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.
Li, et. al. Informational [Page 1]RFC 2281 Cisco HSRP March 1998
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.
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RFC 2281 Cisco HSRP March 1998
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.
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RFC 2281 Cisco HSRP March 1998
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:
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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:
Li, et. al. Informational [Page 5]RFC 2281 Cisco HSRP March 1998
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.
Li, et. al. Informational [Page 6]RFC 2281 Cisco HSRP March 1998
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
Li, et. al. Informational [Page 7]RFC 2281 Cisco HSRP March 1998
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.
Li, et. al. Informational [Page 8]RFC 2281 Cisco HSRP March 1998
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.
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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.
Li, et. al. Informational [Page 10]RFC 2281 Cisco HSRP March 1998
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.
Li, et. al. Informational [Page 11]RFC 2281 Cisco HSRP March 1998
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.
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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.
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RFC 2281 Cisco HSRP March 1998
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.
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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.
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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
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RFC 2281 Cisco HSRP March 1998
10. Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
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.
Li, et. al. Informational [Page 17]
