Internet DRAFT - draft-ietf-dhc-concat
draft-ietf-dhc-concat
Network Working Group Ted Lemon
Internet Draft Nominum, Inc.
Obsoletes: draft-ietf-dhc-concat-05.txt Stuart Cheshire
Category: Standards Track Apple Computer, Inc.
September, 2002
Expires March, 2003
Encoding Long Options in DHCPv4
<draft-ietf-dhc-concat-05.txt>
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC2026.
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Abstract
This document specifies the processing rules for 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.
Introduction
This document updates RFC2131 [1] 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 RFC2131.
The text in section 4.1 that reads "Options may appear only
once, unless otherwise specified in the options document."
should considered to be deleted.
The DHCP protocol [1] specifies objects called "options" that
are encoded in the DHCPv4 packet to pass information between
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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 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. DHCP protocol agents MUST use this method whenever they
receive a DHCP packet containing more than one of a certain type
of option.
Terminology
DHCP
Throughout this document, the acronym "DHCP" is used to refer
to the Dynamic Host Configuration Protocol as specified in
RFC2131 [1] and RFC2132 [2].
DHCPv4
We have used the term "DHCPv4" in the abstract for this
document to distinguish between the DHCP protocol for IPv4 as
defined in RFC2131 and RFC2132 and the DHCP protocol for IPv6,
which, at the time that this document is being written, is
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.
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,
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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].
Applicability
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 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 [1]
and DHCP Options and BOOTP vendor extensions [2] 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 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
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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 concatena-
tion-requiring options, and never split non-concatenation-
requiring options. This is permissible. However, an implement-
ation which supports any concatenation-requiring option MUST be
capable of concatenating received options for both concatena-
tion-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.
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 RFC2131 [1].
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 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.
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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 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 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.
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.
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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 |
+----+---+---+---+---+---+---+---+
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 [1] and in Authentication for DHCP
Messages [5].
Note that the authentication option itself can be split; in such
cases implementations must be careful when setting the authenti-
cation field to zero (prior to generation or verification of the
MAC) as it may be split across multiple options.
References
[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.
[5] R. Droms, W. Arbaugh, "Authentication for DHCP Messages",
RFC3118, Cisco Systems, University of Maryland, June 2001.
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Author Information
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
Expiration
This document will expire on February 28, 2003.
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