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Keywords: IPv6 transition
Independent Submission T. Tsou
Request for Comments: 6654 Huawei Technologies (USA)
Category: Informational C. Zhou
ISSN: 2070-1721 T. Taylor
Huawei Technologies
Q. Chen
China Telecom
July 2012
Gateway-Initiated IPv6 Rapid Deployment on IPv4 Infrastructures (GI 6rd)
Abstract
This document proposes an alternative IPv6 Rapid Deployment on IPv4
Infrastructures (6rd) deployment model to that of RFC 5969. The
basic 6rd model allows IPv6 hosts to gain access to IPv6 networks
across an IPv4 access network using 6-in-4 tunnels. 6rd requires
support by a device (the 6rd customer edge, or 6rd-CE) on the
customer site, which must also be assigned an IPv4 address. The
alternative model described in this document initiates the 6-in-4
tunnels from an operator-owned Gateway collocated with the operator's
IPv4 network edge rather than from customer equipment, and hence is
termed "Gateway-initiated 6rd" (GI 6rd). The advantages of this
approach are that it requires no modification to customer equipment
and avoids assignment of IPv4 addresses to customer equipment. The
latter point means less pressure on IPv4 addresses in a high-growth
environment.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6654.
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RFC 6654 Gateway-Initiated 6rd July 2012
Copyright Notice
Copyright (c) 2012 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.
Table of Contents
1. Introduction ....................................................2
2. Problem Statement ...............................................3
3. Proposed Solution ...............................................4
3.1. Prefix Delegation ..........................................5
3.2. Relevant Differences from Basic 6rd ........................6
4. Security Considerations .........................................7
5. Acknowledgements ................................................7
6. References ......................................................7
6.1. Normative References .......................................7
6.2. Informative References .....................................7
1. Introduction
6rd [RFC5969] provides a transition tool for connecting IPv6 devices
across an IPv4 network to an IPv6 network, at which point the packets
can be routed natively. The network topology is shown in Figure 1.
+--------------+ +-----------------+ +---------+
| | | | | |
+-----+ +-----+ | Provider +--------+ | |
|IPv6 | | 6rd |__| IPv4 | Border |__| IPv6 |
|Host | | CE | | Network | Router | | Network |
+-----+ +-----+ | +--------+ | |
| Customer LAN | | | | |
+--------------+ +-----------------+ +---------+
Figure 1: 6rd Deployment Topology
In Figure 1, the CE is the customer edge router. It is provisioned
with a delegated IPv6 prefix, but it is also configured with an IPv4
address so that it is reachable through the IPv4 network. If a
public IPv4 address is provisioned to every customer, it will
aggravate the pressure due to the IPv4 address shortage for operators
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RFC 6654 Gateway-Initiated 6rd July 2012
faced with a high rate of growth in the number of broadband
subscribers to their network. The use of private addresses with 6rd
avoids this particular difficulty but brings other complications.
2. Problem Statement
Consider an operator facing a high subscriber growth rate. As a
result of this growth rate, the operator faces pressure on its stock
of available public IPv4 addresses. For this reason, the operator is
motivated to offer IPv6 access as quickly as possible. Figure 2
shows the sort of network situation envisioned in the present
document.
+----+ +-------------------+ +----------------+
|Host|\ | | | |
+----+ \_+---+ +----+ Metro +----+ | Backbone |
_|CPE|----| GW | Network | BR |--| Network |
+----+ / +---+ +----+ (IPv4) +----+ | (IPv6) |
|Host|/ | | | |
+----+ +-------------------+ +----------------+
Host = IPv6 customer host device
CPE = customer edge device (customer-provided)
GW = provider edge device (Gateway)
BR = border router (dual stack)
Specialized GW and BR functions are described in the next section.
Figure 2: Typical Network Scenario for IPv6 Transition
The backbone network will be the first part of the operator's network
to support IPv6. The metro network is not so easily upgraded to
support IPv6, since many devices need to be modified and there may be
some impact to existing services. Thus, any means of providing IPv6
access has to minimize the changes required to devices in the metro
network.
In contrast to the situation described for basic 6rd [RFC5569],
the operator is assumed to have no control over the capabilities
of the IP devices on the customer premises. As a result, the
operator cannot assume that any of these devices are capable of
supporting 6rd.
If the customer equipment is in bridged mode and IPv6 is deployed to
sites via a Service Provider's (SP's) IPv4 network, the IPv6-only
host needs an IPv6 address to visit the IPv6 service. In this
scenario, 6to4 [RFC3056] or 6rd can be used. However, each IPv6-only
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RFC 6654 Gateway-Initiated 6rd July 2012
host may need one corresponding IPv4 address when using a public IPv4
address in 6to4 or 6rd, which puts great address pressure on the
operators.
If the CPE in the above figure is acting in bridging mode, each host
behind it needs to be directly assigned an IPv6 prefix so it can
access IPv6 services. If the CPE is acting in routing mode, only the
CPE needs to be assigned an IPv6 prefix, and it delegates prefixes to
the hosts behind it.
If the Gateway supports IPv4 only, then an IPv4 address must also be
assigned to each host (bridging mode) or to the CPE (routing mode).
Both of these cases, but the bridging mode in particular, put
pressure on the provider's stock of IPv4 addresses.
If the Gateway is dual stack, an arrangement may be possible whereby
all communication between the Gateway and the customer site uses IPv6
and the need to assign IPv4 addresses to customer devices is avoided.
A possible solution is presented in the next section.
3. Proposed Solution
For basic 6rd [RFC5969], the 6rd CE initiates the 6-in-4 tunnel to
the dual-stack border router (i.e., the 6rd Border Relay in 6rd
terminology) to carry its IPv6 traffic. To avoid the requirement for
customer premises equipment to fulfill this role, it is necessary to
move the tunneling function to a network device. This document
identifies a functional element, termed the 6rd Gateway, to perform
this task. In what follows, the 6rd Gateway and 6rd Border Relay are
referred to simply as the Gateway and Border Relay, respectively.
The functions of the Gateway are as follows:
o to generate and allocate Gateway-initiated 6rd delegated prefixes
for IPv6-capable customer devices, as described in Section 3.1;
o to forward outgoing IPv6 packets through a tunnel to a Border
Relay, which extracts and forwards them to an IPv6 network as
for 6rd;
o to extract incoming IPv6 packets tunneled from the Border Relay
and forward them to the correct user device.
In the proposed solution, there is only one tunnel initiated from
each Gateway to the Border Relay, which greatly reduces the number of
tunnels the Border Relay has to handle. The deployment scenario
consistent with the problem statement in Section 2 collocates the
Gateway with the IP edge of the access network. This is shown in
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RFC 6654 Gateway-Initiated 6rd July 2012
Figure 2 and is the typical placement of the Broadband Network
Gateway (BNG) in a fixed broadband network. By assumption, the metro
network beyond the BNG is IPv4. Transport between the customer site
and the Gateway is over Layer 2.
The elements of the proposed solution are as follows:
o The IPv6 prefix assigned to the customer site contains the
compressed IPv4 address of the network-facing side of the Gateway,
plus a manually provisioned or Gateway-generated customer site
identifier. This is illustrated in Figure 3.
o The Border Relay is able to route incoming IPv6 packets to the
correct Gateway by extracting the compressed Gateway address from
the IPv6 destination address of the incoming packet, expanding it
to a full 32-bit IPv4 address, and setting it as the destination
address of the encapsulated packet.
o The Gateway can route incoming packets to the correct link after
decapsulation using a mapping from either the full IPv6 prefix or
the customer site identifier extracted from that prefix to the
appropriate link.
3.1. Prefix Delegation
Referring back to Figure 2, prefix assignment to the customer
equipment occurs in the normal fashion through the Gateway/IP edge,
using either DHCPv6 or Stateless Address Autoconfiguration (SLAAC).
Figure 3 illustrates the structure of the assigned prefix, and how
the components are derived, within the context of a complete address.
+--------------------+-----------+
| 32-bit Gateway IPv4 address |
+--------------------+-----------+
|<---IPv4MaskLen --->| o bits | Gateway or manually
/ / generated value, unique
Configured / / / for the Gateway
| / / |
| / / V
| V p bits | o bits | n bits |m bits | 64 bits |
+----------------+------------+---------+-------+----------------+
| | Gateway |Customer | | |
| Common prefix | Identifier | Site |subnet | interface ID |
| | | Index | ID | |
+----------------+------------+---------+-------+----------------+
|<------ GI 6rd delegated prefix ------>|
Figure 3: Gateway-Initiated 6rd Address Format for a Customer Site
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The common prefix, i.e., the first p bits of the GI 6rd delegated
prefix, is configured in the Gateway. This part of the prefix is
common across multiple customers and multiple Gateways. Multiple
common prefix values may be used in a network either for service
separation or for scalability.
The Gateway Identifier is equal to the o low-order bits of the
Gateway IPv4 address on the virtual link to the Border Relay. The
number of bits o is equal to (32 - IPv4MaskLen), where the latter is
the length of the IPv4 prefix from which the Gateway IPv4 addresses
are derived. The value of IPv4MaskLen is configured in both the
Gateways and the Border Relays.
The Customer Site Index is effectively a sequence number assigned to
an individual customer site served by the Gateway. The value of the
index for a given customer site must be unique across the Gateway.
The length n of the Customer Site Index is provisioned in the Gateway
and must be large enough to accommodate the number of customer sites
that the Gateway is expected to serve.
To give a numerical example, consider a 6rd domain containing ten
million IPv6-capable customer devices (a rather high number given
that 6rd is meant for the early stages of IPv6 deployment). The
estimated number of 6rd Gateways needed to serve this domain would be
on the order of 3,300, each serving 30,000 customer devices.
Assuming best-case compression for the Gateway addresses, the Gateway
Identifier field has length o = 12 bits. If 6-in-4 tunneling is
being used, this best case is more likely to be achievable than it
would be if the IPv4 addresses belonged to the customer devices. The
customer device index, which is a more controllable parameter, has
length n = 15 bits.
Overall, these figures suggest that the length p of the common prefix
can be 29 bits for a /56 delegated prefix, or 21 bits if /48
delegated prefixes need to be allocated.
3.2. Relevant Differences from Basic 6rd
A number of the points in [RFC5969] apply, with the simple
substitution of the Gateway for the 6rd CE. When it comes to
configuration, the definition of IPv4MaskLen changes, and there are
other differences as indicated in the previous section. Since
special configuration of customer equipment is not required, the 6rd
DHCPv6 option is inapplicable.
Since the link for the customer site to the network now extends only
as far as the Gateway, Neighbor Unreachability Detection on the part
of customer devices is similarly limited in scope.
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RFC 6654 Gateway-Initiated 6rd July 2012
4. Security Considerations
No further security considerations are raised in this document to
those described in the Security Considerations section of [RFC5969].
5. Acknowledgements
Thanks to Ole Troan for his technical comments on an early version of
this document.
6. References
6.1. Normative References
[RFC5969] Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4
Infrastructures (6rd) -- Protocol Specification",
RFC 5969, August 2010.
6.2. Informative References
[RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains
via IPv4 Clouds", RFC 3056, February 2001.
[RFC5569] Despres, R., "IPv6 Rapid Deployment on IPv4
Infrastructures (6rd)", RFC 5569, January 2010.
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Authors' Addresses
Tina Tsou
Huawei Technologies (USA)
2330 Central Expressway
Santa Clara, CA 95050
USA
EMail: Tina.Tsou.Zouting@huawei.com
Cathy Zhou
Huawei Technologies
Bantian, Longgang District
Shenzhen 518129
P.R. China
EMail: cathy.zhou@huawei.com
Tom Taylor
Huawei Technologies
Ottawa, Ontario
Canada
EMail: tom.taylor.stds@gmail.com
Qi Chen
China Telecom
109 Zhongshan Ave. West
Tianhe District, Guangzhou 510630
P.R. China
EMail: chenqi.0819@gmail.com
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