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RFC 5026 - Mobile IPv6 Bootstrapping in Split Scenario


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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.

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

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.

   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).

   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 [9].  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      |  |
                      |  |             |  |
                      |  +-------------+  |
                      |                   |
                      +-------------------+

                                 +--+
                                 |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.

   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

   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.

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

   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

   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

   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 [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

   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).

   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

   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.

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:

   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

   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].

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.

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

   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.

   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.

   [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.

   [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

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