Network Working Group                                          Ted Lemon
Internet Draft                                             Nominum, Inc.

New draft                                                 February, 2001
                                                    Expires August, 2001

                      Encoding Long DHCP Options

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   This document is an Internet-Draft.  Internet-Drafts are working
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     This draft specifies how DHCP options in a DHCP packet can be
     aggregated so that DHCP protocol agents can send options that are
     more than 255 bytes in length.


     The DHCP protocol [1] specifies objects called "options" that are
     encoded in the DHCP 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.

     In some cases it may be useful to send DHCP options that are
     longer than 255 bytes, however.  RFC2131 [1] 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 an ordering that can be used by DHCP protocol
     agents when sending and receiving options with more than 255
     bytes.  DHCP protocol agents MAY use the ordering described here
     to encode existing options if such options exceed 255 bytes in
     length.  The main purpose of this specification, however, is to
     provide a specification that new DHCP option drafts can


     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
     Encoding agent
        The DHCP protocol agent that is composing a DHCP packet to
     Decoding agent
        The DHCP protocol agent that is processing a DHCP packet it
        has received.
        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 rendesvous with DHCP servers, and so
     Option overload
        The DHCP packet format is based on the BOOTP packet format
        defined in [4].   When used by DHCP protocol agents, BOOTP
        packets have three fields that can contain options.   These
        are the actual option buffer, the server name buffer, and the
        filename buffer.   The DHCP options specification [2] defines
        the DHCP Overload option, which specifies which of these three
        buffers is actually being used in any given DHCP message to
        store DHCP options.

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 [3].


     This specification applies in any case where a DHCP protocol
     agent is encoding a packet containing options, and some of those
     options must be broken into parts.  This can occur either because
     such an option is longer than 255 bytes, or 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.  DHCP protocol
     agents encoding an option in this case MAY use the algorithm
     specified here.

     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.   DHCP protocol agents
     that are decoding such options MAY follow the algorithm specified

     The rationale for making this optional is that existing
     implementations have not been subject to a requirement to encode
     or decode options in this way, and it is not our intention to
     potentially cause existing implementations to be in violation of
     this specification.

The aggregate option buffer

     This behaviour only applies in cases where this specification is
     applicable, as previously defined in the Applicability section.
     DHCP options can be stored in the DHCP packet in three seperate
     portions of the packet.   These are the optional parameters
     field, the sname field, and the file field, as described in [1].
     This complicates the description of the option splitting
     mechanism because there are three seperate buffers into which
     split options may be stored.

     To further complicate matters, an option that doesn't fit into
     one buffer can't overlap the boundary into another buffer - 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 [1].

     The aggregate option buffer is made up of the optional parameters
     field, the file field, and the sname field, in that order.
     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
     in such a way that they cross either boundary between the three
     fields in the aggregate buffer.

Encoding agent behaviour

     Encoding agents MAY split options as described in this
     specification.   Encoding agents supporting options that require
     this MUST split options as described here, if it is necessary to
     do so in order to encode the entire contents of the option.

     Options MAY 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 output
     buffer in sequential order - the first split portion MUST be
     stored first in the aggregate option buffer, then the second
     portion, and so on.

     Each split portion of an option MUST be stored in the aggregate
     option buffer in the same way that a full option would be stored
     - each portion contains a single byte option type code, and then
     a single byte indicating the length of the portion of the option
     payload being stored in that portion, and then the payload
     itself.  The length byte MUST NOT include itself or the option
     code.  For any given option being split, the option code MUST be
     the same.

     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.

Decoding agent behaviour

     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 MAY consider them to be a split option as described
     here.  Decoding agents that support options that require the
     behaviour described in this draft MUST consider such options to
     be split.

     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 ordered as described above under encoding agent
     behaviour.   The decoding agent MUST 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.


Consider an option, option 224, with a value of "".   Normally,
this would be encoded as a single option, as follows:

   | 224 |  8  | 'i' | 's' | 'c' | '.' | 'o' | 'r' | 'g' | '.' |

If an encoding agent needed to split the option in order to fit it
into the option buffer, it could encode it as two seperate options, as
follows, and store it in the aggregate option buffer in the following

   | 224 |  5  | 'i' | 's' | 'c' | '.' | 'o' |

   | 224 |  3  | 'r' | 'g' | '.' |

Security Considerations

     DHCP currently provides no authentication or security mechanisms.
     Potential exposures to attack are discussed in section 7 of the DHCP
     protocol specification [1]. The Classless Static Routes option can
     be used to misdirect network traffic by providing incorrect IP
     addresses for routers.


 [1] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
     Bucknell University, March 1997.
 [2] Alexander, S. and Droms, R., "DHCP Options and BOOTP Vendor
     Extensions", RFC 2132, Silicon Graphics, Inc., Bucknell
     University, March 1997.
 [3] Bradner, S., "Key words for use in RFCs to indicate requirement
     levels", RFC 2119, Harvard University, March 1997.
 [4] Mogul, J., Postel, J., "Internet Standard Subnetting
     Procedure", RFC950, Stanford University, USC/Information
     Sciences Institute, August 1985.

Author Information

Ted Lemon
Nominum, Inc.
950 Charter Street
Redwood City, CA 94043


   This document will expire on August 31, 2001.

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