1 2 3RFC: 793 4 5 6 7 8 9 10 11 TRANSMISSION CONTROL PROTOCOL 12 13 14 DARPA INTERNET PROGRAM 15 16 PROTOCOL SPECIFICATION 17 18 19 20 September 1981 21 22 23 24 25 26 27 28 29 30 31 32 33 34 prepared for 35 36 Defense Advanced Research Projects Agency 37 Information Processing Techniques Office 38 1400 Wilson Boulevard 39 Arlington, Virginia 22209 40 41 42 43 44 45 46 47 by 48 49 Information Sciences Institute 50 University of Southern California 51 4676 Admiralty Way 52 Marina del Rey, California 90291 53 54 55 56September 1981 57 Transmission Control Protocol 58 59 60 61 TABLE OF CONTENTS 62 63 PREFACE ........................................................ iii 64 651. INTRODUCTION ..................................................... 1 66 67 1.1 Motivation .................................................... 1 68 1.2 Scope ......................................................... 2 69 1.3 About This Document ........................................... 2 70 1.4 Interfaces .................................................... 3 71 1.5 Operation ..................................................... 3 72 732. PHILOSOPHY ....................................................... 7 74 75 2.1 Elements of the Internetwork System ........................... 7 76 2.2 Model of Operation ............................................ 7 77 2.3 The Host Environment .......................................... 8 78 2.4 Interfaces .................................................... 9 79 2.5 Relation to Other Protocols ................................... 9 80 2.6 Reliable Communication ........................................ 9 81 2.7 Connection Establishment and Clearing ........................ 10 82 2.8 Data Communication ........................................... 12 83 2.9 Precedence and Security ...................................... 13 84 2.10 Robustness Principle ......................................... 13 85 863. FUNCTIONAL SPECIFICATION ........................................ 15 87 88 3.1 Header Format ................................................ 15 89 3.2 Terminology .................................................. 19 90 3.3 Sequence Numbers ............................................. 24 91 3.4 Establishing a connection .................................... 30 92 3.5 Closing a Connection ......................................... 37 93 3.6 Precedence and Security ...................................... 40 94 3.7 Data Communication ........................................... 40 95 3.8 Interfaces ................................................... 44 96 3.9 Event Processing ............................................. 52 97 98GLOSSARY ............................................................ 79 99 100REFERENCES .......................................................... 85 101 102 103 104 105 106 107 108 109 110 111 112 [Page i] 113 114 115 September 1981 116Transmission Control Protocol 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171[Page ii] 172 173 174September 1981 175 Transmission Control Protocol 176 177 178 179 PREFACE 180 181 182 183This document describes the DoD Standard Transmission Control Protocol 184(TCP). There have been nine earlier editions of the ARPA TCP 185specification on which this standard is based, and the present text 186draws heavily from them. There have been many contributors to this work 187both in terms of concepts and in terms of text. This edition clarifies 188several details and removes the end-of-letter buffer-size adjustments, 189and redescribes the letter mechanism as a push function. 190 191 Jon Postel 192 193 Editor 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 [Page iii] 231 232 233 234 235RFC: 793 236Replaces: RFC 761 237IENs: 129, 124, 112, 81, 23855, 44, 40, 27, 21, 5 239 240 TRANSMISSION CONTROL PROTOCOL 241 242 DARPA INTERNET PROGRAM 243 PROTOCOL SPECIFICATION 244 245 246 247 1. INTRODUCTION 248 249The Transmission Control Protocol (TCP) is intended for use as a highly 250reliable host-to-host protocol between hosts in packet-switched computer 251communication networks, and in interconnected systems of such networks. 252 253This document describes the functions to be performed by the 254Transmission Control Protocol, the program that implements it, and its 255interface to programs or users that require its services. 256 2571.1. Motivation 258 259 Computer communication systems are playing an increasingly important 260 role in military, government, and civilian environments. This 261 document focuses its attention primarily on military computer 262 communication requirements, especially robustness in the presence of 263 communication unreliability and availability in the presence of 264 congestion, but many of these problems are found in the civilian and 265 government sector as well. 266 267 As strategic and tactical computer communication networks are 268 developed and deployed, it is essential to provide means of 269 interconnecting them and to provide standard interprocess 270 communication protocols which can support a broad range of 271 applications. In anticipation of the need for such standards, the 272 Deputy Undersecretary of Defense for Research and Engineering has 273 declared the Transmission Control Protocol (TCP) described herein to 274 be a basis for DoD-wide inter-process communication protocol 275 standardization. 276 277 TCP is a connection-oriented, end-to-end reliable protocol designed to 278 fit into a layered hierarchy of protocols which support multi-network 279 applications. The TCP provides for reliable inter-process 280 communication between pairs of processes in host computers attached to 281 distinct but interconnected computer communication networks. Very few 282 assumptions are made as to the reliability of the communication 283 protocols below the TCP layer. TCP assumes it can obtain a simple, 284 potentially unreliable datagram service from the lower level 285 protocols. In principle, the TCP should be able to operate above a 286 wide spectrum of communication systems ranging from hard-wired 287 connections to packet-switched or circuit-switched networks. 288 289 290 [Page 1] 291 292 293 September 1981 294Transmission Control Protocol 295Introduction 296 297 298 299 TCP is based on concepts first described by Cerf and Kahn in [1]. The 300 TCP fits into a layered protocol architecture just above a basic 301 Internet Protocol [2] which provides a way for the TCP to send and 302 receive variable-length segments of information enclosed in internet 303 datagram "envelopes". The internet datagram provides a means for 304 addressing source and destination TCPs in different networks. The 305 internet protocol also deals with any fragmentation or reassembly of 306 the TCP segments required to achieve transport and delivery through 307 multiple networks and interconnecting gateways. The internet protocol 308 also carries information on the precedence, security classification 309 and compartmentation of the TCP segments, so this information can be 310 communicated end-to-end across multiple networks. 311 312 Protocol Layering 313 314 +---------------------+ 315 | higher-level | 316 +---------------------+ 317 | TCP | 318 +---------------------+ 319 | internet protocol | 320 +---------------------+ 321 |communication network| 322 +---------------------+ 323 324 Figure 1 325 326 Much of this document is written in the context of TCP implementations 327 which are co-resident with higher level protocols in the host 328 computer. Some computer systems will be connected to networks via 329 front-end computers which house the TCP and internet protocol layers, 330 as well as network specific software. The TCP specification describes 331 an interface to the higher level protocols which appears to be 332 implementable even for the front-end case, as long as a suitable 333 host-to-front end protocol is implemented. 334 3351.2. Scope 336 337 The TCP is intended to provide a reliable process-to-process 338 communication service in a multinetwork environment. The TCP is 339 intended to be a host-to-host protocol in common use in multiple 340 networks. 341 3421.3. About this Document 343 344 This document represents a specification of the behavior required of 345 any TCP implementation, both in its interactions with higher level 346 protocols and in its interactions with other TCPs. The rest of this 347 348 349[Page 2] 350 351 352September 1981 353 Transmission Control Protocol 354 Introduction 355 356 357 358 section offers a very brief view of the protocol interfaces and 359 operation. Section 2 summarizes the philosophical basis for the TCP 360 design. Section 3 offers both a detailed description of the actions 361 required of TCP when various events occur (arrival of new segments, 362 user calls, errors, etc.) and the details of the formats of TCP 363 segments. 364 3651.4. Interfaces 366 367 The TCP interfaces on one side to user or application processes and on 368 the other side to a lower level protocol such as Internet Protocol. 369 370 The interface between an application process and the TCP is 371 illustrated in reasonable detail. This interface consists of a set of 372 calls much like the calls an operating system provides to an 373 application process for manipulating files. For example, there are 374 calls to open and close connections and to send and receive data on 375 established connections. It is also expected that the TCP can 376 asynchronously communicate with application programs. Although 377 considerable freedom is permitted to TCP implementors to design 378 interfaces which are appropriate to a particular operating system 379 environment, a minimum functionality is required at the TCP/user 380 interface for any valid implementation. 381 382 The interface between TCP and lower level protocol is essentially 383 unspecified except that it is assumed there is a mechanism whereby the 384 two levels can asynchronously pass information to each other. 385 Typically, one expects the lower level protocol to specify this 386 interface. TCP is designed to work in a very general environment of 387 interconnected networks. The lower level protocol which is assumed 388 throughout this document is the Internet Protocol [2]. 389 3901.5. Operation 391 392 As noted above, the primary purpose of the TCP is to provide reliable, 393 securable logical circuit or connection service between pairs of 394 processes. To provide this service on top of a less reliable internet 395 communication system requires facilities in the following areas: 396 397 Basic Data Transfer 398 Reliability 399 Flow Control 400 Multiplexing 401 Connections 402 Precedence and Security 403 404 The basic operation of the TCP in each of these areas is described in 405 the following paragraphs. 406 407 408 [Page 3] 409 410 411 September 1981 412Transmission Control Protocol 413Introduction 414 415 416 417 Basic Data Transfer: 418 419 The TCP is able to transfer a continuous stream of octets in each 420 direction between its users by packaging some number of octets into 421 segments for transmission through the internet system. In general, 422 the TCPs decide when to block and forward data at their own 423 convenience. 424 425 Sometimes users need to be sure that all the data they have 426 submitted to the TCP has been transmitted. For this purpose a push 427 function is defined. To assure that data submitted to a TCP is 428 actually transmitted the sending user indicates that it should be 429 pushed through to the receiving user. A push causes the TCPs to 430 promptly forward and deliver data up to that point to the receiver. 431 The exact push point might not be visible to the receiving user and 432 the push function does not supply a record boundary marker. 433 434 Reliability: 435 436 The TCP must recover from data that is damaged, lost, duplicated, or 437 delivered out of order by the internet communication system. This 438 is achieved by assigning a sequence number to each octet 439 transmitted, and requiring a positive acknowledgment (ACK) from the 440 receiving TCP. If the ACK is not received within a timeout 441 interval, the data is retransmitted. At the receiver, the sequence 442 numbers are used to correctly order segments that may be received 443 out of order and to eliminate duplicates. Damage is handled by 444 adding a checksum to each segment transmitted, checking it at the 445 receiver, and discarding damaged segments. 446 447 As long as the TCPs continue to function properly and the internet 448 system does not become completely partitioned, no transmission 449 errors will affect the correct delivery of data. TCP recovers from 450 internet communication system errors. 451 452 Flow Control: 453 454 TCP provides a means for the receiver to govern the amount of data 455 sent by the sender. This is achieved by returning a "window" with 456 every ACK indicating a range of acceptable sequence numbers beyond 457 the last segment successfully received. The window indicates an 458 allowed number of octets that the sender may transmit before 459 receiving further permission. 460 461 462 463 464 465 466 467[Page 4] 468 469 470September 1981 471 Transmission Control Protocol 472 Introduction 473 474 475 476 Multiplexing: 477 478 To allow for many processes within a single Host to use TCP 479 communication facilities simultaneously, the TCP provides a set of 480 addresses or ports within each host. Concatenated with the network 481 and host addresses from the internet communication layer, this forms 482 a socket. A pair of sockets uniquely identifies each connection. 483 That is, a socket may be simultaneously used in multiple 484 connections. 485 486 The binding of ports to processes is handled independently by each 487 Host. However, it proves useful to attach frequently used processes 488 (e.g., a "logger" or timesharing service) to fixed sockets which are 489 made known to the public. These services can then be accessed 490 through the known addresses. Establishing and learning the port 491 addresses of other processes may involve more dynamic mechanisms. 492 493 Connections: 494 495 The reliability and flow control mechanisms described above require 496 that TCPs initialize and maintain certain status information for 497 each data stream. The combination of this information, including 498 sockets, sequence numbers, and window sizes, is called a connection. 499 Each connection is uniquely specified by a pair of sockets 500 identifying its two sides. 501 502 When two processes wish to communicate, their TCP's must first 503 establish a connection (initialize the status information on each 504 side). When their communication is complete, the connection is 505 terminated or closed to free the resources for other uses. 506 507 Since connections must be established between unreliable hosts and 508 over the unreliable internet communication system, a handshake 509 mechanism with clock-based sequence numbers is used to avoid 510 erroneous initialization of connections. 511 512 Precedence and Security: 513 514 The users of TCP may indicate the security and precedence of their 515 communication. Provision is made for default values to be used when 516 these features are not needed. 517 518 519 520 521 522 523 524 525 526 [Page 5] 527 528 529 September 1981 530Transmission Control Protocol 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585[Page 6] 586 587 588September 1981 589 Transmission Control Protocol 590 591 592 593 2. PHILOSOPHY 594 5952.1. Elements of the Internetwork System 596 597 The internetwork environment consists of hosts connected to networks 598 which are in turn interconnected via gateways. It is assumed here 599 that the networks may be either local networks (e.g., the ETHERNET) or 600 large networks (e.g., the ARPANET), but in any case are based on 601 packet switching technology. The active agents that produce and 602 consume messages are processes. Various levels of protocols in the 603 networks, the gateways, and the hosts support an interprocess 604 communication system that provides two-way data flow on logical 605 connections between process ports. 606 607 The term packet is used generically here to mean the data of one 608 transaction between a host and its network. The format of data blocks 609 exchanged within the a network will generally not be of concern to us. 610 611 Hosts are computers attached to a network, and from the communication 612 network's point of view, are the sources and destinations of packets. 613 Processes are viewed as the active elements in host computers (in 614 accordance with the fairly common definition of a process as a program 615 in execution). Even terminals and files or other I/O devices are 616 viewed as communicating with each other through the use of processes. 617 Thus, all communication is viewed as inter-process communication. 618 619 Since a process may need to distinguish among several communication 620 streams between itself and another process (or processes), we imagine 621 that each process may have a number of ports through which it 622 communicates with the ports of other processes. 623 6242.2. Model of Operation 625 626 Processes transmit data by calling on the TCP and passing buffers of 627 data as arguments. The TCP packages the data from these buffers into 628 segments and calls on the internet module to transmit each segment to 629 the destination TCP. The receiving TCP places the data from a segment 630 into the receiving user's buffer and notifies the receiving user. The 631 TCPs include control information in the segments which they use to 632 ensure reliable ordered data transmission. 633 634 The model of internet communication is that there is an internet 635 protocol module associated with each TCP which provides an interface 636 to the local network. This internet module packages TCP segments 637 inside internet datagrams and routes these datagrams to a destination 638 internet module or intermediate gateway. To transmit the datagram 639 through the local network, it is embedded in a local network packet. 640 641 The packet switches may perform further packaging, fragmentation, or 642 643 644 [Page 7] 645 646 647 September 1981 648Transmission Control Protocol 649Philosophy 650 651 652 653 other operations to achieve the delivery of the local packet to the 654 destination internet module. 655 656 At a gateway between networks, the internet datagram is "unwrapped" 657 from its local packet and examined to determine through which network 658 the internet datagram should travel next. The internet datagram is 659 then "wrapped" in a local packet suitable to the next network and 660 routed to the next gateway, or to the final destination. 661 662 A gateway is permitted to break up an internet datagram into smaller 663 internet datagram fragments if this is necessary for transmission 664 through the next network. To do this, the gateway produces a set of 665 internet datagrams; each carrying a fragment. Fragments may be 666 further broken into smaller fragments at subsequent gateways. The 667 internet datagram fragment format is designed so that the destination 668 internet module can reassemble fragments into internet datagrams. 669 670 A destination internet module unwraps the segment from the datagram 671 (after reassembling the datagram, if necessary) and passes it to the 672 destination TCP. 673 674 This simple model of the operation glosses over many details. One 675 important feature is the type of service. This provides information 676 to the gateway (or internet module) to guide it in selecting the 677 service parameters to be used in traversing the next network. 678 Included in the type of service information is the precedence of the 679 datagram. Datagrams may also carry security information to permit 680 host and gateways that operate in multilevel secure environments to 681 properly segregate datagrams for security considerations. 682 6832.3. The Host Environment 684 685 The TCP is assumed to be a module in an operating system. The users 686 access the TCP much like they would access the file system. The TCP 687 may call on other operating system functions, for example, to manage 688 data structures. The actual interface to the network is assumed to be 689 controlled by a device driver module. The TCP does not call on the 690 network device driver directly, but rather calls on the internet 691 datagram protocol module which may in turn call on the device driver. 692 693 The mechanisms of TCP do not preclude implementation of the TCP in a 694 front-end processor. However, in such an implementation, a 695 host-to-front-end protocol must provide the functionality to support 696 the type of TCP-user interface described in this document. 697 698 699 700 701 702 703[Page 8] 704 705 706September 1981 707 Transmission Control Protocol 708 Philosophy 709 710 711 7122.4. Interfaces 713 714 The TCP/user interface provides for calls made by the user on the TCP 715 to OPEN or CLOSE a connection, to SEND or RECEIVE data, or to obtain 716 STATUS about a connection. These calls are like other calls from user 717 programs on the operating system, for example, the calls to open, read 718 from, and close a file. 719 720 The TCP/internet interface provides calls to send and receive 721 datagrams addressed to TCP modules in hosts anywhere in the internet 722 system. These calls have parameters for passing the address, type of 723 service, precedence, security, and other control information. 724 7252.5. Relation to Other Protocols 726 727 The following diagram illustrates the place of the TCP in the protocol 728 hierarchy: 729 730 731 +------+ +-----+ +-----+ +-----+ 732 |Telnet| | FTP | |Voice| ... | | Application Level 733 +------+ +-----+ +-----+ +-----+ 734 | | | | 735 +-----+ +-----+ +-----+ 736 | TCP | | RTP | ... | | Host Level 737 +-----+ +-----+ +-----+ 738 | | | 739 +-------------------------------+ 740 | Internet Protocol & ICMP | Gateway Level 741 +-------------------------------+ 742 | 743 +---------------------------+ 744 | Local Network Protocol | Network Level 745 +---------------------------+ 746 747 Protocol Relationships 748 749 Figure 2. 750 751 It is expected that the TCP will be able to support higher level 752 protocols efficiently. It should be easy to interface higher level 753 protocols like the ARPANET Telnet or AUTODIN II THP to the TCP. 754 7552.6. Reliable Communication 756 757 A stream of data sent on a TCP connection is delivered reliably and in 758 order at the destination. 759 760 761 762 [Page 9] 763 764 765 September 1981 766Transmission Control Protocol 767Philosophy 768 769 770 771 Transmission is made reliable via the use of sequence numbers and 772 acknowledgments. Conceptually, each octet of data is assigned a 773 sequence number. The sequence number of the first octet of data in a 774 segment is transmitted with that segment and is called the segment 775 sequence number. Segments also carry an acknowledgment number which 776 is the sequence number of the next expected data octet of 777 transmissions in the reverse direction. When the TCP transmits a 778 segment containing data, it puts a copy on a retransmission queue and 779 starts a timer; when the acknowledgment for that data is received, the 780 segment is deleted from the queue. If the acknowledgment is not 781 received before the timer runs out, the segment is retransmitted. 782 783 An acknowledgment by TCP does not guarantee that the data has been 784 delivered to the end user, but only that the receiving TCP has taken 785 the responsibility to do so. 786 787 To govern the flow of data between TCPs, a flow control mechanism is 788 employed. The receiving TCP reports a "window" to the sending TCP. 789 This window specifies the number of octets, starting with the 790 acknowledgment number, that the receiving TCP is currently prepared to 791 receive. 792 7932.7. Connection Establishment and Clearing 794 795 To identify the separate data streams that a TCP may handle, the TCP 796 provides a port identifier. Since port identifiers are selected 797 independently by each TCP they might not be unique. To provide for 798 unique addresses within each TCP, we concatenate an internet address 799 identifying the TCP with a port identifier to create a socket which 800 will be unique throughout all networks connected together. 801 802 A connection is fully specified by the pair of sockets at the ends. A 803 local socket may participate in many connections to different foreign 804 sockets. A connection can be used to carry data in both directions, 805 that is, it is "full duplex". 806 807 TCPs are free to associate ports with processes however they choose. 808 However, several basic concepts are necessary in any implementation. 809 There must be well-known sockets which the TCP associates only with 810 the "appropriate" processes by some means. We envision that processes 811 may "own" ports, and that processes can initiate connections only on 812 the ports they own. (Means for implementing ownership is a local 813 issue, but we envision a Request Port user command, or a method of 814 uniquely allocating a group of ports to a given process, e.g., by 815 associating the high order bits of a port name with a given process.) 816 817 A connection is specified in the OPEN call by the local port and 818 foreign socket arguments. In return, the TCP supplies a (short) local 819 820 821[Page 10] 822 823 824September 1981 825 Transmission Control Protocol 826 Philosophy 827 828 829 830 connection name by which the user refers to the connection in 831 subsequent calls. There are several things that must be remembered 832 about a connection. To store this information we imagine that there 833 is a data structure called a Transmission Control Block (TCB). One 834 implementation strategy would have the local connection name be a 835 pointer to the TCB for this connection. The OPEN call also specifies 836 whether the connection establishment is to be actively pursued, or to 837 be passively waited for. 838 839 A passive OPEN request means that the process wants to accept incoming 840 connection requests rather than attempting to initiate a connection. 841 Often the process requesting a passive OPEN will accept a connection 842 request from any caller. In this case a foreign socket of all zeros 843 is used to denote an unspecified socket. Unspecified foreign sockets 844 are allowed only on passive OPENs. 845 846 A service process that wished to provide services for unknown other 847 processes would issue a passive OPEN request with an unspecified 848 foreign socket. Then a connection could be made with any process that 849 requested a connection to this local socket. It would help if this 850 local socket were known to be associated with this service. 851 852 Well-known sockets are a convenient mechanism for a priori associating 853 a socket address with a standard service. For instance, the 854 "Telnet-Server" process is permanently assigned to a particular 855 socket, and other sockets are reserved for File Transfer, Remote Job 856 Entry, Text Generator, Echoer, and Sink processes (the last three 857 being for test purposes). A socket address might be reserved for 858 access to a "Look-Up" service which would return the specific socket 859 at which a newly created service would be provided. The concept of a 860 well-known socket is part of the TCP specification, but the assignment 861 of sockets to services is outside this specification. (See [4].) 862 863 Processes can issue passive OPENs and wait for matching active OPENs 864 from other processes and be informed by the TCP when connections have 865 been established. Two processes which issue active OPENs to each 866 other at the same time will be correctly connected. This flexibility 867 is critical for the support of distributed computing in which 868 components act asynchronously with respect to each other. 869 870 There are two principal cases for matching the sockets in the local 871 passive OPENs and an foreign active OPENs. In the first case, the 872 local passive OPENs has fully specified the foreign socket. In this 873 case, the match must be exact. In the second case, the local passive 874 OPENs has left the foreign socket unspecified. In this case, any 875 foreign socket is acceptable as long as the local sockets match. 876 Other possibilities include partially restricted matches. 877 878 879 880 [Page 11] 881 882 883 September 1981 884Transmission Control Protocol 885Philosophy 886 887 888 889 If there are several pending passive OPENs (recorded in TCBs) with the 890 same local socket, an foreign active OPEN will be matched to a TCB 891 with the specific foreign socket in the foreign active OPEN, if such a 892 TCB exists, before selecting a TCB with an unspecified foreign socket. 893 894 The procedures to establish connections utilize the synchronize (SYN) 895 control flag and involves an exchange of three messages. This 896 exchange has been termed a three-way hand shake [3]. 897 898 A connection is initiated by the rendezvous of an arriving segment 899 containing a SYN and a waiting TCB entry each created by a user OPEN 900 command. The matching of local and foreign sockets determines when a 901 connection has been initiated. The connection becomes "established" 902 when sequence numbers have been synchronized in both directions. 903 904 The clearing of a connection also involves the exchange of segments, 905 in this case carrying the FIN control flag. 906 9072.8. Data Communication 908 909 The data that flows on a connection may be thought of as a stream of 910 octets. The sending user indicates in each SEND call whether the data 911 in that call (and any preceeding calls) should be immediately pushed 912 through to the receiving user by the setting of the PUSH flag. 913 914 A sending TCP is allowed to collect data from the sending user and to 915 send that data in segments at its own convenience, until the push 916 function is signaled, then it must send all unsent data. When a 917 receiving TCP sees the PUSH flag, it must not wait for more data from 918 the sending TCP before passing the data to the receiving process. 919 920 There is no necessary relationship between push functions and segment 921 boundaries. The data in any particular segment may be the result of a 922 single SEND call, in whole or part, or of multiple SEND calls. 923 924 The purpose of push function and the PUSH flag is to push data through 925 from the sending user to the receiving user. It does not provide a 926 record service. 927 928 There is a coupling between the push function and the use of buffers 929 of data that cross the TCP/user interface. Each time a PUSH flag is 930 associated with data placed into the receiving user's buffer, the 931 buffer is returned to the user for processing even if the buffer is 932 not filled. If data arrives that fills the user's buffer before a 933 PUSH is seen, the data is passed to the user in buffer size units. 934 935 TCP also provides a means to communicate to the receiver of data that 936 at some point further along in the data stream than the receiver is 937 938 939[Page 12] 940 941 942September 1981 943 Transmission Control Protocol 944 Philosophy 945 946 947 948 currently reading there is urgent data. TCP does not attempt to 949 define what the user specifically does upon being notified of pending 950 urgent data, but the general notion is that the receiving process will 951 take action to process the urgent data quickly. 952 9532.9. Precedence and Security 954 955 The TCP makes use of the internet protocol type of service field and 956 security option to provide precedence and security on a per connection 957 basis to TCP users. Not all TCP modules will necessarily function in 958 a multilevel secure environment; some may be limited to unclassified 959 use only, and others may operate at only one security level and 960 compartment. Consequently, some TCP implementations and services to 961 users may be limited to a subset of the multilevel secure case. 962 963 TCP modules which operate in a multilevel secure environment must 964 properly mark outgoing segments with the security, compartment, and 965 precedence. Such TCP modules must also provide to their users or 966 higher level protocols such as Telnet or THP an interface to allow 967 them to specify the desired security level, compartment, and 968 precedence of connections. 969 9702.10. Robustness Principle 971 972 TCP implementations will follow a general principle of robustness: be 973 conservative in what you do, be liberal in what you accept from 974 others. 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 [Page 13] 999 1000 1001 September 1981 1002Transmission Control Protocol 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057[Page 14] 1058 1059 1060September 1981 1061 Transmission Control Protocol 1062 1063 1064 1065 3. FUNCTIONAL SPECIFICATION 1066 10673.1. Header Format 1068 1069 TCP segments are sent as internet datagrams. The Internet Protocol 1070 header carries several information fields, including the source and 1071 destination host addresses [2]. A TCP header follows the internet 1072 header, supplying information specific to the TCP protocol. This 1073 division allows for the existence of host level protocols other than 1074 TCP. 1075 1076 TCP Header Format 1077 1078 1079 0 1 2 3 1080 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 1081 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1082 | Source Port | Destination Port | 1083 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1084 | Sequence Number | 1085 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1086 | Acknowledgment Number | 1087 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1088 | Data | |U|A|P|R|S|F| | 1089 | Offset| Reserved |R|C|S|S|Y|I| Window | 1090 | | |G|K|H|T|N|N| | 1091 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1092 | Checksum | Urgent Pointer | 1093 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1094 | Options | Padding | 1095 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1096 | data | 1097 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1098 1099 TCP Header Format 1100 1101 Note that one tick mark represents one bit position. 1102 1103 Figure 3. 1104 1105 Source Port: 16 bits 1106 1107 The source port number. 1108 1109 Destination Port: 16 bits 1110 1111 The destination port number. 1112 1113 1114 1115 1116 [Page 15] 1117 1118 1119 September 1981 1120Transmission Control Protocol 1121Functional Specification 1122 1123 1124 1125 Sequence Number: 32 bits 1126 1127 The sequence number of the first data octet in this segment (except 1128 when SYN is present). If SYN is present the sequence number is the 1129 initial sequence number (ISN) and the first data octet is ISN+1. 1130 1131 Acknowledgment Number: 32 bits 1132 1133 If the ACK control bit is set this field contains the value of the 1134 next sequence number the sender of the segment is expecting to 1135 receive. Once a connection is established this is always sent. 1136 1137 Data Offset: 4 bits 1138 1139 The number of 32 bit words in the TCP Header. This indicates where 1140 the data begins. The TCP header (even one including options) is an 1141 integral number of 32 bits long. 1142 1143 Reserved: 6 bits 1144 1145 Reserved for future use. Must be zero. 1146 1147 Control Bits: 6 bits (from left to right): 1148 1149 URG: Urgent Pointer field significant 1150 ACK: Acknowledgment field significant 1151 PSH: Push Function 1152 RST: Reset the connection 1153 SYN: Synchronize sequence numbers 1154 FIN: No more data from sender 1155 1156 Window: 16 bits 1157 1158 The number of data octets beginning with the one indicated in the 1159 acknowledgment field which the sender of this segment is willing to 1160 accept. 1161 1162 Checksum: 16 bits 1163 1164 The checksum field is the 16 bit one's complement of the one's 1165 complement sum of all 16 bit words in the header and text. If a 1166 segment contains an odd number of header and text octets to be 1167 checksummed, the last octet is padded on the right with zeros to 1168 form a 16 bit word for checksum purposes. The pad is not 1169 transmitted as part of the segment. While computing the checksum, 1170 the checksum field itself is replaced with zeros. 1171 1172 The checksum also covers a 96 bit pseudo header conceptually 1173 1174 1175[Page 16] 1176 1177 1178September 1981 1179 Transmission Control Protocol 1180 Functional Specification 1181 1182 1183 1184 prefixed to the TCP header. This pseudo header contains the Source 1185 Address, the Destination Address, the Protocol, and TCP length. 1186 This gives the TCP protection against misrouted segments. This 1187 information is carried in the Internet Protocol and is transferred 1188 across the TCP/Network interface in the arguments or results of 1189 calls by the TCP on the IP. 1190 1191 +--------+--------+--------+--------+ 1192 | Source Address | 1193 +--------+--------+--------+--------+ 1194 | Destination Address | 1195 +--------+--------+--------+--------+ 1196 | zero | PTCL | TCP Length | 1197 +--------+--------+--------+--------+ 1198 1199 The TCP Length is the TCP header length plus the data length in 1200 octets (this is not an explicitly transmitted quantity, but is 1201 computed), and it does not count the 12 octets of the pseudo 1202 header. 1203 1204 Urgent Pointer: 16 bits 1205 1206 This field communicates the current value of the urgent pointer as a 1207 positive offset from the sequence number in this segment. The 1208 urgent pointer points to the sequence number of the octet following 1209 the urgent data. This field is only be interpreted in segments with 1210 the URG control bit set. 1211 1212 Options: variable 1213 1214 Options may occupy space at the end of the TCP header and are a 1215 multiple of 8 bits in length. All options are included in the 1216 checksum. An option may begin on any octet boundary. There are two 1217 cases for the format of an option: 1218 1219 Case 1: A single octet of option-kind. 1220 1221 Case 2: An octet of option-kind, an octet of option-length, and 1222 the actual option-data octets. 1223 1224 The option-length counts the two octets of option-kind and 1225 option-length as well as the option-data octets. 1226 1227 Note that the list of options may be shorter than the data offset 1228 field might imply. The content of the header beyond the 1229 End-of-Option option must be header padding (i.e., zero). 1230 1231 A TCP must implement all options. 1232 1233 1234 [Page 17] 1235 1236 1237 September 1981 1238Transmission Control Protocol 1239Functional Specification 1240 1241 1242 1243 Currently defined options include (kind indicated in octal): 1244 1245 Kind Length Meaning 1246 ---- ------ ------- 1247 0 - End of option list. 1248 1 - No-Operation. 1249 2 4 Maximum Segment Size. 1250 1251 1252 Specific Option Definitions 1253 1254 End of Option List 1255 1256 +--------+ 1257 |00000000| 1258 +--------+ 1259 Kind=0 1260 1261 This option code indicates the end of the option list. This 1262 might not coincide with the end of the TCP header according to 1263 the Data Offset field. This is used at the end of all options, 1264 not the end of each option, and need only be used if the end of 1265 the options would not otherwise coincide with the end of the TCP 1266 header. 1267 1268 No-Operation 1269 1270 +--------+ 1271 |00000001| 1272 +--------+ 1273 Kind=1 1274 1275 This option code may be used between options, for example, to 1276 align the beginning of a subsequent option on a word boundary. 1277 There is no guarantee that senders will use this option, so 1278 receivers must be prepared to process options even if they do 1279 not begin on a word boundary. 1280 1281 Maximum Segment Size 1282 1283 +--------+--------+---------+--------+ 1284 |00000010|00000100| max seg size | 1285 +--------+--------+---------+--------+ 1286 Kind=2 Length=4 1287 1288 1289 1290 1291 1292 1293[Page 18] 1294 1295 1296September 1981 1297 Transmission Control Protocol 1298 Functional Specification 1299 1300 1301 1302 Maximum Segment Size Option Data: 16 bits 1303 1304 If this option is present, then it communicates the maximum 1305 receive segment size at the TCP which sends this segment. 1306 This field must only be sent in the initial connection request 1307 (i.e., in segments with the SYN control bit set). If this 1308 option is not used, any segment size is allowed. 1309 1310 Padding: variable 1311 1312 The TCP header padding is used to ensure that the TCP header ends 1313 and data begins on a 32 bit boundary. The padding is composed of 1314 zeros. 1315 13163.2. Terminology 1317 1318 Before we can discuss very much about the operation of the TCP we need 1319 to introduce some detailed terminology. The maintenance of a TCP 1320 connection requires the remembering of several variables. We conceive 1321 of these variables being stored in a connection record called a 1322 Transmission Control Block or TCB. Among the variables stored in the 1323 TCB are the local and remote socket numbers, the security and 1324 precedence of the connection, pointers to the user's send and receive 1325 buffers, pointers to the retransmit queue and to the current segment. 1326 In addition several variables relating to the send and receive 1327 sequence numbers are stored in the TCB. 1328 1329 Send Sequence Variables 1330 1331 SND.UNA - send unacknowledged 1332 SND.NXT - send next 1333 SND.WND - send window 1334 SND.UP - send urgent pointer 1335 SND.WL1 - segment sequence number used for last window update 1336 SND.WL2 - segment acknowledgment number used for last window 1337 update 1338 ISS - initial send sequence number 1339 1340 Receive Sequence Variables 1341 1342 RCV.NXT - receive next 1343 RCV.WND - receive window 1344 RCV.UP - receive urgent pointer 1345 IRS - initial receive sequence number 1346 1347 1348 1349 1350 1351 1352 [Page 19] 1353 1354 1355 September 1981 1356Transmission Control Protocol 1357Functional Specification 1358 1359 1360 1361 The following diagrams may help to relate some of these variables to 1362 the sequence space. 1363 1364 Send Sequence Space 1365 1366 1 2 3 4 1367 ----------|----------|----------|---------- 1368 SND.UNA SND.NXT SND.UNA 1369 +SND.WND 1370 1371 1 - old sequence numbers which have been acknowledged 1372 2 - sequence numbers of unacknowledged data 1373 3 - sequence numbers allowed for new data transmission 1374 4 - future sequence numbers which are not yet allowed 1375 1376 Send Sequence Space 1377 1378 Figure 4. 1379 1380 1381 1382 The send window is the portion of the sequence space labeled 3 in 1383 figure 4. 1384 1385 Receive Sequence Space 1386 1387 1 2 3 1388 ----------|----------|---------- 1389 RCV.NXT RCV.NXT 1390 +RCV.WND 1391 1392 1 - old sequence numbers which have been acknowledged 1393 2 - sequence numbers allowed for new reception 1394 3 - future sequence numbers which are not yet allowed 1395 1396 Receive Sequence Space 1397 1398 Figure 5. 1399 1400 1401 1402 The receive window is the portion of the sequence space labeled 2 in 1403 figure 5. 1404 1405 There are also some variables used frequently in the discussion that 1406 take their values from the fields of the current segment. 1407 1408 1409 1410 1411[Page 20] 1412 1413 1414September 1981 1415 Transmission Control Protocol 1416 Functional Specification 1417 1418 1419 1420 Current Segment Variables 1421 1422 SEG.SEQ - segment sequence number 1423 SEG.ACK - segment acknowledgment number 1424 SEG.LEN - segment length 1425 SEG.WND - segment window 1426 SEG.UP - segment urgent pointer 1427 SEG.PRC - segment precedence value 1428 1429 A connection progresses through a series of states during its 1430 lifetime. The states are: LISTEN, SYN-SENT, SYN-RECEIVED, 1431 ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, 1432 TIME-WAIT, and the fictional state CLOSED. CLOSED is fictional 1433 because it represents the state when there is no TCB, and therefore, 1434 no connection. Briefly the meanings of the states are: 1435 1436 LISTEN - represents waiting for a connection request from any remote 1437 TCP and port. 1438 1439 SYN-SENT - represents waiting for a matching connection request 1440 after having sent a connection request. 1441 1442 SYN-RECEIVED - represents waiting for a confirming connection 1443 request acknowledgment after having both received and sent a 1444 connection request. 1445 1446 ESTABLISHED - represents an open connection, data received can be 1447 delivered to the user. The normal state for the data transfer phase 1448 of the connection. 1449 1450 FIN-WAIT-1 - represents waiting for a connection termination request 1451 from the remote TCP, or an acknowledgment of the connection 1452 termination request previously sent. 1453 1454 FIN-WAIT-2 - represents waiting for a connection termination request 1455 from the remote TCP. 1456 1457 CLOSE-WAIT - represents waiting for a connection termination request 1458 from the local user. 1459 1460 CLOSING - represents waiting for a connection termination request 1461 acknowledgment from the remote TCP. 1462 1463 LAST-ACK - represents waiting for an acknowledgment of the 1464 connection termination request previously sent to the remote TCP 1465 (which includes an acknowledgment of its connection termination 1466 request). 1467 1468 1469 1470 [Page 21] 1471 1472 1473 September 1981 1474Transmission Control Protocol 1475Functional Specification 1476 1477 1478 1479 TIME-WAIT - represents waiting for enough time to pass to be sure 1480 the remote TCP received the acknowledgment of its connection 1481 termination request. 1482 1483 CLOSED - represents no connection state at all. 1484 1485 A TCP connection progresses from one state to another in response to 1486 events. The events are the user calls, OPEN, SEND, RECEIVE, CLOSE, 1487 ABORT, and STATUS; the incoming segments, particularly those 1488 containing the SYN, ACK, RST and FIN flags; and timeouts. 1489 1490 The state diagram in figure 6 illustrates only state changes, together 1491 with the causing events and resulting actions, but addresses neither 1492 error conditions nor actions which are not connected with state 1493 changes. In a later section, more detail is offered with respect to 1494 the reaction of the TCP to events. 1495 1496 NOTE BENE: this diagram is only a summary and must not be taken as 1497 the total specification. 1498 1499 1500 1501 1502 1503 1504 1505 1506 1507 1508 1509 1510 1511 1512 1513 1514 1515 1516 1517 1518 1519 1520 1521 1522 1523 1524 1525 1526 1527 1528 1529[Page 22] 1530 1531 1532September 1981 1533 Transmission Control Protocol 1534 Functional Specification 1535 1536 1537 1538 1539 +---------+ ---------\ active OPEN 1540 | CLOSED | \ ----------- 1541 +---------+<---------\ \ create TCB 1542 | ^ \ \ snd SYN 1543 passive OPEN | | CLOSE \ \ 1544 ------------ | | ---------- \ \ 1545 create TCB | | delete TCB \ \ 1546 V | \ \ 1547 +---------+ CLOSE | \ 1548 | LISTEN | ---------- | | 1549 +---------+ delete TCB | | 1550 rcv SYN | | SEND | | 1551 ----------- | | ------- | V 1552 +---------+ snd SYN,ACK / \ snd SYN +---------+ 1553 | |<----------------- ------------------>| | 1554 | SYN | rcv SYN | SYN | 1555 | RCVD |<-----------------------------------------------| SENT | 1556 | | snd ACK | | 1557 | |------------------ -------------------| | 1558 +---------+ rcv ACK of SYN \ / rcv SYN,ACK +---------+ 1559 | -------------- | | ----------- 1560 | x | | snd ACK 1561 | V V 1562 | CLOSE +---------+ 1563 | ------- | ESTAB | 1564 | snd FIN +---------+ 1565 | CLOSE | | rcv FIN 1566 V ------- | | ------- 1567 +---------+ snd FIN / \ snd ACK +---------+ 1568 | FIN |<----------------- ------------------>| CLOSE | 1569 | WAIT-1 |------------------ | WAIT | 1570 +---------+ rcv FIN \ +---------+ 1571 | rcv ACK of FIN ------- | CLOSE | 1572 | -------------- snd ACK | ------- | 1573 V x V snd FIN V 1574 +---------+ +---------+ +---------+ 1575 |FINWAIT-2| | CLOSING | | LAST-ACK| 1576 +---------+ +---------+ +---------+ 1577 | rcv ACK of FIN | rcv ACK of FIN | 1578 | rcv FIN -------------- | Timeout=2MSL -------------- | 1579 | ------- x V ------------ x V 1580 \ snd ACK +---------+delete TCB +---------+ 1581 ------------------------>|TIME WAIT|------------------>| CLOSED | 1582 +---------+ +---------+ 1583 1584 TCP Connection State Diagram 1585 Figure 6. 1586 1587 1588 [Page 23] 1589 1590 1591 September 1981 1592Transmission Control Protocol 1593Functional Specification 1594 1595 1596 15973.3. Sequence Numbers 1598 1599 A fundamental notion in the design is that every octet of data sent 1600 over a TCP connection has a sequence number. Since every octet is 1601 sequenced, each of them can be acknowledged. The acknowledgment 1602 mechanism employed is cumulative so that an acknowledgment of sequence 1603 number X indicates that all octets up to but not including X have been 1604 received. This mechanism allows for straight-forward duplicate 1605 detection in the presence of retransmission. Numbering of octets 1606 within a segment is that the first data octet immediately following 1607 the header is the lowest numbered, and the following octets are 1608 numbered consecutively. 1609 1610 It is essential to remember that the actual sequence number space is 1611 finite, though very large. This space ranges from 0 to 2**32 - 1. 1612 Since the space is finite, all arithmetic dealing with sequence 1613 numbers must be performed modulo 2**32. This unsigned arithmetic 1614 preserves the relationship of sequence numbers as they cycle from 1615 2**32 - 1 to 0 again. There are some subtleties to computer modulo 1616 arithmetic, so great care should be taken in programming the 1617 comparison of such values. The symbol "=<" means "less than or equal" 1618 (modulo 2**32). 1619 1620 The typical kinds of sequence number comparisons which the TCP must 1621 perform include: 1622 1623 (a) Determining that an acknowledgment refers to some sequence 1624 number sent but not yet acknowledged. 1625 1626 (b) Determining that all sequence numbers occupied by a segment 1627 have been acknowledged (e.g., to remove the segment from a 1628 retransmission queue). 1629 1630 (c) Determining that an incoming segment contains sequence numbers 1631 which are expected (i.e., that the segment "overlaps" the 1632 receive window). 1633 1634 1635 1636 1637 1638 1639 1640 1641 1642 1643 1644 1645 1646 1647[Page 24] 1648 1649 1650September 1981 1651 Transmission Control Protocol 1652 Functional Specification 1653 1654 1655 1656 In response to sending data the TCP will receive acknowledgments. The 1657 following comparisons are needed to process the acknowledgments. 1658 1659 SND.UNA = oldest unacknowledged sequence number 1660 1661 SND.NXT = next sequence number to be sent 1662 1663 SEG.ACK = acknowledgment from the receiving TCP (next sequence 1664 number expected by the receiving TCP) 1665 1666 SEG.SEQ = first sequence number of a segment 1667 1668 SEG.LEN = the number of octets occupied by the data in the segment 1669 (counting SYN and FIN) 1670 1671 SEG.SEQ+SEG.LEN-1 = last sequence number of a segment 1672 1673 A new acknowledgment (called an "acceptable ack"), is one for which 1674 the inequality below holds: 1675 1676 SND.UNA < SEG.ACK =< SND.NXT 1677 1678 A segment on the retransmission queue is fully acknowledged if the sum 1679 of its sequence number and length is less or equal than the 1680 acknowledgment value in the incoming segment. 1681 1682 When data is received the following comparisons are needed: 1683 1684 RCV.NXT = next sequence number expected on an incoming segments, and 1685 is the left or lower edge of the receive window 1686 1687 RCV.NXT+RCV.WND-1 = last sequence number expected on an incoming 1688 segment, and is the right or upper edge of the receive window 1689 1690 SEG.SEQ = first sequence number occupied by the incoming segment 1691 1692 SEG.SEQ+SEG.LEN-1 = last sequence number occupied by the incoming 1693 segment 1694 1695 A segment is judged to occupy a portion of valid receive sequence 1696 space if 1697 1698 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND 1699 1700 or 1701 1702 RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND 1703 1704 1705 1706 [Page 25] 1707 1708 1709 September 1981 1710Transmission Control Protocol 1711Functional Specification 1712 1713 1714 1715 The first part of this test checks to see if the beginning of the 1716 segment falls in the window, the second part of the test checks to see 1717 if the end of the segment falls in the window; if the segment passes 1718 either part of the test it contains data in the window. 1719 1720 Actually, it is a little more complicated than this. Due to zero 1721 windows and zero length segments, we have four cases for the 1722 acceptability of an incoming segment: 1723 1724 Segment Receive Test 1725 Length Window 1726 ------- ------- ------------------------------------------- 1727 1728 0 0 SEG.SEQ = RCV.NXT 1729 1730 0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND 1731 1732 >0 0 not acceptable 1733 1734 >0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND 1735 or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND 1736 1737 Note that when the receive window is zero no segments should be 1738 acceptable except ACK segments. Thus, it is be possible for a TCP to 1739 maintain a zero receive window while transmitting data and receiving 1740 ACKs. However, even when the receive window is zero, a TCP must 1741 process the RST and URG fields of all incoming segments. 1742 1743 We have taken advantage of the numbering scheme to protect certain 1744 control information as well. This is achieved by implicitly including 1745 some control flags in the sequence space so they can be retransmitted 1746 and acknowledged without confusion (i.e., one and only one copy of the 1747 control will be acted upon). Control information is not physically 1748 carried in the segment data space. Consequently, we must adopt rules 1749 for implicitly assigning sequence numbers to control. The SYN and FIN 1750 are the only controls requiring this protection, and these controls 1751 are used only at connection opening and closing. For sequence number 1752 purposes, the SYN is considered to occur before the first actual data 1753 octet of the segment in which it occurs, while the FIN is considered 1754 to occur after the last actual data octet in a segment in which it 1755 occurs. The segment length (SEG.LEN) includes both data and sequence 1756 space occupying controls. When a SYN is present then SEG.SEQ is the 1757 sequence number of the SYN. 1758 1759 1760 1761 1762 1763 1764 1765[Page 26] 1766 1767 1768September 1981 1769 Transmission Control Protocol 1770 Functional Specification 1771 1772 1773 1774 Initial Sequence Number Selection 1775 1776 The protocol places no restriction on a particular connection being 1777 used over and over again. A connection is defined by a pair of 1778 sockets. New instances of a connection will be referred to as 1779 incarnations of the connection. The problem that arises from this is 1780 -- "how does the TCP identify duplicate segments from previous 1781 incarnations of the connection?" This problem becomes apparent if the 1782 connection is being opened and closed in quick succession, or if the 1783 connection breaks with loss of memory and is then reestablished. 1784 1785 To avoid confusion we must prevent segments from one incarnation of a 1786 connection from being used while the same sequence numbers may still 1787 be present in the network from an earlier incarnation. We want to 1788 assure this, even if a TCP crashes and loses all knowledge of the 1789 sequence numbers it has been using. When new connections are created, 1790 an initial sequence number (ISN) generator is employed which selects a 1791 new 32 bit ISN. The generator is bound to a (possibly fictitious) 32 1792 bit clock whose low order bit is incremented roughly every 4 1793 microseconds. Thus, the ISN cycles approximately every 4.55 hours. 1794 Since we assume that segments will stay in the network no more than 1795 the Maximum Segment Lifetime (MSL) and that the MSL is less than 4.55 1796 hours we can reasonably assume that ISN's will be unique. 1797 1798 For each connection there is a send sequence number and a receive 1799 sequence number. The initial send sequence number (ISS) is chosen by 1800 the data sending TCP, and the initial receive sequence number (IRS) is 1801 learned during the connection establishing procedure. 1802 1803 For a connection to be established or initialized, the two TCPs must 1804 synchronize on each other's initial sequence numbers. This is done in 1805 an exchange of connection establishing segments carrying a control bit 1806 called "SYN" (for synchronize) and the initial sequence numbers. As a 1807 shorthand, segments carrying the SYN bit are also called "SYNs". 1808 Hence, the solution requires a suitable mechanism for picking an 1809 initial sequence number and a slightly involved handshake to exchange 1810 the ISN's. 1811 1812 The synchronization requires each side to send it's own initial 1813 sequence number and to receive a confirmation of it in acknowledgment 1814 from the other side. Each side must also receive the other side's 1815 initial sequence number and send a confirming acknowledgment. 1816 1817 1) A --> B SYN my sequence number is X 1818 2) A <-- B ACK your sequence number is X 1819 3) A <-- B SYN my sequence number is Y 1820 4) A --> B ACK your sequence number is Y 1821 1822 1823 1824 [Page 27] 1825 1826 1827 September 1981 1828Transmission Control Protocol 1829Functional Specification 1830 1831 1832 1833 Because steps 2 and 3 can be combined in a single message this is 1834 called the three way (or three message) handshake. 1835 1836 A three way handshake is necessary because sequence numbers are not 1837 tied to a global clock in the network, and TCPs may have different 1838 mechanisms for picking the ISN's. The receiver of the first SYN has 1839 no way of knowing whether the segment was an old delayed one or not, 1840 unless it remembers the last sequence number used on the connection 1841 (which is not always possible), and so it must ask the sender to 1842 verify this SYN. The three way handshake and the advantages of a 1843 clock-driven scheme are discussed in [3]. 1844 1845 Knowing When to Keep Quiet 1846 1847 To be sure that a TCP does not create a segment that carries a 1848 sequence number which may be duplicated by an old segment remaining in 1849 the network, the TCP must keep quiet for a maximum segment lifetime 1850 (MSL) before assigning any sequence numbers upon starting up or 1851 recovering from a crash in which memory of sequence numbers in use was 1852 lost. For this specification the MSL is taken to be 2 minutes. This 1853 is an engineering choice, and may be changed if experience indicates 1854 it is desirable to do so. Note that if a TCP is reinitialized in some 1855 sense, yet retains its memory of sequence numbers in use, then it need 1856 not wait at all; it must only be sure to use sequence numbers larger 1857 than those recently used. 1858 1859 The TCP Quiet Time Concept 1860 1861 This specification provides that hosts which "crash" without 1862 retaining any knowledge of the last sequence numbers transmitted on 1863 each active (i.e., not closed) connection shall delay emitting any 1864 TCP segments for at least the agreed Maximum Segment Lifetime (MSL) 1865 in the internet system of which the host is a part. In the 1866 paragraphs below, an explanation for this specification is given. 1867 TCP implementors may violate the "quiet time" restriction, but only 1868 at the risk of causing some old data to be accepted as new or new 1869 data rejected as old duplicated by some receivers in the internet 1870 system. 1871 1872 TCPs consume sequence number space each time a segment is formed and 1873 entered into the network output queue at a source host. The 1874 duplicate detection and sequencing algorithm in the TCP protocol 1875 relies on the unique binding of segment data to sequence space to 1876 the extent that sequence numbers will not cycle through all 2**32 1877 values before the segment data bound to those sequence numbers has 1878 been delivered and acknowledged by the receiver and all duplicate 1879 copies of the segments have "drained" from the internet. Without 1880 such an assumption, two distinct TCP segments could conceivably be 1881 1882 1883[Page 28] 1884 1885 1886September 1981 1887 Transmission Control Protocol 1888 Functional Specification 1889 1890 1891 1892 assigned the same or overlapping sequence numbers, causing confusion 1893 at the receiver as to which data is new and which is old. Remember 1894 that each segment is bound to as many consecutive sequence numbers 1895 as there are octets of data in the segment. 1896 1897 Under normal conditions, TCPs keep track of the next sequence number 1898 to emit and the oldest awaiting acknowledgment so as to avoid 1899 mistakenly using a sequence number over before its first use has 1900 been acknowledged. This alone does not guarantee that old duplicate 1901 data is drained from the net, so the sequence space has been made 1902 very large to reduce the probability that a wandering duplicate will 1903 cause trouble upon arrival. At 2 megabits/sec. it takes 4.5 hours 1904 to use up 2**32 octets of sequence space. Since the maximum segment 1905 lifetime in the net is not likely to exceed a few tens of seconds, 1906 this is deemed ample protection for foreseeable nets, even if data 1907 rates escalate to l0's of megabits/sec. At 100 megabits/sec, the 1908 cycle time is 5.4 minutes which may be a little short, but still 1909 within reason. 1910 1911 The basic duplicate detection and sequencing algorithm in TCP can be 1912 defeated, however, if a source TCP does not have any memory of the 1913 sequence numbers it last used on a given connection. For example, if 1914 the TCP were to start all connections with sequence number 0, then 1915 upon crashing and restarting, a TCP might re-form an earlier 1916 connection (possibly after half-open connection resolution) and emit 1917 packets with sequence numbers identical to or overlapping with 1918 packets still in the network which were emitted on an earlier 1919 incarnation of the same connection. In the absence of knowledge 1920 about the sequence numbers used on a particular connection, the TCP 1921 specification recommends that the source delay for MSL seconds 1922 before emitting segments on the connection, to allow time for 1923 segments from the earlier connection incarnation to drain from the 1924 system. 1925 1926 Even hosts which can remember the time of day and used it to select 1927 initial sequence number values are not immune from this problem 1928 (i.e., even if time of day is used to select an initial sequence 1929 number for each new connection incarnation). 1930 1931 Suppose, for example, that a connection is opened starting with 1932 sequence number S. Suppose that this connection is not used much 1933 and that eventually the initial sequence number function (ISN(t)) 1934 takes on a value equal to the sequence number, say S1, of the last 1935 segment sent by this TCP on a particular connection. Now suppose, 1936 at this instant, the host crashes, recovers, and establishes a new 1937 incarnation of the connection. The initial sequence number chosen is 1938 S1 = ISN(t) -- last used sequence number on old incarnation of 1939 connection! If the recovery occurs quickly enough, any old 1940 1941 1942 [Page 29] 1943 1944 1945 September 1981 1946Transmission Control Protocol 1947Functional Specification 1948 1949 1950 1951 duplicates in the net bearing sequence numbers in the neighborhood 1952 of S1 may arrive and be treated as new packets by the receiver of 1953 the new incarnation of the connection. 1954 1955 The problem is that the recovering host may not know for how long it 1956 crashed nor does it know whether there are still old duplicates in 1957 the system from earlier connection incarnations. 1958 1959 One way to deal with this problem is to deliberately delay emitting 1960 segments for one MSL after recovery from a crash- this is the "quite 1961 time" specification. Hosts which prefer to avoid waiting are 1962 willing to risk possible confusion of old and new packets at a given 1963 destination may choose not to wait for the "quite time". 1964 Implementors may provide TCP users with the ability to select on a 1965 connection by connection basis whether to wait after a crash, or may 1966 informally implement the "quite time" for all connections. 1967 Obviously, even where a user selects to "wait," this is not 1968 necessary after the host has been "up" for at least MSL seconds. 1969 1970 To summarize: every segment emitted occupies one or more sequence 1971 numbers in the sequence space, the numbers occupied by a segment are 1972 "busy" or "in use" until MSL seconds have passed, upon crashing a 1973 block of space-time is occupied by the octets of the last emitted 1974 segment, if a new connection is started too soon and uses any of the 1975 sequence numbers in the space-time footprint of the last segment of 1976 the previous connection incarnation, there is a potential sequence 1977 number overlap area which could cause confusion at the receiver. 1978 19793.4. Establishing a connection 1980 1981 The "three-way handshake" is the procedure used to establish a 1982 connection. This procedure normally is initiated by one TCP and 1983 responded to by another TCP. The procedure also works if two TCP 1984 simultaneously initiate the procedure. When simultaneous attempt 1985 occurs, each TCP receives a "SYN" segment which carries no 1986 acknowledgment after it has sent a "SYN". Of course, the arrival of 1987 an old duplicate "SYN" segment can potentially make it appear, to the 1988 recipient, that a simultaneous connection initiation is in progress. 1989 Proper use of "reset" segments can disambiguate these cases. 1990 1991 Several examples of connection initiation follow. Although these 1992 examples do not show connection synchronization using data-carrying 1993 segments, this is perfectly legitimate, so long as the receiving TCP 1994 doesn't deliver the data to the user until it is clear the data is 1995 valid (i.e., the data must be buffered at the receiver until the 1996 connection reaches the ESTABLISHED state). The three-way handshake 1997 reduces the possibility of false connections. It is the 1998 1999 2000 2001[Page 30] 2002 2003 2004September 1981 2005 Transmission Control Protocol 2006 Functional Specification 2007 2008 2009 2010 implementation of a trade-off between memory and messages to provide 2011 information for this checking. 2012 2013 The simplest three-way handshake is shown in figure 7 below. The 2014 figures should be interpreted in the following way. Each line is 2015 numbered for reference purposes. Right arrows (-->) indicate 2016 departure of a TCP segment from TCP A to TCP B, or arrival of a 2017 segment at B from A. Left arrows (<--), indicate the reverse. 2018 Ellipsis (...) indicates a segment which is still in the network 2019 (delayed). An "XXX" indicates a segment which is lost or rejected. 2020 Comments appear in parentheses. TCP states represent the state AFTER 2021 the departure or arrival of the segment (whose contents are shown in 2022 the center of each line). Segment contents are shown in abbreviated 2023 form, with sequence number, control flags, and ACK field. Other 2024 fields such as window, addresses, lengths, and text have been left out 2025 in the interest of clarity. 2026 2027 2028 2029 TCP A TCP B 2030 2031 1. CLOSED LISTEN 2032 2033 2. SYN-SENT --> <SEQ=100><CTL=SYN> --> SYN-RECEIVED 2034 2035 3. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED 2036 2037 4. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK> --> ESTABLISHED 2038 2039 5. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK><DATA> --> ESTABLISHED 2040 2041 Basic 3-Way Handshake for Connection Synchronization 2042 2043 Figure 7. 2044 2045 In line 2 of figure 7, TCP A begins by sending a SYN segment 2046 indicating that it will use sequence numbers starting with sequence 2047 number 100. In line 3, TCP B sends a SYN and acknowledges the SYN it 2048 received from TCP A. Note that the acknowledgment field indicates TCP 2049 B is now expecting to hear sequence 101, acknowledging the SYN which 2050 occupied sequence 100. 2051 2052 At line 4, TCP A responds with an empty segment containing an ACK for 2053 TCP B's SYN; and in line 5, TCP A sends some data. Note that the 2054 sequence number of the segment in line 5 is the same as in line 4 2055 because the ACK does not occupy sequence number space (if it did, we 2056 would wind up ACKing ACK's!). 2057 2058 2059 2060 [Page 31] 2061 2062 2063 September 1981 2064Transmission Control Protocol 2065Functional Specification 2066 2067 2068 2069 Simultaneous initiation is only slightly more complex, as is shown in 2070 figure 8. Each TCP cycles from CLOSED to SYN-SENT to SYN-RECEIVED to 2071 ESTABLISHED. 2072 2073 2074 2075 TCP A TCP B 2076 2077 1. CLOSED CLOSED 2078 2079 2. SYN-SENT --> <SEQ=100><CTL=SYN> ... 2080 2081 3. SYN-RECEIVED <-- <SEQ=300><CTL=SYN> <-- SYN-SENT 2082 2083 4. ... <SEQ=100><CTL=SYN> --> SYN-RECEIVED 2084 2085 5. SYN-RECEIVED --> <SEQ=100><ACK=301><CTL=SYN,ACK> ... 2086 2087 6. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED 2088 2089 7. ... <SEQ=101><ACK=301><CTL=ACK> --> ESTABLISHED 2090 2091 Simultaneous Connection Synchronization 2092 2093 Figure 8. 2094 2095 The principle reason for the three-way handshake is to prevent old 2096 duplicate connection initiations from causing confusion. To deal with 2097 this, a special control message, reset, has been devised. If the 2098 receiving TCP is in a non-synchronized state (i.e., SYN-SENT, 2099 SYN-RECEIVED), it returns to LISTEN on receiving an acceptable reset. 2100 If the TCP is in one of the synchronized states (ESTABLISHED, 2101 FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT), it 2102 aborts the connection and informs its user. We discuss this latter 2103 case under "half-open" connections below. 2104 2105 2106 2107 2108 2109 2110 2111 2112 2113 2114 2115 2116 2117 2118 2119[Page 32] 2120 2121 2122September 1981 2123 Transmission Control Protocol 2124 Functional Specification 2125 2126 2127 2128 2129 2130 TCP A TCP B 2131 2132 1. CLOSED LISTEN 2133 2134 2. SYN-SENT --> <SEQ=100><CTL=SYN> ... 2135 2136 3. (duplicate) ... <SEQ=90><CTL=SYN> --> SYN-RECEIVED 2137 2138 4. SYN-SENT <-- <SEQ=300><ACK=91><CTL=SYN,ACK> <-- SYN-RECEIVED 2139 2140 5. SYN-SENT --> <SEQ=91><CTL=RST> --> LISTEN 2141 2142 2143 6. ... <SEQ=100><CTL=SYN> --> SYN-RECEIVED 2144 2145 7. SYN-SENT <-- <SEQ=400><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED 2146 2147 8. ESTABLISHED --> <SEQ=101><ACK=401><CTL=ACK> --> ESTABLISHED 2148 2149 Recovery from Old Duplicate SYN 2150 2151 Figure 9. 2152 2153 As a simple example of recovery from old duplicates, consider 2154 figure 9. At line 3, an old duplicate SYN arrives at TCP B. TCP B 2155 cannot tell that this is an old duplicate, so it responds normally 2156 (line 4). TCP A detects that the ACK field is incorrect and returns a 2157 RST (reset) with its SEQ field selected to make the segment 2158 believable. TCP B, on receiving the RST, returns to the LISTEN state. 2159 When the original SYN (pun intended) finally arrives at line 6, the 2160 synchronization proceeds normally. If the SYN at line 6 had arrived 2161 before the RST, a more complex exchange might have occurred with RST's 2162 sent in both directions. 2163 2164 Half-Open Connections and Other Anomalies 2165 2166 An established connection is said to be "half-open" if one of the 2167 TCPs has closed or aborted the connection at its end without the 2168 knowledge of the other, or if the two ends of the connection have 2169 become desynchronized owing to a crash that resulted in loss of 2170 memory. Such connections will automatically become reset if an 2171 attempt is made to send data in either direction. However, half-open 2172 connections are expected to be unusual, and the recovery procedure is 2173 mildly involved. 2174 2175 If at site A the connection no longer exists, then an attempt by the 2176 2177 2178 [Page 33] 2179 2180 2181 September 1981 2182Transmission Control Protocol 2183Functional Specification 2184 2185 2186 2187 user at site B to send any data on it will result in the site B TCP 2188 receiving a reset control message. Such a message indicates to the 2189 site B TCP that something is wrong, and it is expected to abort the 2190 connection. 2191 2192 Assume that two user processes A and B are communicating with one 2193 another when a crash occurs causing loss of memory to A's TCP. 2194 Depending on the operating system supporting A's TCP, it is likely 2195 that some error recovery mechanism exists. When the TCP is up again, 2196 A is likely to start again from the beginning or from a recovery 2197 point. As a result, A will probably try to OPEN the connection again 2198 or try to SEND on the connection it believes open. In the latter 2199 case, it receives the error message "connection not open" from the 2200 local (A's) TCP. In an attempt to establish the connection, A's TCP 2201 will send a segment containing SYN. This scenario leads to the 2202 example shown in figure 10. After TCP A crashes, the user attempts to 2203 re-open the connection. TCP B, in the meantime, thinks the connection 2204 is open. 2205 2206 2207 2208 TCP A TCP B 2209 2210 1. (CRASH) (send 300,receive 100) 2211 2212 2. CLOSED ESTABLISHED 2213 2214 3. SYN-SENT --> <SEQ=400><CTL=SYN> --> (??) 2215 2216 4. (!!) <-- <SEQ=300><ACK=100><CTL=ACK> <-- ESTABLISHED 2217 2218 5. SYN-SENT --> <SEQ=100><CTL=RST> --> (Abort!!) 2219 2220 6. SYN-SENT CLOSED 2221 2222 7. SYN-SENT --> <SEQ=400><CTL=SYN> --> 2223 2224 Half-Open Connection Discovery 2225 2226 Figure 10. 2227 2228 When the SYN arrives at line 3, TCP B, being in a synchronized state, 2229 and the incoming segment outside the window, responds with an 2230 acknowledgment indicating what sequence it next expects to hear (ACK 2231 100). TCP A sees that this segment does not acknowledge anything it 2232 sent and, being unsynchronized, sends a reset (RST) because it has 2233 detected a half-open connection. TCP B aborts at line 5. TCP A will 2234 2235 2236 2237[Page 34] 2238 2239 2240September 1981 2241 Transmission Control Protocol 2242 Functional Specification 2243 2244 2245 2246 continue to try to establish the connection; the problem is now 2247 reduced to the basic 3-way handshake of figure 7. 2248 2249 An interesting alternative case occurs when TCP A crashes and TCP B 2250 tries to send data on what it thinks is a synchronized connection. 2251 This is illustrated in figure 11. In this case, the data arriving at 2252 TCP A from TCP B (line 2) is unacceptable because no such connection 2253 exists, so TCP A sends a RST. The RST is acceptable so TCP B 2254 processes it and aborts the connection. 2255 2256 2257 2258 TCP A TCP B 2259 2260 1. (CRASH) (send 300,receive 100) 2261 2262 2. (??) <-- <SEQ=300><ACK=100><DATA=10><CTL=ACK> <-- ESTABLISHED 2263 2264 3. --> <SEQ=100><CTL=RST> --> (ABORT!!) 2265 2266 Active Side Causes Half-Open Connection Discovery 2267 2268 Figure 11. 2269 2270 In figure 12, we find the two TCPs A and B with passive connections 2271 waiting for SYN. An old duplicate arriving at TCP B (line 2) stirs B 2272 into action. A SYN-ACK is returned (line 3) and causes TCP A to 2273 generate a RST (the ACK in line 3 is not acceptable). TCP B accepts 2274 the reset and returns to its passive LISTEN state. 2275 2276 2277 2278 TCP A TCP B 2279 2280 1. LISTEN LISTEN 2281 2282 2. ... <SEQ=Z><CTL=SYN> --> SYN-RECEIVED 2283 2284 3. (??) <-- <SEQ=X><ACK=Z+1><CTL=SYN,ACK> <-- SYN-RECEIVED 2285 2286 4. --> <SEQ=Z+1><CTL=RST> --> (return to LISTEN!) 2287 2288 5. LISTEN LISTEN 2289 2290 Old Duplicate SYN Initiates a Reset on two Passive Sockets 2291 2292 Figure 12. 2293 2294 2295 2296 [Page 35] 2297 2298 2299 September 1981 2300Transmission Control Protocol 2301Functional Specification 2302 2303 2304 2305 A variety of other cases are possible, all of which are accounted for 2306 by the following rules for RST generation and processing. 2307 2308 Reset Generation 2309 2310 As a general rule, reset (RST) must be sent whenever a segment arrives 2311 which apparently is not intended for the current connection. A reset 2312 must not be sent if it is not clear that this is the case. 2313 2314 There are three groups of states: 2315 2316 1. If the connection does not exist (CLOSED) then a reset is sent 2317 in response to any incoming segment except another reset. In 2318 particular, SYNs addressed to a non-existent connection are rejected 2319 by this means. 2320 2321 If the incoming segment has an ACK field, the reset takes its 2322 sequence number from the ACK field of the segment, otherwise the 2323 reset has sequence number zero and the ACK field is set to the sum 2324 of the sequence number and segment length of the incoming segment. 2325 The connection remains in the CLOSED state. 2326 2327 2. If the connection is in any non-synchronized state (LISTEN, 2328 SYN-SENT, SYN-RECEIVED), and the incoming segment acknowledges 2329 something not yet sent (the segment carries an unacceptable ACK), or 2330 if an incoming segment has a security level or compartment which 2331 does not exactly match the level and compartment requested for the 2332 connection, a reset is sent. 2333 2334 If our SYN has not been acknowledged and the precedence level of the 2335 incoming segment is higher than the precedence level requested then 2336 either raise the local precedence level (if allowed by the user and 2337 the system) or send a reset; or if the precedence level of the 2338 incoming segment is lower than the precedence level requested then 2339 continue as if the precedence matched exactly (if the remote TCP 2340 cannot raise the precedence level to match ours this will be 2341 detected in the next segment it sends, and the connection will be 2342 terminated then). If our SYN has been acknowledged (perhaps in this 2343 incoming segment) the precedence level of the incoming segment must 2344 match the local precedence level exactly, if it does not a reset 2345 must be sent. 2346 2347 If the incoming segment has an ACK field, the reset takes its 2348 sequence number from the ACK field of the segment, otherwise the 2349 reset has sequence number zero and the ACK field is set to the sum 2350 of the sequence number and segment length of the incoming segment. 2351 The connection remains in the same state. 2352 2353 2354 2355[Page 36] 2356 2357 2358September 1981 2359 Transmission Control Protocol 2360 Functional Specification 2361 2362 2363 2364 3. If the connection is in a synchronized state (ESTABLISHED, 2365 FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT), 2366 any unacceptable segment (out of window sequence number or 2367 unacceptible acknowledgment number) must elicit only an empty 2368 acknowledgment segment containing the current send-sequence number 2369 and an acknowledgment indicating the next sequence number expected 2370 to be received, and the connection remains in the same state. 2371 2372 If an incoming segment has a security level, or compartment, or 2373 precedence which does not exactly match the level, and compartment, 2374 and precedence requested for the connection,a reset is sent and 2375 connection goes to the CLOSED state. The reset takes its sequence 2376 number from the ACK field of the incoming segment. 2377 2378 Reset Processing 2379 2380 In all states except SYN-SENT, all reset (RST) segments are validated 2381 by checking their SEQ-fields. A reset is valid if its sequence number 2382 is in the window. In the SYN-SENT state (a RST received in response 2383 to an initial SYN), the RST is acceptable if the ACK field 2384 acknowledges the SYN. 2385 2386 The receiver of a RST first validates it, then changes state. If the 2387 receiver was in the LISTEN state, it ignores it. If the receiver was 2388 in SYN-RECEIVED state and had previously been in the LISTEN state, 2389 then the receiver returns to the LISTEN state, otherwise the receiver 2390 aborts the connection and goes to the CLOSED state. If the receiver 2391 was in any other state, it aborts the connection and advises the user 2392 and goes to the CLOSED state. 2393 23943.5. Closing a Connection 2395 2396 CLOSE is an operation meaning "I have no more data to send." The 2397 notion of closing a full-duplex connection is subject to ambiguous 2398 interpretation, of course, since it may not be obvious how to treat 2399 the receiving side of the connection. We have chosen to treat CLOSE 2400 in a simplex fashion. The user who CLOSEs may continue to RECEIVE 2401 until he is told that the other side has CLOSED also. Thus, a program 2402 could initiate several SENDs followed by a CLOSE, and then continue to 2403 RECEIVE until signaled that a RECEIVE failed because the other side 2404 has CLOSED. We assume that the TCP will signal a user, even if no 2405 RECEIVEs are outstanding, that the other side has closed, so the user 2406 can terminate his side gracefully. A TCP will reliably deliver all 2407 buffers SENT before the connection was CLOSED so a user who expects no 2408 data in return need only wait to hear the connection was CLOSED 2409 successfully to know that all his data was received at the destination 2410 TCP. Users must keep reading connections they close for sending until 2411 the TCP says no more data. 2412 2413 2414 [Page 37] 2415 2416 2417 September 1981 2418Transmission Control Protocol 2419Functional Specification 2420 2421 2422 2423 There are essentially three cases: 2424 2425 1) The user initiates by telling the TCP to CLOSE the connection 2426 2427 2) The remote TCP initiates by sending a FIN control signal 2428 2429 3) Both users CLOSE simultaneously 2430 2431 Case 1: Local user initiates the close 2432 2433 In this case, a FIN segment can be constructed and placed on the 2434 outgoing segment queue. No further SENDs from the user will be 2435 accepted by the TCP, and it enters the FIN-WAIT-1 state. RECEIVEs 2436 are allowed in this state. All segments preceding and including FIN 2437 will be retransmitted until acknowledged. When the other TCP has 2438 both acknowledged the FIN and sent a FIN of its own, the first TCP 2439 can ACK this FIN. Note that a TCP receiving a FIN will ACK but not 2440 send its own FIN until its user has CLOSED the connection also. 2441 2442 Case 2: TCP receives a FIN from the network 2443 2444 If an unsolicited FIN arrives from the network, the receiving TCP 2445 can ACK it and tell the user that the connection is closing. The 2446 user will respond with a CLOSE, upon which the TCP can send a FIN to 2447 the other TCP after sending any remaining data. The TCP then waits 2448 until its own FIN is acknowledged whereupon it deletes the 2449 connection. If an ACK is not forthcoming, after the user timeout 2450 the connection is aborted and the user is told. 2451 2452 Case 3: both users close simultaneously 2453 2454 A simultaneous CLOSE by users at both ends of a connection causes 2455 FIN segments to be exchanged. When all segments preceding the FINs 2456 have been processed and acknowledged, each TCP can ACK the FIN it 2457 has received. Both will, upon receiving these ACKs, delete the 2458 connection. 2459 2460 2461 2462 2463 2464 2465 2466 2467 2468 2469 2470 2471 2472 2473[Page 38] 2474 2475 2476September 1981 2477 Transmission Control Protocol 2478 Functional Specification 2479 2480 2481 2482 2483 2484 TCP A TCP B 2485 2486 1. ESTABLISHED ESTABLISHED 2487 2488 2. (Close) 2489 FIN-WAIT-1 --> <SEQ=100><ACK=300><CTL=FIN,ACK> --> CLOSE-WAIT 2490 2491 3. FIN-WAIT-2 <-- <SEQ=300><ACK=101><CTL=ACK> <-- CLOSE-WAIT 2492 2493 4. (Close) 2494 TIME-WAIT <-- <SEQ=300><ACK=101><CTL=FIN,ACK> <-- LAST-ACK 2495 2496 5. TIME-WAIT --> <SEQ=101><ACK=301><CTL=ACK> --> CLOSED 2497 2498 6. (2 MSL) 2499 CLOSED 2500 2501 Normal Close Sequence 2502 2503 Figure 13. 2504 2505 2506 2507 TCP A TCP B 2508 2509 1. ESTABLISHED ESTABLISHED 2510 2511 2. (Close) (Close) 2512 FIN-WAIT-1 --> <SEQ=100><ACK=300><CTL=FIN,ACK> ... FIN-WAIT-1 2513 <-- <SEQ=300><ACK=100><CTL=FIN,ACK> <-- 2514 ... <SEQ=100><ACK=300><CTL=FIN,ACK> --> 2515 2516 3. CLOSING --> <SEQ=101><ACK=301><CTL=ACK> ... CLOSING 2517 <-- <SEQ=301><ACK=101><CTL=ACK> <-- 2518 ... <SEQ=101><ACK=301><CTL=ACK> --> 2519 2520 4. TIME-WAIT TIME-WAIT 2521 (2 MSL) (2 MSL) 2522 CLOSED CLOSED 2523 2524 Simultaneous Close Sequence 2525 2526 Figure 14. 2527 2528 2529 2530 2531 2532 [Page 39] 2533 2534 2535 September 1981 2536Transmission Control Protocol 2537Functional Specification 2538 2539 2540 25413.6. Precedence and Security 2542 2543 The intent is that connection be allowed only between ports operating 2544 with exactly the same security and compartment values and at the 2545 higher of the precedence level requested by the two ports. 2546 2547 The precedence and security parameters used in TCP are exactly those 2548 defined in the Internet Protocol (IP) [2]. Throughout this TCP 2549 specification the term "security/compartment" is intended to indicate 2550 the security parameters used in IP including security, compartment, 2551 user group, and handling restriction. 2552 2553 A connection attempt with mismatched security/compartment values or a 2554 lower precedence value must be rejected by sending a reset. Rejecting 2555 a connection due to too low a precedence only occurs after an 2556 acknowledgment of the SYN has been received. 2557 2558 Note that TCP modules which operate only at the default value of 2559 precedence will still have to check the precedence of incoming 2560 segments and possibly raise the precedence level they use on the 2561 connection. 2562 2563 The security paramaters may be used even in a non-secure environment 2564 (the values would indicate unclassified data), thus hosts in 2565 non-secure environments must be prepared to receive the security 2566 parameters, though they need not send them. 2567 25683.7. Data Communication 2569 2570 Once the connection is established data is communicated by the 2571 exchange of segments. Because segments may be lost due to errors 2572 (checksum test failure), or network congestion, TCP uses 2573 retransmission (after a timeout) to ensure delivery of every segment. 2574 Duplicate segments may arrive due to network or TCP retransmission. 2575 As discussed in the section on sequence numbers the TCP performs 2576 certain tests on the sequence and acknowledgment numbers in the 2577 segments to verify their acceptability. 2578 2579 The sender of data keeps track of the next sequence number to use in 2580 the variable SND.NXT. The receiver of data keeps track of the next 2581 sequence number to expect in the variable RCV.NXT. The sender of data 2582 keeps track of the oldest unacknowledged sequence number in the 2583 variable SND.UNA. If the data flow is momentarily idle and all data 2584 sent has been acknowledged then the three variables will be equal. 2585 2586 When the sender creates a segment and transmits it the sender advances 2587 SND.NXT. When the receiver accepts a segment it advances RCV.NXT and 2588 sends an acknowledgment. When the data sender receives an 2589 2590 2591[Page 40] 2592 2593 2594September 1981 2595 Transmission Control Protocol 2596 Functional Specification 2597 2598 2599 2600 acknowledgment it advances SND.UNA. The extent to which the values of 2601 these variables differ is a measure of the delay in the communication. 2602 The amount by which the variables are advanced is the length of the 2603 data in the segment. Note that once in the ESTABLISHED state all 2604 segments must carry current acknowledgment information. 2605 2606 The CLOSE user call implies a push function, as does the FIN control 2607 flag in an incoming segment. 2608 2609 Retransmission Timeout 2610 2611 Because of the variability of the networks that compose an 2612 internetwork system and the wide range of uses of TCP connections the 2613 retransmission timeout must be dynamically determined. One procedure 2614 for determining a retransmission time out is given here as an 2615 illustration. 2616 2617 An Example Retransmission Timeout Procedure 2618 2619 Measure the elapsed time between sending a data octet with a 2620 particular sequence number and receiving an acknowledgment that 2621 covers that sequence number (segments sent do not have to match 2622 segments received). This measured elapsed time is the Round Trip 2623 Time (RTT). Next compute a Smoothed Round Trip Time (SRTT) as: 2624 2625 SRTT = ( ALPHA * SRTT ) + ((1-ALPHA) * RTT) 2626 2627 and based on this, compute the retransmission timeout (RTO) as: 2628 2629 RTO = min[UBOUND,max[LBOUND,(BETA*SRTT)]] 2630 2631 where UBOUND is an upper bound on the timeout (e.g., 1 minute), 2632 LBOUND is a lower bound on the timeout (e.g., 1 second), ALPHA is 2633 a smoothing factor (e.g., .8 to .9), and BETA is a delay variance 2634 factor (e.g., 1.3 to 2.0). 2635 2636 The Communication of Urgent Information 2637 2638 The objective of the TCP urgent mechanism is to allow the sending user 2639 to stimulate the receiving user to accept some urgent data and to 2640 permit the receiving TCP to indicate to the receiving user when all 2641 the currently known urgent data has been received by the user. 2642 2643 This mechanism permits a point in the data stream to be designated as 2644 the end of urgent information. Whenever this point is in advance of 2645 the receive sequence number (RCV.NXT) at the receiving TCP, that TCP 2646 must tell the user to go into "urgent mode"; when the receive sequence 2647 number catches up to the urgent pointer, the TCP must tell user to go 2648 2649 2650 [Page 41] 2651 2652 2653 September 1981 2654Transmission Control Protocol 2655Functional Specification 2656 2657 2658 2659 into "normal mode". If the urgent pointer is updated while the user 2660 is in "urgent mode", the update will be invisible to the user. 2661 2662 The method employs a urgent field which is carried in all segments 2663 transmitted. The URG control flag indicates that the urgent field is 2664 meaningful and must be added to the segment sequence number to yield 2665 the urgent pointer. The absence of this flag indicates that there is 2666 no urgent data outstanding. 2667 2668 To send an urgent indication the user must also send at least one data 2669 octet. If the sending user also indicates a push, timely delivery of 2670 the urgent information to the destination process is enhanced. 2671 2672 Managing the Window 2673 2674 The window sent in each segment indicates the range of sequence 2675 numbers the sender of the window (the data receiver) is currently 2676 prepared to accept. There is an assumption that this is related to 2677 the currently available data buffer space available for this 2678 connection. 2679 2680 Indicating a large window encourages transmissions. If more data 2681 arrives than can be accepted, it will be discarded. This will result 2682 in excessive retransmissions, adding unnecessarily to the load on the 2683 network and the TCPs. Indicating a small window may restrict the 2684 transmission of data to the point of introducing a round trip delay 2685 between each new segment transmitted. 2686 2687 The mechanisms provided allow a TCP to advertise a large window and to 2688 subsequently advertise a much smaller window without having accepted 2689 that much data. This, so called "shrinking the window," is strongly 2690 discouraged. The robustness principle dictates that TCPs will not 2691 shrink the window themselves, but will be prepared for such behavior 2692 on the part of other TCPs. 2693 2694 The sending TCP must be prepared to accept from the user and send at 2695 least one octet of new data even if the send window is zero. The 2696 sending TCP must regularly retransmit to the receiving TCP even when 2697 the window is zero. Two minutes is recommended for the retransmission 2698 interval when the window is zero. This retransmission is essential to 2699 guarantee that when either TCP has a zero window the re-opening of the 2700 window will be reliably reported to the other. 2701 2702 When the receiving TCP has a zero window and a segment arrives it must 2703 still send an acknowledgment showing its next expected sequence number 2704 and current window (zero). 2705 2706 The sending TCP packages the data to be transmitted into segments 2707 2708 2709[Page 42] 2710 2711 2712September 1981 2713 Transmission Control Protocol 2714 Functional Specification 2715 2716 2717 2718 which fit the current window, and may repackage segments on the 2719 retransmission queue. Such repackaging is not required, but may be 2720 helpful. 2721 2722 In a connection with a one-way data flow, the window information will 2723 be carried in acknowledgment segments that all have the same sequence 2724 number so there will be no way to reorder them if they arrive out of 2725 order. This is not a serious problem, but it will allow the window 2726 information to be on occasion temporarily based on old reports from 2727 the data receiver. A refinement to avoid this problem is to act on 2728 the window information from segments that carry the highest 2729 acknowledgment number (that is segments with acknowledgment number 2730 equal or greater than the highest previously received). 2731 2732 The window management procedure has significant influence on the 2733 communication performance. The following comments are suggestions to 2734 implementers. 2735 2736 Window Management Suggestions 2737 2738 Allocating a very small window causes data to be transmitted in 2739 many small segments when better performance is achieved using 2740 fewer large segments. 2741 2742 One suggestion for avoiding small windows is for the receiver to 2743 defer updating a window until the additional allocation is at 2744 least X percent of the maximum allocation possible for the 2745 connection (where X might be 20 to 40). 2746 2747 Another suggestion is for the sender to avoid sending small 2748 segments by waiting until the window is large enough before 2749 sending data. If the the user signals a push function then the 2750 data must be sent even if it is a small segment. 2751 2752 Note that the acknowledgments should not be delayed or unnecessary 2753 retransmissions will result. One strategy would be to send an 2754 acknowledgment when a small segment arrives (with out updating the 2755 window information), and then to send another acknowledgment with 2756 new window information when the window is larger. 2757 2758 The segment sent to probe a zero window may also begin a break up 2759 of transmitted data into smaller and smaller segments. If a 2760 segment containing a single data octet sent to probe a zero window 2761 is accepted, it consumes one octet of the window now available. 2762 If the sending TCP simply sends as much as it can whenever the 2763 window is non zero, the transmitted data will be broken into 2764 alternating big and small segments. As time goes on, occasional 2765 pauses in the receiver making window allocation available will 2766 2767 2768 [Page 43] 2769 2770 2771 September 1981 2772Transmission Control Protocol 2773Functional Specification 2774 2775 2776 2777 result in breaking the big segments into a small and not quite so 2778 big pair. And after a while the data transmission will be in 2779 mostly small segments. 2780 2781 The suggestion here is that the TCP implementations need to 2782 actively attempt to combine small window allocations into larger 2783 windows, since the mechanisms for managing the window tend to lead 2784 to many small windows in the simplest minded implementations. 2785 27863.8. Interfaces 2787 2788 There are of course two interfaces of concern: the user/TCP interface 2789 and the TCP/lower-level interface. We have a fairly elaborate model 2790 of the user/TCP interface, but the interface to the lower level 2791 protocol module is left unspecified here, since it will be specified 2792 in detail by the specification of the lowel level protocol. For the 2793 case that the lower level is IP we note some of the parameter values 2794 that TCPs might use. 2795 2796 User/TCP Interface 2797 2798 The following functional description of user commands to the TCP is, 2799 at best, fictional, since every operating system will have different 2800 facilities. Consequently, we must warn readers that different TCP 2801 implementations may have different user interfaces. However, all 2802 TCPs must provide a certain minimum set of services to guarantee 2803 that all TCP implementations can support the same protocol 2804 hierarchy. This section specifies the functional interfaces 2805 required of all TCP implementations. 2806 2807 TCP User Commands 2808 2809 The following sections functionally characterize a USER/TCP 2810 interface. The notation used is similar to most procedure or 2811 function calls in high level languages, but this usage is not 2812 meant to rule out trap type service calls (e.g., SVCs, UUOs, 2813 EMTs). 2814 2815 The user commands described below specify the basic functions the 2816 TCP must perform to support interprocess communication. 2817 Individual implementations must define their own exact format, and 2818 may provide combinations or subsets of the basic functions in 2819 single calls. In particular, some implementations may wish to 2820 automatically OPEN a connection on the first SEND or RECEIVE 2821 issued by the user for a given connection. 2822 2823 2824 2825 2826 2827[Page 44] 2828 2829 2830September 1981 2831 Transmission Control Protocol 2832 Functional Specification 2833 2834 2835 2836 In providing interprocess communication facilities, the TCP must 2837 not only accept commands, but must also return information to the 2838 processes it serves. The latter consists of: 2839 2840 (a) general information about a connection (e.g., interrupts, 2841 remote close, binding of unspecified foreign socket). 2842 2843 (b) replies to specific user commands indicating success or 2844 various types of failure. 2845 2846 Open 2847 2848 Format: OPEN (local port, foreign socket, active/passive 2849 [, timeout] [, precedence] [, security/compartment] [, options]) 2850 -> local connection name 2851 2852 We assume that the local TCP is aware of the identity of the 2853 processes it serves and will check the authority of the process 2854 to use the connection specified. Depending upon the 2855 implementation of the TCP, the local network and TCP identifiers 2856 for the source address will either be supplied by the TCP or the 2857 lower level protocol (e.g., IP). These considerations are the 2858 result of concern about security, to the extent that no TCP be 2859 able to masquerade as another one, and so on. Similarly, no 2860 process can masquerade as another without the collusion of the 2861 TCP. 2862 2863 If the active/passive flag is set to passive, then this is a 2864 call to LISTEN for an incoming connection. A passive open may 2865 have either a fully specified foreign socket to wait for a 2866 particular connection or an unspecified foreign socket to wait 2867 for any call. A fully specified passive call can be made active 2868 by the subsequent execution of a SEND. 2869 2870 A transmission control block (TCB) is created and partially 2871 filled in with data from the OPEN command parameters. 2872 2873 On an active OPEN command, the TCP will begin the procedure to 2874 synchronize (i.e., establish) the connection at once. 2875 2876 The timeout, if present, permits the caller to set up a timeout 2877 for all data submitted to TCP. If data is not successfully 2878 delivered to the destination within the timeout period, the TCP 2879 will abort the connection. The present global default is five 2880 minutes. 2881 2882 The TCP or some component of the operating system will verify 2883 the users authority to open a connection with the specified 2884 2885 2886 [Page 45] 2887 2888 2889 September 1981 2890Transmission Control Protocol 2891Functional Specification 2892 2893 2894 2895 precedence or security/compartment. The absence of precedence 2896 or security/compartment specification in the OPEN call indicates 2897 the default values must be used. 2898 2899 TCP will accept incoming requests as matching only if the 2900 security/compartment information is exactly the same and only if 2901 the precedence is equal to or higher than the precedence 2902 requested in the OPEN call. 2903 2904 The precedence for the connection is the higher of the values 2905 requested in the OPEN call and received from the incoming 2906 request, and fixed at that value for the life of the 2907 connection.Implementers may want to give the user control of 2908 this precedence negotiation. For example, the user might be 2909 allowed to specify that the precedence must be exactly matched, 2910 or that any attempt to raise the precedence be confirmed by the 2911 user. 2912 2913 A local connection name will be returned to the user by the TCP. 2914 The local connection name can then be used as a short hand term 2915 for the connection defined by the <local socket, foreign socket> 2916 pair. 2917 2918 Send 2919 2920 Format: SEND (local connection name, buffer address, byte 2921 count, PUSH flag, URGENT flag [,timeout]) 2922 2923 This call causes the data contained in the indicated user buffer 2924 to be sent on the indicated connection. If the connection has 2925 not been opened, the SEND is considered an error. Some 2926 implementations may allow users to SEND first; in which case, an 2927 automatic OPEN would be done. If the calling process is not 2928 authorized to use this connection, an error is returned. 2929 2930 If the PUSH flag is set, the data must be transmitted promptly 2931 to the receiver, and the PUSH bit will be set in the last TCP 2932 segment created from the buffer. If the PUSH flag is not set, 2933 the data may be combined with data from subsequent SENDs for 2934 transmission efficiency. 2935 2936 If the URGENT flag is set, segments sent to the destination TCP 2937 will have the urgent pointer set. The receiving TCP will signal 2938 the urgent condition to the receiving process if the urgent 2939 pointer indicates that data preceding the urgent pointer has not 2940 been consumed by the receiving process. The purpose of urgent 2941 is to stimulate the receiver to process the urgent data and to 2942 indicate to the receiver when all the currently known urgent 2943 2944 2945[Page 46] 2946 2947 2948September 1981 2949 Transmission Control Protocol 2950 Functional Specification 2951 2952 2953 2954 data has been received. The number of times the sending user's 2955 TCP signals urgent will not necessarily be equal to the number 2956 of times the receiving user will be notified of the presence of 2957 urgent data. 2958 2959 If no foreign socket was specified in the OPEN, but the 2960 connection is established (e.g., because a LISTENing connection 2961 has become specific due to a foreign segment arriving for the 2962 local socket), then the designated buffer is sent to the implied 2963 foreign socket. Users who make use of OPEN with an unspecified 2964 foreign socket can make use of SEND without ever explicitly 2965 knowing the foreign socket address. 2966 2967 However, if a SEND is attempted before the foreign socket 2968 becomes specified, an error will be returned. Users can use the 2969 STATUS call to determine the status of the connection. In some 2970 implementations the TCP may notify the user when an unspecified 2971 socket is bound. 2972 2973 If a timeout is specified, the current user timeout for this 2974 connection is changed to the new one. 2975 2976 In the simplest implementation, SEND would not return control to 2977 the sending process until either the transmission was complete 2978 or the timeout had been exceeded. However, this simple method 2979 is both subject to deadlocks (for example, both sides of the 2980 connection might try to do SENDs before doing any RECEIVEs) and 2981 offers poor performance, so it is not recommended. A more 2982 sophisticated implementation would return immediately to allow 2983 the process to run concurrently with network I/O, and, 2984 furthermore, to allow multiple SENDs to be in progress. 2985 Multiple SENDs are served in first come, first served order, so 2986 the TCP will queue those it cannot service immediately. 2987 2988 We have implicitly assumed an asynchronous user interface in 2989 which a SEND later elicits some kind of SIGNAL or 2990 pseudo-interrupt from the serving TCP. An alternative is to 2991 return a response immediately. For instance, SENDs might return 2992 immediate local acknowledgment, even if the segment sent had not 2993 been acknowledged by the distant TCP. We could optimistically 2994 assume eventual success. If we are wrong, the connection will 2995 close anyway due to the timeout. In implementations of this 2996 kind (synchronous), there will still be some asynchronous 2997 signals, but these will deal with the connection itself, and not 2998 with specific segments or buffers. 2999 3000 In order for the process to distinguish among error or success 3001 indications for different SENDs, it might be appropriate for the 3002 3003 3004 [Page 47] 3005 3006 3007 September 1981 3008Transmission Control Protocol 3009Functional Specification 3010 3011 3012 3013 buffer address to be returned along with the coded response to 3014 the SEND request. TCP-to-user signals are discussed below, 3015 indicating the information which should be returned to the 3016 calling process. 3017 3018 Receive 3019 3020 Format: RECEIVE (local connection name, buffer address, byte 3021 count) -> byte count, urgent flag, push flag 3022 3023 This command allocates a receiving buffer associated with the 3024 specified connection. If no OPEN precedes this command or the 3025 calling process is not authorized to use this connection, an 3026 error is returned. 3027 3028 In the simplest implementation, control would not return to the 3029 calling program until either the buffer was filled, or some 3030 error occurred, but this scheme is highly subject to deadlocks. 3031 A more sophisticated implementation would permit several 3032 RECEIVEs to be outstanding at once. These would be filled as 3033 segments arrive. This strategy permits increased throughput at 3034 the cost of a more elaborate scheme (possibly asynchronous) to 3035 notify the calling program that a PUSH has been seen or a buffer 3036 filled. 3037 3038 If enough data arrive to fill the buffer before a PUSH is seen, 3039 the PUSH flag will not be set in the response to the RECEIVE. 3040 The buffer will be filled with as much data as it can hold. If 3041 a PUSH is seen before the buffer is filled the buffer will be 3042 returned partially filled and PUSH indicated. 3043 3044 If there is urgent data the user will have been informed as soon 3045 as it arrived via a TCP-to-user signal. The receiving user 3046 should thus be in "urgent mode". If the URGENT flag is on, 3047 additional urgent data remains. If the URGENT flag is off, this 3048 call to RECEIVE has returned all the urgent data, and the user 3049 may now leave "urgent mode". Note that data following the 3050 urgent pointer (non-urgent data) cannot be delivered to the user 3051 in the same buffer with preceeding urgent data unless the 3052 boundary is clearly marked for the user. 3053 3054 To distinguish among several outstanding RECEIVEs and to take 3055 care of the case that a buffer is not completely filled, the 3056 return code is accompanied by both a buffer pointer and a byte 3057 count indicating the actual length of the data received. 3058 3059 Alternative implementations of RECEIVE might have the TCP 3060 3061 3062 3063[Page 48] 3064 3065 3066September 1981 3067 Transmission Control Protocol 3068 Functional Specification 3069 3070 3071 3072 allocate buffer storage, or the TCP might share a ring buffer 3073 with the user. 3074 3075 Close 3076 3077 Format: CLOSE (local connection name) 3078 3079 This command causes the connection specified to be closed. If 3080 the connection is not open or the calling process is not 3081 authorized to use this connection, an error is returned. 3082 Closing connections is intended to be a graceful operation in 3083 the sense that outstanding SENDs will be transmitted (and 3084 retransmitted), as flow control permits, until all have been 3085 serviced. Thus, it should be acceptable to make several SEND 3086 calls, followed by a CLOSE, and expect all the data to be sent 3087 to the destination. It should also be clear that users should 3088 continue to RECEIVE on CLOSING connections, since the other side 3089 may be trying to transmit the last of its data. Thus, CLOSE 3090 means "I have no more to send" but does not mean "I will not 3091 receive any more." It may happen (if the user level protocol is 3092 not well thought out) that the closing side is unable to get rid 3093 of all its data before timing out. In this event, CLOSE turns 3094 into ABORT, and the closing TCP gives up. 3095 3096 The user may CLOSE the connection at any time on his own 3097 initiative, or in response to various prompts from the TCP 3098 (e.g., remote close executed, transmission timeout exceeded, 3099 destination inaccessible). 3100 3101 Because closing a connection requires communication with the 3102 foreign TCP, connections may remain in the closing state for a 3103 short time. Attempts to reopen the connection before the TCP 3104 replies to the CLOSE command will result in error responses. 3105 3106 Close also implies push function. 3107 3108 Status 3109 3110 Format: STATUS (local connection name) -> status data 3111 3112 This is an implementation dependent user command and could be 3113 excluded without adverse effect. Information returned would 3114 typically come from the TCB associated with the connection. 3115 3116 This command returns a data block containing the following 3117 information: 3118 3119 local socket, 3120 3121 3122 [Page 49] 3123 3124 3125 September 1981 3126Transmission Control Protocol 3127Functional Specification 3128 3129 3130 3131 foreign socket, 3132 local connection name, 3133 receive window, 3134 send window, 3135 connection state, 3136 number of buffers awaiting acknowledgment, 3137 number of buffers pending receipt, 3138 urgent state, 3139 precedence, 3140 security/compartment, 3141 and transmission timeout. 3142 3143 Depending on the state of the connection, or on the 3144 implementation itself, some of this information may not be 3145 available or meaningful. If the calling process is not 3146 authorized to use this connection, an error is returned. This 3147 prevents unauthorized processes from gaining information about a 3148 connection. 3149 3150 Abort 3151 3152 Format: ABORT (local connection name) 3153 3154 This command causes all pending SENDs and RECEIVES to be 3155 aborted, the TCB to be removed, and a special RESET message to 3156 be sent to the TCP on the other side of the connection. 3157 Depending on the implementation, users may receive abort 3158 indications for each outstanding SEND or RECEIVE, or may simply 3159 receive an ABORT-acknowledgment. 3160 3161 TCP-to-User Messages 3162 3163 It is assumed that the operating system environment provides a 3164 means for the TCP to asynchronously signal the user program. When 3165 the TCP does signal a user program, certain information is passed 3166 to the user. Often in the specification the information will be 3167 an error message. In other cases there will be information 3168 relating to the completion of processing a SEND or RECEIVE or 3169 other user call. 3170 3171 The following information is provided: 3172 3173 Local Connection Name Always 3174 Response String Always 3175 Buffer Address Send & Receive 3176 Byte count (counts bytes received) Receive 3177 Push flag Receive 3178 Urgent flag Receive 3179 3180 3181[Page 50] 3182 3183 3184September 1981 3185 Transmission Control Protocol 3186 Functional Specification 3187 3188 3189 3190 TCP/Lower-Level Interface 3191 3192 The TCP calls on a lower level protocol module to actually send and 3193 receive information over a network. One case is that of the ARPA 3194 internetwork system where the lower level module is the Internet 3195 Protocol (IP) [2]. 3196 3197 If the lower level protocol is IP it provides arguments for a type 3198 of service and for a time to live. TCP uses the following settings 3199 for these parameters: 3200 3201 Type of Service = Precedence: routine, Delay: normal, Throughput: 3202 normal, Reliability: normal; or 00000000. 3203 3204 Time to Live = one minute, or 00111100. 3205 3206 Note that the assumed maximum segment lifetime is two minutes. 3207 Here we explicitly ask that a segment be destroyed if it cannot 3208 be delivered by the internet system within one minute. 3209 3210 If the lower level is IP (or other protocol that provides this 3211 feature) and source routing is used, the interface must allow the 3212 route information to be communicated. This is especially important 3213 so that the source and destination addresses used in the TCP 3214 checksum be the originating source and ultimate destination. It is 3215 also important to preserve the return route to answer connection 3216 requests. 3217 3218 Any lower level protocol will have to provide the source address, 3219 destination address, and protocol fields, and some way to determine 3220 the "TCP length", both to provide the functional equivlent service 3221 of IP and to be used in the TCP checksum. 3222 3223 3224 3225 3226 3227 3228 3229 3230 3231 3232 3233 3234 3235 3236 3237 3238 3239 3240 [Page 51] 3241 3242 3243 September 1981 3244Transmission Control Protocol 3245Functional Specification 3246 3247 3248 32493.9. Event Processing 3250 3251 The processing depicted in this section is an example of one possible 3252 implementation. Other implementations may have slightly different 3253 processing sequences, but they should differ from those in this 3254 section only in detail, not in substance. 3255 3256 The activity of the TCP can be characterized as responding to events. 3257 The events that occur can be cast into three categories: user calls, 3258 arriving segments, and timeouts. This section describes the 3259 processing the TCP does in response to each of the events. In many 3260 cases the processing required depends on the state of the connection. 3261 3262 Events that occur: 3263 3264 User Calls 3265 3266 OPEN 3267 SEND 3268 RECEIVE 3269 CLOSE 3270 ABORT 3271 STATUS 3272 3273 Arriving Segments 3274 3275 SEGMENT ARRIVES 3276 3277 Timeouts 3278 3279 USER TIMEOUT 3280 RETRANSMISSION TIMEOUT 3281 TIME-WAIT TIMEOUT 3282 3283 The model of the TCP/user interface is that user commands receive an 3284 immediate return and possibly a delayed response via an event or 3285 pseudo interrupt. In the following descriptions, the term "signal" 3286 means cause a delayed response. 3287 3288 Error responses are given as character strings. For example, user 3289 commands referencing connections that do not exist receive "error: 3290 connection not open". 3291 3292 Please note in the following that all arithmetic on sequence numbers, 3293 acknowledgment numbers, windows, et cetera, is modulo 2**32 the size 3294 of the sequence number space. Also note that "=<" means less than or 3295 equal to (modulo 2**32). 3296 3297 3298 3299[Page 52] 3300 3301 3302September 1981 3303 Transmission Control Protocol 3304 Functional Specification 3305 3306 3307 3308 A natural way to think about processing incoming segments is to 3309 imagine that they are first tested for proper sequence number (i.e., 3310 that their contents lie in the range of the expected "receive window" 3311 in the sequence number space) and then that they are generally queued 3312 and processed in sequence number order. 3313 3314 When a segment overlaps other already received segments we reconstruct 3315 the segment to contain just the new data, and adjust the header fields 3316 to be consistent. 3317 3318 Note that if no state change is mentioned the TCP stays in the same 3319 state. 3320 3321 3322 3323 3324 3325 3326 3327 3328 3329 3330 3331 3332 3333 3334 3335 3336 3337 3338 3339 3340 3341 3342 3343 3344 3345 3346 3347 3348 3349 3350 3351 3352 3353 3354 3355 3356 3357 3358 [Page 53] 3359 3360 3361 September 1981 3362Transmission Control Protocol 3363Functional Specification 3364 OPEN Call 3365 3366 3367 3368 OPEN Call 3369 3370 CLOSED STATE (i.e., TCB does not exist) 3371 3372 Create a new transmission control block (TCB) to hold connection 3373 state information. Fill in local socket identifier, foreign 3374 socket, precedence, security/compartment, and user timeout 3375 information. Note that some parts of the foreign socket may be 3376 unspecified in a passive OPEN and are to be filled in by the 3377 parameters of the incoming SYN segment. Verify the security and 3378 precedence requested are allowed for this user, if not return 3379 "error: precedence not allowed" or "error: security/compartment 3380 not allowed." If passive enter the LISTEN state and return. If 3381 active and the foreign socket is unspecified, return "error: 3382 foreign socket unspecified"; if active and the foreign socket is 3383 specified, issue a SYN segment. An initial send sequence number 3384 (ISS) is selected. A SYN segment of the form <SEQ=ISS><CTL=SYN> 3385 is sent. Set SND.UNA to ISS, SND.NXT to ISS+1, enter SYN-SENT 3386 state, and return. 3387 3388 If the caller does not have access to the local socket specified, 3389 return "error: connection illegal for this process". If there is 3390 no room to create a new connection, return "error: insufficient 3391 resources". 3392 3393 LISTEN STATE 3394 3395 If active and the foreign socket is specified, then change the 3396 connection from passive to active, select an ISS. Send a SYN 3397 segment, set SND.UNA to ISS, SND.NXT to ISS+1. Enter SYN-SENT 3398 state. Data associated with SEND may be sent with SYN segment or 3399 queued for transmission after entering ESTABLISHED state. The 3400 urgent bit if requested in the command must be sent with the data 3401 segments sent as a result of this command. If there is no room to 3402 queue the request, respond with "error: insufficient resources". 3403 If Foreign socket was not specified, then return "error: foreign 3404 socket unspecified". 3405 3406 3407 3408 3409 3410 3411 3412 3413 3414 3415 3416 3417[Page 54] 3418 3419 3420September 1981 3421 Transmission Control Protocol 3422 Functional Specification 3423OPEN Call 3424 3425 3426 3427 SYN-SENT STATE 3428 SYN-RECEIVED STATE 3429 ESTABLISHED STATE 3430 FIN-WAIT-1 STATE 3431 FIN-WAIT-2 STATE 3432 CLOSE-WAIT STATE 3433 CLOSING STATE 3434 LAST-ACK STATE 3435 TIME-WAIT STATE 3436 3437 Return "error: connection already exists". 3438 3439 3440 3441 3442 3443 3444 3445 3446 3447 3448 3449 3450 3451 3452 3453 3454 3455 3456 3457 3458 3459 3460 3461 3462 3463 3464 3465 3466 3467 3468 3469 3470 3471 3472 3473 3474 3475 3476 [Page 55] 3477 3478 3479 September 1981 3480Transmission Control Protocol 3481Functional Specification 3482 SEND Call 3483 3484 3485 3486 SEND Call 3487 3488 CLOSED STATE (i.e., TCB does not exist) 3489 3490 If the user does not have access to such a connection, then return 3491 "error: connection illegal for this process". 3492 3493 Otherwise, return "error: connection does not exist". 3494 3495 LISTEN STATE 3496 3497 If the foreign socket is specified, then change the connection 3498 from passive to active, select an ISS. Send a SYN segment, set 3499 SND.UNA to ISS, SND.NXT to ISS+1. Enter SYN-SENT state. Data 3500 associated with SEND may be sent with SYN segment or queued for 3501 transmission after entering ESTABLISHED state. The urgent bit if 3502 requested in the command must be sent with the data segments sent 3503 as a result of this command. If there is no room to queue the 3504 request, respond with "error: insufficient resources". If 3505 Foreign socket was not specified, then return "error: foreign 3506 socket unspecified". 3507 3508 SYN-SENT STATE 3509 SYN-RECEIVED STATE 3510 3511 Queue the data for transmission after entering ESTABLISHED state. 3512 If no space to queue, respond with "error: insufficient 3513 resources". 3514 3515 ESTABLISHED STATE 3516 CLOSE-WAIT STATE 3517 3518 Segmentize the buffer and send it with a piggybacked 3519 acknowledgment (acknowledgment value = RCV.NXT). If there is 3520 insufficient space to remember this buffer, simply return "error: 3521 insufficient resources". 3522 3523 If the urgent flag is set, then SND.UP <- SND.NXT-1 and set the 3524 urgent pointer in the outgoing segments. 3525 3526 3527 3528 3529 3530 3531 3532 3533 3534 3535[Page 56] 3536 3537 3538September 1981 3539 Transmission Control Protocol 3540 Functional Specification 3541SEND Call 3542 3543 3544 3545 FIN-WAIT-1 STATE 3546 FIN-WAIT-2 STATE 3547 CLOSING STATE 3548 LAST-ACK STATE 3549 TIME-WAIT STATE 3550 3551 Return "error: connection closing" and do not service request. 3552 3553 3554 3555 3556 3557 3558 3559 3560 3561 3562 3563 3564 3565 3566 3567 3568 3569 3570 3571 3572 3573 3574 3575 3576 3577 3578 3579 3580 3581 3582 3583 3584 3585 3586 3587 3588 3589 3590 3591 3592 3593 3594 [Page 57] 3595 3596 3597 September 1981 3598Transmission Control Protocol 3599Functional Specification 3600 RECEIVE Call 3601 3602 3603 3604 RECEIVE Call 3605 3606 CLOSED STATE (i.e., TCB does not exist) 3607 3608 If the user does not have access to such a connection, return 3609 "error: connection illegal for this process". 3610 3611 Otherwise return "error: connection does not exist". 3612 3613 LISTEN STATE 3614 SYN-SENT STATE 3615 SYN-RECEIVED STATE 3616 3617 Queue for processing after entering ESTABLISHED state. If there 3618 is no room to queue this request, respond with "error: 3619 insufficient resources". 3620 3621 ESTABLISHED STATE 3622 FIN-WAIT-1 STATE 3623 FIN-WAIT-2 STATE 3624 3625 If insufficient incoming segments are queued to satisfy the 3626 request, queue the request. If there is no queue space to 3627 remember the RECEIVE, respond with "error: insufficient 3628 resources". 3629 3630 Reassemble queued incoming segments into receive buffer and return 3631 to user. Mark "push seen" (PUSH) if this is the case. 3632 3633 If RCV.UP is in advance of the data currently being passed to the 3634 user notify the user of the presence of urgent data. 3635 3636 When the TCP takes responsibility for delivering data to the user 3637 that fact must be communicated to the sender via an 3638 acknowledgment. The formation of such an acknowledgment is 3639 described below in the discussion of processing an incoming 3640 segment. 3641 3642 3643 3644 3645 3646 3647 3648 3649 3650 3651 3652 3653[Page 58] 3654 3655 3656September 1981 3657 Transmission Control Protocol 3658 Functional Specification 3659RECEIVE Call 3660 3661 3662 3663 CLOSE-WAIT STATE 3664 3665 Since the remote side has already sent FIN, RECEIVEs must be 3666 satisfied by text already on hand, but not yet delivered to the 3667 user. If no text is awaiting delivery, the RECEIVE will get a 3668 "error: connection closing" response. Otherwise, any remaining 3669 text can be used to satisfy the RECEIVE. 3670 3671 CLOSING STATE 3672 LAST-ACK STATE 3673 TIME-WAIT STATE 3674 3675 Return "error: connection closing". 3676 3677 3678 3679 3680 3681 3682 3683 3684 3685 3686 3687 3688 3689 3690 3691 3692 3693 3694 3695 3696 3697 3698 3699 3700 3701 3702 3703 3704 3705 3706 3707 3708 3709 3710 3711 3712 [Page 59] 3713 3714 3715 September 1981 3716Transmission Control Protocol 3717Functional Specification 3718 CLOSE Call 3719 3720 3721 3722 CLOSE Call 3723 3724 CLOSED STATE (i.e., TCB does not exist) 3725 3726 If the user does not have access to such a connection, return 3727 "error: connection illegal for this process". 3728 3729 Otherwise, return "error: connection does not exist". 3730 3731 LISTEN STATE 3732 3733 Any outstanding RECEIVEs are returned with "error: closing" 3734 responses. Delete TCB, enter CLOSED state, and return. 3735 3736 SYN-SENT STATE 3737 3738 Delete the TCB and return "error: closing" responses to any 3739 queued SENDs, or RECEIVEs. 3740 3741 SYN-RECEIVED STATE 3742 3743 If no SENDs have been issued and there is no pending data to send, 3744 then form a FIN segment and send it, and enter FIN-WAIT-1 state; 3745 otherwise queue for processing after entering ESTABLISHED state. 3746 3747 ESTABLISHED STATE 3748 3749 Queue this until all preceding SENDs have been segmentized, then 3750 form a FIN segment and send it. In any case, enter FIN-WAIT-1 3751 state. 3752 3753 FIN-WAIT-1 STATE 3754 FIN-WAIT-2 STATE 3755 3756 Strictly speaking, this is an error and should receive a "error: 3757 connection closing" response. An "ok" response would be 3758 acceptable, too, as long as a second FIN is not emitted (the first 3759 FIN may be retransmitted though). 3760 3761 3762 3763 3764 3765 3766 3767 3768 3769 3770 3771[Page 60] 3772 3773 3774September 1981 3775 Transmission Control Protocol 3776 Functional Specification 3777CLOSE Call 3778 3779 3780 3781 CLOSE-WAIT STATE 3782 3783 Queue this request until all preceding SENDs have been 3784 segmentized; then send a FIN segment, enter CLOSING state. 3785 3786 CLOSING STATE 3787 LAST-ACK STATE 3788 TIME-WAIT STATE 3789 3790 Respond with "error: connection closing". 3791 3792 3793 3794 3795 3796 3797 3798 3799 3800 3801 3802 3803 3804 3805 3806 3807 3808 3809 3810 3811 3812 3813 3814 3815 3816 3817 3818 3819 3820 3821 3822 3823 3824 3825 3826 3827 3828 3829 3830 [Page 61] 3831 3832 3833 September 1981 3834Transmission Control Protocol 3835Functional Specification 3836 ABORT Call 3837 3838 3839 3840 ABORT Call 3841 3842 CLOSED STATE (i.e., TCB does not exist) 3843 3844 If the user should not have access to such a connection, return 3845 "error: connection illegal for this process". 3846 3847 Otherwise return "error: connection does not exist". 3848 3849 LISTEN STATE 3850 3851 Any outstanding RECEIVEs should be returned with "error: 3852 connection reset" responses. Delete TCB, enter CLOSED state, and 3853 return. 3854 3855 SYN-SENT STATE 3856 3857 All queued SENDs and RECEIVEs should be given "connection reset" 3858 notification, delete the TCB, enter CLOSED state, and return. 3859 3860 SYN-RECEIVED STATE 3861 ESTABLISHED STATE 3862 FIN-WAIT-1 STATE 3863 FIN-WAIT-2 STATE 3864 CLOSE-WAIT STATE 3865 3866 Send a reset segment: 3867 3868 <SEQ=SND.NXT><CTL=RST> 3869 3870 All queued SENDs and RECEIVEs should be given "connection reset" 3871 notification; all segments queued for transmission (except for the 3872 RST formed above) or retransmission should be flushed, delete the 3873 TCB, enter CLOSED state, and return. 3874 3875 CLOSING STATE 3876 LAST-ACK STATE 3877 TIME-WAIT STATE 3878 3879 Respond with "ok" and delete the TCB, enter CLOSED state, and 3880 return. 3881 3882 3883 3884 3885 3886 3887 3888 3889[Page 62] 3890 3891 3892September 1981 3893 Transmission Control Protocol 3894 Functional Specification 3895STATUS Call 3896 3897 3898 3899 STATUS Call 3900 3901 CLOSED STATE (i.e., TCB does not exist) 3902 3903 If the user should not have access to such a connection, return 3904 "error: connection illegal for this process". 3905 3906 Otherwise return "error: connection does not exist". 3907 3908 LISTEN STATE 3909 3910 Return "state = LISTEN", and the TCB pointer. 3911 3912 SYN-SENT STATE 3913 3914 Return "state = SYN-SENT", and the TCB pointer. 3915 3916 SYN-RECEIVED STATE 3917 3918 Return "state = SYN-RECEIVED", and the TCB pointer. 3919 3920 ESTABLISHED STATE 3921 3922 Return "state = ESTABLISHED", and the TCB pointer. 3923 3924 FIN-WAIT-1 STATE 3925 3926 Return "state = FIN-WAIT-1", and the TCB pointer. 3927 3928 FIN-WAIT-2 STATE 3929 3930 Return "state = FIN-WAIT-2", and the TCB pointer. 3931 3932 CLOSE-WAIT STATE 3933 3934 Return "state = CLOSE-WAIT", and the TCB pointer. 3935 3936 CLOSING STATE 3937 3938 Return "state = CLOSING", and the TCB pointer. 3939 3940 LAST-ACK STATE 3941 3942 Return "state = LAST-ACK", and the TCB pointer. 3943 3944 3945 3946 3947 3948 [Page 63] 3949 3950 3951 September 1981 3952Transmission Control Protocol 3953Functional Specification 3954 STATUS Call 3955 3956 3957 3958 TIME-WAIT STATE 3959 3960 Return "state = TIME-WAIT", and the TCB pointer. 3961 3962 3963 3964 3965 3966 3967 3968 3969 3970 3971 3972 3973 3974 3975 3976 3977 3978 3979 3980 3981 3982 3983 3984 3985 3986 3987 3988 3989 3990 3991 3992 3993 3994 3995 3996 3997 3998 3999 4000 4001 4002 4003 4004 4005 4006 4007[Page 64] 4008 4009 4010September 1981 4011 Transmission Control Protocol 4012 Functional Specification 4013SEGMENT ARRIVES 4014 4015 4016 4017 SEGMENT ARRIVES 4018 4019 If the state is CLOSED (i.e., TCB does not exist) then 4020 4021 all data in the incoming segment is discarded. An incoming 4022 segment containing a RST is discarded. An incoming segment not 4023 containing a RST causes a RST to be sent in response. The 4024 acknowledgment and sequence field values are selected to make the 4025 reset sequence acceptable to the TCP that sent the offending 4026 segment. 4027 4028 If the ACK bit is off, sequence number zero is used, 4029 4030 <SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK> 4031 4032 If the ACK bit is on, 4033 4034 <SEQ=SEG.ACK><CTL=RST> 4035 4036 Return. 4037 4038 If the state is LISTEN then 4039 4040 first check for an RST 4041 4042 An incoming RST should be ignored. Return. 4043 4044 second check for an ACK 4045 4046 Any acknowledgment is bad if it arrives on a connection still in 4047 the LISTEN state. An acceptable reset segment should be formed 4048 for any arriving ACK-bearing segment. The RST should be 4049 formatted as follows: 4050 4051 <SEQ=SEG.ACK><CTL=RST> 4052 4053 Return. 4054 4055 third check for a SYN 4056 4057 If the SYN bit is set, check the security. If the 4058 security/compartment on the incoming segment does not exactly 4059 match the security/compartment in the TCB then send a reset and 4060 return. 4061 4062 <SEQ=SEG.ACK><CTL=RST> 4063 4064 4065 4066 [Page 65] 4067 4068 4069 September 1981 4070Transmission Control Protocol 4071Functional Specification 4072 SEGMENT ARRIVES 4073 4074 4075 4076 If the SEG.PRC is greater than the TCB.PRC then if allowed by 4077 the user and the system set TCB.PRC<-SEG.PRC, if not allowed 4078 send a reset and return. 4079 4080 <SEQ=SEG.ACK><CTL=RST> 4081 4082 If the SEG.PRC is less than the TCB.PRC then continue. 4083 4084 Set RCV.NXT to SEG.SEQ+1, IRS is set to SEG.SEQ and any other 4085 control or text should be queued for processing later. ISS 4086 should be selected and a SYN segment sent of the form: 4087 4088 <SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK> 4089 4090 SND.NXT is set to ISS+1 and SND.UNA to ISS. The connection 4091 state should be changed to SYN-RECEIVED. Note that any other 4092 incoming control or data (combined with SYN) will be processed 4093 in the SYN-RECEIVED state, but processing of SYN and ACK should 4094 not be repeated. If the listen was not fully specified (i.e., 4095 the foreign socket was not fully specified), then the 4096 unspecified fields should be filled in now. 4097 4098 fourth other text or control 4099 4100 Any other control or text-bearing segment (not containing SYN) 4101 must have an ACK and thus would be discarded by the ACK 4102 processing. An incoming RST segment could not be valid, since 4103 it could not have been sent in response to anything sent by this 4104 incarnation of the connection. So you are unlikely to get here, 4105 but if you do, drop the segment, and return. 4106 4107 If the state is SYN-SENT then 4108 4109 first check the ACK bit 4110 4111 If the ACK bit is set 4112 4113 If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, send a reset (unless 4114 the RST bit is set, if so drop the segment and return) 4115 4116 <SEQ=SEG.ACK><CTL=RST> 4117 4118 and discard the segment. Return. 4119 4120 If SND.UNA =< SEG.ACK =< SND.NXT then the ACK is acceptable. 4121 4122 second check the RST bit 4123 4124 4125[Page 66] 4126 4127 4128September 1981 4129 Transmission Control Protocol 4130 Functional Specification 4131SEGMENT ARRIVES 4132 4133 4134 4135 If the RST bit is set 4136 4137 If the ACK was acceptable then signal the user "error: 4138 connection reset", drop the segment, enter CLOSED state, 4139 delete TCB, and return. Otherwise (no ACK) drop the segment 4140 and return. 4141 4142 third check the security and precedence 4143 4144 If the security/compartment in the segment does not exactly 4145 match the security/compartment in the TCB, send a reset 4146 4147 If there is an ACK 4148 4149 <SEQ=SEG.ACK><CTL=RST> 4150 4151 Otherwise 4152 4153 <SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK> 4154 4155 If there is an ACK 4156 4157 The precedence in the segment must match the precedence in the 4158 TCB, if not, send a reset 4159 4160 <SEQ=SEG.ACK><CTL=RST> 4161 4162 If there is no ACK 4163 4164 If the precedence in the segment is higher than the precedence 4165 in the TCB then if allowed by the user and the system raise 4166 the precedence in the TCB to that in the segment, if not 4167 allowed to raise the prec then send a reset. 4168 4169 <SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK> 4170 4171 If the precedence in the segment is lower than the precedence 4172 in the TCB continue. 4173 4174 If a reset was sent, discard the segment and return. 4175 4176 fourth check the SYN bit 4177 4178 This step should be reached only if the ACK is ok, or there is 4179 no ACK, and it the segment did not contain a RST. 4180 4181 If the SYN bit is on and the security/compartment and precedence 4182 4183 4184 [Page 67] 4185 4186 4187 September 1981 4188Transmission Control Protocol 4189Functional Specification 4190 SEGMENT ARRIVES 4191 4192 4193 4194 are acceptable then, RCV.NXT is set to SEG.SEQ+1, IRS is set to 4195 SEG.SEQ. SND.UNA should be advanced to equal SEG.ACK (if there 4196 is an ACK), and any segments on the retransmission queue which 4197 are thereby acknowledged should be removed. 4198 4199 If SND.UNA > ISS (our SYN has been ACKed), change the connection 4200 state to ESTABLISHED, form an ACK segment 4201 4202 <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK> 4203 4204 and send it. Data or controls which were queued for 4205 transmission may be included. If there are other controls or 4206 text in the segment then continue processing at the sixth step 4207 below where the URG bit is checked, otherwise return. 4208 4209 Otherwise enter SYN-RECEIVED, form a SYN,ACK segment 4210 4211 <SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK> 4212 4213 and send it. If there are other controls or text in the 4214 segment, queue them for processing after the ESTABLISHED state 4215 has been reached, return. 4216 4217 fifth, if neither of the SYN or RST bits is set then drop the 4218 segment and return. 4219 4220 4221 4222 4223 4224 4225 4226 4227 4228 4229 4230 4231 4232 4233 4234 4235 4236 4237 4238 4239 4240 4241 4242 4243[Page 68] 4244 4245 4246September 1981 4247 Transmission Control Protocol 4248 Functional Specification 4249SEGMENT ARRIVES 4250 4251 4252 4253 Otherwise, 4254 4255 first check sequence number 4256 4257 SYN-RECEIVED STATE 4258 ESTABLISHED STATE 4259 FIN-WAIT-1 STATE 4260 FIN-WAIT-2 STATE 4261 CLOSE-WAIT STATE 4262 CLOSING STATE 4263 LAST-ACK STATE 4264 TIME-WAIT STATE 4265 4266 Segments are processed in sequence. Initial tests on arrival 4267 are used to discard old duplicates, but further processing is 4268 done in SEG.SEQ order. If a segment's contents straddle the 4269 boundary between old and new, only the new parts should be 4270 processed. 4271 4272 There are four cases for the acceptability test for an incoming 4273 segment: 4274 4275 Segment Receive Test 4276 Length Window 4277 ------- ------- ------------------------------------------- 4278 4279 0 0 SEG.SEQ = RCV.NXT 4280 4281 0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND 4282 4283 >0 0 not acceptable 4284 4285 >0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND 4286 or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND 4287 4288 If the RCV.WND is zero, no segments will be acceptable, but 4289 special allowance should be made to accept valid ACKs, URGs and 4290 RSTs. 4291 4292 If an incoming segment is not acceptable, an acknowledgment 4293 should be sent in reply (unless the RST bit is set, if so drop 4294 the segment and return): 4295 4296 <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK> 4297 4298 After sending the acknowledgment, drop the unacceptable segment 4299 and return. 4300 4301 4302 [Page 69] 4303 4304 4305 September 1981 4306Transmission Control Protocol 4307Functional Specification 4308 SEGMENT ARRIVES 4309 4310 4311 4312 In the following it is assumed that the segment is the idealized 4313 segment that begins at RCV.NXT and does not exceed the window. 4314 One could tailor actual segments to fit this assumption by 4315 trimming off any portions that lie outside the window (including 4316 SYN and FIN), and only processing further if the segment then 4317 begins at RCV.NXT. Segments with higher begining sequence 4318 numbers may be held for later processing. 4319 4320 second check the RST bit, 4321 4322 SYN-RECEIVED STATE 4323 4324 If the RST bit is set 4325 4326 If this connection was initiated with a passive OPEN (i.e., 4327 came from the LISTEN state), then return this connection to 4328 LISTEN state and return. The user need not be informed. If 4329 this connection was initiated with an active OPEN (i.e., came 4330 from SYN-SENT state) then the connection was refused, signal 4331 the user "connection refused". In either case, all segments 4332 on the retransmission queue should be removed. And in the 4333 active OPEN case, enter the CLOSED state and delete the TCB, 4334 and return. 4335 4336 ESTABLISHED 4337 FIN-WAIT-1 4338 FIN-WAIT-2 4339 CLOSE-WAIT 4340 4341 If the RST bit is set then, any outstanding RECEIVEs and SEND 4342 should receive "reset" responses. All segment queues should be 4343 flushed. Users should also receive an unsolicited general 4344 "connection reset" signal. Enter the CLOSED state, delete the 4345 TCB, and return. 4346 4347 CLOSING STATE 4348 LAST-ACK STATE 4349 TIME-WAIT 4350 4351 If the RST bit is set then, enter the CLOSED state, delete the 4352 TCB, and return. 4353 4354 4355 4356 4357 4358 4359 4360 4361[Page 70] 4362 4363 4364September 1981 4365 Transmission Control Protocol 4366 Functional Specification 4367SEGMENT ARRIVES 4368 4369 4370 4371 third check security and precedence 4372 4373 SYN-RECEIVED 4374 4375 If the security/compartment and precedence in the segment do not 4376 exactly match the security/compartment and precedence in the TCB 4377 then send a reset, and return. 4378 4379 ESTABLISHED STATE 4380 4381 If the security/compartment and precedence in the segment do not 4382 exactly match the security/compartment and precedence in the TCB 4383 then send a reset, any outstanding RECEIVEs and SEND should 4384 receive "reset" responses. All segment queues should be 4385 flushed. Users should also receive an unsolicited general 4386 "connection reset" signal. Enter the CLOSED state, delete the 4387 TCB, and return. 4388 4389 Note this check is placed following the sequence check to prevent 4390 a segment from an old connection between these ports with a 4391 different security or precedence from causing an abort of the 4392 current connection. 4393 4394 fourth, check the SYN bit, 4395 4396 SYN-RECEIVED 4397 ESTABLISHED STATE 4398 FIN-WAIT STATE-1 4399 FIN-WAIT STATE-2 4400 CLOSE-WAIT STATE 4401 CLOSING STATE 4402 LAST-ACK STATE 4403 TIME-WAIT STATE 4404 4405 If the SYN is in the window it is an error, send a reset, any 4406 outstanding RECEIVEs and SEND should receive "reset" responses, 4407 all segment queues should be flushed, the user should also 4408 receive an unsolicited general "connection reset" signal, enter 4409 the CLOSED state, delete the TCB, and return. 4410 4411 If the SYN is not in the window this step would not be reached 4412 and an ack would have been sent in the first step (sequence 4413 number check). 4414 4415 4416 4417 4418 4419 4420 [Page 71] 4421 4422 4423 September 1981 4424Transmission Control Protocol 4425Functional Specification 4426 SEGMENT ARRIVES 4427 4428 4429 4430 fifth check the ACK field, 4431 4432 if the ACK bit is off drop the segment and return 4433 4434 if the ACK bit is on 4435 4436 SYN-RECEIVED STATE 4437 4438 If SND.UNA =< SEG.ACK =< SND.NXT then enter ESTABLISHED state 4439 and continue processing. 4440 4441 If the segment acknowledgment is not acceptable, form a 4442 reset segment, 4443 4444 <SEQ=SEG.ACK><CTL=RST> 4445 4446 and send it. 4447 4448 ESTABLISHED STATE 4449 4450 If SND.UNA < SEG.ACK =< SND.NXT then, set SND.UNA <- SEG.ACK. 4451 Any segments on the retransmission queue which are thereby 4452 entirely acknowledged are removed. Users should receive 4453 positive acknowledgments for buffers which have been SENT and 4454 fully acknowledged (i.e., SEND buffer should be returned with 4455 "ok" response). If the ACK is a duplicate 4456 (SEG.ACK < SND.UNA), it can be ignored. If the ACK acks 4457 something not yet sent (SEG.ACK > SND.NXT) then send an ACK, 4458 drop the segment, and return. 4459 4460 If SND.UNA < SEG.ACK =< SND.NXT, the send window should be 4461 updated. If (SND.WL1 < SEG.SEQ or (SND.WL1 = SEG.SEQ and 4462 SND.WL2 =< SEG.ACK)), set SND.WND <- SEG.WND, set 4463 SND.WL1 <- SEG.SEQ, and set SND.WL2 <- SEG.ACK. 4464 4465 Note that SND.WND is an offset from SND.UNA, that SND.WL1 4466 records the sequence number of the last segment used to update 4467 SND.WND, and that SND.WL2 records the acknowledgment number of 4468 the last segment used to update SND.WND. The check here 4469 prevents using old segments to update the window. 4470 4471 4472 4473 4474 4475 4476 4477 4478 4479[Page 72] 4480 4481 4482September 1981 4483 Transmission Control Protocol 4484 Functional Specification 4485SEGMENT ARRIVES 4486 4487 4488 4489 FIN-WAIT-1 STATE 4490 4491 In addition to the processing for the ESTABLISHED state, if 4492 our FIN is now acknowledged then enter FIN-WAIT-2 and continue 4493 processing in that state. 4494 4495 FIN-WAIT-2 STATE 4496 4497 In addition to the processing for the ESTABLISHED state, if 4498 the retransmission queue is empty, the user's CLOSE can be 4499 acknowledged ("ok") but do not delete the TCB. 4500 4501 CLOSE-WAIT STATE 4502 4503 Do the same processing as for the ESTABLISHED state. 4504 4505 CLOSING STATE 4506 4507 In addition to the processing for the ESTABLISHED state, if 4508 the ACK acknowledges our FIN then enter the TIME-WAIT state, 4509 otherwise ignore the segment. 4510 4511 LAST-ACK STATE 4512 4513 The only thing that can arrive in this state is an 4514 acknowledgment of our FIN. If our FIN is now acknowledged, 4515 delete the TCB, enter the CLOSED state, and return. 4516 4517 TIME-WAIT STATE 4518 4519 The only thing that can arrive in this state is a 4520 retransmission of the remote FIN. Acknowledge it, and restart 4521 the 2 MSL timeout. 4522 4523 sixth, check the URG bit, 4524 4525 ESTABLISHED STATE 4526 FIN-WAIT-1 STATE 4527 FIN-WAIT-2 STATE 4528 4529 If the URG bit is set, RCV.UP <- max(RCV.UP,SEG.UP), and signal 4530 the user that the remote side has urgent data if the urgent 4531 pointer (RCV.UP) is in advance of the data consumed. If the 4532 user has already been signaled (or is still in the "urgent 4533 mode") for this continuous sequence of urgent data, do not 4534 signal the user again. 4535 4536 4537 4538 [Page 73] 4539 4540 4541 September 1981 4542Transmission Control Protocol 4543Functional Specification 4544 SEGMENT ARRIVES 4545 4546 4547 4548 CLOSE-WAIT STATE 4549 CLOSING STATE 4550 LAST-ACK STATE 4551 TIME-WAIT 4552 4553 This should not occur, since a FIN has been received from the 4554 remote side. Ignore the URG. 4555 4556 seventh, process the segment text, 4557 4558 ESTABLISHED STATE 4559 FIN-WAIT-1 STATE 4560 FIN-WAIT-2 STATE 4561 4562 Once in the ESTABLISHED state, it is possible to deliver segment 4563 text to user RECEIVE buffers. Text from segments can be moved 4564 into buffers until either the buffer is full or the segment is 4565 empty. If the segment empties and carries an PUSH flag, then 4566 the user is informed, when the buffer is returned, that a PUSH 4567 has been received. 4568 4569 When the TCP takes responsibility for delivering the data to the 4570 user it must also acknowledge the receipt of the data. 4571 4572 Once the TCP takes responsibility for the data it advances 4573 RCV.NXT over the data accepted, and adjusts RCV.WND as 4574 apporopriate to the current buffer availability. The total of 4575 RCV.NXT and RCV.WND should not be reduced. 4576 4577 Please note the window management suggestions in section 3.7. 4578 4579 Send an acknowledgment of the form: 4580 4581 <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK> 4582 4583 This acknowledgment should be piggybacked on a segment being 4584 transmitted if possible without incurring undue delay. 4585 4586 4587 4588 4589 4590 4591 4592 4593 4594 4595 4596 4597[Page 74] 4598 4599 4600September 1981 4601 Transmission Control Protocol 4602 Functional Specification 4603SEGMENT ARRIVES 4604 4605 4606 4607 CLOSE-WAIT STATE 4608 CLOSING STATE 4609 LAST-ACK STATE 4610 TIME-WAIT STATE 4611 4612 This should not occur, since a FIN has been received from the 4613 remote side. Ignore the segment text. 4614 4615 eighth, check the FIN bit, 4616 4617 Do not process the FIN if the state is CLOSED, LISTEN or SYN-SENT 4618 since the SEG.SEQ cannot be validated; drop the segment and 4619 return. 4620 4621 If the FIN bit is set, signal the user "connection closing" and 4622 return any pending RECEIVEs with same message, advance RCV.NXT 4623 over the FIN, and send an acknowledgment for the FIN. Note that 4624 FIN implies PUSH for any segment text not yet delivered to the 4625 user. 4626 4627 SYN-RECEIVED STATE 4628 ESTABLISHED STATE 4629 4630 Enter the CLOSE-WAIT state. 4631 4632 FIN-WAIT-1 STATE 4633 4634 If our FIN has been ACKed (perhaps in this segment), then 4635 enter TIME-WAIT, start the time-wait timer, turn off the other 4636 timers; otherwise enter the CLOSING state. 4637 4638 FIN-WAIT-2 STATE 4639 4640 Enter the TIME-WAIT state. Start the time-wait timer, turn 4641 off the other timers. 4642 4643 CLOSE-WAIT STATE 4644 4645 Remain in the CLOSE-WAIT state. 4646 4647 CLOSING STATE 4648 4649 Remain in the CLOSING state. 4650 4651 LAST-ACK STATE 4652 4653 Remain in the LAST-ACK state. 4654 4655 4656 [Page 75] 4657 4658 4659 September 1981 4660Transmission Control Protocol 4661Functional Specification 4662 SEGMENT ARRIVES 4663 4664 4665 4666 TIME-WAIT STATE 4667 4668 Remain in the TIME-WAIT state. Restart the 2 MSL time-wait 4669 timeout. 4670 4671 and return. 4672 4673 4674 4675 4676 4677 4678 4679 4680 4681 4682 4683 4684 4685 4686 4687 4688 4689 4690 4691 4692 4693 4694 4695 4696 4697 4698 4699 4700 4701 4702 4703 4704 4705 4706 4707 4708 4709 4710 4711 4712 4713 4714 4715[Page 76] 4716 4717 4718September 1981 4719 Transmission Control Protocol 4720 Functional Specification 4721USER TIMEOUT 4722 4723 4724 4725 USER TIMEOUT 4726 4727 For any state if the user timeout expires, flush all queues, signal 4728 the user "error: connection aborted due to user timeout" in general 4729 and for any outstanding calls, delete the TCB, enter the CLOSED 4730 state and return. 4731 4732 RETRANSMISSION TIMEOUT 4733 4734 For any state if the retransmission timeout expires on a segment in 4735 the retransmission queue, send the segment at the front of the 4736 retransmission queue again, reinitialize the retransmission timer, 4737 and return. 4738 4739 TIME-WAIT TIMEOUT 4740 4741 If the time-wait timeout expires on a connection delete the TCB, 4742 enter the CLOSED state and return. 4743 4744 4745 4746 4747 4748 4749 4750 4751 4752 4753 4754 4755 4756 4757 4758 4759 4760 4761 4762 4763 4764 4765 4766 4767 4768 4769 4770 4771 4772 4773 4774 [Page 77] 4775 4776 4777 September 1981 4778Transmission Control Protocol 4779 4780 4781 4782 4783 4784 4785 4786 4787 4788 4789 4790 4791 4792 4793 4794 4795 4796 4797 4798 4799 4800 4801 4802 4803 4804 4805 4806 4807 4808 4809 4810 4811 4812 4813 4814 4815 4816 4817 4818 4819 4820 4821 4822 4823 4824 4825 4826 4827 4828 4829 4830 4831 4832 4833[Page 78] 4834 4835 4836September 1981 4837 Transmission Control Protocol 4838 4839 4840 4841 GLOSSARY 4842 4843 4844 48451822 4846 BBN Report 1822, "The Specification of the Interconnection of 4847 a Host and an IMP". The specification of interface between a 4848 host and the ARPANET. 4849 4850ACK 4851 A control bit (acknowledge) occupying no sequence space, which 4852 indicates that the acknowledgment field of this segment 4853 specifies the next sequence number the sender of this segment 4854 is expecting to receive, hence acknowledging receipt of all 4855 previous sequence numbers. 4856 4857ARPANET message 4858 The unit of transmission between a host and an IMP in the 4859 ARPANET. The maximum size is about 1012 octets (8096 bits). 4860 4861ARPANET packet 4862 A unit of transmission used internally in the ARPANET between 4863 IMPs. The maximum size is about 126 octets (1008 bits). 4864 4865connection 4866 A logical communication path identified by a pair of sockets. 4867 4868datagram 4869 A message sent in a packet switched computer communications 4870 network. 4871 4872Destination Address 4873 The destination address, usually the network and host 4874 identifiers. 4875 4876FIN 4877 A control bit (finis) occupying one sequence number, which 4878 indicates that the sender will send no more data or control 4879 occupying sequence space. 4880 4881fragment 4882 A portion of a logical unit of data, in particular an internet 4883 fragment is a portion of an internet datagram. 4884 4885FTP 4886 A file transfer protocol. 4887 4888 4889 4890 4891 4892 [Page 79] 4893 4894 4895 September 1981 4896Transmission Control Protocol 4897Glossary 4898 4899 4900 4901header 4902 Control information at the beginning of a message, segment, 4903 fragment, packet or block of data. 4904 4905host 4906 A computer. In particular a source or destination of messages 4907 from the point of view of the communication network. 4908 4909Identification 4910 An Internet Protocol field. This identifying value assigned 4911 by the sender aids in assembling the fragments of a datagram. 4912 4913IMP 4914 The Interface Message Processor, the packet switch of the 4915 ARPANET. 4916 4917internet address 4918 A source or destination address specific to the host level. 4919 4920internet datagram 4921 The unit of data exchanged between an internet module and the 4922 higher level protocol together with the internet header. 4923 4924internet fragment 4925 A portion of the data of an internet datagram with an internet 4926 header. 4927 4928IP 4929 Internet Protocol. 4930 4931IRS 4932 The Initial Receive Sequence number. The first sequence 4933 number used by the sender on a connection. 4934 4935ISN 4936 The Initial Sequence Number. The first sequence number used 4937 on a connection, (either ISS or IRS). Selected on a clock 4938 based procedure. 4939 4940ISS 4941 The Initial Send Sequence number. The first sequence number 4942 used by the sender on a connection. 4943 4944leader 4945 Control information at the beginning of a message or block of 4946 data. In particular, in the ARPANET, the control information 4947 on an ARPANET message at the host-IMP interface. 4948 4949 4950 4951[Page 80] 4952 4953 4954September 1981 4955 Transmission Control Protocol 4956 Glossary 4957 4958 4959 4960left sequence 4961 This is the next sequence number to be acknowledged by the 4962 data receiving TCP (or the lowest currently unacknowledged 4963 sequence number) and is sometimes referred to as the left edge 4964 of the send window. 4965 4966local packet 4967 The unit of transmission within a local network. 4968 4969module 4970 An implementation, usually in software, of a protocol or other 4971 procedure. 4972 4973MSL 4974 Maximum Segment Lifetime, the time a TCP segment can exist in 4975 the internetwork system. Arbitrarily defined to be 2 minutes. 4976 4977octet 4978 An eight bit byte. 4979 4980Options 4981 An Option field may contain several options, and each option 4982 may be several octets in length. The options are used 4983 primarily in testing situations; for example, to carry 4984 timestamps. Both the Internet Protocol and TCP provide for 4985 options fields. 4986 4987packet 4988 A package of data with a header which may or may not be 4989 logically complete. More often a physical packaging than a 4990 logical packaging of data. 4991 4992port 4993 The portion of a socket that specifies which logical input or 4994 output channel of a process is associated with the data. 4995 4996process 4997 A program in execution. A source or destination of data from 4998 the point of view of the TCP or other host-to-host protocol. 4999 5000PUSH 5001 A control bit occupying no sequence space, indicating that 5002 this segment contains data that must be pushed through to the 5003 receiving user. 5004 5005RCV.NXT 5006 receive next sequence number 5007 5008 5009 5010 [Page 81] 5011 5012 5013 September 1981 5014Transmission Control Protocol 5015Glossary 5016 5017 5018 5019RCV.UP 5020 receive urgent pointer 5021 5022RCV.WND 5023 receive window 5024 5025receive next sequence number 5026 This is the next sequence number the local TCP is expecting to 5027 receive. 5028 5029receive window 5030 This represents the sequence numbers the local (receiving) TCP 5031 is willing to receive. Thus, the local TCP considers that 5032 segments overlapping the range RCV.NXT to 5033 RCV.NXT + RCV.WND - 1 carry acceptable data or control. 5034 Segments containing sequence numbers entirely outside of this 5035 range are considered duplicates and discarded. 5036 5037RST 5038 A control bit (reset), occupying no sequence space, indicating 5039 that the receiver should delete the connection without further 5040 interaction. The receiver can determine, based on the 5041 sequence number and acknowledgment fields of the incoming 5042 segment, whether it should honor the reset command or ignore 5043 it. In no case does receipt of a segment containing RST give 5044 rise to a RST in response. 5045 5046RTP 5047 Real Time Protocol: A host-to-host protocol for communication 5048 of time critical information. 5049 5050SEG.ACK 5051 segment acknowledgment 5052 5053SEG.LEN 5054 segment length 5055 5056SEG.PRC 5057 segment precedence value 5058 5059SEG.SEQ 5060 segment sequence 5061 5062SEG.UP 5063 segment urgent pointer field 5064 5065 5066 5067 5068 5069[Page 82] 5070 5071 5072September 1981 5073 Transmission Control Protocol 5074 Glossary 5075 5076 5077 5078SEG.WND 5079 segment window field 5080 5081segment 5082 A logical unit of data, in particular a TCP segment is the 5083 unit of data transfered between a pair of TCP modules. 5084 5085segment acknowledgment 5086 The sequence number in the acknowledgment field of the 5087 arriving segment. 5088 5089segment length 5090 The amount of sequence number space occupied by a segment, 5091 including any controls which occupy sequence space. 5092 5093segment sequence 5094 The number in the sequence field of the arriving segment. 5095 5096send sequence 5097 This is the next sequence number the local (sending) TCP will 5098 use on the connection. It is initially selected from an 5099 initial sequence number curve (ISN) and is incremented for 5100 each octet of data or sequenced control transmitted. 5101 5102send window 5103 This represents the sequence numbers which the remote 5104 (receiving) TCP is willing to receive. It is the value of the 5105 window field specified in segments from the remote (data 5106 receiving) TCP. The range of new sequence numbers which may 5107 be emitted by a TCP lies between SND.NXT and 5108 SND.UNA + SND.WND - 1. (Retransmissions of sequence numbers 5109 between SND.UNA and SND.NXT are expected, of course.) 5110 5111SND.NXT 5112 send sequence 5113 5114SND.UNA 5115 left sequence 5116 5117SND.UP 5118 send urgent pointer 5119 5120SND.WL1 5121 segment sequence number at last window update 5122 5123SND.WL2 5124 segment acknowledgment number at last window update 5125 5126 5127 5128 [Page 83] 5129 5130 5131 September 1981 5132Transmission Control Protocol 5133Glossary 5134 5135 5136 5137SND.WND 5138 send window 5139 5140socket 5141 An address which specifically includes a port identifier, that 5142 is, the concatenation of an Internet Address with a TCP port. 5143 5144Source Address 5145 The source address, usually the network and host identifiers. 5146 5147SYN 5148 A control bit in the incoming segment, occupying one sequence 5149 number, used at the initiation of a connection, to indicate 5150 where the sequence numbering will start. 5151 5152TCB 5153 Transmission control block, the data structure that records 5154 the state of a connection. 5155 5156TCB.PRC 5157 The precedence of the connection. 5158 5159TCP 5160 Transmission Control Protocol: A host-to-host protocol for 5161 reliable communication in internetwork environments. 5162 5163TOS 5164 Type of Service, an Internet Protocol field. 5165 5166Type of Service 5167 An Internet Protocol field which indicates the type of service 5168 for this internet fragment. 5169 5170URG 5171 A control bit (urgent), occupying no sequence space, used to 5172 indicate that the receiving user should be notified to do 5173 urgent processing as long as there is data to be consumed with 5174 sequence numbers less than the value indicated in the urgent 5175 pointer. 5176 5177urgent pointer 5178 A control field meaningful only when the URG bit is on. This 5179 field communicates the value of the urgent pointer which 5180 indicates the data octet associated with the sending user's 5181 urgent call. 5182 5183 5184 5185 5186 5187[Page 84] 5188 5189 5190September 1981 5191 Transmission Control Protocol 5192 5193 5194 5195 REFERENCES 5196 5197 5198 5199[1] Cerf, V., and R. Kahn, "A Protocol for Packet Network 5200 Intercommunication", IEEE Transactions on Communications, 5201 Vol. COM-22, No. 5, pp 637-648, May 1974. 5202 5203[2] Postel, J. (ed.), "Internet Protocol - DARPA Internet Program 5204 Protocol Specification", RFC 791, USC/Information Sciences 5205 Institute, September 1981. 5206 5207[3] Dalal, Y. and C. Sunshine, "Connection Management in Transport 5208 Protocols", Computer Networks, Vol. 2, No. 6, pp. 454-473, 5209 December 1978. 5210 5211[4] Postel, J., "Assigned Numbers", RFC 790, USC/Information Sciences 5212 Institute, September 1981. 5213 5214 5215 5216 5217 5218 5219 5220 5221 5222 5223 5224 5225 5226 5227 5228 5229 5230 5231 5232 5233 5234 5235 5236 5237 5238 5239 5240 5241 5242 5243 5244 5245 5246 [Page 85] 5247 5248