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Network Working Group R. Kalin Request for Comments: 60 MIT

                                                          13 July 1970
                     A Simplified NCP Protocol


 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
 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.


 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
 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
 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 a

Kalin [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
 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 assignment

Kalin [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
 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 by RFC 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

Kalin [Page 7] RFC 60 A Simplified NCP Protocol 13 July 1970


 [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]

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