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The Internet Protocol [IP]

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  1.  
  2.  
  3. RFC: 791
  4.  
  5.  
  6.  
  7.  
  8.  
  9.  
  10.  
  11. INTERNET 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
  55.  
  56. September 1981
  57. Internet Protocol
  58.  
  59.  
  60.  
  61. TABLE OF CONTENTS
  62.  
  63. PREFACE ........................................................ iii
  64.  
  65. 1. INTRODUCTION ..................................................... 1
  66.  
  67. 1.1 Motivation .................................................... 1
  68. 1.2 Scope ......................................................... 1
  69. 1.3 Interfaces .................................................... 1
  70. 1.4 Operation ..................................................... 2
  71.  
  72. 2. OVERVIEW ......................................................... 5
  73.  
  74. 2.1 Relation to Other Protocols ................................... 9
  75. 2.2 Model of Operation ............................................ 5
  76. 2.3 Function Description .......................................... 7
  77. 2.4 Gateways ...................................................... 9
  78.  
  79. 3. SPECIFICATION ................................................... 11
  80.  
  81. 3.1 Internet Header Format ....................................... 11
  82. 3.2 Discussion ................................................... 23
  83. 3.3 Interfaces ................................................... 31
  84.  
  85. APPENDIX A: Examples & Scenarios ................................... 34
  86. APPENDIX B: Data Transmission Order ................................ 39
  87.  
  88. GLOSSARY ............................................................ 41
  89.  
  90. REFERENCES .......................................................... 45
  91.  
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  112. [Page i]
  114.  
  115. September 1981
  116. Internet Protocol
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  171. [Page ii]
  173.  
  174. September 1981
  175. Internet Protocol
  176.  
  177.  
  178.  
  179. PREFACE
  180.  
  181.  
  182.  
  183. This document specifies the DoD Standard Internet Protocol. This
  184. document is based on six earlier editions of the ARPA Internet Protocol
  185. Specification, and the present text draws heavily from them. There have
  186. been many contributors to this work both in terms of concepts and in
  187. terms of text. This edition revises aspects of addressing, error
  188. handling, option codes, and the security, precedence, compartments, and
  189. handling restriction features of the internet protocol.
  190.  
  191. Jon Postel
  192.  
  193. Editor
  194.  
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  230. [Page iii]
  233.  
  234. September 1981
  235.  
  236.  
  237. RFC: 791
  238. Replaces: RFC 760
  239. IENs 128, 123, 111,
  240. 80, 54, 44, 41, 28, 26
  241.  
  242. INTERNET PROTOCOL
  243.  
  244. DARPA INTERNET PROGRAM
  245. PROTOCOL SPECIFICATION
  246.  
  247.  
  248.  
  249. 1. INTRODUCTION
  250.  
  251. 1.1. Motivation
  252.  
  253. The Internet Protocol is designed for use in interconnected systems of
  254. packet-switched computer communication networks. Such a system has
  255. been called a "catenet" [1]. The internet protocol provides for
  256. transmitting blocks of data called datagrams from sources to
  257. destinations, where sources and destinations are hosts identified by
  258. fixed length addresses. The internet protocol also provides for
  259. fragmentation and reassembly of long datagrams, if necessary, for
  260. transmission through "small packet" networks.
  261.  
  262. 1.2. Scope
  263.  
  264. The internet protocol is specifically limited in scope to provide the
  265. functions necessary to deliver a package of bits (an internet
  266. datagram) from a source to a destination over an interconnected system
  267. of networks. There are no mechanisms to augment end-to-end data
  268. reliability, flow control, sequencing, or other services commonly
  269. found in host-to-host protocols. The internet protocol can capitalize
  270. on the services of its supporting networks to provide various types
  271. and qualities of service.
  272.  
  273. 1.3. Interfaces
  274.  
  275. This protocol is called on by host-to-host protocols in an internet
  276. environment. This protocol calls on local network protocols to carry
  277. the internet datagram to the next gateway or destination host.
  278.  
  279. For example, a TCP module would call on the internet module to take a
  280. TCP segment (including the TCP header and user data) as the data
  281. portion of an internet datagram. The TCP module would provide the
  282. addresses and other parameters in the internet header to the internet
  283. module as arguments of the call. The internet module would then
  284. create an internet datagram and call on the local network interface to
  285. transmit the internet datagram.
  286.  
  287. In the ARPANET case, for example, the internet module would call on a
  288.  
  289.  
  290. [Page 1]
  292.  
  293. September 1981
  294. Internet Protocol
  295. Introduction
  296.  
  297.  
  298.  
  299. local net module which would add the 1822 leader [2] to the internet
  300. datagram creating an ARPANET message to transmit to the IMP. The
  301. ARPANET address would be derived from the internet address by the
  302. local network interface and would be the address of some host in the
  303. ARPANET, that host might be a gateway to other networks.
  304.  
  305. 1.4. Operation
  306.  
  307. The internet protocol implements two basic functions: addressing and
  308. fragmentation.
  309.  
  310. The internet modules use the addresses carried in the internet header
  311. to transmit internet datagrams toward their destinations. The
  312. selection of a path for transmission is called routing.
  313.  
  314. The internet modules use fields in the internet header to fragment and
  315. reassemble internet datagrams when necessary for transmission through
  316. "small packet" networks.
  317.  
  318. The model of operation is that an internet module resides in each host
  319. engaged in internet communication and in each gateway that
  320. interconnects networks. These modules share common rules for
  321. interpreting address fields and for fragmenting and assembling
  322. internet datagrams. In addition, these modules (especially in
  323. gateways) have procedures for making routing decisions and other
  324. functions.
  325.  
  326. The internet protocol treats each internet datagram as an independent
  327. entity unrelated to any other internet datagram. There are no
  328. connections or logical circuits (virtual or otherwise).
  329.  
  330. The internet protocol uses four key mechanisms in providing its
  331. service: Type of Service, Time to Live, Options, and Header Checksum.
  332.  
  333. The Type of Service is used to indicate the quality of the service
  334. desired. The type of service is an abstract or generalized set of
  335. parameters which characterize the service choices provided in the
  336. networks that make up the internet. This type of service indication
  337. is to be used by gateways to select the actual transmission parameters
  338. for a particular network, the network to be used for the next hop, or
  339. the next gateway when routing an internet datagram.
  340.  
  341. The Time to Live is an indication of an upper bound on the lifetime of
  342. an internet datagram. It is set by the sender of the datagram and
  343. reduced at the points along the route where it is processed. If the
  344. time to live reaches zero before the internet datagram reaches its
  345. destination, the internet datagram is destroyed. The time to live can
  346. be thought of as a self destruct time limit.
  347.  
  348.  
  349. [Page 2]
  351.  
  352. September 1981
  353. Internet Protocol
  354. Introduction
  355.  
  356.  
  357.  
  358. The Options provide for control functions needed or useful in some
  359. situations but unnecessary for the most common communications. The
  360. options include provisions for timestamps, security, and special
  361. routing.
  362.  
  363. The Header Checksum provides a verification that the information used
  364. in processing internet datagram has been transmitted correctly. The
  365. data may contain errors. If the header checksum fails, the internet
  366. datagram is discarded at once by the entity which detects the error.
  367.  
  368. The internet protocol does not provide a reliable communication
  369. facility. There are no acknowledgments either end-to-end or
  370. hop-by-hop. There is no error control for data, only a header
  371. checksum. There are no retransmissions. There is no flow control.
  372.  
  373. Errors detected may be reported via the Internet Control Message
  374. Protocol (ICMP) [3] which is implemented in the internet protocol
  375. module.
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  408. [Page 3]
  410.  
  411. September 1981
  412. Internet Protocol
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  467. [Page 4]
  469.  
  470. September 1981
  471. Internet Protocol
  472.  
  473.  
  474.  
  475. 2. OVERVIEW
  476.  
  477. 2.1. Relation to Other Protocols
  478.  
  479. The following diagram illustrates the place of the internet protocol
  480. in the protocol hierarchy:
  481.  
  482.  
  483. +------+ +-----+ +-----+ +-----+
  484. |Telnet| | FTP | | TFTP| ... | ... |
  485. +------+ +-----+ +-----+ +-----+
  486. | | | |
  487. +-----+ +-----+ +-----+
  488. | TCP | | UDP | ... | ... |
  489. +-----+ +-----+ +-----+
  490. | | |
  491. +--------------------------+----+
  492. | Internet Protocol & ICMP |
  493. +--------------------------+----+
  494. |
  495. +---------------------------+
  496. | Local Network Protocol |
  497. +---------------------------+
  498.  
  499. Protocol Relationships
  500.  
  501. Figure 1.
  502.  
  503. Internet protocol interfaces on one side to the higher level
  504. host-to-host protocols and on the other side to the local network
  505. protocol. In this context a "local network" may be a small network in
  506. a building or a large network such as the ARPANET.
  507.  
  508. 2.2. Model of Operation
  509.  
  510. The model of operation for transmitting a datagram from one
  511. application program to another is illustrated by the following
  512. scenario:
  513.  
  514. We suppose that this transmission will involve one intermediate
  515. gateway.
  516.  
  517. The sending application program prepares its data and calls on its
  518. local internet module to send that data as a datagram and passes the
  519. destination address and other parameters as arguments of the call.
  520.  
  521. The internet module prepares a datagram header and attaches the data
  522. to it. The internet module determines a local network address for
  523. this internet address, in this case it is the address of a gateway.
  524.  
  525.  
  526. [Page 5]
  528.  
  529. September 1981
  530. Internet Protocol
  531. Overview
  532.  
  533.  
  534.  
  535. It sends this datagram and the local network address to the local
  536. network interface.
  537.  
  538. The local network interface creates a local network header, and
  539. attaches the datagram to it, then sends the result via the local
  540. network.
  541.  
  542. The datagram arrives at a gateway host wrapped in the local network
  543. header, the local network interface strips off this header, and
  544. turns the datagram over to the internet module. The internet module
  545. determines from the internet address that the datagram is to be
  546. forwarded to another host in a second network. The internet module
  547. determines a local net address for the destination host. It calls
  548. on the local network interface for that network to send the
  549. datagram.
  550.  
  551. This local network interface creates a local network header and
  552. attaches the datagram sending the result to the destination host.
  553.  
  554. At this destination host the datagram is stripped of the local net
  555. header by the local network interface and handed to the internet
  556. module.
  557.  
  558. The internet module determines that the datagram is for an
  559. application program in this host. It passes the data to the
  560. application program in response to a system call, passing the source
  561. address and other parameters as results of the call.
  562.  
  563.  
  564. Application Application
  565. Program Program
  566. \ /
  567. Internet Module Internet Module Internet Module
  568. \ / \ /
  569. LNI-1 LNI-1 LNI-2 LNI-2
  570. \ / \ /
  571. Local Network 1 Local Network 2
  572.  
  573.  
  574.  
  575. Transmission Path
  576.  
  577. Figure 2
  578.  
  579.  
  580.  
  581.  
  582.  
  583.  
  584.  
  585. [Page 6]
  587.  
  588. September 1981
  589. Internet Protocol
  590. Overview
  591.  
  592.  
  593.  
  594. 2.3. Function Description
  595.  
  596. The function or purpose of Internet Protocol is to move datagrams
  597. through an interconnected set of networks. This is done by passing
  598. the datagrams from one internet module to another until the
  599. destination is reached. The internet modules reside in hosts and
  600. gateways in the internet system. The datagrams are routed from one
  601. internet module to another through individual networks based on the
  602. interpretation of an internet address. Thus, one important mechanism
  603. of the internet protocol is the internet address.
  604.  
  605. In the routing of messages from one internet module to another,
  606. datagrams may need to traverse a network whose maximum packet size is
  607. smaller than the size of the datagram. To overcome this difficulty, a
  608. fragmentation mechanism is provided in the internet protocol.
  609.  
  610. Addressing
  611.  
  612. A distinction is made between names, addresses, and routes [4]. A
  613. name indicates what we seek. An address indicates where it is. A
  614. route indicates how to get there. The internet protocol deals
  615. primarily with addresses. It is the task of higher level (i.e.,
  616. host-to-host or application) protocols to make the mapping from
  617. names to addresses. The internet module maps internet addresses to
  618. local net addresses. It is the task of lower level (i.e., local net
  619. or gateways) procedures to make the mapping from local net addresses
  620. to routes.
  621.  
  622. Addresses are fixed length of four octets (32 bits). An address
  623. begins with a network number, followed by local address (called the
  624. "rest" field). There are three formats or classes of internet
  625. addresses: in class a, the high order bit is zero, the next 7 bits
  626. are the network, and the last 24 bits are the local address; in
  627. class b, the high order two bits are one-zero, the next 14 bits are
  628. the network and the last 16 bits are the local address; in class c,
  629. the high order three bits are one-one-zero, the next 21 bits are the
  630. network and the last 8 bits are the local address.
  631.  
  632. Care must be taken in mapping internet addresses to local net
  633. addresses; a single physical host must be able to act as if it were
  634. several distinct hosts to the extent of using several distinct
  635. internet addresses. Some hosts will also have several physical
  636. interfaces (multi-homing).
  637.  
  638. That is, provision must be made for a host to have several physical
  639. interfaces to the network with each having several logical internet
  640. addresses.
  641.  
  642.  
  643.  
  644. [Page 7]
  646.  
  647. September 1981
  648. Internet Protocol
  649. Overview
  650.  
  651.  
  652.  
  653. Examples of address mappings may be found in "Address Mappings" [5].
  654.  
  655. Fragmentation
  656.  
  657. Fragmentation of an internet datagram is necessary when it
  658. originates in a local net that allows a large packet size and must
  659. traverse a local net that limits packets to a smaller size to reach
  660. its destination.
  661.  
  662. An internet datagram can be marked "don't fragment." Any internet
  663. datagram so marked is not to be internet fragmented under any
  664. circumstances. If internet datagram marked don't fragment cannot be
  665. delivered to its destination without fragmenting it, it is to be
  666. discarded instead.
  667.  
  668. Fragmentation, transmission and reassembly across a local network
  669. which is invisible to the internet protocol module is called
  670. intranet fragmentation and may be used [6].
  671.  
  672. The internet fragmentation and reassembly procedure needs to be able
  673. to break a datagram into an almost arbitrary number of pieces that
  674. can be later reassembled. The receiver of the fragments uses the
  675. identification field to ensure that fragments of different datagrams
  676. are not mixed. The fragment offset field tells the receiver the
  677. position of a fragment in the original datagram. The fragment
  678. offset and length determine the portion of the original datagram
  679. covered by this fragment. The more-fragments flag indicates (by
  680. being reset) the last fragment. These fields provide sufficient
  681. information to reassemble datagrams.
  682.  
  683. The identification field is used to distinguish the fragments of one
  684. datagram from those of another. The originating protocol module of
  685. an internet datagram sets the identification field to a value that
  686. must be unique for that source-destination pair and protocol for the
  687. time the datagram will be active in the internet system. The
  688. originating protocol module of a complete datagram sets the
  689. more-fragments flag to zero and the fragment offset to zero.
  690.  
  691. To fragment a long internet datagram, an internet protocol module
  692. (for example, in a gateway), creates two new internet datagrams and
  693. copies the contents of the internet header fields from the long
  694. datagram into both new internet headers. The data of the long
  695. datagram is divided into two portions on a 8 octet (64 bit) boundary
  696. (the second portion might not be an integral multiple of 8 octets,
  697. but the first must be). Call the number of 8 octet blocks in the
  698. first portion NFB (for Number of Fragment Blocks). The first
  699. portion of the data is placed in the first new internet datagram,
  700. and the total length field is set to the length of the first
  701.  
  702.  
  703. [Page 8]
  705.  
  706. September 1981
  707. Internet Protocol
  708. Overview
  709.  
  710.  
  711.  
  712. datagram. The more-fragments flag is set to one. The second
  713. portion of the data is placed in the second new internet datagram,
  714. and the total length field is set to the length of the second
  715. datagram. The more-fragments flag carries the same value as the
  716. long datagram. The fragment offset field of the second new internet
  717. datagram is set to the value of that field in the long datagram plus
  718. NFB.
  719.  
  720. This procedure can be generalized for an n-way split, rather than
  721. the two-way split described.
  722.  
  723. To assemble the fragments of an internet datagram, an internet
  724. protocol module (for example at a destination host) combines
  725. internet datagrams that all have the same value for the four fields:
  726. identification, source, destination, and protocol. The combination
  727. is done by placing the data portion of each fragment in the relative
  728. position indicated by the fragment offset in that fragment's
  729. internet header. The first fragment will have the fragment offset
  730. zero, and the last fragment will have the more-fragments flag reset
  731. to zero.
  732.  
  733. 2.4. Gateways
  734.  
  735. Gateways implement internet protocol to forward datagrams between
  736. networks. Gateways also implement the Gateway to Gateway Protocol
  737. (GGP) [7] to coordinate routing and other internet control
  738. information.
  739.  
  740. In a gateway the higher level protocols need not be implemented and
  741. the GGP functions are added to the IP module.
  742.  
  743.  
  744. +-------------------------------+
  745. | Internet Protocol & ICMP & GGP|
  746. +-------------------------------+
  747. | |
  748. +---------------+ +---------------+
  749. | Local Net | | Local Net |
  750. +---------------+ +---------------+
  751.  
  752. Gateway Protocols
  753.  
  754. Figure 3.
  755.  
  756.  
  757.  
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  761.  
  762. [Page 9]
  764.  
  765. September 1981
  766. Internet Protocol
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  821. [Page 10]
  823.  
  824. September 1981
  825. Internet Protocol
  826.  
  827.  
  828.  
  829. 3. SPECIFICATION
  830.  
  831. 3.1. Internet Header Format
  832.  
  833. A summary of the contents of the internet header follows:
  834.  
  835.  
  836. 0 1 2 3
  837. 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
  838. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  839. |Version| IHL |Type of Service| Total Length |
  840. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  841. | Identification |Flags| Fragment Offset |
  842. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  843. | Time to Live | Protocol | Header Checksum |
  844. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  845. | Source Address |
  846. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  847. | Destination Address |
  848. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  849. | Options | Padding |
  850. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  851.  
  852. Example Internet Datagram Header
  853.  
  854. Figure 4.
  855.  
  856. Note that each tick mark represents one bit position.
  857.  
  858. Version: 4 bits
  859.  
  860. The Version field indicates the format of the internet header. This
  861. document describes version 4.
  862.  
  863. IHL: 4 bits
  864.  
  865. Internet Header Length is the length of the internet header in 32
  866. bit words, and thus points to the beginning of the data. Note that
  867. the minimum value for a correct header is 5.
  868.  
  869.  
  870.  
  871.  
  872.  
  873.  
  874.  
  875.  
  876.  
  877.  
  878.  
  879.  
  880. [Page 11]
  882.  
  883. September 1981
  884. Internet Protocol
  885. Specification
  886.  
  887.  
  888.  
  889. Type of Service: 8 bits
  890.  
  891. The Type of Service provides an indication of the abstract
  892. parameters of the quality of service desired. These parameters are
  893. to be used to guide the selection of the actual service parameters
  894. when transmitting a datagram through a particular network. Several
  895. networks offer service precedence, which somehow treats high
  896. precedence traffic as more important than other traffic (generally
  897. by accepting only traffic above a certain precedence at time of high
  898. load). The major choice is a three way tradeoff between low-delay,
  899. high-reliability, and high-throughput.
  900.  
  901. Bits 0-2: Precedence.
  902. Bit 3: 0 = Normal Delay, 1 = Low Delay.
  903. Bits 4: 0 = Normal Throughput, 1 = High Throughput.
  904. Bits 5: 0 = Normal Relibility, 1 = High Relibility.
  905. Bit 6-7: Reserved for Future Use.
  906.  
  907. 0 1 2 3 4 5 6 7
  908. +-----+-----+-----+-----+-----+-----+-----+-----+
  909. | | | | | | |
  910. | PRECEDENCE | D | T | R | 0 | 0 |
  911. | | | | | | |
  912. +-----+-----+-----+-----+-----+-----+-----+-----+
  913.  
  914. Precedence
  915.  
  916. 111 - Network Control
  917. 110 - Internetwork Control
  918. 101 - CRITIC/ECP
  919. 100 - Flash Override
  920. 011 - Flash
  921. 010 - Immediate
  922. 001 - Priority
  923. 000 - Routine
  924.  
  925. The use of the Delay, Throughput, and Reliability indications may
  926. increase the cost (in some sense) of the service. In many networks
  927. better performance for one of these parameters is coupled with worse
  928. performance on another. Except for very unusual cases at most two
  929. of these three indications should be set.
  930.  
  931. The type of service is used to specify the treatment of the datagram
  932. during its transmission through the internet system. Example
  933. mappings of the internet type of service to the actual service
  934. provided on networks such as AUTODIN II, ARPANET, SATNET, and PRNET
  935. is given in "Service Mappings" [8].
  936.  
  937.  
  938.  
  939. [Page 12]
  941.  
  942. September 1981
  943. Internet Protocol
  944. Specification
  945.  
  946.  
  947.  
  948. The Network Control precedence designation is intended to be used
  949. within a network only. The actual use and control of that
  950. designation is up to each network. The Internetwork Control
  951. designation is intended for use by gateway control originators only.
  952. If the actual use of these precedence designations is of concern to
  953. a particular network, it is the responsibility of that network to
  954. control the access to, and use of, those precedence designations.
  955.  
  956. Total Length: 16 bits
  957.  
  958. Total Length is the length of the datagram, measured in octets,
  959. including internet header and data. This field allows the length of
  960. a datagram to be up to 65,535 octets. Such long datagrams are
  961. impractical for most hosts and networks. All hosts must be prepared
  962. to accept datagrams of up to 576 octets (whether they arrive whole
  963. or in fragments). It is recommended that hosts only send datagrams
  964. larger than 576 octets if they have assurance that the destination
  965. is prepared to accept the larger datagrams.
  966.  
  967. The number 576 is selected to allow a reasonable sized data block to
  968. be transmitted in addition to the required header information. For
  969. example, this size allows a data block of 512 octets plus 64 header
  970. octets to fit in a datagram. The maximal internet header is 60
  971. octets, and a typical internet header is 20 octets, allowing a
  972. margin for headers of higher level protocols.
  973.  
  974. Identification: 16 bits
  975.  
  976. An identifying value assigned by the sender to aid in assembling the
  977. fragments of a datagram.
  978.  
  979. Flags: 3 bits
  980.  
  981. Various Control Flags.
  982.  
  983. Bit 0: reserved, must be zero
  984. Bit 1: (DF) 0 = May Fragment, 1 = Don't Fragment.
  985. Bit 2: (MF) 0 = Last Fragment, 1 = More Fragments.
  986.  
  987. 0 1 2
  988. +---+---+---+
  989. | | D | M |
  990. | 0 | F | F |
  991. +---+---+---+
  992.  
  993. Fragment Offset: 13 bits
  994.  
  995. This field indicates where in the datagram this fragment belongs.
  996.  
  997.  
  998. [Page 13]
  1000.  
  1001. September 1981
  1002. Internet Protocol
  1003. Specification
  1004.  
  1005.  
  1006.  
  1007. The fragment offset is measured in units of 8 octets (64 bits). The
  1008. first fragment has offset zero.
  1009.  
  1010. Time to Live: 8 bits
  1011.  
  1012. This field indicates the maximum time the datagram is allowed to
  1013. remain in the internet system. If this field contains the value
  1014. zero, then the datagram must be destroyed. This field is modified
  1015. in internet header processing. The time is measured in units of
  1016. seconds, but since every module that processes a datagram must
  1017. decrease the TTL by at least one even if it process the datagram in
  1018. less than a second, the TTL must be thought of only as an upper
  1019. bound on the time a datagram may exist. The intention is to cause
  1020. undeliverable datagrams to be discarded, and to bound the maximum
  1021. datagram lifetime.
  1022.  
  1023. Protocol: 8 bits
  1024.  
  1025. This field indicates the next level protocol used in the data
  1026. portion of the internet datagram. The values for various protocols
  1027. are specified in "Assigned Numbers" [9].
  1028.  
  1029. Header Checksum: 16 bits
  1030.  
  1031. A checksum on the header only. Since some header fields change
  1032. (e.g., time to live), this is recomputed and verified at each point
  1033. that the internet header is processed.
  1034.  
  1035. The checksum algorithm is:
  1036.  
  1037. The checksum field is the 16 bit one's complement of the one's
  1038. complement sum of all 16 bit words in the header. For purposes of
  1039. computing the checksum, the value of the checksum field is zero.
  1040.  
  1041. This is a simple to compute checksum and experimental evidence
  1042. indicates it is adequate, but it is provisional and may be replaced
  1043. by a CRC procedure, depending on further experience.
  1044.  
  1045. Source Address: 32 bits
  1046.  
  1047. The source address. See section 3.2.
  1048.  
  1049. Destination Address: 32 bits
  1050.  
  1051. The destination address. See section 3.2.
  1052.  
  1053.  
  1054.  
  1055.  
  1056.  
  1057. [Page 14]
  1059.  
  1060. September 1981
  1061. Internet Protocol
  1062. Specification
  1063.  
  1064.  
  1065.  
  1066. Options: variable
  1067.  
  1068. The options may appear or not in datagrams. They must be
  1069. implemented by all IP modules (host and gateways). What is optional
  1070. is their transmission in any particular datagram, not their
  1071. implementation.
  1072.  
  1073. In some environments the security option may be required in all
  1074. datagrams.
  1075.  
  1076. The option field is variable in length. There may be zero or more
  1077. options. There are two cases for the format of an option:
  1078.  
  1079. Case 1: A single octet of option-type.
  1080.  
  1081. Case 2: An option-type octet, an option-length octet, and the
  1082. actual option-data octets.
  1083.  
  1084. The option-length octet counts the option-type octet and the
  1085. option-length octet as well as the option-data octets.
  1086.  
  1087. The option-type octet is viewed as having 3 fields:
  1088.  
  1089. 1 bit copied flag,
  1090. 2 bits option class,
  1091. 5 bits option number.
  1092.  
  1093. The copied flag indicates that this option is copied into all
  1094. fragments on fragmentation.
  1095.  
  1096. 0 = not copied
  1097. 1 = copied
  1098.  
  1099. The option classes are:
  1100.  
  1101. 0 = control
  1102. 1 = reserved for future use
  1103. 2 = debugging and measurement
  1104. 3 = reserved for future use
  1105.  
  1106.  
  1107.  
  1108.  
  1109.  
  1110.  
  1111.  
  1112.  
  1113.  
  1114.  
  1115.  
  1116. [Page 15]
  1118.  
  1119. September 1981
  1120. Internet Protocol
  1121. Specification
  1122.  
  1123.  
  1124.  
  1125. The following internet options are defined:
  1126.  
  1127. CLASS NUMBER LENGTH DESCRIPTION
  1128. ----- ------ ------ -----------
  1129. 0 0 - End of Option list. This option occupies only
  1130. 1 octet; it has no length octet.
  1131. 0 1 - No Operation. This option occupies only 1
  1132. octet; it has no length octet.
  1133. 0 2 11 Security. Used to carry Security,
  1134. Compartmentation, User Group (TCC), and
  1135. Handling Restriction Codes compatible with DOD
  1136. requirements.
  1137. 0 3 var. Loose Source Routing. Used to route the
  1138. internet datagram based on information
  1139. supplied by the source.
  1140. 0 9 var. Strict Source Routing. Used to route the
  1141. internet datagram based on information
  1142. supplied by the source.
  1143. 0 7 var. Record Route. Used to trace the route an
  1144. internet datagram takes.
  1145. 0 8 4 Stream ID. Used to carry the stream
  1146. identifier.
  1147. 2 4 var. Internet Timestamp.
  1148.  
  1149.  
  1150.  
  1151. Specific Option Definitions
  1152.  
  1153. End of Option List
  1154.  
  1155. +--------+
  1156. |00000000|
  1157. +--------+
  1158. Type=0
  1159.  
  1160. This option indicates the end of the option list. This might
  1161. not coincide with the end of the internet header according to
  1162. the internet header length. This is used at the end of all
  1163. options, not the end of each option, and need only be used if
  1164. the end of the options would not otherwise coincide with the end
  1165. of the internet header.
  1166.  
  1167. May be copied, introduced, or deleted on fragmentation, or for
  1168. any other reason.
  1169.  
  1170.  
  1171.  
  1172.  
  1173.  
  1174.  
  1175. [Page 16]
  1177.  
  1178. September 1981
  1179. Internet Protocol
  1180. Specification
  1181.  
  1182.  
  1183.  
  1184. No Operation
  1185.  
  1186. +--------+
  1187. |00000001|
  1188. +--------+
  1189. Type=1
  1190.  
  1191. This option may be used between options, for example, to align
  1192. the beginning of a subsequent option on a 32 bit boundary.
  1193.  
  1194. May be copied, introduced, or deleted on fragmentation, or for
  1195. any other reason.
  1196.  
  1197. Security
  1198.  
  1199. This option provides a way for hosts to send security,
  1200. compartmentation, handling restrictions, and TCC (closed user
  1201. group) parameters. The format for this option is as follows:
  1202.  
  1203. +--------+--------+---//---+---//---+---//---+---//---+
  1204. |10000010|00001011|SSS SSS|CCC CCC|HHH HHH| TCC |
  1205. +--------+--------+---//---+---//---+---//---+---//---+
  1206. Type=130 Length=11
  1207.  
  1208. Security (S field): 16 bits
  1209.  
  1210. Specifies one of 16 levels of security (eight of which are
  1211. reserved for future use).
  1212.  
  1213. 00000000 00000000 - Unclassified
  1214. 11110001 00110101 - Confidential
  1215. 01111000 10011010 - EFTO
  1216. 10111100 01001101 - MMMM
  1217. 01011110 00100110 - PROG
  1218. 10101111 00010011 - Restricted
  1219. 11010111 10001000 - Secret
  1220. 01101011 11000101 - Top Secret
  1221. 00110101 11100010 - (Reserved for future use)
  1222. 10011010 11110001 - (Reserved for future use)
  1223. 01001101 01111000 - (Reserved for future use)
  1224. 00100100 10111101 - (Reserved for future use)
  1225. 00010011 01011110 - (Reserved for future use)
  1226. 10001001 10101111 - (Reserved for future use)
  1227. 11000100 11010110 - (Reserved for future use)
  1228. 11100010 01101011 - (Reserved for future use)
  1229.  
  1230.  
  1231.  
  1232.  
  1233.  
  1234. [Page 17]
  1236.  
  1237. September 1981
  1238. Internet Protocol
  1239. Specification
  1240.  
  1241.  
  1242.  
  1243. Compartments (C field): 16 bits
  1244.  
  1245. An all zero value is used when the information transmitted is
  1246. not compartmented. Other values for the compartments field
  1247. may be obtained from the Defense Intelligence Agency.
  1248.  
  1249. Handling Restrictions (H field): 16 bits
  1250.  
  1251. The values for the control and release markings are
  1252. alphanumeric digraphs and are defined in the Defense
  1253. Intelligence Agency Manual DIAM 65-19, "Standard Security
  1254. Markings".
  1255.  
  1256. Transmission Control Code (TCC field): 24 bits
  1257.  
  1258. Provides a means to segregate traffic and define controlled
  1259. communities of interest among subscribers. The TCC values are
  1260. trigraphs, and are available from HQ DCA Code 530.
  1261.  
  1262. Must be copied on fragmentation. This option appears at most
  1263. once in a datagram.
  1264.  
  1265. Loose Source and Record Route
  1266.  
  1267. +--------+--------+--------+---------//--------+
  1268. |10000011| length | pointer| route data |
  1269. +--------+--------+--------+---------//--------+
  1270. Type=131
  1271.  
  1272. The loose source and record route (LSRR) option provides a means
  1273. for the source of an internet datagram to supply routing
  1274. information to be used by the gateways in forwarding the
  1275. datagram to the destination, and to record the route
  1276. information.
  1277.  
  1278. The option begins with the option type code. The second octet
  1279. is the option length which includes the option type code and the
  1280. length octet, the pointer octet, and length-3 octets of route
  1281. data. The third octet is the pointer into the route data
  1282. indicating the octet which begins the next source address to be
  1283. processed. The pointer is relative to this option, and the
  1284. smallest legal value for the pointer is 4.
  1285.  
  1286. A route data is composed of a series of internet addresses.
  1287. Each internet address is 32 bits or 4 octets. If the pointer is
  1288. greater than the length, the source route is empty (and the
  1289. recorded route full) and the routing is to be based on the
  1290. destination address field.
  1291.  
  1292.  
  1293. [Page 18]
  1295.  
  1296. September 1981
  1297. Internet Protocol
  1298. Specification
  1299.  
  1300.  
  1301.  
  1302. If the address in destination address field has been reached and
  1303. the pointer is not greater than the length, the next address in
  1304. the source route replaces the address in the destination address
  1305. field, and the recorded route address replaces the source
  1306. address just used, and pointer is increased by four.
  1307.  
  1308. The recorded route address is the internet module's own internet
  1309. address as known in the environment into which this datagram is
  1310. being forwarded.
  1311.  
  1312. This procedure of replacing the source route with the recorded
  1313. route (though it is in the reverse of the order it must be in to
  1314. be used as a source route) means the option (and the IP header
  1315. as a whole) remains a constant length as the datagram progresses
  1316. through the internet.
  1317.  
  1318. This option is a loose source route because the gateway or host
  1319. IP is allowed to use any route of any number of other
  1320. intermediate gateways to reach the next address in the route.
  1321.  
  1322. Must be copied on fragmentation. Appears at most once in a
  1323. datagram.
  1324.  
  1325. Strict Source and Record Route
  1326.  
  1327. +--------+--------+--------+---------//--------+
  1328. |10001001| length | pointer| route data |
  1329. +--------+--------+--------+---------//--------+
  1330. Type=137
  1331.  
  1332. The strict source and record route (SSRR) option provides a
  1333. means for the source of an internet datagram to supply routing
  1334. information to be used by the gateways in forwarding the
  1335. datagram to the destination, and to record the route
  1336. information.
  1337.  
  1338. The option begins with the option type code. The second octet
  1339. is the option length which includes the option type code and the
  1340. length octet, the pointer octet, and length-3 octets of route
  1341. data. The third octet is the pointer into the route data
  1342. indicating the octet which begins the next source address to be
  1343. processed. The pointer is relative to this option, and the
  1344. smallest legal value for the pointer is 4.
  1345.  
  1346. A route data is composed of a series of internet addresses.
  1347. Each internet address is 32 bits or 4 octets. If the pointer is
  1348. greater than the length, the source route is empty (and the
  1349.  
  1350.  
  1351.  
  1352. [Page 19]
  1354.  
  1355. September 1981
  1356. Internet Protocol
  1357. Specification
  1358.  
  1359.  
  1360.  
  1361. recorded route full) and the routing is to be based on the
  1362. destination address field.
  1363.  
  1364. If the address in destination address field has been reached and
  1365. the pointer is not greater than the length, the next address in
  1366. the source route replaces the address in the destination address
  1367. field, and the recorded route address replaces the source
  1368. address just used, and pointer is increased by four.
  1369.  
  1370. The recorded route address is the internet module's own internet
  1371. address as known in the environment into which this datagram is
  1372. being forwarded.
  1373.  
  1374. This procedure of replacing the source route with the recorded
  1375. route (though it is in the reverse of the order it must be in to
  1376. be used as a source route) means the option (and the IP header
  1377. as a whole) remains a constant length as the datagram progresses
  1378. through the internet.
  1379.  
  1380. This option is a strict source route because the gateway or host
  1381. IP must send the datagram directly to the next address in the
  1382. source route through only the directly connected network
  1383. indicated in the next address to reach the next gateway or host
  1384. specified in the route.
  1385.  
  1386. Must be copied on fragmentation. Appears at most once in a
  1387. datagram.
  1388.  
  1389. Record Route
  1390.  
  1391. +--------+--------+--------+---------//--------+
  1392. |00000111| length | pointer| route data |
  1393. +--------+--------+--------+---------//--------+
  1394. Type=7
  1395.  
  1396. The record route option provides a means to record the route of
  1397. an internet datagram.
  1398.  
  1399. The option begins with the option type code. The second octet
  1400. is the option length which includes the option type code and the
  1401. length octet, the pointer octet, and length-3 octets of route
  1402. data. The third octet is the pointer into the route data
  1403. indicating the octet which begins the next area to store a route
  1404. address. The pointer is relative to this option, and the
  1405. smallest legal value for the pointer is 4.
  1406.  
  1407. A recorded route is composed of a series of internet addresses.
  1408. Each internet address is 32 bits or 4 octets. If the pointer is
  1409.  
  1410.  
  1411. [Page 20]
  1413.  
  1414. September 1981
  1415. Internet Protocol
  1416. Specification
  1417.  
  1418.  
  1419.  
  1420. greater than the length, the recorded route data area is full.
  1421. The originating host must compose this option with a large
  1422. enough route data area to hold all the address expected. The
  1423. size of the option does not change due to adding addresses. The
  1424. intitial contents of the route data area must be zero.
  1425.  
  1426. When an internet module routes a datagram it checks to see if
  1427. the record route option is present. If it is, it inserts its
  1428. own internet address as known in the environment into which this
  1429. datagram is being forwarded into the recorded route begining at
  1430. the octet indicated by the pointer, and increments the pointer
  1431. by four.
  1432.  
  1433. If the route data area is already full (the pointer exceeds the
  1434. length) the datagram is forwarded without inserting the address
  1435. into the recorded route. If there is some room but not enough
  1436. room for a full address to be inserted, the original datagram is
  1437. considered to be in error and is discarded. In either case an
  1438. ICMP parameter problem message may be sent to the source
  1439. host [3].
  1440.  
  1441. Not copied on fragmentation, goes in first fragment only.
  1442. Appears at most once in a datagram.
  1443.  
  1444. Stream Identifier
  1445.  
  1446. +--------+--------+--------+--------+
  1447. |10001000|00000010| Stream ID |
  1448. +--------+--------+--------+--------+
  1449. Type=136 Length=4
  1450.  
  1451. This option provides a way for the 16-bit SATNET stream
  1452. identifier to be carried through networks that do not support
  1453. the stream concept.
  1454.  
  1455. Must be copied on fragmentation. Appears at most once in a
  1456. datagram.
  1457.  
  1458.  
  1459.  
  1460.  
  1461.  
  1462.  
  1463.  
  1464.  
  1465.  
  1466.  
  1467.  
  1468.  
  1469.  
  1470. [Page 21]
  1472.  
  1473. September 1981
  1474. Internet Protocol
  1475. Specification
  1476.  
  1477.  
  1478.  
  1479. Internet Timestamp
  1480.  
  1481. +--------+--------+--------+--------+
  1482. |01000100| length | pointer|oflw|flg|
  1483. +--------+--------+--------+--------+
  1484. | internet address |
  1485. +--------+--------+--------+--------+
  1486. | timestamp |
  1487. +--------+--------+--------+--------+
  1488. | . |
  1489. .
  1490. .
  1491. Type = 68
  1492.  
  1493. The Option Length is the number of octets in the option counting
  1494. the type, length, pointer, and overflow/flag octets (maximum
  1495. length 40).
  1496.  
  1497. The Pointer is the number of octets from the beginning of this
  1498. option to the end of timestamps plus one (i.e., it points to the
  1499. octet beginning the space for next timestamp). The smallest
  1500. legal value is 5. The timestamp area is full when the pointer
  1501. is greater than the length.
  1502.  
  1503. The Overflow (oflw) [4 bits] is the number of IP modules that
  1504. cannot register timestamps due to lack of space.
  1505.  
  1506. The Flag (flg) [4 bits] values are
  1507.  
  1508. 0 -- time stamps only, stored in consecutive 32-bit words,
  1509.  
  1510. 1 -- each timestamp is preceded with internet address of the
  1511. registering entity,
  1512.  
  1513. 3 -- the internet address fields are prespecified. An IP
  1514. module only registers its timestamp if it matches its own
  1515. address with the next specified internet address.
  1516.  
  1517. The Timestamp is a right-justified, 32-bit timestamp in
  1518. milliseconds since midnight UT. If the time is not available in
  1519. milliseconds or cannot be provided with respect to midnight UT
  1520. then any time may be inserted as a timestamp provided the high
  1521. order bit of the timestamp field is set to one to indicate the
  1522. use of a non-standard value.
  1523.  
  1524. The originating host must compose this option with a large
  1525. enough timestamp data area to hold all the timestamp information
  1526. expected. The size of the option does not change due to adding
  1527.  
  1528.  
  1529. [Page 22]
  1531.  
  1532. September 1981
  1533. Internet Protocol
  1534. Specification
  1535.  
  1536.  
  1537.  
  1538. timestamps. The intitial contents of the timestamp data area
  1539. must be zero or internet address/zero pairs.
  1540.  
  1541. If the timestamp data area is already full (the pointer exceeds
  1542. the length) the datagram is forwarded without inserting the
  1543. timestamp, but the overflow count is incremented by one.
  1544.  
  1545. If there is some room but not enough room for a full timestamp
  1546. to be inserted, or the overflow count itself overflows, the
  1547. original datagram is considered to be in error and is discarded.
  1548. In either case an ICMP parameter problem message may be sent to
  1549. the source host [3].
  1550.  
  1551. The timestamp option is not copied upon fragmentation. It is
  1552. carried in the first fragment. Appears at most once in a
  1553. datagram.
  1554.  
  1555. Padding: variable
  1556.  
  1557. The internet header padding is used to ensure that the internet
  1558. header ends on a 32 bit boundary. The padding is zero.
  1559.  
  1560. 3.2. Discussion
  1561.  
  1562. The implementation of a protocol must be robust. Each implementation
  1563. must expect to interoperate with others created by different
  1564. individuals. While the goal of this specification is to be explicit
  1565. about the protocol there is the possibility of differing
  1566. interpretations. In general, an implementation must be conservative
  1567. in its sending behavior, and liberal in its receiving behavior. That
  1568. is, it must be careful to send well-formed datagrams, but must accept
  1569. any datagram that it can interpret (e.g., not object to technical
  1570. errors where the meaning is still clear).
  1571.  
  1572. The basic internet service is datagram oriented and provides for the
  1573. fragmentation of datagrams at gateways, with reassembly taking place
  1574. at the destination internet protocol module in the destination host.
  1575. Of course, fragmentation and reassembly of datagrams within a network
  1576. or by private agreement between the gateways of a network is also
  1577. allowed since this is transparent to the internet protocols and the
  1578. higher-level protocols. This transparent type of fragmentation and
  1579. reassembly is termed "network-dependent" (or intranet) fragmentation
  1580. and is not discussed further here.
  1581.  
  1582. Internet addresses distinguish sources and destinations to the host
  1583. level and provide a protocol field as well. It is assumed that each
  1584. protocol will provide for whatever multiplexing is necessary within a
  1585. host.
  1586.  
  1587.  
  1588. [Page 23]
  1590.  
  1591. September 1981
  1592. Internet Protocol
  1593. Specification
  1594.  
  1595.  
  1596.  
  1597. Addressing
  1598.  
  1599. To provide for flexibility in assigning address to networks and
  1600. allow for the large number of small to intermediate sized networks
  1601. the interpretation of the address field is coded to specify a small
  1602. number of networks with a large number of host, a moderate number of
  1603. networks with a moderate number of hosts, and a large number of
  1604. networks with a small number of hosts. In addition there is an
  1605. escape code for extended addressing mode.
  1606.  
  1607. Address Formats:
  1608.  
  1609. High Order Bits Format Class
  1610. --------------- ------------------------------- -----
  1611. 0 7 bits of net, 24 bits of host a
  1612. 10 14 bits of net, 16 bits of host b
  1613. 110 21 bits of net, 8 bits of host c
  1614. 111 escape to extended addressing mode
  1615.  
  1616. A value of zero in the network field means this network. This is
  1617. only used in certain ICMP messages. The extended addressing mode
  1618. is undefined. Both of these features are reserved for future use.
  1619.  
  1620. The actual values assigned for network addresses is given in
  1621. "Assigned Numbers" [9].
  1622.  
  1623. The local address, assigned by the local network, must allow for a
  1624. single physical host to act as several distinct internet hosts.
  1625. That is, there must be a mapping between internet host addresses and
  1626. network/host interfaces that allows several internet addresses to
  1627. correspond to one interface. It must also be allowed for a host to
  1628. have several physical interfaces and to treat the datagrams from
  1629. several of them as if they were all addressed to a single host.
  1630.  
  1631. Address mappings between internet addresses and addresses for
  1632. ARPANET, SATNET, PRNET, and other networks are described in "Address
  1633. Mappings" [5].
  1634.  
  1635. Fragmentation and Reassembly.
  1636.  
  1637. The internet identification field (ID) is used together with the
  1638. source and destination address, and the protocol fields, to identify
  1639. datagram fragments for reassembly.
  1640.  
  1641. The More Fragments flag bit (MF) is set if the datagram is not the
  1642. last fragment. The Fragment Offset field identifies the fragment
  1643. location, relative to the beginning of the original unfragmented
  1644. datagram. Fragments are counted in units of 8 octets. The
  1645.  
  1646.  
  1647. [Page 24]
  1649.  
  1650. September 1981
  1651. Internet Protocol
  1652. Specification
  1653.  
  1654.  
  1655.  
  1656. fragmentation strategy is designed so than an unfragmented datagram
  1657. has all zero fragmentation information (MF = 0, fragment offset =
  1658. 0). If an internet datagram is fragmented, its data portion must be
  1659. broken on 8 octet boundaries.
  1660.  
  1661. This format allows 2**13 = 8192 fragments of 8 octets each for a
  1662. total of 65,536 octets. Note that this is consistent with the the
  1663. datagram total length field (of course, the header is counted in the
  1664. total length and not in the fragments).
  1665.  
  1666. When fragmentation occurs, some options are copied, but others
  1667. remain with the first fragment only.
  1668.  
  1669. Every internet module must be able to forward a datagram of 68
  1670. octets without further fragmentation. This is because an internet
  1671. header may be up to 60 octets, and the minimum fragment is 8 octets.
  1672.  
  1673. Every internet destination must be able to receive a datagram of 576
  1674. octets either in one piece or in fragments to be reassembled.
  1675.  
  1676. The fields which may be affected by fragmentation include:
  1677.  
  1678. (1) options field
  1679. (2) more fragments flag
  1680. (3) fragment offset
  1681. (4) internet header length field
  1682. (5) total length field
  1683. (6) header checksum
  1684.  
  1685. If the Don't Fragment flag (DF) bit is set, then internet
  1686. fragmentation of this datagram is NOT permitted, although it may be
  1687. discarded. This can be used to prohibit fragmentation in cases
  1688. where the receiving host does not have sufficient resources to
  1689. reassemble internet fragments.
  1690.  
  1691. One example of use of the Don't Fragment feature is to down line
  1692. load a small host. A small host could have a boot strap program
  1693. that accepts a datagram stores it in memory and then executes it.
  1694.  
  1695. The fragmentation and reassembly procedures are most easily
  1696. described by examples. The following procedures are example
  1697. implementations.
  1698.  
  1699. General notation in the following pseudo programs: "=<" means "less
  1700. than or equal", "#" means "not equal", "=" means "equal", "<-" means
  1701. "is set to". Also, "x to y" includes x and excludes y; for example,
  1702. "4 to 7" would include 4, 5, and 6 (but not 7).
  1703.  
  1704.  
  1705.  
  1706. [Page 25]
  1708.  
  1709. September 1981
  1710. Internet Protocol
  1711. Specification
  1712.  
  1713.  
  1714.  
  1715. An Example Fragmentation Procedure
  1716.  
  1717. The maximum sized datagram that can be transmitted through the
  1718. next network is called the maximum transmission unit (MTU).
  1719.  
  1720. If the total length is less than or equal the maximum transmission
  1721. unit then submit this datagram to the next step in datagram
  1722. processing; otherwise cut the datagram into two fragments, the
  1723. first fragment being the maximum size, and the second fragment
  1724. being the rest of the datagram. The first fragment is submitted
  1725. to the next step in datagram processing, while the second fragment
  1726. is submitted to this procedure in case it is still too large.
  1727.  
  1728. Notation:
  1729.  
  1730. FO - Fragment Offset
  1731. IHL - Internet Header Length
  1732. DF - Don't Fragment flag
  1733. MF - More Fragments flag
  1734. TL - Total Length
  1735. OFO - Old Fragment Offset
  1736. OIHL - Old Internet Header Length
  1737. OMF - Old More Fragments flag
  1738. OTL - Old Total Length
  1739. NFB - Number of Fragment Blocks
  1740. MTU - Maximum Transmission Unit
  1741.  
  1742. Procedure:
  1743.  
  1744. IF TL =< MTU THEN Submit this datagram to the next step
  1745. in datagram processing ELSE IF DF = 1 THEN discard the
  1746. datagram ELSE
  1747. To produce the first fragment:
  1748. (1) Copy the original internet header;
  1749. (2) OIHL <- IHL; OTL <- TL; OFO <- FO; OMF <- MF;
  1750. (3) NFB <- (MTU-IHL*4)/8;
  1751. (4) Attach the first NFB*8 data octets;
  1752. (5) Correct the header:
  1753. MF <- 1; TL <- (IHL*4)+(NFB*8);
  1754. Recompute Checksum;
  1755. (6) Submit this fragment to the next step in
  1756. datagram processing;
  1757. To produce the second fragment:
  1758. (7) Selectively copy the internet header (some options
  1759. are not copied, see option definitions);
  1760. (8) Append the remaining data;
  1761. (9) Correct the header:
  1762. IHL <- (((OIHL*4)-(length of options not copied))+3)/4;
  1763.  
  1764.  
  1765. [Page 26]
  1767.  
  1768. September 1981
  1769. Internet Protocol
  1770. Specification
  1771.  
  1772.  
  1773.  
  1774. TL <- OTL - NFB*8 - (OIHL-IHL)*4);
  1775. FO <- OFO + NFB; MF <- OMF; Recompute Checksum;
  1776. (10) Submit this fragment to the fragmentation test; DONE.
  1777.  
  1778. In the above procedure each fragment (except the last) was made
  1779. the maximum allowable size. An alternative might produce less
  1780. than the maximum size datagrams. For example, one could implement
  1781. a fragmentation procedure that repeatly divided large datagrams in
  1782. half until the resulting fragments were less than the maximum
  1783. transmission unit size.
  1784.  
  1785. An Example Reassembly Procedure
  1786.  
  1787. For each datagram the buffer identifier is computed as the
  1788. concatenation of the source, destination, protocol, and
  1789. identification fields. If this is a whole datagram (that is both
  1790. the fragment offset and the more fragments fields are zero), then
  1791. any reassembly resources associated with this buffer identifier
  1792. are released and the datagram is forwarded to the next step in
  1793. datagram processing.
  1794.  
  1795. If no other fragment with this buffer identifier is on hand then
  1796. reassembly resources are allocated. The reassembly resources
  1797. consist of a data buffer, a header buffer, a fragment block bit
  1798. table, a total data length field, and a timer. The data from the
  1799. fragment is placed in the data buffer according to its fragment
  1800. offset and length, and bits are set in the fragment block bit
  1801. table corresponding to the fragment blocks received.
  1802.  
  1803. If this is the first fragment (that is the fragment offset is
  1804. zero) this header is placed in the header buffer. If this is the
  1805. last fragment ( that is the more fragments field is zero) the
  1806. total data length is computed. If this fragment completes the
  1807. datagram (tested by checking the bits set in the fragment block
  1808. table), then the datagram is sent to the next step in datagram
  1809. processing; otherwise the timer is set to the maximum of the
  1810. current timer value and the value of the time to live field from
  1811. this fragment; and the reassembly routine gives up control.
  1812.  
  1813. If the timer runs out, the all reassembly resources for this
  1814. buffer identifier are released. The initial setting of the timer
  1815. is a lower bound on the reassembly waiting time. This is because
  1816. the waiting time will be increased if the Time to Live in the
  1817. arriving fragment is greater than the current timer value but will
  1818. not be decreased if it is less. The maximum this timer value
  1819. could reach is the maximum time to live (approximately 4.25
  1820. minutes). The current recommendation for the initial timer
  1821. setting is 15 seconds. This may be changed as experience with
  1822.  
  1823.  
  1824. [Page 27]
  1826.  
  1827. September 1981
  1828. Internet Protocol
  1829. Specification
  1830.  
  1831.  
  1832.  
  1833. this protocol accumulates. Note that the choice of this parameter
  1834. value is related to the buffer capacity available and the data
  1835. rate of the transmission medium; that is, data rate times timer
  1836. value equals buffer size (e.g., 10Kb/s X 15s = 150Kb).
  1837.  
  1838. Notation:
  1839.  
  1840. FO - Fragment Offset
  1841. IHL - Internet Header Length
  1842. MF - More Fragments flag
  1843. TTL - Time To Live
  1844. NFB - Number of Fragment Blocks
  1845. TL - Total Length
  1846. TDL - Total Data Length
  1847. BUFID - Buffer Identifier
  1848. RCVBT - Fragment Received Bit Table
  1849. TLB - Timer Lower Bound
  1850.  
  1851. Procedure:
  1852.  
  1853. (1) BUFID <- source|destination|protocol|identification;
  1854. (2) IF FO = 0 AND MF = 0
  1855. (3) THEN IF buffer with BUFID is allocated
  1856. (4) THEN flush all reassembly for this BUFID;
  1857. (5) Submit datagram to next step; DONE.
  1858. (6) ELSE IF no buffer with BUFID is allocated
  1859. (7) THEN allocate reassembly resources
  1860. with BUFID;
  1861. TIMER <- TLB; TDL <- 0;
  1862. (8) put data from fragment into data buffer with
  1863. BUFID from octet FO*8 to
  1864. octet (TL-(IHL*4))+FO*8;
  1865. (9) set RCVBT bits from FO
  1866. to FO+((TL-(IHL*4)+7)/8);
  1867. (10) IF MF = 0 THEN TDL <- TL-(IHL*4)+(FO*8)
  1868. (11) IF FO = 0 THEN put header in header buffer
  1869. (12) IF TDL # 0
  1870. (13) AND all RCVBT bits from 0
  1871. to (TDL+7)/8 are set
  1872. (14) THEN TL <- TDL+(IHL*4)
  1873. (15) Submit datagram to next step;
  1874. (16) free all reassembly resources
  1875. for this BUFID; DONE.
  1876. (17) TIMER <- MAX(TIMER,TTL);
  1877. (18) give up until next fragment or timer expires;
  1878. (19) timer expires: flush all reassembly with this BUFID; DONE.
  1879.  
  1880. In the case that two or more fragments contain the same data
  1881.  
  1882.  
  1883. [Page 28]
  1885.  
  1886. September 1981
  1887. Internet Protocol
  1888. Specification
  1889.  
  1890.  
  1891.  
  1892. either identically or through a partial overlap, this procedure
  1893. will use the more recently arrived copy in the data buffer and
  1894. datagram delivered.
  1895.  
  1896. Identification
  1897.  
  1898. The choice of the Identifier for a datagram is based on the need to
  1899. provide a way to uniquely identify the fragments of a particular
  1900. datagram. The protocol module assembling fragments judges fragments
  1901. to belong to the same datagram if they have the same source,
  1902. destination, protocol, and Identifier. Thus, the sender must choose
  1903. the Identifier to be unique for this source, destination pair and
  1904. protocol for the time the datagram (or any fragment of it) could be
  1905. alive in the internet.
  1906.  
  1907. It seems then that a sending protocol module needs to keep a table
  1908. of Identifiers, one entry for each destination it has communicated
  1909. with in the last maximum packet lifetime for the internet.
  1910.  
  1911. However, since the Identifier field allows 65,536 different values,
  1912. some host may be able to simply use unique identifiers independent
  1913. of destination.
  1914.  
  1915. It is appropriate for some higher level protocols to choose the
  1916. identifier. For example, TCP protocol modules may retransmit an
  1917. identical TCP segment, and the probability for correct reception
  1918. would be enhanced if the retransmission carried the same identifier
  1919. as the original transmission since fragments of either datagram
  1920. could be used to construct a correct TCP segment.
  1921.  
  1922. Type of Service
  1923.  
  1924. The type of service (TOS) is for internet service quality selection.
  1925. The type of service is specified along the abstract parameters
  1926. precedence, delay, throughput, and reliability. These abstract
  1927. parameters are to be mapped into the actual service parameters of
  1928. the particular networks the datagram traverses.
  1929.  
  1930. Precedence. An independent measure of the importance of this
  1931. datagram.
  1932.  
  1933. Delay. Prompt delivery is important for datagrams with this
  1934. indication.
  1935.  
  1936. Throughput. High data rate is important for datagrams with this
  1937. indication.
  1938.  
  1939.  
  1940.  
  1941.  
  1942. [Page 29]
  1944.  
  1945. September 1981
  1946. Internet Protocol
  1947. Specification
  1948.  
  1949.  
  1950.  
  1951. Reliability. A higher level of effort to ensure delivery is
  1952. important for datagrams with this indication.
  1953.  
  1954. For example, the ARPANET has a priority bit, and a choice between
  1955. "standard" messages (type 0) and "uncontrolled" messages (type 3),
  1956. (the choice between single packet and multipacket messages can also
  1957. be considered a service parameter). The uncontrolled messages tend
  1958. to be less reliably delivered and suffer less delay. Suppose an
  1959. internet datagram is to be sent through the ARPANET. Let the
  1960. internet type of service be given as:
  1961.  
  1962. Precedence: 5
  1963. Delay: 0
  1964. Throughput: 1
  1965. Reliability: 1
  1966.  
  1967. In this example, the mapping of these parameters to those available
  1968. for the ARPANET would be to set the ARPANET priority bit on since
  1969. the Internet precedence is in the upper half of its range, to select
  1970. standard messages since the throughput and reliability requirements
  1971. are indicated and delay is not. More details are given on service
  1972. mappings in "Service Mappings" [8].
  1973.  
  1974. Time to Live
  1975.  
  1976. The time to live is set by the sender to the maximum time the
  1977. datagram is allowed to be in the internet system. If the datagram
  1978. is in the internet system longer than the time to live, then the
  1979. datagram must be destroyed.
  1980.  
  1981. This field must be decreased at each point that the internet header
  1982. is processed to reflect the time spent processing the datagram.
  1983. Even if no local information is available on the time actually
  1984. spent, the field must be decremented by 1. The time is measured in
  1985. units of seconds (i.e. the value 1 means one second). Thus, the
  1986. maximum time to live is 255 seconds or 4.25 minutes. Since every
  1987. module that processes a datagram must decrease the TTL by at least
  1988. one even if it process the datagram in less than a second, the TTL
  1989. must be thought of only as an upper bound on the time a datagram may
  1990. exist. The intention is to cause undeliverable datagrams to be
  1991. discarded, and to bound the maximum datagram lifetime.
  1992.  
  1993. Some higher level reliable connection protocols are based on
  1994. assumptions that old duplicate datagrams will not arrive after a
  1995. certain time elapses. The TTL is a way for such protocols to have
  1996. an assurance that their assumption is met.
  1997.  
  1998.  
  1999.  
  2000.  
  2001. [Page 30]
  2003.  
  2004. September 1981
  2005. Internet Protocol
  2006. Specification
  2007.  
  2008.  
  2009.  
  2010. Options
  2011.  
  2012. The options are optional in each datagram, but required in
  2013. implementations. That is, the presence or absence of an option is
  2014. the choice of the sender, but each internet module must be able to
  2015. parse every option. There can be several options present in the
  2016. option field.
  2017.  
  2018. The options might not end on a 32-bit boundary. The internet header
  2019. must be filled out with octets of zeros. The first of these would
  2020. be interpreted as the end-of-options option, and the remainder as
  2021. internet header padding.
  2022.  
  2023. Every internet module must be able to act on every option. The
  2024. Security Option is required if classified, restricted, or
  2025. compartmented traffic is to be passed.
  2026.  
  2027. Checksum
  2028.  
  2029. The internet header checksum is recomputed if the internet header is
  2030. changed. For example, a reduction of the time to live, additions or
  2031. changes to internet options, or due to fragmentation. This checksum
  2032. at the internet level is intended to protect the internet header
  2033. fields from transmission errors.
  2034.  
  2035. There are some applications where a few data bit errors are
  2036. acceptable while retransmission delays are not. If the internet
  2037. protocol enforced data correctness such applications could not be
  2038. supported.
  2039.  
  2040. Errors
  2041.  
  2042. Internet protocol errors may be reported via the ICMP messages [3].
  2043.  
  2044. 3.3. Interfaces
  2045.  
  2046. The functional description of user interfaces to the IP is, at best,
  2047. fictional, since every operating system will have different
  2048. facilities. Consequently, we must warn readers that different IP
  2049. implementations may have different user interfaces. However, all IPs
  2050. must provide a certain minimum set of services to guarantee that all
  2051. IP implementations can support the same protocol hierarchy. This
  2052. section specifies the functional interfaces required of all IP
  2053. implementations.
  2054.  
  2055. Internet protocol interfaces on one side to the local network and on
  2056. the other side to either a higher level protocol or an application
  2057. program. In the following, the higher level protocol or application
  2058.  
  2059.  
  2060. [Page 31]
  2062.  
  2063. September 1981
  2064. Internet Protocol
  2065. Specification
  2066.  
  2067.  
  2068.  
  2069. program (or even a gateway program) will be called the "user" since it
  2070. is using the internet module. Since internet protocol is a datagram
  2071. protocol, there is minimal memory or state maintained between datagram
  2072. transmissions, and each call on the internet protocol module by the
  2073. user supplies all information necessary for the IP to perform the
  2074. service requested.
  2075.  
  2076. An Example Upper Level Interface
  2077.  
  2078. The following two example calls satisfy the requirements for the user
  2079. to internet protocol module communication ("=>" means returns):
  2080.  
  2081. SEND (src, dst, prot, TOS, TTL, BufPTR, len, Id, DF, opt => result)
  2082.  
  2083. where:
  2084.  
  2085. src = source address
  2086. dst = destination address
  2087. prot = protocol
  2088. TOS = type of service
  2089. TTL = time to live
  2090. BufPTR = buffer pointer
  2091. len = length of buffer
  2092. Id = Identifier
  2093. DF = Don't Fragment
  2094. opt = option data
  2095. result = response
  2096. OK = datagram sent ok
  2097. Error = error in arguments or local network error
  2098.  
  2099. Note that the precedence is included in the TOS and the
  2100. security/compartment is passed as an option.
  2101.  
  2102. RECV (BufPTR, prot, => result, src, dst, TOS, len, opt)
  2103.  
  2104. where:
  2105.  
  2106. BufPTR = buffer pointer
  2107. prot = protocol
  2108. result = response
  2109. OK = datagram received ok
  2110. Error = error in arguments
  2111. len = length of buffer
  2112. src = source address
  2113. dst = destination address
  2114. TOS = type of service
  2115. opt = option data
  2116.  
  2117.  
  2118.  
  2119. [Page 32]
  2121.  
  2122. September 1981
  2123. Internet Protocol
  2124. Specification
  2125.  
  2126.  
  2127.  
  2128. When the user sends a datagram, it executes the SEND call supplying
  2129. all the arguments. The internet protocol module, on receiving this
  2130. call, checks the arguments and prepares and sends the message. If the
  2131. arguments are good and the datagram is accepted by the local network,
  2132. the call returns successfully. If either the arguments are bad, or
  2133. the datagram is not accepted by the local network, the call returns
  2134. unsuccessfully. On unsuccessful returns, a reasonable report must be
  2135. made as to the cause of the problem, but the details of such reports
  2136. are up to individual implementations.
  2137.  
  2138. When a datagram arrives at the internet protocol module from the local
  2139. network, either there is a pending RECV call from the user addressed
  2140. or there is not. In the first case, the pending call is satisfied by
  2141. passing the information from the datagram to the user. In the second
  2142. case, the user addressed is notified of a pending datagram. If the
  2143. user addressed does not exist, an ICMP error message is returned to
  2144. the sender, and the data is discarded.
  2145.  
  2146. The notification of a user may be via a pseudo interrupt or similar
  2147. mechanism, as appropriate in the particular operating system
  2148. environment of the implementation.
  2149.  
  2150. A user's RECV call may then either be immediately satisfied by a
  2151. pending datagram, or the call may be pending until a datagram arrives.
  2152.  
  2153. The source address is included in the send call in case the sending
  2154. host has several addresses (multiple physical connections or logical
  2155. addresses). The internet module must check to see that the source
  2156. address is one of the legal address for this host.
  2157.  
  2158. An implementation may also allow or require a call to the internet
  2159. module to indicate interest in or reserve exclusive use of a class of
  2160. datagrams (e.g., all those with a certain value in the protocol
  2161. field).
  2162.  
  2163. This section functionally characterizes a USER/IP interface. The
  2164. notation used is similar to most procedure of function calls in high
  2165. level languages, but this usage is not meant to rule out trap type
  2166. service calls (e.g., SVCs, UUOs, EMTs), or any other form of
  2167. interprocess communication.
  2168.  
  2169.  
  2170.  
  2171.  
  2172.  
  2173.  
  2174.  
  2175.  
  2176.  
  2177.  
  2178. [Page 33]
  2180.  
  2181. September 1981
  2182. Internet Protocol
  2183.  
  2184.  
  2185.  
  2186. APPENDIX A: Examples & Scenarios
  2187.  
  2188. Example 1:
  2189.  
  2190. This is an example of the minimal data carrying internet datagram:
  2191.  
  2192.  
  2193. 0 1 2 3
  2194. 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
  2195. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2196. |Ver= 4 |IHL= 5 |Type of Service| Total Length = 21 |
  2197. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2198. | Identification = 111 |Flg=0| Fragment Offset = 0 |
  2199. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2200. | Time = 123 | Protocol = 1 | header checksum |
  2201. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2202. | source address |
  2203. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2204. | destination address |
  2205. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2206. | data |
  2207. +-+-+-+-+-+-+-+-+
  2208.  
  2209. Example Internet Datagram
  2210.  
  2211. Figure 5.
  2212.  
  2213. Note that each tick mark represents one bit position.
  2214.  
  2215. This is a internet datagram in version 4 of internet protocol; the
  2216. internet header consists of five 32 bit words, and the total length of
  2217. the datagram is 21 octets. This datagram is a complete datagram (not
  2218. a fragment).
  2219.  
  2220.  
  2221.  
  2222.  
  2223.  
  2224.  
  2225.  
  2226.  
  2227.  
  2228.  
  2229.  
  2230.  
  2231.  
  2232.  
  2233.  
  2234.  
  2235.  
  2236.  
  2237. [Page 34]
  2239.  
  2240. September 1981
  2241. Internet Protocol
  2242.  
  2243.  
  2244.  
  2245. Example 2:
  2246.  
  2247. In this example, we show first a moderate size internet datagram (452
  2248. data octets), then two internet fragments that might result from the
  2249. fragmentation of this datagram if the maximum sized transmission
  2250. allowed were 280 octets.
  2251.  
  2252.  
  2253. 0 1 2 3
  2254. 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
  2255. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2256. |Ver= 4 |IHL= 5 |Type of Service| Total Length = 472 |
  2257. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2258. | Identification = 111 |Flg=0| Fragment Offset = 0 |
  2259. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2260. | Time = 123 | Protocol = 6 | header checksum |
  2261. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2262. | source address |
  2263. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2264. | destination address |
  2265. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2266. | data |
  2267. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2268. | data |
  2269. \ \
  2270. \ \
  2271. | data |
  2272. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2273. | data |
  2274. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2275.  
  2276. Example Internet Datagram
  2277.  
  2278. Figure 6.
  2279.  
  2280.  
  2281.  
  2282.  
  2283.  
  2284.  
  2285.  
  2286.  
  2287.  
  2288.  
  2289.  
  2290.  
  2291.  
  2292.  
  2293.  
  2294.  
  2295.  
  2296. [Page 35]
  2298.  
  2299. September 1981
  2300. Internet Protocol
  2301.  
  2302.  
  2303.  
  2304. Now the first fragment that results from splitting the datagram after
  2305. 256 data octets.
  2306.  
  2307.  
  2308. 0 1 2 3
  2309. 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
  2310. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2311. |Ver= 4 |IHL= 5 |Type of Service| Total Length = 276 |
  2312. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2313. | Identification = 111 |Flg=1| Fragment Offset = 0 |
  2314. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2315. | Time = 119 | Protocol = 6 | Header Checksum |
  2316. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2317. | source address |
  2318. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2319. | destination address |
  2320. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2321. | data |
  2322. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2323. | data |
  2324. \ \
  2325. \ \
  2326. | data |
  2327. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2328. | data |
  2329. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2330.  
  2331. Example Internet Fragment
  2332.  
  2333. Figure 7.
  2334.  
  2335.  
  2336.  
  2337.  
  2338.  
  2339.  
  2340.  
  2341.  
  2342.  
  2343.  
  2344.  
  2345.  
  2346.  
  2347.  
  2348.  
  2349.  
  2350.  
  2351.  
  2352.  
  2353.  
  2354.  
  2355. [Page 36]
  2357.  
  2358. September 1981
  2359. Internet Protocol
  2360.  
  2361.  
  2362.  
  2363. And the second fragment.
  2364.  
  2365.  
  2366. 0 1 2 3
  2367. 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
  2368. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2369. |Ver= 4 |IHL= 5 |Type of Service| Total Length = 216 |
  2370. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2371. | Identification = 111 |Flg=0| Fragment Offset = 32 |
  2372. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2373. | Time = 119 | Protocol = 6 | Header Checksum |
  2374. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2375. | source address |
  2376. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2377. | destination address |
  2378. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2379. | data |
  2380. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2381. | data |
  2382. \ \
  2383. \ \
  2384. | data |
  2385. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2386. | data |
  2387. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2388.  
  2389. Example Internet Fragment
  2390.  
  2391. Figure 8.
  2392.  
  2393.  
  2394.  
  2395.  
  2396.  
  2397.  
  2398.  
  2399.  
  2400.  
  2401.  
  2402.  
  2403.  
  2404.  
  2405.  
  2406.  
  2407.  
  2408.  
  2409.  
  2410.  
  2411.  
  2412.  
  2413.  
  2414. [Page 37]
  2416.  
  2417. September 1981
  2418. Internet Protocol
  2419.  
  2420.  
  2421.  
  2422. Example 3:
  2423.  
  2424. Here, we show an example of a datagram containing options:
  2425.  
  2426.  
  2427. 0 1 2 3
  2428. 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
  2429. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2430. |Ver= 4 |IHL= 8 |Type of Service| Total Length = 576 |
  2431. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2432. | Identification = 111 |Flg=0| Fragment Offset = 0 |
  2433. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2434. | Time = 123 | Protocol = 6 | Header Checksum |
  2435. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2436. | source address |
  2437. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2438. | destination address |
  2439. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2440. | Opt. Code = x | Opt. Len.= 3 | option value | Opt. Code = x |
  2441. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2442. | Opt. Len. = 4 | option value | Opt. Code = 1 |
  2443. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2444. | Opt. Code = y | Opt. Len. = 3 | option value | Opt. Code = 0 |
  2445. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2446. | data |
  2447. \ \
  2448. \ \
  2449. | data |
  2450. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2451. | data |
  2452. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2453.  
  2454. Example Internet Datagram
  2455.  
  2456. Figure 9.
  2457.  
  2458.  
  2459.  
  2460.  
  2461.  
  2462.  
  2463.  
  2464.  
  2465.  
  2466.  
  2467.  
  2468.  
  2469.  
  2470.  
  2471.  
  2472.  
  2473. [Page 38]
  2475.  
  2476. September 1981
  2477. Internet Protocol
  2478.  
  2479.  
  2480.  
  2481. APPENDIX B: Data Transmission Order
  2482.  
  2483. The order of transmission of the header and data described in this
  2484. document is resolved to the octet level. Whenever a diagram shows a
  2485. group of octets, the order of transmission of those octets is the normal
  2486. order in which they are read in English. For example, in the following
  2487. diagram the octets are transmitted in the order they are numbered.
  2488.  
  2489.  
  2490. 0 1 2 3
  2491. 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
  2492. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2493. | 1 | 2 | 3 | 4 |
  2494. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2495. | 5 | 6 | 7 | 8 |
  2496. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2497. | 9 | 10 | 11 | 12 |
  2498. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  2499.  
  2500. Transmission Order of Bytes
  2501.  
  2502. Figure 10.
  2503.  
  2504. Whenever an octet represents a numeric quantity the left most bit in the
  2505. diagram is the high order or most significant bit. That is, the bit
  2506. labeled 0 is the most significant bit. For example, the following
  2507. diagram represents the value 170 (decimal).
  2508.  
  2509.  
  2510. 0 1 2 3 4 5 6 7
  2511. +-+-+-+-+-+-+-+-+
  2512. |1 0 1 0 1 0 1 0|
  2513. +-+-+-+-+-+-+-+-+
  2514.  
  2515. Significance of Bits
  2516.  
  2517. Figure 11.
  2518.  
  2519. Similarly, whenever a multi-octet field represents a numeric quantity
  2520. the left most bit of the whole field is the most significant bit. When
  2521. a multi-octet quantity is transmitted the most significant octet is
  2522. transmitted first.
  2523.  
  2524.  
  2525.  
  2526.  
  2527.  
  2528.  
  2529.  
  2530.  
  2531.  
  2532. [Page 39]
  2534.  
  2535. September 1981
  2536. Internet Protocol
  2537.  
  2538.  
  2539.  
  2540.  
  2541.  
  2542.  
  2543.  
  2544.  
  2545.  
  2546.  
  2547.  
  2548.  
  2549.  
  2550.  
  2551.  
  2552.  
  2553.  
  2554.  
  2555.  
  2556.  
  2557.  
  2558.  
  2559.  
  2560.  
  2561.  
  2562.  
  2563.  
  2564.  
  2565.  
  2566.  
  2567.  
  2568.  
  2569.  
  2570.  
  2571.  
  2572.  
  2573.  
  2574.  
  2575.  
  2576.  
  2577.  
  2578.  
  2579.  
  2580.  
  2581.  
  2582.  
  2583.  
  2584.  
  2585.  
  2586.  
  2587.  
  2588.  
  2589.  
  2590.  
  2591. [Page 40]
  2593.  
  2594. September 1981
  2595. Internet Protocol
  2596.  
  2597.  
  2598.  
  2599. GLOSSARY
  2600.  
  2601.  
  2602.  
  2603. 1822
  2604. BBN Report 1822, "The Specification of the Interconnection of
  2605. a Host and an IMP". The specification of interface between a
  2606. host and the ARPANET.
  2607.  
  2608. ARPANET leader
  2609. The control information on an ARPANET message at the host-IMP
  2610. interface.
  2611.  
  2612. ARPANET message
  2613. The unit of transmission between a host and an IMP in the
  2614. ARPANET. The maximum size is about 1012 octets (8096 bits).
  2615.  
  2616. ARPANET packet
  2617. A unit of transmission used internally in the ARPANET between
  2618. IMPs. The maximum size is about 126 octets (1008 bits).
  2619.  
  2620. Destination
  2621. The destination address, an internet header field.
  2622.  
  2623. DF
  2624. The Don't Fragment bit carried in the flags field.
  2625.  
  2626. Flags
  2627. An internet header field carrying various control flags.
  2628.  
  2629. Fragment Offset
  2630. This internet header field indicates where in the internet
  2631. datagram a fragment belongs.
  2632.  
  2633. GGP
  2634. Gateway to Gateway Protocol, the protocol used primarily
  2635. between gateways to control routing and other gateway
  2636. functions.
  2637.  
  2638. header
  2639. Control information at the beginning of a message, segment,
  2640. datagram, packet or block of data.
  2641.  
  2642. ICMP
  2643. Internet Control Message Protocol, implemented in the internet
  2644. module, the ICMP is used from gateways to hosts and between
  2645. hosts to report errors and make routing suggestions.
  2646.  
  2647.  
  2648.  
  2649.  
  2650. [Page 41]
  2652.  
  2653. September 1981
  2654. Internet Protocol
  2655. Glossary
  2656.  
  2657.  
  2658.  
  2659. Identification
  2660. An internet header field carrying the identifying value
  2661. assigned by the sender to aid in assembling the fragments of a
  2662. datagram.
  2663.  
  2664. IHL
  2665. The internet header field Internet Header Length is the length
  2666. of the internet header measured in 32 bit words.
  2667.  
  2668. IMP
  2669. The Interface Message Processor, the packet switch of the
  2670. ARPANET.
  2671.  
  2672. Internet Address
  2673. A four octet (32 bit) source or destination address consisting
  2674. of a Network field and a Local Address field.
  2675.  
  2676. internet datagram
  2677. The unit of data exchanged between a pair of internet modules
  2678. (includes the internet header).
  2679.  
  2680. internet fragment
  2681. A portion of the data of an internet datagram with an internet
  2682. header.
  2683.  
  2684. Local Address
  2685. The address of a host within a network. The actual mapping of
  2686. an internet local address on to the host addresses in a
  2687. network is quite general, allowing for many to one mappings.
  2688.  
  2689. MF
  2690. The More-Fragments Flag carried in the internet header flags
  2691. field.
  2692.  
  2693. module
  2694. An implementation, usually in software, of a protocol or other
  2695. procedure.
  2696.  
  2697. more-fragments flag
  2698. A flag indicating whether or not this internet datagram
  2699. contains the end of an internet datagram, carried in the
  2700. internet header Flags field.
  2701.  
  2702. NFB
  2703. The Number of Fragment Blocks in a the data portion of an
  2704. internet fragment. That is, the length of a portion of data
  2705. measured in 8 octet units.
  2706.  
  2707.  
  2708.  
  2709. [Page 42]
  2711.  
  2712. September 1981
  2713. Internet Protocol
  2714. Glossary
  2715.  
  2716.  
  2717.  
  2718. octet
  2719. An eight bit byte.
  2720.  
  2721. Options
  2722. The internet header Options field may contain several options,
  2723. and each option may be several octets in length.
  2724.  
  2725. Padding
  2726. The internet header Padding field is used to ensure that the
  2727. data begins on 32 bit word boundary. The padding is zero.
  2728.  
  2729. Protocol
  2730. In this document, the next higher level protocol identifier,
  2731. an internet header field.
  2732.  
  2733. Rest
  2734. The local address portion of an Internet Address.
  2735.  
  2736. Source
  2737. The source address, an internet header field.
  2738.  
  2739. TCP
  2740. Transmission Control Protocol: A host-to-host protocol for
  2741. reliable communication in internet environments.
  2742.  
  2743. TCP Segment
  2744. The unit of data exchanged between TCP modules (including the
  2745. TCP header).
  2746.  
  2747. TFTP
  2748. Trivial File Transfer Protocol: A simple file transfer
  2749. protocol built on UDP.
  2750.  
  2751. Time to Live
  2752. An internet header field which indicates the upper bound on
  2753. how long this internet datagram may exist.
  2754.  
  2755. TOS
  2756. Type of Service
  2757.  
  2758. Total Length
  2759. The internet header field Total Length is the length of the
  2760. datagram in octets including internet header and data.
  2761.  
  2762. TTL
  2763. Time to Live
  2764.  
  2765.  
  2766.  
  2767.  
  2768. [Page 43]
  2770.  
  2771. September 1981
  2772. Internet Protocol
  2773. Glossary
  2774.  
  2775.  
  2776.  
  2777. Type of Service
  2778. An internet header field which indicates the type (or quality)
  2779. of service for this internet datagram.
  2780.  
  2781. UDP
  2782. User Datagram Protocol: A user level protocol for transaction
  2783. oriented applications.
  2784.  
  2785. User
  2786. The user of the internet protocol. This may be a higher level
  2787. protocol module, an application program, or a gateway program.
  2788.  
  2789. Version
  2790. The Version field indicates the format of the internet header.
  2791.  
  2792.  
  2793.  
  2794.  
  2795.  
  2796.  
  2797.  
  2798.  
  2799.  
  2800.  
  2801.  
  2802.  
  2803.  
  2804.  
  2805.  
  2806.  
  2807.  
  2808.  
  2809.  
  2810.  
  2811.  
  2812.  
  2813.  
  2814.  
  2815.  
  2816.  
  2817.  
  2818.  
  2819.  
  2820.  
  2821.  
  2822.  
  2823.  
  2824.  
  2825.  
  2826.  
  2827. [Page 44]
  2829.  
  2830. September 1981
  2831. Internet Protocol
  2832.  
  2833.  
  2834.  
  2835. REFERENCES
  2836.  
  2837.  
  2838.  
  2839. [1] Cerf, V., "The Catenet Model for Internetworking," Information
  2840. Processing Techniques Office, Defense Advanced Research Projects
  2841. Agency, IEN 48, July 1978.
  2842.  
  2843. [2] Bolt Beranek and Newman, "Specification for the Interconnection of
  2844. a Host and an IMP," BBN Technical Report 1822, Revised May 1978.
  2845.  
  2846. [3] Postel, J., "Internet Control Message Protocol - DARPA Internet
  2847. Program Protocol Specification," RFC 792, USC/Information Sciences
  2848. Institute, September 1981.
  2849.  
  2850. [4] Shoch, J., "Inter-Network Naming, Addressing, and Routing,"
  2851. COMPCON, IEEE Computer Society, Fall 1978.
  2852.  
  2853. [5] Postel, J., "Address Mappings," RFC 796, USC/Information Sciences
  2854. Institute, September 1981.
  2855.  
  2856. [6] Shoch, J., "Packet Fragmentation in Inter-Network Protocols,"
  2857. Computer Networks, v. 3, n. 1, February 1979.
  2858.  
  2859. [7] Strazisar, V., "How to Build a Gateway", IEN 109, Bolt Beranek and
  2860. Newman, August 1979.
  2861.  
  2862. [8] Postel, J., "Service Mappings," RFC 795, USC/Information Sciences
  2863. Institute, September 1981.
  2864.  
  2865. [9] Postel, J., "Assigned Numbers," RFC 790, USC/Information Sciences
  2866. Institute, September 1981.
  2867.  
  2868.  
  2869.  
  2870.  
  2871.  
  2872.  
  2873.  
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