Internet Engineering Task Force B. Beser
INTERNET DRAFT Juniper Networks
DHC Working Group P. Duffy (ed.)
Document: draft-ietf-dhc-packetcable-05.txt Cisco Systems
December 2002
DHCP Option for CableLabs Client Configuration
1. 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-
Drafts.
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
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
2. Abstract
This document defines a 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.
3. Table of Contents
1. Status of this Memo..........................................1
2. Abstract.....................................................1
3. Table of Contents............................................1
4. Conventions used in this document............................2
5. Terminology..................................................2
6. Introduction.................................................2
7. CableLabs Client Configuration Option Format.................4
8. CableLabs Client Configuration Option: Sub-Option Definitions 5
8.1. TSP's DHCP Server Address Sub-Options.......................5
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8.2. TSP's Provisioning Server Address Sub-Option................5
8.3. TSP's AS-REQ/AS-REP Backoff and Retry.......................6
8.4. TSP's AP-REQ/AP-REP Backoff and Retry.......................7
8.5. TSP's Kerberos Realm Name Sub-Option........................7
8.6. TSP's Ticket Granting Server Utilization Sub-Option.........8
8.7. TSP's Provisioning Timer Sub-Option.........................8
9. Informational Description of CCC Option Usage................8
10. IANA Considerations..........................................9
11. Legacy Use Information.......................................9
12. Procedure for Adding Additional Sub-options..................9
13. Security Considerations......................................9
14. References..................................................10
15. Acknowledgments.............................................11
16. Author's Addresses..........................................11
17. Full Copyright Statement....................................11
4. 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].
5. 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.
6. 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.
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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 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 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 CM and TSP DHCP servers will
populate this option into DHCP responses. See section 9 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.
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7. CableLabs Client Configuration Option Format
The option begins with a tag octet containing the option code (code
TBD). 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:
+------+--------+--------------+--------------+---+--------------+
| Code | 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] SHOULD 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 |
+---------+---------+--------------------------------------------+
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8. 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.
8.1. 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 |
+-----+-----+-----+-----+-----+-----+
8.2. 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 |
+-----+-----+-----+-----+-----+-----+-----+
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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.
8.3. 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 seconds.
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.
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8.4. 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.
8.5. 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 |
+-----+-----+-----+-----+ +-----+
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8.6. 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.
8.7. 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 and a value between 1 and 30
(minutes, inclusive) MUST be used. If any other value is specified,
the MTA MUST treat the sub-option as non-populated.
Code Len Value
+-----+-----+---------+
| 8 | 1 | (1..30) |
+-----+-----+---------+
9. 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.
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10. IANA Considerations
IANA has assigned a value of TBD for the DHCP option code described
in this document.
11. 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 when IANA issues an official option
number.
12. 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. The initial sub-option codes are described in
sections of this document.
IANA is requested to register codes for future CableLabs Client
Configuration Sub-options with an "Expert Review" approval policy as
described in RFC 2434 [2]. Future proposed sub-options will be
assigned a numeric code chosen by CableLabs, which will be
documented in the Internet Drafts that describe the sub-options. The
code assignment will be reviewed by a designated expert from the
IETF prior to publication in an RFC.
13. 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
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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.
14. References
14.1. Normative
1. S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
2. T. Narten and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", RFC 2434, October 1998.
3. P. Mockapetris, "Domain Names - Implementation and Specification
", RFC 1035, November 1987
4. T. Lemon and S. Cheshire, "Encoding Long Options in the Dynamic
Host Configuration Protocol (DHCPv4)", RFC 3396, November 2002.
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.
14.2. Informational
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
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9. R. Droms and W. Arbaugh, "Authentication for DHCP Messages", RFC
3118, June 2001
15. 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); and Rich Woundy (AT&T Broadband).
16. Author's 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
17. Full Copyright Statement
Copyright (C) The Internet Society (2002). 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 assigns.
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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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