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revision 1.91 by hemppah, Thu Feb 27 12:08:58 2003 UTC revision 1.92 by hemppah, Thu Feb 27 13:15:52 2003 UTC
# Line 366  where $ps$ = $summaryindex(provider(s)) Line 366  where $ps$ = $summaryindex(provider(s))
366    
367  \section{Tightly structured}  \section{Tightly structured}
368    
369    In recents months, several tightly structured overlays has been proposed.
370    This list includes CAN \cite{ratnasamy01can}, Chord \cite{stoica01chord},
371    Kademlia \cite{maymounkov02kademlia}, Kelips \cite{gupta03kelips},
372    Koorde \cite{kaashoek03koorde}, ODHDHT \cite{naor03simpledht},
373    Pastry \cite{rowston01pastry}, Peernet \cite{eriksson03peernet},
374    Skip Graphs \cite{AspnesS2003}, SkipNet \cite{harvey03skipnet2},
375    Symphony \cite{gurmeet03symphony}, SWAN \cite{bonsma02swan}, Tapestry
376    \cite{zhao01tapestry} and Viceroy \cite{malkhi02viceroy}. While there
377    are significat differences among proposed systems, they all have in common
378    that participating peers are assigned \emph{peer identifiers} from
379    a large \emph{identifier space}. Furthermore, application-specific
380    data items are also assigned globally unique identifiers, \emph{keys},
381    which are selected from the same identifier space. The form of identifier
382    space differs between proposed systems. Circular identifier space (and variants)
383    is most widely used. For instance, Chord, Koorde, Pastry, SWAN, Tapestry
384    and Viceroy use a circular identifier space of $n$-bit integers modulo $2^{n}$. The
385    value of $n$ varies among approaches. Again, CAN uses a $d$-dimensional cartesian
386    to implement identifier space.
387    
388    To store data into tightly structured overlay, each application-specific
389    key is \emph{mapped} by the overlay to a existing peer in the overlay. Each
390    peer in the structured overlay maintains a \emph{routing table}, which consists
391    of identifiers and IP addresses of other peers in the overlay. These are peer's
392    neighbors in the overlay network.Figure \ref{fig:structured_hashing}
393    illustrates the process of data to key mapping in tightly strucuted overlays.
394    
395    \begin{figure}
396    \centering
397    \includegraphics[width=12cm, height=6cm]{structured_overlay.eps}
398    \caption{Basic idea of structured overlay}
399    \label{fig:structured_hashing}
400    \end{figure}
401    
402    All messages are routed across overlay links towards peers, whose
403    peer identifier is gradually ''closer'' to the key's identifier
404    in the identifier space. Distance can be measured by numerical
405    difference between identifiers (e.g., Chord), the number of
406    same prefix bits between identifiers (e.g., Pastry and Tapestry),
407    bit-wise exclusive or (XOR) (e.g., Kademlia). However, in all
408    previously schemes, each hop in the overlay shortens the path
409    between current peer working with query and the key which was
410    looked up.
411    
412    
413    
414  -service is data block, node/peer is a physical computer  -service is data block, node/peer is a physical computer
415  -*servers* self-organize towards a lookup network  -*servers* self-organize towards a lookup network
416  -DHTs can be thought as a 'structured overlay random graphs'  -DHTs can be thought as a 'structured overlay random graphs'
# Line 460  maintenance overhead Line 505  maintenance overhead
505  load balance  load balance
506    
507    
 \cite{zhao01tapestry}  
508    
 \cite{rowston01pastry}  
509    
 \cite{stoica01chord}  
510    
 \cite{ratnasamy01can}  
   
 \cite{maymounkov02kademlia}  
511    
512  \cite{freedman02trie}  \cite{freedman02trie}
513    
514  \cite{plaxton97accessingnearby}  \cite{plaxton97accessingnearby}
515    
 \cite{malkhi02viceroy}  
516    
 \cite{bonsma02swan}  
517    
518  \cite{AspnesS2003}  
519  \cite{78977}  \cite{78977}
520    
 \cite{gurmeet03symphony}  
521    
 \cite{eriksson03peernet}  
522    
523  \cite{harvey03skipnet2}  
524    
525    
526  \cite{garciamolina03sil}  \cite{garciamolina03sil}
# Line 498  load balance Line 533  load balance
533    
534  \cite{pias03lighthouse}  \cite{pias03lighthouse}
535    
 \cite{naor03simpledht}  
536    
 \cite{gupta03kelips}  
   
 \cite{kaashoek03koorde}  
537  \cite{debruijn46graph}  \cite{debruijn46graph}
538    
539    
# Line 542  load balance Line 573  load balance
573    
574  \cite{kubiatowicz00oceanstore}  \cite{kubiatowicz00oceanstore}
575    
 \begin{figure}  
 \centering  
 \includegraphics[width=10cm, height=6cm]{structured_overlay.eps}  
 %\includegraphics[width=10cm, height=6cm]{structured_query.eps}  
 \caption{Basic idea of structured overlay}  
 \label{fig:structured_hashing}  
 \end{figure}  
576    
577    
578    

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