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RFC 4881

Low-Latency Handoffs in Mobile IPv4

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Top   ToC   RFC4881 - Page 1
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 (C) The IETF Trust (2007).


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
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      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
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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.
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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.
Top   ToC   RFC4881 - Page 5
      seamless handoff - L3 handoff that is both low latency and low

      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:
Top   ToC   RFC4881 - Page 6
      - 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.
Top   ToC   RFC4881 - Page 7
   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.
Top   ToC   RFC4881 - Page 8
   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)
Top   ToC   RFC4881 - Page 9
   | 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.
Top   ToC   RFC4881 - Page 10

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.
Top   ToC   RFC4881 - Page 11

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.
Top   ToC   RFC4881 - Page 12
      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.
Top   ToC   RFC4881 - Page 13
      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

   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
Top   ToC   RFC4881 - Page 14
   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.
Top   ToC   RFC4881 - Page 15
   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.
Top   ToC   RFC4881 - Page 16
   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

   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
Top   ToC   RFC4881 - Page 17
   Identifier Extension, and are described in Sections 9.5 and 9.6.
   These two extensions SHOULD NOT be present in the same PrRtSol

   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
Top   ToC   RFC4881 - Page 18
   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.
Top   ToC   RFC4881 - Page 19

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
Top   ToC   RFC4881 - Page 20
   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.
Top   ToC   RFC4881 - Page 21
   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

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
Top   ToC   RFC4881 - Page 22
   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

      - 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

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