Internet Engineering Task Force (IETF) J. Chroboczek
Request for Comments: 8965 IRIF, University of Paris-Diderot
Category: Informational January 2021
ISSN: 2070-1721
Applicability of the Babel Routing Protocol
Abstract
Babel is a routing protocol based on the distance-vector algorithm
augmented with mechanisms for loop avoidance and starvation
avoidance. This document describes a number of niches where Babel
has been found to be useful and that are arguably not adequately
served by more mature protocols.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
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). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see 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
https://www.rfc-editor.org/info/rfc8965.
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Table of Contents
1. Introduction and Background
1.1. Technical Overview of the Babel Protocol
2. Properties of the Babel Protocol
2.1. Simplicity and Implementability
2.2. Robustness
2.3. Extensibility
2.4. Limitations
3. Successful Deployments of Babel
3.1. Heterogeneous Networks
3.2. Large-Scale Overlay Networks
3.3. Pure Mesh Networks
3.4. Small Unmanaged Networks
4. Security Considerations
5. References
5.1. Normative References
5.2. Informative References
Acknowledgments
Author's Address
1. Introduction and Background
Babel [RFC8966] 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. This document describes 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 the Enhanced Interior Gateway Routing Protocol
(EIGRP) [DUAL] [RFC7868], known as "feasibility", which avoids
routing loops and therefore makes counting to infinity impossible.
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
the Destination-Sequenced Distance-Vector Routing Protocol (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 carried 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
This section describes 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, and starvation avoidance) rely on some reasonably weak
properties of the network and the metric being used. The most
significant are:
causality: the "happens-before" relation is acyclic (intuitively,
a control message is not received before it has been sent);
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);
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, for example, reflecting 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:
robust with respect to unusual networks: an unusual network (non-
transitive links, unstable link costs, etc.) is likely not to
violate the assumptions of the protocol;
robust with respect to novel metrics: an unusual metric
(continuous, constantly fluctuating, etc.) is likely not to
violate the assumptions of the protocol.
Section 3 gives examples of successful deployments of Babel that
illustrate these properties.
These robustness properties have important consequences for the
applicability of the protocol: Babel works (more or less efficiently)
in a range of circumstances where traditional routing protocols don't
work well (or at all).
2.3. Extensibility
Babel's packet format has a number of features that make the protocol
extensible (see Appendix D of [RFC8966]), 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. 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
assumptions.
Notable extensions deployed to date include:
* source-specific routing (also known as Source-Address Dependent
Routing, SADR) [BABEL-SS] allows forwarding to take a packet's
source address into account, thus enabling a cheap form of
multihoming [SS-ROUTING];
* 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
in production (and their usefulness is yet to be demonstrated):
* frequency-aware routing [BABEL-Z] aims to minimise radio
interference in wireless networks;
* ToS-aware routing [BABEL-TOS] allows routing to take a packet's
Type of Service (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, Babel relies on periodic updates to
clear pathologies after a mobility event or in the presence of heavy
packet loss. The use of periodic updates makes Babel unsuitable in
at least two kinds of environments:
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 protocol that uses a reliable transport (such as
OSPF, IS-IS, or EIGRP);
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 the Ad
hoc On-Demand Distance Vector (AODV) routing protocol [RFC3561], the
IPv6 Routing Protocol for Low-Power and Lossy Networks (RPL)
[RFC6550], or the Lightweight On-demand Ad hoc Distance-vector
Routing Protocol - Next Generation (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 second algorithm described in Section 3.5.5 of [RFC8966]
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 makes Babel slow to recover from a topology change in networks
that perform automatic route aggregation.
3. Successful Deployments of Babel
This section gives a few examples of environments where Babel has
been successfully deployed.
3.1. Heterogeneous Networks
Babel is able to deal with both classical, prefix-based ("Internet-
style") routing and flat ("mesh-style") routing over non-transitive
link technologies. Just like traditional distance-vector protocols,
Babel is able to carry prefixes of arbitrary length, to suppress
redundant announcements by applying the split-horizon optimisation
where applicable, and can be configured to filter out redundant
announcements (manual aggregation). Just like specialised mesh
protocols, Babel doesn't by default assume that links are transitive
or symmetric, can dynamically compute metrics based on an estimation
of link quality, and carries large numbers of host routes efficiently
by omitting common prefixes.
Because of these properties, Babel has seen a number of successful
deployments in medium-sized heterogeneous networks, networks that
combine a wired, aggregated backbone with meshy wireless bits at the
edges.
Efficient operation in heterogeneous networks requires the
implementation to distinguish between wired and wireless links, and
to perform link quality estimation on wireless links.
3.2. Large-Scale Overlay Networks
The algorithms used by Babel (loop avoidance, hysteresis, delayed
updates) allow it to remain stable 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 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 the
Optimized Link State Routing Protocol with Expected Transmission
Count (OLSR-ETX) and OLSRv2 (OLSR Version 2) [RFC7181] (equipped
e.g., with the Directional Airtime (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
transit.
4. 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.
At the time of writing, the Babel protocol is usually 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 Wi-Fi Protected Access 2 (WPA2)). If Babel is being
run over an unprotected 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.
Message Authentication Code (MAC) authentication for Babel (Babel-
MAC) [RFC8967] is a simple and easy to implement mechanism that only
guarantees authenticity, integrity, and replay protection of the
routing traffic and only supports symmetric keying with a small
number of keys (typically just one or two). Babel-DTLS [RFC8968] 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, optional replay protection, and the ability to use
asymmetric keys.
Due to its simplicity, Babel-MAC should be the preferred security
mechanism in most deployments, with Babel-DTLS available for networks
that require its additional features.
In addition to the above, the information that a mobile Babel node
announces to the whole routing domain is often sufficient to
determine a mobile node's physical location with reasonable
precision. This might make Babel unapplicable in scenarios where a
node's location is considered confidential.
5. References
5.1. Normative References
[RFC8966] Chroboczek, J. and D. Schinazi, "The Babel Routing
Protocol", RFC 8966, DOI 10.17487/RFC8966, January 2021,
<https://www.rfc-editor.org/info/rfc8966>.
5.2. Informative References
[BABEL-RTT]
Jonglez, B. and J. Chroboczek, "Delay-based Metric
Extension for the Babel Routing Protocol", Work in
Progress, Internet-Draft, draft-jonglez-babel-rtt-
extension-02, 11 March 2019, <https://tools.ietf.org/html/
draft-jonglez-babel-rtt-extension-02>.
[BABEL-SS] Boutier, M. and J. Chroboczek, "Source-Specific Routing in
Babel", Work in Progress, Internet-Draft, draft-ietf-
babel-source-specific-07, 28 October 2020,
<https://tools.ietf.org/html/draft-ietf-babel-source-
specific-07>.
[BABEL-TOS]
Chouasne, G. and J. Chroboczek, "TOS-Specific Routing in
Babel", Work in Progress, Internet-Draft, draft-chouasne-
babel-tos-specific-00, 3 July 2017,
<https://tools.ietf.org/html/draft-chouasne-babel-tos-
specific-00>.
[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,
<https://tools.ietf.org/html/draft-chroboczek-babel-
diversity-routing-01>.
[BRIDGING-LAYERS]
Murray, D., Dixon, M., and T. Koziniec, "An Experimental
Comparison of Routing Protocols in Multi Hop Ad Hoc
Networks", In Proceedings of ATNAC,
DOI 10.1109/ATNAC.2010.5680190, October 2010,
<https://doi.org/10.1109/ATNAC.2010.5680190>.
[DELAY-BASED]
Jonglez, B., Boutier, M., and J. Chroboczek, "A delay-
based routing metric", March 2014,
<http://arxiv.org/abs/1403.3488>.
[DSDV] Perkins, C. and P. Bhagwat, "Highly Dynamic Destination-
Sequenced Distance-Vector Routing (DSDV) for Mobile
Computers", ACM SIGCOMM '94: Proceedings of the Conference
on Communications Architectures, Protocols and
Applications, pp. 234-244, DOI 10.1145/190314.190336,
October 1994, <https://doi.org/10.1145/190314.190336>.
[DUAL] Garcia-Luna-Aceves, J. J., "Loop-Free Routing Using
Diffusing Computations", IEEE/ACM Transactions on
Networking, Volume 1, Issue 1, DOI 10.1109/90.222913,
February 1993, <https://doi.org/10.1109/90.222913>.
[LOADng] Clausen, T. H., Verdiere, A. C. D., 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)", Work
in Progress, Internet-Draft, draft-clausen-lln-loadng-15,
4 July 2016,
<https://tools.ietf.org/html/draft-clausen-lln-loadng-15>.
[METAROUTING]
Griffin, T. G. and J. L. Sobrinho, "Metarouting", ACM
SIGCOMM Computer Communication Review, Volume 35, Issue 4,
DOI 10.1145/1090191.1080094, August 2005,
<https://doi.org/10.1145/1090191.1080094>.
[REAL-WORLD]
Abolhasan, M., Hagelstein, B., and J. C.-P. Wang, "Real-
world performance of current proactive multi-hop mesh
protocols", 15th Asia-Pacific Conference on
Communications, DOI 10.1109/APCC.2009.5375690, October
2009, <https://doi.org/10.1109/APCC.2009.5375690>.
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, DOI 10.17487/RFC1195,
December 1990, <https://www.rfc-editor.org/info/rfc1195>.
[RFC2453] Malkin, G., "RIP Version 2", STD 56, RFC 2453,
DOI 10.17487/RFC2453, November 1998,
<https://www.rfc-editor.org/info/rfc2453>.
[RFC3561] Perkins, C., Belding-Royer, E., and S. Das, "Ad hoc On-
Demand Distance Vector (AODV) Routing", RFC 3561,
DOI 10.17487/RFC3561, July 2003,
<https://www.rfc-editor.org/info/rfc3561>.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
<https://www.rfc-editor.org/info/rfc5340>.
[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,
DOI 10.17487/RFC6550, March 2012,
<https://www.rfc-editor.org/info/rfc6550>.
[RFC7181] Clausen, T., Dearlove, C., Jacquet, P., and U. Herberg,
"The Optimized Link State Routing Protocol Version 2",
RFC 7181, DOI 10.17487/RFC7181, April 2014,
<https://www.rfc-editor.org/info/rfc7181>.
[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,
<https://www.rfc-editor.org/info/rfc7779>.
[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
2016, <https://www.rfc-editor.org/info/rfc7868>.
[RFC8967] Dô, C., Kolodziejak, W., and J. Chroboczek, "MAC
Authentication for the Babel Routing Protocol", RFC 8967,
DOI 10.17487/RFC8967, January 2021,
<https://www.rfc-editor.org/info/rfc8967>.
[RFC8968] Décimo, A., Schinazi, D., and J. Chroboczek, "Babel
Routing Protocol over Datagram Transport Layer Security",
RFC 8968, DOI 10.17487/RFC8968, January 2021,
<https://www.rfc-editor.org/info/rfc8968>.
[SS-ROUTING]
Boutier, M. and J. Chroboczek, "Source-specific routing",
In Proceedings of the IFIP Networking Conference,
DOI 10.1109/IFIPNetworking.2015.7145305, May 2015,
<http://arxiv.org/pdf/1403.0445>.
Acknowledgments
The author is indebted to Jean-Paul Smetz and Alexander Vainshtein
for their input to this document.
Author's Address
Juliusz Chroboczek
IRIF, University of Paris-Diderot
Case 7014
75205 Paris CEDEX 13
France
Email: jch@irif.fr
