7. Type-Length-Value Objects 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Value (if any) (+padding (if any)) | .. | (variable # of bytes) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | (optional nested TLVs) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Each TLV is encoded as: o a 2-byte Type field o a 2-byte Length field, which contains the length of the Value field in bytes; 0 means no value o the value itself (if any) o padding bytes with a value of zero up to the next 4-byte boundary if the Length is not divisible by 4
While padding bytes MUST NOT be included in the number stored in the Length field of the TLV, if the TLV is enclosed within another TLV, then the padding is included in the enclosing TLV's Length value. Each TLV that does not define optional fields or variable-length content MAY be sent with additional sub-TLVs appended after the TLV to allow for extensibility. When handling such TLV types, each node MUST accept received TLVs that are longer than the fixed fields specified for the particular type and ignore the sub-TLVs with either unknown types or types not supported within that particular TLV. If any sub-TLVs are present, the Length field of the TLV describes the number of bytes from the first byte of the TLV's own Value (if any) to the last (padding) byte of the last sub-TLV. For example, type=123 (0x7b) TLV with value 'x' (120 = 0x78) is encoded as: 007B 0001 7800 0000. If it were to have a sub-TLV of type=124 (0x7c) with value 'y', it would be encoded as 007B 000C 7800 0000 007C 0001 7900 0000. In this section, the following special notation is used: .. = octet string concatenation operation. H(x) = non-cryptographic hash function specified by the DNCP profile. In addition to the TLV types defined in this document, TLV Types 11-31 and 512-767 are unassigned and may be sequentially registered, starting at 11, by Standards Action [RFC5226] by extensions to DNCP that may be applicable in multiple DNCP profiles. 7.1. Request TLVs 7.1.1. Request Network State TLV 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Type: Request network state (1)| Length: >= 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ This TLV is used to request response with a Network State TLV (Section 7.2.2) and all Node State TLVs (Section 7.2.3) (without node data).
7.1.2. Request Node State TLV 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type: Request node state (2) | Length: > 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Node Identifier | | (length fixed in DNCP profile) | ... | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ This TLV is used to request a Node State TLV (Section 7.2.3) (including node data) for the node with the matching node identifier. 7.2. Data TLVs 7.2.1. Node Endpoint TLV 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type: Node endpoint (3) | Length: > 4 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Node Identifier | | (length fixed in DNCP profile) | ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Endpoint Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ This TLV identifies both the local node's node identifier, as well as the particular endpoint's endpoint identifier. Section 4.2 specifies when it is sent.
7.2.2. Network State TLV 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type: Network state (4) | Length: > 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | H(sequence number of node 1 .. H(node data of node 1) .. | | .. sequence number of node N .. H(node data of node N)) | | (length fixed in DNCP profile) | ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ This TLV contains the current network state hash calculated by its sender (Section 4.1 describes the algorithm). 7.2.3. Node State TLV 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type: Node state (5) | Length: > 8 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Node Identifier | | (length fixed in DNCP profile) | ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Milliseconds Since Origination | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | H(Node Data) | | (length fixed in DNCP profile) | ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | (optionally) Node Data (a set of nested TLVs) | ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ This TLV represents the local node's knowledge about the published state of a node in the DNCP network identified by the Node Identifier field in the TLV. Every node, including the node publishing the node data, MUST update the Milliseconds Since Origination whenever it sends a Node State TLV based on when the node estimates the data was originally published. This is, e.g., to ensure that any relative timestamps contained within the published node data can be correctly offset and
interpreted. Ultimately, what is provided is just an approximation, as transmission delays are not accounted for. Absent any changes, if the originating node notices that the 32-bit Milliseconds Since Origination value would be close to overflow (greater than 2^32 - 2^16), the node MUST republish its TLVs even if there is no change. In other words, absent any other changes, the TLV set MUST be republished roughly every 48 days. The actual node data of the node may be included within the TLV as well as in the optional Node Data field. The set of TLVs MUST be strictly ordered based on ascending binary content (including TLV type and length). This enables, e.g., efficient state delta processing and no-copy indexing by TLV type by the recipient. The node data content MUST be passed along exactly as it was received. It SHOULD be also verified on receipt that the locally calculated H(Node Data) matches the content of the field within the TLV, and if the hash differs, the TLV SHOULD be ignored. 7.3. Data TLVs within Node State TLV These TLVs are published by the DNCP nodes and are therefore only encoded in the Node Data field of Node State TLVs. If encountered outside Node State TLV, they MUST be silently ignored. 7.3.1. Peer TLV 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type: Peer (8) | Length: > 8 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Peer Node Identifier | | (length fixed in DNCP profile) | ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Peer Endpoint Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | (Local) Endpoint Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ This TLV indicates that the node in question vouches that the specified peer is reachable by it on the specified local endpoint. The presence of this TLV at least guarantees that the node publishing it has received traffic from the peer recently. For guaranteed up- to-date bidirectional reachability, the existence of both nodes' matching Peer TLVs needs to be checked.
7.3.2. Keep-Alive Interval TLV 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type: Keep-alive interval (9) | Length: >= 8 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Endpoint Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Interval | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ This TLV indicates a non-default interval being used to send keep- alives as specified in Section 6.1. Endpoint identifier is used to identify the particular (local) endpoint for which the interval applies on the sending node. If 0, it applies for ALL endpoints for which no specific TLV exists. Interval specifies the interval in milliseconds at which the node sends keep-alives. A value of zero means no keep-alives are sent at all; in that case, some lower-layer mechanism that ensures the presence of nodes MUST be available and used. 8. Security and Trust Management If specified in the DNCP profile, either DTLS [RFC6347] or TLS [RFC5246] may be used to authenticate and encrypt either some (if specified optional in the profile) or all unicast traffic. The following methods for establishing trust are defined, but it is up to the DNCP profile to specify which ones may, should, or must be supported. 8.1. Trust Method Based on Pre-Shared Key A trust model based on Pre-Shared Key (PSK) is a simple security management mechanism that allows an administrator to deploy devices to an existing network by configuring them with a predefined key, similar to the configuration of an administrator password or Wi-Fi Protected Access (WPA) key. Although limited in nature, it is useful to provide a user-friendly security mechanism for smaller networks.
8.2. PKI-Based Trust Method A PKI-based trust model enables more advanced management capabilities at the cost of increased complexity and bootstrapping effort. However, it allows trust to be managed in a centralized manner and is therefore useful for larger networks with a need for an authoritative trust management. 8.3. Certificate-Based Trust Consensus Method For some scenarios -- such as bootstrapping a mostly unmanaged network -- the methods described above may not provide a desirable trade-off between security and user experience. This section includes guidance for implementing an opportunistic security [RFC7435] method that DNCP profiles can build upon and adapt for their specific requirements. The certificate-based consensus model is designed to be a compromise between trust management effort and flexibility. It is based on X.509 certificates and allows each DNCP node to provide a trust verdict on any other certificate, and a consensus is found to determine whether a node using this certificate or any certificate signed by it is to be trusted. A DNCP node not using this security method MUST ignore all announced trust verdicts and MUST NOT announce any such verdicts by itself, i.e., any other normative language in this subsection does not apply to it. The current effective trust verdict for any certificate is defined as the one with the highest priority from all trust verdicts announced for said certificate at the time. 8.3.1. Trust Verdicts Trust verdicts are statements of DNCP nodes about the trustworthiness of X.509 certificates. There are 5 possible trust verdicts in order of ascending priority: 0 (Neutral): no trust verdict exists, but the DNCP network should determine one. 1 (Cached Trust): the last known effective trust verdict was Configured or Cached Trust. 2 (Cached Distrust): the last known effective trust verdict was Configured or Cached Distrust.
3 (Configured Trust): trustworthy based upon an external ceremony or configuration. 4 (Configured Distrust): not trustworthy based upon an external ceremony or configuration. Trust verdicts are differentiated in 3 groups: o Configured verdicts are used to announce explicit trust verdicts a node has based on any external trust bootstrap or predefined relations a node has formed with a given certificate. o Cached verdicts are used to retain the last known trust state in case all nodes with configured verdicts about a given certificate have been disconnected or turned off. o The Neutral verdict is used to announce a new node intending to join the network, so a final verdict for it can be found. The current effective trust verdict for any certificate is defined as the one with the highest priority within the set of trust verdicts announced for the certificate in the DNCP network. A node MUST be trusted for participating in the DNCP network if and only if the current effective trust verdict for its own certificate or any one in its certificate hierarchy is (Cached or Configured) Trust, and none of the certificates in its hierarchy have an effective trust verdict of (Cached or Configured) Distrust. In case a node has a configured verdict, which is different from the current effective trust verdict for a certificate, the current effective trust verdict takes precedence in deciding trustworthiness. Despite that, the node still retains and announces its configured verdict. 8.3.2. Trust Cache Each node SHOULD maintain a trust cache containing the current effective trust verdicts for all certificates currently announced in the DNCP network. This cache is used as a backup of the last known state in case there is no node announcing a configured verdict for a known certificate. It SHOULD be saved to a non-volatile memory at reasonable time intervals to survive a reboot or power outage. Every time a node (re)joins the network or detects the change of an effective trust verdict for any certificate, it will synchronize its cache, i.e., store new effective trust verdicts overwriting any previously cached verdicts. Configured verdicts are stored in the cache as their respective cached counterparts. Neutral verdicts are never stored and do not override existing cached verdicts.
8.3.3. Announcement of Verdicts A node SHOULD always announce any configured verdicts it has established by itself, and it MUST do so if announcing the configured verdict leads to a change in the current effective trust verdict for the respective certificate. In absence of configured verdicts, it MUST announce Cached Trust verdicts it has stored in its trust cache, if one of the following conditions applies: o The stored trust verdict is Cached Trust, and the current effective trust verdict for the certificate is Neutral or does not exist. o The stored trust verdict is Cached Distrust, and the current effective trust verdict for the certificate is Cached Trust. A node rechecks these conditions whenever it detects changes of announced trust verdicts anywhere in the network. Upon encountering a node with a hierarchy of certificates for which there is no effective trust verdict, a node adds a Neutral Trust- Verdict TLV to its node data for all certificates found in the hierarchy and publishes it until an effective trust verdict different from Neutral can be found for any of the certificates, or a reasonable amount of time (10 minutes is suggested) with no reaction and no further authentication attempts has passed. Such trust verdicts SHOULD also be limited in rate and number to prevent denial-of-service attacks.
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.
184.108.40.206. Trust by Identification A node implementing certificate-based trust MUST provide an interface to retrieve the current set of effective trust verdicts, fingerprints, and names of all certificates currently known and set configured verdicts to be announced. Alternatively, it MAY provide a companion DNCP node or application with these capabilities with which it has a pre-established trust relationship. 220.127.116.11. 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). 18.104.22.168. 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. 22.214.171.124. Trust on First Use A node that is not associated with any other DNCP node MAY trust the certificate of the first DNCP node it can successfully establish a connection with. This method MUST NOT be used when the node has already associated with any other DNCP node. 9. DNCP Profile-Specific Definitions Each DNCP profile MUST specify the following aspects: o Unicast and optionally a multicast transport protocol(s) to be used. If a multicast-based node and status discovery is desired, a datagram-based transport supporting multicast has to be available. o How the chosen transport(s) is secured: Not at all, optionally, or always with the TLS scheme defined here using one or more of the methods, or with something else. If the links with DNCP nodes can be sufficiently secured or isolated, it is possible to run DNCP in a secure manner without using any form of authentication or encryption.
o Transport protocols' parameters such as port numbers to be used or multicast addresses to be used. Unicast, multicast, and secure unicast may each require different parameters, if applicable. o When receiving TLVs, what sort of TLVs are ignored in addition -- as specified in Section 4.4 -- e.g., for security reasons. While the security of the node data published within the Node State TLVs is already ensured by the base specification (if secure unicast transport is used, Node State TLVs are sent only via unicast as multicast ones are ignored on receipt), if a profile adds TLVs that are sent outside the node data, a profile should indicate whether or not those TLVs should be ignored if they are received via multicast or non-secured unicast. A DNCP profile may define the following DNCP TLVs to be safely ignored: * Anything received over multicast, except Node Endpoint TLV (Section 7.2.1) and Network State TLV (Section 7.2.2). * Any TLVs received over unreliable unicast or multicast at a rate that is that is too high; Trickle will ensure eventual convergence given the rate slows down at some point. o How to deal with node identifier collision as described in Section 4.4. Main options are either for one or both nodes to assign new node identifiers to themselves or to notify someone about a fatal error condition in the DNCP network. o Imin, Imax, and k ranges to be suggested for implementations to be used in the Trickle algorithm. The Trickle algorithm does not require these to be the same across all implementations for it to work, but similar orders of magnitude help implementations of a DNCP profile to behave more consistently and to facilitate estimation of lower and upper bounds for convergence behavior of the network. o Hash function H(x) to be used, and how many bits of the output are actually used. The chosen hash function is used to handle both hashing of node data and producing network state hash, which is a hash of node data hashes. SHA-256 defined in [RFC6234] is the recommended default choice, but a non-cryptographic hash function could be used as well. If there is a hash collision in the network state hash, the network will effectively be partitioned to partitions that believe they are up to date but are actually no longer converged. The network will converge either when some node data anywhere in the network changes or when conflicting Node State TLVs get transmitted across the partition (either caused by "Trickle-Driven Status Updates" (Section 4.3) or as part of the "Processing of Received TLVs" (Section 4.4)). If a node publishes
node data with a hash that collides with any previously published node data, the update may not be (fully) propagated, and the old version of node data may be used instead. o DNCP_NODE_IDENTIFIER_LENGTH: The fixed length of a node identifier (in bytes). o Whether to send keep-alives, and if so, whether it is per-endpoint (requires multicast transport) or per-peer. Keep-alive also has associated parameters: * DNCP_KEEPALIVE_INTERVAL: How often keep-alives are to be sent by default (if enabled). * DNCP_KEEPALIVE_MULTIPLIER: How many times the DNCP_KEEPALIVE_INTERVAL (or peer-supplied keep-alive interval value) node may not be heard from to be considered still valid. This is just a default used in absence of any other configuration information or particular per-endpoint configuration. o Whether to support dense multicast-enabled link optimization (Section 6.2) or not. For some guidance on choosing transport and security options, please see Appendix B. 10. Security Considerations DNCP-based protocols may use multicast to indicate DNCP state changes and for keep-alive purposes. However, no actual published data TLVs will be sent across that channel. Therefore, an attacker may only learn hash values of the state within DNCP and may be able to trigger unicast synchronization attempts between nodes on a local link this way. A DNCP node MUST therefore rate limit its reactions to multicast packets. When using DNCP to bootstrap a network, PKI-based solutions may have issues when validating certificates due to potentially unavailable accurate time or due to the inability to use the network to either check Certificate Revocation Lists or perform online validation. The Certificate-based trust consensus mechanism defined in this document allows for a consenting revocation; however, in case of a compromised device, the trust cache may be poisoned before the actual revocation happens allowing the distrusted device to rejoin the network using a different identity. Stopping such an attack might require physical intervention and flushing of the trust caches.
11. IANA Considerations IANA has set up a registry for the (decimal 16-bit) "DNCP TLV Types" under "Distributed Node Consensus Protocol (DNCP)". The registration procedure is Standards Action [RFC5226]. The initial contents are: 0: Reserved 1: Request network state 2: Request node state 3: Node endpoint 4: Network state 5: Node state 6: Reserved for future use (was: Custom) 7: Reserved for future use (was: Fragment count) 8: Peer 9: Keep-alive interval 10: Trust-Verdict 11-31: Unassigned 32-511: Reserved for per-DNCP profile use 512-767: Unassigned 768-1023: Reserved for Private Use [RFC5226] 1024-65535: Reserved for future use
12. References 12.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 2008, <http://www.rfc-editor.org/info/rfc5226>. [RFC6206] Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko, "The Trickle Algorithm", RFC 6206, DOI 10.17487/RFC6206, March 2011, <http://www.rfc-editor.org/info/rfc6206>. [RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, DOI 10.17487/RFC6234, May 2011, <http://www.rfc-editor.org/info/rfc6234>. 12.2. Informative References [RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July 2003, <http://www.rfc-editor.org/info/rfc3315>. [RFC3493] Gilligan, R., Thomson, S., Bound, J., McCann, J., and W. Stevens, "Basic Socket Interface Extensions for IPv6", RFC 3493, DOI 10.17487/RFC3493, February 2003, <http://www.rfc-editor.org/info/rfc3493>. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008, <http://www.rfc-editor.org/info/rfc5246>. [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, January 2012, <http://www.rfc-editor.org/info/rfc6347>. [RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection Most of the Time", RFC 7435, DOI 10.17487/RFC7435, December 2014, <http://www.rfc-editor.org/info/rfc7435>.
Appendix A. Alternative Modes of Operation Beyond what is described in the main text, the protocol allows for other uses. These are provided as examples. A.1. Read-Only Operation If a node uses just a single endpoint and does not need to publish any TLVs, full DNCP node functionality is not required. Such a limited node can acquire and maintain a view of the TLV space by implementing the processing logic as specified in Section 4.4. Such node would not need Trickle, peer-maintenance, or even keep-alives at all, as the DNCP nodes' use of it would guarantee eventual receipt of network state hashes, and synchronization of node data, even in the presence of unreliable transport. A.2. Forwarding Operation If a node with a pair of endpoints does not need to publish any TLVs, it can detect (for example) nodes with the highest node identifier on each of the endpoints (if any). Any TLVs received from one of them would be forwarded verbatim as unicast to the other node with the highest node identifier. Any tinkering with the TLVs would remove guarantees of this scheme working; however, passive monitoring would obviously be fine. This type of simple forwarding cannot be chained, as it does not send anything proactively. Appendix B. DNCP Profile Additional Guidance This appendix explains implications of design choices made when specifying the DNCP profile to use particular transport or security options. B.1. Unicast Transport -- UDP or TCP? The node data published by a DNCP node is limited to 64 KB due to the 16-bit size of the length field of the TLV it is published within. Some transport choices may decrease this limit; if using, e.g., UDP datagrams for unicast transport, the upper bound of the node data size is whatever the nodes and the underlying network can pass to each other as DNCP does not define its own fragmentation scheme. A profile that chooses UDP has to be limited to small node data (e.g., somewhat smaller than IPv6 default MTU if using IPv6) or specify a minimum that all nodes have to support. Even then, if using non-link-local communications, there is some concern about what middleboxes do to fragmented packets. Therefore, the use of stream
transport such as TCP is probably a good idea if either non-link-local communication is desired or fragmentation is expected to cause problems. TCP also provides some other facilities, such as a relatively long built-in keep-alive, which in conjunction with connection closes occurring from eventual failed retransmissions may be sufficient to avoid the use of in-protocol keep-alive defined in Section 6.1. Additionally, it is reliable, so there is no need for Trickle on such unicast connections. The major downside of using TCP instead of UDP with DNCP-based profiles lies in the loss of control over the time at which TLVs are received; while unreliable UDP datagrams also have some delay, TLVs within reliable stream transport may be delayed significantly due to retransmissions. This is not a problem if no relative time-dependent information is stored within the TLVs in the DNCP-based protocol; for such a protocol, TCP is a reasonable choice for unicast transport if it is available. B.2. (Optional) Multicast Transport Multicast is needed for dynamic peer discovery and to trigger unicast exchanges; for that, unreliable datagram transport (=typically UDP) is the only transport option defined within this specification, although DNCP-based protocols may themselves define some other transport or peer discovery mechanism (e.g., based on Multicast DNS (mDNS) or DNS). If multicast is used, a well-known address should be specified and for, e.g., IPv6, respectively, the desired address scopes. In most cases, link-local and possibly site-local are useful scopes. B.3. (Optional) Transport Security In terms of provided security, DTLS and TLS are equivalent; they also consume a similar amount of state on the devices. While TLS is on top of a stream protocol, using DTLS also requires relatively long session caching within the DTLS layer to avoid expensive reauthentication/authorization steps if and when any state within the DNCP network changes or per-peer keep-alive (if enabled) is sent. TLS implementations (at the time of writing the specification) seem more mature and available (as open source) than DTLS ones. This may be due to a long history of use with HTTPS.
Some libraries seem not to support multiplexing between insecure and secure communication on the same port, so specifying distinct ports for secured and unsecured communication may be beneficial. Appendix C. Example Profile This is the DNCP profile of SHSP, an experimental (and for the purposes of this document fictional) home automation protocol. The protocol itself is used to make a key-value store published by each of the nodes available to all other nodes for distributed monitoring and control of a home infrastructure. It defines only one additional TLV type: a key=value TLV that contains a single key=value assignment for publication. o Unicast transport: IPv6 TCP on port EXAMPLE-P1 since only absolute timestamps are used within the key=value data and since it focuses primarily on Linux-based nodes that support both protocols as well. Connections from and to non-link-local addresses are ignored to avoid exposing this protocol outside the secure links. o Multicast transport: IPv6 UDP on port EXAMPLE-P2 to link-local scoped multicast address ff02:EXAMPLE. At least one node per link in the home is assumed to facilitate node discovery without depending on any other infrastructure. o Security: None. It is to be used only on trusted links (WPA2-x wireless, physically secure wired links). o Additional TLVs to be ignored: None. No DNCP security is specified, and no new TLVs are defined outside of node data. o Node identifier length (DNCP_NODE_IDENTIFIER_LENGTH): 32 bits that are randomly generated. o Node identifier collision handling: Pick new random node identifier. o Trickle parameters: Imin = 200 ms, Imax = 7, k = 1. It means at least one multicast per link in 25 seconds in stable state (0.2 * 2^7). o Hash function H(x) + length: SHA-256, only 128 bits used. It's relatively fast, and 128 bits should be plenty to prevent random conflicts (64 bits would most likely be sufficient, too). o No in-protocol keep-alives (Section 6.1); TCP keep-alive is to be used. In practice, TCP keep-alive is seldom encountered anyway, as changes in network state cause packets to be sent on the
unicast connections, and those that fail sufficiently many retransmissions are dropped much before the keep-alive actually would fire. o No support for dense multicast-enabled link optimization (Section 6.2); SHSP is a simple protocol for a few nodes (network wide, not even to mention on a single link) and therefore would not provide any benefit. Acknowledgements Thanks to Ole Troan, Pierre Pfister, Mark Baugher, Mark Townsley, Juliusz Chroboczek, Jiazi Yi, Mikael Abrahamsson, Brian Carpenter, Thomas Clausen, DENG Hui, and Margaret Cullen for their contributions to the document. Thanks to Kaiwen Jin and Xavier Bonnetain for their related research work. Authors' Addresses Markus Stenberg Independent Helsinki 00930 Finland Email: email@example.com Steven Barth Independent Halle 06114 Germany Email: firstname.lastname@example.org