| RFC 9080 | Homenet Babel Profile | August 2021 |
| Chroboczek | Standards Track | [Page] |
This document defines the exact subset of the Babel routing protocoland its extensions that is required by an implementation of the Homenetprotocol suite, as well as the interactions between the Home NetworkingControl Protocol (HNCP) and Babel.¶
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 7841.¶
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained athttps://www.rfc-editor.org/info/rfc9080.¶
Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved.¶
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The core of the Homenet protocol suite consists of the Home NetworkingControl Protocol (HNCP)[RFC7788], a protocol used forflooding configuration information and assigning prefixes to links,combined with the Babel routing protocol[RFC8966].Babel is an extensible, flexible, and modular protocol: minimalimplementations of Babel have been demonstrated that consist of a fewhundred lines of code, while the "large" implementation includes supportfor a number of extensions and consists of over ten thousand lines ofC code.¶
This document consists of two parts. The first specifies the exactsubset of the Babel protocol and its extensions that is required by animplementation of the Homenet protocol suite. The second specifies howHNCP interacts with Babel.¶
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 BCP 14[RFC2119][RFC8174] when, and only when, they appear in all capitals, as shown here.¶
The Babel routing protocol and its extensions are defined in a numberof documents:¶
A Homenet implementation of BabelMUST encapsulate Babel controltraffic in IPv6 packets sent to the IANA-assigned port 6696 and either theIANA-assigned multicast group ff02::1:6 or to a link-local unicastaddress.¶
Rationale: Since Babel is able to carry bothIPv4 and IPv6 routes over either IPv4 or IPv6, choosing the protocol usedfor carrying control traffic is a matter of preference. Since IPv6 hassome features that make implementations somewhat simpler and more reliable(notably properly scoped and reasonably stable link-local addresses), werequire carrying control data over IPv6.¶
A Homenet implementation of BabelSHOULD implement the IPv4subset of the protocol defined in the body of RFC 8966. Use of othertechniques for acquiring IPv4 connectivity (such as multiple layers ofNAT) is strongly discouraged.¶
Rationale: Support for IPv4 will likely remainnecessary for years to come, and even in pure IPv6 deployments, includingcode for supporting IPv4 has very little cost. Since HNCP makes it easyto assign distinct IPv4 prefixes to the links in a network, it is notnecessary to resort to multiple layers of NAT, with all of itsproblems.¶
A Homenet implementation of BabelMUST implement source-specificrouting for IPv6, as defined in RFC 9079[RFC9079].¶
Rationale: Source-specific routing is anessential component of the Homenet architecture. Source-specific routingfor IPv4 is not required, since HNCP arranges things so that a singlenonspecific IPv4 default route is announced (Section 6.5 of [RFC7788]).¶
A Homenet implementation of Babel must use metrics that areof a similar magnitude to the values suggested inAppendix A of [RFC8966].In particular, itSHOULD assign costs that are no lessthan 256 to wireless links andSHOULD assign costs between 32 and 196 tolossless wired links.¶
Rationale: If two implementations of Babelchoose very different values for link costs, combining routers fromdifferent vendors will cause suboptimal routing.¶
A Homenet implementation of BabelSHOULD distinguish betweenwired and wireless links; if it is unable to determine whether a link iswired or wireless, itSHOULD make the worst-case hypothesis that the linkis wireless. ItSHOULD dynamically probe the quality of wireless linksand derive a suitable metric from its quality estimation.Appendix A of [RFC8966] gives an example of a suitable algorithm.¶
Rationale: Support for wireless transit links isa distinguishing feature of Homenet, and one that is requested by ourusers. In the absence of dynamically computed metrics, the routingprotocol attempts to minimise the number of links crossed by a route andtherefore prefers long, lossy links to shorter, lossless ones. Inwireless networks, "hop-count routing is worst-path routing".¶
While it would be desirable to perform link-quality probing on somewired link technologies, notably power-line networks, these kinds of linkstend to be difficult or impossible to detect automatically, and we are notaware of any published link-quality algorithms for them. Hence, we do notrequire link-quality estimation for wired links of any kind.¶
A Homenet implementation of BabelMAY perform route selection byapplying hysteresis to route metrics, as suggested inSection 3.6 of [RFC8966]and described in detail in Section III.E of[DELAY-BASED]. However, hysteresis is not required, and theimplementation may simply pick the route with the smallest metric.¶
Rationale: Hysteresis is only useful incongested and highly dynamic networks. In a typical home network, which is stableand uncongested, the feedback loop that hysteresis compensates for doesnot occur.¶
A Homenet implementation of Babel may include support for otherextensions to the protocol, as long as they are known to interoperate withboth the core protocol and source-specific routing.¶
Rationale: A number of extensions to the Babelrouting protocol have been defined over the years; however, they areuseful in fairly specific situations, such as routing over global-scaleoverlay networks[BABEL-RTT] or multi-hop wireless networkswith multiple radio frequencies[BABEL-Z]. Hence, with theexception of source-specific routing, no extensions are required forHomenet.¶
The Homenet architecture cleanly separates configuration, which is doneby HNCP, from routing, which is done by Babel. While the coupling betweenthe two protocols is deliberately kept to a minimum, some interactions areunavoidable.¶
All the interactions between HNCP and Babel consist of HNCP causingBabel to perform an announcement on its behalf (under no circumstancesdoes Babel cause HNCP to perform an action). How this is realised is animplementation detail that is outside the scope of this document; while itcould conceivably be done using a private communication channel betweenHNCP and Babel, in existing implementations, HNCP installs a route in theoperating system's kernel that is later picked up by Babel using theexisting redistribution mechanisms.¶
If an HNCP node receives a DHCPv6 prefix delegation for prefixP and publishes an External-Connection TLV containing a Delegated-PrefixTLV with prefix P and no Prefix-Policy TLV, then itMUST announcea source-specific default route with source prefix P over Babel.¶
Rationale: Source-specific routes are the maintool that Homenet uses to enable optimal routing in the presence ofmultiple IPv6 prefixes. External connections with nontrivial prefixpolicies are explicitly excluded from this requirement, since their exactbehaviour is application specific.¶
If an HNCP node receives a DHCPv4 lease with an IPv4 address andwins the election for NAT gateway, then itMUST act as a NAT gateway andMUST announce a (nonspecific) IPv4 default route over Babel.¶
Rationale: The Homenet stack does not usesource-specific routing for IPv4; instead, HNCP elects a single NATgateway and publishes a single default route towards that gateway([RFC7788],Section 6.5).¶
An HNCP node that receives a DHCPv6 prefix delegationMAYannounce a nonspecific IPv6 default route over Babel in addition to thesource-specific default route mandated by requirementREQ7.¶
Rationale: Since the source-specific defaultroute is more specific than the nonspecific default route, the formerwill override the latter if all nodes implement source-specific routing.Announcing an additional nonspecific route is allowed, since doing thatcauses no harm and might simplify operations in some circumstances,e.g., when interoperating with a routing protocol that does notsupport source-specific routing.¶
An HNCP node that receives a DHCPv4 lease with an IPv4 address andwins the election for NAT gatewaySHOULD NOT announce a source-specificIPv4 default route.¶
Rationale: Homenet does not require support for IPv4source-specific routing. Announcing IPv4 source-specific routes will notcause routing pathologies (blackholes or routing loops), but it mightcause packets sourced in different parts of the Homenet to followdifferent paths, with all the confusion that this entails.¶
Both HNCP and Babel carry their control data in IPv6 packets witha link-local source address, and implementations are required to droppackets sent from a global address. Hence, they are only susceptible toattacks from a directly connected link on which the HNCP and Babelimplementations are listening.¶
The security of a Homenet network relies on having a set of "Internal","Ad Hoc", and "Hybrid" interfaces (Section 5.1 of [RFC7788])that are assumed to be connected to links that are secured at a lowerlayer. HNCP and Babel packets are only accepted when they originate onthese trusted links. "External" and "Guest" interfaces are connected tolinks that are not trusted, and any HNCP or Babel packets that arereceived on such interfaces are ignored. ("Leaf" interfaces are a specialcase since they are connected to trusted links, but HNCP and Babel trafficreceived on such interfaces is ignored.) This implies that the securityof a Homenet network depends on the reliability of the border discoveryprocedure described inSection 5.3 of [RFC7788].¶
If untrusted links are used for transit, which isNOT RECOMMENDED,then any HNCP and Babel traffic that is carried over such linksMUST besecured using an upper-layer security protocol. While both HNCP and Babelsupport cryptographic authentication, at the time of writing, no protocolfor autonomous configuration of HNCP and Babel security has been defined.¶
This document has no IANA actions.¶
A number of people have helped with defining the requirements listed inthis document. I am especially indebted toBarbara Stark andMarkus Stenberg.¶