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The quality of frame-based real-time services, such as voice-over-IP, would suffer as a consequence.Ģ. The non-deterministic inflation imposed by LAPS byte stuffing could make tightly controlled frame delay variation (with acceptable absolute delays) very difficult if not impossible to achieve. Service-Impacting Effects of Non-Deterministic Inflation This definitely invalidates the assumption of random data and skews the distribution towards the worst-case of a doubling of frame size. Malicious users can generate frames with a high density of octet values which must be 'escaped'. The inflation problem is aggravated by the possibility of emulation attacks. The assumption of random data could easily be invalidated by applications that happen to produce the escaped octet values more frequently than a random process would.
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The tail of this distribution is extremely long: it reaches zero only after a potential doubling of the frame size. Standard deviation is: s (sigma) = sqrt (npq) = sqrt (F*127) / 128 Mean of the distribution is: m ( mu) = np = F*(1/128) bytes Probability of 'failure': q = 1 - p = 127/128 M is the mean of the distribution of the number of bytes that must be escaped in the original F-byte frameĪssuming random data the number of bytes 'escaped' per F-byte frame follows a binomial distribution with: Where F is the un-escaped frame length in bytes Since two byte patterns are replaced by pairs of bytes, the probability that a random data byte will be 'escaped' is p = 1/128.įor a frame F bytes long prior to 'escaping', the average frame-length after 'escaping' will be: F' = F + m bytes Consequently, the frame length of the payload is inflated in a non-deterministic manner. Because of its special meaning, the 'escape' character must also be 'escaped'. This is done by replacing each occurrence of the flag pattern with a sequence of two bytes: a special 'escape' byte, followed by a slightly modified version of the flag pattern. To ensure the flag pattern is unique, any occurrences of it must be removed from the data prior to encapsulation. This is an important attribute, since it permits predictable performance.įor byte-oriented LAPS, frame delineation uses a simple flag mechanism: a unique one-byte pattern is used to detect both beginning and end of each frame. The throughput capacity is independent of the data contents. Deterministic vs Statistical ThroughputĪll currently defined Ethernet physical layers provide a deterministic throughput capacity. Three aspects of throughput are discussed: deterministic versus statistical behaviour, the effect of frame inflation on throughput, and the effect of delineation performance on throughput.ġ. This positive feedback mechanism makes frame loss performance an important parameter for packet-based traffic in general.
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The re-transmission requests will then generate more traffic. Any frames lost due to bit errors within the frame payload or due to loss of frame delineation will usually trigger a request for re-transmission at a higher layer. Packet based traffic generally requires received frames to be error-free. The following elaborates on these issues. The simplified LCP is laudable, but the similarities to PPP/HDLC/SDH mean that LAPS shares the same drawbacks in throughput performance and the service effects of flag/byte-stuffing delineation. Uses the 'Address' field to identify among IPv4, IPv6, etc.Uses a much-simpler version of Link Control Protocol (no 'Protocol' field, so no LCP frames only 2 states instead of "16 events, 12 actions, and 11 LCP frame formats").Supports only point-to-point Layer 2 topology (i.e.Uses the same byte-stuffing / flag pattern delineation mechanism.LAPS is a modified version of PPP with the following similarities: Some comments on LAPS (Draft X.86, April 2000) to get the ball rolling.