Mobile IP Working Group Jahanzeb Faizan
Internet-Draft Hesham El-Rewini
Expires: August, 2004 Southern Methodist University
Mohammad Khalil
Nortel Networks
February, 2004
Problem Statement: Home Agent Reliability
draft-jfaizan-mipv6-ha-reliability-01.txt
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
In Mobile IPv6, the Mobile Node is dependent on a single Home Agent
for the seamless roaming over the Internet. Mobile IPv6 also allows
deployment of multiple Home Agents on the home link for providing
continuous service to Mobile Node in case of Home Agent failure. But
switching of service from the failed Home Agent to another functional
Home Agent on the home link is problematic and the base Mobile IPv6
specifications does not currently have well-described solutions. This
document aims to describe and illustrate these problems, and propose
some guidelines for possible solutions.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Overview of the Problem . . . . . . . . . . . . . . . . . 3
1.2 Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Mobile IPv6 Deployment Scenario . . . . . . . . . . . . . . . .5
3. Problem statement . . . . . . . . . . . . . . . . . . . . . . .5
3.1 Failure . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1.1 Home Agent Failure . . . . . . . . . . . . . . . . 6
3.1.2 Home Link Failure . . . . . . . . . . . . . . . . .6
3.2 Failure Detection . . . . . . . . . . . . . . . . . . . . 6
3.3 Recovery . . . . . . . . . . . . . . . . . . . . . . . . .7
3.4 IPsec Security Association with new Home Agent . . . . . .7
3.4.1 Dynamic Keying . . . . . . . . . . . . . . . . . . 7
3.4.2 Manual Keying . . . . . . . . . . . . . . . . . . .7
3.5 Correct Ordering . . . . . . . . . . . . . . . . . . . . .8
3.6 Load Balancing . . . . . . . . . . . . . . . . . . . . . .8
4. Solution Guidelines . . . . . . . . . . . . . . . . . . . . . .8
4.1 Security Implications . . . . . . . . . . . . . . . . . . 8
4.2 IPsec Security with new Home Agent . . . . . . . . . . . .8
4.3 Seamless failure . . . . . . . . . . . . . . . . . . . . .8
4.4 Mobile Node functionality . . . . . . . . . . . . . . . . 9
4.5 Messages over air interface . . . . . . . . . . . . . . . 9
4.6 Home Agent addition and failure . . . . . . . . . . . . . 9
4.7 Load Balancing. . . . . . . . . . . . . . . . . . . . . . 9
References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . .10
Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . .10
Intellectual Property and Copyright Statements . . . . . . . .10
Appendix: Changes from the previous version. . . . . . . . . .11
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1. Introduction
Mobile IPv6[1] is designed to allow a Mobile Node(MN) to change its
point of IP subnet attachment in the Internet at the network or IP
layer. MN is always identified by it Home Address regardless of its
current location. Its mobility is not limited by conventional IP
network boundaries. In Mobile IPv6 system the Home Agent(HA) remains
at conventional IPv6 subnet called the home link and when the MN is
at the home link then the packets sent to it are routed through
conventional IPv6[5] routing mechanisms. When the MN is not at home
link it registers its remote point of attachment address called
Care-of Address with the HA. This allows HA to forward packets,
addressed to the MN at its home link, to its current location.
In Mobile IPv6 system, as currently specified, a single HA services
multiple MNs. Mobile IPv6 also allows deployment of multiple HAs on
the same link so that if the serving HA fails then any other HA
on the link can provide service to the MN.
The goal of this draft is to:
o Articulate the problems resulting from the failure of a serving
HA and switching of service to another HA.
o Specify a set of framework guidelines to evaluate proposed
solutions.
1.1 Overview of the Problem
In Mobile IPv6, MN registers and establishes a connection with only
one HA. The MN is reliant on this HA for its connectivity. Thus the
HA represents the possibility of a single point of failure for Mobile
IPv6. A HA may be responsible for multiple MNs on the home link. The
failure of a single HA may then result in the loss of connectivity
for numerous MNs located throughout the Internet. Thus the HA and MN
taken together have a shared fate. A MN cannot afford the loss of its
HA. To overcome this problem Mobile IPv6 allows deployment of
multiple HAs on the home link so that upon the failure of serving HA,
another HA can take over the functions of failed HA and thus provide
continuous service to the MN(s) registered with failed HA. This
transfer of service from the failed HA to a new working HA is
problematic and the current specification of Mobile IPv6 does not
provide solution to these problems.
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1.2 Terminology
Following terms are not re-defined. They are included for the
convenience of the readers.
Mobile IPv6
Mobile IP for IPv6 [1]
Mobile Node (MN)
A node that can change its point of attachment from one link
to another, while still being reachable via its home address.
IP
Internet Protocol Version 6 (IPv6).[5]
Home Address
A unicast routable address assigned to a MN, used as the
permanent address of the MN. This address is within the MN's
home link. Standard IP routing mechanisms will deliver
packets destined for a MN's home address to its home
link.MNs can have multiple home addresses, for instance when
there are multiple home prefixes on the home link.
Home Link
The link on which a MN's home subnet prefix is defined.
Home Agent (HA)
A router on a MN's home link with which the MN has registered
its current Care-of address. While the MN is away from home,
the HA intercepts packets on the home link destined to the
MN's home address, encapsulates them, and tunnels them to the
MN's registered Ccare-of address.
Care-of Address
A unicast routable address associated with a MN while
visiting a foreign link; the subnet prefix of this IP address
is a foreign subnet prefix. Among the multiple
Care-of addresses that a MN may have at any given time (e.g.,
with different subnet prefixes), the one registered with the
MN's HA for a given home address is called its "primary"
care-of address.
IPsec Security Association
An IPSec security association is a cooperative relationship
formed by the sharing of cryptographic keying material and
associated context. Security associations are simplex. That
is, two security associations are needed to protect
bidirectional traffic between two nodes, one for each
direction.
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Home Registration
A registration between the MN and its HA, authorized by the
use of IPsec.
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 [6].
2. Mobile IPv6 Deployment Scenario
This section describes a basic deployment scenario where multiple
HAs, referred as HAs 1..n, have to coexist on the same home link to
provide continuous service to MN in case of failure of the serving
HA. MN runs Mobile IPv6 MN functionality with the mobility signaling
messages protected by IPsec. Also all the HAs 1..n run Mobile IPv6 HA
functionality along with IPsec server software. Initially MN is
registered and has IPv6 tunnel with HA_1.
..Foreign Network.. ......Home Network............
. . . .
. +----+ . . +-------+ .
. |MN | .<=========> . | HA_1 | .
. | | . . +-------+ +-------+ .
. +----+ . . ..... | HA_n | .
. . . +-------+ +-------+ .
. . . | HA_2 | .
. . . +-------+ .
................... ..............................
Figure 1
3. Problem statement
This section uses the scenario discussed in section 2 to describe the
problems associated with the failure of serving HA and as the result
of this switching of service to another HA on the home link. Consider
the failure of HA_1. and switching of service to a new HA_x
(where x = 2..n) on the same home link. This whole process of failure
detection and switching is problematic. The problems are discussed
in the following sub-sections.
3.1 Failure
The following sub-sections introduce two possible scenarios of
failure.
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3.1.1 Home Agent Failure
There could be single or multiple HAs failure on the home link. Since
MN could register only with a single HA on the home link which is
HA_1 in our scenario, so failure of multiple HAs is not going to
effect the normal operation of Mobile IPv6. We are only concerned
with the serving HA failure on the home link.
3.1.2 Home Link Failure
There could be failure of home link which will make it inaccessible
to the MN. If this occurs then even the serving HA_1 is operational,
to the MN it would appear that its serving HA_1 has failed.
3.2 Failure Detection
Transfer of service from the failed HA_1 to new HA_x will occur after
the detection of failure by MN. MN could detect the failure of HA_1
under certain conditions. These are listed below.
o When MN sends Binding Update(BU) message to the failed HA_1 and
does not receive matching Binding Acknowledgment(BA) message, it
will retransmit BUs until timeout occurs. Upon this MN will come to
know about the failure of HA_1.
o Similarly when MN sends Mobile Prefix Solicitation(MPS) message to
the failed HA_1 and does not receive Mobile Prefix Advertisement,
it will retransmit MPSs until timeout occurs and that's how it will
come to know that HA_1 has failed.
According to Mobile IPv6 MN after sending first BU or MPS message to
failed HA_1 will wait for a initial timeout period which is set to
INITIAL_BINDACK_TIMEOUT (1 second) in case of BU and
INITIAL_SOLICIT_TIMER (3 seconds) in case of MPS. This timeout period
will be doubled for each subsequent BU or MPS message until value of
MAX_BINACK_TIMEOUT (32 seconds) is reached. MN MAY send infinite BUs
or MPSs to failed HA_1 before the final timeout occurs.
So the detection of failed HA_1 will be delayed by a considerable
amount of time. Also there will be many messages transmitted over the
air interface during this period. Moreover BU and MPS are not
periodic rather on demand. MN will send BU only to register new
Care-of Address or to extend the lifetime of existing registration
with its serving HA. Similarly MN will send MPS only when its serving
HA's address is about to become invalid. As a result MN will suffer
packet loss and disconnectivity problems. This could have noticeable
performance implications on real-time applications.
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3.3 Recovery
Once the failure is detected, according to the current specifications
of Mobile IPv6 MN will try to register its Care-of Address with any
other HA on the home link. For this MN must know which other HAs are
available on the home link. MN MAY start Dynamic Home Agent
discovery(DHAD)[1] protocol and as a result will get a list of
available HAs on the home link. MN could then select HA_x (in our
scenario) on the list as its potential serving HA. MN will send BU
message to HA_x setting Home Registration(H) bit.
But this recovery mechanism is problematic. If there is only one
HA available on the home link then according to current
specifications of Mobile IPv6 even if the retransmission parameter
MAX_BINACK_TIMEOUT (32 seconds) is reached MN will continue to send
BU messages to the HA_1 until it receives valid BA message and this
will never happen because HA_1 has failed. This makes the MN enter
into an endless loop.
Even if there are multiple HAs exist (as in our scenario), besides
failure detection, there is an extra burden on MN to perform
Home Registration with the new HA and in some cases multiple
Home Registrations if there are unsuccessful attempts. Also if there
is no information about the available HAs on the home link then MN
has to perform DHAD. All these factors together result in extra
messages overhead on the air interface, service interruption and
burden on MN.
3.4 IPsec Security Association with new Home Agent
According to the current specifications of Mobile IPv6 MN and HA_x
MUST use IPsec Security Associations to protect the integrity and
authenticity of the BUs and BAs. There are two methods of
establishing such Associations.
3.4.1 Dynamic Keying
If MN and the new HA_x does not have existing Security Association to
protect the BU, IKE[2] (referred as Dynamic Keying) will be
initiated according to the guidelines defined in [3]. The latency
caused by IKE transactions might cause performance degradation.
3.4.2 Manual Keying
The problem of Dynamic Keying can be avoided by Manual Keying. It
involves out-of-band entry of Security Associations in MN and HA. MN
can be statically configured for a set of HAs among HAs 1..n and
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corresponding Security Associations before launching MN in the
Mobile IPv6 network. This will allow MN to register with any other
HA and use appropriate Security Associations upon the failure of it's
serving HA. But this policy is not flexible enough to accommodate the
dynamic nature of home link.
3.5 Correct Ordering
Upon the HA_1 failure the sequence number information in the Binding
Cache of HA_1 will also be lost. The new HA_x to which MN will switch
will not have the knowledge about the sequence number of last sent BU
by the MN. This introduces new security vulnerabilities to the
Mobile IPv6.
3.6 Load Balancing
Mobile IPv6 does not include any specification about how the HAs
on home link will do load balancing among them. This is important for
utilizing the services of all HAs on the home link efficiently.
4. Solution Guidelines
This section describes guidelines for a solution to the above
mentioned problems. The sub-sections discuss the guidelines in a
decreasing order of importance.
4.1 Security Implications
The solution MUST NOT introduce any new security vulnerabilities to
the Mobile IPv6.
4.2 IPsec Security with new Home Agent
The solution SHOULD provide a mechanism to quickly establish IPsec
Security Association between the MN and the new HA such that the
service interruption is minimal.
4.3 Seamless failure
It is recommended that the failure of HA should be transparent from
the MN. This will contribute in minimizing the period of service
interruption.
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4.4 Mobile Node functionality
The solution SHOULD cause minimal modification to the MN operation
as it is defined by Mobile IPv6.
4.5 Messages over air interface
The solution SHOULD use minimal new messages.
4.6 Home Agent addition and failure
The solution SHOULD provide recovery mechanism for the failed HA.
Also any new HA added on the home link SHOULD be ready to serve in
minimum amount of time possible.
4.7 Load Balancing
The solution SHOULD provide load balancing mechanism for the HAs on
the home link. It could be of centralized or distributed nature.
References
[1] Perkins, C., Johnson, D. and J. Arkko, "Mobility Support in
IPv6", draft-ietf-mobileip-ipv6-24 (work in progress), August
2003.
[2] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.
[3] Arkko, J., Devarapalli, V. and F. Dupont, "Using IPsec to
Protect Mobile IPv6 Signaling between Mobile Nodes and Home
Agents", draft-ietf-mobileip-mipv6-ha-ipsec-06 (work in
progress), June 2003.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[5] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[6] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
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Authors' Addresses
Jahanzeb Faizan
Southern Methodist University
Computer Science and Engineering Department.
6425 N Ownby Dr., SIC #300D
Dallas, TX, 75205, USA
Phone +1 214-768-3712, Fax +1 214-768-3085
EMail: jfaizan@smu.edu
Hesham El-Rewini
Southern Methodist University
Computer Science and Engineering Department.
6425 N Ownby Dr., SIC #306C
Dallas, TX, 75205, USA
Phone +1 214-768-3278, Fax +1 214-768-3085
EMail: rewini@engr.smu.edu
Mohammad Khalil
Nortel Networks
Richardson, TX, USA
Phone: +1 972-685-0564
EMail: mkhalil@nortelnetworks
Acknowledgements
The authors would like to thank Vijay Devarapalli and Ryuji Wakikawa
for their continuous feedback and helping us improve this draft.
Funding for the RFC Editor function is currently provided by the
Internet Society.
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Appendix: Changes from Previous Version
The following changes have been made to this document from version
00:
o Addition of types of failure, correct ordering and load balancing
sections in the problem statement.
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o Also failure detection and recovery sections are explained in
more detail in the problem statement.
o IPsec Security Associations with the new Home Agent section is
organized into Dynamic and Manual Keying sub-sections.
o Load balancing requirement is added in the solution guidelines
section.
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