Network Working Group                                          Ted Lemon
Internet Draft                                             Nominum, Inc.
                                                         Stuart Cheshire
                                                    Apple Computer, Inc.

Obsoletes: draft-ietf-dhc-concat-01.txt                    October, 2001
                                                     Expires April, 2002

                      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 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.   This
     method also MUST be used whenever a decoding agent receives a DHCP
     packet containing more than one of a certain type of option.


     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 rendezvous with DHCP servers, and so
     Option overload
        The DHCP packet format is based on the BOOTP packet format
        defined in RFC951 [4].   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 [2]
        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.

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 RFC2119 [3].


     This specification applies when a DHCP agent is encoding a packet
     containing options, and 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 algo-
     rithm specified here.   It can also occur because there is not suff-
     icient 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 seperate instances of the same option in the
     way that we specify here.

The aggregate option buffer

     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 RFC2131 [1].
     This complicates the description of the option splitting
     mechanism because there are three seperate 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 RFC2131 [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.

     The encoding agent is free to choose to use either or both of 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.

Encoding agent behaviour

     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

     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 RFC2131 [1].

     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 RFC2132 [2].   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.

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 MUST consider them to be split portions of an option as
     described in the preceding section.

     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 behaviour.

     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 seperate object.


     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 seperate
     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 |

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 RFC2131 [1].


 [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] Croft, W., Gilmore, J., "BOOTSTRAP PROTOCOL (BOOTP)", RFC951,
     Stanford University, Sun Microsystems, September 1985.

Author Information

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

Stuart Cheshire
Apple Computer, Inc.
1 Infinite Loop
California 95014
Phone: +1 408 974 3207


   This document will expire on April 31, 2002.

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