Introduction - Peer-to-Peer Computing: Applications, Architecture, Protocols, and Challenges - page 3

successfully tackled, security issues such as confidentiality and data integrity
are still notoriously hard to solve. This is because without a central author-
ity such as a certification authority (CA), keys distribution among peers is
very di cult to handle. Consequently, it is di cult to realize communication
confidentiality. On the other hand, peers' communications and topology con-
trol rely very much on reliable updates among peers to maintain a consistent
routing table. Yet, again without the help of a central authority, such update
messages' integrity can be easily compromised by some malicious peers.
In summary, P2P systems present a unique combination of challenges listed
below.
Highly Decentralized Organization. The advent of P2P systems is due
to the ever increasing desire of moving away from centralized control,
in both aspects of accessing computing resources and accessing informa-
tion. Thus, it is very di cult, if not impossible, to coordinate the peers
in an organized manner. This in turn leads to an inevitable detachment
of data from the sources. Essentially, when a peer wants to access a data
item or some service, it cannot target a particular “server” but a swarm
of potential suppliers. As such, redundancy is intrinsic in a P2P system.
Absolute Autonomy. Every peer is autonomous and its behaviors are not
under any centralized controller. A peer does not even need to follow
any “protocol” but is instead “enticed” with some incentive schemes to
cooperate. Indeed, a Byzantine behavior model should be assumed for an
arbitrary peer. Consequently, it is di cult to deduce system performance
from a bottom-up perspective. Instead, it can only be deduced from a
holistic emergent angle. Finally, autonomy of peers also implies possibly
malicious actions can be carried out by an arbitrary peer, exacerbating
security concerns.
Possibly Unstructured Networking. From a networking point of view,
although structured network topologies have been widely considered
(detailed in Chapter 3), currently a mesh or random swarm network-
ing is the norm. This is because maintaining a structured topology goes
directly against the autonomy of peers. Thus, such a structured network
architecture is only realized at a system level, e.g., connecting the track-
ers in a BitTorrent network, but not at the user level. As a result, it is
di cult to provide quality-of-service (QoS) guarantees to users.
Unreliable Communication Environment. Application level networking
is used among peers, and thus, the connections can be unreliable. For
instance, peer dynamics (i.e., peers joining and leaving) can lead to some
broken connections. Thus, similar to the lack of structured topology, it is
di cult to provide QoS guarantees. Similarly, in a P2P wireless network
(e.g., a wireless sensor network), the communication links among peers
are also highly unreliable. Consequently, a more fault-tolerant commu-
nication paradigm has to be devised for such a P2P network.