Network Working Group T. Ts'o, Editor

Network Working Group T. Ts'o, Editor
Request for Comments: 2941 VA Linux Systems
Obsoletes: 1416 J. Altman
Category: Standards Track Columbia University
September 2000

Telnet Authentication Option

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2000). All Rights Reserved.

Abstract

This document describes the authentication option to the telnet [1]
protocol as a generic method for negotiating an authentication type
and mode including whether encryption should be used and if
credentials should be forwarded. While this document summarizes
currently utilized commands and types it does not define a specific
authentication type. Separate documents are to be published defining
each authentication type.

This document updates a previous specification of the telnet
authentication option, RFC 1416 [2], so that it can be used to
securely enable the telnet encryption option [3].

1. Command Names and Codes

AUTHENTICATION 37

Authentication Commands
IS 0
SEND 1
REPLY 2
NAME 3

Authentication Types
NULL 0
KERBEROS_V4 1

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RFC 2941 Telnet Authentication Option September 2000

KERBEROS_V5 2
SPX* 3
MINK* 4
SRP 5
RSA*[also used by SRA*] 6
SSL* 7
[unassigned] 8
[unassigned] 9
LOKI* 10
SSA* 11
KEA_SJ 12
KEA_SJ_INTEG 13
DSS 14
NTLM* 15

Authentication types followed by (*) were never submitted to the
IETF for consideration as an Internet standard.

Following historical practice, future authentication type numbers
and authentication modifiers will be assigned by the IANA under a
First Come First Served policy as outlined by RFC 2434 [4].
Despite the fact that authentication type numbers are allocated
out of an 8-bit number space (as are most values in the telnet
specification) it is not anticipated that the number space is or
will become in danger of being exhausted. However, if this
should become an issue, when over 50% of the number space becomes
allocated, the IANA shall refer allocation requests to either the
IESG or a designated expert for approval. IANA is instructed not
to issue new suboption values without submission of documentation
of their use.

Modifiers
AUTH_WHO_MASK 1
AUTH_CLIENT_TO_SERVER 0
AUTH_SERVER_TO_CLIENT 1

AUTH_HOW_MASK 2
AUTH_HOW_ONE_WAY 0
AUTH_HOW_MUTUAL 2

ENCRYPT_MASK 20
ENCRYPT_OFF 0
ENCRYPT_USING_TELOPT 4
ENCRYPT_AFTER_EXCHANGE 16
ENCRYPT_RESERVED 20

INI_CRED_FWD_MASK 8
INI_CRED_FWD_OFF 0

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RFC 2941 Telnet Authentication Option September 2000

INI_CRED_FWD_ON 8

2. Command Meanings

This document makes reference to a "server" and a "client". For the
purposes of this document, the "server" is the side of the connection
that performed the passive TCP open (TCP LISTEN state), and the
"client" is the side of the connection that did the active open.

IAC WILL AUTHENTICATION

The client side of the connection sends this command to indicate
that it is willing to send and receive authentication information.

IAC DO AUTHENTICATION

The servers side of the connection sends this command to indicate
that it is willing to send and receive authentication information.

IAC WONT AUTHENTICATION

The client side of the connection sends this command to indicate
that it refuses to send or receive authentication information; the
server side must send this command if it receives a DO
AUTHENTICATION command.

IAC DONT AUTHENTICATION

The server side of the connection sends this command to indicate
that it refuses to send or receive authentication information; the
client side must send this command if it receives a WILL
AUTHENTICATION command.

IAC SB AUTHENTICATION SEND authentication-type-pair-list IAC SE

The sender of this command (the server) requests that the remote
side send authentication information for one of the authentication
types listed in "authentication-type-pair-list". The
"authentication-type-pair-list" is an ordered list of
"authentication-type" pairs. Only the server side (DO
AUTHENTICATION) is allowed to send this.

IAC SB AUTHENTICATION IS authentication-type-pair <auth data> IAC SE

The sender of this command (the client) is sending the
authentication information for authentication type
"authentication-type-pair". Only the client side (WILL
AUTHENTICATION) is allowed to send this.

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RFC 2941 Telnet Authentication Option September 2000

IAC SB AUTHENTICATION REPLY authentication-type-pair <auth data> IAC
SE

The sender of this command (the server) is sending a reply to the
the authentication information received in a previous IS command.
Only the server side (DO AUTHENTICATION) is allowed to send this.

IAC SB AUTHENTICATION NAME remote-user IAC SE

This optional command is sent to specify the account name on the
remote host that the user wishes to be authorized to use. Note
that authentication may succeed, and the authorization to use a
particular account may still fail. Some authentication mechanisms
may ignore this command.

The "authentication-type-pair" is two octets, the first is the
authentication type, and the second is a modifier to the type. The
authentication type may or may not include built-in encryption. For
instance, when the Kerberos 4 authentication type is negotiated
encryption must be negotiated with the telnet ENCRYPT option.
However, the SSL and KEA_SJ authentication types provide an encrypted
channel as part of a successful telnet AUTH option negotiation.

There are currently five one bit fields defined in the modifier. The
first two of these bits are processed as a pair, the AUTH_WHO_MASK
bit and the AUTH_HOW_MASK bit. There are four possible combinations
of these two bits:

AUTH_CLIENT_TO_SERVER
AUTH_HOW_ONE_WAY

The client will send authentication information about the local
user to the server. If the negotiation is successful, the
server will have authenticated the user on the client side of
the connection.

AUTH_SERVER_TO_CLIENT
AUTH_HOW_ONE_WAY

The server will authenticate itself to the client. If the
negotiation is successful, the client will know that it is
connected to the server that it wants to be connected to.

AUTH_CLIENT_TO_SERVER
AUTH_HOW_MUTUAL

The client will send authentication information about the local
user to the server, and then the server will authenticate

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RFC 2941 Telnet Authentication Option September 2000

itself to the client. If the negotiation is successful, the
server will have authenticated the user on the client side of
the connection, and the client will know that it is connected
to the server that it wants to be connected to.

AUTH_SERVER_TO_CLIENT
AUTH_HOW_MUTUAL

The server will authenticate itself to the client, and then the
client will authenticate itself to the server. If the
negotiation is successful, the client will know that it is
connected to the server that it wants to be connected to, and
the server will know that the client is who it claims to be.

The third and fifth bits in the modifier are the ENCRYPT_MASK
bits. These bits are used to determine if and how encryption
should be enabled. Of the four possible combinations only three
are currently defined:

ENCRYPT_OFF

Encryption will not be used for this session. TELOPT
ENCRYPT SHOULD NOT be negotiated. This mode MUST be used
with all AUTH types that do not provide a shared secret to
be used as a session key.

ENCRYPT_USING_TELOPT

Encryption will be negotiated via the use of TELOPT ENCRYPT.
Immediately after authentication has completed TELOPT
ENCRYPT MUST be negotiated in both directions. This is
required to occur before credentials forwarding; other
telnet options are negotiated; or any user data is
transmitted. A failure to successfully negotiate TELOPT
ENCRYPT in either direction MUST result in immediate session
termination.

ENCRYPT_AFTER_EXCHANGE

Encryption will be activated in both directions immediately
after the successful exchange of the shared secret to be
used as the session key. The encryption algorithm to be
used MUST be implied by the AUTH type.

The fourth bit field in the modifier is the INI_CRED_FWD_MASK bit.
This bit is either set to INI_CRED_FWD_ON or INI_CRED_FWD_OFF.
This bit is set by the client to advise the server to expect
forwarded credentials from the client.

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RFC 2941 Telnet Authentication Option September 2000

INI_CRED_FWD_OFF

The client will not be forwarding credentials to the server.
This mode must be used if the selected authentication method
does not support credentials forwarding.

INI_CRED_FWD_ON

Once authentication, and perhaps encryption, completes, the
client will immediately forward authentication credentials
to the server.

The motivation for this advisory bit is that the server may wish
to wait until the forwarded credentials have been sent before
starting any operating system specific login procedures which may
depend on these credentials. Note that credentials forwarding may
not be supported by all authentication mechanisms. It is a
protocol error to set this bit if the underlying authentication
mechanism does not support credentials forwarding.

Credentials forwarding MUST NOT be performed if
AUTH_CLIENT_TO_SERVER|AUTH_HOW_ONE_WAY was used since the identity
of the server can not be assured. Credentials SHOULD NOT be
forwarded if the telnet connection is not protected using some
encryption or integrity protection services.

Note that older implementations of the telnet authentication
option will not understand the ENCRYPT_MASK and INI_CRED_FWD_MASK
bits. Hence an implementation wishing to offer these bits should
offer authentication type pairs with these bits both set and not
set if backwards compatibility is required.

3. Default Specification

The default specification for this option is

WONT AUTHENTICATION DONT AUTHENTICATION

meaning there will not be any exchange of authentication information.

4. Motivation

One of the deficiencies of the Telnet protocol is that in order to
log into remote systems, users have to type their passwords, which
are passed in clear text through the network. If the connections go
through untrusted networks, there is the possibility that passwords
will be compromised by someone watching the packets while in transit.

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RFC 2941 Telnet Authentication Option September 2000

The purpose of the AUTHENTICATION option is to provide a framework
for the passing of authentication information through the TELNET
session, and a mechanism to enable encryption of the data stream as a
side effect of successful authentication or via subsequent use of the
telnet ENCRYPT option. This means that: 1) the users password will
not be sent in clear text across the network, 2) if the front end
telnet process has the appropriate authentication information, it can
automatically send the information, and the user will not have to
type any password. 3) once authentication has succeeded, the data
stream can be encrypted to provide protection against active attacks.

It is intended that the AUTHENTICATION option be general enough that
it can be used to pass information for any authentication and
encryption system.

5. Security Implications

The ability to negotiate a common authentication mechanism between
client and server is a feature of the authentication option that
should be used with caution. When the negotiation is performed, no
authentication has yet occurred. Therefore each system has no way of
knowing whether or not it is talking to the system it intends. An
intruder could attempt to negotiate the use of an authentication
system which is either weak, or already compromised by the intruder.

If the authentication type requires that encryption be enabled as a
separate optional negotiation (the ENCRYPT option), it will provide a
window of vulnerability from when the authentication completes, up to
and including the negotiation to turn on encryption by an active
attacker. An active attack is one where the underlying TCP stream
can be modified or taken over by the active attacker. If the server
only offers authentication type pairs that include the
ENCRYPT_USING_TELOPT set in the ENCRYPT_MASK field, this will avoid
the window of vulnerability, since both parties will agree that
telnet ENCRYPT option must be successfully negotiated immediately
following the successful completion of telnet AUTH.

Other authentication types link the enabling of encryption as a side
effect of successful authentication. This will also provide
protection against the active attacker. The ENCRYPT_AFTER_EXCHANGE
bit allows these authentication types to negotiate encryption so that
it can be made optional.

Another opportunity for active attacks is presented when encryption
may be turned on and off without re-authentication. Once encryption
is disabled, an attacker may hijack the telnet stream, and interfere
with attempts to restart encryption. Therefore, a client SHOULD NOT

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RFC 2941 Telnet Authentication Option September 2000

support the ability to turn off encryption. Once encryption is
disabled, if an attempt to re-enable encryption fails, the client
MUST terminate the telnet connection.

It is important that in both cases the authentication type pair be
integrity protected at the end of the authentication exchange. This
must be specified for each authentication type to ensure that the
result of the telnet authentication option negotiation is agreed to
by both the client and the server. Some authentication type
suboptions may wish to include all of the telnet authentication
negotiation exchanges in the integrity checksum, to fully protect the
entire exchange.

Each side MUST verify the consistency of the auth-type-pairs in each
message received. Any variation in the auth-type-pair MUST be
treated as a fatal protocol error.

6. Implementation Rules

WILL and DO are used only at the beginning of the connection to
obtain and grant permission for future negotiations.

The authentication is only negotiated in one direction; the server
must send the "DO", and the client must send the "WILL". This
restriction is due to the nature of authentication; there are three
possible cases; server authenticates client, client authenticates
server, and server and client authenticate each other. By only
negotiating the option in one direction, and then determining which
of the three cases is being used via the suboption, potential
ambiguity is removed. If the server receives a "DO", it must respond
with a "WONT". If the client receives a "WILL", it must respond with
a "DONT".

Once the two hosts have exchanged a DO and a WILL, the server is free
to request authentication information. In the request, a list of
supported authentication types is sent. Only the server may send
requests ("IAC SB AUTHENTICATION SEND authentication-type-pair-list
IAC SE"). Only the client may transmit authentication information
via the "IAC SB AUTHENTICATION IS authentication-type ... IAC SE"
command. Only the server may send replies ("IAC SB AUTHENTICATION
REPLY authentication-type ... IAC SE"). As many IS and REPLY
suboptions may be exchanged as are needed for the particular
authentication scheme chosen.

If the client does not support any of the authentication types listed
in the authentication-type-pair-list, a type of NULL should be used
to indicate this in the IS reply. Note that if the client responds
with a type of NULL, the server may choose to close the connection.

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RFC 2941 Telnet Authentication Option September 2000

When the server has concluded that authentication cannot be
negotiated with the client it should send IAC DONT AUTH to the
client.

The order of the authentication types MUST be ordered to indicate a
preference for different authentication types, the first type being
the most preferred, and the last type the least preferred.

As long as the server is WILL AUTH it may request authentication
information at any time. This is done by sending a new list of
supported authentication types. Requesting authentication
information may be done as a way of verifying the validity of the
client's credentials after an extended period of time or to negotiate
a new session key for use during encryption.

7. User Interface

Normally protocol specifications do not address user interface
specifications. However, due to the fact that the user will probably
want to be able to configure the authentication and encryption and
know whether or not the negotiations succeeded, some guidance needs
to be given to implementors to provide some minimum level of user
control.

The user MUST be able to specify whether or not authentication is to
be used, and whether or not encryption is to used if the
authentication succeeds. There SHOULD be at least four settings,
REQUIRE, PROMPT, WARN and DISABLE. Setting the authentication switch
to REQUIRE means that if the authentication fails, then an
appropriate error message must be displayed and the TELNET connection
must be terminated. Setting the authentication switch to PROMPT
means that if the authentication fails, then an appropriate error
message must be displayed and the user must be prompted for
confirmation before continuing the TELNET session. Setting the
authentication switch to WARN means that if the authentication fails,
then an appropriate error message must be displayed before continuing
the TELNET session. Setting the authentication switch to DISABLE
means that authentication will not be attempted. The encryption
switch SHOULD have the same settings as the authentication switch;
however its settings are only used when authentication succeeds. The
default setting for both switches should be WARN. Both of these
switches may be implemented as a single switch, though having them
separate gives more control to the user.

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RFC 2941 Telnet Authentication Option September 2000

8. Example

The following is an example of use of the option:

Client Server
IAC DO AUTHENTICATION
IAC WILL AUTHENTICATION
[ The server is now free to request authentication information.
]
IAC SB AUTHENTICATION SEND
KERBEROS_V4 CLIENT|MUTUAL
KERBEROS_V4 CLIENT|ONE_WAY IAC
SE
[ The server has requested mutual Kerberos authentication, but is
willing to do just one-way Kerberos authentication. The client
will now respond with the name of the user that it wants to log
in as, and the Kerberos ticket. ]
IAC SB AUTHENTICATION NAME "joe"
IAC SE
IAC SB AUTHENTICATION IS
KERBEROS_V4 CLIENT|MUTUAL AUTH 4
7 1 67 82 65 89 46 67 7 9 77 0
48 24 49 244 109 240 50 208 43
35 25 116 104 44 167 21 201 224
229 145 20 2 244 213 220 33 134
148 4 251 249 233 229 152 77 2
109 130 231 33 146 190 248 1 9
31 95 94 15 120 224 0 225 76 205
70 136 245 190 199 147 155 13
IAC SE
[ The server responds with an ACCEPT command to state that the
authentication was successful. ]
IAC SB AUTHENTICATION REPLY
KERBEROS_V4 CLIENT|MUTUAL ACCEPT
IAC SE
[ Next, the client sends across a CHALLENGE to verify that it is
really talking to the right server. ]
IAC SB AUTHENTICATION IS
KERBEROS_V4 CLIENT|MUTUAL
CHALLENGE xx xx xx xx xx xx xx
xx IAC SE
[ Lastly, the server sends across a RESPONSE to prove that it
really is the right server.

IAC SB AUTHENTICATION REPLY
KERBEROS_V4 CLIENT|MUTUAL
RESPONSE yy yy yy yy yy yy yy yy
IAC SE

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RFC 2941 Telnet Authentication Option September 2000

The following is an example of use of the option with encryption
negotiated via telnet ENCRYPT:

Client Server
IAC DO AUTHENTICATION
IAC WILL AUTHENTICATION
[ The server is now free to request authentication information.
]
IAC SB AUTHENTICATION SEND
KERBEROS_V4
CLIENT|MUTUAL|ENCRYPT_USING_TELOPT
KERBEROS_V4 CLIENT|ONE_WAY IAC
SE
[ The server has requested mutual Kerberos authentication, but is
willing to do just one-way Kerberos authentication. In both
cases it is willing to encrypt the data stream. The client
will now respond with the name of the user that it wants to log
in as, and the Kerberos ticket. ]
IAC SB AUTHENTICATION NAME "joe"
IAC SE
IAC SB AUTHENTICATION IS
KERBEROS_V4
CLIENT|MUTUAL|ENCRYPT_USING_TELOPT
AUTH 4 7 1 67 82 65 89 46 67 7 9
77 0 48 24 49 244 109 240 50 208
43 35 25 116 104 44 167 21 201
224 229 145 20 2 244 213 220 33
134 148 4 251 249 233 229 152 77
2 109 130 231 33 146 190 248 1 9
31 95 94 15 120 224 0 225 76 205
70 136 245 190 199 147 155 13
IAC SE
[ The server responds with an ACCEPT command to state that the
authentication was successful. ]
IAC SB AUTHENTICATION REPLY
KERBEROS_V4
CLIENT|MUTUAL|ENCRYPT_USING_TELOPT
ACCEPT IAC SE
[ Next, the client sends across a CHALLENGE to verify that it is
really talking to the right server. ]
IAC SB AUTHENTICATION IS
KERBEROS_V4
CLIENT|MUTUAL|ENCRYPT_USING_TELOPT

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RFC 2941 Telnet Authentication Option September 2000

CHALLENGE xx xx xx xx xx xx xx
xx IAC SE
[ The server sends across a RESPONSE to prove that it really is
the right server. ]
IAC SB AUTHENTICATION REPLY
KERBEROS_V4
CLIENT|MUTUAL|ENCRYPT_USING_TELOPT
RESPONSE yy yy yy yy yy yy yy yy
IAC SE
[ At this point, the client and server begin to negotiate the
telnet ENCRYPT option in each direction for a secure channel.
If the option fails in either direction for any reason the
connection must be immediately terminated. ]

The following is an example of use of the option with integrated
encryption:

Client Server
IAC DO AUTHENTICATION
IAC WILL AUTHENTICATION
[ The server is now free to request authentication information.
]
IAC SB AUTHENTICATION SEND
KEA_SJ
CLIENT|MUTUAL|ENCRYPT_AFTER_EXCHANGE
IAC SE
[ The server has requested mutual KEA authentication with
SKIPJACK encryption. The client will now respond with the name
of the user that it wants to log in as and the KEA cert. ]
IAC SB AUTHENTICATION NAME "joe"
IAC SE IAC SB AUTHENTICATION IS
KEA_SJ
CLIENT|MUTUAL|ENCRYPT_AFTER_EXCHANGE
'1' CertA||Ra IAC SE
[ The server responds with its KEA Cert. ]
IAC SB AUTHENTICATION REPLY
KEA_SJ
CLIENT|MUTUAL|ENCRYPT_AFTER_EXCHANGE
'2'
CertB||Rb||IVb||Encrypt(NonceB)
IAC SE
[ Next, the client sends across a CHALLENGE to verify that it is
really talking to the right server. ]
IAC SB AUTHENTICATION IS KEA_SJ
CLIENT|MUTUAL|ENCRYPT_AFTER_EXCHANGE
'3' IVa||Encrypt( NonceB xor
0x0C18 || NonceA ) IAC SE

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RFC 2941 Telnet Authentication Option September 2000

[ At this point, the client begins to encrypt the outgoing data
stream, and the server, after receiving this command, begins to
decrypt the incoming data stream. Lastly, the server sends
across a RESPONSE to prove that it really is the right server.
]
IAC SB AUTHENTICATION REPLY
KEA_SJ
CLIENT|MUTUAL|ENCRYPT_AFTER_EXCHANGE
'4' Encrypt( NonceA xor 0x0C18 )
IAC SE
[ At this point, the server begins to encrypt its outgoing data
stream, and the client, after receiving this command, begins to
decrypt its incoming data stream. ]

It is expected that any implementation that supports the Telnet
AUTHENTICATION option will support all of this specification.

9. Security Considerations

This memo describes a general framework for adding authentication and
encryption to the telnet protocol. The actual authentication
mechanism is described in the authentication suboption
specifications, and the security of the authentication option is
dependent on the strengths and weaknesses of the authentication
suboption.

It should be noted that the negotiation of the authentication type
pair is not protected, thus allowing an attacker to force the result
of the authentication to the weakest mutually acceptable method.
(For example, even if both sides of the negotiation can accept a
"strong" mechanism and a "40-bit" mechanism, an attacker could force
selection of the "40-bit" mechanism.) An implementation should
therefore only accept an authentication mechanism to be negotiated if
it is willing to trust it as being secure.

It should also be noted that the negotiation of the username in the
IAC SB AUTHENTICATION NAME name IAC SE message is not protected.
Implementations should verify the value by a secure method before
using this untrusted value.

11. Acknowledgements

Many people have worked on this document over the span of many years.
Dave Borman was a document editor and author of much of the original
text. Other folks who have contributed ideas and suggestions to this
text include: David Carrel, Jeff Schiller, and Richard Basch.

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RFC 2941 Telnet Authentication Option September 2000

10. References

[1] Postel, J. and J. Reynolds, "Telnet Protocol Specification", STD
8, RFC 854, May 1983.

[2] Borman D., "Telnet Authentication Option", RFC 1416, February
1993.

[3] Ts'o, T., "Telnet Data Encryption Option", RFC 2946, September
2000.

[4] Alvestrand, H. and T. Narten, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

12. Authors' Addresses

Theodore Ts'o, Editor
VA Linux Systems
43 Pleasant St.
Medford, MA 02155

Phone: (781) 391-3464
EMail: [email protected]

Jeffrey Altman
Columbia University
Watson Hall Room 716
612 West 115th Street
New York NY 10025

Phone: +1 (212) 854-1344
EMail: [email protected]

Mailing List: [email protected]

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RFC 2941 Telnet Authentication Option September 2000

13. Full Copyright Statement

Copyright (C) The Internet Society (2000). 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.

Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.

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