DHCP CableLabs option: Dynamic Host Configuration Protocol (DHCP) Option for CableLabs Client Configuration

Network Working Group                                           B. Beser
Request for Comments: 3495                              Juniper Networks
Category: Standards Track                                  P. Duffy, Ed.
                                                           Cisco Systems
                                                              March 2003


           Dynamic Host Configuration Protocol (DHCP) Option
                  for CableLabs Client Configuration

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 Dynamic Host Configuration Protocol (DHCP)
   option that will be used to configure various devices deployed within
   CableLabs architectures.  Specifically, the document describes DHCP
   option content that will be used to configure one class of CableLabs
   client device: a PacketCable Media Terminal Adapter (MTA).  The
   option content defined within this document will be extended as
   future CableLabs client devices are developed.

Table of Contents

   1.  Conventions used in this document............................  2
   2.  Terminology..................................................  2
   3.  Introduction.................................................  3
   4.  CableLabs Client Configuration Option Format.................  4
   5.  CableLabs Client Configuration Option: Sub-Option Definitions  5
       5.1.  TSP's DHCP Server Address Sub-Options..................  5
       5.2.  TSP's Provisioning Server Address Sub-Option...........  6
       5.3.  TSP's AS-REQ/AS-REP Backoff and Retry..................  6
       5.4.  TSP's AP-REQ/AP-REP Backoff and Retry..................  7
       5.5.  TSP's Kerberos Realm Name Sub-Option...................  8
       5.6.  TSP's Ticket Granting Server Utilization Sub-Option....  8
       5.7.  TSP's Provisioning Timer Sub-Option....................  8
   6.  Informational Description of CCC Option Usage................  9
   7.  IANA Considerations..........................................  9
   8.  Legacy Use Information.......................................  9
   9.  Procedure for Adding Additional Sub-options..................  9
   10. Security Considerations...................................... 10
   11. References................................................... 10
       11.1. Normative References................................... 10
       11.2. Informative References................................. 11
   12. Acknowledgments.............................................. 11
   13. Authors' Addresses........................................... 12
   14. Full Copyright Statement..................................... 13

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 [1].

2.  Terminology

   Definitions of terms used throughout this document:

      *  "Telephony Service Provider" or "TSP"

   The business entity from which a subscriber receives telephony
   service.

   See RFC 2131 [6] for additional DHCP terminology.

3.  Introduction

   Cable Television Laboratories, Inc. (CableLabs) is a non-profit
   research and development consortium that serves the cable television
   industry via design and specification of new and emerging broadband
   service architectures.  Several CableLabs initiatives define DHCP
   clients that have specific DHCP configuration requirements.  One such
   initiative is the PacketCable project.

   The PacketCable project is aimed at architecting, qualifying, and
   supporting Internet-based multimedia services over cable-based packet
   networks.  These new multimedia services will include telephony and
   videoconferencing, delivered using the basic Internet Protocol (IP)
   technology that is used to send data via the Internet.

   PacketCable 1.0 provides Voice over IP (VoIP) service delivery.  The
   VoIP service is supported at the customer site by two key components:
   a Cable Modem (CM) and a Media Terminal Adapter (MTA).  The CM
   converts the cable RF signals to/from various IP voice protocols,
   while the MTA converts the VoIP protocols into analog telephony
   compatible with a common telephone.

   The CM and MTA may be packaged together or separately.  If packaged
   together, the unit is referred to as an Embedded-MTA (EMTA - depicted
   in Figure 1).  If packaged separately, the MTA is referred to as a
   Standalone MTA (SMTA).

             |----------------------------------------------|
             |                                              |
             |   |-----------|           |-------------|    |
             |   |           |           |             |    |
   Telephony |   |  Media    | internal  |   Cable     |    | RF Link
   ----------|---| Terminal  |===========|   Modem     |----|-------
   Link      |   | Adapter   | connection|             |    |
             |   |-----------|           |-------------|    |
             |                                              |
             |----------------------------------------------|

                  Figure 1. PacketCable 1.0 Embedded-MTA

   The CM and MTA are distinct IP devices: each has its own MAC address
   and IP configuration.  The CM and MTA utilize the DHCP protocol to
   obtain IP configuration.  It is assumed that the CM and MTA may be
   administered by different business entities.  The CM communicates
   with and is configured by the Data Access Provider's (DAP's) DHCP
   servers.  Likewise, the MTA communicates with and is configured by
   the Telephony Service Provider's (TSP's) DHCP servers.

   The PacketCable architecture requires that the business entity
   controlling the configuration of the CM also determines which
   business entities control the configuration of the MTA.  This is
   similar to the example found in the PSTN system: individuals can pick
   their long distance carriers even though the ultimate control of
   their telephone remains with the local carrier.

   Due to specific needs of the MTA configuration process (described in
   [7]), a new CableLabs Client Configuration (CCC) option is needed for
   the DHCP protocol.  Both CM and MTA DHCP clients will request this
   option.  When requested, both the DAP and TSP DHCP servers will
   populate this option into DHCP responses.  See section 6 for further
   operational details.

   It should be noted that, although the CCC option will be initially
   deployed to support PacketCable VOIP applications, the CCC option
   will also be used to support various non VOIP applications.  Use of
   the CCC option does not necessarily mean that the service provider is
   a TSP.

4.  CableLabs Client Configuration Option Format

   The option begins with a tag octet containing the option code (code
   122).  A length octet follows the tag octet.  The value of the length
   octet does not include itself or the tag octet.  The length octet is
   followed by "length" octets of sub-option content (total length, not
   sub-option count).  The option layout is depicted below:

      +------+--------+--------------+--------------+---+--------------+
      | 122  | Length | Sub-option 1 | Sub-option 2 |...| Sub-option n |
      +------+--------+--------------+--------------+---+--------------+

   When the total length of a CCC option exceeds 255 octets, the
   procedure outlined in [4] MUST be employed to split the option into
   multiple, smaller options.

   A sub-option begins with a tag octet containing the sub-option code.
   A length octet follows the tag octet.  The value of the length octet
   does not include itself or the tag octet.  The length octet is
   followed by "length" octets of sub-option information.  The sub-
   option layout is depicted below:

      +-------------------+--------+------------------------+
      | Sub-option Code   | Length | Sub-option information |
      +-------------------+--------+------------------------+
   The sub-option codes are summarized below.

      +---------+---------+--------------------------------------------+
      |  Sub-   | Sent to | Description                                |
      | option  |         |                                            |
      |  Code   |         |                                            |
      +===================+============================================+
      |    1    |  CM     | TSP's Primary DHCP Server Address          |
      +---------+---------+--------------------------------------------+
      |    2    |  CM     | TSP's Secondary DHCP Server Address        |
      +---------+---------+--------------------------------------------+
      |    3    |  MTA    | TSP's Provisioning Server Address          |
      +---------+---------+--------------------------------------------+
      |    4    |  MTA    | TSP's AS-REQ/AS-REP Backoff and Retry      |
      +---------+---------+--------------------------------------------+
      |    5    |  MTA    | TSP's AP-REQ/AP-REP Backoff and Retry      |
      +---------+---------+--------------------------------------------+
      |    6    |  MTA    | TSP's Kerberos Realm Name                  |
      +---------+---------+--------------------------------------------+
      |    7    |  MTA    | TSP's Ticket Granting Server Utilization   |
      +---------+---------+--------------------------------------------+
      |    8    |  MTA    | TSP's Provisioning Timer Value             |
      +---------+---------+--------------------------------------------+
      | 9 - 255 |         | Reserved for future extensions             |
      +---------+---------+--------------------------------------------+

5.  CableLabs Client Configuration Option: Sub-Option Definitions

   The following sections provide detailed descriptions of each sub-
   option.  There are a few general formatting rules:

   -  Fully Qualified Domain Names (FQDNs) MUST be encoded per RFC 1035
      [3] section 3.1.  Note that a terminating 0 is required.  Also
      note that compression, as described in RFC 1035 [3] section 4.1.4,
      MUST NOT be applied.

   -  IPv4 addresses MUST be encoded as 4 binary octets in network
      byte-order (high order byte first).

   -  All multi-octet quantities MUST be encoded per network byte-
      ordering.

5.1. TSP's DHCP Server Address Sub-Options

   The TSP DHCP Server Address sub-options identify the DHCP servers
   from which an MTA is permitted to accept a DHCP OFFER.  Sub-option 1
   is the address of the TSP's primary DHCP server.  Sub-option 2 is the
   address of the TSP's secondary DHCP server.

   The sub-option length MUST be 4 and the sub-option MUST include the
   DHCP server's IPv4 address as follows:

        Code  Len          Address
      +-----+-----+-----+-----+-----+-----+
      | 1/2 |  4  |  a1 |  a2 |  a3 |  a4 |
      +-----+-----+-----+-----+-----+-----+

5.2. TSP's Provisioning Server Address Sub-Option

   This option contains the address of the TSP's Provisioning server.
   MTAs communicate with the Provisioning server at various stages in
   their provisioning process.

   The address can be configured as either an IPv4 address or as an
   FQDN.  The encoding of sub-option 3 will adhere to one of 2 formats.

   1. IPv4 address.  The sub-option length MUST be 5.  The length octet
      MUST be followed by a single octet that indicates the specific
      address type that follows.  This type octet MUST be set to 1 to
      indicate an IPv4 address.  The type octet MUST be followed by 4
      octets of IPv4 address:

       Code   Len   Type        Address
      +-----+-----+-----+-----+-----+-----+-----+
      |  3  |  5  |  1  |  a1 |  a2 |  a3 |  a4 |
      +-----+-----+-----+-----+-----+-----+-----+

   2. FQDN.  The length octet MUST be followed by a single octet that
      indicates the specific address type that follows.  This type octet
      MUST be set to 0 to indicate an FQDN.  The type octet MUST be
      followed by the encoded FQDN:

       Code   Len   Type            FQDN
      +-----+-----+-----+-----+-----+   +-----+
      |  3  | n+1 |  0  |  f1 |  f2 |...|  fn |
      +-----+-----+-----+-----+-----+   +-----+

   It is not anticipated that additional type codes, beyond IPv4 (1) and
   FQDN (0), will be required.  Thus, IANA will not be required to
   maintain a registry of type codes.

5.3. TSP's AS-REQ/AS-REP Backoff and Retry

   This sub-option configures an MTA's Kerberos AS-REQ/AS-REP timeout,
   backoff, and retry mechanism.

   RFC 1510 [5] does not define a backoff/retry mechanism to be employed
   when an AS-REQ/AS-REP message exchange fails.  This sub-option
   contains parameters required by the backoff/retry mechanism outlined
   in [8].

   The encoding of this sub-option is depicted below:

      Code Len   Nom Timeout     Max Timeout     Max Retries
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      | 4 |12 |n1 |n2 |n3 |n4 |m1 |m2 |m3 |m4 |r1 |r2 |r3 |r4 |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+

   The length octet of this sub-option MUST contain the value 12.

   The length octet MUST be followed by 4 octets containing the AS-
   REQ/AS-REP nominal (initial) timeout value.  This value is a 32 bit
   unsigned quantity in units of milliseconds.

   The next 4 octets MUST contain the AS-REQ/AS-REP maximum timeout
   value.  This value is a 32 bit unsigned quantity in units of seconds.

   The final 4 octets MUST contain the AS-REQ/AS-REP maximum retry
   count.  This value is a 32 bit unsigned quantity.

5.4. TSP's AP-REQ/AP-REP Backoff and Retry

   This sub-option configures an MTA's Kerberos AP-REQ/AP-REP timeout,
   backoff, and retry mechanism.

   RFC 1510 [5] does not define a backoff/retry mechanism to be employed
   when an AP-REQ/AP-REP message exchange fails.  This sub-option
   contains parameters required by the backoff/retry mechanism outlined
   in [8].

   The encoding of this sub-option is depicted below:

      Code Len   Nom Timeout     Max Timeout     Max Retries
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+
      | 5 |12 |n1 |n2 |n3 |n4 |m1 |m2 |m3 |m4 |r1 |r2 |r3 |r4 |
      +---+---+---+---+---+---+---+---+---+---+---+---+---+---+

   The length octet of this sub-option MUST contain the value 12.

   The length octet MUST be followed by 4 octets containing the AP-
   REQ/AP-REP nominal (initial) timeout value.  This value is a 32 bit
   unsigned quantity in units of seconds.

   The next 4 octets MUST contain the AP-REQ/AP-REP maximum timeout
   value.  This value is a 32 bit unsigned quantity in units of seconds.

   The final 4 octets MUST contain the AP-REQ/AP-REP maximum retry
   count.  This value is a 32 bit unsigned quantity.

5.5. TSP's Kerberos Realm Name Sub-Option

   The PacketCable architecture requires an MTA to authenticate itself
   to the TSP's network via the Kerberos protocol.  A Kerberos Realm
   name is required at the MTA to permit a DNS lookup for the address of
   the TSP's Kerberos Key Distribution Center (KDC) entity.

   The Kerberos Realm name MUST be encoded per the domain style realm
   name described in RFC 1510 [5].  This realm name MUST be all capital
   letters and conform to the syntax described in RFC 1035 [3] section
   3.1.  The sub-option is encoded as follows:

       Code   Len   Kerberos Realm Name
      +-----+-----+-----+-----+   +-----+
      |  6  |  n  |  k1 |  k2 |...|  kn |
      +-----+-----+-----+-----+   +-----+

5.6. TSP's Ticket Granting Server Utilization Sub-Option

   This sub-option encodes a boolean value which indicates whether an
   MTA should or should not utilize a TGT (Ticket Granting Ticket) when
   obtaining a service ticket for one of the PacketCable application
   servers.  The encoding is as follows:

       Code   Len   Value
      +-----+-----+-----+
      |  7  |  1  | 1/0 |
      +-----+-----+-----+

   The length MUST be 1.  The last octet contains a Boolean value which
   MUST be either 0 or 1.  A value of 1 MUST be interpreted as true.  A
   value of 0 MUST be interpreted as false.

5.7. TSP's Provisioning Timer Sub-Option

   The provisioning timer defines the maximum time allowed for the MTA
   provisioning process to complete.  If this timer expires before the
   MTA has completed the provisioning process, the MTA should reset the
   timer and re-start its provisioning process from the beginning.

   The sub-option length MUST be 1.  The value octet specifies 0 to 255
   minutes.  A value of 0 means the timer MUST be disabled.

       Code   Len    Value
      +-----+-----+---------+
      |  8  |  1  | (0..255)|
      +-----+-----+---------+

6.  Informational Description of CCC Option Usage.

   Cablelabs client devices issue DHCP requests that include DHCP
   options 55 (Parameter Request List) and 60 (Vendor Class Identifier).
   Option 55 will request the CCC option from the DHCP server.  Option
   60 will indicate the specific Cablelabs client device type, thus
   directing the DHCP server to populate specific CCC sub-option content
   in its responses.  The details of which CCC sub-options are populated
   for each specific client type are specified in various Cablelabs
   project specifications.  For example, specific usage of the CCC
   option for the PacketCable project is described in [7].

   Note that client devices never populate the CCC option in their DHCP
   requests.

7.  IANA Considerations

   IANA has assigned a value of 122 for the DHCP option code described
   in this document.

8.  Legacy Use Information

   The CableLabs Client Configuration option initially used the site-
   specific option value of 177 (0xB1).  The use of the site-specific
   option is to be deprecated now that IANA has issued an official
   option number.

9.  Procedure for Adding Additional Sub-options

   IANA is requested to maintain a new number space of "CableLabs Client
   Configuration Sub-options", located in the BOOTP-DHCP Parameters
   Registry (http://www.iana.org/assignments/bootp-dhcp-parameters).
   The initial sub-option codes are described in section 4 of this
   document.

   IANA is requested to register codes for future CableLabs Client
   Configuration Sub-options via an "IETF Consensus" approval policy as
   described in RFC 2434 [2].

10.  Security Considerations

   Potential exposures to attack in the DHCP protocol are discussed in
   section 7 of the DHCP protocol specification [6] and in
   Authentication for DHCP Messages [9].

   The CCC option can be used to misdirect network traffic by providing
   incorrect DHCP server addresses, incorrect provisioning server
   addresses, and incorrect Kerberos realm names to a Cablelabs client
   device.  This misdirection can lead to several threat scenarios.  A
   Denial of Service (DoS) attack can result from address information
   being simply invalid.  A man-in-the-middle attack can be mounted by
   providing addresses to a potential snooper.  A malicious TSP can
   steal customers from the customer selected TSP, by altering the
   Kerberos realm designation.

   These threats are mitigated by several factors.

   Within the cable delivery architecture required by PacketCable, 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.

   Assuming that server addresses and Kerberos realm name were
   successfully spoofed to the point that a malicious client device was
   able to contact a KDC, the client device must still present valid
   certificates to the KDC before being service enabled.  Given the
   computational overhead of the certificate validation process, this
   situation could present a DoS opportunity.

   Finally, it is possible for a malicious (although certified) TSP to
   redirect a customer from the customer's selected TSP.  It is assumed
   that all TSP's permitted onto an access providers network are trusted
   entities that will cooperate to insure peaceful coexistence.  If a
   TSP is found to be redirecting customers, this should be handled as
   an administrative matter between the access provider and the TSP.

11.  References

11.1. Normative References

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

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

   [3] Mockapetris, P., "Domain Names - Implementation and
       Specification", STD 13, RFC 1035, November 1987.

   [4] Lemon, T. and S. Cheshire, "Encoding Long Options in the Dynamic
       Host Configuration Protocol (DHCPv4)", RFC 3396, November 2002.

   [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.

11.2. Informative References

   [7] "PacketCable MTA Device Provisioning Specification", PKT-SP-
       PROV-I05-021127.  http://www.packetcable.com/specifications.html

   [8] "PacketCable Security Specification", PKT-SP-SEC-I07-021127,
       http://www.packetcable.com/specifications.html

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

12.  Acknowledgments

   The authors would like to extend a heartfelt thanks to 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).

   The authors would also like to extend a special "thank you" to Rich
   Woundy (Comcast) for his thoughtful inputs.

13.  Authors' Addresses

   Burcak Beser
   Juniper Networks
   1194 North Matilda Avenue
   Sunnyvale, CA, 94089

   EMail: burcak@juniper.net


   Paul Duffy
   Cisco Systems
   300 Apollo Drive
   Chelmsford, MA, 01824

   EMail: paduffy@cisco.com

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