rfc675.txt

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Network Working Group                                        Vinton Cerf
Request for Comments: 675                                    Yogen Dalal
NIC: 2                                                     Carl Sunshine
INWG: 72                                                   December 1974


         SPECIFICATION OF INTERNET TRANSMISSION CONTROL PROGRAM

                         December 1974 Version


1.  INTRODUCTION

   This document describes the functions to be performed by the
   internetwork Transmission Control Program [TCP] and its interface to
   programs or users that require its services. Several basic
   assumptions are made about process to process communication and these
   are listed here without further justification. The interested reader
   is referred to [CEKA74, TOML74, BELS74, DALA74, SUNS74] for further
   discussion.

   The authors would like to acknowledge the contributions of R.
   Tomlinson (three way handshake and Initial Sequence Number
   Selection), D. Belsnes, J. Burchfiel, M. Galland, R. Kahn, D. Lloyd,
   W. Plummer, and J. Postel all of whose good ideas and counsel have
   had a beneficial effect (we hope) on this protocol design.  In the
   early phases of the design work, R. Metcalfe, A. McKenzie, H.
   Zimmerman, G. LeLann, and M. Elie were most helpful in explicating
   the various issues to be resolved. Of course, we remain responsible
   for the remaining errors and misstatements which no doubt lurk in the
   nooks and crannies of the text.

   Processes are viewed as the active elements of all HOST computers in
   a network. Even terminals and files or other I/O media are viewed as
   communicating through the use of processes. Thus, all network
   communication is viewed as inter-process communication.

   Since a process may need to distinguish among several communication
   streams between itself and another process [or processes], we imagine
   that each process may have a number of PORTs through which it
   communicates with the ports of other processes.

   Since port names are selected independently by each operating system,
   TCP, or user, they may not be unique. To provide for unique names at
   each TCP, we concatenate a NETWORK identifier, and a TCP identifier
   with a port name to create a SOCKET name which will be unique
   throughout all networks connected together.




Cerf, Dalal & Sunshine                                          [Page 1]

RFC 675              Specification of Internet TCP         December 1974


   A pair of sockets form a CONNECTION which can be used to carry data
   in either direction [i.e. full duplex]. The connection is uniquely
   identified by the <local socket, foreign socket> address pair, and
   the same local socket can participate in multiple connections to
   different foreign sockets [see Section 2.2].

   Processes exchange finite length LETTERS as a way of communicating;
   thus, letter boundaries are significant. However, the length of a
   letter may be such that it must be broken into FRAGMENTS before it
   can be transmitted to its destination. We assume that the fragments
   will normally be reassembled into a letter before being passed to the
   receiving process. Throughout this document, it is legitimate to
   assume that a fragment contains all or a part of a letter, but that a
   fragment never contains parts of more than one letter.

   We specifically assume that fragments are transmitted from Host to
   Host through means of a PACKET SWITCHING NETWORK [PSN] [ROWE70,
   POUZ73]. This assumption is probably unnecessary, since a circuit
   switched network could also be used, but for concreteness, we
   explicitly assume that the hosts are connected to one or more PACKET
   SWITCHES [PS] of a PSN [HEKA7O, POUZ74, SCWI71].

   Processes make use of the TCP by handing it letters. The TCP breaks
   these into fragments, if necessary, and then embeds each fragment in
   an INTERNETWORK PACKET. Each internetwork packet is in turn embedded
   in a LOCAL PACKET suitable for transmission from the host to one of
   its serving PS. The packet switches may perform further formatting or
   other operations to achieve the delivery of the local packet to the
   destination Host.

   The term LOCAL PACKET is used generically here to mean the formatted
   bit string exchanged between a host and a packet switch. The format
   of bit strings exchanged between the packet switches in a PSN will
   generally not be of concern to us. If an internetwork packet is
   destined for a TCP in a foreign PSN, the packet is routed to a
   GATEWAY which connects the origin PSN with an intermediate or the
   destination PSN. Routing of internetwork packets to the GATEWAY may
   be the responsibility of the source TCP or the local PSN, depending
   upon the PSN Implementation.

   One model of TCP operation is to imagine that there is a basic
   GATEWAY associated with each TCP which provides an interface to the
   local network. This basic GATEWAY performs routing and packet
   reformatting or embedding, and may also implement congestion and
   error control between the TCP and GATEWAYS at or intermediate to the
   destination TCP.





Cerf, Dalal & Sunshine                                          [Page 2]

RFC 675              Specification of Internet TCP         December 1974


   At a GATEWAY between networks, the internetwork packet is unwrapped
   from its local packet format and examined to determine through which
   network the internetwork packet should travel next. The internetwork
   packet is then wrapped in a local packet format suitable to the next
   network and passed on to a new packet switch.

   A GATEWAY is permitted to break up the fragment carried by an
   internetwork packet into smaller fragments if this is necessary for
   transmission through the next network. To do this, the GATEWAY
   produces a set of internetwork packets, each carrying a new fragment.
   The packet format is designed so that the destination TCP may treat
   fragments created by the source TCP or by intermediate GATEWAYS
   nearly identically.

   The TCP is responsible for regulating the flow of internetwork
   packets to and from the processes it serves, as a way of preventing
   its host from becoming saturated or overloaded with traffic. The TCP
   is also responsible for retransmitting unacknowledged packets, and
   for detecting duplicates. A consequence of this error
   detection/retransmission scheme is that the order of letters received
   on a given connection is also maintained [CEKA74, SUNS74]. To perform
   these functions, the TCP opens and closes connections between ports
   as described in Section 4.3. The TCP performs retransmission,
   duplicate detection, sequencing, and flow control on all
   communication among the processes it serves.

2.  The TCP INTERFACE to the USER

2.1  The TCP as a POST OFFICE

   The TCP acts in many ways like a postal service since it provides a
   way for processes to exchange letters with each other. It sometimes
   happens that a process may offer some service, but not know in
   advance what its correspondents' addresses are. The analogy can be
   drawn with a mail order house which opens a post office box which can
   accept mail from any source. Unlike the post box, however, once a
   letter from a particular correspondent arrives, a port becomes
   specific to the correspondent until the owner of the port declares
   otherwise.

   In addition to acting like a postal service, the TCP insures end-to-
   end acknowledgment, error correction, duplicate detection,
   sequencing, and flow control.








Cerf, Dalal & Sunshine                                          [Page 3]

RFC 675              Specification of Internet TCP         December 1974


2.2  Sockets and Addressing

   We have borrowed the term SOCKET from the ARPANET terminology
   [CACR70, MCKE73]. In general, a socket is the concatenation of a
   NETWORK identifier, TCP identifier, and PORT identifier. A CONNECTION
   is fully specified by the pair of SOCKETS at each end since the same
   local socket may participate in many connections to different foreign
   sockets.

   Once the connections is specified in the OPEN command [see section
   2.3.2], the TCP supplies a [short] Local Connection Name by which the
   user refers to the connection in subsequent commands. In particular
   this facilitates using connections with initially unspecified foreign
   sockets.

   TCP's are free to associate ports with processes however they choose.
   However, several basic concepts seem necessary in an implementation.
   There must be well known sockets [WKS] which the TCP associates only
   with the "appropriate" processes by some means. We envision that
   processes may "own" sockets, and that processes can only initiate
   connections on the sockets they own [means for implementing ownership
   is a local issue, but we envision a Request Port user call, or a
   method of uniquely allocating a group of ports to a given process,
   e.g. by associating the high order bits of a port name with a given
   process.]

   Once initiated, a connection may be passed to another process that
   does not own the local socket [e.g. from logger to service process].
   Strictly speaking this is a reconnection issue which might be more
   elegantly handled by a general reconnection protocol as discussed in
   section 3.3. To simplify passing a connection within a single TCP,
   such "invisible" switches may be allowed as in TENEX systems.

   Of course, each connection is associated with exactly one process,
   and any attempt to reference that connection by another process will
   be signaled as an error by the TCP. This prevents stealing data from
   or inserting data into another process' data stream.

   A connection is initiated by the rendezvous of an arriving
   internetwork packet and a waiting Transmission Control Block [TCB]
   created by a user OPEN, SEND, INTERPUPT, or RECEIVE call [see section
   2.3]. The matching of local and foreign socket identifiers determines
   when a successful connection has been initiated. The connection
   becomes established when sequence numbers have been synchronized in
   both directions as described in section 4.3.2.






Cerf, Dalal & Sunshine                                          [Page 4]

RFC 675              Specification of Internet TCP         December 1974


   It is possible to specify a socket only partially by setting the PORT
   identifier to zero or setting both the TCP and PORT identifiers to
   zero. A socket of all zero is called UNSPECIFIED. The purpose behind
   unspecified sockets is to provide a sort of "general delivery"
   facility [useful for logger type processes with well known sockets].

   There are bounds on the degree of unspecificity of socket
   identifiers. TCB's must have fully specified local sockets, although
   the foreign socket may be fully or partly unspecified. Arriving
   packets must have fully specified sockets.

   We employ the following notation:

    x.y.z = fully specified socket with x=net, y=TCP, z=port

    x.y.u = as above, but unspecified port

    x.u.u = as above, but unspecified TCP and port

    u.u.u = completely unspecified

    with respect to implementation, u = 0 [zero]

    We illustrate the principles of matching by giving all cases of
    incoming packets which match with existing TCB's. Generally, both
    the local (foreign) socket of the TCB and the foreign (local) socket
    of the packet must match.

          TCB local   TCB foreign     Packet local    Packet foreign

    (a)     a.b.c       e.f.g           e.f.g           a.b.c

    (b)     a.b.c       e.f.u           e.f.g           a.b.c

    (c)     a.b.c       e.u.u           e.f.g           a.b.c

    (d)     a.b.c       u.u.u           e.f.g           a.b.c

    There are no other legal combinations of socket identifiers which
    match. Case (d) is typical of the ARPANET well known socket idea in
    which the well known socket (a.b.c) LISTENS for a connection from
    any (u.u.u) socket. Cases (b) and (c) can be used to restrict
    matching to a particular TCP or net.








Cerf, Dalal & Sunshine                                          [Page 5]

RFC 675              Specification of Internet TCP         December 1974


2.3  TCP USER CALLS

2.3.1  A Note on Style

    The following sections functionally define the USER/TCP interface.
    The notation used is similar to most procedure or function calls in
    high level languages, but this usage is not meant to rule out trap
    type service calls [e.g. SVC's, UUO's, EMT's,...].

    The user calls described below specify the basic functions the TCP
    will perform to support interprocess communication. Individual
    implementations should define their own exact format, and may
    provide combinations or subsets of the basic functions in single
    calls. In particular, some implementations may wish to automatically
    OPEN a connection on the first SEND, RECEIVE, or INTERRUPT issued by
    the user for a given connection.

    In providing interprocess communication facilities, the TCP must not
    only accept commands, but also return information to the processes
    it serves. This communication consists of:

    (a) general information about a connection [interrupts, remote
        close, binding of unspecified foreign socket].

    (b) replies to specific user commands indicating success or various
        types of failure.

   Although the means for signaling user processes and the exact format
   of replies will vary from one implementation to another, it would
   promote common understanding and testing if a common set of codes
   were adopted. Such a set of Event Codes is described in section 2.4.

   With respect to error messages, references to "local" and "foreign"
   are ambiguous unless it is known whether these refer to the world as
   seen by the sender or receiver of the error message. The authors
   attempted several different approaches and finally settled on the
   convention that these references would be as seen by the receiver of
   the message.

2.3.2  OPEN CONNECTION

   Format: OPEN(local port, foreign socket [, timeout])

   We assume that the local TCP is aware of the identity of the
   processes it serves and will check the authority of the process to