSol. In these cases, when the MN does not receive a notification
or confirmation of a PRE-REGISTRATION handoff, the MN MUST perform a
standard Mobile IPv4 Registration as soon as it connects to the nFA
(L2-LU) as described in Section 8.1.1.
8.2. POST-REGISTRATION Signaling Failure Recovery
Failure occurs in POST-REGISTRATION when either the HRqst or HRply
message is dropped. The effects of the failure and the recovery
procedure depend on which message is dropped, and whether the handoff
is source or target triggered. Since all of the POST-REGISTRATION
signaling is going over the fixed network, it can be expected that
spontaneous dropping of packets in the absence of congestion and
router failure should be a relatively rare event. Nevertheless,
failure recovery mechanisms SHOULD be implemented.
8.2.1. HRqst Message Dropped
If the HRqst message is dropped, the effect is the same for both
source- and target-triggered handoffs. 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).
El Malki, Ed. Experimental PAGE 44
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
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 that 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 an FA Advertisement to the MN as soon as its L2 link is up
(L2-LU at nFA) and MAY drop the tunneled packets. The nFA SHOULD
send an ICMP Destination Unreachable [9] message to the oFA. When
the oFA receives this message, it will terminate the tunnel and stop
forwarding packets. If reverse tunneling was requested, the nFA MUST
NOT reverse tunnel because it has not received handoff confirmation.
In source-triggered handoff, the nFA assumes that the handoff is
confirmed because it has sent the HRply, but the oFA has not received
it so it does not have confirmation. Without failure recovery, the
MN could move to the nFA without the oFA being able to start the
forward tunnel for the MN's packets, and the MN would not be able to
initiate a Mobile IPv4 Registration because it does not know that the
handoff has failed. In this situation, the oFA MUST send out an
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 canceled. 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.
El Malki, Ed. Experimental PAGE 45
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
9. Generalized Link Layer and IPv4 Address (LLA) Extension
This section defines the Generalized Link Layer and IPv4 Address
(LLA) Extension, used by any node that needs to communicate link
layer and IPv4 addresses. The format of the extension relies on
sub-types, where each sub-type defines its own sub-structure. This
document 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
138 (skippable) [1] - when used in Registration Requests
140 (skippable) [1] - when used in Agent Advertisements
Length
The length of the Link Layer Address + the one-octet Sub-Type
field
Sub-Type
This field contains the Link Layer sub-type identifier
LLA
Contains the Link Layer Address
In this document, seven sub-types are defined:
1 3GPP2 International Mobile Station Identity and
Connection ID [13]
2 3GPP International Mobile Subscriber Identity [15]
3 Ethernet 48-bit MAC address [5]
4 64-bit Global ID, EUI-64 [6]
5 Solicited IPv4 Address
6 Access Point Identifier
7 FA IPv4 Address
The following subsections describe the extensions.
El Malki, Ed. Experimental PAGE 46
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
9.1. 3GPP2 IMSI Link Layer Address and Connection ID Extension
The IMSI Link Layer Address Extension contains the International
Mobile Station Identity (IMSI).
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
1 (skippable) [1]
Length
The length of the IMSI field + the one-octet Sub-Type field
Sub-Type
1
IMSI
Contains the IMSI, in the form:
<IMSI>:<Connection Id>
Where the <IMSI> is an ASCII-based representation of the
International Mobile Station Identity, most significant
digit first, ":" is ASCII 0x3a, and the Connection ID is the
ASCII representation of a small, decimal number used for
distinguishing different link-layer connections from the
same mobile device.
El Malki, Ed. Experimental PAGE 47
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
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
2 (skippable) [1]
Length
The length of the IMSI field + the one-octet Sub-Type field
Sub-Type
2
IMSI
Contains the IMSI, a number composed of 15 digits or less,
coded as described in [15].
El Malki, Ed. Experimental PAGE 48
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
9.3. Ethernet Link Layer Address Extension
The Ethernet Link Layer Address Extension contains the 48-bit
Ethernet MAC Address, as defined in [5].
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type | MAC ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
3 (skippable) [1]
Length
7 (includes the Sub-Type field)
Sub-Type
3
MAC
Contains the 48-bit Ethernet MAC Address.
El Malki, Ed. Experimental PAGE 49
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
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
4 (skippable) [1]
Length
9 (includes the Sub-Type field)
Sub-Type
4
MAC
Contains the 64-bit Global Identifier Address.
El Malki, Ed. Experimental PAGE 50
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
9.5. Solicited IPv4 Address Extension
The 32-bit Solicited IPv4 Address Extension contains the IPv4 address
of the agent (FA) being solicited. This extension MAY be present in
an ICMP Agent Solicitation as explained in Section 3.3.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type | IPv4 addr ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
5 (skippable) [1]
Length
5 (includes the Sub-Type field)
Sub-Type
5
IPv4 Address
Contains the 32-bit IPv4 Address of the solicited node.
El Malki, Ed. Experimental PAGE 51
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
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
6 (skippable) [1]
Length
5 (includes the Sub-Type field)
Sub-Type
6
AP ID
Contains the 32-bit Access Point Identifier.
El Malki, Ed. Experimental PAGE 52
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
9.7. FA IPv4 Address Extension
The 32-bit FA IPv4 Address Extension contains the IPv4 address of the
agent (FA). This extension MAY be present in a Registration Request
message to identify the oFA as explained in Section 5.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type | IPv4 addr ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
7 (skippable) [1]
Length
5 (includes the Sub-Type field)
Sub-Type
7
IPv4 Address
Contains the 32-bit IPv4 Address of the FA node.
10. IANA Considerations
This document defines one new extension to Mobile IPv4 Control
messages and one new extension to Mobile IPv4 Router Discovery
messages already maintained by IANA. This document also defines a
new Mobile IPv4 Control message type to be used between FAs. To
ensure correct interoperation based on this specification, IANA must
reserve values in the Mobile IPv4 number space for two new extensions
and one new message type. IANA must also manage the numbering spaces
created by the two new extensions, the message type, and its
associated Code field.
10.1. New Extension Values
Section 9 introduces two extensions.
Generalized Link Layer and IPv4 Address (LLA) Extension for Router
Discovery messages: A new Mobile IPv4 extension that follows after
Mobile IPv4 ICMP Router Discovery messages (e.g., Mobile IP Agent
Advertisements). The type value of this extension belongs to the
El Malki, Ed. Experimental PAGE 53
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
Mobile IPv4 number space for Router Discovery messages maintained by
IANA. The value assigned by IANA is 140. This new extension uses
the Link Layer and IPv4 Address Identifier (LLA) sub-type numbering
space that requires IANA management (see Section 10.2).
Generalized Link Layer and IPv4 Address (LLA) Extension for Mobile IP
Control messages: A new Mobile IPv4 extension appended to Mobile IP
Control messages (e.g., Registration Request). The type value of
this extension belongs to the Mobile IPv4 number space for extensions
to Mobile IPv4 Control messages maintained by IANA. It MUST be in
the skippable (128-255) range as defined in [1]. The value assigned
is 138 by IANA. This new extension uses the Link Layer and IP
Address Identifier (LLA) sub-type numbering space that requires IANA
management (see Section 10.2).
10.2. Generalized Link Layer and IP Address Identifier (LLA)
Sub-type Values
This section describes the sub-type values that are applicable to
both the Generalized LLA Extensions for Mobile IP Control and Router
Discovery messages. This specification makes use of the sub-type
values 1-7, and all other values other than zero (reserved) are
available for assignment via IETF consensus [14]. The seven sub-type
values defined in this specification are:
1 3GPP2 International Mobile Station Identity and
Connection ID [13]
2 3GPP International Mobile Subscriber Identity [15]
3 Ethernet 48-bit MAC address [5]
4 64-bit Global ID, EUI-64 [6]
5 Solicited IPv4 Address
6 Access Point Identifier
7 FA IPv4 Address
10.3. New Message Type and Code
Sections 4.5 and 4.6 define two new Mobile IPv4 message types:
Handoff Request and Handoff Reply. These require two type numbers to
be assigned by IANA from the Mobile IPv4 Control message type address
space. The Handoff Reply message also introduces its own Code field
that requires IANA to manage a new Code address space. This
specification makes use of the Code values 0-1, where 0 identifies a
successful handoff and 1 defines a generic handoff failure. All
other values are available for assignment via IETF consensus [14].
El Malki, Ed. Experimental PAGE 54
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
Code Values for Mobile IP Handoff Reply Messages
0 Successful Handoff
1 Generic Handoff Failure
2-255 Unallocated
11. Security Considerations
For the PRE-REGISTRATION method, as discussed in Section 3.8, the oFA
and nFA MUST share a security association to authenticate and
integrity protect messages transported between them. In addition,
oFA must be authorized to solicit nFA based on the security
association. The minimal requirement to establish a security
association between FAs is that both FAs support manual pre-
configuration of security associations involving shared keys. Other
mechanisms to establish security associations using IKE [16] based on
shared secrets or public keys may also be used. The inter-FA ICMP
messages (solicitations and advertisements) MUST be authenticated and
integrity protected using ESP [10]. The default ESP authentication
algorithm for use in this specification is HMAC-SHA1-96 [12]. The
absence of this security would allow denial-of-service attacks from
malicious nodes at any distance from the FA. To secure Registration
Request and Reply messages, PRE-REGISTRATION uses the security
mechanisms already described in [1] and optionally [11].
POST-REGISTRATION introduces a new change to Mobile IPv4, which is
the possibility that an MN may receive packets from an FA with which
it has not yet performed a Mobile IPv4 Registration. It is not
recommended that the MN drop packets from unknown FAs since it would
effectively eliminate the advantages of POST-REGISTRATION. From a
security viewpoint, dropping packets from unknown FAs would not
provide significant protection for an MN from any attack. This is
because any malicious host may use the MN's home address to send
packets to the MN through its current known FA; therefore, processing
packets received from unknown FAs would not provide worse security
than with normal Mobile IPv4.
In a similar way to PRE-REGISTRATION, in POST-REGISTRATION, oFA and
nFA MUST share a security association required to protect the Handoff
Request and Reply messages. The minimal requirement to establish a
security association between FAs is that the FAs support manual pre-
configuration of security associations involving shared keys. Other
mechanisms to establish security associations using IKE [16] based on
shared secrets or public keys may also be used. The Handoff Request
and Reply messages MUST be authenticated using the FA-FA
authentication extension [11] that uses the default algorithm
specified in [7]. The absence of this security would allow
impersonation attacks and denial-of-service attacks.
El Malki, Ed. Experimental PAGE 55
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
The minimal requirement is that all FAs involved in low latency
handoffs MUST support manual pre-configuration of peer-to-peer
security associations with neighboring FAs, involving shared secrets
and are already required to support the default algorithms of [1].
Other mechanisms to establish security associations using IKE [16]
based on shared or public keys may also be used.
Since the techniques outlined in this document depend on particular
L2 information (triggers) to optimize performance, some level of L2
security is assumed. Both PRE- and POST-REGISTRATION techniques
depend on L2 triggers, but the L2 security implications are different
for the two techniques.
In particular, in POST-REGISTRATION, the L2 triggers initiate the
establishment of tunnels that route IPv4 packets for the MN to its
new location. Therefore, the L2 triggers MUST be secured against any
tampering by malicious nodes, either mobile or within the wired
network. The L2 addresses or IPv4 addresses for the MN and the FAs
that appear in the L2 triggers MUST correspond to the actual nodes
that are participating in the handoff. If there is any possibility
that tampering may occur, the recipient of an L2 trigger MUST have
some way of authenticating the L2 information. Wireless networks
that do not provide such features will be subject to impersonation
attacks, where malicious nodes could cause FAs to believe that an MN
has moved to other service areas or to allow a bogus MN to obtain
unauthorized service from an FA prior to performing a Mobile IPv4
Registration. In POST-REGISTRATION, the L2 triggers would typically
be sent between a wireless base station and the FA. No standard
protocol exists at this time to communicate the L2 trigger
information, but it is important that any future protocol used for
this purpose provides adequate security. If the wireless base
station and FA were integrated, then this security threat would not
apply. Also the layer 2 control messages on the wireless link must
be secured appropriately to prevent a malicious node from running
impersonation attacks and causing unwanted L2 triggers to be
generated. Integrity and replay protection would be required to
avoid impersonation threats and resource consumption threats where a
malicious node replays old messages to cause resource consumption.
This depends on the type of L2 security of the wireless link. For
example, in cellular technologies, the control messages are secured,
although the type of security varies depending on the cellular
standard, but this is not typically the case in WLAN IEEE 802.11
networks.
In PRE-REGISTRATION, the security of L2 triggers has different
implications. The PRE-REGISTRATION technique depends on Mobile IPv4
security between MN and FA, so the same security considerations in
[1] apply. Should malicious nodes be able to generate or modify L2
El Malki, Ed. Experimental PAGE 56
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
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, and the
default limit is 10 per second.
12. Acknowledgements
The authors want to thank Lennart Bang, Bryan Hartwell, Joel
Hortelius, Gianluca Verin, and Jonathan Wood for valuable comments
and suggestions on the whole document. The authors also thank the
Mobile IPv4 WG chairs, Phil Roberts and Basavaraj Patil, for their
input.
13. References
13.1. Normative References
[1] Perkins, C., Ed., "IP Mobility Support for IPv4", RFC 3344,
August 2002.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Montenegro, G., Ed., "Reverse Tunneling for Mobile IP, revised",
RFC 3024, January 2001.
[4] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina,
"Generic Routing Encapsulation (GRE)", RFC 2784, March 2000.
[5] Plummer, D., "Ethernet Address Resolution Protocol: Or
Converting Network Protocol Addresses to 48.bit Ethernet Address
for Transmission on Ethernet Hardware", STD 37, RFC 826,
November 1982.
[6] IEEE, "Guidelines for 64-bit Global Identifier (EUI-64)
Registration Authority",
http://standards.ieee.org/regauth/oui/tutorials/EUI64.html,
March 1997.
[7] Perkins, C., Calhoun, P., and J. Bharatia, "Mobile IPv4
Challenge/Response Extensions (Revised)", RFC 4721, January
2007.
El Malki, Ed. Experimental PAGE 57
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
[8] Deering, S., Ed., "ICMP Router Discovery Messages", RFC 1256,
September 1991.
[9] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792,
September 1981.
[10] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303,
December 2005.
[11] Fogelstroem, E., Jonsson, A., and C. Perkins, "Mobile IPv4
Regional Registration", RFC 4857, June 2007.
[12] Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96 within ESP
and AH", RFC 2404, November 1998.
13.2. Informative References
[13] TIA/EIA/IS-2000.
[14] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
[15] 3GPP TS 23.003 (www.3gpp.org).
[16] Kaufman, C., Ed., "Internet Key Exchange (IKEv2) Protocol", RFC
4306, December 2005.
El Malki, Ed. Experimental PAGE 58
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
Appendix A - Gateway Foreign Agents
The Mobile IPv4 Regional Registration specification [11] introduces
the Gateway Foreign Agent (GFA), as a mobility agent that two FAs
providing service to an MN have in common. Figure A.1 provides an
example of an MN's initial registration through the GFA. If this is
the first registration message, the message MUST be forwarded to the
HA. All packets sent to the MN will be delivered to the GFA, which
in turn will forward the packets to the FA servicing the MN.
RegReq +-----+ RegReq
+----------->| oFA |--------------+
| +-----+ |
| v
+----+ +-----+ RegReq +----+
| MN | | GFA |<------->| HA |
+----+ +-----+ +----+
+-----+
| nFA |
+-----+
Figure A.1 - Initial Registrations through GFA
If the MN moves to an nFA that is serviced by a GFA common with oFA,
the MN MAY issue a Regional Registration Request (see Figure A.2).
The Regional Registration message does not need to be forwarded to
the HA, since the MN's traffic can still be delivered to the same
GFA. This optimized approach effectively reduces the latency
involved in the registration process.
+-----+
| oFA |
+-----+
+----+ +-----+ +----+
| MN | | GFA | | HA |
+----+ +-----+ +----+
| ^
| +-----+ |
+------------>| nFA |-------------+
RegRegReq +-----+ RegRegReq
Figure A.2 - Regional Registration through GFA
Note that the GFA may also be the MN's first-hop router.
El Malki, Ed. Experimental PAGE 59
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
Appendix B - Low-Latency Handoffs for Multiple-Interface MNs
For MNs that have two wireless network interfaces, either on the same
wireless network or on wireless networks having different wireless L2
technologies, the techniques discussed in this document may be
unnecessary if the Mobile IPv4 stack on the MN allows switching an
IPv4 address binding between interfaces. This Appendix discusses how
multiple wireless interfaces can aid low-latency handoff.
+------+ +---------+
| HA |--------| (GFA) |
+------+ +---------+
/ \
... ...
/ \
/ \
+------+ +------+
| oFA | | nFA |
+------+ +------+
| |
+------+ +------+
| RN1 | | RN2 |
+------+ +------+
+------+
| MN | --------->
+------+
Movement
Figure B.1 - Network Model for Mobile IPv4 with Multi-Access
Figure B.1 illustrates the normal and hierarchical MIPv4 models. As
shown in the figure, assume that the MN is connected to Radio Network
1 (RN1) and is registered with oFA through which it is receiving
traffic. Suppose MN enters the coverage area of RN2 and nFA and that
it prefers connectivity to this network for reasons beyond the scope
of this document (e.g., user preferences, cost, QoS available, etc.).
The MN activates the interface to RN2 but continues communicating
through RN1. The MN may solicit advertisements from nFA through the
interface connected to RN1 to speed up the handoff process, provided
there is no TTL restriction, or it can solicit advertisements through
the interface connected to RN2 if it has been configured for IPv4
traffic.
Once the MN is registered with nFA and is successfully receiving and
transmitting through the new network, it tears down the interface to
RN1. If the MN has enough time to complete this procedure without
incurring degraded service or disconnection, the MN would experience
a seamless multi-access handoff, but it may not be possible in all
El Malki, Ed. Experimental PAGE 60
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
cases, due to network coverage or for other reasons. Should multiple
interface handoff be possible, then the low-latency methods described
in this document are not necessary.
In order to support the possible failure of the connectivity with the
new network (RN2/nFA) in the short period following handoff, the MN
may use the S bit in its Mobile IPv4 Registration Request to maintain
simultaneous bindings with both its existing (HA or GFA) binding with
oFA and a new binding with nFA.
Appendix C - PRE-REGISTRATION Message Summary
This appendix contains a quick reference for IPv4 and layer 2
addresses to be used in PRE-REGISTRATION messages.
Proxy Router Advertisement (PrRtAdv)
This is a standard Router/Agent Advertisement [1] with the following
characteristics:
Source IPv4 Address: nFA IPv4 Address
Source Layer 2 Address: oFA L2 Address
Destination IPv4 Address: MN IPv4 Address (from PrRtSol)
Destination Layer 2 Address: MN L2 Address (from PrRtSol)
LLA Extension (defined in this spec): containing nFA Layer 2
Address.
Proxy Router Solicitation (PrRtSol)
This is a standard Router/Agent Solicitation [1] with the following
characteristics:
Source IPv4 Address: MN Address
Source Layer 2 Address: MN Address
Destination IPv4 Address: oFA Address (from source address of
previous Router Advertisement or PrRtAdv)
Destination Layer 2 Address: oFA Address (from source address of
previous Router Advertisement or PrRtAdv LLA)
LLA Extension (defined in this spec): depends on the layer 2
technology (e.g., typically for WLAN, this would be the BSSID of
the new WLAN Access Point)
Registration Request (as seen on MN-oFA link)
This is a Mobile IPv4 Registration Request message [1] with the
following characteristics:
Source IPv4 Address: MN Address
Source Layer 2 Address: MN Address
Destination IPv4 Address: nFA Address (from source addr of
PrRtAdv)
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RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
Destination Layer 2 Address: Default Router (i.e., oFA Address)
LLA Extension (defined in this spec): containing the MN's L2
address that must be used by nFA. This will typically be an
Ethernet MAC address but other types can be used as specified in
Section 9 of this document.
Although this is not mandated, an MN implementation may set the S bit
(see Section 6) in Registration Request messages to improve the
handoff and avoid problems due to failed layer 2 handoffs and layer 2
ping-pong effects between two base stations.
Registration Reply (as seen on oFA-MN link)
This is a Mobile IPv4 Registration Reply message [1] with the
following characteristics:
Source IPv4 Address: nFA Address
Source Layer 2 Address: oFA Address
Destination IPv4 Address: MN Address (from source of Registration
Request)
Destination Layer 2 Address: MN Address (from source of
Registration Request)
El Malki, Ed. Experimental PAGE 62
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
Contributing Authors
Pat Calhoun
Cisco Systems
EMail: pcalhoun@cisco.com
Tom Hiller
Lucent Technologies
EMail: tom.hiller@lucent.com
James Kempf
NTT DoCoMo USA Labs
EMail: kempf@docomolabs-usa.com
Peter J. McCann
Motorola Labs
EMail: pete.mccann@motorola.com
Ajoy Singh
Motorola
EMail: asingh1@email.mot.com
Hesham Soliman
Elevate Technologies
EMail: Hesham@elevatemobile.com
Sebastian Thalanany
US Cellular
EMail: Sebastian.thalanany@uscellular.com
Editor's Address
Karim El Malki
Athonet
EMail: karim@athonet.com
El Malki, Ed. Experimental PAGE 63
RFC 4881 Low-Latency Mobile IPv4 Handoffs June 2007
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El Malki, Ed. Experimental PAGE 64
Low-Latency Handoffs in Mobile IPv4
RFC TOTAL SIZE: 151124 bytes
PUBLICATION DATE: Friday, June 29th, 2007
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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