I saw that address and control fields are constant in PPP protocol. What is the need for even having a field that has constant value in it if we had not have this field then wouldn't it have saved some overhead?


1 Answer 1


The address and control fields were designed with future changes in mind, and other values "may be defined at a later time, or by prior agreement." (RFC 1662 "PPP in HDLC-like Framing", section 3.1)

You're absolutely right of course, this is overhead. The immediately following section 3.2 explains that they are able to be compressed:


When using the standard HDLC-like framing, the Address and Control fields contain the hexadecimal values 0xff and 0x03 respectively. When other Address or Control field values are in use, Address- and-Control-Field-Compression MUST NOT be negotiated.

On transmission, compressed Address and Control fields are simply omitted.

On reception, the Address and Control fields are decompressed by examining the first two octets. [details follow]

Address-and-Control-Field-Compression is an option which can be negotiated by the Link Control Protocol defined in RFC 1661 "The Point-to-Point Protocol" section 6.

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