By allowing intermediate nodes in a communications network
to perform non-trivial operations on packets and thus mix data
from multiple streams, network coding breaks with the ruling
store and forward networking paradigm and opens a number of
challenging questions. Following a brief overview of the basic
principles behind emerging network coding protocols, we focus
on the prevailing layered architecture and discuss the pros
and cons of applying network coding ideas at the different layers
of the internet protocol stack. While addressing throughput,
robustness, reliability and security concerns, we identify
a number of research challenges deemed relevant towards the
practical implementation of network coding in future communication
networks.