1. RFC 9080
Internet Engineering Task Force (IETF)                     J. Chroboczek
Request for Comments: 9080             IRIF, University of Paris-Diderot
Category: Standards Track                                    August 2021
ISSN: 2070-1721

             Homenet Profile of the Babel Routing Protocol


   This document defines the exact subset of the Babel routing protocol
   and its extensions that is required by an implementation of the
   Homenet protocol suite, as well as the interactions between the Home
   Networking Control Protocol (HNCP) and Babel.

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 7841.

   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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Table of Contents

   1.  Introduction
     1.1.  Requirements Language
     1.2.  Background
   2.  The Homenet Profile of Babel
     2.1.  Requirements
     2.2.  Optional Features
   3.  Interactions between HNCP and Babel
     3.1.  Requirements
     3.2.  Optional Features
   4.  Security Considerations
   5.  IANA Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Author's Address

1.  Introduction

   The core of the Homenet protocol suite consists of the Home
   Networking Control Protocol (HNCP) [RFC7788], a protocol used for
   flooding configuration information and assigning prefixes to links,
   combined with the Babel routing protocol [RFC8966].  Babel is an
   extensible, flexible, and modular protocol: minimal implementations
   of Babel have been demonstrated that consist of a few hundred lines
   of code, while the "large" implementation includes support for a
   number of extensions and consists of over ten thousand lines of C

   This document consists of two parts.  The first specifies the exact
   subset of the Babel protocol and its extensions that is required by
   an implementation of the Homenet protocol suite.  The second
   specifies how HNCP interacts with Babel.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

1.2.  Background

   The Babel routing protocol and its extensions are defined in a number
   of documents:

   *  RFC 8966 [RFC8966] defines the Babel routing protocol.  It allows
      Babel's control data to be carried either over link-local IPv6 or
      over IPv4 and in either case allows announcing both IPv4 and IPv6
      routes.  It leaves link cost estimation, metric computation, and
      route selection to the implementation.  Distinct implementations
      of Babel [RFC8966] will interoperate, in the sense that they will
      maintain a set of loop-free forwarding paths.  However, if they
      implement conflicting options, they might not be able to exchange
      a full set of routes.  In the worst case, an implementation that
      only implements the IPv6 subset of the protocol and an
      implementation that only implements the IPv4 subset of the
      protocol will not exchange any routes.  In addition, if
      implementations use conflicting route selection policies,
      persistent oscillations might occur.

   *  The informative Appendix A of [RFC8966] suggests a simple and
      easy-to-implement algorithm for cost and metric computation that
      has been found to work satisfactorily in a wide range of

   *  While RFC 8966 does not provide an algorithm for route selection,
      its Section 3.6 suggests selecting the route with the smallest
      metric with some hysteresis applied.  An algorithm that has been
      found to work well in practice is described in Section III.E of

   *  Four documents define optional extensions to Babel: authentication
      based on Hashed Message Authentication Code (HMAC) [RFC8967],
      source-specific routing [RFC9079], delay-based routing
      [BABEL-RTT], and ToS-specific (Type of Service) routing
      [ToS-SPECIFIC].  All of these extensions interoperate with the
      core protocol as well as with each other.

2.  The Homenet Profile of Babel

2.1.  Requirements

   REQ1:   A Homenet implementation of Babel MUST encapsulate Babel
           control traffic in IPv6 packets sent to the IANA-assigned
           port 6696 and either the IANA-assigned multicast group
           ff02::1:6 or to a link-local unicast address.

              Rationale: Since Babel is able to carry both IPv4 and IPv6
              routes over either IPv4 or IPv6, choosing the protocol
              used for carrying control traffic is a matter of
              preference.  Since IPv6 has some features that make
              implementations somewhat simpler and more reliable
              (notably properly scoped and reasonably stable link-local
              addresses), we require carrying control data over IPv6.

   REQ2:   A Homenet implementation of Babel MUST implement the IPv6
           subset of the protocol defined in the body of RFC 8966.

              Rationale: Support for IPv6 routing is an essential
              component of the Homenet architecture.

   REQ3:   A Homenet implementation of Babel SHOULD implement the IPv4
           subset of the protocol defined in the body of RFC 8966.  Use
           of other techniques for acquiring IPv4 connectivity (such as
           multiple layers of NAT) is strongly discouraged.

              Rationale: Support for IPv4 will likely remain necessary
              for years to come, and even in pure IPv6 deployments,
              including code for supporting IPv4 has very little cost.
              Since HNCP makes it easy to assign distinct IPv4 prefixes
              to the links in a network, it is not necessary to resort
              to multiple layers of NAT, with all of its problems.

   REQ4:   A Homenet implementation of Babel MUST implement source-
           specific routing for IPv6, as defined in RFC 9079 [RFC9079].

              Rationale: Source-specific routing is an essential
              component of the Homenet architecture.  Source-specific
              routing for IPv4 is not required, since HNCP arranges
              things so that a single nonspecific IPv4 default route is
              announced (Section 6.5 of [RFC7788]).

   REQ5:   A Homenet implementation of Babel must use metrics that are
           of a similar magnitude to the values suggested in Appendix A
           of [RFC8966].  In particular, it SHOULD assign costs that are
           no less than 256 to wireless links and SHOULD assign costs
           between 32 and 196 to lossless wired links.

              Rationale: If two implementations of Babel choose very
              different values for link costs, combining routers from
              different vendors will cause suboptimal routing.

   REQ6:   A Homenet implementation of Babel SHOULD distinguish between
           wired and wireless links; if it is unable to determine
           whether a link is wired or wireless, it SHOULD make the
           worst-case hypothesis that the link is wireless.  It SHOULD
           dynamically probe the quality of wireless links and 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 is a
              distinguishing feature of Homenet, and one that is
              requested by our users.  In the absence of dynamically
              computed metrics, the routing protocol attempts to
              minimise the number of links crossed by a route and
              therefore prefers long, lossy links to shorter, lossless
              ones.  In wireless networks, "hop-count routing is worst-
              path routing".

              While it would be desirable to perform link-quality
              probing on some wired link technologies, notably power-
              line networks, these kinds of links tend to be difficult
              or impossible to detect automatically, and we are not
              aware of any published link-quality algorithms for them.
              Hence, we do not require link-quality estimation for wired
              links of any kind.

2.2.  Optional Features

   OPT1:   A Homenet implementation of Babel MAY perform route selection
           by applying hysteresis to route metrics, as suggested in
           Section 3.6 of [RFC8966] and described in detail in
           Section III.E of [DELAY-BASED].  However, hysteresis is not
           required, and the implementation may simply pick the route
           with the smallest metric.

              Rationale: Hysteresis is only useful in congested and
              highly dynamic networks.  In a typical home network, which
              is stable and uncongested, the feedback loop that
              hysteresis compensates for does not occur.

   OPT2:   A Homenet implementation of Babel may include support for
           other extensions to the protocol, as long as they are known
           to interoperate with both the core protocol and source-
           specific routing.

              Rationale: A number of extensions to the Babel routing
              protocol have been defined over the years; however, they
              are useful in fairly specific situations, such as routing
              over global-scale overlay networks [BABEL-RTT] or multi-
              hop wireless networks with multiple radio frequencies
              [BABEL-Z].  Hence, with the exception of source-specific
              routing, no extensions are required for Homenet.

3.  Interactions between HNCP and Babel

   The Homenet architecture cleanly separates configuration, which is
   done by HNCP, from routing, which is done by Babel.  While the
   coupling between the two protocols is deliberately kept to a minimum,
   some interactions are unavoidable.

   All the interactions between HNCP and Babel consist of HNCP causing
   Babel to perform an announcement on its behalf (under no
   circumstances does Babel cause HNCP to perform an action).  How this
   is realised is an implementation detail that is outside the scope of
   this document; while it could conceivably be done using a private
   communication channel between HNCP and Babel, in existing
   implementations, HNCP installs a route in the operating system's
   kernel that is later picked up by Babel using the existing
   redistribution mechanisms.

3.1.  Requirements

   REQ7:   If an HNCP node receives a DHCPv6 prefix delegation for
           prefix P and publishes an External-Connection TLV containing
           a Delegated-Prefix TLV with prefix P and no Prefix-Policy
           TLV, then it MUST announce a source-specific default route
           with source prefix P over Babel.

              Rationale: Source-specific routes are the main tool that
              Homenet uses to enable optimal routing in the presence of
              multiple IPv6 prefixes.  External connections with
              nontrivial prefix policies are explicitly excluded from
              this requirement, since their exact behaviour is
              application specific.

   REQ8:   If an HNCP node receives a DHCPv4 lease with an IPv4 address
           and wins the election for NAT gateway, then it MUST act as a
           NAT gateway and MUST announce a (nonspecific) IPv4 default
           route over Babel.

              Rationale: The Homenet stack does not use source-specific
              routing for IPv4; instead, HNCP elects a single NAT
              gateway and publishes a single default route towards that
              gateway ([RFC7788], Section 6.5).

   REQ9:   If an HNCP node assigns a prefix P to an attached link and
           announces P in an Assigned-Prefix TLV, then it MUST announce
           a route towards P over Babel.

              Rationale: Prefixes assigned to links must be routable
              within the Homenet.

3.2.  Optional Features

   OPT3:   An HNCP node that receives a DHCPv6 prefix delegation MAY
           announce a nonspecific IPv6 default route over Babel in
           addition to the source-specific default route mandated by
           requirement REQ7.

              Rationale: Since the source-specific default route is more
              specific than the nonspecific default route, the former
              will override the latter if all nodes implement source-
              specific routing.  Announcing an additional nonspecific
              route is allowed, since doing that causes no harm and
              might simplify operations in some circumstances, e.g.,
              when interoperating with a routing protocol that does not
              support source-specific routing.

   OPT4:   An HNCP node that receives a DHCPv4 lease with an IPv4
           address and wins the election for NAT gateway SHOULD NOT
           announce a source-specific IPv4 default route.

              Rationale: Homenet does not require support for IPv4
              source-specific routing.  Announcing IPv4 source-specific
              routes will not cause routing pathologies (blackholes or
              routing loops), but it might cause packets sourced in
              different parts of the Homenet to follow different paths,
              with all the confusion that this entails.

4.  Security Considerations

   Both HNCP and Babel carry their control data in IPv6 packets with a
   link-local source address, and implementations are required to drop
   packets sent from a global address.  Hence, they are only susceptible
   to attacks from a directly connected link on which the HNCP and Babel
   implementations 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 lower layer.  HNCP and Babel packets are only accepted when they
   originate on these trusted links.  "External" and "Guest" interfaces
   are connected to links that are not trusted, and any HNCP or Babel
   packets that are received on such interfaces are ignored.  ("Leaf"
   interfaces are a special case since they are connected to trusted
   links, but HNCP and Babel traffic received on such interfaces is
   ignored.)  This implies that the security of a Homenet network
   depends on the reliability of the border discovery procedure
   described in Section 5.3 of [RFC7788].

   If untrusted links are used for transit, which is NOT RECOMMENDED,
   then any HNCP and Babel traffic that is carried over such links MUST
   be secured using an upper-layer security protocol.  While both HNCP
   and Babel support cryptographic authentication, at the time of
   writing, no protocol for autonomous configuration of HNCP and Babel
   security has been defined.

5.  IANA Considerations

   This document has no IANA actions.

6.  References

6.1.  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,

   [RFC7788]  Stenberg, M., Barth, S., and P. Pfister, "Home Networking
              Control Protocol", RFC 7788, DOI 10.17487/RFC7788, April
              2016, <https://www.rfc-editor.org/info/rfc7788>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8966]  Chroboczek, J. and D. Schinazi, "The Babel Routing
              Protocol", RFC 8966, DOI 10.17487/RFC8966, January 2021,

   [RFC9079]  Boutier, M. and J. Chroboczek, "Source-Specific Routing in
              the Babel Routing Protocol", RFC 9079,
              DOI 10.17487/RFC9079, August 2021,

6.2.  Informative References

              Jonglez, B. and J. Chroboczek, "Delay-based Metric
              Extension for the Babel Routing Protocol", Work in
              Progress, Internet-Draft, draft-ietf-babel-rtt-extension-
              00, 26 April 2019, <https://datatracker.ietf.org/doc/html/

   [BABEL-Z]  Chroboczek, J., "Diversity Routing for the Babel Routing
              Protocol", Work in Progress, Internet-Draft, draft-
              chroboczek-babel-diversity-routing-01, 15 February 2016,

              Jonglez, B., Boutier, M., and J. Chroboczek, "A delay-
              based routing metric", March 2014,

   [RFC8967]  Dô, C., Kolodziejak, W., and J. Chroboczek, "MAC
              Authentication for the Babel Routing Protocol", RFC 8967,
              DOI 10.17487/RFC8967, January 2021,

              Chouasne, G. and J. Chroboczek, "TOS-Specific Routing in
              Babel", Work in Progress, Internet-Draft, draft-chouasne-
              babel-tos-specific-00, 3 July 2017,


   A number of people have helped with defining the requirements listed
   in this document.  I am especially indebted to Barbara Stark and
   Markus Stenberg.

Author's Address

   Juliusz Chroboczek
   IRIF, University of Paris-Diderot
   Case 7014
   75205 Paris CEDEX 13

   Email: jch@irif.fr
  1. RFC 9080