[Gzz-commits] gzz/Documentation/misc/hemppah-progradu mastert...

[Hermanni Hyytiälä]

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Module name:    gzz
Changes by:     Hermanni Hyytiälä <address@hidden>      03/03/05 02:45:34

Modified files:
        Documentation/misc/hemppah-progradu: masterthesis.tex 

Log message:
        Error fixes, updates, typos

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http://savannah.gnu.org/cgi-bin/viewcvs/gzz/gzz/Documentation/misc/hemppah-progradu/masterthesis.tex.diff?tr1=1.111&tr2=1.112&r1=text&r2=text

Patches:
Index: gzz/Documentation/misc/hemppah-progradu/masterthesis.tex
diff -u gzz/Documentation/misc/hemppah-progradu/masterthesis.tex:1.111 
gzz/Documentation/misc/hemppah-progradu/masterthesis.tex:1.112
--- gzz/Documentation/misc/hemppah-progradu/masterthesis.tex:1.111      Tue Mar 
 4 10:02:09 2003
+++ gzz/Documentation/misc/hemppah-progradu/masterthesis.tex    Wed Mar  5 
02:45:33 2003
@@ -27,9 +27,9 @@
 
 \tyyppi{Master's Thesis}
 
-\keywords{Peer-to-Peer, P2P, security, Distributed systems}
+\keywords{Peer-to-Peer, P2P, security, Distributed systems, Hypermedia systems}
 
-\avainsanat{Vertaisverkot, P2P, tietoturva, hajautetut järjestelmät}
+\avainsanat{Vertaisverkot, P2P, tietoturva, hajautetut järjestelmät, 
hypermedia-järjestelmät}
 
 \contactinformation{\\
 Hermanni Hyytiälä\\
@@ -307,9 +307,9 @@
 \end{figure}  
 
 Previously presented improvements are only partial solutions. Obviously, more
-research is required to make loosely structured approach's data lookup more
-scalable and effective. More advanced techniques to improve loosely strcutured
-systems' data lookup is presented in chapter 3.
+research is required to make data lookup of loosely structured approach more
+scalable and effective. More advanced techniques to improve data lookup of 
+loosely strcutured systems is presented in chapter 3.
 
 
 \subsection{Sketch of formal definition}
@@ -329,7 +329,7 @@
 
 \section{Tightly structured}
 
-Partly due to loosely structured systems' scalability problems, several 
tightly 
+Partly due to scalability problems of loosely structured systems, several 
tightly 
 structured overlays has been proposed.
 This list includes CAN \cite{ratnasamy01can}, Chord \cite{stoica01chord}, 
 Kademlia \cite{maymounkov02kademlia}, Kelips \cite{gupta03kelips}, 
@@ -344,12 +344,13 @@
 data items are also assigned globally unique identifiers, \emph{keys}, 
 which are selected from the same identifier space. The form of identifier
 space differs between proposed systems. Circular identifier space (and 
variants) 
-is most widely used. For instance, Chord, Koorde, Pastry, SWAN, Tapestry
-and Viceroy use a circular identifier space of $n$-bit integers modulo 
$2^{n}$. The
-value of $n$ varies among approaches. Again, CAN uses a $d$-dimensional 
cartesian 
+is most widely used. For instance, Chord \cite{stoica01chord}, Koorde 
\cite{kaashoek03koorde}, 
+Pastry \cite{rowston01pastry}, SWAN \cite{bonsma02swan}, Tapestry 
\cite{zhao01tapestry}
+and Viceroy \cite{malkhi02viceroy} use a circular identifier space of $n$-bit 
integers modulo $2^{n}$. The
+value of $n$ varies among approaches. Again, CAN \cite{ratnasamy01can} uses a 
$d$-dimensional cartesian 
 to implement identifier space.
 
-Stoica et al. \cite{balakrishanarticle03lookupp2p} have listed 
+Stoica et al.. \cite{balakrishanarticle03lookupp2p} have listed 
 four requirements for tightly structured overlays, which have to be 
 addressed in order to perform data lookups in tightly structured overlays. 
 First, mapping of keys to peers must be done in a load-balanced
@@ -423,7 +424,7 @@
 \begin{figure}
 \centering
 \includegraphics[width=10cm, height=6cm]{structured_query.eps}
-\caption{Simplified structured system's query}
+\caption{Simplified data lookup of tightly structured system}
 \label{fig:structured_query}
 \end{figure}
 
@@ -431,7 +432,7 @@
 \begin{figure}
 \centering
 \includegraphics[width=10cm, height=8cm]{kademlia_lookup.eps}
-\caption{Kademlia's lookup process}
+\caption{Data lookup process of Kademlia}
 \label{fig:kademlia_lookup}
 \end{figure}
 
@@ -510,12 +511,12 @@
 have very little in common. Indeed, the only thing they share is the fact that 
no other peer is more
 important than other peer in the Peer-to-Peer network. Fault tolerance 
\emph{may} may
 be an area, in which approaches have similar properties (e.g., single point of 
failure).
-However, both approaches' fault-tolerance properties are currently only 
initial calculations, or 
+However, fault-tolerance properties of both approaches are currently only 
initial calculations, or 
 experimented in simulation environments. In real-life, measuring fault 
tolerance is much more 
 challenging task and requires more research to get reliable answers.
  
 Thus, there are significant differences between loosely structured and tightly 
structured approaches.
-The most important aspect is the performance and scalability. While loosely 
structured approach's performance
+The most important aspect is the performance and scalability. While 
performance of loosely structured approach 
 is not always even linear, generally tightly structured approach can perform 
all internal operations in
 poly-logarithmic time\footnote{However, it is unknown whether all proposed 
algorithms can preserve 
 logarithmic properties in real-life applications or not.}. 
@@ -694,7 +695,7 @@
 \parbox{37pt}{$O$($d$)} &
 \parbox{37pt}{$O(dn^{\frac{1}{d}})$} &
 \parbox{85pt}{2$d$} &
-\parbox{85pt}{System's performance may decrease if nodes are not homogeneous 
and nodes join and leave the system in a dynamic manner, where $d$ is the 
dimension of virtual key space}
+\parbox{85pt}{The performance of system may decrease if nodes are not 
homogeneous and nodes join and leave the system in a dynamic manner, where $d$ 
is the dimension of virtual key space}
 \\ \hline
 
 \parbox{37pt}{Chord \cite{stoica01chord}} &
@@ -702,7 +703,7 @@
 \parbox{37pt}{$O(\log{n}$} &
 \parbox{37pt}{$O(\log{n})$} &
 \parbox{85pt}{2$(\log{n})$} &
-\parbox{85pt}{System's performance may decrease if nodes are not homogeneous 
and nodes join and leave the system in a dynamic manner}
+\parbox{85pt}{The performance of system may decrease if nodes are not 
homogeneous and nodes join and leave the system in a dynamic manner}
 \\ \hline
 
 
@@ -738,7 +739,7 @@
 \parbox{37pt}{$O$($\sqrt{n}$)} &
 \parbox{37pt}{$O(1)$} &
 \parbox{85pt}{$\frac{n}{\sqrt{n}} + c*(\sqrt{n}-1) + \frac{Totalnumber of 
files}{\sqrt{n}}$, where n is the number of nodes and c the number of 
contacts/foreign affinity group} &
-\parbox{85pt}{Insert/delete overhead is constant and performed background, 
System's performance may decrease if nodes are not homogeneous and nodes join 
and leave the system in a dynamic manner}
+\parbox{85pt}{Insert/delete overhead is constant and performed background, the 
performance of system may decrease if nodes are not homogeneous and nodes join 
and leave the system in a dynamic manner}
 \\ \hline
 
 \parbox{37pt}{Koorde \cite{kaashoek03koorde}} &
@@ -763,7 +764,7 @@
 \parbox{37pt}{$O(\log{n})$} &
 \parbox{37pt}{$O(\log{n})$} &
 \parbox{85pt}{$(2^{b - 1})\frac{\log{n}}{b}$, where $b$ is a configurable 
parameter for tuning digit-fixing properties (routing table)} &
-\parbox{85pt}{System's performance may decrease if nodes are not homogeneous 
and nodes join and leave the system in a dynamic manner, based on Plaxton's 
algorithm}
+\parbox{85pt}{The performance of system performance may decrease if nodes are 
not homogeneous and nodes join and leave the system in a dynamic manner, based 
on Plaxton's algorithm}
 \\ \hline
 
 
@@ -829,7 +830,7 @@
 \parbox{37pt}{$O(\log{n})$} &
 \parbox{37pt}{$O(\log{n})$} &
 \parbox{85pt}{$(2^{b - 1})\frac{\log{n}}{b}$, where $b$ is a configurable 
parameter for tuning digit-fixing properties (routing table)} &
-\parbox{85pt}{System's performance may decrease if nodes are not homogeneous 
and nodes join and leave the system in a dynamic manner, based on Plaxton's 
algorithm}
+\parbox{85pt}{The performance of system performance may decrease if nodes are 
not homogeneous and nodes join and leave the system in a dynamic manner, based 
on Plaxton's algorithm}
 \\ \hline
 
 \parbox{37pt}{Viceroy \cite{malkhi02viceroy}} &
@@ -837,7 +838,7 @@
 \parbox{37pt}{$O(1)$} &
 \parbox{37pt}{$O(\log{n})$} &
 \parbox{85pt}{11} &
-\parbox{85pt}{System's performance may decrease if nodes are not homogeneous 
and nodes join and leave the system in a dynamic manner, not necessarily 
fault-tolerant because of constant degree of neighbors}
+\parbox{85pt}{The performance of system performance may decrease if nodes are 
not homogeneous and nodes join and leave the system in a dynamic manner, not 
necessarily fault-tolerant because of constant degree of neighbors}
 \\ \hline
 
 
@@ -872,8 +873,8 @@
 Since Napster \cite{napsterurl} and Gnutella \cite{gnutellaurl} was first time 
introduced 
 to public, researchers' main concern has been scalability problem of loosely 
structured 
 approach. However, people often misunderstand the scalability problem of 
loosely structured 
-approach; loosely structured systems' \emph{network} is scalable, but the 
\emph{query model} is not. 
-Tightly structured system's main concern is to make overlay's data lookup 
+approach; \emph{network} of loosely structured systems is scalable, but the 
\emph{query model} is not. 
+The main concern of tightly structured system is to make overlay's data lookup 
 routing more flexible againts hostile attacks. Another key problems in tightly 
structured 
 approach are the lack of keyword searches and support for heterogeneous peers.
 
@@ -893,9 +894,9 @@
 Fail-stop attack, Spam attack \cite{naor03simpledht}, Byzantine problem 
\cite{357176} and \cite{296824}, and
 general Distrubuted Denial of Service attack. 
 
-In Sybil attack model, hostile entity presents multpile 
+In Sybil attack model, hostile entity presents multiple 
 entities. Therefore, one hostile entity can control a large fraction of the 
Peer-to-Peer system. Optimal
-possible solution to Sybil attack would be that system could \emph{distinct} 
system's entities reliably. Unfortunately,
+possible solution to Sybil attack would be that system could \emph{distinct} 
entities of system reliably. Unfortunately,
 currently there no realizable techiques for this task. Partial solutions for 
Sybil is attack is to replicate
 and fragment data randomly among several participating peer. However, both 
suggestions assume that two different 
 remote entities are actually different; Sybil attacks are still possible and 
therefore, would need centralized 
@@ -914,17 +915,22 @@
 attack, hostile or faulty peer may produce false information of the data. 
Possible solution againts this attack
 is that peer should not trust to single entity. Instead peer should get 
information from multiple entities and trust 
 on majority's opinion. However, Spam attack is combined with Sybil attack, 
obviously previously mentioned solution
-won't work. Again, more research is required to solve this attack model 
reliability. Naor et al \cite{naor03simpledht}
+won't work. Again, more research is required to solve this attack model 
reliability. Naor et al. \cite{naor03simpledht}
  has proposed a partial solution againts Spam attack with \emph{faulty} peers 
(not hostile).
 
 Traditional overload of targeted peers is best known form of distrubuted 
Denial of Service attack (DDoS). For example, 
 hostile entity can attempt to burden targetted peers with garbage packets. As 
a implication, peers may act
 incorrectly or stop working. DDoS attack may be very severe, especially if 
rate of replication and caching 
-in Peer-to-Peer system is low. This may lead to data loss in the Peer-to-Peer 
system. Daswani et al 
+in Peer-to-Peer system is low. This may lead to data loss in the Peer-to-Peer 
system. Daswani et al. 
 \cite{daswani02queryflooddos} has done research regarding to this subject. 
Authors suggest efficient load balancing 
-policies for Peer-to-Peer system in order to prevent massive system failures. 
Sit et al \cite{sit02securitycons} 
+policies for Peer-to-Peer system in order to prevent massive system failures. 
Sit et al. \cite{sit02securitycons} 
 suggests that identifier assignment algorithm for peers would assign 
identifier with respect to network topology 
-and replicas should be located physically to different locations. 
+and replicas should be located physically to different locations.
+
+As stated in \cite{naor03simpledht}, an important aspect is that when it comes 
to general security aspects and 
+byzantine faults in any Peer-to-Peer system, there should be a clear 
distinction between attacks on the 
+algorihms assuming the construction of overlay is correct, and attacks on the 
construction itself. Clearly, Sybil
+and Spam attack belongs to the first category, and rest of the attacks to the 
latter category.
 
 \subsection{Trust, data authenticity and integrity}
 
@@ -961,7 +967,7 @@
 of anonymity in which no one can link author to a specific document. In 
publisher-anonymity system,
 no one is able to link publisher to a specific document. Reader-anonymity 
means that a specific
 document cannot be linked to document's readers. This form of anonymity 
protects the privacy of a
-system's users. Furthermore, in peer-anonymity means that no peer can be 
linked to a specific document, i.e.
+users of the system. Furthermore, in peer-anonymity means that no peer can be 
linked to a specific document, i.e.
 no one is able to determine the peer, where document was originally published. 
Document-anonymity
 means that peer doesn't know which data it is currently hosting. Finally, 
query-anonymity refers is form
 of document-anonymity; when other peers performs data lookups, peer doesn't 
know which data it servers
@@ -977,8 +983,8 @@
 the peers responsible to given data in Peer-to-Peer system. Of course, when we 
know the peers responsible
 for the data, the anonymity of peer is lost. Fortunately, there are partial 
solutions to previously
 mentioned situations, i.e. \emph{pseudonym} which is a partial form of 
anonymity. For instance, pseudonym can used for 
-addressing peer-anonymity by providing anonymous-like identifiers to peers 
(e.g., tightly structured system's 
-peer identifiers).
+addressing peer-anonymity by providing anonymous-like identifiers to peers 
(e.g., peer identifiers of tightly 
+structured system).
 
 Anonymity is widely used in those Peer-to-Peer system in which data 
publication and non-censorship are important properties
 of the system. These include
@@ -1002,7 +1008,7 @@
 there has been a lot of violation of copyright laws by users of Peer-to-Peer 
filesharing systems. As a 
 consequence, some lawsuits has been created againts the companies how have 
build popular file-sharing programs.
 
-To our knowledge, Nejdl et al \cite{nejdl03accesscontrol} have proposed very 
recently first practical solution to access 
+To our knowledge, Nejdl et al. \cite{nejdl03accesscontrol} have proposed very 
recently first practical solution to access 
 control problem in Peer-to-Peer systems. They use RDF-based schema policies to 
restrict access to certain
 data. Unfortunately, their current prototype works only in loosely structured 
systems.
 
@@ -1041,18 +1047,18 @@
  approach is not very efficient, since proposals create a lot of additional 
network traffic when
 in function.
 
-Additionally, Lynch et al. \cite{lynch02atomicdataaccess} propose a solution 
to secure routing table 
+Additionally, Lynch et al.. \cite{lynch02atomicdataaccess} propose a solution 
to secure routing table 
 maintenance, but their solution seems to have to major problems 
\cite{castro02securitystructured}. First,
 the solution is very expensive even without faulty or hostile entities. 
Second, each group of replicas
 in their solution must have less than 1/3 of its peer faulty. Thus, this 
feature results in a low
 probability of succesfull routing.
 
-Aspnes et al in \cite{aspnes02faultrouting} and Kaashoek et all in 
\cite{kaashoek03koorde} formally 
+Aspnes et al. in \cite{aspnes02faultrouting} and Kaashoek et al.l in 
\cite{kaashoek03koorde} formally 
 prove the lower and upper bounds for space requirements of locating a specific 
date item in 
 Peer-to-Peer system. They show that to provide high degree of fault tolerance 
and efficiency, each 
 participating peer must maintain $O(\log{n})$ neighbors. 
 
-Fiat et al in \cite{fiat02censorship}, \cite{saia02dynamicfaultcontentnetwork} 
and Datar in \cite{datar02butterflies}  
+Fiat et al. in \cite{fiat02censorship}, 
\cite{saia02dynamicfaultcontentnetwork} and Datar in \cite{datar02butterflies}  
 describe tightly structured overlay with analytical results in the presence of 
hostile entities. However,
 none of these proposals doesn't address an efficient, dynamic tightly 
structured overlay and multiple rounds
 of hostile attack. Also, above mentioned propsals are not very efficient. In 
\cite{fiat02censorship}, each node 
@@ -1091,7 +1097,7 @@
 originator starts a flood with small TTL value. If the search is not succesful,
 the query originator increases the TTL value and performs another flood. This 
 process is repeated until the desired data is found or maximumum depth $D$ 
-has been reached. Expanding ring, proposed by Shenker et al., 
\cite{lv02searchreplication}, 
+has been reached. Expanding ring, proposed by Shenker et al.., 
\cite{lv02searchreplication}, 
 is similar to iterative deepening techique. With these techniques, search 
 may not be fast when desired data item requires many consecutive flooding 
rounds.
 
@@ -1118,7 +1124,7 @@
 depth-first traversal and peers' routing tables are dynamically built
 using caching. This is an outcome of Freenet's main design priciples, 
 i.e., anonymity. Additional improvements to Freenet's data lookup using
-''small-world phenomenon'' has been proposed by Zhang et al. 
\cite{zhang02using}.
+''small-world phenomenon'' has been proposed by Zhang et al.. 
\cite{zhang02using}.
 
 
 Since tightly structured systems have efficient data lookup at the application 
level overlay,
@@ -1155,19 +1161,23 @@
 better performance. 
 
 Some studies have been concentraded on SQL-like queries \cite{harren02complex}
-in tightly structured overlays. Another approaches includes adapting loosely 
structured approache's
-data lookup model into tightly structured systems 
\cite{ansaryefficientbroadcast03}, \cite{chord:om_p-meng}.
+in tightly structured overlays. Another approaches includes adaption of data 
lookup model of loosely 
+structured approach into tightly structured systems 
\cite{ansaryefficientbroadcast03}, \cite{chord:om_p-meng}.
 Additional studies include additional layer upon overlay network 
\cite{kronfol02fasdsearch}, 
 \cite{joseph02p2players} and range queries \cite{andrzejak02rangequeries}.
 
 Many techniques have been developed in order to provide more efficient search 
indexing. As 
-studies queries follow Zipf-like distributions \cite{breslau98implications} 
caching and precomputation
+studies queries follow Zipf-like distributions\footnote{Zipf distribution is a 
variant of power-law function. 
+Zipf-distribution can be used in observation of frequency of occurrence event 
$E$, as a function of the rank 
+$i$ when the rank is determined by the frequency of occurrence, $E_i \sim 
\frac{1}{i^{a}}$, where the exponent 
+$a$ is close to unity.} \cite{breslau98implications} caching and precomputation
 can be done for optimizting search indices \cite{li03feasibility}. Regular 
compression algorithms,
 Bloom filters \cite{362692}, vector space models 
\cite{CuencaAcuna2002DSIWorkshop} and view 
 trees \cite{Bhattacharjee03resultcache} can be used for even better 
optimizations. Authors 
 in \cite{li03feasibility} use Gap compression \cite{wittengigabytes}, Adaptive 
Set Intersection \cite{338634}  
 and clustering with their search optimizations.
 
+
 While it is expected that web-like searches can be layered on top of tightly 
structured overlay, much
 more research is required to make indexing and searching more efficient.
 
@@ -1182,8 +1192,8 @@
 neighbors on behalf of peer itself and maps data items randomly throughout the 
overlay network. However,
 Peer-to-Peer system is \emph{never} in ''ideal'' state as it is always 
evolving system.
 
-Current research has been focused on tightly structured systems' system 
management, since all presented
-tightly structured approache's algorithms have been analyzed under static 
simulation environments. Furthermore, propsed tightly structured
+Current research has been focused on system management of tightly structured 
systems, since all presented
+algorithms of tightly structured approach have been analyzed under static 
simulation environments. Furthermore, propsed tightly structured
 overlays are configured statically to achieve the desired reliability even in 
uncommon and adverse environment
 \cite{rowston03controlloingreliability}. The most important factor for
 future research is to get real-life experiences from tightly structured 
system, when there are frequent
@@ -1198,36 +1208,36 @@
 more efficient analytical tools for modelling complex Peer-to-Peer system. 
 
 Some research has been done with regard to load balancing properties of 
tightly structured
-overlays. Byers et al. suggest "power of two choices" whereby an item is 
stored at the less loaded 
-of two (or more) random alternatives \cite{byers03dhtbalancing}. Rao et al. 
uses virtual servers
+overlays. Byers et al.. suggest "power of two choices" whereby an item is 
stored at the less loaded 
+of two (or more) random alternatives \cite{byers03dhtbalancing}. Rao et al.. 
uses virtual servers
 to control load balance in Peer-to-Peer systems \cite{rao03loadbalancing}. 
Their work rests on
 idea which was originally introduced by Chord system.
 
 Also, query and routing hotspots may be an issue in tightly structured 
overlays \cite{ratnasamy02routing}. 
 Hotspots happen, when specific key is being requested extremely often in 
tightly structured overlays. Recent study 
-by Freedman et al. tries to reduce hot spots in the system by performing 
\emph{sloppy} hashing 
+by Freedman et al.. tries to reduce hot spots in the system by performing 
\emph{sloppy} hashing 
 \cite{sloppy:iptps03}. Another key feature of their work is that peers 
self-organize into clusters, 
 therefore enabling peers to find nearby data without looking up data from 
distant peers.
 
 An implicit assumption of almost every tightly structured system is that there 
is random, uniform 
 distribution of peer and key identifiers. Even if participating peers are 
extremely heterogeneous in
 face of computing power, or network bandwidth, data items are distributed 
uniformly. Clearly, this
-a serious problem of tightly structured overlays , since measurement study by 
Saroiu et al. shows 
+a serious problem of tightly structured overlays , since measurement study by 
Saroiu et al.. shows 
 that there extreme heterogeneity among participating peers in already deployed 
Peer-to-Peer systems.
 \cite{saroiu02measurementstudyp2p}. Symphony seems to be the first tightly 
structured overlay system 
-which support hetergeneity. However, Zhao et al. have proposed a secondary 
layer a top of structured overlay
+which support hetergeneity. However, Zhao et al.. have proposed a secondary 
layer a top of structured overlay
 to support hetergeneity better \cite{zhao02brocade}. 
 
-Research has been done on self-organization. Ledlie et al. propose techniques 
for forming and maintaining
+Research has been done on self-organization. Ledlie et al.. propose techniques 
for forming and maintaining
 groups in highly dynamic environment \cite{ledlie02selfp2p}. Unfortunately 
their work relies on idea that
 participating peers would create multiple hierarchical groups; it's not clear 
whether this approach
-is fault-tolerant and suitable to Peer-to-Peer environment. More promising 
work has been done by Rowston et al.
+is fault-tolerant and suitable to Peer-to-Peer environment. More promising 
work has been done by Rowston et al..
 in \cite{rowston03controlloingreliability}. Authors propose techiques for 
self-tuning, dealing with
 uncommon conditions (e.g., network partition and high failure rates). 
Moreover, authors arque that
 these techniques, the concerns over the tightly structured overlay maintenance 
costs are no more
 an open issue.
 
-Finally, little research has been done regarding self-monitoring and data 
availability. Zhang et al.
+Finally, little research has been done regarding self-monitoring and data 
availability. Zhang et al..
 describe a arbitrary data structure on top of tightly structured overlay 
\cite{zhang03somo}. They
 call their proposal as \emph{data overlay}, since it support several 
fundamental data structures.
 Authors use this data overlay to build Self-Organized Metadata Overlay (SOMO), 
which can be used
@@ -1259,10 +1269,10 @@
 
 Somewhat surprisingly little research has been in this area, especially when 
considering 
 the possible impact of this \emph{unwanted socical behaviour} to performance 
of Peer-to-Peer 
-system. Problem is addressed by Golle et al. \cite{golle01incentivesp2p}. Some 
+system. Problem is addressed by Golle et al.. \cite{golle01incentivesp2p}. 
Some 
 research has been focused on semantic properties of the overlay in order to 
increase 
-cooperation among participating peers \cite{crespo02semanticoverlay}. 
Ramanathan et al. 
-\cite{ramanathan02goodpeers} and Bernstein et al. \cite{bernstein03selection} 
use 
+cooperation among participating peers \cite{crespo02semanticoverlay}. 
Ramanathan et al.. 
+\cite{ramanathan02goodpeers} and Bernstein et al.. \cite{bernstein03selection} 
use 
 empirical metrics and decision trees when teaching peers to make better 
decisions
 when contacting other peers in Peer-to-Peer system. Alpine \cite{alpineurl} is 
an example of
 Peer-to-Peer system, which uses empirical metrics for peer selection.
@@ -1272,7 +1282,7 @@
 
 Very little research has been done on simulating the \emph{global} 
Peer-to-Peer system. Presumably, this
 is due to complex nature of Peer-to-Peer system, which makes comprehensive 
simulations very 
-diffucult. Floyd et al. has been studying the simulation of the Internet in 
\cite{504642}. Authors
+diffucult. Floyd et al.. has been studying the simulation of the Internet in 
\cite{504642}. Authors
 state that simulating the Internet is very challenging task, because of 
Internet's heterogeneity
 and rapid change. Obviously, these factors exist also in Peer-to-Peer system 
even with higher
 rates.
@@ -1532,7 +1542,7 @@
 \parbox{90pt}{Byzantine faults \cite{296824}} &
 \parbox{110pt}{Faulty nodes may behave arbitrarily} &
 \parbox{110pt}{Byzantine replication algorithms -> get information from 
multiple entities, trust majority's opinion} &
-\parbox{110pt}{Much research has been done on this field, practical solutions, 
decreases system's, performance slighly}
+\parbox{110pt}{Much research has been done on this field, practical solutions, 
decreases the performance of system slighly}
 \\ \hline
 
 \caption{Performance and usability problems in Peer-to-Peer.}