A tribute to Jon Postel

Martin Cooper mjc at cooper.org.uk
Thu Dec 10 16:08:24 UTC 1998

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I was saddened to learn of the recent passing of Jon Postel, to whom
so many people owe so great a debt of gratitude for his contribution
to shaping the Internet which so many people use and earn their
living through today.

In pondering how I should remember him, I settled upon his
robustness principle, which is as equally good advice when
applied to life as to internetworking.  


1.3.2 Robustness Principle

   At every layer of the protocols, there is a general rule (from
   [TRANS:2] by Jon Postel) whose application can lead to enormous
   benefits in robustness and interoperability:

                      Be conservative in what you do,
                be liberal in what you accept from others.

Also, whilst browsing the Bibliography of the O'Reilly Sendmail V8
text recently, I was amused to find a reference to the following
early RFC, published in the year I was born, which illustrates Jon's
great vision wonderfully appropriately.


Network Working Group                                     Jon Postel  (SRI-ARC)
Request for Comments: 706                                              Nov 1975
NIC #33861

                    On the Junk Mail Problem

In the ARPA Network Host/IMP interface protocol there is no
mechanism for the Host to selectively refuse messages. This means
that a Host which desires to receive some particular messages must
read all messages addressed to it. Such a Host could be sent many
messages by a malfunctioning Host. This would constitute a denial of
service to the normal users of this Host. Both the local users and
the network communication could suffer. The services denied are the
processor time consumed in examining the undesired messages and
rejecting them, and the loss of network thruput or increased delay
due to the unnecessary busyness of the network.

It would be useful for a Host to be able to decline messages from
sources it believes are misbehaving or are simply annoying. If the
Host/IMP interface protocol allowed the Host to say to the IMP
"refuse messages from Host X", the IMPs could discard the unwanted
messages at their earliest opportunity returning a "refused" notice
to the offending Host.

How the IMPs might do this is an open issue -- here are two

The destination IMP would keep a list (per local Host) of sources
to refuse (this has the disadvantage of keeping the network

The destination IMP on receiving the "refuse messages from Host
X" message forwards the message to the source IMP (the IMP local
to Host X). That IMP keeps a list (per local Host) of
destinations that are refusing messages from this source Host.

This restriction on messages might be removed by a destination Host
either by sending a "accept messages from Host X" message to the
IMP, or by resetting its Host/IMP interface.

A Host might make use of such a facility by measuring, per source,
the number of undesired messages per unit time, if this measure
exceeds a threshold then the Host could issue the "refuse messages
from Host X" message to the IMP.


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