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RFC: 793 TRANSMISSION CONTROL PROTOCOL DARPA INTERNET PROGRAM PROTOCOL SPECIFICATION September 1981 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 September 1981 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 ........................................ 9 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 .................................... 30 3.5 Closing a Connection ......................................... 37 3.6 Precedence and Security ...................................... 40 3.7 Data Communication ........................................... 40 3.8 Interfaces ................................................... 44 3.9 Event Processing ............................................. 52 GLOSSARY ............................................................ 79 REFERENCES .......................................................... 85 [Page i] September 1981 Transmission Control Protocol [Page ii] September 1981 Transmission Control Protocol PREFACE This document describes the DoD Standard Transmission Control Protocol (TCP). There have been nine 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 clarifies several details and removes the end-of-letter buffer-size adjustments, and redescribes the letter mechanism as a push function. Jon Postel Editor [Page iii] RFC: 793 Replaces: RFC 761 IENs: 129, 124, 112, 81, 55, 44, 40, 27, 21, 5 TRANSMISSION CONTROL PROTOCOL DARPA INTERNET PROGRAM PROTOCOL SPECIFICATION 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 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 focuses its attention primarily 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. 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] September 1981 Transmission Control Protocol Introduction TCP is based on concepts first described by Cerf and Kahn in [1]. The TCP fits into a layered protocol architecture just above a basic Internet Protocol [2] 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. The internet protocol also deals with any fragmentation or reassembly of the TCP segments 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. Some 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 [Page 2] September 1981 Transmission Control Protocol Introduction section offers a very brief view of the protocol interfaces and 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 data 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, a minimum functionality is required at the TCP/user interface for any valid implementation. The interface between TCP and lower level protocol is essentially unspecified except that it is assumed there is a mechanism whereby the two levels 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. The lower level protocol which is assumed throughout this document is the Internet Protocol [2]. 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] September 1981 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 general, the TCPs decide when to block and forward data at their own convenience. Sometimes users need to be sure that all the data they have submitted to the TCP has been transmitted. For this purpose a push function is defined. To assure that data submitted to a TCP is actually transmitted the sending user indicates that it should be pushed through to the receiving user. A push causes the TCPs to promptly forward and deliver data up to that point to the receiver. The exact push point might not be visible to the receiving user and the push function does not supply a record boundary marker. 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 correct delivery of data. TCP recovers from internet communication system errors. 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. The window indicates an allowed number of octets that the sender may transmit before receiving further permission. [Page 4] September 1981 Transmission Control Protocol Introduction 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 and host addresses from the internet communication layer, this forms a socket. A pair of sockets uniquely identifies each connection. That is, a socket may be simultaneously used in multiple connections. 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 between unreliable hosts and 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] September 1981 Transmission Control Protocol [Page 6] September 1981 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. The term packet is used generically here to mean the data of one transaction between a host and its network. The format of data blocks exchanged within the 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 user's buffer and notifies the receiving user. The TCPs include control information in the segments which they use to ensure reliable ordered data transmission. The model of internet communication is that there is an internet protocol module associated with each TCP which provides an interface to the local network. This internet module packages TCP segments inside internet datagrams and routes these datagrams to a destination internet module 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 [Page 7] September 1981 Transmission Control Protocol Philosophy other operations to achieve the delivery of the local packet to the destination internet module. 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, or to the final destination. 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 further broken into smaller fragments at subsequent gateways. The internet datagram fragment format is designed so that the destination internet module can reassemble fragments into internet datagrams. A destination internet module 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 (or internet module) 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 an 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. 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 in this document. [Page 8] September 1981 Transmission Control Protocol Philosophy 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 & ICMP | Gateway Level +-------------------------------+ | +---------------------------+ | Local Network Protocol | Network Level +---------------------------+ Protocol Relationships Figure 2. 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. [Page 9] September 1981 Transmission Control Protocol Philosophy 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 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 containing data, 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 receiving TCP has taken the responsibility to do so. To govern the flow of data between TCPs, a flow control mechanism is employed. The receiving TCP reports a "window" to the sending TCP. This window specifies the number of octets, starting with the acknowledgment number, that the 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 TCP they might not be unique. To provide for unique addresses within 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. A 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 are necessary in any implementation. 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 initiate connections only 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 [Page 10] September 1981 Transmission Control Protocol Philosophy 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 passive OPEN request means that the process wants to accept incoming connection requests rather than attempting to initiate a connection. Often the process requesting a passive OPEN will accept a connection request from any caller. In this case a foreign socket of all zeros is used to denote an unspecified socket. Unspecified foreign sockets are allowed only on passive OPENs. A service process that wished to provide services for unknown other processes would 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 is permanently assigned to a particular socket, and other sockets are 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. (See [4].) Processes can issue passive OPENs and wait for matching active OPENs from other processes and be informed by the TCP when connections have been established. Two processes which issue active OPENs to each other at the same time will be correctly connected. This flexibility is critical for the support of distributed computing in which components act asynchronously with respect to each other. There are two principal cases for matching the sockets in the local passive OPENs and an foreign active OPENs. In the first case, the local passive OPENs has fully specified the foreign socket. In this case, the match must be exact. In the second case, the local passive OPENs has left the foreign socket unspecified. In this case, any foreign socket is acceptable as long as the local sockets match. Other possibilities include partially restricted matches. [Page 11] September 1981 Transmission Control Protocol Philosophy If there are several pending passive OPENs (recorded in TCBs) with the same local socket, an foreign active OPEN will be matched to a TCB with the specific foreign socket in the foreign active OPEN, if such a TCB exists, before selecting a TCB with an unspecified foreign socket. The procedures to establish connections utilize the synchronize (SYN) control flag and involves 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 each 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. The sending user indicates in each SEND call whether the data in that call (and any preceeding calls) should be immediately pushed through to the receiving user by the setting of the PUSH flag. 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 push function is signaled, then it must send all unsent data. When a receiving TCP sees the PUSH flag, it must not wait for more data from the sending TCP before passing the data to the receiving process. There is no necessary relationship between push functions and segment boundaries. The data in any particular segment may be the result of a single SEND call, in whole or part, or of multiple SEND calls. The purpose of push function and the PUSH flag is to push data through from the sending user to the receiving user. It does not provide a record service. There is a coupling between the push function and the use of buffers of data that cross the TCP/user interface. Each time a PUSH 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 data arrives that fills the user's buffer before a PUSH is seen, the data is passed to the user in buffer size units. 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 [Page 12] September 1981 Transmission Control Protocol Philosophy 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 will take action to process the 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 must properly mark outgoing segments with the security, compartment, and precedence. Such TCP modules must 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 will follow a general principle of robustness: be conservative in what you do, be liberal in what you accept from others. [Page 13] September 1981 Transmission Control Protocol [Page 14] September 1981 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 [2]. 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|P|R|S|F| | | Offset| Reserved |R|C|S|S|Y|I| Window | | | |G|K|H|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] September 1981 Transmission Control Protocol Functional Specification Sequence Number: 32 bits The sequence number of the first data octet in this segment (except when SYN is present). If SYN is present the sequence number is the initial sequence number (ISN) and the first data octet is ISN+1. 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. Once a connection is established this is always sent. Data Offset: 4 bits The number of 32 bit words in the TCP Header. This indicates where the data begins. The TCP header (even one including options) is an integral number of 32 bits long. Reserved: 6 bits Reserved for future use. Must be zero. Control Bits: 6 bits (from left to right): URG: Urgent Pointer field significant ACK: Acknowledgment field significant PSH: Push Function 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 [Page 16] September 1981 Transmission Control Protocol Functional Specification prefixed to the TCP header. This pseudo header contains the Source Address, the Destination Address, the Protocol, and TCP length. 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 length plus the data length in octets (this is not an explicitly transmitted quantity, but is computed), and it does not count the 12 octets of the pseudo header. 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 is 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 must be header padding (i.e., zero). A TCP must implement all options. [Page 17] September 1981 Transmission Control Protocol Functional Specification Currently defined options include (kind indicated in octal): Kind Length Meaning ---- ------ ------- 0 - End of option list. 1 - No-Operation. 2 4 Maximum Segment 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. Maximum Segment Size +--------+--------+---------+--------+ |00000010|00000100| max seg size | +--------+--------+---------+--------+ Kind=2 Length=4 [Page 18] September 1981 Transmission Control Protocol Functional Specification Maximum Segment Size Option Data: 16 bits If this option is present, then it communicates the maximum receive segment size at the TCP which sends this segment. This field must only be sent in the initial connection request (i.e., in segments with the SYN control bit set). If this option is not used, any segment size is allowed. 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 next SND.WND - send window SND.UP - send urgent pointer SND.WL1 - segment sequence number used for last window update SND.WL2 - segment acknowledgment number used for last window update ISS - initial send sequence number Receive Sequence Variables RCV.NXT - receive next RCV.WND - receive window RCV.UP - receive urgent pointer IRS - initial receive sequence number [Page 19] September 1981 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. The send window is the portion of the sequence space labeled 3 in 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. The receive window is the portion of the sequence space labeled 2 in 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] September 1981 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, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT, 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 received and sent a connection request. ESTABLISHED - represents an open connection, data received can be delivered to the user. The normal state for the data transfer phase of the connection. 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. 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. LAST-ACK - represents waiting for an acknowledgment of the connection termination request previously sent to the remote TCP (which includes an acknowledgment of its connection termination request). [Page 21] September 1981 Transmission Control Protocol Functional Specification TIME-WAIT - represents waiting for enough time to pass to be sure the remote TCP received the acknowledgment of its connection termination request. CLOSED - represents no connection state at all. 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, ACK, RST and FIN flags; and timeouts. The state diagram in figure 6 illustrates only 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. NOTE BENE: this diagram is only a summary and must not be taken as the total specification. [Page 22] September 1981 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 \ +---------+ | rcv ACK of FIN ------- | CLOSE | | -------------- snd ACK | ------- | V x V snd FIN V +---------+ +---------+ +---------+ |FINWAIT-2| | CLOSING | | LAST-ACK| +---------+ +---------+ +---------+ | rcv ACK of FIN | rcv ACK of FIN | | rcv FIN -------------- | Timeout=2MSL -------------- | | ------- x V ------------ x V \ snd ACK +---------+delete TCB +---------+ ------------------------>|TIME WAIT|------------------>| CLOSED | +---------+ +---------+ TCP Connection State Diagram Figure 6. [Page 23] September 1981 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. Numbering of octets within a segment is that the first data octet immediately following the header is the lowest numbered, and the following octets are numbered consecutively. 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 symbol "=<" means "less than or equal" (modulo 2**32). 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). [Page 24] September 1981 Transmission Control Protocol Functional Specification In response to sending data the TCP will receive acknowledgments. The following comparisons are needed to process the acknowledgments. SND.UNA = oldest unacknowledged sequence number SND.NXT = next sequence number to be sent SEG.ACK = acknowledgment from the receiving TCP (next sequence number expected by the receiving TCP) SEG.SEQ = first sequence number of a segment SEG.LEN = the number of octets occupied by the data in the segment (counting SYN and FIN) SEG.SEQ+SEG.LEN-1 = last sequence number of a segment A new acknowledgment (called an "acceptable ack"), is one for which the inequality below holds: SND.UNA < SEG.ACK =< SND.NXT A segment on the retransmission queue is fully acknowledged if the sum of its sequence number and length is less or equal than the acknowledgment value in the incoming segment. When data is received the following comparisons are needed: RCV.NXT = next sequence number expected on an incoming segments, and is the left or lower edge of the receive window RCV.NXT+RCV.WND-1 = last sequence number expected on an incoming segment, and is the right or upper edge of the receive window 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 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND [Page 25] September 1981 Transmission Control Protocol Functional Specification The first part of this test checks to see if the beginning of the segment falls in the window, the second part of the test checks to see if the end of the segment falls in the window; if the segment passes either part of the test it contains data in the 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: 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 < RCV.NXT+RCV.WND or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND Note that when the receive window is zero no segments should be acceptable except ACK segments. Thus, it is be possible for a TCP to maintain a zero receive window while transmitting data and receiving ACKs. However, even when the receive window is zero, a TCP must process the RST and URG fields of all incoming segments. 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 (SEG.LEN) includes both data and sequence space occupying controls. When a SYN is present then SEG.SEQ is the sequence number of the SYN. [Page 26] September 1981 Transmission Control Protocol Functional Specification 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 from this is -- "how does the TCP identify duplicate segments from previous 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 one incarnation of a connection from being used while the same sequence numbers may still be present in the network from an earlier incarnation. 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 the Maximum Segment Lifetime (MSL) and that the MSL is less than 4.55 hours 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 segments carrying a control bit called "SYN" (for synchronize) and the initial sequence numbers. As a shorthand, segments 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. The synchronization requires each side to send it's own initial sequence number and to receive a confirmation of it in acknowledgment from the other side. Each side must also receive the other side's initial sequence number and send a confirming acknowledgment. 1) A --> B SYN my sequence number is X 2) A <-- B ACK your sequence number is X 3) A <-- B SYN my sequence number is Y 4) A --> B ACK your sequence number is Y [Page 27] September 1981 Transmission Control Protocol Functional Specification Because steps 2 and 3 can be combined in a single message this is called the three way (or three message) handshake. 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 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. The TCP Quiet Time Concept This specification provides that hosts which "crash" without retaining any knowledge of the last sequence numbers transmitted on each active (i.e., not closed) connection shall delay emitting any TCP segments for at least the agreed Maximum Segment Lifetime (MSL) in the internet system of which the host is a part. In the paragraphs below, an explanation for this specification is given. TCP implementors may violate the "quiet time" restriction, but only at the risk of causing some old data to be accepted as new or new data rejected as old duplicated by some receivers in the internet system. TCPs consume sequence number space each time a segment is formed and entered into the network output queue at a source host. The duplicate detection and sequencing algorithm in the TCP protocol relies on the unique binding of segment data to sequence space to the extent that sequence numbers will not cycle through all 2**32 values before the segment data bound to those sequence numbers has been delivered and acknowledged by the receiver and all duplicate copies of the segments have "drained" from the internet. Without such an assumption, two distinct TCP segments could conceivably be [Page 28] September 1981 Transmission Control Protocol Functional Specification assigned the same or overlapping sequence numbers, causing confusion at the receiver as to which data is new and which is old. Remember that each segment is bound to as many consecutive sequence numbers as there are octets of data in the segment. Under normal conditions, TCPs keep track of the next sequence number to emit and the oldest awaiting acknowledgment so as to avoid mistakenly using a sequence number over before its first use has been acknowledged. This alone does not guarantee that old duplicate data is drained from the net, so the sequence space has been made very large to reduce the probability that a wandering duplicate will cause trouble upon arrival. At 2 megabits/sec. it takes 4.5 hours to use up 2**32 octets of sequence space. Since the maximum segment lifetime in the net is not likely to exceed a few tens of seconds, this is deemed ample protection for foreseeable nets, even if data rates escalate to l0's of megabits/sec. At 100 megabits/sec, the cycle time is 5.4 minutes which may be a little short, but still within reason. The basic duplicate detection and sequencing algorithm in TCP can be defeated, however, if a source TCP does not have any memory of the sequence numbers it last used on a given connection. For example, if the TCP were to start all connections with sequence number 0, then upon crashing and restarting, a TCP might re-form an earlier connection (possibly after half-open connection resolution) and emit packets with sequence numbers identical to or overlapping with packets still in the network which were emitted on an earlier incarnation of the same connection. In the absence of knowledge about the sequence numbers used on a particular connection, the TCP specification recommends that the source delay for MSL seconds before emitting segments on the connection, to allow time for segments from the earlier connection incarnation to drain from the system. Even hosts which can remember the time of day and used it to select initial sequence number values are not immune from this problem (i.e., even if time of day is used to select an initial sequence number for each new connection incarnation). Suppose, for example, that a connection is opened starting with sequence number S. Suppose that this connection is not used much and that eventually the initial sequence number function (ISN(t)) takes on a value equal to the sequence number, say S1, of the last segment sent by this TCP on a particular connection. Now suppose, at this instant, the host crashes, recovers, and establishes a new incarnation of the connection. The initial sequence number chosen is S1 = ISN(t) -- last used sequence number on old incarnation of connection! If the recovery occurs quickly enough, any old [Page 29] September 1981 Transmission Control Protocol Functional Specification duplicates in the net bearing sequence numbers in the neighborhood of S1 may arrive and be treated as new packets by the receiver of the new incarnation of the connection. The problem is that the recovering host may not know for how long it crashed nor does it know whether there are still old duplicates in the system from earlier connection incarnations. One way to deal with this problem is to deliberately delay emitting segments for one MSL after recovery from a crash- this is the "quite time" specification. Hosts which prefer to avoid waiting are willing to risk possible confusion of old and new packets at a given destination may choose not to wait for the "quite time". Implementors may provide TCP users with the ability to select on a connection by connection basis whether to wait after a crash, or may informally implement the "quite time" for all connections. Obviously, even where a user selects to "wait," this is not necessary after the host has been "up" for at least MSL seconds. To summarize: every segment emitted occupies one or more sequence numbers in the sequence space, the numbers occupied by a segment are "busy" or "in use" until MSL seconds have passed, upon crashing a block of space-time is occupied by the octets of the last emitted segment, if a new connection is started too soon and uses any of the sequence numbers in the space-time footprint of the last segment of the previous connection incarnation, there is a potential sequence number overlap area which could cause confusion at the receiver. 3.4. Establishing a connection The "three-way handshake" is the procedure used to establish a connection. This procedure normally is initiated by one TCP and responded to by another TCP. The procedure also works if two TCP simultaneously initiate the procedure. When simultaneous attempt occurs, each 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 follow. 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 [Page 30] September 1981 Transmission Control Protocol Functional Specification implementation of a trade-off between memory and messages to provide information for this checking. The simplest three-way handshake is shown in figure 7 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. 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 7. In line 2 of figure 7, 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!). [Page 31] September 1981 Transmission Control Protocol Functional Specification Simultaneous initiation is only slightly more complex, as is shown in figure 8. Each TCP cycles from CLOSED to SYN-SENT to SYN-RECEIVED to ESTABLISHED. 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=100><ACK=301><CTL=SYN,ACK> ... 6. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED 7. ... <SEQ=101><ACK=301><CTL=ACK> --> ESTABLISHED Simultaneous Connection Synchronization Figure 8. 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. 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, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT), it aborts the connection and informs its user. We discuss this latter case under "half-open" connections below. [Page 32] September 1981 Transmission Control Protocol Functional Specification TCP A TCP B 1. CLOSED LISTEN 2. SYN-SENT --> <SEQ=100><CTL=SYN> ... 3. (duplicate) ... <SEQ=90><CTL=SYN> --> SYN-RECEIVED 4. SYN-SENT <-- <SEQ=300><ACK=91><CTL=SYN,ACK> <-- SYN-RECEIVED 5. SYN-SENT --> <SEQ=91><CTL=RST> --> LISTEN 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 9. As a simple example of recovery from old duplicates, consider figure 9. 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 [Page 33] September 1981 Transmission Control Protocol Functional Specification 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 indicates 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 10. After TCP A crashes, the user attempts to re-open the connection. TCP B, in the meantime, thinks the connection is open. 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. SYN-SENT CLOSED 7. SYN-SENT --> <SEQ=400><CTL=SYN> --> Half-Open Connection Discovery Figure 10. When the SYN arrives at line 3, TCP B, being in a synchronized state, and the incoming segment outside the window, 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 [Page 34] September 1981 Transmission Control Protocol Functional Specification continue to try to establish the connection; the problem is now reduced to the basic 3-way handshake of figure 7. 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 11. 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. 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 11. In figure 12, 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 12. [Page 35] September 1981 Transmission Control Protocol Functional Specification 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) must be sent whenever a segment arrives which apparently is not intended for the current connection. A reset must not be sent if it is not clear that this is the case. There are three groups of states: 1. If the connection does not exist (CLOSED) then a reset is sent in response to any incoming segment except another reset. In particular, SYNs addressed to a non-existent connection are rejected by this means. If the incoming segment has an ACK field, the reset takes its sequence number from the ACK field of the segment, otherwise the reset has sequence number zero and the ACK field is set to the sum of the sequence number and segment length of the incoming segment. The connection remains in the CLOSED state. 2. If the connection is in any non-synchronized state (LISTEN, SYN-SENT, SYN-RECEIVED), and the incoming segment acknowledges something not yet sent (the segment carries an unacceptable ACK), or if an incoming segment has a security level or compartment which does not exactly match the level and compartment requested for the connection, a reset is sent. If our SYN has not been acknowledged and the precedence level of the incoming segment is higher than the precedence level requested then either raise the local precedence level (if allowed by the user and the system) or send a reset; or if the precedence level of the incoming segment is lower than the precedence level requested then continue as if the precedence matched exactly (if the remote TCP cannot raise the precedence level to match ours this will be detected in the next segment it sends, and the connection will be terminated then). If our SYN has been acknowledged (perhaps in this incoming segment) the precedence level of the incoming segment must match the local precedence level exactly, if it does not a reset must be sent. If the incoming segment has an ACK field, the reset takes its sequence number from the ACK field of the segment, otherwise the reset has sequence number zero and the ACK field is set to the sum of the sequence number and segment length of the incoming segment. The connection remains in the same state. [Page 36] September 1981 Transmission Control Protocol Functional Specification 3. If the connection is in a synchronized state (ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT), any unacceptable segment (out of window sequence number or unacceptible acknowledgment number) must elicit only an empty acknowledgment segment containing the current send-sequence number and an acknowledgment indicating the next sequence number expected to be received, and the connection remains in the same state. If an incoming segment has a security level, or compartment, or precedence which does not exactly match the level, and compartment, and precedence requested for the connection,a reset is sent and connection goes to the CLOSED state. The reset takes its sequence number from the ACK field of the incoming segment. Reset Processing In all states except SYN-SENT, all reset (RST) segments are validated by checking their SEQ-fields. A reset is valid if its sequence number is in the window. In the SYN-SENT state (a RST received in response to an initial SYN), the RST 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 signal a user, 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. Users must keep reading connections they close for sending until the TCP says no more data. [Page 37] September 1981 Transmission Control Protocol Functional Specification 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. Note 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 will respond with a CLOSE, upon which the TCP can send a FIN to the other TCP after sending any remaining data. The TCP then waits until its own FIN is acknowledged whereupon it deletes the connection. If an ACK is not forthcoming, after the user 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 38] September 1981 Transmission Control Protocol Functional Specification TCP A TCP B 1. ESTABLISHED ESTABLISHED 2. (Close) FIN-WAIT-1 --> <SEQ=100><ACK=300><CTL=FIN,ACK> --> CLOSE-WAIT 3. FIN-WAIT-2 <-- <SEQ=300><ACK=101><CTL=ACK> <-- CLOSE-WAIT 4. (Close) TIME-WAIT <-- <SEQ=300><ACK=101><CTL=FIN,ACK> <-- LAST-ACK 5. TIME-WAIT --> <SEQ=101><ACK=301><CTL=ACK> --> CLOSED 6. (2 MSL) CLOSED Normal Close Sequence Figure 13. TCP A TCP B 1. ESTABLISHED ESTABLISHED 2. (Close) (Close) FIN-WAIT-1 --> <SEQ=100><ACK=300><CTL=FIN,ACK> ... FIN-WAIT-1 <-- <SEQ=300><ACK=100><CTL=FIN,ACK> <-- ... <SEQ=100><ACK=300><CTL=FIN,ACK> --> 3. CLOSING --> <SEQ=101><ACK=301><CTL=ACK> ... CLOSING <-- <SEQ=301><ACK=101><CTL=ACK> <-- ... <SEQ=101><ACK=301><CTL=ACK> --> 4. TIME-WAIT TIME-WAIT (2 MSL) (2 MSL) CLOSED CLOSED Simultaneous Close Sequence Figure 14. [Page 39] September 1981 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 ports. The precedence and security parameters used in TCP are exactly those defined in the Internet Protocol (IP) [2]. Throughout this TCP specification the term "security/compartment" is intended to indicate the security parameters used in IP including security, compartment, user group, and handling restriction. A connection attempt with mismatched security/compartment values or a lower precedence value must be rejected by sending a reset. Rejecting a connection due to too low a precedence only occurs after an acknowledgment of the SYN has been received. 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. The security paramaters may be used even in a non-secure environment (the values would indicate unclassified data), thus hosts in non-secure environments must be prepared to receive the security parameters, though they need not send them. 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 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 [Page 40] September 1981 Transmission Control Protocol Functional Specification acknowledgment it advances SND.UNA. The extent to which the values of these variables differ is a measure of the delay in the communication. The amount by which the variables are advanced is the length of the data in the segment. Note that once in the ESTABLISHED state all segments must carry current acknowledgment information. The CLOSE user call implies a push function, as does the FIN control flag in an incoming segment. Retransmission Timeout Because of the variability of the networks that compose an internetwork system and the wide range of uses of TCP connections the retransmission timeout must be dynamically determined. One procedure for determining a retransmission time out is given here as an illustration. An Example Retransmission Timeout Procedure Measure the elapsed time between sending a data octet with a particular sequence number and receiving an acknowledgment that covers that sequence number (segments sent do not have to match segments received). This measured elapsed time is the Round Trip Time (RTT). Next compute a Smoothed Round Trip Time (SRTT) as: SRTT = ( ALPHA * SRTT ) + ((1-ALPHA) * RTT) and based on this, compute the retransmission timeout (RTO) as: RTO = min[UBOUND,max[LBOUND,(BETA*SRTT)]] where UBOUND is an upper bound on the timeout (e.g., 1 minute), LBOUND is a lower bound on the timeout (e.g., 1 second), ALPHA is a smoothing factor (e.g., .8 to .9), and BETA is a delay variance factor (e.g., 1.3 to 2.0). 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. 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 must tell the user to go into "urgent mode"; when the receive sequence number catches up to the urgent pointer, the TCP must tell user to go [Page 41] September 1981 Transmission Control Protocol Functional Specification into "normal mode". If the urgent pointer is updated while the user is in "urgent 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 must be added to the segment sequence number to yield the urgent pointer. The absence of this flag indicates that there is no urgent data outstanding. To send an urgent indication the user must also send at least one data octet. If the sending user also indicates a push, 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 numbers the sender of the window (the data receiver) is currently prepared to accept. There is an assumption that this is related 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, it will be discarded. 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 from the user and send at least one octet of new data even if the send window is zero. The sending TCP must 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. When the receiving TCP has a zero window and a segment arrives it must still send an acknowledgment showing its next expected sequence number and current window (zero). The sending TCP packages the data to be transmitted into segments [Page 42] September 1981 Transmission Control Protocol Functional Specification which fit the current window, and may repackage segments on the retransmission queue. Such repackaging is not required, but may be helpful. 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. A refinement to avoid this problem is to act on the window information from segments that carry the highest acknowledgment number (that is segments with acknowledgment number equal or greater than the highest previously received). The window management procedure has significant influence on the communication performance. The following comments are suggestions to implementers. Window Management Suggestions Allocating a very small window causes data to be transmitted in many small segments when better performance is achieved using fewer large segments. One suggestion for avoiding small windows is for the receiver to defer updating a window until the additional allocation is at least X percent of the maximum allocation possible for the connection (where X might be 20 to 40). Another suggestion is for the sender to avoid sending small segments by waiting until the window is large enough before sending data. If the the user signals a push function then the data must be sent even if it is a small segment. Note that the acknowledgments should not be delayed or unnecessary retransmissions will result. One strategy would be to send an acknowledgment when a small segment arrives (with out updating the window information), and then to send another acknowledgment with new window information when the window is larger. The segment sent to probe a zero window may also begin a break up of transmitted data into smaller and smaller segments. If a segment containing a single data octet sent to probe a zero window is accepted, it consumes one octet of the window now available. If the sending TCP simply sends as much as it can whenever the window is non zero, the transmitted data will be broken into alternating big and small segments. As time goes on, occasional pauses in the receiver making window allocation available will [Page 43] September 1981 Transmission Control Protocol Functional Specification result in breaking the big segments into a small and not quite so big pair. And after a while the data transmission will be in mostly small segments. The suggestion here is that the TCP implementations need to actively attempt to combine small window allocations into larger windows, since the mechanisms for managing the window tend to lead to many small windows in the simplest minded implementations. 3.8. Interfaces There are of course two interfaces of concern: the user/TCP interface and the TCP/lower-level interface. We have a fairly elaborate model of the user/TCP interface, but the interface to the lower level protocol module is left unspecified here, since it will be specified in detail by the specification of the lowel level protocol. For the case that the lower level is IP we note some of the parameter values that TCPs might use. User/TCP Interface The following 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 must 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 44] September 1981 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. Open Format: OPEN (local port, foreign socket, active/passive [, timeout] [, precedence] [, security/compartment] [, options]) -> 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 the lower level protocol (e.g., IP). 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 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 timeout, if present, permits the caller to set up a timeout for all data submitted to TCP. If data is not successfully delivered to the destination within the timeout period, the TCP will abort the connection. The present global default is five minutes. The TCP or some component of the operating system will verify the users authority to open a connection with the specified [Page 45] September 1981 Transmission Control Protocol Functional Specification precedence or security/compartment. The absence of precedence or security/compartment specification in the OPEN call indicates the default values must 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.Implementers may want to give the user control of this precedence negotiation. For example, the user might be allowed to specify that the precedence must be exactly matched, or that any attempt to raise the precedence be confirmed by the user. A local connection name will be returned to the user by the TCP. 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, PUSH 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 PUSH flag is set, the data must be transmitted promptly to the receiver, and the PUSH bit will be set in the last TCP segment created from the buffer. If the PUSH flag is not set, the data may be combined with data from subsequent SENDs for transmission efficiency. If the URGENT flag is set, segments sent to the destination TCP will have the urgent pointer set. The receiving TCP will signal the urgent condition to the receiving process if 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 process the urgent data and to indicate to the receiver when all the currently known urgent [Page 46] September 1981 Transmission Control Protocol Functional Specification 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. 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, the current user 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 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. 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 buffers. In order for the process to distinguish among error or success indications for different SENDs, it might be appropriate for the [Page 47] September 1981 Transmission Control Protocol Functional Specification buffer address to be returned along with the coded response to the SEND request. TCP-to-user signals are discussed below, indicating the information which should be returned to the calling process. Receive Format: RECEIVE (local connection name, buffer address, byte count) -> byte count, urgent flag, push flag 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 segments arrive. This strategy permits increased throughput at the cost of a more elaborate scheme (possibly asynchronous) to notify the calling program that a PUSH has been seen or a buffer filled. If enough data arrive to fill the buffer before a PUSH is seen, the PUSH flag will not be set in the response to the RECEIVE. The buffer will be filled with as much data as it can hold. If a PUSH is seen before the buffer is filled the buffer will be returned partially filled and PUSH indicated. If there is urgent data the user will have been informed as soon as it arrived via a TCP-to-user signal. The receiving user should thus be in "urgent 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 "urgent mode". Note that data following the urgent pointer (non-urgent data) cannot be delivered to the user in the same buffer with preceeding urgent data unless the boundary is clearly marked for the user. To distinguish among several outstanding RECEIVEs and to take care of the case that a buffer is not completely filled, the return code is accompanied by both a buffer pointer and a byte count indicating the actual length of the data received. Alternative implementations of RECEIVE might have the TCP [Page 48] September 1981 Transmission Control Protocol Functional Specification allocate buffer storage, or the TCP might share a ring buffer with the user. 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 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 push function. Status Format: STATUS (local connection name) -> status data 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, [Page 49] September 1981 Transmission Control Protocol Functional Specification foreign socket, local connection name, receive window, send window, connection state, number of buffers awaiting acknowledgment, number of buffers pending receipt, urgent state, precedence, security/compartment, and 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. 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 Push flag Receive Urgent flag Receive [Page 50] September 1981 Transmission Control Protocol Functional Specification TCP/Lower-Level 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 (IP) [2]. If the lower level protocol is IP it 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: routine, Delay: normal, Throughput: normal, Reliability: normal; or 00000000. 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. If the lower level is IP (or other protocol that provides this feature) and source routing is used, the interface must allow the route information to be communicated. This is especially important so that the source and destination addresses used in the TCP checksum be the originating source and ultimate destination. It is also important to preserve the return route to answer connection requests. Any lower level protocol will have to provide the source address, destination address, and protocol fields, and some way to determine the "TCP length", both to provide the functional equivlent service of IP and to be used in the TCP checksum. [Page 51] September 1981 Transmission Control Protocol Functional Specification 3.9. Event Processing The processing depicted in this section is an example of one possible implementation. Other implementations may have slightly different processing sequences, but they should differ from those in this section only in detail, not in substance. 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 TIME-WAIT 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 (modulo 2**32). [Page 52] September 1981 Transmission Control Protocol Functional Specification 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. 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. Note that if no state change is mentioned the TCP stays in the same state. [Page 53] September 1981 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. Note that some parts of the foreign socket may be unspecified in a passive OPEN and are to be filled in by the parameters of the incoming SYN segment. 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 passive enter the LISTEN state and return. 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. A SYN segment of the form <SEQ=ISS><CTL=SYN> is sent. Set SND.UNA to ISS, SND.NXT 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 If active and 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. 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 must be sent with the data segments 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". [Page 54] September 1981 Transmission Control Protocol Functional Specification OPEN Call SYN-SENT STATE SYN-RECEIVED STATE ESTABLISHED STATE FIN-WAIT-1 STATE FIN-WAIT-2 STATE CLOSE-WAIT STATE CLOSING STATE LAST-ACK STATE TIME-WAIT STATE Return "error: connection already exists". [Page 55] September 1981 Transmission Control Protocol Functional Specification SEND Call SEND Call CLOSED STATE (i.e., TCB does not exist) If the user does not 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. 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 must be sent with the data segments 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 SYN-RECEIVED STATE Queue the data for transmission after entering ESTABLISHED state. If no space to queue, respond with "error: insufficient resources". ESTABLISHED STATE CLOSE-WAIT STATE Segmentize the buffer and send it with a piggybacked acknowledgment (acknowledgment value = RCV.NXT). If there is insufficient space to remember this buffer, simply return "error: insufficient resources". If the urgent flag is set, then SND.UP <- SND.NXT-1 and set the urgent pointer in the outgoing segments. [Page 56] September 1981 Transmission Control Protocol Functional Specification SEND Call FIN-WAIT-1 STATE FIN-WAIT-2 STATE CLOSING STATE LAST-ACK STATE TIME-WAIT STATE Return "error: connection closing" and do not service request. [Page 57] September 1981 Transmission Control Protocol Functional Specification RECEIVE Call RECEIVE Call CLOSED STATE (i.e., TCB does not exist) If the user does not 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 FIN-WAIT-1 STATE FIN-WAIT-2 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 "push seen" (PUSH) 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. [Page 58] September 1981 Transmission Control Protocol Functional Specification RECEIVE Call CLOSE-WAIT STATE Since the remote side has already sent FIN, RECEIVEs must be satisfied by text already on hand, but not yet delivered to the user. If no text is awaiting delivery, the RECEIVE will get a "error: connection closing" response. Otherwise, any remaining text can be used to satisfy the RECEIVE. CLOSING STATE LAST-ACK STATE TIME-WAIT STATE Return "error: connection closing". [Page 59] September 1981 Transmission Control Protocol Functional Specification CLOSE Call CLOSE Call CLOSED STATE (i.e., TCB does not exist) If the user does not 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 are returned with "error: closing" responses. Delete TCB, enter CLOSED state, and return. SYN-SENT STATE Delete the TCB and return "error: closing" responses to any queued SENDs, or RECEIVEs. SYN-RECEIVED STATE If no SENDs have been issued and there is no pending data to send, then form a FIN segment and send it, and enter FIN-WAIT-1 state; otherwise queue for processing after entering ESTABLISHED 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 60] September 1981 Transmission Control Protocol Functional Specification CLOSE Call CLOSE-WAIT STATE Queue this request until all preceding SENDs have been segmentized; then send a FIN segment, enter CLOSING state. CLOSING STATE LAST-ACK STATE TIME-WAIT STATE Respond with "error: connection closing". [Page 61] September 1981 Transmission Control Protocol Functional Specification ABORT Call ABORT Call CLOSED STATE (i.e., TCB does not exist) If the user should not 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, enter CLOSED state, and return. SYN-SENT STATE All queued SENDs and RECEIVEs should be given "connection reset" notification, delete the TCB, enter CLOSED state, and return. SYN-RECEIVED STATE ESTABLISHED STATE FIN-WAIT-1 STATE FIN-WAIT-2 STATE CLOSE-WAIT STATE Send a reset segment: <SEQ=SND.NXT><CTL=RST> All queued SENDs and RECEIVEs should be given "connection reset" notification; all segments queued for transmission (except for the RST formed above) or retransmission should be flushed, delete the TCB, enter CLOSED state, and return. CLOSING STATE LAST-ACK STATE TIME-WAIT STATE Respond with "ok" and delete the TCB, enter CLOSED state, and return. [Page 62] September 1981 Transmission Control Protocol Functional Specification STATUS Call STATUS Call CLOSED STATE (i.e., TCB does not exist) If the user should not 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. CLOSE-WAIT STATE Return "state = CLOSE-WAIT", and the TCB pointer. CLOSING STATE Return "state = CLOSING", and the TCB pointer. LAST-ACK STATE Return "state = LAST-ACK", and the TCB pointer. [Page 63] September 1981 Transmission Control Protocol Functional Specification STATUS Call TIME-WAIT STATE Return "state = TIME-WAIT", and the TCB pointer. [Page 64] September 1981 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 RST An incoming RST should be ignored. Return. second 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. The RST should be formatted as follows: <SEQ=SEG.ACK><CTL=RST> Return. third 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. <SEQ=SEG.ACK><CTL=RST> [Page 65] September 1981 Transmission Control Protocol Functional Specification SEGMENT ARRIVES If the SEG.PRC is greater than the TCB.PRC then if allowed by the user and the system set TCB.PRC<-SEG.PRC, if not allowed send a reset and return. <SEQ=SEG.ACK><CTL=RST> If the SEG.PRC is less than the TCB.PRC then continue. Set RCV.NXT 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: <SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK> SND.NXT is 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. If the listen was not fully specified (i.e., the foreign socket was not fully specified), then the unspecified fields should be filled in now. fourth other text or control Any other control or text-bearing segment (not containing SYN) must 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 the ACK bit If the ACK bit is set If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, send a reset (unless the RST bit is set, if so drop the segment and return) <SEQ=SEG.ACK><CTL=RST> and discard the segment. Return. If SND.UNA =< SEG.ACK =< SND.NXT then the ACK is acceptable. second check the RST bit [Page 66] September 1981 Transmission Control Protocol Functional Specification SEGMENT ARRIVES If the RST bit is set If the ACK was acceptable then signal the user "error: connection reset", drop the segment, enter CLOSED state, delete TCB, and return. Otherwise (no ACK) drop the segment and return. third check the security and precedence If the security/compartment in the segment does not exactly match the security/compartment in the TCB, send a reset If there is an ACK <SEQ=SEG.ACK><CTL=RST> Otherwise <SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK> If there is an ACK The precedence in the segment must match the precedence in the TCB, if not, send a reset <SEQ=SEG.ACK><CTL=RST> If there is no ACK If the precedence in the segment is higher than the precedence in the TCB then if allowed by the user and the system raise the precedence in the TCB to that in the segment, if not allowed to raise the prec then send a reset. <SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK> If the precedence in the segment is lower than the precedence in the TCB continue. If a reset was sent, discard the segment and return. fourth check the SYN bit This step should be reached only if the ACK is ok, or there is no ACK, and it the segment did not contain a RST. If the SYN bit is on and the security/compartment and precedence [Page 67] September 1981 Transmission Control Protocol Functional Specification SEGMENT ARRIVES are acceptable then, RCV.NXT is set to SEG.SEQ+1, IRS is set to SEG.SEQ. SND.UNA should be advanced to equal SEG.ACK (if there is an ACK), and any segments on the retransmission queue which are thereby acknowledged should be removed. If SND.UNA > ISS (our SYN has been ACKed), change the connection state to ESTABLISHED, 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 there are other controls or text in the segment then continue processing at the sixth step below where the URG bit is checked, otherwise return. Otherwise enter SYN-RECEIVED, form a SYN,ACK segment <SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK> and send it. If there are other controls or text in the segment, queue them for processing after the ESTABLISHED state has been reached, return. fifth, if neither of the SYN or RST bits is set then drop the segment and return. [Page 68] September 1981 Transmission Control Protocol Functional Specification SEGMENT ARRIVES