    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.

    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
       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.

    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
          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

    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
       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.

    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
       [5].

    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
           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.

    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
       Director.

    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
       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

    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
       English.

       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
       "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.

    Lemon & Cheshire            Standards Track                     [Page 9]