Internet Engineering Task Force P. Duffy
INTERNET DRAFT Cisco Systems
DHC Working Group January 2003
Document: draft-ietf-dhc-pktc-kerb-tckt-00.txt
Kerberos Ticket Control Sub-option
for the CableLabs Client Configuration Option.
Status of this Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC 2026.
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document defines a new sub-option for the CableLabs Client
Configuration (CCC) Option. This new sub-option will be used to
direct CableLabs Client Devices (CCDs) to invalidate locally
persisted Kerberos tickets.
1. Conventions used in this document
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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 [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.
KTC - Kerberos Ticket Control. The CCC sub-option described in
this document.
MTA - Media Terminal Adapter. The CCD specific to the PacketCable
architecture.
PacketCable - multimedia architecture developed by CableLabs. See
[6] 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 employ the Kerberos protocol
[3] to support CCD authentication and establishment of security
associations between CCDs and application servers.
CCDs are permitted to locally persist Kerberos tickets. Thus a
power-cycled CCD is enabled to avoid expensive ticket acquisition
for locally persisted, non-expired tickets. This feature greatly
reduces the Kerberos overhead of a deployment.
This sub-option allows the service provider to control the
lifetime of Kerberos tickets persisted locally on a CCD. The
service provider requires this capability to support operational
functions such as disabling a subscriber's service, forcing re-
establishment of security associations, or for testing and remote
diagnostic of CCDs.
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4. Kerberos 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
+-----+-----+-----+-----+
| TBD | 2 | m1 | m2 |
+-----+-----+-----+-----+
The length MUST be 2. The TCM field is encoded as an unsigned 16
bit quantity per network byte-ordering rules. Each bit of the TCM
is assigned to a specific server or server group. A bit value of 0
means the CCD MUST NOT invalidate the locally persisted ticket for
the server/server group. A bit value of 1 means the CCD MUST
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 persist Kerberos tickets, it MUST
ignore this sub-option.
5. IANA Considerations
IANA is requested to assign a sub-option code to this sub-option
from the "CableLabs Client Configuration" sub-option number space
(maintained within the BOOTP-DHCP Parameters Registry).
6. Security Considerations
Potential DHCP protocol attack exposure is discussed in section 7
of the DHCP protocol specification [4] and in Authentication for
DHCP Messages [5]. Additional CCC attack exposure is discussed
in [1].
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The KTC sub-option could be used to disrupt a CableLabs
architecture deployment. In the specific case of PacketCable
[6], a deployment could be disrupted if a large number of MTAs
are reset/power cycled, initiate their provisioning flow [7], and
are instructed by a malicious DHCP server to invalidate all
Kerberos 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 Kerberos infrastructure.
However, the scenario described above is unlikely to occur.
Within the cable delivery architecture required by PacketCable
(and other CableLabs architectures), the DHCP client is connected
to a network through a cable modem and the CMTS (head-end). The
CMTS is explicitly configured with a set of DHCP servers to which
DHCP requests are forwarded. Further, a correctly configured
CMTS will only allow downstream traffic from specific IP
addresses/ranges.
7. References
7.1. Normative
[1] B. Beser and P. Duffy, "DHCP Option for CableLabs Client
Configuration", http://www.ietf.org/internet-drafts/draft-ietf-
dhc-packetcable-06.txt, January 2003.
[2] S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
7.2. Informational
[3] J. Kohl and C. Neuman, "The Kerberos Network Authentication
Service (V5)", RFC 1510, September 1993.
[4] R. Droms, "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[5] R. Droms and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001
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[6] "PacketCable Architecture Framework Technical Report", PKT-
TR-ARCH-V01-991201,
http://www.packetcable.com/specifications.html
[7] "PacketCable MTA Device Provisioning Specification", PKT-SP-
PROV-I05-021127. http://www.packetcable.com/specifications.html
8. 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 (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).
9. Author's Addresses
Paul Duffy
Cisco Systems
300 Apollo Drive
Chelmsford, MA, 01824
Email: paduffy@cisco.com
10. Full Copyright Statement
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