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Network Working Group                                           R. KalinRequest for Comments: 60                                             MIT                                                            13 July 1970A Simplified NCP ProtocolAbstract   This RFC defines a new NCP protocol that is simple enough to be   implemented on a very small computer, yet can be extended for   efficient operation on large timesharing machines. Because worst case   storage requirements can be predicted, a conservative implementation   can be freed of complicated resource allocation and storage control   procedures. A general error recovery procedure is also defined.Overview and Rational   The central premise of this proposal is an insistence that all user-   to-user connections be bi-directional. For those familiar with   communication theory, this appears most reasonable. All communication   requires a cyclical flow of information. To deny a simple association   between a message and its reply makes protocol unnecessarily   complicated and turns simple mechanisms of flow control into   nightmares.   It is proposed that a bi-directional connection, or duplex link, be   identified by a pair of socket numbers, one for each end. This is   half the number presently required. Associated with the connection   are some number of "crates" or message containers. These crates   travel back and forth over the link carrying network messages from   one side to the other. Buffers are allocated at each end of the link   to hold crates and the messages that they carry. Worst case buffer   requirements are equal to the number of crates in circulation, or the   "capacity" of the link.Details   A message buffer has four states which follow one another cyclically.   They are:   1) empty,   2) filled with a message-laden crate to be unloaded,   3) filled with an empty crate, and   4) filled with a message-laden crate to be sent.Kalin                                                           [Page 1]

RFC 60                  A Simplified NCP Protocol           13 July 1970   Normally state transitions correspond to message arrival, message   removal, message insertion and message transmission.   For a process to be an NCP it must:   1) be able to make initial contact with foreign hosts via the control   link and, if necessary, delete user-to-user links left over from the   previous system incarnation.   2) be able to create user-to-user links.   3) be able to interface users with these links.   4) be able to delete user-to-user links.   The first of the four functions shall not be discussed here except to   point out that it contains critical races that can not be resolved   without making assumptions about maximum message propagation delays.   Since within the ARPA network, bounds on message turnaround time do   not exist, the approach chosen must necessarily be tender. The other   three functions are discussed first from the viewpoint of one   interested in implementing a minimal NCP. Then extensions and   improvements are proposed that are suitable for larger machines.   Any NCP must be capable of creating a duplex link between a local   user process and a remote one. The current protocol accomplishes this   by queuing a potentially unbounded number of RFC's and waiting for   the user to examine the queue to determine with whom he wishes to   talk.  There is no guarantee that the user will ever look at the   queue and there is no way to limit the size of the queue. The   overflow error message suggested fails in the respect because it   admits that the RFC will only be sent again. The picture need not be   this bleak. The following network conversation demonstrates how   connections can be made without using queues or relying on user   process attention.   Suppose that a local user process and a remote user process wish to   establish a new connection. The remote process asks its NCP to listen   for a connection request and gives it the socket identifier for its   end. Optionally it can give both socket identifiers. The user process   at the local end asks its NCP to send a request for a duplex link   (RFDL). It specifies both socket identifiers of the proposed link.   The local NCP sends a RFDL over the control link with the following   format:   RFDL <my socket> <your socket> <max number buffers> <spare>Kalin                                                           [Page 2]

RFC 60                  A Simplified NCP Protocol           13 July 1970   The third argument is normally supplied by the local NCP and   indicates the maximum number of buffers the NCP will consider   allocating to this duplex link. If buffers are in user storage the   count may be given by the user in a call made to the NCP.   The RFDL is received at the remote host and the remote NCP compares   <my socket> and <your socket> against the socket identifiers supplied   by unmatched listens issued to it. For listens in which just a single   identifier was given only <your socket> must match. If both socket   identifiers were given, they both must match. If a match is found an   acknowledgement message with the following format is sent back by the   NCP:   ACDL <your socket> <my socket> <number buffers> <spare>   The <number buffers> parameter is equal to the smaller of <max number   buffers> as specified in the RFDL and the number of message buffers   agreeable to the remote NCP. If no match is found the error message   returned is an ACDL in which <number buffers> equals zero. Note that   the RFDL mechanism is similar to a RFC mechanism in which the bound   on queue size is one and connection acceptance is done entirely by   the NCP.   The two varieties of a listen correspond to two modes of channel   operation. The single parameter variety, as typified by a LOGIN   process, is to be used by programs that will "talk with anyone who   happens to dial their number". Screening of contacts for   appropriateness is left to the user process. The double parameter   listen is used by user programs who know with whom they will   communicate and do not wish to be bothered by random RFDL's from   other sources. Given the way in which socket name space is   partitioned, it is impossible to get a matching RFDL from any process   but the one intended.   Message buffers for the connection are allocated in the remote host   before it sends the ACDL and in the local host at the time the ACDL   is received. The number of buffers at each end is equal to the   <number buffers> parameter in the ACDL. The state of all remote   buffers is "empty" and of all local buffers "filled with empty   crate". After buffers are allocated the local user process is   notified that it is able to start sending messages.   The type of interface presented by the NCP between the user process   and the newly created duplex link is a decision local to that host. A   simple but complete interface would provide two calls to be made to   the NCP. GETMESSAGE would return the next message from the link   complete with marking, text and padding. PUTMESSAGE would take aKalin                                                           [Page 3]

RFC 60                  A Simplified NCP Protocol           13 July 1970   message, marking and text only, and buffer it for transmission. The   obvious logical errors would be reported.   We suggest that message alignment be left to the user. On most   machines it is a simple but time consuming operation. If done in the   NCP there is no guarantee that the user will not have to readjust it   himself. It is usually not possible to know a priori whether the text   portion should be right adjusted to a word boundary, left adjusted to   a word boundary, aligned to the end of the last message, or   fragmented in some exotic way.   Within this protocol message boundaries are used to provide storage   allocation information. If not required by the user this information   can be forgotten and the user interface can be made to appear as a   bit stream. Though welcomed by purists, such a strategy may produce   complications when attempting to synchronize both ends of a link.   Links are deleted by removing empty crates from them and reclaiming   the buffers allocated to the crates removed. Only buffers with crates   in can be reclaimed; empty buffers must remain available to receive   messages that may arrive. When no crates are left, no buffers remain,   and the socket identifiers can be forgotten. When empty crates are   removed, a decrement size message is sent to the foreign NCP to allow   it to reduce its buffer allocation:   DEC <my socket> <your socket> <number of buffers dropped>   A reply is solicited from the foreign NCP to affirm the deletions or   to complain of an error. Possible errors include "no such link" and   "impossible number of buffers dropped".   The option to close a link can be given to a user process by   providing either of two system calls. NOMOREOUTPUT declares that no   more messages will be sent by the local user process. All local   buffers for the link that contain empty crates are reclaimed by the   NCP. DEC messages are sent to the foreign NCP. As crates are emptied,   via GETMESSAGE calls, their buffers are reclaimed too. As an   alternative, the call KILLMESSAGE can be implemented. This call can   be used in place of a PUTMESSAGE. Instead of filling an empty crate   with a message to be sent, KILLMESSAGE will cause the crate to be   reclaimed and a DEC control message sent.   In situations where the user process has died, or for some other   reason can not close the link, more drastic measures must be taken.   For these situations, the ABEND control message is defined:   ABEND <my socket> <your socket>Kalin                                                           [Page 4]

RFC 60                  A Simplified NCP Protocol           13 July 1970   After sending an ABEND the issuing NCP starts to close the link. All   buffers containing input are destroyed. A DEC is issued for these and   the previously empty buffers. If messages arrive on the link, they   are destroyed and a DEC is issued. Any ABEND received for the link is   ignored.   When the remote NCP receives the ABEND, it stops sending messages   over the link and refuses new messages from the user process at its   end.  Empty buffers are reclaimed. Pending output messages are   destroyed and their buffers reclaimed. Input messages are fed to the   user process as long as it will accept them. Buffers are reclaimed as   input is accepted. DEC's are issued to cover all buffers reclaimed.   When the user process will take no more input, input messages are   destroyed and their buffers reclaimed. Eventually all buffers will be   reclaimed at both ends of the link. At such time the connection can   be considered closed and the socket numbers used can be reassigned   without ambiguity.   Under this proposed protocol the above four functions constitute all   that must be part of a network NCP. If buffers are allocated only   when free ones exist there can be no "overflow" errors nor is there   any need to place further constraints on message flow. For any user   message that arrives buffer room is guaranteed. All control messages   can be processed without requiring additional storage to be   allocated.  Attempts by a user process to issue too many listens can   be thwarted by local control procedures.   Inefficiencies in storage will result when the number of outstanding   connections gets large. One price of coding simplicity is a fifty   percent utilization of buffer space. On large hosts it may prove   advantageous to implement some of the following NCP extensions. With   more complicated flow control procedures, it becomes possible for an   NCP to allocate more buffer space than actually exists and still not   to get into trouble. Other extensions provide message compression,   improved throughput and user transparent error recovery.   Because some extensions require the cooperation of foreign hosts and   assume that they have implemented more than the minimal NCP it is   proposed that an inquiry control message be used to find out what   extensions the foreign host has implemented. The response to an INQ   will be a control message defining a host profile. If an "undefined   error" message is returned, the foreign host is assumed to have only   a minimal NCP.   A simple extension is to define a control message that will replace   user RFNM's. A user RFNM is a null text message sent, for example, as   a reply when a file is transferred via a duplex link. They are   inefficient since they tie up an entry in the IMP's link assignmentKalin                                                           [Page 5]

RFC 60                  A Simplified NCP Protocol           13 July 1970   table and degrade network throughput. A more efficient solution is to   send a special message over the control link. In this way one short   message can replace several user messages.   URFNM <my socket> <your socket> <number of user RFNM's>   Because the control link is concurrent with the return side of the   user link, URFNM's can not be substituted for user RFNM's when there   are other messages to be sent on the return link. Otherwise ordering   will be lost and with it user transparency.   Throughput can also be increased with a mechanism to add additional   crates on a duplex link. This might be at user instigation or be a   decision of the NCP.   INC <my socket> <your socket> <number buffers desired>   The foreign host replies to an increase request by returning an INCR.   INCR <my socket> <your socket> <number buffers to be added>   If the foreign NCP is unable to meet the additional buffer demand,   <number buffers to be added> will be less than <number buffers   desired> and possibly zero. The initial state of all local buffers   added is "filled with empty crate" and of all foreign buffers   "empty".   The spare argument in the RFDL and ACDL could be used to declare the   maximum sized message that will actually be sent in that direction. A   perceptive NCP could observe this information and allocate smaller   buffers. A lesser NCP could ignore it and always assume maximum   length messages. For example, if the field were zero then only user   RFNM's would be sent. A smart NCP would allocate no storage at all.   If the NCP retains a copy of each user message sent over the network   until a reply is returned, an automatic error recovery procedure can   be implemented. Because the capacity of the link is always known, an   NCP can determine whether there are messages in transit. This is done   by first sending a STOP message to the foreign NCP:   STOP <my socket> <your socket>   The STOP message tells the foreign NCP to temporarily stop   transmitting messages over the selected link. Unlike CEASE-ON-LINK   there is no guarantee as to how many messages will be sent before the   STOP takes effect. The local NCP then sends a link inquiry message:   LINQ <my socket> <your socket>Kalin                                                           [Page 6]

RFC 60                  A Simplified NCP Protocol           13 July 1970   The reply gives the number of crates at the foreign end of the link.   The LINQ message is repeated until this number plus the number of   local crates equals the capacity of the link. At this time no   messages are in transit and the two ends of the link have been   synchronized.  Messages can now be identified relative to the   synchronization point.  Thus the local NCP can send a control message   asking, for example, that the third to last message be retransmitted.   The foreign NCP is able to identify which message this is and to   retransmit it. Once all errors have been recovered a START control   message, similar in format to the STOP, is sent to the foreign NCP   and normal operation continues. The entire recovery procedure can be   transparent to both user processes.   It is expected that the larger hosts will not adhere strictly to the   worst case storage allocation requirements. Rather they will allocate   more buffers than they have and reply on statistics to keep them out   of trouble most of the time. Such conduct is perfectly permissible as   long as it is transparent to foreign hosts. The protocol allows an   NCP to lie about storage allocation as long as he is not caught. In   situations where detection appears imminent some of the following   control mechanisms will need to be applied. They are listed in   increasing order of power.   a) Do not send out any user RFNM's or other short messages. There is   a good chance that they will be replaced by longer messages that will   strain buffer capacity even more.   b) Try not to accept any new messages from the IMP. Block local   processes attempting to issue messages.   c) Issue DEC's to free up buffer space. Do not allocate more than one   buffer to RFDL's and refuse INC's.   d) Fake errors in messages waiting for local user action. Do this   only if the host that sent it has implemented error recovery. This   will free buffer space and allow you to recover later. This final   measure is admittedly a last resort, but it should be powerful enough   to control any emergency.   It is the hope of the author that the above protocol presents an   attractive alternative to that proposed byRFC 54 and its additions.   Although it appears at a late date, it should not be more than a   minor jolt to implementation efforts. It is simple enough to be   implemented quickly. If adopted, a majority of the present sites   could be talking intelligently with one another by the end of the   summer.Kalin                                                           [Page 7]

RFC 60                  A Simplified NCP Protocol           13 July 1970References   [1] Crocker, S.D., Postel, J., Newkirk, J. and Kraley, M., "Official   protocol proffering",RFC 54, June 1970.Author's Address   Richard Kalin   MIT Lincoln Laboratory         [ This RFC was put into machine readable form for entry ]           [ into the online RFC archives by Ian Redfern 4/97 ]Kalin                                                           [Page 8]