Network Working Group G. Giaretta, Ed.
Request for Comments: 5026 Qualcomm
Category: Standards Track J. Kempf
DoCoMo Labs USA
V. Devarapalli, Ed.
Azaire Networks
October 2007
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.
Abstract
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.
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RFC 5026 MIP6 Bootstrapping in Split Scenario October 2007
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
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1. Introduction
Mobile IPv6 [1] 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 [7].
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 [7], 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).
2. Terminology
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 [2].
General mobility terminology can be found in [8]. 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.
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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.
Split scenario
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 [7], 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 [7] 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 [7] 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).
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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)
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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 [9]. This is conceptually similar to Figure
1, since the MSP == MSA, except the Home Agent, and potentially the
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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 | |
| | | |
| +-------------+ |
| |
+-------------------+
+--+
|MN|
+--+
Figure 3 -- Split Scenario with PKI
For more details on the use of PKI for IKEv2 authentication, please
refer to [3] and [10].
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 [7], 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.
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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 [1] 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 [4]. 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 [3]. 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 [3]).
The solution defined in this document also allows the Mobile Node
to auto-configure its Home Address based on stateless auto-
configuration [11], Cryptographically Generated Addresses [12], or
privacy addresses [13].
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
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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 [14]. 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 [15] for details.
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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 [4] 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 [4] 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
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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 [3]. 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 [3] and [5]. 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
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for the actual home address allocation. This is explained in detail
in [3].
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) [12] or based on the privacy extensions for
stateless auto-configuration [13]. 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 [5], 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
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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.
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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 [16], a list of goals for such a communication is
provided. [17] and [18] 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
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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 [6]. 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 [6]. 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 [16].
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
[6]. 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).
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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)
<-----------------------
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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)
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Option Length
8-bit unsigned integer indicating the length of the option
excluding the type and length fields
Status
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:
0 DNS update performed
128 Reason unspecified
129 Administratively prohibited
130 DNS update failed
R flag
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.
Reserved
MUST be set to 0.
MN identity
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.
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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.
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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:
1. A legitimate client obtains a bogus Home Agent address from a
bogus DNS server. This is sometimes called a "pharming" attack.
2. 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 [5], 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
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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 [19] or TSIG/TKEY [20] [21], 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
[22].
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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 [1] 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:
o 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 [3] .
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 [16] for a discussion of this interface.
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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 [19] or TSIG/TKEY
[20][21]. 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 [16] 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 [16] 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
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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 [23].
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 ([1]): DNS-UPDATE-TYPE, 17
(Section 8.1)
o from the "IKEv2 Configuration Payload Attribute Types" namespace
([5]): MIP6_HOME_PREFIX attribute, 16 (Section 8.2)
o from the "IKEv2 Notify Payload Error Types" namespace ([5]):
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.
0 DNS update performed
128 Reason unspecified
129 Administratively prohibited
130 DNS update failed
Future values of the Status field in the DNS Update mobility option
can be allocated using Standards Action or IESG approval.
11. Contributors
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.
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The design team members can be reached at:
Gerardo Giaretta, gerardog@qualcomm.com
Basavaraj Patil, basavaraj.patil@nokia.com
Alpesh Patel, alpesh@cisco.com
Jari Arkko, jari.arkko@kolumbus.fi
James Kempf, kempf@docomolabs-usa.com
Yoshihiro Ohba, yohba@tari.toshiba.com
Gopal Dommety, gdommety@cisco.com
Alper Yegin, alper.yegin@samsung.com
Vijay Devarapalli, vijay.devarapalli@azairenet.com
Kuntal Chowdury, kchowdury@starentnetworks.com
Junghoon Jee, jhjee@etri.re.kr
Julien Bournelle, julien.bournelle@gmail.com
12. Acknowledgements
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. References
13.1. Normative References
[1] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in
IPv6", RFC 3775, June 2004.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Devarapalli, V. and F. Dupont, "Mobile IPv6 Operation with
IKEv2 and the Revised IPsec Architecture", RFC 4877, April
2007.
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[4] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[5] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC
4306, December 2005.
[6] 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
[7] Patel, A. and G. Giaretta, "Problem Statement for bootstrapping
Mobile IPv6 (MIPv6)", RFC 4640, September 2006.
[8] Manner, J. and M. Kojo, "Mobility Related Terminology", RFC
3753, June 2004.
[9] Adams, C., Farrell, S., Kause, T., and T. Mononen, "Internet
X.509 Public Key Infrastructure Certificate Management Protocol
(CMP)", RFC 4210, September 2005.
[10] Korver, B., "The Internet IP Security PKI Profile of
IKEv1/ISAKMP, IKEv2, and PKIX", RFC 4945, August 2007.
[11] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[12] Aura, T., "Cryptographically Generated Addresses (CGA)", RFC
3972, March 2005.
[13] Narten, T., Draves, R., and S. Krishnan, "Privacy Extensions
for Stateless Address Autoconfiguration in IPv6", RFC 4941,
September 2007.
[14] Droms, R., "DNS Configuration options for Dynamic Host
Configuration Protocol for IPv6 (DHCPv6)", RFC 3646, December
2003.
[15] Chowdhury, K. and A. Yegin, "MIP6-bootstrapping for the
Integrated Scenario", Work in Progress, June 2007.
[16] Giaretta, G., "AAA Goals for Mobile IPv6", Work in Progress,
September 2006.
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RFC 5026 MIP6 Bootstrapping in Split Scenario October 2007
[17] Chowdhury, K., "RADIUS Mobile IPv6 Support", Work in Progress,
March 2007.
[18] Bournelle, J., "Diameter Mobile IPv6: HA <-> HAAA Support",
Work in Progress, May 2007.
[19] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"DNS Security Introduction and Requirements", RFC 4033, March
2005.
[20] Vixie, P., Gudmundsson, O., Eastlake, D., and B. Wellington,
"Secret Key Transaction Authentication for DNS (TSIG)", RFC
2845, May 2000.
[21] Eastlake, D., "Secret Key Establishment for DNS (TKEY RR)", RFC
2930, September 2000.
[22] Jang, H., "DHCP Option for Home Information Discovery in
MIPv6", Work in Progress, June 2007.
[23] 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.
Authors' Addresses
Gerardo Giaretta
Qualcomm
EMail: gerardog@qualcomm.com
James Kempf
DoCoMo Labs USA
181 Metro Drive
San Jose, CA 95110
USA
EMail: kempf@docomolabs-usa.com
Vijay Devarapalli
Azaire Networks
3121 Jay Street
Santa Clara, CA 95054
USA
EMail: vijay.devarapalli@azairenet.com
Giaretta, et al. Standards Track [Page 27]
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Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
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Giaretta, et al. Standards Track [Page 28]