Goodput

In computer networks, goodput is the application-level throughput (i.e. the number of useful information bits delivered by the network to a certain destination per unit of time). The amount of data considered excludes bits as well as retransmitted data packets. This is related to the amount of time from the first bit of the first packet sent (or delivered) until the last bit of the last packet is delivered.

For example, if a file is transferred, the goodput that the user experiences corresponds to the file size in bits divided by the file transfer time. The goodput is always lower than the throughput (the gross bit rate that is transferred physically), which generally is lower than network access connection speed (the channel capacity or bandwidth).

Examples of factors that cause lower goodput than throughput are:

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 IP over Ethernet, which is 1500 bytes. Each packet requires 20 bytes of header information and 20 bytes of header information, so only 1460 bytes are available per packet for the file transfer data itself (Linux and Mac OS X are further limited to 1448 bytes as they also carry a 12-byte time stamp). Furthermore, the data is 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), or 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.

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