DHC Working Group                                                 Q. Sun
Internet-Draft                                                    Y. Cui
Intended status: Standards Track                     Tsinghua University
Expires: August 18, 23, 2014                                    M. Siodelski
                                                             S. Krishnan
                                                               I. Farrer
                                                     Deutsche Telekom AG
                                                       February 14, 19, 2014

                      DHCPv4 over DHCPv6 Transport


   IPv4 connectivity is still needed as networks migrate towards IPv6.
   Users require IPv4 configuration even if the uplink to their service
   provider supports IPv6 only.  This document describes a mechanism for
   obtaining IPv4 configuration information dynamically in IPv6 networks
   by carrying DHCPv4 messages over DHCPv6 transport.  Two new DHCPv6
   messages and two new DHCPv6 options are defined for this purpose.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on August 18, 23, 2014.

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   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Architecture Overview . . . . . . . . . . . . . . . . . . . .   3
   5.  New DHCPv6 Messages . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Message Types . . . . . . . . . . . . . . . . . . . . . .   5
     5.2.  Message Formats . . . . . . . . . . . . . . . . . . . . .   5
     5.3.  DHCPv4-query Message Flags  . . . . . . . . . . . . . . .   6
     5.4.  DHCPv4-response Message Flags . . . . . . . . . . . . . .   7
   6.  New DHCPv6 Options  . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  DHCPv4 Message Option Format  . . . . . . . . . . . . . .   7
     6.2.  4o6 Server Address Option Format  . . . . . . . . . . . .   8
   7.  Use of the DHCPv4-query Unicast Flag  . . . . . . . . . . . .   9
   8.  DHCP 4o6 Client Behavior  . . . . . . . . . . . . . . . . . .   9
   9.  Relay Agent Behavior  . . . . . . . . . . . . . . . . . . . .  11
   10. DHCP 4o6 Server Behavior  . . . . . . . . . . . . . . . . . .  11
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  12
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12  13
   13. Contributors List . . . . . . . . . . . . . . . . . . . . . .  12  13
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     14.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13  14

1.  Introduction

   As the migration towards IPv6 continues, IPv6-only networks will
   become more prevalent.  In such networks, IPv4 connectivity will
   continue to be provided as a service over IPv6-only networks.  In
   addition to provisioning IPv4 addresses for clients of this service,
   other IPv4 configuration parameters may also be needed (e.g.
   addresses of IPv4-only services).

   This document describes a transport mechanism to carry DHCPv4
   messages using the DHCPv6 protocol for the dynamic provisioning of
   IPv4 addresses and other DHCPv4 specific configuration parameters
   across IPv6-only networks.  It leverages the existing DHCPv4
   infrastructure, e.g. failover, DNS updates, DHCP leasequery, etc.

   When IPv6 multicast is used to transport 4o6 messages, another
   benefit is that the operator can gain information about the
   underlying IPv6 network the 4o6 client is connected to from the the
   DHCPv6 relay agents the request has passed through.

2.  Requirements Language

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

3.  Terminology

   This document makes use of the following terms:

   CPE:                            Customer Premises Equipment (also
                                   known as Customer Provided
                                   Equipment), which provides access for
                                   devices connected to a Local Area
                                   Network (typically at the customer's
                                   site/home) to the Internet Service
                                   Provider's network.

   DHCP 4o6 client (or client):    A DHCP client supporting both the
                                   DHCPv6 protocol [RFC3315] as well as
                                   the DHCPv4 over DHCPv6 protocol
                                   described in this document.  Such a
                                   client is capable of requesting IPv6
                                   configuration using DHCPv6 and IPv4
                                   configuration using DHCPv4 over

   DHCP 4o6 server (or server):    A DHCP server that is capable of
                                   processing DHCPv4 packets
                                   encapsulated in the DHCPv4 Message
                                   option (defined below).

   DHCPv4 over DHCPv6:             A protocol described in this
                                   document, used to carry DHCPv4
                                   messages in the payload of DHCPv6

4.  Architecture Overview

   The architecture described here addresses a typical use case, where a
   DHCP client's uplink supports IPv6 only and the Service Provider's
   network supports IPv6 and limited IPv4 services.  In this scenario,
   the client can only use the IPv6 network to access IPv4 services, so
   IPv4 services must be configured using IPv6 as the underlying network

   Although the purpose of this document is to address the problem of
   communication between the DHCPv4 client and the DHCPv4 server, the
   mechanism that it describes does not restrict the transported
   messages types to DHCPv4 only.  As the DHCPv4 message is a special
   type of BOOTP message, BOOTP messages can also be transported using
   the same mechanism.

   DHCP clients may be running on CPE devices, end hosts or any other
   device that supports the DHCP client function.  At the time of
   writing, DHCP clients on CPE devices are comparatively easier to
   modify than those implemented on end hosts.  As a result, this
   document uses the CPE as an example for describing the mechanism.
   This does not preclude any end-host, or other device requiring IPv4
   configuration, from implementing DHCPv4 over DHCPv6 in the future.

   This mechanism works by carrying DHCPv4 messages encapsulated within
   DHCPv6 messages.  Figure 1, below, illustrates one possible
   deployment architecture.

   The DHCP 4o6 client implements a new DHCPv6 message called
   DHCPv4-query, which contains a new option called the DHCPv4 Message
   option encapsulating a DHCPv4 message sent by the client.  The format
   of this option is described in Section 6.1.

   The DHCPv6 message can be transmitted either via DHCPv6 Relay Agents
   or directly to the DHCP 4o6 server.  The server replies with a
   DHCPv4-response message, which is a new DHCPv6 message carrying the
   DHCPv4 response encapsulated in the DHCPv4 Message option.

                 _____________             _____________
                /             \           /             \
                |             |           |             |
       +--------+-+  IPv6   +-+-----------+-+  IPv6   +-+--------+
       | DHCP 4o6 | network |    DHCPv6     | network | DHCP 4o6 |
       |  client  +---------+  Relay Agent  +---------+  Server  |
       | (on CPE) |         |               |         |          |
       +--------+-+         +-+-----------+-+         +-+--------+
                |             |           |             |
                \_____________/           \_____________/

                      Figure 1: Architecture Overview

   By default, the DHCPv4-over-DHCPv6 function MUST be disabled on the
   client.  Before the client can use DHCPv4 over DHCPv6, it MUST obtain
   the necessary IPv6 configuration.  The client requests the 4o6 Server
   Address option from the server by sending the option code in Option
   Request option as described in [RFC3315].  If the server responds
   with the 4o6 Server Address option, it is an indication to the client
   to attempt using DHCPv4 over DHCPv6 to obtain IPv4 configuration.

   The client obtains the address(es) of the DHCP 4o6 server(s) from the
   4o6 Server Address option and uses them to communicate with the DHCP
   4o6 servers as described in Section 8.  If the 4o6 Server Address
   option contains no addresses (is empty), the client uses the well-
   known All_DHCP_Relay_Agents_and_Servers multicast address to
   communicate with the DHCP 4o6 server(s).

   Before applying for an IPv4 address via a DHCPv4-query message, the
   client must identify a suitable network interface for the address.
   Once the request is acknowledged by the server, the client can
   configure the address and other relevant parameters on this
   interface.  The mechanism for determining a suitable interface is out
   of the scope of the document.

5.  New DHCPv6 Messages

   Two new DHCPv6 messages carry DHCPv4 messages between the client and
   the server using the DHCPv6 protocol: DHCPv4-query and
   DHCPv4-response.  This section describes the structures of these

5.1.  Message Types

   DHCPV4-QUERY (TBD):     The DHCP 4o6 client sends a DHCPv4-query
                           message to a DHCP 4o6 server.  The DHCPv4
                           Message option carried by this message
                           contains a DHCPv4 message that the DHCP 4o6
                           client uses to request IPv4 configuration
                           parameters from the server.

   DHCPv4-RESPONSE (TBD):  A DHCP 4o6 server sends a DHCPv4-response
                           message to a DHCP 4o6 client.  It contains a
                           DHCPv4 Message option carrying a DHCPv4
                           message in response to a DHCPv4 message
                           received by the server in the DHCPv4 Message
                           option of the DHCPv4-query message.

5.2.  Message Formats

   Both DHCPv6 messages defined in this document share the following

      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
     |    msg-type   |                     flags                     |
     |                                                               |
     .                            options                            .
     .                           (variable)                          .
     |                                                               |

     Figure 2: The format of DHCPv4-query and DHCPv4-response messages

   msg-type        Identifies the message type.  It can be either
                   DHCPV4-QUERY (TBD) or DHCPV4-RESPONSE (TBD)
                   corresponding to the contained DHCPv4-query or
                   DHCPv4-response, respectively.

   flags           Specifies flags providing additional information
                   required by the server to process the DHCPv4 message
                   encapsulated in the DHCPv4-query message, or required
                   by the client to process a DHCPv4 message
                   encapsulated in the DHCPv4-response message.

   options         Options carried by the message.  The DHCPv4 Message
                   Option (described in Section 6.1) MUST be carried by
                   the message.  Only DHCPv6 options for IPv4
                   configuration may be included in this field.  It MUST
                   NOT contain DHCPv6 options related solely to IPv6, or
                   IPv6-only service configuration.

5.3.  DHCPv4-query Message Flags

   The "flags" field of the DHCPv4-query is used to carry additional
   information that may be used by the server to process the
   encapsulated DHCPv4 message.  Currently only one bit of this field is
   used.  Remaining bits are reserved for the future use.  The "flags"
   field has the following format:

          0                   1                   2
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
         |U|                    MBZ                      |

                    Figure 3: DHCPv4-query flags format

   U               Unicast Flag.  If set to 1, it indicates that the
                   DHCPv4 message encapsulated within the DHCPv4-query
                   message would be sent to a unicast address if it was
                   sent using IPv4.  If this flag is set to 0, it
                   indicates that the DHCPv4 message would be sent to
                   the broadcast address if it was sent using IPv4.  The
                   usage of the flag is described in detail in
                   Section 7.

   MBZ             Bits MUST be set to zero when sending and MUST be
                   ignored when receiving.

5.4.  DHCPv4-response Message Flags

   This document introduces no flags to be carried in the "flags" field
   of the DHCPv4-response message.  They are all reserved for the future
   use.  The DHCP 4o6 server MUST set all bits of this field to 0 and
   the DHCP 4o6 client MUST ignore the content in this field.

6.  New DHCPv6 Options

6.1.  DHCPv4 Message Option Format

   The DHCPv4 Message option carries a DHCPv4 message that is sent by
   the client or the server.  Such messages exclude any IP or UDP

   The format of the DHCPv4 Message option is:

      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
     |       OPTION_DHCPV4_MSG       |           option-len          |
     |                                                               |
     .                        DHCPv4-message                         .
     .                                                               .
     .                                                               .

                  Figure 4: DHCPv4 Message option Format

   option-code     OPTION_DHCPV4_MSG (TBD).

   option-len      Length of the DHCPv4 message.

   DHCPv4-message  The DHCPv4 message sent by the client or the server.
                   In a DHCPv4-query message it contains a DHCPv4
                   message sent by a client.  In a DHCPv4-response
                   message it contains a DHCPv4 message sent by a server
                   in response to a client.

6.2.  4o6 Server Address Option Format

   The 4o6 Server Address option is sent by a server to a client
   requesting IPv6 configuration using DHCPv6 [RFC3315].  It carries a
   list of DHCP 4o6 server's IPv6 addresses that the client should
   contact to obtain IPv4 configuration.  This list may include either
   multicast or and unicast addresses.  The client sends its requests to
   all unique addresses carried in this option.

   This option may also carry no IPv6 addresses, which instructs the
   client to use the All_DHCP_Relay_Agents_and_Servers multicast address
   as the destination address.

   The presence of this option in the server's response indicates to the
   client that it should use DHCPv4 over DHCPv6 to obtain IPv4
   configuration.  If the option is absent, the client MUST NOT enable
   DHCPv4-over-DHCPv6 function.

   The format of the 4o6 Server Address option is:

      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
     | OPTION_DHCP4_O_DHCP6_SERVER   |           option-len          |
     |                                                               |
     .                        IPv6 Address(es)                       .
     .                                                               .
     .                                                               .

                Figure 5: 4o6 Servers Address Option Format

   option-code     OPTION_DHCP4_O_DHCP6_SERVER (TBD).

   option-len      Length of the IPv6 address(es) carried by the option,
                   i.e. multiple of 16 octets.  Minimal length of this
                   option is 0.

   IPv6 Address    Zero or more IPv6 addresses of the DHCP 4o6

7.  Use of the DHCPv4-query Unicast Flag

   A DHCPv4 client conforming to [RFC2131] may send its DHCPREQUEST
   message to either a broadcast or unicast address depending on its
   state.  For example, a client in the RENEWING state uses a unicast
   address to contact the DHCPv4 server to renew its lease.  A client in
   the REBINDING state uses a broadcast address.  If there is a DHCPv4
   relay agent in the middle, a client in the RENEWING state may send a
   DHCPREQUEST message to the unicast address of the relay agent.  In
   such a case, the server is unable to determine whether the client
   sent the message to a unicast or broadcast address and thus the
   server may be unable to correctly determine the client's state.
   [RFC5010] introduced the "Flags Suboption" that relay agents add to
   relayed messages to indicate whether broadcast or unicast was used by
   the client.

   In DHCPv4 over DHCPv6, IPv6 is used to deliver DHCPv4 messages to the
   DHCP 4o6 server.  There is no relation between the outer IPv6 address
   and the inner DHCPv4 message.  As a result, the server is unable to
   determine whether the received DHCPv4 messages should have been sent
   using broadcast or unicast in IPv4 by checking the IPv6 address.
   This is similar to the case addressed by [RFC5010].

   In order to allow the server to determine the client's state, the
   "Unicast" flag is carried in the DHCPv4-query message.  The client
   MUST set this flag to 1 when the DHCPv4 message would have been sent
   to the unicast address if using DHCPv4 over IPv4.  This flag MUST be
   set to 0 if the DHCPv4 client would have sent the message to the
   broadcast address in IPv4.  The choice whether a given message should
   be sent to a broadcast or unicast address is made based on the
   [RFC2131] and its extensions.

   Note: The "Unicast" flag reflects how the DHCPv4 packet would have
   been sent; not how the DHCPv6 packet itself is sent.

8.  DHCP 4o6 Client Behavior

   The DHCPv4-over-DHCPv6 function MUST be disabled by default.  The
   client MUST obtain the necessary IPv6 configuration (stateless or
   stateful) before using DHCPv4 over DHCPv6.  The client intending to
   use DHCPv4 over DHCPv6 MUST request the 4o6 Server Address option
   using Option Request option (ORO) in every Solicit, Request, Renew,
   Rebind and Information-request message.

   The server MAY include the 4o6 Server Address option in its response
   to the client.  If the client receives this option, it MUST enable
   the DHCPv4-over-DHCPv6 function.  The client MUST NOT enable the
   DHCPv4-over-DHCPv6 function if the server does not include the 4o6
   Server Address option in its response.  If the client does not
   receive this option and DHCPv4 over DHCPv6 is already enabled, the
   client MUST disable the DHCPv4-over-DHCPv6 function.

   If the client receives the 4o6 Server Address option and there is a
   DHCPv4 client active on the interface over which that DHCPv6 option
   was received, it MUST stop the DHCPv4 client from sending messages
   using [RFC2131].

   If the client receives a 4o6 Server Address option that contains no
   IP addresses, i.e. the option is empty, the client MUST send its
   requests to the All_DHCP_Relay_Agents_and_Servers multicast address.
   If there is a list of IP addresses in the option, the client SHOULD
   send requests to each unique address carried by the option.

   If the client obtained stateless IPv6 configuration by sending
   Information-request message to the server, the client MUST follow the
   rules in [RFC4242] to periodically refresh the DHCPv4-over-DHCPv6
   configuration (i.e. list of DHCP 4o6 servers) as well as other
   configuration data.  The client which obtained stateful IPv6
   configuration will refresh the status of DHCPv4-over-DHCPv6 function
   when extending a lifetime of acquired IPv6 address (Renew and Rebind

   The client MUST employ an IPv6 address of an appropriate scope to
   source the DHCPv4-query message from.  When the client sends a
   DHCPv4-query message to the multicast address, it MUST use a link-
   local address as the source address as described in [RFC3315].  When
   the client sends a DHCPv4-query message using unicast, the source
   address MUST be an address of appropriate scope, acquired in advance.

   The client generates a DHCPv4 message and stores it verbatim in the
   DHCPv4 Message option carried by the DHCPv4-query message.  The
   client MUST put exactly one DHCPv4 Message option into a single
   DHCPv4-query message.  The client MUST NOT request the 4o6 Server
   Address option in the DHCPv4-query message.

   The client MUST follow rules defined in Section 7 when setting the
   Unicast flag based on the DHCPv4 destination.

   On receiving a DHCPv4-response message, the client MUST look for the
   DHCPv4 Message option within this message.  If this option is not
   found, the DHCPv4-response message is discarded.  If the DHCPv4
   Message option is present, the client extracts the DHCPv4 message it
   contains and processes it as described in section 4.4 of [RFC2131].

   When dealing with IPv4 configuration, the client MUST follow the
   normal DHCPv4 retransmission requirements and strategy as specified
   in section 4.1 of [RFC2131].  There are no explicit transmission
   parameters associated with a DHCPv4-query message, as this is
   governed by the DHCPv4 [RFC2131] "state machine".

   The client MUST implement [RFC4361] to ensure that the device
   correctly identifies itself.

9.  Relay Agent Behavior

   When a DHCPv6 relay agent receives a DHCPv4-query message, it may not
   recognize this message.  The unknown message can be forwarded as
   described in [I-D.ietf-dhc-dhcpv6-unknown-msg].

   Additionally, the DHCPv6 relay agent MAY allow the configuration of a
   dedicated DHCPv4 over DHCPv6 specific destination address(es),
   differing from the address(es) of the DHCPv6-only server(s).  To
   implement this function, the relay checks the received DHCPv6 message
   type and forwards according to the following logic:

   1.  If the message type is DHCPV4-QUERY, the packet is relayed to the
       configured DHCP 4o6 Server's address(es) in the form of normal
       DHCPv6 packet (i.e. DHCPv6/UDP/IPv6).

   2.  For any other DHCPv6 message type, forward according to section
       20 of [RFC3315].

   The above logic only allows for separate relay destinations
   configured on the relay agent closest to the client (single relay
   hop).  Multiple relaying hops are not considered in the case of
   separate relay destinations.

10.  DHCP 4o6 Server Behavior

   When the server receives a DHCPv4-query message from a client, it
   searches for the DHCPv4 Message option.  The server discards the
   packet without this option.  The server MAY notify an administrator
   about the receipt of a malformed packet.  The mechanism for this
   notification is out of scope for this document.

   If the server finds a valid DHCPv4 Message option, it extracts the
   original DHCPv4 message.  Since the DHCPv4 message and is encapsulated in
   the contents of DHCPv6 message, it lacks the "flags" field carried
   in information which is typically used
   by the DHCPv4-query message and uses them DHCPv4 server, implementing [RFC2131], to generate make address
   allocation decisions, e.g. giaddr for relayed messages and IPv4
   address of the appropriate
   DHCPv4 response (server interface which the server using to client message).  The response is
   generated as described in [RFC2131] communicate with
   directly connected client.  Therefore, the exception that DHCP 4o6 server allocates
   addresses according to the local address assignment policies
   determined by the server SHOULD use administrator.  For example, if the information carried in
   DHCPv4-query message has been sent via a relay, the "flags" server MAY use
   the link-address field of the
   DHCPv4-query Relay-forward message as a lookup for
   the IPv4 subnet to find out whether assign DHCPv4 address from.  If the client's DHCPv4-query
   message would
   have has been sent to from a directly connected client, the broadcast or unicast server
   MAY use IPv6 source address if of the message to determine the
   appropriate IPv4 has been
   used.  This is useful subnet to use for DHCPv4 address assignment.

   The server may also act as a DHCPv4 relay agent and forward the
   DHCPv4 packet to a "normal" DHCPv4 server.  In this case, the server
   would need to determine set the state giaddr to one of its own addresses and add
   Relay Agent Information option (82), including a Link Selection
   suboption [RFC3527] with the
   client. IPv4 subnet to assign a DHCPv4 address
   from, as mentioned above.  There are other complexities with this
   solution as enough information needs to be retained (or included in a
   Relay Agent Information option) to be able to return the response
   back to the client; how this might be done is outside the scope of
   this document.

   The server SHOULD use "flags" field of the "flags" DHCPv4-query message to
   create a response (server to client DHCPv4 message).  The use of this
   field is described in detail in Section 7.

   When an appropriate DHCPv4 response is generated, created, the 4o6 Server server places it
   in the payload of a DHCPv4 Message option, which it puts into the
   DHCPv4-response message.

   If the DHCPv4-query message was received directly by the server, the
   DHCPv4-response message MUST be unicast from the interface on which
   the original message was received.

   If the DHCPv4-query message was received in a Relay-forward message,
   the server creates a Relay-reply message with the DHCPv4-response
   message in the payload of a Relay Message option, and responds as
   described in section 20.3 of [RFC3315].

11.  Security Considerations

   In this specification, DHCPv4 messages are encapsulated in the newly
   defined option and messages.  This is similar to the handling of the
   current relay agent messages.  In order to bypass firewalls or
   network authentication gateways, a malicious attacker may leverage
   this feature to convey other messages using DHCPv6, i.e. use DHCPv6
   as a form of encapsulation.  However, the potential risk from this is
   no more severe than that with the current DHCPv4 and DHCPv6 practice.

   It is possible for a rogue server to reply with a 4o6 Server Address
   Option containing duplicated IPv6 addresses, which could cause an
   amplification attack.  To avoid this, the client MUST check if there
   are duplicate IPv6 addresses in a 4o6 Server Address Option when
   receiving one.  The client MUST ignore any but the first instance of
   each address.

12.  IANA Considerations

   IANA is requested to allocate two DHCPv6 option codes for use by
   Codes" table, and two DHCPv6 message type codes for the DHCPV4-QUERY
   and DHCPV4-RESPONSE from the "DHCP Message Codes" table of the
   Dynamic Host Configuration Protocol for IPv6 (DHCPv6) Registry.  Both
   tables can be found at http://www.iana.org/assignments/

13.  Contributors List

   Many thanks to Ted Lemon, Bernie Volz, Tomek Mrugalski, Yuchi Chen
   and Cong Liu, for their great contributions to the draft.

14.  References

14.1.  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.

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

   [RFC4242]  Venaas, S., Chown, T., and B. Volz, "Information Refresh
              Time Option for Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 4242, November 2005.

   [RFC4361]  Lemon, T. and B. Sommerfeld, "Node-specific Client
              Identifiers for Dynamic Host Configuration Protocol
              Version Four (DHCPv4)", RFC 4361, February 2006.

14.2.  Informative References

              Cui, Y., Sun, Q., and T. Lemon, "Handling Unknown DHCPv6
              Messages", draft-ietf-dhc-dhcpv6-unknown-msg-05 (work in
              progress), February 2014.

   [RFC3527]  Kinnear, K., Stapp, M., Johnson, R., and J. Kumarasamy,
              "Link Selection sub-option for the Relay Agent Information
              Option for DHCPv4", RFC 3527, April 2003.

   [RFC5010]  Kinnear, K., Normoyle, M., and M. Stapp, "The Dynamic Host
              Configuration Protocol Version 4 (DHCPv4) Relay Agent
              Flags Suboption", RFC 5010, September 2007.

Authors' Addresses

   Qi Sun
   Tsinghua University
   Beijing  100084

   Phone: +86-10-6278-5822
   Email: sunqi@csnet1.cs.tsinghua.edu.cn

   Yong Cui
   Tsinghua University
   Beijing  100084

   Phone: +86-10-6260-3059
   Email: yong@csnet1.cs.tsinghua.edu.cn

   Marcin Siodelski
   950 Charter Street
   Redwood City, CA  94063

   Phone: +1 650 423 1431
   Email: msiodelski@gmail.com

   Suresh Krishnan

   Email: suresh.krishnan@ericsson.com
   Ian Farrer
   Deutsche Telekom AG
   GTN-FM4,Landgrabenweg 151
   Bonn, NRW  53227

   Email: ian.farrer@telekom.de