RFC 5726
Mobile IPv6 Location Privacy Solutions
Pages: 48
Experimental
Experimental
Part 1 of 3 – Pages 1 to 20
Internet Research Task Force (IRTF) Y. Qiu Request for Comments: 5726 Institute for Infocomm Research Category: Experimental F. Zhao, Ed. ISSN: 2070-1721 Google R. Koodli Cisco Systems February 2010 Mobile IPv6 Location Privacy SolutionsAbstract
Mobile IPv6 (RFC 3775) enables a mobile node to remain reachable while it roams on the Internet. However, the location and movement of the mobile node can be revealed by the IP addresses used in signaling or data packets. In this document, we consider the Mobile IPv6 location privacy problem described in RFC 4882, and propose efficient and secure techniques to protect location privacy of the mobile node. This document is a product of the IP Mobility Optimizations (MobOpts) Research Group. Status of This Memo This document is not an Internet Standards Track specification; it is published for examination, experimental implementation, and evaluation. This document defines an Experimental Protocol for the Internet community. This document is a product of the Internet Research Task Force (IRTF). The IRTF publishes the results of Internet-related research and development activities. These results might not be suitable for deployment. This RFC represents the consensus of the IP Mobility Optimizations Research Group of the Internet Research Task Force (IRTF). Documents approved for publication by the IRSG are not a candidate for any level of Internet Standard; see Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc5726.
Copyright Notice Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document.
Table of Contents
1. Introduction ....................................................5 2. Conventions and Terminology .....................................6 2.1. Conventions ................................................6 2.2. Terminology ................................................6 3. Requirements ....................................................8 4. Solution Overview ...............................................9 5. Reverse-Tunneled Correspondent Binding Update ..................11 5.1. The Procedure .............................................12 5.2. Route-Optimized Payload Packets ...........................14 5.3. Mobile Node Operation .....................................15 5.3.1. Conceptual Data Structures .........................15 5.3.2. Reverse-Tunneled Correspondent Binding Update to the Correspondent Node ...................15 5.3.3. Reverse-Tunneled Correspondent Binding Acknowledgement from the Correspondent Node ........16 5.3.4. Route-Optimized Payload Packets ....................16 5.3.5. Receiving ICMP Error Message .......................17 5.3.6. Binding Error from the Correspondent Node ..........17 5.3.7. Binding Refresh Request from the Correspondent Node .................................17 5.4. Home Agent Operation ......................................17 5.5. Correspondent Node Operation ..............................18 5.5.1. Conceptual Data Structures .........................18 5.5.2. Reverse-Tunneled Correspondent Binding Update from the Mobile Node ........................18 5.5.3. Reverse-tunneled Correspondent Binding Acknowledgement to the Mobile Node .................18 5.5.4. Route-Optimized Payload Packets ....................18 5.5.5. ICMP Error Message to the Mobile Node ..............19 5.5.6. Binding Error to the Mobile Node ...................19 5.5.7. Binding Refresh Request to the Mobile Node .........19 5.6. Summary ...................................................20 6. IP Address Location Privacy Solution Using the Pseudo Home Address ...................................................20 6.1. Home Binding Update .......................................20 6.1.1. Pseudo Home Address Registration ...................20 6.1.2. Home De-Registration ...............................21 6.2. Correspondent Binding Update Using the Pseudo Home Address ...................................................22 6.2.1. Return Routability Procedure .......................22 6.2.2. Route-Optimized Correspondent Binding Update .......24 6.2.3. Reverse-tunneled Correspondent Binding Update ......25 6.2.4. Using Different Pseudo Home Addresses with Different Correspondent Nodes ......................25 6.3. Payload Packets ...........................................25 6.3.1. Reverse Tunneling Mode .............................25
6.3.2. Route Optimization Mode ............................26
6.4. Prefix Discovery ..........................................26
6.5. Mobile Node Operation .....................................26
6.5.1. Conceptual Data Structures .........................26
6.5.2. Binding Update to the Home Agent ...................27
6.5.3. Binding Acknowledgement from the Home Agent ........27
6.5.4. Home Test Init to the Home Agent ...................28
6.5.5. Home Test from the Home Agent ......................28
6.5.6. Route-Optimized Payload Packets ....................29
6.5.7. Receiving Binding Refresh Request ..................29
6.6. Home Agent Operation ......................................29
6.6.1. Conceptual Data Structures .........................30
6.6.2. Binding Update from the Mobile Node ................30
6.6.3. Binding Acknowledgement to the Mobile Node .........31
6.6.4. Home Test Init from the Mobile Node ................31
6.6.5. Home Test to the Mobile Node .......................32
6.7. Correspondent Node Operation ..............................32
7. Extensions to Mobile IPv6 ......................................32
7.1. Encrypted Home Address Destination Option .................32
7.2. Encrypted Home Address Routing Header .....................33
7.3. Pseudo Home Address Mobility Option .......................34
7.4. Pseudo Home Address Acknowledgement Mobility Option .......35
8. Security Considerations ........................................37
8.1. Home Binding Update .......................................37
8.2. Correspondent Binding Update ..............................38
8.3. Route-Optimized Payload Packets ...........................38
9. Related Work ...................................................39
10. IANA Considerations ...........................................40
11. Conclusion ....................................................40
12. Acknowledgements ..............................................41
13. References ....................................................41
13.1. Normative References .....................................41
13.2. Informative References ...................................42
Appendix A. Profiling Attack: Discussion ..........................44
A.1. The Care-of Address ........................................44
A.2. Profiling on the Encrypted Home Address ....................44
A.3. The IPsec SPI ..............................................45
A.4. The IPsec Sequence Number ..................................45
A.5. The Regular Interval of Signaling Messages..................46
A.6. The Sequence Number in the Binding Update Message ..........46
A.7. Multiple Concurrent Sessions ...............................46
A.8. Summary ....................................................47
1. Introduction
The IP address location privacy problem is concerned with unwittingly revealing the current location of a mobile node to eavesdroppers and to communicating parties. In the presence of mobility as specified in Mobile IPv6 [6], there are two related problems: disclosing the care-of address to a correspondent node, and revealing the home address to an eavesdropper (please see the terminology below). A detailed description of the location privacy problem can be found in RFC 4882 [11]. This document assumes that the reader is familiar with the basic operation of Mobile IPv6 specified in RFC 3775, as well as the location privacy problem described in RFC 4882. In order to protect location privacy, a mobile node must not disclose the binding between its care-of address and its home address. In this document, we propose a set of extensions to the Mobile IPv6 specification to address the IP address location privacy problem. Related to the IP address location privacy is "profiling", where the activities of a mobile node are linked and then analyzed. Profiled activities may contribute to compromising a mobile node's location privacy, especially when combined with additional information. Furthermore, once location privacy is compromised, it may lead to more targeted profiling. Solutions to thwart profiling are important; however, they are not central to this document. We discuss profiling in the appendix. We propose two IP address location privacy solutions in this document. With the first solution (as described in Section 5), the mobile node can communicate with the correspondent node by using the real home address without location privacy being breached by eavesdroppers. This is done by using parameters generated during the return routability procedure to mask the real home address, which provides an evolution towards location privacy protection based on return routability messages already specified in RFC 3775. With the second solution (as described in Section 6), an IPsec tunnel mode security association with a non-null encryption algorithm is negotiated to encrypt signaling messages (including the real home address therein) exchanged between the mobile node and the home agent, for example, during the home binding update procedure. Furthermore, during the return routability procedure and the correspondent binding update procedure, a "pseudo home address" (the definition of this new term and many other commonly used mobility related terms is provided in Section 2) is used to replace the real home address in various messages, which allows the mobile node to hide its real home address from both the correspondent node and eavesdroppers without the need for additional extensions to the correspondent node. Moreover, the mobile node may mask the pseudo
home address by using the mechanism specified in Section 5 to further enhance location privacy protection. Each of these two solutions can be implemented on its own without relying on the other. The solutions presented in this document are designed based on the following assumptions. First, we focus on location privacy issues arising when the mobile node attaches to a foreign link; location privacy issues when the mobile node attaches to its home link, if any, are outside the scope of this document. Second, we assume that IPsec [2] is used to secure mobility signaling messages exchanged between the mobile node and the home agent; therefore, location privacy solutions when other security mechanisms are used are beyond the scope of this document. Third, we assume that eavesdroppers are passive attackers, e.g., an eavesdropper along the path traversed by traffic flows from or to the mobile node. We make this assumption because messages generated by active attackers can either be discarded based on local policy at a mobile node or the mobile node could choose to treat such messages like those of any other correspondent nodes. Thus, specific threats to location privacy posed by active attackers are also beyond the scope of this document. Fourth, in order to simplify analysis, we assume that both the correspondent node and the home agent are fixed nodes; if either is mobile, the same analysis and solutions for the mobile node may also apply. Finally, the same solution applies to each of the care-of addresses if a mobile node maintains more than one care-of address. This document represents the consensus of the MobOpts Research Group. It has been reviewed by the Research Group members active in the specific area of work. At the request of their chairs, this document has been comprehensively reviewed by multiple active contributors to the IETF Mobile IP related working groups.2. Conventions and Terminology
2.1. Conventions
The keywords "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 [1].2.2. Terminology
In this document, we introduce two new terms, "pseudo home address" and "encrypted home address". The definition of these two terms is provided in the following.
o Pseudo Home Address (pHoA): A unicast IPv6 address formed to
replace the real home address used in certain Mobile IPv6
signaling or data packets. Without explicit indication, the
pseudo home address looks like a regular IPv6 address [5].
o Encrypted Home Address (eHoA): The output when applying an
encryption algorithm to the real home address or the pseudo home
address with additional inputs, e.g., a key. The real home
address can be recovered from the encrypted home address by using
a decryption algorithm.
In addition, we use commonly adopted mobility-related terms as
defined in [6] and [11] throughout this document. Some of these
terms are provided below for easier reference. Nevertheless, we
assume that readers are familiar with the basic operation of the
Mobile IPv6 protocol as defined in RFC 3775 [6], RFC 3776 [7], and
RFC 4877 [8].
o Mobile Node (MN): A Mobile IPv6 compliant mobile node that can
roam on the Internet
o Correspondent Node (CN): An IPv6 node that communicates with the
mobile node
o Home Network: The network where the mobile node is normally
present when it is not roaming
o Visited Network: The network that the mobile node uses to access
the Internet when it is roaming
o Home Agent (HA): A router on the mobile node's home network that
provides forwarding support when the mobile node is roaming
o Home Address (HoA): The mobile node's unicast IP address valid on
its home network
o Care-of Address (CoA): The mobile node's unicast IP address valid
on the visited network
o Return Routability (RR): A procedure which enables secure binding
between the care-of address and the home address when no pre-
existing security association exists between the mobile node and
the correspondent node
o Home Test Init (HoTI) / Home Test (HoT) / Care-of Test Init (CoTI)
/ Care-of Test (CoT): Messages used during the return routability
procedure
o Binding Update (BU): A message used by the mobile node to securely
bind its care-of address to its home address at the correspondent
node or the home agent
o Binding Acknowledgement (BA): A response to the Binding Update
o Message Authentication Code (MAC): The value, which is computed
using HMAC_SHA1 in this document, that protects both a message's
integrity and its authenticity
o Route Optimization: A mechanism that allows direct routing of
packets between a roaming mobile node and its correspondent node,
without having to traverse the home network
o Reverse Tunneling or Bidirectional Tunneling: A mechanism used for
packet forwarding between a roaming mobile node and its
correspondent node via its home agent
3. Requirements
In this section, we describe the requirements that should be met by
the Mobile IPv6 location privacy solutions, hereafter referred to as
"the solution". These are some of the basic requirements set forth
in order to make the solution readily implementable by those familiar
with Mobile IPv6 and the related security protocols used with it
(such as IKEv2 [4] and IPsec).
R01: The solution must follow the framework and architecture of IPv6
and Mobile IPv6 (as specified in RFC 3775, RFC 3776, and RFC
4877).
R02: The solution must not interfere with the operation of IPsec.
This means that the principles and the operation specified in
RFC 3776 and RFC 4877 need to be followed. For example, the
IPsec security association and policy must be identified by the
real home address.
R03: The solution should provide back-compatibility in order for
different Mobile IPv6 entities to work together even though they
may have different capabilities. This requires the mobile node
to be able to detect whether the home agent or the correspondent
node supports the use of the location privacy solutions.
R04: The overhead resulting from the solution, in terms of payloads
or messages transmitted and memory, should be kept minimal.
4. Solution Overview
The IP address location privacy solutions proposed in this document intend to conceal the binding between the mobile node's real home address and its care-of address from eavesdroppers and the correspondent node. In this section, we present an overview of the proposed solutions. With the Mobile IPv6 specification, during the home binding update procedure, both the real home address and the care-of address are in the cleartext when either the IPsec tunnel mode or the IPsec transport mode is used with no encryption. As described in Section 6.1, the solution to prevent the real home address being leaked to eavesdroppers on the MN-HA path during the home binding update procedure is to set up an IPsec tunnel mode security association with a non-null encryption algorithm to encrypt home binding signaling messages and the real home address therein. This method is also used to enable location privacy protection during other mobility signaling message exchanges between the home agent and the mobile node, such as the prefix discovery procedure (see Section 6.4). When communicating with the correspondent node with the reverse tunneling mode, the mobile node can hide its current location from the correspondent node and eavesdroppers along the HA-CN path, since the care-of address is not included in payload packets transmitted on that path. Also, an IPsec security association with a non-null encryption algorithm established between the mobile node and the home agent can conceal the real home address carried in payload packets from eavesdroppers along the MN-HA path. In order to communicate with a correspondent node in the route optimization mode, the mobile node needs to perform the return routability procedure followed by the correspondent binding update procedure. With the current Mobile IPv6 specification, the real home address and the care-of address in the correspondent Binding Update message and payload packets are visible to eavesdroppers. Therefore, in order to send and receive packets through the optimized route and protect location privacy at the same time, the mobile node needs to disclose its care-of address and conceal its real home address. There are two different scenarios and we propose a different solution for each scenario. One scenario is that the correspondent node is able to obtain the mobile node's real home address and initiates communication with the mobile node by using the real home address. In this case, the mobile node needs to continue to use the real home address with the correspondent node in order to maintain session continuity, and to
conceal the real home address from eavesdroppers. The solution for this scenario (hereinafter referred to as "reverse-tunneled correspondent binding update") is described in Section 5. With this solution, the mobile node exchanges the same return routability signaling messages as defined in RFC 3775 with the correspondent node and then derives a privacy management key from keygen tokens and uses this key to encrypt the real home address. Finally, it reverse- tunnels an extended correspondent Binding Update message via the home agent to register the encrypted home address and the real home address at the correspondent node. After the correspondent registration, the mobile node and the correspondent node use the registered encrypted home address, instead of the real home address in payload packets exchanged via the optimized route. The encrypted home address is different for different correspondent nodes since the privacy management key would be different. The other scenario is that the mobile node prefers to conceal its real home address from both the correspondent node and the eavesdroppers (typically the mobile node initiates communication in this case, since the correspondent node does not know the real home address). The solution for this scenario is described in Section 6.2. With this solution, the mobile node first obtains a home keygen token generated based on the pseudo home address during the home address test procedure. Subsequently, the mobile node sends the correspondent Binding Update message to register the binding between the pseudo home address and the care-of address at the correspondent node via the optimized route. After the correspondent registration, the mobile node and the correspondent node use the registered pseudo home address, instead of the real home address, in payload packets exchanged via the optimized route. Note that the use of the pseudo home address is completely transparent to the correspondent node. Furthermore, it is feasible to throttle "profiling" on the pseudo home address by using a combination of these two solutions. That is, the mobile node uses the pseudo home address in the extended home address test procedure to obtain a home keygen token; then, it uses the pseudo home address instead of the real home address in the reverse-tunneled correspondent binding update procedure. With this solution, the encrypted pseudo home address used in route optimized payload packets looks different to eavesdroppers each time, after a new round of the return routability procedure is completed. Before a pseudo home address is used with a correspondent node, it MUST be registered with the home agent during the home registration procedure. The mobile node indicates the requested pseudo home address in a new mobility option, called the Pseudo Home Address option (see Section 7.3), carried in the home Binding Update message,
and the home agent indicates the status of pseudo home address registration in another new mobility option, called Pseudo Home Address Acknowledgement option (see Section 7.4), carried in the home Binding Acknowledgement message. The pseudo home address MUST be routable in order for the home agent to intercept packets destined at this pseudo home address. It is statistically difficult for other nodes to derive the real home address from the pseudo home address. A detailed description of pseudo home address generation is provided in Section 6.1.1.1. With extensions introduced in this document, a mobile node is able to discover whether the home agent and the correspondent node support the location privacy solutions or not. When present in the home Binding Update message, the Pseudo Home Address mobility option indicates that the mobile node requests the use of the location privacy solutions. If such a Binding Update message is valid and the home agent supports the location privacy solutions for this particular mobile node, it responds with the Pseudo Home Address Acknowledgement mobility option in the Binding Acknowledgement message. Otherwise, if the home agent does not support the location privacy solutions, it does not include the Pseudo Home Address Acknowledgement mobility option in the Binding Acknowledgement message. Similarly, the presence of the Encrypted Home Address destination option in the correspondent Binding Update message indicates to the correspondent node that the mobile node requests the use of the location privacy solutions. If such a Binding Update message is valid and the correspondent node supports the location privacy solutions for this particular mobile node, it responds with the Encrypted Home Address routing header in the correspondent Binding Acknowledgement message to the mobile node. If the correspondent node does not support the location privacy solutions, it rejects the mobile node's request by returning an ICMP Parameter Problem message with Code value set to 2. Furthermore, a home agent that recognizes such extensions but does not wish to provide location privacy protection MAY redirect the mobile node to another home agent. If the request for using the location privacy solutions is rejected, the mobile node may either proceed without location privacy protection, or try with a different home agent or a correspondent node, or abort the operation.5. Reverse-Tunneled Correspondent Binding Update
In this section, we describe a solution that protects location privacy against eavesdroppers when the mobile node uses the real home address during communication with the correspondent node via the optimized route. Note that this solution does not require any change to return routability signaling messages. The detailed description is as follows.
5.1. The Procedure
After the return routability procedure is completed, if the mobile node needs to protect location privacy, and at the same time still uses the real home address with the correspondent node, the mobile node derives a privacy management key, Kpm, from the Kbm, where Kpm = HMAC_SHA1 (Kbm, 0). The mobile node uses Kpm to generate the encrypted home address as follows. encrypted home address = Enc(Kpm, the home address) Where Enc() is a symmetric key encryption algorithm. AES is the default encryption algorithm. Kpm changes upon every change of Kbm, which itself changes when return routability is run (e.g., upon change of care-of address, expiry of keygen token, etc.). So, Kpm is not re-used when a care-of address changes. The mobile node generates a correspondent Binding Update message and reverse-tunnels this message to the correspondent node via the home agent. The format of this message after encapsulation is: IPv6 header (source = care-of address, destination = home agent) ESP header in tunnel mode IPv6 header (source = home address, destination = correspondent node) Destination option header Encrypted Home Address option (encrypted home address) Parameters: Alternative Care-of Address option (care-of address) sequence number (within the Binding Update message header) home nonce index (within the Nonce Indices option) care-of nonce index (within the Nonce Indices option) First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent | BU))) This packet is protected by the IPsec security association with a non-null encryption algorithm. If the home agent can process this packet successfully, it forwards the following packet to the correspondent node. IPv6 header (source = home address, destination = correspondent node) Destination option header Encrypted Home Address option (encrypted home address)
Parameters:
Alternative Care-of Address option (care-of address)
sequence number (within the Binding Update message header)
home nonce index (within the Nonce Indices option)
care-of nonce index (within the Nonce Indices option)
First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent
| BU)))
After receiving a reverse-tunneled correspondent Binding Update
message, the correspondent node performs the operation as described
in Section 5.5. If the correspondent Binding Update message is
processed successfully and an acknowledgement is requested, the
correspondent node constructs a Binding Acknowledgement message shown
below.
IPv6 header (source = correspondent node,
destination = home address)
Encrypted Home Address routing header
encrypted home address
Parameters:
sequence number (within the Binding Update message header)
First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent
| BA)))
Upon receiving this Binding Acknowledgement message, the home agent
applies the IPsec security association with a non-null encryption
algorithm to this message and forwards the following packet to the
mobile node.
IPv6 header (source = home agent,
destination = care-of address)
ESP header in tunnel mode
IPv6 header (source = correspondent node,
destination = home address)
Encrypted Home Address routing header
encrypted home address
Parameters:
sequence number (within the Binding Update message header)
First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent
| BA)))
The reverse-tunneled correspondent binding update procedure is
completed after the mobile node processes the received Binding
Acknowledgement message. Note that when the mobile node communicates
with a different correspondent node, the encrypted home address looks
different.
To delete an established Binding Cache entry at the correspondent
node, the mobile node reverse-tunnels the following Binding Update
message via the home agent. Note that the Encrypted Home Address
option is optional during the correspondent binding de-registration
and only the home keygen token is used to generate Kbm and Kpm, if
needed, in this case.
IPv6 header (source = care-of address,
destination = home agent)
ESP header in tunnel mode
IPv6 header (source = home address,
destination = correspondent node)
Destination option header (optional)
Encrypted Home Address option (encrypted home address)
Parameters:
Alternative Care-of Address option (care-of address)
sequence number (within the Binding Update message header)
home nonce index (within the Nonce Indices option)
care-of nonce index (within the Nonce Indices option)
First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent
| BU)))
If an acknowledgement is requested, the correspondent node returns
the following Binding Acknowledgement message to the mobile node.
IPv6 header (source = correspondent node,
destination = home address)
Encrypted Home Address routing header (optional)
encrypted home address
Parameters:
sequence number (within the Binding Update message header)
First (96, HMAC_SHA1 (Kbm, (care-of address | correspondent
| BA)))
Since the destination IP address in this message is the home address,
the home agent will receive this message and forward it to the mobile
node via the reverse tunnel.
5.2. Route-Optimized Payload Packets
After the correspondent registration is completed successfully,
subsequent payload packets are exchanged via the optimized route
between the mobile node and the correspondent node. In such packets,
only the encrypted home address carried in the Encrypted Home Address
destination option and the Encrypted Home Address routing header are
visible to eavesdroppers.
The format of payload packets sent from the mobile node to the
correspondent node is:
IPv6 header (source = care-of address,
destination = correspondent node)
Destination option header
Encrypted Home Address option (encrypted home address)
Payload
The format of payload packets sent from the correspondent node to the
mobile node is:
IPv6 header (source = correspondent node,
destination = care-of address)
Encrypted Home Address routing header
encrypted home address
Payload
5.3. Mobile Node Operation
5.3.1. Conceptual Data Structures
The Binding Update List entry for the correspondent registration is
extended with a new field to store the current encrypted home address
used with a particular correspondent node. The encrypted home
address is stored when the mobile node sends a reverse-tunneled
correspondent Binding Update message, and the state of the
corresponding Binding Update List entry is updated when the mobile
node successfully processes the correspondent Binding Acknowledgement
message. Note that the encrypted home address field is not valid in
the Binding Update List entry for the home registration.
Given that the encrypted home address is 128 bits long, it is
expected that each encrypted home address or the combination of the
encrypted home address and the correspondent node's IP address stored
in the Binding Update List is unique. Therefore, the mobile node can
use the encrypted home address (or use it together with the
correspondent node's IP address) as a primary key to look up the
Binding Update List.
5.3.2. Reverse-Tunneled Correspondent Binding Update to the
Correspondent Node
After the return routability procedure, if the mobile node chooses to
use the location privacy solution with the correspondent node, e.g.,
based on the mobile node's configuration, it generates the encrypted
home address, updates or creates a new correspondent Binding Update
List entry to store the encrypted home address, then forwards the
correspondent Binding Update message through the reverse tunnel established with the home agent. Note that the MAC is generated in the same way as specified in RFC 3775, and it does not cover the encrypted home address.5.3.3. Reverse-Tunneled Correspondent Binding Acknowledgement from the Correspondent Node
When the mobile node receives a Binding Acknowledgement message from the correspondent node in response to a previously sent reverse- tunneled correspondent Binding Update message, if this Binding Acknowledgement message contains an Encrypted Home Address routing header, the mobile node considers that the correspondent node supports the location privacy solution. The mobile node authenticates this message based on RFC 3775. If authentication is successful, the mobile node decrypts the encrypted home address and compares the result with the real home address, or compares the encrypted home address with the one stored in the Binding Update List entry. If they match, the mobile node considers that the correspondent registration is successful and updates the state of the corresponding Binding Update List entry. If they do not match, the mobile node MAY start the correspondent binding update procedure again.5.3.4. Route-Optimized Payload Packets
In order to maintain session continuity, upper layers of the IP stack in the mobile node still use the real home address, even after the reverse-tunneled correspondent registration. A possible way of implementation is as follows. When the Mobile IP sublayer at the mobile node receives a packet from the upper layer, the normal processing as specified in RFC 3775 is performed. Subsequently, the Home Address option is replaced with the Encrypted Home Address option carrying the encrypted home address stored in the corresponding Binding Update List entry, and then the mobile node forwards the packet to the correspondent node via the optimized route. On the other hand, when the mobile node receives a payload packet carrying the Encrypted Home Address routing header, the mobile node uses the encrypted home address (optionally together with the IP address of the correspondent node) to look up the Binding Update List. If an entry is found, the mobile node accepts this packet, replaces the Encrypted Home Address option with the Home Address option carrying the real home address, and continues with processing based on RFC 3775. If no entry is found, the mobile node silently drops the received packet.
5.3.5. Receiving ICMP Error Message
The mobile node may receive an ICMP Parameter Problem, Code 2, message forwarded by the home agent via the bidirectional tunnel, for example, when the correspondent node does not support the use of the Encrypted Home Address option. If such a message is received, the mobile node SHOULD not attempt to use the location privacy solution with the correspondent node. The mobile node may choose either not to communicate with the correspondent node, or to communicate without location privacy protection.5.3.6. Binding Error from the Correspondent Node
When the mobile node communicates with a correspondent node by using the encrypted home address, a Binding Error message with the Status field set as 1 (unknown binding for Home Address destination option) may be received by the mobile node if there is no valid Binding Cache entry established at the correspondent node. Note that we do not specify a new Status value to be used in this case because the implementation of the Binding Update List entry can contain an indication of whether an encrypted home address is currently used with the correspondent node. Upon receiving the Binding Error message, the mobile node can find out which encrypted home address is invalid by looking at the Home Address field of the Binding Error message. The mobile node may then perform the correspondent binding update procedure to establish a valid binding for the encrypted home address.5.3.7. Binding Refresh Request from the Correspondent Node
When the mobile node receives a Binding Refresh Request message sent from the correspondent node and forwarded by the home agent via the bidirectional tunnel, the mobile node needs to perform the correspondent binding update procedure to refresh the binding for the encrypted home address at the correspondent node.5.4. Home Agent Operation
With the solution described in this section (i.e., Section 5), there is no new home agent operation to be specified. That is, the home agent behaves based on RFC 3775 when processing signaling or data packets.
5.5. Correspondent Node Operation
5.5.1. Conceptual Data Structures
The Binding Cache entry is extended with a new field to store the current encrypted home address used with a particular mobile node. The encrypted home address is stored when the correspondent node successfully processes a reverse-tunneled correspondent Binding Update message. Given that the encrypted home address is 128 bits long, it is expected that each encrypted home address or the combination of the care-of address and the encrypted home address stored in the Binding Cache entry is unique. Therefore, the correspondent node can use the encrypted home address (or use it together with the care-of address) as a primary key to look up the Binding Cache.5.5.2. Reverse-Tunneled Correspondent Binding Update from the Mobile Node
When receiving a reverse-tunneled Binding Update message with the Encrypted Home Address option, if the correspondent node supports the location privacy solution, it verifies the message by using the same method as defined in RFC 3775. If this verification succeeds, the correspondent node generates Kpm and uses it to decrypt the encrypted home address, and compares the result with the source IP address. If they match, the correspondent node stores the encrypted home address in the corresponding Binding Cache entry.5.5.3. Reverse-tunneled Correspondent Binding Acknowledgement to the Mobile Node
If an acknowledgement to the reverse-tunneled correspondent Binding Update message is requested by the mobile node, the correspondent node returns a Binding Acknowledgement message with the Encrypted Home Address routing header, if it supports the location privacy solution. The MAC in the Binding Acknowledgement message is generated in the same way as specified in RFC 3775 and does not cover the encrypted home address carried in the Encrypted Home Address routing header.5.5.4. Route-Optimized Payload Packets
In order to maintain session continuity, upper layers of the IP stack in the correspondent node still use the real home address, even after the reverse-tunneled correspondent registration.
A possible way of implementation is as follows. When the IP layer at the correspondent node finishes processing the packet received from the upper layer based on RFC 3775, the Type 2 routing header together with the real home address therein is replaced with the Encrypted Home Address routing header with the encrypted home address found in the corresponding Binding Cache entry. Then, this packet is forwarded to the mobile node via the optimized route. On the other hand, when the correspondent node receives a payload packet with the Encrypted Home Address option, it uses the encrypted home address (optionally together with the care-of address of the mobile node) to look up the Binding Cache. If there is an entry, the correspondent node replaces the Encrypted Home Address option with the Home Address option carrying the real home address before forwarding the packet to the upper layer. If no matching entry is found, the correspondent node sends a Binding Error message to the source IP address, i.e., the care-of address of the mobile node.5.5.5. ICMP Error Message to the Mobile Node
When receiving a reverse-tunneled correspondent Binding Update message with the Encrypted Home Address option, if the correspondent node does not support location privacy extensions, it sends an ICMP Parameter Problem, Code 2, message to the source IP address (i.e., the home address of the mobile node) and the home agent then forwards this ICMP message to the mobile node via the bidirectional tunnel.5.5.6. Binding Error to the Mobile Node
When the correspondent node receives a payload packet with the Encrypted Home Address option for which there is no valid Binding Cache entry, it returns a Binding Error message with the Status code set as 1 back to the source IP address of the packet. Furthermore, the Home Address field in the Binding Error message MUST be copied from the Encrypted Home Address field in the Encrypted Home Address destination option of the offending packet, or set to the unspecified address if no such option appears in the packet.5.5.7. Binding Refresh Request to the Mobile Node
When the correspondent node realizes that a Binding Cache entry is about to expire, it sends a Binding Refresh Request message to the real home address of the mobile node stored in the Binding Cache entry.
5.6. Summary
With the solution in Section 5, the real home address is visible in the Binding Update and Binding Acknowledgement messages along the HA-CN path. Like Mobile IPv6 itself, it has not been designed to change the communications between the home network and the correspondent node; the same issues would affect non-mobile hosts as well. This solution meets all the requirements set forth for the location privacy solutions and provides a simple way to provide location privacy protection while allowing the use of the real home address with the correspondent node.
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