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

DHCPv4 over DHCPv6 Transport
draft-ietf-dhc-dhcpv4-over-dhcpv6-05

Abstract

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

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on August 18, 2014.

Copyright Notice

Copyright (c) 2014 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

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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 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 DHCPv6.
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 messages.

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 protocol.

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 messages.

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 format:

   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 headers.

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 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 Server(s).

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 messages).

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 and the contents of the "flags" field carried in the DHCPv4-query message and uses them to generate the appropriate DHCPv4 response (server to client message). The response is generated as described in [RFC2131] with the exception that the server SHOULD use the information carried in the "flags" field of the DHCPv4-query message to find out whether the client's message would have been sent to the broadcast or unicast address if IPv4 has been used. This is useful for the server to determine the state of the client. The use of the "flags" field is described in detail in Section 7.

When an appropriate DHCPv4 response is generated, the 4o6 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 OPTION_DHCPV4_MSG, OPTION_DHCP4_O_DHCP6_SERVER from the "DHCP Option 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/dhcpv6-parameters/dhcpv6-parameters.xml.

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

[I-D.ietf-dhc-dhcpv6-unknown-msg] Cui, Y., Sun, Q. and T. Lemon, "Handling Unknown DHCPv6 Messages", Internet-Draft draft-ietf-dhc-dhcpv6-unknown-msg-05, February 2014.
[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 P.R.China Phone: +86-10-6278-5822 EMail: sunqi@csnet1.cs.tsinghua.edu.cn
Yong Cui Tsinghua University Beijing, 100084 P.R.China Phone: +86-10-6260-3059 EMail: yong@csnet1.cs.tsinghua.edu.cn
Marcin Siodelski 950 Charter Street Redwood City, CA 94063 USA Phone: +1 650 423 1431 EMail: msiodelski@gmail.com
Suresh Krishnan Ericsson EMail: suresh.krishnan@ericsson.com
Ian Farrer Deutsche Telekom AG GTN-FM4,Landgrabenweg 151 Bonn, NRW 53227 Germany EMail: ian.farrer@telekom.de