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APPENDIX B
NCP Transport service
The ARPANET Host-to-Host Protocol [4] (implemented by the Network
Control Program) may be used in the ARPANET.
Connection Establishment
The SMTP transmission channel is established via NCP between
the sender process socket U and receiver process socket L. The
Initial Connection Protocol [5] is followed resulting in a pair
of simplex connections. This pair of connections is used as
the transmission channel. This protocol is assigned the
contact socket 25 (31 octal), that is L=25.
Data Transfer
The NCP data connections are established in 8-bit byte mode.
The SMTP data is 7-bit ASCII characters. Each character is
transmitted as an 8-bit byte with the high-order bit cleared to
zero.