Distributed Node Consensus Protocol
draft-ietf-homenet-dncp-09
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 7787.
|
|
|---|---|---|---|
| Authors | Markus Stenberg , Steven Barth | ||
| Last updated | 2015-09-17 (Latest revision 2015-08-05) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Mark Townsley | ||
| Shepherd write-up | Show Last changed 2015-06-05 | ||
| IESG | IESG state | Became RFC 7787 (Proposed Standard) | |
| Consensus boilerplate | Yes | ||
| Telechat date |
(None)
Needs a YES. Needs 10 more YES or NO OBJECTION positions to pass. |
||
| Responsible AD | Terry Manderson | ||
| Send notices to | homenet-chairs@ietf.org, draft-ietf-homenet-dncp@ietf.org, draft-ietf-homenet-dncp.shepherd@ietf.org, draft-ietf-homenet-dncp.ad@ietf.org, "Mark Townsley" <mark@townsley.net> | ||
| IANA | IANA review state | IANA OK - Actions Needed |
draft-ietf-homenet-dncp-09
Homenet Working Group M. Stenberg
Internet-Draft S. Barth
Intended status: Standards Track Independent
Expires: February 6, 2016 August 5, 2015
Distributed Node Consensus Protocol
draft-ietf-homenet-dncp-09
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 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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 6, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 6
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Hash Tree . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Data Transport . . . . . . . . . . . . . . . . . . . . . 8
4.3. Trickle-Driven Status Updates . . . . . . . . . . . . . . 9
4.4. Processing of Received TLVs . . . . . . . . . . . . . . . 10
4.5. Adding and Removing Peers . . . . . . . . . . . . . . . . 12
4.6. Data Liveliness Validation . . . . . . . . . . . . . . . 13
5. Data Model . . . . . . . . . . . . . . . . . . . . . . . . . 14
6. Optional Extensions . . . . . . . . . . . . . . . . . . . . . 15
6.1. Keep-Alives . . . . . . . . . . . . . . . . . . . . . . . 15
6.1.1. Data Model Additions . . . . . . . . . . . . . . . . 16
6.1.2. Per-Endpoint Periodic Keep-Alives . . . . . . . . . . 16
6.1.3. Per-Peer Periodic Keep-Alives . . . . . . . . . . . . 17
6.1.4. Received TLV Processing Additions . . . . . . . . . . 17
6.1.5. Peer Removal . . . . . . . . . . . . . . . . . . . . 17
6.2. Support For Dense Multicast-Enabled Links . . . . . . . . 17
7. Type-Length-Value Objects . . . . . . . . . . . . . . . . . . 18
7.1. Request TLVs . . . . . . . . . . . . . . . . . . . . . . 19
7.1.1. Request Network State TLV . . . . . . . . . . . . . . 19
7.1.2. Request Node State TLV . . . . . . . . . . . . . . . 19
7.2. Data TLVs . . . . . . . . . . . . . . . . . . . . . . . . 20
7.2.1. Node Endpoint TLV . . . . . . . . . . . . . . . . . . 20
7.2.2. Network State TLV . . . . . . . . . . . . . . . . . . 20
7.2.3. Node State TLV . . . . . . . . . . . . . . . . . . . 21
7.3. Data TLVs within Node State TLV . . . . . . . . . . . . . 22
7.3.1. Peer TLV . . . . . . . . . . . . . . . . . . . . . . 22
7.3.2. Keep-Alive Interval TLV . . . . . . . . . . . . . . . 22
8. Security and Trust Management . . . . . . . . . . . . . . . . 23
8.1. Pre-Shared Key Based Trust Method . . . . . . . . . . . . 23
8.2. PKI Based Trust Method . . . . . . . . . . . . . . . . . 23
8.3. Certificate Based Trust Consensus Method . . . . . . . . 23
8.3.1. Trust Verdicts . . . . . . . . . . . . . . . . . . . 24
8.3.2. Trust Cache . . . . . . . . . . . . . . . . . . . . . 25
8.3.3. Announcement of Verdicts . . . . . . . . . . . . . . 25
8.3.4. Bootstrap Ceremonies . . . . . . . . . . . . . . . . 26
9. DNCP Profile-Specific Definitions . . . . . . . . . . . . . . 27
10. Security Considerations . . . . . . . . . . . . . . . . . . . 29
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
12.1. Normative references . . . . . . . . . . . . . . . . . . 30
12.2. Informative references . . . . . . . . . . . . . . . . . 30
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Appendix A. Alternative Modes of Operation . . . . . . . . . . . 31
A.1. Read-only Operation . . . . . . . . . . . . . . . . . . . 31
A.2. Forwarding Operation . . . . . . . . . . . . . . . . . . 31
Appendix B. Some Questions and Answers [RFC Editor: please
remove] . . . . . . . . . . . . . . . . . . . . . . 31
Appendix C. Changelog [RFC Editor: please remove] . . . . . . . 32
Appendix D. Draft Source [RFC Editor: please remove] . . . . . . 33
Appendix E. Acknowledgements . . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
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 hash tree is used. It is formed by first
calculating a hash for the dataset 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.
1.1. Applicability
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 roughly be
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
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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.
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 may be unsuitable since it does not
define any delta synchronization scheme but always transmits the
complete updated TLV set verbatim.
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.
DNCP-based a protocol which 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 which 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 DNCP-based protocol(s)
with 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
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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.
Address an identifier used as source or destination of a
DNCP message flow, e.g., a tuple (IPv6 address, UDP
port) for an IPv6 UDP transport.
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 a 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.
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
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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.
Bidirectionally a peer is locally unidirectionally reachable if a
reachable recent and 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.
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 the nodes in the DNCP network and
maintains the liveliness of published node data by ensuring that the
publishing node was - at least recently - bidirectionally reachable.
New potential peers can be discovered autonomously on multicast-
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enabled links, their addresses may be manually configured or they may
be found by some other means defined in a later specification.
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.
Each leaf represents one recently bidirectionally reachable DNCP node
(see Section 4.6), and is represented by a tuple consisting of the
node's 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
whether the view of the network of two or more nodes is consistent
and shared.
The node data hashes in the leaves and the root network state hash
are updated on-demand and whenever any locally stored per-node state
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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 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. 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 speeds 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:
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* If only reliable unicast transport is used, Trickle is not used
at all. Where Trickle reset has been specified, a single
Network State TLV (Section 7.2.2) is sent instead 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 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 multicast-enabled link optimization
(Section 6.2).
4.3. Trickle-Driven Status Updates
The Trickle algorithm [RFC6206] 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. 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 which 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.
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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.
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 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 multiple access link,
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 breaks.
A DNCP node MUST process TLVs received from any valid 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.
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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
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 re-publish its own node data with a sequence number
significantly (e.g., 1000) greater than the received one, 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 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 sequence number is less than that within the TLV).
+ The local information is potentially incorrect (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 H(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.
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+ 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 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).
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 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.
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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
The topology graph MUST be traversed 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 topology graph traversal starts with the local node marked as
reachable. 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 meet all of the following criteria:
o The origination time (in milliseconds) of R's node data is greater
than current time in - 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 removed
immediately after the topology graph traversal, however it is
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RECOMMENDED to keep them at least briefly to improve the speed of
DNCP network state convergence and to reduce the number of redundant
state transmissions between nodes.
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
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 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.
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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.
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
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 (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.
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For every endpoint that a keep-alive is specified for in the DNCP
profile, the endpoint-specific keep-alive interval MUST be
maintained. By default, it is DNCP_KEEPALIVE_INTERVAL. If there is
a local value that is preferred for that for any reason
(configuration, energy conservation, media type, ..), it can be
substituted instead. If a non-default keep-alive interval is used on
any endpoint, a DNCP node MUST publish appropriate Keep-Alive
Interval TLV(s) (Section 7.3.2) 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 interval started, as specified in the step 2 of Section 4.2
of [RFC6206]. The actual sending time SHOULD be further delayed by a
random timespan in [0, Imin/2].
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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 the step 2
of Section 4.2 of [RFC6206].
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. 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, 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 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 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 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
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
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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.
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 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.
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 unsigned and 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. Each TLV which does not define optional fields or
variable-length content MAY be sent with additional nested TLVs
appended after the required TLV fields - and padding (if applicable)
to allow for extensibility. In this case the length field includes
the length of the original TLV, the length of the padding that are
inserted before the embedded TLVs and the length of the added TLVs.
Therefore, each node MUST accept received TLVs that are longer than
the fixed fields specified and ignore embedded TLVs it does not
understand.
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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 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value |
..
| (variable # of bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (Optional nested TLVs) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
For example, type=123 (0x7b) TLV with value 'x' (120 = 0x78) is
encoded as: 007B 0001 7800 0000. If it were to have sub-TLV of
type=124 (0x7c) with value 'y', it would be encoded as 007B 0009 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 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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
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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: 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. Section 4.2 specifies
when it is sent.
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) |
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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) |
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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
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 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 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
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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 therefore only
encoded within the 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.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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.
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.
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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.
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
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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.
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
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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:
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.
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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
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.
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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:
* 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.
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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 inability to use the network to either check
Certificate 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 the (decimal 16-bit) "DNCP TLV
Types" under "Distributed Node Consensus Protocol (DNCP)", with the
following initial contents: ([RFC Editor: please remove] ideally as
http://www.iana.org/assignments/dncp-registry)
0: Reserved
1: Request network state
2: Request node state
3: Node endpoint
4: Network state
5: Node state
6: Reserved (was: Custom)
7: Reserved (was: Fragment count)
8: Peer
9: Keep-alive interval
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10: Trust-Verdict
11-31: Free - policy of 'standards action' should be used
32-511: Reserved for per-DNCP profile use
512-767: Free - policy of 'standards action' should be used
768-1023: Private use
1024-65535: Reserved for future protocol evolution (for example,
DNCP version 2)
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>.
[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>.
[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>.
[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>.
[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
[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>.
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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 limited
node can acquire and maintain 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 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 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. Some Questions and Answers [RFC Editor: please remove]
Q: 32-bit endpoint id?
A: Here, it would save 32 bits per peer 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 C. Changelog [RFC Editor: please remove]
draft-ietf-homenet-dncp-09:
o Reserved 1024+ TLV types for future versions (=versioning
mechanism); private use section moved from 192-255 to 512-767.
o Added applicability statement and clarified some text based on
reviews.
draft-ietf-homenet-dncp-08:
o Removed fragmentation as it is somewhat underspecified and
unimplemented. It may be specified in some future extension draft
or new version of DNCP.
o Added generic sub-TLV extensibility mechanism.
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:
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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.
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 D. 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!
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Appendix E. 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 draft.
Authors' Addresses
Markus Stenberg
Independent
Helsinki 00930
Finland
Email: markus.stenberg@iki.fi
Steven Barth
Independent
Halle 06114
Germany
Email: cyrus@openwrt.org
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