Mipshop Working Group Rajeev Koodli, Editor INTERNET DRAFT Nokia Research Center 14 April 2006 Fast Handovers for Mobile IPv6 draft-ietf-mipshop-fmipv6-rev-00.txt By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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 document is a submission of the IETF MIPSHOP WG. Comments should be directed to the MIPSHOP WG mailing list, mipshop@ietf.org. 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 due to both 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 a protocol to improve handover latency due to Mobile IPv6 procedures. This document does not address improving the link switching latency. Koodli (Editor) Expires 14 October 2006 [Page i] Internet Draft Fast Handovers 14 April 2006 Contents Abstract i 1. Introduction 2 2. Terminology 2 3. Protocol Overview 4 3.1. Addressing the Handover Latency . . . . . . . . . . . . . 4 3.2. Protocol Operation . . . . . . . . . . . . . . . . . . . 6 3.3. Protocol Operation of Network-initiated Handover . . . . 8 4. Protocol Details 9 5. Miscellaneous 13 5.1. Handover Capability Exchange . . . . . . . . . . . . . . 13 5.2. Determining New Care of Address . . . . . . . . . . . . . 14 5.3. Packet Loss . . . . . . . . . . . . . . . . . . . . . . . 14 5.4. DAD Handling . . . . . . . . . . . . . . . . . . . . . . 14 5.5. Fast or Erroneous Movement . . . . . . . . . . . . . . . 15 6. Message Formats 16 6.1. New Neighborhood Discovery Messages . . . . . . . . . . . 16 6.1.1. Router Solicitation for Proxy Advertisement (RtSolPr) . . . . . . . . . . . . . . . . . . . . 16 6.1.2. Proxy Router Advertisement (PrRtAdv) . . . . . . 18 6.2. Inter-Access Router Messages . . . . . . . . . . . . . . 21 6.2.1. Handover Initiate (HI) . . . . . . . . . . . . . 21 6.2.2. Handover Acknowledge (HAck) . . . . . . . . . . . 23 6.3. New Mobility Header Messages . . . . . . . . . . . . . . 25 6.3.1. Fast Binding Update (FBU) . . . . . . . . . . . . 26 6.3.2. Fast Binding Acknowledgment (FBack) . . . . . . . 27 6.3.3. Fast Neighbor Advertisement (FNA) . . . . . . . . 29 6.4. New Options . . . . . . . . . . . . . . . . . . . . . . . 30 6.4.1. IP Address Option . . . . . . . . . . . . . . . . 30 6.4.2. New Router Prefix Information Option . . . . . . 31 6.4.3. Link-layer Address (LLA) Option . . . . . . . . . 32 6.4.4. Mobility Header Link-layer Address (MH-LLA) Option 34 6.4.5. Neighbor Advertisement Acknowledgment (NAACK) . . 34 7. Configurable Parameters 36 8. Security Considerations 36 9. IANA Considerations 37 Koodli (Editor) Expires 14 October 2006 [Page ii] Internet Draft Fast Handovers 14 April 2006 10. Acknowledgments 38 11. Normative References 38 12. Author's Address 39 13. Contributors 39 A. Change Log 39 Intellectual Property Statement 40 Disclaimer of Validity 41 Copyright Statement 41 Acknowledgment 41 Koodli (Editor) Expires 14 October 2006 [Page 1] Internet Draft Fast Handovers 14 April 2006 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 involve movement detection, IP address configuration, and location update. The combined handover latency is often sufficient to affect 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 regardless of 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]. 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 an IP subnet that offers wireless connectivity to a MN. An Access Point Identifier (AP-ID) refers the AP's L2 address. Sometimes, AP-ID is also referred to as a Base Station Subsystem ID (BSSID). Access Router (AR) The MN's default router Koodli (Editor) Expires 14 October 2006 [Page 2] Internet Draft Fast Handovers 14 April 2006 Previous Access Router (PAR) The MN's default router prior to its handover 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's subnet New CoA (NCoA) The MN's Care of Address valid on NAR's subnet Handover A process of terminating existing connectivity and obtaining new IP connectivity. Router Solicitation for Proxy Advertisement (RtSolPr) A message from the MN to the PAR requesting information for a potential handover Proxy Router Advertisement (PrRtAdv) A message from the PAR to the MN that provides information about neighboring links facilitating expedited movement detection. The message also acts as a trigger for network-initiated handover. (AP-ID, AR-Info) tuple Contains an access router's L2 and IP addresses, and prefix valid on the interface to which the Access Point (identified by AP-ID) is attached. The triplet [Router's L2 address, Router's IP address and Prefix] is called ``AR-Info''. Assigned Addressing A particular type of NCoA configuration in which the NAR assigns an IPv6 address for the MN. The method by which NAR manages its address pool is not specified in this document. 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, Koodli (Editor) Expires 14 October 2006 [Page 3] Internet Draft Fast Handovers 14 April 2006 and to confirm use of NCoA when the MN has not received FBACK Handover Initiate (HI) A message from the PAR to the NAR regarding a MN's handover Handover Acknowledge (HAck) A message from the NAR to the PAR as a response to HI v +------------+ +-+ | Previous | < | | ---------- | Access | ------ > ----\ +-+ | Router | < \ MN | (PAR) | \ | +------------+ +---------------+ | ^ IP | Correspondent | | | Network | Node | V | +---------------+ v / v +------------+ / +-+ | New | < / | | ---------- | Access | ------ > ----/ +-+ | Router | < MN | (NAR) | +------------+ Figure 1: Reference Scenario for Handover 3. Protocol Overview 3.1. Addressing the Handover Latency The ability to immediately send packets from a new subnet link depends on the ``IP connectivity'' latency, which in turn depends on the movement detection latency and the new CoA configuration latency. Once a MN is IP-capable on the new subnet link, it can send a Binding Update to its Home Agent and one or more correspondents. Once its correspondents successfully process the Binding Update, which typically involves the Return Routability procedure, the MN can receive packets at the new CoA. So, the ability to receive packets Koodli (Editor) Expires 14 October 2006 [Page 4] Internet Draft Fast Handovers 14 April 2006 from correspondents directly at its new CoA depends on the Binding Update latency as well as the IP connectivity 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 while connected to its current router. 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 L2 address. The ``Router Solicitation for Proxy Advertisement (RtSolPr)'' and ``Proxy Router Advertisement (PrRtAdv)'' messages 6.1 are used for aiding movement detection. Through the RtSolPr and PrRtAdv messages, the MN also formulates a prospective new CoA (NCoA), when it is still present on the 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 support the mechanism specified in this document to avoid potential address conflicts and SHOULD use them. In order to reduce the Binding Update latency, the protocol specifies a binding 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 result is that PAR begins tunneling packets arriving for PCoA to NCoA. Such a tunnel remains active until the MN completes the 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 Koodli (Editor) Expires 14 October 2006 [Page 5] Internet Draft Fast Handovers 14 April 2006 the MN's correspondent). Such a reverse tunnel ensures that packets containing PCoA as source IP address 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). Forwarding support for PCoA is provided through a reverse tunnel between the MN and the PAR. 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. A neighbor discovery operation involving a neighbor's address resolution (i.e., Neighbor Solicitation and Neighbor Advertisement) typically results 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 period of one second each. In order to 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 expedited 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 addresses are assigned by the access router. 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 supported and SHOULD be used. The access routers MUST have necessary security association established by means outside the scope of this document. 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] Koodli (Editor) Expires 14 October 2006 [Page 6] Internet Draft Fast Handovers 14 April 2006 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 of the MN. 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) when FBU is sent from NAR's link. 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 determine whether NCoA is acceptable to NAR through the exchange of HI and HAck messages. When assigned addressing (i.e., addresses are assigned by the router) is used, the proposed NCoA in FBU is carried in HI, and NAR MAY assign the proposed NCoA. Such an 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. 2. The MN does not receive FBack on the previous link. One reason for this is that the MN has not sent the FBU. The other is that the MN has left the link after sending the FBU, which itself 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 Koodli (Editor) Expires 14 October 2006 [Page 7] Internet Draft Fast Handovers 14 April 2006 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 rare but the undesirable outcome resulting from address collision. 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 is 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 is characterized as ``reactive'' mode of operation. Note that the reactive mode also 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 unsolicited, i.e., without the MN first sending RtSolPr. This mode is described in Section 3.3. 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. In such networks, the PAR can send an unsolicited PrRtAdv containing the link layer 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 PrRtAdv, 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. The unsolicited PrRtAdv also allows 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 topology map of links and subnets. Such usage of PrRtAdv is decoupled from the actual handover. See Section 6.1.2. Koodli (Editor) Expires 14 October 2006 [Page 8] Internet Draft Fast Handovers 14 April 2006 MN PAR NAR | | | |------RtSolPr------->| | |<-----PrRtAdv--------| | | | | |------FBU----------->|--------HI--------->| | |<------HAck---------| | <--FBack---|--FBack---> | | | | disconnect forward | | packets===============>| | | | | | | connect | | | | | |--------- FNA --------------------------->| |<=================================== deliver packets | | Figure 2: ``Predictive'' Fast Handover 4. Protocol Details All description makes use of Figure 1 as the reference. After discovering one or more 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 can send RtSolPr at any time, e.g., when one or more new access points are discovered. The MN can also send RtSolPr more than once during its attachment to PAR. The trigger for sending RtSolPr can originate 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 referred to as ``L2 triggers'', are outside the scope of this document. Nevertheless, they serve as events that invoke this protocol. For instance, when a ``link up'' indication is obtained on the new link, protocol messages (e.g., FNA) can be immediately transmitted. Implementations SHOULD make use of such triggers whenever available. Koodli (Editor) Expires 14 October 2006 [Page 9] Internet Draft Fast Handovers 14 April 2006 MN PAR NAR | | | |------RtSolPr------->| | |<-----PrRtAdv--------| | | | | disconnect | | | | | | | | connect | | |------FNA[FBU]-------|------------------->| | |<-----FBU-----------| | |------FBack-------->| | forward | | packets===============>| | | | |<=================================== deliver packets | | Figure 3: ``Reactive'' Fast Handover The RtSolPr message contains one or more AP-IDs. A wildcard requests all available tuples. 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 access point, it MUST respond indicating that the new access point 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 access point is connected to the PAR's current interface (to which MN is attached), PAR MUST respond with a Code value indicating that the new access point is connected to the current interface, but not send any prefix information. This scenario could arise, for example, when several wireless access points are bridged into a wired network. No further protocol action is necessary. 3. If the new access point is known and the PAR has information about it, then PAR MUST respond indicating that the new access point is known and supply the [AP-ID, AR-Info] tuple. If the new Koodli (Editor) Expires 14 October 2006 [Page 10] Internet Draft Fast Handovers 14 April 2006 access point 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 access point, the PAR SHOULD supply neighborhood [AP-ID, AR-Info] tuples subject to path MTU restrictions (i.e., provide any `n' tuples without exceeding the link MTU). When further protocol action is necessary, some implementations MAY choose to begin buffering copies of incoming packets at PAR. If such FIFO buffering is used, PAR MUST continue forwarding the packets to PCoA (i.e., buffer and forward). Such buffering can be useful when the MN leaves without sending the FBU message from the PAR's link. The PAR SHOULD stop buffering after processing the FBU message. The size of the buffer is an implementation-specific consideration. The method by which Access Routers exchange information about their neighbors and thereby allow construction of Proxy Router Advertisements with information about neighboring subnets is 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, use of above messages is optional on such links. After a PrRtAdv message is processed, the MN sends FBU and includes the proposed NCoA. The MN SHOULD send FBU from PAR's link whenever ``anticipation'' of handover is feasible. When anticipation is not feasible or when it has not received an FBack, the MN sends FBU immediately after attaching to NAR's link. This FBU SHOULD be encapsulated in a FNA message. The encapsulation allows NAR to discard the (inner) FBU packet if an address conflict is detected as a result of (outer) FNA packet processing (see FNA processing below). 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. In order to determine the NAR's address for the HI message, the PAR can perform longest prefix match of NCoA (in FBU) with the prefix list of neighboring access routers. When the source IP address of FBU is PCoA, i.e., the FBU is sent from the PAR's link, the HI message MUST have a Code value set to 0. See Section 6.2.1. When the source IP address of FBU is not PCoA, i.e., the FBU is sent from the NAR's link, the HI message MUST have a Code value of 1. See Section 6.2.1. The HI message contains the PCoA, link-layer address and the NCoA of the MN. In response to processing a HI message with Code 0, the NAR Koodli (Editor) Expires 14 October 2006 [Page 11] Internet Draft Fast Handovers 14 April 2006 1. determines whether NCoA supplied in the HI message is a valid address for use, and if it is, starts proxying [6] the address for PROXY_ND_LIFETIME during which the MN is expected to connect to NAR. The NAR MAY use the link-layer address to verify if a corresponding IP address exists in its forwarding tables. 2. allocates NCoA for the MN when assigned addressing is used, creates a proxy neighbor cache entry and begins defending it. The NAR MAY allocate the NCoA proposed in HI. 3. MAY create a host route entry for PCoA (on the interface to which the MN is attaching to) in case NCoA cannot be accepted or assigned. 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. provides the status of handover request in Handover Acknowledge (HAck) message. When the Code value in HI is 1, NAR MUST skip the above operations since it would have performed those operations during FNA processing. However, it SHOULD be prepared to process any other options which may be defined in the future. Sending a HI message with Code 1 allows NAR to, loosely speaking, validate the neighbor cache entry it creates for the MN during FNA processing. That is, NAR can make use of the knowledge that its trusted peer (i.e., PAR) has a trust relationship with the MN. If HAck contains an assigned NCoA, FBack MUST include it, and the MN MUST use the address provided in FBack. The PAR MAY send FBack to previous link as well to facilitate faster reception in the event the MN be still present there. The 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 the NCoA corresponding to the FNA message is acceptable, the NAR MUST, 1. delete its proxy neighbor cache entry, if any is present. Koodli (Editor) Expires 14 October 2006 [Page 12] Internet Draft Fast Handovers 14 April 2006 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. When the NCoA corresponding to the FNA message is not acceptable, the NAR MUST 1. discard the inner (FBU) packet. 2. send a Router Advertisement with the NAACK option in which it MAY include an alternate NCoA for use. This message MUST be sent to the source IP address present in FNA using the same Layer 2 address present in FNA. If the MN receives a Router Advertisement with a NAACK option, it MUST use the IP address, if any, provided in the NAACK option. Otherwise, the MN should configure another NCoA. 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. Once the MN has confirmed its NCoA, it SHOULD send a Neighbor Advertisement message. This message allows MN's neighbors to update their neighbor cache entries with the MN's addresses. For data forwarding, the PAR tunnels packets to the MN using its global IP address valid on the interface to which the MN was attached. The MN reverse tunnels its packets to the same global address of PAR. The tunnel end-point addresses must be configured accordingly. When PAR receives a reverse tunneled packet, it must verify if a secure binding exists for the MN identified by PCoA in the tunneled packet, before forwarding the packet. 5. Miscellaneous 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 and stop sending any more RtSolPr messages. Even if a MN's current access router is capable of fast handover, the new access router to which the MN attaches may be incapable of fast Koodli (Editor) Expires 14 October 2006 [Page 13] Internet Draft Fast Handovers 14 April 2006 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 MUST verify if NCoA present in HI is already in use. In any case, NAR MUST respond to HI using a HAck, in which it may include another NCoA to use, especially when assigned address configuration is used. If there is a CoA present in HAck, PAR MUST include it 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, which may not be exactly co-ordinated with fast handover signaling. Furthermore, the arrival pattern of packets is dependent on many factors, including application characteristics, network queuing behaviors etc. Hence, packets may arrive at NAR before the MN is able to establish its link there. 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 requests this feature (for the MN), it SHOULD also provide its own support for buffering. 5.4. DAD Handling Duplicate Address Detection (DAD) was defined in [7] 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 auto-configuration adds delays to a handover. The probability of an interface identifier duplication on the same subnet is very low, however it cannot be ignored. In this draft certain precautions are proposed to minimize the effects of a duplicate address occurrence. Koodli (Editor) Expires 14 October 2006 [Page 14] Internet Draft Fast Handovers 14 April 2006 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 the address configuration procedure according to [6] 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 occur at a rate suitable 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. The signaling overhead over the air and in the network may increase significantly, especially in the case of rapid movement between several 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. If it finds a match it SHOULD tear down 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 Koodli (Editor) Expires 14 October 2006 [Page 15] Internet Draft Fast Handovers 14 April 2006 lifetime of such tunnels should be enough to allow a MN to update all its active bindings. The default lifetime of the tunnel should be the same as the lifetime value in the FBU message. The effect of erroneous movement is typically limited to loss of packets since routing can change and the PAR may forward packets towards another router before the MN actually connects to that router. If the MN discovers itself on an unanticipated access router, a Fast Binding Update to the PAR SHOULD be sent. Since Fast Binding Updates are authenticated, they supersede the existing binding and packets MUST be redirected to the new confirmed location of the MN. 6. Message Formats All the ICMPv6 messages have a common Type specified in [4]. The messages are distinguished based on the Subtype field (see below). The values for the Subtypes are specified in Section 9. For all the ICMPv6 messages, the checksum is defined in [2]. 6.1. New Neighborhood Discovery Messages 6.1.1. Router Solicitation for Proxy Advertisement (RtSolPr) Mobile Nodes send Router Solicitation for Proxy Advertisement in order to prompt routers for Proxy Router Advertisements. All the link-layer address options have the format defined in 6.4.3. 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 | Code | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Subtype | Reserved | Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options ... +-+-+-+-+-+-+-+-+-+-+-+- Figure 4: Router Solicitation for Proxy (RtSolPr) Message IP Fields: Koodli (Editor) Expires 14 October 2006 [Page 16] Internet Draft Fast Handovers 14 April 2006 Source Address An IP address assigned to the sending interface Destination Address The address of the Access Router or the all routers multicast address. Hop Limit 255. See RFC 2461. 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. See RFC 2402. ICMP Fields: Type The Experimental Mobility Protocol Type. See [4]. Code 0 Checksum The ICMPv6 checksum. Subtype 2 Reserved MUST be set to zero by the sender and ignored by the receiver. Identifier MUST be set by the sender so that replies can be matched to this Solicitation. Valid Options: Source Link-layer Address When known, the link-layer address of the sender SHOULD be included using the Link-Layer Address option. See LLA option format below. New Access Point Link-layer Address The link-layer address or identification of the access 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. Koodli (Editor) Expires 14 October 2006 [Page 17] Internet Draft Fast Handovers 14 April 2006 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. Including the source LLA option allows the receiver to record the sender's L2 address so that neighbor discovery, when the receiver needs to send packets back to the sender (of RtSolPr message), can be avoided. When a wildcard is used for New Access Point LLA, no other New Access Point LLA options must be present. A Proxy Router Advertisement (PrRtAdv) message should be received by the MN as a response to RtSolPr. 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 milliseconds if RTT is not known, it SHOULD resend RtSolPr message. Subsequent retransmissions can be up to RTSOLPR_RETRIES, but MUST use an exponential backoff in which the timeout period (i.e., 2xRTT or 100 milliseconds) is doubled prior to 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. When RtSolPr messages are sent more than once, they MUST be rate limited with MAX_RTSOLPR_RATE per second. During each use of RtSolPr, exponential backoff is used for retransmissions. 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. All the link-layer address options have the format defined in 6.4.3. 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 Advertisement or the address of the node the Access Router is instructing to handover. Koodli (Editor) Expires 14 October 2006 [Page 18] Internet Draft Fast Handovers 14 April 2006 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Subtype | Reserved | Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options ... +-+-+-+-+-+-+-+-+-+-+-+- Figure 5: Proxy Router Advertisement (PrRtAdv) Message Hop Limit 255. See RFC 2461. 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. See RFC 2402. ICMP Fields: Type The Experimental Mobility Protocol Type. See [4]. Code 0, 1, 2, 3 or 4. See below. Checksum The ICMPv6 checksum. Subtype 3 Reserved MUST be set to zero by the sender and ignored by the receiver. Identifier Copied from Router Solicitation for Proxy Advertisement or set to Zero if unsolicited. Valid Options in the following order: Source Link-layer Address When known, the link-layer address of the sender SHOULD be included using the Link-Layer Address option. See LLA option format below. New Access Point Link-layer Address Koodli (Editor) Expires 14 October 2006 [Page 19] Internet Draft Fast Handovers 14 April 2006 The link-layer address or identification of the access point is copied from RtSolPr message. This option MUST be present. New Router's Link-layer Address The link-layer address of the Access Router for which this message is proxied for. This option MUST be included when Code is 0 or 1. New Router's IP Address The IP address of NAR. This option MUST be included when Code is 0 or 1. New Router Prefix Information Option. Specifies the prefix of the Access Router the message is proxied for and is used for address auto-configuration. This option MUST be included when Code is 0 or 1. However, when this prefix is the same as what is used in the New Router's IP Address option (above), the Prefix Information option need not be present. New CoA Option MAY be present when PrRtAdv is sent unsolicited. PAR MAY compute new CoA using NAR's prefix information and the MN's L2 address, or by any other means. 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. Currently, Code values 0, 1, 2, 3 and 4 are defined. A Proxy Router Advertisement with Code 0 means that the MN should use the [AP-ID, AR-Info] tuple (present in the options above) for movement detection and NCoA formulation. The Option-Code field in the New Access Point LLA option in this case is 1 reflecting the LLA of the access point for which the rest of the options are related. Multiple tuples may be present. A Proxy Router Advertisement with Code 1 means that the message is sent unsolicited. If a New CoA option is present following the New Router Prefix Information option, the MN SHOULD use the supplied NCoA and send FBU immediately or else stand to lose service. This message acts as a network-initiated handover trigger. See Section 3.3. The Option-Code field in the New Access Point LLA option (see below) in Koodli (Editor) Expires 14 October 2006 [Page 20] Internet Draft Fast Handovers 14 April 2006 this case is 1 reflecting the LLA of the access point for which the rest of the options are related. A Proxy Router Advertisement with Code 2 means that no new router information is present. Each New Access Point LLA option contains an Option-Code value (described below) which indicates a specific outcome. - When the Option-Code field in the New Access Point LLA option is 5, handover to that access point does not require change of CoA. No other options are required in this case. - When the Option-Code field in the New Access Point LLA option is 6, 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 other options are required in this case. - When the Option-Code field in the New Access Point LLA option is 7, it means that the NAR does not support fast handover. The MN MUST stop fast handover protocol operations. No other options are required in this case. A Proxy Router Advertisement with Code 3 means that new router information is present only for a subset of access points requested. The Option-Code field values (defined above including a value of 1) distinguish different outcomes for individual access points. A Proxy Router Advertisement with Code 4 means that the subnet information regarding neighboring access points is sent unsolicited, but the message is not a handover trigger, unlike when the message is sent with Code 1. Multiple tuples may be present. 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. 6.2. Inter-Access Router Messages 6.2.1. Handover Initiate (HI) The Handover Initiate (HI) is an ICMPv6 message sent by an Access Router (typically PAR) to another Access Router (typically NAR) to initiate the process of a MN's handover. IP Fields: Koodli (Editor) Expires 14 October 2006 [Page 21] Internet Draft Fast Handovers 14 April 2006 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Subtype |S|U| Reserved | Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options ... +-+-+-+-+-+-+-+-+-+-+-+- Figure 6: Handover Initiate (HI) Message Source Address The IP address of the PAR Destination Address The IP address of the NAR Hop Limit 255. See RFC 2461. Authentication Header The authentication header MUST be used when this message is sent. See RFC 2402. ICMP Fields: Type The Experimental Mobility Protocol Type. See [4]. Code 0 or 1. See below Checksum The ICMPv6 checksum. Subtype 4 S Assigned address configuration flag. When set, this message requests a new CoA to be returned by the destination. May be set when Code = 0. MUST be 0 when Code = 1. U Buffer flag. When set, the destination SHOULD buffer any packets towards the node indicated in the options of this message. Used when Code = 0, SHOULD be set to 0 when Code = 1. Koodli (Editor) Expires 14 October 2006 [Page 22] Internet Draft Fast Handovers 14 April 2006 Reserved MUST be set to zero by the sender and ignored by the receiver. Identifier MUST be set by the sender so replies can be matched to this message. Valid Options: Link-layer address of MN The link-layer address of the MN that is undergoing handover to the destination (i.e., NAR). This option MUST be included so that the destination can recognize the MN. 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 assign this address. The PAR uses a Code value of 0 when it processes an FBU with PCoA as source IP address. The PAR uses a Code value of 1 when it processes an FBU whose source IP address is not PCoA. If Handover Acknowledge (HAck) message is not received as a response in a short time period but no less than twice the typical round trip time (RTT) between source and destination, or 100 milliseconds if RTT is not known, the Handover Initiate SHOULD be re-sent. Subsequent retransmissions can be up to HI_RETRIES, but MUST use exponential backoff in which the timeout period (i.e., 2xRTT or 100 milliseconds) is doubled during each instance of retransmission. 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. IP Fields: Source Address Koodli (Editor) Expires 14 October 2006 [Page 23] Internet Draft Fast Handovers 14 April 2006 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Subtype | Reserved | Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options ... +-+-+-+-+-+-+-+-+-+-+-+- Figure 7: Handover Acknowledge (HAck) Message 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. See RFC 2461. Authentication Header The authentication header MUST be used when this message is sent. See RFC 2402. ICMP Fields: Type The Experimental Mobility Protocol Type. See [4]. Code 0: Handover Accepted, NCoA valid 1: Handover Accepted, NCoA not valid 2: Handover Accepted, NCoA in use 3: Handover Accepted, NCoA assigned (used in Assigned addressing) 4: Handover Accepted, NCoA not assigned (used in Assigned addressing) 5: Handover Accepted, use PCoA 128: Handover Not Accepted, reason unspecified 129: Administratively prohibited 130: Insufficient resources Koodli (Editor) Expires 14 October 2006 [Page 24] Internet Draft Fast Handovers 14 April 2006 Checksum The ICMPv6 checksum. Subtype 5 Reserved MUST be set to zero by the sender and ignored by the receiver. Identifier Copied from the corresponding field in the Handover Initiate message this message is in response to. 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 a 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 3. The NAR MAY provide support for PCoA (instead of accepting or assigning NCoA), using a host route entry to forward packets to the PCoA, and using a tunnel to the PAR to forward packets from the MN (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. The NAR indicates forwarding support for PCoA using Code value 5 in the HAck message. Subsequently, PAR establishes a tunnel to NAR in order to forward packets arriving for PCoA. When responding to a HI message containing a Code value 1, the Code values 1, 2, and 4 in the HAck message are not relevant. Finally, the new access router can always refuse handover, in which case it should indicate the reason in one of the available Code values. 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. Koodli (Editor) Expires 14 October 2006 [Page 25] Internet Draft Fast Handovers 14 April 2006 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. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 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 Acknowledgment message. `H' flag MUST be set to one. See [3]. `L' flag See [3]. `K' flag See [3]. Reserved This field is unused. MUST be set zero. Sequence Number See [3]. Lifetime See [3]. Koodli (Editor) Expires 14 October 2006 [Page 26] Internet Draft Fast Handovers 14 April 2006 Mobility Options MUST contain alternate CoA option set to NCoA IP address when FBU is sent from PAR'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 NCoA on the NAR according to Neighbor Discovery and IPv6 Address Configuration protocols. 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 so that PAR is 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. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Status |K| Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sequence # | Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . . . Mobility options . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 9: Fast Binding Acknowledgment (FBack) Message Koodli (Editor) Expires 14 October 2006 [Page 27] Internet Draft Fast Handovers 14 April 2006 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: 0 Fast Binding Update accepted 1 Fast Binding Update accepted but NCoA is invalid. Use NCoA supplied in ``alternate'' 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 sender of this message will retain a binding for traffic redirection. Mobility Options MUST contain ``alternate'' CoA if Status is 1. Koodli (Editor) Expires 14 October 2006 [Page 28] Internet Draft Fast Handovers 14 April 2006 6.3.3. Fast Neighbor Advertisement (FNA) A MN sends a Fast Neighbor Advertisement to announce itself to the NAR. 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 Mobility Header Link-Layer Address of the MN in MH-LLA option format. See Section 6.4.4. The MN sends Fast Neighbor Advertisement to the NAR, as soon as it regains connectivity on the new link. 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. 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. Koodli (Editor) Expires 14 October 2006 [Page 29] Internet Draft Fast Handovers 14 April 2006 6.4. New Options All the options are of the form shown in Figure 11. 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 | Option-Code | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ ... ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 11: Option Format The Type values are defined from the Neighbor Discovery options space. The Length field is in units of 8 octets, except for the Mobility Header Link-Layer Address option, whose Length field is in units of octets in accordance with [3], Section 6.2. And, Option-Code provides additional information for each of the options (See individual options below). 6.4.1. IP Address Option This option is sent in the Proxy Router Advertisement, the Handover Initiate, and Handover Acknowledge messages. Type To be assigned by IANA Length The size of this option in 8 octets including the Type, Option-Code and Length fields. Option-Code 1 Old Care-of Address 2 New Care-of Address 3 NAR's IP address Prefix Length The Length of the IPv6 Address Prefix. Koodli (Editor) Expires 14 October 2006 [Page 30] Internet Draft Fast Handovers 14 April 2006 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 | Option-Code | Prefix Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + IPv6 Address + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 12: IPv6 Address Option Reserved MUST be set to zero by the sender and MUST be ignored by the receiver. 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. Type To be assigned by IANA Length The size of this option in 8 octets including the Type, Option-Code and Length fields. Option-Code 0 Prefix Length Koodli (Editor) Expires 14 October 2006 [Page 31] Internet Draft Fast Handovers 14 April 2006 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 | Option-Code | Prefix Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + Prefix + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 13: New Router Prefix Information Option 8-bit unsigned integer. The number of leading bits in the Prefix that are valid. The value ranges from 0 to 128. Reserved MUST be set to zero by the sender and MUST be ignored by the receiver. 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) Option Type To be assigned by IANA Length The size of this option in 8 octets including the Type, Option-Code and Length fields. Option-Code Koodli (Editor) Expires 14 October 2006 [Page 32] Internet Draft Fast Handovers 14 April 2006 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 | Option-Code | LLA... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 14: Link-Layer Address Option 0 wildcard requesting resolution for all nearby access points 1 Link-layer Address of the New Access Point 2 Link-layer Address of the MN 3 Link-layer Address of the NAR (i.e., Proxied Originator) 4 Link-layer Address of the source of RtSolPr or PrRtAdv message 5 The access point identified by the LLA belongs to the current interface of the router 6 No prefix information available for the access point identified by the LLA 7 No fast handovers support available for the access point identified by the LLA LLA The variable length link-layer address. Depending on the size of individual LLA option, appropriate padding MUST be used to ensure that the entire option size is a multiple of 8 octects. The New Access Point Link Layer address contains the link-layer address of the access point for which handover is about to be attempted. This is used in the Router Solicitation for Proxy Advertisement message. The MN Link-Layer address option contains the link-layer address of a MN. It is used in the Handover Initiate message. The NAR (i.e., Proxied Originator) Link-Layer address option contains the Link Layer address of the Access Router for which the Proxy Router Solicitation message refers to. Koodli (Editor) Expires 14 October 2006 [Page 33] Internet Draft Fast Handovers 14 April 2006 6.4.4. Mobility Header Link-layer Address (MH-LLA) Option This option is identical to the LLA option, but is carried in the Mobility Header messages, e.g., FNA. In the future, other Mobility Header messages may also make use of this option. The format of the option when LLA is 6 bytes is shown in Figure 15. When LLA size is different, the option MUST be aligned appropriately. (See Section 6.2 in [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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option-Code | Pad0=0 | LLA | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LLA | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 15: Mobility Header Link-Layer Address Option Type To be assigned by IANA Length The size of this option in octets not including the Type, Length and Option-Code fields. Option-Code 2 Link-layer Address of the MN LLA The variable length link-layer address. 6.4.5. Neighbor Advertisement Acknowledgment (NAACK) Type To be assigned by IANA. Length 8-bit unsigned integer. Length of the option, in 8 Koodli (Editor) Expires 14 October 2006 [Page 34] Internet Draft Fast Handovers 14 April 2006 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 | Option-Code | Status | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 16: Neighbor Advertisement Acknowledgment Option octets. The length is 1 when a new CoA is not supplied. The length is 3 when a new CoA is present (immediately following the Reserved field) Option-Code 0 Status 8-bit unsigned integer indicating the disposition of the Fast Neighbor Advertisement message. The following Status values are currently defined: 1 The New CoA is invalid 2 The New CoA is invalid, use the supplied CoA. The New CoA MUST be present following the Reserved field. 128 Link Layer Address unrecognized Reserved MUST be set to zero by the sender and MUST be ignored by the receiver. Since the NAACK option is carried in a Router Advertisement, the Length field is units of 8 octets unlike in other options. The NAR responds to FNA with the NAACK option to notify the MN to use a different NCoA if there is address collision. If the NCoA is invalid, the Router Advertisement MUST use the NCoA as the destination address but use the L2 address present in FNA. The MN SHOULD use the NCoA if it is supplied 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. Koodli (Editor) Expires 14 October 2006 [Page 35] Internet Draft Fast Handovers 14 April 2006 7. Configurable Parameters Parameter Name Default Value Definition ------------------- ---------------------- ------- RTSOLPR_RETRIES 3 Section6.1.1 MAX_RTSOLPR_RATE 3 Section6.1.1 FBU_RETRIES 3 Section 4 PROXY_ND_LIFETIME 1.5 seconds Section 6.2.2 HI_RETRIES 3 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, future work may specify this security association 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 inadvertent redirection: in this case, the FBU is secured, but the target of binding happens to be an unsuspecting node either due to inadvertent 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 Koodli (Editor) Expires 14 October 2006 [Page 36] Internet Draft Fast Handovers 14 April 2006 order to do this, HI and HAck message exchanges are to be used. When NAR accepts NCoA in HI (with Code = 0), 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 inadvertent 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, the extent of this vulnerability is arguably small. 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. IANA Considerations This document defines four new experimental ICMPv6 messages which use the Experimental Mobility Protocol ICMPv6 format [4]. These require four new Subtype value assignments out of the Experimental Mobility Protocol Subtype Registry [4] as follows: Subtype Description Reference ------- ----------- --------- 2 RtSolPr Section 6.1.1 3 PrRtAdv Section 6.1.2 4 HI Section 6.2.1 5 HAck Section 6.2.2 The document defines four new Neighbor Discovery [6] options which need Type assignment from IANA. Option-Type Description Reference ----------- ----------- --------- TBD IP Address Option Section 6.4.1 TBD New Router Prefix Information Option Section 6.4.2 TBD Link-layer Address Option Section 6.4.3 TBD Neighbor Advertisement Acknowledgment Option Section 6.4.5 Koodli (Editor) Expires 14 October 2006 [Page 37] Internet Draft Fast Handovers 14 April 2006 The document defines three new Mobility Header messages which need type allocation from the Mobility Header Types registry at http://www.iana.org/assignments/mobility-parameters: 1. Fast Binding Update, described in Section 6.3.1 2. Fast Binding Acknowledgment, described in Section 6.3.2, and 3. Fast Neighbor Advertisement, described in Section 6.3.3 The document defines a new Mobility Option which needs type assignment from the Mobility Options Type registry at http://www.iana.org/assignments/mobility-parameters: 1. Mobility Header Link-Layer Address option, described in Section 6.4.4. 10. Acknowledgments The editor would like to thank all those who have provided feedback on this specification, but can only mention a few here: Vijay Devarapalli, Youn-Hee Han, Suvidh Mathur, Gabriel Montenegro, Takeshi Ogawa, Sun Peng, YC Peng, Domagoj Premec, and Jonathan Wood. The editor would like to acknowledge contribution from James Kempf to improve this specification. The editor would also like to thank [mipshop] working group chair Gabriel Montenegro and the erstwhile [mobile ip] working group chairs Basavaraj Patil and Phil Roberts for providing much support for this work. 11. Normative References 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. [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. E. Perkins, and J. Arkko, ``Mobility Support in IPv6'', Request for Comments (Proposed Standard) 3775, Internet Engineering Task Force, June 2004. Koodli (Editor) Expires 14 October 2006 [Page 38] Internet Draft Fast Handovers 14 April 2006 [4] J. Kempf, ``Instructions for Seamoby and Experimental Mobility Protocol IANA Allocations (work in progress)'', Internet Engineering Task Force, June 2004. [5] S. Kent and R. Atkinson, ``IP Authentication Header'', Request for Comments (Draft Standard) 2402, Internet Engineering Task Force, November 1998. [6] T. Narten, E. Nordmark, and W. Simpson, ``Neighbor Discovery for IP Version 6 (IPv6)'', Request for Comments (Draft Standard) 2461, Internet Engineering Task Force, December 1998. [7] S. Thomson and T. Narten, ``IPv6 Stateless Address Autoconfiguration'', Request for Comments (Draft Standard) 2462, Internet Engineering Task Force, December 1998. 12. Author's Address Rajeev Koodli, Editor Nokia Research Center 313 Fairchild Drive Mountain View, CA 94043 USA Phone: +1 650 625 2359 Fax: +1 650 625 2502 E-Mail: Rajeev.Koodli@nokia.com 13. Contributors This document has its origins in 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. A. Change Log The following revisions have been done since IESG review in Sep 04. - Added IPSec AH reference. - Changed options format to make use of RFC 2461 options Type space. Revised IANA Considerations section accordingly. - Added exponential backoff for retransmissions. Added rate limiting for RtSolPr message. Koodli (Editor) Expires 14 October 2006 [Page 39] Internet Draft Fast Handovers 14 April 2006 - Replaced ``attachment point'' with ``access point'' for consistency. - Clarified [AP-ID, AR-Info] in Section 2. Clarified use of Prefix Information Option in Section 6.1.2. - Separated MH-LLA from LLA to future-proof LLA option. The following changes refer up to version 02 (under mipshop). The Section numbers refer to version 06 (under mobile ip). - New ICMPv6 format incorporated. ID Nits conformance. - Last Call comments incorporated - 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 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this Koodli (Editor) Expires 14 October 2006 [Page 40] Internet Draft Fast Handovers 14 April 2006 specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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. Copyright Statement Copyright (C) The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Koodli (Editor) Expires 14 October 2006 [Page 41]