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get honored, and the return value of the actual transfer is returned to them,
but no redundant transfers are performed.
In some other cases, different requests are made to transfer different parts
of the same file to the same destination. These requests are merged into one
request, and only one transfer command is issued. But again, all of the re-
quests get honored, and the appropriate return value is returned to all of
them.
4.4 High-Performance Wide Area Data Transfers
An important set of challenges appears when dealing with data transfers over
wide area networks. Perhaps the most visible issue in this context is that
of the transport protocol. Data transfers over wide area, and in particular
over high-capacity network links, make the performance limitations of the
TCP (Transmission Control Protocol) visible. In this section, we will discuss
these limitations and various alternatives in the following sections. Much of
the work related to wide area data transfer is focused on file or disk-to-disk
transfer. We will present other types of scenarios, how they are different,
and what the challenges and applicable solutions are. This section concludes
with a summary of remaining challenges and possible directions for future
research.
4.4.1 TCP Characteristics and Limitations
Traditionally, applications using wide area networks have been designed to
use the transmission control protocol (TCP) for reliable data communication.
TCP, defined in RFC (Requests for Comments) 793 * but with numerous
updates published since, provides byte stream semantics for end-to-end data
transport. The two communicating entities create a TCP connection that
can be used to send bytes in order and reliably (this defines byte stream
semantics). Another important feature of TCP is that it implements con-
gestion avoidance. Congestion appears when multiple concurrent streams go
through the same link or network element (router) or when a stream tra-
verses a low-capacity link. Congestion avoidance is necessary since if conges-
tion occurs, the overall utility of the network would be reduced because of
the capacity wasted with retransmissions and transmission of data that even-
tually is dropped. TCP's congestion avoidance mechanism is based on two
algorithms: slow start and congestion avoidance. These algorithms utilize the
notion of a congestion window , whose size is modified in response to packed
* http://www.faqs.org/rfcs/rfc768.html. Accessed July 16, 2009.
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