INTERNET-DRAFT S. Knight
July 28, 1997 D. Weaver
Ascend Communications, Inc.
D. Whipple
Microsoft, Inc.
R. Hinden
D. Mitzel
Ipsilon Networks, Inc.
Virtual Router Redundancy Protocol
<draft-ietf-vrrp-spec-01.txt>
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
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This internet draft expires on January 29, 1998.
Abstract
This memo defines the Virtual Router Redundancy Protocol (VRRP).
VRRP specifies an election protocol that dynamically assigns
responsibility for a virtual IP address to a single router among a
collection of VRRP routers. The VRRP router controlling the virtual
IP address is called the Master router, and forwards packets sent to
the virtual IP address. The election process provides dynamic fail
over in the forwarding responsibility should the Master become
unavailable. The virtual IP address can then be used as the default
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first hop router by end-hosts. The advantage gained from using the
VRRP virtual IP address is a higher availability default path without
requiring configuration of dynamic routing or router discovery
protocols on every end-host.
This memo describes the features and theory of operation of VRRP.
The protocol processing and state machine that guarantee convergence
to a single Master router is presented. Also issues related to MAC
address mapping, handling ARP requests, generating ICMP redirects,
and security issues are addressed.
Table of Contents
1. Introduction...............................................3
2. Scope......................................................4
3. Definitions................................................6
4. Sample Configurations......................................8
4.1 Sample Configuration 1................................8
4.2 Sample Configuration 2................................9
5. Protocol..................................................10
5.1 VRRP Packet Format...................................10
5.2 IP Field Descriptions................................10
5.3 VRRP Field Descriptions..............................11
6. Protocol State Machine....................................14
6.1 Parameters.............................................14
6.2 Timers.................................................14
6.3 State Transition Diagram..............................15
6.4 State Descriptions....................................15
7. Sending and Receiving VRRP Packets........................18
7.1 Receiving VRRP Packets................................18
7.2 Transmitting Packets...................................18
7.3 Virtual MAC Address....................................19
8. Host Operation............................................19
8.1 Host ARP Requests....................................19
9. Operational Issues........................................19
9.1 ICMP Redirects.........................................19
9.2 Proxy ARP..............................................19
9.3 Network Management.....................................19
10. Operation over FDDI and Token Ring.......................20
11. Security Considerations...................................21
11.1 No Authentication.....................................21
11.2 Simple Text Password..................................21
11.3 IP Authentication Header..............................21
12. References................................................23
13. Authors' Addresses........................................23
14. Acknowledgments...........................................24
15. Changes from Previous Drafts..............................25
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1. Introduction
There are a number of methods that an end-host can use to determine
its first hop router towards a particular IP destination. These
include running (or snooping) a dynamic routing protocol such as
Routing Information Protocol [RIP] or OSPF version 2 [OSPF], running
an ICMP router discovery client [DISC] or using a statically
configured default route.
Running a dynamic routing protocol on every end-host may be
infeasible for a number of reasons, including administrative
overhead, processing overhead, security issues, or lack of a protocol
implementation for some platforms. Neighbor or router discovery
protocols may require active participation by all hosts on a network,
leading to large timer values to reduce protocol overhead in the face
of large numbers of hosts. This can result in a significant delay in
the detection of a lost (i.e., dead) neighbor, which may introduce
unacceptably long "black hole" periods.
The use of a statically configured default route is quite popular; it
minimizes configuration and processing overhead on the end-host and
is supported by virtually every IP implementation. This mode of
operation is likely to persist as dynamic host configuration
protocols [DHCP] are deployed, which typically provide configuration
for an end-host IP address and default gateway. However, this
creates a single point of failure. Loss of the default router
results in a catastrophic event, isolating all end-hosts that are
unable to detect any alternate path that may be available.
The Virtual Router Redundancy Protocol (VRRP) is designed to
eliminate the single point of failure inherent in the static default
routed environment. VRRP specifies an election protocol that
dynamically assigns responsibility for a virtual IP address to a
single router among a collection of VRRP routers. The VRRP router
controlling the virtual IP address is called the Master router, and
forwards packets sent to the virtual IP address. The election
process provides dynamic fail-over in the forwarding responsibility
should the Master become unavailable. The virtual IP address can
then be used as the default first hop router by end-hosts. The
advantage gained from using the VRRP virtual IP address is a higher
availability default path without requiring configuration of dynamic
routing or router discovery protocols on every end-host.
VRRP provides a function similar to a Cisco Systems, Inc. proprietary
protocol named Hot Standby Router Protocol (HSRP) [HSRP].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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document are to be interpreted as described in [RFC 2119].
1.1 Scope
The remainder of this document describes the features, design goals,
and theory of operation of VRRP. The message formats, protocol
processing rules and state machine that guarantee convergence to a
single Master router are presented. Finally, operational issues
related to MAC address mapping, handling of ARP requests, generation
of ICMP redirect messages, and security issues are addressed.
This protocol is intended for use with IPv4 routers only. A separate
specification will be produced if it is decided that similar
functionality is desirable in an IPv6 environment.
1.2 Definitions
Cluster The set of routers participating in VRRP to emulate a
virtual router.
Master Router The VRRP router controlling the virtual IP address
and assuming the responsibility of forwarding packets
sent to the virtual router.
Backup Router The set of routers in the quiescent state with regard
to the virtual router operation. This set includes
all active VRRP routers within a cluster that are not
the Master router.
2.0 Required Features
This section outlines the set of features that were considered
mandatory and that guided the design of VRRP.
2.1 Virtual IP Management
Management of the virtual IP address is the primary function of the
virtual router protocol. While providing election of a Master router
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
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capable of supporting IP traffic.
- Provide for election of multiple virtual routers on a network for
load balancing or in support of multiple logical IP 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.
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.
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2.5 Efficient Operation over Extended LANs
Sending IP packets on a multiaccess LAN requires mapping from the
virtual IP 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 assumes that each router has a consistent set of routes. The
mechanism used to learn or configure this routing state and ensure
its consistency is beyond the scope of this specification.
VRRP specifies an election protocol to provide the virtual router
function described earlier. All protocol messaging is performed
using IP multicast datagrams, thus the protocol can operate over a
variety of multiaccess LAN technologies supporting IP 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 messages sent by
the Master router to enable bridge learning in an extended LAN.
A virtual router is identified by its virtual IP address, and
associated with a VRRP cluster. The virtual IP address must not
match the real IP address of any host or the virtual IP address of
any other VRRP cluster on the LAN. Each VRRP router assigned to the
cluster must be configured with the same virtual IP address and must
have a real IP address with a prefix matching the virtual router
address. In addition, each VRRP router is assigned a priority to
indicate the preference for Master election. Multiple virtual
routers can be elected on a network by associating them with
different VRRP clusters, and a single router can participate in
multiple VRRP clusters by maintaining independent state machines for
each cluster.
To minimize network traffic, only the Master router sends periodic
Advertisement messages. A Backup router will not attempt to pre-empt
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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 pre-emption attempts. 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 11.0 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 in a VRRP cluster (i.e., a Master
and one Backup), 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 VRRP network.
+--------------------------+
| Cluster X |
| |
| +-------+ +-------+ |
| | MRX | | BRX | |
| | | | | |
| |(P=200)| |(P=100)| |
| | | | | |
| +-------+ +-------+ |
Real IP 1 ---------->* *<---------- Real IP 2
| | * | |
+-------------^------------+
| | |
-------------------+------|-----+-----+-------------+------
| ^ ^
Virtual IP --(VIPX)-+ (VIPX) (VIPX)
| |
+--+--+ +--+--+
| H1 | | H2 |
+-----+ +-----+
Legend:
---+---+---+-- = 802 network, Ethernet or FDDI
H = Host computer
MR = Master Router (Priority=200)
BR = Backup Router (Priority=100)
* = IP Address
VIP = default router for hosts (Virtual IP)
The above configuration shows a typical VRRP scenario. In this
configuration, the end-hosts install a default route to the virtual
IP address (VIPX), and the routers run VRRP to elect the Master
router. The router on the left (MRX) becomes the Master router
because it has the highest priority and the router on the right (BRX)
becomes the backup router.
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4.2 Sample Configuration 2
The following figure shows a configuration with two clusters.
+--------------------------+
| Cluster X and Cluster Y |
| |
| +-----+ +-----+ |
| | MRX | | BRX | |
| | & | | & | |
| | BRY | | MRY | |
| +-----+ +-----+ |
Real IP 1 ---------->* *<---------- Real IP 2
| | * * | |
+---------^------^---------+
| | | |
------------------+--|------|--+-----+--------+--------+--------+--
| | ^ ^ ^ ^
Virtual IP --(VIPX)-+ | (VIPX) (VIPX) (VIPY) (VIPY)
| | | | |
Virtual IP --(VIPY)--------+ +--+--+ +--+--+ +--+--+ +--+--+
| H1 | | H2 | | H3 | | H4 |
+-----+ +-----+ +--+--+ +--+--+
Legend:
---+---+---+-- = 802 network, Ethernet or FDDI
H = Host computer
MR = Master Router
BR = Backup Router
* = IP Address
VIP = default router for hosts (Virtual IP)
In the above configuration, half of the hosts install a default route
to cluster X's virtual IP address (VIPX), and the other half of the
hosts install a default route to cluster Y's virtual IP address
(VIPY). This has the effect of load balancing the outgoing traffic,
while also providing full redundancy.
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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 IP address.
VRRP packets are sent encapsulated in IP packets. They are sent to
an IPv4 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 IP 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 | Version | VRRP Cluster | Priority | Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1 | Auth Type | Adver Int | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2 | Virtual IP address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3 | Authentication Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.2 IP Field Descriptions
5.2.1 Source Address
The real IP address of the interface the packet is being sent from.
5.2.2 Destination Address
The VRRP IP multicast address assigned by the IANA. It is defined to
be:
224.0.0.(TBD IANA assignment)
This is a link local scope multicast address. Routers MUST NOT
forward a datagram with this destination address regardless of its
TTL.
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5.2.3 TTL
The TTL MUST be set to 255. A VRRP router receiving a packet with
the TTL not equal to 255 MUST discard the packet.
5.2.4 Protocol
The VRRP IP protocol number assigned by the IANA. It is defined to
be (TBD).
5.3 VRRP Field Descriptions
5.3.1 Version
The version field specifies the VRRP protocol version of this packet.
This document defines version 1.
5.3.2 VRRP Cluster
The VRRP Cluster field specifies the cluster this packet applies to.
Note: The interface may participate in more than one VRRP cluster
simultaneously, perhaps serving as Master in one cluster, while
simultaneously serving as backup in other clusters.
5.3.3 Priority
The priority field specifies the router's priority for the Virtual IP
address and cluster. Higher values equal higher priority. This
field is an 8 bit unsigned field, giving 1 as the minimum priority,
and 255 as the maximum priority. The default priority is 100
(decimal).
The priority value zero (0) has special meaning indicating that the
current Master has stopped running VRRP. This is used to trigger
Backup routers to quickly transition to Master without having to wait
for the current Master to timeout.
In the event that two or more routers within a cluster have equal
priority, and that priority is the highest priority for the cluster,
initially the router with the higher real interface IP address
(interpreted as a 32 bit unsigned integer) will become Master. Any
router joining the cluster with the same priority will not become
Master even if it has a higher IP address unless the current Master
goes down.
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5.3.4 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.
5.3.5 Authentication Type
The authentication type field identifies the authentication method
being utilized. The authentication type field is an 8 bit number. 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
5.3.5.1 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.
5.3.5.2 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.
5.3.5.3 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 HMAC: Keyed-Hashing for Message
Authentication [HMAC]. Keys may be either configured manually or via
a key distribution protocol.
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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.
5.3.6 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.
5.3.7 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. For
computing the checksum, the checksum field is set to zero.
5.3.8 Virtual IP address
The virtual IP address field specifies the Virtual IP (VIP) address
associated with the particular cluster. This field is used for
troubleshooting misconfigured routers.
The VIP MUST be an IP address assigned from the subnet that the
interface is attached and does not match any hosts real IP or cluster
VIP address.
5.3.9 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 with an authentication string that does not match its
configured authentication string SHOULD 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
Cluster_ID Cluster identifier. Configured item. There
is no default.
Priority Priority value for this cluster. Configured
item. Range is between 1-255. Default is
100 (decimal).
Virtual_IP Virtual IP Address for this cluster.
Configured item.
Advertisement_Interval Time interval between ADVERTISEMENTS in
seconds. Default is 1 second.
Skew_Time Calculated time to skew Master_Down_Interval
in seconds. Defined to be:
( (256 - Priority) / 256 )
Master_Down_Interval Time interval for Backup to declare Master
down in seconds. Defined to be:
(3 * Advertisement_Interval) + Skew_time
Preempt_Mode Configuration switch controlling whether a
higher priority VRRP router preempts a lower
priority VRRP Master. Values are True to
preempt and False to not preempt. Default
is True.
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 |
+---------------+ +---------------+
| |---------------------->| |
| 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.
6.4.1 Initialize
{Initialize} is the state a virtual router takes when VRRP is
inactive. The purpose of this state is to wait for a Startup event.
If a Startup event is received, then:
- Set the Master_Down_Timer to Master_Down_Interval
- Transition to the {Backup} state
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, an virtual router MUST do the following:
- MUST NOT respond to ARP requests for the virtual router IP address
- MUST discard packets with a destination link layer MAC address
equal to the virtual router MAC address
- MUST not accept packets addressed to the Virtual IP address
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- 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 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 physical
router for the Virtual IP address.
While in this state, a virtual router MUST do the following:
- MUST respond to ARP requests for the VIP address with the virtual
router MAC address
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- Must accept and forward packets with a destination link layer MAC
address equal to the virtual router MAC address
- Must accept packets addressed to the VIP address
- 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 IP Address of the sender is greater than
the local IP Address, then:
o Cancel Adver_Timer
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
The following actions MUST be performed when a VRRP packet is
received:
- Verify that the IP TTL is 255.
- Verify that the received packet length is greater than or equal
to the VRRP header length
- Verify the VRRP checksum
- Verify the VRRP version
- Perform authentication specified by Auth Type
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.
- Verify that the Cluster identifier and the VIP are valid on the
receiving interface
- Verify that the VIP in packet is same as the configured VIP for
this cluster
If any one of the above checks fails, the receiver MUST discard the
packet.
- 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 an
error occurred.
7.2 Transmitting Packets
The following operations MUST be performed prior to 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
- Send the VRRP packet to the VRRP IP multicast group
Note: VRRP packets are transmitted with the virtual MAC address as
the source MAC address to ensure that learning bridges correctly
determine the LAN segment the virtual router is attached to.
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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-XX-XX-{cluster id} (in hex in internet standard bit-order)
The first three octets are derived from the IANA's OUI. The next two
octets (to be assigned by the IANA) indicate the address block
assigned to the VRRP protocol. {cluster id} is the VRRP cluster
identifier. This mapping provides for up to 255 VRRP clusters on a
network.
8. Host Operation
8.1 Host ARP Requests
When a host sends an ARP request for the virtual IP address, the
Master router MUST respond to the ARP request with the virtual MAC
address for the virtual router. This allows the client to always use
the same MAC address regardless of the current Master router. The
request MUST be handled as a standard ARP reply.
9. Operational Issues
9.1 ICMP Redirects
VRRP operation relies on hosts only using the Virtual IP address. It
is important that client hosts do not learn the real IP address of
any VRRP router on the LAN segment. Consequently VRRP routers MUST
NOT send ICMP Redirects on any interface they are running VRRP on.
9.2 Proxy ARP
If Proxy ARP is to be used on a router running VRRP, then the VRRP
router must advertise the Virtual Router MAC address in the Proxy ARP
message. Doing otherwise could cause hosts to learn the real IP
address of the VRRP routers.
9.3 Network Management
It is important that network management tools (e.g., SNMP, Telnet,
etc.) always use the real IP addresses of a VRRP router. This
ensures that network management is aware of the status of the real
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routers (e.g., to detect that a router has failed so that it can be
repaired).
10. Operation over FDDI and Token Ring
10.1 Operation over FDDI
FDDI interfaces strip 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 stripped off the ring during the Master
convergence, and convergence will fail.
To avoid this an implementations 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 stripping any ADVERTISEMENTS it did not originate.
10.2 Operation over Token Ring
Token Ring has several characteristics which make running VRRP
problematic. This includes:
- No general multicast mechanism. Required use of "functional
addresses" as a substitute, which may collide with other usage of
the same "functional addresses".
- Token Ring interfaces may have a limited ability to receive on
multiple MAC addresses.
- In order to switch to a new master located on a different physical
ring from the previous master when using source route bridges, a
mechanism is required to update cached source route information.
Due the these issues and the limited knowledge about the detailed
operation of Token Ring by the authors, this version of VRRP does not
work over Token Ring networks. This may be remedied in new version
of this document, or in a separate document.
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11. 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 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.
11.1 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 in a
single cluster on a link).
11.2 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 link. It protects against routers
inadvertently becoming a member of a VRRP cluster. A new router must
first be configured with the correct password before it can become a
member of the VRRP cluster. This type of authentication does not
protect against hostile attacks where the password can be learned by
a node snooping VRRP packets on the link. The Simple Text
Authentication combined with the TTL check makes it difficult for a
VRRP packet to be sent from another link to disrupt VRRP operation.
This type of authentication is RECOMMENDED when there is minimal risk
of nodes on the link actively disrupting VRRP operation.
11.3 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 HMAC: Keyed-Hashing for Message
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Authentication [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 the link. 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 ARP replies) which are independent from
VRRP and are not protected.
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12. References
[AUTH] Atkinson, R., "IP Authentication Header", RFC-1826, August
1995.
[DISC] Deering, S., "ICMP Router Discovery Messages", RFC-1256,
September 1991.
[DHCP] Droms, R., "Dynamic Host Configuration Protocol", RFC-1541,
October 1993.
[HMAC] Krawczyk, H., M. Bellare, R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC-2104, February 1997.
[HSRP] Li, T., B. Cole, P. Morton, D. Li, "Hot Standby Router
Protocol (HSRP)", Internet Draft, <draft-li-hsrp-00.txt>,
June 1997.
[OSPF] Moy, J., "OSPF version 2", RFC-1583, July 1997.
[RIP] Hedrick, C., "Routing Information Protocol" , RFC-1058,
June 1988.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC-2119, BCP14, March 1997.
13. Author's Addresses
Steven Knight Phone: +1 612 943-8990
Ascend Communications EMail: Steven.Knight@ascend.com
High Performance Network Division
10250 Valley View Road, Suite 113
Eden Prairie, MN USA 55344
USA
Douglas Weaver Phone: +1 612 943-8990
Ascend Communications EMail: Doug.Weaver@ascend.com
High Performance Network Division
10250 Valley View Road, Suite 113
Eden Prairie, MN USA 55344
USA
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David Whipple Phone: +1 206 703-3876
Microsoft Corporation EMail: dwhipple@microsoft.com
One Microsoft Way
Redmond, WA USA 98052-6399
USA
Robert Hinden Phone: +1 408 990-2004
Ipsilon Networks, Inc. EMail: hinden@ipsilon.com
232 Java Drive
Sunnyvale, CA 94089
USA
Danny Mitzel Phone: +1 408 990-2037
Ipsilon Networks, Inc. EMail: mitzel@ipsilon.com
232 Java Drive
Sunnyvale, CA 94089
USA
14. Acknowledgments
The authors would like to thank Glen Zorn, and Michael Lane, Clark
Bremer, Hal Peterson, Peter Hunt, Tony Li, Barbara Denny, and Steve
Bellovin for their comments and suggestions.
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15. Changes from Previous Drafts
Changes from <draft-ietf-vrrp-spec-00.txt>
- Added Preempt_Mode to allow user control over preemption
independent of configured priorities.
- Rewrote authentication section and expanded security
considerations.
- Expanded State Description section and removed State Table which
become redundant and impossible to edit.
- Changed authentication to be on a per interface basis (not per
cluster).
- Clarified text on disabling ICMP Redirects.
- Added text on FDDI and Token Ring issues.
- Added HSRP acknowledgment.
- Rewrote Introduction, Required Features, and VRRP Overview
sections.
- Many small text clarifications.
Changes from <draft-hinden-vrrp-00.txt>
- Changed default behavior to stay with current master when
priorities are equal. This behavior can be changed by configuring
explicit priorities.
- Changed Master state behavior to not send Advertisements when
receiving Advertisement with lower priority. Change reduces worst
case election message overhead to "n", where "n" is number of
configured equal priority VRRP routers.
- Added Skew_Time parameter and changed receiving advertisement with
zero priority behavior to cause resulting advertisement sent to be
skewed by priority.
- Changed sending behavior to send VRRP packets with VMAC as source
MAC and added text describing why this is important for bridged
environments.
- Changed definition of VMAC to be in IANA assigned unicast MAC
block.
- Added Advertisement Interval to VRRP header.
- Added text regarding ICMP Redirects, Proxy ARP, and network
management issues.
- Various small text clarifications.
draft-ietf-vrrp-spec-01.txt [Page 25]
