Distributed Prefix Assignment Algorithm
draft-ietf-homenet-prefix-assignment-07

Network Working Group                                         P. Pfister
Internet-Draft                                               B. Paterson
Intended status: Standards Track                           Cisco Systems
Expires: December 17, 2015                                      J. Arkko
                                                                Ericsson
                                                           June 15, 2015

                Distributed Prefix Assignment Algorithm
                draft-ietf-homenet-prefix-assignment-07

Abstract

   This document specifies a distributed algorithm for automatic prefix
   assignment.  Thus it provides an alternative to manual or centralized
   prefix and address assignment techniques.  Given a set of delegated
   prefixes, it ensures that 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

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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on December 17, 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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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents

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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Subroutine Specific Terminology . . . . . . . . . . . . .   5
   3.  Applicability Statement . . . . . . . . . . . . . . . . . . .   6
   4.  Algorithm Specification . . . . . . . . . . . . . . . . . . .   8
     4.1.  Prefix Assignment Algorithm Subroutine  . . . . . . . . .   8
     4.2.  Overriding and Destroying Existing Assignments  . . . . .  10
     4.3.  Other Events  . . . . . . . . . . . . . . . . . . . . . .  12
   5.  Prefix Selection Considerations . . . . . . . . . . . . . . .  12
   6.  Implementation Capabilities and Node Behavior . . . . . . . .  14
   7.  Algorithm Parameters  . . . . . . . . . . . . . . . . . . . .  15
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  17
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     11.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Appendix A.  Static Configuration Example . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   This document specifies a distributed algorithm for automatic prefix
   assignment.  The algorithm provides a generic alternative to
   centralized (human or software based) approaches for network prefixes
   and addresses assignment.  Although it does not require to be
   configured to operate properly, it supports custom configuration by
   means of variable priority assignments, and can therefore be used in
   fully autonomic as well as professionally managed networks.

   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
   satisfied after a finite convergence period:

   1.  At most one prefix from each delegated prefix is assigned to each
       link.

   2.  Assigned prefixes are non-overlapping (i.e., an assigned prefix
       never includes another assigned prefix).

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   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.

   Each assignment has a priority specified by the Node making the
   assignment, allowing for custom assignment policies.  When multiple
   Nodes assign different prefixes from the same delegated prefix to the
   same link, or when multiple Nodes assign overlapping prefixes (to the
   same link or to different links), the assignment with the greatest
   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
   within a given time window, 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",
   and "SHOULD", are to be interpreted as described in [RFC2119].

   This document makes use of the following terminology.  The terms
   defined here are ordered in such a way as to avoid forward
   references, and therefore are not sorted alphabetically.

   Node:   An entity executing the algorithm specified in this document
      and able to communicate with other Nodes using the Flooding
      Mechanism.

   Flooding Mechanism:   A mechanism allowing participating Nodes to
      reliably share information with all other participating Nodes.

   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.

   Shared Link:   A Link multiple Nodes may be connected to.  Most of
      the time, a Shared Link is a multi-access link or point-to-point
      link, virtual or physical, requiring prefixes to be assigned to
      it.

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   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] requesting router MAY be considered as a different
      Private Link.

   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., using
      the alphanumeric order).

   Flooding Delay:   A value which MUST be provided by the Flooding
      Mechanism and SHOULD be a deterministic or likely upper bound on
      the information propagation delay among participating Nodes.

   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 that 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 (i.e., overlapping or from the same Delegated
      Prefix and assigned to the same link), all Advertised Prefixes but
      the one with the greatest priority will eventually be removed.  In
      case of a 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 is never 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 purpose outside of 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.

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   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.

   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
      with a timer counting down to zero.  It is used to reduce the
      probability of colliding assignments made by multiple Nodes by
      delaying the creation of new Assigned Prefixes or the
      advertisement of adopted Assigned Prefixes by a random amount of
      time.

   Renumbering:   Event occurring when an Assigned Prefix which was
      applied is destroyed.  Renumbering is undesirable as it usually
      implies reconfiguring routers or hosts.

2.1.  Subroutine Specific Terminology

   In addition to the terms defined in Section 2, the subroutine
   specified in Section 4 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 by the Node
      executing the algorithm.  At some point in time, the Current
      Assignment from different Nodes may differ, but the algorithm
      ensures that eventually, all Nodes directly connected to a Shared
      Link have the same Current Assignment for any given Delegated
      Prefix.

   Precedence:   An Advertised Prefix takes precedence over an Assigned
      Prefix if and only if one of the following conditions is met:

      *  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 identical,
         and the Node ID from the Node advertising the Advertised Prefix
         is strictly greater than the local Node ID.

   Best Assignment:   For a given Delegated Prefix and Link, the Best
      Assignment is the unique Advertised Prefix (if any) that:

      *  Includes or is included in the Delegated Prefix (i.e., the
         Advertised Prefix is a sub-prefix of the Delegated Prefix, or
         the Delegated Prefix is a sub-prefix of the Advertised Prefix).

      *  Is assigned on the given Link.

      *  Has the greatest Advertised Prefix Priority among Advertised
         Prefixes fulfilling the two preceding conditions (and, in case
         of a tie, the prefix advertised by the Node with the greatest
         Node ID among all prefixes with greatest priority).

      *  Takes 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
      the following two 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 Assigned Prefix.

3.  Applicability Statement

   Each Node MUST have a set of non-overlapping Delegated Prefixes
   (i.e., which do not include each other).  This set MAY change over
   time and be different from one Node to another at some point, but
   Nodes MUST eventually have 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 any 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]

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   in order to manage both IPv4 and IPv6 prefix assignment using a
   single prefix space.

   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 is advertising them.

   o  Specify whether an Advertised Prefix was assigned to a directly
      connected Shared Link, and if so, on which one.

   o  Provide a Flooding Delay value, which SHOULD represent a
      deterministic or likely upper bound on the information propagation
      delay among participating Nodes.  Whenever the Flooding Mechanism
      is unable to adhere to the provided Flooding Delay, renumbering
      may happen.  As such a delay often depends on the size of the
      network, it MAY change over time and MAY be different from one
      Node to another.  Furthermore, the process of selecting this value
      is subject to a tradeoff between convergence speed and lower
      renumbering probability (e.g., the value 0 may be used when
      renumbering is harmless), and is therefore out of scope of this
      document.

   The algorithm ensures that whenever the Flooding Delay is provided
   and respected, and in the absence of any topology change or Delegated
   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
   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.

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4.  Algorithm Specification

   This section specifies the behavior of Nodes implementing the prefix
   assignment algorithm.  The terms 'Current Assignment', 'Precedence',
   'Best Assignment' and 'Valid' are used as defined in Section 2.1.

4.1.  Prefix Assignment Algorithm Subroutine

   This section specifies the prefix assignment algorithm subroutine.
   It is defined for a given Delegated Prefix and Link pair and takes a
   BackoffTriggered boolean as parameter (indicating whether the
   subroutine execution was triggered by the Backoff Timer or by another
   event).

   For a given Delegated Prefix and Link pair, the subroutine MUST be
   run with the BackoffTriggered boolean set to false 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.

   Furthermore, for a given Delegated Prefix and Link pair, the
   subroutine MUST be run with the BackoffTriggered boolean set to true
   whenever:

   o  The Backoff Timer associated with the considered Delegated Prefix
      and 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
   subroutine instead of executing it immediately, e.g., while receiving
   an update from the Flooding Mechanism, or for security reasons (see
   Section 8).  Even if other events occur in the meantime, the
   subroutine MUST be run only once.  It is also assumed that, whenever
   one of these events is the Backoff Timer firing, the subroutine is
   executed with the BackoffTriggered boolean set to true.

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   In order to execute the subroutine for a given Delegated Prefix and
   Link pair, first look for the Best Assignment and Current Assignment
   associated with the Delegated Prefix and 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 and Link pair is desired (As any result would be valid,
       the process of making this decision is out of the scope of this
       document) and do the following:

       *  If it is not desired, stop the execution of the subroutine.

       *  Else if the Backoff Timer is running, stop the execution of
          the subroutine.

       *  Else if the BackoffTriggered boolean is set to false, 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 subroutine.

       *  Else, continue the execution of the subroutine.

       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.

       *  Set as 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.

       *  Set as not applied.

       *  The Apply Timer set to '2 * Flooding Delay'.

       *  Set as not published.

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   3.  If there is a Current Assignment but no Best Assignment:

       *  If the Current Assignment is not valid, destroy it, and
          execute the subroutine again with the BackoffTriggered boolean
          set to false.

       *  If the Current Assignment is valid and published, stop the
          execution of the subroutine.

       *  If the Current Assignment is valid and not published, the Node
          MUST either:

          +  Adopt the prefix by canceling 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 go to case 1.

   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 subroutine 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.

4.2.  Overriding and Destroying Existing Assignments

   In addition to the behaviors specified in Section 4.1, the following
   procedures MAY be used in order to provide additional behavior
   options (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:

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      *  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 is 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.

      *  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:

      *  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 adhere to this rule
         MUST implement a mechanism which detects whether the
         distributed algorithm does not converge and, whenever this
         would happen, stop creating overriding Assigned Prefixes which
         do not adhere to this rule.  The specifications for such safety
         procedures are out of the scope of this document.

   Removing an Assigned Prefix:   A Node MAY destroy any Assigned Prefix
      which is published.  Such an event reflects the desire of a Node
      to not assign a prefix from a given Delegated Prefix to a given
      Link anymore.  In order to ensure algorithm convergence, such a
      procedure MUST NOT be executed unless there was a change in the
      Node configuration.  Furthermore, whenever an Assigned Prefix is
      destroyed in this way, the prefix assignment algorithm subroutine
      MUST be run for the Delegated Prefix and Link pair associated with
      the destroyed Assigned Prefix.

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   The two procedures specified in this section are OPTIONAL.  They
   could be used for various purposes, e.g., for providing custom prefix
   assignment configuration or reacting to prefix space exhaustion (by
   overriding short Assigned Prefixes and assigning longer ones).

4.3.  Other Events

   When the Apply Timer fires, the associated Assigned Prefix MUST be
   applied.

   When the Backoff Timer associated with a given Delegated Prefix and
   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
   (e.g., when the Delegated Prefix expires), 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 subroutine specified in
   Section 4.1 requires a new prefix to be selected, the prefix MUST be
   selected either:

   o  Among prefixes included in the considered Delegated Prefix which
      were previously assigned and applied on the 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
      Section 7) prefixes included in the considered Delegated Prefix
      and not including or included in any Assigned or Advertised
      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
      configuration.

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   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 of the form Prefix/PrefixLength is
      available if and only if it satisfies the three following
      conditions:

      *  It is included in the considered Delegated Prefix.

      *  It does not include and is not included in any Assigned or
         Advertised Prefix.

      *  It is equal to the considered Delegated Prefix or Prefix/
         (PrefixLength-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
   criteria into account:

   Prefix Stability:   In some cases, it is desirable that the selected
      prefix should remain 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 prevent
      any shorter prefix from being assigned.  For this reason, the set
      of RANDOM_SET_SIZE candidates is created from available prefixes
      with longest prefix lengths and, in case of a tie, preferring
      numerically 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
       pick a prefix length such that:

       *  There is at least one candidate prefix.

       *  The prefix length is chosen large 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.  Generate a set of prefixes of desired length, which are pseudo-
       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.

   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 customizable, 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 adopts existing ones.

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      Such behavior does not require the implementation of the
      considerations specified in Section 5 or Section 4.2.  The Node
      never checks the validity of existing assignments, 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 4.2.  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 4.2.  It is
      suited when the administrator desires some particular prefix to be
      assigned on a given Link, or some Link to be assigned prefixes
      with a greater priority when there are not enough prefixes
      available for all Links.

   Note that if all Nodes directly connected to some Link are listener
   Nodes or none of these Nodes are willing to make an assignment from a
   given Delegated Prefix to the given Link, no prefix from the given
   Delegated Prefix will ever be assigned to the Link (and such existing
   prefixes will be removed).  This situation may be detected by
   watching whether no prefix from a given Delegated Prefix has been
   assigned to the Link for longer than BACKOFF_MAX_DELAY plus the
   Flooding Delay.

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, reduced control traffic (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

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   the collision probability and the lesser the amount of control
   traffic, but the greater 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.

8.  Security Considerations

   The prefix assignment algorithm functions on top of two distinct
   mechanisms, the Flooding Mechanism and the Node ID assignment
   mechanism.

      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 algorithm.

      An attacker able to prevent other Nodes from accessing a portion
      or the whole set of Advertised Prefixes may compromise the
      correctness of the algorithm.

      An attacker able to cause repetitive Node ID changes may cause
      traffic to be generated in 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 algorithm or cause the result to violate the correctness
      conditions.

   Whenever the security of the Flooding Mechanism and Node ID
   assignment mechanism cannot be ensured, the convergence of the
   algorithm may be prevented.  In environments where such attacks may
   be performed, the execution of the prefix assignment algorithm
   subroutine SHOULD be rate limited, as specified in Section 4.1.

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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 algorithm, 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 4.1 are given a
   rule priority of 0.

   Whenever the configuration is changed or the prefix assignment
   algorithm subroutine is run: For each Link/Delegated Prefix pair,
   look for the configuration rule with the greatest 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 4.2) 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 as specified in Section 4.2.  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
   algorithm.

Authors' Addresses

   Pierre Pfister
   Cisco Systems
   Paris
   France

   Email: pierre.pfister@darou.fr

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   Benjamin Paterson
   Cisco Systems
   Paris
   France

   Email: paterson.b@gmail.com

   Jari Arkko
   Ericsson
   Jorvas  02420
   Finland

   Email: jari.arkko@piuha.net

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