Protocol experiment (RFC0700)
Original Publication Date: 1974-Aug-01
Included in the Prior Art Database: 2000-Sep-13
Internet Society Requests For Comment (RFCs)
E. Mader: AUTHOR [+3]
In early February, 1974 the main line printer on BBN's TENEX system failed and it was decided to use the PDP-11 line printer via the ARPANET both for the direct purpose of obtaining listings and also the indirect purpose of studying network protocols.
NWG/RFC 700 August 1974
INWG Experiments Note 1
A Protocol Experiment
Eric R. Mader
William W. Plummer
Raymond S. Tomlinson
In early February, 1974 the main line printer on BBN's TENEX system
failed and it was decided to use the PDP-11 line printer via the ARPANET
both for the direct purpose of obtaining listings and also the indirect
purpose of studying network protocols.
II. The Basic Protocol
The design was based on the protocol described by Cerf and Kahn in INWG
Note #39. Familiarity with that document is assumed. The following is
a brief sketch of the protocol. Not all features described in this
section have been implemented. See Section VI.
At any instant, the sender has two pointers into the stream of bytes to
be sent. Bytes to the left of the LEFT pointer have already been sent
and acknowledged. Bytes in the "window" between the LEFT and RIGHT
pointers have been sent (zero or more times), but no indication of
successful transmission has been received. Bytes to the right of RIGHT
remain to be considered at some time in the future.
In operation the sender is constantly sending bytes from the input data
stream resulting in the RIGHT pointer advancing. Positive
acknowledgements produced by the receiver cause the LEFT edge of the
window to move towards the RIGHT edge.
LEFT and RIGHT are actually numerical byte positions within the data
stream. The low order 16 bits of RIGHT are sent with each message as a
sequence number so that the receiver can identify which part of the data
stream it is receiving in case messages are not received in the same
order they were transmitted. The receiver has a finite amount of buffer
space available in which it can reassemble an image of the data in the
transmitter's window. The receiver discards any messages which have
sequence numbers outside of its buffer area. However, messages to the
left of LEFT must be acknowledged even though they are discarded.
Otherwise, a lost ACK would cause the sender to retransmit (and the
receiver ingore) the message indefinitely. Messages received with bad
checksums are also discarded.
As "good" messages are received, the holes are filled in the receiver's
buffer and continuous segments at the left edge are passed to the
physical line printer (in our case). The receiver informs the sender of
this action by sending an ACK (acknowledgement) message. This message
specifies the sequence number of the byte it would like to receive next
(the new value of LEFT in the sender) and the current amount of buffer
space it has available (new maximum window width in the sender). The
sender ignores ACK's to the left of LEFT and to the right of RIGHT.
Thus, both the sender and receiver are prepared to handle ...