Network Working Group
Request for Comments: 381
NIC: 11151
J. McQuillan
D. Walden
Bolt Beranek and Newman Inc.
26 July 1972

Three Aids To Improved Network Operation

1. Scheduled Software Maintenance

As the ARPA Network has grown larger, we have found it difficult to find times when necessary new software can be slipped into the network without disrupting anyone. For instance, there is always intrasite traffic between the machines at MIT, and there is almost always traffic between the AMES TIP and IMP--the sun never sets on the ARPA Network. To minimize unscheduled disruptions and to simultaneously let us do what we have to do, we propose to schedule 7 A.M. - 8 A.M. eastern time every Tuesday as a time when the IMPs can be reloaded. We will probably not use this period every Tuesday, but we do reserve this period every Tuesday. The above period is in addition to the several hours a month already scheduled at each site for hardware preventative maintenance.

Because a network user may not know when his machine is scheduled for maintenance or because he may forget and work through the Tuesday morning software period, we propose to generalize the IMP-Going-Down IMP-to-Host control message so it may be used to remind the user. This message (described in detail below) will contain information that the IMP is going down in m times five minutes, for n times 5 minutes, for a given reason. Hosts (and the TIP) should use this information to remind all their Network users that the IMP will be going down after the stated interval.

Occasionally there is an emergency reason for restarting or reloading an IMP. For instance, while three Hosts at a site are functioning well, one Host cannot communicate with the IMP. This sort of situation sometimes requires the IMP to be restarted. Such a restart will be preceded by several minutes by an IMP-Going-Down Message to allow working users to save their work in such a way that they can restart once the IMP is back up.

In both of these cases, as well as cases where an IMP is performing so poorly that is must be shut down quickly, a type 2 IMP-to-HOST message will be transmitted to the HOST about 30 seconds before the IMP goes down. Finally, of course, there may be occasions when the IMP crashes so quickly that no warning is given, but the IMP will never be intentionally shut down in this way.

2. IMP-to-Host Communication

There have long been complaints that the IMP-to-Host error messages were not precise enough or were just plain ambiguous. In RFC #312 we proposed some additional error messages. These and other IMP-to-Host message changes will be made on August 14, 1972 and we encourage Hosts to modify their NCP's as appropriate by then. Unmodified NCPs will probably continue to work after this change, but each site should look into this question carefully. The table below lists all the IMP-to-Host messages and clearly indicates the changes which will be made.

   Type      Old Meaning             New Meaning
    0        Regular Messages        Same
    1        Error without           Error in Leader of Host-to-
             identification          IMP Message
                                          Bits 31,32=00 - IMP's
                                          error flip-flop set on
                                          the first 32 bits of a
                                          Host-to-IMP message which
                                          the IMP therefore cannot
                                     Bits 31,32=01 - Host-to-IMP
                                          message too short (less
                                          than 32 bits)
                                     Bits 31,32=10 - illegal
                                          Host-to-IMP code
    2       IMP Going Down           IMP Going Down
                                          Bits 17-32 coded as follows:
                                          All bits zero - going down in
                                          30 sec.
                                     Bits 17,18=01 - scheduled
                                          hardware PM
                                     Bits 17,18=10 - scheduled
                                          software reload
                                     Bits 17,18=11 - emergency
                                          reload or restart
                                     Bits 19-22 - how soon the
                                          IMP is going down - in
                                          5 minute units
                                     Bits 23-32 - how long the IMP
                                          will be down - in 5
                                          minute units
    3       Blocked Link             Unassigned
    4       NOF                      Same
    5       RFNM                     Same
    6       Link Table Full          Unassigned
    7       Destination Dead         Destination Dead
                                        Bit 32=0 - the destination
                                          IMP is dead, or cannot be
                                          reached, or does not exist
                                        Bit 32=1 - the destination
                                          Host is dead or does not
    8       Error with identi-       Error in Data of Host-to IMP
            fication                 Message
                                        IMP's error flip-flop set
                                        on the data bits of a Host-
                                        to-IMP message identified
                                        by the given source and link
    9       Incomplete Transmission  Incomplete Transmission
                                        Bits 31,32=00 - the destination
                                           Host did not take the message
                                           for a long time
                                        Bits 31,32=01 - Host-to-IMP
                                           message too long (more
                                           than 8095 bits)
                                        Bits 31,32=10 - Host-to IMP
                                           message too slow.  The
                                           last message took more
                                           than 15 secs. between
                                           the first bit and the
                                           last bit, and was discarded
                                        Bits 31,32=11 - Host-to-
                                           IMP message lost in the
   10       Unassigned                 IMP-Host Interface Reset
                                           The IMP's ready line has
                                           been dropped and pending
                                           output to the Host discarded
                                           (probably because the Host
                                           has not taken messages from
                                           the IMP for a long time).
                                           The IMP will return a type 1
                                           message of subtype 0 at the
                                           completion of the next Host-
                                           to-IMP message.

These changes can be summarized as follows:

  1. There is now one and only one IMP-to-Host message in response to each Host-to-IMP regular message.
  1. Message types 1, 2, 7 and 9 now carry additional information.
  1. Message type 10 has been added.
  1. Message types 3 and 6 have been discarded.

3. Network News Service

We have instituted a Network news service. TIP users get the news by typing the TIP command @NEWS. Users of other Host can get the news by ICPing to socket 15600031 (octal) at the BBN Tenex.

If you have further suggestions for improving the operation of the Network, we request your comments.

[ This RFC was put into machine readable form for entry ]

[ into the online RFC archives by Lorrie Shiota 08/00]