Internet DRAFT - draft-baker-6man-multi-homed-host
draft-baker-6man-multi-homed-host
IPv6 Maintenance F. Baker
Internet-Draft Cisco Systems
Updates: 4861 (if approved) B. Carpenter
Intended status: Standards Track Univ. of Auckland
Expires: March 6, 2016 September 3, 2015
Host routing in a multi-prefix network
draft-baker-6man-multi-homed-host-03
Abstract
This note describes expected IPv6 host behavior in a network that has
more than one prefix, each allocated by an upstream network that
implements 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.
This host behavior may interact with source address selection in a
given implementation, but logically follows it. 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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 6, 2016.
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Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction and Applicability . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Sending context expected by the host . . . . . . . . . . . . 3
2.1. Expectations the host has of the network . . . . . . . . 3
2.2. Expectations of multihomed networks . . . . . . . . . . . 5
3. Reasonable expectations of the host . . . . . . . . . . . . . 5
3.1. Default Router Selection . . . . . . . . . . . . . . . . 5
3.2. Source Address Selection . . . . . . . . . . . . . . . . 5
3.3. Redirects . . . . . . . . . . . . . . . . . . . . . . . . 6
3.4. History . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Residual issues . . . . . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction and Applicability
This note 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 implements 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
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out of scope. The question here is how the host interacts with that
network.
BCP 38 filtering by ISPs is not the only scenario where such behavior
is valuable. The combination of existing recommendations for home
gateways [RFC6092] [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 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 the host to be aware
whether an ingress filter, a stateful firewall, or any other type of
filter is in place. Therefore, the only safe 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 5.2, 6.2.3, 6.3.4 and 8 of RFC
4861 will need to extend their implementations accordingly. This
specification is fully consistent with [RFC6724] and implementers
will need to add support for its Rule 5.5. Hosts that do not support
these features may fail to communicate in the presence of filters as
described above.
1.1. 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 SLAAC [RFC4862]
[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) it will be generally the case that the
configured addresses match one of the prefixes advertised in a Router
Advertisement that are supposed to be in that link.
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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 1: 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
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the setting of the L and A flags in the PIO. In some circumstances
both L and A might be zero.
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 a
router implementation rejects such a PIO as a configuration error.
Newer implementations that support this mechanism will need to be
updated accordingly: a host SHOULD NOT ignore a PIO simply because
both L and A flags are cleared; a router SHOULD be able to send such
a PIO.
2.2. Expectations of multihomed networks
The direct implication of Section 2.1 is that routing protocols used
in multihomed networks SHOULD be capable of source-prefix based
egress routing, and that multihomed networks SHOULD deploy them.
3. Reasonable expectations of the host
3.1. Default Router Selection
Default Router Selection is modified as follows: A host SHOULD select
default routers for each prefix it is assigned an address in.
Routers that have advertised the prefix in its Router Advertisement
message SHOULD be preferred over routers that do not advertise the
prefix.
As a result of doing so, 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
thing.
3.2. Source Address Selection
There is an interaction with Default Address Selection [RFC6724].
Rule 5.5 of that specification states that the source address used to
send to a given destination address should if possible be chosen from
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a prefix known to be advertised by the first-hop router for that
destination. This selection rule would be applicable in a host
following the recommendation in the previous paragraph.
3.3. Redirects
There is potential for adverse interaction with any off-link Redirect
(Redirect for a GUA 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 may
need to contain appropriate source-specific entries.
3.4. History
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 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 the 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 1, which happens to be one of the authors' 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 multi-homed). In that case, there is no way for the "wrong"
router to detect the existence of the "right" router, or to route to
it.
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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 memo asks the IANA for no new parameters.
6. Security Considerations
This document does not create any new security or privacy exposures.
It is intended to avoid connectivity issues in the presence of BCP 38
ingress filters or stateful firewalls combined with multihoming.
There might be a small privacy improvement, however: 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. Acknowledgements
Comments were received from Jinmei Tatuya and Ole Troan, who have
suggested important text, plus Mikael Abrahamsson, Steven Barth,
Juliusz Chroboczek, Toerless Eckert, David Farmer, Pierre Pfister,
Mark Smith, Dusan Mudric, and James Woodyatt.
8. References
8.1. 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,
<http://www.rfc-editor.org/info/rfc2119>.
[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>.
[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,
<http://www.rfc-editor.org/info/rfc4861>.
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[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,
<http://www.rfc-editor.org/info/rfc6724>.
8.2. Informative References
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
May 2000, <http://www.rfc-editor.org/info/rfc2827>.
[RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
2003, <http://www.rfc-editor.org/info/rfc3315>.
[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March
2004, <http://www.rfc-editor.org/info/rfc3704>.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862,
DOI 10.17487/RFC4862, September 2007,
<http://www.rfc-editor.org/info/rfc4862>.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
<http://www.rfc-editor.org/info/rfc4941>.
[RFC6092] Woodyatt, J., Ed., "Recommended Simple Security
Capabilities in Customer Premises Equipment (CPE) for
Providing Residential IPv6 Internet Service", RFC 6092,
DOI 10.17487/RFC6092, January 2011,
<http://www.rfc-editor.org/info/rfc6092>.
[RFC7084] Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic
Requirements for IPv6 Customer Edge Routers", RFC 7084,
DOI 10.17487/RFC7084, November 2013,
<http://www.rfc-editor.org/info/rfc7084>.
[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,
<http://www.rfc-editor.org/info/rfc7217>.
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Appendix A. Change Log
Initial Version: 2015-08-05
Version 01: Update text on PIOs, added text on Redirects, and
clarified the concept of a "simple" network, 2015-08-13.
Version 02: Clarifications after WG discussions, 2015-08-19.
Version 03: More clarifications after more WG discussions,
especially adding stateful firewalls, uRPF, and more precise
discussion of RFC 4861, 2015-09-03.
Authors' Addresses
Fred Baker
Cisco Systems
Santa Barbara, California 93117
USA
Email: fred@cisco.com
Brian Carpenter
Department of Computer Science
University of Auckland
PB 92019
Auckland 1142
New Zealand
Email: brian.e.carpenter@gmail.com
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