Network Working Group R. Hinden
Internet-Draft Nokia
Expires: August 23, 2007 J.Cruz, Editor
Cisco Systems
February 23, 2007
Virtual Router Redundancy Protocol for IPv6
<draft-ietf-vrrp-ipv6-spec-08.txt>
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This memo defines the Virtual Router Redundancy Protocol (VRRP) for
IPv6. It is version three (3) of the protocol. It is based on the
original version of VRRP (version 2) for IPv4 that is defined in
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RFC2338.
VRRP specifies an election protocol that dynamically assigns
responsibility for a virtual router to one of the VRRP routers on a
LAN. 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. The advantage gained from using VRRP for IPv6 is a
quicker switch over to back up routers than can be obtained with
standard IPv6 Neighbor Discovery [ND] mechanisms.
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Table of Contents
1. Introduction................................................3
2. Required Features...........................................5
3. VRRP Overview...............................................6
4. Sample Configurations.......................................8
5. Protocol...................................................10
5.1 VRRP Packet Format....................................10
5.2 IP Field Descriptions.................................11
5.3 VRRP Field Descriptions...............................11
6. Protocol State Machine....................................13
6.1 Parameters per Virtual Router.........................13
6.2 Timers................................................13
6.3 State Transition Diagram..............................15
6.4 State Descriptions....................................15
7. Sending and Receiving VRRP Packets........................19
7.1 Receiving VRRP Packets................................19
7.2 Transmitting Packets..................................19
7.3 Virtual MAC Address...................................20
7.4 IPv6 Interface Identifiers............................20
8. Operational Issues........................................21
8.1 ICMPv6 Redirects......................................21
8.2 ND Neighbor Solicitation..............................21
8.3 Router Advertisements.................................21
8.4 Potential Forwarding Loop.............................22
8.5 Recommendations regarding setting priority values.....22
9. Operation over FDDI, Token Ring, and ATM LANE.............22
9.1 Operation over FDDI...................................22
9.2 Operation over Token Ring.............................22
9.3 Operation over ATM LANE...............................25
10. Security Considerations...................................25
11. Intellectual Property.....................................26
12. Acknowledgments...........................................26
13. IANA Considerations.......................................27
14. Normative References......................................27
15. Informative References....................................28
16. Changes from RFC2338......................................29
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1. Introduction
IPv6 hosts on a LAN will usually learn about one or more default
routers by receiving Router Advertisements sent using the IPv6
Neighbor Discovery protocol [ND]. The Router Advertisements are
multicast periodically at a rate that the hosts will learn about the
default routers in a few minutes. They are not sent frequently enough
to rely on the absence of the router advertisement to detect router
failures.
Neighbor Discovery (ND) includes a mechanism called Neighbor
Unreachability Detection to detect the failure of a neighbor node
(router or host) or the forwarding path to a neighbor. This is done
by sending unicast ND Neighbor Solicitation messages to the neighbor
node. To reduce the overhead of sending Neighbor Solicitations, they
are only sent to neighbors to which the node is actively sending
traffic and only after there has been no positive indication that the
router is up for a period of time. Using the default parameters in
ND, it will take a host about 38 seconds to learn that a router is
unreachable before it will switch to another default router. This
delay would be very noticeable to users and cause some transport
protocol implementations to timeout.
While the ND unreachability detection could be speeded up by changing
the parameters to be more aggressive (note that the current lower
limit for this is 5 seconds), this would have the downside of
significantly increasing the overhead of ND traffic. Especially when
there are many hosts all trying to determine the reachability of a
one of more routers.
The Virtual Router Redundancy Protocol for IPv6 provides a much
faster switch over to an alternate default router than can be
obtained using standard ND procedures. Using VRRP a backup router
can take over for a failed default router in around three seconds
(using VRRP default parameters). This is done with out any
interaction with the hosts and a minimum amount of VRRP traffic.
VRRP specifies an election protocol that dynamically assigns
responsibility for a virtual router to one of the VRRP routers on a
LAN. 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.
VRRP provides a function similar to the proprietary protocols Hot
Standby Router Protocol (HSRP) [HSRP] and IP Standby Protocol
[IPSTB].
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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 [RFC2119].
1.1 Scope
The remainder of this document describes the features, design goals,
and theory of operation of VRRP for IPv6. The message formats,
protocol processing rules and state machine that guarantee
convergence to a single Virtual Router Master are presented.
Finally, operational issues related to MAC address mapping, handling
of Neighbor Discovery requests, generation of ICMPv6 redirect
messages, and security issues are addressed.
This protocol is intended for use with IPv6 routers only. VRRP for
IPv4 is defined in [VRRP-V4].
1.2 Definitions
VRRP Router A router running the Virtual Router Redundancy
Protocol. It may participate in one or more
virtual routers.
Virtual Router An abstract object managed by VRRP that acts
as a default router for hosts on a shared LAN.
It consists of a Virtual Router Identifier and
an a set of associated IPv6 address(es) across
a common LAN. A VRRP Router may backup one or
more virtual routers.
IPv6 Address Owner The VRRP router that has the virtual router's
IPv6 address(es) as real interface address.
This is the router that, when up, will respond
to packets addressed to the IPv6 address(es)
for ICMPv6 pings, TCP connections, etc.
Virtual Router Master The VRRP router that is assuming the
responsibility of forwarding packets sent to
the IPv6 address(es) associated with the
virtual router, and answering ND requests for
these IPv6 address(es). Note that if the IPv6
address owner is available, then it will
always become the Master.
Virtual Router Backup The set of VRRP routers available to assume
forwarding responsibility for a virtual router
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should the current Master fail.
2.0 Required Features
This section outlines the set of features that were considered
mandatory and that guided the design of VRRP.
2.1 IPv6 Address Backup
Backup of an IPv6 address(es) is the primary function of the Virtual
Router Redundancy Protocol. While providing election of a Virtual
Router Master and the additional functionality described below, the
protocol should strive to:
- Minimize the duration of black holes.
- Minimize the steady state bandwidth overhead and processing
complexity.
- Function over a wide variety of multiaccess LAN technologies
capable of supporting IPv6 traffic.
- Provide for election of multiple virtual routers on a network for
load balancing
- Support of multiple logical IPv6 subnets on a single LAN segment.
2.2 Preferred Path Indication
A simple model of Master election among a set of redundant routers is
to treat each router with equal preference and claim victory after
converging to any router as Master. However, there are likely to be
many environments where there is a distinct preference (or range of
preferences) among the set of redundant routers. For example, this
preference may be based upon access link cost or speed, router
performance or reliability, or other policy considerations. The
protocol should allow the expression of this relative path preference
in an intuitive manner, and guarantee Master convergence to the most
preferential router currently available.
2.3 Minimization of Unnecessary Service Disruptions
Once Master election has been performed then any unnecessary
transitions between Master and Backup routers can result in a
disruption in service. The protocol should ensure after Master
election that no state transition is triggered by any Backup router
of equal or lower preference as long as the Master continues to
function properly.
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Some environments may find it beneficial to avoid the state
transition triggered when a router becomes available that is
preferred over the current Master. It may be useful to support an
override of the immediate convergence to the preferred path.
2.4 Efficient Operation over Extended LANs
Sending IPv6 packets on a multiaccess LAN requires mapping from an
IPv6 address to a MAC address. The use of the virtual router MAC
address in an extended LAN employing learning bridges can have a
significant effect on the bandwidth overhead of packets sent to the
virtual router. If the virtual router MAC address is never used as
the source address in a link level frame then the station location is
never learned, resulting in flooding of all packets sent to the
virtual router. To improve the efficiency in this environment the
protocol should: 1) use the virtual router MAC as the source in a
packet sent by the Master to trigger station learning; 2) trigger a
message immediately after transitioning to Master to update the
station learning; and 3) trigger periodic messages from the Master to
maintain the station learning cache.
3.0 VRRP Overview
VRRP specifies an election protocol to provide the virtual router
function described earlier. All protocol messaging is performed
using IPv6 multicast datagrams, thus the protocol can operate over a
variety of multiaccess LAN technologies supporting IPv6 multicast.
Each VRRP virtual router has a single well-known MAC address
allocated to it. This document currently only details the mapping to
networks using the IEEE 802 48-bit MAC address. The virtual router
MAC address is used as the source in all periodic VRRP messages sent
by the Master router to enable bridge learning in an extended LAN.
A virtual router is defined by its virtual router identifier (VRID)
and a set of IPv6 address(es). A VRRP router may associate a virtual
router with its real address on an interface, and may also be
configured with additional virtual router mappings and priority for
virtual routers it is willing to backup. The mapping between VRID
and its IPv6 address(es) must be coordinated among all VRRP routers
on a LAN. However, there is no restriction against reusing a VRID
with a different address mapping on different LANs. The scope of
each virtual router is restricted to a single LAN.
To minimize network traffic, only the Master for each virtual router
sends periodic VRRP Advertisement messages. A Backup router will not
attempt to preempt the Master unless it has higher priority. This
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eliminates service disruption unless a more preferred path becomes
available. It's also possible to administratively prohibit all
preemption attempts. The only exception is that a VRRP router will
always become Master of any virtual router associated with address it
owns. If the Master becomes unavailable then the highest priority
Backup will transition to Master after a short delay, providing a
controlled transition of the virtual router responsibility with
minimal service interruption.
The VRRP protocol design provides rapid transition from Backup to
Master to minimize service interruption, and incorporates
optimizations that reduce protocol complexity while guaranteeing
controlled Master transition for typical operational scenarios. The
optimizations result in an election protocol with minimal runtime
state requirements, minimal active protocol states, and a single
message type and sender. The typical operational scenarios are
defined to be two redundant routers and/or distinct path preferences
among each router. A side effect when these assumptions are violated
(i.e., more than two redundant paths all with equal preference) is
that duplicate packets may be forwarded for a brief period during
Master election. However, the typical scenario assumptions are
likely to cover the vast majority of deployments, loss of the Master
router is infrequent, and the expected duration in Master election
convergence is quite small ( << 1 second ). Thus the VRRP
optimizations represent significant simplifications in the protocol
design while incurring an insignificant probability of brief network
degradation.
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4. Sample Configurations
4.1 Sample Configuration 1
The following figure shows a simple network with two VRRP routers
implementing one virtual router. Note that this example is provided
to help understand the protocol, but is not expected to occur in
actual practice.
+-----------+ +-----------+
| Rtr1 | | Rtr2 |
|(MR VRID=1)| |(BR VRID=1)|
| | | |
VRID=1 +-----------+ +-----------+
IPv6 A -------->* *<--------- IPv6 B
| |
| |
------------------+------------+-----+--------+--------+--------+--
^ ^ ^ ^
| | | |
(IPv6 A) (IPv6 A) (IPv6 A) (IPv6 A)
| | | |
+--+--+ +--+--+ +--+--+ +--+--+
| H1 | | H2 | | H3 | | H4 |
+-----+ +-----+ +--+--+ +--+--+
Legend:
---+---+---+-- = Ethernet, Token Ring, or FDDI
H = Host computer
MR = Master Router
BR = Backup Router
* = IPv6 Address
(IPv6) = default router for hosts
Eliminating all mention of VRRP (VRID=1) from the figure above leaves
it as a typical IPv6 deployment. Each router has a link-local IPv6
address on the LAN interface (Rtr1 is assigned IPv6 Link-Local A and
Rtr2 is assigned IPv6 Link-Local B), and each host learns a default
route from Router Advertisements through one of the routers (in this
example they all use Rtr1's IPv6 Link-Local A).
Moving to the VRRP environment, each router has the exact same Link-
Local IPv6 address. Rtr1 is said to be the IPv6 address owner of
IPv6 A, and Rtr2 is the IPv6 address owner of IPv6 B. A virtual
router is then defined by associating a unique identifier (the
virtual router ID) with the address owned by a router. Finally, the
VRRP protocol manages virtual router fail over to a backup router.
The example above shows a virtual router configured to cover the IPv6
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address owned by Rtr1 (VRID=1,IPv6_Address=A). When VRRP is enabled
on Rtr1 for VRID=1 it will assert itself as Master, with
priority=255, since it is the IPv6 address owner for the virtual
router IPv6 address. When VRRP is enabled on Rtr2 for VRID=1 it will
transition to Backup, with priority=100, since it is not the IPv6
address owner. If Rtr1 should fail then the VRRP protocol will
transition Rtr2 to Master, temporarily taking over forwarding
responsibility for IPv6 A to provide uninterrupted service to the
hosts.
Note that in this example IPv6 B is not backed up, it is only used by
Rtr2 as its interface address. In order to backup IPv6 B, a second
virtual router must be configured. This is shown in the next
section.
4.2 Sample Configuration 2
The following figure shows a configuration with two virtual routers
with the hosts splitting their traffic between them. This example is
expected to be common in actual practice.
+-----------+ +-----------+
| Rtr1 | | Rtr2 |
|(MR VRID=1)| |(BR VRID=1)|
|(BR VRID=2)| |(MR VRID=2)|
VRID=1 +-----------+ +-----------+ VRID=2
IPv6 A -------->* *<---------- IPv6 B
| |
| |
------------------+------------+-----+--------+--------+--------+--
^ ^ ^ ^
| | | |
(IPv6 A) (IPv6 A) (IPv6 B) (IPv6 B)
| | | |
+--+--+ +--+--+ +--+--+ +--+--+
| H1 | | H2 | | H3 | | H4 |
+-----+ +-----+ +--+--+ +--+--+
Legend:
---+---+---+-- = Ethernet, Token Ring, or FDDI
H = Host computer
MR = Master Router
BR = Backup Router
* = IPv6 Address
(IPv6) = default router for hosts
In the example above, half of the hosts have learned a default route
through Rtr1's IPv6 A and half are using Rtr2's IPv6 B. The
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configuration of virtual router VRID=1 is exactly the same as in the
first example (see section 4.1), and a second virtual router has been
added to cover the IPv6 address owned by Rtr2 (VRID=2,
IPv6_Address=B). In this case Rtr2 will assert itself as Master for
VRID=2 while Rtr1 will act as a backup. This scenario demonstrates a
deployment providing load splitting when both routers are available
while providing full redundancy for robustness.
5.0 Protocol
The purpose of the VRRP packet is to communicate to all VRRP routers
the priority and the state of the Master router associated with the
Virtual Router ID.
VRRP packets are sent encapsulated in IPv6 packets. They are sent to
the IPv6 multicast address assigned to VRRP.
5.1 VRRP Packet Format
This section defines the format of the VRRP packet and the relevant
fields in the IPv6 header.
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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5.2 IPv6 Field Descriptions
5.2.1 Source Address
The IPv6 link-local address of the interface the packet is being sent
from.
5.2.2 Destination Address
The IPv6 multicast address as assigned by the IANA for VRRP is:
FF02:0:0:0:0:0:XXXX:XXXX
This is a link-local scope multicast address. Routers MUST NOT
forward a datagram with this destination address regardless of its
Hop Limit.
5.2.3 Hop Limit
The Hop Limit MUST be set to 255. A VRRP router receiving a packet
with the Hop Limit not equal to 255 MUST discard the packet.
5.2.4 Next Header
The IPv6 Next Header protocol assigned by the IANA for VRRP is 112
(decimal).
5.3 VRRP Field Descriptions
5.3.1 Version
The version field specifies the VRRP protocol version of this packet.
This document defines version 3.
5.3.2 Type
The type field specifies the type of this VRRP packet. The only
packet type defined in this version of the protocol is:
1 ADVERTISEMENT
A packet with unknown type MUST be discarded.
5.3.3 Virtual Rtr ID (VRID)
The Virtual Router Identifier (VRID) field identifies the virtual
router this packet is reporting status for.
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5.3.4 Priority
The priority field specifies the sending VRRP router's priority for
the virtual router. Higher values equal higher priority. This field
is an 8 bit unsigned integer field.
The priority value for the VRRP router that owns the IPv6 address
associated with the virtual router MUST be 255 (decimal).
VRRP routers backing up a virtual router MUST use priority values
between 1-254 (decimal). The default priority value for VRRP routers
backing up a virtual router is 100 (decimal).
The priority value zero (0) has special meaning indicating that the
current Master has stopped participating in VRRP. This is used to
trigger Backup routers to quickly transition to Master without having
to wait for the current Master to timeout.
5.3.5 Count IPv6 Addr
The number of IPv6 addresses contained in this VRRP advertisement.
The minimum value is 1.
5.3.5 Rsvd
This field MUST be set to zero on transmission and ignored on
reception.
5.3.6 Advertisement Interval (Adver Int)
The Advertisement interval is a 12-bit field that indicates the time
interval (in centiseconds) between ADVERTISEMENTS. The default is
100 centiseconds (1 second). This field is used for troubleshooting
misconfigured routers.
5.3.7 Checksum
The checksum field is used to detect data corruption in the VRRP
message.
The checksum is the 16-bit one's complement of the one's complement
sum of the entire VRRP message starting with the version field and a
"pseudo-header" as defined in section 8.1 of RFC2460 [IPv6]. The
next header field in the "pseudo-header" should be set to 112
(decimal) for VRRP. For computing the checksum, the checksum field
is set to zero. See RFC1071 for more detail [CKSM].
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5.3.8 IPv6 Address(es)
One or more IPv6 addresses associated associated with the virtual
router. The number of addresses included is specified in the "Count
IP Addr" field. The first address must be the IPv6 link-local
address associated with the virtual router. These fields are used
for troubleshooting misconfigured routers. If more than one address
is sent it is recommended that all routers be configured to send
these addresses in the same order to make it easier to do this
comparison.
6. Protocol State Machine
6.1 Parameters per Virtual Router
VRID Virtual Router Identifier. Configurable
item in the range 1-255 (decimal). There is
no default.
Priority Priority value to be used by this VRRP
router in Master election for this virtual
router. The value of 255 (decimal) is
reserved for the router that owns the IPv6
address associated with the virtual router.
The value of 0 (zero) is reserved for Master
router to indicate it is releasing
responsibility for the virtual router. The
range 1-254 (decimal) is available for VRRP
routers backing up the virtual router. The
default value is 100 (decimal).
IPv6_Addresses One or more IPv6 addresses associated with
this virtual router. Configured item. No
default. The first address must be the
Link-Local address associated with the
virtual router.
Advertisement_Interval Time interval between ADVERTISEMENTS
(centiseconds). Default is 100 centiseconds
(1 second).
Master_Adver_Interval Advertisement interval contained in
ADVERTISEMENTS received from the Master
(centiseconds). This value is saved by
virtual routers in Backup state and used to
compute Skew_Time and Master_Down_Interval.
The initial value is same as
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Advertisement_Interval.
Skew_Time Time to skew Master_Down_Interval in
centiseconds. Calculated as:
(((256 - priority) *
Master_Adver_Interval) / 256).
Master_Down_Interval Time interval for Backup to declare Master
down (centiseconds). Calculated as:
(3 * Master_Adver_Interval) + Skew_time
Preempt_Mode Controls whether a higher priority Backup
router preempts a lower priority Master.
Values are True to allow preemption and
False to prohibit preemption. Default is
True.
Note: Exception is that the router that owns
the IPv6 address associated with the virtual
router always preempts independent of the
setting of this flag.
Accept_Mode Controls whether a virtual router in Master
state will accept packets addressed to the
address owner's IPv6 address as its own if
it is not the IPv6 address owner. Default
is False.
6.2 Timers
Master_Down_Timer Timer that fires when ADVERTISEMENT has not
been heard for Master_Down_Interval.
Adver_Timer Timer that fires to trigger sending of
ADVERTISEMENT based on
Advertisement_Interval.
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6.3 State Transition Diagram
+---------------+
+--------->| |<-------------+
| | Initialize | |
| +------| |----------+ |
| | +---------------+ | |
| | | |
| V V |
+---------------+ +---------------+
| |---------------------->| |
| Master | | Backup |
| |<----------------------| |
+---------------+ +---------------+
6.4 State Descriptions
In the state descriptions below, the state names are identified by
{state-name}, and the packets are identified by all upper case
characters.
A VRRP router implements an instance of the state machine for each
virtual router election it is participating in.
6.4.1 Initialize
The purpose of this state is to wait for a Startup event. If a
Startup event is received, then:
- If the Priority = 255 (i.e., the router owns the IPv6 address
associated with the virtual router)
o Send an ADVERTISEMENT
o Send an unsolicited ND Neighbor Advertisement with the Router
Flag (R) set, the Solicited Flag (S) unset, the Override flag
(O) set, the Target Address set to the IPv6 link-local address
of the Virtual Router, and the Target Link Layer address set to
the virtual router MAC address.
o Set the Adver_Timer to Advertisement_Interval
o Transition to the {Master} state
else
o Set Master_Adver_Interval to Advertisement_Interval
o Set the Master_Down_Timer to Master_Down_Interval
o Transition to the {Backup} state
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endif
6.4.2 Backup
The purpose of the {Backup} state is to monitor the availability and
state of the Master Router.
While in this state, a VRRP router MUST do the following:
- MUST NOT respond to ND Neighbor Solicitation messages for the IPv6
address(es) associated with the virtual router.
- MUST NOT send ND Router Advertisement messages for the virtual
router.
- MUST discard packets with a destination link layer MAC address
equal to the virtual router MAC address.
- MUST NOT accept packets addressed to the IPv6 address(es)
associated with the virtual router.
- If a Shutdown event is received, then:
o Cancel the Master_Down_Timer
o Transition to the {Initialize} state
endif
- If the Master_Down_Timer fires, then:
o Send an ADVERTISEMENT
o Compute and join the Solicited-Node multicast address [ADD-ARH]
for the IPv6 address(es) addresses associated with the the
Virtual Router.
o Send an unsolicited ND Neighbor Advertisement with the Router
Flag (R) set, the Solicited Flag (S) unset, the Override flag
(O) set, the Target Address set to the IPv6 link-local address
of the Virtual Router, and the Target Link Layer address set to
the virtual router MAC address.
o Set the Adver_Timer to Advertisement_Interval
o Transition to the {Master} state
endif
- If an ADVERTISEMENT is received, then:
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If the Priority in the ADVERTISEMENT is Zero, then:
o Set the Master_Down_Timer to Skew_Time
else:
If Preempt_Mode is False, or If the Priority in the
ADVERTISEMENT is greater than or equal to the local
Priority, then:
o Set Master_Adver_Interval to Adver Interval contained in
the ADVERTISEMENT.
o Reset the Master_Down_Timer to Master_Down_Interval
else:
o Discard the ADVERTISEMENT
endif
endif
endif
6.4.3 Master
While in the {Master} state the router functions as the forwarding
router for the IPv6 address associated with the virtual router.
While in this state, a VRRP router MUST do the following:
- MUST be a member of the Solicited-Node multicast address for the
IPv6 link-local address associated with the virtual router.
- MUST respond to ND Neighbor Solicitation message for the IPv6
address(es) associated with the virtual router.
- MUST send ND Router Advertisements for the virtual router.
- MUST respond to ND Router Solicitation message for the virtual
router.
- MUST forward packets with a destination link layer MAC address
equal to the virtual router MAC address.
- MUST accept packets addressed to the IPv6 address(es) associated
with the virtual router if it is the IPv6 address owner or if
Accept_Mode is True. Otherwise, MUST NOT accept these packets.
- If a Shutdown event is received, then:
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o Cancel the Adver_Timer
o Send an ADVERTISEMENT with Priority = 0
o Transition to the {Initialize} state
endif
- If the Adver_Timer fires, then:
o Send an ADVERTISEMENT
o Reset the Adver_Timer to Advertisement_Interval
endif
- If an ADVERTISEMENT is received, then:
If the Priority in the ADVERTISEMENT is Zero, then:
o Send an ADVERTISEMENT
o Reset the Adver_Timer to Advertisement_Interval
else:
If the Priority in the ADVERTISEMENT is greater than the
local Priority,
or
If the Priority in the ADVERTISEMENT is equal to the local
Priority and the IPv6 Address of the sender is greater than
the local IPv6 Address, then:
o Cancel Adver_Timer
o Set Master_Adver_Interval to Adver Interval contained in
the ADVERTISEMENT.
o Set Master_Down_Timer to Master_Down_Interval
o Transition to the {Backup} state
else:
o Discard ADVERTISEMENT
endif
endif
endif
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7. Sending and Receiving VRRP Packets
7.1 Receiving VRRP Packets
Performed the following functions when a VRRP packet is received:
- MUST verify that the IPv6 Hop Limit is 255.
- MUST verify the VRRP version is 3
- MUST verify that the received packet contains the complete VRRP
packet (including fixed fields, and IPv6 Address.
- MUST verify the VRRP checksum
- MUST verify that the VRID is configured on the receiving
interface and the local router is not the IPv6 Address owner
(Priority equals 255 (decimal)).
If any one of the above checks fails, the receiver MUST discard the
packet, SHOULD log the event and SHOULD indicate via network
management that an error occurred.
- MAY verify that the IPv6 Address matches the IPv6_Address
configured for the VRID.
If the above check fails, the receiver SHOULD log the event and
SHOULD indicate via network management that a misconfiguration was
detected. If the packet was not generated by the address owner
(Priority does not equal 255 (decimal)), the receiver MUST drop the
packet, otherwise continue processing.
- MUST verify that the Adver Interval in the packet is the same as
the locally configured for this virtual router
If the above check fails, the receiver SHOULD log the event and
SHOULD indicate via network management that a misconfiguration was
detected. However, the packet is not discarded. If the virtual
router is in Backup state, it uses the received Adver Interval to re-
calculate its Master_Down_Interval.
7.2 Transmitting VRRP Packets
The following operations MUST be performed when transmitting a VRRP
packet.
- Fill in the VRRP packet fields with the appropriate virtual
router configuration state
- Compute the VRRP checksum
- Set the source MAC address to Virtual Router MAC Address
- Set the source IPv6 address to interface link-local IPv6 address
- Set the IPv6 protocol to VRRP
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- Send the VRRP packet to the VRRP IP multicast group
Note: VRRP packets are transmitted with the virtual router MAC
address as the source MAC address to ensure that learning bridges
correctly determine the LAN segment the virtual router is attached
to.
7.3 Virtual Router MAC Address
The virtual router MAC address associated with a virtual router is an
IEEE 802 MAC Address in the following format:
00-00-5E-00-02-{VRID} (in hex in internet standard bit-order)
The first three octets are derived from the IANA's OUI. The next two
octets (00-02) indicate the address block assigned to the VRRP for
IPv6 protocol. {VRID} is the VRRP Virtual Router Identifier. This
mapping provides for up to 255 VRRP routers on a network.
7.4 IPv6 Interface Identifiers
IPv6 Routers running VRRP MUST create their Interface Identifiers in
the normal manner (e.g., RFC2464 "Transmission of IPv6 Packets over
Ethernet"). They MUST NOT use the Virtual Router MAC address to
create the Modified EUI-64 identifiers.
This VRRP specification describes how to advertise and resolve the
VRRP routers IPv6 link local address into the Virtual Router MAC
address.
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8. Operational Issues
8.1 ICMPv6 Redirects
ICMPv6 Redirects may be used normally when VRRP is running between a
group of routers [ICMPv6]. This allows VRRP to be used in
environments where the topology is not symmetric (e.g., the VRRP
routers do not connect to the same destinations).
The IPv6 source address of an ICMPv6 redirect should be the address
the end host used when making its next hop routing decision. If a
VRRP router is acting as Master for virtual router(s) containing
addresses it does not own, then it must determine which virtual
router the packet was sent to when selecting the redirect source
address. One method to deduce the virtual router used is to examine
the destination MAC address in the packet that triggered the
redirect.
8.2 ND Neighbor Solicitation
When a host sends an ND Neighbor Solicitation message for the virtual
router IPv6 address, the Master virtual router MUST respond to the ND
Neighbor Solicitation message with the virtual MAC address for the
virtual router. The Master virtual router MUST NOT respond with its
physical MAC address. This allows the client to always use the same
MAC address regardless of the current Master router.
When a Master virtual router sends an ND Neighbor Solicitation
message for a host's IPv6 address, the Master virtual router MUST
include the virtual MAC address for the virtual router if it sends a
source link-layer address option in the neighbor solicitation
message. It MUST NOT use its physical MAC address in the source
link-layer address option.
When a VRRP router restarts or boots, it SHOULD not send any ND
messages with its physical MAC address for the IPv6 address it owns,
it should only send ND messages that include Virtual MAC addresses.
This may entail:
- When configuring an interface, VRRP routers should send an
unsolicitated ND Neighbor Advertisement message containing the
virtual router MAC address for the IPv6 address on that interface.
- At system boot, when initializing interfaces for VRRP operation;
delay all ND Router and Neighbor Advertisements and Solicitation
messages until both the IPv6 address and the virtual router MAC
address are configured.
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8.3 Router Advertisements
When a backup VRRP router has become Master for a virtual router, it
is responsible for sending Router Advertisements for the virtual
router as specified in section 6.4.3. The backup routers must be
configured to send the same Router Advertisement options as the
address owner.
Router Advertisement options that advertise special services (e.g.,
Home Agent Information Option) that are present in the address owner,
should not be sent by the address owner unless the backup routers are
prepared to assume these services in full and have a complete and
synchronized database for this service.
8.4 Potential Forwarding Loop
A VRRP router SHOULD not forward packets addressed to the IPv6
Address it becomes Master for if it is not the owner. Forwarding
these packets would result in unnecessary traffic. Also in the case
of LANs that receive packets they transmit (e.g., token ring) this
can result in a forwarding loop that is only terminated when the IPv6
TTL expires.
One such mechanism for VRRP routers is to add/delete a reject host
route for each adopted IPv6 address when transitioning to/from MASTER
state.
8.5 Recommendations regarding setting priority values
A priority value of 255 designates a particular router as the "IPv6
address owner". Care must be taken not to configure more than one
router on the link in this way for a single VRID.
Routers with priority 255 will, as soon as they start up, preempt all
lower priority routers. Configure no more than one router on the
link with priority 255, especially if preemption is set. If no
router has this priority, and preemption is disabled, then no
preemption will occur.
When there are multiple Backup routers, their priority values should
be uniformly distributed. For example, if one Backup routers has the
default priority of 100 and another BR is added, a priority of 50
would be a better choice for it than 99 or 100 to facilitate faster
convergence.
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9. Operation over FDDI, Token Ring, and ATM LANE
9.1 Operation over FDDI
FDDI interfaces remove from the FDDI ring frames that have a source
MAC address matching the device's hardware address. Under some
conditions, such as router isolations, ring failures, protocol
transitions, etc., VRRP may cause there to be more than one Master
router. If a Master router installs the virtual router MAC address
as the hardware address on a FDDI device, then other Masters'
ADVERTISEMENTS will be removed from the ring during the Master
convergence, and convergence will fail.
To avoid this an implementation SHOULD configure the virtual router
MAC address by adding a unicast MAC filter in the FDDI device, rather
than changing its hardware MAC address. This will prevent a Master
router from removing any ADVERTISEMENTS it did not originate.
9.2 Operation over Token Ring
Token ring has several characteristics that make running VRRP
difficult. These include:
- In order to switch to a new master located on a different bridge
token ring segment from the previous master when using source
route bridges, a mechanism is required to update cached source
route information.
- No general multicast mechanism supported across old and new token
ring adapter implementations. While many newer token ring adapters
support group addresses, token ring functional address support is
the only generally available multicast mechanism. Due to the
limited number of token ring functional addresses these may
collide with other usage of the same token ring functional
addresses.
Due to these difficulties, the preferred mode of operation over token
ring will be to use a token ring functional address for the VRID
virtual MAC address. Token ring functional addresses have the two
high order bits in the first MAC address octet set to B'1'. They
range from 03-00-00-00-00-80 to 03-00-02-00-00-00 (canonical format).
However, unlike multicast addresses, there is only one unique
functional address per bit position. The functional addresses
addresses 03-00-00-10-00-00 through 03-00-02-00-00-00 are reserved
by the Token Ring Architecture [TKARCH] for user-defined
applications. However, since there are only 12 user-defined token
ring functional addresses, there may be other non-IP protocols using
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the same functional address. Since the Novell IPX [IPX] protocol uses
the 03-00-00-10-00-00 functional address, operation of VRRP over
token ring will avoid use of this functional address. In general,
token ring VRRP users will be responsible for resolution of other
user-defined token ring functional address conflicts.
VRIDs are mapped directly to token ring functional addresses. In
order to decrease the likelihood of functional address conflicts,
allocation will begin with the largest functional address. Most non-
IP protocols use the first or first couple user-defined functional
addresses and it is expected that VRRP users will choose VRIDs
sequentially starting with 1.
VRID Token Ring Functional Address
---- -----------------------------
1 03-00-02-00-00-00
2 03-00-04-00-00-00
3 03-00-08-00-00-00
4 03-00-10-00-00-00
5 03-00-20-00-00-00
6 03-00-40-00-00-00
7 03-00-80-00-00-00
8 03-00-00-01-00-00
9 03-00-00-02-00-00
10 03-00-00-04-00-00
11 03-00-00-08-00-00
Or more succinctly, octets 3 and 4 of the functional address are
equal to (0x4000 >> (VRID - 1)) in non-canonical format.
Since a functional address cannot be used used as a MAC level source
address, the real MAC address is used as the MAC source address in
VRRP advertisements. This is not a problem for bridges since packets
addressed to functional addresses will be sent on the spanning-tree
explorer path [802.1D].
The functional address mode of operation MUST be implemented by
routers supporting VRRP on token ring.
Additionally, routers MAY support unicast mode of operation to take
advantage of newer token ring adapter implementations that support
non-promiscuous reception for multiple unicast MAC addresses and to
avoid both the multicast traffic and usage conflicts associated with
the use of token ring functional addresses. Unicast mode uses the
same mapping of VRIDs to virtual MAC addresses as Ethernet. However,
one important difference exists. ND request/reply packets contain the
virtual MAC address as the source MAC address. The reason for this is
that some token ring driver implementations keep a cache of MAC
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address/source routing information independent of the ND cache.
Hence, these implementations need have to receive a packet with the
virtual MAC address as the source address in order to transmit to
that MAC address in a source-route bridged network.
Unicast mode on token ring has one limitation that should be
considered. If there are VRID routers on different source-route
bridge segments and there are host implementations that keep their
source-route information in the ND cache and do not listen to
gratuitous NDs, these hosts will not update their ND source-route
information correctly when a switch-over occurs. The only possible
solution is to put all routers with the same VRID on the same source-
bridge segment and use techniques to prevent that bridge segment from
being a single point of failure. These techniques are beyond the
scope this document.
For both the multicast and unicast mode of operation, VRRP
advertisements sent to 224.0.0.18 should be encapsulated as described
in [RFC1469].
9.3 Operation over ATM LANE
Operation of VRRP over ATM LANE on routers with ATM LANE interfaces
and/or routers behind proxy LEC's are beyond the scope of this
document.
10. Security Considerations
VRRP for IPv6 does not currently include any type of authentication.
Earlier versions of the VRRP (for IPv4) specification included
several types of authentication ranging from none to strong.
Operational experience and further analysis determined that these did
not provide sufficient security to overcome the vulnerability of
misconfigured secrets causing multiple masters to be elected. Due to
the nature of the VRRP protocol, even if VRRP messages are
cryptographically protected, it does not prevent hostile routers from
behaving as if they are a VRRP master, creating multiple masters.
Authentication of VRRP messages could have prevented a hostile router
from causing all properly functioning routers from going into backup
state. However, having multiple masters can cause as much disruption
as no routers, which authentication cannot prevent. Also, even if a
hostile router could not disrupt VRRP, it can disrupt ARP and create
the same effect as having all routers go into backup.
It should be noted that these attacks are not worse and are a subset
of the attacks that any node attached to a LAN can do independently
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of VRRP. The kind of attacks a malicious node on a LAN can do
include promiscuously receiving packets for any router's MAC address,
sending packets with the router's MAC address as the source MAC
addresses in the L2 header to tell the L2 switches to send packets
addressed to the router to the malicious node instead of the router,
send redirects to tell the hosts to send their traffic somewhere
else, send unsolicited ND replies, answer ND requests, etc., etc.
All of this can be done independently of implementing VRRP. VRRP
does not add to these vulnerabilities.
Independent of any authentication type VRRP includes a mechanism
(setting TTL=255, checking on receipt) that protects against VRRP
packets being injected from another remote network. This limits most
vulnerabilities to local attacks.
VRRP does not provide any confidentiality. Confidentiality is not
necessary for the correct operation of VRRP and there is no
information in the VRRP messages that must be kept secret from other
nodes on the LAN.
If SEcure Neighbor Discovery (SEND) [SEND] is deployed, VRRP
authentication could be usefully added, because misconfiguration of
secrets will not be an issue. Routers with different secrets will
have different IP addresses, and therefore there will be no issue
with multiple masters with the same IP (and MAC) addresses. Also,
SEND will prevent malicious routers from sending misleading ND
messages.
11. Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
draft-ietf-vrrp-ipv6-spec-08.txt [Page 27]
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copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights.
12. Acknowledgments
This specification is based on RFC2238. The authors of RFC2238 are
S. Knight, D. Weaver, D. Whipple, R. Hinden, D. Mitzel, P. Hunt, P.
Higginson, M. Shand, and A. Lindem.
The author of this document would also like to thank Erik Nordmark,
Thomas Narten, Steve Deering, Radia Perlman, Danny Mitzel, Mukesh
Gupta, Don Provan, Mark Hollinger, John Cruz, and Melissa Johnson for
their helpful suggestions.
13. IANA Considerations
VRRP for IPv6 needs an IPv6 link-local scope multicast address
assigned by the IANA for this specification. The IPv6 multicast
address should be of the following form:
FF02:0:0:0:0:0:XXXX:XXXX
The values assigned address should be entered into section 5.2.2.
A convenient assignment of this link-local scope multicast would be:
FF02:0:0:0:0:0:0:12
as this would be consistent with the IPv4 assignment for VRRP.
The IANA should also reserve a block of IANA Ethernet unicast
addresses from:
00-00-5E-00-02-00 to 00-00-5E-00-02-FF in hex
for VRRP for IPv6. Similar assignments are documented in:
http://www.iana.org/assignments/ethernet-numbers
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14. Normative References
[802.1D] International Standard ISO/IEC 10038: 1993, ANSI/IEEE Std
802.1D, 1993 edition.
[ADD-ARH] Hinden, R., S. Deering, "IP Version 6 Addressing
Architecture", RFC4291, February 2006.
[ICMPv6] Conta, A., S. Deering, M. Gupta, "Internet Control Message
Protocol (ICMPv6) for the Internet Protocol Version 6
(IPv6) Specification", RFC4443, March 2006.
[IPv6] Deering, S., R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC2460, December 1998.
[IPX] Novell Incorporated., "IPX Router Specification", Version
1.10, October 1992.
[ND] Narten, T., E. Nordmark, W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC2461, December 1998.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC2119, BCP0014, March 1997.
[SEND] Arkko, J., Kempf, J., Sommerfeld, B., Zill, B. and P.
Nikander, "SEcure Neighbor Discovery (SEND)", RFC3971,
March 2005.
[TKARCH] IBM Token-Ring Network, Architecture Reference, Publication
SC30-3374-02, Third Edition, (September, 1989).
[VRRP-V4] Hinder, R., "Virtual Router Redundancy Protocol (VRRP)",
RFC3768, April 2004.
15. Informative References
[HSRP] Li, T., B. Cole, P. Morton, D. Li, "Cisco Hot Standby
Router Protocol (HSRP)", RFC2281, March 1998.
[IPSTB] Higginson, P., M. Shand, "Development of Router Clusters to
Provide Fast Failover in IP Networks", Digital Technical
Journal, Volume 9 Number 3, Winter 1997.
[CKSM] Braden, R., D. Borman, C. Partridge, "Computing the
Internet Checksum", RFC1071, September 1988.
[RFC1469] Pusateri, T., "IP Multicast over Token Ring Local Area
draft-ietf-vrrp-ipv6-spec-08.txt [Page 29]
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Networks", RFC1469, June 1993.
16. Changes from RFC2338
- Added new subsection (8.3) that provided more detail on sending ND
Router Advertisements.
- Added new subsection (8.5) with recommendations about setting
priority values and it's relationship to the preempt flag.
- Changed rules for receiving VRRP packets to not drop the packet if
the Adver Interval is not consistent with the local configuration
for the virtual router. Only log and notify network management.
Moreover, use the Master's Adver Interval to compute
Master_Down_Interval and Skew_Time.
- Reduced granularity of the Advertisement_Interval to centiseconds
(i.e., 1/100 of a second). Changes include:
o Made Adver Int field in the header 12-bits to allow range from
1 to 4096 centiseconds.
o Change Skew_Timer calculation to skew over one
Advertisement_Interval.
- Added switch (Accept_Mode) to control whether a virtual router in
Master state will accept packets addresses to the address owner's
IPv6 address as its own if it is not the IPv6 address owner.
- Changed VMAC assignments to a separate block of IANA Ethernet
addresses and added this to the IANA considerations section.
- Removed different authentication methods, header fields, and
updated the security considerations section to explain the reasons
for doing this.
- General rewrite to change protocol to provide virtual router
functionality from IPv4 to IPv6. Specific changes include:
o Increment VRRP version to 3.
o Change packet format to support an 128-bit IPv6 address.
o Rewrote text to specify IPv6 Neighbor Discovery mechanisms
instead of ARP.
o Changed state machine actions to use Neighbor Discovery
mechanisms. This includes sending unsolicited Neighbor
Advertisements, Receiving Neighbor Solicitations, joining the
appropriate solicited node multicast group, sending Router
Advertisements, and receiving Router Solicitations.
- Revised the section 4 examples text with a clearer description of
mapping of IPv6 address owner, priorities, etc.
- Clarify the section 7.1 text describing address list validation.
- Corrected text in Preempt_Mode definition.
- Changed authentication to be per Virtual Router instead of per
Interface.
- Added new subsection (9.3) stating that VRRP over ATM LANE is
beyond the scope of this document.
- Clarified text describing received packet length check.
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- Clarified text describing received authentication check.
- Clarified text describing VRID verification check.
- Added new subsection (8.3) describing need to not forward packets
for adopted IPv6 addresses.
- Added clarification to the security considerations section. Added
reference to SEND.
- Added reference for computing the internet checksum.
- Updated references and author information.
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Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Authors' Addresses
Robert Hinden
Nokia, Inc.
313 Fairchild Drive
Mountain View, CA 94043
USA
Phone: +1 650 625-2004
EMail: bob.hinden@nokia.com
John Cruz
Cisco Systems, Inc.
3600 Cisco Way
San Jose, CA 95134
USA
Phone: +1 408 527 1034
Email: johcruz@cisco.com
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