Network Working Group                                      J. Chroboczek
Internet-Draft                         IRIF, University of Paris-Diderot
Intended status: Informational                              July 7, 2019
Expires: January 8, 2020

              Applicability of the Babel routing protocol


   Babel is a routing protocol based on the distance-vector algorithm
   augmented with mechanisms for loop avoidance and starvation
   avoidance.  In this document, we describe a number of niches where
   Babel has been found to be useful and that are arguably not
   adequately served by more mature protocols.

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Table of Contents

   1.  Introduction and background . . . . . . . . . . . . . . . . .   2
     1.1.  Technical overview of the Babel protocol  . . . . . . . .   2
   2.  Properties of the Babel protocol  . . . . . . . . . . . . . .   3
     2.1.  Simplicity and implementability . . . . . . . . . . . . .   3
     2.2.  Robustness  . . . . . . . . . . . . . . . . . . . . . . .   3
     2.3.  Extensibility . . . . . . . . . . . . . . . . . . . . . .   4
     2.4.  Limitations . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Successful deployments of Babel . . . . . . . . . . . . . . .   6
     3.1.  Hybrid networks . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Large scale overlay networks  . . . . . . . . . . . . . .   7
     3.3.  Pure mesh networks  . . . . . . . . . . . . . . . . . . .   7
     3.4.  Small unmanaged networks  . . . . . . . . . . . . . . . .   7
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informational References  . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction and background

   Babel [RFC6126bis] is a routing protocol based on the familiar
   distance-vector algorithm (sometimes known as distributed Bellman-
   Ford) augmented with mechanisms for loop avoidance (there is no
   "counting to infinity") and starvation avoidance.  In this document,
   we describe a number of niches where Babel is useful and that are
   arguably not adequately served by more mature protocols such as OSPF
   [RFC5340] and IS-IS [RFC1195].

1.1.  Technical overview of the Babel protocol

   At its core, Babel is a distance-vector protocol based on the
   distributed Bellman-Ford algorithm, similar in principle to RIP
   [RFC2453], but with two important extensions: provisions for sensing
   of neighbour reachability, bidirectional reachability and link
   quality, and support for multiple address families (e.g., IPv6 and
   IPv4) in a single protocol instance.

   Algorithms of this class are simple to understand and simple to
   implement, but unfortunately they do not work very well -- they
   suffer from "counting to infinity", a case of pathologically slow
   convergence in some topologies after a link failure.  Babel uses a
   mechanism pioneered by EIGRP [DUAL] [RFC7868], known as
   "feasibility", which avoids routing loops and therefore makes
   counting to infinity impossible.

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   Feasibility is a conservative mechanism, one that not only avoids all
   looping routes but also rejects some loop-free routes.  Thus, it can
   lead to a situation known as "starvation", where a router rejects all
   routes to a given destination, even those that are loop-free.  In
   order to recover from starvation, Babel uses a mechanism pioneered by
   DSDV [DSDV] and known as "sequenced routes".  In Babel, this
   mechanism is generalised to deal with prefixes of arbitrary length
   and routes announced at multiple points in a single routing domain
   (DSDV was a pure mesh protocol, and only dealt with host routes).

   In DSDV, the sequenced routes algorithm is slow to react to a
   starvation episode.  In Babel, starvation recovery is accelerated by
   using explicit requests (known as "seqno requests" in the protocol)
   that signal a starvation episode and cause a new sequenced route to
   be propagated in a timely manner.  In the absence of packet loss,
   this mechanism is provably complete and clears the starvation in time
   proportional to the diameter of the network, at the cost of some
   additional signalling traffic.

2.  Properties of the Babel protocol

   In this section, we describe the properties of the Babel protocol as
   well as its known limitations.

2.1.  Simplicity and implementability

   Babel is a conceptually simple protocol.  It consists of a familiar
   algorithm (distributed Bellman-Ford) augmented with three simple and
   well-defined mechanisms (feasibility, sequenced routes and explicit
   requests).  Given a sufficiently friendly audience, the principles
   behind Babel can be explained in 15 minutes, and a full description
   of the protocol can be done in 52 minutes (one microcentury).

   An important consequence is that Babel is easy to implement.  At the
   time of writing, there exist four independent, interoperable
   implementations, including one that was reportedly written and
   debugged in just two nights.

2.2.  Robustness

   The fairly strong properties of the Babel protocol (convergence, loop
   avoidance, starvation avoidance) rely on some reasonably weak
   properties of the network and the metric being used.  The most
   significant are:

   o  causality: the "happens-before" relation is acyclic (intuitively,
      a control message is not received before it has been sent);

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   o  strict monotonicity of the metric: for any metric M and link
      cost C, M < C + M (intuitively, this implies that cycles have a
      strictly positive metric);

   o  left-distributivity of the metric: for any metrics M and M' and
      cost C, if M <= M', then C + M <= C + M' (intuitively, this
      implies that a good choice made by a neighbour B of a node A is
      also a good choice for A).

   See [METAROUTING] for more information about these properties and
   their consequences.

   In particular, Babel does not assume a reliable transport, it does
   not assume ordered delivery, it does not assume that communication is
   transitive, and it does not require that the metric be discrete
   (continuous metrics are possible, reflecting for example packet loss
   rates).  This is in contrast to link-state routing protocols such as
   OSPF [RFC5340] or IS-IS [RFC1195], which incorporate a reliable
   flooding algorithm and make stronger requirements on the underlying
   network and metric.

   These weak requirements make Babel a robust protocol:

   o  robust with respect to bugs: an implementation bug does most
      likely not violate the properties on which Babel relies; in our
      (extensive) experience, bugs tend to slow down convergence or
      cause sub-optimal routing, but do not cause the network to

   o  robust with respect to unusual networks: an unusual network (non-
      transitive links, unstable metrics, etc.) does most likely not
      violate the assumptions of the protocol;

   o  robust with respect to novel metrics: no matter how strange your
      metric (continuous, constantly fluctuating, etc.), it does most
      likely not violate the assumptions of the protocol.

   These robustness properties have important consequences for the
   applicability of the protocol: Babel works (more or less efficiently)
   in a wide range of circumstances where traditional routing protocols
   give up.

2.3.  Extensibility

   Babel's packet format has a number of features that make the protocol
   extensible (see Appendix C of [RFC6126bis]), and a number of
   extensions have been designed to make Babel work better in situations
   that were not envisioned when the protocol was initially designed.

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   The ease of extensibility is not an accident, but a consequence of
   the design of the protocol: it is reasonably easy to check whether a
   given extension violates the assumptions on which Babel relies.

   All of the extensions designed to date interoperate with the base
   protocol and with each other.  This, again, is a consequence of the
   protocol design: in order to check that two extensions to the Babel
   protocol are interoperable, it is enough to verify that the
   interaction of the two does not violate the base protocol's

   Notable extensions deployed to date include:

   o  source-specific routing (SADR) [BABEL-SS] allows forwarding to
      take a packet's source address into account, thus enabling a cheap
      form of multihoming [SS-ROUTING];

   o  RTT-based routing [BABEL-RTT] minimises link delay, which is
      useful in overlay network (where both hop count and packet loss
      are poor metrics).

   Some other extensions have been designed, but have not seen
   deployment yet (and their usefulness is yet to be demonstrated):

   o  frequency-aware routing [BABEL-Z] aims to minimise radio
      interference in wireless networks;

   o  ToS-aware routing [BABEL-TOS] allows routing to take a packet's
      ToS marking into account for selected routes without incurring the
      full cost of a multi-topology routing protocol.

2.4.  Limitations

   Babel has some undesirable properties that make it suboptimal or even
   unusable in some deployments.

2.4.1.  Periodic updates

   The main mechanisms used by Babel to reconverge after a topology
   change are reactive: triggered updates, triggered retractions and
   explicit requests.  However, in the presence of heavy packet loss,
   Babel relies on periodic updates to clear pathologies.  This reliance
   on periodic updates makes Babel unsuitable in at least two kinds of

   o  large, stable networks: since Babel sends periodic updates even in
      the absence of topology changes, in well-managed, large, stable
      networks the amount of control traffic will be reduced by using a

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      protocol that uses a reliable transport (such as OSPF, IS-IS or

   o  low-power networks: the periodic updates use up battery power even
      when there are no topology changes and no user traffic, which
      makes Babel wasteful in low-power networks.

2.4.2.  Full routing table

   While there exist techniques that allow a Babel speaker to function
   with a partial routing table (e.g., by learning just a default route
   or, more generally, performing route aggregation), Babel is designed
   around the assumption that every router has a full routing table.  In
   networks where some nodes are too constrained to hold a full routing
   table, it might be preferable to use a protocol that was designed
   from the outset to work with a partial routing table (such as AODVv2
   [AODVv2], RPL [RFC6550] or LOADng [LOADng]).

2.4.3.  Slow aggregation

   Babel's loop-avoidance mechanism relies on making a route unreachable
   after a retraction until all neighbours have been guaranteed to have
   acted upon the retraction, even in the presence of packet loss.
   Unless the optional algorithm described in Section 3.5.5 of
   [RFC6126bis] is implemented, this entails that a node is unreachable
   for a few minutes after the most specific route to it has been
   retracted.  This delay may make Babel slow to recover from a topology
   change in networks that perform automatic route aggregation.

3.  Successful deployments of Babel

   In this section, we give a few examples of environments where Babel
   has been successfully deployed.

3.1.  Hybrid networks

   Babel is able to deal with both classical, prefix-based ("Internet-
   style") routing and flat ("mesh-style") routing over non-transitive
   link technologies.  Because of that, it has seen a number of
   successful deployments in medium-sized hybrid networks, networks that
   combine a wired, aggregated backbone with meshy wireless bits at the
   edges.  No other routing protocol known to us is similarly robust and
   efficient in this particular kind of topology.

   Efficient operation in hybrid networks requires the implementation to
   distinguish wired and wireless links, and to perform link quality
   estimation on wireless links.

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3.2.  Large scale overlay networks

   The algorithms used by Babel (loop avoidance, hysteresis, delayed
   updates) allow it to remain stable and efficient in the presence of
   unstable metrics, even in the presence of a feedback loop.  For this
   reason, it has been successfully deployed in large scale overlay
   networks, built out of thousands of tunnels spanning continents,
   where it is used with a metric computed from links' latencies.

   This particular application depends on the extension for RTT-
   sensitive routing [DELAY-BASED].

3.3.  Pure mesh networks

   While Babel is a general-purpose routing protocol, it has been
   repeatedly shown to be competitive with dedicated routing protocols
   for wireless mesh networks [REAL-WORLD] [BRIDGING-LAYERS].  Although
   this particular niche is already served by a number of mature
   protocols, notably OLSR-ETX and OLSRv2 [RFC7181] (equipped e.g. with
   the DAT metric [RFC7779]), Babel has seen a moderate amount of
   successful deployment in pure mesh networks.

3.4.  Small unmanaged networks

   Because of its small size and simple configuration, Babel has been
   deployed in small, unmanaged networks (e.g., home and small office
   networks), where it serves as a more efficient replacement for RIP
   [RFC2453], over which it has two significant advantages: the ability
   to route multiple address families (IPv6 and IPv4) in a single
   protocol instance, and good support for using wireless links for

4.  IANA Considerations

   This document requires no IANA actions.  [RFC Editor: please remove
   this section before publication.]

5.  Security Considerations

   As is the case in all distance-vector routing protocols, a Babel
   speaker receives reachability information from its neighbours, which
   by default is trusted by all nodes in the routing domain.

   In most deployments, the Babel protocol is run over a network that is
   secured either at the physical layer (e.g., physically protecting
   Ethernet sockets) or at the link layer (using a protocol such as WiFi
   Protected Access (WPA2)).  If Babel is being run over an unprotected

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   network, then the routing traffic needs to be protected using a
   sufficiently strong cryptographic mechanism.

   At the time of writing, two such mechanisms have been defined.
   Babel-HMAC [HMAC] is a simple and easy to implement mechanism that
   only guarantees authenticity and integrity of the routing traffic,
   and only supports symmetric keying with a small number of keys
   (typically just one or two), but is invulnerable to replay even in
   the absence of persistent state.  Babel-DTLS [DTLS] is a more complex
   mechanism, that requires some minor changes to be made to a typical
   Babel implementation and depends on a DTLS stack being available, but
   inherits all of the features of DTLS, notably confidentiality and the
   ability to use asymmetric keys.

   Due to its simplicity, Babel-HMAC should be the preferred security
   mechanism in most deployments, with Babel-DTLS available for networks
   that require its additional features.

6.  Acknowledgments

   The author is indebted to Jean-Paul Smetz and Alexander Vainshtein
   for their input to this document.

7.  References

7.1.  Normative References

              Chroboczek, J. and D. Schinazi, "The Babel Routing
              Protocol", Internet Draft draft-ietf-babel-rfc6126bis-07,
              November 2018.

7.2.  Informational References

   [AODVv2]   Perkins, C., Ratliff, S., Dowdell, J., Steenbrink, L., and
              V. Mercieca, "Ad Hoc On-demand Distance Vector Version 2
              (AODVv2) Routing", draft-ietf-manet-aodvv2-16 (work in
              progress), May 2016.

              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.

              Boutier, M. and J. Chroboczek, "Source-Specific Routing in
              Babel", draft-ietf-babel-source-specific-04 (work in
              progress), October 2018.

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              Chouasne, G. and J. Chroboczek, "TOS-Specific Routing in
              Babel", draft-chouasne-babel-tos-specific-00 (work in
              progress), July 2017.

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

              Murray, D., Dixon, M., and T. Koziniec, "An Experimental
              Comparison of Routing Protocols in Multi Hop Ad Hoc
              Networks", Proc. ATNAC 2010, 2010.

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

   [DSDV]     Perkins, C. and P. Bhagwat, "Highly Dynamic Destination-
              Sequenced Distance-Vector Routing (DSDV) for Mobile
              Computers", ACM SIGCOMM'94 Conference on Communications
              Architectures, Protocols and Applications 234-244, 1994.

   [DTLS]     Decimo, A., Schinazi, D., and J. Chroboczek, "Babel
              Routing Protocol over Datagram Transport Layer Security",
              draft-ietf-babel-dtls-07 (work in progress), July 2019.

   [DUAL]     Garcia Luna Aceves, J., "Loop-Free Routing Using Diffusing
              Computations", IEEE/ACM Transactions on Networking 1:1,
              February 1993.

   [HMAC]     Do, C., Kolodziejak, W., and J. Chroboczek, "HMAC
              authentication for the Babel routing protocol", draft-
              ietf-babel-hmac-07 (work in progress), June 2019.

   [LOADng]   Clausen, T., Verdiere, A., Yi, J., Niktash, A., Igarashi,
              Y., Satoh, H., Herberg, U., Lavenu, C., Lys, T., and J.
              Dean, "The Lightweight On-demand Ad hoc Distance-vector
              Routing Protocol - Next Generation (LOADng)", draft-
              clausen-lln-loadng-15 (work in progress), January 2017.

              Griffin, T. and J. Sobrinho, "Metarouting", 2005.

              In Proceedings of the 2005 conference on Applications,
              technologies, architectures, and protocols for computer
              communications (SIGCOMM'05).

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              Abolhasan, M., Hagelstein, B., and J. Wang, "Real-world
              performance of current proactive multi-hop mesh
              protocols", Asia-Pacific Conference on Communication 2009,

   [RFC1195]  Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
              dual environments", RFC 1195, December 1990.

   [RFC2453]  Malkin, G., "RIP Version 2", STD 56, RFC 2453, November

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, July 2008.

   [RFC6550]  Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
              Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
              JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
              Low-Power and Lossy Networks", RFC 6550, March 2012.

   [RFC7181]  Clausen, T., Dearlove, C., Jacquet, P., and U. Herberg,
              "The Optimized Link State Routing Protocol Version 2",
              RFC 7181, April 2014.

   [RFC7779]  Rogge, H. and E. Baccelli, "Directional Airtime Metric
              Based on Packet Sequence Numbers for Optimized Link State
              Routing Version 2 (OLSRv2)", RFC 7779,
              DOI 10.17487/RFC7779, April 2016.

   [RFC7868]  Savage, D., Ng, J., Moore, S., Slice, D., Paluch, P., and
              R. White, "Cisco's Enhanced Interior Gateway Routing
              Protocol (EIGRP)", RFC 7868, DOI 10.17487/RFC7868, May

              Boutier, M. and J. Chroboczek, "Source-Specific Routing",
              August 2014, <>.

              In Proc.  IFIP Networking 2015.

Author's Address

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


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