IPv6 Maintenance F. Baker
Internet-Draft Cisco Systems
Intended status: Standards Track B. Carpenter
Expires: February 14, 2016 Univ. of Auckland
August 13, 2015
Host routing in a multi-prefix network
draft-baker-6man-multi-homed-host-01
Abstract
This note describes expected host behavior in a network that has more
than one prefix, each allocated by an upstream network that
implements BCP 38 filtering, when the host has multiple routers to
choose from.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. Expectations the host has of the network . . . . . . . . . . 2
3. Reasonable expectations of the host . . . . . . . . . . . . . 4
4. Expectations of multihomed networks . . . . . . . . . . . . . 5
5. Residual issues . . . . . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 5
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
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] 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.
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. 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 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 on-link in that subnet.
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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.
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
Figure 1: Two simple networks
Because 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 BCP
38 filters. Even if there was, it would be an indirect route, rather
than a direct route originating with the host; this is not "wrong",
but can be inefficient and prone to failure. Therefore the host
would do well to select the appropriate router itself.
Since the host derives fundamental default routing information from
the Route Advertisement, this implies that, in any network with hosts
using multiple prefixes, each prefix SHOULD be advertised via on-link
Prefix Information Options [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.
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3. Reasonable expectations of the host
Modern hosts maintain a fair bit of history, in terms of what has
historically 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 includes a next hop address for use when a packet is
sent to the indicated address.
When a host makes a successful exchange with a remote address or
prefix using a particular source address, and the host has received a
prefix that matches that source address in an RA, then the host
SHOULD include the prefix in such history. On subsequent attempts to
communicate with that remote address, if it has an address in that
prefix at that time, a host MAY use an address in the remembered
prefix for the session.
A host SHOULD select a "default gateway" for each source prefix it
uses to form an address or is assigned an address in. That router
SHOULD be one of the routers advertising the prefix in its RA. As a
result of doing so, when a host emits a datagram using a source
address in one of those prefixes and has no history directing it
otherwise, it SHOULD send it to the indicated "default gateway". In
the "simplest" network described in Section 2, 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.
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
a prefix known to be advertised by the next-hop router for that
destination. This selection rule would be applicable in a host
following the recommendation in the previous paragraph.
There is potential for adverse interaction with any off-link Redirect
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 needs to contain appropriate source-specific entries.
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4. Expectations of multihomed networks
The direct implication of Section 2 is that routing protocols used in
multihomed networks SHOULD be capable of source-prefix based egress
routing, and that multihomed networks SHOULD deploy them.
5. Residual issues
In an HNCP network, in which one router on each LAN advertises all
prefixes and the others do not, 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 LAN
but in a different subnet). In some scenarios, where the local
network is a highly constrained or lossy wireless network, this extra
hop may be a significant performance handicap.
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.
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.
6. IANA Considerations
This memo asks the IANA for no new parameters.
7. Security Considerations
This document does not create any new security or privacy exposures.
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.
8. Acknowledgements
Comments were received from Jinmei Tatuya, who has suggested
important text, plus Ole Troan, Pierre Pfister, Toerless Eckert,
Mikael Abrahamsson, and Juliusz Chroboczek.
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9. References
9.1. 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>.
9.2. 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>.
[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>.
[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>.
[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>.
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[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>.
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.
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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