DOD STANDARD INTERNET PROTOCOL January 1980 prepared for Defense Advanced Research Projects Agency Information Processing Techniques Office 1400 Wilson Boulevard Arlington, Virginia 22209 by Information Sciences Institute University of Southern California 4676 Admiralty Way Marina del Rey, California 90291 12
`
`Page 1 of 40
`
`SAMSUNG EXHIBIT 1028
`
`

`

`January 1980 Internet Protocol PREFACE This document specifies the DoD Standard Internet Protocol. This document is based on five earlier editions of the ARPA Internet Protocol Specification, and the present text draws heavily from them. There have been many contributors to this work both in terms of concepts and in terms of text. This edition revises the details security, compartmentation, and precedence features of the internet protocol. Jon Postel Editor 13
`
`Page 2 of 40
`
`

`

`January 1980 RFC: 760 IEN: i28 Replaces: IENs 123~ !Ii~ 80, 54~ 44, 41, 28, 26 DOD STANDARD INTERNET PROTOCOL i~ INTRODUCTION I.I. Motivation j The Internet Protocol is designed for use in interconnected systems of packet-switched ~,.~r~ ..... ~ communication networks. Such a system has been called a ~'catenet ~' [I]. The [nternet protocol provides for transmitting blocks of data called datag~ams from sources to destinations~ where sources and destinations are hosts identified by fixed length addresses~ The internet protocol also provides for fragmentation and reassembiy of long datagrams, if necessary, for transmission through ~'small packet ~' networks° 1,,2, Scope The internet protocol is spec[fica!ly limited in scope to provide the functions necessary to deliver a package of bits {an internet datagram) from a source to a destination over an interconnected system of networks. There are no mechanisms to promote data reliabiiity~ flow control, sequencing~ or other services commonly found in host-to-host protocols~ 1.3. Interfaces This protocol :is called on by host-to-host protocols in an internet environment, This protocol calls on local netwock protocols to carry the internet datagram to the next gateway or destination host, For example, a TCP module would call on the inte~n~t module to take a ! TCP segment (including the TCP h=ad=r and user data) as the data o ~.~? module would provide the portion of an internet datagram The '~ addresses and other parameters :in the internet header to the internet j module as arguments of the call, The internet module would then create an internet datagram and call on the local network interface to transmit the internet datagram. In the ARPANET caset for example, the internet module would call on a local net module which would add the 1822 leader [2] to the internet datagram creating an ARPANET message to transmit to the IMP. The ARPANET address would be derived from the internet address by the local network interface and would be the address of some host in the ARPANET, that host might be a gateway to other networks. 14
`
`Page 3 of 40
`
`

`

`Internet Protocol Introduction January 1980 1o4o Operation The internet protocol implements two basic functions: fragmentation° addressing and The internet modules use the addresses carried in the internet header to transmit internet datagrams toward their destinations. The selection of a path for transmission is called routing. The internet modules use fields in the internet header to fragment and reassemble internet datagrams when necessary for transmission through ~'small packet" networks. The model of operation is that an internet module resides in each host engaged in internet communication and in each gateway that interconnects networks. These modules share common rules for interpreting address fields and for fragmenting and assembling internet datagrams, in addition, these modules (especially in gateways) may have procedures for making routing decisions and other functions. The internet protocol treats each internet datagram as an independent entity unrelated to any other internet datagram. There are no connections or logical circuits (virtual or otherwise). The internet protocol uses four key mechanisms in providing its service: Type of Service, Time to Live, Options, and Header Checksum. The Type of Service is used to indicate the quality of the service desired; this may be thought of as selecting among Interactivew Bulk, or Real Time, for example. The type of service is an abstract or generalized set of parameters which characterize the service choices provided in the networks that make up the internet. This type of service indication is to be used by gateways to select the actual transmission parameters for a particular network, the network to be used for the next hop, or the next gateway when routing an internet datagramo The Time to Live is an indication of the lifetime of an internet datagram. It is set by the sender of the datagram and reduced at the points along the route where it is processed. If the time to live reaches zero before the internet datagram reaches its destinations the internet datagram is destroyed. The time to live can be thought of as a self destruct time limit. The Options provide for control, functions needed or useful in some situations but unnecessary for the most common communications. The 15
`
`Page 4 of 40
`
`

`

`January 1980 Internet Protocol i Introduction options include provisions for timestamps, error reports~ and special routing. The Header Checksum provides a verification that the information used in processing internet datagram has been transmitted correctly° The data may contain errors. If the header checksum fails, the internet datagram is discarded at once by the entity which detects the error. The internet protocol does not provide a reliable communication facility. There are no acknowledgments either end-to-end or hop-by-hop. There is no error control for data, only a header checksum. There are no retransmissions. There is no flow control. 16
`
`Page 5 of 40
`
`

`

`January 1980 Internet Protocol 2. OVERVIEW 2.1. Relation to Other Protocols The following @iagram illustrates the place of the internet protocol in the protocol hierarchy: ITelnetl [ FTP I Ivoicel -.. I I I 1 1 1 4 ~- ÷ ÷ + ..... + I TcPl I RTPI ... 1 1 I I I 4 I- I Internet Protocol I 4 ......... + I I Local Network Protocol I Protocol Relationships Figure i. Internet protocol interfaces on one side to the higher level host-to-host protocols and on the other side to the local network protocol. 2.2. Model of Operation The model of operation for transmitting a datagram from one application program to another is illustrated by the following scenario: We suppose that this transmission will involve one intermediate gateway. The sending application program prepares its data and calls on its local internet module to send that data as a datagram and passes t} destination address and other parameters as arguments of the call. The internet module prepares a datagram header and attaches the dal 17
`
`Page 6 of 40
`
`

`

`Internet Protocol Overview January 1980 to ito The internet module determines a local network address for this internet address~ in this case it is the address of a gateway~ It sends this datagram and the local network address to the local network interface° The local network interface creates a local network header~ and attaches the datagram to its then sends the result via the local network. The datagram arrives at a gateway host wrapped in the local network headers the local network interface strips off this header~ and turns the datagram over to the internet module~ The internet module determines from the internet address that the datagram should be forwarded to another host in a second network. The internet module determines a local net address for the destination host. It calls on the local network interface for that network to send the datagram~ This local network interface creates a local network header and attaches the datagram sending the result to the destination host° At this destination host the datagram is stripped of the local net header by the local network interface and handed to the internet module~ The internet module determines that the datagram is for an application program in this host. It passes the data to the application program in response to a system calls passing the source address and other parameters as results of the cali~ Application Program \ Internet Module \ LNI-i \ Local Network 1 Application Program / Internet Module Internet Module / \ / LNI-I LNI-2 LNI-2 / \ / Local Network 2 Transmission Path Figure 2 18
`
`Page 7 of 40
`
`

`

`January 1980 Internet Protocol Overview 2o3~ Function Description The function or purpose of Internet Protocol is to move datagrams through an interconnected set of networks. This is done by passing the datagrams from one internet module to another until the destination is reached~ The internet modules reside in hosts and gateways in the internet system. The datagrams are routed from one internet module to another through individual networks based on the interpretation of an internet address° Thusr one important mechanism of the internet protocol is the internet address. In the routing of messages from one internet module to another, datagrams may need to traverse a network whose maximum packet size is smaller than the size of the datagram. To overcome this difficulty~ a fragmentation mechanism is provided in the internet protocol. Addressing A distinction is made between namesp addresses, and routes [3]. A name indicates what we seek° An address indicates where it is. A route indicates how to get there. The internet protocol deals primarily with addresses. It is the task of higher level (i.e., host-to-host or application) protocols to make the mapping from names to addresses, The inte~net module maps internet addresses to local net addresses. It is the task of lower level (i,e., local net or gateways) procedures to make the mapping from local net addresses to routes. Addresses are fixed length of four octets (32 bits). An address begins with a one octet network number, followed by a three octet local address. This three octet field is called the "rest ~' fieid~ Care must be taken in mapping internet addresses to local net addresses; a single physical host must be able to act as if it were several distinct hosts to the extent of using several distinct internet addresses. A host should also be able to have several physical interfaces (multi-homing). That is, a host should be allowed several physical interfaces to the network with each having several logical internet addresses. E --pl,~s of address mappings may be found in ~ference [4] Fr agmentat ion Fragmentation of an internet datagram may be necessary when it originates in a local net that allows a large packet size and must 19
`
`Page 8 of 40
`
`

`

`Internet Protocol Overview January 1980 traverse a local net that limits packets to a smaller size to reach its destination. An internet datagram can be marked "don't fragment." Any internet datagram so marked is not to be internet fragmented under any circumstances. If internet datagram marked don't fragment cannot be delivered to its destination without fragmenting it, it is to be discarded instead. Fragmentation, transmission and reassembly across a local network which is invisible to the internet protocol module is called intranet fragmentation and may be used [5]. The internet fragmentation and reassembly procedure needs to be able to break a datagram into an almost arbitrary number of pieces that can be later reassembled. The receiver of the fragments uses the identification field to ensure that fragments of different datagrams are not mixed. The fragment offset field tells the receiver the position of a fragment in the original datagram. The fragment offset and length determine the portion of the original datagram covered by this fragment. The more-fragments flag indicates (by being reset) the last fragment. These fields provide sufficient information to reassemble datagrams. The identification field is used to distinguish the fragments of one datagram from those of another. The originating protocol module of an internet datagram sets the identification field to a value that must be unique for that source-destination pair and protocol for the time the datagram will be active in the internet system. The originating protocol module of a complete datagram sets the more-fragments flag to zero and the fragment offset to zero. To fragment a long internet datagram, an internet protocol module (for example, in a gateway), creates two new internet datagrams and copies the contents of the internet header fields from the long datagram into both new internet headers. The data of the long datagram is divided into two portions on a 8 octet (64 bit) boundary (the second portion might not be an integral multiple of 8 octets, but the first must be). Call the number of 8 octet blocks in the first portion NFB (for Number of Fragment Blocks). The first portion of the data is placea in the first new internet datagram, and the total length field is set to the length of the first datagram. The more-fragments flag is set to one. The second portion of the data is placed in the second new internet datagram, and the total length field is set to the length of the second datagram. The more-fragments flag carries the same value as the long datagram. The fragment offset field of the second new internet 20
`
`Page 9 of 40
`
`

`

`January 1980 Internet Protocol Overview datagram is set to the value of that field in the long datagram plus NFB. This procedure can be generalized for an n-way split, rather than the two-way split described. To assemble the fragments o~ an internet datagram, an internet protocol module (for example at a destination host) combines internet datagram that all have the same value for the four fields: identification, source, destination, and protocol. The combination is done by placing the data portion of each fragment in the relative position indicated by the fragment offset in that fragment's internet header. The first fragment will have the fragment offset zero, and the last fragment will have the more-fragments flag reset to zero. 21
`
`Page 10 of 40
`
`

`

`January 1980 Internet Protocol 3o SPECIFICATION 3.1. Internet Header Format A summary of the contents of the internet header follows: 0 ! 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 ,F-+-+-+-+-+-+-+-+-+-+- +- +-+-+-+-+-+-+-+-+- +-+- +- +- +- +- +- +- +-+-+-+ jVersionl IHL IType of Service I Total Length +_+_+_+_+_+-+_+-+_ +-+_+-+_+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1 identification IFlags I Fragment Offset +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1 Time to Live I Protocol 1 lleader Checksum I- - + - + - + - + - + - +- + - + - I- - -~ - F -,F - ~ -+ - + - I- - + - ~I- - + - + - + - + - + - + - + - + - + - + - + - + - + - + I Source Address + - + - + - + -+ - + - + -~ - ~ -~ - ~ -+ -+ -~ -~ -+ -+ -+ -~ _ + - + - + - + - + - + - + - + - + - + -+ -+ - + - + 1 Destination Addcess -~- +- + - + - + -+- F -,F--F--F- + - F -+- F-F - + - F-+ -+ -+ - + - + - + - + - F - + -+ -F -+ -+ -+ - + I I ~adding I Opt ions i - + - + -+ - .~ _ + - + - + - + - ~ - .~ _ + - + - + - +-+ - + - + - + - + - + - + - + - + - + - + -+ - + ,-. ~ - + - _~ _ .~ _ + Example internet Datagram Header Figure 3. Note that each ~;~ku~ mark represents one bit position. Version: 4 bits The Version field indicates the format of the internet headers document descEioes version 4. L,. ~1 1 S rHL:.~ 4 bits Internet Header Length is the length of the internet header in 32 bit words~ and thus points to the beginning of the data. Note that the minimum value for a co~-rect header is 5. 22
`
`Page 11 of 40
`
`

`

`Internet Protocol Specification January 1980 Type of Service: 8 bits The Type of Service provides an indication of the abstract parameters of the quality of service desired. These parameters are to be used to guide the selection of the actual service parameters when transmitting a datagram through a particular network. Several networks offer service precedence~ which somehow treats high precedence traffic as more important than other traffic. A few networks offer a Stream service, whereby one can achieve a smoother service at some cost. Typically this involves the reservation of resources within the network. Another choice involves a low-delay vs. high-reliability trade off. Typically networks invoke more complex (and delay producing) mechanisms as the need for reliability increases. Bits 0-2: Bit 3: Bits 4-5: Bit 6: Bits 7: Precedence. Stream or Datagram. Reliability. Speed over Reliability. Speed. 0 1 2 P RECEDENCE 3 4 5 ..... 4 STRM ] RELIABILITY ~ ..... 4 7 SPEED I ÷ PRECEDENCE lll-Flash Override ll0-Flash llX-Immediate 01X-Priority 00X-Routine STRM RELIABILITY i-STREAM ll-highest 0-DTGRM 10-higher 01-iower 00-1owest S/R SPEED 1-speed 1-high 0-rlblt 0-low The type of service is used to specify the treatment of the datagram during its transmission through the internet system. In the discussion (section 3.2) below~ a chart shows the relationship of the internet type of service to the actual service provided on the ARPANET, the SATNET, and the PRNET. Total Length: 16 bits Total Length is the length of the datagram, measured in octets, • d including internet header and data. This flel~ allows the length of a datagram tc be up to 65,535 octets. Such long datagrams are impractical for most hosts and networks. All hosts must be prepared to accept datagrams of up to 576 octets (whether they arrive whole 23
`
`Page 12 of 40
`
`

`

`January 1980 Internet Protoco~ Specificati0 or in fragments). It is recommended that hosts only send datagrams larger than 576 octets if they have assurance that the destination is prepared to accept the larger datagrams. The number 576 is selected to allow a reasonable sized data block to ] be transmitted in addition to the required header information. For example, this size allows a data block of 512 octets plus 64 header octets to fit in a datagram. The maximal internet header is 60 octets, and a typical internet header is 20 octets, allowing a margin for headers of higher level protocols. Identification: 16 bits An identifying value assigned by the sender to aid in assembling the fragments of a datagram. Flags: 3 bits Various Control Flags. Bit 0: reserved, must be zero Bit i: Don't Fragment This Datagram (DF). Bit 2: More Fragments Flag (MF). 0 1 2 +---+___+___+ +---+---+___+ Fragment Offset: 13 bits This field indicates where in the datagram this fragment belongs. The fragment offset is measured in units of 8 octets (64 bits). The first fragment has offset zero. Time to Live: 8 bits This field indicates the maximum time the datagram is allowed to remain the internet system. If this field contains the value zero, then the datagram should be destroyed. This field is modified in internet header processing. The time is measured in units of seconds. The intention is to cause undeliverable datagrams to be discarded. 24
`
`Page 13 of 40
`
`

`

`Internet Protocol Specification January 1980 Protocol: 8 bits This field indicates the next level protocol used in the data portion of the internet datagram. The values for various protocols are specified in reference [6]. Header Checksum: 16 bits A checksum on the header only. Since some header fields may change (e.g., time to live), this is recomputed and verified at each point that the internet header is processed. The checksum algorithm is: The checksum field is the 16 bit one's complement of the one's complement sum of all 16 bit words in the header. For purposes of computing the checksum, the value of the checksum field is zero. This is a simple to compute checksum and experimental evidence indicates it is adequate, but it is provisional and may be replaced by a CRC procedure, depending on further experience. Source Address: 32 bits The source address. The first octet is the Source Network, and the following three octets are the Source Local Address. Destination Address: 32 bits The destination address. The first octet is the Destination Network, and the following three octets are the Destination Local Address. 25
`
`Page 14 of 40
`
`

`

`January 1980 l Internet Protocol Specification i Options: variable The option field is variable in length. options. Case i: Case 2: There may be zero or more There are two cases for the format of an option: A single octet of option-type. An option-type octet, an option-length octet, and the actual option-data octets. The option-length octet counts the option-type octet and the option-length octet as well as the option-data octets. The option-type octet is viewed as having 3 fields: 1 bit reserved, must be zero 2 bits option class, 5 bits option number. The option classes are: 0 = control 1 = internet error 2 = experimental debugging and measurement 3 = reserved for future use 26
`
`Page 15 of 40
`
`

`

`Internet Protocol Specification January 1980 The following internet options are defined: CLASS NUMBER LENGTH DESCRIPTION 0 0 - 0 1 - 0 2 4 0 3 var o 0 7 var. 0 8 4 1 1 var. 2 4 6 2 5 6 End of Option lists This option occupies only 1 octet; ~t has no length octet. No Operation° This option occupies only 1 octet; it has no length octet. Security. Used to carry Security~ and user group (TCC) information compatible with DOD requirements. Source Routing. Used tc route the internet datagram based on information supplied by the source. Return Route. Used to record the route an internet datagram takes. Stream ID. Used to carry the stream identifier. General Error Report. Used to report errors in internet datagram processing. Internet Timestamp. Satellite Timestamp. Specific Option Definitions End of Option List looooooool Type=0 This option indicates the end of the option list. This might not coincide with the end of the internet header according to the internet header length. This is used at the end of all options, not the end of each option, and need only be used if the end of the options would not otherwise coincide with the end of the internet header. May be copied, introduced, or deleted on fragmentation. 27
`
`Page 16 of 40
`
`

`

`January 1980 Internet Protocol , Specification No Operation I000000011 ÷ + Type=l This option may be used between options, for example, to align the beginning of a subsequent option on a 32 bit boundary. May be copied, introduced, or deleted on fragmentation. Security This option provides a way for DOD hosts to send security and TCC (closed user groups) parameters through networks whose transport leader does not contain fields for this information° The format for this option is as follows: loooooololOOOOOlOOlOOOOOOSSl Type=2 Length=4 TCC Security: 2 bits Specifies one of 4 levels of security ll-top secret 10-secret 01-confidential 00-unclassified Transmission Control Code: 8 bits Provides a means to compartmentalize traffic and define controlled communities of interest among subscribers. Note that this option does not require processing by the internet module but does require that this information be passed to higher level protocol modules. The security and TCC information might be used to supply class level and compartment information for transmitting datagrams into or through AUTODIN II. Must be copied on fragmentation. 28
`
`Page 17 of 40
`
`

`

`Internet Protocol Specification January 1980 Source Route -+ ÷ t + // + 1000000111 length I source route I -~ + t + // + Type=3 The source route option provides a means for the source of an internet datagram to supply routing information to be used by the gateways in forwarding the datagram to the destination. The option begins with the option type code. The second octet is the option length which includes the option type code and the length octet, as well as length-2 octets of source route data. A source route is composed of a series of internet addresses. Each internet address is 32 bits or 4 octets. The length defaults to two, which indicates the source route is empty and the remaining routing is to be based on the destination address field. If the address in destination address field has been reached and this option's length is not two, the next address in the source route replaces the address in the destination address field, and is deleted from the source route and this option's length is reduced by four. (The Internet Header Length Field must be changed also.) Must be copied on fragmentation. Return Route + ........ 4 + + II + I000001111 length I return route I 4 + t + II + Type=7 The return route option provides a means to record the route of an internet datagram. The option begins with the option type code. The second octet is the option length which includes the option type code and the length octet, as well as length-2 octets of return route data. A return route is composed of a series of internet addresses. The length defaults to two, which indicates the return route is empty. 29
`
`Page 18 of 40
`
`

`

`January 1980 Internet Protocol Specification When an internet module routes a datagram it checks to see if the return route option is present° If it is~ it inserts its own internet address as known in the environment into which this datagram is being forwarded into the return route at the front of the address string and increments the length by four° Not copied on fragmentationt goes in first fragment only. Stream Identifier 4 t ~- + F 100001000 100000010 I Stream ID I Type=8 Length=4 This option provides a way for the 16-bit SATNET stream identifier to be carried through networks that do not support the stream concept. Must be copied on fragmentation. General Error Report 4 ) .I 4 .... II .... + id I I ) ÷ .... // .... + IOOlOOOOll length lerr code I 4 ! t 4 Type=33 The general error report is used to report an error detected in processing an internet datagram tc the source internet module of that datagram. The "err code" indicates the type of error detected, and the "id" is copied from the identification field of the datagram in error, additional octets of error information may be present depending on the err code. If an internet datagram containing the general error report option is found to be in error or must be discarded, no error report is sent. ERR CODE: 0 - Undetermined Error, used when no information is available about the type of error or the error does not fit any defined class. Following the id should be as much of the datagram (starting with the internet header) as fits in the option space. 1 - Datagram Discardeds used when specific information is 3o
`
`Page 19 of 40
`
`

`

`Internet Protocol Specification January 1980 available about the reason for discarding the datagram can be reported° Eollowing the id should be the original (4-octets) destination address~ and the (1-octet) reason. Reason Description 2 3 No Reason No One Wants It - No higher level protocol or application program at destination wants this datagram. Fragmentation Needed & DF - Cannot deliver with out fragmenting and has don't fragment bit set. Reassembly Problem - Destination could not reassemble due to missing fragments when time to live expired. Gateway Congestion - Gateway discarded datagram due to congestion. The error report is placed in a datagram with the following values in the internet header fields: Version: Same as the datagram in error. IHL: As computed. Type of Service: Zero. Total Length: As computed. Identification: A new identification is selected. Flags: Zero. Fragment Offset: Zero. Time to Live: Sixty. Protocol: Same as the datagram in error, Header Checksum: As computed. Source Address: Address of the error reporting module. Destination Address: Source address of the datagram in error, Options: The General Error Report Option. Padding: As needed. Not copied on fragmentation, goes with first fragment. Internet Timestamp 4 # ~ ~ + --4 101000100[000001001 time in milliseconds I Type=68 Length=6 The data of the timestamp is a 32 bit time measured in milliseconds. 31
`
`Page 20 of 40
`
`

`

`January 1980 Internet Protocol Specification Not copied on fragmentation, goes with first fragment Satellite Timestamp 10100010lI000001001 time in milliseconds Type=69 Length=6 The data of the timestamp is a 32 bit time measured in milliseconds. Not copied on fragmentation, goes with first fragment Padding: variable The internet header padding is used to ensure that the internet header ends on a 32 bit boundary. The padding is zero. 3.2. Discussion The implementation of a protocol must be robust. Each implementation must expect to interoperate with others created by different individuals. While the goal of this specification is to be explicit about the protocol there is the possibility of differing interpretations. In general, an implementation should be conservative in its sending behavior, and liberal in its receiving behavior. That is, it should be careful to send well-formed datagrams, but should accept any datagram that it can interpret (e.g., not object to technical errors where the meaning is still clear). The basic internet service is datagram oriented and provides for the fragmentation of datagrams at gateways, with reassembly taking place at the destination internet protocol module in the destination host. Of course, fragmentation and reassembly of datagrams within a network or by private agreement between the gateways of a network is also allowed since this is transparent to the internet protocols and the higher-level protocols. This transparent type of fragmentation and reassembly is termed "network-dependent" (or intranet) fragmentation and is not discussed further here. Internet addresses distinguish sources and destinations to the host level and provide a protocol field as well. It is assumed that each protocol will provide for whatever multiplexing is necessary within a host. 32
`
`Page 21 of 40
`
`

`

`Internet Protocol Specification January 1980 Addressing The 8 bit network numberr which is the first octet of the address, has a value as specified in reference [6]. The 24 bit local address, assigned by the local network, should allow for a single physical host to act as several distinct internet hosts. That is, there should be mapping between internet host addresses and network/host interfaces that allows several internet addresses to correspond to one interface. It should also be allowed for a host to have several physical interfaces and to treat the datagrams from several of them as if they were all addressed to a single host. Address mappings between internet addresses and addresses for ARPANET, SATNET, PRNET, and other networks are described in reference [4]. Fragmentation and Reassembly. The internet identification field (ID) is used together with the source and destination address, and the protocol fields, to identify datagram fragments for reassembly. The More Fragments flag bit (MF) is set if the datagram is not the last fragment. The Fragment Offset field identifies the fragment location, relative to the beginning of the original unfragmented datagram. Fragments are counted in units of 8 octets. The fragmentation strategy is designed so than an unfragmented datagram has all zero fragmentation information (MF = 0, fragment offset = 0). If an internet datagram is fragmented, its data portion must be broken on 8 octet boundaries. This format allows 2"'13 = 8192 fragments of 8 octets each for a total of 65,536 octets. Note that this is consistent with the the datagram total length field. When fragmentation occurs, some options are copied, but others remain with the first fragment only. Every internet module must be able to forward a datagram of 68 octets without further fragmentation. This is because an internet header may be up to 60 octets, and the minimum fragment is 8 octets. Every internet destination must be able to receive a datagram of 576 octets either in one piece or in fragments to be reassembled. 33
`
`Page 22 of 40
`
`

`

`January 1980 E Internet Protocol[ Specification The fields which may be affected by fragmentation include: (i) options field (2) more fragments flag (3) fragment offset (4) internet header length field (5) total length field (6) header checksum If the Don't Fragment flag (DF) bit is set, then internet fragmentation of this datagram is NOT permittedr although it may be discarded. This can be used to prohibit fragmentation in cases where the receiving host does not have sufficient resources to reassemble internet fragments~ General notation in the following pseudo programs: "=<" means "less than or equal", "#" means "not equal", "=" means "equal", "<-" means "is set to". Also, "x to y" includes x and excludes y; for example, "4 to 7" would include 4r 5~ and 6 (but not 7)° Fragmentation Procedure The maximum sized datagram that can be transmit