/* Apologies to those who receive multiple copies of this owing to their presence on multiple mailing lists. */ 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 possibilities: The destination IMP would keep a list (per local Host) of sources to refuse (this has the disadvantage of keeping the network busy). 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. */