Network Working Group J. Chroboczek
Internet-Draft IRIF, University of Paris-Diderot
Intended status: Informational April 7, 2018
Expires: October 9, 2018
Applicability of the Babel routing protocol
draft-ietf-babel-applicability-03
Abstract
Where we argue that, although OSPF and IS-IS are fine protocols,
there exists a space where the Babel routing protocol (RFC 6126bis)
is useful.
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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 . . . . . . . . . . . . . . 6
3.3. Pure mesh networks . . . . . . . . . . . . . . . . . . . 7
3.4. Small unmanaged networks . . . . . . . . . . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. Informational References . . . . . . . . . . . . . . . . . . 7
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 argue that there exist niches where Babel is useful and that are
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 traditional distance-vector protocol based on
the distributed Bellman-Ford algorithm, similar in principle to RIP
[RFC2453], but with two obvious 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.
Feasibility is a conservative mechanism, one that not only avoids all
looping routes but also rejects some loop-free routes. Thus, it can
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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. While
Babel is a young protocol, there exist four independent
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 rather weak properties
of the network and the metric being used. The most significant are:
o causality: a control message is not received before it has been
sent (more precisely, the "happens-before" relation is acyclic);
o strict monotonicity of the metric: M < C + M;
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o left-distributivity of the metric: if M <= M', then
C + M <= C + M'.
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 traditional link-state routing
protocols such as OSPF [RFC5340] or IS-IS [RFC1195], which are
layered over 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
probably 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
collapse;
o robust with respect to unusual networks: an unusual network (non-
transitive links, unstable metrics, etc.) does most probably 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
probably 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.
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.
Remarkably enough, 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 the
interoperability of two implementations of Babel, it is enough to
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verify that the interaction of the two does not violate the
protocol's assumptions.
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
deployments:
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
protocol that relies on a reliable transport (such as OSPF, IS-IS
or EIGRP);
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.
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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
succesful 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.
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
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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
transit.
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. A number of attacks are possible if this
information is not suitably protected, either by a lower-layer
mechanism or by an extension to the protocol itself (e.g. [RFC7298]).
Implementors and deployers must be aware of the insecure nature of
the base protocol, and must take suitable measures to ensure that the
protocol is deployed as securely as required by the application.
6. Informational References
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[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.
[BABEL-RTT]
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.
[BABEL-SS]
Boutier, M. and J. Chroboczek, "Source-Specific Routing in
Babel", draft-ietf-babel-source-specific-03 (work in
progress), August 2018.
[BABEL-TOS]
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.
[BRIDGING-LAYERS]
Murray, D., Dixon, M., and T. Koziniec, "An Experimental
Comparison of Routing Protocols in Multi Hop Ad Hoc
Networks", Proc. ATNAC 2010, 2010.
[DELAY-BASED]
Jonglez, B. 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 Conference on Communications
Architectures, Protocols and Applications 234-244, 1994.
[DUAL] Garcia Luna Aceves, J., "Loop-Free Routing Using Diffusing
Computations", IEEE/ACM Transactions on Networking 1:1,
February 1993.
[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.
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[REAL-WORLD]
Abolhasan, M., Hagelstein, B., and J. Wang, "Real-world
performance of current proactive multi-hop mesh
protocols", Asia-Pacific Conference on Communication 2009,
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
1998.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, July 2008.
[RFC6126bis]
Chroboczek, J. and D. Schinazi, "The Babel Routing
Protocol", Internet Draft draft-ietf-babel-rfc6126bis-04,
October 2017.
[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.
[RFC7298] Ovsienko, D., "Babel Hashed Message Authentication Code
(HMAC) Cryptographic Authentication", RFC 7298,
DOI 10.17487/RFC7298, July 2014,
<http://www.rfc-editor.org/info/rfc7298>.
[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
2016.
[SS-ROUTING]
Boutier, M. and J. Chroboczek, "Source-Specific Routing",
August 2014, <http://arxiv.org/pdf/1403.0445>.
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In Proc. IFIP Networking 2015.
Author's Address
Juliusz Chroboczek
IRIF, University of Paris-Diderot
Case 7014
75205 Paris Cedex 13
France
Email: jch@irif.fr
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