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:

The so-called "short leader" format, used from 1970 until roughly the end of 1975 (I don't know the exact date), although it continued to be supported after that date, but hosts using it could not reach all ofthe other hosts in the network at some point shortly thereafter.
The so-called "long leader" format, introduced around the end of 1975, which allowed i) more IMPs in the network, ii) more hosts per IMP, and iii) various other improvements.

Here are the short leader formats:

Host->IMP

IMP->Host

and the long leader formats:
IMP->Host

Host->IMP

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

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Last updated: 15/May/2012