MEXT (MIP6) Working Group R. Wakikawa (Editor)
Internet-Draft Toyota ITC
Intended status: Standards Track July 14, 2008
Expires: January 15, 2009
Home Agent Reliability Protocol
draft-ietf-mip6-hareliability-04.txt
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Copyright (C) The IETF Trust (2008).
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Abstract
The home agent can be a single point of failure when Mobile IPv6 is
operated in a system. It is critical to provide home agent
reliability in the event of a home agent crashing or becoming
unavailable. This would allow another home agent to take over and
continue providing service to the mobile nodes. This document
describes the problem scope briefly and provides a mechanism of home
agent failure detection, home agent state transfer, and home agent
switching for home agent redundancy and reliability.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Problem Statement and Requirements . . . . . . . . . . . . . . 6
4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 8
4.1. Home Agent Virtual Switch . . . . . . . . . . . . . . . . 8
4.2. Home Agent Hard Switch . . . . . . . . . . . . . . . . . . 9
4.3. Home Agent Management . . . . . . . . . . . . . . . . . . 10
5. Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1. New Mobility Header Messages . . . . . . . . . . . . . . . 11
5.1.1. State Synchronization Message . . . . . . . . . . . . 11
5.1.2. Home Agent Control Message . . . . . . . . . . . . . . 13
5.1.3. Home Agent Hello Message . . . . . . . . . . . . . . . 15
5.1.4. Home Agent Switch Message . . . . . . . . . . . . . . 16
5.2. New Mobility Options . . . . . . . . . . . . . . . . . . . 17
5.2.1. IP address Option . . . . . . . . . . . . . . . . . . 17
5.2.2. Binding Cache Information Option . . . . . . . . . . . 17
5.2.3. AAA Information Option . . . . . . . . . . . . . . . . 18
6. Home Agent Configuration . . . . . . . . . . . . . . . . . . . 20
6.1. Network Configuration . . . . . . . . . . . . . . . . . . 20
6.2. Address Configuration for Virtual Switch . . . . . . . . . 21
6.3. Address Configuration for Hard Switch . . . . . . . . . . 21
7. Home Agent Common Operation . . . . . . . . . . . . . . . . . 22
7.1. Home Agent List Management . . . . . . . . . . . . . . . . 22
7.2. Detecting Home Agent Failure . . . . . . . . . . . . . . . 22
7.3. Processing Hello Messages . . . . . . . . . . . . . . . . 23
7.3.1. Requesting Hello Message . . . . . . . . . . . . . . . 24
7.3.2. Sending Hello Message . . . . . . . . . . . . . . . . 24
7.3.3. Receiving Hello Message . . . . . . . . . . . . . . . 25
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7.4. Processing State Synchronization Messages . . . . . . . . 26
7.4.1. Requesting State of a Particular Mobile Node(s) . . . 26
7.4.2. Synchronizing State . . . . . . . . . . . . . . . . . 27
7.4.3. Reliable Transmission by Explicit Acknowledgement . . 28
7.5. Processing Home Agent Control Messages . . . . . . . . . . 29
7.5.1. Standby Home Agent becomes an Active Home Agent . . . 29
7.5.2. Active Home Agent becomes in-active . . . . . . . . . 30
7.6. Interworking with VRRP . . . . . . . . . . . . . . . . . . 31
7.7. Retransmissions and Rate Limiting . . . . . . . . . . . . 32
8. Home Agent Virtual Switch . . . . . . . . . . . . . . . . . . 34
8.1. Consideration of Routing and Neighbor Discovery
Protocol . . . . . . . . . . . . . . . . . . . . . . . . . 34
8.2. Home Agent Recovery . . . . . . . . . . . . . . . . . . . 34
9. Home Agent Hard Switch . . . . . . . . . . . . . . . . . . . . 35
9.1. Home Agent Recovery . . . . . . . . . . . . . . . . . . . 35
9.2. Sending Home Agent Switch Messages . . . . . . . . . . . . 35
9.3. Notification of Home Agent Switch Completion . . . . . . . 36
9.4. Mobile Node Operation . . . . . . . . . . . . . . . . . . 36
9.4.1. Home Agent Addresses Discovery . . . . . . . . . . . . 36
9.4.2. IKE/IPsec pre-establishment to Home Agents . . . . . . 37
9.4.3. Receiving Home Agent Switch message . . . . . . . . . 37
10. Security Considerations . . . . . . . . . . . . . . . . . . . 39
11. Protocol Constants . . . . . . . . . . . . . . . . . . . . . . 41
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 42
13. Additional Authors . . . . . . . . . . . . . . . . . . . . . . 43
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 43
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 44
15.1. Normative References . . . . . . . . . . . . . . . . . . . 44
15.2. Informative References . . . . . . . . . . . . . . . . . . 44
Appendix A. Change Log From Previous Versions . . . . . . . . . . 46
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 46
Intellectual Property and Copyright Statements . . . . . . . . . . 47
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1. Introduction
In Mobile IPv6 [RFC-3775] and NEMO Basic Support [RFC-3963], if a
home agent loses the binding cache state, due to failure or some
other reason, it results in a loss of service for the mobile nodes.
It is beneficial to provide high availability and redundancy for a
home agent so that mobile nodes can avail of uninterrupted service
even when one home agent crashes or loses state. The Home Agent
Reliability protocol is designed to synchronize the Mobile IPv6
states between active and standby home agents as VRRP[RFC-3768] or
HSRP [RFC-2281]. A home agent maintains not only binding cache but
also IPsec and IKE related states per mobile node. Mobile IPv6
mandates IPsec encryption for signaling of home binding registration
(BU and BA) and return routability (HoTI and HoT) as described in
[RFC-3776]. However, IPsec states synchronization is out of scope in
this document. The scope of Home Agent Reliability protocol is
limited to the management of Mobile IPv6 related states. Thus, we
define two different approaches such as Home Agent Virtual Switch and
Home Agent Hard Switch depending on the capability of IPsec state
synchronization. In both cases, a mobile node maintains only one
home binding at any given time.
Home Agent Virtual Switch
The Home Agent Virtual Switch operation can be used only if IPsec
state synchronization mechanism is available (outside of Home
Agent Reliability Protocol). The IPsec state per mobile node MUST
be shared between the active home agent and standby home agents in
the background. The active and the standby home agents are
addressed by the same home agent address, although only the active
home agent is accessible with the home agent address. Each mobile
node negotiates just one Security Association with the active home
agent. In case there is a failure of the active home agent, the
standby home agent takes over without the mobile node being aware
of the change in the home agent.
Home Agent Hard Switch
In the Home Agent Hard Switch, IPsec/IKE states synchronization is
not required. The home agents are addressed by different IP
addresses. When an active home agent fails, a mobile node will
receive a notification (Home Agent Switch message [RFC-5142]) from
a standby home agent, and send a Binding Update to the standby
home agent. In order for the mobile node to receive the Home
Agent Switch message securely from the standby home agent, the
mobile node needs to establish an IPsec SA with both the active
and the standby home agents beforehand.
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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].
In this document, the term mobile node refers to both a mobile node
[RFC-3775] and a mobile router [RFC-3963].
Mobility related terms used in this document are defined in [RFC-
3775] and [RFC-3753]. In addition or in replacement of these, the
following terms are defined or redefined:
Active Home Agent
A home agent that is currently serving the mobile nodes.
Standby Home Agent
A home agent which will serve the mobile nodes when the active
home agent fails.
Failed Home Agent
A home agent that is not available due to hardware or software
failure, system maintenance, etc.
Redundant Home Agent Set
A group of active and standby home agent(s). The Group Identifier
is used to identify a redundant home agent set.
Virtual Home Agent Address
A home agent address shared among home agents in a redundant home
agent set and used only in the Home Agent Virtual Switch case.
The address is only acitivated on an active home agent.
Home Agent Preference
This preference value is originally defined for Dynamic Home Agent
Address Discovery (DHAAD) in RFC3775. This protocol re-uses this
preference value for home agent selection when an active home
agent has failed. However, an operator can also define an
independent value used only for the home agent reliability
protocol if the operator wants to have different preference values
for DHAAD and the home agent reliability protocol. A home agent
SHOULD NOT use the same preference value of other home agents.
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3. Problem Statement and Requirements
In Mobile IPv6 [RFC-3775], a mobile node registers and establishes a
binding with only one home agent. Thus the home agent represents the
possibility of a single point of failure for Mobile IPv6. A home
agent is responsible for multiple mobile nodes on its home link. The
failure of the home agent may then result in the loss of connectivity
for numerous mobile nodes located throughout the Internet. To
overcome this problem, Mobile IPv6 allows deployment of multiple home
agents on the home link so that upon the failure of a home agent, a
mobile node can re-establish its connection through a new home agent.
However, the base Mobile IPv6 specification does not address home
agent failover and dynamic transfer of service from one home agent to
another. This transfer of service from the failed home agent to a
new active home agent requires coordination or pre-configuration
among the home agents regarding security associations, transfer of
mobile node bindings, and other service information for reliable
Mobile IPv6 service in a deployment scenario.
For the home agent reliability solution, we define the following
requirements:
Reliable Home agent service
Multiple home agents are available for a home prefix and one of
them actively serves the mobile nodes. A standby home agent takes
over when the active home agent becomes unavailable. The transfer
of the MN-HA association should be transparent to applications and
should not take longer than the care-of-addresses update procedure
described in Mobile IPv6 [RFC-3775].
Availability of a redundant home agent set
Availability of an active home agent address and a standby home
agent address at the bootstrapping period for the mobile node is
assumed.
State Synchronization
The information for mobile nodes must be able to be synchronized
between an active home agent and standby home agents. This
includes the Binding Cache, AAA information, other Mobile IPv6 and
NEMO related information. Note that the Home Agent Reliability
protocol exchanges only running states of mobile nodes.
Therefore, we do not have any specific operation for synchronizing
the configuration information. For instance, when Mobile IPv6 is
operated with Authentication protocol, the synchronizing the
configurations of the Authentication protocol is out of scope in
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this document. Operators MAY correctly configure in multiple home
agents.
Consideration of IPsec/IKE transfer
An active home agent maintains several IPsec and IKE states for
mobile nodes. These states are synchronized within the redundant
home agent set. The details are described in Section 10.
Secured Message Exchanges
The messages used between the home agents to transfer binding
cache information MAY be authenticated and encrypted.
Failure Detection
Redundant home agents must actively check for possible failure of
an active home agent. If a home agent supports an existing
failure detection mechanism such as VRRP[RFC-3768] or HSRP [RFC-
2281], it can re-use that mechanism to detect the home agent
failure. On the other hand, periodic Hello messages are
introduced to detect active home agent's service availability in
this document.
Failure Notification
If necessary, a mobile node is notified about the active home
agent failure by the standby home agent.
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4. Protocol Overview
4.1. Home Agent Virtual Switch
A mobile node remains unaware about the change in the active home
agent since the home agents have replicated all session state
including IPsec/IKE/ESP states. IPsec/IKE/ESP state transfer is out
of scope of this document.
MN HA1(active) HA2(Standby)
| | .
|<--------->| | 1. IKE exchange (with HoA assignment)
|---------->| . 2. Binding Update
|<----------| . 3. Binding Acknowledgment
| | .
| |<--------->. 4. State exchanges (binding cache/IPsec)
| | .
| X . HA1 FAILURE
| X .
| X<----------. 5. Failure Detection
| X | 6. HA2 takes over the HA1
| X |
| X | RECOVERY COMPLETE
Figure 1: Overview of Home Agent Virtual Switch
The operations of the Home Agent Virtual Switch mode are shown in
Figure 1. A mobile node first attempts the IKE exchange for Security
Association (SA) setup and home address assignment (1). After
binding registration is done (2, 3), the active home agent pushes all
the states of its mobile nodes with a state synchronization message
(4). The standby home agent(s) is not active unless it takes over
from a failed home Agent.
When the active home agent's failure is detected (5), the standby
home agent activates the virtual home agent address on its interface
and takes over for the failed home agent. All the home agents in the
redundant home agent set share a virtual home agent address and the
routing will ensure only the active home agent will be reachable
using that virtual home agent address. The standby home agent can
serve all the mobile nodes for which the states are synchronized,
without any further message exchange, because it has all the
necessary information which it obtained from the failed home agent.
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4.2. Home Agent Hard Switch
The overview of the Home Agent Hard Switch is shown in Figure 2.
This mode is not transparent to the mobile node when the active home
agent failure occurs.
MN HA1(active) HA2(Standby)
| | |
|<--------->| | 1. IKE exchange (with HoA assignment)
|---------->| | 2. Binding Update
|<----------| | 3. Binding Acknowledgment
|<--------------------->| 4. IKE exchange (without HoA assignment)
| | |
| |<--------->. 5. State exchanges (binding cache)
| | |
| X | HA1 FAILURE
| X |
| X<----------| 6. Failure Detection
|<----------------------| 7. Sending Home Agent
| X | Switch message
|<--------------------->| 8. Binding Registration
| X |
| X | RECOVERY COMPLETE
Figure 2: Overview of Home Agent Hard Switch
The mobile node establishes IPsec/IKE state with all the home agents
in the redundant home agent set beforehand (1 and 4), however it
registers its binding only with the active home agent (2 and 3).
When an active home agent fails, a standby home agent uses a pre-
existing IPsec SA to notify the mobile node about the failure by
securely sending a Home Agent Switch message. In order to discover
home agent addresses, two different mechanisms are defined, as
described in Section 9.4.1. The active home agent synchronizes the
required states of the mobile nodes, such as Binding Cache and AAA
information, with other standby home agents periodically (5). The
mobile node MUST NOT request a home address(es) assignment through
the IKE exchange to the standby home agent when it establishes an SA
with it (4).
When the standby home agent detects the failure of the active home
agent (6), it sends a Home Agent Switch message to all the mobile
nodes that were registered with the failed home agent (7). The Home
Agent Switch message must be encrypted by a pre-established IPsec SA.
After the switch message, the mobile node MUST send a binding update
to the new active home agent in order to update the Mobile IPv6
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tunnel end points (8).
4.3. Home Agent Management
HA1(active) HA2 HA3 .. HAn
| | | |
|------->| | | 1. HA1 sends SwitchBack Request
|<-------| | | 2. HA2 sends SwitchBack Reply
| | | |
|<-------| | | 3. HA2 sends SwitchCompl (optional)
(standby) (active) | | HA2 BECOMES ACTIVE HA
| | | |
SYSTEM MAINTENANCE, ETC.
| | | |
|------->| | | 4. HA1 sends SwitchOver Request
|<-------| | | 5. HA2 sends SwitchOver Reply
| | | |
|------->| | | 6. HA1 sends SwitchCompl (optional)
(active) (standby) | | 7. HA1 returns to active HA
| | | | HA1 BECOMES ACTIVE AGAIN
Figure 3: Manual Home Agent Change
In some scenarios the active home agent may need to stop serving
mobile nodes for system maintenance. This specification provides for
a manual home agent switch by using SwitchBack Request and Reply
messages. As shown in Figure 3, the active home agent (HA1) sends a
SwitchBack Request message to a standby home agent (HA2). As soon as
HA2 receives the message, it becomes the active home agent. HA2 will
acknowledge the message by sending a SwitchBack Reply message to HA1.
HA1 becomes a standby home agent when it receives the SwitchBack
Reply. After the downtime, HA1 sends a SwitchOver Request to HA2 in
order to become the active home agent again.
The SwitchCompl message is used only in the Home Agent Hard Switch.
As shown in Section 9, it takes certain time to complete the home
agent switch. Thus, the old active home agent continues serving the
received packets for the mobile nodes during the switch process. As
soon as the new home agent completes the recovery, it sends
SwitchCompl message to the previous active home agent. SwitchCompl
is an optional operation in this specification.
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5. Messages
5.1. New Mobility Header Messages
5.1.1. State Synchronization Message
This message is used to exchange state corresponding to a particular
mobile node(s). It MUST be unicasted and MUST be authenticated by
IPsec ESP. This message has the MH Type value TBD.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type |A| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
. .
. Mobility Options .
. .
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: State Synchronization Message
Type
8-bit unsigned integer. It can be assigned one of the following
values:
0: Request
This message is called State Synchronization Request (SS-REQ)
and used to solicit the active state corresponding to a
particular mobile node.
1: Reply
The message is called State Synchronization Reply (SS-REP) and
is used between the home agents in the redundant home agent set
to exchange binding cache information and any other information
related to providing mobility service to the mobile nodes
either periodically or in response to a SS-REQ.
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2: Reply-Ack
The message is called State Synchronization Reply-Ack (SS-ACK)
and is used to acknowledge to the SS-REP. This message is
optional and is specially used when the links between home
agents are not reliable.
Ack flag
This flag is valid only for SS-REP. If the sender requires
explicit acknowledgment by SS-ACK, it MUST set this flag.
Reserved
This field is unused. It MUST be initialized to zero by the
sender and MUST be ignored by the receiver.
Identifier
A 16-bit identifier to aid in matching state synchronization
message. The identifier should never be set to 0. It should
always be more than 1.
Mobility Options
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 are described in [RFC-3775]. The
receiver MUST ignore and skip any options which it does not
understand. This message requires at least one mobility option,
therefore, there is no default length for this message.
One of the following options is mandatory in SS-REQ message.
Multiple same options can be stored in the same SS-REQ message,
(ex. two IP address options for two mobile nodes):
* IP Address Option (Sub-type: Home Address) defined in [RFC-
5268]. If a home agent wants the Binding Cache information for
a particular mobile node, it includes the mobile node's home
address in an IPv6 Address Option. If a home agent want to
solicit all the active mobile nodes' states, it can include the
unspecified address (0::0) in an IPv6 address option.
* Mobile Network Prefix Option. If a home agent wants to know
the forwarding state setup for a particular Mobile Network
Prefix, it includes a Mobile Network Prefix Option as defined
in [RFC-3963].
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* Vendor Specific Mobility Option. If a home agent wants vendor
specific information, it can solicit with this option as
defined in [RFC-5094].
One of the following options is mandatory in SS-REP:
* Binding Cache Information Option
* AAA Information Option
* Vendor Specific Mobility Option
5.1.2. Home Agent Control Message
This message is used to control the status of a home agent to either
active or standby. This message MUST be unicasted between home
agents and MUST be authenticated and encrypted by IPsec ESP. The
Home Agent Control message has the MH Type value TBD. If no options
are present in this message, no padding is necessary and the Header
Len field will be set to 1.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Status |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Mobility Options .
. .
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Home Agent Control Message
Type
8-bit unsigned integer. It can be assigned one of the following
values:
0: SwitchOver Request (SO-REQ)
It is unicasted by a standby home agent that desires to become
the active home agent. The receiver of the message MUST
transition to standby state as soon as the message is received
and validated successfully.
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1: SwitchOver Reply (SO-REP)
It is used to acknowledge the receipt of the corresponding SO-
REQ.
2: SwitchBack Request (SB-REQ)
It is unicasted by an active home agent that desires to become
a standby home agent. The receiver of this message SHOULD
transition to active state as soon as the message is received
and validated successfully.
3: SwitchBack Reply (SB-REP)
It is used to acknowledge the receipt of the corresponding SB-
REQ.
4: Switch Complete (SW-COMP)
This message is used to indicate the completion of switch over,
(i.e. sending home agent switch messages and receiving binding
update messages from all the served mobile nodes).
Status
8-bit unsigned integer indicating the disposition of a SO-REQ or
SB-REQ. This field is only valid in SO-REP and SB-REP. The
following Status values are defined:
0: Success
128: Reason unspecified
129: Administratively prohibited
130: Not active home agent (The receiver of SO-REQ is not the
active home agent)
131: Not standby home agent (The receiver of SB-REQ is already
the active home agent)
132: Not in same redundant home agent set
Mobility Options
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No options are defined in this specification
5.1.3. Home Agent Hello Message
The Home Agent Hello (HA-HELLO) message MUST be either unicasted or
multicasted to carry home agent information among the redundant home
agent set. The HA-Hello message is defined for two purpose: 1) an
alive check and 2) home agent information exchange. A HA-HELLO
SHOULD be authenticated and encrypted by IPsec ESP when it is
unicasted. If a HA-Hello message is multicasted, IPsec ESP cannot be
applied. In this case the redundant home agent set should be located
in a secure network. Alternatively, all the home agents MUST have a
secure channel with each other. The HA-Hello has the MH Type value
TBD. If no options are present in this message, 0 octets of padding
are necessary and the Header Len field will be set to 2.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence # |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Agent Preference | Home Agent Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hello Interval | Group ID |A|R| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility Options .
. .
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Home Agent Hello Message
Sequence #
16-bit unsigned integer. The Sequence number of the HA-Hello
message can be used to verify whether this Hello message is the
latest one or not.
Home Agent Preference
16-bit unsigned integer. The preference for the home agent
sending the HA-Hello message. This preference is the same as the
Home Agent Preference value of the Home Agent Information option
as defined in [RFC-3775]. However, operators MAY use a different
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preference value for this operation.
Home Agent Lifetime
16-bit unsigned integer. The lifetime for the home agent sending
the HA-Hello message. This lifetime is the same as the Home Agent
Lifetime value of the Home Agent Information option as defined in
[RFC-3775].
Hello Interval
16-bit unsigned integer. The interval for the home agent sending
this Hello message.
Group Identifier
8-bit unsigned integer. This value is used to identify a
particular redundant home agent set.
A flag
Active Home Agent flag. If this flag is set, the sender of this
HA-Hello message is an active home agent.
R flag
HA-HELLO requesting flag. If this flag is set, the receiver of
this HA-Hello message must send back a HA-Hello message to the
sender.
Reserved
This field is unused. It MUST be initialized to zero by the
sender and MUST be ignored by the receiver.
Mobility Options
No valid options are defined in this specification.
5.1.4. Home Agent Switch Message
This message is defined in Section 9.4.3. The Home Agent Reliability
protocol extends this message for the Home Agent Hard Switch.
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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 |I| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
. Home Agent Addresses .
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. Mobility options .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Home Agent Switch Message
IPsec Re-key (I)
The IPsec re-key (I) bit is set to indicate that the mobile node
SHOULD start an IPsec re-key with the home agent specified in the
Home Agent Addresses field. This flag is used when a failed home
agent recovers and needs to re-establish IPsec SA/IKE state with a
mobile node. When this flag is set, the mobile node does not
switch the home agent, but only re-key the SA.
Reserved
The reserve field is reduced from 8 to 7 bits
5.2. New Mobility Options
5.2.1. IP address Option
This option is already defined in the Fast Handovers for Mobile IPv6
(FMIP) specification [RFC-5268]. This document introduces new Sub-
Type values for home agent address and Home Address.
Option-Code
* 4: Home Address
5.2.2. Binding Cache Information Option
The binding cache information option has an alignment requirement of
8n+2. The Binding Cache Information option is only valid in a State
Synchronization message. Its format is as follows:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = TBD | Length = 40 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| Home Address |
+ +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Care-of Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
. Mobile Network Prefix Option .
. .
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Binding Cache Information Option
The fields of Home Address, Care-of Address, Flags, Sequence Number,
and Lifetime are copied from the registered binding of a particular
mobile node or mobile router. The 8-bit Reserved field MUST be set
to zero. If the R-flag is set to indicate this binding cache entry
is for a mobile router, then this option will be immediately followed
by one or more Mobile Network Prefix options.
5.2.3. AAA Information Option
This option is used to carry the AAA state of the mobile node's
Mobile IPv6 sessions. The AAA state information can be carried in
RADIUS or Diameter AVP formats including the user and session info.
This information option is only valid in a State Synchronization
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message.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = TBD | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
. AAA AVPs .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Vendor Specific Inforamtion Option
Type
8-bit Type value. The value is TBD.
Length
8-bit length value.
AAA AVPs
Series of TLV encoded AAA AVPs (including vendor specific AVPs)
carrying AAA-related information for each Mobile IPv6 and IPsec/
IKE session.
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6. Home Agent Configuration
6.1. Network Configuration
The Home Agent Reliability protocol supports two different
configurations for standby home agents. Standby home agents can be
placed on the same home link or on a different link.
HA1 HA2 HA3 HA4 .... HAn
| | | | |
--------+------+------+------+--------+---------
Home Link
Figure 10: Local Recovery Configuration
Figure 10 depicts the configuration where home agents serving the
same home network are located on the same link. For example, HA2,
HA3 and HA4 are standby home agents of HA1. This is the same as what
Mobile IPv6 defines for home agent configuration.
---------IGP------>|<---BGP--->|<-----IGP---------
HA1 HA2 HA3 HA4
| | | |
--------+------+-----+ R---R---R +-----+------+-------
Home Link Routers Recovery Link
(region-1) (region-2)
Figure 11: Global Recovery Configuration
Figure 11 illustrates when standby home agents are located on a
different link (illustrated as Recovery Link in Figure 11). Most
large operators have a very stringent requirement on network
availability even in the worst type of disaster or outage. For
example, HAs in region-1 are backed up by HAs in region-2. These two
regions are geographically separated. If region-1 suffers a downtime
due to any reason, all the sessions will be seamlessly taken over by
the nodes in region-2. This is called geographic redundancy. This
is a well-known configuration for Telecommunications operators. It
can achieve home agent recovery even if the entire home link fails.
In Figure 11, HA3 and HA4 are standby home agents of HA1 and HA2. In
this case, HA3 and HA4 cannot receive packets meant for the home
network until the route on the Routers is changed. The routing must
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be also updated to direct the packets meant for the home link to the
recovery link.
6.2. Address Configuration for Virtual Switch
Each standby home agent obtains its individual IPv6 address from its
attached link. This IPv6 address is used by the home agent
reliability protocol to exchange information with the associated home
agents. The link between home agents should be secured.
The virtual home agent's IPv6 address which is known by the mobile
node is shared with the standby home agents. When a home agent
fails, the standby home agent activates the IPv6 address of the
failed home agent and becomes the active home agent. The standby
home agent should not activate the IPv6 address until it knows the
active home agent is no longer reachable at the address, otherwise
address duplication will occur. To guarantee transparency of the
home agent virtual switch to mobile nodes located on the home link,
the neighbor cache of the home agent IP address MUST be carefully
operated. See Section 8.1 in detail.
6.3. Address Configuration for Hard Switch
Each standby home agent obtains its individual IPv6 address from its
attached link. This IPv6 address is used by the home agent
reliability protocol to exchange information with the associated home
agents. The link between home agents should be secured.
Each home agent configures itself with a different IPv6 address from
the same home prefix. This IPv6 address can be used for the Home
Agent Reliability protocol if the standby home agents are located at
the same link of the active home agent (Figure 10). In case of
Figure 11, the router must carefully route packets to the standby
home agents as described in Section 8.1. Once a mobile node
registers its binding with the active home agent, it may solicit an
IPsec/IKE exchange with standby home agents. These packets must be
routed to the recovery link. This can be achieved by installing host
routes for the standby home agents on the recovery link of the
router.
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7. Home Agent Common Operation
7.1. Home Agent List Management
In Mobile IPv6 [RFC-3775], each home agent periodically sends router
advertisements with the Home Address Information option [RFC-3775]
when there are multiple home agents present on a link. This
information is managed in a home agent list. For the Home Agent
Reliability Protocol, HA-HELLO messages are used to manage the home
agent list. There are several reasons to use HA-HELLO message
instead of Router Advertisement such as:
1. In the Home Agent Virtual Switch mode, if the standby home agents
send unsolicited Router Advertisements to the home link, the
mobile nodes attached to the home link are aware of the presence
of standby home agents. However, the standby home agents must be
hidden until the active home agent fails. HA-Hello messages are
exchanged only between home agents.
2. As shown in Section 6.1, standby home agents are not always
configured at the same link. In this case, Router Advertisements
cannot be sent to the recovery link unless the home link and the
recovery link are connected (ex. L2TP). HA-HELLO can be sent
beyond the link.
3. The Home Agent Reliability protocol defines to manage additional
information such as Group ID and Active/Standby Status of the
home agents in the home agent list.
In Mobile IPv6, Router Advertisement are to carry the home agent
information to both mobile nodes on the home link and the home
agents. On the other hand, in the Home Agent Reliability protocol,
HA-Hello is to exchange the information among the home agents and the
Router Advertisement is used to notify the information to the mobile
nodes. The home agents SHOULD NOT process the Home Agent Information
option carried by Router Advertisement if HA-HELLO is available.
Operators can define different values to the parameters of the home
agent information for HA-HELLO and Router Advertisement. The
management operation of the home agent list is unchanged and defined
in [RFC-3775].
7.2. Detecting Home Agent Failure
The active and standby home agents can monitor each other in several
ways. One method is to reuse other failure detection mechanisms
defined in VRRP[RFC-3768] and HSRP [RFC-2281] (see Section 7.6).
However, VRRP and HSRP cannot detect the case where the system is
running but the Mobile IPv6 stack is not operational. In the Home
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Agent Reliability protocol, a new message called HA-HELLO is
periodically exchanged in the redundant home agent set as a heart-
beat. If HA-HELLO is implemented as a part of Mobile IPv6 stack, it
can detect the home agent failure (Mobile IPv6 stack failure). This
HA-HELLO can also be exchanged frequently enough to detect the
failure promptly and does not introduce any processing overhead to
the mobile node attached to the home link.
Failure events used in the Home Agent Reliability protocol are listed
below.
Loss of HA-HELLO
In the event that a standby home agent does not receive any HA-
HELLO from its peer which is currently the active home agent for a
configurable duration, the standby home agent assumes the active
home agent's failure. Any home agents can also request the home
agent information of the other home agent in the same redundant
home agent set by sending HA-HELLO with R-flag set. If HA-HELLO
is not replied from the target home agent within a configurable
amount of time, that home agent peer is considered to have failed.
The detail of the Hello message is described in Section 7.3.
Monitored Server Failure by the Active Home Agent
There may be number of critical servers such as AAA server in the
network that are essential for the ongoing Mobile IPv6 sessions at
the home agent. Operators can have a policy in place that the
active home agent is treated as a failed home agent upon detecting
that the link to such servers has failed.
Routing Peer/Link Failure
Operators may require the home agent to detect its next-hop
routing peer failure. If the next-hop routing failure is fatal in
nature, or due to some other routing policies, the active home
agent is treated as a failed home gent and the recovering
operation should be started.
7.3. Processing Hello Messages
HA-HELLO MUST be either unicasted or multicasted. A new multicast
address (all_homeagent_multicast_address) will be assigned by the
IANA. When all the home agents in a redundant home agent set are
configured on a same home link, they MUST join the
all_homeagent_multicast_address. On the other hand, if a home
recovery link is separately defined as described in Figure 11, each
home agent SHOULD unicast HA-HELLO.
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7.3.1. Requesting Hello Message
A home agent can solicit HA-HELLO to a particular home agent in the
same redundant home agent set by unicasting HA-HELLO with the R-flag
set. The sender MUST fill the fields of the HA-HELLO with its home
agent information. If a home agent needs to request HA-HELLO to all
the home agents, it sends the HA-HELLO with R-flag set to the
all_homeagent_multicast_address. Requesting HA-HELLO SHOULD be
operated when:
1. A new home agent needs to collect the information of all the
other home agents in the same redundant home agent set. The HA-
HELLO with R-flag set is multicasted to
all_homeagent_multicast_address.
2. A home agent entry in the redundant home agent list is about to
be removed due to home agent lifetime expiration. The HA-HELLO
with R-flag set is unicasted to the home agent whose lifetime is
soon expired.
3. HA-HELLO has not been received during the specified hello
interval. The HA-HELLO with R-flag set is unicasted to the
target home agent.
7.3.2. Sending Hello Message
Each home agent periodically sends HA-HELLO for the home agent's
failure detection. The interval time is configured at each home
agent. Each home agent MUST also send a HA-HELLO in following case:
1. when a home agent receives a HA-HELLO with the R-flag set
2. When a home agent detects its local information such as home
agent preference, home agent lifetime, and registration status
change.
3. When a new home agent boots up, it SHOULD solicit Hello messages
by multicasting a Hello message with the R-flag set in parallel
with sending its own Hello message.
When a home agent sends HA-HELLO, the following rule MUST be applied.
o Whenever a home agent generates HA-HELLO, it MUST increment in the
Sequence Number. The Sequence Number SHOULD be initialized to
zero for the first Hello message. To accomplish sequence number
rollover, if the sequence number has already been assigned to be
the largest possible number representable as a 16-bit unsigned
integer, then when it is incremented it will then have a value of
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zero (0).
o It MUST also specify its own Group ID in HA-HELLO.
o If a home agent is the active home agent, it MUST set the A-flag
in HA-HELLO.
o In the Home Agent Hard Switch mode, the source IPv6 address of HA-
HELLO MUST be the home agent address.
o In the Home Agent Virtual Switch mode, the home agent local
address MUST be used.
7.3.3. Receiving Hello Message
When a home agent receives HA-HELLO, it SHOULD verify the HA-HELLO as
follows:
o If the HA-HELLO is not protected by IPsec ESP, it SHOULD be
discarded. Note that, only if the HA-HELLO is sent on a dedicated
link between the home agents, IPsec protection might not be always
required. This depends on the operational policy.
o If HA-HELLO is sent from non global IPv6 address, it MUST be
discarded.
o If the source IPv6 address of HA-HELLO is not belong to one of the
home agents in the redundant home agent set, the HA-HELLO MUST be
ignored.
o If the Group ID field of the received HA-HELLO and the receiver's
Group ID is different, HA-HELLO MUST be discarded. HA-HELLO MUST
NOT be sent to home agents whose Group ID is different from the
sender.
o If the Sequence Number value in the HA-HELLO is equal to or less
than the last received Sequence Number value stored in the home
agent list entry, the HA-HELLO MUST be discarded.
o HA-HELLO satisfying all of above tests MUST be processed by
receiver. The receiver copies home agent information in HA-HELLO
to the corresponding home agent list entry. The home agent
address of the sender is retrieved from the Source Address field
of the IPv6 header of the HA-HELLO.
o If the home agent lifetime field in the HA-HELLO is set to 0, the
receiver removes the sender from the home agents list.
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o If the R-flag is set in the received HA-HELLO, the receiver MUST
send a new HA-HELLO to the originator as described in
Section 7.3.2.
7.4. Processing State Synchronization Messages
It is necessary for standby home agents to synchronize the state
information of each mobile node registered with the active home
agent. In the Home Agent Hard Switch mode, it is not necessary for
the home agents to synchronize the complete binding cache
information. The standby home agent needs the mapping information of
the active home agent and the mobile node. The information is used
to send the Home Agent Switch messages to all the mobile node served
by the failed home agent.
7.4.1. Requesting State of a Particular Mobile Node(s)
When a home agent needs the state information for a particular mobile
node or a subset of mobile nodes, it sends a SS-REQ message
constructed as follows:
o It MUST set the Type field to 0 (Request).
o It MUST set a random value in the Identifier field that does not
coincide with any other currently pending Requests.
o It MUST include an IP address mobility option(s) which subtype is
set to the home address if the target is mobile node(s).
o It MUST include a Mobile Network Prefix mobility option(s) for
mobile router(s).
o It MUST set the unspecified address (0::0) in the Home Address
mobility option if it solicits the state of all the mobile nodes
and mobile routers registering at the receiver of SS-REQ
(i.e.destination of SS-REQ).
o In the Home Agent Virtual Switch, the sender of the SS-REQ MUST be
a home agent local address of one of the home agents in the same
redundant home agent set.
o In the Home Agent Hard Switch, the sender of the SS-REQ MUST be a
home agent address of one of the home agents in the same redundant
home agent set.
o The destination of the SS-REQ MUST be the active home agent for
the requesting home address or mobile network prefix. The standby
home agent MUST NOT reply the SS-RREP to the sender.
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When a home agent receives the SS-REQ, it MUST verify if SS-REQ is
constructed with the above rules. If SS-REQ satisfy all the above
tests, the receiver of the SS-REQ MUST reply SS-REP including the
state information of the requested mobile node(s) and/or mobile
network prefix(es) as described in Section Section 7.4.2.
7.4.2. Synchronizing State
State synchronization messages SHOULD be sent when:
1. The active home agent receives SS-REQ.
2. The active home agent creates a binding cache entry for a
particular mobile node.
3. The active home agent deletes a binding cache entry for a
particular mobile node.
The active home agent MAY additionally send state synchronization
message in following cases:
1. The active home agent update the state information for all
sessions that changed since the last update in a periodic
interval
2. Only for the Home Agent Virtual Switch mode, the active home
agent updates a binding cache entry for a particular mobile node
whenever the binding cache entry is updated. In the Home Agent
Hard Switch mode, standby home agents only need the mapping
information of a home address of the mobile node/router and the
home agent address of the active home agent to which the mobile
node/router is currently registering. This mapping is used to
send a Home Agent Switch message.
If an active home agent sends a State Synchronization message
whenever the local state information changes, such as a binding cache
change, the number of the State Synchronization messages sent can be
quite large.
All the state information of the requested mobile nodes is stored in
the SS-REP. Following rules must be applied when the active home
constructs SS-REP.
o If the SS-REP is sent in response to the SS-REQ, the active home
agent MUST copy the Identifier field of the State Synchronization
request message to the Identifier field in the SS-REP. Otherwise,
it MUST set the Identifier field to 0.
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o When the active home agent stores the state of multiple mobile
nodes in a SS-REP, a Binding Cache Information option is used as a
separator. For each mobile node, a Binding Cache Information
option is placed first, followed by any other options such as AAA
option. When the next Binding Cache Information option is reached
in the State Synchronization message, it indicates the information
of a different mobile node.
o If the unspecified address is found in the Home Address mobility
option carried with the SS-REQ, the active home agent MUST return
the state of all the active mobile nodes and mobile routers by the
SS-REP. The IP fragmentation can be occurred depending on the
total size of all the states.
o A SS-REP MUST be authenticated and encrypted by IPsec ESP.
o The destination and source home agents MUST belong to the same
redundant home agent set.
o In the Home Agent Hard Switch, the IPv6 source address MUST be set
to the home agent address of the sender.
o In the Home Agent Virtual Switch, the IPv6 source address MUST be
set to the home agent local address of the sender.
When a home agent receives a SS-REP, it MUST verify whether the SS-
REP is constructed with the above rules or not. If the SS-REP does
not satisfy all the rules above, it is discarded. Otherwise, the
following operation must be taken.
o The receiver of SS-REP MUST update its binding cache and all other
necessary information such as AAA and vendor specific information
in the particular database.
o In the Home Agent Hard Switch mode, the receiver MUST record the
IPv6 address of the sender as the active home agent of the mobile
node.
7.4.3. Reliable Transmission by Explicit Acknowledgement
Signaling messages of the Home Agent Reliability protocol are not
guaranteed reliable transmission due to the Mobility Header use.
This is not always critical, because the link between home agents is
carefully managed as stable and reliable. However, operators may
need more explicit notification to confirm the message exchanges
between home agents. This specification provides an optional
acknowledgment to SS-REP messages.
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If an active home agent requires an acknowledgment of SS-REP, it set
the Ack flag in the SS-REP. The receiver of such SS-REP will send
back a SS-ACK. The receiver MUST copy the Identifier value received
in the SS-REP into SS-ACK in order to match the SS-REP and SS-ACK.
7.5. Processing Home Agent Control Messages
7.5.1. Standby Home Agent becomes an Active Home Agent
When a standby home agent decides to become an active home agent, the
standby home agent sends a SO-REQ to the active home agent. This
message MUST be unicasted to the active home agent and MUST be
encrypted and authenticated by IPsec ESP. The active home Agent MUST
NOT generate this message.
When an active home agent receives a SO-REQ, it MUST operate the
following verification and operations:
o If the SO-REQ is not protected by IPsec, it MUST be discarded.
o If the receiver of the SO-REQ is not an active home agent, it MUST
send a SO-REP with the Status field set to 130 (Not active home
agent).
o If the sender home agent does not belong to the same redundant
home agent set, a SO-REP message MUST be sent to the sender with
the Status field set to 132 (Not in same redundant home agent
set).
o If the receiver is an active home agent, there is case where the
active home agent cannot be standby home agent. In such case, the
active home agent can reply a SO-REP with the Status field set to
129 (Administratively prohibited).
o Otherwise, the active home agent MUST become a standby home agent
and reply with a SO-REP message with the Status field set to 0
(Success).
The SO-REP MUST be also protected by IPsec ESP. Otherwise, the
message MUST be silently discarded. If the receiver of SO-REP did
not send a SO-REQ message (i.e. unexpected SO-REP), the message MUST
be ignored. If the Status field of the SwitchOver Reply message is 0
(Success), the receiving standby home agent immediately becomes an
active home agent as described in Section 8.2. If the value in the
Status field is greater than 128 an error has occurred. In this
case, the receiver MUST NOT attempt to be an active home agent.
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7.5.2. Active Home Agent becomes in-active
When an active home agent decides to become a standby home agent, it
sends a SB-REQ to one of standby home agent. The reason for the
active home agent to send this message can be administrative
intervention, and events like Monitored Server Failure by the active
home agent or Routing Peer/Link Failure. This message MUST be
unicasted to one of the standby home agents and MUST be encrypted and
authenticated by IPsec ESP. A standby home agent MUST NOT generate
this message.
When a home agent receives a SwitchBack Request message, it first
verifies the message.
o If the SwitchBack Request message is not protected by IPsec ESP,
it MUST be discarded.
o If the sender home agent of the SB-REQ is not an active home
agent, the receiver MUST reply a SB-REP with the Status field is
set to 130 (Not active home agent).
o If the sending home agent does not belong to the same redundant
home agent set, a SB-REP MUST be sent in which the Status field
set to 132 (Not in same redundant home agent set).
o Otherwise, the receiving home agent MUST send a SB-REP with the
Status field is set to 0 (Success).
After sending the SwitchBack reply, it MUST NOT become an active home
agent immediately. This is because the active home agent is still
active until it receives the SB-REP which is acknowledging the SB-
REQ. The standby home agent SHOULD change to active at least after
LINK_TRAVERSAL_TIME.
If a home agent receives a SB-REP, it MUST be protected by IPsec ESP,
otherwise the message MUST be silently discarded. If the receiving
home agent did not send a SB-REQ matched to the received SB-REP, the
message MUST be silently discarded. If the Status field of the SB-
REP is 0 (Success), the active home agent immediately becomes a
standby home agent. The sender home agent of SB-REP becomes active
home agent as described in Section 8.2. If the value in the Status
field is greater than 128, the receiver of SB-REP (active home agent)
cannot become a standby home agent and MUST continue to be an active
home agent.
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7.6. Interworking with VRRP
VRRP and HSRP specify an election protocol that dynamically assigns
responsibility for a virtual router to one of the VRRP routers on a
LAN. This operation is similar to the Home Agent Virtual Switch
operation. For example, the VRRP router controlling the IP
address(es) associated with a virtual router is called the Master,
and forwards packets sent to these IP addresses. The election
process provides dynamic fail over in the forwarding responsibility
should the Master become unavailable. Although VRRP is used to
guarantee home agent address reachability, it cannot be used for
state synchronization and explicit switching of Master and Backup.
Thus, the Home Agent Reliability protocol cannot be replaced by VRRP.
This section explains how VRRP can interwork with the Home Agent
Reliability protocol.
When VRRP is available, VRRP can replace the Hello message described
in Section 5.1.3. However, some of information is missed by using
VRRP. After receiving a VRRP message, each home agent SHOULD process
the message and store the information as if it receives Home Agent
Hello messages Section 7.3.3. The Home Agents SHOULD still perform
binding cache synchronization as described in Section 7.4 and SHOULD
support the Home Agent Switch message as described in Section 9.2.
In addition to this, VRRP is useful only if all home agents are
located on the same link. If the home agents are topologically
separated, the Home Agent Reliability protocol MUST be used.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| Type | Virtual Rtr ID| Priority |Count IPv6 Addr|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|(rsvd) | Adver Int | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| IPv6 Address(es) |
+ +
+ +
+ +
+ +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Figure 12: VRRP Packet Format
The message format of VRRP is described in Figure 12. Each field is
mapped as follows:
Virtual Rtr ID
Group ID is stored in the Virtual Rtr ID field.
Priority
Home Agent Preference is stored in the Priority field. Note that
VRRP only has 8 bits for the Priority field. Therefore, values
larger than 255 MUST NOT be assigned to the preference value.
Count IPv6 IPv6 Addr
This field MUST be always be 1.
Advert Int
This field MUST be mapped to the Hello Interval field of the Home
Agent Hello message, though it only has 12 bytes.
IPv6 address
A home agent address is stored in this field.
Home Agent Lifetime, Sequence Number and Flags field are missing in
the VRRP packet format. Therefore, operators SHOULD use the same
statically configured value for Home Agent Lifetime. Each home agent
does not check freshness of received VRRP message because of no
sequence number. If VRRP is used, a home agent cannot determine the
active home agent from the VRRP message due to lack of A flag, and
cannot request a VRRP advertisement to other home agents.
7.7. Retransmissions and Rate Limiting
Standby and active home agents are responsible for retransmissions
and rate limiting of a SS-REQ, SO-REQ, SB-REQ messages for which they
expect a response. The home agent MUST determine a value for the
initial transmission timer:
o If the home agent sends a SS-REQ message, it SHOULD use an initial
retransmission interval of INITIAL_STATE_SYNC_REQ_TIMER.
o If a standby home agent sends a SO-REQ message, it SHOULD use an
initial retransmission interval of INITIAL_SWICHOVER_REQ_TIMER.
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o If an active home agent sends a SB-REQ message, it SHOULD use an
initial retransmission interval of INITIAL_SWICHBACK_REQ_TIMER .
If the sending home agent fails to receive a valid matching response
within the selected initial retransmission interval, it SHOULD
retransmit the message until a response is received. All of the
above constants are specified in Section 11.
The retransmission MUST use an exponential backoff process as
described in [RFC-3775] until either the home agent receives a
response, or the timeout period reaches the value
MAC_HARELIABILITY_TIMEOUT (16sec). The home agent SHOULD use a
separate back-off process for different message types and different
destinations. The rate limiting of Mobility Header messages is the
same as one in [RFC-3775]. A home agent MUST NOT send Mobility
Header Messages to a particular home agent more than MAX_UPDATE_RATE
(3) times a second, which is specified in [RFC-3775].
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8. Home Agent Virtual Switch
8.1. Consideration of Routing and Neighbor Discovery Protocol
This section gives a brief explanation of how a home agent interacts
with routing and Neighbor Discovery Protocol (NDP) when the Home
Agent Virtual Switch mode is used.
When a standby home agent becomes active in the Home Agent Virtual
Switch mode, it MUST start to advertise the home agent address and
the home prefix of the home addresses serviced by the redundant home
agent set into the routing infrastructure. This operation is
normally done using a route selector such as BGP or an OSPF modifier.
For example, we can use the AS_Path prepend operation for BGP, and
the Metric field in OSPF for the route selection. When each home
agent participates in OSPF routing, each home agent should be
configured with the appropriate metric matched to the home agent
preference value. When the active home agent fails, OSPF detects the
failure and can dynamically switch the route to the standby home
Agent based on the OSPF cost value. If this cost conflicts with the
home agent preference value due to configuration errors, the routers
on the home link may not route packets to the desired standby home
agent. In order to change the OSPF cost correctly and dynamically,
The operator takes other existing approaches. For example, most of
router vendors have a private MIB to set the cost via SNMP, though
this is a vendor-specific function.
When an active home agent activates a home agent address, it SHOULD
use a virtual MAC address as introduced in [RFC-3768]. When the
active home agent is changed, the neighbor cache of the active home
agent is not necessarily updated on mobile nodes located on the home
link. Otherwise, the new home agent MUST update the neighbor cache
entry for the home agent address on all the mobile nodes located on
the home link. In addition, Mobile IPv6 uses proxy NDP to intercept
packets meant for mobile nodes which are away from the home link.
However, it is unnecessary for the new active home agent to overwrite
the existing proxy neighbor entries of the mobile nodes.
8.2. Home Agent Recovery
After detecting the active home agent has failed, the standby home
agent whose preference value is the highest MUST take over the failed
home agent. The standby home agent MUST activate the virtual home
agent address. If a virtual MAC address as introduced in [RFC-3768]
is used, the standby home agent MUST start using that virtual MAC
address as well. Since all the necessary state has already been
transferred to this standby home agent before the active home agent
failed, it can immediately start acting as the active home agent.
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9. Home Agent Hard Switch
9.1. Home Agent Recovery
After detecting the active home agent has failed, the standby home
agent whose preference value is the highest MUST take over the failed
home agent. The standby home agent MUST send a Home Agent Switch
message to all the mobile nodes that were registered at the failed
home agent as described in Section 9.2, using the pre-established
IPsec SA. The standby Home Agent MUST set its own address in the
Home Agent Address field in the Home Agent Switch message so that it
will receive the binding update from the mobile node as an
acknowledgment of the sent Home Agent Switch message. The home agent
switch-over is complete when it receives binding updates from all the
mobile nodes. It is important to remark that sending Home Agent
Switch messages to all the mobile nodes at once may bring non-
negligible overhead to the home agent.
This overhead cannot be avoided if the active home agent suddenly
stop serving mobile node because of unexpected reasons (crash,
network trouble, etc). However, if this switch over is operated
under the administrative operation (maintenance, etc), the previous
active home agent may continue serving the mobile nodes until the
switch over is completed. Until the mobile node sends a binding
update to the new active home agent, it still sends the packet to the
previous home agent in the Home Agent Hard Switch. Therefore, the
new active home agent can notify the completion of switch-over to the
previous active home agent by using Home Agent Control message as
described in Section 9.3. As soon as this message is received, the
previous active home agent can be shutdown or detached from the
network safely.
9.2. Sending Home Agent Switch Messages
The standby home agent which is going to be active MUST send a Home
Agent Switch message as defined in [RFC-5142] to all the mobile nodes
that were being served by the failed home agent. The Home Agent
Switch message must be securely sent to the mobile node by using
IPsec ESP. The standby home agent MUST include only its own home
agent address in the Home Agent Switch message. If there are a large
number of mobile nodes served by the failed home agent, the overhead
sending Home Agent Switch messages is high. Until a mobile node
receives this Home Agent Switch messages, the mobile node's
communication is discontinued. Therefore, until the standby home
agent completes sending the Home Agent Switch message to all the
mobile nodes and receives Binding Updates from all the mobile node,
the failed home agent SHOULD serve mobile nodes if possible. This is
the case when the active home agent is replaced by administrative
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operation with the Home Agent Control messages as described in
Section 9.3.
When a failed home agent recovers, it MUST re-establish an IPsec SA
with each mobile node served by its redundant home agent set.
Otherwise, it cannot be either a standby or active home agent for the
mobile nodes. Therefore, as soon as the active home agent detects
the recovery of the failed home agent, it sends a Home Agent Switch
message with the I-flag set to all the mobile nodes serving by other
home agents in the same redundant home agent set, and includes the
recovered home agent address in the Home Agent Addresses field. The
mobile node will re-key the SA, but it will not change the home agent
by this home agent switch message which I-flag is set.
9.3. Notification of Home Agent Switch Completion
If the new active home agent completes the switch-over as described
in Section 8.2, it SHOULD send a SW-COMP to the previous active home
agent in the Home Agent Hard Switch case. Until the previous home
agent receives this message, it SHOULD continue serving any mobile
nodes that are registered with it. Once the previous home agent
receives the SW-COMP message, it can stop home agent services.
9.4. Mobile Node Operation
9.4.1. Home Agent Addresses Discovery
In the Home Agent Hard Switch mode, a mobile node authenticates
itself to two or more home agents and creates IPsec SAs with them
during bootstrapping. When the active home agent fails, another home
agent can use the pre-existing SA to notify the mobile node about the
failure by sending a Home Agent Switch message.
In order to discover multiple home agent addresses, two different
mechanisms are defined in the bootstrapping solution in the split
scenario [RFC-5026]. One is DNS lookup by home agent Name, the other
is DNS lookup by Service Name. DHCPv6 can also be used in the
integrated scenario [ID-BOOTINT] to provide home agent provisioning
to mobile nodes.
In the split scenario, a mobile node can use DNS lookup by Service
Name to discover the home agents, as defined in [RFC-5026]. For
example, if home agent reliability is required by a mobile node, DNS
lookup by Service Name method is recommended for the mobile node to
discover multiple home agents addresses. Therefore, mobile nodes
will query the DNS SRV records with a service name of mip6 and
protocol name of ipv6. The DNS SRV records includes multiple home
agent addresses and different preference values and weights. The
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mobile node SHOULD choose two or more home agents from the home
agents list according to their preference value. Then the mobile
node should authenticate itself to these home agents via an IKEv2
exchange.
In the integrated scenario, a mobile node can use DHCPv6 to get home
agent provisioning from an MSP or ASP, as already defined in [ID-
BOOTINT]. The only requirement is that the DHCPv6 response must
include multiple home agents' information in order to support home
agent reliability.
9.4.2. IKE/IPsec pre-establishment to Home Agents
After a mobile node obtains multiple home agent addresses, it needs
to trigger multiple IKE exchanges with the multiple home agents
selected from the home agent list. Since both IKEv1 and IKEv2 can be
used to bootstrap Mobile IPv6, this solution does not introduce any
new operations to co-operate with IKEv1 or IKEv2. It should initiate
IKE for home agents as soon as home registration is complete.
The mobile node MUST follow the standard IKEv2 exchange in the
bootstrapping solution of the split scenario [RFC-5026]. Home
Address configuration maybe also be included, if necessary, for the
first IKE exchange. After its Home Address is assigned or approved
by the first home agent, mobile node SHOULD register itself with the
second home agent with IKE using the same Home Address. Therefore,
no home address configuration should be used in the second IKEv2
procedure. Note that the mobile node only sends a Binding Update
message to the first home agent.
9.4.3. Receiving Home Agent Switch message
A mobile node must follow the operation specified in [RFC-5142] when
it receives a Home Agent Switch message.
If the I-flag is set in the received Home Agent Switch message, the
mobile node MUST re-key the SA with the home agent addresses stored
in the Home Agent Addresses field. The mobile node MUST NOT change
its active home agent when the I-flag is set. If the home agent
address is not known from the bootstrapping described in
Section 9.4.1, the mobile node MUST NOT start an IKE session with the
unknown home agent. Instead, it SHOULD re-start home agent discovery
again to update its home agent address information.
When the mobile node receives a Home Agent Switch message without
I-flag set, and if the message contains the IPv6 address of a standby
home agent, it MUST select the standby home agent in the switch
message as the active home agent and send a new Binding Update
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message to it. Note that the standby home agent address in the Home
Agent Switch MUST be equal to the sender of the Home Agent Switch
message. The standby Home agent expects the Binding Update as an
acknowledgment of the Home Agent Switch message. The mobile node
already has a pre-established SA with the standby home agents and
should use that SA to send the Binding Update. If the address stored
in the Home agent address field is different from the sender, the
mobile node MUST send a binding update to the sender.
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10. Security Considerations
Since Mobile IPv6 operation requires ESP in transport mode between
the mobile node and the home agent, we will discuss the ESP field
synchronization issues between the mobile node and the redundant set
of home agents. This synchronization is required only for Home Agent
Virtual Switch mode. Most of fields should be synchronized based on
[RFC-4301]. The ESP header has the following fields:
SPI
This field identifies the SAD at the receiver.
The mobile node negotiates only one IPsec SA. Hence, the SPI
value will remain unchanged upon home agent failover.
Sequence Number
This field is used for "anti-replay" feature of ESP. The
transmitter must include this monotonically increasing number.
The receiver may process the sequence number based on local
policy.
The mobile node and the redundant home agent set will have the
same set of sequence numbers for transmit and receive. Hence,
synchronization of the sequence number field is mandatory in this
mode of operation.
The SA1, SA2, SA3, SA4 could be synchronized between the home
agents as these messages are not sent continuously. Moreover for
the Binding Update case, if the mobile node is in the middle of
sending a Binding Update to an active home agent for a binding
refresh, and the active home agent is not available at that
moment, the mobile node will not get any response from the active
home agent. After a standby home agent becomes active, the mobile
node will retry and it will receive the Binding Update from the
mobile node with a sequence number that is +n from its last known
sequence number for SA1. For the Binding Acknowledgment case
(SA2), the standby home agent SHOULD add a random number to the
last known sequence number over and above the replay window to
ensure that the packet passes the replay check at the mobile node.
The same applies to HoTi and HoT messages with SA3 and SA4. Note
that this windowing of the sequence numbers for Mobile IPv6
signaling is only needed to cover the corner cases when Binding
Update or HoTi is in-flight and the active home agent fails.
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The technique explained above should work for user data packets if
ESP is used to encrypt user data traffic as well. The actual
switchover time and the routing infrastructure convergence time is
the only latency that the user may perceive.
Initialization Vector
Since the Initialization Vector will be delivered in each exchange
between a mobile node and home agent, this field is not
necessarily synchronized between home agents.
Others
Other fields should be synchronized based on RFC4301 [RFC-4301]
In the Home Agent Hard Switch mode, the standby home agent needs to
send a Home Agent Switch message using IPsec encryption. Since the
mobile node has pre-established an IPsec SA with both the active and
standby home agents, the standby home agent can send the message to
the mobile node with the pre-established IPsec SA.
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11. Protocol Constants
INITIAL_STATE_SYNC_REQ_TIMER: 3sec
INITIAL_SWICHOVER_REQ_TIMER: 1sec
INITIAL_SWICHBACK_REQ_TIMER 1sec
LINK_TRAVERSAL_TIME 150msec
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12. IANA Considerations
o The values for following mobility header message MUST be assigned
by IANA.
* State Synchronization Message
* Home Agent Control Message
* Home Agent Hello Message
* Home Agent Switch Message
o The values for following mobility options MUST be assigned by
IANA.
1. Binding Cache Information Option
2. AAA Information Option
o New Option Code for the IP address option defined in [RFC-5268]
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13. Additional Authors
This document is a result of discussions in the Mobile IPv6 Home
Agent Reliability Design Team. The members of the design team that
are listed below are authors that have contributed to this document:
Samita Chakrabarti
samita.chakrabarti@azairenet.com
Kuntal Chowdhury
kchowdhury@starentnetworks.com
Hui Deng
denghui@chinamobile.com
Vijay Devarapalli
vijay.devarapalli@azairenet.com
Sri Gundavelli
sgundave@cisco.com
Brian Haley
brian.haley@hp.com
Behcet Sarikaya
bsarikaya@huawei.com
Ryuji Wakikawa
ryuji@sfc.wide.ad.jp
14. Acknowledgements
This document includes a lot of text from [ID-LOCALHAHA] and [ID-
HAHA]. Therefore the authors of these two documents are
acknowledged. We would also like to thank the authors of the home
agent reliability problem statement [ID-PS-HARELIABILITY] for
describing the problem succinctly and Alice Qin for her work on the
hello protocol.
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15. References
15.1. 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., Arkko, J., "Mobility Support in
IPv6", RFC 3775, June 2004.
[RFC-3963] Devarapalli, V., Wakikawa, R., Petrescu, A., and P.
Thubert, "Network Mobility (NEMO) Basic Support Protocol", RFC 3963,
January 2005.
[RFC-5094] Devarapalli, V., "Mobile IPv6 Vendor Specific Option", RFC
5094, October 2007.
[RFC-5142] Haley, B., "Mobility Header Home Agent Switch Message",
RFC-5142, November 2007.
[RFC-5026] Giaretta, G., "Mobile IPv6 bootstrapping in split
scenario", RFC 5026, October 2007.
[ID-BOOTINT] Chowdhury, K. and A. Yegin, "MIP6-bootstrapping via
DHCPv6 for the Integrated Scenario",
draft-ietf-mip6-bootstrapping-integrated-dhc-06 (work in progress),
April 2008.
15.2. Informative References
[RFC-3768] Hinden, R., "Virtual Router Redundancy Protocol (VRRP)",
RFC 3768, April 2004.
[RFC-2281] Li, T., Cole, B., Morton, P., and D. Li, "Cisco Hot
Standby Router Protocol (HSRP)", RFC 2281, March 1998.
[RFC-3776] Arkko, J., Devarapalli, V., and F. Dupont, "Using IPsec to
Protect Mobile IPv6 Signaling Between Mobile Nodes and Home Agents",
RFC 3776, June 2004.
[RFC-4301] Kent, S. and Atkinson, R., "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC-3753] Manner, J. and M. Kojo, "Mobility Related Terminology",
RFC 3753, June 2004.
[RFC-5268] Koodli, R., "Mobile IPv6 Fast Handovers", RFC 5268, June
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2008.
[ID-HAHA] Wakikawa, R., "Inter Home Agents Protocol Specification",
draft-wakikawa-mip6-nemo-haha-spec-01 (expired), March 2006.
[ID-LOCALHAHA] Devarapalli, V., "Local HA to HA protocol",
draft-devarapalli-mip6-nemo-local-haha-01 (expired), March 2006.
[ID-PS-HARELIABILITY] Faizan, J., "Problem Statement: Home Agent
Reliability", draft-jfaizan-mipv6-ha-reliability-01 (expired),
February 2004.
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Appendix A. Change Log From Previous Versions
Changes from draft-ietf-mip6-hareliability-03
o Only Editorial Update and No Technical Change
Author's Address
Ryuji Wakikawa
Toyota ITC
6-6-20 Akasaka, Minato-ku
Tokyo 107-0052
Japan
Phone: +81-3-5561-8276
Fax: +81-3-5561-8292
Email: ryuji@jp.toyota-itc.com
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