Homenet Working Group M. Stenberg
Internet-Draft
Intended status: Standards Track S. Barth
Expires: December 22, 2015
June 20, 2015
Distributed Node Consensus Protocol
draft-ietf-homenet-dncp-06
Abstract
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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on December 22, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Merkle Tree . . . . . . . . . . . . . . . . . . . . . . . 6
4.2. Data Transport . . . . . . . . . . . . . . . . . . . . . 7
4.3. Trickle-Driven Status Updates . . . . . . . . . . . . . . 8
4.4. Processing of Received TLVs . . . . . . . . . . . . . . . 9
4.5. Adding and Removing Peers . . . . . . . . . . . . . . . . 11
4.6. Data Liveliness Validation . . . . . . . . . . . . . . . 11
5. Data Model . . . . . . . . . . . . . . . . . . . . . . . . . 12
6. Optional Extensions . . . . . . . . . . . . . . . . . . . . . 13
6.1. Keep-Alives . . . . . . . . . . . . . . . . . . . . . . . 13
6.1.1. Data Model Additions . . . . . . . . . . . . . . . . 14
6.1.2. Per-Endpoint Periodic Keep-Alives . . . . . . . . . . 14
6.1.3. Per-Peer Periodic Keep-Alives . . . . . . . . . . . . 14
6.1.4. Received TLV Processing Additions . . . . . . . . . . 15
6.1.5. Neighbor Removal . . . . . . . . . . . . . . . . . . 15
6.2. Support For Dense Broadcast Links . . . . . . . . . . . . 15
6.3. Node Data Fragmentation . . . . . . . . . . . . . . . . . 16
7. Type-Length-Value Objects . . . . . . . . . . . . . . . . . . 17
7.1. Request TLVs . . . . . . . . . . . . . . . . . . . . . . 17
7.1.1. Request Network State TLV . . . . . . . . . . . . . . 17
7.1.2. Request Node State TLV . . . . . . . . . . . . . . . 18
7.2. Data TLVs . . . . . . . . . . . . . . . . . . . . . . . . 18
7.2.1. Node Endpoint TLV . . . . . . . . . . . . . . . . . . 18
7.2.2. Network State TLV . . . . . . . . . . . . . . . . . . 18
7.2.3. Node State TLV . . . . . . . . . . . . . . . . . . . 19
7.3. Data TLVs within Node State TLV . . . . . . . . . . . . . 20
7.3.1. Fragment Count TLV . . . . . . . . . . . . . . . . . 20
7.3.2. Neighbor TLV . . . . . . . . . . . . . . . . . . . . 20
7.3.3. Keep-Alive Interval TLV . . . . . . . . . . . . . . . 21
8. Security and Trust Management . . . . . . . . . . . . . . . . 22
8.1. Pre-Shared Key Based Trust Method . . . . . . . . . . . . 22
8.2. PKI Based Trust Method . . . . . . . . . . . . . . . . . 22
8.3. Certificate Based Trust Consensus Method . . . . . . . . 22
8.3.1. Trust Verdicts . . . . . . . . . . . . . . . . . . . 23
8.3.2. Trust Cache . . . . . . . . . . . . . . . . . . . . . 24
8.3.3. Announcement of Verdicts . . . . . . . . . . . . . . 24
8.3.4. Bootstrap Ceremonies . . . . . . . . . . . . . . . . 25
9. DNCP Profile-Specific Definitions . . . . . . . . . . . . . . 26
10. Security Considerations . . . . . . . . . . . . . . . . . . . 28
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11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 29
12.1. Normative references . . . . . . . . . . . . . . . . . . 29
12.2. Informative references . . . . . . . . . . . . . . . . . 29
Appendix A. Some Questions and Answers [RFC Editor: please
remove] . . . . . . . . . . . . . . . . . . . . . . 29
Appendix B. Changelog [RFC Editor: please remove] . . . . . . . 30
Appendix C. Draft Source [RFC Editor: please remove] . . . . . . 31
Appendix D. Acknowledgements . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31
1. Introduction
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.
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 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) 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 neighbors are present, transitively, the whole
network state also stays up-to-date.
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.
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2. Terminology
DNCP profile a definition of the set of rules and values listed
in Section 9 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_.
DNCP node a single node which runs a protocol based on a DNCP
profile.
Link a link-layer media over which directly connected
nodes can communicate.
DNCP network 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.
Node identifier an opaque fixed-length identifier consisting of
DNCP_NODE_IDENTIFIER_LENGTH bytes which uniquely
identifies a DNCP node within a DNCP network.
Interface a node's attachment to a particular link.
Endpoint 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 Section 4.2.
Endpoint a 32-bit opaque value, which identifies a
identifier 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 [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 a particular local and remote
endpoint pair.
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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.
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. The hash function and the
length of the hash value are defined in the DNCP
profile.
Network state a hash value which 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.
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.
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
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 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,
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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.
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.
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.
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.
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.
4. Operation
4.1. Merkle Tree
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.
Each leaf represents one recently bidirectionally reachable node (see
Section 4.6), 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 Node
State TLV (Section 7.2.3). 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
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whether the view of the network of two or more nodes is consistent
and shared.
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 Neighbor
TLV (Section 7.3.2)s and - when combined with remote data - results
in awareness of bidirectional reachability changes.
4.2. Data Transport
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 TLS [RFC5246] on top of TCP or
DTLS [RFC6347] 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.
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 Endpoint TLV (Section 7.2.1). When it is sent
varies, depending on the underlying transport:
o If using a stream transport, the TLV SHOULD be sent only once
within the stream.
o If using datagram transport, it MUST be included in every
datagram.
Bidirectional peer relationship is not necessary for read-only access
to the DNCP state, but it is required to be able to publish data.
Given the assorted transport options as well as potential endpoint
configuration, a DNCP endpoint may be used in various transport
modes:
Unicast:
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* If only reliable unicast transport is employed, Trickle is not
used at all. Where Trickle reset occurs, a single Network
State TLV (Section 7.2.2) is sent instead to every unicast
peer. Additionally, recently changed Node State TLV
(Section 7.2.3)s MAY be included.
* 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 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 every now and
then, 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.
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 broadcast link optimization (Section 6.2).
4.3. Trickle-Driven Status Updates
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 Section 9.
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.
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.
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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 also 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 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.
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.
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 MUST NOT contain the optional
node data part 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
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
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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 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.
* 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
(local update sequence number is less than that within the
TLV).
+ The local information is potentially incorrect (local update
sequence number matches but the node data hash differs).
+ There is no data for that node altogether.
Then:
+ If the TLV does not contain node data, and the hash of the
node data differs, 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 if present, update sequence number,
relative time) to match the received TLV.
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.
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o Any other TLV: TLVs not recognized by the receiver MUST be
silently ignored.
If secure unicast transport is configured for an endpoint, any Node
State TLVs received over insecure multicast MUST be silently ignored.
4.5. Adding and Removing Peers
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 Neighbor TLV (Section 7.3.2) 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 employed to ensure its presence.
When the peer is no longer present, the Neighbor TLV and the local
DNCP peer state MUST be removed.
If the local endpoint is in the Multicast-Listen+Unicast transport
mode, a Neighbor TLV (Section 7.3.2) MUST NOT be published for the
peers not having the highest node identifier.
4.6. Data Liveliness Validation
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.
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.
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
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DNCP network state convergence and to reduce the number of redundant
state transmissions between nodes.
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.
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 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
particular DNCP profile.
o Node data: the set of TLV tuples published by that particular
node. As they are transmitted ordered (see Node State TLV
(Section 7.2.3) for details), maintaining the order within the
data structure here may be reasonable.
o 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 Section 4.4.
o 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 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 value uniquely identifying
it within the local node.
o Trickle instance: the endpoint's Trickle instance with parameters
I, T, and c (only on an endpoint in Multicast+Unicast transport
mode).
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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 Range of addresses: the DNCP nodes that are allowed to connect.
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 a unicast-only endpoint with
unreliable unicast transport) .
6. Optional Extensions
This section specifies extensions to the core protocol that a DNCP
profile may use.
6.1. Keep-Alives
Trickle-driven status updates (Section 4.3) 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.
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 or per-peer 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
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any endpoint, a DNCP node MUST publish appropriate Keep-Alive
Interval TLV(s) (Section 7.3.3) within its node data.
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 which 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 which indicates the last time
a consistent Network State TLV (Section 7.2.2) 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.
o Last sent: If per-peer keep-alives are enabled, a timestamp which
indicates the last time a Network State TLV (Section 7.2.2) was
sent to 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 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].
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 transmission time 't' in [I/2, I] MUST be
randomly chosen.
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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) which is consistent
with the locally calculated network state hash, the Last contact
timestamp for the peer MUST be updated.
6.1.5. Neighbor Removal
For every peer on every endpoint, the endpoint-specific keep-alive
interval must be calculated by looking for Keep-Alive Interval TLVs
(Section 7.3.3) 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.
6.2. Support For Dense Broadcast Links
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 Neighbor TLV
(Section 7.3.2) 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).
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.
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
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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.
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.
6.3. Node Data Fragmentation
A DNCP profile may be required to support node data which would not
fit the maximum size of a single Node State TLV (Section 7.2.3)
(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.
The data within Node State TLVs of all fragments MUST be valid, as
specified in Section 7.2.3. 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
Section 7.2.3.
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.
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If using fragmentation, the data liveliness validation defined in
Section 4.6 is extended so that if a Fragment Count TLV
(Section 7.3.1) 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.
7. Type-Length-Value Objects
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.
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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
For example, type=123 (0x7b) TLV with value 'x' (120 = 0x78) is
encoded as: 007B 0001 7800 0000.
In this section, the following special notation is used:
.. = octet string concatenation operation.
H(x) = non-cryptographic hash function specified by DNCP profile.
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: REQ-NETWORK-STATE (1) | Length: 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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).
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: REQ-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. It is used when
bidirectional peering is desired, as described in the Section 4.2.
7.2.2. Network State TLV
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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) |
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This TLV contains the current locally calculated network state hash,
see Section 4.1 for how it is calculated.
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) |
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Update Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Milliseconds Since Origination |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| H(node data) |
| (length fixed in DNCP profile) |
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|(optionally) Nested TLVs containing node information |
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 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.
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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.
The actual node data of the node may be included within the TLV as
well. In a DNCP profile which supports fragmentation, described in
Section 6.3, 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.
7.3. Data TLVs within Node State TLV
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.
7.3.1. Fragment Count 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: FRAGMENT-COUNT (7) | Length: > 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Count |
| (length fixed in DNCP profile) |
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
If the DNCP profile supports node data fragmentation as specified in
Section 6.3, 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.
7.3.2. Neighbor TLV
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
7.3.3. 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 specified in Section 6.1.
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.
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.
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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. Pre-Shared Key Based Trust Method
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.
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. 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.
8.3. Certificate Based Trust Consensus Method
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.
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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.
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
relation 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.
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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.
8.3.3. Announcement of Verdicts
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:
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.
Trust verdicts are announced using Trust-Verdict TLVs:
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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 |
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. Final byte MUST have value of 0.
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 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.
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
trust verdicts to be announced. Alternatively it MAY provide a
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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 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.
9. DNCP Profile-Specific Definitions
Each DNCP profile MUST specify the following aspects:
o 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.
o 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.
o 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.
o When receiving TLVs, what sort of TLVs are ignored in addition -
as specified in Section 4.4 - e.g., for security reasons. A DNCP
profile may safely define the following DNCP TLVs to be safely
ignored:
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* 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 too
high rate; 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 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.
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 specific data, and network state hash, which is a
hash of node specific data hashes. SHA-256 defined in [RFC6234]
is the recommended default choice, but a non-cryptographic hash
function could be used as well.
o DNCP_NODE_IDENTIFIER_LENGTH: The fixed length of a node identifier
(in bytes).
o Whether to send keep-alives, and if so, whether per-endpoint
(requires multicast transport), or per-peer. Keep-alive has also
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) 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.
o Whether to support fragmentation, and if so, the number of bytes
reserved for fragment count in the node identifier.
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10. Security Considerations
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.
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.
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.
11. IANA Considerations
IANA should set up a registry for DNCP TLV types, with the following
initial contents:
0: Reserved
1: Request network state
2: Request node state
3: Node endpoint
4: Network state
5: Node state
6: Reserved (was: Custom)
7: Fragment count
8: Neighbor
9: Keep-alive interval
10: Trust-Verdict
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32-191: Reserved for per-DNCP profile use
192-255: Reserved for per-implementation experimentation. How
collision is avoided is out of scope of this document.
For the rest of the values (11-31, 256-65535), policy of 'standards
action' should be used.
12. References
12.1. Normative references
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6206] Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko,
"The Trickle Algorithm", RFC 6206, March 2011.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
12.2. Informative references
[RFC3493] Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
Stevens, "Basic Socket Interface Extensions for IPv6", RFC
3493, February 2003.
[RFC6234] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011.
Appendix A. Some Questions and Answers [RFC Editor: please remove]
Q: 32-bit endpoint id?
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.
Q: Why have topology information at all?
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.
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Appendix B. Changelog [RFC Editor: please remove]
draft-ietf-homenet-dncp-06:
o Removed custom TLV.
o Made keep-alive multipliers local implementation choice, profiles
just provide guidance on sane default value.
o Removed the DNCP_GRACE_INTERVAL as it is really implementation
choice.
o Simplified the suggested structures in data model.
o Reorganized the document and provided an overview section.
draft-ietf-homenet-dncp-04:
o 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.
draft-ietf-homenet-dncp-03:
o Renamed connection -> endpoint.
o !!! Backwards incompatible change: Renumbered TLVs, and got rid of
node data TLV; instead, node data TLV's contents are optionally
within node state TLV.
draft-ietf-homenet-dncp-02:
o Changed DNCP "messages" into series of TLV streams, allowing
optimized round-trip saving synchronization.
o Added fragmentation support for bigger node data and for chunking
in absence of reliable L2 and L3 fragmentation.
draft-ietf-homenet-dncp-01:
o 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.
o Facilitated (simple) read-only clients by making Node Connection
TLV optional if just using DNCP for read-only purposes.
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o Added text describing how to deal with "dense" networks, but left
actual numbers and mechanics up to DNCP profiles and (local)
configurations.
draft-ietf-homenet-dncp-00: Split from pre-version of draft-ietf-
homenet-hncp-03 generic parts. Changes that affect implementations:
o TLVs were renumbered.
o 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).
o 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.
o Instead of 'ping', use 'keep-alive' (optional) for dead peer
detection. Different message used!
Appendix C. Draft Source [RFC Editor: please remove]
As usual, this draft is available at https://github.com/fingon/ietf-
drafts/ in source format (with nice Makefile too). Feel free to send
comments and/or pull requests if and when you have changes to it!
Appendix D. Acknowledgements
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.
Authors' Addresses
Markus Stenberg
Helsinki 00930
Finland
Email: markus.stenberg@iki.fi
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Steven Barth
Halle 06114
Germany
Email: cyrus@openwrt.org
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