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RFC 2281 - Cisco Hot Standby Router Protocol (HSRP)


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Network Working Group                                              T. Li
Request for Comments: 2281                              Juniper Networks
Category: Informational                                          B. Cole
                                                        Juniper Networks
                                                               P. Morton
                                                           Cisco Systems
                                                                   D. Li
                                                           Cisco Systems
                                                              March 1998

                Cisco Hot Standby Router Protocol (HSRP)

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

IESG Note

   This document reflects an existing deployed protocol.  The IETF does
   have a working group which is in the process of producing a standards
   track protocol to address the same issues.

Abstract

   The memo specifies the Hot Standby Router Protocol (HSRP).  The goal
   of the protocol is to allow hosts to appear to use a single router
   and to maintain connectivity even if the actual first hop router they
   are using fails.  Multiple routers participate in this protocol and
   in concert create the illusion of a single virtual router.  The
   protocol insures that one and only one of the routers is forwarding
   packets on behalf of the virtual router.  End hosts forward their
   packets to the virtual router.

   The router forwarding packets is known as the active router.  A
   standby router is selected to replace the active router should it
   fail. The protocol provides a mechanism for determining active and
   standby routers, using the IP addresses on the participating routers.
   If an active router fails a standby router can take over without a
   major interruption in the host's connectivity.  This memo also
   discusses the ARP, MAC address, and security issues with this
   protocol.

TABLE OF CONTENTS

   1   Introduction ..............................................  2
   2   Conditions of Use .........................................  3
   3   Scope .....................................................  4
   3.1 Terminology ...............................................  4
   4   Definitions ...............................................  4
   5   Protocol ..................................................  4
   5.1 Packet formats ............................................  4
   5.2 Operational parameters ....................................  7
   5.3 States ....................................................  8
   5.4 Timers ....................................................  9
   5.5 Events ....................................................  9
   5.6 Actions ................................................... 10
   5.7 State Transitions.......................................... 11
   6   MAC address considerations ................................ 13
   6.1 General ................................................... 13
   6.2 Address Filter ............................................ 14
   6.3 ICMP Redirect ............................................. 14
   6.4 Proxy ARP ................................................. 15
   7   Security Considerations ................................... 15
   8   References ................................................ 15
   9   Authors' Addresses ........................................ 16
   10  Full Copyright Statement .................................. 17

1. Introduction

   The Hot Standby Router Protocol, HSRP, provides a mechanism which is
   designed to support non-disruptive failover of IP traffic in certain
   circumstances.  In particular, the protocol protects against the
   failure of the first hop router when the source host cannot learn the
   IP address of the first hop router dynamically.  The protocol is
   designed for use over multi-access, multicast or broadcast capable
   LANs (e.g., Ethernet).  HSRP is not intended as a replacement for
   existing dynamic router discovery mechanisms and those protocols
   should be used instead whenever possible [1].  A large class of
   legacy host implementations that do not support dynamic discovery are
   capable of configuring a default router.  HSRP provides failover
   services to those hosts.

   All of the routers participating in HSRP are assumed to be running
   appropriate IP routing protocols and have a consistent set of routes.
   The discussion of which protocols are appropriate and whether routing
   is consistent in any given situation is beyond the scope of this
   specification.

   Using HSRP, a set of routers work in concert to present the illusion
   of a single virtual router to the hosts on the LAN.  This set is
   known as an HSRP group or a standby group.  A single router elected
   from the group is responsible for forwarding the packets that hosts
   send to the virtual router.  This router is known as the active
   router.  Another router is elected as the standby router.  In the
   event that the active router fails, the standby assumes the packet
   forwarding duties of the active router.  Although an arbitrary number
   of routers may run HSRP, only the active router forwards the packets
   sent to the virtual router.

   To minimize network traffic, only the active and the standby routers
   send periodic HSRP messages once the protocol has completed the
   election process.  If the active router fails, the standby router
   takes over as the active router.  If the standby router fails or
   becomes the active router, another router is elected as the standby
   router.

   On a particular LAN, multiple hot standby groups may coexist and
   overlap.  Each standby group emulates a single virtual router.  For
   each standby group, a single well-known MAC address is allocated to
   the group, as well as an IP address.  The IP address SHOULD belong to
   the primary subnet in use on the LAN, but MUST differ from the
   addresses allocated as interface addresses on all routers and hosts
   on the LAN, including virtual IP addresses assigned to other HSRP
   groups.

   If multiple groups are used on a single LAN, load splitting can be
   achieved by distributing hosts among different standby groups.

   The remainder of this specification discusses the operation of a
   single standby group.  In the case of multiple groups, each group
   operates independently of other groups on the LAN and according to
   this specification.  Note that individual routers may participate in
   multiple groups.  In this case, the router maintains separate state
   and timers for each group.

2  Conditions of Use

   US Patent number 5,473,599 [2], assigned to Cisco Systems, Inc. may
   be applicable to HSRP.  If an implementation requires the use of any
   claims of patent no. 5,473,599, Cisco will license such claims on
   reasonable, nondiscriminatory terms for use in practicing the
   standard.  More specifically, such license will be available for a
   one-time, paid up fee.

3  Scope

   This document describes the packets, messages, states, and events
   used to implement the protocol.  It does not discuss network
   management or internal implementation issues.

3.1   Terminology

   The language conventions of RFC 2119 [3] are used in this document.

4  Definitions

   Active Router   - the router that is currently forwarding packets
                     for the virtual router

   Standby Router  - the primary backup router

   Standby Group   - the set of routers participating in HSRP that
                     jointly emulate a virtual router

   Hello Time      - the interval between successive HSRP Hello
                     messages from a given router

   Hold Time       - the interval between the receipt of a Hello
                     message and the presumption that the sending
                     router has failed

5  Protocol

   Within a standby group, the routers periodically advertise state
   information using various messages.

5.1  Packet formats

   The standby protocol runs on top of UDP, and uses port number 1985.
   Packets are sent to multicast address 224.0.0.2 with TTL 1.

   Routers use their actual IP address as the source address for
   protocol packets, not the virtual IP address.  This is necessary so
   that the HSRP routers can identify each other.

   The format of the data portion of the UDP datagram is:

                          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     |   Op Code     |     State     |   Hellotime   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Holdtime    |   Priority    |     Group     |   Reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Authentication  Data                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Authentication  Data                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Virtual IP Address                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Version:  1 octet

      The version of the HSRP messages.  This document describes version
      0.

   Op Code:  1 octet

      The Op Code describes the type of message contained in this
      packet.  Possible values are:

         0 - Hello
         1 - Coup
         2 - Resign

      Hello messages are sent to indicate that a router is running and
      is capable of becoming the active or standby router.

      Coup messages are sent when a router wishes to become the active
      router.

      Resign messages are sent when a router no longer wishes to be the
      active router.

   State:  1 octet

      Internally, each router in the standby group implements a state
      machine.  The State field describes the current state of the
      router sending the message.  Details on the individual states are
      described below.  Possible values are:

         0 - Initial
         1 - Learn
         2 - Listen
         4 - Speak
         8 - Standby
        16 - Active

   Hellotime:  1 octet

      This field is only meaningful in Hello messages.  It contains the
      approximate period between the Hello messages that the router
      sends.  The time is given in seconds.

      If the Hellotime is not configured on a router, then it MAY be
      learned from the Hello message from the active router.  The
      Hellotime SHOULD only be learned if no Hellotime is configured and
      the Hello message is authenticated.  A router that sends a Hello
      message MUST insert the Hellotime that it is using in the
      Hellotime field in the Hello message.  If the Hellotime is not
      learned from a Hello message from the active router and it is not
      manually configured, a default value of 3 seconds is RECOMMENDED.

   Holdtime:  1 octet

      This field is only meaningful in Hello messages.  It contains the
      amount of time that the current Hello message should be considered
      valid.  The time is given in seconds.

      If a router sends a Hello message, then receivers should consider
      that Hello message to be valid for one Holdtime.  The Holdtime
      SHOULD be at least three times the value of the Hellotime and MUST
      be greater than the Hellotime.  If the Holdtime is not configured
      on a router, then it MAY be learned from the Hello message from
      the active router.  The Holdtime SHOULD only be learned if the
      Hello message is authenticated.  A router that sends a Hello
      message MUST insert the Holdtime that it is using in the Holdtime
      field in the Hello message.

      A router which is in active state MUST NOT learn new values for
      the Hellotime and the Holdtime from other routers, although it may
      continue to use values which it learned from the previous active
      router.  It MAY also use the Hellotime and Holdtime values learned
      through manual configuration.  The active router MUST NOT use one
      configured time and one learned time.  If the Holdtime is not
      learned and it is not manually configured, a default value of 10
      seconds is RECOMMENDED.

   Priority:  1 octet

      This field is used to elect the active and standby routers.  When
      comparing priorities of two different routers, the router with the
      numerically higher priority wins.  In the case of routers with
      equal priority the router with the higher IP address wins.

   Group:   1 octet

      This field identifies the standby group.  For Token Ring, values
      between 0 and 2 inclusive are valid.  For other media values
      between 0 and 255 inclusive are valid.

   Authentication Data:    8 octets

      This field contains a clear-text 8 character reused password.

      If no authentication data is configured, the RECOMMENDED default
      value is 0x63 0x69 0x73 0x63 0x6F 0x00 0x00 0x00.

   Virtual IP Address:     4 octets

      The virtual IP address used by this group.

      If the virtual IP address is not configured on a router, then it
      MAY be learned from the Hello message from the active router.  An
      address SHOULD only be learned if no address was configured and
      the Hello message is authenticated.

5.2  Operational parameters

   The following information MUST be known to each router in the standby
   group.  The mechanisms used to determine this information are outside
   of the scope of this document.

      Standby group number

      Virtual MAC address

      Priority

      Authentication Data

      Hellotime

      Holdtime

   The following information MUST be known to at least one router in
   each standby group and MAY be known by any of the other routers in
   the group.

      Virtual IP Address

   The following information MAY be configured on any router:

      Preemption capability

         If a router has higher priority than the active router and
         preemption is configured, it MAY take over as the active router
         using a Coup message.

5.3 States

   Each router in the group participates in the protocol by implementing
   a simple state machine.  This specification describes the externally
   visible behavior of this state machine.  Implementations MAY vary
   their internal implementations within the functional description of
   the state machine.

   All routers begin in the Initial state.  This section discusses the
   intent of each state.  For specific details on the actions taken in
   each state, please see the state transition table in section 5.7.

   1. Initial

      This is the starting state and indicates that HSRP is not running.
      This state is entered via a configuration change or when an
      interface first comes up.

   2. Learn

      The router has not determined the virtual IP address, and not yet
      seen an authenticated Hello message from the active router.  In
      this state the router is still waiting to hear from the active
      router.

   3. Listen

      The router knows the virtual IP address, but is neither the active
      router nor the standby router.  It listens for Hello messages from
      those routers.

   4. Speak

      The router sends periodic Hello messages and is actively
      participating in the election of the active and/or standby router.
      A router cannot enter Speak state unless it has the virtual IP
      address.

   5. Standby

      The router is a candidate to become the next active router and
      sends periodic Hello messages.  Excluding transient conditions,
      there MUST be at most one router in the group in Standby state.

   6. Active

      The router is currently forwarding packets that are sent to the
      group's virtual MAC address.  The router sends periodic Hello
      messages.  Excluding transient conditions, there MUST be at most
      one router in Active state in the group.

5.4 Timers

   Each router maintains three timers, an Active timer, a Standby timer,
   and a Hello timer.

   The Active timer is used to monitor the active router.  The active
   timer is started anytime an authenticated Hello message is seen from
   the active router.  It is set to expire in the Holdtime seen in the
   Hello message.

   The Standby timer is used to monitor the standby router The Standby
   timer is started anytime an authenticated Hello message is seen from
   the standby router.  It is set to expire in the Holdtime seen in the
   Hello message.

   The Hello timer expires once per Hellotime period.  If the router is
   in Speak, Standby, or Active states, it should generate a Hello
   message upon Hello timer expiry.  The Hello timer MUST be jittered.

5.5 Events

   These are the events in the HSRP finite state machine.

      a - HSRP is configured on an enabled interface.

      b - HSRP is disabled on an interface or the interface is disabled.

      c - Active timer expiry.  The Active timer was set to the Holdtime
      when the last Hello message was seen from the active router.

      d - Standby timer expiry.  The Standby timer was set to the
      Holdtime when the last Hello message was seen from the standby
      router.

      e - Hello timer expiry.  The periodic timer for sending Hello
      messages has expired.

      f - Receipt of a Hello message of higher priority from a router in
      Speak state.

      g - Receipt of a Hello message of higher priority from the active
      router.

      h - Receipt of a Hello message of lower priority from the active
      router.

      i - Receipt of a Resign message from the active router.

      j - Receipt of a Coup message from a higher priority router.

      k - Receipt of a Hello message of higher priority from the standby
      router.

      l - Receipt of a Hello message of lower priority from the standby
      router.

5.6 Actions

   This section specifies the actions to be taken as part of the state
   machine.

      A  Start Active Timer
         If this action occurred as the result of the receipt of a an
         authenticated Hello message from the active router, the Active
         timer is set to the Holdtime field in the Hello message.
         Otherwise the Active timer is set to the current Holdtime value
         in use by this router.  The Active timer is then started.

      B  Start Standby Timer
         If this action occurred as the result of the receipt of an
         authenticated Hello message from the standby router, the
         Standby timer is set to the Holdtime field in the Hello
         message.  Otherwise the Standby timer is set to the current
         hold time value in use by this router.  The Standby timer is
         then started.

      C  Stop Active Timer
         The Active timer is stopped.

      D  Stop Standby Timer
         The Standby timer is stopped.

      E  Learn Parameters
         This action is taken when an authenticated message is received
         from the active router.  If the virtual IP address for this
         group was not manually configured, the virtual IP address MAY
         be learned from the message.  The router MAY learn Hellotime
         and Holdtime values from the message.

      F  Send Hello Message
         The router sends a Hello message with its current State,
         Hellotime and Holdtime.

      G  Send Coup Message
         The router sends a Coup message to inform the active router
         that there is a higher priority router available.

      H  Send Resign Message
         The router sends a Resign message to allow another router to
         become the active router.

      I  Send Gratuitous ARP Message
         The router broadcasts an ARP response packet advertising the
         group's virtual IP address and virtual MAC address.  The packet
         is sent using the virtual MAC address as the source MAC address
         in the link layer header, as well as within the ARP packet.

5.7 State Transitions

   This table describes the state transitions of the state machine.  For
   each event and current state of the router, the router MUST perform
   the set of actions specified and transition to the designated state.
   If no action is specified, no action should be taken.  If no state
   change is specified, no state change should be performed.

   The notation used in this table has the specified set of actions
   listed as letters corresponding to the actions listed in section 5.6.
   The next state is listed as a number as specified in section 5.3.  A
   slash ('/') separates the actions and states.  Certain state
   transitions have alternatives which depend on external state.
   Alternatives are separated by a '|'.  See the attached notes for
   details on these transitions.

                                                 States
+-----+----------+----------+----------+----------+----------+----------+
|     |    1     |     2    |    3     |    4     |    5     |     6    |
|     |  Initial |  Learn   |  Listen  |  Speak   |  Standby |   Active |
+-----+----------+----------+----------+----------+----------+----------+
|Event|                                                                 |
+-----+----------+----------+----------+----------+----------+----------+
|  a  |  AB/2|3+ |          |          |          |          |          |
+-----+----------+----------+----------+----------+----------+----------+
|  b  |          |   CD/1   |   CD/1   |   CD/1   |   CD/1   |   CDH/1  |
+-----+----------+----------+----------+----------+----------+----------+
|  c  |          |          |   AB/4   |          |  CDFI/6  |          |
+-----+----------+----------+----------+----------+----------+----------+
|  d  |          |          |   B/4    |   D/5    |          |          |
+-----+----------+----------+----------+----------+----------+----------+
|  e  |          |          |          |    F     |    F     |    F     |
+-----+----------+----------+----------+----------+----------+----------+
|  f  |          |          |          |   B/3    |   B/3    |          |
+-----+----------+----------+----------+----------+----------+----------+
|  g  |          |   EAB/3  |    EA    |   EA     |   EA     |   AB/4   |
+-----+----------+----------+----------+----------+----------+----------+
|  h  |          |   EAB/3  | A|BGFI/6*| A|BGFI/6*| A|BGFI/6*|    G     |
+-----+----------+----------+----------+----------+----------+----------+
|  i  |          |          |   AB/4   |   A      |  CFI/6   |          |
+-----+----------+----------+----------+----------+----------+----------+
|  j  |          |          |          |          |          |   ABH/4  |
+-----+----------+----------+----------+----------+----------+----------+
|  k  |          |          |    B     |   B/3    |  B/3     |    B     |
+-----+----------+----------+----------+----------+----------+----------+
|  l  |          |          |    B/4   |   D/5    |          |    B     |
+-----+----------+----------+----------+----------+----------+----------+

   Notes

   +   If the virtual IP address is configured, set state 3 (Listen) If
   the virtual IP address is not configured, set state 2 (Learn).  In
   either case do actions A and B.

   *   If the router is configured to preempt do actions B, G, F, and I
   and set state to 6 (Active).  If the router is not configured to
   preempt do actions A with no state change.

6  MAC Address Considerations

6.1 General

   Each HSRP group has an associated well known virtual MAC address.  On
   token ring networks, these addresses are actually functional
   addresses.  The three addresses 0xC0 0x00 0x00 0x01 0x00 0x00, 0xC0
   0x00 0x00 0x02 0x00 0x00, and 0xC0 0x00 0x00 0x04 0x00 0x00
   correspond to groups 0, 1, and 2 respectively.

   On other media, the virtual MAC addresses are 0x00 0x00 0x0C 0x07
   0xAC XX where XX represents the HSRP group number.  Routers which
   implement HSRP SHOULD use well-known HSRP MAC addresses as the
   group's virtual MAC address whenever possible.

   The active router MUST accept and forward traffic that is destined
   for the group's virtual MAC address.  It MUST stop accepting or
   forwarding such traffic when the router leaves the Active state.

   If and only if the router is in the Active state, the router MUST use
   the group's virtual MAC address as the source MAC address for its
   Hello messages.  This is necessary in order to allow learning bridges
   to be able to determine which LAN segment the virtual MAC address
   currently belongs to.

   For each group, there is one virtual IP address and one virtual MAC
   address.  This is a desirable situation, since the ARP table entries
   in the end stations do not need to change over time as the HSRP
   active router moves from one router to another.

   Additionally, for HSRP to work in bridging environments, the bridges
   must be able to quickly update themselves as the virtual MAC address
   "moves".  Although learning bridges typically are able to do this,
   some have been known to have problems with this.  It is RECOMMENDED
   that only true learning bridges be used with HSRP.

   The movement of the virtual MAC address can cause further undesirable
   side effects in environments where additional state is tied to the
   MAC address.  For example on Token Ring, if Source Route Bridging is
   in use, a RIF will be stored with the virtual MAC address in a host's
   RIF cache.  The RIF indicates the path and final ring used to reach
   the MAC address.  As routers transition into Active state, they will
   not be able to affect the RIF caches on the hosts on the bridged
   ring.  This may lead to packets being bridged to the ring for the
   previous active router.

   In such circumstances, a router MAY use its normal MAC addresses as
   the virtual MAC address.  This method of operation is strongly
   discouraged.  In this mode, the virtual IP address will map to a
   different MAC address over time.  This can create problems for end
   stations, since ARP tables assume a relatively static mapping between
   MAC address and IP address.  These ARP tables are normally updated
   when the end stations receive the gratuitous ARP responses generated
   by a router that enters the active state.

6.2 Address Filter

   As noted, routers currently emulating a virtual router adopt their
   group's MAC and IP addresses.  MAC addresses are typically provided
   in an address filter or 'list' of MAC addresses in a router's
   interface controller.  It is desirable for routers to be able to add
   one or more virtual MAC addresses to their controllers' MAC address
   filter while maintaining their primary MAC addresses.

   Unfortunately, some interface controllers support address filtering
   for only one unicast MAC address.  Or, in the case of Token Ring, the
   functional address which HSRP should use is already in use for some
   other protocol.  In these cases, such routers can still implement
   HSRP, but the protocol must change the interface's primary MAC
   address when assuming or relinquishing control as the active router.

   This is potentially problematic because some traffic may otherwise
   wish to use the router's primary MAC address.  However, the problem
   MAY be mitigated by having the router send out gratuitous ARP packets
   regarding its non-HSRP IP addresses.  Through this, other network
   entities using IP should update their ARP tables to reflect that the
   router is now using a group virtual MAC address rather than its
   primary MAC address.

   Some protocols may not be able to run simultaneously with the standby
   protocol due to the interface primary MAC address change.  For
   example, DECnet phase IV and HSRP will not be able to run at the same
   time on some equipment.

6.3 ICMP Redirect

   While running HSRP, it is important to prevent the host from
   discovering the primary MAC addresses of the routers in its standby
   group.  Thus, any protocol that informs a host of a router's primary
   address should be disabled.  Thus, routers participating in HSRP on
   an interface MUST NOT send ICMP redirects on that interface.

6.4 Proxy ARP

   Typically, hosts learn the HSRP virtual IP address through the
   configuration of their default router.  These hosts then send packets
   for destinations outside of the LAN to the virtual IP address.  In
   some environments, hosts may instead make use of proxy ARP in order
   to route off of the LAN.  In this case, the hosts use the MAC address
   that is supplied in proxy ARP responses.  HSRP functionality is
   maintained if the proxy ARP responses specify the HSRP virtual MAC
   address.

   If an HSRP router is configured to support proxy ARP with HSRP, then
   the router MUST specify the HSRP virtual MAC address in any proxy ARP
   responses it generates.  These proxy ARP responses MUST not be
   suppressed based upon HSRP state.  Suppression based upon state could
   result in lack of any proxy ARP response being generated, since these
   proxy ARP responses may be suppressed due to other reasons, such as
   split-horizon rules.

7. Security Considerations

   This protocol does not provide security.  The authentication field
   found within the message is useful for preventing misconfiguration.
   The protocol is easily subverted by an active intruder on the LAN.
   This can result in a packet black hole and a denial-of-service
   attack.  It is difficult to subvert the protocol from outside the LAN
   as most routers will not forward packets addressed to the all-routers
   multicast address (224.0.0.2).

8. References

   [1] Deering, S., "ICMP Router Discovery Messages", RFC 1256,
   September 1991.

   [2] United States Patent.  Patent Number : 5,473,599.  Standby Router
   Protocol.  Date of Patent: Dec. 5, 1995.

   [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
   Levels", BCP 14, RFC 2119, March 1997.

9. Authors' Addresses

   Tony Li
   Juniper Networks, Inc.
   3260 Jay St.
   Santa Clara, CA 95054

   Phone: (408) 327-1900
   EMail: tli@juniper.net

   Bruce Cole
   Juniper Networks, Inc.
   3260 Jay St.
   Santa Clara, CA 95054

   Phone: (408) 327-1900
   EMail: cole@juniper.net

   Phil Morton
   Cisco Systems
   170 Tasman Dr.
   San Jose, CA 95143

   Phone: (408) 526-7632
   EMail: pmorton@cisco.com

   Dawn Li
   Cisco Systems
   170 Tasman Dr.
   San Jose, CA 95143

   Phone: (408) 527-2014
   EMail: dawnli@cisco.com

10.  Full Copyright Statement

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   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

 

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