PPP General Frame Format
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PPP uses the HDLC basic framing structure, which includes two fields that are needed in HDLC but not in PPP due to how the latter operates: the Address and Control fields. Why bother sending two bytes that have the same value for every frame and aren't used for anything? Originally they were maintained for compatibility, but this reduces efficiency.
To avoid wasting two bytes in every frame, it is possible during initial link setup using LCP for the two devices on the link to negotiate a feature called Address and Control Field Compression (ACFC) using the LCP option by that same name. When enabled, this feature simply causes these two fields not to be sent for most PPP frames (but not LCP control frames.) In fact, the feature would be better named Address and Control Field Suppression because the fields are just suppressed: compressed down to nothing.
Now, even when devices agree to use field compression, they must still be capable of receiving both compressed and uncompressed frames. They differentiate one from the other by looking at the first two bytes after the initial Flag field. If they contain the value 0xFF03, they must be the Address and Control fields; otherwise, those fields were suppressed. (The value 0xFF03 is not a valid Protocol field value, so there is no chance of ambiguity.)
Similarly, it is also possible for the two devices on the link to negotiate compression of the Protocol field, so it takes only one byte instead of two. This is done generally by dropping the first byte if it is zero, a process called Protocol Field Compression (PFC). Recall that the first byte must be even and the second odd. Thus, a receiving device examines the evenness of the first byte of the Protocol field in each frame. If it is odd, this means that a leading byte of zeroes in the Protocol field has been suppressed, because the first byte of a full two-byte Protocol value must be even.
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