The Size of Packets

Summary

The ISP Column A column on things Internet Other Formats: The Size of Packets October 2024 Geoff Huston We've now been running packet-switched networks for many decades, and these days it's packets and not virtual circuits lie behind most of the world's digital communications service. But some very fundamental questions remain unanswered in this packet-switched world. Perhaps the most basic questions is: "How big should a packet be?" And, surprisingly enough, there is no clear answer! The pragmatic default Internet answer these days is that an Internet packet is between 20 and 1,500 octets in size. Any bigger and the packet is likely to encounter packet fragmentation with its attendant issues of heightened risk of packet discard. Any smaller and the IP packet header is fatally truncated. So most hosts and applications stick inside the lanes and send packets within this size range. This was not always the case. In September 1981 RFC 791, the Internet Protocol Specification, was published. This specification had the advice that IP hosts must be prepared to accept IP packets of up to 576 octets (whether they arrive whole or in fragments). Packets larger than 576 octets were to be used only if the sending host had some assurance that the destination (and the active network elements along the packet’s forwarding path) were prepared to accept the larger datagrams. The document explains the rationale for this choice: "The number 576 is selected to allow a reasonable sized data block to be transmitted in addition to the required header information. For example, this size allows a data block of 512 octets plus 64 header octets to fit in a datagram. The maximal [IPv4] internet header is 60 octets, and a typical internet header is 20 octets, allowing a margin for headers of higher level protocols." Enter Ethernet The original work on a radically different form of high speed networking for local area networks occurred in the mid 1970’s, and the original published description, “...