Encoding Long Options in the Dynamic Host Configuration Protocol (DHCPv4)
RFC 3396

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.

Abstract

   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.

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   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
   option.

2. Terminology

   DHCP
      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].

   DHCPv4
      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
      development.

   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
      received.

   Options
      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.

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   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
      options.

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

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   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
   placed.

   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.

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   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.

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   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
   [5].

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   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
       1997.

   [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.

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12. Authors' Addresses

   Ted Lemon
   Nominum, Inc.
   2385 Bay Road
   Redwood City, CA 94043
   USA

   EMail: mellon@nominum.com

   Stuart Cheshire
   Apple Computer, Inc.
   1 Infinite Loop
   Cupertino
   California 95014
   USA

   Phone: +1 408 974 3207
   EMail: rfc@stuartcheshire.org

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13. Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

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Acknowledgement

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

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