draft-ietf-homenet-prefix-assignment-02.txt   draft-ietf-homenet-prefix-assignment-03.txt 
Network Working Group P. Pfister Network Working Group P. Pfister
Internet-Draft B. Paterson Internet-Draft B. Paterson
Intended status: Standards Track Cisco Systems Intended status: Standards Track Cisco Systems
Expires: July 9, 2015 J. Arkko Expires: August 12, 2015 J. Arkko
Ericsson Ericsson
January 5, 2015 February 8, 2015
Distributed Prefix Assignment Algorithm Distributed Prefix Assignment Algorithm
draft-ietf-homenet-prefix-assignment-02 draft-ietf-homenet-prefix-assignment-03
Abstract Abstract
This document specifies a distributed algorithm for automatic prefix This document specifies a distributed algorithm for automatic prefix
assignment. Given a set of delegated prefixes, it ensures at most assignment. Given a set of delegated prefixes, it ensures that at
one prefix is assigned from each delegated prefix to each link. most one prefix is assigned from each delegated prefix to each link.
Nodes may assign available prefixes to the links they are directly Nodes may assign available prefixes to the links they are directly
connected to, or for other private purposes. The algorithm connected to, or for other private purposes. The algorithm
eventually converges and ensures that all assigned prefixes do not eventually converges and ensures that all assigned prefixes do not
overlap. overlap.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 9, 2015. This Internet-Draft will expire on August 12, 2015.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Applicability statement . . . . . . . . . . . . . . . . . . . 5 3. Applicability statement . . . . . . . . . . . . . . . . . . . 5
4. Algorithm Specification . . . . . . . . . . . . . . . . . . . 6 4. Algorithm Specification . . . . . . . . . . . . . . . . . . . 6
4.1. Algorithm Terminology . . . . . . . . . . . . . . . . . . 6 4.1. Algorithm Terminology . . . . . . . . . . . . . . . . . . 6
4.2. Prefix Assignment Algorithm Routine . . . . . . . . . . . 7 4.2. Prefix Assignment Algorithm Routine . . . . . . . . . . . 7
4.3. Overriding and Destroying Existing Assignments . . . . . 10 4.3. Overriding and Destroying Existing Assignments . . . . . 10
4.4. Other Events . . . . . . . . . . . . . . . . . . . . . . 11 4.4. Other Events . . . . . . . . . . . . . . . . . . . . . . 11
5. Prefix Selection Considerations . . . . . . . . . . . . . . . 11 5. Prefix Selection Considerations . . . . . . . . . . . . . . . 12
6. Implementation Capabilities and Node Behavior . . . . . . . . 13 6. Implementation Capabilities and Node Behavior . . . . . . . . 14
7. Algorithm Parameters . . . . . . . . . . . . . . . . . . . . 14 7. Algorithm Parameters . . . . . . . . . . . . . . . . . . . . 14
8. Security Considerations . . . . . . . . . . . . . . . . . . . 15 8. Security Considerations . . . . . . . . . . . . . . . . . . . 15
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 16 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
11.1. Normative References . . . . . . . . . . . . . . . . . . 16 11.1. Normative References . . . . . . . . . . . . . . . . . . 16
11.2. Informative References . . . . . . . . . . . . . . . . . 16 11.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix A. Static Configuration Example . . . . . . . . . . . . 16 Appendix A. Static Configuration Example . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction 1. Introduction
This document specifies a distributed algorithm for automatic prefix This document specifies a distributed algorithm for automatic prefix
assignment. Given a set of delegated prefixes, nodes may assign assignment. Given a set of delegated prefixes, nodes may assign
available prefixes to links they are directly connected to, or for available prefixes to links they are directly connected to, or for
their private use. The algorithm ensures that the following their private use. The algorithm ensures that the following
assertions are eventually true: assertions are eventually true:
1. At most one prefix from each delegated prefix is assigned to each 1. At most one prefix from each delegated prefix is assigned to each
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mechanism ensures that all messages are propagated to all nodes mechanism ensures that all messages are propagated to all nodes
faster than a given timing upper bound, the algorithm also ensures faster than a given timing upper bound, the algorithm also ensures
that all assigned prefixes used for networking operations (e.g., host that all assigned prefixes used for networking operations (e.g., host
configuration) remain unchanged, unless another node assigns an configuration) remain unchanged, unless another node assigns an
overlapping prefix with a higher assignment priority, or the topology overlapping prefix with a higher assignment priority, or the topology
changes and renumbering cannot be avoided. changes and renumbering cannot be avoided.
2. Terminology 2. Terminology
In this document, the key words "MAY", "MUST, "MUST NOT", "OPTIONAL", In this document, the key words "MAY", "MUST, "MUST NOT", "OPTIONAL",
"RECOMMENDED", "SHOULD", and "SHOULD NOT", are to be interpreted as and "SHOULD", are to be interpreted as described in [RFC2119].
described in [RFC2119].
This document makes use of the following terminology: This document makes use of the following terminology:
Link: An object the distributed algorithm will assign prefixes to. Link: An object the distributed algorithm will assign prefixes to.
A Node may only assign prefixes to Links it is directly connected A Node may only assign prefixes to Links it is directly connected
to. A Link is either Shared or Private. to. A Link is either Shared or Private.
Private Link: A Private Link is an abstract concept defined for the Private Link: A Private Link is an abstract concept defined for the
sake of this document. It allows nodes to make assignments for sake of this document. It allows nodes to make assignments for
their private use or delegation. For instance, every DHCPv6-PD their private use or delegation. For instance, every DHCPv6-PD
[RFC3633] client MAY be considered as a different Private Link. [RFC3633] client MAY be considered as a different Private Link.
Shared Link: A Link multiple nodes are connected to. Most of the Shared Link: A Link multiple nodes may be connected to. Most of
time, a Shared Link would consist in a multi-access link or point- the time, a Shared Link would consist in a multi-access link or
to-point link, virtual or physical, requiring prefixes to be point-to-point link, virtual or physical, requiring prefixes to be
assigned to. assigned to.
Delegated Prefix: A prefix provided to the algorithm and used as a Delegated Prefix: A prefix provided to the algorithm and used as a
prefix pool for Assigned Prefixes. prefix pool for Assigned Prefixes.
Node ID: A value identifying a given participating node. The set Node ID: A value identifying a given participating node. The set
of identifiers MUST be strictly and totally ordered (e.g., of identifiers MUST be strictly and totally ordered (e.g., using
alphanumeric order). the alphanumeric order).
Flooding Mechanism: A mechanism implementing reliable broadcast and Flooding Mechanism: A mechanism implementing reliable broadcast and
used to advertise published Assigned Prefixes. used to advertise published Assigned Prefixes.
Flooding Delay: Optional value provided by the Flooding Mechanism Flooding Delay: Value which SHOULD be provided by the Flooding
indicating a deterministic or likely upper bound of the Mechanism indicating a deterministic or likely upper bound of the
information propagation delay. When the Flooding Mechanism does information propagation delay. When the Flooding Mechanism does
not provide a value, it is set to DEFAULT_FLOODING_DELAY not provide a value, it is set to DEFAULT_FLOODING_DELAY
(Section 7). (Section 7).
Advertised Prefix: A prefix advertised by another node and Advertised Prefix: A prefix advertised by another node and
delivered to the local node by the Flooding Mechanism. It has an delivered to the local node by the Flooding Mechanism. It has an
Advertised Prefix Priority and, when assigned to a directly Advertised Prefix Priority and, when assigned to a directly
connected Shared Link, is associated with a Shared Link. connected Shared Link, is associated with a Shared Link.
Advertised Prefix Priority: A value that defines the priority of an Advertised Prefix Priority: A value that defines the priority of an
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algorithm. Each Assigned Prefix is associated with a timer (Apply algorithm. Each Assigned Prefix is associated with a timer (Apply
Timer) used to apply the Assigned Prefix. An Assigned Prefix is Timer) used to apply the Assigned Prefix. An Assigned Prefix is
unapplied when destroyed. unapplied when destroyed.
Published (Assigned Prefix): The Assigned Prefix is advertised Published (Assigned Prefix): The Assigned Prefix is advertised
through the Flooding Mechanism as assigned to its associated Link. through the Flooding Mechanism as assigned to its associated Link.
A published Assigned Prefix MUST have an Advertised Prefix A published Assigned Prefix MUST have an Advertised Prefix
Priority. It will appear as an Advertised Prefix to other nodes, Priority. It will appear as an Advertised Prefix to other nodes,
once received through the Flooding Mechanism. once received through the Flooding Mechanism.
Prefix Adoption: When an Advertised Prefix which does not conflict
with any other Advertised Prefix or published Assigned Prefix
stops being advertised, any other node connected to the same Link
MAY, after some random delay, start advertising the same prefix.
This procedure is called adoption and provides seamless assignment
transfer from a node to another, e.g., in case of node failure.
Backoff Timer: Every Delegated Prefix and Link pair is associated Backoff Timer: Every Delegated Prefix and Link pair is associated
with a timer counting down to zero. It is used to avoid multiple with a timer counting down to zero. It is used to avoid multiple
nodes from making colliding assignments by delaying the creation nodes from making colliding assignments by delaying the creation
of new Assigned Prefixes or the advertisement of adopted Assigned of new Assigned Prefixes or the advertisement of adopted Assigned
Prefixes by a random amount of time. Prefixes by a random amount of time.
Renumbering: Event occuring when an Assigned Prefix which was Renumbering: Event occurring when an Assigned Prefix which was
applied is destroyed. It is undesirable as it usually implies applied is destroyed. It is undesirable as it usually implies
reconfiguring routers or hosts. reconfiguring routers or hosts.
3. Applicability statement 3. Applicability statement
Each node MUST have a set of disjoint Delegated Prefixes. It MAY Each node MUST have a set of disjoint Delegated Prefixes. This set
change over time and be different from one node to another at some MAY change over time and be different from one node to another at
point, but nodes MUST eventually agree on the same set of disjoint some point, but nodes MUST eventually have the same set of disjoint
Delegated Prefixes. Delegated Prefixes.
Given this set of disjoint Delegated Prefixes, nodes may assign Given this set of disjoint Delegated Prefixes, nodes may assign
available prefixes from each Delegated Prefix to the Links they are available prefixes from each Delegated Prefix to the Links they are
directly connected to. The algorithm ensures that at most one prefix directly connected to. The algorithm ensures that at most one prefix
from a given Delegated Prefix is assigned to a given Link. 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 The algorithm can be applied to any address space and can be used to
manage multiple address spaces simultaneously. For instance, an manage multiple address spaces simultaneously. For instance, an
implementation can make use of IPv4-mapped IPv6 addresses [RFC4291] implementation can make use of IPv4-mapped IPv6 addresses [RFC4291]
in order to manage both IPv4 and IPv6 prefix assignment in order to manage both IPv4 and IPv6 prefix assignment using a
simultaneously. single prefix space.
The algorithm supports dynamically changing topologies: The algorithm supports dynamically changing topologies:
o Nodes may join or leave the set of participating nodes. o Nodes may join or leave the set of participating nodes.
o Nodes may join or leave Links. o Nodes may join or leave Links.
o Links may be joined or split. o Links may be joined or split.
All nodes MUST run a common Flooding Mechanism in order to share All nodes MUST run a common Flooding Mechanism in order to share
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The Flooding Mechanism MUST: The Flooding Mechanism MUST:
o Provide a way to flood Assigned Prefixes assigned to a directly o Provide a way to flood Assigned Prefixes assigned to a directly
connected Link along with their respective Advertised Prefix connected Link along with their respective Advertised Prefix
Priority and the Node ID of the node which advertises it. Priority and the Node ID of the node which advertises it.
o Specify whether an Advertised Prefix was assigned to a directly o Specify whether an Advertised Prefix was assigned to a directly
connected Shared Link, and if so, on which one. connected Shared Link, and if so, on which one.
In addition, a Flooding Delay SHOULD be specified and respected in In addition, a Flooding Delay SHOULD be specified and respected in
order to avoid undesired renumbering. If not specified, or whenever order to avoid renumbering. If not specified, or whenever the
the Flooding Mechanism is unable to respect the provided delay, Flooding Mechanism is unable to respect the provided delay,
renumbering may happen. As such delay often depends on the size of renumbering may happen. As such delay often depends on the size of
the network, it MAY change over time and MAY be different from one the network, it MAY change over time and MAY be different from one
node to another. node to another.
The algorithm ensures that whenever the Flooding Delay is provided The algorithm ensures that whenever the Flooding Delay is provided
and respected, and in the absence of topology change or delegated and respected, and in the absence of topology change or delegated
prefix removal, renumbering never happens. prefix removal, renumbering never happens.
Each node MUST have a Node ID. Node IDs MAY change over time and be 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 the same on multiple nodes at some point, but each node MUST
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assignment algorithm. assignment algorithm.
4.1. Algorithm Terminology 4.1. Algorithm Terminology
The algorithm makes use of the following terms: The algorithm makes use of the following terms:
Current Assignment: For a given Delegated Prefix and Link, the Current Assignment: For a given Delegated Prefix and Link, the
Current Assignment is the Assigned Prefix (if any) included in the Current Assignment is the Assigned Prefix (if any) included in the
Delegated Prefix and assigned to the given Link. 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 Precedence: An Advertised Prefix takes precedence over an Assigned
Prefix if and only if: Prefix if and only if:
* The Assigned Prefix is not published. * The Assigned Prefix is not published.
* The Assigned Prefix is published and the Advertised Prefix * The Assigned Prefix is published and the Advertised Prefix
Priority from the Advertised Prefix is strictly greater than Priority from the Advertised Prefix is strictly greater than
the Advertised Prefix Priority from the Assigned Prefix. the Advertised Prefix Priority from the Assigned Prefix.
* The Assigned Prefix is published, the priorities are equal, and * The Assigned Prefix is published, the priorities are identical,
the Node ID from the node advertising the Advertised Prefix is and the Node ID from the node advertising the Advertised Prefix
strictly greater than the local Node ID. is strictly greater than the local Node ID.
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 assigned on the given Link (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 associated with
the same Link and Delegated Prefix (if any).
Valid (Assigned Prefix) An Assigned Prefix is valid if and only if Valid (Assigned Prefix) An Assigned Prefix is valid if and only if
the two following conditions are met: the two following conditions are met:
* No Advertised Prefix including or included in the Assigned * No Advertised Prefix including or included in the Assigned
Prefix takes precedence over the Assigned Prefix. Prefix takes precedence over the Assigned Prefix.
* No Advertised Prefix including or included in the same * No Advertised Prefix including or included in the same
Delegated Prefix as the Assigned Prefix and assigned to the Delegated Prefix as the Assigned Prefix and assigned to the
same Link takes precedence over the considered Assigned Prefix. same Link takes precedence over the Assigned Prefix.
4.2. Prefix Assignment Algorithm Routine 4.2. Prefix Assignment Algorithm Routine
This section specifies the prefix assignment algorithm routine. It This section specifies the prefix assignment algorithm routine. It
is defined for a given Delegated Prefix/Link pair and may be run is defined for a given Delegated Prefix/Link pair and may be run
either as triggered by the Backoff Timer, or not. either as triggered by the Backoff Timer, or not.
For a given Delegated Prefix and Link pair, the routine MUST be run For a given Delegated Prefix and Link pair, the routine MUST be run
as not triggered by the Backoff Timer whenever: as not triggered by the Backoff Timer whenever:
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the given pair. This case MAY be ignored if the creation of a new the given pair. This case MAY be ignored if the creation of a new
Assigned Prefix associated with the considered pair is not Assigned Prefix associated with the considered pair is not
desired. desired.
o The considered Delegated Prefix is added. o The considered Delegated Prefix is added.
o The considered Link is added. o The considered Link is added.
o The Node ID is modified. o The Node ID is modified.
Additionaly, for a given Delegated Prefix and Link pair, the routine Additionally, for a given Delegated Prefix and Link pair, the routine
MUST be run as triggered by the Backoff Timer whenever: MUST be run as triggered by the Backoff Timer whenever:
o The Backoff Timer associated with the considered Delegated Prefix/ o The Backoff Timer associated with the considered Delegated Prefix/
Link pair fires while there is no Current Assignment associated Link pair fires while there is no Current Assignment associated
with the given pair. with the given pair.
When such an event occurs, a node MAY delay the execution of the When such an event occurs, a node MAY delay the execution of the
routine instead of executing it immediately, e.g. while receiving an routine instead of executing it immediately, e.g. while receiving an
update from the Flooding Mechanism, or for security reasons (see update from the Flooding Mechanism, or for security reasons (see
Section 8). Even though other events occur in the meantime, the Section 8). Even though other events occur in the meantime, the
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When the prefix assignment algorithm routine requires an assignment When the prefix assignment algorithm routine requires an assignment
to be created or adopted, the chosen Advertised Prefix Priority is to be created or adopted, the chosen Advertised Prefix Priority is
unspecified (any value would be valid). The values to be used in unspecified (any value would be valid). The values to be used in
such situations MAY be specified by other documents making use of the such situations MAY be specified by other documents making use of the
prefix assignment algorithm or be left as an implementation specific prefix assignment algorithm or be left as an implementation specific
choice. choice.
4.3. Overriding and Destroying Existing Assignments 4.3. Overriding and Destroying Existing Assignments
In addition to the behavior specified in Section 4.2, the following In addition to the behaviors specified in Section 4.2, the following
procedures MAY be used in order to provide more advanced behavior procedures MAY be used in order to provide more advanced behavior
(Section 6): (Section 6):
Overriding Existing Assignments: For any given Link and Delegated Overriding Existing Assignments: For any given Link and Delegated
Prefix, a node MAY create a new Assigned Prefix using a chosen Prefix, a node MAY create a new Assigned Prefix using a chosen
prefix and Advertised Prefix Priority such that: prefix and Advertised Prefix Priority such that:
* The chosen prefix is included in or is equal to the considered * The chosen prefix is included in or is equal to the considered
Delegated Prefix. Delegated Prefix.
* The Current Assignment, if any, as well as all existing * The Current Assignment, if any, as well as all existing
Assigned Prefixes which include or are included inside the Assigned Prefixes which include or are included inside the
chosen prefix are destroyed. chosen prefix, are destroyed.
* It is not applied. * It is not applied.
* The Apply Timer set to '2 * Flooding Delay'. * The Apply Timer set to '2 * Flooding Delay'.
* It is published. * It is published.
* The Advertised Prefix Priority is greater than the Advertised * The Advertised Prefix Priority is greater than the Advertised
Prefix Priority from all Advertised Prefixes which include or Prefix Priority from all Advertised Prefixes which include or
are included in the chosen prefix. are included in the chosen prefix.
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* Such overriding assignments MUST NOT be created unless there * Such overriding assignments MUST NOT be created unless there
was a change in the node configuration, a Link was added, or an was a change in the node configuration, a Link was added, or an
Advertised Prefix was added or removed. Advertised Prefix was added or removed.
* The chosen Advertised Prefix Priority for the new Assigned * The chosen Advertised Prefix Priority for the new Assigned
Prefix SHOULD be greater than all priorities from the destroyed Prefix SHOULD be greater than all priorities from the destroyed
Assigned Prefixes. If not, simple topologies with only two Assigned Prefixes. If not, simple topologies with only two
nodes may not converge. Nodes which do not respect this rule nodes may not converge. Nodes which do not respect this rule
MUST implement a mechanism which detects whether the MUST implement a mechanism which detects whether the
distributed algorithm do not converge and, whenever this would distributed algorithm do not converge and, whenever this would
happen, stop creating overriding Assigned Prefixes causing the happen, stop creating overriding Assigned Prefixes which do not
destruction of other Assigned Prefixes. The specifications for hold this rule. The specifications for such safety procedures
such safety procedures are out of the scope of this document. are out of the scope of this document.
Removing an Assigned Prefix: A node MAY destroy any Assigned Prefix Removing an Assigned Prefix: A node MAY destroy any Assigned Prefix
which is published. Such an event reflects the desire from a node 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 to not assign a prefix from a given Delegated Prefix to a given
Link anymore. In order to ensure algorithm convergence, such Link anymore. In order to ensure algorithm convergence, such
procedure MUST NOT be executed unless there was a change in the procedure MUST NOT be executed unless there was a change in the
node configuration. Additionally, whenever an Assigned Prefix is node configuration. Additionally, whenever an Assigned Prefix is
destroyed this way, the prefix assignment algorithm routine MUST destroyed this way, the prefix assignment algorithm routine MUST
be run for the Delegated Prefix/Link pair associated with the be run for the Delegated Prefix/Link pair associated with the
deleted Assigned Prefix. deleted Assigned Prefix.
These procedures are optional. They could be used for diverse These procedures are OPTIONAL. They could be used for diverse
purposes, e.g., for providing custom prefix assignment configuration purposes, e.g., for providing custom prefix assignment configuration
or reacting to prefix space exhaustion (by overriding short Assigned or reacting to prefix space exhaustion (by overriding short Assigned
Prefixes and assigning longer ones). Prefixes and assigning longer ones).
4.4. Other Events 4.4. Other Events
When the Apply Timer fires, the associated prefix MUST be applied. When the Apply Timer fires, the associated Assigned Prefix MUST be
applied.
When the Backoff Timer associated with a given Delegated Prefix/Link When the Backoff Timer associated with a given Delegated Prefix/Link
pair fires while there is a Current Assignment associated with the pair fires while there is a Current Assignment associated with the
same pair, the Current Assignment MUST be published with some same pair, the Current Assignment MUST be published with some
associated Advertised Prefix Priority and, if the prefix is not associated Advertised Prefix Priority and, if the prefix is not
applied, the Apply Timer MUST be set to '2 * Flooding Delay'. applied, the Apply Timer MUST be set to '2 * Flooding Delay'.
When a Delegated Prefix is removed from the set of Delegated When a Delegated Prefix is removed from the set of Delegated
Prefixes, all Assigned Prefixes included in the removed Delegated Prefixes, all Assigned Prefixes included in the removed Delegated
Prefix MUST be destroyed. Prefix MUST be destroyed.
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When a Link is removed, all Assigned Prefixes assigned to that Link When a Link is removed, all Assigned Prefixes assigned to that Link
MUST be destroyed. MUST be destroyed.
5. Prefix Selection Considerations 5. Prefix Selection Considerations
When the prefix assignment algorithm routine specified in Section 4.2 When the prefix assignment algorithm routine specified in Section 4.2
requires a new prefix to be selected, the prefix MUST be selected requires a new prefix to be selected, the prefix MUST be selected
either: either:
o Among prefixes which were previously assigned and applied on the o Among prefixes which were previously assigned and applied on the
considered Link. considered Link. For that purpose, Applied Prefixes may be stored
in stable storage along with their associated Link.
o Randomly, picked in a set of at least RANDOM_SET_SIZE (see o Randomly, picked in a set of at least RANDOM_SET_SIZE (see
Section 7) candidate prefixes. If less than RANDOM_SET_SIZE Section 7) candidate prefixes. If less than RANDOM_SET_SIZE
candidates can be found, the prefix MUST be picked among all candidates can be found, the prefix MUST be picked among all
candidates. candidates.
o Based on some custom selection process specified in the o Based on some custom selection process specified in the
configuration. configuration.
A simple implementation MAY randomly pick the prefix among all A simple implementation MAY randomly pick the prefix among all
available prefixes, but this strategy is inefficient in terms of available prefixes, but this strategy is inefficient in terms of
address space use as a few long prefixes may exhaust the pool of address space use as a few long prefixes may exhaust the pool of
available short prefixes. available short prefixes.
The rest of this section describes a more efficient approach which 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 MAY be applied any time a node needs to pick a prefix for a new
assignment. The two following definitions are used: assignment. The two following definitions are used:
Available prefix: The prefix A/N is available if and only if A/N 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 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 Prefix but A/(N-1) does include an Assigned or Advertised Prefix
Advertised Prefix (or N equals 0 and there is no Assigned or (or N equals 0 and there is no Assigned or Advertised Prefixes at
Advertised Prefixes at all). all).
Candidate prefix: A prefix which is included in or is equal to an Candidate prefix: A prefix which is included in or is equal to an
available prefix. available prefix.
The procedure described in this section takes the three following The procedure described in this section takes the three following
criteria into account: criteria into account:
Stability: In some cases, it is desirable that the selected prefix Stability: In some cases, it is desirable that the selected prefix
remains the same across executions and reboots. For this purpose, remains the same across executions and reboots. For this purpose,
prefixes previously applied on the Link or pseudo-random prefixes prefixes previously applied on the Link or pseudo-random prefixes
generated based on node and Link specific values may be generated based on node and Link specific values may be
considered. considered.
Randomness: When no stored or pseudo-random prefix is chosen, a Randomness: When no stored or pseudo-random prefix is chosen, a
prefix may be randomly picked among RANDOM_SET_SIZE candidates of prefix may be randomly picked among RANDOM_SET_SIZE candidates of
desired length. If less than RANDOM_SET_SIZE candidates can be desired length. If less than RANDOM_SET_SIZE candidates can be
found, the prefix is picked among all candidates. found, the prefix is picked among all candidates.
Addressing-space usage efficiency: In the process of assigning Addressing-space usage efficiency: In the process of assigning
prefixes, a small set of badly chosen long prefixes may easily prefixes, a small set of badly chosen long prefixes may prevent
prevent any shorter prefix from being assigned. For this reason, any shorter prefix from being assigned. For this reason, the set
the set of RANDOM_SET_SIZE candidates is created from the set of of RANDOM_SET_SIZE candidates is created from the set of available
available prefixes with longest prefix lengths and, in case of prefixes with longest prefix lengths and, in case of tie,
tie, prefer small prefix values. preferring small prefix values.
When executing the procedure, do as follows: When executing the procedure, do as follows:
1. For each prefix stored in stable-storage, check if the prefix is 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 included in or equal to an available prefix. If so, pick that
prefix and stop. prefix and stop.
2. For each prefix length, count the number of available prefixes of 2. For each prefix length, count the number of available prefixes of
the given length. the given length.
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6. Choose a random prefix from the set of selected candidates. 6. Choose a random prefix from the set of selected candidates.
The complexity of this procedure is equivalent to the complexity of The complexity of this procedure is equivalent to the complexity of
iterating over available prefixes. Such operation may be iterating over available prefixes. Such operation may be
accomplished in linear time, e.g., by storing Advertised and Assigned accomplished in linear time, e.g., by storing Advertised and Assigned
Prefixes in a binary trie. Prefixes in a binary trie.
6. Implementation Capabilities and Node Behavior 6. Implementation Capabilities and Node Behavior
Implementations of the prefix assignment algorithm may vary from very Implementations of the prefix assignment algorithm may vary from very
basic to highly customisable, enabling different types of fully basic to highly customizable, enabling different types of fully
interoperable behaviors. The three following behaviors are given as interoperable behaviors. The three following behaviors are given as
examples: examples:
Listener: The node only acts upon assignments made by other nodes, Listener: The node only acts upon assignments made by other nodes,
i.e, it never creates new assignments nor adopt existing ones. i.e, it never creates new assignments nor adopt existing ones.
Such behavior does not require the implementation of the Such behavior does not require the implementation of the
considerations specified in Section 5 or Section 4.3. The node considerations specified in Section 5 or Section 4.3. The node
never checks existing assignments validity, which makes this never checks existing assignments validity, which makes this
behavior particularly suited to lightweight devices which can rely behavior particularly suited to lightweight devices which can rely
on more capable neighbors to make assignments on directly on more capable neighbors to make assignments on directly
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situations where there is no preference over which prefix should situations where there is no preference over which prefix should
be assigned to which Link, and there is no priority between be assigned to which Link, and there is no priority between
different Links. different Links.
Advanced: The node is capable of assigning new prefixes, adopting Advanced: The node is capable of assigning new prefixes, adopting
existing ones, making overriding assignments and destroying existing ones, making overriding assignments and destroying
existing ones. Such behavior requires the implementation of the existing ones. Such behavior requires the implementation of the
considerations specified in Section 5 and Section 4.3. It is considerations specified in Section 5 and Section 4.3. It is
suited when the administrator desires some particular prefix to be suited when the administrator desires some particular prefix to be
assigned on a given Link, or some Links to be assigned prefixes assigned on a given Link, or some Links to be assigned prefixes
with a higher priority. with a greater priority.
7. Algorithm Parameters 7. Algorithm Parameters
This document does not provide values for ADOPT_MAX_DELAY, This document does not provide values for ADOPT_MAX_DELAY,
BACKOFF_MAX_DELAY and RANDOM_SET_SIZE. The algorithm ensures BACKOFF_MAX_DELAY and RANDOM_SET_SIZE. The algorithm ensures
convergence and correctness for any chosen values, even when these convergence and correctness for any chosen values, even when these
are different from node to node. They MAY be adjusted depending on are different from node to node. They MAY be adjusted depending on
the context, providing a tradeoff between convergence time, efficient the context, providing a tradeoff between convergence time, efficient
addressing, low verbosity (less traffic is generated by the Flooding addressing, low verbosity (less traffic is generated by the Flooding
Mechanism), and low collision probability. Mechanism), and low collision probability.
ADOPT_MAX_DELAY (respectively BACKOFF_MAX_DELAY) represents the ADOPT_MAX_DELAY (respectively BACKOFF_MAX_DELAY) represents the
maximum backoff time a node may wait before adopting an assignment maximum backoff time a node may wait before adopting an assignment
(respectively making a new assignment). BACKOFF_MAX_DELAY MUST be (respectively making a new assignment). BACKOFF_MAX_DELAY MUST be
greater than or equal to ADOPT_MAX_DELAY. The greater greater than or equal to ADOPT_MAX_DELAY. The greater
ADOPT_MAX_DELAY and (BACKOFF_MAX_DELAY - ADOPT_MAX_DELAY), the lower ADOPT_MAX_DELAY and (BACKOFF_MAX_DELAY - ADOPT_MAX_DELAY), the lower
the collision probability and the verbosity, but the longer the the collision probability and the verbosity, but the greater the
convergence time. convergence time.
RANDOM_SET_SIZE represents the desired size of the set a random RANDOM_SET_SIZE represents the desired size of the set a random
prefix will be picked from. The greater RANDOM_SET_SIZE, the better prefix will be picked from. The greater RANDOM_SET_SIZE, the better
the convergence time and the lower the collision probability, but the the convergence time and the lower the collision probability, but the
worse the addressing-space usage efficiency. worse the addressing-space usage efficiency.
When the Flooding Mechanism does not provide a Flooding Delay, it is When the Flooding Mechanism does not provide a Flooding Delay, it is
set to DEFAULT_FLOODING_DELAY. As participating nodes do not need to set to DEFAULT_FLOODING_DELAY. As participating nodes do not need to
agree on a common Flooding Delay value, this default value MAY be agree on a common Flooding Delay value, this default value MAY be
different from one node to another. If the context in which the different from one node to another. If the context in which the
algorithm is used does not suffer from renumbering, the value 0 MAY algorithm is used does not suffer from renumbering, the value 0 MAY
be used. Otherwise it depends on the Flooding Mechanism properties be used. Otherwise it depends on the Flooding Mechanism properties
and the desired renumbering probability, and is therefore out of and the desired renumbering probability, and is therefore out of
scope of this document. scope of this document.
8. Security Considerations 8. Security Considerations
The prefix assignment algorithm functions on top of two distinct The prefix assignment algorithm functions on top of two distinct
mechanisms, the Flooding Mechanism and the Node ID assignment mechanisms, the Flooding Mechanism and the Node ID assignment
mechanism. In order to operate securely: mechanism.
An attacker able to publish Advertised Prefixes through the An attacker able to publish Advertised Prefixes through the
flooding mechanism may perform the following attacks: flooding mechanism may perform the following attacks:
* Publish a single overriding assignment for a whole Delegated * Publish a single overriding assignment for a whole Delegated
Prefix or for the whole address space, thus preventing any node Prefix or for the whole address space, thus preventing any node
from assigning prefixes to Links. from assigning prefixes to Links.
* Quickly publish and remove Advertised Prefixes, generating * Quickly publish and remove Advertised Prefixes, generating
traffic at the Flooding Mechanism layer and causing multiple traffic at the Flooding Mechanism layer and causing multiple
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