Goodput - Example

Example

Imagine that a file is being transferred using HTTP over a switched Ethernet connection with a total channel capacity of 100 megabits per second. The file cannot be transferred over Ethernet as a single continuous stream; instead, it must be broken down into individual chunks. These chunks must be no larger than the maximum transmission unit of Ethernet, which is 1500 bytes. Each packet requires 20 bytes of IP header information and 20 bytes of TCP header information, so only 1460 bytes are available per packet for the file transfer data itself (Unix systems, Linux, and Mac OS X are further limited to 1448 bytes as they also carry a 12 bytes time stamp). Furthermore, the data are transmitted over Ethernet in a frame, which imposes a 26 byte overhead per packet. Given these overheads, the maximum goodput is 1460/1526 × 100 Mbit/s which is 95.67 megabits per second or 11.959 megabytes per second.

Note that this example doesn't consider some additional Ethernet overhead, such as the interframe gap (a minimum of 96 bit times), nor collisions (which have a variable impact, depending on the network load). TCP itself also adds the overhead of acknowledgements (which along with the round-trip delay time and the TCP window size in effect will rate-limit each individual TCP connection, see bandwidth-delay product). This example also does not consider the overhead of the HTTP protocol itself, which becomes relevant when transferring small files.