IRIF, University of Paris-Diderot

Case 7014 Paris CEDEX 13 75205 France jch@irif.fr

distance-vector loop starvation Bellman-Ford routing routing protocol wireless mesh network IGP 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 .

Copyright Notice Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents () in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• .  Introduction and Background
  • .  Technical Overview of the Babel Protocol
• .  Properties of the Babel Protocol
  • .  Simplicity and Implementability
  • .  Robustness
  • .  Extensibility
  • .  Limitations
• .  Successful Deployments of Babel
  • .  Heterogeneous Networks
  • .  Large-Scale Overlay Networks
  • .  Pure Mesh Networks
  • .  Small Unmanaged Networks
• .  Security Considerations
• .  References
  • .  Normative References
  • .  Informative References
• Acknowledgments
• Author's Address

Introduction and Background Babel 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 and IS-IS .

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 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) , 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) 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.

Properties of the Babel Protocol This section describes the properties of the Babel protocol as well as its known limitations.

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.

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 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 or IS-IS , 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.

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).

Extensibility Babel's packet format has a number of features that make the protocol extensible (see ), 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) allows forwarding to take a packet's source address into account, thus enabling a cheap form of multihoming ;
• RTT-based routing 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 aims to minimise radio interference in wireless networks;
• ToS-aware routing 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.

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

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.

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 , the IPv6 Routing Protocol for Low-Power and Lossy Networks (RPL) , or the Lightweight On-demand Ad hoc Distance-vector Routing Protocol - Next Generation (LOADng) ).

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 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.

Successful Deployments of Babel This section gives a few examples of environments where Babel has been successfully deployed.

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.

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 .

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 . 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) (equipped e.g., with the Directional Airtime (DAT) metric ), Babel has seen a moderate amount of successful deployment in pure mesh networks.

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 , 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.

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) 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 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.

References Normative References Informative References ENS Lyon IRIF, University of Paris-Diderot This document defines an extension to the Babel routing protocol that uses symmetric delay in metric computation and therefore makes it possible to prefer lower latency links to higher latency ones. Work in Progress IRIF, University of Paris-Diderot IRIF, University of Paris-Diderot Source-specific routing (also known as Source-Address Dependent Routing, SADR) is an extension to traditional next-hop routing where packets are forwarded according to both their destination and their source address. This document describes an extension for source- specific routing to the Babel routing protocol. Work in Progress PPS, University of Paris-Diderot PPS, University of Paris-Diderot This document describes an extension to the Babel routing protocol to support TOS-specific routing. This version is using mandatory sub- TLVs. Work in Progress This document defines an extension to the Babel routing protocol that allows routing updates to carry radio frequency information, and therefore makes it possible to use radio diversity information for route selection. Work in Progress In Proceedings of ATNAC ACM SIGCOMM '94: Proceedings of the Conference on Communications Architectures, Protocols and Applications, pp. 234-244 IEEE/ACM Transactions on Networking, Volume 1, Issue 1 This document describes the Lightweight Ad hoc On-Demand - Next Generation (LOADng) distance vector routing protocol, a reactive routing protocol intended for use in Mobile Ad hoc NETworks (MANETs). Work in Progress ACM SIGCOMM Computer Communication Review, Volume 35, Issue 4 15th Asia-Pacific Conference on Communications This memo specifies an integrated routing protocol, based on the OSI Intra-Domain IS-IS Routing Protocol, which may be used as an interior gateway protocol (IGP) to support TCP/IP as well as OSI. This allows a single routing protocol to be used to support pure IP environments, pure OSI environments, and dual environments. This specification was developed by the IS-IS working group of the Internet Engineering Task Force. [STANDARDS-TRACK] This document specifies an extension of the Routing Information Protocol (RIP) to expand the amount of useful information carried in RIP messages and to add a measure of security. [STANDARDS-TRACK] The Ad hoc On-Demand Distance Vector (AODV) routing protocol is intended for use by mobile nodes in an ad hoc network. It offers quick adaptation to dynamic link conditions, low processing and memory overhead, low network utilization, and determines unicast routes to destinations within the ad hoc network. It uses destination sequence numbers to ensure loop freedom at all times (even in the face of anomalous delivery of routing control messages), avoiding problems (such as "counting to infinity") associated with classical distance vector protocols. This memo defines an Experimental Protocol for the Internet community. This document describes the modifications to OSPF to support version 6 of the Internet Protocol (IPv6). The fundamental mechanisms of OSPF (flooding, Designated Router (DR) election, area support, Short Path First (SPF) calculations, etc.) remain unchanged. However, some changes have been necessary, either due to changes in protocol semantics between IPv4 and IPv6, or simply to handle the increased address size of IPv6. These modifications will necessitate incrementing the protocol version from version 2 to version 3. OSPF for IPv6 is also referred to as OSPF version 3 (OSPFv3). Changes between OSPF for IPv4, OSPF Version 2, and OSPF for IPv6 as described herein include the following. Addressing semantics have been removed from OSPF packets and the basic Link State Advertisements (LSAs). New LSAs have been created to carry IPv6 addresses and prefixes. OSPF now runs on a per-link basis rather than on a per-IP-subnet basis. Flooding scope for LSAs has been generalized. Authentication has been removed from the OSPF protocol and instead relies on IPv6's Authentication Header and Encapsulating Security Payload (ESP). Even with larger IPv6 addresses, most packets in OSPF for IPv6 are almost as compact as those in OSPF for IPv4. Most fields and packet- size limitations present in OSPF for IPv4 have been relaxed. In addition, option handling has been made more flexible. All of OSPF for IPv4's optional capabilities, including demand circuit support and Not-So-Stubby Areas (NSSAs), are also supported in OSPF for IPv6. [STANDARDS-TRACK] Low-Power and Lossy Networks (LLNs) are a class of network in which both the routers and their interconnect are constrained. LLN routers typically operate with constraints on processing power, memory, and energy (battery power). Their interconnects are characterized by high loss rates, low data rates, and instability. LLNs are comprised of anything from a few dozen to thousands of routers. Supported traffic flows include point-to-point (between devices inside the LLN), point-to-multipoint (from a central control point to a subset of devices inside the LLN), and multipoint-to-point (from devices inside the LLN towards a central control point). This document specifies the IPv6 Routing Protocol for Low-Power and Lossy Networks (RPL), which provides a mechanism whereby multipoint-to-point traffic from devices inside the LLN towards a central control point as well as point-to-multipoint traffic from the central control point to the devices inside the LLN are supported. Support for point-to-point traffic is also available. [STANDARDS-TRACK] This specification describes version 2 of the Optimized Link State Routing Protocol (OLSRv2) for Mobile Ad Hoc Networks (MANETs). This document specifies a Directional Airtime (DAT) link metric for usage in Optimized Link State Routing version 2 (OLSRv2). This document describes the protocol design and architecture for Enhanced Interior Gateway Routing Protocol (EIGRP). EIGRP is a routing protocol based on Distance Vector technology. The specific algorithm used is called "DUAL", a Diffusing Update Algorithm as referenced in "Loop-Free Routing Using Diffusing Computations" (Garcia-Luna-Aceves 1993). The algorithm and procedures were researched, developed, and simulated by SRI International. In Proceedings of the IFIP Networking Conference

Acknowledgments The author is indebted to and for their input to this document.

Author's Address IRIF, University of Paris-Diderot

Case 7014 Paris CEDEX 13 75205 France jch@irif.fr