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Network Working Group P. Duffy
Request for Comments: 3594 Cisco Systems
Category: Standards Track September 2003


PacketCable Security Ticket Control Sub-Option
for the DHCP CableLabs Client Configuration (CCC) 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 (2003). All Rights Reserved.

Abstract

This document defines a new sub-option for the DHCP CableLabs Client
Configuration (CCC) Option. This new sub-option will be used to
direct CableLabs Client Devices (CCDs) to invalidate security tickets
stored in CCD non volatile memory (i.e., locally persisted security
tickets).

1. 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 BCP 14, RFC 2119 [2].

2. Terminology

Definitions of terms/acronyms used throughout this document:

CCC - CableLabs Client Configuration option, described in [1].

CCD - CableLabs Client Device. A PacketCable MTA is an example of a
CCD.

STC - Security Ticket Control. The CCC sub-option described in this
document.





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MTA - Media Terminal Adapter. The CCD specific to the PacketCable
architecture.

PacketCable - multimedia architecture developed by CableLabs. See
[8] for full details.

3. Introduction

The CableLabs Client Configuration Option [1] defines several
sub-options used to configure devices deployed into CableLabs
architectures. These architectures implement the PacketCable
Security Specification [4] (based on Kerberos V5 [5]), to support CCD
authentication and establishment of security associations between
CCDs and application servers.

CCDs are permitted to retain security tickets in local persistent
storage. Thus a power-cycled CCD is enabled to avoid expensive
ticket acquisition for locally persisted, non-expired tickets. This
feature greatly reduces the security overhead of a deployment.

This sub-option allows the service provider to control the lifetime
of tickets persisted locally on a CCD. The service provider requires
this capability to support operational functions such as forcing re-
establishment of security associations, remote testing, and remote
diagnostic of CCDs.

It should be noted that, although based on the Kerberos V5 RFC [5],
the PacketCable Security Specification is not a strict implementation
of this RFC. See [4] for details of the PacketCable Security
Specification.

4. Security Ticket Control Sub-option

This sub-option defines a Ticket Control Mask (TCM) that instructs
the CCD to validate/invalidate specific application server tickets.
The sub-option is encoded as follows:

Code Len TCM
+-----+-----+-----+-----+
| 9 | 2 | m1 | m2 |
+-----+-----+-----+-----+

The length MUST be 2. The TCM field is encoded as an unsigned 16 bit
quantity per network byte order. Each bit of the TCM is assigned to
a specific server or server group. A bit value of 0 means the CCD
MUST apply normal invalidation rules (defined in [4]) to the locally





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persisted ticket for the server/server group. A bit value of 1 means
the CCD MUST immediately invalidate the locally persisted ticket for
the server/server group.

Bit #0 is the least significant bit of the field. The bit positions
are assigned as follows:

Bit #0 - the PacketCable Provisioning Server used by the CCD.

Bit #1 - the group of all PacketCable Call Management Servers used
by the CCD.

Bit #2 - #15. Reserved and MUST be set to 0.

If a CCD does not locally store tickets, it MUST ignore this
sub-option. Bit values not known to the CCD MUST be ignored.

5. IANA Considerations

IANA has assigned a sub-option code to this sub-option from the
"CableLabs Client Configuration" sub-option number space (maintained
within the BOOTP-DHCP Parameters Registry).

IANA has also set-up a new registry and will maintain a new number
space of "CableLabs Client Configuration Option Ticket Control Mask
Bit Definitions", located in the BOOTP-DHCP Parameters Registry. The
initial bit definitions are described in section 4 of this document.
IANA will register future bit mask definitions via an "IETF
Consensus" approval policy as described in RFC 2434 [3].

6. Security Considerations

Potential DHCP protocol attack exposure is discussed in section 7 of
the DHCP protocol specification [6] and in Authentication for DHCP
Messages [7]. Additional CCC attack exposure is discussed in [1].

The STC sub-option could be used to disrupt a CableLabs architecture
deployment. In the specific case of PacketCable [8], a deployment
could be disrupted if a large number of MTAs are reset/power cycled,
initiate their provisioning flow [9], and are instructed by a
malicious DHCP server to invalidate all security tickets. This could
lead to a Denial of Service (DoS) condition as this large set of MTAs
simultaneously attempt to authenticate and obtain tickets from the
security infrastructure.

However, the scenario described above is unlikely to occur. Within
the cable delivery architecture required by the various CableLabs
projects, the DHCP client is connected to a network through a cable



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modem and the CMTS (head-end router). The CMTS is explicitly
configured with a set of valid DHCP server addresses to which DHCP
requests are forwarded. Further, a correctly configured CMTS will
only allow DHCP downstream traffic from specific DHCP server
addresses.

It should be noted that the downstream filtering of DHCP packets will
not prevent spoofed DHCP servers behind the CMTS, but the network
infrastructure behind the CMTS is assumed to be closely controlled by
the service provider.

7. Intellectual Property Statement

The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.



















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8. References

8.1. Normative

[1] Beser, B. and P. Duffy, "DHCP Option for CableLabs Client
Configuration", RFC 3495, March 2003.

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

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

[4] "PacketCable Security Specification", PKT-SP-SEC-I09-030728,
http://www.packetcable.com/downloads/specs/
PKT-SP-SEC-I09-030728.pdf

8.2. Informative

[5] Kohl, J. and C. Neuman, "The Kerberos Network Authentication
Service (V5)", RFC 1510, September 1993.

[6] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March
1997.

[7] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages", RFC
3118, June 2001

[8] "PacketCable 1.0 Architecture Framework Technical Report",
PKT-TR-ARCH-V01-991201,
http://www.packetcable.com/downloads/specs/
pkt-tr-arch-v01-991201.pdf

[9] "PacketCable MTA Device Provisioning Specification",
PKT-SP-PROV-I07-030728,
http://www.packetcable.com/downloads/specs/
PKT-SP-PROV-I07-030728.pdf














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9. Acknowledgments

The author would like to acknowledge the effort of all those who
contributed to the development of the PacketCable Provisioning
specifications:

Sumanth Channabasappa (Alopa Networks); Angela Lyda, Rick Morris,
Rodney Osborne (Arris Interactive); Steven Bellovin and Chris
Melle (AT&T); Eugene Nechamkin (Broadcom); John Berg, Maria
Stachelek, Matt Osman, Venkatesh Sunkad (CableLabs); Klaus
Hermanns, Azita Kia, Michael Thomas, Paul Duffy (Cisco); Deepak
Patil (Com21); Jeff Ollis, Rick Vetter (General
Instrument/Motorola); Roger Loots, David Walters (Lucent); Peter
Bates (Telcordia); Patrick Meehan (Tellabs); Satish Kumar, Itay
Sherman, Roy Spitzer (Telogy/TI), Aviv Goren (Terayon);
Prithivraj Narayanan (Wipro), and Burcak Beser (Juniper
Networks).

10. Author's Address

Paul Duffy
Cisco Systems
1414 Massachusetts Avenue
Boxborough, MA 01719

EMail: paduffy@cisco.com

























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11. Full Copyright Statement

Copyright (C) The Internet Society (2003). 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 assignees.

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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