Internet DRAFT - draft-huang-dhcp-su
draft-huang-dhcp-su
Internet Area L.Huang
Internet Draft China Mobile
Intended status: Informational X.Cheng&L.Lin
Expires: September 2, 2009 BUPT
March 2, 2009
Extension of DHCP Relay Agent Information Option
draft-huang-dhcp-su-00.txt
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Internet-Draft DHCP Option Extension February 2009
Abstract
This Internet draft describes an extension of DHCP Relay Agent
Information option for the IP address assignment diversity and the
server-to-client replies forwarding convenience.
Table of Contents
1. Introduction................................................2
2. Extension of DHCP Relay Agent Information Option.............2
2.1. Purpose................................................3
2.2. Mechanism..............................................3
3. Security Considerations......................................5
4. IANA Considerations.........................................5
5. References..................................................5
Author's Addresses.............................................6
1. Introduction
As defined in RFC 3046, Relay Agent Information option is inserted by
the DHCP relay agent (or downstream network element) when forwarding
client-originated DHCP packets to a DHCP server. Servers recognizing
the Relay Agent Information option may use the information to
implement IP address or other parameter assignment policies. The DHCP
Server echoes the option back verbatim to the relay agent in server-
to-client replies, and the relay agent strips the option before
forwarding the reply to the client.
How to implement more flexible IP address assignment policies with
the Relay Agent Information option? This document define an extension
of DHCP Relay Agent Information option for the IP address assignment
diversity and the server-to-client replies forwarding convenience.
2. Extension of DHCP Relay Agent Information Option
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2.1. Purpose
According to RFC 3046, Relay Agent Information Option is mainly
responsible for:
(1) More flexible IP address assignment policies;
(2) More better server-to-client replies forwarding.
Does Relay Agent Information option gain its ends? DHCP Server may
implement IP address assignment policies with the circuit information
carried in Relay Agent Information option. However, such information
isn't legible enough.
Inserted more earlier, the Relay Agent Information option is more
specific. But the option can't precisely locate the client even it's
inserted in the device closest to the client. Although Relay Agent
Information option can be used to relay DHCP responses back to the
proper circuit, this only works in the network element which inserts
the option.
As the result, the circuit information in Relay Agent Information
option can't fully support the IP address assignment policies. What
the DHCP server needs is to locate the client level by level with the
whole path information form client to server. Its benefit is that IP
address assignment policies of different granularities can be made.
When transmitting the server-to-client replies, all network elements
can make use of the Relay Agent Information option.
2.2. Mechanism
The "Relay Agent Information" option is organized as a single DHCP
option that contains one or more "sub-options" that convey
information known by the relay agent. The initial sub-options are
defined for a relay agent that is co-located in a public circuit
access unit. The Agent Information field consists of a sequence of
SubOpt/Length/Value tuples for each sub-option.
Such definition in RFC 3046 has some drawbacks:
(1) Relay Agent Information option can only be added by one device
in the network, other network elements can only forward the
message when finding Relay Agent Information option in it.
(2) Sub-option is used to define a kind of information, which means
sub-options of the same type value can't coexist in one Relay
Agent Information option.
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Based on the demand of the new Relay Agent Information option, we
extend this option as following:
There is still unique Relay Agent Information option in a DHCP
message, but this option can be modified by multiple network elements
between DHCP client and DHCP server. For the sake of distinguishing
the information added by different devices, we define a new sub-
option whose type field is y and len field is 0, which are used to
identify the end of modification of one device. When a network
element receives a DHCP message, if it wants to add Relay Agent
Information option, it must check whether Relay Agent Information is
already existed in current message. If there is no Relay Agent
Information option in the message, the network element adds this kind
of option according to RFC 3046; otherwise, the network element adds
a new option information after existed option in the sub-options form
that are defined in RFC 3046 or other RFCs, such as sub-
option1(circuit) and sub-option2(remote). Regardless of the two
situations, devices must plus a sub-option(type x, len 0) that is
defined above to indicate the end of the modification and increase
corresponding number in length field of Relay Agent Information
option.
For example, if all network elements add sub-option1 and sub-option2,
DHCP Relay Agent Information option may be following format:
Relay Agent Information option:
+-------+------+-----+
|Code(x)|Len(N)|Value|
+-------+------+-----+
Value:
+---+----+--------+---+----+--------+---+---+
| 1 | N1 | a1~aN1 | 2 | N2 | b1~bN2 | y | 0 |
+---+----+--------+---+----+--------+---+---+
| 1 | N3 | c1~cN3 | 2 | N4 | d1~dN4 | y | 0 |
+---+----+--------+---+----+--------+---+---+
/ ... \
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\ ... /
+---+----+--------+---+----+--------+---+---+
| 1 | N51| y1~yN3 | 2 | N52| z1~zN52| y | 0 |
+---+----+--------+---+----+--------+---+---+
DHCP server receiving the DHCP message can obtain layer Relay Agent
Information option information and it can allocate different IP
address according to the different strategy of allocating granularity
constituted by option. Then, Relay Agent Information option is
replicated in DHCP response to the DHCP client.
Network elements (e.g.: DHCP relay agent or downstream network
element) strip its own sub-options when receiving the DHCP response
packets and then relay DHCP responses back to the proper circuit by
reading the sub-options.
3. Security Considerations
This document doesn't propose any new protocol.
4. IANA Considerations
This document requires a new number for DHCP option code x and a new
DHCP sub-option code y described in section 2.2.
5. References
[RFC2131] R. Droms, Bucknell University, " Dynamic Host Configuration
Protocol ", RFC 2131, March 1997.
[RFC2132] S. Alexander, Silicon Graphics, Inc., and R. Droms, " DHCP
Options and BOOTP Vendor Extensions ", RFC 2132, March 1997.
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[RFC3046] M. Patrick, Motorola BCS, " DHCP Relay Agent Information
Option ", RFC 3046, January 2001.
Author's Addresses
Lu Huang
China Mobile
53A,Xibianmennei Ave.,
Xuanwu District,
Beijing 100053
China
Email: huanglu@chinamobile.com
Xu Cheng
Beijing University of Posts and Telecommunications
10,Xitucheng Road.,
Haidian District,
Beijing 100876
China
Email: chengx@buptnet.edu.cn
Lin Lin
Beijing University of Posts and Telecommunications
10,Xitucheng Road.,
Haidian District,
Beijing 100876
China
Email: linl@buptnet.edu.cn
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