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Independent Submission                                       A. McKenzie
Request for Comments: 6529                                    S. Crocker
Category: Historic                                            April 2012
ISSN: 2070-1721

                Host/Host Protocol for the ARPA Network


   This document reproduces the Host/Host Protocol developed by the ARPA
   Network Working Group during 1969, 1970, and 1971.  It describes a
   protocol used to manage communication between processes residing on
   independent Hosts.  It addresses issues of multiplexing multiple
   streams of communication (including addressing, flow control,
   connection establishment/disestablishment, and other signaling) over
   a single hardware interface.  It was the official protocol of the
   ARPA Network from January 1972 until the switch to TCP/IP in January
   1983.  It is offered as an RFC at this late date to help complete the
   historical record available through the RFC series.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for the historical record.

   This document defines a Historic Document for the Internet community.
   This is a contribution to the RFC Series, independent of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

Table of Contents

   1. Introduction ....................................................3
   2. A Few Comments on Nomenclature and Key Concepts .................4
   3. Host/Host Protocol Document .....................................5
      (with its own table of contents on page 7)
   4. Security Considerations ........................................34

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

1.  Introduction

   The Host/Host Protocol for the ARPA Network was created during 1969,
   1970, and 1971 by the Network Working Group, chaired by Steve
   Crocker, a graduate student at UCLA.  Many of the RFCs with numbers
   less than 72, plus RFCs 102, 107, 111, 124, 132, 154, and 179 dealt
   with the development of this protocol.  The first official document
   defining the protocol was issued by Crocker on August 3, 1970 as
   "Host-Host Protocol Document No. 1" (see citation in RFC 65), which
   was based on RFC 54 by Crocker, Postel, Newkirk, and Kraley.
   Revision of Document No. 1 began in mid-February 1971, as discussed
   in RFC 102.  Although McKenzie is listed as the author of the January
   1972 document, which superseded Document No. 1, it is more correct to
   say McKenzie was the person who compiled and edited the document.
   Most or all of the ideas in the document originated with others.

   At the time "Host-Host Protocol Document No. 1" was issued it was not
   given an RFC number because it was not to be viewed as a "request for
   comments" but as a standard for implementation.  It was one of a set
   of such standards maintained as a separate set of documentation by
   the Network Information Center (NIC) at Stanford Research Institute
   (SRI).  The January 1972 version (NIC 8246) reproduced here also
   followed that approach.  It has been noted by many that all
   subsequent standards were issued as RFCs, and the absence of the
   Host/Host Protocol specification from the RFC series creates a
   curious gap in the historical record.  It is to fill that gap that
   this RFC is offered.

   In 1972, most ARPA Network documents, RFCs and others, were prepared
   and distributed in hard copy.  The Host/Host Protocol document was
   typed on a typewriter (probably an IBM Selectric), which had
   interchangeable print elements, and used both italic and boldface
   fonts in addition to the regular font.  Diagrams were drawn by a
   graphic artist and pasted into the typed document.  Since RFCs are
   constrained to use a single typeface, we have tried to indicate
   boldface by the use of either all capitals or by a double underline,
   and to indicate italics by the use of underscores around words in
   place of spaces.  The resulting document is a bit more difficult to
   read, but preserves the emphases of the original.  Of course, the
   pagination has changed, and we hope we have correctly modified all of
   the page numbers.  There were three footnotes in the original
   document and we have moved these into the text, set off by
   indentation and square brackets.  A .pdf image of the original
   document can be found at

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

2.  A Few Comments on Nomenclature and Key Concepts

   In the protocol definition, "RFC" is used to mean "Request for
   Connection", which refers to either a "Sender to Receiver" or a
   "Receiver to Sender" request to initiate a connection.  In
   retrospect, this seems like an unnecessarily confusing choice of

   At the time this protocol was defined, it was given the
   undistinguished name "Host-Host Protocol."  The acronym "NCP" meant
   "Network Control Program" and referred to the code that had to be
   added to the operating system within each host to enable it to
   interact with its Interface Message Processor (IMP) and manage
   multiple connections.  Over time, and particularly in the context of
   the change from this protocol to TCP/IP, this protocol was commonly
   called "NCP" and the expansion changed to "Network Control Protocol."

   This protocol was superseded by TCP.  In this document, the protocol
   is referred to as a second layer (or "level") protocol, whereas in
   current writings TCP is usually referred to as a layer 4 protocol.
   When this protocol was created, it was expected that over time new
   layers would be created on top of, below, and even in between
   existing layers.

   This protocol used a separate channel (the control link) to manage
   connections.  This was abandoned in future protocols.

   In this design, there was no checksum or other form of error control
   except for the RST.  There had been in earlier versions, but it was
   removed at the insistence of the IMP designers who argued vigorously
   that the underlying network of IMPs would never lose a packet or
   deliver one with errors.  Although the IMP network was generally
   quite reliable, there were instances where the interface between the
   IMP and the host could drop bits, and, of course, experience with
   congestion control as the network was more heavily used made it clear
   that the host layer would have to deal with occasional losses in
   transmission.  These changes were built into TCP.

   Uncertainty about timing constraints in the design of protocols is
   evident in this document and remains a source of ambiguity,
   limitation, and error in today's design processes.

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

3.  Host/Host Protocol Document

                              Host/Host Protocol
                                   for the
                                 ARPA Network

   Prepared for the Network Working Group by
      Alex McKenzie
      January 1972

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012


   This document specifies a protocol for use in communication between
   Host computers on the ARPA Network.  In particular, it provides for
   connection of independent processes in different Hosts, control of
   the flow of data over established connections, and several ancillary
   functions.  Although basically self-contained, this document
   specifies only one of several ARPA Network protocols; all protocol
   specifications are collected in the document
   _Current_Network_Protocols,_ NIC #7104.

   This document supersedes NIC #7147 of the same title.  Principal
   differences between the documents include:

      - prohibition of spontaneous RET, ERP, and RRP commands
      - a discussion of the problem of unanswered CLS commands (page 16)
      - a discussion of the implications of queueing and not queueing
        RFCs (page 14)
      - the strong recommendation that received ERR commands be logged,
        and some additional ERR specifications.

   In addition to the above, several minor editorial changes have been

   Although there are many individuals associated with the network who
   are knowledgeable about protocol issues, individuals with questions
   pertaining to Network protocols should initially contact one of the

      Steve Crocker
      Advanced Research Projects Agency
      1400 Wilson Boulevard
      Arlington, Virginia 22209
      (202) 694-5921 or 5922

      Alex McKenzie
      Bolt Beranek and Newman Inc.
      50 Moulton Street
      Cambridge, Massachusetts 02133
      (617) 491-1350 ext. 441

      Jon Postel
      University of California at Los Angeles
      Computer Science Department
      3732 Boelter Hall
      Los Angeles, California 90024
      (213) 325-2363

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

                              TABLE OF CONTENTS

   I.    INTRODUCTION..................................................8
         An overview of the multi-leveled protocol structure in the ARPA

   II.   COMMUNICATION CONCEPTS.......................................10
         Definitions of terminology and a description of the overall
         strategy used in Host-to-Host communication.

   III.  NCP FUNCTIONS................................................13
         The meat of the document for the first-time reader.  Host-to-
         Host "commands" are introduced with descriptions of conditions
         of their use, discussion of possible problems, and other
         background material.

               Connection Establishment..........................13
               Connection Termination............................15
               Flow Control......................................17
               Test Inquiry......................................20

   IV.   DECLARATIVE SPECIFICATIONS...................................23
         Details for the NCP implementer.  A few additional "commands"
         are introduced, and those described in Section III are
         reviewed.  Formats and code and link assignments are specified.

               Message Format....................................23
               Link Assignment...................................25
               Control Messages..................................25
               Control Commands..................................25
               Opcode Assignment.................................31
               Control Command Summary...........................31

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

                             I.  INTRODUCTION

   The ARPA Network provides a capability for geographically separated
   computers, called Hosts, to communicate with each other.  The Host
   computers typically differ from one another in type, speed, word
   length, operating system, etc.  Each Host computer is connected into
   the network through a local small computer called an _Interface_
   _Message_Processor_(IMP)._  The complete network is formed by
   interconnecting these IMPs, all of which are virtually identical,
   through wideband communications lines supplied by the telephone
   company.  Each IMP is programmed to store and forward messages to the
   neighboring IMPs in the network.  During a typical operation, a Host
   passes a message to its local IMP; the first 32 bits of this message
   include the "network address" of a destination Host.  The message is
   passed from IMP to IMP through the Network until it finally arrives
   at the destination IMP, which in turn passes it along to the
   destination Host.

   Specifications for the physical and logical message transfer between
   a Host and its local IMP are contained in Bolt Beranek and Newman
   (BBN) Report No. 1822.  These specifications are generally called the
   _first_level_protocol_ or Host/IMP Protocol.  This protocol is not by
   itself, however, sufficient to specify meaningful communication
   between processes running in two dissimilar Hosts.  Rather, the
   processes must have some agreement as to the method of initiating
   communication, the interpretation of transmitted data, and so forth.
   Although it would be possible for such agreements to be reached by
   each pair of Hosts (or processes) interested in communication, a more
   general arrangement is desirable in order to minimize the amount of
   implementation necessary for Network-wide communication.
   Accordingly, the Host organizations formed a Network Working Group
   (NWG) to facilitate an exchange of ideas and to formulate additional
   specifications for Host-to-Host communications.

   The NWG has adopted a "layered" approach to the specification of
   communications protocol.  The inner layer is the Host/IMP protocol.
   The next layer specifies methods of establishing communications
   paths, managing buffer space at each end of a communications path,
   and providing a method of "interrupting" a communications path.  This
   protocol, which will be used by all higher-level protocols, is known
   as the _second_level_protocol,_ or Host/Host protocol.  (It is worth
   noting that, although the IMP sub-network provides a capability for
   _message_switching,_ the Host/Host protocol is based on the concept
   of _line_switching._)  Examples of further layers of protocol
   currently developed or anticipated include:

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

   1) An _Initial_Connection_Protocol_ (ICP) which provides a convenient
      standard method for several processes to gain simultaneous access
      to some specific process (such as the "logger") at another Host.

   2) A _Telecommunication_Network_ (TELNET) protocol which provides for
      the "mapping" of an arbitrary keyboard-printer terminal into a
      Network Virtual Terminal (NVT), to facilitate communication
      between a terminal user at one Host site and a terminal-serving
      process at some other site which "expects" to be connected to a
      (local) terminal logically different from the (remote) terminal
      actually in use.  The TELNET protocol specifies use of the ICP to
      establish the communication path between the terminal user and the
      terminal-service process.

   3) A _Data_Transfer_ protocol to specify standard methods of
      formatting data for shipment through the network.

   4) A _File_Transfer_ protocol to specify methods for reading,
      writing, and updating files stored at a remote Host.  The File
      Transfer protocol specifies that the actual transmission of data
      should be performed in accordance with the Data Transfer protocol.

   5) A _Graphics_ protocol to specify the means for exchanging graphics
      display information.

   6) A _Remote_Job_Service_ (RJS) protocol to specify methods for
      submitting input to, obtaining output from, and exercising control
      over Hosts which provide batch processing facilities.

   The remainder of this document describes and specifies the Host/Host,
   or second level, protocol as formulated by the Network Working Group.

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

                        II.  COMMUNICATION CONCEPTS

   The IMP sub-network imposes a number of physical restrictions on
   communications between Hosts; these restrictions are presented in BBN
   Report Number 1822.  In particular, the concepts of leaders,
   messages, padding, links, and message types are of interest to the
   design of Host/Host protocol.  The following discussion assumes that
   the reader is familiar with these concepts.

   Although there is little uniformity among the Hosts in either
   hardware or operating systems, the notion of multiprogramming
   dominates most of the systems.  These Hosts can each concurrently
   support several users, with each user running one or more processes.
   Many of these processes may want to use the network concurrently, and
   thus a fundamental requirement of the Host/Host protocol is to
   provide for process-to-process communication over the network.  Since
   the first level protocol only takes cognizance of Hosts, and since
   the several processes in execution within a Host are usually
   independent, it is necessary for the second level protocol to provide
   a richer addressing structure.

   Another factor which influenced the Host/Host protocol design is the
   expectation that typical process-to-process communication will be
   based, not on a solitary message, but rather upon a sequence of
   messages.  One example is the sending of a large body of information,
   such as a data base, from one process to another.  Another example is
   an interactive conversation between two processes, with many

   These considerations led to the introduction of the notions of
   connections, a Network Control Program, a "control link", "control
   commands", connection byte size, message headers, and sockets.

   A _connection_ is an extension of a link.  A connection couples two
   processes so that output from one process is input to the other.
   Connections are defined to be simplex (i.e., unidirectional), so two
   connections are necessary if a pair of processes are to converse in
   both directions.

   Processes within a Host are envisioned as communicating with the rest
   of the network through a _Network_Control_Program_ (NCP), resident in
   that Host, which implements the second level protocol.  The primary
   function of the NCP is to establish connections, break connections,
   and control data flow over the connections.  We will describe the NCP
   as though it were part of the operating system of a Host supporting
   multiprogramming, although the actual method of implementing the NCP
   may be different in some Hosts.

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

   In order to accomplish its tasks, the NCP of one Host must
   communicate with the NCPs of other Hosts.  To this end, a particular
   link between each pair of Hosts has been designated as the
   _control_link._  Messages transmitted over the control link are
   called _control_messages_*, and must always be interpreted by an NCP
   as a sequence of one or more _control_commands_.  For example, one
   kind of control command is used to initiate a connection, while
   another kind carries notification that a connection has been

      [*Note that in BBN Report Number 1822, messages of non-zero type
      are called control messages, and are used to control the flow of
      information between a Host and its IMP.  In this document, the
      term "control message" is used for a message of type zero
      transmitted over the control link.  The IMPs take no special
      notice of these messages.]

   The concept of a message, as used above, is an artifact of the IMP
   sub-network; network message boundaries may have little intrinsic
   meaning to communicating processes.  Accordingly, it has been decided
   that the NCP (rather than each transmitting process) should be
   responsible for segmenting interprocess communication into network
   messages.  Therefore, it is a principal of the second level protocol
   that no significance may be inferred from message boundaries by a
   receiving process.  _The_only_exception_to_this_principle_is_in_

   Since message boundaries are selected by the transmitting NCP, the
   receiving NCP must be prepared to concatenate successive messages
   from the network into a single (or differently divided) transmission
   for delivery to the receiving process.  The fact that Hosts have
   different word sizes means that a message from the network might end
   in the middle of a word at the receiving end, and thus the
   concatenation of the next message might require the receiving Host to
   carry out extensive bit-shifting.  Because bit-shifting is typically
   very costly in terms of computer processing time, the protocol
   includes the notions of connection byte size and message headers.

   As part of the process of establishing a connection, the processes
   involved must agree on a _connection_byte_size._  Each message sent
   over the connection must then contain an integral number of bytes of
   this size.  Thus the pair of processes involved in a connection can
   choose a mutually convenient byte size, for example, the least common
   multiple of their Host word lengths.  It is important to note that
   the ability to choose a byte size _must_ be available to the
   processes involved in the connection; an NCP is prohibited from
   imposing an arbitrary byte size on any process running in its own

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

   Host.  In particular, an outer layer of protocol may specify a byte
   size to be used by that protocol.  If some NCP is unable to handle
   that byte size, then the outer layer of protocol will not be
   implementable on that Host.

   The IMP sub-network requires that the first 32 bits of each message
   (called the leader) contain addressing information, including
   destination Host address and link number.  The second level protocol
   extends the required information at the beginning of each message to
   a total of 72 bits; these 72 bits are called the _message_header._  A
   length of 72 bits is chosen since most Hosts either can work
   conveniently with 8-bit units of data or have word lengths of 18 or
   36 bits; 72 is the least common multiple of these lengths.  Thus, the
   length chosen for the message header should reduce bit-shifting
   problems for many Hosts.  In addition to the leader, the message
   header includes a field giving the byte size used in the message, a
   field giving the number of bytes in the message, and "filler" fields.
   The format of the message header is fully described in Section IV.

   Another major concern of the second level protocol is a method for
   reference to processes in other Hosts.  Each Host has some internal
   scheme for naming processes, but these various schemes are typically
   different and may even be incompatible.  Since it is not practical to
   impose a common internal process naming scheme, a standard
   intermediate name space is used, with a separate portion of the name
   space allocated to each Host.  Each Host must have the ability to map
   internal process identifiers into its portion of this name space.

   The elements of the name space are called _sockets._  A socket forms
   one end of a connection, and a connection is fully specified by a
   pair of sockets.  A socket is identified by a Host number and a
   32-bit socket number.  The same 32-bit number in different Hosts
   represents different sockets.

   A socket is either a _receive_socket_ or a _send_socket,_ and is so
   marked by its low-order bit (0 = receive; 1 = send).  This property
   is called the socket's _gender._  The sockets at either end of a
   connection must be of opposite gender.  Except for the gender, second
   level protocol places no constraints on the assignment of socket
   numbers within a Host.

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

                            III.  NCP FUNCTIONS

   The functions of the NCP are to establish connections, terminate
   connections, control flow, transmit interrupts, and respond to test
   inquiries.  These functions are explained in this section, and
   control commands are introduced as needed.  In Section IV the formats
   of all control commands are presented together.

   Connection Establishment

   The commands used to establish a connection are STR (sender-to-
   receiver) and RTS (receiver- to-sender).

           8*         32               32           8
        | STR |   send socket  | receive socket | size |

      [*The number shown above each control command field is the length
      of that field in bits.]

           8          32               32           8
        | RTS | receive socket |  send socket   | link |

   The STR command is sent from a prospective sender to a prospective
   receiver, and the RTS from a prospective receiver to a prospective
   sender.  The send socket field names a socket local to the
   prospective sender; the receive socket field names a socket local to
   the prospective receiver.  In the STR command, the "size" field
   contains an unsigned binary number (in the range 1 to 255; zero is
   prohibited) specifying the byte size to be used for all messages over
   the connection.  In the RTS command, the "link" field specifies a
   link number; all messages over the connection must be sent over the
   link specified by this number.  These two commands are referred to as
   requests-for-connection (RFCs).  An STR and an RTS match if the
   receive socket fields match and the send socket fields match.  A
   connection is established when a matching pair of RFCs have been
   exchanged.  _Hosts_are_prohibited_from_establishing_more_than_one_

   With respect to a particular connection, the Host containing the send
   socket is called the _sending_Host_ and the Host containing the
   receive socket is called the _receiving_Host._  A Host may connect
   one of its receive sockets to one of its send sockets, thus becoming
   both the sending Host and the receiving Host for that connection.

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

   These terms apply only to data flow; control messages will, in
   general, be transmitted in both directions.

   A Host sends an RFC either to request a connection, or to accept a
   foreign Host's request.  Since RFC commands are used both for
   requesting and for accepting the establishment of a connection, it is
   possible for either of two cooperating processes to initiate
   connection establishment.  As a consequence, a family of processes
   may be created with connection-initiating actions built-in, and the
   processes within this family may be started up (in different Hosts)
   in arbitrary order provided that appropriate queueing is performed by
   the Hosts involved (see below).

   _There_is_no_prescribed_lifetime_for_an_RFC._  A Host is permitted to
   queue incoming RFCs and withhold a response for an arbitrarily long
   time, or, alternatively, to reject requests (see Connection
   Termination below) immediately if it does not have a matching RFC
   outstanding.  It may be reasonable, for example, for an NCP to queue
   an RFC that refers to some currently unused socket until a local
   process takes control of that socket number and tells the NCP to
   accept or reject the request.  Of course, the Host which sent the RFC
   may be unwilling to wait for an arbitrarily long time, so it may
   abort the request.  On the other hand, some NCP implementations may
   not include any space for queueing RFCs, and thus can be expected to
   reject RFCs unless the RFC sequence was initiated locally.


   The decision to queue, or not queue, incoming RFCs has important
   implications which NCP implementers must not ignore.  Each RFC which
   is queued, of course, requires a small amount of memory in the Host
   doing the queueing.  If each incoming RFC is queued until a local
   process seizes the local socket and accepts (or rejects) the RFC, but
   no local process ever seizes the socket, the RFC must be queued
   "forever."  Theoretically this could occur infinitely many times
   (there is no reason not to queue several RFCs for a single local
   socket, letting the local process decide which, if any, to accept)
   thus requiring infinite storage for the RFC queue.  On the other
   hand, if no queueing is performed the cooperating processes described
   above will be able to establish a desired connection only by accident
   (when they are started up such that one issues its RFC while the RFC
   of the other is in transit in the network -- clearly an unlikely

   Perhaps the most reasonable solution to the problems posed above is
   for _each_ NCP to give processes running in its own Host two options
   for attempting to initiate connections.  The first option would allow
   a process to cause an RFC to be sent to a specified remote socket;

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

   with the NCP notifying the process as to whether the RFC were
   accepted or rejected by the remote Host.  The second option would
   allow a process to tell _its_own_ NCP to "listen" for an RFC to a
   specified local socket from some remote socket (the process might
   also specify the particular remote socket and/or Host it wishes to
   communicate with) and to accept the RFC (i.e., return a matching RFC)
   if and when it arrives.  Note that this also involves queueing (of
   "listen" requests), but it is internal queueing which is susceptible
   to reasonable management by the local Host.  If this implementation
   were available, one of two cooperating processes could "listen" while
   the other process caused a series of RFCs to be sent to the
   "listening" socket until one was accepted.  Thus, no queueing of
   incoming RFCs would be required, although it would do no harm.

   _queueing_facility._  This is not, however, an absolute requirement
   of the protocol.

   Connection Termination

   The command used to terminate a connection is CLS (close).

           8        32            32
        | CLS |  my socket  | your socket |

   The "my socket" field contains the socket local to the sender of the
   CLS command.  The "your socket" field contains the socket local to
   the receiver of the CLS command.  _Each_side_must_send_and_receive_a_

   It is not necessary for a connection to be established (i.e., for
   _both_ RFCs to be exchanged) before connection termination begins.
   For example, if a Host wishes to refuse a request for connection, it
   sends back a CLS instead of a matching RFC.  The refusing Host then
   waits for the initiating Host to acknowledge the refusal by returning
   a CLS.  Similarly, if a Host wishes to abort its outstanding request
   for a connection, it sends a CLS command.  The foreign Host is
   obliged to acknowledge the CLS with its own CLS.  Note that even
   though the connection was never established, CLS commands must be
   exchanged before the sockets are free for other use.

   After a connection is established, CLS commands sent by the receiver

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

   and sender have slightly different effects.  CLS commands sent by the
   sender indicate that no more messages will be sent over the
   connection.  _This_command_must_not_be_sent_if_there_is_a_message_
   _in_transit_over_the_connection._  A CLS command sent by the receiver
   acts as a demand on the sender to terminate transmission.  However,
   since there is a delay in getting the CLS command to the sender, the
   receiver must expect more input.

   A Host should "quickly" acknowledge an incoming CLS so the foreign
   Host can purge its tables.  However, _there_is_no_prescribed_time_

   Because the CLS command is used both to initiate closing, aborting
   and refusing a connection, and to acknowledge closing, aborting and
   refusing a connection, race conditions can occur.  However, they do
   not lead to ambiguous or erroneous results, as illustrated in the
   following examples.

      EXAMPLE 1: Suppose that Host A sends Host B a request for
      connection, and then A sends a CLS to Host B because it is tired
      of waiting for a reply.  However, just when A sends its CLS to B,
      B sends a CLS to A to refuse the connection.  A will "believe" B
      is acknowledging the abort, and B will "believe" A is
      acknowledging its refusal, but the outcome will be correct.

      EXAMPLE 2: Suppose that Host A sends Host B an RFC followed by a
      CLS as in example 1.  In this case, however, B sends a matching
      RFC to A just when A sends its CLS.  Host A may "believe" that the
      RFC is an attempt (on the part of B) to establish a new connection
      or may understand the race condition; in either case it can
      discard the RFC since its socket is not yet free.  Host B will
      "believe" that the CLS is breaking an _established_ connection,
      but the outcome is correct since a matching CLS is the required
      response, and both A and B will then terminate the connection.

   Every NCP implementation is faced with the problem of what to do if a
   matching CLS is not returned "quickly" by a foreign Host (i.e., if
   the foreign Host appears to be violating protocol in this respect).
   One naive answer is to hold the connection in a partially closed
   state "forever" waiting for a matching CLS.  There are two
   difficulties with this solution.  First, the socket involved may be a
   "scarce resource" such as the "logger" socket specified by an Initial
   Connection Protocol (see NIC # 7101) which the local Host cannot
   afford to tie up indefinitely.  Second, a partially broken (or
   malicious) process in a foreign Host may send an unending stream of
   RFCs which the local Host wishes to refuse by sending CLS commands
   and waiting for a match.  This could, in worst cases, require 2^32 !
   socket pairs to be stored before duplicates began to appear.

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

   Clearly, no Host is prepared to store (or search) this much

   A second possibility sometimes suggested is for the Host which is
   waiting for matching CLS commands (Host A) to send a RST (see page
   20) to the offending Host (Host B), thus allowing all tables to be
   reinitialized at both ends.  This would be rather unsatisfactory to
   any user at Host A who happened to be performing useful work on Host
   B via network connections, since these connections would also be
   broken by the RST.

   Most implementers, recognizing these problems, have adopted some
   unofficial timeout period after which they "forget" a connection even
   if a matching CLS has not been received.  The danger with such an
   arrangement is that if a second connection between the same pair of
   sockets is later established, and a CLS finally arrives for the first
   connection, the second connection is likely to be closed.  This
   situation can only arise, however, if one Host violates protocol in
   two ways; first by failing to respond quickly to an incoming CLS, and
   second by permitting establishment of a connection involving a socket
   which it believes is already in use.  It has been suggested that the
   network adopt some standard timeout period, but the NWG has been
   unable to arrive at a period which is both short enough to be useful
   and long enough to be acceptable to every Host.  Timeout periods in
   current use seem to range between approximately one minute and
   approximately five minutes.  _It_must_be_emphasized_that_all_timeout_

   Flow Control

   After a connection is established, the sending Host sends messages
   over the agreed-upon link to the receiving Host.  The receiving NCP
   accepts messages from its IMP and queues them for its various
   processes.  Since it may happen that the messages arrive faster than
   they can be processed, some mechanism is required which permits the
   receiving Host to quench the flow from the sending Host.

   The flow control mechanism requires the receiving Host to allocate
   buffer space for each connection and to notify the sending Host of
   how much space is available.  The sending Host keeps track of how
   much room is available and never sends more data than it believes the
   receiving Host can accept.

   To implement this mechanism, the sending Host keeps two counters

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

   associated with each connection, a _message_counter_ and a
   _bit_counter._  Each counter is initialized to zero when the
   connection is established and is increased by allocate (ALL) control
   commands sent from the receiving Host as described below.  When
   sending a message, the NCP of the sending Host subtracts one from the
   message counter and the _text_length_ (defined below) from the bit
   counter.  The sender is prohibited from sending if either counter
   would be decremented below zero.  The sending Host may also return
   all or part of the message or bit space allocation with a return
   (RET) command upon receiving a give-back (GVB) command from the
   receiving Host (see below).

   The _text_length_ of a message is defined as the product of the
   connection byte size and the byte count for the message; both of
   these quantities appear in the message header.  Messages with a zero
   byte count, hence a zero text length, are specifically permitted.
   Messages with zero text length do not use bit space allocation, but
   do use message space allocation.  The flow control mechanisms do not
   pertain to the control link, since connections are never explicitly
   established over this link.

   The control command used to increase the sender's bit counter and
   message counter is ALL (allocate).

           8      8       16           32
        | ALL | link | msg space | bit space |

   This command is sent only from the receiving Host to the sending
   Host, and is legal only when a connection using the link number
   appearing in the "link" field is established.  The "msg space" field
   and the "bit space" field are defined to be unsigned binary integers
   specifying the amounts by which the sender's message counter and bit
   counter (respectively) are to be incremented.  The receiver is
   prohibited from incrementing the sender's counter above (2^16 - 1),
   or the sender's bit counter above (2^32 - 1).  In general, this rule
   will require the receiver to maintain counters which are incremented
   and decremented according to the same rules as the sender's counters.

   The receiving Host may request that the sending Host return all or
   part of its current allocation.  The control command for this request
   is GVB (give-back).

           8      8    8    8
        | GVB | link | fm | fb |

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

   This command is sent only from the receiving Host to the sending
   Host, and is legal only when a connection using the link number in
   the "link" field is established.  The fields fm and fb are defined as
   the fraction (in 128ths) of the current message space allocation and
   bit space allocation (respectively) to be returned.  If either of the
   fractions is equal to or greater than one, _all_ of the corresponding
   allocation must be returned.  Fractions are used since, with messages
   in transit, the sender and receiver may not agree on the actual
   allocation at every point in time.

   Upon receiving a GVB command, the sending Host must return _at_
   _least_* the requested portions of the message and bit space
   allocations.  (A sending Host is prohibited from spontaneously
   returning portions of the message and bit space allocations.)  The
   control command for performing this function is RET (return).

      [*In particular, fractional returns must be rounded up, not

           8      8       16           32
        | RET | link | msg space | bit space |

   This command is sent only from the sending Host to the receiving
   Host, and is legal only when a connection using the link number in
   the "link" field is established and a GVB command has been received
   from the receiving Host.  The "msg space" field and the "bit space"
   field are defined as unsigned binary integers specifying the amounts
   by which the sender's message counter and bit counter (respectively)
   have been decremented due to the RET activity (i.e., the amounts of
   message and bit space allocation being returned).  NCPs are obliged
   to answer a GVB with a RET "quickly"; however, there is _no_
   prescribed time period in which the answering RET must be sent.

   Some Hosts will allocate only as much space as they can guarantee for
   each link.  These Hosts will tend to use the GVB command only to
   reclaim space which is being filled very slowly or not at all.  Other
   Hosts will allocate more space than they have, so that they may use
   their space more efficiently.  Such a Host will then need to use the
   GVB command when the input over a particular link comes faster than
   it is being processed.


   The second level protocol has included a mechanism by which the
   transmission over a connection may be "interrupted." The meaning of

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

   the "interrupt" is not defined at this level, but is made available
   for use by outer layers of protocol.  The interrupt command sent from
   the receiving Host to the sending Host is INR (interrupt-by-

           8      8
        | INR | link |

   The interrupt command sent from the sending Host to the receiving
   Host is INS (interrupt-by-sender).

           8      8
        | INS | link |

   The INR and INS commands are legal only when a connection using the
   link number in the "link" field is established.

   Test Inquiry

   It may sometimes be useful for one Host to determine if some other
   Host is capable of carrying on network conversations.  The control
   command to be used for this purpose is ECO (echo).

           8      8
        | ECO | data |

   The "data" field may contain any bit configuration chosen by the Host
   sending the ECO.  Upon receiving an ECO command an NCP must respond
   by returning the data to the sender in an ERP (echo-reply) command.

           8      8
        | ERP | data |

   A Host should "quickly" respond (with an ERP command) to an incoming
   ECO command.  However, there is no prescribed time period, after the
   receipt of an ECO, in which the ERP must be returned.  A Host is
   prohibited from sending an ERP when no ECO has been received, or from
   sending an ECO to a Host while a previous ECO to that Host remains

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

   "unanswered."  Any of the following constitute an "answer" to an ECO:
   information from the local IMP that the ECO was discarded by the
   network (e.g., IMP/Host message type 7 - Destination Dead), ERP, RST,
   or RRP (see below).


   Occasionally, due to lost control messages, system "crashes", NCP
   errors, or other factors, communication between two NCPs will be
   disrupted.  One possible effect of any such disruption might be that
   neither of the involved NCPs could be sure that its stored
   information regarding connections with the other Host matched the
   information stored by the NCP of the other Host.  In this situation,
   an NCP may wish to reinitialize its tables and request that the other
   Host do likewise; for this purpose the protocol provides the pair of
   control commands RST (reset) and RRP (reset-reply).

        | RST |

        | RRP |

   The RST command is to be interpreted by the Host receiving it as a
   signal to purge its NCP tables of any entries which arose from
   communication with the Host which sent the RST.  The Host sending the
   RST should likewise purge its NCP tables of any entries which arise
   from communication with the Host to which the RST was sent.  The Host
   receiving the RST should acknowledge receipt by returning an RRP.
   _responding_to_RST)_until_the_second_Host_returns_an_RRP._  In fact,
   to avoid synchronization errors, the first Host _should_not_
   communicate with the second until the RST is answered.  Of course, if
   the IMP subnetwork returns a "Destination Dead" (type 7) message in
   response to the control message containing the RST, an RRP should not
   be expected.  If both NCPs decide to send RSTs at approximately the
   same time, then each Host will receive an RST and each must answer
   with an RRP, even though its own RST has not yet been answered.

   Some Hosts may choose to "broadcast" RSTs to the entire network when
   they "come up." One method of accomplishing this would be to send an

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

   RST command to each of the 256 possible Host addresses; the IMP
   subnetwork would return a "Destination Dead" (type 7) message for
   each non-existent Host, as well as for each Host actually "dead."

   Hosts are prohibited from sending an RRP when no RST has been
   received.  Further, Hosts may send only one RST in a single control
   message and should wait a "reasonable time" before sending another
   RST to the same Host.  Under these conditions, a single RRP
   constitutes an "answer" to _all_ RSTs sent to that Host, and any
   other RRPs arriving from that Host should be discarded.

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012


   Message Format

   All Host-to-Host messages (i.e., messages of type zero) shall have a
   header 72 bits long consisting of the following fields (see Figure

      Bits 1-32   Leader - The contents of this field must be
                  constructed according to the specifications contained
                  in BBN Report Number 1822.

      Bits 33-40  Field M1 - Must be zero.

      Bits 41-48  Field S - Connection byte size.  This size must be
                  identical to the byte size in the STR used in
                  establishing the connection.  If this message is being
                  transmitted over the control link the connection byte
                  size must be 8.

      Bits 49-64  Field C - Byte Count.  This field specifies the number
                  of bytes in the text portion of the message.  A zero
                  value in the C field is explicitly permitted.

      Bits 65-72  Field M2 - Must be zero.

   Following the header, the message shall consist of a text field of C
   bytes, where each byte is S bits in length.  Following the text there
   will be field M3 followed by padding.  The M3 field is zero or more
   bits long and must be all zero; this field may be used to fill out a
   message to a word boundary.

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

   |<---------------------------32 bits--------------------------->|
   |<----8 bits--->|<----8 bits--->|<-----------16 bits----------->|

   |                                                               |
   |                             LEADER                            |
   |                                                               |
   |               |               |                               |
   |    FIELD M1   |    FIELD S    |            FIELD C            |
   |               |               |                               |
   |               |               ^                               |
   |    FIELD M2   |               |                               |
   |               |               |                               |
   +---------------+               |                               |
   |                               |                               |
   |                               |                               |
   |                               |                               |
   |                               |                               |
   |                             TEXT                              |
   |                               |                               |
   |                               |                               |
   |                               |                               |
   |                               |                               |
   |                               |          +--------------------+
   |                               |          |                    |
   |                               |          |      FIELD M3      |
   |                               V          |                    |
   |                                   |
   |      10-----------------0         |<-------PADDING
   |                                   |

                               Figure 1

   The message header must, among other things, enable the NCP at the
   receiving Host to identify correctly the connection over which the
   message was sent.  Given a set of messages from Host A to Host B, the
   only field in the header under the control of the NCP at Host B is
   the link number (assigned via the RTS control command).  Therefore,
   each NCP must insure that, at a given point in time, for each
   connection for which it is the receiver, a unique link is assigned.
   Recall that the link is specified by the sender's address and the
   link number; thus a unique link number must be assigned to each
   connection to a given Host.

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

   Link Assignment

      Links are assigned as follows:

      Link number    Assignment
      ===========    ==========

      0              Control link

      2-71           Available for connections

      1, 72-190      Reserved - not for current use

      191            To be used only for measurement work under the
                     direction of the Network Measurement Center at UCLA

      192-255        Available for private experimental use.

   Control Messages

   Messages sent over the control link have the same format as other
   Host-to-Host messages.  The connection byte size (Field S in the
   message header) must be 8.  Control messages may not contain more
   than 120 bytes of text; thus the value of the byte count (Field C in
   the message header) must be less than or equal to 120.

   Control messages must contain an integral number of control commands.
   A single control command may not be split into parts which are
   transmitted in different control messages.

   Control Commands

   Each control command begins with an 8-bit _opcode._  These opcodes
   have values of 0, 1, ...  to permit table lookup upon receipt.
   Private experimental protocols should be tested using opcodes of 255,
   254, ...  Most of the control commands are more fully explained in
   Section III.

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

   NOP - No operation

        | NOP |

   The NOP command may be sent at any time and should be discarded by
   the receiver.  It may be useful for formatting control messages.

   RST - Reset

        | RST |

   The RST command is used by one Host to inform another that all
   information regarding previously existing connections, including
   partially terminated connections, between the two Hosts should be
   purged from the NCP tables of the Host receiving the RST.  Except for
   responding to RSTs, the Host which sent the RST is not obliged to
   communicate further with the other Host until an RRP is received in

   RRP - Reset reply

        | RRP |

   The RRP command must be sent in reply to an RST command.

   RTS - Request connection, receiver to sender

           8          32               32           8
        | RTS | receive socket |  send socket   | link |

   The RTS command is used to establish a connection and is sent from
   the Host containing the receive socket to the Host containing the
   send socket.  The link number for message transmission over the

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

   connection is assigned with this command; the "link" field must be
   between 2 and 71, inclusive.

   STR - Request connection, sender to receiver

           8          32               32           8
        | STR |   send socket  | receive socket | size |

   The STR command is used to establish a connection and is sent from
   the Host containing the send socket to the Host containing the
   receive socket.  The connection byte size is assigned with this
   command; the size must be between 1 and 255, inclusive.

   CLS - Close

           8        32            32
        | CLS |  my socket  | your socket |

   The CLS command is used to terminate a connection.  A connection need
   not be completely established before a CLS is sent.

   ALL - Allocate

           8      8       16           32
        | ALL | link | msg space | bit space |

   The ALL command is sent from a receiving Host to a sending Host to
   increase the sending Host's space counters.  This command may be sent
   only while the connection is established.  The receiving Host is
   prohibited from incrementing the Host's message counter above
   (2^16 - 1) or bit counter above (2^32 - 1).

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

   GVB - Give back

           8      8    8    8
        | GVB | link | fm | fb |
                       ^    ^
                       |    +--- bit fraction
                       +-------- message fraction

   The GVB command is sent from a receiving Host to a sending Host to
   request that the sending Host return all or part of its message space
   and/or bit space allocations.  The "fractions" specify what portion
   (in 128ths) of each allocation must be returned.  This command may be
   sent only while the connection is established.

   RET - Return

           8      8       16           32
        | RET | link | msg space | bit space |

   The RET command is sent from the sending Host to the receiving Host
   to return all or a part of its message space and/or bit space
   allocations in response to a GVB command.  This command may be sent
   only while the connection is established.

   INR - Interrupt by receiver

           8      8
        | INR | link |

   The INR command is sent from the receiving Host to the sending Host
   when the receiving process wants to interrupt the sending process.
   This command may be sent only while the connection is established.

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

   INS - Interrupt by sender

           8      8
        | INS | link |

   The INS command is sent from the sending Host to the receiving Host
   when the sending process wants to interrupt the receiving process.
   This command may be sent only while the connection is established.

   ECO - Echo request

           8      8
        | ECO | data |

   The ECO command is used only for test purposes.  The data field may
   be any bit configuration convenient to the Host sending the ECO

   ERP - Echo reply

           8      8
        | ERP | data |

   The ERP command must be sent in reply to an ECO command.  The data
   field must be identical to the data field in the incoming ECO

   ERR - Error detected

           8      8                    80
        +-----+------+---------------------------- ~ -------------+
        | ERR | code |                data                        |
        +-----+------+---------------------------- ~ -------------+

   The ERR command may be sent whenever a second level protocol error is
   detected in the input from another Host.  In the case that the error
   condition has a predefined error code, the "code" field specifies the
   specific error, and the data field gives parameters.  For other

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

   errors the code field is zero and the data field is idiosyncratic to
   the sender.  Implementers of Network Control Programs are expected to
   publish timely information on their ERR commands.

   The usefulness of the ERR command is compromised if it is merely
   discarded by the receiver.  Thus, sites are urged to record incoming
   ERRs if possible, and to investigate their cause in conjunction with
   the sending site.  The following codes are defined.  Additional codes
   may be defined later.

      a. Undefined (Error code = 0)
        The "data" field is idiosyncratic to the sender.

      b. Illegal opcode (Error code = 1)
        An illegal opcode was detected in a control message.  The "data"
        field contains the ten bytes of the control message beginning
        with the byte containing the illegal opcode.  If the remainder
        of the control message contains less than ten bytes, fill will
        be necessary; the value of the fill is zeros.

      c. Short parameter space (Error code = 2)
        The end of a control message was encountered before all the
        required parameters of the control command being decoded were
        found.  The "data" field contains the command in error; the
        value of any fill necessary is zeros.

      d. Bad parameters (Error code = 3)
        Erroneous parameters were found in a control command.  For
        example, two receive or two send sockets in an STR, RTS, or CLS;
        a link number outside the range 2 to 71 (inclusive); an ALL
        containing a space allocation too large.  The "data" field
        contains the command in error; the value of any fill necessary
        is zeros.

      e. Request on a non-existent socket (Error code = 4)
        A request other than STR or RTS was made for a socket (or link)
        for which no RFC has been transmitted in either direction.  This
        code is meant to indicate to the NCP receiving it that functions
        are being performed out of order.  The "data" field contains the
        command in error; the value of any fill necessary is zeros.

      f. Socket (link) not connected (Error code = 5)
        There are two cases:

        1.  A control command other than STR or RTS refers to a socket
           (or link) which is not part of an established connection.
           This code would be used when one RFC had been transmitted,
           but the matching RFC had not.  It is meant to indicate the

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

           failure of the NCP receiving it to wait for a response to an
           RFC.  The "data" field contains the command in error; the
           value of any fill necessary is zeros.

        2. A message was received over a link which is not currently
           being used for any connection.  The contents of the "data"
           field are the message header followed by the first eight bits
           of text (if any) or zeros.

   Opcode Assignment

   Opcodes are defined to be eight-bit unsigned binary numbers.  The
   values assigned to opcodes are:

      NOP = 0
      RTS = 1
      STR = 2
      CLS = 3
      ALL = 4
      GVB = 5
      RET = 6
      INR = 7
      INS = 8
      ECO = 9
      ERP = 10
      ERR = 11
      RST = 12
      RRP = 13

   Control Command Summary

        | NOP |

           8          32               32           8
        | RTS | receive socket |  send socket   | link |

           8          32               32           8
        | STR |   send socket  | receive socket | size |

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

           8        32            32
        | CLS |  my socket  | your socket |

           8      8       16           32
        | ALL | link | msg space | bit space |

           8      8    8    8
        | GVB | link | fm | fb |

           8      8       16           32
        | RET | link | msg space | bit space |

           8      8
        | INR | link |

           8      8
        | INS | link |

           8      8
        | ECO | data |

           8      8
        | ERP | data |

           8      8                    80
        +-----+------+---------------------------- ~ -------------+
        | ERR | code |                data                        |
        +-----+------+---------------------------- ~ -------------+

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

        | RST |

        | RRP |

   [ This is the end of the January 1972 document. ]

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RFC 6529         Host-Host Protocol for the ARPA Network      April 2012

4.  Security Considerations

   This document does not discuss any security considerations.

Authors' Addresses

   Alexander McKenzie
   PMB #4334, PO Box 2428
   Pensacola, FL 32513
   EMail: amckenzie3@yahoo.com

   Steve Crocker
   5110 Edgemoor Lane
   Bethesda, MD 20814
   EMail: steve@stevecrocker.com

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