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revision 1.205 by hemppah, Mon May 26 09:07:05 2003 UTC revision 1.206 by hemppah, Mon Jun 2 06:50:41 2003 UTC
# Line 821  we describe the listed properties of Pee Line 821  we describe the listed properties of Pee
821    
822  \chapter{Open Problems in Peer-to-Peer}  \chapter{Open Problems in Peer-to-Peer}
823    
824  In this chapter, we discuss open problems in Peer-to-Peer research.  In this chapter we discuss open problems in Peer-to-Peer research.
825  Note that the open problems list considered here is not meant  Note that the unsolved problems considered do not represent
826  to be an exhaustive survey of \emph{all} open problems in Peer-to-Peer domain;  an exhaustive survey of \emph{all} unsolved problems in Peer-to-Peer domain. In this chapter
827  we focus our attention to some issues related to security, scalability, usability and performance.  we focus our attention on some issues related to security, scalability, usability and performance.
828    
829    
830  \section{Overview}  \section{Overview}
831    
832  Partly due to the non-maturity of modern Peer-to-Peer technology, there are several  Partly due to the non-maturity of modern Peer-to-Peer technology, there are several
833  open problems to be solved. Also, many techniques developed for traditional distributed  problems to be solved. Also, many techniques developed for traditional distributed
834  systems may no longer apply with Peer-to-Peer systems, e.g., load balancing techiques \cite{byers03dhtbalancing}.  systems may no longer apply with Peer-to-Peer systems, e.g., load balancing techiques \cite{byers03dhtbalancing}.
835    
836  Different problems apply to both the loosely structured and the tightly structured approach.  Different problems apply to both the loosely structured and the tightly structured approach.
837  For instance, since the introduction of Gnutella \cite{gnutellaurl}, the main concern has been the scalability problem of loosely structured  For instance, since the introduction of Gnutella \cite{gnutellaurl}, the main concern has been the scalability problem of loosely structured
838  systems. However, the scalability problem of the loosely structured is often misunderstood;  systems. However, the scalability problem of the loosely structured is often misunderstood;
839  \emph{the network overlay} of loosely structured systems is scalable, but the \emph{data lookup model} is not as  \emph{the network overlay} of loosely structured systems is scalable, but the \emph{data lookup model} is not, because
840  the data lookup process creates lot of extra network traffic (e.g., \cite{yang02improvingsearch}).  the data lookup process creates lot of extra network traffic (e.g., \cite{yang02improvingsearch}).
841    
842  In tightly structured systems the main concern is to make overlay's data lookup process  In tightly structured systems the main objective is to make overlay's data lookup process
843  more fault tolerant against hostile attacks (e.g., \cite{castro02securerouting}). Other key problems in tightly structured  more fault tolerant against hostile attacks (e.g., \cite{castro02securerouting}). Other key problems in tightly structured
844  systems are the lack of keyword searches \cite{harren02complex, ansaryefficientbroadcast03}, support for heterogeneous peers  systems are the lack of keyword searches \cite{harren02complex, ansaryefficientbroadcast03}, support for heterogeneous peers
845  \cite{rowston03controlloingreliability} and load balancing \cite{balakrishanarticle03lookupp2p, byers03dhtbalancing}.  \cite{rowston03controlloingreliability} and load balancing \cite{balakrishanarticle03lookupp2p, byers03dhtbalancing}.
846    
847  \section{Security problems}  \section{Security problems}
848    
849  In this section we describe security problems related to Peer-to-Peer domain. First, we discuss attacks  In this section we describe security problems related to the Peer-to-Peer domain. First, we discuss well-known attacks
850  and lack of trust in Peer-to-Peer systems. Then, we describe anonymity, access control, hostile entities  on Peer-to-Peer systems. Then, we discuss the common lack of trust in Peer-to-Peer system, and related issues of anonymity, access control, hostile entities
851  and secure query routing problems. Finally, we briefly cover external security threats.  and secure query routing. Finally, we briefly cover external security threats.
852    
853  \subsection{Attacks}  \subsection{Attacks}
854    
# Line 861  the Fail-stop attack, the Spam attack \c Line 861  the Fail-stop attack, the Spam attack \c
861  the Distributed Denial of Service attack.  the Distributed Denial of Service attack.
862    
863  In the Sybil attack model \cite{douceur02sybil}, a hostile entity presents multiple  In the Sybil attack model \cite{douceur02sybil}, a hostile entity presents multiple
864  entities, i.e., when a peer communicates with a subset of other participating entities to perform an operation, a peer communicates  entities, i.e., when a peer communicates with a subset of other participating entities to perform an operation whereas a peer communicates
865  only with the same hostile entity. Hostile entity can control a large fraction of Peer-to-Peer system while  only with the same hostile entity. A hostile entity can control a large fraction of a Peer-to-Peer system while
866  repressing the redundancy of the system. Authors argue in  \cite{douceur02sybil} that without a centralized authority, Sybil attacks are always possible in a Peer-to-Peer  repressing the redundancy of the system. Authors argue in  \cite{douceur02sybil} that without a centralized authority, Sybil attacks are always possible in a Peer-to-Peer
867  system except under extreme and unrealistic assumptions of resource parity and coordination among entities. Unrealistic assumptions include: all entities  system except under extreme and unrealistic assumptions of resource parity and coordination among entities. Unrealistic assumptions include: all entities
868  should be nearly homogeneous, all identities can be validated simultaneously by all  should be nearly homogeneous; all identities can be validated simultaneously by all
869  entities across the system and when accepting identities that are not directly validated, the required number of certificates exceeds  entities across the system; and, when accepting identities that are not directly validated, the required number of certificates exceeds
870  the number of systemwide failures \cite{douceur02sybil}. Castro et al. \cite{castro02securerouting} suggest the use of cryptographic content hashes in the  the number of systemwide failures \cite{douceur02sybil}. Castro et al. \cite{castro02securerouting} suggest the use of cryptographic content hashes in the
871  creation process of peer identifier against the Sybil attack. According to authors, in this technique the IP address of a peer can be verified by the other peer.  creation process of peer identifier against the Sybil attack. According to the authors, in this technique the IP address of a peer can be verified by the other peer.
872  They call this method as a one form of \emph{self-certifying data}.  They characterize this method as a form of \emph{self-certifying data}.
873    
874  In the Fail-stop attack model, cited in \cite{naor03simpledht}, a faulty peer is deleted from the Peer-to-Peer system. Thus,  In the Fail-stop attack model, cited in \cite{naor03simpledht}, a faulty peer is deleted from the Peer-to-Peer system. Thus,
875  a specific data item can be lost from the system temporarily (or permanently). The reason for the faultiness of a peer can be a  a specific data item can be lost from the system temporarily or permanently. The reason for the faultiness of a peer can be a
876  software failure or a hostile attack. The Byzantine attack model \cite{357176} is closely related to Fail-stop model. In the Byzantine attack model  software failure or a hostile attack. The Byzantine attack model \cite{357176} is closely related to the Fail-stop model. In the Byzantine attack model,
877  $3f + 1$ is the minimum number of peers that allow system to provide the safety and liveness properties when up to $f$ peers are faulty \cite{357176}.    $3f + 1$ is the minimum number of peers that allow the system to provide the safety and liveness properties when up to $f$ peers are faulty \cite{357176}.  
878  The Byzantine model can be seen as more severe than Fail-stop model as there are no restrictions over the behavior of faulty peers, e.g., the cooperation  The Byzantine model can be seen as more severe than the Fail-stop model because there are no restrictions over the behavior of faulty peers, e.g., the cooperation
879  between multiple \emph{malicious} faulty peers is possible \cite{357176}. A practical solution for the Byzantine failures have been  between multiple \emph{malicious} faulty peers is possible \cite{357176}. Castro et al. \cite{296824} have proposed a practical solution for the Byzantine failures.
880  proposed by Castro et al. \cite{296824}. Authors use in their work replication algorithm to tolerate Byzantine faults and cryptographic  The authors use in their work replication algorithm to tolerate Byzantine faults and cryptographic
881  certificate techniques to prevent spoofing and replays to detect corrupted messages.  certificate techniques to prevent spoofing and replays to detect corrupted messages.
882    
883  The Spam generating attack \cite{naor03simpledht} is another known attack model against Peer-to-Peer system. In the Spam  The Spam generating attack \cite{naor03simpledht} is another known attack model against a Peer-to-Peer system. In the Spam
884  attack, a hostile or faulty peer may produce false information of the data, or refuses to (or is not able to) reply to requests.  attack, a hostile or faulty peer may produce false data information, or refuses to (or is not able to) reply to requests.
885  Naor et al. \cite{naor03simpledht} have proposed a partial solution against Spam attack in a \emph{faulty} peer environment (not hostile).  Naor et al. \cite{naor03simpledht} have proposed a partial solution against Spam attack in a \emph{faulty} peer environment (not hostile).
886    
887  Overloading of targeted peers is a form of Distributed Denial of Service attack (DDoS) (see, e.g., \cite{372148}). For instance,  Overloading of targeted peers is a form of Distributed Denial of Service attack (DDoS) (see, e.g., \cite{372148}). For instance,

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