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RFC 6148 - DHCPv4 Lease Query by Relay Agent Remote ID


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Internet Engineering Task Force (IETF)                       P. Kurapati
Request for Comments: 6148                              Juniper Networks
Updates: 4388                                                 R. Desetti
Category: Standards Track                                       B. Joshi
ISSN: 2070-1721                                Infosys Technologies Ltd.
                                                           February 2011

              DHCPv4 Lease Query by Relay Agent Remote ID

Abstract

   Some relay agents extract lease information from the DHCP messages
   exchanged between the client and DHCP server.  This lease information
   is used by relay agents for various purposes like antispoofing and
   prevention of flooding.  RFC 4388 defines a mechanism for relay
   agents to retrieve the lease information from the DHCP server when
   this information is lost.  The existing lease query mechanism is
   data-driven, which means that a relay agent can initiate the lease
   query only when it starts receiving data to and from the clients.  In
   certain scenarios, this model is not scalable.  This document first
   looks at issues in the existing mechanism and then proposes a new
   query type, query by Remote ID, to address these issues.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6148.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   publication of this document.  Please review these documents
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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
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   Without obtaining an adequate license from the person(s) controlling
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   than English.

Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................4
   3. Motivation ......................................................6
   4. Protocol Details ................................................7
      4.1. Sending the DHCPLEASEQUERY Message .........................7
      4.2. Responding to the DHCPLEASEQUERY Message ...................8
      4.3. Building a DHCPLEASEACTIVE or DHCPLEASEUNKNOWN Message .....8
      4.4. Determining the IP Address to Be Used in Response ..........9
      4.5. Sending a DHCPLEASEACTIVE or DHCPLEASEUNKNOWN Message ......9
      4.6. Receiving a DHCPLEASEACTIVE or DHCPLEASEUNKNOWN Message ....9
      4.7. Receiving No Response to the DHCPLEASEQUERY Message .......10
      4.8. Lease-Binding Data Storage Requirements ...................10
      4.9. Using the DHCPLEASEQUERY Message with Multiple
           DHCP Servers ..............................................10
   5. RFC 4388 Considerations ........................................11
   6. Security Considerations ........................................11
   7. Acknowledgments ................................................11
   8. References .....................................................12
      8.1. Normative References ......................................12
      8.2. Informative References ....................................12

1.  Introduction

   DHCP relay agents snoop DHCP messages and append a Relay Agent
   Information option before relaying them to the configured DHCP
   server.  In this process, some relay agents also glean the lease
   information sent by the server and maintain this locally.  This
   information is used to prevent spoofing attempts from clients and
   also sometimes to install routing information.  When a relay agent
   reboots, this information is lost.  RFC 4388 [RFC4388] has defined a
   mechanism to retrieve this lease information from the DHCP server.
   The existing query types defined by RFC 4388 [RFC4388] are data-
   driven.  When a client sends data upstream, the relay agent can query
   the server about the related lease information, based on the source
   MAC/IP address.  These mechanisms do not scale well when there are
   thousands of clients connected to the relay agent.  In the data-
   driven model, lease query does not provide the full and consolidated
   active lease information associated with a given connection/circuit,
   which will result in inefficient anti-spoofing.  The relay agent also
   has to contend with considerable resources for negative caching,
   especially under spoofing attacks.

   We need a mechanism for a relay agent to retrieve the consolidated
   lease information for a given connection/circuit before upstream
   traffic is sent by the clients.

              +--------+
              |  DHCP  |     +--------------+
              | Server |-...-|    DSLAM     |
              |        |     |  Relay Agent |
              +--------+     +--------------+
                                |        |
                            +------+   +------+
                            |Modem1|   |Modem2|
                            +------+   +------+
                               |        |    |
                            +-----+  +-----+ +-----+
                            |Node1|  |Node2| |Node3|
                            +-----+  +-----+ +-----+

                                 Figure 1

   For example, when a DSLAM (Digital Subscriber Line Access
   Multiplexer) acting as a relay agent is rebooted, it should query the
   server for the lease information for all the connections/circuits.
   Also, as shown in the above figure, there could be multiple clients
   on one DSL circuit.  The relay agent should get the lease information
   of all the clients connected to a DSL circuit.  This is possible by
   introducing a new query type based on the Remote ID sub-option of the
   Relay Agent Information option.  This document talks about the
   motivation for the new query type and the method to perform it.

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  Access Concentrator

      An access concentrator is a router or switch at the broadband
      access provider's edge of a public broadband access network.  This
      document assumes that the access concentrator includes the DHCP
      relay agent functionality.

   o  DHCP client

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

   o  DHCP relay agent

      A DHCP relay agent is a third-party agent that transfers Bootstrap
      Protocol (BOOTP) and DHCP messages between clients and servers
      residing on different subnets, per RFC 951 [RFC951] and RFC 1542
      [RFC1542].

   o  DHCP server

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

   o  Fast path

      Fast path refers to data transfer that happens through a network
      processor or an Application Specific Integrated Circuit (ASIC)
      programmed to forward the data at very high speeds.

   o  Gleaning

      Gleaning is the extraction of location information from DHCP
      messages as the messages are forwarded by the DHCP relay agent
      function.

   o  Location information

      Location information is information needed by the access
      concentrator to forward traffic to a broadband-accessible node.
      This information includes knowledge of the node's hardware
      address, the port or virtual circuit that leads to the node,
      and/or the hardware address of the intervening subscriber modem.

   o  MAC address

      In the context of a DHCP packet, a MAC address consists of the
      following fields: hardware type ("htype"), hardware length
      ("hlen"), and client hardware address ("chaddr").

   o  Slow path

      Slow path refers to data transfer that happens through the control
      plane.  This has very limited buffers to store data, and the
      speeds are very low compared to the fast path data transfer.

   o  Upstream

      Upstream is the direction from the broadband subscriber towards
      the access concentrator.

3.  Motivation

   Consider an access concentrator (e.g., DSLAM) working also as a DHCP
   relay agent.  A "fast path" and a "slow path" generally exist in most
   networking boxes.  Fast path processing is done in a network
   processor or an ASIC.  Slow path processing is done in a normal
   processor.  As much as possible, regular data forwarding should be
   done in the fast path.  Slow path processing should be reduced, as it
   may become a bottleneck.

   For an access concentrator having multiple access ports, multiple IP
   addresses may be assigned to a single port using DHCP, and the number
   of clients on a port may be unknown.  The access concentrator may
   also not know the network portions of the IP addresses that are
   assigned to its DHCP clients.

   The access concentrator gleans IP address or other information from
   DHCP negotiations for antispoofing and other purposes.  The
   antispoofing itself is done in the fast path.  The access
   concentrator keeps track of only one list of IP addresses: the list
   of IP addresses that are assigned by the DHCP servers; upstream
   traffic from all other IP addresses is dropped.  If a client starts
   its data transfer after its DHCP negotiations have been gleaned by
   the access concentrator, no legitimate packets will be dropped
   because of antispoofing.  In other words, antispoofing is effective
   (no legitimate packets are dropped, and all spoofed packets are
   dropped) and efficient (antispoofing is done in the fast path).  The
   intention is to achieve similar effective and efficient antispoofing
   in the lease query scenario also, when an access concentrator loses
   its gleaned information (for example, because of a reboot).

   After a deep analysis, we found that the three existing query types
   supported by RFC 4388 [RFC4388] do not provide effective and
   efficient antispoofing for the above scenario, and a new mechanism is
   required.

   The existing query types necessitate a data-driven approach: the
   lease queries can only be performed when the access concentrator
   receives data.  This results in

   o  increased outage time for clients

   o  excessive negative caching, consuming a lot of resources under a
      spoofing attack

   o  antispoofing being done in the slow path instead of the fast path

4.  Protocol Details

   This section talks about the protocol details for query by Remote ID.
   Most of the message handling is similar to RFC 4388 [RFC4388], and
   this section highlights only the differences.  Readers are advised to
   go through RFC 4388 [RFC4388] before going through this section for
   complete understanding of the protocol.

   When used in this document, the unqualified term "DHCPLEASEQUERY"
   indicates a lease query by Remote ID, unless otherwise specified.

   RFC 3046 [RFC3046] defines two sub-options for the Relay Agent
   Information option.  Sub-option 1 corresponds to the Circuit ID that
   identifies the local circuit of the access concentrator.  This
   sub-option is unique to the relay agent.  Sub-option 2 corresponds to
   the Remote ID that identifies the remote node connected to the access
   concentrator.  The Remote ID is globally unique in the network and is
   configured per circuit/connection in the relay agent.

   This document defines a new query type based on the Remote ID
   sub-option.  Suppose that the access concentrator (e.g., DSLAM) lost
   the lease information when it was rebooted.  When the access
   concentrator comes up, it initiates (for each connection/circuit) a
   DHCP lease query by Remote ID as defined in this section.  For this
   query, the requester supplies an option 82 that includes only a
   Remote ID sub-option in the DHCPLEASEQUERY message.  The Remote ID is
   normally pre-provisioned in the access concentrator per circuit/
   connection and hence will remain available to the access concentrator
   after reboot.

   The DHCP server MUST reply with a DHCPLEASEACTIVE message if there is
   an active lease corresponding to the Remote ID that is present in the
   DHCPLEASEQUERY message.  Otherwise, the server MUST reply with a
   DHCPLEASEUNKNOWN message.  Servers that do not implement DHCP lease
   query based on Remote ID SHOULD simply not respond.

4.1.  Sending the DHCPLEASEQUERY Message

   The lease query defined in this document will mostly be used by
   access concentrators, but it may also be used by other authorized
   elements in the network.  The DHCPLEASEQUERY message uses the DHCP
   message format as described in RFC 2131 [RFC2131], and uses message
   number 10 in the DHCP Message Type option (option 53).  The
   DHCPLEASEQUERY message has the following pertinent message contents:

   o  There MUST be a Relay Agent Information option (option 82) with
      only a Remote ID sub-option (sub-option 2) in the DHCPLEASEQUERY
      message.

   o  The Parameter Request List option [RFC2132] MUST be populated by
      the access concentrator with the Associated-IP option code.  The
      giaddr field and other option codes listed in the Parameter
      Request List option are set as explained in Section 6.2 of
      RFC 4388 [RFC4388].

   o  The ciaddr field MUST be set to zero.

   o  The values of htype, hlen, and chaddr MUST be set to zero.

   o  The Client Identifier option (option 61) MUST NOT appear in the
      packet.

   The DHCPLEASEQUERY message SHOULD be sent to a DHCP server that is
   known to possess authoritative information concerning the Remote ID.
   The DHCPLEASEQUERY message MAY be sent to more than one DHCP server,
   and in the absence of information concerning which DHCP server might
   possess authoritative information concerning the Remote ID, it SHOULD
   be sent to all DHCP servers configured for the associated relay agent
   (if any are known).

4.2.  Responding to the DHCPLEASEQUERY Message

   There are two possible responses to a DHCPLEASEQUERY message:

   o  DHCPLEASEUNKNOWN

      The DHCPLEASEUNKNOWN message indicates that the client associated
      with the Remote ID sub-option of the DHCPLEASEQUERY message is not
      allocated any lease or it is not managed by the server.

   o  DHCPLEASEACTIVE

      The DHCPLEASEACTIVE message indicates that the server not only
      knows the client specified in the DHCPLEASEQUERY message, but also
      knows that there is an active lease for that client.

4.3.  Building a DHCPLEASEACTIVE or DHCPLEASEUNKNOWN Message

   A DHCPLEASEACTIVE message is built by populating information
   pertaining to the client associated with the IP address specified in
   the ciaddr field.

   In the case where more than one IP address has been involved in a
   DHCP message exchange with the client specified by the Remote ID,
   then the list of all those IP addresses MUST be returned in the
   Associated-IP option, whether or not that option was requested as
   part of the Parameter Request List option.  This is intended for
   maintaining backwards compatibility with RFC 4388 [RFC4388].

   All other options specified in the Parameter Request List [RFC2132]
   are processed as mentioned in Section 6.4.2 of RFC 4388 [RFC4388].

   In a DHCPLEASEUNKNOWN response message, the DHCP server MUST echo the
   option 82 received in the DHCPLEASEQUERY message.  No other option is
   included in the message.

4.4.  Determining the IP Address to Be Used in Response

   The IP address placed in the ciaddr field of a DHCPLEASEACTIVE
   message MUST be the IP address with the latest client-last-
   transaction-time associated with the client described by the Remote
   ID specified in the DHCPLEASEQUERY message.

   If there is only a single IP address that fulfills this criteria,
   then it MUST be placed in the ciaddr field of the DHCPLEASEACTIVE
   message.

   In the case where more than one IP address has been accessed by the
   client specified by the Remote ID, then the DHCP server MUST return
   the IP address returned to the client in the most recent transaction
   with the client, unless the DHCP server has been configured by the
   server administrator to use some other preference mechanism.

4.5.  Sending a DHCPLEASEACTIVE or DHCPLEASEUNKNOWN Message

   The server unicasts the DHCPLEASEACTIVE or DHCPLEASEUNKNOWN message
   to the address specified in the giaddr field of the DHCPLEASEQUERY
   message.

4.6.  Receiving a DHCPLEASEACTIVE or DHCPLEASEUNKNOWN Message

   When a DHCPLEASEACTIVE message is received in response to the
   DHCPLEASEQUERY message, it means that there is currently an active
   lease associated with the Remote ID in the DHCP server.  The access
   concentrator SHOULD use the information in the htype, hlen, and
   chaddr fields of the DHCPLEASEACTIVE message as well as the Relay

   Agent Information option included in the packet to refresh its
   location information for this IP address.  An access concentrator is
   likely to query by IP address for all the IP addresses specified in
   the Associated-IP option in the response, if any, at this point in
   time.

   When a DHCPLEASEUNKNOWN message is received by an access concentrator
   that had sent out a DHCPLEASEQUERY message, it means that the DHCP
   server does not have definitive information concerning the DHCP
   client specified in the Remote ID sub-option of the DHCPLEASEQUERY
   message.  The access concentrator MAY store this information for
   future use.  However, another DHCPLEASEQUERY message to the same DHCP
   server SHOULD NOT be attempted with the same Remote ID sub-option.

   For lease query by Remote ID, the impact of negative caching is
   greatly reduced, as the response leads to "definitive" information on
   all the nodes connected behind the connection.  Note that in the case
   of the data-driven approach [RFC4388], a node spoofing several IP
   addresses can lead to negative caching of greater magnitude.  Another
   important change that this document brings is the removal of periodic
   lease queries generated from negative caching caused by
   DHCPLEASEUNKNOWN messages.  Since the information obtained through
   query by Remote ID is complete, there is no need to attempt lease
   query again for the same connection.

4.7.  Receiving No Response to the DHCPLEASEQUERY Message

   The condition of an access concentrator receiving no response to a
   DHCPLEASEQUERY message is handled in the same manner as suggested in
   RFC 4388 [RFC4388].

4.8.  Lease-Binding Data Storage Requirements

   Implementation Note:

   To generate replies for a lease query by Remote ID efficiently, a
   DHCP server should index the lease-binding data structures using
   Remote ID.

4.9.  Using the DHCPLEASEQUERY Message with Multiple DHCP Servers

   This scenario is handled in the same way it is done in RFC 4388
   [RFC4388].

5.  RFC 4388 Considerations

   This document is compatible with RFC 4388-based [RFC4388]
   implementations, which means that a client that supports this
   extension can work with a server not supporting this document,
   provided it uses RFC 4388-defined query types.  Also, a server
   supporting this document can work with a client not supporting this
   query type.  However, there are some changes that this document
   proposes with respect to RFC 4388 [RFC4388].  Implementers extending
   RFC 4388 [RFC4388] implementations to support this document should
   take note of the following points:

   o  There may be cases where a query by IP address/MAC address/Client
      Identifier has an option 82 containing a Remote ID.  In that case,
      the query will still be recognized as a query by IP address/MAC
      address/Client Identifier as specified by RFC 4388 [RFC4388].

   o  Section 6.4 of RFC 4388 [RFC4388] suggests that a DHCPLEASEUNKNOWN
      message MUST NOT have any other option present.  But for a query
      by Remote ID, option 82 MUST be present in the reply.

6.  Security Considerations

   This document inherits the security concerns present in the original
   lease query protocol specification (RFC 4388 [RFC4388]).

   This specification introduces one additional issue, beyond those
   described in RFC 4388 [RFC4388].  A query by Remote ID will result in
   the server replying with consolidated lease-binding information.
   Such a query, if done from an unauthorized source, may lead to a leak
   of lease-binding information.  It is critical to deploy
   authentication techniques mentioned in RFC 3118 [RFC3118] to prevent
   such unauthorized lease queries.

7.  Acknowledgments

   Copious amounts of text in this document are derived from RFC 4388
   [RFC4388].  Kim Kinnear, Damien Neil, Stephen Jacob, Ted Lemon, Ralph
   Droms, and Alfred Hoenes provided valuable feedback on this document.

8.  References

8.1.  Normative References

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

   [RFC4388]  Woundy, R. and K. Kinnear, "Dynamic Host Configuration
              Protocol (DHCP) Leasequery", RFC 4388, February 2006.

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

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

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

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

8.2.  Informative References

   [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.

Authors' Addresses

   Pavan Kurapati
   Juniper Networks
   Embassy Prime Buildings, C.V. Raman Nagar
   Bangalore  560 093
   India

   EMail: kurapati@juniper.net
   URI:   http://www.juniper.net/

   D.T.V Ramakrishna Rao
   Infosys Technologies Ltd.
   44 Electronics City, Hosur Road
   Bangalore  560 100
   India

   EMail: ramakrishnadtv@infosys.com
   URI:   http://www.infosys.com/

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

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

 

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