Internet DRAFT - draft-fbnvv-v6ops-site-multihoming
draft-fbnvv-v6ops-site-multihoming
IPv6 Operations (v6ops) Working Group N. Buraglio
Internet Draft Energy Sciences Network
Intended status: Informational K. Frank
Expires: July 2024 P. Nero
Private
P. Volpato
E. Vasilenko
Huawei Technologies
January 29, 2024
IPv6 Site connection to many Carriers
draft-fbnvv-v6ops-site-multihoming-03
Abstract
Carrier resilience is a typical business requirement. IPv4
deployments have traditionally solved this challenge through private
internal site addressing in combination with separate NAT engines
attached to multiple redundant carriers. IPv6 brings support for
true end-to-end connectivity on the Internet, and hence NAT is the
least desirable option in such deployments. Native IPv6 solutions
for carrier resiliency, however, have drawbacks. This document
discusses all currently-available options to organize carrier
resiliency for a site, their strengths and weaknesses, and provides
a history of past IETF efforts approaching the issue. The views
presented here are the summary of discussions on the v6ops mailing
list and are not just the personal opinion of the authors.
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 https://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 July 2024.
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Copyright Notice
Copyright (c) 2024 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
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Table of Contents
1. Terminology and pre-requisite..................................3
2. Problem statement..............................................4
2.1. General issues with network translation...................7
3. Problem history for the host-driven solution...................7
4. Solution requirements.........................................10
5. Available Solutions...........................................12
5.1. PI-based.................................................12
5.2. PA-based solution........................................15
5.2.1. Bind() case on the application side.................16
5.2.2. Getaddrinfo() case on the application side..........17
5.2.3. PA-based solution conclusion........................18
5.3. Shifting the problem to the centralized site.............20
5.4. ULA with NPTv6...........................................23
5.5. ULA with NAT66...........................................26
5.6. Application proxy........................................28
6. Conclusion....................................................32
7. Security Considerations.......................................35
8. IANA Considerations...........................................35
9. References....................................................35
9.1. Normative References.....................................35
9.2. Informative References...................................37
Appendix A.......................................................39
Acknowledgments..................................................40
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1. Terminology and pre-requisite
Terminology is inherited from [ND] and [SLAAC]. Additional terms:
RIR (Regional Internet Registry): an organization that manages
Internet numbering resources (such as IP addresses and autonomous
system (AS) numbers) within a geographical region of the world.
LIR (Local Internet Registry): an organization (usually a carrier
or an Enterprise/Academic institution) that receives its
allocation of IP addresses from the Regional Internet Registry
(RIR), then assigns parts of that allocation to its customers.
PA (provider-assigned or provider-aggregatable) address space: a
block of IP addresses assigned to the end customer by a carrier
or other owner that received it inside a bigger block from an RIR
or LIR. The principal characteristic of the PA address block is
that it is aggregated in the bigger block (not announced
separately) in the Internet routing tables.
PI (provider-independent) address space: a block of IP addresses
assigned by RIR or LIR to the end customer with the possibility
for independent announcements in the Internet routing tables.
DHCP-PD: IPv6 Prefix delegation by DHCP from the carrier to the
client to be used for numbering throughout the client site (As
currently defined by [DHCPv6] Section 6.3).
End-to-end connectivity: The possibility to initiate the connection
from any direction, including the case of complex protocols with
many logically related sessions.
ALG (Application-level gateway or Application-layer gateway): a
component that monitors the data payload of connections to
discover and fix application-level referrals (embedded IP
addresses).
Subscriber-only services: The resource that is filtered to the
public Internet and available only for some portion of the
Internet, typically only for subscribers of the particular
carrier.
Multi-homing solution: The set of architectural, configurational and
operational network and application changes necessary to be
applied to a single-homed site plan in order to let all hosts in
the site gain access to at least two paths towards the Internet,
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effectively solving the Problem Statement, satisfying as many
Solution Requirements as possible, and adding as little
complexity as possible compared to a single-homed site.
2. Problem statement
Always-on connectivity is a key requirement for the vast majority of
businesses. [IAB report] predicts that there "might be as many as 10
million multihomed sites by 2050". Unfortunately, several issues may
affect the connection of a business to its upstream service
provider. For example, the carrier's network, the network gateway,
or the first-mile infrastructure may experience issues. It is
especially true now after many recent examples of massive carrier
outages.
A redundant connection to the carrier is then the norm for business.
A simple topology that showcases this key requirement is shown in
Figure 1. Note that the topology could be more complex as shown, for
example, in Figure 3 [MHMP Enterprise].
+------+ _________
| | / \
+---| CPE1 |----/ Carrier \
2001:db8:0:1001::xx | | |\ \ 1 /
+------+ | +------+ \ \_________/
| | | \
| Host |----+ \
| | | \
+------+ | +------+ \_________
2001:db8:0:2001::yy | | | / \
+---| CPE2 |----/ Carrier \
| | \ 2 /
+------+ \_________/
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Figure 1: The basic Carrier Resiliency topology
Without entering too much into details, resilience is generally
achieved by employing redundant elements. Two Customer Premises
Equipment (CPE) systems are usually employed. Very often, each CPE
connects the business to a different carrier. In some cases, a CPE
may even connect to two different carriers, to achieve a higher
level of protection against network failures.
It is very desirable to have paths with diversity. Unfortunately, it
is not always possible for the enterprise to understand what exactly
is shared between paths offered by different carriers. The duct the
fibers run through may even be shared in many cases, with the risk
of both strands being damaged at the same time. Let's assume for the
discussion in this document that the paths available for the site to
reach the Internet are sufficiently diverse.
In literature, a topology such as that shown in Figure 1 based on
IPv6 connectivity is often referred to as Multi-Home, Multi-Prefix
(MHMP). The name implies that a network is multi-homed to different
carriers, receiving from them different network prefixes.
It is important to mention that the topology in Figure 1 or all
Figures of [MHMP Enterprise] assume redundant Carriers' connection
to the router upstream. Direct Carrier connection to the host (for
example 3GPP modem) is not in the scope of this document that has
"site connection" in the name.
The problem of providing network protection with IPv6 was thoroughly
discussed in [Local Protection]. This document is an overview of the
most commonly used methods to facilitate the desired protection in
modern IPv6 networks, along with a discussion of the advantages and
disadvantages of each method.
In IPv4 environments, such a scenario is implemented through
independent NAT translation on every CPE to the carrier in
combination with private address space on-site. [Local Protection]
has a good list of benefits. The solution was initially adopted due
to the shortage of globally-reachable address space. Later, security
and carrier resiliency were identified as additional benefits. Due
to the prevalence of a solution based around address translation in
IPv4, demands are often voiced for such a mechanism in IPv6 as well,
running the risk of being chosen by network designers without
evaluating alternative options.
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[Local Protection] has a list of IPv6 tools that replace all
functionality of the NAT solution except address conservation, which
is not a necessity in IPv6. Time has shown that all IPv6 tools have
been accepted as valid replacements except IPv6 "multi-homing and
renumbering". Discussions about this last problem are ongoing - see
[Multi-Homing] [MHMP Enterprise] [MHMP] [flash renumbering].
This document considers 6 solutions with current operational
deployment (more may be added in future versions of the draft):
1. Static PI address space for the site, routed by multiple
carriers. This method allows the allocation of Provider
Independent (PI) addresses on-site while routing announcements
are propagated by carriers on behalf of the client. It will be
discussed in section 5.1.
2. Dynamic PA addresses distribution and withdrawal from carriers.
An [IPv6] host gets different Provider Aggregatable (PA)
addresses for its interfaces, possibly from different carriers.
It is the host's responsibility to properly choose the
combination of a source address and the relevant next hop. This
solution is primarily based on the interaction of [ND] and
[SASA] and will be discussed in section 5.2.
3. Shifting Internet access resilience to a central site. A branch
site is granted redundant connectivity to a central hub
location where the aspects related to resilient Internet
connectivity are handled. The methods is discussed in section
5.3.
4. Static ULA address space for the site with NPTv6 translation.
[NPTv6] is adopted in combination with Unique Local Addresses
(ULA). This method is discussed in section 5.4.
5. Static ULA address space for the site with NAT66 translation.
In this case NAT66 is combined with Unique Local Addresses
(ULA). The solution is discussed in section 5.5.
6. Relying on an application proxy to handle path decisions. In
this case, internal hosts are only required to send traffic to
a central device in the site, which then independently selects
which carrier to use to reach the Internet. This method is
discussed in section 5.6.
The IETF consensus to preserve end-to-end connectivity does not
favor the general acceptance of solutions 3 and 4, yet they are
reported in this document because they offer a degree of support for
the requirements listed in section 4.
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2.1. General issues with network translation
Part of the architectures described in this document rely on
different forms of translation of the source IP address of internal
hosts to offer redundant carrier connectivity. It is important to
point out that, while these options have been included due to having
a degree of real-world deployment, they incur a large number of
drawbacks unrelated to carrier redundancy which have been
extensively documented.
From standards as well as practical perspectives, the [IAB request]
to avoid the use of address and prefix translation has been heavily
based on the long list of [NAT implications]. Some or all of the
problems caused by network translation apply to the solutions in
sections 5.4 and 5.5 breaking of the end-to-end model, the
impossibility of initiating sessions from the outside, breaking
IPSec encryption, breaking of application-layer referrals to the
addresses, single point of failure, redundancy challenges (state
replication), and performance challenges (stateful processing).
The current document and its authors make no endorsement for the use
of either NPTv6 or NAT66; rather, the intent is to illustrate and
summarize operational alternatives currently in use when true route-
based multi-homing, as exemplified by the use of PI in section 5.1,
is not available.
Network administrators looking to implement multi-homing as
described in sections 5.4 or 5.5 are encouraged to consider [IAB
request] and [NAT implications] before making a final decision.
3. Problem history for the host-driven solution
Client applications typically utilize getaddrinfo() to establish
communication and to perform source address selection. It was
initially assumed in [SASA] and implemented by getaddrinfo() that
the next hop is chosen before the source address.
Bind() permits overriding this order, but it is typically used only
in server-side applications.
This specific process is the reason why in the case of a network
fault, when a redundant CPE/link is promoted to the primary role,
some specific destinations may become unreachable, causing the
solutions listed hereafter to leave some unresolved scenarios.
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The initial discussion on carrier resiliency occurred in the
[multi6] and [Shim6] working groups. [multi6] requirements were
reused in this document. [shim6] proposed to separate the location
and identity properties of addresses. [Shim6] did not gain market
acceptance. It is not currently supported by available software and
hardware options.
The host-based solution is highly dependent on the availability of
the information that the prefix is stale. [6renum] has a deep
discussion on how to best react in cases where there is missing
information, including the use of other mechanisms to automatically
resolve the problem. The best way to provide warnings about stale
prefixes is still in discussion [flash renumbering].
The next notable step was the addition of rule 5.5 to [SASA] to
prefer the selection of a source address covered by a prefix
advertised by the chosen next hop router. This allows the packet to
be on the path to the PA-owning carrier and avoid packet filtering
due to the application of [BCP38].
Concerning Figure 1, if connectivity to Carrier 1 were lost then the
host would select a source address in the prefix belonging to
Carrier 2 to communicate with a certain destination outside of its
local segment, thus achieving resiliency.
On the other hand, this method is not a solution when only a
particular source address is permitted (i.e., not filtered) to
access a particular outside resource (e.g., "subscriber-only
services") or when any type of deterministic traffic distribution
policy is desired, because the random next hop choice would in turn
lead to a random choice of source address.
[MHMP] then discussed the problem thoroughly, attempting to use only
already available tools. It was suggested that the solution must
push related policies to the host as 1) "Routing Information
Options" of [Route Preferences] and 2) [Policy by DHCP] to modify
policies in the host's [SASA] selection algorithm. This solution has
not gained market acceptance due to complexity reasons. Moreover,
DHCPv6 is not universally implemented, being notably absent from
some of the most widely deployed client platforms.
Additionally, [HNCP] prescribes deprecating delegated prefixes (by
setting their preferred lifetime to zero) when the router has
information about loss of reachability to the carrier that sourced
the prefix. This is particularly important when renumbering occurs
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(the PA prefix may change after disconnection and re-connection to
the carrier).
The next solution was proposed in [Multi-Homing]. The document
incorrectly assumed (errata 7009 and 7010 are published) that the
source address is chosen first in the typical scenario when a client
initiates outbound communications. [Multi-Homing] section 3.2
proposed to prefer next hops from those routers that advertise the
prefix covering the already selected source address. Hence, [Multi-
Homing] unblocked the possibility for an application to use bind()
to select the source address first since section 3.2 contains vital
instructions on how to choose the next hop in that condition. It is
important to note that the more common scenario of choosing the next
hop before the source address is not solved by [Multi-Homing].
Further progress in the problem discussion was made in [MHMP
Enterprise], which discusses potentially complex on-site topologies
and the source routing that is needed in such a scenario; it may be
considered a comprehensive guide covering the source routing aspect
of the complex site with carrier resiliency. Unfortunately, at this
time it is not yet possible to use it in practice due to the lack of
any market-accepted solutions to split and distribute PA prefixes
throughout the complex site. All other solutions (PI, ULA+NAT66,
ULA+NPTv6) do not require source routing. [MHMP Enterprise] might
become vital in the future if a solution were to be adopted for
splitting and propagating PA prefixes through the complex site;
however, such a solution is currently unavailable.
Restrictions to the list of applicable source addresses for a
specific next hop (rule 5 or 5.5 of [SASA]) may not have been
implemented in certain host operating systems. [Conditional PIO] can
in this case mitigate the problem through selective PIO
announcements to a particular host. This represents a valuable
transition mechanism until rule 5.5 of [SASA] will be universally
applied.
[DNS Options] allows the router to supply the addresses of many
different DNS resolvers, including those of completely unrelated
carriers. It is not possible, however, to provide information to
clients regarding which DNS resolver is related to which particular
prefix; such information might be crucial in scenarios where traffic
steering policies are required for successful communication
(including when accessing filtered resources such as "subscriber-
only services").
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Finally, [Provisioning domains] permit virtualization of the router
on the link, representing one physical device as many logical ones
with fully separate sets of link parameters. This solution is
valuable in some scenarios to deliver more diverse information to
the host but does little to assist it with choosing the proper
combination of next hop and source address that is still restricted
by [SASA]. The challenge remains the same independent of how many
physical or logical routers are present on the link. Moreover,
virtualization of a single router on a link having two uplinks to
different carriers creates a problem, because the host could
randomly choose the wrong combination of source address and next hop
announced by one of the virtual routers.
Yet, [Provisioning domains] remains valuable in scenarios where
several different routers are behind a single router, as well as
when multiple physical routers are present on the same link (i.e.,
the problem of host choice already exists). This is because, in
contrast to [DNS Options], [Provisioning domains] retains the
information that associates prefixes with the DNS resolvers of their
respective carrier.
4. Solution requirements
Solution Requirements are reflecting section 3.1 of [multi6] except
for "Transport-Layer Survivability" (preservation of the session
during the carrier disruption) which is not considered mandatory.
1. Site resiliency to an arbitrary number of carriers, with an
arbitrary number of routers on the link.
2. End-to-end connectivity wherever possible.
3. Possibility for internal communication using any prefixes
distributed by local routers, irrespective of the status of the
connectivity to the carriers that distributed such prefixes.
4. The speed of convergence for the prefix deprecation on the site
after connectivity is lost to any particular carrier should be
comparable to the speed of routing convergence on the site. If
flash renumbering affects the convergence of a particular
solution then such a solution should have mechanisms to
guarantee the desired level of convergence performance despite
flash renumbering.
5. Support for sites with complex topologies, including multiple
internal on-site hops requiring many routers and links.
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6. Access to resources from the carrier's "subscriber-only
services" is permitted only when using the address space
distributed by the particular carrier. There may exist a need
for the host to check DNS resolvers from all carriers before it
can even discover the restricted resource. A given host may
thus need to choose the correct source address that would be
accepted by the particular carrier.
7. Possibility for traffic steering between different paths
(including both internal to the site, and the Internet) based
on bandwidth, cost, load, latency, packet loss, hop count, etc.
(e.g. application traffic/path engineering) for both outbound
and inbound packets.
It is out of scope for this document to evaluate issues related to
perimeter security; every system should assume an insecure
environment, which is already the case since the host is
establishing frequent connections to the open Internet.
Different environments have diverse security policies, needs, and
obligations that may be shaped by internal policy, regulatory
compliance, or national security requirements, and as such will not
be discussed in detail in this document.
Privacy is furthermore assumed to be protected by [Temporary
addressing] and is also kept outside of carrier resiliency design
considerations. Address-based privacy considerations are not
affected by carrier resiliency mechanisms and techniques.
All solutions to the problem statement in this document would have
different cost advantages and disadvantages. The associated costs
may greatly vary for different geographies, market segments, and
organization sizes, but the choices should be ultimately driven by
any needs and requirements set upon the organization by their
respective governing bodies, with input from the appropriate subject
matter experts.
It is recognized that cost is often a determining factor in IT and
networking decisions, but it is also considered out of the scope of
this document.
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5. Available Solutions
5.1. PI-based
While building or expanding an IPv6 addressing plan, it may become
necessary for an organization to procure or acquire additional
network resources; it is reasonably straightforward to obtain
additional Provider-Independent GUA address space and ASN from an
RIR or LIR. However, not all organizations have the in-house
expertise, desire, or capability to execute such a plan.
PI prefixes allow the creation of an address plan that would never
need renumbering, due to the non-dynamic and non-conditional nature
of the prefix allocation. Hence, the address plan may be stable
enough to be manually provisioned over all routers and links even in
a complex topology.
The long-term supportability and network convergence time of this
solution are excellent because there is no need for renumbering;
losing one of the Internet connectivities simply implies normal
routing updates with default routing status withdrawal by the
affected router.
It may, however, be a more involved process to get the PI prefix
routed by the carrier because such type of customer attachment is
typically charged more by the carrier due to the more complicated
nature of the connection and configuration (i.e. dynamic protocol
configuration, configuration of appropriate prefix filtering, and
equipment to support the necessary protocols). It generally requires
a manual procedure on the carrier's side, and hence it is not
possible to set this up without their cooperation.
For faster convergence, routers should announce a "default" on-site
and then add some more specific prefixes if needed. Additional
destination prefixes facilitate the distribution of traffic between
the carriers.
In cases where internal routing hops are present between the edge
devices, such announcements likely require other protocols in
addition to Neighbor Discovery.
As for the network interface of the internal host (the last hop),
such an announcement comes in the form of default router status in
[ND], and [Route Preferences] for the more specific routes.
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The crucial requirement in the PI-based solution is that routers
have to continuously track connectivity to carriers to be able to
deprecate the "default" as well as any more specific announcements
when such connectivity is lost.
When multi-homing with PI space there are multiple entry points into
the local network for traffic destined to every single address in
the site, and this opens up discussion on how to load balance
incoming traffic to such a site. An example can split the prefix
into many smaller prefixes and announce them separately to different
carriers. This requires that the initial prefix is of adequate size,
to avoid ending up with announcements smaller than the longest
acceptable prefix length within the default-free zone, which is
conventionally /48. This advanced incoming traffic engineering is
possible only with PI address space.
Where traffic symmetry over the WAN is important, however, such as
in environments performing application-based traffic steering, PI
can fail to guarantee the necessary control over returning traffic.
Traffic engineering using routing advertisements provides benefits
for simple active/passive or active/active connectivity needs, but
it can also be insufficient; routing advertisements are not granular
or fast enough to make things work in scenarios where different
application traffic must be steered towards different uplinks based
on upper-layer information. As an example, an administrator might
need FTP traffic generated by an internal host to always exit
through the secondary Internet link, and for the return traffic to
also arrive back on that same link; allowing the downstream
component of FTP connections to occupy bandwidth on the primary link
might be undesirable, and impact the performance of more sensitive
protocols running over the primary link. Even in the extreme case
where /128 prefixes for the internal host were advertised,
engineering different return paths based on upper-layer information
would not be feasible in a deterministic manner for the same source
address.
In addition, environments, where the edge devices (commonly,
perimeter firewalls) are stateful might experience packet drops due
to TCP session flows being split across Internet links. This would
be especially true in the case where multiple stateful devices were
deployed, one for each Internet link; the devices would thus only
see a portion of the TCP sessions and never consider them fully
established, or miss enough segments that the next arriving sequence
number might be considered out of the window from their standpoint.
Both conditions could cause the devices to drop segments. Note that
these symptoms would be exaggerated for TCP traffic but also
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possible with other protocols such as UDP, if the stateful devices
had a way to track the sessions and allowed inbound UDP traffic only
when a matching tuple already existed in their session tables.
On the other hand, a crucial advantage of using a prefix that is
globally reachable through multiple Internet links is the seamless
failover of transport-layer sessions across the links without having
to re-establish them. Conventionally, in IPv4 environments, such a
failover would have been guaranteed to break existing communication
due to the ubiquity of PA addressing combined with NAT44, and the
consequent change in source address as seen by the remote endpoint.
This issue would also be felt when leveraging IPv6 multi-homing in a
way that caused source address changes upon failover, either due to
NAT (see sections 5.4 and 5.5) or the prefix of the existing
connection becoming unusable (see section 5.2).
The biggest problem with PI addresses today remains the fact that
the widespread practice of using PI space would bloat the Internet
routing table by at least 10 times (reaching up to 10M entries in
the default-free zone), which would greatly impact the cost and
scalability of Internet routers. Admittedly, however, this problem
would not be an immediate concern for any enterprise company
implementing a PI-based solution.
The implementation of RPKI (for prefix ownership protection) is also
among the additional challenges that enterprises making use of PI
may face in the future, requiring skilled personnel to the setup and
operation of BGP.
Advantages:
- Preserves end-to-end communication,
- Does not require any special functionality from the host,
- Straightforward design,
- Allows aggregation of multiple uplinks for increased throughput,
- Supports sites with complex topology,
- Seamless link failover without transport session re-establishment,
- Supports outbound traffic steering.
Disadvantages:
- Hard to implement especially for smaller entities, as it requires
knowledge of advanced routing protocols like BGP and availability
of advanced networking hardware (it is not as simple as plugging
two independent CPEs into the same switch as the client and having
it work); moreover, skills related to RPKI may be needed in the
future,
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- In case of provider failure a prefix may not be correctly
deprecated from global routing, and this could lead to complex
error scenarios and downtime,
- The need to pay and liaise with a RIR or LIR for the PI address
space,
- Needs carriers to accept PI prefix advertisements,
- Carriers typically charge significantly more for such type of
attachment; often SD-WAN contracts are required instead of
business DSL contracts,
- The return traffic path is not guaranteed to mirror the outbound
path,
- Bloats the Internet routing table with a potential 10 million new
routes.
5.2. PA-based solution
Let us assume that a site is connected to the Internet through many
carriers, with every carrier delegating a PA prefix. How the prefix
is delivered to the site is unimportant for the current example. It
may be dynamic (most likely using DHCP-PD) or static (offline
agreement with carrier).
We will also assume for this example that the local CPE performs no
proprietary functions and that hosts are provisioned with an address
from each PA allocation.
As soon as a router becomes aware that the path to the carrier that
delegated a prefix is down, all more specific prefixes under that
prefix must be deprecated (preferred lifetime set to zero) in all
subnets. This is specified as requirement L-13 of [HNCP] but is not
currently requested in other standards. Support for the [HNCP]
protocol itself is not mandatory.
The PA-based solution is the most affected by renumbering, a highly
probable occurrence since 1/3 of carriers dynamically rotate
prefixes leased by DHCP-PD after any re-connection. Measures
specified in section 4.1 of [6renum] (i.e. usage of FQDN, DHCP+DNS,
ULA, DHCP-PD, Parameterized Address Configuration, transition
period, etc) alleviate the negative effects of renumbering.
[Node Requirements] does not make DHCP mandatory for address
assignment. Some popular client operating systems do not support
DHCP, even though they "should", according to [Node Requirements].
Hence, for the general case, we should assume that address
assignment and configuration are done with [SLAAC]. Note that the
logic below would not change if DHCP-IA [DHCPv6] were to be used for
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address assignment. A similar statement applies to the address
acquisition by DHCP-PD [DHCPv6], [PDperDevice], the size of the
address pool received by hosts does not help to resolve the problem
of source address and next hop choice, [SASA] restrictions remain
the same.
As a result, we have a host with many IP addresses that needs to
choose the source address and the next hop before any communication
attempts it may wish to make. This is because, if the source address
of one carrier were to be used to send a packet to a different
carrier, then the other carrier would filter it out in suspicion of
an address spoofing attack.
5.2.1. Bind() case on the application side
Potentially, the application on the host may use bind() to choose
the source address first using any logic pre-programmed into the
application. This application-driven solution is the only current
way to access the "subscriber-only services" of the particular
carrier and for steering traffic based on cost, latency, packet
loss, hop count, etc.
After the application selects the proper source address with bind(),
it must choose the next hop to serve it.
There are two options to accomplish this:
1. [Multi-Homing] section 3.2 has an augmentation to [ND] section
6.3.6 asking to prefer default routers that advertised the
particular prefix already used for the source address.
2. [Route Preferences] to install the external prefix into the
host's routing table.
It may be valuable to implement [Provisioning domains] to supply
along with DNS resolvers the relationship of such resolvers to each
prefix provided by the different carriers, as unrelated resolvers
may respond with unusable or missing information when queried for
"subscriber-only services". In any case, IP stacks of host OSes are,
in practice, not capable of accepting and using this additional
information - so it would not play any role in the decision-making
process.
The implementer should check that the functionality mentioned in
this paragraph is supported on routers and applications before
relying on it in designs. Unfortunately, [Provisioning domains] and
[Multi-Homing] have low acceptance on the market, and hence the
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application-driven solution for carrier resiliency has a low
probability of having been implemented, and at best would be
inconsistent.
5.2.2. Getaddrinfo() case on the application side
Let us now consider the more typical case in which the application
is simplified in its networking aspects. In this case, the
application would use getaddrinfo(), which is typically compliant
with [SASA] on all OSes. This solution is referred to as "host-
driven" within this section.
For historical reasons, getaddrinfo() selects the next hop first. By
default, the next hop is chosen in a random round-robin manner
between all available routers on the particular link (under [ND]
section 6.3.6). If the routers and host support the "Default Router
Preference" or "Route Preference" fields of [Route Preferences] in
the RA header or the "Routing Information Option", priority can be
assigned to certain default routers over others using a simple High,
Medium, Low value; this functionality, however, is not universally
implemented.
Rule 5.5 of [SASA] section 5 will then prefer, in the list of
available source addresses, those inside prefixes that are already
advertised by the chosen next hop. Hence, the random next hop leads
to a random source address choice among those available.
Such behavior may block the possibility of accessing the
"subscriber-only services" of a particular carrier (traffic would be
filtered due to having a source address belonging to a different
carrier) as well as prevent traffic steering by any sort of policy.
Moreover, even if [Provisioning domains] were to be implemented, the
recommendation for random next hop choice would prevent effective
use of it.
Simple Internet connectivity with carrier resiliency could be
achieved in this manner, as carrier resiliency would work on a basic
level albeit with unpredictable traffic distribution between the
carriers. All that is needed for this is to support rule 5.5 of
[SASA] section 5.
Importantly, in addition to supporting rule 5.5 of [SASA], the
router should also support the L-13 requirement of [HNCP] to
deprecate prefixes individually and not the default router status
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itself. This is the only way for convergence to take place
effectively in the case of connectivity loss to the PA carrier.
[MHMP] discusses that it is possible to achieve traffic steering
supplying policies by two mechanisms:
1. "Routing Information Options" of [Route Preferences] to
influence next hop choice
2. [Policy by DHCP] to modify policies in the [SASA] source
address selection algorithm.
The latter method is considered impractical because of its
complexity and general lack of DHCPv6 support in many commonly
deployed operating systems.
5.2.3. PA-based solution conclusion
One of the drawbacks of the PA-based solution is that it can fail to
meet the requirements for real-time traffic steering based on
measured link quality and upper-layer information. While PA-based
traffic steering using the methods described above can be sufficient
in some environments, it must be kept in mind that the decision of
which link to exit from is ultimately left to the host in the form
of source address selection. In environments performing application-
based traffic steering, it is instead crucial that the steering
decision be made by the edge device, due to it having a policy table
based on network, transport, and application-layer aspects of
packets as well as real-time link quality metrics. This policy
represents the intent of the administrator and, ultimately, the
business requirements of the organization that owns the network; it
may for various reasons not discussed in this document have been
deemed unfeasible to replicate and apply this policy table directly
at the host level inside the clients. In the PA-based solution, the
host has access to prefixes from all available Internet links and
can assign itself routable addresses from them, and is thus free to
ignore any policy configured on the edge device. The gateway is
forced to forward the packet in a way that honors the source address
chosen by the host, lest the packet be dropped upstream due to
implementations of [BCP38]. Once the traffic leaves the network
through the correct uplink, however, the return path is guaranteed
to be symmetric due to the address selected by the host being routed
only to one of the Internet links; this could be considered to
satisfy requirement 7 of section 4 with regards to the inbound
portion of traffic steering.
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The host stack (in [MPTCP] case) or the host application (in
[MPQUIC] case) may resolve the MHMP problem in the PA environment by
choosing the source address first (using bind()) as discussed in
section 5.2.1. [MPTCP] socket interface extensions explain in
sections 5.3.1 or 5.3.3 how to bind many local addresses to the
future or current sessions. [MPQUIC] does not have a clear
discussion on binding to local addresses, but the [MPQUIC] generally
transfers the majority of tasks to the application layer where
possible.
Finally, in the case where a site has many links and routers
(complex topology) then source routing or other connection tracking
mechanisms in the internal network are mandatory to deliver the
traffic to the carrier owning the source address of the packet. This
aspect is properly discussed in [MHMP Enterprise], albeit it is not
yet of any use in the PA-based scenario because the delegated PA
prefix should since the beginning be dynamically split into smaller
prefixes and propagated to all links throughout the site, and there
is currently no accepted and available method to do this. Such a
method should also assist with prefix deprecation in the case where
connectivity to the carrier is lost. This is not supported at the
moment because neither [HNCP] nor DHCP-PD have gained acceptance for
this purpose. There is thus a chance that if this problem is
addressed then [MHMP Enterprise] might become very important for
sites with complex topologies.
Advantages:
- No need to own and operate a registered, Provider-Independent
address space,
- Preserves end-to-end communication,
- In simple networks, it may be as easy as plugging all CPE routers
and client devices into the same L2 domain (i.e. Switch).
Disadvantages:
- Scalability issues, easy for simple networks, but exponentially
more difficult in complex networks,
- Prefixes may not get deprecated when the CPE itself fails, as
opposed to just the link,
- Not all issues are resolved yet, only the simplest scenario is
possible (simple topology and unpredictable traffic distribution),
- Carriers may frequently change the prefix (flash renumbering), and
this could disrupt communication,
- Sites with complex topologies are not well supported yet,
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- Traffic steering by any policies (including the capability to
access "subscriber-only services") is not supported yet.
5.3. Shifting the problem to the centralized site
Another possible approach is shifting the Internet access resiliency
problem to a central site when the branch has a redundant, private
WAN connectivity provisioned through any or multiple available
methods (SDH/PDH/OTN links, MPLS VPNs, customer-managed overlays).
In this scenario, the backhaul towards the central site (which then
performs the ultimate handoff to the Internet) happens over
physically or virtually dedicated links, and the actual addressing
solution offered by the carrier serving the branch becomes
irrelevant; in the extreme case the branch carrier does not provide
an IP service at all (as in the case of optical transport or a
layer-2 VPN) or the provided addressing is only used to establish
tunnels (in the case of overlays). It is not uncommon for the branch
edge device to not even have a default route configured towards any
of the carrier next hops, instead configuring a default route
through the overlay or having as next hop a loopback interface
address reachable through the overlay itself.
The reasoning behind this choice is based on several factors and
commonly involves one or more of the following assumptions:
1. Branch site Multi-Homing is mainly a matter of first-mile
redundancy due to the increased difficulty of providing stable
connectivity to remote sites compared to a large central site.
Suppose the traffic can be made to traverse the first mile in
an optimal environment (because the entire path is under the
network administrator's control, at least at the network
level). In that case, the relatively high-quality Internet
circuits found at the central site can be managed using more
traditional and resource-intensive techniques (for example, by
significantly increasing capacity and carrier diversity, tuning
routing advertisements, and using ECMP).
2. Obtaining enterprise-class connectivity, where the customer has
the option to announce their own address space dynamically to
the carriers, can be complex and cannot thus be done for every
site.
3. Managing a geographically-distributed Internet breakout may
pose greater operational, financial, and security-related
challenges when the proper orchestration tools are not
employed.
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It is typically much easier to arrange the resiliency over internal
WAN links. This is primarily because the addressing structure and
the path selection are under the control of a single entity
throughout the LAN and the private WAN. This can allow, for example,
to number the entire internetwork using a single type of addressing,
recreating the benefits of the solution in section 5.1 while
avoiding some of its disadvantages, namely:
- "In case of provider failure, a prefix may not be correctly
deprecated": in the tunnel-based solution, all network devices are
under the control of the same entity experiencing the hypothetical
issue, simplifying troubleshooting and speeding up resolution.
- "Need carriers to accept PI prefix advertisements": in the tunnel-
based solution, this needs to be done only once, at the central
site, instead of at every site implementing the solution.
- "Carriers typically charge significantly more for such type of
attachment": see the previous observation.
- "Return traffic path not guaranteed to mirror the outbound path":
path symmetry in the more critical first mile can be guaranteed by
the network administrator through configuring the network devices
at the far end of the WAN to honor the original link choice of the
incoming sessions in a stateful manner, or by applying the same
deterministic forwarding algorithms on devices at both ends.
Several vendors provide this functionality out of the box in
certain products. Symmetry is thus guaranteed where it matters,
i.e., where traffic must traverse links having vastly different
characteristics and quality (it is not uncommon for remote sites
to be served by a primary DSL/fiber link and a secondary, much
more limited cellular link).
The tunnel-based solution described in this section may also be
implemented as a scheme in which the central site is not owned by
the organization at all and is instead part of a service offered by
a tunnel broker somewhere on the Internet. Such a choice can be
appealing due to factors such as outsourcing of operational burden
and the possibility of superior performance due to the broker having
a globally distributed and fine-tuned network of "hubs," to the
closest of which each site can then connect to.
Another advantage of having control over the path crossing the first
mile of the branch site lies in the possibility of applying error-
correcting algorithms to the traffic; several vendors offer this
functionality which, although proprietary, can be made to work by
placing compatible devices at the branch edge and the central site,
usually terminating the tunnels comprising the overlay. Such
techniques typically include forward error correction, compression,
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packet duplication, and deduplication across multiple low-quality
links, to prevent or lessen packet loss across the overlay. These
techniques, however, cannot improve other metrics such as latency.
The principal downside of the tunnel-based solution is not making
use of the "local Internet breakout": users at the branch site are
almost guaranteed to experience worse performance towards Internet
destinations compared to solutions listed in sections 5.1-5.2, 5.4-
5.5 due to the traffic having to be backhauled to the central site
first.
However, it should be noted that as long as the centralized site
uses a solution similar to 5.1 or 5.2, it'll also enable
communication that otherwise would have failed or required complex
use-case-specific workarounds.
In fact, despite the alleviating factors discussed above, shifting
the problem to a different area of the network might not be
considered a technical solution at all because the central site
would face the same fundamental challenges, and it would ultimately
have the same options for multi-homing as discussed in sections 5.1-
5.2, 5.4-5.5. As such, what is described in this section could be
considered a non-technical solution for a small site.
Enterprise WAN design in itself remains outside the scope of this
document.
Advantages:
- Shifting the problem to a different location may help solve it
more efficiently,
- The simplest solution for the small site,
- Supported CE products (Wireguard),
- No need to renumber, hence no issues with prefix deprecation,
- No need for special support on hosts,
- Traffic steering is easy to implement, including traffic symmetry
requirements or active/passive failover,
- A centralized Internet gateway simplifies perimeter security,
- Possibility of applying WAN optimization techniques to the first
section of the path toward the Internet,
- Multiple free or paid tunnel brokers exist with different SLAs,
- Avoid unnecessarily polluting the global routing table, and may
also get better AS paths because of the aggregations compared to
using a PI.
Disadvantages:
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- Looping the Internet traffic through the centralized site might
increase latency, and additional links on the traffic path may
contribute to jitter and packet loss,
- More bandwidth is needed for rented WAN links,
- Side effects related to tunnelings, such as encapsulation
processing and overhead,
- Convergence time in case of underlay network failures may be
affected by the need to re-establish the tunnels and routing
neighborships of the overlay,
- The central site becomes a single point of failure for the
Internet access of the entire organization,
- Some or all of the disadvantages listed in sections 5.1-5.2, 5.4-
5.5 apply, depending on the specific solution selected to solve
the multi-homing issue at the central site. These may include end-
to-end connectivity and traffic steering issues toward Internet
destinations.
5.4. ULA with NPTv6
[ULA] allows for the creation and use of local, non-globally
reachable, and not centrally assigned address space that is
sufficiently random to be treated like globally unique addressing
within a given organization or environment. However, [ULA] has
notable limits concerning the number of sites that one prefix may
span, and well-documented usability limitations when considering
address selection details across large, diverse network
environments.
Organizations requiring larger than a /48 prefix are often better
served applying for and receiving a global PI allocation that is
right-sized for their needs. There may exist creative tweaks for
expanding ULA beyond its normal size, but that is outside the scope
of this document.
There is also one significant detail of [ULA] address space that is
important to note in the presence of a dual-stacked environment, as
[ULA] is prioritized below GUA and IPv4 address space on the hosts
according to [SASA] section 2.1. Hence, in a dual-stack environment,
it is necessary to modify the [SASA] policy table to insert the /48
prefix with higher precedence, as recommended in section 10.6 of
[SASA]. Automation for such configuration is OS-specific, and in
some cases may not be possible.
It is not mandatory to have [ULA] for the solution described in this
section, registered GUA (PI addresses in particular) may be used
too, but this has a low chance of happening as PI address space is
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much more widely deployed as a routed policy directly to carriers or
other upstream service providers.
Network Prefix translation [NPTv6] is the unique IPv6 technology
that enables a lightweight version of NAT with a 1:1 stateless
relationship between addresses on the "inside" and "outside". It is
formally an experimental protocol despite having a long list of
production deployments. A stateless algorithmic relationship permits
to have asymmetric routing and easy redundancy if multiple gateways
are implemented facing a single carrier.
Like any NAT it may create a challenge for protocols that embed IP
addresses at the application level. It may require the usage of an
external [STUN] server for address translation, or monitoring of the
session by an ALG.
Additionally, when crossing NAT environments, protocols such as
IPSec require or fall back to "NAT traversal" schemes, which
typically work by encapsulating the original session into UDP.
Application support for this with IPv6 is often poor because a main
talking point for the adoption of IPv6 is that "NAT traversal" is no
longer required and that this simplifies the application logic.
However, in practice, this is not always the case, and legacy
applications may still require these techniques to operate.
Contrary to other forms of NAT, with NPTv6 there is no need to
generate or retain translation logs because translation is stateless
and deterministic.
Two-thirds of carriers lease a permanent prefix to subscribers, and
such prefix would thus remain the same after the uplink is
disconnected and re-connected. For this reason, it is possible to
initiate an inbound session from the "outside" stably and
continuously; the NPTv6 engine can apply the required translation on
all outbound and inbound packets regardless of whether they are part
of a new or existing connection. For other carriers which do not
lease prefixes permanently, some additional efforts are needed to
dynamically update DNS records, and use those to establish inbound
connections. The possibility of connection initiation in any
direction (when firewall rules allow it) is considered valuable by
some engineers, while others value the one-way connectivity typical
of NAT that is lost with NPTv6.
Similarly, other NAT-related problems are not present with NPTv6
because it does not require manipulation of the transport layer.
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[NPTv6] is partially acting against the [IAB request] to preserve
the end-to-end transparency of the Internet which is important for
the Internet's future flexibility.
Together, [ULA] and [NPTv6] may effectively mimic the typical IPv4
carrier resilience practices: the organization might only have [ULA]
inside its network (no GUA), and every site could have many
redundant connections through separate [NPTv6] engines on every
border gateway, making use of the actual PA space provided by each
carrier on the external side of the translation to enable global
reachability towards Internet destinations.
There is, however, a principal difference from the typical IPv4 NAT
solution in that [NPTv6] needs an equally-sized prefix on the
"inside" and "outside". While it is typically possible to get a /56
or /60 from the fixed broadband carrier, it is significantly less
common to be delegated more than a /64 by the mobile carrier; hence,
if the carrier is mobile then only a simplified site with one
internal /64 subnet is feasible.
For faster convergence, routers should announce a "default" on-site
and then add some more specific prefixes if needed. Additional
destination prefixes facilitate the distribution of traffic between
the carriers.
In cases where internal routing hops are present between the edge
devices, such announcements likely require other protocols in
addition to Neighbor Discovery.
As for the network interface of the internal host (the last hop),
such an announcement comes in the form of default router status in
[ND], and [Route Preferences] for the more specific routes. The last
one, in particular, is essential to be able to access "subscriber-
only services".
The crucial requirement remains that routers must continuously track
connectivity to carriers to be able to deprecate the "default" as
well as any more specific announcements when such connectivity is
lost.
Advantages:
- No need for official address space, the ULA prefix is pseudo-
randomly self-generated,
- Easy to implement, similar in practice to current IPv4 carrier
resiliency techniques,
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- Potential for traffic distribution policy between different
carriers.
Disadvantages:
- It is challenging to automate ULA prioritization above IPv4 on
hosts,
- NPTv6 breaks some applications with address referrals at the
application level, some additional solutions are needed (STUN,
ALG),
- Custom distribution policies are needed for access to filtered
resources ("subscriber-only services"),
- Session initiation from the outside is practical only in cases
where the carrier prefix is stable or DNS records are dynamically
updated,
- Currently limited to one subnet per site in mobile environments,
- May hinder overall IPv6 adoption as IPv6 with NPTv6 loses the end-
to-end connectivity advantage,
- For applications, the drawbacks are similar to ULA with NAT66
(section 5.4).
5.5. ULA with NAT66
The observations regarding the use of ULA from the first paragraph
of the previous section apply to this section as well. It is
important to be mindful of the requirement and effort necessary to
prioritize ULA above IPv4 in the [SASA] policy table of hosts.
It is not mandatory to have [ULA] for the solution described in this
section, alternatively, registered GUA (PI addresses in particular)
may be used, but this brings its own set of requirements and effort,
as PI address space is much more useful when routed directly to
carriers. However, not all carriers may support this, or they might
charge significantly more to do so.
The [IAB request] to avoid the use of NAT has been heavily based on
the long list of [NAT implications]. In the case of stateful NAT66,
the full list of problems caused by NAT is applicable: breaking of
the end-to-end model, the impossibility of initiating sessions from
the outside, breaking IPSec encryption, breaking of application-
layer referrals to the addresses, single point of failure,
redundancy challenges (state replication), and performance
challenges (stateful processing).
Hence, it is easy to understand the IETF consensus for not having a
stateful NAT standard for IPv6. There is no RFC for NAT66. A
proprietary implementation may furthermore create interoperability
challenges.
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Those details aside, NAT has operational advantages: it avoids
renumbering in case of PA address space change, and is historically
the most common solution for carrier resiliency, especially in small
to medium-sized environments that may not have the resources,
availability, or expertise to leverage a PI-based solution with
upstream carriers. Hence, it is supported by many products, both
commercial and open source; one example is [nftables NAT66].
Another issue with NAT is the lack of UPnPv6 standardization and
implementation. With IPv4, applications behind a NAT44 can
dynamically request to know their public IP as well as new port
forwardings, thanks to UPnP. With IPv6 this is not available, and
the end-user experience with NAT66 is thus likely to be worse than
with NAT44.
For faster convergence, routers should announce a "default" on-site
and then additionally some more specific prefixes if needed.
Additional destination prefixes facilitate the distribution of
traffic between the carriers.
In cases where internal routing hops are present between the edge
devices, such announcements likely require other protocols in
addition to Neighbor Discovery.
As for the network interface of the internal host (the last hop),
such an announcement comes in the form of default router status in
[ND], and [Route Preferences] for the more specific routes. The last
one, in particular, is essential to be able to access "subscriber-
only services".
The crucial requirement remains that routers must continuously track
connectivity to carriers to be able to deprecate the "default" as
well as any more specific announcements when such connectivity is
lost.
Advantages:
- No need for official address space, as the ULA prefix is pseudo-
randomly self-generated,
- Easy to implement,
- Equivalent in practice to current IPv4 carrier resiliency
techniques,
- NAT may be a normative requirement in itself (this is highly
questionable, but brought forward in many discussions),
- Support for sites with complex topologies,
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- Potential for traffic distribution policy between different
carriers.
Disadvantages:
- It is challenging to automate ULA prioritization above IPv4 on
hosts,
- NAT breaks some applications with address referrals at the
application level,
- Custom distribution policies needed for access to filtered
resources ("subscriber-only services"),
- Some additional solutions are needed (STUN, ALG),
- Session initiation from the outside is blocked in practice (needs
complex configuration),
- NAT implies the requirement to keep logs for compliance and
troubleshooting,
- Expensive, due to the higher costs of stateful processing,
- May hinder overall IPv6 adoption,
- For applications, the drawbacks are similar to ULA with NPTv6
(section 5.3).
5.6. Application proxy
Another possible approach is shifting the Internet access resiliency
problem to an explicit application proxy similar to the one
described in [IP Proxy], that can be placed anywhere in the network.
Traditionally, enterprise networks have often relied on proxies to
enable Internet access for clients. The primary motivation for this
has not been to provide network functionality but to enforce policy,
authenticate users, and perform traffic filtering. The caveats for
this mechanism to work are essentially two, namely that both the
clients and the applications they are accessing support the use of
an application proxy. "Transparent" proxies which sit in the traffic
path, similar to those described in Section 4 of [IP Proxy], have
also been employed to negate the requirement for clients to be
configured to use the application proxy. With regards to the Site
Multi-homing problem, however, this configuration is functionally
equivalent to the scenario in 5.4 while having no advantages
compared to it; for this reason, this section deals exclusively with
explicitly configured proxies accepting traffic destined to an
address owned by them.
By proxying connections at the transport layer and above, thus
splitting the connection into 2 parts, there is an inherent
possibility to use a different source address for the second,
upstream half of the connection; in an IPv4 environment, this can in
some cases replace address translation in environments where private
address space is used by the clients. A similar possibility exists
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in an IPv6 environment, allowing end hosts to communicate with the
Internet even if they only possess ULA or otherwise non-globally
reachable addresses as long as they can reach the application proxy.
The use of a proxy in an IPv6 site with multi-homing requirements
thus allows for the following configuration:
- Two or more Internet gateways from diverse carriers, receiving and
advertising PA GUA address space on the network,
- An application proxy provisioned with a ULA address as well as PA
addresses from each Internet uplink,
- Clients provisioned with ULA addresses to communicate with
internal destinations and the proxy, and only optionally PA
addresses.
In such a configuration clients support, and are configured for, the
proxying of outbound connections through the explicit application
proxy. The connection to the proxy then uses ULA addresses
exclusively, and the second part of the connection from the
application proxy out to the Internet can use a viable PA source
address selected from the available prefixes. In this manner,
clients are relieved of the duty to select a GUA source as well as
tracking of the health and validity of related prefixes. This leaves
the source GUA selection, and in turn the link selection, to the
proxy itself. In this case, tracking of the health of each Internet
link needs to be performed in only one host, i.e. the proxy, and
such tracking is facilitated further in the case where the proxy and
the Internet gateways are the same devices.
Available technologies for signaling and establishing proxied
connections include [HTTP], [SOCKS], and, more recently, [MASQUE],
which allow the application proxy host to interject at various
layers of the stack up to the application itself, supporting TCP,
UDP, and even the forwarding of raw IP datagrams. Discussion of the
low-level workings of such technologies is out of the scope of this
document.
Owing to the extensive past usage of Web proxies and the dominance
of HTTP and HTTPS in the outbound component of enterprise network
traffic, a client in the enterprise can typically perform nearly all
of its interactions with the Internet through a proxy, but several
direct connections are often required for some applications; in the
scenario described in this section, the client is free to reach
Internet destinations in such a manner if it is provisioned with one
or more of the actual PA prefixes coming from the Internet links,
and if such traffic is not restricted by other on-path devices such
as firewalls. Using its ULA addresses may even work for direct
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communication, with the caveats outlined in sections 5.4 and 5.5.
For any traffic generated by clients which bypasses the proxy, all
considerations listed in sections 5.2, 5.4, and 5.5 apply.
The device hosting the application proxy itself is in a situation
comparable to the one described in section 5.2, but the process or
daemon which performs the proxy functions can be placed arbitrarily
close or even inside the gateways. This mitigates a number of the
disadvantages listed in 5.2, namely:
- "Prefixes may not get deprecated when the CPE itself fails": the
application proxy can track the health of each prefix using active
polling and other proprietary methods.
- "Carriers may frequently change the prefix (flash renumbering)":
renumbering is simplified as it only needs to occur on one host,
and any patches and upgrades of the network stack required to
support newer and more robust renumbering mechanisms are
simplified. Additionally, if the application proxy is the gateway
itself, it can become aware of any new prefixes without having to
employ any kind of network signaling.
- "Sites with complex topologies are not well supported": the
application proxy can be positioned in the subnet and link which
is shared by the internal interface of all the site's Internet
gateways, recreating the "simple topology" condition. Clients
reach the proxy using ULA and conventional routing is sufficient
in the more complex, internal part of the network.
- "Traffic steering by any policies is not supported yet": link
selection is centralized in the application proxy, which can
enforce steering policies of various kinds.
Communication inbound from the Internet toward clients is impossible
in a purely proxy-based scenario and requires that clients be
provisioned with GUAs in addition to the ULA they employ to reach
the proxy. All considerations in 5.2 are applicable in this case.
Additionally, if an application supports and the clients opt to use
the proxy for it, outbound usage of such an application results in a
completely different mechanism (an application proxy) and a
different GUA address than the one used for inbound sessions. It is
however likely that the host inside the site is acting purely as a
client for such an application, and any peer-to-peer or server
applications running on the host are elected for bypassing the
proxy.
Modern client operating systems offer several standardized ways of
configuring the use of an application proxy, including manual and
automatic techniques in which the proxy, as well as the list of
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destinations to be proxied, is auto-discovered by the client upon
joining the network. Discussion of these techniques is out of the
scope of this document, but they constitute a hard limit on the
applicability of proxies as described in this section. If the number
of hosts that do not support automatic configuration is large,
provisioning of the proxy may become impractical or impossible. This
is further compounded by the fact that an ever-increasing number of
hosts in enterprise networks is headless and/or unmanaged, such as
IoT devices, and may not even support proxies at all or not support
a way of configuring them. Any such device would need to resort to
one of the other options described in this document to gain Internet
access and benefit from multi-homing. Network designers may find
that, in such cases, the need to mix and match solutions has
operational drawbacks that outweigh the advantages of using a proxy.
It is also crucial to keep in mind that the use of an application
proxy does not technically constitute a solution to the issue of
IPv6 multi-homing, which is a network problem. This is obvious for
two reasons:
- The solution operates at the transport layer and above,
- The device hosting the application proxy faces the same
fundamental challenges and ultimately has the same options for
multi-homing as discussed in sections 5.1-5.2, 5.4-5.5. As such,
what is described in this section could be considered an upper-
layer workaround to a network problem.
Advantages:
- No NAT, communication is terminated and re-established at a higher
layer using a different source address, and the client is aware of
this,
- No need to own and operate a registered, Provider-Independent
address space,
- ULA usage is ULA to ULA, avoiding the need to engineer ULA
prioritization above IPv4 in the [SASA] policy table,
- Organizations may already be using an application proxy,
- Supports sites with complex topology,
- Supports outbound traffic steering,
- Potential for traffic distribution policy between different
carriers.
Disadvantages:
- No E2E unless the client has GUA as well, and even then outbound
and inbound use different mechanisms,
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- Proxy provisioning and discovery,
- The application proxy is an additional point of failure in the
network, and ensuring its redundancy could bring increased costs
and complexity,
- Requires explicit application and/or protocol support,
- Requires explicit client support,
- While not NAT66, sessions must still be tracked in a stateful
manner by the application proxy,
- Splitting the flow of data at the application layer is
computationally expensive and can incur performance penalties far
over a traditional network hop.
6. Conclusion
Not all requirements can be satisfied by every solution:
+--+--------------------------------+----+-----+---------+---------+
| | Requirement | PI | PA |ULA+NPTv6| ULA+NAT |
+--+--------------------------------+----+-----+---------+---------+
| 1| Carriers Resiliency | + | + | + | + |
| 2| End-to-End Connectivity | + | + | +/- | - |
| 3| Internal Connectivity | + | + | +/- | +/- |
| 4| Convergence speed | + | +/- | + | + |
| 5| Complex Topology support | + | - | +/- | + |
| 6| Subscriber-only services | - | - | +/- | +/- |
| 7| Traffic Steering on Router | +/-| - | + | + |
| 7| Traffic Steering on Host OS | - | - | - | - |
| 7| Traffic Steering on Application| - | - | - | - |
+--+--------------------------------+----+-----+---------+---------+
The table above shows partial support for end-to-end connectivity
for the ULA+NPTv6 solution because, while it does allow initiating
connectivity in any direction, employing address references at the
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application layer requires extra steps, for example in the form of
an ALG or [STUN].
Internal connectivity is marked as partial support in the ULA
solutions due to the complexity involved in promoting the ULA
address space above IPv4 in the [SASA] policy table of hosts.
Convergence speed is partially satisfied by the PA-based solution
because [HNCP] or DHCP-PD have not been adopted by the market, and
they would be needed for prefix deprecation propagation over a
complex site.
Complex topology support is marked as partially satisfied by the
ULA+NPTv6 solution because it is not possible in practice to get an
external prefix larger than /64 from mobile carriers.
Support for "subscriber-only services" is marked as partial in the
ULA solutions because it needs a routing announcement as a "Routing
Information Option" of [Route Preferences], which is not widely
supported.
Traffic steering on routers is marked as partially supported for the
PI-based solution because of the high complexity involved in
organizing the steering of incoming traffic. NAT/NPTv6-based
solutions connect ingress traffic steering to egress which makes
them simpler in this regard.
Some of the functionality reflected in the table above may be
improved in the future, but a roadmap (active IETF draft) is not
available at the time of writing.
Theoretically, from a purely technical point of view, the solutions
in section 5 are ordered by the number of requirements satisfied,
from most to least: PI is the best, PA-based is more complex, NPTv6
breaks the end-to-end Internet model, and so on. In the real world,
though, the company could have non-technical requirements that
override the technical ones. For example, an organization might find
that the tunnel broker solution described in 5.5 fits their use-case
best, even though it doesn't solve the issue so much as outsource it
to a different organization (the tunnel-based solution in 5.5 cannot
be properly evaluated in the requirements table due to it
technically not being a solution, and anything added to the table
would merely reflect the choice of multi-homing techniques at the
central site). The table above is, in this sense, not complete - it
should be enriched with non-technical requirements as perceived by
the network owner.
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For many network owners, the main deciding factor may be the desire
to have end-to-end connectivity, as it is the most notable advantage
of deploying IPv6 compared to IPv4. This may be especially important
when managing resources that need to be exposed to the Internet.
If this is needed then PI or PA-based solutions are applicable, and
network owners must undertake some additional steps to implement
them:
- obtain a PI prefix from a RIR or LIR,
- pay a premium for the advertisement of the PI prefixes through the
carriers (as these often impose different tariffs for such an
attachment circuit).
If both of these challenges are deemed acceptable then the PI-based
solution is preferable: simple, reliable, and scalable. The
universal adoption of PI by companies of all sizes would, however,
create a burden for Internet routing. Routing tables in the default-
free zone could reach up to 10M entries due to small allocations
being advertised directly by end sites, without any ties to any
particular carrier, and thus no summarization being possible at the
carrier level. To support this, routers would need much larger
amounts of specialized hardware memory, and this would not only
disqualify all current hardware but it would also multiply expenses
related to Internet routers in the future. If many organizations
were to implement this option, the Internet would break for
everyone. In any case, this issue is unlikely to be considered a
priority by an enterprise organization whose decision-making process
is already constrained by many other factors.
If at least one of the aforementioned challenges is not acceptable
then PA may be the solution of choice, even with all the
restrictions and complexities discussed in section 5.2. A notable
exception to this is if complex topology support is a requirement
and the site is served by a mobile carrier (due to the
unavailability of prefixes larger than /64 on such connections).
It may also be the case that end-to-end connectivity is not a
necessity, and may even be undesired. Here, the ULA+NPTv6 solution
(discussed in section 5.4) satisfies a greater amount of
requirements in the majority of situations, apart from cases where
NAT66 is a strict (non-technical) requirement or the site has a
combination of complex topology and mobile connectivity (problematic
due to the small assignments on the WAN side and the 1:1 mechanism
of NPTv6).
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For sites having a complex topology (many links and routers), a PA-
based solution is not an option yet, because it would need automatic
PA address distribution over the site and neither [HNCP] nor DHCP-PD
have gained market acceptance for this task.
The logical steps in the design process would then be like the ones
above, but after having evaluated the PI-based solution the next
option would be ULA+NPTv6.
Previous availability of PI space or perceived NAT66 regulatory
requirements might also be primary factors, and then the logic may
yet again be different.
For further use cases that have not been discussed in this document,
it is possible to get a general expectation of compatibility using
the table above.
It is however recommended to read section 5 to fine-tune the custom
requirements matrix and grade each solution accordingly.
7. Security Considerations
This document is informational.
Hence, it may not create additional security challenges.
8. IANA Considerations
This document has no request to IANA.
9. References
9.1. Normative References
[BCP38] P. Ferguson, D. Senie, "Network Ingress Filtering: Defeating
Denial of Service Attacks which employ IP Source Address
Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827, May
2000, <https://www.rfc-editor.org/info/rfc2827>.
[IPv6] S. Deering, R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 8200, DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[ND] T. Narten, E. Nordmark, W. Simpson, H. Soliman, "Neighbor
Discovery for IP version 6 (IPv6)", RFC 4861, DOI
10.17487/RFC4861, September 2007, <https://www.rfc-
editor.org/info/rfc4861>.
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[SLAAC] S. Thomson, T. Narten, T. Jinmei, "IPv6 Stateless Address
Autoconfiguration", RFC 4862, DOI 10.17487/RFC4862,
September 2007, <https://www.rfc-editor.org/info/rfc4862>.
[NAT Implications] T. Hain, "Architectural Implications of NAT", RFC
2993, DOI 10.17487/RFC2993, November 2000,
<https://www.rfc-editor.org/info/rfc2993>.
[Local Protection] G. Van de Velde, T. Hain, R. Droms, B. Carpenter,
E. Klein, "Local Network Protection for IPv6", RFC 4864,
DOI 10.17487/RFC4864, May 2007, <https://www.rfc-
editor.org/info/rfc4864>.
[IAB request] D. Thaler, L. Zhang, G. Lebovitz, "IAB Thoughts on
IPv6 Network Address Translation", RFC 5902, DOI
10.17487/RFC5902, July 2010, <https://www.rfc-
editor.org/info/rfc5902>.
[NPTv6] M. Wasserman, F. Baker, "IPv6-to-IPv6 Network Prefix
Translation", RFC 6296, DOI 10.17487/RFC6296, June 2011,
<https://www.rfc-editor.org/info/rfc6296>.
[ULA] R. Hinden, B. Haberman, "Unique Local IPv6 Unicast Addresses",
RFC 4193, DOI 10.17487/RFC4193, October 2005,
<https://www.rfc-editor.org/info/rfc4193>.
[SASA] D. Thaler, R. Draves, A. Matsumoto, T. Chown, "Default
Address Selection for Internet Protocol Version 6 (IPv6)",
RFC 6724, DOI 10.17487/RFC6724, September 2012,
<https://www.rfc-editor.org/info/rfc6724>.
[HNCP] M. Stenberg, S. Barth, P. Pfister, "Home Networking Control
Protocol", RFC 7788, DOI 10.17487/RFC7788, April 2016,
<https://www.rfc-editor.org/info/rfc7788>.
[Node Requirements] T. Chown, J. Loughney, T. Winters, "IPv6 Node
Requirements", BCP 220, RFC 8504, DOI 10.17487/RFC8504,
January 2019, <https://www.rfc-editor.org/info/rfc8504>.
[MHMP Enterprise] F. Baker, C. Bowers, J. Linkova, "Enterprise
Multihoming Using Provider-Assigned IPv6 Addresses without
Network Prefix Translation: Requirements and Solutions",
RFC 8678 DOI 10.17487/RFC8678, December 2019,
<https://www.rfc-editor.org/info/rfc8678>.
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[Multi-Homing] F. Baker, B. Carpenter, "First-Hop Router Selection
by Hosts in a Multi-Prefix Network", RFC 8028, DOI
10.17487/RFC8028, November 2016, <https://www.rfc-
editor.org/info/rfc8028>.
9.2. Informative References
[IAB report] D. Meyer, L. Zhang, K. Fall, "Report from the IAB
Workshop on Routing and Addressing", RFC 4984, DOI
10.17487/RFC4984, September 2007, <https://www.rfc-
editor.org/info/rfc4984>.
[Shim6] E. Nordmark, M. Bagnulo, "Shim6: Level 3 Multihoming Shim
Protocol for IPv6", RFC 5533, DOI 10.17487/RFC5533, June
2009, <https://www.rfc-editor.org/info/rfc5533>.
[STUN] M. Petit-Huguenin, G. Salgueiro, J. Rosenberg, D. Wing, R.
Mahy, P. Matthews, "Session Traversal Utilities for NAT
(STUN)", RFC 8489, DOI 10.17487/RFC8489, February 2020,
<https://www.rfc-editor.org/info/rfc8489>.
[MHMP] O. Troan, D. Miles, S. Matsushima, T. Okimoto, D. Wing, "IPv6
Multihoming without Network Address Translation", RFC
7157, DOI 10.17487/RFC7157, March 2014, <https://www.rfc-
editor.org/info/rfc7157>.
[Route Preferences] R. Draves, D. Thaler, "Default Router
Preferences and More-Specific Routes", RFC 4191, DOI
10.17487/RFC4191, November 2005, <https://www.rfc-
editor.org/info/rfc4191>.
[Policy by DHCP] A. Matsumoto, T. Fujisaki, T. Chown, "Distributing
Address Selection Policy Using DHCPv6", RFC 7078 DOI
10.17487/RFC7078, January 2014, <https://www.rfc-
editor.org/info/rfc7078>.
[DNS Options] J. Jeong, S. Park, L. Beloeil, S. Madanapalli, "IPv6
Router Advertisement Options for DNS Configuration", RFC
8106 DOI 10.17487/RFC8106, March 2017, <https://www.rfc-
editor.org/info/rfc8106>.
[Conditional PIO] J. Linkova, M. Stucchi, "Using Conditional Router
Advertisements for Enterprise Multihoming", RFC 8475 DOI
10.17487/RFC8475, October 2018, <https://www.rfc-
editor.org/info/rfc8475>.
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[Provisioning domains] P. Pfister, E. Vyncke, T. Pauly, D. Schinazi,
W. Shao, "Discovering Provisioning Domain Names and Data",
RFC 8801 DOI 10.17487/RFC8801, July 2020,
<https://www.rfc-editor.org/info/rfc8801>.
[DHCPv6] T. Mrugalski, M. Siodelski, B. Volz, A. Yourtchenko, M.
Richardson, S. Jiang, T. Lemon, T. Winters, " Dynamic Host
Configuration Protocol for IPv6 (DHCPv6)", RFC 8415 DOI
10.17487/RFC8415, November 2018, <https://www.rfc-
editor.org/info/rfc8415>.
[Temporary addressing] F. Gont, S. Krishnan, T. Narten, R. Draves,
"Temporary Address Extensions for Stateless Address
Autoconfiguration in IPv6", RFC 8981 DOI 10.17487/RFC8981,
February 2021, <https://www.rfc-editor.org/info/rfc8981>.
[6renum] S. Jiang, B. Liu, B. Carpenter, "IPv6 Enterprise Network
Renumbering Scenarios, Considerations, and Methods", RFC
6879 DOI 10.17487/RFC6879, February 2013,
<https://www.rfc-editor.org/info/rfc6879>.
[multi6] J. Abley, B. Black, V. Gill, "Goals for IPv6 Site-
Multihoming Architectures", RFC 3582 DOI 10.17487/RFC3582,
August 2003, <https://www.rfc-editor.org/info/rfc3582>.
[flash renumbering] F. Gont, J. Zorz, R. Patterson, "Improving the
Robustness of Stateless Address Autoconfiguration (SLAAC)
to Flash Renumbering Events", draft-ietf-6man-slaac-renum-
04 (work in progress), February 2023.
[nftables NAT66] Marco Cilloni, "NAT66: The good, the bad, the
ugly", <https://blog.apnic.net/2018/02/02/nat66-good-bad-
ugly>.
[IP Proxy] Chatel, M., "Classical versus Transparent IP Proxies",
RFC 1919, DOI 10.17487/RFC1919, March 1996,
<https://www.rfc-editor.org/info/rfc1919>.
[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed.,
"HTTP Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110,
June 2022, <https://www.rfc-editor.org/info/rfc9110>.
[SOCKS] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and L.
Jones, "SOCKS Protocol Version 5", RFC 1928, DOI
10.17487/RFC1928, March 1996, <https://www.rfc-
editor.org/info/rfc1928>.
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[MASQUE] Pauly, T., Ed., Schinazi, D., Chernyakhovsky, A.,
Kuhlewind, M., and M. Westerlund, "Proxying IP in HTTP",
RFC 9484, DOI 10.17487/RFC9484, October 2023,
<https://www.rfc-editor.org/info/rfc9484>.
[MPTCP] A. Ford, C. Raiciu, M. Handley, O. Bonaventure, C. Paasch,
"TCP Extensions for Multipath Operation with Multiple
Addresses", RFC 8684 DOI 10.17487/RFC8684, March 2020,
<https://www.rfc-editor.org/info/rfc8684>.
[MPQUIC] Y. Liu, Y. Ma, Q. De Coninck, O. Bonaventure, C. Huitema,
M. Kuehlewind, "Multipath Extension for QUIC", draft-ietf-
quic-multipath-06 (work in progress), July 2023.
[PDperDevice] L. Colitti, J. Linkova, X. Ma, "Using DHCPv6-PD to
Allocate Unique IPv6 Prefix per Client in Large Broadcast
Networks", draft-ietf-v6ops-dhcp-pd-per-device-06 (work in
progress), November 2023.
Appendix A
A survey was proposed to collect information on multi-homing
solutions adopted in field deployment.
As of IETF 118 (Prague, November 4-10 2023), 17 respondents provided
answers to the following questions.
Question 1: How many CPEs are connected to ISPs on the average site?
15 responses:
1.1. One CPE: 6.7%
1.2. Two CPEs: 80.0%
1.3. More than two: 13.3%
Question 2: How many uplinks are configured per CPEs/CEs in your
average site?
15 responses:
2.1. One (every CPE has just 1 uplink toward a certain ISP): 53.3%
2.2. Two or more (a CPE is connected at least with 2 different
ISPs): 46.7%
Question 3: Which configuration do you support?
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14 responses:
3.1. Active/active: 78.6%
3.2. Active/standby: 21.4%
Question 4: Do your CPEs/CEs implement VRRP or any other dynamic
exchange of redundancy control information?
15 responses:
4.1. Yes: 53.3%
4.2. No: 46.7%
Question 5: If the answer to the previous is "No", which method do
you employ for supporting IPv6 multi-homing?
10 responses:
5.1. PI addressing: 40.0%
5.2. PA addressing (GUA), each ISP assigns a prefix to every CPE/CE:
30.0%
5.3. ULA/GUA intra-site with ALG at the border: 0.0%
5.4. ULA/GUA intra-site with NPTv6 at the border: 10.0%
5.5. ULA/GUA intra-site with NAT66 at the border: 10.0%
5.6. Other: 10.0%
5.6.1 PA addressing (GUA) with address from CPE. ULA intra-site.
NAT66 at border for mismatches: 10.0%
Question 6: Which method do you employ for supporting IPv4 multi-
homing (if any)?
15 responses:
6.1. Public Addressing: 53.3%
6.2. NAT44: 46.7%
Acknowledgments
Thanks to v6ops working group for problem discussion.
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Authors' Addresses
Klaus Frank
Email: klaus.frank@posteo.de
Nick Buraglio
Energy Sciences Network
Email: buraglio@forwardingplane.net
Paolo Nero
Email: oselists@gmail.com
Paolo Volpato
Huawei Technologies
Email: paolo.volpato@huawei.com
Eduard Vasilenko
Huawei Technologies
Email: vasilenko.eduard@huawei.com
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