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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
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as "work in progress."

   To learn the current status of any Internet-Draft, please check the
   "1id-abstracts.txt" listing contained in the Internet- Drafts Shadow
   Directories on ds.internic.net (US East Coast), nic.nordu.net
   (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific
   Rim).

   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.










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