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IEN: 124 DOD STANDARD TRANSMISSION CONTROL PROTOCOL December 1979 prepared for Defense Advanced Research Projects Agency Information Processing Techniques Office 1400 Wilson Boulevard Arlington, Virginia 22209 by Information Sciences Institute University of Southern California 4676 Admiralty Way Marina del Rey, California 90291 December 1979 Transmission Control Protocol TABLE OF CONTENTS PREFACE ........................................................ iii 1. INTRODUCTION ..................................................... 1 1.1 Motivation .................................................... 1 1.2 Scope ......................................................... 2 1.3 About This Document ........................................... 2 1.4 Interfaces .................................................... 3 1.5 Operation ..................................................... 3 2. PHILOSOPHY ....................................................... 7 2.1 Elements of the Internetwork System ........................... 7 2.2 Model of Operation ............................................ 7 2.3 The Host Environment .......................................... 8 2.4 Interfaces .................................................... 9 2.5 Relation to Other Protocols ................................... 9 2.6 Reliable Communication ....................................... 10 2.7 Connection Establishment and Clearing ........................ 10 2.8 Data Communication ........................................... 12 2.9 Precedence and Security ...................................... 13 2.10 Robustness Principle ......................................... 13 3. FUNCTIONAL SPECIFICATION ........................................ 15 3.1 Header Format ................................................ 15 3.2 Terminology .................................................. 19 3.3 Sequence Numbers ............................................. 24 3.4 Establishing a connection .................................... 29 3.5 Closing a Connection ......................................... 35 3.6 Precedence and Security ...................................... 38 3.7 Data Communication ........................................... 38 3.8 Interfaces ................................................... 42 3.9 Event Processing ............................................. 52 GLOSSARY ............................................................ 75 REFERENCES .......................................................... 83 [Page i] December 1979 Transmission Control Protocol [Page ii] December 1979 Transmission Control Protocol PREFACE This document describes the DoD Standard Transmission Control Protocol (TCP). There have been seven earlier editions of the ARPA TCP specification on which this standard is based, and the present text draws heavily from them. There have been many contributors to this work both in terms of concepts and in terms of text. This edition incorporates the addition of security, compartmentation, and precedence concepts into the TCP specification. Jon Postel Editor [Page iii] December 1979 IEN:124 Replaces: IENs 112, 81, 55, 44, 40, 27, 21, 5 DOD STANDARD TRANSMISSION CONTROL PROTOCOL 1. INTRODUCTION The Transmission Control Protocol (TCP) is intended for use as a highly reliable host-to-host protocol between hosts in packet-switched computer communication networks, and especially in interconnected systems of such networks. This document describes the functions to be performed by the Transmission Control Protocol, the program that implements it, and its interface to programs or users that require its services. 1.1. Motivation Computer communication systems are playing an increasingly important role in military, government, and civilian environments. This document primarily focuses its attention on military computer communication requirements, especially robustness in the presence of communication unreliability and availability in the presence of congestion, but many of these problems are found in the civilian and government sector as well. As strategic and tactical computer communication networks are developed and deployed, it is essential to provide means of interconnecting them and to provide standard interprocess communication protocols which can support a broad range of applications. In anticipation of the need for such standards, the Deputy Undersecretary of Defense for Research and Engineering has declared the Transmission Control Protocol (TCP) described herein to be a basis for DoD-wide inter-process communication protocol standardization. TCP is a connection-oriented, end-to-end reliable protocol designed to fit into a layered hierarchy of protocols which support multi-network applications. The TCP provides for reliable inter-process communication between pairs of processes in host computers attached to distinct but interconnected computer communication networks. Very few assumptions are made as to the reliability of the communication protocols below the TCP layer. At most, the TCP assumes it can obtain a simple, potentially unreliable datagram service from the lower level protocols. In principle, the TCP should be able to operate above a wide spectrum of communication systems ranging from hard-wired connections to packet-switched or circuit-switched networks. [Page 1] December 1979 Transmission Control Protocol Introduction The TCP fits into a layered protocol architecture just above a basic Internet Protocol [1] which provides a way for the TCP to send and receive variable-length segments of information enclosed in internet datagram "envelopes". The internet datagram provides a means for addressing source and destination TCPs in different networks, and the internet protocol also deals with any fragmentation or reassembly of the TCP segments which might be required to achieve transport and delivery through multiple networks and interconnecting gateways. The internet protocol also carries information on the precedence, security classification and compartmentation of the TCP segments, so this information can be communicated end-to-end across multiple networks. Protocol Layering +---------------------+ | higher-level | +---------------------+ | TCP | +---------------------+ | internet protocol | +---------------------+ |communication network| +---------------------+ Figure 1 Much of this document is written in the context of TCP implementations which are co-resident with higher level protocols in the host computer. As a practical matter, many computer systems will be connected to networks via front-end computers which house the TCP and internet protocol layers, as well as network specific software. The TCP specification describes an interface to the higher level protocols which appears to be implementable even for the front-end case, as long as a suitable host-to-front end protocol is implemented. 1.2. Scope The TCP is intended to provide a reliable process-to-process communication service in a multinetwork environment. The TCP is intended to be a host-to-host protocol in common use in multiple networks. 1.3. About this Document This document represents a specification of the behavior required of any TCP implementation, both in its interactions with higher level protocols and in its interactions with other TCPs. The rest of this section offers a very brief view of the protocol interfaces and [Page 2] December 1979 Transmission Control Protocol Introduction operation. Section 2 summarizes the philosophical basis for the TCP design. Section 3 offers both a detailed description of the actions required of TCP when various events occur (arrival of new segments, user calls, errors, etc.) and the details of the formats of TCP segments. 1.4. Interfaces The TCP interfaces on one side to user or application processes and on the other side to a lower level protocol such as Internet Protocol. The interface between an application process and the TCP is illustrated in reasonable detail. This interface consists of a set of calls much like the calls an operating system provides to an application process for manipulating files. For example, there are calls to open and close connections and to send and receive letters on established connections. It is also expected that the TCP can asynchronously communicate with application programs. Although considerable freedom is permitted to TCP implementors to design interfaces which are appropriate to a particular operating system environment, this TCP specification requires a certain minimum functionality to be achieved at the TCP/user interface for any valid implementation. The interface between TCP and lower level protocol is essentially unspecified except that it is assume there is a mechanism whereby the two can asynchronously pass information to each other. Typically, one expects the lower level protocol to specify this interface. TCP is designed to work in a very general environment of interconnected networks. Therefore, the lower level protocol which is assumed throughout this document is the Internet Protocol. 1.5. Operation As noted above, the primary purpose of the TCP is to provide reliable, securable logical circuit or connection service between pairs of processes. To provide this service on top of a less reliable internet communication system requires facilities in the following areas: Basic Data Transfer Reliability Flow Control Multiplexing Connections Precedence and Security The basic operation of the TCP in each of these areas is described in the following paragraphs. [Page 3] December 1979 Transmission Control Protocol Introduction Basic Data Transfer: The TCP is able to transfer a continuous stream of octets in each direction between its users by packaging some number of octets into segments for transmission through the internet system. In this stream mode, the TCPs decide when to block and forward data at their own convenience. For users who desire a record-oriented service, the TCP also permits the user to submit records, called letters, for transmission. When the sending user indicates a record boundary (end-of-letter), this causes the TCPs to promptly forward and deliver data up to that point to the receiver. Reliability: The TCP must recover from data that is damaged, lost, duplicated, or delivered out of order by the internet communication system. This is achieved by assigning a sequence number to each octet transmitted, and requiring a positive acknowledgment (ACK) from the receiving TCP. If the ACK is not received within a timeout interval, the data is retransmitted. At the receiver, the sequence numbers are used to correctly order segments that may be received out of order and to eliminate duplicates. Damage is handled by adding a checksum to each segment transmitted, checking it at the receiver, and discarding damaged segments. As long as the TCPs continue to function properly and the internet system does not become completely partitioned, no transmission errors will affect the users. All errors in the internet communication system are recovered by the TCP. Flow Control: TCP provides a means for the receiver to govern the amount of data sent by the sender. This is achieved by returning a "window" with every ACK indicating a range of acceptable sequence numbers beyond the last segment successfully received. For stream mode, the window indicates an allowed number of octets that the sender may transmit before receiving further permission. It is also possible for the TCP to operate in a mode where buffer sizes and letter boundaries are incorporated in flow control. Multiplexing: To allow for many processes within a single Host to use TCP communication facilities simultaneously, the TCP provides a set of addresses or ports within each host. Concatenated with the network [Page 4] December 1979 Transmission Control Protocol Introduction and host addresses from the internet communication layer, this forms a socket. A pair of sockets uniquely identifies each connection. That is, different connections may have a common socket on one side, but the sockets on the other sides must be different. The binding of ports to processes is handled independently by each Host. However, it proves useful to attach frequently used processes (e.g., a "logger" or timesharing service) to fixed sockets which are made known to the public. These services can then be accessed through the known addresses. Establishing and learning the port addresses of other processes may involve more dynamic mechanisms. Connections: The reliability and flow control mechanisms described above require that TCPs initialize and maintain certain status information for each data stream. The combination of this information, including sockets, sequence numbers, and window sizes, is called a connection. Each connection is uniquely specified by a pair of sockets identifying its two sides. When two processes wish to communicate, their TCP's must first establish a connection (initialize the status information on each side). When their communication is complete, the connection is terminated or closed to free the resources for other uses. Since connections must be established over the unreliable internet communication system, a handshake mechanism with clock-based sequence numbers is used to avoid erroneous initialization of connections. Precedence and Security: The users of TCP may indicate the security and precedence of their communication. Provision is made for default values to be used when these features are not needed. [Page 5] December 1979 Transmission Control Protocol [Page 6] December 1979 Transmission Control Protocol 2. PHILOSOPHY 2.1. Elements of the Internetwork System The internetwork environment consists of hosts connected to networks which are in turn interconnected via gateways. It is assumed here that the networks may be either local networks (e.g., the ETHERNET) or large networks (e.g., the ARPANET), but in any case are based on packet switching technology. The active agents that produce and consume messages are processes. Various levels of protocols in the networks, the gateways, and the hosts support an interprocess communication system that provides two-way data flow on logical connections between process ports. We specifically assume that data is transmitted from host to host through means of a set of networks. When we say network, we have in mind a packet switched network (PSN). This assumption is probably unnecessary, since a circuit switched network or a hybrid combination of the two could also be used; but for concreteness, we explicitly assume that the hosts are connected to one or more packet switches of a PSN. The term packet is used generically here to mean the data of one transaction between a host and a packet switch. The format of data blocks exchanged between the packet switches in a network will generally not be of concern to us. Hosts are computers attached to a network, and from the communication network's point of view, are the sources and destinations of packets. Processes are viewed as the active elements in host computers (in accordance with the fairly common definition of a process as a program in execution). Even terminals and files or other I/O devices are viewed as communicating with each other through the use of processes. Thus, all 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. 2.2. Model of Operation Processes transmit data by calling on the TCP and passing buffers of data as arguments. The TCP packages the data from these buffers into segments and calls on the internet module to transmit each segment to the destination TCP. The receiving TCP places the data from a segment into the receiving users buffer and notifies the receiving user. The TCPs include control information in the segments which they use to ensure reliable ordered data transmission. [Page 7] December 1979 Transmission Control Protocol Philosophy The model of internet communication is that there is a basic gateway (or internet protocol module) associated with each TCP which provides an interface to the local network. This basic gateway packages TCP segments inside internet datagrams and routes these datagrams to a destination or intermediate gateway. To transmit the datagram through the local network, it is embedded in a local network packet. The packet switches may perform further packaging, fragmentation, or other operations to achieve the delivery of the local packet to the destination gateway. At a gateway between networks, the internet datagram is "unwrapped" from its local packet and examined to determine through which network the internet datagram should travel next. The internet datagram is then "wrapped" in a local packet suitable to the next network and routed to the next gateway. A gateway is permitted to break up an internet datagram into smaller internet datagram fragments if this is necessary for transmission through the next network. To do this, the gateway produces a set of internet datagrams; each carrying a fragment. Fragments may be broken into smaller ones at intermediate gateways. The internet datagram fragment format is designed so that the destination gateway can reassemble fragments into internet datagrams. A destination gateway unwraps the segment from the datagram (after reassembling the datagram, if necessary) and passes it to the destination TCP. This simple model of the operation glosses over many details. One important feature is the type of service. This provides information to the gateway to guide it in selecting the service parameters to be used in traversing the next network. Included in the type of service information is the precedence of the datagram. Datagrams may also carry security information to permit host and gateways that operate in multilevel secure environments to properly segregate datagrams for security considerations. 2.3. The Host Environment The TCP is assumed to be a module in a time sharing operating system. The users access the TCP much like they would access the file system. The TCP may call on other operating system functions, for example, to manage data structures. The actual interface to the network is assumed to be controlled by a device driver module. The TCP does not call on the network device driver directly, but rather calls on the internet datagram protocol module which may in turn call on the device driver. [Page 8] December 1979 Transmission Control Protocol Philosophy Though it is assumed here that processes are supported by the host operating system, the mechanisms of TCP do not preclude implementation of the TCP in a front-end processor. However, in such an implementation, a host-to-front-end protocol must provide the functionality to support the type of TCP-user interface described above. 2.4. Interfaces The TCP/user interface provides for calls made by the user on the TCP to OPEN or CLOSE a connection, to SEND or RECEIVE data, or to obtain STATUS about a connection. These calls are like other calls from user programs on the operating system, for example, the calls to open, read from, and close a file. The TCP/internet interface provides calls to send and receive datagrams addressed to TCP modules in hosts anywhere in the internet system. These calls have parameters for passing the address type of service, precedence, security, and other control information. 2.5. Relation to Other Protocols The following diagram illustrates the place of the TCP in the protocol hierarchy: +------+ +-----+ +-----+ +-----+ |Telnet| | FTP | |Voice| ... | | Application Level +------+ +-----+ +-----+ +-----+ | | | | +-----+ +-----+ +-----+ | TCP | | RTP | ... | | Host Level +-----+ +-----+ +-----+ | | | +-------------------------------+ | Internet Protocol | Gateway Level +-------------------------------+ | +---------------------------+ | Local Network Protocol | Network Level +---------------------------+ | Protocol Relationships Figure 2. [Page 9] December 1979 Transmission Control Protocol Philosophy It is expected that the TCP will be able to support higher level protocols efficiently. It should be easy to interface higher level protocols like the ARPANET Telnet or AUTODIN II THP to the TCP. 2.6. Reliable Communication A stream of data sent on a TCP connection is delivered reliably and in order at the destination. Transmission is made reliable via the use of sequence numbers and acknowledgments. Conceptually, each octet of data is assigned a sequence number. The sequence number of the first octet of data in a segment is the sequence number transmitted with that segment and is called the segment sequence number. Segments also carry an acknowledgment number which is the sequence number of the next expected data octet of transmissions in the reverse direction. When the TCP transmits a segment, it puts a copy on a retransmission queue and starts a timer; when the acknowledgment for that data is received, the segment is deleted from the queue. If the acknowledgment is not received before the timer runs out, the segment is retransmitted. An acknowledgment by TCP does not guarantee that the data has been delivered to the end user, but only that the TCP has taken the responsibility to do so. To govern the flow of data into a TCP, a flow control mechanism is employed. The the data receiving TCP reports a window to the sending TCP. This window is the number of octets starting with the acknowledgment number that the data receiving TCP is currently prepared to receive. 2.7. Connection Establishment and Clearing To identify the separate data streams that a TCP may handle, the TCP provides a port identifier. Since port identifiers are selected independently by each operating system, TCP, or user, they might not be unique. To provide for unique addresses at each TCP, we concatenate an internet address identifying the TCP with a port identifier to create a socket which will be unique throughout all networks connected together. A connection is fully specified by the pair of sockets at the ends, since the same local socket may participate in many connections to different foreign sockets. A connection can be used to carry data in both directions, that is, it is "full duplex". TCPs are free to associate ports with processes however they choose. However, several basic concepts seem necessary in any implementation. [Page 10] December 1979 Transmission Control Protocol Philosophy There must be well-known sockets which the TCP associates only with the "appropriate" processes by some means. We envision that processes may "own" ports, and that processes can only initiate connections on the ports they own. (Means for implementing ownership is a local issue, but we envision a Request Port user command, 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.) A connection is specified in the OPEN call by the local port and foreign socket arguments. In return, the TCP supplies a (short) local connection name by which the user refers to the connection in subsequent calls. There are several things that must be remembered about a connection. To store this information we imagine that there is a data structure called a Transmission Control Block (TCB). One implementation strategy would have the local connection name be a pointer to the TCB for this connection. The OPEN call also specifies whether the connection establishment is to be actively pursued, or to be passively waited for. A foreign socket of all zeros is called unspecified. The purpose behind unspecified sockets is to provide a sort of "general delivery" facility (useful for processes offering services). This is allowed only for passive OPENs. A service process that wished to provide services for unknown other processes could issue a passive OPEN request with an unspecified foreign socket. Then a connection could be made with any process that requested a connection to this local socket. It would help if this local socket were known to be associated with this service. Well-known sockets are a convenient mechanism for a priori associating a socket address with a standard service. For instance, the "Telnet-Server" process might be permanently assigned to a particular socket, and other sockets might be reserved for File Transfer, Remote Job Entry, Text Generator, Echoer, and Sink processes (the last three being for test purposes). A socket address might be reserved for access to a "Look-Up" service which would return the specific socket at which a newly created service would be provided. The concept of a well-known socket is part of the TCP specification, but the assignment of sockets to services is outside this specification. Processes can issue passive OPENs and wait for matching calls from other processes and be informed by the TCP when connections have been established. Two processes which issue calls to each other at the same time are correctly connected. This flexibility is critical for the support of distributed computing in which components act asynchronously with respect to each other. [Page 11] December 1979 Transmission Control Protocol Philosophy There are two cases for matching the sockets in the local request and an incoming segment. In the first case, the local request has fully specified the foreign socket. In this case, the match must be exact. In the second case, the local request has left the foreign socket unspecified. In this case, any foreign socket is acceptable as long as the local sockets match. If there are several pending passive OPENs (recorded in TCBs) with the same local socket, an incoming segment should be matched to a request with the specific foreign socket in the segment, if such a request exists, before selecting a request with an unspecified foreign socket. The procedures to establish and clear connections utilize synchronize (SYN) and finis (FIN) control flags and involve an exchange of three messages. This exchange has been termed a three-way hand shake [3]. A connection is initiated by the rendezvous of an arriving segment containing a SYN and a waiting TCB entry created by a user OPEN command. The matching of local and foreign sockets determines when a connection has been initiated. The connection becomes "established" when sequence numbers have been synchronized in both directions. The clearing of a connection also involves the exchange of segments, in this case carrying the FIN control flag. 2.8. Data Communication The data that flows on a connection may be thought of as a stream of octets, or as a sequence of records. In TCP the records are called letters and are of variable length. The sending user indicates in each SEND call whether the data in that call completes a letter by the setting of the end-of-letter parameter. The length of a letter may be such that it must be broken into segments before it can be transmitted to its destination. We assume that the segments will normally be reassembled into a letter before being passed to the receiving process. A segment may contain all or a part of a letter, but a segment never contains parts of more than one letter. The end of a letter is marked by the appearance of an EOL control flag in a segment. A sending TCP is allowed to collect data from the sending user and to send that data in segments at its own convenience, until the end of letter is signaled then it must send all unsent data. When a receiving TCP has a complete letter, it must not wait for more data from the sending TCP before passing the letter to the receiving process. There is a coupling between letters as sent and the use of buffers of data that cross the TCP/user interface. Each time an end-of-letter [Page 12] December 1979 Transmission Control Protocol Philosophy (EOL) flag is associated with data placed into the receiving user's buffer, the buffer is returned to the user for processing even if the buffer is not filled. If a letter is longer than the user's buffer, the letter is passed to the user in buffer size units, the last of which may be only partly full. The TCP is responsible for regulating the flow of segments on the connections, as a way of preventing itself from becoming saturated or overloaded with traffic. This is done using a window flow control mechanism. The data receiving TCP reports to the data sending TCP a window which is the range of sequence numbers of data octets that data receiving TCP is currently prepared to accept. TCP also provides a means to communicate to the receiver of data that at some point further along in the data stream than the receiver is currently reading there is urgent data. TCP does not attempt to define what the user specifically does upon being notified of pending urgent data, but the general notion is that the receiving process should take action to read through the end urgent data quickly. 2.9. Precedence and Security The TCP makes use of the internet protocol type of service field and security option to provide precedence and security on a per connection basis to TCP users. Not all TCP modules will necessarily function in a multilevel secure environment, some may be limited to unclassified use only, and others may operate at only one security level and compartment. Consequently, some TCP implementations and services to users may be limited to a subset of the multilevel secure case. TCP modules which operate in a multilevel secure environment should properly mark outgoing segments with the security, compartment, and precedence. Such TCP modules should also provide to their users or higher level protocols such as Telnet or THP an interface to allow them to specify the desired security level, compartment, and precedence of connections. 2.10. Robustness Principle TCP implementations should follow a general principle of robustness: be conservative in what you do, be liberal in what you accept from others. [Page 13] December 1979 Transmission Control Protocol [Page 14] December 1979 Transmission Control Protocol 3. FUNCTIONAL SPECIFICATION 3.1. Header Format TCP segments are sent as internet datagrams. The Internet Protocol header carries several information fields, including the source and destination host addresses [1]. A TCP header follows the internet header, supplying information specific to the TCP protocol. This division allows for the existence of host level protocols other than TCP. TCP Header Format 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Port | Destination Port | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Acknowledgment Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Data | |U|A|E|R|S|F| | | Offset| Reserved |R|C|O|S|Y|I| Window | | | |G|K|L|T|N|N| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Checksum | Urgent Pointer | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ TCP Header Format Note that one tick mark represents one bit position. Figure 3. Source Port: 16 bits The source port number. Destination Port: 16 bits The destination port number. [Page 15] December 1979 Transmission Control Protocol Functional Specification Sequence Number: 32 bits The sequence number of the first data octet in this segment. Acknowledgment Number: 32 bits If the ACK control bit is set this field contains the value of the next sequence number the sender of the segment is expecting to receive. Data Offset: 4 bits The number of 32 bit words in the TCP Header. This indicates where the data begins. The TCP header including options is an integral number of 32 bits long. Reserved: 6 bits Reserved for future use. Must be zero. Control Bits: 8 bits (from left to right): URG: Urgent Pointer field significant ACK: Acknowledgment field significant EOL: End of Letter RST: Reset the connection SYN: Synchronize sequence numbers FIN: No more data from sender Window: 16 bits The number of data octets beginning with the one indicated in the acknowledgment field which the sender of this segment is willing to accept. Checksum: 16 bits The checksum field is the 16 bit one's complement of the one's complement sum of all 16 bit words in the header and text. If a segment contains an odd number of header and text octets to be checksummed, the last octet is padded on the right with zeros to form a 16 bit word for checksum purposes. The pad is not transmitted as part of the segment. While computing the checksum, the checksum field itself is replaced with zeros. The checksum also covers a 96 bit pseudo header conceptually prefixed to the TCP header. This pseudo header contains the Source Address, the Destination Address, the Protocol, and TCP length. [Page 16] December 1979 Transmission Control Protocol Functional Specification This gives the TCP protection against misrouted segments. This information is carried in the Internet Protocol and is transferred across the TCP/Network interface in the arguments or results of calls by the TCP on the IP. +--------------------------+ | Source Address | +--------------------------+ | Destination Address | +--------------------------+ | zero | PTCL | TCP Length | +--------------------------+ The TCP Length is the TCP header plus the data length in octets (this is not a explicitly transmitted quantity, but is computed from the total length, and the header length). Urgent Pointer: 16 bits This field communicates the current value of the urgent pointer as a positive offset from the sequence number in this segment. The urgent pointer points to the sequence number of the octet following the urgent data. This field should only be interpreted in segments with the URG control bit set. Options: variable Options may occupy space at the end of the TCP header and are a multiple of 8 bits in length. All options are included in the checksum. An option may begin on any octet boundary. There are two cases for the format of an option: Case 1: A single octet of option-kind. Case 2: An octet of option-kind, an octet of option-length, and the actual option-data octets. The option-length counts the two octets of option-kind and option-length as well as the option-data octets. Note that the list of options may be shorter than the data offset field might imply. The content of the header beyond the End-of-Option option should be header padding (i.e., zero). A TCP must implement all options. [Page 17] December 1979 Transmission Control Protocol Functional Specification Currently defined options include (kind indicated in octal): Kind Length Meaning ---- ------ ------- 0 - End of option list. 1 - No-Operation. 100 - Reserved. 105 4 Buffer Size. Specific Option Definitions End of Option List +--------+ |00000000| +--------+ Kind=0 This option code indicates the end of the option list. This might not coincide with the end of the TCP header according to the Data Offset field. This is used at the end of all options, not the end of each option, and need only be used if the end of the options would not otherwise coincide with the end of the TCP header. No-Operation +--------+ |00000001| +--------+ Kind=1 This option code may be used between options, for example, to align the beginning of a subsequent option on a word boundary. There is no guarantee that senders will use this option, so receivers must be prepared to process options even if they do not begin on a word boundary. Buffer Size +--------+--------+---------+--------+ |01000101|00000100| buffer size | +--------+--------+---------+--------+ Kind=105 Length=4 [Page 18] December 1979 Transmission Control Protocol Functional Specification Buffer Size Option Data: 16 bits If this option is present, then it communicates the receive buffer size at the TCP which sends this segment. This field should only be sent in segments with the SYN control bit set. If this option is not used, the default buffer size of one octet is assumed. Padding: variable The TCP header padding is used to ensure that the TCP header ends and data begins on a 32 bit boundary. The padding is composed of zeros. 3.2. Terminology Before we can discuss very much about the operation of the TCP we need to introduce some detailed terminology. The maintenance of a TCP connection requires the remembering of several variables. We conceive of these variables being stored in a connection record called a Transmission Control Block or TCB. Among the variables stored in the TCB are the local and remote socket numbers, the security and precedence of the connection, pointers to the user's send and receive buffers, pointers to the retransmit queue and to the current segment. In addition several variables relating to the send and receive sequence numbers are stored in the TCB. Send Sequence Variables SND.UNA - send unacknowledged SND.NXT - send sequence SND.WND - send window SND.BS - send buffer size SND.UP - send urgent pointer SND.WL - send sequence number used for last window update SND.LBB - send last buffer beginning ISS - initial send sequence number Receive Sequence Variables RCV.NXT - receive sequence RCV.WND - receive window RCV.BS - receive buffer size RCV.UP - receive urgent pointer RCV.LBB - receive last buffer beginning IRS - initial receive sequence number [Page 19] December 1979 Transmission Control Protocol Functional Specification The following diagrams may help to relate some of these variables to the sequence space. Send Sequence Space 1 2 3 4 ----------|----------|----------|---------- SND.UNA SND.NXT SND.UNA +SND.WND 1 - old sequence numbers which have been acknowledged 2 - sequence numbers of unacknowledged data 3 - sequence numbers allowed for new data transmission 4 - future sequence numbers which are not yet allowed Send Sequence Space Figure 4. Receive Sequence Space 1 2 3 ----------|----------|---------- RCV.NXT RCV.NXT +RCV.WND 1 - old sequence numbers which have been acknowledged 2 - sequence numbers allowed for new reception 3 - future sequence numbers which are not yet allowed Receive Sequence Space Figure 5. There are also some variables used frequently in the discussion that take their values from the fields of the current segment. [Page 20] December 1979 Transmission Control Protocol Functional Specification Current Segment Variables SEG.SEQ - segment sequence number SEG.ACK - segment acknowledgment number SEG.LEN - segment length SEG.WND - segment window SEG.UP - segment urgent pointer SEG.PRC - segment precedence value A connection progresses through a series of states during its lifetime. The states are: LISTEN, SYN-SENT, SYN-RECEIVED, ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, TIME-WAIT, CLOSE-WAIT, CLOSING, and the fictional state CLOSED. Closed is fictional because it represents the state when there is no TCB, and therefore, no connection. Briefly the meanings of the states are: LISTEN - represents waiting for a connection request from any remote TCP and port. SYN-SENT - represents waiting for a matching connection request after having sent a connection request. SYN-RECEIVED - represents waiting for a confirming connection request acknowledgment after having both sent and received a connection request. ESTABLISHED - represents an open connection, ready to transmit and receive data segments. FIN-WAIT-1 - represents waiting for a connection termination request from the remote TCP, or an acknowledgment of the connection termination request previously sent. FIN-WAIT-2 - represents waiting for a connection termination request from the remote TCP. TIME-WAIT - represents waiting for enough time to pass to be sure the remote TCP received the acknowledgment of its connection termination request. CLOSE-WAIT - represents waiting for a connection termination request from the local user. CLOSING - represents waiting for a connection termination request acknowledgment from the remote TCP. CLOSED - represents no connection state at all. [Page 21] December 1979 Transmission Control Protocol Functional Specification A TCP connection progresses from one state to another in response to events. The events are the user calls, OPEN, SEND, RECEIVE, CLOSE, ABORT, and STATUS; the incoming segments, particularly those containing the SYN and FIN flags; and timeouts. The Glossary contains a more complete list of terms and their definitions. The state diagram in figure 6 only illustrates state changes, together with the causing events and resulting actions, but addresses neither error conditions nor actions which are not connected with state changes. In a later section, more detail is offered with respect to the reaction of the TCP to events. [Page 22] December 1979 Transmission Control Protocol Functional Specification +---------+ ---------\ active OPEN | CLOSED | \ ----------- +---------+<---------\ \ create TCB | ^ \ \ snd SYN passive OPEN | | CLOSE \ \ ------------ | | ---------- \ \ create TCB | | delete TCB \ \ V | \ \ +---------+ CLOSE | \ | LISTEN | ---------- | | +---------+ delete TCB | | rcv SYN | | SEND | | ----------- | | ------- | V +---------+ snd SYN,ACK / \ snd SYN +---------+ | |<----------------- ------------------>| | | SYN | rcv SYN | SYN | | RCVD |<-----------------------------------------------| SENT | | | snd ACK | | | |------------------ -------------------| | +---------+ rcv ACK of SYN \ / rcv SYN,ACK +---------+ | -------------- | | ----------- | x | | snd ACK | V V | CLOSE +---------+ | ------- | ESTAB | | snd FIN +---------+ | CLOSE | | rcv FIN V ------- | | ------- +---------+ snd FIN / \ snd ACK +---------+ | FIN |<----------------- ------------------>| CLOSE | | WAIT-1 |------------------ -------------------| WAIT | +---------+ rcv FIN \ / CLOSE +---------+ | rcv ACK of FIN ------- | | ------- | -------------- snd ACK | | snd FIN V x V V +---------+ +---------+ |FINWAIT-2| | CLOSING | +---------+ +---------+ | rcv FIN | rcv ACK of FIN | ------- Timeout=2MSL | -------------- V snd ACK ------------ V delete TCB +---------+ delete TCB +---------+ |TIME WAIT|----------------->| CLOSED | +---------+ +---------+ TCP Connection State Diagram Figure 6. [Page 23] December 1979 Transmission Control Protocol Functional Specification 3.3. Sequence Numbers A fundamental notion in the design is that every octet of data sent over a TCP connection has a sequence number. Since every octet is sequenced, each of them can be acknowledged. The acknowledgment mechanism employed is cumulative so that an acknowledgment of sequence number X indicates that all octets up to but not including X have been received. This mechanism allows for straight-forward duplicate detection in the presence of retransmission. It is essential to remember that the actual sequence number space is finite, though very large. This space ranges from 0 to 2**32 - 1. Since the space is finite, all arithmetic dealing with sequence numbers must be performed modulo 2**32. This unsigned arithmetic preserves the relationship of sequence numbers as they cycle from 2**32 - 1 to 0 again. There are some subtleties to computer modulo arithmetic, so great care should be taken in programming the comparison of such values. The typical kinds of sequence number comparisons which the TCP must perform include: (a) Determining that an acknowledgment refers to some sequence number sent but not yet acknowledged. (b) Determining that all sequence numbers occupied by a segment have been acknowledged (e.g., to remove the segment from a retransmission queue). (c) Determining that an incoming segment contains sequence numbers which are expected (i.e., that the segment "overlaps" the receive window). On send connections the following comparisons are needed: older sequence numbers newer sequence numbers SND.UNA SEG.ACK SND.NXT | | | ----|----XXXXXXX------XXXXXXXXXX---------XXXXXX----|---- | | | | | | | | | Segment 1 Segment 2 Segment 3 <----- sequence space -----> Sending Sequence Space Information Figure 7. [Page 24] December 1979 Transmission Control Protocol Functional Specification SND.UNA = oldest unacknowledged sequence number SND.NXT = next sequence number to be sent SEG.ACK = acknowledgment (next sequence number expected by the acknowledging TCP) SEG.SEQ = first sequence number of a segment SEG.SEQ+SEG.LEN-1 = last sequence number of a segment An acceptable acknowledgment, SEG.ACK, is one for which the inequality below holds: 0 < (SEG.ACK - SND.UNA) =< (SND.NXT - SND.UNA) or: SND.UNA < SEG.ACK =< SND.NXT Note that all arithmetic is modulo 2**32 and that comparisons are unsigned. "=<" means "less than or equal". Similarly, the determination that a particular segment has been fully acknowledged can be made if the inequality below holds: 0 < (SEG.SEQ+SEG.LEN-1 - SND.UNA) < (SEG.ACK - SND.UNA) SEG.LEN is the number of octets occupied by the data in the segment. It is important to note that SEG.LEN must be non-zero; segments which do not occupy any sequence space (e.g., empty acknowledgment segments) are never placed on the retransmission queue, so would not go through this particular test. [Page 25] December 1979 Transmission Control Protocol Functional Specification On receive connections the following comparisons are needed: older sequence numbers newer sequence numbers RCV.NXT RCV.NXT+RCV.WND | | ---------XXX|XXX------XXXXXXXXXX---------XXX|XX--------- | | | | | | | | Segment 1 Segment 2 Segment 3 <----- sequence space -----> Receiving Sequence Space Information Figure 8. RCV.NXT = next sequence number expected on incoming segments RCV.NXT+RCV.WND = last sequence number expected on incoming segments, plus one SEG.SEQ = first sequence number occupied by the incoming segment SEG.SEQ+SEG.LEN-1 = last sequence number occupied by the incoming segment A segment is judged to occupy a portion of valid receive sequence space if 0 =< (SEG.SEQ+SEG.LEN-1 - RCV.NXT) < (RCV.NXT+RCV.WND - RCV.NXT) SEG.SEQ+SEG.LEN-1 is the last sequence number occupied by the segment; RCV.NXT is the next sequence number expected on an incoming segment; and RCV.NXT+RCV.WND is the right edge of the receive window. Actually, it is a little more complicated than this. Due to zero windows and zero length segments, we have four cases for the acceptability of an incoming segment: [Page 26] December 1979 Transmission Control Protocol Functional Specification Segment Receive Test Length Window ------- ------- ------------------------------------------- 0 0 SEG.SEQ = RCV.NXT 0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND >0 0 not acceptable >0 >0 RCV.NXT < SEG.SEQ+SEG.LEN =< RCV.NXT+RCV.WND Note that the acceptance test for a segment, since it requires the end of a segment to lie in the window, is somewhat more restrictive than is absolutely necessary. If at least the first sequence number of the segment lies in the receive window, or if some part of the segment lies in the receive window, then the segment might be judged acceptable. Thus, in figure 8, at least segments 1 and 2 are acceptable by the strict rule, and segment 3 may or may not be, depending on the strictness of interpretation of the rule. Note that when the receive window is zero no segments should be acceptable except ACK segments. Thus, it should be possible for a TCP to maintain a zero receive window while transmitting data and receiving ACKs. We have taken advantage of the numbering scheme to protect certain control information as well. This is achieved by implicitly including some control flags in the sequence space so they can be retransmitted and acknowledged without confusion (i.e., one and only one copy of the control will be acted upon). Control information is not physically carried in the segment data space. Consequently, we must adopt rules for implicitly assigning sequence numbers to control. The SYN and FIN are the only controls requiring this protection, and these controls are used only at connection opening and closing. For sequence number purposes, the SYN is considered to occur before the first actual data octet of the segment in which it occurs, while the FIN is considered to occur after the last actual data octet in a segment in which it occurs. The segment length includes both data and sequence space occupying controls. Initial Sequence Number Selection The protocol places no restriction on a particular connection being used over and over again. A connection is defined by a pair of sockets. New instances of a connection will be referred to as incarnations of the connection. The problem that arises owing to this is -- "how does the TCP identify duplicate segments from previous [Page 27] December 1979 Transmission Control Protocol Functional Specification incarnations of the connection?" This problem becomes apparent if the connection is being opened and closed in quick succession, or if the connection breaks with loss of memory and is then reestablished. To avoid confusion we must prevent segments from being emitted with sequence numbers which duplicate those which are still in the network. We want to assure this, even if a TCP crashes and loses all knowledge of the sequence numbers it has been using. When new connections are created, an initial sequence number (ISN) generator is employed which selects a new 32 bit ISN. The generator is bound to a (possibly fictitious) 32 bit clock whose low order bit is incremented roughly every 4 microseconds. Thus, the ISN cycles approximately every 4.55 hours. Since we assume that segments will stay in the network no more than tens of seconds or minutes, at worst, we can reasonably assume that ISN's will be unique. For each connection there is a send sequence number and a receive sequence number. The initial send sequence number (ISS) is chosen by the data sending TCP, and the initial receive sequence number (IRS) is learned during the connection establishing procedure. For a connection to be established or initialized, the two TCPs must synchronize on each other's initial sequence numbers. This is done in an exchange of connection establishing messages carrying a control bit called "SYN" (for synchronize) and the initial sequence numbers. As a shorthand, messages carrying the SYN bit are also called "SYNs". Hence, the solution requires a suitable mechanism for picking an initial sequence number and a slightly involved handshake to exchange the ISN's. A "three way handshake" is necessary because sequence numbers are not tied to a global clock in the network, and TCPs may have different mechanisms for picking the ISN's. The receiver of the first SYN has no way of knowing whether the segment was an old delayed one or not, unless it remembers the last sequence number used on the connection (which is not always possible), and so it must ask the sender to verify this SYN. The "three way handshake" and the advantages of a "clock-driven" scheme are discussed in [3]. Knowing When to Keep Quiet To be sure that a TCP does not create a segment that carries a sequence number which may be duplicated by an old segment remaining in the network, the TCP must keep quiet for a maximum segment lifetime (MSL) before assigning any sequence numbers upon starting up or recovering from a crash in which memory of sequence numbers in use was lost. For this specification the MSL is taken to be 2 minutes. This is an engineering choice, and may be changed if experience indicates [Page 28] December 1979 Transmission Control Protocol Functional Specification it is desirable to do so. Note that if a TCP is reinitialized in some sense, yet retains its memory of sequence numbers in use, then it need not wait at all; it must only be sure to use sequence numbers larger than those recently used. It should be noted that this strategy does not protect against spoofing or other replay type duplicate message problems. 3.4. Establishing a connection The "three-way handshake" is essentially a unidirectional attempt to establish a connection, i.e., there is an initiator and a responder. The TCP can also establish a connection when a simultaneous initiation occurs. A simultaneous attempt occurs when one TCP receives a "SYN" segment which carries no acknowledgment after it has sent a "SYN". Of course, the arrival of an old duplicate "SYN" segment can potentially make it appear, to the recipient, that a simultaneous connection initiation is in progress. Proper use of "reset" segments can disambiguate these cases. Several examples of connection initiation are offered below. Although these examples do not show connection synchronization using data-carrying segments, this is perfectly legitimate, so long as the receiving TCP doesn't deliver the data to the user until it is clear the data is valid (i.e., the data must be buffered at the receiver until the connection reaches the ESTABLISHED state). The three-way handshake reduces the possibility of false connections. It is the implementation of a trade-off between memory and messages to provide information for this checking. The simplest three-way handshake is shown in figure 9 below. The figures should be interpreted in the following way. Each line is numbered for reference purposes. Right arrows (-->) indicate departure of a TCP segment from TCP A to TCP B, or arrival of a segment at B from A. Left arrows (<--), indicate the reverse. Ellipsis (...) indicates a segment which is still in the network (delayed). An "XXX" indicates a segment which is lost or rejected. Comments appear in parentheses. TCP states represent the state AFTER the departure or arrival of the segment (whose contents are shown in the center of each line). Segment contents are shown in abbreviated form, with sequence number, control flags, and ACK field. Other fields such as window, addresses, lengths, and text have been left out in the interest of clarity. [Page 29] December 1979 Transmission Control Protocol Functional Specification TCP A TCP B 1. CLOSED LISTEN 2. SYN-SENT --> <SEQ=100><CTL=SYN> --> SYN-RECEIVED 3. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED 4. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK> --> ESTABLISHED 5. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK><DATA> --> ESTABLISHED Basic 3-Way Handshake for Connection Synchronization Figure 9. In line 2 of figure 9, TCP A begins by sending a SYN segment indicating that it will use sequence numbers starting with sequence number 100. In line 3, TCP B sends a SYN and acknowledges the SYN it received from TCP A. Note that the acknowledgment field indicates TCP B is now expecting to hear sequence 101, acknowledging the SYN which occupied sequence 100. At line 4, TCP A responds with an empty segment containing an ACK for TCP B's SYN; and in line 5, TCP A sends some data. Note that the sequence number of the segment in line 5 is the same as in line 4 because the ACK does not occupy sequence number space (if it did, we would wind up ACKing ACK's!). Simultaneous initiation is only slightly more complex, as is shown in figure 10. Each TCP cycles from CLOSED to SYN-SENT to SYN-RECEIVED to ESTABLISHED. The principle reason for the three-way handshake is to prevent old duplicate connection initiations from causing confusion. To deal with this, a special control message, reset, has been devised. A TCP which receives a reset message first verifies that the ACK field of the reset acknowledges something the TCP sent (otherwise, the message is ignored). If the receiving TCP is in a non-synchronized state (i.e., SYN-SENT, SYN-RECEIVED), it returns to LISTEN on receiving an acceptable reset. If the TCP is in one of the synchronized states (ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, TIME-WAIT, CLOSE-WAIT, CLOSING), it aborts the connection and informs its user. We discuss this latter case under "half-open" connections below. [Page 30] December 1979 Transmission Control Protocol Functional Specification TCP A TCP B 1. CLOSED CLOSED 2. SYN-SENT --> <SEQ=100><CTL=SYN> ... 3. SYN-RECEIVED <-- <SEQ=300><CTL=SYN> <-- SYN-SENT 4. ... <SEQ=100><CTL=SYN> --> SYN-RECEIVED 5. SYN-RECEIVED --> <SEQ=101><ACK=301><CTL=ACK> ... 6. ESTABLISHED <-- <SEQ=301><ACK=101><CTL=ACK> <-- SYN-RECEIVED 7. ... <SEQ=101><ACK=301><CTL=ACK> --> ESTABLISHED Simultaneous Connection Synchronization Figure 10. TCP A TCP B 1. CLOSED LISTEN 2. SYN-SENT --> <SEQ=100><CTL=SYN> ... 3. (duplicate) ... <SEQ=1000><CTL=SYN> --> SYN-RECEIVED 4. SYN-SENT <-- <SEQ=300><ACK=1001><CTL=SYN,ACK> <-- SYN-RECEIVED 5. SYN-SENT --> <SEQ=1001><CTL=RST> --> LISTEN (ACK is ok) 6. ... <SEQ=100><CTL=SYN> --> SYN-RECEIVED 7. SYN-SENT <-- <SEQ=400><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED 8. ESTABLISHED --> <SEQ=101><ACK=401><CTL=ACK> --> ESTABLISHED Recovery from Old Duplicate SYN Figure 11. As a simple example of recovery from old duplicates, consider [Page 31] December 1979 Transmission Control Protocol Functional Specification figure 11. At line 3, an old duplicate SYN arrives at TCP B. TCP B cannot tell that this is an old duplicate, so it responds normally (line 4). TCP A detects that the ACK field is incorrect and returns a RST (reset) with its SEQ field selected to make the segment believable. TCP B, on receiving the RST, returns to the LISTEN state. When the original SYN (pun intended) finally arrives at line 6, the synchronization proceeds normally. If the SYN at line 6 had arrived before the RST, a more complex exchange might have occurred with RST's sent in both directions. Half-Open Connections and Other Anomalies An established connection is said to be "half-open" if one of the TCPs has closed or aborted the connection at its end without the knowledge of the other, or if the two ends of the connection have become desynchronized owing to a crash that resulted in loss of memory. Such connections will automatically become reset if an attempt is made to send data in either direction. However, half-open connections are expected to be unusual, and the recovery procedure is mildly involved. If at site A the connection no longer exists, then an attempt by the user at site B to send any data on it will result in the site B TCP receiving a reset control message. Such a message should indicate to the site B TCP that something is wrong, and it is expected to abort the connection. Assume that two user processes A and B are communicating with one another when a crash occurs causing loss of memory to A's TCP. Depending on the operating system supporting A's TCP, it is likely that some error recovery mechanism exists. When the TCP is up again, A is likely to start again from the beginning or from a recovery point. As a result, A will probably try to OPEN the connection again or try to SEND on the connection it believes open. In the latter case, it receives the error message "connection not open" from the local (A's) TCP. In an attempt to establish the connection, A's TCP will send a segment containing SYN. This scenario leads to the example shown in figure 12. After TCP A crashes, the user attempts to re-open the connection. TCP B, in the meantime, thinks the connection is open. [Page 32] December 1979 Transmission Control Protocol Functional Specification TCP A TCP B 1. (CRASH) (send 300,receive 100) 2. CLOSED ESTABLISHED 3. SYN-SENT --> <SEQ=400><CTL=SYN> --> (??) 4. (!!) <-- <SEQ=300><ACK=100><CTL=ACK> <-- ESTABLISHED 5. SYN-SENT --> <SEQ=100><CTL=RST> --> (Abort!!) 6. CLOSED 7. SYN-SENT --> <SEQ=400><CTL=SYN> --> CLOSED 8. (Abort!!)<-- <SEQ=xxx><ACK=401><CTL=RST,ACK> <-- CLOSED 9. CLOSED CLOSED Half-Open Connection Discovery Figure 12. When the SYN arrives at line 3, TCP B, being in a synchronized state, responds with an acknowledgment indicating what sequence it next expects to hear (ACK 100). TCP A sees that this segment does not acknowledge anything it sent and, being unsynchronized, sends a reset (RST) because it has detected a half-open connection. TCP B aborts at line 5. TCP A will continue to retransmit its SYN; and if the user at TCP B re-opens the connection, eventually everything will work out. In the figure TCP B does not reopen the connection, and in line 8 sends a reset to reject the offered connection. An interesting alternative case occurs when TCP A crashes and TCP B tries to send data on what it thinks is a synchronized connection. This is illustrated in figure 13. In this case, the data arriving at TCP A from TCP B (line 2) is unacceptable because no such connection exists, so TCP A sends a RST. The RST is acceptable so TCP B processes it and aborts the connection. [Page 33] December 1979 Transmission Control Protocol Functional Specification TCP A TCP B 1. (CRASH) (send 300,receive 100) 2. (??) <-- <SEQ=300><ACK=100><DATA=10><CTL=ACK> <-- ESTABLISHED 3. --> <SEQ=100><CTL=RST> --> (ABORT!!) Active Side Causes Half-Open Connection Discovery Figure 13. In figure 14, we find the two TCPs A and B with passive connections waiting for SYN. An old duplicate arriving at TCP B (line 2) stirs B into action. A SYN-ACK is returned (line 3) and causes TCP A to generate a RST (the ACK in line 3 is not acceptable). TCP B accepts the reset and returns to its passive LISTEN state. TCP A TCP B 1. LISTEN LISTEN 2. ... <SEQ=Z><CTL=SYN> --> SYN-RECEIVED 3. (??) <-- <SEQ=X><ACK=Z+1><CTL=SYN,ACK> <-- SYN-RECEIVED 4. --> <SEQ=Z+1><CTL=RST> --> (return to LISTEN!) 5. LISTEN LISTEN Old Duplicate SYN Initiates a Reset on two Passive Sockets Figure 14. A variety of other cases are possible, all of which are accounted for by the following rules for RST generation and processing. Reset Generation As a general rule, reset (RST) should be sent whenever a segment arrives which apparently is not intended for the current or a future incarnation of the connection. A reset should not be sent if it is not clear that this is the case. Thus, if any segment arrives for a nonexistent connection, a reset should be sent. If a segment ACKs [Page 34] December 1979 Transmission Control Protocol Functional Specification something which has never been sent on the current connection, then one of the following two cases applies. 1. If the connection is in any non-synchronized state (LISTEN, SYN-SENT, SYN-RECEIVED) or if the connection does not exist, a reset (RST) should be formed and sent for any segment that acknowledges something not yet sent. The RST should take its SEQ field from the ACK field of the offending segment (if the ACK control bit was set), and its ACK bit should be reset (zero), except to refuse a initial SYN. A reset is also sent if an incoming segment has a security level or compartment which does not exactly match the level and compartment requested for the connection. If the precedence of the incoming segment is less than the precedence level requested a reset is sent. 2. If the connection is in a synchronized state (ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, TIME-WAIT, CLOSE-WAIT, CLOSING), any unacceptable segment should elicit only an empty acknowledgment segment containing the current send-sequence number and an acknowledgment indicating the next sequence number expected to be received. Reset Processing All reset (RST) segments are validated by checking their SEQ-fields. A reset is valid if its sequence number is the next expected sequence number. In the case of a RST received in response to an initial SYN any sequence number is acceptable if the ACK field acknowledges the SYN. The receiver of a RST first validates it, then changes state. If the receiver was in the LISTEN state, it ignores it. If the receiver was in SYN-RECEIVED state and had previously been in the LISTEN state, then the receiver returns to the LISTEN state, otherwise the receiver aborts the connection and goes to the CLOSED state. If the receiver was in any other state, it aborts the connection and advises the user and goes to the CLOSED state. 3.5. Closing a Connection CLOSE is an operation meaning "I have no more data to send." The notion of closing a full-duplex connection is subject to ambiguous interpretation, of course, since it may not be obvious how to treat the receiving side of the connection. We have chosen to treat CLOSE in a simplex fashion. The user who CLOSEs may continue to RECEIVE until he is told that the other side has CLOSED also. Thus, a program could initiate several SENDs followed by a CLOSE, and then continue to RECEIVE until signaled that a RECEIVE failed because the other side has CLOSED. We assume that the TCP will unilaterally inform a user, [Page 35] December 1979 Transmission Control Protocol Functional Specification even if no RECEIVEs are outstanding, that the other side has closed, so the user can terminate his side gracefully. A TCP will reliably deliver all buffers SENT before the connection was CLOSED so a user who expects no data in return need only wait to hear the connection was CLOSED successfully to know that all his data was received at the destination TCP. There are essentially three cases: 1) The user initiates by telling the TCP to CLOSE the connection 2) The remote TCP initiates by sending a FIN control signal 3) Both users CLOSE simultaneously Case 1: Local user initiates the close In this case, a FIN segment can be constructed and placed on the outgoing segment queue. No further SENDs from the user will be accepted by the TCP, and it enters the FIN-WAIT-1 state. RECEIVEs are allowed in this state. All segments preceding and including FIN will be retransmitted until acknowledged. When the other TCP has both acknowledged the FIN and sent a FIN of its own, the first TCP can ACK this FIN. It should be noted that a TCP receiving a FIN will ACK but not send its own FIN until its user has CLOSED the connection also. Case 2: TCP receives a FIN from the network If an unsolicited FIN arrives from the network, the receiving TCP can ACK it and tell the user that the connection is closing. The user should respond with a CLOSE, upon which the TCP can send a FIN to the other TCP. The TCP then waits until its own FIN is acknowledged whereupon it deletes the connection. If an ACK is not forthcoming, after a timeout the connection is aborted and the user is told. Case 3: both users close simultaneously A simultaneous CLOSE by users at both ends of a connection causes FIN segments to be exchanged. When all segments preceding the FINs have been processed and acknowledged, each TCP can ACK the FIN it has received. Both will, upon receiving these ACKs, delete the connection. [Page 36] December 1979 Transmission Control Protocol Functional Specification TCP A TCP B 1. ESTABLISHED ESTABLISHED 2. (Close) FIN-WAIT-1 --> <SEQ=100><CTL=FIN> --> CLOSE-WAIT 3. FIN-WAIT-2 <-- <SEQ=300><ACK=101><CTL=ACK> <-- CLOSE-WAIT 4. (Close) TIME-WAIT <-- <SEQ=301><CTL=FIN> <-- CLOSING 5. TIME-WAIT --> <SEQ=100><ACK=301><CTL=ACK> --> CLOSED 6. (2 MSL) CLOSED Normal Close Sequence Figure 15. TCP A TCP B 1. ESTABLISHED ESTABLISHED 2. (Close) (Close) FIN-WAIT-1 --> <SEQ=100><CTL=FIN> ... FIN-WAIT-1 <-- <SEQ=300><CTL=FIN> <-- ... <SEQ=100><CTL=FIN> --> 3. CLOSING --> <SEQ=100><ACK=301><CTL=ACK> ... CLOSING <-- <SEQ=300><ACK=101><CTL=ACK> <-- ... <SEQ=100><ACK=301><CTL=ACK> --> 4. CLOSED CLOSED Simultaneous Close Sequence Figure 16. [Page 37] December 1979 Transmission Control Protocol Functional Specification 3.6. Precedence and Security The intent is that connection be allowed only between ports operating with exactly the same security and compartment values and at the higher of the precedence level requested by the two parts. The precedence levels are: flash override - 111 flash - 110 immediate - 10X priority - 01X routine - 00X The security levels are: top secret - 11 secret - 10 confidential - 01 unclassified - 00 The compartments are assigned by the Defense Communications Agency. The defaults are precedence: routine, security: unclassified, compartment: zero. A host which does not implement precedence or security feature should clear these fields to zero for segments it sends. A connection attempt with mismatched security/compartment values or a lower precedence value should be rejected by sending a reset. Note that TCP modules which operate only at the default value of precedence will still have to check the precedence of incoming segments and possibly raise the precedence level they use on the connection. 3.7. Data Communication Once the connection is established data is communicated by the exchange of segments. Because segments may be lost due to errors (checksum test failure), or network congestion, TCP uses retransmission (after a timeout) to ensure delivery of every segment. Duplicate segments may arrive due to network or TCP retransmission. As discussed in the section on sequence numbers the TCP performs certain tests on the sequence and acknowledgment numbers in the segments to verify their acceptability. The sender of data keeps track of the next sequence number to use in the variable SND.NXT. The receiver of data keeps track of the next [Page 38] December 1979 Transmission Control Protocol Functional Specification sequence number to expect in the variable RCV.NXT. The sender of data keeps track of the oldest unacknowledged sequence number in the variable SND.UNA. If the data flow is momentarily idle and all data sent has been acknowledged then the three variables will be equal. When the sender creates a segment and transmits it the sender advances SND.NXT. When the receiver accepts a segment it advances RCV.NXT and sends an acknowledgment. When the data sender receives an acknowledgment it advances SND.UNA. The extent to which the values of these variables differ is a measure of the delay in the communication. Normally the amount by which the variables are advanced is the length of the data in the segment. However, when letters are used there are special provisions for coordination the sequence numbers, the letter boundaries, and the receive buffer boundaries. End of Letter Sequence Number Adjustments There is provision in TCP for the receiver of data to optionally communicate to the sender of data on a connection at the time of the connection synchronization the receiver's buffer size. If this is done the receiver must use this fixed size of buffers for the lifetime of the connection. If a buffer size is communicated then there is a coordination between receive buffers, letters, and sequence numbers. Each time a buffer is completed either due to being filled or due to an end of letter, the sequence number is incremented through the end of that buffer. That is, whenever an EOL is transmitted, the sender advances its send sequence number, SND.NXT, by an amount sufficient to consume all the unused space in the receiver's buffer. The amount of space consumed in this fashion is subtracted from the send window just as is the space consumed by actual data. And, whenever an EOL is received, the receiver advances its receive sequence number, RCV.NXT, by an amount sufficient to consume all the unused space in the receiver's buffer. The amount of space consumed in this fashion is subtracted from the receive window just as is the space consumed by actual data. [Page 39] December 1979 Transmission Control Protocol Functional Specification older sequence numbers newer sequence numbers | Buffer 1 | Buffer 2 | | ----+-------------------------------+----------------- XXXXXXXXXXXXXXXXXXXXX+++++++++++ | | | |<-----SEG.LEN------>| | | | | | | | SEG.SEQ A B XXX - data octets from segment +++ - phantom data <----- sequence space -----> End of Letter Adjustment Figure 17. In the case illustrated above, if the segment does not carry an EOL flag, the next value of SND.NXT or RCV.NXT will be A. If it does carry an EOL flag, the next value will be B. The exchange of buffer size and sequencing information is done in units of octets. If no buffer size is stated, then the buffer size is assumed to be 1 octet. The receiver tells the sender the size of the buffer in a SYN segment that contains the 16 bit buffer size data in an option field in the TCP header. Each EOL advances the sequence number (SN) to the next buffer boundary While LBB < SEG.SEQ+SEG.LEN Do LBB <- LBB + BS End SN <- LBB where LBB is the Last Buffer Beginning, and BS is the buffer size. The CLOSE user call implies an end of letter, as does the FIN control flag in an incoming segment. The Communication of Urgent Information The objective of the TCP urgent mechanism is to allow the sending user to stimulate the receiving user to accept some urgent data and to permit the receiving TCP to indicate to the receiving user when all the currently known urgent data has been received by the user. [Page 40] December 1979 Transmission Control Protocol Functional Specification This mechanism permits a point in the data stream to be designated as the end of "urgent" information. Whenever this point is in advance of the receive sequence number (RCV.NXT) at the receiving TCP, that TCP should tell the user to go into "urgent mode"; when the receive sequence number catches up to the urgent pointer, the TCP should tell user to go into "normal mode". If the urgent pointer is updated while the user is in "read fast" mode, the update will be invisible to the user. The method employs a urgent field which is carried in all segments transmitted. The URG control flag indicates that the urgent field is meaningful and should be added to the segment sequence number to yield the urgent pointer. The absence of this flag indicates that the urgent pointer has not changed. To send an urgent indication the user must also send at least one data octet. If the sending user also indicates end of letter, timely delivery of the urgent information to the destination process is enhanced. Managing the Window The window sent in each segment indicates the range of sequence number the sender of the window (the data receiver) is currently prepared to accept. There is an assumption that this is somehow related to to the currently available data buffer space available for this connection. Indicating a large window encourages transmissions. If more data arrives than can be accepted, this will result in excessive retransmissions, adding unnecessarily to the load on the network and the TCPs. Indicating a small window may restrict the transmission of data to the point of introducing a round trip delay between each new segment transmitted. The mechanisms provided allow a TCP to advertise a large window and to subsequently advertise a much smaller window without having accepted that much data. This, so called "shrinking the window," is strongly discouraged. The robustness principle dictates that TCPs will not shrink the window themselves, but will be prepared for such behavior on the part of other TCPs. The sending TCP must be prepared to accept and send at least one octet of new data even if the send window is zero. The sending TCP should regularly retransmit to the receiving TCP even when the window is zero. Two minutes is recommended for the retransmission interval when the window is zero. This retransmission is essential to guarantee that when either TCP has a zero window the re-opening of the window will be reliably reported to the other. [Page 41] December 1979 Transmission Control Protocol Functional Specification Users must keep reading connections they close for sending until the TCP says no more data. In a connection with a one-way data flow, the window information will be carried in acknowledgment segments that all have the same sequence number so there will be no way to reorder them if they arrive out of order. This is not a serious problem, but it will allow the window information to be on occasion temporarily based on old reports from the data receiver. 3.8. Interfaces There are of course two interfaces of concern: the user/TCP interface and the TCP/IP interface. We have a fairly elaborate model of the user/TCP interface, but only a sketch of the interface to the lower level protocol module. User/TCP Interface The functional description of user commands to the TCP is, at best, fictional, since every operating system will have different facilities. Consequently, we must warn readers that different TCP implementations may have different user interfaces. However, all TCPs must provide a certain minimum set of services to guarantee that all TCP implementations can support the same protocol hierarchy. This section specifies the functional interfaces required of all TCP implementations. TCP User Commands The following sections functionally characterize a 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., SVCs, UUOs, EMTs). The user commands described below specify the basic functions the TCP must 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 or RECEIVE issued by the user for a given connection. [Page 42] December 1979 Transmission Control Protocol Functional Specification In providing interprocess communication facilities, the TCP must not only accept commands, but must also return information to the processes it serves. The latter consists of: (a) general information about a connection (e.g., 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 below. Open Format: OPEN (local port, foreign socket, active/passive [, buffer size] [, timeout] [, precedence] [, security/compartment]) -> local connection name 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 local network and TCP identifiers for the source address will either be supplied by the TCP or by the processes that serve it (e.g., the program which interfaces the TCP network). 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 the active/passive flag is set to passive, then this is a call to LISTEN for an incoming connection. A passive open may have either a fully specified foreign socket to wait for a particular connection or an unspecified foreign socket to wait for any call. A fully specified passive call can be made active by the subsequent execution of a SEND. A full-duplex transmission control block (TCB) is created and partially filled in with data from the OPEN command parameters. On an active OPEN command, the TCP will begin the procedure to synchronize (i.e., establish) the connection at once. The buffer size, if present, indicates that the caller will always receive data from the connection in that size of buffers. This buffer size is a measure of the buffer between the user and [Page 43] December 1979 Transmission Control Protocol Functional Specification the local TCP. The buffer size between the two TCPs may be different. The timeout, if present, permits the caller to set up a timeout for all buffers transmitted on the connection. If a buffer is not successfully delivered to the destination within the timeout period, the TCP will abort the connection. The present global default is 30 seconds. The buffer retransmission rate may vary; most likely, it will be related to the measured time for responses from the remote TCP. The TCP or some component of the operating system will verify the users authority to open a connection with the specified precedence or security/compartment. The absence of precedence or security/compartment specification in the OPEN call indicates the default values should be used. TCP will accept incoming requests as matching only if the security/compartment information is exactly the same and only if the precedence is equal to or higher than the precedence requested in the OPEN call. The precedence for the connection is the higher of the values requested in the OPEN call and received from the incoming request, and fixed at that value for the life of the connection. 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. Send Format: SEND(local connection name, buffer address, byte count, EOL flag, URGENT 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 TCP segment created from the [Page 44] December 1979 Transmission Control Protocol Functional Specification buffer. If the EOL flag is not set, subsequent SENDs will appear to be part of the same letter. If the URGENT flag is set, segments resulting from this call will have the urgent pointer set to indicate that some of the data associated with this call is urgent. This facility, for example, can be used to simulate "break" signals from terminals or error or completion codes from I/O devices. The semantics of this signal to the receiving process are unspecified. The receiving TCP will signal the urgent condition to the receiving process as long as the urgent pointer indicates that data preceding the urgent pointer has not been consumed by the receiving process. The purpose of urgent is to stimulate the receiver to accept some urgent data and to indicate to the receiver when all the currently known urgent data has been received. The number of times the sending user's TCP signals urgent will not necessarily be equal to the number of times the receiving user will be notified of the presence of urgent data. 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 segment arriving for the local socket), then the designated buffer 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 a timeout is specified, then the current timeout for this connection is changed to the new one. 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. However, this simple method is both highly subject to deadlocks (for example, both sides of the connection might try to do SENDs before doing any RECEIVEs) and offers poor performance, so it is not recommended. 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. [Page 45] December 1979 Transmission Control Protocol Functional Specification Multiple SENDs are served in first come, first served order, so the TCP will queue those it cannot service immediately. We have implicitly assumed an asynchronous user interface in which a SEND later elicits some kind of SIGNAL or pseudo-interrupt from the serving TCP. An alternative is to return a response immediately. For instance, SENDs might return immediate local acknowledgment, even if the segment sent had not been acknowledged by the distant TCP. We could optimistically assume eventual success. If we are wrong, the connection will close anyway due to the timeout. In implementations of this kind (synchronous), there will still be some asynchronous signals, but these will deal with the connection itself, and not with specific segments or letters. NOTA BENE: In order for the process to distinguish among error or success indications for different SENDs, it might be appropriate for the buffer address to be returned along with the coded response to the SEND request. We will offer an example event code format below, showing the information which should be returned to the calling process. Receive 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 the buffer was filled, or some error occurred, but this scheme is highly subject to deadlocks. A more sophisticated implementation would permit several RECEIVEs to be outstanding at once. These would be filled as letters, segments or fragments 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 or a buffer filled. If insufficient buffer space is given to reassemble a complete letter, the EOL flag will not be set in the response to the RECEIVE. The buffer will be filled with as much data as it can hold. The last buffer required to hold the letter is returned with EOL signaled. [Page 46] December 1979 Transmission Control Protocol Functional Specification 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. If a buffer size was given in the OPEN call, then all buffers presented in RECEIVE calls must be of exactly that size, or an error indication will be returned. The URGENT flag will be set only if the receiving user has previously been informed via a general event, that urgent data is waiting. The receiving user should thus be in "read-fast" mode. If the URGENT flag is on, additional urgent data remains. If the URGENT flag is off, this call to RECEIVE has returned all the urgent data, and the user may now leave "read-fast" mode. 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. Alternative implementations of RECEIVE might have the TCP allocate buffer storage, or the TCP might share a ring buffer with the user. Variations of this kind will produce obvious variation in user interface to the TCP. Close 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. Closing connections is intended to be a graceful operation in the sense that outstanding SENDs will be transmitted (and retransmitted), as flow control permits, until all have been serviced. Thus, it should be acceptable to make several SEND calls, followed by a CLOSE, and expect all the data to be sent to the destination. It should also be clear that users should continue to RECEIVE on CLOSING connections, since the other side may be trying to transmit the last of its data. Thus, CLOSE means "I have no more to send" but does not mean "I will not receive any more." It may happen (if the user level protocol is not well thought out) that the closing side is unable to get rid [Page 47] December 1979 Transmission Control Protocol Functional Specification of all its data before timing out. In this event, CLOSE turns into ABORT, and the closing TCP gives up. The user may CLOSE the connection at any time on his own initiative, or in response to various prompts from the TCP (e.g., 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 error responses. Close also implies end of letter. Status Format: STATUS(local connection name) This is an implementation dependent user command and could be excluded without adverse effect. Information returned would typically come from the TCB associated with the connection. 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 buffers awaiting acknowledgment, number of buffers pending receipt (including partial ones), receive buffer size, urgent state, precedence, security/compartment, and default transmission timeout. Depending on the state of the connection, or on the implementation itself, 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. [Page 48] December 1979 Transmission Control Protocol Functional Specification Abort Format: ABORT (local connection name) This command causes all pending SENDs and RECEIVES to be aborted, the TCB to be removed, and a special RESET message to be sent to the TCP on the other side of the connection. Depending on the implementation, users may receive abort indications for each outstanding SEND or RECEIVE, or may simply receive an ABORT-acknowledgment. TCP-to-User Messages It is assumed that the operating system environment provides a means for the TCP to asynchronously signal the user program. When the TCP does signal a user program, certain information is passed to the user. Often in the specification the information will be an error message. In other cases there will be information relating to the completion of processing a SEND or RECEIVE or other user call. The following information is provided: Local Connection Name Always Response String Always Buffer Address Send & Receive Byte count (counts bytes received) Receive End-of-Letter flag Receive End-of-Urgent flag Receive TCP/Network Interface The TCP calls on a lower level protocol module to actually send and receive information over a network. One case is that of the ARPA internetwork system where the lower level module is the Internet Protocol [1]. In most cases the following simple interface would be adequate. [Page 49] December 1979 Transmission Control Protocol Functional Specification The following two calls satisfy the requirements for the TCP to internet protocol module communication: SEND (dest, TOS, TTL, BufPTR, len, Id, DF, options => result) where: dest = destination address TOS = type of service TTL = time to live BufPTR = buffer pointer len = length of buffer Id = Identifier DF = Don't Fragment options = option data result = response OK = datagram sent ok Error = error in arguments or local network error Note that the precedence is included in the TOS and the security/compartment is passed as an option. RECV (BufPTR => result, source, dest, prot, TOS, len) where: BufPTR = buffer pointer result = response OK = datagram received ok Error = error in arguments source = source address dest = destination address prot = protocol TOS = type of service options = option data len = length of buffer Note that the precedence is in the TOS, and the security/compartment is an option. When the TCP sends a segment, it executes the SEND call supplying all the arguments. The internet protocol module, on receiving this call, checks the arguments and prepares and sends the message. If the arguments are good and the segment is accepted by the local network, the call returns successfully. If either the arguments are bad, or the segment is not accepted by the local network, the call returns unsuccessfully. On unsuccessful returns, a reasonable report should be made as to the cause of the [Page 50] December 1979 Transmission Control Protocol Functional Specification problem, but the details of such reports are up to individual implementations. When a segment arrives at the internet protocol module from the local network, either there is a pending RECV call from TCP or there is not. In the first case, the pending call is satisfied by passing the information from the segment to the TCP. In the second case, the TCP is notified of a pending segment. The notification of a TCP may be via a pseudo interrupt or similar mechanism, as appropriate in the particular operating system environment of the implementation. A TCP's RECV call may then either be immediately satisfied by a pending segment, or the call may be pending until a segment arrives. We note that the Internet Protocol provides arguments for a type of service and for a time to live. TCP uses the following settings for these parameters: Type of Service = Precedence: none, Package: stream, Reliability: higher, Preference: speed, Speed: higher; or 00011111. Time to Live = one minute, or 00111100. Note that the assumed maximum segment lifetime is two minutes. Here we explicitly ask that a segment be destroyed if it cannot be delivered by the internet system within one minute. [Page 51] December 1979 Transmission Control Protocol Functional Specification 3.9. Event Processing The activity of the TCP can be characterized as responding to events. The events that occur can be cast into three categories: user calls, arriving segments, and timeouts. This section describes the processing the TCP does in response to each of the events. In many cases the processing required depends on the state of the connection. Events that occur: User Calls OPEN SEND RECEIVE CLOSE ABORT STATUS Arriving Segments SEGMENT ARRIVES Timeouts USER TIMEOUT RETRANSMISSION TIMEOUT The model of the TCP/user interface is that user commands receive an immediate return and possibly a delayed response via an event or pseudo interrupt. In the following descriptions, the term "signal" means cause a delayed response. Error responses are given as character strings. For example, user commands referencing connections that do not exist receive "error: connection not open". Please note in the following that all arithmetic on sequence numbers, acknowledgment numbers, windows, et cetera, is modulo 2**32 the size of the sequence number space. Also note that "=<" means less than or equal to. A natural way to think about processing incoming segments is to imagine that they are first tested for proper sequence number (i.e., that their contents lie in the range of the expected "receive window" in the sequence number space) and then that they are generally queued and processed in sequence number order. [Page 52] December 1979 Transmission Control Protocol Functional Specification When a segment overlaps other already received segments we reconstruct the segment to contain just the new data, and adjust the header fields to be consistent. [Page 53] December 1979 Transmission Control Protocol Functional Specification OPEN Call OPEN Call CLOSED STATE (i.e., TCB does not exist) Create a new transmission control block (TCB) to hold connection state information. Fill in local socket identifier, foreign socket, precedence, security/compartment, and user timeout information. Verify the security and precedence requested are allowed for this user, if not return "error: precedence not allowed" or "error: security/compartment not allowed." If active and the foreign socket is unspecified, return "error: foreign socket unspecified"; if active and the foreign socket is specified, issue a SYN segment. An initial send sequence number (ISS) is selected and a SYN segment of the form <SEQ=ISS><CTL=SYN> is sent. Set SND.UNA to ISS, SND.NXT to ISS+1, SND.LBB to ISS+1, enter SYN-SENT state, and return. If the caller does not have access to the local socket specified, return "error: connection illegal for this process". If there is no room to create a new connection, return "error: insufficient resources". LISTEN STATE SYN-SENT STATE SYN-RECEIVED STATE ESTABLISHED STATE FIN-WAIT-1 STATE FIN-WAIT-2 STATE TIME-WAIT STATE CLOSE-WAIT STATE CLOSING STATE Return "error: connection already exists". [Page 54] December 1979 Transmission Control Protocol Functional Specification SEND Call SEND Call CLOSED STATE (i.e., TCB does not exist) If the user should no have access to such a connection, then return "error: connection illegal for this process". Otherwise, return "error: connection does not exist". LISTEN STATE If the foreign socket is specified, then change the connection from passive to active, select an ISS, send a SYN segment, set SND.UNA to ISS, SND.NXT to ISS+1 and SND.LBB to ISS+1. Enter SYN-SENT state. Data associated with SEND may be sent with SYN segment or queued for transmission after entering ESTABLISHED state. The urgent bit if requested in the command should be sent with the first data segment sent as a result of this command. If there is no room to queue the request, respond with "error: insufficient resources". If Foreign socket was not specified, then return "error: foreign socket unspecified". SYN-SENT STATE Queue for processing after the connection is ESTABLISHED. Typically, nothing can be sent yet, anyway, because the send window has not yet been set by the other side. If no space, return "error: insufficient resources". SYN-RECEIVED STATE Queue for later processing after entering ESTABLISHED state. If no space to queue, respond with "error: insufficient resources". ESTABLISHED STATE Segmentize the buffer, send or queue it for output, with a piggybacked acknowledgment (acknowledgment value = SND.UNA) with the data (this is not required, but there is no advantage in not doing so). If there is insufficient space to remember this buffer, simply return "error: insufficient resources". If remote buffer size is not one octet, then, if this is the end of a letter, do the following end-of-letter/buffer-size adjustment processing: [Page 55] December 1979 Transmission Control Protocol Functional Specification SEND Call if EOL = 0 then SND.NXT <- SEG.SEQ + SEG.LEN if EOL = 1 then While SND.LBB < SEG.SEQ + SEG.LEN Do SND.LBB <- SND.LBB + SND.BS End SND.NXT <- SND.LBB If the urgent flag is set, then SND.UP <- SND.NXT-1 and set the urgent pointer in the outgoing segment. FIN-WAIT-1 STATE FIN-WAIT-2 STATE TIME-WAIT STATE Return "error: connection closing" and do not service request. CLOSE-WAIT STATE Segmentize any text to be sent and queue for output. If there is insufficient space to remember the SEND, return "error: insufficient resources" CLOSING STATE Respond with "error: connection closing" [Page 56] December 1979 Transmission Control Protocol Functional Specification RECEIVE Call RECEIVE Call CLOSED STATE (i.e., TCB does not exist) If the user should no have access to such a connection, return "error: connection illegal for this process". Otherwise return "error: connection does not exist". LISTEN STATE SYN-SENT STATE SYN-RECEIVED STATE Queue for processing after entering ESTABLISHED state. If there is no room to queue this request, respond with "error: insufficient resources". ESTABLISHED STATE If insufficient incoming segments are queued to satisfy the request, queue the request. If there is no queue space to remember the RECEIVE, respond with "error: insufficient resources". Reassemble queued incoming segments into receive buffer and return to user. Mark "end of letter" (EOL) if this is the case. If RCV.UP is in advance of the data currently being passed to the user notify the user of the presence of urgent data. When the TCP takes responsibility for delivering data to the user that fact must be communicated to the sender via an acknowledgment. The formation of such an acknowledgment is described below in the discussion of processing an incoming segment. FIN-WAIT-1 STATE FIN-WAIT-2 STATE Reassemble and return a letter, or as much as will fit, in the user buffer. Queue the request if it cannot be serviced immediately. [Page 57] December 1979 Transmission Control Protocol Functional Specification RECEIVE Call TIME-WAIT STATE CLOSE-WAIT STATE Since the remote side has already sent FIN, RECEIVEs must be satisfied by text already reassembled, but not yet delivered to the user. If no reassembled segment text is awaiting delivery, the RECEIVE should get a "error: connection closing" response. Otherwise, any remaining text can be used to satisfy the RECEIVE. CLOSING STATE Return "error: connection closing" [Page 58] December 1979 Transmission Control Protocol Functional Specification CLOSE Call CLOSE Call CLOSED STATE (i.e., TCB does not exist) If the user should no have access to such a connection, return "error: connection illegal for this process". Otherwise, return "error: connection does not exist". LISTEN STATE Any outstanding RECEIVEs should be returned with "error: closing" responses. Delete TCB, return "ok". SYN-SENT STATE Delete the TCB and return "error: closing" responses to any queued SENDs, or RECEIVEs. SYN-RECEIVED STATE Queue for processing after entering ESTABLISHED state or segmentize and send FIN segment. If the latter, enter FIN-WAIT-1 state. ESTABLISHED STATE Queue this until all preceding SENDs have been segmentized, then form a FIN segment and send it. In any case, enter FIN-WAIT-1 state. FIN-WAIT-1 STATE FIN-WAIT-2 STATE Strictly speaking, this is an error and should receive a "error: connection closing" response. An "ok" response would be acceptable, too, as long as a second FIN is not emitted (the first FIN may be retransmitted though). [Page 59] December 1979 Transmission Control Protocol Functional Specification CLOSE Call TIME-WAIT STATE Strictly speaking, this is an error and should receive a "error: connection closing" response. An "ok" response would be acceptable, too. However, since the FIN has been sent and acknowledged, nothing should be sent (or retransmitted). CLOSE-WAIT STATE Queue this request until all preceding SENDs have been segmentized; then send a FIN segment, enter CLOSING state. CLOSING STATE Respond with "error: connection closing" [Page 60] December 1979 Transmission Control Protocol Functional Specification ABORT Call ABORT Call CLOSED STATE (i.e., TCB does not exist) If the user should no have access to such a connection, return "error: connection illegal for this process". Otherwise return "error: connection does not exist". LISTEN STATE Any outstanding RECEIVEs should be returned with "error: connection reset" responses. Delete TCB, return "ok". SYN-SENT STATE Delete the TCB and return "reset" responses to any queued SENDs, or RECEIVEs. SYN-RECEIVED STATE Send a RST of the form: <SEQ=SND.NXT><ACK=RCV.NXT><CTL=RST,ACK> and return any unprocessed SENDs, or RECEIVEs with "reset" code, delete the TCB. ESTABLISHED STATE Send a reset segment: <SEQ=SND.NXT><ACK=RCV.NXT><CTL=RST,ACK> All queued SENDs and RECEIVEs should be given "reset" responses; all segments queued for transmission (except for the RST formed above) or retransmission should be flushed, delete the TCB. [Page 61] December 1979 Transmission Control Protocol Functional Specification ABORT Call FIN-WAIT-1 STATE FIN-WAIT-2 STATE A reset segment (RST) should be formed and sent: <SEQ=SND.NXT><ACK=RCV.NXT><CTL=RST,ACK> Outstanding SENDs, RECEIVEs, CLOSEs, and/or segments queued for retransmission, or segmentizing, should be flushed, with "connection reset" notification to the user, delete the TCB. TIME-WAIT STATE Respond with "ok" and delete the TCB. CLOSE-WAIT STATE Flush any pending SENDs and RECEIVEs, returning "connection reset" responses for them. Form and send a RST segment: <SEQ=SND.NXT><ACK=RCV.NXT><CTL=RST,ACK> Flush all segment queues and delete the TCB. CLOSING STATE Respond with "ok" and delete the TCB; flush any remaining segment queues. If a CLOSE command is still pending, respond "error: connection reset". [Page 62] December 1979 Transmission Control Protocol Functional Specification STATUS Call STATUS Call CLOSED STATE (i.e., TCB does not exist) If the user should no have access to such a connection, return "error: connection illegal for this process". Otherwise return "error: connection does not exist". LISTEN STATE Return "state = LISTEN", and the TCB pointer. SYN-SENT STATE Return "state = SYN-SENT", and the TCB pointer. SYN-RECEIVED STATE Return "state = SYN-RECEIVED", and the TCB pointer. ESTABLISHED STATE Return "state = ESTABLISHED", and the TCB pointer. FIN-WAIT-1 STATE Return "state = FIN-WAIT-1", and the TCB pointer. FIN-WAIT-2 STATE Return "state = FIN-WAIT-2", and the TCB pointer. TIME-WAIT STATE Return "state = TIME-WAIT and the TCB pointer. CLOSE-WAIT STATE Return "state = CLOSE-WAIT", and the TCB pointer. CLOSING STATE Return "state = CLOSING", and the TCB pointer. [Page 63] December 1979 Transmission Control Protocol Functional Specification SEGMENT ARRIVES SEGMENT ARRIVES If the state is CLOSED (i.e., TCB does not exist) then all data in the incoming segment is discarded. An incoming segment containing a RST is discarded. An incoming segment not containing a RST causes a RST to be sent in response. The acknowledgment and sequence field values are selected to make the reset sequence acceptable to the TCP that sent the offending segment. If the ACK bit is off, sequence number zero is used, <SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK> If the ACK bit is on, <SEQ=SEG.ACK><CTL=RST> Return. If the state is LISTEN then first check for an ACK Any acknowledgment is bad if it arrives on a connection still in the LISTEN state. An acceptable reset segment should be formed for any arriving ACK-bearing segment, except another RST. The RST should be formatted as follows: <SEQ=SEG.ACK><CTL=RST> Return. An incoming RST should be ignored. Return. if there was no ACK then check for a SYN If the SYN bit is set, check the security. If the security/compartment on the incoming segment does not exactly match the security/compartment in the TCB then send a reset and return and check the precedence. If the SEG.PRC is less than the TCB.PRC then send a reset and return. If the SEG.PRC is greater than the TCB.PRC then set TCB.PRC<-SEG.PRC. Now RCV.NXT and RCV.LBB are set to SEG.SEQ+1, IRS is set to SEG.SEQ and any other control or text should be queued for processing later. ISS should be selected and a SYN segment sent of the form: [Page 64] December 1979 Transmission Control Protocol Functional Specification SEGMENT ARRIVES <SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK> SND.NXT and SND.LBB are set to ISS+1 and SND.UNA to ISS. The connection state should be changed to SYN-RECEIVED. Note that any other incoming control or data (combined with SYN) will be processed in the SYN-RECEIVED state, but processing of SYN and ACK should not be repeated. This segment may also include data and control bits (e.g., URG, EOL) which were queued for transmission. if there was no SYN but there was other text or control Any other control or text-bearing segment (not containing SYN) should have an ACK and thus would be discarded by the ACK processing. An incoming RST segment could not be valid, since it could not have been sent in response to anything sent by this incarnation of the connection. So you are unlikely to get here, but if you do, drop the segment, and return. If the state is SYN-SENT then first check for an ACK If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, or the security/compartment in the segment does not exactly match the security/compartment in the TCB, or the precedence in the segment is less than the precedence in the TCB, send a reset <SEQ=SEG.ACK><CTL=RST> and discard the segment. Return. If SND.UNA =< SEG.ACK =< SND.NXT and the security/compartment and precedence are acceptable then the ACK is acceptable. SND.UNA should be advanced to equal SEG.ACK, and any segments on the retransmission queue which are thereby acknowledged should be removed. if the ACK is ok (or there is no ACK), check the RST bit If the RST bit is set then signal the user "error: connection reset", enter CLOSED state, drop the segment, delete TCB, and return. if the ACK is ok (or there is no ACK) and it was not a RST, check the SYN bit [Page 65] December 1979 Transmission Control Protocol Functional Specification SEGMENT ARRIVES If the SYN bit is on and the security/compartment and precedence are acceptable then, RCV.NXT and RCV.LBB are set to SEG.SEQ+1, IRS is set to SEG.SEQ. If SND.UNA > ISS (our SYN has been ACKed), change the connection state to ESTABLISHED, otherwise enter SYN-RECEIVED. In any case, form an ACK segment: <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK> and send it. Data or controls which were queued for transmission may be included. If SEG.PRC is greater than TCB.PRC set TCB.PRC<-SEG.PRC. If there are other controls or text in the segment then continue processing at the fifth step below where the URG bit is checked, otherwise return. [Page 66] December 1979 Transmission Control Protocol Functional Specification SEGMENT ARRIVES Otherwise, first check sequence number SYN-RECEIVED STATE ESTABLISHED STATE FIN-WAIT-1 STATE FIN-WAIT-2 STATE TIME-WAIT STATE CLOSE-WAIT STATE CLOSING STATE Segments are processed in sequence. Initial tests on arrival are used to discard old duplicates, but further processing is done in SEG.SEQ order. If a segment's contents straddle the boundary between old and new, only the new parts should be processed. There are four cases for the acceptability test for an incoming segment: Segment Receive Test Length Window ------- ------- ------------------------------------------- 0 0 SEG.SEQ = RCV.NXT 0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND >0 0 not acceptable >0 >0 RCV.NXT < SEG.SEQ+SEG.LEN =< RCV.NXT+RCV.WND Note that the test above guarantees that the last sequence number used by the segment lies in the receive-window. If the RCV.WND is zero, no segments will be acceptable, but special allowance should be made to accept valid ACKs. If an incoming segment is not acceptable, an acknowledgment should be sent in reply: <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK> If the incoming segment is unacceptable, drop it and return. [Page 67] December 1979 Transmission Control Protocol Functional Specification SEGMENT ARRIVES second check security and precedence If the security/compartment and precedence in the segment do not exactly match the security/compartment and precedence in the TCB then form a reset and return. Note this check is placed following the sequence check to prevent a segment from an old connection between these parts with a different security or precedence from causing an abort of the current connection. third check the ACK field, SYN-RECEIVED STATE If the RST bit is off and SND.UNA < SEG.ACK =< SND.NXT then set SND.UNA <- SEG.ACK, remove any acknowledged segments from the retransmission queue, and enter ESTABLISHED state. If the segment acknowledgment is not acceptable, form a reset segment, <SEQ=SEG.ACK><CTL=RST> and send it, unless the incoming segment is an RST (or there is no ACK), in which case, it should be discarded, then return. ESTABLISHED STATE If SND.UNA < SEG.ACK =< SND.NXT then, set SND.UNA <- SEG.ACK. Any segments on the retransmission queue which are thereby entirely acknowledged are removed. Users should receive positive acknowledgments for buffers which have been SENT and fully acknowledged (i.e., SEND buffer should be returned with "ok" response). If the ACK is a duplicate, it can be ignored. If the segment passes the sequence number and acknowledgment number tests, the send window should be updated. If SND.WL =< SEG.SEQ, set SND.WND <- SEG.WND and set SND.WL <- SEG.SEQ. If the remote buffer size is not one, then the end-of-letter/buffer-size adjustment to sequence numbers may have an effect on the next expected sequence number to be acknowledged. It is possible that the remote TCP will acknowledge with a SEG.ACK equal to a sequence number of an [Page 68] December 1979 Transmission Control Protocol Functional Specification SEGMENT ARRIVES octet that was skipped over at the end of a letter. This a mild error on the remote TCPs part, but not cause for alarm. FIN-WAIT-1 STATE FIN-WAIT-2 STATE In addition to the processing for the ESTABLISHED state, if the retransmission queue is empty, the user's CLOSE can be acknowledged ("ok") but do not delete the TCB. TIME-WAIT STATE The only thing that can arrive in this state is a retransmission of the remote FIN. Acknowledge it, and restart the 2 MSL timeout. CLOSE-WAIT STATE CLOSING STATE Do the same processing as for the ESTABLISHED state. fourth check the RST bit, SYN-RECEIVED STATE If the RST bit is set then, if the segment has passed sequence and acknowledgment tests, it is valid. If this connection was initiated with a passive OPEN (i.e., came from the LISTEN state), then return this connection to LISTEN state. The user need not be informed. If this connection was initiated with an active OPEN (i.e., came from SYN-SENT state) then the connection was refused, signal the user "connection refused". In either case, all segments on the retransmission queue should be removed. ESTABLISHED FIN-WAIT-1 FIN-WAIT-2 CLOSE-WAIT CLOSING STATE If the RST bit is set then, any outstanding RECEIVEs and SEND should receive "reset" responses. All segment queues should be flushed. Users should also receive an unsolicited general "connection reset" signal. Enter the CLOSED state, delete the TCB, and return. [Page 69] December 1979 Transmission Control Protocol Functional Specification SEGMENT ARRIVES TIME-WAIT Enter the CLOSED state, delete the TCB, and return. fifth, check the SYN bit, SYN-RECEIVED ESTABLISHED STATE If the SYN bit is set, check the segment sequence number against the receive window. The segment sequence number must be in the receive window; if not, ignore the segment. If the SYN is on and SEG.SEQ = RCV.NXT-1, then everything is ok and no action is needed; but if they are not equal, there is an error and a reset must be sent. If a reset must be sent it is formed as follows: <SEQ=SEG.ACK><CTL=RST> The connection must be aborted as if a RST had been received. FIN-WAIT STATE-1 FIN-WAIT STATE-2 TIME-WAIT STATE CLOSE-WAIT STATE CLOSING STATE This case should not occur, since a duplicate of the SYN which started the current connection incarnation will have been filtered in the SEG.SEQ processing. Other SYN's will have been rejected by this test as well (see SYN processing for ESTABLISHED state). sixth, check the URG bit, ESTABLISHED STATE FIN-WAIT-1 STATE FIN-WAIT-2 STATE If the URG bit is set, RCV.UP <- max(RCV.UP,SEG.UP), and signal the user that the remote side has urgent data if the urgent pointer (RCV.UP) is in advance of the data consumed. If the user has already been signaled (or is still in the "urgent mode") for this continuous sequence of urgent data, do not signal the user again. [Page 70] December 1979 Transmission Control Protocol Functional Specification SEGMENT ARRIVES TIME-WAIT STATE CLOSE-WAIT STATE CLOSING This should not occur, since a FIN has been received from the remote side. Ignore the URG. seventh, process the segment text, ESTABLISHED STATE Once in the ESTABLISHED state, it is possible to deliver segment text to user RECEIVE buffers. Text from segments can be moved into buffers until either the buffer is full or the segment is empty. If the segment empties and carries an EOL flag, then the user is informed, when the buffer is returned, that an EOL has been received. If buffer size is not one octet, then do the following end-of-letter/buffer-size adjustment processing: if EOL = 0 then RCV.NXT <- SEG.SEQ + SEG.LEN if EOL = 1 then While RCV.LBB < SEG.SEQ+SEG.LEN Do RCV.LBB <- RCV.LBB + RCV.BS End RCV.NXT <- RCV.LBB When the TCP takes responsibility for delivering the data to the user it must also acknowledge the receipt of the data. Send an acknowledgment of the form: <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK> This acknowledgment should be piggybacked on a segment being transmitted if possible without incurring undue delay. [Page 71] December 1979 Transmission Control Protocol Functional Specification SEGMENT ARRIVES FIN-WAIT-1 STATE FIN-WAIT-2 STATE If there are outstanding RECEIVEs, they should be satisfied, if possible, with the text of this segment; remaining text should be queued for further processing. If a RECEIVE is satisfied, the user should be notified, with "end-of-letter" (EOL) signal, if appropriate. TIME-WAIT STATE CLOSE-WAIT STATE This should not occur, since a FIN has been received from the remote side. Ignore the segment text. eighth, check the FIN bit, Send an acknowledgment for the FIN. Signal the user "connection closing", and return any pending RECEIVEs with same message. Note that FIN implies EOL for any segment text not yet delivered to the user. If the current state is ESTABLISHED, enter the CLOSE-WAIT state. If the current state is FIN-WAIT-1, enter the CLOSING state. If the current state is FIN-WAIT-2, enter the TIME-WAIT state. and return. [Page 72] December 1979 Transmission Control Protocol Functional Specification USER TIMEOUT USER TIMEOUT For any state if the user timeout expires, flush all queues, signal the user "error: connection aborted due to user timeout" in general and for any outstanding calls, delete the TCB, and return. RETRANSMISSION TIMEOUT For any state if the retransmission timeout expires on a segment in the retransmission queue, send the segment at the front of the retransmission queue again, reinitialize the retransmission timer, and return. [Page 73] December 1979 Transmission Control Protocol [Page 74] December 1979 Transmission Control Protocol GLOSSARY 1822 BBN Report 1822, "The Specification of the Interconnection of a Host and an IMP". The specification of interface between a host and the ARPANET. ACK A control bit (acknowledge) occupying no sequence space, which indicates that the acknowledgment field of this segment specifies the next sequence number the sender of this segment is expecting to receive, hence acknowledging receipt of all previous sequence numbers. ARPANET message The unit of transmission between a host and an IMP in the ARPANET. The maximum size is about 1012 octets (8096 bits). ARPANET packet A unit of transmission used internally in the ARPANET between IMPs. The maximum size is about 126 octets (1008 bits). buffer size An option (buffer size) used to state the receive data buffer size of the sender of this option. May only be sent in a segment that also carries a SYN. connection A logical communication path identified by a pair of sockets. datagram A message sent in a packet switched computer communications network. Destination Address The destination address, usually the network and host identifiers. EOL A control bit (End of Letter) occupying no sequence space, indicating that this segment ends a logical letter with the last data octet in the segment. If this end of letter causes a less than full buffer to be released to the user and the connection buffer size is not one octet then the end-of-letter/buffer-size adjustment to the receive sequence number must be made. [Page 75] December 1979 Transmission Control Protocol Glossary FIN A control bit (finis) occupying one sequence number, which indicates that the sender will send no more data or control occupying sequence space. fragment A portion of a logical unit of data, in particular an internet fragment is a portion of an internet datagram. FTP A file transfer protocol. header Control information at the beginning of a message, segment, fragment, packet or block of data. host A computer. In particular a source or destination of messages from the point of view of the communication network. Identification An Internet Protocol field. This identifying value assigned by the sender aids in assembling the fragments of a datagram. IMP The Interface Message Processor, the packet switch of the ARPANET. internet address A source or destination address specific to the host level. internet datagram The unit of data exchanged between an internet module and the higher level protocol together with the internet header. internet fragment A portion of the data of an internet datagram with an internet header. IP Internet Protocol. IRS The Initial Receive Sequence number. The first sequence number used by the sender on a connection. [Page 76] December 1979 Transmission Control Protocol Glossary ISN The Initial Sequence Number. The first sequence number used on a connection, (either ISS or IRS). Selected on a clock based procedure. ISS The Initial Send Sequence number. The first sequence number used by the sender on a connection. leader Control information at the beginning of a message or block of data. In particular, in the ARPANET, the control information on an ARPANET message at the host-IMP interface. left sequence This is the next sequence number to be acknowledged by the data receiving TCP (or the lowest currently unacknowledged sequence number) and is sometimes referred to as the left edge of the send window. letter A logical unit of data, in particular the logical unit of data transmitted between processes via TCP. local packet The unit of transmission within a local network. module An implementation, usually in software, of a protocol or other procedure. MSL Maximum Segment Lifetime, the time a TCP segment can exist in the internetwork system. octet An eight bit byte. Options An Option field may contain several options, and each option may be several octets in length. The options are used primarily in testing situations; for example, to carry timestamps. Both the Internet Protocol and TCP provide for options fields. packet A package of data with a header which may or may not be [Page 77] December 1979 Transmission Control Protocol Glossary logically complete. More often a physical packaging than a logical packaging of data. port The portion of a socket that specifies which logical input or output channel of a process is associated with the data. process A program in execution. A source or destination of data from the point of view of the TCP or other host-to-host protocol. PSN A Packet Switched Network. For example, the ARPANET. RCV.BS receive buffer size, the remote buffer size RCV.LBB receive last buffer beginning RCV.NXT receive next sequence number RCV.UP receive urgent pointer RCV.WND receive window receive last buffer beginning This is the sequence number of the first octet of the most recent buffer. This value is use in calculating the next sequence number when a segment contains an end of letter indication. receive next sequence number This is the next sequence number the local TCP is expecting to receive. receive window This represents the sequence numbers the local (receiving) TCP is willing to receive. Thus, the local TCP considers that segments overlapping the range RCV.NXT to RCV.NXT + RCV.WND - 1 carry acceptable data or control. Segments containing sequence numbers entirely outside of this range are considered duplicates and discarded. [Page 78] December 1979 Transmission Control Protocol Glossary RST A control bit (reset), occupying no sequence space, indicating that the receiver should delete the connection without further interaction. The receiver can determine, based on the sequence number and acknowledgment fields of the incoming segment, whether it should honor the reset command or ignore it. In no case does receipt of a segment containing RST give rise to a RST in response. RTP Real Time Protocol: A host-to-host protocol for communication of time critical information. Rubber EOL An end of letter (EOL) requiring a sequence number adjustment to align the beginning of the next letter on a buffer boundary. SEG.ACK segment acknowledgment SEG.LEN segment length SEG.PRC segment precedence value SEG.SEQ segment sequence SEG.UP segment urgent pointer field SEG.WND segment window field segment A logical unit of data, in particular a TCP segment is the unit of data transfered between a pair of TCP modules. segment acknowledgment The sequence number in the acknowledgment field of the arriving segment. segment length The amount of sequence number space occupied by a segment, including any controls which occupy sequence space. [Page 79] December 1979 Transmission Control Protocol Glossary segment sequence The number in the sequence field of the arriving segment. send last buffer beginning This is the sequence number of the first octet of the most recent buffer. This value is use in calculating the next sequence number when a segment contains an end of letter indication. send sequence This is the next sequence number the local (sending) TCP will use on the connection. It is initially selected from an initial sequence number curve (ISN) and is incremented for each octet of data or sequenced control transmitted. send window This represents the sequence numbers which the remote (receiving) TCP is willing to receive. It is the value of the window field specified in segments from the remote (data receiving) TCP. The range of sequence numbers which may be emitted by a TCP lies between SND.NXT and SND.UNA + SND.WND - 1. SND.BS send buffer size, the local buffer size SND.LBB send last buffer beginning SND.NXT send sequence SND.UNA left sequence SND.UP send urgent pointer SND.WL send sequence number at last window update SND.WND send window socket An address which specifically includes a port identifier, that is, the concatenation of an Internet Address with a TCP port. [Page 80] December 1979 Transmission Control Protocol Glossary Source Address The source address, usually the network and host identifiers. SYN A control bit in the incoming segment, occupying one sequence number, used at the initiation of a connection, to indicate where the sequence numbering will start. TCB Transmission control block, the data structure that records the state of a connection. TCB.PRC The precedence of the connection. TCP Transmission Control Protocol: A host-to-host protocol for reliable communication in internetwork environments. TOS Type of Service, an Internet Protocol field. Type of Service An Internet Protocol field which indicates the type of service for this internet fragment. URG A control bit (urgent), occupying no sequence space, used to indicate that the receiving user should be notified to do urgent processing as long as there is data to be consumed with sequence numbers less than the value indicated in the urgent pointer. urgent pointer A control field meaningful only when the URG bit is on. This field communicates the value of the urgent pointer which indicates the data octet associated with the sending user's urgent call. [Page 81] December 1979 Transmission Control Protocol [Page 82] December 1979 Transmission Control Protocol REFERENCES [1] Postel, J. (ed.), "DOD Standard Internet Protocol," Defense Advanced Research Projects Agency, Information Processing Techniques Office, IEN 123, December 1979. [2] Feinler, E. and J. Postel, ARPANET Protocol Handbook, Network Information Center, Stanford Research Institute, Menlo Park, CA, January 1978. [3] Dalal, Y. and C. Sunshine, "Connection Management in Transport Protocols," Computer Networks, Vol. 2, No. 6, pp. 454-473, December 1978. [Page 83] December 1979 Transmission Control Protocol [Page 84]