tcp-ip@ucbvax.ARPA (06/04/85)
From: Mike Brescia <brescia@bbnccv> - arpanet message size Your local arpanet/milnet imp will accept messages from your host up to 1008-octets-less-one-bit (8063 bits) beyond the AHIP (arpanet host-imp protocol) leader. For those sites which use interfaces that handle data in units of 2 octets (vax or pdp11 and lhdh interface), this sets a practical limit of 1006 bytes. - significance of 126 octets The arpanet imps handle data internally and forward them among themselves using buffers and packets of "1008 bits or 126 bytes". Also, messages 126 octets or shorter are treated differently in the imp end-to-end exchanges (exchanges between the imp connected to the source host and that at the destination). Briefly, the "single packet" messages (126 or shorter) are sent directly, while longer "multi-packet" messages are sent only after the source imp has sent a short allocation request to the destination imp and gotten a reply. This is one reason why a data measurement a few years ago showed a throughput peak at packet size of 126, then smaller throughput until the packet size was increased beyond 4*126 octets. The imp end-to-end algorithms are currently being redesigned to relax these allocation delays, allow more than 8 messages at a time between hosts or gateways (mentioned in some other notes), and remove some other causes of host blocking where the imp delays accepting data from the host.