Why not just calculate a packet rate per second? Or am I missing something obvious?
No, that is a good question. One reason for including the packet size s is discussed in Section 5.3 of RFC 4342: "The packet size s is used in the TCP throughput equation. A CCID 3 implementation MAY calculate s as the segment size averaged over multiple round trip times -- for example, over the most recent four loss intervals, for loss intervals as defined in Section 6.1. Alternately, a CCID 3 implementation MAY use the Maximum Packet Size to derive s. In this case, s is set to the Maximum Segment Size (MSS), the maximum size in bytes for the data segment, not including the default DCCP and IP packet headers. Each packet transmitted then counts as one MSS, regardless of the actual segment size, and the TCP throughput equation can be interpreted as specifying the sending rate in packets per second." Thus, an implementation MAY calculate the allowed sending rate in bytes per second, using for s the average segment size. Or an implementation may use the MSS for s, and in fact calculate the allowed sending rate simply in packets per second. This would be a purely local implementation decision. - Sally http://www.icir.org/floyd/
