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rfc:rfc7787

Internet Engineering Task Force (IETF) M. Stenberg Request for Comments: 7787 S. Barth Category: Standards Track Independent ISSN: 2070-1721 April 2016

                Distributed Node Consensus Protocol

Abstract

 This document describes the Distributed Node Consensus Protocol
 (DNCP), a generic state synchronization protocol that uses the
 Trickle algorithm and hash trees.  DNCP is an abstract protocol and
 must be combined with a specific profile to make a complete
 implementable protocol.

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/rfc7787.

Copyright Notice

 Copyright (c) 2016 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.

Stenberg & Barth Standards Track [Page 1] RFC 7787 Distributed Node Consensus Protocol April 2016

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   1.1.  Applicability . . . . . . . . . . . . . . . . . . . . . .   4
 2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   8
 3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
 4.  Operation . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   4.1.  Hash Tree . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.1.1.  Calculating Network State and Node Data Hashes  . . .  10
     4.1.2.  Updating Network State and Node Data Hashes . . . . .  10
   4.2.  Data Transport  . . . . . . . . . . . . . . . . . . . . .  10
   4.3.  Trickle-Driven Status Updates . . . . . . . . . . . . . .  12
   4.4.  Processing of Received TLVs . . . . . . . . . . . . . . .  13
   4.5.  Discovering, Adding, and Removing Peers . . . . . . . . .  15
   4.6.  Data Liveliness Validation  . . . . . . . . . . . . . . .  16
 5.  Data Model  . . . . . . . . . . . . . . . . . . . . . . . . .  17
 6.  Optional Extensions . . . . . . . . . . . . . . . . . . . . .  19
   6.1.  Keep-Alives . . . . . . . . . . . . . . . . . . . . . . .  19
     6.1.1.  Data Model Additions  . . . . . . . . . . . . . . . .  20
     6.1.2.  Per-Endpoint Periodic Keep-Alives . . . . . . . . . .  20
     6.1.3.  Per-Peer Periodic Keep-Alives . . . . . . . . . . . .  20
     6.1.4.  Received TLV Processing Additions . . . . . . . . . .  21
     6.1.5.  Peer Removal  . . . . . . . . . . . . . . . . . . . .  21
   6.2.  Support for Dense Multicast-Enabled Links . . . . . . . .  21
 7.  Type-Length-Value Objects . . . . . . . . . . . . . . . . . .  22
   7.1.  Request TLVs  . . . . . . . . . . . . . . . . . . . . . .  23
     7.1.1.  Request Network State TLV . . . . . . . . . . . . . .  23
     7.1.2.  Request Node State TLV  . . . . . . . . . . . . . . .  24
   7.2.  Data TLVs . . . . . . . . . . . . . . . . . . . . . . . .  24
     7.2.1.  Node Endpoint TLV . . . . . . . . . . . . . . . . . .  24
     7.2.2.  Network State TLV . . . . . . . . . . . . . . . . . .  25
     7.2.3.  Node State TLV  . . . . . . . . . . . . . . . . . . .  25
   7.3.  Data TLVs within Node State TLV . . . . . . . . . . . . .  26
     7.3.1.  Peer TLV  . . . . . . . . . . . . . . . . . . . . . .  26
     7.3.2.  Keep-Alive Interval TLV . . . . . . . . . . . . . . .  27
 8.  Security and Trust Management . . . . . . . . . . . . . . . .  27
   8.1.  Trust Method Based on Pre-Shared Key  . . . . . . . . . .  27
   8.2.  PKI-Based Trust Method  . . . . . . . . . . . . . . . . .  28
   8.3.  Certificate-Based Trust Consensus Method  . . . . . . . .  28
     8.3.1.  Trust Verdicts  . . . . . . . . . . . . . . . . . . .  28
     8.3.2.  Trust Cache . . . . . . . . . . . . . . . . . . . . .  29
     8.3.3.  Announcement of Verdicts  . . . . . . . . . . . . . .  30
     8.3.4.  Bootstrap Ceremonies  . . . . . . . . . . . . . . . .  31
 9.  DNCP Profile-Specific Definitions . . . . . . . . . . . . . .  32
 10. Security Considerations . . . . . . . . . . . . . . . . . . .  34
 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  35

Stenberg & Barth Standards Track [Page 2] RFC 7787 Distributed Node Consensus Protocol April 2016

 12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  36
   12.1.  Normative References . . . . . . . . . . . . . . . . . .  36
   12.2.  Informative References . . . . . . . . . . . . . . . . .  36
 Appendix A.  Alternative Modes of Operation . . . . . . . . . . .  38
   A.1.  Read-Only Operation . . . . . . . . . . . . . . . . . . .  38
   A.2.  Forwarding Operation  . . . . . . . . . . . . . . . . . .  38
 Appendix B.  DNCP Profile Additional Guidance . . . . . . . . . .  38
   B.1.  Unicast Transport -- UDP or TCP?  . . . . . . . . . . . .  38
   B.2.  (Optional) Multicast Transport  . . . . . . . . . . . . .  39
   B.3.  (Optional) Transport Security . . . . . . . . . . . . . .  39
 Appendix C.  Example Profile  . . . . . . . . . . . . . . . . . .  40
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  41
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  41

1. Introduction

 DNCP is designed to provide a way for each participating node to
 publish a small set of TLV (Type-Length-Value) tuples (at most 64 KB)
 and to provide a shared and common view about the data published by
 every currently bidirectionally reachable DNCP node in a network.
 For state synchronization, a hash tree is used.  It is formed by
 first calculating a hash for the data set published by each node,
 called node data, and then calculating another hash over those node
 data hashes.  The single resulting hash, called network state hash,
 is transmitted using the Trickle algorithm [RFC6206] 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, once the maximum
 Trickle interval per link or unicast connection has been reached)
 makes DNCP very thrifty when updates happen rarely.
 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 peers are present, transitively, the whole network
 state also stays up to date.

Stenberg & Barth Standards Track [Page 3] RFC 7787 Distributed Node Consensus Protocol April 2016

1.1. Applicability

 DNCP is useful for cases like autonomous bootstrapping, discovery,
 and negotiation of embedded network devices like routers.
 Furthermore, it can be used as a basis to run distributed algorithms
 like [RFC7596] or use cases as described in Appendix C.  DNCP is
 abstract, which allows it to be tuned to a variety of applications by
 defining profiles.  These profiles include choices of:
  1. unicast transport: a datagram or stream-oriented protocol (e.g.,

TCP, UDP, or the Stream Control Transmission Protocol (SCTP)) for

    generic protocol operation.
  1. optional transport security: whether and when to use security

based on Transport Layer Security (TLS) or Datagram Transport

    Layer Security (DTLS), if supported over the chosen transport.
  1. optional multicast transport: a multicast-capable protocol like

UDP allowing autonomous peer discovery or more efficient use of

    multiple access links.
  1. communication scopes: using either hop by hop only relying on

link-local addressing (e.g., for LANs), addresses with broader

    scopes (e.g., over WANs or the Internet) relying on an existing
    routing infrastructure, or a combination of both (e.g., to
    exchange state between multiple LANs over a WAN or the Internet).
  1. payloads: additional specific payloads (e.g., IANA standardized,

enterprise-specific, or private use).

  1. extensions: possible protocol extensions, either as predefined in

this document or specific for a particular use case.

 However, there are certain cases where the protocol as defined in
 this document is a less suitable choice.  This list provides an
 overview while the following paragraphs provide more detailed
 guidance on the individual matters.
  1. large amounts of data: nodes are limited to 64 KB of published

data.

  1. very dense unicast-only networks: nodes include information about

all immediate neighbors as part of their published data.

  1. predominantly minimal data changes: node data is always

transported as is, leading to a relatively large transmission

    overhead for changes affecting only a small part of it.

Stenberg & Barth Standards Track [Page 4] RFC 7787 Distributed Node Consensus Protocol April 2016

  1. frequently changing data: DNCP with its use of Trickle is

optimized for the steady state and less efficient otherwise.

  1. large amounts of very constrained nodes: DNCP requires each node

to store the entirety of the data published by all nodes.

 The topology of the devices is not limited and automatically
 discovered.  When relying on link-local communication exclusively,
 all links having DNCP nodes need to be at least transitively
 connected by routers running the protocol on multiple endpoints in
 order to form a connected network.  However, there is no requirement
 for every device in a physical network to run the protocol.
 Especially if globally scoped addresses are used, DNCP peers do not
 need to be on the same or even neighboring physical links.
 Autonomous discovery features are usually used in local network
 scenarios; however, with security enabled, DNCP can also be used over
 unsecured public networks.  Network size is restricted merely by the
 capabilities of the devices, i.e., each DNCP node needs to be able to
 store the entirety of the data published by all nodes.  The data
 associated with each individual node identifier is limited to about
 64 KB in this document; however, protocol extensions could be defined
 to mitigate this or other protocol limitations if the need arises.
 DNCP is most suitable for data that changes only infrequently to gain
 the maximum benefit from using Trickle.  As the network of nodes
 grows, or the frequency of data changes per node increases, 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.
 The suitability of DNCP for a particular application can be roughly
 evaluated by considering the expected average network-wide state
 change interval A_NC_I; it is computed by dividing the mean interval
 at which a node originates a new TLV set by the number of
 participating nodes.  If keep-alives are used, A_NC_I is the minimum
 of the computed A_NC_I and the keep-alive interval.  If A_NC_I is
 less than the (application-specific) Trickle minimum interval, DNCP
 is most likely unsuitable for the application as Trickle will not be
 utilized most of the time.
 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.  Nevertheless, if doing so does not raise
 A_NC_I above the (sensibly chosen) Trickle interval parameters for a
 particular application, using DNCP is probably not suitable for the
 application.

Stenberg & Barth Standards Track [Page 5] RFC 7787 Distributed Node Consensus Protocol April 2016

 Another consideration is the size of the published TLV set by a node
 compared to the size of deltas in the TLV set.  If the TLV set
 published by a node is very large, and has frequent small changes,
 DNCP as currently specified in this specification may be unsuitable
 as it lacks a delta synchronization scheme to keep implementation
 simple.
 DNCP can be used in networks where only unicast transport is
 available.  While DNCP uses the least amount of bandwidth when
 multicast is utilized, even in pure unicast mode, the use of Trickle
 (ideally with k < 2) results in a protocol with an exponential
 backoff timer and fewer transmissions than a simpler protocol not
 using Trickle.

2. Terminology

 DNCP profile      the values for the set of parameters given in
                   Section 9.  They are prefixed with DNCP_ in this
                   document.  The profile also specifies the set of
                   optional DNCP extensions to be used.  For a simple
                   example DNCP profile, see Appendix C.
 DNCP-based        a protocol that provides a DNCP profile, according
 protocol          to Section 9, and zero or more TLV assignments from
                   the per-DNCP profile TLV registry as well as their
                   processing rules.
 DNCP node         a single node that runs a DNCP-based protocol.
 Link              a link-layer media over which directly connected
                   nodes can communicate.
 DNCP network      a set of DNCP nodes running a DNCP-based
                   protocol(s) with a matching DNCP profile(s).  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.
 Node identifier   an opaque fixed-length identifier consisting of
                   DNCP_NODE_IDENTIFIER_LENGTH bytes that uniquely
                   identifies a DNCP node within a DNCP network.
 Interface         a node's attachment to a particular link.
 Address           an identifier used as the source or destination of
                   a DNCP message flow, e.g., a tuple (IPv6 address,
                   UDP port) for an IPv6 UDP transport.

Stenberg & Barth Standards Track [Page 6] RFC 7787 Distributed Node Consensus Protocol April 2016

 Endpoint          a locally configured termination point for
                   (potential or established) DNCP message flows.  An
                   endpoint is the source and destination for separate
                   unicast message flows to individual nodes and
                   optionally for multicast messages to all thereby
                   reachable nodes (e.g., for node discovery).
                   Endpoints are usually in one of the transport modes
                   specified in Section 4.2.
 Endpoint          a 32-bit opaque and locally unique value, which
 identifier        identifies a particular endpoint of a particular
                   DNCP node.  The value 0 is reserved for DNCP and
                   DNCP-based protocol purposes and not used to
                   identify an actual endpoint.  This definition is in
                   sync with the interface index definition in
                   [RFC3493], as the non-zero small positive integers
                   should comfortably fit within 32 bits.
 Peer              another DNCP node with which a DNCP node
                   communicates using at least one particular local
                   and remote endpoint pair.
 Node data         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.
 Origination time  the (estimated) time when the node data set with
                   the current sequence number was published.
 Node state        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 (i.e., since the origination
                   time).  The hash function and the length of the
                   hash value are defined in the DNCP profile.
 Network state     a hash value that represents the current state of
 hash              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 Section 4.1.
 Trust verdict     a statement about the trustworthiness of a
                   certificate announced by a node participating in
                   the certificate-based trust consensus mechanism.

Stenberg & Barth Standards Track [Page 7] RFC 7787 Distributed Node Consensus Protocol April 2016

 Effective trust   the trust verdict with the highest priority within
 verdict           the set of trust verdicts announced for the
                   certificate in the DNCP network.
 Topology graph    the undirected graph of DNCP nodes produced by
                   retaining only bidirectional peer relationships
                   between nodes.
 Bidirectionally   a peer is locally unidirectionally reachable if a
 reachable         consistent multicast or any unicast DNCP message
                   has been received by the local node (see Section
                   4.5).  If said peer in return also considers the
                   local node unidirectionally reachable, then
                   bidirectionally reachability is established.  As
                   this process is based on publishing peer
                   relationships and evaluating the resulting topology
                   graph as described in Section 4.6, this information
                   is available to the whole DNCP network.
 Trickle instance  a distinct Trickle [RFC6206] algorithm state kept
                   by a node (Section 5) and related to an endpoint or
                   a particular (peer, endpoint) tuple with Trickle
                   variables I, t, and c.  See Section 4.3.

2.1. Requirements Language

 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 RFC
 2119 [RFC2119].

3. Overview

 DNCP operates primarily using unicast exchanges between nodes, and it
 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.
 DNCP is based on exchanging TLVs (Section 7) and defines a set of
 mandatory and optional ones for its operation.  They are categorized
 into TLVs for requesting information (Section 7.1), transmitting data
 (Section 7.2), and being published as data (Section 7.3).  DNCP-based
 protocols usually specify additional ones to extend the capabilities.
 DNCP discovers the topology of the nodes in the DNCP network and
 maintains the liveliness of published node data by ensuring that the
 publishing node is bidirectionally reachable.  New potential peers
 can be discovered autonomously on multicast-enabled links; their

Stenberg & Barth Standards Track [Page 8] RFC 7787 Distributed Node Consensus Protocol April 2016

 addresses may be manually configured or they may be found by some
 other means defined in the particular DNCP profile.  The DNCP profile
 may specify, for example, a well-known anycast address or provision
 the remote address to contact via some other protocol such as DHCPv6
 [RFC3315].
 A hash tree of height 1, rooted in itself, is maintained by each node
 to represent the state of all currently reachable nodes (see
 Section 4.1), and the Trickle algorithm is used to trigger
 synchronization (see Section 4.3).  The need to check peer nodes for
 state changes is thereby determined by comparing the current root of
 their respective hash trees, i.e., their individually calculated
 network state hashes.
 Before joining a DNCP network, a node starts with a hash tree that
 has only one leaf if the node publishes some TLVs, and no leaves
 otherwise.  It then announces the network state hash calculated from
 the hash tree by means of the Trickle algorithm on all its configured
 endpoints.
 When an update is detected by a node (e.g., by receiving a different
 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 hash 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.
 Whenever a node's local copy of any node data and its hash 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.

4. Operation

4.1. Hash Tree

 Each DNCP node maintains an arbitrary width hash tree of height 1.
 The root of the tree represents the overall network state hash and is
 used to determine whether the view of the network of two or more
 nodes is consistent and shared.  Each leaf represents one
 bidirectionally reachable DNCP node.  Every time a node is added or
 removed from the topology graph (Section 4.6), it is likewise added
 or removed as a leaf.  At any time, the leaves of the tree are
 ordered in ascending order of the node identifiers of the nodes they
 represent.

Stenberg & Barth Standards Track [Page 9] RFC 7787 Distributed Node Consensus Protocol April 2016

4.1.1. Calculating Network State and Node Data Hashes

 The network state hash and the node data hashes are calculated using
 the hash function defined in the DNCP profile (Section 9) and
 truncated to the number of bits specified therein.
 Individual node data hashes are calculated by applying the function
 and truncation on the respective node's node data as published in the
 Node State TLV.  Such node data sets are always ordered as defined in
 Section 7.2.3.
 The network state hash is calculated by applying the function and
 truncation on the concatenated network state.  This state is formed
 by first concatenating each node's sequence number (in network byte
 order) with its node data hash to form a per-node datum for each
 node.  These per-node data are then concatenated in ascending order
 of the respective node's node identifier, i.e., in the order that the
 nodes appear in the hash tree.

4.1.2. Updating Network State and Node Data Hashes

 The network state hash and the node data hashes are updated on-demand
 and whenever any locally stored per-node state changes.  This
 includes local unidirectional reachability encoded in the published
 Peer TLVs (Section 7.3.1) and -- when combined with remote data --
 results in awareness of bidirectional reachability changes.

4.2. Data Transport

 DNCP has few requirements for the underlying transport; it requires
 some way of transmitting either a 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 that 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 TLS [RFC5246] on top of TCP or DTLS
 [RFC6347] on top of UDP -- is also required.  They provide for
 integrity protection and confidentiality of the node data, as well as
 authentication and authorization using the schemes defined in
 "Security and Trust Management" (Section 8).  A specific definition
 of the transport(s) in use and its parameters MUST be provided by the
 DNCP profile.
 TLVs (Section 7) 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.  DNCP does not fragment or

Stenberg & Barth Standards Track [Page 10] RFC 7787 Distributed Node Consensus Protocol April 2016

 reassemble TLVs; thus, it MUST be ensured that the underlying
 transport performs these operations should they be necessary.  If
 this document indicates sending one or more TLVs, then the sending
 node does not need to keep track of the packets sent after handing
 them over to the respective transport, i.e., reliable DNCP operation
 is ensured merely by the explicitly defined timers and state machines
 such as Trickle (Section 4.3).  TLVs in general are handled
 individually and statelessly (and thus do not need to be sent in any
 particular order) with one exception: To form bidirectional peer
 relationships, DNCP requires identification of the endpoints used for
 communication.  As bidirectional peer relationships are required for
 validating liveliness of published node data as described in
 Section 4.6, a DNCP node MUST send a Node Endpoint TLV
 (Section 7.2.1).  When it is sent varies, depending on the underlying
 transport, but conceptually it should be available whenever
 processing a Network State TLV:
 o  If using a stream transport, the TLV MUST be sent at least once
    per connection but SHOULD NOT be sent more than once.
 o  If using a datagram transport, it MUST be included in every
    datagram that also contains a Network State TLV (Section 7.2.2)
    and MUST be located before any such TLV.  It SHOULD also be
    included in any other datagram to speed up initial peer detection.
 Given the assorted transport options as well as potential endpoint
 configuration, a DNCP endpoint may be used in various transport
 modes:
 Unicast:
  • If only reliable unicast transport is used, Trickle is not used

at all. Whenever the locally calculated network state hash

       changes, a single Network State TLV (Section 7.2.2) is sent to
       every unicast peer.  Additionally, recently changed Node State
       TLVs (Section 7.2.3) MAY be included.
  • If only unreliable unicast transport is used, Trickle state is

kept per peer, and it is used to send Network State TLVs

       intermittently, as specified in Section 4.3.
 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 periodically, as
    specified in Section 4.3.  Additionally, per-endpoint keep-alives
    MAY be defined in the DNCP profile, as specified in Section 6.1.2.

Stenberg & Barth Standards Track [Page 11] RFC 7787 Distributed Node Consensus Protocol April 2016

 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 dense multicast-enabled link optimization
    (Section 6.2).

4.3. Trickle-Driven Status Updates

 The Trickle algorithm [RFC6206] is used to ensure protocol
 reliability over unreliable multicast or unicast transports.  For
 reliable unicast transports, its actual algorithm is unnecessary and
 omitted (Section 4.2).  DNCP maintains multiple Trickle states as
 defined in Section 5.  Each such state can be based on different
 parameters (see below) and is responsible for ensuring that a
 specific peer or all peers on the respective endpoint are regularly
 provided with the node's current locally calculated network state
 hash for state comparison, i.e., to detect potential divergence in
 the perceived network state.
 Trickle defines 3 parameters: Imin, Imax, and k.  Imin and Imax
 represent the minimum value for I and the maximum number of doublings
 of Imin, where I 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 Section 9.
 The Trickle state for all Trickle instances defined in Section 5 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 the receipt of
 more recent data from another node as explained in Section 4.1.  A
 node MUST NOT reset its Trickle state merely based on receiving a
 Network State TLV (Section 7.2.2) with a network state hash that is
 different from its locally calculated one.
 Every time a particular Trickle instance indicates that an update
 should be sent, the node MUST send a Network State TLV
 (Section 7.2.2) if and only if:
 o  the endpoint is in Multicast+Unicast transport mode, in which case
    the TLV MUST be sent over multicast.
 o  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.

Stenberg & Barth Standards Track [Page 12] RFC 7787 Distributed Node Consensus Protocol April 2016

 A (sub)set of all Node State TLVs (Section 7.2.3) 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 secure
 unicast.

4.4. Processing of Received TLVs

 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 Section 9 for what may be safely defined
 to be ignored in a profile.  Any 'reply' mentioned in the steps below
 denotes the sending of the specified TLV(s) to the originator of the
 TLV being processed.  All such replies MUST be sent using unicast.
 If the TLV being replied to was received via multicast and it was
 sent to a multiple access link, the reply MUST be delayed by a random
 time span in [0, Imin/2], to avoid potential simultaneous replies
 that may cause problems on some links, unless specified differently
 in the DNCP profile.  The sending of replies MAY also be rate limited
 or omitted for a short period of time by an implementation.  However,
 if the TLV is not forbidden by the DNCP profile, an implementation
 MUST reply to retransmissions of the TLV with a non-zero probability
 to avoid starvation, which would break the state synchronization.
 A DNCP node MUST process TLVs received from any valid (e.g.,
 correctly scoped) address, as specified by the DNCP profile and the
 configuration of a particular endpoint, whether this address is known
 to be the address of a peer 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.
 Upon receipt of:
 o  Request Network State TLV (Section 7.1.1): The receiver MUST reply
    with a Network State TLV (Section 7.2.2) and a Node State TLV
    (Section 7.2.3) for each node data used to calculate the network
    state hash.  The Node State TLVs SHOULD NOT contain the optional
    node data part to avoid redundant transmission of node data,
    unless explicitly specified in the DNCP profile.
 o  Request Node State TLV (Section 7.1.2): If the receiver has node
    data for the corresponding node, it MUST reply with a Node State
    TLV (Section 7.2.3) for the corresponding node.  The optional node
    data part MUST be included in the TLV.
 o  Network State TLV (Section 7.2.2): 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

Stenberg & Barth Standards Track [Page 13] RFC 7787 Distributed Node Consensus Protocol April 2016

    the sender, the receiver MUST reply with a Request Network State
    TLV (Section 7.1.1).  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 Request Network State
    TLV (Section 7.1.1) 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.
 o  Node State TLV (Section 7.2.3):
  • If the node identifier matches the local node identifier and

the TLV has a greater sequence number than its current local

       value, or the same sequence number and a different hash, the
       node SHOULD republish its own node data with a sequence number
       significantly greater than the received one (e.g., 1000) to
       reclaim the node identifier.  This difference is needed in
       order to ensure that it is higher than any potentially
       lingering copies of the node state in the network.  This may
       occur normally once due to the local node restarting and not
       storing the most recently used sequence number.  If this occurs
       more than once or for nodes not republishing 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.
  • If the node identifier does not match the local node

identifier, and one or more of the following conditions are

       true:
       +  The local information is outdated for the corresponding node
          (the local sequence number is less than that within the
          TLV).
       +  The local information is potentially incorrect (the local
          sequence number matches but the node data hash differs).
       +  There is no data for that node altogether.
       Then:
       +  If the TLV contains the Node Data field, it SHOULD also be
          verified by ensuring that the locally calculated hash of the
          node data matches the content of the H(Node Data) field
          within the TLV.  If they differ, the TLV SHOULD be ignored
          and not processed further.

Stenberg & Barth Standards Track [Page 14] RFC 7787 Distributed Node Consensus Protocol April 2016

       +  If the TLV does not contain the Node Data field, and the
          H(Node Data) field within the TLV differs from the local
          node data hash for that node (or there is none), the
          receiver MUST reply with a Request Node State TLV
          (Section 7.1.2) for the corresponding node.
       +  Otherwise, the receiver MUST update its locally stored state
          for that node (node data based on the Node Data field if
          present, sequence number, and relative time) to match the
          received TLV.
    For comparison purposes of the 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 where (a % b) represents the remainder of a modulo b
    and (a & b) represents bitwise conjunction of a and b is
    RECOMMENDED unless the DNCP profile defines another.
 o  Any other TLV: TLVs not recognized by the receiver MUST be
    silently ignored unless they are sent within another TLV (for
    example, TLVs within the Node Data field of a Node State TLV).
    TLVs within the Node Data field of the Node State TLV not
    recognized by the receiver MUST be retained for distribution to
    other nodes and for calculation of the node data hash as described
    in Section 7.2.3 but are ignored for other purposes.
 If secure unicast transport is configured for an endpoint, any Node
 State TLVs received over insecure multicast MUST be silently ignored.

4.5. Discovering, Adding, and Removing Peers

 Peer relations are established between neighbors using one or more
 mutually connected endpoints.  Such neighbors exchange information
 about network state and published data directly, and through
 transitivity, this information then propagates throughout the
 network.
 New peers are discovered using the regular unicast or multicast
 transport defined in the DNCP profile (Section 9).  This process is
 not distinguished from peer addition, i.e., an unknown peer is simply
 discovered by receiving regular DNCP protocol TLVs from it, and
 dedicated discovery messages or TLVs do not exist.  For unicast-only
 transports, the individual node's transport addresses are
 preconfigured or obtained using an external service discovery
 protocol.  In the presence of a multicast transport, messages from
 unknown peers are handled in the same way as multicast messages from
 peers that are already known; thus, new peers are simply discovered
 when sending their regular DNCP protocol TLVs using multicast.

Stenberg & Barth Standards Track [Page 15] RFC 7787 Distributed Node Consensus Protocol April 2016

 When receiving a Node Endpoint TLV (Section 7.2.1) on an endpoint
 from an unknown peer:
 o  If received over unicast, the remote node MUST be added as a peer
    on the endpoint, and a Peer TLV (Section 7.3.1) MUST be created
    for it.
 o  If received over multicast, the node MAY be sent a (possibly rate-
    limited) unicast Request Network State TLV (Section 7.1.1).
 If keep-alives specified in Section 6.1 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 used to ensure its presence.  If
 the peer does not send keep-alives, and no means to verify presence
 of the peer are available, the peer MUST be considered no longer
 present, and it SHOULD NOT be added back as a peer until it starts
 sending keep-alives again.  When the peer is no longer present, the
 Peer TLV and the local DNCP peer state MUST be removed.  DNCP does
 not define an explicit message or TLV for indicating the termination
 of DNCP operation by the terminating node; however, a derived
 protocol could specify an extension, if the need arises.
 If the local endpoint is in the Multicast-Listen+Unicast transport
 mode, a Peer TLV (Section 7.3.1) MUST NOT be published for the peers
 not having the highest node identifier.

4.6. Data Liveliness Validation

 Maintenance of the hash tree (Section 4.1) and thereby network state
 hash updates depend on up-to-date information on bidirectional node
 reachability derived from the contents of a topology graph.  This
 graph changes whenever nodes are added to or removed from the network
 or when bidirectional connectivity between existing nodes is
 established or lost.  Therefore, the graph MUST be updated either
 immediately or with a small delay shorter than the DNCP profile-
 defined Trickle Imin whenever:
 o  A Peer TLV or a whole node is added or removed, or
 o  The origination time (in milliseconds) of some node's node data is
    less than current time - 2^32 + 2^15.
 The artificial upper limit for the origination time is used to
 gracefully avoid overflows of the origination time and allow for the
 node to republish its data as noted in Section 7.2.3.

Stenberg & Barth Standards Track [Page 16] RFC 7787 Distributed Node Consensus Protocol April 2016

 The topology graph update starts with the local node marked as
 reachable and all other nodes marked as unreachable.  Other nodes are
 then iteratively marked as reachable using the following algorithm: A
 candidate not-yet-reachable node N with an endpoint NE is marked as
 reachable if there is a reachable node R with an endpoint RE that
 meets all of the following criteria:
 o  The origination time (in milliseconds) of R's node data is greater
    than current time - 2^32 + 2^15.
 o  R publishes a Peer TLV with:
  • Peer Node Identifier = N's node identifier
  • Peer Endpoint Identifier = NE's endpoint identifier
  • Endpoint Identifier = RE's endpoint identifier
 o  N publishes a Peer TLV with:
  • Peer Node Identifier = R's node identifier
  • Peer Endpoint Identifier = RE's endpoint identifier
  • Endpoint Identifier = NE's endpoint identifier
 The algorithm terminates when no more candidate nodes fulfilling
 these criteria can be found.
 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 forgotten
 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.  This reduces the number of queries
 needed to reconverge during both initial network convergence and when
 a part of the network loses and regains bidirectional connectivity
 within that time period.

5. Data Model

 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 optional extensions
 (Section 6) describe additional data requirements, and some optional
 parts of the core protocol may also require more.

Stenberg & Barth Standards Track [Page 17] RFC 7787 Distributed Node Consensus Protocol April 2016

 A DNCP node has:
 o  A data structure containing data about the most recently sent
    Request Network State TLVs (Section 7.1.1).  The simplest option
    is keeping a timestamp of the most recent request (required to
    fulfill reply rate limiting specified in Section 4.4).
 A DNCP node has the following for every DNCP node in the DNCP
 network:
 o  Node identifier: the unique identifier of the node.  The length,
    how it is produced, and how collisions are handled is up to the
    DNCP profile.
 o  Node data: the set of TLV tuples published by that particular
    node.  As they are transmitted in a particular order (see Node
    State TLV (Section 7.2.3) for details), maintaining the order
    within the data structure here may be reasonable.
 o  Latest 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 Section 4.4.
 o  Origination time: the (estimated) time when the current TLV set
    with the current sequence number was published.  It is used to
    populate the Milliseconds Since Origination field in a Node State
    TLV (Section 7.2.3).  Ideally, it also has millisecond accuracy.
 Additionally, a DNCP node has a set of endpoints for which DNCP is
 configured to be used.  For each such endpoint, a node has:
 o  Endpoint identifier: the 32-bit opaque locally unique value
    identifying the endpoint within a node.  It SHOULD NOT be reused
    immediately after an endpoint is disabled.
 o  Trickle instance: the endpoint's Trickle instance with parameters
    I, T, and c (only on an endpoint in Multicast+Unicast transport
    mode).
 and one (or more) of the following:
 o  Interface: the assigned local network interface.
 o  Unicast address: the DNCP node it should connect with.
 o  Set of addresses: the DNCP nodes from which connections are
    accepted.

Stenberg & Barth Standards Track [Page 18] RFC 7787 Distributed Node Consensus Protocol April 2016

 For each remote (peer, endpoint) pair detected on a local endpoint, a
 DNCP node has:
 o  Node identifier: the unique identifier of the peer.
 o  Endpoint identifier: the unique endpoint identifier used by the
    peer.
 o  Peer address: the most recently used address of the peer
    (authenticated and authorized, if security is enabled).
 o  Trickle instance: the particular peer's Trickle instance with
    parameters I, T, and c (only on an endpoint in unicast mode, when
    using an unreliable unicast transport).

6. Optional Extensions

 This section specifies extensions to the core protocol that a DNCP
 profile may specify to be used.

6.1. Keep-Alives

 While DNCP provides mechanisms for discovery and adding new peers on
 an endpoint (Section 4.5), 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 as noted in
 Section 4.6.
 If keep-alives are not specified in the DNCP profile, the rest of
 this subsection MUST be ignored.
 A DNCP profile MAY specify either per-endpoint (sent using multicast
 to all DNCP nodes connected to a multicast-enabled link) or per-peer
 (sent using unicast to each peer individually) keep-alive support.
 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 an appropriate Keep-Alive
 Interval TLV(s) (Section 7.3.2) within its node data.

Stenberg & Barth Standards Track [Page 19] RFC 7787 Distributed Node Consensus Protocol April 2016

6.1.1. Data Model Additions

 The following additions to the Data Model (Section 5) are needed to
 support keep-alives:
 For each configured endpoint that has per-endpoint keep-alives
 enabled:
 o  Last sent: If a timestamp that indicates the last time a Network
    State TLV (Section 7.2.2) was sent over that interface.
 For each remote (peer, endpoint) pair detected on a local endpoint, a
 DNCP node has:
 o  Last contact timestamp: A timestamp that indicates the last time a
    consistent Network State TLV (Section 7.2.2) was received from the
    peer over multicast or when anything was received over unicast.
    Failing to update it for a certain amount of time as specified in
    Section 6.1.5 results in the removal of the peer.  When adding a
    new peer, it is initialized to the current time.
 o  Last sent: If per-peer keep-alives are enabled, a timestamp that
    indicates the last time a Network State TLV (Section 7.2.2) was
    sent to that point-to-point peer.  When adding a new peer, it is
    initialized to the current time.

6.1.2. Per-Endpoint Periodic Keep-Alives

 If per-endpoint keep-alives are enabled on an endpoint in
 Multicast+Unicast transport mode, and if no traffic containing a
 Network State TLV (Section 7.2.2) has been sent to a particular
 endpoint within the endpoint-specific keep-alive interval, a Network
 State TLV (Section 7.2.2) MUST be sent on that endpoint, and a new
 Trickle interval started, as specified in step 2 of Section 4.2 of
 [RFC6206].  The actual sending time SHOULD be further delayed by a
 random time span in [0, Imin/2].

6.1.3. Per-Peer Periodic Keep-Alives

 If per-peer keep-alives are enabled on a unicast-only endpoint, and
 if no traffic containing a Network State TLV (Section 7.2.2) has been
 sent to a particular peer within the endpoint-specific keep-alive
 interval, a Network State TLV (Section 7.2.2) MUST be sent to the
 peer, and a new Trickle interval started, as specified in step 2 of
 Section 4.2 of [RFC6206].

Stenberg & Barth Standards Track [Page 20] RFC 7787 Distributed Node Consensus Protocol April 2016

6.1.4. Received TLV Processing Additions

 If a TLV is received over unicast from the peer, the Last contact
 timestamp for the peer MUST be updated.
 On receipt of a Network State TLV (Section 7.2.2) that is consistent
 with the locally calculated network state hash, the Last contact
 timestamp for the peer MUST be updated in order to maintain it as a
 peer.

6.1.5. Peer Removal

 For every peer on every endpoint, the endpoint-specific keep-alive
 interval must be calculated by looking for Keep-Alive Interval TLVs
 (Section 7.3.2) published by the node, and if none exist, use the
 default value of DNCP_KEEPALIVE_INTERVAL.  If the peer's Last contact
 timestamp has not been updated for at least a locally chosen
 potentially endpoint-specific keep-alive multiplier (defaults to
 DNCP_KEEPALIVE_MULTIPLIER) times the peer's endpoint-specific keep-
 alive interval, the Peer TLV for that peer and the local DNCP peer
 state MUST be removed.

6.2. Support for Dense Multicast-Enabled Links

 This optimization is needed to avoid a state space explosion.  Given
 a large set of DNCP nodes publishing data on an endpoint that uses
 multicast on a link, every node will add a Peer TLV (Section 7.3.1)
 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 used, there will be O(N^2) keep-alives running on the link
 if the liveliness of peers is not ensured using some other way (e.g.,
 TCP connection lifetime, Layer 2 notification, or per-endpoint keep-
 alive).
 An upper bound for the number of peers 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
 it MAY also be chosen at runtime.  The main consideration when
 selecting a bound (if any) for a particular type of link should be
 whether it supports multicast traffic and whether a too large number
 of peers case is likely to happen during the use of that 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.

Stenberg & Barth Standards Track [Page 21] RFC 7787 Distributed Node Consensus Protocol April 2016

 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 used, 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.
 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.  See Section 4.2 for
 implications on the specific endpoint behavior.  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 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 form peer
 relationships with all peers as specified in Section 4.5.

7. Type-Length-Value Objects

  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 (if any) (+padding (if any))              |
 ..
 |                     (variable # of bytes)                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     (optional nested TLVs)                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Each TLV is encoded as:
 o  a 2-byte Type field
 o  a 2-byte Length field, which contains the length of the Value
    field in bytes; 0 means no value
 o  the value itself (if any)
 o  padding bytes with a value of zero up to the next 4-byte boundary
    if the Length is not divisible by 4

Stenberg & Barth Standards Track [Page 22] RFC 7787 Distributed Node Consensus Protocol April 2016

 While padding bytes MUST NOT be included in the number stored in the
 Length field of the TLV, if the TLV is enclosed within another TLV,
 then the padding is included in the enclosing TLV's Length value.
 Each TLV that does not define optional fields or variable-length
 content MAY be sent with additional sub-TLVs appended after the TLV
 to allow for extensibility.  When handling such TLV types, each node
 MUST accept received TLVs that are longer than the fixed fields
 specified for the particular type and ignore the sub-TLVs with either
 unknown types or types not supported within that particular TLV.  If
 any sub-TLVs are present, the Length field of the TLV describes the
 number of bytes from the first byte of the TLV's own Value (if any)
 to the last (padding) byte of the last sub-TLV.
 For example, type=123 (0x7b) TLV with value 'x' (120 = 0x78) is
 encoded as: 007B 0001 7800 0000.  If it were to have a sub-TLV of
 type=124 (0x7c) with value 'y', it would be encoded as 007B 000C 7800
 0000 007C 0001 7900 0000.
 In this section, the following special notation is used:
    .. = octet string concatenation operation.
    H(x) = non-cryptographic hash function specified by the DNCP
    profile.
 In addition to the TLV types defined in this document, TLV Types
 11-31 and 512-767 are unassigned and may be sequentially registered,
 starting at 11, by Standards Action [RFC5226] by extensions to DNCP
 that may be applicable in multiple DNCP profiles.

7.1. Request TLVs

7.1.1. Request Network State TLV

  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: Request network state (1)|          Length: >= 0         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 This TLV is used to request response with a Network State TLV
 (Section 7.2.2) and all Node State TLVs (Section 7.2.3) (without node
 data).

Stenberg & Barth Standards Track [Page 23] RFC 7787 Distributed Node Consensus Protocol April 2016

7.1.2. Request Node State TLV

  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: Request node state (2)  |          Length: > 0          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Node Identifier                        |
 |                  (length fixed in DNCP profile)               |
 ...
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 This TLV is used to request a Node State TLV (Section 7.2.3)
 (including node data) for the node with the matching node identifier.

7.2. Data TLVs

7.2.1. Node Endpoint TLV

  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                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 This TLV identifies both the local node's node identifier, as well as
 the particular endpoint's endpoint identifier.  Section 4.2 specifies
 when it is sent.

Stenberg & Barth Standards Track [Page 24] RFC 7787 Distributed Node Consensus Protocol April 2016

7.2.2. Network State TLV

  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(sequence number of node 1 .. H(node data of node 1) ..  |
 |    .. sequence number of node N .. H(node data of node N))    |
 |                  (length fixed in DNCP profile)               |
 ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 This TLV contains the current network state hash calculated by its
 sender (Section 4.1 describes the algorithm).

7.2.3. Node State TLV

  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)               |
 ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                       Sequence Number                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                Milliseconds Since Origination                 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         H(Node Data)                          |
 |                  (length fixed in DNCP profile)               |
 ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       (optionally) Node Data (a set of nested TLVs)           |
 ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 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.
 Every node, including the node publishing the node data, 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

Stenberg & Barth Standards Track [Page 25] RFC 7787 Distributed Node Consensus Protocol April 2016

 interpreted.  Ultimately, what is provided is just an approximation,
 as transmission delays are not accounted for.
 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 republish its TLVs even if
 there is no change.  In other words, absent any other changes, the
 TLV set MUST be republished roughly every 48 days.
 The actual node data of the node may be included within the TLV as
 well as in the optional Node Data field.  The 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.  The
 node data content MUST be passed along exactly as it was received.
 It SHOULD be also verified on receipt that the locally calculated
 H(Node Data) matches the content of the field within the TLV, and if
 the hash differs, the TLV SHOULD be ignored.

7.3. Data TLVs within Node State TLV

 These TLVs are published by the DNCP nodes and are therefore only
 encoded in the Node Data field of Node State TLVs.  If encountered
 outside Node State TLV, they MUST be silently ignored.

7.3.1. Peer TLV

  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: Peer (8)          |          Length: > 8          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      Peer Node Identifier                     |
 |                  (length fixed in DNCP profile)               |
 ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    Peer Endpoint Identifier                   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                   (Local) Endpoint Identifier                 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 This TLV indicates that the node in question vouches that the
 specified peer 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 peer recently.  For guaranteed up-
 to-date bidirectional reachability, the existence of both nodes'
 matching Peer TLVs needs to be checked.

Stenberg & Barth Standards Track [Page 26] RFC 7787 Distributed Node Consensus Protocol April 2016

7.3.2. Keep-Alive Interval TLV

  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                            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 This TLV indicates a non-default interval being used to send keep-
 alives as specified in Section 6.1.
 Endpoint identifier is used to identify the particular (local)
 endpoint for which the interval applies on the sending node.  If 0,
 it applies for ALL endpoints for which no specific TLV exists.
 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 the
 presence of nodes MUST be available and used.

8. Security and Trust Management

 If specified in the DNCP profile, either DTLS [RFC6347] or TLS
 [RFC5246] 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.

8.1. Trust Method Based on Pre-Shared Key

 A trust model based on Pre-Shared Key (PSK) is a simple security
 management mechanism that allows an administrator to deploy devices
 to an existing network by configuring them with a predefined key,
 similar to the configuration of an administrator password or Wi-Fi
 Protected Access (WPA) key.  Although limited in nature, it is useful
 to provide a user-friendly security mechanism for smaller networks.

Stenberg & Barth Standards Track [Page 27] RFC 7787 Distributed Node Consensus Protocol April 2016

8.2. PKI-Based Trust Method

 A PKI-based trust model enables more advanced management capabilities
 at the cost of increased complexity and bootstrapping effort.
 However, it allows trust to be managed in a centralized manner and is
 therefore useful for larger networks with a need for an authoritative
 trust management.

8.3. Certificate-Based Trust Consensus Method

 For some scenarios -- such as bootstrapping a mostly unmanaged
 network -- the methods described above may not provide a desirable
 trade-off between security and user experience.  This section
 includes guidance for implementing an opportunistic security
 [RFC7435] method that DNCP profiles can build upon and adapt for
 their specific requirements.
 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.
 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.
 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.

8.3.1. Trust Verdicts

 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:
    0 (Neutral): no trust verdict exists, but the DNCP network should
    determine one.
    1 (Cached Trust): the last known effective trust verdict was
    Configured or Cached Trust.
    2 (Cached Distrust): the last known effective trust verdict was
    Configured or Cached Distrust.

Stenberg & Barth Standards Track [Page 28] RFC 7787 Distributed Node Consensus Protocol April 2016

    3 (Configured Trust): trustworthy based upon an external ceremony
    or configuration.
    4 (Configured Distrust): not trustworthy based upon an external
    ceremony or configuration.
 Trust verdicts are differentiated in 3 groups:
 o  Configured verdicts are used to announce explicit trust verdicts a
    node has based on any external trust bootstrap or predefined
    relations a node has formed with a given certificate.
 o  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.
 o  The Neutral verdict is used to announce a new node intending to
    join the network, so a final verdict for it can be found.
 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.

8.3.2. Trust Cache

 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.
 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.

Stenberg & Barth Standards Track [Page 29] RFC 7787 Distributed Node Consensus Protocol April 2016

8.3.3. Announcement of Verdicts

 A node SHOULD always announce any configured verdicts it has
 established by itself, and it MUST do so if announcing the configured
 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:
 o  The stored trust verdict is Cached Trust, and the current
    effective trust verdict for the certificate is Neutral or does not
    exist.
 o  The stored trust verdict is Cached Distrust, and the current
    effective trust verdict for the certificate is Cached Trust.
 A node rechecks these conditions whenever it detects changes of
 announced trust verdicts anywhere in the network.
 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.

Stenberg & Barth Standards Track [Page 30] RFC 7787 Distributed Node Consensus Protocol April 2016

 Trust verdicts are announced using Trust-Verdict TLVs:
  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: > 36           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Verdict    |                 (reserved)                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 |                                                               |
 |                                                               |
 |                      SHA-256 Fingerprint                      |
 |                                                               |
 |                                                               |
 |                                                               |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                          Common Name                          |
    Verdict represents the numerical index of the trust verdict.
    (reserved) is reserved for future additions and MUST be set to 0
    when creating TLVs and ignored when parsing them.
    SHA-256 Fingerprint contains the SHA-256 [RFC6234] hash value of
    the certificate in DER format.
    Common name contains the variable-length (1-64 bytes) common name
    of the certificate.

8.3.4. Bootstrap Ceremonies

 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 peer 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.

Stenberg & Barth Standards Track [Page 31] RFC 7787 Distributed Node Consensus Protocol April 2016

8.3.4.1. Trust by Identification

 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 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.

8.3.4.2. Preconfigured Trust

 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).

8.3.4.3. Trust on Button Press

 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.

8.3.4.4. Trust on First Use

 A node that 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.

9. DNCP Profile-Specific Definitions

 Each DNCP profile MUST specify the following aspects:
 o  Unicast and optionally a multicast transport protocol(s) to be
    used.  If a multicast-based node and status discovery is desired,
    a datagram-based transport supporting multicast has to be
    available.
 o  How the chosen transport(s) is 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.

Stenberg & Barth Standards Track [Page 32] RFC 7787 Distributed Node Consensus Protocol April 2016

 o  Transport protocols' parameters such as port numbers to be used or
    multicast addresses to be used.  Unicast, multicast, and secure
    unicast may each require different parameters, if applicable.
 o  When receiving TLVs, what sort of TLVs are ignored in addition --
    as specified in Section 4.4 -- e.g., for security reasons.  While
    the security of the node data published within the Node State TLVs
    is already ensured by the base specification (if secure unicast
    transport is used, Node State TLVs are sent only via unicast as
    multicast ones are ignored on receipt), if a profile adds TLVs
    that are sent outside the node data, a profile should indicate
    whether or not those TLVs should be ignored if they are received
    via multicast or non-secured unicast.  A DNCP profile may define
    the following DNCP TLVs to be safely ignored:
  • Anything received over multicast, except Node Endpoint TLV

(Section 7.2.1) and Network State TLV (Section 7.2.2).

  • Any TLVs received over unreliable unicast or multicast at a

rate that is that is too high; Trickle will ensure eventual

       convergence given the rate slows down at some point.
 o  How to deal with node identifier collision as described in
    Section 4.4.  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.
 o  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 help implementations of a
    DNCP profile to behave more consistently and to facilitate
    estimation of lower and upper bounds for convergence behavior of
    the network.
 o  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 data and producing network state hash, which is a
    hash of node data hashes.  SHA-256 defined in [RFC6234] is the
    recommended default choice, but a non-cryptographic hash function
    could be used as well.  If there is a hash collision in the
    network state hash, the network will effectively be partitioned to
    partitions that believe they are up to date but are actually no
    longer converged.  The network will converge either when some node
    data anywhere in the network changes or when conflicting Node
    State TLVs get transmitted across the partition (either caused by
    "Trickle-Driven Status Updates" (Section 4.3) or as part of the
    "Processing of Received TLVs" (Section 4.4)).  If a node publishes

Stenberg & Barth Standards Track [Page 33] RFC 7787 Distributed Node Consensus Protocol April 2016

    node data with a hash that collides with any previously published
    node data, the update may not be (fully) propagated, and the old
    version of node data may be used instead.
 o  DNCP_NODE_IDENTIFIER_LENGTH: The fixed length of a node identifier
    (in bytes).
 o  Whether to send keep-alives, and if so, whether it is per-endpoint
    (requires multicast transport) or per-peer.  Keep-alive also has
    associated parameters:
  • DNCP_KEEPALIVE_INTERVAL: How often keep-alives are to be sent

by default (if enabled).

  • DNCP_KEEPALIVE_MULTIPLIER: How many times the

DNCP_KEEPALIVE_INTERVAL (or peer-supplied keep-alive interval

       value) 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.
 o  Whether to support dense multicast-enabled link optimization
    (Section 6.2) or not.
 For some guidance on choosing transport and security options, please
 see Appendix B.

10. Security Considerations

 DNCP-based protocols 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 MUST therefore rate limit its reactions to
 multicast packets.
 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 the inability to use the network to either
 check Certificate Revocation Lists or perform online validation.
 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.

Stenberg & Barth Standards Track [Page 34] RFC 7787 Distributed Node Consensus Protocol April 2016

11. IANA Considerations

 IANA has set up a registry for the (decimal 16-bit) "DNCP TLV Types"
 under "Distributed Node Consensus Protocol (DNCP)".  The registration
 procedure is Standards Action [RFC5226].  The initial contents are:
    0: Reserved
    1: Request network state
    2: Request node state
    3: Node endpoint
    4: Network state
    5: Node state
    6: Reserved for future use (was: Custom)
    7: Reserved for future use (was: Fragment count)
    8: Peer
    9: Keep-alive interval
    10: Trust-Verdict
    11-31: Unassigned
    32-511: Reserved for per-DNCP profile use
    512-767: Unassigned
    768-1023: Reserved for Private Use [RFC5226]
    1024-65535: Reserved for future use

Stenberg & Barth Standards Track [Page 35] RFC 7787 Distributed Node Consensus Protocol April 2016

12. References

12.1. 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>.
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            DOI 10.17487/RFC5226, May 2008,
            <http://www.rfc-editor.org/info/rfc5226>.
 [RFC6206]  Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko,
            "The Trickle Algorithm", RFC 6206, DOI 10.17487/RFC6206,
            March 2011, <http://www.rfc-editor.org/info/rfc6206>.
 [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
            (SHA and SHA-based HMAC and HKDF)", RFC 6234,
            DOI 10.17487/RFC6234, May 2011,
            <http://www.rfc-editor.org/info/rfc6234>.

12.2. Informative References

 [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
            C., and M. Carney, "Dynamic Host Configuration Protocol
            for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
            2003, <http://www.rfc-editor.org/info/rfc3315>.
 [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
            Stevens, "Basic Socket Interface Extensions for IPv6",
            RFC 3493, DOI 10.17487/RFC3493, February 2003,
            <http://www.rfc-editor.org/info/rfc3493>.
 [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.2", RFC 5246,
            DOI 10.17487/RFC5246, August 2008,
            <http://www.rfc-editor.org/info/rfc5246>.
 [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
            Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
            January 2012, <http://www.rfc-editor.org/info/rfc6347>.
 [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
            Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
            December 2014, <http://www.rfc-editor.org/info/rfc7435>.

Stenberg & Barth Standards Track [Page 36] RFC 7787 Distributed Node Consensus Protocol April 2016

 [RFC7596]  Cui, Y., Sun, Q., Boucadair, M., Tsou, T., Lee, Y., and I.
            Farrer, "Lightweight 4over6: An Extension to the Dual-
            Stack Lite Architecture", RFC 7596, DOI 10.17487/RFC7596,
            July 2015, <http://www.rfc-editor.org/info/rfc7596>.

Stenberg & Barth Standards Track [Page 37] RFC 7787 Distributed Node Consensus Protocol April 2016

Appendix A. Alternative Modes of Operation

 Beyond what is described in the main text, the protocol allows for
 other uses.  These are provided as examples.

A.1. Read-Only Operation

 If a node uses just a single endpoint and does not need to publish
 any TLVs, full DNCP node functionality is not required.  Such a
 limited node can acquire and maintain a view of the TLV space by
 implementing the processing logic as specified in Section 4.4.  Such
 node would not need Trickle, peer-maintenance, or even keep-alives at
 all, as the DNCP nodes' use of it would guarantee eventual receipt of
 network state hashes, and synchronization of node data, even in the
 presence of unreliable transport.

A.2. Forwarding Operation

 If a node with a pair of endpoints does not need to publish any TLVs,
 it can detect (for example) nodes with the highest node identifier on
 each of the endpoints (if any).  Any TLVs received from one of them
 would be forwarded verbatim as unicast to the other node with the
 highest node identifier.
 Any tinkering with the TLVs would remove guarantees of this scheme
 working; however, passive monitoring would obviously be fine.  This
 type of simple forwarding cannot be chained, as it does not send
 anything proactively.

Appendix B. DNCP Profile Additional Guidance

 This appendix explains implications of design choices made when
 specifying the DNCP profile to use particular transport or security
 options.

B.1. Unicast Transport – UDP or TCP?

 The node data published by a DNCP node is limited to 64 KB due to the
 16-bit size of the length field of the TLV it is published within.
 Some transport choices may decrease this limit; if using, e.g., UDP
 datagrams for unicast transport, the upper bound of the node data
 size is whatever the nodes and the underlying network can pass to
 each other as DNCP does not define its own fragmentation scheme.  A
 profile that chooses UDP has to be limited to small node data (e.g.,
 somewhat smaller than IPv6 default MTU if using IPv6) or specify a
 minimum that all nodes have to support.  Even then, if using
 non-link-local communications, there is some concern about what
 middleboxes do to fragmented packets.  Therefore, the use of stream

Stenberg & Barth Standards Track [Page 38] RFC 7787 Distributed Node Consensus Protocol April 2016

 transport such as TCP is probably a good idea if either
 non-link-local communication is desired or fragmentation is expected
 to cause problems.
 TCP also provides some other facilities, such as a relatively long
 built-in keep-alive, which in conjunction with connection closes
 occurring from eventual failed retransmissions may be sufficient to
 avoid the use of in-protocol keep-alive defined in Section 6.1.
 Additionally, it is reliable, so there is no need for Trickle on such
 unicast connections.
 The major downside of using TCP instead of UDP with DNCP-based
 profiles lies in the loss of control over the time at which TLVs are
 received; while unreliable UDP datagrams also have some delay, TLVs
 within reliable stream transport may be delayed significantly due to
 retransmissions.  This is not a problem if no relative time-dependent
 information is stored within the TLVs in the DNCP-based protocol; for
 such a protocol, TCP is a reasonable choice for unicast transport if
 it is available.

B.2. (Optional) Multicast Transport

 Multicast is needed for dynamic peer discovery and to trigger unicast
 exchanges; for that, unreliable datagram transport (=typically UDP)
 is the only transport option defined within this specification,
 although DNCP-based protocols may themselves define some other
 transport or peer discovery mechanism (e.g., based on Multicast DNS
 (mDNS) or DNS).
 If multicast is used, a well-known address should be specified and
 for, e.g., IPv6, respectively, the desired address scopes.  In most
 cases, link-local and possibly site-local are useful scopes.

B.3. (Optional) Transport Security

 In terms of provided security, DTLS and TLS are equivalent; they also
 consume a similar amount of state on the devices.  While TLS is on
 top of a stream protocol, using DTLS also requires relatively long
 session caching within the DTLS layer to avoid expensive
 reauthentication/authorization steps if and when any state within the
 DNCP network changes or per-peer keep-alive (if enabled) is sent.
 TLS implementations (at the time of writing the specification) seem
 more mature and available (as open source) than DTLS ones.  This may
 be due to a long history of use with HTTPS.

Stenberg & Barth Standards Track [Page 39] RFC 7787 Distributed Node Consensus Protocol April 2016

 Some libraries seem not to support multiplexing between insecure and
 secure communication on the same port, so specifying distinct ports
 for secured and unsecured communication may be beneficial.

Appendix C. Example Profile

 This is the DNCP profile of SHSP, an experimental (and for the
 purposes of this document fictional) home automation protocol.  The
 protocol itself is used to make a key-value store published by each
 of the nodes available to all other nodes for distributed monitoring
 and control of a home infrastructure.  It defines only one additional
 TLV type: a key=value TLV that contains a single key=value assignment
 for publication.
 o  Unicast transport: IPv6 TCP on port EXAMPLE-P1 since only absolute
    timestamps are used within the key=value data and since it focuses
    primarily on Linux-based nodes that support both protocols as
    well.  Connections from and to non-link-local addresses are
    ignored to avoid exposing this protocol outside the secure links.
 o  Multicast transport: IPv6 UDP on port EXAMPLE-P2 to link-local
    scoped multicast address ff02:EXAMPLE.  At least one node per link
    in the home is assumed to facilitate node discovery without
    depending on any other infrastructure.
 o  Security: None.  It is to be used only on trusted links (WPA2-x
    wireless, physically secure wired links).
 o  Additional TLVs to be ignored: None.  No DNCP security is
    specified, and no new TLVs are defined outside of node data.
 o  Node identifier length (DNCP_NODE_IDENTIFIER_LENGTH): 32 bits that
    are randomly generated.
 o  Node identifier collision handling: Pick new random node
    identifier.
 o  Trickle parameters: Imin = 200 ms, Imax = 7, k = 1.  It means at
    least one multicast per link in 25 seconds in stable state (0.2 *
    2^7).
 o  Hash function H(x) + length: SHA-256, only 128 bits used.  It's
    relatively fast, and 128 bits should be plenty to prevent random
    conflicts (64 bits would most likely be sufficient, too).
 o  No in-protocol keep-alives (Section 6.1); TCP keep-alive is to be
    used.  In practice, TCP keep-alive is seldom encountered anyway,
    as changes in network state cause packets to be sent on the

Stenberg & Barth Standards Track [Page 40] RFC 7787 Distributed Node Consensus Protocol April 2016

    unicast connections, and those that fail sufficiently many
    retransmissions are dropped much before the keep-alive actually
    would fire.
 o  No support for dense multicast-enabled link optimization
    (Section 6.2); SHSP is a simple protocol for a few nodes (network
    wide, not even to mention on a single link) and therefore would
    not provide any benefit.

Acknowledgements

 Thanks to Ole Troan, Pierre Pfister, Mark Baugher, Mark Townsley,
 Juliusz Chroboczek, Jiazi Yi, Mikael Abrahamsson, Brian Carpenter,
 Thomas Clausen, DENG Hui, and Margaret Cullen for their contributions
 to the document.
 Thanks to Kaiwen Jin and Xavier Bonnetain for their related research
 work.

Authors' Addresses

 Markus Stenberg
 Independent
 Helsinki  00930
 Finland
 Email: markus.stenberg@iki.fi
 Steven Barth
 Independent
 Halle  06114
 Germany
 Email: cyrus@openwrt.org

Stenberg & Barth Standards Track [Page 41]

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