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Network Working Group                                     Jon Postel  (SRI-ARC)Request for Comments: 706                                              Nov 1975NIC #33861                    On the Junk Mail ProblemIn the ARPA Network Host/IMP interface protocol there is nomechanism for the Host to selectively refuse messages. This meansthat a Host which desires to receive some particular messages mustread all messages addressed to it. Such a Host could be sent manymessages by a malfunctioning Host. This would constitute a denial ofservice to the normal users of this Host. Both the local users andthe network communication could suffer. The services denied are theprocessor time consumed in examining the undesired messages andrejecting them, and the loss of network thruput or increased delaydue to the unnecessary busyness of the network.It would be useful for a Host to be able to decline messages fromsources it believes are misbehaving or are simply annoying. If theHost/IMP interface protocol allowed the Host to say to the IMP"refuse messages from Host X", the IMPs could discard the unwantedmessages at their earliest opportunity returning a "refused" noticeto the offending Host.How the IMPs might do this is an open issue -- here are twopossibilities:The destination IMP would keep a list (per local Host) of sourcesto refuse (this has the disadvantage of keeping the networkbusy).The destination IMP on receiving the "refuse messages from HostX" message forwards the message to the source IMP (the IMP localto Host X). That IMP keeps a list (per local Host) ofdestinations that are refusing messages from this source Host.This restriction on messages might be removed by a destination Hosteither by sending a "accept messages from Host X" message to theIMP, 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 measureexceeds a threshold then the Host could issue the "refuse messagesfrom Host X" message to the IMP.