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draft-ietf-v6ops-ipv6-multihoming-without-ipv6nat-01.txt
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Internet Engineering Task Force O. Troan, Ed.
Internet-Draft Cisco
Intended status: Informational D. Miles
Expires: March 3, 2012 Alcatel-Lucent
S. Matsushima
Softbank Telecom
T. Okimoto
NTT West
D. Wing
Cisco
August 31, 2011
IPv6 Multihoming without Network Address Translation
draft-ietf-v6ops-ipv6-multihoming-without-ipv6nat-01
Abstract
Network Address and Port Translation (NAPT) works well for conserving
global addresses and addressing multihoming requirements, because an
IPv4 NAPT router implements three functions: source address
selection, next-hop resolution and optionally DNS resolution. For
IPv6 hosts one approach could be the use of IPv6 NAT. However, NAT
should be avoided, if at all possible, to permit transparent end-to-
end connectivity. In this document, we analyze the use cases of
multihoming. We also describe functional requirements and possible
solutions for multihoming without the use of NAT in IPv6 for hosts
and small IPv6 networks that would otherwise be unable to meet
minimum IPv6 allocation criteria. Nevertheless, we mention that the
possible needs for IPv6 NAT in the transition phase to the fully
deployment of the proposed solutions.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 3, 2012.
Troan, et al. Expires March 3, 2012 [Page 1]
Internet-Draft IPv6 Multihoming without NAT August 2011
Copyright Notice
Copyright (c) 2011 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. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. IPv6 multihomed network scenarios . . . . . . . . . . . . . . 5
3.1. Classification of network scenarios for multihomed host . 5
3.2. Multihomed network environment . . . . . . . . . . . . . . 8
3.3. Problem Statement . . . . . . . . . . . . . . . . . . . . 9
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1. End-to-End transparency . . . . . . . . . . . . . . . . . 10
4.2. Policy distribution . . . . . . . . . . . . . . . . . . . 11
4.3. Scalability . . . . . . . . . . . . . . . . . . . . . . . 11
5. Problem statement and analysis . . . . . . . . . . . . . . . . 11
5.1. Source address selection . . . . . . . . . . . . . . . . . 12
5.2. Next-hop selection . . . . . . . . . . . . . . . . . . . . 12
5.3. DNS recursive name server selection . . . . . . . . . . . 13
6. Implementation approach . . . . . . . . . . . . . . . . . . . 14
6.1. Source address selection . . . . . . . . . . . . . . . . . 14
6.2. Next-hop selection . . . . . . . . . . . . . . . . . . . . 14
6.3. DNS recursive name server selection . . . . . . . . . . . 15
7. Considerations for host without multi-prefix support . . . . . 16
7.1. IPv6 NAT . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.2. Co-existence consideration . . . . . . . . . . . . . . . . 17
8. Security Considerations . . . . . . . . . . . . . . . . . . . 17
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 19
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
11.1. Normative References . . . . . . . . . . . . . . . . . . . 19
11.2. Informative References . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
Troan, et al. Expires March 3, 2012 [Page 2]
Internet-Draft IPv6 Multihoming without NAT August 2011
1. Introduction
IPv6 provides enough globally unique addresses to permit every
conceivable host on the Internet to be uniquely addressed without the
requirement for Network Address Port Translation (NAPT [RFC3022]),
offering a renaissance in end-to-end transparent connectivity.
Unfortunately, this may not be possible due to the necessity of NAT
even in IPv6, because of multihoming. Though there are some
mechanisms to implement multihoming, such as BGP multihoming
[RFC4116] in network level, and SCTP based multihoming [RFC4960] in
application transport level, there is no mechanism in IPv6 that
serves as a replacement for NAT based multihoming in IPv4. In IPv4,
for a host or a small network, NAT based multihoming is easily
deployable and an already deployed technique. Some of the same
reasons for IPv4 NATs may be applicable to IPv6.
Whenever a host or small network (which does not meet minimum IPv6
allocation criteria) is connected to multiple upstream networks, an
IPv6 address is assigned by each respective service provider
resulting in hosts with multiple global scope IPv6 addresses with
different prefixes. As each service provider is allocated a
different address space from its Internet Registry, it in-turn
assigns a different address space to the end-user network or host.
For example, a remote access user's host or router may use a VPN to
simultaneously connect to a remote network and retain a default route
to the Internet for other purposes.
In IPv4 a common solution to the multihoming problem is to employ
NAPT on a border router and use private address space for individual
host addressing. The use of NAPT allows hosts to have exactly one IP
address visible on the public network and the combination of NAPT
with provider-specific outside addresses (one for each uplink) and
destination-based routing insulates a host from the impacts of
multiple upstream networks. The border router may also implement a
DNS cache or DNS policy to resolve address queries from hosts.
It is our goal to avoid the IPv6 equivalent of NAT. So, the goals
for IPv6 multihoming defined in [RFC3582] do not match the goals of
this document. Also regardless of what the IPv6 NAT's specification
is, we are trying to avoid any form of network address translation
technique that may not be visible for either of the end hosts. To
reach this goal, mechanisms are needed for end-user hosts to have
multiple address assignments and resolve issues such as which address
to use for sourcing traffic to which destination:
Troan, et al. Expires March 3, 2012 [Page 3]
Internet-Draft IPv6 Multihoming without NAT August 2011
o If multiple routers exist on a single link the host must select
the appropriate next-hop for each connected network. Each router
is in turn connected to a different service provider network,
which provides independent address assignment. Routing protocols
that would normally be employed for router-to-router network
advertisement seem inappropriate for use by individual hosts.
o Source address selection also becomes difficult whenever a host
has more than one address within the same address scope. Current
address selection criteria may result in hosts using an arbitrary
or random address when sourcing upstream traffic. Unfortunately,
for the host, the appropriate source address is a function of the
upstream network for which the packet is bound for. If an
upstream service provider uses IP anti-spoofing or ingress
filtering, it is conceivable that the packets that have an
inappropriate source address for the upstream network would never
reach their destination.
o In a multihomed environment, different DNS scopes or partitions
may exist in each independent upstream network. A DNS query sent
to an arbitrary upstream DNS recursive name servier may result in
incorrect or poisoned responses.
In short, while IPv6 facilitates hosts having more than one address
in the same address scope, the application of this causes significant
issues for a host from routing, source address selection and DNS
resolution perspectives. A possible consequence of assigning a host
multiple identically-scoped addresses is severely impaired IP
connectivity.
If a host connects to a network behind an IPv4 NAPT, the host has one
private address in the local network. There is no confusion. The
NAT becomes the gateway of the host and forwards the packet to an
appropriate network when it is multihomed. It also operates a DNS
cache server, which receives all DNS inquires, and gives a correct
answer to the host.
In this document, we analyze the use cases of multihoming. We also
describe functional requirements and possible solutions for
multihoming without the use of NAT in IPv6 for hosts and small IPv6
networks that would otherwise be unable to meet minimum IPv6
allocation criteria. Nevertheless, we mention that the possible
needs for IPv6 NAT in the transition phase to the fully deployment of
the proposed solutions.
Troan, et al. Expires March 3, 2012 [Page 4]
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2. Terminology
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].
IPv6 NAT The terms "NAT66" and "IPv6 NAT" refer to NPTv6
[RFC6296].
NAPT Network Address Port Translation as described
in [RFC3022]. In other contexts, NAPT is often
pronounced "NAT" or written as "NAT".
Multihomed with multi-prefix (MHMP) A host implementation which
supports the mechanisms described in this
document. Namely source address selection
policy, next-hop selection and DNS selection
policy.
3. IPv6 multihomed network scenarios
In this section, we classify three scenarios of the multihoming
environment.
3.1. Classification of network scenarios for multihomed host
Scenario 1:
In this scenario, two or more routers are present on a single link
shared with the host(s). Each router is in turn connected to a
different service provider network, which provides independent
address assignment and DNS recursive name servers. A host in this
environment would be offered multiple prefixes and DNS recursive name
servers advertised from the two different routers.
Troan, et al. Expires March 3, 2012 [Page 5]
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+------+ ___________
| | / \
+---| rtr1 |=====/ network \
| | | \ 1 /
+------+ | +------+ \___________/
| | |
| hosts|-----+
| | |
+------+ | +------+ ___________
| | | / \
+---| rtr2 |=====/ network \
| | \ 2 /
+------+ \___________/
Figure 1: single uplink, multiple next-hop, multiple prefix
(Scenario 1)
Figure 1 illustrates the host connecting to rtr1 and rtr2 via a
shared link. Networks 1 and 2 are reachable via rtr1 and rtr2
respectively. When the host sends packets to network 1, the next-hop
to network 1 is rtr1. Similarly, rtr2 is the next-hop to network 2.
- e.g., broadband service (Internet, VoIP, IPTV, etc.)
Scenario 2:
In this scenario, a single gateway router connects the host to two or
more upstream service provider networks. This gateway router would
receive prefix delegations and a different set of DNS recursive name
servers from each independent service provider network. The gateway
in turn advertises the provider prefixes to the host, and for DNS,
may either act as a lightweight DNS cache server or may advertise the
complete set of service provider DNS recursive name servers to the
hosts.
Troan, et al. Expires March 3, 2012 [Page 6]
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+------+ ___________
+-----+ | | / \
| |=======| rtr1 |=====/ network \
| |port1 | | \ 1 /
+------+ | | +------+ \___________/
| | | |
| hosts|-----| GW |
| | | rtr |
+------+ | | +------+ ___________
| |port2 | | / \
| |=======| rtr2 |=====/ network \
+-----+ | | \ 2 /
+------+ \___________/
Figure 2: single uplink, single next-hop, multiple prefix
(Scenario 2)
Figure 2 illustrates the host connected to GW rtr. GW rtr connects
to networks 1 and 2 via port1 and 2 respectively. When the host
sends packets to either network 1 or 2, the next-hop is GW rtr. When
the packets are sent to network 1 (network 2), GW rtr forwards the
packets to port1 (port2).
- e.g, Internet + VPN/Application Service Provider (ASP)
Scenario 3:
In this scenario, a host has more than one active interface that
connects to different routers and service provider networks. Each
router provides the host with a different address prefix and set of
DNS recursive name servers, resulting in a host with a unique address
per link/interface.
Troan, et al. Expires March 3, 2012 [Page 7]
Internet-Draft IPv6 Multihoming without NAT August 2011
+------+ +------+ ___________
| | | | / \
| |-----| rtr1 |=====/ network \
| | | | \ 1 /
| | +------+ \___________/
| |
| host |
| |
| | +------+ ___________
| | | | / \
| |=====| rtr2 |=====/ network \
| | | | \ 2 /
+------+ +------+ \___________/
Figure 3: Multiple uplink, multiple next-hop, multiple prefix
(Scenario 3)
Figure 3 illustrates the host connecting to rtr1 and rtr2 via a
direct connection or a virtual link. When the host sends packets
network 1, the next-hop to network 1 is rtr1. Similarly, rtr2 is the
next-hop to network 2.
- e.g., Mobile Wifi + 3G, ISP A + ISP B
3.2. Multihomed network environment
In an IPv6 multihomed network, a host is assigned two or more IPv6
addresses and DNS recursive name servers from independent service
provider networks. When this multihomed host attempts to connect
with other hosts, it may incorrectly resolve the next-hop router, use
an inappropriate source address, or use a DNS response from an
incorrect service provider that may result in impaired IP
connectivity.
Multihomed networks in IPv4 have been commonly implemented through
the use of a gateway router with NAPT function (scenario 2 with
NAPT). An analysis of the current IPv4 NAPT and DNS functions within
the gateway router should provide a baseline set of requirements for
IPv6 multihomed environments. A destination prefix/route is often
used on the gateway router to separate traffic between the networks.
Troan, et al. Expires March 3, 2012 [Page 8]
Internet-Draft IPv6 Multihoming without NAT August 2011
+------+ ___________
| | / \
+---| rtr1 |=====/ network \
| | | \ 1 /
+------+ +-----+ | +------+ \___________/
| IPv4 | | | |
| host |-----| GW |---+
| | | rtr | |
+------+ +-----+ | +------+ ___________
(NAPT&DNS) | | | / \
(private +---| rtr2 |=====/ network \
address | | \ 2 /
space) +------+ \___________/
Figure 4: IPv4 Multihomed environment with Gateway Router performing
NAPT
3.3. Problem Statement
A multihomed IPv6 host has one or more assigned IPv6 addresses and
DNS recursive name servers from each upstream service provider,
resulting in the host having multiple valid IPv6 addresses and DNS
recursive name servers. The host must be able to resolve the
appropriate next-hop, the correct source address and DNS recursive
name server to use based on the destination prefix. To prevent IP
spoofing, operators will often implement ingress filtering to discard
traffic with an inappropriate source address, making it essential for
the host to correctly resolve these three items before sourcing the
first packet.
IPv6 has mechanisms for the provision of multiple routers on a single
link and multiple address assignments to a single host. However,
when these mechanisms are applied to the three scenarios in
Section 3.1 a number of connectivity issues are identified:
Scenario 1:
The host has been assigned an address from each router and recognizes
both rtr1 and rtr2 as valid default routers (in the default routers
list).
o The source address selection policy on the host does not
deterministically resolve a source address. Ingress filtering or
filter policies will discard traffic with source addresses that
the operator did not assign.
o The host will select one of the two routers as the active default
router. No traffic is sent to the other router.
Troan, et al. Expires March 3, 2012 [Page 9]
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Scenario 2:
The host has been assigned two different addresses from the single
gateway router. The gateway router is the only default router on the
link.
o The source address selection policy on the host does not
deterministically resolve a source address. Ingress filtering or
filter policies will discard traffic with source addresses that
the operator did not assign.
o The gateway router does not have an autonomous mechanism for
determining which traffic should be sent to which network. If the
gateway router is implementing host functions (i.e., processing
Router Advertisement) then two valid default routers may be
recognized.
Scenario 3:
A host has two separate interfaces and on each interface a different
address is assigned. Each link has its own router.
o The host does not have enough information for determining which
traffic should be sent to which upstream routers. The host will
select one of the two routers as the active default router, and no
traffic is sent to the other router. The default address
selection rules select the address assigned to the outgoing
interface as the source address. So, if a host has an appropriate
routing table, an appropriate source address will be selected.
All scenarios:
o In network deployments utilizing local namespaces, the host may
choose to communicate with a "wrong" DNS recursive server unable
to serve a local namespace.
4. Requirements
This section describes requirements that any solution multi-address
and multi-uplink architectures need to meet.
4.1. End-to-End transparency
One of the major design goals for IPv6 is to restore the end-to-end
transparency of the Internet. If NAT mechanism is applied to IP
communication between hosts, it is required to apply complex NAT
traversal mechanism to establish bi-directional IP communication.
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Essentially, extra NAT traversal meachanism does not need to be
implemented on application, on an environment with end-to-end
transparency. Therefore, The IPv6 multihoming solution SHOULD
guarantee end-to-end transparency by avoiding IPv6 NAT.
4.2. Policy distribution
The solution SHOULD have a function to provide a policy on sites/
nodes. In particular, a network service provider has to control his
or her user nodes such as CPE devices. All nodes are not necessarily
controlled evenly with policy providing. It is required to identify
a nodes and provide indepenent policy by each node.
The providing mechanisms should have:
o a function to distribute policies to nodes dynamically to update
their behavior. When the network environment changes and the
nodes' behavior has to be changed, a network administrator can
modify the behavior of the nodes.
o a function to control every node centrally. A site administrator
or a service provider could determine or could have an effect on
the behavior at their users' hosts.
o a function to control node-specific behavior. Even when multiple
nodes are on the same subnet, the mechanism should be able to
provide a method for the network administrator to make nodes
behave differently. For example, each node may have a different
set of assigned prefixes. In such a case, the appropriate
behavior may be different.
4.3. Scalability
The solution will have to be able to manage a large number of sites/
nodes. In services for residential users, provider edge devices have
to manage thousands of sites. In such environments, sending packets
periodically to each site may affect edge system performance.
5. Problem statement and analysis
The problems described in Section 3 can be classified into these
three types:
o Wrong source address selection
o Wrong next-hop selection
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o Wrong DNS server selection
This section reviews the problem statements presented above and the
proposed functional requirements to resolve the issues.
5.1. Source address selection
A multihomed IPv6 host will typically have different addresses
assigned from each service provider either on the same link
(scenarios 1 & 2) or different links (scenario 3). When the host
wishes to send a packet to any given destination, the current source
address selection rules [RFC3484] may not deterministically resolve
the correct source address when the host addressing was via Router
Advertisement (RA) or DHCPv6.
[I-D.ietf-6man-addr-select-considerations] describes the use of the
policy table [RFC3484] to resolve this problem, but there is no
mechanism defined to disseminate the policy table information to a
host. A proposal is in [I-D.ietf-6man-addr-select-opt] to provide a
DHCPv6 mechanism for host policy table management.
Again, by employing DHCPv6, the server could restrict address
assignment (of additional prefixes) only to hosts that support policy
table management.
Scenario 1: "Host" needs to support the solution for this problem.
Scenario 2: "Host" needs to support the solution for this problem.
Scenario 3: If "Host" support the next-hop selection solution, there
is no need to support the address selection functionality on the
host.
It is noted that the service providers (i.e., ISP and enterprise/VPN)
must also support [I-D.ietf-6man-addr-select-opt].
5.2. Next-hop selection
A multihomed IPv6 host or gateway may have multiple uplinks to
different service providers. Here each router would use Router
Advertisements [RFC4861] for distributing default route/next-hop
information to the host or gateway router.
In this case, the host or gateway router may select any valid default
router from the default routers list, resulting in traffic being sent
to the wrong router and discarded by the upstream service provider.
Using the above scenarios as an example, whenever the host wishes to
reach a destination in network 2 and there is no connectivity between
networks 1 and 2 (as is the case for a walled-garden or closed
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service), the host or gateway router does not know whether to forward
traffic to rtr1 or rtr2 to reach a destination in network 2. The
host or gateway router may choose rtr1 as the default router, and
traffic fails to reach the destination server. The host or gateway
router requires route information for each upstream service provider,
but the use of a routing protocol between the gateway and the two
routers causes both configuration and scaling issues. For IPv4
hosts, the gateway router is often pre-configured with static route
information or uses of Classless Static Route Options [RFC3442] for
DHCPv4. Extensions to Router Advertisements through Default Router
Preference and More-Specific Routes [RFC4191] provides for link-
specific preferences but does not address per-host configuration in a
multi-access topology because of its reliance on Router
Advertisements. A DHCPv6 option, such as that in
[I-D.ietf-mif-dhcpv6-route-option], is preferred for host-specific
configuration. By employing a DHCPv6 solution, a DHCPv6 server could
restrict address assignment (of additional prefixes) only to hosts
that support more advanced next-hop and address selection
requirements.
Scenario 1: "Host" needs to support the solution for this problem.
Scenario 2: "GW rtr" needs to support the solution for this problem.
Scenario 3: "Host" needs to support the solution for this problem.
It is noted that the service providers (i.e., ISP and enterprise/VPN)
must also support [I-D.ietf-mif-dhcpv6-route-option].
5.3. DNS recursive name server selection
A multihomed IPv6 host or gateway router may be provided multiple DNS
recursive name servers through DHCPv6 [RFC3646] or RA [RFC6106].
When the host or gateway router sends a DNS query, it would normally
choose one of the available DNS recursive name servers for the query.
In the IPv6 gateway router scenario, the Broadband Forum [TR124]
required that the query be sent to all DNS recursive name servers,
and the gateway waits for the first reply. In IPv6, given our use of
specific destination-based policy for both routing and source address
selection, it is desirable to extend a policy-based concept to DNS
recursive name server selection. Doing so can minimize DNS recursive
name server load and avoid issues where DNS recursive name servers in
different networks have connectivity issues, or the DNS recursive
name server are not publicly accessible. In the worst case, a DNS
query for a name from a local namespace may not be resolved correctly
if sent towards a DNS server not aware of said local namespace,
resulting in a lack of connectivity.
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It is not issue of Domain Name System model itself, but an IPv6
multihomed host or gateway router should have the ability to select
appropriate DNS recursive name servers for each service based on the
domain space for the destination, and each service should provide
rules specific to that network. [I-D.ietf-mif-dns-server-selection]
proposes a solution for distributing DNS server selection policy
using a DHCPv6 option.
Scenario 1: "Host" needs to support the solution for this problem.
Scenario 2: "GW rtr" needs to support the solution for this problem.
Scenario 3: "Host" needs to support the solution for this problem.
It is noted that the service providers (i.e., ISP and enterprise/VPN)
must also support [I-D.ietf-mif-dns-server-selection].
6. Implementation approach
As mentioned in Section 5, in the multi-prefix environment, we have
three problems in source address selection, next-hop selection, and
DNS recursive name server selection. In this section, possible
solution mechanisms for each problem are introduced and evaluated
against the requirements in Section 4.
6.1. Source address selection
Possible solutions and their evaluation are summarized in
[I-D.ietf-6man-addr-select-considerations]. When those solutions are
examined against the requirements in Section 4, the proactive
approaches, such as the policy table distribution mechanism and the
routing hints mechanism, are more appropriate in that they can
propagate the network administrator's policy directly. The policy
distribution mechanism has an advantage with regard to the host's
protocol stack impact and the static nature of the assumed target
network environment.
6.2. Next-hop selection
As for the source address selection problem, both a policy-based
approach and a non policy-based approach are possible with regard to
the next-hop selection problem. Because of the same requirements,
the policy propagation-based solution mechanism, whatever the policy,
should be more appropriate.
Routing information is a typical example of policy related to next-
hop selection. If we assume source address-based routing at hosts or
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intermediate routers, the pairs of source prefixes and next-hops can
be another example of next-hop selection policy.
The routing information-based approach has a clear advantage in
implementation and is already commonly used.
The existing proposed or standardized routing information
distribution mechanisms are routing protocols, such as RIPng and
OSPFv3, the RA extension option defined in [RFC4191], the DHCPv6
route information option proposed in
[I-D.ietf-mif-dhcpv6-route-option], and the [TR069] standardized at
BBF.
The RA-based mechanism has difficulty in per-host routing information
distribution. The dynamic routing protocols such as RIPng are not
usually used between the residential users and ISP networks because
of their scalability implications. The DHCPv6 mechanism does not
have these difficulties and has the advantage of its relaying
functionality. It is commonly used and is thus easy to deploy.
[TR069], mentioned above, is a possible solution mechanism for
routing information distribution to customer-premises equipment
(CPE). It assumes, however, IP reachability to the Auto
Configuration Server (ACS) is established. Therefore, if the CPE
requires routing information to reach the ACS, [TR069] cannot be used
to distribute this information.
6.3. DNS recursive name server selection
As in the above two problems, a policy-based approach and non policy-
based approach are possible. In a non policy-based approach, a host
or a home gateway router is assumed to send DNS queries to several
DNS recursive name servers at once or to select one of the available
servers.
In the non policy-based approach, by making a query to a DNS
recursive name server in a different service provider to that which
hosts the service, a user could be directed to unexpected IP address
or receive an invalid response, and thus cannot connect to the
service provider's private and legitimate service. For example, some
DNS recursive name servers reply with different answers depending on
the source address of the DNS query, which is sometimes called split-
horizon. When the host mistakenly makes a query to a different
provider's DNS recursive name server to resolve a FQDN of another
provider's private service, and the DNS recursive name server adopts
the split-horizon configuration, the queried server returns an IP
address of the non-private side of the service. Another problem with
this approach is that it causes unnecessary DNS traffic to the DNS
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recursive name servers that are visible to the users.
The alternative of a policy-based approach is documented in
[I-D.ietf-mif-dns-server-selection], where several pairs of DNS
recursive name server addresses and DNS domain suffixes are defined
as part of a policy and conveyed to hosts in a new DHCP option. In
an environment where there is a home gateway router, that router can
act as a DNS recursive name server, interpret this option and
distribute DNS queries to the appropriate DNS servers according to
the policy.
7. Considerations for host without multi-prefix support
This section presents an alternative approach to mitigate the problem
in a multihomed network. This approach will help IPv6 hosts that are
not capable of the enhancements for the source address selection
policy, next-hop selection policy, and DNS selection policy described
in Section 6.
7.1. IPv6 NAT
In a typical IPv4 multihomed network deployment, IPv4 NAPT is
practically used and it can eventually avoid assigning multiple
addresses to the hosts and solve the next-hop selection problem. In
a similar fashion, IPv6 NAT can be used as a last resort for IPv6
multihomed network deployments where one needs to assign a single
IPv6 address to a host.
__________
/ \
+---/ Internet \
gateway router | \ /
+------+ +---------------------+ | \__________/
| | | | | WAN1 +--+
| host |-----|LAN| Router |--------|
| | | | |NAT|WAN2+--+
+------+ +---------------------+ | __________
| / \
+---/ ASP \
\ /
\__________/
Figure 5: Legacy Host
The gateway router also has to support the two features, next-hop
selection and DNS server selection, shown in Section 6.
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The implementation and issues of IPv6 NAT are out of the scope of
this document. They may be covered by another document under
discussion [RFC6296].
7.2. Co-existence consideration
To allow the co-existence of non-MHMP hosts and MHMP hosts (i.e.
hosts supporting multi-prefix with the enhancements for the source
address selection), GW-rtr may need to treat those hosts separately.
An idea to achieve this is that GW-rtr identifies the hosts, and then
assigns a single prefix to non-MHMP hosts and assigns multiple
prefixes to MHMP hosts. In this case, GW-rtr can perform IPv6 NAT
only for the traffic from non-MHMP hosts if its source address is not
appropriate.
Another idea is that GW-rtr assigns multiple prefixes to the both
hosts, and it performs IPv6 NAT for the traffic from non-MHMP hosts
if its source address is not appropriate.
In scenario 1 and 3, the non-MHMP hosts can be placed behind the NAT
box. In this case, the non-MHMP host can access the service through
the NAT box.
The implementation of identifying non-MHMP hosts and NAT policy is
outside the scope of this document.
8. Security Considerations
This document requires that the solutions for MHMP should have policy
providing functions. New security threats can be introduced
depending on what kind and what form of the policy. The threats can
be categorized in two parts: the policy receiver side and the policy
distributor side.
A policy receiver may receive an evil policy from a policy
distributor. A policy distributor should expect some hosts in its
network do not follow the distributed policy. The security threats
related to IPv6 multihoming are described in [RFC4218]. Those
threats that are specific to MHMP solutions are enumerated below.
Threats related to the policy distributor side:
Policy collision can happen. When multiple policy distributor
exists, a policy receiver may not follow one or each of the
received policy. Especially when a policy conflicts with
another policy, a policy receiver cannot implement each of the
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policy. To solve or mitigate this issue, policy prioritization
rule should be defined, such as preference for the policy
received on a trusted interface. Another solution is to
preclude the functionality of multiple policy acceptance at the
receiver side. In this case, a policy distributor should
cooperate with other policy distributors, and a single
representative provider should distribute a merged policy.
Apart from the policy collision, a network service provider
should expect the existence of hosts that will not obey the
received policy. A possible solutions is to ingress-filter
those packets that do not match the distributed policy and drop
them. About the route selection, packet forwarding or
redirection can be another possible solution. About the source
address selection, IPv6 NAT can be another possible solution.
Threats related to the policy receiver side:
A policy receiver are exposed to the threats of unauthorized
policy, which can lead to session hijack, DoS, wiretapping and
phishing. Unauthorized policy here means a policy distributed
from an entity that does not have rights to do so. Usually,
only a site administrator and a network service provider have
rights to distribute these policies just as well as IP address
assignment and DNS server address notification. Regarding
source address selection, unauthorized policy can expose an IP
address that will not usually be exposed to an external server,
which can be a privacy problem. To solve or mitigate this
problem of unauthorized policy, one approach is limiting on use
of these policy distribution mechanisms, as described in the
section 4.4 of [I-D.ietf-mif-dns-server-selection]. For
example, a policy should be preferred or accepted when the
policy is delivered across a secure, trusted channel such as 3G
connection in cellular services. The proposed solutions are
baed on DHCP, so the limitation of local site communication,
which is often used in WiFi access services, should be another
solution or mitigation for this problem. About DNS server
selection issue, DNSSEC can be another solution. About source
address selection, the ingress filter at the network service
provider router can be a solution.
Another threat is the leakage of the policy and privacy issues
resulting from that. Especially when each client is
distributed its own policy from the network service provider,
the policy can give a hint of which service the client