Sequence Numbers Do Not Guarantee Loop Freedom; AODV Can Yield Routing Loops

TL;DR

This paper demonstrates that the AODV routing protocol, widely believed to be loop-free due to sequence numbers, can actually form routing loops, challenging existing assumptions and proofs.

Contribution

It provides a counterexample showing AODV can create loops, revealing that sequence numbers alone do not ensure loop freedom in routing protocols.

Findings

AODV can form routing loops despite using sequence numbers

Existing proofs of AODV's loop freedom rely on unverified assumptions

Sequence numbers alone are insufficient to guarantee loop freedom

Abstract

In the area of mobile ad-hoc networks and wireless mesh networks, sequence numbers are often used in routing protocols to avoid routing loops. It is commonly stated in protocol specifications that sequence numbers are sufficient to guarantee loop freedom if they are monotonically increased over time. A classical example for the use of sequence numbers is the popular Ad hoc On-Demand Distance Vector (AODV) routing protocol. The loop freedom of AODV is not only a common belief, it has been claimed in the abstract of its RFC and at least two proofs have been proposed. AODV-based protocols such as AODVv2 (DYMO) and HWMP also claim loop freedom due to the same use of sequence numbers. In this paper we show that AODV is not a priori loop free; by this we counter the proposed proofs in the literature. In fact, loop freedom hinges on non-evident assumptions to be made when resolving ambiguities occurring in the RFC. Thus, monotonically increasing sequence numbers, by themselves, do not guarantee loop freedom.