Network Working Group                                      J. Chroboczek
Internet-Draft                         IRIF, University of Paris-Diderot
Intended status: Experimental                               July 3, 2017
Expires: January 4, 2018

             Homenet profile of the Babel routing protocol


   This document defines the subset of the Babel routing protocol
   [RFC6126] and its extensions that a Homenet router must implement, as
   well as the interactions between HNCP [RFC7788] and Babel.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Background  . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  The Homenet profile of Babel  . . . . . . . . . . . . . . . .   3
     2.1.  Requirements  . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Non-requirements  . . . . . . . . . . . . . . . . . . . .   4
   3.  Interactions between HNCP and Babel . . . . . . . . . . . . .   5
     3.1.  Requirements  . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Non-requirements  . . . . . . . . . . . . . . . . . . . .   6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   5.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   The core of the Homenet protocol suite consists of HNCP [RFC7788], a
   protocol used for flooding configuration information and assigning
   prefixes to links, combined with the Babel routing protocol
   [RFC6126].  Babel is an extensible, flexible and modular protocol:
   minimal implementations of Babel have been demonstrated that consist
   of a few hundred of lines of code, while the "large" implementation
   includes support for a number of extensions and consists of over ten
   thousand lines of C code.

   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.  Background

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

   o  The body of RFC 6126 [RFC6126] defines the core, unextended
      protocol.  It allows Babel's control data to be carried over
      either link-local IPv6 or 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 core RFC 6126 Babel
      will interoperate and maintain a set of loop-free forwarding
      paths, but given conflicting metrics or route selection policies
      may give rise to persistent oscillations.

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   o  The informative Appendix A of RFC 6126 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 topologies.

   o  While RFC 6126 does not provide an algorithm for route selection,
      its Section 3.6 suggests selecting the route with smallest metric
      with some hysteresis applied.  An algorithm that has been found to
      work well in practice is described in Section III.E of

   o  The extension mechanism for Babel is defined in RFC 7557

   o  Four RFCs and Internet-Drafts define optional extensions to Babel:
      HMAC-based authentication [RFC7298], source-specific routing
      [BABEL-SS], radio interference aware routing [BABEL-Z], and delay-
      based routing [BABEL-RTT].  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 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 6126.

      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 6126.  Use of other
   techniques for acquiring IPv4 connectivity (such as multiple layers
   of NAT) is strongly discouraged.

      Rationale: support for IPv4 will remain necessary for years to
      come, and even in pure IPv6 deployments, including code for

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      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

   REQ4: a Homenet implementation of Babel MUST implement source-
   specific routing for IPv6, as defined in draft-boutier-babel-source-
   specific [BABEL-SS].  This implies that it MUST implement the
   extension mechanism defined in RFC 7557.

      Rationale: source-specific routing is an essential component of
      the Homenet architecture.  The extension mechanism is required by
      source-specific routing.  Source-specific routing for IPv4 is not
      required, since HNCP arranges things so that a single non-specific
      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
   RFC 6126.  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 sub-optimal 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.  The algorithm described in Appendix A of RFC 6126 MAY be

      Rationale: support for wireless transit links is a "killer
      feature" of Homenet, something that is requested by our users and
      easy to explain to our bosses.  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".

2.2.  Non-requirements

   NR1: a Homenet implementation of Babel MAY perform route selection by
   applying hysteresis to route metrics, as suggested in Section 3.6 of
   RFC 6126 and described in detail in Section III.E of [BABEL-RTT].
   However, it MAY simply pick the route with the smallest metric.

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

   NR2: 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

3.  Interactions between HNCP and Babel

   The Homenet architecture cleanly separates between configuration,
   which is done by HNCP, and 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 (in particular, 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, existing
   implementations have HNCP install a route in the operating system's
   kernel which 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

      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 non-trivial prefix policies
      are explicitly excluded from this requirement, since their exact
      behaviour is application-specific.

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   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 (non-specific) IPv4 default route over

      Rationale: the Homenet architecture 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]
      7788 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

3.2.  Non-requirements

   NR3: an HNCP node that receives a DHCPv6 prefix delegation MAY
   announce a non-specific 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 non-specific default route, the former will
      override the latter if all nodes implement source-specific
      routing.  Announcing an additional non-specific 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.

   NR4: 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.

      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.

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   The security of a Homenet network relies on having a set of trusted
   "internal" links that are secured at a lower layer (either physically
   or at the link layer); HNCP and Babel packets are only accepted when
   they originate on these trusted links (see Section 5 of [RFC7788]).
   External, leaf and guest links are not trusted, and any HNCP or Babel
   packets that are received on such links are ignored.

   If untrusted links are used for transit, which is NOT RECOMMENDED,
   and therefore need to carry HNCP and Babel traffic, then HNCP and
   Babel 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.  Acknowledgments

   A number of people have helped with definining the requirements
   listed in this document.  I am especially indebted to Markus Stenberg
   for his help.

6.  References

6.1.  Normative References

              Boutier, M. and J. Chroboczek, "Source-Specific Routing in
              Babel", draft-boutier-babel-source-specific-01 (work in
              progress), January 2015.

   [RFC6126]  Chroboczek, J., "The Babel Routing Protocol", RFC 6126,
              February 2011.

   [RFC7557]  Chroboczek, J., "Extension Mechanism for the Babel Routing
              Protocol", RFC 7557, May 2015.

   [RFC7788]  Stenberg, M., Barth, S., and P. Pfister, "Home Networking
              Control Protocol", RFC 7788, DOI 10.17487/RFC7788, April

6.2.  Informative References

              Jonglez, B. and J. Chroboczek, "Delay-based Metric
              Extension for the Babel Routing Protocol", draft-jonglez-
              babel-rtt-extension-01 (work in progress), May 2015.

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   [BABEL-Z]  Chroboczek, J., "Diversity Routing for the Babel Routing
              Protocol", draft-chroboczek-babel-diversity-routing-01
              (work in progress), February 2016.

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

              Available online from

   [RFC7298]  Ovsienko, D., "Babel Hashed Message Authentication Code
              (HMAC) Cryptographic Authentication", RFC 7298, July 2014.

Author's Address

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


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