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TCP Transport service
The Transmission Control Protocol  is used in the ARPA
Internet, and in any network following the US DoD standards for
The SMTP transmission channel is a TCP connection established
between the sender process port U and the receiver process port
L. This single full duplex connection is used as the
transmission channel. This protocol is assigned the service
port 25 (31 octal), that is L=25.
The TCP connection supports the transmission of 8-bit bytes.
The SMTP data is 7-bit ASCII characters. Each character is
transmitted as an 8-bit byte with the high-order bit cleared to