The Port Control Protocol in Dual-Stack Lite environments
draft-dupont-pcp-dslite-02

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 November 01, 2012.

Copyright Notice

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1. Disclaimer

This is a temporary version to avoid unwanted expire.

2. Introduction

The 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 were proposed: the plain and the encapsulation modes. This document selects the plain mode as the one to use.

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].

3. Plain Mode

In the plain mode the B4, the customer end-point of the DS-Lite IPv6 tunnel, implements a PCP proxy ([I-D.bpw-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 summary: 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.

4. IANA Considerations

This document makes no request of IANA.

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

5. Security Considerations

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

At the opposite the encapsulation mode Appendix Appendix A by default is fully transparent for the B4: PCP requests are blindly encapsulated as any other IPv4 packets to the Internet. So to apply a policy on them requires heavier and far less flexible tools.

6. 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.

7. References

7.1. Normative References

7.2. 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 5)
• they run an InterWorking Function (IWF) for other protocols ([I-D.bpw-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. Disclaimer
• 2. Introduction
• 3. Plain Mode
• 4. IANA Considerations
• 5. Security Considerations
• 6. Acknowledgments
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Appendix A. Encapsulation Mode
• Appendix B. Justification
• Authors' Addresses