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In computer networks (including wireless networks), goodput is the application level throughput, i.e. the number of useful bits per unit of time forwarded by the network from a certain source address to a certain destination, excluding protocol overhead, and excluding retransmitted data packets.
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 generally 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 may cause lower goodput than throughput are:
- Protocol overhead; Typically, transport layer, network layer and sometimes datalink layer protocol overhead is included in the throughput, but is excluded from the goodput.
- Transport layer flow control and congestion avoidance, for example TCP slow start, may cause a lower goodput than the maximum throughput.
- Retransmission of lost or corrupt packets due to transport layer automatic repeat request (ARQ), caused by bit errors or packet dropping in congested switches and routers, is included in the datalink layer or network layer throughput but not in the goodput.
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 contiguous stream, instead it must be broken down into individual segments, called packets. These packets 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. Furthermore, the packets are transmitted over Ethernet in a frame which imposes a 38 byte overhead per packet. Given these overheads, the maximum goodput is 1460/1538 × 100 Mbit/s which is 94.92 megabits per second or 11.866 megabytes per second.
Note that this example doesn't consider some additional overhead, such as the space required between frames (a minimum of 96 bit times), nor collisions (which have a variable impact, depending on the network load).
