Enterprise Multihoming Using Provider-Assigned IPv6 Addresses without Network Prefix Translation: Requirements and SolutionsSanta Barbara93117CaliforniaUnited States of AmericaFredBaker.IETF@gmail.comJuniper NetworksSunnyvale94089CaliforniaUnited States of Americacbowers@juniper.netGoogle1 Darling Island RdPyrmontNSW2009Australiafurry@google.com
Routing Area
Routing Working GroupConnecting an enterprise site to multiple ISPs over IPv6 using
provider-assigned addresses is difficult without the use of some form of
Network Address Translation (NAT). Much has been written on this topic
over the last 10 to 15 years, but it still remains a problem without a
clearly defined or widely implemented solution. Any multihoming solution
without NAT requires hosts at the site to have addresses from each ISP
and to select the egress ISP by selecting a source address for outgoing
packets. It also requires routers at the site to take into account those
source addresses when forwarding packets out towards the ISPs.This document examines currently available mechanisms for providing a solution
to this problem for a broad range of enterprise topologies.
It covers the behavior of routers to forward traffic by taking into account
source address, and it covers the behavior of hosts to select appropriate
default source addresses. It also covers any possible role that routers might
play in providing information to hosts to help them select appropriate
source addresses. In the process of exploring potential solutions, this
document also makes explicit requirements for how the solution would be
expected to behave from the perspective of an enterprise site network
administrator.Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of 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 candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any
errata, and how to provide feedback on it may be obtained at
.
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Table of Contents
. Introduction
. Requirements Language
. Terminology
. Enterprise Multihoming Use Cases
. Simple ISP Connectivity with Connected SERs
. Simple ISP Connectivity Where SERs Are Not Directly Connected
. Mechanisms for Hosts To Choose Good Default Source Addresses in a Multihomed Site
. Default Source Address Selection Algorithm on Hosts
. Selecting Default Source Address When Both Uplinks Are Working
. Distributing Default Address Selection Policy Table with DHCPv6
. Controlling Default Source Address Selection with Router Advertisements
. Controlling Default Source Address Selection with ICMPv6
. Summary of Methods for Controlling Default Source Address Selection to Implement Routing Policy
. Selecting Default Source Address When One Uplink Has Failed
. Controlling Default Source Address Selection with DHCPv6
. Controlling Default Source Address Selection with Router Advertisements
. Controlling Default Source Address Selection with ICMPv6
. Summary of Methods for Controlling Default Source Address Selection on the Failure of an Uplink
. Selecting Default Source Address upon Failed Uplink Recovery
. Controlling Default Source Address Selection with DHCPv6
. Controlling Default Source Address Selection with Router Advertisements
. Controlling Default Source Address Selection with ICMP
. Summary of Methods for Controlling Default Source Address Selection upon Failed Uplink Recovery
. Selecting Default Source Address When All Uplinks Have Failed
. Controlling Default Source Address Selection with DHCPv6
. Controlling Default Source Address Selection with Router Advertisements
. Controlling Default Source Address Selection with ICMPv6
. Summary of Methods for Controlling Default Source Address Selection When All Uplinks Failed
. Summary of Methods for Controlling Default Source Address Selection
. Solution Limitations
. Connections Preservation
. Other Configuration Parameters
. DNS Configuration
. Deployment Considerations
. Deploying SADR Domain
. Hosts-Related Considerations
. Other Solutions
. Shim6
. IPv6-to-IPv6 Network Prefix Translation
. Multipath Transport
. IANA Considerations
. Security Considerations
. References
. Normative References
. Informative References
Acknowledgements
Authors' Addresses
IntroductionSite multihoming, the connection of a subscriber network to multiple
upstream networks using redundant uplinks, is a common enterprise
architecture for improving the reliability of its Internet connectivity.
If the site uses provider-independent (PI) addresses, all traffic
originating from the enterprise can use source addresses from the PI
address space. Site multihoming with PI addresses is commonly used with
both IPv4 and IPv6, and does not present any new technical
challenges.It may be desirable for an enterprise site to connect to multiple
ISPs using provider-assigned (PA) addresses instead of PI addresses.
Multihoming with provider-assigned addresses is typically less expensive
for the enterprise relative to using provider-independent addresses as it does not
require obtaining and maintaining PI address space nor does it require
running BGP between the enterprise and the ISPs (for small/medium networks,
running BGP might be not only undesirable but also impossible, especially if
residential-type ISP connections are used).
PA multihoming is also a practice that should be facilitated and encouraged
because it does not add to the size of the Internet routing table,
whereas PI multihoming does. Note that PA is also used to mean
"provider-aggregatable". In this document, we assume that
provider-assigned addresses are always provider-aggregatable.With PA multihoming, for each ISP connection, the site is assigned a
prefix from within an address block allocated to that ISP by its
National or Regional Internet Registry. In the simple case of two ISPs
(ISP-A and ISP-B), the site will have two different prefixes assigned to
it (prefix-A and prefix-B). This arrangement is problematic. First,
packets with the "wrong" source address may be dropped by one of the
ISPs. In order to limit denial-of-service attacks using spoofed source
addresses, recommends that ISPs
filter traffic from customer sites to allow only traffic with a source
address that has been assigned by that ISP. So a packet sent from a
multihomed site on the uplink to ISP-B with a source address in prefix-A
may be dropped by ISP-B.However, even if ISP-B does not implement BCP 38, or ISP-B adds
prefix-A to its list of allowed source addresses on the uplink from the
multihomed site, two-way communication may still fail. If the packet
with a source address in prefix-A was sent to ISP-B because the uplink to
ISP-A failed, then if ISP-B does not drop the packet, and the packet
reaches its destination somewhere on the Internet, the return packet
will be sent back with a destination address in prefix-A. The return
packet will be routed over the Internet to ISP-A, but it will not be
delivered to the multihomed site because the site uplink with ISP-A has failed.
Two-way communication would require some arrangement for ISP-B to
advertise prefix-A when the uplink to ISP-A fails.Note that the same may be true of a provider that does not
implement BCP 38, even if his upstream provider does, or of a provider
that has no corresponding route to deliver the ingress traffic
to the multihomed site. The issue is not that the immediate provider implements ingress
filtering; it is that someone upstream does (so egress traffic is blocked) or lacks a route (causing blackholing of the ingress traffic).
Another issue with asymmetric traffic flow (when the egress traffic
leaves the site via one ISP, but the return traffic enters the site
via another uplink) is related to stateful firewalls/middleboxes.
Keeping state in that case might be problematic, even impossible.
With IPv4, this problem is commonly solved by using private address
space described in within the multihomed site and
Network Address Translation (NAT) or Network Address/Port Translation
(NAPT) on the uplinks to the ISPs. However, one of the goals of IPv6 is
to eliminate the need for and the use of NAT or NAPT. Therefore,
requiring the use of NAT or NAPT for an enterprise site to multihome
with provider-assigned addresses is not an attractive solution. describes a translation solution
specifically tailored to meet the requirements of multihoming with
provider-assigned IPv6 addresses. With the IPv6-to-IPv6 Network Prefix
Translation (NPTv6) solution, an enterprise can use either
Unique Local Addresses or the prefix assigned
by one of the ISPs within its site. As traffic leaves the site on an uplink to an ISP,
the source address is translated in a predictable and reversible manner
to an address within the prefix assigned by the ISP on that uplink.
is currently classified as
Experimental, and it has been implemented by several vendors.
See for more discussion of NPTv6.This document defines routing requirements for enterprise multihoming.
This document focuses on the following general class of
solutions.Each host at the enterprise has multiple addresses, at least one from
each ISP-assigned prefix. As discussed in
and in , each host
is responsible for choosing the source address that is applied to each packet it
sends. A host is expected to be able to respond dynamically to the failure of an
uplink to a given ISP by no longer sending packets with the source
address corresponding to that ISP. Potential mechanisms for
communicating network changes to the host are Neighbor
Discovery Router Advertisements , DHCPv6 , and ICMPv6 .The routers in the enterprise network are responsible for ensuring
that packets are delivered to the "correct" ISP uplink based on source
address. This requires that at least some routers in the site network
are able to take into account the source address of a packet when
deciding how to route it. That is, some routers must be capable of some
form of Source Address Dependent Routing (SADR), if only as described in
. At a minimum, the routers connected to the ISP
uplinks (the site exit routers or SERs) must be capable of Source
Address Dependent Routing. Expanding the connected domain of routers
capable of SADR from the site exit routers deeper into the site network
will generally result in more efficient routing of traffic with external
destinations.This document is organized as follows. looks in more detail at the
enterprise networking environments in which this solution is expected to
operate. The discussion of uses the concepts of
Source-Prefix-Scoped Routing advertisements and forwarding tables
and describes how
Source-Prefix-Scoped Routing advertisements are used to generate
source-prefix-scoped forwarding tables. A detailed
description of generating source-prefix-scoped forwarding tables is provided in
.
discusses existing and proposed mechanisms for hosts to
select the default source address to be used by applications.
It also discusses the requirements for routing that are needed to
support these enterprise network scenarios and the mechanisms by
which hosts are expected to update default source addresses based
on network state.
discusses deployment considerations, while discusses other solutions.Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
"MAY", and "OPTIONAL" in this document are to be interpreted as
described in BCP 14
when, and only when, they appear in all capitals, as shown here.
Terminology
PA (provider-assigned or provider-aggregatable) address space:
a block of IP addresses assigned by a Regional Internet Registry (RIR)
to a Local Internet Registry (LIR), used to create allocations to end sites.
Can be aggregated and present in the routing table as one route.
PI (provider-independent) address space:
a block of IP addresses assigned by a Regional Internet Registry
(RIR) directly to end site / end customer.
ISP:
Internet Service Provider
LIR (Local Internet Registry):
an organization (usually an ISP or an enterprise/academic) that receives
its allocation of IP addresses from its Regional Internet Registry, then assigns parts of that allocation to its customers.
RIR (Regional Internet Registry):
an organization that manages the Internet number resources (such
as IP addresses and autonomous system (AS) numbers)
within a geographical region of the world.
SADR (Source Address Dependent Routing):
routing that takes into account the source address of a packet in addition to the packet destination address.
SADR domain:
a routing domain in which some (or all) routers exchange source-dependent routing information.
Source-Prefix-Scoped Routing/Forwarding Table:
a routing (or forwarding) table that contains routing (or forwarding) information only
applicable to packets with source addresses from the specific prefix.
Unscoped Routing/Forwarding Table:
a routing (or forwarding) table that can be used to route/forward packets with any source address.
SER (Site Edge Router):
a router that connects the site to an ISP (terminates an ISP uplink).
LLA (Link-Local Address):
an IPv6 unicast address from the fe80::/10 prefix .
ULA (Unique Local IPv6 Unicast Address):
an IPv6 unicast address from the FC00::/7 prefix. They are globally unique and intended for
local communications .
GUA (Global Unicast Address):
a globally routable IPv6 address of the global scope .
SLAAC (IPv6 Stateless Address Autoconfiguration):
a stateless process of configuring the network stack on IPv6 hosts .
RA (Router Advertisement):
a message sent by an IPv6 router to advertise its presence to hosts together with various
network-related parameters required for hosts to perform SLAAC .
PIO (Prefix Information Option):
a part of an RA message containing information about IPv6 prefixes that could be used by hosts
to generate global IPv6 addresses .
RIO (Route Information Option):
a part of an RA message containing information about more specific IPv6 prefixes reachable
via the advertising router .
Enterprise Multihoming Use CasesSimple ISP Connectivity with Connected SERsWe start by looking at a scenario in which a site has connections
to two ISPs, as shown in . The
site is assigned the prefix 2001:db8:0:a000::/52 by ISP-A and prefix
2001:db8:0:b000::/52 by ISP-B. We consider three hosts in the site.
H31 and H32 are on a LAN that has been assigned subnets
2001:db8:0:a010::/64 and 2001:db8:0:b010::/64. H31 has been assigned
the addresses 2001:db8:0:a010::31 and 2001:db8:0:b010::31. H32 has
been assigned 2001:db8:0:a010::32 and 2001:db8:0:b010::32. H41 is on a
different subnet that has been assigned 2001:db8:0:a020::/64 and
2001:db8:0:b020::/64.We refer to a router that connects the site to an ISP as a site
edge router (SER). Several other routers provide connectivity among the
internal hosts (H31, H32, and H41), as well as connect the internal
hosts to the Internet through SERa and SERb. In this example, SERa and
SERb share a direct connection to each other.
In , we consider a scenario in which this
is not the case.For the moment, we assume that the hosts are able to select
suitable source addresses through some mechanism that
doesn't involve the routers in the site network. Here, we focus on
the primary task of the routed site network, which is to get packets
efficiently to their destinations, while sending a packet to the ISP
that assigned the prefix that matches the source address of the
packet. In , we examine what role
the routed network may play in helping hosts select suitable
source addresses for packets.With this solution, routers will need some form of Source Address
Dependent Routing, which will be new functionality. It would be useful
if an enterprise site does not need to upgrade all routers to support
the new SADR functionality in order to support PA multihoming. We
consider whether this is possible and examine the trade-offs of not having
all routers in the site support SADR functionality.In the topology in , it is
possible to support PA multihoming with only SERa and SERb being
capable of SADR. The other routers can continue to forward based only
on destination address, and exchange routes that only consider
destination address. In this scenario, SERa and SERb communicate
source-scoped routing information across their shared connection. When
SERa receives a packet with a source address matching prefix
2001:db8:0:b000::/52, it forwards the packet to SERb, which forwards
it on the uplink to ISP-B. The analogous behavior holds for traffic
that SERb receives with a source address matching prefix
2001:db8:0:a000::/52.In , when only SERa and SERb
are capable of source address dependent routing, PA multihoming will
work. However, the paths over which the packets are sent will
generally not be the shortest paths. The forwarding paths will
generally be more efficient when more routers are capable of SADR. For
example, if R4, R2, and R6 are upgraded to support SADR, then they can
exchange source-scoped routes with SERa and SERb. They will then know
to send traffic with a source address matching prefix
2001:db8:0:b000::/52 directly to SERb, without sending it to SERa
first.Simple ISP Connectivity Where SERs Are Not Directly ConnectedIn , we modify the topology
slightly by inserting R7, so that SERa and SERb are no longer directly
connected. With this topology, it is not enough to just enable SADR
routing on SERa and SERb to support PA multihoming. There are two
solutions to enable PA multihoming in this topology.One option is to effectively modify the topology by creating a
logical tunnel between SERa and SERb by using Generic Routing
Encapsulation (GRE) , for example. Although
SERa and SERb are not directly connected physically in this topology,
they can be directly connected logically by a tunnel.The other option is to enable SADR functionality on R7. In this
way, R7 will exchange source-scoped routes with SERa and SERb, making
the three routers act as a single SADR domain. This illustrates the
basic principle that the minimum requirement for the routed site
network to support PA multihoming is having all of the site exit
routers be part of a connected SADR domain. Extending the connected
SADR domain beyond that point can produce more efficient forwarding
paths.Enterprise Network Operator ExpectationsBefore considering a more complex scenario, let's look in more
detail at the reasonably simple multihoming scenario in to understand what can reasonably
be expected from this solution. As a general guiding principle, we
assume an enterprise network operator will expect a multihomed network
to behave as close to a single-homed network as possible. So a
solution that meets those expectations where possible is a good
thing.For traffic between internal hosts and for traffic from outside the
site to internal hosts, an enterprise network operator would expect
there to be no visible change in the path taken by this traffic, since
this traffic does not need to be routed in a way that depends on
source address. It is also reasonable to expect that internal hosts
should be able to communicate with each other using either of their
source addresses without restriction. For example, H31 should be able
to communicate with H41 using a packet with S=2001:db8:0:a010::31,
D=2001:db8:0:b020::41, regardless of the state of uplink to ISP-B.These goals can be accomplished by having all of the routers in the
network continue to originate normal unscoped destination routes for
their connected networks. If we can arrange it so that these unscoped
destination routes are used for forwarding this traffic, then we will
have accomplished multihoming's goal of not affecting the forwarding
of traffic destined for internal hosts.For traffic destined for external hosts, it is reasonable to expect
that traffic with a source address from the prefix assigned by ISP-A
to follow the path that the traffic would follow if there were no
connection to ISP-B. This can be accomplished by having SERa originate
a source-scoped route of the form (S=2001:db8:0:a000::/52, D=::/0).
If all of the routers in the site support SADR, then the path of
traffic exiting via ISP-A can match that expectation. If some routers
don't support SADR, then it is reasonable to expect that the path for
traffic exiting via ISP-A may be different within the site. This is a
trade-off that the enterprise network operator may decide to make.It is important to understand the behavior of this multihoming solution
when an uplink to one of the ISPs fails. To simplify this discussion,
we assume that all routers in the site support SADR. We start by
looking at the operation of the network when the uplinks to both ISP-A and
ISP-B are functioning properly. SERa originates a source-scoped route
of the form (S=2001:db8:0:a000::/52, D=::/0), and SERb originates a
source-scoped route of the form (S=2001:db8:0:b000::/52, D=::/0).
These routes are distributed through the routers in the site, and they
establish within the routers two sets of forwarding paths for traffic
leaving the site. One set of forwarding paths is for packets with
source addresses in 2001:db8:0:a000::/52. The other set of forwarding
paths is for packets with source addresses in 2001:db8:0:b000::/52. The
normal destination routes, which are not scoped to these two source
prefixes, play no role in the forwarding. Whether a packet exits the
site via SERa or via SERb is completely determined by the source
address applied to the packet by the host. So for example, when host
H31 sends a packet to host H101 with (S=2001:db8:0:a010::31,
D=2001:db8:0:1234::101), the packet will only be sent out the link
from SERa to ISP-A.Now consider what happens when the uplink from SERa to ISP-A fails.
The only way for the packets from H31 to reach H101 is for H31 to
start using the source address for ISP-B. H31 needs to send the
following packet: (S=2001:db8:0:b010::31, D=2001:db8:0:1234::101).This behavior is very different from the behavior that occurs with
site multihoming using PI addresses or with PA addresses using NAT. In
these other multihoming solutions, hosts do not need to react to
network failures several hops away in order to regain Internet access.
Instead, a host can be largely unaware of the failure of an uplink to
an ISP. When multihoming with PA addresses and NAT, existing sessions
generally need to be reestablished after a failure since the external
host will receive packets from the internal host with a new source
address. However, new sessions can be established without any action
on the part of the hosts. Multihoming with PA addresses and NAT has created
the expectation of a fairly quick and simple recovery from network failures.
Alternatives should to be evaluated in terms of the speed and complexity
of the recovery mechanism.Another significant difference between this multihoming solution
and multihoming with either PI addresses or with
PA addresses using NAT is the ability of the enterprise network
operator to route traffic over different ISPs based on destination
address. We still consider the fairly simple network of
and assume that uplinks to both
ISPs are functioning. Assume that the site is multihomed using PA
addresses and NAT, and that SERa and SERb each originate a normal
destination route for D=::/0, with the route origination dependent on
the state of the uplink to the respective ISP.Now suppose it is observed that an important application running
between internal hosts and external host H101 experiences much better
performance when the traffic passes through ISP-A (perhaps because
ISP-A provides lower latency to H101). When multihoming this site with
PI addresses or with PA addresses and NAT, the enterprise network
operator can configure SERa to originate into the site network a
normal destination route for D=2001:db8:0:1234::/64 (the destination
prefix to reach H101) that depends on the state of the uplink to
ISP-A. When the link to ISP-A is functioning, the destination route
D=2001:db8:0:1234::/64 will be originated by SERa, so traffic from all
hosts will use ISP-A to reach H101 based on the longest destination
prefix match in the route lookup.Implementing the same routing policy is more difficult with the PA
multihoming solution described in this document since it doesn't use
NAT. By design, the only way to control where a packet exits this
network is by setting the source address of the packet. Since the
network cannot modify the source address without NAT, the host must
set it. To implement this routing policy, each host needs to use the
source address from the prefix assigned by ISP-A to send traffic
destined for H101. Mechanisms have been proposed to allow hosts to
choose the source address for packets in a fine-grained manner. We
will discuss these proposals in .
However, an enterprise network administrator would not expect to
interact with host operating systems to ensure that a particular
source address is chosen for a particular
destination prefix in order to implement this routing policy.More Complex ISP ConnectivityThe previous sections considered two variations of a simple
multihoming scenario in which the site is connected to two ISPs offering
only Internet connectivity. It is likely that many actual enterprise
multihoming scenarios will be similar to this simple example. However,
there are more complex multihoming scenarios that we would like this
solution to address as well.It is fairly common for an ISP to offer a service in addition to
Internet access over the same uplink. Two variations of this are
reflected in . In addition to Internet
access, ISP-A offers a service that requires the site to access host
H51 at 2001:db8:0:5555::51. The site has a single physical and logical
connection with ISP-A, and ISP-A only allows access to H51 over that
connection. So when H32 needs to access the service at H51, it needs to
send packets with (S=2001:db8:0:a010::32, D=2001:db8:0:5555::51), and
those packets need to be forwarded out the link from SERa to ISP-A.ISP-B illustrates a variation on this scenario. In addition to
Internet access, ISP-B also offers a service that requires the site
to access host H61. The site has two connections to two different
parts of ISP-B (shown as SERb1 and SERb2 in
). ISP-B expects Internet traffic to use the
uplink from SERb1, while it expects traffic destined for
the service at H61 to use the uplink from SERb2. For either uplink,
ISP-B expects the ingress traffic to have a source address matching
the prefix that it assigned to the site, 2001:db8:0:b000::/52.As discussed before, we rely completely on the internal host to set
the source address of the packet properly. In the case of a packet
sent by H31 to access the service in ISP-B at H61, we expect the
packet to have the following addresses: (S=2001:db8:0:b010::31,
D=2001:db8:0:6666::61). The routed network has two potential ways of
distributing routes so that this packet exits the site on the uplink
at SERb2.We could just rely on normal destination routes, without using
source-prefix-scoped routes. If we have SERb2 originate a normal
unscoped destination route for D=2001:db8:0:6666::/64, the packets
from H31 to H61 will exit the site at SERb2 as desired. We should not
have to worry about SERa needing to originate the same route, because
ISP-B should choose a globally unique prefix for the service at
H61.The alternative is to have SERb2 originate a source-prefix-scoped
destination route of the form (S=2001:db8:0:b000::/52,
D=2001:db8:0:6666::/64). From a forwarding point of view, the use of
the source-prefix-scoped destination route would result in traffic
with source addresses corresponding only to ISP-B being sent to SERb2.
Instead, the use of the unscoped destination route would result in
traffic with source addresses corresponding to ISP-A and ISP-B being
sent to SERb2, as long as the destination address matches the
destination prefix. It seems like either forwarding behavior would be
acceptable.However, from the point of view of the enterprise network
administrator trying to configure, maintain, and troubleshoot this
multihoming solution, it seems much clearer to have SERb2 originate
the source-prefix-scoped destination route corresponding to the service
offered by ISP-B. In this way, all of the traffic leaving the site is
determined by the source-prefix-scoped routes, and all of the traffic
within the site or arriving from external hosts is determined by the
unscoped destination routes. Therefore, for this multihoming solution
we choose to originate source-prefix-scoped routes for all traffic
leaving the site.ISPs and Provider-Assigned PrefixesWhile we expect that most site multihoming involves connecting to
only two ISPs, this solution allows for connections to an arbitrary
number of ISPs. However, when evaluating scalable
implementations of the solution, it would be reasonable to assume that
the maximum number of ISPs that a site would connect to is five (topologies with two redundant routers,
each having two uplinks to different ISPs, plus a tunnel to a head office acting as fifth one are not unheard of).It is also useful to note that the prefixes assigned to the site by
different ISPs will not overlap. This must be the case, since the
provider-assigned addresses have to be globally unique.Simplified TopologiesThe topologies of many enterprise sites using this multihoming
solution may in practice be simpler than the examples that we have
used. The topology in could be
further simplified by directly connecting all hosts to the LAN
that connects the two site exit routers, SERa and SERb. The topology
could also be simplified by connecting both uplinks to ISP-A and ISP-B
to the same site exit router. However, it is the aim of this
document to provide a solution that applies to a broad range of
enterprise site network topologies, so this document focuses on providing
a solution to the more general case. The simplified cases will also be
supported by this solution, and there may even be optimizations that
can be made for simplified cases. This solution, however, needs to
support more complex topologies.We are starting with the basic assumption that enterprise site
networks can be quite complex from a routing perspective. However,
even a complex site network can be multihomed to different ISPs with
PA addresses using IPv4 and NAT. It is not reasonable to expect an
enterprise network operator to change the routing topology of the site
in order to deploy IPv6.Generating Source-Prefix-Scoped Forwarding TablesSo far, we have described in general terms how the SADR-capable routers in this
solution forward traffic by using both normal unscoped destination routes and
source-prefix-scoped destination routes. Here we give a precise method
for generating a source-prefix-scoped forwarding table on a router that
supports SADR.
Compute the next-hops for the source-prefix-scoped destination
prefixes using only routers in the connected SADR domain. These are
the initial source-prefix-scoped forwarding table entries.
Compute the next-hops for the unscoped destination prefixes using
all routers in the IGP. This is the unscoped forwarding table.
For a given source-prefix-scoped forwarding table T (scoped to
source prefix P), consider a source-prefix-scoped forwarding
table T', whose source prefix P' contains P. We call T the more
specific source-prefix-scoped forwarding table, and T' the less
specific source-prefix-scoped forwarding table. We select
entries in forwarding table T' to augment forwarding table T
based on the following rules. If a destination prefix of an
entry in forwarding table T' exactly matches the destination
prefix of an existing entry in forwarding table T (including
destination prefix length), then do not add the entry to
forwarding table T. If the destination prefix does NOT match an
existing entry, then add the entry to forwarding table T.
As the unscoped forwarding table is considered to be scoped
to ::/0, this process will propagate routes from the unscoped
forwarding table to forwarding table T. If there exist multiple
source-prefix-scoped forwarding tables whose source prefixes
contain P, these source-prefix-scoped forwarding tables should be
processed in order from most specific to least specific.
The forwarding tables produced by this process are used in the following
way to forward packets.
Select the most specific (longest prefix match) source-prefix-scoped forwarding table
that matches the source address of the packet (again, the unscoped
forwarding table is considered to be scoped to ::/0).
Look up the destination address of the packet in
the selected forwarding table to
determine the next-hop for the packet.
The following example illustrates how this process is used to create
a forwarding table for each provider-assigned source prefix. We consider
the multihomed site network in .
Initially we assume that all of the routers in the site network support
SADR. shows the routes that are
originated by the routers in the site network.Each SER originates destination routes that are scoped to the source
prefix assigned by the ISP to which the SER connects. Note that the SERs
also originate the corresponding unscoped destination route. This is not
needed when all of the routers in the site support SADR. However, it is
required when some routers do not support SADR. This will be discussed
in more detail later.We focus on how R8 constructs its source-prefix-scoped forwarding
tables from these route advertisements. R8 computes the next hops for
destination routes that are scoped to the source prefix
2001:db8:0:a000::/52. The results are shown in the first table in
. (In this example, the next hops are
computed assuming that all links have the same metric.) Then, R8
computes the next hops for destination routes that are scoped to the
source prefix 2001:db8:0:b000::/52. The results are shown in the second
table in . Finally, R8 computes
the next hops for the unscoped destination prefixes. The results are
shown in the third table in .
The final step is for R8 to augment the more specific source-prefix-scoped
forwarding tables with entries from less specific source-prefix-scoped
forwarding tables. The unscoped forwarding table is considered as being
scoped to ::/0, so both 2001:db8:0:a000::/52 and 2001:db8:0:b000::/52
are more specific prefixes of ::/0. Therefore, entries in the unscoped
forwarding table will be evaluated to be added to these two
more specific source-prefix-scoped forwarding tables. If a forwarding
entry from the less specific source-prefix-scoped forwarding table
has the exact same destination prefix (including destination prefix length)
as the forwarding entry from the more specific source-prefix-scoped forwarding table,
then the existing forwarding entry in the more specific source-prefix-scoped forwarding table wins.
As an example of how the source-prefix-scoped forwarding entries are
augmented, we consider how the two
entries in the first table in
(the table for source prefix = 2001:db8:0:a000::/52) are augmented with
entries from the third table in
(the table of unscoped or scoped to ::/0 forwarding entries). The first four unscoped
forwarding entries (D=2001:db8:0:a010::/64, D=2001:db8:0:b010::/64,
D=2001:db8:0:a020::/64, and D=2001:db8:0:b020::/64) are not an exact
match for any of the existing entries in the forwarding table for source
prefix 2001:db8:0:a000::/52. Therefore, these four entries are added to
the final forwarding table for source prefix 2001:db8:0:a000::/52. The
result of adding these entries is reflected in the first four entries the
first table in .The next less specific source-prefix-scoped (scope is ::/0) forwarding table entry is for
D=2001:db8:0:5555::/64. This entry is an exact match for the existing
entry in the forwarding table for the more specific source prefix 2001:db8:0:a000::/52.
Therefore, we do not replace the existing entry with the entry from the
unscoped forwarding table. This is reflected in the fifth entry in the
first table in . (Note that since
both scoped and unscoped entries have R7 as the next hop, the result of
applying this rule is not visible.)The next less specific source-prefix-scoped (scope is ::/0) forwarding table entry is for
D=2001:db8:0:6666::/64. This entry is not an exact match for any
existing entries in the forwarding table for source prefix
2001:db8:0:a000::/52. Therefore, we add this entry. This is reflected in
the sixth entry in the first table in .The next less specific source-prefix-scoped (scope is ::/0) forwarding table entry
is for D=::/0. This entry is
an exact match for the existing entry in the forwarding table for the more specific source
prefix 2001:db8:0:a000::/52. Therefore, we do not overwrite the existing
source-prefix-scoped entry, as can be seen in the last entry in the
first table in .The forwarding tables produced by this process at R8 have the desired
properties. A packet with a source address in 2001:db8:0:a000::/52 will
be forwarded based on the first table in . If the packet is destined for the
Internet at large or the service at D=2001:db8:0:5555/64, it will be
sent to R7 in the direction of SERa. If the packet is destined for an
internal host, then the first four entries will send it to R2 or R5 as
expected. Note that if this packet has a destination address
corresponding to the service offered by ISP-B (D=2001:db8:0:5555::/64),
then it will get forwarded to SERb2. It will be dropped by SERb2 or by
ISP-B, since the packet has a source address that was not assigned by
ISP-B. However, this is expected behavior. In order to use the service
offered by ISP-B, the host needs to originate the packet with a source
address assigned by ISP-B.In this example, a packet with a source address that doesn't match
2001:db8:0:a000::/52 or 2001:db8:0:b000::/52 must have originated from
an external host. Such a packet will use the unscoped forwarding table
(the last table in ). These
packets will flow exactly as they would in absence of multihoming.We can also modify this example to illustrate how it supports
deployments in which not all routers in the site support SADR. Continuing
with the topology shown in , suppose
that R3 and R5 do not support SADR. Instead they are only capable of
understanding unscoped route advertisements. The SADR routers in the
network will still originate the routes shown in . However, R3 and R5 will only
understand the unscoped routes as shown in .With these unscoped route advertisements, R5 will produce the
forwarding table shown in .As all SERs belong to the SADR domain, any traffic that needs to exit the site will eventually hit a SADR-capable router. To prevent routing loops involving SADR-capable and non-SADR-capable routers, traffic that enters the SADR-capable domain does not leave the domain until it exits the site. Therefore all SADR-capable routers within the domain MUST be logically connected.Note that the mechanism described here for converting
source-prefix-scoped destination prefix routing advertisements into
forwarding state is somewhat different from that proposed in . The method
described in this document is functionally equivalent, but it is based on application of existing mechanisms for the described scenarios.Mechanisms for Hosts To Choose Good Default Source Addresses in a Multihomed SiteUntil this point, we have made the assumption that hosts are able to
choose the correct source address using some unspecified mechanism. This
has allowed us to focus on what the routers in a multihomed site
network need to do in order to forward packets to the correct ISP based
on source address. Now we look at possible mechanisms for hosts to
choose the correct source address. We also look at what role, if any,
the routers may play in providing information that helps hosts to choose
source addresses.
It should be noted that this section discusses how hosts could select
the default source address for new connections. Any connection that
already exists on a host is bound to a specific source address that
cannot be changed.
discusses the connections preservation issue in more detail.
Any host that needs to be able to send traffic using the uplinks to a given ISP
is expected to be configured with an address from the prefix
assigned by that ISP. The host will control which ISP is used for its
traffic by selecting one of the addresses configured on the host as the
source address for outgoing traffic. It is the responsibility of the
site network to ensure that a packet with the source address from an ISP
is now sent on an uplink to that ISP.If all of the ISP uplinks are working, then the host's choice of source address
may be driven by the desire to load share across ISP
uplinks, or it may be driven by the desire to take advantage of certain
properties of a particular uplink or ISP (if some information about various
path properties has been made available to the host somehow. See
as an example). If any of the ISP uplinks is
not working, then the choice of source address by the host can cause
packets to get dropped.How a host selects a suitable source address
in a multihomed site is not a solved problem. We do not attempt to solve
this problem in this document. Instead we discuss the current state of
affairs with respect to standardized solutions and the implementation of
those solutions. We also look at proposed solutions for this
problem.An external host initiating communication with a host internal to a
PA-multihomed site will need to know multiple addresses for that host in
order to communicate with it using different ISPs to the multihomed
site (knowing just one address would undermine all benefits of redundant connectivity provided by multihoming). These addresses are typically learned through DNS. (For
simplicity, we assume that the external host is single-homed.) The
external host chooses the ISP that will be used at the remote multihomed
site by setting the destination address on the packets it transmits. For
a session originated from an external host to an internal host, the
choice of source address used by the internal host is simple. The
internal host has no choice but to use the destination address in the
received packet as the source address of the transmitted packet.For a session originated by a host inside the multihomed site,
the decision of which source address to select is more complicated. We
consider three main methods for hosts to get information about the
network. The two proactive methods are Neighbor Discovery Router
Advertisements and DHCPv6. The one reactive method we consider is
ICMPv6. Note that we are explicitly excluding the possibility of having
hosts participate in, or even listen directly to, routing protocol
advertisements.First we look at how a host is currently expected to select the
default source and destination addresses to be used for a new connection.Default Source Address Selection Algorithm on Hosts defines the algorithms that hosts are
expected to use to select source and destination addresses for
packets. It defines an algorithm for selecting a source address and a
separate algorithm for selecting a destination address. Both of these
algorithms depend on a policy table. defines
a default policy that produces certain behavior.The rules in the two algorithms in depend
on many different properties of addresses. While these are needed for
understanding how a host should choose addresses in an arbitrary
environment, most of the rules are not relevant for understanding how
a host should choose among multiple source addresses in a multihomed
environment when sending a
packet to a remote host. Returning to the example in , we look at what the default algorithms in
say about the source address that internal
host H31 should use to send traffic to external host H101, somewhere
on the Internet.There is no choice to be made with respect to destination address.
H31 needs to send a packet with D=2001:db8:0:1234::101 in order to
reach H101. So H31 has to choose between using S=2001:db8:0:a010::31
or S=2001:db8:0:b010::31 as the source address for this packet. We go
through the rules for source address selection in . Rule 1 (Prefer same address) is not useful to
break the tie between source addresses because neither one of the candidate
source addresses equals the destination address. Rule 2 (Prefer appropriate scope) is also not useful in this scenario because both
source addresses and the destination address have global scope.Rule 3 (Avoid deprecated addresses) applies to an address that has
been autoconfigured by a host using stateless address
autoconfiguration as defined in . An address
autoconfigured by a host has a preferred lifetime and a valid
lifetime. The address is preferred until the preferred lifetime
expires, after which it becomes deprecated. A deprecated address is not
used if there is a preferred address of the appropriate scope available.
When the valid lifetime expires, the address cannot be used at all. The
preferred and valid lifetimes for an autoconfigured address are set
based on the corresponding lifetimes in the Prefix Information Option
in Neighbor Discovery Router Advertisements. In this scenario, a
possible tool to control source address selection in this scenario
would be for a host to deprecate an address by having routers on that
link, R1 and R2 in , send Router Advertisement messages
containing PIOs with the Preferred Lifetime value for the deprecated
source prefix set to zero. This is a rather blunt tool, because it discourages or prohibits the use
of that source prefix for all destinations. However, it may be useful in some scenarios.
For example, if all uplinks to a particular ISP fail, it is desirable to prevent hosts from
using source addresses from that ISP address space.
Rule 4 (Avoid home addresses) does not apply here because we are
not considering Mobile IP.Rule 5 (Prefer outgoing interface) is not useful in this scenario
because both source addresses are assigned to the same interface.Rule 5.5 (Prefer addresses in a prefix advertised by the next-hop) is not
useful in the scenario when both R1 and R2 will advertise both source
prefixes. However, this rule may potentially allow a host to select the
correct source prefix by selecting a next-hop. The most obvious way
would be to make R1 advertise itself as a default router and send
PIO for 2001:db8:0:a010::/64, while R2 advertises itself as a
default router and sends PIO for 2001:db8:0:b010::/64. We'll discuss
later how Rule 5.5 can be used to influence a source address selection
in single-router topologies (e.g., when H41 is sending traffic using R3
as a default gateway).Rule 6 (Prefer matching label) refers to the label value determined
for each source and destination prefix as a result of applying the
policy table to the prefix. With the default policy table defined in
, Label(2001:db8:0:a010::31) =
5, Label(2001:db8:0:b010::31) = 5, and Label(2001:db8:0:1234::101) =
5. So with the default policy, Rule 6 does not break the tie. However,
the algorithms in are defined in such a way
that non-default address selection policy tables can be used.
defines a way to distribute a non-default address
selection policy table to hosts using DHCPv6. So even though the
application of Rule 6 to this scenario using the default policy table
is not useful, Rule 6 may still be a useful tool.Rule 7 (Prefer temporary addresses) has to do with the technique
described in to periodically randomize the
interface portion of an IPv6 address that has been generated using
stateless address autoconfiguration. In general, if H31 were using
this technique, it would use it for both source addresses, for example,
creating temporary addresses 2001:db8:0:a010:2839:9938:ab58:830f and
2001:db8:0:b010:4838:f483:8384:3208, in addition to
2001:db8:0:a010::31 and 2001:db8:0:b010::31. So this rule would prefer
the two temporary addresses, but it would not break the tie between
the two source prefixes from ISP-A and ISP-B.Rule 8 (Use longest matching prefix) dictates that, between two
candidate source addresses, the host selects the one that has
longest common prefix length with the destination address. For example, if H31 were
selecting the source address for sending packets to H101, this rule
would not break the tie between candidate source addresses
2001:db8:0:a101::31 and 2001:db8:0:b101::31 since the common prefix length
with the destination is 48. However, if H31 were selecting the source
address for sending packets to H41 address 2001:db8:0:a020::41, then this
rule would result in using 2001:db8:0:a101::31 as a source
(2001:db8:0:a101::31 and 2001:db8:0:a020::41 share the common prefix
2001:db8:0:a000::/58, while for 2001:db8:0:b101::31 and
2001:db8:0:a020::41, the common prefix is 2001:db8:0:a000::/51).
Therefore Rule 8 might be useful for selecting the correct source
address in some, but not all, scenarios (for example if ISP-B services
belong to 2001:db8:0:b000::/59, then H31 would always use
2001:db8:0:b010::31 to access those destinations).So we can see that of the eight source address selection rules from
, four actually apply to our basic site
multihoming scenario. The rules that are relevant to this scenario are
summarized below.
Rule 3: Avoid deprecated addresses.
Rule 5.5: Prefer addresses in a prefix advertised by the
next-hop.
Rule 6: Prefer matching label.
Rule 8: Prefer longest matching prefix.
The two methods that we discuss for controlling the source address
selection through the four relevant rules above are SLAAC Router
Advertisement messages and DHCPv6.We also consider a possible role for ICMPv6 for getting
traffic-driven feedback from the network. With the source address
selection algorithm discussed above, the goal is to choose the correct
source address on the first try, before any traffic is sent. However,
another strategy is to choose a source address, send the packet, get
feedback from the network about whether or not the source address is
correct, and try another source address if it is not.We consider four scenarios in which a host needs to select the correct
source address. In the first scenario, both uplinks are working. In
the second scenario, one uplink has failed. The third scenario is a
situation in which one failed uplink has recovered. The last scenario is failure of both (all)
uplinks.
It should be noted that
only defines the behavior of IPv6
hosts to select default addresses that applications and upper-layer
protocols can use. Applications and upper-layer protocols can make their
own choices on selecting source addresses.
The mechanism proposed in this document attempts to ensure that the subset of source addresses available for applications and upper-layer protocols is selected with the up-to-date network state in mind. The rest of the document discusses various aspects of the default source address selection defined in , calling it for the sake of brevity "the source address selection".
Selecting Default Source Address When Both Uplinks Are WorkingAgain we return to the topology in . Suppose that the site administrator wants
to implement a policy by which all hosts need to use ISP-A to reach
H101 at D=2001:db8:0:1234::101. So for example, H31 needs to select
S=2001:db8:0:a010::31.Distributing Default Address Selection Policy Table with DHCPv6This policy can be implemented by using DHCPv6 to distribute an
address selection policy table that assigns the same label to
destination addresses that match 2001:db8:0:1234::/64 as it does to
source addresses that match 2001:db8:0:a000::/52. The following two
entries accomplish this.This requires that the hosts implement ,
the basic source and destination address framework, along with , the DHCPv6 extension for distributing a
non-default policy table. Note that it does NOT require that the
hosts use DHCPv6 for address assignment. The hosts could still use
stateless address autoconfiguration for address configuration, while
using DHCPv6 only for policy table distribution (see ). However this method has a number of
disadvantages:
DHCPv6 support is not a mandatory requirement for IPv6 hosts ,
so this method might not work for all devices.
Network administrators are required to explicitly configure
the desired network access policies on DHCPv6 servers. While it might be feasible in the scenario
of a single multihomed network, such approach might have some scalability issues, especially if the centralized
DHCPv6 solution is deployed to serve a large number of multihomed sites.
Controlling Default Source Address Selection with Router AdvertisementsNeighbor Discovery currently has two mechanisms to communicate
prefix information to hosts. The base specification for Neighbor
Discovery (see ) defines the Prefix
Information Option (PIO) in the Router Advertisement (RA) message.
When a host receives a PIO with the A-flag set, it can use the
prefix in the PIO as the source prefix from which it assigns itself an
IP address using stateless address autoconfiguration (SLAAC)
procedures described in . In the example of
, if the site network is using
SLAAC, we would expect both R1 and R2 to send RA messages with PIOs
with the A-flag set for both source prefixes 2001:db8:0:a010::/64 and
2001:db8:0:b010::/64. H31 would then use the
SLAAC procedure to configure itself with 2001:db8:0:a010::31 and
2001:db8:0:b010::31.Whereas a host learns about source prefixes from PIO messages,
hosts can learn about a destination prefix from a Router
Advertisement containing a Route Information Option (RIO), as
specified in . The destination prefixes in
RIOs are intended to allow a host to choose the router that it uses
as its first hop to reach a particular destination prefix.As currently standardized, neither PIO nor RIO options contained
in Neighbor Discovery Router Advertisements can communicate the
information needed to implement the desired routing policy. PIOs
communicate source prefixes, and RIOs communicate destination
prefixes. However, there is currently no standardized way to
directly associate a particular destination prefix with a particular
source prefix. proposes a Source
Address Dependent Route Information option for Neighbor Discovery
Router Advertisements that would associate a source prefix with
a destination prefix. The details of might need tweaking to address
this use case. However, in order to be able to use Neighbor
Discovery Router Advertisements to implement this routing policy, an
extension is needed that would allow R1 and R2 to explicitly communicate to H31
an association between S=2001:db8:0:a000::/52 and D=2001:db8:0:1234::/64.However, Rule 5.5 of the default source address selection algorithm (discussed
in ),
together with default router preference
(specified in )
and RIO, can be used to influence a source
address selection on a host as described below. Let's look at source
address selection on the host H41. It receives RAs from R3 with PIOs
for 2001:db8:0:a020::/64 and 2001:db8:0:b020::/64. At that point, all
traffic would use the same next-hop (R3 link-local address) so Rule
5.5 does not apply. Now let's assume that R3 supports SADR and has
two scoped forwarding tables, one scoped to S=2001:db8:0:a000::/52
and another scoped to S=2001:db8:0:b000::/52. If R3 generates two
different link-local addresses for its interface facing H41 (one for
each scoped forwarding table, LLA_A and LLA_B), R3 will send
two different RAs: one from LLA_A that includes a PIO for
2001:db8:0:a020::/64, and another from LLA_B that includes a PIO
for 2001:db8:0:b020::/64. Now it is possible to influence H41 source
address selection for destinations that follow the default route by
setting the default router preference in RAs. If it is desired that H41
reaches H101 (or any destination in the Internet) via ISP-A, then
RAs sent from LLA_A should have the default router preference set to 01
(high priority), while RAs sent from LLA_B should have the preference
set to 11 (low). LLA_A would then be chosen as a next-hop for H101,
and therefore (per Rule 5.5) 2001:db8:0:a020::41 would be
selected as the source address. If, at the same time, it is desired
that H61 is accessible via ISP-B, then R3 should include a RIO for
2001:db8:0:6666::/64 in its RA sent from LLA_B. H41 would choose
LLA_B as a next-hop for all traffic to H61, and then per Rule 5.5,
2001:db8:0:b020::41 would be selected as a source address.If in the above-mentioned scenario it is desirable that all
Internet traffic leaves the network via ISP-A, and the link to ISP-B
is used for accessing ISP-B services only (not as an ISP-A link
backup), then RAs sent by R3 from LLA_B should have their Router Lifetime
values set to zero and should include RIOs for ISP-B address space. It would
instruct H41 to use LLA_A for all Internet traffic but to use LLA_B as
a next-hop while sending traffic to ISP-B addresses.The description of the mechanism above assumes SADR support by the
first-hop routers as well as SERs. However, a first-hop router can still
provide a less flexible version of this mechanism even without
implementing SADR. This could be done by providing configuration knobs on the
first-hop router that allow it to generate different link-local addresses
and to send individual RAs for each prefix.
The mechanism described above relies on Rule 5.5 of the
default source address selection algorithm defined in
.
states that
"A host SHOULD select default routers for each prefix it is
assigned an address in." It also recommends that
hosts should implement Rule 5.5. of .
Hosts following the recommendations specified in
therefore should be able to benefit from
the solution described in this document. No standards need to be
updated in regards to host behavior. Controlling Default Source Address Selection with ICMPv6We now discuss how one might use ICMPv6 to implement the routing
policy to send traffic destined for H101 out the uplink to ISP-A,
even when uplinks to both ISPs are working. If H31 started sending
traffic to H101 with S=2001:db8:0:b010::31 and
D=2001:db8:0:1234::101, it would be routed through SER-b1 and out
the uplink to ISP-B. SERb1 could recognize that this traffic is
not following the desired routing policy and react by sending an
ICMPv6 message back to H31.In this example, we could arrange things so that SERb1 drops the
packet with S=2001:db8:0:b010::31 and D=2001:db8:0:1234::101, and
then sends to H31 an ICMPv6 Destination Unreachable message with
Code 5 (Source address failed ingress/egress policy). When H31
receives this packet, it would then be expected to try another
source address to reach the destination. In this example, H31 would
then send a packet with S=2001:db8:0:a010::31 and
D=2001:db8:0:1234::101, which will reach SERa and be forwarded out
the uplink to ISP-A.However, we would also want it to be the case that SERb1 does not
enforce this routing policy when the uplink from SERa to ISP-A has
failed. This could be accomplished by having SERa originate a
source-prefix-scoped route for (S=2001:db8:0:a000::/52,
D=2001:db8:0:1234::/64), and have SERb1 monitor the presence of that
route. If that route is not present (because SERa has stopped
originating it), then SERb1 will not enforce the routing policy, and
it will forward packets with S=2001:db8:0:b010::31 and
D=2001:db8:0:1234::101 out its uplink to ISP-B.We can also use this source-prefix-scoped route originated by
SERa to communicate the desired routing policy to SERb1. We can
define an EXCLUSIVE flag to be advertised together with the IGP
route for (S=2001:db8:0:a000::/52, D=2001:db8:0:1234::/64). This
would allow SERa to communicate to SERb that SERb should reject
traffic for D=2001:db8:0:1234::/64 and respond with an ICMPv6
Destination Unreachable Code 5 message, as long as the route for
(S=2001:db8:0:a000::/52, D=2001:db8:0:1234::/64) is present. The definition of an EXCLUSIVE flag for SADR
advertisements in IGPs would require future standardization work.
Finally, if we are willing to extend ICMPv6 to support this
solution, then we could create a mechanism for SERb1 to tell the
host which source address it should be using to successfully forward
packets that meet the policy. In its current form, when SERb1 sends
an ICMPv6 Destination Unreachable Code 5 message, it is basically
saying, "This source address is wrong. Try another source address."
In the absence of a clear indication which address to try next, the host
will iterate over all addresses assigned to the interface (e.g., various
privacy addresses), which would lead to significant delays and degraded user experience.
It would be better if the ICMPv6 message could say, "This source
address is wrong. Instead use a source address in
S=2001:db8:0:a000::/52". However, using ICMPv6 for signaling source address information
back to hosts introduces new challenges. Most routers currently have
software or hardware limits on generating ICMP messages. A site
administrator deploying a solution that relies on the SERs
generating ICMP messages could try to improve the performance of
SERs for generating ICMP messages. However, in a large network, it
is still likely that ICMP message generation limits will be reached.
As a result, hosts would not receive ICMPv6 back, which in turn leads
to traffic blackholing and poor user experience. To improve the
scalability of ICMPv6-based signaling, hosts SHOULD cache the
preferred source address (or prefix) for the given destination
(which in turn might cause issues in the case of the corresponding
ISP uplink failure - see ). In
addition, the same source prefix SHOULD be used for other
destinations in the same /64 as the original destination address.
The source prefix to the destination mapping SHOULD have a specific lifetime. Expiration of the
lifetime SHOULD trigger the source address selection algorithm
again.
Using ICMPv6 Destination Unreachable Messages with Code 5 to influence source address selection
introduces some security challenges, which are discussed in .
As currently standardized in , the ICMPv6
Destination Unreachable Message with Code 5 would allow for the
iterative approach to retransmitting packets using different source addresses.
As currently defined, the ICMPv6 message does not provide
a mechanism to communicate information about which source prefix
should be used for a retransmitted packet. The current document does not
define such a mechanism, but it might be a useful extension
to define in a different document. However, this approach has some security implications,
such as an ability for an attacker to send spoofed ICMPv6 messages
to signal an invalid/unreachable source prefix, causing a DoS-type attack.Summary of Methods for Controlling Default Source Address Selection to Implement Routing PolicySo to summarize this section, we have looked at three methods for
implementing a simple routing policy where all traffic for a given
destination on the Internet needs to use a particular ISP, even when
the uplinks to both ISPs are working.The default source address selection policy cannot distinguish
between the source addresses needed to enforce this policy, so a
non-default policy table using associating source and destination
prefixes using label values would need to be installed on each host.
A mechanism exists for DHCPv6 to distribute a non-default policy
table, but such solution would heavily rely on DHCPv6 support by the host
operating system. Moreover, there is no mechanism to translate
desired routing/traffic engineering policies into policy tables on
DHCPv6 servers. Therefore using DHCPv6 for controlling the address
selection policy table is not recommended and SHOULD NOT be
used.At the same time, Router Advertisements provide a reliable
mechanism to influence the source address selection process via PIO, RIO,
and default router preferences. As all those options have been
standardized by the IETF and are supported by various operating systems,
no changes are required on hosts. First-hop routers in the
enterprise network need to be capable of sending different RAs for
different SLAAC prefixes (either based on scoped forwarding tables
or based on preconfigured policies).SERs can enforce the routing policy by sending ICMPv6 Destination
Unreachable messages with Code 5 (Source address failed
ingress/egress policy) for traffic sent with the wrong
source address. The policy distribution could be automated by defining
an EXCLUSIVE flag for the source-prefix-scoped route, which could then be
set on the SER that originates the route. As ICMPv6 message
generation can be rate limited on routers, it SHOULD NOT be used as
the only mechanism to influence source address selection on hosts.
While hosts SHOULD select the correct source address for a given
destination, the network SHOULD signal any source address issues back
to hosts using ICMPv6 error messages.Selecting Default Source Address When One Uplink Has FailedNow we discuss whether DHCPv6, Neighbor Discovery Router Advertisements,
and ICMPv6 can help a host choose the right source address when an
uplink to one of the ISPs has failed. Again we look at the scenario in
. This time we look at traffic from
H31 destined for external host H501 at D=2001:db8:0:5678::501. We
initially assume that the uplink from SERa to ISP-A is working and
that the uplink from SERb1 to ISP-B is working.We assume there is no particular routing policy desired, so H31 is
free to send packets with S=2001:db8:0:a010::31 or
S=2001:db8:0:b010::31 and have them delivered to H501. For this
example, we assume that H31 has chosen S=2001:db8:0:b010::31 so that
the packets exit via SERb to ISP-B. Now we see what happens when the
link from SERb1 to ISP-B fails. How should H31 learn that it needs to
start sending packets to H501 with S=2001:db8:0:a010::31 in order
to start using the uplink to ISP-A? We need to do this in a way that
doesn't prevent H31 from still sending packets with
S=2001:db8:0:b010::31 in order to reach H61 at
D=2001:db8:0:6666::61.Controlling Default Source Address Selection with DHCPv6For this example, we assume that the site network in
has a centralized DHCP server and that all
routers act as DHCP relay agents. We assume that both of the
addresses assigned to H31 were assigned via DHCP.We could try to have the DHCP server monitor the state of the
uplink from SERb1 to ISP-B in some manner and then tell H31 that it
can no longer use S=2001:db8:0:b010::31 by setting its valid
lifetime to zero. The DHCP server could initiate this process by
sending a Reconfigure message to H31 as described in
. Or the DHCP server can assign addresses
with short lifetimes in order to force clients to renew them
often.This approach would prevent H31 from using S=2001:db8:0:b010::31
to reach a host on the Internet. However, it would also prevent
H31 from using S=2001:db8:0:b010::31 to reach H61 at
D=2001:db8:0:6666::61, which is not desirable.Another potential approach is to have the DHCP server monitor the
uplink from SERb1 to ISP-B and control the choice of source address
on H31 by updating its address selection policy table via the
mechanism in . The DHCP server could
initiate this process by sending a Reconfigure message to H31. Note
that requires that Reconfigure messages use
DHCP authentication. DHCP authentication could be avoided by using
short address lifetimes to force clients to send Renew messages to
the server often. If the host does not obtain its IP addresses from
the DHCP server, then it would need to use the Information Refresh
Time option defined in .If the following policy table can be installed on H31 after the
failure of the uplink from SERb1, then the desired routing behavior
should be achieved based on source and destination prefix being
matched with label values.The described solution has a number of significant drawbacks,
some of them already discussed in .
DHCPv6 support is not required for an IPv6 host, and there are
operating systems that do not support DHCPv6. Besides that, it
does not appear that has been widely
implemented on host operating systems.
does not clearly specify this kind
of a dynamic use case in which the address selection policy needs to be
updated quickly in response to the failure of a link. In a large
network, it would present scalability issues as many hosts need
to be reconfigured in a very short period of time.
Updating DHCPv6 server configuration each time an ISP's
uplink changes its state introduces some scalability issues, especially
for mid/large distributed-scale enterprise networks. In addition to that,
the policy table needs to be manually configured by administrators, which makes
that solution prone to human error.
No mechanism exists for making DHCPv6 servers aware of
network topology/routing changes in the network. In general,
having DHCPv6 servers monitor network-related events sounds like a
bad idea as it requires completely new functionality beyond the scope of the
DHCPv6 role.
Controlling Default Source Address Selection with Router AdvertisementsThe same mechanism as discussed in can be used to control the source
address selection in the case of an uplink failure. If a particular
prefix should not be used as a source for any destination, then the
router needs to send an RA with the Preferred Lifetime field for that
prefix set to zero.Let's consider a scenario in which all uplinks are operational, and
H41 receives two different RAs from R3: one from LLA_A with a PIO for
2001:db8:0:a020::/64 and the default router preference set to 11 (low), and
another one from LLA_B with a PIO for 2001:db8:0:a020::/64, the default
router preference set to 01 (high), and a RIO for 2001:db8:0:6666::/64.
As a result, H41 uses 2001:db8:0:b020::41 as a source address for
all Internet traffic, and those packets are sent by SERs to ISP-B. If
SERb1's uplink to ISP-B fails, the desired behavior is that H41 stops
using 2001:db8:0:b020::41 as a source address for all destinations
but H61. To achieve that, R3 should react to SERb1's uplink failure
(which could be detected as the disappearance of scoped route
(S=2001:db8:0:b000::/52, D=::/0)) by withdrawing
itself as a default router. R3 sends a new RA from LLA_B with the Router
Lifetime value set to zero (which means that it should not be used as
the default router). That RA still contains a PIO for 2001:db8:0:b020::/64
(for SLAAC purposes) and a RIO for 2001:db8:0:6666::/64 so that H41 can
reach H61 using LLA_B as a next-hop and 2001:db8:0:b020::41 as a
source address. For all traffic following the default route, LLA_A
will be used as a next-hop and 2001:db8:0:a020::41 as a source
address.If all of the uplinks to ISP-B have failed, source addresses from
ISP-B address space should not be used. In such a failure scenario,
the forwarding table scoped S=2001:db8:0:b000::/52 contains no
entries, indicating that R3 can instruct hosts to stop using source
addresses from 2001:db8:0:b000::/52 by sending RAs containing PIOs
with Preferred Lifetime values set to zero.Controlling Default Source Address Selection with ICMPv6Now we look at how ICMPv6 messages can provide information back
to H31. We assume again that, at the time of the failure, H31 is
sending packets to H501 using (S=2001:db8:0:b010::31,
D=2001:db8:0:5678::501). When the uplink from SERb1 to ISP-B fails,
SERb1 would stop originating its source-prefix-scoped route for the
default destination (S=2001:db8:0:b000::/52, D=::/0) as well as its
unscoped default destination route. With these routes no longer in
the IGP, traffic with (S=2001:db8:0:b010::31,
D=2001:db8:0:5678::501) would end up at SERa based on the unscoped
default destination route being originated by SERa. Since that
traffic has the wrong source address to be forwarded to ISP-A, SERa
would drop it and send a Destination Unreachable message with Code 5
(Source address failed ingress/egress policy) back to H31. H31 would
then know to use another source address for that destination and
would try with (S=2001:db8:0:a010::31, D=2001:db8:0:5678::501). This
would be forwarded to SERa based on the source-prefix-scoped default
destination route still being originated by SERa, and SERa would
forward it to ISP-A. As discussed above, if we are willing to extend
ICMPv6, SERa can even tell H31 what source address it should use to
reach that destination. The expected host behavior has been
discussed in .
Using ICMPv6 would have the same scalability/rate limiting issues
that are discussed in .
An ISP-B uplink failure immediately makes source addresses from 2001:db8:0:b000::/52
unsuitable for external communication and might trigger a large
number of ICMPv6 packets being sent to hosts in that subnet.
Summary of Methods for Controlling Default Source Address Selection on the Failure of an UplinkIt appears that DHCPv6 is not particularly well suited to quickly
changing the source address used by a host when an uplink
fails, which eliminates DHCPv6 from the list of
potential solutions. On the other hand, Router Advertisements
provide a reliable mechanism to dynamically provide hosts with a
list of valid prefixes to use as source addresses as well as to prevent
the use of particular prefixes. While no additional new features are
required to be implemented on hosts, routers need to be able to send
RAs based on the state of scoped forwarding table entries and to
react to network topology changes by sending RAs with particular
parameters set.It seems that the use of ICMPv6 Destination Unreachable messages generated by
the SER (or any SADR-capable) routers, together with the use of RAs
to signal source address selection errors back to hosts, has the
potential to provide a support mechanism. However, scalability issues
may arise in large networks when topology suddenly changes.
Therefore, it is highly desirable that hosts are able to
select the correct source address in the case of uplink failure, with
ICMPv6 being an additional mechanism to signal unexpected failures
back to hosts.The current behaviors of different host operating systems upon receipt of
an ICMPv6 Destination Unreachable message with Code 5 (Source
address failed ingress/egress policy) is not clear to the authors.
Information from implementers, users, and testing would be quite
helpful in evaluating this approach.Selecting Default Source Address upon Failed Uplink RecoveryThe next logical step is to look at the scenario when a failed
uplink on SERb1 to ISP-B comes back up, so the hosts can start using
source addresses belonging to 2001:db8:0:b000::/52 again.Controlling Default Source Address Selection with DHCPv6The mechanism to use DHCPv6 to instruct the hosts (H31 in our
example) to start using prefixes from ISP-B space (e.g.,
S=2001:db8:0:b010::31 for H31) to reach hosts on the Internet is
quite similar to one discussed in and shares the same
drawbacks.Controlling Default Source Address Selection with Router AdvertisementsLet's look at the scenario discussed in .
If the uplink(s) failure caused
the complete withdrawal of prefixes from the 2001:db8:0:b000::/52
address space by setting the Preferred Lifetime value to zero, then the
recovery of the link should just trigger the sending of a new RA with a
non-zero Preferred Lifetime. In another scenario discussed in
, the failure
of the SERb1 uplink to ISP-B
leads to the disappearance of the (S=2001:db8:0:b000::/52,
D=::/0) entry from the forwarding table scoped to
S=2001:db8:0:b000::/52 and, in turn, causes R3 to send RAs
with the Router Lifetime set to zero from LLA_B. The recovery of the SERb1 uplink to ISP-B leads to the reappearance
of the scoped forwarding entry (S=2001:db8:0:b000::/52, D=::/0).
That reappearance acts as a signal for R3 to advertise itself as
a default router for ISP-B address space domain (to send RAs from LLA_B
with non-zero Router Lifetime).
Controlling Default Source Address Selection with ICMPIt looks like ICMPv6 provides a rather limited functionality to
signal back to hosts that particular source addresses have become
valid again. Unless the changes in the uplink specify a particular
(S,D) pair, hosts can keep using the same source address even after
an ISP uplink has come back up. For example, after the uplink from
SERb1 to ISP-B had failed, H31 received ICMPv6 Code 5 message (as
described in ) and
allegedly started using (S=2001:db8:0:a010::31,
D=2001:db8:0:5678::501) to reach H501. Now when the SERb1 uplink
comes back up, the packets with that (S,D) pair are still routed to
SERa1 and sent to the Internet. Therefore, H31 is not informed that
it should stop using 2001:db8:0:a010::31 and start using
2001:db8:0:b010::31 again. Unless SERa has a policy configured to
drop packets (S=2001:db8:0:a010::31, D=2001:db8:0:5678::501) and
send ICMPv6 back if the SERb1 uplink to ISP-B is up, H31 will be unaware
of the network topology change and keep using S=2001:db8:0:a010::31
for Internet destinations, including H51.One of the possible options may be using a scoped route with an
EXCLUSIVE flag as described in .
SERa1 uplink recovery would
cause the (S=2001:db8:0:a000::/52, D=2001:db8:0:1234::/64) route to
reappear in the routing table. In the absence of that, route packets to H101 are sent
to ISP-B (as ISP-A uplink was down) with source addresses from 2001:db8:0:b000::/52.
When the route reappears, SERb1 rejects those packets and sends ICMPv6 back as
discussed in . Practically,
it might lead to scalability issues, which have been already
discussed in
and .Summary of Methods for Controlling Default Source Address Selection upon Failed Uplink RecoveryOnce again, DHCPv6 does not look like a reasonable choice to
manipulate the source address selection process on a host in the case of
network topology changes. Using Router Advertisement provides the
flexible mechanism to dynamically react to network topology changes
(if routers are able to use routing changes as a trigger for sending
out RAs with specific parameters). ICMPv6 could be considered as a
supporting mechanism to signal incorrect source address back to
hosts, but it should not be considered as the only mechanism to control
the address selection in multihomed environments.Selecting Default Source Address When All Uplinks Have FailedOne particular tricky case is a scenario when all uplinks have
failed. In that case, there is no valid source address to be used for
any external destinations when it might be desirable to have
intra-site connectivity.Controlling Default Source Address Selection with DHCPv6From the DHCPv6 perspective, uplinks failure should be treated as two
independent failures and processed as described in
. At this stage, it is quite
obvious that it would result in a quite complicated policy table that
would need to be explicitly configured by administrators and therefore
seems to be impractical.Controlling Default Source Address Selection with Router AdvertisementsAs discussed in , an
uplink failure causes the scoped default entry to disappear from the
scoped forwarding table and triggers RAs with zero Router Lifetimes.
Complete disappearance of all scoped entries for a given source
prefix would cause the prefix to be withdrawn from hosts by setting the
Preferred Lifetime value to zero in the PIO. If all uplinks (SERa, SERb1
and SERb2) fail, hosts either lose their default routers and/or
have no global IPv6 addresses to use as a source. (Note that 'uplink
failure' might mean 'IPv6 connectivity failure with IPv4 still being
reachable', in which case, hosts might fall back to IPv4 if there is
IPv4 connectivity to destinations). As a result, intra-site
connectivity is broken. One of the possible ways to solve it is to
use ULAs.In addition to GUAs, all hosts have ULA addresses assigned, and these addresses are
used for intra-site communication even if there is no GUA assigned
to a host. To avoid accidental leaking of packets with ULA sources,
SADR-capable routers SHOULD have a scoped forwarding table for ULA
source for internal routes but MUST NOT have an entry for D=::/0 in
that table. In the absence of (S=ULA_Prefix; D=::/0), first-hop
routers will send dedicated RAs from a unique link-local source
LLA_ULA with a PIO from ULA address space, a RIO for the ULA prefix, and
Router Lifetime set to zero. The behavior is consistent with the
situation when SERb1 lost the uplink to ISP-B (so there is no
Internet connectivity from 2001:db8:0:b000::/52 sources), but those
sources can be used to reach some specific destinations. In the case
of ULA, there is no Internet connectivity from ULA sources, but they
can be used to reach other ULA destinations. Note that ULA usage
could be particularly useful if all ISPs assign prefixes via
DHCP prefix delegation. In the absence of ULAs, upon the
failure of all uplinks, hosts would lose all
their GUAs upon prefix-lifetime expiration, which again makes
intra-site communication impossible.
It should be noted that Rule 5.5 (prefer a prefix advertised by the selected next-hop)
takes precedence over the Rule 6 (prefer matching label, which ensures that GUA source addresses
are preferred over ULAs for GUA destinations). Therefore if ULAs are used, the network
administrator needs to ensure that, while the site has Internet connectivity, hosts do not
select a router that advertises ULA prefixes as their default router.
Controlling Default Source Address Selection with ICMPv6In the case of the failure of all uplinks, all SERs will drop outgoing IPv6
traffic and respond with ICMPv6 error messages. In a large network
in which many hosts attempt to reach Internet destinations,
the SERs need to generate an ICMPv6 error for every packet they
receive from hosts, which presents the same scalability issues
discussed in .Summary of Methods for Controlling Default Source Address Selection When All Uplinks FailedAgain, combining SADR with Router Advertisements seems to be the
most flexible and scalable way to control the source address
selection on hosts.Summary of Methods for Controlling Default Source Address SelectionThis section summarizes the scenarios and options discussed above.While DHCPv6 allows administrators to manipulate source address
selection policy tables, this method has a number of significant
disadvantages that eliminate DHCPv6 from a list of potential
solutions:
It requires hosts to support DHCPv6 and its extension
.
A DHCPv6 server needs to monitor network state and detect routing
changes.
The use of policy tables requires manual configuration and might be extremely
complicated, especially in the case of a distributed network in which a large
number of remote sites are being served by centralized DHCPv6 servers.
Network topology/routing policy changes could trigger
simultaneous reconfiguration of large number of hosts, which
presents serious scalability issues.
The use of Router Advertisements to influence the source address
selection on hosts seem to be the most reliable, flexible, and scalable
solution. It has the following benefits:
No new (non-standard) functionality needs to be implemented on
hosts (except for RIO support ,
which is not widely implemented at the time of this writing).
No changes in RA format.
Routers can react to routing table changes by sending RAs, which
would minimize the failover time in the case of network topology
changes.
Information required for source address selection is broadcast
to all affected hosts in the case of a topology change event, which
improves the scalability of the solution (compared to DHCPv6
reconfiguration or ICMPv6 error messages).
To fully benefit from the RA-based solution, first-hop routers need
to implement SADR, belong to the SADR domain, and be able to send dedicated RAs per scoped
forwarding table as discussed above, reacting to network changes by
sending new RAs. It should be noted that the proposed solution would
work even if first-hop routers are not SADR-capable but still able
to send individual RAs for each ISP prefix and react to topology changes
as discussed above (e.g., via configuration knobs). The RA-based solution relies heavily on hosts correctly implementing
the default address selection algorithm as defined in .
While the basic, and the most common, multihoming scenario (two or more Internet
uplinks, no 'walled gardens') would work for any host supporting the minimal
implementation of , more complex use cases (such as
'walled garden' and other scenarios when some ISP resources can only be reached from
that ISP address space) require that hosts support Rule 5.5 of the default address
selection algorithm. There is some evidence that not all host OSes
have that rule implemented currently. However, it should be noted that
states that Rule 5.5 should be implemented.
The ICMPv6 Code 5 error message SHOULD be used to complement an RA-based
solution to signal incorrect source address selection back to hosts,
but it SHOULD NOT be considered as the standalone solution.
To prevent scenarios when hosts in multihomed environments incorrectly
identify on-link/off-link destinations, hosts SHOULD treat ICMPv6 Redirects
as discussed in . Solution LimitationsConnections Preservation
The proposed solution is not designed to preserve connection state in the
case of an uplink failure. When all uplinks to an ISP go down, all transport connections
established to/from that ISP address space will be interrupted (unless the transport
protocol has specific multihoming support). That behavior is similar to the scenario
of IPv4 multihoming with NAT when an uplink failure causes all connections to be NATed
to completely different public IPv4 addresses. While it does sound suboptimal, it is
determined by the nature of PA address space: if all uplinks to the particular ISP
have failed, there is no path for the ingress traffic to reach the site, and the egress
traffic is supposed to be dropped by the ingress filters .
The only potential way to overcome this limitation would be to run BGP with all ISPs
and to advertise all site prefixes to all uplinks - a solution that shares all the drawbacks
of using the PI address space without sharing its benefits. Networks willing and capable
of running BGP and using PI are out of scope of this document.
It should be noted that in the case of IPv4 NAT-based multihoming, uplink
recovery could cause connection interruptions as well (unless packet forwarding is
integrated with the tracking of existing NAT sessions so that the egress interface for the existing
sessions is not changed). However, the proposed solution has the benefit of preserving
the existing sessions during and after the restoration of the failed uplink. Unlike the uplink
failure event, which causes all addresses from the affected prefix to be deprecated,
the recovery would just add new, preferred addresses to a host without making any
addresses unavailable. Therefore, connections established to and from those addresses
do not have to be interrupted.
While it's desirable for active connections to survive ISP failover events,
such events affect the reachability of IP addresses assigned to hosts in sites using
PA address space. Unless the transport (or higher-level protocols) is capable
of surviving the host renumbering, the active connections will be broken. The proposed
solution focuses on minimizing the impact of failover on new connections and on
multipath-aware protocols.
Another way to preserve connection state is to use multipath
transport as discussed in .
Other Configuration ParametersDNS ConfigurationIn a multihomed environment, each ISP might provide their own list of
DNS servers. For example, in the topology shown in
, ISP-A might provide
H51 2001:db8:0:5555::51 as a recursive DNS server, while ISP-B might provide
H61 2001:db8:0:6666::61 as a recursive DNS server (RDNSS).
defines IPv6 Router Advertisement options to allow
IPv6 routers to advertise a list of RDNSS addresses
and a DNS Search List (DNSSL) to IPv6 hosts. Using RDNSS together with 'scoped' RAs
as described above would allow a first-hop router (R3 in ) to send
DNS server addresses and search lists provided by each ISP (or the corporate DNS server
addresses if the enterprise is running its own DNS servers. As discussed below, the DNS
split-horizon problem is too hard to solve without running a local DNS server).As discussed in , failure of all ISP uplinks
would cause deprecation of all addresses assigned to a host from the address space
of all ISPs.
If any intra-site IPv6 connectivity is still desirable (most likely to be the case for
any mid/large-scale network), then ULAs should be used as discussed in
.
In such a scenario, the enterprise network should run its own
recursive DNS server(s) and provide its ULA addresses to hosts via
RDNSS in RAs sent for ULA-scoped
forwarding table as described in .There are some scenarios in which the final outcome of the name resolution might be different
depending on:
which DNS server is used;
which source address the client uses to send a DNS query to the server (DNS split horizon).
There is no way currently to instruct a host to use a particular DNS server from the configured servers list
for resolving a particular name. Therefore, it does not seem feasible to solve the problem of DNS server selection
on the host (it should be noted that this particular issue is protocol-agnostic and happens for IPv4 as well). In such
a scenario, it is recommended that the enterprise run its own local recursive DNS server.To influence host source address selection for packets sent to a particular DNS server,
the following requirements must be met:
The host supports RIO as defined in .
The routers send RIOs for routes to DNS server addresses.
For example, if it is desirable that host H31 reaches the ISP-A DNS server H51 2001:db8:0:5555::51
using its source address 2001:db8:0:a010::31, then both R1 and R2 should send RIOs containing the route to 2001:db8:0:5555::51
(or covering route) in their 'scoped' RAs, containing LLA_A as the default router address and the PIO for SLAAC prefix 2001:db8:0:a010::/64.
In that case, host H31 (if it supports Rule 5.5) would select LLA_A as a next-hop and then choose 2001:db8:0:a010::31 as the source address
for packets to the DNS server.
It should be noted that explicitly prohibits using DNS information if the RA Router Lifetime
has expired:
An RDNSS address or a DNSSL domain name MUST be used only as
long as both the RA router Lifetime (advertised by a Router
Advertisement message) and the corresponding option
Lifetime have not expired.
Therefore, hosts might ignore RDNSS information provided
in ULA-scoped RAs, as those RAs would have Router Lifetime values set
to zero. However, , which
obsoletes RFC 6106, has removed that requirement.
As discussed above, the DNS split-horizon problem and the selection of the correct
DNS server in a multihomed environment are not easy problems to solve. The proper solution would
require hosts to support the concept of multiple provisioning domains (PvD, a set of
configuration information associated with a network, ).
Deployment ConsiderationsThe solution described in this document requires certain mechanisms to
be supported by the network infrastructure and hosts. It requires some
routers in the enterprise site to support some form of
SADR. It also requires hosts to be able to learn when the uplink to an ISP changes
its state so that the hosts can use appropriate source addresses. Ongoing work to create
mechanisms to accomplish this are discussed in this document, but they
are still works in progress.
Deploying SADR Domain
The proposed solution does not prescribe particular details regarding deploying an SADR domain within a multihomed enterprise network. However the following guidelines could be applied:
The SADR domain is usually limited by the multihomed site border.
The minimal deployable scenario requires enabling SADR on all SERs and including them into a single SADR domain.
As discussed in , extending the connected SADR domain beyond the SERs to the first-hop routers can produce more efficient forwarding paths and allow the network to fully benefit from SADR. It would also simplify the operation of the SADR domain.
During the incremental SADR domain expansion from the SERs down towards first-hop routers, it's important to ensure that, at any given moment, all SADR-capable routers within the domain are logically connected (see ).
Hosts-Related Considerations
The solution discussed in this document relies on the default address
selection algorithm, Rule 5.5 .
While considers this rule as optional, the
more recent states that
"A host SHOULD select default routers for each prefix it is
assigned an address in." It also recommends
that hosts should implement Rule 5.5. of .
Therefore, while hosts compliant with RFC 8028 already have a mechanism to learn
about state changes to ISP uplinks and to select the source addresses
accordingly, many hosts do not support such a mechanism yet.
It should be noted that a multihomed enterprise network utilizing
multiple ISP prefixes can be considered as a typical multiple
provisioning domain (mPvD) scenario, as described in .
This document defines a way for the network to provide
the PvD information to hosts indirectly, using the existing mechanisms.
At the same time, takes one step further
and describes a comprehensive mechanism for hosts to discover the whole
set of configuration information associated with different PvDs/ISPs.
complements this
document in terms of enabling hosts to learn about ISP uplink
states and to select the corresponding source addresses.
Other SolutionsShim6The Shim6 protocol , specified by the Shim6
working group, allows a host at a multihomed site to
communicate with an external host and to exchange information about
possible source and destination address pairs that they can use to
communicate. The Shim6 working group also specified the REAchability Protocol
(REAP) to detect failures in the path between working
address pairs and to find new working address pairs. A fundamental
requirement for Shim6 is that both internal and external hosts need to
support Shim6. That is, both the host internal to the multihomed site
and the host external to the multihomed site need to support Shim6 in
order for there to be any benefit for the internal host to run Shim6.
The Shim6 protocol specification was published in 2009, but it has not
been widely implemented. Therefore Shim6 is not considered as a viable solution
for enterprise multihoming.IPv6-to-IPv6 Network Prefix TranslationIPv6-to-IPv6 Network Prefix Translation (NPTv6) is not the focus of this document.
NPTv6 suffers from the same fundamental issue as any other approaches to address
translation: it breaks end-to-end connectivity. Therefore
NPTv6 is not considered as a desirable solution, and this document intentionally
focuses on solving the enterprise multihoming problem without any form of address translation.
With increasing interest and ongoing work in bringing path awareness to
transport- and application-layer protocols, hosts might be able to
determine the properties of the various network paths and choose among
the paths that are available to them. As selecting the correct source address is one
of the possible mechanisms that path-aware hosts may utilize, address
translation negatively affects hosts' path-awareness, which makes NTPv6
an even more undesirable solution.
Multipath Transport
Using multipath transport (such as Multipath TCP (MPTCP)
or multipath capabilities in QUIC) might solve the problems discussed in
since a multipath
transport would allow hosts to use multiple
source addresses for a single connection and to switch between those source
addresses when a particular address becomes unavailable or a new address
is assigned to the host interface. Therefore, if all hosts in the
enterprise network use only multipath transport for all
connections, the signaling solution described in
might not be needed (it should be noted
that Source Address Dependent Routing would still be required to
deliver packets to the correct uplinks).
At the time this document was written, multipath transport alone
could not be considered a solution for the problem of selecting the source
address in a multihomed environment. There are a significant number of
hosts that do not use multipath transport currently, and it seems
unlikely that the situation will change in the foreseeable
future (even if new releases of operating systems support multipath protocols,
there will be a long tail of legacy hosts). The solution for enterprise multihoming needs to work for the
least common denominator: hosts without multipath transport support. In
addition, not all protocols are using multipath transport. While
multipath transport would complement the solution described in , it could not be considered as a sole
solution to the problem of source address selection in multihomed
environments.
On the other hand, PA-based multihoming could provide additional
benefits to multipath protocols, should those protocols be deployed in the network. Multipath
protocols could leverage source address selection to achieve maximum path diversity (and potentially improved performance).
Therefore, the deployment of multipath protocols should not be considered
as an alternative to the approach proposed in this document. Instead, both solutions complement
each other, so deploying multipath protocols in a PA-based multihomed network proves mutually beneficial.
IANA ConsiderationsThis document has no IANA actions.Security Considerations discusses a mechanism for
controlling source address selection on hosts using ICMPv6 messages.
Using ICMPv6 to influence source address selection allows an attacker to exhaust the list of candidate source
addresses on the host by sending spoofed ICMPv6 Code 5 for all
prefixes known on the network (therefore preventing a victim from
establishing communication with the destination host).
Another possible attack vector is using ICMPv6 Destination Unreachable
Messages with Code 5 to steer the egress
traffic towards the particular ISP, so the attacker can benefit from
their ability doing traffic sniffing/interception
in that ISP network.
To prevent those attacks, hosts SHOULD verify that the original packet
header included in the ICMPv6 error message was actually sent by the host (to ensure that the
ICMPv6 message was triggered by a packet sent by the host).
As ICMPv6 Destination Unreachable Messages with Code 5 could be originated by any
SADR-capable router within the domain (or even come from the Internet), the Generalized TTL
Security Mechanism (GTSM) cannot be applied.
Filtering such ICMPv6 messages at the site border cannot be recommended as it would break
the legitimate end-to-end error signaling mechanism for which ICMPv6 was designed.
The security considerations of using stateless address autoconfiguration are discussed in .
ReferencesNormative ReferencesNetwork Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address SpoofingThis paper discusses a simple, effective, and straightforward method for using ingress traffic filtering to prohibit DoS (Denial of Service) attacks which use forged IP addresses to be propagated from 'behind' an Internet Service Provider's (ISP) aggregation point. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Address Allocation for Private InternetsThis document describes address allocation for private internets. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Key words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Default Router Preferences and More-Specific RoutesThis document describes an optional extension to Router Advertisement messages for communicating default router preferences and more-specific routes from routers to hosts. This improves the ability of hosts to pick an appropriate router, especially when the host is multi-homed and the routers are on different links. The preference values and specific routes advertised to hosts require administrative configuration; they are not automatically derived from routing tables. [STANDARDS-TRACK]Unique Local IPv6 Unicast AddressesThis document defines an IPv6 unicast address format that is globally unique and is intended for local communications, usually inside of a site. These addresses are not expected to be routable on the global Internet. [STANDARDS-TRACK]IP Version 6 Addressing ArchitectureThis specification defines the addressing architecture of the IP Version 6 (IPv6) protocol. The document includes the IPv6 addressing model, text representations of IPv6 addresses, definition of IPv6 unicast addresses, anycast addresses, and multicast addresses, and an IPv6 node's required addresses.This document obsoletes RFC 3513, "IP Version 6 Addressing Architecture". [STANDARDS-TRACK]Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) SpecificationThis document describes the format of a set of control messages used in ICMPv6 (Internet Control Message Protocol). ICMPv6 is the Internet Control Message Protocol for Internet Protocol version 6 (IPv6). [STANDARDS-TRACK]Neighbor Discovery for IP version 6 (IPv6)This document specifies the Neighbor Discovery protocol for IP Version 6. IPv6 nodes on the same link use Neighbor Discovery to discover each other's presence, to determine each other's link-layer addresses, to find routers, and to maintain reachability information about the paths to active neighbors. [STANDARDS-TRACK]IPv6 Stateless Address AutoconfigurationThis document specifies the steps a host takes in deciding how to autoconfigure its interfaces in IP version 6. The autoconfiguration process includes generating a link-local address, generating global addresses via stateless address autoconfiguration, and the Duplicate Address Detection procedure to verify the uniqueness of the addresses on a link. [STANDARDS-TRACK]IPv6 Router Advertisement Options for DNS ConfigurationThis document specifies IPv6 Router Advertisement options to allow IPv6 routers to advertise a list of DNS recursive server addresses and a DNS Search List to IPv6 hosts. [STANDARDS-TRACK]IPv6-to-IPv6 Network Prefix TranslationThis document describes a stateless, transport-agnostic IPv6-to-IPv6 Network Prefix Translation (NPTv6) function that provides the address-independence benefit associated with IPv4-to-IPv4 NAT (NAPT44) and provides a 1:1 relationship between addresses in the "inside" and "outside" prefixes, preserving end-to-end reachability at the network layer. This document defines an Experimental Protocol for the Internet community.Default Address Selection for Internet Protocol Version 6 (IPv6)This document describes two algorithms, one for source address selection and one for destination address selection. The algorithms specify default behavior for all Internet Protocol version 6 (IPv6) implementations. They do not override choices made by applications or upper-layer protocols, nor do they preclude the development of more advanced mechanisms for address selection. The two algorithms share a common context, including an optional mechanism for allowing administrators to provide policy that can override the default behavior. In dual-stack implementations, the destination address selection algorithm can consider both IPv4 and IPv6 addresses -- depending on the available source addresses, the algorithm might prefer IPv6 addresses over IPv4 addresses, or vice versa.Default address selection as defined in this specification applies to all IPv6 nodes, including both hosts and routers. This document obsoletes RFC 3484. [STANDARDS-TRACK]Distributing Address Selection Policy Using DHCPv6RFC 6724 defines default address selection mechanisms for IPv6 that allow nodes to select an appropriate address when faced with multiple source and/or destination addresses to choose between. RFC 6724 allows for the future definition of methods to administratively configure the address selection policy information. This document defines a new DHCPv6 option for such configuration, allowing a site administrator to distribute address selection policy overriding the default address selection parameters and policy table, and thus allowing the administrator to control the address selection behavior of nodes in their site.Multiple Provisioning Domain ArchitectureThis document is a product of the work of the Multiple Interfaces Architecture Design team. It outlines a solution framework for some of the issues experienced by nodes that can be attached to multiple networks simultaneously. The framework defines the concept of a Provisioning Domain (PvD), which is a consistent set of network configuration information. PvD-aware nodes learn PvD-specific information from the networks they are attached to and/or other sources. PvDs are used to enable separation and configuration consistency in the presence of multiple concurrent connections.First-Hop Router Selection by Hosts in a Multi-Prefix NetworkThis document describes expected IPv6 host behavior in a scenario that has more than one prefix, each allocated by an upstream network that is assumed to implement BCP 38 ingress filtering, when the host has multiple routers to choose from. It also applies to other scenarios such as the usage of stateful firewalls that effectively act as address-based filters. Host behavior in choosing a first-hop router may interact with source address selection in a given implementation. However, the selection of the source address for a packet is done before the first-hop router for that packet is chosen. Given that the network or host is, or appears to be, multihomed with multiple provider-allocated addresses, that the host has elected to use a source address in a given prefix, and that some but not all neighboring routers are advertising that prefix in their Router Advertisement Prefix Information Options, this document specifies to which router a host should present its transmission. It updates RFC 4861.IPv6 Router Advertisement Options for DNS ConfigurationThis document specifies IPv6 Router Advertisement (RA) options (called "DNS RA options") to allow IPv6 routers to advertise a list of DNS Recursive Server Addresses and a DNS Search List to IPv6 hosts.This document, which obsoletes RFC 6106, defines a higher default value of the lifetime of the DNS RA options to reduce the likelihood of expiry of the options on links with a relatively high rate of packet loss.Ambiguity of Uppercase vs Lowercase in RFC 2119 Key WordsRFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.Dynamic Host Configuration Protocol for IPv6 (DHCPv6)This document describes the Dynamic Host Configuration Protocol for IPv6 (DHCPv6): an extensible mechanism for configuring nodes with network configuration parameters, IP addresses, and prefixes. Parameters can be provided statelessly, or in combination with stateful assignment of one or more IPv6 addresses and/or IPv6 prefixes. DHCPv6 can operate either in place of or in addition to stateless address autoconfiguration (SLAAC).This document updates the text from RFC 3315 (the original DHCPv6 specification) and incorporates prefix delegation (RFC 3633), stateless DHCPv6 (RFC 3736), an option to specify an upper bound for how long a client should wait before refreshing information (RFC 4242), a mechanism for throttling DHCPv6 clients when DHCPv6 service is not available (RFC 7083), and relay agent handling of unknown messages (RFC 7283). In addition, this document clarifies the interactions between models of operation (RFC 7550). As such, this document obsoletes RFC 3315, RFC 3633, RFC 3736, RFC 4242, RFC 7083, RFC 7283, and RFC 7550.Informative ReferencesDestination/Source RoutingThis note specifies using packets' source addresses in route lookups as additional qualifier to be used in hop-by-hop routing decisions. This applies to IPv6 [RFC2460] in general with specific considerations for routing protocol left for separate documents. There is nothing precluding similar operation in IPv4, but this is not in scope of this document. Note that destination/source routing, source/destination routing, SADR, source-specific routing, source-sensitive routing, S/D routing and D/S routing are all used synonymously.Work in ProgressDiscovering Provisioning Domain Names and DataProvisioning Domains (PvDs) are defined as consistent sets of network configuration information. This allows hosts to manage connections to multiple networks and interfaces simultaneously, such as when a home router provides connectivity through both a broadband and cellular network provider. This document defines a mechanism for explicitly identifying PvDs through a Router Advertisement (RA) option. This RA option announces a PvD identifier, which hosts can compare to differentiate between PvDs. The option can directly carry some information about a PvD and can optionally point to additional PvD information that can be retrieved using HTTP over TLS.Work in ProgressIP Network Address Translator (NAT) Terminology and ConsiderationsThis document attempts to describe the operation of NAT devices and the associated considerations in general, and to define the terminology used to identify various flavors of NAT. This memo provides information for the Internet community.Ingress Filtering for Multihomed NetworksBCP 38, RFC 2827, is designed to limit the impact of distributed denial of service attacks, by denying traffic with spoofed addresses access to the network, and to help ensure that traffic is traceable to its correct source network. As a side effect of protecting the Internet against such attacks, the network implementing the solution also protects itself from this and other attacks, such as spoofed management access to networking equipment. There are cases when this may create problems, e.g., with multihoming. This document describes the current ingress filtering operational mechanisms, examines generic issues related to ingress filtering, and delves into the effects on multihoming in particular. This memo updates RFC 2827. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Privacy Extensions for Stateless Address Autoconfiguration in IPv6Nodes use IPv6 stateless address autoconfiguration to generate addresses using a combination of locally available information and information advertised by routers. Addresses are formed by combining network prefixes with an interface identifier. On an interface that contains an embedded IEEE Identifier, the interface identifier is typically derived from it. On other interface types, the interface identifier is generated through other means, for example, via random number generation. This document describes an extension to IPv6 stateless address autoconfiguration for interfaces whose interface identifier is derived from an IEEE identifier. Use of the extension causes nodes to generate global scope addresses from interface identifiers that change over time, even in cases where the interface contains an embedded IEEE identifier. Changing the interface identifier (and the global scope addresses generated from it) over time makes it more difficult for eavesdroppers and other information collectors to identify when different addresses used in different transactions actually correspond to the same node. [STANDARDS-TRACK]The Generalized TTL Security Mechanism (GTSM)The use of a packet's Time to Live (TTL) (IPv4) or Hop Limit (IPv6) to verify whether the packet was originated by an adjacent node on a connected link has been used in many recent protocols. This document generalizes this technique. This document obsoletes Experimental RFC 3682. [STANDARDS-TRACK]Shim6: Level 3 Multihoming Shim Protocol for IPv6This document defines the Shim6 protocol, a layer 3 shim for providing locator agility below the transport protocols, so that multihoming can be provided for IPv6 with failover and load-sharing properties, without assuming that a multihomed site will have a provider-independent IPv6 address prefix announced in the global IPv6 routing table. The hosts in a site that has multiple provider- allocated IPv6 address prefixes will use the Shim6 protocol specified in this document to set up state with peer hosts so that the state can later be used to failover to a different locator pair, should the original one stop working. [STANDARDS-TRACK]Failure Detection and Locator Pair Exploration Protocol for IPv6 MultihomingThis document specifies how the level 3 multihoming Shim6 protocol (Shim6) detects failures between two communicating nodes. It also specifies an exploration protocol for switching to another pair of interfaces and/or addresses between the same nodes if a failure occurs and an operational pair can be found. [STANDARDS-TRACK]TCP Extensions for Multipath Operation with Multiple AddressesTCP/IP communication is currently restricted to a single path per connection, yet multiple paths often exist between peers. The simultaneous use of these multiple paths for a TCP/IP session would improve resource usage within the network and, thus, improve user experience through higher throughput and improved resilience to network failure.Multipath TCP provides the ability to simultaneously use multiple paths between peers. This document presents a set of extensions to traditional TCP to support multipath operation. The protocol offers the same type of service to applications as TCP (i.e., reliable bytestream), and it provides the components necessary to establish and use multiple TCP flows across potentially disjoint paths. This document defines an Experimental Protocol for the Internet community.IPv6 Support for Generic Routing Encapsulation (GRE)Generic Routing Encapsulation (GRE) can be used to carry any network- layer payload protocol over any network-layer delivery protocol. Currently, GRE procedures are specified for IPv4, used as either the payload or delivery protocol. However, GRE procedures are not specified for IPv6.This document specifies GRE procedures for IPv6, used as either the payload or delivery protocol.IANA Considerations for IPv6 Neighbor Discovery Prefix Information Option FlagsThe Prefix Information Option (PIO) in the IPv6 Neighbor Discovery Router Advertisement message defines an 8-bit flag field; this field has two flags defined, and the remaining 6 bits are reserved (Reserved1). RFC 6275 defines a flag from this field without creating an IANA registry or updating RFC 4861. The purpose of this document is to create an IANA registry for the PIO flags. This document updates RFC 4861.IPv6 Node RequirementsThis document defines requirements for IPv6 nodes. It is expected that IPv6 will be deployed in a wide range of devices and situations. Specifying the requirements for IPv6 nodes allows IPv6 to function well and interoperate in a large number of situations and deployments.This document obsoletes RFC 6434, and in turn RFC 4294.Source Address Dependent Route Information Option for Router AdvertisementsThis document defines the Source Address Dependent Route Information option for Router Advertisements, enabling source address dependent routes to be installed in hosts by neighboring routers. It also adds a new flag to the existing Route Information option for backward compatibility purposes.Work in ProgressAcknowledgementsThe original outline was suggested by .
The authors would like to thank the following people (in alphabetical
order) for their review and feedback:
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Authors' AddressesSanta Barbara93117CaliforniaUnited States of AmericaFredBaker.IETF@gmail.comJuniper NetworksSunnyvale94089CaliforniaUnited States of Americacbowers@juniper.netGoogle1 Darling Island RdPyrmontNSW2009Australiafurry@google.com