A Technical History of the ARPANET -
A Technical Tour
Overview of Host-to-Host protocols
Details of Host-to-Host protocols
Overview of the Host-to-Host Protocols
The way that processes within the hosts communicated with one another through
the network was know as the "host to host protocol". The host to
host protocol was known as the second level protocol that resided above the
HOST-IMP level protocol. This protocol was implemented by the NCP (network
control program) which was part of a host's operating system. The NCP was
responsible for connection establishment, connection termination and flow
control. Over time the term "NCP" became synonymous with the
host to host protocol. The NCP became was the first transport layer
protocol of the ARPANET (TCP later succeeded it.). Links were used for
exchanging information about connections. Specific links were designated,
i.e. link 1 between each pair of hosts was originally designed as the control
link (later became link 0). The control link allowed the NCP to accomplish
its functions and every connection had one. Traffic (messages sent) on the
control link consisted only of control commands. Multiple commands were
allowed to be sent at one time. Another major task of the NCP was to break
down the interprocess communication into messages. These messages were
then sent to the NCP of the receiving host were it was also the job of the NCP
to put back together the messages into the original interprocess communication.
What was the motivation for the host to host protocol?
To provide for process-to-process communication over the
network. This became a requirement because users were running multiple
independent processes that wanted to use the network concurrently.
The expectation that typical process-to-process communication will be based, not
on a solitary message, but rather upon a sequence of messages. For
instance, processes might want to share large bodies of information, such as a
Explore the details of the Host-to-Host protocols
- Protocol Data Unit - Variable length
messages, with max size of 8096 bits.
- Addressing - Accomplished with
- Multiplexing - Coming soon.
- Fragmentation and Reassembly - The NCP was in charge of splitting
up the interprocess communications into data chunks called
messages. These messages were then handed over to the IMPs
where they were transported to the destination Host. The NCP
of the destination Host was then able to recombine the messages
to form the original interprocess communication.
- Connection Management - Details.
- Error Detection/Recovery - Details
- Flow Control - When the Host sends a
message on the network, a connection (link) is created. Only one
message can be transmitted on a given link at one time.
- Congestion Control - See congestion control
for the Host-to-IMP protocol.