payloadlength

Payload length, often written as payloadlength, is a parameter used in networking to denote the size of the data carried by a protocol message, excluding the header. It is measured in bytes and indicates how many octets of payload follow the header fields in the same protocol data unit. The concept is central to framing, routing, and segmentation, because it informs parsing, validation, and reassembly.

In IPv6, the header includes a 16-bit Payload Length field. This value specifies the length, in octets, of the IPv6 payload following the IPv6 header. The field does not include the 40-byte IPv6 header. If extension headers are present, their combined length plus any encapsulated upper-layer payload are included in this value. The maximum payload length is 65535 octets. Values of zero are allowed and indicate that there is no IPv6 payload beyond the header chain.

Other protocols define payload length differently. In UDP, the Length field in the UDP header includes both the header (8 bytes) and the data, so the application data length equals UDP Length minus 8. TCP has no dedicated payload length field; the remaining data length is determined from the IP layer and the TCP segment boundaries.

Calculation and interpretation of payload length can affect fragmentation, MTU considerations, and error checking. Mismatches between advertised payload length and actual data can indicate corruption or tampering.

See also: IPv6, UDP, TCP, MTU, fragmentation.