4. Overview of Mobile IPv6 4.1. Basic Operation A mobile node is always expected to be addressable at its home address, whether it is currently attached to its home link or is away from home. The "home address" is an IP address assigned to the mobile node within its home subnet prefix on its home link. While a mobile node is at home, packets addressed to its home address are routed to the mobile node's home link, using conventional Internet routing mechanisms. While a mobile node is attached to some foreign link away from home, it is also addressable at one or more care-of addresses. A care-of address is an IP address associated with a mobile node that has the subnet prefix of a particular foreign link. The mobile node can acquire its care-of address through conventional IPv6 mechanisms, such as stateless or stateful auto-configuration. As long as the mobile node stays in this location, packets addressed to this care-of address will be routed to the mobile node. The mobile node may also accept packets from several care-of addresses, such as when it is moving but still reachable at the previous link.
The association between a mobile node's home address and care-of address is known as a "binding" for the mobile node. While away from home, a mobile node registers its primary care-of address with a router on its home link, requesting this router to function as the "home agent" for the mobile node. The mobile node performs this binding registration by sending a "Binding Update" message to the home agent. The home agent replies to the mobile node by returning a "Binding Acknowledgement" message. The operation of the mobile node is specified in Section 11, and the operation of the home agent is specified in Section 10. 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. Mobile nodes can provide information about their current location to correspondent nodes. This happens through the correspondent registration. As a part of this procedure, a return routability test is performed in order to authorize the establishment of the binding. The operation of the correspondent node is specified in Section 9. There are two possible modes for communications between the mobile node and a correspondent node. The first mode, bidirectional tunneling, does not require Mobile IPv6 support from the correspondent node and is available even if the mobile node has not registered its current binding with the correspondent node. Packets from the correspondent node are routed to the home agent and then tunneled to the mobile node. Packets to the correspondent node are tunneled from the mobile node to the home agent ("reverse tunneled") and then routed normally from the home network to the correspondent node. In this mode, the home agent uses proxy Neighbor Discovery to intercept any IPv6 packets addressed to the mobile node's home address (or home addresses) on the home link. Each intercepted packet is tunneled to the mobile node's primary care-of address. This tunneling is performed using IPv6 encapsulation . The second mode, "route optimization", requires the mobile node to register its current binding at the correspondent node. Packets from the correspondent node can be routed directly to the care-of address of the mobile node. When sending a packet to any IPv6 destination, the correspondent 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 a new type of IPv6 routing header  (see Section 6.4) to route the packet to the mobile node by way of the care-of address indicated in this binding.
Routing packets directly to the mobile node's care-of address allows the shortest communications path to be used. It also eliminates congestion at the mobile node's home agent and home link. In addition, the impact of temporary failures of the home agent or networks on the path to or from the home agent is reduced. When routing packets directly to the mobile node, the correspondent node sets the Destination Address in the IPv6 header to the care-of address of the mobile node. A new type of IPv6 routing header (see Section 6.4) is also added to the packet to carry the desired home address. Similarly, the mobile node sets the Source Address in the packet's IPv6 header to its current care-of addresses. The mobile node adds a new IPv6 "Home Address" destination option (see Section 6.3) to carry its home address. The inclusion of home addresses in these packets makes the use of the care-of address transparent above the network layer (e.g., at the transport layer). Mobile IPv6 also provides support for multiple home agents, and a limited support for the reconfiguration of the home network. In these cases, the mobile node may not know the IP address of its own home agent, and even the home subnet prefixes may change over time. A mechanism known as "dynamic home agent address discovery" allows a mobile node to dynamically discover the IP address of a home agent on its home link, even when the mobile node is away from home. Mobile nodes can also learn new information about home subnet prefixes through the "mobile prefix discovery" mechanism. These mechanisms are described starting in Section 6.5. This document is written under the assumption that the mobile node is configured with the home prefix for the mobile node to be able to discover a home agent and configure a home address. This might be limiting in deployments where the home agent and the home address for the mobile node need to be assigned dynamically. Additional mechanisms have been specified for the mobile node to dynamically configure a home agent, a home address, and the home prefix. These mechanisms are described in "Mobile IPv6 Bootstrapping in Split Scenario"  and "MIP6-bootstrapping for the Integrated Scenario" . 4.2. New IPv6 Protocol Mobile IPv6 defines a new IPv6 protocol, using the Mobility Header (see Section 6.1). This header is used to carry the following messages: Home Test Init Home Test
Care-of Test Init Care-of Test These four messages are used to perform the return routability procedure from the mobile node to a correspondent node. This ensures authorization of subsequent Binding Updates, as described in Section 5.2.5. Binding Update A Binding Update 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". Binding Acknowledgement A Binding Acknowledgement is used to acknowledge receipt of a Binding Update, if an acknowledgement was requested in the Binding Update (e.g., the Binding Update was sent to a home agent), or an error occurred. Binding Refresh Request A Binding Refresh Request is used by a correspondent node to request that a mobile node re-establish its binding with the correspondent node. This message is typically used when the cached binding is in active use but the binding's lifetime is close to expiration. The correspondent node may use, for instance, recent traffic and open transport layer connections as an indication of active use. Binding Error The Binding Error is used by the correspondent node to signal an error related to mobility, such as an inappropriate attempt to use the Home Address destination option without an existing binding. The Binding Error message is also used by the home agent to signal an error to the mobile node, if it receives an unrecognized Mobility Header Message Type from the mobile node. 4.3. New IPv6 Destination Option Mobile IPv6 defines a new IPv6 destination option, the Home Address destination option. This option is described in detail in Section 6.3.
4.4. New IPv6 ICMP Messages Mobile IPv6 also introduces four new ICMP message types, two for use in the dynamic home agent address discovery mechanism, and two for renumbering and mobile configuration mechanisms. As described in Sections 10.5 and 11.4.1, the following two new ICMP message types are used for home agent address discovery: o Home Agent Address Discovery Request, described in Section 6.5. o Home Agent Address Discovery Reply, described in Section 6.6. The next two message types are used for network renumbering and address configuration on the mobile node, as described in Section 10.6: o Mobile Prefix Solicitation, described in Section 6.7. o Mobile Prefix Advertisement, described in Section 6.8. 4.5. Conceptual Data Structure Terminology This document describes the Mobile IPv6 protocol in terms of the following conceptual data structures: Binding Cache A cache of bindings for other nodes. This cache is maintained by home agents and correspondent nodes. The cache contains both "correspondent registration" entries (see Section 9.1) and "home registration" entries (see Section 10.1). Binding Update List This list is maintained by each mobile node. The list has an item for every binding that the mobile node has or is trying to establish with a specific other node. Both correspondent and home registrations are included in this list. Entries from the list are deleted as the lifetime of the binding expires. See Section 11.1.
Home Agents List Home agents need to know which other home agents are on the same link. This information is stored in the Home Agents List, as described in more detail in Section 10.1. The list is used for informing mobile nodes during dynamic home agent address discovery. 4.6. Unique-Local Addressability This specification requires that home and care-of addresses MUST be unicast routable addresses. Unique-local IPv6 unicast addresses (ULAs, RFC 4193 ) may be usable on networks that use such non- globally routable addresses, but this specification does not define when such usage is safe and when it is not. Mobile nodes may not be able to distinguish between their home site and the site at which they are currently located. This can make it hard to prevent accidental attachment to other sites, because the mobile node might use the ULA at another site, which could not be used to successfully send packets to the mobile node's home agent (HA). This would result in unreachability between the mobile node (MN) and the HA, when unique-local IPv6 routable addresses are used as care-of addresses. Similarly, CNs outside the MN's own site will not be reachable when ULAs are used as home addresses. Therefore, unique-local IPv6 unicast addresses SHOULD NOT be used as home or care-of addresses when other address choices are available. If such addresses are used, however, according to RFC 4193 , they are treated as any global unicast IPv6 address so, for the remainder of this specification, use of unique-local IPv6 unicast addresses is not differentiated from other globally unique IPv6 addresses. 5. Overview of Mobile IPv6 Security This specification provides a number of security features. These include the protection of Binding Updates both to home agents and correspondent nodes, the protection of mobile prefix discovery, and the protection of the mechanisms that Mobile IPv6 uses for transporting data packets. Binding Updates are protected by the use of IPsec extension headers, or by the use of the Binding Authorization Data option. This option employs a binding management key, Kbm, which can be established through the return routability procedure. Mobile prefix discovery is protected through the use of IPsec extension headers. Mechanisms related to transporting payload packets -- such as the Home Address destination option and type 2 routing header -- have been specified in a manner that restricts their use in attacks.
5.1. Binding Updates to Home Agents The mobile node and the home agent MUST use an IPsec security association to protect the integrity and authenticity of the Binding Updates and Acknowledgements. Both the mobile nodes and the home agents MUST support and SHOULD use the Encapsulating Security Payload (ESP)  header in transport mode and MUST use a non-NULL payload authentication algorithm to provide data origin authentication, connectionless integrity, and optional anti-replay protection. Note that Authentication Header (AH)  is also possible but for brevity not discussed in this specification. In order to protect messages exchanged between the mobile node and the home agent with IPsec, appropriate security policy database entries must be created. A mobile node must be prevented from using its security association to send a Binding Update on behalf of another mobile node using the same home agent. This MUST be achieved by having the home agent check that the given home address has been used with the right security association. Such a check is provided in the IPsec processing, by having the security policy database entries unequivocally identify a single security association for protecting Binding Updates between any given home address and home agent. In order to make this possible, it is necessary that the home address of the mobile node is visible in the Binding Updates and Acknowledgements. The home address is used in these packets as a source or destination, or in the Home Address destination option or the type 2 routing header. As with all IPsec security associations in this specification, manual configuration of security associations MUST be supported. The shared secrets used MUST be random and unique for different mobile nodes, and MUST be distributed off-line to the mobile nodes. Automatic key management with the Internet Key Exchange Protocol version 2 (IKEv2)  MAY be supported as described in . Section 11.3.2 discusses how IKEv2 connections to the home agent need a careful treatment of the addresses used for transporting IKEv2. This is necessary to ensure that a Binding Update is not needed before the IKEv2 exchange that is needed for securing the Binding Update. More detailed descriptions and examples using IPsec to protect communications between the mobile node and the home agent have been published .
5.2. Binding Updates to Correspondent Nodes The protection of Binding Updates sent to correspondent nodes does not require the configuration of security associations or the existence of an authentication infrastructure between the mobile nodes and correspondent nodes. Instead, a method called the return routability procedure is used to ensure that the right mobile node is sending the message. This method does not protect against attackers who are on the path between the home network and the correspondent node. However, attackers in such a location are capable of performing the same attacks even without Mobile IPv6. The main advantage of the return routability procedure is that it limits the potential attackers to those having an access to one specific path in the Internet, and avoids forged Binding Updates from anywhere else in the Internet. For a more in-depth explanation of the security properties of the return routability procedure, see Section 15. Also, consult . The integrity and authenticity of the Binding Update messages to correspondent nodes are protected by using a keyed-hash algorithm. The binding management key, Kbm, is used to key the hash algorithm for this purpose. Kbm is established using data exchanged during the return routability procedure. The data exchange is accomplished by use of node keys, nonces, cookies, tokens, and certain cryptographic functions. Section 5.2.5 outlines the basic return routability procedure. Section 5.2.6 shows how the results of this procedure are used to authorize a Binding Update to a correspondent node. 5.2.1. Node Keys Each correspondent node has a secret key, Kcn, called the "node key", which it uses to produce the keygen tokens sent to the mobile nodes. The node key MUST be a random number, 20 octets in length. The node key allows the correspondent node to verify that the keygen tokens used by the mobile node in authorizing a Binding Update are indeed its own. This key MUST NOT be shared with any other entity. A correspondent node MAY generate a fresh node key at any time; this avoids the need for secure persistent key storage. Procedures for optionally updating the node key are discussed later in Section 5.2.7.
o The "home keygen token" and "care-of keygen token" are 64-bit values sent by the correspondent node to the mobile node via the home agent (via the Home Test message) and the care-of address (by the Care-of Test message), respectively. The mobile node should set the home init or care-of init cookie to a newly generated random number in every Home or Care-of Test Init message it sends. The cookies are used to verify that the Home Test or Care-of Test message matches the Home Test Init or Care-of Test Init message, respectively. These cookies also serve to ensure that parties who have not seen the request cannot spoof responses. Home and care-of keygen tokens are produced by the correspondent node based on its currently active secret key (Kcn) and nonces, as well as the home or care-of address (respectively). A keygen token is valid as long as both the secret key (Kcn) and the nonce used to create it are valid. 5.2.4. Cryptographic Functions By default in this specification, the function used to compute hash values is SHA-1 , which is considered to offer sufficient protection for Mobile IPv6 control messages (see Section 15.10). Message Authentication Codes (MACs) are then computed using HMAC_SHA1 . HMAC_SHA1(K,m) denotes such a MAC computed on message m with key K. 5.2.5. Return Routability Procedure The return routability procedure enables the correspondent node to obtain some reasonable assurance that the mobile node is in fact addressable at its claimed care-of address as well as at its home address. Only with this assurance is the correspondent node able to accept Binding Updates from the mobile node, which would then instruct the correspondent node to direct that mobile node's data traffic to its claimed care-of address. This is done by testing whether packets addressed to the two claimed addresses are routed to the mobile node. The mobile node can pass the test only if it is able to supply proof that it received certain data (the "keygen tokens") that the correspondent node sends to those addresses. These data are combined by the mobile node into a binding management key, denoted Kbm. The figure below shows the message flow for the return routability procedure.
Mobile node Home agent Correspondent node | | | Home Test Init (HoTI) | | |------------------------->|------------------------->| | | | | Care-of Test Init (CoTI) | |---------------------------------------------------->| | | | | Home Test (HoT) | |<-------------------------|<-------------------------| | | | | Care-of Test (CoT) | |<----------------------------------------------------| | | The Home and Care-of Test Init messages are sent at the same time. The procedure requires very little processing at the correspondent node, and the Home and Care-of Test messages can be returned quickly, perhaps nearly simultaneously. These four messages form the return routability procedure. Home Test Init A mobile node sends a Home Test Init message to the correspondent node (via the home agent) to acquire the home keygen token. The contents of the message can be summarized as follows: * Source Address = home address * Destination Address = correspondent * Parameters: + home init cookie The Home Test Init message conveys the mobile node's home address to the correspondent node. The mobile node also sends along a home init cookie that the correspondent node must return later. The Home Test Init message is reverse tunneled through the home agent. (The headers and addresses related to reverse tunneling have been omitted from the above discussion of the message contents.) The mobile node remembers these cookie values to obtain some assurance that its protocol messages are being processed by the desired correspondent node.
Care-of Test Init The mobile node sends a Care-of Test Init message to the correspondent node (directly, not via the home agent) to acquire the care-of keygen token. The contents of this message can be summarized as follows: * Source Address = care-of address * Destination Address = correspondent * Parameters: + care-of init cookie The Care-of Test Init message conveys the mobile node's care-of address to the correspondent node. The mobile node also sends along a care-of init cookie that the correspondent node must return later. The Care-of Test Init message is sent directly to the correspondent node. Home Test The Home Test message is sent in response to a Home Test Init message. It is sent via the home agent. The contents of the message are: * Source Address = correspondent * Destination Address = home address * Parameters: + home init cookie + home keygen token + home nonce index When the correspondent node receives the Home Test Init message, it generates a home keygen token as follows: home keygen token := First (64, HMAC_SHA1 (Kcn, (home address | nonce | 0))) where | denotes concatenation. The final "0" inside the HMAC_SHA1 function is a single zero octet, used to distinguish home and care-of cookies from each other.
The home keygen token is formed from the first 64 bits of the MAC. The home keygen token tests that the mobile node can receive messages sent to its home address. Kcn is used in the production of home keygen token in order to allow the correspondent node to verify that it generated the home and care-of nonces, without forcing the correspondent node to remember a list of all tokens it has handed out. The Home Test message is sent to the mobile node via the home network, where it is presumed that the home agent will tunnel the message to the mobile node. This means that the mobile node needs to already have sent a Binding Update to the home agent, so that the home agent will have received and authorized the new care-of address for the mobile node before the return routability procedure. For improved security, the data passed between the home agent and the mobile node is made immune to inspection and passive attacks. Such protection is gained by encrypting the home keygen token as it is tunneled from the home agent to the mobile node as specified in Section 10.4.6. The security properties of this additional security are discussed in Section 15.4.1. The home init cookie from the mobile node is returned in the Home Test message, to ensure that the message comes from a node on the route between the home agent and the correspondent node. The home nonce index is delivered to the mobile node to later allow the correspondent node to efficiently find the nonce value that it used in creating the home keygen token. Care-of Test This message is sent in response to a Care-of Test Init message. This message is not sent via the home agent; it is sent directly to the mobile node. The contents of the message are: * Source Address = correspondent * Destination Address = care-of address * Parameters: + care-of init cookie + care-of keygen token + care-of nonce index
When the correspondent node receives the Care-of Test Init message, it generates a care-of keygen token as follows: care-of keygen token := First (64, HMAC_SHA1 (Kcn, (care-of address | nonce | 1))) Here, the final "1" inside the HMAC_SHA1 function is a single octet containing the hex value 0x01, and is used to distinguish home and care-of cookies from each other. The keygen token is formed from the first 64 bits of the MAC, and sent directly to the mobile node at its care-of address. The care-of init cookie from the Care-of Test Init message is returned to ensure that the message comes from a node on the route to the correspondent node. The care-of nonce index is provided to identify the nonce used for the care-of keygen token. The home and care-of nonce indices MAY be the same, or different, in the Home and Care-of Test messages. When the mobile node has received both the Home and Care-of Test messages, the return routability procedure is complete. As a result of the procedure, the mobile node has the data it needs to send a Binding Update to the correspondent node. The mobile node hashes the tokens together to form a 20-octet binding key Kbm: Kbm = SHA-1 (home keygen token | care-of keygen token) A Binding Update may also be used to delete a previously established binding (Section 6.1.7). In this case, the care-of keygen token is not used. Instead, the binding management key is generated as follows: Kbm = SHA-1(home keygen token) Note that the correspondent node does not create any state specific to the mobile node, until it receives the Binding Update from that mobile node. The correspondent node does not maintain the value for the binding management key Kbm; it creates Kbm when given the nonce indices and the mobile node's addresses. 5.2.6. Authorizing Binding Management Messages After the mobile node has created the binding management key (Kbm), it can supply a verifiable Binding Update to the correspondent node. This section provides an overview of this registration. The figure below shows the message flow.
Mobile node Correspondent node | | | Binding Update (BU) | |---------------------------------------------->| | (MAC, seq#, nonce indices, care-of address) | | | | | | Binding Acknowledgement (BA) (if sent) | |<----------------------------------------------| | (MAC, seq#, status) | Binding Update To authorize a Binding Update, the mobile node creates a binding management key Kbm from the keygen tokens as described in the previous section. The contents of the Binding Update include the following: * Source Address = care-of address * Destination Address = correspondent * Parameters: + home address (within the Home Address destination option if different from the Source Address) + sequence number (within the Binding Update message header) + home nonce index (within the Nonce Indices option) + care-of nonce index (within the Nonce Indices option) + First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent | BU))) The Binding Update contains a Nonce Indices option, indicating to the correspondent node which home and care-of nonces to use to recompute Kbm, the binding management key. The MAC is computed as described in Section 6.2.7, using the correspondent node's address as the destination address and the Binding Update message itself ("BU" above) as the Mobility Header (MH) Data. Once the correspondent node has verified the MAC, it can create a Binding Cache entry for the mobile.
Binding Acknowledgement The Binding Update is in some cases acknowledged by the correspondent node. The contents of the message are as follows: * Source Address = correspondent * Destination Address = care-of address * Parameters: + sequence number (within the Binding Update message header) + First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent | BA))) The Binding Acknowledgement contains the same sequence number as the Binding Update. The MAC is computed as described in Section 6.2.7, using the correspondent node's address as the destination address and the message itself ("BA" above) as the MH Data. Bindings established with correspondent nodes using keys created by way of the return routability procedure MUST NOT exceed MAX_RR_BINDING_LIFETIME seconds (see Section 12). The value in the Source Address field in the IPv6 header carrying the Binding Update is normally also the care-of address that is used in the binding. However, a different care-of address MAY be specified by including an Alternate Care-of Address mobility option in the Binding Update (see Section 6.2.5). When such a message is sent to the correspondent node and the return routability procedure is used as the authorization method, the Care-of Test Init and Care-of Test messages MUST have been performed for the address in the Alternate Care-of Address option (not the Source Address). The nonce indices and MAC value MUST be based on information gained in this test. Binding Updates may also be sent to delete a previously established binding. In this case, generation of the binding management key depends exclusively on the home keygen token and the care-of nonce index is ignored. 5.2.7. Updating Node Keys and Nonces Correspondent nodes generate nonces at regular intervals. It is recommended to keep each nonce (identified by a nonce index) acceptable for at least MAX_TOKEN_LIFETIME seconds (see Section 12) after it has been first used in constructing a return routability
message response. However, the correspondent node MUST NOT accept nonces beyond MAX_NONCE_LIFETIME seconds (see Section 12) after the first use. As the difference between these two constants is 30 seconds, a convenient way to enforce the above lifetimes is to generate a new nonce every 30 seconds. The node can then continue to accept tokens that have been based on the last 8 (MAX_NONCE_LIFETIME / 30) nonces. This results in tokens being acceptable MAX_TOKEN_LIFETIME to MAX_NONCE_LIFETIME seconds after they have been sent to the mobile node, depending on whether the token was sent at the beginning or end of the first 30-second period. Note that the correspondent node may also attempt to generate new nonces on demand, or only if the old nonces have been used. This is possible, as long as the correspondent node keeps track of how long a time ago the nonces were used for the first time, and does not generate new nonces on every return routability request. Due to resource limitations, rapid deletion of bindings, or reboots the correspondent node may not in all cases recognize the nonces that the tokens were based on. If a nonce index is unrecognized, the correspondent node replies with an error code in the Binding Acknowledgement (either 136, 137, or 138 as discussed in Section 6.1.8). The mobile node can then retry the return routability procedure. An update of Kcn SHOULD be done at the same time as an update of a nonce, so that nonce indices can identify both the nonce and the key. Old Kcn values have to be therefore remembered as long as old nonce values. Given that the tokens are normally expected to be usable for MAX_TOKEN_LIFETIME seconds, the mobile node MAY use them beyond a single run of the return routability procedure until MAX_TOKEN_LIFETIME expires. After this the mobile node SHOULD NOT use the tokens. A fast moving mobile node MAY reuse a recent home keygen token from a correspondent node when moving to a new location, and just acquire a new care-of keygen token to show routability in the new location. While this does not save the number of round-trips due to the simultaneous processing of home and care-of return routability tests, there are fewer messages being exchanged, and a potentially long round-trip through the home agent is avoided. Consequently, this optimization is often useful. A mobile node that has multiple home addresses MAY also use the same care-of keygen token for Binding Updates concerning all of these addresses.
5.2.8. Preventing Replay Attacks The return routability procedure also protects the participants against replayed Binding Updates through the use of the sequence number and a MAC. Care must be taken when removing bindings at the correspondent node, however. Correspondent nodes must retain bindings and the associated sequence number information at least as long as the nonces used in the authorization of the binding are still valid. Alternatively, if memory is very constrained, the correspondent node MAY invalidate the nonces that were used for the binding being deleted (or some larger group of nonces that they belong to). This may, however, impact the ability to accept Binding Updates from mobile nodes that have recently received keygen tokens. This alternative is therefore recommended only as a last measure. 5.2.9. Handling Interruptions to Return Routability In some scenarios, such as simultaneous mobility, where both correspondent host and mobile host move at the same time, or in the case where the correspondent node reboots and loses data, route optimization may not complete, or relevant data in the binding cache might be lost. o Return Routability signaling MUST be sent to the correspondent node's home address if it has one (i.e., not to the correspondent nodes care-of address if the correspondent node is also mobile). o If Return Routability signaling timed out after MAX_RO_FAILURE attempts, the mobile node MUST revert to sending packets to the correspondent node's home address through its home agent. The mobile node may run the bidirectional tunneling in parallel with the return routability procedure until it is successful. Exponential backoff SHOULD be used for retransmission of return routability messages. The return routability procedure may be triggered by movement of the mobile node or by sustained loss of end-to-end communication with a correspondent node (e.g., based on indications from upper layers) that has been using a route optimized connection to the mobile node. If such indications are received, the mobile node MAY revert to bidirectional tunneling while restarting the return routability procedure.
5.3. Dynamic Home Agent Address Discovery Dynamic home agent address discovery has been designed for use in deployments where security is not needed. For this reason, no security solution is provided in this document for dynamic home agent address discovery. 5.4. Mobile Prefix Discovery The mobile node and the home agent SHOULD use an IPsec security association to protect the integrity and authenticity of the Mobile Prefix Solicitations and Advertisements. Both the mobile nodes and the home agents MUST support and SHOULD use the Encapsulating Security Payload (ESP) header in transport mode with a non-NULL payload authentication algorithm to provide data origin authentication, connectionless integrity, and optional anti-replay protection. 5.5. Payload Packets Payload packets exchanged with mobile nodes can be protected in the usual manner, in the same way as stationary hosts can protect them. However, Mobile IPv6 introduces the Home Address destination option, a routing header, and tunneling headers in the payload packets. In the following we define the security measures taken to protect these, and to prevent their use in attacks against other parties. This specification limits the use of the Home Address destination option to the situation where the correspondent node already has a Binding Cache entry for the given home address. This avoids the use of the Home Address option in attacks described in Section 15.1. Mobile IPv6 uses a type of routing header specific to Mobile IPv6. This type provides the necessary functionality but does not open vulnerabilities discussed in Section 15.1 and RFC 5095 . Tunnels between the mobile node and the home agent are protected by ensuring proper use of source addresses, and optional cryptographic protection. The mobile node verifies that the outer IP address corresponds to its home agent. The home agent verifies that the outer IP address corresponds to the current location of the mobile node (Binding Updates sent to the home agents are secure). The home agent identifies the mobile node through the source address of the inner packet. (Typically, this is the home address of the mobile node, but it can also be a link-local address, as discussed in Section 10.4.2. To recognize the latter type of addresses, the home
agent requires that the Link-Local Address Compatibility (L) was set in the Binding Update.) These measures protect the tunnels against vulnerabilities discussed in Section 15.1. For traffic tunneled via the home agent, additional IPsec ESP encapsulation MAY be supported and used. If multicast group membership control protocols or stateful address autoconfiguration protocols are supported, payload data protection MUST be supported.