Goodput

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 include:

• Protocol overhead: Typically, transport layer, network layer and sometimes data link 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 data link layer or network layer throughput but not in the goodput.

Example

For an application using the common Transmission Control Protocol (TCP) over IPv4 over Ethernet, the minimum protocol overhead consists of

• Ethernet physical layer: 20 bytes preamble, start frame delimiter and interpacket gap
• Ethernet data link layer: 18 bytes frame headers and frame check sequence
• IPv4 network layer: 20 bytes packet headers
• TCP transport layer: 20 bytes segment headers

for a total of 78 bytes: 38 bytes for Ethernet, and 40 bytes for IPv4 and TCP.

An IP packet over Ethernet may have a size of up to 1500 bytes – the maximum transmission unit for Ethernet. That means that a TCP segment can transport up 1500 - 40 = 1460 bytes, while Ethernet needs to transport 1500 + 38 = 1538 bytes. The overall efficiency is 1460 / 1538 = 94.9%. Accordingly, the maximum goodput for any application running over 100 Mbit/s Ethernet with TCP/IP is 94.9 Mbit/s or 11.9 MB/s. For Gigabit Ethernet, the same calculation results in 949 Mbit/s, or 119 MB/s.

When large objects or files (megabytes) are transmitted, the overhead of the application itself may be ignored since most protocols only add a single header of limited size for each object or file. When only small objects are transmitted, the overhead increases in proportion and becomes more significant.