Request for Comments: 5026
Category: Standards Track
DoCoMo Labs USA
V. Devarapalli, Ed.
Mobile IPv6 Bootstrapping in Split Scenario
Status of This Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
A Mobile IPv6 node requires a Home Agent address, a home address, and IPsec security associations with its Home Agent before it can start utilizing Mobile IPv6 service. RFC 3775 requires that some or all of these are statically configured. This document defines how a Mobile IPv6 node can bootstrap this information from non-topological information and security credentials pre-configured on the Mobile Node. The solution defined in this document solves the split scenario described in the Mobile IPv6 bootstrapping problem statement in RFC 4640. The split scenario refers to the case where the Mobile Node's mobility service is authorized by a different service provider than basic network access. The solution described in this document is also generically applicable to any bootstrapping case, since other scenarios are more specific realizations of the split scenario.
Table of Contents
1. Introduction ....................................................3 2. Terminology .....................................................3 3. Split Scenario ..................................................4 4. Components of the Solution ......................................7 5. Protocol Operations .............................................9 5.1. Home Agent Address Discovery ...............................9 5.1.1. DNS Lookup by Home Agent Name ......................10 5.1.2. DNS Lookup by Service Name .........................10 5.2. IPsec Security Associations Setup .........................11 5.3. Home Address Assignment ...................................11 5.3.1. Home Address Assignment by the Home Agent ..........11 5.3.2. Home Address Auto-Configuration by the Mobile Node ........................................12 5.4. Authorization and Authentication with MSA .................14 6. Home Address Registration in the DNS ...........................14 7. Summary of Bootstrapping Protocol Flow .........................16 8. Option and Attribute Format ....................................17 8.1. DNS Update Mobility Option ................................17 8.2. MIP6_HOME_PREFIX Attribute ................................19 9. Security Considerations ........................................20 9.1. HA Address Discovery ......................................20 9.2. Home Address Assignment through IKEv2 .....................22 9.3. SA Establishment Using EAP through IKEv2 ..................22 9.4. Backend Security between the HA and AAA Server ............22 9.5. Dynamic DNS Update ........................................23 10. IANA Considerations ...........................................24 11. Contributors ..................................................24 12. Acknowledgements ..............................................25 13. References ....................................................25 13.1. Normative References .....................................25 13.2. Informative References ...................................26
Mobile IPv6  requires the Mobile Node to know its Home Agent Address, its own Home Address, and the cryptographic materials (e.g., shared keys or certificates) needed to set up IPsec security associations with the Home Agent (HA) in order to protect Mobile IPv6 signaling. This is generally referred to as the Mobile IPv6 bootstrapping problem .
The Mobile IPv6 base protocol does not specify any method to automatically acquire this information, which means that network administrators are normally required to manually set configuration data on Mobile Nodes and HAs. However, in real deployments, manual configuration does not scale as the Mobile Nodes increase in number.
As discussed in , several bootstrapping scenarios can be identified depending on the relationship between the network operator that authenticates a mobile node for granting network access service (Access Service Authorizer, ASA) and the service provider that authorizes Mobile IPv6 service (Mobility Service Authorizer, MSA). This document describes a solution to the bootstrapping problem that is applicable in a scenario where the MSA and the ASA are different entities (i.e., a split scenario).
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 .
General mobility terminology can be found in . The following additional terms are used here:
Access Service Authorizer (ASA)
A network operator that authenticates a mobile node and establishes the mobile node's authorization to receive Internet service.
Access Service Provider (ASP)
A network operator that provides direct IP packet forwarding to and from the end host.
Mobility Service Authorizer (MSA)
A service provider that authorizes Mobile IPv6 service.
Mobility Service Provider (MSP)
A service provider that provides Mobile IPv6 service. In order to obtain such service, the mobile node must be authenticated and prove authorization to obtain the service.
A scenario where mobility service and network access service are authorized by different entities. This implies that MSA is different from ASA.
3. Split Scenario
In the problem statement description , there is a clear assumption that mobility service and network access service can be separate. This assumption implies that mobility service and network access service may be authorized by different entities. As an example, the service model defined in  allows an enterprise network to deploy a Home Agent and offer Mobile IPv6 service to a user, even if the user is accessing the Internet independent of its enterprise account (e.g., by using his personal WiFi hotspot account at a coffee shop).
Therefore, in this document it is assumed that network access and mobility service are authorized by different entities, which means that authentication and authorization for mobility service and network access will be considered separately. This scenario is called split scenario.
Moreover, the model defined in  separates the entity providing the service from the entity that authenticates and authorizes the user. This is similar to the roaming model for network access. Therefore, in the split scenario, two different cases can be identified depending on the relationship between the entity that provides the mobility service (i.e., Mobility Service Provider, MSP) and the entity that authenticates and authorizes the user (i.e., Mobility Service Authorizer, MSA).
Figure 1 depicts the split scenario when the MSP and the MSA are the same entity. This means that the network operator that provides the Home Agent authenticates and authorizes the user for mobility service.
Mobility Service Provider and Authorizer +-------------------------------------------+ | | | +-------------+ +--+ | | | MSA/MSP AAA | <-------------> |HA| | | | server | AAA protocol +--+ | | +-------------+ | | | +-------------------------------------------+ +--+ |MN| +--+
Figure 1 -- Split Scenario (MSA == MSP)
Figure 2 shows the split scenario in case the MSA and the MSP are two different entities. This might happen if the Mobile Node is far from its MSA network and is assigned a closer HA to optimize performance or if the MSA cannot provide any Home Agent and relies on a third party (i.e., the MSP) to grant mobility service to its users. Notice that the MSP might or might not also be the network operator that is providing network access (i.e., ASP, Access Service Provider).
Mobility Service Authorizer +-------------+ | MSA AAA | | server | +-------------+ ^ | AAA protocol | | Mobility Service | Provider +--------|----------------------------------+ | V | | +-------------+ +--+ | | | MSP AAA | <-------------> |HA| | | | server | AAA protocol +--+ | | +-------------+ | | | +-------------------------------------------+ +--+ |MN| +--+
Figure 2 -- Split Scenario (MSA != MSP)
Note that Figure 1 and Figure 2 assume the use of an Authentication, Authorization, and Accounting (AAA) protocol to authenticate and authorize the Mobile Node for mobility service. However, since the Internet Key Exchange Protocol (IKEv2) allows an Extensible Authentication Protocol (EAP) client authentication only and the server authentication needs to be performed based on certificates or public keys, the Mobile Node potentially requires a Certificate Revocation List check (CRL check) even though an AAA protocol is used to authenticate and authorize the Mobile Node for mobility service.
If, instead, a Public Key Infrastructure (PKI) is used, the Mobile Node and HA use certificates to authenticate each other and there is no AAA server involved . This is conceptually similar to Figure 1, since the MSP == MSA, except the Home Agent, and potentially the
Mobile Node, may require a certificate revocation list check (CRL check) with the Certification Authority (CA). The CA may be either internal or external to the MSP. Figure 3 illustrates this.
Certification Authority +-------------+ | CA | | server | +-------------+ ^ | CRL Check | | Mobility Service | Provider and Authorizer +--------|----------+ | V | | +-------------+ | | | HA | | | | | | | +-------------+ | | | +-------------------+
Figure 3 -- Split Scenario with PKI
For more details on the use of PKI for IKEv2 authentication, please refer to  and .
The split scenario is the simplest model that can be identified, since no assumptions about the access network are made. This implies that the mobility service is bootstrapped independently from the authentication protocol for network access used (e.g., EAP or even open access). For this reason, the solution described in this document and developed for this scenario could also be applied to the integrated access-network deployment model , even if it might not be optimized.
4. Components of the Solution
The bootstrapping problem is composed of different sub-problems that can be solved independently in a modular way. The components are identified and a brief overview of their solution follow.
HA address discovery
The Mobile Node needs to discover the address of its Home Agent. The main objective of a bootstrapping solution is to minimize the data pre-configured on the Mobile Node. For this reason, the DHAAD defined in  may not be applicable in real deployments since it is required that the Mobile Node is pre-configured with the home network prefix and does not allow an operator to load balance by having Mobile Nodes dynamically assigned to Home Agents located in different subnets. This document defines a solution for Home Agent address discovery that is based on Domain Name Service (DNS), introducing a new DNS SRV record . The unique information that needs to be pre-configured on the Mobile Node is the domain name of the MSP.
IPsec Security Associations setup
Mobile IPv6 requires that a Mobile Node and its Home Agent share an IPsec SA in order to protect binding updates and other Mobile IPv6 signaling. This document provides a solution that is based on IKEv2 and follows what is specified in . The IKEv2 peer authentication can be performed both using certificates and using EAP depending on the network operator's deployment model.
Home Address (HoA) assignment
The Mobile Node needs to know its Home Address in order to bootstrap Mobile IPv6 operation. The Home Address is assigned by the Home Agent during the IKEv2 exchange (as described in ). The solution defined in this document also allows the Mobile Node to auto-configure its Home Address based on stateless auto- configuration , Cryptographically Generated Addresses , or privacy addresses .
Authentication and Authorization with MSA
The user must be authenticated in order for the MSP to grant the service. Since the user credentials can be verified only by the MSA, this authorization step is performed by the MSA. Moreover, the mobility service must be explicitly authorized by the MSA based on the user's profile. These operations are performed in different ways depending on the credentials used by the Mobile Node during the IKEv2 peer authentication and on the backend infrastructure (PKI or AAA).
An optional part of bootstrapping involves providing a way for the Mobile Node to have its Fully Qualified Domain Name (FQDN) updated in the DNS with a dynamically assigned home address. While not all applications will require this service, many networking applications use the FQDN to obtain an address for a node prior to starting IP traffic with it. The solution defined in this document specifies that the dynamic DNS update is performed by the Home Agent or through the AAA infrastructure, depending on the trust relationship in place.
5. Protocol Operations
This section describes in detail the procedures needed to perform Mobile IPv6 bootstrapping based on the components identified in the previous section.
5.1. Home Agent Address Discovery
Once a Mobile Node has obtained network access, it can perform Mobile IPv6 bootstrapping. For this purpose, the Mobile Node queries the DNS server to request information on Home Agent service. As mentioned before in the document, the Mobile Node should be pre- configured with the domain name of the Mobility Service Provider.
The Mobile Node needs to obtain the IP address of a DNS server before it can send a DNS request. This can be configured on the Mobile Node or obtained through DHCPv6 from the visited link . In any case, it is assumed that there is some kind of mechanism by which the Mobile Node is configured with a DNS server since a DNS server is needed for many other reasons.
Two options for DNS lookup for a Home Agent address are identified in this document: DNS lookup by Home Agent Name and DNS lookup by service name.
This document does not provide a specific mechanism to load balance different Mobile Nodes among Home Agents. It is possible for an MSP to achieve coarse-grained load balancing by dynamically updating the SRV RR priorities to reflect the current load on the MSP's collection of Home Agents. Mobile Nodes then use the priority mechanism to preferentially select the least loaded HA. The effectiveness of this technique depends on how much of a load it will place on the DNS servers, particularly if dynamic DNS is used for frequent updates.
While this document specifies a Home Agent Address Discovery solution based on DNS, when the ASP and the MSP are the same entity, DHCP may be used. See  for details.
5.1.1. DNS Lookup by Home Agent Name
In this case, the Mobile Node is configured with the Fully Qualified Domain Name of the Home Agent. As an example, the Mobile Node could be configured with the name "ha1.example.com", where "example.com" is the domain name of the MSP granting the mobility service.
The Mobile Node constructs a DNS request by setting the QNAME to the name of the Home Agent. The request has QTYPE set to "AAAA" so that the DNS server sends the IPv6 address of the Home Agent. Once the DNS server replies to this query, the Mobile Node knows its Home Agent address and can run IKEv2 in order to set up the IPsec SAs and get a Home Address.
Note that the configuration of a home agent FQDN on the mobile node can also be extended to dynamically assign a local home agent from the visited network. Such dynamic assignment would be useful, for instance, from the point of view of improving routing efficiency in bidirectional tunneling mode. Enhancements or conventions for dynamic assignment of local home agents are outside the scope of this specification.
5.1.2. DNS Lookup by Service Name
RFC 2782  defines the service resource record (SRV RR) that allows an operator to use several servers for a single domain, to move services from host to host, and to designate some hosts as primary servers for a service and others as backups. Clients ask for a specific service/protocol for a specific domain and get back the names of any available servers.
RFC 2782  also describes the policies to choose a service agent based on the preference and weight values. The DNS SRV record may contain the preference and weight values for multiple Home Agents available to the Mobile Node in addition to the Home Agent FQDNs. If multiple Home Agents are available in the DNS SRV record, then the Mobile Node is responsible for processing the information as per policy and for picking one Home Agent. If the Home Agent of choice does not respond to the IKE_SA_INIT messages or if IKEv2 authentication fails, the Mobile Node SHOULD try the next Home Agent on the list. If none of the Home Agents respond, the Mobile Node SHOULD try again after a period of time that is configurable on the Mobile Node. If IKEv2 authentication fails with all Home Agents, it is an unrecoverable error on the Mobile Node.
The service name for Mobile IPv6 Home Agent service, as required by RFC 2782, is "mip6" and the protocol name is "ipv6". Note that a transport name cannot be used here because Mobile IPv6 does not run over a transport protocol.
The SRV RR has a DNS type code of 33. As an example, the Mobile constructs a request with QNAME set to "_mip6._ipv6.example.com" and QTYPE to SRV. The reply contains the FQDNs of one or more servers that can then be resolved in a separate DNS transaction to the IP addresses. However, if there is room in the SRV reply, it is RECOMMENDED that the DNS server also return the IP addresses of the Home Agents in AAAA records as part of the additional data section (in order to avoid requiring an additional DNS round trip to resolve the FQDNs).
5.2. IPsec Security Associations Setup
As soon as the Mobile Node has discovered the Home Agent Address, it establishes an IPsec Security Association with the Home Agent itself through IKEv2. The detailed description of this procedure is provided in . If the Mobile Node wants the HA to register the Home Address in the DNS, it MUST use the FQDN as the initiator identity in the IKE_AUTH step of the IKEv2 exchange (IDi). This is needed because the Mobile Node has to prove it is the owner of the FQDN provided in the subsequent DNS Update Option. See Sections 6 and 9 for a more detailed analysis on this issue.
The IKEv2 Mobile Node to Home Agent authentication can be performed using either IKEv2 public key signatures or the Extensible Authentication Protocol (EAP). The details about how to use IKEv2 authentication are described in  and . The choice of an IKEv2 peer authentication method depends on the deployment. The Mobile Node should be configured with the information on which IKEv2 authentication method to use. However, IKEv2 restricts the Home Agent to Mobile Node authentication to use public key signature-based authentication.
5.3. Home Address Assignment
Home Address assignment is performed during the IKEv2 exchange. The Home Address can be assigned directly by the Home Agent or it can be auto-configured by the Mobile Node.
5.3.1. Home Address Assignment by the Home Agent
When the Mobile Node runs IKEv2 with its Home Agent, it can request a HoA through the Configuration Payload in the IKE_AUTH exchange by including an INTERNAL_IP6_ADDRESS attribute. When the Home Agent processes the message, it allocates a HoA and sends it a CFG_REPLY message. The Home Agent could consult a DHCP server on the home link for the actual home address allocation. This is explained in detail in .
5.3.2. Home Address Auto-Configuration by the Mobile Node
With the type of assignment described in the previous section, the Home Address cannot be generated based on Cryptographically Generated Addresses (CGAs)  or based on the privacy extensions for stateless auto-configuration . However, the Mobile Node might want to have an auto-configured HoA based on these mechanisms. It is worthwhile to mention that the auto-configuration procedure described in this section cannot be used in some possible deployments, since the Home Agents might be provisioned with pools of allowed Home Addresses.
In the simplest case, the Mobile Node is provided with a pre- configured home prefix and home prefix length. In this case, the Mobile Node creates a Home Address based on the pre-configured prefix and sends it to the Home Agent, including an INTERNAL_IP6_ADDRESS attribute in a Configuration Payload of type CFG_REQUEST. If the Home Address is valid, the Home Agent replies with a CFG_REPLY, including an INTERNAL_IP6_ADDRESS with the same address. If the Home Address provided by the Mobile Node is not valid, the Home Agent assigns a different Home Address including an INTERNAL_IP6_ADDRESS attribute with a new value. According to , the Mobile Node MUST use the address sent by the Home Agent. Later, if the Mobile Node wants to use an auto-configured Home Address (e.g., based on CGA), it can run Mobile Prefix Discovery, obtain a prefix, auto-configure a new Home Address, and then perform a new CREATE_CHILD_SA exchange.
If the Mobile Node is not provided with a pre-configured Home Prefix, the Mobile cannot simply propose an auto-configured HoA in the Configuration Payload since the Mobile Node does not know the home prefix before the start of the IKEv2 exchange. The Mobile Node must obtain the home prefix and the home prefix length before it can configure a home address.
One simple solution would be for the Mobile Node to just assume that the prefix length on the home link is 64 bits and extract the home prefix from the Home Agent's address. The disadvantage with this solution is that the home prefix cannot be anything other than /64. Moreover, this ties the prefix on the home link and the Home Agent's address, but, in general, a Home Agent with a particular address should be able to serve a number of prefixes on the home link, not just the prefix from which its address is configured.
Another solution would be for the Mobile Node to assume that the prefix length on the home link is 64 bits and send its interface identifier to the Home Agent in the INTERNAL_IP6_ADDRESS attribute within the CFG_REQ payload. Even though this approach does not tie the prefix on the home link and the Home Agent's address, it still requires that the home prefix length is 64 bits.
For this reason, the Mobile Node needs to obtain the home link prefixes through the IKEv2 exchange. In the Configuration Payload during the IKE_AUTH exchange, the Mobile Node includes the MIP6_HOME_PREFIX attribute in the CFG_REQUEST message. The Home Agent, when it processes this message, MUST include in the CFG_REPLY payload prefix information for one prefix on the home link. This prefix information includes the prefix length (see Section 8.2). The Mobile Node auto-configures a Home Address from the prefix returned in the CFG_REPLY message and runs a CREATE_CHILD_SA exchange to create security associations for the new Home Address.
As mentioned before in this document, there are deployments where auto-configuration of the Home Address cannot be used. In this case, when the Home Agent receives a CFG_REQUEST that includes a MIP6_HOME_PREFIX attribute in the subsequent IKE response, it includes a Notify Payload type "USE_ASSIGNED_HoA" and the related Home Address in a INTERNAL_IP6_ADDRESS attribute. If the Mobile Node gets a "USE_ASSIGNED_HoA" Notify Payload in response to the Configuration Payload containing the MIP6_HOME_PREFIX attribute, it looks for an INTERNAL_IP6_ADDRESS attribute and MUST use the address contained in it in the subsequent CREATE_CHILD_SA exchange.
When the Home Agent receives a Binding Update for the Mobile Node, it performs proxy DAD for the auto-configured Home Address. If DAD fails, the Home Agent rejects the Binding Update. If the Mobile Node receives a Binding Acknowledgement with status 134 (DAD failed), it MUST stop using the current Home Address, configure a new HoA, and then run IKEv2 CREATE_CHILD_SA exchange to create security associations based on the new HoA. The Mobile Node does not need to run IKE_INIT and IKE_AUTH exchanges again. Once the necessary security associations are created, the Mobile Node sends a Binding Update for the new Home Address.
It is worth noting that with this mechanism, the prefix information carried in MIP6_HOME_PREFIX attribute includes only one prefix and does not carry all the information that is typically present when received through a IPv6 router advertisement. Mobile Prefix Discovery, specified in RFC 3775, is the mechanism through which the Mobile Node can get all prefixes on the home link and all related information. That means that MIP6_HOME_PREFIX attribute is only used for Home Address auto-configuration and does not replace the usage of Mobile Prefix Discovery for the purposes detailed in RFC 3775.
5.4. Authorization and Authentication with MSA
In a scenario where the Home Agent is discovered dynamically by the Mobile Node, it is very likely that the Home Agent is not able to verify by its own the credentials provided by the Mobile Node during the IKEv2 exchange. Moreover, the mobility service needs to be explicitly authorized based on the user's profile. As an example, the Home Agent might not be aware of whether the mobility service can be granted at a particular time of the day or when the credit of the Mobile Node is going to expire.
Due to all these reasons, the Home Agent may need to contact the MSA in order to authenticate the Mobile Node and authorize mobility service. This can be accomplished based on a Public Key Infrastructure if certificates are used and a PKI is deployed by the MSP and MSA. On the other hand, if the Mobile Node is provided with other types of credentials, the AAA infrastructure must be used.
The definition of this backend communication is out of the scope of this document. In , a list of goals for such a communication is provided.  and  define the RADIUS and Diameter extensions, respectively, for the backend communication.
6. Home Address Registration in the DNS
In order that the Mobile Node is reachable through its dynamically assigned Home Address, the DNS needs to be updated with the new Home Address. Since applications make use of DNS lookups on FQDN to find a node, the DNS update is essential for providing IP reachability to the Mobile Node, which is the main purpose of the Mobile IPv6 protocol. The need for DNS updating is not discussed in RFC 3775 since it assumes that the Mobile Node is provisioned with a static Home Address. However, when a dynamic Home Address is assigned to the Mobile Node, any existing DNS entry becomes invalid and the Mobile Node becomes unreachable unless a DNS update is performed.
Since the DNS update must be performed securely in order to prevent attacks or modifications from malicious nodes, the node performing this update must share a security association with the DNS server. Having all possible Mobile Nodes sharing a security association with the DNS servers of the MSP might be cumbersome from an administrative perspective. Moreover, even if a Mobile Node has a security association with a DNS server of its MSP, an address authorization issue comes into the picture. A detailed analysis of possible threats against DNS update is provided in Section 9.5.
Therefore, due to security and administrative reasons, it is RECOMMENDED that the Home Agent perform DNS entry updates for the
Mobile Node. For this purpose, the Mobile Node MAY include a new mobility option in the Binding Update, the DNS Update option, with the flag R not set in the option. This option is defined in Section 8 and includes the FQDN that needs to be updated. After receiving the Binding Update, the Home Agent MUST update the DNS entry with the identifier provided by the Mobile Node and the Home Address included in the Home Address Option. The procedure for sending a dynamic DNS update message is specified in . The dynamic DNS update SHOULD be performed in a secure way; for this reason, the usage of TKEY and TSEC or DNSSEC is recommended (see Section 9.5 for details). As soon as the Home Agent has updated the DNS, it MUST send a Binding Acknowledgement message to the Mobile Node, including the DNS Update mobility option with the correct value in the Status field (see Section 8.1).
This procedure can be performed directly by the Home Agent if the Home Agent has a security association with the domain specified in the Mobile Node's FQDN.
On the other hand, if the Mobile Node wants to be reachable through a FQDN that belongs to the MSA, the Home Agent and the DNS server that must be updated belong to different administrative domains. In this case, the Home Agent may not share a security association with the DNS server and the DNS entry update can be performed by the AAA server of the MSA. In order to accomplish this, the Home Agent sends to the AAA server the FQDN-HoA pair through the AAA protocol. This message is proxied by the AAA infrastructure of the MSP and is received by the AAA server of the MSA. The AAA server of the MSA performs the DNS update based on . Notice that, even in this case, the Home Agent is always required to perform a DNS update for the reverse entry, since this is always performed in the DNS server of the MSP. The detailed description of the communication between Home Agent and AAA is out of the scope of this document. More details are provided in .
A mechanism to remove stale DNS entries is needed. A DNS entry becomes stale when the related Home Address is no longer used by the Mobile Node. To remove a DNS entry, the Mobile Node includes in the Binding Update the DNS Update mobility option, with the flag R set in the option. After receiving the Binding Update, the Home Agent MUST remove the DNS entry identified by the FQDN provided by the Mobile Node and the Home Address included in the Home Address Option. The procedure for sending a dynamic DNS update message is specified in . As mentioned above, the dynamic DNS update SHOULD be performed in a secure way; for this reason, the usage of TKEY and TSEC or DNSSEC is recommended (see Section 9.5 for details).
If there is no explicit de-registration BU from the Mobile Node, then the HA MAY use the binding cache entry expiration as a trigger to remove the DNS entry.
7. Summary of Bootstrapping Protocol Flow
The message flow of the whole bootstrapping procedure when the dynamic DNS update is performed by the Home Agent is depicted below.
+----+ +----+ +-----+
| MN | | HA | | DNS |
+----+ +----+ +-----+
IKEv2 exchange (HoA configuration) <======================> BU (DNS update option) -----------------------> DNS update <-------------------> BA (DNS update option) <-----------------------
On the contrary, the figure below shows the message flow of the whole bootstrapping procedure when the dynamic DNS update is performed by the AAA server of the MSA.
+----+ +----+ +---+ +---+
| MN | | HA | |AAA| |DNS|
+----+ +----+ +---+ +---+
IKEv2 exchange (HoA configuration) <======================> BU (DNS update option) -----------------------> AAA request (FQDN, HoA) <--------------> DNS update <-----------> AAA answer (FQDN, HoA) <--------------> BA (DNS update option) <-----------------------
Notice that even in this last case, the Home Agent is always required to perform a DNS update for the reverse entry, since this is always performed in the DNS server of the MSP. This is not depicted in the figure above.
8. Option and Attribute Format
8.1. DNS Update Mobility Option
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Type | Option Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Status |R| Reserved | MN identity (FQDN) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Option Type DNS-UPDATE-TYPE (17) Option Length
8-bit unsigned integer indicating the length of the option excluding the type and length fields
8-bit unsigned integer indicating the result of the dynamic DNS update procedure. This field MUST be set to 0 and ignored by the receiver when the DNS Update mobility option is included in a Binding Update message. When the DNS Update mobility option is included in the Binding Acknowledgement message, values of the Status field less than 128 indicate that the dynamic DNS update was performed successfully by the Home Agent. Values greater than or equal to 128 indicate that the dynamic DNS update was not completed by the HA. The following Status values are currently defined:
- DNS update performed
- Reason unspecified
- Administratively prohibited
- DNS update failed
If set, the Mobile Node is requesting the HA to remove the DNS entry identified by the FQDN specified in this option and the HoA of the Mobile Node. If not set, the Mobile Node is requesting the HA to create or update a DNS entry with its HoA and the FQDN specified in the option.
MUST be set to 0.
The identity of the Mobile Node in FQDN format to be used by the Home Agent to send a Dynamic DNS update. It is a variable length field.
8.2. MIP6_HOME_PREFIX Attribute
The MIP6_HOME_PREFIX attribute is carried in IKEv2 Configuration Payload messages. This attribute is used to convey the home prefix from which the Mobile Node auto-configures its home address.
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |R| Attribute Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Prefix Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Home Prefix | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Prefix Length | +-+-+-+-+-+-+-+-+ Reserved (1 bit)
This bit MUST be set to zero and MUST be ignored on receipt.
Attribute Type (15 bits)
A unique identifier for the MIP6_HOME_PREFIX attribute (16).
Length (2 octets)
Length in octets of Value field (Home Prefix, Prefix Lifetime and Prefix Length). This can be 0 or 21.
Prefix Lifetime (4 octets)
The lifetime of the Home Prefix.
Home Prefix (16 octets)
The prefix of the home link through which the Mobile Node may auto-configure its Home Address.
Prefix Length (1 octet)
The length in bits of the home prefix specified in the field Home Prefix.
When the MIP6_HOME_PREFIX attribute is included by the Mobile Node in the CFG_REQUEST payload, the value of the Length field is 0. When the MIP6_HOME_PREFIX attribute is included in the CFG_REPLY payload by the Home Agent, the value of the Length field is 21 and the attribute contains also the home prefix information.
9. Security Considerations
9.1. HA Address Discovery
Use of DNS for address discovery carries certain security risks. DNS transactions in the Internet are typically done without any authentication of the DNS server by the client or of the client by the server. There are two risks involved:
- A legitimate client obtains a bogus Home Agent address from a bogus DNS server. This is sometimes called a "pharming" attack.
- An attacking client obtains a legitimate Home Agent address from a legitimate server.
The risk in Case 1 is mitigated because the Mobile Node is required to conduct an IKE transaction with the Home Agent prior to performing a Binding Update to establish Mobile IPv6 service. According to the IKEv2 specification , the responder must present the initiator with a valid certificate containing the responder's public key, and the responder to initiator IKE_AUTH message must be protected with an authenticator calculated using the public key in the certificate. Thus, an attacker would have to set up both a bogus DNS server and a bogus Home Agent, and provision the Home Agent with a certificate that a victim Mobile Node could verify. If the Mobile Node can detect that the certificate is not trustworthy, the attack will be foiled when the Mobile Node attempts to set up the IKE SA.
The risk in Case 2 is limited for a single Mobile Node to Home Agent transaction if the attacker does not possess proper credentials to authenticate with the Home Agent. The IKE SA establishment will fail when the attacking Mobile Node attempts to authenticate itself with the Home Agent. Regardless of whether the Home Agent utilizes EAP or host-side certificates to authenticate the Mobile Node, the authentication will fail unless the Mobile Node has valid credentials.
Another risk exists in Case 2 because the attacker may be attempting to propagate a Denial of Service (DoS) attack on the Home Agent. In that case, the attacker obtains the Home Agent address from the DNS, then propagates the address to a network of attacking hosts that bombard the Home Agent with traffic. This attack is not unique to the bootstrapping solution, however, it is actually a risk that any Mobile IPv6 Home Agent installation faces. In fact, the risk is faced by any service in the Internet that distributes a unicast globally routable address to clients. Since Mobile IPv6 requires that the Mobile Node communicate through a globally routable unicast address of a Home Agent, it is possible that the Home Agent address could be propagated to an attacker by various means (theft of the Mobile Node, malware installed on the Mobile Node, evil intent of the Mobile Node owner him/herself, etc.) even if the home address is manually configured on the Mobile Node. Consequently, every Mobile IPv6 Home Agent installation will likely be required to mount anti- DoS measures. Such measures include overprovisioning of links to and from Home Agents and of Home Agent processing capacity, vigilant monitoring of traffic on the Home Agent networks to detect when traffic volume increases abnormally indicating a possible DoS attack, and hot spares that can quickly be switched on in the event an attack is mounted on an operating collection of Home Agents. DoS attacks of moderate intensity should be foiled during the IKEv2 transaction. IKEv2 implementations are expected to generate their cookies without any saved state, and to time out cookie generation parameters frequently, with the timeout value increasing if a DoS attack is suspected. This should prevent state depletion attacks, and should assure continued service to legitimate clients until the practical limits on the network bandwidth and processing capacity of the Home Agent network are reached.
Explicit security measures between the DNS server and host, such as DNSSEC  or TSIG/TKEY  , can mitigate the risk of 1) and 2), but these security measures are not widely deployed on end nodes. These security measures are not sufficient to protect the Home Agent address against DoS attack, however, because a node having a legitimate security association with the DNS server could nevertheless intentionally or inadvertently cause the Home Agent address to become the target of DoS.
Finally, notice that the assignment of a home agent from the serving network access provider's (local home agent) or a home agent from a nearby network (nearby home agent) may set up the potential to compromise a mobile node's location privacy. A home address anchored at such a home agent contains some information about the topological location of the mobile node. Consequently, a mobile node requiring location privacy should not disclose this home address to nodes that are not authorized to learn the mobile node's location, e.g., by updating DNS with the new home address.
Security considerations for discovering HA using DHCP are covered in .
9.2. Home Address Assignment through IKEv2
Mobile IPv6 bootstrapping assigns the home address through the IKEv2 transaction. The Mobile Node can either choose to request an address, similar to DHCP, or the Mobile Node can request a prefix on the home link, then auto-configure an address.
RFC 3775  requires that a Home Agent check authorization of a home address received during a Binding Update. Therefore, the home agent MUST authorize each home address allocation and use. This authorization is based on linking the mobile node identity used in the IKEv2 authentication process and the home address. Home agents MUST implement at least the following two modes of authorization:
- Configured home address(es) for each mobile node. In this mode, the home agent or infrastructure nodes behind it know what addresses a specific mobile node is authorized to use. The mobile node is allowed to request these addresses, or if the mobile node requests any home address, these addresses are returned to it.
o First-come-first-served (FCFS). In this mode, the home agent maintains a pool of "used" addresses, and allows mobile nodes to request any address, as long as it is not used by another mobile node.
Addresses MUST be marked as used for at least as long as the binding exists, and are associated with the identity of the mobile node that made the allocation.
In addition, the home agent MUST ensure that the requested address is not the authorized address of any other mobile node, i.e., if both FCFS and configured modes use the same address space.
9.3. SA Establishment Using EAP through IKEv2
Security considerations for authentication of the IKE transaction using EAP are covered in  .
9.4. Backend Security between the HA and AAA Server
Some deployments of Mobile IPv6 bootstrapping may use an AAA server to handle authorization for mobility service. This process has its own security requirements, but the backend protocol for a Home Agent to an AAA server interface is not covered in this document. Please see  for a discussion of this interface.
9.5. Dynamic DNS Update
If a Home Agent performs dynamic DNS update on behalf of the Mobile Node directly with the DNS server, the Home Agent MUST have a security association of some type with the DNS server. The security association MAY be established either using DNSSEC  or TSIG/TKEY . A security association is REQUIRED even if the DNS server is in the same administrative domain as the Home Agent. The security association SHOULD be separate from the security associations used for other purposes, such as AAA.
In the case where the Mobility Service Provider is different from the Mobility Service Authorizer, the network administrators may want to limit the number of cross-administrative domain security associations. If the Mobile Node's FQDN is in the Mobility Service Authorizer's domain, since a security association for AAA signaling involved in mobility service authorization is required in any case, the Home Agent can send the Mobile Node's FQDN to the AAA server under the HA-AAA server security association, and the AAA server can perform the update. In that case, a security association is required between the AAA server and DNS server for the dynamic DNS update. See  for a deeper discussion of the Home Agent to AAA server interface.
Regardless of whether the AAA server or Home Agent performs DNS update, the authorization of the Mobile Node to update a FQDN MUST be checked prior to the performance of the update. It is an implementation issue as to how authorization is determined. However, in order to allow this authorization step, the Mobile Node MUST use a FQDN as the IDi in IKE_AUTH step of the IKEv2 exchange. The FQDN MUST be the same as the FQDN that will be provided by the Mobile Node in the DNS Update Option.
In case EAP over IKEv2 is used to set-up the IPsec SA, the Home Agent gets authorization information about the Mobile Node's FQDN via the AAA back end communication performed during IKEv2 exchange. The outcome of this step will give the Home Agent the necessary information to authorize the DNS update request of the Mobile Node. See  for details about the communication between the AAA server and the Home Agent needed to perform the authorization.
In case certificates are used in IKEv2, the authorization information about the FQDN for DNS update MUST be present in the certificate provided by the Mobile Node. Since the IKEv2 specification does not include a required certificate type, it is not possible to specify precisely how the Mobile Node's FQDN should appear in the certificate. However, as an example, if the certificate type is "X.509 Certificate - Signature" (one of the recommended types), then the FQDN may appear in the subjectAltName attribute extension .
10. IANA Considerations
This document defines a new Mobility Option and a new IKEv2 Configuration Attribute Type.
The following values have been assigned:
o from the "Mobility Option" namespace (): DNS-UPDATE-TYPE, 17 (Section 8.1)
- from the "IKEv2 Configuration Payload Attribute Types" namespace (): MIP6_HOME_PREFIX attribute, 16 (Section 8.2)
- from the "IKEv2 Notify Payload Error Types" namespace (): USE_ASSIGNED_HoA error type, 42 (Section 5.3.2)
This document creates a new name space "Status Codes (DNS Update Mobility Option)" for the status field in the DNS Update mobility option. The current values are described in Section 8.1 and are listed below.
- DNS update performed
- Reason unspecified
- Administratively prohibited
- DNS update failed
Future values of the Status field in the DNS Update mobility option can be allocated using Standards Action or IESG approval.
This contribution is a joint effort of the bootstrapping solution design team of the MIP6 WG. The contributors include Basavaraj Patil, Alpesh Patel, Jari Arkko, James Kempf, Yoshihiro Ohba, Gopal Dommety, Alper Yegin, Junghoon Jee, Vijay Devarapalli, Kuntal Chowdury, and Julien Bournelle.
The design team members can be reached at:
Gerardo Giaretta, firstname.lastname@example.org Basavaraj Patil, email@example.com Alpesh Patel, firstname.lastname@example.org Jari Arkko, email@example.com James Kempf, firstname.lastname@example.org Yoshihiro Ohba, email@example.com Gopal Dommety, firstname.lastname@example.org Alper Yegin, email@example.com Vijay Devarapalli, firstname.lastname@example.org Kuntal Chowdury, email@example.com Junghoon Jee, firstname.lastname@example.org Julien Bournelle, email@example.com
The authors would like to thank Rafa Lopez, Francis Dupont, Jari Arkko, Kilian Weniger, Vidya Narayanan, Ryuji Wakikawa, and Michael Ye for their valuable comments.
13.1. Normative References
 Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in IPv6", RFC 3775, June 2004.  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  Devarapalli, V. and F. Dupont, "Mobile IPv6 Operation with IKEv2 and the Revised IPsec Architecture", RFC 4877, April 2007.  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000.  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC 4306, December 2005.  Vixie, P., Thomson, S., Rekhter, Y., and J. Bound, "Dynamic Updates in the Domain Name System (DNS UPDATE)", RFC 2136, April 1997.
13.2. Informative References
 Patel, A. and G. Giaretta, "Problem Statement for bootstrapping Mobile IPv6 (MIPv6)", RFC 4640, September 2006.  Manner, J. and M. Kojo, "Mobility Related Terminology", RFC 3753, June 2004.  Adams, C., Farrell, S., Kause, T., and T. Mononen, "Internet X.509 Public Key Infrastructure Certificate Management Protocol (CMP)", RFC 4210, September 2005.  Korver, B., "The Internet IP Security PKI Profile of IKEv1/ISAKMP, IKEv2, and PKIX", RFC 4945, August 2007.  Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, September 2007.  Aura, T., "Cryptographically Generated Addresses (CGA)", RFC 3972, March 2005.  Narten, T., Draves, R., and S. Krishnan, "Privacy Extensions for Stateless Address Autoconfiguration in IPv6", RFC 4941, September 2007.  Droms, R., "DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646, December 2003.  Chowdhury, K. and A. Yegin, "MIP6-bootstrapping for the Integrated Scenario", Work in Progress, June 2007.  Giaretta, G., "AAA Goals for Mobile IPv6", Work in Progress, September 2006.  Chowdhury, K., "RADIUS Mobile IPv6 Support", Work in Progress, March 2007.  Bournelle, J., "Diameter Mobile IPv6: HA <-> HAAA Support", Work in Progress, May 2007.  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, March 2005.  Vixie, P., Gudmundsson, O., Eastlake, D., and B. Wellington, "Secret Key Transaction Authentication for DNS (TSIG)", RFC 2845, May 2000.  Eastlake, D., "Secret Key Establishment for DNS (TKEY RR)", RFC 2930, September 2000.  Jang, H., "DHCP Option for Home Information Discovery in MIPv6", Work in Progress, June 2007.  Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 3280, April 2002.
firstname.lastname@example.org James Kempf DoCoMo Labs USA 181 Metro Drive San Jose, CA 95110 USA EMail: email@example.com Vijay Devarapalli Azaire Networks 3121 Jay Street Santa Clara, CA 95054 USA EMail: firstname.lastname@example.org
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