Internet Engineering Task Force T. Tsou
Internet-Draft Huawei Technologies (USA)
Intended status: Standards Track C. Zhou
Expires: January 8, 2012 T. Taylor
Huawei Technologies
O. Troan
Cisco
Q. Chen
China Telecom
July 7, 2011
"Gateway-Initiated" 6rd
draft-tsou-softwire-gwinit-6rd-03
Abstract
This document proposes an alternative 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-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. The advantages of this
approach is 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
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This Internet-Draft will expire on January 8, 2012.
Copyright Notice
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Copyright (c) 2011 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . . 3
3. Proposed Solution . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Prefix Delegation . . . . . . . . . . . . . . . . . . . . . 5
3.2. Relevant Differences From Basic 6rd . . . . . . . . . . . . 7
3.3. Security Considerations . . . . . . . . . . . . . . . . . . 7
3.4. IANA Considerations . . . . . . . . . . . . . . . . . . . . 7
4. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Normative References . . . . . . . . . . . . . . . . . . . 7
4.2. informative References . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7
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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 also with an IPv4 address so that
it is reachable through the IPv4 network. If public IPv4 address is
provisioned to every customer, it will aggravate the pressure due to
IPv4 address shortage for operators faced with a high rate of growth
in the number of broadband subscribers to their network. It is out
of scope of this document if private IPv4 address is provisioned.
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.
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 be unable to manage 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
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host needs a IPv6 address to visit the IPv6 service. In this
scenario, 6to4 or 6RD can be used. However, each IPv6-only host may
need one corresponding IPv4 address when using public IPv4 address in
6to4 or 6rd, which brings great address pressure to the operators.
If the customer equipment is in routing mode, the operator has an
opportunity to avoid assigning IPv4 addresses to sites running IPv6
only. Some other means is available for routing IPv6 traffic through
the IPv4 network to that site. The GateWay in the existing IPv4
access network should be updated to support IPv6. But the metro
network does not need to be updated.
In 6rd scenario, reachability between CEs in 6RD should go to BR.
But in this Gateway-initiated 6rd case, it does not need to go to BR
which only needs gateway to gateway traffic. How the interaction
between GW and GW works is for further elaboration.
3. Proposed Solution
For basic 6rd, the 6rd-CE described in [RFC5969] initiates the 6-in-4
tunnel to the Border Router 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 PE to
perform this task. The functions of 6rd PE are:
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 6rd 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 Router, which greatly reduces the number
of tunnels the Border Router 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
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.
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+-------+ +-------------------+ +---------+
+-----+ | | | | | |
|IPv6 | | | +---------+ IPv4 +--------+ | IPv6 |
|Cust |_|Access |_| Gateway | Metro | Border |_| core |
|site | |network| |(IP edge)| network | Router | | network |
+-----+ | | +---------+ +--------+ | |
| | | | | |
+-------+ +-------------------+ +---------+
Figure 2: Gateway-Initiated 6rd At the IP Edge
The elements of the proposed solution are these:
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 below.
o The Border Router 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 SLAAC. Figure 3 illustrates the structure of
the assigned prefix, and how the components are derived, within the
context of a complete address.
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+--------------------+-----------+
| 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
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 Router. 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 Routers.
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
in 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 IPv6-in-IPv4 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.
More controllably, the customer device index has length n = 15 bits.
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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, Neighbour Unreachability Detection on the part
of customer devices is similarly limited in scope.
3.3. Security Considerations
No change from [RFC5969].
3.4. IANA Considerations
This memo makes no request of IANA.
4. References
4.1. Normative References
[RFC5969] Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4
Infrastructures (6rd) -- Protocol Specification",
RFC 5969, August 2010.
4.2. informative References
[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
Phone:
Email: tena@huawei.com
Cathy Zhou
Huawei Technologies
Bantian, Longgang District
Shenzhen 518129
P.R. China
Phone:
Email: cathyzhou@huawei.com
Tom Taylor
Huawei Technologies
1852 Lorraine Ave.t
Ottawa, Ontario K1H 6Z8
Canada
Phone:
Email: tom111.taylor@bell.net
Ole Troan
Cisco
Phone:
Email: ot@cisco.com
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Qi Chen
China Telecom
109, Zhongshan Ave. West,
Tianhe District, Guangzhou 510630
P.R. China
Phone:
Email: chenqi.0819@gmail.com
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