A simple data link protocol for high-speed packet networks

Abstract

Many popular point-to-point data link protocols use the high-level data link control (HDLC) framing mechanism, which delineates protocol data units (PDUs) by means of a special bit pattern or flag. When such a flag occurs in the payload portion of a frame, an escape byte used to pad the transmitted byte stream enables the receiver to differentiate between a true framing flag and an occurrence of the flag pattern in the user information. The need to process each byte in the incoming byte stream to identify the flag pattern makes this frame delineation method increasingly more complex and expensive to implement as the interface speed increases. In addition, the byte stuffing operation performed when the flag pattern appears in the user information stream expands the carried traffic, interfering with quality of service (QoS) management procedures and making the link susceptible to malicious attacks. These factors limit the scalability and QoS management capabilities of the flag-based data link protocols, particularly at high link rates. The increasing popularity of the packet over SONET (POS) protocol stack makes it desirable to find alternative framing protocols. In this paper, we present a simple data link (SDL) protocol. Based on using a Length Indicator field and a header cyclic redundancy check (CRC), rather than a flag, to delineate frames, SDL is inherently scalable to high speeds and provides constant transmission overhead. We discuss the error control, delineation, and resynchronization mechanisms used by SDL, and we describe a link scrambler designed to protect against malicious attacks that attempt to generate very low bit transition density on the line. We also present additional link control functions designed to make SDL suitable for next-generation multiservice networks.