One document matched: draft-ietf-homenet-dncp-06.xml
<?xml version='1.0' ?>
<!--
Created: Mon Nov 18 17:55:22 2013 mstenber
split from draft-ietf-homenet-hncp-03-pre - generic parts
TBD: Where should we note that DNCP is fundamentally about individual TLVs,
and they can be queued (and jittered) freely?
-->
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<?rfc autobreaks="yes"?>
<?rfc compact="yes"?>
<?rfc strict='yes'?>
<?rfc subcompact="no"?>
<?rfc symrefs="yes"?>
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<rfc
ipr='trust200902'
docName='draft-ietf-homenet-dncp-06'
category='std'
>
<front>
<title abbrev="Distributed Node Consensus Protocol">
Distributed Node Consensus Protocol
</title>
<author initials="M" surname="Stenberg" fullname="Markus Stenberg">
<address>
<postal>
<street/>
<city>Helsinki</city>
<code>00930</code>
<country>Finland</country>
</postal>
<email>markus.stenberg@iki.fi</email>
</address>
</author>
<author initials="S" surname="Barth" fullname="Steven Barth">
<address>
<postal>
<street/>
<city>Halle</city>
<code>06114</code>
<country>Germany</country>
</postal>
<email>cyrus@openwrt.org</email>
</address>
</author>
<date month="June" year="2015" />
<area>Internet</area>
<workgroup>Homenet Working Group</workgroup>
<keyword>Homenet</keyword>
<abstract>
<t>This document describes the Distributed Node Consensus Protocol
(DNCP), a generic state synchronization protocol which uses Trickle
and Merkle trees. DNCP leaves some details unspecified or
provides alternative options. Therefore, only profiles which specify
those missing parts define actual implementable DNCP based
protocols.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>DNCP is designed to provide a way for each participating node to
publish a set of TLV (Type-Length-Value) tuples, and to provide
a shared and common view about the data published by every currently
or recently bidirectionally reachable DNCP node in a network.</t>
<t>For state synchronization a Merkle tree is used. It is formed by
first calculating a hash for the dataset, called node data, published
by each node, and then calculating another hash over those node data hashes.
The single resulting hash, called network state hash, is transmitted using the
<xref target="RFC6206">Trickle algorithm</xref> to ensure that all nodes
share the same view of the current state of the published data within the
network. The use of Trickle with only short network state hashes sent
infrequently (in steady state) makes DNCP very thrifty when updates happen
rarely.</t>
<t>For maintaining liveliness of the topology and the data within it,
a combination of Trickled network state, keep-alives, and "other"
means of ensuring reachability are used. The core idea is that if
every node ensures its neighbors are present, transitively, the whole
network state also stays up-to-date.</t>
<t>DNCP is most suitable for data that changes only infrequently to gain
the maximum benefit from using Trickle. As the network of nodes, or
the rate of data changes grows over a given time interval, Trickle
is eventually used less and less and the benefit of using DNCP
diminishes. In these cases Trickle just provides extra complexity
within the specification and little added value. If constant rapid
state changes are needed, the preferable choice is to use an
additional point-to-point channel whose address or locator is
published using DNCP.</t>
</section>
<section title="Terminology">
<texttable suppress-title="true" style="none" align="left">
<ttcol width="25%" /><ttcol width="75%" />
<c>DNCP profile</c>
<c>a definition of the set of rules and values listed in <xref
target="profile-bits"/> specifying the behavior of a DNCP based
protocol, such as the transport method in use. In this document,
any DNCP profile specific parameter with a profile-specific
fixed value is prefixed with DNCP_.</c>
<c /><c />
<c>DNCP node</c>
<c>a single node which runs a protocol based on a DNCP
profile.</c>
<c /><c />
<c>Link</c>
<c>a link-layer media over which directly connected nodes can
communicate.</c>
<c>DNCP network</c>
<c>a set of DNCP nodes running the same DNCP profile.
The set consists of nodes that have discovered each other using the
transport method defined in the DNCP profile, via multicast on local
links, and/or by using unicast communication.
</c>
<c /><c />
<c>Node identifier</c>
<c>an opaque fixed-length identifier consisting of
DNCP_NODE_IDENTIFIER_LENGTH bytes which uniquely identifies a DNCP
node within a DNCP network.</c>
<c /><c />
<c /><c />
<c>Interface</c>
<c>a node's attachment to a particular link.</c>
<c /><c />
<c>Endpoint</c>
<c>a locally configured communication endpoint of a DNCP node, such
as a network socket. It is either bound to an Interface for multicast
and unicast communication, or configured for explicit unicast
communication with a predefined set of remote addresses. Endpoints
are usually in one of the transport modes specified in <xref
target="dt" />. </c>
<c /><c />
<c>Endpoint identifier</c>
<c>a 32-bit opaque value, which identifies a particular endpoint of
a particular DNCP node. The value 0 is reserved for DNCP and DNCP
profile purposes and not used to identify an actual
endpoint. This definition is in sync with the interface index
definition in <xref target="RFC3493"/>, as the non-zero small
positive integers should comfortably fit within 32 bits.</c>
<c /><c />
<c>Peer</c>
<c>another DNCP node with which a DNCP node communicates using a
particular local and remote endpoint pair.</c>
<c /><c />
<c>Node data</c>
<c>a set of TLVs published and owned by a node in the DNCP
network. Other nodes pass it along as-is, even if they cannot
fully interpret it.</c>
<c /><c />
<c>Node state</c>
<c>a set of metadata attributes for node data. It includes a sequence
number for versioning, a hash value for comparing equality of stored
node data, and a timestamp indicating the time passed since its last
publication. The hash function and the length of the hash value are
defined in the DNCP profile.</c>
<c /><c />
<c>Network state hash</c>
<c>a hash value which represents the current state of the network.
The hash function and the length of the hash value are defined in
the DNCP profile.
Whenever a node is added, removed or updates its published node data
this hash value changes as well.
For calculation, please see <xref target="merkle" />.
</c>
<c /><c />
<c>Trust verdict</c>
<c>a statement about the trustworthiness of a
certificate announced by a node participating in the certificate
based trust consensus mechanism.</c>
<c /><c />
<c>Effective trust verdict</c>
<c>the trust verdict with the highest priority within the set of
trust verdicts announced for the certificate in the DNCP network.</c>
<c /><c />
<c>Topology graph</c>
<c>the undirected graph of DNCP nodes produced by
retaining only bidirectional peer relationships between nodes.</c>
</texttable>
<section anchor="kwd" title='Requirements Language'>
<t>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" in this document are to
be interpreted as described in <xref target='RFC2119'>RFC 2119</xref>.
</t>
</section>
</section>
<section title="Overview">
<t>DNCP operates primarily using unicast exchanges between nodes, and
may use multicast for Trickle-based shared state dissemination and
topology discovery. If used in pure unicast mode with unreliable
transport, Trickle is also used between peers.</t>
<t>DNCP discovers the topology of its nodes and maintains the
liveliness of published node data by ensuring that the publishing
node was - at least recently - bidirectionally reachable. This is
determined, e.g., by a recent and consistent multicast or unicast TLV
exchange with its peers. New potential peers can be discovered
autonomously on multicast-enabled links, their addresses may be
manually configured or they may be found by some other means defined
in a later specification.</t>
<t>A Merkle tree is maintained by each node to represent the state of all
currently reachable nodes and the Trickle algorithm is used to trigger
synchronization. Consistency among neighboring nodes
is thereby determined by comparing the current root of their respective
trees, i.e., their individually calculated network state hashes.</t>
<t>Before joining a DNCP network, a node starts with a Merkle tree
(and therefore a calculated network state hash) only consisting of
the node itself. It then announces said hash by means of the Trickle
algorithm on all its configured endpoints.</t>
<t>When an update is detected by a node (e.g., by receiving an
inconsistent network state hash from a peer) the originator of the
event is requested to provide a list of the state of all nodes,
i.e., all the information it uses to calculate its own Merkle
tree. The node uses the list to determine whether its own information
is outdated and - if necessary - requests the actual node data that
has changed.</t>
<t>Whenever a node's local copy of any node data and its Merkle tree are
updated (e.g., due to its own or another node's node state changing or
due to a peer being added or removed) its Trickle instances are reset
which eventually causes any update to be propagated to all of its
peers.</t>
</section>
<section title="Operation">
<section title="Merkle Tree" anchor="merkle">
<t>Each DNCP node maintains a Merkle tree of height 1 to manage
state updates of individual DNCP nodes, the leaves of the tree, and
the network as a whole, the root of the tree.</t>
<t>Each leaf represents one recently bidirectionally reachable node
(see <xref target="liveliness" />), and is represented by a tuple
consisting of the node's update sequence number in network byte
order concatenated with the hash-value of the node's ordered node
data published in the <xref target="node-state">Node State
TLV</xref>. These leaves are ordered in ascending order of the
respective node identifiers.
The root of the tree - the network state hash - is represented by
the hash-value calculated over all such leaf tuples concatenated in
order. It is used to determine whether the view of the network of
two or more nodes is consistent and shared.</t>
<t>The leaves and the root network state hash are updated on-demand
and whenever any locally stored per-node state changes. This
includes local unidirectional reachability encoded in the published
<xref target="neighbor">Neighbor TLV</xref>s and - when combined
with remote data - results in awareness of bidirectional
reachability changes.</t>
</section>
<section anchor="dt" title="Data Transport">
<t>DNCP has relatively few requirements for the underlying
transport; it requires some way of transmitting either unicast
datagram or stream data to a peer and, if used in multicast mode, a
way of sending multicast datagrams.
As multicast is used only to identify potential new DNCP nodes and
to send status messages which merely notify that a unicast exchange
should be triggered, the multicast transport does not have to be
secured.
If unicast security is desired and one of the built-in security
methods is to be used, support for some TLS-derived transport
scheme - such as <xref target="RFC5246">TLS</xref> on top of TCP or
<xref target="RFC6347">DTLS</xref> on top of UDP - is also
required.
A specific definition of the transport(s) in use and their parameters
MUST be provided by the DNCP profile.</t>
<t>TLVs are sent across the transport as is, and they SHOULD be
sent together where, e.g., MTU considerations do not recommend
sending them in multiple batches. TLVs in general are handled
individually and statelessly, with one exception. To form
bidirectional peer relationships DNCP requires identification of
the endpoints used for communication. A DNCP node desiring
bidirectional peer relationship therefore MUST send an <xref
target="endpoint">Endpoint TLV</xref>. When it is sent varies,
depending on the underlying transport:
<list style="symbols">
<t>If using a stream transport, the TLV SHOULD be sent only once
within the stream. </t>
<t>If using datagram transport, it MUST be included in every
datagram.</t>
</list>
Bidirectional peer
relationship is not necessary for read-only access to the DNCP
state, but it is required to be able to publish data.
</t>
<t>Given the assorted transport options as well as potential
endpoint configuration, a DNCP endpoint may be used in various
transport modes:
<list style="hanging">
<t hangText="Unicast:">
<list style="symbols">
<t>If only reliable unicast transport is employed, Trickle is
not used at all. Where Trickle reset occurs, a single
<xref target="net-state">Network State TLV</xref> is sent
instead to every unicast peer. Additionally, recently changed
<xref target="node-state">Node State TLV</xref>s MAY be
included. </t>
<t>If only unreliable unicast transport is employed, Trickle
state is kept per each peer and it is used to send Network
State TLVs every now and then, as specified in <xref
target="trickle-updates" />.</t>
</list>
</t>
<t hangText="Multicast+Unicast:"> If multicast datagram transport
is available on an endpoint, Trickle state is only maintained for the
endpoint as a whole. It is used to send Network State TLVs every
now and then, as specified in <xref target="trickle-updates"
/>. Additionally, per-endpoint keep-alives MAY be defined
in the DNCP profile, as specified in <xref target="pe-ka" />.</t>
<t hangText="MulticastListen+Unicast:">
Just like Unicast, except multicast transmissions are listened to
in order to detect changes of the highest node identifier.
This mode is used only if the DNCP profile supports <xref
target="broadcast">dense broadcast link optimization</xref>.</t>
</list>
</t>
</section>
<section title="Trickle-Driven Status Updates"
anchor="trickle-updates">
<t>The Trickle algorithm has 3 parameters: Imin, Imax and k. Imin
and Imax represent the minimum and maximum values for I, which is
the time interval during which at least k Trickle updates must be
seen on an endpoint to prevent local state transmission. The
actual suggested Trickle algorithm parameters are DNCP profile
specific, as described in <xref target="profile-bits"/>.</t>
<t>The Trickle state for all Trickle instances is considered
inconsistent and reset if and only if the locally calculated
network state hash changes. This occurs either due to a change in
the local node's own node data, or due to receipt of more recent
data from another node.</t>
<t>Every time a particular Trickle instance indicates that an
update should be sent, the node MUST send a <xref
target="net-state">Network State TLV</xref> if and only if:
<list style="symbols">
<t>the endpoint is in Multicast+Unicast transport mode, in which
case the TLV MUST be sent over multicast.</t>
<t>the endpoint is NOT in Multicast+Unicast transport mode, and the
unicast transport is unreliable, in which case the TLV MUST be sent
over unicast.</t>
</list>
</t>
<t>A (sub)set of all <xref target="node-state">Node State
TLVs</xref> MAY also be included, unless it is defined as
undesirable for some reason by the DNCP profile, or to avoid
exposure of the node state TLVs by transmitting them within
insecure multicast when using also secure unicast.</t>
</section>
<section title="Processing of Received TLVs" anchor="reception">
<t>This section describes how received TLVs are processed. The DNCP
profile may specify when to ignore particular TLVs, e.g., to modify
security properties - see <xref target="profile-bits" /> for
what may be safely defined to be ignored in a profile.
Any 'reply' mentioned in the steps below denotes sending of the
specified TLV(s) over unicast to the originator of the TLV being
processed. If the TLV being replied to was received via multicast
and it was sent to a link with shared bandwidth, the reply SHOULD
be delayed by a random timespan in [0, Imin/2], to avoid potential
simultaneous replies that may cause problems on some links. Sending
of replies MAY also be rate-limited or
omitted for a short period of time by an implementation. However, an
implementation MUST eventually reply to similar repeated requests,
as otherwise state synchronization would break.</t>
<t>A DNCP node MUST process TLVs received from any valid address,
as specified by a given DNCP profile and the configuration of a
particular endpoint, whether this address is known to be the
address of a neighbor or not. This provision satisfies the needs of
monitoring or other host software that needs to discover the DNCP
topology without adding to the state in the network.</t>
<t>Upon receipt of:
<list style="symbols">
<t><xref target="req-net-state">Request Network State TLV</xref>:
The receiver MUST reply with a <xref target="net-state">Network
State TLV</xref> and a <xref target="node-state">Node State
TLV</xref> for each node data used to calculate the network state
hash. The Node State TLVs MUST NOT contain the optional node
data part unless explicitly specified in the DNCP profile.</t>
<t><xref target="req-node-state">Request Node State TLV</xref>:
If the receiver has node data for the corresponding node, it MUST
reply with a <xref target="node-state">Node State TLV</xref> for
the corresponding node. The optional node data part MUST be
included in the TLV.</t>
<t><xref target="net-state">Network State TLV</xref>:
If the network state hash differs from the locally calculated
network state hash, and the receiver is unaware of any particular
node state differences with the sender, the receiver MUST reply
with a <xref target="req-net-state">Request Network State
TLV</xref>. These replies MUST be rate limited to only at most
one reply per link per unique network state hash within Imin. The
simplest way to ensure this rate limit is a timestamp indicating
requests, and sending at most one <xref target="req-net-state">
Request Network State TLV</xref> per Imin.
To facilitate faster state synchronization, if a Request Network
State TLV is sent in a reply, a local, current Network State TLV
MAY also be sent.</t>
<t><xref target="node-state">Node State TLV</xref>:
<list style="symbols">
<t>If the node identifier matches the local node identifier and
the TLV has a greater update sequence number than its current
local value, or the same update sequence number and a different
hash, the node SHOULD re-publish its own node data with an
update sequence number significantly (e.g., 1000) greater than
the received one, to reclaim the node identifier.
This may occur normally once due to the local
node restarting and not storing the most recently used update
sequence number. If this occurs more than once or for nodes
not re-publishing their own node data, the DNCP profile
MUST provide guidance on how to handle these situations as
it indicates the existence of another active node with the same
node identifier.</t>
<t>If the node identifier does not match the local node
identifier, and one or more of the following conditions are
true:
<list style="symbols">
<t>The local information is outdated for the corresponding node
(local update sequence number is less than that within the
TLV).</t>
<t>The local information is potentially incorrect (local update
sequence number matches but the node data hash differs).</t>
<t>There is no data for that node altogether.</t>
</list>
Then:
<list style="symbols">
<t>If the TLV does not contain node data, and the hash of the
node data differs, the receiver MUST reply with a <xref
target="req-node-state">Request Node State TLV</xref> for the
corresponding node.</t>
<t>Otherwise the receiver MUST update its locally stored
state for that node (node data if present, update sequence
number, relative time) to match the received TLV.</t>
</list>
</t>
</list>
For comparison purposes of the update sequence number,
a looping comparison function MUST be used to avoid problems in
case of overflow. The comparison function
a < b <=> (a - b) % 2^32 & 2^31 != 0
is RECOMMENDED unless the DNCP profile defines another.
</t>
<t>Any other TLV:
TLVs not recognized by the receiver MUST be silently ignored.</t>
</list>
</t>
<t>If secure unicast transport is configured for an endpoint, any
Node State TLVs received over insecure multicast MUST be silently
ignored.</t>
</section>
<section title="Adding and Removing Peers">
<t>When receiving a <xref target="endpoint">Node Endpoint
TLV</xref> on an endpoint from an unknown peer:
<list style="symbols">
<t>If received over unicast, the remote node MUST be added as a
peer on the endpoint and a <xref target="neighbor">Neighbor
TLV</xref> MUST be created for it.
</t>
<t>If received over multicast, the node MAY be sent a (possibly
rate-limited) unicast <xref target="req-net-state">Request
Network State TLV</xref>.</t>
</list>
</t>
<t>If keep-alives specified in <xref target="ka" /> are NOT sent by
the peer (either the DNCP profile does not specify the use of
keep-alives or the particular peer chooses not to send
keep-alives), some other existing local transport-specific
means (such as Ethernet carrier-detection or TCP keep-alive)
MUST be employed to ensure its presence.
When the peer is no longer present, the Neighbor
TLV and the local DNCP peer state MUST be removed.</t>
<t>If the local endpoint is in the Multicast-Listen+Unicast
transport mode, a <xref target="neighbor">Neighbor TLV</xref> MUST
NOT be published for the peers not having the highest node
identifier.</t>
</section>
<section anchor="liveliness" title="Data Liveliness Validation">
<t>When a Neighbor TLV or a whole node is added or removed, the
topology graph MUST be traversed either immediately or with a
small delay shorter than the DNCP profile-defined Trickle Imin.</t>
<t>The topology graph traversal starts with the local node. The
edges to be traversed are identified by looking for Neighbor TLVs
on both nodes, that have the other node's node identifier in the
Neighbor Node Identifier, and local and neighbor endpoint
identifiers swapped. Each node reached is marked currently
reachable.</t>
<t>DNCP nodes that have not been reachable in the most recent
topology graph traversal MUST NOT be used for calculation of the
network state hash, be provided to any applications that need to
use the whole TLV graph, or be provided to remote nodes. They
MAY be removed immediately after the topology graph traversal, however
it is RECOMMENDED to keep them at least briefly to improve the
speed of DNCP network state convergence and to reduce the number of
redundant state transmissions between nodes.</t>
</section>
</section>
<section anchor="dm" title="Data Model">
<t>This section describes the local data structures a minimal
implementation might use. This section is provided only as a
convenience for the implementor. Some of the <xref
target="ext">optional extensions</xref> describe additional data
requirements, and some optional parts of the core protocol may also
require more.</t>
<t>A DNCP node has:
<list style="symbols">
<t>A data structure containing data about the most recently sent
<xref target="req-net-state">Request Network State TLVs</xref>.
The simplest option is keeping a timestamp of the most recent request
(required to fulfill reply rate limiting specified in <xref target="reception" />).</t>
</list>
</t>
<t>A DNCP node has for every DNCP node in the DNCP network:
<list style="symbols">
<t>Node identifier: the unique identifier of the node. The length,
how it is produced, and how collisions are handled, is up to the
particular DNCP profile.</t>
<t>Node data: the set of TLV tuples published by that particular
node. As they are transmitted ordered (see <xref
target="node-state">Node State TLV</xref> for details), maintaining
the order within the data structure here may be reasonable. </t>
<t>Latest update sequence number: the 32-bit sequence number that
is incremented any time the TLV set is published. The comparison
function used to compare them is described in <xref
target="reception" />.</t>
<t>Origination time: the (estimated) time when the
current TLV set with the current update sequence number was
published.
It is used to populate the Milliseconds Since Origination field in
a <xref target="node-state">Node State TLV</xref>. Ideally it also
has millisecond accuracy.
</t>
</list>
</t>
<t>Additionally, a DNCP node has a set of endpoints for which DNCP
is configured to be used. For each such endpoint, a node has:
<list style="symbols">
<t>Endpoint identifier: the 32-bit opaque value uniquely
identifying it within the local node.</t>
<t>Trickle instance: the endpoint's Trickle instance with
parameters I, T, and c (only on an endpoint in Multicast+Unicast
transport mode).</t>
</list>
</t>
<t>and one (or more) of the following:
<list style="symbols">
<t>Interface: the assigned local network interface.</t>
<t>Unicast address: the DNCP node it should connect with.</t>
<t>Range of addresses: the DNCP nodes that are allowed to connect.</t>
</list>
</t>
<t>For each remote (peer, endpoint) pair detected on a
local endpoint, a DNCP node has:
<list style="symbols">
<t>Node identifier: the unique identifier of the peer.</t>
<t>Endpoint identifier: the unique endpoint identifier used by the
peer.</t>
<t>Peer address: the most recently used address of the peer
(authenticated and authorized, if security is enabled).</t>
<t>Trickle instance: the particular peer's Trickle instance with
parameters I, T, and c (only on a unicast-only endpoint with
unreliable unicast transport) .</t>
</list>
</t>
</section>
<section anchor="ext" title="Optional Extensions">
<t>This section specifies extensions to the core protocol that a DNCP
profile may use.</t>
<section anchor="ka" title="Keep-Alives">
<t><xref target="trickle-updates">Trickle-driven status
updates</xref> provide a mechanism for handling of new peer
detection on an endpoint, as well as state change
notifications. Another mechanism may be needed to get rid of old,
no longer valid peers if the transport or lower layers do not
provide one.</t>
<t>If keep-alives are not specified in the DNCP profile, the rest
of this subsection MUST be ignored.</t>
<t>A DNCP profile MAY specify either per-endpoint or per-peer
keep-alive support. </t>
<t>For every endpoint that a keep-alive is specified for in the
DNCP profile, the endpoint-specific keep-alive interval MUST be
maintained. By default, it is DNCP_KEEPALIVE_INTERVAL. If there is a
local value that is preferred for that for any reason (configuration,
energy conservation, media type, ..), it can be substituted
instead. If a non-default keep-alive interval is used on any
endpoint, a DNCP node MUST publish appropriate <xref
target="ka-interval">Keep-Alive Interval TLV(s)</xref> within its
node data.</t>
<section title="Data Model Additions">
<t>The following additions to the <xref target="dm">Data
Model</xref> are needed to support keep-alives:</t>
<t>For each configured endpoint that has per-endpoint keep-alives
enabled:
<list style="symbols">
<t>Last sent: If a timestamp which indicates the last time a
<xref target="net-state">Network State TLV</xref> was sent over
that interface.</t>
</list>
</t>
<t>For each remote (peer, endpoint) pair detected on a
local endpoint, a DNCP node has:
<list style="symbols">
<t>Last contact timestamp: a timestamp which indicates the last
time a consistent <xref target="net-state">Network State
TLV</xref> was received from the peer over multicast, or anything
was received over unicast. When adding a new peer, it is
initialized to the current time.</t>
<t>Last sent: If per-peer keep-alives are enabled, a timestamp
which indicates the last time a <xref
target="net-state">Network State TLV</xref> was sent to to that
point-to-point peer. When adding a new peer, it is initialized
to the current time.</t>
</list>
</t>
</section>
<section anchor="pe-ka" title="Per-Endpoint Periodic Keep-Alives">
<t>If per-endpoint keep-alives are enabled on an endpoint in
Multicast+Unicast transport mode, and if no traffic containing a
<xref target="net-state">Network State TLV</xref> has been sent
to a particular endpoint within the endpoint-specific keep-alive
interval, a <xref target="net-state">Network State TLV</xref>
MUST be sent on that endpoint, and a new Trickle transmission
time 't' in [I/2, I] MUST be randomly chosen. The actual sending
time SHOULD be further delayed by a random timespan in [0,
Imin/2].</t>
</section>
<section title="Per-Peer Periodic Keep-Alives">
<t>If per-peer keep-alives are enabled on a unicast-only
endpoint, and if no traffic containing a <xref
target="net-state">Network State TLV</xref> has been sent to a
particular peer within the endpoint-specific keep-alive interval,
a <xref target="net-state">Network State TLV</xref> MUST be sent to
the peer and a new Trickle transmission time 't' in [I/2, I] MUST
be randomly chosen.</t>
</section>
<section title="Received TLV Processing Additions">
<t>If a TLV is received over unicast from the peer, the Last
contact timestamp for the peer MUST be updated.</t>
<t>On receipt of a <xref target="net-state">Network State TLV</xref>
which is consistent with the locally calculated network state hash,
the Last contact timestamp for the peer MUST be updated.</t>
</section>
<section title="Neighbor Removal">
<t>For every peer on every endpoint, the endpoint-specific
keep-alive interval must be calculated by looking for <xref
target="ka-interval">Keep-Alive Interval TLVs</xref> published by
the node, and if none exist, using the default value of
DNCP_KEEPALIVE_INTERVAL. If the peer's last contact
timestamp has not been updated for at least locally chosen
potentially endpoint-specific keep-alive multiplier (defaults to
DNCP_KEEPALIVE_MULTIPLIER) times the peer's endpoint-specific
keep-alive interval, the Neighbor TLV for that peer and the local
DNCP peer state MUST be removed.</t>
</section>
</section>
<section anchor="broadcast" title="Support For Dense Broadcast Links">
<t>This optimization is needed to avoid a state space explosion.
Given a large set of DNCP nodes publishing data on an endpoint
that actually uses multicast on a link, every node will add a
<xref target="neighbor">Neighbor TLV</xref> for each peer.
While Trickle limits the amount of traffic on the link in
stable state to some extent, the total amount of data that is added
to and maintained in the DNCP network given N nodes on a
multicast-enabled link is O(N^2). Additionally if per-peer
keep-alives are employed, there will be O(N^2) keep-alives running
on the link if liveliness of peers is not ensured using some other
way (e.g., TCP connection lifetime, layer 2 notification,
per-endpoint keep-alive). </t>
<t>An upper bound for the number of neighbors that are allowed for
a particular type of link that an endpoint in Multicast+Unicast
transport mode is used on SHOULD be provided by a DNCP profile, but
MAY also be chosen at runtime.
Main consideration when selecting a bound (if any)
for a particular type of link should be whether it supports
broadcast traffic, and whether a too large number of neighbors case
is likely to happen during the use of that particular DNCP profile
on that particular type of link. If neither is likely, there is little
point specifying support for this for that particular link
type.</t>
<t>If a DNCP profile does not support this extension at all, the
rest of this subsection MUST be ignored. This is because when this
extension is employed, the state within the DNCP network only
contains a subset of the full topology of the network. Therefore
every node must be aware of the potential of it being used in a
particular DNCP profile.</t>
<t>If the specified upper bound is exceeded for some endpoint in
Multicast+Unicast transport mode and if the node does not have the
highest node identifier on the link, it SHOULD treat the endpoint
as a unicast endpoint connected to the node that has the highest
node identifier detected on the link, therefore transitioning to
Multicast-listen+Unicast transport mode. The nodes in
Multicast-listen+Unicast transport mode MUST keep listening to
multicast traffic to both receive messages from the node(s) still
in Multicast+Unicast mode, and as well to react to nodes with a
greater node identifier appearing. If the highest node identifier
present on the link changes, the remote unicast address of the
endpoints in Multicast-Listen+Unicast transport mode MUST be
changed. If the node identifier of the local node is the highest
one, the node MUST switch back to, or stay in Multicast+Unicast
mode, and normally form peer relationships with all peers.</t>
</section>
<section anchor="fragmentation" title="Node Data Fragmentation">
<t>A DNCP profile may be required to support node data which would
not fit the maximum size of a single <xref target="node-state">Node
State TLV</xref> (roughly 64KB of payload), or use a datagram-only
transport with a limited MTU and no reliable support for
fragmentation. To handle such cases, a DNCP profile MAY specify a
fixed number of trailing bytes in the node identifier to represent
a fragment number indicating a part of a node's node data. The
profile MAY also specify an upper bound for the size of a single
fragment to accommodate limitations of links in the network. Note
that the maximum size of fragment also constrains the maximum size
of a single TLV published by a node. </t>
<t>The data within Node State TLVs of all fragments MUST be valid,
as specified in <xref target="node-state" />. The locally used node
data for a particular node MUST be produced by concatenating node
data in each fragment, in ascending fragment number order. The
locally used concatenated node data MUST still follow the ordering
described in <xref target="node-state" />.</t>
<t>Any transmitted node identifiers used to identify the own or any
other node MUST have the fragment number 0. For algorithm purposes,
the relative time since the most recent fragment change MUST be
used, regardless of fragment number. Therefore, even if just some
of the node data fragments change, they all are considered
refreshed if one of them is.</t>
<t>If using fragmentation, the data liveliness validation defined
in <xref target="liveliness" /> is extended so that if a <xref
target="fragment-count">Fragment Count TLV</xref> is present within
the fragment number 0, all fragments up to fragment number
specified in the Count field are also considered reachable if the
fragment number 0 itself is reachable based on graph
traversal. </t>
</section>
</section>
<section anchor="tlvs" title="Type-Length-Value Objects">
<t>
Each TLV is encoded as a 2 byte type field, followed by a 2 byte
length field (of the value excluding header, in bytes, 0 meaning no value)
followed by the value itself, if any. Both type and length fields
in the header as well as all integer fields inside the value -
unless explicitly stated otherwise - are represented in network
byte order. Padding bytes with value zero MUST be added up to the
next 4 byte boundary if the length is not divisible by 4. These
padding bytes MUST NOT be included in the number stored in the length
field.
</t>
<figure>
<artwork>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value |
| (variable # of bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
</figure>
<t>
For example, type=123 (0x7b) TLV with value 'x' (120 =
0x78) is encoded as: 007B 0001 7800 0000.
</t>
<t>In this section, the following special notation is used:
<list>
<t>.. = octet string concatenation operation.</t>
<t>H(x) = non-cryptographic hash function specified by DNCP
profile. </t>
</list>
</t>
<section title="Request TLVs">
<section anchor="req-net-state" title="Request Network State TLV">
<figure>
<artwork>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type: REQ-NETWORK-STATE (1) | Length: 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
</figure>
<t>This TLV is used to request response with a <xref
target="net-state">Network State TLV</xref> and all <xref
target="node-state">Node State TLVs</xref> (without node
data).</t>
</section>
<section anchor="req-node-state" title="Request Node State TLV">
<figure>
<artwork>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type: REQ-NODE-STATE (2) | Length: >0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Node Identifier |
| (length fixed in DNCP profile) |
...
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
</figure>
<t>This TLV is used to request a <xref target="node-state">
Node State TLV</xref> (including node data) for the node
with the matching node identifier.</t>
</section>
</section>
<section title="Data TLVs">
<section anchor="endpoint" title="Node Endpoint TLV">
<figure>
<artwork>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type: NODE-ENDPOINT (3) | Length: > 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Node Identifier |
| (length fixed in DNCP profile) |
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Endpoint Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
</figure>
<t>This TLV identifies both the local node's node identifier, as
well as the particular endpoint's endpoint identifier. It is used
when bidirectional peering is desired, as described in the <xref
target="dt" />. </t>
</section>
<section anchor="net-state" title="Network State TLV">
<figure>
<artwork>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type: NETWORK-STATE (4) | Length: > 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| H(update number of node 1 .. H(node data of node 1) .. |
| .. update number of node N .. H(node data of node N)) |
| (length fixed in DNCP profile) |
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
</figure>
<t>This TLV contains the current locally calculated network state
hash, see <xref target="merkle" /> for how it is calculated.</t>
</section>
<section anchor="node-state" title="Node State TLV">
<figure>
<artwork>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type: NODE-STATE (5) | Length: > 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Node Identifier |
| (length fixed in DNCP profile) |
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Update Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Milliseconds Since Origination |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| H(node data) |
| (length fixed in DNCP profile) |
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|(optionally) Nested TLVs containing node information |
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
</figure>
<t>This TLV represents the local node's knowledge about the
published state of a node in the DNCP network identified by the
Node Identifier field in the TLV.</t>
<t>Every node, including the originating one, MUST update the
Milliseconds Since Origination whenever it sends a Node
State TLV based on when the node estimates the data was
originally published. This is, e.g., to ensure that any relative
timestamps contained within the published node data can be
correctly offset and interpreted. Ultimately, what is provided is
just an approximation, as transmission delays are not accounted
for. </t>
<t>Absent any changes, if the originating node notices that the
32-bit milliseconds since origination value would be close to
overflow (greater than 2^32-2^16), the node MUST re-publish its
TLVs even if there is no change. In other words, absent any other
changes, the TLV set MUST be re-published roughly every 48
days.</t>
<!-- TODO: (MSt) ^ is there better place for this paragraph? -->
<t>The actual node data of the node may be included within the
TLV as well. In a DNCP profile which supports fragmentation,
described in <xref target="fragmentation" />, the TLV data may be
only partial but it MUST contain full individual TLVs.
This set of TLVs MUST be strictly ordered based on ascending
binary content (including TLV type and length). This enables,
e.g., efficient state delta processing and no-copy indexing by TLV
type by the recipient.</t>
</section>
</section>
<section title="Data TLVs within Node State TLV">
<t>These TLVs are published by the DNCP nodes, and therefore only
encoded within the Node State TLVs. If encountered outside Node
State TLV, they MUST be silently ignored.</t>
<section anchor="fragment-count"
title="Fragment Count TLV">
<figure>
<artwork>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type: FRAGMENT-COUNT (7) | Length: > 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Count |
| (length fixed in DNCP profile) |
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
</figure>
<t>If the DNCP profile supports node data fragmentation as
specified in <xref target="fragmentation" />, this TLV indicates
that the node data is encoded as a sequence of Node State
TLVs. Following Node State TLVs with Node Identifiers up to Count
greater than the current one MUST be considered reachable and part
of the same logical set of node data that this TLV is within. The
fragment portion of the Node Identifier of the Node State TLV
this TLV appears in MUST be zero.</t>
</section>
<section anchor="neighbor"
title="Neighbor TLV">
<figure>
<artwork>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type: NEIGHBOR (8) | Length: > 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor Node Identifier |
| (length fixed in DNCP profile) |
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor Endpoint Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Local Endpoint Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
</figure>
<t>This TLV indicates that the node in question vouches that the
specified neighbor is reachable by it on the specified local
endpoint.
The presence of this TLV at least guarantees that the node
publishing it has received traffic from the neighbor
recently. For guaranteed up-to-date bidirectional reachability,
the existence of both nodes' matching Neighbor TLVs needs to be
checked. </t>
</section>
<section anchor="ka-interval"
title="Keep-Alive Interval TLV">
<figure>
<artwork>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type: KEEP-ALIVE-INTERVAL (9) | Length: 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Endpoint Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
</artwork>
</figure>
<t>This TLV indicates a non-default interval being used to send
keep-alives specified in <xref target="ka" />.</t>
<t>Endpoint identifier is used to identify the particular
endpoint for which the interval applies. If 0, it applies for
ALL endpoints for which no specific TLV exists.</t>
<t>Interval specifies the interval in milliseconds at which the
node sends keep-alives. A value of zero means no keep-alives are
sent at all; in that case, some lower layer mechanism that
ensures presence of nodes MUST be available and used. </t>
</section>
</section>
</section>
<section title="Security and Trust Management">
<t>If specified in the DNCP profile, either <xref
target="RFC6347">DTLS</xref> or <xref target="RFC5246">TLS</xref> may
be used to authenticate and encrypt either some (if specified
optional in the profile), or all unicast traffic. The following
methods for establishing trust are defined, but it is up to the DNCP
profile to specify which ones may, should or must be supported.</t>
<section title="Pre-Shared Key Based Trust Method">
<t>A PSK-based trust model is a simple security management
mechanism that allows an administrator to deploy devices to an
existing network by configuring them with a pre-defined key,
similar to the configuration of an administrator password or
WPA-key. Although limited in nature it is useful to provide a
user-friendly security mechanism for smaller networks. </t>
</section>
<section title="PKI Based Trust Method">
<t>A PKI-based trust-model enables more advanced management
capabilities at the cost of increased complexity and
bootstrapping effort. It however allows trust to be managed in a
centralized manner and is therefore useful for larger networks
with a need for an authoritative trust management.</t>
</section>
<section title="Certificate Based Trust Consensus Method">
<t>The certificate-based consensus model is designed to be a
compromise between trust management effort and flexibility. It is
based on X.509-certificates and allows each DNCP node to provide a
trust verdict on any other certificate and a consensus is found to
determine whether a node using this certificate or any
certificate signed by it is to be trusted. </t>
<t>A DNCP node not using this security method MUST ignore all
announced trust verdicts and MUST NOT announce any such verdicts
by itself, i.e., any other normative language in this subsection
does not apply to it.</t>
<t>The current effective trust verdict for any certificate is
defined as the one with the highest priority from all trust
verdicts announced for said certificate at the time.</t>
<section title="Trust Verdicts">
<t>Trust verdicts are statements of DNCP nodes about the
trustworthiness of X.509-certificates. There are 5 possible
trust verdicts in order of ascending priority:
<list>
<t>0 (Neutral): no trust verdict exists but the DNCP network
should determine one.</t>
<t>1 (Cached Trust): the last known effective trust verdict was
Configured or Cached Trust.</t>
<t>2 (Cached Distrust): the last known effective trust verdict
was Configured or Cached Distrust.</t>
<t>3 (Configured Trust): trustworthy based upon an external
ceremony or configuration.</t>
<t>4 (Configured Distrust): not trustworthy based upon an
external ceremony or configuration.</t>
</list>
</t>
<t>
Trust verdicts are differentiated in 3 groups:
<list style="symbols">
<t>Configured verdicts are used to announce explicit
trust verdicts a node has based on any external trust
bootstrap or predefined relation a node has formed with a
given certificate.</t>
<t>Cached verdicts are used to retain the last known trust
state in case all nodes with configured verdicts about a
given certificate have been disconnected or turned off.</t>
<t>The Neutral verdict is used to announce a new node
intending to join the network so a final verdict for it can
be found.</t>
</list>
</t>
<t>
The current effective trust verdict for any certificate is
defined as the one with the highest priority within the set of
trust verdicts announced for the certificate in the DNCP
network.
A node MUST be trusted for participating in the DNCP network if
and only if the current effective trust verdict for its own
certificate or any one in its certificate hierarchy is (Cached
or Configured) Trust and none of the certificates in its
hierarchy have an effective trust verdict of (Cached or
Configured) Distrust.
In case a node has a configured verdict, which is different
from the current effective trust verdict for a certificate, the
current effective trust verdict takes precedence in deciding
trustworthiness. Despite that, the node still retains and
announces its configured verdict.
</t>
</section>
<section title="Trust Cache">
<t>Each node SHOULD maintain a trust cache containing the current
effective trust verdicts for all certificates currently announced
in the DNCP network. This cache is used as a backup of the last
known state in case there is no node announcing a configured
verdict for a known certificate. It SHOULD be saved to a
non-volatile memory at reasonable time intervals to survive a
reboot or power outage.</t>
<t>Every time a node (re)joins the network or detects the change
of an effective trust verdict for any certificate, it will
synchronize its cache, i.e., store new effective trust verdicts
overwriting any previously cached verdicts. Configured verdicts
are stored in the cache as their respective cached counterparts.
Neutral verdicts are never stored and do not override existing
cached verdicts.</t>
</section>
<section title="Announcement of Verdicts">
<t>A node SHOULD always announce any configured trust verdicts it
has established by itself, and it MUST do so if announcing the
configured trust verdict leads to a change in the current
effective trust verdict for the respective certificate. In
absence of configured verdicts, it MUST announce cached trust
verdicts it has stored in its trust cache, if one of the
following conditions applies:
<list style="symbols">
<t>The stored trust verdict is Cached Trust and the current
effective trust verdict for the certificate is Neutral or does
not exist.</t>
<t>The stored trust verdict is Cached Distrust and the current
effective trust verdict for the certificate is Cached
Trust.</t>
</list>
A node rechecks these conditions whenever it detects changes of
announced trust verdicts anywhere in the network.
</t>
<t>Upon encountering a node with a hierarchy of certificates for
which there is no effective trust verdict, a node adds a Neutral
Trust-Verdict-TLV to its node data for all certificates found in
the hierarchy, and publishes it until an effective trust verdict
different from Neutral can be found for any of the certificates,
or a reasonable amount of time (10 minutes is suggested) with no
reaction and no further authentication attempts has passed. Such
trust verdicts SHOULD also be limited in rate and number to
prevent denial-of-service attacks.</t>
<t>Trust verdicts are announced using Trust-Verdict TLVs:
<figure>
<artwork>
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type: Trust-Verdict (10) | Length: 37-100 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Verdict | (reserved) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| |
| SHA-256 Fingerprint |
| |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Common Name |
</artwork>
</figure>
<list>
<t>Verdict represents the numerical index of the trust
verdict.</t>
<t>(reserved) is reserved for future additions and MUST be set
to 0 when creating TLVs and ignored when parsing them.</t>
<t>SHA-256 Fingerprint contains the <xref
target="RFC6234">SHA-256</xref> hash value of the certificate
in DER-format.</t>
<t>Common Name contains the variable-length (1-64 bytes) common
name of the certificate. Final byte MUST have value of 0.</t>
</list>
</t>
</section>
<section title="Bootstrap Ceremonies">
<t>The following non-exhaustive list of methods describes
possible ways to establish trust relationships between
DNCP nodes and node certificates. Trust establishment is a
two-way process in which the existing network must trust the
newly added node and the newly added node must trust at least
one of its neighboring nodes.
It is therefore necessary that both the newly added node and an
already trusted node perform such a ceremony to successfully
introduce a node into the DNCP network. In all cases an
administrator MUST be provided with external means to identify
the node belonging to a certificate based on its fingerprint
and a meaningful common name.</t>
<section title="Trust by Identification">
<t>A node implementing certificate-based trust MUST provide
an interface to retrieve the current set of effective trust
verdicts, fingerprints and names of all certificates currently
known and set configured trust verdicts to be
announced. Alternatively it MAY provide a companion DNCP node
or application with these capabilities with which it has a
pre-established trust relationship.</t>
</section>
<section title="Preconfigured Trust">
<t>A node MAY be preconfigured to trust a certain set of
node or CA certificates. However such trust relationships
MUST NOT result in unwanted or unrelated trust for nodes not
intended to be run inside the same network (e.g., all other
devices by the same manufacturer).</t>
</section>
<section title="Trust on Button Press">
<t>A node MAY provide a physical or virtual interface to put
one or more of its internal network interfaces temporarily into
a mode in which it trusts the certificate of the first
DNCP node it can successfully establish a connection
with.</t>
</section>
<section title="Trust on First Use">
<t>A node which is not associated with any other DNCP node MAY
trust the certificate of the first DNCP node it can
successfully establish a connection with. This method MUST NOT
be used when the node has already associated with any other
DNCP node.</t>
</section>
</section>
</section>
</section>
<section anchor="profile-bits" title="DNCP Profile-Specific Definitions">
<!-- TBD - Pierre has a point - should define guidance on these,
perhaps -->
<t>Each DNCP profile MUST specify the following aspects:
<list style="symbols">
<t>Unicast and optionally multicast transport protocol(s) to be
used. If multicast-based node and status discovery is desired, a
datagram-based transport supporting multicast has to be available.
</t>
<t>How the chosen transport(s) are secured: Not at all, optionally
or always with the TLS scheme defined here using one or more of the
methods, or with something else. If the links with DNCP nodes can
be sufficiently secured or isolated, it is possible to run DNCP in
a secure manner without using any form of authentication or
encryption.</t>
<t>Transport protocols' parameters such as port numbers to be used,
or multicast address to be used. Unicast, multicast, and secure
unicast may each require different parameters, if applicable. </t>
<t>When receiving TLVs, what sort of TLVs are ignored in addition -
as specified in <xref target="reception" /> - e.g., for security
reasons.
A DNCP profile may safely define the following DNCP TLVs to be safely
ignored:
<list style="symbols">
<t>Anything received over multicast, except <xref
target="endpoint">Node Endpoint TLV</xref> and <xref
target="net-state">Network State TLV</xref>.
</t>
<t>Any TLVs received over unreliable unicast or multicast at too
high rate; Trickle will ensure eventual convergence given the
rate slows down at some point.</t>
</list>
</t>
<t>How to deal with node identifier collision as described in <xref
target="reception" />. Main options are either for one or both
nodes to assign new node identifiers to themselves, or to notify
someone about a fatal error condition in the DNCP network.</t>
<t>Imin, Imax and k ranges to be suggested for implementations to
be used in the Trickle algorithm. The Trickle algorithm does not
require these to be the same across all implementations for it to
work, but similar orders of magnitude helps implementations of a DNCP
profile to behave more consistently and to facilitate estimation of
lower and upper bounds for convergence behavior of the network.</t>
<t>Hash function H(x) to be used, and how many bits of the output
are actually used. The chosen hash function is used to handle both
hashing of node specific data, and network state hash, which is a
hash of node specific data hashes. SHA-256 defined in <xref
target="RFC6234" /> is the recommended default choice, but a
non-cryptographic hash function could be used as well.</t>
<t>DNCP_NODE_IDENTIFIER_LENGTH: The fixed length of a node
identifier (in bytes).</t>
<t>Whether to send keep-alives, and if so, whether per-endpoint
(requires multicast transport), or per-peer. Keep-alive has also
associated parameters:
<list style="symbols">
<t>DNCP_KEEPALIVE_INTERVAL: How often keep-alives are to be
sent by default (if enabled).</t>
<t>DNCP_KEEPALIVE_MULTIPLIER: How many times the
DNCP_KEEPALIVE_INTERVAL (or peer-supplied keep-alive interval
value) a node may not be heard from to be considered still
valid. This is just a default used in absence of any other
configuration information, or particular per-endpoint
configuration.</t>
</list>
</t>
<t>Whether to support fragmentation, and if so, the number of bytes
reserved for fragment count in the node identifier.</t>
</list>
</t>
</section>
<section title="Security Considerations">
<t>DNCP profiles may use multicast to indicate DNCP state changes and
for keep-alive purposes. However, no actual published data TLVs will
be sent across that channel. Therefore an attacker may only learn
hash values of the state within DNCP and may be able to trigger
unicast synchronization attempts between nodes on a local link this
way. A DNCP node should therefore rate-limit its reactions to
multicast packets.</t>
<t>When using DNCP to bootstrap a network, PKI based solutions may have
issues when validating certificates due to potentially unavailable
accurate time, or due to inability to use the network to either check
Certifcate Revocation Lists or perform on-line validation.</t>
<t>The Certificate-based trust consensus mechanism defined in this
document allows for a consenting revocation, however in case of a
compromised device the trust cache may be poisoned before the actual
revocation happens allowing the distrusted device to rejoin the network
using a different identity. Stopping such an attack might require
physical intervention and flushing of the trust caches. </t>
</section>
<section anchor="iana" title="IANA Considerations">
<t>IANA should set up a registry for DNCP TLV types,
with the following initial contents:</t>
<t>0: Reserved</t>
<t>1: Request network state</t>
<t>2: Request node state</t>
<t>3: Node endpoint</t>
<t>4: Network state</t>
<t>5: Node state</t>
<t>6: Reserved (was: Custom)</t>
<t>7: Fragment count</t>
<t>8: Neighbor</t>
<t>9: Keep-alive interval</t>
<t>10: Trust-Verdict</t>
<t>32-191: Reserved for per-DNCP profile use</t>
<t>192-255: Reserved for per-implementation experimentation. How
collision is avoided is out of scope of this document.</t>
<t>For the rest of the values (11-31, 256-65535), policy of 'standards
action' should be used.</t>
</section>
</middle>
<back>
<references title="Normative references">
<?rfc include="reference.RFC.2119.xml"?>
<?rfc include="reference.RFC.6206.xml"?>
<?rfc include="reference.RFC.6347.xml"?>
<?rfc include="reference.RFC.5246.xml"?>
</references>
<references title="Informative references">
<?rfc include="reference.RFC.3493.xml"?>
<?rfc include="reference.RFC.6234.xml"?>
</references>
<section title="Some Questions and Answers [RFC Editor: please remove]">
<t>Q: 32-bit endpoint id?</t>
<t>A: Here, it would save 32 bits per neighbor if it was 16 bits (and
less is not realistic). However, TLVs defined elsewhere would not
seem to even gain that much on average. 32 bits is also used for
ifindex in various operating systems, making for simpler
implementation.</t>
<t>Q: Why have topology information at all?</t>
<t>A: It is an alternative to the more traditional seq#/TTL-based flooding
schemes. In steady state, there is no need to, e.g., re-publish every now
and then.</t>
</section>
<section title="Changelog [RFC Editor: please remove]">
<t>draft-ietf-homenet-dncp-06:
<list style="symbols">
<t>Removed custom TLV.</t>
<t>Made keep-alive multipliers local implementation choice, profiles
just provide guidance on sane default value.</t>
<t>Removed the DNCP_GRACE_INTERVAL as it is really
implementation choice.</t>
<t>Simplified the suggested structures in data model.</t>
<t>Reorganized the document and provided an overview section.</t>
</list>
</t>
<t>draft-ietf-homenet-dncp-04:
<list style="symbols">
<t>Added mandatory rate limiting for network state requests, and
optional slightly faster convergence mechanism by including current
local network state in the remote network state requests.</t>
</list>
</t>
<t>draft-ietf-homenet-dncp-03:
<list style="symbols">
<t>Renamed connection -> endpoint.</t>
<t>!!! Backwards incompatible change: Renumbered TLVs, and got rid
of node data TLV; instead, node data TLV's contents are optionally
within node state TLV.</t>
</list>
</t>
<t>draft-ietf-homenet-dncp-02:
<list style="symbols">
<t>Changed DNCP "messages" into series of TLV streams, allowing
optimized round-trip saving synchronization.</t>
<t>Added fragmentation support for bigger node data and for chunking
in absence of reliable L2 and L3 fragmentation.</t>
</list>
</t>
<t>draft-ietf-homenet-dncp-01:
<list style="symbols">
<t>Fixed keep-alive semantics to consider unicast requests also
updates of most recently consistent, and added proactive unicast
request to ensure even inconsistent keep-alive messages eventually
triggering consistency timestamp update.</t>
<t>Facilitated (simple) read-only clients by making Node Connection
TLV optional if just using DNCP for read-only purposes.</t>
<t>Added text describing how to deal with "dense" networks, but left
actual numbers and mechanics up to DNCP profiles and (local)
configurations.</t>
</list>
</t>
<t>draft-ietf-homenet-dncp-00: Split from pre-version of
draft-ietf-homenet-hncp-03 generic parts. Changes that affect
implementations:
<list style="symbols">
<t>TLVs were renumbered.</t>
<t>TLV length does not include header (=-4). This facilitates,
e.g., use of DHCPv6 option parsing libraries (same encoding), and
reduces complexity (no need to handle error values of length less
than 4).</t>
<t>Trickle is reset only when locally calculated network state hash
is changes, not as remote different network state hash is seen. This
prevents, e.g., attacks by multicast with one multicast packet to force
Trickle reset on every interface of every node on a link.</t>
<t>Instead of 'ping', use 'keep-alive' (optional) for dead peer
detection. Different message used!</t>
</list>
</t>
</section>
<section title="Draft Source [RFC Editor: please remove]">
<t>As usual, this draft is available at <eref
target="https://github.com/fingon/ietf-drafts/">
https://github.com/fingon/ietf-drafts/</eref>
in source format (with nice Makefile too). Feel free to send comments
and/or pull requests if and when you have changes to it! </t>
</section>
<section title="Acknowledgements">
<t>Thanks to Ole Troan, Pierre Pfister, Mark Baugher, Mark Townsley,
Juliusz Chroboczek, Jiazi Yi, Mikael Abrahamsson, Brian Carpenter and
Thomas Clausen for their contributions to the draft.</t>
</section>
</back>
</rfc>
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