Network Working Group
Request for Comments: 4241
Category: Informational
Y. Shirasaki
S. Miyakawa
T. Yamasaki
NTT Communications
A. Takenouchi
December 2005

A Model of IPv6/IPv4 Dual Stack Internet Access Service

Status of This Memo

This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.

Copyright Notice

Copyright © The Internet Society (2005).


This RFC is not a candidate for any level of Internet Standard. The IETF disclaims any knowledge of the fitness of this RFC for any purpose and notes that the decision to publish is not based on IETF review apart from IESG review for conflict with IETF work. The RFC Editor has chosen to publish this document at its discretion. See RFC 3932 for more information.


   This memo is a digest of the user network interface specification of
   NTT Communications' dual stack ADSL access service, which provide a
   IPv6/IPv4 dual stack services to home users.  In order to simplify
   user setup, these services have a mechanism to configure IPv6
   specific parameters automatically.  The memo focuses on two basic
   parameters:  the prefix assigned to the user and the addresses of
   IPv6 DNS servers, and it specifies a way to deliver these parameters
   to Customer Premises Equipment (CPE) automatically.

1. Introduction

   This memo is a digest of the user network interface specification of
   NTT Communications' dual stack ADSL access service, which provide
   IPv6/IPv4 dual stack services to home users.  In order to simplify
   user setup, these services have a mechanism to configure IPv6
   specific parameters automatically.  The memo focuses on two basic
   parameters:  the prefix assigned to the user and the addresses of
   IPv6 DNS servers, and it specifies a way to deliver these parameters
   to Customer Premises Equipment (CPE) automatically.

This memo covers two topics: an architecture for IPv6/IPv4 dual stack access service and an automatic configuration function for IPv6- specific parameters.

The architecture is mainly targeted at a leased-line ADSL service for home users. It assumes that there is a Point-to-Point Protocol (PPP) logical link between Customer Premises Equipment (CPE) and Provider Edge (PE) equipment. In order to exclude factors that are specific to access lines, this architecture only specifies PPP and its upper layers. To satisfy [RFC3177], the prefix length that is delegated to the CPE is /48, but /64 is also a possible option.

In this architecture, IPv6/IPv4 dual stack service is specified as follows.

  • IPv6 and IPv4 connectivities are provided over a single PPP logical link.
  • IPv6 connectivity is independent of IPv4 connectivity. IPV6CP and IPCP work independently over a single PPP logical link.

Figure 1 shows an outline of the service architecture. NTT Communications has been providing a commercial service based on this architecture since the Summer 2002.

          |                                             _____________
   [HOST]-+ +-----------+               +----------+   /             \
          | | Customer  |   ADSL line   | Provider |  | ISP core and  |
          +-+ Premises  +---------------+   Edge   |--| The internet  |
          | | Equipment | to subscriber +-----+----+   \_____________/
   [HOST]-+ +-----------+                     |         |   |
          |                             +-----+------+  | +-+----------+
                                        | AAA server |  | | DNS server |
                                        +------------+  | +------------+
                                                      | NTP server etc.|
    Figure 1: Dual Stack Access Service Architecture  +----------------+

The automatic configuration function aims at simplification of user setup. Usually, users have to configure at least two IPv6-specific parameters: prefix(es) assigned to them [RFC3769] and IPv6 DNS servers' addresses. The function is composed of two sub-functions:

  • Delegation of prefix(es) to be used in the user site.
  • Notification of IPv6 DNS server addresses and/or other server addresses.

Section 2 of this memo details the user/network interface. Section 3 describes an example connection sequence.

2. User/Network Interface

This section describes details of the user/network interface specification. Only PPP over Ethernet (PPPoE) and its upper layers are mentioned; the other layers, such as Ethernet and lower layers, are out of scope. IPv4-related parameter configuration is also out of scope.

2.1. Below the IP Layer

The service uses PPP connection and Challenge Handshake Authentication Protocol (CHAP) authentication to identify each CPE. The CPE and PE handle both the PPP Internet Protocol Control Protocol (IPCP) [RFC1332] and the Internet Protocol V6 Control Protocol (IPV6CP) [RFC2472] identically and simultaneously over a single PPP connection. This means either the CPE or the PE can open/close any Network Control Protocol (NCP) session at any time without any side- effect for the other. It is intended that users can choose among three services: IPv4 only, IPv6 only, and IPv4/IPv6 dual stack. A CPE connected to an ADSL line discovers a PE with the PPPoE mechanism [RFC2516].

Note that, because CPE and PE can negotiate only their interface identifiers with IPV6CP, PE and CPE can use only link-local-scope addresses before the prefix delegation mechanism described below is run.

2.2. IP Layer

After IPV6CP negotiation, the CPE initiates a prefix delegation request. The PE chooses a global-scope prefix for the CPE with information from an Authentication, Authorization, and Accounting (AAA) server or local prefix pools, and it delegates the prefix to the CPE. Once the prefix is delegated, the prefix is subnetted and assigned to the local interfaces of the CPE. The CPE begins sending router advertisements for the prefixes on each link. Eventually, hosts can acquire global-scope prefixes through conventional IPv6 stateless [RFC2462] or stateful auto-configuration mechanisms ([RFC3315], etc.) and begin to communicate using global-scope addresses.

2.3. Prefix Delegation

The PE delegates prefixes to CPE using Dynamic Host Configuration Protocol for IPv6 (DHCPv6) [RFC3315] with the prefix delegation options [RFC3633]. The sequence for prefix delegation is as follows:

  • The CPE requests prefix(es) from a PE by sending a DHCPv6 Solicit message that has a link-local source address negotiated by IPV6CP, mentioned in the previous section, and includes an IA_PD option.
  • An AAA server provides prefix(es) to the PE or the PE chooses prefix(es) from its local pool, and the PE returns an Advertise message that contains an IA_PD option and IA_PD Prefix options. The prefix-length in the IA_PD Prefix option is 48.

IA_PD option and IA_PD Prefix options for the chosen prefix(es) back to the PE.

  • The PE confirms the prefix(es) in the Request message in a Reply message.

If IPV6CP is terminated or restarted by any reason, CPE must initiate a Rebind/Reply message exchange as described in [RFC3633].

2.4. Address Assignment

The CPE assigns global-scope /64 prefixes, subnetted from the delegated prefix, to its downstream interfaces. When the delegated prefix has an infinite lifetime, the preferred and valid lifetimes of assigned /64 prefixes should be the default values in [RFC2461].

Because a link-local address is already assigned to the CPE's upstream interface, global-scope address assignment for that interface is optional.

2.5. Routing

The CPE and PE use static routing between them, and no routing protocol traffic is necessary.

The CPE configures its PPPoE logical interface or the link-local address of PE as the IPv6 default gateway, automatically after the prefix delegation exchange.

When the CPE receives packets that are destined for the addresses in the delegated /48 prefix, the CPE must not forward the packets to a PE. The CPE should return ICMPv6 Destination Unreachable message to a source address or silently discard the packets, when the original packet is destined for the unassigned prefix in the delegated prefix. (For example, the CPE should install a reject route or null interface as next hop for the delegated prefix.)

2.6. Obtaining Addresses of DNS Servers

The service provides IPv6 recursive DNS servers in the ISP site. The PE notifies the global unicast addresses of these servers with the Domain Name Server option that is described in [RFC3646], in Advertise/Reply messages on the prefix delegation message exchange.

Devices connected to user network may learn a recursive DNS server address with the mechanism described in [RFC3736].

The CPE may serve as a local DNS proxy server and include its address in the DNS server address list. This is easy to implement, because it is analogous to IPv4 SOHO router ( is a DNS proxy server and a default router in most sites).

2.7. Miscellaneous Information

The PE may notify other IPv6-enabled server addresses, such as Network Time Protocol servers [RFC4075], SIP servers [RFC3319], etc., in an Advertise/Reply message on the prefix delegation message exchange, if those are available.

2.8. Connectivity Monitoring

ICMPv6 Echo Request will be sent to the user network for connectivity monitoring in the service. The CPE must return a single IPv6 Echo Reply packet when it receives an ICMPv6 Echo Request packet. The health-check packets are addressed to a subnet-router anycast address for the delegated prefix.

The old document of APNIC IPv6 address assignment policy required that APNIC could ping the subnet anycast address to check address usage.

   To achieve this requirement, for example, once the prefix
   2001:db8:ffff::/48 is delegated, the CPE must reply to the ICMPv6
   Echo Request destined for 2001:db8:ffff:: any time that IPV6CP and
   DHCPv6-PD are up for the upstream direction.  Because some
   implementations couldn't reply when 2001:db8:ffff::/64 was assigned
   to its downstream physical interface and the interface was down, such
   an implementation should assign 2001:db8:ffff::/64 for the loopback
   interface, which is always up, and 2001:db8:ffff:1::/64,
   2001:db8:ffff:2::/64, etc., to physical interfaces.

3. An Example of Connection Sequence

         CPE                      PE
          |                       |
          |----------PADI-------->| \
          |<---------PADO---------|  | PPPoE
          |----------PADR-------->|  | Discovery Stage
          |<---------PADS---------| /
          |                       |
          |---Configure-Request-->| \
          |<--Configure-Request---|  | PPP Link Establishment Phase
          |<----Configure-Ack-----|  | (LCP)
          |-----Configure-Ack---->| /
          |                       |
          |<------Challenge-------| \
          |-------Response------->|  | PPP Authentication Phase (CHAP)
          |<-------Success--------| /
          |                       |
          |---Configure-Request-->| \
          |<--Configure-Request---|  |
          |<----Configure-Nak-----|  | PPP Network Layer Protocol Phase
          |<----Configure-Ack-----|  | (IPCP)
          |---Configure-Request-->|  |
          |<----Configure-Ack-----| /
          |                       |
          |---Configure-Request-->| \
          |<--Configure-Request---|  | PPP Network Layer Protocol Phase
          |<----Configure-Ack-----|  | (IPV6CP)
          |-----Configure-Ack---->| /
          |                       |
          |--------Solicit------->| \
          |<------Advertise-------|  | DHCPv6
          |--------Request------->|  |
          |<--------Reply---------| /
          |                       |

Figure 2: Example of Connection Sequence

Figure 2 is an example of a normal link-up sequence, from start of PPPoE to start of IPv6/IPv4 communications. IPv4 communication becomes available after IPCP negotiation. IPv6 communication with link-local scope addresses becomes possible after IPV6CP negotiation. IPv6 communication with global-scope addresses becomes possible after prefix delegation and conventional IPv6 address configuration mechanism. IPCP is independent of IPV6CP and prefix delegation.

4. Security Considerations

In this architecture, the PE and CPE trust the point-to-point link between them; they trust that there is no man-in-the-middle and they trust PPPoE authentication. Because of this, DHCP authentication is not considered necessary and is not used.

The service provides an always-on global-scope prefix for users. Each device connected to user network has global-scope addresses. Without any packet filters, devices might be accessible from outside the user network in that case. The CPE and each device involved in the service should have functionality to protect against unauthorized accesses, such as a stateful inspection packet filter. The relationship between CPE and devices connected to the user network for this problem should be considered in the future.

5. Acknowledgements

Thanks are given for the input and review by Tatsuya Sato, Hideki Mouri, Koichiro Fujimoto, Hiroki Ishibashi, Ralph Droms, Ole Troan, Pekka Savola, and IPv6-ops-IAJapan members.

6. References

6.1. Normative References

   [RFC3177] IAB and IESG, "IAB/IESG Recommendations on IPv6 Address
             Allocations to Sites", RFC 3177, September 2001.
   [RFC1332] McGregor, G., "The PPP Internet Protocol Control Protocol
             (IPCP)", RFC 1332, May 1992.
   [RFC2472] Haskin, D. and E. Allen, "IP Version 6 over PPP", RFC 2472,
             December 1998.
   [RFC2516] Mamakos, L., Lidl, K., Evarts, J., Carrel, D., Simone, D.,
             and R. Wheeler, "A Method for Transmitting PPP Over
             Ethernet (PPPoE)", RFC 2516, February 1999.
   [RFC2462] Thomson, S. and T. Narten,  "IPv6 Stateless Address
             Autoconfiguration", RFC 2462, December 1998.
   [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.
   [RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
             Host Configuration Protocol (DHCP) version 6", RFC 3633,
             December 2003.  RFC 3633, December 2003.
   [RFC2461] Narten, T., Nordmark, E. and W. Simpson, "Neighbor
             Discovery for IP Version 6 (IPv6)", RFC 2461, December
   [RFC3646] Droms, R., "DNS Configuration options for Dynamic Host
             Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
             December 2003.
   [RFC3736] Droms, R., "Stateless Dynamic Host Configuration Protocol
             (DHCP) Service for IPv6", RFC 3736, April 2004.
   [RFC4075] Kalusivalingam, V., "Simple Network Time Protocol (SNTP)
             Configuration Option for DHCPv6", RFC 4075, May 2005.
   [RFC3319] Schulzrinne, H. and B. Volz, "Dynamic Host Configuration
             Protocol (DHCPv6) Options for Session Initiation Protocol
             (SIP) Servers", RFC 3319, July 2003.

6.2. Informative References

   [RFC3769] Miyakawa, S. and R. Droms, "Requirements for IPv6 Prefix
             Delegation", RFC 3769, June 2004.

Authors' Addresses

Yasuhiro Shirasaki
NTT Communications Corporation
Tokyo Opera City Tower 21F
3-20-2 Nishi-Shinjuku, Shinjuku-ku
Tokyo 163-1421, Japan


   Shin Miyakawa, Ph. D
   NTT Communications Corporation
   Tokyo Opera City Tower 21F
   3-20-2 Nishi-Shinjuku, Shinjuku-ku
   Tokyo 163-1421, Japan


   Toshiyuki Yamasaki
   NTT Communications Corporation
   1-1-6 Uchisaiwaicho, Chiyoda-ku
   Tokyo 100-8019, Japan
   Ayako Takenouchi
   NTT Cyber Solutions Laboratories, NTT Corporation
   3-9-11 Midori-Cho, Musashino-Shi
   Tokyo 180-8585, Japan

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