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A Technical History of the ARPANET -
A Technical Tour
Host-to-IMP
Overview of Host-to-IMP protocols
Details of Host-to-IMP protocols
Overview of the Host-to-IMP Protocols
"With IMP-HOST protocol, a HOST has operating rules which permit it to send
messages to specified HOSTs on the ARPANET and to be informed of the dispensation
of those messages. In particular, the IMP-HOST protocol constrains HOSTs in
their transmissions so that they can make good use of available communications
capacity without denying such availability to other HOSTs" [Crocker 1972, "Function-oriented..."]
The Network Working Group (NWG) used a layered approach to the specification
of the communication protocols. [Completion Report, III-60] The higher layers, such as programs like telnet and FTP
would make use of the host-to-host layer, which would in turn make use of the
host-IMP layer. The hosts never see the IMP-IMP protocol implementation and
have no need to know about the inner workings of the IMP subnetwork. This layered
approach permits later development of ad hoc protocols at the top of the layered protocol
stack. [Completion Report, III-62] (See figure below)
The way the host and IMP communicated back-and-forth was considered the host-IMP
protocol. The host would pass its messages to its IMP that were
to be delivered to another host on the network. The destination IMP would then send
acknowledgements back to the host on the status of the messages. If a message
was successfully received by the destination host, the destination IMP would send a
RFNM (ready for next message) and if a
message was lost in the network the destination IMP would send an incomplete transmission
message. The destination IMP was responsible for the reassembly of the received
packets into the original message and sending that fully assembled message to
its host. An IMP was also able to block incoming messages from its
host for various reasons.
Explore the details of the Host-to-IMP protocols
- Protocol Data Unit - Variable length
messages with a max size of 8096 bits. This is the same as the
Host-to-Host message, with the exception of control messages
sent from the Host to its attached IMP.
- Addressing - A Host only has one IMP that it can communicate with,
the local IMP. Therefore, no addressing is required for Host to IMP
communication.
- Multiplexing - The message ID was used for multiplexing several
independent data streams.
- Fragmentation and Reassembly - None at this layer. All fragmentation
and reassembly occurred at the Source
IMP-to-Destination IMP layer.
- Routing - Not applicable. There is only one route between a local
Host and a local IMP.
- Communication - Details
- Error Detection/Recovery - Details
- Flow Control - Details
- Congestion Control - Details
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