Network Working Group Glenn Stump, IBM
INTERNET DRAFT Ralph Droms, Bucknell University
March 1997
Expires September 1997
The User Class Option for DHCP
<draft-ietf-dhc-userclass-01.txt>
Status of this Memo
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1. Abstract
This option is used by a DHCP client to optionally identify the type
or category of user or applications it represents. The information
contained in this option is an NVT ASCII text object that represents
the user class of which the client is a member.
2. Definitions
Throughout this document, the words that are used to define the
significance of particular requirements are capitalized. These words
are:
o "MUST"
This word or the adjective "REQUIRED" means that the
item is an absolute requirement of this specification.
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o "MUST NOT"
This phrase means that the item is an absolute prohibition
of this specification.
o "SHOULD"
This word or the adjective "RECOMMENDED" means that there
may exist valid reasons in particular circumstances to ignore
this item, but the full implications should be understood and
the case carefully weighed before choosing a different course.
o "SHOULD NOT"
This phrase means that there may exist valid reasons in
particular circumstances when the listed behavior is acceptable
or even useful, but the full implications should be understood
and the case carefully weighed before implementing any behavior
described with this label.
o "MAY"
This word or the adjective "OPTIONAL" means that this item is
truly optional. One vendor may choose to include the item
because a particular marketplace requires it or because it
enhances the product, for example; another vendor may omit the
same item.
This document also uses the following terms:
o "DHCP client"
A DHCP client or "client" is an Internet host using DHCP to obtain
configuration parameters such as a network address.
o "DHCP server"
A DHCP server of "server"is an Internet host that returns
configuration parameters to DHCP clients.
o "binding"
A binding is a collection of configuration parameters, including
at least an IP address, associated with or "bound to" a DHCP
client. Bindings are managed by DHCP servers.
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3. User Class Information
This option is used by a DHCP client to optionally identify the type
or category of user or applications it represents. The information
contained in this option is an NVT ASCII text object that represents
the user class of which the client is a member.
DHCP administrators may define specific user class identifiers to
convey information about a client's software configuration or about
its user's preferences. For example, an identifier may specify that
a particular DHCP client is a member of the class "accounting
auditors", which have special service needs such as a particular
database server.
Servers not equipped to interpret the user class specified by a
client MUST ignore it (although it may be reported). Otherwise,
servers SHOULD respond with the set of options corresponding to the
user class specified by the client. Further, if the server responds
with the set of options corresponding to the given user class, it
MUST return this option (with the given user class value) to the
client.
Clients which do not receive information for the user class requested
SHOULD make an attempt to operate without it, although they may do so
(and may announce they are doing so) in a degraded mode.
The code for this option is 77. The minimum length for this option
is two.
Code Len text1
+-----+-----+-----+-----+-----
| 77 | N | c1 | c2 | ...
+-----+-----+-----+-----+-----
Implemention Note: Simulating Multiple User Classes
Although the user class option field only permits one NVT string, the
working group envisions that multiple classes can be simulated by
creating combination classes which map into a single class NVT
string. For example, suppose a site desires to create multiple
logical user classes, including:
"mobile" -- These hosts receive short lease times
and are assumed to dynamically update
their own DNS records
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"engineer" -- These hosts are assigned a high-
performance NFS file server
For the above two classes, then, a combination class could look
something like:
"mobeng" -- hosts of this mobile-engineer combination
class get assigned a high-performance
file server and a short lease time, and
a DNS proxy A record update is not attempted
on their behalf.
Thus, by mapping combinations of classes into single class names, you
can effectively implement multiple user classing at a site using only
the single NVT string field.
Implementation Note: Serving Competing Option Values
When servicing a request from a client of a particular user class, a
DHCP server makes decisions about what collection of options to
include in its response. These decisions are expected to consider
options and values designated for the client host by virtue of its
subnet/location, vendor class, user class, or client id.
In cases where multiple option values are possible for return to the
client due to multiple, overlapping "affiliations", DHCP server
policy may dictate which values take precedence over others. A DHCP
server implementation SHOULD provide flexibility in specifying DHCP
option precedence policy so that DHCP administrators can customize a
DHCP system to best suit their network environments.
If flexibility in a server's option precedence policy is not
implemented by a vendor (or is perhaps implemented but not exercised
by an administrator), a recommended default policy is that option
values of specific affiliations override those of less specific.
That is, an option value designated for a specific client --
sometimes known as a "reserved binding" -- SHOULD override option
values designated for the client's user or vendor class, which SHOULD
override option values designated for the client's vendor class,
which SHOULD override option values for the client's subnet.
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Security Considerations
Security issues are not discussed in this document.
References
[DHCP] Droms, R., "Dynamic Host Configuration Protocol", RFC1541,
October 1993
Acknowledgments
Author Information
Glenn Stump
IBM Networking Software Solutions
4205 South Miami Blvd.
RTP, NC 27709
Phone: (919) 254-5616
email: glennstump@vnet.ibm.com
Ralph Droms
Computer Science Department
323 Dana Engineering
Bucknell University
Lewisburg, PA 17837
Phone: (717) 524-1145
email: droms@bucknell.edu
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