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Network Working Group T. Lemon Request for Comments: 3396 Nominum, Inc. Updates: 2131 S. Cheshire Category: Standards Track Apple Computer, Inc.

                                                         November 2002
                       Encoding Long Options
        in the Dynamic Host Configuration Protocol (DHCPv4)

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 (2002).  All Rights Reserved.


 This document specifies the processing rules for Dynamic Host
 Configuration Protocol (DHCPv4) options that appear multiple times in
 the same message.  Multiple instances of the same option are
 generated when an option exceeds 255 octets in size (the maximum size
 of a single option) or when an option needs to be split apart in
 order to take advantage of DHCP option overloading.  When multiple
 instances of the same option appear in the options, file and/or sname
 fields in a DHCP packet, the contents of these options are
 concatenated together to form a single option prior to processing.

1. Introduction

 This document updates RFC 2131 [3] by clarifying the rules for option
 concatenation specified in section 4.1.  It is expected that the
 reader will be familiar with this portion of RFC 2131.  The text in
 section 4.1 that reads "Options may appear only once, unless
 otherwise specified in the options document."  should be considered
 as deleted.
 The DHCP protocol [3] specifies objects called "options" that are
 encoded in the DHCPv4 packet to pass information between DHCP
 protocol agents.  These options are encoded as a one-byte type code,
 a one-byte length, and a buffer consisting of the number of bytes
 specified in the length, from zero to 255.

Lemon & Cheshire Standards Track [Page 1] RFC 3396 Encoding Long Options in DHCPv4 November 2002

 However, in some cases it may be useful to send options that are
 longer than 255 bytes.  RFC 2131 [3] specifies that when more than
 one option with a given type code appears in the DHCP packet, all
 such options should be concatenated together.  It does not, however,
 specify the order in which this concatenation should occur.
 We specify here the ordering that MUST be used by DHCP protocol
 agents when sending options with more than 255 bytes.  This method
 also MUST be used for splitting options that are shorter than 255
 bytes, if for some reason the encoding agent needs to do so.  DHCP
 protocol agents MUST use this method whenever they receive a DHCP
 packet containing more than one occurrence of a certain type of

2. Terminology

    Throughout this document, the acronym "DHCP" is used to refer to
    the Dynamic Host Configuration Protocol as specified in RFC 2131
    [3] and RFC 2132 [4].
    We have used the term "DHCPv4" in the abstract for this document
    to distinguish between the DHCP protocol for IPv4 as defined in
    RFC 2131 and RFC 2132 and the DHCP protocol for IPv6, which, at
    the time that this document was written, was still under
 DHCP protocol agents
    This refers to any device on the network that sends or receives
    DHCP packets - any DHCP client, server or relay agent.  The nature
    of these devices is not important to this specification.
 Encoding agent
    The DHCP protocol agent that is composing a DHCP packet to send.
 Decoding agent
    The DHCP protocol agent that is processing a DHCP packet it has
    DHCP options are collections of data with type codes that indicate
    how the options should be used.  Options can specify information
    that is required for the DHCP protocol, IP stack configuration
    parameters for the client, information allowing the client to
    rendezvous with DHCP servers, and so on.

Lemon & Cheshire Standards Track [Page 2] RFC 3396 Encoding Long Options in DHCPv4 November 2002

 Option overload
    The DHCP packet format is based on the BOOTP packet format defined
    in RFC 951 [1].  When used by DHCP protocol agents, BOOTP packets
    have three fields that can contain options.  These are the
    optional parameters field, the sname field, and the filename
    field.  The DHCP options specification [4] defines the DHCP
    Overload option, which specifies which of these three fields is
    actually being used in any given DHCP message to store DHCP

3. Requirements Language

 In this document, the key words "MAY", "MUST, "MUST NOT", "OPTIONAL",
 "RECOMMENDED", "SHOULD", and "SHOULD NOT", are to be interpreted as
 described in BCP 14, RFC 2119 [2].

4. Applicability

 This specification applies when a DHCP agent is encoding a packet
 containing options, where some of those options must be broken into
 parts.  This need can occur for two reasons.  First, it can occur
 because the value of an option that needs to be sent is longer than
 255 bytes.  In this case, the encoding agent MUST follow the
 algorithm specified here.  It can also occur because there is not
 sufficient space in the current output buffer to store the option,
 but there is space for part of the option, and there is space in
 another output buffer for the rest.  In this case, the encoding agent
 MUST either use this algorithm or not send the option at all.
 This specification also applies in any case where a DHCP protocol
 agent has received a DHCP packet that contains more than one instance
 of an option of a given type.  In this case, the agent MUST
 concatenate these separate instances of the same option in the way
 that we specify here.
 This option updates the Dynamic Host Configuration Protocol [3] and
 DHCP Options and BOOTP vendor extensions [4] documents.  However,
 because many currently-deployed DHCP protocol agents do not implement
 option concatenation, DHCP protocol agents should be careful not to
 transmit split options unless either it will not matter if the
 recipient cannot correctly reassemble the options, or it is certain
 that the recipient implements concatenation.
 Let us divide all DHCP options into two categories - those that, by
 definition, require implementation of the mechanisms defined in this
 document, and those that do not.  We will refer to the former as
 concatenation-requiring options, and the latter as non-
 concatenation-requiring options.  In order for an option to be a

Lemon & Cheshire Standards Track [Page 3] RFC 3396 Encoding Long Options in DHCPv4 November 2002

 concatenation-requiring option, the protocol specification that
 defines that option must require implementation of option splitting
 and option concatenation as described in this document, by
 specifically referencing this document.
 A DHCP protocol agent SHOULD NOT split an option as described in this
 document unless it has no choice, or it knows that its peer can
 properly handle split options.  A peer is assumed to properly handle
 split options if it has provided or requested at least one
 concatenation-requiring option.  Alternatively, the administrator of
 the agent generating the option can specifically configure the agent
 to assume that the recipient can correctly concatenate options split
 as described in this document.
 Some implementors may find it easiest to only split concatenation-
 requiring options, and never split non-concatenation-requiring
 options.  This is permissible.  However, an implementation which
 supports any concatenation-requiring option MUST be capable of
 concatenating received options for both concatenation-requiring and
 non-concatenation-requiring options.
 No restrictions apply to option concatenation when a DHCP agent
 receives a DHCP message.  Any DHCP protocol agent that implements the
 mechanisms described in this document can assume that when it
 receives two options of the same type, it should concatenate them.

5. The Aggregate Option Buffer

 DHCP options can be stored in the DHCP packet in three separate
 portions of the packet.  These are the optional parameters field, the
 sname field, and the file field, as described in RFC 2131 [3].  This
 complicates the description of the option splitting mechanism because
 there are three separate fields into which split options may be
 To further complicate matters, an option that doesn't fit into one
 field can't overlap the boundary into another field - the encoding
 agent must instead break the option into two parts and store one part
 in each buffer.
 To simplify this discussion, we will talk about an aggregate option
 buffer, which will be the aggregate of the three buffers.  This is a
 logical aggregation - the buffers MUST appear in the locations in the
 DHCP packet described in RFC 2131 [3].
 The aggregate option buffer is made up of the optional parameters
 field, the file field, and the sname field, in that order.

Lemon & Cheshire Standards Track [Page 4] RFC 3396 Encoding Long Options in DHCPv4 November 2002

 WARNING: This is not the physical ordering of these fields in the
 DHCP packet.
 Options MUST NOT be stored in the aggregate option buffer in such a
 way that they cross either boundary between the three fields in the
 aggregate buffer.
 The encoding agent is free to choose to use either or both the sname
 field and file field.  If the encoding agent does not choose to use
 either or both of these two fields, then they MUST NOT be considered
 part of the aggregate option buffer in that case.

6. Encoding Agent Behavior

 Encoding agents decide to split options based on the reasons we have
 described in the preceding section entitled "applicability".
 Options can be split on any octet boundary.  No split portion of an
 option that has been split can contain more than 255 octets.  The
 split portions of the option MUST be stored in the aggregate option
 buffer in sequential order - the first split portion MUST be stored
 first in the aggregate option buffer, then the second portion, and so
 on.  The encoding agent MUST NOT attempt to specify any semantic
 information based on how the option is split.
 Note that because the aggregate option buffer does not represent the
 physical ordering of the DHCP packet, if an option were split into
 three parts and each part went into one of the possible option
 fields, the first part would go into the optional parameters field,
 the second part would go into the file field, and the third part
 would go into the sname field.  This maintains consistency with
 section 4.1 of RFC 2131 [3].
 Each split portion of an option MUST be stored in the aggregate
 option buffer as if it were a normal variable-length option as
 described in RFC 2132 [4].  The length fields of each split portion
 of the option MUST add up to the total length of the option data.
 For any given option being split, the option code field in each split
 portion MUST be the same.

7. Decoding Agent Behavior

 When a decoding agent is scanning an incoming DHCP packet's option
 buffer and finds two or more options with the same option code, it
 MUST consider them to be split portions of an option as described in
 the preceding section.

Lemon & Cheshire Standards Track [Page 5] RFC 3396 Encoding Long Options in DHCPv4 November 2002

 In the case that a decoding agent finds a split option, it MUST treat
 the contents of that option as a single option, and the contents MUST
 be reassembled in the order that was described above under encoding
 agent behavior.
 The decoding agent should ensure that when the option's value is
 used, any alignment issues that are particular to the machine
 architecture on which the decoding agent is running are accounted for
 - there is no requirement that the encoding agent align the options
 in any particular way.
 There is no semantic meaning to where an option is split - the
 encoding agent is free to split the option at any point, and the
 decoding agent MUST reassemble the split option parts into a single
 object, and MUST NOT treat each split portion of the option as a
 separate object.

8. Example

 Consider an option, Bootfile name (option code 67), with a value of
 "/diskless/foo".  Normally, this would be encoded as a single option,
 as follows:
    | 67 | 13 | / | d | i | s | k | l | e | s | s | / | f | o | o |
 If an encoding agent needed to split the option in order to fit it
 into the option buffer, it could encode it as two separate options,
 as follows, and store it in the aggregate option buffer in the
 following sequence:
    | 67 | 7 | / | d | i | s | k | l | e |
    | 67 | 6 | s | s | / | f | o | o |

9. Security Considerations

 This document raises no new security issues.  Potential exposures to
 attack in the DHCP protocol are discussed in section 7 of the DHCP
 protocol specification [3] and in Authentication for DHCP Messages

Lemon & Cheshire Standards Track [Page 6] RFC 3396 Encoding Long Options in DHCPv4 November 2002

 Note that the authentication option itself can be split; in such
 cases implementations must be careful when setting the authentication
 field to zero (prior to generation or verification of the MAC) as it
 may be split across multiple options.

10. References

10.1. Normative References

 [1] Croft, W. and J. Gilmore, "BOOTSTRAP PROTOCOL (BOOTP)", RFC 951,
     September 1985.
 [2] Bradner, S., "Key words for use in RFCs to indicate requirement
     levels", BCP 14, RFC 2119, March 1997.
 [3] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March
 [4] Alexander, S. and Droms, R., "DHCP Options and BOOTP Vendor
     Extensions", RFC 2132, March 1997.

10.2. Informative References

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

11. 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

Lemon & Cheshire Standards Track [Page 7] RFC 3396 Encoding Long Options in DHCPv4 November 2002

12. Authors' Addresses

 Ted Lemon
 Nominum, Inc.
 2385 Bay Road
 Redwood City, CA 94043
 Stuart Cheshire
 Apple Computer, Inc.
 1 Infinite Loop
 California 95014
 Phone: +1 408 974 3207

Lemon & Cheshire Standards Track [Page 8] RFC 3396 Encoding Long Options in DHCPv4 November 2002

13. 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
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an


 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Lemon & Cheshire Standards Track [Page 9]

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