ARPANET Technical Information: Packet Formats

Packet formats

There were two different kinds of packet formats used in the ARPANET: those used for "packets" sent between IMPs (these are more like 'frames' in modern parlance, since a single user "message" might be broken up into several "packets" for transmission through the network), and those used for packets (technically, "messages") sent between IMPs and their connected hosts.

IMP-IMP packets

Documentation on these is scanty, since they were internal to the IMP "subnetwork" (another word with a somewhat peculiar meaning back then: it means the collection of IMPs and point-point lines connecting them), and so BB&N was free to change them whenever it wanted to.

Here are two that I have found, though:

1970 - as documented in the 1970 FJCC paper

Circa 1977 - as documented in the 1977 Computer Networks paper

(Documentation of what these fields mean/do to be added later.)

Host->IMP and IMP->Host messages

To save space in the the headers, Host->IMP and IMP->Host messages have a slightly different (albeit basically identical) format: Host->IMP messages contain only a destination address, and IMP->Host messages have only a source address.

There were two versions of these headers:

Here are the short leader formats:


and the long leader formats:


(Documentation of what these fields mean/do to be added later.)

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Last updated: 15/May/2012