Serial link protocol design: A critique of the X.25 standard, level 2

There are certain technical design principles for link communication protocols which, if followed, result in a protocol that is less complex in both concept and implementation, but at the same time provides better service, than if the principles are not followed. These principles include modularization into sub-protocols, symmetry between the nodes on the link, and use of the state-exchange model of a conversation rather than the command-response model. The principles are described, the extent to which they are followed by the standard protocol X.25, level 2, is examined, and a protocol adhering to them is presented.