Provisioning IPv4 Configuration Over IPv6 Only Networks
draft-ietf-dhc-v4configuration-02

Abstract

As IPv6 becomes more widely adopted, some service providers are taking the approach of deploying IPv6 only networks, without dual-stack functionality for IPv4. However, access to IPv4 based services is still an ongoing requirement and approaches such as IPv4-in-IPv6 softwire tunnels are being developed to meet this need.

In order to provision end-user's hosts with the necessary IPv4 configuration, a number of different mechanisms have been proposed. This memo discusses the benefits and drawbacks of each, with the aim of recommending a single approach as the basis for future work.

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.

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 April 3, 2014.

Copyright Notice

Copyright (c) 2013 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
• 1.1. Overview of IPv4 Parameter Configuration Approaches
• 1.2. DHCPv4o6 Based Provisioning - Functional Overview
• 1.3. DHCPv6 Based Provisioning - Functional Overview
• 1.4. DHCPv6+DHCPv4oSW Based Provisioning - Functional Overview
• 1.5. DHCPv4oSW Based Provisioning - Functional Overview
• 1.6. DHCPv4oDHCPv6 Based Provisioning - Functional Overview
• 2. Requirements for the Solution Evaluation
• 3. Comparison of the Five Approaches
• 3.1. DHCPv6 Based Provisioning
• 3.1.1. Pros
• 3.1.2. Cons
• 3.2. DHCPv4o6 Based Provisioning
• 3.2.1. Pros
• 3.2.2. Cons
• 3.3. DHCPv6+DHCPv4oSW Based Provisioning
• 3.3.1. Pros
• 3.3.2. Cons
• 3.4. DHCPv4oSW Based Provisioning
• 3.4.1. Pros
• 3.4.2. Cons
• 3.5. DHCPv4oDHCPv6 Based Provisioning
• 3.5.1. Pros
• 3.5.2. Cons
• 4. Conclusion
• 5. IANA Considerations
• 6. Security Considerations
• 6.1. DHCPv4oIPv6
• 6.2. DHCPv6
• 6.3. DHCPv6+DHCPv4oSW
• 6.4. DHCPv4oSW
• 6.5. DHCPv4oDHCPv6
• 7. Acknowledgements
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Authors' Addresses

1. Introduction

A service provider with an IPv6-only network must also be able to provide customers with access to the Internet and other services over IPv4. Softwire based IPv4-in-IPv6 tunneling mechanisms are an obvious example of this, such as the ones described in:

• [I-D.ietf-softwire-lw4over6]
• [I-D.ietf-softwire-map]
• [I-D.ietf-softwire-unified-cpe]

A general trend here is to relocate NAT44 functionality and IPv4 address sharing from the centralized tunnel concentrator to the CPE in order to achieve better scalability. This results in the need to provision a number of configuration parameters to the CPE, such as the external public IPv4 address and a restricted port-range to use for NAT. These parameters are pre-requisites for successfully configuring the client for providing IPv4 based connectivity.

In order to configure customer's devices for softwire functionality, a dynamic provisioning mechanism is necessary. In IPv4 only networks, DHCPv4 has often been used to provide IPv4 configuration, but in an IPv6 only network, DHCPv4 messages cannot be transported natively.

Although IPv4-in-IPv6 softwire tunnel clients are currently the only use-case for DHCP based configuration of IPv4 parameters in IPv6 only networks, a suitable approach must not be limited to only supporting the configuration of softwires or other specific underlying IPv4 over IPv6 architectures or mechanisms. DHCPv4 options may also need to be conveyed to clients for configuring IPv4 based services, e.g. SIP server addresses.

This document describes and compares five different methods which have been proposed as solutions to this problem.

1.1. Overview of IPv4 Parameter Configuration Approaches

The following approaches for transporting IPv4 configuration parameters over IPv6 only networks have been suggested:

1. Adapt DHCPv4 format messages to be transported over IPv6 as described in [I-D.ietf-dhc-dhcpv4-over-ipv6]. For brevity, this is referred to as DHCPv4o6.
2. Extend DHCPv6 with new options for IPv4 configuration, such as [I-D.ietf-softwire-map-dhcp] describes.
3. Use DHCPv6 as above for external IPv4 address and source port configuration. Use DHCPv4 over IPv4 messages within an IPv6 softwire for configuring additional parameters. This is referred to as DHCPv6+DHCPv4oSW.
4. Use DHCPv4 over IPv4 messages transported over a broadcast capable IPv4overIPv6 transport (e.g. a softwire) for all IPv4 configuration including the external IPv4 address and source port configuration. This is referred to as DHCPv4oSW.
5. Use DHCPv4 format messages, transporting them within a new DHCPv6 message type as described in [I-D.ietf-dhc-dhcpv4-over-dhcpv6]. This is referred to as DHCPv4oDHCPv6.

At the time of writing, working examples of the first two approaches have been developed and successfully tested in several different operators networks. The fourth approach has been tested successfully in a lab environment. The remaining two methods are still theoretical.

The following sections provide describe each of the approaches in more detail.

1.2. DHCPv4o6 Based Provisioning - Functional Overview

In order to receive IPv4 configuration parameters, IPv4-only clients initiate and exchange DHCPv4 messages with the DHCPv4 server. In order adapt this to an IPv6-only network, an existing DHCPv4 client implements a 'Client Relay' (CRA) function, which takes DHCPv4 messages and puts them into UDPv6 and IPv6.

As the mechanism involves unicast based communications, the IPv6 address of the server must be provisioned to the client. This option is described in [I-D.mrugalski-softwire-dhcpv4-over-v6-option].

The DHCPv4o6 server must provide an IPv6 interface to the client. This interface may be directly on the server and/or via an intermediary 'Transport Relay Agent' device can act as the gateway between the IPv4 and IPv6 domains.

For the dynamic allocation of IPv4 addresses, the DHCPv4 server needs to be extended to support the new DHCPv4o6 functionality, such as the storing the IPv6 address of DHCPv4o6 clients and implementing the CRA6ADDR option.

This approach currently uses functional elements for ingress and egress of the IPv6-only transport domain--the CRA on the host and the TRA or TSV on the server. As a result, this approach has sometimes been referred to as a tunneling approach. However, relay agent encapsulation is not a tunnel, since it carries only DHCP traffic; it would be more accurate to describe it as an encapsulation based transport.

It is worth noting that there is no technical reason for using relay encapsulation for DHCPv4o6; this approach was taken because the authors of the draft originally imagined that it might be used to provide configuration information for an unmodified DHCPv4 client. However, this turns out not to be a viable approach: in order for this to work, there would have to be IPv4 routing on the local link to which the client is connected. In that case, there's no need for DHCPv4o6.

Given that this is the case, there is no technical reason why DHCPv4o6 can't simply use the IPv6 transport directly, without any relay encapsulation. This would greatly simplify the specification and the implementation, and would still address the requirements stated in this document.

[I-D.ietf-dhc-dhcpv4-over-ipv6] decribes this solution in detail.

The protocol stack is as follows:

DHCPv4/UDPv6/IPv6

1.3. DHCPv6 Based Provisioning - Functional Overview

In this approach, DHCPv6 [RFC3315] would be extended with new DHCPv6 options for configuring all IPv4 based services and functions, (i.e. IPv4 address assignment and any necessary DHCPv4 options). DHCPv4 options needed by IPv4 clients connected to the IPV6 network are updated as new DHCPv6 native options carrying IPv4 configuration parameters. IPv4 address leasing would also need to be managed by the DHCPv6 server.

At the time of writing, it is not known how many such options would need to be ported from DHCPv4 to DHCPv6.

An example of this approach is described in [I-D.ietf-softwire-map-dhcp], where a DHCPv6 message is used to convey the parameters necessary for IPv4 in IPv6 softwire configuration.

The protocol stack is as follows:

DHCPv6/UDPv6/IPv6

1.4. DHCPv6+DHCPv4oSW Based Provisioning - Functional Overview

In this approach, the configuration of IPv4 address and source ports (if required) is carried out using DHCPv6 as described in section 1.3 above. Any additional IPv4 configuration parameters which are required are then provisioned using a DHCPv4 messages transported within IPv6 in the configured softwire in the same manner as any other IPv4 based traffic. Broadcast based DHCPv4 DHCPDISCOVER messages (necessary for IPv4 address assignment) can not be transported as they are not compatible with the softwire architecture.

On receipt at the tunnel concentrator (e.g. MAP Border Router or a Lightweight 4over6 lwAFTR), the DHCPv4 message removed from the softwire and forwarded to the DHCPv4 server in the same way as any other IPv4 packet is handled.

As the client is already configured with its external IPv4 address and source ports (using DHCPv6 or a well-known IPv4 address for DS-Lite clients), the messages exchanged between the DHCPv4 client and server would be strictly DHCPINFORM/DHCPACK messages, for the configuration of additional IPv4 parameters.

For this approach to function, a mechanism for the DHCPv4 client to learn the IPv4 address of the DHCPv4 server is needed. This could be done by defining a well-known IPv4 address for the DHCPv4 server, implementing a DHCPv4 relay function within the tunnel concentrator or other configuration methods.

From a transport perspective, the key difference between this method and DHCPv4o6 (described above) is that here, the DHCPv4 message is put into UDPv4 and IPv4 and then put into the IPv6 softwire, instead of directly placing the DHCPv4 message into UDPv6 and IPv6.

Currently, this approach is only theoretical and does not have a corresponding Internet Draft providing more detail.

The protocol stack used for obtaining an IPv4 address and source ports (if required) is as follows:

DHCPv6/UDPv6/IPv6

The protocol stack used for obtaining additional IPv4 configuration is as follows:

DHCPv4/UDPv4/IPv4/IPv6

1.5. DHCPv4oSW Based Provisioning - Functional Overview

[I-D.troan-dhc-dhcpv4osw] describes a method for complete client configuration using DHCPv4 transported across a broadcast capable link layer transport, such as a softwire. This differs from DHCPv6+DHCPv4oSW in that it could support all DHCPv4 message types and is not limited to just DHCPINFORM/DHCPACK messages. This means that it can be used for the assignment of IPv4 addresses as well as other DHCPv4 options.

This functions by running a DHCPv4 client on the link layer interface (e.g. the softwire tunnel interface). As the link layer must support broadcasts, DHCPDISCOVER and other broadcast DHCPv4 messages can be supported. The DHCPv4 message flow is then the same as described in section 3.1 of [RFC2131].

In this approach, either the tunnel concentrator must also be the DHCPv4 server or it must act as a DHCPv4 relay so that the broadcast DHCPDISCOVER/DHCPREQUEST messages can be decapsulated and forwarded to the DHCPv4 server. If it is functioning as a relay, then the DHCPv4 Relay Information Option (option 82) is used to convey the client's source IPv6 address. This is also used by the relay for routing return DHCPv4 packets.

The DHCPv4oSW client may be configured with a shared IPv4 address with restricted layer 4 source ports. This will normally exclude the well-known TCP/UDP ports in the range 0-1023, so the DHCPv4oSW client must be updated to source BOOTP/DHCP requests from a port taken from the range allocated to the client instead of UDP port 67. Likewise, the DHCPv4oSW server must use the L4 source port from a client's message as the destination port for the response.

The protocol stack used for obtaining DHCPv4 based configuration is:

DHCPv4/UDPv4/IPv4/IPv6

1.6. DHCPv4oDHCPv6 Based Provisioning - Functional Overview

[I-D.ietf-dhc-dhcpv4-over-dhcpv6] describes the transport of DHCPv4 messages within two new DHCPv6 messages types: BOOTREQUESTV6 and BOOTREPLYV6. These messages types must be implemented in both the DHCPv4oDHCPv6 client and server.

In this approach, the configuration of stateless IPv4 addresses and source ports (if required) is carried out using DHCPv6 as described in section 1.3 above. Dynamic IPv4 addressing, and/or any additional IPv4 configuration, is provided using DHCPv4 messages carried (without IPv4/UDPv4 headers) within a new OPTION_BOOTP_MSG DHCPv6 option.

OPTION_BOOTP_MSG enables the client and server to send BOOTP/DHCPv4 messages verbatim across the IPv6 network. When a DHCPv4oDHCPv6 server receives a DHCPv6 request containing OPTION_BOOT_MSG within a BOOTREQUESTV6 message, it passes it to the DHCPv4 server engine. Likewise, the DHCPv4 server place its DHCPv4 response in the payload of OPTION_BOOTP_MSG and puts this into a BOOTPRPLYV6 message.

As the DHCPv4 messages are carried within DHCPv6 multicast messages, using the All_DHCP_Relay_Agents_and_Servers, they can be relayed in exactly the same way as any other DHCPv6 multicasted message. Optionally, DHCPv6 relays could be updated so that they forward the BOOTREQUESTV6 message to a different destination address, allowing for the separation of DHCPv4 and DHCPv6 provisioning infrastructure.

The protocol stack used for obtaining dynamic v4 addressing or additional IPv4 configuraion is as follows:

DHCPv4/DHCPv6/UDPv6/IPv6

2. Requirements for the Solution Evaluation

The following requirements have been defined for the evalution of the different approaches:

1. Minimize the amount of work necessary to implement the solution through re-use of existing standards and implementations as much as possible.
2. Provide a method of supporting all DHCPv4 options so that they can be utilised without the need for further standardation.
3. Allow for the dynamic leasing of IPv4 addresses to clients. This allows for more efficient use of limited IPv4 resources.
4. Enable the separation of IPv4 and IPv6 host configuration infrastructure, i.e. independent DHCPv4 and DHCPv6 servers.
5. Avoid leaving legacy IPv4 options in DHCPv6.
6. Provide a flexible architecture to give operators the option of only deploying the functional elements necessary for their specific requirements.
7. Not restricted to specific IPv4 over IPv6 transport mechanisms or architectures.
8. 
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   

3. Comparison of the Five Approaches

The table below provides an evaluation comparison of how the different approaches meet the solution requirements described above.

The following sections of the document provide more details of the pros and cons relevant to each of the approaches.

3.1. DHCPv6 Based Provisioning

3.1.1. Pros

1. Simpler, in that no additional functional elements are required except the DHCPv6 client and server.
2. A single protocol is used to deliver configuration information for IPv4 and IPv6.
3. A single provisioning point for all configuration parameters.
4. Implementations already exist, proving that the approach works.

3.1.2. Cons

1. Any required DHCPv4 options must be ported to DHCPv6, which will require re-development work for each option. All functional elements in the DHCPv6 implementation (clients, servers, relays) would need to be updated for each change.
2. Means that DHCPv4 'legacy' options, which will be of decreasing relevance in the future will remain in DHCPv6 for the lifetime of the protocol.
3. Each time that a DHCPv4 option is ported to DHCPv6, all clients and servers would need to be updated to implement the new option.
4. Does not provide an architecture for keeping IPv4 and IPv6 domains separated.
5. Does not provide a mechanism for dynamic IPv4 address leasing. A DHCPv4 lease lifetime management mechanism would need to be added to DHCPv6 for this.

3.2. DHCPv4o6 Based Provisioning

3.2.1. Pros

1. Implemention makes all existing DHCPv4 options available with no further ongoing development work necessary.
2. IPv4 and IPv6 based provisioning can be separated from each other if required, allowing flexibility in network design.
3. Easy to implement through minor adaptation of existing DHCPv4 client, relay and server code.
4. Simple, in that no additional functional elements are necessary except the DHCPv4o6 client relay and server. If a TSV is used, then the TRA is not required.
5. Suitable for the provisioning of dynamic IPv4 configuration as the existing DHCPv4 leasing mechanism can be used.
6. Implementations already exist, proving that the approach works.

3.2.2. Cons

1. More complex, in that there are more new functional elements (CRA, DHCPv4o6 server and optionally TRA) within the architecture than are necessary in DHCPv6 based provisioning.
2. A new DHCPv6 option is necessary in order to provision the IPv6 address of the DHCPv4 server to the end device.
3. The DHCPv4 client host needs to be updated to implement the IPv6 encapsulation and decapsulation function (i.e An HCRA). Otherwise a separate On-Link CRA (LCRA) functional element must be deployed.
4. A DHCPv4 server must be deployed and maintained.
5. The DHCPv4 server needs to be updated to implement new DHCPv4o6 functionality.

3.3. DHCPv6+DHCPv4oSW Based Provisioning

3.3.1. Pros

1. Once implemented, all existing DHCPv4 options will be be available with no further ongoing development work necessary.
2. Uses the existing DHCPv4 and DHCPv6 architectures in order to provide IPv4 configuration in an IPv6 only environment.
3. DHCPv4 and DHCPv6 based provisioning can be separated from each other if required, allowing flexibility in network design.

3.3.2. Cons

1. More complex, in that there are more new functional elements within the architecture than are necessary in DHCPv6 based provisioning.
2. IPv4 over IPv6 softwire approaches that distribute NAT to the CPE and allow for IP address sharing (MAP-E & LW4o6) forbid the use of reserved TCP/UDP ports (e.g. 0-1024). Every DHCPv4 client sharing the same address needs to have a UDP listener running on UDP port 68. To resolve this would require significant rework to either the softwire mechanisms and/or the DHCPv4 client implementatioIn.
3. From the current specification, DHCPINFORM is not suitable for use over a softwire. Additional work, such as the development of 'shims' would be necessary
4. The current DHCPINFORM specification has a number of unclear points, such as those described in [I-D.ietf-dhc-dhcpinform-clarify]. Substantial work would be required to resolve this.
5. Links the deployment of IPv4 configuration over IPv6 to a softwire implementation (e.g. requiring a softwire concentrator to act as a DHCPv4 relay). Whilst softwires are the only application for this functionality at the moment, this may not always be the case.
6. A new mechanism must be defined in order to provide the DHCPv4 client with the IPv4 address of the DHCPv4 server so that unicast DHCPINFORM messages can be sent.
7. As only DHCPINFORM/DHCPACK DHCPv4 message types are supported, dynamic IPv4 address leasing (using DHCPDISCOVER messages) can not be used.
8. Restricted to underlying hub-and-spoke IPv4 over IPv6 architectures. The 'hub' is necessary for the location of the DHCPv4 relay, as all traffic. An underlying mesh architecture does not have such a location to deploy the relay.
9. The approach is unproven as no existing implementations exist.

3.4. DHCPv4oSW Based Provisioning

3.4.1. Pros

1. Once implemented, all existing DHCPv4 options will be be available with no further ongoing development work necessary.
2. Uses the existing DHCPv4 architecture in order to provide IPv4 configuration in an IPv6 only environment.
3. DHCPv4 and DHCPv6 based provisioning can be separated from each other if required, allowing flexibility in network design.

3.4.2. Cons

1. Requires the DHCPv4 client, DHCPv4 server and softwire concentrator (or other relaying device) to be modified.
2. Requires the DHCPv4 client and server to be updated to use dynamic ports taken from the restricted port set allocated to the client instead of the well-known DHCPv4 ports.
3. The DHCPv4 client must be modified to identify the properties of the interface it is configuring and request parameters accordingly (e.g. restricted port-sets cannot be used on Ethernet transport interfaces but are allowed for a softwire transport)
4. May not be suitable for configuring translation based approaches (e.g. MAP-T)
5. Restricted to underlying hub-and-spoke IPv4 over IPv6 architectures. The 'hub' is necessary for the location of the DHCPv4 relay, as all traffic, including DHCPDISCOVER messages will pass through it. An underlying mesh architecture does not have such a location to deploy the relay.

3.5. DHCPv4oDHCPv6 Based Provisioning

3.5.1. Pros

1. Once implemented, all existing DHCPv4 options will be be available with no further ongoing development work necessary.
2. Uses the existing DHCPv4 and DHCPv6 architectures in order to provide IPv4 configuration in an IPv6 only environment.
3. DHCPv4 and DHCPv6 based provisioning can be separated from each other if required, allowing flexibility in network design.
4. Suitable for the provisioning of dynamic IPv4 configuration as the existing DHCPv4 leasing mechanism can be used.

3.5.2. Cons

1. More complex, in that there are more new functional elements within the architecture than are necessary in DHCPv6 based provisioning.
2. DHCPv6 clients needs to be updated to implement the new DHCPv6 message types (BOOTPREQUESTv6 and BOOTPREPLYv6).
3. The DHCPv6 server needs to be updated to implement new DHCPv4oDHCPv6 message types and functionality.
4. The approach is currently unproven as no existing implementations exist.

4. Conclusion

Whilst all of the approaches described here will require some development work in order to realize, it is clear from the above analysis that the most sustainable approach capitalizes on existing DHCPv4 implementations and include them as new DHCPv6 message types. The main rationale for this is that it enables all of DHCPv4's existing options to be migrated for use over IPv6 in a single step.

Porting of all necessary DHCPv4 options to DHCPv6 would require ongoing development work, re-implementing existing DHCPv4 functionality in DHCPv6. This will result in having legacy DHCPv4 options in DHCPv6, which will no longer be useful once IPv4 is completely abandoned.

Therefore, the DHCPv6 approach is not suitable for delivering IPv4 configuration parameters in an efficient, ongoing manner.

The dynamic leasing of IPv4 addresses is fundamental to the efficient use of remaining IPv4 resources. This will become increasingly important in the future, so a mechanism which supports this is necessary. DHCPv4oSW does not provide this function and so is not recommended.

The DHCPv4o6 approach requires a DHCPv4 server (with DHCPv4o6 functionality) for all deployment scenarios, even when DHCPv4 specific functionality (e.g. sending DHCPv4 options) is not required by the operator.

Therefore, this memo recommends DHCPv4oDHCPv6 [I-D.ietf-dhc-dhcpv4-over-dhcpv6] as the best underlying approach for provisioning IPv4 parameters over an IPv6 only network.

5. IANA Considerations

This document makes no request of IANA.

Note to RFC Editor: this section may be removed on publication as an RFC.

6. Security Considerations

The following sections provide pointers to the documented security considerations associated with each approach.

6.1. DHCPv4oIPv6

Security considerations associated with this approach are described in Section 8 of [I-D.ietf-dhc-dhcpv4-over-ipv6].

6.2. DHCPv6

Security considerations associated with this approach are described in Section 23 of [RFC3315].

6.3. DHCPv6+DHCPv4oSW

There is currently no document describing this mechanism, so no security considerations have been documented.

6.4. DHCPv4oSW

At the time of writing,[I-D.troan-dhc-dhcpv4osw] does not list any security considerations.

6.5. DHCPv4oDHCPv6

Security considerations associated with this approach are described in Section 10 of [RFC3315].

7. Acknowledgements

Thanks to Ted Lemon, Tomek Mrugalski, Ole Troan and Francis Dupont for their input and reviews.

8. References

8.1. Normative References

8.2. Informative References

Authors' Addresses