One document matched: draft-josefsson-krb-tcp-expansion-00.txt
Network Working Group S. Josefsson
Internet-Draft November 12, 2005
Updates: 4120 (if approved)
Expires: May 16, 2006
Kerberos 5 TCP/IP Expansion Mechanism
draft-josefsson-krb-tcp-expansion-00
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
An expansion mechanism for the Kerberos 5 TCP/IP Transport is
described.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions used in this document . . . . . . . . . . . . . . . 3
3. Expansion Mechanism for TCP/IP transport . . . . . . . . . . . 3
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
6. Copying conditions . . . . . . . . . . . . . . . . . . . . . . 5
7. Normative References . . . . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6
Intellectual Property and Copyright Statements . . . . . . . . . . 7
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1. Introduction
The Kerberos 5 [3] specification reserve the high order bit in the
length field for TCP/IP transport for future expansion. This
document update [3] to describe the behaviour when that bit is set.
The required behaviour for clients and KDCs regarding the reserved
bit was specified as follows in section 7.2.2 of [3]:
Each request (KRB_KDC_REQ) and response (KRB_KDC_REP or
KRB_ERROR) sent over the TCP stream is preceded by the
length of the request as 4 octets in network byte order.
The high bit of the length is reserved for future
expansion and MUST currently be set to zero. If a KDC
that does not understand how to interpret a set high bit
of the length encoding receives a request with the high
order bit of the length set, it MUST return a KRB-ERROR
message with the error KRB_ERR_FIELD_TOOLONG and MUST
close the TCP stream.
This document describe how the high bit is used to implement an
expansion mechanism. This expansion mechanism is intended for
features that are specific for the TCP/IP transport.
2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
3. Expansion Mechanism for TCP/IP transport
Kerberos 5 require Key Distribution Centers (KDCs) to accept requests
over TCP. Each request and response is prefixed by 4 octets,
encoding an integer in network byte order, that indicate the length
of the packet. The high bit of the 4 octet length field was reserved
for future expansion. Servers that do not understand how to
interpret a set high bit are required to return a KRB-ERROR with the
KRB_ERR_FIELD_TOOLONG error code, and to close the TCP stream.
We will use the reserved bit to provide an expansion mechanism. When
the reserved high bit is set, the remaining 31 bits of the 4 octets
are treated as an extensible typed hole, and thus form a 31 bit
integer enumerating various expansions. Each of the values indicate
a specific extended operation mode, which are to be described
elsewhere.
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If the KDC do not understand a requested expansion, it MUST return a
KRB-ERROR with a KRB_ERR_FIELD_TOOLONG value (prefixed by the 4 octet
length integer, with the high bit clear, as usual) and close the TCP
stream.
The following table specify the meaning of the 31 lower bits in the 4
octet field, when the high bit is set:
0 RESERVED.
1...2147483647 AVAILABLE for registration, through IANA.
2147483648 RESERVED.
Each expansion mechanism MUST describe the structure of protocol data
beyond the length field, and also the behaviour of the client and
KDC.
4. Security Considerations
Because the initial length field is not protected, it is possible for
an active attacker (i.e., one that is able to modify traffic between
the client and the KDC) to make it appear to the client that the
server do not support this expansion. Client and KDC policies can be
used to reject connections that do not use any expansion.
5. IANA Considerations
IANA needs to create a new registry for "Kerberos 5 TCP/IP
Expansions". The initial contents of this registry should be:
[[RFC Editor: Replace xxxx below with the number of this RFC.]]
Decimal Meaning Reference
------- ------- ---------
0 RESERVED. RFC XXXX
1...2147483647 AVAILABLE for registration.
2147483648 RESERVED. RFC XXXX
IANA will register new values on a First Come First Served basis, as
defined in BCP 64 [2]. Changing the RESERVED values (0 and
2147483648) will require IETF Consensus.
While the registration procedures do not require expert review,
authors of expansion are encouraged to seek community review and
comment whenever that is feasible. Authors may seek community review
by posting a specification of their proposed mechanism as an
Internet-Draft. Expansions intended for widespread use should be
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standardized through the normal IETF process, when appropriate.
6. Copying conditions
The author grants third parties the irrevocable right to copy, use
and distribute this document, with or without modification, in any
medium, without royalty. The author requests that any citation or
excerpt of unmodified text reference this document. If the text is
modified in any way other than translation, any claim of endorsement
by the IETF or status within its document series must be removed.
7. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
[3] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos
Network Authentication Service (V5)", RFC 4120, July 2005.
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Author's Address
Simon Josefsson
Email: simon@josefsson.org
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