IMP-IMP and HOST-HOST Control Links
A proposal to segregate control traffic between network processors from regular Host communication.
Cerf, V., "IMP-IMP and HOST-HOST Control Links", RFC 18, DOI 10.17487/RFC0018, September 1969, <https://www.rfc-editor.org/info/rfc18>.
@misc{rfc18,
series = {Request for Comments},
number = 18,
howpublished = {RFC 18},
publisher = {RFC Editor},
doi = {10.17487/RFC0018},
url = {https://www.rfc-editor.org/info/rfc18},
author = {V. Cerf},
title = {{IMP-IMP and HOST-HOST Control Links}},
pagetotal = 1,
year = 1969,
month = sep,
} RFC text
Network Working Group University of California Los Angeles
Request for Comments: 18 Vint Cerf
Category: September 1969
It is suggested that link 1 be used for the HOST-HOST control link
and link 0 be used for IMP-IMP control.
This will facilitate communication between Hosts and reduce delays
due to interference with IMP-IMP communications.
[This RFC has been restored to on-line status by Elmar K. Bins]
AI ELI5
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§1Segregating Control Traffic
Vint Cerf proposes that the ARPANET reserve specific links for control communications. Link 0 would carry only IMP-to-IMP control messages, and link 1 would carry Host-to-Host control messages. Regular user data would use other links. This separation prevents control messages from being delayed when the network becomes congested with user traffic.
Like having a special highway for emergency vehicles so they do not get stuck in traffic jams.
§2Link Assignment
The proposal specifies two dedicated links: link 0 for IMP-to-IMP control, carrying messages about network state and routing decisions between the network processors themselves, and link 1 for Host-to-Host control, enabling direct communication between user systems without router involvement. This two-level separation addresses both inter-router coordination and user-level communication.
Think of it as a three-lane road: one lane just for trucks talking to trucks, one just for cars talking to cars, and all other lanes for regular traffic.
§3Lasting Principle
This early insight into network design—that control traffic and data traffic should be managed separately—became a fundamental principle of networking. Modern networks maintain separate control planes (where routing and management decisions happen) from data planes (where user traffic flows). This separation ensures that the network can manage itself even when congested or under attack.
The idea that control messages need special treatment became the foundation for how networks manage themselves when things get busy.