    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.