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IETF RFC 5142
Mobility Header Home Agent Switch Message
Last modified on Friday, January 25th, 2008
 Permanent link to RFC 5142
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Network Working Group B. Haley
Request for Comments: 5142 Hewlett-Packard
Category: Standards Track V. Devarapalli
Azaire Networks
H. Deng
China Mobile
J. Kempf
DoCoMo USA Labs
January 2008
Mobility Header Home Agent Switch Message
 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
This document specifies a new Mobility Header message type that can
be used between a home agent and mobile node to signal to a mobile
node that it should acquire a new home agent.
Haley, et al. Standards Track PAGE 1 
RFC 5142 Home Agent Switch Message January 2008
Table of Contents
1. Introduction ....................................................2
2. Terminology .....................................................3
3. Scenarios .......................................................3
3.1. Overloaded .................................................3
3.2. Load Balancing .............................................3
3.3. Maintenance ................................................3
3.4. Functional Load Balancing ..................................3
3.5. Home Agent Renumbering .....................................4
4. Home Agent Switch Message .......................................4
5. Home Agent Operation ............................................6
5.1. Sending Home Agent Switch Messages .........................6
5.2. Retransmissions ............................................7
5.3. Mobile Node Errors .........................................7
6. Mobile Node Operation ...........................................8
6.1. Receiving Home Agent Switch Messages .......................8
6.2. Selecting a Home Agent .....................................9
7. Operational Considerations ......................................9
8. Protocol Constants .............................................10
9. IANA Considerations ............................................10
10. Security Considerations .......................................10
11. References ....................................................11
11.1. Normative References .....................................11
11.2. Informative References ...................................11
Acknowledgments ...................................................11
1. Introduction
RFC 3775 [RFC 3775] contains no provision to allow a home agent to
inform a mobile node that it needs to stop acting as the home agent
for the mobile node. For example, a home agent may wish to handoff
some of its mobile nodes to another home agent because it has become
overloaded or it is going offline.
This protocol describes a signaling message, called the Home Agent
Switch message, that can be used to send a handoff notification
between a home agent and mobile node.
The Home Agent Switch message does not attempt to solve all general
problems related to changing the home agent of a mobile node. In
particular, this protocol does not attempt to solve:
o The case where the Home Address of a mobile node must change in
order to switch to a new home agent. This operation should be
avoided using this message.
Haley, et al. Standards Track PAGE 2
RFC 5142 Home Agent Switch Message January 2008
o Determining when a home agent should actively move mobile nodes
to another home agent. This decision should be made by a
backend protocol, for example, as described in [hareliability].
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 [RFC 2119].
3. Scenarios
Here are some example scenarios where a home agent signaling message
would be useful.
3.1. Overloaded
There are a number of reasons a home agent might be considered
overloaded. One might be that it is at, or near, its limit on the
number of home bindings it is willing to accept. Another is that it
has reached a pre-determined level of system resource usage --
memory, cpu cycles, etc. In either case, it would be desirable for a
home agent to reduce the number of home bindings before a failure
occurs.
3.2. Load Balancing
A home agent might know of other home agents that are not as heavily
loaded as itself, learned through some other mechanism outside the
scope of this document. An operator may wish to try and balance this
load so that a failure would disrupt a smaller percentage of mobile
nodes.
3.3. Maintenance
Most operators do periodic maintenance in order to maintain
reliability. If a home agent is being shutdown for maintenance, it
would be desirable to inform mobile nodes so they do not lose
mobility service.
3.4. Functional Load Balancing
A Mobile IPv6 home agent provides mobile nodes with two basic
services. It acts as a rendezvous server where correspondent nodes
can find the current care-of address for the mobile node, and as an
overlay router to tunnel traffic to/from the mobile node at its
current care-of address.
Haley, et al. Standards Track PAGE 3
RFC 5142 Home Agent Switch Message January 2008
A mobility service provider could have two sets of home agents to
handle the two functions. The rendezvous function could be handled
by a machine specialized for high-speed transaction processing, while
the overlay router function could be handled by a machine with high
data throughput.
A mobile node would start on the rendezvous server home agent and
stay there if it does route optimization. However, if the original
home agent detects that the mobile node is not doing route
optimization, but instead reverse-tunneling traffic, it could
redirect the mobile node to a home agent with better data throughput.
3.5. Home Agent Renumbering
Periodically, a mobility service provider may want to shut-down home
agent services at a set of IPv6 addresses and bring service back up
at a new set of addresses. Note that this may not involve anything
as complex as IPv6 network renumbering [RFC 4192]; it may just involve
changing the addresses of the home agents. With a signaling message,
the service provider could inform mobile nodes to look for a new home
agent.
4. Home Agent Switch Message
The Home Agent Switch message is used by the home agent to signal to
the mobile node that it needs to stop acting as the home agent for
the mobile node, and that it should acquire a new home agent. Home
Agent Switch messages are sent as described in Section 5.
The message described below follows the Mobility Header format
specified in Section 6.1 of [RFC 3775]:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Proto | Header Len | MH Type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
. .
. Message Data .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Haley, et al. Standards Track PAGE 4
RFC 5142 Home Agent Switch Message January 2008
The Home Agent Switch Message uses the MH Type value (12). When this
value is indicated in the MH Type field, the format of the Message
Data field in the Mobility Header is as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|# of Addresses | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
. .
. Home Agent Addresses .
. .
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
. .
. Mobility Options .
. .
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
# of Addresses
An 8-bit unsigned integer indicating the number of IPv6 home agent
addresses in the message. If set to zero, the mobile node MUST
perform home agent discovery.
Reserved
An 8-bit field reserved for future use. The value MUST be
initialized to zero by the sender, and MUST be ignored by the
receiver.
Home Agent Addresses
A list of alternate home agent addresses for the mobile node. The
number of addresses present in the list is indicated by the "# of
Addresses" field in the Home Agent Switch message.
Haley, et al. Standards Track PAGE 5
RFC 5142 Home Agent Switch Message January 2008
Mobility Options
A Variable-length field of such length that the complete Mobility
Header is an integer multiple of 8 octets long. This field
contains zero or more TLV-encoded mobility options. The encoding
and format of defined options MUST follow the format specified in
Section 6.2 of [RFC 3775]. The receiver MUST ignore and skip any
options that it does not understand.
The Binding Refresh Advice mobility option defined in Section
6.2.4 of [RFC 3775] is valid for the Home Agent Switch message.
If no home agent addresses and no options are present in this
message, no padding is necessary and the Header Len field in the
Mobility Header will be set to zero.
5. Home Agent Operation
5.1. Sending Home Agent Switch Messages
When sending a Home Agent Switch message, the sending node constructs
the packet as it would any other Mobility Header, except:
o The MH Type field MUST be set to (12).
o If alternative home agent addresses are known, the sending home
agent SHOULD include them in the list of suggested alternate
home agents. The home agent addresses field should be
constructed as described in Section 10.5.1 of [RFC 3775], which
will randomize addresses of the same preference in the list.
o The "# of Addresses" field MUST be filled-in corresponding to
the number of home agent addresses included in the message. If
no addresses are present, the field MUST be set to zero,
forcing the mobile node to perform home agent discovery by some
other means.
o If the home agent is able to continue offering services to the
mobile node for some period of time, it MAY include a Binding
Refresh Advice mobility option indicating the time (in units of
4 seconds) until the binding will be deleted.
The Home Agent Switch message MUST use the home agent to mobile node
IPsec ESP (Encapsulating Security Payload) authentication SA
(Security Association) for integrity protection.
Haley, et al. Standards Track PAGE 6
RFC 5142 Home Agent Switch Message January 2008
A home agent SHOULD send a Home Agent Switch message when a known
period of unavailability is pending so the mobile node has sufficient
time to find another suitable home agent.
The sending node does not need to be the current home agent for the
mobile node, for example as described in [hareliability], but it MUST
have a security association with the mobile node so the message is
not rejected. In this case, the Home Agent Switch message SHOULD
only contain the address of the home agent sending the message in the
Home Agent Addresses field, which implies that the mobile node should
switch to using the sender as its new home agent.
5.2. Retransmissions
If the home agent does not receive a response from the mobile node --
either a Binding Update message to delete its home binding if it is
the current home agent, or a Binding Update message to create a home
binding if it is not the current home agent -- then it SHOULD
retransmit the message until a response is received. The initial
value for the retransmission timer is INITIAL-HA-SWITCH-TIMEOUT.
The retransmissions by the home agent MUST use an exponential back-
off mechanism, in which the timeout period is doubled upon each
retransmission, until either the home agent gets a response from the
mobile node to delete its binding, or the timeout period reaches the
value MAX-HA-SWITCH-TIMEOUT. The home agent MAY continue to send
these messages at this slower rate indefinitely.
If the home agent included a Binding Refresh Advice mobility option,
then it SHOULD delay any retransmissions until at least one half of
the time period has expired, or INITIAL-HA-SWITCH-TIMEOUT, whichever
value is less.
5.3. Mobile Node Errors
If a mobile node does not understand how to process a Home Agent
Switch message, it will send a Binding Error message as described in
Section 6.1.
If a mobile node is unreachable, in other words, it still has a home
binding with the home agent after reaching the timeout period of MAX-
HA-SWITCH-TIMEOUT, the home agent SHOULD NOT make any conclusions
about its status.
In either case, the home agent SHOULD attempt to continue providing
services until the lifetime of the binding expires.
Haley, et al. Standards Track PAGE 7
RFC 5142 Home Agent Switch Message January 2008
Attempts by the mobile node to extend the binding lifetime with a
Binding Update message SHOULD be rejected, and a Binding
Acknowledgement SHOULD be returned with status value 129
(Administratively prohibited) as specified in Section 6.1.8 of
[RFC 3775].
6. Mobile Node Operation
6.1. Receiving Home Agent Switch Messages
Upon receiving a Home Agent Switch message, the Mobility Header MUST
be verified as specified in [RFC 3775], specifically:
o The Checksum, MH type, Payload Proto, and Header Len fields
MUST meet the requirements of Section 9.2 of [RFC 3775].
o The packet MUST be covered by the home agent to mobile node
IPsec ESP authentication SA for integrity protection.
If the packet is dropped due to the above tests, the receiving node
MUST follow the processing rules as Section 9.2 of [RFC 3775] defines.
For example, it MUST send a Binding Error message with the Status
field set to 2 (unrecognized MH Type value) if it does not support
the message type.
Upon receipt of a Home Agent Switch message, the mobile node MUST
stop using its current home agent for services and MUST delete its
home binding by sending a Binding Update message as described in
Section 11.7.1 of [RFC 3775]. This acts as an acknowledgement of the
Home Agent Switch message. Alternately, if the sender of the message
is not the current home agent, sending a Binding Update message to
create a home binding will act as an acknowledgement of the Home
Agent Switch message. Retransmissions of Binding Update messages
MUST use the procedures described in Section 11.8 of [RFC 3775].
If a Binding Refresh Advice mobility option is present, the mobile
node MAY delay the deletion of its home binding and continue to use
its current home agent until the calculated time period has expired.
If the Home Agent Switch message contains a list of alternate home
agent addresses, the mobile node SHOULD select a new home agent as
described in Section 6.2, and establish the necessary IPsec security
associations with the new home agent by whatever means required as
part of the mobile node/home agent bootstrapping protocol for the
home agent's mobility service provider. If no alternate home agent
addresses are included in the list, the mobile node MUST first
perform home agent discovery.
Haley, et al. Standards Track PAGE 8
RFC 5142 Home Agent Switch Message January 2008
6.2. Selecting a Home Agent
In most cases, the home agent addresses in the Home Agent Switch
message will be of other home agents on the home link of the mobile
node (the computed prefix is the same). In this case, the mobile
node SHOULD select a new home agent from the addresses as they are
ordered in the list. If the first address in the list is unable to
provide service, then the subsequent addresses in the list should be
tried in-order.
In the case that the home agent addresses in the Home Agent Switch
message are not all home agents on the home link of the mobile node
(the computed prefix is different), the mobile node SHOULD select one
with its home network prefix first, if available, followed by home
agents with other prefixes. Choosing a home agent with a different
prefix might require a change of the home address for the mobile
node, which could cause a loss of connectivity for any connections
using the current home address.
7. Operational Considerations
This document does not specify how an operator might use the Home
Agent Switch message in its network. However, the following
requirements are placed on its usage:
o The use of this message needs to take into account possible
signaling overhead, congestion, load from the mechanism itself,
and the resulting registration to another home agent. A home
agent may provide service for thousands, if not millions, of
mobile nodes. Careless application of the Home Agent Switch
message may cause the new home agent, or some other parts of
the network, to suffer. As a result, it is REQUIRED that
applications of this message either employ a feedback loop
between resources of the new home agent and the sending of
additional Home Agent Switch messages, or apply a maximum rate
at which mobile nodes can be informed of the switch that is far
below the designated capacity of new registrations that the set
of home agents can process. If no other information is
available, this maximum rate should default to MAX-HA-SWITCH-
TRANSMIT-RATE.
o In general, switching the home agent of a mobile node should
only be done when absolutely necessary, since it might cause a
service disruption if the switch to a new home agent fails, the
new home agent is itself under an overload condition, or the
network connection of the new home agent is congested.
Haley, et al. Standards Track PAGE 9
RFC 5142 Home Agent Switch Message January 2008
Similarly, the path characteristics via the new home agent may
be different, which may cause temporary difficulties for end-
to-end transport layer operation.
o If this message is being used for load-balancing between a set
of home agents, they should all be configured with the same set
of prefixes so a home agent switch does not require a change of
the home address for a mobile node. That operation is NOT
RECOMMENDED and should be avoided.
8. Protocol Constants
INITIAL-HA-SWITCH-TIMEOUT 5 seconds
MAX-HA-SWITCH-TIMEOUT 20 seconds
MAX-HA-SWITCH-TRANSMIT-RATE 1 per second
9. IANA Considerations
IANA has assigned a new Mobility Header type for the following new
message described in Section 4:
(12) Home Agent Switch message
10. Security Considerations
As with other messages in [RFC 3775], the Home Agent Switch message
MUST use the home agent to mobile node ESP encryption SA for
confidentiality protection, and MUST use the home agent to mobile
node ESP authentication SA for integrity protection.
The Home Agent Switch message MAY use the IPsec ESP SA in place for
Binding Updates and Acknowledgements, as specified in Section 5.1 of
[RFC 3775], in order to reduce the number of configured security
associations. This also gives the message authenticity protection.
Some operators may not want to reveal the list of home agents to on-
path listeners. In such a case, the Home Agent Switch message should
use the home agent to mobile node IPsec ESP encryption SA for
confidentiality protection.
Haley, et al. Standards Track PAGE 10
RFC 5142 Home Agent Switch Message January 2008
11. References
11.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility
Support in IPv6", RFC 3775, June 2004.
11.2. Informative References
[RFC 4192] Baker, F., Lear, E., and R. Droms, "Procedures for
Renumbering an IPv6 Network without a Flag Day", RFC
4192, September 2005.
[hareliability] Wakikawa, R., Ed., "Home Agent Reliability Protocol",
Work in Progress, November 2007.
Acknowledgments
We would like to thank the authors of a number of previous documents
that contributed content to this RFC:
o Ryuji Wakikawa, Pascal Thubert, and Vijay Devarapalli,
"Inter Home Agents Protocol Specification", March 2006.
o Hui Deng, Brian Haley, Xiaodong Duan, Rong Zhang, and Kai Zhang,
"Load Balance for Distributed Home Agents in Mobile IPv6",
October 2004.
o James Kempf, "Extension to RFC 3775 for Alerting the Mobile Node
to Home Agent Unavailability", October 2005.
o Brian Haley and Sri Gundavelli, "Mobility Header Signaling
Message", September 2007.
Thanks also to Kilian Weniger, Jixing Liu, Alexandru Petrescu, Jouni
Korhonen, and Wolfgang Fritsche for their review and feedback.
Haley, et al. Standards Track PAGE 11
RFC 5142 Home Agent Switch Message January 2008
Author's Addresses
Brian Haley
Hewlett-Packard Company
110 Spitbrook Road
Nashua, NH 03062, USA
EMail: brian.haley@hp.com
Vijay Devarapalli
Azaire Networks
3121 Jay Street
Santa Clara, CA 95054 USA
EMail: vijay.devarapalli@azairenet.com
James Kempf
DoCoMo USA Labs
181 Metro Drive
Suite 300
San Jose, CA 95110 USA
EMail: kempf@docomolabs-usa.com
Hui Deng
China Mobile
53A, Xibianmennei Ave.
Xuanwu District
Beijing 100053
China
EMail: denghui@chinamobile.com
Haley, et al. Standards Track PAGE 12
RFC 5142 Home Agent Switch Message January 2008
Full Copyright Statement
Copyright © The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
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Haley, et al. Standards Track PAGE 13
Mobility Header Home Agent Switch Message
RFC TOTAL SIZE: 27460 bytes
PUBLICATION DATE: Friday, January 25th, 2008
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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