MIP4 K. Leung
Internet-Draft G. Dommety
Expires: August 30, 2006 P. Yegani
Cisco Systems
February 26, 2006
Mobility Management using Proxy Mobile IPv4
draft-leung-mip4-proxy-mode-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
Mobile IPv4 is a standard mobility protocol that enables IPv4 device
to roam between networks. The mobile device has the Mobile IP
function to signal its location to the routing anchor, known as the
Home Agent. However, there are many IPv4 devices without such
capability due to various reasons. This document describes a
solution based on Proxy Mobile IPv4, which enables some other entity
to provide mobility support on behalf of an unaltered and mobility-
unaware IPv4 device.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions used in this document . . . . . . . . . . . . . . 3
3. Solution Benefits . . . . . . . . . . . . . . . . . . . . . . 4
4. Overview of Proxy Mobile IPv4 . . . . . . . . . . . . . . . . 5
4.1. Mobility Signaling for Mobile Station . . . . . . . . . . 5
4.1.1. Registration Sequencing . . . . . . . . . . . . . . . 6
4.1.2. Resource Cleanup . . . . . . . . . . . . . . . . . . . 7
4.2. Establishment of Bi-Directional Tunnel . . . . . . . . . . 8
5. Appearance of Being at Home Network . . . . . . . . . . . . . 8
5.1. ARP Considerations . . . . . . . . . . . . . . . . . . . . 8
5.2. ICMP Considerations . . . . . . . . . . . . . . . . . . . 9
5.3. DHCP Considerations . . . . . . . . . . . . . . . . . . . 9
5.4. PPP IPCP Considerations . . . . . . . . . . . . . . . . . 10
5.5. Link-Local Multicast and Broadcast Considerations . . . . 10
6. Mobility Proxy Agent Operation . . . . . . . . . . . . . . . . 10
7. Home Agent Operation . . . . . . . . . . . . . . . . . . . . . 11
7.1. Processing Proxy Registration Request . . . . . . . . . . 11
8. Mobile Station Operation . . . . . . . . . . . . . . . . . . . 11
8.1. Initial Network Access . . . . . . . . . . . . . . . . . . 12
8.2. Moving to a New MPA . . . . . . . . . . . . . . . . . . . 12
8.3. Packet forwarding . . . . . . . . . . . . . . . . . . . . 12
8.4. Moving to a Different Domain . . . . . . . . . . . . . . . 13
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
10. Security Considerations . . . . . . . . . . . . . . . . . . . 13
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . . . 16
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1. Introduction
There are many IPv4 devices which don't have or can't be enabled with
Mobile IP [5] functionality. For them, Proxy Mobile IPv4 provides
mobility support without being "touched". The scheme is based on an
external node acting as a proxy Mobile Node that registers the
location of the device and maintains reachability while the device is
on the network. The Foreign Agent and Home Agent perform their
normal roles. The following examples depict possible scenarios:
1. An Access Point or Base Station performs registration when a
mobile station is associated on the airlink.
2. An Access Router or Foreign Agent performs registration when a
mobile station is detected.
3. A wireless mobile termination device performs registration for an
attached host.
In the first two cases, the proxy node is a network element and the
signaling can be considered part of the mobility management function
of the domain. The third case is when the proxy node moves along
with the mobile device and may be considered as a part of the mobile
device. Some could argue that this is not a proxy mode because the
Mobile Node is the wireless mobile termination device. But the Home
Address is "owned" by the host, meaning that packets to and from this
IP address is received and sent by this host, respectively. The
wireless mobile termination device is providing mobility for this IP
address unbeknownst to the host. An example of this is cdma2000's
Network Mode on the mobile termination unit. For cdma2000, the true
Mobile IP mode would be the Relay Mode, which is when the host
operates as the Mobile Node. Anyways, this mode of operation is well
understood and commonly deployed. The aim of this document is to
describe how the network elements can provide mobility management for
the mobile devices.
Similar to Mobile IPv4, the proxy node initiating the registration
procedure needs to know the Home Agent serving the mobile device and
have the mobility security association with that Home Agent. If the
proxy node and HA are in the same administrative domain, a common
mobility security association between them may be used for the mobile
devices.
2. Conventions used in this document
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
The following new terminology and abbreviations are introduced in
this document and all other general mobility related terms as
defined in Mobile IPv4 specification [5].
Mobility Station (MS)
Any IPv4 node that has the ability to physically access or roam
across different networks. The Mobile Station does not have
the Mobile IPv4 protocol stack.
Mobility Proxy Agent (MPA)
The Mobile IPv4 entity that offers proxy mobility service for a
Mobile Station by performing registration function on the
host's behalf. As mentioned before, this may be the Access
Point, Base Station, Mobile Terminal, Access Router, or Access
Gateway.
3. Solution Benefits
Proxy Mobile IPv4 is designed to satisfy the requirements listed
below. In addition, the solution leverages well-studied
specification and highly available implementations. Only incremental
enhancements are added to Mobile IPv4 protocol to enrich its breadth
to support both client-based and network-based mobility. For
example, a Home Agent can anchor Mobile Stations that have Mobile IP
as well as the hosts without such capability.
The network-based Mobility Management solution defined in this
document provides the following benefits:
1. Support for Unmodified Hosts
An overwhelming majority of IPv4 hosts do not have Mobile IP
capability. Providing mobility for them is achievable using
Proxy Mobile IPv4. This is accomplished without "touching"
the user's devices running on a myriad of operating systems
and networking stacks.
2. Airlink Consumption
Mobility-related signaling over the air-link is eliminated.
Considering that Network Address Translation (NAT) is
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ubiquitous in IPv4 networks, a mobile node needs to send
keepalives at short intervals to properly maintain the NAT
states [6]. This can be performed by the MPA in the network
which doesn't consume any air-link bandwidth.
3. Reduction of Signaling Overhead in the Network
The MPA can aggregate multiple MSes on the same tunnel. Thus
the frequent keepalives needed to maintain the NAT states can
be reduced significantly. The signaling load on the network
diminishes which increases the overall capacity.
4. Support for Heterogeneous Wireless Link Technologies
One aspect is how to adopt the scheme to an access technology.
Since Proxy Mobile IPv4 is based on a heterogeneous mobility
protocol, it can be used for any type of access network.
The other aspect is how to support roaming across different
access technologies. As long as the MPA can use the same NAI
to identify the MS for various access networks, roaming
between them is possible.
5. Support for IPv4 and IPv6 Host
As IPv6 increases in popularity, the host will likely be dual
stack. Adding IPv6 support to the host for Proxy Mobile IPv4
involves adopting the works in [10] and [11]. This method
allows both IPv4 and IPv6 mobility to be set up by a common
protocol, not two as would be required by the use of Mobile
IPv4 and Mobile IPv6.
4. Overview of Proxy Mobile IPv4
4.1. Mobility Signaling for Mobile Station
After network access authentication, MPA exchange registration
messages with the Home Agent to set up proper routing and tunneling
of packets from/to the Mobile Station. As specified in RFC 3344 [5],
a Foreign Agent may be used in the registration procedure. However,
for the remainder of this document, MPA is described to use direct
registration to the Home Agent. The difference is covered in RFC
3344 [5] and can be presumed to be adaquately understood (e.g. the
tunnel is between FA and HA instead of MPA and HA for FA registration
and direct registration, respectively).
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4.1.1. Registration Sequencing
MS MPA-1 MPA-2 HA
|MS @ MPA-1 | | |
x-----------x | |
| | Proxy Reg | |
| | Request | |
1)| o---------------------->|
| | | |
2)| | | o Create MBE
| | | | MS @ MPA-1
| | | Proxy Reg |
| | | Reply |
| | | Seq=X |
3)| |<----------------------o
| | | |
|MS moved to MPA-2 | |
x-----------------------x |
| | | Proxy Reg |
| | | Request |
4)| | o---------->|
| | | |
5)| | | o Update MBE
| | | | MS @ MPA-2
| | | |
| | | Proxy Reg |
| | | Reply |
| | | Seq=X+1 |
6)| | |<----------o
| | | |
Figure 1: Maintaining Sequencing for Registrations
In the case where each MPA act independently, the Home Agent is
responsible to maintain the order of sequence of registrations. The
MPA sends registration to the HA when the MS is associated. Re-
registration is needed to extend the lifetime. But the registrations
from previous MPA can cause out of order problem. In order to
efficiently maintain the right sequence, the HA assigns a sequence
number in the Registration Reply. Subsequent registration requests
from the MPA contains the value. The HA knows which registration is
new and ignore stale registrations. In addition, HA should send
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Registration Revocation to previous MPA to inform the MPA that the MS
has moved elsewhere. This eliminates stale registrations from MPA
which is not the current one serving the MS.
4.1.2. Resource Cleanup
MS New MPA HA Previous MPA
| | Proxy | |
| | Reg | |
| | Request | |
1)| o---------->| |
| | | |
| | | Update |
| | | existing |
2)| | o MBE for MS|
| | | |
| | | Reg |
| | | Revocation|
3)| | o---------->|
| | | |
4)| | | o Remove visitor
| | | | entry for MS
| | | |
| | | | Reg
| | | | Revoc Ack
5)| | |<----------o
| | | |
| | Proxy | |
| | Reg | |
| | Reply | |
6)| o<----------o |
Figure 2: Registration Revocation for Previous MPA
MPA which no longer serve the Mobile Station needs to remove any
associated mobility states and relinquish resources which are no
longer needed.
When the Home Agent receives a Proxy Registration Request for a
Mobile Station with an existing MBE, a Registration Revocation [3543]
is sent to the previous MPA (in steps #1 through #3). The previous
MPA removes the visitor entry for the Mobile Station before sending
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acknowledgement to the Home Agent (in steps #4 and #5). In parallel,
the Home Agent sends the Proxy Registration Reply to the new MPA (in
step #6). At the end of sequence of events, only states for the MS
are in the new MPA and the Home Agent.
4.2. Establishment of Bi-Directional Tunnel
After receiving a successful Registration Reply for the Proxy
Registration Request, the MPA sets up a tunnel to the Mobile
Station's Home Agent.
The bi-directional tunnel between the MPA and the Home Agent allows
packets to flow in both directions, while the mobile station is
connected to its visited link. The tunnel endpoints are the LMAP and
the Home Agent. All traffic to and from the Mobile Station is sent
through this bi-directional tunnel.
While the MPA is serving a Mobile Station, it MUST be able to
intercept all packets sent from the Mobile Station and forward them
out the MPA-Home Agent tunnel created for supporting that Mobile
Station. Typically, forwarding is based on Layer 2 information such
as the source MAC address or ingress PPP interface. This allows MSes
with overlapping IP addresses to be supported.
Any packets received on the tunnel from Home Agent, the MPA
decapsulates before forwarding to the Mobile Station on its link.
Typically, the forwarding is based on the Destination IP address and
HA indicator such as tunnel interface identifier or HA address. This
allows MSes with overlapping IP addresses to be supported.
5. Appearance of Being at Home Network
Since the Mobile Station is not aware of mobility and does not
participate in handover signaling, the network elements deceive the
host to believe that it is stationary yet directing its traffic to
the location where it has actually moved. An unmodified host uses
ARP [8] to learn the MAC address of other nodes on the subnet,
obtains an IP address and other host configuration via DHCP [2], and
sends link-local multicast/broadcasts. The network's response to the
host has to be equivalent to the situation when host is always on the
home subnet.
5.1. ARP Considerations
For IEEE 802 type of access networks (e.g. WLAN, WiMAX), the Mobile
Station sends ARP request for host and default gateway on its subnet.
The purpose of maintaining an ARP entry is to allow the delivery of
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the packet from MS to the IP node on the link. For Proxy Mobile IP,
the objective is to get the packet from the MS to the Home Agent.
For CNs with the same home network but roaming elsewhere, the HA will
tunnel the packet to them. Otherwise, the HA forwards the traffic
via normal routing.
The method to deliver the packet to the HA depends on whether the MPA
is on the BS or AR. If the MPA is in the Base Station, the ARP entry
in the MS serves no purpose since the packet will be tunneled to the
HA by the BS. If the MPA is in the Access Router, then the Base
Station needs to rewrite the destination MAC address properly to
reach the AR. Alternatively, a common MAC address - listened to by
all participating AR - is sent to MS in response to an ARP Request.
MPA@BS: MS <-> BS/MPA <==============> HA
MS has ARP entries for default gateway and hosts on subnet which
are set to some pseudo MAC address that is never used.
MPA@AR: MS <-> BS <-> AR/MPA <===> HA
MS has ARP entries for default gateway and hosts on subnet which
are set to some pseudo MAC address which is rewritten by BS or a
common MAC address for ARs.
Figure 3: ARP Entry Management
5.2. ICMP Considerations
In some cases, the Mobile Station sends an ICMP Query [9] with IP TTL
set to 1 destined to the default gateway. This check is used to
detect if default gateway is still reachable on the link. The MPA
should respond with ICMP Reply when it is providing mobility service.
5.3. DHCP Considerations
Proxy Mobile IP allows the Mobile Station to operate as a normal IPv4
host using the standard IP address configuration via DHCP.
The MS can obtain an IP address from DHCP in two scenarios: 1) Home
network is where MS booted up and 2) Home network is anchored
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elsewhere.
For case #1, MS boots up and obtains an IP address via DHCP normally.
Proxy Mobile IP is not involved in this step. When MS moves to a new
AR, the AR detects the IP address of the MS and exchanges
registration messages with the HA to set up the routing.
For case #2, MS boots up and initiates DHCP. The BS or AR that
performed authentication appends the Subnet Selection Option [7]
containing HA's subnet. When MPA detects the DHCP Ack from the DHCP
Server, it sends registration message to set up the routing. Another
method is for MPA to tunnel the DHCP messages to the HA which acts as
a DHCP Relay Agent.
MS unicast the DHCP Request to renew its IP address directly to the
DHCP server. As long as the tunnel between MPA and HA has already
been set up, the renewal messages can be exchanged between the DHCP
client in MS and DHCP server.
5.4. PPP IPCP Considerations
When MS access the network via PPP [3], LCP CHAP is used to
authenticate the user. After authentication, the NAS (which is the
LMAP) sends the proxy Registration Request to the Home Agent, which
responds with the Home Address in the Registration Reply. The NAS
informs the MS to use the Home Address during IPCP [4]. When MS
moves to a new NAS, the same procedure happens and MS has the same IP
address for communication.
5.5. Link-Local Multicast and Broadcast Considerations
MPA may tunnel all packets destined to Link-Local Multicast or
Broadcast to the HA. The HA looks up the hosts which are in the same
subnet and send a duplicated packet to each of them.
6. Mobility Proxy Agent Operation
The role of the MPA is performing the functions of a Mobile Node. It
sends Registration Request to the Home Agent (via the Foreign Agent
when available) to set up routing. When there is no FA, MPA operates
in Collocated Care-of Address mode and provides tunneling support.
FA can provide tunneling when it is used during registration in
accordance to RFC 3344.
The MPA needs to know the following information for sending a
registration.
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1. NAI
2. MN-HA Mobility Security Association
3. Home Agent Address
This information can be downloaded from AAA server or configured by
administrator. One example is configuration of subnet to HA mapping.
When an MS associates with the Base Station, the MPA registers to the
HA base on the IP address of the MS.
7. Home Agent Operation
The Home Agent has the functionalities described in RFC 3344. In
addition, the following feature is introduced.
7.1. Processing Proxy Registration Request
Upon the receipt of a Proxy Registration Request from a MPA, the HA
looks up the MBE indexed by the NAI. If MBE exists, HA compares the
Sequence Numbers between the message and MBE. If the value in the
message is zero or greater than or equal to the MBE, HA accepts the
registration. HA replies with a sequence number that is one greater
than larger value of either the MBE or Registration Request. If the
registration is denied, then HA sends error code "Administratively
prohibited (65)".
8. Mobile Station Operation
As per this specification, a mobile station would function as a
normal IPv4 host. The required behavior of the node will be
consistent with the base IPv4 specification [1]. The mobile station
will have the ability to retain its IPv4 address as it moves from one
point of network attachment to the other with out ever requiring it
to participate in any mobility related signaling.
The mobile station when boots up for the first time can obtain an
IPv4 address using DHCP.
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As the mobile station roams, it is always able to communicate using
the DHCP leased IP address on the home network. The MPA on the
currently attached network signals to the HA to ensure proper
forwarding path for MS's traffic.
8.1. Initial Network Access
When the Mobile Station access the network for the first time and
attaches to a network on the MPA, it will present its identity in the
form of NAI to the network as part of the network access
authentication process.
Once the address configuration is complete, the Mobile Station will
always be able to use that IP address anywhere in network.
8.2. Moving to a New MPA
When a Mobile Station moves to a new MPA from another MPA, the
following occurs:
The Mobile Station will perform a network access authentication with
the attached MPA. If the authentication fails, the Mobile Station
will not be able to use the link. After a succesful authentication,
the MPA will have the identifier and the other profile data of the
Mobile Station.
Once the network access authentication process is complete, the
Mobile Station may sense a change in the Link Layer and use ARP,
DHCP, and/or ICMP to detect if it is still on the same subnet. These
mechanisms are handled by the network as described in section 5
"Appearance of Being At Home Network".
8.3. Packet forwarding
All packets that are be sent from the Mobile Station to the
corresponding node will be sent as normal IPv4 packets setting the
Source Address of the IPv4 header to the Home Address and the
Destination Address to the corresponding node's IP address. The
default gateway for the Mobile Station will always be its HA. The
ARP Entry for HA address is a pseudo HA MAC address.
Similarly, all packets sent to the Mobile Node's Home Address by the
corresponding node will be received by the Mobile Station in the
original form (without any tunneling overhead) on its link.
No special operation is required by the Mobile Station to either send
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or receive packets.
8.4. Moving to a Different Domain
The scheme defined in this document, provides mobility support to the
Mobile Station within that domain. Once the Mobile Station obtains
an assigned Home Address, it can continue to use the same address
roaming between MPAs in the network with its HA providing the
topological anchor point for that address. However, the Mobile
Station that roams across domains is required to have the Mobile IPv4
stack and operate as a normal MIPv4 mobile node to achieve mobility
across domains.
9. IANA Considerations
This document defines a new Sequencing Extension.
10. Security Considerations
The functionality in this document is protected by the Authentication
Extensions described in RFC 3344 [5]. Access Authentication and
Authorization MUST be performed prior to Proxy Mobile IP
registration. The Identity (NAI) that is used during the Access
Authentication and Authorization is used to as the NAI in the Proxy
Mobile IP Registration. In order to protect the Registration Request
and Registration Reply, each MPA needs to have the MN-HA SA to
register the MS. The SA can be provisioned by the administrator or
obtained during the Access Authentication.
11. Acknowledgements
12. References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[3] Simpson, W., "The Point-to-Point Protocol (PPP) for the
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Transmission of Multi-protocol Datagrams over Point-to-Point
Links", RFC 1331, May 1992.
[4] McGregor, G., "The PPP Internet Protocol Control Protocol
(IPCP)", RFC 1332, May 1992.
[5] Perkins, C., "IP Mobility Support for IPv4", RFC 3344,
August 2002.
[6] Levkowetz, H. and S. Vaarala, "Mobile IP Traversal of Network
Address Translation (NAT) Devices", RFC 3519, May 2003.
[7] Waters, G., "The IPv4 Subnet Selection Option for DHCP",
RFC 3011, November 2000.
[8] Plummer, D., "An Ethernet Address Resolution Protocol",
RFC 826, November 1982.
[9] Postel, J., "Internet Control Message Protocol", RFC 792,
September 1981.
[10] Calhoun, P., Hiller, T., and P. McCann, "IPv6 over Mobile
IPv4", draft-mccann-mobileip-ipv6mipv4-00.txt (work in
progress), August 2001.
[11] Tsirtsis, G. and H. Soliman, "Dual Stack Mobile IPv4",
draft-tsirtsis-v4v6-mipv4-00.txt (work in progress), August
2003.
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Authors' Addresses
Kent Leung
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
US
Email: kleung@cisco.com
Gopal Dommety
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
US
Email: gdommety@cisco.com
Parviz Yegani
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
US
Email: pyegani@cisco.com
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