Mobile IP Working Group MIPv4 Handoffs Design Team INTERNET-DRAFT Karim El Malki (Editor) Expires: December 2002 Pat R. Calhoun Tom Hiller James Kempf Peter J. McCann Ajoy Singh Hesham Soliman Sebastian Thalanany June 2002 Low Latency Handoffs in Mobile IPv4 Status of this memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/lid-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This document is a product of the Mobile IP WG. Abstract Mobile IPv4 describes how a Mobile Node can perform IP-layer handoffs between subnets served by different Foreign Agents. In certain cases, the latency involved in these handoffs can be above the threshold required for the support of delay-sensitive or real-time services. The aim of this document is to present two methods to achieve low- latency Mobile IP handoffs. In addition, a combination of these two methods is described. The described techniques allow greater support MIPv4 handoffs design team [Page 1] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 for real-time services on a Mobile IPv4 network by minimising the period of time when a Mobile Node is unable to send or receive IP packets due to the delay in the Mobile IP Registration process. TABLE OF CONTENTS 1. Introduction.....................................................3 1.1. Terminology ................................................3 1.2. The Techniques .............................................5 1.3. L2 triggers ................................................7 1.4. Requirements language ......................................9 2. Requirements.....................................................9 3. The PRE-REGISTRATION Handoff Method..............................9 3.1. Operation ..................................................9 3.2. Network-Initiated Handoff .................................12 3.3. Mobile-Initiated Handoff ..................................14 3.4. Obtaining and Proxying nFA Advertisements .................15 3.4.1. Inter-FA Solicitation................................15 3.4.2. Tunneled nFA Advertisements..........................16 3.5. Caching Router Advertisements .............................16 3.6. Movement Detection and MN Considerations ..................17 3.7. L2 Address Considerations .................................18 3.8. Applicability of PRE-REGISTRATION Handoff .................19 4. The POST-REGISTRATION Handoff Method............................20 4.1. Two Party Handoff .........................................20 4.2. Three Party Handoff .......................................25 4.3. Renewal or Termination of Tunneling Service ...............29 4.4. When will the MN perform a Mobile IP Registration? ........30 4.5. Handoff Request (HRqst) Message format ....................31 4.6. Handoff Reply (HRply) Message .............................33 4.7. Handoff To Third (HTT) Message ............................35 4.8. Applicability of POST-REGISTRATION Handoff Method .........35 5. Combined Handoff Method.........................................36 6. Layer 2 and Layer 3 Handoff timing Considerations...............37 7. Reverse Tunneling Support.......................................37 8. Handoff Signaling Failure Recovery..............................37 8.1. PRE-REGISTRATION Signaling Failure Recovery ...............37 8.1.1. Failure of ProxyRtSol and ProxyRtAdv.................38 8.1.2. Failure of Inter-FA solicitation and advertisement...38 8.2. POST-REGISTRATION Signaling Failure Recovery ..............38 8.2.1. HRqst Message Dropped................................39 8.2.2. HRply Message Dropped................................39 9. Generalized Link Layer Address Extension........................40 9.1. 3GPP2 IMSI Link Layer Address and Connection ID Extension 41 9.2. 3GPP IMSI Link Layer Address Extension ...................41 9.3. Ethernet Link Layer Address Extension ....................42 9.4. IEEE 64-Bit Global Identifier (EUI-64) Address Extension .43 9.5. Solicited IP Address Extension ...........................43 9.6. Access Point Identifier Extension ........................44 El Malki (Editor) et. al. [Page 2] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 10. IANA Considerations............................................44 11. Security Considerations........................................45 12. Acknowledgements...............................................46 13. References.....................................................46 14. Authors' Addresses.............................................47 15. Full Copyright Statement.......................................49 Appendix A - Gateway Foreign Agents................................51 Appendix B - Low Latency Handoffs for Multiply-Interfaced MNs......52 1. Introduction Mobile IPv4 [1] describes how a Mobile Node (MN) can perform IP-layer handoff between subnets served by different Foreign Agents (FAs). In certain cases, the latency involved in handoff can be above the threshold required for the support of delay-sensitive or real-time services. The aim of this document is to present two methods to achieve low-latency Mobile IP handoff during movement between FAs. The presented techniques allow greater support for real-time services on a Mobile IPv4 network by minimising the period of time when a MN is unable to send or receive IP packets due to the delay in the Mobile IP Registration process. In the rest of this section, terminology used throughout the document is presented, the handoff techniques are briefly described, and the use of link layer information is outlined. In Section 2, a brief description of requirements is presented. Section 3 describes the details of the PRE-REGISTRATION handoff technique, while Section 4 describes the details of the POST-REGISTRATION handoff technique. In Section 5, a combined method using the two handoff techniques together is presented. Section 6 discusses Layer 2 and Layer 3 handoff timing considerations. Section 7 discusses reverse tunneling support, Section 8 describes mechanisms to recover from message failures while Section 9 describes protocol extensions required by the handoff techniques. Sections 10 and 11 discuss IANA and security considerations. Finally the two appendices discuss additional material related to the handoff techniques. Appendix A gives a short introduction to Regional Registrations [2] which can be used together with low latency handoffs. Appendix B discusses low latency handoff when a MN has multiple wireless L2 interfaces, in which case the techniques in this document may not be necessary. 1.1. Terminology This section presents a few terms used throughout the document. oFA - old Foreign Agent, the FA involved in handling a MN's care of address prior to an L3 handoff. El Malki (Editor) et. al. [Page 3] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 nFA - new Foreign Agent, the FA anticipated to be handling a MN's care of address after completion of an L3 handoff. aFA - anchor Foreign Agent, the FA that is currently handling the network end of the BET in POST-REGISTRATION. L2 handoff - Movement of a MN's point of Layer 2 (L2) connection from one wireless access point to another. L3 handoff - Movement of a MN between FAs which involves changing the care-of address (CoA) at Layer 3 (L3). L2 trigger - Information from L2 that informs L3 of particular events before and after L2 handoff. The descriptions of L2 triggers in this document are not specific to any particular L2, but rather represent generalizations of L2 information available from a wide variety of L2 protocols. L2-MT - An L2 trigger that occurs at the MN informing of movement to a certain nFA (Mobile Trigger). L2-ST or source trigger - An L2 trigger that occurs at oFA, informing the oFA that L2 handoff is about to occur. L2-TT or target trigger - An L2 trigger that occurs at nFA, informing the nFA that a MN is about to be handed off to nFA. L2-LU - An L2 trigger that occurs at the MN or nFA, informing that the L2 link between MN and nFA is established. L2-LD - An L2 trigger that occurs at the oFA, informing the oFA that the L2 link between MN and oFA is lost. low latency handoff - L3 handoff in which the period of time during which the MN is unable to receive packets is minimized. low loss handoff - L3 handoff in which the number of packets dropped or delayed is minimized. Low loss handoff is often called smooth handoff. seamless handoff - L3 handoff that is both low latency and low loss. bi-directional edge tunnel (BET) - A bidirectional tunnel established between two FAs for purposes of temporarily routing a MN's traffic to/from it on a new subnet without requiring the MN to change CoA. El Malki (Editor) et. al. [Page 4] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 ping-ponging - Rapid back and forth movement between two wireless access points often due to failure of L2 handoff. Ping-ponging can occur if radio conditions for both the old and new access points are about equivalent and less than optimal for establishing a good, low-error L2 connection. network-initiated handoff - L3 handoff in which oFA or nFA initiates the handoff. mobile-initiated handoff - L3 handoff in which the MN initiates the handoff. IP address identifier - An IP address of a MN or FA, or an L2 identifier that allows an FA to deduce the IP address of a MN or FA. If the IP address identifier is an L2 identifier, it may be specific to the L2 technology. 1.2. The Techniques Mobile IP was originally designed without any assumptions about the underlying link layers over which it would operate so that it could have the widest possible applicability. This approach has the advantage of facilitating a clean separation between L2 and L3 of the protocol stack, but it has negative consequences for handoff latency. The strict separation between L2 and L3 results in the following built-in sources of delay: - The MN may only communicate with a directly connected FA. This implies that a MN may only begin the registration process after an L2 handoff to nFA (new FA) has completed. - The registration process takes some non-zero time to complete as the Registration Requests propagate through the network. During this period of time the MN is not able to send or receive IP packets. This document presents techniques for reducing these built-in delay components of Mobile IP. The techniques can be divided into two general categories, depending on which of the above problems they are attempting to address: - Allow the MN to communicate with the nFA while still connected to the oFA. - Provide for data delivery to the MN at the nFA even before the formal registration process has completed. The first category of techniques allows the MN to "pre-build" its registration state on the nFA prior to an underlying L2 handoff. The second category of techniques allow for service to continue El Malki (Editor) et. al. [Page 5] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 uninterrupted while the handoff is being processed by the network. Three methods are presented in this draft to achieve low-latency L3 handoff, one for each category described above and one as a combination of the two: - PRE-REGISTRATION handoff method, - POST-REGISTRATION handoff method, - combined handoff method. The PRE-REGISTRATION handoff method allows the MN to be involved in an anticipated IP-layer handoff. The MN is assisted by the network in performing an L3 handoff before it completes the L2 handoff. The L3 handoff can be either network-initiated or mobile-initated. Accordingly, L2 triggers are used both in the MN and in the FA to trigger particular L3 handoff events. The PRE-REGISTRATION method coupled to L2 mobility helps to achieve seamless handoffs between FAs. The basic Mobile IPv4 concept involving advertisement followed by registration is supported and the PRE-REGISTRATION handoff method relies on Mobile IP security. No new messages are proposed, except for an extension to the Agent Solicitation message in the mobile- initiated case. The POST-REGISTRATION handoff method proposes extensions to the Mobile IP protocol to allow the oFA (old FA) and nFA (new FA) to utilize L2 triggers to set up a bi-directional tunnel between oFA and nFA that allows the MN to continue using its oFA while on nFA's subnet. This enables a rapid establishment of service at the new point of attachment which minimizes the impact on real-time applications. The MN must eventually perform a formal Mobile IP registration after L2 communication with the new FA is established, but this can be delayed as required by the MN or FA. Until the MN performs registration, the FAs will setup and move bidirectional tunnels as required to give the MN continued connectivity. The combined method involves running a PRE-REGISTRATION and a POST- REGISTRATION handoff in parallel. If the PRE-REGISTRATION handoff can be performed before the L2 handoff completes, the combined method resolves to a PRE-REGISTRATION handoff. However, if the PRE- REGISTRATION handoff does not complete within an access technology dependent time period, the oFA starts forwarding traffic for the MN to the nFA as specified in the POST-REGISTRATION handoff method. This provides for a useful backup mechanism when completion of a PRE- 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 El Malki (Editor) et. al. [Page 6] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 interface). However, the case of multiply-interfaced MNs needs special consideration, since the handoff methods described in this document may not be required in all cases (see Appendix B). 1.3. L2 triggers An L2 trigger is a signal of an L2 event. In this document, the L2 events relate to the L2 handoff process. One possible event is early notice of an upcoming change in the L2 point of attachment of the mobile node to the access network. Another possible event is the completion of relocation of the mobile node's L2 point of attachment to a new L2 access point. This information comes from L2 programmatically or is derived from L2 messages. Although the protocols outlined in this document make use of specific L2 information, Mobile IP should be kept independent of any specific L2. L2 triggers are an abstraction mechanism for a technology specific trigger. Therefore, an L2 trigger that is made available to the Mobile IPv4 stack is assumed to be generic and technology independent. The precise format of these triggers is not covered in this document, but the information required to be contained in the L2 triggers for low latency handoffs is specified. In order to properly abstract from the L2, it is assumed that one of the three entities - the MN, oFA, or nFA - is made aware of the need for an L2 handoff, and that the nFA or MN can optionally also be made aware that an L2 handoff has completed. A specific L2 will often dictate when a trigger is received and which entity will receive it. Certain L2s provide advance triggers on the network-side, while others provide advance triggers on the MN. Also, the particular timing of the trigger with respect to the actual L2 handoff may differ from technology to technology. For example, some wireless links may provide such a trigger well in advance of the actual handoff. In contrast, other L2s may provide little or no information in anticipation of the L2 handoff. An L2 trigger may be categorized according to whether it is received by the MN, oFA, or nFA. Table 1 gives such a categorization along with information expected to be contained in the trigger. The methods presented in this document operate based on different types of L2 triggers as shown in Table 1. Once the L2 trigger is received, the handoff processes described hereafter are initiated. The three triggers: L2-ST, L2-TT and L2-MT are independent of each other and MUST NOT occur together since each will trigger a different type of handoff behaviour. El Malki (Editor) et. al. [Page 7] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 +-------------+----------------------+------------------------------+ | L2 trigger | Mobile | Source | | | Trigger | Trigger | | | (L2-MT) | (L2-ST) | +-------------+----------------------+------------------------------+ | Recipient | MN | oFA | +-------------+----------------------+--------------+---------------+ | Method | PRE | PRE | POST | | | mobile- | network- | source | | | initiated | initiated | trigger | +-------------+----------------------+--------------+---------------+ | When? | sufficiently before | sufficiently | sufficiently | | | the L2 handover | before L2 | before L2 | | | so that MN can | handover for | handover for | | | solicit ProxyRtAdv | FA to send | oFA & nFA to | | | from oFA. | proxyRtAdv | exchange | | | | to MN. | HRq/HRy. | +-------------+----------------------+--------------+---------------+ | Parameters | nFA IP address | nFA IP address identifier | | | identifier | MN IP address identifier | | | | | +-------------+----------------------+------------------------------+ +------------+------------------------+-------------+---------------+ | L2 trigger | Target | Link-Up | Link-Down | | | Trigger | | | | | (L2-TT) | | | | | | (L2-LU) | (L2-LD) | |------------+------------------------+-------------+---------------+ | Recipient | nFA | MN or nFA | oFA | |------------+------------+-----------+-------------+---------------+ | Method | PRE | POST | PRE & POST | POST | | | network | target | | | | | initiated | trigger | | | |------------+------------------------+-------------+---------------+ | When? | | when radio | when radio | | | same as | link between| link between | | | source trigger | MN & nFA is| MN and oFA | | | | established | is lost | |------------+------------------------+-------------+---------------+ | Parameters | oFA IP address id | @MN: nFA IP | MN IP address | | | MN IP address id. | or L2 addr. | identifier | |------------+------------------------+-------------+---------------+ Table 1 - L2 Triggers El Malki (Editor) et. al. [Page 8] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 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 [3]. 2. Requirements The following requirements are applicable to low-latency handoff techniques and are supported by the methods in this document: - to provide low-latency and low loss handoff for real time services, - to have no dependence on a wireless L2 technology, - to support inter- and intra-access technology handoffs, - to limit wireless bandwidth usage. 3. The PRE-REGISTRATION Handoff Method The PRE-REGISTRATION handoff method is based on the original concept of Mobile IP handoff as specified in [1], in which: - an advertisement for an FA is received by an MN, - the advertisement allows the MN to perform movement detection, - the MN registers with the FA. It reuses the basic messages specified in [1]. The PRE-REGISTRATION method allows both the MN and FA to initiate handoff. In both cases, abiding by the basic Mobile IP handoff concept allows the MN to choose with which FA to register. The PRE-REGISTRATION method can make use of L2 triggers on either the FA or MN side, depending on whether network-initiated or mobile-initiated handoff occurs. PRE- REGISTRATION also supports both the normal Mobile IP model [1] in which the MN is receiving packets from a Home Agent (HA) and the Regional Registration model [2] in which the MN receives packets from a Gateway Foreign Agent (GFA). It also supports movement where a new AAA transaction must occur to authenticate the MN with the new domain. 3.1. Operation The overall PRE-REGISTRATION Handoff mechanism is summarised in Figure 1 below: El Malki (Editor) et. al. [Page 9] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 +---------+ | HA (GFA)|<---------+ +---------+ | 4. (Reg)RegReq | 5. (Reg)RegReply v +-----+ 1a. RtSol +-----+ | | -----------------> | nFA | | oFA | 1b. RtAdv | | +-----+ <----------------- +-----+ ^ | ^ (2a. ProxyRtSol) | | 2b / | | ProxyRtAdv / 3. (Reg)RegReq | | / | v --------------- +-----+ / | MN | +-----+ - - - - - -> Movement Figure 1 - PRE-REGISTRATION Handoff Protocol The following steps provide more detail on the protocol: 1. Messages 1a is a Router Solicitation (RtSol) from oFA to nFA. Message 1b is a Router (Agent) Advertisement (RtAdv) from nFA to oFA. These messages SHOULD occur in advance of the PRE- REGISTRATION Handoff in order not to delay the handoff. For this to occur, oFA SHOULD solicit and cache advertisements from neighbouring nFAs, thus decoupling the timing of this exchange from the rest of the PRE-REGISTRATION Handoff. When the L3 handoff is initiated by a target L2 trigger at nFA (L2-TT), message 1b equals message 2b and is sent unsolicited directly to MN (tunneled by nFA to MN through oFA) instead of being relayed by oFA. 2. Message 2a is a Proxy Router Solicitation (PrRtSol). It is different from a normal Router Solicitation since it is actually soliciting an advertisement from a router different from the one receiving this message. The presence of message 2a indicates that the handoff is mobile-initiated and its absence means that the handoff is network-initiated. In mobile-initiated handoff, message 2a occurs if there is an L2 trigger in the MN to solicit for a Proxy Router Advertisement (PrRtAdv). When message 2a is received by the oFA, the oFA MUST return the Proxy Router Advertisement (Agent Advertisement) in message 2b. In network-initiated handoff, the L2 trigger occurs at oFA and oFA MUST relay the Agent Advertisement in message 2b without the need for the MN to El Malki (Editor) et. al. [Page 10] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 solicit. Note that it is also possible for nFA to advertise directly to the MN in the network-initiated target-trigger case (section 3.2). In all cases message 2b is simply nFA's agent advertisement. 3. The MN performs movement detection upon receipt of either a solicited or unsolicited Agent Advertisement and, if appropriate, it sends a Registration Request (RegReq) message [1] in message 3 to nFA. When a local Gateway Foreign Agent (GFA) is present this message MAY be a Regional Registration Request (RegRegReq) [2]. Message 3 is routed through oFA since the MN is not directly connected to nFA prior to the L2 handoff. 4. Messages 4 and 5 complete the standard Mobile IP Registration [1] or Regional Registration [2] initiated with message 3. In the network-initiated target-triggered case, the Registration Reply in message 5 SHOULD be sent by nFA to the MN both through oFA and directly on-link. This is necessary since the MN may have to detach from oFA, due to the wireless L2 connection, before it received the Reply. Figures 2 and 3 illustrate this tunneling, though it is not shown in Figure 1. Tunneling can take place either at L3 or L2. In the mobile-initiated and network-initiated source-triggered cases the nFA will not have the oFA's address. Therefore the Reply MUST be unicast by nFA to the MN on-link as soon as the MN connects to nFA (L2-UP). The MN's L2 address is obtained using the extensions in Section 9, as described in 3.7. 5. If the Registration is successful then packets for the MN are now tunnelled from the HA (or GFA) to the nFA where the MN has moved to. PRE-REGISTRATION is not dependent on Regional Registration extensions [2]. However if the HA is at a distance (in terms of delay) from the nFA, the use of a local GFA reduces the time required for the handoff procedure to complete. The time at which the L2 trigger is received by the oFA or MN, thereby triggering the PRE-REGISTRATION handoff, compared to the time at which the actual L2 handoff occurs is important for the optimal performance of the low latency handoff. That is, in the optimal case the L2 trigger will be received, the four messaging steps of PRE-REG described above will be completed (i.e. Registration Request processed by HA or GFA) and the first packet redirected by the HA (or GFA) to nFA will reach the MN at the moment in which the MN's L2 link to nFA is fully established. The MN would therefore not suffer any El Malki (Editor) et. al. [Page 11] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 disruption due to the L3 handoff. This may require particular implementation techniques and deployment, such as L2 techniques, buffering and bicasting, but these are outside the scope of this document. In addition further handoff smoothing considerations may be required to prevent the loss of packets in-flight between HA (or GFA) and oFA while the MN performs a PRE-REGISTRATION handoff. These are also outside the scope of this document. Figures 2, 3, and 4 contain message timing diagrams for both the network-initiated and mobile-initiated PRE-REGISTRATION handoff procedures. 3.2. Network-Initiated Handoff As described in Table 1, a PRE-REGISTRATION handoff can be initated at oFA by a source trigger or at nFA by a target trigger. A source- triggered network-initiated handoff occurs when an L2 trigger is received at the oFA informing it of a certain MN's upcoming movement from oFA to nFA. The L2 trigger contains information such as the MN's IP address identifier (i.e. the IP address itself or an identifier which can be resolved to the IP address) and the nFA's IP address identifier. An identifier may be specific to the wireless technology (e.g. Access Point ID). A target-triggered network-initiated handoff occurs when an L2 trigger is received at the nFA informing it of a certain MN's upcoming movement from oFA. This type of trigger is also shown in Table 1. The L2 trigger contains information such as the MN's IP address identifier and the oFA's IP address identifier. In a source-triggered handoff, when oFA receives the trigger (L2-ST) it MUST send message 2b, the Proxy Router Advertisement (PrRtAdv), to the MN. The PrRtAdv is nFA's agent advertisement [1] with one of the link-layer extensions described in sections 9.3 or 9.6. The use of the contents of this extension is described in section 3.7. Messages 1a and 1b SHOULD be exchanged by oFA and nFA before the L2 trigger is received (see section 3.4.1). Message 2a is not used. In a target- triggered handoff, when nFA receives the trigger (L2-TT) it MUST tunnel an Agent Advertisement to the MN through oFA to initiate the L3 handoff. The inner Advertisement is unicast by nFA to MN, thus nFA treats the target-trigger as a Router Solicitation. This Advertisement is tunneled to oFA which functions as a normal router, decapsulating the Advertisement and forwarding it to the MN. This messages MUST be authenticated to prevent attacks (see section 3.4.2). Figures 2 and 3 contain message timing diagrams describing the PRE- REGISTRATION network-initiated handoff for source and target triggers. El Malki (Editor) et. al. [Page 12] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 MN oFA nFA HA/GFA | |<~~~~~~ L2-Source | | | | Trigger | | |<--------------------| | | | ProxyRtAdv | | | | | | | |---------------------------------------->| | | RegReq or | | | | RegRegReq | |------------------->| | (routed via oFA) | | RegReq or RegRegReq| | | | | | | |<-------------------| | | | (Reg)RegReply | | | | | |<----------------------------------------| | | | (Reg)RegReply | | | | (sent to MN when it attaches to nFA) | Figure 2 - PRE-REGISTRATION Handoff Message Timing Diagram (Network-Initiated, Source Trigger) MN oFA nFA HA/GFA | | L2-Target~~~~~~~~>| | | | Trigger | | | | | | | |...................| | |<--------------------------------------- | | | (ProxyRtAdv) |...................| | | | Tunneled Agent | | | | Advertisement | | | | | | |---------------------------------------->| | | RegReq. or | | | | RegRegReq | |------------------->| | (routed via oFA) | | RegReq or RegRegReq| | | | | | | |<-------------------| | | | (Reg)RegReply | | | | | |<----------------------------------------| | | | (Reg)RegReply | | | | (sent to MN when it attaches to nFA) | Figure 3 - PRE-REGISTRATION Handoff Message Timing Diagram (Network-Initiated, Target Trigger) El Malki (Editor) et. al. [Page 13] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 3.3. Mobile-Initiated Handoff As shown in Table 1, a mobile-initiated handoff occurs when an L2 trigger is received at the MN informing that it will shortly move to nFA. The L2 trigger contains information such as the nFA's IP address identifier (i.e. nFA's IP address or an identifier which can be resolved to the nFA's IP address). The message timing diagram is shown in Figure 4. MN oFA nFA HA/GFA |<~~~~~ L2-Trigger | | | | | | | |-------------------->| | | | ProxyRtSol | | | | | | | |<--------------------| | | | ProxyRtAdv | | | | | | | |---------------------------------------->| | | RegReq or | | | | RegRegReq | |------------------->| | (routed via oFA) | | RegReq or RegRegReq| | | | | | | |<-------------------| | | | (Reg)RegReply | |<----------------------------------------| | | | (Reg)RegReply | | | | (sent to MN when it attaches to nFA) | Figure 4 - PRE-REGISTRATION Handoff Message Timing Diagram (Mobile-Initiated) As a consequence of the L2 trigger (L2-MT) the MN MUST send message 1a, the Proxy Router Solicitation (PrRtSol). This message is a unicast agent solicitation to oFA for a Proxy Router Advertisement (PrRtAdv). This solicitation MUST have a TTL=1 as in [1]. The Proxy Router Advertisement Solicitation unicast to oFA is an agent solicitation with a special extension. The solicitation MUST have an extension containing an IP address identifier because the MN is soliciting another specific FA's advertisement from the oFA. This specific FA will be the MN's nFA. The IP address identifier contains the IP address of the nFA or an identifier that can be used by the oFA to resolve to nFA's IP address. If the identifier is not an IP address, it MAY be specific to the underlying wireless technology, for example, an Access Point or Base Station ID. The extension is a subtype of the Generalised Link-Layer Address extension described in Section 9. Two extension subtypes have been defined to contain the nFA's IP address and an access point identifier. They are called the Solicited Agent IP Address Extension and the Access Point Identifier El Malki (Editor) et. al. [Page 14] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 Extension, and are described in Sections 9.5 and 9.6. These two extensions SHOULD NOT be present in the same PrRtSol message. When oFA receives the PrRtSol message it MUST reply to the MN with the Proxy Router Advertisement (PrRtAdv, message 2b). The PrRtAdv is simply the agent advertisement for the requested nFA, proxied by oFA. In order to expedite the handoff, the actual nFA advertisement SHOULD be cached by the oFA following a previous exchange with nFA, shown in messages 1a and 1b, as specified in Section 3.5. The PrRtAdv message MUST contain the nFA's L2 address (using the LLA extension in 9.2). This is further described in section 3.7. 3.4. Obtaining and Proxying nFA Advertisements Since L2 triggers are involved in initiating PRE-REGISTRATION handoff, the trigger timing SHOULD be arranged such that a full L3 PRE-REGISTRATION handoff can complete before the L2 handoff process completes. That is, the L2 handoff should be completed after the MN's Registration with the nFA is performed (message 3 in Fig.1). The Registration MAY be transmitted more than once to reduce the probability that it is lost due to errors on the wireless link. A PRE-REGISTRATION handoff in this case requires the MN to receive an agent advertisement from the nFA through the old wireless access point. How to achieve this is discussed in the following subsections. Messages exchanged between FAs MUST be authenticated to prevent attacks. The minimal requirement is that all FAs involved in low latency handoffs MUST support manual pre-configuration of security associations with other neighbouring FAs, involving shared keys and the default algorithms of [1]. 3.4.1. Inter-FA Solicitation This applies to the network-initiated source-triggered (L2-ST) and mobile-initiated (L2-MT) cases only. Inter-FA solicitation assumes that oFA has access to the IP address of the nFA. The IP address of nFA is obtained by means of an L2 trigger at oFA in the network- initiated case (see Section 3.2) or by means of the extension to the Proxy Router Solicitation (PrRtSol) from the MN in the mobile- initiated case (see Section 3.3). Once the oFA has access to the address of the nFA for a specific MN, it MUST send a unicast agent solicitation to nFA. The nFA replies to the oFA by unicasting an Agent Advertisement with appropriate extensions. This method removes the TTL limitation of [1] for Mobile IP messages (i.e. TTL=1 is not applicable here). The TTL limitation cannot be applied since oFA and nFA may be more than one hop away and since it is unnecessary for a secured unicast message. The ICMP solicitations and advertisements MUST be authenticated. These messages MUST be protected using ESP [16] to prevent attacks. An FA El Malki (Editor) et. al. [Page 15] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 MUST NOT accept ICMP solicitations or advertisements from sources which are not authenticated. As a practical matter, oFA SHOULD pre-solicit and cache advertisements from known neighboring FAs (see section 3.5), in order to prevent having to perform the above solicitation during an actual handoff procedure. 3.4.2. Tunneled nFA Advertisements This applies to the network-initiated target-triggered (L2-TT) case only. Following a target trigger (L2-TT) the nFA MUST send a tunneled agent advertisement to the MN through oFA. Tunneling nFA advertisments assumes that the nFA is aware of the IP address for oFA and the MN. These IP addresses are obtained by means of the IP address identifiers in an L2 trigger at nFA in the network-initiated case (see Section 3.2). However in [1] the TTL must be 1 on Agent Advertisements from the nFA. Therefore tunneling advertisements is applicable if the TTL limitation of [1] is relaxed. For this purpose, a pre-established security association between oFA and nFA MUST be in place to authenticate this message and relax the TTL limitation. If the implementation requires this, a tunnel SHOULD be configured when the inter-FA security association is established. The tunneled ICMP advertisement MUST be secured using tunnel mode ESP [16] between nFA and oFA. An FA MUST NOT accept tunneled packets from sources which are not authenticated. 3.5. Caching Router Advertisements In the mobile-initiated (L2-MT) case and the network-initiated source-triggered (L2-ST) case, the message exchange 1 in Figure 1 could impose an additional latency on the L3 handoff process if done as part of the handoff procedure. In order to remove this source of latency, the inter-FA Router Solicitation and Advertisement exchange SHOULD be performed in advance of handoff. A process SHOULD be in place at the oFA to solicit its neighbouring nFAs at a predefined time interval (MIN_SOLICITATION_INTERVAL). This interval SHOULD NOT be set too small to avoid unnecessary consumption of network bandwidth and nFA processing resources. The minimum value of MIN_SOLICITATION_INTERVAL is 1 sec. If the FA Challenge/Response mechanism in [9] is used then the MIN_SOLICITATION_INTERVAL MUST be set to a value smaller then the window of time in which a challenge remains valid so that the nFA challenge does not expire before the MN issues the Registration Request. Therefore the MIN_SOLICITATION_INTERVAL in oFA MUST be set to a value equal to (0.5 * nFA's CHALLENGE_WINDOW * nFA's Agent advertisement interval). The CHALLENGE_WINDOW and Agent advertisement interval are defined in [9] 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 El Malki (Editor) et. al. [Page 16] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 document. To allow advertisement caching in certain implementations and in cases where the nFA advertisement interval is very small, it MAY be necessary for the implementation in nFA to allow different CHALLENGE_WINDOW and agent advertisement interval settings for its nFA-oFA interface. The oFA SHOULD cache the most recent advertisement from its neighbouring nFAs. This advertisement MUST be sent to the MN in message 2b with a TTL=1. The oFA SHOULD also have a mechanism in place to create a list of neighbouring nFAs. The minimum requirement for each FA is that it SHOULD allow manual configuration of a list of nFA addresses which an MN could possibly perform an L3 handoff to. The FA addresses in this list will depend on deployment and radio coverage. It is also possible to specify another protocol to achieve nFA discovery, but it is outside the scope of this document. 3.6. Movement Detection and MN Considerations When the MN receives an Agent Advertisement with a Mobility Agent extension, it performs actions according to the following movement detection mechanism: the MN MUST be "Eager" to perform new bindings. This means that the MN MUST perform Registrations with any new FA from which it receives an advertisement (i.e. MN is Eager), as long as there are no locally-defined policies in the MN that discourage the use of the discovered FA. For example, the MN could have a policy based on the cost of service. The method by which the MN determines whether the FA is a new FA is described in [1] and MAY use an FA-NAI extension [2]. 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 both the PRE- REGISTRATION procedure has completed (Registration Reply is received) as described in [1]. Overall the MN behaves as described in [1] with the following additions: the specified movement detection mechanism mentioned above the ability to use the L2-MT to initiate an agent solicitation with a special extension (PrRtSol). When moving from a PRE-REGISTRATION network to a normal Mobile IP [1] network the MN will no longer receive PrRtAdv messages (agent advertisements with the LLA extension). If the MN still receives L2- MTs then it will attempt to send PrRtSol messages. The FA will either ignore the solicitation or will reply with a normal agent advertisement [1]. In the absence of a PrRtSol, when receiving a normal agent advertisement the MN MUST resort to normal Mobile IP behaviour [1]. If the MN does not receive a PrRtAdv in reply to its PrRtSol, it SHOULD retransmit the PrRtSol message once after PRE_SOL_INTERVAL seconds and then for another PRE_SOL_ATTEMPTS times with exponential backoff of the transmission interval. If a PrRtAdv El Malki (Editor) et. al. [Page 17] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 is not received within PRE_SOL_INTERVAL seconds after the last PrRtSol attempt, the MN MUST resort to normal Mobile IP behaviour [1]. The default values for PRE_SOL_ATTEMPTS is 2 and the default value for PRE_SOL_INTERVAL is 1 second. It should be noted that the performance of the movement detection mechanism mandated in PRE- REGISTRATION MAY have sub-optimal behaviour on the other Mobile IP [1] network. Instead when the MN moves from a normal Mobile IP [1] network to a PRE-REGISTRATION network, the MN will start receiving L2-MTs or PrRtAdv messages. When the MN receives L2-MTs or PrRtAdv messages it MUST follow the PRE-REGISTRATION procedure. If there is uncertainty as to which mode to choose (e.g. MN receives messages from both PRE-REGISTRATION and normal FAs) the MN SHOULD choose PRE- REGISTRATION. 3.7. L2 Address Considerations Some special considerations should be taken with respect to the wireless system on which this handoff method is being implemented. Consider an Ethernet-like system (e.g. IEEE 802.11) for example. In PRE-REGISTRATION the MN is registering with an FA (nFA) that is not its current first-hop router, therefore the L2 address of the Ethernet frame containing the MN's Registration Request reaching the nFA is not the MN's address. Therefore the FA MUST NOT use the Ethernet address of the incoming Registration Request as the MN's L2 address as specified in [1]. This applies to the cases where the wireless access points are bridges or routers and independently of whether the FA is implemented in the wireless access points themselves. In this case the MN's Registration Request (or Regional Registration Request) MUST use an L2 address extension to the Registration message when the MN is performing a registration. Such an L2 address is either the same L2 address that remains constant as the MN moves, or it is the MN's L2 address at nFA. To communicate its L2 address, the MN includes a Generalised Link Layer Extension (see Section 9.3) with its Registration Request (or Regional Registration Request) message. If this extension is present the FA MUST use the L2 address contained in the extension to communicate with the MN. For the same reasons, the MN MUST NOT use the source L2 address of the Agent Advertisement message (PrRtAdv) as its default router's L2 address. Therefore the oFA/nFA MUST include a Generalised Link Layer Extension (see Section 9.3) with its Agent Advertisement (PrRtAdv) messages. If a particular wireless L2 technology has defined a special L2 interface to the wireless network that allows the FA to resolve the mapping between an MN's IP address and an L2 address without the need to use the extension, the L2 address extension would not be needed. El Malki (Editor) et. al. [Page 18] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 3.8. Applicability of PRE-REGISTRATION Handoff The PRE-REGISTRATON Handoff method is applicable to scenarios where a period of service disruption due to layer 3 is not acceptable, for example when performing real-time communications, and therefore where an anticipation of the layer 3 handoff is required. Security for the PRE-REGISTRATION handoff method is based on the same security model as [1] including the use of AAA. A prerequisite for PRE-REGISTRATION is that the FA or MN are able to obtain an L2 trigger informing them of a pending L2 handoff procedure. The target of the L2 handoff is another access point or radio network that is in the coverage area of a new FA. The L2 trigger information may be in the form of IP address identifiers which may need to be resolved to IP addresses using methods that may be specific to the wireless network and are not considered here. If, for example, the oFA or MN determines that the IP address of the new FA is oFA's address then the PRE-REGISTRATION handoff SHOULD NOT be initiated. The L2 trigger must allow enough time for the PRE-REGISTRATION handoff procedure to be performed. In many wireless L2 technologies, the L2 handoff procedure involves a number of message exchanges before the effective L2 handoff is performed. For such technologies, PRE-REGISTRATION handoff can be initiated at the beginning of the L2 handoff procedure and completed before the L2 handoff is completed. It is efficient to engineer the network such that this succession of events is ensured. The PRE-REGISTRATION Handoff method is applicable in the following cases: - when the MN has locally defined policies that determine a preference for one access over another, for example due to service cost within the same or different technology, and therefore where it is necessary to allow the MN to select the appropriate FA with which to connect, - when L3 cannot rely upon L2 security between the MN and the FA to make modifications to IP routing and therefore authenticated Mobile IP messages are required, - when the trigger to initiate the handoff is received at the MN. In the first case it is necessary to involve eventual local MN policies in the movement detection procedure as described in 3.6. El Malki (Editor) et. al. [Page 19] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 4. The POST-REGISTRATION Handoff Method The POST-REGISTRATION handoff method uses bi-directional edge tunnels (BETs) or unidirectional tunnels to perform low latency change in the L2 point of attachment for the MN without requiring any involvement by the MN. Figure 5 illustrates the basic POST-REGISTRATION handoff. Following a successful Mobile IP Registration between MN and oFA, the oFA becomes the mobility anchor point for the MN, called the anchor FA (aFA). When the MN moves from oFA to nFA, rather than performing signaling over the wireless link to register with the nFA, the MN can defer the L3 handoff and continue to use it's aFA (i.e. oFA in this case). If the MN moves to a third FA before registering with the nFA, in certain cases described later, the third FA signals aFA to move the wireless link end of the BET from nFA to it. The network end of the BET remains anchored at aFA until the MN performs the Mobile IP Registration. +------+ | CN | +------+ | ... | +------+ BET +------+ | aFA |==========| nFA | +------+ +------+ | wireless link | movement +------+ ---------> | MN | +------+ Figure 5 - POST-REGISTRATION Concept Messages between oFA/aFA and nFA MUST be authenticated. The minimal requirement is that all FAs involved in low latency handoffs MUST support manual pre-configuration of security associations with other neighbouring FAs, involving shared keys and the default algorithms of [1]. POST-REGISTRATION FAs MUST implement the inter-FA authentication extension (FA-FA authentication extension) specified in [2] and MAY additionally use other security mechanisms. 4.1. Two Party Handoff Two party handoff occurs when the MN moves from oFA, where the MN performed a Mobile IP Registration, to nFA. In the normal case, this movement would result in a new Mobile IP Registration at nFA. However in POST-REGISTRATION, the MN and nFA MAY delay this but maintain El Malki (Editor) et. al. [Page 20] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 connectivity using the BET (or alternatively unidirectional tunnel) between oFA and nFA. The protocol is shown in Figure 6. 1a) L2-ST ~~~~> +------+ 2) HRqst +------+ <~~~ 1b) L2-TT | oFA |<-------->| nFA | 4a) L2-LD~> +------+ 3) HRply +------+ <~~~ 4b) L2-LU ^ ^ old L2 | | new L2 +-------+ +-----+ | | | | V V +------+ movement 4c) L2-LU ---> | MN | ---------> +------+ Figure 6 - Two Party Handoff (POST-REGISTRATION) The following describes the progress of a two party handoff. The numbered items refer to steps in Figure 6. To identify the difference between a source triggered HRqst/HRply message for tunnel creation, a target triggered HRqst/HRply message for tunnel creation and HRqst/HRply to extend or terminate a BET (or unidirectional tunnel), the message will be identified respectively by (s), (t) and (r). 1) Either the oFA or nFA receives an L2 trigger informing it that a certain MN is about to move from oFA to nFA. The two cases are: a) The L2 trigger is a source trigger (L2-ST) at oFA. The trigger contains the MN's L2 address and an IP identifier (the IP address itself or an L2 address that can be resolved to the IP address) for nFA. b) The L2 trigger is a target trigger (L2-TT) at nFA. The trigger contains the MN's L2 address and an IP identifier for oFA. 2) The FA receiving the trigger sends a Handoff Request (HRqst) to the other FA. There two cases: a) If oFA is sending the HRqst, the H bit is set and the N bit is unset, indicating it is an HRqst(s). The HRqst(s) contains the lifetime of the tunnel the oFA is willing to support, the home network IP address of the MN, the MN's HA address and an LLA option with the MN's L2 address. If the lifetime is zero and the T bit is not set, the oFA is not willing to tunnel any packets for MN. A positive lifetime and a set T bit indicate that the oFA is willing to tunnel for the indicated time. Section 4.6 describes El Malki (Editor) et. al. [Page 21] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 the HRqst(s) and Section 9 describes the LLA option. b) If nFA is sending the HRqst, the N bit is set and the H bit is unset, indicating it is an HRqst(t). If the T bit is set, nFA has requested a reverse tunnel and the HRqst(t) contains the lifetime of the tunnel the nFA is requesting. The HRqst(t) also contains an LLA option with the MN's L2 address. The MN's home network IP address and HA address are not sent, unless they are discovered by some means outside the scope of this document (for example, as part of the L2-TT). Section 4.6 describes the HRqst(t). 3) The FA receiving the HRqst sends a Handoff Reply (HRply) to the other FA. There are two cases: a) If oFA is sending the HRply, the N bit is set and the H and R bits are unset, indicating that the reply is in response to a HRqst(t), i.e. it is an HRply(t). If the T bit is set, the HRply(t) contains the tunnel lifetime the oFA is willing to provide, otherwise, the tunnel lifetime is set to zero, indicating that the oFA is not willing to provide tunnel service. If both HRply(t) and HRqst(t) have the T bit set and non-zero lifetime a BET is established. The HRply(t) also contains the MN's home subnet IP address, the MN's HA address, and an LLA option containing the MN's L2 address. Section 4.7 describes the HRply(t). b) If nFA is sending the HRply, the H bit is set and the N and R bits are unset, indicating the reply is in response to a HRqst(s), i.e. it is an HRply(s). If the T bit is set, the nFA indicates that it requests a reverse tunnel, and the lifetime field is set with the requested tunnel lifetime. The T Bit can be set in HRply only if the oFA had set the T bit in the corresponding HRqst or if the nFA requires to reverse tunnel incoming packets to oFA because ingress filtering is enabled on its network. This establishes a BET. The tunnel lifetime requested by the nFA must be less than or equal to the tunnel lifetime offered by oFA in the HRqst(s). Section 4.7 describes the HRply(s). 4) The point during the L2 handoff in which the MN is no longer connected on a certain link is signaled by an L2-LD trigger at oFA and MN. Completion of L2 handoff is signaled by an L2-LU trigger at nFA and MN. Each node handles the trigger in the following way: a) When the oFA receives the L2-LD trigger, it begins forwarding MN-bound packets through the forward tunnel to nFA. El Malki (Editor) et. al. [Page 22] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 b) When the nFA receives the L2-LU trigger, it begins delivering packets tunneled from the oFA to the MN and forwards any outbound packets from MN to the next hop using normal routing mechanisms or through the reverse tunnel to oFA or HA. c) When the MN receives the L2-LU, it MAY intiate the Mobile IP Registration process by soliciting an Agent Advertisement as described in [1]. If the Registration is successful the nFA takes over the role of anchor FA (aFA) from the oFA. Alternatively the MN MAY defer the Mobile IP Registration (see section 4.4). 5) The oFA becomes an aFA if the MN moves to a third FA before having performed a Mobile IP Registration with nFA. 6) Should L2 handoff fail in Step 4 (due to L2 reasons) and a ping- pong situation arise, the oFA may be able to determine this case through the trigger mechanism (i.e. FA sees successive L2-ST/L2- TT followed by L2-LD and then L2-LU). The FA which originated the HRqst can in this case cancel the tunnel by sending an HRqst(r) to the other FA with lifetime zero. It will then simply continue delivering packets to MN exactly as if no handoff had been pending. Section 4.6 describes the HRqst(r). If in the HRqst/HRply the oFA has set the B bit 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 either no security between the nFA and the MN's HA is required, or there is an existing nFA-HA security association in place. The actual timing of BET or unidirectional tunnel placement depends on the available L2 triggers. The forward tunnel from oFA to nFA is constructed using one of the tunneling procedures described in [1] for the HA to FA tunnel with the difference that the ends of the tunnel are at the oFA and nFA, respectively. The reverse tunnel from nFA to oFA is constructed as described in [4] with the difference that the network end of the tunnel is at the oFA instead of the HA. If both forward and reverse tunnels are established then a BET has been established. With optimal L2 trigger information, as described above, the FAs can setup the BET immediately when the L2 handoff is initiated, start tunneling MN-bound data when the link to the MN goes down and the nFA can use the link up trigger to start delivering packets. In the absence of optimal L2 trigger information, the HRply can act as the trigger to start tunneling MN-bound data, but in this case, the period of packet delivery disruption to the MN could still be present and additional measures may be required to provide uninterrupted service. Additonally, particular implementation and deployment scenarios could require that techniques be employed to El Malki (Editor) et. al. [Page 23] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 smooth handoff by providing a means to convey packets arriving during the L2 handoff. The exact techniques involved in smoothing are currently under discussion by the working group and are outside the scope of this document. Figures 7 and 8 show timing diagrams for source trigger (L2-ST) and target trigger (L2-TT) two party handoff, respectively. MN nFA oFA | | | | | HRqst(s) |<~~~ L2-ST | |<------------------| | | HRply(s) | | |------------------>| | | | --------------------------------------------<~~~ L2-LD L2 Handoff --------------------------------------------<~~~ L2-LU | | | |<------------------->| | | MN's traffic | | Figure 7 - Two Party Source Trigger Handoff Timing MN nFA oFA | | | | L2-TT ~~~>| HRqst(t) | | |------------------>| | | HRply(t) | | |<------------------| | | | --------------------------------------------<~~~ L2-LD L2 Handoff --------------------------------------------<~~~ L2-LU | | | |<------------------->| | | MN's traffic | | Figure 8 - Two Party Target Trigger Handoff Timing 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). El Malki (Editor) et. al. [Page 24] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 2) The MN completes the process by performing a Mobile IP Registration with the current FA. This may be initiated by the FA which sends an Agent Advertisement or by the MN which solicits for an Agent Advertisement as in [1]. 3) The MN moves to a third FA (see section 4.2) 4.2. Three Party Handoff Three party handoff is applicable when an MN that has already established an aFA and is receiving tunneled packets through its current FA moves to a new FA without performing a Mobile IP Registration. The need for this function depends on the wireless system in which POST-REGISTRATION is being implemented. For radio L2 protocols in which it is possible for the MN to move so rapidly from one FA to another such that a probability exists that the Mobile IP Registration with nFA will not complete before the MN moves on, HTT SHOULD be implemented. Certain wireless systems and implementations do not allow such fast movement between FAs and may force the Mobile IP Registration to occur soon after L2 handoff, in which case three party handoff is not applicable. If this three party handoff feature is not implemented, the FA SHOULD send an Agent Advertisement to the MN after L2 handoff has completed (L2-LU at nFA) and/or the MN SHOULD solicit a Router Advertisement after L2 handoff (L2-LU at MN). +------+ +--->| 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 Router Solicitations and FA does not send Agent Advertisements) or it may result from rapid movement El Malki (Editor) et. al. [Page 25] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 between oFA and nFA that does not allow enough time for the registration to complete. This scenario is shown in Figure 9. In this case, oFA must inform nFA (i.e. the third FA) to contact aFA about moving the radio end of the tunnel. This is performed with the Handoff To Third (HTT) message. The general idea behind the three party handoff procedure is that the oFA supplies nFA with the same information it would have obtained via an L2-TT if handoff had occurred from aFA to nFA, then the nFA performs an HRqst(t)/HRply(t) sequence with aFA in order to move the BET to nFA. When the L2 handoff is complete, oFA sends an HRqst(r) to aFA to terminate the previous BET. The following describes the progress of a three party handoff. The numbered items refer to steps in Figure 9. 1) Either the oFA or nFA receives an L2 trigger informing it that a certain MN is about to be moved. The two cases are: a) The L2 trigger is a source trigger (L2-ST) at oFA. The trigger contains the MN's L2 address and an IP identifier (IP address or L2 address that can be mapped to an IP address) for nFA. b) The L2 trigger is a target trigger (L2-TT) at nFA. The trigger contains the MN's L2 address and an IP identifier for oFA. 2) The oFA and nFA exchange a HTT/HRply or HRqst/HTT pair. HTT is indicated by setting both the H and N bits in the HRqst or HRply. The HTT message MUST NOT have any tunnel flags set, because the tunnel is negotiated between the aFA and nFA, not oFA and nFA. There are two cases: a) The L2 trigger is an L2-ST. The oFA sends HTT to nFA containing the MN's home IP address, the MN's HA address, an LLA containing the aFA's IP address, and an LLA containing the L2 address of the MN. This is enough information for nFA to perform a target triggered handoff with aFA. The nFA responds with a HRply(s). Section 4.8 describes the HTT. b) The L2 trigger is an L2-TT. The nFA sends HRqst(t) to oFA, exactly as if a two party handoff were occurring. The oFA responds with HTT containing the same information as in a) above. This is enough information for nFA to perform a target triggered handoff with aFA. 3) Upon receipt of the HTT, the nFA first checks its Visitor Cache to see whether it is already tunneling for MN. If so, Step 6 is performed. If not, nFA performs a target triggered handoff with El Malki (Editor) et. al. [Page 26] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 aFA, exactly as in Section 4.1, exchanging a HRqst(t)/HRply(t) pair. Because aFA receives no L2 trigger indicating when L2 handoff starts, it may start tunneling to nFA upon transmission of the HRply(t). 4) Once the L2 handoff is underway and the MN gets disconnected at L2, aFA and oFA exchange messages canceling tunnel service between aFA and oFA and allowing aFA to start the tunnel with nFA. a) The point in the L2 handoff process where the MN gets disconnected from oFA is signaled at oFA by L2-LD. b) The oFA exchanges a HRqst(r)/HRply(r) pair having lifetime zero with aFA. This cancels tunnel service between oFA and aFA. If aFA has not already established a tunnel to nFA, it must do so immediately upon receipt of the HRqst(r). The aFA provides tunneling service exactly as described in Section 4.1 Step 4a. 5) Completion of L2 handoff is signaled by an L2-LU trigger at nFA and/or MN. The nFA and MN handle the trigger in the following ways: a) The nFA provides packet delivery service to the MN exactly as described in Section 4.1, Step 4b. b) The MN either defers or initiates Mobile IP Registration when it receives the L2-LU, as in Section 4.1 6) In the special case where nFA and aFA are the same (i.e. the MN is moving back to the original anchor FA), aFA recognizes that it is tunneling to oFA when it checks its Visitor Cache in Step 3. In this case, there is no need for aFA to send the HRqst(t)/HRply(t) in Step 3. Upon receipt of the L2-LU trigger on handoff completion, the aFA begins routing packets to MN and the tunnel to nFA is torn down. The oFA still exchanges the HRqst(r)/HRply(r) with aFA in Step 4b because oFA cannot know a priori that aFA and nFA are the same, but they are redundant. Unlike two party handoff, the timing of BET establishment between aFA and nFA cannot fully depend on the availability of L2 trigger information because aFA does not receive an L2 trigger signalling L2 handoff. The two timing extremes at which aFA can place the BET with nFA are: 1) At the earliest, aFA MAY start tunneling packets using the BET to nFA after sending the HRply(t) to nFA in response to the request for target-triggered handoff 2) At the latest, aFA MAY start tunneling packets using the BET to El Malki (Editor) et. al. [Page 27] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 nFA and tear down the BET with oFA when receiving the HRqst(r) from oFA indicating the MN has disconnected. In addition, aFA MAY continue tunneling to oFA if 1) is selected, until the HRqst(r) is received. If 1) is selected and the aFA continues to tunnel to oFA, the result may be duplicated packets at the MN, because the MN will receive packets through oFA on the old L2 until it disconnects (L2-LD). If 2) is selected, the additional latency will add to the MN's L3 service disruption period. Of course, aFA can choose to place the BET some time between 1) and 2) if reliable bounds are available on the duration of time between L2- ST/L2-TT and the MN's disconnection (L2-LD). The exact selection of when to establish the BET is likely to be influenced by network engineering and implementation considerations, including whether a handoff smoothing solution is deployed, and is beyond the scope of this specification. Figures 10 and 11 show timing diagrams for source trigger (L2-ST) and target trigger (L2-TT) three party handoff, respectively. 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 (Editor) et. al. [Page 28] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 MN nFA oFA aFA | | | | | |<~~~ L2-TT | | | | | | | |------------->| | | | HRqst(t) | | | | | | | |<-------------| | | | HTT | | | | | | | |------------------------------>| | | HRqst(t) | | | | | | | |<------------------------------| | | HRply(t) | | | | | | ----------------------------------<~~~ L2-LD | |--------------->| L2 Handoff | HRqst(r) | | | |<---------------| | HRply(r) | | | ----------------------------------<~~~ L2-LU | | | | | | | | | |<-------------->| | | | MN's traffic | | | | | | | Figure 11 - Three Party Target Trigger Handoff Timing 4.3. Renewal or Termination of Tunneling Service To prevent a BET from expiring when its lifetime runs out, the MN's current FA signals the aFA to either renew or terminate the BET. This may be the case when the MN defers Mobile IP Registration. If no such signal is received, the aFA will terminate the BET when the lifetime expires. In addition, the current FA or aFA may need to terminate the BET prior to the lifetime expiring. In order to avoid error conditions in which tunnels do not expire even though the MN to which they apply is no longer reachable, FAs SHOULD set the tunnel lifetime field to some value other that 0xffff, which indicates "good until cancelled". Figure 12 illustrates the message exchange that occurs between the FA needing to terminate or extend the tunnel (designated FA(1) in the figure) and the other FA (designated FA(2) in the figure). The HRqst(r)/HRply(r) is indicated by setting the R bit in the HRqst/HRply messages. If the HRqst(r) is renewing a BET then it El Malki (Editor) et. al. [Page 29] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 contains a non-zero lifetime, otherwise if the lifetime is set to zero it indicates tunnel termination. The aFA has complete control over whether a tunnel is extended or terminated, and it MAY reply to a request for extension with a shorter lifetime than was requested. HRqst(r) +------+ <-------- +------+ | FA(2)| ---------> | FA(1)| +------+ HRply(r) +------+ Figure 12 - BET Renewal or Termination 4.4. When will the MN perform a Mobile IP Registration? The MN/FA have control over when to perform the Mobile IP Registration. Although the MN/FA may decide to defer Mobile IP Registration for a certain period, three possible events can lead to the need to terminate tunneling service. If this occurs the MN MUST perform the Mobile IP Registration. These events are: 1) The end of life for the BET is pending and a request by the current FA to aFA for renewal has been denied, or alternatively the current FA or aFA needs to terminate the BET prematurely. The FA in this case MUST initiate the Mobile IP Registration by sending an Agent Advertisement to the MN as in [1]. 2) The MN itself decides to perform a Mobile IP Registration and initiates it by sending an Agent solicitation as in [1]. 3) During a source triggered handoff, the oFA attempts to perform BET handoff but nFA is not capable of performing it. The FA in this case MUST initiate the Mobile IP Registration by sending the MN an Agent Advertisement as in [1]. Note that this situation will never arise during target triggered handoff because an HRqst(t) will not be sent to oFA by an nFA that doesn't support POST-REGISTRATION. Some detailed scenarios relating to case 2) will be described hereafter. According to [1], when using an FA care-of address the MN MAY use the FA as its default router. Otherwise it MUST choose its default router from those advertised in the ICMP Router Advertisement portion of the Agent Advertisement. Here we assume that the FA router is also the MN's default router. In POST-REGISTRATION, when both a forward and reverse tunnel are established between oFA and nFA (i.e. a BET) and the MN has moved to nFA, the oFA MUST continue sending Router Advertisements to the MN. This is to refresh the MN's default router entry. The Router Advertisements are tunnelled from oFA to nFA through the forward tunnel and MUST be unicast to the MN. Similarly to PRE-REGISTRATION, tunneling of Advertisements is possible only if the TTL limitation of [1] is relaxed. If this is not possible then El Malki (Editor) et. al. [Page 30] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 the nFA MUST advertise to the MN as soon as it's link to the nFA is up (L2-UP). The MN MUST perform a Mobile IP registration [1] when it receives an Agent Advertisement following a POST-REGISTRATION handoff. Instead, when the forward tunnel is established but not the reverse tunnel, oFA MUST NOT advertise to the MN. In this case, as described previously, it is possible that the MN will not receive Router Advertisements for extended periods of time. According to [14] hosts will remove default router entries if the lifetime of the Router Advertisement expires and no further advertisements are received. Note that the ICMP Router Advertisement lifetime is not related to the Registration Lifetime in the Mobility Agent Advertisement extension [1]. To avoid this disruption the MN MUST solicit the default router (i.e. FA) before the lifetime of its active default router entry runs out, or alternatively the FA MUST advertise as soon as the MN-nFA link is up (L2-UP). This effectively means that the MN will at most be able to defer Mobile IP Registration for as long as the remaining lifetime of the active default router, as configured in the ICMP Router Advertisements. The MN MUST perform a Mobile IP registration [1] when it receives an Agent Advertisement following a POST-REGISTRATION handoff. 4.5. Handoff Request (HRqst) Message format This is a new Mobile IP message carried on UDP (destination port 434) [1]. The UDP header is followed by the fields below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type |H|N|R|M|G|T|B| Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Lifetime | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MN Home Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | HA Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + Identification + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Extensions ... +-+-+-+-+-+-+-+- Type TBD (Handoff Request) H Source triggered handoff request. When set and the N bit is unset, indicates that the request El Malki (Editor) et. al. [Page 31] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 was the result of an L2-ST at oFA. N Target triggered handoff request. When set and the H bit is unset, indicates that the request was the result of an L2-TT at nFA. R Set if the request is an HRqst(r), i.e. a request to renew the tunnel. Neither the H nor the N bit are set. M The FA issuing the HRqst will use Minimal Encapsulation as defined in [1,5] for the tunnel. G The FA issuing the HRqst will use GRE [5] Encapsulation as defined in [1,5] for the tunnel. When this flag is set the HRqst may require extensions containing the GRE type and key fields, but they are outside the scope of this document. T For an HRqst(s), indicates that the oFA is willing to support both forward and reverse tunnel service. For an HRqst(t), indicates that the nFA is requesting reverse tunnel service. B When sent in an HRqst(s), indicates that the MN has requested a reverse tunnel to the HA and that the nFA SHOULD use reverse tunnel to the HA if it will not be reverse tunneling to the oFA. Lifetime The lifetime, in seconds, for which tunnel service for the MN will be maintained. If this is an HRqst(t), then the lifetime represents a request by nFA for a reverse tunnel. If this is an HRqst(s), then the lifetime represents the maximum amount of time that oFA is willing to maintain the both the forward and reverse tunnel. If this is an HRqst(r), then the lifetime Represents a request for the amount of time to renew the tunnel's lifetime. A value of 0 on an HRqst(s) indicates that the oFA is unwilling to grant any tunnel service. A value of 0 on an HRqst(t) indicates that the nFA does not require reverse tunnel service. A value of 0 on an HRqst(r) indicates that the tunnel should be terminated immediately. A value of 0xffff indicates infinity. MN Home Address For HRqst(s), the home address of the MN. HA Addr For HRqst(s), the HA address of the mobile node. El Malki (Editor) et. al. [Page 32] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 Identification As in defined in [1]. Extensions The Message MUST include an LLA (see Section 9) containing the MN's L2 address and an L2 address that can be mapped to an IP address for the FA. This Message MUST contain the FA-FA Authentication Extension [2] that is used to secure the HRqst message. 4.6. Handoff Reply (HRply) Message This is a new Mobile IP message carried on UDP (destination port 434) [1]. The UDP header is followed by the fields below. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type |H|N|R|M|G|T|B| Reserved | Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Lifetime | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MN Home Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | HA Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + Identification + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Extensions ... +-+-+-+-+-+-+-+- Type TBD (Handoff Reply) Code A value indicating the result of the Handoff Request. Only two codes are currently supported, 0, indicating success, and a nonzero value, indicating that the handoff cannot be performed. Lifetime The lifetime, in seconds, for which the bi-directional tunnel for the MN will be maintained. If this is an HRply(s), then the lifetime represents a request by nFA, and it can be any value up to the maximum value sent in the HRqst(s). Larger values are assumed to default to OFA's maximum. If this is an HRply(t), then the lifetime represents the maximum amount of time that the oFA will grant to the nFA. If this is a HRply(r), then the lifetime represents the amount of time by which the tunnel life will be El Malki (Editor) et. al. [Page 33] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 extended. If the Code field indicates that handoff failed, the Lifetime field will be ignored and SHOULD be set to zero. A value of 0 on an HRply(t) indicates that the oFA is unwilling to grant service. A value of 0 on an HRply(s) indicates that the nFA does not require service. A value of 0 on HRply(r) indicates that the tunnel lifetime will be terminated. A value of 0xffff indicates infinite lifetime. H Source triggered handoff reply. When set and the N bit is unset, indicates that the reply is in response to an HRqst(s). N Target triggered handoff reply. When set and the H bit is unset, indicates that the reply is in response to an HRqst(t). R Set if the reply is an HRply(r). Neither the H nor the N bit are set. M The FA issuing the HRqst will use Minimal Encapsulation as defined in [1,5] for the tunnel. G The FA issuing the HRqst will use GRE [5] Encapsulation as defined in [1,5] for the tunnel. When this flag is set the HRply may require extensions containing the GRE type and key fields, but they are outside the scope of this document. T For an HRply(s), indicates that the nFA is Requesting to reverse tunnel service. For an HRply(t), indicates that the oFA is willing to provide both forward and reverse tunnel service. B When sent in an HRply(t), indicates that the MN has requested a reverse tunnel to the HA and that the nFA SHOULD use reverse tunnel to the HA if it will not be reverse tunneling to the oFA. It can be set in HRply(t) only if the T bit was unset in the corresponding HRqst(t). MN Home Address For HRply(t), the home address of the MN. HA Addr For HRply(t), the HA address of the mobile node. Identification As in defined in [1]. Extensions This Message MUST contain the FA-FA Authentication Extension [2] that is used to El Malki (Editor) et. al. [Page 34] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 secure the HRply message. 4.7. Handoff To Third (HTT) Message The Handoff to Third message has the same format as the Handoff Request and Handoff Reply Messages, except both the H and N bits are set. If the HTT message is in response to a L2-ST and is sent to initiate a handoff, then, with the exception of the H and N bits, the message has the same fields set and includes the same extensions as an HRqst(s). If the HTT message is sent in response to an HRqst(t), then, with the exception of the H and N bits, the message has the same fields set and includes the same extensions as an HRply(t). The tunnel bits MUST NOT be set in the HTT message because BET construction is not negotiated between oFA and nFA, it is negotiated between nFA and aFA in the ensuing HRqst(t)/HRply(t). In addition, the HTT MUST contain the following extensions in the specified order: Solicited IP Address Option: containing aFA's Address LLA Option: containing the L2 address of the MN. 4.8. Applicability of POST-REGISTRATION Handoff Method The POST-REGISTRATION handoff approach allows FAs to communicate directly about a pending handoff, and does not require any IP layer messages to be sent to or from a MN prior to the L2 handoff event. Therefore, it eliminates a possible source of handoff latency. This may be required when the link layer imposes hard deadlines on the time at which a handoff must occur, such as when a MN is rapidly moving out of a radio coverage area. Consequently, POST-REGISTRATION is primarily of interest in handoff between FAs that support the same radio access technology. Handoff between heterogeneous radio technologies will, of necessity, require interaction between the MN and the network, and so is not a domain of applicability for POST- REGISTRATION. Because a POST-REGISTRATION handoff is triggered by an unspecified mechanism that informs the oFA or nFA that an L2 handoff is pending, the POST-REGISTRATION approach is only applicable to networks where such a mechanism is available. For example, an L2 may provide indications of radio signal quality that cause the oFA or nFA to send the POST-REGISTRATION handoff messages. Any such indications must also provide each FA involved in the handoff with the identity of the other, so that messages can be sent to the right place. This may involve mapping L2 information onto FA IP addresses. Also, the FAs involved in a handoff must have pre-provisioned security arrangements so that the POST-REGISTRATION messages can be authenticated. If a El Malki (Editor) et. al. [Page 35] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 handoff is to be completed as a result of the POST-REGISTRATION messaging, any L2 handoff indications must also be securely authenticated so that traffic to the old point of attachment is not improperly halted. POST-REGISTRATION handoff is appropriate in the following cases: - L2 triggers are available on the network to indicate that L2 handoff is pending. - Pre-provisioned security mechanisms are in place to allow fast and secure messaging between the FAs and between the MN and an FA. - Access point choice by the MN is not a concern or choice requires user intervention and therefore is not on the critical path for handoff. 5. Combined Handoff Method The combined method uses both PRE-REGISTRATION and POST-REGISTRATION handoff by running the PRE-REGISTRATION method and in parallel exchanging the POST-REGISTRATION handoff messages between oFA and nFA. The only case not considered already in the POST-REGISTRATION method is mobile-initiated handoff. In the mobile-initiated case, the Handoff Request message is initated by the oFA or nFA when it receives the Registration Request from the MN. The combined method follows the PRE-REGISTRATION Handoff when it is successful before the completion of the MN's L2 handoff. However, if PRE-REGISTRATION does not complete prior to the expiration of a timer on one or the other of the FAs, POST-REGISTRATION handoff is used. Using POST-REGISTRATION handoff insulates the MN from delays caused by errors such as loss of one of the Mobile IP messages involved in PRE-REGISTRATION. The start of POST-REGISTRATION is gated by the expiration of a timer on the FAs. The timer is started at oFA following a source-trigger, at nFA following the target-trigger, or at oFA and nFA following the receipt of the Registration Request from the MN in the mobile- initiated case. The timer is reset if the Registration Reply message is received by the appropriate FA and sent to the MN. Although the POST-REGISTRATION Handoff Request and Handoff Reply messages are exchanged in advance, no forwarding of traffic between oFA and nFA is performed unless the timer expires. The timer should be set to a value that allows forwarding between oFA and nFA to begin before the MN completes the L2 handoff to nFA. El Malki (Editor) et. al. [Page 36] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 6. Layer 2 and Layer 3 Handoff timing Considerations In the optimal cases considered in the PRE-REGISTRATION and POST- REGISTRATION handoffs it was assumed that a timely L2 trigger would be received in such a way that packets could be delivered to the MN via its nFA immediately upon connection. In this way the MN would not suffer disruption due to the L3 handoff. However such precise timing of the L2 trigger and handoff mechanism with respect to the actual L2 handoff event will not be possible in all wireless systems and may depend on particular implementation techniques. Therefore some uncertainty may exist at L3 as to exactly when the L2 connection between the MN and the nFA becomes fully established and can be used for L3 traffic. It is possible that in certain implementations traffic will be re-reouted too early or too late with respect to the moment when the connection between the MN and the nFA becomes fully established. The techniques which will solve this problem and allow the MN to receive traffic independently of the timing of the L2 handoff event are currently under study by the Mobile IP WG but are outside the scope of this document. 7. Reverse Tunneling Support The handoff methods all support reverse tunneling. The MN may request reverse tunneling [4] by setting the 'T' bit in its Registration Request. In the case of POST-REGISTRATION, if the MN had requested Reverse Tunneling previously at oFA, the Handoff message from oFA (see Section 4) includes the 'T' bit enabled to inform nFA to establish a BET for the visitor entry. Typically, the 'T' bit will always be set to ensure that any delays in the MN receiving its new care of address do not result in any delay in uplink packet transmission from the MN, but local policies and particular L2 technologies may allow the reverse tunnel to be turned off unless the MN specifically requests it. 8. Handoff Signaling Failure Recovery In general and to a greater extent in wireless networks, packets carrying handoff signaling may be dropped or lost due to errors on the link. In this section, we consider mechanisms for recovery from handoff signaling failures. 8.1. PRE-REGISTRATION Signaling Failure Recovery Failure of PRE-REGISTRATION signaling breaks down into three cases: 1) Loss of messages ProxyRtSol and ProxyRtAdv on the air link. 2) Loss of the solicitation by an FA to obtain another neighbouring FA's Advertisment or loss of the neighbouring FA's El Malki (Editor) et. al. [Page 37] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 advertisement. 3) Failure of the standard Mobile IP Registration. Of these, case 3) is handled by standard Mobile IP mechanisms described in RFC 2002 [1]. Case 2) is relatively unlikely because spontaneous packet drop rates on the fixed network are caused by congestion or router failure and likely to be low. Since bit error rates on wireless links are higher than on fixed links, case 1) is more likely to occur. In the following subsections, the cases 1) and 2) are considered. 8.1.1. Failure of ProxyRtSol and ProxyRtAdv PRE-REGISTRATION handoff can fail in network-initiated handoff when the ProxyRtAdv sent by oFA in response to the source trigger (L2-ST) or the advertisement sent by nFA in response to the target trigger (L2-TT) fails to reach the MN. PRE-REGISTRATION handoff can also fail in mobile-initiated handoff when either the ProxyRtSol sent from the MN or return ProxyRtAdv sent from the oFA are dropped. To reduce the probability that ProxyRtAdv and ProxyRtSol are lost the MN and FA MAY transmit multiple copies of these messages. Sholuld these messages fail anyway, in both cases the MN connects to the nFA without having received any prior signaling. When this happens the MN MUST solicit an FA Advertisement when it connects to nFA at L2 (L2-UP) and perform standard Mobile IP registration on the nFA as specified in [1]. 8.1.2. Failure of Inter-FA solicitation and advertisement The solicitation from an FA to another neighbouring FA may fail or the corresponding advertisement from the neighbouring FA may be lost. To reduce the probability that these messages are lost, the FAs MAY transmit multiple copies of these messages. If a failure occurs anyway, the FA soliciting the Agent Advertisment is unable to send a ProxyRtAdv in response to a source trigger or to a mobile-initiated ProxyRtSol. In these cases, when the MN does not receive a notification or confirmation of a PRE-REGISTRATION handoff, the MN MUST perform a standard Mobile IP registration as soon as it connects to the nFA (L2-UP) as specified in 8.1.1 and [1]. 8.2. POST-REGISTRATION Signaling Failure Recovery Failure occurs in POST-REGISTRATION when either the HRqst or HRply message is dropped. The effects of the failure and the recovery procedure depend on which message is dropped, and whether the handover is source or target triggered. Since all of the POST- REGISTRATION signaling is going over the fixed network, it can be expected that spontaneous dropping of packets in the absence of congestion and router failure should be a relatively rare event. Nevertheless, failure recovery mechanisms SHOULD be implemented. El Malki (Editor) et. al. [Page 38] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 8.2.1. HRqst Message Dropped If the HRqst message is dropped, the effect is the same for both source and target-triggered handoff. In either case, the FA to which the HRqst was destined will never respond with an HRply message. If the handoff is source-triggered, then the nFA never learns of the handoff, and the oFA never receives confirmation. If the handoff is target-triggered, then the oFA never learns of the handoff, and the nFA never receives confirmation. The recovery procedure in this case is as follows: the oFA MUST NOT construct a forward tunnel when the MN moves off-link (L2-LD) if the handoff is source-triggered, and the nFA MUST NOT construct a reverse tunnel if the handoff is target-triggered. If the nFA was not informed of the handoff by an HRqst message (corresponding to failure of source-triggered handoff) or if the handoff was not confirmed by an HRply message (corresponding to failure of target-triggered handoff) the nFA MUST unicast an Agent Advertisement to the MN as soon as its L2 connection is established (L2-LU at nFA). 8.2.2. HRply Message Dropped If the HRply message is dropped, the FA sending the HRply will assume that the handoff has been confirmed, but the FA that is expecting to receive the HRply does not receive confirmation. In this case, the failure recovery procedure is different for source-triggered and target-triggered handoffs. In a target-triggered handoff, the oFA assumes the handoff is confirmed because it has sent the HRply, but the nFA has not received it so it does not have confirmation. The oFA starts tunneling packets to the nFA when the MN moves from its link (L2-LD). The nFA MUST send a FA Advertisement to the MN as soon as its L2 link is up (L2-UP at nFA) and MAY drop the tunneled packets. The nFA SHOULD send an ICMP Destination Unreachable [15] message to the oFA. When the oFA receives this message it will terminate the tunnel and stop forwarding packets. If reverse tunneling was requested the nFA MUST NOT reverse tunnel because it has not received confirmation of the handoff. In source-triggered handoff, the nFA assumes the handoff is confirmed because it has sent the HRply, but the oFA has not received it so it does not have confirmation. Without failure recovery, the MN could move to the nFA without the oFA being able to start the forward tunnel for the MN's packets, and the MN would not be able to initiate a Mobile IP registration because it does not know that the handoff has failed. In this situation, the oFA MUST send out a HRqst message to the nFA with lifetime zero as soon as the MN leaves its link (L2- LD). The oFA SHOULD continue to retransmit the HRqst message, with exponential backoff for CONFIG-HFAIL seconds or until it receives an HRply acknowledging the request to cancel the tunnel. The default El Malki (Editor) et. al. [Page 39] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 value for CONFIG-HFAIL is 10 seconds. When the nFA receives the HRqst, it MUST immediately send an Agent Advertisement to the MN, as is the case whenever a tunnel is cancelled. In addition, the oFA MUST also drop any packets received through the reverse tunnel from the nFA. The oFA SHOULD NOT send the ICMP Destination Unreachable message to the nFA because the nFA has been informed by the HRqst message to cancel the tunnel. However, if the nFA receives an ICMP Destination unreachable message for the tunnel prior to receiving the HRqst canceling the tunnel, it MUST send an FA Advertisement to the MN and cancel the tunnel. 9. Generalized Link Layer Address Extension This section defines the Generalized Link Layer Address (LLA) Extension, used by any node that needs to communicate Link Layer Addresses. The format of the extension relies on sub-types, where each sub-type defines its own sub-structure. This draft defines six sub-types. Future RFCs should allocate their own sub-type and define their own address formats. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Sub-Type | LLA ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type TBD (skippable) [1] - when used for Mobile IP Registrations TBD (skippable) [1] - when used for Router Advertisements Length The length of the Link Layer Address + the one octet Sub-Type field Sub-Type This field contains the Link Layer sub-type identifier LLA Contains the Link Layer Address In this document, six subtypes are defined: 1 3GPP2 International Mobile Station Identity and Connection ID [6] 2 3GPP International Mobile Subscriber Identity [13] El Malki (Editor) et. al. [Page 40] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 3 Ethernet 48 bit MAC address [7] 4 64 bit Global ID, EUI-64 [8] 5 Solicited IP Address 6 Access Point Identifier The following subsections describe the extensions. 9.1. 3GPP2 IMSI Link Layer Address and Connection ID Extension The IMSI Link Layer Address Extension contains the International Mobile Station Identity. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Sub-Type | IMSI ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type TBD (skippable) [1] Length The length of the IMSI field + the one octet Sub-Type field Sub-Type 1 IMSI Contains the IMSI, in the form: : Where the is an ASCII-based representation of the International Mobile Station Identifier, most significant digit first, ":" is ASCII 0x3a, and the Connection ID is the ASCII representation of a small, decimal number used for distinguishing different link-layer connections from the same device. 9.2. 3GPP IMSI Link Layer Address Extension The IMSI Link Layer Address Extension contains the International Mobile Station Identity. El Malki (Editor) et. al. [Page 41] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Sub-Type | IMSI ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type TBD (skippable) [1] Length The length of the IMSI field + the one octet Sub-Type field Sub-Type 2 IMSI Contains the IMSI, a number composed of 15-digits or less, coded as described in [13]. 9.3. Ethernet Link Layer Address Extension The Ethernet Link Layer Address Extension contains the 48 bit Ethernet MAC Address, as defined in [7]. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Sub-Type | MAC ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type TBD (skippable) [1] Length 7 (includes the Sub-Type field) Sub-Type 3 MAC Contains the 48 bit Ethernet MAC Address. El Malki (Editor) et. al. [Page 42] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 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 [8]. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Sub-Type | MAC ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type TBD (skippable) [1] Length 9 (includes the Sub-Type field) Sub-Type 4 MAC Contains the 64-Bit Global Identifier Address. 9.5. Solicited IP Address Extension The 32-bit Solicited IP Address Extension contains the IP address of the agent (FA) being solicited. This extension MAY be present in an ICMP Agent Solicitation as explained in Section 3.3. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Sub-Type | IP addr ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type TBD (skippable) [1] Length 5 (includes the Sub-Type field) Sub-Type 5 El Malki (Editor) et. al. [Page 43] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 IP Address Contains the 32-Bit IP Address of the solicited node. 9.6. Access Point Identifier Extension The 32-bit Access Point Identifier Extension contains an Identifier of the Access Point to which the MN will move. This may be a wireless L2 identifier. The MN is able to solicit an advertisement from the FA servicing a certain Access Point by using this extension with Agent Solicitations as explained in Section 3.3. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Sub-Type | AP ID... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type TBD (skippable) [1] Length 5 (includes the Sub-Type field) Sub-Type 6 AP ID Contains the 32-Bit Access Point Identifier. 10. IANA Considerations Section 9 introduces the Generalized Link Layer Address Extension numbering space that requires IANA management. This specification makes use of the subtype values 1-6, and all other values other than zero (0) are available for assignment via IETF consensus [12]. The numbers for the Generalized Link Layer Address Extension are taken from the numbering space defined for Mobile IP registration and Router Advertisement extensions in RFC 2002 [1]. The same Generalized Link Layer Address Extensions are used in both Mobile IP Registration and Router Advertisement messages, which have different extension numbering spaces defined in [1]. Therefore two separate Generalized Link Layer Address Extension numbering spaces are required having the same sub-type values. The Generalized Link Layer Address Extension numbering MUST NOT conflict with any numbers used in RFC 2002 [1], El Malki (Editor) et. al. [Page 44] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 RFC 2344 [4], RFC 2356 [10], RFC 2794 [11] and RFC 3012 [9]. In the POST-REGISTRATION Handoffs method, Sections 4.4 and 4.5 require numbers assigned from the Mobile IP control message type address space. The numbers assigned MUST NOT conflict with [1] and [2]. 11. Security Considerations A security consideration for PRE-REGISTRATION method, as discussed in Section 3.8, is that oFA and nFA MUST share a security association to authenticate messages transported between them and oFA must be authorized to solicit nFA. The inter-FA messages (solicitations and advertisements) MUST be authenticated using ESP [16]. The absence of this security would allow denial of service attacks from malicious nodes at any distance from the FA. Otherwise, PRE-REGISTRATION uses the security mechanisms described in [1] and [2]. POST-REGISTRATION introduces a new change to Mobile IP, which is the possibility that a MN may receive packets from an FA with which it has not yet registered. In the event that the MN does not wish to receive packets from unknown FAs, it MAY drop them. In a similar way to PRE-REGISTRATION, oFA and nFA MUST share a security association required to protect the Handoff Request and Reply messages. The Handoff Request and Reply messages MUST be authenticated using the FA-FA authentication extension [2]. The absence of this security would allow impersonation attacks and denial of service attacks. The minimal requirement is that all FAs involved in low latency handoffs MUST support manual pre-configuration of security associations with neighbouring FAs, involving shared keys and the default algorithms of [1]. Since the techniques outlined in this document depend on particular L2 information (triggers) to optimize performance, some level of L2 security is assumed. Both PRE and POST-REGISTRATION techniques depend on L2 triggers, but the L2 security implications are different for the two techniques. In particular, in POST-REGISTRATION the L2 triggers initiate the establishment of tunnels which route IP packets for the MN to its new location. Therefore the L2 triggers MUST be secured against any tampering by malicious nodes, either mobile or within the wired network. The L2 addresses or IP addresses for the MN and the FAs that appear in the L2 triggers MUST correspond to the actual nodes that are participating in the handover. If there is any possibility that tampering may occur, the recipient of an L2 trigger MUST have some way of authenticating the L2 information. Provided the L2 triggers are so secured, the nodes involved in a handover can reject any traffic from a node whose L2 address or IP address was not received in a trigger, yet tries to send traffic. Wireless networks that do not provide such features will be subject to impersonation El Malki (Editor) et. al. [Page 45] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 attacks, where malicious nodes could cause FAs to believe that a MN has moved to other service areas or to allow a bogus MN to obtain unauthorized service from an FA prior to performing a Mobile IP registration. In PRE-REGISTRATION the security of L2 triggers has different implications. The PRE-REGISTRATION technique depends on Mobile IP security between MN and FA, so the same security considerations in [1] apply. Should malicious nodes be able to generate or modify L2 trigger information (i.e. L2-ST or L2-TT), this would cause advertisements to be sent to the MN. They would consume wireless resources and processing in the MN, but would not allow an impersonation attack. In order to prevent such denial of service attacks, there should be a limit on the number of advertisements that an FA (oFA) will relay to the MN as a result of the reception of L2 triggers. This number will depend on the L2 technology. In order to prevent any such denial of service attacks the L2 triggers SHOULD be secured. 12. Acknowledgements Thanks to the Mobile IP WG chairs, Phil Roberts and Basavaraj Patil, for their input and to Jonathan Wood for valuable comments on PRE- REGISTRATION. 13. References [1] C. Perkins, Editor, "IP Mobility Support for IPv4", RFC3220, January 2002. [2] E. Gustafsson, A. Jonsson and C. Perkins, "Mobile IP Regional Tunnel Management ", draft-ietf-mobileip-reg-tunnel-06 (work in progress), March 2002. [3] S. Bradner. "Key words for use in RFCs to Indicate Requirement Levels". BCP 14, RFC 2119, March 1997. [4] G. Montenegro, "Reverse Tunneling for Mobile IP, revised", RFC 3024, January 2001. [5] D. Farinacci, T. Li, S. Hanks, and P. Traina, "Generic Routing Encapsulation (GRE)", RFC 2784, Internet Engineering Task Force, March 2000. [6] TIA/EIA/IS-2000. [7] D. Plummer, "An Ethernet Address Resolution Protocol - or Converting Network Protocol Addresses to 48.bit Ethernet Address for Transmission on Ethernet Hardware", RFC 826, Symbolics,Inc., November 1982. El Malki (Editor) et. al. [Page 46] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 [8] IEEE, "Guidelines for 64-bit Global Identifier (EUI-64) Registration Authority", http://standards.ieee.org/regauth/oui/tutorials/EUI64.html, March 1997. [9] C. Perkins, P. Calhoun, "Mobile IP Challenge/Response Extensions", RFC 3012, November 2000. [10] G. Montenegro and V. Gupta, "Sun's SKIP Firewall Traversal for Mobile IP", RFC 2356, June 1998. [11] P. Calhoun, C. Perkins, "Mobile IP Network Access Identifier Extension", RFC 2794, March 2000. [12] T. Narten, H, Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. [13] 3GPP TS 23.003 (www.3gpp.org). [14] S. Deering, "ICMP Router Discovery", RFC1256, September 1991 [15] J. Postel, "Internet Control Message Protocol," RFC 792, September 1981. [16] S.Kent, R. Atkinson, "IP Encapsulating Security Payload (ESP)", RFC 2406, November 1998. 14. Authors' Addresses The document editor and authors may be contacted at the addresses below: Karim El Malki Ericsson Radio Systems AB LM Ericssons Vag. 8 126 25 Stockholm SWEDEN Phone: +46 8 7195803 Fax: +46 8 7190170 E-mail: Karim.El-Malki@era.ericsson.se Pat Calhoun Black Storm Networks 250 Cambridge Ave. Suite 200 Palo Alto, CA 94306 USA El Malki (Editor) et. al. [Page 47] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 Phone: +1 650 617 2932 E-mail: pcalhoun@bstormnetworks.com Tom Hiller Lucent Technologies Rm 2F-218 263 Shuman Blvd Naperville, IL 60566-7050 USA Phone: +1 630 979 7673 Fax: +1 630 979 7673 E-Mail: tom.hiller@lucent.com James Kempf NTT DoCoMo USA Labs 181 Metro Drive, Suite 300 San Jose, CA 95110 USA Phone: +1 408 451 4711 EMail: kempf@docomolabs-usa.com Peter J. McCann Lucent Technologies Rm 2Z-305 263 Shuman Blvd Naperville, IL 60566-7050 USA Phone: +1 630 713 9359 Fax: +1 630 713 4982 E-Mail: mccap@lucent.com Ajoy Singh Motorola 1501 West Shure Drive Arlington Heights, IL o 60004 USA Phone: +1 847 632 6941 E-mail: asingh1@email.mot.com Hesham Soliman Ericsson Radio Systems Torshamnsgatan 23, Kista El Malki (Editor) et. al. [Page 48] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 Stockholm SWEDEN Phone: +46 8 4046619 Fax: +46 8 4043630 E-mail: Hesham.Soliman@era.ericsson.se Sebastian Thalanany Motorola 1475 West Shure Drive Arlington Heights, IL - 60004 USA Phone: +1 847 435 9296 E-mail: sthalan1@email.mot.com The working group can be contacted via the current chairs: Basavaraj Patil Phil Roberts Nokia Corporation Megisto Systems Inc. 6000 Connection Drive Suite 120, 20251 Century Blvd Irving, TX 75039 Germantown MD 20874 USA USA Phone: +1 972-894-6709 Phone: +1 847-202-9314 EMail: Raj.Patil@nokia.com EMail: proberts@megisto.com Fax : +1 972-894-5349 15. Full Copyright Statement Copyright (C) The Internet Society (2001). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in anyway, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided onan "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING El Malki (Editor) et. al. [Page 49] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE." El Malki (Editor) et. al. [Page 50] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 Appendix A - Gateway Foreign Agents The Mobile IP Regional Registration specification [2] introduces the Gateway Foreign Agent (GFA), as a mobility agent that two FAs providing service to a MN have in common. Figure A.1 provides an example of a MN's initial registration through the GFA. If this is the first registration message, the message MUST be forwarded to the HA. All packets destined for the mobile will be delivered to the GFA, which in turn will forward the packets to the FA servicing the MN. Reg Req +-----+ Reg Req +----------->| oFA |--------------+ | +-----+ | | v +----+ +-----+ Reg Req +----+ | MN | | GFA |<------->| HA | +----+ +-----+ +----+ +-----+ | nFA | +-----+ Figure A.1 - Initial Registrations through GFA If the MN moves to a nFA that is serviced by a GFA common with oFA, the MN MAY issue a Regional Registration Request (see Figure A.2). The Regional Registration message does not need to be forwarded to the HA, since the MN's traffic can still be delivered to the same GFA. This optimized approach effectively reduces the latency involved in the registration process. +-----+ | oFA | +-----+ +----+ +-----+ +----+ | MN | | GFA | | HA | +----+ +-----+ +----+ | ^ | +-----+ | +------------>| nFA |-------------+ Regional Reg +-----+ Regional Reg Figure A.2 - Regional Registration through GFA Note that the GFA may also be the MN's first-hop router. El Malki (Editor) et. al. [Page 51] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 Appendix B - Low Latency Handoffs for Multiply-Interfaced MNs For MNs that have two wireless network interfaces, either on the same wireless network or on wireless networks having different wireless L2 technologies, the techniques discussed in this document may be unnecessary if the Mobile IP stack on the MN allows switching an IP address binding between interfaces. This Appendix discusses how multiple wireless interfaces can aid low latency handoff. Figure B.1 illustrates the normal and hierarchical MIPv4 models. As shown in the figure, assume that the MN is connected to Radio Network 1 (RN1) and is registered with oFA through which it is receiving traffic. Suppose MN enters the coverage area of RN2 and nFA and that it prefers connectivity to this network for reasons beyond the scope of this document (e.g. user preferences, cost, QoS available etc.). The MN activates the interface to RN2 but continues communicating through RN1. The MN may solicit advertisements from nFA through the interface connected to RN1 to speed up the handoff process, provided there is no TTL restriction, or it can solicit advertisements through the interface connected to RN2 if it has been configured for IP traffic. +------+ +---------+ | HA |--------| (GFA) | +------+ +---------+ / \ / \ ... ... / \ / \ +------+ +------+ | oFA | | nFA | +------+ +------+ | | +------+ +------+ | RN1 | | RN2 | +------+ +------+ +------+ | MN | ---------> +------+ Movement Figure B.1 - Network Model for Mobile IPv4 With Multi-Access Once the MN is registered with nFA and is successfully receiving and transmitting through the new network, it tears down the interface to El Malki (Editor) et. al. [Page 52] INTERNET-DRAFT Low Latency Mobile IPv4 Handoffs June 2002 RN1. If the MN has enough time to complete this procedure without incurring degraded service or disconnection, the MN would experience a seamless multi-access handoff but it may not be possible in all cases, due to network coverage or for other reasons. Should multiple interface handoff be possible then the low latency methods described in this document are not necessary. In order to support the possible failure of the connectivity with the new network (RN2/nFA) in the short period following handoff, the MN may use the "S" bit in its Mobile IP Registration Request to maintain simultaneous bindings both its existing (HA or GFA) binding with oFA and a new binding with nFA. El Malki (Editor) et. al. [Page 53]