Internet Engineering Task Force (IETF) F. Baker
Request for Comments: 8028
Updates: 4861 B. Carpenter
Category: Standards Track Univ. of Auckland
ISSN: 2070-1721 November 2016 First-Hop Router Selection by Hosts in a Multi-Prefix Network
This 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
Status of This Memo
This is an Internet Standards Track document.
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). Further information on
Internet Standards is available in 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
1. Introduction and Applicability
This document describes the expected behavior of an IPv6 [RFC2460]
host in a network that has more than one prefix, each allocated by an
upstream network that is assumed to implement BCP 38 [RFC2827]
ingress filtering, and in which the host is presented with a choice
of routers. It expects that the network will implement some form of
egress routing, so that packets sent to a host outside the local
network from a given ISP's prefix will go to that ISP. If the packet
is sent to the wrong egress, it is liable to be discarded by the BCP
38 filter. However, the mechanics of egress routing once the packet
leaves the host are out of scope. The question here is how the host
interacts with that network.
Various aspects of this issue, and possible solution approaches, are
discussed in "IPv6 Multihoming without Network Address Translation"
BCP 38 filtering by ISPs is not the only scenario where such behavior
is valuable. Implementations that combine existing recommendations,
such as [RFC6092] and [RFC7084] can also result in such filtering.
Another case is when the connections to the upstream networks include
stateful firewalls, such that return packets in a stream will be
discarded if they do not return via the firewall that created the
state for the outgoing packets. A similar cause of such discards is
unicast reverse path forwarding (uRPF) [RFC3704].
In this document, the term "filter" is used for simplicity to cover
all such cases. In any case, one cannot assume that the host is
aware whether an ingress filter, a stateful firewall, or any other
type of filter is in place. Therefore, the only known consistent
solution is to implement the features defined in this document.
Note that, apart from ensuring that a message with a given source
address is given to a first-hop router that appears to know about the
prefix in question, this specification is consistent with [RFC4861].
Nevertheless, implementers of Sections 6.2.3, 6.3.4, 6.3.6, and 8.1
of RFC 4861 should extend their implementations accordingly. This
specification is fully consistent with [RFC6724] and depends on
support for its Rule 5.5 (see Section 3.3). Hosts that do not
support these features may fail to communicate in the presence of
filters as described above.
1.1. Host Model
It could be argued that the proposal in this document, which is to
send messages using a source address in a given prefix to the router
that advertised the prefix in its Router Advertisement (RA), is a
form of the Strong End System (ES, e.g., Host) model, discussed in
Section 184.108.40.206 of [RFC1122]. In short, [RFC1122] identifies two
basic models. First, the "strong host" model describes the host as a
set of hosts in one chassis, each of which uses a single address on a
single interface and always both sends and receives on that
interface. Alternatively, the "weak host" model treats the host as
one system with zero or more addresses on every interface and is
capable of using any interface for any communication. As noted
there, neither model is completely satisfactory. For example, a host
with a link-local-only interface and a default route pointing to that
interface will necessarily send packets using that interface but with
a source address derived from some other interface, and will
therefore be a de facto weak host. If the router upstream from such
a host implements BCP 38 Ingress Filtering [RFC2827], such as by
implementing uRPF on each interface, the router might prevent
communication by weak hosts.
| MIF Router +---/--- Other interfaces
| | Two interfaces with subnets
| | from a common prefix
| MIF Host |
Figure 1: Hypothetical MIF Interconnection
The proposal also differs slightly from the language in [RFC1122] for
the Strong Host model. The proposal is that the packet will go to a
router that advertised a given prefix but that does not specify what
interface that might happen on. Hence, if the router is a multi-
interface (MIF) router and it is using a common prefix spanning two
or more LANs shared by the host (as in Figure 1), the host might use
either of those LANs, according to this proposal. The Strong Host
model is not stated in those terms, but in terms of the interface
used. A strong host would treat such an MIF router as two separate
routers when obeying the rules from RFC 1122 as they apply in the
(A) A host MUST silently discard an incoming datagram whose
destination address does not correspond to the physical
interface through which it is received.
(B) A host MUST restrict itself to sending (non-source-routed) IP
datagrams only through the physical interface that corresponds
to the IP source address of the datagrams.
However, when comparing the presumptive route lookup mechanisms in
each model, this proposal is indeed most similar to the Strong Host
model, as is any source/destination routing paradigm.
Strong: route (src IP addr, dest IP addr, TOS) -> gateway
Weak: route (dest IP addr, TOS) -> gateway, interface
In the hypothetical MIF model suggested in Figure 1, the address
fails to identify a single interface, but it does identify a single
1.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Sending Context Expected by the Host
2.1. Expectations the Host Has of the Network
A host receives prefixes in a Router Advertisement [RFC4861], which
goes on to identify whether they are usable by Stateless Address
Autoconfiguration (SLAAC) [RFC4862] with any type of interface
identifier [RFC4941] [RFC7217]. When no prefixes are usable for
SLAAC, the Router Advertisement would normally signal the
availability of DHCPv6 [RFC3315] and the host would use it to
configure its addresses. In the latter case (or if both SLAAC and
DHCPv6 are used on the same link for some reason), the configured
addresses generally match one of the prefixes advertised in a Router
Advertisement that are supposed to be on-link for that link.
The simplest multihomed network implementation in which a host makes
choices among routers might be a LAN with one or more hosts on it and
two or more routers, one for each upstream network, or a host that is
served by disjoint networks on separate interfaces. In such a
network, especially the latter, there is not necessarily a routing
protocol, and the two routers may not even know that the other is a
router as opposed to a host, or may be configured to ignore its
presence. One might expect that the routers may or may not receive
each other's RAs and form an address in the other router's prefix
(which is not per [RFC4862], but is implemented by some stub router
implementations). However, all hosts in such a network might be
expected to create an address in each prefix so advertised.
+---------+ +---------+ +---------+ +---------+
| ISP | | ISP | | ISP | | ISP |
+----+----+ +----+----+ +----+----+ +----+----+
| | | |
| | | |
+----+----+ +----+----+ +----+----+ +----+----+
| Router | | Router | | Router | | Router |
+----+----+ +----+----+ +----+----+ +----+----+
| | | |
+------+------+ | +--------+ |
| +--+ Host +--+
| Host |
Common LAN Case Disjoint LAN Case
(Multihomed Network) (Multihomed Host)
Figure 2: Two Simple Networks
If there is no routing protocol among those routers, there is no
mechanism by which packets can be deterministically forwarded between
the routers (as described in BCP 84 [RFC3704]) in order to avoid
filters. Even if there was routing, it would result in an indirect
route, rather than a direct route originating with the host; this is
not "wrong", but can be inefficient. Therefore, the host would do
well to select the appropriate router itself.
Since the host derives fundamental default routing information from
the Router Advertisement, this implies that, in any network with
hosts using multiple prefixes, each prefix SHOULD be advertised via a
Prefix Information Option (PIO) [RFC4861] by one of the attached
routers, even if addresses are being assigned using DHCPv6. A router
that advertises a prefix indicates that it is able to appropriately
route packets with source addresses within that prefix, regardless of
the setting of the L and A flags in the PIO.
In some circumstances, both L and A might be zero. If SLAAC is not
wanted (A=0) and there is no reason to announce an on-link prefix
(L=0), a PIO SHOULD be sent to inform hosts that they should use the
router in question as the first hop for packets with source addresses
in the PIO prefix. An example case is the MIF router in Figure 1,
which could send PIOs with A=L=0 for the common prefix. Although
this does not violate the existing standard [RFC4861], such a PIO has
not previously been common, and it is possible that existing host
implementations simply ignore such a PIO or that existing router
implementations are not capable of sending such a PIO. Newer
implementations that support this mechanism should be updated
o A host SHOULD NOT ignore a PIO simply because both L and A flags
are cleared (extending Section 6.3.4 of [RFC4861]).
o A router SHOULD be able to send such a PIO (extending
Section 6.2.3 of [RFC4861]).
2.2. Expectations of Multihomed Networks
Networking equipment needs to support source/destination routing for
at least some of the routes in the Forwarding Information Base (FIB),
such as default egress routes differentiated by source prefix.
Installation of source/destination routes in the FIB might be
accomplished using static routes, Software-Defined Networking (SDN)
technologies, or dynamic routing protocols.
3. Reasonable Expectations of the Host
3.1. Interpreting Router Advertisements
As described in [RFC4191] and [RFC4861], a Router Advertisement may
contain zero or more Prefix Information Options (PIOs) or zero or
more Route Information Options (RIOs). In their original intent,
these indicate general information to a host: "the router whose
address is found in the source address field of this packet is one of
your default routers", "you might create an address in this prefix",
or "this router would be a good place to send traffic directed to a
given destination prefix". In a multi-prefix network with multiple
exits, the host's characterization of each default router SHOULD
include the prefixes it has announced (extending Section 6.3.4 of
[RFC4861]). In other words, the PIO is reinterpreted to also imply
that the advertising router would be a reasonable first hop for any
packet using a source address in any advertised prefix, regardless of
Default Router Preference.
( ISP A ) - + Bob-A +--+ +-----+
+-------+ / +---------+ +--+ |
| | / | | |
| Alice +--/--( The Internet ) | Bob |
| | \ | | |
+-------+ \ +---------+ +--+ |
( ISP B ) - + Bob-B +--+ +-----+
Figure 3: PIOs, RIOs, and Default Routes
The implications bear consideration. Imagine, Figure 3, that hosts
Alice and Bob are in communication. Bob's network consists of at
least Bob (the computer), two routers (Bob-A and Bob-B), and the
links between them; it may be much larger, for example, a campus or
corporate network. Bob's network is therefore multihomed, and Bob's
first-hop routers are Bob-A (to the upstream ISP A advertising prefix
PA) and Bob-B (to the upstream network B and advertising prefix PB).
We assume that Bob is not applying Rule 5.5 of [RFC6724]. If Bob is
responding to a message from Alice, his choice of source address is
forced to be the address Alice used as a destination (which we may
presume to have been in prefix PA). Hence, Bob either created or was
assigned an address in PA, and can only reasonably send traffic using
it to Bob-A as a first-hop router. If there are several routers in
Bob's network advertising the prefix PA (referred to as "Bob-Ax"
routers), then Bob should choose its first-hop router only from among
those routers. From among the multiple Bob-Ax routers, Bob should
choose the first-hop router based on the criteria specified in
Section 3 of [RFC4191]. If none of the Bob-Ax routers has advertised
an RA with a non-zero Router Lifetime or an RIO with a non-zero Route
Lifetime that includes Alice, but router Bob-B has, it is irrelevant.
Bob is using the address allocated in PA and courts a BCP 38 discard
if he doesn't send the packet to Bob-A.
In the special case that Bob is initiating the conversation, an RIO
might, however, influence source address choice. Bob could
presumably use any address allocated to him, in this case, his
address in PA or PB. If Bob-B has advertised an RIO for Alice's
prefix and Bob-A has not, Bob MAY take that fact into account in
address selection -- choosing an address that would allow him to make
use of the RIO.
3.2. Default Router Selection
Default Router Selection (Section 6.3.6 of [RFC4861]) is extended as
follows: A host SHOULD select default routers for each prefix it is
assigned an address in. Routers that have advertised the prefix in
their Router Advertisement message SHOULD be preferred over routers
that do not advertise the prefix, regardless of Default Router
Preference. Note that this document does not change the way in which
default router preferences are communicated [RFC4191].
If no router has advertised the prefix in an RA, normal routing
metrics will apply. An example is a host connected to the Internet
via one router, and at the same time connected by a VPN to a private
domain that is also connected to the global Internet.
As a result of this, when a host sends a packet using a source
address in one of those prefixes and has no history directing it
otherwise, it SHOULD send it to the indicated default router. In the
"simplest" network described in Section 2.1, that would get it to the
only router that is directly capable of getting it to the right ISP.
This will also apply in more complex networks, even when more than
one physical or virtual interface is involved.
In more complex cases, wherein routers advertise RAs for multiple
prefixes whether or not they have direct or isolated upstream
connectivity, the host is dependent on the routing system already.
If the host gives the packet to a router advertising its source
prefix, it should be able to depend on the router to do the right
3.3. Source Address Selection
There is an interaction with Default Address Selection [RFC6724]. A
host following the recommendation in the previous section will store
information about which next hops advertised which prefixes. Rule
5.5 of RFC 6724 states that the source address used to send to a
given destination address should, if possible, be chosen from a
prefix known to be advertised by the next-hop router for that
destination. Therefore, this selection rule SHOULD be implemented in
a host following the recommendation in the previous section.
There is potential for adverse interaction with any off-link Redirect
(Redirect for a destination that is not on-link) message sent by a
router in accordance with Section 8 of [RFC4861]. Hosts SHOULD apply
off-link redirects only for the specific pair of source and
destination addresses concerned, so the host's Destination Cache
might need to contain appropriate source-specific entries. This
extends the validity check specified in Section 8.1 of [RFC4861].
Some modern hosts maintain history, in terms of what has previously
worked or not worked for a given address or prefix and in some cases
the effective window and Maximum Segment Size (MSS) values for TCP or
other protocols. This might include a next-hop address for use when
a packet is sent to the indicated address.
When such a host makes a successful exchange with a remote
destination using a particular address pair, and the host has
previously received a PIO that matches the source address, then the
host SHOULD include the prefix in such history, whatever the setting
of the L and A flags in the PIO. On subsequent attempts to
communicate with that destination, if it has an address in that
prefix at that time, a host MAY use an address in the remembered
prefix for the session.
4. Residual Issues
Consider a network where routers on a link run a routing protocol and
are configured with the same information. Thus, on each link, all
routers advertise all prefixes on that link. The assumption that
packets will be forwarded to the appropriate egress by the local
routing system might cause at least one extra hop in the local
network (from the host to the wrong router, and from there to another
router on the same link).
In a slightly more complex situation such as the disjoint LAN case of
Figure 2, for example, a home plus corporate home-office
configuration, the two upstream routers might be on different LANs
and therefore different subnets (e.g., the host is itself
multihomed). In that case, there is no way for the "wrong" router to
detect the existence of the "right" router, or to route to it.
In such a case, it is particularly important that hosts take the
responsibility to memorize and select the best first hop as described
in Section 3.
5. IANA Considerations
This document does not request any registry actions.
6. Security Considerations
This document is intended to avoid connectivity issues in the
presence of BCP 38 ingress filters or stateful firewalls combined
with multihoming. It does not, in itself, create any new security or
privacy exposures. However, since the solution is designed to ensure
that routing occurs correctly in situations where it previously
failed, this might result in unexpected exposure of networks that
were previously unreachable.
There might be a small privacy improvement: with the current
practice, a multihomed host that sends packets with the wrong address
to an upstream router or network discloses the prefix of one upstream
to the other upstream network. This practice reduces the probability
of that occurrence.
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <http://www.rfc-editor.org/info/rfc2460>.
[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191,
November 2005, <http://www.rfc-editor.org/info/rfc4191>.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
[RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
[RFC7217] Gont, F., "A Method for Generating Semantically Opaque
Interface Identifiers with IPv6 Stateless Address
Autoconfiguration (SLAAC)", RFC 7217,
DOI 10.17487/RFC7217, April 2014,
Comments were received from Jinmei Tatuya and Ole Troan, who have
suggested important text, plus Mikael Abrahamsson, Steven Barth,
Carlos Bernardos Cano, Chris Bowers, Zhen Cao, Juliusz Chroboczek,
Toerless Eckert, David Farmer, Bob Hinden, Ben Laurie, Dusan Mudric,
Tadahisa Okimoto, Pierre Pfister, Behcet Sarikaya, Mark Smith, and
Santa Barbara, California 93117
United States of America
Department of Computer Science
University of Auckland