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task, or thread, depending on what the underlying operating system supports. How this step is
performed depends on the operating system.
This new server handles this client's entire request. When complete, this new server terminates.
C3. Go back to step Cl.
The advantage of a concurrent server is that the server just spawns other servers to handle the
client requests. Each client has, in essence, its own server. Assuming the operating system
allows multiprogramming, multiple clients are serviced concurrently.
The reason we categorize servers, and not clients, is because a client normally can't tell whether
it's talking to an iterative server or a concurrent server.
As a general rule, TCP servers are concurrent, and UDP servers are iterative, but there are a few
exceptions. We'll look in detail at the impact of UDP on its servers in Section 11.12, and the
impact of TCP on its servers in Section 18.11. 1.9 Port Numbers
We said that TCP and UDP identify applications using 16-bit port numbers. How are these port
Servers are normally known by their well-known port number. For example, every TCP/IP
implementation that provides an FTP server provides that service on TCP port 21. Every Telnet
server is on TCP port 23. Every implementation of TFTP (the Trivial File Transfer Protocol) is
on UDP port 69. Those services that can be provided by any implementation of TCP/IP have
well-known port numbers between 1 and 1023. The well-known ports are managed by the
Internet Assigned Numbers Authority (IANA).
Until 1992 the well-known ports were between I and 255. Ports between 256 and 1023 were normally used by
Unix systems for Unix-specific services-that is, services found on a Unix system, but probably not found on
other operating systems. The IANA now manages the ports between 1 and 1023.
An example of the difference between an Internet-wide service and a Unix-specific service is the difference
between Telnet and Riogin. Both allow us to login across a network to another host. Telnet is a TCP/IP standard file://...
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- Spring '12