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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.
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.
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.
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.
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.
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 use the connection specified. Depending upon the implementation of the TCP, the source network and TCP identifiers will either be supplied by the TCP or by the processes that serve it [e.g. the
program which interfaces the TCP to its packet switch or the packet switch itself]. These considerations are the result of concern about security, to the extent that no TCP be able to masquerade as another one, and so on. Similarly, no process can masquerade as another without the collusion of the TCP. If no foreign socket is specified [i.e. the foreign socket parameter is 0 or not present], then this constitutes a LISTENING local socket which can accept communication from any foreign socket. Provision is also made for partial specification of foreign sockets as described in section 2.2. If the specified connection is already OPEN, an error is returned, otherwise a full-duplex transmission control block [TCB] is created and partially filled in with data from the OPEN command parameters. The TCB format is described in more detail in section 4.2.2. No network traffic is generated by the OPEN command. The first SEND or INTERRUPT by the local user or the foreign user will cause the TCP to synchronize the connection. The timeout, if present, permits the caller to set up a timeout for all letters transmitted on the connection. If a letter is not successfully transmitted within the timeout period, the user is notified and may ignore the condition [TCP will continue trying to transmit] or direct the TCP to close the connection. The present global default is 30 seconds, and connections which are set up without specifying another timeout will retransmit every letter for at least 30 seconds before notifying the user. The retransmission rate may vary, and is the responsibility of the TCP and not the user. Most likely, it will be related to the measured time for responses to return from letters sent. Depending on the TCP implementation, either a local connection name will be returned to the user by the TCP, or the user will specify this local connection name (in which case another parameter is needed in the call). The local connection name can then be used as a short hand term for the connection defined by the <local socket, foreign socket> pair. Responses from the TCP which may occur as a result of this call are detailed in section 2.4. 2.3.3 SEND LETTER Format: SEND(local connection name, buffer address, byte count, EOL flag [, timeout])
This call causes the data contained in the indicated user buffer to be sent on the indicated connection. If the connection has not been opened, the SEND is considered an error. Some implementations may allow users to SEND first, in which case an automatic OPEN would be done. If the calling process is not authorized to use this connection, an error is returned. If the EOL flag is set, the data is the End Of a Letter, and the EOL bit will be set in the last packet created from the buffer. If the EOL f1ag is not set, subsequent SEND's will appear as part of the same letter. This extended letter facility should be used sparingly because some TCP's may delay processing packets until an entire letter is received. If no foreign socket was specified in the OPEN, but the connection is established [e.g. because a listening connection has become specific due to a foreign letter arriving for the local port] then the designated letter is sent to the implied foreign socket. In general, users who make use of OPEN with an unspecified foreign socket can make use of SEND without ever explicitly knowing the foreign socket address. However, if a SEND is attempted before the foreign socket becomes specified, an error will be returned. Users can use the STATUS call to determine the status of the connection. In some implementations the TCP may notify the user when an unspecified socket is bound. If the timeout is specified, then the current default timeout for this connection is changed to the new one. This can affect not only all letters sent including and after this one, but also those which have not yet been sent, since the timeout is kept in the TCB and not associated with each letter sent. Of course, a time is maintained for each internetwork packet formed so as to determine how long each of these has been on the retransmission queue. In the simplest implementation, SEND would not return control to the sending process until either the transmission was complete or the timeout had been exceeded. This simple method is highly subject to deadlocks and is not recommended. [For example both sides of the connection try to do SEND's before doing any RECEIVE's.] A more sophisticated implementation would return immediately to allow the process to run concurrently with network I/O, and, furthermore, to allow multiple SENDs to be in progress concurrently. Multiple SENDs are served in first come, first served order, so the TCP will queue those it cannot service immediately.
NOTA BENE: In order for the process to distinguish among error or success indications for different letters, the buffer address should be returned along with the coded response to the SEND request. We will offer an example event code format in section 2.4, showing the information which should be returned to the calling process. The semantics of the INTERRUPT call are described later, but this call can have an effect on letters which have been given to the TCP but not yet sent. In particular, all such letters are flushed by the source TCP. Thus one of the responses to a SEND may be "flushed due to interrupt." Responses from the TCP which may occur as a result of this call are detailed in section 2.4. 2.3.4 RECEIVE LETTER Format: RECEIVE(local connection name, buffer address, byte count) This command allocates a receiving buffer associated with the specified connection. If no OPEN precedes this command or the calling process is not authorized to use this connection, an error is returned. In the simplest implementation, control would not return to the calling program until either a letter was received, or some error occurred, but this scheme is highly subject to deadlocks [see section 2.3.3]. A more sophisticated implementation would permit several RECEIVE's to be outstanding at once, These would be filled as letters arrive. This strategy permits increased throughput, at the cost of a more elaborate scheme [possibly asynchronous] to notify the calling program that a letter has been received. If insufficient buffer space is given to reassemble a complete letter, an indication that the buffer holds a partial letter will be given; the buffer will be filled with as much data as it can hold. The remaining parts of a partly delivered letter will be placed in buffers as they are made available via successive RECEIVES. If a number of RECEIVES are outstanding, they may be filled with parts of a single long letter or with at most one letter each. The event codes associated with each RECEIVE will indicate what is contained in the buffer. To distinguish among several outstanding RECEIVES, and to take care of the case that a letter is smaller than the buffer supplied, the event code is accompanied by both a buffer pointer and a byte count indicating the actual length of the letter received.
The semantics of the INTERRUPT system call are discussed later, but this call can have an effect on outstanding RECEIVES. When the TCP receives an INTERRUPT, it will flush all data currently queued up awaiting receipt by the receiving process. If no data is waiting, but several buffers have been made available by anticipatory RECEIVE commands, these buffers are returned to the process with an error indicating that any data that might have been placed in those buffers has been flushed. This enables the receiving process to synchronize its RECEIVES with the interrupt. That is, the process can distinguish between RECEIVES issued before the receipt of the INTERRUPT and these issued afterwards. Responses from the TCP which may occur as a result of this call are detailed in section 2.4. 2.3.5 CLOSE CONNECTION Format: CLOSE(local connection name) This command causes the connection specified to be closed. If the connection is not open or the calling process is not authorized to use this connection, an error is returned. Any unfilled receive buffers or pending send buffers will be returned to the user with event codes indicating they were aborted due to the CLOSE. Users should wait for event codes for each SEND before closing the connection if they wish to be certain that all letters were successfully delivered. The user may CLOSE the connection at any time on his own initiative, or in response to various prompts from the TCP [remote close executed, transmission timeout exceeded, destination inaccessible]. Because closing a connection requires communication with the foreign TCP, connections may remain in the closing state for a short time. Attempts to reopen the connection before the TCP replies to the CLOSE command will result in errors. Responses from the TCP which may occur as a result of this call are detailed in section 2.4. 2.3.6 INTERRUPT Format: INTERRUPT(local connection name) A special control signal is sent to the destination indicating an interrupt condition. This facility can be used to simulate "break" signals from terminals or error or completion codes from I/O devices, for example. The semantics of this signal to the receiving process
are unspecified. The receiving TCP will signal the interrupt to the receiving process immediately upon receipt, and will also flush any outstanding letters waiting to be delivered. Since it is possib1e to tell where in the letter stream this command was invoked, it is possible for the receiving TCP to flush only preceding data. The sending TCP will flush any letters pending transmission, returning a special error code to indicate the flush. If the connection is not open or the calling process is not authorized to use this connection, an error is returned. Responses from the TCP which may occur as a result of this call are detailed in section 2.4. 2.3.7 STATUS Format: STATUS(local connection name) This command returns a data block containing the following information: local socket, foreign socket, local connection name, receive window, send window, connection state, number of letters awaiting acknowledgment, number of letters pending receipt [including partial ones], default transmission timeout Depending on the state of the connection, some of this information may not be available or meaningful. If the calling process is not authorized to use this connection, an error is returned. This prevents unauthorized processes from gaining information about a connection. Responses from the TCP which may occur as a result of this call are detailed in section 2.4. 2.4 TCP TO USER MESSAGES 2.4.1 TYPE CODES All messages include a type code which identifies the type of user call to which the message applies. Types are: 0 - General message, does not apply to a particular user call 1 - Applies to OPEN 2 - Applies to CLOSE
3 - Applies to INTERRUPT 10 - Applies to SEND 20 - Applies to RECEIVE 30 - Applies to STATUS 2.4.2 MESSAGE FORMAT [notional] All messages include the following three fields: Type code Local connection name Event code For message types 0-3 [General, Open, Close, Interrupt] only these three fields are necessary. For message type 10 [Send] one additional field is necessary: Buffer address For message type 20 [Receive] three additional fields are necessary: Buffer address Byte count End-of-letter flag For message type 30 [status] additional data might include; Local socket, foreign socket Send window [measures buffer space at foreign TCP] Receive window [measures buffer space at local TCP] Connection state [see section 4.3.6] Number of letters awaiting acknowledgment Number of letters awaiting receipt Retransmission timeout
2.4.3 EVENT CODES The event code specifies the particular event that the TCP wishes to communicate to the user. In addition to the event code, three flags may be useful to classify the event into major categories and facilitate event processing by the user: E flag: set if event is an error L/F flag: indicates whether event was generated by Local TCP, or Foreign TCP or network P/T flag: indicates whether the event is Permanent or Temporary [retry may succeed] Events are encoded into 8 bits with the high order bits set to indicate the state of the E, L/F, and P/T flags, respectively. Events specified so far are listed below with their codes and flag settings. A * means a flag does not apply or can take both values for this event. Additional events may be defined in the course of experimentation. 0 0** general success 1 ELP connection illegal for this process 2 OF* unspecified foreign socket has become bound 3 ELP connection not open 4 ELT no room for TCB 5 ELT foreign socket unspecified 6 ELP connection already open EFP unacceptable SYN [or SYN/ACK] arrived at foreign TCP. Note: This is not a misprint, the local meaning is different from foreign. 7 EFP connection does not exist at foreign TCP 8 EFT foreign TCP inaccessible [may have subcases] 9 ELT retransmission timeout
10 E*P buffer flushed due to interrupt 11 OF* interrupt to user 12 **P connection closing 13 E** general error 14 E*P connection reset Possible events for each message type are as follows: Type 0[general]: 2,11,12,14 Type 1[open]: 0,1,4,6,13 Type 2[close]: 0,1,3,13 Type 3[interrupt]: 0,1,3,5,7,8,9,12,13 Type 10[send]: 0,1,3,5,7,8,9,10,11,12,13 Type 20[receive]: 0,1,3,10,12,13 Type 30[status]: 0,1,13 Note that events 6(foreign), 7, 8 are generated at the foreign TCP or in the network[s], and these same codes are used in the error field of the internet packet [see section 4.2.1]. 3. HIGHER LEVEL PROTOCOLS 3.1 INTRODUCTION It is envisioned that the TCP will be able to support higher level protocols efficiently. It should be easy to interface existing ARPANET protocols like TELNET and FTP to the TCP. 3.2 WELL KNOWN SOCKETS At some point, a set of well known 24 bit port numbers must be picked. The type of service associated with the well known ports might include: (a) Logger (b) FTP (File transfer protocol)
(c) RJE (Remote job entry) (d) Host status (e) TTY Test (f) HELP - descriptive, interactive system documentation WE RESERVE WELL KNOWN SOCKET 0 (24 bits of 0) for global messages destined for a particular TCP but not related to any particular connection. We imagine that this socket would be used for unusual TCP synchronization (e.g. RESET ALL) or for testing purposes (e.g. sending letters to TRASHCAN or ECHO). This does not conflict with the usage that if a socket is 0, it is unspecified, since no user can SEND, CLOSE, or INTERRUPT on socket 0. 3.3 RECONNECTION PROTOCOL (RCP) Port identifiers fall into two categories: permanent and transient. For example, a Logger process is generally assigned a port identifier that is fixed and well known. Transient processes will in general have ID's which are dynamically assigned. In the distributed processing environment of the network, two processes that don't have well known port identifiers may often wish to communicate. This can be achieved with the help of a well known process using a reconnection protocol. Such a protocol is briefly outlined using the communication facilities provided by the TCP. It essentially provides a mechanism by which port identifiers are exchanged in order to establish a connection between a pair of sockets. Such a protoco1 can be used to achieve the dynamic establishment of new connections in order to have multiple processes solving a problem cooperatively, or to provide a user process access to a server process via a logger, when the logger's end of the connection can not be invisibly passed to the server process. A paper on this subject by R. Schantz [SCHA74] discusses some of the issues associated with reconnection, and some of the ideas contained therein went into the design of the protocol outlined below. In the ARPANET, a protocol was implemented which would allow a process to connect to a well known socket, thus making an implicit request for service, and then be switched to another socket so that the well known socket could be freed for use by others. Since sockets
in our TCP are permitted to have connections with more than one foreign socket, this facility may not be explicitly needed (i.e. connections <A,B> and <A,C> are distinguishable). However. the well known socket may be in one network and the actual service socket(s) may be in another network (or at least in another TCP). Thus, the invisible switching of a connection from one port to another within a TCP may not be sufficient as an "Initial Connection Protocol". We imagine that a process wishes to use socket N1.T1.Q to access well known socket N2.T2.P. However, the process associated with socket N2.T2.P will actually start up a new process somewhere which will use N3.T3.S as its server socket. The N(i) and T(i) may be distinct or the same. The user will send to N2.T2.P the relevant user information such as user name, password, and account. The server will start up the server process and send to N1.T1.Q the actual service socket ldentif1er: N3.T3.S. The connection (N1.TI.Q,N2.T2.P) can then be closed, and the user can do a RECEIVE on (N1.T1.Q,N3.T3.S). The serving process can SEND on (N3.T3.S,N1.T1.Q). There are many variations on this scheme, some involving the user process doing a RECEIVE on a different socket (e.g. (N1.T1.X,U.U.U)) with the server doing SEND on (N3.T3.S,N1.T1.X). Without showing all the detail of synchronization of sequence numbers and the like, we can illustrate the exchange as shown below. USER SERVER 1. RECEIVE(N2.T2.P,U.U.U) 1. SEND (N1.T1.Q,N2.T2.P)==> <== 2. SEND(N2.T2.P,N1.T1.Q) With "N3.T3.S" as data 2. RECEIVE(N1.T1.Q,N2.T2.P) 3. CLOSE(N1.T1.Q,N2.T2.P)==> <:= 3. CLOSE(N2.T2.P,N1.T1.Q) 4. RECEIVE(N1.T1.Q,N3.T3.S) <== 4. SEND(N3.T3.S,N1.T1.Q) At this point, a connection is open between N1.T1.Q and N3.T3.S. A variation might be to have the user do an extra RECEIVE on (N1.T1.X,U.U.U) and have the data "N1.T1.X" be sent in the first user SEND. Then, the server can start up the real serving process and do a
SEND on (N3.T3.S,N1.T1.X) without having to send the "N3.T3.S" data to the user. Or perhaps both server and receiver exchange this data, to assure security of the ultimate connection (i.e. some wild process might try to connect to N1.T1.X if it is merely RECEIVING on foreign socket U.U.U.). We do not propose any specific reconnection protocol here, but leave this to further deliberation, since it is really a user level protocol issue. 4. TCP IMPLEMENTATION 4.1 INTRODUCTION Conceptually, the TCP is made up of several processes. Some of these deal with USER/TCP commands, and others with packets arriving from the network. The TCP also has an internal measurement facility which can be activated remotely. Any particular TCP could be viewed in a number of ways. It could be implemented as an independent process, servicing many user processes. It could be viewed as a set of re-entrant library routines which share a common interface to the local PSN, and common buffer storage. It could even be viewed as a set of processes, some handling the user, some the input of packets from the net, and some the output of packets to the net. 4.2 TCP DATA STRUCTURES 4.2.1 INTERNETWORK PACKET FONMAT 8 bits: Internet information 2 bits: Reserved for local PSN use 2 bits: Header format (11 in binary) 4 bits: Protocol version number 8 bits: Header length in octets (32 is the current value) 16 bits: Length of text in octets 32 bits: Packet sequence number 32 bits: Acknowledgment number (i.e. sequence number of next octet expected).
16 bits: Window size (in octets) 16 bits: Control Information Listed from high to low order: SYN: Request to synchronize sending sequence numbers ACK: There is a valid acknowledgment in the 32 bit ACK field FIN: Sender will stop SENDing and RECEIVEing on this connection DSN: The sender has stopped using sequence numbers and wants to initiate a new sequence number for sending. EOS: This packet is the end of a segment and therefore has a checksum in the 16 bit checksum field. If this bit is not set, the 16 bit checksum field is to be ignored. The bit is usually set, but if fragmentation at a GATEWAY occurs, the packets preceding the last one will not have checksums, and the last packet will have the checksum for the entire original fragment (segment) as it was calculated by the sending TCP. EOL: This packet contains the last fragment of a letter. The EOS bit will always be set in this case. INT: The sender wants to INTERRUPT on this connection. XXX: six (6) unused control bits OD: three (3) bits of control dispatch: 000: Null (the control octet contents should be ignored} 001: Event Code is present in the control octet. These were defined in section 2.4.3. 010: Special Functions 011: Reject (codes as yet undefined) 1XX: Unused 8 bits: Control Data Octet If CD is 000 then this octet is to be ignored.
If CD is 001, this octet contains event codes defined in section 2.4.3 If CD is 010, this octet contains a special function code as defined below: 0: RESET all connections between Source and Destination TCPs l: RESET the specific connection referenced in this packet 2: ECHO return packet to sender with the special function code ECHOR (Echo Reply). 3: QUERY Query status of connection referenced in this packet 4: STATUS Reply to QUERY with requested status. 5: ECHOR Echo Reply 6: TRASH Discard packet without acknowledgment >6: Unused Note: Special function packets not pertaining to a particular connection [RESET all, ECHO, ECHOR, and TRASH] are normally sent using socket zero as described in section 3.2. If CD is 01l, this octet contains an as yet undefined REJECT code. If CD is 1XX, this octet is undefined. 4 bits: Length of destination network address in 4 bit units (current value is 1) 4 bits: Destination network address 1010-1111 are addresses of ARPANET, UCL, CYCLADES, NPL, CADC, and EPSS respectively. 16 bits: Destination TCP address 8 bits: Padding 4 bits: length of source network address in 4 bit units (current value is 1) 4 bits: source network address (as for destination address)