| IETF Mobile IP Working Group | C. Perkins (Ed.) |
| Internet-Draft | Tellabs Inc. |
| Obsoletes: 3775 (if approved) | D. Johnson |
| Intended status: Standards Track | Rice University |
| J. Arkko | |
| Ericsson | |
| Mar 11, 2011 |
Mobility Support in IPv6
draft-ietf-mext-rfc3775bis-13.txt
This document specifies Mobile IPv6, a protocol which allows nodes to remain reachable while moving around in the IPv6 Internet. 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. To support this operation, Mobile IPv6 defines a new IPv6 protocol and a new destination option. All IPv6 nodes, whether mobile or stationary, can communicate with mobile nodes. This document obsoletes RFC 3775.
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This document specifies a protocol which allows nodes to remain reachable while moving around in the IPv6 Internet. Without specific support for mobility in IPv6 [RFC2460], packets destined to a mobile node would not be able to reach it while the mobile node is away from its home link. 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 defined in this document, known as Mobile IPv6, allows a mobile node to move from one link to another without changing the mobile node's "home address". Packets may be routed to the mobile node using this address regardless of the mobile node's current point of attachment to the Internet. The mobile node may also 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 network-layer mobility management problem. Some mobility management applications -- for example, handover among wireless transceivers, each of which covers only a very small geographic area -- have been solved using link-layer techniques. For example, in many current wireless LAN products, link-layer mobility mechanisms allow a "handover" of a mobile node from one cell to another, re-establishing link-layer connectivity to the node in each new location.
Mobile IPv6 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:
This document obsoletes RFC 3775. Issues with the original document have been observed during integration, testing and deployment of RFC 3775. A more detailed list of the changes since RFC 3775 may be found in Appendix Appendix B.
The design of Mobile IP support in IPv6 (Mobile IPv6) benefits both from the experiences gained from the development of Mobile IP support in IPv4 (Mobile IPv4) [RFC3344] [RFC2003] [RFC2004], and from the opportunities provided by IPv6. Mobile IPv6 thus shares many features with Mobile IPv4, but is integrated into IPv6 and offers many other improvements. This section summarizes the major differences between Mobile IPv4 and Mobile IPv6:
The key words "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 [RFC2119].
These terms are intended to be compatible with the definitions given in RFC 3753[RFC3753]. However, if there is any conflict, the definitions given here should be considered to supersede those in RFC 3753.
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 [RFC2473].
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 [RFC2460] (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 from 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 needs 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" [RFC5026] and "MIP6 bootstrapping for the Integrated Scenario" [I-D.ietf-mip6-bootstrapping-integrated-dhc].
Mobile IPv6 defines a new IPv6 protocol, using the Mobility Header (see Section 6.1). This Header is used to carry the following messages:
Mobile IPv6 defines a new IPv6 destination option, the Home Address destination option. This option is described in detail in Section 6.3.
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 Section 10.5 and Section 11.4.1, the following two new ICMP message types are used for home agent address discovery:
The next two message types are used for network renumbering and address configuration on the mobile node, as described in Section 10.6:
This document describes the Mobile IPv6 protocol in terms of the following conceptual data structures:
This specification requires that home and care-of addresses MUST be unicast routable addresses. Unique-local IPv6 unicast addresses (ULAs) RFC4193 [RFC4193] 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 HA. This would result in unreachability between the 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 RFC4193 [RFC4193], 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.
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 which restricts their use in attacks.
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) [RFC4303] 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) [RFC4302] 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 IKEv2 [RFC5996] MAY be supported as described in [RFC4877].
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 which 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 [RFC3776][RFC4877].
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 assure 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 [RFC4225]
The integrity and authenticity of the Binding Update messages to correspondent nodes is 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.
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.
Each correspondent node also generates nonces at regular intervals. The nonces should be generated by using a random number generator that is known to have good randomness properties [RFC4086]. A correspondent node may use the same Kcn and nonce with all the mobiles it is in communication with.
Each nonce is identified by a nonce index. When a new nonce is generated, it must be associated with a new nonce index; this may be done, for example, by incrementing the value of the previous nonce index, if the nonce index is used as an array pointer into a linear array of nonces. However, there is no requirement that nonces be stored that way, or that the values of subsequent nonce indices have any particular relationship to each other. The index value is communicated in the protocol, so that if a nonce is replaced by new nonce during the run of a protocol, the correspondent node can distinguish messages that should be checked against the old nonce from messages that should be checked against the new nonce. Strictly speaking, indices are not necessary in the authentication, but allow the correspondent node to efficiently find the nonce value that it used in creating a keygen token.
Correspondent nodes keep both the current nonce and a small set of valid previous nonces whose lifetime has not yet expired. Expired values MUST be discarded, and messages using stale or unknown indices will be rejected.
The specific nonce index values cannot be used by mobile nodes to determine the validity of the nonce. Expected validity times for the nonces values and the procedures for updating them are discussed later in Section 5.2.7.
A nonce is an octet string of any length. The recommended length is 64 bits.
The return routability address test procedure uses cookies and keygen tokens as opaque values within the test init and test messages, 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.
By default in this specification, the function used to compute hash values is SHA-1 [FIPS.180-1.1995], 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 [RFC2104][FIPS.180-1.1995]. HMAC_SHA1(K,m) denotes such a MAC computed on message m with key K.
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") which 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.
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.
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 below figure 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) |
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 which 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.
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.
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.
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.
The mobile node may run the bidirectional tunnelling 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 optimised connection to the mobile node. If such indications are received, the mobile node MAY revert to bi-directional tunnelling while re-starting the return routability procedure.
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.
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.
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 [RFC5095].
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.
The Mobility Header is an extension header used by mobile nodes, correspondent nodes, and home agents in all messaging related to the creation and management of bindings. The subsections within this section describe the message types that may be sent using the Mobility Header.
Mobility Header messages MUST NOT be sent with a type 2 routing header, except as described in Section 9.5.4 for Binding Acknowledgement. Mobility Header messages also MUST NOT be used with a Home Address destination option, except as described in Section 11.7.1 and Section 11.7.2 for Binding Update. Binding Update List or Binding Cache information (when present) for the destination MUST NOT be used in sending Mobility Header messages. That is, Mobility Header messages bypass both the Binding Cache check described in Section 9.3.2 and the Binding Update List check described in Section 11.3.1 which are normally performed for all packets. This applies even to messages sent to or from a correspondent node which is itself a mobile node.
The Mobility Header is identified by a Next Header value of 135 in the immediately preceding header, and has the following format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Proto | Header Len | MH Type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
. .
. Message Data .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Mobile IPv6 also defines a number of "mobility options" for use within these messages; if included, any options MUST appear after the fixed portion of the message data specified in this document. The presence of such options will be indicated by the Header Len field within the message. When the Header Len value is greater than the length required for the message specified here, the remaining octets are interpreted as mobility options. These options include padding options that can be used to ensure that other options are aligned properly, and that the total length of the message is divisible by 8. The encoding and format of defined options are described in Section 6.2.
Alignment requirements for the Mobility Header are the same as for any IPv6 protocol Header. That is, they MUST be aligned on an 8-octet boundary.
The Binding Refresh Request (BRR) message requests a mobile node to update its mobility binding. This message is sent by correspondent nodes according to the rules in Section 9.5.5. When a mobile node receives a packet containing a Binding Refresh Request message it processes the message according to the rules in Section 11.7.4.
The Binding Refresh Request message uses the MH Type value 0. When this value is indicated in the MH Type field, the format of the Message Data field in the Mobility Header is as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
If no actual options are present in this message, no padding is necessary and the Header Len field will be set to 0.
A mobile node uses the Home Test Init (HoTI) message to initiate the return routability procedure and request a home keygen token from a correspondent node (see Section 11.6.1). The Home Test Init message uses the MH Type value 1. When this value is indicated in the MH Type field, the format of the Message Data field in the Mobility Header is as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Home Init Cookie +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility Options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
If no actual options are present in this message, no padding is necessary and the Header Len field will be set to 1.
This message is tunneled through the home agent when the mobile node is away from home. Such tunneling SHOULD employ IPsec ESP in tunnel mode between the home agent and the mobile node. This protection is indicated by the IPsec security policy database. The protection of Home Test Init messages is unrelated to the requirement to protect regular payload traffic, which MAY use such tunnels as well.
A mobile node uses the Care-of Test Init (CoTI) message to initiate the return routability procedure and request a care-of keygen token from a correspondent node (see Section 11.6.1). The Care-of Test Init message uses the MH Type value 2. When this value is indicated in the MH Type field, the format of the Message Data field in the Mobility Header is as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Care-of Init Cookie +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility Options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
If no actual options are present in this message, no padding is necessary and the Header Len field will be set to 1.
The Home Test (HoT) message is a response to the Home Test Init message, and is sent from the correspondent node to the mobile node (see Section 5.2.5). The Home Test message uses the MH Type value 3. When this value is indicated in the MH Type field, the format of the Message Data field in the Mobility Header is as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Nonce Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Home Init Cookie +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Home Keygen Token +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
If no actual options are present in this message, no padding is necessary and the Header Len field will be set to 2.
The Care-of Test (CoT) message is a response to the Care-of Test Init message, and is sent from the correspondent node to the mobile node (see Section 11.6.2). The Care-of Test message uses the MH Type value 4. When this value is indicated in the MH Type field, the format of the Message Data field in the Mobility Header is as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Care-of Nonce Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Care-of Init Cookie +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Care-of Keygen Token +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility Options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
If no actual options are present in this message, no padding is necessary and the Header Len field will be set to 2.
The Binding Update (BU) message is used by a mobile node to notify other nodes of a new care-of address for itself. Binding Updates are sent as described in Section 11.7.1 and Section 11.7.2.
The Binding Update uses the MH Type value 5. When this value is indicated in the MH Type field, the format of the Message Data field in the Mobility Header is as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence # |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|A|H|L|K| Reserved | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
If no options are present in this message, 4 octets of padding are necessary and the Header Len field will be set to 1.
The care-of address is specified either by the Source Address field in the IPv6 header or by the Alternate Care-of Address option, if present. The care-of address MUST be a unicast routable address. IPv6 Source Address MUST be a topologically correct source address. Binding Updates for a care-of address which is not a unicast routable address MUST be silently discarded.
The deletion of a binding MUST be indicated by setting the Lifetime field to 0. In deletion, the generation of the binding management key depends exclusively on the home keygen token, as explained in Section 5.2.5.
Correspondent nodes SHOULD NOT delete the Binding Cache entry before the lifetime expires, if any application hosted by the correspondent node is still likely to require communication with the mobile node. A Binding Cache entry that is de-allocated prematurely might cause subsequent packets to be dropped from the mobile node, if they contain the Home Address destination option. This situation is recoverable, since a Binding Error message is sent to the mobile node (see Section 6.1.9); however, it causes unnecessary delay in the communications.
The Binding Acknowledgement is used to acknowledge receipt of a Binding Update (Section 6.1.7). This packet is sent as described in Section 9.5.4 and Section 10.3.1.
The Binding Acknowledgement has the MH Type value 6. When this value is indicated in the MH Type field, the format of the Message Data field in the Mobility Header is as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status |K| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence # | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
If no options are present in this message, 4 octets of padding are necessary and the Header Len field will be set to 1.
The Binding Error (BE) message 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; see Section 9.3.3 for details.
The Binding Error message uses the MH Type value 7. When this value is indicated in the MH Type field, the format of the Message Data field in the Mobility Header is as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Home Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Mobility Options .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
If no actual options are present in this message, no padding is necessary and the Header Len field will be set to 2.
Mobility messages can include zero or more mobility options. This allows optional fields that may not be needed in every use of a particular Mobility Header, as well as future extensions to the format of the messages. Such options are included in the Message Data field of the message itself, after the fixed portion of the message data specified in the message subsections of Section 6.1.
The presence of such options will be indicated by the Header Len of the Mobility Header. If included, the Binding Authorization Data option (Section 6.2.7) MUST be the last option and MUST NOT have trailing padding. Otherwise, options can be placed in any order.
Mobility options are encoded within the remaining space of the Message Data field of a mobility message, using a 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 | Option Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following subsections specify the Option types which are currently defined for use in the Mobility Header.
Implementations MUST silently ignore any mobility options that they do not understand.
Mobility options may have alignment requirements. Following the convention in IPv6, these options are aligned in a packet so that multi-octet values within the Option Data field of each option fall on natural boundaries (i.e., fields of width n octets are placed at an integer multiple of n octets from the start of the header, for n = 1, 2, 4, or 8) [RFC2460].
The Pad1 option does not have any alignment requirements. Its format is as follows:
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Type = 0 |
+-+-+-+-+-+-+-+-+
NOTE! the format of the Pad1 option is a special case - it has neither Option Length nor Option Data fields.
The Pad1 option is used to insert one octet of padding in the Mobility Options area of a Mobility Header. If more than one octet of padding is required, the PadN option, described next, should be used rather than multiple Pad1 options.
The PadN option does not have any alignment requirements. Its format is as follows:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
| Type = 1 | Option Length | Option Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
The PadN option is used to insert two or more octets of padding in the Mobility Options area of a mobility message. For N octets of padding, the Option Length field contains the value N-2, and the Option Data consists of N-2 zero-valued octets. PadN Option data MUST be ignored by the receiver.
The Binding Refresh Advice option has an alignment requirement of 2n. Its format 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 = 2 | Length = 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Refresh Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Binding Refresh Advice option is only valid in the Binding Acknowledgement, and only on Binding Acknowledgements sent from the mobile node's home agent in reply to a home registration. The Refresh Interval is measured in units of four seconds, and indicates remaining time until the mobile node SHOULD send a new home registration to the home agent. The Refresh Interval MUST be set to indicate a smaller time interval than the Lifetime value of the Binding Acknowledgement.
The Alternate Care-of Address option has an alignment requirement of 8n+6. Its format 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 = 3 | Length = 16 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Alternate Care-of Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Normally, a Binding Update specifies the desired care-of address in the Source Address field of the IPv6 header. However, this is not possible in some cases, such as when the mobile node wishes to indicate a care-of address which it cannot use as a topologically correct source address (Section 6.1.7 and Section 11.7.2) or when the used security mechanism does not protect the IPv6 header (Section 11.7.1).
The Alternate Care-of Address option is provided for these situations. This option is valid only in Binding Update. The Alternate Care-of Address field contains an address to use as the care-of address for the binding, rather than using the Source Address of the packet as the care-of address.
The Nonce Indices option has an alignment requirement of 2n. Its format 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 = 4 | Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Nonce Index | Care-of Nonce Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Nonce Indices option is valid only in the Binding Update message sent to a correspondent node, and only when present together with a Binding Authorization Data option. When the correspondent node authorizes the Binding Update, it needs to produce home and care-of keygen tokens from its stored random nonce values.
The Home Nonce Index field tells the correspondent node which nonce value to use when producing the home keygen token.
The Care-of Nonce Index field is ignored in requests to delete a binding. Otherwise, it tells the correspondent node which nonce value to use when producing the care-of keygen token.
The Binding Authorization Data option does not have alignment requirements as such. However, since this option must be the last mobility option, an implicit alignment requirement is 8n + 2. The format of this 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 = 5 | Option Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| Authenticator |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Binding Authorization Data option is valid in the Binding Update and Binding Acknowledgement.
The Option Length field contains the length of the authenticator in octets.
The Authenticator field contains a cryptographic value which can be used to determine that the message in question comes from the right authority. Rules for calculating this value depends on the used authorization procedure.
For the return routability procedure, this option can appear in the Binding Update and Binding Acknowledgements. Rules for calculating the Authenticator value are the following:
Mobility Data = care-of address | correspondent | MH Data Authenticator = First (96, HMAC_SHA1 (Kbm, Mobility Data))
Where | denotes concatenation. "Care-of address" is the care-of address which will be registered for the mobile node if the Binding Update succeeds, or the home address of the mobile node if this option is used in de-registration. Note also that this address might be different from the source address of the Binding Update message, if the Alternative Care-of Address mobility option is used, or when the lifetime of the binding is set to zero.
The "correspondent" is the IPv6 address of the correspondent node. Note that, if the message is sent to a destination which is itself mobile, the "correspondent" address may not be the address found in the Destination Address field of the IPv6 header; instead the home address from the type 2 Routing header should be used.
"MH Data" is the content of the Mobility Header, excluding the Authenticator field itself. The Authenticator value is calculated as if the Checksum field in the Mobility Header was zero. The Checksum in the transmitted packet is still calculated in the usual manner, with the calculated Authenticator being a part of the packet protected by the Checksum. Kbm is the binding management key, which is typically created using nonces provided by the correspondent node (see Section 9.4). Note that while the contents of a potential Home Address destination option are not covered in this formula, the rules for the calculation of the Kbm do take the home address in account. This ensures that the MAC will be different for different home addresses.
The first 96 bits from the MAC result are used as the Authenticator field.
The Home Address option is carried by the Destination Option extension header (Next Header value = 60). It is used in a packet sent by a mobile node while away from home, to inform the recipient of the mobile node's home address.
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 +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The alignment requirement [RFC2460] for the Home Address option is 8n+6.
The three highest-order bits of the Option Type field are encoded to indicate specific processing of the option [RFC2460]; for the Home Address option, these three bits are set to 110. This indicates the following processing requirements:
The Home Address option MUST be placed as follows:
For each IPv6 packet header, the Home Address Option MUST NOT appear more than once. However, an encapsulated packet [RFC2473] MAY contain a separate Home Address option associated with each encapsulating IP header.
The inclusion of a Home Address destination 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 of the receiver's Binding Cache and MUST NOT cause any changes in the routing of subsequent packets sent by this receiving node.
Mobile IPv6 defines a new routing header variant, the type 2 routing header, to allow the packet to be routed directly from a correspondent to the mobile node's care-of address. The mobile node's care-of address is inserted into the IPv6 Destination Address field. Once the packet arrives at the care-of address, the mobile node retrieves its home address from the routing header, and this is used as the final destination address for the packet.
The new routing header uses a different type than defined for "regular" IPv6 source routing, enabling firewalls to apply different rules to source routed packets than to Mobile IPv6. This routing header type (type 2) is restricted to carry only one IPv6 address. All IPv6 nodes which process this routing header MUST verify that the address contained within is the node's own home address in order to prevent packets from being forwarded outside the node. The IP address contained in the routing header, since it is the mobile node's home address, MUST be a unicast routable address. Furthermore, if the scope of the home address is smaller than the scope of the care-of address, the mobile node MUST discard the packet (see Section 4.6).
The type 2 routing header has the following format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len=2 | Routing Type=2|Segments Left=1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Home Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
For a type 2 routing header, the Hdr Ext Len MUST be 2. The Segments Left value describes the number of route segments remaining; i.e., number of explicitly listed intermediate nodes still to be visited before reaching the final destination. Segments Left MUST be 1. The ordering rules for extension headers in an IPv6 packet are described in Section 4.1 of RFC 2460 [RFC2460]. The type 2 routing header defined for Mobile IPv6 follows the same ordering as other routing headers. If another routing header is present along with a type 2 routing header, the type 2 routing header should follow the other routing header. A packet containing such nested encapsulation should be created as if the inner (type 2) routing header was constructed first and then treated as an original packet by header construction process for the other routing header.
In addition, the general procedures defined by IPv6 for routing headers suggest that a received routing header MAY be automatically "reversed" to construct a routing header for use in any response packets sent by upper-layer protocols, if the received packet is authenticated [6]. This MUST NOT be done automatically for type 2 routing headers.
The ICMP Home Agent Address Discovery Request message is used by a mobile node to initiate the dynamic home agent address discovery mechanism, as described in Section 11.4.1. The mobile node sends the Home Agent Address Discovery Request message to the Mobile IPv6 Home-Agents anycast address [RFC2526] for its own home subnet prefix. (Note that the currently defined anycast addresses may not work with all prefix lengths other than those defined in RFC 4291 [RFC4291] [RFC3627].)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Source Address of the Home Agent Address Discovery Request message packet is typically one of the mobile node's current care-of addresses. At the time of performing this dynamic home agent address discovery procedure, it is likely that the mobile node is not registered with any home agent. Therefore, neither the nature of the address nor the identity of the mobile node can be established at this time. The home agent MUST then return the Home Agent Address Discovery Reply message directly to the Source Address chosen by the mobile node.
The ICMP Home Agent Address Discovery Reply message is used by a home agent to respond to a mobile node that uses the dynamic home agent address discovery mechanism, as described in Section 10.5.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
. .
. Home Agent Addresses .
. .
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The ICMP Mobile Prefix Solicitation Message is sent by a mobile node to its home agent while it is away from home. The purpose of the message is to solicit a Mobile Prefix Advertisement from the home agent, which will allow the mobile node to gather prefix information about its home network. This information can be used to configure and update home address(es) according to changes in prefix information supplied by the home agent.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP Fields:
ICMP Fields:
The Mobile Prefix Solicitation messages may have options. These options MUST use the option format defined in Neighbor Discovery (RFC 4861 [RFC4861]). This document does not define any option types for the Mobile Prefix Solicitation message, but future documents may define new options. Home agents MUST silently ignore any options they do not recognize and continue processing the message.
A home agent will send a Mobile Prefix Advertisement to a mobile node to distribute prefix information about the home link while the mobile node is traveling away from the home network. This will occur in response to a Mobile Prefix Solicitation with an Advertisement, or by an unsolicited Advertisement sent according to the rules in Section 10.6.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier |M|O| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP Fields:
ICMP Fields:
The Mobile Prefix Advertisement messages may have options. These options MUST use the option format defined in Neighbor Discovery (RFC 4861 [RFC4861]). This document defines one option which may be carried in a Mobile Prefix Advertisement message, but future documents may define new options. Mobile nodes MUST silently ignore any options they do not recognize and continue processing the message.
If the Advertisement is sent in response to a Mobile Prefix Solicitation, the home agent MUST copy the Identifier value from that message into the Identifier field of the Advertisement.
The home agent MUST NOT send more than one Mobile Prefix Advertisement message per second to any mobile node.
The M and O bits MUST be cleared if the Home Agent DHCPv6 support is not provided. If such support is provided then they are set in concert with the home network's administrative settings.
Mobile IPv6 modifies the format of the Router Advertisement message [RFC4861] by the addition of a single flag bit to indicate that the router sending the Advertisement message is serving as a home agent on this link. 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 [RFC4861]:
Mobile IPv6 requires knowledge of a router's global address in building a Home Agents List as part of the dynamic home agent address discovery mechanism.
However, Neighbor Discovery [RFC4861] 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 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 [RFC4861]:
In a Router Advertisement, a home agent MUST, and all other routers MAY, include at least one Prefix Information option with the Router Address (R) bit set. Neighbor Discovery (RFC 4861 [RFC4861]) specifies that, when 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 Neighbor Discovery options. Also, when sending unsolicited multicast Router Advertisements more frequently than the limit specified in RFC 4861, the sending router need not include all options in each of these Advertisements. However, in both of these cases the router SHOULD include at least one Prefix Information option with the Router Address (R) bit set in each such advertisement, if this bit is set in some advertisement sent by the router.
In addition, the following requirement can assist mobile nodes in movement detection. Barring changes in the prefixes for the link, routers that send multiple Router Advertisements with the Router Address (R) bit set in some of the included Prefix Information options SHOULD provide at least one option and router address which stays the same in all of the Advertisements.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 11.5.1.
This option MUST be silently ignored for other Neighbor Discovery messages.
Mobile IPv6 defines a new Home Agent Information option, used in Router Advertisements 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 (see Section 7.1) is not set. If this option is not included in a Router Advertisement in which the Home Agent (H) bit is set, the lifetime for this home agent MUST be considered to be the same as the Router Lifetime in the Router Advertisement. If multiple Advertisements are being sent instead of a single larger unsolicited multicast Advertisement, all of the multiple Advertisements with the Router Address (R) bit set MUST include this option with the same contents, otherwise this option MUST be omitted from all Advertisements.
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.
The Neighbor Discovery protocol specification [RFC4861] 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:
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.
One method which can provide for faster movement detection, is to increase the rate at which unsolicited Router Advertisements are sent. Mobile IPv6 relaxes this limit such that routers MAY send unsolicited multicast Router Advertisements more frequently. This method can be applied 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).
Routers supporting mobility SHOULD be able to be configured with a smaller MinRtrAdvInterval value and MaxRtrAdvInterval value to allow sending of unsolicited multicast Router Advertisements more often. The minimum allowed values are:
In the case where the minimum intervals and delays are used, the mean time between unsolicited multicast router advertisements is 50ms. Use of these modified limits MUST be configurable (see also the configuration variable MinDelayBetweenRas in Section 13 which may also have to be modified accordingly). Systems where these values are available MUST NOT default to them, and SHOULD default to values specified in Neighbor Discovery (RFC 4861 [RFC4861]). Knowledge of the type of network interface and operating environment SHOULD be taken into account in configuring these limits for each network interface. This is important with some wireless links, where increasing the frequency of multicast beacons can cause considerable overhead. Routers SHOULD adhere to the intervals specified in RFC 4861 [RFC4861], if this overhead is likely to cause service degradation.
Additionally, the possible low values of MaxRtrAdvInterval may cause some problems with movement detection in some mobile nodes. To ensure that this is not a problem, Routers SHOULD add 20ms to any Advertisement Intervals sent in RAs, which are below 200 ms, in order to account for scheduling granularities on both the MN and the Router.
Note that multicast Router Advertisements are not always required in certain wireless networks that have limited bandwidth. Mobility detection or link changes in such networks may be done at lower layers. Router advertisements in such networks SHOULD be sent only when solicited. In such networks it SHOULD be possible to disable unsolicited multicast Router Advertisements on specific interfaces. The MinRtrAdvInterval and MaxRtrAdvInterval in such a case can be set to some high values.
Home agents MUST include the Source Link-Layer Address option in all Router Advertisements they send. This simplifies the process of returning home, as discussed in Section 11.5.5.
Note that according to Neighbor Discovery (RFC 4861 [RFC4861]), AdvDefaultLifetime is by default based on the value of MaxRtrAdvInterval. AdvDefaultLifetime is used in the Router Lifetime field of Router Advertisements. Given that this field is expressed in seconds, a small MaxRtrAdvInterval value can result in a zero value for this field. To prevent this, routers SHOULD keep AdvDefaultLifetime in at least one second, even if the use of MaxRtrAdvInterval would result in a smaller value.
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.
The requirements are set for the following groups of nodes:
It is outside the scope of this specification to specify which of these groups are mandatory in IPv6. We only describe what is mandatory for a node that supports, for instance, route optimization. Other specifications are expected to define the extent of IPv6.
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. There are no Mobile IPv6 specific MUST requirements for such nodes, and basic IPv6 techniques are sufficient. If a mobile node attempts to set up route optimization with a node with only basic IPv6 support, an ICMP error will signal that the node does not support such optimizations (Section 11.3.5), and communications will flow through the home agent .
An IPv6 node MUST NOT support the Home Address destination option, type 2 routing header, or the Mobility Header unless it fully supports the requirements listed in the next sections for either route optimization, mobile node, or home agent functionality.
Nodes that implement route optimization are a subset of all IPv6 nodes on the Internet. The ability of a correspondent node to participate in route optimization is essential for the efficient operation of the IPv6 Internet, for the following reasons:
These effects combine to enable much better performance and robustness for communications between mobile nodes and IPv6 correspondent nodes. Route optimization introduces a small amount of additional state for the peers, some additional messaging, and up to 1.5 roundtrip delays before it can be turned on. However, it is believed that the benefits far outweigh the costs in most cases. Section 11.3.1 discusses how mobile nodes may avoid route optimization for some of the remaining cases, such as very short-term communications.
The following requirements apply to all correspondent nodes that support route optimization:
All IPv6 routers, even those not serving as a home agent for Mobile IPv6, have an effect on how well mobile nodes can communicate:
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 that serve as a home agent:
Finally, the following requirements apply to all IPv6 nodes capable of functioning as mobile nodes:
IPv6 nodes with route optimization support maintain a Binding Cache of bindings for other nodes. A separate Binding Cache SHOULD be maintained by each IPv6 node for each of its unicast routable addresses. 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 [RFC4861]. When sending a packet, the Binding Cache is searched before the Neighbor Discovery conceptual Destination Cache [RFC4861].
Each Binding Cache entry conceptually contains the following fields:
Binding Cache entries not marked as home registrations MAY be replaced at any time by any reasonable local cache replacement policy but SHOULD NOT be unnecessarily deleted. The Binding Cache for any one of a node's IPv6 addresses 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.
Mobility Header processing MUST observe the following rules:
Subsequent checks depend on the particular Mobility Header.
This section describes how the correspondent node sends packets to the mobile node, and receives packets from it.
Packets containing a Home Address option MUST be dropped if the given home address is not a unicast routable address.
Mobile nodes can include a Home Address destination option in a packet if they believe the correspondent node has a Binding Cache entry for the home address of a mobile node. If the Next Header value of the Destination Option is one of the following: {50 (ESP), 51 (AH), 135 (Mobility Header)}, the packet SHOULD be processed normally. Otherwise, the packet MUST be dropped if there is no corresponding Binding Cache entry. A corresponding Binding Cache entry MUST have the same home address as appears in the Home Address destination option, and the currently registered care-of address MUST be equal to the source address of the packet.
If the packet is dropped due to the above tests, the correspondent node MUST send the Binding Error message as described in Section 9.3.3. The Status field in this message should be set to 1 (unknown binding for Home Address destination option).
The correspondent node MUST process the option in a manner consistent with exchanging the Home Address field from the Home Address option into the IPv6 header and replacing the original value of the Source Address field there. After all IPv6 options have been processed, it MUST be possible for upper layers to process the packet without the knowledge that it came originally from a care-of address or that a Home Address option was used.
The use of IPsec Authentication Header (AH) for the Home Address option is not required, except that if the IPv6 header of a packet is covered by AH, then the 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 AH computation. 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.
When attempting to verify AH authentication data in a packet that contains a Home Address option, the receiving node MUST calculate the AH authentication data as if the following were true: The Home Address option contains the care-of address, and the source IPv6 address field of the IPv6 header contains the home address. This conforms with the calculation specified in Section 11.3.2.
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 type 2 routing header to route the packet to this mobile node (the destination node) by way of its care-of address. However, the sending node MUST NOT do this in the following cases:
When calculating authentication data in a packet that contains a type 2 routing header, the correspondent node MUST calculate the AH authentication data as if the following were true: The routing header contains the care-of address, the destination IPv6 address field of the IPv6 header contains the home address, and the Segments Left field is zero. The IPsec Security Policy Database lookup MUST based on the mobile node's home address.
For instance, assuming there are no additional routing headers in this packet beyond those needed by Mobile IPv6, the correspondent node could set the fields in the packet's IPv6 header and routing header as follows:
The IP layer will insert the routing header before performing any necessary IPsec processing. Once all IPsec processing has been performed, the node swaps the IPv6 destination field with the Home Address field in the routing header, sets the Segments Left field to one, and sends the packet. This ensures the AH calculation is done on the packet in the form it will have on the receiver after advancing the routing header.
Following the definition of a type 2 routing header in Section 6.4, 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).
Note that following the above conceptual model in an implementation creates some additional requirements for path MTU discovery since the layer that determines the packet size (e.g., TCP and applications using UDP) needs to be aware of the size of the headers added by the IP layer on the sending node.
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 to the mobile node's current primary care-of address.
Section 9.2 and Section 9.3.1 describe error conditions that lead to a need to send a Binding Error message.
A Binding Error message is sent directly to the address that appeared in the IPv6 Source Address field of the offending packet. If the Source Address field does not contain a unicast address, the Binding Error message MUST NOT be sent.
The Home Address field in the Binding Error message MUST be copied from the Home Address field in the Home Address destination option of the offending packet, or set to the unspecified address if no such option appeared in the packet.
Note that the IPv6 Source Address and Home Address field values discussed above are the values from the wire, i.e., before any modifications possibly performed as specified in Section 9.3.1.
Binding Error messages SHOULD be subject to rate limiting in the same manner as is done for ICMPv6 messages [RFC4443].
When the correspondent node has a Binding Cache entry for a mobile node, all traffic destined to the mobile node goes directly to the current care-of address of the mobile node using a routing header. Any ICMP error message caused by packets on their way to the care-of address will be returned in the normal manner 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 through the mobile node's home link. Any ICMP error message caused by the packet on its way to the mobile node while in the tunnel, will be transmitted to the mobile node's home agent. By the definition of IPv6 encapsulation [RFC2473], the home agent MUST relay certain ICMP error messages back to the original sender of the packet, which in this case is the correspondent node.
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. Note that if the mobile node continues to send packets with the Home Address destination option to this correspondent node, they will be dropped due to the lack of a binding. For this reason it is important that only persistent ICMP messages lead to the deletion of the Binding Cache entry.
This subsection specifies actions taken by a correspondent node during the return routability procedure.
Upon receiving a Home Test Init message, the correspondent node verifies the following:
Any packet carrying a Home Test Init message which fails to satisfy this test MUST be silently ignored.
Otherwise, in preparation for sending the corresponding Home Test Message, the correspondent node checks that it has the necessary material to engage in a return routability procedure, as specified in Section 5.2. The correspondent node MUST have a secret Kcn and a nonce. If it does not have this material yet, it MUST produce it before continuing with the return routability procedure.
Section 9.4.3 specifies further processing.
Upon receiving a Care-of Test Init message, the correspondent node verifies the following:
Any packet carrying a Care-of Test Init message which fails to satisfy this test MUST be silently ignored.
Otherwise, in preparation for sending the corresponding Care-of Test Message, the correspondent node checks that it has the necessary material to engage in a return routability procedure in the manner described in Section 9.4.1.
Section 9.4.4 specifies further processing.
The correspondent node creates a home keygen token and uses the current nonce index as the Home Nonce Index. It then creates a Home Test message (Section 6.1.5) and sends it to the mobile node at the latter's home address.
The correspondent node creates a care-of keygen token and uses the current nonce index as the Care-of Nonce Index. It then creates a Care-of Test message (Section 6.1.6) and sends it to the mobile node at the latter's care-of address.
This section explains how the correspondent node processes messages related to bindings. These messages are:
Before accepting a Binding Update, the receiving node MUST validate the Binding Update according to the following tests:
When the Home Registration (H) bit is not set, the following are also required:
If the Home Registration (H) bit is set, the Nonce Indices mobility option MUST NOT be present.
If the mobile node sends a sequence number which is not greater than the sequence number from the last valid Binding Update for this home address, then the receiving node MUST send back a Binding Acknowledgement with status code 135, and the last accepted sequence number in the Sequence Number field of the Binding Acknowledgement.
If a binding already exists for the given home address and the home registration flag has a different value than the Home Registration (H) bit in the Binding Update, then the receiving node MUST send back a Binding Acknowledgement with status code 139 (registration type change disallowed). The home registration flag stored in the Binding Cache entry MUST NOT be changed.
If the receiving node no longer recognizes the Home Nonce Index value, Care-of Nonce Index value, or both values from the Binding Update, then the receiving node MUST send back a Binding Acknowledgement with status code 136, 137, or 138, respectively.
Packets carrying Binding Updates that fail to satisfy all of these tests for any reason other than insufficiency of the Sequence Number, registration type change, or expired nonce index values, MUST be silently discarded.
If the Binding Update is valid according to the tests above, then the Binding Update is processed further as follows:
The specified care-of address MUST be determined as follows:
The home address for the binding MUST be determined as follows:
This section describes the processing of a valid Binding Update that requests a node to cache a 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 home address, or update its existing Binding Cache entry for this home address, if such an entry already exists. The lifetime for the Binding Cache entry is initialized from the Lifetime field specified in the Binding Update, although this lifetime MAY be reduced by the node caching the binding; the lifetime for the Binding Cache entry MUST NOT be greater than the Lifetime value specified in the Binding Update. Any Binding Cache entry MUST be deleted after the expiration of its lifetime.
Note that if the mobile node did not request a Binding Acknowledgement, then it is not aware of the selected shorter lifetime. The mobile node may thus use route optimization and send packets with the Home Address destination option. As discussed in Section 9.3.1, such packets will be dropped if there is no binding. This situation is recoverable, but can cause temporary packet loss.
The correspondent node MAY refuse to accept a new Binding Cache entry if it does not have sufficient resources. A new entry MAY also be refused if the correspondent node believes its resources are utilized more efficiently in some other purpose, such as serving another mobile node with higher amount of traffic. In both cases the correspondent node SHOULD return a Binding Acknowledgement with status value 130.
This section describes the processing of a valid Binding Update that requests a node to delete a binding when the Home Registration (H) bit is not set in the Binding Update.
Any existing binding for the given home address MUST be deleted. A Binding Cache entry for the home address MUST NOT be created in response to receiving the Binding Update.
If the Binding Cache entry was created by use of return routability nonces, the correspondent node MUST ensure that the same nonces are not used again with the particular home and care-of address. If both nonces are still valid, the correspondent node has to remember the particular combination of nonce indexes, addresses, and sequence number as illegal until at least one of the nonces has become too old.
A Binding Acknowledgement may be sent to indicate receipt of a Binding Update as follows:
If the node accepts the Binding Update and creates or updates an entry for this binding, the Status field in the Binding Acknowledgement MUST be set to a value less than 128. Otherwise, the Status field MUST be set to a value greater than or equal to 128. Values for the Status field are described in Section 6.1.8 and in the IANA registry of assigned numbers [RFC3232].
If the Status field in the Binding Acknowledgement contains the value 136 (expired home nonce index), 137 (expired care-of nonce index), or 138 (expired nonces) then the message MUST NOT include the Binding Authorization Data mobility option. Otherwise, the Binding Authorization Data mobility option MUST be included, and MUST meet the specific authentication requirements for Binding Acknowledgements as defined in Section 5.2.
If the Source Address field of the IPv6 header that carried the Binding Update does not contain a unicast address, the Binding Acknowledgement MUST NOT be sent and the Binding Update packet MUST be silently discarded. Otherwise, the acknowledgement MUST be sent to the Source Address. Unlike the treatment of regular packets, this addressing procedure does not use information from the Binding Cache. However, a routing header is needed in some cases. If the Source Address is the home address of the mobile node, i.e., the Binding Update did not contain a Home Address destination option, then the Binding Acknowledgement MUST be sent to that address and the routing header MUST NOT be used. Otherwise, the Binding Acknowledgement MUST be sent using a type 2 routing header which contains the mobile node's home address.
If a Binding Cache entry being deleted is still in active use when sending packets to a mobile node, then the next packet sent to the mobile node will be routed normally to the mobile node's home link. Communication with the mobile node continues, but the tunneling from the home network 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 Refresh Request message to the mobile node in an attempt to avoid this overhead and latency due to deleting and recreating the Binding Cache entry. This message is always sent to the home address of the mobile node.
The correspondent node MAY retransmit Binding Refresh Request messages as long as the rate limitation is applied. The correspondent node MUST stop retransmitting when it receives a Binding Update.
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. 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.
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.
A node MAY choose to drop any entry already in its Binding Cache in order to make space for a new entry. For example, a "least-recently used" (LRU) strategy for cache entry replacement among entries should work well, unless the size of the Binding Cache is substantially insufficient. When entries are deleted, the correspondent node MUST follow the rules in Section 5.2.8 in order to guard the return routability procedure against replay attacks.
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 through the mobile node's home link. The mobile node can detect this and establish a new binding if necessary.
However, if the mobile node believes that the binding still exists, it may use route optimization and send packets with the Home Address destination option. This can create temporary packet loss, as discussed earlier, in the context of binding lifetime reductions performed by the correspondent node (Section 9.5.2).
Each home agent MUST maintain a Binding Cache and Home Agents List.
The rules for maintaining a Binding Cache are the same for home agents and correspondent nodes and have already been described in Section 9.1.
The Home Agents List is maintained by each home agent, recording information about each router on the same link that is acting as a home agent. This list is used by the dynamic home agent address discovery mechanism. A router is known to be acting as a home agent, if it sends a Router Advertisement in which the Home Agent (H) bit is set. When the lifetime for a list entry (defined below) expires, that entry is removed from the Home Agents List. The Home Agents List is similar to the Default Router List conceptual data structure maintained by each host for Neighbor Discovery [RFC4861]. The Home Agents List MAY be implemented in any manner consistent with the external behavior described in this document.
Each home agent maintains a separate Home Agents List for each link on which it is serving as a home agent. A new entry is created or an existing entry is updated in response to receipt of a valid Router Advertisement in which the Home Agent (H) bit is set. Each Home Agents List entry conceptually contains the following fields:
All IPv6 home agents MUST observe the rules described in Section 9.2 when processing Mobility Headers.
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 9.5.1. Furthermore, it MUST authenticate the Binding Update as described in Section 5.1. An authorization step specific for the home agent is also needed to ensure that only the right node can control a particular home address. This is provided through the home address unequivocally identifying the security association that must be used.
This section describes the processing of a valid and authorized Binding Update when it requests the registration of the mobile node's primary care-of address.
To begin processing the Binding Update, the home agent MUST perform the following sequence of tests:
If home agent accepts the Binding Update, it MUST then create a new entry in its Binding Cache for this mobile node or update its existing Binding Cache entry, if such an entry already exists. The Home Address field as received in the Home Address option provides the home address of the mobile node.
The home agent MUST mark this Binding Cache entry as a home registration to indicate that the node is serving as a home agent for this binding. Binding Cache entries marked as a home registration MUST be excluded from the normal cache replacement policy used for the Binding Cache (Section 9.6) and MUST NOT be removed from the Binding Cache until the expiration of the Lifetime period.
Unless this home agent already has a binding for the given home address, the home agent MUST perform Duplicate Address Detection [RFC4862] on the mobile node's home link before returning the Binding Acknowledgement. This ensures that no other node on the home link was using the mobile node's home address when the Binding Update arrived. If this Duplicate Address Detection fails for the given home address or an associated link local address, then the home agent MUST reject the complete Binding Update and MUST return a Binding Acknowledgement to the mobile node, in which the Status field is set to 134 (Duplicate Address Detection failed). When the home agent sends a successful Binding Acknowledgement to the mobile node, the home agent assures to the mobile node that its address(es) will be kept unique by the home agent for as long as the lifetime was granted for the binding.
The specific addresses, which are to be tested before accepting the Binding Update and later to be defended by performing Duplicate Address Detection, depend on the setting of the Link-Local Address Compatibility (L) bit, as follows:
The lifetime of the Binding Cache entry depends on a number of factors:
Regardless of the setting of the Acknowledge (A) bit in the Binding Update, the home agent MUST return a Binding Acknowledgement to the mobile node constructed as follows:
The rules for selecting the Destination IP address (and possibly routing header construction) for the Binding Acknowledgement to the mobile node are the same as in Section 9.5.4.
In addition, the home agent MUST follow the procedure defined in Section 10.4.1 to intercept packets on the mobile node's home link addressed to the mobile node, while the home agent is serving as the home agent for this mobile node. The home agent MUST also be prepared to accept reverse tunneled packets from the new care-of address of the mobile node, as described in Section 10.4.5. Finally, the home agent MUST also propagate new home network prefixes, as described in Section 10.6.
A binding may need to be de-registered when the mobile node returns home or when the mobile node knows that it will not have any care-of addresses in the visited network.
A Binding Update is validated and authorized in the manner described in the previous section; note that when the mobile node de-registers when it is at home, it MAY choose to omit the Home Address destination option, in which case the mobile node's home address is the source IP address of the de-registration Binding Update. This section describes the processing of a valid Binding Update that requests the receiving node to no longer serve as its home agent, de-registering its primary care-of address.
To begin processing the Binding Update, the home agent MUST perform the following test:
If the home agent does not reject the Binding Update as described above, then the home agent MUST return a Binding Acknowledgement to the mobile node, constructed as follows:
The rules for selecting the Destination IP address (and, if required, routing header construction) for the Binding Acknowledgement to the mobile node are the same as in the previous section. When the Status field in the Binding Acknowledgement is greater than or equal to 128 and the Source Address of the Binding Update is on the home link, and the Binding Update came from a mobile node on the same link, the home agent MUST send it to the mobile node's link layer address (retrieved either from the Binding Update or through Neighbor Solicitation).
When a mobile node sends a Binding Update to refresh the binding from the visited link and soon after moves to the home link and sends a de-registration Binding Update, a race condition can happen if the first Binding Update gets delayed. The delayed Binding Update can cause the home agent to create a new Binding Cache entry for a mobile node that had just attached to the home link and successfully deleted the binding. This would prevent the mobile node from using its home address from the home link.
In order to prevent this, the home agent SHOULD NOT remove the Binding Cache entry immediately after receiving the deregistration Binding Update from the mobile node. It SHOULD mark the Binding Cache entry as invalid, and MUST stop intercepting packets on the mobile node's home link that are addressed to the mobile node (Section 10.4.1). The home agent should wait for MAX_DELETE_BCE_TIMEOUT (Section 12) seconds before removing the Binding Cache entry completely. In the scenario described above, if the home agent receives the delayed Binding Update that the mobile node sent from the visited link, it would reject the message since the sequence number would be less than the last received deregistration Binding Update from the home link. The home agent would then send a Binding Acknowledgment with status '135' (Sequence number out of window) to the care of address on the visited link. The mobile node can continue using the home address from the home link.
While a node is serving as the home agent for a mobile node it MUST attempt to intercept packets on the mobile node's home link that are addressed to the mobile node.
In order to do this, when a node begins serving as the home agent it MUST have performed Duplicate Address Detection (as specified in Section 10.3.1), and subsequently it MUST multicast onto the home link a Neighbor Advertisement message [RFC4861] on behalf of the mobile node. For the home address specified in the Binding Update, the home agent sends a Neighbor Advertisement message [RFC4861] to the all-nodes multicast address on the home link to advertise the home agent's own link-layer address for this IP address on behalf of the mobile node. If the Link-Layer Address Compatibility (L) flag has been specified in the Binding Update, the home agent MUST do the same for the link-local address of the mobile node.
All fields in each Neighbor Advertisement message SHOULD be set in the same way they would be set by the mobile node if it was sending this Neighbor Advertisement [RFC4861] while at home, with the following exceptions:
Any node on the home link that receives one of the Neighbor Advertisement messages (described above) will update its Neighbor Cache to associate the mobile node's address with the home agent's link layer address, causing it to transmit any future packets normally destined to the mobile node to the mobile node's home agent. Since multicasting on the local link (such as Ethernet) is typically not guaranteed to be reliable, the home agent MAY retransmit this Neighbor Advertisement message up to MAX_NEIGHBOR_ADVERTISEMENT (see [RFC4861]) times to increase its reliability. It is still possible that some nodes on the home link will not receive any of the Neighbor Advertisements, but these nodes will eventually be able to detect the link-layer address change for the mobile node's address through use of Neighbor Unreachability Detection [RFC4861].
While a node is serving as a home agent for some mobile node, the home agent uses IPv6 Neighbor Discovery [RFC4861] to intercept unicast packets on the home link addressed to the mobile node. In order to intercept packets in this way, the home agent MUST act as a proxy for this mobile node and reply to any received Neighbor Solicitations for it. When a home agent receives a Neighbor Solicitation, it MUST check if the Target Address specified in the message matches the address of any mobile node for which it has a Binding Cache entry marked as a home registration.
If such an entry exists in the home agent's Binding Cache, the home agent MUST reply to the Neighbor Solicitation with a Neighbor Advertisement giving the home agent's own link-layer address as the link-layer address for the specified Target Address. In addition, the Router (R) bit in the Advertisement MUST be set to zero. Acting as a proxy in this way allows other nodes on the mobile node's home link to resolve the mobile node's address and for the home agent to defend these addresses on the home link for Duplicate Address Detection [RFC4861].
For any packet sent to a mobile node from the mobile node's home agent (in which the home agent is the original sender of the packet), the home agent is operating as a correspondent node of the mobile node for this packet and the procedures described in Section 9.3.2 apply. The home agent then uses a routing header to route the packet to the mobile node by way of the primary care-of address in the home agent's Binding Cache.
While the mobile node is away from home, the home agent intercepts any packets on the home link addressed to the mobile node's home address, as described in Section 10.4.1. In order to forward each intercepted packet to the mobile node, the home agent MUST tunnel the packet to the mobile node using IPv6 encapsulation [RFC2473]. When a home agent encapsulates an intercepted packet for forwarding to the mobile node, the home agent sets the Source Address in the new tunnel IP header to the home agent's own IP address and sets the Destination Address in the tunnel IP header to the mobile node's primary care-of address. When received by the mobile node, normal processing of the tunnel header [RFC2473] will result in decapsulation and processing of the original packet by the mobile node.
However, packets addressed to the mobile node's link-local address MUST NOT be tunneled to the mobile node. Instead, these packets MUST be discarded and the home agent SHOULD return an ICMP Destination Unreachable, Code 3, message to the packet's Source Address (unless this Source Address is a multicast address).
Interception and tunneling of the following multicast addressed packets on the home network are only done if the home agent supports multicast group membership control messages from the mobile node as described in the next section. Tunneling of multicast packets to a mobile node follows similar limitations to those defined above for unicast packets addressed to the mobile node's link-local address. Multicast packets addressed to a multicast address with link-local scope [RFC4291], to which the mobile node is subscribed, MUST NOT be tunneled to the mobile node. These packets SHOULD be silently discarded (after delivering to other local multicast recipients). Multicast packets addressed to a multicast address with a scope larger than link-local, but smaller than global (e.g., site-local and organization-local [RFC4291]), to which the mobile node is subscribed, SHOULD NOT be tunneled to the mobile node. Multicast packets addressed with a global scope, to which the mobile node has successfully subscribed, MUST be tunneled to the mobile node.
Before tunneling a packet to the mobile node, the home agent MUST perform any IPsec processing as indicated by the security policy data base.
This section is a prerequisite for the multicast data packet forwarding, described in the previous section. If this support is not provided, multicast group membership control messages are silently ignored.
In order to forward multicast data packets from the home network to all the proper mobile nodes, the home agent SHOULD be capable of receiving tunneled multicast group membership control information from the mobile node in order to determine which groups the mobile node has subscribed to. These multicast group membership messages are Listener Report messages specified in MLD [RFC2710] or in other protocols such as [RFC3810].
The messages are issued by the mobile node, but sent through the reverse tunnel to the home agent. These messages are issued whenever the mobile node decides to enable reception of packets for a multicast group or in response to an MLD Query from the home agent. The mobile node will also issue multicast group control messages to disable reception of multicast packets when it is no longer interested in receiving multicasts for a particular group.
To obtain the mobile node's current multicast group membership the home agent must periodically transmit MLD Query messages through the tunnel to the mobile node. These MLD periodic transmissions will ensure the home agent has an accurate record of the groups in which the mobile node is interested despite packet losses of the mobile node's MLD group membership messages.
All MLD packets are sent directly between the mobile node and the home agent. Since all of these packets are destined to a link-scope multicast address and have a hop limit of 1, there is no direct forwarding of such packets between the home network and the mobile node. The MLD packets between the mobile node and the home agent are encapsulated within the same tunnel header used for other packet flows between the mobile node and home agent.
Note that at this time, even though a link-local source is used on MLD packets, no functionality depends on these addresses being unique, nor do they elicit direct responses. All MLD messages are sent to multicast destinations. To avoid ambiguity on the home agent, due to mobile nodes which may choose identical link-local source addresses for their MLD function, it is necessary for the home agent to identify which mobile node was actually the issuer of a particular MLD message. This may be accomplished by noting which tunnel such an MLD arrived by, which IPsec SA was used, or by other distinguishing means.
This specification puts no requirement on how the functions in this section and the multicast forwarding in Section 10.4.2 are to be achieved. At the time of this writing it was thought that a full IPv6 multicast router function would be necessary on the home agent, but it may be possible to achieve the same effects through a "proxy MLD" application coupled with kernel multicast forwarding. This may be the subject of future specifications.
This section describes how home agents support the use of stateful address autoconfiguration mechanisms such as DHCPv6 [RFC3315] from the mobile nodes. If this support is not provided, then the M and O bits must remain cleared on the Mobile Prefix Advertisement Messages. Any mobile node which sends DHCPv6 messages to the home agent without this support will not receive a response.
If DHCPv6 is used, packets are sent with link-local source addresses either to a link-scope multicast address or a link-local address. Mobile nodes desiring to locate a DHCPv6 service may reverse tunnel standard DHCPv6 packets to the home agent. Since these link-scope packets cannot be forwarded onto the home network, it is necessary for the home agent to either implement a DHCPv6 relay agent or a DHCPv6 server function itself. The arriving tunnel or IPsec SA of DHCPv6 link-scope messages from the mobile node must be noted so that DHCPv6 responses may be sent back to the appropriate mobile node. DHCPv6 messages sent to the mobile node with a link-local destination must be tunneled within the same tunnel header used for other packet flows.
Unless a binding has been established between the mobile node and a correspondent node, traffic from the mobile node to the correspondent node goes through a reverse tunnel. Home agents MUST support reverse tunneling as follows:
The return routability procedure, described in Section 5.2.5, assumes that the confidentiality of the Home Test Init and Home Test messages is protected as they are tunneled between the home agent and the mobile node. Therefore, the home agent MUST support tunnel mode IPsec ESP for the protection of packets belonging to the return routability procedure. Support for a non-null encryption transform and authentication algorithm MUST be available. It is not necessary to distinguish between different kinds of packets during the return routability procedure.
Security associations are needed to provide this protection. When the care-of address for the mobile node changes as a result of an accepted Binding Update, special treatment is needed for the next packets sent using these security associations. The home agent MUST set the new care-of address as the destination address of these packets, as if the outer header destination address in the security association had changed.
The above protection SHOULD be used with all mobile nodes. The use is controlled by configuration of the IPsec security policy database both at the mobile node and at the home agent.
As described earlier, the Binding Update and Binding Acknowledgement messages require protection between the home agent and the mobile node. The Mobility Header protocol carries both these messages as well as the return routability messages. From the point of view of the security policy database these messages are indistinguishable. When IPsec is used to protect return routability signaling or payload packets, this protection MUST only be applied to the return routability packets entering the IPv6 encapsulated tunnel interface between the mobile node and the home agent. This can be achieved, for instance, by defining the security policy database entries specifically for the tunnel interface. That is, the policy entries are not generally applied on all traffic on the physical interface(s) of the nodes, but rather only on traffic that enters the tunnel. This makes use of per-interface security policy database entries [RFC4301] specific to the tunnel interface (the node's attachment to the tunnel [RFC2460]).
This section describes an optional mechanism by which a home agent can help mobile nodes to discover the addresses of other home agents on the mobile node's home network. The home agent keeps track of the other home agents on the same link and responds to queries sent by the mobile node.
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; the mobile node uses the list as described in Section 11.4.1. The information for the list is learned through receipt of the periodic unsolicited multicast Router Advertisements, in a manner similar to the Default Router List conceptual data structure maintained by each host for Neighbor Discovery [RFC4861]. In the construction of the Home Agents List, the Router Advertisements are from each (other) home agent on the link and the Home Agent (H) bit is set in them.
On receipt of a valid Router Advertisement, as defined in the processing algorithm specified for Neighbor Discovery [RFC4861], the home agent performs the following steps in addition to any steps already required of it by Neighbor Discovery:
A home agent SHOULD maintain an entry in its Home Agents List for each valid home agent address until that entry's lifetime expires, after which time the entry MUST be deleted.
As described in Section 11.4.1, a mobile node attempts dynamic home agent address discovery by sending an ICMP Home Agent Address Discovery Request message to the Mobile IPv6 Home-Agents anycast address [RFC2526] for its home IP subnet prefix. A home agent receiving a Home Agent Address Discovery Request message that serves this subnet SHOULD return an ICMP Home Agent Address Discovery Reply message to the mobile node with the Source Address of the Reply packet set to one of the global unicast addresses of the home agent. The Home Agent Addresses field in the Reply message is constructed as follows:
Mobile IPv6 arranges to propagate relevant prefix information to the mobile node when it is away from home, so that it may be used in mobile node home address configuration and in network renumbering. In this mechanism, mobile nodes away from home receive Mobile Prefix Advertisement messages. These messages include Prefix Information Options for the prefixes configured on the home subnet interface(s) of the home agent.
If there are multiple home agents, differences in the advertisements sent by different home agents can lead to an inability to use a particular home address when changing to another home agent. In order to ensure that the mobile nodes get the same information from different home agents, it is preferred that all of the home agents on the same link be configured in the same manner.
To support this, the home agent monitors prefixes advertised by itself and other home agents on the home link. In Neighbor Discovery (RFC 4861 [RFC4861]) it is acceptable for two routers to advertise different sets of prefixes on the same link. For home agents, the differences should be detected for a given home address because the mobile node communicates only with one home agent at a time and the mobile node needs to know the full set of prefixes assigned to the home link. All other comparisons of Router Advertisements are as specified in Section 6.2.7 of RFC 4861.
A home agent serving a mobile node will schedule the delivery of the new prefix information to that mobile node when any of the following conditions occur:
MUST:
SHOULD:
MAY:
The home agent uses the following algorithm to determine when to send prefix information to the mobile node.
The list of prefixes is sent in its entirety in all cases.
If the home agent has already scheduled the transmission of a Mobile Prefix Advertisement to the mobile node, then the home agent will replace the advertisement with a new one to be sent at the scheduled time.
Otherwise, the home agent computes a fresh value for RAND_ADV_DELAY which offsets from the current time for the scheduled transmission. First calculate the maximum delay for the scheduled Advertisement:
MaxScheduleDelay = min (MaxMobPfxAdvInterval, Preferred Lifetime),
where MaxMobPfxAdvInterval is as defined in Section 12. Then compute the final delay for the advertisement:
RAND_ADV_DELAY = MinMobPfxAdvInterval + (rand() % abs(MaxScheduleDelay - MinMobPfxAdvInterval))
Here rand() returns a random integer value in the range of 0 to the maximum possible integer value. This computation is expected to alleviate bursts of advertisements when prefix information changes. In addition, a home agent MAY further reduce the rate of packet transmission by further delaying individual advertisements, when necessary to avoid overwhelming local network resources. The home agent SHOULD periodically continue to retransmit an unsolicited Advertisement to the mobile node, until it is acknowledged by the receipt of a Mobile Prefix Solicitation from the mobile node.
The home agent MUST wait PREFIX_ADV_TIMEOUT (see Section 12) before the first retransmission and double the retransmission wait time for every succeeding retransmission until a maximum number of PREFIX_ADV_RETRIES attempts (see Section 12) has been tried. If the mobile node's bindings expire before the matching Binding Update has been received, then the home agent MUST NOT attempt any more retransmissions, even if not all PREFIX_ADV_RETRIES have been retransmitted. In the mean time, if the mobile node sends another Binding Update without returning home, then the home agent SHOULD begin transmitting the unsolicited Advertisement again.
If some condition, as described above, occurs on the home link and causes another Prefix Advertisement to be sent to the mobile node, before the mobile node acknowledges a previous transmission, the home agent SHOULD combine any Prefix Information options in the unacknowledged Mobile Prefix Advertisement into a new Advertisement. The home agent then discards the old Advertisement.
When sending a Mobile Prefix Advertisement to the mobile node, the home agent MUST construct the packet as follows:
As described in Section 10.3.1, the lifetime returned by the home agent in a Binding Acknowledgement MUST NOT be greater than the remaining valid lifetime for the subnet prefix in the mobile node's home address. This limit on the binding lifetime serves to prohibit use of a mobile node's home address after it becomes invalid.
Each mobile node MUST maintain a Binding Update List.
The Binding Update List records information for each Binding Update sent by this mobile node, in which the lifetime of the binding has not yet expired. The Binding Update List includes all bindings sent by the mobile node either to its home agent or correspondent nodes. It also contains Binding Updates which are waiting for the completion of the return routability procedure before they can be sent. However, for multiple Binding Updates sent to the same destination address, the Binding Update List contains only the most recent Binding Update (i.e., with the greatest Sequence Number value) sent to that destination. 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 Binding Update List is used to determine whether a particular packet is sent directly to the correspondent node or tunneled via the home agent (see Section 11.3.1).
The Binding Update list also conceptually contains the following data related to running the return routability procedure. This data is relevant only for Binding Updates sent to correspondent nodes.
All IPv6 mobile nodes MUST observe the rules described in Section 9.2 when processing Mobility Headers.
While a mobile node is away from home, it continues to use its home address, as well as also using one or more care-of addresses. When sending a packet while away from home, a mobile node MAY choose among these in selecting the address that it will use as the source of the packet, as follows:
Detailed operation of these cases is described later in this section and also discussed in [RFC3484].
For packets sent by a mobile node while it is at home, no special Mobile IPv6 processing is required. Likewise, if the mobile node uses any address other than one of its home addresses as the source of a packet sent while away from home, no special Mobile IPv6 processing is required. In either case, the packet is simply addressed and transmitted in the same way as any normal IPv6 packet.
For packets sent by the mobile node sent while away from home using the mobile node's home address as the source, special Mobile IPv6 processing of the packet is required. This can be done in the following two ways:
This section sketches the interaction between outbound Mobile IPv6 processing and outbound IP Security (IPsec) processing for packets sent by a mobile node while away from home. Any specific implementation MAY use algorithms and data structures other than those suggested here, but its processing MUST be consistent with the effect of the operation described here and with the relevant IPsec specifications. In the steps described below, it is assumed that IPsec is being used in transport mode [RFC4301] and that the mobile node is using its home address as the source for the packet (from the point of view of higher protocol layers or applications, as described in Section 11.3.1):
When an automated key management protocol is used to create new security associations for a peer, it is important to ensure that the peer can send the key management protocol packets to the mobile node. This may not be possible if the peer is the home agent of the mobile node and the purpose of the security associations would be to send a Binding Update to the home agent. Packets addressed to the home address of the mobile node cannot be used before the Binding Update has been processed. For the default case of using IKEv2 [RFC5996] as the automated key management protocol, such problems can be avoided by the following requirements when communicating with its home agent:
The Key Management Mobility Capability (K) bit in Binding Updates and Acknowledgements can be used to avoid the need to rerun IKEv2 upon movements.
While away from home, a mobile node will receive packets addressed to its home address, by one of two methods:
For packets received by the first method, the mobile node MUST check that the IPv6 source address of the tunneled packet is the IP address of its home agent. In this method, the mobile node may also send a Binding Update to the original sender of the packet as described in Section 11.7.2 and subject to the rate limiting defined in Section 11.8. The mobile node MUST also process the received packet in the manner defined for IPv6 encapsulation [RFC2473], which will result in the encapsulated (inner) packet being processed normally by upper-layer protocols within the mobile node as if it had been addressed (only) to the mobile node's home address.
For packets received by the second method, the following rules will result in the packet being processed normally by upper-layer protocols within the mobile node as if it had been addressed to the mobile node's home address.
A node receiving a packet addressed to itself (i.e., one of the node's addresses is in the IPv6 destination field) follows the next header chain of headers and processes them. When it encounters a type 2 routing header during this processing, it performs the following checks. If any of these checks fail, the node MUST silently discard the packet.
Once the above checks have been performed, the node swaps the IPv6 destination field with the Home Address field in the routing header, decrements segments left by one from the value it had on the wire, and resubmits the packet to IP for processing the next header. Conceptually, this follows the same model as in RFC 2460. However, in the case of type 2 routing header this can be simplified since it is known that the packet will not be forwarded to a different node.
The definition of AH requires the sender to calculate the AH integrity check value of a routing header in the same way it appears in the receiver after it has processed the header. Since IPsec headers follow the routing header, any IPsec processing will operate on the packet with the home address in the IP destination field and segments left being zero. Thus, the AH calculations at the sender and receiver will have an identical view of the packet.
A mobile node that is connected to its home link functions in the same way as any other (stationary) node. Thus, when it is at home, a mobile node functions identically to other multicast senders and receivers. Therefore, this section describes the behavior of a mobile node that is not on its home link.
In order to receive packets sent to some multicast group, a mobile node must join that multicast group. One method, in which a mobile node MAY join the group, is via a (local) multicast router on the foreign link being visited. In this case, the mobile node MUST use its care-of address and MUST NOT use the Home Address destination option when sending MLD packets [RFC2710].
Alternatively, a mobile node MAY join multicast groups via a bi-directional tunnel to its home agent. The mobile node tunnels its multicast group membership control packets (such as those defined in [RFC2710] or in [RFC3810]) to its home agent, and the home agent forwards multicast packets down the tunnel to the mobile node. A mobile node MUST NOT tunnel multicast group membership control packets until (1) the mobile node has a binding in place at the home agent, and (2) the latter sends at least one multicast group membership control packet via the tunnel. Once this condition is true, the mobile node SHOULD assume it does not change as long as the binding does not expire.
A mobile node that wishes to send packets to a multicast group also has two options:
Note that direct sending from the foreign link is only applicable while the mobile node is at that foreign link. This is because the associated multicast tree is specific to that source location and any change of location and source address will invalidate the source specific tree or branch and the application context of the other multicast group members.
This specification does not provide mechanisms to enable such local multicast session to survive hand-off and to seamlessly continue from a new care-of address on each new foreign link. Any such mechanism, developed as an extension to this specification, needs to take into account the impact of fast moving mobile nodes on the Internet multicast routing protocols and their ability to maintain the integrity of source specific multicast trees and branches.
While the use of bidirectional tunneling can ensure that multicast trees are independent of the mobile nodes movement, in some case such tunneling can have adverse affects. The latency of specific types of multicast applications (such as multicast based discovery protocols) will be affected when the round-trip time between the foreign subnet and the home agent is significant compared to that of the topology to be discovered. In addition, the delivery tree from the home agent in such circumstances relies on unicast encapsulation from the agent to the mobile node. Therefore, bandwidth usage is inefficient compared to the native multicast forwarding in the foreign multicast system.
Any node that does not recognize the Mobility header will return an ICMP Parameter Problem, Code 1, message to the sender of the packet. If the mobile node receives such an ICMP error message in response to a return routability procedure or Binding Update, it SHOULD record in its Binding Update List that future Binding Updates SHOULD NOT be sent to this destination. Such Binding Update List entries SHOULD be removed after a period of time in order to allow for retrying route optimization.
New Binding Update List entries MUST NOT be created as a result of receiving ICMP error messages.
Correspondent nodes that have participated in the return routability procedure MUST implement the ability to correctly process received packets containing a Home Address destination option. Therefore, correctly implemented correspondent nodes should always be able to recognize Home Address options. If a mobile node receives an ICMP Parameter Problem, Code 2, message from some node indicating that it does not support the Home Address option, the mobile node SHOULD log the error and then discard the ICMP message.
When a mobile node receives a packet containing a Binding Error message, it should first check if the mobile node has a Binding Update List entry for the source of the Binding Error message. If the mobile node does not have such an entry, it MUST ignore the message. This is necessary to prevent a waste of resources on, e.g., return routability procedure due to spoofed Binding Error messages.
Otherwise, if the message Status field was 1 (unknown binding for Home Address destination option), the mobile node should perform one of the following three actions:
If the message Status field was 2 (unrecognized MH Type value), the mobile node should perform one of the following two actions:
Sometimes when the mobile node needs to send a Binding Update to its home agent to register its new primary care-of address, as described in Section 11.7.1, the mobile node may not know the address of any router on its home link that can serve as a home agent for it. For example, some nodes on its home link may have been reconfigured while the mobile node has been away from home, such that the router that was operating as the mobile node's home agent has been replaced by a different router serving this role.
In this case, the mobile node MAY attempt to discover the address of a suitable home agent on its home link. To do so, the mobile node sends an ICMP Home Agent Address Discovery Request message to the Mobile IPv6 Home-Agents anycast address [RFC2526] for its home subnet prefix. As described in Section 10.5, the home agent on its home link that receives this Request message will return an ICMP Home Agent Address Discovery Reply message. This message gives the addresses for the home agents operating on the home link.
The mobile node, upon receiving this Home Agent Address Discovery Reply message, MAY then send its home registration Binding Update to any of the unicast IP addresses listed in the Home Agent Addresses field in the Reply. For example, the mobile node MAY attempt its home registration to each of these addresses, in turn, until its registration is accepted. The mobile node sends a Binding Update to an address and waits for the matching Binding Acknowledgement, moving on to the next address if there is no response. The mobile node MUST, however, wait at least InitialBindackTimeoutFirstReg seconds (see Section 13) before sending a Binding Update to the next home agent. In trying each of the returned home agent addresses, the mobile node SHOULD try each of them in the order they appear in the Home Agent Addresses field in the received Home Agent Address Discovery Reply message. In order to do this, the mobile node SHOULD store the list of home agents for later use in case the home agent currently managing the mobile node's care-of address forwarding should become unavailable. The list MAY be stored, along with any available lifetime information for the home agent addresses, in nonvolatile memory to survive reboots by the mobile node.
If the mobile node has a current registration with some home agent (the Lifetime for that registration has not yet expired), then the mobile node MUST attempt any new registration first with that home agent. If that registration attempt fails (e.g., timed out or rejected), the mobile node SHOULD then reattempt this registration with another home agent. If the mobile node knows of no other suitable home agent, then it MAY attempt the dynamic home agent address discovery mechanism described above.
If, after a mobile node transmits a Home Agent Address Discovery Request message to the Home Agents Anycast address, it does not receive a corresponding Home Agent Address Discovery Reply message within INITIAL_DHAAD_TIMEOUT (see Section 12) seconds, the mobile node MAY retransmit the same Request message to the same anycast address. This retransmission MAY be repeated up to a maximum of DHAAD_RETRIES (see Section 12) attempts. Each retransmission MUST be delayed by twice the time interval of the previous retransmission.
When a mobile node has a home address that is about to become invalid, it SHOULD send a Mobile Prefix Solicitation to its home agent in an attempt to acquire fresh routing prefix information. The new information also enables the mobile node to participate in renumbering operations affecting the home network, as described in Section 10.6.
The mobile node MUST use the Home Address destination option to carry its home address. The mobile node MUST support and SHOULD use IPsec to protect the solicitation. The mobile node MUST set the Identifier field in the ICMP header to a random value.
As described in Section 11.7.2, Binding Updates sent by the mobile node to other nodes MUST use a lifetime no greater than the remaining lifetime of its home registration of its primary care-of address. The mobile node SHOULD further limit the lifetimes that it sends on any Binding Updates to be within the remaining valid lifetime (see Section 10.6.2) for the prefix in its home address.
When the lifetime for a changed prefix decreases, and the change would cause cached bindings at correspondent nodes in the Binding Update List to be stored past the newly shortened lifetime, the mobile node MUST issue a Binding Update to all such correspondent nodes.
These limits on the binding lifetime serve to prohibit use of a mobile node's home address after it becomes invalid.
Section 10.6 describes the operation of a home agent to support boot time configuration and renumbering a mobile node's home subnet while the mobile node is away from home. The home agent sends Mobile Prefix Advertisements to the mobile node while away from home, giving "important" Prefix Information options that describe changes in the prefixes in use on the mobile node's home link.
The Mobile Prefix Solicitation is similar to the Router Solicitation used in Neighbor Discovery [RFC4861], except it is routed from the mobile node on the visited network to the home agent on the home network by usual unicast routing rules.
When a mobile node receives a Mobile Prefix Advertisement, it MUST validate it according to the following test:
Any received Mobile Prefix Advertisement not meeting these tests MUST be silently discarded.
For an accepted Mobile Prefix Advertisement, the mobile node MUST process Managed Address Configuration (M), Other Stateful Configuration (O), and the Prefix Information Options as if they arrived in a Router Advertisement [RFC4861] on the mobile node's home link. (This specification does not, however, describe how to acquire home addresses through stateful protocols.) Such processing may result in the mobile node configuring a new home address, although due to separation between preferred lifetime and valid lifetime, such changes should not affect most communications by the mobile node, in the same way as for nodes that are at home.
This specification assumes that any security associations and security policy entries that may be needed for new prefixes have been pre-configured in the mobile node. Note that while dynamic key management avoids the need to configure new security associations, it is still necessary to add policy entries to protect the communications involving the home address(es). Mechanisms for setting up these entries are outside the scope of this specification.
The primary goal of movement detection is to detect L3 handovers. This section does not attempt to specify a fast movement detection algorithm which will function optimally for all types of applications, link-layers and deployment scenarios; instead, it describes a generic method that uses the facilities of IPv6 Neighbor Discovery, including Router Discovery and Neighbor Unreachability Detection. At the time of this writing, this method is considered well enough understood to recommend for standardization, however it is expected that future versions of this specification or other specifications may contain updated versions of the movement detection algorithm that have better performance.
Generic movement detection uses Neighbor Unreachability Detection to detect when the default router is no longer bi-directionally reachable, in which case the mobile node must discover a new default router (usually on a new link). However, this detection only occurs when the mobile node has packets to send, and in the absence of frequent Router Advertisements or indications from the link-layer, the mobile node might become unaware of an L3 handover that occurred. Therefore, the mobile node should supplement this method with other information whenever it is available to the mobile node (e.g., from lower protocol layers).
When the mobile node detects an L3 handover, it performs Duplicate Address Detection [RFC4862] on its link-local address, selects a new default router as a consequence of Router Discovery, and then performs Prefix Discovery with that new router to form new care-of address(es) as described in Section 11.5.3. It then registers its new primary care-of address with its home agent as described in Section 11.7.1. After updating its home registration, the mobile node then updates associated mobility bindings in correspondent nodes that it is performing route optimization with as specified in Section 11.7.2.
Due to the temporary packet flow disruption and signaling overhead involved in updating mobility bindings, the mobile node should avoid performing an L3 handover until it is strictly necessary. Specifically, when the mobile node receives a Router Advertisement from a new router that contains a different set of on-link prefixes, if the mobile node detects that the currently selected default router on the old link is still bi-directionally reachable, it should generally continue to use the old router on the old link rather than switch away from it to use a new default router.
Mobile nodes can use the information in received Router Advertisements to detect L3 handovers. In doing so the mobile node needs to consider the following issues:
In addition, the mobile node should consider the following events as indications that an L3 handover may have occurred. Upon receiving such indications, the mobile node needs to perform Router Discovery to discover routers and prefixes on the new link, as described in Section 6.3.7 of Neighbor Discovery (RFC 4861 [RFC4861]).
When an MN detects that it has arrived on a new link using the movement detection algorithm in use (Section 11.5.1,) or on bootstrapping, it performs the following steps to determine if it is on the home link.
After detecting that it has moved a mobile node SHOULD generate a new primary care-of address using normal IPv6 mechanisms. This SHOULD also be done when the current primary care-of address becomes deprecated. A mobile node MAY form a new primary care-of address at any time, but a mobile node MUST NOT send a Binding Update about a new care-of address to its home agent more than MAX_UPDATE_RATE times within a second.
In addition, a mobile node MAY form new non-primary care-of addresses even when it has not switched to a new default router. A mobile node can have only one primary care-of address at a time (which is registered with its home agent), but it MAY have an additional care-of address for any or all of the prefixes on its current link. Furthermore, since a wireless network interface may actually allow a mobile node to be reachable on more than one link at a time (i.e., within wireless transmitter range of routers on more than one separate link), a mobile node MAY have care-of addresses on more than one link at a time. The use of more than one care-of address at a time is described in Section 11.5.4.
As described in Section 4, in order to form a new care-of address, a mobile node MAY use either stateless [RFC4862] or stateful (e.g., DHCPv6 [RFC3315]) Address Autoconfiguration. If a mobile node needs to use a source address (other than the unspecified address) in packets sent as a part of address autoconfiguration, it MUST use an IPv6 link-local address rather than its own IPv6 home address.
RFC 4862 [RFC4862] specifies that in normal processing for Duplicate Address Detection, the node SHOULD delay sending the initial Neighbor Solicitation message by a random delay between 0 and MAX_RTR_SOLICITATION_DELAY. Since delaying DAD can result in significant delays in configuring a new care-of address when the Mobile Node moves to a new link, the Mobile Node preferably SHOULD NOT delay DAD when configuring a new care-of address. The Mobile Node SHOULD delay according to the mechanisms specified in RFC 4862 unless the implementation has a behavior that desynchronizes the steps that happen before the DAD in the case that multiple nodes experience handover at the same time. Such desynchronizing behaviors might be due to random delays in the L2 protocols or device drivers, or due to the movement detection mechanism that is used.
As described in Section 11.5.3, a mobile node MAY use more than one care-of address at a time. Particularly in the case of many wireless networks, a mobile node effectively might be reachable through multiple links at the same time (e.g., with overlapping wireless cells), on which different on-link subnet prefixes may exist. The mobile node MUST ensure that its primary care-of address always has a prefix that is advertised by its current default router. After selecting a new primary care-of address, the mobile node MUST send a Binding Update containing that care-of address to its home agent. The Binding Update MUST have the Home Registration (H) and Acknowledge (A) bits set its home agent, as described on Section 11.7.1.
To assist with smooth handovers, a mobile node SHOULD retain its previous primary care-of address as a (non-primary) care-of address, and SHOULD still accept packets at this address, even after registering its new primary care-of address with its home agent. This is reasonable, since the mobile node could only receive packets at its previous primary care-of address if it were indeed still connected to that link. If the previous primary care-of address was allocated using stateful Address Autoconfiguration [RFC3315], the mobile node may not wish to release the address immediately upon switching to a new primary care-of address.
Whenever a mobile node determines that it is no longer reachable through a given link, it SHOULD invalidate all care-of addresses associated with address prefixes that it discovered from routers on the unreachable link which are not in the current set of address prefixes advertised by the (possibly new) current default router.
A mobile node detects that it has returned to its home link through the movement detection algorithm in use (Section 11.5.2), when the mobile node detects that its home subnet prefix is again on-link. To be able to send and receive packets using its home address from the home link, the mobile node MUST send a Binding Update to its home agent to instruct its home agent to no longer intercept or tunnel packets for it. Until the mobile node sends such a de-registration Binding Update, it MUST NOT attempt to send and receive packets using its home address from the home link. The home agent will continue to intercept all packets sent to the mobile's home address and tunnel them to the previously registered care-of address.
In this home registration, the mobile node MUST set the Acknowledge (A) and Home Registration (H) bits, set the Lifetime field to zero, and set the care-of address for the binding to the mobile node's own home address. The mobile node MUST use its home address as the source address in the Binding Update.
When sending this Binding Update to its home agent, the mobile node must be careful in how it uses Neighbor Solicitation [RFC4861] (if needed) to learn the home agent's link-layer address, since the home agent will be currently configured to intercept packets to the mobile node's home address using Proxy Neighbor Discovery (Proxy ND). In particular, the mobile node is unable to use its home address as the Source Address in the Neighbor Solicitation until the home agent stops defending the home address.
Neighbor Solicitation by the mobile node for the home agent's address will normally not be necessary, since the mobile node has already learned the home agent's link-layer address from a Source Link-Layer Address option in a Router Advertisement. However, if there are multiple home agents it may still be necessary to send a solicitation. In this special case of the mobile node returning home, the mobile node MUST multicast the packet, and in addition set the Source Address of this Neighbor Solicitation to the unspecified address (0:0:0:0:0:0:0:0). The target of the Neighbor Solicitation MUST be set to the mobile node's home address. The destination IP address MUST be set to the Solicited-Node multicast address [RFC4291]. The home agent will send a multicast Neighbor Advertisement back to the mobile node with the Solicited flag (S) set to zero. In any case, the mobile node SHOULD record the information from the Source Link-Layer Address option or from the advertisement, and set the state of the Neighbor Cache entry for the home agent to REACHABLE.
The mobile node then sends its Binding Update to the home agent's link-layer address, instructing its home agent to no longer serve as a home agent for it. By processing this Binding Update, the home agent will cease defending the mobile node's home address for Duplicate Address Detection and will no longer respond to Neighbor Solicitations for the mobile node's home address. The mobile node is then the only node on the link receiving packets at the mobile node's home address. In addition, when returning home prior to the expiration of a current binding for its home address, and configuring its home address on its network interface on its home link, the mobile node MUST NOT perform Duplicate Address Detection on its own home address, in order to avoid confusion or conflict with its home agent's use of the same address. This rule also applies to the derived link-local address of the mobile node, if the Link Local Address Compatibility (L) bit was set when the binding was created. If the mobile node returns home after the bindings for all of its care-of addresses have expired, then it SHOULD perform DAD.
After the Mobile Node sends the Binding Update, it MUST be prepared to reply to Neighbor Solicitations for its home address. Such replies MUST be sent using a unicast Neighbor Advertisement to the sender's link-layer address. It is necessary to reply, since sending the Binding Acknowledgement from the home agent may require performing Neighbor Discovery, and the mobile node may not be able to distinguish Neighbor Solicitations coming from the home agent from other Neighbor Solicitations. Note that a race condition exists where both the mobile node and the home agent respond to the same solicitations sent by other nodes; this will be only temporary, however, until the Binding Update is accepted.
After receiving the Binding Acknowledgement for its Binding Update to its home agent, the mobile node MUST multicast onto the home link (to the all-nodes multicast address) a Neighbor Advertisement [RFC4861], to advertise the mobile node's own link-layer address for its own home address. The Target Address in this Neighbor Advertisement MUST be set to the mobile node's home address, and the Advertisement MUST include a Target Link-layer Address option specifying the mobile node's link-layer address. The mobile node MUST multicast such a Neighbor Advertisement for each of its home addresses, as defined by the current on-link prefixes, including its link-local address. The Solicited Flag (S) in these Advertisements MUST NOT be set, since they were not solicited by any Neighbor Solicitation. The Override Flag (O) in these Advertisements MUST be set, indicating that the Advertisements SHOULD override any existing Neighbor Cache entries at any node receiving them.
Since multicasting on the local link (such as Ethernet) is typically not guaranteed to be reliable, the mobile node MAY retransmit these Neighbor Advertisements [RFC4861] up to MAX_NEIGHBOR_ADVERTISEMENT times to increase their reliability. It is still possible that some nodes on the home link will not receive any of these Neighbor Advertisements, but these nodes will eventually be able to recover through use of Neighbor Unreachability Detection [RFC4861].
Note that the tunnel via the home agent typically stops operating at the same time that the home registration is deleted.
This section defines the rules that the mobile node must follow when performing the return routability procedure. Section 11.7.2 describes the rules when the return routability procedure needs to be initiated.
A mobile node that initiates a return routability procedure MUST send (in parallel) a Home Test Init message and a Care-of Test Init messages. However, if the mobile node has recently received (see Section 5.2.7) one or both home or care-of keygen tokens, and associated nonce indices for the desired addresses, it MAY reuse them. Therefore, the return routability procedure may in some cases be completed with only one message pair. It may even be completed without any messages at all, if the mobile node has a recent home keygen token and has previously visited the same care-of address so that it also has a recent care-of keygen token. If the mobile node intends to send a Binding Update with the Lifetime set to zero and the care-of address equal to its home address - such as when returning home - sending a Home Test Init message is sufficient. In this case, generation of the binding management key depends exclusively on the home keygen token (Section 5.2.5).
A Home Test Init message MUST be created as described in Section 6.1.3.
A Care-of Test Init message MUST be created as described in Section 6.1.4. When sending a Home Test Init or Care-of Test Init message the mobile node MUST record in its Binding Update List the following fields from the messages:
Note that a single Care-of Test Init message may be sufficient even when there are multiple home addresses. In this case the mobile node MAY record the same information in multiple Binding Update List entries.
Upon receiving a packet carrying a Home Test message, a mobile node MUST validate the packet according to the following tests:
Any Home Test message not satisfying all of these tests MUST be silently ignored. Otherwise, the mobile node MUST record the Home Nonce Index and home keygen token in the Binding Update List. If the Binding Update List entry does not have a care-of keygen token, the mobile node SHOULD continue waiting for the Care-of Test message.
Upon receiving a packet carrying a Care-of Test message, a mobile node MUST validate the packet according to the following tests:
Any Care-of Test message not satisfying all of these tests MUST be silently ignored. Otherwise, the mobile node MUST record the Care-of Nonce Index and care-of keygen token in the Binding Update List. If the Binding Update List entry does not have a home keygen token, the mobile node SHOULD continue waiting for the Home Test message.
If after receiving either the Home Test or the Care-of Test message and performing the above actions, the Binding Update List entry has both the home and the care-of keygen tokens, the return routability procedure is complete. The mobile node SHOULD then proceed with sending a Binding Update as described in Section 11.7.2.
Correspondent nodes from the time before this specification was published may not support the Mobility Header protocol. These nodes will respond to Home Test Init and Care-of Test Init messages with an ICMP Parameter Problem code 1. The mobile node SHOULD take such messages as an indication that the correspondent node cannot provide route optimization, and revert back to the use of bidirectional tunneling.
The mobile node MUST support the protection of Home Test and Home Test Init messages as described in Section 10.4.6.
When IPsec is used to protect return routability signaling or payload packets, the mobile node MUST set the source address it uses for the outgoing tunnel packets to the current primary care-of address. The mobile node starts to use a new primary care-of address immediately after sending a Binding Update to the home agent to register this new address.
In order to change its primary care-of address as described in Section 11.5.1 and Section 11.5.3, a mobile node MUST register this care-of address with its home agent in order to make this its primary care-of address.
Also, if the mobile node wants the services of the home agent beyond the current registration period, the mobile node should send a new Binding Update to it well before the expiration of this period, even if it is not changing its primary care-of address. However, if the home agent returned a Binding Acknowledgement for the current registration with Status field set to 1 (accepted but prefix discovery necessary), the mobile node should not try to register again before it has learned the validity of its home prefixes through mobile prefix discovery. This is typically necessary every time this Status value is received, because information learned earlier may have changed.
To register a care-of address or to extend the lifetime of an existing registration, the mobile node sends a packet to its home agent containing a Binding Update, with the packet constructed as follows:
The Acknowledge (A) bit in the Binding Update requests the home agent to return a Binding Acknowledgement in response to this Binding Update. As described in Section 6.1.8, the mobile node SHOULD retransmit this Binding Update to its home agent until it receives a matching Binding Acknowledgement. Once reaching a retransmission timeout period of MAX_BINDACK_TIMEOUT, the mobile node SHOULD restart the process of delivering the Binding Update, but trying instead the next home agent returned during dynamic home agent address discovery (see Section 11.4.1). If there was only one home agent, the mobile node instead SHOULD continue to periodically retransmit the Binding Update at this rate until acknowledged (or until it begins attempting to register a different primary care-of address). See Section 11.8 for information about retransmitting Binding Updates.
With the Binding Update, the mobile node requests the home agent to serve as the home agent for the given home address. Until the lifetime of this registration expires, the home agent considers itself the home agent for this home address.
Each Binding Update MUST be authenticated as coming from the right mobile node, as defined in Section 5.1. The mobile node MUST use its home address - either in the Home Address destination option or in the Source Address field of the IPv6 header - in Binding Updates sent to the home agent. This is necessary in order to allow the IPsec policies to be matched with the correct home address.
When sending a Binding Update to its home agent, the mobile node MUST also create or update the corresponding Binding Update List entry, as specified in Section 11.7.2.
The last Sequence Number value sent to the home agent in a Binding Update is stored by the mobile node. If the sending mobile node has no knowledge of the correct Sequence Number value, it may start at any value. If the home agent rejects the value, it sends back a Binding Acknowledgement with a status code 135, and the last accepted sequence number in the Sequence Number field of the Binding Acknowledgement. The mobile node MUST store this information and use the next Sequence Number value for the next Binding Update it sends.
If the mobile node has additional home addresses, then the mobile node SHOULD send an additional packet containing a Binding Update to its home agent to register the care-of address for each such other home address.
The home agent will only perform DAD for the mobile node's home address when the mobile node has supplied a valid binding between its home address and a care-of address. If some time elapses during which the mobile node has no binding at the home agent, it might be possible for another node to autoconfigure the mobile node's home address. Therefore, the mobile node MUST treat the creation of a new binding with the home agent using an existing home address, the same as creation of a new home address. In the unlikely event that the mobile node's home address is autoconfigured as the IPv6 address of another network node on the home network, the home agent will reply to the mobile node's subsequent Binding Update with a Binding Acknowledgement containing a Status of 134 (Duplicate Address Detection failed). In this case, the mobile node MUST NOT attempt to re-use the same home address. It SHOULD continue to register the care-of addresses for its other home addresses, if any. Mechanisms outlined in "Mobile IPv6 Bootstrapping in Split Scenario" [RFC5026] allow mobile nodes to acquire new home addresses to replace the one for which Status 134 was received.
When the mobile node is assured that its home address is valid, it can initiate a correspondent registration with the purpose of allowing the correspondent node to cache the mobile node's current care-of address. This procedure consists of the return routability procedure followed by a registration.
This section defines when the correspondent registration is to be initiated and the rules to follow while it is being performed.
After the mobile node has sent a Binding Update to its home agent, registering a new primary care-of address (as described in Section 11.7.1), the mobile node SHOULD initiate a correspondent registration for each node that already appears in the mobile node's Binding Update List. The initiated procedures can be used to either update or delete binding information in the correspondent node.
For nodes that do not appear in the mobile node's Binding Update List, the mobile node MAY initiate a correspondent registration at any time after sending the Binding Update to its home agent. Considerations regarding when (and if) to initiate the procedure depend on the specific movement and traffic patterns of the mobile node and are outside the scope of this document.
In addition, the mobile node MAY initiate the correspondent registration in response to receiving a packet that meets all of the following tests:
If a mobile node has multiple home addresses, it becomes important to select the right home address to use in the correspondent registration. The used home address MUST be the Destination Address of the original (inner) packet.
The peer address used in the procedure MUST be determined as follows:
Note that the validity of the original packet is checked before attempting to initiate a correspondent registration. For instance, if a Home Address destination option appeared in the original packet, then rules in Section 9.3.1 are followed.
A mobile node MAY also choose to keep its topological location private from certain correspondent nodes, and thus need not initiate the correspondent registration.
Upon successfully completing the return routability procedure, and after receiving a successful Binding Acknowledgement from the Home Agent, a Binding Update MAY be sent to the correspondent node.
In any Binding Update sent by a mobile node, the care-of address (either the Source Address in the packet's IPv6 header or the Care-of Address in the Alternate Care-of Address mobility option of the Binding Update) MUST be set to one of the care-of addresses currently in use by the mobile node or to the mobile node's home address. A mobile node MAY set the care-of address differently for sending Binding Updates to different correspondent nodes.
A mobile node MAY also send a Binding Update to such a correspondent node, instructing it to delete any existing binding for the mobile node from its Binding Cache, as described in Section 6.1.7. Even in this case a successful completion of the return routability procedure is required first.
If the care-of address is not set to the mobile node's home address, the Binding Update requests that the correspondent node create or update an entry for the mobile node in the correspondent node's Binding Cache. This is done in order to record a care-of address for use in sending future packets to the mobile node. In this case, the value specified in the Lifetime field sent in the Binding Update SHOULD be less than or equal to the remaining lifetime of the home registration and the care-of address specified for the binding. The care-of address given in the Binding Update MAY differ from the mobile node's primary care-of address.
If the Binding Update is sent to the correspondent node, requesting the deletion of any existing Binding Cache entry it has for the mobile node, the care-of address is set to the mobile node's home address and the Lifetime field set to zero. In this case, generation of the binding management key depends exclusively on the home keygen token (Section 5.2.5). The care-of nonce index SHOULD be set to zero in this case. In keeping with the Binding Update creation rules below, the care-of address MUST be set to the home address if the mobile node is at home, or to the current care-of address if it is away from home.
If the mobile node wants to ensure that its new care-of address has been entered into a correspondent node's Binding Cache, the mobile node needs to request an acknowledgement by setting the Acknowledge (A) bit in the Binding Update.
A Binding Update is created as follows:
Each Binding Update MUST have a Sequence Number greater than the Sequence Number value sent in the previous Binding Update to the same destination address (if any). The sequence numbers are compared modulo 2**16, as described in Section 9.5.1. There is no requirement, however, that the Sequence Number value strictly increase by 1 with each new Binding Update sent or received, as long as the value stays within the window. 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. If the sending mobile node has no Binding Update List entry, the Sequence Number SHOULD start at a random value. The mobile node MUST NOT use the same Sequence Number in two different Binding Updates to the same correspondent node, even if the Binding Updates provide different care-of addresses.
The mobile node is responsible for the completion of the correspondent registration, as well as any retransmissions that may be needed (subject to the rate limitation defined in Section 11.8).
Upon receiving a packet carrying a Binding Acknowledgement, a mobile node MUST validate the packet according to the following tests:
Any Binding Acknowledgement not satisfying all of these tests MUST be silently ignored.
When a mobile node receives a packet carrying a valid Binding Acknowledgement, the mobile node MUST examine the Status field as follows:
The treatment of a Binding Refresh Advice mobility option within the Binding Acknowledgement depends on where the acknowledgement came from. This option MUST be ignored if the acknowledgement came from a correspondent node. If it came from the home agent, the mobile node uses the Refresh Interval field in the option as a suggestion that it SHOULD attempt to refresh its home registration at the indicated shorter interval.
If the acknowledgement came from the home agent, the mobile node examines the value of the Key Management Mobility Capability (K) bit. If this bit is not set, the mobile node SHOULD discard key management protocol connections, if any, to the home agent. The mobile node MAY also initiate a new key management connection.
If this bit is set, the mobile node SHOULD move its own endpoint in the key management protocol connections to the home agent, if any. The mobile node's new endpoint should be the new care-of address.
When a mobile node receives a packet containing a Binding Refresh Request message, if the mobile node has a Binding Update List entry for the source of the Binding Refresh Request, and the mobile node wants to retain its Binding Cache entry at the correspondent node, then the mobile node should start a return routability procedure. If the mobile node wants to have its Binding Cache entry removed, it can either ignore the Binding Refresh Request and wait for the binding to time out, or at any time, it can delete its binding from a correspondent node with an explicit Binding Update with a zero lifetime and the care-of address set to the home address. If the mobile node does not know if it needs the Binding Cache entry, it can make the decision in an implementation dependent manner, such as based on available resources.
Note that the mobile node should be careful to not respond to Binding Refresh Requests for addresses not in the Binding Update List to avoid being subjected to a denial of service attack.
If the return routability procedure completes successfully, a Binding Update message SHOULD be sent, as described in Section 11.7.2. The Lifetime field in this Binding Update SHOULD be set to a new lifetime, extending any current lifetime remaining from a previous Binding Update sent to this node (as indicated in any existing Binding Update List entry for this node), and the lifetime SHOULD again be less than or equal to the remaining lifetime of the home registration and the care-of address specified for the binding. When sending this Binding Update, the mobile node MUST update its Binding Update List in the same way as for any other Binding Update sent by the mobile node.
The mobile node is responsible for retransmissions and rate limiting in the return routability procedure, registrations, and in solicited prefix discovery.
When the mobile node sends a Mobile Prefix Solicitation, Home Test Init, Care-of Test Init or Binding Update for which it expects a response, the mobile node has to determine a value for the initial retransmission timer:
If the mobile node fails to receive a valid matching response within the selected initial retransmission interval, the mobile node SHOULD retransmit the message until a response is received.
The retransmissions by the mobile node MUST use an exponential back-off process in which the timeout period is doubled upon each retransmission, until either the node receives a response or the timeout period reaches the value MAX_BINDACK_TIMEOUT. The mobile node MAY continue to send these messages at this slower rate indefinitely.
The mobile node SHOULD start a separate back-off process for different message types, different home addresses and different care-of addresses. However, in addition an overall rate limitation applies for messages sent to a particular correspondent node. This ensures that the correspondent node has a sufficient amount of time to respond when bindings for multiple home addresses are registered, for instance. The mobile node MUST NOT send Mobility Header messages of a particular type to a particular correspondent node more than MAX_UPDATE_RATE times within a second.
Retransmitted Binding Updates MUST use a Sequence Number value greater than that used for the previous transmission of this Binding Update. Retransmitted Home Test Init and Care-of Test Init messages MUST use new cookie values.
DHAAD_RETRIES 4 retransmissions INITIAL_BINDACK_TIMEOUT 1 second INITIAL_DHAAD_TIMEOUT 3 seconds INITIAL_SOLICIT_TIMER 3 seconds MAX_BINDACK_TIMEOUT 32 seconds MAX_DELETE_BCE_TIMEOUT 10 seconds MAX_NONCE_LIFETIME 240 seconds MAX_TOKEN_LIFETIME 210 seconds MAX_RO_FAILURE 3 retries MAX_RR_BINDING_LIFETIME 420 seconds MAX_UPDATE_RATE 3 times PREFIX_ADV_RETRIES 3 retransmissions PREFIX_ADV_TIMEOUT 3 seconds
MaxMobPfxAdvInterval Default: 86,400 seconds MinDelayBetweenRAs Default: 3 seconds, Min: 0.03 seconds MinMobPfxAdvInterval Default: 600 seconds InitialBindackTimeoutFirstReg Default: 1.5 seconds
Home agents MUST allow the first three variables to be configured by system management, and mobile nodes MUST allow the last variable to be configured by system management.
The default value for InitialBindackTimeoutFirstReg has been calculated as 1.5 times the default value of RetransTimer, as specified in Neighbor Discovery (RFC 4861 [RFC4861]) times the default value of DupAddrDetectTransmits, as specified in Stateless Address Autoconfiguration (RFC 4862 [RFC4862])
The value MinDelayBetweenRAs overrides the value of the protocol constant MIN_DELAY_BETWEEN_RAS, as specified in Neighbor Discovery (RFC 4861 [RFC4861]). This variable SHOULD be set to MinRtrAdvInterval, if MinRtrAdvInterval is less than 3 seconds.
This document defines a new IPv6 protocol, the Mobility Header, described in Section 6.1. This protocol has been assigned protocol number 135.
This document also creates a new name space "Mobility Header Type", for the MH Type field in the Mobility Header. The current message types are described starting from Section 6.1.2, and are the following:
Future values of the MH Type can be allocated using Standards Action or IESG Approval [RFC5226].
Furthermore, each mobility message may contain mobility options as described in Section 6.2. This document defines a new name space "Mobility Option" to identify these options. The current mobility options are defined starting from Section 6.2.2 and are the following:
Future values of the Option Type can be allocated using Standards Action or IESG Approval [RFC5226].
Finally, this document creates a third new name space "Status Code" for the Status field in the Binding Acknowledgement message. The current values are listed in Section 6.1.8 and are the following:
Future values of the Status field can be allocated using Standards Action or IESG Approval [RFC5226].
All fields labeled "Reserved" are only to be assigned through Standards Action or IESG Approval.
This document also defines a new IPv6 destination option, the Home Address option, described in Section 6.3. This option has been assigned the Option Type value 0xC9.
This document also defines a new IPv6 type 2 routing header, described in Section 6.4. The value 2 has been allocated by IANA.
In addition, this document defines four ICMP message types, two used as part of the dynamic home agent address discovery mechanism, and two used in lieu of Router Solicitations and Advertisements when the mobile node is away from the home link. These messages have been assigned ICMPv6 type numbers from the informational message range:
This document also defines two new Neighbor Discovery [RFC4861] options, which have been assigned Option Type values within the option numbering space for Neighbor Discovery messages:
Any mobility solution must protect itself against misuses of the mobility features and mechanisms. In Mobile IPv6, most of the potential threats are concerned with false Bindings, usually resulting in Denial-of-Service attacks. Some of the threats also pose potential for Man-in-the-Middle, Hijacking, Confidentiality, and Impersonation attacks. The main threats this protocol protects against are the following:
As a fundamental service in an IPv6 stack, Mobile IPv6 is expected to be deployed in most nodes of the IPv6 Internet. The above threats should therefore be considered as being applicable to the whole Internet.
It should also be noted that some additional threats result from movements as such, even without the involvement of mobility protocols. Mobile nodes must be capable to defend themselves in the networks that they visit, as typical perimeter defenses applied in the home network no longer protect them.
This specification provides a series of features designed to mitigate the risk introduced by the threats listed above. The main security features are the following:
The support for encrypted reverse tunneling (see Section 11.3.1) allows mobile nodes to defeat certain kinds of traffic analysis.
Protecting those Binding Updates that are sent to home agents and those that are sent to arbitrary correspondent nodes requires very different security solutions due to the different situations. Mobile nodes and home agents are naturally expected to be subject to the network administration of the home domain.
Thus, they can and are supposed to have a security association that can be used to reliably authenticate the exchanged messages. See Section 5.1 for the description of the protocol mechanisms, and Section 15.3 below for a discussion of the resulting level of security.
It is expected that Mobile IPv6 route optimization will be used on a global basis between nodes belonging to different administrative domains. It would be a very demanding task to build an authentication infrastructure on this scale. Furthermore, a traditional authentication infrastructure cannot be easily used to authenticate IP addresses because IP addresses can change often. It is not sufficient to just authenticate the mobile nodes; Authorization to claim the right to use an address is needed as well. Thus, an "infrastructureless" approach is necessary. The chosen infrastructureless method is described in Section 5.2, and Section 15.4 discusses the resulting security level and the design rationale of this approach.
Specific rules guide the use of the Home Address destination option, the routing header, and the tunneling headers in the payload packets. These rules are necessary to remove the vulnerabilities associated with their unrestricted use. The effect of the rules is discussed in Section 15.7, Section 15.8, and Section 15.9.
Denial-of-Service threats against Mobile IPv6 security mechanisms themselves concern mainly the Binding Update procedures with correspondent nodes. The protocol has been designed to limit the effects of such attacks, as will be described in Section 15.4.5.
Signaling between the mobile node and the home agent requires message integrity. This is necessary to assure the home agent that a Binding Update is from a legitimate mobile node. In addition, correct ordering and anti-replay protection are optionally needed.
IPsec ESP protects the integrity of the Binding Updates and Binding Acknowledgements by securing mobility messages between the mobile node and the home agent.
IPsec can provide anti-replay protection only if dynamic keying is used (which may not always be the case). IPsec does not guarantee correct ordering of packets, only that they have not been replayed. Because of this, sequence numbers within the Mobile IPv6 messages are used to ensure correct ordering (see Section 5.1). However, if the 16 bit Mobile IPv6 sequence number space is cycled through, or the home agent reboots and loses its state regarding the sequence numbers, replay and reordering attacks become possible. The use of dynamic keying, IPsec anti-replay protection, and the Mobile IPv6 sequence numbers can together prevent such attacks. It is also recommended that use of non-volatile storage be considered for home agents, to avoid losing their state.
A sliding window scheme is used for the sequence numbers. The protection against replays and reordering attacks without a key management mechanism works when the attacker remembers up to a maximum of 2**15 Binding Updates.
The above mechanisms do not show that the care-of address given in the Binding Update is correct. This opens the possibility for Denial-of-Service attacks against third parties. However, since the mobile node and home agent have a security association, the home agent can always identify an ill-behaving mobile node. This allows the home agent operator to discontinue the mobile node's service, and possibly take further actions based on the business relationship with the mobile node's owner.
Note that the use of a single pair of manually keyed security associations conflicts with the generation of a new home address [RFC4941] for the mobile node, or with the adoption of a new home subnet prefix. This is because IPsec security associations are bound to the used addresses. While certificate-based automatic keying alleviates this problem to an extent, it is still necessary to ensure that a given mobile node cannot send Binding Updates for the address of another mobile node. In general, this leads to the inclusion of home addresses in certificates in the Subject AltName field. This again limits the introduction of new addresses without either manual or automatic procedures to establish new certificates. Therefore, this specification restricts the generation of new home addresses (for any reason) to those situations where a security association or certificate for the new address already exists.
Support for IKEv2 has been specified as optional. The following should be observed about the use of manual keying:
The use of IKEv2 with Mobile IPv6 is documented in more detail in [RFC4877]. The following should be observed regarding the use of IKEv2:
The motivation for designing the return routability procedure was to have sufficient support for Mobile IPv6, without creating significant new security problems. The goal for this procedure was not to protect against attacks that were already possible before the introduction of Mobile IPv6.
The next sections will describe the security properties of the used method, both from the point of view of possible on-path attackers who can see those cryptographic values that have been sent in the clear (Section 15.4.2 and Section 15.4.3) and from the point of view of other attackers (Section 15.4.6).
The chosen infrastructureless method verifies that the mobile node is "live" (that is, it responds to probes) at its home and care-of addresses. Section 5.2 describes the return routability procedure in detail. The procedure uses the following principles:
For further information about the design rationale of the return routability procedure, see [I-D.aura-mipv6-bu-attacks] [I-D.roe-mobileip-updateauth] [I-D.nordmark-mobileip-bu3way] [RFC4225]. The mechanisms used have been adopted from these documents.
The return routability procedure protects Binding Updates against all attackers who are unable to monitor the path between the home agent and the correspondent node. The procedure does not defend against attackers who can monitor this path. Note that such attackers are in any case able to mount an active attack against the mobile node when it is at its home location. The possibility of such attacks is not an impediment to the deployment of Mobile IPv6 because these attacks are possible regardless of whether or not Mobile IPv6 is in use.
This procedure also protects against Denial-of-Service attacks in which the attacker pretends to be mobile, but uses the victim's address as the care-of address. This would cause the correspondent node to send the victim some unexpected traffic. This procedure defends against these attacks by requiring at least the passive presence of the attacker at the care-of address or on the path from the correspondent to the care-of address. Normally, this will be the mobile node.
This section discusses the protection offered by the return routability method by comparing it to the security of regular IPv6 communications. We will divide vulnerabilities into three classes: (1) those related to attackers on the local network of the mobile node, home agent, or the correspondent node, (2) those related to attackers on the path between the home network and the correspondent node, and (3) off-path attackers, i.e., the rest of the Internet.
We will now discuss the vulnerabilities of regular IPv6 communications. The on-link vulnerabilities of IPv6 communications include Denial-of-Service, Masquerading, Man-in-the-Middle, Eavesdropping, and other attacks. These attacks can be launched through spoofing Router Discovery, Neighbor Discovery and other IPv6 mechanisms. Some of these attacks can be prevented with the use of cryptographic protection in the packets.
A similar situation exists with on-path attackers. That is, without cryptographic protection, the traffic is completely vulnerable.
Assuming that attackers have not penetrated the security of the Internet routing protocols, attacks are much harder to launch from off-path locations. Attacks that can be launched from these locations are mainly Denial-of-Service attacks, such as flooding and/or reflection attacks. It is not possible for an off-path attacker to become a Man-in-the-Middle.
Next, we will consider the vulnerabilities that exist when IPv6 is used together with Mobile IPv6 and the return routability procedure. On the local link, the vulnerabilities are the same as those in IPv6, but Masquerade and Man-in-the-Middle attacks can now also be launched against future communications, and not just against current communications. If a Binding Update was sent while the attacker was present on the link, its effects remain for the lifetime of the binding. This happens even if the attacker moves away from the link. In contrast, an attacker who uses only plain IPv6 generally has to stay on the link in order to continue the attack. Note that in order to launch these new attacks, the IP address of the victim must be known. This makes this attack feasible, mainly in the context of well-known interface IDs, such as those already appearing in the traffic on the link or registered in the DNS.
On-path attackers can exploit similar vulnerabilities as in regular IPv6. There are some minor differences, however. Masquerade, Man-in-the-Middle, and Denial-of-Service attacks can be launched with just the interception of a few packets, whereas in regular IPv6 it is necessary to intercept every packet. The effect of the attacks is the same regardless of the method, however. In any case, the most difficult task an attacker faces in these attacks is getting on the right path.
The vulnerabilities for off-path attackers are the same as in regular IPv6. Those nodes that are not on the path between the home agent and the correspondent node will not be able to receive the home address probe messages.
In conclusion, we can state the following main results from this comparison:
For a more in-depth discussion of these issues, see [RFC4225].
The return routability procedure also protects the participants against replayed Binding Updates. The attacker is unable replay the same message due to the sequence number which is a part of the Binding Update. It is also unable to modify the Binding Update since the MAC verification would fail after such a modification.
Care must be taken when removing bindings at the correspondent node, however. If a binding is removed while the nonce used in its creation is still valid, an attacker could replay the old Binding Update. Rules outlined in Section 5.2.8 ensure that this cannot happen.
The return routability procedure has protection against resource exhaustion Denial-of-Service attacks. The correspondent nodes do not retain any state about individual mobile nodes until an authentic Binding Update arrives. This is achieved through the construct of keygen tokens from the nonces and node keys that are not specific to individual mobile nodes. The keygen tokens can be reconstructed by the correspondent node, based on the home and care-of address information that arrives with the Binding Update. This means that the correspondent nodes are safe against memory exhaustion attacks except where on-path attackers are concerned. Due to the use of symmetric cryptography, the correspondent nodes are relatively safe against CPU resource exhaustion attacks as well.
Nevertheless, as [I-D.aura-mipv6-bu-attacks] describes, there are situations in which it is impossible for the mobile and correspondent nodes to determine if they actually need a binding or whether they just have been fooled into believing so by an attacker. Therefore, it is necessary to consider situations where such attacks are being made.
Even if route optimization is a very important optimization, it is still only an optimization. A mobile node can communicate with a correspondent node even if the correspondent refuses to accept any Binding Updates. However, performance will suffer because packets from the correspondent node to the mobile node will be routed via the mobile's home agent rather than a more direct route. A correspondent node can protect itself against some of these resource exhaustion attacks as follows. If the correspondent node is flooded with a large number of Binding Updates that fail the cryptographic integrity checks, it can stop processing Binding Updates. If a correspondent node finds that it is spending more resources on checking bogus Binding Updates than it is likely to save by accepting genuine Binding Updates, then it may silently discard some or all Binding Updates without performing any cryptographic operations.
Layers above IP can usually provide additional information to help determine whether there is a need to establish a binding with a specific peer. For example, TCP knows if the node has a queue of data that it is trying to send to a peer. An implementation of this specification is not required to make use of information from higher protocol layers, but some implementations are likely to be able to manage resources more effectively by making use of such information.
We also require that all implementations be capable of administratively disabling route optimization.
Attackers can try to break the return routability procedure in many ways. Section 15.4.2 discusses the situation where the attacker can see the cryptographic values sent in the clear, and Section 15.4.3 discusses the impact this has on IPv6 communications. This section discusses whether attackers can guess the correct values without seeing them.
While the return routability procedure is in progress, 64 bit cookies are used to protect spoofed responses. This is believed to be sufficient, given that to blindly spoof a response a very large number of messages would have to be sent before success would be probable.
The tokens used in the return routability procedure provide together 128 bits of information. This information is used internally as input to a hash function to produce a 160 bit quantity suitable for producing the keyed hash in the Binding Update using the HMAC_SHA1 algorithm. The final keyed hash length is 96 bits. The limiting factors in this case are the input token lengths and the final keyed hash length. The internal hash function application does not reduce the entropy.
The 96 bit final keyed hash is of typical size and is believed to be secure. The 128 bit input from the tokens is broken in two pieces, the home keygen token and the care-of keygen token. An attacker can try to guess the correct cookie value, but again this would require a large number of messages (an the average 2**63 messages for one or 2**127 for two). Furthermore, given that the cookies are valid only for a short period of time, the attack has to keep a high constant message rate to achieve a lasting effect. This does not appear practical.
When the mobile node is returning home, it is allowed to use just the home keygen token of 64 bits. This is less than 128 bits, but attacking it blindly would still require a large number of messages to be sent. If the attacker is on the path and capable of seeing the Binding Update, it could conceivably break the keyed hash with brute force. However, in this case the attacker has to be on the path, which appears to offer easier ways for denial-of-service than preventing route optimization.
The dynamic home agent address discovery function could be used to learn the addresses of home agents in the home network.
The ability to learn addresses of nodes may be useful to attackers because brute-force scanning of the address space is not practical with IPv6. Thus, they could benefit from any means which make mapping the networks easier. For example, if a security threat targeted at routers or even home agents is discovered, having a simple ICMP mechanism to easily find out possible targets may prove to be an additional (though minor) security risk.
This document does not define any authentication mechanism for dynamic home agent address discovery messages. Therefore the home agent cannot verify the home address of the mobile node that requested the list of home agents.
Apart from discovering the address(es) of home agents, attackers will not be able to learn much from this information, and mobile nodes cannot be tricked into using wrong home agents, as all other communication with the home agents is secure.
In cases where additional security is needed, one may consider instead the use of MIPv6 bootstrapping [RFC5026], (based on DNS SRV Resource Records [RFC2782]) in conjunction with security mechanisms suggested in these specifications. In that solution, security is provided by the DNSSEC [RFC4033] framework. The needed pre-configured data on the mobile node for this mechanism is the domain name of the mobile service provider, which is marginally better than the home subnet prefix. For the security, a trust anchor which dominates the domain is needed.
The mobile prefix discovery function may leak interesting information about network topology and prefix lifetimes to eavesdroppers; for this reason, requests for this information have to be authenticated. Responses and unsolicited prefix information needs to be authenticated to prevent the mobile nodes from being tricked into believing false information about the prefixes and possibly preventing communications with the existing addresses. Optionally, encryption may be applied to prevent leakage of the prefix information.
Tunnels between the mobile node and the home agent can be protected by ensuring proper use of source addresses, and optional cryptographic protection. These procedures are discussed in Section 5.5.
Binding Updates to the home agents are secure. When receiving tunneled traffic, the home agent verifies that the outer IP address corresponds to the current location of the mobile node. This acts as a weak form of protection against spoofing packets that appear to come from the mobile node. This is particularly useful, if no end-to-end security is being applied between the mobile and correspondent nodes. The outer IP address check prevents attacks where the attacker is controlled by ingress filtering. It also prevents attacks when the attacker does not know the current care-of address of the mobile node. Attackers who know the care-of address and are not controlled by ingress filtering could still send traffic through the home agent. This includes attackers on the same local link as the mobile node is currently on. But such attackers could send packets that appear to come from the mobile node without attacking the tunnel; the attacker could simply send packets with the source address set to the mobile node's home address. However, this attack does not work if the final destination of the packet is in the home network, and some form of perimeter defense is being applied for packets sent to those destinations. In such cases it is recommended that either end-to-end security or additional tunnel protection be applied, as is usual in remote access situations.
Home agents and mobile nodes may use IPsec ESP to protect payload packets tunneled between themselves. This is useful for protecting communications against attackers on the path of the tunnel.
When a Unique-Local Address (ULA) RFC4193 [RFC4193] is used as a home address, reverse tunneling can be used to send local traffic from another location. Administrators should be aware of this when allowing such home addresses. In particular, the outer IP address check described above is not sufficient against all attackers. The use of encrypted tunnels is particularly useful for these kinds of home addresses.
When the mobile node sends packets directly to the correspondent node, the Source Address field of the packet's IPv6 header is the care-of address. Therefore, ingress filtering [RFC2827] works in the usual manner even for mobile nodes, as the Source Address is topologically correct. The Home Address option is used to inform the correspondent node of the mobile node's home address.
However, the care-of address in the Source Address field does not survive in replies sent by the correspondent node unless it has a binding for this mobile node. Also, not all attacker tracing mechanisms work when packets are being reflected through correspondent nodes using the Home Address option. For these reasons, this specification restricts the use of the Home Address option. It may only be used when a binding has already been established with the participation of the node at the home address, as described in Section 5.5 and Section 6.3. This prevents reflection attacks through the use of the Home Address option. It also ensures that the correspondent nodes reply to the same address that the mobile node sends traffic from.
No special authentication of the Home Address option is required beyond the above, but note that if the IPv6 header of a packet is covered by IPsec Authentication Header, then that authentication covers the Home Address option as well. Thus, even when authentication is used in the IPv6 header, the security of the Source Address field in the IPv6 header is not compromised by the presence of a Home Address option. Without authentication of the packet, any field in the IPv6 header, including the Source Address field or any other part of the packet and the Home Address option can be forged or modified in transit. In this case, the contents of the Home Address option is no more suspect than any other part of the packet.
The definition of the type 2 routing header is described in Section 6.4. This definition and the associated processing rules have been chosen so that the header cannot be used for what is traditionally viewed as source routing. In particular, the Home Address in the routing header will always have to be assigned to the home address of the receiving node; otherwise the packet will be dropped.
Generally, source routing has a number of security concerns. These include the automatic reversal of unauthenticated source routes (which is an issue for IPv4, but not for IPv6). Another concern is the ability to use source routing to "jump" between nodes inside, as well as outside a firewall. These security concerns are not issues in Mobile IPv6, due to the rules mentioned above.
In essence the semantics of the type 2 routing header is the same as a special form of IP-in-IP tunneling where the inner and outer source addresses are the same.
This implies that a device which implements the filtering of packets should be able to distinguish between a type 2 routing header and other routing headers, as required in Section 8.3. This is necessary in order to allow Mobile IPv6 traffic while still having the option of filtering out other uses of routing headers.
This document relies on hash-based message authentication codes (HMAC) computed using the SHA-1 [FIPS.180-1.1995] hash algorithm for the home keygen token, care-of keygen token, as well as the authentication fields in the binding update and binding authorization data (see Section 5.2.4). While SHA-1 has been deprecated for some cryptographic mechanisms, SHA-1 is considered secure for the foreseeable future when used as specified here. For additional details, see [I-D.turner-sha0-sha1-seccon].
Work done by Tuomas Aura, Mike Roe, Greg O'Shea, Pekka Nikander, Erik Nordmark, and Michael Thomas shaped the return routability protocols described in [I-D.roe-mobileip-updateauth].
Significant contributions were made by members of the Mobile IPv6 Security Design Team, including (in alphabetical order) Gabriel Montenegro, Erik Nordmark and Pekka Nikander.
We would like to thank the members of the Mobile IP, Mobility Extensions for IPv6, and IPng Working Groups for their comments and suggestions on this work. We would particularly like to thank (in alphabetical order) Fred Baker, Josh Broch, Samita Chakrabarti, Robert Chalmers, Noel Chiappa, Jean-Michel Combes, Greg Daley, Vijay Devarapalli, Rich Draves, Francis Dupont, Ashutosh Dutta, Arnaud Ebalard, Wesley Eddy, Thomas Eklund, Jun-Ichiro Itojun Hagino, Brian Haley, Marc Hasson, John Ioannidis, James Kempf, Rajeev Koodli, Suresh Krishnan, Krishna Kumar, T.J. Kniveton, Joe Lau, Aime Le Rouzic, Julien Laganier, Jiwoong Lee, Benjamin Lim, Vesa-Matti Mantyla, Kevin Miles, Glenn Morrow, Ahmad Muhanna, Thomas Narten, Karen Nielsen, Simon Nybroe, David Oran, Mohan Parthasarathy, Basavaraj Patil, Brett Pentland, Lars Henrik Petander, Alexandru Petrescu, Mattias Petterson, Ken Powell, Ed Remmell, Phil Roberts, Patrice Romand, Luis A. Sanchez, Pekka Savola, Jeff Schiller, Arvind Sevalkar, Keiichi Shima, Tom Soderlund, Hesham Soliman, Jim Solomon, Tapio Suihko, Dave Thaler, Pascal Thubert, Benny Van Houdt, Jon-Olov Vatn, Ryuji Wakikawa, Kilian Weniger, Carl E. Williams, Vladislav Yasevich, Alper Yegin, and Xinhua Zhao, for their detailed reviews of earlier versions of this document. Their suggestions have helped to improve both the design and presentation of the protocol.
We would also like to thank the participants of the Mobile IPv6 testing event (1999), implementors who participated in Mobile IPv6 interoperability testing at Connectathons (2000, 2001, 2002, and 2003), and the participants at the ETSI interoperability testing (2000, 2002). Finally, we would like to thank the TAHI project who has provided test suites for Mobile IPv6.
This document does not specify how to piggyback payload packets on the binding related messages. However, it is envisioned that this can be specified in a separate document when issues such as the interaction between piggybacking and IPsec are fully resolved (see also Appendix Appendix A.3). The return routability messages can indicate support for piggybacking with a new mobility option.
Due to the concerns about opening reflection attacks with the Home Address destination option, this specification requires that this option be verified against the Binding Cache, i.e., there must be a Binding Cache entry for the Home Address and Care-of Address.
Future extensions may be specified that allow the use of unverified Home Address destination options in ways that do not introduce security issues.
While the return routability procedure provides a good level of security, there exist methods that have even higher levels of security. Secondly, as discussed in Section 15.4, future enhancements of IPv6 security may cause a need to also improve the security of the return routability procedure. Using IPsec as the sole method for authorizing Binding Updates to correspondent nodes is also possible. The protection of the Mobility Header for this purpose is easy, though one must ensure that the IPsec SA was created with appropriate authorization to use the home address referenced in the Binding Update. For instance, a certificate used by IKEv2 to create the security association might contain the home address. A future specification may specify how this is done.
Future specifications may improve the efficiency of Neighbor Discovery tasks, which could be helpful for fast movements. One factor is currently being looked at: the delays caused by the Duplicate Address Detection mechanism. Currently, Duplicate Address Detection needs to be performed for every new care-of address as the mobile node moves, and for the mobile node's link-local address on every new link. In particular, the need and the trade-offs of re-performing Duplicate Address Detection for the link-local address every time the mobile node moves on to new links will need to be examined. Improvements in this area are, however, generally applicable and progress independently from the Mobile IPv6 specification.
Future functional improvements may also be relevant for Mobile IPv6 and other applications. For instance, mechanisms that would allow recovery from a Duplicate Address Detection collision would be useful for link-local, care-of, and home addresses.
The following issues were identified during the evolution of the current document. Discussion about most of the issues can be found on the [mext] working group page http://trac.tools.ietf.org/wg/mext/trac/report/6