12.3
TCP/IP AND RELATED PROTOCOLS
325
Figure 12.6
Protocol layers showing the relationship of TCP with other layered protocols.
reliable, ordered, full duplex, and flow controlled. TCP is designed to support a wide
range of upper-layer protocols (ULPs). The ULP can channel continuous streams of data
through TCP for delivery to peer ULPs. The TCP breaks the streams into portions that
are encapsulated together with appropriate addressing and control information to form a
segment--the unit of exchange between TCPs. In turn, the TCP passes the segments to
the network layer for transmission through the communication system to the peer TCP.
As shown in Figure 12.6, the layer below the TCP in the protocol hierarchy is com-
monly the IP layer. The IP layer provides a way for the TCP to send and receive
variable-length segments of information enclosed in Internet datagram "envelopes." The
Internet datagram provides a means for addressing source and destination TCPs in dif-
ferent networks. The IP also deals with fragmentation or reassembly of TCP segments
required to achieve transport and delivery through the multiple networks and intercon-
necting gateways (routers). The IP also carries information on the precedence, security
classification, and compartmentation of the TCP segments, so this information can be
communicated end-to-end across multiple networks.
12.3.4.2
TCP Mechanisms
. TCP builds its services on top of the network layer's
potentially unreliable services with mechanisms such as error detection, positive acknowl-
edgments, sequence numbers, and flow control. These mechanisms require certain address-
ing and control information to be initialized and maintained during data transfer. This
collection of information is called a TCP connection. The following paragraphs describe
the purpose and operation of the major TCP mechanisms.
Par Mechanism. TCP uses a positive acknowledgment with retransmission (PAR)
mechanism to recover from the loss of a segment by the lower layers. The strategy
with PAR is for sending a TCP to retransmit a segment at timed intervals until a posi-
tive acknowledgment is returned. The choice of retransmission interval affects efficiency.
An interval that is too long reduces data throughput while one that is too short floods
Summary :
12.3 TCP/IP AND RELATED PROTOCOLS 325 Figure 12.6 Protocol layers showing the relationship of TCP with other layered protocols. The TCP breaks the streams into portions that are encapsulated together with appropriate addressing and control information to form a segment--the unit of exchange between TCPs. In turn, the TCP passes the segments to the network layer for transmission through the communication system to the peer TCP.
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information,segments,layer,mechanisms,par,data,control,layers,through,protocols,networks,protocol,internet