INTERNET-DRAFT R. Hinden/Nokia
December 5, 2002
Virtual Router Redundancy Protocol for IPv6
<draft-ietf-vrrp-ipv6-spec-03.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of [RFC2026].
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
To view the list Internet-Draft Shadow Directories, see
http://www.ietf.org/shadow.html.
This internet draft expires on June 5, 2003.
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
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 associated with a
virtual router is called the Master, and forwards packets sent to
this IP address. 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...................................................11
5.1 VRRP Packet Format....................................11
5.2 IP Field Descriptions.................................11
5.3 VRRP Field Descriptions...............................12
6. Protocol State Machine....................................15
6.1 Parameters per Virtual Router.........................15
6.2 Timers................................................16
6.3 State Transition Diagram..............................16
6.4 State Descriptions....................................16
7. Sending and Receiving VRRP Packets........................21
7.1 Receiving VRRP Packets................................21
7.2 Transmitting Packets..................................21
7.3 Virtual MAC Address...................................22
7.4 IPv6 Interface Identifiers............................22
8. Operational Issues........................................23
8.1 ICMPv6 Redirects......................................23
8.2 ND Neighbor Solicitation..............................23
8.3 Potential Forwarding Loop.............................24
9. Operation over FDDI, Token Ring, and ATM LANE.............24
9.1 Operation over FDDI...................................24
9.2 Operation over Token Ring.............................24
9.3 Operation over ATM LANE...............................25
10. Security Considerations...................................26
10.1 No Authentication....................................27
10.2 Simple Text Password.................................27
10.3 IP Authentication Header.............................28
11. Intellectual Property.....................................28
12. Acknowledgments...........................................29
13. IANA Considerations.......................................29
14. References................................................29
15. Authors' Address..........................................31
16. Changes from RFC2338......................................31
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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 associated with a
virtual router is called the Master, and forwards packets sent to
this IP address. The election process provides dynamic fail over in
the forwarding responsibility should the Master become unavailable.
VRRP provides a function similar to a Cisco Systems, Inc. proprietary
protocol named Hot Standby Router Protocol (HSRP) [HSRP] and to a
Digital Equipment Corporation, Inc. proprietary protocol named 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 [RFC 2119].
The IESG/IETF take no position regarding the validity or scope of any
intellectual property right or other rights that might be claimed to
pertain to the implementation or use of the technology, or the extent
to which any license under such rights might or might not be
available. See the IETF IPR web page at http://www.ietf.org/ipr.html
for additional information.
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 IPv6 address 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 as real interface address. This
is the router that, when up, will respond to
packets addressed to the IPv6 addresses 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 associated with the virtual
router, and answering ND requests for this
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IPv6 address. 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
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 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.
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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.
Some environments may find it beneficial to avoid the state
transition triggered when a router becomes available that is more
preferential than the current Master. It may be useful to support an
override of the immediate convergence to the preferred path.
2.4 Extensible Security
The virtual router functionality is applicable to a wide range of
internetworking environments that may employ different security
policies. The protocol should require minimal configuration and
overhead in the insecure operation, provide for strong authentication
when increased security is required, and allow integration of new
security mechanisms without breaking backwards compatible operation.
2.5 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.
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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 an IPv6 address. 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 it's
IPv6 address 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
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.
VRRP defines three types of authentication providing simple
deployment in insecure environments, added protection against
misconfiguration, and strong sender authentication in security
conscious environments. Analysis of the protection provided and
vulnerability of each mechanism is deferred to Section 10.0 Security
Considerations. In addition new authentication types and data can be
defined in the future without affecting the format of the fixed
portion of the protocol packet, thus preserving backward compatible
operation.
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
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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.
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
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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
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.
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4.2 Sample Configuration 2
The following figure shows a configuration with two virtual routers
with the hosts spitting 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
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.
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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 | (reserved) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Auth Type | Adver Int | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ IPv6 Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication Data (1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication Data (2) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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
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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.
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).
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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 Reserved
This field MUST be set to zero on transmission and ignored on
reception.
5.3.6 Authentication Type
The authentication type field identifies the authentication method
being utilized. Authentication type is unique on a Virtual Router
basis. The authentication type field is an 8 bit unsigned integer.
A packet with unknown authentication type or that does not match the
locally configured authentication method MUST be discarded.
The authentication methods currently defined are:
0 - No Authentication
1 - Simple Text Password
2 - IP Authentication Header
5.3.6.1 No Authentication
The use of this authentication type means that VRRP protocol
exchanges are not authenticated. The contents of the Authentication
Data field should be set to zero on transmission and ignored on
reception.
5.3.6.2 Simple Text Password
The use of this authentication type means that VRRP protocol
exchanges are authenticated by a clear text password. The contents
of the Authentication Data field should be set to the locally
configured password on transmission. There is no default password.
The receiver MUST check that the Authentication Data in the packet
matches its configured authentication string. Packets that do not
match MUST be discarded.
Note that there are security implications to using Simple Text
password authentication, and one should see the Security
Consideration section of this document.
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5.3.6.3 IP Authentication Header
The use of this authentication type means the VRRP protocol exchanges
are authenticated using the mechanisms defined by the IP
Authentication Header [AUTH] using "The Use of HMAC-MD5-96 within ESP
and AH" [HMAC]. Keys may be either configured manually or via a key
distribution protocol.
If a packet is received that does not pass the authentication check
due to a missing authentication header or incorrect message digest,
then the packet MUST be discarded. The contents of the
Authentication Data field should be set to zero on transmission and
ignored on reception.
5.3.7 Advertisement Interval (Adver Int)
The Advertisement interval indicates the time interval (in seconds)
between ADVERTISEMENTS. The default is 1 second. This field is used
for troubleshooting misconfigured routers.
5.3.8 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].
5.3.9 IPv6 Address
The IPv6 link-local address associated with the virtual router.
5.3.10 Authentication Data
The authentication string is currently only utilized for simple text
authentication, similar to the simple text authentication found in
the Open Shortest Path First routing protocol [OSPF]. It is up to 8
characters of plain text. If the configured authentication string is
shorter than 8 bytes, the remaining space MUST be zero-filled. Any
VRRP packet received with an authentication string that does not
match the locally configured authentication string MUST be discarded.
The authentication string is unique on a per interface basis.
There is no default value for this field.
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6. Protocol State Machine
6.1 Parameters per Virtual Router
VRID Virtual Router Identifier. Configured 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_Address The IPv6 link-local address associated with
this virtual router. Configured item. No
default.
Advertisement_Interval Time interval between ADVERTISEMENTS
(seconds). Default is 1 second.
Skew_Time Time to skew Master_Down_Interval in
seconds. Calculated as:
( (256 - Priority) / 256 )
Master_Down_Interval Time interval for Backup to declare Master
down (seconds). Calculated as:
(3 * Advertisement_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.
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Authentication_Type Type of authentication being used. Values
are defined in section 5.3.6.
Authentication_Data Authentication data specific to the
Authentication_Type being used.
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.
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:
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- 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 the Master_Down_Timer to Master_Down_Interval
o Transition to the {Backup} state
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 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 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
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- If the Master_Down_Timer fires, then:
o Send an ADVERTISEMENT
o Compute and join the Solicited-Node multicast address [ADD-ARH]
for the link-local IPv6 address of 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:
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 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
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address 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 NOT accept packets addressed to the IPv6 address associated
with the virtual router if it is not the IPv6 address owner.
- MUST accept packets addressed to the IPv6 address associated with
the virtual router if it is the IPv6 address owner.
- If a Shutdown event is received, then:
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
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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, IPv6 Address, and Authentication
Data).
- 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)).
- MUST verify that the Auth Type matches the locally configured
authentication method for the virtual router and perform that
authentication method
If any one of the above checks fails, the receiver MUST discard the
packet, SHOULD log the event and MAY 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 MAY
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 MUST discard the packet,
SHOULD log the event and MAY indicate via network management that a
misconfiguration was detected.
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
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- 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
- 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-00-01-{VRID} (in hex in internet standard bit-order)
The first three octets are derived from the IANA's OUI. The next two
octets (00-01) indicate the address block assigned to the VRRP
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.
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.
It may be useful to disable Redirects for specific cases where VRRP
is being used to load share traffic between a number of routers in a
symmetric topology.
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 routers sends an ND Neighbor Solicitation
message for a hosts 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;
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delay all ND Router and Neighbor Advertisements and Solicitation
messages until both the IPv6 address and the virtual router MAC
address are configured.
8.3 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.
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.
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- 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
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.
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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
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.
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10. Security Considerations
VRRP is designed for a range of internetworking environments that may
employ different security policies. The protocol includes several
authentication methods ranging from no authentication, simple clear
text passwords, and strong authentication using IP Authentication
with MD5 HMAC. The details on each approach including possible
attacks and recommended environments follows.
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.
The security measures discussed in the following sections only
provide various kinds of authentication. No confidentiality is
provided. 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.
10.1 No Authentication
The use of this authentication type means that VRRP protocol
exchanges are not authenticated. This type of authentication SHOULD
only be used in environments were there is minimal security risk and
little chance for configuration errors (e.g., two VRRP routers on a
LAN).
10.2 Simple Text Password
The use of this authentication type means that VRRP protocol
exchanges are authenticated by a simple clear text password.
This type of authentication is useful to protect against accidental
misconfiguration of routers on a LAN. It protects against routers
inadvertently backing up another router. A new router must first be
configured with the correct password before it can run VRRP with
another router. This type of authentication does not protect against
hostile attacks where the password can be learned by a node snooping
VRRP packets on the LAN. The Simple Text Authentication combined
with the TTL check makes it difficult for a VRRP packet to be sent
from another LAN to disrupt VRRP operation.
This type of authentication is RECOMMENDED when there is minimal risk
of nodes on a LAN actively disrupting VRRP operation. If this type
of authentication is used the user should be aware that this clear
text password is sent frequently, and therefore should not be the
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same as any security significant password.
10.3 IP Authentication Header
The use of this authentication type means the VRRP protocol exchanges
are authenticated using the mechanisms defined by the IP
Authentication Header [AUTH] using "The Use of HMAC-MD5-96 within ESP
and AH", [HMAC]. This provides strong protection against
configuration errors, replay attacks, and packet
corruption/modification.
This type of authentication is RECOMMENDED when there is limited
control over the administration of nodes on a LAN. While this type
of authentication does protect the operation of VRRP, there are other
types of attacks that may be employed on shared media links (e.g.,
generation of bogus ND replies) that are independent from VRRP and
are not protected.
Specifically, although securing VRRP prevents unauthorized machines
from taking part in the election protocol, it does not protect hosts
on the network from being deceived. For example, a gratuitous ND
reply from what purports to be the virtual router's IPv6 address can
redirect traffic to an unauthorized machine. Similarly, individual
connections can be diverted by means of forged ICMPv6 Redirect
messages.
11. 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, and Steve Deering for their his helpful suggestions.
12. 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.
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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.
13. 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", Internet Draft, <draft-ietf-ipngwg-addr-
arch-v3-11.txt>, October 2002.
[AUTH] Kent, S., R. Atkinson, "IP Authentication Header", RFC2402,
November 1998.
[CKSM] Braden, R., D. Borman, C. Partridge, "Computing the
Internet Checksum", RFC1071, September 1988.
[HMAC] Madson, C., R. Glenn, "The Use of HMAC-MD5-96 within ESP
and AH", RFC2403, November 1998.
[HSRP] Li, T., B. Cole, P. Morton, D. Li, "Cisco Hot Standby
Router Protocol (HSRP)", RFC2281, March 1998.
[ICMPv6] Conta, A., S. Deering, "Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6)",
RFC2463, December 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.
[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.
[OSPF] Moy, J., "OSPF version 2", RFC2328, STD0054, April 1998.
[RIP] Malkin, G., "RIP Version 2", RFC2453, STD0056, November
draft-ietf-vrrp-ipv6-spec-03.txt [Page 29]
INTERNET-DRAFT VRRP for IPv6 December 5, 2002
1998.
[RFC1469] Pusateri, T., "IP Multicast over Token Ring Local Area
Networks", RFC1469, June 1993.
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", RFC2026, BCP00009, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC2119, BCP0014, March 1997.
[TKARCH] IBM Token-Ring Network, Architecture Reference, Publication
SC30-3374-02, Third Edition, (September, 1989).
[VRRP-V4] Knight, S., et. al., "Virtual Router Redundancy Protocol",
RFC2338, April 1998.
14. Author's Address
Robert Hinden Phone: +1 650 625-2004
Nokia EMail: hinden@iprg.nokia.com
313 Fairchild Drive
Mountain View, CA 94043
USA
15. Changes from RFC2338
- 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 Change to only support one router address (instead of multiple
addresses). This included removing address count field from
header.
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
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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.
- Clarified text describing received authentication check.
- Clarified text describing VRID verification check.
- Added new subsection (8.4) describing need to not forward packets
for adopted IPv6 addresses.
- Added clarification to the security considerations section.
- Added reference for computing the internet checksum.
- Updated references and author information.
- Removed IPR section as no IPR claims have been made agains this
draft.
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