The DHCPv4 Relay Agent Identifier Suboption

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Document Type Active Internet-Draft (dhc WG)
Authors Bharat Joshi  , D.T.V. Rao  , Mark Stapp 
Last updated 2013-01-15
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DHC                                                             B. Joshi
Internet-Draft                                        D. Ramakrishna Rao
Intended status: Standards Track                            Infosys Ltd.
Expires: July 19, 2013                                          M. Stapp
                                                     Cisco Systems, Inc.
                                                        January 15, 2013

              The DHCPv4 Relay Agent Identifier Suboption


   This document defines a new Relay Agent Identifier suboption for the
   Dynamic Host Configuration Protocol's (DHCP) Relay Agent Information
   option.  The suboption carries a value that uniquely identifies the
   relay agent device within the administrative domain.  The value is
   normally administratively-configured in the relay agent.  The
   suboption allows a DHCP relay agent to include the identifier in the
   DHCP messages it sends.

Status of this Memo

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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Example Use-Cases . . . . . . . . . . . . . . . . . . . . . . . 3
     3.1.  Bulk Leasequery . . . . . . . . . . . . . . . . . . . . . . 3
     3.2.  Industrial Ethernet . . . . . . . . . . . . . . . . . . . . 3
   4.  Suboption Format  . . . . . . . . . . . . . . . . . . . . . . . 4
   5.  Identifier Stability  . . . . . . . . . . . . . . . . . . . . . 4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     9.2.  Informative References  . . . . . . . . . . . . . . . . . . 8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8

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1.  Introduction

   The Dynamic Host Configuration Protocol for IPv4 (DHCPv4) [RFC2131]
   provides IP addresses and configuration information for IPv4 clients.
   It includes a relay agent capability, in which network elements
   receive broadcast messages from clients and forward them to DHCP
   servers as unicast messages.  In many network environments, relay
   agents add information to the DHCP messages before forwarding them,
   using the Relay Agent Information option [RFC3046].  Servers that
   recognize the relay agent information option echo it back in their

   This specification introduces a Relay Agent Identifier (Relay-Id)
   suboption for the Relay Agent Information option.  The Relay-Id
   suboption carries a sequence of octets that is intended to uniquely
   identify the relay agent within the administrative domain.  In this
   document, an administrative domain consist of all DHCP servers and
   relay agents that communicate with each other.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   DHCPv4 terminology is defined in [RFC2131], and the DHCPv4 Relay
   Agent Information Option in [RFC3046].

3.  Example Use-Cases

3.1.  Bulk Leasequery

   There has been quite a bit of recent interest in extending the DHCP
   Leasequery protocol [RFC4388] to accommodate some additional
   situations.  There is a recent document
   [I-D.ietf-dhc-dhcpv4-bulk-leasequery] proposing a variety of
   enhancements to the existing Leasequery protocol.  The document
   describes a use-case where a relay agent queries DHCP servers using
   the Relay Identifier to retrieve all the leases allocated through the
   relay agent.

3.2.  Industrial Ethernet

   DHCP typically identifies clients based on information in their DHCP
   messages - such as the Client-Identifier option, or the value of the
   chaddr field.  In some networks, however, the location of a client -

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   its point of attachment to the network - is a more useful identifier.
   In factory-floor networks (commonly called 'Industrial' networks),
   for example, the role a device plays is often fixed and based on its
   location.  Using manual address configuration is possible (and is
   common) but it would be beneficial if DHCP configuration could be
   applied to these networks.

   One way to provide connection-based identifiers for industrial
   networks is to have the network elements acting as DHCP relay agents
   supply information that a DHCP server could use as a client
   identifier.  A straightforward way to form identifier information is
   to combine something that is unique within the scope of the network
   element, such as a port/slot value, with something that uniquely
   identifies that network element, such as a Relay Agent Identifier.

4.  Suboption Format

   Format of the Relay Agent Identifier suboption:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |SUBOPT_RELAY_ID|    length     |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
      .                                                               .
      .                   identifier (variable)                       .
      .                                                               .



      length            the number of octets in the suboption
                        (excluding the suboption ID and length fields);
                        the minimum length is one.

      identifier        the identifying data.

5.  Identifier Stability

   If the relay identifier is to be meaningful it has to be stable.  A
   relay agent SHOULD use a single identifier value consistently.  The

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   identifier used by a relay device SHOULD be committed to stable
   storage, unless the relay device can regenerate the value upon

   If the relay-id configured in a relay agent is not unique within its
   administrative domain, resource allocation problems may occur as the
   DHCP server attempts to allocate the same resource to devices behind
   two different relay agents.  Therefore, relay-id configured in a
   relay agent MUST be unique within its administrative domain.  To aid
   in ensuring uniqueness of relay-ids, relay agents SHOULD make their
   relay identifiers visible to their administrators via their user
   interface, through a log entry, through a MIB field, or through some
   other mechanism.

   Implementors of relay agents should note that the identifier needs to
   be present in all DHCP message types where its value is being used by
   the DHCP server.  The relay agent may not be able to add the Relay
   Agent Information option to all messages - such as RENEW messages
   sent as IP unicasts.  In some deployments that might mean that the
   server has to be willing to continue to associate the relay
   identifier it has last seen with a lease that is being RENEWed.
   Other deployments may prefer to use the Server Identifier Override
   suboption [RFC5107] to permit the relay device to insert the Relay
   Agent Information option into all relayed messages.

   Handling situations where a relay agent device is replaced is another
   aspect of stability.  One of the use-cases for the relay identifier
   is to permit a server to associate clients' lease bindings with the
   relay device connected to the clients.  If the relay device is
   replaced, because it has failed or been upgraded, it may be desirable
   for the new device to continue to provide the same relay identifier
   as the old device.  Therefore if a relay agent supports relay-id, the
   relay-id should be administratively configurable.


       Administrators should take special care to ensure that relay-ids
       configured in their relay agents are not duplicated. Some
       implementation advice is offered to administrators with regard
       to configuration of relay-ids, detection and consequences of
       duplicate relay-ids.

       Configuration of Relay-IDs:

       Various strategies may be used to configure relay-ids. Any
       proposed strategy should be evaluated in terms of whether it can
       ensure unique relay-ids in the administrative domain. It should

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       be noted that relay-ids configured using the strategy must also
       satisfy requirements as stated in the rest of this document
       (especially Section 5). One strategy that may be used is relay-id
       on a relay agent may re-use an existing identifier or set of
       identifiers that are already guaranteed to be unique (e.g.,
       UUID [RFC4122] or IP address).

       Consequences and Detection of Duplication of Relay-IDs:

       This document only defines relay-id suboption but not its
       use-cases.  Consequences of duplication of relay-ids depend on
       how relay-ids are used. Administrators should create mechanisms
       to detect duplication of relay-ids.

       Some mechanisms to detect duplication can be created based on
       use-cases of relay-id. For example, DHCP servers use various
       decision criteria during allocation of IP addresses and other
       resources. If relay-id is part of the decision criteria, DHCP
       server will attempt, but fail, to allocate the same resource
       (typically an IP address) to two devices on the opposite side
       of the two relay agents with duplicate IDs. In most cases this
       won't happen, because the DHCP server isn't configured that way;
       in the cases where it does happen, DHCP server should log the

       It should be emphasized that these mechanisms may not be
       fool-proof at indicating duplication of relay-ids as the cause
       (the failures may be caused because of other reasons as well.)
       But they serve as a first step in the analysis towards detection
       of duplication relay-ids.

       In contrast, the following approach is suggested as a general
       mechanism to detect duplication of relay-ids. Network management
       systems collect various types of information from the devices
       under their control. As part of this, they should also collect
       relay-id configured for each relay-agent (it becomes easy to do
       if relay-id is exposed as a MIB field). At the network management
       subsystem that has visibility into the entire administrative
       domain, it should have back-end tools to check for duplicate
       relay ids in the collected information.

6.  Security Considerations

   Security issues with the Relay Agent Information option and its use
   by servers in address assignment are discussed in [RFC3046] and
   [RFC4030].  The DHCP Relay Agent Information option depends on a
   trusted relationship between the DHCP relay agent and the DHCP

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   server, as described in Section 5 of RFC 3046.  While the
   introduction of fraudulent DHCP relay agent information options can
   be prevented by a perimeter defense that blocks these options unless
   the DHCP relay agent is trusted, a deeper defense using the
   authentication suboption for DHCP relay agent information option
   [RFC4030] SHOULD be deployed as well.  It also helps in avoiding
   duplication of relay identifiers by malicious entities.  However,
   implementation of authentication suboption for DHCP relay agent
   information option [RFC4030] is not a must to support relay-id

7.  IANA Considerations

   We request that IANA assign a new suboption code from the registry of
   DHCP Agent Sub-Option Codes maintained in

      Relay Agent Identifier Suboption [TBA]

8.  Acknowledgments

   Thanks to Bernie Volz, David W. Hankins, Pavan Kurapati and Ted Lemon
   for providing valuable suggestions.

9.  References

9.1.  Normative References

   [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.

   [RFC4030]  Stapp, M. and T. Lemon, "The Authentication Suboption for
              the Dynamic Host Configuration Protocol (DHCP) Relay Agent
              Option", RFC 4030, March 2005.

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9.2.  Informative References

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              July 2005.

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

   [RFC5107]  Johnson, R., Kumarasamy, J., Kinnear, K., and M. Stapp,
              "DHCP Server Identifier Override Suboption", RFC 5107,
              February 2008.

              Kinnear, K., Stapp, M., Joshi, B., and N. Russell, "Bulk
              DHCPv4 Lease Query",
              draft-ietf-dhc-dhcpv4-bulk-leasequery-07 (work in
              progress), October 2012.

Authors' Addresses

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


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


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   Mark Stapp
   Cisco Systems, Inc.
   1414 Massachusetts Ave.
   Boxborough, MA  01719

   Phone: +1 978 936 0000

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