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\section{Related Work}

Previous research has addressed some of the issues that XORP faces and
provides useful reference points in understanding XORP.  Earlier
projects such GateD~\cite{gated} and Zebra~\cite{zebra} provide user
level routing protocol implementations.  GateD is a monolithic
application, and whilst the distributions are stable, this structure
means experimental work can interfere with existing stable protocols.
Zebra is a more modular system, but provides a more rigid
inter-process communication system with a lesser degree degree of
access control on inter-process communication than we envisage for
XORP.

Forwarding plane projects, such as Click~\cite{click} and Scout, 
provide a means
for a XORP router to use a high performance forwarding plane on
commodity PC hardware and provide a potential path for more customized
forwarding planes in future.  At present XORP is targetting Click and
native FreeBSD / Linux forwarding planes on PC's.

The Multi-Threaded Routing Toolkit~\cite{mrt} (MRT) project provides
analysis and simulation daemons for routers to use.  XORP is presently
exploring the use of the MRT applications for testing and validation
purposes.