Internet DRAFT - draft-axu-addr-sel
draft-axu-addr-sel
6man A. Suhonen
Internet-Draft Tampere University of Technology,
Updates: 3484 (if approved) Finland
Intended status: Experimental July 12, 2010
Expires: January 13, 2011
Address Selection Using Source Address Specific Routing Tables
draft-axu-addr-sel-01
Abstract
RFC 3484 defines two algorithms for default source and destination
address selection, but it has several shortcomings. This document
specifies an alternate address selection algorithm that uses routing
tables as policy input.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 13, 2011.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. Filter Algorithm . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Scope Recognition . . . . . . . . . . . . . . . . . . . . 6
3. Precedences and Labels . . . . . . . . . . . . . . . . . . . . 6
4. Routing Table and Address Properties . . . . . . . . . . . . . 7
4.1. Local Scope . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Link Local Scope . . . . . . . . . . . . . . . . . . . . . 8
4.3. Autoconfiguration for Global Scope . . . . . . . . . . . . 8
4.4. Limited Scope . . . . . . . . . . . . . . . . . . . . . . 9
4.5. Transition Technique Routing Tables . . . . . . . . . . . 9
4.6. IPv4 Compatible Routing Tables . . . . . . . . . . . . . . 10
4.7. Mobility . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.8. Reachability Information . . . . . . . . . . . . . . . . . 10
4.9. Additional Filter Constraints . . . . . . . . . . . . . . 10
4.10. Forwarding . . . . . . . . . . . . . . . . . . . . . . . . 11
4.11. Dynamic Routing Protocols . . . . . . . . . . . . . . . . 11
5. RFC3484 Rule Comparison . . . . . . . . . . . . . . . . . . . 11
5.1. Source Address Selection Rules . . . . . . . . . . . . . . 12
5.1.1. Prefer Same Address . . . . . . . . . . . . . . . . . 12
5.1.2. Prefer Appropriate Scope . . . . . . . . . . . . . . . 12
5.1.3. Avoid Deprecated Addresses . . . . . . . . . . . . . . 12
5.1.4. Prefer Home Address . . . . . . . . . . . . . . . . . 12
5.1.5. Prefer Outgoing Interface . . . . . . . . . . . . . . 12
5.1.6. Prefer Matching Label . . . . . . . . . . . . . . . . 12
5.1.7. Prefer Public Addresses . . . . . . . . . . . . . . . 12
5.1.8. Use Longest Matching Prefix . . . . . . . . . . . . . 12
5.2. Destination Address Selection Rules . . . . . . . . . . . 13
5.2.1. Avoid Unusable Destinations . . . . . . . . . . . . . 13
5.2.2. Prefer Matching Scope . . . . . . . . . . . . . . . . 13
5.2.3. Avoid Deprecated Addresses . . . . . . . . . . . . . . 13
5.2.4. Prefer Home Address . . . . . . . . . . . . . . . . . 13
5.2.5. Prefer Matching Label . . . . . . . . . . . . . . . . 13
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5.2.6. Prefer Higher Precedence . . . . . . . . . . . . . . . 13
5.2.7. Prefer Native Transport . . . . . . . . . . . . . . . 13
5.2.8. Prefer Smaller Scope . . . . . . . . . . . . . . . . . 13
5.2.9. Use Longest Matching Prefix . . . . . . . . . . . . . 14
5.2.10. Leave Order Unchanged . . . . . . . . . . . . . . . . 14
6. RFC5220 Concerns . . . . . . . . . . . . . . . . . . . . . . . 14
6.1. Multiple Routers on a Single Interface . . . . . . . . . . 14
6.2. Ingress Filtering Problem . . . . . . . . . . . . . . . . 14
6.3. Half-Closed Network Problem . . . . . . . . . . . . . . . 14
6.4. Combined Use of Global and ULA . . . . . . . . . . . . . . 15
6.5. Site Renumbering . . . . . . . . . . . . . . . . . . . . . 15
6.6. Multicast Source Address Selection . . . . . . . . . . . . 15
6.7. Temporary Address Selection . . . . . . . . . . . . . . . 15
6.8. IPv4 or IPv6 Prioritization . . . . . . . . . . . . . . . 16
6.9. ULA and IPv4 Dual-Stack Environment . . . . . . . . . . . 16
6.10. ULA or Global Prioritization . . . . . . . . . . . . . . . 16
7. RFC5221 Requirements . . . . . . . . . . . . . . . . . . . . . 16
7.1. Effectiveness . . . . . . . . . . . . . . . . . . . . . . 16
7.2. Timing . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.3. Dynamic Behavior Update . . . . . . . . . . . . . . . . . 16
7.4. Node-Specific Behavior . . . . . . . . . . . . . . . . . . 17
7.5. Application-Specific Behavior . . . . . . . . . . . . . . 17
7.6. Multiple Interface . . . . . . . . . . . . . . . . . . . . 17
7.7. Central Control . . . . . . . . . . . . . . . . . . . . . 17
7.8. Next-Hop Selection . . . . . . . . . . . . . . . . . . . . 17
7.9. Compatibility with RFC 3493 . . . . . . . . . . . . . . . 17
7.10. Compatibility and Interoperability with RFC 3484 . . . . . 18
7.11. Security . . . . . . . . . . . . . . . . . . . . . . . . . 18
8. Implementation Issues and Other Concerns . . . . . . . . . . . 18
8.1. Low Memory and Power Concerns . . . . . . . . . . . . . . 18
8.2. Differing Larger Scopes . . . . . . . . . . . . . . . . . 18
8.3. Connection Pooling . . . . . . . . . . . . . . . . . . . . 19
8.4. Using Just One Table with Tags . . . . . . . . . . . . . . 19
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
11. Security Considerations . . . . . . . . . . . . . . . . . . . 20
11.1. RFC5220 Considerations . . . . . . . . . . . . . . . . . . 20
11.2. RFC5221 Requirements . . . . . . . . . . . . . . . . . . . 20
11.2.1. List of threats introduced by new
address-selection mechanism . . . . . . . . . . . . . 20
11.2.2. List of recommendations in which security
mechanism should be applied . . . . . . . . . . . . . 20
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
12.1. Normative References . . . . . . . . . . . . . . . . . . . 21
12.2. Informative References . . . . . . . . . . . . . . . . . . 21
Appendix A. Routing Table Example . . . . . . . . . . . . . . . . 22
A.1. Before . . . . . . . . . . . . . . . . . . . . . . . . . . 22
A.2. After Conversion . . . . . . . . . . . . . . . . . . . . . 23
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Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
[RFC3484] defines default address selection rules for IPv6 and for
IPv4 in relation to IPv6. Several shortcomings in the original
address selection rules have been identified in [RFC5220] and its
sister document [RFC5221] specifies some requirements for any
attempts to update the original address selection algorithm.
A further concern comes from multipath protocols. When SCTP
[RFC4960], for example, finds that its active source destination
address pair is no longer functional, it will need to start searching
for a new one. If the multipath protocol doesn't respect address
selection policy, it may cause similar security incidents as the old
address selection algorithm. A multipath protocol should also
consult the algorithm during the session and not only on connection
setup. SHIM6 [RFC5533] has similar concerns.
The communicating hosts may both have a dozen addresses so it might
take unacceptably long to iterate through all combinations before
finding a functional pair. On the other hand, many of the invalid
combinations could be filtered out using this algorithm, making the
process noticeably faster.
This algorithm always performs address selection on source-
destination address pairs with additional information such as next
hop attached. Preferences are also associated with address pairs
instead of single addresses.
1.1. Terminology
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].
An autoconfiguration method is any process or protocol that acquires
or creates an IP address for a link. Examples of autoconfiguration
methods include RARP [RFC0903], DHCPv4 [RFC2131], ICMPv6 RA
[RFC4862], DHCPv6 [RFC3315], Teredo [RFC4380], 6to4 [RFC3056], ISATAP
[RFC5214], IPv4 Link Local [RFC3927], PPP [RFC1661] and mDNS.
An autoconfiguration agent is a process or daemon that executes an
autoconfiguration method on the host.
Autoconfiguration methods may also produce other information such as
default routes and DNS resolvers. The complete collection of
information produced by an autoconfiguration method is called an
autoconfiguration blob in this document.
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2. Filter Algorithm
When a host has several addresses, they SHOULD each be associated
with their own routing tables. The first stage in selecting source
and destination addresses SHOULD be to filter out combinations where
the routing table attached with the source (local) address does not
have a valid route for the destination (remote) address. If more
than one route matches, the most specific route SHOULD be checked for
validity.
If a destination address can't be found from the routing table for a
given source address the system MUST discard that destination address
for that source address.
If none of the possible destination addresses can be found in the
routing table for a source address, then that source address MUST be
discarded for those destination addresses.
2.1. Scope Recognition
One side effect of this filter algorithm is that it doesn't need to
know anything about scopes. The routing tables associated with
source address candidates will determine what destination addresses
they are usable with. This effect is demonstrated below and later in
this document.
Whenever scopes are mentioned in this draft, they are always
mentioned in the context of generating policy input for the
algorithm.
3. Precedences and Labels
Each routing table has a default precedence, meaning all routes added
to that table will have that precedence in the absence of a specific
precedence.
This precedence MUST be used to sort the source and destination
address pairs after the filtering stage according to preference.
Higher precedence values have higher preference. In effect, the
precedence is for the address pair, not for a single address.
When two or more address pairs have the same precedence, their
destination prefix lengths MUST be compared and the longer prefixes
MUST be considered more preferable.
When two or more address pairs are still equal, their destination
metrics in the routing table MUST be compared and address pairs with
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better metrics MUST be considered more preferable.
If there are still ties, they MAY be broken by some Equal Cost Multi-
Path load sharing techniques. Otherwise the algorithm SHOULD output
the equal address pairs in the same order as they appeared in its
input.
The label abstraction used by the original RFC [RFC3484] loosely
corresponds to the routing table abstraction in this algorithm. If a
database table join is performed on the source address policy table
and the destination address policy table as defined by [RFC3484] on
the label field and the precedences are added together, the result
resembles the policy tables used by this algorithm.
This algorithm normally performs both source and destination address
selection simultaneously and efficiently.
In order to perform source address selection, only one destination
address SHOULD be presented to the algorithm, which will then look
for the address in all tables and sort the source addresses where it
was found according to the precedences.
In order to perform destination address selection, only one source
address SHOULD be presented to the algorithm along with the set of
destination addresses. The algorithm will then look for all the
given destination addresses in the table associated with the source
address and sort the results according to the precedences.
4. Routing Table and Address Properties
FIXME: come up with a better section title
This section details how source address specific routing tables
should be populated to be usable as input data for this algorithm.
4.1. Local Scope
The routing tables associated with localhost addresses (127.0.0.1 and
::1) SHOULD only have routes to localhost address space. (127.0.0.0/8
and ::1/128) In addition, if the host supports node local multicast,
a route for the node local scope multicast space MAY also appear in
this table. (e.g. ff01::/16, ff11::/16)
The routing tables associated with any other addresses assigned to
the host SHOULD have a host route for the address itself pointing to
the loopback interface.
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The default precedence for all local scope route entries SHOULD be
500.
4.2. Link Local Scope
The routing table associated with a link local address (e.g.
169.254.123.45%le0) SHOULD only have one external unicast route, the
link local network for that link (e.g. 169.254.0.0%le0/16). In
addition, if the host supports multicast on this link, a route for
the link local scope multicast space MAY also appear in this table.
(e.g. ff02::/16, ff12::/16)
This means that the link local source address is usable only with
other link local destination addresses on the same link.
The default precedence for all link local scope route entries SHOULD
be 500.
4.3. Autoconfiguration for Global Scope
When global scope addresses are assigned to interfaces dynamically
through stateless or stateful autoconfiguration the process MUST
yield a default route. That default route SHOULD be placed only into
the routing table associated with that address. In addition, if the
host and network support multicast, a route for the global scope
multicast space SHOULD also appear in this table. (e.g. ff0e::/16,
ff1e::/16)
This usually means that the next hop of that default route will only
be useable with the source address learned from that default router.
Some autoconfiguration methods (see [RFC3442] and [RFC4191]) can be
used to communicate other routes in addition to the default route.
Those routes SHOULD likewise be added only into the routing table
associated with the address configured using that same interchange.
The default precedence for all global scope route entries SHOULD be
400.
The system MAY automatically add depreference routes to global scope
routing tables. These routes will cover address space reserved for
transition techniques, such as 2002::/16 (FIXME: add xrefs) and
2001::/32. They SHOULD have the same next-hop information as the
default route in the same table, but their precedence SHOULD be 150.
The system MAY automatically add blackhole routes to global scope
routing tables for illegal address combinations. An example of such
an illegal combination is IPv6 prefix 2002:a00::/24, which
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corresponds to 6to4 addresses generated from IPv4 addresses inside
10.0.0.0/8 which can't be used on the Internet.
Another example of automatically generated extra routes is fc00::/7
for ULA addresses as defined by [RFC4193]. However, the routing
table for a ULA should only have a route for the address space it's
usable in, and that route should be more specific than the default
routes for globally usable source addresses, it should win according
to the longest matching prefix rule.
4.4. Limited Scope
The routing tables for site local addresses SHOULD have routes for
site local address space. They SHOULD NOT have the default route, so
that they would be automatically eliminated when selecting address
pairs for site external communication.
However, if the site edge automatically translates limited scope
addresses to global addresses, the routing tables associated with
limited scope addresses MAY have the default route.
This algorithm effectively treats all addresses that aren't
associated with a default route as limited scope. The system MAY
automatically recognize addresses within the ULA prefix fc00::/7
[RFC4193] and treat them as limited scope.
The default precedence for limited scope addresses SHOULD be the same
as global scope addresses (400), but it SHOULD be simple for the
system or network administration to change this setting.
In addition, if the host and network support multicast, a route for
the site local scope multicast space MAY also appear in this table.
(e.g. ff05::/16, ff15::/16)
4.5. Transition Technique Routing Tables
The default precedence for all route entries for source addresses
generated through transition techniques SHOULD be 300.
The transition table SHOULD NOT of course have a depreference route
for its own address space. Instead, the precedence of the route for
its own address space SHOULD be 350. This is to make using the same
transition technique source address more preferable than some
different transition technique.
Individual transition techniques or the system administrator MAY
specify different default precedences to establish relative
preferences between transition techniques or the proxies/servers
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associated with them.
4.6. IPv4 Compatible Routing Tables
The precedence for all IPv4 compatible global scope route entries
SHOULD be easily configurable. The default precedence should be 400.
If administration wishes to promote the use of IPv6, then the IPv4
entries should have a precedence of 200.
4.7. Mobility
The precedence of route entries in the tables for home addresses and
care-of addresses SHOULD be easily configurable. The default
precedence for home addresses should be 425 and for care-of addresses
it should be 400. If an address is simultaneously a home address and
a care-of address, then the precedence should be 450. When the host
is ''at one home'', that address will be used, and when the host is
visiting ''at the other home'', the home address of that other home
will be preferred.
4.8. Reachability Information
If the next-hop information associated with a route in any table has
been found unreachable or the link is down the precedence of the
affected routes MAY be temporarily dropped to zero until they work
again.
4.9. Additional Filter Constraints
The address selection algorithm MAY also be given additional filter
constraints, such as "use only link#3" or "do not use next-hop
10.0.0.1". [RFC5014] specifies an interface that does something very
similar.
Work is going on in the MIF-wg [I-D.blanchet-mif-problem-statement]
to tie address selection and next-hop selection with DNS resolver
selection and other similar resources. That is, when using the DNS
resolvers received from one autoconfiguration agent, the host SHOULD
also always use the default route received from the same
autoconfiguration agent.
This algorithm supports those efforts by making it possible to
restrict a process to one routing table for both address resolution
and selection.
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4.10. Forwarding
If a host is configured to forward packets between networks, it
SHOULD combine the routing tables for the networks in question into
one. Link local scope tables MUST NOT be combined.
If the host has multiple addresses from different global scope
prefixes then system administration MAY specify which addresses are
combined to form routing tables. The resulting functionality
resembles the VRF functionality found in some modern routers.
One purpose behind this algorithm is to move source routing burden
from the network to the host. So if a router wants to advertise two
(or more) prefixes on the subnet, but to keep their routing separate,
it should use different link local and link layer addresses when
advertising them. It can then choose the correct VRF to forward a
packet depending on which link layer address it received it on.
4.11. Dynamic Routing Protocols
Hosts don't usually run dynamic routing protocols, but since they
sometimes do, this subsection is included for completeness. Dynamic
routing protocols can be used to convey address selection
configuration information for this algorithm.
Dynamic routing protocol instances are usually bound to links or
interfaces. With this algorithm network administrators MAY bind
routing protocol instances to specific addresses or prefixes on a
link and the routing tables associated with them. The routing
protocol instance MUST update only the routing table it is associated
with.
A reasonable default setting is that all addresses that are not link
local are associated with the routing protocol instance. Thus, they
will share a routing table.
If the network administration wants to separate traffic belonging to
different upstream operator prefixes, it may wish to run separate
routing protocol instances throughout the network for different
upstream prefixes.
5. RFC3484 Rule Comparison
The algorithm defined by [RFC3484] uses a set of rules to perform its
function. Those rules are compared to this algorithm in this
section.
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5.1. Source Address Selection Rules
5.1.1. Prefer Same Address
The interface address is added with a higher metric to its address
specific routing table than any other routes. This ensures that this
algorithm conforms to this rule.
5.1.2. Prefer Appropriate Scope
The routes of different scopes are assigned different precedences.
They correspond to the scope values of the original algorithm.
5.1.3. Avoid Deprecated Addresses
If precedences for deprecated addresses are zeroed, they should
automatically be depreferred against any other addresses.
5.1.4. Prefer Home Address
The higher precedences assigned to home addresses make them always
preferable when compared to almost everything else, apart from link
local addresses. If a host has two home addresses in different
networks, the rules presented will make it prefer the correct address
depending on the current location of the mobile node.
5.1.5. Prefer Outgoing Interface
The metric for the route on the table for the address on the outgoing
interface should be better than on other interfaces, so all else
being equal, it should break the tie to ensure that this rule is met.
5.1.6. Prefer Matching Label
This algorithm doesn't have labels at all. However, automatically
added depreference routes will take care of this rule.
5.1.7. Prefer Public Addresses
The section on temporary address selection (Section 6.7) already
deals with this rule.
5.1.8. Use Longest Matching Prefix
This is a debated rule so reproducing it is also questionable.
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5.2. Destination Address Selection Rules
5.2.1. Avoid Unusable Destinations
This algorithm conforms to this rule by design.
5.2.2. Prefer Matching Scope
The routes of different scopes are assigned different precedences.
The routes of matching scopes are assigned higher precedences than
routes of differing scopes.
5.2.3. Avoid Deprecated Addresses
If precedences for deprecated addresses are zeroed, they should
automatically be depreferred against any other addresses.
5.2.4. Prefer Home Address
The higher precedences assigned to home addresses make them always
preferable when compared to almost everything else, apart from link
local addresses. If a host has two home addresses in different
networks, the rules presented will make it prefer the correct address
depending on the current
5.2.5. Prefer Matching Label
This algorithm doesn't have labels at all. However, automatically
added depreference routes will take care of this rule.
5.2.6. Prefer Higher Precedence
This algorithm primarily compares precedence values only. All the
rules above are encoded as precedence values into the routing tables.
5.2.7. Prefer Native Transport
This rule essentially is a special case of the matching scope and
matching label rules. The special routing table generation rules for
transition mechanisms make sure that this algorithm behaves the same
way as the original algorithm.
5.2.8. Prefer Smaller Scope
The routes of different scopes are assigned different precedences.
They correspond to the scope values of the original algorithm.
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5.2.9. Use Longest Matching Prefix
This is a debated rule so reproducing it is also questionable.
5.2.10. Leave Order Unchanged
This new algorithm conforms to this rule.
6. RFC5220 Concerns
[RFC5220] presents several problems and issues with the original
default address selection algorithm. The following subsections
address these issues.
6.1. Multiple Routers on a Single Interface
This problem was one of the starting points for the development of
this algorithm. This algorithm solves the problem by having separate
routing tables for addresses learned from different routers.
6.2. Ingress Filtering Problem
This algorithm will always choose the correct link and next-hop
address for each source address. However, if several source
addresses share the same next-hop on the same link, then there's
nothing the algorithm can do. The problem has to be fixed inside the
router announcing the prefixes.
6.3. Half-Closed Network Problem
This problem was one of the starting points for the development of
this algorithm. This algorithm solves the problem by having separate
routing tables for different addresses.
The default assumption is that the auto-configuration method supplies
a default route for all globally usable addresses. The routing
tables of source addresses usable only within a closed network SHOULD
NOT have a default route. They SHOULD only have routes to the
networks they are usable within.
System or network administration MUST specify allowed or disallowed
connections by modifying the auto-configuration input or the routing
tables.
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6.4. Combined Use of Global and ULA
This algorithm solves the problem by having separate routing tables
for different addresses. Scope of address usage is controlled by the
routing tables.
Implementations MAY recognize ULA addresses and other limited scope
addresses as scopes of their own, and treat them properly when
autogenerating the routing tables.
System or network administration MUST specify allowed or disallowed
address pair selection by modifying the auto-configuration input or
the routing tables.
6.5. Site Renumbering
When the autoconfiguration client discovers that a prefix or address
has been deprecated, it SHOULD drop the route precedences for all the
routes associated with the deprecated resource to zero.
When such deprecated routing information finally times out and is no
longer in use, the routing table associated with it MAY be removed
entirely.
6.6. Multicast Source Address Selection
Multicast address selection works the same way as unicast address
selection. The source address candidate routing tables SHOULD have
only the appropriate multicast scope routes.
6.7. Temporary Address Selection
A temporary addresses MAY be associated with routing tables of its
own, instead of sharing a routing table with the address used to
generate the temporary address.
The precedences for the table for a temporary address would be lower
than that of a similar but permanent address. Clients wishing to
make use of the temporary address would add appropriate constraints
to their address selection.
Alternatively, if the system or network administration wishes that
the host use a temporary address with some certain destination
network, a route to that network could be added to the routing table
for the temporary address with a higher than normal precedence.
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6.8. IPv4 or IPv6 Prioritization
This is a configuration issue with the routing tables. The algorithm
itself doesn't dictate policy for sites.
6.9. ULA and IPv4 Dual-Stack Environment
This special case is easily handled by omitting the default route
from the routing table for ULA addresses. This would result in site-
external destination IPv6 addresses not having any usable source
addresses and thus they would never be considered by this algorithm.
6.10. ULA or Global Prioritization
Already covered in Section 6.4.
7. RFC5221 Requirements
[RFC5221] defines a set of requirements for the address selection
algorithm. The subsection headings used in that document have been
copied here and an explanation of how this algorithm deals with each
issue is given.
7.1. Effectiveness
The effectiveness of the proposed solution to solve problems
presented in [RFC5220] is covered by Section 6.
7.2. Timing
This algorithm relies on other methods and protocols to submit
address selection configuration and information and to place it in
the routing table. These other methods include auto-configuration
and routing protocols.
Once the routing table is updated, the address selection algorithm
will start making decisions based on the new information.
7.3. Dynamic Behavior Update
From the point of view of this algorithm, this problem is a feature
of auto-configuration methods. If the autoconfiguration methods
rewrite routing tables, the address selection algorithm will always
use the updated information when it's invoked.
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7.4. Node-Specific Behavior
From the point of view of this algorithm, this problem is a feature
of autoconfiguration methods. This algorithm will happily make
address selection decisions according to any input it is given.
7.5. Application-Specific Behavior
Additional filter constraints from Section 4.9 can be used to
influence address selection per application.
7.6. Multiple Interface
This algorithm doesn't differenciate between cases where a host has
multiple interfaces and where it has multiple prefixes on a single
interface. If it solves a problem satisfactorily for one case, it
solves it identically for the other case as well.
7.7. Central Control
This algorithm doesn't specify new methods for central control. It
does, however, work well with other protocols that provide methods of
central control, such as routing protocols.
7.8. Next-Hop Selection
The next-hop and interface used is a side product of the source
address specific routing table lookup, which is performed in the
filtering stage.
A very pleasing feature of this algorithm is that there can be
multiple routers advertising different prefixes on the same subnet,
and this algorithm will still select proper address pairs and next-
hops to satisfy any SAVI requirements.
7.9. Compatibility with RFC 3493
FIXME TBD
On first impression, this algorithm shouldn't have any impact on the
Socket API. Then again, routing table index could be referenced as
part of some process.
Solaris, for example, creates new alias-interfaces for each new
address assigned to a physical interface. So if_index could also be
used to uniquely identify a source address specific routing table on
that platform. Other operating systems do not work the same way.
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7.10. Compatibility and Interoperability with RFC 3484
When a host implementing this address selection algorithm and a host
implementing the [RFC3484] algorithm interact, this algorithm will
become constrained by the choices made by the peer.
One key difference between this algorithm and the [RFC3484] algorithm
is that this algorithm considers all valid source address candidates,
where as the original algorithm chooses only one source address per
every destination address. This difference can be easily overcome by
an extra filter rule, that accepts only the highest precedence
source-destination address pair for every given destination address.
7.11. Security
Security issues raised in [RFC5221] are covered by Section 11.2.
8. Implementation Issues and Other Concerns
Some popular operating systems already implement all the features
required to implement this algorithm. In such cases all that is
required is to integrate the features together.
The trickiest feature required by this algorithm is probably support
for multiple routing tables. This may also create backward
compatibility issues in some implementations. More discussion may be
required here.
8.1. Low Memory and Power Concerns
The biggest worry is that creating lots of routing tables will waste
memory and power. However, when compared to the old way (see
Appendix A), memory consumption doesn't explode. Every route that
was present in the monolithic routing table will usually be present
in only one source address specific routing table.
CGAs (ADD XREF) MAY reuse the same routing table.
8.2. Differing Larger Scopes
The default route for global scope addresses is 0::0/0, but this
route will also cover addresses of potentially incompatible scopes.
For example, the basic algorithm would accept a link local
destination address with a global scope source address.
One way to prevent this would be to add blackhole routes into the
routing tables of global scope addresses for address space belonging
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to incompatible scopes. The filter algorithm SHOULD treat a
blackhole route as an indication that no valid route was found for
addresses matching the blackhole in that table.
8.3. Connection Pooling
When trying to establish a new connection, the stack MAY send open
packets to all source/destination/nexthop combinations that pass the
filter stage at a pace of three per second until it receives a
response.
When the connection is established the addresses are fixed (for non-
multipathing protocols, such as TCP).
If the peer also responds to the other connection attempts after the
first connection is established, those connections MAY either be
reset immediately, or the stack MAY pool them for a short while in an
incomplete handshake state, in case some application tries to open an
identical socket.
This would benefit applications such as web browsers, mail transfer
agents and database clients, which routinely create more than one
connection between the same two hosts and the same destination port.
It would also benefit dual stacked or multi-homed hosts where some of
the addresses or networks are misconfigured and don't work.
8.4. Using Just One Table with Tags
It is possible to implement this algorithm with just one routing
table, if tags or bitfields are used to identify which routing table
each route really belongs to.
However, since a less specific route in one table can have higher
precedence than a more specific route in another table, care must be
taken in the implementation.
It is also possible to implement this algorithm without interfering
with the actual routing table at all, by just mirroring all the
routing table information and changes in a policy table used by this
algorithm only.
9. Acknowledgements
This document was written using the template derived from an initial
version written by Pekka Savola and contributed by him to the xml2rfc
project.
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Thanks to the following people for giving feedback during the writing
of this document: Jari Arkko, Jan Melen, Arifumi Matsumoto, James
Morse, Tim Chown, Brian E Carpenter, .
10. IANA Considerations
This document has no IANA Actions.
11. Security Considerations
11.1. RFC5220 Considerations
Section 4 of [RFC5220] raises a concern that a malicious attacker can
gather information about addresses connected to the target host by
triggering the address selection algorithm on the target host by
various methods and listening to what candidates it produces.
This algorithm doesn't completely remove that possibility, but due to
the filtering stage, the attacker can only gain information on
addresses routable to the address used by the attacker.
11.2. RFC5221 Requirements
Section 3 of [RFC5221] lists two security concerns which are dealt
with in subsections below.
11.2.1. List of threats introduced by new address-selection mechanism
This specification relies on existing autoconfiguration methods and
routing protocols to distribute address selection hints. Each of
those SHOULD have their own methods to combat leakage, hijacking and
denial of service.
11.2.2. List of recommendations in which security mechanism should be
applied
This specification relies on existing autoconfiguration methods and
routing protocols to distribute address selection hints. Each of
those SHOULD have their own methods to ensure integrity,
authentication and authorization.
12. References
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12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3442] Lemon, T., Cheshire, S., and B. Volz, "The Classless
Static Route Option for Dynamic Host Configuration
Protocol (DHCP) version 4", RFC 3442, December 2002.
[RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", RFC 4191, November 2005.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[RFC5220] Matsumoto, A., Fujisaki, T., Hiromi, R., and K. Kanayama,
"Problem Statement for Default Address Selection in Multi-
Prefix Environments: Operational Issues of RFC 3484
Default Rules", RFC 5220, July 2008.
[RFC5221] Matsumoto, A., Fujisaki, T., Hiromi, R., and K. Kanayama,
"Requirements for Address Selection Mechanisms", RFC 5221,
July 2008.
12.2. Informative References
[I-D.blanchet-mif-problem-statement]
Blanchet, M. and P. Seite, "Multiple Interfaces Problem
Statement", draft-blanchet-mif-problem-statement-01 (work
in progress), June 2009.
[RFC0903] Finlayson, R., Mann, T., Mogul, J., and M. Theimer,
"Reverse Address Resolution Protocol", STD 38, RFC 903,
June 1984.
[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
RFC 1661, July 1994.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, March 1997.
[RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains
via IPv4 Clouds", RFC 3056, February 2001.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
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and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3927] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
Configuration of IPv4 Link-Local Addresses", RFC 3927,
May 2005.
[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
[RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
Network Address Translations (NATs)", RFC 4380,
February 2006.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[RFC5014] Nordmark, E., Chakrabarti, S., and J. Laganier, "IPv6
Socket API for Source Address Selection", RFC 5014,
September 2007.
[RFC5214] Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
March 2008.
[RFC5533] Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
Shim Protocol for IPv6", RFC 5533, June 2009.
Appendix A. Routing Table Example
This section demonstrates how this algorithm affects the routing
table of a multi-homed host. Appendix A.1 shows the routing table
using only methods without this algorithm. Appendix A.2 shows the
routing tables produced on the same host if this algorithm is
applied.
A.1. Before
This routing table was initially copied from a system running Linux
2.6.25. The addresses were then greatly simplified to make the table
fit better on the page.
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+--------------------------------+----------+--------+--------+
| Network | Next-Hop | Link | Metric |
+--------------------------------+----------+--------+--------+
| 2001::/32 | :: | teredo | 256 |
| 2001:db8:1::/64 | :: | eth0 | 256 |
| 2001:db8:2::/64 | :: | eth1 | 256 |
| fe80::/64 | :: | teredo | 256 |
| fe80::/64 | :: | eth0 | 256 |
| fe80::/64 | :: | eth1 | 256 |
| ::/0 | :: | teredo | 1029 |
| ::/0 | fe80::13 | eth0 | 1024 |
| ::/0 | fe80::ce | eth1 | 1024 |
| ::/0 | :: | lo | -1 !U |
| ::1/128 | :: | lo | 0 |
| 2001:db8:1:0:a00:ff:fedc:a/128 | :: | lo | 0 |
| 2001:db8:2:0:200:ff:fec4:b/128 | :: | lo | 0 |
| 2001:0:c200:201::3/128 | :: | lo | 0 |
| fe80::a00:ff:fedc:a/128 | :: | lo | 0 |
| fe80::200:ff:fec4:b/128 | :: | lo | 0 |
| fe80::ffff:ffff:ffff/128 | :: | lo | 0 |
+--------------------------------+----------+--------+--------+
Table 1: Routing Table w/o Modifications
"!U" after metric denotes unreachable or blackhole routes.
A.2. After Conversion
These tables contain and implement just the basic idea. Thus the
combined size of these tables is equal to Table 1. Optional
improvements are presented in the next subsection.
+---------+----------+------+-------+
| Network | Next-Hop | Link | Prec |
+---------+----------+------+-------+
| ::/0 | :: | lo | -1 !U |
| ::1/128 | :: | lo | 500 |
+---------+----------+------+-------+
Table 2: Routing Table for ::1
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+------------------------+----------+--------+------+
| Network | Next-Hop | Link | Prec |
+------------------------+----------+--------+------+
| 2001::/32 | :: | teredo | 350 |
| ::/0 | :: | teredo | 300 |
| 2001:0:c200:201::3/128 | :: | lo | 500 |
+------------------------+----------+--------+------+
Table 3: Routing Table for 2001:0:c200:201::3%teredo
+--------------------------+----------+--------+------+
| Network | Next-Hop | Link | Prec |
+--------------------------+----------+--------+------+
| fe80::/64 | :: | teredo | 500 |
| fe80::ffff:ffff:ffff/128 | :: | lo | 500 |
+--------------------------+----------+--------+------+
Table 4: Routing Table for fe80::ffff:ffff:ffff%teredo
+--------------------------------+----------+------+------+
| Network | Next-Hop | Link | Prec |
+--------------------------------+----------+------+------+
| 2001:db8:1::/64 | :: | eth0 | 400 |
| ::/0 | fe80::13 | eth0 | 400 |
| 2001:db8:1:0:a00:ff:fedc:a/128 | :: | lo | 500 |
+--------------------------------+----------+------+------+
Table 5: Routing Table for 2001:db8:1:0:a00:ff:fedc:a%eth0
+-------------------------+----------+------+------+
| Network | Next-Hop | Link | Prec |
+-------------------------+----------+------+------+
| fe80::/64 | :: | eth0 | 500 |
| fe80::a00:ff:fedc:a/128 | :: | lo | 500 |
+-------------------------+----------+------+------+
Table 6: Routing Table for fe80::a00:ff:fedc:a%eth0
+--------------------------------+----------+------+------+
| Network | Next-Hop | Link | Prec |
+--------------------------------+----------+------+------+
| 2001:db8:2::/64 | :: | eth1 | 400 |
| 2001:db8:2:0:200:ff:fec4:b/128 | :: | lo | 500 |
| ::/0 | fe80::ce | eth1 | 400 |
+--------------------------------+----------+------+------+
Table 7: Routing Table for 2001:db8:2:0:200:ff:fec4:b%eth1
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+-------------------------+----------+------+------+
| Network | Next-Hop | Link | Prec |
+-------------------------+----------+------+------+
| fe80::/64 | :: | eth1 | 500 |
| fe80::200:ff:fec4:b/128 | :: | lo | 500 |
+-------------------------+----------+------+------+
Table 8: Routing Table for fe80::200:ff:fec4:b%eth1
Author's Address
Aleksi Suhonen
Tampere University of Technology, Finland
Korkeakoulunkatu 1
Tampere 33720
FI
Phone: +358 45 670 2048
Email: i-d-2010@ssd.axu.tm
Suhonen Expires January 13, 2011 [Page 25]
