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Versions: 00 01 02 RFC 3594

   Internet Engineering Task Force                 P. Duffy
   INTERNET DRAFT                                  Cisco Systems
   DHC Working Group                               May 2003
   Document: draft-ietf-dhc-pktc-kerb-tckt-02.txt
              PacketCable Security 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.
      Internet-Drafts are working documents of the Internet Engineering
      Task Force (IETF), its areas, and its working groups.  Note that
      other groups may also distribute working documents as Internet-
      Internet-Drafts are draft documents valid for a maximum of six
      months and may be updated, replaced, or obsoleted by other
      documents at any time.  It is inappropriate to use Internet-
      Drafts as reference material or to cite them other than as "work
      in progress."
      The list of current Internet-Drafts can be accessed at
      The list of Internet-Draft Shadow Directories can be accessed at
   Copyright Notice
      Copyright (C) The Internet Society (2003).  All Rights Reserved.
      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 security tickets.
   1.   Conventions used in this document
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      The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
      "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.
      STC - Security Ticket Control.  The CCC sub-option described in
      this document.
      MTA - Media Terminal Adapter. The CCD specific to the PacketCable
      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.
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      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
      | TBD |  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 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-
   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).
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      IANA is also requested to 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 is requested to 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 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.   References
   7.1.  Normative
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      [1] B. Beser and P. Duffy, "DHCP Option for CableLabs Client
      Configuration", RFC 3495, March 2003.
      [2] S. Bradner, "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.
      [3] T. Narten and H. Alvestrand, "Guidelines for Writing an IANA
      Considerations Section in RFCs", RFC 2434, October 1998.
      [4] "PacketCable Security Specification", PKT-SP-SEC-I08-030415,
   7.2.  Informational
      [5] J. Kohl and C. Neuman, "The Kerberos Network Authentication
      Service (V5)", RFC 1510, September 1993.
      [6] R. Droms, "Dynamic Host Configuration Protocol", RFC 2131,
      March 1997.
      [7] R. Droms and W. Arbaugh, "Authentication for DHCP Messages",
      RFC 3118, June 2001
      [8] "PacketCable Architecture Framework Technical Report", PKT-
      [9] "PacketCable MTA Device Provisioning Specification", PKT-SP-
      PROV-I06-030415.  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
      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
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      Bates (Telcordia); Patrick Meehan (Tellabs); Satish Kumar, Itay
      Sherman, Roy Spitzer (Telogy/TI), Aviv Goren (Terayon);
      Prithivraj Narayanan (Wipro), and Burcak Beser (Juniper
   9.   Author's Addresses
      Paul Duffy
      Cisco Systems
      300 Apollo Drive
      Chelmsford, MA, 01824
      Email: paduffy@cisco.com
   10.  Full Copyright Statement
      Copyright (C) The Internet Society (2003).  All Rights Reserved.
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      The limited permissions granted above are perpetual and will not
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      This document and the information contained herein is provided on
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      Funding for the RFC Editor function is currently provided by the
      Internet Society.
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