Network Working Group P. Pfister
Internet-Draft B. Paterson
Intended status: Standards Track Cisco Systems
Expires: July 9, 2015 J. Arkko
Ericsson
January 5, 2015
Distributed Prefix Assignment Algorithm
draft-ietf-homenet-prefix-assignment-02
Abstract
This document specifies a distributed algorithm for automatic prefix
assignment. Given a set of delegated prefixes, it ensures at most
one prefix is assigned from each delegated prefix to each link.
Nodes may assign available prefixes to the links they are directly
connected to, or for other private purposes. The algorithm
eventually converges and ensures that all assigned prefixes do not
overlap.
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
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This Internet-Draft will expire on July 9, 2015.
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
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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
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Applicability statement . . . . . . . . . . . . . . . . . . . 5
4. Algorithm Specification . . . . . . . . . . . . . . . . . . . 6
4.1. Algorithm Terminology . . . . . . . . . . . . . . . . . . 6
4.2. Prefix Assignment Algorithm Routine . . . . . . . . . . . 7
4.3. Overriding and Destroying Existing Assignments . . . . . 10
4.4. Other Events . . . . . . . . . . . . . . . . . . . . . . 11
5. Prefix Selection Considerations . . . . . . . . . . . . . . . 11
6. Implementation Capabilities and Node Behavior . . . . . . . . 13
7. Algorithm Parameters . . . . . . . . . . . . . . . . . . . . 14
8. Security Considerations . . . . . . . . . . . . . . . . . . . 15
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
11.1. Normative References . . . . . . . . . . . . . . . . . . 16
11.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix A. Static Configuration Example . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction
This document specifies a distributed algorithm for automatic prefix
assignment. Given a set of delegated prefixes, nodes may assign
available prefixes to links they are directly connected to, or for
their private use. The algorithm ensures that the following
assertions are eventually true:
1. At most one prefix from each delegated prefix is assigned to each
link.
2. Assigned prefixes are not included in and do not include other
assigned prefixes.
3. Assigned prefixes do not change in the absence of topology or
configuration changes.
In the rest of this document the two first conditions are referred to
as the correctness conditions of the algorithm while the third
condition is referred to as its convergence condition.
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Each assignment has a priority specified by the node making the
assignment, allowing for more advanced assignment policies. When
multiple nodes assign different prefixes from the same delegated
prefix to the same link, or when multiple nodes assign overlapping
prefixes, the assignment with the highest priority is kept and other
assignments are removed.
The prefix assignment algorithm requires that participating nodes
share information through a flooding mechanism. If the flooding
mechanism ensures that all messages are propagated to all nodes
faster than a given timing upper bound, the algorithm also ensures
that all assigned prefixes used for networking operations (e.g., host
configuration) remain unchanged, unless another node assigns an
overlapping prefix with a higher assignment priority, or the topology
changes and renumbering cannot be avoided.
2. Terminology
In this document, the key words "MAY", "MUST, "MUST NOT", "OPTIONAL",
"RECOMMENDED", "SHOULD", and "SHOULD NOT", are to be interpreted as
described in [RFC2119].
This document makes use of the following terminology:
Link: An object the distributed algorithm will assign prefixes to.
A Node may only assign prefixes to Links it is directly connected
to. A Link is either Shared or Private.
Private Link: A Private Link is an abstract concept defined for the
sake of this document. It allows nodes to make assignments for
their private use or delegation. For instance, every DHCPv6-PD
[RFC3633] client MAY be considered as a different Private Link.
Shared Link: A Link multiple nodes are connected to. Most of the
time, a Shared Link would consist in a multi-access link or point-
to-point link, virtual or physical, requiring prefixes to be
assigned to.
Delegated Prefix: A prefix provided to the algorithm and used as a
prefix pool for Assigned Prefixes.
Node ID: A value identifying a given participating node. The set
of identifiers MUST be strictly and totally ordered (e.g.,
alphanumeric order).
Flooding Mechanism: A mechanism implementing reliable broadcast and
used to advertise published Assigned Prefixes.
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Flooding Delay: Optional value provided by the Flooding Mechanism
indicating a deterministic or likely upper bound of the
information propagation delay. When the Flooding Mechanism does
not provide a value, it is set to DEFAULT_FLOODING_DELAY
(Section 7).
Advertised Prefix: A prefix advertised by another node and
delivered to the local node by the Flooding Mechanism. It has an
Advertised Prefix Priority and, when assigned to a directly
connected Shared Link, is associated with a Shared Link.
Advertised Prefix Priority: A value that defines the priority of an
Advertised Prefix received from the Flooding Mechanism or a
published Assigned Prefix. Whenever multiple Advertised Prefixes
are conflicting, all Advertised Prefixes but the one with the
greatest priority will eventually be removed. In case of tie, the
assignment advertised by the node with the greatest Node ID is
kept and others are removed. In order to ensure convergence, the
range of priority values MUST have an upper bound.
Assigned Prefix: A prefix included in a Delegated Prefix and
assigned to a Shared or Private Link. It represents a local
decision to assign a given prefix from a given Delegated Prefix to
a given Link. The algorithm ensures that there never is more than
one Assigned Prefix per Delegated Prefix and Link pair. When
destroyed, an Assigned Prefix is set as not applied, ceases to be
advertised, and is removed from the set of Assigned Prefixes.
Applied (Assigned Prefix): When an Assigned Prefix is applied, it
MAY be used (e.g., for host configuration, routing protocol
configuration, prefix delegation). When not applied, it MUST NOT
be used for any other purposes than the prefix assignment
algorithm. Each Assigned Prefix is associated with a timer (Apply
Timer) used to apply the Assigned Prefix. An Assigned Prefix is
unapplied when destroyed.
Published (Assigned Prefix): The Assigned Prefix is advertised
through the Flooding Mechanism as assigned to its associated Link.
A published Assigned Prefix MUST have an Advertised Prefix
Priority. It will appear as an Advertised Prefix to other nodes,
once received through the Flooding Mechanism.
Backoff Timer: Every Delegated Prefix and Link pair is associated
with a timer counting down to zero. It is used to avoid multiple
nodes from making colliding assignments by delaying the creation
of new Assigned Prefixes or the advertisement of adopted Assigned
Prefixes by a random amount of time.
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Renumbering: Event occuring when an Assigned Prefix which was
applied is destroyed. It is undesirable as it usually implies
reconfiguring routers or hosts.
3. Applicability statement
Each node MUST have a set of disjoint Delegated Prefixes. It MAY
change over time and be different from one node to another at some
point, but nodes MUST eventually agree on the same set of disjoint
Delegated Prefixes.
Given this set of disjoint Delegated Prefixes, nodes may assign
available prefixes from each Delegated Prefix to the Links they are
directly connected to. The algorithm ensures that at most one prefix
from a given Delegated Prefix is assigned to a given Link.
The algorithm can be applied to any address space and can be used to
manage multiple address spaces simultaneously. For instance, an
implementation can make use of IPv4-mapped IPv6 addresses [RFC4291]
in order to manage both IPv4 and IPv6 prefix assignment
simultaneously.
The algorithm supports dynamically changing topologies:
o Nodes may join or leave the set of participating nodes.
o Nodes may join or leave Links.
o Links may be joined or split.
All nodes MUST run a common Flooding Mechanism in order to share
published Assigned Prefixes. The set of participating nodes is
defined as the set of nodes participating in the Flooding Mechanism.
The Flooding Mechanism MUST:
o Provide a way to flood Assigned Prefixes assigned to a directly
connected Link along with their respective Advertised Prefix
Priority and the Node ID of the node which advertises it.
o Specify whether an Advertised Prefix was assigned to a directly
connected Shared Link, and if so, on which one.
In addition, a Flooding Delay SHOULD be specified and respected in
order to avoid undesired renumbering. If not specified, or whenever
the Flooding Mechanism is unable to respect the provided delay,
renumbering may happen. As such delay often depends on the size of
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the network, it MAY change over time and MAY be different from one
node to another.
The algorithm ensures that whenever the Flooding Delay is provided
and respected, and in the absence of topology change or delegated
prefix removal, renumbering never happens.
Each node MUST have a Node ID. Node IDs MAY change over time and be
the same on multiple nodes at some point, but each node MUST
eventually have a Node ID which is unique among the set of
participating nodes.
4. Algorithm Specification
This section specifies the behavior of nodes implementing the prefix
assignment algorithm.
4.1. Algorithm Terminology
The algorithm makes use of the following terms:
Current Assignment: For a given Delegated Prefix and Link, the
Current Assignment is the Assigned Prefix (if any) included in the
Delegated Prefix and assigned to the given Link.
Best Assignment: For a given Delegated Prefix and Link, the Best
Assignment is (if any) the Advertised Prefix:
* Including or included in the Delegated Prefix.
* Assigned on the given Link.
* Having the greatest Advertised Prefix Priority among Advertised
Prefixes (and, in case of tie, the prefix advertised by the
node with the greatest Node ID among all prefixes with greatest
priority).
* Taking precedence over the Current Assignment (if any)
associated with the same Link and Delegated Prefix.
Precedence: An Advertised Prefix takes precedence over an Assigned
Prefix if and only if:
* The Assigned Prefix is not published.
* The Assigned Prefix is published and the Advertised Prefix
Priority from the Advertised Prefix is strictly greater than
the Advertised Prefix Priority from the Assigned Prefix.
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* The Assigned Prefix is published, the priorities are equal, and
the Node ID from the node advertising the Advertised Prefix is
strictly greater than the local Node ID.
Valid (Assigned Prefix) An Assigned Prefix is valid if and only if
the two following conditions are met:
* No Advertised Prefix including or included in the Assigned
Prefix takes precedence over the Assigned Prefix.
* No Advertised Prefix including or included in the same
Delegated Prefix as the Assigned Prefix and assigned to the
same Link takes precedence over the considered Assigned Prefix.
4.2. Prefix Assignment Algorithm Routine
This section specifies the prefix assignment algorithm routine. It
is defined for a given Delegated Prefix/Link pair and may be run
either as triggered by the Backoff Timer, or not.
For a given Delegated Prefix and Link pair, the routine MUST be run
as not triggered by the Backoff Timer whenever:
o An Advertised Prefix including or included in the considered
Delegated Prefix is added or removed.
o An Assigned Prefix included in the considered Delegated Prefix and
associated with a different Link than the considered Link was
destroyed, while there is no Current Assignment associated with
the given pair. This case MAY be ignored if the creation of a new
Assigned Prefix associated with the considered pair is not
desired.
o The considered Delegated Prefix is added.
o The considered Link is added.
o The Node ID is modified.
Additionaly, for a given Delegated Prefix and Link pair, the routine
MUST be run as triggered by the Backoff Timer whenever:
o The Backoff Timer associated with the considered Delegated Prefix/
Link pair fires while there is no Current Assignment associated
with the given pair.
When such an event occurs, a node MAY delay the execution of the
routine instead of executing it immediately, e.g. while receiving an
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update from the Flooding Mechanism, or for security reasons (see
Section 8). Even though other events occur in the meantime, the
routine MUST be run only once. It is also assumed that, whenever one
of these events is the Backoff Timer firing, the routine is executed
as triggered by the Backoff Timer.
In order to execute the routine for a given Delegated Prefix/Link
pair, first look for the Best Assignment and Current Assignment
associated with the Delegated Prefix/Link pair, then execute the
corresponding case:
1. If there is no Best Assignment and no Current Assignment: Decide
whether the creation of a new assignment for the given Delegated
Prefix/Link pair is desired (As any result would be valid, the
way the decision is taken is out of the scope of this document)
and do the following:
* If it is not desired, stop the execution of the routine.
* Else if the Backoff Timer is running, stop the execution of
the routine.
* Else if the routine was not executed as triggered by the
Backoff Timer, set the Backoff Timer to some random delay
between ADOPT_MAX_DELAY and BACKOFF_MAX_DELAY (see Section 7)
and stop the execution of the routine.
* Else, continue the execution of the routine.
Select a prefix for the new assignment (see Section 5 for
guidance regarding prefix selection). This prefix MUST be
included in or be equal to the considered Delegated Prefix and
MUST NOT include or be included in any Advertised Prefix. If a
suitable prefix is found, use it to create a new Assigned Prefix:
* Assigned to the considered Link.
* Not applied.
* The Apply Timer set to '2 * Flooding Delay'.
* Published with some selected Advertised Prefix Priority.
2. If there is a Best Assignment but no Current Assignment: Cancel
the Backoff Timer and use the prefix from the Best Assignment to
create a new Assigned Prefix:
* Assigned to the considered Link.
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* Not applied.
* The Apply Timer set to '2 * Flooding Delay'.
* Not published.
3. If there is a Current Assignment but no Best Assignment:
* If the Current Assignment is not valid, destroy it, and
execute the routine again, as not triggered by the Backoff
Timer.
* If the Current Assignment is valid and published, stop the
execution of the routine.
* If the Current Assignment is valid and not published, the node
MAY either:
+ Adopt the prefix by cancelling the Apply Timer and set the
Backoff Timer to some random delay between 0 and
ADOPT_MAX_DELAY (see Section 7). This procedure is used to
avoid renumbering when the node advertising the prefix left
the Shared Link.
+ Destroy it and execute case 1 in order to create a
different assignment.
4. If there is a Current Assignment and a Best Assignment:
* Cancel the Backoff Timer.
* If the two prefixes are identical, set the Current Assignment
as not published. If the Current Assignment is not applied
and the Apply Timer is not set, set the Apply Timer to '2 *
Flooding Delay'.
* If the two prefixes are not identical, destroy the Current
Assignment and go to case 2.
When the prefix assignment algorithm routine requires an assignment
to be created or adopted, any Advertised Prefix Priority value can be
used. Other documents MAY provide restrictions over this value
depending on the context the algorithm is operating in, or leave it
as implementation-specific.
When the prefix assignment algorithm routine requires an assignment
to be created or adopted, the chosen Advertised Prefix Priority is
unspecified (any value would be valid). The values to be used in
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such situations MAY be specified by other documents making use of the
prefix assignment algorithm or be left as an implementation specific
choice.
4.3. Overriding and Destroying Existing Assignments
In addition to the behavior specified in Section 4.2, the following
procedures MAY be used in order to provide more advanced behavior
(Section 6):
Overriding Existing Assignments: For any given Link and Delegated
Prefix, a node MAY create a new Assigned Prefix using a chosen
prefix and Advertised Prefix Priority such that:
* The chosen prefix is included in or is equal to the considered
Delegated Prefix.
* The Current Assignment, if any, as well as all existing
Assigned Prefixes which include or are included inside the
chosen prefix are destroyed.
* It is not applied.
* The Apply Timer set to '2 * Flooding Delay'.
* It is published.
* The Advertised Prefix Priority is greater than the Advertised
Prefix Priority from all Advertised Prefixes which include or
are included in the chosen prefix.
In order to ensure algorithm convergence:
* Such overriding assignments MUST NOT be created unless there
was a change in the node configuration, a Link was added, or an
Advertised Prefix was added or removed.
* The chosen Advertised Prefix Priority for the new Assigned
Prefix SHOULD be greater than all priorities from the destroyed
Assigned Prefixes. If not, simple topologies with only two
nodes may not converge. Nodes which do not respect this rule
MUST implement a mechanism which detects whether the
distributed algorithm do not converge and, whenever this would
happen, stop creating overriding Assigned Prefixes causing the
destruction of other Assigned Prefixes. The specifications for
such safety procedures are out of the scope of this document.
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Removing an Assigned Prefix: A node MAY destroy any Assigned Prefix
which is published. Such an event reflects the desire from a node
to not assign a prefix from a given Delegated Prefix to a given
Link anymore. In order to ensure algorithm convergence, such
procedure MUST NOT be executed unless there was a change in the
node configuration. Additionally, whenever an Assigned Prefix is
destroyed this way, the prefix assignment algorithm routine MUST
be run for the Delegated Prefix/Link pair associated with the
deleted Assigned Prefix.
These procedures are optional. They could be used for diverse
purposes, e.g., for providing custom prefix assignment configuration
or reacting to prefix space exhaustion (by overriding short Assigned
Prefixes and assigning longer ones).
4.4. Other Events
When the Apply Timer fires, the associated prefix MUST be applied.
When the Backoff Timer associated with a given Delegated Prefix/Link
pair fires while there is a Current Assignment associated with the
same pair, the Current Assignment MUST be published with some
associated Advertised Prefix Priority and, if the prefix is not
applied, the Apply Timer MUST be set to '2 * Flooding Delay'.
When a Delegated Prefix is removed from the set of Delegated
Prefixes, all Assigned Prefixes included in the removed Delegated
Prefix MUST be destroyed.
When one Delegated Prefix is replaced by another one that includes or
is included in the deleted Delegated Prefix, all Assigned Prefixes
which were included in the deleted Delegated Prefix but are not
included in the added Delegated Prefix MUST be destroyed. Others MAY
be kept.
When a Link is removed, all Assigned Prefixes assigned to that Link
MUST be destroyed.
5. Prefix Selection Considerations
When the prefix assignment algorithm routine specified in Section 4.2
requires a new prefix to be selected, the prefix MUST be selected
either:
o Among prefixes which were previously assigned and applied on the
considered Link.
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o Randomly, picked in a set of at least RANDOM_SET_SIZE (see
Section 7) candidate prefixes. If less than RANDOM_SET_SIZE
candidates can be found, the prefix MUST be picked among all
candidates.
o Based on some custom selection process specified in the
configuration.
A simple implementation MAY randomly pick the prefix among all
available prefixes, but this strategy is inefficient in terms of
address space use as a few long prefixes may exhaust the pool of
available short prefixes.
The rest of this section describes a more efficient approach which
MAY be applied any time a node needs to pick a prefix for a new
assignment. The two following definitions are used:
Available prefix: The prefix A/N is available if and only if A/N
does not include and is not included in any Assigned or Advertised
Prefix but A/(N-1) does include or is included in an Assigned or
Advertised Prefix (or N equals 0 and there is no Assigned or
Advertised Prefixes at all).
Candidate prefix: A prefix which is included in or is equal to an
available prefix.
The procedure described in this section takes the three following
criteria into account:
Stability: In some cases, it is desirable that the selected prefix
remains the same across executions and reboots. For this purpose,
prefixes previously applied on the Link or pseudo-random prefixes
generated based on node and Link specific values may be
considered.
Randomness: When no stored or pseudo-random prefix is chosen, a
prefix may be randomly picked among RANDOM_SET_SIZE candidates of
desired length. If less than RANDOM_SET_SIZE candidates can be
found, the prefix is picked among all candidates.
Addressing-space usage efficiency: In the process of assigning
prefixes, a small set of badly chosen long prefixes may easily
prevent any shorter prefix from being assigned. For this reason,
the set of RANDOM_SET_SIZE candidates is created from the set of
available prefixes with longest prefix lengths and, in case of
tie, prefer small prefix values.
When executing the procedure, do as follows:
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1. For each prefix stored in stable-storage, check if the prefix is
included in or equal to an available prefix. If so, pick that
prefix and stop.
2. For each prefix length, count the number of available prefixes of
the given length.
3. If the desired prefix length was not specified, select one. The
available prefixes count computed previously may be used to help
picking a prefix length such that:
* There is at least one candidate prefix.
* The prefix length is chosen great enough to not exhaust the
address space.
Let N be the chosen prefix length.
4. Iterate over available prefixes starting with prefixes of length
N down to length 0 and create a set of RANDOM_SET_SIZE candidate
prefixes of length exactly N included in or equal to available
prefixes. The end goal here is to create a set of
RANDOM_SET_SIZE candidate prefixes of length N included in a set
of available prefixes of maximized prefix length. In case of a
tie, smaller prefix values (as defined by the bit-wise
lexicographical order) are preferred.
5. For each pseudo-random prefix, check if the prefix is equal to a
candidate prefix. If so, pick that prefix and stop.
6. Choose a random prefix from the set of selected candidates.
The complexity of this procedure is equivalent to the complexity of
iterating over available prefixes. Such operation may be
accomplished in linear time, e.g., by storing Advertised and Assigned
Prefixes in a binary trie.
6. Implementation Capabilities and Node Behavior
Implementations of the prefix assignment algorithm may vary from very
basic to highly customisable, enabling different types of fully
interoperable behaviors. The three following behaviors are given as
examples:
Listener: The node only acts upon assignments made by other nodes,
i.e, it never creates new assignments nor adopt existing ones.
Such behavior does not require the implementation of the
considerations specified in Section 5 or Section 4.3. The node
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never checks existing assignments validity, which makes this
behavior particularly suited to lightweight devices which can rely
on more capable neighbors to make assignments on directly
connected Shared Links.
Basic: The node is capable of assigning new prefixes or adopting
prefixes which do not conflict with any other existing assignment.
Such behavior does not require the implementation of the
considerations specified in Section 4.3. It is suited to
situations where there is no preference over which prefix should
be assigned to which Link, and there is no priority between
different Links.
Advanced: The node is capable of assigning new prefixes, adopting
existing ones, making overriding assignments and destroying
existing ones. Such behavior requires the implementation of the
considerations specified in Section 5 and Section 4.3. It is
suited when the administrator desires some particular prefix to be
assigned on a given Link, or some Links to be assigned prefixes
with a higher priority.
7. Algorithm Parameters
This document does not provide values for ADOPT_MAX_DELAY,
BACKOFF_MAX_DELAY and RANDOM_SET_SIZE. The algorithm ensures
convergence and correctness for any chosen values, even when these
are different from node to node. They MAY be adjusted depending on
the context, providing a tradeoff between convergence time, efficient
addressing, low verbosity (less traffic is generated by the Flooding
Mechanism), and low collision probability.
ADOPT_MAX_DELAY (respectively BACKOFF_MAX_DELAY) represents the
maximum backoff time a node may wait before adopting an assignment
(respectively making a new assignment). BACKOFF_MAX_DELAY MUST be
greater than or equal to ADOPT_MAX_DELAY. The greater
ADOPT_MAX_DELAY and (BACKOFF_MAX_DELAY - ADOPT_MAX_DELAY), the lower
the collision probability and the verbosity, but the longer the
convergence time.
RANDOM_SET_SIZE represents the desired size of the set a random
prefix will be picked from. The greater RANDOM_SET_SIZE, the better
the convergence time and the lower the collision probability, but the
worse the addressing-space usage efficiency.
When the Flooding Mechanism does not provide a Flooding Delay, it is
set to DEFAULT_FLOODING_DELAY. As participating nodes do not need to
agree on a common Flooding Delay value, this default value MAY be
different from one node to another. If the context in which the
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algorithm is used does not suffer from renumbering, the value 0 MAY
be used. Otherwise it depends on the Flooding Mechanism properties
and the desired renumbering probability, and is therefore out of
scope of this document.
8. Security Considerations
The prefix assignment algorithm functions on top of two distinct
mechanisms, the Flooding Mechanism and the Node ID assignment
mechanism. In order to operate securely:
An attacker able to publish Advertised Prefixes through the
flooding mechanism may perform the following attacks:
* Publish a single overriding assignment for a whole Delegated
Prefix or for the whole address space, thus preventing any node
from assigning prefixes to Links.
* Quickly publish and remove Advertised Prefixes, generating
traffic at the Flooding Mechanism layer and causing multiple
executions of the prefix assignment algorithm in all
participating nodes.
* Publish and remove Advertised Prefixes in order to prevent the
convergence of the execution.
An attacker able to prevent other nodes from accessing a portion
or the whole set of Advertised Prefixes may compromise the
correctness of the execution.
An attacker able to cause repetitive Node ID changes may induce
traffic generation from the Flooding Mechanism and multiple
executions of the prefix assignment algorithm in all participating
nodes.
An attacker able to publish Advertised Prefixes using a Node ID
used by another node may prevent the correctness and convergence
of the execution.
Whenever the security of the Flooding Mechanism and Node ID
assignment mechanism could not be ensured, the convergence of the
execution may be prevented. In environments where such attacks may
be performed, the execution of the prefix assignment algorithm
routine SHOULD be rate limited, as specified in Section 4.2.
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9. IANA Considerations
This document has no actions for IANA.
10. Acknowledgments
The authors would like to thank those who participated in the
previous document's version development as well as the present one.
In particular, the authors would like to thank Tim Chown, Fred Baker,
Mark Townsley, Lorenzo Colitti, Ole Troan, Ray Bellis, Markus
Stenberg, Wassim Haddad, Joel Halpern, Samita Chakrabarti, Michael
Richardson, Anders Brandt, Erik Nordmark, Laurent Toutain, Ralph
Droms, Acee Lindem and Steven Barth for interesting discussions and
document review.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
11.2. Informative References
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
Host Configuration Protocol (DHCP) version 6", RFC 3633,
December 2003.
Appendix A. Static Configuration Example
This section describes an example of how custom configuration of the
prefix assignment algorithm may be implemented.
The node configuration is specified as a finite set of rules. A rule
is defined as:
o A prefix to be used.
o A Link on which the prefix may be assigned.
o An Assigned Prefix Priority (smallest possible Assigned Prefix
Priority if the rule may not override other Assigned Prefixes).
o A rule priority (0 if the rule may not override existing
Advertised Prefixes).
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In order to ensure the convergence of the execution, the Assigned
Prefix Priority MUST be an increasing function (not necessarily
strictly) of the configuration rule priority (i.e. the greater is the
configuration rule priority, the greater the Assigned Prefix Priority
must be).
Each Assigned Prefix is associated with a rule priority. Assigned
Prefixes which are created as specified in Section 4.2 are given a
rule priority of 0.
Whenever the configuration is changed or the prefix assignment
algorithm routine is run: For each Link/Delegated Prefix pair, look
for the configuration rule with the highest configuration rule
priority such that:
o The prefix specified in the configuration rule is included in the
considered Delegated Prefix.
o The Link specified in the configuration rule is the considered
Link.
o All the Assigned Prefixes which would need to be destroyed in case
a new Assigned Prefix is created from that configuration rule (as
specified in Section 4.3) have an associated rule priority which
is strictly lower than the one of the considered configuration
rule.
o The assignment would be valid when published with an Advertised
Prefix Priority equal to the one specified in the configuration
rule.
If a rule is found, a new Assigned Prefix is created based on that
rule in conformance with Section 4.3. The new Assigned Prefix is
associated with the Advertised Prefix Priority and the rule priority
specified in the considered configuration rule.
Note that the use of rule priorities ensures the convergence of the
execution.
Authors' Addresses
Pierre Pfister
Cisco Systems
Paris
France
Email: pierre.pfister@darou.fr
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Benjamin Paterson
Cisco Systems
Paris
France
Email: benjamin@paterson.fr
Jari Arkko
Ericsson
Jorvas 02420
Finland
Email: jari.arkko@piuha.net
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