Internet Engineering Task Force (IETF) P. Pfister
Request for Comments: 7695 B. Paterson
Category: Standards Track Cisco Systems
ISSN: 2070-1721 J. Arkko
Ericsson
November 2015
Distributed Prefix Assignment Algorithm
Abstract
This document specifies a distributed algorithm for dividing a set of
prefixes in a manner that allows for automatic assignment of sub-
prefixes that are unique and non-overlapping. Used in conjunction
with a protocol that provides flooding of information among a set of
participating nodes, prefix configuration within a network may be
automated.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7695.
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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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.
Pfister, et al. Standards Track [Page 1]RFC 7695 Prefix Assignment Algorithm November 2015
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Subroutine-Specific Terminology . . . . . . . . . . . . . 6
3. Applicability Statement . . . . . . . . . . . . . . . . . . . 7
4. Algorithm Specification . . . . . . . . . . . . . . . . . . . 9
4.1. Prefix Assignment Algorithm Subroutine . . . . . . . . . 9
4.2. Overriding and Destroying Existing Assignments . . . . . 12
4.3. Other Events . . . . . . . . . . . . . . . . . . . . . . 13
5. Prefix Selection Considerations . . . . . . . . . . . . . . . 14
6. Implementation Capabilities and Node Behavior . . . . . . . . 16
7. Algorithm Parameters . . . . . . . . . . . . . . . . . . . . 17
8. Security Considerations . . . . . . . . . . . . . . . . . . . 17
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
11.1. Normative References . . . . . . . . . . . . . . . . . . 18
11.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. Static Configuration Example . . . . . . . . . . . . 19
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
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 prefix
and address assignment. Although it does not have 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 configured networks. This document focuses on
the algorithm itself and therefore context-specific considerations
(such as the process of selecting a prefix value and length when
making a new assignment) are out of scope.
The algorithm makes use of a flooding mechanism allowing
participating nodes to advertise prefixes assigned to the links to
which they are directly connected or for other purposes, e.g., for
private assignment or prefix delegation. Advertising a prefix
therefore serves two purposes. It is a claim that a prefix is in
use, meaning that no other node may advertise an overlapping prefix
(unless it has a greater priority). And, it is a way for other nodes
to know which prefixes have been assigned to the links to which they
are directly connected.
Pfister, et al. Standards Track [Page 2]RFC 7695 Prefix Assignment Algorithm November 2015
The algorithm is given a set of delegated prefixes and 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).
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. Definitions
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" 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 try 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.
Pfister, et al. Standards Track [Page 3]RFC 7695 Prefix Assignment Algorithm November 2015
Flooding Mechanism: A mechanism allowing participating Nodes to
reliably share information with all other participating Nodes.
Link: An object to which the distributed algorithm will assign
prefixes. A Node may only assign prefixes to Links to which it is
directly connected. A Link is either Shared or Private.
Shared Link: A Link to which multiple Nodes may be connected. 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.
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). The mechanism used to assign Node IDs,
whether manual or automated, is out of scope for this document.
Flooding Delay: A value that 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.
Pfister, et al. Standards Track [Page 4]RFC 7695 Prefix Assignment Algorithm November 2015
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.
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 from the Flooding Mechanism.
Destroy (an Assigned Prefix): Local action of removing an Assigned
Prefix from the set of Assigned Prefixes. If applied, the prefix
is unapplied. If published, the prefix stops being advertised
through the Flooding Mechanism.
Prefix Adoption: When an Advertised Prefix that 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. By delaying the creation of
new Assigned Prefixes or the advertisement of adopted Assigned
Prefixes by a random amount of time, it reduces the probability of
colliding assignments made by multiple Nodes.
Renumbering: Event occurring when an Assigned Prefix that was
applied is destroyed. Renumbering is undesirable as it usually
implies reconfiguring routers or hosts.
Pfister, et al. Standards Track [Page 5]RFC 7695 Prefix Assignment Algorithm November 2015
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.
* 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 computed as 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).
Pfister, et al. Standards Track [Page 6]RFC 7695 Prefix Assignment Algorithm November 2015
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
Although the algorithm was primarily designed as an autonomic prefix
assignment tool for home networks, it is applicable to other areas.
In particular, it can operate without any kind of configuration as
well as use advanced prefix assignment rules. Additionally, it can
be applied to any address space and can be used to manage multiple
address spaces simultaneously. For instance, an implementation can
make use of IPv4-mapped IPv6 addresses [RFC4291] in order to manage
both IPv4 and IPv6 prefix assignment using a single prefix space.
Each Node MUST have a set of non-overlapping Delegated Prefixes
(i.e., that 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 non-overlapping Delegated
Prefixes.
Given this set of non-overlapping 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. Prefixes may also be assigned for private use. For example,
an assigned prefix may be delegated to some other entity that does
not implement this algorithm [RFC3633], or associated with a high
priority in order to prevent other nodes from assigning any
overlapping prefix [RFC6603].
The algorithm supports dynamically changing topologies and therefore
will converge if the topology remains unmodified for a long enough
period of time. (That time depends on the Flooding Mechanism
properties.) Nevertheless, some topology changes may induce
renumbering, while others do not. In particular, Nodes joining the
set of participating Nodes do not cause renumbering. Similarly,
Nodes leaving the network may be handled without renumbering by using
the prefix adoption procedure. On the other hand, Links that merge
or split may break correctness conditions, and therefore cause
renumbering.
Pfister, et al. Standards Track [Page 7]RFC 7695 Prefix Assignment Algorithm November 2015
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 that is advertising them.
o Specify whether an Advertised Prefix is assigned to a directly
connected Shared Link, and if so, which one. This information
also needs to be updated in case of Links that merge or split.
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 for this
document.
The algorithm ensures that whenever the Flooding Delay is provided
and held, and in the absence of any topology change or Delegated
Prefix removal, renumbering only happens when a Node deliberately
overrides an existing assignment. In the absence of such deliberate
override, the algorithm converges within an absolute worst-case
timespan of (2 * Flooding Delay * L) seconds, where L is the number
of links.
Each Node MUST have a Node ID. In the situation where multiple nodes
have the same Node ID, the algorithm will not suffer, assuming there
are no colliding assignments. However, in order for collisions to be
resolved, that situation MUST be transient.
Finally, leaving the Flooding Mechanism or Node ID assignment process
unsecured makes the network vulnerable to denial-of-service attacks,
as detailed in Section 8. Additionally, as this algorithm requires
all Nodes to know which Node has made which assignment, it may be
unsuitable depending on privacy requirements among participating
Nodes.
Pfister, et al. Standards Track [Page 8]RFC 7695 Prefix Assignment Algorithm November 2015
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). The subroutine also makes use of the two following
configuration parameters: ADOPT_MAX_DELAY and BACKOFF_MAX_DELAY,
which are defined in Section 7.
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.
o An Assigned Prefix included in the considered Delegated Prefix and
associated with the considered Link is destroyed outside of the
context of the subroutine, as specified in Section 4.2.
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.
Pfister, et al. Standards Track [Page 9]RFC 7695 Prefix Assignment Algorithm November 2015
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 if one of
these events is the firing of the Backoff Timer while there is no
Current Assignment associated with the given pair, the subroutine is
executed with the BackoffTriggered boolean set to true.
In order to execute the subroutine for a given Delegated Prefix and
Link pair, first get the Current Assignment and compute the Best
Assignment associated with the Delegated Prefix and Link pair, then
execute the steps depending on the following cases:
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 scope for 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.
Pfister, et al. Standards Track [Page 10]RFC 7695 Prefix Assignment Algorithm November 2015
2. If there is a Best Assignment but no Current Assignment: First,
check if the Best Assignment is equal to or included in the
Delegated Prefix. If not, stop the execution of the subroutine.
Otherwise, 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.
* With the Apply Timer set to (2 * Flooding Delay).
* Set as not published.
3. If there is a Current Assignment but no Best Assignment:
* If the Current Assignment is not valid, destroy it, and
execute the 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, and it SHOULD therefore be preferred.
+ Destroy it and go to case 1, allowing a different prefix to
be assigned, or the prefix to be removed. When the Current
Assignment is applied, this causes renumbering.
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.
Pfister, et al. Standards Track [Page 11]RFC 7695 Prefix Assignment Algorithm November 2015
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 in which the algorithm is
operating 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:
* 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 that 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 that include or
are included in the chosen prefix.
* The Advertised Prefix Priority is greater than the Advertised
Prefix Priority from all Advertised Prefixes that 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 that do not adhere to this rule
Pfister, et al. Standards Track [Page 12]RFC 7695 Prefix Assignment Algorithm November 2015
MUST implement a mechanism that detects if the distributed
algorithm does not converge and, when this occurs, they MUST
stop creating overriding Assigned Prefixes that do not adhere
to this rule. The specifications for such safety procedures
are out of scope for this document.
Removing an Assigned Prefix: A Node MAY destroy any Assigned Prefix
that 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.
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
that 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.
Pfister, et al. Standards Track [Page 13]RFC 7695 Prefix Assignment Algorithm November 2015
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 that
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 from a set of prefixes, where the set is of at
least RANDOM_SET_SIZE (see Section 7). The prefixes are those
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.
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 that 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 that is included in or
is equal to an available prefix.
The procedure described in this section takes the three following
criteria into account:
Pfister, et al. Standards Track [Page 14]RFC 7695 Prefix Assignment Algorithm November 2015
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
pseudorandom prefixes generated based on Node- and Link-specific
values may be considered.
Randomness: When no stored or pseudorandom 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, numerically
small prefix values are preferred.
When executing the procedure, do as follows:
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.
Pfister, et al. Standards Track [Page 15]RFC 7695 Prefix Assignment Algorithm November 2015
5. Generate a set of prefixes of desired length, which are
pseudorandomly chosen based on Node- and Link-specific values.
For each pseudorandom 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 tree.
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.
Such behavior does not require the implementation of the
considerations specified in Section 4.2 or 5. The Node never
checks the validity of existing assignments, which makes this
behavior particularly suited to lightweight devices that 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 that 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 Sections 4.2 and 5. It is suitable
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
Pfister, et al. Standards Track [Page 16]RFC 7695 Prefix Assignment Algorithm November 2015
Delegated Prefix will ever be assigned to the Link. This situation
may be detected by monitoring whether any 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, control traffic (generated by the Flooding Mechanism),
and collision probability.
ADOPT_MAX_DELAY represents the maximum backoff time a Node may wait
before adopting an assignment; BACKOFF_MAX_DELAY represents the
maximum backoff time a Node may wait before making a new assignment.
BACKOFF_MAX_DELAY MUST be greater than or equal to ADOPT_MAX_DELAY.
The greater ADOPT_MAX_DELAY and (BACKOFF_MAX_DELAY -
ADOPT_MAX_DELAY), the lower the collision probability and the lesser
the amount of control traffic, but the greater the convergence time.
RANDOM_SET_SIZE represents the desired size of the set from which a
random prefix will be picked. 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.
Pfister, et al. Standards Track [Page 17]RFC 7695 Prefix Assignment Algorithm November 2015
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 impede the ability to resolve prefix
assignment collisions.
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.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI
10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
9.2. Informative References
[RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
Host Configuration Protocol (DHCP) version 6", RFC 3633,
DOI 10.17487/RFC3633, December 2003,
<http://www.rfc-editor.org/info/rfc3633>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <http://www.rfc-editor.org/info/rfc4291>.
[RFC6603] Korhonen, J., Ed., Savolainen, T., Krishnan, S., and O.
Troan, "Prefix Exclude Option for DHCPv6-based Prefix
Delegation", RFC 6603, DOI 10.17487/RFC6603, May 2012,
<http://www.rfc-editor.org/info/rfc6603>.
Pfister, et al. Standards Track [Page 18]RFC 7695 Prefix Assignment Algorithm November 2015
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 (the 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).
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 the
configuration rule priority is, the greater the Assigned Prefix
Priority must be).
Each Assigned Prefix is associated with a rule priority. Assigned
Prefixes that 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 that 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 that 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.
Pfister, et al. Standards Track [Page 19]RFC 7695 Prefix Assignment Algorithm November 2015
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.
Acknowledgments
The authors would like to thank those who participated in the
development of draft versions of this document as well as the present
document. 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, Steven Barth, and Juliusz Chroboczek for
interesting discussions and document review.
Authors' Addresses
Pierre Pfister
Cisco Systems
Paris
France
Email: pierre.pfister@darou.fr
Benjamin Paterson
Cisco Systems
Paris
France
Email: paterson.b@gmail.com
Jari Arkko
Ericsson
Jorvas 02420
Finland
Email: jari.arkko@piuha.net
Pfister, et al. Standards Track [Page 20]
