Protocol Overhead
On ATM/DSL (aka PPPoEoA): The amount of overhead added by PPPoEoA on the DSL side of things depends on the packet size because of (i) the absorbing effect of ATM cell-padding which completely cancels out overheads of PPPoEoA in some cases, (ii) the PPPoE overhead can cause an entire additional 53 byte ATM cell to be required, and (iii) IP fragmentation can be induced (which also invokes the first two effects). However ignoring ATM and IP fragmentation for the moment, the basic additional protocol header overheads for AAL5 payload are typically: 2 bytes (for PPP) + 6 (for PPPoE) + 18 (Ethernet framing, variable) + 10 (RFC 2684, variable) = 36 bytes. For very small packets this overhead is even greater because of Ethernet frame padding. However more realistically, the typical DSL/ATM overhead is either zero or 53 bytes as in case (ii) where the ATM cell payload maximum of 48 bytes is exceeded, so requiring an additional 53 byte cell. The 36 byte figure deduced earlier can be slightly reduced by various means: discarding Ethernet FCS loses another 4 bytes, for example, bringing the total down to 32. Compare this with a vastly more header-efficient protocol, PPPoA, with a fixed 10 byte overhead inside the AAL5 payload (ie. on the DSL side).
On Ethernet: On the Ethernet side of things, additional overhead for PPPoE over Ethernet is a fixed 2 + 6 = 8 bytes as before, unless IP fragmentation is produced.
Read more about this topic: Point-to-point Protocol Over Ethernet
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