draft-ietf-homenet-prefix-assignment-04.txt   draft-ietf-homenet-prefix-assignment-05.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: September 23, 2015 J. Arkko Expires: October 10, 2015 J. Arkko
Ericsson Ericsson
March 22, 2015 April 8, 2015
Distributed Prefix Assignment Algorithm Distributed Prefix Assignment Algorithm
draft-ietf-homenet-prefix-assignment-04 draft-ietf-homenet-prefix-assignment-05
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 that at assignment. Given a set of delegated prefixes, it ensures that at
most 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.
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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 September 23, 2015. This Internet-Draft will expire on October 10, 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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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 . . . . . . . . . . . . . . . 12 5. Prefix Selection Considerations . . . . . . . . . . . . . . . 12
6. Implementation Capabilities and Node Behavior . . . . . . . . 14 6. Implementation Capabilities and Node Behavior . . . . . . . . 14
7. Algorithm Parameters . . . . . . . . . . . . . . . . . . . . 14 7. Algorithm Parameters . . . . . . . . . . . . . . . . . . . . 15
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 . . . . . . . . . . . . . . . . . . . . . . . . . 17
11.1. Normative References . . . . . . . . . . . . . . . . . . 16 11.1. Normative References . . . . . . . . . . . . . . . . . . 17
11.2. Informative References . . . . . . . . . . . . . . . . . 16 11.2. Informative References . . . . . . . . . . . . . . . . . 17
Appendix A. Static Configuration Example . . . . . . . . . . . . 17 Appendix A. Static Configuration Example . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18 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:
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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] requesting router MAY be considered as a different [RFC3633] requesting router MAY be considered as a different
Private Link. Private Link.
Shared Link: A Link multiple nodes may be connected to. Most of Shared Link: A Link multiple nodes may be connected to. Most of
the time, a Shared Link would consist in a multi-access link or the time, a Shared Link is a multi-access link or point-to-point
point-to-point link, virtual or physical, requiring prefixes to be link, virtual or physical, requiring prefixes to be assigned to
assigned to. it.
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., using of identifiers MUST be strictly and totally ordered (e.g., using
the alphanumeric order). the alphanumeric order).
Flooding Mechanism: A mechanism allowing participating nodes to Flooding Mechanism: A mechanism allowing participating nodes to
reliably share information with all other participating nodes. reliably share information with all other participating nodes.
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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 renumbering. If not specified, or whenever the order to avoid renumbering. If not specified, or whenever 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 only happens when a node deliberately
overrides an existing assignment.
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
eventually have a Node ID which is unique among the set of eventually have a Node ID which is unique among the set of
participating nodes. participating nodes.
4. Algorithm Specification 4. Algorithm Specification
This section specifies the behavior of nodes implementing the prefix This section specifies the behavior of nodes implementing the prefix
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 by the node Delegated Prefix and assigned to the given Link by the node
executing the algorithm. At some point in time, Current executing the algorithm. At some point in time, Current
Assignment from different nodes may differ, but the algorithm Assignment from different nodes may differ, but the algorithm
ensures that eventually, all nodes directly connected to a Link ensures that eventually, all nodes directly connected to a Shared
have the same Current Assignment for any given Delegated Prefix. Link have the same Current Assignment for any given 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 one of the following conditions is met:
* 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 identical, * The Assigned Prefix is published, the priorities are identical,
and the Node ID from the node advertising the Advertised Prefix and the Node ID from the node advertising the Advertised Prefix
is 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 Best Assignment: For a given Delegated Prefix and Link, the Best
Assignment is (if any) the Advertised Prefix: Assignment is the Advertised Prefix (if any):
* Including or included in the Delegated Prefix. * Including or included in the Delegated Prefix.
* Assigned on the given Link. * Assigned on the given Link.
* Having the greatest Advertised Prefix Priority among Advertised * Having the greatest Advertised Prefix Priority among Advertised
Prefixes assigned on the given Link (and, in case of tie, the Prefixes fulfilling the two preceding conditions (and, in case
prefix advertised by the node with the greatest Node ID among of tie, the prefix advertised by the node with the greatest
all prefixes with greatest priority). Node ID among all prefixes with greatest priority).
* Taking precedence over the Current Assignment associated with * Taking precedence over the Current Assignment associated with
the same Link and Delegated Prefix (if any). 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 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 and Link pair and may be run
either as triggered by the Backoff Timer, or not. either as triggered by the Backoff Timer or as triggered by another
event.
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:
o An Advertised Prefix including or included in the considered o An Advertised Prefix including or included in the considered
Delegated Prefix is added or removed. Delegated Prefix is added or removed.
o An Assigned Prefix included in the considered Delegated Prefix and o An Assigned Prefix included in the considered Delegated Prefix and
associated with a different Link than the considered Link was associated with a different Link than the considered Link was
destroyed, while there is no Current Assignment associated with destroyed, while there is no Current Assignment associated with
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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.
Additionally, 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 and Link pair fires while there is no Current Assignment
with the given pair. associated 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
routine MUST be run only once. It is also assumed that, whenever one 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 of these events is the Backoff Timer firing, the routine is executed
as triggered by the Backoff Timer. as triggered by the Backoff Timer.
In order to execute the routine for a given Delegated Prefix/Link In order to execute the routine for a given Delegated Prefix and Link
pair, first look for the Best Assignment and Current Assignment pair, first look for the Best Assignment and Current Assignment
associated with the Delegated Prefix/Link pair, then execute the associated with the Delegated Prefix and Link pair, then execute the
corresponding case: corresponding case:
1. If there is no Best Assignment and no Current Assignment: Decide 1. If there is no Best Assignment and no Current Assignment: Decide
whether the creation of a new assignment for the given Delegated whether the creation of a new assignment for the given Delegated
Prefix/Link pair is desired (As any result would be valid, the Prefix and Link pair is desired (As any result would be valid,
way the decision is taken is out of the scope of this document) the way the decision is made is out of the scope of this
and do the following: document) and do the following:
* If it is not desired, stop the execution of the routine. * If it is not desired, stop the execution of the routine.
* Else if the Backoff Timer is running, stop the execution of * Else if the Backoff Timer is running, stop the execution of
the routine. the routine.
* Else if the routine was not executed as triggered by the * Else if the routine was not executed as triggered by the
Backoff Timer, set the Backoff Timer to some random delay Backoff Timer, set the Backoff Timer to some random delay
between ADOPT_MAX_DELAY and BACKOFF_MAX_DELAY (see Section 7) between ADOPT_MAX_DELAY and BACKOFF_MAX_DELAY (see Section 7)
and stop the execution of the routine. and stop the execution of the routine.
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3. If there is a Current Assignment but no Best Assignment: 3. If there is a Current Assignment but no Best Assignment:
* If the Current Assignment is not valid, destroy it, and * If the Current Assignment is not valid, destroy it, and
execute the routine again, as not triggered by the Backoff execute the routine again, as not triggered by the Backoff
Timer. Timer.
* If the Current Assignment is valid and published, stop the * If the Current Assignment is valid and published, stop the
execution of the routine. execution of the routine.
* If the Current Assignment is valid and not published, the node * If the Current Assignment is valid and not published, the node
MAY either: MUST either:
+ Adopt the prefix by cancelling the Apply Timer and set the + Adopt the prefix by cancelling the Apply Timer and set the
Backoff Timer to some random delay between 0 and Backoff Timer to some random delay between 0 and
ADOPT_MAX_DELAY (see Section 7). This procedure is used to ADOPT_MAX_DELAY (see Section 7). This procedure is used to
avoid renumbering when the node advertising the prefix left avoid renumbering when the node advertising the prefix left
the Shared Link. the Shared Link.
+ Destroy it and execute case 1 in order to create a + Destroy it and go to case 1.
different assignment.
4. If there is a Current Assignment and a Best Assignment: 4. If there is a Current Assignment and a Best Assignment:
* Cancel the Backoff Timer. * Cancel the Backoff Timer.
* If the two prefixes are identical, set the Current Assignment * If the two prefixes are identical, set the Current Assignment
as not published. If the Current Assignment is not applied as not published. If the Current Assignment is not applied
and the Apply Timer is not set, set the Apply Timer to '2 * and the Apply Timer is not set, set the Apply Timer to '2 *
Flooding Delay'. Flooding Delay'.
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* 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.
* The Advertised Prefix Priority is greater than the Advertised
Prefix Priority from all Advertised Prefixes which include or
are included in the considered Delegated Prefix and are
assigned to the considered Link.
In order to ensure algorithm convergence: In order to ensure algorithm convergence:
* 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
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hold this rule. The specifications for such safety procedures hold this rule. The specifications for 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 and 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 various
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 Assigned Prefix MUST be When the Apply Timer fires, the associated Assigned Prefix MUST be
applied. applied.
When the Backoff Timer associated with a given Delegated Prefix/Link When the Backoff Timer associated with a given Delegated Prefix and
pair fires while there is a Current Assignment associated with the Link pair fires while there is a Current Assignment associated with
same pair, the Current Assignment MUST be published with some the 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.
When one Delegated Prefix is replaced by another one that includes or When one Delegated Prefix is replaced by another one that includes or
is included in the deleted Delegated Prefix, all Assigned Prefixes is included in the deleted Delegated Prefix, all Assigned Prefixes
which were included in the deleted Delegated Prefix but are not which were included in the deleted Delegated Prefix but are not
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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 included in the considered Delegated Prefix which
considered Link. For that purpose, Applied Prefixes may be stored were previously assigned and applied on the considered Link. For
in stable storage along with their associated Link. 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) prefixes included in the considered Delegated Prefix
candidates can be found, the prefix MUST be picked among all and not including or included in any Assigned or Advertised
candidates. Prefix. If less than RANDOM_SET_SIZE candidates are found, the
prefix MUST be picked among all 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 it
does not include and is not included in any Assigned or Advertised holds the three following conditions:
Prefix but A/(N-1) does include 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 * It is included in the considered Delegated Prefix.
available prefix.
* It does not include and is not included in any Assigned or
Advertised Prefix but
* It is equal to the considered Delegated Prefix or A/(N-1)
includes an Assigned or Advertised Prefix.
Candidate prefix: A prefix of desired length which is included in
or is equal to an 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 prevent prefixes, a small set of badly chosen long prefixes may prevent
any shorter prefix from being assigned. For this reason, the set any shorter prefix from being assigned. For this reason, the set
of RANDOM_SET_SIZE candidates is created from the set of available of RANDOM_SET_SIZE candidates is created from available prefixes
prefixes with longest prefix lengths and, in case of tie, with longest prefix lengths and, in case of tie, preferring small
preferring small prefix values. 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.
3. If the desired prefix length was not specified, select one. The 3. If the desired prefix length was not specified, select one. The
available prefixes count computed previously may be used to help available prefixes count computed previously may be used to help
picking a prefix length such that: picking a prefix length such that:
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4. Iterate over available prefixes starting with prefixes of 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 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 of length exactly N included in or equal to available
prefixes. The end goal here is to create a set of prefixes. The end goal here is to create a set of
RANDOM_SET_SIZE candidate prefixes of length N included in a set RANDOM_SET_SIZE candidate prefixes of length N included in a set
of available prefixes of maximized prefix length. In case of a of available prefixes of maximized prefix length. In case of a
tie, smaller prefix values (as defined by the bit-wise tie, smaller prefix values (as defined by the bit-wise
lexicographical order) are preferred. lexicographical order) are preferred.
5. For each pseudo-random prefix, check if the prefix is equal to a 5. Generate a set of prefixes of desired length, which are pseudo-
candidate prefix. If so, pick that prefix and stop. randomly chosen based on Node and Link specific values. 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. 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 customizable, 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 adopts 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
connected Shared Links. connected Shared Links.
Basic: The node is capable of assigning new prefixes or adopting Basic: The node is capable of assigning new prefixes or adopting
prefixes which do not conflict with any other existing assignment. prefixes which do not conflict with any other existing assignment.
Such behavior does not require the implementation of the Such behavior does not require the implementation of the
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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. 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
executions of the prefix assignment algorithm in all executions of the prefix assignment algorithm in all
participating nodes. participating nodes.
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