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Versions: (draft-joshi-dhc-l2ra) 00 01 02 03 04 05 06

DHC Working Group                                               B. Joshi
Internet-Draft                                               P. Kurapati
Expires: October 23, 2009                      Infosys Technologies Ltd.
                                                          April 21, 2009


                    Layer 2 Relay Agent Information
                       draft-ietf-dhc-l2ra-04.txt

Status of this Memo

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   This Internet-Draft will expire on October 23, 2009.

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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.



































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Need of Layer 2 Relay Agent  . . . . . . . . . . . . . . . . .  6
   4.  Layer 2 Relay Agent in various network scenarios . . . . . . .  7
     4.1.  DHCP server and client on same subnet  . . . . . . . . . .  7
       4.1.1.  Client-server interaction  . . . . . . . . . . . . . .  7
       4.1.2.  Issues due to introduction of Layer 2 Relay Agent  . .  9
     4.2.  Multiple DHCP server and Client on same subnet . . . . . .  9
       4.2.1.  Client-server interaction  . . . . . . . . . . . . . . 10
       4.2.2.  Issues due to introduction of Layer 2 Relay Agent  . . 10
     4.3.  DHCP server on another subnet with one Layer 3 Relay
           Agent  . . . . . . . . . . . . . . . . . . . . . . . . . . 11
       4.3.1.  Client-server interaction  . . . . . . . . . . . . . . 11
       4.3.2.  Issues due to introduction of Layer 2 Relay Agent  . . 13
   5.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 14
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     8.1.  Normative Reference  . . . . . . . . . . . . . . . . . . . 17
     8.2.  Informative Reference  . . . . . . . . . . . . . . . . . . 17
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18




























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1.  Introduction

   DHCP Relay Agents eliminate the necessity of having a DHCP server on
   each physical network.  Relay Agents populate the 'giaddr' field and
   also append the 'Relay Agent Information' option to the DHCP
   messages.  DHCP servers use this option for IP address and other
   parameter assignment policies.  These DHCP Relay Agents are typically
   an IP routing aware device and are referred as Layer 3 Relay Agents.

   In some network configurations, there is a need for Layer 2 devices
   to append the Relay Agent Information option as they are closer to
   the end hosts.  These Layer 2 devices are typically operating only as
   bridges for the network and may not have an IPv4 address on the
   network in question.  Lacking a valid IPv4 source address, they
   cannot relay packets directly to a DHCP server located on another
   network.  These Layer 2 devices append the Relay Agent Information
   option and broadcast the DHCP message.  A Layer 3 Relay Agent relays
   it to the DHCP server.

   This document provides information about where a Layer 2 Relay Agent
   fits in and how it is used.  This document also looks at various
   network scenarios with Layer 2 Relay Agents and discusses various
   issues caused by the introduction of Layer 2 Relay Agents.




























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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   This document uses the following terms:

   o  "DHCP client"

   A DHCP client is an Internet host using DHCP to obtain configuration
   parameters such as a network address.

   o  "Layer 3 Relay Agent"

   A Layer 3 Relay Agent is a third-party agent that transfers Bootstrap
   Protocol (BOOTP) and DHCP messages between clients and servers
   residing on different subnets, per [RFC951] and [RFC1542].

   o  "BRAS"

   BRAS or Broadband Remote Access Server is a network element which
   acts as an aggregation device terminating end user sessions.  BRAS is
   usually the first IP edge device in a Layer 2 Access Network
   architecture.

   o  "DHCP server"

   A DHCP server is an Internet host that returns configuration
   parameters to DHCP clients.

   o  "Unnumbered Interfaces"

   An interface with no IP address associated with it.  IP packets
   generated from this interface may use a local loopback address which
   may be shared with other unnumbered interfaces.















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3.  Need of Layer 2 Relay Agent

   A Layer 2 device intercepts DHCP messages for the following reasons:

   1.  In some network deployments like xDSL, the subscriber aggregation
       devices (also known as Access Concentrator or a DSLAM in case of
       DSL) are configured to act as bridges.  As these devices are
       closest to the subscriber, they are in the best position to
       provide a unique Relay Agent Information option to enforce
       policies in the DHCP server.

   2.  In some network deployments, a Layer 2 device can append Relay
       Agent Information in DHCP messages so that it can use this
       information to forward the DHCP Replies to the specific port on
       which the request was received.

   3.  In some networks, the Layer 2 Switch which is closest to the end
       users, snoops the DHCP messages.  These switches extract DHCP
       Lease Information and use this information to install packet
       filters.  This helps in preventing Layer 2 and Layer 3 spoofing
       attempts by the subscribers.  A point to note here is that in
       cases where switches maintain the Lease Information, they have to
       intercept unicast DHCP messages as well to keep this information
       up to date.

   4.  NOTE: Please send an email to the authors if you are aware of any
       other functionality of Layer 2 Relay Agent.  It will be helpful
       in updating this list.  This note will be removed before moving
       this draft for IESG review.






















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4.  Layer 2 Relay Agent in various network scenarios

   This section describes the various network scenarios where a Layer 2
   Relay Agent fits in.  It also describes how it handles different DHCP
   messages.

4.1.  DHCP server and client on same subnet

   In certain network configurations, a DHCP server may reside on the
   same subnet as the DHCP clients.  A Layer 2 aggregation device
   resides between the DHCP clients and DHCP server.  The following
   points describe how this Layer 2 device handles various DHCP messages
   if it acts as a Layer 2 Relay Agent.  Figure 1 shows a typical
   network setup.


   +--------+
   | End    |           +--------+     |
   | Host#1 +-----------|        |     |     +-----------+
   +--------+           | Layer  +-----|     |           |
                        |   2    |     +-----| DHCP      |
   +--------+           | device |     |     | Server#1  |
   | End    +-----------|  #1    |     |     +-----------+
   | Host#2 |           +--------+     |
   +--------+                          |
                                       |
   +--------+                          |
   | End    |           +--------+     |
   | Host#3 +-----------|        |     |
   +--------+           | Layer  +-----|
                        |   2    |     |
   +--------+           | device |     |
   | End    +-----------|  #2    |
   | Host#n |           +--------+
   +--------+


                                 Figure 1

4.1.1.  Client-server interaction

   The following summary of protocol message exchanges between clients
   and DHCP servers describes how they are handled in a Layer 2 Relay
   Agent.

   1.  The client (End Host #1) broadcasts a DHCPDISCOVER message on its
       local physical subnet.  Layer 2 Relay Agent #1 intercepts this
       message, appends the Relay Agent Information option and



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       broadcasts it to all the ports except the one on which it was
       received.  The Relay Agent Information option could be created as
       suggested in RFC 3046 [RFC3046].  The Layer 2 Relay Agent does
       not set the 'giaddr' field.

   2.  Layer 2 device #2 would also receive this DHCPDISCOVER message
       from Layer 2 device #1.  If it is configured as Layer 2 Relay
       Agent, it intercepts this message but does not append another
       Relay Agent Information option to the message.  It may discard
       this message if it is coming from an untrusted entity.
       Otherwise, it will broadcast this on all the ports except the one
       on which the message was received.

   3.  The DHCP server responds with a DHCPOFFER message after applying
       its local policies.  It echoes back the Relay Agent Information
       option in the DHCPOFFER message.  The DHCP server can either
       unicast the reply to the MAC address of End Host #1 or broadcast
       the reply.  If the reply is broadcast, the Layer 2 Relay Agent
       intercepts this message and removes the Relay Agent Information
       option.  It identifies the outgoing port using the Relay Agent
       Information option and forwards the message to the identified
       interface.  A Layer 2 Relay Agent may be configured to intercept
       unicast DHCP messages.  In such a case, the Layer 2 Relay Agent
       intercepts unicast DHCP messages and handles them similar to
       broadcast messages.

   4.  The same DHCPOFFER message will be received by Layer 2 Device #2.
       If it is configured as Layer 2 Relay Agent, it broadcasts this
       message normally without removing the Relay Agent option since it
       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
       request message.

   5.  The client receives this DHCPOFFER message and it broadcasts a
       DHCPREQUEST message.  Layer 2 Relay Agent #1 handles this message
       similar to how it handles a DHCPDISCOVER message.

   6.  The server receives the DHCPREQUEST message from the client and
       responds with a DHCPACK/DHCPNACK message.  A DHCP server may
       unicast the DHCPACK message.  The Layer 2 Relay Agent processes
       the DHCPACK message similar to a DHCPOFFER message.

   7.  The Layer 2 Relay Agent processes a DHCPNAK messages similar to a
       DHCPACK message.

   8.  The Layer 2 Relay Agent processes a DHCPDECLINE message similar
       to a DHCPDISCOVER message.




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   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
       internal configuration.  If Layer 2 Relay Agents intercept these
       messages, they append a Relay Agent Information option and
       forward the message towards the DHCP server.  They also intercept
       the reply messages and remove the Relay Agent Information option
       before forwarding them.

4.1.2.  Issues due to introduction of Layer 2 Relay Agent

   1.  A DHCP server should be able to handle a DHCP message that
       contains the Relay Agent Information option without 'giaddr'
       field set in the message.  Some existing DHCP server
       implementations do not echo back the Relay Agent Information
       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
       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 Agent Information option in a DHCP reply.

   2.  There is a case when the DHCP client receives a unicast reply
       message like DHCPACK with a Relay Agent Information option.  This
       may happen when the DHCP server unicasts the DHCPACK message and
       the Layer 2 Relay Agent is configured not to intercept unicast
       messages.  In such a case, the DHCP client can ignore the Relay
       Agent Information option.

   3.  A DHCP server should be able to handle a unicast DHCP message
       containing a Relay Agent Information option.  Some existing DHCP
       server implementations do not echo back the Relay Agent
       Information option in responses to unicast messages.

4.2.  Multiple DHCP server and Client on same subnet

   In certain network scenarios, there could be multiple DHCP servers on
   the same subnet.  Figure 2 shows a typical network setup.















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   +--------+
   | End    |           +--------+     |
   | Host#1 +-----------|        |     |     +-----------+
   +--------+           | Layer  +-----|     |           |
                        |   2    |     +-----| DHCP      |
   +--------+           | device |     |     | Server#1  |
   | End    +-----------|  #1    |     |     +-----------+
   | Host#2 |           +--------+     |
   +--------+                          |
                                       |     +-----------+
   +--------+                          |     | DHCP      |
   | End    |           +--------+     |-----| Server #2 |
   | Host#3 +-----------|        |     |     |           |
   +--------+           | Layer  +-----|     +-----------+
                        |   2    |     |
   +--------+           | device |
   | End    +-----------|  #2    |
   | Host#n |           +--------+
   +--------+


                                 Figure 2

4.2.1.  Client-server interaction

   The message exchanges are the same as explained in 4.1.1.  However,
   due to the introduction of multiple DHCP servers the below additional
   message exchange may happen.

   1.  When Host #1 sends DHCPDISCOVER, it will be received by both DHCP
       Servers connected to Layer 2 Relay Agent #1 and both servers will
       respond with a DHCPOFFER.  So instead of one DHCPOFFER message,
       the Layer 2 Relay Agent would receive two messages.  The
       processing of DHCP messages in the Layer 2 Relay Agents remains
       the same.

4.2.2.  Issues due to introduction of Layer 2 Relay Agent

   1.  Layer 2 relay agents which maintain persistent state, such as
       updating filters or client registration, must be prepared to
       handle potentially conflicting responses from different DHCP
       Servers.  Some Layer 2 relay agents may use "the most recent DHCP
       packet" to update this persistent state but this may not
       necessarily reflect the actual state of the client.  The above is
       possible when two DHCP servers acknowledge the request of a DHCP
       client with the same address but different lease times.  In this
       case, if the relay agent selects the server reply with the
       shorter lease time, it would expire its state possibly before the



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       client even has a chance to renew it.  Therefore, Layer 2 relay
       agents SHOULD select the longest lease time of two conflicting
       but similar replies, by discarding replies that shorten the lease
       time.

   2.  Other issues are the same as described in section 4.1.2.

4.3.  DHCP server on another subnet with one Layer 3 Relay Agent

   In certain network scenarios, there could be a Layer 3 Relay Agent
   which relays the DHCP messages from one subnet to a DHCP server on
   another subnet and vice versa.  In typical deployments, the Access
   Concentrator acts as Layer 2 Relay Agent and the IP edge device (BRAS
   or IP Services Switch) acts as Layer 3 Relay Agent.


   +--------+
   | End    |        +--------+     |                   |
   | Host#1 +--------|        |     |  +-----------+    |
   +--------+        | Layer  +-----|  |           |    |
                     |   2    |     +--| Layer 3   |----|
   +--------+        | device |     |  | Relay     |    |
   | End    +--------|  #1    |     |  | Agent #1  |    |
   | Host#2 |        +--------+     |  +-----------+    |  +---------+
   +--------+                       |                   |  |         |
                                    |                   +--| DHCP    |
   +--------+                       |                   |  | Server  |
   | End    |        +--------+     |                   |  |   #1    |
   | Host#3 +--------|        |     |                      +---------+
   +--------+        | Layer  +-----|
                     |   2    |     |
   +--------+        | device |     |
   | End    +--------|  #2    +
   | Host#n |        +--------+
   +--------+



                                 Figure 3

4.3.1.  Client-server interaction

   As far as DHCP message processing is concerned, the presence of Layer
   3 Relay Agents is transparent to Layer 2 Relay Agents.  So all the
   messages are handled in the same way as defined in section 4.1.1 for
   the Layer 2 Relay Agent.

   The Layer 3 Relay Agents are configured to trust/untrust an entity



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   based on specific criteria (For example : VLAN/interface on which the
   message was received).  If the DHCP message coming from the client
   has a relay agent option present, the Layer 3 Relay Agent checks if
   it is coming in on a trusted interface.  If it is coming from a
   trusted interface, it will set the 'giaddr' field to one of the local
   interface addresses and unicasts it to the configured server(s).  If
   the message is coming from an untrusted interface, the Layer 3 Relay
   Agent discards the message.

   Typical message processing in this scenario is given below.

   1.  When the client sends a DHCPDISCOVER message, the Layer 2 Relay
       Agent forwards it as described in section 4.1.1.  The Layer 3
       Relay Agent receives this message and finds that it contains a
       Relay Agent Information option.  It verifies whether the message
       is from a trusted entity or not.  If it is from a trusted entity,
       the Layer 2 Relay Agent populates the 'giaddr' field as it deems
       appropriate and relays the message to the DHCP server.

   2.  The DHCP Server processes the message in the same way as
       described in section 4.1 and unicasts the DHCPOFFER to the Layer
       3 Relay Agent on the address specified in the 'giaddr' field.

   3.  The Layer 3 Relay Agent processes the DHCPOFFER and identifies
       the outgoing interface.  It resets the 'giaddr' field and
       broadcasts the message on the identified outgoing interface.

   4.  The client receives the DHCPOFFER and generates a DHCPREQUEST
       message.  The Layer 2 Relay Agent processes it as described in
       section 4.1.1.  The Layer 3 Relay Agent receives the DHCPREQUEST
       message and processes it similar to the DHCPDISCOVER message
       described in step #1.

   5.  The DHCP Server processes the DHCPREQUEST and unicasts the DHCP
       ACK message to the layer 3 Relay Agent if the 'broadcast' flag is
       set, or directly to the client if the 'broadcast' flag is not
       set.  If the Layer 3 Relay Agent receives this message, it
       processes it similar to the DHCPOFFER as described in step #3.

   6.  In the case of unicast messages (For example: DHCPREQUEST in case
       of DHCPRENEW), a Layer 3 Relay Agent may or may not intercept the
       message.  If it intercepts a unicast DHCP request message, it
       populates the 'giaddr' field and relays the message to the DHCP
       server.  When the DHCP server sends a reply for this request
       message, it resets the 'giaddr' field, identifies the outgoing
       interface, and forwards the reply on the identified interface.





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4.3.2.  Issues due to introduction of Layer 2 Relay Agent

   Though the processing of DHCP messages remains the same in Layer 2
   Relay Agents, we see some more issues when a Layer 3 Relay Agent is
   present to relay the DHCP messages to the DHCP server.

   1.  When a Layer 2 Relay Agent is configured to intercept unicast
       messages as well, it appends a Relay Agent Information option
       before forwarding the request message.  A Layer 3 Relay Agent may
       not intercept these unicast messages.  Due to this, a DHCP server
       may not echo back the Relay Agent Information option because the
       'giaddr' field is not populated.

   2.  Existing Layer 3 Relay Agents populate the 'giaddr' field with
       the IP address of the interface on which the request was
       received.  This helps the Layer 3 Relay Agent to identify the
       outgoing interface for the DHCP replies.  In some cases, a Layer
       3 Relay Agent may use unnumbered interfaces.  In this case, it
       has to use a system wide IP address to populate the 'giaddr'
       field.  Due to this, it becomes difficult to identify the correct
       outgoing interface for the messages received from the DHCP
       server.  In these cases, some existing Layer 3 Relay Agent
       implementations maintain an internal state for each DHCP message
       and use this state to identify the outgoing interface.

   3.  A DHCP server uses certain parameters to differentiate the RENEW
       and REBIND state of a client.  A DHCP client unicasts a RENEW
       request to the DHCP server, so the DHCP server sees a DHCPREQUEST
       without 'giaddr' and Relay Agent Information option as a RENEW
       request.  On the other hand, a REBIND request is broadcast and so
       the DHCP server expects it to contain 'giaddr' field and a Relay
       Agent Information option.  If the Layer 2 Relay Agent is
       configured to intercept unicast messages, it will append a Relay
       Agent Information option to the unicast DHCP message.  Because of
       this, it could be difficult for the DHCP server to differentiate
       between a RENEWING and REBINDING state.















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5.  Acknowledgments

   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
   some of the existing implementations.  Thanks to Ted Lemon, Mukund
   Kamath, Alfred Hoenes and Stefaan De Cnodder for reviewing the draft
   and providing valuable suggestions.












































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6.  Security Considerations

   o  A Layer 2 Relay Agent should always be configured to identify a
      trustable entity so that it appends a Relay Agent Information
      option to a DHCP message coming from a trustable entity and
      forwards it.  If a DHCP message is received from a non-trustable
      entity, the Layer 2 Relay Agent should discard it and may report
      to the administrator.

   o  The introduction of Layer 2 Relay Agents does not introduce any
      new security issues.  Security issues pertaining to Relay Agents
      in general apply to Layer 2 Relay Agents as well.







































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7.  IANA Considerations

   This document does not introduce any new namespaces for the IANA to
   manage and does not request any new code point assignments.















































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8.  References

8.1.  Normative Reference

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, March 1997.

   [RFC3046]  Patrick, M., "DHCP Relay Agent Information Option",
              RFC 3046, January 2001.

   [RFC3118]  Droms, R. and B. Arbaugh, "Authentication for DHCP
              Messages", RFC 3118, June 2001.

   [RFC3232]  Reynolds, J., "Assigned Numbers", RFC 3232, January 2002.

8.2.  Informative Reference

   [RFC951]   Croft, B. and J. Gilmore, "Bootstrap Protocol (BOOTP)",
              RFC 951, September 1985.

   [RFC1542]  Wimer, W., "Clarifications and Extensions for the
              Bootstrap Protocol", RFC 1542, October 1993.

   [RFC2132]  Droms, R. and S. Alexander, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, March 1997.























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Authors' Addresses

   Bharat Joshi
   Infosys Technologies Ltd.
   44 Electronics City, Hosur Road
   Bangalore  560 100
   India

   Email: bharat_joshi@infosys.com
   URI:   http://www.infosys.com/


   Pavan Kurapati
   Infosys Technologies Ltd.
   44 Electronics City, Hosur Road
   Bangalore  560 100
   India

   Email: pavan_kurapati@infosys.com
   URI:   http://www.infosys.com/































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