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Network Working Group                                       Jon PostelRFC # 690                                                   USC-ISINIC # 32699                                                 June 6, 1975Comments on the proposed Host/IMP Protocol ChangeThis is a set of comments on Dave Walden'sRFC 687 suggesting a set ofchanges to the host--imp protocol.  Dave's points are reproduced herewith my comments underneath.1. Expanded Leader Size.The leader will be expanded from two to five16-bit words.  This will provide space for necessary field expansionsand additions.    The existing protocols set the host header at 40 bits so that taken    together with the leader the length was 72 bits; a nice boundary for    both 8 bit and 36 bit machines.  This suggestion would result in a    prefix of 80 + 40 = 120 bits, not so nice (unless the host header is    extended to 64 bits for a total prefix of 144 bits).2. Expanded Address Field.The address field will be expanded to 24bit, 16 bits of IMP address and 8 bits of host address.  This expansionis more than adequate for any foreseeable ARPA Network growth.    Just a few years ago 256 seemed like a lot of hosts, perhaps, a    extensible scheme might be more appropriate. (I concede 16,777,216,    is big)3. New Message Length Field.A new field will be added which will allowthe source host to optionally specify the message length (in bits) tothe IMP subnetwork.  The IMP subnetwork may be able to use thisinformation (when available) to better utilize network buffer storage.The destination host may also be able to use this information to betterutilize its buffer storage.  This field will be 13 bits wide.    This sound very useful, but if we every want to have longer messages    than now the field should be wider, say 16 bits.4. Expanded Handling Type Field.The handling type field which now isused to distinguish between priority and non-priority message streams,etc., will be expanded to eight bits.  This expanded field will providefor the possibility of a number of parallel message streams havingdifferent handling characteristics between pairs of hosts; e.g.,priority, non-priority, varying numbers of packets per message (seebelow), unordered messages (i.e. the present type-3 messages), a messagestream requiring guaranteed capacity, etc, Note that only some of thesePostel                                                          [Page 1]

RFC 690    Comments on the proposed Host/IMP Protocol Change   June 1975facilities will be available in the near term.    This sounds like a good extension.5. Source Host Control of Packets per Message.The possibility willexist for the source host to specify a message stream which will use agiven number of packets per multi-packet message (e.g. two packets permessage or five packets per message).  Since the IMP network will nothave to use eight packet-buffers for reassembly purposes, as at present,this may result in better services for such messages.  This will helpusers who need both low delay and high throughput.    This seems strange, why not use the message length (as provided in 3    above) to determine the number of packets needed for this message.6. Unordered (type-3) Message Change.Unordered messages will beindicated by a handling type rather than by a message type as atpresent.  This is compatible with the need to check the host accesscontrol capabilities of all messages.  This will provide a slightbackward incompatibility for the three or so hosts which presently usetype-3 messages in their research.    Good, a current special case becomes a general facility.7. Change in Format of Fake Host Addresses.The For/From IMP bit willbe eliminated.  The fake host addresses will be the four highest hostnumbers (e.g. IMP Teletype will be host 252).    Another change for the better.8. Addition of a Parameter to the IMP to Host NOP.The IMP to host NOPwill have added to it a parameter specifying the address (IMP and hostnumber) of the host.    Ah, a clever touch, very handy.9. Backward Compatibility.The old and new formats will be supported inparallel in the IMPs for the foreseeable future to allow gradualphaseover of host software.   A host will be able to specify to its IMPwhether the old or new formats are to be used; thus, it will be possiblefor the host to specify switching back and forth between the two modesfor debugging purposes.  The specification of the mode to be used willbe possible via a proper choice of format in the host to IMP NOPmessage;  The IMP will use the mode of the Host to IMP NOP message theIMP has received.  Further, a host may select to use either the old ornew format without needing to know more about the other format messagethan to discard them should they arrive.  The IMP will initialize bysending several NOP messages of each type to give the hosts its choice.Postel                                                          [Page 2]

RFC 690    Comments on the proposed Host/IMP Protocol Change   June 1975Although a host not implementing the new format will not be able toaddress hosts on IMPs with IMP-number greater than 63, the IMPs willwherever possible do the conversion necessary to permit hosts using theold format to communicate with hosts using the new format and thereverse.  Finally, it will be possible to convert the leader format fromold to new or the reverse without knowledge of the message type.    This sounds difficult to implement, but it is all in the imp, so    fine.  Of course, something along these lines is crucial in an    operating environment.  But I am beginning to get concerned about    changes to host--host protocol and network control programs.[What happened to 10?]11. Non-blocking Host Interface.A mechanism will be provided whichallows the IMP to refuse a message from a host without blocking the hostinterface.  This mechanism will permit the IMP to gather the necessaryresources to send the refused message and then ask the host to resendthe message.  Finally, the host will be permitted to ask to be able tosend a message and be notified when it is possible without requiring themessage to actually be sent and refused.    This is another welcome addition.12. Maximum Message Length.The maximum number of bits of data in amessage may be reduced by a few bits.    I don't see why, but it doesn't matter much.On the whole a fine set of suggestion, though I am concerned aboutchanges to host--host protocol implied here or made more desirable bythese suggestions.  A rough guess is that there is easily a couple ofperson-months of system programmer time for each operating system on thenet implied here.  Say 24 systems times 2 person-months each equals 48person-months equals 4 person-years.  And this may be the lower bound.       [ This RFC was put into machine readable form for entry ]       [ into the online RFC archives by Alex McKenzie with    ]       [ support from GTE, formerly BBN Corp.            11/99 ]Postel                                                          [Page 3]