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RFC 6221 - Lightweight DHCPv6 Relay Agent


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Internet Engineering Task Force (IETF)                     D. Miles, Ed.
Request for Comments: 6221                                      S. Ooghe
Updates: 3315                                             Alcatel-Lucent
Category: Standards Track                                         W. Dec
ISSN: 2070-1721                                            Cisco Systems
                                                             S. Krishnan
                                                             A. Kavanagh
                                                                Ericsson
                                                                May 2011

                     Lightweight DHCPv6 Relay Agent

Abstract

   This document proposes a Lightweight DHCPv6 Relay Agent (LDRA) that
   is used to insert relay agent options in DHCPv6 message exchanges
   identifying client-facing interfaces.  The LDRA can be implemented in
   existing access nodes (such as Digital Subscriber Link Access
   Multiplexers (DSLAMs) and Ethernet switches) that do not support IPv6
   control or routing functions.

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/rfc6221.

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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
      1.1. Requirements Language ......................................3
   2. Background ......................................................3
   3. Terminology .....................................................3
   4. Server Considerations ...........................................5
   5. Message Format ..................................................5
      5.1. Relay-Forward Message ......................................5
      5.2. Relay-Reply Message ........................................6
      5.3. Mandatory DHCP Options .....................................6
           5.3.1. Relay-Message Option ................................6
           5.3.2. Interface-ID Option .................................6
   6. Agent Behaviour .................................................7
      6.1. Relaying a Client Message ..................................7
           6.1.1. Client Message Validation ...........................8
           6.1.2. Trusted and Untrusted Interfaces ....................8
      6.2. Relaying a Relay-Reply Message from the Network ............8
   7. Network Topology ................................................9
      7.1. Client and Server on Same Link .............................9
      7.2. Client and Server behind Relay Agent ......................11
      7.3. Relay Agent in Front of LDRA ..............................12
   8. Contributors ...................................................15
   9. Security Considerations ........................................15
   10. References ....................................................15
      10.1. Normative References .....................................15
      10.2. Informative References ...................................16

1.  Introduction

   DHCPv6 Relay Agents [RFC3315] are deployed to forward DHCPv6 messages
   between clients and servers when they are not on the same IPv6 link
   and are often implemented alongside a routing function in a common
   node.  A Lightweight DHCPv6 Relay Agent (LDRA) allows Relay Agent
   Information to be inserted by an access node that performs a link-
   layer bridging (i.e., non-routing) function.  An LDRA resides on the
   same IPv6 link as the client and a DHCPv6 Relay Agent or server, and
   is functionally the equivalent of the Layer 2 Relay Agent proposed
   for DHCPv4 operation in [L2RA].

   Unlike a DHCPv6 Relay Agent specified in [RFC3315], an LDRA does not
   implement any IPv6 control functions (e.g., ICMPv6) or have any
   routing capability in the node.

1.1.  Requirements Language

   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].

2.  Background

   A variety of different link-layer network topologies exist for the
   aggregation of IPv6 nodes into one or more routers.  In Layer 2
   aggregation networks (IEEE 802.1D bridging or similar) that have many
   nodes on a single link, a DHCPv6 server or DHCP Relay Agent would
   normally be unaware of how a DHCP client is attached to the network.
   The LDRA allows Relay Agent Information, including the Interface-ID
   option [RFC3315], to be inserted by the access node so that it may be
   used by the DHCPv6 server for client identification.  A typical
   application in a broadband service provider could be equivalent to a
   Layer 2 DHCP Relay Agent as described in the Broadband Forum TR-101
   report [TR-101] and in [L2RA].

3.  Terminology

   Access Node               A device that combines many interfaces onto
                             one link.  An access node is not IP-aware
                             in the data path.

   Address                   An IP layer identifier for an interface or
                             set of interfaces.

   Client-facing             An interface on the access node that
                             carries traffic towards the DHCPv6 client.

   Host                      A non-routing IPv6 node that is
                             participating in a DHCPv6 message exchange.

   IP                        Internet Protocol Version 6 (IPv6).

   LDRA                      Lightweight DHCPv6 Relay Agent.

   Lightweight Relay Agent   A function on the access node that
                             intercepts DHCP messages between clients
                             and servers.  The function exists as a bump
                             in the wire on the IP link.

   Link                      A communication facility or medium over
                             which nodes can communicate at the link
                             layer.

   Link-local address        An IP address having only local scope,
                             indicated by having the address prefix
                             fe80::/10, that can be used to reach
                             neighbouring nodes attached to the same
                             link.  Every interface has a link-local
                             address.

   Network-facing            An interface on the access node that
                             carries traffic towards the DHCPv6
                             server(s).

   Node                      A device that implements IPv6.

   Router                    A node that forwards packets not directly
                             addressed to itself.

   Relay Agent               A node that acts as an intermediary to
                             deliver DHCP messages between clients and
                             servers and being on the same link as the
                             client.

   Unspecified address       An IPv6 address that is comprised entirely
                             of zeros.

4.  Server Considerations

   This document updates the behaviour specified in Section 11 of DHCP
   for IPv6 [RFC3315].  RFC 3315 states, in part:

      If the server receives the message from a forwarding relay agent,
      then the client is on the same link as the one to which the
      interface, identified by the link-address field in the message
      from the relay agent, is attached.

   DHCP server implementations conforming to this specification MUST,
   for the purposes of address selection, ignore any link-address field
   whose value is zero.  In the above text from RFC 3315, "link-address"
   refers to both the link-address field of the Relay-Forward message,
   and the link-address fields in any Relay-Forward messages that may be
   nested within the Relay-Forward message.

5.  Message Format

   The Lightweight DHCPv6 Relay Agent (LDRA) exchanges DHCP messages
   between clients and servers using the message formats established in
   [RFC3315].

   To maintain interoperability with existing DHCP relays and servers,
   the message format is unchanged from [RFC3315].  The LDRA implements
   the same message types as a normal DHCPv6 Relay Agent.  They are:

   o  Relay-Forward Messages

   o  Relay-Reply Messages

5.1.  Relay-Forward Message

   The Relay-Forward message is created by any DHCPv6 Relay Agent,
   including an LDRA, to forward messages between clients and servers or
   other relay agents.  These messages are built as specified in
   [RFC3315].

   The Relay-Forward message contains relay agent parameters that
   identify the client-facing interface on which any reply messages
   should be forwarded.  These parameters are link-address, peer-
   address, and Interface-ID.  The link-address parameter MUST be set to
   the unspecified address.  The peer-address parameter MUST be set as
   specified in Section 6.1.  The Interface-ID Relay Agent option MUST
   be included in the Relay-Forward message.  The LDRA MAY insert
   additional relay agent options.

5.2.  Relay-Reply Message

   The Relay-Reply message is constructed by a DHCPv6 server to send
   parameters to a DHCP client when a relay agent is present between the
   server and the client.  The Relay-Reply message may be sent after an
   initial Relay-Forward message as the parameters link-address, peer-
   address, and Interface-ID, as well as the relay agent's IP address,
   are learnt from the Relay-Forward message.

   The server MUST include the Interface-ID option in the Relay-Reply
   Message to indicate to the LDRA the interface on which the
   decapsulated message should be forwarded.

5.3.  Mandatory DHCP Options

   Parameters are exchanged between the DHCP client, Relay Agent, and
   server through the use of DHCP options.  There is a set of mandatory
   DHCP options that MUST be included by the LDRA in all Relay-Forward
   messages.  These are the:

   o  Relay-Message Option

   o  Interface-ID Option

5.3.1.  Relay-Message Option

   A DHCPv6 Relay Agent relays messages between clients and servers or
   other relay agents through Relay-Forward and Relay-Reply message
   types.  The original client DHCP message (i.e., the packet payload,
   excluding UDP and IP headers) is encapsulated in a Relay Message
   option [RFC3315].

   If a Relay-Message would exceed the MTU of the outgoing interface, it
   MUST be discarded, and an error condition SHOULD be logged.

5.3.2.  Interface-ID Option

   The LDRA MUST include the Interface-ID option [RFC3315] in all Relay-
   Forward messages.  When an LDRA receives a Relay-Reply message with
   an Interface-ID option present and link-address unspecified, the LDRA
   MUST relay the decapsulated message to the client on the interface
   identified in the Interface-ID option.

   Servers MAY use the Interface-ID for parameter assignment policies.
   The format of the Interface-ID is outside the scope of this
   contribution.  The Interface-ID SHOULD be considered an opaque value;
   i.e., the server SHOULD NOT try to parse the contents of the
   Interface-ID option.  The LDRA SHOULD use the same Interface-ID value

   for a given interface, and this value SHOULD be retained across
   restarts.  This is because if the Interface-ID changes, a server will
   not be able to use it reliably in parameter assignment policies.

6.  Agent Behaviour

   The LDRA MUST have each of its interfaces configured as either
   client-facing or network-facing.  The LDRA uses the notion of client-
   facing and network-facing interfaces to process DHCPv6 messages.

6.1.  Relaying a Client Message

   The LDRA MUST intercept and process all IP traffic received on any
   client-facing interface that has:

   o  destination IP address set to All_DHCP_Relay_Agents_and_Servers
      (ff02::1:2);

   o  protocol type UDP; and

   o  destination port 547.

   The LDRA MUST also prevent the original message from being forwarded
   on the network-facing interface.

   The lightweight relay agent adds any other options it is configured
   or required to include in the Relay-Forward message.  The LDRA MUST
   set the link-address field of the Relay-Forward message to the
   Unspecified Address (::) and MUST include the Interface-ID option in
   all DHCP Relay-Forward messages.

   If the message received on the client-facing interface is a Relay-
   Forward message, the LDRA MUST set the hop-count field in the newly
   created Relay-Forward message to the value of the hop-count field in
   the received message, incremented by 1 as specified in [RFC3315].

   The LDRA MUST copy the IP destination and link-layer destination
   addresses from the client-originated message into the IP destination
   address and link-layer destination address of the Relay-Forward
   message.

   The LDRA MUST copy the IP source address from the client-originated
   message into the peer-address field of the Relay-Forward message.
   The LDRA MUST copy the link-layer source address from the client-
   originated message into the link-layer source address of the Relay-
   Forward message.

6.1.1.  Client Message Validation

   On receipt of a DHCP message on a client-facing interface, the LDRA
   MUST discard a message if it is of one of the following message
   types:

   o  ADVERTISE (2)

   o  REPLY (7)

   o  RECONFIGURE (10)

   o  RELAY-REPL (13)

   Options contained in the DHCPv6 message MUST NOT be validated by the
   LDRA, making it the responsibility of the DHCP server to check
   message option validity and allow new options to be introduced
   without changes on the LDRA.

6.1.2.  Trusted and Untrusted Interfaces

   In [RFC3046], DHCPv4 Relay Agents had their client-facing interfaces
   set to "trusted" and "untrusted".  An LDRA MUST implement a
   configuration setting for all client-facing interfaces, marking them
   either as trusted or as untrusted.  This setting SHOULD be
   configurable per interface.  When a client-facing interface is deemed
   untrusted, the LDRA MUST discard any message of type RELAY-FORW (12)
   received from the client-facing interface.

6.2.  Relaying a Relay-Reply Message from the Network

   The LDRA MUST intercept and process all IP traffic received on the
   network-facing interface that has:

   o  a link-local scoped source address;

   o  a link-local scoped destination address;

   o  protocol type UDP; and

   o  destination port 547

   An LDRA MUST inspect the DHCP message type and only forward Relay-
   Reply messages.  Other DHCP message types MUST be silently discarded.

   The Relay-Reply message is considered valid by the LDRA if it passes
   the validity checks to be performed by a relay agent per [RFC3315]
   and

   -  the Interface-ID option is present, and the value corresponds to a
      valid interface in the access node;

   -  the Relay-Reply peer-address and the destination IP address are
      identical, and it is a link-local scoped address when no IP
      address is configured on the LDRA; and

   -  the link-address is the Unspecified Address when no IP address is
      configured on the LDRA.

   If the Relay-Reply message is valid, the LDRA copies the peer-address
   into the destination IP address field.  The LDRA SHOULD forward the
   packet to the correct client-facing interface using the destination
   link-layer (Media Access Control (MAC)) address or the Interface-ID
   in the Relay-Reply.  The LDRA SHOULD NOT retransmit the packet on any
   other interface.  The contents of the Relay Message option are put
   into an IP/UDP packet and then forwarded to the client.

   The LDRA MUST copy the link-layer and IP source address from the
   Relay-Reply message into the IP/UDP packet that is forwarded to the
   client.

7.  Network Topology

   The LDRA intercepts any DHCPv6 message received on client-facing
   interfaces with the traffic pattern specified in Section 6.1.  The
   LDRA MUST NOT forward the original client message to a network-facing
   interface; it MUST process the message and add the appropriate Relay-
   Forward options as described in previous sections.

7.1.  Client and Server on Same Link

   The access node acts as a bridge; it has no information about any IP
   prefixes that are valid on the link.  Thus, a server should consider
   address and parameter assignment as if the client DHCP message were
   not relayed.

                 +--------+
   Client -------|        |
                 | Access |
   Client -------|  Node  |-----+
                 | (LDRA) |     |
   Client -------|        |     |
                 +--------+     |
                                |      +--------+
                                |------| Server |
                                |      +--------+
                 +--------+     |
   Client -------|        |     |
                 | Access |     |
   Client -------|  Node  |-----+
                 | (LDRA) |
   Client -------|        |
                 +--------+

          <--------- IPv6 Link -------->

   For example, if a client sent a DHCP Solicit message that was relayed
   by the LDRA to the server, the server would receive the following
   Relay-Forward message from the LDRA:

   src-ip:              client link-local address

   dst-ip:              All_DHCP_Relay_Agents_and_Servers

     msg-type:          RELAY-FORW

     hop-count:         0

     link-address:      Unspecified_Address

     peer-address:      client link-local address

     Interface-ID Option:

       interface-id:    LDRA-inserted interface-id

     Relay-Message Option, which contains:

       msg-type:        SOLICIT

       Solicit Message Options: <from client>

7.2.  Client and Server behind Relay Agent

   The client and server are on different IPv6 links, separated by one
   or more relay agents that will typically act as a router.  The LDRA
   will send Relay-Forward messages upstream towards the second relay
   agent, which in turn will process the messages.

                 +--------+
   Client -------|        |
                 | Access |
   Client -------|  Node  |-----+
                 | (LDRA) |     |
   Client -------|        |     |
                 +--------+     |
                                |      +--------+       +--------+
                                |------| RelayB |-------| Server |
                                |      +--------+       +--------+
                 +--------+     |
   Client -------|        |     |
                 | Access |     |
   Client -------|  Node  |-----+
                 | (LDRA) |
   Client -------|        |
                 +--------+

          <------- IPv6 Link A ------->      <--IPv6 Link B-->

   For example, if a client sent a DHCP Solicit message that was relayed
   by the LDRA to another relay agent and then to the server, the server
   would receive the following Relay-Forward message from the LDRA:

   src-ip:              relayB

   dst-ip:              server

     msg-type:          RELAY-FORW

     hop-count:         1

     link-address:      relayB address from link A

     peer-address:      client link-local address

     Relay-Message Option, which contains:

       msg-type:        RELAY-FORW

       hop-count:       0

       link-address:    Unspecified_Address

       peer-address:    client link-local address

       Interface-ID Option:

         interface-id:  LDRA-inserted interface-id

       Relay-Message Option, which contains:

         msg-type:      SOLICIT

         Solicit Message Options: <from client>

7.3.  Relay Agent in Front of LDRA

   The client and server are on different IPv6 links, separated by one
   or more relay agents that will typically act as a router, and there
   is an [RFC3315] Relay Agent on the client-facing interface of the
   LDRA.  The LDRA will send Relay-Forward messages upstream towards the
   second relay agent, which in turn will process the messages.

                 +--------+
   RelayC -------|        |
                 | Access |
   RelayC -------|  Node  |-----+
                 | (LDRA) |     |
   RelayC -------|        |     |
                 +--------+     |
                                |      +--------+       +--------+
                                |------| RelayB |-------| Server |
                                |      +--------+       +--------+
                 +--------+     |
   RelayC -------|        |     |
                 | Access |     |
   RelayC -------|  Node  |-----+
                 | (LDRA) |
   RelayC -------|        |
                 +--------+

         <------- IPv6 Link A ------->      <--IPv6 Link B-->

   For example, if a client sent a DHCP Solicit message that was relayed
   by RelayC and the LDRA to another relay agent, RelayB, and then to
   the server, the server would receive the following Relay-Forward
   message:

   src-ip:               relayB

   dst-ip:               server

     msg-type:           RELAY-FORW

     hop-count:          2

     link-address:       relayB address from link A

     peer-address:       relayC

     Relay-Message Option, which contains:

       msg-type:         RELAY-FORW

       hop-count:        1

       link-address:     Unspecified_Address

       peer-address:     relayC

       Interface-ID Option:

         interface-id:   LDRA-inserted interface-id

       Relay-Message Option, which contains:

         msg-type:       RELAY-FORW

         hop-count:      0

         link-address:   global or Unspecified_Address

         peer-address:   end client address

         Interface-ID Option: (if required)

           interface-id: relayC-inserted Interface-ID

         Relay-Message Option, which contains:

           msg-type:      SOLICIT

           Solicit Message Options: <from end client>

8.  Contributors

   The authors would like to thank the following for their support:
   Lieven Levrau, Alastair Johnson, Robert Haylock, Mickey Vucic, Ludwig
   Pauwels, Fernando Cuervo, John Kaippallimalil, Fredrik Garneij,
   Alfred Hoenes, Ted Lemon, Tatuya Jinmei, David Hankins, and Ralph
   Droms.

   Comments are solicited and should be addressed to the DHC WG mailing
   list (dhcwg@ietf.org) and/or the authors.

9.  Security Considerations

   The security issues pertaining to DHCPv6 Relay Agents as specified in
   Section 23 of [RFC3315] are also applicable to LDRAs.  The LDRA
   SHOULD implement some form of rate-limiting on client-originated
   traffic in order to prevent excessive process utilisation.  The
   traffic to be rate-limited can be easily identified since the LDRA
   listens only to client-originated IPv6 traffic sent to the
   All_DHCPv6_Servers_and_Relay_Agents address on UDP port 547 and does
   not process any other client-originated traffic.  As DHCP is session-
   oriented, messages in excess of the rate-limit may be silently
   discarded.

   The hop-count-based determination of the trustworthiness of the LDRA
   can be easily defeated by a rogue relay agent on the network-facing
   interface of the LDRA.

10.  References

10.1.  Normative References

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

   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, July 2003.

10.2.  Informative References

   [L2RA]     Joshi, B. and P. Kurapati, "Layer 2 Relay Agent
              Information", Work in Progress, April 2011.

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

   [TR-101]   The Broadband Forum, "Migration to Ethernet-Based DSL
              Aggregation", Technical Report TR-101, April 2006.

Authors' Addresses

   David Miles (editor)
   Alcatel-Lucent
   L3 / 215 Spring St.
   Melbourne, Victoria  3000
   Australia

   Phone: +61 3 9664 3308
   EMail: david.miles@alcatel-lucent.com

   Sven Ooghe
   Alcatel-Lucent
   Copernicuslaan 50
   2018 Antwerp,
   Belgium

   EMail: sven.ooghe@alcatel-lucent.com

   Wojciech Dec
   Cisco Systems
   Haarlerberdweg 13-19
   1101 CH Amsterdam,
   The Netherlands

   EMail: wdec@cisco.com

   Suresh Krishnan
   Ericsson
   8400 Blvd. Decarie
   Town of Mount Royal, Quebec
   Canada

   EMail: suresh.krishnan@ericsson.com

   Alan Kavanagh
   Ericsson
   8400 Blvd. Decarie
   Town of Mount Royal, Quebec
   Canada

   EMail: alan.kavanagh@ericsson.com

 

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