draft-ietf-dhc-l2ra-04.txt   draft-ietf-dhc-l2ra-05.txt 
DHC Working Group B. Joshi DHC Working Group B. Joshi
Internet-Draft P. Kurapati Internet-Draft Infosys Technologies Ltd.
Expires: October 23, 2009 Infosys Technologies Ltd. Intended status: Informational P. Kurapati
April 21, 2009 Expires: October 9, 2011 Juniper Networks
April 7, 2011
Layer 2 Relay Agent Information Layer 2 Relay Agent Information
draft-ietf-dhc-l2ra-04.txt draft-ietf-dhc-l2ra-05.txt
Abstract
In some networks, DHCP servers rely on Relay Agent Information option
appended by Relay Agents for IP address and other parameter
assignment policies. This works fine when end hosts are directly
connected to Relay Agents. In some network configurations, one or
more Layer 2 devices may reside between DHCP clients and Relay agent.
In these network scenarios, it is difficult to use the Relay Agent
Information option for IP address and other parameter assignment
policies effectively. So there is a need for the device that is
closest to the end hosts to append a Relay Agent Information option
in DHCP messages. These devices are typically known as Layer 2 Relay
Agents.
This document aims to describe the network scenarios where a Layer 2
Relay Agent is in use and also how it handles DHCP messages.
Status of this Memo Status of this Memo
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In some networks, DHCP servers rely on Relay Agent Information option
appended by Relay Agents for IP address and other parameter
assignment policies. This works fine when end hosts are directly
connected to Relay Agents. In some network configurations, one or
more Layer 2 devices may reside between DHCP clients and Relay agent.
In these network scenarios, it is difficult to use the Relay Agent
Information option for IP address and other parameter assignment
policies effectively. So there is a need for the device that is
closest to the end hosts to append a Relay Agent Information option
in DHCP messages. These devices are typically known as Layer 2 Relay
Agents.
This document aims to describe the network scenarios where a Layer 2
Relay Agent is in use and also how it handles DHCP messages.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Need of Layer 2 Relay Agent . . . . . . . . . . . . . . . . . 6 3. Need of Layer 2 Relay Agent . . . . . . . . . . . . . . . . . 5
4. Layer 2 Relay Agent in various network scenarios . . . . . . . 7 4. Layer 2 Relay Agent in various network scenarios . . . . . . . 6
4.1. DHCP server and client on same subnet . . . . . . . . . . 7 4.1. DHCP server and client on same subnet . . . . . . . . . . 6
4.1.1. Client-server interaction . . . . . . . . . . . . . . 7 4.1.1. Client-server interaction . . . . . . . . . . . . . . 6
4.1.2. Issues due to introduction of Layer 2 Relay Agent . . 9 4.1.2. Issues due to introduction of Layer 2 Relay Agent . . 8
4.2. Multiple DHCP server and Client on same subnet . . . . . . 9 4.2. Multiple DHCP server and Client on same subnet . . . . . . 8
4.2.1. Client-server interaction . . . . . . . . . . . . . . 10 4.2.1. Client-server interaction . . . . . . . . . . . . . . 9
4.2.2. Issues due to introduction of Layer 2 Relay Agent . . 10 4.2.2. Issues due to introduction of Layer 2 Relay Agent . . 9
4.3. DHCP server on another subnet with one Layer 3 Relay 4.3. DHCP server on another subnet with one Layer 3 Relay
Agent . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Agent . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.3.1. Client-server interaction . . . . . . . . . . . . . . 11 4.3.1. Client-server interaction . . . . . . . . . . . . . . 10
4.3.2. Issues due to introduction of Layer 2 Relay Agent . . 13 4.3.2. Issues due to introduction of Layer 2 Relay Agent . . 12
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
6. Security Considerations . . . . . . . . . . . . . . . . . . . 15 6. Security Considerations . . . . . . . . . . . . . . . . . . . 14
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
8.1. Normative Reference . . . . . . . . . . . . . . . . . . . 17 8.1. Normative Reference . . . . . . . . . . . . . . . . . . . 16
8.2. Informative Reference . . . . . . . . . . . . . . . . . . 17 8.2. Informative Reference . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction 1. Introduction
DHCP Relay Agents eliminate the necessity of having a DHCP server on DHCP Relay Agents eliminate the necessity of having a DHCP server on
each physical network. Relay Agents populate the 'giaddr' field and each physical network. Relay Agents populate the 'giaddr' field and
also append the 'Relay Agent Information' option to the DHCP also append the 'Relay Agent Information' option to the DHCP
messages. DHCP servers use this option for IP address and other messages. DHCP servers use this option for IP address and other
parameter assignment policies. These DHCP Relay Agents are typically parameter assignment policies. These DHCP Relay Agents are typically
an IP routing aware device and are referred as Layer 3 Relay Agents. an IP routing aware device and are referred as Layer 3 Relay Agents.
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message normally without removing the Relay Agent option since it message normally without removing the Relay Agent option since it
had not added the same. A Layer 2 Relay Agent uses the Relay had not added the same. A Layer 2 Relay Agent uses the Relay
Agent Information option to find out if it had appended it to the Agent Information option to find out if it had appended it to the
request message. request message.
5. The client receives this DHCPOFFER message and it broadcasts a 5. The client receives this DHCPOFFER message and it broadcasts a
DHCPREQUEST message. Layer 2 Relay Agent #1 handles this message DHCPREQUEST message. Layer 2 Relay Agent #1 handles this message
similar to how it handles a DHCPDISCOVER message. similar to how it handles a DHCPDISCOVER message.
6. The server receives the DHCPREQUEST message from the client and 6. The server receives the DHCPREQUEST message from the client and
responds with a DHCPACK/DHCPNACK message. A DHCP server may responds with a DHCPACK/DHCPNAK message. If DHCP server
unicast the DHCPACK message. The Layer 2 Relay Agent processes broadcasts the DHCPACK message, Layer 2 Relay Agent processes it
the DHCPACK message similar to a DHCPOFFER message. similar to a DHCPOFFER message. If DHCP server unicasts the
DHCPACK message to the client, Layer 2 Relay agent intercepts the
same and processes the message similar to the broadcasted DHCPACK
message.
7. The Layer 2 Relay Agent processes a DHCPNAK messages similar to a 7. The Layer 2 Relay Agent processes a DHCPNAK messages similar to a
DHCPACK message. DHCPACK message.
8. The Layer 2 Relay Agent processes a DHCPDECLINE message similar 8. The Layer 2 Relay Agent processes a DHCPDECLINE message similar
to a DHCPDISCOVER message. to a DHCPDISCOVER message.
9. The DHCP client can unicast some of the DHCP messages. The Layer 9. The DHCP client can unicast some of the DHCP messages. The Layer
2 Relay Agent may or may not intercept these messages based on 2 Relay Agent may or may not intercept these messages based on
internal configuration. If Layer 2 Relay Agents intercept these internal configuration. If Layer 2 Relay Agents intercept these
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field set in the message. Some existing DHCP server field set in the message. Some existing DHCP server
implementations do not echo back the Relay Agent Information implementations do not echo back the Relay Agent Information
option if giaddr is not set. This may lead to issues at Layer 2 option if giaddr is not set. This may lead to issues at Layer 2
Relay Agents as they will not be able to identify the outgoing Relay Agents as they will not be able to identify the outgoing
port correctly and would broadcast it to all ports. Some Layer 2 port correctly and would broadcast it to all ports. Some Layer 2
Relay Agents discard the reply messages if they do not find a Relay Agents discard the reply messages if they do not find a
Relay Agent Information option in a DHCP reply. Relay Agent Information option in a DHCP reply.
2. There is a case when the DHCP client receives a unicast reply 2. There is a case when the DHCP client receives a unicast reply
message like DHCPACK with a Relay Agent Information option. This message like DHCPACK with a Relay Agent Information option. This
may happen when the DHCP server unicasts the DHCPACK message and can happen only when the DHCP server unicasts the DHCPACK message
the Layer 2 Relay Agent is configured not to intercept unicast and the Layer 2 Relay Agent is not configured to intercept
messages. In such a case, the DHCP client can ignore the Relay unicast messages. Most of the Layer 2 Relay Agents, that are
Agent Information option. deployed today, are configured to intercept the unicast DHCP
messages and hence this behaviour may not be seen in the real
world deployments.
3. A DHCP server should be able to handle a unicast DHCP message 3. A DHCP server should be able to handle a unicast DHCP message
containing a Relay Agent Information option. Some existing DHCP containing a Relay Agent Information option. Some existing DHCP
server implementations do not echo back the Relay Agent server implementations do not echo back the Relay Agent
Information option in responses to unicast messages. Information option in responses to unicast messages.
4.2. Multiple DHCP server and Client on same subnet 4.2. Multiple DHCP server and Client on same subnet
In certain network scenarios, there could be multiple DHCP servers on In certain network scenarios, there could be multiple DHCP servers on
the same subnet. Figure 2 shows a typical network setup. the same subnet. Figure 2 shows a typical network setup.
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received. This helps the Layer 3 Relay Agent to identify the received. This helps the Layer 3 Relay Agent to identify the
outgoing interface for the DHCP replies. In some cases, a Layer outgoing interface for the DHCP replies. In some cases, a Layer
3 Relay Agent may use unnumbered interfaces. In this case, it 3 Relay Agent may use unnumbered interfaces. In this case, it
has to use a system wide IP address to populate the 'giaddr' has to use a system wide IP address to populate the 'giaddr'
field. Due to this, it becomes difficult to identify the correct field. Due to this, it becomes difficult to identify the correct
outgoing interface for the messages received from the DHCP outgoing interface for the messages received from the DHCP
server. In these cases, some existing Layer 3 Relay Agent server. In these cases, some existing Layer 3 Relay Agent
implementations maintain an internal state for each DHCP message implementations maintain an internal state for each DHCP message
and use this state to identify the outgoing interface. and use this state to identify the outgoing interface.
3. A DHCP server uses certain parameters to differentiate the RENEW 3. The DHCP server uses certain parameters to differentiate the
and REBIND state of a client. A DHCP client unicasts a RENEW RENEW and REBIND state of a client. A DHCPREQUEST without
request to the DHCP server, so the DHCP server sees a DHCPREQUEST 'giaddr' and the Relay Agent Information option is treated as
without 'giaddr' and Relay Agent Information option as a RENEW RENEW request while DHCPREQUEST with 'giaddr' and Relay Agent
request. On the other hand, a REBIND request is broadcast and so Information option is treated as REBIND request. In a network
the DHCP server expects it to contain 'giaddr' field and a Relay configuration where both Layer 2 Relay Agent and Layer 3 Relay
Agent Information option. If the Layer 2 Relay Agent is Agent are configured to intercept the unicast DHCP messages, the
configured to intercept unicast messages, it will append a Relay DHCP server will receive RENEW request with Relay Agent
Agent Information option to the unicast DHCP message. Because of Information option and 'giaddr' field set. Since REBIND request
this, it could be difficult for the DHCP server to differentiate will also have Relay Agent Information option and 'giaddr' field
between a RENEWING and REBINDING state. set, it becomes difficult for the DHCP server to differentiate
between RENEW and REBIND requests.
5. Acknowledgments 5. Acknowledgements
This document is the result of a discussion on DHC WG mailing list. This document is the result of a discussion on DHC WG mailing list.
Thanks to David W. Hankins and Michael Wacker for providing inputs on Thanks to David W. Hankins and Michael Wacker for providing inputs on
some of the existing implementations. Thanks to Ted Lemon, Mukund some of the existing implementations. Thanks to Ted Lemon, Mukund
Kamath, Alfred Hoenes and Stefaan De Cnodder for reviewing the draft Kamath, Alfred Hoenes, Ramesh and Stefaan De Cnodder for reviewing
and providing valuable suggestions. the draft and providing valuable suggestions.
6. Security Considerations 6. Security Considerations
o A Layer 2 Relay Agent should always be configured to identify a o A Layer 2 Relay Agent should always be configured to identify a
trustable entity so that it appends a Relay Agent Information trustable entity so that it appends a Relay Agent Information
option to a DHCP message coming from a trustable entity and option to a DHCP message coming from a trustable entity and
forwards it. If a DHCP message is received from a non-trustable forwards it. If a DHCP message is received from a non-trustable
entity, the Layer 2 Relay Agent should discard it and may report entity, the Layer 2 Relay Agent should discard it and may report
to the administrator. to the administrator.
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Bharat Joshi Bharat Joshi
Infosys Technologies Ltd. Infosys Technologies Ltd.
44 Electronics City, Hosur Road 44 Electronics City, Hosur Road
Bangalore 560 100 Bangalore 560 100
India India
Email: bharat_joshi@infosys.com Email: bharat_joshi@infosys.com
URI: http://www.infosys.com/ URI: http://www.infosys.com/
Pavan Kurapati Pavan Kurapati
Infosys Technologies Ltd. Juniper Networks
44 Electronics City, Hosur Road Embassy Prime Buildings, C.V. Raman Nagar
Bangalore 560 100 Bangalore 560 093
India India
Email: pavan_kurapati@infosys.com Email: kurapati@juniper.net
URI: http://www.infosys.com/ URI: http://www.juniper.net/
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