The Port Control Protocol in Dual-Stack Lite environments
draft-dupont-pcp-dslite-05

Abstract

This document specifies the so-called "plain mode" for the use of the Port Control Protocol (PCP) in Dual-Stack Lite (DS-Lite) environments.

Status of This Memo

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This Internet-Draft will expire on October 25, 2013.

Copyright Notice

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1. Introduction

Dual-Stack Lite (DS-Lite, [RFC6333]) is a technology which enables a broadband service provider to share IPv4 addresses among customers by combining two well-known technologies: IP in IP (IPv4-in-IPv6) and Network Address Translation (NAT).

Typically, the home gateway embeds a Basic Bridging BroadBand (B4) capability that encapsulates IPv4 traffic into a IPv6 tunnel to the carrier-grade NAT, named the Address Family Transition Router (AFTR). AFTRs are run by service providers.

The Port Control Protocol (PCP, [I-D.ietf-pcp-base] allows customer applications to create mappings in a NAT for new inbound communications destined to machines located behind a NAT. In a DS-Lite environment, PCP servers control AFTR devices.

Two different modes of operations have been proposed: the plain and the encapsulation modes. This document recommends use of the plain mode.

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. Plain Mode

In the plain mode the B4, the customer end-point of the DS-Lite IPv6 tunnel, implements a PCP proxy ([I-D.ietf-pcp-proxy]) function and uses UDP over IPv6 with the AFTR to send PCP requests and receive PCP responses.

The B4 MUST source PCP requests with the IPv6 address of its DS-Lite tunnel end-point and MUST use a THIRD PARTY option either empty or carrying the IPv4 internal address of the mappings.

In the plain mode the PCP discovery ([I-D.ietf-pcp-base] section 7.1 "General PCP Client: Generating a Request") is changed into:

1. if a PCP server is configured (e.g., in a configuration file or via DHCPv6), that single configuration source is used as the list of PCP Server(s), else;
2. use the IPv6 address of the AFTR.

To summarize, the first rule remains the same with the precision that DHCP is DHCPv6, in the second rule the default router list is replaced by the AFTR.

3. IANA Considerations

This document makes no request of IANA.

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

4. Security Considerations

A DS-Lite PCP deployment could be secure under the Simple Threat Model described in the Security Considerations of the base PCP specification, even though the B4 device makes PCP mapping requests on behalf of internal clients using the THIRD_PARTY option.

To meet the requirements of the Simple Threat Model the DS-Lite PCP server MUST be configured to only allow the B4 device to make THIRD_PARTY requests, and only on behalf of other Internal Hosts sharing the same DS-Lite IPv6 tunnel. The B4 must ensure that the internal IP address in the THIRD_PARTY option corresponds to the IP source address in the IP Header of the PCP request (or proxied UPnP request) that triggered the THIRD_PARTY request. The B4 device MUST guard against spoofed packets being injected into the IPv6 tunnel using the B4 device's IPv4 source address, so the DS-Lite PCP Server can trust that packets received over the DS-Lite IPv6 tunnel with the B4 device's source IPv4 address do in fact originate from the B4 device. The B4 device is in a position to enforce this requirement, because it is the DS-Lite IPv6 tunnel endpoint.

Allowing the B4 device to use the THIRD_PARTY option to create mappings for hosts reached via the IPv6 tunnel terminated by the B4 device is acceptable, because the B4 device is capable of creating these mappings implicitly and can prevent others from spoofing these mappings.

If the conditions described above cannot be ensured, a PCP Authentication mechanism must be implemented to meet the requirements of the Advanced Security Model, as discussed in the PCP specification.

The plain mode provides a control point inside the home network where any policy on PCP requests can be applied, e.g.:

• restrict the use of THIRD PARTY options to the B4
• apply an access-list on internal addresses and/or ports

Therefore, use of the PCP Simple Security model will generally be acceptable within plain mode implementations.

On the other hand, the encapsulation mode Appendix A defaults to being fully transparent for the B4: PCP requests are blindly encapsulated as any other IPv4 packets to the Internet. This makes it more difficult to apply policy to PCP requests, and will generally require implementation of a PCP authentication protocol to meet the Security Considerations of the base PCP specification.

5. Acknowledgments

Reinaldo Penno who checks the validity of the argument about the relative complexity of the encapsulation mode at the AFTR side.

Christian Jacquenet and Mohammed Boucadair who proposed improvements to the document, including the PCP server discovery by Mohammed.

Sam Hartman for his help with the Security Considerations text.

6. References

6.1. Normative References

6.2. Informative References

Appendix A. Encapsulation Mode

The encapsulation mode deals at the B4 side with PCP traffic as any IPv4 traffic: it is encapsulated to and decapsulated from the AFTR over the DS-Lite IPv4 over IPv6 tunnel.

At the AFTR side things are a bit more complex because the PCP server needs the context, here the source IPv6 address, for both to manage mappings and to send back response. So the AFTR MUST tag PCP requests with the source IPv6 address after decapsulation and before forwarding them to the PCP server, and use the same tag to encapsulate PCP responses to correct B4s. (the term "tag" is used to describe the private convention between the AFTR and the PCP server).

Appendix B. Justification

We believe most customers will run a PCP proxy on the B4 because:

• they want a control point where to apply security (Section 4)
• they run an InterWorking Function (IWF) for other protocols ([I-D.ietf-pcp-upnp-igd-interworking]) on the B4 so the proxy is just part of a bigger system

BTW when the home network has only one node (dual-stack capable with embedded B4 element) attached, it is the PCP client.

For a PCP proxy to use IPv4 (encapsulation mode) or IPv6 (plain mode) does not make a sensible difference, so from an implementation point of view the real difference is on the PCP server / AFTR side: the encapsulation mode require an Application Level Gateway (ALG) to tag PCP request with the corresponding customer after decapsulation, when the plain mode is fully transparent.

Authors' Addresses

Table of Contents

• 1. Introduction
• 2. Plain Mode
• 3. IANA Considerations
• 4. Security Considerations
• 5. Acknowledgments
• 6. References
• 6.1. Normative References
• 6.2. Informative References
• Appendix A. Encapsulation Mode
• Appendix B. Justification
• Authors' Addresses