Mobile IP Working Group Rajeev Koodli, Editor INTERNET DRAFT Nokia Research Center 10 October 2003 Fast Handovers for Mobile IPv6 draft-ietf-mobileip-fast-mipv6-08.txt Status of This Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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 to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at: http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at: http://www.ietf.org/shadow.html. This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind. Distribution of this memo is unlimited. Abstract Mobile IPv6 enables a Mobile Node to maintain its connectivity to the Internet when moving from an Access Router to another, a process referred to as handover. During handover, there is a period when the Mobile Node is unable to send or receive packets both due to link switching delay and IP protocol operations. This ``handover latency'' resulting from standard Mobile IPv6 procedures, namely movement detection, new Care of Address configuration and Binding Update, is often unacceptable to real-time traffic such as Voice over IP. Reducing the handover latency could be beneficial to non real-time, throughput-sensitive applications as well. This document specifies enhancements to reduce the handover latency due to standard Mobile IPv6 procedures. This document does not address improving the link switching latency. Koodli (Editor) Expires 10 April 2004 [Page i] Internet Draft Fast Handovers 10 October 2003 Contents Status of This Memo i Abstract i 1. Introduction 2 2. Terminology 2 3. Protocol Overview 3 3.1. Addressing the Handover Latency . . . . . . . . . . . . . 3 3.2. Protocol Operation . . . . . . . . . . . . . . . . . . . 6 3.3. Protocol Operation of Network-initiated Handover . . . . 8 4. Protocol Details 9 5. Other Considerations 13 5.1. Handover Capability Exchange . . . . . . . . . . . . . . 13 5.2. Determining New Care of Address . . . . . . . . . . . . . 13 5.3. Packet Loss . . . . . . . . . . . . . . . . . . . . . . . 13 5.4. DAD Handling . . . . . . . . . . . . . . . . . . . . . . 14 5.5. Fast or Erroneous Movement . . . . . . . . . . . . . . . 14 6. Message Formats 15 6.1. New Neighbor Discovery Messages . . . . . . . . . . . . . 15 6.1.1. Router Solicitation for Proxy . . . . . . . . . . 15 6.1.2. Proxy Router Advertisement (PrRtAdv) . . . . . . 17 6.2. Inter-Access Router Messages . . . . . . . . . . . . . . 20 6.2.1. Handover Initiate (HI) . . . . . . . . . . . . . 20 6.2.2. Handover Acknowledge (HAck) . . . . . . . . . . . 22 6.3. New Mobility Header Messages . . . . . . . . . . . . . . 24 6.3.1. Fast Binding Update (FBU) . . . . . . . . . . . . 24 6.3.2. Fast Binding Acknowledgment (FBack) . . . . . . . 25 6.3.3. Fast Neighbor Advertisement (FNA) . . . . . . . . 27 6.4. New ICMP Options . . . . . . . . . . . . . . . . . . . . 28 6.4.1. IP Address Option . . . . . . . . . . . . . . . . 28 6.4.2. New Router Prefix Information Option . . . . . . 29 6.4.3. Link-layer Address (LLA) . . . . . . . . . . . . 30 6.4.4. Neighbor Advertisement Acknowledgment (NAACK) . . 31 7. Configurable Parameters 32 8. Security Considerations 32 9. Contributors 33 Koodli (Editor) Expires 10 April 2004 [Page ii] Internet Draft Fast Handovers 10 October 2003 10. Acknowledgments 33 A. Change Log 34 B. Contact Information 34 Koodli (Editor) Expires 10 April 2004 [Page 1] Internet Draft Fast Handovers 10 October 2003 1. Introduction Mobile IPv6 [3] describes the protocol operations for a mobile node to maintain connectivity to the Internet during its handover from one access router to another. These operations broadly involve movement detection, IP address configuration, and location update phases. The combined handover latency could be (especially) appreciable for real-time applications. Throughput-sensitive applications can also benefit from reducing this latency. This document describes a protocol to reduce the handover latency. This specification addresses the following problem: how to allow a mobile node to send packets as soon as it detects a new subnet link, and how to deliver packets to a mobile node as soon as its attachment is detected by the new access router. The protocol defines IP protocol messages necessary for its operation on any link technology. It does this without depending on specific link-layer features while allowing link-specific customizations. By definition, this specification considers handovers that inter-work with Mobile IP: once attached to its new access router, a MN engages in Mobile IP operations including Return Routability [3]. Hence, there are no special requirements for a mobile node to behave differently with respect to its standard Mobile IP operations. 2. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", "OPTIONAL", and "silently ignore" in this document are to be interpreted as described in RFC 2119 [1]. The following terminology and abbreviations are used in this document. The reference handover scenario is illustrated in Figure 1. Mobile Node (MN) A Mobile IPv6 host Access Point (AP) A Layer 2 device connected to a subnet that offers wireless connectivity to a MN. Access Router (AR) The MN's default router Previous Access Router (PAR) The MN's default router prior to its handover Koodli (Editor) Expires 10 April 2004 [Page 2] Internet Draft Fast Handovers 10 October 2003 New Access Router (NAR) The MN's anticipated default router subsequent to its handover Previous CoA (PCoA) The MN's Care of Address valid on PAR. The MN may reuse this address while attached to NAR until it finishes Mobile IP operations. New CoA (NCoA) The MN's Care of Address valid on NAR Handover A process of terminating existing connectivity and obtaining new IP connectivity. Router Solicitation for Proxy (RtSolPr) A message from the MN to the PAR requesting information for a potential handover Proxy Router Advertisement (PrRtAdv) A message from the PAR indicating a MN to undergo handover Fast Binding Update (FBU) A message from the MN instructing its PAR to redirect its traffic (towards NAR) Fast Binding Acknowledgment (FBACK) A message from the PAR in response to FBU Fast Neighbor Advertisement (FNA) A message from the MN to the NAR to announce attachment and to confirm use of NCoA when the MN has not received FBACK Handover Initiate (HI) A message from the PAR to the NAR to initiate handover Handover Acknowledge (HACK) A message from the NAR to the PAR as a response to HI 3. Protocol Overview 3.1. Addressing the Handover Latency The ability to quickly send packets from a new subnet link depends on the ``IP connectivity'' latency, which in turn depends on the Koodli (Editor) Expires 10 April 2004 [Page 3] Internet Draft Fast Handovers 10 October 2003 v +------------+ +-+ | Previous | < | | ---------- | Access | ------ > ----\ +-+ | Router | < \ MN | (PAR) | \ | +------------+ +---------------+ | ^ IP | Correspondent | | | Network | Node | V | +---------------+ v / v +------------+ / +-+ | New | < / | | ---------- | Access | ------ > ----/ +-+ | Router | < MN | (NAR) | +------------+ Figure 1: Reference Scenario for Handover movement detection latency and the new CoA configuration latency. Once a MN is IP-capable on the new subnet link, it can send Binding Update to its Home Agent and one or more correspondents. Once its correspondents successfully process the Binding Update, which involves the Return Routability procedure, the MN can receive packets at new CoA. So, the ability to receive packets from correspondents directly at its new CoA depends on the Binding Update latency. The protocol enables a MN to quickly detect that it has moved to a new subnet by providing the new access point and the associated subnet prefix information when the MN is still connected to its current subnet (i.e., PAR in Figure 1). For instance, a MN may discover available access points using link-layer specific mechanisms (e.g., a ``scan'' in WLAN) and then request subnet information corresponding to one or more of those discovered access points. The MN may do this after performing router discovery. The MN may also do this at any time during its sojourn on its current point of attachment. The result of resolving an identifier associated with an access point is a [AP-ID, AR-Info] tuple, which a MN can use in readily detecting movement: when attachment to an access point with AP-ID takes place, the MN knows the corresponding new router's co-ordinates including its prefix, IP address and MAC address. The ``Router Solicitation for Proxy (RtSolPr)'' and ``Proxy Router Koodli (Editor) Expires 10 April 2004 [Page 4] Internet Draft Fast Handovers 10 October 2003 Advertisement (PrRtAdv)'' messages are used for aiding in movement detection. Through the RtSolPr and PrRtAdv messages, the MN also formulates a prospective new CoA (NCoA), again when still present on PAR's link. Hence, the latency due to new prefix discovery subsequent to handover is eliminated. Furthermore, this prospective address can be used immediately after attaching to the new subnet link (i.e., NAR's link) when the MN has received a ``Fast Binding Acknowledgment (FBack)'' message prior to its movement. In the event it moves without receiving an FBack, the MN can still start using NCoA after announcing its attachment through a ``Fast Neighbor Advertisement (FNA)'' message; NAR responds to FNA in case the tentative address is already in use. In this way, NCoA configuration latency is reduced. Under some limited conditions where the probability of address collision is considered insignificant, it may be possible to use NCoA immediately after attaching to the new link. Even so, all implementations MUST implement the mechanism specified in this document to avoid potential address conflicts and SHOULD support them. In order to reduce the Binding Update latency, the protocol specifies a tunnel establishment typically between the Previous CoA (PCoA) and NCoA. A MN sends a ``Fast Binding Update'' message to its Previous Access Router to establish this tunnel. When feasible, the MN SHOULD send FBU from PAR's link. Otherwise, it should send it immediately after detecting attachment to NAR. Subsequent sections describe the protocol mechanics. In any case, the net result is that PAR begins tunneling packets arriving for PCoA to NCoA. Such a tunnel would remain active until the MN completes Binding Update with its correspondents. In the opposite direction, the MN SHOULD reverse tunnel packets to PAR, again until it completes Binding Update. And, PAR SHOULD forward the inner packet in the tunnel to its destination (i.e., to the MN's correspondent). Such a reverse tunnel ensures that packets containing PCoA as source IP addres are not dropped due to ingress filtering. Readers may observe that even though the MN is IP-capable on the new link, it cannot use NCoA directly with its correspondents without the correspondents first establishing a binding cache entry (for NCoA). Setting up a tunnel alone does not ensure that the MN receives packets as soon as attaching to a new subnet link, unless NAR can detect the MN's presence. Neighbor discovery operation involving a neighbor's address resolution (i.e., Neighbor Solicitation and Neighbor Advertisement) typically result in considerable delay, sometimes lasting multiple seconds. For instance, when arriving packets trigger NAR to send Neighbor Solicitation before the MN attaches, subsequent re-transmissions of address resolution are separated by a default time of one second each. In order to Koodli (Editor) Expires 10 April 2004 [Page 5] Internet Draft Fast Handovers 10 October 2003 circumvent this delay, a MN announces its attachment through the FNA message that allows NAR to consider MN to be reachable. If there is no existing entry, FNA allows NAR to create one. If NAR already has an entry, FNA updates the entry while taking potential address conflicts into consideration. Through tunnel establishment for PCoA and fast advertisement, the protocol provides expeditious forwarding of packets to the MN. The protocol also provides the following important functionalities. The access routers can exchange messages to confirm that a proposed NCoA is acceptable. For instance, when a MN sends FBU from PAR's link, FBack can be delivered after NAR considers NCoA acceptable to use. This is especially useful when stateful address configuration is used. The NAR can also rely on its trust relationship with PAR before providing forwarding support for the MN. That is, it may create a forwarding entry for NCoA subject to ``approval'' from PAR which it trusts. Finally, the access routers could transfer network-resident contexts, such as access control, QoS, header compression, in conjunction with handover. For all these operations, the protocol provides ``Handover Initiate (HI)'' and ``Handover Acknowledge (HAck)'' messages. Both of these messages MUST be implemented and SHOULD be supported. 3.2. Protocol Operation The protocol begins when a MN sends RtSolPr to its access router to resolve one or more Access Point Identifiers to subnet-specific information. In response, the access router (e.g., PAR in Figure 1) sends a PrRtAdv message which contains one or more [AP-ID, AR-Info] tuples. The MN may send RtSolPr at any convenient time, for instance as a response to some link-specific event (a ``trigger'') or simply after performing router discovery. However, the expectation is that prior to sending RtSolPr, the MN has discovered the available APs by link-specific methods. The RtSolPr and PrRtAdv messages do not establish any state at the access router, and their packet formats are defined in Section 6.1. With the information provided in the PrRtAdv message, the MN formulates a prospective NCoA and sends an FBU message. The purpose of FBU is to authorize PAR to bind PCoA to NCoA, so that arriving packets can be tunneled to the new location. The FBU SHOULD be sent from PAR's link whenever feasible. For instance, an internal link-specific trigger could enable FBU transmission from the previous link. When it is not feasible, FBU is sent from the new link. Care must be taken to ensure that NCoA used in FBU does not conflict with an address already in use by some other node on link. For this, FBU encapsulation within FNA MUST be implemented and SHOULD be used (See below). Koodli (Editor) Expires 10 April 2004 [Page 6] Internet Draft Fast Handovers 10 October 2003 The format and semantics of FBU processing are specified in Section 6.3.1. Depending on whether an FBack is received or not on the previous link, which clearly depends on whether FBU was sent in the first place, there are two modes of operation. 1. The MN receives FBack on the previous link. This means that packet tunneling would already be in progress by the time the MN handovers to NAR. The MN SHOULD send FNA immediately after attaching to NAR, so that arriving as well as buffered packets can be forwarded to the MN right away. Before sending FBack to MN, PAR can verify if NCoA is acceptable to NAR through the exchange of HI and HAck messages. When stateful assignment is used, the proposed NCoA in FBU is carried in HI, and NAR MAY consider assigning the proposed NCoA. In any case, the assigned NCoA MUST be returned in HAck, and PAR MUST in turn provide the assigned NCoA in FBack. If there is an assigned NCoA returned in FBack, the MN MUST use the assigned address (and not the proposed address in FBU) upon attaching to NAR. The HI and HAck protocol exchange to verify NCoA acceptability, among others noted in Section 3.1, can be used even when stateful assignment is not used. 2. The MN does not receive FBack on the previous link. One obvious reason for this is that the MN has not sent the FBU. The other reason is that the MN has left the link after sending the FBU (which may be lost) but before receiving an FBack. Without receiving an FBack in the latter case, the MN cannot ascertain whether PAR has successfully processed the FBU. Hence, it (re)sends an FBU as soon as it attaches to NAR. In order to enable NAR to forward packets immediately (when FBU has been processed) and to allow NAR to verify if NCoA is acceptable, the MN SHOULD encapsulate FBU in FNA. If NAR detects that NCoA is in use when processing FNA, for instance while creating a neighbor entry, it MUST discard the inner FBU packet and send a Router Advertisement with ``Neighbor Advertisement Acknowledge (NAACK)'' option in which NAR MAY include an alternate IP address for the MN to use. This discarding avoids the undesirable outcome of address collision, even though the chances of such a collision are extremely low. Detailed FNA processing rules are specified in Section 6.3.3. The scenario in which a MN sends FBU and receives FBack on PAR's link is illustrated in Figure 2. For convenience, this scenario may be characterized as ``predictive'' mode of operation. The scenario in which the MN sends FBU from NAR's link is illustrated in Figure 3. For convenience, this scenario may be characterized as ``reactive'' Koodli (Editor) Expires 10 April 2004 [Page 7] Internet Draft Fast Handovers 10 October 2003 mode of operation. Note however that the reactive mode includes the case when FBU has been sent from PAR's link but FBack has not been received yet. Finally, the PrRtAdv message may be sent gratuitously, i.e., without the MN first sending RtSolPr. MN PAR NAR | | | |------RtSolPr------->| | |<-----PrRtAdv--------| | | | | |------FBU----------->|--------HI--------->| | |<------HAck---------| | <--FBack---|--FBack---> | | | | disconnect forward | | packets===============>| | | | | | | connect | | | | | |--------- FNA --------------------------->| |<=================================== deliver packets | | Figure 2: ``Predictive'' Fast Handover 3.3. Protocol Operation of Network-initiated Handover In some wireless technologies, the handover control may reside in the network even though the decision to undergo handover may be co-operatingly arrived at between the MN and the network. On such network, it is possible for the PAR to send an unsolicited PrRtAdv containing the link address, IP address and subnet prefixes of the NAR when the network decides that a handover is imminent. The MN MUST process this PrRtAdv to configure a new care of address on the new subnet, and MUST send an FBU to PAR prior to switching to the new link. After transmitting the FBU, the PAR MUST continue to forward packets to the MN on its current link until the FBU is received. The rest of the operation is the same as that described in Section 3.2. Koodli (Editor) Expires 10 April 2004 [Page 8] Internet Draft Fast Handovers 10 October 2003 MN PAR NAR | | | |------RtSolPr------->| | |<-----PrRtAdv--------| | | | | disconnect | | | | | | | | connect | | |------FNA[FBU]-------|------------------->| | |<-----FBU-----------| | |------FBack-------->| | forward | | packets===============>| | | | |<=================================== deliver packets | | Figure 3: ``Reactive'' Fast Handover An alternative use of the unsolicited PrRtAdv is to allow the network to inform the MN about geographically adjacent subnets without the MN having to explicitly request that information. This can reduce the amount of wireless traffic required for the MN to obtain a neighborhood links and subnet topology map. Such usage of PrRtAdv is decoupled from the actual handover. Exactly how to reconcile this function with the use of PrRtAdv as a handover trigger is a topic for future experimental work. 4. Protocol Details All description makes use of Figure 1 as the reference. After discovering nearby access points, the MN sends RtSolPr in order to resolve access point identifiers to subnet router information. A convenient time to do this is after performing router discovery. However, the MN MAY send RtSolPr at any time during its attachment to PAR. The trigger for sending RtSolPr can emanate from a link-specific event, such as the promise of better signal strength from another access point coupled with fading signal quality with the current access point. Such events, often broadly called as ``L2 triggers'', Koodli (Editor) Expires 10 April 2004 [Page 9] Internet Draft Fast Handovers 10 October 2003 are outside the scope of this document. Nevertheless, they serve as important events that invoke the protocol. For instance, when a ``link up'' indication is obtained on the new link, the protocol messages (e.g., FNA) can be immediately transmitted. Implementations SHOULD make use of such triggers whenever available. The RtSolPr message may contain one or more AP-IDs. A wildcard requests all available tuples. When the MN has knowledge of an impending handover, it MAY set the `U' bit in RtSolPr to request PAR to start buffering its packets. As a response to RtSolPr, PAR sends a PrRtAdv message which indicates one of the following possible conditions. 1. If the PAR does not have an entry corresponding to the new attachment point, it MUST respond indicating that the new point of attachment is unknown. The MN MUST stop fast handover protocol operations on the current link. The MN MAY send an FBU from its new link. 2. If the new point of attachment is connected to the PAR itself, PAR MUST respond indicating that the point of attachment is known but is connected to itself. This could happen, for example, when the wireless access points are bridged into a wired network. No further protocol action is necessary. 3. If the new point of attachment is known and the PAR has information about it, then PAR MUST respond indicating that the point of attachment is known. The message MUST contain the NCoA that the MN should use or the network prefix that should be used to form NCoA. If the new point of attachment is known, but does not support fast handover, the PAR MUST indicate this with Code 3 (See Section 6.1.2). 4. If a wildcard is supplied as an identifier for the new point of attachment, the PAR SHOULD supply neighborhood [AP, AR] tuples subject to path MTU restrictions (i.e., provide any `n' tuples without exceeding the link MTU). If the `U' bit is set in RtSolPr, PAR MAY buffer incoming packets in FIFO order but MUST continue forwarding packets to PCoA on PAR's link. It SHOULD stop buffering after processing the FBU message. The size of the buffer and the duration of buffering are implementation-specific considerations. If the new point of attachment belongs to itself, PAR MAY chose to ignore buffering. The method by which Access Routers exchange information about their neighbors and thereby allow construction of PrRtAdvs with information about the new subnet is outside the scope of this Koodli (Editor) Expires 10 April 2004 [Page 10] Internet Draft Fast Handovers 10 October 2003 document. Furthermore, this document assumes that the access routers share necessary security association established by means outside the scope of this document. The RtSolPr and PrRtAdv messages MUST be implemented by a MN and an access router that supports fast handovers. However, when the parameters necessary for the MN to send packets immediately upon attaching to the NAR are supplied by the link layer handover mechanism itself, the above messages are optional on such link layers. Sometime after PrRtAdv is received, the MN sends FBU in which it includes the proposed NCoA. The MN SHOULD send FBU from PAR's link whenever feasible, i.e., whenever ``anticipation'' of handover is feasible. When anticipation is not feasible or if has not received an FBack yet, the MN sends FBU immediately after attaching to NAR's link. In addition, this FBU SHOULD be encapsulated in a FNA message in order to allow NAR to first verify if NCoA is in use already before it can forward the (inner) FBU packet to PAR. In response to FBU, PAR establishes a binding between PCoA (``Home Address'') and NCoA, and sends FBack to MN. Prior to establishing this binding, PAR SHOULD send a HI message to NAR, and receive HAck in response. The HI message contains the PCoA, link-layer address and the NCoA of the MN. In response to processing the HI message, the NAR 1. verifies if NCoA supplied in the HI message is a valid address for use, and if it is, starts proxying the address. 2. allocates NCoA for the MN when stateful address configuration is used, creates a proxy neighbor cache entry and begins defending it. The NAR MAY consider NCoA proposed in HI for allocation. 3. MAY create a host route entry for PCoA in case NCoA cannot be accepted or assigned such that the entry allows it to forward packets to the MN. This host route entry SHOULD be implemented such that until the MN's presence is detected, either through explicit announcement by the MN or by other means, arriving packets do not invoke neighbor discovery. The NAR MAY also set up a reverse tunnel to PAR in this case. 4. SHOULD start buffering packets if the `U' bit is set. 5. provides the status of handover request in Handover Acknowledge (HAck) message. If HAck contains an assigned NCoA, such as in stateful allocation, FBack MUST include it, and the MN MUST use the address provided in FBack. The PAR SHOULD send FBack to previous link as well. The Koodli (Editor) Expires 10 April 2004 [Page 11] Internet Draft Fast Handovers 10 October 2003 result of FBU and FBack processing is that PAR begins tunneling MN's packets to NCoA. If the MN does not receive an FBack message even after re-transmitting FBU for FBU_RETRIES, it must assume that fast handover support is not available and stop the protocol operation. As soon as the MN establishes link connectivity with the NAR, it SHOULD send a Fast Neighbor Advertisement (FNA) message (See 6.3.3). If the MN has not received an FBack by the time FNA is being sent, it SHOULD encapsulate the FBU in FNA and send them together. When it is acceptable to use NCoA corresponding to the FNA message, the NAR MUST 1. delete the proxy neighbor cache entry, if any is present. 2. create a neighbor cache entry and set its state to REACHABLE without over-writing an existing entry for a different layer 2 address. 3. forward any buffered packets 4. enable the host route entry, if any is present, for PCoA. If NAR detects that NCoA is in use by another node while processing the FNA, it MUST 1. discard the inner (FBU) packet. 2. send a Router Advertisement with the NAACK option, and NAR MAY include an alternate NCoA for use If the MN receives a Router Advertisement with a NAACK option, it MUST revert to configuring a new address, and SHOULD use the address, if any is, provided in the NAACK option. Subsequently, the MN SHOULD send an FBU using the new CoA. As a special case, the address supplied in NAACK could be PCoA itself, in which case the MN MUST not send any more FBUs. If no address is present in NAACK option, the MN MUST follow [5] in order to configure a new CoA and SHOULD still send FBU to PAR. Once the MN has confirmed its NCoA, it SHOULD send a Neighbor Advertisement message. This message updates its neighbor's cache entries with the MN's link-layer address. Koodli (Editor) Expires 10 April 2004 [Page 12] Internet Draft Fast Handovers 10 October 2003 5. Other Considerations 5.1. Handover Capability Exchange The MN expects a PrRtAdv in response to its RtSolPr message. If the MN does not receive a PrRtAdv message even after RTSOLPR_RETRIES, it must assume that PAR does not support the fast handover protocol. Similarly, if PAR sends PrRtAdv gratuitously to a MN, but does not receive an FBU even after GR_PRRTADV_RETRIES, it must assume that the MN does not support the fast handover protocol. Even if a MN's current access router is capable of fast handover, the access router into which the MN is handing over to may not be capable of fast handover. This is indicated to the MN during ``runtime'', through the PrRtAdv message with a Code value of 3. See Section 6.1.2. 5.2. Determining New Care of Address Typically, the MN formulates its prospective NCoA using the information provided in a PrRtAdv message, and sends FBU. The PAR MUST use the NCoA present in FBU in its HI message. The NAR SHOULD verify if NCoA present in HI is already in use. In any case, NAR MUST respond to HI in the form of a HAck in which it may include a different NCoA to use, especially when stateful address configuration is used. If there is an address present in HAck, PAR MUST convey it to the MN in the FBack message. If PrRtAdv message carries a NCoA, the MN MUST use it as its prospective NCoA. 5.3. Packet Loss Handover involves link switching. It is understandably difficult to exactly co-ordinate fast handover signaling with link switching. Furthermore, arrival pattern of packets is dependent on many factors, including application characteristics, network queuing behavior etc. Hence, packets may arrive at NAR before the MN is able to attach to it. These packets will be lost unless they are buffered by the NAR. Similarly, if the MN attaches to NAR and then sends an FBU message, packets arriving at PAR will be lost unless they are buffered. This protocol provides an option to indicate request for buffering at the NAR in the HI message. When the PAR does request this feature, it SHOULD also provide its own support for buffering. Koodli (Editor) Expires 10 April 2004 [Page 13] Internet Draft Fast Handovers 10 October 2003 5.4. DAD Handling Duplicate Address Detection (DAD) was defined in [5] to avoid address duplication on links when stateless address auto- configuration is used. The use of DAD to verify the uniqueness of an IPv6 address configured through stateless autoconfiguration adds delays to a MIPv6 handover. The probability of an interface identifier duplication on the same subnet is very low, however it can not be ignored. In this draft certain precautions are proposed to minimize the effects of a duplicate address occurrence. On links wherein the uniqueness of the interface identifier is ensured within the subnet (by means beyond the scope of this document), DAD handling procedures SHOULD be avoided. In some cases the NAR may already have the knowledge required to assess whether the MN's address is a duplicate or not before the MN moves to the new subnet. For example, the NAR can have a list of all nodes on its subnet, perhaps for access control, and by searching this list, it can confirm whether the MN's address is a duplicate or not. The result of this search is sent back to the PAR in the HACK message. If such knowledge is not available at the NAR, it may indicate this by not confirming NCoA in the HACK message. The NAR may also indicate this in the NAACK option as a response to the FNA message. In such cases, the MN would have to follow stateless address configuration rules after attaching to the NAR. 5.5. Fast or Erroneous Movement Although this specification is for fast handover, the protocol has its limits in terms of how fast a MN can move. A special case of fast movement is ping-pong, where a MN moves between the same two access points rapidly. Another instance of the same problem is erroneous movement i.e., the MN receives information prior to a handover that it is moving to a new access point and but it is either moved to a different one or it aborts movement altogether. All of the above behaviors are usually the result of link layer idiosyncrasies and thus are often tackled at the link layer itself. IP layer mobility, however, introduces its own limits. IP layer handovers should be in a frequency that allows enough time for the MN to update the binding of, at least, its HA and preferably that of every CN with which it is in communication. A MN that moves faster than necessary for this signaling to complete, which may be of the order of few seconds, may start losing packets and ultimately connectivity. The signaling overhead over the air and in the network may increase significantly, especially in the case of rapid movement Koodli (Editor) Expires 10 April 2004 [Page 14] Internet Draft Fast Handovers 10 October 2003 between two access routers. To avoid the signaling overhead, the following measures are suggested. A MN returning to the PAR before updating the necessary bindings when present on NAR MUST send a Fast Binding Update with Home Address equal to the MN's PCoA and a lifetime of zero, to the PAR. The MN should have a security association with the PAR since it performed a fast handover from it to the NAR. The PAR, on receiving this Fast Binding Update, will check its set of outgoing (temporary fast handover) tunnels and if it finds a match it SHOULD drop that tunnel; i.e., stop forwarding packets for this MN and start delivering packets directly to the node instead. The MN SHOULD NOT make any attempt to use any of the fast handover mechanisms described in this specification and SHOULD revert back to standard Mobile IPv6. Temporary tunnels for the purposes of fast handovers should use short lifetimes (in the order of a small number of seconds or less). The lifetime of such tunnels should be enough to allow a MN to update all its active bindings. Erroneous movement is not likely to damage the network but it may cause loss of packets since routing can change and the PAR may forward packets towards another AR before the MN actually connects to that AR. If the MN discovers itself on an unanticipated AR, a Fast Binding Update to the PAR SHOULD be sent. Since Fast Binding Updates are authenticated, they MUST supersede the existing binding and packets SHOULD be redirected to the new confirmed location of the MN. No attempt should be made to recover packets from the AR the MN was supposed to connect to. 6. Message Formats 6.1. New Neighbor Discovery Messages 6.1.1. Router Solicitation for Proxy Mobile Nodes send Router Solicitation for Proxy in order to prompt routers for Proxy Router Advertisements. For all the ICMP messages, the checksum is defined in [2]. IP Fields: Source Address An IP address assigned to the sending interface Destination Address The address of the Access Router or the all routers Koodli (Editor) Expires 10 April 2004 [Page 15] Internet Draft Fast Handovers 10 October 2003 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 | Code | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Identifier |U| Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options ... +-+-+-+-+-+-+-+-+-+-+-+- Figure 4: Router Solicitation for Proxy (RtSolPr) Message multicast address. Hop Limit 255 Authentication Header If a Security Association for the IP Authentication Header exists between the sender and the destination address, then the sender SHOULD include this header. ICMP Fields: Type TBA Code 0 Checksum The ICMP checksum. Identifier MUST be set by the sender so that replies can be matched to this Solicitation. `U' bit If set, requests access router to also buffer arriving packets Reserved MUST be set to zero by the sender and ignored by the receiver. Valid Options: Source link-layer address The link-layer address of the sender, if known. Koodli (Editor) Expires 10 April 2004 [Page 16] Internet Draft Fast Handovers 10 October 2003 It SHOULD be included on link layers that have addresses. New Attachment Point Link-Layer Address The link-layer address or identification of the attachment point for which the MN requests routing advertisement information. It MUST be included in all RtSolPr messages. More than one such address or identifier can be present. This field can also be a wildcard address with all bits set to zero. Future versions of this protocol may define new option types. Receivers MUST silently ignore any options that they do not recognize and continue processing the rest of the message. A MN sends this message if it wishes to initiate fast handover. It indicates its destination with the New Attachment Point Link-Layer Address. A Proxy Router Advertisement message should be received in response. If such a message is not received in a short time period but no less than twice the typical round trip time (RTT) over the access link or 100 ms if RTT is not known, it SHOULD resend RtSolPr message at most RTSOLPR_RETRIES, waiting for the same time during each instance of retransmission. If Proxy Router Advertisement is not received by the time the MN disconnects from the PAR, the MN SHOULD send FBU immediately after configuring a new CoA. 6.1.2. Proxy Router Advertisement (PrRtAdv) Access routers send out Proxy Router Advertisement message gratuitously if the handover is network-initiated or as a response to RtSolPr message from a MN, providing the link-layer address, IP address and subnet prefixes of neighboring routers. This avoids the MN having to solicit Router Advertisement when it connects to the new subnet. IP Fields: Source Address MUST be the link-local address assigned to the interface from which this message is sent. Destination Address The Source Address of an invoking Router Solicitation for Proxy or the address of the node the Access Router is instructing to handover. Hop Limit 255 Koodli (Editor) Expires 10 April 2004 [Page 17] Internet Draft Fast Handovers 10 October 2003 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 | Code | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Identifier | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options ... +-+-+-+-+-+-+-+-+-+-+-+- Figure 5: Proxy Router Advertisement (PrRtAdv) Message Authentication Header If a Security Association for the IP Authentication Header exists between the sender and the destination address, then the sender SHOULD include this header. ICMP Fields: Type TBA Code 0, 1, or 2 Checksum The ICMP checksum. Identifier Copied from Router Solicitation for Proxy or set to Zero if unsolicited. Reserved MUST be set to zero by the sender and ignored by the receiver. Valid Options: Source link-layer address The link-layer address of the sender, if known. It SHOULD be included on link layers that have addresses. Link-layer address of proxied originator (i.e., NAR) The link-layer address of the Access Router for Koodli (Editor) Expires 10 April 2004 [Page 18] Internet Draft Fast Handovers 10 October 2003 which this message is proxied for. This option MUST be included when Code is 0. New Router Prefix Information Option. These options specify the prefixes of the Access Router the message is proxied for and are used for address autoconfiguration. This option SHOULD be included when Code is 0. New CoA Option When PrRtAdv is sent gratuitously, this option MUST be included. Otherwise, this option MAY be present. Future versions of this protocol may define new option types. Receivers MUST silently ignore any options they do not recognize and continue processing the message. A Proxy Router Advertisement with Code 0 but without a New CoA Option means that the MN SHOULD construct a NCoA out of its Interface ID (used in the Destination Address in this Proxy Router Advertisement) and the Prefix in the New Router Prefix Information Option (See Section 6.4.2. A Proxy Router Advertisement with Code 0 and a New CoA Option means that the MN SHOULD use the supplied NCoA or else stand to loose service. A Proxy Router Advertisement with Code 1 is sent if handover to the New Attachment Point, as indicated by the New Attachment Point Link Layer address in the corresponding RtSolPr message, does not require change of CoA. No options are required in this case. A Proxy Router Advertisement with Code 2 means that the PAR is not aware of the Prefix Information requested. The MN SHOULD attempt to send FBU as soon as it regains connectivity with the NAR. No options are required in this case. A Proxy Router Advertisement with Code 3 means that the NAR does not support fast handover. The MN MUST stop fast handover protocol operations. No options are required in this case. When a wildcard AP identifier is supplied in the RtSolPr message, the PrRtAdv message should include any 'n' [Access Point Identifier, Link-layer address option, Prefix Information Option] tuples corresponding to the PAR's neighborhood. Koodli (Editor) Expires 10 April 2004 [Page 19] Internet Draft Fast Handovers 10 October 2003 6.2. Inter-Access Router Messages 6.2.1. Handover Initiate (HI) The Handover Initiate (HI) is a new ICMPv6 message sent by an Access Router (typically PAR) to another Access Router (typically NAR) to initiate the process of a MN's handover. 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 | Code | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Identifier |S|U| Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- Figure 6: Handover Initiate (HI) Message IP Fields: Source Address The IP address of the PAR Destination Address The IP address of the NAR Hop Limit 255 Authentication Header A Security Association MUST exist between the sender and the receiver of this message. This message MUST be authenticated and so the authentication header MUST be used when this message is sent. ICMP Fields: Type TBA Code 0 Koodli (Editor) Expires 10 April 2004 [Page 20] Internet Draft Fast Handovers 10 October 2003 Checksum The ICMP checksum. Identifier MUST be set by the sender so replies can be matched to this message. S Stateful address configuration flag. When set, this message requests a new CoA to be returned by the destination. U Buffer flag. When set, the destination SHOULD buffer any packets towards the node indicated in the options of this message. Reserved MUST be set to zero by the sender and ignored by the receiver. Valid Options: Link-layer address of MN The link-layer address of the MN that is undergoing handover to the destination. This option SHOULD be included to help the destination recognize the MN when it connects to the destination. Previous Care of Address The IP address used by the MN while attached to the originating router. This option SHOULD be included so that host route can be established in case necessary. New Care of Address The IP address the MN wishes to use when connected to the destination. When the `S' bit is set, NAR MAY use this address in its stateful assignment. If Handover Acknowledge (HAck) message is not received as a response, the Handover Initiate SHOULD be re-sent up to HI_RETRIES times using a short retransmission timer with a value no less than twice the round trip time between source and destination or 100 ms if RTT is not known. Koodli (Editor) Expires 10 April 2004 [Page 21] Internet Draft Fast Handovers 10 October 2003 6.2.2. Handover Acknowledge (HAck) The Handover Acknowledgment message is a new ICMPv6 message that MUST be sent (typically by NAR to PAR) as a reply to the Handover Initiate message. 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 | Code | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Identifier |H|T|R| Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- Figure 7: Handover Acknowledge (HAck) Message IP Fields: Source Address Copied from the destination address of the Handover Initiate Message to which this message is a response. Destination Address Copied from the source address of the Handover Initiate Message to which this message is a response. Hop Limit 255 Authentication Header A Security Association MUST exist between the sender and the receiver of this message. This message MUST be authenticated and so the authentication header MUST be used when this message is sent. ICMP Fields: Type TBA Koodli (Editor) Expires 10 April 2004 [Page 22] Internet Draft Fast Handovers 10 October 2003 Code 0: Handover Accepted, NCoA valid 1: Handover Accepted, NCoA not valid 2: Handover Accepted, NCoA in use 3: Handover Accepted, NCoA assigned (Stateful) 4: Handover Accepted, NCoA not assigned (Stateful) 128: Handover Not Accepted, reason unspecified 129: Administratively prohibited 130: Insufficient resources Checksum The ICMP checksum. Identifier Copied from the corresponding field in the Handover Initiate message this message is in response to. Reserved MUST be set to zero by the sender and ignored by the receiver. Valid Options: New Care of Address If the S flag in the Handover Initiate message is set, this option MUST be used to provide NCoA the MN should use when connected to this router. This option MAY be included even when `S' bit is not set, e.g., Code 2 above. Upon receiving the HI message, the NAR MUST respond with a Handover Acknowledge message. If the `S' flag is set in the HI message, the NAR SHOULD include the New Care of Address option and a Code of 3. If the `S' flag is not set in the HI message, the NAR MUST check the validity of the NCoA when sent with HI, and reply with appropriate Code values enumerated above. If NCoA is valid, the NAR SHOULD insert NCoA in its Proxy Neighbor Cache and defend NCoA for PROXY_ND_LIFETIME period of time during which the MN is expected to connect to NAR. If the NCoA is not valid or not assigned, the NAR MUST respond with an appropriate Code value. The NAR MAY provide support for PCoA (instead of accepting or assigning NCoA) and establish a host route entry for PCoA, and set up a tunnel to the PAR to forward MN's packets sent with PCoA as source IP address. This host route entry SHOULD be used to forward packets once the NAR detects that the particular MN is attached to its link. Finally, the new access router can always refuse handover, in which case it should indicate the reason in one of the available Code values. Koodli (Editor) Expires 10 April 2004 [Page 23] Internet Draft Fast Handovers 10 October 2003 6.3. New Mobility Header Messages Mobile IPv6 uses a new IPv6 header type called Mobility Header [3]. The Fast Binding Update, Fast Binding Acknowledgment and Fast Neighbor Advertisement messages use the Mobility Header. 6.3.1. Fast Binding Update (FBU) The Fast Binding Update message is identical to the Mobile IPv6 Binding Update (BU) message. However, the processing rules are slightly different. The Mobility Header Type value for FBU is 8. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sequence # | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |A|H|L|K| Reserved | Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . . . Mobility options . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 8: Fast Binding Update (FBU) Message IP fields: Source address The PCoA or NCoA Destination Address The IP address of the Previous Access Router `A' flag MUST be set to one to request PAR to send a Fast Binding Acknowledgement message. `H' flag MUST be set to one. See [3]. `L' flag See [3]. `K' flag See [3]. Koodli (Editor) Expires 10 April 2004 [Page 24] Internet Draft Fast Handovers 10 October 2003 Reserved This field is unused. MUST be set zero. Sequence Number See [3]. Lifetime See [3]. Mobility Options MUST contain alternate CoA option set to NCoAIP address when sent FBU is sent from PAR's link. MAY contain alternate CoA option set to NCoA when FBU is sent from NAR's link. The MN sends FBU message any time after receiving a PrRtAdv message. If the MN moves prior to receiving a PrRtAdv message, it SHOULD send a FBU to the PAR after configuring a new CoA on the NAR. The source IP address is PCoA when FBU is sent from PAR's link, and the source IP address is NCoA when sent from NAR's link. When FBU is sent from NAR's link, it SHOULD be encapsulated within FNA. The FBU MUST also include the Home Address Option and the Home Address is PCoA. A FBU message MUST be protected in a way appropriate for MN and PAR communication. That is, PAR MUST be able to determine that the FBU message is sent by a genuine MN. 6.3.2. Fast Binding Acknowledgment (FBack) The Fast Binding Acknowledgment message is sent by the PAR to acknowledge receipt of a Fast Binding Update message in which the `A' bit is set. The Fast Binding Acknowledgment message SHOULD NOT be sent to the MN before the PAR receives a HAck message from the NAR. The Fast Binding Acknowledgment MAY also be sent to the MN on the old link. The Mobility Header Type value for FBACK is 9. IP fields: Source address The IP address of the Previous Access Router Destination Address The NCoA Status 8-bit unsigned integer indicating the disposition of the Fast Binding Update. Values of the Status field less than 128 indicate that the Binding Update was accepted by the receiving node. The following such Status values are currently defined: Koodli (Editor) Expires 10 April 2004 [Page 25] Internet Draft Fast Handovers 10 October 2003 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Status |K| Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sequence # | Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . . . Mobility options . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 9: Fast Binding Acknowledgment (FBack) Message 0 Fast Binding Update accepted 1 Fast Binding Update accepted but NCoA is invalid. Use NCoA supplied in ``alternate'' (e.g., stateful) CoA Values of the Status field greater than or equal to 128 indicate that the Binding Update was rejected by the receiving node. The following such Status values are currently defined: 128 Reason unspecified 129 Administratively prohibited 130 Insufficient resources 131 Incorrect interface identifier length `K' flag See [3]. Reserved An unused field. MUST be set to zero. Sequence Number Copied from FBU message for use by the MN in matching this acknowledgment with an outstanding FBU. Lifetime The granted lifetime in seconds for which the Koodli (Editor) Expires 10 April 2004 [Page 26] Internet Draft Fast Handovers 10 October 2003 sender of this message will retain a binding for traffic redirection. Mobility Options MUST contain ``alternate'' CoA if Status is 1. 6.3.3. Fast Neighbor Advertisement (FNA) A MN sends a Fast Neighbor Advertisement to announce itself to the NAR. The Mobility Header Type value for FBACK is 10. When the Mobility Header Type is FNA, the Payload Proto field may be set to IPv6 in order to assist FBU encapsulation. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . Mobility Options . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 10: Fast Neighbor Advertisement (FNA) Message IP fields: Source address NCoA Destination Address NAR's IP address Mobility Options MUST contain the Link-Layer Address The MN sends Fast Neighbor Advertisement to the NAR, as soon as it regains connectivity on the new link to announce its attachment so that arriving or buffered packets can be immediately forwarded. If NAR is proxying NCoA, it creates a neighbor cache entry in REACHABLE state. If there is no entry at all, it creates one and sets it to REACHABLE. If there is an entry in INCOMPLETE state without a link-layer address, it sets it to REACHABLE. During the process of creating a neighbor cache entry, NAR can also detect if NCoA is in use, thus avoiding address collisions. Since FBU is encapsulated within FNA when sent from NAR's link, NAR drops FBU in case it detects any collision. Koodli (Editor) Expires 10 April 2004 [Page 27] Internet Draft Fast Handovers 10 October 2003 The combination of NCoA (present in source IP address) and the Link-Layer Address (present as a Mobility Option) SHOULD be used to distinguish the MN from other nodes. 6.4. New ICMP Options 6.4.1. IP Address Option This option is sent in the Proxy Router Advertisement, the Handover Initiate, and Handover Acknowledge messages. The Proxy Router Advertisement and Handover Acknowledgment messages only contain the NCoA while the Handover Initiate message may include both NCoA and PCoA. The format is based on [4]. 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 | Prefix Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + IPv6 Address + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 11: IPv6 Address Option Type TBA Length size of this option units of 8 octets (i.e., 3) Sub-Type 1 Old Care-of Address 2 New Care-of Address Koodli (Editor) Expires 10 April 2004 [Page 28] Internet Draft Fast Handovers 10 October 2003 Prefix Length The Length of the IPv6 Address Prefix IPv6 address The IP address for the unit defined by the Type field. 6.4.2. New Router Prefix Information Option This option is sent in the PrRtAdv message in order to provide the prefix information valid on the NAR. 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 | Prefix Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + Prefix + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 12: New Router Prefix Information Option Type TBA Length The length of the option (including the type, sub-type and length fields) in units of 8 octets. Sub-Type 0 Prefix Length 8-bit unsigned integer. The number of leading bits in the Prefix that are valid. The value ranges from 0 to 128. Koodli (Editor) Expires 10 April 2004 [Page 29] Internet Draft Fast Handovers 10 October 2003 Prefix An IP address or a prefix of an IP address. The Prefix Length field contains the number of valid leading bits in the prefix. The bits in the prefix after the prefix length are reserved and MUST be initialized to zero by the sender and ignored by the receiver. 6.4.3. Link-layer Address (LLA) 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 ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 13: Link-Layer Address Option Type TBA Length The length of the option (including the type, sub-type and length fields) in units of 8 octets. Sub-Type 0 wildcard requesting resolution for all nearby access points 1 Link-layer Address of the new Attachment Point. 2 Link-layer Address of the MN. 3 Link-layer Address of the Proxied Originator LLA The variable length link-layer address. The New Attachment Point Link Layer address contains the link-layer address of the attachment point for which handover is about to be attempted. This is used in the Router Solicitation for Proxy message. The MN Link-Layer address option contains the link-layer address of a MN. It is used in the Handover Initiate message. Koodli (Editor) Expires 10 April 2004 [Page 30] Internet Draft Fast Handovers 10 October 2003 The Proxied Originator Link-Layer address option contains the Link Layer address of the Access Router for which the Proxy Router Solicitation message refers to. These options MUST be silently ignored when used with other Neighbor Discovery messages. 6.4.4. Neighbor Advertisement Acknowledgment (NAACK) 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 | Status | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 14: Neighbor Advertisement Acknowledgment Option Type TBA Length 8-bit unsigned integer. Length of the option, in 8 octets, excluding the Option Type and Option Length fields. Sub-Type 0 Status 8-bit unsigned integer indicating the disposition of the Fast Binding Update. Values of the Status field less than 128 indicate that the Binding Update was accepted by the receiving node. The following such Status values are currently defined: 0 The New CoA is valid 1 The New CoA is invalid 2 The New CoA is invalid, use the supplied CoA 128 Link Layer Address unrecognized The NAR responds to FNA with the NAACK option to notify the MN to use a different NCoA when there is address collision. If the NCoA is invalid, the Router Advertisement MUST use the PCoA as the destination address, available from the HI message. The MN SHOULD Koodli (Editor) Expires 10 April 2004 [Page 31] Internet Draft Fast Handovers 10 October 2003 use the NCoA supplied along with the NAACK option. If the NAACK indicates that the Link Layer Address is unrecognized the MN MUST NOT use the NCoA or the PCoA and SHOULD start immediately the process of acquiring a NCoA at the NAR. In the future, new option types may be defined. 7. Configurable Parameters Parameter Name Default Value Definition ------------------- ---------------------- ------- RTSOLPR_RETRIES 2 GR_PRRTADV_RETRIES 2 FBU_RETRIES 3 Section 4 PROXY_ND_LIFETIME 1 second Section 6.2.2 HI_RETRIES 4 Section 6.2.1 8. Security Considerations The following security vulnerabilities are identified, and suggested solutions mentioned. 1. Insecure FBU: in this case, packets meant for one address could be stolen, or redirected to some unsuspecting node. This concern is the same as that in a MN and Home Agent relationship. Hence, the PAR MUST ensure that the FBU packet arrived from a node that legitimately owns the PCoA. The access router and its hosts may use any available mechanism to establish a security association which MUST be used to secure FBU. The current version of this protocol does not specify how this security association is established. However, the future work, either as part of this document or in a separate document, may specify this security establishment. If an access router can ensure that the source IP address in an arriving packet could only have originated from the node whose link-layer address is in the router's neighbor cache, then a bogus node cannot use a victim's IP address for malicious redirection of traffic. Such an operation is recommended at least on neighbor discovery messages including the RtSolPr message. 2. Secure FBU, malicious or inadvertant redirection: in this case, the FBU is secured, but the target of binding happens to be an Koodli (Editor) Expires 10 April 2004 [Page 32] Internet Draft Fast Handovers 10 October 2003 unsuspecting node either due to inadvertant operation or due to malicious intent. This vulnerability can lead to a MN with genuine security association with its access router redirecting traffic to an incorrect address. However, the target of malicious traffic redirection is limited to an interface on an access router with which the PAR has a security association. The PAR MUST verify that the NCoA to which PCoA is being bound actually belongs to NAR's prefix. In order to do this, HI and HAck message exchanges are to be used. When NAR accepts NCoA in HI, it proxies NCoA so that any arriving packets are not sent on the link until the MN attaches and announces itself through FNA. So, any inadvertant or malicious redirection to a host is avoided. It is still possible to jam NAR's buffer with redirected traffic. However, since NAR's handover state corresponding to NCoA has a finite (and short) lifetime corresponding to a small multiple of anticipated handover latency, this loophole is not significant. 3. Sending FBU from NAR's link: a malicious node may send FBU from NAR's link providing an unsuspecting node's address as NCoA. Since FBU is encapsulated in FNA, NAR should detect the collision with an address in use when processing FNA, and it then drops FBU. When NAR is unable to detect address collisions, there is a vulnerability that redirection can affect an unsuspecting node. 9. Contributors This document originated from the fast handover design team effort. The members of this design team in alphabetical order were; Gopal Dommety, Karim El-Malki, Mohammed Khalil, Charles Perkins, Hesham Soliman, George Tsirtsis and Alper Yegin. 10. Acknowledgments The editor would like to thank all the interested folks who have provided feedback on this specification. The editor would like to acknowledge the contribution from James Kempf to improve this specification. And, the working group chairs have been very supportive. References [1] S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. Request for Comments (Best Current Practice) 2119, Internet Engineering Task Force, March 1997. Koodli (Editor) Expires 10 April 2004 [Page 33] Internet Draft Fast Handovers 10 October 2003 [2] A. Conta and S. Deering. Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification. Request for Comments (Draft Standard) 2463, Internet Engineering Task Force, December 1998. [3] D. Johnson, C. Perkins, and J. Arkko. Mobility Support in IPv6 (work in progress). Internet Draft, Internet Engineering Task Force, 2002. [4] M. Khalil, R. Narayanan, H. Akhtar, and E. Qaddoura. Mobile IP Extensions Rationalization (MIER) (work in progress). Internet Draft, Internet Engineering Task Force. draft-ietf-mobileip-mier-05.txt, February 2000. [5] S. Thomson and T. Narten. IPv6 Stateless Address Autoconfiguration. Request for Comments (Draft Standard) 2462, Internet Engineering Task Force, December 1998. A. Change Log The following changes have taken place since the previous version. The Section numbers refer to version 06. - Revised the security considerations section in v07 - Refined and added a section on network-initiated handover v07 - Section 3 format change - Section 4 format change (i.e., no subsections). - Description in Section 4.4 merged with ``Fast or Erroneous Movement'' - Section 4.5 deprecated - Section 4.6 deprecated - Revision of some message formats in Section 6 B. Contact Information The design team member's contact information: Gopal Dommety Cisco Systems, Inc. Koodli (Editor) Expires 10 April 2004 [Page 34] Internet Draft Fast Handovers 10 October 2003 170 West Tasman Drive San Jose, CA 95134 Phone:+1 408 525 1404 E-Mail: gdommety@cisco.com 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 Mohamed Khalil Nortel Networks E-Mail: mkhalil@nortelnetworks.com Charles E. Perkins Communications Systems Lab Nokia Research Center 313 Fairchild Drive Mountain View, California 94043 USA Phone: +1-650 625-2986 E-Mail: charliep@iprg.nokia.com Fax: +1 650 625-2502 Hesham Soliman Flarion Technologies E-mail: H.Soliman@flarion.com George Tsirtsis Flarion Technologies E-Mail: G.Tsirtsis@flarion.com Alper E. Yegin NTT DoCoMo Labs USA E-mail: alper@docomolabs-usa.com The editor's contact information: Rajeev Koodli Nokia Research Center 313 Fairchild Drive Mountain View, CA 94043 USA Phone: +1 650 625 2359 Fax: +1 650 625 2502 Koodli (Editor) Expires 10 April 2004 [Page 35] Internet Draft Fast Handovers 10 October 2003 E-Mail: Rajeev.Koodli@nokia.com Koodli (Editor) Expires 10 April 2004 [Page 36]