2.14 Summary
UDP is a simple, unreliable, connectionless
protocol, while TCP is a complex, reliable, connection-oriented
protocol. SCTP combines some of the features of both protocols,
providing additional features beyond those found in TCP. While most
applications on the Internet use TCP (the Web, Telnet, FTP, and
email), there is a need for all three transport layers. In
Section 22.4, we
will discuss the reasons to choose UDP instead of TCP. In Section 23.12, we
will discuss the reasons to choose SCTP instead of TCP.
TCP establishes connections using a three-way
handshake and terminates connections using a four-packet exchange.
When a TCP connection is established, it goes from the CLOSED state
to the ESTABLISHED state, and when it is terminated, it goes back
to the CLOSED state. There are 11 states in which a TCP connection
can be, and a state transition diagram gives the rules on how to go
between the states. Understanding this diagram is essential to
diagnosing problems using the netstat command and
understanding what happens when an application calls functions such
as connect, accept, and close.
TCP's TIME_WAIT state is a continual source of
confusion with network programmers. This state exists to implement
TCP's full-duplex connection termination (i.e., to handle the case
of the final ACK being lost), and to allow old duplicate segments
to expire in the network.
SCTP establishes an association by using a
four-way handshake and terminates connections using a three-packet
exchange. When an SCTP association is established, it goes from the
CLOSED state to the ESTABLISHED state, and when it is terminated,
it goes back to the CLOSED state. There are eight states in which
an SCTP association can be, and a state transition diagram gives
the rules on how to go between the states. SCTP does not need the
TIME_WAIT state as TCP does due to its use of verification
tags.
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