IETF Mobile IP Working Group David B. Johnson INTERNET-DRAFT Carnegie Mellon University Charles Perkins Sun Microsystems 4 August 1998 Mobility Support in IPv6 Status of This Memo This document is a submission by the Mobile IP Working Group of the Internet Engineering Task Force (IETF). Comments should be submitted to the Working Group mailing list at "mobile-ip@SmallWorks.COM". Distribution of this memo is unlimited. This document is an Internet-Draft. 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." To view the entire list of current Internet-Drafts, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). Abstract This document specifies the operation of mobile computers using IPv6. Each mobile node is always identified by its home address, regardless of its current point of attachment to the Internet. While situated away from its home, a mobile node is also associated with a care-of address, which provides information about the mobile node's current location. IPv6 packets addressed to a mobile node's home address are transparently routed to its care-of address. The protocol enables IPv6 nodes to cache the binding of a mobile node's home address with its care-of address, and to then send any packets destined for the mobile node directly to it at this care-of address. Johnson and Perkins Expires 4 February 1999 [Page i] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 Contents Status of This Memo i Abstract i 1. Introduction 1 2. Comparison with Mobile IP for IPv4 3 3. Terminology 6 3.1. General Terms . . . . . . . . . . . . . . . . . . . . . . 6 3.2. Mobile IPv6 Terms . . . . . . . . . . . . . . . . . . . . 7 3.3. Specification Language . . . . . . . . . . . . . . . . . 8 4. Overview of Mobile IPv6 9 4.1. Basic Operation . . . . . . . . . . . . . . . . . . . . . 9 4.2. New IPv6 Destination Options . . . . . . . . . . . . . . 11 4.3. Conceptual Data Structures . . . . . . . . . . . . . . . 13 4.4. Binding Management . . . . . . . . . . . . . . . . . . . 16 5. New IPv6 Destination Options 19 5.1. Binding Update Option Format . . . . . . . . . . . . . . 19 5.2. Binding Acknowledgement Option Format . . . . . . . . . . 23 5.3. Binding Request Option Format . . . . . . . . . . . . . . 27 5.4. Home Address Option Format . . . . . . . . . . . . . . . 28 6. Modifications to IPv6 Neighbor Discovery 30 6.1. Modified Router Advertisement Message Format . . . . . . 30 6.2. Modified Prefix Information Option Format . . . . . . . . 31 6.3. New Advertisement Interval Option Format . . . . . . . . 33 6.4. New Home Agent Information Option Format . . . . . . . . 34 6.5. Changes to Sending Router Advertisements . . . . . . . . 36 6.6. Changes to Sending Router Solicitations . . . . . . . . . 37 7. Requirements for IPv6 Nodes 39 7.1. Requirements for All IPv6 Hosts and Routers . . . . . . . 39 7.2. Requirements for All IPv6 Routers . . . . . . . . . . . . 39 7.3. Requirements for IPv6 Home Agents . . . . . . . . . . . . 39 7.4. Requirements for IPv6 Mobile Nodes . . . . . . . . . . . 40 8. Correspondent Node Operation 42 8.1. Receiving Packets from a Mobile Node . . . . . . . . . . 42 8.2. Receiving Binding Updates . . . . . . . . . . . . . . . . 42 8.3. Requests to Cache a Binding . . . . . . . . . . . . . . . 43 8.4. Requests to Delete a Binding . . . . . . . . . . . . . . 44 Johnson and Perkins Expires 4 February 1999 [Page ii] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 8.5. Sending Binding Acknowledgements . . . . . . . . . . . . 44 8.6. Sending Binding Requests . . . . . . . . . . . . . . . . 45 8.7. Cache Replacement Policy . . . . . . . . . . . . . . . . 45 8.8. Receiving ICMP Error Messages . . . . . . . . . . . . . . 46 8.9. Sending Packets to a Mobile Node . . . . . . . . . . . . 47 9. Home Agent Operation 49 9.1. Receiving Router Advertisement Messages . . . . . . . . . 49 9.2. Dynamic Home Agent Address Discovery . . . . . . . . . . 50 9.3. Primary Care-of Address Registration . . . . . . . . . . 51 9.4. Primary Care-of Address De-registration . . . . . . . . . 54 9.5. Intercepting Packets for a Mobile Node . . . . . . . . . 54 9.6. Tunneling Intercepted Packets to a Mobile Node . . . . . 56 9.7. Renumbering the Home Subnet . . . . . . . . . . . . . . . 57 10. Mobile Node Operation 60 10.1. Sending Packets While Away from Home . . . . . . . . . . 60 10.2. Receiving Packets While Away from Home . . . . . . . . . 62 10.3. Movement Detection . . . . . . . . . . . . . . . . . . . 63 10.4. Forming New Care-of Addresses . . . . . . . . . . . . . . 66 10.5. Sending Binding Updates to the Home Agent . . . . . . . . 67 10.6. Dynamic Home Agent Address Discovery . . . . . . . . . . 68 10.7. Sending Binding Updates to Correspondent Nodes . . . . . 69 10.8. Sending Binding Updates to the Previous Default Router . 71 10.9. Retransmitting Binding Updates . . . . . . . . . . . . . 72 10.10. Rate Limiting for Sending Binding Updates . . . . . . . . 72 10.11. Receiving Binding Acknowledgements . . . . . . . . . . . 72 10.12. Receiving Binding Requests . . . . . . . . . . . . . . . 73 10.13. Receiving ICMP Error Messages . . . . . . . . . . . . . . 74 10.14. Receiving Tunneled Router Advertisements . . . . . . . . 74 10.15. Using Multiple Care-of Addresses . . . . . . . . . . . . 75 10.16. Routing Multicast Packets . . . . . . . . . . . . . . . . 76 10.17. Returning Home . . . . . . . . . . . . . . . . . . . . . 76 11. Constants 78 12. IANA Considerations 79 13. Security Considerations 80 13.1. Binding Updates, Acknowledgements, and Requests . . . . . 80 13.2. Home Address Option . . . . . . . . . . . . . . . . . . . 80 13.3. General Mobile Computing Issues . . . . . . . . . . . . . 81 Johnson and Perkins Expires 4 February 1999 [Page iii] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 Changes from Previous Draft 83 Acknowledgements 85 References 86 Chair's Address 88 Authors' Addresses 89 Johnson and Perkins Expires 4 February 1999 [Page iv] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 1. Introduction This document specifies the operation of mobile computers using Internet Protocol Version 6 (IPv6) [5]. Without specific support for mobility in IPv6, packets destined to a mobile node (host or router) would not be able to reach it while the mobile node is away from its home link (the link on which its home IPv6 subnet prefix is in use), since routing is based on the subnet prefix in a packet's destination IP address. In order to continue communication in spite of its movement, a mobile node could change its IP address each time it moves to a new link, but the mobile node would then not be able to maintain transport and higher-layer connections when it changes location. Mobility support in IPv6 is particularly important, as mobile computers are likely to account for a majority or at least a substantial fraction of the population of the Internet during the lifetime of IPv6. The protocol operation defined here, known as Mobile IPv6, allows a mobile node to move from one link to another without changing the mobile node's IP address. A mobile node is always addressable by its "home address", an IP address assigned to the mobile node within its home subnet prefix on its home link. Packets may be routed to the mobile node using this address regardless of the mobile node's current point of attachment to the Internet, and the mobile node may continue to communicate with other nodes (stationary or mobile) after moving to a new link. The movement of a mobile node away from its home link is thus transparent to transport and higher-layer protocols and applications. The Mobile IPv6 protocol is just as suitable for mobility across homogeneous media as for mobility across heterogeneous media. For example, Mobile IPv6 facilitates node movement from one Ethernet segment to another as well as it facilitates node movement from an Ethernet segment to a wireless LAN cell, with the mobile node's IP address remaining unchanged in spite of such movement. One can think of the Mobile IPv6 protocol as solving the "macro" mobility management problem. More "micro" mobility management applications -- for example, handoff among wireless transceivers, each of which covers only a very small geographic area -- are possibly more suited to other solutions. For example, in many current wireless LAN products, link-layer mobility mechanisms allow a "handoff" of a mobile node from one cell to another, reestablishing link-layer connectivity to the node in each new location. As long as such handoff occurs only within cells of the mobile node's home link, such link-layer mobility mechanisms are likely to offer faster convergence and lower overhead than Mobile IPv6. Extensions to the Mobile IPv6 protocol are also possible to support a more local, hierarchical form of mobility management, but such extensions are beyond the scope of this document. Johnson and Perkins Expires 4 February 1999 [Page 1] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 The protocol specified in this document solves the problem of transparently routing packets to and from mobile nodes while away from home. However, it does not attempt to solve all general problems related to the use of mobile computers or wireless networks. In particular, this protocol does not attempt to solve: - Handling links with partial reachability, such as typical wireless networks. Some aspects of this problem are addressed by the movement detection procedure described in Section 10.3, but no attempt has been made to fully solve this problem in its general form. Most aspects of this problem can be solved by the workaround of restricting such networks to only one router per link, although there are still possible hidden terminal problems when two nodes on the same link (on opposite sides of the router) attempt to communicate directly. - Access control on a link being visited by a mobile node. This is a general problem any time an untrusted node is allowed to connect to any link layer. It is independent whether the connecting node uses Mobile IP, DHCP [2], or just "borrows" an IP address on the link. Johnson and Perkins Expires 4 February 1999 [Page 2] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 2. Comparison with Mobile IP for IPv4 The design of Mobile IP support in IPv6 (Mobile IPv6) represents a natural combination of the experiences gained from the development of Mobile IP support in IPv4 (Mobile IPv4) [15, 14, 16], together with the opportunities provided by the design and deployment of a new version of IP itself (IPv6) and the new protocol features offered by IPv6. Mobile IPv6 thus shares many features with Mobile IPv4, but the protocol is now fully integrated into IP and provides many improvements over Mobile IPv4. This section summarizes the major differences between Mobile IPv4 and Mobile IPv6: - Support for what is known in Mobile IPv4 as "Route Optimization" [17] is now built in as a fundamental part of the protocol, rather than being added on as a optional set of extensions that may not be supported by all nodes as in Mobile IPv4. This integration of Route Optimization functionality allows direct routing from any correspondent node to any mobile node, without needing to pass through the mobile node's home network and be forwarded by its home agent, and thus eliminates the problem of "triangle routing" present in the base Mobile IPv4 protocol [15]. This integration also allows the Mobile IPv4 "registration" functionality and the Mobile IPv4 Route Optimization functionality to be performed by a single protocol rather than two separate (and different) protocols. - Support is also integrated into Mobile IPv6 -- and into IPv6 itself -- for allowing mobile nodes and Mobile IP to coexist efficiently with routers that perform "ingress filtering" [6]. A mobile node now uses its care-of address as the Source Address in the IP header of packets it sends, allowing the packets to pass normally through ingress filtering routers. The home address of the mobile node is carried in the packet in a Home Address destination option, allowing the use of the care-of address in the packet to be transparent above the IP layer. The ability to correctly process a Home Address option in a received packet is required in all IPv6 nodes, whether mobile nor stationary, whether host or router. - The use of the care-of address as the Source Address in each packet's IP header also simplifies routing of multicast packets sent by a mobile node. With Mobile IPv4, the mobile node had to tunnel multicast packets to its home agent in order to transparently use its home address as the source of the multicast packets. With Mobile IPv6, the use of the Home Address option allows the home address to be used but still be compatible with multicast routing that is based in part on the packet's Source Address. Johnson and Perkins Expires 4 February 1999 [Page 3] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 - There is no longer any need to deploy special routers as "foreign agents" as are used in Mobile IPv4. In Mobile IPv6, mobile nodes make use of the enhanced features of IPv6, such as Neighbor Discovery [13] and Address Autoconfiguration [22], to operate in any location away from home without any special support required from its local router. - Unlike Mobile IPv4, Mobile IPv6 utilizes IPsec [8, 9, 10] for all security requirements (sender authentication, data integrity protection, and replay protection) for Binding Updates (which serve the role of both registration and Route Optimization in Mobile IPv4). Mobile IPv4 relies on its own security mechanisms for these functions, based on statically configured "mobility security associations". - The movement detection mechanism in Mobile IPv6 provides bidirectional confirmation of a mobile node's ability to communicate with its default router in its current location (packets that the router sends are reaching the mobile node, and packets that the mobile node sends are reaching the router). This confirmation provides a detection of the "black hole" situation that may exist in some wireless environments where the link to the router does not work equally well in both directions, such as when the mobile node has moved out of good wireless transmission range from the router. The mobile node may then attempt to find a new router and begin using a new care-of address if its link to its current router is not working well. In contrast, in Mobile IPv4, only the forward direction (packets from the router are reaching the mobile node) is confirmed, allowing the black hole condition to persist. - Most packets sent to a mobile node while away from home in Mobile IPv6 are tunneled using an IPv6 Routing header rather than IP encapsulation, whereas Mobile IPv4 must use encapsulation for all packets. The use of a Routing header requires less additional header bytes to be added to the packet, reducing the overhead of Mobile IP packet delivery. To avoid modifying the packet in flight, however, packets intercepted and tunneled by a mobile node's home agent in Mobile IPv6 must still use encapsulation for tunneling. - While a mobile node is away from home, its home agent intercepts any packets for the mobile node that arrive at the home network, using IPv6 Neighbor Discovery [13] rather than ARP [18] as is used in Mobile IPv4. The use of Neighbor Discovery improves the robustness of the protocol (e.g., due to the Neighbor Advertisement "override" bit) and simplifies implementation of Mobile IP due to the ability to not be concerned with any particular link layer as is required in ARP. Johnson and Perkins Expires 4 February 1999 [Page 4] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 - The use of IPv6 encapsulation (and the Routing header) removes the need in Mobile IPv6 to manage "tunnel soft state", which was required in Mobile IPv4 due to limitations in ICMP for IPv4. Due to the definition of ICMP for IPv6, the use of tunnel soft state is no longer required in IPv6 for correctly relaying ICMP error messages from within the tunnel back to the original sender of the packet. - The dynamic home agent address discovery mechanism in Mobile IPv6 uses IPv6 anycast and returns a single reply to the mobile node, rather than the corresponding Mobile IPv4 mechanism that used IPv4 directed broadcast and returned a separate reply from each home agent on the mobile node's home link. The Mobile IPv6 mechanism is more efficient and more reliable, since only one packet need be sent back to the mobile node and since the mobile node is less likely to lose one of the replies because no "implosion" of replies is required by the protocol. - Mobile IPv6 defines an Advertisement Interval option on Router Advertisements (equivalent to Agent Advertisements in Mobile IPv4), allowing a mobile node to decide for itself how many Router Advertisements (Agent Advertisements) it is willing to miss before declaring its current router unreachable. - The use of IPv6 destination options allows all Mobile IPv6 control traffic to be piggybacked on any existing IPv6 packets, whereas in Mobile IPv4 and its Route Optimization extensions, separate UDP packets were required for each control message. Johnson and Perkins Expires 4 February 1999 [Page 5] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 3. Terminology 3.1. General Terms IP Internet Protocol Version 6 (IPv6). node A device that implements IP. router A node that forwards IP packets not explicitly addressed to itself. host Any node that is not a router. link A communication facility or medium over which nodes can communicate at the link layer, such as an Ethernet (simple or bridged). A link is the layer immediately below IP. interface A node's attachment to a link. subnet prefix A bit string that consists of some number of initial bits of an IP address. interface identifier A number used to identify a node's interface on a link. The interface identifier is the remaining low-order bits in the node's IP address after the subnet prefix. link-layer address A link-layer identifier for an interface, such as IEEE 802 addresses on Ethernet links. packet An IP header plus payload. Johnson and Perkins Expires 4 February 1999 [Page 6] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 3.2. Mobile IPv6 Terms home address An IP address assigned to a mobile node within its home link. home subnet prefix The IP subnet prefix corresponding to a mobile node's home address. home link The link on which a mobile node's home subnet prefix is defined. Standard IP routing mechanisms will deliver packets destined for a mobile node's home address to its home link. mobile node A node that can change its point of attachment from one link to another, while still being reachable via its home address. movement A change in a mobile node's point of attachment to the Internet such that it is no longer connected to the same link as it was previously. If a mobile node is not currently attached to its home link, the mobile node is said to be "away from home". correspondent node A peer node with which a mobile node is communicating. The correspondent node may be either mobile or stationary. foreign subnet prefix Any IP subnet prefix other than the mobile node's home subnet prefix. foreign link Any link other than the mobile node's home link. home agent A router on a mobile node's home link with which the mobile node has registered its current care-of address. While the mobile node is away from home, the home agent intercepts packets on the home link destined to the mobile node's home Johnson and Perkins Expires 4 February 1999 [Page 7] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 address, encapsulates them, and tunnels them to the mobile node's registered care-of address. care-of address An IP address associated with a mobile node while visiting a foreign link; the subnet prefix of this IP address is a foreign subnet prefix. Among the multiple care-of addresses that a mobile node may have at a time (e.g., with different subnet prefixes), the one registered with the mobile node's home agent is called its "primary" care-of address. binding The association of the home address of a mobile node with a care-of address for that mobile node, along with the remaining lifetime of that association. 3.3. Specification Language The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [3]. Johnson and Perkins Expires 4 February 1999 [Page 8] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 4. Overview of Mobile IPv6 4.1. Basic Operation A mobile node is always addressable by its home address, whether it is currently attached to its home link or is away from home. While a mobile node is at home, packets addressed to its home address are routed to it using conventional Internet routing mechanisms in the same way as if the node were never mobile. Since the subnet prefix of a mobile node's home address is the subnet prefix (or one of the subnet prefixes) on the mobile node's home link (it is the mobile node's home subnet prefix), packets addressed to it will be routed to its home link. While a mobile node is attached to some foreign link away from home, it is also addressable by one or more care-of addresses, in addition to its home address. A care-of address is an IP address associated with a mobile node while visiting a particular foreign link. The subnet prefix of a mobile node's care-of address is the subnet prefix (or one of the subnet prefixes) on the foreign link being visited by the mobile node; if the mobile node is connected to this foreign link while using that care-of address, packets addressed to this care-of address will be routed to the mobile node in its location away from home. The association between a mobile node's home address and care-of address is known as a "binding" for the mobile node. A mobile node typically acquires its care-of address through stateless [22] or stateful (e.g., DHCPv6 [2]) address autoconfiguration, according to the methods of IPv6 Neighbor Discovery [13]. Other methods of acquiring a care-of address are also possible, such as static pre-assignment by the owner or manager of a particular foreign link, but details of such other methods are beyond the scope of this document. While away from home, a mobile node registers one of its care-of addresses with a router on its home link, requesting this router to function as the "home agent" for the mobile node. This binding registration is done by the mobile node sending to the home agent a packet containing a "Binding Update" destination option; the home agent then replies to the mobile node by returning a packet containing a "Binding Acknowledgement" destination option. The care-of address in this binding registered with its home agent is known as the mobile node's "primary care-of address". The mobile node's home agent thereafter uses proxy Neighbor Discovery to intercept any IPv6 packets addressed to the mobile node's home address (or home addresses) on the home link, and tunnels each intercepted packet to the mobile node's primary care-of address. To tunnel each intercepted packet, the home agent encapsulates the Johnson and Perkins Expires 4 February 1999 [Page 9] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 packet using IPv6 encapsulation [4], with the outer IPv6 header addressed to the mobile node's primary care-of address. Section 10.15 discusses the reasons why it may be desirable for a mobile node to use more than one care-of address at the same time. However, a mobile node's primary care-of address is distinct among these in that the home agent maintains only a single care-of address registered for each mobile node, and always tunnels a mobile node's packets intercepted from its home link to this mobile node's registered primary care-of address. The home agent thus need not implement any policy to determine which of possibly many care-of addresses to which to tunnel each intercepted packet, leaving the mobile node entirely in control of this policy by which of its care-of addresses it registers with its home agent. It is possible that while a mobile node is away from home, some nodes on its home link may be reconfigured, such that the router that was operating as the mobile node's home agent is replaced by a different router serving this role. In this case, the mobile node may not know the IP address of its own home agent. Mobile IPv6 provides a mechanism, known as "dynamic home agent address discovery", that allows a mobile node to dynamically discover the IP address of a home agent on its home link with which it may register its care-of address while away from home. The mobile node sends a Binding Update to the "Home-Agents anycast address" for its own home subnet prefix and thus reaches one of the (possibly many) routers on its home link currently operating as a home agent. This home agent rejects the mobile node's Binding Update, but returns in the Binding Acknowledgement in response a list of all home agents on the home link. This list of home agents is maintained by each home agent on the home link through use of the Home Agent (H) bit in each home agent's periodic unsolicited multicast Router Advertisements. The Binding Update and Binding Acknowledgement destination options, together with a "Binding Request" destination option, are also used to allow IPv6 nodes communicating with a mobile node, to dynamically learn and cache the mobile node's binding. When sending a packet to any IPv6 destination, a node checks its cached bindings for an entry for the packet's destination address. If a cached binding for this destination address is found, the node uses an IPv6 Routing header [5] (instead of IPv6 encapsulation) to route the packet to the mobile node by way of the care-of address indicated in this binding. If, instead, the sending node has no cached binding for this destination address, the node sends the packet normally (with no Routing header), and the packet is subsequently intercepted and tunneled by the mobile node's home agent as described above. Any node communicating with a mobile node is referred to in this document as a "correspondent node" of the mobile node, and may itself be either a stationary node or a mobile node. Johnson and Perkins Expires 4 February 1999 [Page 10] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 Since a Binding Update, Binding Acknowledgement, and Binding Request are each represented in a packet as an IPv6 destination option [5], they may be included in any IPv6 packet. Any of these options can be sent in either of two ways: - A Binding Update, Binding Acknowledgement, or Binding Request can be included within any IPv6 packet carrying any payload such as TCP [20] or UDP [19]. - A Binding Update, Binding Acknowledgement, or Binding Request can be sent as a separate IPv6 packet containing no payload. In this case, the Next Header field in the last extension header in the packet is set to the value 59, to indicate "No Next Header" [5]. Mobile IPv6 also defines one additional IPv6 destination option. When a mobile node sends a packet while away from home, it will generally set the Source Address in the packet's IPv6 header to one of its current care-of addresses, and will also include a "Home Address" destination option in the packet, giving the mobile node's home address. Many routers implement security policies such as "ingress filtering" [6] that do not allow forwarding of packets that appear to have a Source Address that is not topologically correct. By using the care-of address as the IPv6 header Source Address, the packet will be able to pass normally through such routers, yet ingress filtering rules will still be able to locate the true topological source of the packet in the same way as packets from non-mobile nodes. By also including the Home Address option in each packet, the sending mobile node can communicate its home address to the correspondent node receiving this packet, allowing the use of the care-of address to be transparent above the Mobile IPv6 support level (e.g., at the transport layer). The inclusion of a Home Address option in a packet affects only the correspondent node's receipt of this single packet; no state is created or modified in the correspondent node as a result of receiving a Home Address option in a packet. 4.2. New IPv6 Destination Options As discussed in general in Section 4.1, the following four new IPv6 destination options are defined for Mobile IPv6: Binding Update A Binding Update option is used by a mobile node to notify a correspondent node or the mobile node's home agent of its current binding. The Binding Update sent to the mobile node's home agent to register its primary care-of address is marked as a "home registration". Any packet that includes a Binding Update option MUST also include either an AH [8] or Johnson and Perkins Expires 4 February 1999 [Page 11] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 ESP [9] header providing sender authentication, data integrity protection, and replay protection. The Binding Update option is described in detail in Section 5.1. Binding Acknowledgement A Binding Acknowledgement option is used to acknowledge receipt of a Binding Update, if an acknowledgement was requested in the Binding Update. Any packet that includes a Binding Acknowledgement option MUST also include either an AH [8] or ESP [9] header providing sender authentication, data integrity protection, and replay protection. The Binding Acknowledgement option is described in detail in Section 5.2. Binding Request A Binding Request option is used to request a mobile node to send to the requesting node a Binding Update containing the mobile node's current binding. This option is typically used by a correspondent node to refresh a cached binding for a mobile node, when the cached binding is in active use but the binding's lifetime is close to expiration. No authentication is required for the Binding Request option. The Binding Request option is described in detail in Section 5.3. Home Address A Home Address option is used in a packet sent by a mobile node to inform the recipient of that packet of the mobile node's home address. For packets sent by a mobile node while away from home, the mobile node generally uses one of its care-of addresses as the Source Address in the packet's IPv6 header. By including a Home Address option in the packet, the correspondent node receiving the packet is able to substitute the mobile node's home address for this care-of address when processing the packet, thus making the use of the care-of address transparent to the correspondent node. If the IP header of a packet carrying a Home Address option is covered by authentication, then the Home Address option MUST also be covered by this authentication, but no other authentication is required for the Home Address option. The Home Address option is described in detail in Section 5.4. Extensions to the format of these options MAY be included after the fixed portion of the option data specified in this document. The presence of such extensions will be indicated by the Option Length field within the option. When the Option Length is greater than the length required for the option specified here, the remaining octets are interpreted as extensions. Currently, no extensions have been defined. Johnson and Perkins Expires 4 February 1999 [Page 12] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 4.3. Conceptual Data Structures This document describes the Mobile IPv6 protocol in terms of the following three conceptual data structures: Binding Cache A cache, maintained by each IPv6 node, of bindings for other nodes. The Binding Cache MAY be implemented in any manner consistent with the external behavior described in this document, for example by being combined with the node's Destination Cache as maintained by Neighbor Discovery [13]. When sending a packet, the Binding Cache is searched before the Neighbor Discovery conceptual Destination Cache [13] (i.e., any Binding Cache entry for this destination SHOULD take precedence over any Destination Cache entry for the same destination). Each Binding Cache entry conceptually contains the following fields: - The home address of the mobile node for which this is the Binding Cache entry. This field is used as the key for searching the Binding Cache for the destination address of a packet being sent. If the destination address of the packet matches the home address in the Binding Cache entry, this entry SHOULD be used in routing that packet. - The care-of address for the mobile node indicated by the home address field in this Binding Cache entry. If the destination address of a packet being routed by a node matches the home address in this entry, the packet SHOULD be routed to this care-of address, as described in Section 8.9, for packets originated by this node, or in Section 9.6, if this node is the mobile node's home agent and the packet was intercepted by it on the home link. - A lifetime value, indicating the remaining lifetime for this Binding Cache entry. The lifetime value is initialized from the Lifetime field in the Binding Update that created or last modified this Binding Cache entry. Once the lifetime on this entry expires, the entry MUST be deleted from the Binding Cache. - A flag indicating whether or not this Binding Cache entry is a "home registration" entry. - The value of the Prefix Length field received in the Binding Update that created or last modified this Binding Cache entry. This field is only valid if the "home registration" flag is set on this Binding Cache entry. Johnson and Perkins Expires 4 February 1999 [Page 13] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 - The maximum value of the Sequence Number field received in previous Binding Updates for this mobile node home address. The Sequence Number field is 16 bits long, and all comparisons between Sequence Number values MUST be performed modulo 2**16. - Recent usage information for this Binding Cache entry, as needed to implement the cache replacement policy in use in the Binding Cache and to assist in determining whether a Binding Request should be sent when the lifetime on this entry nears expiration. - The time at which a Binding Request was last sent for this entry, as needed to implement the rate limiting restriction for sending Binding Requests. An entry in a node's Binding Cache for which the node is serving as a home agent is marked as a "home registration" entry and SHOULD NOT be deleted by the home agent until the expiration of its binding lifetime. Other Binding Cache entries MAY be replaced at any time by any reasonable local cache replacement policy but SHOULD NOT be unnecessarily deleted. Any node's Binding Cache may contain at most one entry for each mobile node home address. The contents of a node's Binding Cache MUST NOT be changed in response to a Home Address option in a received packet. Binding Update List A list, maintained by each mobile node, recording information for each Binding Update sent by this mobile node, for which the Lifetime sent in that Binding Update has not yet expired. The Binding Update List includes all bindings sent by the mobile node: those to correspondent nodes, to the mobile node's home agent, and to a previous default router of the mobile node. The Binding Update List MAY be implemented in any manner consistent with the external behavior described in this document. Each Binding Update List entry conceptually contains the following fields: - The IP address of the node to which a Binding Update was sent. This node might still have a Binding Cache entry created or updated from this Binding Update, if the Binding Update was successfully received by that node (e.g., not lost by the network) and if that node has not deleted the entry before its expiration (e.g., to reclaim space in its Binding Cache for other entries). - The home address for which that Binding Update was sent. This will be one of the mobile node's home addresses for Johnson and Perkins Expires 4 February 1999 [Page 14] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 most Binding Updates (Sections 10.5 and 10.7), but will be the mobile node's previous care-of address for Binding Updates sent to the mobile node's previous default router (Section 10.8). - The care-of address sent in that Binding Update. This value is necessary for the mobile node to determine if it has sent a Binding Update giving its new care-of address to this destination after changing its care-of address. - The remaining lifetime of that binding. This lifetime is initialized from the Lifetime value sent in the Binding Update and is decremented until it reaches zero, at which time this entry MUST be deleted from the Binding Update List. - The maximum value of the Sequence Number field sent in previous Binding Updates to this destination. The Sequence Number field is 16 bits long, and all comparisons between Sequence Number values MUST be performed modulo 2**16. - The time at which a Binding Update was last sent to this destination, as needed to implement the rate limiting restriction for sending Binding Updates. - The state of any retransmissions needed for this Binding Update, if the Acknowledge (A) bit was set in this Binding Update. This state includes the time remaining until the next retransmission attempt for the Binding Update, and the current state of the exponential back-off mechanism for retransmissions. - A flag that, when set, indicates that future Binding Updates should not be sent to this destination. The mobile node sets this flag in the Binding Update List entry when it receives an ICMP Parameter Problem, Code 2, error message in response to a Binding Update sent to that destination, as described in Section 10.13. Home Agents List A list, maintained by each home agent, recording information about each other home agent on a link on which this node is serving as a home agent; each home agent maintains a separate Home Agents List for each such link on which it is serving. This list is used in the dynamic home agent address discovery mechanism. The information for the list is learned through receipt of the periodic unsolicited multicast Router Advertisements from each other home agent on the link, in which the Home Agent (H) bit is set, in a manner similar to the Johnson and Perkins Expires 4 February 1999 [Page 15] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 Default Router List conceptual data structure maintained by each host for Neighbor Discovery [13]. The Home Agents List MAY be implemented in any manner consistent with the external behavior described in this document. Each Home Agents List entry conceptually contains the following fields: - The IP address of another router on the home link that this node currently believes is operating as a home agent for this link. A new entry is created or an existing entry is updated in the Home Agents List in response to receipt of a valid Router Advertisement in which the Home Agent (H) bit is set. - The remaining lifetime of this Home Agents List entry. If a Home Agent Information Option is present in a Router Advertisement received from a home agent, the lifetime of the Home Agents List entry representing this home agent is initialized from the Home Agent Lifetime field in the option; otherwise, the lifetime is initialized from the Router Lifetime field in the received Router Advertisement. The Home Agents List entry lifetime is decremented until it reaches zero, at which time this entry MUST be deleted from the Home Agents List. - The preference for this home agent, for use in ordering the Home Agents List returned in a Binding Acknowledgement; higher values indicate a more preferable home agent. The preference value is taken from the Home Agent Preference field (a signed, twos-complement integer) in the received Router Advertisement, if the Router Advertisement contains a Home Agent Information Option, and is otherwise set to the default value of 0. 4.4. Binding Management When a mobile node configures a new care-of address and decides to use this new address as its primary care-of address, the mobile node registers this new binding with its home agent by sending the home agent a Binding Update. The mobile node indicates that an acknowledgement is needed for this Binding Update and continues to periodically retransmit it until acknowledged. The home agent acknowledges the Binding Update by returning a Binding Acknowledgement to the mobile node. When a mobile node receives a packet tunneled to it from its home agent, the mobile node assumes that the original sending correspondent node has no Binding Cache entry for the mobile node, since the correspondent node would otherwise have sent the packet directly to the mobile node using a Routing header. The mobile node Johnson and Perkins Expires 4 February 1999 [Page 16] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 thus returns a Binding Update to the correspondent node, allowing it to cache the mobile node's binding for routing future packets to it. Although the mobile node may request an acknowledgement for this Binding Update, it need not, since subsequent packets from the correspondent node will continue to be intercepted and tunneled by the mobile node's home agent, effectively causing any needed Binding Update retransmission. A correspondent node with a Binding Cache entry for a mobile node may refresh this binding, for example if the binding's lifetime is near expiration, by sending a Binding Request to the mobile node. Normally, a correspondent node will only refresh a Binding Cache entry in this way if it is actively communicating with the mobile node and has indications, such as an open TCP connection to the mobile node, that it will continue this communication in the future. When a mobile node receives a Binding Request, it replies by returning a Binding Update to the node sending the Binding Request. A mobile node may use more than one care-of address at the same time, although only one care-of address may be registered for it at its home agent as its primary care-of address. The mobile node's home agent will tunnel all intercepted packets for the mobile node to its (single) registered primary care-of address, but the mobile node will accept packets that it receives at any of its current care-of addresses. Use of more than one care-of address by a mobile node may be useful, for example, to improve smooth handoff when the mobile node moves from one wireless link to another. If each of these wireless links is connected to the Internet through a separate base station, such that the wireless transmission range from the two base stations overlap, the mobile node may be able to remain connected to both links while in the area of overlap. In this case, the mobile node could acquire a new care-of address on the new link before moving out of transmission range and disconnecting from the old link. The mobile node may thus still accept packets at its old care-of address while it works to update its home agent and correspondent nodes, notifying them of its new care-of address on the new link. Since correspondent nodes cache bindings, it is expected that correspondent nodes usually will route packets directly to the mobile node's care-of address, so that the home agent is rarely involved with packet transmission to the mobile node. This is essential for scalability and reliability, and for minimizing overall network load. By caching the care-of address of a mobile node, optimal routing of packets can be achieved from the correspondent node to the mobile node. Routing packets directly to the mobile node's care-of address also eliminates congestion at the mobile node's home agent and home link. In addition, the impact of any possible failure of the home agent, the home link, or intervening networks leading to or from the Johnson and Perkins Expires 4 February 1999 [Page 17] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 home link is reduced, since these nodes and links are not involved in the delivery of most packets to the mobile node. Johnson and Perkins Expires 4 February 1999 [Page 18] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 5. New IPv6 Destination Options 5.1. Binding Update Option Format The Binding Update destination option is used by a mobile node to notify other nodes of a new care-of address for itself. As a destination option, it MAY be included in any existing packet being sent to this same destination or MAY be sent in a packet by itself; a packet containing a Binding Update is sent in the same way as any packet sent by a mobile node (Section 10.1). The Binding Update option is encoded in type-length-value (TLV) format as follows: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Type | Option Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |A|H|C| Reserved| Prefix Length | Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + Care-of Address + | (only present if C bit set) | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Option Type 195 ??? Option Length 8-bit unsigned integer. Length of the option, in octets, excluding the Option Type and Option Length fields. For the current definition of the Binding Update option, the minimum value for this field is 8; the length is 24 if the Care-of Address Present (C) bit is set. Acknowledge (A) The Acknowledge (A) bit is set by the sending mobile node to request a Binding Acknowledgement (Section 5.2) be returned upon receipt of the Binding Update. Johnson and Perkins Expires 4 February 1999 [Page 19] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 Home Registration (H) The Home Registration (H) bit is set by the sending mobile node to request the receiving node to act as this node's home agent. The destination of the packet carrying this option MUST be that of a router sharing the same subnet prefix as the home address of the mobile node in the binding (given by the Home Address field in the Home Address option in the packet). Care-of Address Present (C) The Care-of Address Present (C) bit indicates the presence of the Care-of Address field in the Binding Update. The care-of address for this binding is either the address in the Care-of Address field in the Binding Update, if this bit is set, or the Source Address in the packet's IPv6 header, if this bit is not set. Reserved Sent as 0; ignored on reception. Prefix Length The Prefix Length field is valid only for a "home registration" Binding Update. This field MUST be zero if the Home Registration (H) bit is not set in the Binding Update. The Prefix Length field is set by the sending mobile node to the (nonzero) length of its subnet prefix in its home address (given in the Home Address option in the packet) to request its home agent to use the interface identifier in the mobile node's home address (the remaining low-order bits after the indicated subnet prefix) to form all other home addresses for the mobile node on the home link. The home agent becomes the home agent not only for the individual home address given in this binding, but also for all other home addresses for this mobile node formed from this interface identifier. That is, for each on-link prefix on the home link, the home agent uses the interface identifier to form other valid addresses for the mobile node on the home link, and acts as a home agent also for those addresses. In addition, the home agent forms the link-local address and site-local address corresponding to this interface identifier, and defends each for purposes of Duplicate Address Detection. Details of this operation are described in Section 9.3. Sequence Number Used by the receiving node to sequence Binding Updates and by the sending node to match a returned Binding Acknowledgement Johnson and Perkins Expires 4 February 1999 [Page 20] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 with this Binding Update. Each Binding Update sent by a mobile node MUST use a Sequence Number greater than the Sequence Number value sent in the previous Binding Update (if any) to the same destination address (modulo 2**16). Lifetime 32-bit unsigned integer. The number of seconds remaining before the binding must be considered expired. A value of all one bits (0xffffffff) indicates infinity. A value of zero indicates that the Binding Cache entry for the mobile node should be deleted. Care-of Address This field in the Binding Update is optional and is only present when the Care-of Address Present (C) bit is set. If present, it gives the care-of address of the mobile node for this binding. For most Binding Updates sent, it is expected that this field will not be present, and instead that the care-of address for the binding will be given by the Source Address field in the packet's IPv6 header. Any packet that includes a Binding Update option MUST also include a Home Address option. The home address of the mobile node in the binding given in the Binding Update option is indicated by the Home Address field in the Home Address option in the packet. Any packet that includes a Binding Update option MUST also include either an AH [8] or ESP [9] header providing sender authentication, data integrity protection, and replay protection. If the care-of address in the binding (either the Care-of Address field in the Binding Update option or the Source Address field in the packet's IPv6 header) is equal to the home address of the mobile node, the Binding Update option indicates that any existing binding for the mobile node MUST be deleted. Likewise, if the Lifetime field in the Binding Update option is equal to 0, the Binding Update option indicates that any existing binding for the mobile node MUST be deleted. In each of these cases, a Binding Cache entry for the mobile node MUST NOT be created in response to receiving the Binding Update. The last Sequence Number value sent to a destination in a Binding Update is stored by the mobile node in its Binding Update List entry for that destination; the last Sequence Number value received from a mobile node in a Binding Update is stored by a correspondent node in its Binding Cache entry for that mobile node. Thus, the mobile node's and the correspondent node's knowledge of the last sequence number expire at the same time. If the sending mobile node has no Johnson and Perkins Expires 4 February 1999 [Page 21] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 Binding Update List entry, the Sequence Number may start at any value; if the receiving correspondent node has no Binding Cache entry for the sending mobile node, it MUST accept any Sequence Number value in a received Binding Update from this mobile node. The three highest-order bits of the Option Type are encoded to indicate specific processing of the option [5]. For the Binding Update option, these three bits are set to 110, indicating that any IPv6 node processing this option that does not recognize the Option Type must discard the packet and, only if the packet's Destination Address was not a multicast address, return an ICMP Parameter Problem, Code 2, message to the packet's Source Address; and that the data within the option cannot change en-route to the packet's final destination. Extensions to the Binding Update option format may be included after the fixed portion of the Binding Update option specified above. The presence of such extensions will be indicated by the Option Length field. When the Option Length is greater than the length defined above, the remaining octets are interpreted as extensions. Currently, no extensions have been defined. Johnson and Perkins Expires 4 February 1999 [Page 22] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 5.2. Binding Acknowledgement Option Format The Binding Acknowledgement destination option is used to acknowledge receipt of a Binding Update option (Section 5.1). When a node receives a packet containing a Binding Update option, with this node being the destination of the packet (only the destination node processes the option since it is a destination option), this node MUST return a Binding Acknowledgement to the source of the packet, if the Acknowledge (A) bit is set in the Binding Update. As a destination option, this node MAY included it in any existing packet being sent to the mobile node or MAY send it in a packet by itself; a packet containing a Binding Acknowledgement is sent in the same way as any packet to a mobile node (Section 8.9). The Binding Acknowledgement option is encoded in type-length-value (TLV) format as follows: 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 +-+-+-+-+-+-+-+-+ | Option Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Length | Status | Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Refresh | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + . . . Home Agents List . . . + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Option Type 2 ??? Option Length 8-bit unsigned integer. Length of the option, in octets, excluding the Option Type and Option Length fields. This field MUST be set to 11, except when the Status field is equal to 135 (dynamic home agent address discovery response), in which case this field MUST be set to 11 + 16 * N, where N is the number of IP addresses included in the Home Agents List field; the Home Johnson and Perkins Expires 4 February 1999 [Page 23] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 Agents List field MUST NOT be included in the option if the Status field is not set to 135. Status 8-bit unsigned integer indicating the disposition of the 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 Binding Update accepted 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 Poorly formed Binding Update 130 Administratively prohibited 131 Insufficient resources 132 Home registration not supported 133 Not home subnet 134 Sequence Number field value too small 135 Dynamic home agent address discovery response 136 Incorrect interface identifier length 137 Not home agent for this mobile node Up-to-date values of the Status field are to be specified in the most recent "Assigned Numbers" [21]. Sequence Number The Sequence Number in the Binding Acknowledgement is copied from the Sequence Number field in the Binding Update being acknowledged, for use by the mobile node in matching this Acknowledgement with an outstanding Binding Update. Lifetime The granted lifetime for which this node will attempt to retain the entry for this mobile node in its Binding Cache. If the node sending the Binding Acknowledgement is serving as the mobile node's home agent, the Lifetime period also indicates the period for which this node will continue this service; if the mobile node requires home agent service from this node beyond this period, the mobile node MUST send a new Binding Update to it before the expiration of this period (even if it is not changing its primary care-of address), in order to extend the lifetime. The value of this field is undefined Johnson and Perkins Expires 4 February 1999 [Page 24] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 if the Status field indicates that the Binding Update was rejected. Refresh The recommended interval at which the mobile node SHOULD send a new Binding Update to this node in order to "refresh" the mobile node's binding in this node's Binding Cache. This refreshing of the binding is useful in case the node fails and loses its cache state. The Refresh period is determined by the node sending the Binding Acknowledgement (the node caching the binding). If this node is serving as the mobile node's home agent, the Refresh value may be set, for example, based on whether the node stores its Binding Cache in volatile storage or in nonvolatile storage. If the node sending the Binding Acknowledgement is not serving as the mobile node's home agent, the Refresh period SHOULD be set equal to the Lifetime period in the Binding Acknowledgement; even if this node loses this cache entry due to a failure of the node, packets from it can still reach the mobile node through the mobile node's home agent, causing a new Binding Update to this node to allow it to recreate this cache entry. The value of this field is undefined if the Status field indicates that the Binding Update was rejected. Home Agents List A list of home agents on the home link for the mobile node to which this Binding Acknowledgement is sent. This field MUST NOT be present (zero addresses listed) unless the Binding Acknowledgement is sent in response to an anycast Binding Update sent by this mobile node attempting dynamic home agent address discovery. In this case, the Status field MUST be set to 135 (dynamic home agent address discovery response). The construction of the Home Agents List field in a Binding Acknowledgement is defined in Section 9.2. Any packet that includes a Binding Acknowledgement option MUST also include either an AH [8] or ESP [9] header providing sender authentication, data integrity protection, and replay protection. If the node returning the Binding Acknowledgement accepted the Binding Update for which the Acknowledgement is being returned (the value of the Status field in the Acknowledgement is less than 128), this node will have an entry for the mobile node in its Binding Cache and MUST use this entry (which includes the care-of address received in the Binding Update) in sending the packet containing the Binding Acknowledgement to the mobile node. The details of sending this packet to the mobile node are the same as for sending any packet to a mobile node using a binding, and are described in Section 8.9. The Johnson and Perkins Expires 4 February 1999 [Page 25] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 packet is sent using a Routing header, routing the packet to the mobile node by way of its care-of address recorded in the Binding Cache entry. If the node returning the Binding Acknowledgement instead rejected the Binding Update (the value of the Status field in the Acknowledgement is greater than or equal to 128), this node MUST similarly use a Routing header in sending the packet containing the Binding Acknowledgement, as described in Section 8.9, but MUST NOT use its Binding Cache in forming the IP header or Routing header in this packet. Rather, the care-of address used by this node in sending the packet containing the Binding Acknowledgement MUST be copied from the care-of address received in the rejected Binding Update; this node MUST NOT modify its Binding Cache in response to receiving this rejected Binding Update and MUST ignore its Binding Cache in sending the packet in which it returns this Binding Acknowledgement. The packet is sent using a Routing header, routing the packet to the home address of the rejected Binding Update by way of the care-of address indicated in the packet containing the Binding Update. When sending a Binding Acknowledgement to reject a Binding Update, the Binding Acknowledgement MUST be sent in an IPv6 packet containing no payload (with the Next Header field in the last extension header in the packet set to indicate "No Next Header" [5]). The three highest-order bits of the Option Type are encoded to indicate specific processing of the option [5]. For the Binding Acknowledgement option, these three bits are set to 000, indicating that any IPv6 node processing this option that does not recognize the Option Type must skip over this option and continue processing the header, and that the data within the option cannot change en-route to the packet's final destination. Johnson and Perkins Expires 4 February 1999 [Page 26] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 5.3. Binding Request Option Format The Binding Request destination option is used to request a mobile node's binding from the mobile node. As a destination option, it MAY be included in any existing packet being sent to the mobile node or MAY be sent in a packet by itself; a packet containing a Binding Request option is sent in the same way as any packet to a mobile node (Section 8.9). When a mobile node receives a packet containing a Binding Request option, it SHOULD return a Binding Update (Section 5.1) to the source of the Binding Request. The Binding Request option is encoded in type-length-value (TLV) format as follows: 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Type | Option Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Option Type 3 ??? Option Length 8-bit unsigned integer. Length of the option, in octets, excluding the Option Type and Option Length fields. For the current definition of the Binding Request option, this field MUST be set to 0. The three highest-order bits of the Option Type are encoded to indicate specific processing of the option [5]. For the Binding Request option, these three bits are set to 000, indicating that any IPv6 node processing this option that does not recognize the Option Type must skip over this option and continue processing the header, and that the data within the option cannot change en-route to the packet's final destination. Extensions to the Binding Request option format may be included after the fixed portion of the Binding Request option specified above. The presence of such extensions will be indicated by the Option Length field. When the Option Length is greater than 0 octets, the remaining octets are interpreted as extensions. Currently, no extensions have been defined. Johnson and Perkins Expires 4 February 1999 [Page 27] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 5.4. Home Address Option Format The Home Address destination option is used in a packet sent by a mobile node while away from home, to inform the recipient of that packet of the mobile node's home address. For packets sent by a mobile node while away from home, the mobile node generally uses one of its care-of addresses as the Source Address in the packet's IPv6 header. By including a Home Address option in the packet, the correspondent node receiving the packet is able to substitute the mobile node's home address for this care-of address when processing the packet, thus making the use of the care-of address transparent to the correspondent node. The Home Address option is encoded in type-length-value (TLV) format as follows: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Type | Option Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + Home Address + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Option Type 196 ??? Option Length 8-bit unsigned integer. Length of the option, in octets, excluding the Option Type and Option Length fields. For the current definition of the Home Address option, this field MUST be set to 16. Home Address The home address of the mobile node sending the packet. The inclusion of a Home Address option in a packet affects the receiving node's processing of only this single packet; no state is created or modified in the receiving node as a result of receiving a Home Address option in a packet. In particular, the presence of a Home Address option in a received packet MUST NOT alter the contents Johnson and Perkins Expires 4 February 1999 [Page 28] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 of the receiver's Binding Cache and MUST NOT cause any changes in the routing of subsequent packets sent by this receiving node. No authentication of the Home Address option is required, except that if the IPv6 header of a packet is covered by authentication, then that authentication MUST also cover the Home Address option; this coverage is achieved automatically by the definition of the Option Type code for the Home Address option, since it indicates that the data within the option cannot change en-route to the packet's final destination, and thus the option is included in the authentication computation. If the packet carries no IP authentication, then the contents of the Home Address option, as well as the Source Address field or any other field in the IPv6 header, may have been forged or altered during transit. Upon receipt of a packet containing a Home Address option, the receiving node replaces the Source Address in the IPv6 header with the Home Address in the Home Address option. By requiring that any authentication of the IPv6 header also cover the Home Address option, the security of the Source Address field in the IPv6 header is not compromised by the presence of a Home Address option. Security issues related to the Home Address option are discussed further in Section 13. The three highest-order bits of the Option Type are encoded to indicate specific processing of the option [5]. For the Home Address option, these three bits are set to 110, indicating that any IPv6 node processing this option that does not recognize the Option Type must discard the packet and, only if the packet's Destination Address was not a multicast address, return an ICMP Parameter Problem, Code 2, message to the packet's Source Address; and that the data within the option cannot change en-route to the packet's final destination. Extensions to the Home Address option format may be included after the fixed portion of the Home Address option specified above. The presence of such extensions will be indicated by the Option Length field. When the Option Length is greater than 8 octets, the remaining octets are interpreted as extensions. Currently, no extensions have been defined. Johnson and Perkins Expires 4 February 1999 [Page 29] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 6. Modifications to IPv6 Neighbor Discovery 6.1. Modified Router Advertisement Message Format Mobile IPv6 modifies the format of the Router Advertisement message [13] by the addition of a single flag bit for use in the dynamic home agent address discovery mechanism (Sections 9.2 and 10.6). The format of the Router Advertisement message is as follows: 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cur Hop Limit |M|O|H| Reserved| Router Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reachable Time | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Retrans Timer | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options ... +-+-+-+-+-+-+-+-+-+-+-+- This format represents the following changes over that originally specified for Neighbor Discovery [13]: Home Agent (H) The Home Agent (H) bit is set in a Router Advertisement to indicate that the router sending this Router Advertisement is also functioning as a Mobile IP home agent. Reserved Reduced from a 6-bit field to a 5-bit field to account for the addition of the Home Agent (H) bit. Johnson and Perkins Expires 4 February 1999 [Page 30] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 6.2. Modified Prefix Information Option Format Mobile IPv6 requires knowledge of a router's global address for two reasons: - To allow a home agent (a router) to learn the address of all other home agents on the link for which it is proving home agent service, for use in building its Home Agents List as part of the dynamic home agent address discovery mechanism (Sections 9.2 and 10.6). - To allow a mobile node to send a Binding Update to its previous default router, after moving to a new subnet and acquiring a new care-of address (Section 10.8). However, Neighbor Discovery [13] only advertises a router's link-local address, by requiring this address to be used as the IP Source Address of each Router Advertisement. Mobile IPv6 extends Neighbor Discovery to allow a router to easily and efficiently advertise its global address, by the addition of a single flag bit in the format of a Prefix Information option for use in Router Advertisement messages. The format of the Prefix Information option is as follows: 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 | Prefix Length |L|A|R|Reserved1| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Valid Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Preferred Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + Prefix + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ This format represents the following changes over that originally specified for Neighbor Discovery [13]: Johnson and Perkins Expires 4 February 1999 [Page 31] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 Router Address (R) 1-bit router address flag. When set, indicates that the Prefix field, in addition to advertising the indicated prefix, contains a complete IP address assigned to the sending router. This router IP address has the same scope and conforms to the same lifetime values as the advertised prefix. This use of the Prefix field is compatible with its use in advertising the prefix itself, since prefix advertisement uses only the leading number Prefix bits specified by the Prefix Length field. Interpretation of this flag bit is thus independent of the processing required for the On-Link (L) and Autonomous Address-Configuration (A) flag bits. Reserved1 Reduced from a 6-bit field to a 5-bit field to account for the addition of the Router Address (R) bit. In a solicited Router Advertisement, a router MUST include at least one Prefix Information option with the Router Address (R) bit set. Neighbor Discovery specifies that, if including all options in a Router Advertisement causes the size of the Advertisement to exceed the link MTU, multiple Advertisements can be sent, each containing a subset of the options [13]. In this case, at least one of these multiple Advertisements begin sent instead of a single larger solicited Advertisement, MUST include a Prefix Information option with the Router Address (R) bit set. All routers SHOULD include at least one Prefix Information option with the Router Address (R) bit set, in each unsolicited multicast Router Advertisement that they send. If multiple Advertisements are being sent instead of a single larger unsolicited multicast Advertisement, at least one of these multiple Advertisements SHOULD include a Prefix Information option with the Router Address (R) bit set. Johnson and Perkins Expires 4 February 1999 [Page 32] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 6.3. New Advertisement Interval Option Format Mobile IPv6 defines a new Advertisement Interval option, used in Router Advertisement messages to advertise the interval at which the sending router sends unsolicited multicast Router Advertisements. The format of the Advertisement Interval option is as follows: 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 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Advertisement Interval | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 6 ??? Length 8-bit unsigned integer. The length of the option (including the type and length fields) in units of 8 octets. The value of this field MUST be 1. Reserved This field is unused. It MUST be initialized to zero by the sender and MUST be ignored by the receiver. Advertisement Interval 32-bit unsigned integer. The maximum time, in milliseconds, between successive unsolicited router Router Advertisement messages sent by this router on this network interface. Using the conceptual router configuration variables defined by Neighbor Discovery [13], this field MUST be equal to the value MaxRtrAdvInterval, expressed in milliseconds. Routers MAY include this option in their Router Advertisements. A mobile node receiving a Router Advertisement containing this option SHOULD utilize the specified Advertisement Interval for that router in its movement detection algorithm, as described in Section 10.3. This option MUST be silently ignored for other Neighbor Discovery messages. Johnson and Perkins Expires 4 February 1999 [Page 33] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 6.4. New Home Agent Information Option Format Mobile IPv6 defines a new Home Agent Information option, used in Router Advertisement messages sent by a home agent to advertise information specific to this router's functionality as a home agent. The format of the Home Agent Information option is as follows: 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 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Home Agent Preference | Home Agent Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 7 ??? Length 8-bit unsigned integer. The length of the option (including the type and length fields) in units of 8 octets. The value of this field MUST be 1. Reserved This field is unused. It MUST be initialized to zero by the sender and MUST be ignored by the receiver. Home Agent Preference 16-bit signed, twos-complement integer. The preference for the home agent sending this Router Advertisement, for use in ordering the Home Agents List returned in a Binding Acknowledgement; higher values mean more preferable. If this option is not included in a Router Advertisement in which the Home Agent (H) bit is set, the preference value for this home agent SHOULD be considered to be 0. Values greater than 0 indicate a home agent more preferable than this default value, and values less than 0 indicate a less preferable home agent. Home Agent Lifetime 16-bit unsigned integer. The lifetime associated with the home agent in units of seconds. The maximum value corresponds to 18.2 hours. A value of 0 MUST NOT be used. The Home Agent Lifetime applies only to this router's usefulness as a home agent; it does not apply to information contained in other message fields or options. If this option is not included in Johnson and Perkins Expires 4 February 1999 [Page 34] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 a Router Advertisement in which the Home Agent (H) bit is set, the lifetime for this home agent SHOULD be considered to be the same as the Router Lifetime specified in the main body of the Router Advertisement message. Home agents MAY include this option in their Router Advertisements. This option MUST NOT be included in a Router Advertisement in which the Home Agent (H) bit (Section 6.1) is not set. This option MUST be silently ignored for other Neighbor Discovery messages. If both the Home Agent Preference and Home Agent Lifetime are set to their default values specified above, this option SHOULD NOT be included in the Router Advertisement messages sent by this home agent. Johnson and Perkins Expires 4 February 1999 [Page 35] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 6.5. Changes to Sending Router Advertisements The Neighbor Discovery protocol specification [13] limits routers to a minimum interval of 3 seconds between sending unsolicited multicast Router Advertisement messages from any given network interface (limited by MinRtrAdvInterval and MaxRtrAdvInterval), stating that: "Routers generate Router Advertisements frequently enough that hosts will learn of their presence within a few minutes, but not frequently enough to rely on an absence of advertisements to detect router failure; a separate Neighbor Unreachability Detection algorithm provides failure detection." This limitation, however, is not suitable to providing timely movement detection for mobile nodes. Mobile nodes detect their own movement by learning the presence of new routers as the mobile node moves into wireless transmission range of them (or physically connects to a new wired network), and by learning that previous routers are no longer reachable. Mobile nodes MUST be able to quickly detect when they move to a link served by a new router, so that they can acquire a new care-of address and send Binding Updates to register this care-of address with their home agent and to notify correspondent nodes as needed. Thus, to provide good support for mobile nodes, Mobile IPv6 relaxes this limit such that routers MAY send unsolicited multicast Router Advertisements more frequently. In particular, on network interfaces where the router is expecting to provide service to visiting mobile nodes (e.g., wireless network interfaces), or on which it is serving as a home agent to one or more mobile nodes (who may return home and need to hear its Advertisements), the home agent SHOULD be configured with a smaller MinRtrAdvInterval value and MaxRtrAdvInterval value, to allow sending of unsolicited multicast Router Advertisements more often. Recommended values for these limits are: - MinRtrAdvInterval 0.5 seconds - MaxRtrAdvInterval 1.5 seconds Use of these modified limits MUST be configurable, and specific knowledge of the type of network interface in use SHOULD be taken into account in configuring these limits for each network interface. When sending unsolicited multicast Router Advertisements more frequently than the standard limit on unsolicited multicast Advertisement frequency, the sending router need not include all options in each of these Advertisements, but it SHOULD include at least one Prefix Information option with the Router Address (R) bit set (Section 6.2) in each. Johnson and Perkins Expires 4 February 1999 [Page 36] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 6.6. Changes to Sending Router Solicitations In addition to the limit on routers sending unsolicited multicast Router Advertisement messages (Section 6.5), Neighbor Discovery defines limits on nodes sending Router Solicitation messages, such that a node SHOULD send no more than 3 Router Solicitations, and that these 3 transmissions SHOULD be spaced at least 4 seconds apart. However, these limits prevent a mobile node from finding a new default router (and thus a new care-of address) quickly as it moves about. Mobile IPv6 relaxes this limit such that, while a mobile node is away from home, it MAY send Router Solicitations more frequently. The following limits for sending Router Solicitations are recommended for mobile nodes while away from home: - A mobile node that is not configured with any current care-of address (e.g., the mobile node has moved since its previous care-of address was configured), MAY send more than the defined Neighbor Discovery limit of MAX_RTR_SOLICITATIONS Router Solicitations. - The rate at which a mobile node sends Router Solicitations MUST be limited, although a mobile node MAY send Router Solicitations more frequently than the defined Neighbor Discovery limit of RTR_SOLICITATION_INTERVAL seconds. The minimum interval MUST be configurable, and specific knowledge of the type of network interface in use SHOULD be taken into account in configuring this limit for each network interface. A recommended minimum interval is 1 second. - After sending at most MAX_RTR_SOLICITATIONS Router Solicitations, a mobile node MUST reduce the rate at which it sends subsequent Router Solicitations. Subsequent Router Solicitations SHOULD be sent using a binary exponential backoff mechanism, doubling the interval between consecutive Router Solicitations, up to a maximum interval. The maximum interval MUST be configurable and SHOULD be chosen appropriately based on the characteristics of the type of network interface in use. - While still searching for a new default router and care-of address, a mobile node MUST NOT increase the rate at which it sends Router Solicitations unless it has received a positive indication (such as from lower network layers) that it has moved to a new link. After successfully acquiring a new care-of address, the mobile node SHOULD also increase the rate at which it will send Router Solicitations when it next begins searching for a new default router and care-of address. Johnson and Perkins Expires 4 February 1999 [Page 37] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 - A mobile node that is currently configured with a care-of address SHOULD NOT send Router Solicitations, until its movement detection algorithm (Section 10.3) determines that it has moved and that its current care-of address might no longer be valid. Johnson and Perkins Expires 4 February 1999 [Page 38] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 7. Requirements for IPv6 Nodes Mobile IPv6 places some special requirements on the functions provided by different types of IPv6 nodes. This section summarizes those requirements, identifying the functionality each requirement is intended to support. Further details on this functionality is provided in the following sections. 7.1. Requirements for All IPv6 Hosts and Routers Since any IPv6 node may at any time be a correspondent node of a mobile node, either sending a packet to a mobile node or receiving a packet from a mobile node, the following requirements apply to ALL IPv6 nodes (whether host or router, whether mobile or stationary): - Every IPv6 node MUST be able to process a Home Address option received in any IPv6 packet. - Every IPv6 node SHOULD be able to process a Binding Update option received in a packet, and to return a Binding Acknowledgement option if the Acknowledge (A) bit is set in the received Binding Update. - Every IPv6 node SHOULD be able to maintain a Binding Cache of the bindings received in accepted Binding Updates. 7.2. Requirements for All IPv6 Routers The following requirements apply to all IPv6 routers, even those not serving as a home agent for Mobile IPv6: - Every IPv6 router SHOULD be able to send an Advertisement Interval option in its Router Advertisements, to aid movement detection by mobile nodes. The use of this option in Router Advertisements MUST be configurable. - Every IPv6 router SHOULD be able to support sending unsolicited multicast Router Advertisements at the faster rate described in Section 6.5. The use of this faster rate MUST be configurable. 7.3. Requirements for IPv6 Home Agents In order for a mobile node to operate correctly while away from home, at least one IPv6 router on the mobile node's home link must function as a home agent for the mobile node. The following additional requirements apply to all IPv6 routers capable of serving as a home agent: Johnson and Perkins Expires 4 February 1999 [Page 39] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 - Every home agent MUST be able to maintain an entry in its Binding Cache for each mobile node for which it is serving as the home agent. Each such Binding Cache entry records the mobile node's binding with its primary care-of address and is marked as a "home registration". - Every home agent MUST be able to intercept packets (using proxy Neighbor Discovery) addressed to a mobile node for which it is currently serving as the home agent, on that mobile node's home link, while the mobile node is away from home. - Every home agent MUST be able to encapsulate such intercepted packets in order to tunnel them to the primary care-of address for the mobile node indicated in its binding in the home agent's Binding Cache. - Every home agent MUST be able to return a Binding Acknowledgement option in response to a Binding Update option received with the Acknowledge (A) bit set. - Every home agent MUST be able to accept packets addressed to the Home-Agents anycast address for the subnet on which it is serving as a home agent, and MUST be able to participate in dynamic home agent address discovery (Section 9.2). 7.4. Requirements for IPv6 Mobile Nodes Finally, the following requirements apply to all IPv6 nodes capable of functioning as mobile nodes: - Every IPv6 mobile node MUST be able to perform IPv6 decapsulation [4]. - Every IPv6 mobile node MUST support sending Binding Update options, as specified in Sections 10.5, 10.7, and 10.8; and MUST be able to receive and process Binding Acknowledgement options, as specified in Section 10.11. - Every IPv6 mobile node MUST support use of the dynamic home agent address discovery mechanism, as described in Section 10.6. - Every IPv6 mobile node MUST maintain a Binding Update List in which it records the IP address of each other node to which it has sent a Binding Update, for which the Lifetime sent in that binding has not yet expired. - Every IPv6 mobile node MUST support receiving a Binding Request option, by responding with a Binding Update option. Johnson and Perkins Expires 4 February 1999 [Page 40] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 - Every IPv6 mobile node MUST support sending packets containing a Home Address option; this option MUST be included in all packets sent while away from home, if the packet would otherwise have been sent with the mobile node's home address as the IP Source Address. Johnson and Perkins Expires 4 February 1999 [Page 41] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 8. Correspondent Node Operation A correspondent node is any node communicating with a mobile node. The correspondent node, itself, may be stationary or mobile, and may possibly also be functioning as a home agent for Mobile IPv6. The procedures in this section thus apply to all IPv6 nodes. 8.1. Receiving Packets from a Mobile Node Packets sent by a mobile node while away from home generally include a Home Address option. When any node receives a packet containing a Home Address option, it MUST process the option in a manner consistent with copying the Home Address field from the Home Address option into the IPv6 header, replacing the original value of the Source Address field there. Further processing of such a packet (e.g., at the transport layer) thus need not know that the original Source Address was a care-of address, or that the Home Address option was used in the packet. Since the sending mobile node uses its home address at the transport layer when sending such a packet, the use of the care-of address and Home Address option is transparent to both the mobile node and the correspondent node above the level of the Home Address option generation and processing. 8.2. Receiving Binding Updates Upon receiving a Binding Update option in some packet, the receiving node MUST validate the Binding Update according to the following tests: - The packet contains a valid AH [8] or ESP [9] header that provides sender authentication, integrity protection, and replay protection. - The packet MUST contain a valid Home Address option. The home address for the binding is specified by the Home Address field of the Home Address option. - The Option Length field in the Binding Update option is greater than or equal to the length specified in Section 5.1. - The Sequence Number field in the Binding Update option is greater than the Sequence Number received in the previous Binding Update for this home address, if any. The Sequence Number comparison is performed modulo 2**16. Johnson and Perkins Expires 4 February 1999 [Page 42] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 Any Binding Update not satisfying all of these tests MUST be silently ignored, and the packet carrying the Binding Update MUST be discarded. If the Binding Update is valid according to the tests above, then the Binding Update is processed further as follows: - If the Destination Address in the packet's IPv6 header is the Home-Agents anycast address for a local prefix and this address is assigned to one of this node's network interfaces, then the mobile node sending this Binding Update is attempting dynamic home agent address discovery. Processing for this type of received Binding Update is described in Section 9.2. (If the Destination Address is not assigned to one of this node's network interfaces, then the packet would have been forwarded as a normal packet and the Binding Update, as a destination option, would not be processed in any way by this node.) - If the Lifetime specified in the Binding Update is nonzero and the specified Care-of Address is not equal to the home address for the binding (as given in the Home Address option in the packet), then this is a request to cache a binding for the mobile node. If the Home Registration (H) bit is set in the Binding Update, the Binding Update is processed according to the procedure specified in Section 9.3; otherwise, it is processed according to the procedure specified in Section 8.3. - If the Lifetime specified in the Binding Update is zero or the specified Care-of Address matches the home address for the binding, then this is a request to delete the mobile node's cached binding. If the Home Registration (H) bit is set in the Binding Update, the Binding Update is processed according to the procedure specified in Section 9.4; otherwise, it is processed according to the procedure specified in Section 8.4. 8.3. Requests to Cache a Binding When a node receives a Binding Update, it MUST validate it and determine the type of Binding Update according to the steps described in Section 8.2. This section describes the processing of a valid Binding Update that requests a node to cache a mobile node's binding, for which the Home Registration (H) bit is not set in the Binding Update. In this case, the receiving node SHOULD create a new entry in its Binding Cache for this mobile node (or update its existing Binding Cache entry for this mobile node, if such an entry already exists). The home address of the mobile node is taken from the Home Address field in the packet's Home Address option. The new Binding Cache Johnson and Perkins Expires 4 February 1999 [Page 43] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 entry records the association between this home address and the care-of address for the binding, as specified in either the Care-of Address field of the Binding Update or in the Source Address field in the packet's IPv6 header. Any Binding Cache entry created or updated in response to processing this Binding Update MUST be deleted after the expiration of the Lifetime period specified in the Binding Update. 8.4. Requests to Delete a Binding When a node receives a Binding Update, it MUST validate it and determine the type of Binding Update according to the steps described in Section 8.2. This section describes the processing of a valid Binding Update that requests a node to delete a mobile node's binding from its Binding Cache, for which the Home Registration (H) bit is not set in the Binding Update. In this case, the receiving node MUST delete any existing entry in its Binding Cache for this mobile node. The home address of the mobile node is taken from the Home Address field in the packet's Home Address option. 8.5. Sending Binding Acknowledgements When any node receives a packet containing a Binding Update option in which the Acknowledge (A) bit is set, it SHOULD return a Binding Acknowledgement option acknowledging receipt of the Binding Update. If the node accepts the Binding Update and creates or updates an entry in its Binding Cache for this binding, the Status field in the Binding Acknowledgement MUST be set to a value less than 128; if the node rejects the Binding Update and does not create or update an entry for this binding, the Status field in the Binding Acknowledgement MUST be set to a value greater than or equal to 128. Specific values for the Status field are described in Section 5.2 and in the most recent "Assigned Numbers" [21]. As described in Section 5.2, the packet in which the Binding Acknowledgement is returned MUST include either an AH [8] or ESP [9] header providing sender authentication, data integrity protection, and replay protection; and the packet MUST be sent using a Routing header in the same way as any other packet sent to a mobile node using a care-of address (even if the binding was rejected), as described in Section 8.9. The packet is routed first to the care-of address contained in the Binding Update being acknowledged, and then to the mobile node's home address. This use of the Routing header ensures that the Binding Acknowledgement will be routed to the current location of the node sending the Binding Update, whether the Binding Update was accepted or rejected. Johnson and Perkins Expires 4 February 1999 [Page 44] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 8.6. Sending Binding Requests Entries in a node's Binding Cache MUST be deleted when their lifetime expires. If such an entry is still in active use in sending packets to a mobile node, the next packet sent to the mobile node will be routed normally to the mobile node's home link, where it will be intercepted and tunneled to the mobile node. The mobile node will then return a Binding Update to the sender, allowing it to create a new Binding Cache entry for sending future packets to the mobile node. Communication with the mobile node continues uninterrupted, but the forwarding of this packet through the mobile node's home agent creates additional overhead and latency in delivering packets to the mobile node. If the sender knows that the Binding Cache entry is still in active use, it MAY send a Binding Request option to the mobile node in an attempt to avoid this overhead and latency due to deleting and recreating the Binding Cache entry. Since a Binding Request is a destination option, it may, for example, be included in any packet already being sent to the mobile node, such as a packet that is part of ongoing TCP communication with the mobile node. When the mobile node receives a packet from some sender containing a Binding Request option, it returns a Binding Update option to that sender, giving its current binding and a new lifetime. 8.7. Cache Replacement Policy Any entry in a node's Binding Cache MUST be deleted after the expiration of the Lifetime specified in the Binding Update from which the entry was created or last updated. Conceptually, a node maintains a separate timer for each entry in its Binding Cache. When creating or updating a Binding Cache entry in response to a received and accepted Binding Update, the node sets the timer for this entry to the specified Lifetime period. When a Binding Cache entry's timer expires, the node deletes the entry. Each node's Binding Cache will, by necessity, have a finite size. A node MAY use any reasonable local policy for managing the space within its Binding Cache, except that any entry marked as a "home registration" (Section 9.3) MUST NOT be deleted from the cache until the expiration of its lifetime period. When attempting to add a new "home registration" entry in response to a Binding Update with the Home Registration (H) bit set, if insufficient space exists (or can be reclaimed) in the node's Binding Cache, the node MUST reject the Binding Update and SHOULD return a Binding Acknowledgement to the sending mobile node, in which the Status field is set to 131 (insufficient resources). When otherwise attempting to add a new entry to its Binding Cache, a node MAY, if needed, choose to drop any entry already in its Binding Cache, other than a "home registration" Johnson and Perkins Expires 4 February 1999 [Page 45] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 entry, in order to make space for the new entry. For example, a "least-recently used" (LRU) strategy for cache entry replacement among entries not marked as a "home registration" is likely to work well. Any binding dropped from a node's Binding Cache due to lack of cache space will be rediscovered and a new cache entry created, if the binding is still in active use by the node for sending packets. If the node sends a packet to a destination for which it has dropped the entry from its Binding Cache, the packet will be routed normally, leading to the mobile node's home link. There, the packet will be intercepted by the mobile node's home agent and tunneled to the mobile node's current primary care-of address. As when a Binding Cache entry is initially created, this indirect routing to the mobile node through its home agent will result in the mobile node sending a Binding Update to this sending node when it receives the tunneled packet, allowing it to add an entry again for this destination mobile node to its Binding Cache. 8.8. Receiving ICMP Error Messages When a correspondent node sends a packet to a mobile node, if the correspondent node has a Binding Cache entry for the destination address of the packet, then the correspondent node uses a Routing header to deliver the packet to the mobile node through the care-of address in the binding recorded in the Binding Cache entry. Any ICMP error message caused by the packet on its way to the mobile node will be returned normally to the correspondent node. On the other hand, if the correspondent node has no Binding Cache entry for the mobile node, the packet will be routed to the mobile node's home link. There, it will be intercepted by the mobile node's home agent, encapsulated, and tunneled to the mobile node's primary care-of address. Any ICMP error message caused by the packet on its way to the mobile node while in the tunnel, will be returned to the mobile node's home agent (the source of the tunnel). By the definition of IPv6 encapsulation [4], this encapsulating node MUST relay certain ICMP error messages back to the original sender of the packet, which in this case is the correspondent node. Likewise, if a packet for a mobile node arrives at the mobile node's previous default router (e.g., the mobile node moved after the packet was sent), the router will encapsulate and tunnel the packet to the mobile node's new care-of address (if it has a Binding Cache entry for the mobile node). As above, any ICMP error message caused by the packet while in this tunnel will be returned to the previous default router (the source of the tunnel), which MUST relay certain ICMP error messages back to the correspondent node [4]. Johnson and Perkins Expires 4 February 1999 [Page 46] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 Thus, in all cases, any meaningful ICMP error messages caused by packets from a correspondent node to a mobile node will be returned to the correspondent node. If the correspondent node receives persistent ICMP Destination Unreachable messages after sending packets to a mobile node based on an entry in its Binding Cache, the correspondent node SHOULD delete this Binding Cache entry. If the correspondent node subsequently transmits another packet to the mobile node, the packet will be routed to the mobile node's home link, intercepted by the mobile node's home agent, and tunneled to the mobile node's primary care-of address using IPv6 encapsulation. The mobile node will then return a Binding Update to the correspondent node, allowing it to recreate a (correct) Binding Cache entry for the mobile node. 8.9. Sending Packets to a Mobile Node Before sending any packet, the sending node SHOULD examine its Binding Cache for an entry for the destination address to which the packet is being sent. If the sending node has a Binding Cache entry for this address, the sending node SHOULD use a Routing header to route the packet to this mobile node (the destination node) by way of the care-of address in the binding recorded in that Binding Cache entry. For example, assuming use of a Type 0 Routing header [5], if no other use of a Routing header is involved in the routing of this packet, the mobile node sets the fields in the packet's IPv6 header and Routing header as follows: - The Destination Address in the packet's IPv6 header is set to the mobile node's care-of address copied from the Binding Cache entry. - The Routing header is initialized to contain a single route segment, with an Address of the mobile node's home address (the original destination address to which the packet was being sent). Following the definition of a Type 0 Routing header [5], this packet will be routed to the mobile node's care-of address, where it will be delivered to the mobile node (the mobile node has associated the care-of address with its network interface). Normal processing of the Routing header by the mobile node will then proceed as follows: - The mobile node swaps the Destination Address in the packet's IPv6 header and the Address specified in the Routing header. This results in the packet's IP Destination Address being set to the mobile node's home address. - The mobile node then resubmits the packet to its IPv6 module for further processing, "looping back" the packet inside the mobile node. Since the mobile node recognizes its own home address as Johnson and Perkins Expires 4 February 1999 [Page 47] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 one of its current IP addresses, the packet is processed further within the mobile node, in the same way then as if the mobile node was at home. If, instead, the sending node has no Binding Cache entry for the destination address to which the packet is being sent, the sending node simply sends the packet normally, with no Routing header. If the destination node is not a mobile node (or is a mobile node that is currently at home), the packet will be delivered directly to this node and processed normally by it. If, however, the destination node is a mobile node that is currently away from home, the packet will be intercepted by the mobile node's home agent and tunneled (using IPv6 encapsulation [4]) to the mobile node's current primary care-of address, as described in Section 9.6. The mobile node will then send a Binding Update to the sending node, as described in Section 10.7, allowing the sending node to create a Binding Cache entry for its use in sending subsequent packets to this mobile node. Johnson and Perkins Expires 4 February 1999 [Page 48] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 9. Home Agent Operation 9.1. Receiving Router Advertisement Messages For each link on which a router provides service as a home agent, the router maintains a Home Agents List recording information about all other home agents on that link. This list is used in the dynamic home agent address discovery mechanism, described in Section 9.2. The information for the list is learned through receipt of the periodic unsolicited multicast Router Advertisements from each other home agent on the link, in which the Home Agent (H) bit is set, in a manner similar to the Default Router List conceptual data structure maintained by each host for Neighbor Discovery [13]. On receipt of a valid Router Advertisement, as defined in the processing algorithm specified for Neighbor Discovery [13], the home agent performs the following steps, in addition to any steps already required of it by Neighbor Discovery: - If the Home Agent (H) bit in the Router Advertisement is not set, skip all of the following steps. There are no special processing steps required by Mobile IP for this Router Advertisement, since the Advertisement was not sent by a home agent. - Otherwise, extract the Source Address from the IP header of the Router Advertisement. This is the link-local IP address on this link of the home agent sending this Advertisement [13]. Determine the global address of the router based on the Prefix Information option received from it in which the Router Address (R) bit is set (Section 6.2). - Determine from the Router Advertisement the preference for this home agent. If the Router Advertisement contains a Home Agent Information Option, then the preference is taken from the Home Agent Preference field in the option; otherwise, the default preference of 0 SHOULD be used. - Determine from the Router Advertisement the lifetime for this home agent. If the Router Advertisement contains a Home Agent Information Option, then the lifetime is taken from the Home Agent Lifetime field in the option; otherwise, the lifetime specified by the Router Lifetime field in the Router Advertisement SHOULD be used. - If the global address of the home agent sending this Advertisement, as determined above, is not already present in the Home Agents List maintained by the receiving home agent, and the lifetime for the sending home agent, also as determined above, is non-zero, create a new entry in the list, and initialize its lifetime and preference to the values determined above. Johnson and Perkins Expires 4 February 1999 [Page 49] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 - If the global address of the home agent sending this Advertisement is already present in the receiving home agent's Home Agents List, reset its lifetime and preference to the values determined above. - If the address is already present in this home agent's Home Agents List and the received home agent lifetime value is zero, immediately delete this entry in the Home Agents List. A home agent SHOULD maintain an entry in its Home Agents List for each such valid home agent address until that entry's lifetime expires, after which time the entry MUST be deleted. 9.2. Dynamic Home Agent Address Discovery When a node receives a Binding Update, it MUST validate it and determine the type of Binding Update according to the steps described in Section 8.2. This section describes the processing of a valid Binding Update that indicates that the mobile node sending it is attempting dynamic home agent address discovery. As described in Section 10.6, a mobile node attempts dynamic home agent address discovery by sending its "home registration" Binding Update to the Home-Agents anycast address for its home IP subnet prefix (the packet MUST also include a Home Address option). A home agent receiving such a Binding Update that is serving this subnet (the home agent is configured with this anycast address on one of its network interfaces) MUST reject the Binding Update and SHOULD return a Binding Acknowledgement indicating this rejection, with the Source Address of the packet carrying the Binding Acknowledgement set to one of the global unicast addresses of the home agent. The Status field in the Binding Acknowledgement MUST be set to 135 (dynamic home agent address discovery response). In this Binding Acknowledgement rejecting the dynamic home agent address discovery Binding Update, this home agent SHOULD set the Home Agents List as follows: - The Home Agents List in this Binding Acknowledgement SHOULD contain the IP address of all home agents currently listed in this home agent's own Home Agents List (Section 4.3). However, if this home agent's own IP address would be placed in the list (as described below) as the first entry in the list, then this home agent SHOULD NOT include its own address in the list in the Binding Acknowledgement. Not placing this home agent's own IP address in the list will cause the receiving mobile node to consider this home agent as the most preferred home agent; otherwise, this home agent will be considered to be preferred in its order given by its place in the list returned. Johnson and Perkins Expires 4 February 1999 [Page 50] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 - The IP addresses in the Home Agents List should be placed in the Home Agents List in the Binding Acknowledgement in order of decreasing preference value, based either on the respective advertised preference from a Home Agent Information option or on the default preference of 0 if no preference is advertised (or on the configured home agent preference for this home agent itself). The home agent with the highest preference SHOULD be listed first, and the home agent with the lowest preference SHOULD be listed last. - Among home agents with equal preference, their IP addresses in the Home Agents List SHOULD be listed in an order randomized with respect to other home agents with equal preference, each time a Binding Acknowledgement with a non-empty Home Agents List is returned by this home agent. - The Option Length field in this Binding Acknowledgement MUST be set to 11 + 16 * N, where N is the number of IP addresses included in the Home Agents List field in the Binding Acknowledgement. The mobile node, upon receiving this Binding Acknowledgement, MAY then resend its Binding Update to the home agent address given as the IP Source Address of the packet carrying the Binding Acknowledgement or to any of the unicast IP addresses listed in the Home Agents List field in the Acknowledgement. For example, the mobile node may re-attempt its home registration with each of these home agents in turn, by sending each a Binding Update and waiting for the matching Binding Acknowledgement, until its registration is accepted by one of these home agents. In trying each of the returned home agent addresses, the mobile node SHOULD try each in the order listed in the Home Agents List in the Binding Acknowledgement. If the home agent identified by the Source Address field in the IP header of the packet carrying the Binding Acknowledgement is not listed in the Home Agents List, it SHOULD be tried before the first address given in the list; otherwise, it SHOULD be tried in the in its listed order. 9.3. Primary Care-of Address Registration When a node receives a Binding Update, it MUST validate it and determine the type of Binding Update according to the steps described in Section 8.2. This section describes the processing of a valid Binding Update that requests the receiving node to serve as its home agent, registering its primary care-of address. To begin processing the Binding Update, the home agent MUST perform the following sequence of tests: Johnson and Perkins Expires 4 February 1999 [Page 51] INTERNET-DRAFT Mobility Support in IPv6 4 August 1998 - If the node is not a router that implements home agent functionality, then the node MUST reject the Binding Update and SHOULD return a Binding Acknowledgement to the mobile node, in which the Status field is set to 132 (home registration not supported). - Else, if the home address for the binding (the Home Address field in the packet's Home Address option) is not an on-link IPv6 address with respect to the home agent's current Prefix List, then the home agent MUST reject the Binding Update and SHOULD return a Binding Acknowledgement to the mobile node, in which the Status field is set to 133 (not home subnet). - Else, if the Prefix Length field is nonzero in the Binding Update and this length differs from the length of the home agent's own knowledge of the corresponding subnet prefix on the home link, then the home agent MUST reject the Binding Update and SHOULD return a Binding Acknowledgement to the mobile node, in which the Status field is set to 136 (incorrect subnet prefix length). - Else, if the home agent chooses to reject the Binding Update for any other rea