wdiff rfc7157.original rfc7157.txt

Internet Engineering Task Force (IETF) O. Troan, Ed.
Internet-Draft
Request for Comments: 7157 Cisco
Intended status:
Category: Informational D. Miles
Expires: August 18, 2014 Alcatel-Lucent
ISSN: 2070-1721 Google Fiber
S. Matsushima
Softbank Telecom
T. Okimoto
NTT West
D. Wing
Cisco
February 14,
March 2014

IPv6 Multihoming without Network Address Translation
draft-ietf-v6ops-ipv6-multihoming-without-ipv6nat-06

Abstract

Network Address and Port Translation (NAPT) works well for conserving
global addresses and addressing multihoming requirements, requirements because an
IPv4 NAPT router implements three functions: source address
selection, next-hop resolution resolution, and optionally (optionally) DNS resolution. For
IPv6 hosts hosts, one approach could be the use of NPTv6. IPv6-to-IPv6 Network
Prefix Translation (NPTv6). However, NAT and NPTv6 should be
avoided, if at all possible, to permit transparent end-to-
end 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 IPv6-allocation criteria. We conclude that DHCPv6 based DHCPv6-based
solutions are suitable to solve the multihoming issues, issues described in
this document, while but NPTv6 may be required as an intermediate solution.

Status of This Memo

This Internet-Draft document is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a maximum candidate for any level of Internet
Standard; see Section 2 of RFC 5741.

Information about the current status of six months this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained 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 August 18, 2014.
http://www.rfc-editor.org/info/rfc7157.

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 Multihomed Network Scenarios . . . . . . . . . . . . . . 5
3.1. Classification of network scenarios Network Scenarios for multihomed host Multihomed Host . 5
3.2. Multihomed network environment Network Environment . . . . . . . . . . . . . 7 8
3.3. Problem Statement . . . . . . . . . . . . . . . . . . . . 8 9
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 9 10
4.1. End-to-End transparency Transparency . . . . . . . . . . . . . . . . . 10
4.2. Scalability . . . . . . . . . . . . . . . . . . . . . . . 10 11
5. Problem statement and analysis Analysis . . . . . . . . . . . . . . . 10 . . . . . . . 11
5.1. Source address selection Address Selection . . . . . . . . . . . . . . . . 10 11
5.2. Next-hop selection Next Hop Selection . . . . . . . . . . . . . . . . . . . 11 12
5.3. DNS recursive name server selection Recursive Name Server Selection . . . . . . . . . . . 12
6. Implementation approach Approach . . . . . . . . . . . . . . . . . . . 12 13
6.1. Source address selection Address Selection . . . . . . . . . . . . . . . . 12 13
6.2. Next-hop selection Next Hop Selection . . . . . . . . . . . . . . . . . . . 13 14
6.3. DNS recursive name server selection Recursive Name Server Selection . . . . . . . . . . . 13 14
6.4. Other algorithms available Algorithms Available in RFCs . . . . . . . . . . . 14 15
7. Considerations for MHMP deployment Deployment . . . . . . . . . . . . . 14 15
7.1. Non-MHMP host consideration Host Consideration . . . . . . . . . . . . . . . 15 16
7.2. Co-existence considerations Coexistence Considerations . . . . . . . . . . . . . . . 15 16
7.3. Policy collision consideration Collision Consideration . . . . . . . . . . . . . 16 17
8. Security Considerations . . . . . . . . . . . . . . . . . . . 16 17
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 18
11. 19
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
11.1. 19
10.1. Normative References . . . . . . . . . . . . . . . . . . 18
11.2. 19
10.2. Informative References . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20

1. Introduction

In this document, we analyze the use cases of multihoming, describe
functional requirements requirements, and describe the problems with IPv6
multihoming. There are two ways to avoid the problems of IPv6
multihoming:

1. IPv6 using IPv6-to-IPv6 network prefix translation (NPTv6, [RFC6296]), (NPTv6) [RFC6296],
or;

2. refining IPv6 specifications to resolve the problems with IPv6
multihoming.

This document concerns itself with the latter, latter and explores the
solution space. We hope this will encourage the development of
solutions to the problem so that, in the long run, NPTv6 can be
avoided.

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]), (NAPT) [RFC3022],
offering a renaissance in end-to-end transparent connectivity.

Unfortunately, this may not be possible in every case, due to the
possible necessity of NAT even in IPv6, because of multihoming.
Though there are mechanisms to implement multihoming, such as BGP
multihoming [RFC4116] at the network level, level and SCTP based multihoming based on
the Stream Control Transmission Protocol (SCTP) [RFC4960] in the
transport layer, there is no mechanism in IPv6 that serves as a
replacement for NAT based NAT-based multihoming in IPv4. In IPv4, for a host
or a small network, NAT based NAT-based multihoming is easily deployable and is
an already deployed already-deployed technique.

Whenever a host or small network (that 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 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 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 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 Also, regardless of what the NPTv6 specification is,
we are trying to avoid any form of network address translation
technique that may not be visible to either of the end hosts. To
reach this goal, several 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:

o If multiple routers exist on a single link link, the host must select
the appropriate next-hop 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 becomes difficult whenever a host has
more than one address of the same address scope. Current address
selection criteria, 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. bound. 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 server 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; host from routing, source address selection selection, and DNS
resolution perspectives. A possible consequence of assigning a host
multiple identically-scoped 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 or DNS proxy, which receives all DNS inquires, and gives
a correct answer to the host.

2. Terminology

NPTv6 IPv6-to-IPv6 Network Prefix Translation as described in
NPTv6
[RFC6296].

NAPT Network Address Port Translation as described in
[RFC3022]. In other contexts, NAPT is often pronounced
"NAT" or written as "NAT".

MHMP Multihomed with multi-prefix (MHMP) multi-prefix. A host implementation which that
supports the mechanisms described in this
document. Namely document;
namely, source address selection policy, next-hop selection next hop
selection, and DNS selection policy.

3. IPv6 multihomed network scenarios Multihomed Network Scenarios

In this section, we classify three scenarios of the multihoming
environment.

3.1. Classification of network scenarios Network Scenarios for multihomed host 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 is, in turn turn, connected to a
different service provider network, that 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.

+------+ ___________
| | / \
+---| rtr1 |=====/ network \
| | | \ 1 /
+------+ | +------+ \___________/
| | |
| hosts|-----+
| | |
+------+ | +------+ ___________
| | | / \
+---| rtr2 |=====/ network \
| | \ 2 /
+------+ \___________/

Figure 1: single uplink, multiple next-hop, multiple prefix 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 rtr2,
respectively. When the host sends packets to network 1, the next-hop next hop
to network 1 is rtr1. Similarly, rtr2 is the next-hop next hop to network 2.

- e.g.,

Example: multiple broadband service providers (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 gateway,
in turn 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.

+------+ ___________
+-----+ | | / \
| |=======| rtr1 |=====/ network \
| |port1 | | \ 1 /
+------+ | | +------+ \___________/
| | | |
| hosts|-----| GW |
| | | rtr |
+------+ | | +------+ ___________
| |port2 | | / \
| |-------| rtr2 |=====/ network \
+-----+ | | \ 2 /
+------+ \___________/

Figure 2: single uplink, single next-hop, multiple prefix 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 2, respectively. As the figure
shows a logical topology of the scenario, the port1 could be a pseudo pseudo-
interface for tunneling, which connects to the network 1 through the network 2,
2 and vice versa. When the host sends packets to either network 1 or
2, the next-hop 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,

Example: Internet + VPN/Application 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.

+------+ +------+ ___________
| | | | / \
| |-----| rtr1 |=====/ network \
| | | | \ 1 /
| | +------+ \___________/
| |
| host |
| |
| | +------+ ___________
| | | | / \
| |=====| rtr2 |=====/ network \
| | | | \ 2 /
+------+ +------+ \___________/

Figure 3: Multiple uplink, multiple next-hop, multiple prefix 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 to
network 1, the next-hop next hop to network 1 is rtr1. Similarly, rtr2 is the
next-hop
next hop to network 2.

- e.g.,

Example: Mobile Wifi + 3G, ISP A + ISP B

3.2. Multihomed network environment 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

In many cases, multihomed networks in IPv4 have been implemented
through the use of a gateway router with NAPT function (scenario 2
with NAPT) in many
cases. 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 prefix/
route is often used on the gateway router to separate traffic between
the networks.

+------+ ___________
| | / \
+---| rtr1 |=====/ network \
| | | \ 1 /
+------+ +-----+ | +------+ \___________/
| IPv4 | | | |
| hosts|-----| GW |---+
| | | rtr | |
+------+ +-----+ | +------+ ___________
(NAPT&DNS) | | | / \
(private +---| rtr2 |=====/ network \
address | | \ 2 /
space) +------+ \___________/

Figure 4: IPv4 Multihomed environment Environment with Gateway Router performing 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, next hop, the correct source address address, and the correct 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 described in
Section 3.1 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.

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 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) Advertisement (RA)), then two valid default routers may be
recognized.

Scenario 3:

A host has two separate interfaces interfaces, and on each interface has a
different address is assigned. Each link has its own router.

o The host does not have enough information for determining to determine 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 Transparency

One of the major design goals for IPv6 is to restore the end-to-end
transparency of the Internet. If NAT is applied to IP communication
between hosts, NAT traversal mechanism mechanisms are required, required to establish bi-
directional
bidirectional IP communication. A In an environment with end-to-end
transparancy, a NAT traversal mechanism does not need to be
implemented in an application, in an environment with end-
to-end transparency. application (e.g., ICE [RFC5245]). Therefore, the
IPv6 multihoming solution should strive to avoid NPTv6 to achieve
end-to-end transparency.

4.2. 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 Analysis

The problems described in Section 3 can be classified into these
three types:

o Wrong source address selection

o Wrong next-hop next hop selection

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 Address Selection

A multihomed IPv6 host will typically have different addresses
assigned from each service provider either on either the same link
(scenarios 1 & and 2) or different links (scenario 3). When the host
wishes to send a packet to any given destination, the current source
address selection rules [RFC6724] may not deterministically select
the correct source address. [RFC7078] describes the use of the
policy table [RFC6724] (as discussed in [RFC6724]) to resolve this problem,
using 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" Host needs to support the solution for this problem.

Scenario 2: "Host" Host needs to support the solution for this problem.

Scenario 3: If "Host" support Host supports the next-hop 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 network's DHCP server and enterprise/VPN) DHCP-forwarding
routers must also support the Address Selection option [RFC7078].

5.2. Next-hop selection Next Hop Selection

A multihomed IPv6 host or gateway may have multiple uplinks to
different service providers. Here Here, each router would use Router
Advertisements [RFC4861] for distributing to distribute 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
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 but
traffic fails will fail 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 In
IPv4, gateway router is routers are often pre-configured with static route
information routes or uses of
use the Classless Static Route Options [RFC3442] for DHCPv4. Extensions An
extension to Router Advertisements through Default Router Preference
and More-Specific Routes [RFC4191] provides for link-
specific link-specific
preferences but does not address per-host configuration in a
multi-access multi-
access topology because of its reliance on Router Advertisements.

Scenario 1: "Host" Host needs to support the solution for this problem.

Scenario 2: "GW rtr" GW rtr needs to support the solution for this problem.

Scenario 3: "Host" Host needs to support the solution for this problem.

5.3. DNS recursive name server selection 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] (BBF)
[TR-124] requires that the query be sent to all DNS recursive name servers,
servers and that the gateway waits wait 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 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 servers 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.

It is not an issue of the 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. [RFC6731] proposes a
solution for distributing DNS server selection policy using a DHCPv6
option.

Scenario 1: "Host" Host needs to support the solution for this problem.

Scenario 2: "GW rtr" GW rtr needs to support the solution for this problem.

Scenario 3: "Host" Host needs to support the solution for this problem.

It is noted that the service providers (i.e., ISP network's DHCP server and enterprise/VPN) DHCP-forwarding
routers must also support the Address Selection option [RFC6731].

6. Implementation approach Approach

As mentioned in Section 5, in the multi-prefix environment, we have
three problems; problems: source address selection, next-hop next hop selection, and DNS
recursive name server selection. In this section, possible solutions
for each problem are introduced and evaluated against the
requirements in Section 4.

6.1. Source address selection Address Selection

The problems of address selection in multi-prefix environments are
summarized in [RFC5220]. When 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 Next Hop Selection

As for the source address selection problem, both a policy-based
approach and a non policy-based non-policy-based approach are possible with regard to
the next-hop 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- next
hop selection. If we assume source address-based source-address-based routing at hosts or
intermediate routers, pairs of source prefixes and next-hops next hops can be
another example of next-hop next hop selection policy.

The routing information-based 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 protocols (such as RIPng Routing
Information Protocol Next Generation (RIPng) and
OSPFv3, OSPFv3), the RA
extension option defined in [RFC4191], and the CPE WAN Management
Protocol (CWMP) [TR069] standardized at BBF.

The RA-based mechanism doesn't handle distribution of per-host
routing information easily. Dynamic routing protocols are not
typically used between residential users and ISPs, because of their
scalability and security implications. The DHCPv6 mechanism does not
have these problems and has the advantage of its relaying relay functionality. It
is commonly used and is thus easy to deploy.

[TR069], mentioned above, is defines a possible solution mechanism for
routing information distribution to customer-premises customer premises equipment
(CPE). It assumes, however, that IP reachability to the Auto
Configuration Server (ACS) is has been established. Therefore, if the
CPE requires routing information to reach the ACS, CWMP [TR069]
cannot be used to distribute this information.

6.3. DNS recursive name server selection Recursive Name Server Selection

Note: Split-horizon DNS is discussed in this section. Split-
horizon DNS is known to cause problems with applications to allow
information leakage. The discussion of split-horizon DNS is not
condoning its use, but rather acknowledging that split-horizon DNS
is used and that its use is another justification for network
address translation. The goal of this document is to encourage
building solutions which that do not need network address translation.

Two solutions appear possible: make split-horizon improve the function of split-
horizon DNS work better (which is discussed below) or meet network administrator's
administrators' requirements without split-horizon DNS (which is
out of scope of this document).

As in the above two problems, a policy-based approach and a non non-
policy-based approach are possible. In a non policy-based 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 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 an unexpected IP
address or receive an invalid response, and thus cannot it could not 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. "split-horizon". When the host mistakenly makes a query to a
different provider's DNS recursive name server to resolve a FQDN Fully
Qualified Domain Name (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 non-
private side of the service. Another problem with this approach is
that it causes unnecessary DNS traffic to the DNS recursive name
servers that are visible to the users.

The alternative of to a policy-based approach is documented in
[RFC6731], 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 option, and distribute DNS queries to the
appropriate DNS servers according to the policy.

6.4. Other algorithms available Algorithms Available in RFCs

The authors of this document are aware of a variety of other
algorithms and architectures, such as shim6 Shim6 [RFC5533] and HIP
[RFC5206], that may be useful in this environment. At the time of
this writing, there is not enough operational experience on which to
base a recommendation. Should such operational experience become
available, this document may be updated in the future.

7. Considerations for MHMP deployment Deployment

This section describes considerations to mitigate possible problem problems
in a network which that implements MHMP described (described in Section 6. 6).

7.1. Non-MHMP host consideration Host Consideration

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 next hop selection problem. In
a similar fashion, NPTv6 can be used as a last resort for IPv6
multihomed network deployments where one needs to assign a single
IPv6 address to a non-MHMP 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 next hop
selection and DNS server selection, shown in Section 6.

The implementation and issues of NPTv6 are out of the scope of this
document. They may be covered by another document under discussion
document, but are discussed in Section 5 of [RFC6296].

7.2. Co-existence considerations Coexistence Considerations

To allow the co-existence coexistence of non-MHMP hosts and MHMP hosts (i.e. (i.e.,
hosts supporting multi-prefix with the enhancements for the source
address selection), GW-rtr GW rtr may need to treat those hosts separately.

An idea for how to achieve this, is that GW-rtr identifies this would be for GW rtr to identify the
hosts, and then assigns assign a single prefix to non-MHMP hosts and assigns assign
multiple prefixes to MHMP hosts. In this case, GW-rtr 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 GW rtr could assign multiple prefixes to both hosts,
hosts and performs perform IPv6 NAT for traffic from non-MHMP hosts if its
source address is not appropriate.

In scenario scenarios 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.

7.3. Policy collision consideration Collision Consideration

When multiple policy distributors exist, a policy receiver may not
follow one or each of the received policy. policies. In particular, when a policy
conflicts with another policy, a policy receiver cannot implement each
both of the policy. policies. To solve or mitigate this issue, it is
required that a
prioritization rule is required to align these the policies along
preference on with the
preferences of a trusted interface. Another solution is to preclude
the functionality of multiple policy the acceptance of multiple policies 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.

This document does not presume specific recommendations for resolving
policy collision. It is expected to that the implementation to will decide
how to resolve the conflicts. If they are not resolved consistently
by different implementations, that could affect interoperability and
security trust boundaries. Future work will be is expected to address the
need for consistent policy resolution to avoid interoperability and
security trust boundary issues.

8. Security Considerations

In today's multi-homed multihomed IPv4 networks, it is difficult to resolve or
coordinate conflicts between the two upstream networks. This problem
persists with IPv6, no matter if the hosts use IPv6 provider-
dependent or provider-independent addresses.

This document requires that the solutions for MHMP should solutions have functions that
provide policy
providing functions. controls. New security threats can be introduced
depending on what the 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 that some hosts in
its network do will not follow the distributed policy. At the time of
this writing, there are no known methods to resolve conflicts between
the host's own policy (policy receiver) and the policies of upstream
providers (policy provider). As this document is analyzing the
problem space, rather than proposing a solution, we note the
following problems:

Threats related to the policy distributor side:

Service

The service provider should expect the existence of hosts that
will not obey the received policy. A possible solutions solution is to
ingress-filter those packets that do not match the distributed
policy and drop them. About the For route selection, packet forwarding
or redirection can be another possible solution.
About the For source
address selection, IPv6 NAT can be another possible solution.

Administrators of

In a multihomed multiple-provider network, nodes in the network
may be administered by different networks organizations. Administrators
might need to control policies (and nodes' behaviors) a node's behavior)
independently of other administrators. It means that the Access control policies
need to have be in place to restrict the administrator's access controls
for such cross-administrative policy access. Administrators
must control only nodes that are part of their own networks, or
some administrators must control to
only the nodes that are part of
their own networks, while others are it is authorized to control
nodes across administrative boundaries. To be success to
cross-administrative policy-control, per-user authorization
might be required with existing AAA and network management
standards. control.

Threats related to the policy receiver side:

For the policy receiver side, who should be trusted to accept
policies is a fundamental issue. How is the trust established,
and how established?
How can the network element be assured that it can
established establish
that trust before the network is fully configured. configured? If a policy
receiver trusts an untrusted network, it will cause
that the
distributing of the unwanted and unauthorized policy that is
described below.

A policy receiver are is exposed to the threats of unauthorized
policy, which can lead to session hijack, falsification, DoS,
wiretapping
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 in
addition to 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 the problem of unauthorized policy, one
approach is limiting on to limit the use of these policy distribution
mechanisms, as described in the section Section 4.4 of [RFC6731]. For
example, a policy should be preferred or accepted when the
policy is verified its integrity and if delivered across
over a secure, trusted channel such as 3G connection in a cellular services. data
connection. The proposed solutions are based 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 For the DNS server selection issue, DNSSEC DNS
Security (DNSSEC) can be another solution. About For 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 clients receive different
policies from the network service provider, that difference
provides hints about the policy host itself and can give a hint of which service be useful to
uniquely identify the client
subscribes. host. Encryption of the communication channel,
channel and separation of the communication channel per host
can be solutions for this problem.

The security threats related to IPv6 multihoming are described in
[RFC4218].

9. IANA Considerations

This document has no IANA actions.

10. Contributors

The following people contributed to this document: Akiko Hattori,
Arifumi Matsumoto, Frank Brockners, Fred Baker, Tomohiro Fujisaki,
Jun-ya Kato, Shigeru Akiyama, Seiichi Morikawa, Mark Townsley,
Wojciech Dec, Yasuo Kashimura, and Yuji Yamazaki. This document has
greatly benefited from inputs by Randy Bush, Brian Carpenter, and
Teemu Savolainen.

11.

10. References

11.1.

10.1. Normative References

[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", RFC 4191, November 2005.

[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.

[RFC6296] Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix
Translation", RFC 6296, June 2011.

[RFC6724] Thaler, D., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, September 2012.

[RFC6731] Savolainen, T., Kato, J., and T. Lemon, "Improved
Recursive DNS Server Selection for Multi-Interfaced
Nodes", RFC 6731, December 2012.

[RFC7078] Matsumoto, A., Fujisaki, T., and T. Chown, "Distributing
Address Selection Policy Using DHCPv6", RFC 7078, January
2014.

11.2.

10.2. Informative References

[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022, January
2001.

[RFC3442] Lemon, T., Cheshire, S., and B. Volz, "The Classless
Static Route Option for Dynamic Host Configuration
Protocol (DHCP) version 4", RFC 3442, December 2002.

[RFC3582] Abley, J., Black, B., and V. Gill, "Goals for IPv6 Site-
Multihoming Architectures", RFC 3582, August 2003.

[RFC3646] Droms, R., "DNS Configuration options for Dynamic Host
Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
December 2003.

[RFC4116] Abley, J., Lindqvist, K., Davies, E., Black, B., and V.
Gill, "IPv4 Multihoming Practices and Limitations", RFC
4116, July 2005.

[RFC4218] Nordmark, E. and T. Li, "Threats Relating to IPv6
Multihoming Solutions", RFC 4218, October 2005.

[RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC
4960, September 2007.

[RFC5206] Nikander, P., Henderson, T., Vogt, C., and J. Arkko, "End-
Host Mobility and Multihoming with the Host Identity
Protocol", RFC 5206, April 2008.

[RFC5220] Matsumoto, A., Fujisaki, T., Hiromi, R., and K. Kanayama,
"Problem Statement for Default Address Selection in Multi-
Prefix Environments: Operational Issues of RFC 3484
Default Rules", RFC 5220, July 2008.

[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245, April
2010.

[RFC5533] Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
Shim Protocol for IPv6", RFC 5533, June 2009.

[RFC6106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
"IPv6 Router Advertisement Options for DNS Configuration",
RFC 6106, November 2010.

[TR-124] The Broadband Forum, "TR-124, Functional Requirements for
Broadband Residential Gateway Devices", Issue: 2, May
2010, <http://www.broadband-forum.org/technical/download/
TR-124_Issue-2.pdf>.

[TR069] The BroadBand Broadband Forum, "TR-069, CPE WAN Management Protocol
v1.1,
v1.1", Version: Issue 1 Amendment 2", 2, December 2007.

[TR124] The BroadBand Forum, "TR-124i2, Functional Requirements
for Broadband Residential Gateway Devices (work in
progress)", May 2010. 2007,
<http://www.broadband-forum.org/technical/download/
TR-069_Amendment-2.pdf>.

Authors' Addresses

Ole Troan (editor)
Cisco
Oslo
Norway

Email:

EMail: ot@cisco.com

David Miles
Alcatel-Lucent
Melbourne
Australia

Email: david.miles@alcatel-lucent.com
Google Fiber
Mountain View, CA
USA

EMail: davidmiles@google.com

Satoru Matsushima
Softbank Telecom
Tokyo
Japan

Email:

EMail: satoru.matsushima@g.softbank.co.jp

Tadahisa Okimoto
NTT West
Osaka
Japan

Email:

EMail: t.okimoto@west.ntt.co.jp

Dan Wing
Cisco
170 West Tasman Drive
San Jose
USA

Email:

EMail: dwing@cisco.com