The ARPANET Host-to-Host Protocol  (implemented by the Network Control Program) may be used in the ARPANET.
The SMTP transmission channel is established via NCP between the sender process socket U and receiver process socket L. The Initial Connection Protocol  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.
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.