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RFC 7788

Home Networking Control Protocol

Pages: 40
Proposed Standard
Updated by:  8375
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Internet Engineering Task Force (IETF)                       M. Stenberg
Request for Comments: 7788                                      S. Barth
Category: Standards Track                                    Independent
ISSN: 2070-1721                                               P. Pfister
                                                           Cisco Systems
                                                              April 2016

                    Home Networking Control Protocol


This document describes the Home Networking Control Protocol (HNCP), an extensible configuration protocol, and a set of requirements for home network devices. HNCP is described as a profile of and extension to the Distributed Node Consensus Protocol (DNCP). HNCP enables discovery of network borders, automated configuration of addresses, name resolution, service discovery, and the use of any routing protocol that supports routing based on both the source and destination address. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at
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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( 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.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1. Requirements Language . . . . . . . . . . . . . . . . . . 7 3. DNCP Profile . . . . . . . . . . . . . . . . . . . . . . . . 7 4. HNCP Versioning and Router Capabilities . . . . . . . . . . . 9 5. Interface Classification . . . . . . . . . . . . . . . . . . 9 5.1. Interface Categories . . . . . . . . . . . . . . . . . . 9 5.2. DHCP-Aided Auto-Detection . . . . . . . . . . . . . . . . 10 5.3. Algorithm for Border Discovery . . . . . . . . . . . . . 11 6. Autonomous Address Configuration . . . . . . . . . . . . . . 12 6.1. Common Link . . . . . . . . . . . . . . . . . . . . . . . 12 6.2. External Connections . . . . . . . . . . . . . . . . . . 13 6.3. Prefix Assignment . . . . . . . . . . . . . . . . . . . . 14 6.3.1. Prefix Assignment Algorithm Parameters . . . . . . . 14 6.3.2. Making New Assignments . . . . . . . . . . . . . . . 16 6.3.3. Applying Assignments . . . . . . . . . . . . . . . . 17 6.3.4. DHCPv6 Prefix Delegation . . . . . . . . . . . . . . 17 6.4. Node Address Assignment . . . . . . . . . . . . . . . . . 17 6.5. Local IPv4 and ULA Prefixes . . . . . . . . . . . . . . . 18 7. Configuration of Hosts and Non-HNCP Routers . . . . . . . . . 19 7.1. IPv6 Addressing and Configuration . . . . . . . . . . . . 19 7.2. DHCPv6 for Prefix Delegation . . . . . . . . . . . . . . 20 7.3. DHCPv4 for Addressing and Configuration . . . . . . . . . 20 7.4. Multicast DNS Proxy . . . . . . . . . . . . . . . . . . . 21 8. Naming and Service Discovery . . . . . . . . . . . . . . . . 21 9. Securing Third-Party Protocols . . . . . . . . . . . . . . . 22
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   10. Type-Length-Value Objects . . . . . . . . . . . . . . . . . .  23
     10.1.  HNCP-Version TLV . . . . . . . . . . . . . . . . . . . .  23
     10.2.  External-Connection TLV  . . . . . . . . . . . . . . . .  24
       10.2.1.  Delegated-Prefix TLV . . . . . . . . . . . . . . . .  25
       10.2.2.  DHCPv6-Data TLV  . . . . . . . . . . . . . . . . . .  27
       10.2.3.  DHCPv4-Data TLV  . . . . . . . . . . . . . . . . . .  27
     10.3.  Assigned-Prefix TLV  . . . . . . . . . . . . . . . . . .  28
     10.4.  Node-Address TLV . . . . . . . . . . . . . . . . . . . .  29
     10.5.  DNS-Delegated-Zone TLV . . . . . . . . . . . . . . . . .  30
     10.6.  Domain-Name TLV  . . . . . . . . . . . . . . . . . . . .  31
     10.7.  Node-Name TLV  . . . . . . . . . . . . . . . . . . . . .  31
     10.8.  Managed-PSK TLV  . . . . . . . . . . . . . . . . . . . .  32
   11. General Requirements for HNCP Nodes . . . . . . . . . . . . .  32
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  34
     12.1.  Interface Classification . . . . . . . . . . . . . . . .  34
     12.2.  Security of Unicast Traffic  . . . . . . . . . . . . . .  35
     12.3.  Other Protocols in the Home  . . . . . . . . . . . . . .  35
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  36
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  37
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  37
     14.2.  Informative References . . . . . . . . . . . . . . . . .  39
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  40
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  40

1. Introduction

The Home Networking Control Protocol (HNCP) is designed to facilitate the sharing of state among home routers to fulfill the needs of the IPv6 homenet architecture [RFC7368], which assumes zero-configuration operation, multiple subnets, multiple home routers, and (potentially) multiple upstream service providers providing (potentially) multiple prefixes to the home network. While RFC 7368 sets no requirements for IPv4 support, HNCP aims to support the dual-stack mode of operation, and therefore the functionality is designed with that in mind. The state is shared as TLVs transported in the DNCP node state among the routers (and potentially advanced hosts) to enable: o Autonomic discovery of network borders (Section 5.3) based on Distributed Node Consensus Protocol (DNCP) topology. o Automated portioning of prefixes delegated by the service providers as well as assigned prefixes to both HNCP and non-HNCP routers (Section 6.3) using [RFC7695]. Prefixes assigned to HNCP routers are used to: * Provide addresses to non-HNCP aware nodes (using Stateless Address Autoconfiguration (SLAAC) and DHCP).
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      *  Provide space in which HNCP nodes assign their own addresses
         (Section 6.4).

   o  Internal and external name resolution, as well as multi-link
      service discovery (Section 8).

   o  Other services not defined in this document that do need to share
      state among homenet nodes and do not cause rapid and constant TLV
      changes (see the following applicability section).

   HNCP is a protocol based on DNCP [RFC7787] and includes a DNCP
   profile that defines transport and synchronization details for
   sharing state across nodes defined in Section 3.  The rest of the
   document defines behavior of the services noted above, how the
   required TLVs are encoded (Section 10), as well as additional
   requirements on how HNCP nodes should behave (Section 11).

1.1. Applicability

While HNCP does not deal with routing protocols directly (except potentially informing them about internal and external interfaces if classification specified in Section 5.3 is used), in homenet environments where multiple IPv6 source prefixes can be present, routing based on the source and destination address is necessary [RFC7368]. Ideally, the routing protocol is also zero configuration (e.g., no need to configure identifiers or metrics), although HNCP can also be used with a manually configured routing protocol. As HNCP uses DNCP as the actual state synchronization protocol, the applicability statement of DNCP applies here as well; HNCP should not be used for any data that changes rapidly and constantly. If such data needs to be published in an HNCP network, 1) a more applicable protocol should be used for those portions, and 2) locators to a server of said protocol should be announced using HNCP instead. An example for this is naming and service discovery (Section 8) for which HNCP only transports DNS server addresses and no actual per- name or per-service data of hosts. HNCP TLVs specified within this document, in steady state, stay constant, with one exception: as Delegated-Prefix TLVs (Section 10.2.1) do contain lifetimes, they force republishing of that data every time the valid or preferred lifetimes of prefixes are updated (significantly). Therefore, it is desirable for ISPs to provide large enough valid and preferred lifetimes to avoid unnecessary HNCP state churn in homes, but even given non-cooperating ISPs, the state churn is proportional only to the number of externally received delegated prefixes and not to the home network size, and it should therefore be relatively low.
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   HNCP assumes a certain level of control over host configuration
   servers (e.g., DHCP [RFC2131]) on links that are managed by its
   routers.  Some HNCP functionality (such as border discovery or some
   aspects of naming) might be affected by existing DHCP servers that
   are not aware of the HNCP-managed network and thus might need to be
   reconfigured to not result in unexpected behavior.

   While HNCP routers can provide configuration to and receive
   configuration from non-HNCP routers, they are not able to traverse
   such devices based solely on the protocol as defined in this
   document, i.e., HNCP routers that are connected only by different
   interfaces of a non-HNCP router will not be part of the same HNCP

   While HNCP is designed to be used by (home) routers, it can also be
   used by advanced hosts that want to do, e.g., their own address
   assignment and routing.

   HNCP is link-layer agnostic; if a link supports IPv6 (link-local)
   multicast and unicast, HNCP will work on it.  Trickle retransmissions
   and keep-alives will handle both packet loss and non-transitive
   connectivity, ensuring eventual convergence.

2. Terminology

The following terms are used as they are defined in [RFC7695]: o Advertised Prefix Priority o Advertised Prefix o Assigned Prefix o Delegated Prefix o Prefix Adoption o Private Link o Published Assigned Prefix o Applied Assigned Prefix o Shared Link
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   The following terms are used as they are defined in [RFC7787]:

   o  DNCP profile

   o  Node identifier

   o  Link

   o  Interface

   (HNCP) node       a device implementing this specification.

   (HNCP) router     a device implementing this specification, which
                     forwards traffic on behalf of other devices.

   Greatest node     when comparing the DNCP node identifiers of
   identifier        multiple nodes, the one that has the greatest value
                     in a bitwise comparison.

   Border            separation point between administrative domains; in
                     this case, between the home network and any other
                     network, i.e., usually an ISP network.

   Internal link     a link that does not cross borders.

   Internal          an interface that is connected to an internal link.

   External          an interface that is connected to a link that is
   interface         not an internal link.

   Interface         a local configuration denoting the use of a
   category          particular interface.  The Interface category
                     determines how an HNCP node should treat the
                     particular interface.  The External and Internal
                     categories mark the interface as out of or within
                     the network border; there are also a number of
                     subcategories of Internal that further affect local
                     node behavior.  See Section 5.1 for a list of
                     interface categories and how they behave.  The
                     Internal or External category may also be auto-
                     detected (Section 5.3).

   Border router     a router announcing external connectivity and
                     forwarding traffic across the network border.
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   Common Link       a set of nodes on a link that share a common view
                     of it, i.e., they see each other's traffic and the
                     same set of hosts.  Unless configured otherwise,
                     transitive connectivity is assumed.

   DHCPv4            refers to the Dynamic Host Configuration Protocol
                     [RFC2131] in this document.

   DHCPv6            refers to the Dynamic Host Configuration Protocol
                     for IPv6 (DHCPv6) [RFC3315] in this document.

   DHCP              refers to cases that apply to both DHCPv4 and
                     DHCPv6 in this document.

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. DNCP Profile

The DNCP profile for HNCP is defined as follows: o HNCP uses UDP datagrams on port 8231 as a transport over link- local scoped IPv6, using unicast and multicast (FF02:0:0:0:0:0:0:11 is the HNCP group address). Received datagrams where either or both of the IPv6 source or destination addresses are not link-local scoped MUST be ignored. Replies to multicast and unicast messages MUST be sent to the IPv6 source address and port of the original message. Each node MUST be able to receive (and potentially reassemble) UDP datagrams with a payload of at least 4000 bytes. o HNCP operates on multicast-capable interfaces only. HNCP nodes MUST assign a non-zero 32-bit endpoint identifier to each interface for which HNCP is enabled. The value 0 is not used in DNCP TLVs but has a special meaning in HNCP TLVs (see Sections 6.4 and 10.3). These identifiers MUST be locally unique within the scope of the node, and using values equivalent to the IPv6 link- local scope identifiers for the given interfaces are RECOMMENDED. o HNCP uses opaque 32-bit node identifiers (DNCP_NODE_IDENTIFIER_LENGTH = 32). A node implementing HNCP SHOULD use a random node identifier. If there is a node identifier collision (as specified in the Node-State TLV handling of Section 4.4 of [RFC7787]), the node MUST immediately generate
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      and use a new random node identifier that is not used by any other
      node at the time, based on the current DNCP network state.

   o  HNCP nodes MUST use the leading 64 bits of the MD5 message digest
      [RFC1321] as the DNCP hash function H(x) used in building the DNCP
      hash tree.

   o  HNCP nodes MUST use DNCP's per-endpoint keep-alive extension on
      all endpoints.  The following parameters are suggested:

      *  Default keep-alive interval (DNCP_KEEPALIVE_INTERVAL): 20

      *  Multiplier (DNCP_KEEPALIVE_MULTIPLIER): 2.1 on virtually
         lossless links works fine, as it allows for one lost keep-
         alive.  If used on a lossy link, a considerably higher
         multiplier, such as 15, should be used instead.  In that case,
         an implementation might prefer shorter keep-alive intervals on
         that link as well to ensure that the timeout (equal to
         which entirely lost nodes time out is low enough.

   o  HNCP nodes use the following Trickle parameters for the per-
      interface Trickle instances:

      *  k SHOULD be 1, as the timer reset when data is updated, and
         further retransmissions should handle packet loss.  Even on a
         non-transitive lossy link, the eventual per-endpoint keep-
         alives should ensure status synchronization occurs.

      *  Imin SHOULD be 200 milliseconds but MUST NOT be lower.  Note:
         earliest transmissions may occur at Imin / 2.

      *  Imax SHOULD be 7 doublings of Imin [RFC6206] but MUST NOT be

   o  HNCP unicast traffic SHOULD be secured using Datagram Transport
      Layer Security (DTLS) [RFC6347] as described in DNCP if exchanged
      over unsecured links.  UDP on port 8232 is used for this purpose.
      A node implementing HNCP security MUST support the DNCP Pre-Shared
      Key (PSK) method, SHOULD support the PKI-based trust method, and
      MAY support the DNCP certificate-based trust consensus method.
      [RFC7525] provides guidance on how to securely utilize DTLS.

   o  HNCP nodes MUST ignore all Node-State TLVs received via multicast
      on a link that has DNCP security enabled in order to prevent
      spoofing of node state changes.
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4. HNCP Versioning and Router Capabilities

Multiple versions of HNCP based on compatible DNCP profiles may be present in the same network when transitioning between HNCP versions, and for troubleshooting purposes, it might be beneficial to identify the HNCP agent version running. Therefore, each node MUST include an HNCP-Version TLV (Section 10.1) indicating the currently supported version in its node data and MUST ignore (except for DNCP synchronization purposes) any TLVs that have a type greater than 32 and that are published by nodes that didn't also publish an HNCP- Version TLV. HNCP routers may also have different capabilities regarding interactions with hosts, e.g., for configuration or service discovery. These are indicated by M, P, H, and L values. The combined "capability value" is a metric indicated by interpreting the bits as an integer, i.e., (M << 12 | P << 8 | H << 4 | L). These values are used to elect certain servers on a Common Link, as described in Section 7. Nodes that are not routers MUST announce the value 0 for all capabilities. Any node announcing the value 0 for a capability is considered to not advertise said capability and thus does not take part in the respective election.

5. Interface Classification

5.1. Interface Categories

HNCP specifies the following categories that interfaces can be configured to be in: Internal category: This declares an interface to be internal, i.e., within the borders of the HNCP network. The interface MUST operate as a DNCP endpoint. Routers MUST forward traffic with appropriate source addresses between their internal interfaces and allow internal traffic to reach external networks. All nodes MUST implement this category, and nodes not implementing any other category implicitly use it as a fixed default. External category: This declares an interface to be external, i.e., not within the borders of the HNCP network. The interface MUST NOT operate as a DNCP endpoint. Accessing internal resources from external interfaces is restricted, i.e., the use of Recommended Simple Security Capabilities in Customer Premises Equipments (CPEs) [RFC6092] is RECOMMENDED. HNCP routers SHOULD announce acquired configuration information for use in the network as described in Section 6.2, if the interface appears to be connected to an external network. HNCP routers MUST implement this category.
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   Leaf category:  This declares an interface used by client devices
      only.  Such an interface uses the Internal category with the
      exception that it MUST NOT operate as a DNCP endpoint.  This
      category SHOULD be supported by HNCP routers.

   Guest category:  This declares an interface used by untrusted client
      devices only.  In addition to the restrictions of the Leaf
      category, HNCP routers MUST filter traffic from and to the
      interface such that connected devices are unable to reach other
      devices inside the HNCP network or query services advertised by
      them unless explicitly allowed.  This category SHOULD be supported
      by HNCP routers.

   Ad Hoc category:  This configures an interface to use the Internal
      category, but no assumption is made about the link's transitivity.
      All other interface categories assume transitive connectivity.
      This affects the Common Link (Section 6.1) definition.  Support
      for this category is OPTIONAL.

   Hybrid category:  This declares an interface to use the Internal
      category while still trying to acquire (external) configuration
      information on it, e.g., by running DHCP clients.  This is useful,
      e.g., if the link is shared with a non-HNCP router under control
      and still within the borders of the same network.  Detection of
      this category automatically in addition to manual configuration is
      out of scope of this document.  Support for this category is

5.2. DHCP-Aided Auto-Detection

Auto-detection of interface categories is possible based on interaction with DHCPv4 [RFC2131] and DHCPv6 Prefix Delegation (DHCPv6-PD) [RFC3633] servers on connected links. HNCP defines special DHCP behavior to differentiate its internal servers from external ones in order to achieve this. Therefore, all internal devices (including HNCP nodes) running DHCP servers on links where auto-detection is used by any HNCP node MUST use the following mechanism based on "The User Class Option for DHCP" [RFC3004] and its DHCPv6 counterpart [RFC3315]: o The device MUST ignore or reject DHCP-Requests containing a DHCP user class consisting of the ASCII string "HOMENET". Not following this rule (e.g., running unmodified DHCP servers) might lead to false positives when auto-detection is used, i.e., HNCP nodes assume an interface to not be internal, even though it was intended to be.
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5.3. Algorithm for Border Discovery

This section defines the interface classification algorithm. It is suitable for both IPv4 and IPv6 (single or dual stack) and detects the category of an interface either automatically or based on a fixed configuration. By determining the category for all interfaces, the network borders are implicitly defined, i.e., all interfaces not belonging to the External category are considered to be within the borders of the network; all others are not. The following algorithm MUST be implemented by any node implementing HNCP. However, if the node does not implement auto-detection, only the first and last step are required. The algorithm works as follows, with evaluation stopping at first match: 1. If a fixed category is configured for an interface, it is used. 2. If a delegated prefix could be acquired by running a DHCPv6 client, it is considered external. The DHCPv6 client MUST have included a DHCPv6 user class consisting of the ASCII string "HOMENET" in all of its requests. 3. If an IPv4 address could be acquired by running a DHCPv4 client on the interface, it is considered external. The DHCPv4 client MUST have included a DHCP user class consisting of the ASCII string "HOMENET" in all of its requests. 4. The interface is considered internal. Note that as other HNCP nodes will ignore the client due to the User Class option, any server that replies is clearly external (or a malicious internal node). An HNCP router SHOULD allow setting the fixed category for each interface that may be connected to either an internal or external device (e.g., an Ethernet port that can be connected to a modem, another HNCP router, or a client). Note that all fixed categories except internal and external cannot be auto-detected and can only be selected using manual configuration. An HNCP router using auto-detection on an interface MUST run the appropriately configured DHCP clients as long as the interface without a fixed category is active (including states where auto- detection considers it to be internal) and rerun the algorithm above to react to conditions resulting in a different interface category. The router SHOULD wait for a reasonable time period (5 seconds as a
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   default), during which the DHCP clients can acquire a lease, before
   treating a newly activated or previously external interface as

6. Autonomous Address Configuration

This section specifies how HNCP nodes configure host and node addresses. At first, border routers share information obtained from service providers or local configuration by publishing one or more External-Connection TLVs (Section 10.2). These contain other TLVs such as Delegated-Prefix TLVs (Section 10.2.1) that are then used for prefix assignment. Finally, HNCP nodes obtain addresses either statelessly or using a specific stateful mechanism (Section 6.4). Hosts and non-HNCP routers are configured using SLAAC, DHCP, or DHCPv6-PD.

6.1. Common Link

HNCP uses the concept of Common Link both in autonomic address configuration and naming and service discovery (Section 8). A Common Link refers to the set of interfaces of nodes that see each other's traffic and presumably also the traffic of all hosts that may use the nodes to, e.g., forward traffic. Common Links are used, e.g., to determine where prefixes should be assigned or which peers participate in the election of a DHCP server. The Common Link is computed separately for each local internal interface, and it always contains the local interface. Additionally, if the local interface is not set to the Ad Hoc category (see Section 5.1), it also contains the set of interfaces that are bidirectionally reachable from the given local interface; that is, every remote interface of a remote node meeting all of the following requirements: o The local node publishes a Peer TLV with: * Peer Node Identifier = remote node's node identifier * Peer Endpoint Identifier = remote interface's endpoint identifier * Endpoint Identifier = local interface's endpoint identifier o The remote node publishes a Peer TLV with: * Peer Node Identifier = local node's node identifier * Peer Endpoint Identifier = local interface's endpoint identifier
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      *  Endpoint Identifier = remote interface's endpoint identifier

   A node MUST be able to detect whether two of its local internal
   interfaces are connected, e.g., by detecting an identical remote
   interface being part of the Common Links of both local interfaces.

6.2. External Connections

Each HNCP router MAY obtain external connection information such as address prefixes, DNS server addresses, and DNS search paths from one or more sources, e.g., DHCPv6-PD [RFC3633], NETCONF [RFC6241], or static configuration. Each individual external connection to be shared in the network is represented by one External-Connection TLV (Section 10.2). Announcements of individual external connections can consist of the following components: Delegated Prefixes: Address space available for assignment to internal links announced using Delegated-Prefix TLVs (Section 10.2.1). Some address spaces might have special properties that are necessary to understand in order to handle them (e.g., information similar to [RFC6603]). This information is encoded using DHCPv6-Data TLVs (Section 10.2.2) inside the respective Delegated-Prefix TLVs. Auxiliary Information: Information about services such as DNS or time synchronization regularly used by hosts in addition to addressing and routing information. This information is encoded using DHCPv6-Data TLVs (Section 10.2.2) and DHCPv4-Data TLVs (Section 10.2.3). Whenever information about reserved parts (e.g., as specified in [RFC6603]) is received for a delegated prefix, the reserved parts MUST be advertised using Assigned-Prefix TLVs (Section 10.3) with the greatest priority (i.e., 15), as if they were assigned to a Private Link. Some connections or delegated prefixes may have a special meaning and are not regularly used for internal or Internet connectivity; instead, they may provide access to special services like VPNs, sensor networks, Voice over IP (VoIP), IPTV, etc. Care must be taken that these prefixes are properly integrated and dealt with in the network, in order to avoid breaking connectivity for devices who are not aware of their special characteristics or to only selectively allow certain devices to use them. Such prefixes are distinguished using Prefix-Policy TLVs (Section Their contents MAY be
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   partly opaque to HNCP nodes, and their identification and usage
   depends on local policy.  However, the following general rules MUST
   be adhered to:

      Special rules apply when making address assignments for prefixes
      with Prefix-Policy TLVs with type 131, as described in
      Section 6.3.2.

      In the presence of any type 1 to 128 Prefix-Policy TLV, the prefix
      is specialized to reach destinations denoted by any such Prefix-
      Policy TLV, i.e., in absence of a type 0 Prefix-Policy TLV, it is
      not usable for general Internet connectivity.  An HNCP router MAY
      enforce this restriction with appropriate packet filter rules.

6.3. Prefix Assignment

HNCP uses the prefix assignment algorithm [RFC7695] in order to assign prefixes to HNCP internal links and uses some of the terminology (Section 2) defined there. HNCP furthermore defines the Assigned-Prefix TLV (Section 10.3), which MUST be used to announce Published Assigned Prefixes.

6.3.1. Prefix Assignment Algorithm Parameters

All HNCP nodes running the prefix assignment algorithm use the following values for its parameters: Node IDs: HNCP node identifiers are used. The comparison operation is defined as bitwise comparison. Set of Delegated Prefixes: The set of prefixes encoded in Delegated-Prefix TLVs that are not strictly included in prefixes encoded in other Delegated-Prefix TLVs. Note that Delegated- Prefix TLVs included in ignored External-Connection TLVs are not considered. It is dynamically updated as Delegated-Prefix TLVs are added or removed. Set of Shared Links: The set of Common Links associated with interfaces with the Internal, Leaf, Guest, or Ad Hoc category. It is dynamically updated as interfaces are added, removed, or switched from one category to another. When multiple interfaces are detected as belonging to the same Common Link, prefix assignment is disabled on all of these interfaces except one.
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   Set of Private Links:   This document defines Private Links as
      representing DHCPv6-PD clients or as a mean to advertise prefixes
      included in the DHCPv6 Exclude Prefix option.  Other
      implementation-specific Private Links may be defined whenever a
      prefix needs to be assigned for a purpose that does not require a
      consensus with other HNCP nodes.

   Set of Advertised Prefixes:   The set of prefixes included in
      Assigned-Prefix TLVs advertised by other HNCP nodes (prefixes
      advertised by the local node are not in this set).  The associated
      Advertised Prefix Priority is the priority specified in the TLV.
      The associated Shared Link is determined as follows:

      *  If the Link Identifier is 0, the Advertised Prefix is not
         assigned on a Shared Link.

      *  If the other node's interface identified by the Link Identifier
         is included in one of the Common Links used for prefix
         assignment, it is considered as assigned on the given Common

      *  Otherwise, the Advertised Prefix is not assigned on a Shared

      Advertised Prefixes as well as their associated priorities and
      associated Shared Links MUST be updated as Assigned-Prefix TLVs
      are added, updated, or removed, and as Common Links are modified.

   ADOPT_MAX_DELAY:   The default value is 0 seconds (i.e., prefix
      adoption is done instantly).

   BACKOFF_MAX_DELAY:   The default value is 4 seconds.

   RANDOM_SET_SIZE:   The default value is 64.

   Flooding Delay:   The default value is 5 seconds.

   Default Advertised Prefix Priority:   When a new assignment is
      created or an assignment is adopted -- as specified in the prefix
      assignment algorithm routine -- the default Advertised Prefix
      Priority to be used is 2.
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6.3.2. Making New Assignments

Whenever the prefix assignment algorithm subroutine (Section 4.1 of [RFC7695]) is run on a Common Link, and whenever a new prefix may be assigned (case 1 of the subroutine: no Best Assignment and no Current Assignment), the decision of whether the assignment of a new prefix is desired MUST follow these rules in order: If the Delegated-Prefix TLV contains a DHCPv6-Data TLV, and the meaning of one of the DHCP options is not understood by the HNCP node, the creation of a new prefix is not desired. This rule applies to TLVs inside Delegated-Prefix TLVs but not to those inside External-Connection TLVs. If the remaining preferred lifetime of the prefix is 0 and there is another delegated prefix of the same IP version used for prefix assignment with a non-zero preferred lifetime, the creation of a new prefix is not desired. If the Delegated-Prefix TLV does not include a Prefix-Policy TLV indicating restrictive assignment (type 131) or if local policy exists to identify it based on, e.g., other Prefix-Policy TLV values and allows assignment, the creation of a new prefix is desired. Otherwise, the creation of a new prefix is not desired. If the considered delegated prefix is an IPv6 prefix, and whenever there is at least one available prefix of length 64, a prefix of length 64 MUST be selected unless configured otherwise. In case no prefix of length 64 would be available, a longer prefix MAY be selected even without configuration. If the considered delegated prefix is an IPv4 prefix (Section 6.5 details how IPv4-delegated prefixes are generated), a prefix of length 24 SHOULD be preferred. In any case, an HNCP router making an assignment MUST support a mechanism suitable to distribute addresses from the considered prefix if the link is intended to be used by clients. In this case, a router assigning an IPv4 prefix MUST announce the L-capability, and a router assigning an IPv6 prefix with a length greater than 64 MUST announce the H-capability as defined in Section 4.
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6.3.3. Applying Assignments

The prefix assignment algorithm indicates when a prefix is applied to the respective Common Link. When that happens, each router connected to said link: MUST forward traffic destined to said prefix to the respective link. MUST participate in the client configuration election as described in Section 7, if the link is intended to be used by clients. MAY add an address from said prefix to the respective network interface as described in Section 6.4, e.g., if it is to be used as source for locally originating traffic.

6.3.4. DHCPv6 Prefix Delegation

When an HNCP router announcing the P-Capability (Section 4) receives a DHCPv6-PD request from a client, it SHOULD assign one prefix per delegated prefix in the network. This set of assigned prefixes is then delegated to the client, after it has been applied as described in the prefix assignment algorithm. Each DHCPv6-PD client MUST be considered as an independent Private Link, and delegation MUST be based on the same set of delegated prefixes as the one used for Common Link prefix assignments; however, the prefix length to be delegated MAY be smaller than 64. The assigned prefixes MUST NOT be given to DHCPv6-PD clients before they are applied and MUST be withdrawn whenever they are destroyed. As an exception to this rule, in order to shorten delays of processed requests, a router MAY prematurely give out a prefix that is advertised but not yet applied if it does so with a valid lifetime of not more than 30 seconds and ensures removal or correction of lifetimes as soon as possible.

6.4. Node Address Assignment

This section specifies how HNCP nodes reserve addresses for their own use. Nodes MAY, at any time, try to reserve a new address from any Applied Assigned Prefix. Each HNCP node SHOULD announce an IPv6 address and -- if it supports IPv4 -- MUST announce an IPv4 address, whenever matching prefixes are assigned to at least one of its Common Links. These addresses are published using Node-Address TLVs and used to locally reach HNCP nodes for other services. Nodes SHOULD NOT create and announce more than one assignment per IP version to avoid cluttering the node data with redundant information unless a special use case requires it.
Top   ToC   RFC7788 - Page 18
   Stateless assignment based on Semantically Opaque Interface
   Identifiers [RFC7217] SHOULD be used for address assignment whenever
   possible (e.g., the prefix length is 64), otherwise (e.g., for IPv4
   if supported) the following method MUST be used instead: For any
   assigned prefix for which stateless assignment is not used, the first
   quarter of the addresses are reserved for HNCP-based address
   assignments, whereas the last three quarters are left to the DHCP
   elected router (Section 4 specifies the DHCP server election
   process).  For example, if the prefix is assigned and
   applied to a Common Link, addresses included in are
   reserved for HNCP nodes, and the remaining addresses are reserved for
   the elected DHCPv4 server.

   HNCP nodes assign addresses to themselves and then (to ensure
   eventual lack of conflicting assignments) publish the assignments
   using the Node-Address TLV (Section 10.4).

   The process of obtaining addresses is specified as follows:

   o  A node MUST NOT start advertising an address if it is already
      advertised by another node.

   o  An assigned address MUST be part of an assigned prefix currently
      applied on a Common Link that includes the interface specified by
      the endpoint identifier.

   o  An address MUST NOT be used unless it has been advertised for at
      least ADDRESS_APPLY_DELAY consecutive seconds and is still
      currently being advertised.  The default value for
      ADDRESS_APPLY_DELAY is 3 seconds.

   o  Whenever the same address is advertised by more than one node, all
      but the one advertised by the node with the greatest node
      identifier MUST be removed.

6.5. Local IPv4 and ULA Prefixes

HNCP routers can create a Unique Local Address (ULA) or private IPv4 prefix to enable connectivity between local devices. These prefixes are inserted in HNCP as if they were delegated prefixes of a (virtual) external connection (Section 6.2). The following rules apply: An HNCP router SHOULD create a ULA prefix if there is no other IPv6 prefix with a preferred time greater than 0 in the network. It MAY also do so if there are other delegated IPv6 prefixes, but none of which is locally generated (i.e., without any Prefix- Policy TLV) and has a preferred time greater than 0. However, it
Top   ToC   RFC7788 - Page 19
      MUST NOT do so otherwise.  In case multiple locally generated ULA
      prefixes are present, only the one published by the node with the
      greatest node identifier is kept among those with a preferred time
      greater than 0 -- if there is any.

      An HNCP router MUST create a private IPv4 prefix [RFC1918]
      whenever it wishes to provide IPv4 Internet connectivity to the
      network and no other private IPv4 prefix with Internet
      connectivity currently exists.  It MAY also enable local IPv4
      connectivity by creating a private IPv4 prefix if no IPv4 prefix
      exists but MUST NOT do so otherwise.  In case multiple IPv4
      prefixes are announced, only the one published by the node with
      the greatest node identifier is kept among those with a Prefix-
      Policy TLV of type 0 -- if there is any.  The router publishing a
      prefix with Internet connectivity MUST forward IPv4 traffic to the
      Internet and perform NAT on behalf of the network as long as it
      publishes the prefix; other routers in the network MAY choose not

   Creation of such ULA and IPv4 prefixes MUST be delayed by a random
   time span between 0 and 10 seconds in which the router MUST scan for
   others trying to do the same.

   When a new ULA prefix is created, the prefix is selected based on the
   configuration, using the last non-deprecated ULA prefix, or generated
   based on [RFC4193].

7. Configuration of Hosts and Non-HNCP Routers

HNCP routers need to ensure that hosts and non-HNCP downstream routers on internal links are configured with addresses and routes. Since DHCP clients can usually only bind to one server at a time, a per-link and per-service election takes place. HNCP routers may have different capabilities for configuring downstream devices and providing naming services. Each router MUST therefore indicate its capabilities as specified in Section 4 in order to participate as a candidate in the election.

7.1. IPv6 Addressing and Configuration

In general, Stateless Address Autoconfiguration [RFC4861] is used for client configuration for its low overhead and fast renumbering capabilities. Therefore, each HNCP router sends Router Advertisements on interfaces that are intended to be used by clients and MUST at least include a Prefix Information Option for each Applied Assigned Prefix that it assigned to the respective link in every such advertisement. However, stateful DHCPv6 can be used in
Top   ToC   RFC7788 - Page 20
   addition by administrative choice to, e.g., collect hostnames and use
   them to provide naming services or whenever stateless configuration
   is not applicable.

   The designated stateful DHCPv6 server for a Common Link (Section 6.1)
   is elected based on the capabilities described in Section 4.  The
   winner is the router (connected to the Common Link) advertising the
   greatest H-capability.  In case of a tie, Capability Values
   (Section 4) are compared, and the router with the greatest value is
   elected.  In case of another tie, the router with the greatest node
   identifier is elected among the routers with tied Capability Values.

   The elected router MUST serve stateful DHCPv6 and SHOULD provide
   naming services for acquired hostnames as outlined in Section 8; all
   other nodes MUST NOT.  Stateful addresses SHOULD be assigned in a way
   that does not hinder fast renumbering even if the DHCPv6 server or
   client do not support the DHCPv6 reconfigure mechanism, e.g., by only
   handing out leases from locally generated (ULA) prefixes and prefixes
   with a length different from 64 and by using low renew and rebind
   times (i.e., not longer than 5 minutes).  In case no router was
   elected, stateful DHCPv6 is not provided.  Routers that cease to be
   elected DHCP servers SHOULD -- when applicable -- invalidate
   remaining existing bindings in order to trigger client

7.2. DHCPv6 for Prefix Delegation

The designated DHCPv6 server for prefix delegation on a Common Link is elected based on the capabilities described in Section 4. The winner is the router (connected to the Common Link) advertising the greatest P-capability. In case of a tie, Capability Values (Section 4) are compared, and the router with the greatest value is elected. In case of another tie, the router with the greatest node identifier is elected among the routers with tied Capability Values. The elected router MUST provide prefix delegation services [RFC3633] on the given link (and follow the rules in Section 6.3.4); all other nodes MUST NOT.

7.3. DHCPv4 for Addressing and Configuration

The designated DHCPv4 server on a Common Link (Section 6.1) is elected based on the capabilities described in Section 4. The winner is the router (connected to the Common Link) advertising the greatest L-capability. In case of a tie, Capability Values (Section 4) are compared, and the router with the greatest value is elected. In case of another tie, the router with the greatest node identifier is elected among the routers with tied Capability Values.
Top   ToC   RFC7788 - Page 21
   The elected router MUST provide DHCPv4 services on the given link;
   all other nodes MUST NOT.  The elected router MUST provide IP
   addresses from the pool defined in Section 6.4 and MUST announce
   itself as router [RFC2132] to clients.

   DHCPv4 lifetimes renew and rebind times (T1 and T2) SHOULD be short
   (i.e., not longer than 5 minutes) in order to provide reasonable
   response times to changes.  Routers that cease to be elected DHCP
   servers SHOULD -- when applicable -- invalidate remaining existing
   bindings in order to trigger client reconfiguration.

7.4. Multicast DNS Proxy

The designated Multicast DNS (mDNS) [RFC6762] proxy on a Common Link is elected based on the capabilities described in Section 4. The winner is the router (connected to the Common Link) advertising the greatest M-capability. In case of a tie, Capability Values (Section 4) are compared, and the router with the greatest value is elected. In case of another tie, the router with the greatest node identifier is elected among the routers with tied Capability Values. The elected router MUST provide an mDNS proxy on the given link and announce it as described in Section 8.

(page 21 continued on part 2)

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